Effects of a novel 2,3-oxidosqualene cyclase inhibitor on the regulation of cholesterol biosynthesis in HepG2 cells.
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.
Mark
M
M
Preclinical Research, Department of Biology, Biberach, Germany.
Muller
P
P
Maier
R
R
Eisele
B
B
D001549:Benzamides / Q000494:pharmacology*; D002784:Cholesterol / Q000096:biosynthesis*; D003514:Cyclohexylamines / Q000494:pharmacology*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D004852:Epoxy Compounds / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic; D006801:Humans; D006903:Hydroxymethylglutaryl CoA Reductases / Q000096:biosynthesis; D000960:Hypolipidemic Agents / Q000494:pharmacology; D019751:Intramolecular Transferases; D007535:Isomerases / Q000037:antagonists & inhibitors*; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008148:Lovastatin / Q000031:analogs & derivatives / Q000494:pharmacology; D019821:Simvastatin; D013185:Squalene / Q000031:analogs & derivatives; D014407:Tumor Cells, Cultured
D001549:Benzamides; D003514:Cyclohexylamines; D004791:Enzyme Inhibitors; D004852:Epoxy Compounds; D000960:Hypolipidemic Agents; C099235:N-(4-chlorobenzoyl)-N-methyl-4-(4-dimethylaminomethylphenyl)cyclohexylamine; D013185:Squalene; D002784:Cholesterol; D008148:Lovastatin; D019821:Simvastatin; D006903:Hydroxymethylglutaryl CoA Reductases; D007535:Isomerases; D019751:Intramolecular Transferases; C021924:lanosterol synthase