- Email: [email protected]
NADPH:Quinone Oxidoreductase-1 as a New Regulatory Enzyme That Increases Melanin Synthesis
commentary
in mammalian gene regulation, it is highly likely that SNPs that alter the regulation of gene expression may function at some distance from the target gene. This concept is exemplified in the context of melanocyte biology by recent association studies into pigmentation regulation in the eye. The presence of a single SNP located within intron 86 of the HERC2 gene was found to be the major determinant of blue/brown eye-color phenotypes in humans (Sturm et al., 2008). Although this finding may have provided impetus to investigate the role of this gene in melanocyte function, prior knowledge of melanocyte biology suggests that this SNP is likely to regulate the expression of the neighboring OCA2 gene, with its role already firmly established in the process of pigmentation. To fully appreciate and extend these findings, the genetic associations and locus interactions of these candidate susceptibility genes must also be examined in the wider context of the autoimmune diseases that accompany vitiligo. CONFLICT OF INTEREST
The authors state no conflict of interest.
References
Boissy RE, Spritz RA (2009) Frontiers and controversies in the pathobiology of vitiligo: separating the wheat from the chaff. Exp Dermatol 18:583–5 Jin Y, Mailloux CM, Gowan K et al. (2007) NALP1 in vitiligo-associated multiple autoimmune disease. N Engl J Med 356:1216–25 Jin Y, Riccardi SL, Gowan K et al. (2010) Finemapping of vitiligo susceptibility loci on chromosomes 7 and 9 and interactions with NLRP1 (NALP1). J Invest Dermatol 130:774–83 Liu L, Li C, Gao J et al. (2009) Genetic polymorphisms of glutathione S-transferase and risk of vitiligo in the Chinese population. J Invest Dermatol 129:2646–52 Spritz RA, Gowan K, Bennett DC et al. (2004) Novel vitiligo susceptibility loci on chromosomes 7 (AIS2) and 8 (AIS3), confirmation of SLEV1 on chromosome 17, and their roles in an autoimmune diathesis. Am J Hum Genet 74:188–91 Sturm RA, Duffy DL, Zhao ZZ et al. (2008) A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color. Am J Hum Genet 82:424–31 Taieb A, Picardo M (2009) Clinical practice. Vitiligo. N Engl J Med 360:160–9 Waterman EA, Gawkrodger DJ, Watson PF et al. (2009) Autoantigens in vitiligo identified by the serological selection of a phage-displayed melanocyte cDNA expression library. J Invest Dermatol 130:230–40
See related article on pg 784
NADPH:Quinone Oxidoreductase-1 as a New Regulatory Enzyme That Increases Melanin Synthesis Yuji Yamaguchi1, Vincent J. Hearing2, Akira Maeda1 and Akimichi Morita1 Most hypopigmenting reagents target the inhibition of tyrosinase, the key enzyme involved in melanin synthesis. In this issue, Choi et al. report that NADPH:quinone oxidoreductase-1 (NQO1) increases melanin synthesis, probably via the suppression of tyrosinase degradation. Because NQO1 was identified by comparing normally pigmented melanocytes with hypopigmented acral lentiginous melanoma cells, these results suggest various hypotheses regarding the carcinogenic origin of the latter. Journal of Investigative Dermatology (2010) 130, 645–647. doi:10.1038/jid.2009.378
Enzymes that regulate skin pigmentation
Melanin synthesis by melanocytes is one of the most important parameters regulating skin pigmentation (Yamaguchi et al., 2007a), and more than 150 pigment genes have now been identified. The cosmetic industry has developed numerous hypopigmenting (depigmenting or whitening) reagents to suppress melanin synthesis in order to meet the needs of customers with hyperpigmenting conditions such as chloasma (melasma) and/ or ephelides (freckles) (Solano et al., 2006). There is also a demand for hyperpigmenting (artificial tanning) reagents to treat vitiligo and other hypopigmenting diseases, and there are societies in which tanning is perceived as beautiful and healthy. Taking together these observations, methods of controlling melanin synthesis have considerable significance for patients with pigmentary disorders and in the general marketplace. Enzymes involved in melanin synthesis include tyrosinase, tyrosinase-related protein-1 (TYRP1), and dopachrome tautomerase (DCT) (Yamaguchi and Hearing, 2009). Among these enzymes in the tyrosinase family, tyrosinase
plays the critical role in melanin synthesis, and various factors, including the copper-transporter ATP7A, have the capacity to modulate its enzymatic activity (Setty et al., 2008). Indeed, most hypopigmenting reagents are intended to suppress tyrosinase activity. Additionally, two variants of oculo cutaneous albinism in humans (OCA1A and OCA1B) are caused by mutations in tyrosinase. Tyrosinase is the rate-limiting enzyme that mediates hydroxylation of tyrosine to dopaquinone and the oxidation of 5,6-dihydroxyindole (DHI) to indole-5,6-quinone, which produces eumelanin (Figure 1). DCT is a tautomerase that converts dopachrome to DHI-2-carboxylic acid (DHICA), whereas TYRP1 is a DHICA oxidase that further stimulates eumelanin synthesis. Consequently, melanogenic enzymes that act within melanosomes are critical for the production of the pigmented biopolymer melanin. NQO1 as an enhancer of tyrosinase activity
Yoon’s group has identified NADPH: quinone oxidoreductase-1 (NQO1)
Department of Geriatric and Environmental Dermatology, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan and 2Laboratory of Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA 1
Correspondence: Yuji Yamaguchi, Department of Geriatric and Environmental Dermatology, Nagoya City University Graduate School of Medical Sciences, 1-Kawasumi, Mizuho-cho, Mizuho-ku, Nagoya 467-8601, Japan. E-mail: [email protected]
www.jidonline.org 645
commentary
Clinical Implications • NQO1 is a newly identified regulatory enzyme that increases tyrosinase activity, probably by suppressing its degradation. • Identification of a novel regulatory enzyme provides new pharmacologic opportunities to treat pigmentary disorders. • Likewise, identification of a novel enzyme may provide leads for the pharmacologic treatment of melanoma.
by comparing hypopigmented acral lentiginous melanoma cells with their normal counterparts, pigmented melanocytes obtained from the back skin of the same patient (Choi et al., this issue). NQO1 is an enzyme that catalyzes the two-electron reduction and the detoxification of quinones (including coenzyme Q10, also known as ubiquinone, and p-benzoquinone) to broad-sense hydroquinones (including ubiquinol and narrowly defined hydroquinone, also known as 1,4-benzenediol). The lower section of Figure 1 shows a representative reaction for NQO1, which may play a role in melanin synthesis, specifically the conversion of dopaquinone to dopa chrome via leukodopachrome. NQO1, one of the phase 2 enzymes, ordinarily protects cells against free radical damage and oxidative stress. Because hydroquinone is a well-known hypopigmenting reagent, NQO1 may negatively regulate skin pigmentation via an increased production of hydroquinone within melanocytes. Indeed, the authors demonstrate that overexpression of NQO1 results in a modest decrease in TYRP1 and microphthalmia-associated transcription factor (MITF, the master regulator of skin pigmentation) at protein levels (Figure 2). On the other hand, melanogenesis increased in concert with increasing levels of NQO1 in the various melanoma cell lines tested. Additionally, the inhibition of NQO1 resulted in a decrease in MITF, tyrosinase, and TYRP1 (but not DCT) at protein levels in tissue culture and in a reduction in melanin pigment using a zebrafish model. Finally, overexpression of NQO1 resulted in an increase in tyrosinase protein and in its enzymatic activity, but not at the
mRNA level in tissue culture. Choi et al. conclude that NQO1 positively regulates melanin synthesis, probably by stabilizing tyrosinase protein. These findings triggered the authors’ interest in elucidating the mechanism(s) by which NQO1 upregulates tyrosinase activity. Keap1 (Kelch-like ECH-associated protein 1) regulates the expression of NQO1 via Nrf2, a member of the NF-E2 family of nuclear basic leucine zipper transcription factors (Figure 2)
(Dinkova-Kostova et al., 2002). Keap1 binds to Nrf2, which translocates to the nucleus in response to carcinogens and oxidants, followed by activation of the antioxidant response element of the NQO1 gene. It seems likely that upregulated NQO1 enhances the expression of tyrosinase independently of its regulation by MITF, because NQO1 does not affect the expression of tyrosinase at the mRNA level or the expression of MITF at either the mRNA or the protein levels. Choi et al. hypothesize that NQO1 suppresses the ubiquitin–proteasome system that would normally degrade tyrosinase. Other factors that affect tyrosinase trafficking in melanocytes may be involved in the selective upregulation of tyrosinase in response to NQO1. NQO1 as a therapeutic target for acral melanoma
To identify NQO1 as an enzyme that increases melanin synthesis, Choi
Eumelanin TYRP1
Tyrosinase
HO
HO
CO2H NH2
HO
Tyrosine
O
HO
N H
DHI
N H
HO
CO2H NH2
HO
Dopaquinone
CO2H
DHICA CO2
Tyrosinase O
HO
DCT
O N H
CO2H
Leukodopachrome
HO
N
CO2H
Dopachrome
Cysteine Pheomelanin
NADP+
NADPH O
O
p-Benzoquinone
OH
NQO1
OH
Hydroquinone
Figure 1. Key players in melanin biosynthesis and NQO1 as an oxidoreductase that mediates the conversion of quinones to hydroquinones. Tyrosinase, DCT, and TYRP1 are enzymes that convert tyrosine (and DHI) to dopaquinone (and eumelanin), dopachrome to DHICA, and DHICA to eumelanin, respectively. Pheomelanin derives from dopaquinone based on the presence of cysteine. NQO1 is an oxidoreductase that mediates the conversion of quinones (including p-benzoquinone and coenzyme Q10) to hydroquinones (including narrowly defined hydroquinone and ubiquinol). DCT, dopachrome tautomerase; DHI, 5,6-dihydroxyindole; DHICA, 5,6-dihydroxyindole-2-carboxylic acid; NADPH, nicotinamide adenine dinuclueotide phosphate; NQO1, NADPH:quinone oxidoreductase-1; TYRP1, tyrosinase-related protein-1. Adapted from PubChem (http://pubchem.ncbi.nlm.nih.gov).
646 Journal of Investigative Dermatology (2010), Volume 130
commentary
important for studying carcinogenesis and metastasis by AM cells.
Melanocyte
Concluding remarks Melanin synthesis
Keap1 1 Nrf2 Keap1
Nrf2
Tyrosinase MITF ?
TYRP1 ?
ARE NQO1
DCT
In summary, Choi et al. (2010) have identified NQO1 as a new pigmentation regulatory enzyme that increases tyrosinase activity, probably by suppressing its degradation. Future studies should elucidate the mechanism(s) by which AM occurs via NQO1 and/ or other factors identified in their microarray analyses at the gene level and in their proteomics analyses at the protein level. CONFLICT OF INTEREST
The authors state no conflict of interest. Figure 2. Scheme summarizing how NQO1 regulates melanin synthesis in melanocytes. NQO1 is regulated by ARE, which is enhanced by Nrf2. Nrf2 migrates into the nucleus from the cytoplasm, where it is released from Keap1 in response to various stimuli. NQO1 does not significantly affect the expression of MITF, TYRP1, or DCT, but it increases the catalytic activity and protein level of tyrosinase and thereby induces melanin synthesis. Vertical blue arrows indicate up- or downregulation in response to NQO1 stimulation; horizontal blue arrow indicates that NQO1 stimulation has no effect on DCT expression. Blue labels indicate data presented by Choi et al. (2010). ARE, antioxidant response element; DCT, dopachrome tautomerase; Keap1, Kelch-like ECHassociated protein-1; MITF, microphthalmia-associated transcription factor; NQO1, NADPH:quinone oxidoreductase-1; Nrf2, an NF-E2 family of nuclear basic leucine zipper transcription factors; TYRP1, tyrosinase-related protein-1.
et al. used two established and highly sensitive analytical approaches— microarrays at the gene level and proteomics at the protein level—and they compared less pigmented cells with normally pigmented cells. In addition to their findings concerning the regulation of skin pigmentation, the investigators elegantly established an acral melanoma (AM) cell line, which has been reported only rarely (Murata et al., 2007). Because AM has a unique phenotype, in that amplifications of narrow regions of the genome (gains of chromosomes 6p or 1q) are often observed in AM patients as compared with non-AM patients (Namiki et al., 2005), comparison with normal melanocytes may elucidate the mechanism(s) by which AM transformation occurs. Skin is heterogeneous (site-specifically different) in terms of the presence or absence of hair, degree of pigmentation, and epidermal thickness. Melanoma types obtained from various body sites reflect different characteristics; BRAF
and N-RAS mutations are observed in melanomas with chronic suninduced damage, but they are rare in melanomas without chronic suninduced damage, in mucosal melanomas, and in AMs (Curtin et al., 2005). Even normal melanocytes are able to adopt different phenotypes in response to external stimuli such as dickkopf 1, an inhibitor of the Wnt/βcatenin signaling pathway (Yamaguchi et al., 2007b). In that sense, the ideal comparisons would have been among primary AM cells, metastatic AM cells, and normal melanocytes located between the two sites (primary and metastatic AM sites). However, Choi et al. identified NQO1 as an AM carcinogenic candidate; to explain the mechanism, they suggested that the loss of NQO1 in AM cells facilitates carcinogenesis due to the loss of its suppressive scavenger effect in the face of oxidative stress caused by reactive oxygen species. Other factors shown in Supplementary Tables 1 and 2 of their article are potentially
References
Choi T-Y, Sohn K-C, Kim J-H et al. (2010) Impact of NAD(P)H:quinone oxidoreductase-1 on pigmentation. J Invest Dermatol 130:784–92 Curtin JA, Fridlyand J, Kageshita T et al. (2005) Distinct sets of genetic alterations in melanoma. N Engl J Med 353:2135–47 Dinkova-Kostova AT, Holtzclaw WD, Cole RN et al. (2002) Direct evidence that sulfhydryl groups of Keap1 are the sensors regulating induction of phase 2 enzymes that protect against carcinogens and oxidants. Proc Natl Acad Sci USA 99:11908–13 Murata H, Ashida A, Takata M et al. (2007) Establishment of a novel melanoma cell line SMYM-PRGP showing cytogenetic and biological characteristics of the radial growth phase of acral melanomas. Cancer Sci 98:958–63 Namiki T, Yanagawa S, Izumo T et al. (2005) Genomic alterations in primary cutaneous melanomas detected by metaphase comparative genomic hybridization with laser capture or manual microdissection: 6p gains may predict poor outcome. Cancer Genet Cytogenet 157:1–11 Setty SR, Tenza D, Sviderskaya EV et al. (2008) Cell-specific ATP7A transport sustains copperdependent tyrosinase activity in melanosomes. Nature 454:1142–6 Solano F, Briganti S, Picardo M et al. (2006) Hypopigmenting agents: an updated review on biological, chemical and clinical aspects. Pigment Cell Res 19:550–71 Yamaguchi Y, Brenner M, Hearing VJ (2007a) The regulation of skin pigmentation. J Biol Chem 282:27557–61 Yamaguchi Y, Passeron T, Watabe H et al. (2007b) The effects of dickkopf 1 on gene expression and Wnt signaling by melanocytes: mechanisms underlying its suppression of melanocyte function and proliferation. J Invest Dermatol 127:1217–25 Yamaguchi Y, Hearing VJ (2009) Physiological factors that regulate skin pigmentation. Biofactors 35:193–9
www.jidonline.org 647
in mammalian gene regulation, it is highly likely that SNPs that alter the regulation of gene expression may function at some distance from the target gene. This concept is exemplified in the context of melanocyte biology by recent association studies into pigmentation regulation in the eye. The presence of a single SNP located within intron 86 of the HERC2 gene was found to be the major determinant of blue/brown eye-color phenotypes in humans (Sturm et al., 2008). Although this finding may have provided impetus to investigate the role of this gene in melanocyte function, prior knowledge of melanocyte biology suggests that this SNP is likely to regulate the expression of the neighboring OCA2 gene, with its role already firmly established in the process of pigmentation. To fully appreciate and extend these findings, the genetic associations and locus interactions of these candidate susceptibility genes must also be examined in the wider context of the autoimmune diseases that accompany vitiligo. CONFLICT OF INTEREST
The authors state no conflict of interest.
References
Boissy RE, Spritz RA (2009) Frontiers and controversies in the pathobiology of vitiligo: separating the wheat from the chaff. Exp Dermatol 18:583–5 Jin Y, Mailloux CM, Gowan K et al. (2007) NALP1 in vitiligo-associated multiple autoimmune disease. N Engl J Med 356:1216–25 Jin Y, Riccardi SL, Gowan K et al. (2010) Finemapping of vitiligo susceptibility loci on chromosomes 7 and 9 and interactions with NLRP1 (NALP1). J Invest Dermatol 130:774–83 Liu L, Li C, Gao J et al. (2009) Genetic polymorphisms of glutathione S-transferase and risk of vitiligo in the Chinese population. J Invest Dermatol 129:2646–52 Spritz RA, Gowan K, Bennett DC et al. (2004) Novel vitiligo susceptibility loci on chromosomes 7 (AIS2) and 8 (AIS3), confirmation of SLEV1 on chromosome 17, and their roles in an autoimmune diathesis. Am J Hum Genet 74:188–91 Sturm RA, Duffy DL, Zhao ZZ et al. (2008) A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color. Am J Hum Genet 82:424–31 Taieb A, Picardo M (2009) Clinical practice. Vitiligo. N Engl J Med 360:160–9 Waterman EA, Gawkrodger DJ, Watson PF et al. (2009) Autoantigens in vitiligo identified by the serological selection of a phage-displayed melanocyte cDNA expression library. J Invest Dermatol 130:230–40
See related article on pg 784
NADPH:Quinone Oxidoreductase-1 as a New Regulatory Enzyme That Increases Melanin Synthesis Yuji Yamaguchi1, Vincent J. Hearing2, Akira Maeda1 and Akimichi Morita1 Most hypopigmenting reagents target the inhibition of tyrosinase, the key enzyme involved in melanin synthesis. In this issue, Choi et al. report that NADPH:quinone oxidoreductase-1 (NQO1) increases melanin synthesis, probably via the suppression of tyrosinase degradation. Because NQO1 was identified by comparing normally pigmented melanocytes with hypopigmented acral lentiginous melanoma cells, these results suggest various hypotheses regarding the carcinogenic origin of the latter. Journal of Investigative Dermatology (2010) 130, 645–647. doi:10.1038/jid.2009.378
Enzymes that regulate skin pigmentation
Melanin synthesis by melanocytes is one of the most important parameters regulating skin pigmentation (Yamaguchi et al., 2007a), and more than 150 pigment genes have now been identified. The cosmetic industry has developed numerous hypopigmenting (depigmenting or whitening) reagents to suppress melanin synthesis in order to meet the needs of customers with hyperpigmenting conditions such as chloasma (melasma) and/ or ephelides (freckles) (Solano et al., 2006). There is also a demand for hyperpigmenting (artificial tanning) reagents to treat vitiligo and other hypopigmenting diseases, and there are societies in which tanning is perceived as beautiful and healthy. Taking together these observations, methods of controlling melanin synthesis have considerable significance for patients with pigmentary disorders and in the general marketplace. Enzymes involved in melanin synthesis include tyrosinase, tyrosinase-related protein-1 (TYRP1), and dopachrome tautomerase (DCT) (Yamaguchi and Hearing, 2009). Among these enzymes in the tyrosinase family, tyrosinase
plays the critical role in melanin synthesis, and various factors, including the copper-transporter ATP7A, have the capacity to modulate its enzymatic activity (Setty et al., 2008). Indeed, most hypopigmenting reagents are intended to suppress tyrosinase activity. Additionally, two variants of oculo cutaneous albinism in humans (OCA1A and OCA1B) are caused by mutations in tyrosinase. Tyrosinase is the rate-limiting enzyme that mediates hydroxylation of tyrosine to dopaquinone and the oxidation of 5,6-dihydroxyindole (DHI) to indole-5,6-quinone, which produces eumelanin (Figure 1). DCT is a tautomerase that converts dopachrome to DHI-2-carboxylic acid (DHICA), whereas TYRP1 is a DHICA oxidase that further stimulates eumelanin synthesis. Consequently, melanogenic enzymes that act within melanosomes are critical for the production of the pigmented biopolymer melanin. NQO1 as an enhancer of tyrosinase activity
Yoon’s group has identified NADPH: quinone oxidoreductase-1 (NQO1)
Department of Geriatric and Environmental Dermatology, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan and 2Laboratory of Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA 1
Correspondence: Yuji Yamaguchi, Department of Geriatric and Environmental Dermatology, Nagoya City University Graduate School of Medical Sciences, 1-Kawasumi, Mizuho-cho, Mizuho-ku, Nagoya 467-8601, Japan. E-mail: [email protected]
www.jidonline.org 645
commentary
Clinical Implications • NQO1 is a newly identified regulatory enzyme that increases tyrosinase activity, probably by suppressing its degradation. • Identification of a novel regulatory enzyme provides new pharmacologic opportunities to treat pigmentary disorders. • Likewise, identification of a novel enzyme may provide leads for the pharmacologic treatment of melanoma.
by comparing hypopigmented acral lentiginous melanoma cells with their normal counterparts, pigmented melanocytes obtained from the back skin of the same patient (Choi et al., this issue). NQO1 is an enzyme that catalyzes the two-electron reduction and the detoxification of quinones (including coenzyme Q10, also known as ubiquinone, and p-benzoquinone) to broad-sense hydroquinones (including ubiquinol and narrowly defined hydroquinone, also known as 1,4-benzenediol). The lower section of Figure 1 shows a representative reaction for NQO1, which may play a role in melanin synthesis, specifically the conversion of dopaquinone to dopa chrome via leukodopachrome. NQO1, one of the phase 2 enzymes, ordinarily protects cells against free radical damage and oxidative stress. Because hydroquinone is a well-known hypopigmenting reagent, NQO1 may negatively regulate skin pigmentation via an increased production of hydroquinone within melanocytes. Indeed, the authors demonstrate that overexpression of NQO1 results in a modest decrease in TYRP1 and microphthalmia-associated transcription factor (MITF, the master regulator of skin pigmentation) at protein levels (Figure 2). On the other hand, melanogenesis increased in concert with increasing levels of NQO1 in the various melanoma cell lines tested. Additionally, the inhibition of NQO1 resulted in a decrease in MITF, tyrosinase, and TYRP1 (but not DCT) at protein levels in tissue culture and in a reduction in melanin pigment using a zebrafish model. Finally, overexpression of NQO1 resulted in an increase in tyrosinase protein and in its enzymatic activity, but not at the
mRNA level in tissue culture. Choi et al. conclude that NQO1 positively regulates melanin synthesis, probably by stabilizing tyrosinase protein. These findings triggered the authors’ interest in elucidating the mechanism(s) by which NQO1 upregulates tyrosinase activity. Keap1 (Kelch-like ECH-associated protein 1) regulates the expression of NQO1 via Nrf2, a member of the NF-E2 family of nuclear basic leucine zipper transcription factors (Figure 2)
(Dinkova-Kostova et al., 2002). Keap1 binds to Nrf2, which translocates to the nucleus in response to carcinogens and oxidants, followed by activation of the antioxidant response element of the NQO1 gene. It seems likely that upregulated NQO1 enhances the expression of tyrosinase independently of its regulation by MITF, because NQO1 does not affect the expression of tyrosinase at the mRNA level or the expression of MITF at either the mRNA or the protein levels. Choi et al. hypothesize that NQO1 suppresses the ubiquitin–proteasome system that would normally degrade tyrosinase. Other factors that affect tyrosinase trafficking in melanocytes may be involved in the selective upregulation of tyrosinase in response to NQO1. NQO1 as a therapeutic target for acral melanoma
To identify NQO1 as an enzyme that increases melanin synthesis, Choi
Eumelanin TYRP1
Tyrosinase
HO
HO
CO2H NH2
HO
Tyrosine
O
HO
N H
DHI
N H
HO
CO2H NH2
HO
Dopaquinone
CO2H
DHICA CO2
Tyrosinase O
HO
DCT
O N H
CO2H
Leukodopachrome
HO
N
CO2H
Dopachrome
Cysteine Pheomelanin
NADP+
NADPH O
O
p-Benzoquinone
OH
NQO1
OH
Hydroquinone
Figure 1. Key players in melanin biosynthesis and NQO1 as an oxidoreductase that mediates the conversion of quinones to hydroquinones. Tyrosinase, DCT, and TYRP1 are enzymes that convert tyrosine (and DHI) to dopaquinone (and eumelanin), dopachrome to DHICA, and DHICA to eumelanin, respectively. Pheomelanin derives from dopaquinone based on the presence of cysteine. NQO1 is an oxidoreductase that mediates the conversion of quinones (including p-benzoquinone and coenzyme Q10) to hydroquinones (including narrowly defined hydroquinone and ubiquinol). DCT, dopachrome tautomerase; DHI, 5,6-dihydroxyindole; DHICA, 5,6-dihydroxyindole-2-carboxylic acid; NADPH, nicotinamide adenine dinuclueotide phosphate; NQO1, NADPH:quinone oxidoreductase-1; TYRP1, tyrosinase-related protein-1. Adapted from PubChem (http://pubchem.ncbi.nlm.nih.gov).
646 Journal of Investigative Dermatology (2010), Volume 130
commentary
important for studying carcinogenesis and metastasis by AM cells.
Melanocyte
Concluding remarks Melanin synthesis
Keap1 1 Nrf2 Keap1
Nrf2
Tyrosinase MITF ?
TYRP1 ?
ARE NQO1
DCT
In summary, Choi et al. (2010) have identified NQO1 as a new pigmentation regulatory enzyme that increases tyrosinase activity, probably by suppressing its degradation. Future studies should elucidate the mechanism(s) by which AM occurs via NQO1 and/ or other factors identified in their microarray analyses at the gene level and in their proteomics analyses at the protein level. CONFLICT OF INTEREST
The authors state no conflict of interest. Figure 2. Scheme summarizing how NQO1 regulates melanin synthesis in melanocytes. NQO1 is regulated by ARE, which is enhanced by Nrf2. Nrf2 migrates into the nucleus from the cytoplasm, where it is released from Keap1 in response to various stimuli. NQO1 does not significantly affect the expression of MITF, TYRP1, or DCT, but it increases the catalytic activity and protein level of tyrosinase and thereby induces melanin synthesis. Vertical blue arrows indicate up- or downregulation in response to NQO1 stimulation; horizontal blue arrow indicates that NQO1 stimulation has no effect on DCT expression. Blue labels indicate data presented by Choi et al. (2010). ARE, antioxidant response element; DCT, dopachrome tautomerase; Keap1, Kelch-like ECHassociated protein-1; MITF, microphthalmia-associated transcription factor; NQO1, NADPH:quinone oxidoreductase-1; Nrf2, an NF-E2 family of nuclear basic leucine zipper transcription factors; TYRP1, tyrosinase-related protein-1.
et al. used two established and highly sensitive analytical approaches— microarrays at the gene level and proteomics at the protein level—and they compared less pigmented cells with normally pigmented cells. In addition to their findings concerning the regulation of skin pigmentation, the investigators elegantly established an acral melanoma (AM) cell line, which has been reported only rarely (Murata et al., 2007). Because AM has a unique phenotype, in that amplifications of narrow regions of the genome (gains of chromosomes 6p or 1q) are often observed in AM patients as compared with non-AM patients (Namiki et al., 2005), comparison with normal melanocytes may elucidate the mechanism(s) by which AM transformation occurs. Skin is heterogeneous (site-specifically different) in terms of the presence or absence of hair, degree of pigmentation, and epidermal thickness. Melanoma types obtained from various body sites reflect different characteristics; BRAF
and N-RAS mutations are observed in melanomas with chronic suninduced damage, but they are rare in melanomas without chronic suninduced damage, in mucosal melanomas, and in AMs (Curtin et al., 2005). Even normal melanocytes are able to adopt different phenotypes in response to external stimuli such as dickkopf 1, an inhibitor of the Wnt/βcatenin signaling pathway (Yamaguchi et al., 2007b). In that sense, the ideal comparisons would have been among primary AM cells, metastatic AM cells, and normal melanocytes located between the two sites (primary and metastatic AM sites). However, Choi et al. identified NQO1 as an AM carcinogenic candidate; to explain the mechanism, they suggested that the loss of NQO1 in AM cells facilitates carcinogenesis due to the loss of its suppressive scavenger effect in the face of oxidative stress caused by reactive oxygen species. Other factors shown in Supplementary Tables 1 and 2 of their article are potentially
References
Choi T-Y, Sohn K-C, Kim J-H et al. (2010) Impact of NAD(P)H:quinone oxidoreductase-1 on pigmentation. J Invest Dermatol 130:784–92 Curtin JA, Fridlyand J, Kageshita T et al. (2005) Distinct sets of genetic alterations in melanoma. N Engl J Med 353:2135–47 Dinkova-Kostova AT, Holtzclaw WD, Cole RN et al. (2002) Direct evidence that sulfhydryl groups of Keap1 are the sensors regulating induction of phase 2 enzymes that protect against carcinogens and oxidants. Proc Natl Acad Sci USA 99:11908–13 Murata H, Ashida A, Takata M et al. (2007) Establishment of a novel melanoma cell line SMYM-PRGP showing cytogenetic and biological characteristics of the radial growth phase of acral melanomas. Cancer Sci 98:958–63 Namiki T, Yanagawa S, Izumo T et al. (2005) Genomic alterations in primary cutaneous melanomas detected by metaphase comparative genomic hybridization with laser capture or manual microdissection: 6p gains may predict poor outcome. Cancer Genet Cytogenet 157:1–11 Setty SR, Tenza D, Sviderskaya EV et al. (2008) Cell-specific ATP7A transport sustains copperdependent tyrosinase activity in melanosomes. Nature 454:1142–6 Solano F, Briganti S, Picardo M et al. (2006) Hypopigmenting agents: an updated review on biological, chemical and clinical aspects. Pigment Cell Res 19:550–71 Yamaguchi Y, Brenner M, Hearing VJ (2007a) The regulation of skin pigmentation. J Biol Chem 282:27557–61 Yamaguchi Y, Passeron T, Watabe H et al. (2007b) The effects of dickkopf 1 on gene expression and Wnt signaling by melanocytes: mechanisms underlying its suppression of melanocyte function and proliferation. J Invest Dermatol 127:1217–25 Yamaguchi Y, Hearing VJ (2009) Physiological factors that regulate skin pigmentation. Biofactors 35:193–9
www.jidonline.org 647