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Departments of 1 Biochemistry and 2 Medicine, Marion Bessin Liver Research Center, Albert Einstein College of Medicine, Bronx, New York 10461; and 3 Department of Medicine and Liver Center, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, New Jersey 07103
Human obesity is
associated with elevated plasma leptin levels. Obesity is also an
important risk factor for cholesterol gallstones, which form as a
result of cholesterol hypersecretion into bile. Because leptin levels
are correlated with gallstone prevalence, we explored the effects of
acute leptin administration on biliary cholesterol secretion using lean
(FA/
) and obese (fa/fa) Zucker rats. Zucker
(fa/fa) rats become obese and hyperleptinemic due to
homozygosity for a missense mutation in the leptin receptor, which
diminishes but does not completely eliminate responsiveness to leptin.
Rats were infused intravenously for 12 h with saline or
pharmacological doses of recombinant murine leptin (5 µg · kg
1 · min
1)
sufficient to elevate plasma leptin concentrations to 500 ng/ml compared with basal levels of 3 and 70 ng/ml in lean and obese rats,
respectively. Obesity was associated with a marked impairment in
biliary cholesterol secretion. In biles of obese compared with lean
rats, bile salt hydrophobicity was decreased whereas
phosphatidylcholine hydrophobicity was increased. High-dose leptin
partially normalized cholesterol secretion in obese rats without
altering lipid compositions, implying that both chronic effects of
obesity and relative resistance to leptin contributed to impaired
biliary cholesterol elimination. In lean rats, acute leptin
administration increased biliary cholesterol secretion rates. Without
affecting hepatic cholesterol contents, leptin downregulated
hepatic activity of 3-hydroxy-3-methylglutaryl-coenzyme A
(HMG-CoA) reductase, upregulated activities of both sterol
27-hydroxylase and cholesterol 7
-hydroxylase, and lowered plasma
very low-density lipoprotein cholesterol concentrations. Increased
biliary cholesterol secretion in the setting of decreased cholesterol
biosynthesis and increased catabolism to bile salts suggests that
leptin promotes elimination of plasma cholesterol.
obesity; liver; bile salts; phospholipids; lipoproteins
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