Melanocortin receptor-mediated mobilization of intracellular free calcium in HEK293 cells

Melanocortin-4 receptor-mediated inhibition of apoptosis in immortalized hypothalamic neurons via mitogen-activated protein kinase

The melanocortin-4 receptor (MC4R) is a seven transmembrane member of the melanocortin receptor family. The GT1-1 cell line exhibits endogenous expression of MC4R. In this study, GT1-1 cells were used to study MC4R signaling pathways and to examine the effects of melanocortin receptor agonist NDP-MSH on apoptosis. MC4R mRNA expression was demonstrated by RT-PCR. Functional melanocortin receptor expression was implied by specific binding of NDP-MSH and cAMP production. NDP-MSH-stimulated GnRH release in a dose-dependent manner. Serum deprivation-induced apoptosis in GT1-1 cells, and the NDP-MSH inhibited this effect. The melanocortin receptor antagonist SHU9119 blocked the antiapoptotic actions of NDP-MSH, and the MAP kinase inhibitor PD98059 significantly attenuated the antiapoptotic effect. NDP-MSH-stimulated ERK1/2 phosphorylation in a dose-dependent manner. ERK1/2 phosphorylation could be abolished by SHU9119. In GT1-1 cells, melanocortin receptor activation causes ERK1/2 phosphorylation. In these cells, MC4R activation is also associated with antiapoptotic effects.

The melanocortin system in leukocyte biology

The melanocortin system is composed of the melanocortin peptides, adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormone, the melanocortin receptors (MCRs), and the endogenous antagonists agouti- and agouti-related protein. Melanocortin peptides exert multiple effects upon the host, including anti-inflammatory and immunomodulatory effects. Leukocytes are a source of melanocortins and a major target for these peptides. Because of reduced translocation of the nuclear factor NF-kappaB to the nucleus, MCR activation by their ligands causes a collective reduction of the most important molecules involved in the inflammatory process. This review examines how melanocortin peptides and their receptors participate in leukocyte biology.

Melanocortin Receptor Ligands: New Horizons for Skin Biology and Clinical Dermatology

The melanocortin (MC) system is probably the best characterized neuropeptide network of the skin. Most cutaneous cell types express MC receptors (MC-Rs) and synthesize MCs, such as alpha-melanocyte-stimulating hormone (alpha-MSH), that act in autocrine and paracrine fashion. In human skin cells, activation of adenylate cyclase by MCs occurs at 10(-6)-10(-9) M doses of the ligand, but effects are induced in some cell types at subnanomolar concentrations. In addition to the pigmentary action of MCs on epidermal melanocytes, the hair follicle is a source and target for MCs. MCs regulate lipogenesis in sebocytes expressing both MC-1R and MC-5R. In adipocytes, lipid metabolism is modulated by agouti signalling protein, a natural MC-1R/MC-4R antagonist. The anti-inflammatory activity of alpha-MSH includes immunomodulatory effects on several resident skin cells and antifibrogenic effects mediated via MC-1R expressed by dermal fibroblasts. In human mast cells, alpha-MSH appears to be proinflammatory due to histamine release. alpha-MSH exhibits cytoprotective activity against UVB-induced apoptosis and DNA damage, a finding that helps explain the increased risk of cutaneous melanoma in individuals with loss of function MC-1R mutations. These findings should improve our understanding of skin physiology and pathophysiology and may offer novel strategies with MCs as future therapeutics for skin diseases.

Activation of the melanocortin-4 receptor mobilizes intracellular free calcium in immortalized hypothalamic neurons

BACKGROUND

At least 4% of childhood obesity is due to mutations in the hypothalamic melanocortin-4 receptor. The melanocortin-4 receptor, a seven transmembrane G-protein-coupled receptor, is important in the regulation of feeding behavior and body weight. The specific pathways of intracellular signaling remain in investigative stages. To further understand its function, we hypothesized that the melanocortin-4 receptor activates the Galphaq/phospholipase C signaling pathway, resulting in alterations of cytoplasmic calcium in immortalized hypothalamic (GT1-1) neurons.

MATERIALS AND METHODS

Changes in intracellular calcium were measured after loading GT1-1 cells with fura-2-AM. Cells were treated with NDP-alphaMSH, an alpha-melanocortin stimulating hormone analogue, and intracellular calcium changes were recorded. Cells treated with NDP-alpha-MSH were also treated with the melanocortin-4 receptor antagonist, SHU-9119. To assess the specific G-protein subunit involved, GT1-1 neurons were treated with the phospholipase C inhibitor U73122 and its inactive analogue, U73433. Experiments were also performed after inhibition of IP3 receptors with 2-aminoethoxydiphenylborate (2APB). Additional experiments were conducted in a calcium-depleted buffer environment. Data were analyzed by ANOVA with statistical significance of P < 0.05.

RESULTS

Agonist treatment (0.01-1000 nm) of GT1-1 neurons resulted in dose-dependent increases in intracellular calcium. SHU-9119 (0.01-1000 nm) abolished the calcium response. Treatment with U73122 (10 microm) attenuated the calcium response, while U73433 (10 microm) had minimal effect. 2APB (200 microm) inhibited the calcium transient, and the use of calcium-free buffer did not affect the amplitude of the calcium spike.

CONCLUSIONS

Our study demonstrates that, upon agonist binding, the melanocortin-4 receptor mediates increases in intracellular calcium through the Galphaq-protein/phospholipase C dependent signaling pathway. Understanding the physiological importance of calcium signaling by the melanocortin-4 receptor may be important for future development of therapeutic targets.

Evidence for a stimulatory action of melanin-concentrating hormone on luteinising hormone release involving MCH1 and melanocortin-5 receptors

The present series of studies aimed to further our understanding of the role of melanin-concentrating hormone (MCH) neurones in the central regulation of luteinising hormone (LH) release in the female rat. LH release was stimulated when MCH was injected bilaterally into the rostral preoptic area (rPOA) or medial preoptic area (mPOA), but not when injected into the zona incerta (ZI), of oestrogen-primed ovariectomised rats. In rats that were steroid-primed to generate a surge-like release of LH, MCH administration into the ZI blocked this rise in LH release: no such effect occurred when MCH was injected into the rPOA or mPOA. In vitro, MCH stimulated gonadotrophin-releasing hormone (GnRH) release from hypothalamic explants. Double-label immunohistochemistry showed GnRH-immunoreactive neurones in the vicinity of and intermingled with immunoreactive MCH processes. MCH is the endogenous ligand of the MCH type 1 receptor (MCH1-R). Previously, we have shown a role for melanocortin-5 receptors (MC5-R) in the stimulatory action of MCH, so we next investigated the involvement of both MCH1-R and/or MC5-R in mediating the actions of MCH on GnRH and hence LH release. The stimulatory action of MCH in the rPOA was inhibited by administration of antagonists for either MCH1-R or MC5-R. However, in the mPOA, the action of MCH was blocked only by the MC5-R antagonist. LH release was stimulated by an agonist for MC5-R injected into the rPOA or mPOA; this was blocked by the MC5-R antagonist but not the MCH1-R antagonist. These results indicate that both MCH1-R and MC5-R are involved in the central control of LH release by MCH.

Melanocortin-5 receptor: a marker of human sebocyte differentiation

Melanocortin receptors (MC1R-MC5R) and their ligands (melanocyte-stimulating hormone (MSH) and adrenocorticotrophin hormone (ACTH)) have been shown to influence physiological functions of cells and organs, including exocrine glands. Since relatively little is known about MC5R expression and function in the human sebaceous gland, we examined expression of MC5R by immunohistochemistry and RT-PCR in human sebaceous cells in vivo and in vitro. In human skin, MC5R was detected only in differentiating, lipid-laden sebaceous cells but not in basal, undifferentiated sebaceous cells. Similarly, in cultured human sebocytes MC5R was only detectable at the onset of differentiation and in fully differentiated cells displaying prominent lipid granules. The lipid profile of the cultured and differentiated human sebocytes was shown to be human sebum-specific using (14)C-acetate labeling and high performance thin layer chromatography. Our studies suggest that MC5R is a marker of human sebocyte differentiation.

Roles of acetylation and other post-translational modifications in melanocortin function and interactions with endorphins

Phylogenetic, developmental, anatomic, and stimulus-specific variations in post-translational processing of POMC are well established. For melanocortins, the role of alpha-N-acetylation and the selective activities of alpha, beta, and gamma forms are of special interest. Acetylation may shift the predominant activity of POMC products between endorphinergic and melanocortinergic actions-which are often in opposition. This review addresses: (1) variations in POMC processing; (2) the influence of acetylation on the functional activity of alpha-MSH; (3) state- and stimulus-dependent effects on the proportional distribution of forms of melanocortins and endorphins; (4) divergent effects of alpha-MSH and beta-endorphin administration; (5) potential roles of beta- and gamma-MSH.

Melanocortin-1 receptor structure and functional regulation

The melanogenic actions of the melanocortins are mediated by the melanocortin-1 receptor (MC1R). MC1R is a member of the G-protein-coupled receptors (GPCR) superfamily expressed in cutaneous and hair follicle melanocytes. Activation of MC1R by adrenocorticotrophin or alpha-melanocyte stimulating hormone is positively coupled to the cAMP signaling pathway and leads to a stimulation of melanogenesis and a switch from the synthesis of pheomelanins to the production of eumelanic pigments. The functional behavior of the MC1R agrees with emerging concepts in GPCR signaling including dimerization, coupling to more than one signaling pathway and a high agonist-independent constitutive activity accounting for inverse agonism phenomena. In addition, MC1R displays unique properties such as an unusually high number of natural variants often associated with clearly visible phenotypes and the occurrence of endogenous peptide antagonists. Therefore MC1R is an ideal model to study GPCR function. Here we review our current knowledge of MC1R structure and function, with emphasis on information gathered from the analysis of natural variants. We also discuss recent data on the regulation of MC1R function by paracrine and endocrine factors and by external stimuli such as ultraviolet light.

Interactions of human melanocortin 4 receptor with nonpeptide and peptide agonists

Specific interactions of human melanocortin-4 receptor (hMC4R) with its nonpeptide and peptide agonists were studied using alanine-scanning mutagenesis. The binding affinities and potencies of two synthetic, small-molecule agonists (THIQ, MB243) were strongly affected by substitutions in transmembrane alpha-helices (TM) 2, 3, 6, and 7 (residues Glu(100), Asp(122), Asp(126), Phe(261), His(264), Leu(265), and Leu(288)). In addition, a I129A mutation primarily affected the binding and potency of THIQ, while F262A, W258A, Y268A mutations impaired interactions with MB243. By contrast, binding affinity and potency of the linear peptide agonist NDP-MSH were substantially reduced only in D126A and H264A mutants. Three-dimensional models of receptor-ligand complexes with their agonists were generated by distance-geometry using the experimental, homology-based, and other structural constraints, including interhelical H-bonds and two disulfide bridges (Cys(40)-Cys(279), Cys(271)-Cys(277)) of hMC4R. In the models, all pharmacophore elements of small-molecule agonists are spatially overlapped with the corresponding key residues (His(6), d-Phe(7), Arg(8), and Trp(9)) of the linear peptide: their charged amine groups interact with acidic residues from TM2 and TM3, similar to His(6) and Arg(6) of NDP-MSH; their substituted piperidines mimic Trp(9) of the peptide and interact with TM5 and TM6, while the d-Phe aromatic rings of all three agonists contact with Leu(133), Trp(258), and Phe(261) residues.

Activation of the cAMP pathway by variant human MC1R alleles expressed in HEK and in melanoma cells

Alpha-melanocyte-stimulating hormone (alpha-MSH) activates the melanocortin-1 receptor (MC1R) on melanocytes to promote a switch from red/yellow pheomelanin synthesis to darker eumelanins via positive coupling to adenylate cyclase. The human MC1R locus is highly polymorphic with the specific variants associated with red hair and fair skin (RHC phenotype) postulated to be loss-of-function receptors. We have examined the ability of MC1R variants to activate the cAMP pathway in stably transfected HEK293 cells. The RHC associated variants, Arg151Cys, Arg160Trp and Asp294His, demonstrated agonist-mediated increases in cAMP and phosphorylation of cAMP-responsive element-binding protein (CREB). Whereas the Asp294His variant showed severely impaired functional responses, the Arg151Cys and Arg160Trp variants retained considerable signaling capacity. Melanoma cells homozygous for either the Arg151Cys variant or consensus sequence both elicited CREB phosphorylation in response to alpha-MSH in the presence of IBMX. The common RHC alleles, Arg151Cys, Arg160Trp and Asp294His, are neither complete loss-of-function receptors nor are they functionally equivalent.

Evidence for direct actions of melanocortin peptides on bone metabolism

Expression of melanocortin-4 receptor (MC4R) mRNA in developing rat limb buds, teeth, and skull bone first indicated a possible role for MC4R in bone metabolism. We therefore investigated whether MC4R mRNA was expressed in the rat osteosarcoma UMR106.06 cell line and in primary rat osteoblast cells. Reverse transcriptase-polymerase chain reaction (RT-PCR), Northern blot analysis, and ribonuclease protection assay (RPA) were used to demonstrate MC4R mRNA expression in UMR106.06 and primary osteoblast cells. MC4R mRNA was found to be localized to the periosteum of mouse bone using in situ hybridization. We also used RT-PCR and rat specific MC2R and MC5R oligonucleotides to amplify the correct size DNA fragments for these melanocortin receptors from rat primary osteoblasts. In conclusion, melanocortin receptor expression in mouse periosteum and rat osteoblasts suggests a direct role for POMC derived peptides in bone development and bone metabolism.

Adrenocorticotropin evokes transient elevations in intracellular free calcium ([Ca2+]i) and increases basal [Ca2+]i in resting chondrocytes through a phospholipase C-dependent mechanism

Both clinical and in vitro evidence points to the involvement of the melanocortin peptide, ACTH, in the terminal differentiation of chondrocytes. Terminal differentiation along the endochondral pathway is responsible for linear growth, but also plays a role in osteoarthritic cartilage degeneration. Chondrocyte terminal differentiation is associated with an incremental increase in chondrocyte basal intracellular free calcium ([Ca(2+)](i)), and ACTH agonism of melanocortin receptors is known to mobilize [Ca(2+)](i.) Using differentiated resting chondrocytes highly expressing type II collagen and aggrecan, we examined the influence of both ACTH and dexamethasone treatment on matrix gene transcription and [Ca(2+)](i). Resting chondrocytes treated concurrently with dexamethasone and ACTH expressed matrix gene transcripts in a pattern consistent with that of rapid terminal differentiation. Using the fluorescent Ca(2+) indicator, fura-2, we determined that ACTH evokes transient increases in [Ca(2+)](i) and elevates basal Ca(2+) levels in resting chondrocytes. The transient increases were initiated intracellularly, were abrogated by the phospholipase C-specific inhibitor, U73122, and were partly attenuated by myo-inositol 1,4,5-triphosphate receptor inhibition via 10 mm caffeine. The initial intracellular release also resulted in store-operated calcium entry, presumably through store-operated channels. Dexamethasone priming increased both the initial ACTH-evoked [Ca(2+)](i) release and the subsequent store-operated calcium entry. These data demonstrate roles for ACTH and glucocorticoid in the regulation of chondrocyte terminal differentiation. Because the actions of ACTH are mediated through known G protein-coupled receptors, the melanocortin receptors, these data may provide a new therapeutic target in the treatment of growth deficiencies and cartilage degeneration.

Functional selectivity of melanocortin 4 receptor peptide and nonpeptide agonists: evidence for ligand-specific conformational states

Agonists of the melanocortin 4 (MC4) receptor have potential pharmaceutical benefit in the treatment of obesity and sexual dysfunction. In this study, we have compared the ability of a number of peptide and nonpeptide agonists to activate a FLAG-tagged human MC4 (FMC4) receptor, as measured by both cAMP accumulation and calcium mobilization using a fluorometric imaging plate reader (FLIPR). In addition, we have analyzed the ability of these agonists to cause receptor internalization, as measured by fluorescence-activated cell sorting analysis. The endogenous agonist alpha-melanocortin-stimulating hormone (alpha-MSH) increased cAMP accumulation, calcium mobilization, and receptor internalization in a dose-dependent manner in human embryonic kidney 293 cells expressing the FMC4 receptor. The activity of the other agonists varied considerably in these assays, and overall, the potency and intrinsic activity of the agonists in the cAMP accumulation assays did not correlate with their potency or intrinsic activity in either the FLIPR or receptor internalization assays. Agonists could be clearly separated into two functional classes based on their structure. Peptide agonists beta-MSH, des-acetyl-alpha-MSH, and [Nle(4), D-Phe(7)]-alpha-melanocortin-stimulating hormone exhibited 80 to 112% of the maximal alpha-MSH response in cAMP accumulation and 62 to 96% in FLIPR assays and were able to cause 75 to 118% of receptor internalization induced by alpha-MSH. Conversely, although the nonpeptide agonists exhibited 73 to 149% of the alpha-MSH response in the cAMP accumulation assays, they were significantly impaired in the FLIPR (7-40%) and receptor internalization (-5-38%) assays. These findings demonstrate an important difference in activation and internalization of the MC4 receptor by nonpeptide versus peptide agonists and provides evidence of agonist-specific conformational states.

Receptor−Antagonist Interactions in the Complexes of Agouti and Agouti-Related Protein with Human Melanocortin 1 and 4 Receptors,

The molecular interactions between human melanocortin receptor-1 and -4 (hMC1R and hMC4R) and their endogenous antagonists, agouti signaling protein (ASIP) and agouti-related protein (AGRP), were assessed by studying the effects of site-directed mutations on the binding affinity of (125)I-ASIP[90-132(L89Y)] and (125)I-AGRP(86-132). Mutations of homologous residues from transmembrane helices (TMHs) 3 and 6 and extracellular loop (EL) 3 (D121A, T124A, F257A, and F277M in hMC1R and D126A, I129A F261A, and M281F in hMC4R) impaired binding of both antagonists to hMC4R and binding of the ASIP fragment to hMC1R. However, the mutations in TMH2 (E94A in hMC1R and E100A in hMC4R), TMH7 (F280A in hMC1R and F284A in hMC4R), and EL2 (Y183S, H184S, and D184H in hMC1R) only significantly affected binding of the ASIP fragment. The dependence of agonist binding on the dithiothreitol concentration followed a monophasic curve for wild-type hMC4R and its C40A, C271A, and C279A mutants and a biphasic curve for hMC1R, suggesting the presence of at least one structurally and functionally essential disulfide bond in both wild-type receptors and the hMC4R mutants. Models of complexes of both receptors with the ASIP fragment and hMC4R with the AGRP fragment were calculated using constraints from the experimental structures of rhodopsin and AGRP fragments, a set of deduced hydrogen bonds, supplemented by two proposed disulfide bridges and receptor-ligand contacts, derived from our mutagenesis data. In the models of the ASIP fragment complexed with both receptors, the core ligand tripeptide, Arg-Phe-Phe, positioned between TMHs 3 and 6, is shifted toward TMHs 2 and 7 relative to its position in the AGRP-hMC4R model, while the N-terminal loop and two central disulfides of the antagonists interact with EL2 of the receptors.

Activation of MAP kinase by MC4-R through PI3 kinase

The melanocortin 4 receptor (MC4-R) is a Galpha s-coupled receptor known to increase cAMP production following agonist stimulation. We demonstrate that the mitogen-activated protein kinases p42 (ERK2) and p44 (ERK1) are also activated by MC4-R following treatment with the MC4-R agonist NDP-alpha-MSH in stably transfected CHO-K1 cells. This time- and dose-dependent response is abolished by the MC4-R antagonist SHU-9119. p42/p44 MAPK activation is blocked by the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002 but not by the protein kinase A (PKA) inhibitor Rp-cAMPS, indicating that that signal activating the p42/p44 MAPK pathway is conveyed through inositol triphosphate.

Melanocortin 1 receptor variants, pigmentation, and skin cancer susceptibility

The melanocortin 1 receptor is a key regulator of variation in normal human pigmentation. Genetic variants of this receptor cause red hair and fair skin, and several case-control studies have demonstrated that these genetic variants increase the risk of skin cancer development in humans. The mechanism whereby the risks of skin cancer are increased is not entirely clear, and may be because of a combination of effects on pigmentation and non-pigmentary pathways.

ACTH enhances chondrogenesis in multipotential progenitor cells and matrix production in chondrocytes

The association of melanocortin peptide overproduction with enhanced linear growth prompted the current investigation of adrenocorticotropin hormone (ACTH) effects on multipotential chondroprogenitor populations and committed chondrocytes in culture. Two multipotential progenitor populations, rat bone marrow stromal cells (BMSC) and the clonal multipotential cell line RCJ3.1, and two committed chondrocyte populations, resting chondrocytes (RC) isolated from the rib of young rats and the chondrocyte restricted cell line RCJ3.1C5.18 (C5.18), were cultured in differentiation medium plus or minus ACTH. Alcian blue stain was used to quantitate proteoglycan matrix production in all populations treated with a range of ACTH concentrations. Changes in proliferation due to ACTH treatment of all cell types were measured using 3H-thymidine incorporation. Differences in matrix production of ACTH-treated and -untreated RC and C5.18 cells were determined using 3H-proline incorporation. Relative transcript expression of the chondrocyte matrix proteins collagen type II (COLL II) and aggrecan (AGR) in treated and untreated cells was analyzed by Northern blot. Collagen type X (COLL X), a marker of hypertrophic differentiation, was measured in committed chondrocytic populations. Western analysis was used to detect the melanocortin-3 receptor (MC3-R), which was a suspected mediator of the ACTH signal. Matrix deposition was dose-dependently increased by ACTH in all cell populations as measured by alcian blue stain. ACTH treatment increased proliferation in multipotential progenitor populations (BMSC and RCJ3.1) while proliferation was decreased in committed chondrocyte populations (RC and C5.18). Total protein and total cell-associated collagen production were significantly increased by ACTH treatment in committed populations. Relative COLL II and AGR transcript expressions were significantly increased in both the RC- and C5.18-committed population and very significantly increased in the progenitor populations. Additionally, collagen type X expression was detected earlier and in greater abundance in ACTH-treated committed chondrocyte populations. Finally, the melanocortin-3 receptor was detected in all examined cell types by Western blot. These data show that ACTH promotes the development of the chondrocyte phenotype from multipotential mesenchymal progenitor populations and increases matrix production and differentiation of committed chondrocytes. These findings, together with the detection of the MC3-R in all of these cell types, indicate a role for the melanocortin system in chondrogenesis.

Liver-specific expression of the agouti gene in transgenic mice promotes liver carcinogenesis in the absence of obesity and diabetes

Background

The agouti protein is a paracrine factor that is normally present in the skin of many species of mammals. Agouti regulates the switch between black and yellow hair pigmentation by signalling through the melanocortin 1 receptor (Mc1r) on melanocytes. Lethal yellow (A^y) and viable yellow (A^vy) are dominant regulatory mutations in the mouse agouti gene that cause the wild-type protein to be produced at abnormally high levels throughout the body. Mice harboring these mutations exhibit a pleiotropic syndrome characterized by yellow coat color, obesity, hyperglycemia, hyperinsulinemia, and increased susceptibility to hyperplasia and carcinogenesis in numerous tissues, including the liver. The goal of this research was to determine if ectopic expression of the agouti gene in the liver alone is sufficient to recapitulate any aspect of this syndrome. For this purpose, we generated lines of transgenic mice expressing high levels of agouti in the liver under the regulatory control of the albumin promoter. Expression levels of the agouti transgene in the liver were quantified by Northern blot analysis. Functional agouti protein in the liver of transgenic mice was assayed by its ability to inhibit binding of the α-melanocyte stimulating hormone (αMSH) to the Mc1r. Body weight, plasma insulin and blood glucose levels were analyzed in control and transgenic mice. Control and transgenic male mice were given a single intraperitoneal injection (10 mg/kg) of the hepatocellular carcinogen, diethylnitrosamine (DEN), at 15 days of age. Mice were euthanized at 36 or 40 weeks after DEN injection and the number of tumors per liver and total liver weights were recorded.

Results

The albumin-agouti transgene was expressed at high levels in the livers of mice and produced a functional agouti protein. Albumin-agouti transgenic mice had normal body weights and normal levels of blood glucose and plasma insulin, but responded to chemical initiation of the liver with an increased number of liver tumors compared to non-transgenic control mice.

Conclusions

The data demonstrate that liver-specific expression of the agouti gene is not sufficient to induce obesity or diabetes, but, in the absence of these factors, agouti continues to promote hepatocellular carcinogenesis.

Gamma-MSH, sodium metabolism, and salt-sensitive hypertension

Alpha-, beta-, and gamma-melanocyte stimulating hormones (MSHs) are melanotropin peptides that are derived from the ACTH/beta-endorphin prohormone proopiomelanocortin (POMC). They have been highly conserved through evolutionary development, although their functions in mammals have remained obscure. The identification in the last decade of a family of five membrane-spanning melanocortin receptors (MC-Rs), for which the melanotropins are the natural ligands, has permitted the characterization of a number of important actions of these peptides, although the physiological function(s) of gamma-MSH have remained elusive. Much evidence indicates that gamma-MSH stimulates sympathetic outflow and raises blood pressure through a central mechanism. However, this review focuses on newer cardiovascular and renal actions of the peptide, acting in most cases through the MC3-R. In rodents, a high-sodium diet (HSD) increases the pituitary abundance of POMC mRNA and of gamma-MSH content and results in a doubling of plasma gamma-MSH concentration. The peptide is natriuretic and acts through renal MC3-Rs, which are also upregulated by the HSD. Thus the system appears designed to participate in the integrated response to dietary sodium excess. Genetic or pharmacologic induction of gamma-MSH deficiency results in marked salt-sensitive hypertension that is corrected by the administration of the peptide, probably through a central site of action. Deletion of the MC3-R also produces salt-sensitive hypertension, which, however, is not corrected by infusion of the hormone. These observations in aggregate suggest the operation of a hormonal system important in blood pressure control and in the regulation of sodium excretion. The relationship of these two actions to each other and the significance of this system in humans are important questions for future research.

Alpha-melanocyte-stimulating hormone stimulates oxytocin release from the dendrites of hypothalamic neurons while inhibiting oxytocin release from their terminals in the neurohypophysis

The peptides alpha-melanocyte stimulating hormone (alpha-MSH) and oxytocin, when administered centrally, produce similar behavioral effects. alpha-MSH induces Fos expression in supraoptic oxytocin neurons, and alpha-MSH melanocortin-4 receptors (MC4Rs) are highly expressed in the supraoptic nucleus, suggesting that alpha-MSH and oxytocin actions are not independent. Here we investigated the effects of alpha-MSH on the activity of supraoptic neurons. We confirmed that alpha-MSH induces Fos expression in the supraoptic nucleus when injected centrally and demonstrated that alpha-MSH also stimulates Fos expression in the nucleus when applied locally by retrodialysis. Thus alpha-MSH-induced Fos expression is not associated with electrophysiological excitation of supraoptic neurons because central injection of alpha-MSH or selective MC4 receptor agonists inhibited the electrical activity of oxytocin neurons in the supraoptic nucleus recorded in vivo. Consistent with these observations, oxytocin secretion into the bloodstream decreased after central injection of alpha-MSH. However, MC4R ligands induced substantial release of oxytocin from dendrites in isolated supraoptic nuclei. Because dendritic oxytocin release can be triggered by changes in [Ca2+]i, we measured [Ca2+]i responses in isolated supraoptic neurons and found that MC4R ligands induce a transient [Ca2+]i increase in oxytocin neurons. This response was still observed in low extracellular Ca2+ concentration and probably reflects mobilization of [Ca2+]i from intracellular stores rather than entry via voltage-gated channels. Taken together, these results show for the first time that a peptide, here alpha-MSH, can induce differential regulation of dendritic release and systemic secretion of oxytocin, accompanied by dissociation of Fos expression and electrical activity.

Gain-of-Function Somatic Cell Lines for Drug Discovery Applications Generated by Homologous Recombination

Gene targeting allows for precise genomic engineering and has been used extensively to generate both loss-of-function and gain-of-function models in mice. Similar manipulation of the genome of somatic cell lines holds high value in basic and applied research, but has been hampered by low recombination frequencies and the subsequent labor-intensive analysis of a large number of cell clones. By combining gene targeting methods with fluorescence-activated cell sorting, gain-of-function cell lines were generated and identified based on a functional readout. To demonstrate the general applicability of this approach to drug discovery, we generated targeted promoter insertion cell lines for two key drug target classes -- the G protein-coupled receptor melanocortin-receptor 4 and the nuclear receptor peroxisome proliferator-activated receptor-gamma. Molecular analysis of the engineered cell clones confirmed the predicted integration of a constitutive promoter into an endogenous allele, and the appropriate pharmacology for these targets validated the use of these gain-of-function cell lines in drug discovery applications, including high-throughput compound screening.

Functional characterization of melanocortin-4 receptor mutations associated with childhood obesity

The melanocortin-4 receptor (MC4R) is a member of the rhodopsin-like G protein-coupled receptor family. The binding of alpha-MSH to the MC4R leads to increased cAMP production. Recent pharmacological and genetic studies have provided compelling evidence that MC4R is an important regulator of food intake and energy homeostasis. Allelic variants of MC4R were reported in some children with early-onset severe obesity. However, few studies have been performed to confirm that these allelic variants result in an impairment of the receptor's function. In this study, we expressed wild-type and variant MC4Rs in HEK293 cells and systematically studied ligand binding, agonist-stimulated cAMP, and cell surface expression. Six of the 11 mutants examined had either decreased (S58C, N62S, Y157S, C271Y) or no (P78L, G98R) ligand binding, with proportional impairments in [Nle4, d-Phe7]-alpha-MSH-stimulated cAMP production. Confocal microscopy confirmed that the observed decreases in hormone binding by these mutants are associated with decreased cell surface expression due to intracellular retention of the mutants. The other five allelic variants (D37V, P48S, V50M, I170V, N274S) were found to be expressed at the cell surface and to bind agonist and respond with increased cAMP production normally. The data on these latter five variants raise the question as to whether they are indeed causative of the obesity or not and, if so, by what mechanism. Our data, therefore, stress the importance of characterizing the properties of MC4R variants associated with early-onset severe obesity. We further propose a classification scheme for mutant MC4Rs based upon their properties.

Why do melanin ornaments signal individual quality? Insights from metal element analysis of barn owl feathers

Melanin-based variation in colour patterns is under strong genetic control and not, or weakly, sensitive to the environment and body condition. Current signalling theory predicts that such traits may not signal honestly phenotypic quality because their production does not entail a significant fitness cost. However, recent studies revealed that in several bird species melanin-based traits covary with phenotypic attributes. In a first move to understand whether such covariations have a physiological basis, we quantified concentrations of five chemical elements in two pigmented plumage traits in the barn owl (Tyto alba). This bird shows continuous variation from immaculate to heavily marked with black spots (plumage spottiness) and from dark reddish-brown to white (plumage coloration), two traits that signal various aspects of individual quality. These two traits are sexually dimorphic with females being spottier and darker coloured than males. We found an enhancement in calcium and zinc concentration within black spots compared with the unspotted feather parts. The degree to which birds were spotted was positively correlated with calcium concentration within spots, whereas the unspotted feather parts of darker reddish-brown birds were more concentrated in zinc. This suggests that two different pigments are responsible for plumage spottiness and plumage coloration. We discuss the implications of our results in light of recent experimental field studies showing that female spottiness signals offspring humoral response towards an artificially administrated antigen, parasite resistance and fluctuating asymmetry of wing feathers.

Evidence for the interaction of protein kinase C and melanocortin 3-receptor signaling pathways

The melanocortin-3 receptor, MC3-R, is abundant in the brain and is activated by gamma-2-melanocyte stimulating hormone (gamma-2-MSH). We have previously reported the translocation of protein kinase C (PKC) in spontaneous hypertensive rat (SHR) brain synaptosomes treated with gamma-2-MSH. In this study, the expression of PKA and the related PKB in SHR brain synaptosomes was analyzed. PKA was detected in total synaptosomal fractions but not in particulate fractions, whereas PKB was not detected in either fraction. We next tested the hypothesis that the PKC pathway is involved in MC3-R signaling in a neuronal, CAD, cell line. Mobilization of intracellular Ca2+ was analyzed by dual fluorescence imaging of Fura-2AM loaded MC3-R transfected cells. An increase in intracellular Ca2+ was observed upon treatment with gamma-2-MSH. A MC3-R-green fluorescent protein (GFP) fusion protein was expressed and shown to localize mainly to the plasma membrane in the soma and to neurites in differentiated CAD cells. Treatment with gamma-2-MSH led to a punctate appearance and co-immunoprecipitation of the receptor fusion protein with protein kinase C-gamma (PKC-gamma). Differentiation of some neuronal cells has been shown to be associated with changes in the expression levels of protein kinase C isoenzymes. Induction of CAD cell differentiation was associated with down-regulation of the atypical PKC-zeta and protein kinase B (PKB/Akt1), that was less pronounced in MC3-R transfected cells. However, the levels of classical PKC isozymes, PKC-alpha, PKC-gamma, and PKC-beta were unchanged. These studies therefore indicate a role for PKC isozymes in gamma-2-MSH/MC3-R receptor signaling and in neuronal cell differentiation.

alpha-MSH and desacetyl-alpha-MSH signaling through melanocortin receptors

The functional significance of N-terminal acetylation of ACTH[1-13]NH(2) is unknown. N-terminal acetylation of ACTH[1-13]NH(2) (known as desacetyl-alpha-MSH) to produce alpha-MSH enhances some activities of ACTH[1-13]NH(2) and virtually eliminates others. To determine whether alpha-MSH and desacetyl-alpha-MSH diverge in their coupling to melanocortin receptors in vitro, we measured the sensitivity of MC1, MC3, MC4, and MC5 receptors stably expressed in HEK293 cells to these peptides, functionally coupling them to adenylyl cyclase and a calcium signaling pathway. alpha-MSH and desacetyl-alpha-MSH similarly coupled these overexpressed receptors to both signaling pathways. In contrast, we discovered that alpha-MSH significantly increased primary rat osteoblast proliferation while for desacetyl-alpha-MSH there was only a trend to do the same. Osteoblast cells expressing very low levels of endogenous melanocortin receptors, in contrast with transfected HEK293 cells overexpressing a single melanocortin receptor, may provide an in vitro model for differentiating between alpha-MSH and desacetyl-alpha-MSH signaling.

Ligand-dependent activation of the melanocortin 5 receptor: cAMP production and ryanodine receptor-dependent elevations of [Ca(2+)](I)

The melanocortins are involved in the regulation of various cognitive and physiological processes such as learning, feeding, immune suppression, pigmentation, and sebum production. Five melanocortin receptors have been identified, of which the melanocortin 5 receptor (MC5R) has the most widespread distribution. This subtype is found in the brain, and at numerous peripheral sites including the skin where it is expressed in the sebaceous glands. The purpose of this study was to identify the peptide that functions as a natural ligand at the MC5R in the skin. alpha-MSH, ACTH1-39, ACTH1-17, ACTH1-10, and ACTH4-10 all increased the production of cAMP in HEK293 cells transfected with the mouse MC5R. alpha-MSH and ACTH1-17 were the most potent in this respect. In addition, all peptides stimulated a rapid and transient increase in [Ca(2+)](i), and, ACTH1-10 was the most potent. The increases in [Ca(2+)](i) were of intracellular origin, but not associated with inositol phosphate production. The elevations in [Ca(2+)](i) were reduced by ruthenium red and procaine and it is therefore possible that they were mediated via ryanodine receptors.