p53 down-regulates CHK1 through p21 and the retinoblastoma protein

Cytoplasmic localization of p21 protects trophoblast giant cells from DNA damage induced apoptosis

Proliferating trophoblast stem cells (TSCs) can differentiate into nonproliferating but viable trophoblast giant cells (TGCs) that are resistant to DNA damage induced apoptosis. Differentiation is associated with selective up-regulation of the Cip/Kip cyclin-dependent kinase inhibitors p57 and p21; expression of p27 remains constant. Previous studies showed that p57 localizes to the nucleus in TGCs where it is essential for endoreplication. Here we show that p27 also remains localized to the nucleus during TSC differentiation where it complements the role of p57. Unexpectedly, p21 localized to the cytoplasm where it was maintained throughout both the G- and S-phases of endocycles, and where it prevented DNA damage induced apoptosis. This unusual status for a Cip/Kip protein was dependent on site-specific phosphorylation of p21 by the Akt1 kinase that is also up-regulated in TGCs. Although cytoplasmic p21 is widespread among cancer cells, among normal cells it has been observed only in monocytes. The fact that it also occurs in TGCs reveals that p57 and p21 serve nonredundant functions, and suggests that the role of p21 in suppressing apoptosis is restricted to terminally differentiated cells.

The CK1 Family: Contribution to Cellular Stress Response and Its Role in Carcinogenesis

Members of the highly conserved and ubiquitously expressed pleiotropic CK1 family play major regulatory roles in many cellular processes including DNA-processing and repair, proliferation, cytoskeleton dynamics, vesicular trafficking, apoptosis, and cell differentiation. As a consequence of cellular stress conditions, interaction of CK1 with the mitotic spindle is manifold increased pointing to regulatory functions at the mitotic checkpoint. Furthermore, CK1 is able to alter the activity of key proteins in signal transduction and signal integration molecules. In line with this notion, CK1 is tightly connected to the regulation and degradation of β-catenin, p53, and MDM2. Considering the importance of CK1 for accurate cell division and regulation of tumor suppressor functions, it is not surprising that mutations and alterations in the expression and/or activity of CK1 isoforms are often detected in various tumor entities including cancer of the kidney, choriocarcinomas, breast carcinomas, oral cancer, adenocarcinomas of the pancreas, and ovarian cancer. Therefore, scientific effort has enormously increased (i) to understand the regulation of CK1 and its involvement in tumorigenesis- and tumor progression-related signal transduction pathways and (ii) to develop CK1-specific inhibitors for the use in personalized therapy concepts. In this review, we summarize the current knowledge regarding CK1 regulation, function, and interaction with cellular proteins playing central roles in cellular stress-responses and carcinogenesis.

Fos-dependent induction of Chk1 protects osteoblasts from replication stress

Stable Fos expression in the osteoblast lineage results in the development of osteosarcomas (OS) in mice, yet the underlying mechanisms are poorly understood. Using a genetic system in which Fos expression can be induced in osteoblasts in a doxycycline-dependent manner and through subsequent RNA sequencing and gene set enrichment analysis, we were able to identify novel transcriptional targets of Fos in osteoblasts. These included a distinct activation of cellular response toward replication stress (RS), exemplified by a Fos-dependent induction of the RS-suppressing Chk1 kinase. Importantly, Fos expression protects osteoblasts from RS and DNA damage likely through upregulation of Chk1 and facilitates transformation by Ras/E1A oncogenes. These data reveal a novel function of Fos in safeguarding genome stability during replication, which is particularly relevant in conditions of oncogene-induced S-phase entry.

The dual roles of geminin during trophoblast proliferation and differentiation

Geminin is a protein involved in both DNA replication and cell fate acquisition. Although it is essential for mammalian preimplantation development, its role remains unclear. In one study, ablation of the geminin gene (Gmnn) in mouse preimplantation embryos resulted in apoptosis, suggesting that geminin prevents DNA re-replication, whereas in another study it resulted in differentiation of blastomeres into trophoblast giant cells (TGCs), suggesting that geminin regulates trophoblast specification and differentiation. Other studies concluded that trophoblast differentiation into TGCs is regulated by fibroblast growth factor-4 (FGF4), and that geminin is required to maintain endocycles. Here we show that ablation of Gmnn in trophoblast stem cells (TSCs) proliferating in the presence of FGF4 closely mimics the events triggered by FGF4 deprivation: arrest of cell proliferation, formation of giant cells, excessive DNA replication in the absence of DNA damage and apoptosis, and changes in gene expression that include loss of Chk1 with up-regulation of p57 and p21. Moreover, FGF4 deprivation of TSCs reduces geminin to a basal level that is required for maintaining endocycles in TGCs. Thus, geminin acts both like a component of the FGF4 signal transduction pathway that governs trophoblast proliferation and differentiation, and geminin is required to maintain endocycles.

miR-16 and miR-26a target checkpoint kinases Wee1 and Chk1 in response to p53 activation by genotoxic stress

The tumour suppressor p53 is a crucial regulator of cell cycle arrest and apoptosis by acting as a transcription factor to regulate a variety of genes. At least in part, this control is exerted by p53 via regulating expression of numerous microRNAs. We identified two abundantly expressed microRNAs, miR-16 and miR-26a, whose expression is regulated by p53 during the checkpoint arrest induced by the genotoxic drug, doxorubicin. Importantly, among the targets of these miRs are two critical checkpoint kinases, Chk1 and Wee1. The p53-dependent augmentation of miR-16 and miR-26a expression levels led to the cell cycle arrest of tumour cells in G1/S and increased apoptosis. Strikingly, the bioinformatics analysis of survival times for patients with breast and prostate cancers has revealed that co-expression of mir-16 and miR-26a correlated with a better survival outcome. Collectively, our data provide a novel mechanism whereby p53 represses Chk1 and Wee1 expression, at least partially, via upregulation of miR-16 and miR-26a and thus sensitizes tumour cells to genotoxic therapies.

p53-dependent gene repression through p21 is mediated by recruitment of E2F4 repression complexes

The p53 tumor suppressor protein is a major sensor of cellular stresses, and upon stabilization, activates or represses many genes that control cell fate decisions. While the mechanism of p53-mediated transactivation is well established, several mechanisms have been proposed for p53-mediated repression. Here, we demonstrate that the cyclin-dependent kinase inhibitor p21 is both necessary and sufficient for the downregulation of known p53-repression targets, including survivin, CDC25C, and CDC25B in response to p53 induction. These same targets are similarly repressed in response to p16 overexpression, implicating the involvement of the shared downstream retinoblastoma (RB)-E2F pathway. We further show that in response to either p53 or p21 induction, E2F4 complexes are specifically recruited onto the promoters of these p53-repression targets. Moreover, abrogation of E2F4 recruitment via the inactivation of RB pocket proteins, but not by RB loss of function alone, prevents the repression of these genes. Finally, our results indicate that E2F4 promoter occupancy is globally associated with p53-repression targets, but not with p53 activation targets, implicating E2F4 complexes as effectors of p21-dependent p53-mediated repression.

Regulation of p53 by jagged1 contributes to angiotensin II-induced impairment of myocardial angiogenesis

Angiotensin II (AngII) is a major contributor to the development of heart failure, however, the molecular and cellular mechanisms still remain elucidative. Inadequate angiogenesis in myocardium leads to transition from cardiac hypertrophy to dysfunction, this study was therefore conducted to examine the effects of AngII on myocardial angiogenesis and the underlying mechanisms. AngII treatment significantly impaired angiogenetic responses, which were determined by counting the capillaries either in matrigel formed by cultured cardiac microvascular endothelial cells (CMVECs) or in myocardium of mice and by measuring the in vitro and in vivo production of VEGF proteins, and stimulated accumulation and phosphorylation of cytosolic p53 which led to increases in phosphorylated p53 and decreases of hypoxia inducible factor (Hif-1) in nucleus. All of these cellular and molecular events induced by AngII in CEMCs and hearts of mice were largely reduced by a p53 inhibitor, pifithrin-α (PFT-α). Interestingly, AngII stimulated the upregulation of Jagged1, a ligand of Notch, but it didn't affect the expression of Delta-like 4 (Dll-4), another ligand of Notch. Inhibition of p53 by PFT-α partly abolished this effect of AngII. Further experiments showed that knockdown ofJagged1 by addition of siRNA to cultured CMVECs dramatically declined AngII-stimulated accumulation and phosphorylation of p53 in cytosol, upregulation of phosphorylated p53 and downregulation of Hif-1 expression in nucleus, decrease of VEGF production and impairment of capillary-like tube formation by the cells. Our data collectively suggest that AngII impairs myocardial angiogenetic responses through p53-dependent downregulation of Hif-1 which is regulated by Jagged1/Notch1 signaling.

Polo-like kinase 4 transcription is activated via CRE and NRF1 elements, repressed by DREAM through CDE/CHR sites and deregulated by HPV E7 protein

Infection by oncogenic viruses is a frequent cause for tumor formation as observed in cervical cancer. Viral oncoproteins cause inactivation of p53 function and false transcriptional regulation of central cell cycle genes. Here we analyze the regulation of Plk4, serving as an example of many cell cycle- and p53-regulated genes. Cell cycle genes are often repressed via CDE and CHR elements in their promoters and activated by NF-Y binding to CCAAT-boxes. In contrast, general activation of Plk4 depends on NRF1 and CRE sites. Bioinformatic analyses imply that NRF1 and CRE are central elements of the transcriptional network controlling cell cycle genes. We identify CDE and CHR sites in the Plk4 promoter, which are necessary for binding of the DREAM (DP, RB-like, E2F4 and MuvB) complex and for mediating repression in G₀/G₁. When cells progress to G₂ and mitosis, DREAM is replaced by the MMB (Myb-MuvB) complex that only requires the CHR element for binding. Plk4 expression is downregulated by the p53-p21^WAF1/CIP1-DREAM signaling pathway through the CDE and CHR sites. Cell cycle- and p53-dependent repression is abrogated by HPV E7 oncoprotein. Together with genome-wide analyses our results imply that many cell cycle genes upregulated in tumors by viral infection are bound by DREAM through CDE/CHR sites.

Dynamics of DNA damage induced pathways to cancer

Chemotherapy is commonly used in cancer treatments, however only 25% of cancers are responsive and a significant proportion develops resistance. The p53 tumour suppressor is crucial for cancer development and therapy, but has been less amenable to therapeutic applications due to the complexity of its action, reflected in 66,000 papers describing its function. Here we provide a systematic approach to integrate this information by constructing a large-scale logical model of the p53 interactome using extensive database and literature integration. The model contains 206 nodes representing genes or proteins, DNA damage input, apoptosis and cellular senescence outputs, connected by 738 logical interactions. Predictions from in silico knock-outs and steady state model analysis were validated using literature searches and in vitro based experiments. We identify an upregulation of Chk1, ATM and ATR pathways in p53 negative cells and 61 other predictions obtained by knockout tests mimicking mutations. The comparison of model simulations with microarray data demonstrated a significant rate of successful predictions ranging between 52% and 71% depending on the cancer type. Growth factors and receptors FGF2, IGF1R, PDGFRB and TGFA were identified as factors contributing selectively to the control of U2OS osteosarcoma and HCT116 colon cancer cell growth. In summary, we provide the proof of principle that this versatile and predictive model has vast potential for use in cancer treatment by identifying pathways in individual patients that contribute to tumour growth, defining a sub population of “high” responders and identification of shifts in pathways leading to chemotherapy resistance.

Roles of Chk1 in cell biology and cancer therapy

The evolutionally conserved DNA damage response (DDR) and cell cycle checkpoints preserve genome integrity. Central to these genome surveillance pathways is a protein kinase, Chk1. DNA damage induces activation of Chk1, which then transduces the checkpoint signal and facilitates cell cycle arrest and DNA damage repair. Significant progress has been made recently toward our understanding of Chk1 regulation and its implications in cancer etiology and therapy. Specifically, a model that involves both spatiotemporal and conformational changes of proteins has been proposed for Chk1 activation. Further, emerging evidence suggests that Chk1 does not appear to be a tumor suppressor; instead, it promotes tumor growth and may contribute to anticancer therapy resistance. Recent data from our laboratory suggest that activating, but not inhibiting, Chk1 in the absence of chemotherapy might represent an innovative approach to suppress tumor growth. These findings suggest unique regulation of Chk1 in cell biology and cancer etiology, pointing to novel strategies for targeting Chk1 in cancer therapy.

Endoreplication

Developmentally programmed polyploidy occurs by at least four different mechanisms, two of which (endoreduplication and endomitosis) involve switching from mitotic cell cycles to endocycles by the selective loss of mitotic cyclin-dependent kinase (CDK) activity and bypassing many of the processes of mitosis. Here we review the mechanisms of endoreplication, focusing on recent results from Drosophila and mice.

p53 can repress transcription of cell cycle genes through a p21(WAF1/CIP1)-dependent switch from MMB to DREAM protein complex binding at CHR promoter elements

The tumor suppressor p53 plays an important role in cell cycle arrest by downregulating transcription. Many genes repressed by p53 code for proteins with functions in G₂/M. A large portion of these genes is controlled by cell cycle-dependent elements (CDE) and cell cycle genes homology regions (CHR) in their promoters. Cyclin B2 is an example of such a gene, with a function at the transition from G₂ to mitosis. We find that p53-dependent downregulation of cyclin B2 promoter activity is dependent on an intact CHR element. In the presence of high levels of p53 or p21^WAF1/CIP1, protein binding to the CHR switches from MMB to DREAM complex by shifting MuvB core-associated proteins from B-Myb to E2F4/DP1/p130. The results suggest a model for p53-dependent transcriptional repression by which p53 directly activates p21^WAF1/CIP1. The inhibitor then prevents further phosphorylation of p130 by cyclin-dependent kinases. The presence of hypophosphorylated pocket proteins shifts the equilibrium for complex formation from MMB to DREAM. In the case of promoters that do not hold CDE or E2F elements, binding of DREAM and MMB solely relies on a CHR site. Thus, p53 can repress target genes indirectly through CHR elements.

"The Octet": Eight Protein Kinases that Control Mammalian DNA Replication

Development of a fertilized human egg into an average sized adult requires about 29 trillion cell divisions, thereby producing enough DNA to stretch to the Sun and back 200 times (DePamphilis and Bell, 2011)! Even more amazing is the fact that throughout these mitotic cell cycles, the human genome is duplicated once and only once each time a cell divides. If a cell accidentally begins to re-replicate its nuclear DNA prior to cell division, checkpoint pathways trigger apoptosis. And yet, some cells are developmentally programmed to respond to environmental cues by switching from mitotic cell cycles to endocycles, a process in which multiple S phases occur in the absence of either mitosis or cytokinesis. Endocycles allow production of viable, differentiated, polyploid cells that no longer proliferate. What is surprising is that among the 516 (Manning et al., 2002) to 557 (BioMart web site) protein kinases encoded by the human genome, only eight regulate nuclear DNA replication directly. These are Cdk1, Cdk2, Cdk4, Cdk6, Cdk7, Cdc7, Checkpoint kinase-1 (Chk1), and Checkpoint kinase-2. Even more remarkable is the fact that only four of these enzymes (Cdk1, Cdk7, Cdc7, and Chk1) are essential for mammalian development. Here we describe how these protein kinases determine when DNA replication occurs during mitotic cell cycles, how mammalian cells switch from mitotic cell cycles to endocycles, and how cancer cells can be selectively targeted for destruction by inducing them to begin a second S phase before mitosis is complete.

p53 negatively regulates Aurora A via both transcriptional and posttranslational regulation

p53 plays an important role in mitotic checkpoint, but what its role is remains enigmatic. Aurora A is a Ser/Thr kinase involved in correcting progression of mitosis. Here, we show that p53 is a negative regulator for Aurora A. We found that p53 deficiency leads to Aurora A elevation. Ectopic expression of p53 or DNA damage-induced expression of p53 can suppress the expression of Aurora A. Mechanistic studies show that p53 is a negative regulator for Aurora A expression through both transcriptional and posttranslational regulation. p53 knockdown in cancer cells reduces the level of p21, which, in turn, increases the activity of CDK2 followed by induction of Rb1 hyperphosphorylation and its dissociation with transcriptional factor E2F3. E2F3 can bind to Aurora A gene promoter, potentiating Aurora A gene expression and p53 deficiency, enhancing the binding of E2F3 on Aurora A promoter. Also, p53 deficiency leads to decelerating Aurora A's turnover rate, due to the fact that p53 deficiency causes the downregulation of Fbw7α, a component of E3 ligase of Aurora A. Consistently, p53 knockdown-mediated Aurora A elevation is mitigated when Fbw7α is ectopically expressed. Thus, p53-mediated Aurora A degradation requires Fbw7α expression. Significantly, inverse correlation between p53 and Aurora A elevation is translated into the deregulation of centrosome amplification. p53 knockdown leads to high percentages of cells with abnormal amplification of centrosome. These data suggest that p53 is an important negative regulator of Aurora A, and that loss of p53 in many types of cancer could lead to abnormal elevation of Aurora A and dysregulated mitosis, which provides a growth advantage for cancer cells.

Differential regulation of p21 (waf1) protein half-life by DNA damage and Nutlin-3 in p53 wild-type tumors and its therapeutic implications

DNA damage induces the canonical p53 pathway including elevation of p21 (waf1) resulting in arrest of cell cycle progression. This can protect cells from subsequent Chk1 inhibition. Some p53 wild-type cancer cells such as HCT116 and U2OS exhibit attenuated p21 (waf1) induction upon DNA damage due to translational inhibition, and are incapable of maintaining arrest upon Chk1 inhibition. The purpose of this study was to determine whether this attenuated p21 (waf1) induction also occurred with the non-DNA damaging agent Nutlin-3 which induces p53 by disrupting binding to its negative regulator MDM2. We find that Nutlin-3 circumvented the attenuated induction of p21 (waf1) protein by increasing its half-life which led to G 1 and G 2 arrest in both cell lines. Interestingly, the p21 (waf1) protein half-life remained short on Nutlin-3 in p53 wild-type MCF10A cells; these cells achieve high p21 (waf1) levels through transcriptional upregulation. Consequently, all three p53 wild-type cells but not p53 mutant MDA-MB-231 cancer cells were protected from subsequent incubation with a combination of DNA damage plus a checkpoint inhibitor.

Pharmacological inhibition of ATM by KU55933 stimulates ATM transcription

Ataxia-telangiectasia mutated (ATM) kinase is a component of a signalling mechanism that determines the process of decision-making in response to DNA damage and involves the participation of multiple proteins. ATM is activated by DNA double-strand breaks (DSBs) through the Mre11–Rad50–Nbs1 (MRN) DNA repair complex, and orchestrates signalling cascades that initiate the DNA damage response. Cells lacking ATM are hypersensitive to insults, particularly genotoxic stress, induced through radiation or radiomimetic drugs. Here, we investigate the degree of ATM activation during time-dependent treatment with genotoxic agents and the effects of ATM on phospho-induction and localization of its downstream substrates. Additionally, we have demonstrated a new cell-cycle-independent mechanism of ATM gene regulation following ATM kinase inhibition with KU5593. Inhibition of ATM activity causes induction of ATM protein followed by oscillation and this mechanism is governed at the transcriptional level. Furthermore, this autoregulatory induction of ATM is also accompanied by a transient upregulation of p53, pATR and E2F1 levels. Since ATM inhibition is believed to sensitize cancer cells to genotoxic agents, this novel insight into the mechanism of ATM regulation might be useful for designing more precise strategies for modulation of ATM activity in cancer therapy.

Increased aldehyde reductase expression mediates acquired radioresistance of laryngeal cancer cells via modulating p53

The main obstacle to cure tumors by radiotherapy has been ascribed to tumor radioresistance. To determine the mechanisms underlying resistance to irradiation, it is essential to compare proteins differentially expressed from radiotherapy-sensitive and -resistant cancer cells. Aldehyde reductase (AKR1A1) was recently identified as increased in radioresistant laryngeal cancer cells by comparative proteomics approach. Here, we provide the mechanism of AKR1A1-mediated radioresistance via p53 regulation in laryngeal cancer cells. AKR1A1 induction was correlated with the radioresistant phenotype of laryngeal cancer HEp-2 cells. AKR1A1 depletion with siRNA significantly enhanced radiation sensitivity of radioresistant HEp-2 cells by promoting radiation-induced cell death and accelerated radiation-mediated inhibition of cell proliferation, without affecting either the PI3K-Akt or MAPK-ERK pathways. Intriguingly, AKR1A1 depletion induced phosphorylation of p53 at serine 15 and G 2/M transition in response to irradiation. We further found that AKR1A1 interacted with p53 and this interaction was dramatically increased in the irradiated radioresistant cells compared with the control cells. AKR1A1 expression also regulated p53 stability in response to irradiation. Furthermore, AKR1A1 depletion only sensitized HCT116 cells expressing p53 to irradiation and not p53-deficient cells. Therefore, our data suggest that radiation-inducible AKR1A1 contributes to acquired radioresistance of laryngeal cancer cells by suppressing p53 activation through inhibitory interaction.

Decreased translation of p21waf1 mRNA causes attenuated p53 signaling in some p53 wild-type tumors

DNA damage induces cell cycle arrest through both Chk1 and the p53 tumor suppressor protein, the latter arresting cells through induction of p21(waf1) protein. Arrest permits cells to repair the damage and recover. The frequent loss of p53 in tumor cells makes them more dependent on Chk1 for arrest and survival. However, some p53 wild type tumor cell lines, such as HCT116 and U2OS, are also sensitive to inhibition of Chk1 due to attenuated p21(waf1) induction upon DNA damage. The purpose of this study is to determine the cause of this attenuated p21(waf1) protein induction. We find that neither the induction of p21(waf1) mRNA nor protein half-life is sufficient to explain the low p21(waf1) protein levels in HCT116 and U2OS cells. The induced mRNA associates with polysomes but little protein is made suggesting these two cell lines have a reduced rate of p21(waf1) mRNA translation. This represents a novel mechanism for disruption of the p53-p21(waf1) pathway as currently known mechanisms involve either mutation of p53 or reduction of p53 protein levels. As a consequence, this attenuated p21(waf1) expression may render some p53 wild type tumors sensitive to a combination of DNA damage plus checkpoint inhibition.

N-(phosphonacetyl)-L-aspartate induces TAp73-dependent apoptosis by modulating multiple Bcl-2 proteins: potential for cancer therapy

p53 is essential for the cellular responses to DNA damage that help to maintain genomic stability. However, the great majority of human cancers undergo disruption of the p53-network. Identification and characterization of molecular components important in both p53-dependent and -independent apoptosis might be useful in developing novel therapies for cancers. In the complete absence of p53, cells treated with N-(phosphonacetyl)-L-aspartate (PALA) continue to synthesize DNA slowly and eventually progress through S phase, suffering severe DNA damage that in turn triggers apoptosis, whereas cells with functional p53 undergo growth arrest. In the present study, we investigated apoptotic signaling in response to PALA and the role of p53 expression in this pathway. We found that treatment of cells lacking p53 with PALA induced TAp73, Noxa, and Bim and inactivation of these proteins with dominant negative plasmids or siRNAs significantly inhibited apoptosis, suggesting that PALA-induced apoptosis was mediated via TAp73-dependent expression of Noxa and Bim. However, PALA treatment inhibited the expression of ΔNp73 only in cells lacking p53 but not in cells expressing p53. In addition, PALA treatment inhibited Bcl-2, and overexpression of Bcl-2 significantly inhibited PALA-induced apoptosis. Moreover, expression of p53 in these cells protected them from PALA-induced apoptosis by activating p21, sustaining the expression of ΔNp73 and inhibiting the induction of Noxa and Bim. Taken together, our study identifies novel but opposing roles for the p53 and TAp73 in the induction of Noxa and Bim and regulation of apoptosis. Our data will help to develop strategies to eliminate cancer cells lacking p53 while protecting normal cells with wild-type p53.

Up-regulation of microRNA-21 correlates with lower kidney cancer survival

Background

MicroRNA-21 is up-regulated in a variety of cancers like, breast, colorectal, lung, head and neck etc. However, the regulation of miR-21 in renal cell carcinoma (RCC) has not yet been studied systematically.

Methods and Results

We measured miR-21 levels in 54 pairs of kidney cancers and their normal matched tissues by real-time PCR. The expression level of miR-21 was correlated with 5 year survival and the pathological stage. Functional studies were done after inhibiting miR-21 in RCC cell lines. We studied in vitro and in vivo effects of the chemo preventive agent genistein on miR-21 expression. In 48 cases (90%), miR-21 was increased. All patients with low miR-21 expression survived 5 years, while with high miR-21 expression, only 50% survived. Higher expression of miR-21 is associated with an increase in the stage of renal cancer. Functional studies after inhibiting miRNA-21 in RCC cell lines show cell cycle arrest, induction of apoptosis and reduced invasive and migratory capabilities. Western blot analysis showed an increase in the expression of p21 and p38 MAP kinase genes and a reduction in cyclin E2. Genistein inhibited the expression of miR-21 in A-498 cells and in the tumors formed after injecting genistein treated A-498 cells in nude mice besides inhibiting tumor formation.

Conclusions

The current study shows a clear correlation between miR-21 expression and clinical characteristics of renal cancer. Thus we believe that miR-21 can be used as a tumor marker and its inhibition may prove to be useful in controlling cancers with up-regulated miR-21.

Importin 7 and exportin 1 link c-Myc and p53 to regulation of ribosomal biogenesis

Members of the β-karyopherin family mediate nuclear import of ribosomal proteins and export of ribosomal subunits, both required for ribosome biogenesis. We report that transcription of the β-karyopherin genes importin 7 (IPO7) and exportin 1 (XPO1), and several additional nuclear import receptors, is regulated positively by c-Myc and negatively by p53. Partial IPO7 depletion triggers p53 activation and p53-dependent growth arrest. Activation of p53 by IPO7 knockdown has distinct features of ribosomal biogenesis stress, with increased binding of Mdm2 to ribosomal proteins L5 and L11 (RPL5 and RPL11). Furthermore, p53 activation is dependent on RPL5 and RPL11. Of note, IPO7 and XPO1 are frequently overexpressed in cancer. Altogether, we propose that c-Myc and p53 counter each other in the regulation of elements within the nuclear transport machinery, thereby exerting opposing effects on the rate of ribosome biogenesis. Perturbation of this balance may play a significant role in promoting cancer.

Cutting edge: Chk1 directs senescence and mitotic catastrophe in recovery from G₂ checkpoint arrest

Besides the well-understood DNA damage response via establishment of G₂ checkpoint arrest, novel studies focus on the recovery from arrest by checkpoint override to monitor cell cycle re-entry. The aim of this study was to investigate the role of Chk1 in the recovery from G₂ checkpoint arrest in HCT116 (human colorectal cancer) wt, p53^–/– and p21^–/– cell lines following H₂O₂ treatment. Firstly, DNA damage caused G₂ checkpoint activation via Chk1. Secondly, overriding G₂ checkpoint led to (i) mitotic slippage, cell cycle re-entry in G₁ and subsequent G₁ arrest associated with senescence or (ii) premature mitotic entry in the absence of p53/p21^WAF1 causing mitotic catastrophe. We revealed subtle differences in the initial Chk1-involved G₂ arrest with respect to p53/p21^WAF1: absence of either protein led to late G₂ arrest instead of the classic G₂ arrest during checkpoint initiation, and this impacted the release back into the cell cycle. Thus, G₂ arrest correlated with downstream senescence, but late G₂ arrest led to mitotic catastrophe, although both cell cycle re-entries were linked to upstream Chk1 signalling. Chk1 knockdown deciphered that Chk1 defines long-term DNA damage responses causing cell cycle re-entry. We propose that recovery from oxidative DNA damage-induced G₂ arrest requires Chk1. It works as cutting edge and navigates cells to senescence or mitotic catastrophe. The decision, however, seems to depend on p53/p21^WAF1. The general relevance of Chk1 as an important determinant of recovery from G₂ checkpoint arrest was verified in HT29 colorectal cancer cells.

Therapeutic implications for the induced levels of Chk1 in Myc-expressing cancer cells

PURPOSE

The transcription factor c-Myc (or "Myc") is a master regulator of pathways driving cell growth and proliferation. MYC is deregulated in many human cancers, making its downstream target genes attractive candidates for drug development. We report the unexpected finding that B-cell lymphomas from mice and patients exhibit a striking correlation between high levels of Myc and checkpoint kinase 1 (Chk1).

EXPERIMENTAL DESIGN

By in vitro cell biology studies as well as preclinical studies using a genetically engineered mouse model, we evaluated the role of Chk1 in Myc-overexpressing cells.

RESULTS

We show that Myc indirectly induces Chek1 transcript and protein expression, independently of DNA damage response proteins such as ATM and p53. Importantly, we show that inhibition of Chk1, by either RNA interference or a novel highly selective small molecule inhibitor, results in caspase-dependent apoptosis that affects Myc-overexpressing cells in both in vitro and in vivo mouse models of B-cell lymphoma.

CONCLUSION

Our data suggest that Chk1 inhibitors should be further evaluated as potential drugs against Myc-driven malignancies such as certain B-cell lymphoma/leukemia, neuroblastoma, and some breast and lung cancers.

The PARP inhibitor PJ34 causes a PARP1-independent, p21 dependent mitotic arrest

Poly(ADP)-ribose polymerase (PARP) inhibitors modify the enzymatic activity of PARP1/2. When certain PARP inhibitors are used either alone or in combination with DNA damage agents they may cause a G2/M mitotic arrest and/or apoptosis in a susceptible genetic context. PARP1 interacts with the cell cycle checkpoint proteins Ataxia Telangectasia Mutated (ATM) and ATM and Rad3-related (ATR) and therefore may influence growth arrest cascades. The PARP inhibitor PJ34 causes a mitotic arrest by an unknown mechanism in certain cell lines, therefore we asked whether PJ34 conditionally activated the checkpoint pathways and which downstream targets were necessary for mitotic arrest. We found that PJ34 produced a concentration dependent G2/M mitotic arrest and differentially affected cell survival in cells with diverse genetic backgrounds. p53 was activated and phosphorylated at Serine15 followed by p21 gene activation through both p53-dependent and -independent pathways. The mitotic arrest was caffeine sensitive and UCN01 insensitive and did not absolutely require p53, ATM or Chk1, while p21 was necessary for maintaining the growth arrest. Significantly, by using stable knockdown cell lines, we found that neither PARP1 nor PARP2 was required for any of these effects produced by PJ34. These results raise questions and cautions for evaluating PARP inhibitor effectiveness, suggesting whether effects should be considered not only on PARP's diverse ADP-ribosylation independent protein interactions but also on homologous proteins that may be producing either overlapping or distinct effect.

Checkpoint kinase 1 prevents cell cycle exit linked to terminal cell differentiation

Trophoblast stem (TS) cells proliferate in the presence of fibroblast growth factor 4, but in its absence, they differentiate into polyploid trophoblast giant (TG) cells that remain viable but nonproliferative. Differentiation is coincident with expression of the cyclin-dependent kinase (CDK)-specific inhibitors p21 and p57, of which p57 is essential for switching from mitotic cell cycles to endocycles. Here, we show that, in the absence of induced DNA damage, checkpoint kinase-1 (CHK1), an enzyme essential for preventing mitosis in response to DNA damage, functions as a mitogen-dependent protein kinase that prevents premature differentiation of TS cells into TG cells by suppressing expression of p21 and p57, but not p27, the CDK inhibitor that regulates mitotic cell cycles. CHK1 phosphorylates p21 and p57 proteins at specific sites, thereby targeting them for degradation by the 26S proteasome. TG cells lack CHK1, and restoring CHK1 activity in TG cells suppresses expression of p57 and restores mitosis. Thus, CHK1 is part of a "G2 restriction point" that prevents premature cell cycle exit in cells programmed for terminal differentiation, a role that CHK2 cannot play.

To repress or not to repress: this is the guardian's question

p53 is possibly the most central tumor suppressor gene of our cells, integrating stress signals to activate a transcriptional program responsible for maintaining cellular homeostasis. Many of the downstream effects of p53 are a consequence of its activity as a transcription factor, resulting in the induction of multiple target genes. In addition to gene activation, however, gene repression is an essential part of the p53 cellular response. Despite extensive research efforts towards the elucidation of p53 functions, the molecular mechanisms and biological consequences of gene repression by p53 have not been studied extensively. We review our current knowledge of the mechanisms and biological consequences of p53 repression, with special attention to recently discovered mechanisms of repression that involve non-coding RNA molecules, an emerging aspect of regulation in the p53 cellular network.

The impact of S- and G2-checkpoint response on the fidelity of G1-arrest by cisplatin and its comparison to a non-cross-resistant platinum(IV) analog

OBJECTIVE

Cisplatin is a DNA-damaging antitumor agent that is highly effective in treating ovarian cancer. It activates the p53/p21 pathway for its cytotoxic mode of action, but it does not induce p21-dependent cell cycle arrest in G1. Therefore, we investigated this paradox, and used the model analog DAP as a positive control for p21-dependent G1-arrest.

METHODS

Studies were conducted in p53-proficient ovarian A2780 tumor cells to examine Cdk activity, cell cycle distribution and DNA damage signaling after cisplatin or DAP in combination with the mitotic inhibitor nocodazole.

RESULTS

Cisplatin consistently induced transient S-phase arrest by inhibiting Cdk2/cyclin A complex in S-phase at 12 h and then a durable G2/M-arrest by inhibiting Cdc2/cyclin B complex at 12-18 h. These inhibitions were associated with Chk1 and Chk2 activation and resultant increase in inhibitory tyrosine phosphorylation of Cdk2 and Cdc2. Cisplatin also potently inhibited G1-phase Cdk4/cyclin D1 and Cdk2/cyclin E activities at ~18 h. In agreement, exposure of cisplatin-treated A2780, HCT-116(p53-/-) and HCT-116(p21-/-) tumor cells to nocodazole revealed limited G1-arrest that was dependent on p53 and p21. In contrast, the durable G1-arrest by DAP, which failed to activate Chk1 and Chk2, was unaffected by nocodazole.

CONCLUSIONS

Cisplatin induced G1-arrest, but at an attenuated level. This was primarily due to orchestration of Cdk inhibition in S-phase first, then in G2, and finally in G1 that effectively blocked cells in G2 and prevented cells from progressing and arresting in G1. These studies demonstrate that cisplatin unequivocally activates G1-checkpoint response, but the fidelity of G1-arrest is compromised by Chk1/2 activation and checkpoint response in S- and G2/M-phase.

One function--multiple mechanisms: the manifold activities of p53 as a transcriptional repressor

Maintenance of genome integrity is a dynamic process involving complex regulation systems. Defects in one or more of these pathways could result in cancer. The most important tumor-suppressor is the transcription factor p53, and its functional inactivation is frequently observed in many tumor types. The tumor suppressive function of p53 is mainly attributed to its ability to regulate numerous target genes at the transcriptional level. While the mechanism of transcriptional induction by p53 is well characterized, p53-dependent repression is not understood in detail. Here, we review the manifold mechanisms of p53 as a transcriptional repressor. We classify two different categories of repressed genes based on the underlying mechanism, and novel mechanisms which involve regulation through noncoding RNAs are discussed. The complete elucidation of p53 functions is important for our understanding of its tumor-suppressor activity and, therefore, represents the key for the development of novel therapeutic approaches.

Gain of cellular adaptation due to prolonged p53 impairment leads to functional switchover from p53 to p73 during DNA damage in acute myeloid leukemia cells

Tumor suppressor p53 plays the central role in regulating apoptosis in response to genotoxic stress. From an evolutionary perspective, the activity of p53 has to be backed up by other protein(s) in case of any functional impairment of this protein, to trigger DNA damage-induced apoptosis in cancer cells. We adopted multiple experimental approaches to demonstrate that in p53-impaired cancer cells, DNA damage caused accumulation of p53 paralogue p73 via Chk-1 that strongly impacted Bax expression and p53-independent apoptosis. On the contrary, when p53 function was restored by ectopic expression, Chk-2 induced p53 accumulation that in turn overshadowed p73 activity, suggesting an antagonistic interaction between p53 family members. To understand such interaction better, p53-expressing cells were impaired differentially for p53 activity. In wild-type p53-expressing cancer cells that were silenced for p53 for several generations, p73 was activated, whereas no such trend was observed when p53 was transiently silenced. Prolonged p53 interference, even in functional p53 settings, therefore, leads to the "gain of cellular adaptation" in a way that alters the cellular microenvironment in favor of p73 activation by altering p73-regulatory proteins, e.g. Chk1 activation and dominant negative p73 down-regulation. These findings not only unveil a hitherto unexplained mechanism underlying the functional switchover from p53 to p73, but also validate p73 as a promising and potential target for cancer therapy in the absence of functional p53.

Alterations in gene expression and sensitivity to genotoxic stress following HdmX or Hdm2 knockdown in human tumor cells harboring wild-type p53

While half of all human tumors possess p53 mutations, inactivation of wild-type p53 can also occur through a variety of mechanisms that do not involve p53 gene mutation or deletion. Our laboratory has been interested in tumor cells possessing wild-type p53 protein and elevated levels of HdmX and/or Hdm2, two critical negative regulators of p53 function. In this study we utilized RNAi to knockdown HdmX or Hdm2 in MCF7 human breast cancer cells, which harbor wild-type p53 and elevated levels of HdmX and Hdm2 then examined gene expression changes and effects on cell growth.Cell cycle and growth assays confirmed that the loss of either HdmX or Hdm2 led to a significant growth inhibition and G1cell cycle arrest. Although the removal of overexpressed HdmX/2 appears limited to an anti-proliferative effect in MCF7cells, the loss of HdmX and/or Hdm2 enhanced cytotoxicity in these same cells exposed to DNA damage. Through the use of Affymetrix GeneChips and subsequent RT-qPCR validations, we uncovered a subset of anti-proliferative p53 target genes activated upon HdmX/2 knockdown. Interestingly, a second set of genes, normally transactivated by E2F1 as cells transverse the G1-S phase boundary, were found repressed in a p21-dependent manner following HdmX/2 knockdown.Taken together, these results provide novel insights into the reactivation of p53 in cells overexpressing HdmX and Hdm2.

DNA damage-induced degradation of Cdc25A does not lead to inhibition of Cdk2 activity in mouse embryonic stem cells

Cyclin-dependent kinase two (Cdk2) is the major regulator of the G1/S transition and the target of an activated G1 checkpoint in somatic cells. In the presence of DNA damage, Cdk2 kinase activity is abrogated by a deficiency of Cdc25A phosphatase, which is marked by Chk1/Chk2 for proteasomal degradation. Embryonic stem cells (ESCs) lack a G1 checkpoint response. In this study, we analyzed the G1 checkpoint pathways in mouse ESCs (mESCs) in the presence of DNA double-strand breaks evoked by ionizing radiation (IR). We show that checkpoint pathways, which operate during G1 phase in somatic cells, are activated in mESCs after IR; however, Cdk2 activity is not abolished. We demonstrate that Cdc25A is degraded in mESCs, but this degradation is not regulated by Chk1 and Chk2 kinases because they are sequestered to the centrosome. Instead, Cdc25A degradation is governed by glycogen synthase kinase-3beta kinase. We hypothesize that Cdc25A degradation does not inhibit Cdk2 activity because a considerable proportion of Cdk2 molecules localize to the cytoplasm and centrosomes in mESCs, where they may be sheltered from regulation by nuclear Cdc25A. Finally, we show that a high Cdk2 activity, which is irresponsive to DNA damage, is the driving force of the rapid escape of mESCs from G1 phase after DNA damage.

Restoration of p53 functions protects cells from concanavalin A-induced apoptosis

A great majority of human cancers encounter disruption of the p53 network. Identification and characterization of molecular components important in both p53-dependent and p53-independent apoptosis might be useful in developing novel therapies. Previously, we reported that concanavalin A (Con A) induced p73-dependent apoptosis of cells lacking functional p53. In the present study, we investigated the mechanism and role of p53 in protection from apoptosis induced by Con A. Treatment with Con A resulted in apoptosis of p53-null ovarian cancer, SKOV3, or Li-Fraumeni syndrome, MDAH041 (041), cells. However, their isogenic pairs, SKP53 and TR9-7, expressing wild-type p53 were much less sensitive and were protected by G(1) arrest. Inhibition of p53 function rendered these cells sensitive to Con A. Con A-induced apoptosis was accompanied by upregulation of forkhead box O1a (FOXO1a) and Bcl-2-interacting mediator (Bim), which were strongly inhibited after p53 expression and rescued after p53 ablation. Moreover, ablation of Bim by short hairpin RNA protected cells from apoptosis. Taken together, our study suggests that Con A induces apoptosis of cells lacking p53 by activating FOXO1a-Bim signaling and that expression of p53 protects these cells by inducing G(1) arrest and by downregulating the expression of both FOXO1a and Bim, identifying a novel cross-talk between FOXO1a and p53 transcription factors.

Ribosomal protein S7 is both a regulator and a substrate of MDM2

MDM2 associates with ribosomal protein S7, and this interaction is required to inhibit MDM2's E3 ligase activity, leading to stabilization of MDM2 and p53. Notably, the MDM2 homolog MDMX facilitates the inhibition of MDM2 E3 ligase activity by S7. Further, ablation of S7 inhibits MDM2 and p53 accumulation induced by different stress signals in some cell types. Thus, ribosomal/nucleolar stress is likely a key integrating event in DNA damage signaling to p53. Interestingly, S7 is itself a substrate for MDM2 E3 ligase activity both in vitro and in vivo. An S7-ubiquitin fusion protein (S7-Ub) selectively inhibits MDM2 degradation of p53 and is unaffected by MDMX. S7-Ub promotes apoptosis to a greater extent than S7 alone. This indicates that MDM2 ubiquitination of S7 is involved in sustaining the p53 response. Thus, S7 functions as both effector and affector of MDM2 to ensure a proper cellular response to different stress signals.

Pro-proliferative FoxM1 is a target of p53-mediated repression

The p53 tumor suppressor protein acts as a transcription factor to modulate cellular responses to a wide variety of stresses. In this study we show that p53 is required for the downregulation of FoxM1, an essential transcription factor that regulates many G2/M-specific genes and is overexpressed in a multitude of solid tumors. After DNA damage, p53 facilitates the repression of FoxM1 mRNA, which is accompanied by a decrease in FoxM1 protein levels. In cells with reduced p53 expression, FoxM1 is upregulated after DNA damage. Nutlin, a small-molecule activator of p53, suppresses FoxM1 levels in two cell lines in which DNA damage facilitates only mild repression. Mechanistically, p53-mediated inhibition of FoxM1 is partially p21 and retinoblastoma (Rb) family dependent, although in some cases p21-independent repression of FoxM1 was also observed. The importance of FoxM1 to cell fate was indicated by the observation that G2/M arrest follows FoxM1 ablation. Finally, our results indicate a potential contribution of p53-mediated repression of FoxM1 for maintenance of a stable G2 arrest.

Cip/Kip cyclin-dependent protein kinase inhibitors and the road to polyploidy

Cyclin-dependent kinases (CDKs) play a central role in the orderly transition from one phase of the eukaryotic mitotic cell division cycle to the next. In this context, p27Kip1 (one of the CIP/KIP family of CDK specific inhibitors in mammals) or its functional analogue in other eukarya prevents a premature transition from G1 to S-phase. Recent studies have revealed that expression of a second member of this family, p57Kip2, is induced as trophoblast stem (TS) cells differentiate into trophoblast giant (TG) cells. p57 then inhibits CDK1 activity, an enzyme essential for initiating mitosis, thereby triggering genome endoreduplication (multiple S-phases without an intervening mitosis). Expression of p21Cip1, the third member of this family, is also induced in during differentiation of TS cells into TG cells where it appears to play a role in suppressing the DNA damage response pathway. Given the fact that p21 and p57 are unique to mammals, the question arises as to whether one or both of these proteins are responsible for the induction and maintenance of polyploidy during mammalian development.

p21 in cancer: intricate networks and multiple activities

One of the main engines that drives cellular transformation is the loss of proper control of the mammalian cell cycle. The cyclin-dependent kinase inhibitor p21 (also known as p21WAF1/Cip1) promotes cell cycle arrest in response to many stimuli. It is well positioned to function as both a sensor and an effector of multiple anti-proliferative signals. This Review focuses on recent advances in our understanding of the regulation of p21 and its biological functions with emphasis on its p53-independent tumour suppressor activities and paradoxical tumour-promoting activities, and their implications in cancer.

Developmentally programmed endoreduplication in animals

Development of a fertilized egg into an adult human requires trillions of cell divisions, the vast majority of which duplicate their genome once and only once. Nevertheless, trophoblast giant cells and megakaryocytes in mammals circumvent this rule by duplicating their genome multiple times without undergoing cell division, a process generally referred to as 'endoreduplication'. In contrast, arthropods such as Drosophila endoreduplicate their genome in most larval tissues, as well as in many adult tissues. Endoreduplication requires that cells prevent entrance into or completion of mitosis and cytokinesis under conditions that permit assembly of prereplication complexes. In addition, cells must prevent induction of apoptosis in response to incomplete DNA replication or DNA damage that may occur during the ensuing sequence of 'endocycles'. Thus, developmentally regulated endoreduplication results in terminal cell differentiation. Recent progress has revealed both differences and similarities in the mechanisms employed by flies and mammals to change from mitotic cell cycles to 'endocycles'. The critical step, however, appears to be switching from a CDK-dependent form of the anaphase promoting complex (APC) to one that functions only in the absence of CDK activity.

Li-Fraumeni syndrome: the genetics and treatment considerations for the sarcoma and associated neoplasms

Li-Fraumeni syndrome is an autosomal dominant disorder first reported by Drs Li and Fraumeni in 1969. Malkin was the first to describe a germline mutation as an underlying defect of Li-Fraumeni syndrome. Cancer risk in mutation carriers has been estimated to be 50% by age 40 and 90% by age 60. Children of affected parents have an approximate 50% risk of inheriting the familial mutation. Functional assays have been established that allow for easy genetic testing for TP53 mutation. Treatment goals center on early detection and surgical resection of affected organ. Targeted therapy for the TP53 gene may hold promise for the future.

New cell or new cycle?

In mammals, trophoblast giant (TG) cell differentiation is characterized by a physiological endoreduplication, resulting in genome size augmentation. A recent study by Ullah and colleagues (pp. 3024-3036), published in this issue of Genes & Development, now elucidates the role of the cyclin-dependent kinase inhibitors (CKIs), p21 and p57, in mammalian endocycle regulation.

Differentiation of trophoblast stem cells into giant cells is triggered by p57/Kip2 inhibition of CDK1 activity

Genome endoreduplication during mammalian development is a rare event for which the mechanism is unknown. It first appears when fibroblast growth factor 4 (FGF4) deprivation induces differentiation of trophoblast stem (TS) cells into the nonproliferating trophoblast giant (TG) cells required for embryo implantation. Here we show that RO3306 inhibition of cyclin-dependent protein kinase 1 (CDK1), the enzyme required to enter mitosis, induced differentiation of TS cells into TG cells. In contrast, RO3306 induced abortive endoreduplication and apoptosis in embryonic stem cells, revealing that inactivation of CDK1 triggers endoreduplication only in cells programmed to differentiate into polyploid cells. Similarly, FGF4 deprivation resulted in CDK1 inhibition by overexpressing two CDK-specific inhibitors, p57/KIP2 and p21/CIP1. TS cell mutants revealed that p57 was required to trigger endoreduplication by inhibiting CDK1, while p21 suppressed expression of the checkpoint protein kinase CHK1, thereby preventing induction of apoptosis. Furthermore, Cdk2(-/-) TS cells revealed that CDK2 is required for endoreduplication when CDK1 is inhibited. Expression of p57 in TG cells was restricted to G-phase nuclei to allow CDK activation of S phase. Thus, endoreduplication in TS cells is triggered by p57 inhibition of CDK1 with concomitant suppression of the DNA damage response by p21.

Growth-inhibitory and tumor- suppressive functions of p53 depend on its repression of CD44 expression

The p53 tumor suppressor is a key mediator of cellular responses to various stresses. Here, we show that under conditions of basal physiologic and cell-culture stress, p53 inhibits expression of the CD44 cell-surface molecule via binding to a noncanonical p53-binding sequence in the CD44 promoter. This interaction enables an untransformed cell to respond to stress-induced, p53-dependent cytostatic and apoptotic signals that would otherwise be blocked by the actions of CD44. In the absence of p53 function, the resulting derepressed CD44 expression is essential for the growth and tumor-initiating ability of highly tumorigenic mammary epithelial cells. In both tumorigenic and nontumorigenic cells, CD44's expression is positively regulated by p63, a paralogue of p53. Our data indicate that CD44 is a key tumor-promoting agent in transformed tumor cells lacking p53 function. They also suggest that the derepression of CD44 resulting from inactivation of p53 can potentially aid the survival of immortalized, premalignant cells.

p53 initiates apoptosis by transcriptionally targeting the antiapoptotic protein ARC

p53 plays an important role in regulating apoptosis. However, the molecular mechanism by which it initiates the apoptotic program still remains to be fully understood. Here, we report that p53 can transcriptionally target the antiapoptotic protein, apoptosis repressor with caspase recruitment domain (ARC). Our results show that reactive oxygen species and anoxia lead to the up-regulation of p53 expression. Concomitantly, ARC is down-regulated at both the protein and mRNA levels. Knockdown of p53 expression can attenuate the decreases in ARC protein and mRNA levels, indicating that ARC down-regulation is a consequence of p53 activation. Strikingly, p53-induced ARC repression occurs in a transcription-dependent manner. We further demonstrate that the p53 up-regulated modulator of apoptosis (PUMA) and Bad are up-regulated in response to the stimulation with reactive oxygen species or anoxia, and p53 is responsible for their up-regulation. ARC can interact with PUMA or Bad via its N terminus. Such an interaction displaces the association of PUMA or Bad with Bcl-2. ARC repression by p53 leads to its failure to counteract the proapoptotic activity of PUMA and Bad. Thus, our data reveal a novel p53 apoptotic pathway in which it initiates apoptosis by transcriptionally repressing ARC.