RB AND C-MYC ACTIVATE EXPRESSION OF THE E-CADHERIN GENE IN EPITHELIAL-CELLS THROUGH INTERACTION WITH TRANSCRIPTION FACTOR AP-2

E-cadherin plays a pivotal role in the biogenesis of the first epithel ium during development, and its down-regulation is associated with met astasis of carcinomas. We recently reported that inactivation of RE fa mily proteins by simian virus 40 large T antigen (LT) in MDCK epitheli al cells results in a mesenchymal conversion associated with invasiven ess and a down-regulation of c-Myc. Reexpression of RE or c-Myc in suc h cells allows the reexpression of epithelial markers including E-cadh erin. Here we show that both RE and c-Myc specifically activate transc ription of the E-cadherin promoter in epithelial cells but not in NIH 3T3 mesenchymal cells. This transcriptional activity is mediated in bo th cases by the transcription factor AP-2. In vitro AP-2 and RE intera ction involves the N-terminal domain of AP-2 and the oncoprotein bindi ng domain and C-terminal domain of RE. In vivo physical interaction be tween RE and AP-2 was demonstrated in MDCK and HaCat cells. In LT-tran sformed MDCK cells, LT, RE, and AP-2 were all coimmunoprecipitated by each of the corresponding antibodies, and a mutation of the RE binding domain of the oncoprotein inhibited its binding to both RE and AP-2. Taken together, our results suggest that there is a tripartite complex between LT, RE, and AP-2 and that the physical and functional interac tions between LT and AP-2 are mediated by RE. Moreover, they define RE and c-Myc as coactivators of AP-2 in epithelial cells and shed new li ght on the significance of the LT-RB complex, linking it to the dediff erentiation processes occurring during tumor progression. These data c onfirm the important role for RE and c-Myc in the maintenance of the e pithelial phenotype and reveal a novel mechanism of gene activation by c-Myc.