A ROLE FOR NUCLEAR PHOSPHATIDYLINOSITOL-SPECIFIC PHOSPHOLIPASE-C IN THE G(2) M PHASE-TRANSITION/

Protein kinase C (PKC) is activated at the nucleus during the G(2) pha se of cell cycle, where it is required for mitosis, However, the mecha nisms controlling cell cycle-dependent activation of nuclear PKC are n ot known, We now report that nuclear levels of the major physiologic P KC activator diacylglycerol (DAG) fluctuate during cell cycle. Specifi cally, nuclear DAG levels in G(2)/M phase cells are 2.5-3-fold higher than in G(1) phase cells, In synchronized cells, nuclear DAG levels ri se to a peak coincident with the G(2)/M phase transition and return to basal levels in G(1) phase cells, This increase in DAG level is suffi cient to stimulate beta(II) PKC-mediated phosphorylation of its mitoti c nuclear envelope substrate lamin B in vitro, Isolated nuclei from G( 2) phase cells contain an active phospholipase activity capable of gen erating DAG in vitro, Nuclear phospholipase activity is inhibited by t he selective phosphatidylinositol-specific phospholipase C (PI-PLC) in hibitor -O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine and neomyci n sulfate, but not by the phosphatidylcholine-PLC selective inhibitor D609 or inhibitors of phospholipase D-mediated DAG generation, Treatme nt of synchronized cells with -O-octadeyl-2-O-methyl-sn-glycero-3-phos phocholine leads to decreased nuclear PI-PLC activity and cell cycle b lockade in the G(2) phase, suggesting a role for nuclear PI-PLC in the G(2)/M phase transition, Our data are consistent with the hypothesis that nuclear PI-PLC generates DAG to activate nuclear beta(II) PKC, wh ose activity is required for mitosis.