Role of brain insulin receptor in control of body weight and reproduction

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 beta-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.

Responsiveness of insulin-induced cardiac sympathetic nerve activation associates with blood pressure regulation in diabetics

To quantitatively evaluate the effect of insulin on cardiac sympathetic nerve activity (SNA) and analyze clinical factors associated with insulin sensitivity for the regulation of SNA in diabetics, 29 Japanese type 2 diabetics without neuropathy were recruited. A 2-h control study and a 2-h hyperinsulinemic euglycemic glucose clamp study were performed. From the power spectral analysis of R-R intervals on ECG during both studies, two major components, the low-frequency (LF) and the high-frequency component (HF), were obtained. Then %LF was calculated as LF/(LF +HF), and the ratio of the average %LF during the last 30 min of the clamp or the control to the average %LF for the entire time for clamp or control (R-%LF) was used as a marker of changes in SNA. R-%LF was significantly higher during the clamp than in the control (1.07 +/- 0.04 vs. 1.03 +/- 0.03, P < 0.05). High responders (individual R-%LF during clamp > or = mean + 2SD in control) showed a higher basal mean blood pressure (BP) before the clamp (89 +/- 3 vs. 82 +/- 2, P < 0.03) but not a higher glucose infusion rate (GIR) compared with low responders (<mean + 2SD). Furthermore, R-%LF showed a positive correlation with basal mean BP (P < 0.02) but not with GIR. These data demonstrate that an acute insulin load stimulates cardiac SNA, and insulin sensitivity in the regulation of SNA may be associated with BP regulation independently of peripheral insulin sensitivity.

Obesity and the risk of spontaneous abortion after oocyte donation

OBJECTIVE

To determine whether obesity increases the risk of spontaneous abortion.

DESIGN

Retrospective study.

SETTING

Oocyte donation program at the Instituto Valenciano de Infertilidad in Spain.

PATIENT(S)

Seven hundred twelve cycles of recipients of ovum donation with known body mass index (BMI), good-quality embryo transfer, and absence of uterine pathology or clinical history of antiphospholipid antibodies or recurrent abortion.

INTERVENTION(S)

Recipients were divided in four BMI (kg/m(2)) groups: lean, with BMI <20 (n = 92; 12.9%); normal, with BMI = 20-24.9 (n = 398; 55.9%); overweight, with BMI = 25-29.9 (n = 172; 24.2%); and obese, with BMI >/=30 (n = 50; 7%). Clinical parameters were compared among the groups.

MAIN OUTCOME MEASURE(S)

Spontaneous abortion rates according to BMI.

RESULT(S)

No difference was found among the four BMI groups in any of the parameters of the cycle analyzed. The overall abortion rate was 15.8% (57 of 360). There were significant differences in abortion rates between the obese (38.1%), and the normal (13.3%) and overweight (15.5%) groups. When several cutoff BMI values were established (20, 25, and 30), only the obese women demonstrated a greater risk of abortion. Compared with the normal population, the obese group showed a significant fourfold increase in the risk of spontaneous abortion.

CONCLUSION(S)

Our findings confirm that obesity (BMI >/=30) is an independent risk factor for spontaneous abortion. Therefore, it would be advisable for obese patients to reduce weight before becoming pregnant.

A nucleoprotein complex containing Sp1, C/EBP beta, and HMGI-Y controls human insulin receptor gene transcription

HMGI-Y is an architectural transcription factor that regulates gene expression in vivo by controlling the formation of stereospecific multiprotein complexes on the AT-rich regions of certain gene promoters. Recently, we demonstrated that HMGI-Y is required for proper transcription of the insulin receptor (IR) gene. Here we provide evidence that transcriptional activation of the human IR promoter requires the assembly of a transcriptionally active multiprotein-DNA complex which includes, in addition to HMGI-Y, the ubiquitously expressed transcription factor Sp1 and the CCAAT-enhancer binding protein beta (C/EBP beta). Functional integrity of this nucleoprotein complex is required for full transactivation of the IR gene by Sp1 and C/EBP beta in cells readily expressing IRs. We show that HMGI-Y physically interacts with Sp1 and C/EBP beta and facilitates the binding of both factors to the IR promoter in vitro. Furthermore, HMGI-Y is needed for transcriptional synergism between these factors in vivo. Repression of HMGI-Y function adversely affects both Sp1- and C/EBP beta-induced transactivation of the IR promoter. Together, these findings demonstrate that HMGI-Y plays significant molecular roles in the transcriptional activities of these factors in the context of the IR gene and provide concordant support for the hypothesis that, in affected individuals, a putative defect in these nuclear proteins may cause decreased IR expression with subsequent impairment of insulin signaling and action.

Adiposity signals and food reward: expanding the CNS roles of insulin and leptin

The hormones insulin and leptin have been proposed to act in the central nervous system (CNS) as adiposity signals as part of a theoretical negative feedback loop that senses the caloric stores of an animal and orchestrates adjustments in energy balance and food intake. Much research has provided support for both the existence of such a feedback loop and the specific roles that insulin and leptin may play. Most studies have focused on hypothalamic sites, which historically are implicated in the regulation of energy balance, and on the brain stem, which is a target for neural and humoral signals relating to ingestive acts. More recent lines of research, including studies from our lab, suggest that in addition to these CNS sites, brain reward circuitry may be a target for insulin and leptin action. These studies are reviewed together here with the goals of providing a historical overview of the findings that have substantiated the originally hypothesized negative feedback model and of opening up new lines of investigation that will build on these findings and allow further refinement of the model of adiposity signal/CNS feedback loop. The understanding of how motivational circuitry and its endocrine or neuroendocrine modulation contributes to normal energy balance regulation should expand possibilities for future therapeutic approaches to obesity and may lead to important insights into mental illnesses such as substance abuse or eating disorders.

Distinct effects of ketone bodies on down-regulation of cell surface insulin receptor and insulin receptor substrate-1 phosphorylation in adrenal chromaffin cells

Treatment (>/=24 h) of cultured bovine adrenal chromaffin cells with ketoacidosis-related concentrations (>/=3 mM) of acetoacetate (but not beta-hydroxybutyrate, acetone, and acidic medium) caused a time- and concentration-dependent reduction of cell surface (125)I-insulin binding by ~38%, with no change in the K(d) value. The reduction of (125)I-insulin binding returned to control nontreated level at 24 h after the washout of acetoacetate-treated cells. Acetoacetate did not increase the internalization rate of cell surface insulin receptor (IR), as measured in the presence of brefeldin A, an inhibitor of cell surface vesicular exit from the trans-Golgi network. Acetoacetate (10 mM for 24 h) lowered cellular levels of the immunoreactive IR precursor molecule (approximately 190 kDa) and IR by 22 and 28%, respectively. Acetoacetate decreased IR mRNA levels by approximately 23% as early as 6 h, producing their maximum plateau reduction at 12 and 24 h. The half-life of IR mRNA was shortened by acetoacetate from 13.6 to 9.5 h. Immunoprecipitation followed by immunoblot analysis revealed that insulin-induced (100 nM for 10 min) tyrosine-phosphorylation of insulin receptor substrate-1 (IRS-1) was attenuated by 56% in acetoacetate-treated cells, with no change in IRS-1 level. These results suggest that chronic treatment with acetoacetate selectively down-regulated the density of cell surface functional IR via lowering IR mRNA levels and IR synthesis, thereby retarding insulin-induced activation of IRS-1.

Relationship between the growth hormone/insulin-like growth factor-I axis, insulin sensitivity, and adrenal androgens in normal prepubertal and pubertal girls

The aim of this study was to analyze the possible implication of changes in the GH/IGF-I axis and in insulin sensitivity for the regulation of adrenal androgen secretion of normal prepubertal and adolescent girls. A total of 61 normal girls were evaluated in prepuberty [Group (Gr)1, n = 33; early (Gr1A, n = 16) and late (Gr1B, n = 17)]; puberty (Gr3, n = 28), early (Gr3A, n = 9) and late (Gr3B, n = 19); and during the transition between prepuberty and puberty (Gr2, n = 26). Insulin sensitivity was estimated by the fasting glucose/insulin ratio (G/I). In Gr1, G/I was significantly higher, and the mean serum IGF-I and serum dehydroepiandrosterone sulfate (DHEAS) were significantly lower than in Gr3 (P < 0.0001). Mean G/I in Gr1A and Gr3A was significantly higher than in Gr1B (P < 0.01) and Gr3B (P < 0.02), respectively, and ratios in Gr1B were also significantly higher than in Gr3A (P < 0.02). However, body mass index (BMI) in Gr1A, Gr1B, and Gr3A was not significantly different, although a significant increment was observed between late prepuberty (Gr1B) and late puberty (Gr3B; P < 0.0001). On the other hand, serum IGF-I levels in Gr1A and Gr3A were significantly lower than those in Gr1B (P < 0.01) and Gr3B (P < 0.02), respectively. The mean serum DHEAS level in Gr1A and Gr3A was significantly lower than in Gr1B (P < 0.01) and Gr3B (P < 0.02), respectively, and the level in Gr1B was also significantly lower than in Gr3A (P < 0.02). Correlation studies within Gr1, Gr2, and Gr3 were also performed. There was a significant positive correlation between serum DHEAS and age and a significant negative correlation between serum DHEAS and G/I in the three groups. However, a significant positive correlation between serum DHEAS and serum IGF-I was only found in Gr1. Furthermore, a significant negative correlation between BMI and the G/I was found in Gr2 and Gr3. Therefore, changes in insulin sensitivity might be involved in adrenal androgen synthesis both in prepuberty and in puberty, as well as during the transition from prepuberty to puberty. Changes in BMI suggest that adiposity might be a mediator of this effect, particularly during late puberty. On the other hand, the GH/IGF axis might be an important metabolic signal involved in the maturational changes of human adrenal androgens during prepuberty, at the time of adrenarche. Indeed, a significant negative correlation between G/I and serum IGF-I was found in Gr1, as well as in Gr2. In conclusion, the findings of this study indicate that the GH/IGF-I axis and insulin resistance might be involved in the mechanism of adrenarche during prepuberty in normal girls. Because these relationships had not been seen in boys, we proposed that prepubertal ovarian estrogens might be responsible for the sex difference. The relationship between insulin resistance and adrenal androgens persists during the transition from prepuberty to puberty, as well as during puberty.

Brain insulin: regulation, mechanisms of action and functions

1. While many questions remained unanswered, it is now well documented that, contrary to earlier views, insulin is an important neuromodulator, contributing to neurobiological processes, in particular energy homeostasis and cognition. A specific role on cognitive functions related to feeding is proposed, and it is suggested that brain insulin from different sources might be involved in the above vital functions in health and disease. 2. A molecule identical to pancreatic insulin, and specific insulin receptors, are found widely distributed in the central nervous system networks related to feeding, reproduction, or cognition. 3. The actions of insulin in the central nervous system may be under both multilevel and multifactorial controls. The amount of blood insulin reaching the brain, brain insulin stores and secretion, potential local biosynthesis and degradation of the peptide, and insulin receptors and signal transduction can be affected by metabolic factors induced by nutrients, hormones, neurotransmitters, and regulatory peptides, peripherally or in the central nervous system. 4. Glucose and serotonin regulate insulin directly in the hypothalamus and may be of importance for its biological effects. Central mechanisms regulating glucose-induced insulin secretion show some analogy with the mechanisms operating in the pancreas. 5. A cross-talk between insulin and leptin receptors has been observed in the brain, and a regulation of central insulin actions, potentially via serotonin modulation, by leptin, galanin, melancortins, and neuropeptide Y (NPY) is suggested. 6. A more complete knowledge of the biological role of insulin in brain function and dysfunction, and of the regulatory mechanisms involved in these processes, constitutes a real advancement in the understanding of the pathophysiology of metabolic and mental diseases and could lead to important medical benefits.

Leptin receptor-deficient obese Zucker rats reduce their food intake in response to a systemic supply of calories from glucose

It has been established that leptin exerts a negative control on food intake, allowing one to maintain stable caloric intake over time. The aim of the present study was to investigate whether leptin regulates food intake when a supply of calories is provided by the systemic route. Experiments were carried out in leptin receptor-deficient obese fa/fa rats and lean Fa/fa controls. In both groups, 48 h of glucose infusion reduced food intake in proportion to caloric supply, resulting in virtually no change in total caloric intake as compared to before the infusion. This hypophagic response was reproduced without adding systemic calories, but by increasing glucose and insulin concentrations specifically in the brain through carotid artery infusion. Concomitant intracerebroventricular administration of 5-(tetradecyloxy)-2-furoic acid, an acetyl CoA carboxylase inhibitor that precludes malonyl-CoA synthesis, abolished the restriction of feeding in carotid-infused lean and obese rats. These data indicate that a supply of calories via glucose infusion induces a hypophagic response independent of leptin signaling in the rat, and support the hypothesis that a rise in central malonyl-CoA, triggered by increased glucose and insulin concentrations, participates in this adaptation. This process could contribute to the limiting of hyperphagia, primarily when leptin signaling is altered, as in the obese state.

Insulin activation of phosphatidylinositol 3-kinase in the hypothalamic arcuate nucleus: a key mediator of insulin-induced anorexia

In peripheral tissues, insulin signaling involves activation of the insulin receptor substrate (IRS)-phosphatidylinositol 3-kinase (PI3K) enzyme system. In the hypothalamus, insulin functions with leptin as an afferent adiposity signal important for the regulation of body fat stores and hepatic glucose metabolism. To test the hypothesis that hypothalamic insulin action involves intracellular PI3K signaling, we used histochemical and biochemical methods to determine the effect of insulin on hypothalamic IRS-PI3K activity. Here, we report that insulin induces tyrosine phosphorylation of the insulin receptor and IRS-1 and -2, increases binding of activated IRS-1 and -2 to the regulatory subunit of PI3K, and activates protein kinase B/Akt, a downstream target of PI3K. Using an immunohistochemical technique to detect PI 3,4,5-triphosphate, the main product of PI3K activity, we further demonstrate that in the arcuate nucleus, insulin-induced PI3K activity occurs preferentially within cells that contain IRS-2. Finally, we show that the food intake- lowering effects of insulin are reversed by intracerebroventricular infusion of either of two PI3K inhibitors at doses that have no independent feeding effects. These findings support the hypothesis that the IRS-PI3K pathway is a mediator of insulin action in the arcuate nucleus and, combined with recent evidence that leptin activates PI3K signaling in the hypothalamus, provide a plausible mechanism for neuronal cross-talk between insulin and leptin signaling.

Physiological and pathophysiological roles of ATP-sensitive K+ channels

ATP-sensitive potassium (K(ATP)) channels are present in many tissues, including pancreatic islet cells, heart, skeletal muscle, vascular smooth muscle, and brain, in which they couple the cell metabolic state to its membrane potential, playing a crucial role in various cellular functions. The K(ATP) channel is a hetero-octamer comprising two subunits: the pore-forming subunit Kir6.x (Kir6.1 or Kir6.2) and the regulatory subunit sulfonylurea receptor SUR (SUR1 or SUR2). Kir6.x belongs to the inward rectifier K(+) channel family; SUR belongs to the ATP-binding cassette protein superfamily. Heterologous expression of differing combinations of Kir6.1 or Kir6.2 and SUR1 or SUR2 variant (SUR2A or SUR2B) reconstitute different types of K(ATP) channels with distinct electrophysiological properties and nucleotide and pharmacological sensitivities corresponding to the various K(ATP) channels in native tissues. The physiological and pathophysiological roles of K(ATP) channels have been studied primarily using K(ATP) channel blockers and K(+) channel openers, but there is no direct evidence on the role of the K(ATP) channels in many important cellular responses. In addition to the analyses of naturally occurring mutations of the genes in humans, determination of the phenotypes of mice generated by genetic manipulation has been successful in clarifying the function of various gene products. Recently, various genetically engineered mice, including mice lacking K(ATP) channels (knockout mice) and mice expressing various mutant K(ATP) channels (transgenic mice), have been generated. In this review, we focus on the physiological and pathophysiological roles of K(ATP) channels learned from genetic manipulation of mice and naturally occurring mutations in humans.

Can perinatal supplementation of long-chain polyunsaturated fatty acids prevent diabetes mellitus?

It is suggested that the negative correlation between breast-feeding and insulin resistance and diabetes mellitus can be related to the presence of significant amounts of long-chain polyunsaturated fatty acids in the human breast milk. Based on this, it is proposed that provision of adequate amounts of long chain polyunsaturated fatty acids during the critical periods of brain growth and development can prevent or postpone the development diabetes mellitus.

Insulin Resistant States and Insulin Signaling

The incidence of type 2 diabetes mellitus (T2D) and obesity is increasing rapidly worldwide, reaching epidemic proportions. Insulin resistance is a key feature in both conditions and plays an important pathophysiological role. Over the last two decades a central role in the origin of insulin resistance has emerged for defects in the intracellular insulin signaling cascade leading to glucose uptake. Herein, we will 1) review insulin signaling pathways leading to glucose uptake, 2) review mouse models of insulin resistance that demonstrate the pathophysiologic importance of specific defects of these pathways and 3) discuss the molecular basis for insulin resistance in some human disease states known to be associated with insulin resistance. Finally, we will briefly mention some novel treatment targets for T2D stemming from this knowledge.

Insulin and leptin revisited: adiposity signals with overlapping physiological and intracellular signaling capabilities

The adipocyte-derived hormone leptin and the pancreatic beta cell-derived hormone insulin each function as afferent signals to the hypothalamus in an endocrine feedback loop that regulates body adiposity. Although these two hormones, and the receptors on which they act, are unrelated and structurally distinct, they exert overlapping effects in the arcuate nucleus, a key hypothalamic area involved in energy homeostasis. Defects in either insulin or leptin signaling in the brain result in hyperphagia, disordered glucose homeostasis, and reproductive dysfunction. To explain this striking physiological overlap, we hypothesize that hypothalamic insulin and leptin signaling converge upon a single intracellular signal transduction pathway, known as the insulin-receptor-substrate phosphatidylinositol 3-kinase pathway. Here we synthesize data from a variety of model systems in which such "cross-talk" between insulin and leptin signal transduction has either been observed or can be inferred, discuss our own data demonstrating that insulin and leptin both activate hypothalamic phosphatidylinositol 3-kinase signaling, and discuss the significance of such convergence with respect to neuronal function in normal individuals and in pathological states such as obesity. Identification of the key early molecular events mediating the action of both insulin and leptin in hypothalamic neurons promises new insight into the regulation of these neurons in health and disease.

Neuropeptides, food intake and body weight regulation: a hypothalamic focus

Energy homeostasis is controlled by a complex neuroendocrine system consisting of peripheral signals like leptin and central signals, in particular, neuropeptides. Several neuropeptides with anorexigenic (POMC, CART, and CRH) as well as orexigenic (NPY, AgRP, and MCH) actions are involved in this complex (partly redundant) controlling system. Starvation as well as overfeeding lead to changes in expression levels of these neuropeptides, which act downstream of leptin, resulting in a physiological response. In this review the role of several anorexigenic and orexigenic (hypothalamic) neuropeptides on food intake and body weight regulation is summarized.

The rat arcuate nucleus integrates peripheral signals provided by leptin, insulin, and a ghrelin mimetic

The hypothalamic circuits controlling food intake and body weight receive and integrate information from circulating satiety signals such as leptin and insulin and also from ghrelin, the only known circulating hormone that stimulates appetite following systemic injection. Activation of arcuate neurons by ghrelin and ghrelin mimetics (the growth hormone secretagogues) is augmented in 48-h-fasted rats compared with fed rats, as reflected by a greater number of cells expressing Fos protein in response to administration of the same maximally effective dose. Here we sought to determine whether this increased responsiveness in fasting might reflect or be influenced by low levels of circulating satiety factors such as leptin or insulin. Chronic central infusion of insulin or leptin during a 48-h fast suppressed the threefold increase in the Fos response to intravenous injection of a maximally effective dose of growth hormone-releasing peptide (GHRP)-6, a synthetic growth hormone secretagogue. This appears to be a direct central action of insulin and leptin because the marked decrease in plasma levels of insulin, leptin, and glucose during fasting were unaffected by central administration of either hormone. Furthermore, the GHRP-6-induced Fos response was twofold greater in obese leptin- and insulin-resistant Zucker rats compared with lean controls. These data provide evidence that the ghrelin-sensitive circuits in the hypothalamus are dynamically regulated by central insulin and leptin action.

Effect of centrally administered insulin on gonadotropin-releasing hormone neuron activity and luteinizing hormone surge in the diabetic female rat

Diabetic female rats have decreased ovulation, sexual behavior, and luteinizing hormone (LH) surges. Peripheral insulin treatment restores the phenotype to normal. We administered central insulin and analyzed serum LH during the time of the LH surge in diabetic and non-diabetic animals to determine if central insulin was sufficient to normalize the phenotype. We assessed the activity and number of hypothalamic gonadotropin-releasing hormone (GnRH) neurons by double label immunocytochemistry for C-FOS and GnRH to determine if decreased GnRH neuron activity or number could account for the diabetes-induced deficits in neuroendocrine function. All animals were ovariectomized and given estradiol and progesterone. Diabetic and control animals were given either intracerebroventricular (ICV) insulin or saline. In experiment I, serial blood collection was performed. In experiment II, animals were sacrificed and their brains were processed for immunocytochemistry during the presumed LH surge. Experiment I showed that diabetic, saline-treated animals were unable to trigger an LH surge. Central insulin restored LH production to control levels. Experiment II revealed similar numbers and activation of GnRH neurons in all four groups. Therefore, the diabetes-induced loss of the LH surge cannot be explained simply by a reduction of GnRH-expressing neurons or by a decrease in GnRH neuronal activity.

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.

The catabolic action of insulin in the brain is mediated by melanocortins

Like leptin, the pancreatic hormone insulin is an important adiposity signal to the brain. We report that the hypothalamic melanocortin system is an important target of the actions of insulin to regulate food intake and body weight. Hypothalamic neurons expressing insulin receptors were found to coexpress the melanocortin precursor molecule pro-opiomelanocortin (POMC), and administration of insulin into the third cerebral ventricle of fasted rats increased expression of POMC mRNA. Finally, a subthreshold dose of the melanocortin antagonist SHU-9119 prevented the reduction in food intake caused by third-ventricular insulin administration. These data suggest that the hypothalamic melanocortin system mediates the anorexic effects of central insulin, as well as of leptin.

Anti-aging effects of caloric restriction: Involvement of neuroendocrine adaptation by peripheral signaling

Many hormonal signals from peripheral tissues contribute to the regulation of energy homeostasis and food intake. These regulators including leptin, insulin, and ghrelin, modulate the orexigenic and anorexigenic neuropeptide expression in hypothalamic nuclei. The anti-aging effects of caloric restriction have been explained from an evolutional viewpoint of the adaptive response of the neuroendocrine and metabolic response systems to maximize survival during periods of food shortage. In organisms, excess energy is stored in adipose tissues as a triglyceride preparation for such survival situations. Adipose tissue has recently been recognized as an endocrine organ, and leptin, as secreted by adipocyte, seems to be an especially important factor for the adaptive response to fasting and neuroendocrine alterations under caloric restriction. In this review, we discuss the potential involvement of neuroendocrine modulators in longevity and the anti-aging effects of caloric restriction.

New pharmacological tools for obesity

Obesity is a multi-factorial, chronic disorder that has reached epidemic proportions in most industrialized countries and is threatening to become a global epidemic. Obese patients are at a higher risk from coronary artery disease, hypertension, hyperlipidemia, diabetes mellitus, certain cancers, cerebrovascular accidents, osteoarthritis, restrictive pulmonary disease, and sleep apnea. Obesity is a particularly challenging clinical condition to treat, because of its complex pathophysiological basis. Indeed, body weight represents the integration of many biological and environmental components. Efforts to develop innovative anti-obesity drugs have been recently intensified. In broad terms, researchers use different distinct strategies: first, to reduce energy intake; second, to increase energy expenditure; third, to alter the partitioning of nutrients between fat and lean tissue. In the present review we concentrate on the first of these strategies, by underlining the new pharmacological tools which are presently studied.

Lessons in obesity from transgenic animals

Many genetic manipulations have created models of obesity, leanness or resistance to dietary obesity in mice, often providing insights into molecular mechanisms that affect energy balance, and new targets for anti-obesity drugs. Since many genes can affect energy balance in mice, polymorphisms in many genes may also contribute to obesity in humans, and there may be many causes of primary leptin resistance. Secondary leptin resistance (due to high leptin levels) can be investigated by combining the ob mutation with other obesity genes. Some transgenic mice have failed to display the expected phenotype, or have even been obese when leanness was expected. Compensatory changes in the expression of other genes during development, or opposing influences of the gene on energy balance, especially in global knockout mice, may offer explanations for such findings. Obesity has been separated from insulin resistance in some transgenic strains, providing new insights into the mechanisms that usually link these phenotypes. It has also been shown that in some transgenic mice, obesity develops without hyperphagia, or leanness without hypophagia, demonstrating that generalised physiological explanations for obesity in individual humans may be inappropriate. Possibly the most important transgenic model of obesity so far created is the Type 1 11beta-hydroxysteroid dehydrogenase over-expressing mouse, since this models the metabolic syndrome in humans. The perspectives into obesity offered by transgenic mouse models should assist clinical researchers in the design and interpretation of their studies in human obesity.

Neuroendocrine regulation of eating behavior

The dual center hypothesis in the central control of energy balance originates from the first observations performed more than 5 decades ago with brain lesioning and stimulation experiments. On the basis of these studies the "satiety center" was located in the ventromedial hypothalamic nucleus, since lesions of this region caused overfeeding and excessive weight gain, while its electrical stimulation suppressed eating. On the contrary, lesioning or stimulation of the lateral hypothalamus elicited the opposite set of responses, thus leading to the conclusion that this area represented the "feeding center". The subsequent expansion of our knowledge of specific neuronal subpopulations involved in energy homeostasis has replaced the notion of specific "centers" controlling energy balance with that of discrete neuronal pathways fully integrated in a more complex neuronal network. The advancement of our knowledge on the anatomical structure and the function of the hypothalamic regions reveals the great complexity of this system. Given the aim of this review, we will focus on the major structures involved in the control of energy balance.

Fructose, weight gain, and the insulin resistance syndrome

This review explores whether fructose consumption might be a contributing factor to the development of obesity and the accompanying metabolic abnormalities observed in the insulin resistance syndrome. The per capita disappearance data for fructose from the combined consumption of sucrose and high-fructose corn syrup have increased by 26%, from 64 g/d in 1970 to 81 g/d in 1997. Both plasma insulin and leptin act in the central nervous system in the long-term regulation of energy homeostasis. Because fructose does not stimulate insulin secretion from pancreatic beta cells, the consumption of foods and beverages containing fructose produces smaller postprandial insulin excursions than does consumption of glucose-containing carbohydrate. Because leptin production is regulated by insulin responses to meals, fructose consumption also reduces circulating leptin concentrations. The combined effects of lowered circulating leptin and insulin in individuals who consume diets that are high in dietary fructose could therefore increase the likelihood of weight gain and its associated metabolic sequelae. In addition, fructose, compared with glucose, is preferentially metabolized to lipid in the liver. Fructose consumption induces insulin resistance, impaired glucose tolerance, hyperinsulinemia, hypertriacylglycerolemia, and hypertension in animal models. The data in humans are less clear. Although there are existing data on the metabolic and endocrine effects of dietary fructose that suggest that increased consumption of fructose may be detrimental in terms of body weight and adiposity and the metabolic indexes associated with the insulin resistance syndrome, much more research is needed to fully understand the metabolic effect of dietary fructose in humans.

SOCS-1 and SOCS-3 block insulin signaling by ubiquitin-mediated degradation of IRS1 and IRS2

Inflammation associates with peripheral insulin resistance, which dysregulates nutrient homeostasis and leads to diabetes. Inflammation induces the expression of SOCS proteins. We show that SOCS1 or SOCS3 targeted IRS1 and IRS2, two critical signaling molecules for insulin action, for ubiquitin-mediated degradation. SOCS1 or SOCS3 bound both recombinant and endogenous IRS1 and IRS2 and promoted their ubiquitination and subsequent degradation in multiple cell types. Mutations in the conserved SOCS box of SOCS1 abrogated its interaction with the elongin BC ubiquitin-ligase complex without affecting its binding to IRS1 or IRS2. The SOCS1 mutants also failed to promote the ubiquitination and degradation of either IRS1 or IRS2. Adenoviral-mediated expression of SOCS1 in mouse liver dramatically reduced hepatic IRS1 and IRS2 protein levels and caused glucose intolerance; by contrast, expression of the SOCS1 mutants had no effect. Thus, SOCS-mediated degradation of IRS proteins, presumably via the elongin BC ubiquitin-ligase, might be a general mechanism of inflammation-induced insulin resistance, providing a target for therapy.

Akt signaling mediates postnatal heart growth in response to insulin and nutritional status

Akt is a serine-threonine kinase that mediates a variety of cellular responses to external stimuli. During postnatal development, Akt signaling in the heart was up-regulated when the heart was rapidly growing and was down-regulated by caloric restriction, suggesting a role of Akt in nutrient-dependent regulation of cardiac growth. Consistent with this notion, reductions in Akt, 70-kDa S6 kinase 1, and eukaryotic initiation factor 4E-binding protein 1 phosphorylation were observed in mice with cardiac-specific deletion of insulin receptor gene, which exhibit a small heart phenotype. In contrast to wild type animals, caloric restriction in these mice had little effect on Akt phosphorylation in the heart. Furthermore, forced expression of Akt1 in these hearts restored 70-kDa S6 kinase 1 and eukaryotic initiation factor 4E-binding protein 1 phosphorylation to normal levels and rescued the small heart phenotype. Collectively, these results indicate that Akt signaling mediates insulin-dependent physiological heart growth during postnatal development and suggest a mechanism by which heart size is coordinated with overall body size as the nutritional status of the organism is varied.

Growth hormone, IGF-1, insulin, SHBG, and estradiol levels in girls before menarche

The timing of pubertal changes depends on the concerted function of the hypothalamic - pituitary - ovarian and other endocrine systems. The somatotropin system and insulin play important roles during the growth and maturation of girls. Our clinical observations have enabled us to determine and implement criteria that split the pre-menarcheal period into three phases with distinct features associated with rising levels of estrogens (pre-estrogenization, onset of estrogenization and full estrogenization). The aim of this work was to determine levels of growth hormone, insulin - like growth factor 1 (IGF-1), sex hormone binding globulins (SHBG), insulin, and estradiol in relation to somatic features in girls during subsequent phases of estrogenization. This prospective study was done in 45 healthy girls. Every three months, we recorded weight, height, BMI, maturation of tertiary sex features, estrogen-related changes in hymen, sonographic dimensions of ovaries and uterus and serum levels of growth hormone, IGF-1, SHBG, insulin, and estradiol. Onset of estrogenization was accompanied by reduction of body mass and slowing down of growth associated with declining levels of growth hormone. These changes were followed by weight gain, pubertal acceleration of growth, rising levels of growth hormone, IGF-1, insulin, and estradiol, and falling levels of SHBG.

Antischizophrenic activity independent of dopamine D2 blockade

Currently, the drug therapy of schizophrenia consists of blockade of central dopamine D2 receptors. There is, however, an urgent medical need for alternative, more effective treatments. Clinical and preclinical literature suggests that stimulation of AMPA-type glutamate receptors may be involved in positive symptoms of schizophrenia, whereas hypofunctionality of NMDA-type glutamate receptors may be involved in negative symptoms and cognitive deficits. Several pharmacological approaches are conceivable to prevent stimulation of AMPA receptors (AMPA receptor blockade, metabotropic glutamate receptors (mGlu(2) receptor) stimulation or lamotrigine-like Na(+)/Ca(2+) channel blockade). Similarly, several pharmacological principles are conceivable to enhance neurotransmission at NMDA receptors (catechol-o-methyl transferase inhibition, glycine uptake blockade, glutathione suppletion and others). In this review, the possible pharmacological approaches and their respective advantages and disadvantages are discussed.

Prevalence of attention deficit/hyperactivity disorder among adults in obesity treatment

BACKGROUND

Bariatric patients showing poor "focus" during treatment more often failed to lose weight or maintain reduced weight. Evaluation of these patients identified a number having attention deficit/hyperactivity disorder (ADHD), evidently a potent factor limiting successful weight control. After searches found no published reports describing comorbid ADHD and obesity, this report was conceived to begin exploring the prevalence and characteristics of these patients.

METHOD

Clinical records of 215 patients receiving obesity treatment during 2000 were reviewed. Data collected and analyzed included age, sex, beginning and ending body mass index (BMI), number of clinic visits, months of treatment, and diagnostic category (ADHD, some ADHD symptoms, non-ADHD). DSM-IV criteria were used, except age of onset was modified to <= 12 years.

RESULTS

Whole sample ADHD prevalence was 27.4% (CI:21.1,32.9), but 42.6% (CI: 36.3% to 48.9%) for BMI >= 40. Mean weight loss among obese patients with ADHD (OB+ADHD) was 2.6 BMI (kg/m2) vs. 4.0 for non-ADHD (NAD) (p < 0.002). For BMI >= 40, OB+ADHD had BMI loss 2.9 vs. 7.0 (NAD) (p < 0.004). OB+ADHD had more clinic visits, with a trend toward longer treatment duration.

CONCLUSIONS

ADHD was highly prevalent among obese patients and highest in those with extreme obesity. Comorbid obesity and ADHD symptoms rendered treatment less successful compared to NAD counterparts. Reasons for the comorbidity are unknown, but may involve brain dopamine or insulin receptor activity. If replicated in further studies, these findings have important implications for treatment of severe and extreme obesity.

The Drosophila insulin/IGF receptor controls growth and size by modulating PtdInsP3 levels

Understanding the control of size is of fundamental biological and clinical importance. Insulin/IGF signaling during development controls growth and size, possibly by coordinating the activities of the Ras and PI 3-kinase signaling pathways. We show that in Drosophila mutating the consensus binding site for the Ras pathway adaptor Drk/Grb2 in Chico/IRS does not interfere with growth whereas mutating the binding sites of the PI 3-kinase adaptor p60 completely abrogates Chico function. Furthermore, we present biochemical and genetic evidence that loss of the homolog of the tumor suppressor gene, Pten, results in increased PtdInsP3 levels and that these increased levels are sufficient to compensate for the complete loss of the Insulin/insulin-like growth factor receptor function. This reduction of Pten activity is also sufficient to vastly increase organism size. These results suggest that PtdInsP3 is a second messenger for growth and that levels of PtdInsP3 during development regulate organismal size.

Nutrient-dependent expression of insulin-like peptides from neuroendocrine cells in the CNS contributes to growth regulation in Drosophila

BACKGROUND

The insulin/IGF-1 signaling pathway controls cellular and organismal growth in many multicellular organisms. In Drosophila, genetic defects in components of the insulin signaling pathway produce small flies that are delayed in development and possess fewer and smaller cells as well as female sterility, reminiscent of the phenotypes of starved flies.

RESULTS

Here we establish a causal link between nutrient availability and insulin-dependent growth. We show that in addition to the Drosophila insulin-like peptide 2 (dilp2) gene, overexpression of dilp1 and dilp3-7 is sufficient to promote growth. Three of the dilp genes are expressed in seven median neurosecretory cells (m-NSCs) in the brain. These m-NSCs possess axon terminals in the larval endocrine gland and on the aorta, from which DILPs may be released into the circulatory system. Although expressed in the same cells, the expression of the three genes is controlled by unrelated cis-regulatory elements. The expression of two of the three genes is regulated by nutrient availability. Genetic ablation of these neurosecretory cells mimics the phenotype of starved or insulin signaling mutant flies.

CONCLUSIONS

These results point to a conserved role of the neuroendocrine axis in growth control in multicellular organisms.

Adipose tissue selective insulin receptor knockout protects against obesity and obesity-related glucose intolerance

Insulin signaling in adipose tissue plays an important role in lipid storage and regulation of glucose homeostasis. Using the Cre-loxP system, we created mice with fat-specific disruption of the insulin receptor gene (FIRKO mice). These mice have low fat mass, loss of the normal relationship between plasma leptin and body weight, and are protected against age-related and hypothalamic lesion-induced obesity, and obesity-related glucose intolerance. FIRKO mice also exhibit polarization of adipocytes into populations of large and small cells, which differ in expression of fatty acid synthase, C/EBP alpha, and SREBP-1. Thus, insulin signaling in adipocytes is critical for development of obesity and its associated metabolic abnormalities, and abrogation of insulin signaling in fat unmasks a heterogeneity in adipocyte response in terms of gene expression and triglyceride storage.

Is type 2 diabetes mellitus a disorder of the brain?

I propose that type 2 diabetes mellitus is due to damage to neurons in the ventromedial hypothalamus or to a defect in the action or properties of insulin or insulin receptors in the brain. These neuronal abnormalities are probably secondary to a marginal deficiency of long-chain polyunsaturated fatty acids during the critical periods of brain growth and development. Hence, supplementation of adequate amounts of long-chain polyunsaturated fatty acids during the third trimester of pregnancy to 2 y postterm can prevent or postpone the development of diabetes mellitus.

Inactivation of muscle insulin and IGF-I receptors and insulin responsiveness

PURPOSE OF REVIEW

This review will outline the recent advances in the area of insulin-stimulated skeletal muscle glucose uptake and its effect on whole body glucose homeostasis, using gene-deletion and transgenic mouse models.

RECENT FINDINGS

Insulin resistance is often the first abnormality detected in cases of type 2 diabetes, and is seen at the level of the peripheral tissues especially muscle. Both the insulin receptor and the insulin-like growth factor I receptor are capable of stimulating glucose uptake into skeletal muscle. One model involves the gene deletion of muscle glucose transport protein 4, which leads to severe insulin resistance and hyperglycemia, and a second model using a transgenic approach abrogates the function of the insulin-like growth factor I receptor and the insulin receptor resulting in severe insulin resistance and progression to diabetes. Both models demonstrate that abrogation of the insulin-like growth factor I receptor and the insulin receptor or a common signalling pathway must be inhibited to cause sufficient insulin resistance to lead to type 2 diabetes; with either glucotoxicity or lipotoxicity being involved in the progression from severe to resistance to full-blown type 2 diabetes.

SUMMARY

Thus, abrogation of insulin-stimulated glucose uptake in skeletal muscle, at least in mice, may lead to severe insulin resistance and diabetes.

The lipids that matter from infant nutrition to insulin resistance

Breast-fed infants showed decreased incidence of obesity, hypertension, diabetes mellitus, and coronary heart disease in later life and higher cognitive function. Breast milk is rich in long-chain polyunsaturated fatty acids (LCPUFAs) and brain preferentially accumulates LCPUFAs during the last trimester of pregnancy and the first few months of life. Breast-fed infants showed significantly lower plasma glucose levels and higher percentage of docosahexaenoic acid and total percentages of LCPUFAs in their skeletal muscle biopsies compared with formula fed. LCPUFAs suppress the production of pro-inflammatory cytokines, regulate the function of several neurotransmitters, enhance the number of insulin receptors in the brain and other tissues, and decrease insulin resistance. LCPUFAs may enhance the production of bone morphogenetic proteins (BMPs), which participate in neurogenesis. It is proposed that the beneficial effects of breast feeding in later life can be attributed to its rich LCPUFA content. It is likely that inadequate breast feeding results in marginal deficiency of LCPUFAs during the critical stages of development, which can lead to insulin resistance. Hence, promoting prolonged breast feeding and/or supplementing LCPUFAs during the critical stages of development may be beneficial in preventing insulin resistance.

Knockout models are useful tools to dissect the pathophysiology and genetics of insulin resistance

OBJECTIVE

The development of type 2 diabetes is linked to insulin resistance coupled with a failure of pancreatic beta-cells to compensate by adequate insulin secretion.

DESIGN

Here, we review studies obtained from genetically engineered mice that provide novel insights into the pathophysiology of insulin resistance.

RESULTS

Knockout models with monogenic impairment in insulin action have highlighted the potential role for insulin signalling molecules in insulin resistance at a tissue-specific level. Polygenic models have strengthened the idea that minor defects in insulin secretion and insulin action, when combined, can lead to diabetes, emphasizing the importance of interactions of different genetic loci in the production of diabetes. Knockout models with tissue-specific alterations in glucose or lipid metabolism have dissected the individual contributions of insulin-responsive organs to glucose homeostasis. They have demonstrated the central role of fat as an endocrine tissue in the maintenance of insulin sensitivity and the development of insulin resistance. Finally, these models have shown the potential role of impaired insulin action in pancreatic beta-cells and brain in the development of insulin deficiency and obesity.

SOCS-6 binds to insulin receptor substrate 4, and mice lacking the SOCS-6 gene exhibit mild growth retardation

SOCS-6 is a member of the suppressor of cytokine signaling (SOCS) family of proteins (SOCS-1 to SOCS-7 and CIS) which each contain a central SH2 domain and a carboxyl-terminal SOCS box. SOCS-1, SOCS-2, SOCS-3, and CIS act to negatively regulate cytokine-induced signaling pathways; however, the actions of SOCS-4, SOCS-5, SOCS-6, and SOCS-7 remain less clear. Here we have used both biochemical and genetic approaches to examine the action of SOCS-6. We found that SOCS-6 and SOCS-7 are expressed ubiquitously in murine tissues. Like other SOCS family members, SOCS-6 binds to elongins B and C through its SOCS box, suggesting that it might act as an E3 ubiquitin ligase that targets proteins bound to its SH2 domain for ubiquitination and proteasomal degradation. We investigated the binding specificity of the SOCS-6 and SOCS-7 SH2 domains and found that they preferentially bound to phosphopeptides containing a valine in the phosphotyrosine (pY) +1 position and a hydrophobic residue in the pY +2 and pY +3 positions. In addition, these SH2 domains interacted with a protein complex consisting of insulin receptor substrate 4 (IRS-4), IRS-2, and the p85 regulatory subunit of phosphatidylinositol 3-kinase. To investigate the physiological role of SOCS-6, we generated mice lacking the SOCS-6 gene. SOCS-6(-/-) mice were born in a normal Mendelian ratio, were fertile, developed normally, and did not exhibit defects in hematopoiesis or glucose homeostasis. However, both male and female SOCS-6(-/-) mice weighed approximately 10% less than wild-type littermates.

Using mouse models to dissect the genetics of obesity

Mice have proved to be powerful models for understanding obesity in humans and farm animals. Single-gene mutants and genetically modified mice have been used successfully to discover genes and pathways that can regulate body weight. For polygenic obesity, the most common pattern of inheritance, many quantitative trait loci (QTLs) have been mapped in crosses between selected and inbred mouse lines. Most QTL effects are additive, and diet, age and gender modify the genetic effects. Congenic, recombinant inbred, advanced intercross, and chromosome substitution strains are needed to map QTLs finely, to identify the genes underlying the traits, and to examine interactions between them.

Molecular insights into insulin action and secretion

Tightly co-ordinated control of both insulin action and secretion is required in order to maintain glucose homeostasis. Gene knockout experiments have helped to define key signalling molecules that affect insulin action, including insulin and insulin-like growth factor-1 (IGF-1) receptors, insulin receptor substrate (IRS) proteins and various downstream effector proteins. beta-cell function is also a tightly regulated process, with numerous factors (including certain signalling molecules) having an impact on insulin production, insulin secretion and beta-cell mass. While signalling molecules play important roles in insulin action and secretion under normal circumstances, abnormal insulin signalling in muscle, adipose tissue, liver and pancreas leads to insulin resistance and beta-cell dysfunction. In particular, the signalling protein IRS-2 may have a central role in linking these abnormalities, although other factors are likely to be involved.