Chang JC, Beuers U, Oude Elferink RP et al.
Tytgat Institute for Liver and Intestinal Research, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
Digestive diseases (Basel, Switzerland). 2017.
Primary biliary cholangitis (PBC; previously referred to as primary biliary cirrhosis) is a chronic fibrosing cholangiopathy with the signature of an autoimmune disease and features of intrahepatic cholestasis. Immunosuppressing treatments are largely unsuccessful. Responsiveness to ursodeoxycholic acid and reduced expression of anion exchanger 2 (AE2) on canalicular membranes and small bile ducts underline the importance of bicarbonate transportation in its disease mechanism. Soluble adenylyl cyclase (sAC; ADCY10) is an evolutionarily conserved bicarbonate sensor that regulates apoptosis, barrier function and TNF signaling. Key Messages: The biliary epithelium defends against the toxic bile by bicarbonate secretion and by maintaining a tight barrier. Passive diffusion of weak acid conjugates (e.g. bile salts and other toxins) across plasma membrane is pH-dependent. Reduced AE2 expression results in both reduced bicarbonate secretion and accumulation of bicarbonate in the cells. Increased intracellular bicarbonate leads to increased sAC activity, which regulates bile salt-induced apoptosis. Reduced bicarbonate secretion causes more bile salts to enter cells, which further increase sAC activity by releasing intracellular Ca2+ store. In vitro studies demonstrate that inhibition of sAC not only corrects sensitization to bile salt-induced apoptosis as a result of AE2 down-regulation but also prevents bile salt-induced apoptosis altogether. Targeting sAC is also likely to slow down disease progression by strengthening the barrier function of biliary epithelia and by reducing oxidative stress as a result of chronic inflammation.sAC is a potential therapeutic target for PBC. More in vitro and in vivo studies are needed to understand how sAC regulates bile salt-induced apoptosis and to establish its therapeutic value in PBC and other cholestatic cholangiopathies.