Protein kinase C-mediated feed back inhibition of the Ca2+ response at the EGF receptor

Activation of the EGF receptor in A431 cells induces the hydrolysis of phosphoinositides and a transient rise of the cytosolic Ca²⁺ concentration, [Ca²⁺], which are completely inhibited by acute pretreatment with activators of protein kinase C, such as phorbol esters. Down regulation of the enzyme (by long-term pretreatment of the cells with phorbol esters) causes the [Ca²⁺]_i response to EGF to increase in magnitude and, especially, to become much more persistent (average t _{1 2} of [Ca²⁺]_i decline 9 min with respect to 2.3 min in controls). These results demonstrate that the activation of protein kinase C induced by EGF in intact A431 cells is sufficient to trigger a feed back, autolimitative regulation of the EGF receptor that might play a prominent physiological role in the definition of the mitogenic activity of the growth factor.