A novel inhibitor of oxidosqualene:lanosterol cyclase inhibits very low-density lipoprotein apolipoprotein B100 (apoB100) production and enhances low-density lipoprotein apoB100 catabolism through marked reduction in hepatic cholesterol content.

OBJECTIVE: Inhibition of 2,3-oxidosqualene:lanosterol cyclase (OSC), an enzyme in the cholesterol synthesis pathway, has the unique ability to inhibit cholesterol synthesis while simultaneously enhancing oxysterol synthesis. Our objectives were to determine, in vivo, if a novel OSC inhibitor reduced low-density lipoprotein (LDL) cholesterol and to define the mechanism(s) involved. METHODS AND RESULTS: Miniature pigs received the OSC inhibitor RO0717625 or placebo and a diet containing fat (34% of energy) and 400 mg per day of cholesterol. Treatment decreased plasma total cholesterol (-20%) and LDL cholesterol (-29%). Apolipoprotein B (apoB) kinetic parameters were determined. Very low-density lipoprotein (VLDL) apoB pool size decreased 22% because of inhibition of VLDL production (-43%). LDL apoB pool size decreased 22% because of a 1.5-fold increase in fractional catabolic rate (FCR). The increased FCR was associated with a 2-fold increase in hepatic LDL receptor mRNA. Hepatic total and microsomal cholesterol were reduced by 16% and 27%, respectively. Plasma lathosterol concentrations decreased 57%, reflecting inhibition of hepatic cholesterol synthesis. Treatment reduced plasma plant sterols and decreased postprandial cholesterol transport in chylomicrons. CONCLUSIONS: A novel OSC inhibitor, RO0717625, decreased VLDL and LDL apoB100 through decreased VLDL production and enhanced LDL clearance. Thus, OSC represents a potential therapeutic target for dyslipidemia.

The monotopic membrane protein human oxidosqualene cyclase is active as monomer.

The monotopic integral membrane protein 2,3-oxidosqualene cyclase (OSC) catalyzes the formation of lanosterol the first sterol precursor of cholesterol in mammals. Therefore, it is an important target for the development of new hypocholesterolemic drugs. Here, we report the overexpression and purification of functional human OSC (hOSC) in Pichia pastoris. The obtained IC(50) for the reference inhibitor Ro 48-8071 is nearly identical for the recombinant hOSC compared to OSC from human liver microsomes. The correlation of analytical ultracentrifugation data and activity measurements showed the highest enzymatic activity for the monomeric hOSC indicating that this would be the natural form. Furthermore, these data helped us to identify the detergent for a successful crystallization of the protein. The availability of this active recombinant human membrane protein is a very important step on the way to a more detailed functional and structural characterization of OSCs.

Enhanced synthesis of the oxysterol 24(S),25-epoxycholesterol in macrophages by inhibitors of 2,3-oxidosqualene:lanosterol cyclase: a novel mechanism for the attenuation of foam cell formation.

Oxysterols are key regulators of lipid metabolism and regulate gene expression by activating the liver X receptor (LXR). LXR plays a vital role in macrophage foam cell formation, a central event in atherosclerosis. It is known that addition of exogenous oxysterols to cultured macrophages activates LXR, leading to increased expression of ABCA1 and cholesterol efflux. In this study, we tested the novel hypothesis that stimulation of endogenous oxysterol synthesis would block foam cell formation induced by atherogenic lipoproteins. Macrophage synthesis of 24(S),25-epoxycholesterol, a potent LXR ligand, increased 60-fold by partial inhibition of 2,3-oxidosqualene:lanosterol cyclase (OSC), a microsomal enzyme in both the cholesterol biosynthetic pathway and the alternative oxysterol synthetic pathway. When macrophages were challenged with human hypertriglyceridemic VLDL (HTG-VLDL), cellular cholesteryl ester accumulation increased 12-fold. This was reduced dramatically, by 65%, after preincubation with an OSC inhibitor (OSCi). The HTG-VLDL-induced accumulation of macrophage TG (70-fold) was unaffected by the OSCi or exogenous 24(S),25-epoxycholesterol, an effect associated with suppression of SREBP-1 processing. By contrast, TO901317, a synthetic LXR agonist, increased cellular TG significantly and markedly increased SREBP-1 processing. OSC inhibition decreased HTG-VLDL uptake through downregulation of LDL-receptor expression, despite substantial inhibition of cholesterol synthesis. Furthermore, OSC inhibition significantly upregulated ABCA1 and ABCG1 expression, which led to enhanced macrophage cholesterol efflux, an effect mediated through LXR activation. Therefore, increased macrophage synthesis of endogenous oxysterols represents a new mechanism for the dual regulation of LXR- and SREBP-responsive genes, an approach that inhibits foam cell formation without detrimental effect on TG synthesis.

Synthesis and structure-activity studies of novel orally active non-terpenoic 2,3-oxidosqualene cyclase inhibitors.

New orally active non-terpenoic inhibitors of human 2,3-oxidosqualene cyclase (hOSC) are reported. The starting point for the optimization process was a set of compounds derived from a fungicide project, which in addition to showing high affinity for OSC from Candida albicans showed also high affinity for human OSC. Common structural elements of these inhibitors are an amine residue and an electrophilic carbonyl C atom embedded in a benzophenone system, which are at a distance of about 10.7 A. Considering that the keto moiety is in a potentially labile position, modifications of the substitution pattern at the benzophenone as well as annelated heteroaryl systems were explored. Our approach combined testing of the compounds first for increased binding affinity and for increased stability in vitro. Most promising compounds were then evaluated for their efficacy in lowering plasma total cholesterol (TC) and plasma low-density lipoprotein cholesterol (LDL-C) in hyperlipidemic hamsters. In this respect, the most promising compounds are the benzophenone derivative 1.fumarate and the benzo[d]isothiazol 24.fumarate, which lowered TC by 40% and 33%, respectively.