Synthesis and secretion of very low density lipoproteins by isolated rat hepatocytes in suspension: Role of diacylglycerol acyltransferase

Isolated rat hepatocytes in suspension secrete very low density lipoproteins (VLDL) at a rate comparable with that of the perfused liver. The apoproteins of these lipoproteins are mainly of the B and E type. The amount of apoprotein C in VLDL secreted by hepatocytes is much less than that present in VLDL obtained from rat serum. Incubation of hepatocytes in the presence of fatty acids stimulates the intracellular synthesis of triacylglycerols and their secretion in VLDL. This stimulation is a linear function of the palmitic acid concentration up to 1.6 mm, the highest concentration tested. Colchicine (50 μm) reduced VLDL secretion by 90%. The stimulation of triacylglycerol synthesis and VLDL secretion upon incubation of hepatocytes with fatty acids is paralleled by an enhanced activity of microsomal diacylglycerol acyltransferase (DGAT, EC 2.3.1.20), the only enzyme exclusively involved in the synthesis of triacylglycerols. A mixture of oleic (0.2 mm) and palmitic (0.2 mm) acid added to the cell medium stimulates the activity of DGAT by 354%. This increase in enzyme activity persisted during cell homogenization and subsequent preparation of microsomes to assay the enzyme. It is concluded that freshly isolated hepatocytes in suspension represent a good system to study triacylglycerol synthesis and VLDL secretion, and that the stimulatory effects of fatty acids on these processes are, at least partially, mediated by enhanced activities of DGAT.