Two waves of the nuclear phospholipase C activity in serum-stimulated HL-60 cells during G1 phase of the cell cycle

Phosphatidylinositol-specific phospholipase C (PI-PLC) is activated in cell nuclei during the cell cycle progression. We have previously demonstrated two peaks of an increase in the nuclear PI-PLC activities in nocodazole-synchronized HL-60 cells. In this study, the activity of nuclear PI-PLC was investigated in serum-stimulated HL-60 cells. In serum-starved HL-60 cells, two peaks of the activity of nuclear PI-PLC were detected at 30 min and 11 h after the re-addition of serum with no parallel increase in PLC activity in cytosol, postnuclear membranes or total cell lysates. An increase in the serine phosphorylation of b splicing variant of PI-PLCβ1 was detected with no change in the amount of PI-PLCβ1b in nuclei isolated at 30 min and 11 h after the addition of serum. PI-PLC inhibitor ET-18-OCH3 and MEK inhibitor PD 98059 completely abolished serum-mediated increase at both time-points. The addition of inhibitors either immediately or 6 h after the addition of serum had inhibitory effects on the number of cells entering S phase. These results demonstrate that two waves of nuclear PI-PLCβ1b activity occur in serum-stimulated cells during G1 phase of the cell cycle and that the later increase in the PLC activity is equally important for the progression into the S phase.