p53's mitochondrial translocation and MOMP action is independent of Puma and Bax and severely disrupts mitochondrial membrane integrity

p53's apoptotic program consists of transcription-dependent and transcription-independent pathways. In the latter, physical interactions between mitochondrial p53 and anti- and pro-apoptotic members of the Bcl2 family of mitochondrial permeability regulators are central. Using isogenic cell systems with defined deficiencies, we characterize in detail how mitochondrial p53 contributes to mitochondrial permeabilization, to what extent its action depends on other key Bcl2 family members and define its release activity. We show that mitochondrial p53 is highly efficient in inducing the release of soluble and insoluble apoptogenic factors by severely disrupting outer and inner mitochondrial membrane integrity. This action is associated with wild-type p53-induced oligomerization of Bax, Bak and VDAC and the formation of a stress-induced endogenous complex between p53 and cyclophilin D, normally located at the inner membrane. Tumor-derived p53 mutants are deficient in activating the Bax/Bak lipid pore. These actions are independent of Puma and Bax. Importantly, the latter distinguishes the mitochondrial from the cytosolic p53 death pathway.

Monoubiquitylation promotes mitochondrial p53 translocation

A major function of the p53 tumor suppressor is the induction of a pleiotropic apoptotic program in response to stress through transcription-dependent and -independent mechanisms. In particular, this includes a direct apoptotic role of p53 at the mitochondria. Stress-induced p53 translocation to the mitochondria with subsequent outer membrane permeabilization is a common early component in p53-mediated apoptosis in normal and transformed cells. However, the mechanism of p53 delivery to the mitochondria remains unknown. Here, we show that the cytoplasm contains a separate and distinct p53 pool that is the major source for p53 translocation to the mitochondria upon its stress-induced stabilization. Using various manipulations that enhance or diminish p53 ubiquitylation, our data provide evidence that Mdm2-mediated monoubiquitylation of p53 greatly promotes its mitochondrial translocation and thus its direct mitochondrial apoptosis. On the other hand, p53 does not require Mdm2 as a shuttler. Upon arrival at the mitochondria, our data suggest that p53 undergoes rapid deubiquitylation by mitochondrial HAUSP via a stress-induced mitochondrial p53-HAUSP complex. This generates the apoptotically active non-ubiquitylated p53. Taken together, we propose a novel model for mitochondrial p53 targeting, whereby a distinct cytoplasmic pool of stabilized monoubiquitylated p53, generated in resting cells by basal levels of Mdm2-type ligases, is subject to a binary switch from a fate of inactivation via subsequent polyubiquitylation and degradation in unstressed cells, to a fate of activation via mitochondrial trafficking.

WT p53, but not tumor-derived mutants, bind to Bcl2 via the DNA binding domain and induce mitochondrial permeabilization

The induction of apoptosis by p53 in response to cellular stress is its most conserved function and crucial for p53 tumor suppression. We recently reported that p53 directly induces oligomerization of the BH1,2,3 effector protein Bak, leading to outer mitochondrial membrane permeabilization (OMMP) with release of apoptotic activator proteins. One important mechanism by which p53 achieves OMMP is by forming an inhibitory complex with the anti-apoptotic BclXL protein. In contrast, the p53 complex with the Bcl2 homolog has not been interrogated. Here we have undertaken a detailed characterization of the p53-Bcl2 interaction using structural, biophysical, and mutational analyses. We have identified the p53 DNA binding domain as the binding interface for Bcl2 using solution NMR. The affinity of the p53-Bcl2 complex was determined by surface plasmon resonance analysis (BIAcore) to have a dominant component KD 535 +/- 24 nm. Moreover, in contrast to wild type p53, endogenous missense mutants of p53 are unable to form complexes with endogenous Bcl2 in human cancer cells. Functionally, these mutants are all completely or strongly compromised in mediating OMMP, as measured by cytochrome c release from isolated mitochondria. These data implicate p53-Bcl2 complexes in contributing to the direct mitochondrial p53 pathway of apoptosis and further support the notion that the DNA binding domain of p53 is a dual function domain, mediating both its transactivation function and its direct mitochondrial apoptotic function.

Stress-induced p53 runs a transcription-independent death program

The transcription-independent p53-mediated apoptotic response has obtained a solid mechanistic basis in recent years. A fraction of stress-induced wild type p53 protein rapidly translocates to mitochondria in response to genotoxic, hypoxic, and oxidative stresses in established cell lines and primary cells, as well as in physiological and pathophysiologic stress responses in the animal. While the groundwork of mechanisms and kinetics of direct mitochondrial p53 activities is laid out, the quantitative contribution of this pathway to total p53-mediated apoptosis and tumor suppression in vivo remains to be elucidated. An update on these efforts is given here.

Stress-induced p53 runs a direct mitochondrial death program: its role in physiologic and pathophysiologic stress responses in vivo

It is now well established that a fraction of stress-induced wtp53 protein rapidly translocates to mitochondria in immortalized and transformed cells in culture. Mitochondrial p53 interacts with anti-apoptotic proteins of the Bcl 2 family at the outer mitochondrial membrane, resulting in membrane permeabilization, release of death effectors such as cytochrome C and subsequent rapid apoptosis. The significance and relevance of this direct mitochondrial p53 program to the overall p53-mediated stress response in vivo is underlined by a number of recent studies in animals and primary cells. They all support a role for this direct pathway in the physiologic and pathophysiologic response to genotoxic and hypoxic insults and occur precisely in those tissues where p53 plays a critical role in mediating apotpotis rather than cell cycle arrest.

In vivo mitochondrial p53 translocation triggers a rapid first wave of cell death in response to DNA damage that can precede p53 target gene activation

p53 promotes apoptosis in response to death stimuli by transactivation of target genes and by transcription-independent mechanisms. We recently showed that wild-type p53 rapidly translocates to mitochondria in response to multiple death stimuli in cultured cells. Mitochondrial p53 physically interacts with antiapoptotic Bcl proteins, induces Bak oligomerization, permeabilizes mitochondrial membranes, and rapidly induces cytochrome c release. Here we characterize the mitochondrial p53 response in vivo. Mice were subjected to gamma irradiation or intravenous etoposide administration, followed by cell fractionation and immunofluorescence studies of various organs. Mitochondrial p53 accumulation occurred in radiosensitive organs like thymus, spleen, testis, and brain but not in liver and kidney. Of note, mitochondrial p53 translocation was rapid (detectable at 30 min in thymus and spleen) and triggered an early wave of marked caspase 3 activation and apoptosis. This caspase 3-mediated apoptosis was entirely p53 dependent, as shown by p53 null mice, and preceded p53 target gene activation. The transcriptional p53 program had a longer lag phase than the rapid mitochondrial p53 program. In thymus, the earliest apoptotic target gene products PUMA, Noxa, and Bax appeared at 2, 4, and 8 h, respectively, while Bid, Killer/DR5, and p53DinP1 remained uninduced even after 20 h. Target gene induction then led to further increase in active caspase 3. Similar biphasic kinetics was seen in cultured human cells. Our results suggest that in sensitive organs mitochondrial p53 accumulation in vivo occurs soon after a death stimulus, triggering a rapid first wave of apoptosis that is transcription independent and may precede a second slower wave that is transcription dependent.

Transdominant DeltaTAp73 isoforms are frequently up-regulated in ovarian cancer. Evidence for their role as epigenetic p53 inhibitors in vivo

Despite strong homology, the roles of TP53 and TP73 in tumorigenesis seem to be fundamentally different. In contrast to TP53, tumor-associated overexpression of TP73 in many different cancers, combined with virtual absence of inactivating mutations and lack of a cancer phenotype in the TP73 null mouse are inconsistent with a suppressor function but instead support an oncogenic function. The discovery of NH(2)-terminally truncated p73 isoforms, collectively called DeltaTAp73, is now the focus of intense interest because they act as potent transdominant inihibitors of wild-type p53 and transactivation-competent TAp73. Therefore, establishing deregulated DeltaTAp73 expression in tumors could be the crucial link to decipher which of the two opposing roles of this bipolar gene is the biologically relevant one. This study is the largest to date and encompasses 100 ovarian carcinomas with complete expression profile of all NH(2)-terminal isoforms, discriminating between TAp73 and DeltaTAp73 (DeltaNp73, DeltaN'p73, Ex2p73, and Ex2/3p73) by isoform-specific real-time reverse transcription-PCR. We find that the set of NH(2)-terminal p73 isoforms distinguishes ovarian cancer patients from healthy controls and thus is a molecular marker for this diagnosis. Ovarian cancers strongly and almost universally overexpress DeltaN'p73 compared with normal tissues (95% of cancers). About one-third of tumors also exhibit concomitant up-regulation of the antagonistic TAp73, whereas only a small subgroup of tumors overexpress DeltaNp73. Thus, deregulation of the E2F1-responsive P1 promoter, rather than the alternate P2 promoter, is mainly responsible for the production of transdominant p53/TAp73 antagonists in ovarian cancer. Tumor stage, grade, presence of metastases, p53 status, and residual disease after resection are significant prognostic markers for overall and recurrence-free survival. A trend is found for better overall survival in patients with low expression of DeltaN'p73/DeltaNp73, compared with patients with high expression. A strong correlation between deregulated DeltaTAp73 and p53 status exists. p53 wild-type cancers exhibit significantly higher deregulation of DeltaN'p73, DeltaNp73, and Ex2/3p73 than p53 mutant cancers. This data strongly supports the hypothesis that overexpression of transdominant p73 isoforms can function as epigenetic inhibitors of p53 in vivo, thereby alleviating selection pressure for p53 mutations in tumors.

Mutational analysis of p53 in human tumors: direct DNA sequencing and SSCP

This chapter describes the techniques necessary to identify p53 genomic mutations in either frozen or paraffin-embedded tumor samples. DNA is extracted from the tumor samples and then used as a template to amplify the p53 coding sequences by polymerase chain reaction (PCR). The PCR products are characterized by agarose gel electrophoresis. Subsequently, mutations can be detected by direct DNA sequencing of the PCR products, followed by acrylamide gel electrophoresis and autoradiography. Alternatively, PCR products containing mutations can be identified by their aberrant mobility on denaturing acrylamide gels, using single-stranded conformation polymorphism (SSCP).

p53 has a direct apoptogenic role at the mitochondria

p53 induces apoptosis by target gene regulation and transcription-independent signaling. However, a mechanism for the latter was unknown. We recently reported that a fraction of induced p53 translocates to the mitochondria of apoptosing tumor cells. Targeting p53 to mitochondria is sufficient to launch apoptosis. Here, we provide evidence that p53 translocation to the mitochondria occurs in vivo in irradiated thymocytes. Further, we show that the p53 protein can directly induce permeabilization of the outer mitochondrial membrane by forming complexes with the protective BclXL and Bcl2 proteins, resulting in cytochrome c release. p53 binds to BclXL via its DNA binding domain. We probe the significance of mitochondrial p53 and show that tumor-derived transactivation-deficient mutants of p53 concomitantly lose the ability to interact with BclXL and promote cytochrome c release. This opens the possibility that mutations might represent "double-hits" by abrogating the transcriptional and mitochondrial apoptotic activity of p53.

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.