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Catecholamines play an important role in controlling white adipose tissue function and development. β- and α2-adrenergic receptors (ARs) couple positively and negatively, respectively, to adenylyl cyclase and are co-expressed in human adipocytes. Previous studies have demonstrated increased adipocyte α2/β-AR balance in obesity, and it has been proposed that increased α2-ARs in adipose tissue with or without decreased β-ARs may contribute mechanistically to the development of increased fat mass. To critically test this hypothesis, adipocyte α2/β-AR balance was genetically manipulated in mice. Human α2A-ARs were transgenically expressed in the adipose tissue of mice that were either homozygous (−/−) or heterozygous (+/−) for a disrupted β3-AR allele. Mice expressing α2-ARs in fat, in the absence of β3-ARs (β3-AR −/− background), developed high fat diet-induced obesity. Strikingly, this effect was due entirely to adipocyte hyperplasia and required the presence of α2-ARs, the absence of β3-ARs, and a high fat diet. Of note, obese α2-transgenic, β3 −/− mice failed to develop insulin resistance, which may reflect the fact that expanded fat mass was due to adipocyte hyperplasia and not adipocyte hypertrophy. In summary, we have demonstrated that increased α2/β-AR balance in adipocytes promotes obesity by stimulating adipocyte hyperplasia. This study also demonstrates one way in which two genes (α2 and β3-AR) and diet interact to influence fat mass.



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