Regulation of synthesis and degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase by low density lipoprotein and 25-hydroxycholesterol in UT-1 cells.

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Abstract
UT-1 cells are a clone of Chinese hamster overy cells that were selected to grow in the presence of compactin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase. These cells have 100- to 1,000-fold more immunoprecipitable reductase than normal. The enzyme activity is rapidly decreased when low density lipoprotein (LDL) or 25-hydroxycholesterol is added to the culture medium. A quantitative immunoprecipitation assay was used to determine whether LDL and 25-hydroxycholesterol inhibit the synthesis or stimulate the degradation of reductase in UT-1 cells. Each of these agents inhibited the incorporation of [35S]methionine into immunoprecipitable reductase by more than 98%. Pulse-chase experiments showed that reductase was degraded with a half-life of 10-13 h in UT-1 cells and that the rate of degradation of preformed enzyme was increased 3-fold by the addition of either LDL or 25-hydroxycholesterol. The predominant mechanism by which LDL and 25-hydroxycholesterol decrease reductase activity in UT-1 cells is a profound suppression of synthesis of the enzyme.

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