Oxytocin has been suggested as a novel therapeutic against obesity, because it induces weight loss and improves glucose tolerance in diet-induced obese rodents. A recent clinical pilot study confirmed the oxytocin-induced weight-reducing effect in obese nondiabetic subjects. Nevertheless, the mechanisms involved and the impact on the main comorbidity associated with obesity, type 2 diabetes, are unknown. Lean and ob/ob mice (model of obesity, hyperinsulinemia, and diabetes) were treated for 2 weeks with different doses of oxytocin, analogues with longer half-life (carbetocin) or higher oxytocin receptor specificity ([Thr4,Gly7]-oxytocin). Food and water intake, body weight, and glycemia were measured daily. Glucose, insulin, and pyruvate tolerance, body composition, several hormones, metabolites, gene expression, as well as enzyme activities were determined. Although no effect of oxytocin on the main parameters was observed in lean mice, the treatment dose-dependently reduced food intake and body weight gain in ob/ob animals. Carbetocin behaved similarly to oxytocin, whereas [Thr4,Gly7]-oxytocin (TGOT) and a low oxytocin dose decreased body weight gain without affecting food intake. The body weight gain-reducing effect was limited to the fat mass only, with decreased lipid uptake, lipogenesis, and inflammation, combined with increased futile cycling in abdominal adipose tissue. Surprisingly, oxytocin treatment of ob/ob mice was accompanied by a worsening of basal glycemia and glucose tolerance, likely due to increased corticosterone levels and stimulation of hepatic gluconeogenesis. These results impose careful selection of the conditions in which oxytocin treatment should be beneficial for obesity and its comorbidities, and their relevance for human pathology needs to be determined.

Presence of brown adipose tissue (BAT), characterized by the expression of the thermogenic uncoupling protein 1 (UCP1), has recently been described in adult humans. UCP1 is expressed in classical brown adipocytes, as well as in “beige cells” in white adipose tissue (WAT). The thermogenic activity of BAT is mainly controlled by the sympathetic nervous system. Endocrine factors, such as fibroblast growth factor 21 (FGF21) and bone morphogenic protein factor-9 (BMP-9), predominantly produced in the liver, were shown to lead to activation of BAT thermogenesis, as well as to “browning” of WAT. This was also observed in response to irisin, a hormone secreted by skeletal muscles. Different approaches were used to delineate the impact of UCP1 on insulin sensitivity. When studied under thermoneutral conditions, UCP1 knockout mice exhibited markedly increased metabolic efficiency due to impaired thermogenesis. The impact of UCP1 deletion on insulin sensitivity in these mice was not reported. Conversely, several studies in both rodents and humans have shown that BAT activation (by cold exposure, β3-agonist treatment, transplantation and others) improves glucose tolerance and insulin sensitivity. Interestingly, similar results were obtained by adipose tissue-specific overexpression of PR-domain-containing 16 (PRDM16) or BMP4 in mice. The mediators of such beneficial effects seem to include FGF21, interleukin-6, BMP8B and prostaglandin D2 synthase. Interestingly, some of these molecules can be secreted by BAT itself, indicating the occurrence of autocrine effects. Stimulation of BAT activity and/or recruitment of UCP1-positive cells are therefore relevant targets for the treatment of obesity/type 2 diabetes in humans.

OBJECTIVE-Human islet amyloid polypeptide (hIAPP) aggregation plays a major role in the development of islet amyloidosis in type 2 diabetes. It is known that extracellular hIAPP oligomers are toxic to pancreatic ␤-cells and associated with apoptosis. We therefore investigated the molecular mechanism by which extracellular hIAPP mediates pancreatic ␤-cell apoptosis. RESEARCH DESIGN AND METHODS-MIN6cells and primary cultures of human pancreatic islets were treated with freshly dissolved hIAPP peptide. Morphology of the cultures was evaluated by electron microscopy. Gene expression was analyzed by microarray, RT-PCR, and immunoblot. Calcium levels were measured in fura-2-loaded cells. Apoptosis was quantified by cytometry.RESULTS-Increased expression of several heat shock proteins and activation of the spliced form of XBP-1, a transcription factor for overexpression of chaperones during endoplasmic reticulum (ER) stress, were detected together with morphological evidence of ER dysfunction. Intracellular calcium overload was detected in association with this process. Moreover, reduction in the proteasome activity, which was detected over time, contributed to the intracellular accumulation of ubiquitinated proteins, leading to a functional suppression of the ubiquitinproteasome pathway. In addition, impairment of the proteasome function contributed to apoptosis, while, despite the presence of hIAPP, cell viability improved when a proteasome activator was overexpressed. The key cytotoxic events induced by extracellular hIAPP were also observed in treated human islets. T he presence of amyloid deposits within the pancreatic islets is a pathophysiological hallmark of type 2 diabetes (1,2). Islet amyloid polypeptide (IAPP), also known as amylin, is the main component of islet amyloid (3,4). It is produced by ␤-cells and cosecreted with insulin in response to nutrient stimuli (5-7). A target region between positions 20 and 29 is thought to be responsible for amyloid fibril formation by the human peptide (8). Several point mutations located in the human IAPP (hIAPP) promoter and missense point mutations on the encoding gene region have been identified by our group and others with different prevalence according to ethnic origin (9,10). However, because the sequence of hIAPP is identical in diabetic and nondiabetic individuals, the process of amyloid formation remains poorly understood. CONCLUSIONS-OurIslet amyloid colocalizes with areas of cell degeneration, and the process of amyloidosis has been associated with progressive loss of pancreatic ␤-cell mass by apoptosis and, thus, much of the pathology of type 2 diabetes (11)(12)(13)(14). A number of studies showed that the toxicity of hIAPP and other amyloidogenic peptides lies in the oligomeric intermediates rather than in the mature fibrils (15-17).The endoplasmic reticulum (ER) integrates protein synthesis and folding and calcium storage and signaling. Several cellular stress conditions can cause ER dysfunction and protein misfolding (18). To overcome ER stress, cells ini...

MicroRNAs (miRNAs) have recently emerged as key regulators of metabolism. However, their potential role in the central regulation of whole-body energy homeostasis is still unknown. In this study we show that the expression of Dicer, an essential endoribonuclease for miRNA maturation, is modulated by nutrient availability and excess in the hypothalamus. Conditional deletion of Dicer in POMC-expressing cells resulted in obesity, characterized by hyperphagia, increased adiposity, hyperleptinemia, defective glucose metabolism and alterations in the pituitary-adrenal axis. The development of the obese phenotype was paralleled by a POMC neuron degenerative process that started around 3 weeks of age. Hypothalamic transcriptomic analysis in presymptomatic POMCDicerKO mice revealed the downregulation of genes implicated in biological pathways associated with classical neurodegenerative disorders, such as MAPK signaling, ubiquitin-proteosome system, autophagy and ribosome biosynthesis. Collectively, our results highlight a key role for miRNAs in POMC neuron survival and the consequent development of neurodegenerative obesity.

Proopiomelanocortin (POMC) neurons are critical sensors of nutrient availability implicated in energy balance and glucose metabolism control. However, the precise mechanisms underlying nutrient sensing in POMC neurons remain incompletely understood. We show that mitochondrial dynamics mediated by Mitofusin 1 (MFN1) in POMC neurons couple nutrient sensing with systemic glucose metabolism. Mice lacking MFN1 in POMC neurons exhibited defective mitochondrial architecture remodeling and attenuated hypothalamic gene expression programs during the fast-to-fed transition. This loss of mitochondrial flexibility in POMC neurons bidirectionally altered glucose sensing, causing abnormal glucose homeostasis due to defective insulin secretion by pancreatic β cells. Fed mice lacking MFN1 in POMC neurons displayed enhanced hypothalamic mitochondrial oxygen flux and reactive oxygen species generation. Central delivery of antioxidants was able to normalize the phenotype. Collectively, our data posit MFN1-mediated mitochondrial dynamics in POMC neurons as an intrinsic nutrient-sensing mechanism and unveil an unrecognized link between this subset of neurons and insulin release.

Background & Aims: PTEN is a dual lipid/protein phosphatase, downregulated in steatotic livers with obesity or HCV infection. Liver-specific PTEN knockout (LPTEN KO) mice develop steatosis, inflammation/fibrosis and hepatocellular carcinoma with aging, but surprisingly also enhanced glucose tolerance. This study aimed at understanding the mechanisms by which hepatic PTEN deficiency improves glucose tolerance, while promoting fatty liver diseases. Methods: Control and LPTEN KO mice underwent glucose/pyruvate tolerance tests and euglycemic-hyperinsulinemic clamps. Body fat distribution was assessed by EchoMRI, CT-scan and dissection analyses. Primary/cultured hepatocytes and insulin-sensitive tissues were analysed ex vivo. Results: PTEN deficiency in hepatocytes led to steatosis through increased fatty acid (FA) uptake and de novo lipogenesis. Although LPTEN KO mice exhibited hepatic steatosis, they displayed increased skeletal muscle insulin sensitivity and glucose uptake, as assessed by euglycemic-hyperinsulinemic clamps. Surprisingly, white adipose tissue (WAT) depots were also drastically reduced. Analyses of key enzymes involved in lipid metabolism further indicated that FA synthesis/esterification was decreased in WAT. In addition, Ucp1 expression and multilocular lipid droplet structures were observed in this tissue, indicating the presence of beige adipocytes. Consistent with a liver to muscle/ adipocyte crosstalk, the expression of liver-derived circulating factors, known to impact on muscle insulin sensitivity and WAT homeostasis (e.g. FGF21), was modulated in LPTEN KO mice.

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Obesity-induced inflammation engenders insulin resistance and type 2 diabetes mellitus (T2DM) but the inflammatory effectors linking obesity to insulin resistance are incompletely understood. Here, we show that hepatic expression of Protein Tyrosine Phosphatase Receptor Gamma (PTPR-γ) is stimulated by inflammation in obese/T2DM mice and positively correlates with indices of inflammation and insulin resistance in humans. NF-κB binds to the promoter of Ptprg and is required for inflammation-induced PTPR-γ expression. PTPR-γ loss-of-function lowers glycemia and insulinemia by enhancing insulin-stimulated suppression of endogenous glucose production. These phenotypes are rescued by re-expression of Ptprg only in liver of mice lacking Ptprg globally. Hepatic PTPR-γ overexpression that mimics levels found in obesity is sufficient to cause severe hepatic and systemic insulin resistance. We propose hepatic PTPR-γ as a link between obesity-induced inflammation and insulin resistance and as potential target for treatment of T2DM.