To fulfill its role as the major energy-storing tissue, adipose has several unique properties that cannot be seen in any other organ, including an almost unlimited capacity to expand in a non-transformed state. As such, the tissue requires potent mechanisms to remodel, acutely and chronically. Adipocytes can rapidly reach the diffusional limit of oxygen during growth; hypoxia is therefore an early determinant that limits healthy expansion. Proper expansion requires a highly coordinated response among many different cell types, including endothelial precursor cells, immune cells, and preadipocytes. There are therefore remarkable similarities between adipose expansion and growth of solid tumors, a phenomenon that presents both an opportunity and a challenge, since pharmacological interventions supporting healthy adipose tissue adaptation can also facilitate tumor growth. IntroductionAdipose tissue (AT) can respond rapidly and dynamically to alterations in nutrient deprivation and excess through adipocyte hypertrophy and hyperplasia, thereby fulfilling its major role in wholebody energy homeostasis. AT remodeling is an ongoing process that is pathologically accelerated in the obese state, and thus, features such as reduced angiogenic remodeling, ECM overproduction, a heightened state of immune cell infiltration and subsequent proinflammatory responses prevail in many obese fat-pads (1). However, not all AT expansion is necessarily associated with pathological changes. The concept of the "metabolically healthy obese" state (2) suggests that some individuals can preserve systemic insulin sensitivity on the basis of "healthy" AT expansion, bypassing all of the aforementioned pathological consequences associated with obesity (3), thereby also avoiding the obesity-associated lipotoxic side effects. Many physiologically relevant processes important for human AT remodeling can be studied in rodent models, with the added advantage that processes related to AT expansion and reduction can occur at an extremely rapid rate. A 24-hour fast in a mouse is associated with a dramatic loss of AT mass and an acute remodeling process that involves rapid infiltration of macrophages; moreover, merely 24 to 48 hours of exposure to a high-fat diet (HFD) can cause a prompt increase in adipocyte size (4). AT is therefore an ideal model system to study rapid alterations in tissue expansion and reduction, as it adapts to a differential nutrient supply. Here, we will focus on key aspects of the intricate dynamics of AT remodeling and subsequent inflammatory consequences that arise from obesity.

examined the innate immune pathway in AbdSc AT from lean, obese, and T2DM subjects, and 4) examined the association of circulating LPS in T2DM subjects. The findings showed that LPS increased TLR-2 protein expression twofold (P Ͻ 0.05). Treatment of AbdSc adipocytes with LPS caused a significant increase in TNF-␣ and IL-6 secretion (IL-6, Control: 2.7 Ϯ 0.5 vs. LPS: 4.8 Ϯ 0.3 ng/ml; P Ͻ 0.001; TNF-␣, Control: 1.0 Ϯ 0.83 vs. LPS: 32.8 Ϯ 6.23 pg/ml; P Ͻ 0.001). NF-B inhibitor reduced IL-6 in AbdSc adipocytes (Control: 2.7 Ϯ 0.5 vs. NF-B inhibitor: 2.1 Ϯ 0.4 ng/ml; P Ͻ 0.001). AbdSc AT protein expression for TLR-2, MyD88, TRAF6, and NF-B was increased in T2DM patients (P Ͻ 0.05), and TLR-2, TRAF-6, and NF-B were increased in LPStreated adipocytes (P Ͻ 0.05). Circulating LPS was 76% higher in T2DM subjects compared with matched controls. LPS correlated with insulin in controls (r ϭ 0.678, P Ͻ 0.0001). Rosiglitazone (RSG) significantly reduced both fasting serum insulin levels (reduced by 51%, P ϭ 0.0395) and serum LPS (reduced by 35%, P ϭ 0.0139) in a subgroup of previously untreated T2DM patients. In summary, our results suggest that T2DM is associated with increased endotoxemia, with AT able to initiate an innate immune response. Thus, increased adiposity may increase proinflammatory cytokines and therefore contribute to the pathogenic risk of T2DM.toll-like receptors; adipocytes; nuclear factor-B; inflammation; insulin OBESITY IS KNOWN TO REPRESENT one of the single most important risk factors for the increased risk of type 2 diabetes mellitus (T2DM) and cardiovascular disease. In addition, an increase in central (visceral) adiposity confers higher metabolic risk. This increased metabolic risk is associated with subclinical inflammation, with several studies demonstrating increased levels of proinflammatory adipocytokines, such as IL-6 and TNF-␣ (32, 33), in patients with obesity and T2DM. Activation of proinflammatory adipocytokines in adipose tissue (AT) is coordinated through NF-B, a key transcription factor in the inflammatory cascade (2,10, 11,18,21,22,33,35,37,38). Adipocytes also secrete adiponectin (29,30,36,41,42), which has been shown to possess anti-inflammatory properties through its action on NF-B and is inversely correlated with obesity and diabetes (29,30,36,41,42). Evidence for the role of NF-B in AT has been shown in studies overexpressing the NF-B activator IKK␤ in mice, which resulted in increased inflammatory cytokine production and the onset of diabetes (7). In contrast, hepatocyte IKK␤ knockout (KO) mice demonstrated a decrease in circulating proinflammatory cytokines (3). This indicates that IKK␤ KO mice do not develop hepatic insulin resistance and glucose intolerance compared with their high-fat diet-fed counterparts. Further studies also illustrate that an inflammatory reaction, induced by the bacterial endotoxin lipopolysaccharide (LPS), is markedly attenuated in the IKK␤ KO mice (3

SUMMARY FGF21, a member of the fibroblast growth factor (FGF) superfamily has recently emerged as a novel regulator of metabolism and energy utilization. However, the exact mechanism(s) whereby FGF21 mediates its actions have not been elucidated. There is considerable evidence that insulin resistance may arise from aberrant accumulation of intracellular lipids in insulin responsive tissues due to lipotoxicity. In particular the sphingolipid ceramide has been implicated in this process. Here, we show that FGF21 rapidly and robustly stimulates adiponectin secretion in rodents, while diminishing accumulation of ceramides in obese animals. Importantly, adiponectin knockout mice are refractory to changes in energy expenditure and ceramide-lowering effects evoked by FGF21 administration. Moreover, FGF21 lowers blood glucose levels and enhances insulin sensitivity in diabetic Lepob/ob mice and diet-induced obese (DIO) mice, only when adiponectin is functionally present. Collectively, these data suggest that FGF21 is a potent regulator of adiponectin secretion, and that FGF21 critically depends on adiponectin to exert its glycemic and insulin sensitizing effects.

Adipose tissue regulates numerous physiological processes, and its dysfunction in obese humans is associated with disrupted metabolic homeostasis, insulin resistance and type 2 diabetes mellitus (T2DM). Although several US-approved treatments for obesity and T2DM exist, these are limited by adverse effects and a lack of effective long-term glucose control. In this Review, we provide an overview of the role of adipose tissue in metabolic homeostasis and assess emerging novel therapeutic strategies targeting adipose tissue, including adipokine-based strategies, promotion of white adipose tissue beiging as well as reduction of inflammation and fibrosis.

We examined rodent models with altered levels of mitoNEET, a protein residing in the mitochondrial outer membrane. Adipocyte-specific overexpression of mitoNEET enhances lipid-uptake and storage, leading to an expansion of adipose tissue mass. Despite the resulting massive obesity, benign aspects of adipose tissue expansion prevail and insulin sensitivity is preserved. MitoNEET inhibits mitochondrial iron transport into the matrix. Since iron is a rate-limiting component for electron transport, mitoNEET reduces β-oxidation rates. This is associated with reduced mitochondrial membrane potential and reduced reactive oxygen species damage, along with higher levels of adiponectin production. Conversely, the reduction of mitoNEET enhances mitochondrial respiratory capacity through enhanced iron content in the matrix, with reduced weight gain on a high fat diet. However, a reduction of mitoNEET also causes heightened oxidative-stress and glucose-intolerance. MitoNEET is therefore a potent regulator of mitochondrial function that profoundly impacts the dynamics of cellular and whole-body lipid homeostasis.

Obese fat pads are frequently undervascularized and hypoxic, leading to increased fibrosis, inflammation, and ultimately insulin resistance. We hypothesized that VEGF-A–induced stimulation of angiogenesis enables sustained and sufficient oxygen and nutrient exchange during fat mass expansion, thereby improving adipose tissue function. Using a doxycycline (Dox)-inducible adipocyte-specific VEGF-A overexpression model, we demonstrate that the local up-regulation of VEGF-A in adipocytes improves vascularization and causes a “browning” of white adipose tissue (AT), with massive up-regulation of UCP1 and PGC1α. This is associated with an increase in energy expenditure and resistance to high fat diet-mediated metabolic insults. Similarly, inhibition of VEGF-A–induced activation of VEGFR2 during the early phase of high fat diet-induced weight gain, causes aggravated systemic insulin resistance. However, the same VEGF-A–VEGFR2 blockade in ob/ob mice leads to a reduced body-weight gain, an improvement in insulin sensitivity, a decrease in inflammatory factors, and increased incidence of adipocyte death. The consequences of modulation of angiogenic activity are therefore context dependent. Proangiogenic activity during adipose tissue expansion is beneficial, associated with potent protective effects on metabolism, whereas antiangiogenic action in the context of preexisting adipose tissue dysfunction leads to improvements in metabolism, an effect likely mediated by the ablation of dysfunctional proinflammatory adipocytes.

Summary The expansion of white adipose tissue (WAT) in obesity involves de novo differentiation of new adipocytes; however, the cellular origin of these cells remains unclear. Here, we utilize Zfp423GFP reporter mice to characterize adipose mural (Pdgfrβ+) cells with varying levels of the preadipocyte commitment factor Zfp423. We find that adipose tissue contains distinct mural populations, with levels of Zfp423 distinguishing adipogenic from inflammatory-like mural cells. Using our “MuralChaser” lineage tracking system, we uncover adipose perivascular cells as developmental precursors of adipocytes formed in obesity, with adipogenesis and precursor abundance regulated in a depot-dependent manner. Interestingly, Pdgfrβ+ cells do not significantly contribute to the initial cold-induced recruitment of beige adipocytes in WAT; it is only after prolonged cold exposure that these cells differentiate into beige adipocytes. These results provide genetic evidence for a mural cell origin of white adipocytes in obesity, and suggest that beige adipogenesis may originate from multiple sources.

Sphingolipids have garnered attention for their role in insulin resistance and lipotoxic cell death. Aberrant accumulation of ceramides correlates with hepatic insulin resistance and steatosis. To further investigate the tissue-specific effects of local changes in ceramidase activity, we have developed transgenic mice inducibly expressing acid ceramidase, to trigger the deacylation of ceramides. This represents the first inducible genetic model that acutely manipulates ceramides in adult mouse tissues. Hepatic overexpression of acid ceramidase prevents hepatic steatosis and prompts improvements in insulin action in liver and adipose tissue. Conversely, overexpression of acid ceramidase within adipose tissue prevents hepatic steatosis and insulin resistance. Induction of ceramidase activity in either tissue promotes a lowering of hepatic ceramides and reduced activation of the ceramide-activated protein kinase C isoform PKC-zeta. These observations suggest the existence of a rapidly acting "crosstalk" between liver and adipose tissue sphingolipids, critically regulating glucose metabolism and hepatic lipid uptake.