CNS melanocortin and leptin effects on stearoyl-CoA desaturase-1 and resistin expression

Comparative Intracerebroventricular and Intrathecal Administration of a Nanomolar Macrocyclic Melanocortin Receptor Agonist MDE6-5-2c (c[Pro-His-DPhe-Arg-Trp-Dap-Ala-DPro]) Decreases Food Intake in Mice

There is a critical need to find safe therapeutics to treat an increasingly obese population and diseases associated with an imbalance in energy homeostasis. The melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) ligands have long been the focus to help scientists understand energy homeostasis and the regulation of feeding behavior. Herein, we use a nanomolar macrocyclic melanocortin receptor agonist ligand MDE6-5-2c (c[Pro-His-DPhe-Arg-Trp-Dap-Ala-DPro) to examine metabolic and energy hemostasis profiles upon intrathecal (IT) administration directly into the spinal cord as compared to intracerebroventricular (ICV) administration directly into the brain. Overall, central ICV administration of MDE6-5-2c resulted in decreased food intake, in a dose-dependent manner, and decreased respiratory exchange ratio (RER). Comparison of IT versus ICV routes of MDE6-5-2c administration resulted in MDE6-5-2c possessing a longer duration of action on both feeding behavior and RER via IT. The C-peptide, ghrelin, GIP, leptin, IL-6, and resistin plasma hormones and biomarkers were compared using IT versus ICV MDE6-5-2c routes of administration. Plasma resistin levels were decreased upon ICV treatment of MDE6-5-2c, as compared to ICV vehicle control treatment. Intrathecal treatment resulted in significantly decreased inflammatory cytokine interleukin-6 (IL-6) levels compared to ICV administration. Investigation of the nonselective MC3R and MC4R macrocyclic agonist MDE6-5-2c molecule revealed differences in food intake, RER, and plasma biomarker profiles based upon ICV or IT routes of administration and characterize this novel molecular chemotype as a molecular probe to study the melanocortin system in vivo.

Stearoyl-CoA desaturase and tumorigenesis

Stearoyl-CoA desaturase (SCD) generates monounsaturated fatty acids (MUFAs) which contribute to cell growth, survival, differentiation, metabolic regulation and signal transduction. Overexpression of SCD is evident and implicated in metabolic diseases such as diabetes and non-alcoholic fatty liver disease. SCD also stimulates canonical Wnt pathway and YAP activation in support of stemness and tumorigenesis. SCD facilitates metabolic reprogramming in cancer which is mediated, at least in part, by regulation of AKT, AMPK, and NF-kB via MUFAs. Our research has revealed the novel positive loop to amplify Wnt signaling through stabilization of LRP5/6 in both hepatic stellate cells and liver tumor-initiating stem cell-like cells. As such, this loop is pivotal in promoting liver fibrosis and liver tumor development. This review summarizes the mechanisms of SCD-mediated tumor promotion described by recent studies and discusses the future prospect for SCD-mediated signaling crosstalk as a potential therapeutic target for cancer.

A Direct in Vivo Comparison of the Melanocortin Monovalent Agonist Ac-His-DPhe-Arg-Trp-NH2 versus the Bivalent Agonist Ac-His-DPhe-Arg-Trp-PEDG20-His-DPhe-Arg-Trp-NH2: A Bivalent Advantage

Bivalent ligands targeting putative melanocortin receptor dimers have been developed and characterized in vitro; however, studies of their functional in vivo effects have been limited. The current report compares the effects of homobivalent ligand CJL-1-87, Ac-His-DPhe-Arg-Trp-PEDG20-His-DPhe-Arg-Trp-NH₂, to monovalent ligand CJL-1-14, Ac-His-DPhe-Arg-Trp-NH₂, on energy homeostasis in mice after central intracerebroventricular (ICV) administration into the lateral ventricle of the brain. Bivalent ligand CJL-1-87 had noteworthy advantages as an antiobesity probe over CJL-1-14 in a fasting-refeeding in vivo paradigm. Treatment with CJL-1-87 significantly decreased food intake compared to CJL-1-14 or saline (50% less intake 2-8 h after treatment). Furthermore, CJL-1-87 treatment decreased the respiratory exchange ratio (RER) without changing the energy expenditure indicating that fats were being burned as the primary fuel source. Additionally, CJL-1-87 treatment significantly lowered body fat mass percentage 6 h after administration (p < 0.05) without changing the lean mass percentage. The bivalent ligand significantly decreased insulin, C-peptide, leptin, GIP, and resistin plasma levels compared to levels after CJL-1-14 or saline treatments. Alternatively, ghrelin plasma levels were significantly increased. Serum stability of CJL-1-87 and CJL-1-14 (T_1/2 = 6.0 and 16.8 h, respectively) was sufficient to permit physiological effects. The differences in binding affinity of CJL-1-14 compared to CJL-1-87 are speculated as a possible mechanism for the bivalent ligand's unique effects. We also provide in vitro evidence for the formation of a MC3R-MC4R heterodimer complex, for the first time to our knowledge, that may be an unexploited neuronal molecular target. Regardless of the exact mechanism, the advantageous ability of CJL-1-87 compared to CJL-1-14 to increase in vitro binding affinity, increase the duration of action in spite of decreased serum stability, decrease in vivo food intake, decrease mice's body fat percent, and differentially affect mouse hormone levels demonstrates the distinct characteristics achieved from the current melanocortin agonist bivalent design strategy.

Resistin increases the expression of NOD2 in mouse monocytes

Previous studies have indicated that resistin, a type of adipokine, contributes to the development of insulin resistance and type 2 diabetes mellitus, and mediates inflammatory reactions. However, a specific receptor for resistin has not yet been identified. In this study, the relationship between resistin and nucleotide-binding oligomerization domain-like receptors, as well as resistin signal transduction, was examined through transfection, quantitative polymerase chain reaction, western blot analysis and ELISA. The mRNA expression of nucleotide-binding oligomerization domain-containing protein 2 (NOD2), a key immune receptor related to insulin resistance, was significantly increased by resistin treatment at concentrations of 100, 150 and 200 ng/ml (P<0.05, P<0.01 and P<0.01, respectively). The mRNA expression of downstream signaling molecules in the NOD2 signaling pathway, receptor-interacting serine/threonine-protein kinase 2 (RIP2; P<0.01 at 6, 12 and 24 h) and inhibitor of NF-κB kinase subunit beta (P<0.01 at 3, 6, 12 and 24 h) were significantly increased by resistin treatment compared with the control. The mRNA expression of key proinflammatory cytokines, tumor necrosis factor α, IL (interleukin)-6 and IL-1β, were also significantly increased by resistin treatment compared with the control (P<0.01). NOD2 knockdown by small interfering RNA (siRNA) significantly decreased the expression of NOD2 and RIP2 (P<0.01), and there was no significant increase in the levels of cytokines, as compared with treatment with control siRNA. These findings indicate that the inflammatory reaction induced by resistin involves the NOD2-nuclear factor (NF)-κB signaling pathway. The inhibition of NF-κB significantly decreased the secretion of key inflammatory cytokines (P<0.01), suggesting that NF-κB signaling mechanisms are essential to the resistin-induced inflammatory response.

Hypothalamic control of hepatic lipid metabolism via the autonomic nervous system

Our body is well designed to store energy in times of nutrient excess, and release energy in times of food deprivation. This adaptation to the external environment is achieved by humoral factors and the autonomic nervous system. Claude Bernard, in the 19th century, showed the importance of the autonomic nervous system in the control of glucose metabolism. In the 20th century, the discovery of insulin and the development of techniques to measure hormone concentrations shifted the focus from the neural control of metabolism to the secretion of hormones, thus functionally "decapitating" the body. Just before the end of the 20th century, starting with the discovery of leptin in 1994, the control of energy metabolism went back to our heads. Since the start of 21st century, numerous studies have reported the involvement of hypothalamic pathways in the control of hepatic insulin sensitivity and glucose production. The autonomic nervous system is, therefore, acknowledged to be one of the important determinants of liver metabolism and a possible treatment target. In this chapter, we review research to date on the hypothalamic control of hepatic lipid metabolism.

The PVH as a site of CB1-mediated stimulation of thermogenesis by MC4R agonism in male rats

The present study was designed to investigate the involvement of the cannabinoid receptor 1 (CB1) in the stimulating effects of the melanocortin-4 receptor (MC4R) agonism on whole-body and brown adipose tissue (BAT) thermogenesis. In a first series of experiments, whole-body and BAT thermogenesis were investigated in rats infused in the third ventricle of the brain with the MC4R agonist melanotan II (MTII) and the CB1 agonist δ9-tetrahydrocannabinol (δ(9)-THC) or the CB1 antagonist AM251. Whole-body thermogenesis was measured by indirect calorimetry and BAT thermogenesis assessed from interscapular BAT (iBAT) temperature. δ(9)-THC blunted the effects of MTII on energy expenditure and iBAT temperature, whereas AM251 tended to potentiate the MTII effects. δ(9)-THC also blocked the stimulating effect of MTII on (14)C-bromopalmitate and (3)H-deoxyglucose uptakes in iBAT. Additionally, δ(9)-THC attenuated the stimulating effect of MTII on the expression of peroxisome proliferator-activated receptor-γ coactivator 1-α (Pgc1α), type II iodothyronine deiodinase (Dio2), carnitine palmitoyltransferase 1B (Cpt1b), and uncoupling protein 1 (Ucp1). In a second series of experiments, we addressed the involvement of the paraventricular hypothalamic nucleus (PVH) in the CB1-mediated effects of MTII on iBAT thermogenesis, which were assessed following the infusion of MTII in the PVH and δ(9)-THC or AM251 in the fourth ventricle of the brain. We demonstrated the ability of δ(9)-THC to blunt MTII-induced iBAT temperature elevation. δ(9)-THC also blocked the PVH effect of MTII on (14)C-bromopalmitate uptake as well as on Pgc1α and Dio2 expression in iBAT. Altogether the results of this study demonstrate the involvement of the PVH in the CB1-mediated stimulating effects of the MC4R agonist MTII on whole-body and BAT thermogenesis.

Role of leptin resistance in the development of obesity in older patients

Obesity is a global epidemic associated with aging-like cellular processes; in both aging and obesity, resistance to hormones such as insulin and leptin can be observed. Leptin is a circulating hormone/cytokine with central and peripheral effects that is released mainly by subcutaneous white adipose tissue. Centrally, leptin controls food intake, energy expenditure, and fat distribution, whereas it controls (among several others) insulin sensitivity, free fatty acids (FFAs) oxidation, and lipolysis in the periphery. Aging is associated with important changes in both the distribution and the composition of adipose tissue. Fat is redistributed from the subcutaneous to the visceral depot and increased inflammation participates in adipocyte dysfunction. This redistribution of adipose tissue in favor of visceral fat influences negatively both longevity and healthy aging as shown in numerous animal models. These modifications observed during aging are also associated with leptin resistance. This resistance blunts normal central and peripheral functions of leptin, which leads to a decrease in neuroendocrine function and insulin sensitivity, an imbalance in energy regulation, and disturbances in lipid metabolism. Here, we review how age-related leptin resistance triggers metabolic disturbances and affects the longevity of obese patients. Furthermore, we discuss the potential impacts of leptin resistance on the decline of brown adipose tissue thermogenesis observed in elderly individuals.

Adipose tissue stearoyl-CoA desaturase 1 index is increased and linoleic acid is decreased in obesity-prone rats fed a high-fat diet

Background

Fatty acid (FA) composition and desaturase indices are associated with obesity and related metabolic conditions. However, it is unclear to what extent desaturase activity in different lipid fractions contribute to obesity susceptibility. Our aim was to test whether desaturase activity and FA composition are linked to an obese phenotype in rats that are either obesity prone (OP) or resistant (OR) on a high-fat diet (HFD).

Methods

Two groups of Sprague–Dawley rats were given ad libitum (AL-HFD) or calorically restricted (HFD-paired; pair fed to calories consumed by chow-fed rats) access to a HFD. The AL-HFD group was categorized into OP and OR sub-groups based on weight gain over 5 weeks. Five different lipid fractions were examined in OP and OR rats with regard to proportions of essential and very long-chain polyunsaturated FAs: linoleic acid (LA), alpha-linolenic acid, eicosapentaenoic acid, docosahexaenoic acid and the stearoyl-CoA desaturase 1 (SCD-1) product 16:1n-7. FA ratios were used to estimate activities of the delta-5-desaturase (20:4n-6/20:3n-6), delta-6-desaturase (18:3n-6/18:2n-6), stearoyl-CoA desaturase 1 (SCD-1; 16:1n-7/16:0, SCD-16 and 18:1n-9/18:0, SCD-18), de novo lipogenesis (16:0/18:2n-6) and FA elongation (18:0/16:0). Fasting insulin, glucose, adiponectin and leptin concentrations were measured in plasma.

Results

After AL-HFD access, OP rats had a significantly higher SCD-16 index and 16:1n-7 proportion, but a significantly lower LA proportion, in subcutaneous adipose tissue (SAT) triacylglycerols, as well as significantly higher insulin and leptin concentrations, compared with OR rats. No differences were found between the two phenotypes in liver (phospholipids; triacylglycerols) or plasma (cholesterol esters; phospholipids) lipid fractions or for plasma glucose or adiponectin concentrations. For the desaturase indices of the HFD-paired rats, the only significant differences compared with the OP or OR rats were higher SCD-16 and SCD-18 indices in SAT triacylglycerols in OP compared with HFD-paired rats.

Conclusion

The higher SCD-16 may reflect higher SCD-1 activity in SAT, which in combination with lower LA proportions may reflect higher insulin resistance and changes in SAT independent of other lipid fractions. Whether a lower SCD-16 index protects against diet-induced obesity is an interesting possibility that warrants further investigation.

Deletion of prolyl carboxypeptidase attenuates the metabolic effects of diet-induced obesity

α-Melanocyte-stimulating hormone (α-MSH) is a critical regulator of energy metabolism. Prolyl carboxypeptidase (PRCP) is an enzyme responsible for its degradation and inactivation. PRCP-null mice (PRCP(gt/gt)) showed elevated levels of brain α-MSH, reduced food intake, and a leaner phenotype compared with wild-type controls. In addition, they were protected against diet-induced obesity. Here, we show that PRCP(gt/gt) animals have improved metabolic parameters compared with wild-type controls under a standard chow diet (SD) as well as on a high-fat diet (HFD). Similarly to when they are exposed to SD, PRCP(gt/gt) mice exposed to HFD for 13 wk showed a leaner phenotype due to decreased fat mass, increased energy expenditure, and locomotor activity. They also showed improved insulin sensitivity and glucose tolerance compared with WT controls and a significant reduction in fasting glucose levels. These improvements occured before changes in body weight and composition were evident, suggesting that the beneficial effect of PRCP ablation is independent of the adiposity levels. In support of a reduced gluconeogenesis, liver PEPCK and G-6-Pase mRNA levels were reduced significantly in PRCP(gt/gt) compared with WT mice. A significant decrease in liver weight and hepatic triglycerides were also observed in PRCP(gt/gt) compared with WT mice. Altogether, our data suggest that PRCP is an important regulator of energy and glucose homeostasis since its deletion significantly improves metabolic parameters in mice exposed to both SD and HFD.

Glycine normalizes hepatic triglyceride-rich VLDL secretion by triggering the CNS in high-fat fed rats

RATIONALE

Dysregulation of hepatic triglyceride (TG)-rich very low-density lipoproteins (VLDL-TG) in obesity and type 2 diabetes contributes to the dyslipidemia that leads to cardiovascular morbidity. The central nervous system (CNS), particularly the hypothalamus, regulates hepatic lipid metabolism. Although the underlying neurocircuitry remains elusive, glycine has been documented to enhance CNS N-methyl-d-aspartate (NMDA) receptor-mediated transmission.

OBJECTIVE

We tested the hypothesis that glycine regulates hepatic VLDL-TG secretion by potentiating NMDA receptor-mediated transmission in the CNS.

METHODS AND RESULTS

Using 10-hour fasted male Sprague-Dawley rats implanted with stereotaxic cannulae into an extrahypothalamic region termed the dorsal vagal complex (DVC) and vascular catheters to enable direct DVC infusion and blood sampling, respectively, the rate of hepatic VLDL-TG secretion was measured following tyloxapol (an inhibitor of lipoprotein lipase) injection. Direct DVC infusion of glycine lowered VLDL-TG secretion, whereas NMDA receptor blocker MK-801 fully negated glycine's effect. NR1 subunit of NMDA receptor antagonist 7-chlorokynurenic acid, adenoviral injection of NR1 short hairpin RNA (shRNA), and hepatic vagotomy also nullified glycine's effect. Finally, DVC glycine normalized the hypersecretion of VLDL-TG induced by high-fat feeding.

CONCLUSIONS

Molecular and pharmacological inhibition of the NR1-containing NMDA receptors in the DVC negated the ability of glycine to inhibit hepatic secretion of VLDL-TG in vivo. Importantly, the hypersecretion of VLDL-TG from the liver induced by a model of high-fat feeding was restored by the hepatic lipid control of CNS glycine sensing. These findings collectively suggest that glycine or glycine analogues may have therapeutic benefits in lowering plasma lipid levels in diabetes and obesity by triggering the CNS.

Nutrigenetics of the lipoprotein metabolism

It is well known that lipid metabolism is a cornerstone in the development of the commonest important chronic diseases worldwide, such as obesity, cardiovascular disease, or metabolic syndrome. In this regard, the area of lipid and lipoprotein metabolism is one of the areas in which the understanding of the development and progression of those metabolic disorders has been studied in greater depth. Thus, growing evidence has demonstrated that while universal recommendations might be appropriate for the general population, in this area there is great variability among individuals, related to a combination of environmental and genetic factors. Moreover, the interaction between genetic and dietary components has helped in understanding this variability. Therefore, with further study into the interaction between the most important genetic markers or single-nucleotide polymorphisms (SNPs) and diet, it may be possible to understand the variability in lipid metabolism, which could lead to an increase in the use of personalized nutrition as the best support to combat metabolic disorders. This review discusses some of the evidence in which candidate SNPs can affect the key players of lipid metabolism and how their phenotypic manifestations can be modified by dietary intake.

Central insulin and leptin-mediated autonomic control of glucose homeostasis

Largely as a result of rising obesity rates, the incidence of type 2 diabetes is escalating rapidly. Type 2 diabetes results from multi-organ dysfunctional glucose metabolism. Recent publications have highlighted hypothalamic insulin- and adipokine-sensing as a major determinant of peripheral glucose and insulin responsiveness. The preponderance of evidence indicates that the brain is the master regulator of glucose homeostasis, and that hypothalamic insulin and leptin signaling in particular play a crucial role in the development of insulin resistance. This review discusses the neuronal crosstalk between the hypothalamus, autonomic nervous system, and tissues associated with the pathogenesis of type 2 diabetes, and how hypothalamic insulin and leptin signaling are integral to maintaining normal glucose homeostasis.

A variant near the melanocortin-4 receptor gene regulates postprandial lipid metabolism in a healthy Caucasian population

The melanocortin-4 receptor (MC4R) is an essential regulator of food intake and energy homeostasis. Previous data suggest an influence of MC4R activity on TAG levels. Thus, the aim of the present study was to determine whether the presence of the rs12970134 polymorphism near the MC4R gene could influence postprandial lipoprotein metabolism in healthy subjects. A total of eighty-eight volunteers were selected, fifty-three homozygous for the common genotype (G/G) and thirty-five carriers for the minor A-allele (G/A and A/A). They were given a fat-rich meal containing 1 g fat and 7 mg cholesterol/kg body weight and vitamin A (60,000 IU/m(2) body surface). Fat accounted for 60 % of energy, and protein and carbohydrates accounted for 15 and 25 % of energy, respectively. Blood samples were taken at time 0, every 1 h until 6 h and every 2·5 h until 11 h. Total cholesterol and TAG in plasma, and cholesterol, TAG and retinyl palmitate in TAG-rich lipoproteins (TRL, large and small TRL) were separated by ultracentrifugation. Individuals carrying the G/G genotype displayed a higher postprandial response of plasma TAG (P = 0·033), total cholesterol (P = 0·019) and large TRL-TAG (P = 0·023) than did carriers of the minor A-allele. Furthermore, G/G subjects showed a greater postprandial response of small TRL-apoB48 than did carriers of the A-allele (P = 0·032). These results suggest that the rs12970134 polymorphism near the MC4R gene region may partly explain the inter-individual differences in postprandial lipoprotein response in healthy subjects.

Regulation of insulin-stimulated glucose uptake in rat white adipose tissue upon chronic central leptin infusion: effects on adiposity

Leptin enhances the glucose utilization in most insulin target tissues and paradoxically decreases it in white adipose tissue (WAT), but knowledge of the mechanisms underlying the inhibitory effect of central leptin on the insulin-dependent glucose uptake in WAT is limited. After 7 d intracerebroventricular leptin treatment (0.2 μg/d) of rats, the overall insulin sensitivity and the responsiveness of WAT after acute in vivo insulin administration were analyzed. We also performed unilateral WAT denervation to clarify the role of the autonomic nervous system in leptin effects on the insulin-stimulated [(3)H]-2-deoxyglucose transport in WAT. Central leptin improved the overall insulin sensitivity but decreased the in vivo insulin action in WAT, including insulin receptor autophosphorylation, insulin receptor substrate-1 tyrosine-phosphorylation, and Akt activation. In this tissue, insulin receptor substrate-1 and glucose transporter 4 mRNA and protein levels were down-regulated after central leptin treatment. Additionally, a remarkable up-regulation of resistin, together with an augmented expression of suppressor of cytokine signaling 3 in WAT, was also observed in leptin-treated rats. As a result, the insulin-stimulated glucose transporter 4 insertion at the plasma membrane and the glucose uptake in WAT were impaired in leptin-treated rats. Finally, denervation of WAT abolished the inhibitory effect of central leptin on glucose transport and decreased suppressor of cytokine signaling 3 and resistin levels in this tissue, suggesting that resistin, in an autocrine/paracrine manner, might be a mediator of central leptin antagonism of insulin action in WAT. We conclude that central leptin, inhibiting the insulin-stimulated glucose uptake in WAT, may regulate glucose availability for triacylglyceride formation and accumulation in this tissue, thereby contributing to the control of adiposity.

Hypothalamic control of lipid metabolism: focus on leptin, ghrelin and melanocortins

The hypothalamus plays a crucial role in the regulation of food intake and energy expenditure. One of the main regulatory factors within the hypothalamus is AMP-activated protein kinase (AMPK), which is involved in a large number of biological actions including the modulation of energy balance. Leptin and ghrelin-induced changes in hypothalamic AMPK lead to important alterations in hypothalamic fatty acid metabolism. Furthermore, it is well known that the hypothalamus controls peripheral lipid metabolism through the sympathetic nervous system, and those actions are independent of food intake. In this short review, we highlight the main molecular pathways triggered by leptin and ghrelin altering both central and peripheral lipid metabolism and, therefore, controlling feeding behavior and energy expenditure.

Hypothalamic control of energy metabolism via the autonomic nervous system

The hypothalamic control of hepatic glucose production is an evident aspect of energy homeostasis. In addition to the control of glucose metabolism by the circadian timing system, the hypothalamus also serves as a key relay center for (humoral) feedback information from the periphery, with the important role for hypothalamic leptin receptors as a striking example. The hypothalamic biological clock uses its projections to the preautonomic hypothalamic neurons to control the daily rhythms in plasma glucose concentration, glucose uptake, and insulin sensitivity. Euglycemic, hyperinsulinemic clamp experiments combined with either sympathetic-, parasympathetic-, or sham-denervations of the autonomic input to the liver have further delineated the hypothalamic pathways that mediate the control of the circadian timing system over glucose metabolism. In addition, these experiments clearly showed both that next to the biological clock peripheral hormones may "use" the preautonomic neurons in the hypothalamus to affect hepatic glucose metabolism, and that similar pathways may be involved in the control of lipid metabolism in liver and white adipose tissue.

Gene expression in hypothalamus, liver, and adipose tissues and food intake response to melanocortin-4 receptor agonist in pigs expressing melanocortin-4 receptor mutations

Transcriptional profiling was used to identify genes and pathways that responded to intracerebroventricular injection of melanocortin-4 receptor (MC4R) agonist [Nle4, d-Phe7]-α-melanocyte stimulating hormone (NDP-MSH) in pigs homozygous for the missense mutation in the MC4R, D298 allele ( n = 12), N298 allele ( n = 12), or heterozygous ( n = 12). Food intake (FI) was measured at 12 and 24 h after treatment. All pigs were killed at 24 h after treatment, and hypothalamus, liver, and back-fat tissue was collected. NDP-MSH suppressed ( P < 0.004) FI at 12 and 24 h in all animals after treatment. In response to NDP-MSH, 278 genes in hypothalamus ( q ≤ 0.07, P ≤ 0.001), 249 genes in liver ( q ≤ 0.07, P ≤ 0.001), and 5,066 genes in fat ( q ≤ 0.07, P ≤ 0.015) were differentially expressed. Pathway analysis of NDP-MSH-induced differentially expressed genes indicated that genes involved in cell communication, nucleotide metabolism, and signal transduction were prominently downregulated in the hypothalamus. In both liver and adipose tissue, energy-intensive biosynthetic and catabolic processes were downregulated in response to NDP-MSH. This included genes encoding for biosynthetic pathways such as steroid and lipid biosynthesis, fatty acid synthesis, and amino acid synthesis. Genes involved in direct energy-generating processes, such as oxidative phosphorylation, electron transport, and ATP synthesis, were upregulated, whereas TCA-associated genes were prominently downregulated in NDP-MSH-treated pigs. Our data also indicate a metabolic switch toward energy conservation since genes involved in energy-intensive biosynthetic and catabolic processes were downregulated in NDP-MSH-treated pigs.

Regulation of lipid metabolism by energy availability: a role for the central nervous system

The central nervous system (CNS) is crucial in the regulation of energy homeostasis. Many neuroanatomical studies have shown that the white adipose tissue (WAT) is innervated by the sympathetic nervous system, which plays a critical role in adipocyte lipid metabolism. Therefore, there are currently numerous reports indicating that signals from the CNS control the amount of fat by modulating the storage or oxidation of fatty acids. Importantly, some CNS pathways regulate adipocyte metabolism independently of food intake, suggesting that some signals possess alternative mechanisms to regulate energy homeostasis. In this review, we mainly focus on how neuronal circuits within the hypothalamus, such as leptin- ghrelin-and resistin-responsive neurons, as well as melanocortins, neuropeptide Y, and the cannabinoid system exert their actions on lipid metabolism in peripheral tissues such as WAT, liver or muscle. Dissecting the complicated interactions between peripheral signals and neuronal circuits regulating lipid metabolism might open new avenues for the development of new therapies preventing and treating obesity and its associated cardiometabolic sequelae.

Hypothalamic nutrient sensing in the control of energy homeostasis

The hypothalamus is a center of convergence and integration of multiple nutrient-related signals. It can sense changes in circulating adiposity hormones, gastric hormones and nutrients, and receives neuroanatomical projections from other nutrient sensors, mainly within the brainstem. The hypothalamus also integrates these signals with various cognitive forebrain-descending information and reward/motivation-related signals coming from the midbrain-dopamine system, to coordinate neuroendocrine, behavioral and metabolic effectors of energy balance. Some of the key nutrient-sensing hypothalamic neurons have been identified in the arcuate, the ventro-medial and the lateral nuclei of the hypothalamus, and the molecular mechanisms underlying intracellular integration of nutrient-related signals in these neurons are currently under intensive investigation. However, little is known about the neural pathways downstream from hypothalamic nutrient sensors, and how they drive effectors of energy homeostasis under physiological conditions. This manuscript will review recent progress from molecular, genetic and neurophysiological studies that identify and characterize the critical intracellular signalling pathways and neurocircuits involved in determining hypothalamic nutrient detection, and link these circuits to behavioral and metabolic effectors of energy balance. We will provide a critical analysis of current data to identify ongoing challenges for future research in this field.

Analysis of the therapeutic functions of novel melanocortin receptor agonists in MC3R- and MC4R-deficient C57BL/6J mice

Melanocortin receptor agonists act in the brain to regulate food intake and body weight and, independently of these actions, affect insulin sensitivity. These experiments investigated the function of novel non-selective melanocortin receptor agonists (BIM-22493, BIM-22511) that cross the blood-brain barrier when administered peripherally. Treatment of diet induced obese C57BL/6J (B6) mice with melanocortin agonists administered peripherally improved obesity, hyperinsulinemia (approximately 50%) and fatty liver disease. Specificity of function was determined using B6 melanocortin-3 and melanocortin-4 receptor knockout mice (MC3RKO, MC4RKO). Chow fed MC4RKO but not MC3RKO used for these tests exhibited obesity, hyperinsulinemia and severe hepatosteatosis associated with increased expression of insulin-stimulated genes involved in lipogenesis. Reduced food intake associated with acute BIM-22493 treatment, and weight loss associated with 14 days of treatment with BIM-22511, required functional MC4R but not MC3R. However, while 14 days of treatment with BIM-22511 did not affect body weight and even increased cumulative food intake in MC4RKO, a significant reduction (approximately 50%) in fasting insulin was still observed. Despite lowering insulin, chronic treatment with BIM-22511 did not improve hepatosteatosis in MC4RKO, and did not affect hepatic lipogenic gene expression. Together, these results demonstrate that peripherally administered melanocortin receptor agonists regulate body weight, liver metabolism and glucose homeostasis through independent pathways. MC4R are necessary for melanocortin agonist-induced weight loss and improvements in liver metabolism, but are not required for improvements in hyperinsulinemia. Agonists with activity at MC4R improve glucose homeostasis at least partially by causing weight loss, however other melanocortin receptors may have potential for treating aberrations in glucose homeostasis associated with obesity.

Role of stearoyl-coenzyme A desaturase in regulating lipid metabolism

PURPOSE OF REVIEW

Stearoyl-coenzyme A desaturase 1 is a delta-9 fatty acid desaturase that catalyzes the synthesis of monounsaturated fatty acids and has emerged as a key regulator of metabolism. This review evaluates the latest advances in our understanding of the pivotal role of stearoyl-coenzyme A desaturase 1 in health and disease.

RECENT FINDINGS

Scd1-deficient mice have reduced lipid synthesis and enhanced lipid oxidation, thermogenesis and insulin sensitivity in various tissues including liver, muscle and adipose tissue due to transcriptional and posttranscriptional effects. These metabolic changes protect Scd1-deficient mice from a variety of dietary, pharmacological and genetic conditions that promote obesity, insulin resistance and hepatic steatosis. Stearoyl-coenzyme A desaturase 1 is required to guard against dietary unsaturated fat deficiency, leptin deficiency-induced diabetes, and palmitate-induced lipotoxic insults in muscle and pancreatic beta-cells. Paradoxical observations of increased muscle stearoyl-coenzyme A desaturase 1 during obesity, starvation and exercise raise questions as to the role of stearoyl-coenzyme A desaturase 1 in this tissue. Mice with a liver-specific loss of stearoyl-coenzyme A desaturase 1, and inhibition of stearoyl-coenzyme A desaturase 1 via antisense or RNA interference, recapitulate only a subset of the phenotypes observed in global Scd1 deficiency, indicating the involvement of multiple tissues.

SUMMARY

Recent studies in humans and animal models have highlighted that modulation of stearoyl-coenzyme A desaturase 1 activity by dietary intervention or genetic manipulation strongly influences several facets of energy metabolism to affect susceptibility to obesity, insulin resistance, diabetes and hyperlipidemia.

Central nervous system regulation of adipocyte metabolism

The white adipose tissue was initially largely known only as an energy storage tissue. It is now well recognized that white adipose tissue is a major endocrine and secretory organ, which releases a wide range of protein signals and factors termed adipokines. The regulation of adipocyte metabolism is an important factor for the understanding of obesity, and some mechanisms are still unknown. Many homeostatic processes, including appetite and food intake, are controlled by neuroendocrine circuits involving the central nervous system. There is substantial evidence demonstrating that the central nervous system also directly regulates adipocyte metabolism. In this review, we discuss the central actions of some peptides with an important role in energy balance regulation on adipocyte metabolism and the physiological relevance of these actions.

Tissue-specific effects of central leptin on the expression of genes involved in lipid metabolism in liver and white adipose tissue

Leptin reduces adiposity and exerts antisteatotic effects on nonadipose tissues. However, the mechanisms underlying leptin effects on lipid metabolism in liver and white adipose tissue have not been fully clarified. Here, we have studied the effects of central leptin administration on key enzymes and transcription factors involved in lipid metabolism in liver and epididymal adipose tissue. Intracerebroventricular leptin infusion for 7 d did not change leptin plasma levels but decreased triacylglyceride content in liver, epididymal adipose tissue, and plasma. In both tissues this treatment markedly decreased the expression of key enzymes of the de novo fatty acid (FA) synthesis such as acetyl-coenzyme A-carboxylase, FA synthase, and stearoyl-coenzyme A desaturase-1, in parallel with a reduction in mRNA expression of sterol regulatory element binding protein-1c in liver and carbohydrate regulatory element binding protein in adipose tissue. In addition, leptin also decreased phosphoenol-pyruvate carboxykinase-C expression in adipose tissue, an enzyme involved in glyceroneogenesis in this tissue. Central leptin administration down-regulates delta-6-desaturase expression in liver and adipose tissue, in parallel with the decrease of the expression of sterol regulatory element binding protein-1c in liver and peroxisome proliferator activated receptor alpha in adipose tissue. Finally, leptin treatment, by regulating adipose triglyceride lipase/hormone sensitive lipase/diacylglycerol transferase 1 expression, also established a new partitioning in the FA-triacylglyceride cycling in adipose tissue, increasing lipolysis and probably the FA efflux from this tissue, and favoring in parallel the FA uptake and oxidation in the liver. These results suggest that leptin, acting at central level, exerts tissue-specific effects in limiting fat tissue mass and lipid accumulation in nonadipose tissues, preventing the development of obesity and type 2 diabetes.

Transgenic MSH overexpression attenuates the metabolic effects of a high-fat diet

To determine whether long-term melanocortinergic activation can attenuate the metabolic effects of a high fat diet, mice overexpressing an NH(2)-terminal POMC transgene that includes alpha- and gamma(3)-MSH were studied on either a 10% low-fat diet (LFD) or 45% high-fat diet (HFD). Weight gain was modestly reduced in transgenic (Tg-MSH) male and female mice vs. wild type (WT) on HFD (P < 0.05) but not LFD. Substantial reductions in body fat percentage were found in both male and female Tg-MSH mice on LFD (P < 0.05) and were more pronounced on HFD (P < 0.001). These changes occurred in the absence of significant feeding differences in most groups, consistent with effects of Tg-MSH on energy expenditure and partitioning. This is supported by indirect calorimetry studies demonstrating higher resting oxygen consumption and lower RQ in Tg-MSH mice on the HFD. Tg-MSH mice had lower fasting insulin levels and improved glucose tolerance on both diets. Histological and biochemical analyses revealed that hepatic fat accumulation was markedly reduced in Tg-MSH mice on the HFD. Tg-MSH also attenuated the increase in corticosterone induced by the HFD. Higher levels of Agrp mRNA, which might counteract effects of the transgene, were measured in Tg-MSH mice on LFD (P = 0.02) but not HFD. These data show that long-term melanocortin activation reduces body weight, adiposity, and hepatic fat accumulation and improves glucose metabolism, particularly in the setting of diet-induced obesity. Our results suggest that long-term melanocortinergic activation could serve as a potential strategy for the treatment of obesity and its deleterious metabolic consequences.

Cholestasis and hypercholesterolemia in SCD1-deficient mice fed a low-fat, high-carbohydrate diet

Stearoyl-coenzyme A desaturase 1-deficient (SCD1(-/-)) mice have impaired MUFA synthesis. When maintained on a very low-fat (VLF) diet, SCD1(-/-) mice developed severe hypercholesterolemia, characterized by an increase in apolipoprotein B (apoB)-containing lipoproteins and the appearance of lipoprotein X. The rate of LDL clearance was decreased in VLF SCD1(-/-) mice relative to VLF SCD1(+/+) mice, indicating that reduced apoB-containing lipoprotein clearance contributed to the hypercholesterolemia. Additionally, HDL-cholesterol was dramatically reduced in these mice. The presence of increased plasma bile acids, bilirubin, and aminotransferases in the VLF SCD1(-/-) mice is indicative of cholestasis. Supplementation of the VLF diet with MUFA- and PUFA-rich canola oil, but not saturated fat-rich hydrogenated coconut oil, prevented these plasma phenotypes. However, dietary oleate was not as effective as canola oil in reducing LDL-cholesterol, signifying a role for dietary PUFA deficiency in the development of this phenotype. These results indicate that the lack of SCD1 results in an increased requirement for dietary unsaturated fat to compensate for impaired MUFA synthesis and to prevent hypercholesterolemia and hepatic dysfunction. Therefore, endogenous MUFA synthesis is essential during dietary unsaturated fat insufficiency and influences the dietary requirement of PUFA.

Leptin suppresses stearoyl-CoA desaturase 1 by mechanisms independent of insulin and sterol regulatory element-binding protein-1c

Stearoyl-CoA desaturase (SCD)1 catalyzes the rate-limiting reaction of monounsaturated fatty acid (MUFA) synthesis and plays an important role in the development of obesity. SCD1 is suppressed by leptin but induced by insulin. We have used animal models to dissect the effects of these hormones on SCD1. In the first model, leptin-deficient ob/ob mice were treated with either leptin alone or with both leptin and insulin to prevent the leptin-mediated fall in insulin. In the second model, mice with a liver-specific knockout of the insulin receptor (LIRKO) and their littermate controls (LOXs) were treated with leptin. As expected, leptin decreased SCD1 transcript, protein, and activity by >60% in ob/ob and LOX mice. However, the effects of leptin were not diminished by the continued presence of hyperinsulinemia in ob/ob mice treated with both leptin and insulin or the absence of insulin signaling in LIRKO mice. Furthermore, genetic knockout of sterol regulatory element-binding protein (SREBP)-1c, the lipogenic transcription factor that mediates the effects of insulin on SCD1, also had no effect on the ability of leptin to decrease either SCD1 transcript or activity. Thus, the effect of leptin on SCD1 in liver is independent of insulin and SREBP-1c, and leptin, rather than insulin, is the major regulator of hepatic MUFA synthesis in obesity-linked diabetes.

Adipokines: implications for female fertility and obesity

Obesity is associated with a diverse set of metabolic disorders, and has reproductive consequences that are complex and not well understood. The adipose tissue-produced leptin has dominated the literature with regards to female fertility complications, but it is pertinent to explore the likely role of other adipokines--adiponectin and resistin--as our understanding of their biological functions emerge. Leptin influences the developing embryo, the functioning of the ovary and the endometrium, interacts with the release and activity of gonadotrophins and the hormones that control their synthesis. In this review such biological actions and potential roles of the adipokines leptin, adiponectin and resistin are explored in relation to female fertility and the complexity of the obese metabolic state.

The 103I variant of the melanocortin 4 receptor is associated with low serum triglyceride levels

CONTEXT

The melanocortin 4 receptor (MC4R) is an essential regulator of energy intake and body weight. Recently, the V103I polymorphism of MC4R has been shown to be negatively associated with body mass index. This suggests that serum lipids and blood pressure in individuals carrying the 103I allele might be influenced as well.

OBJECTIVE

The objective of this study was to determine whether the most common polymorphism of the MC4R, V103I, affects serum lipid levels and/or blood pressure.

DESIGN, SETTING, AND PARTICIPANTS

The study participants were 1173 consecutive patients undergoing cardiac catheterization; they were genotyped for the rs2229616 G-->A substitution at codon 103 (V103I polymorphism) of the MC4R gene. Patients had strictly fasted for at least 12 h before blood samples were drawn. The average age of the patients was 60.9 yr; 72% were males.

MAIN OUTCOME MEASURES

The main outcome measures were body mass index, serum lipids, aortic and systolic blood pressure, and MC4R polymorphism V103I.

RESULTS

Heterozygous carriers of the 103I allele had significantly lower triglyceride levels than individuals homozygous for the wild-type allele (127 vs. 168 mg/dl mean total triglyceride; P = 0.001 or 0.009 after Bonferroni adjustment for seven tests). No homozygous carriers of the 103I allele were present in the study population.

CONCLUSIONS

Our study suggests an influence of MC4R activity on triglyceride levels in cardiovascular patients.

Roles for melanocortins and leptin in adipose tissue apoptosis and fat deposition

Leptin has a wide range of effects on physiological functions related to the regulation of body energy balance. Many of leptin's effects are mediated through neuropeptide-containing neurons and neuropeptide receptors in the hypothalamus. The melanocortin system includes both agonist (alpha-melanocyte stimulating hormone, alphaMSH) and antagonist peptides (agouti related peptide, AGRP). Increased melanocortin receptor stimulation following leptin administration plays an important role in leptin-induced hypophagia and increased sympathetic nervous system activity and is partly responsible for leptin-induced weight loss. However, melanocortins do not appear to mediate some of the more striking centrally-mediated effects of leptin on adipose tissue, including adipose tissue apoptosis, that lead to the extensive depletion of fat.

[Visceral, subcutaneous or intramuscular fat: where is the problem?]

The adipose tissue is a dynamic organ that secrets several factors, denominated adipokines. They are associated, directly or indirectly, in a process that contributes to atherosclerosis, hypertension, insulinic resistance and diabetes type 2, dyslipidemias, presenting the link between adiposity, metabolic syndrome and cardiovascular diseases. In the obesity, body fat depots are increased, presenting eventual elevation in the adipokines expression and secretion. The different fat depots, visceral, abdominal subcutaneous, gluteal-femoral subcutaneous and intramuscular adipose tissue, have different metabolic and endocrine degrees, interfering, therefore, with specific form in the process associated with body adiposity in obese and diabetics subjects. The present study seeks to discuss the endocrine and metabolic role of each adipose tissue compartment, by way to assess their contribution to the complications linked to obesity.

Site and mechanism of leptin action in a rodent form of congenital lipodystrophy

Lipodystrophy is characterized by the complete or partial absence of adipose tissue, insulin resistance, hepatic steatosis, and leptin deficiency. Here, we show that low-dose central leptin corrects the insulin resistance and fatty liver of lipodystrophic aP2-nSREBP-1c mice, while the same dose given peripherally does not. Central leptin also repressed stearoyl-CoA desaturase-1 (SCD-1) RNA and enzymatic activity, which were increased in livers of lipodystrophic mice. aP2-nSREBP-1c mice homozygous for an SCD-1 deletion had markedly reduced hepatic steatosis, increased saturated fatty acids, decreased acetyl-CoA carboxylase activity, and decreased malonyl-CoA levels in the liver. Despite the reduction in hepatic steatosis, these mice remained diabetic. A leptin dose-response curve showed that subcutaneous leptin improved hyperglycemia and hyperinsulinemia in aP2-nSREBP-1c mice at doses that did not substantially alter hepatic steatosis or hepatic SCD enzymatic activity. Leptin treatment at this dose improved insulin-stimulated insulin receptor and insulin receptor substrate 2 (IRS-2) phosphorylation, IRS-2-associated PI3K activity, and Akt activity in liver. Together, these data suggest that CNS-mediated repression of SCD-1 contributes to leptin's antisteatotic actions. Intracerebroventricular leptin improves glucose homeostasis by improving insulin signal transduction in liver, but in this case the effect appears to be independent of SCD-1.

Site and mechanism of leptin action in a rodent form of congenital lipodystrophy

Lipodystrophy is characterized by the complete or partial absence of adipose tissue, insulin resistance, hepatic steatosis, and leptin deficiency. Here, we show that low-dose central leptin corrects the insulin resistance and fatty liver of lipodystrophic aP2-nSREBP-1c mice, while the same dose given peripherally does not. Central leptin also repressed stearoyl-CoA desaturase-1 (SCD-1) RNA and enzymatic activity, which were increased in livers of lipodystrophic mice. aP2-nSREBP-1c mice homozygous for an SCD-1 deletion had markedly reduced hepatic steatosis, increased saturated fatty acids, decreased acetyl-CoA carboxylase activity, and decreased malonyl-CoA levels in the liver. Despite the reduction in hepatic steatosis, these mice remained diabetic. A leptin dose-response curve showed that subcutaneous leptin improved hyperglycemia and hyperinsulinemia in aP2-nSREBP-1c mice at doses that did not substantially alter hepatic steatosis or hepatic SCD enzymatic activity. Leptin treatment at this dose improved insulin-stimulated insulin receptor and insulin receptor substrate 2 (IRS-2) phosphorylation, IRS-2-associated PI3K activity, and Akt activity in liver. Together, these data suggest that CNS-mediated repression of SCD-1 contributes to leptin's antisteatotic actions. Intracerebroventricular leptin improves glucose homeostasis by improving insulin signal transduction in liver, but in this case the effect appears to be independent of SCD-1.