Studies on the mechanism of regulation of bile acid synthesis in humans with some aspects on genetic factors

In the present investigation we studied human bile acid synthesis and its regulation both in vivo and in vitro and also evaluated a possible influence of polymorphisms in the CYP7A1 and CYP8B1 genes on bile acid synthesis in vivo. The following observations were made: • Primary human hepatocytes cultured on matrigel under serum-free conditions produced bile acids consistent with the pattern in vivo, i.e. predominately CA (70%) and CDCA (25%), both conjugated with glycine or taurine. • Both glycine-conjugated and free bile acids suppressed bile acid synthesis and mRNA levels of CYP7A1 in cultures of human primary hepatocytes in the order CDCA>DCA>CA>UDCA. Also mRNA levels of CYP8B1 and CYP27A1 were suppressed but to a much lesser extent. Addition of GCDCA as well as GDCA resulted in higher levels of mRNA for SHP supporting a mechanism by which FXR suppresses CYP7A1 through induction of SHP. • Upregulation of bile acid synthesis in humans by cholestyramine treatment resulted in higher levels of mRNA of not only CYP7A1, but also of HNF-4α. This is consistent with a mechanism in which HNF-4α is an important stimulating nuclear factor involved in bile acid synthesis. • Earlier described polymorphisms in the promoter region of the CYP7A1 gene could not be associated with variation in CYP7A1 enzyme activity in liver biopsies or rate of bile acid synthesis evaluated by serum levels of 7α-hydroxy-4-cholesten-3-one or by isotope dilution kinetics. The genotypes did not differ in experiments with a reporter system for transcriptional activity or in EMSA analysis measuring binding of nuclear extracts. • Differences in ratio of CA and CDCA in gallbladder bile observed in a number of human subjects could not be explained by polymorphisms in the CYP8B1 gene. The results obtained emphasize marked species differences in the mechanisms for regulation of bile acid synthesis. Genetic polymorphism in the key enzymes does not appear to be of major importance for this regulation.