Inositol lipids are regulated during cell cycle progression in the nuclei of murine erythroleukaemia cells

Previous data suggest the existence of discrete pools of inositol lipids, which are components of a nuclear phosphoinositide (PI) cycle. However, it is not known whether the contents of these pools are regulated during cell proliferation. In the present study we demonstrate that the mass levels of three important constituents of the nuclear PI cycle are regulated during the cell cycle. Radioactive label incorporation into PtdIns(4,5)P₂ was seen to increase dramatically as synchronized cells entered S-phase. This did not coincide with any significant changes in the nuclear mass levels of this lipid, suggesting that the rate of turnover of this molecule was increased. Levels of PtdIns4P, the major substrate for PtdIns(4,5)P₂ production by Type I PtdInsP kinases (PIPkins), were regulated during the cell cycle and indicated a complex relationship between these two lipids. An alternative substrate for PtdIns(4,5)P₂, PtdIns5P, phosphorylated by Type II PIPkins, was present in nuclei at much smaller amounts than the PtdIns4P, and thus is unlikely to contribute significantly to PtdIns(4,5)P₂ turnover. However, a large increase in nuclear PtdIns5P mass was observed when murine erythroleukaemia cells are in G₁, and this could represent a potential pool of nuclear inositol lipid that has a specific signalling role. Analysis of extracted lipid fractions indicated the absence of any PtdIns3P in these nuclei.