Involvement of MAP kinase-independent protein kinase C signaling pathway in the EGF-induced p21(WAF1/Cip1) expression and growth inhibition of A431 cells

Previous studies have revealed that the growth inhibition of A431 cells overexpressing epidermal growth factor (EGF) receptors by a high concentration of EGF is mainly due to the expression of cycline dependent kinase (CDK) inhibitor p21(WAF1/Cip1). However, the signal transduction mechanism from the activated EGF receptor to the induction of p21(WAF1/Cip1) gene is still poorly understood. We investigated which signaling pathway plays an important role in p21(WAF1/Cip1) expression and growth inhibition by using specific inhibitors of the signaling molecules. A broad PKC inhibitor, PKC delta inhibitor, but not the conventional PKC inhibitor suppressed the EGF-induced p21(WAF1/Cip1) expression and the growth inhibition of A431 cells. These inhibitors did not alter either the activation of EGF receptor or the stimulation of MAP kinase at detectable levels. Furthermore, we found that the induction of p21(WAF1/Cip1) at the early phase (within 12 hr after stimulation) by a high concentration of EGF was independent of the MAP kinase activation by using dominant negative Ras. These results suggest that PKC, especially PKC delta plays a crucial role in the EGF-induced p21(WAF1/Cip1) expression, resulting in the growth inhibition of A431 cells.