EXPRESSION OF AN ONCOGENIC RAS(HA) GENE IN MURINE KERATINOCYTES INDUCES TYROSINE PHOSPHORYLATION AND REDUCED ACTIVITY OF PROTEIN-KINASE-C DELTA

Murine keratinocytes expressing an oncogenic ras(Ha) gene produce beni gn tumors in vivo and demonstrate altered responses to phorbol esters in vitro. Cultured keratinocytes transduced with the v-ras(Ha) gene (v -ras(Ha) keratinocytes) are resistant to Ca2+-induced terminal differe ntiation, a process that is dependent on protein kinase C (PKC) activa tion in normal keratinocytes. Five PKC isoforms expressed in keratinoc ytes (alpha, delta, epsilon, zeta, and eta) were examined for quantita tive or qualitative changes in v-ras(Ha)-transformed cells. No quantit ative changes were detected, but PKC delta was tyrosine-phosphorylated in v-ras(Ha) keratinocytes and in benign neoplastic keratinocyte cell lines expressing an activated allele of the c-ras(Ha) gene. Analysis of phosphorylated and non-phosphorylated forms of PKC delta from kerat inocytes indicated that phosphorylated PKC delta was not stimulated by phorbol ester treatment. The protein kinase inhibitor staurosporine w as able to induce differentiation in v-ras(Ha) keratinocytes and benig n tumor cell lines, and concomitantly tyrosine phosphorylation of PKC delta decreased. This interaction between tyrosine kinases and PKC del ta in cells expressing an oncogenic ras(Ha) gene may represent a molec ular block to differentiation in neoplastic keratinocytes.