Down-regulation of cholesterol biosynthesis in sitosterolemia: diminished activities of acetoacetyl-CoA thiolase, 3-hydroxy-3-methylglutaryl-CoA synthase, reductase, squalene synthase, and 7-dehydrocholesterol D 7 -reductase in liver and mononuclear leukocytes

Sitosterolemia is a recessively inherited disorder characterized by abnormally increased plasma and tissue plant sterol concentrations. Patients have markedly reduced whole body cholesterol biosynthesis associated with suppressed hepatic, ileal, and mononuclear leukocyte 3-hydroxy3-methylglutaryl coenzyme A (HMG-CoA) reductase, the ratecontrolling enzyme in cholesterol biosynthetic pathway, coupled with significantly increased low density lipoprotein (LDL) receptor expression. To investigate the mechanism of downregulated cholesterol biosynthesis, we assayed several other key enzymes in the cholesterol biosynthetic pathway including acetoacetyl-CoA thiolase, HMG-CoA synthase, squalene synthase, and 7-dehydrocholesterol D 7 -reductase activities in liver and freshly isolated mononuclear leukocytes from four sitosterolemic patients and 19 controls. Hepatic acetoacetylCoA thiolase, HMG-CoA synthase, reductase, and squalene synthase activities were significantly decreased ( P , 0.05) 2 39%, 2 54%, 2 76%, and 2 57%, respectively, and 7-dehydrocholesterol D 7 -reductase activity tended to be lower ( 2 35%) in the sitosterolemic compared with control subjects. The reduced HMG-CoA synthase, reductase, and squalene synthase activities were also found in mononuclear leukocytes from a sitosterolemic patient. Thus, reduced cholesterol synthesis is caused not only by decreased HMG-CoA reductase but also by the coordinate down-regulation of entire pathway of cholesterol biosynthesis. These results suggest that inadequate cholesterol production in sitosterolemia is due to abnormal down-regulation of early, intermediate, and late enzymes in the cholesterol biosynthetic pathway rather than a single inherited defect in the HMG-CoA reductase gene.— Honda, A., G. Salen, L. B. Nguyen, G. S. Tint, A. K. Batta, and S. Shefer. Down-regulation of cholesterol biosynthesis in sitosterolemia: diminished activities of acetoacetyl-CoA thiolase, 3-hydroxy-3methylglutaryl-CoA synthase, reductase, squalene synthase, and 7-dehydrocholesterol D 7 -reductase in liver and mononuclear leukocytes. J. Lipid Res. 1998. 39: 44–50. Supplementary key words LDL receptor • sitosterol Sitosterolemia (1) is a rare recessive inherited disorder characterized clinically by tendon and tuberous xanthomas, accelerated atherosclerosis, hemolytic episodes, and recurrent arthritis and arthralgias. Biochemically, concentrations of plant sterols (sitosterol, campesterol, stigmasterol, avenosterol) and 5 a -saturated stanols are markedly elevated in virtually all tissues but brain, while plasma cholesterol levels may be normal or only moderately increased (1, 2). Plasma and tissue plant sterols are not synthesized endogenously in humans including sitosterolemic subjects (3, 4), but are derived entirely from the diet. Sitosterolemic patients absorb between 15–60% of dietary sitosterol compared to less than 5% absorbed in normal subjects. Further, plant sterols are preferentially excreted by the liver in normal subjects but are retained along with cholesterol in sitosterolemic patients (3–8). Thus, increased intestinal plant sterol absorption is combined with slow hepatic removal to cause abnormally enlarged plant sterol tissue and plasma pools in this disease. Another key biochemical feature of sitosterolemia is Abbreviations: HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; LDL, low density lipoprotein; SRE-1, sterol regulatory element 1. 1 To whom correspondence should be addressed. at P E N N S T A T E U N IV E R S IT Y , on F ebuary 3, 2013 w w w .j.org D ow nladed fom