Detection of functionally active melanocortin receptors and evidence for an immunoregulatory activity of alpha-melanocyte-stimulating hormone in human dermal papilla cells

Therapeutic Effects of Stimulating the Melanocortin Pathway in Regulating Ocular Inflammation and Cell Death

Alpha-melanocyte-stimulating hormone (α-MSH) and its binding receptors (the melanocortin receptors) play important roles in maintaining ocular tissue integrity and immune homeostasis. Particularly extensive studies have demonstrated the biological functions of α-MSH in both immunoregulation and cyto-protection. This review summarizes the current knowledge of both the physiological and pathological roles of α-MSH and its receptors in the eye. We focus on recent developments in the biology of α-MSH and the relevant clinical implications in treating ocular diseases.

The Multifaceted Melanocortin Receptors

The 5 known melanocortin receptors (MCs) have established physiological roles. With the exception of MC2, these receptors can behave unpredictably, and since they are more widely expressed than their established roles would suggest, it is likely that they have other poorly characterized functions. The aim of this review is to discuss some of the less well-explored aspects of the 4 enigmatic members of this receptor family (MC1,3-5) and describe how these are multifaceted G protein-coupled receptors (GPCRs). These receptors appear to be promiscuous in that they bind several endogenous agonists (products of the proopiomelanocortin [POMC] gene) and antagonists but with inconsistent relative affinities and effects. We propose that this is a result of posttranslational modifications that determine receptor localization within nanodomains. Within each nanodomain there will be a variety of proteins, including ion channels, modifying proteins, and other GPCRs, that can interact with the MCs to alter the availability of receptor at the cell surface as well as the intracellular signaling resulting from receptor activation. Different combinations of interacting proteins and MCs may therefore give rise to the complex and inconsistent functional profiles reported for the MCs. For further progress in understanding this family, improved characterization of tissue-specific functions is required. Current evidence for interactions of these receptors with a range of partners, resulting in modulation of cell signaling, suggests that each should be studied within the full context of their interacting partners. The role of physiological status in determining this context also remains to be characterized.

A deep dive into UV-based phototherapy: Mechanisms of action and emerging molecular targets in inflammation and cancer

UV-based phototherapy (including psoralen plus UVA (PUVA), UVB and UVA1) has a long, successful history in the management of numerous cutaneous disorders. Photoresponsive diseases are etiologically diverse, but most involve disturbances in local (and occasionally systemic) inflammatory cells and/or abnormalities in keratinocytes that trigger inflammation. UV-based phototherapy works by regulating the inflammatory component and inducing apoptosis of pathogenic cells. This results in a fascinating and complex network of simultaneous events-immediate transcriptional changes in keratinocytes, immune cells, and pigment cells; the emergence of apoptotic bodies; and the trafficking of antigen-presenting cells in skin-that quickly transform the microenvironment of UV-exposed skin. Molecular elements in this system of UV recognition and response include chromophores, metabolic byproducts, innate immune receptors, neurotransmitters and mediators such as chemokines and cytokines, antimicrobial peptides, and platelet activating factor (PAF) and PAF-like molecules that simultaneously shape the immunomodulatory effects of UV and their interplay with the microbiota of the skin and beyond. Phototherapy's key effects-proapoptotic, immunomodulatory, antipruritic, antifibrotic, propigmentary, and pro-prebiotic-promote clinical improvement in various skin diseases such as psoriasis, atopic dermatitis (AD), graft-versus-host disease (GvHD), vitiligo, scleroderma, and cutaneous T-cell lymphoma (CTCL) as well as prevention of polymorphic light eruption (PLE). As understanding of phototherapy improves, new therapies (UV- and non-UV-based) are being developed that will modify regulatory T-cells (Treg), interact with (resident) memory T-cells and /or utilize agonists and antagonists as well as antibodies targeting soluble molecules such as cytokines and chemokines, transcription factors, and a variety of membrane-associated receptors.

Hormonal regulation in male androgenetic alopecia-Sex hormones and beyond: Evidence from recent genetic studies

Male-pattern hair loss, also termed androgenetic alopecia (AGA), is a highly prevalent age-related condition that is characterized by a distinct pattern of hair loss from the frontotemporal and vertex regions of the scalp. The phenotype is highly heritable and hormone dependent, with androgens being the recognized critical hormonal factor. Numerous molecular genetic studies have focused on genetic variation in and around the gene that encodes the androgen receptor. More recently, however, the availability of high-throughput molecular genetic methods, novel methods of data analysis and sufficiently large sample sizes have rendered possible the systematic investigation of the contribution of other components of the androgen receptor pathway or hormonal pathways beyond the androgen receptor signalling pathways. Over the past decade, genome-wide association studies of increasingly large cohorts have enabled the genome-wide identification of genetic risk factors for AGA, and yielded unprecedented insights into the underlying pathobiology. The present review discusses some of the most intriguing genetic findings on the relevance of (sex)hormonal signalling in AGA.

Immunoglobulin G modulation of the melanocortin 4 receptor signaling in obesity and eating disorders

Melanocortin 4 receptor (MC4R) plays a key role in regulation of appetite activated by its main ligand α-melanocyte-stimulating hormone (α-MSH) in both central and peripheral targets. α-MSH also binds to circulating immunoglobulins (Igs) but the functional significance of such immune complexes (ICs) in MC4R signaling in normal and pathological conditions of altered appetite has remained unknown. To address this question, we analyzed plasma levels, affinity kinetics, and binding epitopes of α-MSH-reactive IgG extracted from plasma samples of female patients with hyperphagic obesity, anorexia nervosa, bulimia nervosa, binge-eating disorder, and healthy controls. Ability of α-MSH/IgG IC to bind and activate human MC4R were studied in vitro and to influence feeding behavior in vivo in rodents. We found that α-MSH-reactive IgG were low in obese but increased in anorectic and bulimic patients and displayed different epitope and kinetics of IC formation. Importantly, while α-MSH/IgG IC from all subjects were binding and activating MC4R, the receptor binding affinity was decreased in obesity. Additionally, α-MSH/IgG IC had lower MC4R-mediated cAMP activation threshold as compared with α-MSH alone in all but not obese subjects. Furthermore, the cellular internalization rate of α-MSH/IgG IC by MC4R-expressing cells was decreased in obese but increased in patients with anorexia nervosa. Moreover, IgG from obese patients prevented central anorexigenic effect of α-MSH. These findings reveal that MC4R is physiologically activated by IC formed by α-MSH/IgG and that different levels and molecular properties of α-MSH-reactive IgG underlie biological activity of such IC relevant to altered appetite in obesity and eating disorders.

Are melanocortin peptides future therapeutics for cutaneous wound healing?

Cutaneous wound healing is a complex process divided into different phases, that is an inflammatory, proliferative and remodelling phase. During these phases, a variety of resident skin cell types but also cells of the immune system orchestrate the healing process. In the last year, it has been shown that the majority of cutaneous cell types express the melanocortin 1 receptor (MC1R) that binds α-melanocyte-stimulating hormone (α-MSH) with high affinity and elicits pleiotropic biological effects, for example modulation of inflammation and immune responses, cytoprotection, antioxidative defense and collagen turnover. Truncated α-MSH peptides such as Lys-Pro-Val (KPV) as well as derivatives like Lys-d-Pro-Thr (KdPT), the latter containing the amino acid sequence 193-195 of interleukin-1β, have been found to possess anti-inflammatory effects but to lack the pigment-inducing activity of α-MSH. We propose here that such peptides are promising future candidates for the treatment of cutaneous wounds and skin ulcers. Experimental approaches in silico, in vitro, ex vivo and in animal models are outlined. This is followed by an unbiased discussion of the pro and contra arguments of such peptides as future candidates for the therapeutic management of cutaneous wounds and a review of the so-far available data on melanocortin peptides and derivatives in wound healing.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

Melanocortin Regulation of Inflammation

Adrenocorticotropic hormone (ACTH), and α-, β-, and γ-melanocyte-stimulating hormones (α-, β-, γ-MSH), collectively known as melanocortins, together with their receptors (melanocortin receptors), are components of an ancient modulatory system. The clinical use of ACTH in the treatment of rheumatoid arthritis started in 1949, originally thought that the anti-inflammatory action was through hypothalamus-pituitary-adrenal axis and glucocorticoid-dependent. Subsequent decades have witnessed extensive attempts in unraveling the physiology and pharmacology of the melanocortin system. It is now known that ACTH, together with α-, β-, and γ-MSHs, also possess glucocorticoid-independent anti-inflammatory and immunomodulatory effects by activating the melanocortin receptors expressed in the brain or peripheral immune cells. This review will briefly introduce the melanocortin system and highlight the action of melanocortins in the regulation of immune functions from in vitro, in vivo, preclinical, and clinical studies. The potential therapeutic use of melanocortins are also summarized.

Study of gene expression alteration in male androgenetic alopecia: evidence of predominant molecular signalling pathways

BACKGROUND

Male androgenetic alopecia (AGA) is the most common form of hair loss in men. It is characterized by a distinct pattern of progressive hair loss starting from the frontal area and the vertex of the scalp. Although several genetic risk loci have been identified, relevant genes for AGA remain to be defined.

OBJECTIVES

To identify biomarkers associated with AGA.

METHODS

Molecular biomarkers associated with premature AGA were identified through gene expression analysis using cDNA generated from scalp vertex biopsies of hairless or bald men with premature AGA, and healthy volunteers.

RESULTS

This monocentric study reveals that genes encoding mast cell granule enzymes, inflammatory mediators and immunoglobulin-associated immune mediators were significantly overexpressed in AGA. In contrast, underexpressed genes appear to be associated with the Wnt/β-catenin and bone morphogenic protein/transforming growth factor-β signalling pathways. Although involvement of these pathways in hair follicle regeneration is well described, functional interpretation of the transcriptomic data highlights different events that account for their inhibition. In particular, one of these events depends on the dysregulated expression of proopiomelanocortin, as confirmed by polymerase chain reaction and immunohistochemistry. In addition, lower expression of CYP27B1 in patients with AGA supports the notion that changes in vitamin D metabolism contributes to hair loss.

CONCLUSIONS

This study provides compelling evidence for distinct molecular events contributing to alopecia that may pave the way for new therapeutic approaches.

Multiplex serum protein analysis reveals potential mechanisms and markers of response to hyperimmune caprine serum in systemic sclerosis

Background

Hyperimmune caprine serum (HICS) is a novel biological therapy with potential benefit for skin in established diffuse cutaneous systemic sclerosis. Here we report multiplex protein analysis of blood samples from a placebo-controlled phase II clinical trial and explore mechanisms of action and markers of response.

Methods

Patients were treated with HICS (n = 10) or placebo (n = 10) over 26 weeks, with follow-up open-label treatment to 52 weeks in 14 patients. Serum or plasma samples at baseline, 26 and 52 weeks were analysed using multiplex or individual immunoassays for 41 proteins. Patterns of change were analysed by clustering using Netwalker 1.0, Pearson coefficient and significance analysis of microarrays (SAM) correction.

Results

Cluster analysis, SAM multiplex testing and paired comparison of individual analytes identified proteins that were upregulated or downregulated during treatment with HICS. There was upregulation of the hypothalamo-pituitary-adrenal axis after HICS treatment evidenced by increases in α-MSH and ACTH in cases treated with HICS. Interestingly, significant increase in PIIINP was associated with HICS treatment and improved MRSS suggesting that this may be a marker of extracellular matrix turnover. Other relevant factors reduced in HICS-treated patients compared with controls, although not reaching statistical significance included COMP, CCL2, IL6, TIMP2, Fractalkine and TGFβ1 levels.

Conclusions

Our results suggest mechanisms of action for HICS, including upregulation of α-MSH, that has been shown to be anti-fibrotic in preclinical models, and possible markers to be included in future trials targeting skin in diffuse cutaneous systemic sclerosis.

Trial registration

Eudract, No. 2007-003122-24. ClinTrials.gov, No. NCT00769028. Registered 7 October 2008.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1252-x) contains supplementary material, which is available to authorized users.

Latitudinal Clines of the Human Vitamin D Receptor and Skin Color Genes

The well-documented latitudinal clines of genes affecting human skin color presumably arise from the need for protection from intense ultraviolet radiation (UVR) vs. the need to use UVR for vitamin D synthesis. Sampling 751 subjects from a broad range of latitudes and skin colors, we investigated possible multilocus correlated adaptation of skin color genes with the vitamin D receptor gene (VDR), using a vector correlation metric and network method called BlocBuster. We discovered two multilocus networks involving VDR promoter and skin color genes that display strong latitudinal clines as multilocus networks, even though many of their single gene components do not. Considered one by one, the VDR components of these networks show diverse patterns: no cline, a weak declining latitudinal cline outside of Africa, and a strong in- vs. out-of-Africa frequency pattern. We confirmed these results with independent data from HapMap. Standard linkage disequilibrium analyses did not detect these networks. We applied BlocBuster across the entire genome, showing that our networks are significant outliers for interchromosomal disequilibrium that overlap with environmental variation relevant to the genes’ functions. These results suggest that these multilocus correlations most likely arose from a combination of parallel selective responses to a common environmental variable and coadaptation, given the known Mendelian epistasis among VDR and the skin color genes.

Bleomycin-induced fibrosis in MC1 signalling-deficient C57BL/6J-Mc1r(e/e) mice further supports a modulating role for melanocortins in collagen synthesis of the skin

The melanocortin-1 receptor (MC1 ) binds α-melanocyte-stimulating hormone (α-MSH) with high affinity and has a physiological role in cutaneous melanin pigmentation. Previously, we reported that human dermal fibroblasts also express functional MC1 . α-MSH suppressed transforming growth factor-β1 - and bleomycin (BLM)-induced collagen synthesis in vitro and in vivo. Using MC1 signalling-deficient C57BL/6J-Mc1r(e/e) mice, we tested as to whether MC1 has a regulatory role on dermal collagen synthesis in the BLM model of scleroderma. Notably, mice with a C57BL/6J genetic background were previously shown to be BLM-non-susceptible. Interestingly, treatment of C57BL/6J-Mc1r(e/e) but not of C57BL/6J-wild-type mice with BLM increased cutaneous collagen type I content at RNA and protein level along with development of skin fibrosis. Cutaneous levels of connective tissue growth factor and monocyte chemotactic protein-1 were also increased in BLM-treated C57BL/6J-Mc1r(e/e) mice. Primary dermal fibroblasts from C57BL/6J-wt mice further expressed MC1 , suggesting that these cells are directly targeted by melanocortins to affect collagen production of the skin. Our findings support the concept that MC1 has an endogenous regulatory function in collagen synthesis and controls the extent of fibrotic stress responses of the skin.

Curbing Inflammation through Endogenous Pathways: Focus on Melanocortin Peptides

The resolution of inflammation is now known to be an active process, armed with a multitude of mediators both lipid and protein in nature. Melanocortins are peptides endowed with considerable promise with their proresolution and anti-inflammatory effects in preclinical models of inflammatory disease, with tissue protective effects. These peptides and their targets are appealing because they can be seen as a natural way of inducing these effects as they harness endogenous pathways of control. Whereas most of the information generated about these mediators derives from several acute models of inflammation (such as zymosan induced peritonitis), there is some indication that these mediators may inhibit chronic inflammation by modulating cytokines, chemokines, and leukocyte apoptosis. In addition, proresolving mediators and their mimics have often been tested alongside therapeutic protocols, hence have been tested in settings more relevant to real life clinical scenarios. We provide here an overview on some of these mediators with a focus on melanocortin peptides and receptors, proposing that they may unveil new opportunities for innovative treatments of inflammatory arthritis.

Multiple sclerosis, relapses, and the mechanism of action of adrenocorticotropic hormone

Relapses in multiple sclerosis (MS) are disruptive and frequently disabling for patients, and their treatment is often a challenge to clinicians. Despite progress in the understanding of the pathophysiology of MS and development of new treatments for long-term management of MS, options for treating relapses have not changed substantially over the past few decades. Corticosteroids, a component of the hypothalamic-pituitary-adrenal axis that modulate immune responses and reduce inflammation, are currently the mainstay of relapse treatment. Adrenocorticotropic hormone (ACTH) gel is another treatment option. Although it has long been assumed that the efficacy of ACTH in treating relapses depends on the peptide’s ability to increase endogenous corticosteroid production, evidence from research on the melanocortin system suggests that steroidogenesis may only partly account for ACTH influences. Indeed, the melanocortin peptides [ACTH and α-, β-, γ-melanocyte-stimulating hormones (MSH)] and their receptors (Melanocortin receptors, MCRs) exert multiple actions, including modulation of inflammatory and immune mediator production. MCRs are widely distributed within the central nervous system and in peripheral tissues including immune cells (e.g., macrophages). This suggests that the mechanism of action of ACTH includes not only steroid-mediated indirect effects, but also direct anti-inflammatory and immune-modulating actions via the melanocortin system. An increased understanding of the role of the melanocortin system, particularly ACTH, in the immune and inflammatory processes underlying relapses may help to improve relapse management.

Selected variants of the melanocortin 4 receptor gene (MC4R) do not confer susceptibility to female pattern hair loss

Female pattern hair loss (FPHL) is a common hair loss disorder in women with a complex mode of inheritance. Its etiopathogenesis is poorly understood. Widespread assumptions of overlapping susceptibility variants between FPHL and male pattern baldness (androgenetic alopecia) and a crucial role of androgens or distinct sexual steroid hormones in the development of FPHL could neither be clearly demonstrated nor completely excluded at the molecular level up to date. Interestingly, recent studies suggested an association of metabolic syndrome-including obesity, hyperlipidaemia, hypertension and diabetes mellitus type 2 or abnormally high fasting blood glucose-with FPHL. Of note, mutations in the melanocortin 4 receptor gene (MC4R) have been identified in patients with morbid obesity. Interestingly, this neuropeptide receptor has been detected amongst others in the dermal papilla of the hair follicle. As almost half of our FPHL patients of German origin present with adipositas and/or obesity, we hypothesized as to whether FPHL could be associated with variants of the MC4R gene. Thus, we genotyped a total of six variants from MC4R in our case-control sample comprising 245 UK patients of German and UK origin. However, based on our present study none of the genotyped MC4R variants displayed any significant association, neither in the overall UK and German samples nor in any subgroup analyses. In summary, these results do not point to an involvement of MC4R in FPHL.

Role of proopiomelanocortin-derived peptides and their receptors in the osteoarticular system: from basic to translational research

Proopiomelanocortin (POMC)-derived peptides such as melanocortins and β-endorphin (β-ED) exert their pleiotropic effects via binding to melanocortin receptors (MCR) and opioid receptors (OR). There is now compelling evidence for the existence of a functional POMC system within the osteoarticular system. Accordingly, distinct cell types of the synovial tissue and bone have been identified to generate POMC-derived peptides like β-ED, ACTH, or α-MSH. MCR subtypes, especially MC1R, MC2R (the ACTH receptor), MC3R, and MC4R, but also the μ-OR and δ-OR, have been detected in various cells of the synovium, cartilage, and bone. The respective ligands of these POMC-derived peptide receptors mediate an increasing number of newly recognized biological effects in the osteoarticular system. These include bone mineralization and longitudinal growth, cell proliferation and differentiation, extracellular matrix synthesis, osteoprotection, and immunomodulation. Importantly, bone formation is also regulated by the central melanocortin system via a complex hormonal interplay with other organs and tissues involved in energy metabolism. Among the POMC-derived peptides examined in cell culture systems from osteoarticular tissue and in animal models of experimentally induced arthritis, α-MSH, ACTH, and MC3R-specific agonists appear to have the most promising antiinflammatory actions. The effects of these melanocortin peptides may be exploited in future for the treatment of patients with inflammatory and degenerative joint diseases.

Hair cortisol: a parameter of chronic stress? Insights from a radiometabolism study in guinea pigs

Measurement of hair cortisol has become popular in the evaluation of chronic stress in various species. However, a sound validation is still missing. Therefore, deposition of radioactivity in hair and excretion into feces and urine after repeated injection of (3)H-cortisol was studied in guinea pigs (n = 8). Each animal was given intraperitoneally 243.6 kBq (3)H-cortisol/day on 3 successive days. After the first injection, all voided excreta were collected for 3 days. After the second injection, hair was shaved off the animals' back and newly grown hair was obtained on day 7. Following methanol extraction, radiolabeled and unlabeled glucocorticoid metabolites (GCM) in fecal and hair samples were characterized by high-performance liquid chromatography (HPLC) and enzyme immunoassays (EIA). In feces, maximum radioactivity was reached 8 h (median) post each injection, whereas maxima in urine were detected in the first samples (median 2.5 h). Metabolites excreted into feces (13.3% ± 3.7) or urine (86.7%) returned nearly to background levels. HPLC of fecal extracts showed minor variation between individuals and sexes. In hair, small amounts of radioactivity were present. However, two EIAs detected large amounts of unlabeled GCM, including high levels at the position of the cortisol standard; radioactivity was absent in this fraction, demonstrating that (3)H-cortisol was metabolized. Furthermore, large amounts of immunoreactivity coinciding with a radioactive peak at the elution position of cortisone were found. These results show for the first time that only small amounts of systemically administered radioactive glucocorticoids are deposited in hair of guinea pigs, while measurement of large amounts of unlabeled GCM strongly suggests local production of glucocorticoids in hair follicles.

Soluble adenylyl cyclase antibody profile as a diagnostic adjunct in the assessment of pigmented lesions

OBJECTIVE

To investigate the usefulness of a novel marker for melanocytic proliferations.

DESIGN

Using a novel monoclonal antibody against soluble adenylyl cyclase (sAC), various benign and malignant melanocytic proliferations were immunostained.

SETTING

Weill Medical College of Cornell University dermatopathology laboratory.

MAIN OUTCOME MEASURES

The results were qualitative, not quantifiable.

RESULTS

The sAC immunostaining produced distinctive patterns that paralleled melanomagenesis. At one pole of the spectrum were benign nevi, including atypical nevi of special sites and recurrent nevi showing a distinct pattern of dotlike Golgi staining, while at the opposite pole was melanoma, in which many cells demonstrated an intense pannuclear expression pattern, often accompanied by loss of the Golgi expression pattern. Melanomas of lentigo maligna and acral lentiginous subtypes exhibited the most striking pannuclear expression, while nodular melanomas showed the least, although with supervening enhanced diffuse cytoplasmic expression. Loss of the Golgi expression pattern was a feature of malignant melanoma.

CONCLUSION

The sAC expression pattern is complex but seems discriminatory, with distinctive and variable staining patterns according to the nature of the lesion biopsied.

Hormones and the pilosebaceous unit

Hormones can exert their actions through endocrine, paracrine, juxtacrine, autocrine and intracrine pathways. The skin, especially the pilosebaceous unit, can be regarded as an endocrine organ meanwhile a target of hormones, because it synthesizes miscellaneous hormones and expresses diverse hormone receptors. Over the past decade, steroid hormones, phospholipid hormones, retinoids and nuclear receptor ligands as well as the so-called stress hormones have been demonstrated to play pivotal roles in controlling the development of pilosebaceous units, lipogenesis of sebaceous glands and hair cycling. Among them, androgen is most extensively studied and of highest clinical significance. Androgen-mediated dermatoses such as acne, androgenetic alopecia and seborrhea are among the most common skin disorders, with most patients exhibiting normal circulating androgen levels. The "cutaneous hyperandrogenism" is caused by in stiu overexpression of the androgenic enzymes and hyperresponsiveness of androgen receptors. Regulation of cutaneous steroidogenesis is analogous to that in gonads and adrenals. More work is needed to explain the regional difference within and between the androgn-mediated dermatoses. The pilosebaceous unit can act as an ideal model for studies in dermato-endocrinology.

The melanocortin system in articular chondrocytes: melanocortin receptors, pro-opiomelanocortin, precursor proteases, and a regulatory effect of alpha-melanocyte-stimulating hormone on proinflammatory cytokines and extracellular matrix components

OBJECTIVE

The pro-opiomelanocortin (POMC)-derived neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) mediates its effects via melanocortin (MC) receptors. This study was carried out to investigate the expression patterns of the MC system and the effects of alpha-MSH in human articular chondrocytes.

METHODS

Articular chondrocytes established from human osteoarthritic joint cartilage were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting for the expression of MC receptors, POMC, and prohormone convertases (PCs). MC-1 receptor (MC-1R) expression in articular cartilage was further studied by immunohistochemistry. Ca(2+) and cAMP assays were used to monitor alpha-MSH signaling, while studies of alpha-MSH function were performed in cultures with chondrocyte micromass pellets stimulated with alpha-MSH. Expression of cytokines and extracellular matrix (ECM) components was determined by real-time RT-PCR, Western immunoblotting, and enzyme-linked immunosorbent assays.

RESULTS

MC-1R expression was detected in articular chondrocytes in vitro and in articular cartilage in situ. In addition, expression of transcripts for MC-2R, MC-5R, POMC, and PCs was detected in articular chondrocytes. Stimulation with alpha-MSH increased the levels of intracellular cAMP, but not Ca(2+), in chondrocytes. Both messenger RNA and protein expression of various proinflammatory cytokines, collagens, matrix metalloproteinases (MMPs), and SOX9 was modulated by alpha-MSH.

CONCLUSION

Human articular chondrocytes are target cells for alpha-MSH. The effects of alpha-MSH on expression of cytokines and MMPs suggest that this neuropeptide plays a role in inflammatory and degenerative processes in cartilage. It is conceivable that inflammatory reactions can be mitigated by the induction of endogenous MCs or administration of alpha-MSH to the affected joints. The induction pattern of regulatory and structural ECM components such as collagens as well as SOX9 and anabolic and catabolic cytokines points to a function of alpha-MSH as a trophic factor in skeletal development during endochondral ossification rather than as a factor in homeostasis of permanent cartilage.

Increased cAMP levels modulate transforming growth factor-beta/Smad-induced expression of extracellular matrix components and other key fibroblast effector functions

cAMP is a key messenger of many hormones and neuropeptides, some of which modulate the composition of extracellular matrix. Treatment of human dermal fibroblasts with dibutyryl cyclic AMP and forskolin antagonized the inductive effects of transforming growth factor-beta (TGF-beta) on the expression of collagen, connective tissue growth factor, tissue inhibitor of matrix metalloproteinase-1, and plasminogen activator inhibitor type I, four prototypical TGF-beta-responsive genes. Increased intracellular cAMP prevented TGF-beta-induced Smad-specific gene transactivation, although TGF-beta-mediated Smad phosphorylation and nuclear translocation remained unaffected. However, increased cAMP levels abolished TGF-beta-induced interaction of Smad3 with its transcriptional co-activator cAMP-response element-binding protein (CREB)-binding protein (CBP)/p300. Overexpression of the transcriptional co-activator CBP/p300 rescued Smad-specific gene transcription in the presence of cAMP suggesting that sequestration of limited amounts of CBP/p300 by the activated cAMP/CREB pathway is the molecular basis of this inhibitory effect. These findings were extended by two functional assays. Increased intracellular cAMP levels suppressed the inductive activity of TGF-beta to contract mechanically unloaded collagen lattices and resulted in an attenuation of fibroblast migration of mechanically induced cell layer wounds. Of note, cAMP and TGF-beta synergistically induced hyaluronan synthase 2 (HAS2) expression and hyaluronan secretion, presumably via putative CREB-binding sites adjacent to Smad-binding sites within the HAS2 promoter. Our findings identify the cAMP pathway as a potent but differential and promoter-specific regulator of TGF-beta-mediated effects involved in extracellular matrix homeostasis.

Hydroxychloroquine modulates metabolic activity and proliferation and induces autophagic cell death of human dermal fibroblasts

Hydroxychloroquine (HCQ) is a commonly used therapeutic agent in skin disorders. Some reports also suggest that HCQ can be useful in fibroblastic diseases of the skin. Here, we investigated the effects of HCQ in human dermal fibroblasts (HDFs). HCQ significantly reduced the metabolic activity and suppressed cell proliferation (IC(50) = approximately 30 microM) of HDFs. The antiproliferative effect of HCQ was associated with decreased activation of the extracellular signal-regulated kinases 1/2 but not with inhibition of the mammalian target of the rapamycin pathway or with dephosphorylation of Akt. HCQ induced a distinct type of cell death in HDFs, characterized by surface exposure of phosphatidylserine but a lack of morphological signs of apoptosis and absence of DNA fragmentation. The HCQ-treated HDFs instead showed autophagic vacuoles with double membranes and digested organelle content. These vacuoles showed light-chain 3 immunostaining, in accordance with increased protein amounts of this autophagy marker. Induction of autophagic cell death by HCQ was also paralleled by increased expression of Beclin-1, a key regulator of autophagy. Our findings indicate that HDFs are target cells of HCQ and form a rationale on the basis of which the in vivo effects of antimalarials can be studied in patients with aberrant fibroblast function.

ACTH: cellular peptide hormone synthesis and secretory pathways

Adrenocorticotrophic hormone (ACTH) is derived from the prohormone, pro-opiomelanocortin (POMC). This precursor undergoes proteolytic cleavage to yield a number of different peptides which vary depending on the tissue. In the anterior pituitary, POMC is processed to ACTH by the prohormone convertase, PC1 and packaged in secretory granules ready for stimulated secretion. In response to stress, corticotrophin releasing hormone (CRH), stimulates release of ACTH from the pituitary cell which in turn causes release of glucocorticoids from the adrenal gland. In tissues, such as the hypothalamus and skin, ACTH is further processed intracellularly to alpha melanocyte stimulating hormone (alphaMSH) which has distinct roles in these tissues. The prohormone, POMC, is itself released from cells and found in the human circulation at concentrations greater than ACTH. While much is known about the tightly regulated synthesis of POMC, there is still a lot to learn about the mechanisms for differentiating secretion of POMC, and the POMC-derived peptides. Understanding what happens to the POMC released from cells will provide new insights into its function.

Alpha-melanocyte-stimulating hormone and related tripeptides: biochemistry, antiinflammatory and protective effects in vitro and in vivo, and future perspectives for the treatment of immune-mediated inflammatory diseases

Alpha-MSH is a tridecapeptide derived from proopiomelanocortin. Many studies over the last few years have provided evidence that alpha-MSH has potent protective and antiinflammatory effects. These effects can be elicited via centrally expressed melanocortin receptors that orchestrate descending neurogenic antiinflammatory pathways. alpha-MSH can also exert antiinflammatory and protective effects on cells of the immune system and on peripheral nonimmune cell types expressing melanocortin receptors. At the molecular level, alpha-MSH affects various pathways implicated in regulation of inflammation and protection, i.e., nuclear factor-kappaB activation, expression of adhesion molecules and chemokine receptors, production of proinflammatory cytokines and mediators, IL-10 synthesis, T cell proliferation and activity, inflammatory cell migration, expression of antioxidative enzymes, and apoptosis. The antiinflammatory effects of alpha-MSH have been validated in animal models of experimentally induced fever; irritant and allergic contact dermatitis, vasculitis, and fibrosis; ocular, gastrointestinal, brain, and allergic airway inflammation; and arthritis, but also in models of organ injury. One obstacle limiting the use of alpha-MSH in inflammatory disorders is its pigmentary effect. Due to its preserved antiinflammatory effect but lack of pigmentary action, the C-terminal tripeptide of alpha-MSH, KPV, has been delineated as an alternative for antiinflammatory therapy. KdPT, a derivative of KPV corresponding to amino acids 193-195 of IL-1beta, is also emerging as a tripeptide with antiinflammatory effects. The physiochemical properties and expected low costs of production render both agents suitable for the future treatment of immune-mediated inflammatory skin and bowel disease, fibrosis, allergic and inflammatory lung disease, ocular inflammation, and arthritis.

The Ca2+-binding capacity of epidermal furin is disrupted by H2O2-mediated oxidation in vitiligo

The Ca(2+)-dependent precursor convertase furin is abundantly expressed in epidermal keratinocytes and melanocytes. In this context, it is noteworthy that proopiomelanocortin (POMC) cleavage is also processed by furin, leading to ACTH, beta-lipotropin, and beta-endorphin. All prohormone convertases including furin are regulated by Ca(2+). Because numerous epidermal peptides and enzymes are affected by H(2)O(2)-mediated oxidation, including the POMC-derived peptides alpha-MSH and beta-endorphin as shown in the epidermis of patients with vitiligo, we here asked the question of whether furin could also be a possible target for this oxidation mechanism by using immunofluorescence, RT-PCR, Western blotting, Ca(2+)-binding studies, and computer modeling. Our results demonstrate significantly decreased in situ immunoreactivity of furin in the epidermis of patients with progressive vitiligo (n = 10), suggesting H(2)O(2)-mediated oxidation. This was confirmed by (45)Ca(2+)-binding studies with human recombinant furin identifying the loss of one Ca(2+)-binding site from the enzyme after oxidation with H(2)O(2). Computer simulation supported alteration of one of the two Ca(2+)-binding sites on furin. Taken together, our results implicate that the Ca(2+)-dependent proteolytic activity of this convertase is targeted by H(2)O(2), which in turn could contribute to the reduced epidermal expression of the POMC-derived peptides alpha-MSH and beta-endorphin as documented earlier in patients with vitiligo.

alpha-MSH related peptides: a new class of anti-inflammatory and immunomodulating drugs

alpha-Melanocyte-stimulating hormone (alpha-MSH) is a tridecapeptide derived from the proopiomelanocortin by post-translational processing. In addition to its effects on melanocytes, alpha-MSH has potent anti-inflammatory effects when administered systemically or locally. The anti-inflammatory effects of alpha-MSH are mediated by direct effects on cells of the immune system as well as indirectly by affecting the function of resident non-immune cells. alpha-MSH affects several pathways implicated in regulation of inflammatory responses such as NF-kappaB activation, expression of adhesion molecules and chemokine receptors, production of pro-inflammatory cytokines and other mediators. Thus alpha-MSH may modulate inflammatory cell proliferation, activity and migration. The anti-inflammatory effects of alpha-MSH have been confirmed by means of animal models of inflammation such as irritant and allergic contact dermatitis, cutaneous vasculitis, asthma, inflammatory bowel disease, rheumatoid arthritis, ocular and brain inflammation. Most of the anti-inflammatory activities of alpha-MSH can be attributed to its C-terminal tripeptide KPV. K(D)PT, a derivative of KPV corresponding to the amino acid 193-195 of IL-1beta, is currently emerging as another tripeptide with potent anti-inflammatory effects. The anti-inflammatory potential together with the favourable physiochemical properties most likely will allow these agents to be developed for the treatment of inflammatory skin, eye and bowel diseases, allergic asthma and arthritis.

Proopiomelanocortin (POMC), the ACTH/melanocortin precursor, is secreted by human epidermal keratinocytes and melanocytes and stimulates melanogenesis

Proopiomelanocortin (POMC) can be processed to ACTH and melanocortin peptides. However, processing is incomplete in some tissues, leading to POMC precursor release from cells. This study examined POMC processing in human skin and the effect of POMC on the melanocortin-1 receptor (MC-1R) and melanocyte regulation. POMC was secreted by both human epidermal keratinocytes (from 5 healthy donors) and matched epidermal melanocytes in culture. Much lower levels of alpha-MSH were secreted and only by the keratinocytes. Neither cell type released ACTH. Cell extracts contained significantly more ACTH than POMC, and alpha-MSH was detected only in keratinocytes. Nevertheless, the POMC processing components, prohormone convertases 1, 2 and regulatory protein 7B2, were detected in melanocytes and keratinocytes. In contrast, hair follicle melanocytes secreted both POMC and alpha-MSH, and this was enhanced in response to corticotrophin-releasing hormone (CRH) acting primarily through the CRH receptor 1. In cells stably transfected with the MC-1R, POMC stimulated cAMP, albeit with a lower potency than ACTH, alpha-MSH, and beta-MSH. POMC also increased melanogenesis and dendricity in human pigment cells. This release of POMC from skin cells and its functional activity at the MC-1R highlight the importance of POMC processing as a key regulatory event in the skin.

Differential expression of HPA axis homolog in the skin

Human skin expresses elements of the hypothalamo-pituitary-adrenal (HPA) axis including pro-opiomelanocortin (POMC), corticotropin releasing hormone (CRH), the CRH receptor-1 (CRH-R1), key enzymes of corticosteroid synthesis and synthesizes glucocorticoids. Expression of these elements is organized in functional, cell type-specific regulatory loops, which imitate the signaling hierarchy of the HPA axis. In melanocytes and fibroblasts CRH-induced CRH-R1 stimulation upregulates POMC expression and production of ACTH through activation of cAMP dependent pathway(s). Melanocytes respond with enhanced production of cortisol and corticosterone, which is dependent on POMC activity. Fibroblasts respond to CRH and ACTH with enhanced production of corticosterone, but not cortisol, which is produced constitutively. Organ-cultured human scalp hair follicles also show a fully functional HPA axis equivalent, including cortisol synthesis and secretion and negative feedback regulation by cortisol on CRH expression. Thus, differential, CRH-driven responses of defined cutaneous cell populations reproduce key features of the central HPA axis at the tissue/single cell levels.

Corticotropin releasing hormone and the skin

Cotricotropin-releasing hormone (CRH) and related peptides are produced in skin that is dependent on species and anatomical location. Local peptide production is regulated by ultraviolet radiation (UVR), glucocorticoids and phase of the hair cycle. The skin also expresses the corresponding receptors (CRH-R1 and CRH-R2), with CRH-R1 being the major receptor in humans. CRH-R1 is expressed in epidermal and dermal compartments, and CRH-R2 predominantly in dermal structures. The gene coding for CRH-R1 generates multiple isoforms through a process modulated by UVR, cyclic adenosine monophosphate (cAMP) and phorbol 12-myristate 13-acetate. The phenotypic effects of CRH in human skin cells are largely mediated by CRH-R1alpha through increases in concentrations of cAMP, inositol triphosphate (IP3), or Ca2+ with subsequent activation of protein kinases A (PKA) and C (PKC) dependent pathways. CRH also modulates the activity of nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-kappaB), activator protein 1 (AP-1) and cAMP responsive element binding protein (CREB). The cellular functions affected by CRH depend on cell type and nutritional status and include modulation of differentiation program(s), proliferation, viability and immune activity. The accumulated evidence indicates that cutaneous CRH is also a component of a local structure organized similarly to the hypothalamo-pituitary-adrenal axis.

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.

N-ethyl-N-nitrosourea-based generation of mouse models for mutant G protein-coupled receptors

Chemical random mutagenesis techniques with the germ line supermutagen N-ethyl-N-nitrosourea (ENU) have been established to provide comprehensive collections of mouse models, which were then mined and analyzed in phenotype-driven studies. Here, we applied ENU mutagenesis in a high-throughput fashion for a gene-driven identification of new mutations. Selected members of the large superfamily of G protein-coupled receptors (GPCR), melanocortin type 3 (Mc3r) and type 4 (Mc4r) receptors, and the orphan chemoattractant receptor GPR33, were used as model targets to prove the feasibility of this approach. Parallel archives of DNA and sperm from mice mutagenized with ENU were screened for mutations in these GPCR, and in vitro assays served as a preselection step before in vitro fertilization was performed to generate the appropriate mouse model. For example, mouse models for inherited obesity were established by selecting fully or partially inactivating mutations in Mc4r. Our technology described herein has the potential to provide mouse models for a GPCR dysfunction of choice within <4 mo and can be extended to other gene classes of interest.

Human mast cells in the neurohormonal network: expression of POMC, detection of precursor proteases, and evidence for IgE-dependent secretion of alpha-MSH

Human mast cells have been shown to release histamine in response to the neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH), but it is unknown whether these cells express proopiomelanocortin (POMC) or POMC-derived peptides. We therefore examined highly purified human skin mast cells and a leukemic mast cell line-1 (HMC-1) for their ability to express POMC and members of the prohormone convertase (PC) family known to process POMC. Furthermore, we investigated whether these cells store and secrete alpha-MSH. Reverse transcriptase-PCR (RT-PCR) analysis revealed that both skin mast cells and HMC-1 cells express POMC mRNA and protein. Expression of the POMC gene at the RNA level in HMC-1 cells could be confirmed by Northern blotting. Transcripts for both PC1 and furin convertase were detectable in skin-derived mast cells and HMC-1 cells, as shown by RT-PCR. In contrast, PC2 transcripts were detected only in skin mast cells, whereas transcripts for paired basic amino acid converting enzyme 4 (PACE4) were present only in HMC-1 cells. Radioimmunoassays performed on cell lysates and cell culture supernatants from human skin-derived mast cells disclosed immunoreactive amounts of alpha-MSH in both fractions. Stimulation with an anti-IgE antibody significantly reduced intracellular alpha-MSH and increased extracellular levels, indicating IgE-mediated secretion of this neuropeptide. Our findings show that human mast cells are active players in the cutaneous POMC system. Mast cell-derived alpha-MSH may contribute to cutaneous hyperpigmentation as seen in patients with urticaria pigmentosa. Moreover, IgE-dependent release of alpha-MSH suggests an immunomodulatory role of this neurohormone during inflammatory and allergic reactions of the skin.

Quantitative analysis of MC1R gene expression in human skin cell cultures

To address the issue of melanocortin-1 receptor (MC1R) expression in non-melanocytic cells, we have quantitatively evaluated the relative expression levels of both MC1R mRNA and protein in a subset of different cell types. Using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) at high cycle numbers, we detected MC1R mRNA in all cell types examined, including human embryonic kidney-293 (HEK 293) cells, a cell type widely used as a negative control in melanocortin expression studies. Quantitative real-time PCR revealed the highest levels of MC1R transcripts were in melanocytic cells, whereas the keratinocyte and fibroblast cell cultures examined had only a low level of expression, similar to that of HEK 293 cells. Antibody mediated detection of MC1R protein in membrane extracts demonstrated exogenous receptor in MC1R transfected cell lines, as well as endogenous MC1R in melanoma cells. However, radioligand binding procedures were required to detect MC1R protein of normal human melanocytes and no surface expression of MC1R was detected in any of the non-melanocytic cells examined. This was consistent with their low level of mRNA, and suggests that, if present, the levels of surface receptor are significantly lower than that in melanocytes. The capacity of such limited levels of MC1R protein to influence non-melanocytic skin cell biology would likely be severely compromised. Indeed, the MC1R agonist [NIe(4), D-Phe(7)] alpha-melanocyte stimulating hormone (NDP-MSH) was unable to elevate intracellular cyclic adenosine monophosphate (cAMP) levels in the keratinocyte and fibroblast cells examined, whereas a robust increase was elicited in melanocytes. Although there are a variety of cell types with detectable MC1R mRNA, the expression of physiologically significant levels of the receptor may be more restricted than the current literature indicates, and within epidermal tissue may be limited to the melanocyte.

Hair melanocytes as neuro-endocrine sensors--pigments for our imagination

We are currently experiencing a spectacular surge in our knowledge of skin function both at the organ and organismal levels, much of this due to a flurry of cutaneous neuroendocrinologic data, that positions the skin as a major sensor of the periphery. As our body's largest organ, the skin incorporates all major support systems including blood, muscle and innervation as well as its role in immuno-competence, psycho-emotion, ultraviolet radiation sensing, endocrine function, etc. It is integral for maintenance of mammalian homeostasis and utilizes locally-produced melanocortins to neutralize noxious stimuli. In particular, the cutaneous pigmentary system is an important stress response element of the skin's sensing apparatus; where stimuli involving corticotrophin-releasing hormone (CRH) and proopiomelanocortin (POMC) peptides help regulate pigmentation in the hair follicle and the epidermis. These pigmentary units are organized into symmetrical functional pigmentary units composed of corticotropin-releasing hormone, and the melanocortin POMC peptides melanocyte stimulating hormone, adrenocorticotropic hormone and also the opiate beta-endorphin. These new findings have led to the concept of "self-similarity" of melanocortin systems based on their expression both at the local (skin) and systemic (CNS) levels, where the only major apparent difference appears to be one of scale. This review explores this concept and describes how the components of the CRH/POMC systems may help regulate the human hair follicle pigmentary unit.