The role of RYBP in the regulation of apoptosis

Abstract

The tumor suppressor Tp53 is the most frequently mutated gene in human cancer. Tp53 encodes a sequence specific transcription factor termed p53 that activates a number of biological programs contributing to tumor suppression, most notably, the pro~qtion of cell cycle arrest and apoptosis. ·To identify new regulators of p5 3 's transcriptional activity, we performed a yeast 2-hyrbid screen and have identified Ring 1 YYl Binding Protein (Rybp) as a novel p53-interacting partner. Consistent with· its role as a transcriptional repressor, we have demonstrated that Rybp inhibits p53-mediated transcription. In addition, Rybp forms a trimeric complex with the critical negative regulator of p53, Mdm2. Mdm2 is an E3 ligase that ubiquitinates p53, targeting it for degradation, and expression of Rybp enhances the Mdm2-mediated ubiquitination. To further investigate the role of Rybp in the regulation of endogenous p53 stability we constructed a recombinant adenovirus expressing Rybp (Ad-Rybp ). Ad-Rybp infection inhibited the accumulation of p53 and the induction of p53 target genes in response to genotoxic stress. However, interpretation of the results was confounded by Ad-Rybp infection reducing global mRNA levels. Despite inhibition of p53; Ad-Rybp was a powerful inducer of apoptosis, and we investigated this in more detail. Analysis of a panel of tumor cell and untransformed cell types revealed that Ad-Rybp infection specifically induces apoptosis in tumor cells but not in normal diploid cells. Furthermore, at a low multiplicity of infection, Ad-Rybp sensitizes tumor cells to apoptosis in the presence of the death receptor ligands, Tumor Necrosis Factor alpha (TNFa) and TNF related apoptosis inducing ligand (TRAIL). These results suggest that the tumor-specific killing properties of Rybp may be exploited for therapeutic advantage.