The role of oestrogen in determining sexual dimorphism in energy balance.

Energy balance is determined by caloric intake and the rate at which energy is expended, with the latter comprising resting energy expenditure, physical activity and adaptive thermogenesis. The regulation of both energy intake and expenditure exhibits clear sexual dimorphism, with young women being relatively protected against weight gain and the development of cardiometabolic diseases. Preclinical studies have indicated that females are more sensitive to the satiety effects of leptin and insulin compared to males. Furthermore, females have greater thermogenic activity than males, whereas resting energy expenditure is generally higher in males than females. In addition to this, in post-menopausal women, the decline in sex steroid concentration, particularly in oestrogen, is associated with a shift in the distribution of adipose tissue and overall increased propensity to gain weight. Oestrogens are known to regulate energy balance and weight homeostasis via effects on both food intake and energy expenditure. Indeed, 17ß-oestradiol treatment increases melanocortin signalling in the hypothalamus to cause satiety. Furthermore, oestrogenic action at the ventromedial hypothalamus has been linked with increased energy expenditure in female mice. We propose that oestrogen action on energy balance is multi-faceted and is fundamental to determining sexual dimorphism in weight control. Furthermore, evidence suggests that the decline in oestrogen levels leads to increased risk of weight gain and development of cardiometabolic disease in women across the menopausal transition.

The role of sleep in PCOS: what we know and what to consider in the future.

INTRODUCTION: Sleep disturbance and clinical sleep conditions disrupt endocrine signals, energy expenditure and nutritional intake. Women with polycystic ovary syndrome (PCOS) are at higher risk of sleep disturbances and clinical conditions. It is possible that sleep may contribute to the exacerbation of PCOS. This review aims to explore the relationship between sleep and chronic disease, particularly in women with PCOS. AREAS COVERED: This review narratively explores what sleep is, how to measure sleep and the possible mechanisms that support the link between sleep in adipose tissue deposition, insulin resistance and the presentation of PCOS. EXPERT OPINION: Research shows that disturbed sleep and clinical sleep conditions disrupt energy expenditure. This may increase adipose tissue deposition and exacerbate insulin resistance which are known to worsen the presentation of PCOS. Further, sleep disturbance in women with PCOS may ameliorate any positive lifestyle changes made after diagnosis. Cognitive behavioural therapy interventions for sleep are a successful strategy for the management of sleep disturbances in the general population. However, such interventions are yet to be trialled in women with PCOS. Given the proposed implications, interventions to improve sleep could provide additional support for women with PCOS to successfully implement lifestyle strategies and should be further investigated.

Glucagon-like peptide-1 control of GnRH secretion in female sheep.

The role of glucagon-like peptide-1 (GLP-1) on gonadotropin-releasing hormone (GnRH) secretion was investigated in ovariectomised (OVX) ewes, in which GnRH and luteinising hormone (LH) secretion had been restrained by treatment with oestrogen and progesterone. Guide tubes for microinjection were placed above the median eminence (ME) and the animals were allowed to recover for 1 month. Jugular venous blood samples were taken via cannulae at 10 min intervals. Vehicle (50 nL) was injected into the ME at 2 h, followed by injection of GLP-1 ((7-36)-amide - 0.5 or 1 nmol) or its receptor agonist, exendin-4 (0.5 nmol) at 4 h (n = 5). Plasma LH levels were quantified as a surrogate measure of GnRH secretion. GLP-1 microinjection into the ME elicited a large amplitude LH pulse in jugular plasma, the effect was greater at the higher dose. Exendin-4 microinjection caused a large, sustained increase in plasma LH levels. To determine how GLP-1 might exert an effect on GnRH secretion, we employed double labelled in situ hybridisation, with RNAScope, for co-localisation of the GLP-1 receptor (GLP-1R) in GnRH, Kisspeptin and NPY cells in the hypothalami of three ewes in the luteal phase of the estrous cycle. GLP1R expression was clearly visible but the receptor was not expressed in GNRH1 or NPY expressing neurons and was visualised in <5% of KISS1 expressing neurons. We conclude that GLP-1 may act at the level of the secretory terminals of GnRH neurons in the ME to stimulate GnRH secretion, the pathway through which such effect is manifested remains unknown.

Effect of sex and sex steroids on brown adipose tissue heat production in humans.

OBJECTIVE: Retrospective studies suggest that women have more active brown adipose tissue (BAT) than men, but little is known of the effect of fluctuating sex steroids across the menstrual cycle on thermogenesis in women. DESIGN: To characterise the effects of sex and sex steroids on BAT activity we recruited healthy weight men (n = 14) and women at two stages of the menstrual cycle (luteal, n = 9; follicular, n = 11). METHODS: Infrared thermography measured supraclavicular temperature to index BAT thermogenesis in response to both cold (immersion of one hand in water at 15°C) and meal (Ensure, 10 kcal/kg body weight) stimuli. RESULTS: Adaptive BAT temperature responses were greater (P < 0.05) in women than men, irrespective of stage of menstrual cycle. Whereas during cold exposure, the increase in BAT temperature was abrogated (P < 0.05) in women during follicular phase compared to men and women during luteal phase. Plasma concentrations of progesterone, 17ß-estradiol, testosterone and cortisol were measured. Regression analyses demonstrated that baseline BAT temperature was positively correlated (P < 0.05) with progesterone levels, but was inversely associated (P < 0.05) with cortisol concentration. Both cold- and meal-induced changes in BAT temperature mildly correlated (P = 0.07; P < 0.05) with 17ß-estradiol levels, but not with testosterone concentrations. CONCLUSIONS: Baseline supraclavicular temperature is elevated in women during the luteal phase of the menstrual cycle, which correlated with elevated progesterone concentrations. Women exhibited greater thermogenic responses than men, irrespective of the state of the menstrual cycle, which was associated with plasma levels of 17ß-estradiol. We conclude that sex steroids may regulate BAT thermogenesis in healthy adults.

Intranasal Targeting of Hypothalamic PTP1B and TCPTP Reinstates Leptin and Insulin Sensitivity and Promotes Weight Loss in Obesity.

The importance of hypothalamic leptin and insulin resistance in the development and maintenance of obesity remains unclear. The tyrosine phosphatases protein tyrosine phosphatase 1B (PTP1B) and T cell protein tyrosine phosphatase (TCPTP) attenuate leptin and insulin signaling and are elevated in the hypothalami of obese mice. We report that elevated PTP1B and TCPTP antagonize hypothalamic leptin and insulin signaling and contribute to the maintenance of obesity. Deletion of PTP1B and TCPTP in the hypothalami of obese mice enhances CNS leptin and insulin sensitivity, represses feeding, and increases browning, to decrease adiposity and improve glucose metabolism. The daily intranasal administration of a PTP1B inhibitor, plus the glucocorticoid antagonist RU486 that decreases TCPTP expression, represses feeding, increases browning, promotes weight loss, and improves glucose metabolism in obese mice. Our findings causally link heightened hypothalamic PTP1B and TCPTP with leptin and insulin resistance and the maintenance of obesity and define a viable pharmacological approach by which to promote weight loss in obesity.

Lipopolysaccharide reduces gonadotrophin-releasing hormone (GnRH) gene expression: role of RFamide-related peptide-3 and kisspeptin.

RFamide-related peptide (RFRP)-3 reduces luteinising hormone (LH) secretion in rodents. Stress has been shown to upregulate the expression of the RFRP gene (Rfrp) with a concomitant reduction in LH secretion, but an effect on expression of the gonadotrophin-releasing hormone (GnRH) gene (Gnrh1) has not been shown. We hypothesised that lipopolysaccharide (LPS)-induced stress affects expression of Rfrp, the gene for kisspeptin (Kiss1) and/or Gnrh1, leading to suppression of LH levels in rats. Intracerebroventricular injections of RFRP-3 (0.1, 1, 5 nmol) or i.v. LPS (15µgkg-1) reduced LH levels. Doses of 1 and 5 nmol RFRP-3 were then administered to analyse gene expression by in situ hybridisation. RFRP-3 (5 nmol) had no effect on Gnrh1 or Kiss1 expression. LPS stress reduced GnRH and Kiss1 expression, without affecting Rfrp1 expression. These data indicate that LPS stress directly or indirectly reduces Gnrh1 expression, but this is unlikely to be due to a change in Rfrp1 expression.

Brown adipose tissue thermogenesis in polycystic ovary syndrome.

OBJECTIVE: Polycystic ovary syndrome (PCOS) is associated with increased obesity with a greater propensity to weight gain and a lack of sustainable lifestyle interventions. Altered brown adipose tissue (BAT) thermogenesis is a potential contributor to obesity in PCOS. BAT activity and modulation have not been studied in PCOS. This observational study explored BAT thermogenesis and its associations in women with and without PCOS. PARTICIPANTS AND METHODS: Cutaneous temperature was recorded from supraclavicular (indicator of BAT activity) and upper arm regions using dataloggers (SubCue, Calgary, Canada) in a cross-sectional substudy, nested within a randomized control trial, of community-recruited premenopausal women with (n = 47, Rotterdam diagnostic criteria) and without (n = 11) PCOS. RESULTS: Complete temperature data were available in 44 PCOS (mean age: 30.0 ± 6.2, mean BMI: 29.3 ± 5.5) and 11 non-PCOS (mean age: 33.0 ± 7.0, mean BMI: 25 ± 3) women. Women with PCOS had lower supraclavicular skin temperature compared to controls overall (33.9 ± 0.7 vs 34.5 ± 1, P < 0.05) and during sleep (34.5 ± 0.6 vs 35.2 ± 0.9, P < 0.001). In the PCOS group, supraclavicular skin temperature overall and over sleep and waking hours correlated inversely with testosterone (r = -0.41 P < 0.05, r = -0.485 P < 0.01 and r = -0.450 P < 0.01 respectively). Testosterone levels explained approximately 15%, 30% and 20% of the variability in supraclavicular skin temperature overall and over sleep and waking hours in women with PCOS, respectively. CONCLUSION: Women with PCOS have lower BAT activity compared to controls. BAT thermogenesis is negatively associated with androgen levels in PCOS.

Adipose and skeletal muscle thermogenesis: studies from large animals.

The balance between energy intake and energy expenditure establishes and preserves a 'set-point' body weight. The latter is comprised of three major components including metabolic rate, physical activity and thermogenesis. Thermogenesis is defined as the cellular dissipation of energy via heat production. This process has been extensively characterised in brown adipose tissue (BAT), wherein uncoupling protein 1 (UCP1) creates a proton leak across the inner mitochondrial membrane, diverting protons away from ATP synthesis and resulting in heat dissipation. In beige adipocytes and skeletal muscle, thermogenesis can occur independent of UCP1. Beige adipocytes have been shown to produce heat via UCP1 as well as via both futile creatine and calcium cycling pathways. On the other hand, the UCP1 homologue UCP3 is abundant in skeletal muscle and post-prandial thermogenesis has been associated with UCP3 and the futile calcium cycling. This review will focus on the differential contributions of adipose tissue and skeletal muscle in determining total thermogenic output and energy expenditure in large mammals. Sheep and pigs do not have a circumscribed brown fat depot but rather possess white fat depots that contain brown and beige adipocytes interspersed amongst white adipose tissue. This is representative of humans, where brown, beige and white adipocytes have been identified in the neck and supraclavicular regions. This review will describe the mechanisms of thermogenesis in pigs and sheep and the relative roles of skeletal muscle and adipose tissue thermogenesis in controlling body weight in larger mammals.

Exercise counteracts the homeostatic decrease in thermogenesis caused by caloric restriction in sheep.

Caloric restriction causes a homeostatic reduction in thermogenesis. We aimed to determine whether exercise could counteract this. We studied four groups of normal-weight ewes ( n = 5), including control sedentary fed ad libitum, exercise fed ad libitum (30 min/d, 5 d/wk), diet-restricted (70% of ad libitum food intake), and combined diet and exercise. Temperature probes implanted in sternal and retroperitoneal adipose tissue and skeletal muscle measured thermogenesis. After the 4-wk intervention, hypothalami were collected for in situ hybridization, and fat and muscle biopsies were collected for real-time PCR and Western blotting. Combined diet and exercise reduced adiposity ( P < 0.05). Caloric restriction alone reduced overnight temperatures in sternal and retroperitoneal fat ( P < 0.05), which was counteracted by exercise ( P < 0.05). Exercise did not induce expression of cellular markers of browning in adipose tissue. There was no effect of diet or exercise on skeletal muscle thermogenesis. Combined diet and exercise increased the expression of neuropeptide Y and agouti-related protein in the hypothalamic arcuate nucleus ( P < 0.05), consistent with reduced adiposity. Gene expressions of key hypothalamic appetite-regulating peptides were not associated with altered thermogenesis. We demonstrate that exercise counteracts the inhibitory effect of caloric restriction to restore thermogenesis in adipose tissue of sheep.-Fuller-Jackson, J.-P., Clarke, I. J., Rao, A., Henry, B. A. Exercise counteracts the homeostatic decrease in thermogenesis caused by caloric restriction in sheep.

Continuous Kisspeptin Restores Luteinizing Hormone Pulsatility Following Cessation by a Neurokinin B Antagonist in Female Sheep.

Pulsatile secretion of the gonadotropin-releasing hormone (GnRH) drives pulsatile secretion of the luteinizing hormone (LH), with evidence that this depends on kisspeptin (Kiss) input to GnRH neurons. Kiss administration causes acute GnRH/LH secretion, and electrophysiological data suggest that Kiss neurons may act in a phasic manner to drive GnRH secretion, but there is not definitive evidence for this. The product of the Kiss-1 gene is proteolytically cleaved to smaller products, and the 10 amino acid C-terminal product (Kiss-10) displays full bioactivity. We have shown previously that continuous delivery of Kiss-10 to anestrous ewes can cause a surge in GnRH secretion and ovulation and increases LH pulse frequency in humans. Here, we tested the hypothesis that continuous Kiss-10 delivery can support pulsatile GnRH/LH secretion in the sheep. Neurokinin B (NKB) provides positive drive to Kiss neurons, so we therefore infused an NKB antagonist (ANT-08) intracerebroventricularly to induce cessation of pulsatile GnRH/LH secretion, with or without concomitant continuous Kiss-10 infusion. ANT-08 suppressed GnRH/LH pulsatility, which was immediately restored with continuous Kiss-10 infusion. These data support the notion that Kiss-10 action is downstream of NKB signaling and that continuous Kiss-10 stimulation of GnRH neurons is sufficient to support a pulsatile pattern of GnRH/LH secretion. This offers further support to the theory that GnRH pulse generation is intrinsic to GnRH neurons and that pulsatile GnRH release can be affected with continuous stimulation by Kiss-10.

Ontogeny and Thermogenic Role for Sternal Fat in Female Sheep.

Brown adipose tissue acting through a unique uncoupling protein (UCP1) has a critical role in preventing hypothermia in newborn sheep but is then thought to rapidly disappear during postnatal life. The extent to which the anatomical location of fat influences postnatal development and thermogenic function in adulthood, particularly following feeding, is unknown, and we examined both in our study. Changes in gene expression of functionally important pathways (i.e., thermogenesis, development, adipogenesis, and metabolism) were compared between sternal and retroperitoneal fat depots together with a representative skeletal muscle over the first month of postnatal life, coincident with the loss of brown fat and the accumulation of white fat. In adult sheep, implanted temperature probes were used to characterize the thermogenic response of fat and muscle to feeding and the effects of reduced or increased adiposity. UCP1 was more abundant in sternal fat than in retroperitoneal fat and was retained only in the sternal depot of adults. Distinct differences in the abundance of gene pathway markers were apparent between tissues, with sternal fat exhibiting some similarities with muscle that were not apparent in the retroperitoneal depot. In adults, the postprandial rise in temperature was greater and more prolonged in sternal fat than in retroperitoneal fat and muscle, a difference that was maintained with altered adiposity. In conclusion, sternal adipose tissue retains UCP1 into adulthood, when it shows a greater thermogenic response to feeding than do muscle and retroperitoneal fat. Sternal fat may be more amenable to targeted interventions that promote thermogenesis in large mammals.

Innate Obesity, Revealed by Selection Markers, Confers Significant Imprint of Hypothalamic Genes Controlling Energy Expenditure.

The incidence of obesity is rapidly escalating and has reached epidemic proportions. In all species, including rodents, humans, and sheep, there is large variation in the degree of weight gain across individuals in response to an obesogenic environment. This individual variation is, at least in part, determined by innate differences in energy expenditure, of which adaptive thermogenesis is a key component. The hypothalamus is essential to the control of body weight and adiposity. Appetite-regulating peptides within the hypothalamus exert reciprocal effects on food intake and energy expenditure, such that neuropeptides that stimulate food intake inhibit thermogenesis and vice versa. This review discusses the role of the hypothalamic neuropeptides in determining innate predisposition to obesity in 3 animal models being obesity-prone and obesity-resistant rodents, genetically lean and obese sheep, and animals selected for high/low cortisol responsiveness. In rodents, leptin resistance is a primary feature of the propensity to become obese. This contrasts that of larger mammals, such as sheep, where altered susceptibility to obesity manifests within the melanocortin and/or orexin pathways. This review highlights fundamental species differences within the hypothalamus that lead to altered susceptibility to weight gain and increased propensity to become obese.

α-Melanocyte stimulating hormone promotes muscle glucose uptake via melanocortin 5 receptors.

OBJECTIVE: Central melanocortin pathways are well-established regulators of energy balance. However, scant data exist about the role of systemic melanocortin peptides. We set out to determine if peripheral α-melanocyte stimulating hormone (α-MSH) plays a role in glucose homeostasis and tested the hypothesis that the pituitary is able to sense a physiological increase in circulating glucose and responds by secreting α-MSH. METHODS: We established glucose-stimulated α-MSH secretion using humans, non-human primates, and mouse models. Continuous α-MSH infusions were performed during glucose tolerance tests and hyperinsulinemic-euglycemic clamps to evaluate the systemic effect of α-MSH in glucose regulation. Complementary ex vivo and in vitro techniques were employed to delineate the direct action of α-MSH via the melanocortin 5 receptor (MC5R)-PKA axis in skeletal muscles. Combined treatment of non-selective/selective phosphodiesterase inhibitor and α-MSH was adopted to restore glucose tolerance in obese mice. RESULTS: Here we demonstrate that pituitary secretion of α-MSH is increased by glucose. Peripheral α-MSH increases temperature in skeletal muscles, acts directly on soleus and gastrocnemius muscles to significantly increase glucose uptake, and enhances whole-body glucose clearance via the activation of muscle MC5R and protein kinase A. These actions are absent in obese mice, accompanied by a blunting of α-MSH-induced cAMP levels in skeletal muscles of obese mice. Both selective and non-selective phosphodiesterase inhibition restores α-MSH induced skeletal muscle glucose uptake and improves glucose disposal in obese mice. CONCLUSION: These data describe a novel endocrine circuit that modulates glucose homeostasis by pituitary α-MSH, which increases muscle glucose uptake and thermogenesis through the activation of a MC5R-PKA-pathway, which is disrupted in obesity.

Stress Increases Gonadotropin Inhibitory Hormone Cell Activity and Input to GnRH Cells in Ewes.

Stress reduces GnRH and gonadotropin secretion in sheep, but the central mechanism for this suppressive effect is unknown. Gonadotropin-inhibitory hormone (GnIH) negatively regulates GnRH neurons and gonadotropes. Here, we measured activity of GnIH neurons and contact of GnIH fibers on GnRH neurons during either chronic "pseudostress" or acute stress in sheep. We also measured GnIH secretion into hypophysial portal blood during pseudostress and acute stress. The pseudostress was daily im injections (0.5 mg) of Synacthen Depot (adrenocorticotropin) or vehicle for 4 weeks, which increased the GnIH cell number and gene expression/cell in the hypothalamus, measured by in situ hybridization. Double label immunohistochemistry showed that Synacthen Depot treatment increased the percentage of GnRH cells in close contact with GnIH fibers but did not alter GnIH levels in hypophysial portal blood. Acute stress protocols were either sequential audiovisual predator stress, followed by insulin-induced hypoglycemia, or a single challenge with lipopolysaccharide (iv). Both of these acute stressors activated a c-Fos response in GnIH cells and increased the contacts of GnIH fibers to GnRH cells. Neither acute stress protocol increased GnIH secretion into hypophysial portal blood. These data show that chronic pseudostress and acute stressors increase the function of GnIH cells as well as the degree to which GnIH cells may provide input to GnRH cells. Thus, GnIH cells may provide a central mechanism whereby stress compromises reproduction. Neither chronic pseudostress nor acute stress elevates secretion of GnIH into portal blood, but stress effects mediated by GnIH cells are directed towards GnRH cell bodies.

Ewes With Divergent Cortisol Responses to ACTH Exhibit Functional Differences in the Hypothalamo-Pituitary-Adrenal (HPA) Axis.

Within any population, the cortisol response to ACTH covers a considerable range. High responders (HRs) exhibit a greater cortisol secretory response to stress or ACTH, compared with individuals classified as low cortisol responders (LRs). We administered ACTH (0.2 µg/kg, iv) to 160 female sheep and selected subpopulations of animals as LR and HR. In the present study, we aimed to characterize the hypothalamo-pituitary-adrenal axis in HR and LR and to identify factors that underlie the differing cortisol responses to ACTH. Hypothalami, pituitaries, and adrenals were collected from nonstressed HR and LR ewes. Expression of genes for CRH, arginine vasopressin (AVP), oxytocin, glucocorticoid receptor, and mineralocorticoid receptor were measured by in situ hybridization in the paraventricular nucleus of the hypothalamus, and proopiomelanocortin (POMC) gene expression was measured in the anterior pituitary. Expression of CRH, AVP, and POMC was higher in HR, with no differences in either glucocorticoid receptor or mineralocorticoid receptor expression. Oxytocin expression was greater in LR. In the adrenal gland, real-time PCR analysis indicated that expression of the ACTH receptor and a range of steroidogenic enzymes was similar in HR and LR. Adrenal weights, the cortex to medulla ratio and adrenal cortisol content were also similar in LR and HR. In conclusion, LR and HR display innate differences in the steady-state expression of CRH, AVP, oxytocin, and POMC, indicating that selection for cortisol responsiveness identifies distinct subpopulations that exhibit innate differences in the gene expression/function of hypothalamo-pituitary-adrenal axis markers.

Cochlear implant users' spectral ripple resolution.

This study revisits the issue of the spectral ripple resolution abilities of cochlear implant (CI) users. The spectral ripple resolution of recently implanted CI recipients (implanted during the last 10 years) were compared to those of CI recipients implanted 15 to 20 years ago, as well as those of normal-hearing and hearing-impaired listeners from previously published data from Henry, Turner, and Behrens [J. Acoust. Soc. Am. 118, 1111-1121 (2005)]. More recently, implanted CI recipients showed significantly better spectral ripple resolution. There is no significant difference in spectral ripple resolution for these recently implanted subjects compared to hearing-impaired (acoustic) listeners. The more recently implanted CI users had significantly better pre-operative speech perception than previously reported CI users. These better pre-operative speech perception scores in CI users from the current study may be related to better performance on the spectral ripple discrimination task; however, other possible factors such as improvements in internal and external devices cannot be excluded.

Stress-induced behavioral and metabolic adaptations lead to an obesity-prone phenotype in ewes with elevated cortisol responses.

The underlying cause of predisposition to obesity is complex but one marker is cortisol responsiveness. Selection of sheep for high (HR) or low (LR) cortisol responses to adrenocorticotropin shows that HR are more likely to become obese. Increased propensity to obesity is associated with reduced skeletal muscle thermogenesis. We sought to determine whether metabolic or behavioral responses to stress also contribute to altered propensity to obesity in LR and HR. Animals (n=5-10/group) were exposed to 3 stressors and we measured food intake and thermogenesis (recorded with dataloggers implanted into muscle). Stressors were hypoglycaemia (0.125 units/kg insulin, IV), a barking dog and immune challenge (200 ng/kg lipopolysaccharide--LPS, IV). LR animals showed a greater catabolic state in response to both immune and psychosocial stressors. LPS reduced (P<0.01) food intake in both groups but LR showed a greater (P<0.05) reduction in food intake and a more substantial (P<0.05) rise in muscle temperature. Introduction of the barking dog reduced (P<0.05) food intake in LR only. These metabolic differences coincided with differences in cortisol responsiveness, where HR animals had increased (P<0.05) cortisol in response to both immune and psychosocial stressors. We also assessed behavior in the following paradigms: 1, isolation in the open field test; 2, response to a human intruder; and 3, food competition. LR had greater (P<0.05) activity, reduced fearfulness and displayed a proactive coping style of behavior. Thus we demonstrate that high cortisol responsiveness identifies animals with stress-induced metabolic and behavioral traits that may contribute to susceptibility to obesity.

Leptin enhances insulin sensitivity by direct and sympathetic nervous system regulation of muscle IGFBP-2 expression: evidence from nonrodent models.

Leptin is produced from white adipose tissue and acts primarily to regulate energy balance. Obesity is associated with leptin resistance and increased circulating levels of leptin. Leptin has recently been shown to influence levels of IGF binding protein-2 (IGFBP-2), a protein that is reduced in obesity and type 2 diabetes. Overexpression of IGFBP-2 protects against obesity and type 2 diabetes. As such, IGFBP-2 signaling may represent a novel pathway by which leptin regulates insulin sensitivity. We sought to investigate how leptin regulates skeletal muscle IGFBP-2 levels and to assess the impact of this on insulin signaling and glucose uptake. In vitro experiments were undertaken in cultured human skeletal myotubes, whereas in vivo experiments assessed the effect of intracerebroventricular leptin on peripheral skeletal muscle IGFBP-2 expression and insulin sensitivity in sheep. Leptin directly increased IGFBP-2 mRNA and protein in human skeletal muscle through both signal transducer and activator of transcription-3 and phosphatidylinositol 3-kinase signaling, in parallel with enhanced insulin signaling. Silencing IGFBP-2 lowered leptin- and insulin-stimulated protein kinase B phosphorylation and glucose uptake. In in vivo experiments, intracerebroventricular leptin significantly increased hind-limb skeletal muscle IGFBP-2, an effect completely blocked by concurrent peripheral infusion of a ß-adrenergic blocking agent. Sheep receiving central leptin showed improvements in glucose tolerance and circulating insulin levels after an iv glucose load. In summary, leptin regulates skeletal muscle IGFBP-2 by both direct peripheral and central (via the sympathetic nervous system) mechanisms, and these likely impact on peripheral insulin sensitivity and glucose metabolism.

High cortisol responses identify propensity for obesity that is linked to thermogenesis in skeletal muscle.

Subjects characterized as cortisol high responders (HRs) consume more calories after stress, but it is unknown whether cortisol responsiveness predicts a propensity for obesity. Female sheep with either high or low cortisol responses to adrenocorticotropin (ACTH) were identified. Body composition was similar in HRs and cortisol low responders (LRs), but the HRs had greater (P<0.01) adiposity than did the LRs (40.5±0.7 vs. 35.8±1.4%) after high-energy feeding, despite comparable food intake. Postprandial thermogenesis in muscle temperature was 0.8 ± 0.08°C higher in the LRs than in the HRs (P<0.01), whereas feeding-induced changes in fat temperature were similar. Leptin and insulin sensitivity were similar in the HRs and LRs. Feeding lowered (P<0.001) the respiratory control ratio in muscle (HRs 9.2±0.8-5.2±1.2; LRs 8.4±0.5-5.2±0.7), indicative of increased uncoupled respiration. Also in muscle, the feeding-induced increases in uncoupling protein (UCP)-3 (fold increase: HRs, 2.4; LRs, 2.0), ryanodine 1 receptor (RyR1; fold increase: HRs 3.1; LRs 2.1), and sarcoendoplasmic reticulum Ca(2+)-dependent ATPase (fold increase: HRs 1.5; LRs 1.6) were equivalent in the HRs and LRs. Sequencing of mitochondrial DNA revealed no haplotypic differences between the 2 groups. We conclude that predisposition to obesity can be predicted by cortisol responsiveness to an ACTH challenge and that the response is due to innate differences in muscle thermogenesis.

Differential effects of acute and chronic estrogen treatment on thermogenic and metabolic pathways in ovariectomized sheep.

Estrogen is protective against weight gain, but the underlying mechanisms are not fully elucidated. We sought to characterize the effects of estrogen on energy expenditure in skeletal muscle and adipose tissue in ovariectomized sheep. Temperature probes were implanted into sc (gluteal) and visceral (retroperitoneal) fat depots and skeletal muscle of the hind limb (vastus lateralis). Food was available from 1100-1600 h to entrain postprandial thermogenesis. We characterized the effects of single (50 µg estradiol benzoate, im) and repeated (25 µg estradiol-17ß, iv) injections as well as chronic (3 × 3 cm estradiol-17ß implants for 7 d) treatment on heat production. A single injection of estrogen increased heat production in visceral fat and skeletal muscle, without an effect on food intake. Increased heat production in skeletal muscle was sustained by repeated estradiol-17ß injections. On the other hand, continuous treatment reduced food intake but had no effect on thermogenesis. To determine possible mechanisms that underpin estradiol-17ß-induced heat production, we measured femoral artery blood flow, the expression of uncoupling protein (UCP) mRNA and the phosphorylation of AMP-activated protein kinase and Akt in fat and muscle. There was little effect of either single or repeated injections of estradiol-17ß on the expression of UCP1, -2, or -3 mRNA in visceral fat or skeletal muscle. Acute injection of estradiol-17ß increased the phosphorylation of AMP-activated protein kinase and Akt in muscle only. Estradiol-17ß treatment did not alter femoral artery blood flow. Thus, the stimulatory effect of estradiol-17ß on thermogenesis in female sheep is dependent upon a pulsatile pattern of treatment and not constant continuous exposure.