Expression of an oncogenic rasHa gene in murine keratinocytes induces tyrosine phosphorylation and reduced activity of protein kinase C delta.

Murine keratinocytes expressing an oncogenic rasHa gene produce benign tumors in vivo and demonstrate altered responses to phorbol esters in vitro. Cultured keratinocytes transduced with the v-rasHa gene (v-rasHa keratinocytes) are resistant to Ca(2+)-induced terminal differentiation, a process that is dependent on protein kinase C (PKC) activation in normal keratinocytes. Five PKC isoforms expressed in keratinocytes (alpha, delta, epsilon, zeta, and eta) were examined for quantitative or qualitative changes in v-rasHa-transformed cells. No quantitative changes were detected, but PKC delta was tyrosine-phosphorylated in v-rasHa keratinocytes and in benign neoplastic keratinocyte cell lines expressing an activated allele of the c-rasHa gene. Analysis of phosphorylated and non-phosphorylated forms of PKC delta from keratinocytes indicated that phosphorylated PKC delta was not stimulated by phorbol ester treatment. The protein kinase inhibitor staurosporine was able to induce differentiation in v-rasHa keratinocytes and benign tumor cell lines, and concomitantly tyrosine phosphorylation of PKC delta decreased. This interaction between tyrosine kinases and PKC delta in cells expressing an oncogenic rasHa gene may represent a molecular block to differentiation in neoplastic keratinocytes.