Aurélie Caillon

Mechanisms of the Anti-Obesity Effects of Oxytocin in Diet-Induced Obese Rats

Apart from its role during labor and lactation, oxytocin is involved in several other functions. Interestingly, oxytocin- and oxytocin receptor-deficient mice develop late-onset obesity with normal food intake, suggesting that the hormone might exert a series of beneficial metabolic effects. This was recently confirmed by data showing that central oxytocin infusion causes weight loss in diet-induced obese mice. The aim of the present study was to unravel the mechanisms underlying such beneficial effects of oxytocin. Chronic central oxytocin infusion was carried out in high fat diet-induced obese rats. Its impact on body weight, lipid metabolism and insulin sensitivity was determined. We observed a dose-dependent decrease in body weight gain, increased adipose tissue lipolysis and fatty acid β-oxidation, as well as reduced glucose intolerance and insulin resistance. The additional observation that plasma oxytocin levels increased upon central infusion suggested that the hormone might affect adipose tissue metabolism by direct action. This was demonstrated using in vitro, ex vivo, as well as in vivo experiments. With regard to its mechanism of action in adipose tissue, oxytocin increased the expression of stearoyl-coenzyme A desaturase 1, as well as the tissue content of the phospholipid precursor, N-oleoyl-phosphatidylethanolamine, the biosynthetic precursor of the oleic acid-derived PPAR-alpha activator, oleoylethanolamide. Because PPAR-alpha regulates fatty acid β-oxidation, we hypothesized that this transcription factor might mediate the oxytocin effects. This was substantiated by the observation that, in contrast to its effects in wild-type mice, oxytocin infusion failed to induce weight loss and fat oxidation in PPAR-alpha-deficient animals. Altogether, these results suggest that oxytocin administration could represent a promising therapeutic approach for the treatment of human obesity and type 2 diabetes.

Chronic mTOR inhibition by rapamycin induces muscle insulin resistance despite weight loss in rats

BACKGROUND AND PURPOSE mTOR inhibitors are currently used as immunosuppressants in transplanted patients and as promising anti‐cancer agents. However, new‐onset diabetes is a frequent complication occurring in patients treated with mTOR inhibitors such as rapamycin (Sirolimus). Here, we investigated the mechanisms associated with the diabetogenic effects of chronic Sirolimus administration in rats and in in vitro cell cultures.

Divergent effects of oxytocin treatment of obese diabetic mice on adiposity and diabetes

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.

Hepatic PTEN deficiency improves muscle insulin sensitivity and decreases adiposity in mice

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. CONCLUSIONS Although steatosis develops in LPTEN KO mice, PTEN deficiency in hepatocytes promotes a crosstalk between liver and muscle, as well as adipose tissue, resulting in enhanced insulin sensitivity, improved glucose tolerance and decreased adiposity.

Central Glucocorticoid Administration Promotes Weight Gain and Increased 11β-Hydroxysteroid Dehydrogenase Type 1 Expression in White Adipose Tissue

Glucocorticoids (GCs) are involved in multiple metabolic processes, including the regulation of insulin sensitivity and adipogenesis. Their action partly depends on their intracellular activation by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). We previously demonstrated that central GC administration promotes hyperphagia, body weight gain, hyperinsulinemia and marked insulin resistance at the level of skeletal muscles. Similar dysfunctions have been reported to occur upon specific overexpression of 11β-HSD1 in adipose tissue. The aim of the present study was therefore to determine whether the effects of central GC infusion may enhance local GC activation in white adipose tissue. Male Wistar and Sprague Dawley (SD) rats were intracerebroventricularly infused with GCs for 2 to 3 days. Body weight, food intake and metabolic parameters were measured, and expression of enzymes regulating 11β-HSD1, as well as that of genes regulated by GCs, were quantified. Central GC administration induced a significant increase in body weight gain and in 11β-HSD1 and resistin expression in adipose tissue. A decrease 11β-HSD1 expression was noticed in the liver of SD rats, as a partial compensatory mechanism. Such effects of GCs are centrally elicited. This model of icv dexamethasone infusion thus appears to be a valuable acute model, that helps delineating the initial metabolic defects occurring in obesity. An impaired downregulation of intracellular GC activation in adipose tissue may be important for the development of insulin resistance.

Alterations in lipid metabolism and thermogenesis with emergence of brown adipocytes in white adipose tissue in diet-induced obesity-resistant Lou/C rats

Recent studies describe the Lou/C rat as a model of resistance to age- and diet-induced obesity and suggest a preferential channeling of nutrients toward utilization rather than storage under standard feeding conditions. The purpose of the present study was to evaluate lipid metabolism of Lou/C and Wistar rats under a high-fat (HF) diet. Four-month-old male Lou/C and Wistar animals were submitted to a 40% HF diet for 5-9 wk. Evolution of food intake, body weight, and body composition, hormonal parameters, and expression of key transcription factors and enzymes involved in lipid metabolism were determined. Wistar rats developed obesity after 5 wk of HF diet, as previously described. Among the various parameters measured, accumulation of intraperitoneal fat was particularly evident in HF-fed Wistar rats. In these animals, thermogenesis was, however, stimulated as a likely compensatory mechanism against the development of obesity. On the contrary, Lou/C animals failed to develop obesity under such a diet, and intraperitoneal fat, not including epididymal and retroperitoneal fat depots, was virtually absent. Enzyme measurements confirmed lipid utilization rather than storage, which was accompanied by the striking emergence of uncoupling protein-1, characteristic of brown adipocytes, in white adipose tissue, particularly in the subcutaneous depot.

Ectopic UCP1 Overexpression in White Adipose Tissue Improves Insulin Sensitivity in Lou/C Rats, a Model of Obesity Resistance

Brown adipose tissue (BAT), characterized by the presence of uncoupling protein 1 (UCP1), has been described as metabolically active in humans. Lou/C rats, originating from the Wistar strain, are resistant to obesity. We previously demonstrated that Lou/C animals express UCP1 in beige adipocytes in inguinal white adipose tissue (iWAT), suggesting a role of this protein in processes such as the control of body weight and the observed improved insulin sensitivity. A β3 adrenergic agonist was administered for 2 weeks in Wistar and Lou/C rats to activate UCP1 and delineate its metabolic impact. The treatment brought about decreases in fat mass and improvements in insulin sensitivity in both groups. In BAT, UCP1 expression increased similarly in response to the treatment in the two groups. However, the intervention induced the appearance of beige cells in iWAT, associated with a marked increase in UCP1 expression, in Lou/C rats only. This increase was correlated with a markedly enhanced glucose uptake measured during euglycemic-hyperinsulinemic clamps, suggesting a role of beige cells in this process. Activation of UCP1 in ectopic tissues, such as beige cells in iWAT, may be an interesting therapeutic approach to prevent body weight gain, decrease fat mass, and improve insulin sensitivity.

Improved leptin sensitivity as a potential candidate responsible for the spontaneous food restriction of the Lou/C rat.

The Lou/C rat, an inbred strain of Wistar origin, was described as a model of resistance to age- and diet-induced obesity. Although such a resistance involves many metabolic parameters described in our previous studies, Lou/C rats also exhibit a spontaneous food restriction due to decreased food consumption during the nocturnal period. We then attempted to delineate the leptin sensitivity and mechanisms implicated in this strain, using different protocols of acute central and peripheral leptin administration. A first analysis of the meal patterns revealed that Lou/C rats eat smaller meals, without any change in meal number compared to age-matched Wistar animals. Although the expression of the recognized leptin transporters (leptin receptors and megalin) measured in the choroid plexus was normal in Lou/C rats, the decreased triglyceridemia observed in these animals is compatible with an increased leptin transport across the blood brain barrier. Improved hypothalamic leptin signaling in Lou/C rats was also suggested by the higher pSTAT3/STAT3 (signal transducer and activator of transcription 3) ratio observed following acute peripheral leptin administration, as well as by the lower hypothalamic mRNA expression of the suppressor of cytokine signaling 3 (SOCS3), known to downregulate leptin signaling. To conclude, spontaneous hypophagia of Lou/C rats appears to be related to improved leptin sensitivity. The main mechanism underlying such a phenomenon consists in improved leptin signaling through the Ob-Rb leptin receptor isoform, which seems to consequently lead to overexpression of brain-derived neurotrophic factor (BDNF) and thyrotropin-releasing hormone (TRH).

The OEA effect on food intake is independent from the presence of PPARα in the intestine and the nodose ganglion, while the impact of OEA on energy expenditure requires the presence of PPARα in mice

BACKGROUND Oleoylethanolamide (OEA) is an endocannabinoid that controls food intake, energy expenditure and locomotor activity. Its anorexigenic effect appears to be mediated by PPARα, but the tissue where the presence of this receptor is required for OEA to inhibit feeding is unknown as yet. Previous studies point to a possible role of proximal enterocytes and neurons of the nodose ganglion. MATERIALS AND METHODS Acute intraperitoneal OEA effects on food intake, energy expenditure, respiratory exchange ratio (RER) and locomotor activity were studied in control mice (PPARα-loxP) and intestinal (Villin-Cre;PPARα-loxP) or nodose ganglion (Phox2B-Cre;PPARα-loxP) specific PPARα knockout mice placed in calorimetric cages. RESULTS OEA administration to both intestinal and nodose ganglion PPARα knockout mice decreased food intake, RER (leading to increased lipid oxidation) and locomotor activity as in control mice. However, while OEA injection acutely decreased energy expenditure in controls, this effect was not observed in mice devoid of PPARα in the intestine. CONCLUSION These results indicate that the OEA effect on food intake is independent from the presence of PPARα in the intestine and the nodose ganglion, while the impact of OEA on energy expenditure requires the presence of PPARα in the intestine.

Inhibitory role of oxytocin on TNFα expression assessed in vitro and in vivo

AIM Oxytocin administration to diet-induced obese (DIO) rodents, monkeys and humans decreases body weight and fat mass with concomitant improvements in glucose metabolism. Moreover, several studies show an immunomodulatory role of oxytocin in a number of settings (such as atherosclerosis, injury, sepsis). This study aims to shed some light on the effects of oxytocin on macrophage polarization and cytokine production, as well as its possible impact on these parameters in adipose tissue in DIO mice with impaired glucose metabolism. METHODS Mouse bone marrow cells were differentiated into macrophages and treated with oxytocin. Macrophage proliferation, cytokine secretion and macrophage populations were determined. For experiments in vivo, DIO mice were treated with oxytocin for 2 weeks. Body weight and composition and glucose tolerance were subsequently followed. At the end of treatment, adipose tissue macrophage populations, plasma cytokine levels and cytokine expression in adipose tissue were determined. RESULTS In bone marrow-derived macrophages, oxytocin induced an anti-inflammatory phenotype (decreased M1/M2 ratio). In M1-derived macrophages, oxytocin decreased TNFα secretion, with no effects on the other cytokines tested nor any effect on cytokine secretion by M2-derived macrophages. Oxytocin treatment in DIO mice in vivo led to decreased body weight accompanied by an improvement in glucose tolerance, with no changes in plasma cytokine levels. In adipose tissue, oxytocin decreased Tnfα expression without modifying the M1/M2 macrophage ratio. CONCLUSION Oxytocin treatment decreases TNFα production both in vitro (in bone marrow-derived macrophages) and in vivo (in epididymal adipose tissue) in DIO mice. This effect may also be contributory to the observed improvement in glucose metabolism.