Recent developments in our understanding of melanocortin system in the regulation of food intake

Morphological study of neuropeptide Y expression in human and mouse anterior insular cortex: Overexpression in the insular cortex and nucleus accumbens in obese mice on a long-term obesogenic diet

BACKGROUND

The anterior lobe of the insular cortex (aINS) is a cortical region that has reciprocal connections with limbic centers such as the anterior cingulate cortex, prefrontal cortex, amygdala and nucleus accumbens (NAc). In fact, the aINS has been involved in the integration of autonomic information for emotional and motivational functions. The compulsive consumption of drugs or high-fat foods induces alterations at both behavioural and brain levels. Brain reward circuits are altered in response to continued intake, in particular the dopaminergic projections from the ventral tegmental area (VTA) to the NAc. The aINS has multiple connections with the components of this system. In recent years, efforts have been made to better understand the fundamental role of the aINS in addiction, making it one of the key centres of interest for research into new treatments for addiction.

OBJECTIVES

The present work focuses on studying 1.- whether the human aINS expresses orexigenic peptides such as neuropeptide Y (NPY), a peptide known to induce hyperphagia, and which has been implicated in the onset and development of obesity, 2.- the long-term effect of an obesogenic diet on NPY expression in the aINS and NAc of C57BL/6 mice.

METHODS

A total of 17 female C57BL/6 J mice were used in this study. Female mice were fed ad libitum with water and, either a standard diet (SD) or a high-fat diet (HFD) to induce obesity. There were seven female mice on the SD and ten on the HFD. The duration of the experiment was 180 days. We also studied 3 human adult brains (1 male and 2 females, mean age 55.7 ± 5.2 years). The morphological study was performed using immunohistochemistry and double immunofluorescence techniques to study the neurochemical profile of NPY neurons of the aINS and NAc of humans and mice.

RESULTS

Our morphological analysis demonstrates for the first time the basal expression of NPY in different layers of the human cortex (II, III, IV, V/VI), in a pattern similar to previous studies in other species. Furthermore, we observed an increase in the number of NPY-positive cells and their intracytoplasmic signal in the aINS and NAc of the obese mice subjected to a long-term obesogenic diet.

CONCLUSIONS

To our knowledge, this is the first study to show the distribution and expression of NPY in the human INS and how its expression is altered after prolonged treatment with an obesogenic diet in obese mice. Our findings may contribute to the understanding of the pathophysiological mechanisms underlying obesity in regions related to the reward system and associated with uncontrolled intake of high-fat foods, thus facilitating the identification of novel therapeutic targets.

Drastic decline in vasoactive intestinal peptide expression in the suprachiasmatic nucleus in obese mice on a long-term high-fat diet

The suprachiasmatic nucleus (SCN) is the main region for the regulation of circadian rhythms. Although the SCN contains a heterogeneous neurochemical phenotype with a wide variety of neuropeptides, a key role has been suggested for the vasoactive intestinal neuropeptide (VIP) as a modulator circadian, reproductive, and seasonal rhythms. VIP is a 28-amino acid polypeptide hormone that belongs to the secretin-glucagon peptide superfamily and shares 68 % homology with the pituitary adenylate cyclase-activating polypeptide (PACAP). VIP acts as an endogenous appetite inhibitor in the central nervous system, where it participates in the control of appetite and energy homeostasis. In recent years, significant efforts have been made to better understand the role of VIP in the regulation of appetite/satiety and energy balance. This study aimed to elucidate the long-term effect of an obesogenic diet on the distribution and expression pattern of VIP in the SCN and nucleus accumbens (NAc) of C57BL/6 mice. A total of 15 female C57BL/6J mice were used in this study. Female mice were fed ad libitum with water and, either a standard diet (SD) or a high-fat diet (HFD) to induce obesity. There were 7 female mice on the SD and 8 on the HFD. The duration of the experiment was 365 days. The morphological study was performed using immunohistochemistry and double immunofluorescence techniques to study the neurochemical profile of VIP neurons of the SCN of C57BL/6 mice. Our data show that HFD-fed mice gained weight and showed reduced VIP expression in neurons of the SCN and also in fibres located in the NAc. Moreover, we observed a loss of neuropeptide Y (NPY) expression in fibres surrounding the SCN. Our findings on VIP may contribute to the understanding of the pathophysiological mechanisms underlying obesity in regions associated with uncontrolled intake of high-fat foods and the reward system, thus facilitating the identification of novel therapeutic targets.

The importance of estradiol for body weight regulation in women

Obesity in women of reproductive age has a number of adverse metabolic effects, including Type II Diabetes (T2D), dyslipidemia, and cardiovascular disease. It is associated with increased menstrual irregularity, ovulatory dysfunction, development of insulin resistance and infertility. In women, estradiol is not only critical for reproductive function, but they also control food intake and energy expenditure. Food intake is known to change during the menstrual cycle in humans. This change in food intake is largely mediated by estradiol, which acts directly upon anorexigenic and orexigenic neurons, largely in the hypothalamus. Estradiol also acts indirectly with peripheral mediators such as glucagon like peptide-1 (GLP-1). Like estradiol, GLP-1 acts on receptors at the hypothalamus. This review describes the physiological and pathophysiological mechanisms governing the actions of estradiol during the menstrual cycle on food intake and energy expenditure and how estradiol acts with other weight-controlling molecules such as GLP-1. GLP-1 analogs have proven to be effective both to manage obesity and T2D in women. This review also highlights the relationship between steroid hormones and women's mental health. It explains how a decline or imbalance in estradiol levels affects insulin sensitivity in the brain. This can cause cerebral insulin resistance, which contributes to the development of conditions such as Parkinson's or Alzheimer's disease. The proper use of both estradiol and GLP-1 analogs can help to manage obesity and preserve an optimal mental health in women by reducing the mechanisms that trigger neurodegenerative disorders.

Mulberry leaf polyphenols alleviated high-fat diet-induced obesity in mice

Mulberry leaf is an important medicinal food plant, which is rich in polyphenol compounds. Mulberry leaf polyphenols (MLP) possess significant lipid-lowering and antioxidant effects, and healthcare functions. In this study, the polyphenol content of mulberry leaf ethanol extract was measured using HPLC. The analysis of mulberry leaf extract resulted in the identification of 14 compounds, of which Chlorogenic acid and Quercitrin were the highest. A high-fat diet (HFD)-induced obese mouse model was developed and treated with MLP for 12 weeks to explore their effect on lipid metabolism in HFD-induced obese mice. The results showed that the MLP could inhibit the weight gain and fat cell volume increase in the HFD-induced obese mice in a dose-dependent manner. Further analysis revealed that the MLP decelerated the fatty acid composition in the adipose tissues of HFD-induced obese mice, and significantly increased the polyunsaturated-to-saturated fatty acid (PUFA/SFA) ratio. The real-time quantitative PCR (RT-qPCR) results indicated that the MLP significantly inhibited the down regulation of uncoupling protein (UCP) 1 (UCP1), UCP3, and PR domain zinc finger protein 16 (PRDM16) caused by the HFD. These beneficial effects of MLP on HFD-induced obese mice might be attributed to their ability to change the fatty acid composition of adipose tissue and increase the expression of thermogenesis genes. Overall, the study results suggested that the MLP could serve as potential lipid-lowering and weight-loss functional food and healthcare products.

Molecular determinants of ACTH receptor for ligand selectivity

The adrenocorticotropic hormone (ACTH) receptor, known as the melanocortin-2 receptor (MC2R), plays a key role in regulating adrenocortical function. ACTH receptor is a subtype of the melanocortin receptor family which is a member of the G-protein coupled receptor (GPCR) superfamily. ACTH receptor has unique characteristics among MCRs. α-MSH, β-MSH, γ-MSH and ACTH are agonists for MCRs but only ACTH is the agonist for ACTH receptor. In addition, the melanocortin receptor accessory protein (MRAP) is required for ACTH receptor expression at cell surface and function. In this review, we summarized the information available on the relationship between ACTH and ACTH receptor and provide the latest understanding of the molecular basis of the ACTH receptor responsible for ligand selectivity and function.

The melanocortin system as a potential target for treating alcohol use disorders: A review of pre-clinical data

The melanocortin (MC) system consists of neuropeptides that are cleaved from the polypeptide precursor proopiomelanocortin (POMC). In the brain, MC neuropeptides signal primarily through the MC-3 and MC-4 receptors, which are widely expressed throughout the brain. While the MC system has been largely studied for its role in food intake and body weight regulation, converging evidence has emerged over approximately the last 20-years showing that alcohol (ethanol), and other drugs of abuse influence the central MC system, and that manipulating MC receptor signalling modulates ethanol intake. Although there is divergent evidence, the wealth of data appears to suggest that activating MC signalling, primarily through the MC-4 receptor, is protective against excessive ethanol consumption. In the present review, we first describe the MC system and then detail how ethanol exposure and consumption alters central MC and MC-receptor expression and levels. This is followed by a review of the data, from pharmacological and genetic studies, which show that manipulations of MC receptor activity alter ethanol intake. We then briefly highlight studies implicating a role for the MC system in modulating neurobiological responses and intake of other drugs of abuse, including amphetamine, cocaine and opioids. Finally, we introduce relatively new observations that the drug, bupropion (BUP), a drug that activates central MC activity, significantly reduces ethanol intake in rodent models when administered alone and in combination with the non-selective opioid receptor antagonist, naltrexone. Phase II clinical trials are currently underway to assess the efficacy of BUP as a treatment for alcohol use disorders.

Amino acid residue L112 in the ACTH receptor plays a key role in ACTH or α-MSH selectivity

The adrenocorticotropic hormone (ACTH) receptor, known as the melanocortin-2 receptor (MC2R), plays a key role in regulating adrenocortical function. MC2R is a subtype of the melanocortin receptor family and ACTH is only agonist for MC2R. Our previous result indicates that ACTH1-17 is the minimal peptide required for MC2R activation but DPhe7-ACTH1-17 has no activity at MC2R. In this study, we examined the molecular basis of the MC2R responsible for ligand selectivity using ACTH analogues and MC2R mutagenesis. Our results indicate that substitution of the 3TM of the MC2R with the corresponding region of the MC3R switches DPhe-ACTH1-17 from no activity to agonist. Further experiment indicates that substitution of the amino acid residue leucine to isoleucine in 112 (L112I) of the 3TM of the MC2R changes both DPhe-ACTH1-17 and ACTH1-15 from no activity to agonists. Surprisingly, mutation L112I switches α-MSH from no activity to agonist, suggesting that this residue plays a key role at MC2R for ligand ACTH or α-MSH selectivity.

Effects of Naltrexone on Energy Balance and Hypothalamic Melanocortin Peptides in Male Mice Fed a High-Fat Diet

The hypothalamic melanocortin system composed of proopiomelanocortin (POMC) and agouti-related protein (AgRP) neurons plays a key role in maintaining energy homeostasis. The POMC-derived peptides, α-MSH and β-EP, have distinct roles in this process. α-MSH inhibits food intake, whereas β-EP, an endogenous opioid, can inhibit POMC neurons and stimulate food intake. A mouse model was used to examine the effects of opioid antagonism with naltrexone (NTX) on Pomc and Agrp gene expression and POMC peptide processing in the hypothalamus in conjunction with changes in energy balance. There were clear stimulatory effects of NTX on hypothalamic Pomc in mice receiving low- and high-fat diets, yet only transient decreases in food intake and body weight gain were noted. The effects on Pomc expression were accompanied by an increase in POMC prohormone levels and a decrease in levels of the processed peptides α-MSH and β-EP. Arcuate expression of the POMC processing enzymes Pcsk1, Pcsk2, and Cpe was not altered by NTX, but expression of Prcp, an enzyme that inactivates α-MSH, increased after NTX exposure. NTX exposure also stimulated hypothalamic Agrp expression, but the effects of NTX on energy balance were not enhanced in Agrp-null mice. Despite clear stimulatory effects of NTX on Pomc expression in the hypothalamus, only modest transient decreases in food intake and body weight were seen. Effects of NTX on POMC processing, and possibly α-MSH inactivation, as well as stimulatory effects on AgRP neurons could mitigate the effects of NTX on energy balance.

Key amino acid residue in Melanocortin-1 receptor (melanocyte α-MSH receptor) for ligand selectivity

The melanocortin-1 receptor (MC1R) is a subtype of the melanocortin receptor family and NDP-α-MSH is a non-selective agonist for MC1R. The core sequence of NDP-α-MSH, His-Phe-Arg-Trp, is important for ligand binding and biological activities at the melanocortin receptor subtypes (MCRs). A recent study indicates that Trp⁹ in NDP-α-MSH plays an important role in ligand selectivity. Deletion of Trp⁹ in NDP-α-MSH (des-Trp⁹-NDP-α-MSH) resulted in loss of agonist activity at MC4R, although remains agonist activity at MC1R. The molecular basis for this receptor ligand selectivity is unknown. In this study we examined what region of the MC1R is responsible for des-NDP-α-MSH selectivity. Our results indicate that (1) substitution of TM3 of MC4R with the corresponding region of MC1R switches des-Trp⁹-NDP-α-MSH from no activity to agonist; (2) des-Trp⁹-NDP-α-MSH exhibits agonistic activity at the L133M mutation of the MC4R; and (3) substitution of non-conserved amino acid residue M128 in TM3 of MC1R significantly reduced des-Trp⁹-NDP-α-MSH agonist activity. Our results demonstrate that amino acid residue 128 in TM3 of MC1R, or amino acid residue L133 in TM3 of the MC4R, play crucial roles in ligand des-Trp⁹-NDP-α-MSH selectivity at MC1R or MC4R.

Melanocortin neurons: Multiple routes to regulation of metabolism

The burden of disability, premature death, escalating health care costs and lost economic productivity due to obesity and its associated complications including hypertension, stroke, cardiovascular disease and type 2 diabetes is staggering [1,2]. A better understanding of metabolic homeostatic pathways will provide us with insights into the biological mechanisms of obesity and how to fundamentally address this epidemic [3-6]. In mammals, energy balance is maintained via a homeostatic system involving both peripheral and central melanocortin systems; changes in body weight reflect an unbalance of the energetic state [7-9]. Although the primary cause of obesity is unknown, there is significant effort to understand the role of the central melanocortin pathway in the brain as it has been shown that deficiency of proopiomelanocortin (POMC) [10,11] and melanocortin 4 receptors (MC4R) [12-15] in both rodents and humans results in severe hyperphagia and obesity [16-23]. In this review, we will summarize how the central melanocortin pathway helps regulate body mass and adiposity within a 'healthy' range through the 'nutrient sensing' network [24-28]. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Molecular signatures of human melanocortin receptors for ligand binding and signaling

Human melanocortin receptors (hMCRs) belong to the seven-transmembrane (TM) domain proteins. There are five hMCR subtypes and each of these receptor subtypes has different patterns of tissue expression and physiological function. The endogenous agonists for hMCRs are α-, β-, and γ-MSH and ACTH and endogenous antagonists are Agouti and AGRP which are the only known naturally occurring antagonists for the receptors. These peptides have their own profiles regarding the relative potency for specific hMCR subtype. Extensive studies have been performed to examine the molecular basis of the hMCRs for different ligand binding affinity and potency. Studies indicate that natural ligand α-MSH utilizes conserved amino acid residues for MCR specific binding (orthosteric binding) while synthetic ligands utilize non-conserved amino acid residues for receptor subtype specific binding (allosteric binding). ACTH is the only endogenous agonist for hMC2R and more amino acid residues at hMC2R are required for ACTH binding and signaling. HMCR computer modeling provides the detailed information of ligand and MCR interaction. This review provides the latest understanding of the molecular basis of the hMCRs for ligand binding and signaling. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Systematic Backbone Conformational Constraints on a Cyclic Melanotropin Ligand Leads to Highly Selective Ligands for Multiple Melanocortin Receptors

Human melanocortin receptors (hMCRs) have been challenging targets to develop ligands that are explicitly selective for each of their subtypes. To modulate the conformational preferences of the melanocortin ligands and improve the biofunctional agonist/antagonist activities and selectivities, we have applied a backbone N-methylation approach on Ac-Nle-c[Asp-His-D-Nal(2')-Arg-Trp-Lys]-NH2 (Ac-Nle(4)-c[Asp(5),D-Nal(2')(7),Lys(10)]-NH2), a nonselective cyclic peptide antagonist at hMC3R and hMC4R and an agonist at hMC1R and hMC5R. Systematic N-methylated derivatives of Ac-Nle(4)-c[Asp(5),D-Nal(2')(7),Lys(10)]-NH2, with all possible backbone N-methylation combinations, have been synthesized and examined for their binding and functional activities toward melanocortin receptor subtypes 1, 3, 4, and 5 (hMCRs). Several N-methylated analogues are selective and potent agonists or antagonists for hMC1R or hMC5R or have selective antagonist activity for hMC3R. The selective hMC1R ligands show strong binding for human melanoma cells. We have also discovered the first universal antagonist (compound 19) for all subtypes of hMCRs.

Molecular characterization of human melanocortin-5 receptor ligand-receptor interaction

The melanocortin-5 receptor (MC5R) is a subtype receptor of the melanocortin receptor (MCR) family, which is expressed centrally, as well as in a variety of peripheral tissues. MC5R has been implicated in many different physiological fields such as lipid metabolism and exocrine function. However, the specific molecular determinants of MC5R responsible for ligand binding and receptor signaling are currently unknown. The aim of this study is to determine the molecular basis of human MC5R (hMC5R) responsible for ligand binding and receptor signaling. Twenty-four single mutations of hMC5R were created and tested. Our results indicate that (1) substituting charged amino acid residue E92 in transmembrane domain 2 (TM2), aspartic acid 115 (D115) and D119 in TM3, and histidine (H) 257 in TM6 with alanine dramatically reduced NDP-α-MSH binding affinity and receptor signaling and (2) substituting aromatic amino acids phenylalanine (F) 195 in TM5, F254 in TM6, and H276 in TM7 with alanine also significantly decreased NDP-α-MSH binding and receptor activity. Combining pharmacological results and computer modeling, our results suggest that D115 and D119 in TM3, F195 in TM5, and F254 in TM6 may form a binding pocket for NDP-α-MSH binding. Our results provide important information about the structural aspects of hMC5R responsible for ligand binding and receptor signaling.

Camp SHINE

According to the National Health and Nutritional Examination Survey (NHANES), in 2008, more than one third of children and adolescents were overweight or obese. This article describes the use of a residential, medically supervised summer camp to help overweight and obese children prevent excessive summertime weight gain. The theoretical framework guiding the program was Rosenstock’s health belief model (HBM). The weight loss results are included as well as information concerning adaptations of physical activities, environmental design considerations, and creating a supportive atmosphere, to meet the unique needs of this population. Data show that weight loss can be achieved for overweight/obese children/teens in residential camps designed to develop healthy behaviors.

Alterations in downstream mediators involved in central control of eating behavior in obese adolescents submitted to a multidisciplinary therapy

OBJECTIVE

The aim of this study was to verify the effects of a multidisciplinary therapy (24 weeks) on neurohormonal control of food intake, specifically in orexigenic (total ghrelin, agouti-related protein [AgRP], neuropeptide Y [NPY], and melanin-concentrating hormone) and anorexigenic factors (leptin, insulin, and alpha-melanocyte stimulating hormone [α-MSH]), in obese adolescents.

METHODS

A total of 88 adolescents (38 boys and 50 girls), including 62 obese and 26 normal-weight, aged 15-19 years were recruited. Obese adolescents were submitted to a 24-week multidisciplinary therapy. AgRP, NPY, melanin-concentrating hormone, leptin, insulin, glucose, α-MSH, total ghrelin, and food intake were measured at three stages (at baseline, after 12 weeks, and after 24 weeks).

RESULTS

At baseline, obese adolescents showed hyperleptinemia (circulating leptin levels, which were, in boys and girls, 40 and 35 times higher than in normal-weight subjects, respectively). After 24 weeks, these values decreased in all obese patients. Our results showed no differences in ghrelin levels between obese and normal-weight adolescents, in both genders. However, obese boys reduced their plasma ghrelin concentration after 24 weeks of therapy (p < .05). The multidisciplinary therapy decreased NPY and AgRP values and increased α-MSH; simultaneously with these changes there was a decrease in total food intake after 24 weeks of therapy.

CONCLUSIONS

We can conclude that the multidisciplinary therapy was efficient to modulate neurohormonal control of food intake in obese adolescents.

Cyclic lactam hybrid α-MSH/Agouti-related protein (AGRP) analogues with nanomolar range binding affinities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the topographical similarities between the pharmacophores of Agouti related protein (AGRP) an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. When employed in two different 23-membered macrocyclic lactam peptide templates, the designed hybrid AGRP/MSH pharmacophore yielded non-competitive ligands with nanomolar range binding affinities. The topography-based pharmacophore hybridization strategy will prove useful in development of unique non-competitive melanocortin receptor modulators.

Structure, function and regulation of the melanocortin receptors

Melanocortin receptors belong to the seven-transmembrane (TM) domain proteins that are coupled to G-proteins and signaled through intracellular cyclic adenosine monophosphate. Many structural features conserved in other G-protein coupled receptors (GPCRs) are found in the melanocortin receptors. There are five melanocortin receptor subtypes and each of the melanocortin receptor subtypes has a different pattern of tissue expression and has its own profile regarding the relative potency of different melanocortin peptides. α-, β-, and γ-MSH and ACTH are known endogenous agonist ligands for the melanocortin receptors. Agouti and AgRP are the only known naturally occurring antagonists of the melanocortin receptors. We have examined the molecular basis of all five human melanocortin receptors for different ligand binding affinities and potencies using chimeric and mutated receptors. Our studies indicate that human melanocortin MC(1) receptor, human melanocortin MC(3) receptor, human melanocortin MC(4) receptor and human melanocortin MC(5) receptor utilize orthosteric sites for non selective agonists, α-MSH and NDP-α-MSH, high affinity binding and utilize allosteric sites for selective agonist or antagonist binding. Furthermore, our results indicate that molecular determinants of human melanocortin MC(2) receptor for ACTH binding and signaling are different from that of other melanocortin receptors. Many studies also indicate that agonists can induce different conformation changes of melanocortin receptors, which then lead to the activation of different signaling pathways, even when the expression level of receptor and the strength of stimulus-response coupling are the same. This finding may provide new information for the design of drugs for targeting melanocortin receptors.

Solid-phase peptide head-to-side chain cyclodimerization: discovery of C(2)-symmetric cyclic lactam hybrid α-melanocyte-stimulating hormone (MSH)/agouti-signaling protein (ASIP) analogues with potent activities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the pharmacophores of the agouti-signaling protein (ASIP), an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. The designed hybrid ASIP/MSH pharmacophore was explored in monomeric cyclic, and cyclodimeric templates. The monomeric cyclic disulfide series yielded peptides with hMC3R-selective non-competitive binding affinities. The direct on-resin peptide lactam cyclodimerization yielded nanomolar range (25-120 nM) hMC1R-selective full and partial agonists in the cyclodimeric lactam series which demonstrates an improvement over the previous attempts at hybridization of MSH and agouti protein sequences. The secondary structure-oriented pharmacophore hybridization strategy will prove useful in development of unique allosteric and orthosteric melanocortin receptor modulators. This report also illustrates the utility of peptide cyclodimerization for the development of novel GPCR peptide ligands.

Melanocortins induce interleukin 6 gene expression and secretion through melanocortin receptors 2 and 5 in 3T3-L1 adipocytes

Interleukin 6 (IL6) is a pleiotropic cytokine that not only affects the immune system, but also plays an active role in many physiological events in various organs. Notably, 35% of systemic IL6 originates from adipose tissues under noninflammatory conditions. Here, we describe a previously unknown function of melanocortins in regulating Il6 gene expression and production in 3T3-L1 adipocytes through membrane receptors which are called melanocortin receptors (MCRs). Of the five MCRs that have been cloned, MC2R and MC5R are expressed during adipocyte differentiation. alpha-Melanocyte-stimulating hormone (alpha-MSH) or ACTH treatment of 3T3-L1 adipocytes induces Il6 gene expression and production in a time- and concentration-dependent manner via various signaling pathways including the protein kinase A, p38 mitogen-activated protein kinase, cJun N-terminal kinase, and IkappaB kinase pathways. Specific inhibition of MC2R and MC5R expression with short interfering Mc2r and Mc5r RNAs significantly attenuated the alpha-MSH-induced increase of intracellular cAMP and both the level of Il6 mRNA and secretion of IL6 in 3T3-L1 adipocytes. Finally, when injected into mouse tail vein, alpha-MSH dramatically increased the Il6 transcript levels in epididymal fat pads. These results suggest that alpha-MSH in addition to ACTH may function as a regulator of inflammation by regulating cytokine production.

Discovery of a spiroindane based compound as a potent, selective, orally bioavailable melanocortin subtype-4 receptor agonist

We report the design, synthesis and properties of spiroindane based compound 1, a potent, selective, orally bioavailable, non-peptide melanocortin subtype-4 receptor agonist. Compound 1 shows excellent erectogenic activity in the rodent models.

Substitution of arginine with proline and proline derivatives in melanocyte-stimulating hormones leads to selectivity for human melanocortin 4 receptor

A new series of melanotropin analogues with His or Arg residues in the core pharmacophores of MTII, SHU9119, and Ac-NDP-gamma-MSH-NH(2) replaced by Pro or trans-/cis-4-guanidinyl-Pro derivatives were designed and synthesized to introduce selectivity toward the human melanocortin 4 receptor (hMC4R). Analogues 1, 2, 3, 6, 7, 8 were found to be hMC4R selective. Second messenger studies have demonstrated that analogues 1 and 2 are insurmountable inhibitors of MTII agonist activity at the hMC4R. Molecular modeling studies suggest that the hMC4R selectivity is due to a beta-turn shift induced by the Pro ring that makes the global minimum structures of these analogues resemble the NMR solution structure of the hASIP melanocortin receptor binding motif. Substitution of His in MTII also provided functional selectivity for the hMC3R or the hMC4R. These findings are important for a better understanding of the selectivity mechanism at the hMC3R/hMC4R and the development of therapeutic ligands selectively targeting the hMC4R.

Amygdalar opioids modulate hypothalamic melanocortin-induced anorexia

We wanted to assess the possibility that opioid activity in the central amygdala (CeA) could modulate the feeding inhibition of melanocortin stimulation of the paraventricular hypothalamus (PVN). The melanocortin system is important in both the acute regulation of satiety and feeding behavior and in the integration of long-term appetite signals. Melanotan II (MTII) is a synthetic MC3R and MC4R agonist which reduces food intake when given intracerebroventricularly (ICV) and into the PVN. Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO), a micro-opioid receptor agonist, increases food intake, while opioid antagonists, like naltrexone (NTX), inhibit food intake after injection into many brain sites involved in appetite regulation, including the CeA. In food-deprived male Sprague-Dawley rats, co-injected intra-PVN MTII partially blocked the orexigenic effect of co-injected intra-CeA DAMGO. Intra-CeA NTX co-injected with intra-PVN MTII reduced food intake significantly more than either alone. NTX administered intra-CeA reduced c-Fos-immunoreactivity (IR) in nucleus accumbens neurons significantly compared to the intra-PVN MTII treated animals, animals co-injected intra-PVN with MTII and intra-CeA with NTX animals, and control animals. Intra-PVN MTII induced c-Fos-IR in significantly more PVN neurons than observed in control animals. Intra-CeA NTX co-injected with intra-PVN MTII induced c-Fos-IR significantly in PVN neurons relative to control and intra-CeA NTX animals. Such data support the significance of opioid action within the CeA as a modulator of the feeding regulation action of melanocortins within the PVN, occurring within the context of a larger appetitive network.

Polymorphisms in uncoupling protein, melanocortin 3 receptor, melanocortin 4 receptor, and pro-opiomelanocortin genes and association with production traits in a commercial broiler line

Because avian uncoupling protein (avUCP), melanocortin 3 receptor (MC3R), melanocortin 4 receptor (MC4R), and pro-opiomelanocortin (POMC) genes may be associated with production traits [e.g., BW, weight gain (WG), and feed conversion ratio (FCR)], male and female broilers from an elite broiler line were screened for polymorphisms in these genes. The PCR-restriction fragment length polymorphism (RFLP) tests were developed to type the missense polymorphisms UCPAla118Val, MC4RSer76Leu, MC3R-Met54Leu, and Gly104Ser and POMCPro61Leu. Of 39 single nucleotide polymorphisms identified in all 4 genes, 24/39 were transitions with 11 having a C to T change. Of the 23 polymorphisms in UCP, 17 represented at least 7 haplotypes in this pedigreed broiler line. The UCP Ala-118Val allele was associated with a) high feed efficiency (FE; P = 0.03) and WG (P = 0.053) in selected males, and b) high BW in selected females (P = 0.07) and unselected males (P = 0.015). The UCPVal118Val allele was found in approximately 10% of the birds that were screened. Five silent substitutions, 3 in MC3R and 2 in MC4R, were also identified. Thirteen polymorphisms were identified in the POMC gene representing at least 3 different alleles. A missense Pro61Leu heterozygote was associated with greater BW in females. The heterozygote MC3R Gly104Ser polymorphism was associated with greater FE in selected males (P = 0.03) and greater BW in unselected males (P = 0.007). The MC4R Ser76Leu heterozygote polymorphism was associated with greater BW than the Leu76 homozygote in females (P = 0.05). From these findings, we hypothesize that UCP, MC3R, MC4R and POMC genes may play important roles and could be candidate loci for production traits such as feed conversion and BW in commercial broiler breeding stock.

Contribution of the transmembrane domain 6 of melanocortin-4 receptor to peptide [Pro5, DNal (2')8]-gamma-MSH selectivity

The melanocortin receptor (MCR) subtype family is a member of the GPCR superfamily and each of them has a different pharmacological profile regarding the relative potency of the endogenous and synthetic melanocortin peptides. Substitution of Trp with DNal (2') in gamma-MSH resulted in the loss of binding affinity and potency at hMC4R. However, the molecular mechanism of this ligand selectivity is unclear. In this study, we utilized chimeric receptors and site-directed mutagenesis approaches to investigate the molecular basis of MC4R responsible for peptide [Pro5, DNal (2')8]-gamma-MSH selectivity. Cassette substitutions of the second, third, fourth, fifth, and sixth TM of the human MC4R (hMC4R) with the homologous regions of hMC1R were constructed and the binding affinity of peptide [Pro5, DNal (2')8]-gamma-MSH at these chimeric receptors was evaluated. Our results indicate that the cassette substitutions of TM2, TM3, TM4 and TM5 of hMC4R with homologous regions of the hMC1R did not significantly increase peptide [Pro5, DNal (2')8]-gamma-MSH binding affinity and potency but substitution of the TM6 of the hMC4R with the same region of the hMC1R significantly enhances [Pro5, DNal (2')8]-gamma-MSH binding affinity and potency. Further site-directed mutagenesis study indicates that four amino acid residues, Phe267, Tyr268, Ile269 and Ser270, in TM6 of the hMC4R may play an important role in [Pro5, DNal (2')-gamma-MSH selective activity at MC4R.

Structure-activity relationship studies on a series of piperazinebenzylalcohols and their ketone and amine analogs as melanocortin-4 receptor ligands

A series of piperazinebenzylalcohols were prepared and studied to compare with their ketone and amine analogs as MC4R antagonists. Several benzylalcohols such as 14a and 14g displayed low nanomolar binding affinities (K(i)<10 nM), and high selectivities over other melanocortin receptor subtypes.

Design and synthesis of 3-arylpyrrolidine-2-carboxamide derivatives as melanocortin-4 receptor ligands

Based on 3-phenylpropionamides, a series of 3-arylpyrrolidine-2-carboxamide derivatives was designed and synthesized to study the effect of cyclizations as melanocortin-4 receptor ligands. It was found that the 2R,3R-pyrrolidine isomer possessed the most potent affinity among the four stereoisomers.

Structure-activity relationships of cyclic lactam analogues of alpha-melanocyte-stimulating hormone (alpha-MSH) targeting the human melanocortin-3 receptor

A variety of dicarboxylic acid linkers introduced between the alpha-amino group of Pro(6) and the -amino group of Lys(10) of the cyclic lactam alpha-melanocyte-stimulating hormone (alpha-MSH)-derived Pro(6)-D-Phe(7)/D-Nal(2')(7)-Arg(8)-Trp(9)-Lys(10)-NH2 pentapeptide template lead to nanomolar range and selective hMC3R agonists and antagonists. Replacement of the Pro(6) residue and the dicarboxylic acid linker with 2,3-pyrazine-dicarboxylic acid furnished a highly selective nanomolar range hMC3R partial agonist (analogue 12, c[CO-2,3-pyrazine-CO-D-Phe-Arg-Trp-Lys]-NH2, EC50 = 27 nM, 70% max cAMP) and an hMC3R antagonist (analogue 13, c[CO-2,3-pyrazine-CO-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 23 nM). Modeling experiments suggest that 2,3-pyrazinedicarboxylic acid stabilizes a beta-turn-like structure with the D-Phe/D-Nal(2') residues, which explains the high potency of the corresponding peptides. Placement of a Nle residue in position 6 produced a hMC3R/hMC5R antagonist (analogue 15, c[CO-(CH 2)2-CO-Nle-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 12 and 17 nM, respectively), similarly to the previously described cyclic gamma-melanocyte-stimulating hormone (gamma-MSH)-derived hMC3R/hMC5R antagonists. These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-3 receptor.

Identification and characterization of pyrrolidine diastereoisomers as potent functional agonists and antagonists of the human melanocortin-4 receptor

A series of trans-4-phenylpyrrolidine-3-carboxamides were synthesized and characterized as potent ligands of the human melanocortin-4 receptor. Interestingly, a pair of diastereoisomers 13b displayed potent functional agonist and antagonist activity, respectively. Thus, the 3S,4R-pyrrolidine 13b-1 possessed a Ki of 1.0 nM and an EC50 of 3.8 nM, while its 3R,4S-isomer 13b-2 exhibited a Ki of 4.7 and an IC50 of 64 nM. Both compounds were highly selective over other melanocortin receptor subtypes. The MC4R agonist 13b-1 also demonstrated efficacy in a diet-induced obesity model in rats.

Melanocortin subtype 4 receptor agonists: Structure–activity relationships about the 4-alkyl piperidine core

SAR about the piperidine core in a series of MC4R agonists is described. A number of alkyl substituents that furnish compounds with good affinity and functional potency are reported.

The synthetic melanocortin (CKPV)2 exerts broad anti-inflammatory effects in human neutrophils

Natural melanocortin peptides exert broad effects on the host and they have remarkable therapeutic potential. However, successful use of melanocortins as therapeutic agents depends on the design of molecules that have more stable pharmacological profiles. The synthetic peptide (CKPV)(2), based on the C-terminal sequence of alpha-melanocyte stimulating hormone (alpha-MSH), has anti-tumor necrosis factor-alpha (TNF-alpha) effects in vitro and in vivo and is a promising candidate to treat inflammation. Because neutrophil activity is a major target for anti-inflammatory therapies, we determined whether (CKPV)(2) modulates human neutrophil functions in vitro. Incubation of freshly-separated human neutrophils with 10(-12)-10(-6)M (CKPV)(2) significantly inhibited activities relevant to the inflammatory reaction. Neutrophil migration toward the two chemoattractants interleukin 8 (IL-8) and N-formyl-methionyl-leucyl-phenylalanine (fMLP) was significantly inhibited by (CKPV)(2). (CKPV)(2) also inhibited reactive oxygen intermediate (ROI) production induced by phorbol 12-myristate 13-acetate (PMA), but not that induced by fMLP. Because these effects of (CKPV)(2) were abolished by the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine (ddAdo), they appear to be cAMP-dependent. Finally, the peptide reduced lipopolysaccharide (LPS)-stimulated expression of TNF-alpha, interleukin-1beta (IL-1beta), interleukin-8 (IL-8), and intercellular adhesion molecule 1 (ICAM-1), as well as TNF-alpha protein release in cell supernatants. The data indicate that (CKPV)(2) modulates broad cAMP-dependent, anti-inflammatory pathways in human neutrophils.

Pyrrolidines as potent functional agonists of the human melanocortin-4 receptor

A series of pyrrolidine derivatives were synthesized and characterized as potent agonists of the human melanocortin-4 receptor. For example, 28c had a K(i) of 13 nM in binding affinity and EC(50) of 6.9 nM in agonist potency with an intrinsic activity of 100% of the endogenous ligand alpha-MSH.

Potent and Selective Peptide Agonists of ?-Melanocyte Stimulating Hormone (?MSH) Action at Human Melanocortin Receptor 5; their Synthesis and Biological Evaluation in vitro

Melanocortin receptors (MC1-5R) and their endogenous ligands (melanocyte-stimulating hormones and adrenocorticotropic hormone) are involved in many physiological processes in humans. Of those receptors, the actions of MC5R are the least understood despite its broad presence in the numerous peripheral tissues and brain. In this study, we describe synthesis and pharmacological properties in vitro (receptor-binding affinity and agonist activity) of several cyclic analogs of alphaMSH which are potent agonists at hMC5R (EC(50) below 1 nM) and of enhanced receptor subtype selectivity (more than 2000-fold versus hMC1b,3R and about 70- to 200-fold versus hMC4R). These compounds are analogs of Ac-Nle(4)-cyclo[Asp(5)-His(6)-D-Nal(2')(7)-Pip(8)-Trp(9)-Lys(10)]-NH(2) (Pip: pipecolic acid) in which His(6) has been replaced with sterically hindered amino acids. They may be useful tools in the elucidation of the MC5R role in skin disorders and in immunomodulatory and in anti-inflammatory actions of alphaMSH.

Potent and selective agonists of alpha-melanotropin (alphaMSH) action at human melanocortin receptor 5; linear analogs of alpha-melanotropin

Alpha-melanotropin, Ac-Ser(1)-Tyr-Ser-Met-Glu-His(6)-Phe(7)-Arg(8)-Trp(9)-Gly-Lys-Pro-Val(13)-NH(2)(1), is a non-selective endogenous agonist for the melanocortin receptor 5; the receptor present in various peripheral tissues and in the brain, cortex and cerebellum. Most of the synthetic analogs of alphaMSH, including a broadly used and more potent the NDP-alphaMSH peptide, Ac-Ser(1)-Tyr-Ser-Nle(4)-Glu-His(6)-D-Phe(7)-Arg(8)-Trp(9)-Gly-Lys-Pro-Val(13)-NH(2), are also not particularly selective for MC5R. To elucidate physiological functions of the melanocortin receptor 5 in rodents and humans, the receptor subtype selective research tools are needed. We report herein syntheses and pharmacological evaluation in vitro of several analogs of NDP-alphaMSH which are highly potent and specific agonists for the human MC5R. The new linear peptides, of structures and solubility properties similar to those of the endogenous ligand alphaMSH, are exemplified by compound 7, Ac-Ser(1)-Tyr-Ser-Met-Glu-Oic(6)-D-4,4'-Bip(7)-Pip(8)-Trp(9)-Gly-Lys-Pro-Val(13)-NH(2) (Oic: octahydroindole-2-COOH, 4,4'-Bip: 4,4'-biphenylalanine, Pip: pipecolic acid), shortly NODBP-alphaMSH, which has an IC(50)=0.74 nM (binding assay) and EC(50)=0.41 (cAMP production assay) at hMC5R nM and greater than 3500-fold selectivity with respect to the melanocortin receptors 1b, 3 and 4. A shorter peptide derived from NODBP-alphaMSH: Ac-Nle-Glu-Oic(6)-D-4,4'-Bip(7)-Pip(8)-Trp(9) -NH(2) (17) was measured to be an agonist only 10-fold less potent at hMC5R than the full length parent peptide. In the structure of this smaller analog, the Nle-Glu-Oic(6)-D-4,4'-Bip(7)-Pip(8) segment was found to be critical for high agonist potency, while the C-terminal Trp(9) residue was shown to be required for high hMC5R selectivity versus hMC1b,3,4R.

Effects of macrocycle size and rigidity on melanocortin receptor-1 and -5 selectivity in cyclic lactam alpha-melanocyte-stimulating hormone analogs

The effects of the linker arm rigidity and size on melanocortin receptor selectivity were explored in a series of compounds using cyclic lactam alpha-melanocyte-stimulating hormone template. A variety of dicarboxylic acid linkers introduced between the alpha-amino group of His(6) and the epsilon-amino group of Lys(10) lead to high-affinity, selective human melanocortin receptor-1 and -5 (hMC1R and hMC5R) antagonists. The incorporation of hydrophilic functions into the linker arm was found to be unfavorable for both binding potency and receptor selectivity. Analogs 8 and 9 containing highly conformationally constrained hydrophobic linkers (m- and p-phthalic acids) were found to be selective nanomolar range hMC1R antagonists (IC(50) = 7 and 4 nm, respectively), whereas the employment of a small conformationally constrained linker (maleic acid) resulted in a high-affinity (IC(50) = 19 nm) and selective hMC5R antagonist (analog 12). These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-1 and -5 receptors.

Development of cyclic gamma-MSH analogues with selective hMC3R agonist and hMC3R/hMC5R antagonist activities

A series of cyclic lactam analogues of gamma-MSH (H-Tyr1-Val2-Met3-Gly4-His5-Phe6-Arg7-Trp8-Asp9-Arg10-Phe11-Gly12-OH) with a bulky hydrophobic residue in the direct proximity to the pharmacophore (Xaa-D-Phe/D-Nal(2')-Arg-Trp) were designed and synthesized by solid-phase methods. A variety of amino acids with a broad range of hydrophobic/hydrophilic properties was introduced in position 5 to further explore their complementary role in receptor selectivity. Biological evaluation of these peptides revealed several analogues with potent hMC3R agonist and hMC3R/hMC5R antagonist activities, and good receptor selectivity. Analogue 4, c[Nle-Arg-D-Phe-Arg-Trp-Glu]-NH2, was found to be a very potent and selective hMC3R agonist (EC50=1.2 nM, 112% act). In addition, analogue 13, c[Nle-Val-D-Nal(2')-Arg-Trp-Glu]-NH2, was identified as an hMC3R/hMC5R antagonist with the best selectivity against the hMC4R in this series (pA2(hMC3R)=8.4; pA2(hMC5R)=8.7). These results indicate the significance of steric factors in melanocortin receptor selectivity and suggest that introduction of bulky residues in the direct proximity to the melanocortin pharmacophore is an effective approach to design of novel hMC3R and hMC5R selective ligands.

The melanocortin system during fasting

This paper sets out to review the implication of the melanocortin system in regulating feeding behavior and energy balance during short- and long-term food deprivation. It is discussed in relation to: (1) body fat exhaustion and the known enhanced drive for refeeding in late fasting and (2) peripheral hormonal status with emphasis on the effect of leptin administration on melanocortin gene expression according to fat store mobilization.

Peripheral influences on central melanocortin neurons

The melanocortins are peptide products of post-translational processing of the pro-opiomelanocortin precursor protein. Melanocortin-expressing neurons are found in the arcuate nucleus of the hypothalamus and the nucleus of the solitary tract in the brain stem. The central melanocortin system is involved in a number of biological functions, including regulation of energy homeostasis. Hypothalamic and brain stem circuits interpret and integrate a number of peripheral inputs to provide a coordinated central response. This review examines the effect of these peripheral signals on central melanocortin signaling.

Structure-activity relationship of linear tetrapeptides Tic-DPhe-Arg-Trp-NH2 at the human melanocortin-4 receptor and effects on feeding behaviors in rat

The melanocortin subtype-4 receptor (MC4R) has been implicated in the control of feeding behavior and body weight regulation. A series of tetrapeptides, based on Tic-DPhe-Arg-Trp-NH2-a mimic of the putative message sequence "His-Phe-Arg-Trp" and modified at the DPhe position, were prepared and pharmacologically characterized for potency and selectivity. Substitution of His with Tic gave peptides with significant increases in selectivity. The effects of the substitution pattern of DPhe were investigated and it has significant influences on potency and the level of the maximum cAMP accumulation. Intracerebroventricular administration of peptide 10 induced significant inhibition of cumulative overnight food intake and feeding duration in rats.