Small-molecule targeting of proliferating cell nuclear antigen chromatin association inhibits tumor cell growth

Forging Ahead through Darkness: PCNA, Still the Principal Conductor at the Replication Fork

Proliferating cell nuclear antigen (PCNA) lies at the center of the faithful duplication of eukaryotic genomes. With its distinctive doughnut-shaped molecular structure, PCNA was originally studied for its role in stimulating DNA polymerases. However, we now know that PCNA does much more than promote processive DNA synthesis. Because of the complexity of the events involved, cellular DNA replication poses major threats to genomic integrity. Whatever predicament lies ahead for the replication fork, PCNA is there to orchestrate the events necessary to handle it. Through its many protein interactions and various post-translational modifications, PCNA has far-reaching impacts on a myriad of cellular functions.

Potent competitive inhibition of human ribonucleotide reductase by a nonnucleoside small molecule

Human ribonucleotide reductase (hRR) is crucial for DNA replication and maintenance of a balanced dNTP pool, and is an established cancer target. Nucleoside analogs such as gemcitabine diphosphate and clofarabine nucleotides target the large subunit (hRRM1) of hRR. These drugs have a poor therapeutic index due to toxicity caused by additional effects, including DNA chain termination. The discovery of nonnucleoside, reversible, small-molecule inhibitors with greater specificity against hRRM1 is a key step in the development of more effective treatments for cancer. Here, we report the identification and characterization of a unique nonnucleoside small-molecule hRR inhibitor, naphthyl salicylic acyl hydrazone (NSAH), using virtual screening, binding affinity, inhibition, and cell toxicity assays. NSAH binds to hRRM1 with an apparent dissociation constant of 37 µM, and steady-state kinetics reveal a competitive mode of inhibition. A 2.66-Å resolution crystal structure of NSAH in complex with hRRM1 demonstrates that NSAH functions by binding at the catalytic site (C-site) where it makes both common and unique contacts with the enzyme compared with NDP substrates. Importantly, the IC₅₀ for NSAH is within twofold of gemcitabine for growth inhibition of multiple cancer cell lines, while demonstrating little cytotoxicity against normal mobilized peripheral blood progenitor cells. NSAH depresses dGTP and dATP levels in the dNTP pool causing S-phase arrest, providing evidence for RR inhibition in cells. This report of a nonnucleoside reversible inhibitor binding at the catalytic site of hRRM1 provides a starting point for the design of a unique class of hRR inhibitors.

Review: The lord of the rings: Structure and mechanism of the sliding clamp loader

Sliding clamps are ring-shaped polymerase processivity factors that act as master regulators of cellular replication by coordinating multiple functions on DNA to ensure faithful transmission of genetic and epigenetic information. Dedicated AAA+ ATPase machines called clamp loaders actively place clamps on DNA, thereby governing clamp function by controlling when and where clamps are used. Clamp loaders are also important model systems for understanding the basic principles of AAA+ mechanism and function. After nearly 30 years of study, the ATP-dependent mechanism of opening and loading of clamps is now becoming clear. Here I review the structural and mechanistic aspects of the clamp loading process, as well as comment on questions that will be addressed by future studies. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 532-546, 2016.

Prognostic value and clinicopathological significance of proliferating cell nuclear antigen expression in gastric cancer: a systematic review and meta-analysis

BACKGROUND

The prognostic significance of proliferating cell nuclear antigen (PCNA) expression in gastric cancer has long been assessed, yet results remain controversial. Therefore, we performed a meta-analysis to assess the prognostic value and clinicopathological significance of PCNA in gastric cancer.

METHODS

A systematic literature search of PubMed, EMBASE, and the Cochrane Library databases was conducted. Summary odds ratios (ORs) and hazard ratios (HRs) with 95% confidence intervals (CIs) were calculated to investigate the correlations between PCNA expression and clinicopathological features, overall survival (OS), and disease-free survival (DFS).

RESULTS

A total of 19 studies involving 2,852 participants were included in our analysis. The pooled HR indicated that high PCNA expression was significantly associated with poor OS (HR 1.66, 95% CI 1.32-2.08) and DFS (HR 1.81, 95% CI 1.40-2.36). Subgroup analysis revealed that the association between PCNA and OS was also significant in Asian and European patients. In addition, the pooled ORs showed that high PCNA expression was significantly associated with deeper tumor invasion (OR 2.37, 95% CI 1.71-3.27), lymph node metastasis (OR 2.49, 95% CI 1.85-3.35), and advanced stage cancer (OR 1.89, 95% CI 1.36-2.63).

CONCLUSION

Our meta-analysis indicates that high PCNA expression might be a prognosticator of poor survival and a promising therapeutic target for gastric cancer patients.

Overexpression of PCNA Attenuates Oxidative Stress-Caused Delay of Gap-Filling during Repair of UV-Induced DNA Damage

UVC irradiation-caused DNA lesions are repaired in mammalian cells solely by nucleotide excision repair (NER), which consists of sequential events including initial damage recognition, dual incision of damage site, gap-filling, and ligation. We have previously shown that gap-filling during the repair of UV-induced DNA lesions may be delayed by a subsequent treatment of oxidants or prooxidants such as hydrogen peroxide, flavonoids, and colcemid. We considered the delay as a result of competition for limiting protein/enzyme factor(s) during repair synthesis between NER and base excision repair (BER) induced by the oxidative chemicals. In this report, using colcemid as oxidative stress inducer, we showed that colcemid-caused delay of gap-filling during the repair of UV-induced DNA lesions was attenuated by overexpression of PCNA but not ligase-I. PCNA knockdown, as expected, delayed the gap-filling of NER but also impaired the repair of oxidative DNA damage. Fen-1 knockdown, however, did not affect the repair of oxidative DNA damage, suggesting repair of oxidative DNA damage is not of long patch BER. Furthermore, overexpression of XRCC1 delayed the gap-filling, and presumably increase of XRCC1 pulls PCNA away from gap-filling of NER for BER, consistent with our hypothesis that delay of gap-filling of NER attributes the competition between NER and BER.

Deviating the level of proliferating cell nuclear antigen in Trypanosoma brucei elicits distinct mechanisms for inhibiting proliferation and cell cycle progression

The DNA replication machinery is spatially and temporally coordinated in all cells to reproduce a single exact copy of the genome per division, but its regulation in the protozoan parasite Trypanosoma brucei is not well characterized. We characterized the effects of altering the levels of proliferating cell nuclear antigen, a key component of the DNA replication machinery, in bloodstream form T. brucei. This study demonstrated that tight regulation of TbPCNA levels was critical for normal proliferation and DNA replication in the parasite. Depleting TbPCNA mRNA reduced proliferation, severely diminished DNA replication, arrested the synthesis of new DNA and caused the parasites to accumulated in G2/M. Attenuating the parasite by downregulating TbPCNA caused it to become hypersensitive to hydroxyurea. Overexpressing TbPCNA in T. brucei arrested proliferation, inhibited DNA replication and prevented the parasite from exiting G2/M. These results indicate that distinct mechanisms of cell cycle arrest are associated with upregulating or downregulating TbPCNA. The findings of this study validate deregulating intra-parasite levels of TbPCNA as a potential strategy for therapeutically exploiting this target in bloodstream form T. brucei.

Stabilization of protein-protein interaction complexes through small molecules

Most of the small molecules that have been identified thus far to modulate protein-protein interactions (PPIs) are inhibitors. Another promising way to interfere with PPI-associated biological processes is to promote PPI stabilization. Even though PPI stabilizers are still scarce, stabilization of PPIs by small molecules is gaining momentum and offers new pharmacological options. Therefore, we have performed a literature survey of PPI stabilization using small molecules. From this, we propose a classification of PPI stabilizers based on their binding mode and the architecture of the complex to facilitate the structure-based design of stabilizers.

STAT1 modification improves therapeutic effects of interferons on lung cancer cells

BACKGROUND

Interferons (IFNs) have potent anti-proliferative, pro-apoptotic, and immunomodulatory activities against cancer. However, the clinical utility of IFNs is limited by toxicity and pharmacokinetics making it difficult to achieve sustained therapeutic levels especially in solid tumors.

METHODS

Signal Transducer and Activator of Transcription 1 (STAT1) or a modified STAT1 (designated STAT1-CC) that is hyper-responsive to IFN were overexpressed in lung cancer SPC-A-1 and H1299 cells using lentiviral vectors. Transduction efficiency was monitored using enhanced green fluorescent protein (EGFP) expression. After transduction, cells were treated with interferon-gamma (IFN-γ) or interferon-beta (IFN-β) and monitored for cell proliferation, migration, and invasiveness using Cell Counting Kit-8 and transwell chamber assays and for apoptosis using Annexin V detection by flow cytometry. In addition, levels of STAT1, STAT1 Tyr-701 phosphorylation (pSTAT1), fibronectin, and β-catenin were determined using western blotting. In the case of IFN-γ stimulation, levels of S100A4, proliferating cell nuclear antigen (PCNA), and c-fos expression were also determined.

RESULTS

We found that expression of STAT1 or STAT1-CC enhanced the effect of IFN-γ and, IFN-β on inhibition of human lung cancer cell proliferation, migration and invasiveness. Moreover, STAT1 and STAT1-CC expression caused increases in pSTAT1 and decreases in fibronectin and β-catenin levels. STAT1-CC showed increased effects compared to STAT1 on IFN-γ induced pSTAT1 and down-regulation of S100A4, PCNA, and c-fos levels.

CONCLUSION

The results show that STAT1-CC exhibited more strength in improving the antitumor response of IFNs in lung cancer cells. Results from this study suggest that combined treatment of IFNs and STAT1-CC might be a feasible approach for the clinical management of lung cancer in the future.

Replication factor C3 is a CREB target gene that regulates cell cycle progression through the modulation of chromatin loading of PCNA

CREB (cyclic AMP response element-binding protein) is a transcription factor overexpressed in normal and neoplastic myelopoiesis and regulates cell cycle progression, although its oncogenic mechanism has not been well characterized. Replication factor C3 (RFC3) is required for chromatin loading of proliferating cell nuclear antigen (PCNA) which is a sliding clamp platform for recruiting numerous proteins in the DNA metabolism. CREB1 expression, which was activated by E2F, was coupled with RFC3 expression during the G1/S progression in the KG-1 acute myeloid leukemia (AML) cell line. There was also a direct correlation between the expression of RFC3 and CREB1 in human AML cell lines as well as in the AML cells from the patients. CREB interacted directly with the CRE site in RFC3 promoter region. CREB-knockdown inhibited primarily G1/S cell cycle transition by decreasing the expression of RFC3 as well as PCNA loading onto the chromatin. Exogenous expression of RFC3 was sufficient to rescue the impaired G1/S progression and PCNA chromatin loading caused by CREB knockdown. These studies suggest that RFC3 may have a role in neoplastic myelopoiesis by promoting the G1/S progression and its expression is regulated by CREB.

Bacterial Sliding Clamp Inhibitors that Mimic the Sequential Binding Mechanism of Endogenous Linear Motifs

The bacterial DNA replication machinery presents new targets for the development of antibiotics acting via novel mechanisms. One such target is the protein-protein interaction between the DNA sliding clamp and the conserved peptide linear motifs in DNA polymerases. We previously established that binding of linear motifs to the Escherichia coli sliding clamp occurs via a sequential mechanism that involves two subsites (I and II). Here, we report the development of small-molecule inhibitors that mimic this mechanism. The compounds contain tetrahydrocarbazole moieties as "anchors" to occupy subsite I. Functional groups appended at the tetrahydrocarbazole nitrogen bind to a channel gated by the side chain of M362 and lie at the edge of subsite II. One derivative induced the formation of a new binding pocket, termed subsite III, by rearrangement of a loop adjacent to subsite I. Discovery of the extended binding area will guide further inhibitor development.

p21 Exploits Residue Tyr151 as a Tether for High-Affinity PCNA Binding

Proliferating cell nuclear antigen (PCNA, processivity factor, sliding clamp) is a ring-shaped protein that tethers proteins to DNA in processes, including DNA replication, DNA repair, and cell-cycle control. Often used as a marker for cell proliferation, PCNA is overexpressed in cancer cells, making it an appealing pharmaceutical target. PCNA interacts with proteins through a PCNA interacting protein (PIP)-box, an eight-amino acid consensus sequence; different binding partners display a wide range of affinities based on function. Of all biological PIP-boxes, p21 has the highest known affinity for PCNA, allowing for inhibition of DNA replication and cell growth under cellular stress. As p21 is one of the few PIP-box sequences to contain a tyrosine rather than a phenylalanine in the eighth conserved position, we probed the significance of the hydroxyl group at this position using a mutational approach. Here we present the cocrystal structure of PCNA bound to a mutant p21 PIP-box peptide, p21Tyr151Phe, with associated isothermal titration calorimetry data. The p21Tyr151Phe peptide showed a 3-fold difference in affinity, as well as differences in entropy and enthalpy of binding. These differences can be attributed to a loss of hydrogen bonding capacity, as well as structural plasticity in the PCNA interdomain connector loop and the hydrophobic cavity of PCNA to which p21 binds. Thus, the hydroxyl group of Tyr151 in p21 acts as a tethering point for ideal packing and surface recognition of the peptide interface, increasing the binding affinity of p21 for PCNA.

Target validation and structure-activity analysis of a series of novel PCNA inhibitors

Proliferating cell nuclear antigen (PCNA) plays an essential role in DNA replication and repair. Tumor cells express high levels of PCNA, identifying it as a potentially ideal target for cancer therapy. Previously, we identified nine compounds termed PCNA inhibitors (PCNA-Is) that bind directly to PCNA, stabilize PCNA trimer structure, reduce chromatin-associated PCNA, and selectively inhibit tumor cell growth. Of these compounds, PCNA-I1 was most potent. The purpose of this study is to further establish targeting of PCNA by PCNA-I1 and to identify PCNA-I1 analogs with superior potencies. We found that PCNA-I1 does not affect the level of chromatin-associated PCNA harboring point mutations at the predicted binding site of PCNA-I1. Forty-six PCNA-I1 analogs with structures of 1-hydrazonomethyl-2-hydroxy (scaffold A), 2-hydrazonomethyl-1-hydroxy (scaffold B), 2-hydrazonomethyl-3-hydroxy (scaffold C), and 4-pyridyl hydrazine (scaffold D) were analyzed for their effects on cell growth in four tumor cell lines and PCNA trimer stabilization. Compounds in scaffold group A and group B showed the highest trimer stabilization and the most potent cell growth inhibitory activities with a significant potency advantage observed in the Z isomers of scaffold A. The absence of trimer stabilization and growth inhibitory effects in compounds of scaffold group D confirms the essentiality of the hydroxynaphthyl substructure. Compounds structure-activity relationship (SAR)-6 and SAR-24 were analyzed for their effects on and found to reduce chromatin-associated PCNA in tumor cells. This study led to the identification of SAR-24, a compound with superior potencies and potentially improved solubility, which will be used for future development of PCNA-targeting cancer therapies.

A Peptide mimicking a region in proliferating cell nuclear antigen specific to key protein interactions is cytotoxic to breast cancer

Proliferating cell nuclear antigen (PCNA) is a highly conserved protein necessary for proper component loading during the DNA replication and repair process. Proteins make a connection within the interdomain connector loop of PCNA, and much of the regulation is a result of the inherent competition for this docking site. If this target region of PCNA is modified, the DNA replication and repair process in cancer cells is potentially altered. Exploitation of this cancer-associated region has implications for targeted breast cancer therapy. In the present communication, we characterize a novel peptide (caPeptide) that has been synthesized to mimic the sequence identified as critical to the cancer-associated isoform of PCNA. This peptide is delivered into cells using a nine-arginine linking mechanism, and the resulting peptide (R9-cc-caPeptide) exhibits cytotoxicity in a triple-negative breast cancer cell line, MDA-MB-436, while having less of an effect on the normal counterparts (MCF10A and primary breast epithelial cells). The novel peptide was then evaluated for cytotoxicity using various in vivo techniques, including ATP activity assays, flow cytometry, and clonogenetic assays. This cytotoxicity has been observed in other breast cancer cell lines (MCF7 and HCC1937) and other forms of cancer (pancreatic and lymphoma). R9-cc-caPeptide has also been shown to block the association of PCNA with chromatin. Alanine scanning of the peptide sequence, combined with preliminary in silico modeling, gives insight to the disruptive ability and the molecular mechanism of action of the therapeutic peptide in vivo.

Antitumor effects of a novel small molecule targeting PCNA chromatin association in prostate cancer

Proliferating cell nuclear antigen (PCNA) plays an essential role in DNA replication and repair. Tumor cells express high levels of PCNA, identifying it as a potentially ideal target for cancer therapy. Previously, we identified nine compounds termed PCNA inhibitors (PCNA-Is) that bind directly to PCNA, stabilize PCNA trimer structure, reduce chromatin-associated PCNA, and selectively inhibit tumor cell growth. Of these compounds, PCNA-I1 is most potent. The purposes of this study were to further investigate the effects of targeting PCNA chromatin association on DNA damage and cytotoxicity and to evaluate the therapeutic potential of PCNA-I1 against tumors in mice. Given the important roles of tumor suppressor p53 in regulating sensitivity of tumor cells to chemotherapeutics, we performed studies in two human prostate cancer cell lines differing in p53 expression: LNCaP cells (wild-type p53) and PC-3 cells (p53-null). PCNA-I1 induced DNA damage and apoptosis in both LNCaP and PC-3 cells and enhanced DNA damage and apoptosis triggered by cisplatin. PCNA-I1 also induced autophagy in PC-3 cells. A short-term pretreatment with PCNA-I1 reduced colony formation by 50% in both cell lines. These data suggest that, unlike many other cytotoxic drugs, the effects of PCNA-I1 on tumor cells do not depend on expression of p53. Intravenous administrations of PCNA-I1 significantly retarded growth of LNCaP tumors of in nude mice without causing detectable effects on mouse body weight and hematology profiles. These data provide proof of concept that targeting PCNA chromatin association could be a novel and effective therapeutic approach for treatment of cancer.

Mitochondrial ferritin, a new target for inhibiting neuronal tumor cell proliferation

Mitochondrial ferritin (FtMt) has a significant effect on the regulation of cytosolic and mitochondrial iron levels. However, because of the deficiency of iron regulatory elements (IRE) in FtMt’s gene sequence, the exact function of FtMt remains unclear. In the present study, we found that FtMt dramatically inhibited SH-SY5Y cell proliferation and tumor growth in nude mice. Interestingly, excess FtMt did not adversely affect the development of drosophila. Additionally, we found that the expression of FtMt in human normal brain tissue was significantly higher than that of neuroblastoma, but not higher than that of neurospongioma. However, the expression of transferrin receptor 1 is completely opposite. We therefore hypothesized that increased expression of FtMt may negatively affect the vitality of neuronal tumor cells. Therefore, we further investigated the underlying mechanisms of FtMt’s inhibitory effects on neuronal tumor cell proliferation. As expected, FtMt overexpression disturbed the iron homeostasis of tumor cells and significantly downregulated the expression of proliferating cell nuclear antigen. Moreover, FtMt affected cell cycle, causing G1/S arrest by modifying the expression of cyclinD1, cyclinE, Cdk2, Cdk4 and p21. Remarkably, FtMt strongly upregulated the expression of the tumor suppressors, p53 and N-myc downstream-regulated gene-1 (NDRG1), but dramatically decreased C-myc, N-myc and p-Rb levels. This study demonstrates for the first time a new role and mechanism for FtMt in the regulation of cell cycle. We thus propose FtMt as a new candidate target for inhibiting neuronal tumor cell proliferation. Appropriate regulation of FtMt expression may prevent tumor cell growth. Our study may provide a new strategy for neuronal cancer therapy.

Aldose reductase inhibition suppresses azoxymethane-induced colonic premalignant lesions in C57BL/KsJ-db/db mice

Type-2 diabetes and obesity-related metabolic abnormalities are major risk factors for the development of colon cancer. In the present study, we examined the effects of polyol pathway enzyme aldose reductase (AR) inhibitor, fidarestat, on the development of azoxymethane (AOM)-induced colonic premalignant lesions in C57BL/KsJ-db/db obese mice. Our results indicate that fidarestat given in the drinking water caused a significant reduction in the total number of colonic premalignant lesions in the AOM treated obese mice. Further, the expression levels of PKC-β2, AKT, COX-2 and iNOS in the colonic mucosa of AOM-treated mice were significantly decreased by fidarestat. The serum levels of IL-1α, IP-10, MIG, TNF-α and VEGF are significantly suppressed in AOM + fidarestat treated obese mice. Fidarestat also decreased the expression of COX-2, iNOS, XIAP, survivin, β-catenin and NF-κB in high glucose-treated HT29 colon cancer cells. In conclusion, our results indicate that fidarestat inhibits the development of colonic premalignant lesions in an obesity-related colon cancer and is chemopreventive to colorectal carcinogenesis in obese individuals.

The Mcm2-7 replicative helicase: a promising chemotherapeutic target

Numerous eukaryotic replication factors have served as chemotherapeutic targets. One replication factor that has largely escaped drug development is the Mcm2-7 replicative helicase. This heterohexameric complex forms the licensing system that assembles the replication machinery at origins during initiation, as well as the catalytic core of the CMG (Cdc45-Mcm2-7-GINS) helicase that unwinds DNA during elongation. Emerging evidence suggests that Mcm2-7 is also part of the replication checkpoint, a quality control system that monitors and responds to DNA damage. As the only replication factor required for both licensing and DNA unwinding, Mcm2-7 is a major cellular regulatory target with likely cancer relevance. Mutations in at least one of the six MCM genes are particularly prevalent in squamous cell carcinomas of the lung, head and neck, and prostrate, and MCM mutations have been shown to cause cancer in mouse models. Moreover various cellular regulatory proteins, including the Rb tumor suppressor family members, bind Mcm2-7 and inhibit its activity. As a preliminary step toward drug development, several small molecule inhibitors that target Mcm2-7 have been recently discovered. Both its structural complexity and essential role at the interface between DNA replication and its regulation make Mcm2-7 a potential chemotherapeutic target.

Proteome analysis of a hepatocyte-specific BIRC5 (survivin)-knockout mouse model during liver regeneration

The Baculoviral IAP repeat-containing protein 5 (BIRC5), also known as inhibitor of apoptosis protein survivin, is a member of the chromosomal passenger complex and a key player in mitosis. To investigate the function of BIRC5 in liver regeneration, we analyzed a hepatocyte-specific BIRC5-knockout mouse model using a quantitative label-free proteomics approach. Here, we present the analyses of the proteome changes in hepatocyte-specific BIRC5-knockout mice compared to wildtype mice, as well as proteome changes during liver regeneration induced by partial hepatectomy in wildtype mice and mice lacking hepatic BIRC5, respectively. The BIRC5-knockout mice showed an extensive overexpression of proteins related to cellular maintenance, organization and protein synthesis. Key regulators of cell growth, transcription and translation MTOR and STAT1/STAT2 were found to be overexpressed. During liver regeneration proteome changes representing a response to the mitotic stimulus were detected in wildtype mice. Mainly proteins corresponding to proliferation, cell cycle and cytokinesis were up-regulated. The hepatocyte-specific BIRC5-knockout mice showed impaired liver regeneration, which had severe consequences on the proteome level. However, several proteins with function in mitosis were found to be up-regulated upon the proliferative stimulus. Our results show that the E3 ubiquitin-protein ligase UHRF1 is strongly up-regulated during liver regeneration independently of BIRC5.

A novel mechanism for regulating the activity of proliferating cell nuclear antigen by a small protein

Proliferating cell nuclear antigen (PCNA) forms a trimeric ring that associates with and influences the activity of many proteins participating in DNA metabolic processes and cell cycle progression. Previously, an uncharacterized small protein, encoded by TK0808 in the archaeon Thermococcus kodakarensis, was shown to stably interact with PCNA in vivo. Here, we show that this protein, designated Thermococcales inhibitor of PCNA (TIP), binds to PCNA in vitro and inhibits PCNA-dependent activities likely by preventing PCNA trimerization. Using hydrogen/deuterium exchange mass spectrometry and site-directed mutagenesis, the interacting regions of PCNA and TIP were identified. Most proteins bind to PCNA via a PCNA-interacting peptide (PIP) motif that interacts with the inter domain connecting loop (IDCL) on PCNA. TIP, however, lacks any known PCNA-interacting motif, suggesting a new mechanism for PCNA binding and regulation of PCNA-dependent activities, which may support the development of a new subclass of therapeutic biomolecules for inhibiting PCNA.

A PCNA-derived cell permeable peptide selectively inhibits neuroblastoma cell growth

Proliferating cell nuclear antigen (PCNA), through its interaction with various proteins involved in DNA synthesis, cell cycle regulation, and DNA repair, plays a central role in maintaining genome stability. We previously reported a novel cancer associated PCNA isoform (dubbed caPCNA), which was significantly expressed in a broad range of cancer cells and tumor tissues, but not in non-malignant cells. We found that the caPCNA-specific antigenic site lies between L126 and Y133, a region within the interconnector domain of PCNA that is known to be a major binding site for many of PCNA's interacting proteins. We hypothesized that therapeutic agents targeting protein-protein interactions mediated through this region may confer differential toxicity to normal and malignant cells. To test this hypothesis, we designed a cell permeable peptide containing the PCNA L126-Y133 sequence. Here, we report that this peptide selectively kills human neuroblastoma cells, especially those with MYCN gene amplification, with much less toxicity to non-malignant human cells. Mechanistically, the peptide is able to block PCNA interactions in cancer cells. It interferes with DNA synthesis and homologous recombination-mediated double-stranded DNA break repair, resulting in S-phase arrest, accumulation of DNA damage, and enhanced sensitivity to cisplatin. These results demonstrate conceptually the utility of this peptide for treating neuroblastomas, particularly, the unfavorable MYCN-amplified tumors.

NRAGE promotes cell proliferation by stabilizing PCNA in a ubiquitin-proteasome pathway in esophageal carcinomas

Neurotrophin receptor-interacting melanoma antigen-encoding gene homolog (NRAGE) is generally recognized as a tumor suppressor as it induces cell apoptosis and suppresses cell metastasis. However, it has recently been reported that NRAGE is overexpressed in lung cancer, melanoma and colon cancer, implicating a complicated role of NRAGE as we have expected. In the study, we aim to elucidate the functional roles and molecular mechanisms of NRAGE in esophageal carcinoma. We found that both NRAGE mRNA and protein were significantly overexpressed in esophageal tumor tissues. Consistently, both in vivo and in vitro analyses demonstrated that knockdown of NRAGE apparently inhibited cell growth, and cell cycle analysis further demonstrated that NRAGE knockdown cells were mainly arrested in G2M cell phase, accompanied with an apparent reduction of S phase. In the process of exploring molecular mechanisms, we found that either knockdown in vitro or knockout in vivo of NRAGE reduced proliferating cell nuclear antigen (PCNA) protein, expression of which could completely rescue the inhibited proliferation in NRAGE defective cells. Furthermore, NRAGE physically interacted with PCNA in esophageal cancer cells through DNA polymerase III subunit, and knockdown of NRAGE facilitated PCNA K48-linked polyubiquitination, leading PCNA to the proteasome-dependent degradation and a ubiquitin-specific protease USP10 was identified to be a key regulator in the process of K48 polyubiquitination in NRAGE-deleted cells. In conclusion, our study highlights a unique role of NRAGE and implies that NRAGE is likely to be an attractive oncotarget in developing novel genetic anticancer therapeutic strategies for esophageal squamous cell carcinomas.

PCNA: a silent housekeeper or a potential therapeutic target?

Proliferating cell nuclear antigen (PCNA) is known as a molecular marker for proliferation given its role in replication. Three identical molecules of PCNA form a molecular sliding clamp around the DNA double helix. This provides an essential platform on which multiple proteins are dynamically recruited and coordinately regulated. Over the past decade, new research has provided a deeper comprehension of PCNA as a coordinator of essential cellular functions for cell growth, death, and maintenance. Although the biology of PCNA in proliferation has been comprehensively reviewed, research progress in unveiling the potential of targeting PCNA for disease treatment has not been systematically discussed. Here we briefly summarize the basic structural and functional characteristics of PCNA, and then discuss new developments in its protein interactions, trimer formation, and signaling regulation that open the door to possible therapeutic targeting of PCNA.

A small molecule inhibitor of monoubiquitinated Proliferating Cell Nuclear Antigen (PCNA) inhibits repair of interstrand DNA cross-link, enhances DNA double strand break, and sensitizes cancer cells to cisplatin

Small molecule inhibitors of proliferating cell nuclear antigen (PCNA)/PCNA interacting protein box (PIP-Box) interactions, including T2 amino alcohol (T2AA), inhibit translesion DNA synthesis. The crystal structure of PCNA in complex with T2AA revealed that T2AA bound to the surface adjacent to the subunit interface of the homotrimer of PCNA in addition to the PIP-box binding cavity. Because this site is close to Lys-164, which is monoubiquitinated by RAD18, we postulated that T2AA would affect monoubiquitinated PCNA interactions. Binding of monoubiquitinated PCNA and a purified pol η fragment containing the UBZ and PIP-box was inhibited by T2AA in vitro. T2AA decreased PCNA/pol η and PCNA/REV1 chromatin colocalization but did not inhibit PCNA monoubiquitination, suggesting that T2AA hinders interactions of pol η and REV1 with monoubiquitinated PCNA. Interstrand DNA cross-links (ICLs) are repaired by mechanisms using translesion DNA synthesis that is regulated by monoubiquitinated PCNA. T2AA significantly delayed reactivation of a reporter plasmid containing an ICL. Neutral comet analysis of cells receiving T2AA in addition to cisplatin revealed that T2AA significantly enhanced formation of DNA double strand breaks (DSBs) by cisplatin. T2AA promoted colocalized foci formation of phospho-ATM and 53BP1 and up-regulated phospho-BRCA1 in cisplatin-treated cells, suggesting that T2AA increases DSBs. When cells were treated by cisplatin and T2AA, their clonogenic survival was significantly less than that of those treated by cisplatin only. These findings show that the inhibitors of monoubiquitinated PCNA chemosensitize cells by inhibiting repair of ICLs and DSBs.

Targeting proliferating cell nuclear antigen and its protein interactions induces apoptosis in multiple myeloma cells

Multiple myeloma is a hematological cancer that is considered incurable despite advances in treatment strategy during the last decade. Therapies targeting single pathways are unlikely to succeed due to the heterogeneous nature of the malignancy. Proliferating cell nuclear antigen (PCNA) is a multifunctional protein essential for DNA replication and repair that is often overexpressed in cancer cells. Many proteins involved in the cellular stress response interact with PCNA through the five amino acid sequence AlkB homologue 2 PCNA-interacting motif (APIM). Thus inhibiting PCNA’s protein interactions may be a good strategy to target multiple pathways simultaneously. We initially found that overexpression of peptides containing the APIM sequence increases the sensitivity of cancer cells to contemporary therapeutics. Here we have designed a cell-penetrating APIM-containing peptide, ATX-101, that targets PCNA and show that it has anti-myeloma activity. We found that ATX-101 induced apoptosis in multiple myeloma cell lines and primary cancer cells, while bone marrow stromal cells and primary healthy lymphocytes were much less sensitive. ATX-101-induced apoptosis was caspase-dependent and cell cycle phase-independent. ATX-101 also increased multiple myeloma cells’ sensitivity against melphalan, a DNA damaging agent commonly used for treatment of multiple myeloma. In a xenograft mouse model, ATX-101 was well tolerated and increased the anti-tumor activity of melphalan. Therefore, targeting PCNA by ATX-101 may be a novel strategy in multiple myeloma treatment.

Anti-cell proliferative efficacy of ferulic acid against 7, 12-dimethylbenz(a) anthracene induced hamster buccal pouch carcinogenesis

The present study was designed to explore the anti-cell proliferative efficacy of ferulic acid by analysing the expression pattern of cell proliferative markers, proliferating cellular nuclear antigen (PCNA) and cyclin D1, in the buccal mucosa of golden Syrian hamsters treated with 7,12-dimethylbenz(a)anthracene (DMBA). Oral squamous cell carcinomas developed in the buccal pouch of hamsters using topical application of 0.5% DMBA three times a week for 14 weeks. Immunohistochemical (PCNA) and RT-PCR (Cyclin D1) analysis revealed over expression of PCNA and cyclin D1 in the buccal mucosa of hamsters treated with DMBA alone (tumor bearing hamsters). Oral administration of ferulic acid at a dose of 40 mg/kg bw to hamsters treated with DMBA not only completely prevented the tumor formation but also down regulated the expression of PCNA and cyclin D1. The results of the present study thus suggests that ferulic acid might have inhibited tumor formation in the buccal mucosa of hamsters treated with DMBA through its anti-cell proliferative potential as evidenced by decreased expression of PCNA and cyclin D1.

Retinoic acid inhibits the cytoproliferative response to weak 50‑Hz magnetic fields in neuroblastoma cells

We previously reported that intermittent exposure to a 50‑Hz magnetic field (MF) at 100 µT stimulates cell proliferation in the human neuroblastoma cell line NB69. The present study aimed to investigate whether the magnetic field-induced growth promotion also occurs at a lower magnetic flux density of 10 µT. To this purpose, NB69 cells were subjected for 42 h to intermittent exposure, 3 h on/3 h off, to a 50‑Hz MF at a 10 or 100 µT magnetic flux density. The field exposure took place either in the presence or in the absence of the antiproliferative agent retinoic acid. At the end of the treatment and/or incubation period, the cell growth was estimated by hemocytometric counting and spectrophotometric analysis of total protein and DNA contents. Potential changes in DNA synthesis were also assessed through proliferating cell nuclear antigen (PCNA) immunolabeling. The results confirmed previously reported data that a 42-h exposure to a 50‑Hz sine wave MF at 100 µT promotes cell growth in the NB69 cell line, and showed that 10 µT induces a similar proliferative response. This effect, which was significantly associated and linearly correlated with PCNA expression, was abolished by the presence of retinoic acid in the culture medium.