Nuclear phosphoinositide 3-kinase C2beta activation during G2/M phase of the cell cycle in HL-60 cells

The activity of nuclear phosphoinositide 3-kinase C2beta (PI3K-C2beta) was investigated in HL-60 cells blocked by aphidicolin at G(1)/S boundary and allowed to progress synchronously through the cell cycle. The activity of immunoprecipitated PI3K-C2beta in the nuclei and nuclear envelopes showed peak activity at 8 h after release from the G(1)/S block, which correlates with G(2)/M phase of the cell cycle. In the nuclei and nuclear envelopes isolated from HL-60 cells at 8 h after release from G(1)/S block, a significant increase in the level of incorporation of radiolabeled phosphate into phosphatidylinositol 3-phosphate (PtdIns(3)P) was observed with no change in the level of radiolabeled PtdIns(4)P, PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3). On Western blots, PI3K-C2beta revealed a single immunoreactive band of 180 kDa, whereas in the nuclei and nuclear envelopes isolated at 8 h after release, the gel shift of 18 kDa was observed. When nuclear envelopes were treated for 20 min with mu-calpain in vitro, the similar gel shift and increase in PI3K-C2beta activity was observed which was completely inhibited by pretreatment with calpain inhibitor calpeptin. The presence of PI3K inhibitor LY 294002 completely abolished the calpain-mediated increase in the activity of PI3K-C2beta but did not prevent the gel shift. When HL-60 cells were released from G(1)/S block in the presence of either calpeptin or LY 294002, the activation of nuclear PI3K-C2beta was completely inhibited. These results demonstrate the calpain-mediated activation of the nuclear PI3K-C2beta during G(2)/M phase of the cell cycle in HL-60 cells.