p300/MDM2 complexes participate in MDM2-mediated p53 degradation

The human papillomavirus type 16 E6 oncoprotein can down-regulate p53 activity by targeting the transcriptional coactivator CBP/p300

The transforming proteins of the small DNA tumor viruses, simian virus 40 (SV40), adenovirus, and human papillomavirus (HPV) target a number of identical cellular regulators whose functional abrogation is required for transformation. However, while both adenovirus E1A and SV40 large T transforming properties also depend on the targeting of the transcriptional coactivator CBP/p300, no such interaction has been described for the HPV oncoprotein E6 or E7. Here, we demonstrate that the HPV-16 E6 protein, previously shown to facilitate the degradation of p53 in a complex with E6-associated protein (E6AP), also targets CBP/p300 in an interaction involving the C-terminal zinc finger of E6 and CBP residues 1808 to 1826. Furthermore, this interaction is limited to E6 proteins of high-risk HPVs associated with cervical cancer that have the capacity to repress p53-dependent transcription. An HPV-16 E6 mutant (L50G) that binds CBP/p300, but not E6AP, is still capable of down-regulating p53 transcriptional activity. Thus, HPV E6 proteins possess two distinct mechanisms by which to abrogate p53 function: the repression of p53 transcriptional activity by targeting the p53 coactivator CBP/p300, and the removal of cellular p53 protein through the proteosome degradation pathway.

Suppression of E1A-mediated transformation by the p50E4F transcription factor

The adenovirus E1A gene can act as an oncogene or a tumor suppressor, with the latter effect generally arising from the induction of apoptosis or the repression of genes that provide oncogenic growth stimuli (e.g., HER-2/c-erbB2/neu) or increased metastatic invasiveness (e.g., metalloproteases). In this study, coexpression of E1A and p50E4F, a cellular transcription factor whose DNA binding activity is stimulated by E1A, suppressed colony formation by NIH 3T3 cells and transformation of primary rat embryo fibroblasts but had no observed effect in the absence of E1A. Domains in p50E4F required for stimulation of the adenovirus E4 promoter were required for the suppressive effect, indicating a transcriptional mechanism. In serum-containing media, retroviral expression of p50E4F in E1A13S/ras-transformed NIH 3T3 fibroblasts had little effect on subconfluent cultures but accelerated a decline in viability after the cultures reached confluence. Cell death occurred by both apoptosis and necrosis, with the predominance of each process determined by culture conditions. In serum-free media, p50E4F accelerated E1A-induced apoptosis. The results suggest that p50E4F sensitizes cells to signals or conditions that cause cell death.

The p53 pathway

Abnormalities of the p53 tumour suppressor gene are among the most frequent molecular events in human and animal neoplasia. Moreover, p53 is one of the most studied proteins in the whole of contemporary biology, with more than 12,500 papers so far written! In this review the choice has been deliberately made not to be fully comprehensive in the coverage of the huge p53 literature. Rather attention is focused on a small number of recent developments which are reviewed in the context of modern models of p53 function. Progress in the analysis of signalling to p53 including phosphorylation cascades, and interactions with proteins such as mdm2 and ARF are highlighted. The plethora of protein-protein interactions is discussed, as are the strategies for defining downstream targets of p53. Finally, the emerging biology of p53 homologues is considered. The need for bridging the gap between reductionist, biochemical and biophysical studies and biological and genetic analysis is emphasized. Only this will provide the needed framework for utilizing the information in clinical care.

Murine double minute (MDM2) blocks p53-coactivator interaction, a new mechanism for inhibition of p53-dependent gene expression

The ability of the p53 tumor suppressor to induce cell cycle arrest and cell death is closely regulated under normal conditions. The transcriptional activity of p53 is negatively controlled by murine double minute (MDM2). p53 requires the coactivator CREB-binding protein (CBP), or its structural homolog, p300, to stimulate transcription of responsive genes. Here we find that the transactivation domain of p53 selectively interacts with the N- and C-terminal regions of CBP/p300. A mutant CBP lacking the N terminus failed to stimulate p53-dependent transactivation. In both p53 null Saos2 cells, and in UV-irradiated MCF7 cells, we observed that MDM2 associates with the N-terminal region of CBP/p300. Because p53 interacts with both MDM2 and CBP/p300 through its trans-activation domain, we examined the role of MDM2 in p53-coactivator interactions. MDM2 blocked CBP/p300 recruitment in vitro and inhibited the interaction of the transactivating region of p53 with both the N- or C-terminal regions of CBP/p300 in a mammalian two-hybrid assay. These observations suggest that MDM2 may be inhibiting p53 trans-activation by shielding its activation domain from the coactivators, a new mechanism for the inhibition of p53-dependent gene expression.

Distinct regulation of p53 and p73 activity by adenovirus E1A, E1B, and E4orf6 proteins

Multiple adenovirus (Ad) early proteins have been shown to inhibit transcription activation by p53 and thereby to alter its normal biological functioning. Since these Ad proteins affect the activity of p53 via different mechanisms, we examined whether this inhibition is target gene specific. In addition, we analyzed whether the same Ad early proteins have a comparable effect on transcription activation by the recently identified p53 homologue p73. Our results show that the large E1B proteins very efficiently inhibited the activity of p53 on the Bax, p21(Waf1), cyclin G, and MDM2 reporter constructs but had no effect on the activation of the same reporter constructs by p73, with the exception of some inhibition of the Bax promoter by Ad12 E1B. The repressive effect of the E1A proteins on p53 activity is less than that seen with the large E1B proteins, but the E1A proteins inhibit the activity of both p53 and p73. We could not detect significant inhibition of p53 functions by E4orf6, but a clear repression of the transcription activation by p73 by this Ad early protein was observed. In addition, we found that stable expression of the Ad5 E1A and that of the E1B protein both caused increased p73 protein expression. The large E1B and the E4orf6 proteins together do not target the p73 protein for rapid degradation after adenoviral infection, as has previously been found for the p53 protein, probably because the large E1B protein does not interact with p73. Our results suggest that the p53 and p73 proteins are both inactivated after Ad infection and transformation but via distinct mechanisms.

Mutations in human ARF exon 2 disrupt its nucleolar localization and impair its ability to block nuclear export of MDM2 and p53

The mammalian ARF-INK4a locus uniquely encodes two cell cycle inhibitors by using separate promoters and alternative reading frames. p16INK4a maintains the retinoblastoma protein in its growth suppressive state while ARF stabilizes p53. We report that human ARF protein predominantly localizes to the nucleolus via a sequence within the exon 2-encoded C-terminal domain and is induced to leave the nucleolus by MDM2. ARF forms nuclear bodies with MDM2 and p53 and blocks p53 and MDM2 nuclear export. Tumor-associated mutations in ARF exon 2 disrupt ARF's nucleolus localization and reduce ARF's ability to block p53 nuclear export and to stabilize p53. Our results suggest an ARF-regulated MDM2-dependent p53 stabilization and link the human tumor-associated mutations in ARF with a functional alteration.

Transcriptional cross talk between NF-kappaB and p53

Many cellular stimuli result in the induction of both the tumor suppressor p53 and NF-kappaB. In contrast to activation of p53, which is associated with the induction of apoptosis, stimulation of NF-kappaB has been shown to promote resistance to programmed cell death. These observations suggest that a regulatory mechanism must exist to integrate these opposing outcomes and coordinate this critical cellular decision-making event. Here we show that both p53 and NF-kappaB inhibit each other's ability to stimulate gene expression and that this process is controlled by the relative levels of each transcription factor. Expression of either wild-type p53 or the RelA(p65) NF-kappaB subunit suppresses stimulation of transcription by the other factor from a reporter plasmid in vivo. Moreover, endogenous, tumor necrosis factor alpha-activated NF-kappaB will inhibit endogenous wild-type p53 transactivation. Following exposure to UV light, however, the converse is observed, with p53 downregulating NF-kappaB-mediated transcriptional activation. Both p53 and RelA(p65) interact with the transcriptional coactivator proteins p300 and CREB-binding protein (CBP), and we demonstrate that these results are consistent with competition for a limiting pool of p300/CBP complexes in vivo. These observations have many implications for regulation of the transcriptional decision-making mechanisms that govern cellular processes such as apoptosis. Furthermore, they suggest a previously unrealized mechanism through which dysregulated NF-kappaB can contribute to tumorigenesis and disease.

Transcription factor E2F-1 is upregulated in response to DNA damage in a manner analogous to that of p53

The transcription factor E2F-1 directs the expression of genes that induce or regulate cell division, and a role for E2F-1 in driving cells into apoptosis is the subject of intense discussion. Recently it has been shown that E2F-1 binds and coprecipitates with the mouse double-minute chromosome 2 protein (Mdm2). A domain of E2F-1 (amino acids 390 to 406) shows striking similarity to the Mdm2 binding domain of the tumor suppressor protein p53. It is known that interaction of Mdm2 with p53 through this domain is required for Mdm2-dependent degradation of p53. We show here that E2F-1 protein is upregulated in response to DNA damage. The kinetics of induction are dependent upon the source of DNA damage, i.e., fast and transient after irradiation with X rays and delayed and stable after irradiation with UVC, and thus match the kinetics of p53 induction in response to DNA damage. We show further that E2F-1 is also upregulated by treatment with the transcription inhibitor actinomycin D and with the kinase inhibitor DRB, as well as by high concentrations of the kinase inhibitor H7, all conditions which also upregulate p53. In our experiments we were not able to see an increase in E2F-1 RNA production but did find an increase in protein stability in UVC-irradiated cells. Upregulation of E2F-1 in response to DNA damage seems to require the presence of wild-type p53, since we did not observe an increase in the level of E2F-1 protein in several cell lines which possess mutated p53. Previous experiments showed that p53 is upregulated after microinjection of an antibody which binds to a domain of Mdm2 that is required for the interaction of Mdm2 with p53. Microinjection of the same antibody also increases the expression of E2F-1 protein, while microinjection of a control antibody does not. Furthermore, microinjection of Mdm2 antisense oligonucleotides upregulates E2F-1 protein, while microinjection of an unrelated oligonucleotide does not. These data suggest that E2F-1 is upregulated in a similar way to p53 in response to DNA damage and that Mdm2 appears to play a major role in this pathway.

MDM2 suppresses p73 function without promoting p73 degradation

The newly identified p53 homolog p73 can mimic the transcriptional activation function of p53. We investigated whether p73, like p53, participates in an autoregulatory feedback loop with MDM2. p73 bound to MDM2 both in vivo and in vitro. Wild-type but not mutant MDM2, expressed in human p53 null osteosarcoma Saos-2 cells, inhibited p73- and p53-dependent transcription driven by the MDM2 promoter-derived p53RE motif as measured in transient-transfection and chloramphenicol acetyltransferase assays and also inhibited p73-induced apoptosis in p53-null human lung adenocarcinoma H1299 cells. MDM2 did not promote the degradation of p73 but instead disrupted the interaction of p73, but not of p53, with p300/CBP by competing with p73 for binding to the p300/CBP N terminus. Both p73alpha and p73beta stimulated the expression of the endogenous MDM2 protein. Hence, MDM2 is transcriptionally activated by p73 and, in turn, negatively regulates the function of this activator through a mechanism distinct from that used for p53 inactivation.

Critical role for Ser20 of human p53 in the negative regulation of p53 by Mdm2

In response to environmental stress, the p53 phosphoprotein is stabilized and activated to inhibit cell growth. p53 stability and activity are negatively regulated by the murine double minute (Mdm2) oncoprotein in an autoregulatory feedback loop. The inhibitory effect of Mdm2 on p53 has to be tightly regulated for proper p53 activity. Phosphorylation is an important level of p53 regulation. In response to DNA damage, p53 is phosphorylated at several N-terminal serines. In this study we examined the role of Ser20, a potential phosphorylation site in human p53, in the regulation of p53 stability and function. Substitution of Ser20 by Ala (p53-Ala20) significantly increases the susceptibility of human p53 to negative regulation by Mdm2 in vivo, as measured by apoptosis and transcription activation assays. Mutation of Ser20 to Ala renders p53 less stable and more prone to Mdm2-mediated degradation. While the in vitro binding of p53 to Mdm2 is not increased by the Ala20 mutation, the same mutation results in a markedly enhanced binding in vivo. This is consistent with the conclusion that phosphorylation of Ser20 in vivo attenuates the binding of wild-type p53 to Mdm2. Peptides bearing non-phosphorylated Ser20 or Ala20 compete with p53 for Mdm2 binding, while a similar peptide with phosphorylated Ser20 does not. This implies a critical role for Ser20 in modulating the negative regulation of p53 by Mdm2, probably through phosphorylation-dependent inhibition of p53-Mdm2 interaction.

c-Abl neutralizes the inhibitory effect of Mdm2 on p53

Upon exposure to stress signals, the p53 tumor suppressor protein is stabilized and induces growth suppression. p53 activities are efficiently inhibited by the Mdm2 oncoprotein through an autoregulatory feedback loop. In addition, Mdm2 promotes p53 degradation, thereby terminating its growth inhibitory signal. Hence, p53 exerts its effects during the interval between p53 activation and the subsequent inhibition by Mdm2. Modulation of this interval by regulatory proteins may determine the extent and duration of p53 activity. Recent studies have shown that the c-Abl protein-tyrosine kinase binds p53 and enhances its transcriptional activity. Here we provide an explanation for the cooperation between these proteins. We demonstrate that c-Abl increases the expression level of the p53 protein. The enhanced expression is achieved by inhibiting Mdm2-mediated degradation of p53. This provides a likely mechanistic explanation for the findings that c-Abl overcomes the inhibitory effects of Mdm2 on p53-mediated transcriptional activation and apoptosis. These results suggest that c-Abl modulates the time window within which p53 remains active. The ability of c-Abl to neutralize the inhibitory effects of Mdm2 on p53 may be important for its growth inhibitory function.

The INK4A/ARF locus and its two gene products

The INK4A/ARF locus on chromosome 9 is one of the sites mutated most frequently in human cancer. Two genes comprising overlapping reading frames encoding p16(INK4a) and p19(ARF) have been discovered at this locus and, remarkably, both play an important role in regulating cell growth, survival and senescence.

Functional role of p35srj, a novel p300/CBP binding protein, during transactivation by HIF-1

Recruitment of p300/CBP by the hypoxia-inducible factor, HIF-1, is essential for the transcriptional response to hypoxia and requires an interaction between the p300/CBP CH1 region and HIF-1alpha. A new p300-CH1 interacting protein, p35srj, has been identified and cloned. p35srj is an alternatively spliced isoform of MRG1, a human protein of unknown function. Virtually all endogenous p35srj is bound to p300/CBP in vivo, and it inhibits HIF-1 transactivation by blocking the HIF-1alpha/p300 CH1 interaction. p35srj did not affect transactivation by transcription factors that bind p300/CBP outside the CH1 region. Endogenous p35srj is up-regulated markedly by the HIF-1 activators hypoxia or deferoxamine, suggesting that it could operate in a negative-feedback loop. In keeping with this notion, a p300 CH1 mutant domain, defective in HIF-1 but not p35srj binding, enhanced endogenous HIF-1 function. In hypoxic cells, p35srj may regulate HIF-1 transactivation by controlling access of HIF-1alpha to p300/CBP, and may keep a significant portion of p300/CBP available for interaction with other transcription factors by partially sequestering and functionally compartmentalizing cellular p300/CBP.

The p53 pathway

Abnormalities of the p53 tumour suppressor gene are among the most frequent molecular events in human and animal neoplasia. Moreover, p53 is one of the most studied proteins in the whole of contemporary biology, with more than 12,500 papers so far written! In this review the choice has been deliberately made not to be fully comprehensive in the coverage of the huge p53 literature. Rather attention is focused on a small number of recent developments which are reviewed in the context of modern models of p53 function. Progress in the analysis of signalling to p53 including phosphorylation cascades, and interactions with proteins such as mdm2 and ARF are highlighted. The plethora of protein-protein interactions is discussed, as are the strategies for defining downstream targets of p53. Finally, the emerging biology of p53 homologues is considered. The need for bridging the gap between reductionist, biochemical and biophysical studies and biological and genetic analysis is emphasized. Only this will provide the needed framework for utilizing the information in clinical care.