Silvana Obici

Endogenous glucose production is inhibited by the adipose-derived protein Acrp30

Intraperitoneal injection of purified recombinant Acrp30 lowers glucose levels in mice. To gain insight into the mechanism(s) of this hypoglycemic effect, purified recombinant Acrp30 was infused in conscious mice during a pancreatic euglycemic clamp. In the presence of physiological hyperinsulinemia, this treatment increased circulating Acrp30 levels by approximately twofold and stimulated glucose metabolism. The effect of Acrp30 on in vivo insulin action was completely accounted for by a 65% reduction in the rate of glucose production. Similarly, glucose flux through glucose-6-phosphatase (G6Pase) decreased with Acrp30, whereas the activity of the direct pathway of glucose-6-phosphate biosynthesis, an index of hepatic glucose phosphorylation, increased significantly. Acrp30 did not affect the rates of glucose uptake, glycolysis, or glycogen synthesis. These results indicate that an acute increase in circulating Acrp30 levels lowers hepatic glucose production without affecting peripheral glucose uptake. Hepatic expression of the gluconeogenic enzymes phosphoenolpyruvate carboxykinase and G6Pase mRNAs was reduced by more than 50% following Acrp30 infusion compared with vehicle infusion. Thus, a moderate rise in circulating levels of the adipose-derived protein Acrp30 inhibits both the expression of hepatic gluconeogenic enzymes and the rate of endogenous glucose production.

Hypothalamic insulin signaling is required for inhibition of glucose production.

Circulating insulin inhibits endogenous glucose production. Here we report that bidirectional changes in hypothalamic insulin signaling affect glucose production. The infusion of either insulin or a small-molecule insulin mimetic in the third cerebral ventricle suppressed glucose production independent of circulating levels of insulin and of other glucoregulatory hormones. Conversely, central antagonism of insulin signaling impaired the ability of circulating insulin to inhibit glucose production. Finally, third-cerebral-ventricle administration of inhibitors of ATP-sensitive potassium channels, but not of antagonists of the central melanocortin receptors, also blunted the effect of hyperinsulinemia on glucose production. These results reveal a new site of action of insulin on glucose production and suggest that hypothalamic insulin resistance can contribute to hyperglycemia in type 2 diabetes mellitus.

Decreasing hypothalamic insulin receptors causes hyperphagia and insulin resistance in rats

We investigated the role of hypothalamic insulin signaling in the regulation of energy balance and insulin action in rats through selective decreases in insulin receptor expression in discrete hypothalamic nuclei. We generated an antisense oligodeoxynucleotide directed against the insulin receptor precursor protein and administered this directly into the third cerebral ventricle. Immunostaining of rat brains after 7-day administration of the oligodeoxynucleotide showed a selective decrease of insulin receptor protein within cells in the medial portion of the arcuate nucleus (decreased by ∼80% as compared to rats treated with a control oligodeoxynucleotide). Insulin receptors in other hypothalamic and extra-hypothalamic areas were not affected. This selective decrease in hypothalamic insulin receptor protein was accompanied by rapid onset of hyperphagia and increased fat mass. During insulin-clamp studies, physiological hyperinsulinemia decreased glucose production by 55% in rats treated with control oligodeoxynucleotides but by only 25% in rats treated with insulin receptor antisense oligodeoxynucleotides. Thus, insulin receptors in discrete areas of the hypothalamus have a physiological role in the control of food intake, fat mass and hepatic action of insulin.

Hypothalamic K(ATP) channels control hepatic glucose production.

Obesity is the driving force behind the worldwide increase in the prevalence of type 2 diabetes mellitus. Hyperglycaemia is a hallmark of diabetes and is largely due to increased hepatic gluconeogenesis. The medial hypothalamus is a major integrator of nutritional and hormonal signals, which play pivotal roles not only in the regulation of energy balance but also in the modulation of liver glucose output. Bidirectional changes in hypothalamic insulin signalling therefore result in parallel changes in both energy balance and glucose metabolism. Here we show that activation of ATP-sensitive potassium (KATP) channels in the mediobasal hypothalamus is sufficient to lower blood glucose levels through inhibition of hepatic gluconeogenesis. Finally, the infusion of a KATP blocker within the mediobasal hypothalamus, or the surgical resection of the hepatic branch of the vagus nerve, negates the effects of central insulin and halves the effects of systemic insulin on hepatic glucose production. Consistent with these results, mice lacking the SUR1 subunit of the KATP channel are resistant to the inhibitory action of insulin on gluconeogenesis. These findings suggest that activation of hypothalamic KATP channels normally restrains hepatic gluconeogenesis, and that any alteration within this central nervous system/liver circuit can contribute to diabetic hyperglycaemia.

Adipose-derived resistin and gut-derived resistin-like molecule–β selectively impair insulin action on glucose production

The adipose-derived hormone resistin is postulated to link obesity to insulin resistance and diabetes. Here, the infusion of either resistin or the resistin-like molecule-beta (RELMbeta) rapidly induced severe hepatic but not peripheral insulin resistance. In the presence of physiologic hyperinsulinemia, the infusion of purified recombinant resistin, increasing circulating resistin levels by approximately twofold to 15-fold, inhibited glucose metabolism such that lower rates of glucose infusion were required to maintain the plasma glucose concentration at basal levels. The effects of resistin and RELMbeta on in vivo insulin action were completely accounted for by a marked increase in the rate of glucose production. These results support the notion that a novel family of fat- and gut-derived circulating proteins modulates hepatic insulin action.

Inhibition of hypothalamic carnitine palmitoyltransferase-1 decreases food intake and glucose production

The enzyme carnitine palmitoyltransferase-1 (CPT1) regulates long-chain fatty acid (LCFA) entry into mitochondria, where the LCFAs undergo β-oxidation. To investigate the mechanism(s) by which central metabolism of lipids can modulate energy balance, we selectively reduced lipid oxidation in the hypothalamus. We decreased the activity of CPT1 by administering to rats a ribozyme-containing plasmid designed specifically to decrease the expression of this enzyme or by infusing pharmacological inhibitors of its activity into the third cerebral ventricle. Either genetic or biochemical inhibition of hypothalamic CPT1 activity was sufficient to substantially diminish food intake and endogenous glucose production. These results indicated that changes in the rate of lipid oxidation in selective hypothalamic neurons signaled nutrient availability to the hypothalamus, which in turn modulated the exogenous and endogenous inputs of nutrients into the circulation.

Lower blood glucose, hyperglucagonemia, and pancreatic α cell hyperplasia in glucagon receptor knockout mice

Glucagon, the counter-regulatory hormone to insulin, is secreted from pancreatic α cells in response to low blood glucose. To examine the role of glucagon in glucose homeostasis, mice were generated with a null mutation of the glucagon receptor (Gcgr−/−). These mice display lower blood glucose levels throughout the day and improved glucose tolerance but similar insulin levels compared with control animals. Gcgr−/− mice displayed supraphysiological glucagon levels associated with postnatal enlargement of the pancreas and hyperplasia of islets due predominantly to α cell, and to a lesser extent, δ cell proliferation. In addition, increased proglucagon expression and processing resulted in increased pancreatic glucogen-like peptide 1 (GLP-1) (1–37) and GLP-1 amide (1–36 amide) content and a 3- to 10-fold increase in circulating GLP-1 amide. Gcgr−/− mice also displayed reduced adiposity and leptin levels but normal body weight, food intake, and energy expenditure. These data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and α and δ cell numbers. Furthermore, the lean phenotype of Gcgr−/− mice suggests glucagon action may be involved in the regulation of whole body composition.

Hypothalamic sensing of circulating fatty acids is required for glucose homeostasis

Increased glucose production is a hallmark of type 2 diabetes and alterations in lipid metabolism have a causative role in its pathophysiology. Here we postulate that physiological increments in plasma fatty acids can be sensed within the hypothalamus and that this sensing is required to balance their direct stimulatory action on hepatic gluconeogenesis. In the presence of physiologically-relevant increases in the levels of plasma fatty acids, negating their central action on hepatic glucose fluxes through (i) inhibition of the hypothalamic esterification of fatty acids, (ii) genetic deletion (Sur1-deficient mice) of hypothalamic KATP channels or pharmacological blockade (KATP blocker) of their activation by fatty acids, or (iii) surgical resection of the hepatic branch of the vagus nerve led to a marked increase in liver glucose production. These findings indicate that a physiological elevation in circulating lipids can be sensed within the hypothalamus and that a defect in hypothalamic lipid sensing disrupts glucose homeostasis.

Standard operating procedures for describing and performing metabolic tests of glucose homeostasis in mice

The Mouse Metabolic Phenotyping Center (MMPC) Consortium was established to address the need to characterize the growing number of mouse models of metabolic diseases, particularly diabetes and obesity. A goal of the MMPC Consortium is to propose standard methods for assessing metabolic phenotypes in mice. In this article, we discuss issues pertaining to the design and performance of various tests of glucose metabolism. We also propose guidelines for the description of methods, presentation of data and interpretation of results. The recommendations presented in this article are based on the experience of the MMPC Consortium and other investigators.

Central melanocortin receptors regulate insulin action

Energy balance and insulin action are tightly coregulated. Leptin regulates energy intake and expenditure partly by modulation of the melanocortin pathway in the hypothalamus. Here we demonstrate potent effects of the melanocortin pathway on insulin action and body distribution of adiposity. Conscious rats received week-long infusions of either a melanocortin receptor agonist, alpha-melanocyte-stimulating hormone (alpha-MSH), or antagonist, SHU9119, in the third cerebral ventricle while food intake was maintained constant in each group. alpha-MSH decreased intra-abdominal fat and markedly enhanced the actions of insulin on both glucose uptake and production, while SHU9119 exerted opposite effects. Our findings elucidate a neuroendocrine network that is likely to play a central role in the coupling of energy intake and insulin action.