Mechanism of regulation of 3‐hydroxy‐3‐methylglutaryl coenzyme A reductase activity by low density lipoprotein in human lymphocytes

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Abstract
The mechanism of action of low density lipoprotein (LDL) on the rate of sterol synthesis and on the activity of its rate‐determining enzyme 3‐hydroxy‐3‐methylglutaryl coenzyme A reductase‐(HMG‐CoA reductase) was studied in lymphocytes freshly isolated from normal subjects and patients heterozygous for familial hypercholesterolaemia.Incubation of normal cells in lipid‐depleted serum led to a substantial rise in the rate of sterol synthesis from [14C]acetate. Addition of cycloheximide (20 μg/ml), a translational inhibitor, reduced sterol synthesis; addition of LDL (100 μg LDL‐cholesterol/ml) had a similar effect as cycloheximide in reducing sterol synthesis with a half‐life of about 3 h.Cordycepin (50 μg/ml) inhibited messenger RNA synthesis by more than 50% but had no inhibitory effect on the induction of sterol synthesis suggesting that the induction of the pathway is independent of newly synthesized messenger RNA.Lymphocytes from patients heterozygous for familial hypercholesterolaemia behaved similarly to cells from normal subjects in that cycloheximide reduced the incorporation of [14C]acetate into sterols, while cordycepin had no effect on the induction of sterol synthesis mediated by lipid‐depleted serum.Since sterol synthesis from [14C]acetate can be taken as a measure of HMG‐CoA reductase activity under our experimental conditions, the results suggest that: (i) the increase HMG‐CoA reductase activity in cells incubated in the presence of lipid‐depleted serum is due to increased de novo synthesis of the enzyme which can rapidly be inhibited by the addition of LDL, and (ii) in lymphocytes from normal subjects and patients heterozygous for familial hypercholesterolaemia the induction of HMG‐CoA reductase by lipid‐depleted serum and, by implication, the subsequent repression of the enzyme by LDL cannot be accounted for by a corresponding increase or decrease in the synthesis of messenger RNA.