Metabolic fate of plasma membrane diacylglycerols in NIH 3T3 fibroblasts.

We have examined the metabolism of three radiolabeled 1,2-diacylglycerols (DGs) in NIH 3T3 fibroblasts. Since the lipids used are not appreciably taken up by the cells, we used a phosphatidylserine (PS)-based liposome fusion system to rapidly associate the lipid species with the plasma membrane. When 1,2-[1-14C]dioleoyl-sn-3-glycerol ([14C]DOG) is delivered in this way, it is rapidly converted predominantly to phosphatidylcholine (PC) and triacylglycerol (TG) and to a lesser extent, to monoacylglycerol (MG) and fatty acids (FA), as well as phosphatidic acid (PA) and phosphatidylinositol (PI). We present evidence that [14C] DOG is largely utilized as an intact molecule rather than being broken down to FA and then incorporated to cell lipids. Examination of the metabolism of 1-stearoyl-2-[1-14C]myristoyl-sn-3-glycerol ([14C]SMG) and 1-stearoyl-2-arachidonoyl-sn-3-glycerol ([14C]SAG) reveal important differences. Both produce substantial labeling of PC but [14C]SMG gives rise to the highest proportion of TG and the lowest of PA and PI, whereas [14C]SAG yields the opposite pattern. When phosphatidic acid labeled on its glycerol backbone (1,2-dioleoyl-sn-[U-14C] glycero-3-phosphate) was supplied to the cells via the liposomes, rapid appearance of labeled DG was found which then decreased with concomitant labeling of cellular PC and TG. Only small amounts of the glycerol backbone were recovered in PI. Our experiments identify three types of processes involved in the metabolism of plasma membrane DGs: (i) transferase-catalyzed conversions to PC and TG, (ii) lipolytic breakdown to MG and FA, and (iii) phosphorylation to PA and then conversion to PI. The relative proportions of each DG species converted to these different products are strongly dependent on the fatty acyl composition of the particular DG molecular species, even though formation of PC is the major event in all cases. Since DGs are important second messengers, our study supports the view that conversion to PC and TG can play a key role in DG signal attenuation.