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

A 2D and 3D melanogenesis model with human primary cells induced by tyrosine

Research on melanogenesis, its regulation in health and disease, and the discovery of new molecules with pigmenting and depigmenting activities use different models. Here we standardize a protocol based on previous ones using primary human melanocytes and keratinocytes in co-cultures, in which melanogenesis was induced under mild conditions by the addition of tyrosine plus ammonium chloride (NH₄Cl). The expression of MITF, TYR, TYRP1, and Melan-A as well as melanin content were measured. Furthermore, we extended this study to a reconstructed 3D model. Pigmentation was visually observable and melanosomes were identified by Fontana-Masson staining by the addition of tyrosine plus NH₄Cl during the stratification phase. The 2D and 3D protocols proposed here circumvent limitations of previous models, using human primary cells and mild conditions for melanogenesis. These protocols offer a viable, robust, simple, and animal-free investigational option for human skin pigmentation studies and screening tests for new compounds that modulate pigmentation.

Identification and characterization of the melanocortin 1 receptor gene (MC1R) in hypermelanistic Chinese tongue sole (Cynoglossus semilaevis)

The Chinese tongue sole (Cynoglossus semilaevis) is a flatfish with distinctive asymmetry in its body coloration. The melanism (hyperpigmentation) in both the blind side and ocular side of C. semilaevis gives it an extremely low commercial value. However, the fundamental molecular mechanism of this melanism remains unclear. Melanocortin 1 receptor (MC1R), a GTP-binding protein-coupled receptor, is considered to play a vital role in the physiology of the vertebrate pigment system. In order to confirm the contribution of MC1R to the body coloration of C. semilaevis, the expression levels of Mc1r mRNA were measured in seven tissue types at different developmental stages of normal and melanistic C. semilaevis. The expression levels of Mc1r mRNA in the heart, brain, liver, kidney, ocular-side skin, and blind-side skin of melanistic C. semilaevis were significantly higher than that of normal C. semilaevis in all developmental stages. Moreover, the knocking down of Mc1r in the C. semilaevis liver cell line (HTLC) increased the expression of the downstream genes microphthalmia transcription factor (Mitf) and tyrosinase-related protein 1 (Tyrp1) in the pigmentation pathway. Thus, the present data suggest that MC1R might play important roles in Tyrp1- and Mitf-mediated pigment synthesis in C. semilaevis.

Melanoma mutations modify melanocyte dynamics in co-culture with keratinocytes or fibroblasts

Melanocytic cell interactions are integral to skin homeostasis, and affect the outcome of multiple diseases, including cutaneous pigmentation disorders and melanoma. By using automated-microscopy and machine-learning-assisted morphology analysis of primary human melanocytes in co-culture, we performed combinatorial interrogation of melanocyte genotypic variants and functional assessment of lentivirus-introduced mutations. Keratinocyte-induced melanocyte dendricity, an indicator of melanocyte differentiation, was reduced in the melanocortin 1 receptor (MC1R) R/R variant strain and by NRAS.Q61K and BRAF.V600E expression, while expression of CDK4.R24C and RAC1.P29S had no detectable effect. Time-lapse tracking of melanocytes in co-culture revealed dynamic interaction phenotypes and hyper-motile cell states that indicated that, in addition to the known role in activating mitogenic signalling, MEK-pathway-activating mutations may also allow melanocytes to escape keratinocyte control and increase their invasive potential. Expanding this combinatorial platform will identify other therapeutic target mutations and melanocyte genetic variants, as well as increase understanding of skin cell interactions.

Ultraviolet Radiations: Skin Defense-Damage Mechanism

UV-radiations are the invisible part of light spectra having a wavelength between visible rays and X-rays. Based on wavelength, UV rays are subdivided into UV-A (320-400 nm), UV-B (280-320 nm) and UV-C (200-280 nm). Ultraviolet rays can have both harmful and beneficial effects. UV-C has the property of ionization thus acting as a strong mutagen, which can cause immune-mediated disease and cancer in adverse cases. Numbers of genetic factors have been identified in human involved in inducing skin cancer from UV-radiations. Certain heredity diseases have been found susceptible to UV-induced skin cancer. UV radiations activate the cutaneous immune system, which led to an inflammatory response by different mechanisms. The first line of defense mechanism against UV radiation is melanin (an epidermal pigment), and UV absorbing pigment of skin, which dissipate UV radiation as heat. Cell surface death receptor (e.g. Fas) of keratinocytes responds to UV-induced injury and elicits apoptosis to avoid malignant transformation. In addition to the formation of photo-dimers in the genome, UV also can induce mutation by generating ROS and nucleotides are highly susceptible to these free radical injuries. Melanocortin 1 receptor (MC1R) has been known to be implicated in different UV-induced damages such as pigmentation, adaptive tanning, and skin cancer. UV-B induces the formation of pre-vitamin D3 in the epidermal layer of skin. UV-induced tans act as a photoprotection by providing a sun protection factor (SPF) of 3-4 and epidermal hyperplasia. There is a need to prevent the harmful effects and harness the useful effects of UV radiations.

cAMP-independent non-pigmentary actions of variant melanocortin 1 receptor: AKT-mediated activation of protective responses to oxidative DNA damage

The melanocortin 1 receptor gene (MC1R), a well-established melanoma susceptibility gene, regulates the amount and type of melanin pigments formed within epidermal melanocytes. MC1R variants associated with increased melanoma risk promote the production of photosensitizing pheomelanins as opposed to photoprotective eumelanins. Wild-type (WT) MC1R activates DNA repair and antioxidant defenses in a cAMP-dependent fashion. Since melanoma-associated MC1R variants are hypomorphic in cAMP signaling, these non-pigmentary actions are thought to be defective in MC1R-variant human melanoma cells and epidermal melanocytes, consistent with a higher mutation load in MC1R-variant melanomas. We compared induction of antioxidant enzymes and DNA damage responses in melanocytic cells of defined MC1R genotype. Increased expression of catalase (CAT) and superoxide dismutase (SOD) genes following MC1R activation was cAMP-dependent and required a WT MC1R genotype. Conversely, pretreatment of melanocytic cells with an MC1R agonist before an oxidative challenge with Luperox decreased (i) accumulation of 8-oxo-7,8-dihydro-2'-deoxyguanine, a major product of oxidative DNA damage, (ii) phosphorylation of histone H2AX, a marker of DNA double-strand breaks, and (iii) formation of DNA breaks. These responses were comparable in cells WT for MC1R or harboring hypomorphic MC1R variants without detectable cAMP signaling. In MC1R-variant melanocytic cells, the DNA-protective responses were mediated by AKT. Conversely, in MC1R-WT melanocytic cells, high cAMP production downstream of MC1R blocked AKT activation and was responsible for inducing DNA repair. Accordingly, MC1R activation could promote repair of oxidative DNA damage by a cAMP-dependent pathway downstream of WT receptor, or via AKT in cells of variant MC1R genotype.

Testing of viable human skin cell dilution cultures as an approach to validating microsampling

Skin biopsies are a valuable technique in the diagnosis of cutaneous inflammatory and neoplastic conditions. We were interested to test the minimal size or equivalent volume by dilution of proteolytically disassociated skin tissue required to allow the isolation and propagation of cutaneous cells, for freezing, storage and biochemical analysis. It was possible to propagate with 100% efficiency fibroblast and melanocytic cells from a 0.1 to 0.5 mm³ equivalent neonatal foreskin sample using a combination of DispaseII and CollagenaseIV. The smallest tissue dilution that allowed melanocytic cell culture was 0.01 mm³, which equated to approximately 16 cells based on the average skin density of melanocytes. However, passaging of cells to create frozen stocks was achieved routinely only from 1 mm³ skin, equating to 1560 cells. Tissue-specific antigen expression of these cultures was tested by western blot of total protein extracts. There was no pigmentation antigen expression in fibroblast cultures; however, smooth muscle actin protein expression was high in fibroblast but absent from melanocytic cell strains. Melanocytic cells expressed pigmentation antigens and E-cadherin, but these were not detected in fibroblasts. Moreover, maturation of these melanocytic cells resulted in a decrease of Dopachrome Tautomerase antigen expression and induction of Tyrosinase protein consistent with the differentiation potential seen in cell cultures derived routinely from large sections of skin tissue.

A case of melanoma in a native Australian murid, the spinifex hopping-mouse (Notomys alexis)

Generally the reporting of diseases and parasites in Australian native murids is rare despite murids making up ~25% of the native mammal fauna of Australia. This paper reports a malignant melanoma in a captive spinifex hopping-mouse (Notomys alexis) and is the first case of melanoma reported in any native Australian murid. With no exposure to ultraviolet radiation the melanoma is proposed to be the result of genetic predisposition and age. As no other reports have been observed in the colony it is likely to be a very rare event in captivity and unlikely to occur in the wild.

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.

MC1R, the cAMP pathway, and the response to solar UV: extending the horizon beyond pigmentation

The melanocortin 1 receptor (MC1R) is a G protein-coupled receptor crucial for the regulation of melanocyte proliferation and function. Upon binding melanocortins, MC1R activates several signaling cascades, notably the cAMP pathway leading to synthesis of photoprotective eumelanin. Polymorphisms in the MC1R gene are a major source of normal variation of human hair color and skin pigmentation, response to ultraviolet radiation (UVR), and skin cancer susceptibility. The identification of a surprisingly high number of MC1R natural variants strongly associated with pigmentary phenotypes and increased skin cancer risk has prompted research on the functional properties of the wild-type receptor and frequent mutant alleles. We summarize current knowledge on MC1R structural and functional properties, as well as on its intracellular trafficking and signaling. We also review the current knowledge about the function of MC1R as a skin cancer, particularly melanoma, susceptibility gene and how it modulates the response of melanocytes to UVR.

UV radiation and the skin

UV radiation (UV) is classified as a "complete carcinogen" because it is both a mutagen and a non-specific damaging agent and has properties of both a tumor initiator and a tumor promoter. In environmental abundance, UV is the most important modifiable risk factor for skin cancer and many other environmentally-influenced skin disorders. However, UV also benefits human health by mediating natural synthesis of vitamin D and endorphins in the skin, therefore UV has complex and mixed effects on human health. Nonetheless, excessive exposure to UV carries profound health risks, including atrophy, pigmentary changes, wrinkling and malignancy. UV is epidemiologically and molecularly linked to the three most common types of skin cancer, basal cell carcinoma, squamous cell carcinoma and malignant melanoma, which together affect more than a million Americans annually. Genetic factors also influence risk of UV-mediated skin disease. Polymorphisms of the melanocortin 1 receptor (MC1R) gene, in particular, correlate with fairness of skin, UV sensitivity, and enhanced cancer risk. We are interested in developing UV-protective approaches based on a detailed understanding of molecular events that occur after UV exposure, focusing particularly on epidermal melanization and the role of the MC1R in genome maintenance.

Human pigmentation genes under environmental selection

Genome-wide association studies and comparative genomics have established major loci and specific polymorphisms affecting human skin, hair and eye color. Environmental changes have had an impact on selected pigmentation genes as populations have expanded into different regions of the globe.

Melanocortin 1 receptor regulates melanoma cell migration by controlling syndecan-2 expression

The melanocortin 1 receptor (MC1R), a key regulator of melanogenesis, is known to control inflammation, acting in concert with the MC1R ligand α-melanocyte-stimulating hormone. Although cell migration is a key event in inflammation, few studies have addressed the function of MC1R in this context. Using highly motile melanoma cells, we found that the expression level of MC1R was associated with the extent of migration of mouse melanoma cells, suggesting that MC1R plays a functional role in controlling this migration. Overexpression of MC1R enhanced melanoma cell migration, whereas the opposite was true when MC1R levels were knocked down using small inhibitory RNAs. Interestingly, MC1R expression enhanced the synthesis of syndecan-2, a cell surface heparan sulfate proteoglycan known to be involved in melanoma cell migration. Knockdown of syndecan-2 expression decreased MC1R-mediated cell migration. Further, MC1R inhibited the activation of p38 MAPK, subsequently enhancing expression of sydnecan-2, in parallel with an increase in the extent of cell migration. Consistently, activation of p38 by H(2)O(2) inhibited syndecan-2 expression and cell migration, whereas inhibition of p38 activation enhanced syndecan-2 expression and cell migration. Finally, we found that α-melanocyte-stimulating hormone inhibited MC1R-mediated cell migration via activation of p38 and inhibition of syndecan-2 expression. Together, the data strongly suggest that MC1R regulates melanoma cell migration via inhibition of syndecan-2 expression.

A UVR-induced G2-phase checkpoint response to ssDNA gaps produced by replication fork bypass of unrepaired lesions is defective in melanoma

UVR is a major environmental risk factor for the development of melanoma. Here we describe a coupled DNA-damage tolerance (DDT) mechanism and G2-phase cell cycle checkpoint induced in response to suberythemal doses of UVR that is commonly defective in melanomas. This coupled response is triggered by a small number of UVR-induced DNA lesions incurred during G1 phase that are not repaired by nucleotide excision repair (NER). These lesions are detected during S phase, but rather than stalling replication, they trigger the DDT-dependent formation of single-stranded DNA (ssDNA) gaps. The ssDNA attracts replication protein A (RPA), which initiates ATR-Chk1 (ataxia telangiectasia and Rad3-related/checkpoint kinase 1) G2-phase checkpoint signaling, and colocalizes with components of the RAD18 and RAD51 postreplication repair pathways. We demonstrate that depletion of RAD18 delays both the resolution of RPA foci and exit from the G2-phase arrest, indicating the involvement of RAD18-dependent postreplication repair in ssDNA gap repair during G2 phase. Moreover, the presence of RAD51 and BRCA1 suggests that an error-free mechanism may also contribute to repair. Loss of the UVR-induced G2-phase checkpoint results in increased UVR signature mutations after exposure to suberythemal UVR. We propose that defects in the UVR-induced G2-phase checkpoint and repair mechanism are likely to contribute to melanoma development.

MC1R variant allele effects on UVR-induced phosphorylation of p38, p53, and DDB2 repair protein responses in melanocytic cells in culture

Variant alleles of the human melanocortin 1 receptor (MC1R) reduce the ability of melanocytes to produce the dark pigment eumelanin, with R alleles being most deficient. Cultured melanocytes of MC1R R/R variant genotype give reduced responses to [Nle(4), D-Phe(7)]α-melanocyte-stimulating hormone (NDP-MSH) ligand stimulation and lower levels of DNA repair than MC1R wild-type strains. p38 controls xeroderma pigmentosum (XP)-C recruitment to DNA damage sites through regulating ubiquitylation of the DNA damage-binding protein 2 (DDB2) protein, and p53 is implicated in the nuclear excision repair process through its regulation of XP-C and DDB2 protein expression. We report the effects of MC1R ligand treatment and UVR exposure on phosphorylation of p38 and p53, and DDB2 protein expression in MC1R variant strains. Wild-type MC1R melanocyte strains grown together with keratinocytes in coculture, when treated with NDP-MSH and exposed to UVR, gave synergistic activation of p38 and p53 phosphorylation, and were not replicated by R/R variant melanocytes, which have lower basal levels of phosphorylated forms of p38. Minor increases in p38 phosphorylation status in R/R variant melanocyte cocultures could be attributed to the keratinocytes alone. We also found that MC1R wild-type strains regulate DDB2 protein levels through p38, but MC1R R/R variant melanocytes do not. This work confirms the important functional role that the MC1R receptor plays in UVR stress-induced DNA repair.

Effect of MC1R variant allele status on MSH-ligand induction of dopachrome tautomerase in melanocytes co-cultured with keratinocytes

A co-culture system of melanocytic cells and keratinocytes was used to examine dendricity and dopachrome tautomerase (DCT) responses in low penetrant 'r' homozygote and 'R/+' heterozygote MC1R variant allele expressing cells compared to that of wild-type (WT) cells. The V60L-/- homozygote r variant cells showed similar responses to ligand as WT MC1R strains, while V92M-/- homozygote r variant cells were generally shown to have greater dendricity and express higher DCT than the WT cells, even at basal levels. The R151C+/- heterozygote cells showed similar responses to WT cells, while the R160W+/- and D294H+/- variant cells were reduced in their responses to NDP-MSH, but still had an active cAMP response with forskolin treatment. These responses are consistent with the dominant negative effect of these alleles on the MC1R WT allele that has previously been demonstrated genetically and biochemically.

Melanocortin MC₁ receptor in human genetics and model systems

The melanocortin MC(1) receptor is a G-protein coupled receptor expressed in the melanocytes of the skin and hair and is known for its key role in the regulation of human pigmentation. Melanocortin MC(1) receptor activation after ultraviolet radiation exposure results in a switch from the red/yellow pheomelanin to the brown/black eumelanin pigment synthesis within cutaneous melanocytes; this pigment is then transferred to the surrounding keratinocytes of the skin. The increase in melanin maturation and uptake results in tanning of the skin, providing a physical protection of skin cells from ultraviolet radiation induced DNA damage. Melanocortin MC(1) receptor polymorphism is widespread within the Caucasian population and some variant alleles are associated with red hair colour, fair skin, poor tanning and increased risk of skin cancer. Here we will discuss the use of mouse coat colour models, human genetic association studies, and in vitro cell culture studies to determine the complex functions of the melanocortin MC(1) receptor and the molecular mechanisms underlying the association between melanocortin MC(1) receptor variant alleles and the red hair colour phenotype. Recent research indicates that melanocortin MC(1) receptor has many non-pigmentary functions, and that the increased risk of skin cancer conferred by melanocortin MC(1) receptor variant alleles is to some extent independent of pigmentation phenotypes. The use of new transgenic mouse models, the study of novel melanocortin MC(1) receptor response genes and the use of more advanced human skin models such as 3D skin reconstruction may provide key elements in understanding the pharmacogenetics of human melanocortin MC(1) receptor polymorphism.

The melanocortin-1 receptor gene polymorphism and association with human skin cancer

The melanocortin-1 receptor (MC1R) is a key gene involved in the regulation of melanin synthesis and encodes a G-protein coupled receptor expressed on the surface of the melanocyte in the skin and hair follicles. MC1R activation after ultraviolet radiation exposure results in the production of the dark eumelanin pigment and the tanning process in humans, providing physical protection against DNA damage. The MC1R gene is highly polymorphic in Caucasian populations with a number of MC1R variant alleles associated with red hair, fair skin, freckling, poor tanning, and increased risk of melanoma and nonmelanoma skin cancer. Variant receptors have shown alterations in biochemical function, largely due to intracellular retention or impaired G-protein coupling, but retain some signaling ability. The association of MC1R variant alleles with skin cancer risk remains after correction for pigmentation phenotype, indicating regulation of nonpigmentary pathways. Notably, MC1R activation has been linked to DNA repair and may also contribute to the regulation of immune responses.

Analysis of cultured human melanocytes based on polymorphisms within the SLC45A2/MATP, SLC24A5/NCKX5, and OCA2/P loci

Single nucleotide polymorphisms (SNPs) within the SLC45A2/MATP, SLC24A5/NCKX5, and OCA2/P genes have been associated with natural variation of pigmentation traits in human populations. Here, we describe the characterization of human primary melanocytic cells genotyped for polymorphisms within the MATP, NCKX5, or OCA2 loci. On the basis of genotype, these cultured cells reflect the phenotypes observed by others in terms of both melanin content and tyrosinase (TYR) activity when comparing skin designated as either "White" or "Black". We found a statistically significant association of MATP-374L (darker skin) with higher TYR protein abundance that was not observed for any NCKX5-111 or OCA2 rs12913832 allele. MATP-374L/L homozygous strains displayed significantly lower MATP transcript levels compared to MATP-374F/F homozygous cells, but this did not reach statistical significance based on NCKX5 or OCA2 genotype. Similarly, we observed significantly increased levels of OCA2 mRNA in rs12913832-T (brown eye) homozygotes compared to rs12913832-C (blue eye) homozygous strains, which was not observed for MATP or NCKX5 gene transcripts. In genotype-phenotype associations performed on a collection of 226 southern European individuals using these same SNPs, we were able to show strong correlations in MATP-L374F, OCA2, and melanocortin-1 receptor with skin, eye, and hair color variation, respectively.

G-protein-coupled receptors and melanoma

G-protein-coupled receptors (GPCR) are the largest family of receptors with over 500 members. Evaluation of GPCR gene expression in primary human tumors identified over-expression of GPCR in several tumor types. Analysis of cancer samples in different disease stages also suggests that some GPCR may be involved in early tumor progression and others may play a critical role in tumor invasion and metastasis. Currently, >50% of drug targets to various human diseases are based on GPCR. In this review, the relationships between several GPCR and melanoma development and/or progression will be discussed. Finally, the possibility of using one or more of these GPCR as therapeutic targets in melanoma will be summarized.

Melanocortin-1 receptor signaling markedly induces the expression of the NR4A nuclear receptor subgroup in melanocytic cells

The melanocortin-1 receptor (MCIR) is a G-protein-coupled receptor expressed primarily in melanocytes and is known to play a pivotal role in the regulation of pigmentation in mammals. In humans MC1R has been found to be highly polymorphic with several functional variants associated with the phenotype of red hair color and fair skin, cutaneous UV sensitivity, and increased risk of developing melanoma and non-melanoma skin cancer. Recent evidence suggests that MC1R plays a photo-protective role in melanocytes in response to UV irradiation. Relatively few genetic targets of MC1R signaling have been identified independent of the pigmentation pathway. Here we show that MC1R signaling in B16 mouse melanoma cells and primary human melanocytes rapidly, and transiently, induces the transcription of the NR4A subfamily of orphan nuclear receptors. Furthermore, primary human melanocytes harboring homozygous RHC variant MC1R alleles exhibited an impaired induction of NR4A genes in response to the potent MC1R agonist (Nle4,D-Phe7)-alpha-melanocyte-stimulating hormone. Using small interference RNA-mediated attenuation of NR4A1 and NR4A2 expression in melanocytes, the ability to remove cyclobutane pyrimidine dimers following UV irradiation appeared to be impaired in the context of MC1R signaling. These data identify the NR4A receptor family as potential mediators of an MC1R-coordinated DNA damage response to UV exposure in melanocytic cells.