Differential Effects of Overexpression of PKCα and PKCδ/ϵ on Cellular E2F Activity in Late G1 Phase

Abstract

Introduction of a reporter gene containing E2F binding sites linked to the luciferase gene permitted us to detect transient cellular E2F activity in late G1 phase rat 3Y1 fibroblasts. Overexpression of three major protein kinase C (PKC) isozymes expressed in 3Y1 cells caused differing effects on E2F activity depending on the isozymes overexpressed. Overexpression of PKCα inhibited E2F activity while the overexpression of PKCδ or PKCϵ enhanced it, suggesting that these PKC isozymes play different roles in the regulation of E2F activity. Consistent with previous findings that the activation of PKC by TPA in late G1 phase results in the inhibition of DNA synthesis (Huang, C., and Ives, H.E., 1987,Nature329,849–850), the addition of TPA in late G1 phase specifically inhibited E2F activity. Overexpression of PKC isozymes resulted in an enhancement of the TPA-induced inhibition of E2F in late G1 phase. This enhancement was observed for all three PKC isozymes examined, suggesting that these PKC isozymes all are potent mediators of the TPA-induced inhibition of E2F activity in late G1 phase.