The liver excretes excess cholesterol in the form of bile acids. Bile acids serve two purposes: to remove unwanted cholesterol from the body and to aid in lipid digestion in the intestine. Bile acids are synthesized in hepatocytes and this production is tightly controlled by the nuclear receptor transcription factors, Liver X Receptors (LXR-α and LXR-β). Activation of LXRs by specific oxysterol derivatives leads to the regulation of bile acid synthesis and reverse cholesterol transport. LXRs control the activation of 7α-hydroxylase, which is the rate limiting enzyme in the pathway for bile acid biosynthesis. This enzyme converts cholesterol into 7-hydroxycholesterol. 7-hydroxycholesterol is converted to one of the two primary bile acids, cholic acid and chenodeoxycholic acid. (see Figure 1) Bile acids are then delivered to the intestines where they aid in absorption of lipids. Some bile acids are lost during this process; however, a majority of bile acids delivered to the intestine are recycled by absorption in the ileum and returned to the liver. Back in the liver glyco- and tauroconjugate bile acids are formed and moved to the gall bladder for storage and use again as digestion aids.
Figure 1.Bile Acid Biosynthesis Pathway.