The role of melanocortins in skin homeostasis

Skin as an endocrine organ: A narrative review

Skin being the largest organ of the body, is equipped with numerous functional properties. Over the past few years, intricate research into the biology of skin has led to a gamut of discoveries. Skin is now regarded as one of the most vital endocrine organs. The skin contains equivalents of the hypothalamo-pituitary-adrenal axis, hypothalamo-pituitary-thyroid axis and the appendages produce multiple hormones such as Vitamin D, sex steroids, retinoids and opioids. In this article, we will explore the role of skin as a target and source of some of the hormones of the human body, and briefly touch on the clinical applications.

Biomimetic proopiomelanocortin suppresses capsaicin-induced sensory irritation in humans

Sensitive skin is a frequently mentioned cosmetic complaint. Addition of a biomimetic of neuromediator has recently appeared as a promising new way to cure skin care product problems. This study was aimed to assess the inhibitory effect of a biomimetic lipopeptide derived from proopiomelanocortin (bPOMC) on capsaicin-induced sensory irritation in human volunteers and also to compare its protective effect with that of the well-known anti irritant strontium chloride. The effect of each test compound was studied on 28 selected healthy volunteers with sensitive skin in accordance with a double-blind vehicle-controlled protocol. From day 1 to day 13 each group was applied the test compound (bPOMC or strontium chloride) to one wing of the nose and the corresponding placebo (vehicle) to the other side twice daily. On days 0 and 14, acute skin irritation was induced by capsaicin solution and quantified using clinical stinging test assessments. Following the application of capsaicin solution, sensory irritation was evaluated using a 4-point numeric scale. The sensations perceived before and after treatment (on days 0 and 14) was calculated for the two zones (test materials and vehicle). Ultimately the percentage of variation between each sample and the placebo and also the inhibitory effect of bPOMC compared to that of strontium chloride were reported. Clinical results showed that after two weeks treatment, the levels of skin comfort reported in the group treated with bPOMC were significantly higher than those obtained in the placebo group and the inhibitory effect of bPOMC was about 47% higher than that of strontium chloride. The results of the present study support the hypothesis that biomimetic peptides may be effective on sensitive skin.

11β-hydroxysteroid dehydrogenase type 1 is expressed in human sebaceous glands and regulates glucocorticoid-induced lipid synthesis and toll-like receptor 2 expression in SZ95 sebocytes

BACKGROUND

Glucocorticoids (GCs) affect the pathophysiology of sebaceous glands, causing development or exacerbation of acne. The availability of GCs is regulated by isoenzymes of 11β-hydroxysteroid dehydrogenase (11βHSD) at tissue-specific levels. 11βHSD type 1 (HSD11β1) is a reductase, catalysing the conversion of cortisone to active cortisol, and is highly expressed in liver and adipose tissue. Recently, HSD11β1 was observed in human skin in keratinocytes and fibroblasts.

OBJECTIVES

To investigate the expression of HSD11β1 in sebaceous glands of normal and acne-involved skin, and to examine the role of HSD11β1 in GC-induced lipid synthesis and toll-like receptor 2 (TLR2) expression in sebocytes.

METHODS

  Expression of HSD11β1 was examined by immunohistochemistry in acne lesional skin and normal skin of healthy volunteers. The cultured SZ95 sebocytes were treated with dexamethasone, and the lipid synthesis and mRNA levels of sterol regulatory element binding protein 1 (SREBP-1) and TLR2 were determined. Use of an HSD11β1 inhibitor and the small interference RNA (siRNA) approach were used to investigate the role of HSD11β1 on the GC regulation of sebocyte functions.

RESULTS

  HSD11β1 was expressed in human sebaceous glands and upregulated in acne lesional skin. HSD11β1 mRNA was enhanced by dexamethasone and cytokines in SZ95 sebocytes. Dexamethasone enhanced lipid synthesis, partially through the transcriptional induction of SREBP-1, and also by increasing TLR2 mRNA levels. Inhibition of HSD11β1 by PF-915275 or siRNA significantly inhibited the GC-induced lipid synthesis and the mRNA expression of SREBP-1 and TLR2.

CONCLUSIONS

  Our results indicate that HSD11β1 plays a key role in the modulation of GC action on sebocytes, including sebum production and TLR2-mediated inflammation, thereby influencing the pathogenesis of acne.

Skin-lightening products revisited

Skin colour typology depends on the amount and location of its chromophores. Among them, eumelanins derived from 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and 5,6-dihydroxyindole (DHI), and phaeomelanins are of utmost importance. These biomolecules result from the multi-step enzymatic and non-enzymatic conversion of tyrosine into melanins. Pigmentation disorders are multiple and depend on alterations in the density in active melanocytes, and on specific abnormalities of any of the complex melanogenesis mechanisms. This review presents some of the main skin-lightening agents with respect to their mechanisms of action and side-effects. Some of the novel compounds may lead to new perspectives in the fields of dermatology and cosmetology. The methods commonly used to assess efficacy of skin-lightening products rely on in vitro models including cell-free enzymatic assays, melanocyte cultures and reconstructed epidermis bioassays. Animal models have little relevance. By contrast, human testing with the support of instrumental evaluations is the most informative.

Skin layer-specific transcriptional profiles in normal and recessive yellow (Mc1re/Mc1re) mice

The melanocortin 1 receptor (Mc1r) plays a central role in cutaneous biology, but is expressed at very low levels by a small fraction of cells in the skin. In humans, loss-of-function MC1R mutations cause fair skin, freckling, red hair, and increased predisposition to melanoma; in mice, Mc1r loss-of-function is responsible for the recessive yellow mutation, associated with pheomelanic hair and a decreased number of epidermal melanocytes. To better understand how Mc1r signaling affects different cutaneous phenotypes, we examined large-scale patterns of gene expression in different skin components (whole epidermal sheets, basal epidermal cells and whole skins) of neonatal (P2.5) normal and recessive yellow mice, starting with a 26K mouse cDNA microarray. From c. 17 000 genes whose levels could be accurately measured in neonatal skin, we identified 883, 2097 and 552 genes that were uniquely expressed in the suprabasal epidermis, basal epidermis and dermis, respectively; specific biologic roles could be assigned for each class. Comparison of normal and recessive yellow mice revealed 69 differentially expressed genes, of which the majority had not been previously implicated in Mc1r signaling. Surprisingly, many of the Mc1r-dependent genes are expressed in cells other than melanocytes, even though Mc1r expression in the skin is confined almost exclusively to epidermal melanocytes. These results reveal new targets for Mc1r signaling, and point to a previously unappreciated role for a Mc1r-dependent paracrine effect of melanocytes on other components of the skin.

Targeting melanocortin receptors as potential novel therapeutics

Adrenocorticotrophic hormone (ACTH(1-39)) and the melanocortins (alpha, beta and gamma-melanocyte-stimulating hormone [MSH]) are derived from a larger precursor molecule known as the pro-opiomelanocortin (POMC) protein. They exert their numerous biological effects by activating 7 transmembrane G-protein coupled receptors (GPCR), leading to adenylyl cyclase activation and subsequent cAMP accumulation within the target cell. To date, 5 melanocortin receptors (MCR) have been identified and termed MC1R to MC5R, they have been shown to have a wide and varied distribution throughout the body, being found in the central nervous system (CNS), periphery and immune cells. Melanocortins have a multitude of actions including: (i) modulating disease pathologies including arthritis, asthma, obesity; (ii) affecting functions, for example erectile dysfunction, skin tanning; and (iii) organ systems, for example cardiovascular system. Recently a mechanistic approach has been identified with alpha-MSH preventing NF-kappaB activation via the preservation and expression of IkappaBalphaprotein. This leads to a reduction of pro-inflammatory mediators including cytokines and inhibition of adhesion molecule expression, with subsequent reduction in leukocyte emigration. Development of selective ligands with an appropriate pharmacokinetic profile will enable a pharmacological evaluation of the potential beneficial effects of the melanocortins. In this review I have discussed the potential mechanistic action for the melanocortins and some of the disease pathologies shown to be modulated. This review proposes targeting the MCR with the ultimate aim of controlling many of the diseases that we face today.

Human hair follicles display a functional equivalent of the hypothalamic-pituitary-adrenal axis and synthesize cortisol

The skin and its major appendages are prominent target organs and potent sources of key players along the classical hypothalamic-pituitary axis, such as corticotropin releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and alpha melanocyte stimulating hormone (alpha-MSH), and even express key steroidogenic enzymes. Therefore, it may have established local stress response systems that resemble the hypothalamic-pituitary-adrenal (HPA) axis. However, functional evidence that this is indeed the case in normal human skin in situ has still been missing. We show that microdissected, organ-cultured human scalp hair follicles respond to CRH stimulation by up-regulating proopiomelanocortin (POMC) transcription and immunoreactivity (IR) for ACTH and alpha-MSH, which must have been processed from POMC. CRH, alpha-MSH, and ACTH also modulate expression of their cognate receptors (CRH-R1, MC1-R, MC2-R). In addition, the strongest stimulus for adrenal cortisol production, ACTH, also up-regulates cortisol-IR in the hair follicles. Isolated human hair follicles secrete substantial levels of cortisol into the culture medium, and this activity is further up-regulated by CRH. CRH also modulates important functional hair growth parameters in vitro (hair shaft elongation, catagen induction, hair keratinocyte proliferation, melanin production). Finally, human hair follicles display HPA axis-like regulatory feedback systems, since the glucocorticoid receptor agonist hydrocortisone down-regulates follicular CRH expression. Thus, even in the absence of endocrine, neural, or vascular systemic connections, normal human scalp hair follicles directly respond to CRH stimulation in a strikingly similar manner to what is seen in the classical HPA axis, including synthesis and secretion of cortisol and activation of prototypic neuroendocrine feedback loops.

Melanocortins in fibroblast biology--current update and future perspective for dermatology

The skin is a target organ and source for proopiomelanocortin (POMC)-derived peptides, such as alpha-melanocyte-stimulating hormone (alpha-MSH), which acts by binding to melanocortin receptors (MC-Rs). Recent progress in our understanding of the cutaneous POMC system has demonstrated that human dermal fibroblasts (HDFs) are a novel target for alpha-MSH. MC-1R is expressed by HDFs in vitro and in situ. MC-1R expression is also detectable in human connective tissue sheath fibroblasts (CTSFs) and in dermal papilla cells (DPCs) of the hair follicle, the latter concomitantly expressing MC-1R and MC-4R in vitro and in situ. Both HDFs and DPCs are capable of generating POMC-derived peptides, although cell-specific differences exist in the expression of prohormone convertases and the amounts of POMC-derived peptides generated. Functional studies have shown that alpha-MSH exerts anti-inflammatory actions in human fibroblastic skin cells by suppressing interleukin-1 (IL-1)-induced IL-8 production, activation of the transcription factor activator protein-1 (AP-1) and induction of intercellular adhesion molecule-1 by interferon-alpha. In addition, alpha-MSH antagonizes the effect of transforming growth factor-beta1 (TGF-beta1) on collagen synthesis in HDFs in vitro and exerts antifibrogenic activity in a mouse model of cutaneous fibrosis. These findings indicate that fibroblastic cells participate in the cutaneous POMC system in which alpha-MSH appears to act as a modulator of inflammatory and fibrogenic responses. The biological activities of alpha-MSH in fibroblastic cells of the skin point towards novel clues in our understanding of the pathophysiology of fibrotic skin disorders and inflammatory diseases of the hair follicle and, finally, suggest innovative therapeutic options for the treatment of these conditions.

Neuroendocrine regulation of sebocytes - a pathogenetic link between stress and acne

A causative link between emotional stress and acne has long been postulated. There is mounting evidence that the molecular mechanism underlying this observation is related to the expression of receptors for several neuroendocrine mediators by the sebaceous gland. Recent and ongoing studies have indicated that human sebocytes express functional receptors for corticotropin-releasing hormone, melanocortins, beta-endorphin, vasoactive intestinal polypeptide, neuropeptide Y and calcitonin gene-related peptide. After ligand binding, these receptors modulate the production of inflammatory cytokines, proliferation, differentiation, lipogenesis and androgen metabolism in sebocytes. By means of their autocrine, paracrine and endocrine actions, these neuroendocrine factors appear to mediate centrally and topically induced stress towards the sebaceous gland, ultimately affecting the clinical course of acne.

[Alpha-melanocyte-stimulating hormone. Its current significance for dermatology]

Alpha-melanocyte-stimulating hormone (alpha-MSH) is a tridecapeptide that was originally characterized as a neuropeptide derived from the pituitary gland. alpha-MSH is synthesized from pro-opiomelanocortin (POMC) by the action of specific prohormone convertases which cleave POMC into alpha-MSH, adrenocorticotropin and beta-endorphin. The various effects of alpha-MSH are mediated via melanocortin receptors. The skin as well as the majority of cutaneous cell types express POMC. Proinflammatory stimuli such as ultraviolet (UV) light induce POMC expression and alpha-MSH secretion. Receptors for alpha-MSH are not only expressed by melanocytes, where they mediate melanogenesis and proliferation, but also by virtually every cutaneous cell type. Accordingly, alpha-MSH elicits a plethora of biological actions in these cell types including immunomodulation, protection from oxidative stress and UV-induced apoptosis, modulation of secretory epithelial function and regulation of extracellular matrix composition. These actions may be exploited in future by using super potent and truncated MSH peptides for the treatment of various skin disorders including inflammatory dermatoses.

Targeting Melanocortin Receptors as a Novel Strategy to Control Inflammation

Adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormones, collectively called melanocortin peptides, exert multiple effects upon the host. These effects range from modulation of fever and inflammation to control of food intake, autonomic functions, and exocrine secretions. Recognition and cloning of five melanocortin receptors (MCRs) has greatly improved understanding of peptide-target cell interactions. Preclinical investigations indicate that activation of certain MCR subtypes, primarily MC1R and MC3R, could be a novel strategy to control inflammatory disorders. As a consequence of reduced translocation of the nuclear factor kappaB to the nucleus, MCR activation causes a collective reduction of the major molecules involved in the inflammatory process. Therefore, anti-inflammatory influences are broad and are not restricted to a specific mediator. Short half-life and lack of selectivity could be an obstacle to the use of the natural melanocortins. However, design and synthesis of new MCR ligands with selective chemical properties are already in progress. This review examines how marshaling MCR could control inflammation.

EEMCO Guidance for the Assessment of Hair Shedding and Alopecia

Knowledge of the hair follicle anatomy and the dynamics of hair cycling is substantial. Recognizing the anagen, catagen and telogen phases as well as teloptosis and the hair eclipse phenomenon clearly characterizes the typical hair chronobiology. Physiological modulators include hormones, neuromediators, miscellaneous biomolecules, seasons, micro-inflammation and ageing. For individuals who present with the complaint of increased hair shedding or alopecia, a host of evaluation techniques are available in addition to history, physical examination and laboratory assessment. Various clinical hair techniques can help in assessing the efficacy of drugs and cosmetics on hair growth. The methods are quite similar to those used to establish a definite diagnosis in dermatological practice. Great strides have been made during the recent decades in the methodology of hair growth trials in dermatology and cosmetology. Clinical evaluations benefit from a few additional specific techniques that enhance the perception of hair (re-) growth, shedding and alopecia. These assessments include the determination of hair patterning and density that may be helped by the 'black-and-white felt' examination. Daily hair counts, the 'hair pull test' and the 'hair feathering test' are also available. Instrumental methods provide reliable quantitative information that is useful if there are adequate controls. Some photographic methods, the trichogram, hair weighing and variants of the hair growth window technique including the phototrichogram, videotrichogram and tractio-phototrichogram provide insight into the complexities of hair cycling and shedding. Skin biopsy is indicated for diagnostic purposes, especially when the hair loss is accompanied by scarring.

Corticotropin-releasing hormone: an autocrine hormone that promotes lipogenesis in human sebocytes

Sebaceous glands may be involved in a pathway conceptually similar to that of the hypothalamic-pituitary-adrenal (HPA) axis. Such a pathway has been described and may occur in human skin and lately in the sebaceous glands because they express neuropeptide receptors. Corticotropin-releasing hormone (CRH) is the most proximal element of the HPA axis, and it acts as central coordinator for neuroendocrine and behavioral responses to stress. To further examine the probability of an HPA equivalent pathway, we investigated the expression of CRH, CRH-binding protein (CRH-BP), and CRH receptors (CRH-R) in SZ95 sebocytes in vitro and their regulation by CRH and several other hormones. CRH, CRH-BP, CRH-R1, and CRH-R2 were detectable in SZ95 sebocytes at the mRNA and protein levels: CRH-R1 was the predominant type (CRH-R1/CRH-R2 = 2). CRH was biologically active on human sebocytes: it induced biphasic increase in synthesis of sebaceous lipids with a maximum stimulation at 10(-7) M and up-regulated mRNA levels of 3 beta- hydroxysteroid dehydrogenase/Delta(5-4) isomerase, although it did not affect cell viability, cell proliferation, or IL-1 beta-induced IL-8 release. CRH, dehydroepiandrosterone, and 17 beta-estradiol did not modulate CRH-R expression, whereas testosterone at 10(-7) M down-regulated CRH-R1 and CRH-R2 mRNA expression at 6 to 24 h, and growth hormone (GH) switched CRH-R1 mRNA expression to CRH-R2 at 24 h. Based on these findings, CRH may be an autocrine hormone for human sebocytes that exerts homeostatic lipogenic activity, whereas testosterone and growth hormone induce CRH negative feedback. The findings implicate CRH in the clinical development of acne, seborrhea, androgenetic alopecia, skin aging, xerosis, and other skin disorders associated with alterations in lipid formation of sebaceous origin.

Evidence for expression of melanocortin-1 receptor in human sebocytes in vitro and in situ

Many lines of evidence indicate that the activity of sebaceous glands can be modulated by neuropeptides. Direct evidence in man, however, is still missing. We show that SZ95 sebocytes, an immortalized human sebaceous gland cell line, express receptors for alpha-melanocyte-stimulating hormone. Reverse transcription polymerase chain reaction with primers against the five melanocortin receptors and immunofluorescence studies using an antibody directed against a peptide corresponding to the amino acids 2-18 of the human melanocortin-1 receptor disclosed specific transcripts and immunoreactivity for melanocortin-1 receptor in these cells. Melanocortin-1 receptor expression was confirmed in sebocytes of normal human skin by immunohistochemistry. In contrast, no immunostaining for the melanocortin-5 receptor could be detected in sebocytes in situ, in accordance with the lack of specific transcripts for this melanocortin receptor in SZ95 sebocytes. As cytokines play an important role in the recruitment of inflammatory cells in acne and related disorders and alpha-melanocyte-stimulating hormone exerts immunomodulatory effects in many other cell types, we investigated the effect of alpha-melanocyte-stimulating hormone on interleukin-8 secretion by SZ95 sebocytes. Treatment with interleukin-1beta resulted in a marked increase in interleukin-8 release that was partially blocked by coincubation with alpha-melanocyte-stimulating hormone in a dose-dependent manner. Taken together, we show here that the melanocortin-1 receptor is expressed in vitro and in situ in human sebocytes. By modulating interleukin-8 secretion, alpha-melanocyte-stimulating hormone may act as a modulator of inflammatory responses in the pilosebaceous unit.

Human skin: an independent peripheral endocrine organ

The historical picture of the endocrine system as a set of discrete hormone-producing organs has been substituted by organs regarded as organized communities in which the cells emit, receive and coordinate molecular signals from established endocrine organs, other distant sources, their neighbors, and themselves. In this wide sense, the human skin and its tissues are targets as well as producers of hormones. Although the role of hormones in the development of human skin and its capacity to produce and release hormones are well established, little attention has been drawn to the ability of human skin to fulfil the requirements of a classic endocrine organ. Indeed, human skin cells produce insulin-like growth factors and -binding proteins, propiomelanocortin derivatives, catecholamines, steroid hormones and vitamin D from cholesterol, retinoids from diet carotenoids, and eicosanoids from fatty acids. Hormones exert their biological effects on the skin through interaction with high-affinity receptors, such as receptors for peptide hormones, neurotransmitters, steroid hormones and thyroid hormones. In addition, the human skin is able to metabolize hormones and to activate and inactivate them. These steps are overtaken in most cases by different skin cell populations in a coordinated way indicating the endocrine autonomy of the skin. Characteristic examples are the metabolic pathways of the corticotropin-releasing hormone/propiomelanocortin axis, steroidogenesis, vitamin D, and retinoids. Hormones exhibit a wide range of biological activities on the skin, with major effects caused by growth hormone/insulin-like growth factor-1, neuropeptides, sex steroids, glucocorticoids, retinoids, vitamin D, peroxisome proliferator-activated receptor ligands, and eicosanoids. At last, human skin produces hormones which are released in the circulation and are important for functions of the entire organism, such as sex hormones, especially in aged individuals, and insulin-like growth factor-binding proteins. Therefore, the human skin fulfils all requirements for being the largest, independent peripheral endocrine organ.