Diet-induced obesity impairs hypothalamic glucose sensing but not glucose hypothalamic extracellular levels, as measured by microdialysis

Resistance and Aerobic Training Were Effective in Activating Different Markers of the Browning Process in Obesity

Lifestyle changes regarding diet composition and exercise training have been widely used as a non-pharmacological clinical strategy in the treatment of obesity, a complex and difficult-to-control disease. Taking the potential of exercise in the browning process and in increasing thermogenesis into account, the aim of this paper was to evaluate the effect of resistance, aerobic, and combination training on markers of browning of white adipose tissue from rats with obesity who were switched to a balanced diet with normal calorie intake. Different types of training groups promote a reduction in the adipose tissue and delta mass compared to the sedentary high-fat diet group (HS). Interestingly, irisin in adipose tissues was higher in the resistance exercise (RE) and aerobic exercise (AE) groups compared to control groups. Moreover, in adipose tissue, the fibroblast growth factor 21 (FGF21), coactivator 1 α (PGC1α), and peroxisome proliferator-activated receptor gamma (PPARγ) were higher in response to resistance training RE compared with the control groups, respectively. Additionally, uncoupling protein 1 (UCP1) showed higher levels in response to group AE compared to the HS group. In conclusion, the browning process in white adipose tissue responds differently toward different training exercise protocols, with resistance and aerobic training efficient in activating different biomarkers of the browning process, upregulating irisin, FGF21, PGC1α, PPARγ, and UCP1 in WAT, which together may suggest an improvement in the thermogenic process in the adipose tissue. Considering the experimental conditions of the present investigation, we suggest future research to pave new avenues to be applied in clinical practices to combat obesity.

Analysis of the Glucose-Dependent Transcriptome in Murine Hypothalamic Cells

Glucose provides vital energy for cells and contributes to gene expression. The hypothalamus is key for metabolic homeostasis, but effects of glucose on hypothalamic gene expression have not yet been investigated in detail. Thus, herein, we monitored the glucose-dependent transcriptome in murine hypothalamic mHypoA-2/10 cells by total RNA-seq analysis. A total of 831 genes were up- and 1390 genes were downregulated by at least 50%. Key genes involved in the cholesterol biosynthesis pathway were upregulated, and total cellular cholesterol levels were significantly increased by glucose. Analysis of single genes involved in fundamental cellular signaling processes also suggested a significant impact of glucose. Thus, we chose ≈100 genes involved in signaling and validated the effects of glucose on mRNA levels by qRT-PCR. We identified Gnai1–3, Adyc6, Irs1, Igfr1, Hras, and Elk3 as new glucose-dependent genes. In line with this, cAMP measurements revealed enhanced noradrenalin-induced cAMP levels, and reporter gene assays elevated activity of the insulin-like growth factor at higher glucose levels. Key data of our studies were confirmed in a second hypothalamic cell line. Thus, our findings link extra cellular glucose levels with hypothalamic lipid synthesis and pivotal intracellular signaling processes, which might be of particular interest in situations of continuously increased glucose levels.

Hypothalamic glucose-sensing mechanisms

Chronic metabolic diseases, including diabetes and obesity, have become a major global health threat of the twenty-first century. Maintaining glucose homeostasis is essential for survival in mammals. Complex and highly coordinated interactions between glucose-sensing mechanisms and multiple effector systems are essential for controlling glucose levels in the blood. The central nervous system (CNS) plays a crucial role in regulating glucose homeostasis. Growing evidence indicates that disruption of glucose sensing in selective CNS areas, such as the hypothalamus, is closely interlinked with the pathogenesis of obesity and type 2 diabetes mellitus. However, the underlying intracellular mechanisms of glucose sensing in the hypothalamus remain elusive. Here, we review the current literature on hypothalamic glucose-sensing mechanisms and discuss the impact of alterations of these mechanisms on the pathogenesis of diabetes.

Prostaglandin in the ventromedial hypothalamus regulates peripheral glucose metabolism

The hypothalamus plays a central role in monitoring and regulating systemic glucose metabolism. The brain is enriched with phospholipids containing poly-unsaturated fatty acids, which are biologically active in physiological regulation. Here, we show that intraperitoneal glucose injection induces changes in hypothalamic distribution and amounts of phospholipids, especially arachidonic-acid-containing phospholipids, that are then metabolized to produce prostaglandins. Knockdown of cytosolic phospholipase A2 (cPLA2), a key enzyme for generating arachidonic acid from phospholipids, in the hypothalamic ventromedial nucleus (VMH), lowers insulin sensitivity in muscles during regular chow diet (RCD) feeding. Conversely, the down-regulation of glucose metabolism by high fat diet (HFD) feeding is improved by knockdown of cPLA2 in the VMH through changing hepatic insulin sensitivity and hypothalamic inflammation. Our data suggest that cPLA2-mediated hypothalamic phospholipid metabolism is critical for controlling systemic glucose metabolism during RCD, while continuous activation of the same pathway to produce prostaglandins during HFD deteriorates glucose metabolism.

The ventromedial hypothalamus regulates systemic glucose metabolism. Here the authors show that cytosolic phospholipase A2 mediated phospholipid metabolism contributes to this regulation in healthy animals but exert deteriorating effects on glucose homeostasis under high-fat-diet feeding.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Circadian-timed dopamine agonist treatment reverses high-fat diet-induced diabetogenic shift in ventromedial hypothalamic glucose sensing

INTRODUCTION

Within the ventromedial hypothalamus (VMH), glucose inhibitory (GI) neurons sense hypoglycaemia while glucose excitatory (GE) neurons sense hyperglycaemia to initiate counter control mechanisms under normal conditions. However, potential electrophysiological alterations of these two neuronal types in vivo in insulin-resistant states have never been simultaneously fully documented. Further, the anti-diabetic effect of dopamine agonism on this VMH system under insulin resistance has not been studied.

METHODS

This study examined the impact of a high-fat diet (HFD) on in vivo electrophysiological recordings from VMH GE and GI neurons and the ability of circadian-timed dopamine agonist therapy to reverse any adverse effect of the HFD on such VMH activities and peripheral glucose metabolism.

RESULTS

HFD significantly inhibited VMH GE neuronal electrophysiological response to local hyperglycaemia (36.3%) and augmented GI neuronal excitation response to local hypoglycaemia (47.0%). Bromocriptine (dopamine agonist) administration at onset of daily activity (but not during the daily sleep phase) completely reversed both VMH GE and GI neuronal aberrations induced by HFD. Such timed treatment also normalized glucose intolerance and insulin resistance. These VMH and peripheral glucose metabolism effects of circadian-timed bromocriptine may involve its known effect to reduce elevated VMH noradrenergic activity in insulin-resistant states as local VMH administration of norepinephrine was observed to significantly inhibit VMH GE neuronal sensing of local hyperglycaemia in insulin-sensitive animals on regular chow diet (52.4%).

CONCLUSIONS

HFD alters VMH glucose sensing in a manner that potentiates hyperglycaemia and this effect on the VMH can be reversed by appropriately circadian-timed dopamine agonist administration.

N1-Methylnicotinamide Improves Hepatic Insulin Sensitivity via Activation of SIRT1 and Inhibition of FOXO1 Acetylation

OBJECTIVE

To explore the effects of N¹-methylnicotinamide (MNAM) on insulin resistance and glucose metabolism in obese type 2 diabetes mellitus (T2DM) mice and regulatory mechanisms of the NAD-dependent deacetylase sirtuin-1 (SIRT1)/forkhead box protein O1 (FOXO1) pathway.

METHODS

Blood glucose and insulin levels were examined in mice. HE and oil red O staining were used to observe the effects of MNAM on liver lipid deposition in ob/ob mice. Real-time PCR and Western blotting were used to detect expression of gluconeogenesis, insulin signaling-related proteins, and SIRT1/FOXO1 pathway-related proteins. L-O2 cells were cultured as a model of insulin resistance, and MNAM and SIRT1 inhibitors were administered in vivo. Residual glucose and insulin signaling-related proteins were detected and the mechanisms associated with the SIRT1/FOXO1 signaling pathway in insulin resistance explored.

RESULTS

MNAM can effectively reduce levels of fasting blood glucose and insulin, improve liver morphology, and reduce lipid accumulation in obese type 2 diabetes mellitus mice. MNAM also downregulates the key proteins in the gluconeogenesis pathway in the liver, upregulates Sirt1 expression, and reduces acetylation of the FOXO1 protein. In vitro, MNAM could promote the glucose uptake capacity of L-O2 cells induced by palmitic acid (PA), a saturated fatty acid that induces IR in various scenarios, including hepatocytes, improving insulin resistance. As Sirt1 expression was inhibited, the reduction of hepatocyte gluconeogenesis and the regulation of the insulin signaling pathway by MNAM were reversed.

CONCLUSION

MNAM activates SIRT1 and inhibits acetylation of FOXO1, which in turn regulates insulin sensitivity in type 2 diabetic mice, leading to a reduction of hepatic glucose output and improvement of insulin resistance.

Resistance Exercise Training Attenuates the Loss of Endogenous GLP-1 Receptor in the Hypothalamus of Type 2 Diabetic Rats

The aim of this study was to investigate the effects of resistance exercise training on hypothalamic GLP-1R levels and its related signaling mechanisms in T2DM. The animals were separated into three groups: a non-diabetic control (CON), diabetic control (DM), and diabetic with resistance exercise (DM + EXE) group. The resistance exercise training group performed ladder climbing (eight repetitions, three days per week for 12 weeks). Body weight was slightly lower in the DM + EXE group than the DM group, but difference between the groups was not significant. Food intake and glucose were significantly lower in the DM + EXE group than in the DM group. The blood insulin concentration was significantly higher and glucagon was significantly lower in the DM + EXE group. The DM + EXE group in the hypothalamus showed significant increases in GLP-1R mRNA, protein kinase A (PKA), glucose transporter 2 (GLUT2), and protein kinase B (AKT) and significant decrease in protein kinase C-iota (PKC-iota). Antioxidant enzymes and apoptosis factors were significantly improved in the DM + EXE group compared with the DM group in the hypothalamus. The results suggest that resistance exercise contributes to improvements the overall health of the brain in diabetic conditions.

Mitochondrial Dynamin-Related Protein 1 (DRP1) translocation in response to cerebral glucose is impaired in a rat model of early alteration in hypothalamic glucose sensing

OBJECTIVE

Hypothalamic glucose sensing (HGS) initiates insulin secretion (IS) via a vagal control, participating in energy homeostasis. This requires mitochondrial reactive oxygen species (mROS) signaling, dependent on mitochondrial fission, as shown by invalidation of the hypothalamic DRP1 protein. Here, our objectives were to determine whether a model with a HGS defect induced by a short, high fat-high sucrose (HFHS) diet in rats affected the fission machinery and mROS signaling within the mediobasal hypothalamus (MBH).

METHODS

Rats fed a HFHS diet for 3 weeks were compared with animals fed a normal chow. Both in vitro (calcium imaging) and in vivo (vagal nerve activity recordings) experiments to measure the electrical activity of isolated MBH gluco-sensitive neurons in response to increased glucose level were performed. In parallel, insulin secretion to a direct glucose stimulus in isolated islets vs. insulin secretion resulting from brain glucose stimulation was evaluated. Intra-carotid glucose load-induced hypothalamic DRP1 translocation to mitochondria and mROS (H2O2) production were assessed in both groups. Finally, compound C was intracerebroventricularly injected to block the proposed AMPK-inhibited DRP1 translocation in the MBH to reverse the phenotype of HFHS fed animals.

RESULTS

Rats fed a HFHS diet displayed a decreased HGS-induced IS. Responses of MBH neurons to glucose exhibited an alteration of their electrical activity, whereas glucose-induced insulin secretion in isolated islets was not affected. These MBH defects correlated with a decreased ROS signaling and glucose-induced translocation of the fission protein DRP1, as the vagal activity was altered. AMPK-induced inhibition of DRP1 translocation increased in this model, but its reversal through the injection of the compound C, an AMPK inhibitor, failed to restore HGS-induced IS.

CONCLUSIONS

A hypothalamic alteration of DRP1-induced fission and mROS signaling in response to glucose was observed in HGS-induced IS of rats exposed to a 3 week HFHS diet. Early hypothalamic modifications of the neuronal activity could participate in a primary defect of the control of IS and ultimately, the development of diabetes.

Glucose-Sensing in the Reward System

Glucose-sensing neurons are neurons that alter their activity in response to changes in extracellular glucose. These neurons, which are an important mechanism the brain uses to monitor changes in glycaemia, are present in the hypothalamus, where they have been thoroughly investigated. Recently, glucose-sensing neurons have also been identified in brain nuclei which are part of the reward system. However, little is known about the molecular mechanisms by which they function, and their role in the reward system. We therefore aim to provide an overview of molecular mechanisms that have been studied in the hypothalamic glucose-sensing neurons, and investigate which of these transporters, enzymes and channels are present in the reward system. Furthermore, we speculate about the role of glucose-sensing neurons in the reward system.

The impact of ageing, fasting and high-fat diet on central and peripheral glucose tolerance and glucose-sensing neural networks in the arcuate nucleus

Obesity and ageing are risk factors for diabetes. In the present study, we investigated the effects of ageing, obesity and fasting on central and peripheral glucose tolerance and on glucose-sensing neuronal function in the arcuate nucleus of rats, with a view to providing insight into the central mechanisms regulating glucose homeostasis and how they change or are subject to dysfunction with ageing and obesity. We show that, following a glucose load, central glucose tolerance at the level of the cerebrospinal fluid (CSF) and plasma is significantly reduced in rats maintained on a high-fat diet (HFD). With ageing, up to 2 years, central glucose tolerance was impaired in an age-dependent manner, whereas peripheral glucose tolerance remained unaffected. Ageing-induced peripheral glucose intolerance was improved by a 24-hour fast, whereas central glucose tolerance was not corrected. Pre-wean, immature animals have elevated basal plasma glucose levels and a delayed increase in central glucose levels following peripheral glucose injection compared to mature animals. Electrophysiological recording techniques revealed an energy-status-dependent role for glucose-excited, inhibited and adapting neurones, along with glucose-induced changes in synaptic transmission. We conclude that ageing affects central glucose tolerance, whereas HFD profoundly affects central and peripheral glucose tolerance and, in addition, glucose-sensing neurones adapt function in an energy-status-dependent manner.

Sweet taste receptor in the hypothalamus: a potential new player in glucose sensing in the hypothalamus

The hypothalamic feeding center plays an important role in energy homeostasis. The feeding center senses the systemic energy status by detecting hormone and nutrient levels for homeostatic regulation, resulting in the control of food intake, heat production, and glucose production and uptake. The concentration of glucose is sensed by two types of glucose-sensing neurons in the feeding center: glucose-excited neurons and glucose-inhibited neurons. Previous studies have mainly focused on glucose metabolism as the mechanism underlying glucose sensing. Recent studies have indicated that receptor-mediated pathways also play a role in glucose sensing. This review describes sweet taste receptors in the hypothalamus and explores the role of sweet taste receptors in energy homeostasis.

Evaluation of circulating sRAGE in osteoporosis according to BMI, adipokines and fracture risk: a pilot observational study

Background

Osteoporosis is a systemic metabolic disease based on age-dependent imbalance between the rates of bone formation and bone resorption. Recent studies on the pathogenesis of this disease identified that bone remodelling impairment, at the base of osteoporotic bone fragility, could be related to protein glycation, in association to oxidative stress. The glycation reactions lead to the generation of glycation end products (AGEs) which, in turn, accumulates into bone, where they binds to the receptor for AGE (RAGE). The aim of this study is to investigate the potential role of circulating sRAGE in osteoporosis, in particular evaluating the correlation of sRAGE with the fracture risk, in association with bone mineral density, the fracture risk marker FGF23, and lipid metabolism.

Results

Circulating level of soluble RAGE correlate with osteopenia and osteoporosis level. Serum sRAGE resulted clearly associated on the one hand to bone fragility and, on the other hand, with BMI and leptin. sRAGE is particularly informative because serum sRAGE is able to provide, as a single marker, information about both the aspects of osteoporotic disease, represented by bone fragility and lipid metabolism.

Conclusions

The measure serum level of sRAGE could have a potential diagnostic role in the monitoring of osteoporosis progression, in particular in the evaluation of fracture risk, starting from the prevention and screening stage, to the osteopenic level to osteoporosis.

Intrauterine Growth Restriction Programs the Hypothalamus of Adult Male Rats: Integrated Analysis of Proteomic and Metabolomic Data

Programming of hypothalamic functions regulating energy homeostasis may play a role in intrauterine growth restriction (IUGR)-induced adulthood obesity. The present study investigated the effects of IUGR on the hypothalamus proteome and metabolome of adult rats submitted to 50% protein-energy restriction throughout pregnancy. Proteomic and metabolomic analyzes were performed by data independent acquisition mass spectrometry and multiple reaction monitoring, respectively. At age 4 months, the restricted rats showed elevated adiposity, increased leptin and signs of insulin resistance. 1356 proteins were identified and 348 quantified while 127 metabolites were quantified. The restricted hypothalamus showed down-regulation of 36 proteins and 5 metabolites and up-regulation of 21 proteins and 9 metabolites. Integrated pathway analysis of the proteomics and metabolomics data indicated impairment of hypothalamic glucose metabolism, increased flux through the hexosamine pathway, deregulation of TCA cycle and the respiratory chain, and alterations in glutathione metabolism. The data suggest IUGR modulation of energy metabolism and redox homeostasis in the hypothalamus of male adult rats. The present results indicated deleterious consequences of IUGR on hypothalamic pathways involved in pivotal physiological functions. These results provide guidance for future mechanistic studies assessing the role of intrauterine malnutrition in the development of metabolic diseases later in life.

Wake-sleep and cardiovascular regulatory changes in rats made obese by a high-fat diet

Obesity is known to be associated with alterations in wake-sleep (WS) architecture and cardiovascular parameters. This study was aimed at assessing the possible influence of diet-induced obesity (DIO) on sleep homeostasis and on the WS state-dependent levels of arterial pressure (AP) and heart rate in the rat. Two groups of age-matched Sprague-Dawley rats were fed either a high-fat hypercaloric diet, leading to DIO, or a normocaloric standard diet (lean controls) for 8 weeks. While under general anesthesia, animals were implanted with instrumentation for the recording of electroencephalogram, electromyogram, arterial pressure, and deep brain temperature. The experimental protocol consisted of 48h of baseline, 12h of gentle handling, enhancing wake and depressing sleep, and 36-h post-handling recovery. Compared to lean controls, DIO rats showed: i) the same amount of rapid-eye movement (REM) and non-REM (NREM) sleep in the rest period, although the latter was characterized by more fragmented episodes; ii) an increase in both REM sleep and NREM sleep in the activity period; iii) a comparable post-handling sleep homeostatic response, in terms of either the degree of Delta power increase during NREM sleep or the quantitative compensation of the REM sleep loss at the end of the 36-h recovery period; iv) significantly higher levels of AP, irrespectively of the different WS states and of the changes in their intensity throughout the experimental protocol. Overall, these changes may be the reflection of a modification in the activity of the hypothalamic areas where WS, autonomic, and metabolic regulations are known to interact.

Prepuberal light phase feeding induces neuroendocrine alterations in adult rats

Feeding patterns are important factors in obesity evolvement. Time-restricted feeding schedules (tRF) during resting phase change energy homeostasis regulation, disrupting the circadian release of metabolism-regulating hormones, such as leptin, insulin and corticosterone and promoting body weight gain. Thyroid (HPT) and adrenal (HPA) axes exhibit a circadian regulation and are involved in energy expenditure, thus studying their parameters in tRF paradigms will elucidate their role in energy homeostasis impairments under such conditions. As tRF in young animals is poorly studied, we subjected prepuberal rats to a tRF either in light (LPF) or in darkness phase (DPF) and analyzed HPT and HPA response when they reach adulthood, as well as their arcuate (ARC) and paraventricular (PVN) hypothalamic nuclei neurons' sensitivity to leptin in subsets of 10-week-old animals after fasting and with i.p. leptin treatment. LPF group showed high body weight and food intake, along with increased visceral fat pads, corticosterone, leptin and insulin serum levels, whereas circulating T₄ decreased. HPA axis hyperactivity was demonstrated by their high PVN Crf mRNA expression; the blunted activity of HPT axis, by the decreased hypophysiotropic PVN Trh mRNA expression. Trh impaired expression to the positive energy balance in LPF, accounted for their ARC leptin resistance, evinced by an increased Npy and Socs3 mRNA expression. We concluded that the hyperphagia of prepuberal LPF animals could account for the HPA axis hyperactivity and for the HPT blocked function due to the altered ARC leptin signaling and impaired NPY regulation on PVN TRH neurons.

Hypothalamic AMP-Activated Protein Kinase Regulates Biphasic Insulin Secretion from Pancreatic β Cells during Fasting and in Type 2 Diabetes

Glucose-stimulated insulin secretion (GSIS) by pancreatic β cells is biphasic. However, the physiological significance of biphasic GSIS and its relationship to diabetes are not yet fully understood. This study demonstrated that impaired first-phase GSIS follows fasting, leading to increased blood glucose levels and brain glucose distribution in humans. Animal experiments to determine a possible network between the brain and β cells revealed that fasting-dependent hyperactivation of AMP-activated protein kinase in the hypothalamus inhibited first-phase GSIS by stimulating the α-adrenergic pancreatic nerve. Furthermore, abnormal excitability of this brain-β cell neural axis was involved in diabetes-related impairment of first-phase GSIS in diabetic animals. Finally, pancreatic denervation improved first-phase GSIS and glucose tolerance and ameliorated severe diabetes by preventing β cell loss in diabetic animals. These results indicate that impaired first-phase GSIS is critical for brain distribution of dietary glucose after fasting. Furthermore, β cells in individuals with diabetes mistakenly sense that they are under conditions that mimic prolonged fasting. The present study provides additional insight into both β cell physiology and the pathogenesis of β cell dysfunction in type 2 diabetes.

•

Fasting-induced hypothalamic AMPK activation inhibited first-phase GSIS by stimulating the α-adrenergic nerve.

•

The brain-pancreas neural axis was involved in β cell dysfunction and glucose intolerance in diabetes.

•

Pancreatic denervation improved first-phase GSIS, glucose tolerance and β cell survival in type 2 diabetic animals.

Glucose-stimulated insulin secretion (GSIS) from pancreatic β cells is biphasic. Furthermore, first-phase GSIS is inhibited in type 2 diabetes. This study revealed that fasting reduced first-phase GSIS by signaling via the brain-pancreatic β cell neural axis, which is essential for maintaining glucose supply to the brain at re-feeding after fasting. Abnormal excitability of this neural axis was also associated with impaired first-phase GSIS in type 2 diabetes. Surgical pancreatic denervation improved diabetes in an animal study. The present data reveal that diabetic β cells exist under conditions that mimic starvation and provide a therapeutic potency of pancreatic denervation against diabetes.

Molecular Mechanisms of Obesity-Induced Osteoporosis and Muscle Atrophy

Obesity and osteoporosis are two alarming health disorders prominent among middle and old age populations, and the numbers of those affected by these two disorders are increasing. It is estimated that more than 600 million adults are obese and over 200 million people have osteoporosis worldwide. Interestingly, both of these abnormalities share some common features including a genetic predisposition, and a common origin: bone marrow mesenchymal stromal cells. Obesity is characterized by the expression of leptin, adiponectin, interleukin 6 (IL-6), interleukin 10 (IL-10), monocyte chemotactic protein-1 (MCP-1), tumor necrosis factor-alpha (TNF-α), macrophage colony stimulating factor (M-CSF), growth hormone (GH), parathyroid hormone (PTH), angiotensin II (Ang II), 5-hydroxy-tryptamine (5-HT), Advance glycation end products (AGE), and myostatin, which exert their effects by modulating the signaling pathways within bone and muscle. Chemical messengers (e.g., TNF-α, IL-6, AGE, leptins) that are upregulated or downregulated as a result of obesity have been shown to act as negative regulators of osteoblasts, osteocytes and muscles, as well as positive regulators of osteoclasts. These additive effects of obesity ultimately increase the risk for osteoporosis and muscle atrophy. The aim of this review is to identify the potential cellular mechanisms through which obesity may facilitate osteoporosis, muscle atrophy and bone fractures.

Glucose Enhances Basal or Melanocortin-Induced cAMP-Response Element Activity in Hypothalamic Cells

Melanocyte-stimulating hormone (MSH)-induced activation of the cAMP-response element (CRE) via the CRE-binding protein in hypothalamic cells promotes expression of TRH and thereby restricts food intake and increases energy expenditure. Glucose also induces central anorexigenic effects by acting on hypothalamic neurons, but the underlying mechanisms are not completely understood. It has been proposed that glucose activates the CRE-binding protein-regulated transcriptional coactivator 2 (CRTC-2) in hypothalamic neurons by inhibition of AMP-activated protein kinases (AMPKs), but whether glucose directly affects hypothalamic CRE activity has not yet been shown. Hence, we dissected effects of glucose on basal and MSH-induced CRE activation in terms of kinetics, affinity, and desensitization in murine, hypothalamic mHypoA-2/10-CRE cells that stably express a CRE-dependent reporter gene construct. Physiologically relevant increases in extracellular glucose enhanced basal or MSH-induced CRE-dependent gene transcription, whereas prolonged elevated glucose concentrations reduced the sensitivity of mHypoA-2/10-CRE cells towards glucose. Glucose also induced CRCT-2 translocation into the nucleus and the AMPK activator metformin decreased basal and glucose-induced CRE activity, suggesting a role for AMPK/CRTC-2 in glucose-induced CRE activation. Accordingly, small interfering RNA-induced down-regulation of CRTC-2 expression decreased glucose-induced CRE-dependent reporter activation. Of note, glucose also induced expression of TRH, suggesting that glucose might affect the hypothalamic-pituitary-thyroid axis via the regulation of hypothalamic CRE activity. These findings significantly advance our knowledge about the impact of glucose on hypothalamic signaling and suggest that TRH release might account for the central anorexigenic effects of glucose and could represent a new molecular link between hyperglycaemia and thyroid dysfunction.

The effect of diet interventions on hypothalamic nutrient sensing pathways in rodents

The hypothalamus plays a fundamental role in regulating homeostatic processes including regulation of food intake. Food intake is driven in part by energy balance, which is sensed by specific brain structures through signaling molecules such as nutrients and hormones. Both circulating glucose and fatty acids decrease food intake via a central mechanism involving the hypothalamus and brain stem. Besides playing a role in signaling energy status, glucose and fatty acids serve as fuel for neurons. This review focuses on the effects of glucose and fatty acids on hypothalamic pathways involved in regulation of energy metabolism as well as on the role of the family of peroxisome proliferator activated receptors (PPARs) which are implicated in regulation of central energy homeostasis. We further discuss the effects of different hypercaloric diets on these pathways.