Phenobarbital reduces EGF receptors and the ability of physiological concentrations of calcium to suppress hepatocyte proliferation.

Optimal proliferation of cultured hepatocytes from normal rats occurs in response to epidermal growth factor (EGF) at an extracellular calcium concentration of 0.4 mM, whereas physiological concentrations of calcium have been shown to decrease hepatocyte proliferation. Exposure of hepatocytes in vivo to phenobarbital (PB, 0.1% in the drinking water) reduced significantly the ability of physiological levels of calcium to suppress hepatocyte proliferation. An increased relative ability of hepatocytes to proliferate at physiological calcium concentrations versus that at 0.4 mM calcium was first seen after 3 days of in vivo PB treatment and this effect was maintained during 2 months of exposure. Hepatocytes from short-term PB-exposed animals (i.e. 3-28 days) proliferated at physiological Ca2+ concentrations 2-3 times better than those from control animals. However, after 2 months of continuous PB exposure, cell growth was reduced significantly at all extracellular Ca2+ concentrations investigated. EGF binding studies demonstrated that the biphasic kinetic effect of PB on hepatocyte proliferation was not correlated directly with the steady decline in EGF receptor number or the lack of significant change in receptor binding affinity with duration of PB exposure. These results imply that the effectiveness of PB in tumour promotion may result from its ability to reduce the absolute magnitude of normal hepatocyte proliferation and to alter the growth regulatory effect of extracellular calcium. Further, the results argue that PB effects on hepatocyte proliferation are not mediated simply through regulation of EGF surface receptor number or binding affinity. Additional events of the EGF-induced cascade necessary for hepatocyte proliferation appear to be modified by PB.