TYRP2-mediated resistance to cis-diamminedichloroplatinum (II) in human melanoma cells is independent of tyrosinase and TYRP1 expression and melanin content

Knockdown of microRNA‑143‑5p by STTM technology affects eumelanin and pheomelanin production in melanocytes

MicroRNAs (miRNAs) serve various roles in the regulation of melanogenesis in mammalian melanocytes that contribute to the development of hair color. The manipulation of the melanocyte action is a new target for genetic improvement. Short tandem target mimic (STTM) is a potent approach for silencing miRNAs in plants and animals. To investigate the function of miR‑143‑5p in melanogenesis, STTM was used to block the expression of miR‑143‑5p (STTM‑miR‑143‑5p). The molecular analysis and luciferase reporter assay identified myosin Va gene (MYO5A) as one of the miR‑143‑5p targets. STTM‑miR‑143‑5p overexpression resulted in an increased expression of downstream melanogenesis genes including microphthalmia‑associated transcription factor (MITF), tyrosinase family members [tyrosinase (TYR) and tyrosinase‑related protein 1 (TYRP1)], melanophilin (MLPH), and Rab27a, thereby contributing to melanocyte pigmentation by promoting total alkali‑soluble melanogenesis (ASM) and eumelanin (EM) contents; conversely, STTM‑miR‑143‑5p overexpression resulted in decreased expression of the tyrosinase‑related protein 2 (TYRP2)/dopachrome tautomerase (DCT), which is responsible for decreased pheomelanin (PM) content in mouse melanocytes. The results indicated that melanin production in melanocytes could be increased by manipulating miR‑143‑5p expression using STTM which resulted in ASM and EM production.

Whole-genome SNP data unravel population structure and signatures of selection for black plumage of indigenous chicken breeds from Jiangxi province, China

Ten indigenous chicken breeds were originally distributed in Jiangxi Province, China, and they define a critical component of Chinese chicken genetic resources. We have investigated the population genetics of seven Jiangxi chicken breeds using 600K chicken BeadChip SNP data. To provide a genome-wide perspective for the population structure of all 10 Jiangxi chicken breeds, we herein genotyped 78 additional individuals from the seven breeds and 63 chickens from three uninvestigated breeds-Yugan Black (YG), Nancheng Black (NC) and Wanzai Yellow using 55K chicken SNP arrays. We then explored merged data of 17 101 SNPs from 235 individuals to infer the population structure of the 10 breeds. We showed that NC and YG are two regional populations of the same breed, as individuals from the two populations clustered together to form a branch separate from the other breeds in the neighbor-joining tree, they always grouped together in multidimensional principal component analyses and they displayed an identical pattern of ancestral lineage composition. Hence, NC and YG should be considered a single breed in the state-supported conservation scheme. Moreover, we conducted a genome scan for signatures of selection for black plumage. bayescan and hapflk analyses of two contrasting groups (three black-feathered breeds vs. six non-black-feathered breeds) consistently detected 25 putative regions under selection. Nine pigmentation- associated genes (DCT, SLC24A5, SLC30A4, MYO5A, CYP19A1, NADK2, SLC45A2, GNAQ and DCP2) reside within these regions, and these genes are interesting candidates for black plumage and provide a starting point for further identification of causative mutations for black feathers in chicken.

Photoperiod and thyroid hormone regulate expression of l-dopachrome tautomerase (Dct), a melanocyte stem-cell marker, in tanycytes of the ovine hypothalamus

The pars tuberalis (PT) of the pituitary is central to the control of seasonal breeding. In mammals, the PT translates the photoperiodic message carried by melatonin into an endocrine thyroid-stimulating hormone output, which controls local thyroid hormone (TH) signalling in tanycytes of the neighbouring hypothalamus. In the present study, we identify l-dopachrome tautomerase (Dct) as a novel marker of ovine tanycytes and show that Dct displays marked seasonal variations in expression, with higher levels during spring and summer. This seasonal profile is photoperiod-dependent because an acute exposure to long days induces Dct expression. In addition, we find that TH also modulates Dct expression. DCT functions as an enzyme in the melanin synthesis pathway within skin melanocytes, whereas expression in other tissues is comparatively low. We demonstrate that both Tyr and Tyrp1, which are enzymes that intervene upstream and downstream of Dct in the melanin synthesis pathway, respectively, are expressed at very low levels in the ovine hypothalamus. This suggests that Dct in tanycytes may not be involved in melanin synthesis. We speculate that DCT function is linked to its protective role towards oxidative stress and/or its function in the control of neural progenitor cell proliferation.

The Role of DCT in HPV16 Infection of HaCaTs

Persistent infection with high-risk human papillomavirus (HPV) genotype is a major factor leading to many human cancers. Mechanisms of HPV entry into host cells and genome trafficking towards the nucleus are incompletely understood. Dopachrome tautomerase (DCT) was identified as a cellular gene required for HPV infection in HeLa cells on a siRNA screen study. Here, we confirm that DCT knockdown significantly decreases HPV infection in the human keratinocyte HaCaT cells as was observed in HeLas. We investigated the effects of DCT knockdown and found that DCT depletion caused increased reactive oxygen species (ROS) levels, DNA damage and altered cell cycle in HaCaT cells. We observed increased viral DNA localization at the endoplasmic reticulum but an overall decrease in infection in DCT knockdown cells. This observation suggests that viral DNA might be retained in the ER due to altered cell cycle, and viral particles are incapable of further movement towards the nucleus in DCT knockdown cells.

Cross-talk between Dopachrome Tautomerase and Caveolin-1 Is Melanoma Cell Phenotype-specific and Potentially Involved in Tumor Progression

l-Dopachrome tautomerase (l-DCT), also called tyrosinase-related protein-2 (TRP-2), is a melanoma antigen overexpressed in most chemo-/radiotherapeutic stress-resistant tumor clones, and caveolin-1 (CAV1) is a main regulator of numerous signaling processes. A structural and functional relationship between DCT and CAV1 is first presented here in two human amelanotic melanoma cell lines, derived from vertical growth phase (MelJuSo) and metastatic (SKMel28) melanomas. DCT co-localizes at the plasma membrane with CAV1 and Cavin-1, another molecular marker for caveolae in both cell phenotypes. Our novel structural model proposed for the DCT-CAV1 complex, in addition to co-immunoprecipitation and mass spectrometry data, indicates a possible direct interaction between DCT and CAV1. The CAV1 control on DCT gene expression, DCT post-translational processing, and subcellular distribution is cell phenotype-dependent. DCT is a modulator of CAV1 stability and supramolecular assembly in both cell phenotypes. During autocrine stimulation, the expressions of DCT and CAV1 are oppositely regulated; DCT increases while CAV1 decreases. Sub-confluent MelJuSo clones DCT(high)/CAV1(low) are proliferating and acquire fibroblast-like morphology, forming massive, confluent clusters as demonstrated by immunofluorescent staining and TissueFAXS quantitative image cytometry analysis. CAV1 down-regulation directly contributes to the expansion of MelJuSo DCT(high) subtype. CAV1 involved in the perpetuation of cell phenotype-overexpressing anti-stress DCT molecule supports the concept that CAV1 functions as a tumor suppressor in early stages of melanoma. DCT is a regulator of the CAV1-associated structures and is possibly a new molecular player in CAV1-mediated processes in melanoma.

DCT protects human melanocytic cells from UVR and ROS damage and increases cell viability

Dopachrome tautomerase (DCT) is involved in the formation of the photoprotective skin pigment eumelanin and has also been shown to have a role in response to apoptotic stimuli and oxidative stress. The effect of DCT on UVR DNA damage responses and survival pathways in human melanocytic cells was examined by knockdown experiments using melanoma cells, neonatal foreskin melanoblasts (MB) in monoculture and in co-culture with human keratinocytes. MB cell strains genotyped as either MC1R WT or MC1R RHC homozygotes, which are known to be deficient in DCT, were transduced with lentivirus vectors for either DCT knockdown or overexpression. We found melanoma cell survival was reduced by DCT depletion and by UVR over time. UVR-induced p53 and pp53-Ser15 levels were reduced with DCT depletion. Knockdown of DCT in MC1R WT and MC1R RHC MB cells reduced their survival after UVR exposure, whereas increased DCT protein levels enhanced survival. DCT depletion reduced p53 and pp53-Ser15 levels in WM266-4 melanoma and MC1R WT MB cells, while MC1R RHC MB cells displayed variable levels. Both MC1R WT and RHC genotypes of MB cells were responsive to UVR at 3 h with increases in both p53 and pp53-Ser15 proteins. MC1R WT MB cell strains in coculture with keratinocytes have an increased cell survival after UVR exposure when compared to those in monoculture, a protective effect which appears to be conferred by the keratinocytes.

Value of dopachrome tautomerase detection in the assessment of melanocytic tumors

Dopachrome tautomerase (DCT) and tyrosinase (Tyr) are melanogenic enzymes and structurally related melanosomal proteins. The present study investigates DCT expression comparatively with Tyr, the most tested melanoma biomarker, aiming to evaluate DCT potential in the assessment of melanocytic tumors and gain insights into the molecular and pathological characterization of DCT-phenotype in tumor progression. DCT and Tyr are simultaneously analyzed in melanoma cell lines by semiquantitative RT-PCR, western blot, and N-glycan analysis, and in cell populations of melanocytic tumors by immunohistofluorescence using a novel anti-hDCT antibody against an extended sequence within DCT luminal domain. DCT, unlike Tyr, is fully processed along the secretory pathway in both pigmented and amelanotic melanoma cells. In 53 nevi and 116 primary malignant melanomas, 81% and 52%, respectively, are DCT+/Tyr+, showing that DCT is a stable antigen, retained by most tumors and partially expressed in Tyr-negative cell populations. The DCT/Tyr disjunction is a process correlated with melanocyte neoplastic transformation and malignant progression. A tumor architecture--DCT-phenotype-containing DCT+/Tyr- cell populations selected into the innermost dermis from double-positive cells is detected in 35% of DCT+/Tyr+ specimens. The DCT-phenotype is associated with enhanced neurotization in benign nevi and with ulceration in thin malignant melanomas. The intradermal DCT+/Tyr- clones in superficial melanomas acquire the expression and specific subcellular distribution of unfavorable prognostic markers. DCT assessment shows specific antigen patterns with potential significance in the outcome of melanocytic lesions, connecting DCT, a mediator of a melanoma stress-resistant pathway, and an antiapoptotic molecule to DCT- phenotypes that are possibly more stable and stress resistant.

A novel form of melanoma apoptosis resistance: melanogenesis up-regulation in apoptotic B16-F0 cells delays ursolic acid-triggered cell death

Melanoma is one of the most aggressive forms of cancer with a continuously growing incidence worldwide and is usually resistant to chemotherapy agents, which is due in part to a strong resistance to apoptosis. The resistance mechanisms are complex and melanoma cells may have diverse possibilities for regulating apoptosis to generate apoptotic deficiencies. In this study, we investigated the relationship between melanogenesis and resistance to apoptosis induced by ursolic acid, a natural chemopreventive agent, in B16-F0 melanoma cells. We demonstrated that cells undergoing apoptosis are able to delay their own death. It appeared that tyrosinase and TRP-1 up-regulation in apoptotic cells and the subsequent production of melanin were clearly implicated in an apoptosis resistance mechanism; while TRP-2, a well known mediator of melanoma resistance to cell death, was repressed. Our results confirm the difficulty of treating melanomas, since, even undergoing apoptosis, cells are nevertheless able to trigger a resistance mechanism to delay death.

Purification and N-glycosylation analysis of melanoma antigen dopachrome tautomerase

Dopachrome tautomerase (DCT) plays a critical role in lowering the oxidative stress resulting from melanogenesis. Levels of DCT are elevated in melanoma cell lines that are especially resistant to chemotherapy and radiation. DCT is processed as a melanoma antigen and is a potential target for immunotherapy. In order to establish a more complete understanding of the role that DCT may play in the etiology and treatment of melanoma skin cancer, isolation of highly pure and properly processed protein is necessary. Purification of native DCT has been problematic due to a hydrophobic transmembrane anchor and interactions with melanin. In this study, DCT was expressed, without its carboxy-terminal transmembrane region using an Sf9 insect cell protein expression system and its recombinant protein was purified by various chromatographic techniques. Analysis of DCT tryptic peptides by MALDI-TOF/TOF determined N-glycosylation as a primary post-translational modification. Our success in the expression of soluble mammalian DCT and the characterization of N-glycosylation sites is a useful reference toward the comprehensive understanding of the structure/function relationship of mammalian DCT.

Heterogeneous SWI/SNF chromatin remodeling complexes promote expression of microphthalmia-associated transcription factor target genes in melanoma

The microphthalmia-associated transcription factor (MITF) promotes melanocyte differentiation and cell cycle arrest. Paradoxically, MITF also promotes melanoma survival and proliferation, acting like a lineage survival oncogene. Thus, it is critically important to understand the mechanisms that regulate MITF activity in melanoma cells. SWI/SNF chromatin remodeling enzymes are multiprotein complexes composed of one of two related ATPases, BRG1 or BRM, and 9-12 associated factors (BAFs). We previously determined that BRG1 interacts with MITF to promote melanocyte differentiation. However, it was unclear whether SWI/SNF enzymes regulate the expression of different classes of MITF target genes in melanoma. In this study, we characterized SWI/SNF subunit expression in melanoma cells and observed down-regulation of BRG1 or BRM, but not concomitant loss of both ATPases. Re-introduction of BRG1 in BRG1 deficient SK-MEL5 cells enhanced expression of differentiation specific MITF target genes and resistance to cisplatin. Down-regulation of the single ATPase, BRM, in SK-MEL5 cells inhibited expression of both differentiation specific and pro-proliferative MITF target genes and inhibited tumorigenicity in vitro. Our data suggest that heterogeneous SWI/SNF complexes composed of either the BRG1 or BRM subunit promote expression of distinct and overlapping MITF target genes and that at least one ATPase is required for melanoma tumorigenicity.

Involvement of ABC transporters in melanogenesis and the development of multidrug resistance of melanoma

Because melanomas are intrinsically resistant to conventional radiotherapy and chemotherapy, many alternative treatment approaches have been developed such as biochemotherapy and immunotherapy. The most common cause of multidrug resistance (MDR) in human cancers is the expression and function of one or more ATP-binding cassette (ABC) transporters that efflux anticancer drugs from cells. Melanoma cells express a group of ABC transporters (such as ABCA9, ABCB1, ABCB5, ABCB8, ABCC1, ABCC2, and ABCD1) that may be associated with the resistance of melanoma cells to a broad range of anticancer drugs and/or of melanocytes to toxic melanin intermediates and metabolites. In this review, we propose a model (termed the ABC-M model) in which the intrinsic MDR of melanoma cells is at least in part because of the transporter systems that may also play a critical role in reducing the cytotoxicity of the melanogenic pathway in melanocytes. The ABC-M model suggests molecular strategies to reverse MDR function in the context of the melanogenic pathway, which could open therapeutic avenues towards the ultimate goal of circumventing clinical MDR in patients with melanoma.

Influence of melanosome dynamics on melanoma drug sensitivity

BACKGROUND

Malignant melanomas are intrinsically resistant to many conventional treatments, such as radiation and chemotherapy, for reasons that are poorly understood. Here we propose and test a model that explains drug resistance or sensitivity in terms of melanosome dynamics.

METHODS

The growth and sensitivity to cisplatin of MNT-1 cells, which are melanotic and enriched with mature stage III and IV melanosomes, and SK-MEL-28 cells, which have only immature stage I and II melanosomes, were compared using clonogenic assays. Differences in pigmentation, melanosome stages, melanosome number, and cellular structures in different cell lines in response to various treatments were examined by electron microscopy. The relative numbers of melanosomes of different stages were compared after treatment with 1-phenyl-2-thiourea. The relationship between drug transporter function and endogenous melanogenic toxicity was assessed by treating cells with the cyclosporin analog PSC-833 and by assessing vacuole formation and cell growth inhibition. All statistical tests were two-sided.

RESULTS

Endogenous melanogenic cytotoxicity, produced by damaged melanosomes, resulted in pronounced cell growth inhibition in MNT-1 cells compared with amelanotic SK-MEL-28 cells. The sensitivity to CDDP of MNT-1 cells was 3.8-fold higher than that of SK-MEL-28 cells (mean IC(50) for SK-MEL-28 and MNT-1 = 2.13 microM and 0.56 microM, respectively; difference = 1.57 microM, 95% confidence interval = 1.45 to 1.69; P = .0017). After treatment with 6.7 microM CDDP for 72 hours, the number of stage II-III melanosomes in surviving MNT-1 cells was 6.8-fold that of untreated cells. Modulation of MNT-1 cells to earlier-stage (II, II-III, III) melanosomes by treatment with the tyrosinase inhibitor 1-phenyl-2-thiourea dramatically increased CDDP resistance. Furthermore, PSC-833 principally suppressed MNT-1 melanotic cell growth via an elevation of autophagosome-like vacuolar structures, possibly by inhibiting melanosome membrane transporters.

CONCLUSIONS

Melanosome dynamics (including their biogenesis, density, status, and structural integrity) regulate the drug resistance of melanoma cells. Manipulation of melanosome functions may be an effective way to enhance the therapeutic activity of anticancer drugs against melanoma.

TRP-2 expression protects HEK cells from dopamine- and hydroquinone-induced toxicity

We previously reported that melanogenic enzyme TRP-2 (or DCT for DOPAchrome tautomerase) expression in WM35 melanoma cells resulted in increased intracellular GSH levels, reduction in DNA damage induced by free radicals, and decreased cell sensitivity to oxidative stress. These effects seemed to depend on a particular cellular context, because none of them were found to occur in HEK epithelial cells. We postulated that the TRP-2 beneficial effect observed in WM35 cells in the oxidative stress situation may relate to quinone metabolization and, more precisely, to the ability of TRP-2 to clear off related toxic metabolites, resulting in a global redox status modification. Here, a comparative protein expression profiling of catecholamine biosynthesis enzymes and detoxification enzymes was conducted in WM35 melanoma cells and in HEK epithelial cells, in comparison with normal human melanocytes. Results showed that WM35 cells, but not HEK cells, expressed enzymes involved in catecholamine biosynthesis, suggesting that their quinone-related toxic metabolites were present in WM35 cells but not in HEK cells. To address the issue of a possible TRP-2 beneficial effect toward quinone toxicity, cell survival experiments were then conducted in HEK cells using dopamine and hydroquinone at toxic concentrations. We showed that TRP-2 expression significantly reduced HEK cell sensitivity to both compounds. This beneficial property of TRP-2 was likely to depend on the integrity of its DOPAchrome tautomerase catalytic site, because both TRP-2(R194Q) and TRP-2(H189G), which have lost their DOPAchrome tautomerase activity, failed to modify the HEK cell response to dopamine and hydroquinone. These results suggest that TRP-2 acts on quinone metabolites other than DOPAchrome, e.g., in the catecholamine pathway, and limits their deleterious effects.

Evaluation of cytotoxic properties of organometallic ferrocifens on melanocytes, primary and metastatic melanoma cell lines

Malignant melanoma is one of the most severe forms of skin cancer, and chemotherapeutic agents currently in use are poorly effective in curing the disease. Here we describe the properties of two organometallic ferrocenyl derivatives, ferrocifen (Fc-OH-Tam) and ferrociphenol (Fc-diOH) that show a specific antiproliferative effect on melanoma cells. After a short incubation period, Fc-OH-Tam is highly cytotoxic on melanoma cells but less toxic on melanocytes. Fc-diOH is slightly toxic at a high concentration but no discrepancy is observed between malignant and normal cells. After a long incubation time the latter is highly toxic for malignant cells but not for normal cells while the former was very highly toxic for primary malignant cells and significantly less toxic for normal cells. We also found that oxidative stress is not implicated in the mechanism of cytotoxicity, since both derivatives neither induce reactive oxygen species (ROS) level in melanocytes nor in melanoma cells. Finally, investigation on hair follicle growth revealed that the two organometallic derivatives induced an irreversible ejection of the hair shaft, thus predicting a potential hair loss side effect if used as a chemotherapeutic treatment.

Melanocytes expressing MC1R polymorphisms associated with red hair color have altered MSH-ligand activated pigmentary responses in coculture with keratinocytes

The occurrence of red hair and pale skin in individuals, which is associated with UV-radiation sensitivity and increased skin cancer risk, is mainly due to polymorphisms in the melanocortin-1 receptor (MC1R) expressed in melanocytes. We have established a serum free human melanocyte-keratinocyte coculture system to study the behavior and functional abilities of melanocytes expressing MC1R red hair color (RHC) variants in order to identify differences from their wild type (WT) counterparts. This model revealed the importance of elevated calcium levels in promoting strong melanocyte interaction with the surrounding keratinocytes and resulted in a dendritic melanocyte morphology similar to that in skin. However, the dendricity response following agonist activation of the MC1R receptor by NDP-MSH peptide, was markedly enhanced in WT melanocytes in comparison to RHC strains. Analysis of mRNA expression and protein levels of the major pigmentation markers following NDP-MSH treatment distinguished the enzyme dopachrome tautomerase as preferentially upregulated in cocultures of WT strains, with negligible or a much reduced response in melanocytes with RHC variant alleles. These results highlight the use of the coculture system in determining fundamental differences in the physiology of melanocytes expressing RHC MC1R receptors and those of WT genotype, which are likely to contribute to the increased skin cancer risk for individuals that carry these variants.

TRP-2 specifically decreases WM35 cell sensitivity to oxidative stress

TRP-2 (dopachrome tautomerase) is a melanogenic enzyme whose expression was recently reported to modulate melanocyte response to different cytotoxic events. Here we studied a possible role of TRP-2 in the oxidative stress response in the amelanotic WM35 melanoma cell line. Cell viability assays showed that TRP-2 overexpression in WM35 cells reduced their sensitivity to oxidative stress. Comet assays linked TRP-2 expression to DNA damage protection, and high-performance liquid chromotography-tandem mass spectrometry experiments showed an increase in intracellular glutathione in TRP-2-overexpressing cells. These effects were specifically reversed when TRP-2 was silenced by RNA interference. Nevertheless, these properties appeared to depend on a particular cell environment because expression of TRP-2 failed to rescue HEK epithelial cells exposed to similar treatments.

Melanosomal sequestration of cytotoxic drugs contributes to the intractability of malignant melanomas

Multidrug resistance mechanisms underlying the intractability of malignant melanomas remain largely unknown. In this study, we demonstrate that the development of multidrug resistance in melanomas involves subcellular sequestration of intracellular cytotoxic drugs such as cis-diaminedichloroplatinum II (cisplatin; CDDP). CDDP is initially sequestered in subcellular organelles such as melanosomes, which significantly reduces its nuclear localization when compared with nonmelanoma/KB-3-1 epidermoid carcinoma cells. The melanosomal accumulation of CDDP remarkably modulates melanogenesis through a pronounced increase in tyrosinase activity. The altered melanogenesis manifested an approximately 8-fold increase in both intracellular pigmentation and extracellular transport of melanosomes containing CDDP. Thus, our experiments provide evidence that melanosomes contribute to the refractory properties of melanoma cells by sequestering cytotoxic drugs and increasing melanosome-mediated drug export. Preventing melanosomal sequestration of cytotoxic drugs by inhibiting the functions of melanosomes may have great potential as an approach to improving the chemosensitivity of melanoma cells.

Cytotoxic T cell targeting of TRP-2 sensitizes human malignant glioma to chemotherapy

Tyrosinase-related protein (TRP)-2 is not only expressed on glioma cells, but is naturally processed and presented by their surface MHC molecules and is recognized by TRP-2-specific cytotoxic T cells. After active immunotherapy, we detected TRP-2-specific cytotoxic T lymphocyte (CTL) activity in patients' peripheral blood mononuclear cells (PBMC). Tumor cells from postvaccination resections showed significantly lower TRP-2 expression and higher sensitivity to carboplatin and temozolomide than those autologous cell lines from prevaccination resections in two patients who demonstrated CTL response to TRP-2. One of two patients underwent treatment with temozolomide after recurrence and responded dramatically. TRP-2-transfected cell line (TRP-2-U373) resulted in significant drug resistance to carboplatin and temozolomide compared to wild-type U-373 (W-U373). There was no significant difference, however, in the mRNA expression of other common drug resistance related proteins, such as BCRP-1, MGMT, MDR-1, MRP-1 and MRP-3, after TRP-2 transfection. TRP-2-U373 tumor cells were immunoselected by a TRP-2-specific CTL line. The immunoselected cells (IS-TRP-2-U373) demonstrated significantly increased sensitivity to carboplatin and temozolomide compared to TRP-2-U373. For the first time, we provide evidence that immunological targeting of tumor-associated antigen TRP-2 significantly increases sensitivity to chemotherapy.

Direct interaction of Sox10 with the promoter of murine Dopachrome Tautomerase (Dct) and synergistic activation of Dct expression with Mitf

The murine dopachrome tautomerase (Dct) gene is expressed early in melanocyte development during embryogenesis, prior to other members of the tyrosinase gene family important for regulating pigmentation. We have used deletion mutants of the Dct promoter, transfections with developmentally relevant transcription factors, and gel shift assays to define transcriptional determinants of Dct expression. Deletion mutagenesis studies show that sequences within the proximal 459 nucleotides are critical for high level expression in melanocytic cells. This region of the promoter contains candidate binding sites for the transcription factors Sox10 and Mitf. Transfections into 293T and NIH3T3 cells show that Sox10 and Mitf independently activate Dct expression, and, when co-transfected, synergistically activate Dct expression. To support the notion that Sox10 acts directly upon the Dct promoter to activate gene expression, direct interaction of Sox10 was demonstrated using gel shifts of oligonucleotide probes derived from promoter sequences within the region required for Sox10-dependent induction. These results suggest that a combinatorial transcription factor interaction is important for expression of Dct in neural crest-derived melanocytes, and support a model for sequential gene activation in melanocyte development whereby Mitf, a Sox10-dependent transcription factor, is expressed initially before an early melanocyte differentiation gene, Dct, is expressed.

Radiation resistance of human melanoma analysed by retroviral insertional mutagenesis reveals a possible role for dopachrome tautomerase

While melanomas are resistant to the cytotoxic effects of radiotherapy, little is known about the molecular mechanisms underlying this intrinsic resistance. Here, we describe the utilization of retroviral insertional mutagenesis to facilitate the analysis of genetic changes that are associated with radioresistance in human melanoma. A radial growth phase human melanoma cell line, WM35, was infected with a replication-defective amphotropic murine retrovirus and subsequently selected for X-ray radiation-resistant variants. Several radiation-resistant clones were independently isolated and characterized. Interestingly, these clones also displayed resistance to ultraviolet radiation and to the chemotherapeutic drug cis-diamminedichloroplatinum(II) (CDDP). By Northern and Western analyses, we showed that the expression of DOPAchrome tautomerase (DCT), also known as tyrosinase-related protein 2 (TYRP2), an enzyme that functions in eumelanin synthesis, was significantly elevated in the radiation-resistant clones relative to the parental WM35 cells. Moreover, the levels of DCT in a variety of human melanoma cell lines correlated with their relative levels of radioresistance and the enforced expression of DCT conferred increased resistance to UV(B) treatment. An analysis of stress signaling induced by radiation as well other cytotoxic stressors showed that resistance associated with DCT overexpression applied specifically to treatments that activate the ERK/MAPK pathway. Indeed, DCT overexpression in a melanoma cell line resulted in increased ERK activity. Moreover, ectopic expression of dominant-active MEK in this melanoma cell line conferred UV(B) resistance suggesting that the ERK/MAPK pathway downstream of DCT may play a critical role in radiation and drug resistance. Overall, given that each gamma- and UV(B)-resistant cell line also exhibited resistance to CDDP and that CDDP-resistant clones showed increased resistance to UV(B) irradiation, these results suggest a common mechanism underlying radio- and chemoresistance, which is mediated by DCT expression.