Expression of 7 alpha-hydroxylase in non-hepatic cells results in liver phenotypic resistance of the low density lipoprotein receptor to cholesterol repression

Abstract

The goal of this study was to understand why the expression of low density lipoprotein (LDL) receptors by the liver is poorly down-regulated by cholesterol. We examined the hypothesis that 7 alpha-hydroxylase may indirectly induce the expression of the LDL receptor by metabolizing, i.e. inactivating oxysterol repressors. Non-hepatic Chinese hamster ovary cells, transfected with a plasmid encoding 7 alpha-hydroxylase, expressed both the mRNA and functional activity of this liver-specific enzyme. In the presence of 5% serum, expression of the LDL receptor by transfected cells was > 20 times that of non-transfected cells despite a 50% increased content of cholesterol ester. Both cell types displayed an almost complete repression of the LDL receptor by the oxysterol 25-hydroxycholesterol, suggesting that transcriptional control of the LDL receptor gene remained intact in the transfected cells. However, only cells expressing 7 alpha-hydroxylase showed a derepression of the LDL receptor with time. This transient sensitivity to 25-hydroxycholesterol repression was attributed to a 3-fold greater rate of metabolism of [3H]25-hydroxycholesterol. The paradoxical induction of LDL receptor mRNA in transfected cells having greater amounts of cholesterol esters suggests that 7 alpha-hydroxylase may preferentially use oxysterols rather than cholesterol as substrates. The combined data are consistent with the proposal that 7 alpha-hydroxylase indirectly induces the LDL receptor gene by metabolizing (inactivating) oxysterol repressors. Liver-specific expression of 7 alpha-hydroxylase can account for the relative resistance of hepatic LDL receptors to down-regulation.