REGULATION OF SURFACE EXPRESSION OF HIGH-AFFINITY RECEPTORS FOR EPIDERMAL GROWTH-FACTOR (EGF) IN HEPATOCYTES BY HORMONES, DIFFERENTIATING AGENTS, AND PHORBOL ESTER

Freshly isolated adult rat hepatocytes exhibit a nonhomogeneous population of epidermal growth factor (EGF) receptors with about 10,000 high-affinity binding sites (K(d) 20 pM) and about 200,000 low-affinity sites (K(d) 600 pM) per cell. With culturing as primary monolayers under conditions where the cells show a marked increase in the sensitivity to the growth-stimulatory effect of EGF, a gradual reduction in the number of EGF receptors and an almost complete loss of high-affinity EGF receptors is seen. Insulin, which promotes growth of hepatocytes in concert with EGF, enhances the down-regulation of these high-affinity receptors. The differentiating (and growth-inhibitory) agent n-butyrate counteracts this down-regulation and preserves the high-affinity receptors. This effect of butyrate is synergistic with the glucocorticoid agent dexamethasone. Another differentiating agent, dimethylsulfoxide (DMSO), also counteracts the down-regulation of high-affinity EGF receptors. Moreover, the tumor promoter, tetradecanoylphorbol acetate (TPA), down-regulates the EGF receptor. This effect is particularly evident when studying the high-affinity receptors up-regulated by prior treatment with butyrate plus dexamethasone. Taken together these results provide strong support for the notion that an inverse relationship exists between expression of high-affinity EGF binding and responsiveness to growth activation by EGF.