Lipoprotein cholesterol uptake mediates up-regulation of bile-acid synthesis by increasing cholesterol 7α-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 7α-hydroxylase expression. However, the effect of lipoprotein cholesterol on sterol 27-hydroxylase expression and the role of different lipoproteins in regulating both enzymes are not well established. We studied the effect of different rabbit lipoproteins on cholesterol 7α-hydroxylase and sterol 27-hydroxylase in cultured rat hepatocytes. β-Migrating very-low-density lipoprotein (βVLDL) and intermediate-density lipoprotein (IDL) caused a significant increase in the intracellular cholesteryl ester content of cells (2.3- and 2-fold, respectively) at a concentration of 200 μg 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. βVLDL and IDL increased bile-acid synthesis (1.9- and 1.6-fold, respectively) by up-regulation of cholesterol 7α-hydroxylase activity (1.7- and 1.5-fold, respectively). Dose- and time-dependent changes in cholesterol 7α-hydroxylase mRNA levels and gene expression underlie the increase in enzyme activity. Incubation of cells with HDL showed no effect. Sterol 27-hydroxylase gene expression was not affected by any of the lipoproteins added. Transient-expression experiments in hepatocytes, transfected with a promoter-reporter construct containing the proximal 348 nucleotides of the rat cholesterol 7α-hydroxylase promoter, showed an enhanced gene transcription (2-fold) with βVLDL, indicating that a sequence important for a cholesterol-induced transcriptional response is located in this part of the cholesterol 7α-hydroxylase gene. The extent of stimulation of cholesterol 7α-hydroxylase is associated with the apo E content of the lipoprotein particle, which is important in the uptake of lipoprotein cholesterol. We conclude that physiological concentrations of cholesterol in apo E-containing lipoproteins increase bile-acid synthesis by stimulating cholesterol 7α-hydroxylase gene transcription, whereas HDL has no effect and sterol 27-hydroxylase is not affected.