Dissection of transcriptional and non-transcriptional p53 activities in the response to genotoxic stress

Regulation and pathological role of p53 in cisplatin nephrotoxicity

Cisplatin is one of the most potent chemotherapy drugs widely used for cancer treatment. However, its use is limited by side effects in normal tissues, particularly the kidneys. Recent studies, using both in vitro and in vivo experimental models, have suggested a critical role for p53 in cisplatin nephrotoxicity. The signaling pathways upstream and downstream of p53 are being investigated and related to renal cell injury and death. Along with the mechanistic studies, renoprotective approaches targeting p53 have been suggested. Further research may integrate p53 signaling with other nephrotoxic signaling pathways, providing a comprehensive understanding of cisplatin nephrotoxicity and leading to the development of effective renoprotective strategies during cancer therapy.

A high-content chemical screen identifies ellipticine as a modulator of p53 nuclear localization

p53 regulates apoptosis and the cell cycle through actions in the nucleus and cytoplasm. Altering the subcellular localization of p53 can alter its biological function. Therefore, small molecules that change the localization of p53 would be useful chemical probes to understand the influence of subcellular localization on the function of p53. To identify such molecules, a high-content screen for compounds that increased the localization of p53 to the nucleus or cytoplasm was developed, automated, and conducted. With this image-based assay, we identified ellipticine that increased the nuclear localization of GFP-mutant p53 protein but not GFP alone in Saos-2 osteosarcoma cells. In addition, ellipticine increased the nuclear localization of endogenous p53 in HCT116 colon cancer cells with a resultant increase in the transactivation of the p21 promoter. Increased nuclear p53 after ellipticine treatment was not associated with an increase in DNA double stranded breaks, indicating that ellipticine shifts p53 to the nucleus through a mechanism independent of DNA damage. Thus, a chemical biology approach has identified a molecule that shifts the localization of p53 and enhances its nuclear activity.

Oligomerization of BAK by p53 utilizes conserved residues of the p53 DNA binding domain

Genotoxic stress triggers a rapid translocation of p53 to the mitochondria, contributing to apoptosis in a transcription-independent manner. Using immunopurification protocols and mass spectrometry, we previously identified the proapoptotic protein BAK as a mitochondrial p53-binding protein and showed that recombinant p53 directly binds to BAK and can induce its oligomerization, leading to cytochrome c release. In this work we describe a combination of molecular modeling, electrostatic analysis, and site-directed mutagenesis to define contact residues between BAK and p53. Our data indicate that three regions within the core DNA binding domain of p53 make contact with BAK; these are the conserved H2 alpha-helix and the L1 and L3 loop. Notably, point mutations in these regions markedly impair the ability of p53 to oligomerize BAK and to induce transcription-independent cell death. We present a model whereby positively charged residues within the H2 helix and L1 loop of p53 interact with an electronegative domain on the N-terminal alpha-helix of BAK; the latter is known to undergo conformational changes upon BAK activation. We show that mutation of acidic residues in the N-terminal helix impair the ability of BAK to bind to p53. Interestingly, many of the p53 contact residues predicted by our model are also direct DNA contact residues, suggesting that p53 interacts with BAK in a manner analogous to DNA. The combined data point to the H2 helix and L1 and L3 loops of p53 as novel functional domains contributing to transcription-independent apoptosis by this tumor suppressor protein.

Cell death in normal and abnormal development

Research over the past 50 years has consistently documented that cell death is an integral part of both normal development and the etiology of birth defects; however, the significance of this cell death has been, until recently, unclear. Research published during the past 15 years has now shown that programmed cell death (PCD) and teratogen-induced cell death are genetically controlled processes (apoptosis) that play important roles in both normal and abnormal development. Therefore, the purpose of this review is to highlight what is known about PCD and teratogen-induced cell death and their relationships to the mechanisms of apoptosis and abnormal development.

Hepatic IGFBP1 is a prosurvival factor that binds to BAK, protects the liver from apoptosis, and antagonizes the proapoptotic actions of p53 at mitochondria

Liver is generally refractory to apoptosis induced by the p53 tumor suppressor protein, but the molecular basis remains poorly understood. Here we show that p53 transcriptional activation leads to enhanced expression of hepatic IGFBP1 (insulin-like growth factor-binding protein-1). Exhibiting a previously unanticipated role, a portion of intracellular IGFBP1 protein localizes to mitochondria where it binds to the proapoptotic protein BAK and hinders BAK activation and apoptosis induction. Interestingly, in many cells and tissues p53 also has a direct apoptotic function at mitochondria that includes BAK binding and activation. When IGFBP1 is in a complex with BAK, formation of a proapoptotic p53/BAK complex and apoptosis induction are impaired, both in cultured cells and in liver. In contrast, livers of IGFBP1-deficient mice exhibit spontaneous apoptosis that is accompanied by p53 mitochondrial accumulation and evidence of BAK oligomerization. These data support the importance of BAK as a mediator of p53's mitochondrial function. The results also identify IGFBP1 as a negative regulator of the BAK-dependent pathway of apoptosis, whose expression integrates the transcriptional and mitochondrial functions of the p53 tumor suppressor protein.

Gadd45 beta is a pro-survival factor associated with stress-resistant tumors

Tumors that acquire resistance against death stimuli constitute a severe problem in the context of cancer therapy. To determine genetic alterations that favor the development of stress-resistant tumors in vivo, we took advantage of polyclonal tumors generated after retroviral infection of newborn Elambda-MYC mice, in which the retroviral integration acts as a mutagen to enhance tumor progression. Tumor cells were cultivated ex vivo and exposed to gamma-irradiation prior to their transplantation into syngenic recipients, thereby providing a strong selective pressure for pro-survival mutations. Secondary tumors developing from stress-resistant tumor stem cells were analysed for retroviral integration sites to reveal candidate genes whose dysregulation confer survival. In addition to the gene encoding the antiapoptotic Bcl-x(L) protein, we identified the gadd45b locus to be a novel common integration site in these tumors, leading to enhanced expression. In accord with a thus far undocumented role of Gadd45beta in tumorigenesis, we showed that NIH3T3 cells overexpressing Gadd45beta form tumors in NOD/SCID mice. Interestingly and differently to other known 'classical' antiapoptotic factors, high Gadd45beta levels did not protect against MYC-, UV- or gamma-irradiation-induced apoptosis, but conferred a strong and specific survival advantage to serum withdrawal.

Teratogen-Induced Activation of p53 in Early Postimplantation Mouse Embryos

Hyperthermia (HS) and 4-hydroperoxycyclophosphamide (4CP) activate the mitochondrial apoptotic pathway in day 9 mouse embryos. Previous microarray analyses Microarray analyses revealed that several p53 target genes are upregulated after exposure to HS or 4CP, suggesting a role for p53 in teratogen-induced apoptosis. To explore the role of p53, we assessed the activation of p53 in day 9 mouse embryos exposed to HS or 4CP in vitro. Both teratogens induced the accumulation of p53 and phosphorylation of p53 at ser-15, two hallmarks of p53 activation. HS and 4CP also induced an increase in Noxa and Puma mRNAs, transcripts of two known proapoptotic p53 target genes; however, these two teratogens did not induce significant increases in NOXA and PUMA proteins, suggesting that p53 does not activate the mitochondrial apoptotic pathway by transcriptionally upregulating the expression of NOXA and PUMA proteins. HS and 4CP also induced the expression of p21 mRNA and protein, suggesting a role for p53 in teratogen-induced cell cycle arrest. Previously, we also showed that HS and 4CP activate the apoptotic pathway in the embryo proper (head and trunk) but not in the heart. We now show that HS and 4CP induce a robust activation of p53 in the embryo proper but an attenuated induction in the heart. HS and 4CP induce the expression of p21 protein in majority of the cells in the embryo; however, expression of NOXA and PUMA proteins were not significantly induced in heads, hearts, or trunks of day 9 embryos. Overall, our results suggest that p53 may play a transcription-dependent role in teratogen-induced cell cycle arrest but a transcription-independent role in teratogen-induced apoptosis in day 9 mouse embryos exposed to HS or 4CP.

Mdm2 inhibitor Nutlin-3a induces p53-mediated apoptosis by transcription-dependent and transcription-independent mechanisms and may overcome Atm-mediated resistance to fludarabine in chronic lymphocytic leukemia

Although TP53 mutations are rare in B-cell chronic lymphocytic leukemia (CLL), Mdm2 overexpression has been reported as an alternative cause of p53 dysfunction. We investigated the potential therapeutic use of nongenotoxic p53 activation by a small-molecule antagonist of Mdm2, Nutlin-3a, in CLL. Nutlin-3a induced significant apoptosis in 30 (91%) of 33 samples from previously untreated patients with CLL; all resistant samples had TP53 mutations. Low levels of Atm (ataxia telangiectasia mutated) or high levels of Mdm2 (murine double minute 2) did not prevent Nutlin-3a from inducing apoptosis. Nutlin-3a used transcription-dependent and transcription-independent pathways to induce p53-mediated apoptosis. Predominant activation of the transcription-independent pathway induced more pronounced apoptosis than that of the transcription-dependent pathway, suggesting that activation of the transcription-independent pathway is sufficient to initiate p53-mediated apoptosis in CLL. Combination treatment of Nutlin-3a and fludarabine synergistically increased p53 levels, and induced conformational change of Bax and apoptosis in wild-type p53 cells but not in cells with mutant p53. The synergistic apoptotic effect was maintained in samples with low Atm that were fludarabine resistant. Results suggest that the nongenotoxic activation of p53 by targeting the Mdm2-p53 interaction provides a novel therapeutic strategy for CLL.

Transcription-independent pro-apoptotic functions of p53

Induction of apoptosis is one of the central activities by which p53 exerts its tumor-suppressing function. Aside from its primary function as a transcription factor, it can promote apoptosis independent of transcription. Recent studies have started to define the mechanisms of non-transcriptional pro-apoptotic p53 activities operating within the intrinsic mitochondria-mediated pathway of apoptosis. So far, two different mechanisms have been described, each of which was assigned to a specific localization of the p53 protein, either in the cytosol or directly at the mitochondria. Although mechanistically different, both transcription-independent modes of apoptosis induction converge, as they both initiate permeabilization of the outer mitochondrial membrane via activation of the pro-apoptotic Bcl-2 family members Bax or Bak.