Effects of tumor-promoting agents 12-O-tetradecanoylphorbol-13-acetate and phenobarbital on DNA synthesis of rat hepatocytes in primary culture.

Both 12-O-tetradecanoylphorbol-13-acetate (TPA) and phenobarbital (PB) enhanced hepatocyte DNA synthesis stimulated with epidermal growth factor (EGF) by 60 to 80% in primary culture when measured by the incorporation of [3H]thymidine. This apparent increase was not due to changes in the specific activity of the deoxythymidine triphosphate (dTTP) pool. TPA enhanced DNA synthesis even at relatively high cell densities, but this was not found with the PB treatment. Although both TPA and PB enhanced DNA synthesis significantly, TPA was most effective when added during the late G1 and/or S phase of the hepatocyte cell cycle, whereas PB treatment was least effective in this period. The binding of EGF was transiently down-regulated by TPA, then restored to control values 6 h later, whereas the binding of this factor was significantly increased at both the 12th and 24th h after PB addition. These results suggest that EGF binding to hepatocytes is not correlated with the enhancement of DNA synthesis by TPA or PB and that the down-regulation of EGF binding is not causally related to the enhancement of DNA synthesis by TPA.