Modulation of 3-hydroxy-3-methylglutaryl-CoA reductase and of acyl-CoA--cholesterol acyltransferase by the transfer of non-esterified cholesterol to rat liver microsomal vesicles.

The incubation of rat liver microsomal fraction with a serum preparation followed by the re-isolation of the microsomal membranes has resulted in an increase in the concentration of non-esterified cholesterol, a considerable decrease in the activity of 3-hydroxy-3-methylglutaryl-CoA reductase and in an increase in the activity of acyl-CoA-cholesterol acyltransferase in the treated microsomal preparation. These effects were related to the concentration of serum in the incubation mixture and to the duration of the incubation. The transfer of non-esterified cholesterol was specific in that the content of protein and the total phospholipids were similar in the original microsomal fraction and the serum-treated microsomal preparation. The incubation of the microsomal fraction with lipoprotein-deficient serum or with no serum resulted in both cases in small changes in the non-esterified cholesterol, the esterified cholesterol and the total phospholipid content in the treated preparations compared with these concentrations in the original microsomal fraction, whereas the activity of acyl-CoA-cholesterol acyltransferase and of 3-hydroxy-3-methylglutaryl-CoA reductase was similar in the lipoprotein-deficient-serum-treated and the buffer-treated microsomal preparations. The activity of 3-hydroxy-3-methylglutaryl-CoA reductase was lower and the activity of acyl-CoA-cholesterol acyltransferase was higher in the lipoprotein-deficient-serum-treated and the buffer-treated microsomal preparations as compared with these activities in the original microsomal fraction. However, the serum-treated microsomal preparation had considerably lower activity of 3-hydroxy-3-methylglutaryl-CoA reductase and considerably higher activity of acyl-CoA-cholesterol acyltransferase than these activities in buffer-treated and in lipoprotein-deficient-serum-treated microsomal preparations.