Vol. 276, Issue 2, G407-G414, February 1999
Bile duct cells: a novel in vitro model for the study of lipid
metabolism and bile acid production
Monika
Zoltowska,
Edgard E.
Delvin,
Khazal
Paradis,
Ernest
Seidman, and
Emile
Levy
Departments of Nutrition, Biochemistry, and Pediatrics, Centre de
Recherche, Hôpital Ste-Justine, Université de
Montréal, Montreal, Quebec, Canada H3T 1C5
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 metabolism. This
cell model closely resembles the in vivo situation because it expresses
the specific phenotypic marker cytokeratin 19 (CK-19), exhibits the
formation of bile duct-like structures, and displays well-formed
microvilli projected from the apical side to central lumen. The BDC
were found to incorporate [14C]oleic acid (in
nmol/mg protein) into triglycerides (121 ± 6), phospholipids (PL;
59 ± 3), and cholesteryl ester (16 ± 1). The medium lipid
content represented 5.90 ± 0.16%
(P < 0.005) of the total
intracellular 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 distribution were apparent between cells and media.
Substantial cholesterol synthesis was observed in BDC, as determined by
the incorporation of
[14C]acetate
suggesting the presence of hydroxymethylglutaryl-CoA (HMG-CoA)
reductase, the rate-limiting enzyme in the cholesterol biosynthetic
pathway. With the use of
[14C]acetate and
[14C]cholesterol as
precursors, both tauro- and glycoconjugates of bile acids were
synthesized, indicating the presence of cholesterol 7
- and
26R-hydroxylases, the key enzymes involved in bile acid 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
reductase (53.0 ± 6.7), cholesterol 7-hydroxylase (15.5 ± 0.5), and acyl-CoA:cholesterol acyltransferase (ACAT; 201.7 ± 10.2) activities (in
pmol · min
1 · mg
protein1) 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 epithelium sterol metabolism and
to study biliary physiology.
biliary epithelium; triglyceride; phospholipid; bile acid; sterol
metabolism; hydroxymethylglutaryl-CoA reductase; cholesterol
7-hydroxylase; acyl-CoA cholesterol acyltranferase
