Lipoprotein regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in rat liver cell cultures

A primary cell culture technique was used to study the effects of lipoproteins on rat hepatocyte 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity. In this system, lipoproteins prepared from normocholesterolemic rat and human plasma, including low density lipoproteins, did not inhibit hepatocyte HMG-CoA reductase activity whereas very low density lipoproteins and high density lipoproteins isolated from the same sources were stimulatory. A lipoprotein was isolated from the plasma of cholesterol-fed rats that did inhibit hepatocyte HMG-CoA reductase activity. An extensive chemical characterization of the inhibitory lipoprotein revealed that it was mainly d less than 1.019 g/ml and had beta mobility on lipoprotein electrophoresis. The lipoprotein was compared to the comparable density fraction in the normocholesterolemic rat plasma and there was no size difference appreciable by negatively stained electron micrographs. However, two important differences in chemical composition were evident: in the inhibitory lipoproteins the per cent of total apoprotein which was in the region of Mr = 35,000 was increased 1.5- to 2-fold, and there was a marked increase in cholesterol ester content. These chemical characteristics may be required for lipoproteins to regulate hepatocyte cholesterol synthesis. Primary cell culture of rat hepatocytes appears to be a useful system in which to study cholesterol metabolism in the liver.