Nuclei contain two differentially regulated pools of diacylglycerol

A number of recent studies have highlighted the presence of a nuclear pool of inositol lipids [1,2] that is regulated during progression through the c ell cycle [1,3], differentiation [1,2] and after DNA damage [2], suggesting that a number of different regulatory pathways impinge upon this pool of l ipids. It has been suggested that the downstream consequence of the activat ion of one of these nuclear phosphoinositide (PI) regulatory pathways is th e generation of nuclear diacylglycerol (DAG) [1,3,4], which is important in the activation of nuclear protein kinase C (PKC) [5-7]. Activation of PKC in turn appears to regulate the progression of cells through G1 and into S phase [4] and through G2 to mitosis [3,8-11]. Although the evidence is enti cing, there is as yet no direct demonstration that nuclear PIs can be hydro lysed to generate nuclear DAG. Previous data in murine erythroleukemia (MEL ) cells have suggested that nuclear phosphoinositidase C beta 1 (PIC-beta 1 ) activity is important in the generation of nuclear DAG. Here, we demonstr ate that the molecular species of nuclear DAG bears little resemblance to t he PI pool and is unlikely to be generated directly by hydrolysis of these inositol lipids. Further, we show that there are in fact two distinct subnu clear pools of DAG; one that is highly disaturated and mono-unsaturated (re presenting more than 90% of the total nuclear DAG) and one that is highly p olyunsaturated and is likely to be derived from the hydrolysis of PI. Analy sis of these pools, either after differentiation or during cell-cycle progr ession, suggests that the pools are independently regulated, possibly by th e regulation of two different nuclear phospholipase Cs (PLCs).