Roles of acyl-coenzyme A:cholesterol acyltransferase-1 and -2

High doses of atorvastatin and simvastatin induce key enzymes involved in VLDL production

Treatments with high doses of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors may induce the expression of sterol regulatory element binding protein (SREBP)-target genes, causing different effects from those attributed to the reduction of hepatic cholesterol content. The aim of this study was to investigate the effects of high doses of statins on the key enzymes involved in VLDL production in normolipidemic rats. To examine whether the effects caused by statin treatment are a consequence of HMG-CoA reductase inhibition, we tested the effect of atorvastatin on these enzymes in mevalonate-fed rats. Atorvastatin and simvastatin enhanced not only HMG-CoA reductase but also the expression of the SREBP-2 gene itself. As a result of the overexpression of SREBP-2 caused by the statin treatment, genes regulated basically by SREBP-1, as FA synthase and acetyl-coenzyme A carboxylase, were also induced and their mRNA levels increased. DAG acyltransferase and microsomal TG transfer protein mRNA levels as well as phosphatidate phosphohydrolase activity were increased by both statins. Simvastatin raised liver cholesterol content, ACAT mRNA levels, and CTP:phosphocholine cytidylyltransferase activity, whereas it reduced liver DAG and phospholipid content. Mevalonate feeding reversed all changes induced by the atorvastatin treatment. These results show that treatment with high doses of statins induces key enzymes controlling rat liver lipid synthesis and VLDL assembly, probably as a result of SREBP-2 overexpression. Despite the induction of the key enzymes involved in VLDL production, both statins markedly reduced plasma TG levels, suggesting that different mechanisms may be involved in the hypotriglyceridemic effect of statins at high or low doses.

Mutant acyl-coenzyme A:cholesterol acyltransferase 1 devoid of cysteine residues remains catalytically active

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) plays important roles in cellular cholesterol homeostasis and in the early stages of atherosclerosis. ACAT1 is an integral membrane protein with multiple transmembrane domains. Human ACAT1 contains nine cysteine residues; its activity is severely inhibited by various thiol-specific modification reagents including p-chloromercuribenzene sulfonic acid, suggesting that certain cysteine residue(s) might be near or at the active site. We constructed various ACAT1 mutants that contained either single cysteine to alanine substitution at various positions, contained a reduced number of cysteines, or contained no cysteine at all. Each of these mutants retained 20% or more of the wild-type ACAT activity. Therefore, cysteine is not essential for ACAT catalysis. For the cysteine-free enzyme, its basic kinetic properties and intracellular localization in Chinese hamster ovary cells were shown to be very similar to those of the wild-type enzyme. The availability of the cysteine-free ACAT1 will facilitate future ACAT structure function studies. Additional studies show that Cys467 is one of the major target sites that leads to p-chloromercuribenzene sulfonic acid-mediated ACAT1 inactivation, suggesting that Cys467 may be near the ACAT active site(s).