Bile acids stimulate cholangiocyte fluid secretion by activation of transmembrane member 16A Cl− channels

Abstract

Bile acids stimulate a bicarbonate‐rich choleresis, in part, through effects on cholangiocytes. Because Cl− channels in the apical membrane of cholangiocytes provide the driving force for secretion and transmembrane member 16A (TMEM16A) has been identified as the Ca2+‐activated Cl− channel in the apical membrane of cholangiocytes, the aim of the present study was to determine whether TMEM16A is the target of bile‐acid–stimulated Cl− secretion and to identify the regulatory pathway involved. In these studies of mouse, rat, and human biliary epithelium exposure to ursodeoxycholic acid (UDCA) or tauroursodeoxycholic acid (TUDCA) rapidly increased the rate of exocytosis, ATP release, [Ca2+]i, membrane Cl− permeability, and transepithelial secretion. Bile‐acid–stimulated Cl− currents demonstrated biophysical properties consistent with TMEM16A and were inhibited by pharmacological or molecular (small‐interfering RNA; siRNA) inhibition of TMEM16A. Bile acid–stimulated Cl− currents were not observed in the presence of apyrase, suramin, or 2‐aminoethoxydiphenyl borate (2‐APB), demonstrating that current activation requires extracellular ATP, P2Y, and inositol 1,4,5‐trisphosphate (IP3) receptors. TUDCA did not activate Cl− currents during pharmacologic inhibition of the apical Na+‐dependent bile acid transporter (ASBT), but direct intracellular delivery of TUDCA rapidly activated Cl− currents. Conclusion: Bile acids stimulate Cl− secretion in mouse and human biliary cells through activation of membrane TMEM16A channels in a process regulated by extracellular ATP and [Ca2+]i. These studies suggest that TMEM16A channels may be targets to increase bile flow during cholestasis. (Hepatology 2018;68:187‐199).

Effects of Endotoxin on Type 3 Inositol 1,4,5‐Trisphosphate Receptor in Human Cholangiocytes

“…First, ITPR3 plays an important role in regulating bicarbonate secretion in cholangiocytes. It is most concentrated in a specialized region of the endoplasmic reticulum (ER) beneath the apical membrane, where it is positioned to release calcium to activate the calcium‐dependent chloride channel, transmembrane member 16a, which, in turn, couples with the chloride‐bicarbonate exchanger, anion exchanger 2, to result in net bicarbonate secretion . This ITPR3 pool is effectively released by nucleotides in bile, which activate apical P2Y receptors, resulting in local formation of InsP3 .…”

“…It is most concentrated in a specialized region of the endoplasmic reticulum (ER) beneath the apical membrane, where it is positioned to release calcium to activate the calcium-dependent chloride channel, transmembrane member 16a, which, in turn, couples with the chloride-bicarbonate exchanger, anion exchanger 2, to result in net bicarbonate secretion. (25) This ITPR3 pool is effectively released by nucleotides in bile, which activate apical P2Y receptors, resulting in local formation of InsP3. (13) Although hepatocytes can secrete ATP into bile, (26,27) activation of the cystic fibrosis transmembrane conductance regulator in FIg.…”

Tauroursodeoxycholate—Bile Acid with Chaperoning Activity: Molecular and Cellular Effects and Therapeutic Perspectives

“…On the other hand, TUDCA was demonstrated to augment the secretory capacity of hepatocytes and cholangiocytes by stimulation of vesicular exocytosis through Ca 2+ -/cPKCα-dependent vesicle fusion process and insertion of key transport proteins into their target membrane [66,70]. Later research revealed that TUDCA exerts its choleretic activity via a dual p38 MAPK /integrin-dependent mechanism of apical carrier insertion [73,76] and stimulation of Cl − secretion in cholangiocytes through the activation of membrane Ca 2+ -activated Cl − channel (TMEM16A) [77]. In contrast, TUDCA exerts its anti-cholestatic properties mainly by enhancing secretory capacity of hepatocytes [78].…”

Calcium-Activated Cl− Channel: Insights on the Molecular Identity in Epithelial Tissues

“…While the majority of this section has been focused on respiratory and colonic epithelium, it is worth noting that TMEM16A has been characterized as the CaCC in several other epithelial tissues [ 73 , 74 , 75 ]. Cell lines of pancreatic ductal cells have been observed to express TMEM16A [ 73 , 75 ].…”