The N-terminal domain of p73 interacts with the CH1 domain of p300/CREB binding protein and mediates transcriptional activation and apoptosis

MDM2 inhibits PCAF (p300/CREB-binding protein-associated factor)-mediated p53 acetylation

Our previous study shows that MDM2, a negative feedback regulator of the tumor suppressor p53, inhibits p300-mediated p53 acetylation. Because PCAF (p300/CREB-binding protein-associated factor) also acetylates and activates p53 after DNA damage, in this study we have examined the effect of MDM2 on PCAF-mediated p53 acetylation. We have found that MDM2 inhibited p53 acetylation by PCAF in vitro. In addition, when overexpressed, MDM2 inhibited PCAF-mediated p53 acetylation in cells. MDM2 interacted with PCAF both in vitro and in cells, as assessed using GST fusion protein interaction and immunoprecipitation assays, respectively. Consistent with the above results, MDM2 significantly repressed the activation of p53 transcriptional activity by PCAF without apparently affecting the level of p53. In addition, MDM2 co-resided with p53 at the p53-responsive mdm2 and p21(waf1/cip1) promoters, inhibiting expression of the endogenous p21(waf1/cip1). These results demonstrate that MDM2 can inhibit PCAF-mediated p53 acetylation and activation.

Physical interaction of p73 with c-Myc and MM1, a c-Myc-binding protein, and modulation of the p73 function

p73 shares high sequence homology with the tumor suppressor p53. Like p53, ectopic overexpression of p73 induces cell cycle arrest and/or apoptosis, and these biological activities are linked to its sequence-specific transactivation function. The COOH-terminal region of p73 is unique and has a function to modulate DNA-binding ability and transactivation activity. To identify and characterize cellular proteins that interact with the COOH-terminal region of p73 alpha and regulate its activity, we employed a yeast-based two-hybrid screen with a human fetal brain cDNA library. We found MM1, a nuclear c-Myc-binding protein, was associated with p73 alpha in both yeast two-hybrid and in vitro pull-down assays. In mammalian cells, MM1 co-immunoprecipitated with p73 alpha, whereas p73 beta and tumor suppressor p53 did not interact with MM1. Overexpression of MM1 in p53-deficient osteosarcoma SAOS-2 cells enhanced the p73 alpha-dependent transcription from the p53/p73-responsive Bax and PG13 promoters, whereas p73 beta- and p53-mediated transcriptional activation was unaffected in the presence of MM1. MM1 also stimulated the p73 alpha-mediated growth suppression in SAOS-2 cells. More importantly, we found that c-Myc was physically associated with p73 alpha and significantly impaired the transcriptional activity of p73 alpha on Bax and p21(waf1) promoters. Expression of MM1 strongly reduced the c-Myc-mediated inhibitory activity on p73 alpha. These results suggest that MM1 may act as a molecular partner for p73 to prevent the c-Myc-mediated inhibitory effect on its activity.

Phosphorylation of forkhead transcription factors by erythropoietin and stem cell factor prevents acetylation and their interaction with coactivator p300 in erythroid progenitor cells

The mammalian forkhead transcription factors, FOXO3a (FKHRL1), FOXO1a (FKHR) and FOXO4 (AFX) are negatively regulated by PKB/Akt kinase. In the present study we examined the engagement of forkhead family of transcription factors in erythropoietin (Epo)- and stem cell factor (SCF)-mediated signal transduction. Our data show that all three forkhead family members, FOXO3a, FOXO1a and FOXO4 are phosphorylated in human primary erythroid progenitors. Experiments performed to determine various upstream signaling pathways contributing to phosphorylation of forkhead family members show that only PI-3-kinase pathway is required for inactivation of FOXO3a. Our data also demonstrate that during Epo deprivation FOXO3a interacts with the transcriptional coactivator p300 and such interaction is disrupted by stimulation of cells with Epo. To determine the domains in FOXO3a, mediating its interaction with p300, we performed GST pull-down assays and found that the N-terminus region containing the first 52 amino acids was sufficient for binding p300. Finally, our data demonstrate that FOXO3a and FOXO1a are acetylated during growth factor deprivation and such acetylation is reversed by stimulation with Epo. Thus mammalian forkhead transcription factors are involved in Epo and SCF signaling in primary erythroid progenitors and may play a role in the induction of apoptotic and mitogenic signals.

DNA damage-dependent acetylation of p73 dictates the selective activation of apoptotic target genes

The tumor suppressor p53 and its close relative p73 are activated in response to DNA damage resulting in either cell cycle arrest or apoptosis. Here, we show that DNA damage induces the acetylation of p73 by the acetyltransferase p300. Inhibiting the enzymatic activity of p300 hampers apoptosis in a p53(-/-) background. Furthermore, a nonacetylatable p73 is defective in activating transcription of the proapoptotic p53AIP1 gene but retains an intact ability to regulate other targets such as p21. Finally, p300-mediated acetylation of p73 requires the protooncogene c-abl. Our results suggest that DNA damage-induced acetylation potentiates the apoptotic function of p73 by enhancing the ability of p73 to selectively activate the transcription of proapoptotic target genes.

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.

Overproduced p73alpha activates a minimal promoter through a mechanism independent of its transcriptional activity

p73, the gene for a protein related to the tumor suppressor p53, encodes several variants which bear distinct carboxy-terminal structures as a result of alternative splicing. We and others showed that these splicing variants have different transcriptional effects on promoters with a p53-binding consensus sequence (p53BCS). Here we show that when transiently overexpressed, p73alpha but not p73beta activated several minimal promoters without the p53BCS, while p73gamma and p73epsilon activated them to a much lesser extent than p73alpha, and p53 suppressed the promoters without p53BCS as reported previously. Moreover, the results of RNase protection and RNA transfection assays suggested that this activation occurred at the transcriptional level. Deletion analysis of p73alpha revealed that the transactivation domain of p73 was not involved in this activity and the C-terminal region of p73alpha which is a specific structure of this variant was essential, suggesting that this phenomenon occurs independent of the transactivation activity of p73alpha and that the C-terminal extension of p73alpha may affect the basal level of transcription.

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.

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.

Evolution of functions within the p53/p63/p73 family

Even though the tumor suppressor gene p53 is highly important in human cancer, as indicated by the fact that it is mutated in about 50% of cases, up to a few years ago no similar proteins had been identified. Recently, two p53 homologues have been identified, p73 and p63, with high amino acid identity suggesting similar functions. Indeed, like p53, p73 as well (i) can bind mdmX, mdm2, p300/CAF and adenovirus E4-orf6 proteins, (ii) can trigger several promoters including p21, bax, mdm2, gadd45, cyclin G, IGFBP3, 14-3-3 sigma, (iii) is able to trigger cell death, (iv) is involved in the DNA damage response, although through a different pathway. Here we analyze the data present in the literature in search of diverging pathways among the p53, p63, p73 family. Both p63 and p73 present two significant structural peculiarities: the presence of an extended non-conserved C-terminus containing a sterile alpha motive (SAM), typical of developmental proteins, and the presence of number of different splicing isoforms differing in the N-terminus or in the absence of the transactivation domain (delta N forms), acting as dominant negative. The mouse knockout of p63 and p73, unlike the ones for p53, shows developmental abnormalities; p63 and p73 are rarely mutated in human cancers; both genes are regulated in different differentiation models. This strongly suggests the involvement of p63 and p73 in development. A picture is emerging showing a gradient of function among p53, p73, p63 ranging from tumor suppression to development.

NBP is the p53 homolog p63

We previously identified a non-p53, p53-responsive DNA element (p53RE)-binding protein named NBP, functionally analogous to p53, from human cervical carcinoma Hela cells. Here we report a biochemical study demonstrating that this activity is the recently cloned p53 analog p63. NBP was purified through conventional and DNA affinity chromatography to apparent homogeneity with a prominent polypeptide migrating in between the 43 and 68 kDa positions on a SDS gel. This polypeptide immunoreacted with monoclonal anti-p63 but not anti-p53 or anti-p73 antibodies. Also, NBP co-purified with p63 through each step of fractionation, as detected with anti-p63 antibodies. DNA-protein complexes formed with purified NBP and p53RE-containing oligomers derived from the p21(waf1) promoter were supershifted by anti-p63 but not anti-p53 antibodies. Thus, these results demonstrate that NBP is encoded by the p53 homolog p63 gene.

A DNA damage-induced p53 serine 392 kinase complex contains CK2, hSpt16, and SSRP1

Phosphorylation of the human p53 protein at Ser-392 has been shown to be responsive to UV but not gamma irradiation. Here we describe identification and purification of a mammalian UV-activated protein kinase complex that phosphorylates Ser-392 of p53 in vitro. This kinase complex contains casein kinase 2 (CK2) and the chromatin transcriptional elongation factor FACT (a heterodimer of hSpt16 and SSRP1). In vitro studies show that FACT alters the specificity of CK2 in the complex such that it selectively phosphorylates p53 over other substrates including casein. In addition, phosphorylation by the kinase complex enhances p53 activity. These results thus provide a potential mechanism for p53 activation by UV irradiation.

p300 does not require its acetylase activity to stimulate p73 function

We previously reported that p73, like p53, utilizes p300 or cAMP-response element-binding protein-binding protein as its coactivator. Here, we extended this work by further examining whether the intrinsic acetylase activity of p300 is necessary for stimulating p73 function. Although p300 acetylated the C-terminal fragment of p73 (amino acids 311-636) in vitro, it was unable to efficiently acetylate the full-length p73. Consistently, p300 did not acetylate p73 in vivo when both the proteins were overexpressed in cells. Also, an acetylase-defective mutant p300 named p300AT2 was able to elevate p73-dependent transcription in cells. p300 associated with p73 when forming DNA-protein complexes and stabilized p73 proteins. These results demonstrate that p300 does not need its acetylase activity to be a coactivator of p73.

Role of the p53-homologue p73 in E2F1-induced apoptosis

Most human cancers harbour aberrations of cell-cycle control, which result in deregulated activity of the E2F transcription factors with concomitant enhanced cell-cycle progression. Oncogenic signalling by E2F1 has recently been linked to stabilization and activation of the tumour suppressor p53 (refs 1,3,4). The p73 protein shares substantial sequence homology and functional similarity with p53 (refs 5-7 ). Hence, several previously considered p53-independent cellular activities may be attributable to p73. Here we provide evidence that E2F1 directly activates transcription of TP73, leading to activation of p53-responsive target genes and apoptosis. Disruption of p73 function by a tumour-derived p53 mutant reduced E2F1-mediated apoptosis. Thus, p73 activation by deregulated E2F1 activity might constitute a p53-independent, anti-tumorigenic safeguard mechanism.

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.

Regulation of transcription of the human presenilin-1 gene by ets transcription factors and the p53 protooncogene

The expression of the human presenilin-1 cellular gene is suppressed by the p53 protooncogene. The rapid kinetic of the down-regulation has suggested that it may result from a primary mechanism. We show here that p53 also suppresses the transcription of a presenilin-1 promoter-chloramphenicol acetyltransferase reporter synthetic gene in transient infection assays in neuroblastoma (SK-N-SH) and hepatoma (HepG2) cell lines. Only a minimum promoter including sequences from -35 to + 6 from the transcription initiation is sufficient to confer down-regulation. We have previously defined a crucial DNA element controlling 90% of the expression of the gene within the same short area, and the identification of the transcription factors involved should also provide insights into the regulation of PS1 by p53. This region contains an Ets transcription factor binding motif, and a 2-base pair alteration within the core sequence (GGAA to TTAA) of the Ets consensus also reduced transcription by more than 90%. We now show that Ets1 and Ets2 indeed transactivate a PS1 promoter-chloramphenicol acetyltransferase reporter including the (-35 to +6) fragment. Furthermore, in vitro translated Ets2 binds specifically to the -10 Ets motif in electrophoretic mobility shift assays. Therefore, Ets1/2 factors bind specifically to the -10 Ets element and activate PS1 transcription. We also show that the coactivator p300 enhances the activation by Ets1 and Ets2 as well as the repression by p53. p300 is known to interact with p53 as well as with Ets1 and Ets2. We show that p53 does not bind directly to the PS1 promoter. Hence the repression of PS1 transcription by p53 is likely to be mediated through protein-protein interactions.

MDM2 inhibits p300-mediated p53 acetylation and activation by forming a ternary complex with the two proteins

p300 acetylates and activates the tumor suppressor p53 after DNA damage. Here, we show that MDM2, a negative-feedback regulator of p53, inhibited p300-mediated p53 acetylation by complexing with these two proteins. First, we purified a p300-MDM2-p53 protein complex from HeLa nuclear extracts, which was inactive in p53 acetylation, but active in histone acetylation. Also, wild-type, but not N-terminally deleted, MDM2 inhibited p53 acetylation by p300 in vitro and in vivo. This inhibition was specific for p53, because MDM2 did not affect acetylation of histones or the C terminus of p73 by p300. Consequently, wild-type, but not the mutant, MDM2 repressed the p300-stimulated sequence-specific DNA-binding and transcriptional activities of p53. These results demonstrate that an additional mechanism of p53 inactivation by MDM2 is to inhibit p53 acetylation by p300.

Physical and functional interaction between p53 mutants and different isoforms of p73

p53 is the most frequently inactivated tumor suppressor gene in human cancer, whereas its homologue, p73, is rarely mutated. Similarly to p53, p73 can promote growth arrest or apoptosis when overexpressed in certain p53-null tumor cells. It has previously been shown that some human tumor-derived p53 mutants can exert gain of function activity. The molecular mechanism underlying this activity remains to be elucidated. We show here that human tumor-derived p53 mutants (p53His175 and p53Gly281) associate in vitro and in vivo with p73 alpha, beta, gamma, and delta. This association occurs under physiological conditions, as verified in T47D and SKBR3 breast cancer cell lines. The core domain of mutant p53 is sufficient for the association with p73, whereas both the specific DNA binding and the oligomerization domains of p73 are required for the association with mutant p53. Furthermore, p53His175 and p53Gly281 mutants markedly reduce the transcriptional activity of the various isoforms of p73. Thus, human tumor-derived p53 mutants can associate with p73 not only physically but also functionally. These findings define a network involving mutant p53 and the various spliced isoforms of p73 that may confer upon tumor cells a selective survival advantage.

The p53/p63/p73 family of transcription factors: overlapping and distinct functions

The p53 gene is the most frequently mutated gene in human cancer. The identification of two homologues, p63 and p73, revealed that p53 is a member of a family of related transcription factors. Given that they share amino acid sequence identity reaching 63% in the DNA-binding domain, p53, p63 and p73 should have redundant functions in the regulation of gene expression. Indeed, p73 can activate p53-regulated genes and suppress growth or induce apoptosis. Moreover, p53 and p73 are both induced by DNA damage - albeit through distinct mechanisms. Other evidence, however, suggests that p63 and p73 are important for regulation of normal development. An extended C-terminal region, not found in p53, is alternatively spliced in p63 and p73. Within this C-terminal extension is a sterile α motif (SAM) previously found in other proteins that regulate development. The p63-deficient mice showed developmental abnormalities. Interestingly, the human p63 gene is mutated in children who have the disease Ectrodactyly, Ectodermal dysplasia and facial Clefts (EEC) syndrome, and the disease phenotype is similar to the one of p63-deficient mice. The p63 and p73 genes are rarely mutated in human cancer, although p73 loss is observed in neuroblastoma and a subtype of T-cell lymphoma. p53, p63 and p73 appear to have overlapping and distinct functions: p53 regulates the stress response to suppress tumors; p63 is essential for ectoderm development; and p73 might regulate both the stress response and development. Because p53 and p73 are linked to different upstream pathways, this family of transcription factors might regulate a common set of genes in response to different extracellular signals and developmental cues.