p53 efficiently suppresses tumor development in the complete absence of its cell-cycle inhibitory and proapoptotic effectors p21, Puma, and Noxa. Cell Rep. 2013;3:1339-1345. [PMID: 23665218 DOI: 10.1016/j.celrep.2013.04.012]

The p53 Target Gene SIVA Enables Non-Small Cell Lung Cancer Development

Although p53 transcriptional activation potential is critical for its ability to suppress cancer, the specific target genes involved in tumor suppression remain unclear. SIVA is a p53 target gene essential for p53-dependent apoptosis, although it can also promote proliferation through inhibition of p53 in some settings. Thus, the role of SIVA in tumorigenesis remains unclear. Here, we seek to define the contribution of SIVA to tumorigenesis by generating Siva conditional knockout mice. Surprisingly, we find that SIVA loss inhibits non-small cell lung cancer (NSCLC) development, suggesting that SIVA facilitates tumorigenesis. Similarly, SIVA knockdown in mouse and human NSCLC cell lines decreases proliferation and transformation. Consistent with this protumorigenic role for SIVA, high-level SIVA expression correlates with reduced NSCLC patient survival. SIVA acts independently of p53 and, instead, stimulates mTOR signaling and metabolism in NSCLC cells. Thus, SIVA enables tumorigenesis in a p53-independent manner, revealing a potential new cancer therapy target.

SIGNIFICANCE

These findings collectively reveal a novel role for the p53 target gene SIVA both in regulating metabolism and in enabling tumorigenesis, independently of p53. Importantly, these studies further identify SIVA as a new prognostic marker and as a potential target for NSCLC cancer therapy.

Ferroptosis as a p53-mediated activity during tumour suppression

Although p53-mediated cell-cycle arrest, senescence and apoptosis serve as critical barriers to cancer development, emerging evidence suggests that the metabolic activities of p53 are also important. Here we show that p53 inhibits cystine uptake and sensitizes cells to ferroptosis, a non-apoptotic form of cell death, by repressing expression of SLC7A11, a key component of the cystine/glutamate antiporter. Notably, p53(3KR), an acetylation-defective mutant that fails to induce cell-cycle arrest, senescence and apoptosis, fully retains the ability to regulate SLC7A11 expression and induce ferroptosis upon reactive oxygen species (ROS)-induced stress. Analysis of mutant mice shows that these non-canonical p53 activities contribute to embryonic development and the lethality associated with loss of Mdm2. Moreover, SLC7A11 is highly expressed in human tumours, and its overexpression inhibits ROS-induced ferroptosis and abrogates p53(3KR)-mediated tumour growth suppression in xenograft models. Our findings uncover a new mode of tumour suppression based on p53 regulation of cystine metabolism, ROS responses and ferroptosis.

Radioresistance in head and neck squamous cell carcinoma: Biological bases and therapeutic implications

Head and neck squamous cell carcinoma (HNSCC) is strongly associated with alcohol and tobacco consumption. Lately, the incidence of human papillomavirus (HPV)-related tumors has shown a significant increase, and HPV-related tumors show distinctive features if compared with the HPV-negative counterpart. Locally advanced HNSCC can be treated with concomitant chemoradiotherapy, but early recurrences sometimes occur. Relapses are often related to an intrinsic radioresistance of the tumors. Alterations in intracellular pathways, primarily involved in cell proliferation, apoptosis, and DNA repair, can lead to radioresistance. Preclinical and clinical evidence highlighted that 3 main pathways, including the epidermal growth factor receptor (EGFR), the phosphotidylinositol-3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR), and the p53 signaling cascades, play a crucial role in radioresistance development. A future approach may consist in the association of radiotherapy (RT) and selective inhibition of the key pathways involved in radioresistance. Phase I, II, and III clinical trials are currently testing these novel treatment strategies.

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.

TP53 mutations in human cancer: database reassessment and prospects for the next decade

More than 50% of human tumors carry TP53 gene mutations and in consequence more than 45,000 somatic and germline mutations have been gathered in the UMD TP53 database (http://p53.fr). Analyses of these mutations have been invaluable for bettering our knowledge on the structure-function relationships within the TP53 protein and the high degree of heterogeneity of the various TP53 mutants in human cancer. In this review, we discuss how with the release of the sequences of thousands of tumor genomes issued from high-throughput sequencing, the description of novel TP53 mutants is now reaching a plateau indicating that we are close to the full set of mutants that target the elusive tumor-suppressive activity of this protein. We performed an extensive and thorough analysis of the TP53 mutation database, focusing particularly on specific sets of mutations that were overlooked in the past because of their low frequencies, for example, synonymous mutations, splice mutations, or mutations-targeting residues subject to posttranslational modifications. We also discuss the evolution of the statistical methods used to differentiate TP53 passenger mutations and artifactual data from true mutations, a process vital to the release of an accurate TP53 mutation database that will in turn be an invaluable tool for both clinicians and researchers.

Antitumor mechanisms when pRb and p53 are genetically inactivated

pRb and p53 are the two major tumor suppressors. Their inactivation is frequent when cancers develop and their reactivation is rationale of most cancer therapeutics. When pRb and p53 are genetically inactivated, cells irreparably lose the antitumor mechanisms afforded by them. Cancer genome studies document recurrent genetic inactivation of RB1 and TP53, and the inactivation becomes more frequent in more advanced cancers. These findings may explain why more advanced cancers are more likely to resist current therapies. Finding successful treatments for more advanced and multi-therapy resistant cancers will depend on finding antitumor mechanisms that remain effective when pRb and p53 are genetically inactivated. Here, we review studies that have begun to make progress in this direction.

MCL-1 but not BCL-XL is critical for the development and sustained expansion of thymic lymphoma in p53-deficient mice

Apoptosis plays a role in normal lymphopoiesis and lymphoid malignancies. Pro-survival MCL-1 is essential for survival of T-cell progenitors, BCL-XL for immature thymocytes, and BCL-2 for mature T cells. Conversely, little is known about the regulators that are required for the survival of T-cell lymphomas. We used constitutive and conditionally gene-targeted mice to investigate which pro-survival BCL-2 family member is required for the sustained survival of thymic lymphomas initiated by loss of p53. Constitutive loss of a single Mcl-1 allele delayed tumor onset. In contrast, lymphomas emerging in p53(-/-) mice in which Mcl-1 could be conditionally deleted had been selected for retention of MCL-1 expression. In contrast, complete loss of BCL-XL had no impact on lymphoma development in p53(-/-) mice. These results demonstrate that thymic lymphomas elicited by loss of p53 must arise from cancer-initiating cells that require MCL-1 for their survival. Acute deletion of both Mcl-1 alleles abrogated the expansion of p53(-/-) lymphomas in mice, whereas inducible loss of BCL-XL had little impact. This reveals that MCL-1 is essential for the sustained survival of these malignant cells and suggests that targeting MCL-1 may be an attractive strategy for the treatment of T-cell lymphoma.

Integrated high-throughput analysis identifies Sp1 as a crucial determinant of p53-mediated apoptosis

The restoration of p53 tumor suppressor function is a promising therapeutic strategy to combat cancer. However, the biological outcomes of p53 activation, ranging from the promotion of growth arrest to the induction of cell death, are hard to predict, which limits the clinical application of p53-based therapies. In the present study, we performed an integrated analysis of genome-wide short hairpin RNA screen and gene expression data and uncovered a previously unrecognized role of Sp1 as a central modulator of the transcriptional response induced by p53 that leads to robust induction of apoptosis. Sp1 is indispensable for the pro-apoptotic transcriptional repression by p53, but not for the induction of pro-apoptotic genes. Furthermore, the p53-dependent pro-apoptotic transcriptional repression required the co-binding of Sp1 to p53 target genes. Our results also highlight that Sp1 shares with p53 a common regulator, MDM2, which targets Sp1 for proteasomal degradation. This uncovers a new mechanism of the tight control of apoptosis in cells. Our study advances the understanding of the molecular basis of p53-mediated apoptosis and implicates Sp1 as one of its key modulators. We found that small molecules reactivating p53 can differentially modulate Sp1, thus providing insights into how to manipulate p53 response in a controlled way.

To be, or not to be: functional dilemma of p53 metabolic regulation

PURPOSE OF REVIEW

In recent years, the emerging role of p53 in metabolic regulation has been a topic of great interest. Although apoptotic and growth arrest functions of p53 remain as important mechanisms for preserving genomic stability, metabolic functions of p53 show increasing potential in contributing to p53-mediated tumor suppression. Numerous recent studies provided further insights into the metabolic functions of p53 and their implications in tumorigenesis.

RECENT FINDINGS

Several novel p53 metabolic targets have been identified that participate in various aspects of metabolism. Although some studies demonstrate the potential tumor suppressive function of p53 metabolic genes, others reveal prosurvival roles of those targets in both tumor and normal cells. Specifically, Tp53-induced glycolysis and apoptosis regulator (TIGAR) has been thought to promote tumor suppression through metabolic fine-tuning, yet, TIGAR-deficient mice display reduction in tumorigenesis. Finally, characterization of the 3KR mouse model underscored the significance of p53 metabolic regulation in tumor suppression, while also alluding to the potential mechanism for selective regulation of p53 metabolic targets.

SUMMARY

Expression of many p53 metabolic genes elicits both antitumor and tumorigenic effects, suggesting that p53 may contribute to cellular protection as well as tumor suppression. Future studies must carefully dissect the duality of p53 metabolic function, which may potentially prove useful in designing cancer therapies.

Identification of genomic locus responsible for experimentally induced testicular teratoma 1 (ett1) on mouse Chr 18

Spontaneous testicular teratomas (STTs) composed by various kinds of tissues are derived from primordial germ cells (PGCs) in the fetal testes of the mouse. In contrast, intra-testicular grafts of the mouse strain (129/Sv-Ter (+/+)) fetal testes possessed the ability to develop the experimental testicular teratomas (ETTs), indistinguishable from the STTs at a morphological level. In this study, linkage analysis was performed for exploration of possible candidate genes involving in ETT development using F2 intercross fetuses derived from [LTXBJ × 129/Sv-Ter (+/+)] F1 hybrids. Linkage analysis with selected simple sequence length polymorphisms along chromosomes 18 and 19, which have been expected to contain ETT-susceptibility loci, demonstrated that a novel recessive candidate gene responsible for ETT development is located in 1.1 Mb region between the SSLP markers D18Mit81 and D18Mit184 on chromosome 18 in the 129/Sv-Ter (+/+) genetic background. Since this locus is different from the previously known loci (including Ter, pgct1, and Tgct1) for STT development, we named this novel gene "experimental testicular teratoma 1 (ett1)". To resolve the location of ett1 independently from other susceptibility loci, ett1 loci was introduced in a congenic strain in which the distal segment of chromosome 18 in LTXBJ strain mice had been replaced by a 1.99 Mbp genomic segment of the 129/Sv-Ter (+/+) mice. Congenic males homozygous for the ett1 loci were confirmed to have the ability to form ETTs, indicating that this locus contain the gene responsible for ETTs. We listed candidate genes included in this region, and discussed about their possible involvement in induction of ETTs.

P53/microRNA-34-induced metabolic regulation: new opportunities in anticancer therapy

MicroRNA-34 (miR-34) is directly regulated by p53, and its potential tumor suppressive roles have been studied extensively. As a p53-induced microRNA, miR-34 functions as a tumor suppressor by playing a role in cell cycle arrest, apoptosis and metabolic regulation. Among these p53/miR-34 associated processes, apoptosis and cell cycle arrest are known as essential for p53/miR-34-mediated tumor suppression. P53-mediated metabolic processes have been shown to play pivotal roles in cancer cell biology. Recent studies have also identified several miR-34 targets involved in p53/miR-34-induced metabolic regulation. However, correlations among these metabolic targets remain to be fully elucidated. In this review, we summarize the current progress in the field of metabolic regulation by the p53/miR-34 axis and propose future directions for the development of metabolic approaches in anticancer therapy.

Restoring the tumour suppressive function of p53 as a parallel strategy in melanoma therapy

The tumour suppressor p53 is a master sensor of stress and it controls the expression of hundreds to thousands of genes with diverse biological functions including cell cycle arrest, apoptosis, and senescence. Consequently p53 is the most mutated gene found in human cancer and p53 mutation rate varies from 5% to 95%. Importantly p53 activity is often inactivated in tumours expressing structurally wild type p53. Thus one of the major challenges in cancer research is to restore the tumour suppressive function of p53. Intensive studies in the past decade have demonstrated that in addition to mutation, p53 activities are largely regulated by cellular factors that control the expression level and/or transcriptional activities of p53. MDM2, MDM4, p14(ARF) and the ASPP family of proteins are among the most studied regulators of p53. With increased understanding of the complexity of p53 regulation, various p53 reactivating approaches are being developed. This review will focus on the recent understanding of p53 inactivation in melanoma and the approaches to reactivate p53 in preclinical studies. Recent success in the therapeutic targeting of the BRAFV600E oncogenic protein was accompanied with subsequent relapse caused by acquired drug resistance. Restoration of the tumour suppressive function of p53 presents a parallel cancer therapeutic opportunity alongside BRAFV600E inhibition. Thus targeted therapy and concurrent reactivation of p53 may be a fertile ground to achieve synergistic killing of the 50% of cancer cells that express structurally wild type p53.

Unravelling mechanisms of p53-mediated tumour suppression

p53 is a crucial tumour suppressor that responds to diverse stress signals by orchestrating specific cellular responses, including transient cell cycle arrest, cellular senescence and apoptosis, which are all processes associated with tumour suppression. However, recent studies have challenged the relative importance of these canonical cellular responses for p53-mediated tumour suppression and have highlighted roles for p53 in modulating other cellular processes, including metabolism, stem cell maintenance, invasion and metastasis, as well as communication within the tumour microenvironment. In this Opinion article, we discuss the roles of classical p53 functions, as well as emerging p53-regulated processes, in tumour suppression.

Drugging the p53 pathway: understanding the route to clinical efficacy

The tumour suppressor p53 is the most frequently mutated gene in human cancer, with more than half of all human tumours carrying mutations in this particular gene. Intense efforts to develop drugs that could activate or restore the p53 pathway have now reached clinical trials. The first clinical results with inhibitors of MDM2, a negative regulator of p53, have shown efficacy but hint at on-target toxicities. Here, we describe the current state of the development of p53 pathway modulators and new pathway targets that have emerged. The challenge of targeting protein-protein interactions and a fragile mutant transcription factor has stimulated many exciting new approaches to drug discovery.

Wild type p53 reactivation: from lab bench to clinic

The p53 tumor suppressor is the most frequently inactivated gene in cancer. Several mouse models have demonstrated that the reconstitution of the p53 function suppresses the growth of established tumors. These facts, taken together, promote the idea of p53 reactivation as a strategy to combat cancer. This review will focus on recent advances in the development of small molecules which restore the function of wild type p53 by blocking its inhibitors Mdm2 and MdmX or their upstream regulators and discuss the impact of different p53 functions for tumor prevention and tumor eradication. Finally, the recent progress in p53 research will be analyzed concerning the role of p53 cofactors and cellular environment in the biological response upon p53 reactivation and how this can be applied in clinic.

p53 activity is selectively licensed in the Drosophila stem cell compartment

Oncogenic stress provokes tumor suppression by p53 but the extent to which this regulatory axis is conserved remains unknown. Using a biosensor to visualize p53 action, we find that Drosophila p53 is selectively active in gonadal stem cells after exposure to stressors that destabilize the genome. Similar p53 activity occurred in hyperplastic growths that were triggered either by the Ras^V12 oncoprotein or by failed differentiation programs. In a model of transient sterility, p53 was required for the recovery of fertility after stress, and entry into the cell cycle was delayed in p53^- stem cells. Together, these observations establish that the stem cell compartment of the Drosophila germline is selectively licensed for stress-induced activation of the p53 regulatory network. Furthermore, the findings uncover ancestral links between p53 and aberrant proliferation that are independent of DNA breaks and predate evolution of the ARF/Mdm2 axis.

DOI:http://dx.doi.org/10.7554/eLife.01530.001

The most common genetic change seen in cancer patients produces a faulty version of the p53 protein, which normally restricts tissue growth. This change promotes cancer because cells can now divide faster and fail to die when they should. Much remains to be learned about how p53 functions to restrain growth. As p53 is found in primitive organisms, and cancer is unlikely to have significantly influenced evolution, suppressing tumor formation was almost certainly not the original function of this gene. Furthermore, p53 works in a different way compared to many other tumour suppressors. Therefore, prevention of cancer is likely to have evolved as a side effect derived from more ancient functions.

Recently, a link between p53 and stem cells has been uncovered. Stem cells are special because they can develop into many different types of cells, and they are crucial for the growth and repair of tissues. To form a particular type of cell, the stem cell divides to create two daughter cells. Commonly, one daughter cell stays in the stem state, whereas the other becomes a particular type of cell, such as a nerve cell or muscle cell. Because of this special property, scientists hypothesize that stem cells have special mechanisms to protect them from DNA damage that might partially depend on p53. This would prevent the spread of damaged genomes that would otherwise occur among daughter cells.

To learn more about how p53 influences stem cells, Wylie, Lu et al. monitored its activity in the gonads of fruit flies, which are a powerful genetic model. They found that damaging DNA activates p53 in stem cells and their daughter cells, but not in other types of cells that have been damaged. In addition, p53 is activated by the uncontrolled growth and division of stem cells in the gonad, even when DNA is not damaged. This is unexpected since molecules linking inappropriate growth to p53 were thought to be present only in mammals. Therefore, it appears that the tumor-suppressing behavior of p53 in mammals was adapted from its more ancient ability to regulate stem cell growth—an ability that evolved before organisms divided into vertebrates and invertebrates.

DOI:http://dx.doi.org/10.7554/eLife.01530.002

Mutant TP53 posttranslational modifications: challenges and opportunities

The wild-type (WT) human p53 (TP53) tumor suppressor can be posttranslationally modified at over 60 of its 393 residues. These modifications contribute to changes in TP53 stability and in its activity as a transcription factor in response to a wide variety of intrinsic and extrinsic stresses in part through regulation of protein-protein and protein-DNA interactions. The TP53 gene frequently is mutated in cancers, and in contrast to most other tumor suppressors, the mutations are mostly missense often resulting in the accumulation of mutant (MUT) protein, which may have novel or altered functions. Most MUT TP53s can be posttranslationally modified at the same residues as in WT TP53. Strikingly, however, codons for modified residues are rarely mutated in human tumors, suggesting that TP53 modifications are not essential for tumor suppression activity. Nevertheless, these modifications might alter MUT TP53 activity and contribute to a gain-of-function leading to increased metastasis and tumor progression. Furthermore, many of the signal transduction pathways that result in TP53 modifications are altered or disrupted in cancers. Understanding the signaling pathways that result in TP53 modification and the functions of these modifications in both WT TP53 and its many MUT forms may contribute to more effective cancer therapies.

Targeting of MCL-1 kills MYC-driven mouse and human lymphomas even when they bear mutations in p53

The transcriptional regulator c-MYC is abnormally overexpressed in many human cancers. Evasion from apoptosis is critical for cancer development, particularly c-MYC-driven cancers. We explored which anti-apoptotic BCL-2 family member (expressed under endogenous regulation) is essential to sustain c-MYC-driven lymphoma growth to reveal which should be targeted for cancer therapy. Remarkably, inducible Cre-mediated deletion of even a single Mcl-1 allele substantially impaired the growth of c-MYC-driven mouse lymphomas. Mutations in p53 could diminish but not obviate the dependency of c-MYC-driven mouse lymphomas on MCL-1. Importantly, targeting of MCL-1 killed c-MYC-driven human Burkitt lymphoma cells, even those bearing mutations in p53. Given that loss of one allele of Mcl-1 is well tolerated in healthy tissues, our results suggest that therapeutic targeting of MCL-1 would be an attractive therapeutic strategy for MYC-driven cancers.

Understanding the non-canonical pathways involved in p53-mediated tumor suppression

In the last three decades since the discovery of p53, it has become increasingly apparent that p53 plays a very important role in tumor suppression. Previously, it was thought that the tumor suppressive functions lied solely in the canonical p53-mediated apoptosis, cell cycle arrest and senescence. However, more recent research has shown that anti-oncogenic activity of p53 can still occur in the absence of these downstream functions. These results suggest that more non-canonical roles of p53 may have a much larger impact on other p53-regulated programs then initially anticipated. Recently, the non-canonical activities of p53 such as cell metabolism, autophagy and necrosis have been the subject of intense study. p53 affects many aspects of cellular metabolism including catabolism, anabolism and reactive oxygen species levels. p53 has a dual role in autophagy regulation. Initiation of autophagy occurs through direct transcription of pro-autophagy genes and inhibition transpires through a transcription-independent mechanism. The role of p53 in these cellular processes is quite complex and evidence suggests that p53 can play both a pro- and anti-oncogenic role in these non-conical pathways. Despite of more than 60,000 publications on p53 in the literature, the mechanisms for p53-mediated tumor suppression apparently needs to be further elucidated.

Tumor suppressor p53 and its mutants in cancer metabolism

Tumor-suppressor p53 plays a key role in tumor prevention. As a transcription factor, p53 transcriptionally regulates its target genes to initiate different biological processes in response to stress, including apoptosis, cell cycle arrest or senescence, to exert its function in tumor suppression. Recent studies have revealed that metabolic regulation is a novel function of p53. Metabolic changes have been regarded as a hallmark of tumors and a key contributor to tumor development. p53 regulates many different aspects of metabolism, including glycolysis, mitochondrial oxidative phosphorylation, pentose phosphate pathway, fatty acid synthesis and oxidation, to maintain the homeostasis of cellular metabolism, which contributes to the role of p53 in tumor suppression. p53 is frequently mutated in human tumors. In addition to loss of tumor suppressive function, tumor-associated mutant p53 proteins often gain new tumorigenic activities, termed gain-of-function of mutant p53. Recent studies have shown that mutant p53 mediates metabolic changes in tumors as a novel gain-of-function to promote tumor development. Here we review the functions and mechanisms of wild-type and mutant p53 in metabolic regulation, and discuss their potential roles in tumorigenesis.

Tumor suppression by p53: is apoptosis important or not?

The mechanisms by which p53 suppresses tumor growth remain ill defined. In this issue of Cell Reports, Timofeev et al. (2013) and Valente et al. (2013) reveal context-dependent contributions of p53-dependent apoptosis to its tumor-suppressive function.

How do I kill thee? Let me count the ways: p53 regulates PARP-1 dependent necrosis

Understanding the impact of the p53 tumor suppressor pathway on the regulation of genome integrity, cancer development, and cancer treatment has intrigued scientists and clinicians for decades. It appears that the p53 pathway is a central node for nearly all cell stress responses, including: gene expression, DNA repair, cell cycle arrest, metabolic adjustments, apoptosis, and senescence. In the past decade, it has become increasingly clear that p53 function is directly regulated by poly(ADP-ribose) polymerase-1 (PARP-1), a nuclear enzyme involved in DNA repair signaling. Here, we will discuss the impact of PARP-1 on p53 function, along with a recently described novel role for the reciprocal regulation of p53 regulated, PARP-1 dependent necrosis following DNA damage.