Membrane protein kinase C activity rapidly increases in quiescent tsRSV-infected NRK cells upon reactivation of the mitogenic v-src protein kinase

The viral src protein kinase, pp60^v-src, is a powerful intracellular mitogen which can initiate and maintain the proliferation of quiescent cells in the absence of any exogenous growth factors. In an attempt to understand how pp60^v-src induces proliferation, we examined the early events in the G₀ to G₁ transition caused by the activation of a thermolabile v-src protein in quiescent, serum-starved tsRSV-transformed NRK cells. The reactivation of pp60^v-src, in the presence of exogenous growth factors, triggered a rapid biphasic surge of membrane-associated protein kinase C (PKC) activity. Unlike TPA-stimulated PKC activity, the pp60^v-src-induced increase in PKC was readily extracted from membranes by EGTA. The down-regulation of PKC activity in these quiescent cells by prolonged exposure to TPA strongly inhibited the ability of the reactivated v-src protein to stimulate DNA replication in serum-deficient medium, suggesting that PKC plays a role in the initial signal by which the viral enzyme induces the G₀ to G₁ transition in NRK cells.