EGF receptor affinity is regulated by intracellular calcium and protein kinase C

Phorbol esters specifically reduce the binding of epidermal growth factor to surface receptors in intact cells, but not when added directly to isolated membranes. We show that after treatment of intact cells with phorbol myristate acetate, 125I-EGF binding is reduced in membranes prepared subsequently. High-affinity binding of 125I-EGF is modulated by an intracellular calcium-dependent regulatory process. Preventing calcium entry with EGTA or enhancing intracellular calcium with A23187 in intact cells modulates EGF receptor affinity in membranes isolated subsequently. Also, EGTA attenuates the usual inhibition of EGF binding caused by phorbol esters. Membrane preparations do not respond to phorbol ester treatment because the calcium- and phospholipid-dependent protein kinase C is removed or inactivated during membrane isolation. Reconstitution of unresponsive membranes with purified C kinase alters phosphorylation of the EGF receptor and restores the inhibitory effect of phorbol esters on 125I-EGF binding previously observed only in intact cells. Thus, activation of the Ca++-dependent enzyme, C kinase, modulates EGF receptor affinity, possibly via altered receptor phosphorylation.