Classically, adult humans have been considered not to possess active brown adipose tissue (BAT). However, positron-emission-tomography has shown fluorodeoxyglucose uptake that is distributed in such a way (e.g., in the neck) that it would seem to be BAT. Until now this has not been supported by direct evidence that these areas truly represented BAT, that is, the presence of the BAT-unique uncoupling protein-1 (UCP1). Samples of adipose tissue from the neck of 35 patients undergoing surgery for thyroid diseases were obtained and analyzed. In 1/3 of the subjects (the younger and leaner), distinct islands composed of UCP1 immunoreactive brown adipocytes could clearly be discerned, accounting for up to 1/3 of all adipocytes. The brown-adipose islands were richly sympathetically innervated (indicating acute central control); adjacent white adipose areas were not. The capillary density was high, implying a high capacity for oxygen delivery. Cells with features of brown adipocyte precursors were found in pericapillary areas. These data demonstrate that human adults indeed possess BAT and thus imply possibilities of future therapeutic strategies for the treatment of obesity, including maintenance of brown adipocytes and stimulation of the growth of preexisting brown precursors.

to occupy distinct anatomical sites in the body. However, previous work, mainly from our lab, supports the notion that WAT and BAT are in fact found together in subcutaneous and visceral fat depots, collectively forming a multidepot organ that we have called the "adipose organ" ( 4,5 ). This fi nding has opened new perspectives in the physiological relationship between BAT and WAT, including the possibility of their reciprocal transformation (transdifferentiation) ( 6-8 ). Harnessing the mechanism of WAT to BAT transdifferentiation could be useful to develop treatments for obesity and type 2 diabetes, because the absence of BAT or its ␤ adrenergic receptors results in obesity ( 9, 10 ) and transgenic mice overexpressing UCP1 in WAT are obesity resistant ( 11 ). Furthermore, treatment of obese rodents with ␤ 3 agonists increases BAT and curbs obesity ( 12, 13 ). Recently, metabolically active BAT has been described in adult humans. Of note, these subjects have a lower body mass index (BMI) and less visceral fat than those without detectable . C57BL/6J mice are obesity-and type 2 diabetes-prone ( 21 ). In fact, earlier work has shown that C57BL/6J mice are more predisposed to store fat in response to a high-fat diet and to develop obesity, hyperglycemia, and hyperinsulinemia than their obesity-resistant A/J counterparts ( 22 ). Furthermore, it has been suggested that the obesity and diabetes resistance of A/J mice may be due to a strong increase in brown adipocytes in some "classic" white adipose depots after cold exposure or treatment with a ␤ 3 adrenergic agonist ( 23, 24 ). Also, a previous work by our group showed that intermuscular fat in the hind legs of C57BL/6J mice contains fewer brown adipocytes than the intermuscular fat of obesity-resistant Sv129 mice (substrain 129/SVPAS SPF/VAF), suggesting the possibility that a difference in BAT amount could explain the susceptibility to obesity and type 2 diabetes of C57BL/6J Abstract White and brown adipocytes are believed to occupy different sites in the body. We studied the anatomical features and quantitative histology of the fat depots in obesity and type 2 diabetes-prone C57BL/6J mice acclimated to warm or cold temperatures. Most of the fat tissue was contained in depots with discrete anatomical features, and most depots contained both white and brown adipocytes. Quantitative analysis showed that cold acclimation induced an increase in brown adipocytes and an almost equal reduction in white adipocytes; however, there were no signifi cant differences in total adipocyte count or any signs of apoptosis or mitosis, in line with the hypothesis of the direct transformation of white into brown adipocytes. The brown adipocyte increase was accompanied by enhanced density of noradrenergic parenchymal nerve fi bers, with a signifi cant correlation between the density of these fi bers and the number of brown adipocytes. Comparison with data from obesity-resistant Sv129 mice disclosed a signifi cantly different brown adipocyte content in C57BL/6J mice, suggesting that th...

In all mammals, white adipose tissue (WAT) and brown adipose tissue (BAT) are found together in several fat depots, forming a multi-depot organ. Adrenergic stimulation induces an increase in BAT usually referred to as "browning". This phenomenon is important because of its potential use in curbing obesity and related disorders; thus, understanding its cellular mechanisms in humans may be useful for the development of new therapeutic strategies. Data in rodents have supported the direct transformation of white into brown adipocytes. Biopsies of pure white omental fat were collected from 12 patients affected by the catecholamine-secreting tumor pheochromocytoma (pheo-patients) and compared with biopsies from controls. Half of the omental fat samples from pheo-patients contained uncoupling protein 1 (UCP1)-immunoreactive-(ir) multilocular cells that were often arranged in a BAT-like pattern endowed with noradrenergic fibers and dense capillary network. Many UCP1-ir adipocytes showed the characteristic morphology of paucilocular cells, which we have been described as cytological marker of transdifferentiation. Electron microscopy showed increased mitochondrial density in multi- and paucilocular cells and disclosed the presence of perivascular brown adipocyte precursors. Brown fat genes, such as UCP1, PR domain containing 16 (PRDM16) and β3-adrenoreceptor, were highly expressed in the omentum of pheo-patients and in those cases without visible morphologic re-arrangement. Of note, the brown determinant PRDM16 was detected by immunohistochemistry only in nuclei of multi- and paucilocular adipocytes. Quantitative electron microscopy and immunohistochemistry for Ki67 suggest an unlikely contribution of proliferative events to the phenomenon. The data support the idea that, in adult humans, white adipocytes of pure white fat that are subjected to adrenergic stimulation are able to undergo a process of direct transformation into brown adipocytes. This article is part of a Special Issue entitled Brown and White Fat: From Signaling to Disease.

Brown adipocytes oxidize fatty acids to produce heat in response to cold or to excessive energy intake; stimulation of brown fat development and function may thus counteract obesity. Brown adipogenesis requires activation of the transcription factor C/EBPβ and recruitment of the zinc finger protein Prdm16, but upstream inducers of these proteins are incompletely defined. Here, we show that genetic inactivation of Plac8, a gene encoding an evolutionarily conserved protein, induces cold intolerance, and late-onset obesity, as well as abnormal morphology and impaired function of brown adipocytes. Using brown preadipocyte lines we show that Plac8 is required for brown fat differentiation, that its overexpression induces C/EBPβ and Prdm16, and that upon induction of differentiation Plac8 associates with C/EBPβ and binds to the C/EBPβ promoter to induce its transcription. Thus, Plac8 is a critical upstream regulator of brown fat differentiation and function that acts, at least in part, by inducing C/EBPβ expression.

Obesity is characterized by insulin-resistance (IR), enhanced lipolysis, and ectopic, inflamed fat. We related the histology of subcutaneous (SAT), visceral fat (VAT), and skeletal muscle to the metabolic abnormalities, and tested their mutual changes after bariatric surgery in type 2 diabetic (T2D) and weight-matched non-diabetic (ND) patients. We measured IR (insulin clamp), lipolysis (2H5-glycerol infusion), ß-cell glucose-sensitivity (ß-GS, mathematical modeling), and VAT, SAT, and rectus abdominis histology (light and electron microscopy). Presurgery, SAT and VAT showed signs of fibrosis/necrosis, small mitochondria, free interstitial lipids, thickened capillary basement membrane. Compared to ND, T2D had impaired ß-GS, intracapillary neutrophils and higher intramyocellular fat, adipocyte area in VAT, crown-like structures (CLS) in VAT and SAT with rare structures (cyst-like) ~10-fold larger than CLS. Fat expansion was associated with enhanced lipolysis and IR. VAT histology and intramyocellular fat were related to impaired ß-GS. Postsurgery, IR and lipolysis improved in all, ß-GS improved in T2D. Muscle fat infiltration was reduced, adipocytes were smaller and richer in mitochondria, and CLS density in SAT was reduced. In conclusion, IR improves proportionally to weight loss but remains subnormal, whilst SAT and muscle changes disappear. In T2D postsurgery, some VAT pathology persists and beta-cell dysfunction improves but is not normalized.