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

Signatures of selection in Angus and Hanwoo beef cattle using imputed whole genome sequence data

In this study, we detected signatures of selection in Hanwoo and Angus beef cattle using allele frequency and haplotype-based methods based on imputed whole genome sequence variants. Our dataset included 13,202 Angus animals with 10,057,633 imputed SNPs and 10,437 Hanwoo animals with 13,241,550 imputed SNPs. The dataset was subset down to 6,873,624 SNPs in common between the two populations to identify within population (runs of homozygosity, extended haplotype homozygosity) and between population signals of selection (allele fixation index, extended haplotype homozygosity). Assuming these selection signals were complementary to each other, they were combined into a decorrelated composite of multiple signals to identify regions under selection for each of the breeds. 27 genomic regions spanning 25.15 Mb and harboring 360 genes were identified in Angus on chromosomes 1,3, 4, 5, 6, 7, 8, 12, 13, 14, 16, 20, 21 and 28. Similarly, in Hanwoo, 59 genes and 17 genomic regions spanning 5.21 Mb on chromosomes 2, 4, 5, 6, 7, 8, 9, 10, 13, 17, 20 and 24 were identified. Apart from a small region on chromosome 13, there was no major overlap of selection signals between the two breeds reflecting their largely different selection histories, environmental challenges, breeding objectives and breed characteristics. Positional candidate genes identified in selected genomic regions in Angus have been previously associated with growth, immunity, reproductive development, feed efficiency and adaptation to environment while the candidate genes identified in Hanwoo included important genes regulating meat quality, fat deposition, cholesterol metabolism, lipid synthesis, neuronal development, and olfactory reception.

Study on the Mechanism of MC5R Participating in Energy Metabolism of Goose Liver

Nutrition and energy levels have an important impact on animal growth, production performance, disease occurrence and health recovery. Previous studies indicate that melanocortin 5 receptor (MC5R) is mainly involved in the regulations of exocrine gland function, lipid metabolism and immune response in animals. However, it is not clear how MC5R participates in the nutrition and energy metabolism of animals. To address this, the widely used animal models, including the overfeeding model and the fasting/refeeding model, could provide an effective tool. In this study, the expression of MC5R in goose liver was first determined in these models. Goose primary hepatocytes were then treated with nutrition/energy metabolism-related factors (glucose, oleic acid and thyroxine), which is followed by determination of MC5R gene expression. Moreover, MC5R was overexpressed in goose primary hepatocytes, followed by identification of differentially expressed genes (DEGs) and pathways subjected to MC5R regulation by transcriptome analysis. At last, some of the genes potentially regulated by MC5R were also identified in the in vivo and in vitro models, and were used to predict possible regulatory networks with PPI (protein-protein interaction networks) program. The data showed that both overfeeding and refeeding inhibited the expression of MC5R in goose liver, while fasting induced the expression of MC5R. Glucose and oleic acid could induce the expression of MC5R in goose primary hepatocytes, whereas thyroxine could inhibit it. The overexpression of MC5R significantly affected the expression of 1381 genes, and the pathways enriched with the DEGs mainly include oxidative phosphorylation, focal adhesion, ECM-receptor interaction, glutathione metabolism and MAPK signaling pathway. Interestingly, some pathways are related to glycolipid metabolism, including oxidative phosphorylation, pyruvate metabolism, citrate cycle, etc. Using the in vivo and in vitro models, it was demonstrated that the expression of some DEGs, including ACSL1, PSPH, HMGCS1, CPT1A, PACSIN2, IGFBP3, NMRK1, GYS2, ECI2, NDRG1, CDK9, FBXO25, SLC25A25, USP25 and AHCY, was associated with the expression of MC5R, suggesting these genes may mediate the biological role of MC5R in these models. In addition, PPI analysis suggests that the selected downstream genes, including GYS2, ECI2, PSPH, CPT1A, ACSL1, HMGCS1, USP25 and NDRG1, participate in the protein-protein interaction network regulated by MC5R. In conclusion, MC5R may mediate the biological effects caused by changes in nutrition and energy levels in goose hepatocytes through multiple pathways, including glycolipid-metabolism-related pathways.

Atypical pituitary hormone-target tissue axis

A long-held belief is that pituitary hormones bind to their cognate receptors in classical target glands to actuate their manifold functions. However, a number of studies have shown that multiple types of pituitary hormone receptors are widely expressed in non-classical target organs. Each pituitary gland-derived hormone exhibits a wide range of nonconventional biological effects in these non-classical target organs. Herein, the extra biological functions of pituitary hormones, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, adrenocorticotrophic hormone, and prolactin when they act on non-classical organs were summarized, defined by the novel concept of an "atypical pituitary hormone-target tissue axis." This novel proposal explains the pathomechanisms of abnormal glucose and lipid metabolism, obesity, hypertension, fatty liver, and atherosclerosis while offering a more comprehensive and systematic insights into the coordinated regulation of environmental factors, genetic factors, and neuroendocrine hormones on human biological functions. The continued exploration of the physiology of the "atypical pituitary hormone-target tissue axis" could enable the identification of novel therapeutic targets for metabolic diseases.

Genetic architecture and selection of Anhui autochthonous pig population revealed by whole genome resequencing

The genetic resources among pigs in Anhui Province are diverse, but their value and potential have yet to be discovered. To illustrate the genetic diversity and population structure of the Anhui pigs population, we resequenced the genome of 150 pigs from six representative Anhui pigs populations and analyzed this data together with the sequencing data from 40 Asian wild boars and commercial pigs. Our results showed that Anhui pigs were divided into two distinct types based on ancestral descent: Wannan Spotted pig (WSP) and Wannan Black pig (WBP) origins from the same ancestor and the other four populations origins from another ancestor. We also identified several potential selective sweep regions associated with domestication characteristics among Anhui pigs, including reproduction-associated genes (CABS1, INSL6, MAP3K12, IGF1R, INSR, LIMK2, PATZ1, MAPK1), lipid- and meat-related genes (SNX19, MSTN, MC5R, PRKG1, CREBBP, ADCY9), and ear size genes (MSRB3 and SOX5). Therefore, these findings expand the catalogue and how these genetic differences among pigs and this newly generated data will be a valuable resource for future genetic studies and for improving genome-assisted breeding of pigs and other domesticated animals.

Pharmacological Melanocortin 5 Receptor Activation Attenuates Glomerular Injury and Proteinuria in Rats With Puromycin Aminonucleoside Nephrosis

Clinical evidence indicates that the melanocortin peptide ACTH is effective in inducing remission of nephrotic glomerulopathies like minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS), including those resistant to steroids. This suggests that a steroid-independent melancortinergic mechanism may contribute. However, the type of melanocortin receptor (MCR) that conveys this beneficial effect as well as the underlying mechanisms remain controversial. Burgeoning evidence suggests that MC5R is expressed in glomeruli and may be involved in glomerular pathobiology. This study aims to test the effectiveness of a novel highly selective MC5R agonist (MC5R-A) in puromycin aminonucleoside (PAN) nephrosis. Upon PAN injury, rats developed evident proteinuria on day 5, denoting an established nephrotic glomerulopathy. Following vehicle treatment, proteinuria continued to persist on day 14 with prominent histologic signs of podocytopathy, marked by ultrastructural glomerular lesions, including extensive podocyte foot process effacement. Concomitantly, there was loss of podocyte homeostatic markers, such as synaptopodin and podocin, and de novo expression of the podocyte injury marker desmin. Treatment with MC5R-A attenuated urine protein excretion and mitigated the loss of podocyte marker proteins, resulting in improved podocyte ultrastructural changes. In vitro in cultured podocytes, MC5R-A prevented the PAN-induced disruption of actin cytoskeleton integrity and apoptosis. MC5R-A treatment in PAN-injured podocytes also reinstated inhibitory phosphorylation and thus averted hyperactivity of GSK3β, a convergent point of multiple podocytopathic pathways. Collectively, pharmacologic activation of MC5R by using the highly selective small-molecule agonist is likely a promising therapeutic strategy to improve proteinuria and glomerular injury in protenuric nephropathies.

Regulation of melanocortin-5 receptor pharmacology by two isoforms of MRAP2 in ricefield eel (Monopterus albus)

The melanocortin-5 receptor (MC5R) has been implicated in the regulation of exocrine gland secretion, immune regulation, and muscle fatty acid oxidation in mammals. Melanocortin-2 receptor accessory protein 2 (MRAP2) can modulate trafficking, ligand binding, and signaling of melanocortin receptors. To explore potential interaction between ricefield eel (Monopterus albus) MC5R and MRAP2s (maMC5R, maMRAP2X1, and maMRAP2X2), herein we studied the pharmacological characteristics of maMC5R and its modulation by maMRAP2s expressed in the human embryonic kidney cells. Three agonists, α-melanocyte-stimulating hormone (α-MSH), ACTH (1-24), and [Nle⁴, D-Phe⁷]-α-MSH, could bind to maMC5R and induce intracellular cAMP production dose-dependently. Compared with human MC5R (hMC5R), maMC5R displayed decreased maximal binding but higher binding affinity to α-MSH or ACTH (1-24). When stimulated with α-MSH or ACTH (1-24), maMC5R showed significantly lower EC₅₀ and maximal response than hMC5R. Two maMRAP2s had no effect on cell surface expression of maMC5R, whereas they significantly increased maximal binding. Only maMRAP2X2 significantly decreased the binding affinity of ACTH (1-24). Both maMRAP2X1 and maMRAP2X2 significantly reduced maMC5R efficacy but did not affect ligand sensitivity. The availability of maMC5R pharmacological characteristics and modulation by maMRAP2s will assist the investigation of its roles in regulating diverse physiological processes in ricefield eel.

Molecular cloning and expression analysis of mc5r like genes (mc5rl) in Ruditapes philippinarum (Manila clam) after aerial exposure and low-temperature stress

The melanocortin-5 receptor (mc5r) plays an important role in exocrine function, lipid metabolism, obesity, and stress response in the vertebrate. However, the functions of the mc5r in mollusks have been rarely investigated. We cloned the full length of Ruditapes philippinarum mc5r like gene (mc5rl) and the sequence structure and phylogenetic relationship of mc5rl were analyzed. Besides, we detected the tissue distribution and the expression pattern of R. philippinarum mc5r like (mc5rl) genes after aerial exposure and low-temperature stress. The full-length cDNA of the mc5rl-1 was 2143 bp, consisting of a 1224 bp open reading frame encoding (ORF) 408 amino acids. Sequence and phylogenetic analyses revealed that the nucleotide and amino acid sequences of Manila clam mc5rl were highly homologous with mc5r of Crassostrea virginica, Crassostrea gigas, Mizuhopecten yessoensis, and Pecten maximus (32%-36%) and low homologous with vertebrates. The results of the distribution of mc5rl genes showed that mc5rl genes were dominant in the mantle, gonad, and hepatopancreas in R. philippinarum. The expression of mc5rl genes was significantly increased after aerial exposure and low-temperature stress in R. philippinarum in hepatopancreas. Aerial exposure and low-temperature stress could induce mc5rl expressed. Mc5rl might serve as a sensor and promote stress response in R. philippinarum. The cloning and expression characteristics of mc5rl will facilitate the investigation of its function in stress response and other physiological processes in R. philippinarum.

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.

Exacerbation of autoimmune uveitis by obesity occurs through the melanocortin 5 receptor

Autoimmune uveitis is a leading cause of blindness with a complex etiology. Obesity is considered a chronic disease with a connection with autoimmune diseases through systemic inflammation. However, an obesity and autoimmune disease connection is not consistently true in rodent models of autoimmune disease. A mouse model of human autoimmune uveitis, experimental autoimmune uveitis (EAU) has been used to better understand the immunobiology of uveitis. In this study, we assessed EAU in a high-fat diet (HFD) obesity model and found that the EAU severity is significantly higher in wild-type mice, but not in HFD melanocortin 5 receptor deficient mice. We find a decrease in CD11b⁺ F4/80⁺ Ly-6C^lo Ly-6G⁺ Mϕs, previously shown to be suppressive, and an enhancement of a Th1 response at the onset of EAU in obese mice. We further demonstrate that at recovery of EAU, obese mice lack regulatory immunity that provides protection from EAU. This report demonstrates that obesity exacerbates autoimmune uveitis and inhibits the promotion of post-EAU regulatory immunity through the melanocortin 5 receptor. The implication of this work is that obesity may contribute to the prevalence of autoimmune uveitis.

Comparative transcriptomic analysis of high and low egg-producing duck ovaries

The egg-laying rate is an important indicator of egg production of laying ducks. Egg production directly impacts the economic benefits of the duck industry. In order to obtain better insight into the molecular mechanisms associated with the process of egg production, comparative transcriptomic analysis of the ovaries of Jinding ducks with high and low egg production was performed using the Illumina HiSeq 2500 system. A total of 843 differentially expressed genes (DEGs) was identified, 367 that were down-regulated and 476 that were up-regulated in high egg production (HEP) ovaries, as compared with low egg production (LEP) ovaries. Some genes, such as MC5R, APOD, ORAI1, and DYRK4, were more active in HEP ovaries, indicating that these genes may play important roles in regulation of egg production. Among these 843 DEGs, 685 were assigned to gene ontology (GO) categories. Of these, 25 genes were related to reproduction, and 30 were related to the reproductive process, including some associated with ovarian follicle development, circadian regulation of gene expression, circadian rhythm, and estrogen receptor binding. Furthermore, some important functional pathways were revealed, such as the steroid biosynthesis pathway, the endocrine and other factor-regulated calcium reabsorption pathways, circadian rhythm, the neuroactive ligand-receptor interaction pathway, fatty acid biosynthesis, and the calcium-signaling pathway, which appear to be much more active in the HEP group, as compared to those of the LEP group. The results of this study provide very useful information that may contribute to future functional studies of genes involved in bird reproduction.

αMSH inhibits adipose inflammation via reducing FoxOs transcription and blocking Akt/JNK pathway in mice

Alpha melanocyte stimulating hormone (αMSH) abates inflammation in multiple tissues, while Forkhead box proteins O (FoxOs) stimulate inflammatory cascade. However, the relationship between αMSH and FoxOs in adipose inflammation remains unclear. In this study, we used LPS-induced inflammation model, attempted to interpret the function of αMSH in inflammation and the interactions with FoxOs. Results indicated that upon inflammatory situation, the secretion of αMSH and the expression of its receptor MC5R were greatly decreased, but FoxOs expressions were elevated. After the treatment with αMSH, LPS-induced adipose inflammation together with FoxOs expressions was significantly reduced. Conversely, when Foxo1, Foxo3a or Foxo4 overexpressed in αMSH treated inflammatory mouse model, all the anti-inflammatory impacts of αMSH were found disappeared. We further studied the mechanisms by which αMSH exerts its anti-inflammatory impacts and how FoxOs reverse αMSH's function. Foxo4 was found as a negative regulator for MC5R transcription in αMSH inhibited inflammation. Moreover, a negative role was found of αMSH in regulating both Akt and JNK signal pathways by observing the enhanced the anti-inflammatory impacts of pathway-specific inhibitors with αMSH treatment. Our findings demonstrate αMSH plays a key role in the prevention of adipose inflammation and inflammatory diseases by down-regulating Akt/JNK signal pathway and negatively interacting with FoxOs, which brings up αMSH as a novel candidate factor in the adipose anti-inflammation process in obesity.

Long Term Osmotic Mini Pump Treatment with Alpha-MSH Improves Myocardial Function in Zucker Diabetic Fatty Rats

The present investigation evaluates the cardiovascular effects of the anorexigenic mediator alpha-melanocyte stimulating hormone (MSH), in a rat model of type 2 diabetes. Osmotic mini pumps delivering MSH or vehicle, for 6 weeks, were surgically implanted in Zucker Diabetic Fatty (ZDF) rats. Serum parameters, blood pressure, and weight gain were monitored along with oral glucose tolerance (OGTT). Echocardiography was conducted and, following sacrifice, the effects of treatment on ischemia/reperfusion cardiac injury were assessed using the isolated working heart method. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity was measured to evaluate levels of oxidative stress, and force measurements were performed on isolated cardiomyocytes to determine calcium sensitivity, active tension and myofilament co-operation. Vascular status was also evaluated on isolated arterioles using a contractile force measurement setup. The echocardiographic parameters ejection fraction (EF), fractional shortening (FS), isovolumetric relaxation time (IVRT), mitral annular plane systolic excursion (MAPSE), and Tei-index were significantly better in the MSH-treated group compared to ZDF controls. Isolated working heart aortic and coronary flow was increased in treated rats, and higher Hill coefficient indicated better myofilament co-operation in the MSH-treated group. We conclude that MSH improves global heart functions in ZDF rats, but these effects are not related to the vascular status.

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.

ACTH Receptor (MC2R) Specificity: What Do We Know About Underlying Molecular Mechanisms?

Coincidentally, the release of this Research Topic in Frontiers in Endocrinology takes place 25 years after the discovery of the adrenocorticotropic hormone receptor (ACTHR) by Mountjoy and colleagues. In subsequent years, following the discovery of other types of mammalian melanocortin receptors (MCRs), ACTHR also became known as melanocortin type 2 receptor (MC2R). At present, five types of MCRs have been reported, all of which share significant sequence similarity at the amino acid level, and all of which specifically bind melanocortins (MCs)-a group of biologically active peptides generated by proteolysis of the proopiomelanocortin precursor. All MCs share an identical -H-F-R-W- pharmacophore sequence. α-Melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH) are the most extensively studied MCs and are derived from the same region. Essentially, α-MSH is formed from the first 13 amino acid residues of ACTH. ACTHR is unique among MCRs because it binds one sole ligand-ACTH, which makes it a very attractive research object for molecular pharmacologists. However, much research has failed, and functional studies of this receptor are lagging behind other MCRs. The reason for these difficulties has already been outlined by Mountjoy and colleagues in their publication on ACTHR coding sequence discovery where the Cloudman S91 melanoma cell line was used for receptor expression because it was a "more sensitive assay system." Subsequent work showed that ACTHR could be successfully expressed only in endogenous MCR-expressing cell lines, since in other cell lines it is retained within the endoplasmic reticulum. The resolution of this methodological problem came in 2005 with the discovery of melanocortin receptor accessory protein, which is required for the formation of functionally active ACTHR. The decade that followed this discovery was filled with exciting research that provided insight into the molecular mechanisms underlying the action of ACTHR. The purpose of this review is to summarize the advances in this fascinating research field.

Role of ACTH in the Interactive/Paracrine Regulation of Adrenal Steroid Secretion in Physiological and Pathophysiological Conditions

In the normal human adrenal gland, steroid secretion is regulated by a complex network of autocrine/paracrine interactions involving bioactive signals released by endothelial cells, nerve terminals, chromaffin cells, immunocompetent cells, and adrenocortical cells themselves. ACTH can be locally produced by medullary chromaffin cells and is, therefore, a major mediator of the corticomedullary functional interplay. Plasma ACTH also triggers the release of angiogenic and vasoactive agents from adrenocortical cells and adrenal mast cells and, thus, indirectly regulates steroid production through modulation of the adrenal blood flow. Adrenocortical neoplasms associated with steroid hypersecretion exhibit molecular and cellular defects that tend to reinforce the influence of paracrine regulatory loops on corticosteroidogenesis. Especially, ACTH has been found to be abnormally synthesized in bilateral macronodular adrenal hyperplasia responsible for hypercortisolism. In these tissues, ACTH is detected in a subpopulation of adrenocortical cells that express gonadal markers. This observation suggests that ectopic production of ACTH may result from impaired embryogenesis leading to abnormal maturation of the adrenogonadal primordium. Globally, the current literature indicates that ACTH is a major player in the autocrine/paracrine processes occurring in the adrenal gland in both physiological and pathological conditions.

Intracellular signaling mechanisms of the melanocortin receptors: current state of the art

The melanocortin system is composed by the agonists adrenocorticotropic hormone and α, β and γ-melanocyte-stimulating hormone, and two naturally occurring antagonists, agouti and agouti-related protein. These ligands act by interaction with a family of five melanocortin receptors (MCRs), assisted by MCRs accessory proteins (MRAPs). MCRs stimulation activates different signaling pathways that mediate a diverse array of physiological processes, including pigmentation, energy metabolism, inflammation and exocrine secretion. This review focuses on the regulatory mechanisms of MCRs signaling, highlighting the differences among the five receptors. MCRs signal through G-dependent and independent mechanisms and their functional coupling to agonists at the cell surface is regulated by interacting proteins, namely MRAPs and β-arrestins. The knowledge of the distinct modulation pattern of MCRs signaling and function may be helpful for the future design of novel drugs able to combine specificity, safety and effectiveness in the course of their therapeutic use.

Replacement of short segments within transmembrane domains of MC2R disrupts retention signal

The proteolysis of the pro-opiomelanocortin precursor results in the formation of melanocortins (MCs), a group of peptides that share the conserved -H-F-R-W- sequence, which acts as a pharmacophore for five subtypes of MC receptors (MCRs). MC type 2 receptor (MC2R; also known as ACTHR) is the most specialized of all the MCRs. It is predominantly expressed in the adrenal cortex and specifically binds ACTH. Unlike other MCRs, it requires melanocortin receptor accessory protein 1 (MRAP) for formation of active receptor and for its transport to the cell membrane. The molecular mechanisms underlying this specificity remain poorly understood. In this study, we used directed mutagenesis to investigate the role of various short MC2R sequence segments in receptor membrane trafficking and specific activation upon stimulation with ligands. The strategy of the study was to replace two to five amino acid residues within one MC2R segment with the corresponding residues of MC4R. In total, 20 recombinant receptors C-terminally fused to enhanced green fluorescent protein were generated and their membrane trafficking efficiencies and cAMP response upon stimulation with α-MSH and ACTH(1-24) were estimated during their stand-alone expression and coexpression with MRAP. Our results indicate that both the motif that determines the ligand-recognition specificity and the intracellular retention signal are formed by a specific extracellular structure, which is supported by the correct alignment of the transmembrane domains. Our results also indicate that the aromatic-residue-rich segment of the second extracellular loop is involved in the effects mediated by the second ACTH pharmacophore (-K-K-R-R-).

Alpha-MSH signalling via melanocortin 5 receptor promotes lipolysis and impairs re-esterification in adipocytes

The melanocortin system has a clear effect on the mobilisation of stored lipids in adipocytes. The aim of the current study was to investigate the role of melanocortin 5 receptor (MC5R) on alpha-melanocyte-stimulating hormone (alpha-MSH)-induced lipolysis in 3T3-L1 adipocytes. To this end, MC5R expression was decreased by small interfering RNA (siRNA), which significantly impaired the alpha-MSH stimulation of lipolysis, as determined by glycerol and nonesterified fatty-acid (NEFA) quantification. The functional role of alpha-MSH/MC5R on triglyceride (TG) hydrolysis was mediated by hormone-sensitive lipase (HSL), adipose triglyceride lipase (ATGL), perilipin 1 (PLIN1) and acetyl-CoA carboxylase (ACC). Immunofluorescence microscopy revealed that phosphorylated HSL clearly surrounded lipid droplets in alpha-MSH-stimulated adipocytes, whereas PLIN1 left the immediate periphery of lipids. These observations were lost when the expression of MC5R was suppressed. In 3T3-L1 adipocytes, alpha-MSH-activated MC5R signals through the cAMP/PKA and MAPK/ERK1/2 pathways. PKA was fundamental for HSL and PLIN1 activation and lipolysis regulation. ERK1/2 inhibition strongly interfered with the release of NEFAs but not glycerol. In addition, the intracellular TG levels, which were decreased after MC5R activation, were restored after ERK1/2 inhibition, indicating that these kinases are involved in NEFA re-esterification rather than lipolysis regulation. This notion is also supported by the observation that the alpha-MSH-mediated activation of phosphoenolpyruvate carboxykinase (PEPCK) was abolished in the presence of ERK1/2 inhibitors. Altogether, these results indicate that alpha-MSH-activated MC5R regulates two tightly coupled pathways in adipocytes: lipolysis and re-esterification. The global effect is a decrease in adipocyte fat mass, which is important for strategies to ameliorate obesity.

Family of melanocortin receptor (MCR) genes in mammals-mutations, polymorphisms and phenotypic effects

The melanocortin receptor gene family consists of five single-exon members, which are located on autosomes. Three genes (MC2R, MC4R and MC5R) are syntenic in the human, mouse, cattle and dog genomes, while in the pig, the syntenic group comprises MC1R, MC2R and MC5R. Two genes (MC1R and MC4R) have been extensively studied due to their function in melanogenesis (MC1R) and energy control (MC4R). Conservative organisation of these genes in five mammalian species (human, mouse, cattle, pig and dog), in terms of the encoded amino acid sequence, is higher in the case of MC4R compared to MC1R. Polymorphisms of these two genes are responsible or associated with variation of pigmentation (MC1R) and adipose tissue deposition (MC4R). Polymorphic variants in MC1R, causing coat colour variation, were described in humans and domestic mammals (cattle, horse, pig, sheep, dog), as well as farm red and arctic foxes. The MC4R gene is very polymorphic in humans and it is well known that some variants cause monogenic obesity or significantly contribute to the development of polygenic obesity. Such relationships are not so evident in domestic mammals; however, at least one missense substitution (298Asp > Asn) in the porcine MC4R significantly contributes, at least in some breeds, to fat tissue accumulation, feed conversion ratio and daily weight gain. Knowledge on the phenotypic effects of polymorphisms of MC2R, MC3R and MC5R in domestic mammals is scarce, probably due to the small number of reports addressing these genes. Thus, further studies focused on these genes should be undertaken.

The melanocortin receptors and their accessory proteins

The five melanocortin receptors (MCRs) named MC1R-MC5R have diverse physiological roles encompassing pigmentation, steroidogenesis, energy homeostasis and feeding behavior as well as exocrine function. Since their identification almost 20 years ago much has been learnt about these receptors. As well as interacting with their endogenous ligands the melanocortin peptides, there is now a growing list of important peptides that can modulate the way these receptors signal, acting as agonists, antagonists, and inverse agonists. The discovery of melanocortin 2 receptor accessory proteins as a novel accessory factor to the MCRs provides further insight into the regulation of these important G protein-coupled receptor.

Functional melanocortin-2 receptors are expressed by mouse aorta-derived mesenchymal progenitor cells

A local melanocortin system is active during tissue injury and inflammation. Thus far this system has been described as autocrine in nature where local production of pro-opiomelanocortin (POMC) peptides by leukocytes feeds back on melanocortin receptor (MC-R) expressing immune cells to quell inflammatory cytokine production. Here we present evidence that POMC peptides may generate extracellular matrix (ECM) changes by inducing matrix production by cells of the mesenchymal lineage through activation of the MC2-R. Using immunoblot, we determined that mouse aorta-derived mesenchymal progenitor cells express both MC2-R and MC3-R. These progenitors respond to treatment with ACTH by increasing collagen matrix synthesis as assessed by picrosirius red stain and (3)H-proline incorporation. ACTH also induces transient increases in intracellular calcium ([Ca(2+)](i)) as assessed using the fluorescent Ca(2+) indicator, fura-2. The ACTH-induced changes in [Ca(2+)](i) are consistent with MC2-R signaling and consist of both an intracellular release and an extracellular influx of Ca(2+). Both mouse aortic mesenchymal progenitors and mouse macrophage cells express POMC and the prohormone convertase 1/3 (PC1/3) indicating they have the potential to contribute to the local production of POMC peptides. These data demonstrate functional MC2-R expression in mouse aorta-derived mesenchymal progenitors and implicate both macrophage and mesenchymal cells as relevant sources of local POMC peptides.

Development of melanoma-targeted polymer micelles by conjugation of a melanocortin 1 receptor (MC1R) specific ligand

The incidence of malignant melanoma is rising faster than that of any other cancer in the United States. Because of its high expression on the surface of melanomas, MC1R has been investigated as a target for selective imaging and therapeutic agents against melanoma. Eight ligands were screened against cell lines engineered to overexpress MC1R, MC4R, or MC5R. Of these, compound 1 (4-phenylbutyryl-His-dPhe-Arg-Trp-NH(2)) exhibited high (0.2 nM) binding affinity for MC1R and low (high nanomolar) affinities for MC4R and MC5R. Functionalization of the ligand at the C-terminus with an alkyne for use in Cu-catalyzed click chemistry was shown not to affect the binding affinity. Finally, formation of the targeted polymer, as well as the targeted micelle formulation, also resulted in constructs with low nanomolar binding affinity.

Nr4a1 siRNA expression attenuates α-MSH regulated gene expression in 3T3-L1 adipocytes

Several recent investigations have underscored the growing role of melanocortin signaling in the peripheral regulation of lipid, glucose, and energy homeostasis. In addition, the melanocortins play a critical role in the central control of satiety. These observations, and the latest reports highlighting the emerging role of the nuclear hormone receptor (NR) 4A subgroup in metabolism, have prompted us to investigate the cross talk between [Nle(4), d-Phe(7)] (NDP)-α-MSH and Nr4a signaling in adipose. We have shown that NDP-MSH strikingly and preferentially induces the expression of the NR4A subgroup (but not any other members of the NR superfamily) in differentiated 3T3-L1 adipocytes. Utilization of quantitative PCR on custom-designed metabolic TaqMan low-density arrays identified the concomitant and marked induction of the mRNAs encoding Il-6, Cox2, Pdk4, and Pck-1 after NDP-MSH treatment. Similar experiments demonstrated that the mRNA expression profile induced by cAMP and NDP-MSH treatment displayed unique but also overlapping properties and suggested that melanocortin-mediated induction of gene expression involves cAMP-dependent and -independent signaling. Nr4a1/Nur77 small interfering RNA (siRNA) expression suppressed NDP-MSH-mediated induction of Nr4a1/Nur77 and Nr4a3/Nor-1 (but not Nr4a2/Nurr1). Moreover, expression of the siRNA-attenuated NDP-MSH mediated induction of the mRNAs encoding Il-6, Cox2/Ptgs2, and Pck-1 expression. In addition, Nur77 siRNA expression attenuated NDP-MSH-mediated glucose uptake. In vivo, ip administration of NDP-MSH to C57 BL/6J (male) mice significantly induced the expression of the mRNA encoding Nur77 and increased IL-6, Cox2, Pck1, and Pdk4 mRNA expression in (inguinal) adipose tissue. We conclude that Nur77 expression is necessary for MSH-mediated induction of gene expression in differentiated adipocytes. Furthermore, this study demonstrates cross talk between MSH and Nr4a signaling in adipocytes.