Unique inhibition of bile salt-induced apoptosis by lecithins and cytoprotective bile salts in immortalized mouse cholangiocytes

Bile duct epithelium is physiologically exposed to high concentrations of bile salts, suggesting the presence of a cytoprotective mechanism( s). The aim of this study was to clarify whether bile salts cause bile duct cell damage and to elucidate the mechanism( s) providing protection against such an action of bile salts. Immortalized mouse cholangiocytes were incubated with taurocholate, taurochenodeoxycholate, glycochenodeoxycholate ( GCDC), taurodeoxycholate, and tauroursodeoxycholate ( TUDC), followed by flow- cytometric analysis and caspase activity assay to evaluate the induction of apoptosis. GCDC time- dependently induced caspase 3 ( 3.4- fold)- and caspase 9 ( 1.4- fold)- mediated apoptosis of cholangiocytes, but this was inhibited by lecithins and TUDC. Further, expression of cholangiocyte bile salt transporters ( apical sodium- dependent bile salt transporter [ Asbt] and multidrug resistance protein 3 [ Mrp3]) was examined by RT- PCR and western blotting, and cholangiocyte bile salt uptake was determined using radiolabeled bile salts. Expression of cholangiocyte Asbt and Mrp3 was increased by bile salts, whereas lecithins interestingly reduced bile salt uptake to inhibit cholangiocyte apoptosis. In conclusion, bile salts themselves cause cholangiocyte apoptosis when absorbed by and retained inside the cell, but this is inhibited by washing out cytotoxic bile salts according to Mrp3, a rescue exporting molecule. Biliary lecithin is seemingly another cytoprotective player against cytotoxic bile salts, reducing their uptake, and this is associated with a reduced expression of Mrp3.