Responses of brown adipose tissue to diet-induced obesity, exercise, dietary restriction and ephedrine treatment

Abstract

Drug-induced weight loss in humans has been associated with undesirable side effects not present in weight loss from lifestyle interventions (caloric restriction or exercise). To investigate the mechanistic differences of weight loss by drug-induced and lifestyle interventions, we examined the gene expression (mRNA) in brown adipose tissue (BAT) and conducted histopathologic assessments in diet-induced obese (DIO) mice given ephedrine (18 mg/kg/day orally), treadmill exercise (10 m/min, 1-h/day), and dietary restriction (DR: 26% dietary restriction) for 7 days. Exercise and DR mice lost more body weight than controls and both ephedrine and exercise reduced percent body fat. All treatments reduced BAT and liver lipid accumulation (i.e., cytoplasmic lipids in brown adipocytes and hepatocytes) and increased oxygen consumption (VO2 ml/kg/h) compared with controls. Mitochondrial biogenesis/function-related genes (TFAM, NRF1 and GABPA) were up-regulated in the BAT of all groups. UCP-1 was up-regulated in exercise and ephedrine groups, whereas MFSD2A was up-regulated in ephedrine and DR groups. PGC-1α up-regulation was observed in exercise and DR groups but not in ephedrine group. In all experimental groups, except for ephedrine, fatty acid transport and metabolism genes were up-regulated, but the magnitude of change was higher in the DR group. PRKAA1 was up-regulated in all groups but not significantly in the ephedrine group. ADRß3 was slightly up-regulated in the DR group only, whereas ESRRA remained unchanged in all groups. Although our data suggest a common pathway of BAT activation elicited by ephedrine treatment, exercise or DR, mRNA changes were indicative of additional nutrient-sensing pathways in exercise and DR.

Introduction

Excess energy intake over expenditure results in obesity, and it has been estimated that more than 50% of the European population are overweight (Body Mass Index, BMI > 25 kg/m²) and 30% are obese (BMI > 30 kg/m²) (Groves, 2006, Information Centre, 2008). In the United States, adult obesity and overweight (combined) prevalence was 68.0% of the population during 2007–2008 (Flegal et al., 2010). Furthermore, complications related to obesity (e.g., cardiovascular disease, type 2 diabetes, hypertension, metabolic syndrome, nonalcoholic fatty liver disease and cancer) continue to burden the health care system (an estimated expense of $147 billion a year) in the United States (Finkelstein et al., 2009, Booth et al., 2011). Common strategies to obtain a healthy BMI include changes in diet and exercise. But these lifestyle changes are not easily achieved, and most importantly, results may be moderate and transient. Recently, brown adipose tissue (BAT) has been proposed as a target to modify energy expenditure. BAT has been shown to maintain core temperature via thermogenesis by dissipating stored energy as heat. Morphologically, brown adipocytes have granular eosinophilic cytoplasm due to high content of large mitochondria with abundant cristae. These adipocytes are densely innervated by the sympathetic nervous system, and sympathetic neuronal release of norepinephrine activates ß-adrenergic receptors (ßARs) expressed on the surface of brown adipocytes, ultimately leading to the production of heat. Heat is generated by uncoupling oxidative phosphorylation from ATP production via uncoupling protein-1 (UCP-1), a BAT-specific proton transporter located in the inner mitochondrial membrane. In rodents, UCP-1 has been shown to play an important role in the regulation of energy expenditure and body weight. BAT activation causes body weight loss in rodents, and is likely to be protective against obesity-associated diseases (Ghorbani et al., 1997, Guerra et al., 1998).

In the past, BAT was thought to exist only in small mammals and infants, but recent data from positron emission tomography and computerized tomography have confirmed the presence of BAT depots in adult humans (Virtanen et al., 2009, Cypess and Kahn, 2010). It is now clear that human BAT depots are activated by cold exposure and adrenergic drugs (Cypess and Kahn, 2010). Mounting evidence suggests that BAT is protective against obesity and weight gain in humans. It has been estimated that as little as 50 g of maximally stimulated human BAT could utilize up to 20% of basal caloric needs (Rothwell and Stock, 1983), and therefore, the ability to stimulate energy expenditure via BAT thermogenesis might have important implications for the treatment of obesity and concurrent diseases in humans. Targeting BAT thermogenesis by drugs is a novel anti-obesity approach; however, it is critical to compare this drug-induced weight loss strategy with weight loss approaches that are generally considered safe, such as diet and exercise. In this study, we examined BAT adaptation to exercise, diet restriction (DR) and ephedrine, a sympathomimetic drug, in diet-induced obese (DIO) mice. Our aim was to better elucidate efficacy and toxicity-driven mechanism of weight loss and to characterize the mechanistic differences among these weight loss strategies by evaluating changes in gene expression (mRNA), body composition, CLAMs (Comprehensive Laboratory Animal Monitoring system) and histopathology in DIO mice.