Regulation of 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase in Hepatoma Tissue Culture Cells by Serum Lipoproteins

Abstract

The rate of 3β-hydroxysterol synthesis in hepatoma tissue culture (HTC) cells can be modified by the quality and quantity of serum lipoprotein in the culture medium. Cells maintained in medium which contained lipoprotein-poor serum have a steady state rate of 3β-hydroxysterol synthesis which is 3- to 4-fold greater than cells grown in medium containing unfractionated dialyzed serum. This increase is also reflected by a similar change in the catalytic activity of the rate-limiting enzyme for sterol biosynthesis (3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (EC 1.1.1.34)). The addition of serum lipoproteins to medium containing lipoprotein-poor serum led to a rapid decrease in catalytic activity of HMG-CoA reductase; a t1/2 for decay of 1½ to 2 hours was calculated from steady state kinetics analysis. Cells incubated in media which contained dialyzed or lipoprotein-poor sera plus cycloheximide showed the same rate of decay in HMG-CoA reductase activity (t1/2: 4 to 5 hours). Also, cycloheximide prevented the increase in catalytic activity of HMG-CoA reductase in cells transferred from medium with whole serum to medium which contained lipoprotein-poor serum. Actinomycin D did not block the effect of added serum lipoprotein on the decay of reductase activity in cells grown on medium poor in lipoprotein, but it did block the increase in catalytic activity of HMG-CoA reductase by cells transferred from medium with unfractionated serum to lipoprotein-poor medium. These results suggest that the regulatory site of action of serum lipoproteins is at a post-transcriptional level and that their net effect is to decrease the rate of synthesis of HMG-CoA reductase.