Involvement of p21 in the PKC-induced regulation of the G2/M cell cycle transition

Activation of protein kinase C (PKC) inhibits cell cycle progression at the G1/S and G2/M transitions. We found that phorbol 12-myristate 13-acetate ( PMA) induced upregulation of p21, not only in MCF-7 cells arrested in the G 1 phase as previously shown, but also in cells delayed in the G2 phase. Thi s increase in p21 in cells accumulated in the G1 and G2/M phases of the cel l cycle after PMA treatment was inhibited by the PKC inhibitor GF109203X. T his indicates that PKC activity is required for PMA-induced p21 upregulatio n and cell cycle arrest in the G1 and G2/M phases of the cell cycle. To fur ther assess the role of p21 in the PKC-induced G2/M cell cycle arrest indep endently of its G1 arrest, we used aphidicolin-synchronised MCF-7 cells. Ou r results show that, in parallel with the inhibition of cdc2 activity, PMA addition enhanced the associations between p21 and either cyclin B or cdc2. Furthermore, we found that after PMA treatment p21 was able to associate w ith the active Tyr-15 dephosphorylated form of cdc2, but this complex was d evoid of kinase activity indicating that p21 may play a role in inhibition of cdc2 induced by PMA. Taken together, these observations provide evidence that p21 is involved in integrating the PKC signaling pathway to the cell cycle machinery at the G2/M cell cycle checkpoint. (C) 1999 Federation of E uropean Biochemical Societies.