Analysis of HSD3B7 knockout mice reveals that a 3alpha-hydroxyl stereochemistry is required for bile acid function

Primary bile acids are synthesized from cholesterol in the liver and thereafter are secreted into the bile and small intestine. Gut flora modify primary bile acids to produce secondary bile acids leading to a chemically diverse bile acid pool that is circulated between the small intestine and liver. A majority of primary and secondary bile acids in higher vertebrates have a 3alpha-hydroxyl group. Here, we characterize a line of knockout mice that cannot epimerize the 3beta-hydroxyl group of cholesterol and as a consequence synthesize a bile acid pool in which 3beta-hydroxylated bile acids predominate. This alteration causes death in 90% of newborn mice and decreases the absorption of dietary cholesterol in surviving adults. Negative feedback regulation of bile acid synthesis mediated by the farnesoid X receptor (FXR) is disrupted in the mutant mice. We conclude that the correct stereochemistry of a single hydroxyl group at carbon 3 in bile acids is required to maintain their physiologic and regulatory functions in mammals.

Transcriptional regulation of hepatobiliary transport systems in health and disease: implications for a rationale approach to the treatment of intrahepatic cholestasis

Hepatobiliary transport systems mediate hepatic uptake and biliary excretion of bile acids, bilirubin and other biliary constituents. Hereditary or acquired defects of these transporters may cause or maintain cholestasis and jaundice under various clinical conditions including progressive familial intrahepatic cholestasis (PFIC) 1-3 or its milder forms, benign recurrent intrahepatic cholestasis (BRIC) 1 and 2 , Dubin-Johnson syndrome, drug and inflammation-induced cholestasis and intrahepatic cholestasis of pregnancy. Moreover, induction of alternative efflux pumps for bile acids/bilirubin and phase I/II detoxifying enzymes may counteract hepatic accumulation of potentially toxic biliary constituents in cholestasis by providing alternative escape routes. Transcriptional and post-transcriptional regulation of hepatobiliary transporters in health and disease is mediated by multiple factors such as bile acids, proinflammatory cytokines, drugs and hormones. Ligand-activated nuclear receptors (NR) and hepatocyte-enriched transcription factors play a critical role in transcriptional transporter regulation. Many hepatobiliary transporter alterations in cholestatic liver disease can now be explained by ligand binding of accumulating cholephiles to NRs. Moreover, NR-mediated actions may be targeted by pharmacological ligands. Understanding the transcriptional mechanisms leading to transporter changes therefore not only represents a key for understanding the pathophysiology of the cholestatic liver disease, but also represents a prerequisite for designing novel therapeutic strategies.