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The activity of nuclear phosphoinositide 3-kinase C2L L (PI3K-C2L L) was investigated in HL-60 cells induced to di¡er-entiate along granulocytic or monocytic lineages. A signi¢cant increase in the activity of immunoprecipitated PI3K-C2L L was observed in the nuclei and nuclear envelopes isolated from alltrans-retinoic acid (ATRA)-di¡erentiated cells which was inhibited by the presence of PI3K inhibitor LY 294002. High-performance liquid chromatography analysis of inositol lipids showed an increased incorporation of radiolabelled phosphate in both PtdIns(3)P and PtdIns(3,4,5)P 3 with no changes in the levels of PtdIns(4)P, PtdIns(3,4)P 2 and PtdIns(4,5)P 2 . Western blot analysis of the PI3K-C2L L immunoprecipitates with anti-P-Tyr antibody revealed a signi¢cant increase in the level of the immunoreactive band corresponding to PI3K-C2L L in the nuclei and nuclear envelopes isolated from ATRA-di¡erentiated cells.

Phospholipase C (PLC) was purified from the membrane-depleted rat liver nuclei. About 60% of the total PLCactivity corresponded to b 1b isoform, 30% to PLC-c 1 and less than 10% to PLC-d 1 . PLC-b 1b and -c 1 were found in the nuclear matrix, while PLC-d 1 was detected in the chromatin. Two peaks of an increase in the total PLC-activity were detected occurring at 6 and 20 h after partial hepatectomy. An early increase in PLC-b 1b activity in the nuclear matrix was associated with serine phosphorylation of the enzyme, while the later increase paralleled the increase in the amount of protein. The increase in the PLC-c 1 activity measured at 6 and 20 h after partial hepatectomy was associated with tyrosine phosphorylation of the enzyme. The activity of PLC-d 1 and the amount of the protein found in the chromatin was increased only at 20 h after partial hepatectomy.

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