Bile duct cells: a novel in vitro model for the study of lipid metabolism and bile acid production

Immortalized bile duct cells (BDC), derived from transgenic mice harboring the SV40 thermosensitive immortalizing mutant gene ts458, were utilized to investigate the role of the biliary epithelium in lipid and sterol metaboli sm. This cell model closely resembles the in vivo situation because it expr esses the specific phenotypic marker cytokeratin 19 (CK-19), exhibits the f ormation of bile duct-like structures, and displays well-formed microvilli projected from the apical side to central lumen. The BDC were found to inco rporate [C-14]oleic acid (in nmol/mg protein) into triglycerides (121 +/- 6 ), phospholipids (PL; 59 +/- 3), and cholesteryl ester (16 +/- I). The medi um lipid content represented 5.90 +/- 0.16% (P < 0.005) of the total intrac ellular production, indicating a limited lipid export capacity. Analysis of PL composition demonstrated the synthesis of all classes of polar lipids, with phosphatidylcholine and phosphatidylethanolamine accounting for 60 +/- 1 and 24 +/- 1%, respectively, of the total. Differences in PL distributio n were apparent between cells and media. Substantial cholesterol synthesis was observed in BDC, as determined by the incorporation of [C-14]acetate su ggesting the presence of hydroxymethylglutaryl-CoA (HMG-CoA) reductase, the rate-limiting enzyme in the cholesterol biosynthetic pathway. With the use of [C-14]acetate and [C-14]cholesterol as precursors, both tauro- and glyc oconjugates of bile acids were synthesized, indicating the presence of chol esterol 7 alpha- and 26R-hydroxylases, the key enzymes involved in bile aci d formation. The transport of bile acids was not limited, as shown by their marked accumulation in the medium (>6-fold of cell content). HMG-CoA reduc tase (53.0 +/- 6.7), cholesterol 7 alpha-hydroxylase (15.5 +/- 0.5), and ac yl-CoA:cholesterol acyltransferase (ACAT; 201.7 +/-. 10.2) activities (in p mol . min(-l) mg protein(-1)) were present in the microsomal fractions. Our data show that biliary epithelial cells actively synthesize lipids and may directly contribute bile acids to the biliary fluid in vivo. This BDC line thus represents an efficient experimental tool to evaluate biliary epithel ium sterol metabolism and to study biliary physiology.