Evidence for specific complex formation between alpha-melanocyte stimulating hormone and 6(R)-L-erythro-5,6,7,8-tetrahydrobiopterin using near infrared Fourier transform Raman spectroscopy

Vitiligo

Vitiligo is an acquired depigmenting disorder that affects 0.5% to 2% of the world population. Three different forms are classified according to the distribution of lesions; namely non-segmental, segmental and mixed vitiligo. Vitiligo is associated with polymorphisms in genes involved in the immune response and in melanogenesis. However, environmental factors are required for the development of manifest disease. In general, the diagnosis is clinical and no laboratory tests or biopsies are required. Metabolic alterations are central to current concepts in pathophysiology. They induce an increased generation of reactive oxygen species and susceptibility to mild exogenous stimuli in the epidermis. This produces a senescent phenotype of skin cells, leads to the release of innate immune molecules, which trigger autoimmunity, and ultimately causes dysfunction and death of melanocytes. Clinical management aims to halt depigmentation, and to either repigment or depigment the skin, depending on the extent of disease. New therapeutic approaches include stimulation of melanocyte differentiation and proliferation through α-melanocyte-stimulating hormone analogues and through epidermal stem cell engineering. Several questions remain unsolved, including the connection between melanocyte depletion and stem cell exhaustion, the underlying degenerative mechanisms and the biological mediators of cell death. Overall, vitiligo is an excellent model for studying degenerative and autoimmune processes and for testing novel approaches in regenerative medicine. For an illustrated summary of this Primer, visit: http://go.nature.com/vIhFSC.

Inhibition of the prohormone convertase subtilisin-kexin isoenzyme-1 induces apoptosis in human melanoma cells

Prohormone convertases (PCs) are endoproteases that process many substrates in addition to hormone precursors. Although overexpression of PCs is linked to carcinogenesis in some solid tumors, the role of subtilisin-kexin isoenzyme-1 (SKI-1) in this context is unknown. We show that SKI-1 is constitutively expressed in human pigment cells with higher SKI activity in seven out of eight melanoma cell lines compared with normal melanocytes. SKI-1 immunoreactivity is also detectable in tumor cells of melanoma metastases. Moreover, tissue samples of the latter display higher SKI-1 mRNA levels and activity than normal skin. From various stimuli tested, 12-O-tetradecanoylphorbol-13-acetate and tunicamycin affected SKI-1 expression. Importantly, SKI-1 inhibition by the cell-permeable enzyme inhibitor decanoyl-RRLL-chloromethylketone (dec-RRLL-CMK) not only suppressed proliferation and metabolic activity of melanoma cells in vitro but also reduced tumor growth of melanoma cells injected intracutaneously into immunodeficient mice. Mechanistic studies revealed that dec-RRLL-CMK induces classical apoptosis of melanoma cells in vitro and affects expression of several SKI-1 target genes including activating transcription factor 6 (ATF6). However, ATF6 gene silencing does not result in apoptosis of melanoma cells, suggesting that dec-RRLL-CMK induces cell death in an ATF6-independent manner. Our findings encourage further studies on SKI-1 as a potential target for melanoma therapy.

Vibrational spectroscopic analyses of unique yellow feather pigments (spheniscins) in penguins

Many animals extract, synthesize and refine chemicals for colour display, where a range of compounds and structures can produce a diverse colour palette. Feather colours, for example, span the visible spectrum and mostly result from pigments in five chemical classes (carotenoids, melanins, porphyrins, psittacofulvins and metal oxides). However, the pigment that generates the yellow colour of penguin feathers appears to represent a sixth, poorly characterized class of feather pigments. This pigment class, here termed 'spheniscin', is displayed by half of the living penguin genera; the larger and richer colour displays of the pigment are highly attractive. Using Raman and mid-infrared spectroscopies, we analysed yellow feathers from two penguin species (king penguin, Aptenodytes patagonicus; macaroni penguin, Eudyptes chrysolophus) to further characterize spheniscin pigments. The Raman spectrum of spheniscin is distinct from spectra of other feather pigments and exhibits 17 distinctive spectral bands between 300 and 1700 cm(-1). Spectral bands from the yellow pigment are assigned to aromatically bound carbon atoms, and to skeletal modes in an aromatic, heterocyclic ring. It has been suggested that the penguin pigment is a pterin compound; Raman spectra from yellow penguin feathers are broadly consistent with previously reported pterin spectra, although we have not matched it to any known compound. Raman spectroscopy can provide a rapid and non-destructive method for surveying the distribution of different classes of feather pigments in the avian family tree, and for correlating the chemistry of spheniscin with compounds analysed elsewhere. We suggest that the sixth class of feather pigments may have evolved in a stem-lineage penguin and endowed modern penguins with a costly plumage trait that appears to be chemically unique among birds.

The redox--biochemistry of human hair pigmentation

The biochemistry of hair pigmentation is a complex field involving a plethora of protein and peptide mechanisms. The in loco factory for melanin formation is the hair follicle melanocyte, but it is common knowledge that melanogenesis results from a fine tuned concerted interaction between the cells of the entire dermal papilla in the anagen hair follicle. The key enzyme is tyrosinase to initiate the active pigmentation machinery. Hence, an intricate understanding from transcription of mRNA to enzyme activity, including enzyme kinetics, substrate supply, optimal pH, cAMP signaling, is a must. Moreover, the role of reactive oxygen species on enzyme regulation and functionality needs to be taken into account. So far our knowledge on the entire hair cycle relies on the murine model of the C57BL/6 mouse. Whether this data can be translated into humans still needs to be shown. This article aims to focus on the effect of H(2)O(2)-redox homeostasis on hair follicle pigmentation via tyrosinase, its substrate supply and signal transduction as well as the role of methionine sulfoxide repair via methionine sulfoxide reductases A and B (MSRA and B).

Regulation of melanogenesis--controversies and new concepts

Despite many efforts, regulation of skin and hair pigmentation is still not fully understood. This article focuses mainly on controversial aspects in pigment cell biology which have emerged over the last decade. The central role of tyrosinase as the key enzyme in initiation of melanogenesis has been closely associated with the 6BH4 dependent phenylalanine hydroxylase (PAH) and tyrosine hydroxylase isoform I (THI) providing evidence for an old concept of the three enzyme theory in the initiation of the pigmentation process. In this context, it is noteworthy that intracellular L-phenylalanine uptake and turnover to L-tyrosine via PAH is vital for substrate supply of THI and tyrosinase. While PAH acts in the cytosol of melanocytes, THI and tyrosinase are sitting side by side in the melanosomal membrane. THI at low pH provides L-3,4-hydroxyphenylalanine L-DOPA which in turn is required for activation of met-tyrosinase. After an intramelanosomal pH change, possibly by the p-protein, has taken place, tyrosinase is subject to control by 6/7BH4 and the proopiomelanocortin (POMC) peptides alpha-MSH melanocyte stimulating hormone and beta-MSH in a receptor independent manner. cAMP is required for the activation of microphthalmia-associated transcription factor to induce expression of tyrosinase, for transcription of THI and for activation of PAH. The redundancy of the cAMP signal is discussed. Finally, we propose a novel mechanism involving H2O2 in the regulation of tyrosinase via p53 through transcription of hepatocyte nuclear factor 1alpha which in turn can also affect the POMC response.

Advances in melanocyte basic science research

This article focuses on recent advances in melanocyte biology and physiology. The major function of this neural crest-derived cell is the production of melanins. A "three enzyme theory" in the initiation of pigmentation is put forward and backed up by recent findings. A receptor-independent role for alpha-MSH and the cofactor (6R)-l-erythro-5,6,7,8-terahydrobiopterin (6BH(4)) in the control of tyrosinase is described. The importance of intramelanosomal pH for melanogenesis is covered. Finally, the redundancy of the cAMP and IP3/DAG/calcium signal in melanocytes together with the downstream events are highlighted. The main message of this article is that the intracellular H(2)O(2)- redox-equilibrium controls melanocyte function in a concentration-dependent manner.

Oxidative stress via hydrogen peroxide affects proopiomelanocortin peptides directly in the epidermis of patients with vitiligo

The human skin holds the capacity for autocrine processing of the proopiomelanocortin (POMC)-derived peptides. Recent data demonstrated the presence and functionality of ACTH, alpha- and beta-melanocyte-stimulating hormone (MSH), and beta-endorphin in the regulation of skin pigmentation, and a role has been put forward for alpha-MSH as an effective antioxidant. In patients with vitiligo, decreased epidermal POMC processing and low alpha-MSH levels were documented previously. These patients accumulate hydrogen peroxide (H2O2) in the 10(-3) M range in their epidermis. Therefore, we examined the involvement of H2O2 on POMC-derived peptides as possible targets for oxidation by this reactive oxygen species. To address this, we employed immunofluorescence labelling, dot blot analysis, Fourier transform Raman spectroscopy, functionality studies, and computer simulation of the peptide structures. We demonstrate H2O2-mediated oxidation of epidermal ACTH, alpha-MSH, and beta-endorphin in vitiligo owing to oxidation of methionine residues in the sequences of these peptides. Moreover, we show that oxidized beta-endorphin loses its function in the promotion of pigmentation in melanocytes. These changes are reversible upon the reduction of H2O2 levels by a pseudocatalase PC-KUS. Moreover, oxidation of alpha-MSH can be prevented by the formation of a 1:1 complex with the abundant cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin. Thus, using vitiligo, we demonstrate that H2O2 can affect pigmentation via epidermal POMC peptide redox homeostasis.

Adrenergic and Cholinergic Control in the Biology of Epidermis: Physiological and Clinical Significance

The presence of an autocrine adrenergic and cholinergic intra/intercellular signal transduction network in the human epidermis contributes significantly to homeostatic and compensatory responses regulating vital functions in keratinocytes and melanocytes. The ligands produced control autocrine and paracrine loops to initiate responses through cognate receptors expressed within the same or adjacent cells. The epidermal adrenergic signal controls calcium homeostasis, cell growth, differentiation, motility, and pigmentation via the beta2 and alpha1 adrenoceptors. The cholinergic system is highly complex comprising both nicotinic and muscarinic receptors with multiple subtypes and this system plays an important role in keratinocyte cell cycle progression, differentiation, directional migration, adhesion, and apoptotic secretion. Moreover, lymphocytes also express adrenergic and cholinergic receptors. Both types of signal transduction receptors are coupled to classical intracellular second messenger pathways, including cAMP-, cGMP-, and calcium-mediated downstream responses. To date, it has been recognized that several dermatoses such as psoriasis, atopic dermatitis, Mal de Meleda, vitiligo, palmoplantar pustulosis, and pemphigus may be mediated, in part, by the non-neuronal adrenergic/cholinergic systems. A detailed understanding of the physiology and pathophysiology of the adrenergic/cholinergic network in the skin could offer the development of specific drugs for novel treatment modalities.

Regulation of tyrosinase by tetrahydropteridines and H2O2

Recently two alternative mechanisms have been put forward for the inhibition of tyrosinase by 6R-l-erythro 5,6,7,8-tetrahydrobiopterin (6BH(4)). Initially allosteric uncompetitive inhibition was demonstrated due to 1:1 binding of 10(-6)M 6BH(4) to a specific domain 28 amino acids away from the Cu(A) active site of the enzyme. Alternatively it was then shown that 10(-3)M 6BH(4) inhibit the reaction by the reduction of the product dopaquinone back to l-dopa. In the study presented herein we have used two structural analogues of 6BH(4) (i.e., 6,7-(R,S)-dimethyl tetrahydrobiopterin and 6-(R,S)-tetrahydromonapterin) confirming classical uncompetitive inhibition due to specific binding of the pyrimidine ring of the pterin moiety to the regulatory domain on tyrosinase. Under these conditions there was no reduction of l-dopaquinone back to l-dopa by both cofactor analogues. Inhibition of tyrosinase by 6BH(4) occurs in the concentration range of 10(-6)M after preactivation with l-tyrosine and this mechanism uncouples the enzyme reaction producing H(2)O(2) from O(2). Moreover, a direct oxidation of 6BH(4) to 7,8-dihydrobiopterin by tyrosinase in the absence of the substrate l-tyrosine was demonstrated. The enzyme was activated by low concentrations of H(2)O(2) (<0.3 x 10(-3)M), but deactivated at concentrations in the range 0.5-5.0 x 10(-3)M. In summary, our results confirm a major role for 6BH(4) in the regulation of human pigmentation.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Indirect oxidation of 6-tetrahydrobiopterin by tyrosinase

6-Tetrahydrobiopterin is known to bind to an allosteric site of tyrosinase to directly inhibit the enzyme. However, simultaneous measurements of ultraviolet-visible absorption spectra and oxygen consumption led us to conclude that the inhibition was due to oxidation of 6-tetrahydrobiopterin by dopaquinone. Immediately after addition of 6-tetrahydrobiopterin, tyrosinase stopped producing dopachrome from either tyrosine or dopa. Duration of inhibition was proportional to the concentration of added 6-tetrahydrobiopterin and the enzyme activity was fully restored after the inhibition. Surprisingly, there was a rapid consumption of oxygen during the inhibition period. In addition, absorption spectra indicated that the only reaction that occurred during the inhibition was oxidation of 6-tetrahydrobiopterin to 7,8-dihydrobiopterin. In the absence of tyrosine or dopa, tyrosinase did not oxidize 6-tetrahydrobiopterin, suggesting that a reaction intermediate between dopa and dopachrome was a target for the inhibition. We propose a new mechanism in which dopa is oxidized to dopaquinone and the latter, instead of producing dopachrome, is reduced back to dopa by 6-tetrahydrobiopterin.

Inhibition of tumor necrosis factor-alpha stimulated NFkappaB/p65 in human keratinocytes by alpha-melanocyte stimulating hormone and adrenocorticotropic hormone peptides

Alpha-melanocyte stimulating hormone (alpha-MSH) has pigmentary, anti-inflammatory, antipyretic, and general immunomodulatory roles. It can oppose several cytokines including tumor necrosis factor-alpha in a number of tissues, including skin. We have previously shown that alpha-MSH can inhibit tumor necrosis factor-alpha stimulated intercellular adhesion molecule 1 upregulation and nuclear factor kappaB (NFkappaB) transcription factor activation in melanocyte and melanoma cells. It is thought, however, that this MSH biology may also extend to other cells of the skin and in this study we extend our work to keratinocytes. We have investigated in detail the ability of three alpha-MSH peptides to inhibit tumor necrosis factor alpha stimulated NFkappaB activation in nonpigmentary HaCaT keratinocytes (alpha-MSH, L-Lys-L-Pro-L-Val, and L-Lys-L-Pro-D-Val) and two adrenocorticotropic hormone (ACTH) peptides (1-17 and 1-39), reported to be present in skin tissue. NFkappaB/p65 activation was analyzed by electrophoretic mobility shift assay and immunofluorescent microscopy. alpha-MSH, L-Lys-L-Pro-L-Val, and L-Lys-L-Pro-D-Val all significantly inhibited tumor necrosis factor alpha stimulated NFkappaB activation, whereas ACTH 1-17 and 1-39 did not, in the HaCaT keratinocytes. MSH peptides and ACTH 1-39 were effective, however, at inhibiting NFkappaB activation in normal human keratinocytes. Immunolabeling of inhibitor kappaBalpha of NFkappaB (IkappaBalpha) revealed an abnormal localization to the nucleus of HaCaT cells, which was unaffected by MSH/ACTH peptides. In contrast, normal human keratinocytes showed a normal IkappaBalpha distribution that responded to MSH/ACTH with nuclear translocation. Our data support previous work on the role of MSH/ACTH peptides as immunomodulatory/anti-inflammatory regulators, and extend this work to keratinocytes identifying a novel IkappaBalpha mechanism and extends findings to ACTH peptides, identifying an abnormal IkappaBalpha mechanism in the immortal HaCaT versus normal keratinocyte.

Melanocyte function and its control by melanocortin peptides

Melanocytes are cells of neural crest origin. In the human epidermis, they form a close association with keratinocytes via their dendrites. Melanocytes are well known for their role in skin pigmentation, and their ability to produce and distribute melanin has been studied extensively. One of the factors that regulates melanocytes and skin pigmentation is the locally produced melanocortin peptide alpha-MSH. The effects of alpha-MSH on melanogenesis are mediated via the MC-1R and tyrosinase, the rate-limiting enzyme in the melanogenesis pathway. Binding of alpha-MSH to its receptor increases tyrosinase activity and eumelanin production, which accounts for the skin-darkening effect of alpha-MSH. Other alpha-MSH-related melanocortin peptides, such as ACTH1-17 and desacetylated alpha-MSH, are also agonists at the MC-1R and could regulate melanocyte function. Recent evidence shows that melanocytes have other functions in the skin in addition to their ability to produce melanin. They are able to secrete a wide range of signal molecules, including cytokines, POMC peptides, catecholamines, and NO in response to UV irradiation and other stimuli. Potential targets of these secretory products are keratinocytes, lymphocytes, fibroblasts, mast cells, and endothelial cells, all of which express receptors for these signal molecules. Melanocytes may therefore act as important local regulators of a range of skin cells. It has been shown that alpha-MSH regulates NO production from melanocytes, and it is possible that the melanocortins regulate the release of other signalling molecules from melanocytes. Therefore, the melanocortin signaling system is one of the important regulators of skin homeostasis.

New aspects on the melanocortins and their receptors

Knowledge of melanocortins and their receptors has increased tremendously over the last few years. The cloning of five melanocortin receptors, and the discovery of two endogenous antagonists for these receptors, agouti and agouti-related peptide, have sparked intense interest in the field. Here we give a comprehensive review of the pharmacology, physiology and molecular biology of the melanocortins and their receptors. In particular, we review the roles of the melanocortins in the immune system, behaviour, feeding, the cardiovascular system and melanoma. Moreover, evidence is discussed suggesting that while many of the actions of the melanocortins are mediated via melanocortin receptors, some appear to be mediated via mechanisms distinct from melanocortin receptors.

Pro-opiomelanocortin-related peptides, prohormone convertases 1 and 2 and the regulatory peptide 7B2 are present in melanosomes of human melanocytes

Recently, it has been shown that alpha-melanocyte stimulating hormone can directly activate tyrosinase by removing the allosteric regulator 6(R)-L-erythro 5,6,7,8 tetrahydrobiopterin resulting in a stable alpha-melanocyte stimulating hormone/6(R)-L-erythro 5,6,7,8 tetrahydrobiopterin complex. As melanin production occurs in the melanosome, a specific organelle of the melanocyte, it seemed important to investigate whether these organelles themselves actually produce pro-opiomelanocortin-related peptides in their acidic environment. The presence of alpha-melanocyte stimulating hormone and adrenocorticotropin in the epidermis and melanocytes has been shown by several investigators. In order to follow possible pro-opiomelanocortin processing in the melanosome, human melanocytes were established in MCDB 153 medium and utilized for immunohistochemistry, immunogold electron microscopy, and western blotting. For this purpose antibodies against alpha-melanocyte stimulating hormone, adrenocorticotropin, prohormone convertases 1 and 2 (PC1 and PC2) and the PC2 regulatory protein 7B2 were used. Our results demonstrated the presence of the entire system for pro-opiomelanocortin processing in the melanosome. Considering the pH optima of these convertases, the results are in agreement with an autocrine intramelanosomal production of pro- opiomelanocortin-related peptides and an autocrine production and recycling of the cofactor 6(R)-L- erythro 5,6,7,8 tetrahydrobiopterin in melanocytes. Based on these novel observations, we would like to propose that the pigmentation process may not necessarily involve a melanocortin-1 receptor-mediated mechanism.