LIVER BILE SALTS Anson Lowe Medicine October 2, 2015 METHODS IN ENZYMOLOGY, VOL. 400 Understand the physiologic function of bile salts The determinants and regulation of bile salts synthesis Enterohepatic recycling Bile salts; enterohepatic recycling; cholesterol Liver Functions • Bilirubin metabolism • Protein Synthesis – Albumin – Coagulation factors (II, V, VII, IX, X) • • • • Bile Salt Metabolism Lipid Metabolism Glycogen storage and gluconeogenesis Drug metabolism/Xenobiotic transformation Bile H20 84% Bile Salts 11.5% Phosphatidyl Choline (lecithin) Bile pigments, protein, inorganic ions 3.0% 1.0% Bile Salt Function Lipid digestion and absorption Essential for the absorption of cholesterol and the fat soluble vitamins; A, D, E, and K. Major route for cholesterol degradation Influences cholesterol synthesis and degradation Bile Salts Synthesized in the liver and excreted into the biliary system Stored in the gallbladder Participate in lipid digestion and absorption in the intestine. Recycled through absorption in the terminal ileum and transport back to the liver Undergraduate Teaching Project ©AGA, Unit 27, slide 54, produced by Milner-Fenwick, Timonium, MD. Bile Salt Synthesis Cholesterol is the starting material ◦ hydrophobic Through a series of hydroxylation steps, cholesterol is turned into either cholic acid or chenodeoxycholic acid Classical Pathway CYP7A1 - this enzyme marks the entry point into the classical pathway. ◦ Hydroxylates the 7a carbon of cholesterol ◦ Expressed only in the liver ◦ Can be regulated Negative feedback based on bile salt concentration CYP7A1 Regulation FXR - a transcription factor characterized by a DNA binding domain and a ligand binding domain that binds chenodeoxycholic and cholic acid. CYP7A1 Regulation Bile salts will bind to FXR, which promotes an association with other transcription factors and results in negative feedback on CYP7A1. ◦ Thus the negative feedback on CYP7A1 inhibits cholesterol breakdown and potentially contributes to higher levels serum cholesterol and increased cardiovascular risk. CYP7A1 Regulation FXR responds to the the level of oxysterols (derived from cholesterol). ◦ Reduces CYP7A1 activity ◦ Promotes cholesterol efflux from the enterocyte CYP7A1 Regulation Approaches for decreasing cholesterol levels and cardiovascular risk. ◦ Lowering the bile salt pool size Resection of the terminal ileum to reduce bile salt recycling Use of bile sequestrants, which bind and removes bile salts in the feces. ◦ Develop agents that inhibit FXR binding to the CYP7A1 DNA regulatory domain. Tu et al. TCB 10:30 (2000) FXR Regulation Positively regulates IBABP (also known as ILLBP) in the enterocyte where bile salts are absorbed and recycled in the terminal ileum. CYP7A1 Deficiency First identified in 2 brothers with a frameshift mutation that disrupts CYP7A1 expression ◦ Early gallstone disease ◦ Hypercholesterolemia Hypothesis: decrease degradation of cholesterol > down regulates LDL receptor > serum cholesterol levels rise ◦ Hypertriglyceridemia Fuchs, Am J Physiol (2003) 284:G551 Abdelmalek et al., J Am Acad Dermatol, 46:161 (2002) Abdelmalek et al., J Am Acad Dermatol, 46:161 (2002) CYP27A1 Deficiency Decrease in total bile acid synthesis, but have normal cholesterol levels Cerebrotendinous xanthomatosis ◦ ◦ ◦ ◦ Tendon xanthomatosis Accumulation of cholesterol in all tissues Progressive neurologic dysfunction Premature atherosclerosis Patients respond to administration of chenodeoxycholic acid ◦ ? mechanism Bile Acid Conjugation Bile Acid Conjugation pKa Cholic Acid 5.0 Glycocholate Acid 3.9 Taurocholate Acid 2.0 Bile Acid Conjugation Bile acid CoA:Amino Acid N-acyltransferase (BAAT) ◦ Conjugation with either glycine or taurine is mediated by BAAT Familial Hypercholanemia (FHC) Mutations in BAAT resulting in high serum bile acid concentrations ◦ Low transport out of the hepatocyte Fat malabsorption ◦ Coagulation defects because of vitamin K malabsorption ◦ steatorrhea Itching - deposition of bile acids in the skin Familial Hypercholanemia (FHC) Treatment ◦ Administration of ursodeoxycholic acid Source of conjugated bile acids for lipid absorption Provides feedback inhibition of bile acid synthesis Undergraduate Teaching Project ©AGA, Unit 27, slide 54, produced by Milner-Fenwick, Timonium, MD. Bacterial Affects on Bile Acid 7a dehydroxylation ◦ Chenodeoxycholic acid > lithocholic acid ◦ Cholic acid > deoxycholic acid Deconjugation > increases hydrophobicity Bile Acid Absorption Conjugated bile acids are reabsorbed by specific uptake mechanisms Unconjugated bile acids are passively absorbed Bile Acid Uptake Conjugated bile acids are taken up in the terminal ileum by a specific sodium dependent transporter, SLC10A2 (IBAT, ileal sodium/bile salt transporter). ◦ SLC10A2 is specifically expressed in the ileum Primary Bile Acid Malabsorption (PBAM) Features ◦ Congenital diarrhea, steatorrhea, reduced plasma cholesterol levels, loss of enterohepatic recirculation of bile acids Mutations in SLC10A2 Primary Bile Acid Malabsorption - SLC10A2 Oelkers et al., J Clin Invest.,(1997) 99:1880 Ileal Lipid-Binding Protein Also known as IBABP, ILLBP, ILBP. Mediates cytoplasmic transport of bile acids Specifically expressed in the ileum Expression is regulated by FXR, which enhances bile acid uptake Wolkoff, A.w. and Cohen, D.E., Am J Physiol 284:G175 (2003) Undergraduate Teaching Project ©AGA, Unit 27, slide 54, produced by Milner-Fenwick, Timonium, MD. Trauner et al., NEJM (1998) 339:1217 Bile Salt Export ATP-dependent transport by the Bile Salt Export Pump (BSEP). Also called SPGP. ◦ Hepatocyte specific expression Progressive Familial Intrahepatic Cholestasis, type 2 (PFIC2) Autosomal recessive characterized by severe cholestasis that results in liver failure and death Mutations in BSEP were found by positional cloning Bile Salt: Hepatocyte > Bile Duct > Gallbladder > Duct Bile Formation Bile Acids Phospholipids Cholesterol Bilirubin Water and bicarbonate Phospholipid (Lecithin) Secretion MDR3 - ATP dependent transporter for phospholipids. Progressive Familial Intrahepatic Cholestasis, type 3 (PFIC3) Most mutations have resulted in a prematurely truncated MDR3 protein Presents with childhood jaundice, icterus, diarrhea, hepatosplenomegaly Liver histology: bile duct proliferation and fibrosis, portal inflammation Therapy: liver transplantation Intrahepatic Cholestasis of Pregnancy (ICP) Usually presents in the 3rd trimester with jaundice Exhibits a higher rate of fetal death Maternal cholestasis resolves after delivery Patients are heterozygotes for a missense mutation in MDR3 Cholesterol Secretion Cholesterol secretion is influenced to an extent by phospholipid secretion ◦ Mice in which only one MDR3 gene was disrupted displayed half the normal phospholipid secretion but normal cholesterol secretion ◦ Homozygous knockouts had no cholesterol secretion Recently defined transporters for cholesterol have been discovered and are likely to play a role Lecithin = phospholipid Copyright ©2002 American Society for Clinical Investigation Feldman: Sleisenger & Fordtran’s Gastrointestinal and Liver Disease, 7th ed. Undergraduate Teaching Project ©AGA, Unit 27, slide 54, produced by Milner-Fenwick, Timonium, MD.