Independent mechanisms for tumor promoters phenobarbital and 12-O-tetradecanoylphorbol-13-acetate in reduction of epidermal growth factor binding by rat hepatocytes.

Primary cultures of hepatocytes derived from adult Fischer 344 rats were used to test for effects of the liver tumor promoter phenobarbital on several components of the epidermal growth factor (EGF) receptor signal transduction pathway. Phenobarbital had no effect on the binding of 125I-labeled EGF by its hepatocyte receptor at 4 degrees C or on EGF-induced receptor down-regulation. However, pretreatment of hepatocytes with phenobarbital (3 mM) at 37 degrees C caused inhibition of subsequent 125I-labeled EGF binding. This response temporally resembled that of hepatocytes to 12-O-tetradecanoylphorbol-13-acetate (TPA) in that maximal inhibition occurred after 1 h of pretreatment but was reversed after longer pretreatment times. The inhibitory effects of phenobarbital and TPA on EGF binding were additive, suggesting that distinct mechanisms mediated the responses to these two tumor promoters. In addition, treatment with TPA, but not phenobarbital, caused a redistribution of the activity of Ca2+/phospholipid-dependent protein kinase C. In untreated and phenobarbital-treated hepatocytes, 20% of protein kinase C activity was isolated with a membranous fraction, while 75% of the activity was membrane associated in TPA-treated hepatocytes. These results demonstrate that phenobarbital, like TPA and other tumor promoters, can modulate the EGF receptor system but suggest that it does so without directly competing with EGF for binding to its receptor or by activating protein kinase C.