The beta-adrenergic receptors and the control of adipose tissue metabolism and thermogenesis

The beta-adrenergic receptors (betaARs) are members of the large family of G protein-coupled receptors. There are three betaAR subtypes (beta1AR, beta2AR beta3AR), each of which is coupled to Galphas and the stimulation of intracellular cAMP levels. While beta1AR and beta2AR are broadly expressed throughout tissues of the body, beta3AR is found predominantly in adipocytes. Stimulation of the betaARs leads to lipolysis in white adipocytes and nonshivering thermogenesis in brown fat. However, in essentially all animal models of obesity, the betaAR system is dysfunctional and the ability to stimulate lipolysis and thermogenesis is impaired. Nevertheless, we and others have shown that selective beta3AR agonists are able to prevent or reverse obesity and the loss of betaAR expression and to stimulate thermogenesis. This chapter will review the current understanding of the role of the sympathetic nervous system and the adipocyte betaARs in models of obesity; the physiologic impact of changes in betaAR expression on body composition and thermogenesis; and the regulation and unique properties of betaAR subtypes in brown and white adipocytes. The latter includes our recent discovery of novel signal transduction mechanisms utilized by beta3AR to activate simultaneously the protein kinase A and MAP kinase pathways. The impact of understanding these pathways and their potential role in modulating adaptive thermogenesis is discussed.