The fellowship of the rings: distinct pools of proliferating cell nuclear antigen trimer at work

UBE2V2 promotes metastasis by regulating EMT and predicts a poor prognosis in lung adenocarcinoma

PURPOSE

As a member of the ubiquitin-conjugating enzyme (E2) family, UBE2V2 demonstrates significant tumorigenicity in many cancers. However, the relationship between UBE2V2 expression and the morbidity of lung adenocarcinoma (LUAD) is still unknown.

METHODS

We detected the mRNA and protein expression of UBE2V2 and analyzed its relationship with clinical parameters as well as survival prognosis based on bioinformatic and immunohistochemistry (IHC) in LUAD. The signaling pathway of UBE2V2 in the development of LUAD was obtained by GSEA. The TIMER database was used to investigate the association between UBE2V2 expression and the level of infiltration of different immune cells. Finally, we explored the effects of UBE2V2 knockdown on the proliferation, apoptosis, and migration of LUAD cells.

RESULTS

The results showed that UBE2V2 was a potential oncogene and might be considered an independent prognostic molecule for LUAD patients based on TCGA prediction (HR: 1.497 p = 0.012) and IHC (HR:1.864 p = 0.044). IHC showed that UBE2V2 was related to the following clinicopathological factors: gender (p = 0.043), stage (p = 0.042), and lymph node metastasis (p = 0.002). Finally, knockdown of UBE2V2 reduced the migration of LUAD cells by regulating EMT-related proteins. Knockdown of UBE2V2 induced LUAD cells to arrest in the G1 phase. Knockdown of UBE2V2 increased LUAD cell apoptosis and decreased proliferation, which might be related to the downregulation of PCNA and upregulation of P53 and ƳH2AX expression. Interestingly, UBE2V2 is negatively correlated with B cells, CD4+ T cells, macrophages, and dendritic cells.

CONCLUSION

UBE2V2 may be a valuable therapeutic target for lung cancer.

Revisiting the Function of p21CDKN1A in DNA Repair: The Influence of Protein Interactions and Stability

The p21CDKN1A protein is an important player in the maintenance of genome stability through its function as a cyclin-dependent kinase inhibitor, leading to cell-cycle arrest after genotoxic damage. In the DNA damage response, p21 interacts with specific proteins to integrate cell-cycle arrest with processes such as transcription, apoptosis, DNA repair, and cell motility. By associating with Proliferating Cell Nuclear Antigen (PCNA), the master of DNA replication, p21 is able to inhibit DNA synthesis. However, to avoid conflicts with this process, p21 protein levels are finely regulated by pathways of proteasomal degradation during the S phase, and in all the phases of the cell cycle, after DNA damage. Several lines of evidence have indicated that p21 is required for the efficient repair of different types of genotoxic lesions and, more recently, that p21 regulates DNA replication fork speed. Therefore, whether p21 is an inhibitor, or rather a regulator, of DNA replication and repair needs to be re-evaluated in light of these findings. In this review, we will discuss the lines of evidence describing how p21 is involved in DNA repair and will focus on the influence of protein interactions and p21 stability on the efficiency of DNA repair mechanisms.

The protective effect of fenofibrate, triptorelin, and their combination against premature ovarian failure in rats

Cyclophosphamide (CP) is a chemotherapy alkylating agent that causes a lot of side effects including premature ovarian failure (POF). This study aimed to evaluate the possible protective effect of fenofibrate (FEN) in CP-induced POF. Rats were randomly divided into five groups as follows: negative control, CP, triptorelin (TRI)-treated, FEN (FEN)-treated, and FEN + TRI-treated. Histological study, collagen area fraction, and immunoexpression of proliferating cell nuclear antigen (PCNA) were evaluated. Also, estrogen, anti-mullerian hormone (AMH), follicle-stimulating hormone (FSH), luteinizing hormone (LH) and ovarian malondialdehyde (MDA), nitric oxide (NOx), reduced glutathione (GSH), superoxide dismutase (SOD), interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-α), and vascular endothelial growth factor (VEGF) were measured. CP significantly reduced ovarian follicle count, as compared with the control group (1.00 ± 0.76 versus 7.75 ± 1.83, respectively). Meanwhile, FEN, either solely or in combination with TRI, significantly increased ovarian follicle count, as compared with the CP group (3.88 ± 0.83 and 5.75 ± 1.39, respectively). As compared with the control group, CP increased the levels of MDA, NOx, IL-10, TNF-α, FSH, LH, and collagen area fraction; however, levels of GSH, SOD, VEGF, AMH, estrogen, and PCNA immunoexpression were reduced with CP. Administration of FEN either solely or in combination with TRI showed significant improvement in all the parameters previously mentioned. FEN can protect the ovary from CP-induced side effects possibly through antioxidant and anti-inflammatory actions.

Targeting Proliferating Cell Nuclear Antigen (PCNA) as an Effective Strategy to Inhibit Tumor Cell Proliferation

Targeting highly proliferating cells is an important issue for many types of aggressive tumors. Proliferating Cell Nuclear Antigen (PCNA) is an essential protein that participates in a variety of processes of DNA metabolism, including DNA replication and repair, chromatin organization and transcription and sister chromatid cohesion. In addition, PCNA is involved in cell survival, and possibly in pathways of energy metabolism, such as glycolysis. Thus, the possibility of targeting this protein for chemotherapy against highly proliferating malignancies is under active investigation. Currently, approaches to treat cells with agents targeting PCNA rely on the use of small molecules or on peptides that either bind to PCNA, or act as a competitor of interacting partners. Here, we describe the status of the art in the development of agents targeting PCNA and discuss their application in different types of tumor cell lines and in animal model systems.

In Situ Analysis of DNA-Protein Complex Formation upon Radiation-Induced DNA Damage

The importance of determining at the cellular level the formation of DNA–protein complexes after radiation-induced lesions to DNA is outlined by the evidence that such interactions represent one of the first steps of the cellular response to DNA damage. These complexes are formed through recruitment at the sites of the lesion, of proteins deputed to signal the presence of DNA damage, and of DNA repair factors necessary to remove it. Investigating the formation of such complexes has provided, and will probably continue to, relevant information about molecular mechanisms and spatiotemporal dynamics of the processes that constitute the first barrier of cell defense against genome instability and related diseases. In this review, we will summarize and discuss the use of in situ procedures to detect the formation of DNA-protein complexes after radiation-induced DNA damage. This type of analysis provides important information on the spatial localization and temporal resolution of the formation of such complexes, at the single-cell level, allowing the study of heterogeneous cell populations.

Niclosamide suppresses acute myeloid leukemia cell proliferation through inhibition of CREB-dependent signaling pathways

CREB (cAMP Response Element Binding protein) is a transcription factor that is overexpressed in primary acute myeloid leukemia (AML) cells and associated with a decreased event-free survival and increased risk of relapse. We recently reported a small molecule inhibitor of CREB, XX-650-23, which inhibits CREB activity in AML cells. Structure-activity relationship analysis for chemical compounds with structures similar to XX-650-23 led to the identification of the anthelminthic drug niclosamide as a potent anti-leukemic agent that suppresses cell viability of AML cell lines and primary AML cells without a significant decrease in colony forming activity of normal bone marrow cells. Niclosamide significantly inhibited CREB function and CREB-mediated gene expression in cells, leading to apoptosis and G1/S cell cycle arrest with reduced phosphorylated CREB levels. CREB knockdown protected cells from niclosamide treatment-mediated cytotoxic effects. Furthermore, treatment with a combination of niclosamide and CREB inhibitor XX-650-23 showed an additive anti-proliferative effect, consistent with the hypothesis that niclosamide and XX-650-23 regulate the same targets or pathways to inhibit proliferation and survival of AML cells. Niclosamide significantly inhibited the progression of disease in AML patient-derived xenograft (PDX) mice, and prolonged survival of PDX mice. Niclosamide also showed synergistic effects with chemotherapy drugs to inhibit AML cell proliferation. While chemotherapy antagonized the cytotoxic potential of niclosamide, pretreatment with niclosamide sensitized cells to chemotherapeutic drugs, cytarabine, daunorubicin, and vincristine. Therefore, our results demonstrate niclosamide as a potential drug to treat AML by inducing apoptosis and cell cycle arrest through inhibition of CREB-dependent pathways in AML cells.

Proliferating cell nuclear antigen in neutrophil fate

The life span of a neutrophil is a tightly regulated process as extended survival is beneficial for pathogen elimination and cell death necessary to prevent cytotoxic content release from activated neutrophils at the inflammatory site. Therefore, the control between survival and death must be a dynamic process. We have previously described that proliferating cell nuclear antigen (PCNA) which is known as a nuclear protein pivotal in DNA synthesis, is a key element in controlling neutrophil survival through its association with procaspases. Contrary to the dogma which asserted that PCNA has a strictly nuclear function, in mature neutrophils, PCNA is present exclusively within the cytosol due to its nuclear export at the end of the granulocytic differentiation. More recent studies are consistent with the notion that the cytosolic scaffold of PCNA is aimed at modulating neutrophil fate rather than simply preventing death. Ultimately, targeting neutrophil survival might have important applications not just in the field of immunology and inflammation, but also in hematology and transfusion. The neutrophil emerges as a unique and powerful cellular model to unravel the basic mechanisms governing the cell cycle-independent functions of PCNA and should be considered as a leader of the pack.

Potential Antitumor Activity and Apoptosis Induction of Glossostemon bruguieri Root Extract against Hepatocellular Carcinoma Cells

Glossostemon bruguieri (moghat) is used as a nutritive and demulcent drink. This study was performed to investigate the antiproliferative effects of moghat root extract (MRE) and its apoptotic mechanism in hepatocellular carcinoma (HCC) cells, HepG2 and Hep3B. MTT assay, morphological changes, apoptosis enzyme linked immunosorbent assay, caspase and apoptotic activation, flow cytometry, and immunoblot analysis were employed. The IC₅₀ of MRE for HepG2 (910 ± 6 μg/ml) and for Hep3B (1510 ± 5 μg/ml) induced significant growth-inhibitory effects against HCC cells, with no cytotoxic effect on normal hepatocytes. MRE treatment induced apoptotic effects to HepG2 cells in a caspase-dependent manner and via upregulating p53/p21 and PCNA. The upregulation of p21 was controlled by p53 expression in HepG2 but not in Hep3B despite upregulation of Bax protein in both cell lines. Interestingly, p21 may be a remarkable switch to G1 arrest in HepG2 cells, but not in Hep3B cells. In addition, Fas- and mitochondria-mediated pathways were found to be involved in MRE-induced apoptosis in Hep3B cells. The GC-MS analysis of MRE revealed two major constituents of pharmaceutical importance: the flavonoid apigenin (17.04%) and the terpenoid squalene (11.32%). The data presented in this paper introduces G. bruguieri as a promising nontoxic herb with therapeutic potential for HCC. To the authors' knowledge, the present study provides the first report on the anticancer activity of MRE on HCC cells.

Assessing Cell Cycle Independent Function of the CDK Inhibitor p21(CDKN¹A) in DNA Repair

The cyclin-dependent kinase (CDK) inhibitor p21(CDKN1A) is a small protein that is able to regulate many important cell functions, often independently of its activity of CDK inhibitor. In addition to cell cycle, this protein regulates cell transcription, apoptosis, cell motility, and DNA repair. In particular, p21 may participate in different DNA repair processes, like the nucleotide excision repair (NER), base excision repair (BER), and double-strand breaks (DSB) repair, because of its ability to interact with DNA repair proteins, such as proliferating cell nuclear antigen (PCNA), a master regulator of many DNA transactions. Although this role has been debated for a long time, the influence of p21 in DNA repair has been now established. However, it remain to be clarified how this role is coupled to proteasomal degradation that has been shown to occur after DNA damage. This chapter describes procedures to study p21 protein recruitment to localized DNA damage sites in the cell nucleus. In particular, we describe a technique based on local irrradiation with UV light through a polycarbonate filter with micropores; an in situ lysis procedure to detect chromatin-bound proteins by immunofluorescence; a cell fractionation procedure to study chromatin association of p21 by Western blot analysis, and p21 protein-protein interactions by an immunoprecipitation assay.

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.

CBP and p300 acetylate PCNA to link its degradation with nucleotide excision repair synthesis

The proliferating cell nuclear antigen (PCNA) protein serves as a molecular platform recruiting and coordinating the activity of factors involved in multiple deoxyribonucleic acid (DNA) transactions. To avoid dangerous genome instability, it is necessary to prevent excessive retention of PCNA on chromatin. Although PCNA functions during DNA replication appear to be regulated by different post-translational modifications, the mechanism regulating PCNA removal and degradation after nucleotide excision repair (NER) is unknown. Here we report that CREB-binding protein (CBP), and less efficiently p300, acetylated PCNA at lysine (Lys) residues Lys13,14,77 and 80, to promote removal of chromatin-bound PCNA and its degradation during NER. Mutation of these residues resulted in impaired DNA replication and repair, enhanced the sensitivity to ultraviolet radiation, and prevented proteolytic degradation of PCNA after DNA damage. Depletion of both CBP and p300, or failure to load PCNA on DNA in NER deficient cells, prevented PCNA acetylation and degradation, while proteasome inhibition resulted in accumulation of acetylated PCNA. These results define a CBP and p300-dependent mechanism for PCNA acetylation after DNA damage, linking DNA repair synthesis with removal of chromatin-bound PCNA and its degradation, to ensure genome stability.

Rapidly cleaving bovine two-cell embryos have better developmental potential and a distinctive mRNA pattern

Mammalian embryos that rapidly reach the two-cell stage in culture have a higher probability of becoming viable blastocysts. Our goal was to separate two-cell bovine embryos based on their zygotic cleavage timing, and to assess their global mRNA levels. Following in vitro fertilization, all embryos that cleaved by 29.5 hpi (early) were cultured separately from those that divided at 46 hpi (late). The blastocyst rates were 46.1 ± 3.7% and 6.1 ± 3.4% for early- and late-cleavers, respectively (P < 0.01). Seven replicates of selected two-cell embryos were collected at each time point for microarray characterization (n = 4) and quantitative reverse-transcriptase PCR (n = 3); the rest were left in culture for blastocyst evaluation. A total of 774 and 594 probes were preferentially present in early- and late-cleaving embryos, respectively (fold change ± 1.5, P < 0.05), with important contrasts related to cell cycle, gene expression, RNA processing, and protein degradation functions. A total of 12 transcripts were assessed by quantitative PCR, of which ATM, ATR, CTNNB1, MSH6, MRE11A, PCNA, APC, CENPE, and GRB2 were in agreement with the hybridization results. Since most of these molecules are directly or indirectly associated with cell-cycle regulation, DNA damage response, and transcription control, our results strongly suggest key roles for those biological functions in mammalian preimplantation development.

DDB2 association with PCNA is required for its degradation after UV-induced DNA damage

DDB2 is a protein playing an essential role in the lesion recognition step of the global genome sub-pathway of nucleotide excision repair (GG-NER) process. Among the proteins involved in the DNA damage response, p21(CDKN1A) (p21) has been reported to participate in NER, but also to be removed by proteolytic degradation, thanks to its association with PCNA. DDB2 is involved in the CUL4-DDB1 complex mediating p21 degradation; however, the direct interaction between DDB2, p21 and PCNA has been never investigated. Here, we show that DDB2 co-localizes with PCNA and p21 at local UV-induced DNA-damage sites, and these proteins co-immunoprecipitate in the same complex. In addition, we provide evidence that p21 is not able to bind directly DDB2, but, to this end, the presence of PCNA is required. Direct physical association of recombinant DDB2 protein with PCNA is mediated by a conserved PIP-box present in the N-terminal region of DDB2. Mutation of the PIP-box resulted in the loss of protein interaction. Interestingly, the same mutation, or depletion of PCNA by RNA interference, greatly impaired DDB2 degradation induced by UV irradiation. These results indicate that DDB2 is a PCNA-binding protein, and that this association is required for DDB2 proteolytic degradation.

Targeting cytosolic proliferating cell nuclear antigen in neutrophil-dominated inflammation

New therapeutic approaches that can accelerate neutrophil apoptosis under inflammatory conditions to enhance the resolution of inflammation are now under study. Neutrophils are deprived of proliferative capacity and have a tightly controlled lifespan to avoid their persistence at the site of injury. We have recently described that the proliferating cell nuclear antigen (PCNA), a nuclear factor involved in DNA replication and repair of proliferating cells is a key regulator of neutrophil survival. The nuclear-to-cytoplasmic relocalization occurred during granulocytic differentiation and is dependent on a nuclear export sequence thus strongly suggesting that PCNA has physiologic cytoplasmic functions. In this review, we will try to put into perspective the physiologic relevance of PCNA in neutrophils. We will discuss key issues such as molecular structure, post-translational modifications, based on our knowledge of nuclear PCNA, assuming that similar principles governing its function are conserved between nuclear and cytosolic PCNA. The example of cystic fibrosis that features one of the most intense neutrophil-dominated pulmonary inflammation will be discussed. We believe that through an intimate comprehension of the cytosolic PCNA scaffold based on nuclear PCNA knowledge, novel pathways regulating neutrophil survival can be unraveled and innovative agents can be developed to dampen inflammation where it proves detrimental.

Nuclear-to-cytoplasmic relocalization of the proliferating cell nuclear antigen (PCNA) during differentiation involves a chromosome region maintenance 1 (CRM1)-dependent export and is a prerequisite for PCNA antiapoptotic activity in mature neutrophils

Neutrophils are deprived of proliferative capacity and have a tightly controlled lifespan to avoid their persistence at the site of injury. We have recently described that the proliferating cell nuclear antigen (PCNA), a nuclear factor involved in DNA replication and repair of proliferating cells, is a key regulator of neutrophil survival. In neutrophils, PCNA was localized exclusively in the cytoplasm due to its nuclear-to-cytoplasmic relocalization during granulocytic differentiation. We showed here that leptomycin B, an inhibitor of the chromosome region maintenance 1 (CRM1) exportin, inhibited PCNA relocalization during granulocytic differentiation of HL-60 and NB4 promyelocytic cell lines and of human CD34(+) primary cells. Using enhanced green fluorescent protein fusion constructs, we have demonstrated that PCNA relocalization involved a nuclear export signal (NES) located from Ile-11 to Ile-23 in the PCNA sequence. However, this NES, located at the inner face of the PCNA trimer, was not functional in wild-type PCNA, but instead, was fully active and leptomycin B-sensitive in the monomeric PCNAY114A mutant. To test whether a defect in PCNA cytoplasmic relocalization would affect its antiapoptotic activity in mature neutrophils, a chimeric PCNA fused with the SV40 nuclear localization sequence (NLS) was generated to preclude its cytoplasmic localization. As expected, neutrophil-differentiated PLB985 cells expressing ectopic SV40NLS-PCNA had an increased nuclear PCNA as compared with cells expressing wild-type PCNA. Accordingly, the nuclear PCNA mutant did not show any antiapoptotic activity as compared with wild-type PCNA. Nuclear-to-cytoplasmic relocalization that occurred during myeloid differentiation is essential for PCNA antiapoptotic activity in mature neutrophils and is dependent on the newly identified monomerization-dependent PCNA NES.

Trypanosoma cruzi DNA replication includes the sequential recruitment of pre-replication and replication machineries close to nuclear periphery

In eukaryotes, many nuclear processes are spatially compartmentalized. Previously, we have shown that in Trypanosoma cruzi, an early-divergent eukaryote, DNA replication occurs at the nuclear periphery where chromosomes remain constrained during the S phase of the cell cycle. We followed Orc1/Cdc6, a pre-replication machinery component and the proliferating cell nuclear antigen (PCNA), a component of replication machinery, during the cell cycle of this protozoon. We found that, at the G(1) stage, TcOrc1/Cdc6 and TcPCNA are dispersed throughout the nuclear space. During the G(1)/S transition, TcOrc1/Cdc6 migrates to a region close to nuclear periphery. At the onset of S phase, TcPCNA is loaded onto the DNA and remains constrained close to nuclear periphery. Finally, in G(2), mitosis and cytokinesis, TcOrc1/Cdc6 and TcPCNA are dispersed throughout the nuclear space. Based on these findings, we propose that DNA replication in T. cruzi is accomplished by the organization of functional machineries in a spatial-temporal manner.

Phosphorylation of human DNMT1: implication of cyclin-dependent kinases

DNA methylation plays a central role in the epigenetic regulation of gene expression during development and progression of cancer diseases. The inheritance of specific DNA methylation patterns are acquired in the early embryo and are specifically maintained after cellular replication via the DNA methyltransferase 1 (DNMT1). Recent studies have suggested that the enzymatic activity of DNMT1 is possibly modulated by phosphorylation of serine/threonine residues located in the N-terminal domain of the enzyme. In the present work, we report that cyclin-dependent kinases (CDKs) 1, 2 and 5 can phosphorylate Ser154 of human DNMT1 in vitro. Further evidence of phosphorylation of endogenous DNMT1 at position 154 by CDKs is also found in 293 cells treated with roscovitine, a specific inhibitor of CDK1, 2 and 5. To determine the importance of Ser154 phosphorylation, a mutant of DNMT1 encoding a single-point mutation at position 154 (S154A) was generated. This mutation induced a severe loss of enzymatic activity when compared to wild type DNMT1. Moreover, after treatment with 5-Aza-2'-Deoxycytidine (5-aza-dC), a faster decline in DNMT1 protein level was observed for HEK-293 cells expressing DNMT1(S154A) as compared to cells expressing wild type DNMT1. Our data suggest that phosphorylation of DNMT1 at Ser154 by CDKs is important for enzymatic activity and protein stability of DNMT1. Considering that tumour-associated cell cycle defects are often mediated by alterations in CDK activity, our results suggest that dysregulation of cell cycle via CDKs could induce abnormal phosphorylation of DNMT1 and lead to DNA hypermethylation often observed in cancer cells.

Regulating neutrophil apoptosis: new players enter the game

Recently, unexpected biological features of polymorphonuclear neutrophils have been revealed. In addition to their pivotal role in the defence against pathogens, neutrophils display a high degree of plasticity and contribute to control of adaptive immune responses. An emerging aspect of neutrophils is their ability to modulate their survival in response to both intrinsic and extrinsic factors. This review focuses on recent advances that have uncovered proliferating cell nuclear antigen (PCNA) and other cell cycle regulatory proteins as novel players regulating neutrophil survival. A better understanding of the mechanisms involved in neutrophil fate might pave the way for the identification of new anti-inflammatory molecules.

Post-translational modification of cellular proteins by ubiquitin and ubiquitin-like molecules: role in cellular senescence and aging

Ubiquitination ofendogenous proteins is one of the key regulatory steps that guides protein degradation through regulation of proteasome activity. During the last years evidence has accumulated that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and age-associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system ofubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination confers also functions different from targeting proteins for degradation. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, post-translational modifications of cellular proteins by covalent attachment of ubiquitin and ubiquitin-like proteins are involved in transcriptional regulation, receptor internalization, DNA repair, stabilization of protein complexes and autophagy. Here, we summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and discuss potential causes and consequences for aging.

Proliferating cell nuclear antigen in the cytoplasm interacts with components of glycolysis and cancer

Proliferating cell nuclear antigen (PCNA) is involved in a wide range of functions in the nucleus. However, a substantial amount of PCNA is also present in the cytoplasm, although their function is unknown. Here we show, through Far-Western blotting and mass spectrometry, that PCNA is associated with several cytoplasmic oncoproteins, including elongation factor, malate dehydrogenase, and peptidyl-prolyl isomerase. Surprisingly, PCNA is also associated with six glycolytic enzymes that are involved in the regulation of steps 4-9 in the glycolysis pathway.

Melanocortin 1 receptor genotype: an important determinant of the damage response of melanocytes to ultraviolet radiation

The melanocortin 1 receptor gene is a main determinant of human pigmentation, and a melanoma susceptibility gene, because its variants that are strongly associated with red hair color increase melanoma risk. To test experimentally the association between melanocortin 1 receptor genotype and melanoma susceptibility, we compared the responses of primary human melanocyte cultures naturally expressing different melanocortin 1 receptor variants to α-melanocortin and ultraviolet radiation. We found that expression of 2 red hair variants abolished the response to α-melanocortin and its photoprotective effects, evidenced by lack of functional coupling of the receptor, and absence of reduction in ultraviolet radiation-induced hydrogen peroxide generation or enhancement of repair of DNA photoproducts, respectively. These variants had different heterozygous effects on receptor function. Microarray data confirmed the observed differences in responses of melanocytes with functional vs. nonfunctional receptor to α-melanocortin and ultraviolet radiation, and identified DNA repair and antioxidant genes that are modulated by α-melanocortin. Our findings highlight the molecular mechanisms by which the melanocortin 1 receptor genotype controls genomic stability of and the mutagenic effect of ultraviolet radiation on human melanocytes.

p21CDKN1A participates in base excision repair by regulating the activity of poly(ADP-ribose) polymerase-1

The cell cycle inhibitor p21(CDKN1A) has been shown to participate in nucleotide excision repair by interacting with PCNA. Here we have investigated whether p21 plays a role in base excision repair (BER), by analyzing p21 interactions with BER factors, and by assessing the response of p21(-/-) human fibroblasts to DNA damage induced by alkylating agents. Absence of p21 protein resulted in a higher sensitivity to alkylation-induced DNA damage, as indicated by reduced clonogenic efficiency, defective DNA repair (assessed by the comet test), and by persistence of histone H2AX phosphorylation. To elucidate the mechanisms at the basis of the function of p21 in BER, we focused on its interaction with poly(ADP-ribose) polymerase-1 (PARP-1), an important player in this repair process. p21 was found to bind the automodification/DNA binding domain of PARP-1, although some interaction occurred also with the catalytic domain after DNA damage. This association was necessary to regulate PARP-1 activity since poly(ADP-ribosylation) induced by DNA damage was higher in p21(-/-) human fibroblasts than in parental p21(+/+) cells, and in primary fibroblasts after p21 knock-down by RNA interference. Concomitantly, recruitment of PARP-1 and PCNA to damaged DNA was greater in p21(-/-) than in p21(+/+) fibroblasts. This accumulation resulted in persistent interaction of PARP-1 with BER factors, such as XRCC1 and DNA polymerase beta, suggesting that prolonged association reduced the DNA repair efficiency. These results indicate that p21 regulates the interaction between PARP-1 and BER factors, to promote efficient DNA repair.

Degradation of p21CDKN1A after DNA damage is independent of type of lesion, and is not required for DNA repair

The inhibitor of cyclin-dependent kinases p21CDKN1A plays a fundamental role in several pathways involved in the DNA damage response, like checkpoint-mediated cell cycle arrest, transcription, apoptosis, and DNA repair. Although p21 protein level is regulated by proteasomal degradation, the relationship of this process with DNA repair pathways is not yet clear. In addition, the role of protein/protein interaction in regulating turnover of p21 protein, is controversial. Here, we show that in human fibroblasts treated with agents inducing lesions repaired through nucleotide excision repair (NER), or base excision repair (BER), p21 degradation was triggered more by the extent, than by the type of DNA damage, or consequent DNA repair pathway. In fact, lowering the amount of DNA damage resulted in an increased stability of p21 protein. Overexpression of p21 was found to obscure degradation, both for p21wt and a p21 mutant unable to bind PCNA (p21PCNA-). However, when expressed to lower levels, turnover of p21 protein after DNA damage was greatly influenced by interaction with PCNA, since p21PCNA- was more efficiently degraded than wild-type protein. Interestingly, a p21 mutant protein unable to localize in the nucleus because of mutations in the NLS region, was not degraded after DNA damage, thus indicating that nuclear localization is necessary for p21 turnover. Removal of p21 was not required for NER activity, since inhibition of p21 degradation by caffeine did not affect the UV-induced recruitment of repair proteins, such as PCNA and DNA polymerase delta, nor significantly influence DNA repair synthesis, as determined by autoradiography. These results indicate that degradation of p21 is not dependent on a particular DNA repair pathway, and is not required for efficient DNA repair.

p21 differentially regulates DNA replication and DNA-repair-associated processes after UV irradiation

Although p21 upregulation is required to block cell-cycle progression following many types of genotoxic insult, UV irradiation triggers p21 proteolysis. The significance of the increased p21 turnover is unclear and might be associated with DNA repair. While the role of p21 in nucleotide excision repair (NER) remains controversial, recent reports have explored its effect on translesion DNA synthesis (TLS), a process that avoids replication blockage during S phase. Herein, we analyze the effect of p21 on different PCNA-driven processes including DNA replication, NER and TLS. Whereas only the CDK-binding domain of p21 is required for cell-cycle arrest in unstressed cells, neither the CDK-binding nor the PCNA-binding domain of p21 is able to block early and late steps of NER. Intriguingly, through its PCNA-binding domain, p21 inhibits the interaction of the TLS polymerase, pol eta (pol eta), with PCNA and impairs the assembly of pol eta foci after UV. Moreover, this obstruction correlates with accumulation of phosphorylated H2AX and increased apoptosis. By showing that p21 is a negative regulator of PCNA-pol eta interaction, our data unveil a link between efficient TLS and UV-induced degradation of p21.

Human hepatocellular carcinoma expresses specific PCNA isoforms: an in vivo and in vitro evaluation

Proliferating cell nuclear antigen (PCNA) is a 36 kDa protein involved in several cellular mechanisms, including DNA synthesis and repair, cell cycle regulation and apoptosis. An alteration in PCNA structure might contribute to DNA-damage accumulation in cancer cells. This study was aimed to evaluate the PCNA pattern of expression, in terms of aggregation status, isoforms and post-translational modifications, in human hepatocellular carcinoma (HCC) and cirrhosis as well as in HCC cell lines. Twelve HCCs and surrounding cirrhotic tissues were analysed, along with HepG2, Hep3B and SNU-398 cell lines. Normal liver specimens and cirrhosis without HCC were included as controls. Both DNA-bound and DNA-unbound PCNA fractions were analysed, and PCNA pattern of expression was displayed on two-dimensional gel electrophoresis followed by western blot. Results were confirmed by mass spectrometry. To compare HCCs vs surrounding tissues, immunolabelling and immunostaining were performed. In 6 of 12 HCCs and in cell lines, we found three major PCNA acidic forms, corresponding to monomers, probably dimers and trimers, and a basic isoform. In the six remaining HCCs, only a PCNA acidic form associated with multiple basic isoforms was detected. Importantly, the PCNA basic form was not found in cirrhotic tissues. To clarify the nature of the detected PCNA isoforms, ubiquitin-specific immunoblotting as well as phosphatase treatment were employed. A PCNA-ubiquitylated form in cell lines and PCNA-phosphorylated isoforms in 6 of 12 HCCs were detected. Finally, in the DNA-bound fraction we detected only an acidic PCNA monomeric form. We conclude that human hepatocellular carcinoma expresses specific PCNA isoforms compared to those found in cirrhosis, implicating a role for PCNA functional alterations in hepatocarcinogenesis.

Interaction of p21(CDKN1A) with PCNA regulates the histone acetyltransferase activity of p300 in nucleotide excision repair

The cell-cycle inhibitor p21^CDKN1A has been suggested to directly participate in DNA repair, thanks to the interaction with PCNA. Yet, its role has remained unclear. Among proteins interacting with both p21 and PCNA, the histone acetyltransferase (HAT) p300 has been shown to participate in DNA repair. Here we report evidence indicating that p21 protein localizes and interacts with both p300 and PCNA at UV-induced DNA damage sites. The interaction between p300 and PCNA is regulated in vivo by p21. Indeed, loss of p21, or its inability to bind PCNA, results in a prolonged binding to chromatin and an increased association of p300 with PCNA, in UV-irradiated cells. Concomitantly, HAT activity of p300 is reduced after DNA damage. In vitro experiments show that inhibition of p300 HAT activity induced by PCNA is relieved by p21, which disrupts the association between recombinant p300 and PCNA. These results indicate that p21 is required during DNA repair to regulate p300 HAT activity by disrupting its interaction with PCNA.

Recruitment of DNA repair synthesis machinery to sites of DNA damage/repair in living human cells

The eukaryotic sliding DNA clamp, proliferating cell nuclear antigen (PCNA), is essential for DNA replication and repair synthesis. In order to load the ring-shaped, homotrimeric PCNA onto the DNA double helix, the ATPase activity of the replication factor C (RFC) clamp loader complex is required. Although the recruitment of PCNA by RFC to DNA replication sites has well been documented, our understanding of its recruitment during DNA repair synthesis is limited. In this study, we analyzed the accumulation of endogenous and fluorescent-tagged proteins for DNA repair synthesis at the sites of DNA damage produced locally by UVA-laser micro-irradiation in HeLa cells. Accumulation kinetics and in vitro pull-down assays of the large subunit of RFC (RFC140) revealed that there are two distinct modes of recruitment of RFC to DNA damage, a simultaneous accumulation of RFC140 and PCNA caused by interaction between PCNA and the extreme N-terminus of RFC140 and a much faster accumulation of RFC140 than PCNA at the damaged site. Furthermore, RFC140 knock-down experiments showed that PCNA can accumulate at DNA damage independently of RFC. These results suggest that immediate accumulation of RFC and PCNA at DNA damage is only partly interdependent.

Aging and the ubiquitinome: traditional and non-traditional functions of ubiquitin in aging cells and tissues

Ubiquitination of endogenous proteins is one of the key regulatory steps of protein degradation followed by regulation of proteasome activity. During the last years evidence has increased that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and aging associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system of ubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination also confers functions different from giving "the kiss of death" to proteins. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, ubiquitination is involved in transcriptional regulation, receptor internalization, DNA repair, and stabilization of protein complexes. This review is therefore the first to summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and to discuss potential causes and consequences for aging.