Bile Acid Metabolism and Signaling in Cholestasis, Inflammation, and Cancer

Abstract

Bile acids are synthesized from cholesterol in the liver. Some cytochrome P450 (CYP) enzymes play key roles in bile acid synthesis. Bile acids are physiological detergent molecules, so are highly cytotoxic. They undergo enterohepatic circulation and play important roles in generating bile flow and facilitating biliary secretion of endogenous metabolites and xenobiotics and intestinal absorption of dietary fats and lipid-soluble vitamins. Bile acid synthesis, transport, and pool size are therefore tightly regulated under physiological conditions. In cholestasis, impaired bile flow leads to accumulation of bile acids in the liver, causing hepatocyte and biliary injury and inflammation. Chronic cholestasis is associated with fibrosis, cirrhosis, and eventually liver failure. Chronic cholestasis also increases the risk of developing hepatocellular or cholangiocellular carcinomas. Extensive research in the last two decades has shown that bile acids act as signaling molecules that regulate various cellular processes. The bile acid-activated nuclear receptors are ligand-activated transcriptional factors that play critical roles in the regulation of bile acid, drug, and xenobiotic metabolism. In cholestasis, these bile acid-activated receptors regulate a network of genes involved in bile acid synthesis, conjugation, transport, and metabolism to alleviate bile acid-induced inflammation and injury. Additionally, bile acids are known to regulate cell growth and proliferation, and altered bile acid levels in diseased conditions have been implicated in liver injury/regeneration and tumorigenesis. We will cover the mechanisms that regulate bile acid homeostasis and detoxification during cholestasis, and the roles of bile acids in the initiation and regulation of hepatic inflammation, regeneration, and carcinogenesis.

Keywords: Cholestasis; Cytochrome P450; Liver cancer; Liver regeneration; Nuclear receptor.

Figure 1. Bile acid synthetic pathways and bile acid structure

Cholesterol is the common precursor for bile acid synthesis via two major bile acid biosynthetic pathways. In the classic pathway, the rate-limiting enzyme cholesterol 7α-hydroxylase (CYP7A1) in the endoplasmic reticulum converts cholesterol into 7α-hydroxycholesterol. The 3β-hydroxysteroid dehydrogenase (3βHSD, HSD3B7) converts 7α-hydroxycholesterol to 7α-hydroxy-4 cholesten-3-one (C4). C4 can be converted to cholic acid (CA) which requires the sterol 12α-hydroxylase (CYP8B1). Without 12α-hydroxylation by CYP8B1, C4 is eventually converted to chenodeoxycholic acid (CDCA). In the classic pathway, the mitochondrial sterol 27-hydroxylase (CYP27A1) catalyzes the steroid side-chain oxidation in both CA and CDCA synthesis. In the alternative pathway, CYP27A1 catalyzes the first step to convert cholesterol to 27-hydroxycholesterol. Oxysterol 7α-hydroxylase (CYP7B1) catalyzes hydroxylation of 27-hydroxycholesterol to 3β, 7α-dihydroxy-5-cholestenoic acid, which eventually is converted to CDCA. In the large intestine, bacterial 7α-dehydroxylase removes a hydroxyl group from C-7 and converts CA to deoxycholic acid (DCA) and CDCA to lithocholic acid (LCA). In mouse liver, most of CDCA is converted to α- and β-muricholic acid (MCA).

Figure 2. Bile acid transporters in the Enterohepatic circulation

Bile acids, after synthesis, are secreted into the bile where bile acids, cholesterol and phospholipids form micelles. Food intake stimulates the gallbladder to release bile acids into the small intestine. Conjugated bile acids are efficiently re-absorbed in the ileum by active transport systems in the ileum, and a small amount of un-conjugated bile acids is reabsorbed by passive diffusion in the small and large intestine. Less than 5% of the bile acids is lost through fecal excretion, which is compensated by de novo synthesis in the liver. At the canalicular membrane of the hepatocytes, the bile salt export pump (BSEP) is the primary bile acid efflux transporter, while the multidrug resistance-associated protein-2 (MRP-2) can also secrete organic substrates including bile acids, bilirubin and glutathione. ABCG5 and ABCG8 form heterodimers and transport cholesterol into the bile, and multidrug resistance-2 (MDR2) is responsible for phospholipid secretion. At the basolateral membrane of the hepatocytes, The Na⁺-dependent taurocholate transporter (NTCP) is mainly responsible for Na+-dependent uptake of conjugated bile acids. The microsomal epoxide hydrolase (mEH) may also mediate Na+-dependent uptake of conjugated bile acids. The organic anion transporters (OATPs) show substrate specificity for unconjugated bile acids. At the basolateral membrane of the hepatocytes, organic solute transporters OSTα and OSTβ heterodimers, MRP3 and MRP4 secrete bile acids into the systemic circulation. In cholestasis, this pathway is induced and leads to subsequent renal bile acid excretion. In the intestine, the apical sodium-dependent bile acid transport (ASBT) mediates bile acid uptake in the ileum. Intracellular bile acids are bound to the intestinal bile acid binding protein (I-BABP) and are transported to the basolateral membrane where bile acids is secreted into the portal circulation by the OSTα and OSTβ heterodimer.

Figure 3. Nuclear receptors

The domain structure of a typical nuclear receptor contains a DNA binding domain (DBD) and a ligand binding domain (LBD). Nuclear receptors recognize consensus DNA sequence AGGTCA half site arranged in direct repeat (DR), everted repeat (ER) and inverted repeat (IR). Ligand binding causes nuclear receptor LBD conformational change, which allows the nuclear receptor to recruit coactivators to replace corepressors. Co-activators facilitate chromatin remodeling and the assembly of general transcriptional machinery, leading to transactivation of the target gene.

Figure 4. FXR regulation of bile acid feedback inhibition of bile acid synthesis and bile acid transport in the enterohepatic system

In hepatocytes, bile acids-activated FXR induces the repressor SHP, which interacts with and represses the trans-activating action of HNF4α and LRH-1, leading to CYP7A1 inhibition. Bile acid/FXR induces SHP to repress NTCP. FXR binds to BSEP gene promoter and induced BSEP and canalicular bile acid secretion. In the intestine, FXR activation inhibits ASBT and induces OSTα and OSTβ, and thus decreases bile acid absorption and promotes basolateral bile acid secretion. Bile acid-activated FXR induces FGF15 (FGF19 in humans). FGF15 binds and activates FGFR4 on the hepatocytes, leading to the inhibition of CYP7A1 gene, a process that may involve ERK1/2 signaling. The downstream target of FGF15/19 has not been well characterized.