EFFECTS OF CYCLOSPORINE ON CHOLESTEROL 27-HYDROXYLATION AND LDL RECEPTOR ACTIVITY IN HEPG2 CELLS

The hypothesis that mitochondrial sterol 27-hydroxylase plays a role i n the sterol-mediated down-regulation of LDL receptor activity was eva luated in HepG2 cells. 27-Hydroxycholesterol was found to be more pote nt at suppressing LDL receptor activity than cholesterol (IC50 values of 8 mu M and 142 mu M for 27-hydroxycholesterol and cholesterol, resp ectively) when the sterols were delivered to cells from 2-hydroxypropy l-beta-cyclodextrin (beta-CD)-solubilized solutions. Cyclosporin, an i mmunosuppressant which has been shown to inhibit the 27-hydroxylation of sterols, was used to assess whether the formation of endogenous 27- hydroxycholesterol was required for the cholesterol-induced suppressio n of LDL receptor activity. Cyclosporin dose-dependently inhibited the 27-hydroxylation of cholesterol by HepG2 mitochondria (K-1 = 0.25 mu M) and HepG2 cell cultures (IC50 = 1 mu M). At 1 mu M, cyclosporin had no effect on LDL receptor activity, and did not prevent the suppressi on of LDL receptor activity caused by: 1) the addition of beta-CD-solu bilized cholesterol, 2) the receptor-mediated uptake of beta-VLDL, or 3) the inhibition of cholesterol esterification. In contrast, 10 mu M cyclosporin was found to inhibit the esterification of cholesterol and to increase the cellular level of free cholesterol resulting in suppr ession of LDL receptor activity. These results suggest that if mitocho ndrial sterol 27-hydroxylase plays a role in the regulation of LDL rec eptor activity, it is not through its effects on the availability and/ or size of the free cholesterol pool regulating LDL receptor activity.