In uncontrolled diabetes, thyroid hormone and sympathetic activators induce thermogenesis without increasing glucose uptake in brown adipose tissue.

Recent advances in human brown adipose tissue (BAT) imaging technology have renewed interest in the identification of BAT activators for the treatment of obesity and diabetes. In uncontrolled diabetes (uDM), activation of BAT is implicated in glucose lowering mediated by intracerebroventricular (icv) administration of leptin, which normalizes blood glucose levels in streptozotocin (STZ)-induced diabetic rats. The potent effect of icv leptin to increase BAT glucose uptake in STZ-diabetes is accompanied by the return of reduced plasma thyroxine (T4) levels and BAT uncoupling protein-1 (Ucp1) mRNA levels to nondiabetic controls. We therefore sought to determine whether activation of thyroid hormone receptors is sufficient in and of itself to lower blood glucose levels in STZ-diabetes and whether this effect involves activation of BAT. We found that, although systemic administration of the thyroid hormone (TR)ß-selective agonist GC-1 increases energy expenditure and induces further weight loss in STZ-diabetic rats, it neither increased BAT glucose uptake nor attenuated diabetic hyperglycemia. Even when GC-1 was administered in combination with a ß(3)-adrenergic receptor agonist to mimic sympathetic nervous system activation, glucose uptake was not increased in STZ-diabetic rats, nor was blood glucose lowered, yet this intervention potently activated BAT. Similar results were observed in animals treated with active thyroid hormone (T3) instead of GC-1. Taken together, our data suggest that neither returning normal plasma thyroid hormone levels nor BAT activation has any impact on diabetic hyperglycemia, and that in BAT, increases of Ucp1 gene expression and glucose uptake are readily dissociated from one another in this setting.

Evidence That in Uncontrolled Diabetes, Hyperglucagonemia Is Required for Ketosis but Not for Increased Hepatic Glucose Production or Hyperglycemia.

Several lines of evidence implicate excess glucagon secretion in the elevated rates of hepatic glucose production (HGP), hyperglycemia, and ketosis characteristic of uncontrolled insulin-deficient diabetes (uDM), but whether hyperglucagonemia is required for hyperglycemia in this setting is unknown. To address this question, adult male Wistar rats received either streptozotocin (STZ) to induce uDM (STZ-DM) or vehicle and remained nondiabetic. Four days later, animals received daily subcutaneous injections of either the synthetic GLP-1 receptor agonist liraglutide in a dose-escalating regimen to reverse hyperglucagonemia or its vehicle for 10 days. As expected, plasma glucagon levels were elevated in STZ-DM rats, and although liraglutide treatment lowered glucagon levels to those of nondiabetic controls, it failed to attenuate diabetic hyperglycemia, elevated rates of glucose appearance (Ra), or increased hepatic gluconeogenic gene expression. In contrast, it markedly reduced levels of both plasma ketone bodies and hepatic expression of the rate-limiting enzyme involved in ketone body production. To independently confirm this finding, in a separate study, treatment of STZ-DM rats with a glucagon-neutralizing antibody was sufficient to potently lower plasma ketone bodies but failed to normalize elevated levels of either blood glucose or Ra. These data suggest that in rats with uDM, hyperglucagonemia is required for ketosis but not for increased HGP or hyperglycemia.

BDNF action in the brain attenuates diabetic hyperglycemia via insulin-independent inhibition of hepatic glucose production.

Recent evidence suggests that central leptin administration fully normalizes hyperglycemia in a rodent model of uncontrolled insulin-deficient diabetes by reducing hepatic glucose production (HGP) and by increasing glucose uptake. The current studies were undertaken to determine whether brain-derived neurotrophic factor (BDNF) action in the brain lowers blood glucose in uncontrolled insulin-deficient diabetes and to investigate the mechanisms mediating this effect. Adult male rats implanted with cannulas to either the lateral cerebral ventricle or the ventromedial hypothalamic nucleus (VMN) received either vehicle or streptozotocin to induce uncontrolled insulin-deficient diabetes. Three days later, animals received daily intracerebroventricular or intra-VMN injections of either BDNF or its vehicle. We found that repeated daily intracerebroventricular administration of BDNF attenuated diabetic hyperglycemia independent of changes in food intake. Instead, using tracer dilution techniques during a basal clamp, we found that BDNF lowered blood glucose levels by potently suppressing HGP, without affecting tissue glucose uptake, an effect associated with normalization of both plasma glucagon levels and hepatic expression of gluconeogenic genes. Moreover, BDNF microinjection directly into the VMN also lowered fasting blood glucose levels in uncontrolled insulin-deficient diabetes, but this effect was modest compared with intracerebroventricular administration. We conclude that central nervous system BDNF attenuates diabetic hyperglycemia via an insulin-independent mechanism. This action of BDNF likely involves the VMN and is associated with inhibition of glucagon secretion and a decrease in the rate of HGP.

Obesity is associated with hypothalamic injury in rodents and humans.

Rodent models of obesity induced by consuming high-fat diet (HFD) are characterized by inflammation both in peripheral tissues and in hypothalamic areas critical for energy homeostasis. Here we report that unlike inflammation in peripheral tissues, which develops as a consequence of obesity, hypothalamic inflammatory signaling was evident in both rats and mice within 1 to 3 days of HFD onset, prior to substantial weight gain. Furthermore, both reactive gliosis and markers suggestive of neuron injury were evident in the hypothalamic arcuate nucleus of rats and mice within the first week of HFD feeding. Although these responses temporarily subsided, suggesting that neuroprotective mechanisms may initially limit the damage, with continued HFD feeding, inflammation and gliosis returned permanently to the mediobasal hypothalamus. Consistent with these data in rodents, we found evidence of increased gliosis in the mediobasal hypothalamus of obese humans, as assessed by MRI. These findings collectively suggest that, in both humans and rodent models, obesity is associated with neuronal injury in a brain area crucial for body weight control.

Leptin activates a novel CNS mechanism for insulin-independent normalization of severe diabetic hyperglycemia.

The brain has emerged as a target for the insulin-sensitizing effects of several hormonal and nutrient-related signals. The current studies were undertaken to investigate mechanisms whereby leptin lowers circulating blood glucose levels independently of insulin. After extending previous evidence that leptin infusion directly into the lateral cerebral ventricle ameliorates hyperglycemia in rats with streptozotocin-induced uncontrolled diabetes mellitus, we showed that the underlying mechanism is independent of changes of food intake, urinary glucose excretion, or recovery of pancreatic ß-cells. Instead, leptin action in the brain potently suppresses hepatic glucose production while increasing tissue glucose uptake despite persistent, severe insulin deficiency. This leptin action is distinct from its previously reported effect to increase insulin sensitivity in the liver and offers compelling evidence that the brain has the capacity to normalize diabetic hyperglycemia in the presence of sufficient amounts of central nervous system leptin.

Fibroblast growth factor 21 action in the brain increases energy expenditure and insulin sensitivity in obese rats.

OBJECTIVE: The hormone fibroblast growth factor 21 (FGF21) exerts diverse, beneficial effects on energy balance and insulin sensitivity when administered systemically to rodents with diet-induced obesity (DIO). The current studies investigate whether central FGF21 treatment recapitulates these effects. RESEARCH DESIGN AND METHODS: After preliminary dose-finding studies, either saline vehicle or recombinant human FGF21 (0.4 microg/day) was infused continuously for 2 weeks into the lateral cerebral ventricle of male Wistar rats rendered obese by high-fat feeding. Study end points included measures of energy balance (body weight, body composition, food intake, energy expenditure, and circulating and hepatic lipids) and glucose metabolism (insulin tolerance test, euglycemic-hyperinsulinemic clamp, and hepatic expression of genes involved in glucose metabolism). RESULTS: Compared with vehicle, continuous intracerebroventricular infusion of FGF21 increased both food intake and energy expenditure in rats with DIO, such that neither body weight nor body composition was altered. Despite unchanged body fat content, rats treated with intracerebroventricular FGF21 displayed a robust increase of insulin sensitivity due to increased insulin-induced suppression of both hepatic glucose production and gluconeogenic gene expression, with no change of glucose utilization. CONCLUSIONS: FGF21 action in the brain increases hepatic insulin sensitivity and metabolic rate in rats with DIO. These findings identify the central nervous system as a potentially important target for the beneficial effects of FGF21 in the treatment of diabetes and obesity.

Leptin deficiency causes insulin resistance induced by uncontrolled diabetes.

OBJECTIVE: Depletion of body fat stores during uncontrolled, insulin-deficient diabetes (uDM) results in markedly reduced plasma leptin levels. This study investigated the role of leptin deficiency in the genesis of severe insulin resistance and related metabolic and neuroendocrine derangements induced by uDM. RESEARCH DESIGN AND METHODS: Adult male Wistar rats remained nondiabetic or were injected with the beta-cell toxin, streptozotocin (STZ) to induce uDM and subsequently underwent subcutaneous implantation of an osmotic minipump containing either vehicle or leptin at a dose (150 microg/kg/day) designed to replace leptin at nondiabetic plasma levels. To control for leptin effects on food intake, another group of STZ-injected animals were pair fed to the intake of those receiving leptin. Food intake, body weight, and blood glucose levels were measured daily, with body composition and indirect calorimetry performed on day 11, and an insulin tolerance test to measure insulin sensitivity performed on day 16. Plasma hormone and substrate levels, hepatic gluconeogenic gene expression, and measures of tissue insulin signal transduction were also measured. RESULTS: Physiologic leptin replacement prevented insulin resistance in uDM via a mechanism unrelated to changes in food intake or body weight. This effect was associated with reduced total body fat and hepatic triglyceride content, preservation of lean mass, and improved insulin signal transduction via the insulin receptor substrate-phosphatidylinositol-3-hydroxy kinase pathway in the liver, but not in skeletal muscle or adipose tissue. Although physiologic leptin replacement lowered blood glucose levels only slightly, it fully normalized elevated plasma glucagon and corticosterone levels and reversed the increased hepatic expression of gluconeogenic enzymes characteristic of rats with uDM. CONCLUSIONS: We conclude that leptin deficiency plays a key role in the pathogenesis of severe insulin resistance and related endocrine disorders in uDM. Treatment of diabetes in humans may benefit from correction of leptin deficiency as well as insulin deficiency.