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

J Lipid Res. 1996 Jan;37(1):148-58.

Abstract

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 "> or =" 10(-9) "< or =" 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-oxidosqualene 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.

MeSH terms

  • Benzamides / pharmacology*
  • 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*
  • Liver / cytology
  • Liver / metabolism*
  • Lovastatin / analogs & derivatives
  • Lovastatin / pharmacology
  • Simvastatin
  • Squalene / analogs & derivatives
  • Tumor Cells, Cultured

Substances

  • Benzamides
  • Cyclohexylamines
  • Enzyme Inhibitors
  • Epoxy Compounds
  • Hypolipidemic Agents
  • N-(4-chlorobenzoyl)-N-methyl-4-(4-dimethylaminomethylphenyl)cyclohexylamine
  • Squalene
  • Cholesterol
  • Lovastatin
  • Simvastatin
  • Hydroxymethylglutaryl CoA Reductases
  • Isomerases
  • Intramolecular Transferases
  • lanosterol synthase