Gene expression of hepatic cholesterol 7α-hydroxylase in the course of puromycin-induced nephrosis

Abstract

Cholesterol conversion to and biliary excretion of bile acids represents the principal pathway of cholesterol catabolism in mammals. Cholesterol 7α-hydroxylase (Ch-7α-H) is the first and the rate limiting step in bile acid production. Recently, Ch-7α-H enzymatic activity has been shown to be normal in rats with established puromycin aminonucleoside-induced nephrosis (NS). To our knowledge, the gene expression of Ch-7α-H in NS has not been investigated. We measured hepatic Ch-7α-H mRNA and protein (by Northern and Western blot analyses) in rats at baseline and longitudinally during the course of induction and chronic phase of puromycin (PAN) induced NS. Groups of placebo-treated (controls) and diet-induced hypercholesterolemic (DHC) rats were included for comparison. The NS and DHC animals exhibited severe hypercholesterolemia of similar magnitude. Hepatic Ch-7α-H transcript and protein remained virtually unchanged throughout the study period in the NS group. In contrast, Ch-7α-H gene expression was markedly up-regulated in the DHC group. These observations suggest that hepatic Ch-7α-H gene expression may be inappropriately low for the degree of the associated hypercholesterolemia in the NS group. It should be noted, however, that hepatic tissue cholesterol concentration was normal in the NS group and greatly increased in the DHC group. This can account for the disparity in Ch-7α-H mRNA levels between the two groups since intracellular rather than extracellular cholesterol modulates Ch-7α-H gene expression. In conclusion, the present study revealed that hepatic Ch-7α-H gene expression remains unchanged during the course of PAN-induced NS in rats. It thus appears that generation and maintenance of hypercholesterolemia in this model of NS does not involve significant alteration of Ch-7α-H gene expression.