Leakage of cytochrome c out of mitochondria is a well-recognized stimulus for apoptosis. Cholangiocytes are thus under a constant threat to become damaged and eliminated by way of apoptosis, although other forms of bile
acid–induced cholangiocyte death cannot be excluded.23 We hypothesize that cholangiocytes protect their check details apical surface against protonated apolar hydrophobic bile acid monomers by maintaining an alkaline pH above the apical membrane (Fig. 1). We think that a vital step in this process is the secretion of HCO at amounts high enough to form a HCO umbrella on the outer surface of the apical membrane. Isoforms of the Cl−/HCO exchanger, AE2, are responsible for the vast majority of biliary HCO secretion. Membrane-bound carboanhydrase
may propagate the HCO umbrella at the apical surface, which keeps the pH of bile high. A recent proteomics study also identified putatively soluble carboanhydrase in human bile.24 The protective HCO umbrella would markedly raise the pH of the luminal fluid near the apical surface and lead to deprotonation of apolar hydrophobic bile acids, rendering them unable to permeate membranes in GS1101 an uncontrolled fashion. This protective function of the biliary HCO umbrella might be equivalent to the protective layer of membrane-bound and secreted mucins in the stomach, the colon or the gallbladder mucosa cells.25 Human gallbladder mucosal cells express various membrane-bound (MUC3, MUC1) and secreted (MUC5B, MUC6, MUC5AC, MUC2) mucins.25 In contrast, cholangiocytes of the smallest intrahepatic bile ductules do not show relevant mucin expression, whereas large bile ducts express MUC3 and MUC5B and may occasionally express MUC1, MUC2, MUC5AC, and MUC6.25 Thus, the biliary HCO umbrella may form the key protective mechanism of human intrahepatic apical cholangiocyte membranes against apolar protonated hydrophobic bile acids. In line with this assumption, AE2 immunoreactivity in human liver has been demonstrated on apical membranes of hepatocytes as well
as small and large cholangiocytes,26 whereas cholangiocytes of small bile ductules in experimental animals have been shown to contribute MCE little to biliary HCO formation.27 We think that this protective mechanism is especially well developed in the human biliary tree as an adaptation to the human bile salt pool characterized by high levels of glycine-conjugated hydrophobic bile salts (pKa 4-5)—in contrast to, for example, the murine bile salt pool, which is dominated by taurine-conjugated hydrophobic bile salts (pKa 1-2)—although it probably also functions at a lower intensity in our evolutionary relatives. This is supported by the observation that rats can dramatically up-regulate their cholangiocyte HCO production.14 Failure to keep bile pH high enough to deprotonate bile acids supposedly has a detrimental effect on cholangiocytes.