Tumour necrosis factor-alpha (TNF-alpha), secreted by cells of the macrophage-monocyte lineage, has a well established role in inflammation and host-defence. The more recent discovery that adipocytes also secrete TNF-alpha has led to a substantial body of research implicating this molecule in the insulin resistance of obesity. However, little is known about the normal regulation of TNF-alpha release from human adipose tissue. In particular, it is not known whether adipocyte production of TNF-alpha is responsive to similar or different molecular regulators than those relevant to macrophages. TNF-alpha release from cultured human adipose tissue and isolated adipocytes was examined using an ELISA. Insulin, cortisol or the thiazolidinedione, BRL 49653, did not have a significant effect on TNF-alpha release from adipose tissue or isolated adipocytes. In contrast, lipopolysaccharide (LPS), a major stimulus of TNF-alpha protein production in monocytes and macrophages, resulted in a fivefold stimulation of TNF-alpha release from human adipose tissue. Significant stimulation of TNF-alpha release was also seen from isolated adipocytes, indicating that the increase in TNF-alpha release from adipose tissue in the presence of LPS is unlikely to be entirely attributable to contaminating monocytes or macrophages. Consistent with this observation was the finding that mRNA for CD14, a known cellular receptor for LPS, is expressed in human adipocytes. The increase in TNF-alpha protein release in response to LPS was blocked by an inhibitor of the matrix metalloproteinase responsible for the cleavage of the membrane-bound proform of TNF-alpha, indicating that this release represented regulated secretion and was not due to cell lysis. In conclusion, the regulation of TNF-alpha protein release from human adipose tissue and isolated adipocytes appears to be similar to its regulation in cell types more traditionally implicated in host defence. The production by the adipocyte of a range of molecules involved in host defence-TNF-alpha, factors D, B and C3, interleukin-6, and macrophage colony-stimulating factor--suggest that this cell type may make a significant contribution to innate immunity.
CP Sewter, JE Digby, F Blows, J Prins and S O'Rahilly
T V Novoselova, R Larder, D Rimmington, C Lelliott, E H Wynn, R J Gorrigan, P H Tate, L Guasti, The Sanger Mouse Genetics Project, S O’Rahilly, A J L Clark, D W Logan, A P Coll and L F Chan
Melanocortin receptor accessory protein 2 (MRAP2) is a transmembrane accessory protein predominantly expressed in the brain. Both global and brain-specific deletion of Mrap2 in mice results in severe obesity. Loss-of-function MRAP2 mutations have also been associated with obesity in humans. Although MRAP2 has been shown to interact with MC4R, a G protein-coupled receptor with an established role in energy homeostasis, appetite regulation and lipid metabolism, the mechanisms through which loss of MRAP2 causes obesity remains uncertain. In this study, we used two independently derived lines of Mrap2 deficient mice (Mrap2 tm1a/tm1a) to further study the role of Mrap2 in the regulation of energy balance and peripheral lipid metabolism. Mrap2 tm1a/tm1a mice have a significant increase in body weight, with increased fat and lean mass, but without detectable changes in food intake or energy expenditure. Transcriptomic analysis showed significantly decreased expression of Sim1, Trh, Oxt and Crh within the hypothalamic paraventricular nucleus of Mrap2 tm1a/tm1a mice. Circulating levels of both high-density lipoprotein and low-density lipoprotein were significantly increased in Mrap2 deficient mice. Taken together, these data corroborate the role of MRAP2 in metabolic regulation and indicate that, at least in part, this may be due to defective central melanocortin signalling.