Glycerolipids make up 10-90% of cellular dry weight and are essential structural components of intracellular membranes and lipid droplets and of bile, milk, surfactant, and the serum lipoproteins. Diacylglycerol, an obligatory intermediate in glycerolipid synthesis, has recently been identified as the probable physiological activator of protein kinase C. Thus, diacylglycerol plays a central role at the branchpoint of triacylglycerol and phospholipid synthesis and also functions as an intracellular signal. Alterations in cellular diacylglycerol content could, via protein kinase C affect a wide range of cellular functions including response to hormones and growth factors, cell differentiation, and enzyme induction. Our discovery that monoacylglycerol acyltransferase (MGAT) varies 700-fold in activity in liver during the rat's lifespan provides a model system in which the capacity for diacylglycerol synthesis may vary greatly. With it we can investigate the regulation of diacylglycerol and glycerolipid synthesis and the possible compartmentalization of diacylglycerol pools. MGAT may prevent casual, unregulated activation of protein kinase C by channelling diacylglycerol towards triacylglycerol synthesis. We will study the regulation of the committed acylation steps, glycerol-P acyltransferase, dihydroxyacetone-P acyltransferase, and MGAT by alternate substrates and the contribution made by each of these major pathways. We will determine why an alternate route of glycerolipid synthesis is required in suckling rat liver and in other tissues with high rates of triacylglycerol synthesis. Regulation of hepatic MGAT activity by thyroid and other hormones and by diet will be studied in perinatal rats and in hepatocytes obtained at different stages of development. We will determine the source of the monoacylglycerol substrate. MGAT will be solubilized and purified by adapting a recently reported method for intestinal MGAT purification. Use of the developing rat liver as a model system to study the regulation of glycerolipid metabolism, may provide insights into the pathogenesis of hepatic steatosis seen in alcoholism, parenteral nutrition, and Reye's Syndrome, and the alterations of lipoprotein metabolism seen in diabetes and atherosclerosis. Finally, understanding the regulation of the basic steps in glycerolipid synthesis will clarify the mechanisms of membrane biogenesis, the formation of lipid droplets, and the biogenesis of the secretory products, bile, milk, pulmonary surfactant and the serum lipoproteins.
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