Role for the p53 homologue p73 in E2F-1-induced apoptosis

Human T-cell leukemia virus type I tax repression of p73beta is mediated through competition for the C/H1 domain of CBP

The Tax protein, encoded by the human T-cell leukemia virus type I (HTLV-I), is required for high level viral transcription and HTLV-I-associated malignant transformation. Although the precise mechanism of malignant transformation by Tax is unclear, it is well established that Tax represses the transcription function of the tumor suppressor p53, possibly accelerating the accumulation of genetic mutations that are critical in HTLV-I-mediated malignant transformation. Tax repression of p53 transcription function appears to occur, at least in part, through competition for the cellular coactivator CBP/p300. In this study, we characterize the effect of Tax on the p53 family member, p73. We demonstrate that Tax also represses the transcription function of p73beta and that the repression is reciprocal in vivo, consistent with the idea that both transcription factors may compete for CBP/p300 in vivo. We provide evidence showing that both Tax and p73 interact strongly with the C/H1 domain of CBP and that their binding to this region is mutually exclusive in vitro. This finding provides evidence supporting the idea that reciprocal transcriptional repression between Tax and p73 is mediated through coactivator competition.

p73beta, a variant of p73, enhances Wnt/beta-catenin signaling in Saos-2 cells

The Wnt/beta-catenin pathway and p53 are very common targets for genetic alterations in colorectal cancer, and relationships between them have been reported. Here, we describe the relation between Wnt/beta-catenin signaling and the p53-related gene p73. p73, but not p53, activated a promoter containing the Tcf-binding sequence in Saos-2 cells, and the degree of activation was positively correlated with that on a p53-responsive promoter. Moreover, p73beta enhanced Wnt/beta-catenin signaling synergistically with Wnt-3a or exogenously expressed beta-catenin, unlike p53, and the enhancement was not caused by the accumulation of beta-catenin. These results show that the effects of p73 on Wnt/beta-catenin signaling differ from those of p53.

Oncogenes induce and activate endogenous p73 protein

The identification of upstream pathways that signal to TP73 is crucial for understanding the biological role of this gene. Since some evidence suggests that TP73 might play a role in tumorigenesis, we asked whether oncogenes can induce and activate endogenous TP73. Here, we show that endogenous p73 alpha and beta proteins are up-regulated in p53-deficient tumor cells in response to overexpressed E2F1, c-Myc, and E1A. E2F1, c-Myc, and E1A-mediated p73 up-regulation leads to activation of the p73 transcription function, as shown by p73-responsive reporter activity and by induction of known endogenous p73 target gene products such as p21 and HDM2. Importantly, E2F1-, c-Myc-, and E1A-mediated activation of endogenous p73 induces apoptosis in SaOs-2 cells. Conversely, inactivation of p73 by a dominant negative p73 inhibitor (p73DD), but not by a mutant p73DD, inhibits oncogene-induced apoptosis. These data show that oncogenes can signal to TP73 in vivo. Moreover, in the absence of p53, oncogenes may enlist p73 to induce apoptosis in tumor cells.

From p63 to p53 across p73

Most genes are members of a family. It is generally believed that a gene family derives from an ancestral gene by duplication and divergence. The tumor suppressor p53 was a striking exception to this established rule. However, two new p53 homologs, p63 and p73, have recently been described [1-6]. At the sequence level, p63 and p73 are more similar to each other than each is to p53, suggesting the possibility that the ancestral gene is a gene resembling p63/p73, while p53 is phylogenetically younger [1,2].The complexity of the family has also been enriched by the alternatively spliced forms of p63 and p73, which give rise to a complex network of proteins involved in the control of cell proliferation, apoptosis and development [1,2,4,7-9]. In this review we will mainly focus on similarities and differences as well as relationships among p63, p73 and p53.

E2F3 contributes both to the inappropriate proliferation and to the apoptosis arising in Rb mutant embryos

The E2F transcription factors are thought to be key downstream targets of the retinoblastoma protein (pRB) tumor suppressor. It is widely believed that E2F1, E2F2, and E2F3 can all activate cellular proliferation but that E2F1 is the specific inducer of apoptosis. Here we show that the E2f3 mutation completely suppresses both the inappropriate proliferation and the p53-dependent apoptosis arising in the Rb mutant embryos. Through the analysis of Rb(-/-);E2f3(+/-) embryos, we have been able to separate E2F3's role in the induction of apoptosis from its ability to induce proliferation. Thus, contrary to the prevailing view of E2F action, E2F3 makes a major contribution to the apoptosis resulting from pRB loss.

Temperature-sensitive mutants of p53 homologs

Two homologs of the p53 tumor suppressor, p63 and p73 have recently been discovered. These proteins have activities similar to p53 in cell culture but have distinct developmental functions in vivo. We found that temperature-sensitive mutants of certain p63 and p73 isoforms can be created by single amino acid substitutions of an alanine residue corresponding to alanine 135 of murine p53. The mutants (p63gamma-Pro167, p73alpha-Leu156 and p73beta-Ile156) can be controlled by temperature shift between 32 degrees C and 39 degrees C. They can be stably expressed in p53-null H1299 cells at 39 degrees C, become transcriptionally activated at 32 degrees C, and induce expression of p53-responsive genes MDM2 and p21WAF1. Activation of p73beta-Ile156 in H1299 cells inhibits cell division but induces significant increase in cell size (hypertrophy), whereas activation of p73alpha-Leu156 and p63gamma-Pro167 induces apoptosis. These mutants may be useful tools for gaining further insight to the functions of p53 homologs.

P63 and P73: P53 mimics, menaces and more

Inactivation of the tumour suppressor p53 is the most common defect in cancer cells. The discovery of its two close relatives, p63 and p73, was therefore both provocative and confounding. Were these new genes tumour suppressors, p53 regulators, or evolutionary spin-offs? Both oncogenic and tumour-suppressor properties have now been attributed to the p53 homologues, perhaps reflecting the complex, often contradictory, protein products encoded by these genes. p63 and p73 are further implicated in many p53-independent pathways, including stem-cell regeneration, neurogenesis and sensory processes.

Defying death after DNA damage

DNA damage frequently triggers death by apoptosis. The irreversible decision to die can be facilitated or forestalled through integration of a wide variety of stimuli from within and around the cell. Here we address some fundamental questions that arise from this model. Why should DNA damage initiate apoptosis in the first place? In damaged cells, what are the alternatives to death and why should they be selected in some circumstances but not others? What signals register DNA damage and how do they impinge on the effector pathways of apoptosis? Is there a suborganellar apoptosome complex effecting the integration of death signals within the nucleus, just as there is in the cytoplasm? And what are the consequences of failure to initiate apoptosis in response to DNA damage?

A common E2F-1 and p73 pathway mediates cell death induced by TCR activation

Strong stimulation of the T-cell receptor (TCR) on cycling peripheral T cells causes their apoptosis by a process called TCR-activation-induced cell death (TCR-AICD). TCR-AICD occurs from a late G1 phase cell-cycle check point independently of the 'tumour suppressor' protein p53. Disruption of the gene for the E2F-1 transcription factor, an inducer of apoptosis, causes significant increases in T-cell number and splenomegaly. Here we show that T cells undergoing TCR-AICD induce the p53-related gene p73, another mediator of apoptosis, which is hypermethylated in lymphomas. Introducing a dominant-negative E2F-1 protein or a dominant-negative p73 protein into T cells protects them from TCR-mediated apoptosis, whereas dominant-negative E2F-2, E2F-4 or p53 does not. Furthermore, E2F-1-null or p73-null primary T cells do not undergo TCR-mediated apoptosis either. We conclude that TCR-AICD occurs from a late G1 cell-cycle checkpoint that is dependent on both E2F-1 and p73 activities. These observations indicate that, unlike p53, p73 serves to integrate receptor-mediated apoptotic stimuli.