Influence of lecithin:cholesterol acyltransferase on cholesterol metabolism in hepatoma cells and hepatocytes.

Cholesterol content and synthesis were measured in rabbit hepatocytes and rat hepatoma cells (Fu5AH) incubated in rabbit serum at concentrations ranging from 2.5% to 50%. Values were compared to controls grown in delipidized serum protein. Cellular cholesterol content varied inversely with the serum concentration, whereas cholesterol synthesis was elevated as serum concentration in the incubation medium was raised. The reduction in cellular cholesterol content and the elevation in synthesis observed with the cells incubated in high concentrations of fresh serum could be correlated with the extent of serum lipoprotein modification by lecithin:cholesterol acyltransferase. Unmodified serum in which LCAT had been inactivated depressed cholesterol synthesis and increased cellular cholesterol content at all concentrations. The presence of active LCAT was not required for the cellular responses, since serum which had been modified before LCAT inactivation also stimulated cholesterol synthesis and decreased content. Qualitatively similar results were obtained with human, rat and rabbit sera. Fractionation of serum demonstrated that the stimulatory activity of LCAT-modified serum was associated primarily with the high-density lipoprotein fraction. Comparative cholesterol flux studies using prelabeled hepatoma cells exposed to either normal or modified high-density lipoproteins demonstrated that cellular cholesterol efflux was somewhat depressed in the presence of the modified lipoprotein whereas cholesterol influx was markedly reduced. These data indicate that LCAT modification of serum lipoproteins alters the relative rates of cholesterol flux with the major effect being on cholesterol uptake. This results in a net loss of cholesterol from the cells accompanied by a stimulation of cholesterol synthesis.