Dysregulation of apoptosis by c-myc in transgenic hepatocytes and effects of growth factors and nongenotoxic carcinogens

Regulation of apoptosis is an important component of multistage hepatocarci nogenesis, The proto-oncogene c-myc has been shown to be important in apopt osis regulation and to be amplified and overexpressed in human and rodent l iver neoplasia. The objectives of the study reported here were to determine whether apoptosis regulation is altered in transgenic hepatocytes that ove rexpress c-myc and whether growth factors or nongenotoxic carcinogens alter apoptosis regulation in c-myc Versus wild-type hepatocytes. Hepatocytes is olated from c-myc transgenic mice had four fold more c-myc RNA and protein (at 12-48 h) in addition to increased apoptosis levels compared with wild-t ype hepatocytes. The increased apoptosis in c-myc hepatocytes was accompani ed by increased p53, bar, and bak and decreased bcl-2 protein levels. Hepat ocytes overexpressing c-myc were more sensitive to apoptosis induced by ble omycin but less sensitive to apoptosis induced by transforming growth facto r (TGF)-beta. Phenobarbital, a potent liver tumor promoter, inhibited apopt osis in c-myc hepatocytes but not in wild-type hepatocytes, decreased p53 a nd bar, and increased bcl-2 protein levels. Nafenopin inhibited apoptosis i n both c-myc and wild-type hepatocytes, whereas 2,3,7,8-tetrachlorodibenzo- p-dioxin did not inhibit apoptosis in either wild-type or c-myc hepatocytes . TGF-alpha inhibited apoptosis and increased bcl-X-L and decreased bak pro tein levels in c-myc hepatocytes but not in wild-type hepatocytes. Insulin- like growth factor-it did not affect apoptosis in c-myc or wild-type hepato cytes. In this study, overexpression of c-myc altered the response to apopt otic stimuli in transgenic hepatocytes. Furthermore, phenobarbital and TGF- alpha inhibited c-myc-induced apoptosis, which may have resulted in a selec tive growth advantage for an initiated cell population and which may be a m echanism for tumor promotion. (C) 1999 Wiley-Liss, Inc.