DNA damage-induced downregulation of Cdc25C is mediated by p53 via two independent mechanisms: one involves direct binding to the cdc25C promoter

The role of DNA damage responses in p53 biology

The tumour suppressor p53 is a central player in cellular DNA damage responses. P53 is upregulated and activated by genotoxic stress and induces a transcriptional programme with effectors promoting apoptosis, cell cycle arrest, senescence and DNA repair. For the best part of the last three decades, these DNA damage-related programmes triggered by p53 were unequivocally regarded as the major if not sole mechanism by which p53 exerts its tumour suppressor function. However, this interpretation has been challenged by a number of recent in vivo studies, demonstrating that mice which are defective in inducing p53-dependent apoptosis, cell cycle arrest and senescence suppress thymic lymphoma as well as wild-type p53 expressing animals. Consequently, the importance of DNA damage responses for p53-mediated tumour suppression has been questioned. In this review, I summarize current knowledge on p53-controlled DNA damage responses and argue that these activities, while their role has certainly changed, remain an important feature of p53 biology with relevance for cancer therapy and tumour suppression.

Cell cycle regulation by checkpoints

Cell cycle checkpoints are surveillance mechanisms that monitor the order, integrity, and fidelity of the major events of the cell cycle. These include growth to the appropriate cell size, the replication and integrity of the chromosomes, and their accurate segregation at mitosis. Many of these mechanisms are ancient in origin and highly conserved, and hence have been heavily informed by studies in simple organisms such as the yeasts. Others have evolved in higher organisms, and control alternative cell fates with significant impact on tumor suppression. Here, we consider these different checkpoint pathways and the consequences of their dysfunction on cell fate.

RAD9 deficiency enhances radiation induced bystander DNA damage and transcriptomal response

BACKGROUND

Radiation induced bystander effects are an important component of the overall response of cells to irradiation and are associated with human health risks. The mechanism responsible includes intra-cellular and inter-cellular signaling by which the bystander response is propagated. However, details of the signaling mechanism are not well defined.

METHODS

We measured the bystander response of Mrad9+/+ and Mrad9-/- mouse embryonic stem cells, as well as human H1299 cells with inherent or RNA interference-mediated reduced RAD9 levels after exposure to 1 Gy α particles, by scoring chromosomal aberrations and micronuclei formation, respectively. In addition, we used microarray gene expression analyses to profile the transcriptome of directly irradiated and bystander H1299 cells.

RESULTS

We demonstrated that Mrad9 null enhances chromatid aberration frequency induced by radiation in bystander mouse embryonic stem cells. In addition, we found that H1299 cells with reduced RAD9 protein levels showed a higher frequency of radiation induced bystander micronuclei formation, compared with parental cells containing inherent levels of RAD9. The enhanced bystander response in human cells was associated with a unique transcriptomic profile. In unirradiated cells, RAD9 reduction broadly affected stress response pathways at the mRNA level; there was reduction in transcript levels corresponding to genes encoding multiple members of the UVA-MAPK and p38MAPK families, such as STAT1 and PARP1, suggesting that these signaling mechanisms may not function optimally when RAD9 is reduced. Using network analysis, we found that differential activation of the SP1 and NUPR1 transcriptional regulators was predicted in directly irradiated and bystander H1299 cells. Transcription factor prediction analysis also implied that HIF1α (Hypoxia induced factor 1 alpha) activation by protein stabilization in irradiated cells could be a negative predictor of the bystander response, suggesting that local hypoxic stress experienced by cells directly exposed to radiation may influence whether or not they will elicit a bystander response in neighboring cells.

p53 Promotes cell survival due to the reversibility of its cell-cycle checkpoints

The tumor suppressor p53 (TP53) has a well-studied role in triggering cell-cycle checkpoint in response to DNA damage. Previous studies have suggested that functional p53 enhances chemosensitivity. In contrast, data are presented to show that p53 can be required for cell survival following DNA damage due to activation of reversible cell-cycle checkpoints. The cellular outcome to DNA damage is determined by the duration and extent of the stimulus in a p53-dependent manner. In response to transient or low levels of DNA damage, p53 triggers a reversible G2 arrest, whereas a sustained p53-dependent cell-cycle arrest and senescence follows prolonged or high levels of DNA damage. Regardless of the length of treatment, p53-null cells arrest in G2, but ultimately adapt and proceed into mitosis. Interestingly, they fail to undergo cytokinesis, become multinucleated, and then die from apoptosis. Upon transient treatment with DNA-damaging agents, wild-type p53 cells reversibly arrest and repair the damage, whereas p53-null cells fail to do so and die. These data indicate that p53 can promote cell survival by inducing reversible cell-cycle arrest, thereby allowing for DNA repair. Thus, transient treatments may exploit differences between wild-type p53 and p53-null cells.

IMPLICATIONS

Although p53 status has been suggested as a clinical predictor of chemotherapeutic efficacy, studies to date have not always supported this. This study demonstrates that p53 is still an important determinant of cell fate in response to chemotherapy, under the appropriate treatment conditions.

Chronic inflammation and cancer: potential chemoprevention through nuclear factor kappa B and p53 mutual antagonism

Activation of nuclear factor-kappa B (NF- κB) as a mechanism of host defense against infection and stress is the central mediator of inflammatory responses. A normal (acute) inflammatory response is activated on urgent basis and is auto-regulated. Chronic inflammation that results due to failure in the regulatory mechanism, however, is largely considered as a critical determinant in the initiation and progression of various forms of cancer. Mechanistically, NF- κB favors this process by inducing various genes responsible for cell survival, proliferation, migration, invasion while at the same time antagonizing growth regulators including tumor suppressor p53. It has been shown by various independent investigations that a down regulation of NF- κB activity directly, or indirectly through the activation of the p53 pathway reduces tumor growth substantially. Therefore, there is a huge effort driven by many laboratories to understand the NF- κB signaling pathways to intervene the function of this crucial player in inflammation and tumorigenesis in order to find an effective inhibitor directly, or through the p53 tumor suppressor. We discuss here on the role of NF- κB in chronic inflammation and cancer, highlighting mutual antagonism between NF- κB and p53 pathways in the process. We also discuss prospective pharmacological modulators of these two pathways, including those that were already tested to affect this mutual antagonism.

Inhibition of KIF22 suppresses cancer cell proliferation by delaying mitotic exit through upregulating CDC25C expression

KIF22 is a microtubule-dependent molecular motor protein with DNA-binding capacity. It is well known that KIF22 plays a critical role in cell mitosis as a motor protein; however, the role of altered KIF22 expression and its transcriptional regulatory function in cancer development have not yet been defined. This study showed that KIF22 was overexpressed in human cancer tissues, and inhibition of KIF22 significantly led to accumulation of cells in the G2/M phases, resulting in suppression of cancer cell proliferation. The investigation of the molecular mechanisms demonstrated that cell division cycle 25C (CDC25C) is a direct transcriptional target of KIF22, and inhibition of KIF22 increased CDC25C expression and cyclin-dependent kinase 1 (CDK1) activity, resulting in delayed mitotic exit. Phosphorylation of KIF22 was required for its transcriptional regulatory function and the reduction of CDK1 activity. Thus, we conclude that inhibition of KIF22 suppresses cancer cell proliferation by delaying mitotic exit through the transcriptional upregulation of CDC25C.

Association of p21 with NF-YA suppresses the expression of Polo-like kinase 1 and prevents mitotic death in response to DNA damage

Polo-like kinase 1 (PLK1) is an important mitotic kinase and its expression is tightly regulated in the cell cycle and in the DNA damage response. PLK1 expression is previously shown to be suppressed by p53 and/or p21. Here, we demonstrate that the CCAAT box in the PLK1 promoter is pivotal for p53/p21-mediated PLK1 repression. Chromatin immunoprecipitation showed that cyclin-dependent kinase 2 (CDK2) associated with the CCAAT box-containing region of PLK1 promoter in unstressed cells, whereas adriamycin (ADR) induced the recruitment of p21 with a concomitant reduction in the occupancy of CDK2 in this region. Expression of p21 inhibited the interaction between CDK2 and the nuclear factor YA (NF-YA) subunit of the CCAAT box-binding transcription factor NF-Y. A mutant p21 that is defective in CDK2 binding was unable to disrupt the CDK2-NF-YA interaction or suppress PLK1 transcription. Co-immunoprecipitation experiments demonstrated the interaction between NF-YA and p21, and in vitro assays showed that p21 could directly bind to NF-YA. Knockdown of NF-YA decreased the amount of PLK1 promoter-associated p21 and abolished p21-mediated PLK1 repression in cells treated with ADR. Depletion of NF-YA diminished the p53-regulated transcriptional activation and suppressed the p53-mediated protection from mitotic death after DNA damage, and these effects of NF-YA deletion were alleviated by PLK1 depletion. Our findings have uncovered a novel p21/NF-YA/PLK1 axis critical for maintaining the checkpoint function of p53 to prevent mitotic death in the DNA damage-induced response.

The transcription factor p53: not a repressor, solely an activator

The predominant function of the tumor suppressor p53 is transcriptional regulation. It is generally accepted that p53-dependent transcriptional activation occurs by binding to a specific recognition site in promoters of target genes. Additionally, several models for p53-dependent transcriptional repression have been postulated. Here, we evaluate these models based on a computational meta-analysis of genome-wide data. Surprisingly, several major models of p53-dependent gene regulation are implausible. Meta-analysis of large-scale data is unable to confirm reports on directly repressed p53 target genes and falsifies models of direct repression. This notion is supported by experimental re-analysis of representative genes reported as directly repressed by p53. Therefore, p53 is not a direct repressor of transcription, but solely activates its target genes. Moreover, models based on interference of p53 with activating transcription factors as well as models based on the function of ncRNAs are also not supported by the meta-analysis. As an alternative to models of direct repression, the meta-analysis leads to the conclusion that p53 represses transcription indirectly by activation of the p53-p21-DREAM/RB pathway.

Dihydromyricetin suppresses the proliferation of hepatocellular carcinoma cells by inducing G2/M arrest through the Chk1/Chk2/Cdc25C pathway

The aim of the present study was to evaluate the antitumor mechanism of dihydromyricetin (DHM). Results showed that DHM significantly inhibited cell viability of HepG2 and Hep3B cells in a dose-dependent manner. DHM induced G2/M cell-cycle arrest in HepG2 and Hep3B cells by altering the expression of cell cycle proteins such as cyclin A, cyclin B1, Cdk1, p53, Cdc25c, p-Cdc25c Chk1 and Chk, which are critical for G2/M transition. Knockdown of p53 and Chk1 in HepG2 cells did not affect G2/M phase arrest caused by DHM. Furthermore, G2/M arrest induced by DHM can be disrupted by Chk2 siRNA. These findings indicate that DHM inhibits the growth of hepatocellular carcinoma (HCC) cells via G2/M phase cell cycle arrest through Chk1/Chk2/Cdc25C pathway. The present study identified effects of DHM in G2/M phase arrest in HCC and described detailed mechanisms of G2/M phase arrest by this agent, which may contribute to its overall cancer preventive efficacy in HCC.

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.

miR-137 regulates the constitutive androstane receptor and modulates doxorubicin sensitivity in parental and doxorubicin-resistant neuroblastoma cells

Chemotherapy is the most common treatment for cancer. However, multidrug resistance (MDR) remains a major obstacle to effective chemotherapy, limiting the efficacy of both conventional chemotherapeutic and novel biologic agents. The constitutive androstane receptor (CAR), a xenosensor, is a key regulator of MDR. It functions in xenobiotic detoxification by regulating the expression of phase I drug-metabolizing enzymes and ATP-binding cassette (ABC) transporters, whose overexpression in cancers and whose role in drug resistance make them potential therapeutic targets for reducing MDR. MicroRNAs (miRNAs) are endogenous negative regulators of gene expression and have been implicated in most cellular processes, including drug resistance. Here, we report the inversely related expression of miR-137 and CAR in parental and doxorubicin-resistant neuroblastoma cells, wherein miR-137 is downregulated in resistant cells. miR-137 overexpression resulted in downregulation of CAR protein and mRNA (via mRNA degradation); it sensitized doxorubicin-resistant cells to doxorubicin (as shown by reduced proliferation, increased apoptosis and increased G2-phase cell cycle arrest) and reduced the in vivo growth rate of neuroblastoma xenografts. We observed similar results in cellular models of hepatocellular and colon cancers, indicating that the doxorubicin-sensitizing effect of miR-137 is not tumor type-specific. Finally, we show for the first time a negative feedback loop whereby miR-137 downregulates CAR expression and CAR downregulates miR-137 expression. Hypermethylation of the miR-137 promoter and negative regulation of miR-137 by CAR contribute in part to reduced miR-137 expression and increased CAR and MDR1 expression in doxorubicin-resistant neuroblastoma cells. These findings demonstrate that miR-137 is a crucial regulator of cancer response to doxorubicin treatment, and they identify miR-137 as a highly promising target to reduce CAR-driven doxorubicin resistance.

Deacetylation of p53 induces autophagy by suppressing Bmf expression

Interferon γ (IFN-γ)-induced cell death is mediated by the BH3-only domain protein, Bik, in a p53-independent manner. However, the effect of IFN-γ on p53 and how this affects autophagy have not been reported. The present study demonstrates that IFN-γ down-regulated expression of the BH3 domain-only protein, Bmf, in human and mouse airway epithelial cells in a p53-dependent manner. p53 also suppressed Bmf expression in response to other cell death-stimulating agents, including ultraviolet radiation and histone deacetylase inhibitors. IFN-γ did not affect Bmf messenger RNA half-life but increased nuclear p53 levels and the interaction of p53 with the Bmf promoter. IFN-γ-induced interaction of HDAC1 and p53 resulted in the deacetylation of p53 and suppression of Bmf expression independent of p53's proline-rich domain. Suppression of Bmf facilitated IFN-γ-induced autophagy by reducing the interaction of Beclin-1 and Bcl-2. Furthermore, autophagy was prominent in cultured bmf(-/-) but not in bmf(+/+) cells. Collectively, these observations show that deacetylation of p53 suppresses Bmf expression and facilitates autophagy.

p53 and the PWWP domain containing effector proteins in chromatin damage repair

In eukaryotic cells, DNA damage repair occurs on a template DNA that is organized with histones to form nucleosomes and chromatin structures. As such, chromatin plays an important role in DNA damage repair. In this review, we will use "chromatin damage repair" as a framework and highlight recent progress in understanding the role of chromatin, chromatin modifiers, chromatin binding effectors (e.g., the PWWP domain proteins), and the p53 tumor suppressor. We view chromatin as an active participant during DNA damage repair.

Systems biology approach identifies the kinase Csnk1a1 as a regulator of the DNA damage response in embryonic stem cells

In pluripotent stem cells, DNA damage triggers loss of pluripotency and apoptosis as a safeguard to exclude damaged DNA from the lineage. An intricate DNA damage response (DDR) signaling network ensures that the response is proportional to the severity of the damage. We combined an RNA interference screen targeting all kinases, phosphatases, and transcription factors with global transcriptomics and phosphoproteomics to map the DDR in mouse embryonic stem cells treated with the DNA cross-linker cisplatin. Networks derived from canonical pathways shared in all three data sets were implicated in DNA damage repair, cell cycle and survival, and differentiation. Experimental probing of these networks identified a mode of DNA damage-induced Wnt signaling that limited apoptosis. Silencing or deleting the p53 gene demonstrated that genotoxic stress elicited Wnt signaling in a p53-independent manner. Instead, this response occurred through reduced abundance of Csnk1a1 (CK1α), a kinase that inhibits β-catenin. Together, our findings reveal a balance between p53-mediated elimination of stem cells (through loss of pluripotency and apoptosis) and Wnt signaling that attenuates this response to tune the outcome of the DDR.

Global effect of inauhzin on human p53-responsive transcriptome

Background

Previously, we reported that Inauhzin (INZ) induces p53 activity and suppresses tumor growth by inhibiting Sirt1. However, it remains unknown whether INZ may globally affect p53-dependent gene expression or not. Herein, we have conducted microarray and real-time PCR analyses of gene expression to determine the global effect of INZ on human p53-responsive transcriptome.

Methodology/Principal Findings

In this study, we conducted microarray analysis followed by PCR validation of general gene expression in HCT116^p53+/+ and HCT116^p53−/− cells treated with or without INZ. Microarray data showed that 324 genes were up-regulated by ≥2.3-fold and 266 genes were down-regulated by ≥2-fold in response to INZ treatment in a p53-dependent manner. GO analysis for these genes further revealed that INZ affects several biological processes, including apoptosis (GO:0006915), cell cycle (GO:0007049), immune system process (GO:0002376), and cell adhesion (GO:0007155), which are in line with p53 functions in cells. Also, pathway and STRING analyses of these genes indicated that the p53-signaling pathway is the most significant pathway responsive to INZ treatment as predicted, since a number of these p53 target genes have been previously reported and some of them were validated by RT-qPCR. Finally, among the 9 tested and highly expressed genes, ACBD4, APOBEC3C, and FLJ14327 could be novel p53 target genes, for they were up-regulated by INZ in HCT116^p53+/+ cells, but not in HCT116^p53−/− cells.

Conclusions/Significance

From our whole genome microarray analysis followed by validation with RT-qPCR, we found that INZ can indeed induce the expression of p53 target genes at a larger scale or globally. Our findings not only verify that INZ indeed activates the p53 signaling pathway, but also provide useful information for identifying novel INZ and/or p53 targets. The global effect of INZ on human p53-responsive transcriptome could also be instrumental to the future design of INZ clinical trials.

The impact of post-transcriptional regulation in the p53 network

The p53 transcription factor regulates the synthesis of mRNAs encoding proteins involved in diverse cellular stress responses such as cell-cycle arrest, apoptosis, autophagy and senescence. In this review, we discuss how these mRNAs are concurrently regulated at the post-transcriptional level by microRNAs (miRNAs) and RNA-binding proteins (RBPs), which consequently modify the p53 transcriptional program in a cell type- and stimulus-specific manner. We also discuss the action of specific miRNAs and RBPs that are direct transcriptional targets of p53 and how they act coordinately with protein-coding p53 target genes to orchestrate p53-dependent cellular responses.

Doxorubicin promotes transcriptional upregulation of Cdc25B in cancer cells by releasing Sp1 from the promoter

Cdc25B phosphatases have a key role in G2/M cell-cycle progression by activating the CDK1-cyclinB1 complexes and functioning as important targets of checkpoints. Overexpression of Cdc25B results in a bypass of the G2/M checkpoint and illegitimate entry into mitosis. It can also cause replicative stress, which leads to genomic instability. Thus, fine-tuning of the Cdc25B expression level is critical for correct cell-cycle arrest in response to DNA damage. In response to genotoxic stress, Cdc25B is mainly regulated by post-transcriptional mechanisms affecting either Cdc25B protein stability or translation. Here, we show that upon DNA damage Cdc25B can be regulated at the transcriptional level. Although ionizing radiation downregulates Cdc25B in a p53-dependent pathway, doxorubicin transcriptionally upregulates Cdc25B in p53-proficient cancer cells. We show that in the presence of wild-type p53, doxorubicin activates the Cdc25B promoter by preventing the binding of Sp1 and increasing the binding of NF-Y on the Cdc25B promoter, thus preventing p53 from downregulating this promoter. Our results highlight the mechanistically distinct regulation of the three Cdc25 phosphatases by checkpoint signalling following doxorubicin treatment.

E2F7, a novel target, is up-regulated by p53 and mediates DNA damage-dependent transcriptional repression

The p53 tumor suppressor protein is a transcription factor that exerts its effects on the cell cycle via regulation of gene expression. Although the mechanism of p53-dependent transcriptional activation has been well-studied, the molecular basis for p53-mediated repression has been elusive. The E2F family of transcription factors has been implicated in regulation of cell cycle-related genes, with E2F6, E2F7, and E2F8 playing key roles in repression. In response to cellular DNA damage, E2F7, but not E2F6 or E2F8, is up-regulated in a p53-dependent manner, with p53 being sufficient to increase expression of E2F7. Indeed, p53 occupies the promoter of the E2F7 gene after genotoxic stress, consistent with E2F7 being a novel p53 target. Ablation of E2F7 expression abrogates p53-dependent repression of a subset of its targets, including E2F1 and DHFR, in response to DNA damage. Furthermore, E2F7 occupancy of the E2F1 and DHFR promoters is detected, and expression of E2F7 is sufficient to inhibit cell proliferation. Taken together, these results show that p53-dependent transcriptional up-regulation of its target, E2F7, leads to repression of relevant gene expression. In turn, this E2F7-dependent mechanism contributes to p53-dependent cell cycle arrest in response to DNA damage.

ATM/ATR checkpoint activation downregulates CDC25C to prevent mitotic entry with uncapped telomeres

Shelterin component TRF2 prevents ATM activation, while POT1 represses ATR signalling at telomeres. Here, we investigate the mechanism of G2/M arrest triggered by telomeres uncapped through TRF2 or POT1 inhibition in human cells. We find that telomere damage-activated ATR and ATM phosphorylate p53, as well as CHK1 and CHK2, thus activating two independent pathways to prevent progression into mitosis with uncapped telomeres. Surprisingly, telomere damage targets the CDC25C phosphatase for proteasome degradation in G2/M. CHK1/CHK2-dependent phosphorylation of CDC25C at Ser 216 is required for CDC25C nuclear export and destruction, which in turn acts to sustain the G2/M arrest elicited by TRF2- or POT1-depleted telomeres. In addition, CDC25C is transcriptionally downregulated by p53 in response to telomere damage. These mechanisms are distinct from the canonical DNA damage response to ionizing radiation, which triggers cell-cycle arrest through CDC25A destruction. Thus, dysfunctional telomeres promote ATM/ATR-dependent degradation of CDC25C phosphatase to block mitotic entry, thereby preventing telomere dysfunction-driven genomic instability.

Genotoxic stress modulates CDC25C phosphatase alternative splicing in human breast cancer cell lines

CDC25 (cell division cycle 25) phosphatases are essential for cell cycle control under normal conditions and in response to DNA damage. They are represented by three isoforms, CDC25A, B and C, each of them being submitted to an alternative splicing mechanism. Alternative splicing of many genes is affected in response to genotoxic stress, but the impact of such a stress on CDC25 splicing has never been investigated. In this study, we demonstrate that genotoxic agents (doxorubicin, camptothecin, etoposide and cisplatin), alter the balance between CDC25C splice variants in human breast cancer cell lines both at the mRNA and protein levels. This modulation occurs during the response to moderate, sub-lethal DNA damage. Our results also suggest that the CDC25C splice variants expression shift induced by a genotoxic stress is dependent on the ATM/ATR signaling but not on p53. This study highlights the modulation of CDC25C alternative splicing as an additional regulatory event involved in cellular response to DNA damage in breast cancer cells.

14-3-3σ mediates G2-M arrest produced by 5-aza-2'-deoxycytidine and possesses a tumor suppressor role in endometrial carcinoma cells

OBJECTIVES

To determine the effect of 5-aza-2'-deoxycytidine (DAC) on human endometrial carcinoma cell (HECC) oncogenicity and demonstrate a molecular mechanism by which DAC modulates HECC oncogenicity.

METHODS

The effect of DAC was tested on HECC RL95-2, AN3, Ishikawa and ECC1 cells. The role of 14-3-3σ on HECC oncogenicity in response to DAC treatment was evaluated in RL95-2 and AN3 cells after forced expression or silencing of 14-3-3σ gene expression.

RESULTS

Treatment of HECC with DAC produced non-cytotoxic cell growth inhibition and G2/M cell cycle arrest. This effect was strongly correlated with increased expression of p21 and 14-3-3σ. Silencing of 14-3-3σ induced cellular proliferation and reduced the effect of DAC on cell cycle arrest in G2/M phases. Conversely, forced expression of 14-3-3σ showed the opposite effect. Furthermore, forced expression of 14-3-3σ in human endometrial cell lines reduced cell growth and colony formation.

CONCLUSIONS

We suggest that 14-3-3σ in HECC suppresses cell proliferation and mediates DAC induced G2/M arrest and inhibition of cell proliferation in HECC.

Downstream and intermediate interactions of synovial sarcoma-associated fusion oncoproteins and their implication for targeted therapy

Synovial sarcoma (SS), an aggressive type of soft tissue tumor, occurs mostly in adolescents and young adults. The origin and molecular mechanism of the development of SS remain only partially known. Over 90% of SS cases are characterized by the t(X;18)(p11.2;q11.2) translocation, which results mainly in the formation of SS18-SSX1 or SS18-SSX2 fusion genes. In recent years, several reports describing direct and indirect interactions of SS18-SSX1/SSX2 oncoproteins have been published. These reports suggest that the fusion proteins particularly affect the cell growth, cell proliferation, TP53 pathway, and chromatin remodeling mechanisms, contributing to SS oncogenesis. Additional research efforts are required to fully explore the protein-protein interactions of SS18-SSX oncoproteins and the pathways that are regulated by these partnerships for the development of effective targeted therapy.

New insights into cancer-related proteins provided by the yeast model

Cancer is a devastating disease with a profound impact on society. In recent years, yeast has provided a valuable contribution with respect to uncovering the molecular mechanisms underlying this disease, allowing the identification of new targets and novel therapeutic opportunities. Indeed, several attributes make yeast an ideal model system for the study of human diseases. It combines a high level of conservation between its cellular processes and those of mammalian cells, with advantages such as a short generation time, ease of genetic manipulation and a wealth of experimental tools for genome- and proteome-wide analyses. Additionally, the heterologous expression of disease-causing proteins in yeast has been successfully used to gain an understanding of the functions of these proteins and also to provide clues about the mechanisms of disease progression. Yeast research performed in recent years has demonstrated the tremendous potential of this model system, especially with the validation of findings obtained with yeast in more physiologically relevant models. The present review covers the major aspects of the most recent developments in the yeast research area with respect to cancer. It summarizes our current knowledge on yeast as a cellular model for investigating the molecular mechanisms of action of the major cancer-related proteins that, even without yeast orthologues, still recapitulate in yeast some of the key aspects of this cellular pathology. Moreover, the most recent contributions of yeast genetics and high-throughput screening technologies that aim to identify some of the potential causes underpinning this disorder, as well as discover new therapeutic agents, are discussed.

Targeting the p53 signaling pathway in cancer therapy - the promises, challenges and perils

INTRODUCTION

Research over the past three decades has identified p53 as a multi-functional transcription factor. p53 influences myriad, highly diverse cellular processes, and represents one of the most important and extensively studied tumor suppressors. Activated by various stresses, p53 blocks cancer progression by provoking transient or permanent growth arrest, by enabling DNA repair, or by advancing cellular death programs. This anti-cancer activity profile, together with genomic and mutational analyses documenting inactivation of p53 in more than 50% of human cancers, motivated drug development efforts to (re-) activate p53 in established tumors.

AREAS COVERED

The complexities of p53 signaling in cancer are summarized, including current strategies and challenges to restore p53's tumor suppressive function in established tumors, to inactivate p53 inhibitors, and to restore wild type function of p53 mutant proteins.

EXPERT OPINION

p53 represents an attractive target for the development of anti-cancer therapies. Whether p53 is 'druggable', however, remains an area of active research and discussion, as p53 has pro-survival functions and chronic p53 activation accelerates aging, which may compromise the long-term homeostasis of an organism. The complex biology and dual functions of p53 in cancer prevention and age-related cellular responses pose significant challenges to the development of p53-targeting cancer therapies.

The NF-Y/p53 liaison: well beyond repression

NF-Y is a sequence-specific transcription factor - TF - targeting the common CCAAT promoter element. p53 is a master TF controlling the response to stress signals endangering genome integrity, often mutated in human cancers. The NF-Y/p53 - and p63, p73 - interaction results in transcriptional repression of a subset of genes within the vast NF-Y regulome under DNA-damage conditions. Recent data shows that NF-Y is also involved in pro-apoptotic activities, either directly, by mediating p53 transcriptional activation, or indirectly, by being targeted by a non coding RNA, PANDA. The picture is subverted in cells carrying Gain-of-function mutant p53, through interactions with TopBP1, a protein also involved in DNA repair and replication. In summary, the connection between p53 and NF-Y is crucial in determining cell survival or death.

p53 regulates Ki-67 promoter activity through p53- and Sp1-dependent manner in HeLa cells

The expression of the human Ki-67 protein, which is strictly associated with cell proliferation, is regulated by a variety of cellular mediators. In this study, we studied the effects of p53 on Ki-67 promoter in HeLa cells using luciferase reporter assay. The results showed that: (1) p53 inhibited Ki-67 promoter activity in a dose-dependent manner, (2) the p53-binding motifs mediated part of the transcriptional repression of Ki-67 promoter through a sequence-specific interaction with p53, (3) p53 was able to repress the Sp1-stimulated Ki-67 promoter activity, and (4) the Sp1-binding sites were responsible for the p53-mediated transcriptional repression of Ki-67 promoter. In conclusion, p53 inhibited Ki-67 promoter activity via p53- and Sp1-dependent pathways, and the interaction between p53 and Sp1 might be involved in the transcriptional regulatory mechanisms.

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.

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.

p53-dependent repression of polo-like kinase-1 (PLK1)

PLK1 is a critical mediator of G₂/M cell cycle transition that is inactivated and depleted as part of the DNA damage-induced G₂/M checkpoint. Here we show that downregulation of PLK1 expression occurs through a transcriptional repression mechanism and that p53 is both necessary and sufficient to mediate this effect. Repression of PLK1 by p53 occurs independently of p21 and of arrest at G₁/S where PLK1 levels are normally repressed in a cell cycle-dependent manner through a CDE/CHR element. Chromatin immunoprecipitation analysis indicates that p53 is present on the PLK1 promoter at two distinct sites termed p53RE1 and p53RE2. Recruitment of p53 to p53RE2, but not to p53RE1, is stimulated in response to DNA damage and/or p53 activation and is coincident with repression-associated changes in the chromatin. Downregulation of PLK1 expression by p53 is relieved by the histone deacetylase inhibitor, trichostatin A, and involves recruitment of histone deacetylase to the vicinity of p53RE2, further supporting a transcriptional repression mechanism. Additionally, wild type, but not mutant, p53 represses expression of the PLK1 promoter when fused upstream of a reporter gene. Silencing of PLK1 expression by RNAi interferes with cell cycle progression consistent with a role in the p53-mediated checkpoint. These data establish PLK1 as a direct transcriptional target of p53, independently of p21, that is required for efficient G₂/M arrest.

Prostaglandin (PG)D(2) and 15-deoxy-Delta(12,14)-PGJ(2), but not PGE(2), mediate shear-induced chondrocyte apoptosis via protein kinase A-dependent regulation of polo-like kinases

Excessive mechanical loading of cartilage producing hydrostatic stress, tensile strain and fluid flow leads to chondrocyte apoptosis and osteoarthritis. High fluid flow induces cyclooxygenase-2 (COX-2) expression in sheared chondrocytes, which suppresses their antioxidant capacity and contributes to apoptosis. The pivotal role of COX-2 in shear-induced chondrocyte apoptosis and the conflicting literature data on the roles of prostaglandin (PG)E(2), PGD(2) and its metabolite 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)) in chondrocyte apoptosis prompted us to analyze which COX-2-derived PG is involved in this process. We show that exogenously added PGD(2) and 15d-PGJ(2), but not PGE(2), diminish the viability of human T/C-28a2 chondrocytes under static conditions. In agreement with these observations, knockdown of L-PGD synthase (L-PGDS) abolishes shear-induced chondrocyte apoptosis. Using cDNA microarrays in conjunction with clustering algorithms, we propose a novel signaling pathway by which high fluid shear mediates COX-2/L-PGDS-dependent chondrocyte apoptosis, which is validated by molecular interventions. We show that L-PGDS controls the downregulation of protein kinase A (PKA), which in turn regulates Polo-like kinase1 (Plk1) and Plk3. Plks target p53, which controls the transcription of p53 effectors (TP53INPs, FAS and Bax) involved in chondrocyte apoptosis. Reconstructing the signaling network regulating chondrocyte apoptosis may provide insights to optimize conditions for culturing artificial cartilage in bioreactors and for developing therapeutic strategies for arthritic disorders.

Transcriptional regulation by p53

Inactivation of p53 is critical for the formation of most tumors. Illumination of the key function(s) of p53 protein in protecting cells from becoming cancerous is therefore a worthy goal. Arguably p53's most important function is to act as a transcription factor that directly regulates perhaps several hundred of the cell's RNA polymerase II (RNAP II)-transcribed genes, and indirectly regulates thousands of others. Indeed p53 is the most well studied mammalian transcription factor. The p53 tetramer binds to its response element where it can recruit diverse transcriptional coregulators such as histone modifying enzymes, chromatin remodeling factors, subunits of the mediator complex, and components of general transcription machinery and preinitiation complex (PIC) to modulate RNAPII activity at target loci (Laptenko and Prives 2006). The p53 transcriptional program is regulated in a stimulus-specific fashion (Murray-Zmijewski et al. 2008; Vousden and Prives 2009), whereby distinct subsets of p53 target genes are induced in response to different p53-activating agents, likely allowing cells to tailor their response to different types of stress. How p53 is able to discriminate between these different loci is the subject of intense research. Here, we describe key aspects of the fundamentals of p53-mediated transcriptional regulation and target gene promoter selectivity.

The central role of CDE/CHR promoter elements in the regulation of cell cycle-dependent gene transcription

The cell cycle-dependent element (CDE) and the cell cycle genes homology region (CHR) control the transcription of genes with maximum expression in G(2) phase and in mitosis. Promoters of these genes are repressed by proteins binding to CDE/CHR elements in G(0) and G(1) phases. Relief from repression begins in S phase and continues into G(2) phase and mitosis. Generally, CDE sites are located four nucleotides upstream of CHR elements in TATA-less promoters of genes such as Cdc25C, Cdc2 and cyclin A. However, expression of some other genes, such as human cyclin B1 and cyclin B2, has been shown to be controlled only by a CHR lacking a functional CDE. To date, it is not fully understood which proteins bind to and control CDE/CHR-containing promoters. Recently, components of the DREAM complex were shown to be involved in CDE/CHR-dependent transcriptional regulation. In addition, the expression of genes regulated by CDE/CHR elements is mostly achieved through CCAAT-boxes, which bind heterotrimeric NF-Y proteins as well as the histone acetyltransferase p300. Importantly, many CDE/CHR promoters are downregulated by the tumor suppressor p53. In this review, we define criteria for CDE/CHR-regulated promoters and propose to distinguish two classes of CDE/CHR-regulated genes. The regulation through transcription factors potentially binding to the CDE/CHR is discussed, and recently discovered links to central pathways regulated by E2F, the pRB family and p53 are highlighted.

p73 and p63 sustain cellular growth by transcriptional activation of cell cycle progression genes

Despite extensive studies on the role of tumor suppressor p53 protein and its homologues, p73 and p63, following their overexpression or cellular stress, very little is known about the regulation of the three proteins in cells during physiologic cell cycle progression. We report a role for p73 and p63 in supporting cellular proliferation through the transcriptional activation of the genes involved in G(1)-S and G(2)-M progression. We found that in MCF-7 cells, p73 and p63, but not p53, are modulated during the cell cycle with a peak in S phase, and their silencing determines a significant suppression of proliferation compared with the control. Chromatin immunoprecipitation analysis shows that in cycling cells, p73 and p63 are bound to the p53-responsive elements (RE) present in the regulatory region of cell cycle progression genes. On the contrary, when the cells are arrested in G(0)-G(1), p73 detaches from the REs and it is replaced by p53, which represses the expression of these genes. When the cells move in S phase, p73 is recruited again and p53 is displaced or is weakly bound to the REs. These data open new possibilities for understanding the involvement of p73 and p63 in cancer. The elevated concentrations of p73 and p63 found in many cancers could cause the aberrant activation of cell growth progression genes and therefore contribute to cancer initiation or progression under certain conditions.

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.

Blinded by the Light: The Growing Complexity of p53

While the tumor suppressor functions of p53 have long been recognized, the contribution of p53 to numerous other aspects of disease and normal life is only now being appreciated. This burgeoning range of responses to p53 is reflected by an increasing variety of mechanisms through which p53 can function, although the ability to activate transcription remains key to p53's modus operandi. Control of p53's transcriptional activity is crucial for determining which p53 response is activated, a decision we must understand if we are to exploit efficiently the next generation of drugs that selectively activate or inhibit p53.

Promyelocytic leukemia protein is required for gain of function by mutant p53

Mutations in the p53 tumor suppressor are the most common genetic events in human cancer. These mutations not only result in a loss of wild-type p53 activity, but can also lead to a gain of new oncogenic properties. Understanding how these gained functions are regulated is in its infancy. In this study, we show that the promyelocytic leukemia (PML) protein is an important regulator of mutant p53. We show that PML interacts with mutant p53. Importantly, PML enhances the transcriptional activity of mutant p53. Unexpectedly, PML is required for the proliferation and colony formation of cancer cells bearing mutant p53. Down-regulation of PML expression inhibits the growth of mutant p53-expressing cancer cells, predominantly by promoting cell cycle arrest. Our results suggest that the tumor suppression function of PML depends on the status of p53. In the context of mutant p53, PML enhances its cancer-promoting activities.

DNA damage induced p53 downregulates Cdc20 by direct binding to its promoter causing chromatin remodeling

CDC20 is a critical molecule in the Spindle Assembly Checkpoint (SAC). It activates the Anaphase promoting complex and helps a dividing cell to proceed towards Anaphase. CDC20 is overexpressed in many tumor cells which cause chromosomal instability. There have been limited reports on the mechanism of SAC's response to genotoxic stress. We show that ectopically expressed p53 or DNA damage induced endogenous p53 can downregulate Cdc20 transcriptionally. We have identified a consensus p53-binding site on the Cdc20 promoter and have shown that it is being used by p53 to bind the promoter and bring about chromatin remodeling thereby repressing Cdc20. Additionally, p53 also downregulates Cdc20 promoter through CDE/CHR element, but in a p21 independent manner. This CDE/CHR element-mediated downregulation occurs only under p53 overexpressed condition but not in the context of DNA damage. The present results suggest that the two CCAAT elements in the Cdc20 promoter are not used by p53 to downregulate its activity, as reported earlier.

Coordinated regulation of cell cycle transcripts by p53-Inducible microRNAs, miR-192 and miR-215

Cell cycle arrest in response to DNA damage is an important antitumorigenic mechanism. MicroRNAs (miRNAs) were recently shown to play key regulatory roles in cell cycle progression. For example, miR-34a is induced in response to p53 activation and mediates G(1) arrest by down-regulating multiple cell cycle-related transcripts. Here we show that genotoxic stress promotes the p53-dependent up-regulation of the homologous miRNAs miR-192 and miR-215. Like miR-34a, activation of miR-192/215 induces cell cycle arrest, suggesting that multiple miRNA families operate in the p53 network. Furthermore, we define a downstream gene expression signature for miR-192/215 expression, which includes a number of transcripts that regulate G(1) and G(2) checkpoints. Of these transcripts, 18 transcripts are direct targets of miR-192/215, and the observed cell cycle arrest likely results from a cooperative effect among the modulations of these genes by the miRNAs. Our results showing a role for miR-192/215 in cell proliferation combined with recent observations that these miRNAs are underexpressed in primary cancers support the idea that miR-192 and miR-215 function as tumor suppressors.

p53 regulates Hsp90beta during arsenite-induced cytotoxicity in glutathione-deficient cells

p53, a tumor suppressor and transcription factor, is a critical modulator in the cellular response to stress. Exposure of glutathione-deficient GCS-2 cells to arsenite significantly phosphorylated and stabilized p53. In addition, p53 transcriptionally repressed Hsp90beta gene expression. Mutation analysis revealed a p53 binding site in the 5' flanking region responsible for the regulation of Hsp90beta gene. Electrophoretic mobility shift assay showed that p53 is bound to Hsp90beta promoter region. ATM kinase, a major determinant in the modulation of p53 specifically affected its phosphorylation at Ser-15. ATM kinase-mediated phosphorylation of p53 is regulated through phosphorylation of Chk2. Down-regulation of ATM and Chk2 by their small interfering RNAs (siRNAs) attenuated the arsenite-induced phosphorylation of p53 and restored Hsp90beta mRNA levels. Taken together, these findings suggest that arsenite acts through ATM and Chk2 to induce phosphorylation of p53. This results in the transcriptional repression of Hsp90beta, under GSH-deficient conditions which may play a role in arsenic-mediated pathogenesis.

Dual regulation of Cdc25A by Chk1 and p53-ATF3 in DNA replication checkpoint control

Eukaryotic cells respond to DNA damage and stalled replication forks by activating signaling pathways that promote cell cycle arrest and DNA repair. A systematic screening of the protein kinase small interfering RNA library reveals that Chk1 and ataxia telangiectasia-mutated (ATM) and Rad3-related (ATR) are the main kinases responsible for intra-S-phase checkpoint upon topoisomerase I inhibitor camptothecin-induced DNA damage. It is well known that ATR-Chk1-mediated protein degradation of Cdc25A protein phosphatase is a crucial mechanism conferring this checkpoint activation. Here we describe another mechanism underlying Cdc25A down-regulation in response to DNA damage that occurs at the transcriptional level. We show that activation of tumor suppressor p53 by DNA damage results in inhibition of Cdc25A transcription as a result of activation of transcriptional repressor ATF3 that directly binds to the Cdc25A promoter. In cells deficient in both Chk1 and p53, Cdc25A down-regulation upon camptothecin-induced DNA damage is completely abolished, leading to severe defects in cell cycle checkpoints and remarkable cell death in mitosis. Our findings reveal two independent mechanisms acting in concert in regulation of Cdc25A in DNA damage response. Although Chk1 affects Cdc25A via rapid phosphorylation and protein turnover, inhibition of Cdc25A transcription by p53-ATF3 is required for the maintenance of cell cycle arrest.

p53-targeted LSD1 functions in repression of chromatin structure and transcription in vivo

Despite years of study focused on the tumor suppressor p53, little is understood about its functions in normal, differentiated cells. We found that p53 directly interacts with lysine-specific demethylase 1 (LSD1) to alter chromatin structure and confer developmental repression of the tumor marker alpha-fetoprotein (AFP). Chromatin immunoprecipitation (ChIP) and sequential ChIP of developmentally staged liver showed that p53 and LSD1 cooccupy a p53 response element, concomitant with dimethylated histone H3 lysine 4 (H3K4me2) demethylation and postnatal repression of AFP transcription. In p53-null mice, LSD1 binding is depleted, H3K4me2 is increased, and H3K9me2 remains unchanged compared to those of the wild type, underscoring the specificity of p53-LSD1 complexes in demethylation of H3K4me2. We performed partial hepatectomy of wild-type mouse liver and induced a regenerative response, which led to a loss of p53, increased H3K4me2, and decreased LSD1 interaction at AFP chromatin, in parallel with reactivation of AFP expression. In contrast, nuclear translocation of p53 in mouse embryonic fibroblasts led to p53 interaction with p21/CIP1 chromatin, without recruitment of LSD1, and to activation of p21/CIP1. These findings reveal that LSD1 is targeted to chromatin by p53, likely in a gene-specific manner, and define a molecular mechanism by which p53 mediates transcription repression in vivo during differentiation.

CDC20, a potential cancer therapeutic target, is negatively regulated by p53

The p53 protein inhibits malignant transformation through direct and indirect regulation of transcription of many genes related to cell cycle, apoptosis and cellular senescence. A number of genes induced by p53 have been well characterized, but biological significance of genes whose expression was suppressed by p53 is still largely undisclosed. To clarify the roles of p53-suppressive genes in carcinogenesis, we analysed two data sets of whole-genome expression profiles, one for cells in which wild-type p53 was exogenously introduced and the other for a large number of clinical cancer tissues. Here, we identified CDC20 that was frequently upregulated in many types of malignancies and remarkably suppressed by ectopic introduction of p53. CDC20 expression was suppressed by genotoxic stresses in p53- and p21-dependent manners through CDE-CHR elements in the CDC20 promoter. Furthermore, small interference RNA (siRNA)-mediated silencing of p53 induced CDC20 expression in normal human dermal fibroblast cells. As we expected, treatment of cancer cells with siRNA against CDC20 induced G(2)/M arrest and suppressed cell growth. Our results indicate that p53 inhibits tumor cell growth through the indirect regulation of CDC20 and that CDC20 might be a good potential therapeutic target for a broad spectrum of human cancer.

Transcriptional regulation by p53: one protein, many possibilities

The p53 tumor suppressor protein is a DNA sequence-specific transcriptional regulator that, in response to various forms of cellular stress, controls the expression of numerous genes involved in cellular outcomes including among others, cell cycle arrest and cell death. Two key features of the p53 protein are required for its transcriptional activities: its ability to recognize and bind specific DNA sequences and to recruit both general and specialized transcriptional co-regulators. In fact, multiple interactions with co-activators and co-repressors as well as with the components of the general transcriptional machinery allow p53 to either promote or inhibit transcription of different target genes. This review focuses on some of the salient features of the interactions of p53 with DNA and with factors that regulate transcription. We discuss as well the complexities of the functional domains of p53 with respect to these interactions.

The dual specificity phosphatase Cdc25B, but not the closely related Cdc25C, is capable of inhibiting cellular proliferation in a manner dependent upon its catalytic activity

Cdc25B and Cdc25C are closely related dual specificity phosphatases that activate cyclin-dependent kinases by removal of inhibitory phosphorylations, thereby triggering entry into mitosis. Cdc25B, but not Cdc25C, has been implicated as an oncogene and been shown to be overexpressed in a variety of human tumors. Surprisingly, ectopic expression of Cdc25B, but not Cdc25C, inhibits cell proliferation in long term assays. Chimeric proteins generated from the two phosphatases show that the anti-proliferative activity is associated with the C-terminal end of Cdc25B. Indeed, the catalytic domain of Cdc25B is sufficient to suppress cell viability in a manner partially dependent upon its C-terminal 26 amino acids that is shown to influence substrate binding. Mutation analysis demonstrates that both the phosphatase activity of Cdc25B as well as its ability to interact with its substrates contribute to the inhibition of cell proliferation. These results demonstrate key differences in the biological activities of Cdc25B and Cdc25C caused by differential substrate affinity and recognition. This also argues that the antiproliferative activity of Cdc25B needs to be overcome for it to act as an oncogene during tumorigenesis.

The effects of a mutant p53 protein on the proliferation and differentiation of PC12 rat phaeochromocytoma cells

PC12 rat phaeochromocytoma cells show neuronal differentiation upon NGF treatment. NGF induces prolonged activation of the Ras/Raf/MEK/ERK pathway in which the 42/44 kDa mitogen-activated protein kinases (MAPKs), ERK 1 and 2 are thought to be the key mediators of the differentiation signals. Activation of ERKs leads to the increased transcription of early response genes resulting in cell cycle arrest. Upon NGF treatment the p53 protein, the most commonly mutated tumor suppressor in human cancers, translocates to the nucleus and may play a role in the mediation of NGF-induced cell cycle arrest and neuronal differentiation. Here we demonstrate that in PC12 cells expressing both wild-type and V143A mutant p53 proteins (p143p53PC12 cells), p53-mediated biological responses are critically influenced. p143p53PC12 cells are not able to cease their proliferation and begin their neuronal differentiation program upon NGF treatment. The presence of mutant p53 also reduces the DNA-binding activity of endogenous p53 and disturbs the regulatory machinery of p53 including both the phosphorylation of ERK 1/2, p38 and SAPK/JNK MAP kinases and itself.