Brown fat cells are specialized to dissipate energy and can counteract obesity; however, the transcriptional basis of their determination is largely unknown. We show here that the zinc-finger protein PRDM16 is highly enriched in brown fat cells compared to white fat cells. When expressed in white fat cell progenitors, PRDM16 activates a robust brown fat phenotype including induction of PGC-1alpha, UCP1, and type 2 deiodinase (Dio2) expression and a remarkable increase in uncoupled respiration. Transgenic expression of PRDM16 at physiological levels in white fat depots stimulates the formation of brown fat cells. Depletion of PRDM16 through shRNA expression in brown fat cells causes a near total loss of the brown characteristics. PRDM16 activates brown fat cell identity at least in part by simultaneously activating PGC-1alpha and PGC-1beta through direct protein binding. These data indicate that PRDM16 can control the determination of brown fat fate.

Abstract␤ 3 -adrenoceptors are involved in metabolism, gut relaxation, and vascular vasodilation. However, their existence and role in the human heart have not been documented. We investigated the effects of several ␤ -adrenoceptor agonists and antagonists on the mechanical properties of ventricular endomyocardial biopsies. In the presence of nadolol, a ␤ 1 -and ␤ 2 -adrenoceptor antagonist, isoprenaline produced consistent negative inotropic effects. Similar negative inotropic effects also resulted from the action of ␤ 3 -adrenoceptor agonists with an order of potency: BRL 37344 Ͼ SR 58611 Ϸ CL 316243 Ͼ CGP 12177. The dose-response curve to BRL 37344-decreasing myocardial contractility was not modified by pretreatment with nadolol, but was shifted to the right by bupranolol, a nonselective ␤ -adrenoceptor antagonist. ␤ 3 -adrenoceptor agonists also induced a reduction in the amplitude and an acceleration in the repolarization phase of the human action potential.

Obesity, i.e. an excess of white adipose tissue (WAT), predisposes to the development of type 2 diabetes and cardiovascular disease. Brown adipose tissue is present in rodents but not in adult humans. It expresses uncoupling protein 1 (UCP1) that allows dissipation of energy as heat. Peroxisome proliferator-activated receptor ␥ (PPAR␥) and PPAR␥ coactivator 1␣ (PGC-1␣) activate mouse UCP1 gene transcription. We show here that human PGC-1␣ induced the activation of the human UCP1 promoter by PPAR␥. Adenovirus-mediated expression of human PGC-1␣ increased the expression of UCP1, respiratory chain proteins, and fatty acid oxidation enzymes in human subcutaneous white adipocytes. Changes in the expression of other genes were also consistent with brown adipocyte mRNA expression profile. PGC-1␣ increased the palmitate oxidation rate by fat cells. Human white adipocytes can therefore acquire typical features of brown fat cells. The PPAR␥ agonist rosiglitazone potentiated the effect of PGC-1␣ on UCP1 expression and fatty acid oxidation. Hence, PGC-1␣ is able to direct human WAT PPAR␥ toward a transcriptional program linked to energy dissipation. However, the response of typical white adipocyte targets to rosiglitazone treatment was not altered by PGC-1␣. UCP1 mRNA induction was shown in vivo by injection of the PGC-1␣ adenovirus in mouse white fat. Alteration of energy balance through an increased utilization of fat in WAT may be a conceivable strategy for the treatment of obesity.

The mobilization of fat stored in adipose tissue is mediated by hormone-sensitive lipase (HSL) and the recently characterized adipose triglyceride lipase (ATGL), yet their relative importance in lipolysis is unknown. We show that a novel potent inhibitor of HSL does not inhibit other lipases. The compound counteracted catecholamine-stimulated lipolysis in mouse adipocytes and had no effect on residual triglyceride hydrolysis and lipolysis in HSL-null mice. In human adipocytes, catecholamine-and natriuretic peptide-induced lipolysis were completely blunted by the HSL inhibitor. When fat cells were not stimulated, glycerol but not fatty acid release was inhibited. HSL and ATGL mRNA levels increased concomitantly during adipocyte differentiation. Abundance of the two transcripts in human adipose tissue was highly correlated in habitual dietary conditions and during a hypocaloric diet, suggesting common regulatory mechanisms for the two genes. Comparison of obese and nonobese subjects showed that obesity was associated with a decrease in catecholamine-induced lipolysis and HSL expression in mature fat cells and in differentiated preadipocytes. In conclusion, HSL is the major lipase for catecholamine-and natriuretic peptide-stimulated lipolysis, whereas ATGL mediates the hydrolysis of triglycerides during basal lipolysis. Decreased catecholamine-induced lipolysis and low HSL expression constitute a possibly primary defect in obesity. Diabetes 54:3190 -3197, 2005 O besity, which is characterized by an excess of fat stores, is the most important risk factor for type 2 diabetes. Adipose tissue lipolysis leads to the hydrolysis of triglycerides and release of free fatty acids (FFAs). Because of the link between elevated circulating FFA levels and the development of insulin resistance and the metabolic syndrome (1,2), adipose tissue lipolysis constitutes a target for the drug industry. Nicotinic acid, which acts by inhibiting adipose tissue lipolysis, was the first extensively used lipid-lowering agent (3). Catecholamines and natriuretic peptides are the major hormones stimulating this catabolic pathway in humans (4). Resistance to catecholamine-induced lipolysis in subcutaneous adipose tissue has been demonstrated in obese adults and children (5,6) and is attributed to decreased expression of lipolytic ␤ 2 -adrenoceptors (7), increased antilipolytic properties of ␣ 2 -adrenoceptors (8), and decreased expression of hormone-sensitive lipase (HSL) (9). It is possible that the HSL defect is the most important factor because it is also observed in nonobese first-degree relatives to obese subjects (10) and because there is a positive relationship between lipolytic capacity and HSL expression in human subcutaneous fat cells (11).The rate-limiting role of HSL in adipose tissue lipolysis has been challenged by the data from HSL knockout mice (12-15). Catecholamine-induced lipolysis is abrogated, but residual basal lipolysis is observed in adipocytes from HSL-null mice. These data suggest the existence of non-HSL lipases in adipos...