Effects of a novel 2,3-oxidosqualene cyclase inhibitor on the regulation of cholesterol biosynthesis in HepG2 cells

• Published in: Journal of lipid research Vol. 37; no. 1; pp. 148 - 158
• Main Authors: Mark, M, Muller, P, Maier, R, Eisele, B
• Format: Journal Article
• Language: English
• Published: BETHESDA Amer Soc Biochemistry Molecular Biology Inc 01.01.1996; Elsevier
• Subjects: Benzamides - pharmacology; Biochemistry & Molecular Biology; Cholesterol - biosynthesis; Cyclohexylamines - pharmacology; Enzyme Inhibitors - pharmacology; Epoxy Compounds - metabolism; Gene Expression Regulation, Enzymologic; Humans; Hydroxymethylglutaryl CoA Reductases - biosynthesis; Hypolipidemic Agents - pharmacology; Intramolecular Transferases; Isomerases - antagonists & inhibitors; Life Sciences & Biomedicine; Liver - cytology; Liver - metabolism; Lovastatin - analogs & derivatives; Lovastatin - pharmacology; Science & Technology; Simvastatin; Squalene - analogs & derivatives; Tumor Cells, Cultured; COENZYME-A REDUCTASE; oxysterols; COMPACTIN ML-236B; COMPETITIVE INHIBITOR; LINE HEP-G2; LOW-DENSITY-LIPOPROTEIN; SQUALENE EPOXIDASE; 3-HYDROXY-3-METHYLGLUTARYL COENZYME; 24,25-epoxylanosterol; FIBROBLASTS; lanosterol; 2,3-monoepoxysqualene; METABOLISM; HMG-CoA reductase; squalene oxidase; 2,3;22,23-diepoxysqualene
• ISSN: 0022-2275; 1539-7262
• DOI: 10.1016/S0022-2275(20)37643-4

Within the cholesterol biosynthesis cascade. the enzyme 2,3-oxidosqualene cyclase [EC 5.4.99.7] is of special interest due to its dual function: cyclization of 2.3-monoepoxysqualene to lanosterol and 2.3:22,23-diepoxysqualene to oxylanosterol. Further determination of the significance of this enzyme for the intracellular cholesterol homeostasis was done with BIBX 79, a new potent, specific inhibitor of this enzyme. In HepG2 cells the effects of BIBX 79 on cholesterol biosynthesis, 2,3-oxidosqualene cyclase as well as HMG-CoA reductase activities were studied. BIBX 79 is a potent inhibitor of sterol biosynthesis in HepG2 cells (IC50 4 x 10(-9) M). No other enzyme within the cholesterol biosynthesis cascade was significantly inhibited as was evidenced by a radio HPLC detection system. In contrast to simvastatin, no direct interaction with HMG-CoA reductase was observed. When incubating HepG2 cells for 16 h with the HMG-CoA reductase inhibitor simvastatin (10(-6)-10(-10) M) HMG-CoA reductase activity was increased up to 180%. BIBX 79 did also affect HMG:CoA reductase activity under these conditions: in concentrations of BIBX 79 greater than or equal to 10(-9) less than or equal to 10(-7) M, where a partial inhibition of 2,3-oxidosqualene cyclase is observed, HMG-CoA reductase activity was decreased. However, higher concentrations of BIBX 79 that totally blocked 2,3-oxidosqualene cyclase led to an increase in HMG-CoA reductase activity. This effect of BIBX 79 on HMG-CoA reductase is thought to be mainly mediated by oxysterols that are formed by the cyclization of 2,3;22,23-diepoxysqualene. 2,3;22,23-Diepoxysqualene is preferentially cyclized by the 2,3:oxidosqualene cyclase and, consequently, only high inhibitor concentrations will also block 2,3;22,23-diepoxysqualene cyclization. Thus, by partial blockade of this enzyme, both an inhibition of lanosterol and subsequently cholesterol formation as well as a concomitant effect on HMG-CoA reductase can be achieved. Both effects complement each other and lead to an effective control of cholesterol biosynthesis. It is therefore concluded that 2,3-osidosqualene cyclase plays a crucial role in the regulation of intracellular cholesterol homeostasis. 2,3-Oxidosqualene cyclase inhibitors offer an attractive approach for novel lipid-lowering agents.