Insulin and Leptin Acutely Regulate Cholesterol Ester Metabolism in Macrophages by Novel Signaling Pathways
- 1Department of Biochemistry and Molecular Biology, University College London, London
- 2School of Biochemistry and Genetics, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne, U.K.
Leptin is produced in adipose tissue and acts in the hypothalamus to regulate food intake. However, recent evidence also indicates a potential for direct roles for leptin in peripheral tissues, including those of the immune system. In this study, we provide direct evidence that macrophages are a target tissue for leptin. We found that J774.2 macrophages express the functional long form of the leptin receptor (ObRb) and that this becomes tyrosine-phosphorylated after stimulation with low doses of leptin. Leptin also stimulates both phosphoinositide 3-kinase (PI 3-kinase) activity and tyrosine phosphorylation of JAK2 and STAT3 in these cells. We investigated the effects of leptin on hormone-sensitive lipase (HSL), which acts as a neutral cholesterol esterase in macrophages and is a rate-limiting step in cholesterol ester breakdown. Leptin significantly increased HSL activity in J774.2 macrophages, and these effects were additive with the effects of cAMP and were blocked by PI 3-kinase inhibitors. Conversely, insulin inhibited HSL in macrophages, but unlike adipocytes, this effect did not require PI 3-kinase. These results indicate that leptin and insulin regulate cholesterol-ester homeostasis in macrophages and, therefore, defects in this process caused by leptin and/or insulin resistance could contribute to the increased incidence of atherosclerosis found associated with obesity and type 2 diabetes.
Address correspondence and reprint requests to Peter R. Shepherd, PhD, Department of Biochemistry and Molecular Biology, University College London, Gower Street, London, WC1E 6BT, U.K. E-mail:.
Received for publication 22 August 2000 and accepted in revised form 14 December 2000.
BSA, bovine serum albumin; CPT-cAMP, 8-(4-chlorophenylthio)-adenosine 3′:5′-cyclic monophosphate; DMEM, Dulbecco’s modified Eagle’s medium; HSL, hormone-sensitive lipase; nCE, neutral cholesterol esterase; PDE, phosphodiesterase; PI 3-kinase, phosphoinositide 3-kinase; PKA, protein kinase A; PKB, protein kinase B.