Leptin regulates insulin sensitivity via phosphatidylinositol-3-OH kinase signaling in mediobasal hypothalamic neurons

Hypothalamic leptin regulation of energy homeostasis and glucose metabolism

Growing evidence suggests that food intake, energy expenditure and endogenous glucose production are regulated by hypothalamic areas that respond to a variety of peripheral signals. Therefore, in response to a reduction in energy stores or circulating nutrients, the brain initiates responses in order to promote positive energy balance to restore and maintain energy and glucose homeostasis. In contrast, in times of nutrient abundance and excess energy storage, key hypothalamic areas activate responses to promote negative energy balance (i.e. reduced food intake and increased energy expenditure) and decreased nutrient availability (reduced endogenous glucose production). Accordingly, impaired responses or 'resistance' to afferent input from these hormonal or nutrient-related signals would be predicted to favour weight gain and insulin resistance and may contribute to the development of obesity and type 2 diabetes.

SH2B1 enhances leptin signaling by both Janus kinase 2 Tyr813 phosphorylation-dependent and -independent mechanisms

Leptin controls body weight by activating its long form receptor (LEPRb). LEPRb binds to Janus kinase 2 (JAK2), a cytoplasmic tyrosine kinase that mediates leptin signaling. We previously reported that genetic deletion of SH2B1 (previously known as SH2-B), a JAK2-binding protein, results in severe leptin-resistant and obese phenotypes, indicating that SH2B1 is a key endogenous positive regulator of leptin sensitivity. Here we show that SH2B1 regulates leptin signaling by multiple mechanisms. In the absence of leptin, SH2B1 constitutively bound, via its non-SH2 domain region(s), to non-tyrosyl-phosphorylated JAK2, and inhibited JAK2. Leptin stimulated JAK2 phosphorylation on Tyr(813), which subsequently bound to the SH2 domain of SH2B1. Binding of the SH2 domain of SH2B1 to phospho-Tyr(813) in JAK2 enhanced leptin induction of JAK2 activity. JAK2 was required for leptin-stimulated phosphorylation of insulin receptor substrate 1 (IRS1), an upstream activator of the phosphatidylinositol 3-kinase pathway. Overexpression of SH2B1 enhanced both JAK2- and JAK2(Y813F)-mediated tyrosine phosphorylation of IRS1 in response to leptin, even though SH2B1 did not enhance JAK2(Y813F) activation. Leptin promoted the interaction of SH2B1 with IRS1. These data suggest that constitutive SH2B1-JAK2 interaction, mediated by the non-SH2 domain region(s) of SH2B1 and the non-Tyr(813) region(s) in JAK2, increases the local concentration of SH2B1 close to JAK2 and inhibits JAK2 activity. Leptin-stimulated SH2B1-JAK2 interaction, mediated by the SH2 domain of SH2B1 and phospho-Tyr(813) in JAK2, promotes JAK2 activation, thus globally enhancing leptin signaling. SH2B1-IRS1 interaction facilitates IRS1 phosphorylation by recruiting IRS1 to JAK2 and/or by protecting IRS1 from dephosphorylation, thus specifically enhancing leptin stimulation of the phosphatidylinositol 3-kinase pathway.

Central interleukin-1 (IL1) signaling is required for pharmacological, but not physiological, effects of leptin on energy balance

Hypothalamic IL1 is suggested to be a critical mediator of the central effects of the adipocyte hormone leptin on energy balance. We hypothesized that IL1 receptor signaling is required for exogenously administered leptin to cause anorexia and weight loss, but not for physiological effects of endogenous leptin signaling on energy balance. To test this hypothesis, we investigated whether chronic hypothalamic over-expression of an IL1 receptor antagonist (AdV-IL1ra) alters food intake and weight gain in normal rats. Our findings demonstrate that impaired IL1 signaling in the CNS did not cause excess weight gain over a period of 11 days (AdV-IL1ra +38.1+/-4.1 g vs. VEH +42.2+/-5.6g; p=0.6) and caused a slightly reduced daily food intake (AdV-IL1ra 29.0+/-1.1 g/day vs. VEH 33.0+/-1.6 g/day; p<0.05). Blocking central IL1 signaling also did not alter the re-feeding response to a prolonged fast, yet was entirely effective in preventing the anorexic effect of exogenously administered leptin (2 mg/kg ip, cumulative food intake at 18 h AdV-IL1ra 30.5+/-1.1 g vs. VEH 26.4+/-1.7 g, p<0.05) and prevented leptin-induced weight loss (AdV-IL1ra -0.1+/-1.3 g vs. VEH -2.7+/-1.9 g, p<0.05). Together these findings suggest that hypothalamic IL1 signaling is required for the pharmacological effects of leptin administration, but that impaired hypothalamic IL1 signaling does not alter the physiological regulation of energy balance.

Hormonal induction of leptin resistance during pregnancy

Despite elevated plasma leptin, food intake is increased during pregnancy leading to fat deposition. We have demonstrated that intracerebroventricular (icv) leptin is unable to suppress food intake in pregnant rats, as it does in non-pregnant animals. Hence, central leptin resistance develops during pregnancy. These changes are physiologically appropriate, providing increased energy reserves to help meet the high metabolic demands of fetal development and lactation. To characterise this central leptin resistance, we have measured levels of leptin receptor (Ob-Rb) mRNA in the hypothalamus, and examined leptin-induced phosphorylation of STAT3 (pSTAT3) in specific regions of the hypothalamus. In addition, to investigate the mechanism underlying pregnancy-induced leptin resistance, we have investigated effects of hormone treatments on hypothalamic responses to leptin in a pseudopregnant rat model. We observed a significant reduction of Ob-Rb mRNA levels in the ventromedial hypothalamic nucleus (VMH) during pregnancy, with no changes detected in other hypothalamic nuclei. Levels of leptin-induced pSTAT3 were specifically suppressed in the VMH and arcuate nucleus of pregnant rats compared to non-pregnant rats. Pseudopregnant rats were hyperphagic but did not become leptin resistant, suggesting that fetal or placental factors are required for the induction of leptin resistance. These data implicate the VMH as a key hypothalamic site involved in hormone-induced leptin resistance during pregnancy, and suggest that placental hormone secretion may mediate the hormone-induced loss of response to leptin.

A preprogalanin cDNA from the turtle pituitary and regulation of its gene expression

Galanin is a hormone 29 or 30 amino acids (aa) long that is widely distributed within the body and exerts numerous biological effects in vertebrates. To fully understand its physiological roles in reptiles, we analyzed preprogalanin cDNA structure and expression in the turtle pituitary. Using the Chinese soft-shell turtle ( Pelodiscus sinensis order Testudines), we obtained a 672-base pair (bp) cDNA containing a 99-bp 5′-untranslated region, a 324-bp preprogalanin coding region, and a 249-bp 3′-untranslated region. The open-reading frame encoded a 108-aa preprogalanin protein with a putative 23-aa signal sequence at the NH2 terminus. Based on the location of putative Lys-Arg dibasic cleavage sites and an amidation signal of Gly-Lys-Arg, we propose that turtle preprogalanin is processed to yield a 29-aa galanin peptide with Gly1 and Thr29 substitutions and a COOH-terminal amidation. Sequence comparison revealed that turtle preprogalanin and galanin-29 had 48–81% and 76–96% aa identities with those of other vertebrates, respectively, suggesting their conservative nature. Expression of the turtle galanin gene was detected in the pituitary, brain, hypothalamus, stomach, liver, pancreas, testes, ovaries, and intestines, but not in the adipose or muscle tissues, suggesting tissue-dependent differences. An in vitro study that used pituitary tissue culture indicated that treatment with 17β-estradiol, testosterone, or gonadotropin-releasing hormone resulted in increased galanin mRNA expression with dose- or time-dependent differences, whereas leptin and neuropeptide Y reduced galanin mRNA levels. These results suggest a hormone-dependent effect on hypophyseal galanin mRNA expression.

Successful weight loss surgery improves eating control and energy metabolism: a review of the evidence

Eating behavior is determined by a balance of memories in terms of reward and punishment to satisfy the urge to consume food. Refilling empty energy stores and hedonistic motivation are rewarding aspects of eating. Overfeeding, associated adverse GI effects, and obesity implicate punishment. In the current review, evidence is given for the hypothesis that bariatric surgery affects control over eating behavior. Moreover, any caloric overload will reduce the feeling of satiety. Durable weight loss after bariatric surgery is probably the result of a new equilibrium between reward and punishment, together with a better signaling of satiation due to beneficial metabolic changes. We propose to introduce three main treatment goals for bariatric surgery: 1) acceptable weight loss, 2) improvement of eating control, and 3) metabolic benefit. To achieve this goal, loss of 50% to 70% of excess weight will be appropriate (i.e. 30% to 40% loss of initial weight), depending on the degree of obesity prior to operation.

Non-alcoholic fatty liver disease: further expression of the metabolic syndrome

Non-alcoholic fatty liver disease has been associated with metabolic disorders, including central obesity, dyslipidemia, hypertension and hyperglycemia. Metabolic syndrome, obesity, and insulin resistance are major risk factors in the pathogenesis of non-alcoholic fatty liver disease. Non-alcoholic fatty liver disease refers to a wide spectrum of liver damage, ranging from simple steatosis to non-alcoholic steatohepatitis, advanced fibrosis and cirrhosis.

Neuronal SH2B1 is essential for controlling energy and glucose homeostasis

SH2B1 (previously named SH2-B), a cytoplasmic adaptor protein, binds via its Src homology 2 (SH2) domain to a variety of protein tyrosine kinases, including JAK2 and the insulin receptor. SH2B1-deficient mice are obese and diabetic. Here we demonstrated that multiple isoforms of SH2B1 (alpha, beta, gamma, and/or delta) were expressed in numerous tissues, including the brain, hypothalamus, liver, muscle, adipose tissue, heart, and pancreas. Rat SH2B1beta was specifically expressed in neural tissue in SH2B1-transgenic (SH2B1(Tg)) mice. SH2B1(Tg) mice were crossed with SH2B1-knockout (SH2B1(KO)) mice to generate SH2B1(TgKO) mice expressing SH2B1 only in neural tissue but not in other tissues. Systemic deletion of the SH2B1 gene resulted in metabolic disorders in SH2B1(KO) mice, including hyperlipidemia, leptin resistance, hyperphagia, obesity, hyperglycemia, insulin resistance, and glucose intolerance. Neuron-specific restoration of SH2B1beta not only corrected the metabolic disorders in SH2B1(TgKO) mice, but also improved JAK2-mediated leptin signaling and leptin regulation of orexigenic neuropeptide expression in the hypothalamus. Moreover, neuron-specific overexpression of SH2B1 dose-dependently protected against high-fat diet-induced leptin resistance and obesity. These observations suggest that neuronal SH2B1 regulates energy balance, body weight, peripheral insulin sensitivity, and glucose homeostasis at least in part by enhancing hypothalamic leptin sensitivity.

Lipin deficiency impairs diurnal metabolic fuel switching

Fatty liver is a common feature of both obesity and lipodystrophy, reflecting compromised adipose tissue function. The lipin-deficient fatty liver dystrophy (fld) mouse is an exception, as there is lipodystrophy without a fatty liver. Using a combination of indirect calorimetry and stable-isotope flux phenotyping, we determined that fld mice exhibit abnormal fuel utilization throughout the diurnal cycle, with increased glucose oxidation near the end of the fasting period and increased fatty acid oxidation during the feeding period. The mechanisms underlying these alterations include a twofold increase compared with wild-type mice in tissue glycogen storage during the fed state, a 40% reduction in hepatic glucose production in the fasted state, and a 27-fold increase in de novo fatty acid synthesis in liver during the fed state. Thus, the inability to store energy in adipose tissue in the fld mouse leads to a compensatory increase in glycogen storage for use during the fasting period and reliance upon hepatic fatty acid synthesis to provide fuel for peripheral tissues during the fed state. The increase in hepatic fatty acid synthesis and peripheral utilization provides a potential mechanism to ameliorate fatty liver in the fld that would otherwise occur as a consequence of adipose tissue dysfunction.

Young adult-specific hyperphagia in diabetic Goto-kakizaki rats is associated with leptin resistance and elevation of neuropeptide Y mRNA in the arcuate nucleus

The present study aimed to examine whether hyperphagia, which is frequently observed in type 1 diabetic patients and model animals, also occurs in type 2 diabetic Goto-Kakizaki (GK) rats and, if so, to explore underlying abnormalities in the hypothalamus. GK rats at postnatal weeks 6-12, compared to control Wistar rats, exhibited hyperphagia, hyperglycaemia, hyperleptinemia and increased visceral fat accumulation, whereas body weight was unaltered. The ability of leptin to suppress feeding was reduced in GK rats compared to Wistar rats of these ages. In GK rats, leptin-induced phosphorylation of signal transducer and activator of transcription 3 was significantly reduced in the cells of the hypothalamic arcuate nucleus (ARC), but not of the ventromedial hypothalamus, whereas the mRNA level of functional leptin receptor was unaltered. By real-time polymerase chain reaction and in situ hybridisation, mRNA levels of neuropeptide Y, but not pro-opiomelanocortin and galanin-like peptide, were significantly increased in the ARC of GK rats at 11 weeks, but not 26 weeks. Following i.c.v. injection of a NPY Y1 antagonist, 1229U91, the amount of food intake in GK rats was indistinguishable from that in Wistar rats, thus eliminating the hyperphagia of GK rats. These results demonstrate that young adult GK rats display hyperphagia in association with leptin resistance and increased NPY mRNA level in the ARC.

Central insulin action in energy and glucose homeostasis

Insulin has pleiotropic biological effects in virtually all tissues. However, the relevance of insulin signaling in peripheral tissues has been studied far more extensively than its role in the brain. An evolving body of evidence indicates that in the brain, insulin is involved in multiple regulatory mechanisms including neuronal survival, learning, and memory, as well as in regulation of energy homeostasis and reproductive endocrinology. Here we review insulin's role as a central homeostatic signal with regard to energy and glucose homeostasis and discuss the mechanisms by which insulin communicates information about the body's energy status to the brain. Particular emphasis is placed on the controversial current debate about the similarities and differences between hypothalamic insulin and leptin signaling at the molecular level.

How the hypothalamus controls glucose production: an update

Recent evidence highlights a crucial role of the brain in the control of glucose homeostasis. The hypothalamus senses and integrates signals of fuel abundance, such as circulating macronutrients (glucose and fatty acids) and nutrient-induced hormones (insulin and leptin). This, in turn, results in the activation of neural pathways that return circulating nutrients to baseline by reducing hepatic glucose production and food intake. In Type 2 diabetes and obesity, the ability of the brain to sense and respond to circulating signals is impaired. In this review, the neuroendocrine circuits that have recently been involved in the regulation of endogenous glucose production in rodents will be described. The study of these neural pathways promises to unveil new targets for the therapy of Type 2 diabetes and obesity.

Leptin receptor signaling and action in the central nervous system

The increasing incidence of obesity in developed nations represents an ever-growing challenge to health care by promoting diabetes and other diseases. The discovery of the hormone, leptin, a decade ago has facilitated the acquisition of new knowledge regarding the regulation of energy balance. A great deal remains to be discovered regarding the molecular and anatomic actions of leptin, however. Here, we discuss the mechanisms by which leptin activates intracellular signals, the roles that these signals play in leptin action in vivo, and sites of leptin action in vivo. Using "reporter" mice, in which LRb-expressing (long form of the leptin receptor) neurons express the histological marker, beta-galactosidase, coupled with the detection of LRb-mediated signal transducer and activator of transcription 3 signaling events, we identified LRb expression in neuronal populations both within and outside the hypothalamus. Understanding the regulation and physiological function of these myriad sites of central leptin action will be a crucial next step in the quest to understand mechanisms of leptin action and energy balance.

Essential role of Pten in body size determination and pancreatic beta-cell homeostasis in vivo

PTEN (phosphatase with tensin homology) is a potent negative regulator of phosphoinositide 3-kinase (PI3K)/Akt signaling, an evolutionarily conserved pathway that signals downstream of growth factors, including insulin and insulin-like growth factor 1. In lower organisms, this pathway participates in fuel metabolism and body size regulation and insulin-like proteins are produced primarily by neuronal structures, whereas in mammals, the major source of insulin is the pancreatic beta cells. Recently, rodent insulin transcription was also shown in the brain, particularly the hypothalamus. The specific regulatory elements of the PI3K pathway in these insulin-expressing tissues that contribute to growth and metabolism in higher organisms are unknown. Here, we report PTEN as a critical determinant of body size and glucose metabolism when targeting is driven by the rat insulin promoter in mice. The partial deletion of PTEN in the hypothalamus resulted in significant whole-body growth restriction and increased insulin sensitivity. Efficient PTEN deletion in beta cells led to increased islet mass without compromise of beta-cell function. Parallel enhancement in PI3K signaling was found in PTEN-deficient hypothalamus and beta cells. Together, we have shown that PTEN in insulin-transcribing cells may play an integrative role in regulating growth and metabolism in vivo.

Lipid phosphatases as a possible therapeutic target in cases of type 2 diabetes and obesity

Phosphatidyl inositol 3-kinase (PI3-kinase) functions as a lipid kinase to produce PI(3,4,5)P(3) from PI(4,5)P(2) in vivo. PI(3,4,5)P(3) is crucial as a lipid second messenger in various metabolic effects of insulin. Lipid phosphatases, src homology 2 domain containing inositol 5'-phosphatase 2 (SHIP2) and skeletal muscle and kidney-enriched inositol phosphatase (SKIP) hydrolyze PI(3,4,5)P(3) to PI(3,4)P(2) and phosphatase and tensin homolog deleted on chromosome ten (PTEN) hydrolyzes PI(3,4,5)P(3) to PI(4,5)P(2). SHIP2 negatively regulates insulin signaling relatively specifically via its 5'-phosphatase activity. Targeted disruption of the SHIP2 gene in mice resulted in increased insulin sensitivity and conferred protection from obesity induced by a high-fat diet. Polymorphisms in the human SHIP2 gene are associated, at least in part, with the insulin resistance of type 2 diabetes. Importantly, inhibition of endogenous SHIP2 through the liver-specific expression of a dominant-negative SHIP2 improves glucose metabolism and insulin resistance in diabetic db/db mice. Overexpression of PTEN and SKIP also inhibited insulin-induced phosphorylation of Akt and the uptake of glucose in cultured cells. Although a homozygous disruption of the PTEN gene in mice results in embryonic lethality, either skeletal muscle or adipose tissue-specific disruption of PTEN ameliorated glucose metabolism without formation of tumors in animal models of diabetes. The role of SKIP in glucose metabolism remains to be further clarified in vivo. Taken together, inhibition of endogenous SHIP2 in the whole body appears to be effective at improving the insulin resistance associated with type 2 diabetes and/or obesity. Inhibition of PTEN in the tissues specifically targeted, including skeletal muscle and fat, may result in an amelioration of insulin resistance in type 2 diabetes, although caution against the formation of tumors is needed.

Critical role of STAT3 in leptin's metabolic actions

Leptin has pleiotropic effects on glucose homeostasis and feeding behavior. Here, we validate the use of a cell-permeable phosphopeptide that blocks STAT3 activation in vivo. The combination of this biochemical approach with stereotaxic surgical techniques allowed us to pinpoint the contribution of hypothalamic STAT3 to the acute effects of leptin on food intake and glucose homeostasis. Leptin's ability to acutely reduce food intake critically depends on intact STAT3 signaling. Likewise, hypothalamic signaling of leptin through STAT3 is required for the acute effects of leptin on liver glucose fluxes. Lifelong obliteration of STAT3 signaling via the leptin receptor in mice (s/s mice) results in severe hepatic insulin resistance that is comparable to that observed in db/db mice, devoid of leptin receptor signaling. Our results demonstrate that the activation of the hypothalamic STAT3 pathway is an absolute requirement for the effects of leptin on food intake and hepatic glucose metabolism.

Central nervous system and control of endogenous glucose production

Recent evidence points to the crucial role of the central nervous system in controlling glucose homeostasis. Hypothalamic centers involved in the regulation of energy balance and endogenous glucose production constantly sense fuel availability by receiving and integrating inputs from circulating nutrients and hormones such as insulin and leptin. In response to these peripheral signals, the hypothalamus sends out efferent impulses that restrain food intake and endogenous glucose production. This promotes energy homeostasis and keeps blood glucose levels in the normal range. Disruption of this intricate neural control is likely to occur in type 2 diabetes and obesity and may contribute to defects of glucose homeostasis and insulin resistance common to both diseases. This review summarizes the latest findings on the hypothalamic control of endogenous glucose production, and focuses on the central effects of circulating macronutrients and nutrient-induced hormones.

Postnatal environment overrides genetic and prenatal factors influencing offspring obesity and insulin resistance

There is growing evidence that the postnatal environment can have a major impact on the development of obesity and insulin resistance in offspring. We postulated that cross-fostering obesity-prone offspring to lean, obesity-resistant dams would ameliorate their development of obesity and insulin resistance, while fostering lean offspring to genetically obese dams would lead them to develop obesity and insulin resistance as adults. We found that obesity-prone pups cross-fostered to obesity-resistant dams remained obese but did improve their insulin sensitivity as adults. In contrast, obesity-resistant pups cross-fostered to genetically obese dams showed a diet-induced increase in adiposity, reduced insulin sensitivity, and associated changes in hypothalamic neuropeptide, insulin, and leptin receptors, which might have contributed to their metabolic defects. There was a selective increase in insulin levels and differences in fatty acid composition of obese dam milk which might have contributed to the increased adiposity, insulin resistance, and hypothalamic changes in obesity-resistant cross-fostered offspring. These results demonstrate that postnatal factors can overcome both genetic predisposition and prenatal factors in determining the development of adiposity, insulin sensitivity, and the brain pathways that mediate these functions.

Forkhead protein FoxO1 mediates Agrp-dependent effects of leptin on food intake

Leptin controls food intake by regulating the transcription of key neuropeptides in the hypothalamus. The mechanism by which leptin regulates gene expression is unclear, however. Here we show that delivery of adenovirus encoding a constitutively nuclear mutant FoxO1, a transcription factor known to control liver metabolism and pancreatic beta-cell function, to the hypothalamic arcuate nucleus of rodents results in a loss of the ability of leptin to curtail food intake and suppress expression of Agrp. Conversely, a transactivation-deficient FoxO1 mutant prevents induction of Agrp by fasting. We also find that FoxO1 and the transcription factor Stat3 exert opposing actions on the expression of Agrp and Pomc through transcriptional squelching. FoxO1 promotes opposite patterns of coactivator-corepressor exchange at the Pomc and Agrp promoters, resulting in activation of Agrp and inhibition of Pomc. Thus, FoxO1 represents a shared component of pathways integrating food intake and peripheral metabolism.