Lipoprotein cholesterol uptake mediates up-regulation of bile-acid synthesis by increasing cholesterol 7 alpha-hydroxylase but not sterol 27-hydroxylase gene expression in cultured rat hepatocytes

Lipoproteins may supply substrate for the formation of bile acids, and the amount of hepatic cholesterol can regulate bile-acid synthesis and increase cholesterol 7 alpha-hydroxylase expression. However, the effect of lipopro tein cholesterol on sterol 27-hydroxylase expression and the role of differ ent lipoproteins in regulating both enzymes are not well established. We st udied the effect of different rabbit lipoproteins on cholesterol 7 alpha-hy droxylase and sterol 27-hydroxylase in cultured rat hepatocytes. beta-Migra ting very-low-density lipoprotein (beta VLDL) and intermediate-density lipo protein (IDL) caused a significant increase in the intracellular cholestery l ester content of cells (2.3- and 2-fold, respectively) at a concentration of 200 mu g of cholesterol/ml, whereas high-density lipoprotein (HDL, 50% v/v), containing no apolipoprotein E (apo E), showed no effect after a 24-h incubation. beta VLDL and IDL increased bile-acid synthesis (1.9- and 1.6- fold, respectively) by up-regulation of cholesterol 7 alpha-hydroxylase act ivity (1.7- and 1.5-fold, respectively). Dose- and time-dependent changes i n cholesterol 7 alpha-hydroxylase mRNA levels and gene expression underlie the increase in enzyme activity. Incubation of cells with HDL showed no eff ect. Sterol 27-hydroxylase gene expression was not affected by any of the l ipoproteins added. Transient-expression experiments in hepatocytes, transfe cted with a promoter-reporter construct containing the proximal 348 nucleot ides of the rat cholesterol 7 alpha-hydroxylase promoter, showed an enhance d gene transcription (2-fold) with beta VLDL, indicating that a sequence im portant for a cholesterol-induced transcriptional response is located in th is part of the cholesterol 7 alpha-hydroxylase gene. The extent of stimulat ion of cholesterol 7 alpha-hydroxylase is associated with the apo E content of the lipoprotein particle, which is important in the uptake of lipoprote in cholesterol. We conclude that physiological concentrations of cholestero l in apo E-containing lipoproteins increase bile-acid synthesis by stimulat ing cholesterol 7 alpha-hydroxylase gene transcription, whereas HDL has no effect and sterol 27-hydroxylase is not affected.