Portal de Periódicos da CAPES

Determination of Melanin Synthetic Pathways

2011; Elsevier BV; Volume: 131; Linguagem: Inglês

10.1038/skinbio.2011.4

1523-1747

Vincent J. Hearing,

RNA regulation and disease

Visible pigmentation of the skin, hair, and eyes depends primarily on the presence of melanin(s) in those tissues. Melanins are produced by specific cells called melanocytes. Not only is the type of melanin produced important, but also its eventual distribution in the tissue dramatically affects visible color, which ultimately determines the functions of the pigment, such as photoprotection (Gilchrest, 2011Gilchrest B.A. Molecular aspects of tanning.J Invest Dermatol. 2011; 131: 14-17Abstract Full Text Full Text PDF Scopus (18) Google Scholar). Clearly, the specification, migration, and differentiation during development of mela-nocyte precursors ("melanoblasts") in specific patterns are essential for eventual pigmentation in adults (Kawakami and Fisher, 2011Kawakami A. Fisher D.E. Key discoveries in melanocyte development.J Invest Dermatol. 2011; 131: 2-4Abstract Full Text Full Text PDF Scopus (22) Google Scholar). Following is a synopsis of critical findings that have led to our current understanding of the biochemical pathways and melanogenic factors involved in melanin synthesis. The key enzyme involved in the synthesis of all types of melanins from the initial precursor tyrosine is tyrosinase (EC 1.14.18.1). Tyrosinases have been described in many species, including mammals and lower animals, plants, and even fungi;in fact, the earliest observations of the catalytic function of tyrosinase were made in extracts of mushrooms (Bourquelot and Bertrand, 1895), which are still widely used today as a highly enriched source of that enzyme. All tyrosinases depend on the binding of copper for their catalytic function (Lerner et al., 1950Lerner A.B. Fitzpatrick T.B. Calkins E. et al.Mammalian tyrosinase; the relationship of copper to enzymatic activity.J Biol Chem. 1950; 187: 793-802Abstract Full Text PDF PubMed Google Scholar; Lerch et al., 1986Lerch K. Huber M. Scheider H.J. et al.Different origins of metal binding sites in binuclear copper proteins, tyrosinase and hemocyanin.J Inorg Chem. 1986; 26: 213-217Google Scholar), although their substrate specificities and physical properties can differ dramatically depending on the species (Lerner et al., 1951Lerner A.B. Fitzpatrick T.B. Calkins E. et al.Mammalian tyrosinase; action on substances structurally related to tyrosine.J Biol Chem. 1951; 191: 799-806Abstract Full Text PDF PubMed Google Scholar; Hearing et al., 1980Hearing V.J. Ekel T.M. Montague P.M. et al.Mammalian tyrosinase. Stoichiometry and measurement of reaction products.Biochim Biophys Acfa. 1980; 611: 251-268Crossref PubMed Scopus (63) Google Scholar). The ratelimiting initial step in the biosynthesis of melanin was initially thought to be the hydroxylation of tyrosine to l-3,4- dihydroxyphenylalanine (DOPA) and its immediate subsequent oxidation to DOPAquinone (DQ). In melanocytic cells, the DQ formed will be spontaneously converted to an orange-colored intermediate known as DOPA-chrome. In vitro, the DOPAchrome will spontaneously lose its carboxylic acid group to form 5,6-dihydroxyindole (DHI), which can then further oxidize and polymerize to form a dense, high-molecular-weight complex now known as DHI-melanin. This was initially reported by Raper, 1926Raper H.S. The tyrosinase-tyrosine reaction: production of L-3.4-dihydroxyphenylalanine from tyrosine.Biochem J. 1926; 20: 735-742Crossref PubMed Google Scholar, and the pathway was later refined by Mason, 1948Mason H.S. he chemistry of melanin: mechanism of the oxidation of dihydroxyphenylalanine by tyrosinase.J Biol Chem. 1948; 172: 83-99Abstract Full Text PDF PubMed Google Scholar; hence, the biosynthetic pathway is frequently referred to as the Raper-Mason pathway. Throughout the 1950s, 1960s, and 1970s, the collaborative research groups at Yale (headed by AB Lerner) and Harvard (headed by TB Fitzpatrick) played key roles in defining the involvement of tyrosinase in the human skin pigmentation (Fitzpatrick et al., 1950Fitzpatrick T.B. Becker Jr, S.W. Lerner A.B. et al.Tyrosinase in human skin: demonstration of its presence and of its role in human melanin formation.Science. 1950; 112: 223-225Crossref PubMed Scopus (68) Google Scholar), how its activities were confined to melanosomes and how those organelles developed (Seiji et al., 1961Seiji M. Fitzpatrick T.B. Birbeck M.S.C. The melanosome: a distinctive subcellular particle of mammalian melanocytes and the site of melanogenesis.J Invest Dermatol. 1961; 36: 243-252Abstract Full Text PDF PubMed Scopus (107) Google Scholar; Szabo et al., 1969Szabo G. Gerald A.B. Pathak M.A. et al.Racial differences in the fate of melanosomes in human epidermis.Nature. 1969; 222: 1081Crossref PubMed Scopus (212) Google Scholar), and the disruptions that occurred in those processes in many skin pigmentary diseases (Breathnach et al., 1965Breathnach A.S. Fitzpatrick T.B. Wyllie L.M.A. Electron microscopy of melanocytes in human piebaldism.J Invest Dermatol. 1965; 45: 28-37Abstract Full Text PDF PubMed Scopus (46) Google Scholar; Kawamura et al., 1971; Lerner and Nordlund, 1978Lerner A.B. Nordlund J.J. Vitiligo what is it? Is it important?.J Am Med Assoc. 1978; 239: 1183-1187Crossref PubMed Scopus (121) Google Scholar; Rees, 2011Rees J.L. The genetics of human pigmentary disorders.J Invest Dermatol. 2011; 131: 12-13Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar; Spritz, 1994Spritz R.A. The genetics of vitiligo.J Invest Dermatol. 1994; 131: 18-20Abstract Full Text Full Text PDF Scopus (37) Google Scholar). Those findings, plus the training of many post-doctoral fellows and clinicians in their groups, played a major role in establishing research centers in Asia, Europe, and the Americas, which still have a strong influence on studies of skin pigmentation and related pigmentary diseases. As summarized in Figure 1, recent revisions of this melanogenic pathway have shown that DOPA is not a distinct intermediate produced initially from tyrosine, but is in fact produced later in the pathway owing to the paired reduction of DQ (Riley, 1999Riley P.A. The great DOPA mystery: the source and significance of DOPA in phase I melanogenesis.Cell Mol Biol (Noisy-le-grand). 1999; 45: 951-960Google Scholar), and that downstream tyrosinase-related enzymes can rearrange the DOPAchrome to form a carboxylated intermediate (DHI-2-car-boxylic acid) known as DHICA, as discussed below. Analysis of the structures of the high-molecular-weight polymers of melanins depended on the development of new techniques to analyze these intractable pigments, and gradual progress was made in defining those structures, initially by Nicolaus's group in Naples and Swan's group in the United Kingdom (Swan, 1963Swan G.A. Chemical structure of melanins.Ann NY Acad Sci. 1963; 100: 1005-1019Crossref PubMed Scopus (51) Google Scholar; Nicolaus et al., 1964Nicolaus R.A. Piattelli M. Fattorusso E. he structure of melanins and melanogenesis. IV. On some natural melanins.Tetrahedron. 1964; 20: 1163-1172Crossref PubMed Scopus (123) Google Scholar). In working with melanins found in nature, it quickly became apparent that there were two major types, the brown-black melanins now collectively known as eumelanins, and the yellow-red melanins now collectively known as pheomelanins. Prota's group in Naples took the lead in defining the structure of pheomelanin and the involvement of sulfur as responsible for its unique color and properties (Prota, 1980Prota G. Recent advances in the chemistry of melanogenesis in mammals.J Invest Dermatol. 1980; 75: 122-127Abstract Full Text PDF PubMed Scopus (363) Google Scholar). Prota and colleagues, as well as Ito and Wakamatsu in Japan (Ito et al., 1984Ito S. Fujita K. Takahashi H. et al.Characterization of melanogenesis in mouse and guinea pig hair by chemical analysis of melanins and of free and bound DOPA and 5-S-cysteinyl-DOPA.J Invest Dermatol. 1984; 83: 12-14Abstract Full Text PDF PubMed Scopus (30) Google Scholar), developed a series of more sensitive and specific assays for eumelanin and pheomelanin intermediates that gradually formed the basis for our understanding of how they are formed in melanocytes and how they are copolymerized in situ. The critical role of sulfhydryl groups in reacting immediately with DQ upon its formation to form various combinations of cysteinylDOPAs and downstream reactions of those intermediates, via cysteinylDOPA-quinones and benzothiazine intermediates, to produce pheomelanins were gradually defined by the groups of Rorsman in Sweden, Ito in Japan, Thody in the United Kingdom, and Prota in Italy (Agrup et al., 1979Agrup G. Hansson C. Rorsman H. et al.Intracellular distribution of DOPA and 5-S-cysteinylDOPA in pigment cells with minimal pigment formation.Acta Dermatol Venereol Suppl. 1979; 59: 355-356Google Scholar; Ito and Fujita, 1985Ito S. Fujita K. Microanalysis of eumelanin and pheomelanin in hair and melanomas by chemical degradation and liquid chromatography.Anal Biochem. 1985; 144: 527-536Crossref PubMed Scopus (347) Google Scholar; Thody et al., 1991Thody A.J. Higgins E.M. Wakamatsu K. et al.Pheomelanin as well as eumelanin is present in human epidermis.J Invest Dermatol. 1991; 97: 340-344Abstract Full Text PDF PubMed Google Scholar; Napolitano et al., 1994Napolitano A. Costantini C. Crescenzi O. et al.Characterization of 1,4-benzothiazine intermediates in the oxidative conversion of 5-S-cysteinyldopa to pheomelanins.Tetrahedron Lett. 1994; 35: 6365-6368Crossref Scopus (30) Google Scholar). Studies characterizing the structural and physical properties of melanins in various tissues have also been led by the groups of Simon in the United States, d'Ischia and Zecca in Italy, and Sarna in Poland (Sarna, 1992Sarna T. Properties and function of the ocular melanina photobiophysical view.J Photochem Photobiol. 1992; 12: 215-258Crossref PubMed Scopus (352) Google Scholar; Zecca et al., 2001Zecca L. Tampellini D. Gerlach M. et al.Substantia nigra neuromelanin: structure, synthesis, and molecular behaviour.J Clin Pathol Mol Pathol. 2001; 54: 414-418Google Scholar; Liu et al., 2005Liu Y. Hong L. Wakamatsu K. et al.Comparison of structural and chemical properties of black and red human hair melanosomes.Photochem Photobiol. 2005; 81: 135-144Crossref PubMed Scopus (162) Google Scholar; Pezzella et al., 2009Pezzella A. Iadonisi A. Valerio S. et al.Disentangling eumelanin "black chromophore'': visible absorption changes as signatures of oxidation state- and aggregation-dependent dynamic interactions in a model water-soluble 5,6- dihydroxyindole polymer.J Am Chem Soc. 2009; 131: 15270-15275Crossref Scopus (116) Google Scholar). The determination to produce eumelanin and/or pheomelanin is regulated physiologically, primarily by the melanocortin 1 receptor (MC1R) as modulated by its opposing ligands, melanocyte stimulating hormone (MSH) and agouti signaling protein (ASIP); this is discussed in more detail by Rees, 2011Rees J.L. The genetics of human pigmentary disorders.J Invest Dermatol. 2011; 131: 12-13Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar. Unexpectedly, in 1980, Pawelek and colleagues reported the novel finding that a biological factor produced in melanocytes was able to prevent the spontaneous decarboxylation of DOPAchrome to DHI, which led to the production of a more soluble and lighter-colored carboxylated melanin, now known as DHICA-melanin (Korner and Pawelek, 1980Korner A.M. Pawelek J.M. DOPAchrome conversion: a possible control point in melanin biosynthesis.J Invest Dermatol. 1980; 75: 192-195Abstract Full Text PDF PubMed Scopus (155) Google Scholar). There was some initial controversy about this point, since that activity could be mimicked in vitro by various divalent metal cations (Palumbo et al., 1987Palumbo A. d'Ischia M. Misuraca G. et al.Effect of metal ions on the rearrangement of DOPAchrome.Biochim Biophys Acta. 1987; 925: 203-209Crossref PubMed Scopus (128) Google Scholar), but with the advent of molecular biology and cloning, the race to clone the tyrosinase gene led indirectly to the identification of two tyrosinase-related proteins (now known as TYRP1 and DCT);one of those was quickly shown to have the enzymatic activity of DOPAchrome tautomerase (Tsukamoto et al., 1992Tsukamoto K. Jackson I.J. Urabe K. et al.A second tyrosinase-related protein, TRP2, is a melanogenic enzyme termed DOPAchrome tauto-merase.Embo J. 1992; 11: 519-526Crossref PubMed Scopus (474) Google Scholar). Orlow and colleagues then demonstrated that the three tyrosinase-related melanogenic enzymes polymerized in a complex within melanocytes that facilitated their physiological interactions (Orlow et al., 1994Orlow S.J. Zhou B.K. Chakraborty A.K. et al.High-molecular-weight forms of tyrosinase and the tyrosinase-related proteins: evidence for a melanogenic complex.J Invest Dermatol. 1994; 103: 196-201Abstract Full Text PDF PubMed Scopus (134) Google Scholar). Because of its critical role in pig-mentation, and the disruptions in normal pigmentation expected to arise from mutations in its encoding gene, Kwon and colleagues initially cloned the gene encoding tyrosinase (Kwon et al., 1987Kwon B.S. Haq A.K. Pomerantz S.H. et al.Isolation and sequence of a cDNA locus for human tyrosinase that maps at the mouse c-albino locus.Proc Natl Acad Sci USA. 1987; 84: 7473-7477Crossref PubMed Scopus (396) Google Scholar) as well as another key melanosomal protein now known to be critically involved in melanosome structure, Pmel17 (Berson et al., 2001). As noted above, cloning of the tyrosinase gene led to the cloning of two other closely related genes, and a number of groups were instrumental in identifying human pigmentary diseases associated with each of those genes, most notably by the laboratories of King and Spritz in the United States (Getting and King, 1994Getting W.S. King R.A. Molecular basis of oculocutaneous albinism.J Invest Dermatol. 1994; 103: 131-136Abstract Full Text PDF Google Scholar; Spritz, 1994Spritz R.A. Molecular genetics of oculocutaneous albinism.Hum Mol Gen. 1994; 3: 1469-1475PubMed Google Scholar) and later by many other groups (reviewed in Hearing and Leong, 2005Hearing V.J. Leong S.P.L. From Melanocyte to Melanoma.in: The Progression to Malignancy. 1st ed. Humana Press, New York2005Google Scholar; Nordlund et al., 2006Nordlund J.J. Boissy R.E. Hearing V.J. et al.The Pigmentary System: Physiology and Pathophysiology. 2nd ed. Blackwell Science, Edinburgh2006Crossref Google Scholar). Interestingly, the four known forms of oculocutaneous albinism result from molecular lesions that disrupt the function of tyrosinase: OCA1, the most severe form, results from mutations in the gene encoding tyrosinase itself, but OCA2, OCA3, and OCA4 (slightly milder forms) result from mutations in genes that affect the processing and trafficking of tyrosinase to melanosomes (P, TYRP1, and MATP, respectively) (Toyofuku et al., 2001Toyofuku K. Wada I. Valencia J.C. et al.Oculocutaneous albinism (OCA) types 1 and 3 are ER retention diseases: mutations in tyrosinase and/or Tyrp1 influence the maturation, degradation of calnexin association of the other.Faseb J. 2001; 15: 2149-2161Crossref PubMed Scopus (130) Google Scholar; Costin et al., 2003Costin G.E. Valencia J.C. Vieira W.D. et al.Tyrosinase processing and intracellular trafficking is disrupted in mouse primary melanocytes carrying the uw mutation: a model for oculocutaneous albinism (OCA) type 4.J Cell Sci. 2003; 116: 3203-3212Crossref PubMed Scopus (146) Google Scholar). Many other pigmentary disorders affect other basic processes in addition to their effects on melanocytes and melanin production, most of them by virtue of disrupting intracellular trafficking pathways involved in organelle biogenesis (including melano-somes) and/or the intracellular transport or transfer of melanosomes to neighboring keratinocytes. Such disorders include Hermansky-Pudlak syndrome, Griscelli syndrome, and Chediak-Higa-shi syndrome. Interested readers are referred to an actively curated web site that lists pigment-related genes and associated diseases (http://www.espcr.org/micemut/). One final consideration that is now becoming more widely accepted is the synthesis and function of melanins in less obscure tissues than the skin, hair, and eyes. Melanocytes do occur, usually as minor populations, in a wide variety of other tissues, and their functions are currently a matter of conjecture and study. Among those other tissues are the inner ear, the substantia nigra, the heart, and adipose tissue. Evidence is accumulating that the melanins play important protective roles in those tissues (Brito and Kos, 2008Brito F.C. Kos L. Timeline and distribution of melanocyte precursors in the mouse heart.Pigment Cell Melanoma Res. 2008; 21: 464-470Crossref Scopus (59) Google Scholar; Zecca et al., 2008Zecca L. Bellei C. Costi P. et al.New melanic pigments in the human brain that accumulate in aging and block environmental toxic metals.Proc Natl Acad Sci USA. 2008; 105: 17567-17572Crossref PubMed Scopus (163) Google Scholar; Randhawa et al., 2009Randhawa M. Huff T. Valencia J.C. et al.Evidence for the ectopic synthesis of melanin in human adipose tissue.Faseb J. 2009; 23: 835-843Crossref Scopus (40) Google Scholar). A recent article sum-marizes many of those studies and provides hints of interesting directions they might take in the future (Brenner and Hearing, 2009Brenner M. Hearing V.J. What are melanocytes really doing all day long ...?.Exp Dermatol. 2009; 18: 799-819Crossref Scopus (213) Google Scholar). Melanins play important roles in human skin for cosmetic appearance, detoxification, and photoprotection, among other functions. In lower species, melanins play critical roles in survival (e.g., in camouflage of prey and predator, in thermal regulation in amphibians, etc.) In humans, melanin content (and thus visible pigmentation of the skin) has a dramatic effect on skin's resistance to UV radiation damage, and the risk of skin cancers in lighter skin is 30- to 40-fold higher than in darker skin. The roles of melanins in other tissues are still being studied and will no doubt provide a wealth of information about their various functions in the skin as well. The author states no conflict of interest. This research was supported in part by the Intramural Research Program of the NIH, National Cancer Institute. Hearing VJ (2011) Determination of melanin synthetic pathways. J Invest Dermatol 131: E8–E11