Mutant p53 protects cells from 12-O-tetradecanoylphorbol-13-acetate-induced death by attenuating activating transcription factor 3 induction

Comparative parallel multi-omics analysis during the induction of pluripotent and trophectoderm states

Following fertilization, it is only at the 32-64-cell stage when a clear segregation between cells of the inner cell mass and trophectoderm is observed, suggesting a 'T'-shaped model of specification. Here, we examine whether the acquisition of these two states in vitro, by nuclear reprogramming, share similar dynamics/trajectories. Using a comparative parallel multi-omics analysis (i.e., bulk RNA-seq, scRNA-seq, ATAC-seq, ChIP-seq, RRBS and CNVs) on cells undergoing reprogramming to pluripotency and TSC state we show that each reprogramming system exhibits specific trajectories from the onset of the process, suggesting 'V'-shaped model. We describe in detail the various trajectories toward the two states and illuminate reprogramming stage-specific markers, blockers, facilitators and TSC subpopulations. Finally, we show that while the acquisition of the TSC state involves the silencing of embryonic programs by DNA methylation, during the acquisition of pluripotency these regions are initially defined but retain inactive by the elimination of H3K27ac.

Nucleolar and spindle‑associated protein 1 promotes non‑small cell lung cancer progression and serves as an effector of myocyte enhancer factor 2D

As a potential oncogene, nucleolar and spindle‑associated protein 1 (NUSAP1) is involved in the regulation of tumor cell proliferation, metastasis and drug resistance. However, the role of NUSAP1 in non‑small cell lung cancer (NSCLC) remains unclear. The present study aimed to investigate the biological function and underlying molecular mechanisms of NUSAP1 in NSCLC. NUSAP1 expression was measured in NSCLC tissues and cell lines via immunohistochemistry and western blotting, respectively. NSCLC cell lines stably inhibiting NUSAP1 were established to investigate its effects on cell proliferation, colony formation and invasion, and on in vivo tumorigenicity. Additionally, the upstream and downstream mechanisms of NUSAP1 in regulating NSCLC progression were investigated. The results indicated that NUSAP1 expression was upregulated in NSCLC tissues and cell lines. High NUSAP1 expression was associated with tumor size, TNM stage, lymph node metastasis and poor patient survival, whereas knockdown of NUSAP1 inhibited NSCLC cell proliferation, colony formation and invasion. Furthermore, downregulation of NUSAP1 decreased the growth of NSCLC xenografts in vivo. In addition, myocyte enhancer factor 2D (MEF2D) directly targeted the NUSAP1 promoter, thereby enhancing the mRNA and protein expression levels of NUSAP1. Moreover, the results demonstrated that MEF2D expression was upregulated in NSCLC tissues and was positively correlated with NUSAP1 expression. MEF2D‑knockdown decreased NSCLC cell proliferation, colony formation and invasion. NUSAP1 upregulation reversed the effects of MEF2D‑knockdown on NSCLC progression. Furthermore, it was observed that MEF2D‑knockdown inhibited the accumulation and nuclear translocation of β‑catenin, thereby repressing the activation of the Wnt/β‑catenin signaling pathway in NSCLC cells, whereas NUSAP1 upregulation rescued the effects of MEF2D‑knockdown on the activation of the Wnt/β‑catenin signaling pathway. In conclusion, the findings of the present study indicated that the MEF2D/NUSAP1 signaling pathway promoted NSCLC progression by inducing the activation of Wnt/β‑catenin signaling, and this novel mechanism may represent a potential treatment target for patients with NSCLC.

Gain-of-Function Mutant p53: All the Roads Lead to Tumorigenesis

The p53 protein is mutated in about 50% of human cancers. Aside from losing the tumor-suppressive functions of the wild-type form, mutant p53 proteins often acquire inherent, novel oncogenic functions, a phenomenon termed mutant p53 gain-of-function (GOF). A growing body of evidence suggests that these pro-oncogenic functions of mutant p53 proteins are mediated by affecting the transcription of various genes, as well as by protein-protein interactions with transcription factors and other effectors. In the current review, we discuss the various GOF effects of mutant p53, and how it may serve as a central node in a network of genes and proteins, which, altogether, promote the tumorigenic process. Finally, we discuss mechanisms by which "Mother Nature" tries to abrogate the pro-oncogenic functions of mutant p53. Thus, we suggest that targeting mutant p53, via its reactivation to the wild-type form, may serve as a promising therapeutic strategy for many cancers that harbor mutant p53. Not only will this strategy abrogate mutant p53 GOF, but it will also restore WT p53 tumor-suppressive functions.

Transcriptional Regulation by Wild-Type and Cancer-Related Mutant Forms of p53

TP53 missense mutations produce a mutant p53 protein that cannot activate the p53 tumor suppressive transcriptional response, which is the primary selective pressure for TP53 mutation. Specific codons of TP53, termed hotspot mutants, are mutated at elevated frequency. Hotspot forms of mutant p53 possess oncogenic properties in addition to being deficient in tumor suppression. Such p53 mutants accumulate to high levels in the cells they inhabit, causing transcriptional alterations that produce pro-oncogenic activities, such as increased pro-growth signaling, invasiveness, and metastases. These forms of mutant p53 very likely use features of wild-type p53, such as interactions with the transcriptional machinery, to produce oncogenic effects. In this review, we discuss commonalities between wild-type and mutant p53 proteins with an emphasis on transcriptional processes.

Emerging Non-Canonical Functions and Regulation by p53: p53 and Stemness

Since its discovery nearly 40 years ago, p53 has ascended to the forefront of investigated genes and proteins across diverse research disciplines and is recognized most exclusively for its role in cancer as a tumor suppressor. Levine and Oren (2009) reviewed the evolution of p53 detailing the significant discoveries of each decade since its first report in 1979. In this review, we will highlight the emerging non-canonical functions and regulation of p53 in stem cells. We will focus on general themes shared among p53's functions in non-malignant stem cells and cancer stem-like cells (CSCs) and the influence of p53 on the microenvironment and CSC niche. We will also examine p53 gain of function (GOF) roles in stemness. Mutant p53 (mutp53) GOFs that lead to survival, drug resistance and colonization are reviewed in the context of the acquisition of advantageous transformation processes, such as differentiation and dedifferentiation, epithelial-to-mesenchymal transition (EMT) and stem cell senescence and quiescence. Finally, we will conclude with therapeutic strategies that restore wild-type p53 (wtp53) function in cancer and CSCs, including RING finger E3 ligases and CSC maintenance. The mechanisms by which wtp53 and mutp53 influence stemness in non-malignant stem cells and CSCs or tumor-initiating cells (TICs) are poorly understood thus far. Further elucidation of p53's effects on stemness could lead to novel therapeutic strategies in cancer research.

Oncogenic Mutant p53 Gain of Function Nourishes the Vicious Cycle of Tumor Development and Cancer Stem-Cell Formation

More than half of human tumors harbor an inactivated p53 tumor-suppressor gene. It is well accepted that mutant p53 shows an oncogenic gain-of-function (GOF) activity that facilitates the transformed phenotype of cancer cells. In addition, a growing body of evidence supports the notion that cancer stem cells comprise a seminal constituent in the initiation and progression of cancer development. Here, we elaborate on the mutant p53 oncogenic GOF leading toward the acquisition of a transformed phenotype, as well as placing mutant p53 as a major component in the establishment of cancer stem cell entity. Therefore, therapy targeted toward cancer stem cells harboring mutant p53 is expected to pave the way to eradicate tumor growth and recurrence.

Mutant p53-R273H mediates cancer cell survival and anoikis resistance through AKT-dependent suppression of BCL2-modifying factor (BMF)

p53 is the most frequently mutated tumor-suppressor gene in human cancers. Unlike other tumor-suppressor genes, p53 mutations mainly occur as missense mutations within the DNA-binding domain, leading to the expression of full-length mutant p53 protein. Mutant p53 proteins not only lose their tumor-suppressor function, but may also gain new oncogenic functions and promote tumorigenesis. Here, we showed that silencing of endogenous p53-R273H contact mutant, but not p53-R175H conformational mutant, reduced AKT phosphorylation, induced BCL2-modifying factor (BMF) expression, sensitized BIM dissociation from BCL-X_L and induced mitochondria-dependent apoptosis in cancer cells. Importantly, cancer cells harboring endogenous p53-R273H mutant were also found to be inherently resistant to anoikis and lack BMF induction following culture in suspension. Underlying these activities is the ability of p53-R273H mutant to suppress BMF expression that is dependent on constitutively active PI3K/AKT signaling. Collectively, these findings suggest that p53-R273H can specifically drive AKT signaling and suppress BMF expression, resulting in enhanced cell survivability and anoikis resistance. These findings open the possibility that blocking of PI3K/AKT will have therapeutic benefit in mutant p53-R273H expressing cancers.

Mechanisms of resistance to apoptosis in the human acute promyelocytic leukemia cell line NB4

Current frontline therapies have improved overall survival in acute promyelocytic leukemia (APL) patients to exceptional rates; however, relapse is still a problem among high-risk and old patients. Therefore, the development of better and safer therapies continues to be a goal in the treatment of this disease. In the present work, we examined three different pathways that hinder cell death in the APL cell line NB4, shedding light on the mechanisms that underlie resistance to apoptosis in these cells and that might help provide them with a proliferative advantage. We found that the proteasome inhibitor MG-132 specifically induces in NB4 cells an Nrf2-mediated antioxidant response which counteracts mitochondria-dependent apoptosis induced by the lipophilic cation dequalinium. More importantly, we also demonstrated that high basal autophagy levels and the gain-of-function of mutant p53 are intrinsic mechanisms of resistance to apoptosis in this cell line. According to our results, the pharmacological inhibition of autophagy and p53 mutants are useful tools to explore resistance to apoptosis in APL and other types of cancer and could be the bases of new therapeutic approaches that improve the efficiency and allow dose reduction of the current treatments.

Protein kinase C-activating tumor promoters modulate the DNA damage response in UVC-irradiated TK6 cells

12-O-Tetradecanoylphorbol-13-acetate (TPA) is a non-genotoxic tumor promoter that dysregulates the protein kinase C (PKC) pathway and causes variable cellular responses to DNA damage in different experimental models. In the present study, we pretreated human lymphoblastoid TK6 cells (wild-type p53) for 72 h with TPA, and five other PKC-activating tumor promoters, to determine how sustained exposure to these chemicals modulates key DNA damage response (DDR) endpoints induced by UVC-irradiation. Here we show that pre-treatment with PKC-activating tumor promoters augmented the sensitivity of TK6 cells to UVC-irradiation characterized by a synergistic increase in apoptosis compared to that induced by either stress alone. In addition, high residual levels of the DNA damage repair signal γH2AX was observed in tumor promoter treated cells indicating a delayed DDR recovery. NH32 (p53-null, isogenic to TK6) cells were resistant to the synergistic effects on apoptosis implicating p53 as a central mediator of the DDR modulating effects. In addition, analysis of p53 target genes in TPA-pre-treated TK6 cells revealed a significant modulation of UVC-induced gene expression that supported a shift toward a pro-apoptotic phenotype. Therefore, sustained exposure to tumor promoting agents modulates the UVC-induced DDR in TK6 cells, which may represent important synergistic interactions that occur during tumor promotion.

Dynamic expression patterns of ATF3 and p53 in the hippocampus of a pentylenetetrazole-induced kindling model

Epilepsy is a common and often deleterious neurological condition. Emerging evidence has demonstrated the roles of innate immunity and the associated inflammatory processes in epilepsy. In a previous study, we found that Toll-like receptors (TLRs) are upregulated and promote mossy fiber sprouting (MFS) in an epileptic model. As downstream effectors of TLRs, the activating transcription factor 3 (ATF3) and p53 proteins were shown to be involved in neurite outgrowth. In the present study, we hypothesized that ATF3 and p53 participate in the process of epilepsy and can affect MFS. To investigate this hypothesis, we examined the expression of ATF3 and p53 in hippocampal tissues of rats kindled by pentylenetetrazole (PTZ) using immunofluorescence, immunohistochemistry and western blotting. MFS was evaluated by Timm staining in the hippocampus. Results from these experiments revealed that expression of ATF3 and p53 is significantly higher (p<0.05) in the CA3 area of the hippocampus in the PTZ-treated group compared to the control group. ATF3 expression gradually increased from 3 days to 4 weeks, peaked at 4 weeks and decreased slightly at 6 weeks in the PTZ group, while the expression of p53 was maintained at similar levels at different time-points following PTZ treatment. No obvious difference in the expression of these proteins was observed between the PTZ and the control group in the dentate gyrus (DG) area (p>0.05). The degree of MFS in the PTZ group peaked at 4 weeks and was maintained at a high level until 6 weeks post-PTZ treatment. In conclusion, ATF3 and p53 may be involved in the occurrence of seizure and play critical roles in MFS in the PTZ kindling model.

The paradigm of mutant p53-expressing cancer stem cells and drug resistance

It is well accepted that expression of mutant p53 involves the gain of oncogenic-specific activities accentuating the malignant phenotype. Depending on the specific cancer type, mutant p53 can contribute to either the early or the late events of the multiphase process underlying the transformation of a normal cell into a cancerous one. This multifactorial system is evident in ~50% of human cancers. Mutant p53 was shown to interfere with a variety of cellular functions that lead to augmented cell survival, cellular plasticity, aberration of DNA repair machinery and other effects. All these effects culminate in the acquisition of drug resistance often seen in cancer cells. Interestingly, drug resistance has also been suggested to be associated with cancer stem cells (CSCs), which reside within growing tumors. The notion that p53 plays a regulatory role in the life of stem cells, coupled with the observations that p53 mutations may contribute to the evolvement of CSCs makes it challenging to speculate that drug resistance and cancer recurrence are mediated by CSCs expressing mutant p53.

Mutant p53 in cancer: new functions and therapeutic opportunities

Many different types of cancer show a high incidence of TP53 mutations, leading to the expression of mutant p53 proteins. There is growing evidence that these mutant p53s have both lost wild-type p53 tumor suppressor activity and gained functions that help to contribute to malignant progression. Understanding the functions of mutant p53 will help in the development of new therapeutic approaches that may be useful in a broad range of cancer types.

The activating transcription factor 3 protein suppresses the oncogenic function of mutant p53 proteins

Mutant p53 proteins (mutp53) often acquire oncogenic activities, conferring drug resistance and/or promoting cancer cell migration and invasion. Although it has been well established that such a gain of function is mainly achieved through interaction with transcriptional regulators, thereby modulating cancer-associated gene expression, how the mutp53 function is regulated remains elusive. Here we report that activating transcription factor 3 (ATF3) bound common mutp53 (e.g. R175H and R273H) and, subsequently, suppressed their oncogenic activities. ATF3 repressed mutp53-induced NFKB2 expression and sensitized R175H-expressing cancer cells to cisplatin and etoposide treatments. Moreover, ATF3 appeared to suppress R175H- and R273H-mediated cancer cell migration and invasion as a consequence of preventing the transcription factor p63 from inactivation by mutp53. Accordingly, ATF3 promoted the expression of the metastasis suppressor SHARP1 in mutp53-expressing cells. An ATF3 mutant devoid of the mutp53-binding domain failed to disrupt the mutp53-p63 binding and, thus, lost the activity to suppress mutp53-mediated migration, suggesting that ATF3 binds to mutp53 to suppress its oncogenic function. In line with these results, we found that down-regulation of ATF3 expression correlated with lymph node metastasis in TP53-mutated human lung cancer. We conclude that ATF3 can suppress mutp53 oncogenic function, thereby contributing to tumor suppression in TP53-mutated cancer.

[The cooperation between p53 and Ras in tumorigenesis]

Inactivation of the tumor suppressor gene and activation of the oncogene cooperate to promote the multistep process of tumorigenesis. p53 is considered to be the most important tumor suppressor gene. p53 is usually found as a result of somatic missense mutation in approximately 50% of human cancers. Ras is found to be one of the most frequently mutated oncogenes in human tumors with the reported frequencies ranging from 30% to 90%. It has been found in many studies synergistic effect between tumor suppressor p53 and oncogene Ras occurs during the multistep process of tumorigenesis. According to the current reports, the cooperative effect between p53 and Ras can be divided into three types: the regulation of p53 for Ras function, , the regulation of Ras for p53, and the cooperation between p53 and Ras to control critical genes that are closely related to tumorigenesis. Understanding their synergistic effects will not only help us further disclose mechanism of tumorigenesis caused by p53 inactivation and Ras activation, but also facilitate personalized treatments and pharmacological target selection for cancer therapy. Therefore, we reviewed the recent progress of synergistic effect be-tween p53 and Ras and its role in tumorigenesis.

Mutant p53R273H attenuates the expression of phase 2 detoxifying enzymes and promotes the survival of cells with high levels of reactive oxygen species

Uncontrolled accumulation of reactive oxygen species (ROS) causes oxidative stress and induces harmful effects. Both high ROS levels and p53 mutations are frequent in human cancer. Mutant p53 forms are known to actively promote malignant growth. However, no mechanistic details are known about the contribution of mutant p53 to excessive ROS accumulation in cancer cells. Herein, we examine the effect of p53(R273H), a commonly occurring mutated p53 form, on the expression of phase 2 ROS-detoxifying enzymes and on the ability of cells to readopt a reducing environment after exposure to oxidative stress. Our data suggest that p53(R273H) mutant interferes with the normal response of human cells to oxidative stress. We show here that, upon oxidative stress, mutant p53(R273H) attenuates the activation and function of NF-E2-related factor 2 (NRF2), a transcription factor that induces the antioxidant response. This effect of mutant p53 is manifested by decreased expression of phase 2 detoxifying enzymes NQO1 and HO-1 and high ROS levels. These findings were observed in several human cancer cell lines, highlighting the general nature of this phenomenon. The failure of p53(R273H) mutant-expressing cells to restore a reducing oxidative environment was accompanied by increased survival, a known consequence of mutant p53 expression. These activities are attributable to mutant p53(R273H) gain of function and might underlie its well-documented oncogenic nature in human cancer.

Mutant p53 gain of function is interwoven into the hallmarks of cancer

Cancer is viewed as being governed by several aberrant biological events defined by Weinberg and Hanahan as 'hallmarks'. In most human cancers the tumour suppressor p53 is mutated, leading to its malfunction and to the acquirement of oncogenic activities, termed 'gain of function'. This commentary links mutant p53 activities to the hallmarks of cancer, describing its involvement in resistance to apoptosis, genomic instability, aberrant cell cycle, invasion and metastasis, tumour microenvironment, and inflammation. Recent work published in The Journal of Pathology by Acin and colleagues, summarized here, reveals an interesting mechanism by which mutant p53 accelerates mitosis entry. Collectively, the growing body of evidence relating mutant p53 and the hallmarks of cancer reinforces the notion that targeting mutant p53 pathways might be beneficial for anti-cancer therapy.

Activating transcription factor-3 (ATF3) functions as a tumor suppressor in colon cancer and is up-regulated upon heat-shock protein 90 (Hsp90) inhibition

Background

Activating transcription factor-3 (ATF3) is involved in the complex process of cellular stress response. However, its exact role in cancer is discussed controversially because both tumor suppressive and oncogenic effects have been described. Here we followed-up on our previous observation that inhibition of Hsp90 may increase ATF3 expression and sought to determine the role of ATF3 in colon cancer.

Methods

Regulation of ATF3 was determined in cancer cells using signaling inhibitors and a heat-shock protein-90 (Hsp90) antagonist. Human HCT116 cancer cells were stably transfected with an ATF3-shRNA or a luciferase-shRNA expression plasmid and alterations in cell motility were assessed in migration assays. The impact of ATF3 down-regulation on cancer growth and metastasis were investigated in a subcutaneous tumor model, a model of hepatic tumor growth and in a model of peritoneal carcinomatosis. Human colon cancer tissues were analyzed for ATF3 expression.

Results

The results show that therapeutic Hsp90 inhibition substantially up-regulates the expression of ATF3 in various cancer cells, including colon, gastric and pancreatic cancer. This effect was evident both in vitro and in vivo. RNAi mediated knock-down of ATF3 in HCT116 colon cancer cells significantly increased cancer cell migration in vitro. Moreover, in xenogenic mouse models, ATF3 knock-down promoted subcutaneous tumor growth and hepatic metastasis, as well as peritoneal carcinomatosis. Importantly, ATF3 expression was lower in human colon cancer specimens, as compared to corresponding normal surrounding tissues, suggesting that ATF3 may represent a down-regulated tumor suppressor in colon cancer.

Conclusion

In conclusion, ATF3 down-regulation in colon cancer promotes tumor growth and metastasis. Considering that blocking Hsp90 induces ATF3 expression, Hsp90 inhibition may represent a valid strategy to treat metastatic colon cancer by up-regulating this anti-metastatic transcription factor.

Cisplatin induces cytotoxicity through the mitogen-activated protein kinase pathways and activating transcription factor 3

The mechanisms underlying the proapoptotic effect of the chemotherapeutic agent, cisplatin, are largely undefined. Understanding the mechanisms regulating cisplatin cytotoxicity may uncover strategies to enhance the efficacy of this important therapeutic agent. This study evaluates the role of activating transcription factor 3 (ATF3) as a mediator of cisplatin-induced cytotoxicity. Cytotoxic doses of cisplatin and carboplatin treatments consistently induced ATF3 expression in five tumor-derived cell lines. Characterization of this induction revealed a p53, BRCA1, and integrated stress response-independent mechanism, all previously implicated in stress-mediated ATF3 induction. Analysis of mitogen-activated protein kinase (MAPK) pathway involvement in ATF3 induction by cisplatin revealed a MAPK-dependent mechanism. Cisplatin treatment combined with specific inhibitors to each MAPK pathway (c-Jun N-terminal kinase, extracellular signal-regulated kinase, and p38) resulted in decreased ATF3 induction at the protein level. MAPK pathway inhibition led to decreased ATF3 messenger RNA expression and reduced cytotoxic effects of cisplatin as measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell viability assay. In A549 lung carcinoma cells, targeting ATF3 with specific small hairpin RNA also attenuated the cytotoxic effects of cisplatin. Similarly, ATF3-/- murine embryonic fibroblasts (MEFs) were shown to be less sensitive to cisplatin-induced cytotoxicity compared with ATF3+/+ MEFs. This study identifies cisplatin as a MAPK pathway-dependent inducer of ATF3, whose expression influences cisplatin's cytotoxic effects.

Mutant p53 gain-of-function in cancer

In its wild-type form, p53 is a major tumor suppressor whose function is critical for protection against cancer. Many human tumors carry missense mutations in the TP53 gene, encoding p53. Typically, the affected tumor cells accumulate excessive amounts of the mutant p53 protein. Various lines of evidence indicate that, in addition to abrogating the tumor suppressor functions of wild-type p53, the common types of cancer-associated p53 mutations also endow the mutant protein with new activities that can contribute actively to various stages of tumor progression and to increased resistance to anticancer treatments. Collectively, these activities are referred to as mutant p53 gain-of-function. This article addresses the biological manifestations of mutant p53 gain-of-function, the underlying molecular mechanisms, and their possible clinical implications.

Activating Transcription Factor 3 regulates in part the enhanced tumour cell cytotoxicity of the histone deacetylase inhibitor M344 and cisplatin in combination

Background

Activating Transcription Factor (ATF) 3 is a key regulator of the cellular integrated stress response whose expression has also been correlated with pro-apoptotic activities in tumour cell models. Combination treatments with chemotherapeutic drugs, such as cisplatin, and histone deacetylase (HDAC) inhibitors have been demonstrated to enhance tumour cell cytotoxicity. We recently demonstrated a role for ATF3 in regulating cisplatin-induced apoptosis and others have shown that HDAC inhibition can also induce cellular stress. In this study, we evaluated the role of ATF3 in regulating the co-operative cytotoxicity of cisplatin in combination with an HDAC inhibitor.

Results

The HDAC inhibitor M344 induced ATF3 expression at the protein and mRNA level in a panel of human derived cancer cell lines as determined by Western blot and quantitative RT-PCR analyses. Combination treatment with M344 and cisplatin lead to increased induction of ATF3 compared with cisplatin alone. Utilizing the MTT cell viability assay, M344 treatments also enhanced the cytotoxic effects of cisplatin in these cancer cell lines. The mechanism of ATF3 induction by M344 was found to be independent of MAPKinase pathways and dependent on ATF4, a known regulator of ATF3 expression. ATF4 heterozygote (+/-) and knock out (-/-) mouse embryonic fibroblast (MEF) as well as chromatin immunoprecipitation (ChIP) assays were utilized in determining the mechanistic induction of ATF3 by M344. We also demonstrated that ATF3 regulates the enhanced cytotoxicity of M344 in combination with cisplatin as evidenced by attenuation of cytotoxicity in shRNAs targeting ATF3 expressing cells.

Conclusion

This study identifies the pro-apoptotic factor, ATF3 as a novel target of M344, as well as a mediator of the co-operative effects of cisplatin and M344 induced tumour cell cytotoxicity.

When mutants gain new powers: news from the mutant p53 field

Ample data indicate that mutant p53 proteins not only lose their tumour suppressive functions, but also gain new abilities that promote tumorigenesis. Moreover, recent studies have modified our view of mutant p53 proteins, portraying them not as inert mutants, but rather as regulated proteins that influence the cancer cell transcriptome and phenotype. This influence is clinically manifested as association of TP53 mutations with poor prognosis and drug resistance in a growing array of malignancies. Here, we review recent studies on mutant p53 regulation, gain-of-function mechanisms, transcriptional effects and prognostic association, with a focus on the clinical implications of these findings.

Differential effects of novel tumour-derived p53 mutations on the transformation of NIH-3T3 cells

The p53 tumour suppressor gene is frequently mutated in human tumours and different tumour-derived mutations have varying effects on cells. The effect of a novel tumour-derived p53 mutation and two recently described mutations from South African breast cancer patients on the growth rate, colony formation, cell cycle arrest after irradiation and response to chemotherapeutic drugs was investigated. None of the p53 mutations had any significant effect on the inherent growth rate of the cells; however, contact inhibition of growth in two of the mutants was lost. These same two mutants formed colonies in soft agar, whereas the third mutant did not. All three of the mutants failed to show a G(1) cell cycle arrest after exposure to 7 Gy of [(60)Co] radiation, albeit to different degrees. Cells expressing the p53 mutants were either more sensitive to cisplatin and melphalan or more resistant than the untransfected cells, depending on the mutation. However, there was no difference in response to daunorubicin treatment. These results demonstrate that different p53 mutations exert varying biological effects on normal cells, with some altering checkpoint activation more effectively than others. The data also suggest that the nature of the p53 mutation influences the sensitivity to cytostatic drugs.

The transcriptional factor Snail simultaneously triggers cell cycle arrest and migration of human hepatoma HepG2

Snail was recently highlighted as a critical transcriptional factor for tumor metastasis. Real time RT/PCR and Western blot analysis demonstrated that Snail mRNA and protein, respectively, were induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) in hepatoma cell HepG2. Blockade of gene expression of Snail by antisense oligodeoxynucleotide and/or siRNA technique can prevent not only the TPA-triggered EMT/cell migration and growth inhibition of HepG2 but also TPA-induced down-regulation of E-cadherin and up-regulation of p15(INK4b). Moreover, the TPA-triggered promoter activation of p15(INK4b) was also prevented. On the other hand, two of the HepG2 clone over-expressing Snail, namely S7 and S15, had a scattered fibroblastic morphology and acquired higher motility than parental HepG2. Also, the proportion of G0/G1 phase of S7 and S15 was higher than that of parental HepG2, consistent with the longer doubling time of both cells. Semiquantitative RT/PCR analysis demonstrated a greatly elevated gene expression of Snail accompanied with decreased E-cadherin and increased p15(INK4b) in both Snail-overexpressing cells. On the transcriptional level, p15(INK4b) promoter activity was 2.6-fold higher in S7 as compared with parental HepG2. Furthermore, electrophoretic mobility of DNA fragments encompassing proximal p15(INK4b) promoter can be retarded by incubation of nuclear extract of S7. Our results demonstrated that Snail play diverse trans-regulatory roles in HepG2. Notably, we suggested that Snail may upregulate p15(INK4b) gene expression by directly activating its promoter.

Mutant p53 attenuates the SMAD-dependent transforming growth factor beta1 (TGF-beta1) signaling pathway by repressing the expression of TGF-beta receptor type II

Both transforming growth factor beta (TGF-beta) and p53 have been shown to control normal cell growth. Acquired mutations either in the TGF-beta signaling pathway or in the p53 protein were shown to induce malignant transformation. Recently, cross talk between wild-type p53 and the TGF-beta pathway was observed. The notion that mutant p53 interferes with the wild-type p53-induced pathway and acts by a "gain-of-function" mechanism prompted us to investigate the effect of mutant p53 on the TGF-beta-induced pathway. In this study, we show that cells expressing mutant p53 lost their sensitivity to TGF-beta1, as observed by less cell migration and a reduction in wound healing. We found that mutant p53 attenuates TGF-beta1 signaling. This was exhibited by a reduction in SMAD2/3 phosphorylation and an inhibition of both the formation of SMAD2/SMAD4 complexes and the translocation of SMAD4 to the cell nucleus. Furthermore, we found that mutant p53 attenuates the TGF-beta1-induced transcription activity of SMAD2/3 proteins. In searching for the mechanism that underlies this attenuation, we found that mutant p53 reduces the expression of TGF-beta receptor type II. These data provide important insights into the molecular mechanisms that underlie mutant p53 "gain of function" pertaining to the TGF-beta signaling pathway.

Transcription regulation by mutant p53

In addition to the loss of wild-type p53 activity, a high percentage of tumor cells accumulate mutant p53 protein isoforms. Whereas the hallmark of the wild-type p53 is its tumor suppressor activities, tumor-associated mutant p53 proteins acquire novel functions enabling them to promote a large spectrum of cancer phenotypes. During the last years, it became clear that tumor-associated mutant p53 proteins are not only distinct from the wild-type p53, but they also represent a heterogeneous population of proteins with a variety of structure-function features. One of the major mechanisms underlying mutant p53 gain of function is the ability to regulate gene expression. Although a large number of specific target genes were identified, the molecular basis for this regulation is not fully elucidated. This review describes the present knowledge about the transcriptional activities of mutant p53 and the mechanisms that might underlie its target gene specificity.

Subcellular localization of Daxx determines its opposing functions in ischemic cell death

This study examined the role of Daxx in ischemic stress. Upon ischemic stress, nuclear export of Daxx to the cytoplasm was observed in primary myocytes as well as in various cell lines. Daxx silencing using siRNAs was detrimental in tethering PML-nuclear body (PML-NB) constituents together. Overexpression of Daxx (W621A) caused nuclear export of p53 independently of PML and promoted ischemic cell death via activation of JNK. Conversely, overexpression of Daxx (S667A) prevented dissociation of PML-NB constituents and protected cells from ischemic death. Collectively, our results demonstrate that the subcellular localization of Daxx determines its role in ischemic cell death.