A Nuclear Receptor Ligand Down-Regulates Cytosolic Phospholipase A2 Expression to Reduce Bile Acid–Induced Cyclooxygenase 2 Activity in Cholangiocytes: Implications of Anticarcinogenic Action of Farnesoid X Receptor Agonists

Bile acids are considered to be involved in the development of biliary tract carcinoma, although the underlying mechanisms are yet to be established. The aims of this study were (1) to investigate the carcinogenic role of bile acids in the biliary system based on the arachidonate–prostanoid pathway and (2) to clarify the therapeutic role of a farnesoid X receptor (FXR) ligand that modifies bile acid metabolism. Immortalized mouse cholangiocytes were incubated with glycochenodeoxycholate (GCDC), taurocholate, taurochenodeoxycholate, taurodeoxycholate, and tauroursodeoxycholate. GCDC induced cyclooxygenase 2 (COX-2) expression (Western blotting, 1.7-fold; RT-PCR, 2.3-fold) and prostaglandin (PG) production (PGE₂, 6.3-fold; PGF_2a, 8.5-fold), whereas cytosolic phospholipase A₂ (cPLA₂) expression and activity were reduced. In contrast, no marked changes were induced by the other bile acids. When the same experiment was performed in the presence of a synthetic FXR ligand (GW4064), cPLA₂ expression and activity were reduced, although COX-2 expression was unchanged. GW4064 also suppressed PG generation by 40%. In conclusion, the present findings suggest a carcinogenic potential of GCDC. A synthetic FXR ligand (GW4064) inhibited the induction of COX-2 activity (detected as PG production) by GCDC, suggesting its anticarcinogenic potential. This effect seemed to be due to down-regulation of cPLA₂. FXR ligands may have therapeutic potential against biliary carcinogenesis, but a delivery system for these agents is still to be developed.