Evidence for a lack of regulation of the assembly and secretion of apolipoprotein B-containing lipoprotein from HepG2 cells by cholesteryl ester.

Although some previous studies have suggested that triglyceride, a major core lipid, plays a key role in the assembly and secretion of apolipoprotein B-containing lipoproteins from HepG2 cells, other reports have indicated the importance of cholesteryl ester, another core lipid. We attempted to better define the roles of triglyceride and cholesteryl ester in the assembly and secretion of apolipoprotein B-containing lipoproteins from HepG2 cells by determining the effects of Sandoz 58-035, a potent acyl-CoA acyltransferase inhibitor, which significantly inhibits cholesteryl synthesis, and Triacsin D, a potent fatty acyl-CoA synthetase inhibitor, which significantly inhibits triglyceride synthesis, on the secretion of apolipoprotein B-containing lipoproteins. Sandoz 58-035 (2 micrograms/ml) decreased very low density lipoproteins (VLDL)-stimulated cellular cholesteryl ester content by 60-80%, and blocked oleate-stimulated cholesteryl ester synthesis by 100%, but did not decrease VLDL- or oleate-stimulated apolipoprotein B secretion. Triacsin D (12.5 microM), which significantly inhibited VLDL and oleate stimulation of triglyceride synthesis, without affecting cholesteryl ester synthesis, blocked the stimulation of apolipoprotein B secretion by both agents. In HepG2 cells transfected with 3-hydroxy-methylglutaryl-CoA reductase cDNA, cholesteryl ester synthesis and mass were increased by 100%, but apolipoprotein B secretion was unaffected. Sandoz 58-035 decreased cholesteryl ester synthesis significantly but did not decrease apolipoprotein B secretion from this cell line. When these transfected cells were incubated with oleate, apolipoprotein B secretion increased; Triacsin D blocked this effect. Finally, sphingomyelinase treatment (which shifts cholesterol from plasma membranes to intracellular pools) increased cholesteryl ester synthesis 4-5-fold, but apolipoprotein B secretion was unaffected. Changes in cellular cholesteryl ester synthesis or mass did not affect the intracellular degradation of newly synthesized apolipoprotein B, but changes in triglyceride synthesis were always associated with corresponding changes in the intracellular degradation of apolipoprotein B. In conclusion, neither long term nor short term changes in cholesteryl ester synthesis or mass regulate the assembly and secretion of apolipoprotein B-containing lipoprotein from HepG2 cells.