Viral Oncoproteins Discriminate between p53 and the p53 Homolog p73

Abstract

p73 is a recently identified member of the p53 family. Previously it was shown that p73 can, when overproduced in p53-defective tumor cells, activate p53-responsive promoters and induce apoptosis. In this report we describe the generation of anti-p73 monoclonal antibodies and confirm that two previously described p73 isoforms are produced in mammalian cells. Furthermore, we show that these two isoforms can bind to canonical p53 DNA-binding sites in electrophoretic mobility shift assays. Despite the high degree of similarity between p53 and p73, we found that adenovirus E1B 55K, simian virus 40 T, and human papillomavirus E6 do not physically interact with p73. The observation that viral oncoproteins discriminate between p53 and p73 suggests that the functions of these two proteins may differ under physiological conditions. Furthermore, they suggest that inactivation of p73 may not be required for transformation.p53 is the most frequently mutated human tumor suppressor gene identified to date. In model systems, restoration of p53 function in tumor cells leads either to a cell cycle block or to apoptosis. These activities have been linked to the ability of p53 to bind to specific DNA sequences and activate transcription. Among the genes that are activated by p53 are the cyclindependent kinase inhibitor p21 and a family of genes involved in oxidative stress (10,36).Recently, Kaghad et al. serendipitously identified a cDNA encoding a novel p53-like protein (22). The protein, called p73, is approximately 60% identical to p53 in the region corresponding to the p53 DNA-binding domain. Furthermore, all of the residues that contact DNA in the p53-DNA crystal structure are conserved in p73 (7). In addition, the N terminus of p73 is similar (ϳ25% identity) to the N-terminal transactivation domain of p53, and the C terminus of p73 contains a region which is comparably similar to a C-terminal oligomerization domain within p53 (34,44,48). Finally, the genomic organizations of p53 and p73 are highly similar, suggesting that they have a common ancestry (22). In keeping with this high degree of similarity, p73 can, at least when overproduced, activate transcription from p53-responsive promoters and induce tumor cell apoptosis (20,22).The p73 gene maps to chromosome 1p36, a region that is frequently deleted in a variety of human cancers (22). Nonetheless, to date no mutations have been identified in the remaining p73 allele in such tumors. Thus, p73, unlike many tumor suppressor gene products such as the retinoblastoma protein (pRB) and p53, does not appear to conform to a two-hit model of tumorigenesis. It has been suggested that this may be due to epigenetic silencing of the p73 allele retained in tumors (22). Alternatively, it is possible that p73 is not a tumor suppressor gene product and is not the relevant target of 1p36 deletions.A number of unrelated DNA tumor viruses can inactivate pRB. Inactivation of pRB leads to derepression of E2F-responsive promoters, which, in turn, allows quiescent cells to enter S phase. In model...

Detection of p73 antibodies in patients with various types of cancer: immunological characterization

“…Although the sequence homology between the central region of p53 and p73 is about 60%, the 2 regions behave differently. This is supported by the observation that SV40 large T antigen binds only to p53 through this central region, whereas p73 does not recognize the viral protein (Marin et al, 1998).…”

Expression of the vascular endothelial growth factor gene is inhibited by p73

“…However, the fact that we have detected a gradual decline in the p73 responsiveness of the VEGF promoter upon consecutive shortening of the promoter from the 5'-end suggests that there may exist additional cis-acting elements upstream of the 785 to 750 promoter region, which may contribute to the repressing e ect of p73 on VEGF promoter activity. It was reported that p73 not only transactivates p53 responsive promoters but also binds to canonical p53 consensus sequences in electrophoretic mobility shift assays (Marin et al, 1998). Remarkably, the VEGF promoter does not possess a p53 binding site.…”

Transcriptional activities of p73 splicing variants are regulated by inter-variant association

“…For example, p73α interacted weakly with p73β, whereas p73β showed strong homotypic interaction in the same system. However, in an immunoprecipitation assay of endogenous p73 proteins using a p73α-specific antibody, p73β was coimmunoprecipitated with p73α [18]. This suggested that at least some p73α forms a complex with p73β.…”