Mammalian tyrosinase: biosynthesis, processing, and modulation by melanocyte-stimulating hormone

Protein kinase C-beta activates tyrosinase by phosphorylating serine residues in its cytoplasmic domain

We have previously shown that protein kinase C-beta (PKC-beta) is required for activation of tyrosinase (Park, H. Y., Russakovsky, V., Ohno, S., and Gilchrest, B. A. (1993) J. Biol. Chem. 268, 11742-11749), the rate-limiting enzyme in melanogenesis. We now examine its mechanism of activation in human melanocytes. In vivo phosphorylation experiments revealed that tyrosinase is phosphorylated through the PKC-dependent pathway and that introduction of PKC-beta into nonpigmented human melanoma cells lacking PKC-beta lead to the phosphorylation and activation of tyrosinase. Preincubation of intact melanosomes with purified active PKC-beta in vitro increased tyrosinase activity 3-fold. By immunoelectron microscopy, PKC-beta but not PKC-alpha was closely associated with tyrosinase on the outer surface of melanosomes. Western blot analysis confirmed the association of PKC-beta with melanosomes. Only the cytoplasmic (extra-melanosomal) domain of tyrosinase, which contains two serines but no threonines, was phosphorylated by the serine/threonine kinase PKC-beta. These two serines at positions 505 and 509 both are present in the C-terminal peptide generated by trypsin digestion of tyrosinase. Co-migration experiments comparing synthetic peptide standards of all three possible phosphorylated tryptic peptides, a diphosphopeptide and two monophosphopeptides, to tyrosinase-phosphorylated in intact melanocytes by PKC-beta and then subjected to trypsin digestion revealed that both serine residues are phosphorylated by PKC-beta. We conclude that PKC-beta activates tyrosinase directly by phosphorylating serine residues at positions 505 and 509 in the cytoplasmic domain of this melanosome-associated protein.

Do hair bulb melanocytes undergo apoptosis during hair follicle regression (catagen)?

The fate of the hair follicle pigmentary unit during the cyclical involution of anagen hair follicles is unknown. Using the C57BL/6 mouse model for hair research, hair follicle melanocytes were examined during the anagen-catagen transformation, comparing spontaneous and pharmacologically induced catagen development. This study shows that both spontaneous catagen and dexamethasone-induced catagen display similar changes in the pigmentary unit. Catagen hair follicles exhibited pigment incontinence in the dermal papilla and in selected outer root sheath keratinocytes. Melanocytes deleted by apoptosis were detected in spontaneous catagen and, more commonly, in dexamethasone-induced catagen, and were identified using transmission electron microscopy by the presence of free premelanosomes in affected cells lacking epithelial specializations, and by the colocalization of TUNEL positivity and tyrosinase-related protein-1 immunoreactivity. By contrast, cyclophosphamide-induced catagen was characterized by the initial retention of melanogenic and dendritic melanocytes in the presence of widespread keratinocyte apoptosis. Melanocyte incontinence and the ectopic distribution of melanin were more severe than in the other forms of catagen. Whereas much of this melanin was extruded, via the hair canal, to the skin surface, hair follicle-derived pigment was also detected within the epidermis, probably derived from pigment-carrying migrating outer root sheath keratinocytes from the proximal hair follicle. Thus, apoptosis may account, at least in part, for the loss of melanogenic melanocytes during spontaneous catagen. Although dexamethasone-induced catagen may provide a useful model for general hair pigmentation research, catagen induced by cyclophosphamide offers an interesting model for studying the response, and relative resistance, of melanocytes to chemical injury.

Autoantibodies to tyrosinase-related protein-1 detected in the sera of vitiligo patients using a quantitative radiobinding assay

In the present study, we describe the in vitro transcription-translation of human tyrosinase-related protein-1 (TRP-1) cDNA and subsequent use of the resulting 35S-labelled TRP-1 in a radioimmunoassay to analyse vitiligo sera for the presence of TRP-1 antibodies. Of 53 vitiligo sera examined in the assay, three (5.7%) were found to be positive for TRP-1 antibodies. In contrast, sera from 20 controls, 10 patients with Hashimoto's thyroiditis and 10 patients with Graves' disease were all negative for TRP-1 antibodies. Although glycosylation of the labelled protein was necessary for its immunoprecipitation by TRP-1-specific monoclonal antibody TA99, this post-translational processing did not affect the binding of any of the sera tested. All three patients positive for TRP-1 antibodies (aged 50-63 years) had had vitiligo of the symmetrical type for more than 1 year, and all of them also had an associated autoimmune disorder: Graves' disease in one and autoimmune hypothyroidism in two. In addition, antibodies to the melanogenic enzymes tyrosinase and tyrosinase-related protein-2 (TRP-2) were present in their serum. Absorption studies indicated that preincubation with COS-7 cell extract containing either expressed TRP-1, tyrosinase or TRP-2 absorbed out the immunoreactivity of the three sera positive in the radioimmunoassay (RIA) with [35S]TRP-1. The results indicate that autoantibodies to TRP-1 cross-react with tyrosinase and TRP-2, suggesting one or more common epitopes between the three proteins.

In vitro modulation of proliferation and melanization of melanoma cells by citrate

B16/F10 murine melanoma cells were grown for 24 and 36 h in Dulbecco's modified Eagle medium in presence of 10-20 mM trisodium citrate. The intracellular melanin concentration and the melanin secreted in the extracellular medium was estimated. It is observed that 20 mM citrate stimulates extracellular melanin secretion in B16/F10 melanoma cells by 200% at 36 h treatment. The intracellular melanin content increased by 90%. This stimulatory effect of citrate was totally abolished when these cells were grown in presence of 1 mM phenyl thiourea, a specific inhibitor of tyrosinase activity. Citrate (0.1-5 mM) had no effect on dopa oxidase activity either at pH 5.0 or at pH 6.8. There was no increase in the tyrosinase specific activity in presence of citrate. The increased melanin synthesis was shown to be due to stimulation of cellular tyrosine hydroxylase activity by citrate. It has been suggested that enhanced melanin synthesis results in an increased production of metabolites that are toxic to the growth of melanoma cells. We have studied the effect of citrate on cellular proliferation. Following 24 and 36 h treatment with citrate, the cells exhibited a dose-dependent decrease in proliferation. In presence of 20 mM citrate the cell number was only up to 50% of the control cultures after 36 h of incubation. The growth retardation was not due to cytotoxicity. Citrate, a natural metabolite, is a unique molecule which may be involved in the regulation of melanin biosynthetic pathway, since it enhances melanogenesis by increasing the hydroxylase activity of tyrosinase which is the regulatory enzyme of this pathway. These observations add further support to the critical role of intramelanosomal pH in regulation of melanogenesis.

Microphthalmia gene product as a signal transducer in cAMP-induced differentiation of melanocytes

Melanocyte differentiation characterized by an increased melanogenesis, is stimulated by alpha-melanocyte-stimulating hormone through activation of the cAMP pathway. During this process, the expression of tyrosinase, the enzyme that controls melanin synthesis is upregulated. We previously showed that cAMP regulates transcription of the tyrosinase gene through a CATGTG motif that binds microphthalmia a transcription factor involved in melanocyte survival. Further, microphthalmia stimulates the transcriptional activity of the tyrosinase promoter and cAMP increases the binding of microphthalmia to the CATGTG motif. These observations led us to hypothesize that microphthalmia mediates the effect of cAMP on the expression of tyrosinase. The present study was designed to elucidate the mechanism by which cAMP regulates microphthalmia function and to prove our former hypothesis, suggesting that microphthalmia is a key component in cAMP-induced melanogenesis. First, we showed that cAMP upregulates the transcription of microphthalmia gene through a classical cAMP response element that is functional only in melanocytes. Then, using a dominant-negative mutant of microphthalmia, we demonstrated that microphthalmia is required for the cAMP effect on tyrosinase promoter. These findings disclose the mechanism by which cAMP stimulates tyrosinase expression and melanogenesis and emphasize the critical role of microphthalmia as signal transducer in cAMP-induced melanogenesis and pigment cell differentiation.

Rac1 mediates dendrite formation in response to melanocyte stimulating hormone and ultraviolet light in a murine melanoma model

Melanocytes are pigment producing cells that reside in the basal layer of the epidermis, and form multiple long dendritic processes that transport melanosomes from the melanocyte cell body to the dendritic tips, and then to keratinocytes. Dendrite formation requires actin polymerization in the newly forming dendrite, and dendrite formation in melanocytes is stimulated by hormones and ultraviolet light. The rho-subfamily of monomeric guanosine triphosphate-binding proteins is implicated in remodeling the cellular actin cytoskeleton, resulting in the formation of filopodia, lamellipodia, and stress fibers, as well as in oncogenesis and activation of the Jun/p38 mitogen activated kinase cascade. In this paper we show that rac1 induces the formation of dendrite-like structures when activated mutants are transiently expressed in B16F1 murine melanoma cells and in four human melanoma cell lines. Activated mutants of cdc42 and rhoA induced the formation of filopodia and stress fibers, respectively, in B16F1 cells, but not dendrites. A dominant negative inhibitor of rac1 abrogated the ability of alpha-melanocyte stimulating hormone, a peptide hormone known to stimulate melanocyte dendrite formation, and ultraviolet light, to induce dendrite formation in B16F1 cells, and alpha-melanocyte stimulating hormone and ultraviolet light stimulated the localization of rac1 to dendrite cell membranes. These results suggest that rac1 is an important signaling intermediate in dendrite formation in B16F1 cells, and that rac1 mediates the well-known ability of alpha-melanocyte stimulating hormone and ultraviolet light to induce dendrite formation.

Expression of tyrosinase and the tyrosinase related proteins in the Mitfvit (vitiligo) mouse eye: implications for the function of the microphthalmia transcription factor

Mitf (Microphthalmia transcription factor), a basic-helix-loop-helix zipper protein, encoded at the microphthalmia (Mitf) locus, regulates the transcription of the gene encoding tyrosinase, the rate-limiting enzyme in melanin biosynthesis, by binding the DNA sequence CATGTG. This binding site is present also in the genes encoding two tyrosinase related proteins, TRP-1 and TRP-2. To gain insight into the function of Mitf in vivo, we determined whether there was a difference in the levels of these proteins in the RPE/choroid of the vitiligo (Mitfvit) mouse, in which there is a mutation of the Mitf gene. This mouse has alteration of RPE pigmentation and function that presumably leads to slow progressive loss of photoreceptor cells. The RPE/choroid was dissected from eyes of vitiligo and C57BL/6 wild-type mice at postnatal ages 2, 4, 7, 10, 14, 21 and 42 days. Extracts of pooled tissues were subjected to electrophoresis and immunoblotting. The levels of tyrosinase, TRP-1 and TRP-2 were determined densitometrically following immunodetection with rabbit antipeptide antisera. In addition, the tyrosine hydroxylase activity of tyrosinase as assayed radiometrically. Levels of TRP-1 were 3-7 fold greater in control RPE/choroid compared with mutants. This marked difference in protein level was observed at the earliest age examined (P2) and persisted throughout the first two weeks. Tyrosinase levels in mutants were similar to controls at P2 and P4, but were reduced at P10 and beyond. Tyrosinase activity was diminished also in mutants by P10. Levels of TRP-2 were similar between mutants and controls, although the typical decrease seen in controls after P14 was attenuated in the mutant mice. There is a significant reduction in the level of TRP-1 in the RPE/choroid of the Mitfvit mouse. The data suggests that transcription of the gene encoding TRP-1 is extremely dependent upon functional Mitf. It provides in vivo evidence that Mitf regulates the transcription of the gene encoding TRP-1 as well as tyrosinase.

Different cis-acting elements are involved in the regulation of TRP1 and TRP2 promoter activities by cyclic AMP: pivotal role of M boxes (GTCATGTGCT) and of microphthalmia

In melanocytes and in melanoma cells, cyclic AMP (cAMP)-elevating agents stimulate melanogenesis and increase the transcription of tyrosinase, the rate-limiting enzyme in melanin synthesis. However, two other enzymes, tyrosinase-related protein 1 (TRP1) and TRP2, are required for a normal melanization process leading to eumelanin synthesis. In B16 melanoma cells, we demonstrated that stimulation of melanogenesis by cAMP-elevating agents results in an increase in tyrosinase, TRP1, and TRP2 expression. cAMP, through a cAMP-dependent protein kinase pathway, stimulates TRP1 and TRP2 promoter activities in both B16 mouse melanoma cells and normal human melanocytes. Regulation of the TRP1 and TRP2 promoters by cAMP involves a M box and an E box. Further, a classical cAMP response element-like motif participates in the cAMP responsiveness of the TRP2 promoter, demonstrating that the TRP2 gene is subjected to different regulatory processes, which could account for its different expression patterns during embryonic development or under specific physiological and pathological conditions. We also found that microphthalmia, a basic helix-loop-helix transcription factor, strongly stimulates the transcriptional activities of the TRP1 and TRP2 promoters, mainly through binding to the M boxes. Additionally, we demonstrated that cAMP increases microphthalmia expression and thereby its binding to TRP1 and TRP2 M boxes. These convergent and compelling results disclose at least a part of the molecular mechanism involved in the regulation of melanogenic gene expression by cAMP and emphasize the pivotal role of microphthalmia in this process.

Mutational analysis of copper binding by human tyrosinase

Tyrosinase (EC 1.14.18.1) is a copper-containing enzyme that catalyzes several reactions in the biosynthesis of melanin pigments and is deficient in patients with type I oculocutaneous albinism (OCA1). Tyrosinase is thought to bind two copper ions, one at each of two conserved sequence motifs, termed CuA and CuB, but to date this has been directly proved only for the Neurospora and mushroom enzyme. Here, we demonstrate that mammalian tyrosinase directly binds copper, and that the CuA and CuB sites are both required for copper binding and for catalytic activity. We show that in human tyrosinase, copper binding by the CuB site is most likely coordinated by residues His363, His367, and His389, and that copper binding may be cooperative, with copper binding at one site facilitating copper binding by the other site. Furthermore, correct folding of the tyrosinase polypeptide appears to be necessary for copper binding, and a number of human OCA1 mutations disrupt copper binding and thus catalytic function of tyrosinase.

Modulation of murine melanocyte function in vitro by agouti signal protein

Molecular and biochemical mechanisms that switch melanocytes between the production of eumelanin or pheomelanin involve the opposing action of two intercellular signaling molecules, alpha-melanocyte-stimulating hormone (MSH) and agouti signal protein (ASP). In this study, we have characterized the physiological effects of ASP on eumelanogenic melanocytes in culture. Following exposure of black melan-a murine melanocytes to purified recombinant ASP in vitro, pigmentation was markedly inhibited and the production of eumelanosomes was decreased significantly. Melanosomes that were produced became pheomelanosome-like in structure, and chemical analysis showed that eumelanin production was significantly decreased. Melanocytes treated with ASP also exhibited time- and dose-dependent decreases in melanogenic gene expression, including those encoding tyrosinase and tyrosinase-related proteins 1 and 2. Conversely, melanocytes exposed to MSH exhibited an increase in tyrosinase gene expression and function. Simultaneous addition of ASP and MSH at approximately equimolar concentrations produced responses similar to those elicited by the hormone alone. These results demonstrate that eumelanogenic melanocytes can be induced in culture by ASP to exhibit features characteristic of pheomelanogenesis in vivo. Our data are consistent with the hypothesis that the effects of ASP on melanocytes are not mediated solely by inhibition of MSH binding to its receptor, and provide a cell culture model to identify novel factors whose presence is required for pheomelanogenesis.

An immunocytochemical study of MHC class I expression on human Langerhans cells and melanocytes

Classical MHC class I glycoproteins (HLA-A, B, and C) present endogenous cytosolic peptide antigen fragments to CD8-positive T-cells. CD8-positive T-cell recognition and destruction of virus-infected cells are dependent on adequate cellular MHC class I expression. Constitutive MHC class I expression is ubiquitous, but known to be deficient on specific differentiated cell types which include hepatocytes, neurones, chondrocytes and myocytes. Although enabling assessment of MHC class I expression on individual cells, limitations of immunocytochemistry were encountered with this assessment on Langerhans cells and melanocytes. These dispersed intraepidermal cells were obscured by adjacent keratinocytes in sections immunostained for MHC class I glycoproteins. Initiatives designed to resolve the issue have included immunoelectron microscopy, cell culture techniques, and animal bone marrow chimera models. Despite the elegance of these techniques, the issue of MHC class I expression on Langerhans cells and melanocytes remains unresolved. In this immunocytochemical study, an alternative strategy was based upon the recognized deficiency of epithelial MHC class I expression within pilosebaceous adnexal units. Langerhans cells and melanocytes were therefore studied within this microenvironment of deficient MHC class I expression, using monomorphic and polymorphic MHC markers. Langerhans cells and melanocytes were demonstrated within pilosebaceous units of scalp skin by immunocytochemistry. Differentiation markers OKT6 (CD1a) and TMH1 defined Langerhans cells and melanocytes, respectively. Monomorphic MHC markers W6/32 and TAL IB5 defined invariant epitopes of HLA class I and II, respectively. Polymorphic MHC class I markers defined the HLA-Bw4 and HLA-Bw6 supertypic determinants. Constitutive MHC class I expression was shown to be deficient on Langerhans cells and melanocytes.

Regulation of melanogenesis in B16 mouse melanoma cells by protein kinase C

Melanogenesis is regulated by a variety of environmental and hormonal factors. In this study, we showed that protein kinase C (PKC) plays a major role in regulating melanogenesis in B16 mouse melanoma cells. Chronic treatment of B16 cells with phorbol dibutyrate resulted in a concentration-dependent loss of density-dependent induction of tyrosinase activity, which correlated positively with a concentration-dependent loss of PKC enzyme activity. In contrast, B16 clones overexpressing PKC alpha had increased tyrosinase activity. Different phorbol derivatives inhibited tyrosinase activity and depleted cellular PKC alpha in a manner that reflected their reported tumor-promoting activity. Western blotting analysis showed that phorbol dibutyrate decreased the amount of the brown locus gene product (TRP-1) by 50% and lowered the amount of the albino locus gene product (tyrosinase) to undetectable levels. None of the phorbol derivatives affected the level of the slaty locus protein (TRP-2). The decrease in tyrosinase and TRP-1 protein levels was found to be due to a decrease in the mRNA encoded by these genes. In addition to inhibiting the density-dependent increase in tyrosinase activity, phorbol dibutyrate inhibited some, but not all, of the 8-bromocyclic AMP-induced increase in tyrosinase activity. This was accompanied by a decrease in the amount of tyrosinase protein induced by 8-bromocyclic AMP. Although 8-bromocyclic AMP did not change the level of TRP-1, it did reverse the decrease in the amount of this protein induced by phorbol dibutyrate. The amount of TRP-2 was not altered by any of these agents. These data suggest that PKC regulates melanogenesis primarily by controlling the constitutive expression of tyrosinase and, to a lesser extent, TRP-1.

Signalling mechanisms of endothelin-induced mitogenesis and melanogenesis in human melanocytes

To understand the signalling mechanisms involved in the dual stimulatory effects of endothelin-1 (ET-1) on DNA synthesis and melanization in cultured human melanocytes, we analysed the biological profile of ET-1 receptor and determined the effects of ET-1 on the protein kinase C, cyclic AMP system and mitogen-activated protein kinase (MAP kinase) in comparison with their relevant stimulants. The photoaffinity labelling of ET-1 receptors with Denny-Jaff reagents revealed an ET-1 receptor with a molecular mass of 51 kDa in human melanocytes. The ET(A) receptor subtype-sensitive antagonist BQ123(50 nM) or pertussis toxin (100 ng/ml) significantly suppressed the ET-1-induced intracellular calcium mobilization, indicating the presence of pertussis toxin-sensitive G-protein-coupled ET(A) receptors. An assay of protein kinase C activity revealed that 10nM ET-1 translocated cytosolic protein kinase C to membrane-bound protein kinase C within 5 min of the start of incubation. In contrast, receptor-mediated melanocyte activation by ET-1 was accompanied by an elevated level of cyclic AMP (4-fold over control) after 10-60 min of incubation, whereas 60 min of incubation of human melanocytes with c-Kit or c-Met ligands such as stem cell factor (10 nM) or basic fibroblast growth factor (10 nM) did not elevate the cyclic AMP level. We have also demonstrated that a specific tyrosine kinase inhibitor, tyrphostin B-42 (10 microM), inhibited the ET-1-induced growth stimulation, suggesting the involvement of the tyrosine kinase pathway in growth stimulation. Consistently, an assay of MAP kinase revealed that ET-1 caused a 10-fold activation of MAP kinase after 5 min of incubation with human melanocytes in a similar way to tyrosine kinase ligands such as stem cell factor and hepatocyte growth factor. Further, the DNA synthesis stimulated by the c-Kit ligand stem cell factor at a concentration of 1 nM was synergistically enhanced by 5 nM ET-1. These results suggest that ET-induced dual cellular events in human melanocytes are closely associated with cross-talk between the protein kinase C and A and tyrosine kinase pathways.

Mitogen-activated protein kinase pathway and AP-1 are activated during cAMP-induced melanogenesis in B-16 melanoma cells

In mammalian melanocytes, melanin synthesis is controlled by tyrosinase, the critical enzyme in the melanogenic pathway. We and others showed that the stimulation of melanogenesis by cAMP is due to an increased tyrosinase expression at protein and mRNA levels. However, the molecular events connecting the rise of intracellular cAMP and the increase in tyrosinase activity remain to be elucidated. In this study, using B16 melanoma cells, we showed that cAMP-elevating agents stimulated mitogen-activated protein (MAP) kinase, p44mapk. This effect was mediated by the activation of MAP kinase kinase. cAMP-elevating agents induced a translocation of p44mapk to the nucleus and an activation of the transcription factor AP-1. cAMP-induced AP-1 contained FOS-related antigen-2 in association with JunD, while after phorbol ester stimulation AP-1 complexes consist mainly of JunD/c-Fos heterodimers. In an attempt to connect these molecular events to the control of tyrosinase expression that appears to be the pivotal point of melanogenesis regulation, we hypothesized that following its activation by cAMP, p44mapk activates AP-1. Then AP-1 could stimulate tyrosinase expression through the interaction with specific DNA sequences present in the mouse tyrosinase promoter.

The pinkeyed-dilution protein and the eumelanin/pheomelanin switch: in support of a unifying hypothesis

The two major types of mammalian melanin are pheomelanin (yellow or red pigment) and eumelanin (black or brown). The agouti (A) and extension (E) loci determine whether follicular melanocytes will deposit pheomelanin or eumelanin within their melanosomes. Mutations at the murine pinkeyed-dilution (P) locus cause a striking reduction in deposition of eumelanic, but not pheomelanic, pigment. The mRNA encoded at the P locus is not expressed in skin that exclusively produces pheomelanic pigment as a result of mutation at the agouti locus. We have suggested, based upon both genetic and biochemical evidence, that three key melanogenic proteins--tyrosinase, tyrosinase-related-protein-1 (TRP-1), and TRP-2, encoded at the albino (C), brown (B), and slaty (Slt) loci, respectively--form a high-molecular-weight "melanogenic complex" within the melanosome. High-molecular-weight forms of tyrosinase, TRP-1 and TRP-2, are absent from eumelanic ocular tissues of p(un)/p(un) mice that fail to produce normal P-locus transcript, even though these mice are genetically normal at the loci that regulate production of the three melanogenic proteins. We have hypothesized that the presence of the p-locus protein is important for the integrity of the melanogenic complex and for the levels of members of the TRP family. We show here that the yellow skins of mice mutant at the agouti or extension loci, as well as the nonyellow skins of pinkeyed-unstable (p(un)/p(un)) mice, demonstrate greatly diminished levels of tyrosinase, TRP-1 and TRP-2, and an absence or markedly decreased proportion of high-molecular-weight forms of melanogenic proteins. We conclude that normal levels of wild-type P-locus protein are necessary for eumelanogenesis and that the absence of this protein may be necessary, but is not sufficient to cause the melanosome to switch to the production of pheomelanin. We discuss the implications of our results in relation to the interacting genetic controls regulating melanogenesis.

Melanocyte differentiation marker gp75, the brown locus protein, can be regulated independently of tyrosinase and pigmentation

Human melanoma arises from epidermal melanocytes and displays remarkable phenotypic heterogeneity. This heterogeneity in part reflects the ability of melanoma cells to undergo differentiation along a pathway parallel to differentiation of normal melanocytes. Tyrosinase, encoded by the albino (c), and the tyrosinase-related protein-1 or gp75, encoded by the brown (b) locus, are two of the best-characterized markers for melanocyte differentiation. Both molecules are glycoproteins expressed in melanosomes, the site of pigment synthesis. We studied the regulation of these proteins in human melanoma cells induced by the polar-planar compound hexamethylene bisacetamide (HMBA). In well-differentiated melanoma cell lines, HMBA induced dendritic morphology and specifically regulated the expression of melanosomal glycoproteins (but not a panel of other molecules expressed by melanoma cells). HMBA specifically down-regulated gp75 expression by rapidly decreasing the steady-state level of gp75 mRNA and gp75 synthesis. HMBA was able to down-regulate gp75 expression even in the presence of cholera toxin, which when added alone induced a two- to threefold increase in gp75 expression. In contrast to uniform down-regulation of gp75 expression, HMBA could either up-regulate or down-regulate tyrosinase expression and pigmentation. Based on the differential regulation of gp75 and tyrosinase, melanoma cells could be classified into two groups. In one group, gp75 expression was coordinately regulated with tyrosinase activity and pigmentation. In the other group, gp75 expression and tyrosinase activity and pigmentation were dissociated (with pigmentation coupling to tyrosinase activity, not to gp75 expression). These results show that in mature melanocytic cells, regulation of gp75 expression follows a pattern that can be independent of regulation of tyrosinase and pigmentation.

Correlation between the number of melanosomes, tyrosinase mRNA levels, and tyrosinase activity in cultured murine melanoma cells in response to various melanogenesis regulatory agents

Tyrosinase is the rate limiting enzyme critically associated with melanin synthesis. The melanosomes are specialized membrane-bound organelles within melanocytic cells in which melanin polymers are ultimately deposited. To determine whether tyrosinase correlates with the number of melanosomes, we examined the relationship between tyrosinase activity, tyrosinase mRNA levels, and the number of melanosomes in B16 murine melanoma cells, using melanogenesis regulatory agents. 12-O-Tetradecanoylphorbol-13-acetate (TPA) or linoleic acid decreased tyrosinase activity, while dibutyryl cyclic adenosine monophosphate (dbcAMP) or palmitic acid increased it. The tyrosinase mRNA levels were not always correlated with tyrosinase activity, i.e., TPA down-regulated, dbcAMP upregulated, while linoleic acid or palmitic acid did not alter the message levels, indicating that fatty acid regulation of melanogenesis was due to post-transcriptional events. The number of melanosomes changed when agents which modulate the tyrosinase gene expression were added, since TPA decreased, dbcAMP increased, and linoleic acid or palmitic acid did not alter their number. These results suggest that the number of melanosomes changed in relation to tyrosinase mRNA level but not to tyrosinase activity in response to melanogenesis regulatory agents.

Tyrosinase isoenzymes: two melanosomal tyrosinases with different kinetic properties and susceptibility to inhibition by calcium

Two forms of tyrosinase from B16 mouse melanoma were identified by nonreducing SDS-PAGE after solubilization of crude melanosomal preparations with the nonionic detergent Brij 35. These forms, named LEMT and HEMT (low and high electrophoretic mobility tyrosinase, respectively), were purified by a combination of differential detergent extraction and chromatographic techniques. They displayed tyrosine hydroxylase and dopa oxidase activity and were stereospecific and sensitive to phenylthiourea, providing that they are true tyrosinases. However, based on its kinetic parameters, HEMT is a much more efficient enzyme. Immunoprecipitation and Western blots performed with the specific antibody alpha PEP1, directed against the b protein carboxyl terminus, suggested that LEMT is identical to the b protein. Both forms of tyrosinase were noncompetitively inhibited by Ca2+ at physiologically relevant concentrations. However, the b protein was apparently more susceptible, since maximal inhibition was reached at lower Ca2+ concentrations for LEMT. Moreover, binding of Ca2+ to the tyrosinases resulted in a noticeable thermal destabilization of the enzymes, which was also more pronounced for LEMT.

Regulation of melanogenesis induced by 5-methoxypsoralen without ultraviolet light in murine melanoma cells

Melanogenesis in melanoma cells can be enhanced by psoralens in the absence of UV light. Melanin biosynthesis is regulated by a number of melanocyte-specific proteins, including tyrosinase, DOPAchrome tautomerase (DCT), and tyrosinase-related protein-1 (TRP-1, gp75). To get more insight on the molecular mechanisms involved in psoralens-induced melanogenesis, we determined tyrosinase and DCT activities as well as mRNA and protein levels of tyrosinase, DCT, and TRP-1 in S91 mouse melanoma cells treated by 5-MOP. High concentration of 5-MOP (5 x 10(-5) M) induced a time-dependent increase of tyrosinase activity and melanin content, which was correlated to an increase of both mRNA and protein levels of tyrosinase. These results demonstrate that the 5-MOP stimulation of melanogenesis is related to increased tyrosinase synthesis. In addition, 5-MOP stimulated TRP-1 synthesis and induced a dose-dependent decrease of DCT activity without any modification in the expression of the protein. We explored then the signalling pathways involved in 5-MOP-induced melanogenesis and, particularly, the role of cyclic AMP and protein kinase C (PKC). A small stimulation of cyclic AMP production was observed in presence of 5-MOP. Furthermore, 1-oleoyl-2-acetylglycerol (OAG), a PKC activator, potentiated the 5-MOP stimulation of tyrosinase activity, while calphostin, a specific PKC inhibitor, inhibited the 5-MOP induction of tyrosinase activity. Phorbol-myristate acetate (PMA), described as a strong activator of PKC, inhibited also the effect of 5-MOP when used at long term. Taken together, these results demonstrate that in murine melanoma cells 5-MOP stimulates melanogenesis by increasing activity and synthesis of tyrosinase. Tyrosinase and TRP-1 expression are coordinately regulated by 5-MOP. Furthermore, a negative correlation between melanogenesis and DCT activity was observed under 5-MOP stimulation. At least, PKA and PKC systems appear to play an important role in the melanogenic effect of 5-MOP.

Molecular characterization of a human tyrosinase-related-protein-2 cDNA. Patterns of expression in melanocytic cells

Pigmentation in mammals is under complex genetic control. Amongst the genes involved in this process, those encoding tyrosinase and the tyrosinase-related-proteins 1 and 2 have been well characterized and share a number of features. Recently, the murine tyrosinase-related-protein-2 gene was shown to encode dopachrome-tautomerase activity and was mapped to the slaty locus. Human tyrosinase and tyrosinase-related-protein-1 genes have been isolated and demonstrate a high degree of similarity with their murine counterparts. However, there has been limited data regarding the existence of a human homologue for tyrosinase-related-protein-2 and its relationship to the other tyrosinase-related proteins. In this study, we report the molecular isolation of a cDNA encoding a human homologue of the murine tyrosinase-related-protein-2/dopachrome tautomerase. We have characterized its expression in human melanocytic cells and have analyzed the relationship between dopachrome tautomerase and tyrosinase activities with the level of visible pigmentation in these cells. TYRP2 has been mapped to the chromosomal region 13q32, thus extending a region of synteny with mouse-chromosome 14.

Repeated UVB irradiations do not have the same potential to promote stimulation of melanogenesis in cultured normal human melanocytes

The major stimulus for human melanin production is ultraviolet (UV) radiation. Little is known about the mechanisms underlying this response and the eventual enzyme regulation resulting from this activation. We treated normal human melanocytes in culture with daily UVB radiations. Cumulative increases in UVB doses resulted in proportional increases in tyrosinase activity over the first few days whereas an intermittent pattern of tyrosinase activation was observed after the fifth day of irradiation. This intermittent pattern consisted of latency periods where no melanogenic response was elicited despite exposure to UVB. Tyrosinase activity in cellular extracts increased shortly after an effective irradiation, peaked at 3 hours and thereafter decreased to below basal levels. Increased tyrosinase activity was associated with increased amounts of both the newly synthesized and mature forms of the enzyme. Decreased tyrosinase activity following an activation period was correlated with decreases in both the expression of tyrosinase mRNA and the amount of the newly synthesized form of the enzyme present in the melanocytes 24 hours after six irradiations. This particular pattern of stimulation of tyrosinase was not observed in S-91 murine melanoma cells after repeated UVB irradiations. Taken together these results may suggest a photo-protective mechanism developed by irradiated normal human melanocytes.

Tyrosinase isoenzymes in mammalian melanocytes. 1. Biochemical characterization of two melanosomal tyrosinases from B16 mouse melanoma

B-16 mouse melanoma melanosomes contain two forms of tyrosinase that can be resolved by SDS/PAGE. These forms interact to different extents with the ion-exchanger DEAE-Sephadex and with hydroxyapatite, and have different affinity for the melanosomal membrane and/or the intraorganular matrix. After partial purification and complete separation of the two tyrosinases, several kinetic parameters were analyzed. The form of lower electrophoretic mobility displayed a higher Km for 3,4-dihydroxy-L-phenylalanine (L-dopa) and L-tyrosine, an absolute requirement for the cofactor L-dopa in its tyrosine hydroxylase activity, and a lower ratio of tyrosine hydroxylation to Dopa oxidation. The form of higher electrophoretic mobility displayed lower values of Km for both substrates and was able to exhibit tyrosine hydroxylase activity after a lag period even in the absence of L-dopa. Both forms were stereospecific for the L isomers and sensitive to the specific tyrosinase inhibitor 2-phenylthiourea. These forms do not appear to result from different degrees of glycosylation, nor from limited proteolysis and are also present in the microsomal fraction of B16 mouse melanoma. They might correspond to different gene products, most likely derived from the b and c loci.

Tyrosinase isoenzymes in mammalian melanocytes. 2. Differential activation by alpha-melanocyte-stimulating hormone

In mouse melanoma melanocytes, alpha-melanocyte-stimulating hormone (MSH) stimulates differentiation, melanin synthesis and tyrosinase activity. However, the molecular mechanisms underlying these events have not yet been characterized. We have studied the activation of tyrosinase by MSH. Treatment of B16 melanoma cells with either theophylline, MSH, or its superpotent analog [Ahx4, DPhe7]MSH promotes a larger induction of tyrosine hydroxylase than of dopa oxidase activity in whole cell extracts. This higher activation of tyrosine hydroxylation was found not only in the melanosomal but also in the microsomal fraction; it appears to be dependent on continued transcription and translation since it can be blocked by actinomycin and cycloheximide. The tyrosinase activity of control and theophylline-treated extracts displayed several kinetic differences, including different Km values for both substrates and requirements for the cofactor L-dopa. SDS/PAGE, followed by a sensitive specific activity stain, demonstrated that melanosomes of control cells contain one lower-electrophoretic-mobility form of tyrosinase, whereas melanosomes of cells treated with either theophylline or MSH display, in addition to the lower-mobility form, a faster-migrating activity band. These tyrosinase forms are not interconvertible by proteolysis or deglycosylation. Their nature is discussed as related to the properties of the previously described low- and high-electrophoretic-mobility tyrosinases (LEMT and HEMT), as well as of the proteins encoded by the c and b loci.

The mouse brown (b) locus protein has dopachrome tautomerase activity and is located in lysosomes in transfected fibroblasts

Many genes mapping to pigmentation loci are involved in the regulation of melanin synthesis in the mouse. The brown (b) locus controls black/brown coat coloration, and its product has significant homology to the key melanogenic enzyme tyrosinase. This has led to suggestions that the b-protein is itself a melanogenic enzyme. In order to investigate its function, we have established lines of mouse fibroblasts stably expressing the b-protein by co-transfection of a b-protein expression vector and a plasmid conferring resistance to the antibiotic G418. The b-protein synthesised by these cells has the expected molecular mass of 75 kDa and reacts with three different anti-b-protein antibodies. We were unable to confirm previous reports that the b-protein has tyrosinase or catalase activity, but detected stereospecific dopachrome tautomerase activity in b-protein-expressing fibroblasts. This dopachrome tautomerase binds to Concanavalin A-Sepharose, and the major product of its action on L-dopachrome is 5,6-dihydroxyindole-2-carboxylic acid. Since this activity is not present in untransfected cells we conclude that the b-protein has dopachrome tautomerase activity. Fibroblasts do not contain melanosomes, the specialised organelles in which the b-protein is located in melanocytes. Nevertheless, indirect immunofluorescence localisation of the b-protein in transfected fibroblasts produces a distinctive pattern of intense juxtanuclear staining combined with punctate cytoplasmic staining. Double-labelling shows co-localisation of the b-protein with the late endosomal/lysosomal markers beta-glucuronidase and LAMP-1, both in transfected fibroblasts and in mouse melanoma cells. These findings are consistent with the hypothesis that melanosomes are closely related to lysosomes.

Differential regulation of tyrosinase activity in skin of white and black individuals in vivo by topical retinoic acid

Tyrosinase activity is a key determinant of melanin production in skin. Because retinoic acid regulates tyrosinase activity in melanoma cells, we analyzed modulation of pigmentation in vivo by retinoic acid. Black and white subjects were either not treated, or treated topically for 4 d under occlusion with vehicle, retinoic acid (0.1%), or the irritant sodium lauryl sulfate (2%). In untreated skin, tyrosinase activity and melanin content were significantly greater (2.3 times, and 3.2 times, respectively) in blacks versus whites. Four days of treatment with topical retinoic acid did not alter tyrosinase activity or melanin content in black skin. In contrast, retinoic acid treatment significantly induced (2.7 times, n = 8) tyrosinase activity, compared to vehicle treatment, in white skin. Melanin content, however, remained unchanged at 4 d. In separate experiments, tyrosinase activity in white subjects (n = 25) was increased 16% (p = 0.01) in sodium lauryl sulfate-treated skin, and 77% (p = 0.0005) in retinoic acid-treated skin, compared to vehicle-treated skin. The effect of retinoic acid on tyrosinase activity could be differentiated from non-specific irritation, because tyrosinase activity in retinoic acid-treated skin was significantly greater (52%, p = 0.004) than sodium lauryl sulfate-treated skin. Similar results were obtained with the dihydroxyphenylalanine reaction done on vehicle, sodium lauryl sulfate-, and retinoic acid-treated white skin. Northern analysis (n = 6) and semi-quantitative polymerase chain reaction (n = 6) demonstrated that retinoic acid treatment did not alter tyrosinase mRNA levels in white skin. Western analysis revealed that induction of tyrosinase activity by retinoic acid also was not associated with increased tyrosinase protein content (n = 9), indicating that regulation of tyrosinase activity by retinoic acid occurs through a post-translational mechanism. These data demonstrate that low tyrosinase activity in white skin in vivo is retinoic acid inducible and high tyrosinase activity in black skin in vivo is neither further induced nor reduced by retinoic acid.

Preparation of purified tyrosinase devoid of dopachrome tautomerase from mammalian malignant melanocytes

Although tyrosinase has been considered for a long time the only enzyme involved in mammalian melanosynthesis, it has been shown that mouse melanoma melanosomes contain high levels of dopachrome tautomerase (DCT2), an enzyme catalyzing DC tautomerization to DHICA. At least in B16 mouse melanoma, DCT is present in higher catalytic amounts than tyrosinase. Moreover, it can be anticipated that tyrosinase and DCT should be very difficult to resolve by most conventional biochemical techniques because of the structural similarity between these enzymes, as predicted from the sequence of their corresponding cDNAs. It is shown that the presence of DCT can cause serious artifacts when tyrosinase activity is determined by most of the currently available methods, such as the Dopa oxidase and melanin formation assays. We describe a simple and convenient method for the preparation of tyrosinase devoid of DCT. The method takes advantage of the different thermal stability of both enzymes. Heating of crude melanosomal extracts at 60 degrees C for 1 hr results in a complete denaturation of DCT, while tyrosinase activity is recovered almost quantitatively. The resulting tyrosinase preparation is considerably purified and the electrophoretic, immunologic and kinetic characteristics of the enzyme appear unaltered. Because if its high yield and simplicity, the method can be used for the microscale partial purification of DCT-free tyrosinase from mammalian malignant melanocytes grown in culture.

Role of tyrosinase as the determinant of pigmentation in cultured human melanocytes

Variations in human pigmentation among different racial groups are due to differences in the production and deposition of melanin in the skin. Although melanin synthesis is known to be controlled by the rate-limiting enzyme tyrosinase, the role of this enzyme as the principal determinant of skin pigmentation is unclear. Results from studies with human melanocyte cultures derived from different racial skin types reveal an excellent correlation between the melanin content of melanocyte cultures and the in situ activity of tyrosinase. Melanocytes derived from black skin have up to 10 times more tyrosinase activity and produce up to 10 times more melanin than melanocytes derived from white skin. However, the higher level of tyrosinase activity in melanocytes derived from black skin is not due to a greater abundance of tyrosinase. Results from immunotitration experiments and Western immunoblots reveal that the number of tyrosinase molecules present in white-skin melanocytes may equal the number found in highly pigmented black skin types. Moreover, approximately equivalent levels of tyrosinase mRNA are present in white and black skin cell strains. In contrast, melanocytes derived from red-haired neonates with low tyrosinase activity contain low numbers of tyrosinase molecules and low levels of tyrosinase mRNA. These results show that tyrosinase activity and melanin production in most light-skinned people is controlled primarily by a post-translational regulation of pre-existing enzyme and not by regulating tyrosinase gene activity. In contrast, melanocytes from red-haired (type I) people have low levels of tyrosinase protein and mRNA, suggesting that transcriptional activity of the tyrosinase gene is suppressed.

Pigment production in murine melanoma cells is regulated by tyrosinase, tyrosinase-related protein 1 (TRP1), DOPAchrome tautomerase (TRP2), and a melanogenic inhibitor

Using antibodies that recognize either tyrosinase, tyrosinase-related protein-1 (TRP1), or tyrosinase-related protein-2 (TRP2, DOPAchrome tautomerase), the quantities of those melanogenic enzymes were analyzed in five melanoma cell lines that possess various degrees of melanin production. All cells except JB/MS-W increased melanin production four to 30 times after 4 d of melanocyte-stimulating hormone (MSH) treatment. Melanin production by JB/MS-W cells was always under background, with or without MSH treatment. There was a positive correlation between quantities and synthetic rates of those melanogenic enzymes and their melanin formation or DOPAchrome tautomerase activities. The activity of a heat-resistant melanogenic inhibitory factor was also analyzed. The results showed, surprisingly, that pigmented cells showed higher levels of melanogenic inhibitors activity. Tyrosinase activity was increased dramatically whereas the level of melanogenic inhibitor was remarkably decreased following MSH treatment. Interestingly, melanogenic inhibitor derived from JB/MS-W cells suppressed not only tyrosinase but also DOPAchrome tautomerase, another enzyme functional in melanin production. These results clearly suggest that melanin production is regulated by a subtle balance between the activities of these enzymes and other factors such as the melanogenic inhibitor.

Subcellular distribution of tyrosinase and tyrosinase-related protein-1: implications for melanosomal biogenesis

Are tyrosinase, encoded at the albino locus, and tyrosinase-related protein-1 (TRP-1), encoded at the brown locus, similarly distributed in melanocytes? We determined the subcellular distribution of tyrosinase and TRP-1 using density fractionation of postnuclear supernatants from mouse melanoma cells of defined genotype followed by immunoblotting with specific antipeptide sera. In highly melanized cells, the majority of tyrosinase cosedimented on Percoll density gradients with visible melanin and with the peak of DOPA incorporation, confirming its presence predominantly in stage III-IV melanosomes. In contrast, the distribution of TRP-1 was limited to a less-dense melanosomal compartment, devoid of melanin. In amelanotic or minimally melanized cells, the majority of tyrosinase shifted into these lighter peaks. To explore a suspected relationship between lysosomes and melanosomes, we analyzed the distribution of lysosome-associated membrane protein-1 (LAMP-1). An overlap in the distribution of LAMP-1 and TRP-1 was demonstrated by immunomicroscopy and confirmed by immunoisolation. LAMP-1 was not present in the dense, melanin-rich melanosomal peak on gradient analysis. TRP-1 from melanoma cells homozygous for the brown mutation is not fully glycosylated, is more rapidly degraded, and is restricted in its distribution compared to its wild-type counterpart. In these mutant cells, all melanosomal compartments contain LAMP-1. Our results demonstrate that in wild-type cells the majority of tyrosinase eventually localizes to stage III-IV melanosomes. TRP-1 is limited to a less dense melanosomal compartment that is also LAMP-1 positive. The existence of this compartment suggests that it may represent a common step in the biogenesis of melanosomes and lysosomes.

Functional properties of cloned melanogenic proteins

Several genes critical to the regulation of melanin production in mammals have recently been cloned and characterized. They map to the albino, brown, and slaty loci in mice, and encode proteins with similar structures and features, but with distinct catalytic capacities. The albino locus encodes tyrosinase, an enzyme with three distinct catalytic activities--tyrosine hydroxylase, 3,4-dihydroxyphenylalanine (DOPA) oxidase and DHI (5,6-dihydroxyindole) oxidase. The brown locus encodes TRP-1 (tyrosinase-related protein-1), which has the same, but greatly reduced, catalytic potential. The slaty locus encodes TRP-2, another tyrosinase related-protein, which has DOPAchrome tautomerase activity. In this study we have examined the enzymatic interactions of these proteins, and their regulation by a novel melanogenic inhibitor. We observed that tyrosinase activity is more stable in the presence of TRP-1 and/or TRP-2, but that the catalytic function of TRP-2 is not affected by the presence of TRP-1 or tyrosinase. Other factors also may influence melanogenesis and a unique melanogenic inhibitor suppresses tyrosinase and DOPAchrome tautomerase activities, but does not affect the spontaneous rate of DOPAchrome decarboxylation to DHI. The results demonstrate the catalytic functions of these proteins and how they stably interact within a melanogenic complex in the melanosome to regulate the quantity and quality of melanin synthesized by the melanocyte.

Alpha-MSH and other melanogenic activators mediate opposite effects on tyrosinase and dopachrome tautomerase in B16/F10 mouse melanoma cells

alpha-MSH was found to decrease the recently characterized dopachrome tautomerase activity in cultures of B16/F10 mouse melanoma cells. Other stimulating agents of melanogenesis, like dibutyryl cyclic AMP, 3-isobutyl-1-methylxanthine, theophylline, retinol, and retinoic acid, caused the same effect. The grade of inhibition depended on the nature of the agent and the time of exposure. In all cases, both melanin production and tyrosinase activity were activated by these treatments, although the grade of tyrosine hydroxylase and dopa oxidase stimulation was different. Moreover, no correlation among the intensities of dopachrome tautomerase inhibition and tyrosinase activation by the tested agents could be obtained. The significance of these results in the regulation of mammalian melanogenesis is discussed.

5,6-Dihydroxyindole-2-carboxylic acid is incorporated in mammalian melanin

The role of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) in the biosynthesis of melanins has been studied by using the incorporation of specifically radiolabelled melanogenic precursors into melanins formed by melanocytes growing in vitro and in vivo. Extracts of mouse melanocytes and intact viable melanocytes were found to incorporate into melanin from 25% to more than 60% of [1-14C]tyrosine. Melanins from melanoma tumours grown in mice were radiolabelled with 3,4-dihydroxy[1-14C]phenylalanine, purified and chemoselectively decarboxylated. Determination of the 14CO2 evolved showed that at least 20% of the precursor incorporated in vivo retains the label in the form of non-aminoacidic aromatic-type carboxyl groups. These results provide the first unambiguous demonstration that DHICA is incorporated in physiologically relevant amounts in mammalian melanins.

Rapid and reversible inhibition of tyrosinase activity by glucosidase inhibitors in human melanoma cells

The dependence of constitutively expressed tyrosinase (dopa oxidase) activity on glycosylation in lightly pigmented human melanoma cells (MM96E) was determined using tunicamycin (TM), which prevents transfer of oligosaccharide chains to nascent protein (core glycosylation), the glucosidase inhibitors castanospermine (CS) and deoxynojirimycin (dNM), and the mannosidase inhibitors deoxymannojirimycin (dMM) and swainsonine (SW). TM caused irreversible inhibition of tyrosinase activity and carbohydrate synthesis as judged by incorporation of 3H-fucose. Tyrosinase in CS- and dNM-treated cells showed 50% loss of activity within 5 h but recovered rapidly when the drugs were removed; dMN and SW had little effect. Expression of the tyrosinase 2B7 epitope and of an 80-kDa melanosomal antigen (B8G3) was inhibited by TM but not by CS, dNM, dMM, or SW. CS and dNM appeared to decrease the half-life of active tyrosinase. Overall, these results indicate that 1) in addition to the requirement for core glycosylation the removal of glucose residues plays a critical role in the formation of active human tyrosinase; 2) glucosidase inhibitors appear to cause an accumulation of inactive tyrosinase and increase the degradation rate of active enzyme; and 3) later stages in oligosaccharide processing are not required for maintaining tyrosinase activity.

Membrane compartmentalization of melanosomal gp75

A melanosomal integral membrane glycoprotein of 75 kD (gp75) has been previously identified as the human homologue of the product specified by the murine brown locus. We presently report that this molecule may be susceptible to limited proteolysis and extrinsic radioiodination in intact, live cells. Consequently, it is suggested that its cellular location might include the plasma membrane and/or a cellular compartment easily accessible to proteases and to chemically catalyzed vectorial iodination. This is of interest in view of the potential applicative value of gp75 as a target for the radioimmunoscintography of melanoma lesions.

Regulation of tyrosinase expression and activity in human melanoma cells via histamine receptors

In cultured human melanoma cells, histamine H1 (mepyramine) and H2 receptor antagonists (cimetidine, ranitidine, impromidine) increased tyrosinase activity, whereas H2 agonists (dimaprit, nordimaprit) decreased activity. Mixtures of agonist and antagonist either decreased or increased tyrosinase activity, depending on the relative concentrations of each drug. Nordimaprit, the most effective inhibitor, lowered tyrosinase activity significantly within 36 h and caused a slower loss of tyrosinase protein as judged by reactivity with two monoclonal antibodies. Prolonged treatment of a melanotic cell line with nordimaprit led to complete loss of pigment, with no loss of the 56-kDa melanosomal antigen 1C11. Cells remained amelanotic for 8 weeks after removal of the drug and, even after 26 weeks, melanin content and tyrosinase expression and activity had not fully recovered. Nordimaprit increased the rate of degradation of tyrosinase and of nordimaprit binding proteins. Whereas nordimprit did not directly inhibit tyrosinase, lysates of treated cells contained an inhibitory activity that partitioned approximately equally across a 10-kDa ultrafiltration membrane. Overall, these results showed that melanogenesis can be controlled via histamine receptors, the mechanism for the H2 agonist nordimaprit consisting of three components: induction of a tyrosinase inhibitor, increased degradation of tyrosinase, and long-term down-regulation of tyrosinase expression.

Pigment content of cultured human melanocytes does not correlate with tyrosinase message level

Tyrosinase is considered to be the rate-limiting enzyme for the biosynthesis of melanin in epidermal melanocytes, and thus tyrosinase activity is thought to be a major regulatory step in melanogenesis. To determine whether the rate of pigment production was controlled at the level of tyrosinase gene expression, we developed a culture system capable of generating large populations of pure human melanocytes and then measured both melanin content as determined spectrophotometrically by absorption at 475 nm and mRNA levels as detected by hybridization with cloned cDNA Pmel 34, encoding human tyrosinase. We examined the relationship between pigment content and tyrosinase mRNA levels among human melanoma and melanocyte lines with very different levels of basal pigmentation; between two clones of a single human melanoma line, one pigmented and one amelanotic; and sequentially in melanocytes before and after simulation with isobutylmethylxanthine to increase melanin content per cell. Using Northern blot analysis and in-situ hybridization we found no correlation between tyrosinase message levels and melanin content, suggesting that posttranscriptional regulation of tyrosinase and/or other events determine the rate of pigment synthesis in human melanocytes.

Evidence for a calcium/calmodulin involvement in density-dependent melanogenesis in murine B16 melanoma cells

Previous work from our laboratory has shown that both cyclic AMP and calcium/calmodulin appear to be involved in the regulation of melanogenesis in murine B16 melanoma cells. In these cells as in murine Cloudman S91 cells, melanogenic responsiveness to melanocyte-stimulating hormone (MSH) varies with cell density in culture. Our objective in this study was to learn more about the intracellular systems involved in the control of melanogenesis, particularly the role played by calcium. The melanogenic response to alpha MSH was compared to the response to drugs affecting intracellular free calcium and calmodulin over a range of cell densities in B16F1 cells. alpha MSH-stimulated melanin production was extremely density-dependent but alpha MSH-stimulated cyclic AMP production was independent of cell density. The melanogenic response to agents that increased intracellular calcium (A23187) or inhibited intracellular calmodulin varied with cell density. A drug (TMB8) that lowered intracellular free calcium, however, increased melanogenesis independently of cell density. At high cell density it was found that an elevation in calcium decreased melanogenesis, whereas agents that reduced calcium or inhibited calmodulin activity increased melanogenesis. At low cell density, however, the inhibitory response to A23187 was lost and in some experiments even stimulated melanogenesis. These data suggest that the calcium/calmodulin signalling system has an inhibitory influence on melanogenesis, and its expression, which depends upon cell density, may also modulate the response to stimulatory agents such as alpha MSH.

Heterogeneity of the MSH receptor among B16 murine melanoma subclones

The heterogeneity of melanotropin receptors on B16 sublines was tested by using photoaffinity crosslinking techniques and the superpotent alpha-MSH derivative [Nle4, D-Phe7, 1'-(2-nitro-4-azido-phenylsulfenyl)-Trp9]-alpha- MSH (NAPS-MSH). Specific crosslinking of this compound to B16-F1, B16-F10, B16-M2R or B16-W4 cells revealed three different subtypes of MSH receptor based on SDS-PAGE analysis. Binding of monoiodinated alpha-MSH to these different subclones is saturable and characteristic for a single class of complexes (0.9 nM less than KD less than 1.6 nM). In this article the nature of the different MSH receptor subtypes as well as their possible correlation to the melanogenic potential of a particular cell line is discussed.

Bromodeoxyuridine- and cyclic AMP-mediated regulation of tyrosinase in Syrian hamster melanoma cells

The thymidine analog 5-bromodeoxyuridine (BrdU) suppresses pigmentation and tyrosinase activity in Syrian hamster melanoma cells W1-1-1. Studies on the molecular mechanism of suppression of pigmentation indicated that BrdU treatment affects the level of tyrosinase gene transcripts. No detectable tyrosinase message was found by Northern blot analysis in cells cultured in the presence of BrdU at concentrations even as low as 0.2 microM. The level of tyrosinase mRNA was found to reflect the level of pigmentation and tyrosinase activity. Studies with dibutyryl cyclic AMP (cAMP) showed that it inhibited pigment synthesis in W1-1-1 cells. With increasing concentrations of cAMP ranging from 10 microM to 300 microM, pigmentation and tyrosinase activity decreased progressively. This inhibition was found to be associated with a corresponding decrease in the level of tyrosinase mRNA. W1-1-1 cells were found not to respond to melanocyte stimulating hormone (MSH). There was no change in pigmentation, tyrosinase activity, or tyrosinase mRNA level in W1-1-1 cells in the presence of MSH. Similarly, theophylline, a phosphodiesterase inhibitor, had no effect on pigmentation or tyrosinase activity in W1-1-1 cells.

Tyrosinase synthesis in different skin types and the effects of alpha-melanocyte-stimulating hormone and cyclic AMP

Tyrosinase synthesis and its regulation in human melanocytes was studied by measuring the incorporation of [35S] methionine into incubated skin biopsies. Tyrosinase was detected in all skin samples with the highest levels in skin type IV and the lowest levels in skin type I. Following psoralen ultraviolet A (PUVA) therapy for several weeks, significant increases in the amounts of tyrosinase were found in skin types III and IV. The presence of alpha-melanocyte-stimulating hormone (alpha-MSH) (100 mumol/l) or the long-acting analogue [Nle4, DPhe7] alpha-MSH (1-10 mumol/l) in the incubation medium failed to alter tyrosinase levels in the skin biopsies taken from patients both before and after receiving PUVA therapy. Bromo-adenosine 3,5-cyclic monophosphate sodium salt (8-bromo-cAMP) (10 mmol/l), on the other hand, increased the amounts of tyrosinase both before and after PUVA, but these effects were only seen in biopsies of type III and IV skin. These results indicate that MSH fails to stimulate tyrosinase synthesis in human melanocytes. Nevertheless, tyrosinase synthesis and its regulation by cyclic AMP-dependent mechanisms could be important control points in the pigmentary response.

The relationship between tyrosinase activity and skin color in human foreskins

Tyrosinase activity was assayed in black and white human foreskin samples by measuring both the hydroxylation of tyrosine to dopa (tyrosine hydroxylase activity) and the conversion of [14C]tyrosine to [14C]melanin (melanin synthesis assay). Enzyme activity was found both in the particulate (75%) and soluble (25%) fractions of the cell. Membrane-bound tyrosinase was readily solubilized by either zwitter-ionic or nonionic detergents. The anionic detergent, sodium cholate, inhibited enzyme activity. Tyrosinase activity in black foreskin homogenates averaged almost three times that in white skin samples (33.8 pmols 3H2O/h/mg skin in black and 12.71 pmoles 3H2O/h/mg skin in white skin), although considerable overlap in activities existed among the two groups. Tyrosinase activities measured with two separate assays, tyrosine hydroxylase and [14C]melanin assays, were similar, suggesting that tyrosine hydroxylase activity is tightly coupled to melanin synthesis. Tyrosinase activity determined by either assay method generally correlated with skin melanin content. Kinetic analysis of tyrosinase from black and white foreskin revealed a Km for tyrosine of 2.5 X 10(-4) M in both skin types. Immunotitration experiments suggested that the difference in tyrosinase activities between white and black skin may be due, not only to different amounts of enzyme present in the melanocytes, but also possibly to differences in the catalytic activities of the enzyme found in melanocytes of black and white skin.