Macrophage 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in sitosterolemia: Effects of increased cellular cholesterol and sitosterol concentrations

Sitosterolemia is a rare, recessively inherited disease characterized clini cally by accelerated atherosclerosis and xanthomas and biochemically by hyp erabsorption and retention of sitosterol and other plant sterols in tissues . Decreased cholesterol biosynthesis and inhibition of 3-hydroxy-3-methylgl uratyl coenzyme A (HMG-CoA) reductase and other enzymes in the biosynthetic pathway have been associated with enhanced low-density lipoprotein (LDL) r eceptor function. We examined the effects of cholesterol and sitosterol on sterol concentrations and composition and HMG-CoA reductase activity in mon ocyte-derived macrophages (MDM) from 12 control and 3 homozygous sitosterol emic subjects. The cells were cultured up to 7 days in media devoid of plan t sterols, but containing increasing amounts of serum cholesterol. Before c ulture, MDM from the homozygous sitosterolemic subjects contained 22% more total sterols than cells from control subjects. Plant sterols and stanols r epresented 15.6% of MDM total sterols in sitosterolemic cells, but only 3.8 % in control cells. After 7 days of culture in 10% delipidated serum (DLS) (20 mug/mL cholesterol, no sitosterol), all plant sterols were eliminated s o that cells from both phenotypes contained only cholesterol. When DLS was replaced with fetal bovine serum (FBS) (300 mug/mL cholesterol), with and w ithout addition of 200 mug/mL LDL, cholesterol levels in MDM from sitostero lemic subjects increased 108% (P < .05) compared with a 65% increase (P < . 04) in control MDM cultured similarly. MDM HMG-CoA reductase activity from the 3 sitosterolemic subjects, which was significantly lower than controls at baseline (24 +/- 3 v 60 +/- 10 pmol/mg/min, P < .05), was not downregula ted by increased cellular cholesterol levels, as observed in control cells. Control MDM were also cultured in medium that contained 10% DLS and was su pplemented with 100 mug/mL cholesterol or sitosterol dissolved in ethanol o r the ethanol vehicle alone. In contrast to cellular cholesterol accumulati on, which significantly downregulated HMG-CoA reductase activity (-53%, P < .05), the increase in cellular sitosterol up to 25.1% of total sterols did not change MDM HMG-CoA reductase activity. Evidence of a normal HMG-CoA re ductase protein in sitosterolemic calls, which was not derepressed upon rem oval of cellular sitosterol, and the failure of cellular sitosterol to inhi bit normal HMG-CoA reductase activity argue against feedback inhibition by sitosterol as a mechanism for low reductase activity in this disease. The l arger accumulation of sterols and inadequate downregulation of HMG-CoA redu ctase in MDM may be mechanisms for foam cell formation and explain, in part , the increased risk of atherosclerosis in sitosterolemia. Copyright (C) 20 01 by W.B. Saunders Company.