Induction of apoptosis in human esophageal cancer cells by sequential transfer of the wild-type p53 and E2F-1 genes: Involvement of p53 accumulation via ARF-mediated MDM2 down-regulation

Transcriptional factor E2F-1 as well as tumor suppressor p53 have been show n to cause apoptosis independently in some types of human cancer cells when overexpressed, Here we report that sequential transfer of the wild-type p5 3 and E2F-1 genes efficiently induces apoptosis in human esophageal cancer cells and that E2F-1 overexpression directly activates expression of p14 (A RF), which inhibits MDM2-mediated p53 degradation, resulting in the stabili zation of p53. Infection of human esophageal cancer cell lines T.Tn and TE8 with adenovirus vector-expressing E2F-1 (Ad-E2F-1) enhanced mRNA and prote in expression of ARF and decreased MDM2 protein expression. Transfection of ARF plasmid decreased MDM2 protein expression, which in turn increased p53 protein expression. Infection of T.Tn and TE8 cells first with adenovirus- expressing wild-type p53 (Ad-p53) and then with Ad-E2F-1 resulted in rapid induction of apoptosis; in contrast, simultaneous infection with Ad-E2F-1 a nd Ad-p53 had no significant antitumor effect. As shown by Western blot ana lysis, infection with suboptimal concentrations of Ad-E2F-1 induced the acc umulation of exogenous p53 transduced by suboptimal concentrations of Ad-p5 3, Moreover, Ad-E2F-1-mediated ARF expression inhibited the up-regulation o f MDM2 by overexpressed p53 in TE8 cells. Thus, overexpression of ectopic E 2F-1 protein may stabilize endogenous as well as ectopic p53 protein via th e E2F-1/ARF/MDM2/p53 regulatory pathway and, in this way, render cells more sensitive to apoptosis, an outcome that has important implications for the treatment of human esophageal cancers.