Studies on the mechanism by which a tumor promoter inhibits binding of epidermal growth factor to cellular receptors

Abstract

This study analyzes the mechanism by which the tumor promoter 12-0-tetradecanoyl-phorbol 13-acetate (TPA) inhibits binding of epidermal growth factor (EGF) to its membrane receptors in HeLa cells. Kinetic studies indicate that inhibition of EGF binding occurs within a few minutes after exposure of cells to TPA; delayed addition of TPA causes a reduction in previously bound EGF. With prolonged exposure to TPA, the cells become refractory to TPA inhibition of EGF binding. Evidence was obtained that TPA acts by causing the dissociation of EGF-receptor complexes present on the cell surface and not by increasing the proteolytic degradation or internalization of EGF. Evidence that the TPA inhibition of EGF-receptor binding is not a direct consequence of TPA binding to the “active site” of EGF receptors was obtained by the differential effects of pH, temperature or exposure time and that TPA does not inhibit EGF binding when added to isolated plasma membrane fragments. Studies with a variety of inhibitors suggest that the TPA inhibition of EGF-receptor binding does not require macromolecular synthesis, energy metabolism, or cytoskeletal changes. Taken together, our results suggest that TPA inhibition of EGF-receptor binding results from TPA-induced changes in the membrane micro-environment of EGF receptors.