DIFFERENTIAL-EFFECTS OF OVEREXPRESSION OF PKC-ALPHA AND PKC-DELTA EPSILON ON CELLULAR E2F ACTIVITY IN LATE G1 PHASE/

Introduction of a reporter gene containing E2F binding sites linked to the luciferase gene permitted us to detect transient cellular E2F act ivity in late G1 phase rat 3Y1 fibroblasts. Overexpression of three ma jor protein kinase C (PKC) isozymes expressed in 3Y1 cells caused diff ering effects on E2F activity depending on the isozymes overexpressed. Overexpression of PKC alpha inhibited E2F activity while the overexpr ession of PKC delta or PKC epsilon enhanced it, suggesting that these PKC isozymes play different roles in the regulation of E2F activity. C onsistent with previous findings that the activation of PKC by TPA in late G1 phase results in the inhibition of DNA synthesis (Huang, C., a nd Ives, H.E., 1987, Nature 329, 849-850), the addition of TPA in late G1 phase specifically inhibited E2F activity. Overexpression of PKC i sozymes resulted in an enhancement of the TPA-induced inhibition of E2 F in late G1 phase. This enhancement was observed for all three PKC is ozymes examined, suggesting that these PKC isozymes all are potent med iators of the TPA-induced inhibition of E2F activity in late G1 phase. (C) 1996 Academic Press, Inc.