p73 function is inhibited by tumor-derived p53 mutants in mammalian cells

The p53 tumor suppressor protein, found mutated in over 50% of all human tumors, is a sequence-specific transcriptional activator. Recent studies have identified a p53 relative, termed p73. We were interested in determining the relative abilities of wild-type and mutant forms of p53 and p73 alpha and -beta isoforms to transactivate various p53-responsive promoters. We show that both p73 alpha and p73 beta activate the transcription of reporters containing a number of p53-responsive promoters in the p53-null cell line H1299. However, a number of significant differences were observed between p53 and p73 and even between p73 alpha and p73 beta. Additionally, a Saccharomyces cerevisiae-based reporter assay revealed a broad array of transcriptional transactivation abilities by both p73 isoforms at 37 degrees C. Recent data have shown that p73 can associate with p53 by the yeast two-hybrid assay. When we examined complex formation in transfected mammalian cells, we found that p73 alpha coprecipitates with. mutant but not wild-type p53. Since many tumor-derived p53 mutants are capable of inhibiting transactivation by wild-type p53, we tested the effects of two representative hot-spot mutants (R175H and R248W) on p73. By cotransfecting p73 alpha along with either p53 mutant and a p53-responsive reporter, we found that both R175H and R248W reduces the transcriptional activity of p73 alpha. This decrease in transcriptional activity is correlated with the reduced ability of p73 alpha to promote apoptosis in the presence of tumor-derived p53 mutants. Our data suggest the possibility that in some turner cells, an outcome of the expression of mutant p53 protein may be to interfere with the endogenous p73 protein.