Resumen de Papel de las sales biliares en la hidrocoleresis estimulada por secretina
Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease of unknown etiopathogenesis, associated with autoimmune phenomena. Immunosuppressives, however, have low efficacy in the treatment of PBC, while ursodeoxycholic acid (UDCA) a choleretic bile salt that in humans elicits a bicarbonate-rich hypercholeresis ameliorates the prognosis of the disease in most patients. We have previously reported that untreated PBC patients have impaired biliary bicarbonate secretion in response to secretin, and that UDCA treatment is able to restore the secretin response. Moreover, untreated PBC patients show diminished liver expression of the Na+-independent Cl¿/ HCO3¿ anion exchanger 2 (AE2), a carrier which is involved in the biliary bicarbonate secretion. More recently, we have reported that secretin is able to increase the bile flow and the biliary bicarbonate secretion in the normal rat when the bile acid pool is maintained through the continuous infusion of taurocholate. Here, we aim to analyze the role of bile acids in the regulation of the secretin-stimulated hydrocholeresis. Thus, the role of UDCA, its conjugate tauroursodeoxycholate acid (TUDCA), and dehydrocholic acid (DHCA), on the secretin-stimulated hydrocholeresis was studied using both an in vivo normal rat model and an in vitro model of 3-dimentional culture of cholangiocytes. Moreover, we analyzed the molecular mechanisms through which bile acids modulate the secretin response in our in vivo and in vitro models.
Our in vivo experiments show that the hydrocholeresis stimulated by secretin in the normal rat varies depending on the type of bile acid employed. Thus, whereas UDCA or its conjugate TUDCA have a synergic effect with secretin on the bile flow, the DHCA-induced hydrocholeresis is not modulated by secretin. Secretin-stimulated bile flow in UDCA-infused normal rats was found to be dependent on microtubule polymerization, and involves PKC, PI3K and MAPK pathways; however, the DHCA-induced hydrocholeresis is microtubule-independent and does not involve the aforementioned signaling pathways in our animal model.
To validate the in vivo data, we employed an in vitro model with 3-dimentional culture of cholangiocytes that allows the analysis of the direct interaction of both bile salts and secretin with the cholangiocytes, excluding the role of hepatocytes. The changes in the area of the cystic structures of cholangiocytes upon the different treatments can be determined. Using this approach, we observed that UDCA needs to be conjugated with taurine to show a synergic effect with secretin, an effect which indeed occurs via a PKC, PI3K, and MAPK dependent mechanisms. Moreover, we could determine that it is also dependent on intracellular calcium and the PKA signaling pathway. On the other hand, the presence of DHCA did not affect the secretin response, suggesting that the DHCA-stimulated hydrocholeresis observed in vivo is cholangiocyte-independent, being mediated mainly through hepatocytes. Finally, the role of Ae2 in the expansion of the cystic structures of cholangiocytes stimulated by the combination of secretin and TUDCA was analyzed by gene knockdown. Our data show that the presence of an siRNA against Ae2 blocks the cystic expansion stimulated by the combination of secretin and TUDCA, suggesting that Ae2 is ultimately the carrier involved in the secretin-stimulated hydrocholeresis mediated by the luminal secretin of bicarbonate. Our present observations expand the understanding of the role of bile acids on the response to secretin, and the molecular mechanisms putatively involved in the therapeutic effects of UDCA.
