EGF-INDUCED AND CPA-INDUCED MITOGENIC STIMULI ARE DIFFERENTIALLY DOWN-REGULATED BY TGF-BETA-1 IN CULTURED RAT HEPATOCYTES

Down-regulation of the mitogenic activity of the rodent liver carcinog en cyproterone acetate (CPA) and of epidermal growth factor (EGF) were compared in cultured rat hepatocytes. Both hepatomitogens produce an increase in the expression of proliferating cell nuclear antigen (PCNA ) and in [H-3]thymidine incorporation in a dose-dependent manner, In c ombination, the two mitogens induced an additive mitogenic response. C oncomitant exposure to the growth inhibitory cytokine transforming gro wth factor beta 1 (TGF-beta 1) resulted in a differential dose-depende nt downregulation of PCNA-expressing cells, The corresponding down-reg ulation of CPA-induced PCNA expression required a 3- to 5-fold higher TGF-beta 1 concentration than for EGF-induced expression, In contrast, CPA-exposed hepatocytes become vulnerable to and EGF-exposed cells pr otected against the apoptosis-inducing activity of TGF-beta 1 (>0.1 ng /ml). Under culture conditions that mimicked a pericentral-equivalent microenvironment (low oxygen tension, low glucagon concentration), PCN A expression was 3-fold lower and CPA-specific resistance was no longe r detectable, It is concluded that EGF and CPA induce their growth sti muli preferentially in the periportal area of the liver but in differe nt hepatocyte sub-populations, which differ in their down-regulation o f premitotic events by TGF-beta 1. At low TGF-beta 1 concentrations, E GF-stimulated cells shift back into a resting cell cycle phase, wherea s CPA-treated hepatocytes are eliminated by apoptosis at higher TGF-be ta 1 concentrations.