During the expansion of fat mass in obesity, vascularization of adipose tissue is insufficient to maintain tissue normoxia. Local hypoxia develops and may result in altered adipokine expression, proinflammatory macrophage recruitment, and insulin resistance. We investigated whether an increase in adipose tissue angiogenesis could protect against obesity-induced hypoxia and, consequently, insulin resistance. Transgenic mice overexpressing vascular endothelial growth factor (VEGF) in brown adipose tissue (BAT) and white adipose tissue (WAT) were generated. Vessel formation, metabolism, and inflammation were studied in VEGF transgenic mice and wild-type littermates fed chow or a high-fat diet. Overexpression of VEGF resulted in increased blood vessel number and size in both WAT and BAT and protection against high-fat diet–induced hypoxia and obesity, with no differences in food intake. This was associated with increased thermogenesis and energy expenditure. Moreover, whole-body insulin sensitivity and glucose tolerance were improved. Transgenic mice presented increased macrophage infiltration, with a higher number of M2 anti-inflammatory and fewer M1 proinflammatory macrophages than wild-type littermates, thus maintaining an anti-inflammatory milieu that could avoid insulin resistance. These studies suggest that overexpression of VEGF in adipose tissue is a potential therapeutic strategy for the prevention of obesity and insulin resistance.

Prevalence of type 2 diabetes (T2D) and obesity is increasing worldwide. Currently available therapies are not suited for all patients in the heterogeneous obese/T2D population, hence the need for novel treatments. Fibroblast growth factor 21 (FGF21) is considered a promising therapeutic agent for T2D/obesity. Native FGF21 has, however, poor pharmacokinetic properties, making gene therapy an attractive strategy to achieve sustained circulating levels of this protein. Here, adeno‐associated viral vectors (AAV) were used to genetically engineer liver, adipose tissue, or skeletal muscle to secrete FGF21. Treatment of animals under long‐term high‐fat diet feeding or of ob/ob mice resulted in marked reductions in body weight, adipose tissue hypertrophy and inflammation, hepatic steatosis, inflammation and fibrosis, and insulin resistance for > 1 year. This therapeutic effect was achieved in the absence of side effects despite continuously elevated serum FGF21. Furthermore, FGF21 overproduction in healthy animals fed a standard diet prevented the increase in weight and insulin resistance associated with aging. Our study underscores the potential of FGF21 gene therapy to treat obesity, insulin resistance, and T2D.

Aims/hypothesis IL-6 is released by the adipose tissue and increased circulating levels in obesity are associated with hyperinsulinaemia and insulin resistance. Short-term experiments suggest that increased IL-6 release by the skeletal muscle following exercise may improve insulin sensitivity. Methods In order to examine the effect of chronically elevated IL-6 levels, we overexpressed Il6 in skeletal muscle in mice using an electro-transfer procedure.Results Circulating IL-6 levels were increased and the animals rapidly lost both weight and body fat, but food intake was unchanged, which is consistent with the finding that IL-6 increased energy expenditure. Insulin levels were inappropriately elevated and combined with hypoglycaemia in spite of reduced 2-deoxy-D-glucose uptake by skeletal muscle. Insulin-stimulated glucose uptake by skeletal muscles ex vivo was reduced, probably due to the decreased amounts of glucose transporter (GLUT)-4. Beta cell insulin content was increased, while apparent beta cell mass was unchanged. Circulating serum amyloid A cluster levels were increased tenfold due to a pronounced proinflammatory state in the liver with infiltration of inflammatory cells. However, no liver steatosis was found, which may be accounted for by concomitant AMP kinase activation. Conclusions/interpretation Chronically elevated IL-6 levels lead to inappropriate hyperinsulinaemia, reduced body weight, impaired insulin-stimulated glucose uptake by the skeletal muscles and marked inflammation in the liver. Thus, the pleiotrophic effects of chronically elevated IL-6 levels preclude any obvious usefulness in treating obesity or its associated metabolic complications in man, despite the fact that weight reduction may be expected.

Adipose tissue is pivotal in the regulation of energy homeostasis through the balance of energy storage and expenditure and as an endocrine organ. An inadequate mass and/or alterations in the metabolic and endocrine functions of adipose tissue underlie the development of obesity, insulin resistance, and type 2 diabetes. To fully understand the metabolic and molecular mechanism(s) involved in adipose dysfunction, in vivo genetic modification of adipocytes holds great potential. Here, we demonstrate that adeno-associated viral (AAV) vectors, especially serotypes 8 and 9, mediated efficient transduction of white (WAT) and brown adipose tissue (BAT) in adult lean and obese diabetic mice. The use of short versions of the adipocyte protein 2 or uncoupling protein-1 promoters or micro-RNA target sequences enabled highly specific, long-term AAV-mediated transgene expression in white or brown adipocytes. As proof of concept, delivery of AAV vectors encoding for hexokinase or vascular endothelial growth factor to WAT or BAT resulted in increased glucose uptake or increased vessel density in targeted depots. This method of gene transfer also enabled the secretion of stable high levels of the alkaline phosphatase marker protein into the bloodstream by transduced WAT. Therefore, AAV-mediated genetic engineering of adipose tissue represents a useful tool for the study of adipose pathophysiology and, likely, for the future development of new therapeutic strategies for obesity and diabetes.

Vascular endothelial growth factor A (VEGF-A) is classically viewed as a key factor in angiogenesis and tissue remodeling. However, recent evidence suggests a potential role of this growth factor in the control of energy metabolism and adipose tissue function. In this regard, we and others have described the effects of the up and downregulation of VEGF-A in adipose tissue on the control of energy homeostasis. VEGF-A overexpression protects against diet-induced obesity and insulin resistance. The observation that VEGF-A overexpression leads to an increase in brown adipose tissue (BAT) thermogenesis and also promotes a “BAT-like” phenotype in white adipose tissue depots is of particular relevance for the understanding of the mechanisms underlying obesity development. In addition, VEGF-A may not only have pro-inflammatory but also anti-inflammatory properties, with a chemotactic activity specific for M2 anti-inflammatory macrophages. This new scientific evidence highlights the importance that VEGF-A actions on metabolism could have on the design of new treatments for obesity, insulin resistance and obesity-related disorders.

We examined the effect of Bone Morphogenetic Protein 4 (BMP4) on energy expenditure in adult mature mice by targeting the liver with adeno-associated viral (AAV) BMP4 vectors to increase circulating levels. We verified the direct effect of BMP4 in inducing a brown oxidative phenotype in differentiating preadipocytes in vitro. AAV-BMP4-treated mice display marked browning of subcutaneous adipocytes, with increased mitochondria and Uncoupling Protein 1 (UCP1). These mice are protected from obesity on a high-fat diet and have increased whole-body energy expenditure, improved insulin sensitivity, reduced liver fat, and reduced adipose tissue inflammation. On a control diet, they show unchanged body weight but improved insulin sensitivity. In contrast, AAV-BMP4-treated mice showed beiging of BAT with reduced UCP1, increased lipids, and reduced hormone-sensitive lipase (HSL). Thus, BMP4 exerts different effects on WAT and BAT, but the overall effect is to enhance insulin sensitivity and whole-body energy expenditure by browning subcutaneous adipose tissue.

Obesity and insulin resistance are associated with increased serum free fatty acids (FFAs). Thus, a reduction in circulating FFAs may increase insulin sensitivity. This could be achieved by increasing FFA reesterification in adipose tissue. Transgenic mice with increased adipose tissue glyceroneogenesis, caused by overexpression of phosphoenolpyruvate carboxykinase (PEPCK), show increased FFA reesterification and develop obesity but are insulin sensitive. Here, we examined whether these transgenic mice were protected from diet-induced insulin resistance. Surprisingly, when fed a high-fat diet for a short period (6 weeks), transgenic mice developed severe obesity and were more hyperinsulinemic, glucose intolerant, and insulin resistant than controls. The high triglyceride accumulation prevented white adipose tissue from buffering the flux of lipids in circulation and led to increased serum triglyceride levels and fat deposition in liver. Furthermore, circulating leptin and FFA concentrations increased to similar levels in transgenic and control mice, while adiponectin levels decreased in transgenic mice compared with controls. In addition, transgenic mice showed fat accumulation in brown adipose tissue, which decreased uncoupling protein-1 expression, suggesting that these mice had impaired diet-induced thermogenesis. These results indicate that increased PEPCK expression in the presence of high-fat feeding may have deleterious effects and lead to severe insulin resistance and type 2 diabetes.

Aims/hypothesis In adipocytes, triacylglycerol synthesis depends on the formation of glycerol 3-phosphate, which originates either from glucose, through glycolysis, or from lactate, through glyceroneogenesis. However, glucose is traditionally viewed as the main precursor of the glycerol backbone and thus, enhanced glucose uptake would be expected to result in increased triacylglycerol synthesis and contribute to obesity. Methods To further explore this issue, we generated a mouse model with chronically increased glucose uptake in adipose tissue by expressing Gck, which encodes the glucokinase enzyme.Results Here we show that the production of high levels of glucokinase led to increased adipose tissue glucose uptake and lactate production, improved glucose tolerance and higher whole-body and skeletal muscle insulin sensitivity. There was no parallel increase in glycerol 3-phosphate synthesis in vivo, fat accumulation or obesity. Moreover, at high glucose concentrations, in cultured fat cells overproducing glucokinase, glycerol 3-phosphate synthesis from pyruvate decreased, while glyceroneogenesis increased in fat cells overproducing hexokinase II. Conclusions/interpretations These findings indicate that the absence of glucokinase inhibition by glucose 6-phosphate probably led to increased glycolysis and blocked S. Muñoz and S. Franckhauser contributed equally to this study. Electronic supplementary material The online version of this article