Treatment of rat intestinal epithelial cells (IEC-6 cells) with
lanosterol 14 alpha-demethylase inhibitors,
ketoconazole and
miconazole, had similar effects on 3-hydroxy-3-methylglutaryl
coenzyme A (
HMG-CoA) reductase activity and
cholesterol biosynthesis but the drugs differed in their ability to prevent the
low density lipoprotein (
LDL) suppression of
reductase activity.
Miconazole, at concentrations that inhibited the metabolism of
lanosterol and epoxylanosterol to the same degree as
ketoconazole, did not prevent
low density lipoprotein action on
reductase activity, whereas
ketoconazole totally abolished the
low density lipoprotein action on
reductase activity. Both drugs caused: 1) a biphasic response in
reductase activity such that at low concentrations (less than 2 microM)
reductase activity was inhibited and at high concentrations (greater than 5 microM) the activity returned to control or higher than control levels; 2) an inhibition of metabolism of
lanosterol to
cholesterol, and 24(S), 25-epoxylanosterol to 24(S), 25-epoxycholesterol. Neither
drug prevented suppression of
reductase activity by 25-hydroxylanosterol, 25-hydroxycholesterol, or
mevalonolactone added to the medium. Each
drug increased the binding, uptake, and degradation of 125I-labeled
LDL and inhibited the re-esterification of free
cholesterol to
cholesteryl oleate and
cholesteryl palmitate. The release of free
cholesterol from [3H]
cholesteryl linoleate LDL could not account for the differential effect of
ketoconazole and
miconazole on the prevention of
low density lipoprotein suppression of
reductase activity. The differential effect of the drugs on
low density lipoprotein suppression of
reductase activity was not unique to IEC-6 cells, but was also observed in several cell lines of different tissue origin such as human skin fibroblast cells (GM-43), human
hepatoblastoma cells (HepG2), and Chinese hamster ovary cells (wild type, K-1; 4 alpha-
methyl sterol oxidase mutant, 215). These observations suggest that the suppressive action of
low density lipoprotein on
reductase activity 1) does not require the de novo synthesis of
cholesterol, or 24(S), 25-epoxysterols; 2) is not mediated via the same mechanism as that of
mevalonolactone; and 3) does not involve cholesteryl reesterification.
Ketoconazole blocks a site in the process of
LDL suppression of
reductase activity that is not affected by
miconazole.