Recently, Almind et al. (2007) showed that the capacity to induce BAT/UCP1 expression around muscles confers resistance to obesity in mice. Similarly, Nagase et al. (1996) showed that treatment of obese yellow KK mice with the β3-AR agonist CL 316243 decreased the body weight and fat of the mice, and induced expression of UCP1 in skeletal muscle [as well as in BAT and white adipose tissue (WAT)]. Even though the authors contend that the β3-AR agonist induced UCP1 in myofibers, we can speculate that in fact the UCP1 mRNA detected in the muscle mass indeed originated from induced brown adipocytes around the muscle fibers, and that the “UCP” signal identified with an UCP1 antibody in the myofibers was in fact increased levels of UCP3 (Boss et al., 1997). These results support, in a more physiological setting, earlier results showing that overexpression of UCP1 in WAT of mice can prevent the development of obesity in genetic as well as dietary models of the disease (Kopecky et al., 1995, 1996a,b). In contrast, lack of BAT or UCP1 (at or close to thermoneutrality) induces obesity and diabetes in mice (Lowell et al., 1993; Hamann et al., 1996; Feldmann et al., 2009). Initially, it was hoped that β3-AR agonists would also enhance energy expenditure in humans but these drugs proved to be ineffective probably due to less than optimal bioavailability and/or pharmacokinetic properties of the compounds (Arch, 2002, 2008). Another likely explanation is that, unlike in rodents, in humans the β3-AR is expressed at much lower levels than the β1-AR and β2-AR in adipose tissues (WAT and BAT; Deng et al., 1996; Arch, 2008). Induction of brown adipocyte formation with drugs in humans, in order to enhance or restore healthy levels of BAT recruitment, is a feasible strategy to enhance energy expenditure but “druggable” molecular targets other than the β3-AR and PPARγ have yet to be identified (Harper et al., 2004; Ravussin and Kozak, 2004).
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Recruitment of