Short communicationG protein-coupled receptor for nicotinic acid in mouse macrophages
Introduction
The ability of the hypolipidemic drug nicotinic acid to elevate high density lipoprotein cholesterol to a greater extent than other drugs is of particular interest in the treatment of atherosclerotic diseases [1]. Nicotinic acid and its metabolically stable derivative acipimox inhibit adenylate cyclase in adipocytes through a pertussis toxin-sensitive G protein [2], [3], which leads to decreased phosphorylation of hormone-sensitive lipase and inhibition of its activity. Skin flushing is a common side effect of nicotinic acid. Flushing is prevented or attenuated by cyclooxygenase inhibitors, which, however, do not affect the lipid-lowering effect of nicotinic acid [4]. This vasodilation has been attributed to the release of prostaglandin D2 in skin [5]. Since the prostaglandin D2 release is not accompanied by a release of histamine, it is assumed that the cell type which releases the prostaglandin is not the mast cell [5]. The formation of prostaglandin D2 has been attributed largely to a glutathione-requiring prostaglandin D synthase localized mainly in antigen-presenting cells [6]. However, there is no direct evidence for the presence of nicotinic acid receptors in these cells in skin. Since the activation of nicotinic acid receptors in skin macrophages might be the reason for the release of prostaglandin D2, the present study was set up to investigate if macrophages express G protein-coupled receptors for nicotinic acid, which have been characterized previously in rat adipocyte and spleen membranes [7], or if a nicotinic acid binding site distinct from this receptor is detectable.
Section snippets
Materials and methods
RAW 264.7 mouse macrophages (European Collection of Cell Cultures) were cultured in Dulbecco’s modified Eagle’s medium (DMEM) with 2 mM glutamine, 100 U/mL penicillin, 200 μg/mL streptomycin and 10% fetal bovine serum (FBS) in 175 cm2 flasks at 37° in the presence of 5% CO2. Confluent cultures were split 1:15 every 3–4 days. Incubations with pertussis toxin (50 ng/mL) were performed for 20 hr. Membranes from adherent and nonadherent cells were prepared separately after mechanical removal of adherent
Results and discussion
Membranes from adherent and nonadherent RAW 264.7 macrophages displayed specific []nicotinic acid binding. The affinity of []nicotinic acid measured in saturation experiments was identical to that previously found for the nicotinic acid receptor in rat adipocyte and spleen membranes [7] (Table 1). The density of []nicotinic acid binding sites in mouse macrophage membranes was considerably lower than in membranes from adipocytes or spleen (Table 1). Competition experiments with
Acknowledgements
The authors are grateful to Eugen Heinz for expert technical assistance. Acipimox was a generous gift from Pharmacia-Upjohn.
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