N-[(1,5,9)-trimethyldecyl]-4 alpha,10-dimethyl-8-aza-trans-decal-3 beta-ol (8-azadecalin 1), a high-energy intermediate analogue for the
2,3-oxidosqualene-lanosterol cyclase, was found to be a powerful (IC50 approximately 0.1 microM) inhibitor of
cholesterol biosynthesis in human
hepatoma HepG2 cells. In analogy with other mammalian cells grown in the presence of cyclase inhibitors, the decrease in C27-sterol formation was accompanied by an accumulation of
2,3-oxidosqualene, 2,3:22, 23-dioxidosqualene, and by the formation of a compound characterized as
24,25-epoxycholesterol, a repressor of
HMG-CoA (3-hydroxy-3-methylglutaryl coenzyme A)
reductase activity. In order to assess the cyclase as a potential pharmacological target for the design of hypocholesterolemic drugs, it is important to test whether inhibitors of this
enzyme are able to act synergistically on the biosynthesis of
cholesterol, i.e. by decreasing the amount of
lanosterol formed and by repressing the regulatory
HMG-CoA reductase via the formation of regulatory
oxysterols. The accumulation of
24,25-epoxycholesterol in relationship to the decrease of C27-sterol biosynthesis and of
HMG-CoA reductase activity showed only a partial correlation: e.g. at [1] = 100 x IC50 only a 50% reduction in
enzyme activity could be attained. In contrast, when HepG2 cells were treated with 2,3:22,23-dioxidosqualene or
24,25-epoxycholesterol, excellent correlations were found between the inhibition of C27-sterol biosynthesis and the repression of
HMG-CoA reductase activity, which was almost complete at the highest concentrations of these
epoxides (10(-5) M). Altogether, our results suggest that treatment of HepG2 cells with a cyclase inhibitor such as 8-azadecalin (1) does not lead to an intracellular accumulation of
repressor molecules high enough to fully trigger a regulatory pathway resulting in a complete down-regulation of
HMG-CoA reductase. At intermediary concentrations of cyclase inhibitors (IC50), however, a synergistic mode of action of these inhibitors seems plausible.