The Role of Pancreatic Preproglucagon in Glucose Homeostasis in Mice.

Glucagon-like peptide 1 (GLP-1) is necessary for normal gluco-regulation, and it has been widely presumed that this function reflects the actions of GLP-1 released from enteroendocrine L cells. To test the relative importance of intestinal versus pancreatic sources of GLP-1 for physiological regulation of glucose, we administered a GLP-1R antagonist, exendin-[9-39] (Ex9), to mice with tissue-specific reactivation of the preproglucagon gene (Gcg). Ex9 impaired glucose tolerance in wild-type mice but had no impact on Gcg-null or GLP-1R KO mice, suggesting that Ex9 is a true and specific GLP-1R antagonist. Unexpectedly, Ex-9 had no effect on blood glucose in mice with restoration of intestinal Gcg. In contrast, pancreatic reactivation of Gcg fully restored the effect of Ex9 to impair both oral and i.p. glucose tolerance. These findings suggest an alternative model whereby islet GLP-1 also plays an important role in regulating glucose homeostasis.

Bariatric surgery emphasizes biological sex differences in rodent hepatic lipid handling.

BACKGROUND: Eighty percent of patients who receive bariatric surgery are women, yet the majority of preclinical studies are in male rodents. Because sex differences drive hepatic gene expression and overall lipid metabolism, we sought to determine whether sex differences were also apparent in these endpoints in response to bariatric surgery. METHODS: Two cohorts of age-matched virgin male and female Long-Evans rats were placed on a high fat diet for 3 weeks and then received either Sham or vertical sleeve gastrectomy (VSG), a surgery which resects 80% of the stomach with no intestinal rearrangement. RESULTS: Each sex exhibited significantly decreased body weight due to a reduction in fat mass relative to Sham controls (p < 0.05). Microarray and follow-up qPCR on liver revealed striking sex differences in gene expression after VSG that reflected a down-regulation of hepatic lipid metabolism and an up-regulation of hepatic inflammatory pathways in females vs. males after VSG. While the males had a significant reduction in hepatic lipids after VSG, there was no reduction in females. Ad lib-fed and fasting circulating triglycerides, and postprandial chylomicron production were significantly lower in VSG relative to Sham animals of both sexes (p < 0.01). However, hepatic VLDL production, highest in sham-operated females, was significantly reduced by VSG in females but not males. CONCLUSIONS: Taken together, although both males and females lose weight and improve plasma lipids, there are large-scale sex differences in hepatic gene expression and consequently hepatic lipid metabolism after VSG.

The role of proximal versus distal stomach resection in the weight loss seen after vertical sleeve gastrectomy.

The mechanisms involved in the weight loss seen after vertical sleeve gastrectomy (VSG) are not clear. The rat stomach has two morphologically and functionally distinct proximal and distal parts. The rat model for VSG involves complete removal of the proximal part and 80% removal of the distal part along the greater curvature. The purpose of this study was to understand the potential independent contributions of removal of these distinct gastric sections to VSG outcomes. We prepared four surgical groups of male Long-Evans rats: VSG, sham surgery (control), selective proximal section removal (PR), and selective distal section removal (DR). Gastric emptying rate (GER) was highest after VSG compared with all other groups. However, PR, in turn, had significantly greater GER compared with both DR and sham groups. The surgery-induced weight loss followed the same pattern with VSG causing the greatest weight loss and PR having greater weight loss compared with DR and sham groups. The results were robust for rats fed regular chow or a high-fat diet. Body mass analysis revealed that the weight loss was due to the loss of fat mass, and there was no change in lean mass after the surgeries. In conclusion, removal of the proximal stomach contributes to most, but not all, of the physiological impact of VSG.

Weight-independent changes in blood glucose homeostasis after gastric bypass or vertical sleeve gastrectomy in rats.

BACKGROUND & AIMS: Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG) reduce weight and improve glucose metabolism in obese patients, although it is not clear if metabolic changes are independent of weight loss. We investigated alterations in glucose metabolism in rats following RYGB or VSG. METHODS: Rats underwent RYGB or VSG and were compared to sham-operated rats fed ad lib or pair-fed to animals that received RYGB. Intraperitoneal glucose tolerance and insulin sensitivity tests were performed to assess glycemic function independent of incretin response. A hyperinsulinemic euglycemic clamp was used to compare tissue-specific changes in insulin sensitivity following each procedure. A mixed-meal tolerance test was used to assess the effect of each surgery on postprandial release of glucagon-like peptide 1 (GLP-1)(7-36) and glucose tolerance, and was also performed in rats given GLP-1 receptor antagonist exendin(9-39). RESULTS: Following RYGB or VSG, glucose tolerance and insulin sensitivity improved in proportion to weight loss. Hepatic insulin sensitivity was significantly better in rats that received RYGB or VSG compared with rats fed ad lib or pair-fed, whereas glucose clearance was similar in all groups. During the mixed-meal tolerance test, plasma levels of GLP-1(7-36) and insulin were greatly and comparably increased in rats that received RYGB and VSG compared with those that were pair-fed or fed ad lib. Administration of a GLP-1 receptor antagonist prevented improvements in glucose and insulin responses after a meal among rats that received RYGB or VSG. CONCLUSIONS: In obese rats, VSG is as effective as RYGB for increasing secretion of GLP-1 and insulin and improving hepatic sensitivity to insulin; these effects are independent of weight loss.

Similar effects of roux-en-Y gastric bypass and vertical sleeve gastrectomy on glucose regulation in rats.

Bariatric surgery is the most efficacious procedure for eliciting weight loss in humans, and many patients undergoing the procedure experience significant lessening of their symptoms of type-2 diabetes in addition to losing weight. We have adapted two bariatric surgical procedures commonly employed in humans to a rat model to begin to understand the mechanisms underlying the improvements in energy homeostasis. Young adult male rats received either roux-en-Y gastric bypass (RYGB) or vertical sleeve gastrectomy (VSG) and were assessed for body weight, food intake and parameters of glucose homeostasis over a 28-week period. Control rats received either a sham surgical procedure or else were unoperated. RYGB and VSG had comparable beneficial effects relative to controls. They ate less food and lost more weight, and they both had improved glucose parameters. The most intriguing aspect of the findings is that the two surgical procedures had such similar effects in spite of quite different rearrangements of the gastrointestinal system.

Perinatal exposure to bisphenol-a and the development of metabolic syndrome in CD-1 mice.

Bisphenol-A (BPA) is an endocrine-disrupting chemical used in the production of plastic food and beverage containers, leading to ubiquitous low-dose human exposure. It has been suggested that exposure to even low doses of BPA during development may be associated with increased susceptibility to obesity and diabetes later in life. Despite growing public concern, the existing empirical data are equivocal, prompting The Endocrine Society, the National Institute of Environmental Health Sciences, and others to call for further research. In this study, we tested the hypothesis that perinatal exposure to an ecologically relevant dose of BPA (1 part per billion via the diet) results in increased susceptibility to high-fat diet-induced obesity and glucose intolerance in adult CD-1 mice. The data did not support this hypothesis. In agreement with previous reports, we find that weanling mice exposed to BPA during gestation and lactation are heavier compared with control mice. We also find that BPA mice are longer than controls at 4 wk of age, but these differences are no longer apparent when the mice reach adulthood, even when tested on a high-fat diet. We conclude that this larger size-for-age represents a faster rate of growth early in development rather than an obese, diabetic phenotype in adulthood.

The roles of leptin receptors on POMC neurons in the regulation of sex-specific energy homeostasis.

Leptin regulates energy homeostasis and reproduction. One key population of leptin receptors (Lepr) are found on proopiomelanocortin (POMC) neurons in the hypothalamic arcuate nucleus, and evidence links the action of gonadal estrogens to these same POMC neurons. To determine whether Lepr on POMC neurons are critical for reproductive capacity or for sex-specific energy and glucose homeostasis, we studied Cre/loxP mice lacking Lepr specifically on POMC neurons (Pomc-Cre, Lepr(flox/flox) mice) and their controls with normal Lepr (Lepr(flox/flox) mice). Pomc-Cre, Lepr(flox/flox) mice maintained normal reproductive capacity and accumulated more body fat than their same sex controls. Ovariectomy (OVX) was performed to investigate the effects of the estrogens and Lepr on POMC neurons on body fat accumulation and glucose tolerance. OVX Pomc-Cre, Lepr(flox/flox) females accumulated more fat than OVX Lepr(flox/flox) females did. Pomc-Cre, Lepr(flox/flox) males were glucose intolerant and insulin insensitive compared with control males. In contrast, control and Pomc-Cre, Lepr(flox/flox) females had similar glucose tolerance before and after OVX. Therefore leptin's action on POMC neurons reduces body fat accumulation, but is not critical for regulation of reproduction. The sex difference in leptin signaling on POMC neurons on glucose tolerance appears independent of ovarian hormones.

Sexually different actions of leptin in proopiomelanocortin neurons to regulate glucose homeostasis.

Leptin regulates energy balance and glucose homeostasis, at least in part, via activation of receptors in the arcuate nucleus of the hypothalamus located in proopiomelanocortin (POMC) neurons. Females have greater sensitivity to central leptin than males, suggested by a greater anorectic effect of central leptin administration in females. We hypothesized that the regulation of energy balance and peripheral glucose homeostasis of female rodents would be affected to a greater extent than in males if the action of leptin in POMC neurons were disturbed. Male and female mice lacking leptin receptors only in POMC neurons gained significantly more body weight and accumulated more body fat. However, female mice gained disproportionately more visceral adiposity than males, and this appeared to be largely the result of differences in energy expenditure. When maintained on a high-fat diet (HFD), both male and female mutants had higher levels of insulin following exogenous glucose challenges. Chow- and HFD-fed males but not females had abnormal glucose disappearance curves following insulin administrations. Collectively, these data indicate that the action of leptin in POMC neurons is sexually different to influence the regulation of energy balance, fat distribution, and glucose homeostasis.

Alcohol drinking in MCH receptor-1-deficient mice.

BACKGROUND: Recently, we demonstrated that exogenous melanin-concentrating hormone (MCH) increases alcohol drinking in rats when administered into the brain. However, because the physiological relevance of this finding is unclear, we tested the hypothesis that endogenous MCH signaling enhances alcohol consumption. METHODS: Alcohol intake was assessed in male and female wildtype (WT), heterozygous (HET), and homozygous MCH receptor-1-deficient (KO) mice. Mice were given 24-hour access to a series of alcohol-containing solutions. Following this, the mice were given limited (1-hour) access to 10% alcohol. Finally, mice were allowed 24-hour access to sucrose/quinine as a caloric control and a means to assess taste preference. A naïve cohort of male WT and KO mice was tested for alcohol clearance following intraperitoneal administration of 3 g/kg alcohol. Another naïve cohort of female mice was utilized to confirm that intracerebroventricular administration of MCH (5 microg) would augment alcohol drinking in mice. RESULTS: Exogenous MCH enhanced 10% alcohol consumption in mice (saline=0.45+/-0.08 g/kg, 5 microg MCH=0.94+/-0.20 g/kg). Male KO mice consumed more 10% alcohol (11.50+/-1.31 g/kg) than WT (6.26+/-1.23 g/kg) and HET mice (6.49+/-1.23 g/kg) during ad libitum access. However, alcohol intake was similar among genotypes during 1 hour daily access. Male KO mice tended to consume less 17.75% sucrose+1.3 mM quinine than controls (WT=10.5+/-3.6, HET=7.5+/-1.7, KO=4.4+/-0.9 g/kg). Alcohol metabolism was similar between WT and KO mice. CONCLUSIONS: The finding that male KO consume more alcohol than WT and HET mice, are reminiscent of the counterintuitive reports that KO mice are hyperphagic and yet eat more when administered exogenous MCH. Changes in taste preference or alcohol metabolism do not appear to be important for the increased alcohol drinking in KO mice.