p53 chromatin epigenetic domain organization and p53 transcription

Epigenetic organization represents an important regulation mechanism of gene expression. In this work, we show that the mouse p53 gene is organized into two epigenetic domains. The first domain is fully unmethylated, associated with histone modifications in active genes, and organized in a nucleosome-free conformation that is deficient in H2a/H2b, whereas the second domain is fully methylated, associated with deacetylated histones, and organized in a nucleosomal structure. In mitotic cells, RNA polymerase is depleted in domain II, which is folded into a higher-order structure and is associated with H1 histone, whereas domain I conformation is preserved. Similar results were obtained for cells treated with inhibitors of associated regulatory factors. These results suggest that depletion of RNA polymerase II is the result of a physical barrier due to the folding of chromatin in domain II. The novel chromatin structure in the first domain during mitosis also suggests a mechanism for marking active genes in successive cell cycles.     

Family Feud in Chemosensitvity: p73 and Mutant p53

The importance of p53 in chemotherapy-induced apoptosis of cancer cells is well established. p53 plays a critical role in the cellular response to DNA damage by regulating genes involved in cell cycle progression, apoptosis, and genomic stability. As a result, p53 tumor status is a critical determinant of both responses to anti-cancer treatment and clinical prognosis. Interestingly, tumors expressing certain mutant forms of p53 (“gain of function”) are particularly resistant to chemotherapy, even when compared to cells that lack any detectable p53. Until recently, the explanation for this enhanced chemoresistance was not clear. Recent studies have shown that the p53 homologues, p73 and p63, are also activated by chemotherapies, leading to tumor cell death. Now the discovery that mutant p53 interacts with p73, and that regulation of this interaction by a p53 polymorphism can modulate chemosensitvity provide a new model for how p53-family interactions can influence the response of tumors to anti-cancer therapies. Since p53 mutations are found in more than 50% of human tumors, strategies aimed at manipulating these interactions may prove useful in enhancing the chemotherapy response, and perhaps, overcoming chemoresistance.     

Involvement of the p53 and p73 transcription factors in neuroAIDS

HIV-associated dementia (HAD) is the most common AIDS-associated neurological disorder and is characterized by the development of synaptodendritic injury to neurons. To advance HAD therapy, it is crucial to identify the mechanisms and factors involved. The viral protein HIV-1 Tat is among those factors and is released by HIV-1-infected cells and can be taken up by adjacent neuronal cells leading to neurotoxic effects. Multiple cellular host proteins have been identified as Tat cofactors in causing neuronal injury. Interestingly, most of these factors function through activation of the p53 pathway. We have now examined the ability of Tat to activate the p53 pathway leading to the induction of endogenous p53 and p73 in neuronal cells. We found that Tat induced p53 and p73 levels in SH-SY5Y cells and that this induction caused retraction of neurites. In the absence of either p53 or p73, Tat failed to induce dendritic retraction or to activate the proapoptotic proteins, such as Bax. Further, we found that p53-accumulation in Tat-treated cells depends on the presence of p73. Therefore, we conclude that Tat contributes to neuronal degeneration through activation of a pathway involving p53 and p73. This information will be valuable for the development of therapeutic agents that affect these pathways to protect CNS neurons and prevent HAD.     

p53, p63 and p73--solos, alliances and feuds among family members

p53 controls crucial stress responses that play a major role in preventing malignant transformation. Hence, inactivation of p53 is the single most common genetic defect in human cancer. With the recent discovery of two close structural homologs, p63 en p73, we are getting a broader view of a fascinating gene family that links developmental biology with tumor biology. While unique roles are apparent for each of these genes, intimate biochemical cross-talk among family members suggests a functional network that might influence many different aspects of individual gene action. The most interesting part of this family network derives from the fact that the p63 and p73 genes are based on the "two-genes-in-one" idea, encoding both agonist and antagonist in the same open reading frame. In this review, we attempt to present an overview of the current status of this fast moving field.     

Ing1 mediates p53 accumulation and chromatin modification in response to oncogenic stress

ING proteins are putative tumor suppressor proteins linked to the p53 pathway and to the chromatin modification machinery. Here we have analyzed the role of the products of the murine Ing1 locus in cellular tumor-protective responses, using mouse primary fibroblasts where the Ing1 locus has been inactivated by the integration of a betageo cassette. We show that Ing1-deficient mouse embryonic fibroblasts display a defective senescence-like antiproliferative response against oncogenic Ras, affecting several senescence-specific markers. This phenotype is accompanied by a reduced accumulation of p53, which can be explained by the reduced basal p53 protein stability in the Ing1-deficient background. Ing1 deficiency also results in defects in the appearance of heterochromatic marks upon expression of oncogenic Ras, suggestive of impaired heterochromatin formation during oncogene-induced senescence. Our results support an important role for the Ing1 locus in protection against oncogenic stress in vivo, both as a mediator of p53 activation and as a regulator of chromatin remodeling processes.     

Novel therapeutic interventions for p53-altered tumors through manipulation of its family members,p63 and p73

TP53 is highly mutated in human cancers, thus targeting this tumor suppressor pathway is highly desirable and will impact many cancer patients.^1,2 Iwakuma T, Lozano G, Flores ER. Li-Fraumeni syndrome: a p53 family affair. Cell Cycle 2005; 4:865-7; PMID:15917654; http://dx.doi.org/10.4161/cc.4.7.1800 Muller PA, Vousden KH. p53 mutations in cancer. Nat Cell Biol 2013; 15:2-8; PMID:23263379; http://dx.doi.org/10.1038/ncb2641  Therapeutic strategies to reactivate the p53-pathway have been challenging,^3,4 Martins CP, Brown-Swigart L, Evan GI. Modeling the therapeutic efficacy of p53 restoration in tumors. Cell 2006; 127:1323-34; PMID:17182091; http://dx.doi.org/10.1016/j.cell.2006.12.007 Ventura A. Restoration of p53 function leads to tumour regression in vivo. Nature 2007; 445:661-5; PMID:17251932; http://dx.doi.org/10.1038/nature05541  and no effective treatment exists.⁵ Khoo KH, Verma CS, Lane DP. Drugging the p53 pathway: understanding the route to clinical efficacy. Nat Rev Drug Discov 2014; 13:217-36; PMID:24577402; http://dx.doi.org/10.1038/nrd4288[Crossref], [PubMed], [Web of Science ®] , [Google Scholar] We utilized the p53-family members, p63 and p73, which are not frequently mutated in cancer, to treat p53-defective cancers. The N-terminal splice variants of p63 and p73 are denoted as the TA and ΔN isoforms. We recently demonstrated that deletion of either ΔNp63 or ΔNp73 in p53-deficient mouse tumors results in tumor regression mediated by metabolic programming. Using this strategy, we identified pramlintide, a synthetic analog of amylin, as an effective treatment for p53 deficient and mutant tumors. Here, we show the utility of using pramlintide, as a potential cancer preventive option for p53-deficient tumors in mouse models. Additionally, we found that in vivo inhibition of both ΔNp63 and ΔNp73 in combination accelerates tumor regression and increases survival of p53-deficient mice. We report that inhibition of both ΔNp63 and ΔNp73 in combination results in upregulation of 3 key metabolic regulators, IAPP, GLS2, and TIGAR resulting in an increase in apoptosis and tumor regression in ΔNp63/ΔNp73/p53 deficient thymic lymphomas. These data highlight the value of generating inhibitors that will simultaneously target ΔNp63 and ΔNp73 to treat cancer patients with alterations in p53.     

Interaction of regulators Mdm2 and Mdmx with transcription factors p53, p63 and p73

The negative regulation of p53, a major human tumor suppressor, by Mdm2 and Mdmx is crucial for the survival of a cell, whereas its aberrant function is a common feature of cancer. Both Mdm proteins act through the spatial occlusion of the p53 transactivation (TA) domain and by the ubiquitination of p53, resulting in its degradation. Two p53 homologues, p63 and p73, have been described in humans. Unlike p53, these proteins regulate developmental processes rather than genome stability. Both p63 and p73 contain TA domains homologous to that of p53, but relatively little is known about their regulation by Mdm2 or Mdmx. Here, we present a detailed characterization of the interaction of Mdm2 and Mdmx with the TA domains of p63 and p73. Earlier reports of Mdm2 and Mdmx interactions with p73 are substantiated by the detailed quantitative characterization reported in this study. Most importantly, earlier contradictions concerning the presumed interaction of the Mdm proteins with p63 are convincingly resolved and for the first time, the affinities of these interactions are determined. Finally, the contribution of these findings to our understanding of the physiological role of these interactions is discussed.