TRANSCRIPTIONAL SQUELCHING BY ECTOPIC EXPRESSION OF E2F-1 AND P53 IS ALLEVIATED BY PROTEASOME INHIBITORS MG-132 AND LACTACYSTIN

The transcription factors p53 and E2F-1 play important roles in the co ntrol of cell cycle progression. In transient transfection experiments , expression of E2F-1, other E2F family members, or p53 squelched tran scription from cotransfected plasmids in a dose-dependent manner, Alth ough the proteasome inhibitors MG-132 and lactacystin markedly increas ed the level of expression of E2F-1 and p53, these inhibitors complete ly alleviated squelching by both proteins, Several observations indica te MG-132 alleviates squelching by influencing the conformation of new ly synthesized p53 and E2F-1 MG-132 increased the fraction of wild typ e p53 bound by a monoclonal antibody which preferentially recognizes m utant conformers of p53, increased binding of hsp70 to p53 and inhibit ed nuclear accumulation of both p53 and E2F-1, but not the pocket prot ein p107, The protease inhibitors ALLN and ALLM did not influence expr ession of E2F-1 or p53, nor did they alleviate squelching by either tr anscription factor, Because MG-132 and lactacycstin are highly specifi c inhibitors of the proteasome protease, our results suggest that the proteasome influences post-translational processes involved in proper folding and cytoplasmic clearing of E2F-1 and p53.