High density lipoproteins and lecithin dispersions increase the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase by increasing the rate of synthesis and decreasing the rate of degradation of the enzyme.

P A Edwards,S F Lan,A M Fogelman

doi:10.1016/s0021-9258(17)39712-0

Abstract

Incubation of rat hepatocytes, prepared from rats fed a normal diet, with either human high density lipoproteins or lecithin dispersions resulted in a dose-and time-dependent increase in the specific activity of 3-hydroxy-3-methylglutaryl-CoA reductase. Incubation of cells with both high density lipoprotein and lecithin dispersions resulted in an additive stimulation of enzymic activity. We used pulse and pulse-chase experiments, followed by immunoprecipitation of the radiolabeled reductase, to demonstrate that high density lipoproteins and lecithin dispersions resulted in increased rates of synthesis and decreased rates of degradation of the reductase. We also demonstrate, by the use of immunoblots, that the increased enzymic activity was paralleled by an equivalent increase in the mass of the reductase. Addition of human low density lipoprotein to rat hepatocytes resulted in a small decrease in both enzymic activity and enzyme synthesis.

Highlights

Incubation of rat hepatocytes, prepared from rats fed a normal diet,witheither human high density lipoproteins or lecithin dispersions resulted in a doseand time-dependent increase in the specific activityof 3-hydroxy-3-methylglutaryl-CoAreductase
Addition of HDL to cultured cells has been shown to result in increased efflux of cellular cholesterol into the medium (16, 19, 20H).ence the stimulation of cholesterogenesis or HMG-CoA reductase activity that occurs when cells are incubated with HDL or lecithin dispersions is associated, under both conditions, with increased efflux of sterol out of the cell
In the present study we have used immunoprecipitation of radiolabeled enzyme to determine apparent rates of enzyme synthesis and degradation in rat hepatocytes incubated inthe absence or presence of lecithin and HDL inorder to delineate the mechanism of enzyme induction

Summary

RESULTS

In both an increase in the relative rate of reductase synthesis (Fig 2) and an increase in the absolute incorporation of [35S]. Addition of HDL or lecithin to hepatocytes prepared from methionine into the enzyme (Fig. 2 4 , inset) even under conrats fed a normal diet resulted in daose- and time-dependent ditions where total cellular protein synthesis was decreased increase in the activity of HMG-CoA reductase When cells were prepared from different rats and lecithin, in the absence or presence of 1p~ mevinolin, to rat incubatedin media alone, 0.07-0.12%of the radioactivity hepatocytes during a 160-min chase affects the relative rate found in total cellular proteins was precipitated of degradation of the reductase; the apparent half-life was by antireductase antibody and migrated as a polypeptide of increased 67% from 89 min in controls to 153 min in experi-.

DISCUSSION

Findings

ENZYME UNITS BLOTTED