Cytoplasmic functions of the tumour suppressor p53

Massively regulated genes: the example of TP53

Intensive study of the TP53 gene over the last three decades has revealed a highly complex network of factors that regulate its performance. The gene has several promoters, alternative splicing occurs and there are alternative translation initiation sites. Up to 10 p53 isoforms have been identified. At the post-translational level, p53 activity depends on its quantity in the cell and on qualitative changes in its structure, intracellular localization, DNA-binding activity and interactions with other proteins. Both accumulation and activation are regulated by an intricate pattern of post-translational modifications, including phosphorylation, acetylation, ubiquitination, sumoylation, neddylation, methylation and glycosylation. The Mdm2 protein, a negative regulator of p53, is the most important determinant of p53 abundance and subcellular localization. Enzymes that post-translationally modify p53 by phosphorylation, methylation and acetylation fine-tune p53 binding to recognition sequences in DNA and p53 interactions with transcription cofactors at promoters of target genes, thereby exerting a discriminatory role in p53 function. This multitude of parameters determining expression, modification, accumulation and localization of p53 proteins may explain how a single gene can display an extensive repertoire of activities. Presumably this is needed, because the p53 protein can have such profound consequences for a cell.

Crystal structure of the p53 core domain bound to a full consensus site as a self-assembled tetramer

Recent studies suggest that p53 binds predominantly to consensus sites composed of two decameric half-sites with zero spacing in vivo. Here we report the crystal structure of the p53 core domain bound to a full consensus site as a tetramer at 2.13A resolution. Comparison with previously reported structures of p53 dimer:DNA complexes and a chemically trapped p53 tetramer:DNA complex reveals that DNA binding by the p53 core domain is a cooperative self-assembling process accompanied by structural changes of the p53 dimer and DNA. Each p53 monomer interacts with its two neighboring subunits through two different protein-protein interfaces. The DNA is largely B-form and shows no discernible bend, but the central base-pairs between the two half-sites display a significant slide. The extensive protein-protein and protein-DNA interactions explain the high cooperativity and kinetic stability of p53 binding to contiguous decameric sites and the conservation of such binding-site configuration in vivo.

Drosophila foxo acts in males to cause sexual-dimorphism in tissue-specific p53 life span effects

Sex-specific selective pressures are hypothesized to lead to sexually antagonistic gene functions that contribute to phenotypes such as aging and cancer. However, relatively little is known about the identity of such genes and possible mechanisms. Here we report that nervous system-specific over-expression of wild-type p53 in Drosophila caused decreased life span in males and increased life span in females. In contrast, tissue-general over-expression produced the opposite pattern: increased life span in males and decreased life span in females. In a foxo null background, p53 life span effects in males were reversed, becoming similar to the effects in females. In contrast, a Sir2 null background tended to reduce the magnitude of p53 effects. The data demonstrate that wild-type p53 over-expression can regulate life span independent of foxo, and suggest that foxo acts in males to produce sexually antagonistic life span effects of p53.

Transgelin induces apoptosis of human prostate LNCaP cells through its interaction with p53

The androgen receptor (AR) and its coregulators have important roles in the carcinogenesis of prostate cancer. p53 is an important tumour suppressor gene, and the absence of a fundamental p53 response may predispose to cancer. Transgelin, known as an ARA54-associated AR inhibitor, can suppress AR function in LNCaP cells. In addition to these effects, we aimed to elucidate the proapoptotic effects of the protein on LNCaP and its underlying mechanisms, especially the interaction between transgelin and p53. Cell counting, flow cytometric analysis and terminal deoxynucleotidyl transferase-dUTP nick-end labelling assays were applied to measure the proapoptotic effect of transgelin. Using western blotting of p53 and double immunofluorescence staining of p53 with transgelin, we show that transfection of transgelin results in increasing cytoplasmic translocation of p53 and upregulation of p53 expression. We also found an interaction between transgelin and p53 in vivo by mammalian two-hybrid and coimmunoprecipitation assays. The activation of the mitochondria-associated apoptosis pathway was observed in LNCaP cells after transfection with transgelin. These results are indicative of p53-mediated mitochondria-associated apoptotic effects of transgelin on LNCaP cells in addition to its known suppressive effects on the AR pathway.

Molecules that modulate Apaf-1 activity

Programmed cell death, apoptosis, is a highly regulated cellular pathway, responsible for the elimination of cells in the organism that are no longer needed or extensively damaged. Defects in the regulation of apoptosis could be at the molecular basis of different diseases, either when it is insufficient or excessive. The formation of the macromolecular complex, apoptosome, is a key event in this pathway, which has also been defined as the intrinsic apoptosis pathway. The apoptosome is a holoenzyme multiprotein complex formed by cytochrome c-activated apoptotic protease-activating factor (Apaf-1), dATP, and procaspase-9. Recent studies have produced a wealth of information about the regulation and functions of Apaf-1, but additional studies aimed at elucidating its role as a signaling device at the crosstalk between different signaling pathways are needed to take advantage for the development of modulators of apoptosis pathways and possible therapeutic applications.

In vivo analysis of p53 tumor suppressor function using genetically engineered mouse models

p53 is a crucial tumor suppressor, as evidenced by the high propensity for p53 mutation during human cancer development. Already more than a decade ago, p53 knockout mice confirmed that p53 is critical for preventing tumorigenesis. More recently, a host of p53 knock-in mouse strains has been generated, with the aim of either more precisely modeling p53 mutations in human cancer or better understanding p53's regulation and downstream activities. In the first category, several mouse strains expressing mutant p53 proteins corresponding to human-tumor-derived mutants have demonstrated that mutant p53 is not equivalent to loss of p53 but additionally exhibits gain-of-function properties, promoting invasive and metastatic phenotypes. The second class of p53 knock-in mouse models expressing engineered p53 mutants has also provided new insight into p53 function. For example, mice expressing p53 mutants lacking specific posttranslational modification sites have revealed that these modifications serve to modulate p53 responses in vivo in a cell-type- and stress-specific manner rather than being absolutely required for p53 stabilization and activation as suggested by in vitro experiments. Additionally, studies of p53 mouse models have established that both p53-driven cell-cycle arrest and apoptosis responses contribute to tumor suppression and that activation of p53 by oncogenic stress imposes an important barrier to tumorigenesis. Finally, the use of mouse strains expressing temporally regulatable p53 has demonstrated that p53 loss is not only required for tumor development but also required for tumor maintenance, suggesting that p53 restoration in human cancer patients may be a promising therapeutic strategy. These sophisticated p53 mouse models have taught us important lessons, and new mouse models will certainly continue to reveal interesting and perhaps surprising aspects of p53's complex biology.

Linking ER Stress to Autophagy: Potential Implications for Cancer Therapy

Different physiological and pathological conditions can perturb protein folding in the endoplasmic reticulum, leading to a condition known as ER stress. ER stress activates a complex intracellular signal transduction pathway, called unfolded protein response (UPR). The UPR is tailored essentially to reestablish ER homeostasis also through adaptive mechanisms involving the stimulation of autophagy. However, when persistent, ER stress can switch the cytoprotective functions of UPR and autophagy into cell death promoting mechanisms. Recently, a variety of anticancer therapies have been linked to the induction of ER stress in cancer cells, suggesting that strategies devised to stimulate its prodeath function or block its prosurvival function, could be envisaged to improve their tumoricidial action. A better understanding of the molecular mechanisms that determine the final outcome of UPR and autophagy activation by chemotherapeutic agents, will offer new opportunities to improve existing cancer therapies as well as unravel novel targets for cancer treatment.

Autophagy regulation by p53

Autophagy is an evolutionarily conserved catabolic pathway that is involved in numerous physiological processes and in multiple pathological conditions including cancer. Autophagy is regulated by an intricate network of signaling cascades that have not yet been entirely disentangled. Accumulating evidence indicates that p53, the best-characterized human tumor suppressor protein, can modulate autophagy in a dual fashion, depending on its subcellular localization. On the one hand, p53 functions as a nuclear transcription factor and transactivates proapoptotic, cell cycle-arresting and proautophagic genes. On the other hand, cytoplasmic p53 can operate at mitochondria to promote cell death and can repress autophagy via poorly characterized mechanisms. This review focuses on the recently discovered function of p53 as a master regulator of autophagy.

Inflammatory cells regulate p53 and caspases in acute pancreatitis

The inflammatory response during pancreatitis regulates necrotic and apoptotic rates of parenchymal cells. Neutrophil depletion by use of anti-polymorphonuclear serum (anti-PMN) increases apoptosis in experimental pancreatitis but the mechanism has not been determined. Our study was designed to investigate signaling mechanisms in pancreatic parenchymal cells regulating death responses with neutrophil depletion. Rats were neutrophil depleted with anti-PMN treatment. Then cerulein pancreatitis was induced, followed by measurements of apoptosis signaling pathways. There was greater activation of executioner caspases-3 in the pancreas with anti-PMN treatment compared with control. There were no differences between these groups of animals in mitochondrial cytochrome c release or in activities of initiator caspase-8 and -9. However, there was greater activation of caspase-2 with anti-PMN treatment during cerulein pancreatitis. The upstream regulation of caspases-2 includes p53, which was increased; the p53 negative regulator, Mdm2, was decreased by anti-PMN treatment during cerulein pancreatitis. In vitro experiments using isolated pancreatic acinar cells a pharmacological inhibitor of Mdm2 increased caspase-2/-3 activities, and an inhibitor of p53 decreased these activities during cholecystokinin-8 treatment. Furthermore, experiments using the AR42J cell line Mdm2 small interfering RNA (siRNA) increased caspase-2/-3 activities, and p53 siRNA decreased these activities during cholecystokinin-8 treatment. These results suggest that during acute pancreatitis the inflammatory response inhibits apoptosis. The mechanism of this inhibition involves caspase-2 and its upstream regulation by p53 and Mdm2. Because previous findings indicate that promotion of apoptosis decreases necrosis and severity of pancreatitis, these results suggest that strategies to inhibit Mdm2 or activate p53 will have beneficial effects for treatment of pancreatitis.

Autophagy mediates pharmacological lifespan extension by spermidine and resveratrol

Although autophagy has widely been conceived as a self-destructive mechanism that causes cell death, accumulating evidence suggests that autophagy usually mediates cytoprotection, thereby avoiding the apoptotic or necrotic demise of stressed cells. Recent evidence produced by our groups demonstrates that autophagy is also involved in pharmacological manipulations that increase longevity. Exogenous supply of the polyamine spermidine can prolong the lifespan of (while inducing autophagy in) yeast, nematodes and flies. Similarly, resveratrol can trigger autophagy in cells from different organisms, extend lifespan in nematodes, and ameliorate the fitness of human cells undergoing metabolic stress. These beneficial effects are lost when essential autophagy modulators are genetically or pharmacologically inactivated, indicating that autophagy is required for the cytoprotective and/or anti-aging effects of spermidine and resveratrol. Genetic and functional studies indicate that spermidine inhibits histone acetylases, while resveratrol activates the histone deacetylase Sirtuin 1 to confer cytoprotection/longevity. Although it remains elusive whether the same histones (or perhaps other nuclear or cytoplasmic proteins) act as the downstream targets of spermidine and resveratrol, these results point to an essential role of protein hypoacetylation in autophagy control and in the regulation of longevity.

Regulation of p53--insights into a complex process

The p53 protein is one of the most important tumor suppressor proteins. Normally, the p53 protein is in a latent state. However, when its activity is required, e.g. upon DNA damage, nucleotide depletion or hypoxia, p53 becomes rapidly activated and initiates transcription of pro-apoptotic and cell cycle arrest-inducing target genes. The activity of p53 is regulated both by protein abundance and by post-translational modifications of pre-existing p53 molecules. In the 30 years of p53 research, a plethora of modifications and interaction partners that modulate p53's abundance and activity have been identified and new ones are continuously discovered. This review will summarize our current knowledge on the regulation of p53 abundance and activity.

AID produces DNA double-strand breaks in non-Ig genes and mature B cell lymphomas with reciprocal chromosome translocations

Cancer-initiating translocations such as those associated with lymphomas require the formation of paired DNA double-strand breaks (DSBs). Activation-induced cytidine deaminase (AID) produces widespread somatic mutation in mature B cells; however, the extent of "off-target" DSB formation and its role in translocation-associated malignancy is unknown. Here, we show that deregulated expression of AID causes widespread genome instability, which alone is insufficient to induce B cell lymphoma; transformation requires concomitant loss of the tumor suppressor p53. Mature B cell lymphomas arising as a result of deregulated AID expression are phenotypically diverse and harbor clonal reciprocal translocations involving a group of Immunoglobulin (Ig) and non-Ig genes that are direct targets of AID. This group includes miR-142, a previously unknown micro-RNA target that is translocated in human B cell malignancy. We conclude that AID produces DSBs throughout the genome, which can lead to lymphoma-associated chromosome translocations in mature B cells.

Mutant p53 reactivation by PRIMA-1MET induces multiple signaling pathways converging on apoptosis

The low molecular weight compound PRIMA-1(MET) reactivates mutant p53 and triggers mutant p53-dependent apoptosis in human tumor cells. We investigated the effect of PRIMA-1(MET) on global gene expression using microarray analysis of Saos-2 cells expressing His273 mutant p53 and parental p53 null Saos-2 cells. PRIMA-1(MET) affected transcription of a significantly larger number of genes in the mutant p53-expressing cells compared to the p53 null cells. Genes affected by PRIMA-1(MET) in a mutant p53-dependent manner include the cell-cycle regulators GADD45B and 14-3-3gamma and the pro-apoptotic Noxa. Several of the affected genes are known p53 target genes and/or contain p53 DNA-binding motifs. We also found mutant p53-dependent disruption of the cytoskeleton, as well as transcriptional activation of the XBP1 gene and cleavage of its mRNA, a marker for endoplasmic reticulum stress. Our data show that PRIMA-1(MET) induces apoptosis through multiple transcription-dependent and -independent pathways. Such integral engagement of multiple pathways leading to apoptosis is consistent with restoration of wild-type properties to mutant p53 and is likely to reduce the risk of drug resistance development in clinical applications of PRIMA-1(MET).

Autophagy genes as tumor suppressors

Autophagy, originally described as a universal lysosome-dependent bulk degradation of cytoplasmic components upon nutrient deprivation, has since been shown to influence diverse aspects of homeostasis and is implicated in a wide variety of pathological conditions, including cancer. The list of autophagy-related (Atg) genes associated with the initiation and progression of human cancer as well as with responses to cancer therapy continues to grow as these genes are being discovered. However, whether Atg genes work through their expected mechanisms of autophagy regulation and/or through as-yet-undefined functions in the development of cancer remains to be further clarified. Here we review recent advances in the knowledge of the molecular basis of autophagy genes and their biological outputs during tumor development. A better understanding of the mechanistic link between cellular autophagy and tumor growth control may ultimately better human cancer treatments.

Using mice to examine p53 functions in cancer, aging, and longevity

The p53 tumor suppressor is a multifaceted transcription factor that responds to a diverse array of stresses that include DNA damage and aberrant oncogene signaling. On activation, p53 prevents the emergence of cancer cells by initiating cell cycle arrest, senescence (terminal cell cycle arrest), or apoptosis. Although its role in assuring longevity by suppressing cancer is well established, recent studies obtained largely from genetically engineered mouse models suggest that p53 may regulate longevity and aging. In some contexts, it appears that altered p53 activity may enhance longevity, and in others, it appears to suppress longevity and accelerate aging phenotypes. Here, we discuss how genetically engineered mouse models have been used to explore antiproliferative functions of p53 in cancer suppression and how mouse models with altered aging phenotypes have shed light on how p53 might influence the aging process.

Transcription-independent p53 apoptosis: an alternative route to death

Apoptosis induced by p53 is firmly established as a central mechanism of tumour suppression. In addition to its complex functions as a nuclear transcription factor, p53 can act in the cytosol and mitochondria to promote apoptosis through transcription-independent mechanisms. Recent studies have shown that physical and functional interactions of p53 with various members of the Bcl-2 family provide the basis for this alternative route of p53-mediated cell death. However, different models of how these interactions promote apoptosis have been proposed. This review focuses on the mechanisms, regulation and physiological roles of transcription-independent p53 activities and highlights recent findings suggesting that the utilisation of these activities provides a promising alternative strategy for p53-based cancer therapy.

p53 and the regulation of hepatocyte apoptosis: implications for disease pathogenesis

The interplay between p53 and apoptosis in diseases such as cancer, neurodegeneration, ischemia and atherosclerosis underscores the need to understand the complexity of p53 networks. Here, we highlight recent studies of p53-induced apoptosis in human diseases, with a focus on the modulation of liver cell apoptosis. In addition, recent work has provided new insights into mechanisms underlying the antiapoptotic functions of the endogenous bile acid ursodeoxycholic acid (UDCA), suggesting that the finely tuned, complex control of p53 by Mdm2 is a key step in the UDCA modulation of deregulated, p53-triggered apoptosis. The effect of targeting cell death signaling proteins has been established in preclinical models of human diseases. Finally, we review recent therapeutic strategies and clinical applications of targeted agents, with a particular emphasis on the potential use of UDCA.

The first 30 years of p53: growing ever more complex

Thirty years ago p53 was discovered as a cellular partner of simian virus 40 large T-antigen, the oncoprotein of this tumour virus. The first decade of p53 research saw the cloning of p53 DNA and the realization that p53 is not an oncogene but a tumour suppressor that is very frequently mutated in human cancer. In the second decade of research, the function of p53 was uncovered: it is a transcription factor induced by stress, which can promote cell cycle arrest, apoptosis and senescence. In the third decade after its discovery new functions of this protein were revealed, including the regulation of metabolic pathways and cytokines that are required for embryo implantation. The fourth decade of research may see new p53-based drugs to treat cancer. What is next is anybody's guess.