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, w hich are components of a nuclear phosphoinositide (PI) cycle. However, it i s not known whether the contents of these pools are regulated during cell p roliferation. In the present study we demonstrate that the mass levels of t hree important constituents of the nuclear PI cycle are regulated during th e cell cycle. Radioactive label incorporation into PtdIns(4,5)P-2 was seen to increase dramatically as synchronized cells entered S-phase. This did no t 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)P-2 production by T ype I PtdInsP kinases (PIPkins), were regulated during the cell cycle and i ndicated a complex relationship between these two lipids. An alternative su bstrate for PtdIns(4,5)P-2, PtdIns5P, phosphorylated by Type II PIPkins, wa s present in nuclei at much smaller amounts than the PtdIns4P, and thus is unlikely to contribute significantly to PtdIns(4,5)P-2 turnover. However, a large increase in nuclear PtdIns5P mass was observed when murine erythrole ukaemia cells are in G(1), and this could represent a potential pool of nuc lear inositol lipid that has a specific signalling role. Analysis of extrac ted lipid fractions indicated the absence of any PtdIns3P in these nuclei.