Platelet-derived growth factor modulates epidermal growth factor receptors by a mechanism distinct from that of phorbol esters.

Platelet-derived growth factor (PDGF) and phorbol 12-myristate 13-acetate (PMA) decrease high affinity binding of 125I-labeled epidermal growth factor (EGF) and potentiate mitogenesis in BALB/c 3T3 cells, and both have been shown to induce the phosphorylation of the EGF receptor at threonine residues. These similarities suggest that the actions of PDGF on EGF binding may be mediated by protein kinase C, the cellular effector of PMA. We show that in density-arrested BALB/c 3T3 cells PDGF and PMA induce a rapid, transient, cycloheximide-independent loss of EGF binding activity. As has been previously shown for PDGF, the ability of PMA to reduce EGF binding was enhanced by cholera toxin, a potent activator of adenylate cyclase. In contrast to PMA, however, PDGF induced a further reduction in EGF binding that was strictly dependent upon continued protein synthesis. Furthermore, PDGF effectively reduced EGF binding in cells refractory to PMA. Cells desensitized to PMA, presumably due to the loss of protein kinase C activity, also remained mitogenically responsive to PDGF. These data suggest that the mechanism by which PDGF modulates EGF binding differs from that of PMA and thus, at least in part, is independent of protein kinase C.