Spirlı̀, Carlo

Intrahepatic bile duct cells (cholangiocytes) play an important role in the secretion and alkalinization of bile. Both Na+/H+exchange (NHE) and Cl−/[Formula: see text]exchange (AE) contribute to these functions, but their functional distribution between the apical and basolateral membrane domains remains speculative. We have addressed this issue in a normal rat cholangiocyte cell line (NRC-1), which maintains a polarized distribution of membrane markers. Gene expression of AE and NHE isoforms was studied by RT-PCR. For functional studies, cells were placed in a chamber that allowed separate perfusion of the apical and basolateral aspect of the epithelial sheet; intracellular pH (pHi) was measured by 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein microfluorometry. In[Formula: see text]-CO2free medium and in the presence of apical amiloride, pHirecovery from an acid load was Na+dependent and was inhibited by basolateral amiloride and by HOE-642 (10 μM), consistent with basolateral localization of the NHE1 isoform, which had clearly expressed mRNA. Apical Na+readmission induced a slow pHirecovery that was inhibited by apical administration of 1 mM HOE-642 or amiloride. Among the apical NHE isoforms, NHE2 but not NHE3 gene expression was detected. The AE1 gene was not expressed, but two different variants of AE2 mRNAs (AE2a and AE2b) were detected; pHiexperiments disclosed AE activities at both sides of the membrane, but only apical AE was activated by cAMP. In conclusion, these studies provide the first functional description of acid-base transporters in a polarized cholangiocyte cell line. NHE1, NHE2, AE2a, and AE2b isoforms are expressed and show different membrane polarity, functional properties, and sensitivity to inhibitors. These observations add a considerable level of complexity to current models of electrolyte transport in cholangiocytes.

In the liver, the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) regulates bile secretion and other functions at the apical membrane of biliary epithelial cells (i.e., cholangiocytes). CF‐related liver disease is a major cause of death in patients with CF. CFTR dysfunction affects innate immune pathways, generating a para‐inflammatory status in the liver and other epithelia. This study investigates the mechanisms linking CFTR to toll‐like receptor 4 activity. We found that CFTR is associated with a multiprotein complex at the apical membrane of normal mouse cholangiocytes, with proteins that negatively control Rous sarcoma oncogene cellular homolog (Src) activity. In CFTR‐defective cholangiocytes, Src tyrosine kinase self‐activates and phosphorylates toll‐like receptor 4, resulting in activation of nuclear factor kappa‐light‐chain‐enhancer of activated B cells and increased proinflammatory cytokine production in response to endotoxins. This Src/nuclear factor kappa‐light‐chain‐enhancer of activated B cells‐dependent inflammatory process attracts inflammatory cells but also generates changes in the apical junctional complex and loss of epithelial barrier function. Inhibition of Src decreased the inflammatory response of CF cholangiocytes to lipopolysaccharide, rescued the junctional defect in vitro, and significantly attenuated endotoxin‐induced biliary damage and inflammation in vivo (Cftr knockout mice). Conclusion: These findings reveal a novel function of CFTR as a regulator of toll‐like receptor 4 responses and cell polarity in biliary epithelial cells; this mechanism is pathogenetic, as shown by the protective effects of Src inhibition in vivo, and may be a novel therapeutic target in CF‐related liver disease and other inflammatory cholangiopathies. (Hepatology 2016;64:2118‐2134).