Reduced Hepatocyte Bicarbonate Output Accelerates Periductal Inflammation and Fibrosis in mdr2-/- Mouse Liver

Background/Aims: Mice deficient for the canalicular phospholipid transporter MDR2 (ABCB4) develop sclerosing cholangitis due to high biliary concentrations of monomeric bile acids. This study determines whether a selective reduction in biliary bicarbonate output, secondary to the deletion of the hepatocyte-expressed carbonic anhydrase CAXIV (Car14) aggravates the bile acid-induced damage observed in the mdr2-/- mouse model. Methods: Bile flow was measured gravimetrically and HCO3- output by microtitration before and during stimulation with intravenously applied tauroursodesoxycholic acid (TUDCA) in car14-/-mdr2-/- (abcb4-/-), car14-/-/mdr2+/+, car14+/+/mdr2-/- and wt mice. Cholangiocyte proliferation, hepatic inflammation and fibrosis was studied by gene and/or protein expression for proinflammatory and profibrotic cytokines, cholangiocyte proliferation markers, and by (immuno) histochemical assessment. The impact of Car14 deficiency was also assessed in a xenobiotic cholangitis model. Results: TUDCA stimulated HCO3-output was significantly increased in 6 week old mdr2-/- mice, and significantly decreased in both car14-/- as well as car14-/-/mdr2-/-mice, compared to wt, while bile flow was unaltered. Both bile flow and HCO3- output were significantly decreased in 11 week old mdr2-/-, and more so in car14-/-/mdr2-/- mice. Loss of Car14 significantly increased inflammatory liver injury and cholangiocyte proliferation, and aggravated liver fibrosis in car14-/-/mdr2-/- mice compared to mdr2-/- mice. In contrast, the absence of Car14 did not affect the hepatic functional and morphological alterations in 3,5-diethoxycarbonyl-1,4-dihydroxychollidine (DDC) fed mice. Conclusions: Car14 deletion reduced biliary HCO3- output and aggravated the functional, inflammatory and morphological alterations in the liver of mdr2-/-mice. These results demonstrate the importance of sufficient hepatocellular bicarbonate output in the protection of the hepatobiliary epithelium against toxic bile acids.