Nuclear phosphatidylinositols decrease during S-phase of the cell cycle in HeLa cells.

In the current study we have measured phosphatidylinositols during the cell cycle. HeLa cells were labeled with [3H]myoinositol to a steady state, synchronized to the G1/S boundary, and the levels of phosphatidylinositol (PtdIns) lipids were measured at various times after release from the block. The levels of total cellular PtdIns, PtdIns(4)P, and PtdIns(4,5)P2 relative to total cellular phospholipid did not vary throughout the cell cycle. We then isolated nuclei from synchronized cells using a non-detergent method and found that the levels of nuclear PtdIns lipids decreased by over 50% at 2 and 4 h after release from the G1/S boundary (S-phase of the cell cycle) and returned to the original levels by 9 h. Separation of individual inositol-containing nuclear lipids showed that PtdIns decreased by 50% while levels of PtdIns(4)P and PtdIns(4,5)P2 decreased by 66%. Levels of the cytoplasmic PtdIns lipids remained constant throughout this period. This experiment indicates that there is specific nuclear. PtdIns turnover that is activated during DNA synthesis.