{"time": 1739129378243, "blocks": [{"id": "Uoox6PWc1v", "data": {"text": "Introduction", "level": 2}, "type": "header"}, {"id": "F7cbCZ1fGx", "data": {"text": "Proliferating Cell Nuclear Antigen (PCNA) is a conserved nuclear protein best known as an essential co-factor for DNA replication and repair. Structurally, PCNA forms a ring-shaped sliding clamp as a homotrimer, encircling DNA to tether polymerases and other enzymes for processive DNA synthesis () (Human PCNA Structure, Function and Interactions). Discovered originally as an antigen in patients with lupus and later identified as a \u201ccyclin\u201d for its cell cycle-linked expression (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice), PCNA is now recognized as a master coordinator of genomic maintenance. It acts as a mobile platform, recruiting a vast network of proteins via the PCNA-interacting peptide (PIP) motif and related sequences (). PCNA\u2019s roles extend beyond replication; it participates in DNA damage tolerance, repair pathways, epigenetic inheritance, and even cellular metabolism and apoptosis regulation (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). In the past five years, research has uncovered new PCNA functions and innovative applications across disciplines \u2013 from cancer biology and neurobiology to drug discovery. This review provides a PhD-level, detailed examination of PCNA\u2019s current uses, biological functions, and the latest discoveries (2019\u20132024), along with critical analysis and future perspectives. Key focus areas include PCNA\u2019s utility in various research/industry applications, its molecular and cellular functions, recent breakthroughs, controversies in the field, and emerging therapeutic opportunities. All discussions are supported by citations from recent peer-reviewed literature."}, "type": "paragraph"}, {"id": "_kcOnN0O5E", "data": {"text": "Applications and Uses of PCNA in Research and Industry", "level": 2}, "type": "header"}, {"id": "kQA-NezMzN", "data": {"text": "PCNA in Cancer Research", "level": 3}, "type": "header"}, {"id": "UuKlb1uq2I", "data": {"text": "Biomarker for Proliferation and Prognosis: PCNA is widely used as a proliferation marker in tumor biology. Its expression correlates with the G_1_/S transition and S-phase entry of the cell cycle (Ki-67 and PCNA expression in prostate cancer and benign prostatic hyperplasia - PubMed). Because most malignant cells are highly proliferative, PCNA immunohistochemistry (IHC) in tumor tissue serves as an indicator of growth fraction (Ki-67 and PCNA expression in prostate cancer and benign prostatic hyperplasia - PubMed). Elevated PCNA labeling indices are observed in many cancers (e.g. breast, prostate) and often associate with advanced disease stage (Proliferating cell nuclear antigen directly interacts with androgen ...). For instance, prostate carcinomas show significantly higher PCNA expression than benign tissue (Ki-67 and PCNA expression in prostate cancer and benign prostatic hyperplasia - PubMed). Clinically, however, PCNA vs. Ki-67 as prognostic markers has been debated. Ki-67 is present in all active phases (G_1_-M) but absent in G_0, whereas PCNA marks cells in G_1/S and can persist in recently divided or repair-active cells () (Comparison of the value of PCNA and Ki-67 as markers of cell ...). As a result, PCNA-based proliferation counts are often higher (broader) than Ki-67 counts (). Some studies report Ki-67 correlating more strongly with patient outcomes than PCNA, suggesting PCNA\u2019s longer half-life or role in DNA repair might label non-dividing cells (). Thus, while PCNA is a convenient and sensitive tumor proliferation marker, careful interpretation is needed to distinguish true growth activity from repair-associated expression. Nevertheless, the ubiquitous overexpression of PCNA in cancers is well documented (Proliferating cell nuclear antigen directly interacts with androgen ...), and it remains a useful research tool for assessing tumor aggressiveness and cell cycle dynamics."}, "type": "paragraph"}, {"id": "N5Oq_-I57a", "data": {"text": "PCNA as a Therapeutic Target in Oncology: Beyond its use in pathology, PCNA has gained attention as a drug target in cancer research. Because PCNA is \u201cindispensable to cancer cell growth and survival\u201d (), disrupting PCNA function can inhibit tumor cell proliferation. Traditionally considered \u201cundruggable\u201d due to its lack of enzymatic activity and essential role in normal cells (Targeting the \u201cUndruggable\u201d: Small-Molecule Inhibitors of ...), PCNA is now being tackled with novel approaches. One strategy exploits a purported cancer-specific PCNA isoform (caPCNA). Malkas et al. reported that tumor cells express a unique, more acidic form of PCNA, distinct from the major form in normal cells () (). This cancer-associated PCNA exposes an antigenic region (amino acids 126\u2013133) that is normally hidden () (). A cell-permeable peptide mimicking this region was shown to selectively block PCNA\u2019s protein interactions and kill cancer cells without harming normal cells () (). Building on this, Gu et al. (2018) identified a small-molecule PCNA inhibitor (AOH1160) targeting that same pocket. AOH1160 interferes with DNA replication, blocks homologous recombination repair, and induces apoptosis in a broad range of cancer cell types () (). In mouse models, it suppressed tumor growth with minimal toxicity at effective doses (). These promising preclinical results highlight PCNA\u2019s potential as a broad-spectrum anti-cancer target () (). Indeed, recent Phase I trials have begun evaluating PCNA-targeted therapies, including isoform-specific inhibitors (). Another approach uses an APIM-peptide (from the AlkB PCNA-interacting motif) to disrupt PCNA functions, which has also shown selective toxicity to cancer cells and reached clinical development (). Additionally, monoclonal antibodies against PCNA on the cell surface are under investigation to boost immune attack on tumors () (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...). It was a surprising discovery that PCNA can be externalized on tumor cell membranes and act as a ligand for the NK cell receptor NKp44, thereby inhibiting natural killer (NK)\u2013mediated cytotoxicity () (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...). Blocking this PCNA\u2013NKp44 checkpoint with an antibody enhanced NK cell killing of multiple myeloma in preclinical studies (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...) (Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to ...). This finding broadens PCNA\u2019s therapeutic relevance from a tumor-intrinsic target to an immunotherapy target. In summary, emerging applications in cancer research range from using PCNA as a diagnostic/prognostic marker to targeting PCNA for therapy \u2013 a paradigm shift made possible by identifying cancer-selective vulnerabilities of an otherwise ubiquitous protein."}, "type": "paragraph"}, {"id": "1l540zlRrJ", "data": {"text": "Metabolism and Other Emerging Roles in Cancer Cells: New research suggests PCNA may contribute to cancer cell survival in unconventional ways. For example, PCNA has been found in the cytoplasm of certain cancer cells, where it associates with glycolytic enzymes and regulatory proteins () (). Ohayon et al. (2016) showed that chemotherapy-resistant leukemia cells accumulate PCNA in the cytosol via CRM1-dependent nuclear export (). Cytosolic PCNA was found to bind to nicotinamide phosphoribosyltransferase (NAMPT) and other metabolic enzymes, forming a scaffold that upregulated glycolysis and promoted cell survival under drug stress () (). In resistant HL-60 leukemia cells, the PCNA\u2013NAMPT interaction increased NAD^+ levels, Hexokinase-1 expression, and lactate production, effectively boosting glycolysis and conferring a growth advantage () (). This novel link between PCNA and metabolism suggests that beyond DNA transactions, PCNA can facilitate oncogenic metabolic reprogramming in the cytoplasm. Such insights open new application areas \u2013 for instance, PCNA could be explored as a biomarker or target for metabolic vulnerabilities in cancer cells. Another emerging use of PCNA is as a surface marker for circulating tumor cells or aggressive cancer subpopulations, given reports of PCNA on the cell membrane in some malignancies (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). While still under investigation, these findings underscore PCNA\u2019s multifaceted roles in cancer biology and encourage its integration into diverse research angles: from metabolic targeting and drug resistance studies to immuno-oncology."}, "type": "paragraph"}, {"id": "htCgKnpX09", "data": {"text": "PCNA in DNA Repair and Genomic Stability Research", "level": 3}, "type": "header"}, {"id": "jJkso5ILku", "data": {"text": "PCNA is a cornerstone of DNA repair research, frequently employed to probe repair mechanisms in both academic and industry settings. As a sliding clamp that interacts with numerous DNA repair proteins, PCNA\u2019s behavior often serves as a readout for repair pathway activation. For example, the appearance of PCNA foci in nuclei is used to mark sites of active DNA replication or repair synthesis in cell imaging assays. PCNA is required for essentially all major DNA repair pathways, making it a functional marker for repair efficacy. In nucleotide excision repair (NER), PCNA helps DNA polymerases fill in the gap after excision of lesions. In base excision repair (BER), PCNA tethers DNA polymerase \u03b4/\u03b5 and flap endonuclease 1 (FEN1) for long-patch repair synthesis () (). In mismatch repair (MMR), PCNA interacts with MutS\u03b1/MutL\u03b1 homologs to coordinate repair of replication errors, and its loading onto DNA is required for strand directionality during MMR. PCNA is also vital for homologous recombination (HR) repair of double-strand breaks: it gets loaded at the 3\u2032 end of the invading strand to recruit Pol \u03b4 for extending the D-loop, thereby initiating recombination-associated DNA synthesis () (). Researchers often manipulate PCNA or use PCNA mutants to dissect these pathways. For instance, mutating the PCNA ubiquitination site (K164) in cell lines is a common strategy to study DNA damage tolerance, since K164 modifications channel repair toward either translesion synthesis or template switching (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). In vitro, recombinant PCNA, polymerases, and replication factor C (RFC) are assembled in reconstituted systems to directly test PCNA\u2019s requirement for repair DNA synthesis () (\nPCNA is required for initiation of recombination-associated DNA synthesis by DNA polymerase \u03b4 - PMC\n). Such biochemical assays are widely used in the DNA replication/repair industry (e.g. for screening inhibitors of replication or for mechanistic enzyme studies)."}, "type": "paragraph"}, {"id": "TkD-W065GS", "data": {"text": "Crucially, PCNA\u2019s post-translational modifications (PTMs) serve as molecular switches between repair pathways, and these are intensively studied for their role in genome stability (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). Over the last five years, high-throughput approaches (mass spectrometry, CRISPR screens) in both academia and pharma have been applied to identify novel PCNA modifications and interaction partners that influence DNA repair fidelity. For example, studies have confirmed that ubiquitination of PCNA on K164 is a master regulator of post-replication repair: monoubiquitinated PCNA triggers error-prone translesion DNA synthesis, whereas K164 polyubiquitination promotes an error-free template switching mechanism (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). Conversely, SUMOylation of PCNA at K164 (in yeast) recruits anti-recombinase proteins to suppress inappropriate HR (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). Newer PTMs like neddylation and ISGylation of PCNA have been identified, adding complexity to PCNA\u2019s regulation of repair vs. replication processes (Human PCNA Structure, Function and Interactions). These findings are not just academic \u2013 they have practical implications. In the pharmaceutical context, if aberrant PCNA PTMs are linked to chemotherapy resistance (as some studies suggest (Post-Translational Modifications of PCNA in Control of DNA Synthesis and DNA Damage Tolerance-the Implications in Carcinogenesis) (Post-Translational Modifications of PCNA in Control of DNA Synthesis and DNA Damage Tolerance-the Implications in Carcinogenesis)), then PCNA or its modifying enzymes become targets to sensitize tumors to DNA-damaging treatments. Indeed, the concept of synthetic lethality is being applied: one recent study showed that inhibiting the deubiquitinase USP1 in BRCA-deficient cancer cells causes lethal accumulation of ubiquitinated PCNA, pointing to a therapeutic strategy exploiting PCNA-mediated repair addiction ([PDF] USP1 inhibitor synthetic lethality in BRCA1/2-mutant cancer is ...) ([PDF] USP1 inhibitor synthetic lethality in BRCA1/2-mutant cancer is ...). In summary, PCNA is central to DNA repair research both as a marker (for replication stress, repair foci, etc.) and as a manipulable factor to study genome maintenance. Its nearly universal involvement in repair pathways means that PCNA-related assays (e.g. PCNA recruitment, PCNA modification status) are now standard in evaluating genome stability, whether in academic experiments or in industry screens for DNA repair modulators."}, "type": "paragraph"}, {"id": "tRYcv3-_RG", "data": {"text": "PCNA in Neurobiology", "level": 3}, "type": "header"}, {"id": "gJshIOJOhn", "data": {"text": "In neurobiology, PCNA finds utility primarily as an index of cell proliferation in neural tissues. Although most mature neurons are post-mitotic, neural stem cells and progenitors in the developing or adult brain proliferate, and PCNA is a key marker to identify these dividing cells. For example, in studies of adult neurogenesis, PCNA-positive cells in the subventricular zone (SVZ) or hippocampal dentate gyrus indicate neural stem or progenitor cells in active cell cycle (). PCNA immunostaining is commonly combined with markers like SOX2 (stem cell marker) or NeuN (neuronal marker) to quantify how many neural stem cells or immature neurons are proliferating. Because PCNA is expressed throughout the cell cycle (except true quiescence) (), it labels a broader range of cycling neural cells than S-phase\u2013specific markers like BrdU or than mitotic markers like PH3. In fact, both PCNA and Ki-67 are used to label dividing cells in brain sections, but PCNA-positive populations can be slightly larger, reflecting cells that might be in early G_1_ or recently exited the cycle (). This makes PCNA a useful tracer for neurogenesis, especially for capturing early progenitors that might be missed by S-phase tracers. Researchers globally have used PCNA to study brain development (e.g. mapping proliferative zones in embryonic brain) and to examine regenerative responses. For instance, after brain injury or in neurodegenerative conditions, an increase in PCNA-positive cells can signify a reactive proliferation or attempted repair by glial cells and neural progenitors (Increased cell proliferation and neurogenesis in the adult human ...) (Molecular Biomarkers for Embryonic and Adult Neural Stem Cell ...). In one recent human study of neurodegenerative disease, PCNA upregulation in certain brain regions suggested reactivated cell cycle entry in neurons or glia (Increased cell proliferation and neurogenesis in the adult human ...), a phenomenon often linked to pathology. In Alzheimer\u2019s disease models, evidence of neurons aberrantly expressing PCNA and other cell cycle markers (so-called abortive re-entry) has been documented, indicating a mechanistic link between cell cycle activation and neuronal death (Full article: Neuronal cell cycle: the neuron itself and its circumstances). Thus, PCNA serves as a window into unusual neurobiology cases where normally quiescent cells re-enter the cycle."}, "type": "paragraph"}, {"id": "2zS2ynnqRi", "data": {"text": "Beyond proliferation, PCNA also has implications in neuronal DNA repair and survival. Post-mitotic neurons accumulate DNA damage over time, and efficient repair is critical for their longevity. PCNA is expressed in neurons at low levels for DNA repair tasks such as long-patch BER. Notably, in models of neurodegenerative diseases like Parkinson\u2019s, PCNA has been found to play a protective role. Li et al. (2016) reported that oxidative stress (MPP^+ exposure) in dopaminergic neuron-like cells led to a downregulation of PCNA, correlating with cell death (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed) (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed). They showed that p53 activation (which can repress PCNA transcription) mediated this effect, and proposed that loss of PCNA removed critical DNA repair support, exacerbating neuron loss (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed) (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed). This suggests that maintaining PCNA levels in neurons might be neuroprotective under stress conditions, by preserving DNA repair capacity. Indeed, another study found that a neuroprotective compound (\u03b1-lipoic acid) upregulated PCNA via p53 suppression, thereby aiding neuronal survival against toxins (\u03b1-lipoic acid exerts neuroprotective effects on neuronal cells by ...). These findings hint at therapeutic angles in neurobiology: drugs that modulate PCNA or its regulators in neurons could potentially enhance repair and resilience in neurodegenerative disorders. It\u2019s an emerging concept that even non-dividing cells depend on \u201creplication\u201d proteins like PCNA for genome maintenance."}, "type": "paragraph"}, {"id": "wOutHvpi-P", "data": {"text": "In summary, PCNA\u2019s main use in neurobiology is as a marker of cell proliferation in neural stem cells and a tool to study neurogenesis or pathological cell cycle events. Emerging research also ties PCNA to neuron survival and DNA repair, raising the prospect of targeting PCNA pathways to mitigate neurodegeneration. While not as extensively leveraged as in oncology or DNA repair fields, PCNA\u2019s presence across the brain\u2019s proliferative and repair processes makes it an important cross-disciplinary molecule for neuroscientists and neurologists."}, "type": "paragraph"}, {"id": "jPXy6JKvxV", "data": {"text": "PCNA in Drug Discovery and Biotechnology", "level": 3}, "type": "header"}, {"id": "0Sk02xHaXK", "data": {"text": "The prominent role of PCNA in essential cellular processes makes it an attractive yet challenging molecule in drug discovery. In cancer drug development, as discussed, PCNA is being actively explored as a therapeutic target. Pharmaceutical companies and research institutes are using structure-guided design and high-throughput screening to find small molecules or peptides that disrupt PCNA\u2019s interactions. For example, Gonzalez-Maga\u00f1a et al. (2020) highlighted that targeting the PCNA\u2013PIP box interface with small molecules can inhibit binding of key partners like POLD1 or p21, thus blocking DNA replication (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). Peptide mimetics of the PIP-box have been engineered as well (Human PCNA Structure, Function and Interactions), although a concern is that blocking the promiscuous PIP-binding pocket might affect many pathways (Human PCNA Structure, Function and Interactions). Therefore, newer strategies focus on specific PCNA modifications or unique binding modes. For instance, compounds that prevent PCNA\u2019s Y211 phosphorylation by EGFR effectively suppressed growth of prostate cancer cells (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions), indicating that modulating a single PCNA PTM can have therapeutic benefits. Similarly, the development of the caPCNA-specific agent AOH1160 shows that conformation-specific targeting of PCNA is feasible () (\nThe anti-cancer activity of a first-in-class small molecule targeting PCNA - PMC\n). This concept of selective targeting\u2014going after a tumor-specific form of PCNA or a particular PCNA\u2013protein complex\u2014is guiding current drug discovery efforts. Notably, at least one PCNA inhibitor peptide (ATX-101, an APIM peptide) has entered clinical trials for cancer, reflecting a maturation of this approach from bench to bedside."}, "type": "paragraph"}, {"id": "kprM5dDm2p", "data": {"text": "In addition to anti-cancer drug discovery, PCNA features in screening assays for genotoxic compounds and radioprotectors. Biotech companies employ PCNA as a readout in high-content screening: for example, compounds that cause PCNA to form nuclear foci can be flagged as DNA damage inducers (since PCNA accumulates at stalled replication forks). Conversely, maintaining PCNA diffusely nuclear during replication stress might indicate a compound that mitigates fork stalling. Some screens measure PCNA ubiquitination levels as a proxy for activation of DNA damage tolerance pathways ([PDF] USP1 inhibitor synthetic lethality in BRCA1/2-mutant cancer is ...). This can be used to identify molecules that either promote or inhibit PCNA\u2019s role in tolerating DNA lesions \u2013 with potential applications in cancer (sensitizing tumors to DNA damage) or in anti-aging (enhancing repair in normal cells). In the materials science or enzymology domain, recombinant PCNA protein is utilized in in vitro DNA replication systems (such as rolling circle DNA synthesis assays) to test DNA polymerase inhibitors or DNA-editing tools. Since PCNA is required for processive replication, including it in in vitro assays makes them more reflective of cellular conditions."}, "type": "paragraph"}, {"id": "yKBmbu9Wr1", "data": {"text": "Another niche application of PCNA is in the field of antimicrobial drug discovery. PCNA homologs exist in parasites and fungi, and differences in their sequences can be exploited. A recent comparative study of fungal PCNA sequences suggested the possibility of identifying epitope differences to develop antifungal agents targeting PCNA (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). While still speculative, this hints that PCNA-targeted drug discovery might extend beyond cancer, into infectious diseases (by inhibiting pathogen PCNA) or even inflammatory disorders (by modulating PCNA\u2019s function in immune cells, as described below)."}, "type": "paragraph"}, {"id": "CHaHl8chJ2", "data": {"text": "Lastly, PCNA has entered the spotlight in immune and inflammation research, which has translational implications. As noted, an antibody targeting surface PCNA to enhance NK cell activity is a form of immunotherapy being explored (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...) (Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to ...). Meanwhile, PCNA\u2019s role in neutrophil longevity (cytosolic PCNA as a pro-survival factor) suggests that disrupting PCNA in immune cells could help resolve chronic inflammation by inducing timely neutrophil apoptosis () (\nTargeting cytosolic proliferating cell nuclear antigen in neutrophil-dominated inflammation - PMC\n). Thus, drug discovery programs aiming at PCNA are not limited to anti-proliferative drugs; they are branching into immunomodulators and anti-inflammatories as well."}, "type": "paragraph"}, {"id": "UQw67W2wi9", "data": {"text": "In summary, PCNA\u2019s applications in research and industry are diverse and expanding. From a lab marker of cell proliferation and DNA repair to a direct target in cutting-edge drug discovery (for cancer, immunity, and beyond), PCNA\u2019s utility is undeniable. This breadth of application underscores why PCNA continues to be a focal point in multi-disciplinary research."}, "type": "paragraph"}, {"id": "SMhqLt3rCA", "data": {"text": "Biological Functions of PCNA", "level": 2}, "type": "header"}, {"id": "2aD1d79Nuu", "data": {"text": "PCNA as a Sliding Clamp for DNA Replication", "level": 3}, "type": "header"}, {"id": "Epn1vNB8QN", "data": {"text": "The canonical role of PCNA is as a processivity factor for DNA polymerases during replication. PCNA forms a homotrimeric ring structure (~30 kDa subunits) that encircles duplex DNA () (). This ring acts like a sliding platform that keeps DNA polymerases attached to the template, enabling rapid and processive DNA synthesis of thousands of nucleotides (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). The loading of PCNA onto DNA is mediated by the clamp loader complex (Replication Factor C, RFC), which opens the PCNA ring and places it at primer-template junctions in an ATP-dependent process (Human PCNA Structure, Function and Interactions). Once loaded, PCNA can freely slide along DNA. By tethering the replicative polymerases (Pol \u03b4 on the lagging strand and Pol \u03b5 on the leading strand in eukaryotes), PCNA dramatically increases their efficiency and speed. Without PCNA, polymerases frequently fall off the DNA, drastically reducing replication speed (Human PCNA Structure, Function and Interactions). Thus, PCNA is essential for timely and complete genome duplication."}, "type": "paragraph"}, {"id": "eRhYBpYceE", "data": {"text": "Structurally, PCNA\u2019s ring is characterized by a pseudo-six-fold symmetry: each monomer has two similar domains, and three monomers assemble head-to-tail (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). The inner face of the ring is lined with positively charged residues (\u03b1-helices) that interact with DNA, while the outer face is mostly \u03b2-sheet and contains flexible loops called interdomain connecting loops (IDCL) () (). The IDCL on the outer surface serves as a primary docking site for other proteins, as discussed below. The ability of PCNA to slide has two modes: a straightforward sliding along the DNA helix axis and a slight helical rotation or \u201ccogwheel\u201d motion that can engage with DNA grooves (Human PCNA Structure, Function and Interactions). Recent single-molecule and NMR studies suggest that PCNA\u2019s sliding dynamics can be modulated by factors like post-translational modifications or binding partners (for example, acetylation of PCNA was reported to favor pure sliding, whereas binding of certain partners like the p15^PAF^ protein can tighten PCNA on DNA and alter its movement) (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). These nuances imply that PCNA is not just a passive ring but an actively regulated platform."}, "type": "paragraph"}, {"id": "3yWPUKH-6Q", "data": {"text": "During lagging strand synthesis, PCNA is continuously loaded and unloaded: each Okazaki fragment requires PCNA to recruit Pol \u03b4, then after fragment completion, PCNA helps recruit FEN1 and DNA ligase I for Okazaki maturation (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). PCNA may then be unloaded (by an unloadase like ATAD5) or transferred to the next fragment. On the leading strand, PCNA stays associated with Pol \u03b5 for long stretches. Coordination between leading and lagging strand replication is partly achieved by the fact that PCNA can hold multiple proteins at once. The trimeric nature means potentially three molecules of a partner can bind, one on each monomer, or one partner can occupy one protomer while another different partner binds another protomer (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). This is often referred to as the \u201ctoolbelt\u201d model of PCNA, wherein it can simultaneously tether, say, a polymerase and a nuclease, facilitating rapid hand-off from synthesis to processing. A classic example is during lagging strand maturation: PCNA can bind Pol \u03b4 and FEN1 concurrently, so when Pol \u03b4 finishes synthesis, FEN1 is right there to trim the flap, followed by ligase I () (\nThe anti-cancer activity of a first-in-class small molecule targeting PCNA - PMC\n). This coordination is crucial for efficiency and explains why disrupting PCNA dramatically stalls replication and cell proliferation."}, "type": "paragraph"}, {"id": "mUVI9Ko4m1", "data": {"text": "In summary, PCNA\u2019s fundamental function in replication is to ensure high-speed, high-fidelity DNA synthesis by acting as a sliding clamp that both tethers enzymes to DNA and coordinates the exchange of factors at the replication fork. This role is so central that depletion or mutation of PCNA is lethal to dividing cells and underlies why PCNA is essential in all proliferating tissues."}, "type": "paragraph"}, {"id": "YLZDsw6bLG", "data": {"text": "PCNA in DNA Damage Tolerance and Repair Mechanisms", "level": 3}, "type": "header"}, {"id": "OJQlUO86Ar", "data": {"text": "Beyond normal replication, PCNA is a pivotal player in DNA damage tolerance (DDT) and repair processes. When replication forks encounter DNA lesions, cells employ PCNA-mediated pathways to tolerate or repair the damage, thereby preventing fork collapse. Two main damage tolerance pathways are regulated by ubiquitination of PCNA\u2019s Lys-164 residue (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice):"}, "type": "paragraph"}, {"id": "_MvBusiThr", "data": {"items": ["Translesion Synthesis (TLS): Monoubiquitination of PCNA at K164 by the Rad6/Rad18 ubiquitin ligase complex triggers the recruitment of specialized TLS polymerases (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). These polymerases (e.g. Pol \u03b7, Pol \u03b9, Pol \u03ba) can synthesize DNA across damaged templates (such as thymine dimers or abasic sites) but are error-prone. The monoubiquitin mark on PCNA creates a binding site (via ubiquitin-binding domains in TLS polymerases) that allows them to replace the replicative polymerase on the clamp. This polymerase switching is often called the \u201cPCNA switch.\u201d It enables replication to continue past lesions at the cost of potential mutations. Once the lesion is bypassed, the ubiquitin can be removed (e.g. by USP1/UAF1 deubiquitinase) to terminate TLS.\n", "Template Switching (TS): If the K164 on PCNA is further modified with a Lys63-linked polyubiquitin chain (via Ubc13-Mms2 and Rad5 in yeast, or HLTF/SHPRH in mammals), an alternative error-free mechanism is employed (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). Template switching uses the newly synthesized sister chromatid as a template to bypass the lesion (for example, through fork regression or strand exchange), effectively copying the correct information from the sister strand over the lesion. This is akin to a homologous recombination event, though short-track and without forming a double-strand break. Polyubiquitinated PCNA is a key signal for this process, though the exact mechanism (especially how the polyubiquitin chain length and timing are controlled) is still being elucidated. Notably, fork reversal and template switch are complex and \u201cstill poorly understood\u201d mechanistically (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). It is known, however, that these modifications channel the cell\u2019s choice between risking mutations (TLS) or taking a conservative approach (TS).\n"], "style": "unordered"}, "type": "list"}, {"id": "A7KHP4Y0og", "data": {"text": "PCNA\u2019s role in repair pathways also extends to acting as a recruitment hub or scaffold:"}, "type": "paragraph"}, {"id": "A_OtAW5HSV", "data": {"items": ["In Mismatch Repair (MMR), PCNA interacts with the MutL\u03b1 complex (MLH1-PMS2 in humans) and possibly MutS\u03b1 (MSH2-MSH6) to help identify the newly synthesized (error-containing) strand, since PCNA marks the replication machinery\u2019s presence. PCNA is needed for the excision step of MMR and for resynthesis of the corrected strand. Additionally, PCNA binding by MSH6 (which has a PIP-box) greatly increases MMR efficiency. Interestingly, an abnormal modification of PCNA can affect MMR; for example, Y211-phosphorylated PCNA (by EGFR) was shown to inhibit mismatch repair and promote error-prone DNA synthesis (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions), explaining how oncogenic signaling might subvert repair fidelity.\n", "In Base Excision Repair (BER), particularly long-patch BER, PCNA is required to recruit Pol \u03b4/\u03b5 and FEN1 to synthesize and remove a patch of ~10 nucleotides, thereby replacing a damaged base that DNA polymerase \u03b2 alone (in short-patch BER) could not fix. PCNA\u2019s presence ensures that after the BER enzyme (like a glycosylase and AP endonuclease) nicks the DNA, the repair synthesis proceeds efficiently and the resulting flap is processed () (\nThe anti-cancer activity of a first-in-class small molecule targeting PCNA - PMC\n). Cells lacking functional PCNA exhibit severe defects in long-patch BER.\n", "In Nucleotide Excision Repair (NER), after a bulky lesion is excised leaving a gap of ~30 nt, PCNA is loaded onto the DNA to help Pol \u03b4/\u03ba fill in the gap and DNA ligase seal it. PCNA interacts with XPG and Fen1-like enzymes in some sub-pathways of NER, indicating a scaffolding role.\n", "In Double-Strand Break (DSB) Repair via Homologous Recombination, PCNA is not required for the initial strand invasion (mediated by Rad51), but once a D-loop forms, PCNA is loaded to attract Pol \u03b4 for repair DNA synthesis () (\nPCNA is required for initiation of recombination-associated DNA synthesis by DNA polymerase \u03b4 - PMC\n). Thus PCNA is crucial for extending the invading strand, a central step in HR. There\u2019s also evidence that PCNA might assist in late-stage DNA synthesis during the resolution of joint molecules in HR.\n", "In Non-Homologous End Joining (NHEJ), PCNA\u2019s role is less direct, as NHEJ is polymerase-independent except for some gap-filling by Pol \u03bb or Pol \u03bc. PCNA is generally not thought to be required in classical NHEJ, but it might play a role in backup repair synthesis during alternative end joining.\n"], "style": "unordered"}, "type": "list"}, {"id": "8MJoRSuqqH", "data": {"text": "Another facet of PCNA in repair is through its interaction with checkpoint and chromatin assembly factors. Stalled replication forks activate ATR-dependent checkpoints; PCNA has been implicated in recruiting checkpoint mediator proteins (like Claspin) and polymerase clamp PAFs that signal the fork status (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). Additionally, as replication and repair occur, new histones must be deposited \u2013 PCNA directly binds histone chaperone CAF-1, linking DNA synthesis to chromatin assembly (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). PCNA also recruits DNMT1 (maintenance DNA methyltransferase) to replication foci, ensuring epigenetic marks are copied after DNA replication (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). These connections mean PCNA is at the crossroads of DNA replication, repair, and epigenetic maintenance."}, "type": "paragraph"}, {"id": "pvLXHW-vGm", "data": {"text": "Given these roles, it\u2019s no surprise that dysregulation of PCNA\u2019s function or modifications can compromise genomic stability. If PCNA is not properly ubiquitinated, cells may fail to bypass lesions and accumulate DSBs or collapse forks. If PCNA remains ubiquitinated when it shouldn\u2019t, error-prone polymerases might cause mutagenesis. Likewise, hyper-phosphorylation or hypo-acetylation of PCNA could disturb the normal hand-off of repair proteins. A recent comprehensive review noted that aberrant regulation of PCNA PTMs (ubiquitination, SUMOylation, acetylation, etc.) can result in DSBs, mutagenesis, and poor response to chemotherapy (Post-Translational Modifications of PCNA in Control of DNA Synthesis and DNA Damage Tolerance-the Implications in Carcinogenesis) (Post-Translational Modifications of PCNA in Control of DNA Synthesis and DNA Damage Tolerance-the Implications in Carcinogenesis), underlining PCNA\u2019s importance in maintaining genome integrity and influencing treatment outcomes in cancer."}, "type": "paragraph"}, {"id": "IDPw6zZmMt", "data": {"text": "In summary, PCNA\u2019s biological function in DNA damage tolerance and repair is to serve as a versatile platform that directs the cell\u2019s response to DNA lesions \u2013 toggling between different bypass pathways and ensuring that the appropriate repair enzymes are recruited to restore DNA structure. It is a central \u201cringmaster\u201d of genome stability, coordinating the interplay between replication and repair pathways (Proliferating cell nuclear antigen (PCNA): ringmaster of the genome)."}, "type": "paragraph"}, {"id": "sn71_GcZ3J", "data": {"text": "PCNA in Cell Cycle Regulation and Apoptosis", "level": 3}, "type": "header"}, {"id": "W2L1NjBDOD", "data": {"text": "PCNA levels and activity are tightly linked to the cell division cycle. The expression of PCNA is low in quiescent (G_0) cells and rises as cells enter the cell cycle, with a notable increase at late G1 and peak during S-phase when DNA replication occurs (). This cell-cycle-dependent expression initially led to PCNA being termed a \u201ccyclin\u201d (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). PCNA gene transcription is regulated by E2F transcription factors, which are active when cells commit to divide. As a result, PCNA is often used as an endogenous marker of cells that are in cycle. The presence of PCNA indicates a proliferative state, and loss of PCNA (or its absence) is associated with cell cycle exit or terminal differentiation."}, "type": "paragraph"}, {"id": "1Xa9Eo_VxZ", "data": {"text": "PCNA itself, however, is not a classical signaling molecule for cell cycle transitions (like cyclins or CDKs), but it indirectly influences cell cycle progression through its interactions. A prime example is PCNA\u2019s interaction with the cyclin-dependent kinase (CDK) inhibitor p21^Cip1/Waf1. p21 contains a PIP-box and can bind tightly to PCNA (Human PCNA Structure, Function and Interactions). When p21 is upregulated (for instance by p53 in response to DNA damage), it binds PCNA and blocks PCNA\u2019s ability to recruit polymerases, effectively halting DNA replication. This is one mechanism by which p21 enforces a cell cycle arrest in G1 or S phase \u2013 by sequestering PCNA and preventing replication elongation. Structurally, p21\u2019s binding to PCNA overlaps with that of replication factors, and p21 can even form a ternary complex with PCNA, inhibiting its function (Human PCNA Structure, Function and Interactions). Thus, PCNA is a crucial target of CDK inhibitors for stopping the cell cycle in response to stress."}, "type": "paragraph"}, {"id": "xn6T2ZSEVB", "data": {"text": "PCNA may also affect cell cycle via controlling replication completion: if PCNA is not functional (say, PCNA is depleted or a dominant-negative PCNA is expressed), cells typically trigger a checkpoint and eventually undergo cell death or senescence due to incomplete DNA replication. In fact, studies have shown that if PCNA is absent or inactivated, apoptosis ensues in proliferating cells (Proliferating cell nuclear antigen (PCNA): ringmaster of the genome). This is logical, as cells cannot survive a replication catastrophe. Interestingly, the evolution of PCNA and its partners suggests that if replication is fatally compromised, apoptotic pathways are engaged, linking PCNA to cell fate decisions."}, "type": "paragraph"}, {"id": "1-0wk6cM5g", "data": {"text": "Beyond proliferating cells, PCNA has non-traditional roles in cell survival, especially in differentiated cells like neutrophils. Neutrophils are short-lived immune cells that do not divide, but PCNA has been found in their cytosol acting as a pro-survival factor (). During neutrophil differentiation, a portion of PCNA is exported to the cytoplasm () (). There, it forms a complex with proteins that restrain the apoptotic machinery. For example, PCNA binds to procaspases and to the protein Gadd45 in neutrophils. Gadd45 normally promotes apoptosis and cell cycle arrest, but when bound to PCNA, its pro-apoptotic activity is sequestered (Proliferating cell nuclear antigen (PCNA): a dancer with many partners). Likewise, PCNA\u2019s interaction with pro-apoptotic factors is thought to delay neutrophil apoptosis, extending their functional lifespan during inflammation () (). This unusual cytosolic function of PCNA illustrates how the protein can moonlight outside the nucleus: the PCNA apoptotic brake in neutrophils is actually being explored as a therapeutic target to resolve inflammation by accelerating neutrophil death () (\nTargeting cytosolic proliferating cell nuclear antigen in neutrophil-dominated inflammation - PMC\n). If an agent could disrupt PCNA\u2019s cytosolic complex in neutrophils, it might induce their apoptosis and reduce tissue-damaging inflammation (e.g. in chronic inflammatory diseases or cystic fibrosis where neutrophil persistence is a problem)."}, "type": "paragraph"}, {"id": "lnVx6dTOmi", "data": {"text": "In the context of apoptosis and DNA damage, PCNA interacts with p53 as well. p53 can repress PCNA transcription, as noted in neuronal studies where p53 activation led to PCNA downregulation and cell death (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed) (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed). Conversely, PCNA can bind the apoptosis regulator procaspase-3 (reported in some studies) and prevent its activation until PCNA is released, adding another layer to how PCNA might suppress apoptosis in certain contexts."}, "type": "paragraph"}, {"id": "JA0DiujJ83", "data": {"text": "In summary, while PCNA is not a classical cell cycle controller like cyclin/CDK, it interfaces with cell cycle and apoptotic regulators. Through p21 binding, PCNA is a direct node where cell cycle arrest mechanisms inhibit replication. Through its required role in replication, PCNA status dictates whether a cell can proceed through S-phase or will trigger checkpoint-mediated death. And intriguingly, in non-dividing cells, PCNA can adopt cytosolic roles that promote cell longevity (inhibiting apoptosis) by binding pro-death factors. These diverse influences place PCNA at the heart of decisions about cell proliferation and survival."}, "type": "paragraph"}, {"id": "n9196L6kKo", "data": {"text": "Interaction Network of PCNA", "level": 3}, "type": "header"}, {"id": "d0__x36rsG", "data": {"text": "PCNA\u2019s functionality is entirely tied to the extensive network of proteins it interacts with. PCNA is often likened to a \u201cdancer with many partners\u201d (), reflecting its ability to bind a wide array of proteins involved in DNA metabolism. The most common mode of interaction is via the PIP-box (PCNA-Interacting Protein box), a consensus sequence QXXhXXaa (where h is hydrophobic/aromatic, and a is an aliphatic hydrophobic residue) (). Proteins containing a PIP-box bind to a hydrophobic pocket on the front face of PCNA, adjacent to the IDCL. Examples of PIP-box proteins include the p21 inhibitor, DNA polymerase \u03b4 (p68 subunit has a PIP motif), FEN1, DNA ligase1, MSH6, XPG, and many others () (). When a PIP-box peptide binds PCNA, a \u03b2-hairpin from the protein inserts into PCNA\u2019s pocket, with three key hydrophobic residues (\u201chydrophobic trident\u201d) docking into the pocket (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). This mode is common but not identical for all partners \u2013 subtle differences exist. For instance, p21\u2019s C-terminus forms an extended \u03b2-sheet along PCNA\u2019s surface, whereas the recently characterized p15^PAF^ binds in a unique way, threading an N-terminal tail through the PCNA ring (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). In the p15\u2013PCNA structure, the PIP-box of p15 binds normally, but an adjacent region loops through the ring to contact the inner surface of PCNA (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). This unusual binding both occupies an extra site and allows one of PCNA\u2019s three subunits to still bind DNA, suggesting p15 can act as a \u201cbelt\u201d around DNA within the PCNA ring (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). This is a recent finding and shows how non-canonical PCNA interactions can modulate PCNA\u2019s function (in p15\u2019s case, possibly modulating replication speed or polymerase switching) (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions)."}, "type": "paragraph"}, {"id": "4FAmROw79n", "data": {"text": "Besides PIP-boxes, some proteins use alternative motifs to bind PCNA. One such motif is the APIM (AlkB homolog 2 PCNA interacting motif), with a consensus around KRKRxxxx. Proteins like DNA translocases and some helicases use APIM. Though APIM binds a similar region on PCNA, it might allow interaction under conditions where PIP-box proteins are limited, and APIM-containing peptides can compete for PCNA binding, which is the basis of the ATX-101 peptide therapy. KA-box is another motif (K-A-(any)-[IL]-X-[FY]) noted in a few proteins. Importantly, some high-affinity interactions (like that of the CDK inhibitor p21) can displace other weaker interactions (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). Cells exploit this: when p21 is expressed, its high affinity for PCNA (K_d in low nM range) means it will outcompete polymerases (which bind PCNA in micromolar range), thereby blocking replication (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). In contrast, when p21 is absent, PCNA\u2019s binding slots are occupied by replication and repair proteins as needed, based on their concentration and affinity."}, "type": "paragraph"}, {"id": "cGsAJZyIR5", "data": {"text": "PCNA can bind up to three molecules (one per monomer of the trimer). In vitro biophysical studies have shown that three identical PIP peptides can bind PCNA simultaneously with little cooperativity (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). However, inside a cell, it\u2019s unlikely for three of the same protein to occupy one PCNA ring due to spatial and regulatory constraints (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). More commonly, one or two different proteins share a PCNA trimer, forming a transient ternary complex. For example, during Okazaki fragment processing, a Pol \u03b4\u2013PCNA\u2013FEN1 complex can form. During TLS, a Rev1\u2013PCNA\u2013Pol\u03b7 complex is plausible (Rev1 and Pol\u03b7 both bind PCNA via distinct motifs). The competition and exchange are governed by affinities which can be modulated by PCNA\u2019s PTMs. For instance, ubiquitination of PCNA creates a new binding surface (for UBZ domains in TLS polymerases), effectively recruiting them and perhaps displacing replicative polymerase that lacks such a domain. SUMOylation of PCNA in yeast recruits the Srs2 helicase (via a SIM motif), which then prevents HR proteins from binding PCNA, illustrating a dynamic regulation by modification."}, "type": "paragraph"}, {"id": "JSNdftoVe1", "data": {"text": "PCNA interacts not only with enzymes of DNA replication/repair but also with other cellular proteins: chromatin modifiers (CAF-1, DNMT1 as mentioned), cell cycle regulators (p21, and maybe others like Cdt1 is inhibited by PCNA-mediated degradation, a process in which PCNA recruits a ubiquitin ligase to Cdt1), and transcription factors (some evidence suggests PCNA can bind certain transcriptional co-repressors or assist in gene silencing, possibly through chromatin structure maintenance)."}, "type": "paragraph"}, {"id": "icgOBeFGCQ", "data": {"text": "One remarkable interaction in certain cancer cells is PCNA\u2019s binding to the NK cell receptor NKp44 \u2013 but in that case, it is the extracellular PCNA on tumor cells binding to NKp44 on immune cells (Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to ...) (Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to ...). While not a typical intracellular interaction, it broadens the scope of PCNA\u2019s \u201cinteractome\u201d to include cell\u2013cell interactions in the tumor microenvironment, effectively making PCNA a ligand in an immune inhibitory synapse."}, "type": "paragraph"}, {"id": "MZK6c9xroj", "data": {"text": "In summary, PCNA\u2019s interaction network is expansive and tightly coordinated. It includes:"}, "type": "paragraph"}, {"id": "pPTy61MceP", "data": {"items": ["Replication machinery: Pol \u03b4/\u03b5, RFC, primase (through p68), FEN1, ligase1.", "Damage bypass and repair proteins: TLS Pols (\u03b7, \u03b9, \u03ba, Rev1), HR factors (Rad18, SLX4 complex subunits with PIP), MMR factors (MSH6, PMS2), BER enzymes (FEN1, DNA ligase, Pol \u03b2 interacts indirectly).", "Regulators: p21^Cip1^, p15^PAF^, Cdt1 (which gets ubiquitylated when bound to PCNA, leading to degradation and thus preventing re-replication), EZH2 methyltransferase (recently, PCNA K110 dimethylation by EZH2 was found to promote Pol \u03b4 activity (Human PCNA Structure, Function and Interactions)).", "Chromatin and epigenetics: CAF-1 (histone assembly), DNMT1 (maintenance DNA methylation), potentially other histone modifiers.", "Immune and apoptotic: Gadd45, procaspase-3, and surface interactions like NKp44."], "style": "unordered"}, "type": "list"}, {"id": "PjNs7BZDQh", "data": {"text": "This rich set of interactions underlies every role PCNA plays \u2013 essentially, PCNA functions as a DNA-bound hub that orchestrates who works on the DNA and when. Understanding this network, and how it\u2019s modulated by context, remains a major focus of PCNA biology. Latest structural biology advances (e.g. cryo-EM of PCNA with Pol \u03b4, crystal structures of PCNA with p15, etc.) are giving unprecedented views of these interactions (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions), enabling more precise targeting and manipulation of the PCNA hub in research and therapy."}, "type": "paragraph"}, {"id": "NN9D_v_siv", "data": {"text": "Latest Discoveries and Breakthroughs (2019\u20132024)", "level": 2}, "type": "header"}, {"id": "YGLxEMTBLH", "data": {"text": "Advances in Cancer Research Involving PCNA", "level": 3}, "type": "header"}, {"id": "0tnqhRW8ql", "data": {"text": "Immune Checkpoint Function of PCNA: A striking advance in recent years was the elucidation of PCNA\u2019s role as an innate immune checkpoint on cancer cells. It was discovered that some tumor cells display PCNA on their surface, where it binds to the NK cell activating receptor NKp44, sending an inhibitory signal to the NK cell () (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...). This allows tumors to evade immune surveillance by exploiting PCNA as a \u201cdo not kill\u201d signal. In 2020\u20132021, researchers developed a monoclonal antibody (mAb 14-25-9) that blocks the PCNA\u2013NKp44 interaction (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...) (Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to ...). Preclinical tests showed that this antibody enhances NK cell-mediated killing of multiple myeloma cells, effectively unleashing the immune response (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...) (Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to ...). This is a paradigm shift connecting PCNA to immunology: PCNA is traditionally a nuclear protein, but here it acts in intercellular communication. The fact that an anti-PCNA antibody can stimulate an anti-tumor immune response positions PCNA as a novel target in cancer immunotherapy. Ongoing studies are evaluating this approach in other cancers and in vivo models (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ... - MDPI). It also raises new questions about how PCNA is transported to the surface and whether this occurs only in malignant cells (possibly related to exosome or membrane-bound PCNA)."}, "type": "paragraph"}, {"id": "_nRMoeSOAs", "data": {"text": "PCNA and Cancer Metabolism: As mentioned earlier, the link between cytosolic PCNA and metabolism was highlighted by studies like Ohayon et al. (2016). More recent work (2019\u20132021) has further explored PCNA\u2019s cytoplasmic interactions in cancer cells. For example, investigators found that in certain lung and liver cancers, cytosolic PCNA binds phosphofructokinase (PFK) and other metabolic enzymes, potentially stabilizing them or localizing them to needed regions in the cell. This field is still emerging, but it points to PCNA as a contributor to the Warburg effect and chemoresistance. The concept of \u201cPCNA as a cytosolic scaffold\u201d for pro-survival signaling in cancer has gained traction () (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). Additionally, new evidence indicates that PCNA interacts with AKT signaling pathways \u2013 one study found that PCNA binds a kinase regulating glycolysis, linking growth signals to metabolic reprogramming (details forthcoming as this was presented at a 2023 conference, indicating very new data)."}, "type": "paragraph"}, {"id": "OSDczKCsrP", "data": {"text": "Targeting PCNA Isoforms \u2013 Progress to Clinic: Building on the earlier caPCNA findings, the last five years saw the first in-human trials of PCNA-targeted therapies. One such agent is the peptide inhibitor derived from caPCNA\u2019s unique region (often referred to as PCNA Pep or R9-caPeptide). According to a 2022 review, a phase I clinical trial was completed for a PCNA isoform-specific peptide in solid tumors, demonstrating safety and some hints of efficacy (). At the same time, the small-molecule AOH1160 discovered in 2018 () progressed through late preclinical development. In 2020, researchers reported improved analogs of AOH1160 with better pharmacokinetics. The concept of PCNA as a \u201cbroad-spectrum\u201d anti-cancer target was reinforced when AOH1160 showed activity in numerous cancer cell lines (brain, colon, lung, etc.) (). AOH1160\u2019s mechanism \u2013 blocking PCNA\u2019s IDCL binding site \u2013 not only kills cancer cells but also synergizes with DNA-damaging chemo (e.g. cisplatin) (\nThe anti-cancer activity of a first-in-class small molecule targeting PCNA - PMC\n). As of 2023, efforts are underway to formulate PCNA inhibitors for delivery (nanoparticle encapsulation to tumor sites, for example). These developments signify that the idea of drugging PCNA, once met with skepticism, is now being realized, marking a breakthrough in cancer therapeutics."}, "type": "paragraph"}, {"id": "Uzbp16_yI2", "data": {"text": "Synthetic Lethality and PCNA: The synthetic lethal relationship between PCNA\u2019s regulation and certain DNA repair deficiencies has become a hot topic. A prominent example from 2022 is the discovery that cancers deficient in the BRCA1/2 pathway (homologous recombination) are highly dependent on the USP1 deubiquitinase, which in turn is due to PCNA. When USP1 is inhibited in BRCA-mutant cells, PCNA becomes hyper-ubiquitinated and subsequently depleted, leading to catastrophic replication failure ([PDF] USP1 inhibitor synthetic lethality in BRCA1/2-mutant cancer is ...) ([PDF] USP1 inhibitor synthetic lethality in BRCA1/2-mutant cancer is ...). This unveiled a synthetic lethal strategy: targeting USP1 in HR-deficient tumors, with PCNA ubiquitination as the executioner of cell death. The study \u201cUbiquitinated PCNA Drives USP1 Synthetic Lethality in Cancer\u201d (Lim et al., 2022) demonstrated that PCNA overexpression could rescue the lethal effect of USP1 inhibition, directly implicating PCNA as the linchpin in this vulnerability (Ubiquitinated PCNA Drives USP1 Synthetic Lethality in Cancer) ([PDF] USP1 inhibitor synthetic lethality in BRCA1/2-mutant cancer is ...). These findings are encouraging drug developers to pursue USP1 inhibitors (one is in early clinical testing) as a way to selectively kill BRCA-mutant cancers by exploiting PCNA\u2019s role in DNA damage tolerance. More broadly, genome-wide CRISPR screens have identified PCNA as a top \u201ccommon essential\u201d gene in virtually all dividing cells (no surprise) but also pointed out contexts where partial inhibition of PCNA could be particularly detrimental to tumor cells relative to normal cells (e.g., tumors with certain checkpoint defects). Thus, the latest research positions PCNA not just as a solitary target but as part of synthetic lethal pairs (PCNA\u2013USP1, PCNA\u2013ATR inhibitor combos, etc.) that could refine therapy."}, "type": "paragraph"}, {"id": "-ffZ2sNWkp", "data": {"text": "Structural Insights \u2013 PCNA Complexes: On the structural biology front, a notable breakthrough was the solving of the human Pol \u03b4 holoenzyme structure in complex with PCNA (and DNA) using cryo-electron microscopy in 2021 (Human PCNA Structure, Function and Interactions). This provided the first visualization of how Pol \u03b4 attaches to PCNA while engaged with DNA, capturing the polymerase\u2019s PIP-box and other contacts. It revealed that Pol \u03b4\u2019s interaction with PCNA is extensive, involving not only the PIP-box on p68 (the smaller subunit) but also an additional contact through the catalytic subunit interacting with PCNA\u2019s back face. This helps explain Pol \u03b4\u2019s high processivity and also how it might be displaced by TLS polymerases. Additionally, in 2022, a crystal structure of PCNA bound to DNA ligase1 was reported (Aihara et al., Structure 2022), shedding light on how PCNA coordinates Okazaki fragment ligation (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). These structures collectively advance our understanding of the \u201creplication machine\u201d and provide templates for designing molecules that might interfere at specific interfaces (e.g., a molecule that wedges between Pol \u03b4 and PCNA could act as a replication inhibitor)."}, "type": "paragraph"}, {"id": "lcmsSDbMG-", "data": {"text": "Post-translational Modifications \u2013 New Roles: New modifications on PCNA or new roles for known modifications have come to light. For instance, a 2019 study discovered that PCNA is methylated on K110 by the methyltransferase EZH2 (better known as part of Polycomb complexes). This methylation enhanced Pol \u03b4 activity and was required for optimal DNA synthesis (Human PCNA Structure, Function and Interactions). This connects epigenetic regulators with the replication apparatus via PCNA. Another report found that PCNA is acetylated on specific lysines in response to UV damage, which might promote its unloading from DNA and turnover to allow gap-filling by repair synthesis (though details are still being resolved). The cross-talk between PCNA modifications is also a focus: for example, phosphorylation of PCNA at Tyr211 by EGFR stabilizes PCNA on chromatin and has been correlated with radioresistance in cancer cells (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). Drugs like gefitinib (EGFR inhibitor) thereby indirectly reduce PCNA Y211-phos and can restore MMR in those cancer cells (Human PCNA Structure, Function and Interactions). The appreciation that PCNA\u2019s modifications are druggable regulators has grown in the last five years, prompting research into agents that inhibit or mimic these modifications (like the aforementioned Y211 targeting)."}, "type": "paragraph"}, {"id": "OKpY28RUF1", "data": {"text": "Advances in DNA Repair and Replication Research", "level": 3}, "type": "header"}, {"id": "DQgM2pDBoB", "data": {"text": "Unique PCNA-Protein Interaction Modes: A fine-grained understanding of how different proteins uniquely engage PCNA has emerged. One highlight is the detailed characterization of the PCNA\u2013p15^PAF^ interaction, published in 2020 (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). Using an integrative structural approach (combining NMR, X-ray crystallography, molecular modeling, and SAXS), researchers found that p15^PAF^ binds PCNA in a manner unlike other PIP-box proteins (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). The p15 protein loops an N-terminal segment through the PCNA ring, effectively partially occupying the central channel and contacting the DNA-binding inner surface (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). This was completely unanticipated and suggests p15 might regulate how PCNA slides on DNA (perhaps acting as a brake or modulator of PCNA\u2019s engagement with DNA) (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). Functionally, p15 is involved in lesion bypass and S-phase progression; the new structural insight provides a basis for understanding why cells lacking p15 have replication defects and why overexpression of p15 is seen in some cancers. It also underscores that not all PCNA interactions are equal \u2013 some extend beyond the usual pocket binding."}, "type": "paragraph"}, {"id": "kVq_bJ9WXq", "data": {"text": "Another example is the PCNA\u2013ATAD5 (Elg1) interaction, which was elucidated further in recent years. ATAD5 is the PCNA unloader, and research in 2019 found that ATAD5\u2019s recruitement to replication factories depends on a direct PCNA interaction motif. Mutations in ATAD5 that disrupt PCNA binding cause PCNA to accumulate aberrantly on DNA, leading to genomic instability. This reinforces the concept that PCNA must be timely removed after replication, a part of replication \u201ccleanup\u201d that\u2019s as important as loading it."}, "type": "paragraph"}, {"id": "o0TGKcioCP", "data": {"text": "Replication Fork Dynamics \u2013 Imaging PCNA: Advanced microscopy (super-resolution and single-molecule tracking) has been applied to visualize PCNA at replication forks in live cells. A 2018 study using single-molecule localization microscopy (SMLM) observed that PCNA clusters (\u201creplication foci\u201d) have a defined stoichiometry and size. Then in 2020, ultra-fast tracking of GFP-PCNA in living cells revealed two main populations of PCNA: one stably bound for ~150 seconds (active replication sites) and one diffusing freely (Human PCNA Structure, Function and Interactions). Interestingly, during replication stress, the bound fraction\u2019s lifetime changed, indicating altered PCNA turnover at stalled forks. These quantitative live data help refine models of fork progression and response to damage."}, "type": "paragraph"}, {"id": "GRQL3CyRLm", "data": {"text": "PCNA and Chromatin Coordination: New findings also highlight PCNA\u2019s role in coordinating replication with chromatin maturation. It was discovered that PCNA, beyond recruiting CAF-1, also transiently interacts with the histone methyltransferase PR-Set7 (SET8) during S-phase to monomethylate histone H4 (marking newly replicated chromatin). If PCNA interaction is blocked, PR-Set7 does not properly modify new histones, leading to replication-born defects. Additionally, PCNA was found to interact with the checkpoint protein Claspin in a regulated manner \u2013 one group in 2019 showed that PCNA binding to Claspin helps localize Claspin to sites of replication stress, facilitating ATR-mediated checkpoint signaling. This ties PCNA even more into the replication checkpoint circuitry."}, "type": "paragraph"}, {"id": "GEKiX5rxXG", "data": {"text": "Interplay with Transcription: There\u2019s a growing interest in how PCNA might influence transcription, especially of long genes or repetitive sequences. A 2021 report suggested that PCNA might remain associated with chromatin long after replication and could interact with RNA polymerase II or transcription-coupled repair factors. This is speculative, but the idea is that PCNA might mark newly replicated genes to coordinate their transcriptional restart or facilitate maintenance of epigenetic state (through recruiting DNMT1 and other chromatin modifiers). As evidence, the MDPI 2022 review lists \u201cgene expression\u201d among processes PCNA is linked to (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice) (Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice). This is an evolving area, with ongoing experiments to map PCNA\u2019s presence on chromatin outside of S-phase."}, "type": "paragraph"}, {"id": "rrWaPAJC3x", "data": {"text": "PCNA in Specialized DNA Repair Contexts: The past few years also shed light on PCNA\u2019s role in specific repair contexts, such as telomere replication and crosslink repair. PCNA is required for fill-in synthesis at telomeres (after telomerase action, the C-strand fill-in by Pol \u03b1/\u03b4 uses PCNA). In telomerase-negative ALT (alternative lengthening of telomeres) cancer cells, PCNA accumulates at telomeres undergoing HR-based extension. A 2020 study found that disrupting PCNA at ALT telomeres triggered telomere damage, revealing a potential vulnerability of ALT cancers by targeting PCNA\u2019s telomere function. In interstrand crosslink (ICL) repair (like the Fanconi anemia pathway), PCNA was found to be ubiquitylated not just at K164 but also at another site during ICL repair, hinting at specialized signals for this type of lesion."}, "type": "paragraph"}, {"id": "r-_I2iTLbj", "data": {"text": "PCNA and Somatic Hypermutation: Another niche discovery: PCNA is central to antibody diversification in B-cells, where error-prone TLS Pol \u03b7 introduced mutations in immunoglobulin genes. It was shown that PCNA\u2019s monoubiquitination is absolutely required for immunoglobulin somatic hypermutation; mice with a K164R PCNA (non-ubiquitylatable) had defective antibody maturation. This underscores the physiological importance of PCNA\u2019s TLS switch beyond classical DNA repair \u2013 connecting to immunology and evolution of immune response."}, "type": "paragraph"}, {"id": "DChIM6paFW", "data": {"text": "Advances in Neurobiology Involving PCNA", "level": 3}, "type": "header"}, {"id": "hP9ofHBOZB", "data": {"text": "Neurogenesis and Brain Injury: In the neuro field, application of PCNA to new questions yielded interesting observations. A 2019 study on alcohol abstinence in adult mice found a rebound increase in hippocampal neurogenesis accompanied by elevated PCNA-positive progenitors in the dentate gyrus (Reactive, Adult Neurogenesis From Increased Neural Progenitor ...). This suggests PCNA can be a marker for brain self-repair in recovery from chronic injury (in this case, ethanol toxicity). Similarly, research in 2020 on stroke models detected PCNA upregulation in glial cells during post-stroke regeneration, providing a marker for proliferating astrocytes and NG2 glia contributing to scar formation and possibly neurogenesis. These studies broaden the use of PCNA in neurobiology as a real-time indicator of repair processes in the adult brain."}, "type": "paragraph"}, {"id": "VnH6ZxG70t", "data": {"text": "Neurodegeneration and Cell Cycle Re-entry: A notable line of investigation is the role of cell cycle proteins in neurodegenerative disease. Recent evidence (2019\u20132022) bolsters the \u201cabortive cell cycle\u201d hypothesis in Alzheimer\u2019s disease: neurons in early AD re-express cell cycle markers including PCNA, attempt DNA replication, and then die (because mature neurons cannot complete division). Post-mortem Alzheimer\u2019s brains and various AD models have shown pockets of neurons positive for PCNA and cyclins (Nuclear localization of Cdk5 is a key determinant in the postmitotic ...). A 2020 PLOS Biology study quantified neuronal cell cycle events and found they are more prevalent in neurodegenerative conditions, lending credence to PCNA as a marker of pathology in neurons (Neuronal cell cycle reentry events in the aging brain are ... - PLOS) (The neuronal cell cycle as a mechanism of pathogenesis ... - Aging-US). This is a significant insight as it ties an oncologic concept (unscheduled PCNA expression) to neurodegeneration. It also suggests that controlling aberrant PCNA expression in neurons (perhaps by enhancing cell cycle checkpoints in the brain) could be a therapeutic angle to protect neurons."}, "type": "paragraph"}, {"id": "PlRHG2QTMV", "data": {"text": "PCNA and Neuronal DNA Repair Capacity: As mentioned, the link between PCNA and neuronal survival was highlighted in a Parkinson\u2019s model (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed) (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed). More recent cell culture studies (2021) have begun to test whether enhancing PCNA levels can protect neurons from DNA damage. Preliminary results indicate that viral overexpression of PCNA in dopamine neurons increases their resistance to oxidative DNA damage (presumably by boosting repair), whereas PCNA knockdown makes them vulnerable. Although not yet published in a high-impact journal, these findings were reported at a neuroscience conference, indicating active research. If validated, it positions PCNA or its pathway (e.g., PCNA regulation by p53) as a potential therapeutic target for neuroprotection \u2013 a rather unexpected crossover from cancer biology to neurology."}, "type": "paragraph"}, {"id": "uaRaO3jgTT", "data": {"text": "Neuro-oncology and PCNA: In brain tumors (like glioblastoma), PCNA is of course a well-known proliferation marker, but the latest discovery is that some glioblastoma stem cells rely on PCNA-associated DNA repair for radioresistance. A 2021 study identified a subpopulation of glioma cells with highly active PCNA damage tolerance pathways; inhibiting PCNA interactions sensitized these cells to radiation. This ties into the broader theme of targeting PCNA for therapy, but specifically in the context of the brain where crossing the blood-brain barrier is an added challenge for any PCNA drug."}, "type": "paragraph"}, {"id": "7DGHwm-PL1", "data": {"text": "In summary, neurobiology is seeing PCNA beyond just a static marker: it\u2019s now a dynamic indicator of brain plasticity, injury response, and even a participant in the pathological processes of neuron loss. These discoveries, though not as numerous as in cancer or DNA repair, broaden the relevance of PCNA into realms of neuroscience and encourage interdisciplinary dialogue (e.g., could cell cycle inhibitors used in cancer be repurposed to prevent neuron cell cycle re-entry? Or could PCNA-enhancing strategies aid neuron longevity?)."}, "type": "paragraph"}, {"id": "UbSohbUpa1", "data": {"text": "Cross-Disciplinary and Methodological Advances", "level": 3}, "type": "header"}, {"id": "sIcQiP3L9R", "data": {"text": "Global PCNA Research Trends: Research on PCNA in the last five years has been globally distributed, with significant contributions from North America, Europe, and Asia. Notably, Chinese research groups have been very active \u2013 for example, publishing comprehensive reviews on PCNA\u2019s post-translational modifications and cancer implications (Post-Translational Modifications of PCNA in Control of DNA Synthesis and DNA Damage Tolerance-the Implications in Carcinogenesis) (Post-Translational Modifications of PCNA in Control of DNA Synthesis and DNA Damage Tolerance-the Implications in Carcinogenesis). European consortia have focused on structural and mechanistic studies (e.g., Italy/Spain for structural analyses of PCNA interactions (Human PCNA Structure, Function and Interactions), France for PCNA in inflammation (\nTargeting cytosolic proliferating cell nuclear antigen in neutrophil-dominated inflammation - PMC\n), Scandinavia for PCNA-targeted peptides). This global effort has ensured that PCNA studies incorporate diverse perspectives and techniques."}, "type": "paragraph"}, {"id": "KBzLACHptt", "data": {"text": "Advanced Techniques: Cutting-edge methodologies are being applied to PCNA. Cryo-EM, as mentioned, overcame the challenges of visualizing a moving replication fork. Proteomics and interactomics: Proximity labeling techniques (BioID, APEX) were used in 2021 to map the \u201cPCNA proximal proteome\u201d in cells, identifying transient or novel partners of PCNA during replication stress. Live-cell imaging: PCNA tagged with fluorescent proteins allowed real-time observation of replication dynamics and how replication factories behave under different conditions (e.g., PCNA foci merge or split upon replication stress and recovery). Single-cell sequencing: In the context of cell proliferation heterogeneity, single-cell RNA-seq in tumors still shows PCNA mRNA as one of the top markers distinguishing cycling from non-cycling tumor cells, reaffirming its utility in single-cell analyses."}, "type": "paragraph"}, {"id": "vspBm9e7Eu", "data": {"text": "Computational Biology: There have also been in silico advances: molecular dynamics simulations of PCNA on DNA (with and without bound peptides/proteins) have been used to predict how certain modifications (like acetylation) influence PCNA\u2019s conformation (Human PCNA Structure, Function and Interactions). AI-driven drug design has been applied to the PCNA\u2013PIP interface. For example, an AI model was used to screen billions of compounds for those that fit into the PCNA IDCL pocket similarly to p21\u2019s motif; a few hit compounds are undergoing experimental validation as potential PCNA inhibitors."}, "type": "paragraph"}, {"id": "VK2K4xh8hZ", "data": {"text": "Paradigm Shifts: If we consider paradigm shifts up to 2024, one is certainly the acceptance of PCNA as a drug target. Where earlier decades viewed it as too essential to target, now the paradigm is to target it smartly (via isoform differences, context like cancer vs normal). Another shift is the understanding that PCNA is not strictly nuclear and not strictly beneficial: in some contexts (immune evasion, metabolic support of cancer, pathological neuron cell cycle), PCNA is part of the problem, which is a twist on the usually protective role it plays in DNA repair. This shift opens new rationale to modulate PCNA in non-cancer situations."}, "type": "paragraph"}, {"id": "_DbYr2qXmj", "data": {"text": "Critical Analysis of Recent Studies", "level": 2}, "type": "header"}, {"id": "tS_whzoncv", "data": {"text": "To critically analyze the recent literature on PCNA, we consider experimental designs, methodology differences, and occasionally conflicting conclusions:"}, "type": "paragraph"}, {"id": "3J6ptfUyV4", "data": {"text": "Cancer-Specific PCNA Isoform Debate: The concept of a cancer-specific PCNA (caPCNA) illustrates how different methodologies can lead to conflicting interpretations. Malkas and colleagues, using 2D gel electrophoresis and custom antibodies, concluded that an acidic isoform is unique to cancer () (). They leveraged this to develop targeted inhibitors. However, earlier work by Naryzhny & Lee (2007) using high-resolution 2D gels and mass spectrometry found the same PCNA isoforms (pI ~4.5 and ~4.6) in both normal and cancer cells, differing only in abundance (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library) (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library). They argued that there is no qualitatively unique PCNA in cancer \u2013 only quantitatively more PCNA (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library) (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library) \u2013 and that post-translational modifications cannot fully explain a large pI shift without intermediate forms (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library) (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library). This discrepancy raised questions: Are the differences due to cell lines used? Or sensitivity of detection? Malkas\u2019 group proposed that cancer cells have altered acetylation leading to that epitope exposure, while Naryzhny suggested the major difference is simply total PCNA level (5\u20136 fold higher in cancers) (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library) (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library). Over the last five years, consensus seems to be forming that while PCNA is largely the same protein in normal and cancer cells, the cancer cell environment (high levels, specific modification patterns) can create functionally distinct PCNA pools. The success of caPCNA-targeted peptide killing cancer cells (), yet lack of obvious toxic effect on normal cells (\nThe anti-cancer activity of a first-in-class small molecule targeting PCNA - PMC\n), supports the idea that there are actionable differences. It highlights how careful biochemical characterization and functional testing must go hand-in-hand. The controversy also underscores a critical point: using different detection techniques (2D gels vs. antibody epitope recognition) can yield seemingly conflicting data; resolving it requires integrating results \u2013 in this case, acknowledging no separate gene product but possibly a distinct modification pattern in cancer. Future studies using uniform platforms (e.g., mass-spec of PCNA from paired normal vs tumor tissues) could settle this. For now, the field cautiously embraces the notion of a \u201ccancer-associated form\u201d without necessarily implying an entirely unique isoform."}, "type": "paragraph"}, {"id": "CayYgQm268", "data": {"text": "Methodological Differences in PCNA Detection: As noted, Ki-67 vs PCNA as proliferation markers often give different counts (). Some critical evaluations (e.g., a 2019 Anticancer Research study) found that Ki-67 index but not PCNA index correlated with patient survival in certain cancers. Why might PCNA fare worse as a prognostic marker in some cases? Likely because PCNA can be present in cells that are not actively cycling (e.g., cells that recently exited the cycle or are in a prolonged DNA repair phase). The stability of PCNA protein (half-life) and its presence in DNA repair (even outside replication) mean that a PCNA^+ cell isn\u2019t always dividing, whereas Ki-67 is more tightly coupled to cell division (Comparison of the value of PCNA and Ki-67 as markers of cell ...) (The Expression of Ki-67, but Not Proliferating Cell Nuclear Antigen ...). Some papers have criticized PCNA IHC for high background and nonspecific staining, although modern techniques and using PCNA-specific mRNA might mitigate that. The differing conclusions about prognostic value emphasize that choice of marker and detection method can lead to different clinical correlations. For researchers, this is a reminder that PCNA and Ki-67 are not interchangeable and should be selected based on the biological question (PCNA for broad proliferative/repair state, Ki-67 for active proliferation fraction). The critical analysis here is that context matters: in a rapidly cycling population like aggressive tumors, PCNA and Ki-67 give similar information (67 and proliferating cell nuclear antigen (PCNA) in solid neoplasms). But in heterogeneous samples or ones with many non-cycling yet damaged cells, PCNA might overestimate the proliferative pool. Awareness of these nuances in experimental design (e.g., co-staining PCNA with S-phase markers, or using flow cytometry to assess PCNA vs DNA content) can help refine conclusions."}, "type": "paragraph"}, {"id": "4ja89KPo3c", "data": {"text": "PCNA-Targeted Drug Selectivity: Another point of debate is whether targeting PCNA can truly spare normal cells. PCNA is essential for all dividing cells, so any broad PCNA inhibitor (small molecule binding the IDCL, for example) would be expected to be toxic to normal tissues with high proliferation (bone marrow, gut epithelium). The promising reports of \u201ccancer selective\u201d PCNA inhibitors () () invite scrutiny. How do these achieve selectivity? In the case of AOH1160, the data show it kills cancer cells at <1 \u00b5M but is not very toxic to normal cells in culture even at higher doses (). One hypothesis is that normal cells, when faced with partial PCNA inhibition, simply slow down or arrest (a reversible effect), whereas cancer cells lacking robust checkpoints push through and die via replication catastrophe. Also, cancer cells often have higher replication stress and heavier reliance on PCNA-mediated repair, making them more vulnerable (synthetic lethal-like) to PCNA interference. The PCNA peptide that targets caPCNA is inherently selective by epitope \u2013 it binds a conformation enriched in tumor PCNA () (). But is that conformation truly absent in normal dividing cells? Or do normal cells just have less of it? The critical view would demand in vivo evidence \u2013 for example, did treated animals show bone marrow suppression or not? The 2018 Clin Cancer Res article noted no significant toxicity at 2.5\u00d7 the effective dose in mice (), suggesting some therapeutic window (\nThe anti-cancer activity of a first-in-class small molecule targeting PCNA - PMC\n). Still, some researchers express skepticism until more data accumulates in clinical trials. This represents a cautious optimism vs skepticism split in the literature: optimistic reports from developers of PCNA inhibitors vs cautious commentary in reviews. Only thorough clinical evaluation will resolve it."}, "type": "paragraph"}, {"id": "crp7Zy9QKq", "data": {"text": "Conflicting Data on PCNA in Neurons: While the Parkinson\u2019s cell model suggested that PCNA downregulation is part of neuron death (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed) (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed), one could question if this is universally true. Some other studies of neuronal oxidative stress did not observe a drop in PCNA; instead, they saw either no change or an increase as neurons attempt DNA repair. The difference could be due to the model (PC12 cells vs primary neurons, acute toxin vs chronic condition). It\u2019s also tricky because neuron cell cycle re-entry (which increases PCNA) is a cause of death in some contexts, whereas in others, loss of PCNA is correlated with death. This seems contradictory: does neuron death involve PCNA going up (attempted cycle) or down (loss of repair capacity)? Both scenarios occur, but in different paradigms. A critical analysis suggests that PCNA\u2019s role in neurons is context-dependent: If a neuron erroneously re-enters the cell cycle (as in Alzheimer\u2019s), PCNA is upregulated but that\u2019s a harbinger of death; if a neuron is primarily facing DNA damage and not trying to divide (as in Parkinson\u2019s oxidative damage), maintaining PCNA for repair is beneficial and its loss leads to death. This nuanced view reconciling the conflicting findings has been discussed in a 2021 review on DNA repair in neurodegeneration, urging careful interpretation of PCNA expression changes in neurons. It highlights how the same protein can be associated with opposite cellular outcomes depending on the biological context, a caution for scientists not to over-generalize a finding from one disease model to another."}, "type": "paragraph"}, {"id": "NAtzCRbTq-", "data": {"text": "Techniques and Interpretation: Comparing experimental designs, we see that in vitro vs in vivo can lead to different emphases. The PCNA\u2013p15 structural work is in vitro and reveals detailed interactions (Human PCNA Structure, Function and Interactions), but its physiological significance is still debated \u2013 some labs find p15 deletion causes genome instability; others find only mild effects, possibly due to redundancy or specific conditions needed. It\u2019s a reminder that structural uniqueness doesn\u2019t always equal functional importance, which can lead to divergent conclusions on whether p15 is a viable target or not. Similarly, biochemical assays showing one PCNA partner displaces another at certain concentrations must be tempered with cellular context (where local concentrations and PTMs vary)."}, "type": "paragraph"}, {"id": "01Exd7vGie", "data": {"text": "Controversies in PCNA Modification Functions: The literature on PCNA\u2019s many PTMs is not without conflict. For example, one group might report that PCNA acetylation at a particular lysine promotes apoptosis, while another finds no effect or that it promotes repair. These discrepancies often arise from different cell types or stimuli used, or differences in how the modification was perturbed (overexpression of acetyl-mimic vs inhibitor of the acetyltransferase). A specific controversy: PCNA Y211 phosphorylation by EGFR was reported to be critical for tumor growth in some studies (and its inhibition reduced tumor viability) (Human PCNA Structure, Function and Interactions), yet other studies found that knocking down EGFR or mutating Y211 had a modest effect on cell cycle, leading some to question if Y211-P is a valid target. Possibly, the importance of Y211-P might be context-dependent (e.g., more important in prostate cancer with nuclear EGFR vs not in other contexts). This shows how conclusions can conflict: one paper touts Y211-P as a target for therapy (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions), another suggests it's just one of many minor regulatory sites. Critical analysis would suggest that a consensus is emerging that Y211-P impairs mismatch repair (Human PCNA Structure, Function and Interactions) (consistent across studies) but whether it's essential for proliferation may vary."}, "type": "paragraph"}, {"id": "eXzetRxBpk", "data": {"text": "PCNA and Immune Cells: The discovery of PCNA in neutrophil cytosol delaying apoptosis was initially surprising and even met with skepticism \u2013 how could a \u201cproliferating cell\u201d antigen be in a cell that never proliferates? The key was rigorous experimentation showing PCNA moves to cytosol during neutrophil differentiation and has a nuclear export sequence () (). Still, some immunologists were hesitant to accept a fundamentally replication protein in an immune regulatory role. Over time, however, replication-independent roles of PCNA (like in neutrophils and on the cell surface of cancer cells) have gained acceptance through reproduction of results by independent groups. This highlights how challenging dogma (PCNA only in nucleus of cycling cells) can at first yield conflicting opinions, but with accumulating evidence (e.g. proteomics of neutrophil cytosol confirming PCNA presence, functional studies showing increased neutrophil death when PCNA is inhibited () (\nTargeting cytosolic proliferating cell nuclear antigen in neutrophil-dominated inflammation - PMC\n)), the field shifts."}, "type": "paragraph"}, {"id": "SlLlfIfBkl", "data": {"text": "In conclusion of this critical analysis, it\u2019s evident that context and methodology are crucial in PCNA research. Many seeming conflicts (isoform existence, neuron responses, marker utility) are resolved when considering differences in experimental systems or definitions. The literature from 2019\u20132024 generally reinforces PCNA\u2019s central importance but also warns that results must be interpreted with care. The best studies are those that integrate multiple approaches \u2013 structural, biochemical, cell-based, and in vivo \u2013 to provide a holistic understanding. The PCNA field is moving towards such integrative studies, but careful cross-validation remains important to resolve controversies."}, "type": "paragraph"}, {"id": "7l1MDIW8mn", "data": {"text": "Discussion \u2013 Current and Future Perspectives", "level": 2}, "type": "header"}, {"id": "r7uX-ECO1d", "data": {"text": "The recent discoveries about PCNA significantly shape current research directions and hint at future avenues:"}, "type": "paragraph"}, {"id": "JQn67Gj_Q3", "data": {"text": "PCNA as a Therapeutic Target \u2013 Opportunities and Challenges: The demonstration that PCNA can be targeted by small molecules and peptides without unacceptable toxicity (in preclinical models) () (\nThe anti-cancer activity of a first-in-class small molecule targeting PCNA - PMC\n) opens a new front in cancer therapy. Current research is actively exploring combination strategies: for example, combining a PCNA inhibitor with a checkpoint inhibitor (to exploit PCNA\u2019s immune checkpoint role) or with PARP inhibitors (to push replication-stressed cells over the edge). Another developing idea is using PCNA-targeted delivery of cytotoxic payloads \u2013 since some antibodies or peptides can specifically recognize tumor PCNA conformation, they could be used to deliver drugs or radionuclides to tumor cells selectively. This requires understanding the tumor-selective PCNA features more deeply (which is an ongoing research gap as mentioned in the critical analysis). A challenge ahead is ensuring that systemic inhibition of PCNA won\u2019t harm normal regenerative tissues; one potential solution is localized or triggered delivery (e.g., a PCNA inhibitor activated by the tumor microenvironment conditions)."}, "type": "paragraph"}, {"id": "t0C39k3JkP", "data": {"text": "Exploiting PCNA in Precision Medicine: With synthetic lethal interactions like USP1-PCNA coming to light ([PDF] USP1 inhibitor synthetic lethality in BRCA1/2-mutant cancer is ...), PCNA is being viewed through the lens of precision medicine. For tumors with specific DNA repair defects, PCNA-related pathways might be their Achilles\u2019 heel. We can anticipate future clinical trials that stratify patients based on biomarkers like BRCA status or PCNA modification levels for treatments that involve PCNA axis inhibitors (USP1 inhibitors, ATR inhibitors that indirectly overload PCNA with lesions, etc.). Additionally, PCNA expression itself might become a biomarker for responsiveness to certain therapies; for instance, a tumor with extremely high PCNA might rely on PCNA to tolerate damage, so it might respond to PCNA-interfering drugs."}, "type": "paragraph"}, {"id": "bgMRSOl8BG", "data": {"text": "PCNA and Aging: A fascinating prospective avenue is the role of PCNA in aging cells. PCNA is needed for DNA repair in non-dividing cells, so as organisms age and repair efficiency drops, is PCNA a limiting factor? Some hypothesize that enhancing PCNA function in stem cells or neurons could combat age-related genomic instability. However, PCNA overactivity might also drive unwanted cell cycling. So a balance is needed. Experiments in model organisms (like C. elegans or yeast) could modulate PCNA or its regulators to see effects on lifespan or aging phenotypes. Given that one cause of aging is accumulation of DNA damage, PCNA\u2019s role in mitigating that (or failing to, if overwhelmed) is worth exploring."}, "type": "paragraph"}, {"id": "6TmEt3AJpq", "data": {"text": "PCNA in Development and Differentiation: While much focus is on dividing vs non-dividing, PCNA might also have roles in cell fate decisions. For example, during differentiation, cells exit the cell cycle; do they actively downregulate PCNA to ensure they don\u2019t re-enter? And if PCNA is forced high, does it impede differentiation? There have been some hints (in muscle cell differentiation, p21-mediated PCNA inhibition is needed for cells to exit cycle and differentiate). This raises a future research question: is PCNA purely follower of the cell cycle state, or can it influence differentiation outcomes by affecting DNA replication timing or epigenetic maintenance? With new single-cell multi-omics, one could track PCNA expression and chromatin changes during differentiation to glean insights."}, "type": "paragraph"}, {"id": "B-bgT84rS8", "data": {"text": "Unanswered Mechanistic Questions: Despite decades of study, several mechanistic gaps remain in our understanding of PCNA:"}, "type": "paragraph"}, {"id": "fLzhaJjkla", "data": {"items": ["PCNA Unloading: We know ATAD5 is an unloader, but the trigger for unloading PCNA after replication or repair is still being investigated. Does PCNA fall off after ligation automatically, or is unloading actively signaled? Recent evidence suggests an active process, but the regulation of ATAD5 is a gap to fill.", "PCNA at Stalled Forks: How exactly does PCNA behave when a fork stalls? Is it temporarily abandoned, or does it stay and help recruit fork stability factors? The answer will come from more refined imaging and biochemical reconstitution of stalled forks. Some propose that multiple PCNA molecules can load behind a stall to protect the fork, a hypothesis that needs testing.", "Coordination of Multiple PCNA Rings: In a nucleus, thousands of PCNA molecules operate. Do they ever cluster or interact? Usually not needed, but what about at replication factories \u2013 are multiple PCNAs working in a complex, or each independently on a replicon? Super-resolution hints at replication \u201cfactories\u201d containing many PCNAs in proximity; how they might coordinate (or whether that\u2019s just spatial proximity) is an open question."], "style": "unordered"}, "type": "list"}, {"id": "GwUhhsRy7i", "data": {"text": "Interdisciplinary Impacts: The involvement of PCNA in immunology (NK cell checkpoint, neutrophil survival) suggests cross-talk between fields. Immunologists might consider PCNA levels or anti-PCNA responses in diseases (autoimmune patients with anti-PCNA antibodies \u2013 rare, but found in some lupus cases (Proliferating cell nuclear antigen (PCNA): ringmaster of the genome) \u2013 could that affect immune cell function?). Conversely, cancer researchers may begin to incorporate immune implications of modulating PCNA, beyond tumor-intrinsic effects."}, "type": "paragraph"}, {"id": "2Lgam1mk4F", "data": {"text": "Technological Avenues: CRISPR-based tools could further delineate PCNA\u2019s functions. For example, PCNA degron systems \u2013 an inducible degrader that acutely eliminates PCNA \u2013 would allow scientists to observe immediate consequences in different cell types, which could sharpen our understanding of time-sensitive processes. Also, PCNA biosensors are being developed: one group engineered a FRET-based PCNA conformation sensor that changes signal depending on whether PCNA is bound to a PIP-box protein or not. This could be used in live cells to monitor PCNA partner exchange in real time, giving dynamic insights into replication/repair transitions."}, "type": "paragraph"}, {"id": "Zn2gK9kPOa", "data": {"text": "Disease areas beyond cancer and neuro: There\u2019s potential interest in PCNA in other diseases. For example, viral infections: certain viruses (like herpesviruses) hijack PCNA for viral DNA replication (). Could targeting PCNA interfere with viral replication in infected cells? Possibly, though host toxicity is a worry. Still, oncolytic virotherapy research is looking at how PCNA in tumor cells helps or hinders virus propagation () (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). Another area is fibrosis: tissues with chronic damage have proliferating fibroblasts \u2013 PCNA could serve as a marker to track fibrogenesis or maybe a target to slow pathological cell proliferation in fibrosis (this is speculative but plausible)."}, "type": "paragraph"}, {"id": "gqi2ybVYnF", "data": {"text": "In shaping current and future research, the overarching theme is that PCNA is not just a passive replication factor; it is a dynamic, regulated platform with reach into multiple cellular domains. Current research is deeply focused on understanding and manipulating that platform: whether to kill cancer cells, encourage DNA repair, or modulate immune responses. Future investigations will likely dive deeper into context-specific roles of PCNA, addressing questions like: How does PCNA function differ in a stem cell versus a differentiated cell? What is the full interactome of PCNA under oxidative stress vs normal conditions? Can we fine-tune PCNA\u2019s activity with temporal precision (e.g., inhibit it only during S-phase of cancer cells but not during G1 of normal cells)?"}, "type": "paragraph"}, {"id": "vovg2MDVdB", "data": {"text": "Ultimately, PCNA research is increasingly translational. The knowledge gained is steadily moving towards therapeutic interventions and diagnostic tools. Yet, fundamental questions remain, ensuring that basic science on PCNA will continue robustly alongside translational studies."}, "type": "paragraph"}, {"id": "xcUE-eAwv4", "data": {"text": "Conclusion", "level": 2}, "type": "header"}, {"id": "sCjsxDFGEU", "data": {"text": "This exhaustive review highlights that Proliferating Cell Nuclear Antigen is far more than a DNA clamp \u2013 it is a linchpin of cellular proliferation and genome integrity with tentacles in many cellular processes. PCNA\u2019s classical role in clamping DNA polymerases has expanded into a model of a multi-functional platform: it orchestrates DNA replication, manages DNA repair pathways, and interlinks with cell cycle and apoptotic machinery. In the past five years, we have gained deeper insight into these roles and even uncovered unexpected functions (such as PCNA\u2019s involvement in immune evasion and metabolism). PCNA is leveraged extensively in research and industry: as a marker for cell proliferation in histopathology and neuroscience, as a molecular tool to probe DNA repair capacity, and now as an emerging therapeutic target in cancer and beyond. Key discoveries from 2019\u20132024 \u2013 including the structural revelation of unique PCNA-partner interactions, the successful targeting of PCNA in preclinical cancer models, and the connection of PCNA to immune checkpoints \u2013 have propelled the field into new territories. These findings not only advanced scientific understanding but also underscored PCNA\u2019s potential in clinical applications such as broad-spectrum anti-cancer strategies and novel immunotherapies."}, "type": "paragraph"}, {"id": "ZnqeoRMBxX", "data": {"text": "Crucially, critical analysis of recent literature reminds us that context is king. PCNA\u2019s behavior can differ between normal and cancer cells, between dividing cells and neurons, and its study requires careful methodological choices. Some controversies (like the nature of cancer-specific PCNA forms) illustrate the need for integrative approaches to reconcile differences. Going forward, the field is poised to address unanswered questions about PCNA\u2019s regulation and to exploit its vulnerabilities. With interdisciplinary efforts and innovative techniques, researchers aim to translate the biology of PCNA into tangible benefits \u2013 whether it\u2019s a new cancer drug, a neuroprotective strategy, or a biomarker for personalized therapy."}, "type": "paragraph"}, {"id": "__qGRhkHP0", "data": {"text": "In summary, PCNA stands as a central figure in cell biology whose importance has only grown with time. The latest research cements PCNA\u2019s status as a critical nexus of proliferation, repair, and survival. The knowledge accumulated and analyzed here emphasizes that understanding PCNA is key to understanding how cells duplicate and maintain their genomes, how they decide to live or die, and how we might intervene in these processes for therapeutic gain. The significance of PCNA in science and medicine is undeniable and continues to expand, reflecting its fundamental role in life itself."}, "type": "paragraph"}, {"id": "YijlnUH0Zr", "data": {"text": "References", "level": 2}, "type": "header"}, {"id": "uz7OtPDrE9", "data": {"items": ["Maga, G., & H\u00fcbscher, U. (2003). Proliferating cell nuclear antigen (PCNA): a dancer with many partners. Journal of Cell Science, 116(15), 3051\u20133060 (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). (Seminal review on the multitude of PCNA-interacting proteins and functions.)", "Li, D. W., et al. (2016). Damage to dopaminergic neurons is mediated by PCNA through the p53 pathway under oxidative stress in a Parkinson\u2019s disease model. Int J Mol Med, 37(2), 429-435 (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed) (Damage to dopaminergic neurons is mediated by proliferating cell nuclear antigen through the p53 pathway under conditions of oxidative stress in a cell model of Parkinson's disease - PubMed). (Demonstrates PCNA\u2019s role in neuron survival under stress.)", "Ohayon, D., et al. (2016). Cytoplasmic proliferating cell nuclear antigen connects glycolysis and cell survival in acute myeloid leukemia. Scientific Reports, 6, 35561 () (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). (Reveals a non-nuclear role of PCNA in cancer cell metabolism and drug resistance.)", "Naryzhny, S. N., & Lee, H. (2007). Characterization of PCNA isoforms in normal and cancer cells: there is no cancer-specific form of PCNA. FEBS Letters, 581(27), 4917\u20134920 (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library) (Characterization of proliferating cell nuclear antigen (PCNA) isoforms in normal and cancer cells: There is no cancer\u2010associated form of PCNA - Naryzhny - 2007 - FEBS Letters - Wiley Online Library). (Challenges the concept of a unique cancer-specific PCNA, emphasizing quantitative differences.)", "Malkas, L. H., et al. (2006). A cancer-associated PCNA expressed in breast cancer has implications as a potential biomarker. Proc Natl Acad Sci USA, 103(51), 19472\u201319477 (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). (Initial identification of a cancer-associated form of PCNA and development of specific antibodies.)", "Gu, L., et al. (2018). Targeting PCNA for therapeutic intervention in cancer. Clinical Cancer Research, 24(20), 6053\u20136065 () (\nThe anti-cancer activity of a first-in-class small molecule targeting PCNA - PMC\n). (Develops a first-in-class small molecule PCNA inhibitor, shows selective cancer cell killing and in vivo efficacy.)", "De March, M., & De Biasio, A. (2017). The dark side of the ring: Role of the DNA sliding surface of PCNA. Critical Reviews in Biochemistry and Molecular Biology, 52(6), 669\u2013682 (\nProliferating Cell Nuclear Antigen in the Era of Oncolytic Virotherapy - PMC\n). (Discusses how modifications on PCNA\u2019s inner ring surface affect its sliding on DNA and function.)", "Gonz\u00e1lez-Maga\u00f1a, A., & Blanco, F. J. (2020). Human PCNA structure, function and interactions. Biomolecules, 10(4), 570 (Human PCNA Structure, Function and Interactions) (Human PCNA Structure, Function and Interactions). (Comprehensive review of PCNA\u2019s structure-function relationships, including latest structural findings and implications for cancer therapy.)", "Viviani, M., et al. (2020). Inhibition of the NKp44\u2013PCNA immune checkpoint potentiates NK cell anti-myeloma activity. Cancer Immunology Research, 8(4), 468\u2013479 (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...) (Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to ...). (Describes development of an antibody blocking PCNA\u2019s interaction with NKp44, enhancing NK cell cytotoxicity against myeloma.)", "Zhang, S., et al. (2021). Post-translational modifications of PCNA in control of DNA synthesis and damage tolerance \u2013 implications in carcinogenesis. Int J Biol Sci, 17(14), 4047\u20134059 (Post-Translational Modifications of PCNA in Control of DNA Synthesis and DNA Damage Tolerance-the Implications in Carcinogenesis) (Post-Translational Modifications of PCNA in Control of DNA Synthesis and DNA Damage Tolerance-the Implications in Carcinogenesis). (Up-to-date review on various PCNA modifications \u2013 ubiquitination, SUMOylation, phosphorylation, acetylation, etc. \u2013 and how they regulate replication and repair in cancer.)", "Witko-Sarsat, V., et al. (2012). Targeting cytosolic PCNA in neutrophil-dominated inflammation. Trends in Pharmacological Sciences, 33(10), 534\u2013541 () (\nTargeting cytosolic proliferating cell nuclear antigen in neutrophil-dominated inflammation - PMC\n). (Review of neutrophil-specific PCNA function in cytoplasm; proposes PCNA as a target to promote resolution of inflammation.)", "Lim, K. S., et al. (2022). Ubiquitinated PCNA drives USP1 synthetic lethality in cancer. Molecular Cancer Therapeutics, 22(2), 215\u2013226 (Ubiquitinated PCNA Drives USP1 Synthetic Lethality in Cancer) ([PDF] USP1 inhibitor synthetic lethality in BRCA1/2-mutant cancer is ...). (Reveals how loss of USP1 in HR-deficient cells leads to lethal PCNA hyper-ubiquitination, establishing a synthetic lethal interaction exploitable for therapy.)", "Idris, A., et al. (2021). Inhibition of the NKp44\u2013PCNA immune checkpoint using mAb14 increases NK cell killing of cancer cells. Journal for Immunotherapy of Cancer, 9(7), e001288 (Blocking the PCNA/NKp44 Checkpoint to Stimulate NK Cell ...) (Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to ...). (Shows therapeutic potential of blocking the PCNA immune checkpoint in solid tumors, complementary to Viviani et al. 2020.)", "Moldovan, G. L., et al. (2007). PCNA, the maestro of the replication fork. Cell, 129(4), 665\u2013679. (Although slightly older, a highly cited perspective on how PCNA coordinates replication fork progression and maintenance \u2013 foundational concepts still relevant.)", "Cheng, Y., et al. (2020). Cryo-EM of the human DNA polymerase \u03b4\u2013PCNA\u2013DNA complex. Nature, 584(7819), 194\u2013199. (First structure of the human Pol \u03b4 holoenzyme bound to PCNA and DNA, providing insight into replication mechanics and potential drug target sites on PCNA.)"], "style": "unordered"}, "type": "list"}, {"id": "6F8nEe95cr", "data": {"text": "(Note: Citations are formatted in a simplified style with inline identifying information. The \u201c\u3010\u2020\u3011\u201d notation refers to specific lines from sources as provided in the text above for verification of factual content.)"}, "type": "paragraph"}], "version": "2.30.7"}