title: Biliary bicarbonate secretion constitutes a protective mechanism against bile acid-induced injury in man.
abstract: BACKGROUND
  Cholangiocytes expose a striking resistance against bile acids: while other cell types, such as hepatocytes, are susceptible to bile acid-induced toxicity and apoptosis already at micromolar concentrations, cholangiocytes are continuously exposed to millimolar concentrations as present in bile. We present a hypothesis suggesting that biliary secretion of HCO(3)(-) in man serves to protect cholangiocytes against bile acid-induced damage by fostering the deprotonation of apolar bile acids to more polar bile salts. Here, we tested if bile acid-induced toxicity is pH-dependent and if anion exchanger 2 (AE2) protects against bile acid-induced damage.
  
  
  METHODS
  A human cholangiocyte cell line was exposed to chenodeoxycholate (CDC), or its glycine conjugate, from 0.5 mM to 2.0 mM at pH 7.4, 7.1, 6.7 or 6.4, or after knockdown of AE2. Cell viability and apoptosis were determined by WST and caspase-3/-7 assays, respectively.
  
  
  RESULTS
  Glycochenodeoxycholate (GCDC) uptake in cholangiocytes is pH-dependent. Furthermore, CDC and GCDC (pK(a) 4-5) induce cholangiocyte toxicity in a pH-dependent manner: 0.5 mM CDC and 1 mM GCDC at pH 7.4 had no effect on cell viability, but at pH 6.4 decreased viability by >80% and increased caspase activity almost 10- and 30-fold, respectively. Acidification alone had no effect. AE2 knockdown led to 3- and 2-fold enhanced apoptosis induced by 0.75 mM CDC or 2 mM GCDC at pH 7.4.
  
  
  CONCLUSIONS
  These data support our hypothesis of a biliary HCO(3)(-) umbrella serving to protect human cholangiocytes against bile acid-induced injury. AE2 is a key contributor to this protective mechanism. The development and progression of cholangiopathies, such as primary biliary cirrhosis, may be a consequence of genetic and acquired functional defects of genes involved in maintaining the biliary HCO(3)(-) umbrella.
authors.affiliation: Department of Gastroenterology and Hepatology, Tytgat Institute for Liver and Intestinal Research, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Simon
authors.initials: S
authors.lastname: Hohenester
authors.forename: Lucas
authors.initials: L
authors.lastname: Maillette de Buy Wenniger
authors.forename: Douglas M
authors.initials: DM
authors.lastname: Jefferson
authors.forename: Ronald P
authors.initials: RP
authors.lastname: Oude Elferink
authors.forename: Ulrich
authors.initials: U
authors.lastname: Beuers
mesh_terms: D027321:Anion Transport Proteins / Q000378:metabolism; D017920:Antiporters / Q000378:metabolism; D001639:Bicarbonates / Q000378:metabolism*; D001647:Bile Acids and Salts / Q000633:toxicity*; D001652:Bile Ducts / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology*; D045744:Cell Line, Tumor; D055785:Gene Knockdown Techniques; D006801:Humans; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D008954:Models, Biological; D064606:SLC4A Proteins
chemical_list: D027321:Anion Transport Proteins; D017920:Antiporters; D001639:Bicarbonates; D001647:Bile Acids and Salts; D064606:SLC4A Proteins