Transient temporal relationship between 1-oleoyl-2-acetyl-sn-glycerol (OAG)-activated synthesis and hydrolysis of polyphosphoinositides: desensitization of phospholipase C and the inositol lipid kinases upon long-term treatment of ascites cells by exogenous OAG.

In ascites tumor cells, phosphoinositide metabolism can be activated by short-term treatment with exogenously added 1-oleoyl-2-acetyl-sn-glycerol (OAG), which is the membrane-permeable analog of diacylglycerides (DAG). Quiescent cells prelabeled with D-myo-2-[3H]inositol and then stimulated with OAG (20 micrograms/ml of medium) reveal transient increases in the liberation of inositol 1,4-bis- and inositol 1,4,5-trisphosphate with peaks at 30 min, and a sustained accumulation of inositol phosphate 30 min after stimulation. The labeling patterns of the corresponding inositol lipids show transient activity profiles for phosphatidylinositol 4-phosphate (PtdIns(4)P) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), and a sustained high activity level for PtdIns 30 min after OAG treatment. These data demonstrate a temporal relationship between synthesis and phospholipase C (PLC)-induced hydrolysis of these lipids. Simultaneous labeling of the cellular inositol phospholipids with [1-14C]arachidonic acid reveals modest accumulations after OAG stimulation. The relative 3H radioactivity distribution between the lipids and their inositol metabolites show that about 10% of the polyphosphoinositide pools are metabolically active. Long-term culturing of the cells (> 24 h) under OAG supplementation produces significant reductions in the catalytic activities of PLC and the PtdIns and PtdIns(4)P-specific kinases which is paralleled by a reduced radioactive labeling of PtdIns(4)P and PtdIns(4,5)P2 under these conditions. These data suggest that diglycerides affect the phosphoinositide metabolism by controlling PLC and phosphoinositide kinase activities probably via modification of membrane properties, and by functioning as modulator of other events.