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

Human TYRP1: Two functions for a single gene?

In the animal kingdom, skin pigmentation is highly variable between species, and it contributes to phenotypes. In humans, skin pigmentation plays a part in sun protection. Skin pigmentation depends on the ratio of the two pigments pheomelanin and eumelanin, both synthesized by a specialized cell population, the melanocytes. In this review, we explore one important factor in pigmentation: the tyrosinase-related protein 1 (TYRP1) gene which is involved in eumelanin synthesis via the TYRP1 protein. Counterintuitively, high TYRP1 mRNA expression is associated with a poor clinical outcome for patients with metastatic melanomas. Recently, we were able to explain this unexpected TYRP1 function by demonstrating that TYRP1 mRNA sequesters microRNA-16, a tumor suppressor miRNA. Here, we focus on actors influencing TYRP1 mRNA abundance, particularly transcription factors, single nucleotide polymorphisms (SNPs), and miRNAs, as they all dictate the indirect oncogenic activity of TYRP1.

Integrated Analysis of mRNA Expression, CpG Island Methylation, and Polymorphisms in the MITF Gene in Ducks (Anas platyrhynchos)

Microphthalmia-associated transcription factor (MITF) is a key regulator for the development and function of melanocytes in skin, eye, and plumage pigmentations. Thus, the MITF was selected as a candidate gene associated with plumage coloration in ducks. This study analyzed the mRNA expression, promoter methylation, and polymorphisms in the MITF gene in ducks with different plumage colors (Putian Black, Putian White, Liancheng White, and Longsheng Jade-green). No expression of the MITF melanin-specific isoform (MITF-M) was detected in white feather bulbs. By contrast, the mRNA expression levels of MITF-M were high in black feather bulbs. Bioinformatics analysis showed that two CpG islands were present in the promoter region of the MITF gene. The methylation level of the second CpG island was significantly lower in black feather bulbs than in white feather bulbs. However, the methylation level of the first CpG island was not different among the feather bulbs with various colors except Liancheng White feather bulbs. The methylation status of the whole CpG island significantly and negatively correlated with the mRNA expression of MITF-M (P < 0.05). Furthermore, four novel SNPs (single nucleotide polymorphisms) were identified in the 5′UTR, exon 4, intron 7, and intron 8 of the MITF gene. Allele T in g.39807T>G and allele G in g.40862G>A were the predominant alleles only found in Putian White, whereas the variant A allele in g.32813G>A exhibited a high allele frequency in Liancheng White. Collectively, these results contributed to the understanding of the function of the MITF gene in duck plumage coloration.

Endothelin Receptor B2 (EDNRB2) Gene Is Associated with Spot Plumage Pattern in Domestic Ducks (Anas platyrhynchos)

Endothelin receptor B subtype 2 (EDNRB2) is a seven-transmembrane G-protein coupled receptor. In this study, we investigated EDNRB2 gene as a candidate gene for duck spot plumage pattern according to studies of chicken and Japanese quail. The entire coding region was cloned by the reverse transcription polymerase chain reaction (RT-PCR). Sequence analysis showed that duck EDNRB2 cDNA contained a 1311bp open reading frame and encoded a putative protein of 436 amino acids residues. The transcript shared 89%-90% identity with the counterparts in other avian species. A phylogenetic tree based on amino acid sequences showed that duck EDNRB2 was evolutionary conserved in avian clade. The entire coding region of EDNRB2 were sequenced in 20 spot and 20 non-spot ducks, and 13 SNPs were identified. Two of them (c.940G>A and c.995G>A) were non-synonymous substitutions, and were genotyped in 647 ducks representing non-spot and spot phenotypes. The c.995G>A mutation, which results in the amino acid substitution of Arg332His, was completely associated with the spot phenotype: all 152 spot ducks were carriers of the AA genotype and the other 495 individuals with non-spot phenotype were carriers of GA or GG genotype, respectively. Segregation in 17 GA×GG and 22 GA×GA testing combinations confirmed this association since the segregation ratios and genotypes of the offspring were in agreement with the hypothesis. In order to investigate the underlying mechanism of the spot phenotype, MITF gene was used as cell type marker of melanocyte progenitor cells while TYR and TYRP1 gene were used as cell type markers of mature melanocytes. Transcripts of MITF, TYR and TYRP1 gene with expected size were identified in all pigmented skin tissues while PCR products were not obtained from non-pigmented skin tissues. It was inferred that melanocytes are absent in non-pigmented skin tissues of spot ducks.

TYRP1 mRNA expression in melanoma metastases correlates with clinical outcome

Background:

Clinical outcome of patients with high-risk melanoma cannot be reliably predicted on the basis of classical histopathological examination. Our study aimed to determine in melanoma metastases a gene expression profile associated with patient survival, and to identify and validate marker(s) of poor clinical outcome.

Methods:

Skin and lymph node metastases from melanoma patients (training population) were used to identify candidate prognostic marker(s) based on DNA microarray analysis. Additional skin metastases (validation population) were used to assess the prognostic value of the first ranked gene by real-time PCR.

Results:

We performed microarray analysis in the training population and generated a list of 278 probe sets associated with a shorter survival. We used the first ranked gene, tyrosinase-related protein 1 (TYRP1), further measured its expression in the validation population by real-time PCR and found it to be significantly correlated with distant metastasis-free survival (DMFS), overall survival (OS) and Breslow thickness. We also found that it was fairly well conserved in the course of the disease regardless of the delay to metastasis occurrence. Finally, although Tyrp1 protein (immunohistochemistry (IHC)) was only detected in about half of the samples, we showed that its expression also correlated with Breslow thickness.

Conclusion:

Our data indicate that TYRP1 mRNA expression level, at least in skin metastases, is a prognostic marker for melanoma, and is particularly useful when prognostic pathology parameters at the primary lesion are lacking. Its conserved expression further supports its use as a target for therapy.

Immune rejection of mouse tumors expressing mutated self

How the immune system recognizes and responds to mutations expressed by cancer cells is a critical issue for cancer immunology. Mutated self-polypeptides are particularly strong tumor-specific rejection antigens for natural tumor immunity, but we know remarkably little about T-cell responses to mutated self during tumor growth in vivo, including levels of response, kinetics, and correlates that predict tumor rejection. To address these questions, a mutated self-antigen, designated tyrosinase-related protein 1 (Tyrp1)-WM, derived from Tyrp1 was expressed in the poorly immunogenic, spontaneously arising B16 melanoma and the immunogenic, chemically induced LiHa fibrosarcoma. Syngeneic mice challenged with LiHa fibrosarcoma cells expressing Tyrp1-WM, but not native Tyrp1, induced specific CD8(+) and CD4(+) T-cell responses against defined mutated epitopes in tumor-draining lymph nodes and in tumors. Subsequently, specific CD8(+) T-cell responses contracted as a minority of tumors progressed. B16 melanomas expressing Tyrp1-WM induced minimal T-cell responses, and no tumor immunity was detected. Treatment with an agonist monoclonal antibody against glucocorticoid-induced tumor necrosis factor receptor family-related gene (GITR) increased the level of CD8(+) T cells recognizing a peptide derived from the Tyrp1-WM sequence and the proportion of mice rejecting tumors. These results show that B16 tumors expressing mutations that generate strongly immunogenic epitopes naturally induce T-cell responses, which are insufficient to reject tumors. Immune modulation, such as inducing GITR signaling, is required to enhance CD8(+) T-cell responses to specific mutations and to lead to tumor rejection.

The Impact of Genomics in Understanding Human Melanoma Progression and Metastasis

Background

Recent technological advances in the analysis of the human genome have opened the door to improving our primitive understanding of the gene expression patterns in cancer. For the first time, we have an overview of the complexities of tumorigenesis and metastatic progression of cancer. The examination of the phenotypic and (epi)genetic changes in cutaneous melanoma has identified several genes deemed central to the development and progression of melanoma.

Methods

A review of the recent literature was performed to determine the role of array-based high-throughput gene expression analysis in understanding the specific genes involved as well as the pathways and the comparative gene expression patterns of primary and metastatic melanoma.

Results

Most studies utilizing gene microarray analysis and other whole genome approaches reveal a wide array of genes and expression patterns in human melanoma. Furthermore, several of the same genes have been found in comparative studies, with some studies attempting correlation with clinical outcome. Several genes have been identified as potential prognostic markers of tumor progression and overall clinical outcome.

Conclusions

High-throughput gene expression analysis has had a major impact in melanoma research. Several gene expression platforms have provided insight into the gene expression patterns in melanoma. Such data will provide the foundations for the future development of prognostic markers and improved targeted therapies for patients with melanoma.

Tyrosinase and ocular diseases: Some novel thoughts on the molecular basis of oculocutaneous albinism type 1

Tyrosinase (TYR) is a multifunctional copper-containing glycoenzyme (approximately 80 kDa), which plays a key role in the rate-limiting steps of the melanin biosynthetic pathway. This membrane-bound protein, possibly evolved by the fusion of two different copper-binding proteins, is mainly expressed in epidermal, ocular and follicular melanocytes. In the melanocytes, TYR functions as an integrated unit with other TYR-related proteins (TYRP1, TYRP2), lysosome-associated membrane protein 1 (LAMP1) and melanocyte-stimulating hormone receptors; thus forming a melanogenic complex. Mutations in the TYR gene (TYR, 11q14-21, MIM 606933) cause oculocutaneous albinism type 1 (OCA1, MIM 203100), a developmental disorder having an autosomal recessive mode of inheritance. In addition, TYR can act as a modifier locus for primary congenital glaucoma (PCG) and it also contributes significantly in the eye developmental process. Expression of TYR during neuroblast division helps in later pathfinding by retinal ganglion cells from retina to the dorsal lateral geniculate nucleus. However, mutation screening of TYR is complicated by the presence of a pseudogene-TYR like segment (TYRL, 11p11.2, MIM 191270), sharing approximately 98% sequence identity with the 3' region of TYR. Thus, in absence of a full-proof strategy, any nucleotide variants identified in the 3' region of TYR could actually be present in TYRL. Interestingly, despite extensive search, the second TYR mutation in 15% of the OCA1 cases remains unidentified. Several possible locations of these "uncharacterized mutations" (UCMs) have been speculated so far. Based on the structure of TYR gene, its sequence context and some experimental evidences, we propose two additional possibilities, which on further investigations might shed light on the molecular basis of UCMs in TYR of OCA1 patients; (i) partial deletion of the exons 4 and 5 region of TYR that is homologous with TYRL and (ii) variations in the polymorphic GA complex repeat located between distal and proximal elements of the human TYR promoter that can modulate the expression of the gene leading to disease pathogenesis.

Pheomelanin Coat Colour Dilution in French Cattle Breeds is not Correlated with the TYR, TYRP1 and DCT Transcription Levels

In this study we report the isolation of full-length cDNAs and the expression patterns of TYR, TYRP1 and DCT in four e/e cattle breeds exhibiting different pheomelanic coat colours ranging from reddish brown to creamy white phenotypes. Predicted proteins encoded by bovine TYR, TYRP1 and DCT display high levels of homology and contain all characteristic domains shared between their mouse and human counterparts. The full expression of these three genes is observed in melanocytes of black areas of E(D)/E(D) Prim'Holstein's animals. On the other hand, e/e melanocytes of animals belonging to the Blonde d'Aquitaine (blond), Limousine (red) and Salers (reddish brown) breeds present different levels of down-regulated TYR and DCT expression and a complete repression of TYRP1. Surprisingly, e/e melanocytes of animals belonging to the Charolais breed (creamy white) present an inverse relationship between TYR, TYRP1 and DCT expression and its lower melanogenic activity. The sum of these results shows that the dilution of the coat colour in French cattle breeds is not correlated with a transcription level of TYR family genes. Other possible modifier loci are suggested.

Selective down-regulation of tyrosinase family gene TYRP1 by inhibition of the activity of melanocyte transcription factor, MITF

Tyrosinase (TYR), tyrosinase-related protein-1 (TYRP1/gp75) and dopachrome tautomerase (DCT/TYRP2) belong to a family of melanocyte-specific gene products involved in melanin pigmentation. During melanocyte development expression of tyrosinase family genes is thought to be orchestrated in part by the binding of a shared basic helix-loop-helix transcription factor MITF to the M box, a regulatory element conserved among these genes. In transformed melanocytes, expression of tyrosinase and TYRPs is highly variable. Whereas TYR expression in melanoma cells is regulated by both transcriptional and post-translational mechanisms, TYRP1/gp75 transcription is often completely extinguished during melanoma tumor progression. In this study, we investigated the mechanisms of selective repression of TYRP1 transcription. Interestingly, in early stage melanoma cells TYRP1 mRNA could be induced by inhibition of protein synthesis. Transient transfection experiments with a minimal TYRP1 promoter showed that the promoter activity correlates with expression of the endogenous TYRP1 gene. Nucleotide deletion analysis revealed novel regulatory sequences that attenuate the M box-dependent MITF activity, but which are not involved in the repression of TYRP1. Gel mobility shift analysis showed that binding of the transcription factor MITF to the TYRP1 M box is selectively inhibited in TYRP1(-) cells. These data suggest that protein factors that modulate the activity of MITF in melanoma cells repress TYRP1 and presumably other MITF target genes.

Secretory lysosomes

Regulated secretion of stored secretory products is important in many cell types. In contrast to professional secretory cells, which store their secretory products in specialized secretory granules, some secretory cells store their secretory proteins in a dual-function organelle, called a secretory lysosome. Functionally, secretory lysosomes are unusual in that they serve both as a degradative and as a secretory compartment. Recent work shows that cells with secretory lysosomes use new sorting and secretory pathways. The importance of these organelles is highlighted by several genetic diseases, in which immune function and pigmentation--two processes that normally involve secretory lysosomes--are impaired.

The promise of microarray technology in melanoma care

BACKGROUND

Genetic aberration is responsible for the development of neoplastic potential in a number of malignancies. These DNA alterations result in significant changes in gene expression that may now be measured and catalogued. The microarray technique screens and identifies expressed genes that may be responsible for tumorigenesis.

METHODS

The authors review the application of the microarray technique in malignant melanoma.

RESULTS

Candidate melanoma suppressor genes have been identified in melanoma cell lines using this technique. Furthermore, molecular classification using gene expression profiling may improve the accuracy of the staging system for determining prognosis.

CONCLUSIONS

The microarray technique is in its initial development for clinical application in a variety of tumor models. Melanoma is an ideal system to study the genetic changes associated with the stepwise progression of malignancy. It may be possible to efficiently screen the entire human genome to identify the particular aberrations in gene expression responsible for tumorigenesis in melanoma.

PDZ domain protein GIPC interacts with the cytoplasmic tail of melanosomal membrane protein gp75 (tyrosinase-related protein-1)

Tyrosinase and tyrosinase-related proteins (TRPs) are a family of melanosomal membrane proteins involved in mammalian pigmentation. Whereas the melanogenic functions of TRPs are localized in their amino-terminal domains that reside within the lumen of melanosomes, the sorting and targeting of these proteins to melanosomes is mediated by signals in their cytoplasmic domains. To identify proteins that interact with the cytoplasmic tail of gp75 (TRP-1), the most abundant melanosomal membrane protein, we performed yeast two-hybrid screening of a melanocyte cDNA library. Here, we show that the cytoplasmic domain of gp75 interacts with a PDZ domain-containing protein. The gp75-interacting protein is identical to GIPC, an RGS (regulator of G protein signaling)/GAIP-interacting protein, and to SEMCAP-1, a transmembrane semaphorin-binding protein. Carboxyl-terminal amino acid residues, Ser-Val-Val, of gp75 are necessary and sufficient for interaction of gp75 with the single PDZ domain in GIPC. Although endogenous and transfected GIPCs bind efficiently to transiently expressed gp75, only a small amount of GIPC is found associated with gp75 at steady state. Using a strategy to selectively synchronize the biosynthesis of endogenous gp75, we demonstrate that only newly synthesized gp75 associates with GIPC, primarily in the juxtanuclear Golgi region. Our data suggest that GIPC/SEMCAP-1 plays a role in biosynthetic sorting of proteins, specifically gp75, to melanosomes.

Expression of microtubule-associated protein 2 in benign and malignant melanocytes: implications for differentiation and progression of cutaneous melanoma

Cutaneous melanocytic neoplasms are known to acquire variable characteristics of neural crest differentiation. Melanocytic nevus cells in the dermis and desmoplastic melanomas often display characteristics of nerve sheath differentiation. The extent and nature of neuronal differentiation characteristics displayed by primary and metastatic melanoma cells are not well understood. Here, we describe induction of a juvenile isoform of microtubule-associated protein 2 (MAP-2c) in cultured metastatic melanoma cells by the differentiation inducer hexamethylene bisacetamide. Up-regulation of this MAP-2 isoform, a marker for immature neurons, is accompanied by extended dendritic morphology and down-regulation of tyrosinase-related protein 1 (TYRP1/gp75), a melanocyte differentiation marker. In a panel of cell lines that represent melanoma tumor progression, MAP-2c mRNA and the corresponding approximately 70-kd protein could be detected predominantly in primary melanomas. Immunohistochemical analysis of 61 benign and malignant melanocytic lesions showed abundant expression of MAP-2 protein in melanocytic nevi and in the in situ and invasive components of primary melanoma, but only focal heterogeneous expression in a few metastatic melanomas. In contrast, MAP-2-positive dermal nevus cells and the invasive cells of primary melanomas were TYRP1-negative. This reciprocal staining pattern in vivo is similar to the in vitro observation that induction of the neuronal marker MAP-2 in metastatic melanoma cells is accompanied by selective extinction of the melanocytic marker TYRP1. Our data show that neoplastic melanocytes, particularly at early stages, retain the plasticity to express the neuron-specific marker MAP-2. These observations are consistent with the premise that both benign and malignant melanocytes in the dermis can express markers of neuronal differentiation.

Expression and Up-regulation of alternatively spliced transcripts of melastatin, a melanoma metastasis-related gene, in human melanoma cells

Loss of expression of a novel suppressor of metastasis, melastatin (MLSN1), has recently been reported to correlate with metastatic potential of melanoma cells. Using differential display analysis, we identified MLSN1 among genes overexpressed in pigmented metastatic human melanoma cells treated with the differentiation inducer hexamethylene bisacetamide (HMBA). In this study, we show that multiple short transcripts of MLSN1 are present in melanocytes and pigmented metastatic melanoma cell lines while the full-length 5. 4-kb mRNA is detectable only in melanocytes. Treatment of pigmented melanoma cells with the differentiation-inducing agent, HMBA, results in up-regulation of the 5.4-kb MLSN1 mRNA as well as short RNAs. Analysis of a panel of nonpigmented primary and metastatic melanoma cell lines showed weak expression of a 1.8-kb mRNA in a few melanoma cell lines. Northern blot and RT-PCR analyses with DNA probes and oligonucleotide primers that correspond to distinct regions of full-length MLSN1 mRNA indicated that the short transcripts contained sequences corresponding primarily to either 5'- or 3'-end of the 5.4-kb mRNA. HMBA appears to up-regulate MLSN1 transcripts derived mainly from the 5'-end. Modulators of cAMP and protein kinase C pathways had no significant effect on MLSN1 expression. Our data show that multiple MLSN1 transcripts, both constitutively expressed and inducible, are present in cultured pigmented melanoma cells, and suggest that MLSN1 expression can be regulated at the level of both transcription and mRNA processing.

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

Tyrosinase-related protein-2 (TYRP2) is a melanocyte-specific enzyme that catalyses the non-decarboxylative tautomerization of L-dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) in the melanin biosynthetic pathway. We have recently demonstrated that the constitutive expression of TYRP2 in human melanoma cells positively correlates with cis-diamminedichloroplatinum (II) (CDDP) resistance, and that the ectopic expression of TYRP2 in CDDP-sensitive cells rendered them more resistant to CDDP treatment. Here, we demonstrate that this correlation between constitutive TYRP2 expression and CDDP resistance applies to a panel of distinct human melanoma cell lines obtained from patients with melanoma at various stages of disease progression. We further show that CDDP resistance correlates only with TYRP2 expression and is associated neither with tyrosinase and TYRP1 expression, nor with cellular melanin content. Together, these results further support the notion that TYRP2 is a novel mediator of CDDP resistance in melanoma cells and suggest that this function of TYRP2 is independent of cellular melanin content and of the other regulatory enzymes of the melanogenic pathway.

Role of microphthalmia transcription factor in regulation of melanocyte differentiation marker TRP-1

Tyrosinase and a family of tyrosinase-related proteins (TRPs) are melanocyte differentiation gene products involved in melanin pigmentation. Members of the tyrosinase family share upstream transcriptional regulatory elements suggesting that expression of these genes is regulated by shared mechanisms. Microphthalmia transcription factor MITF, a melanocyte-specific basic helix-loop-helix protein, has been shown to transactivate tyrosinase and TRP-1 genes in vitro by binding to a shared regulatory sequence known as M box. The role of MITF in concomitant regulation of these genes in vivo is not clear. We showed earlier that in human melanoma cells TRP-1 can be regulated independently of tyrosinase and pigmentation. To investigate the role of MITF in TRP-1 regulation, we studied the effect of pharmacological agents that modulate transcription of tyrosinase and TRP-1 on MITF. In melanoma cells treated with hexamethylene bisacetamide (HMBA), transcription of TRP-1 gene was selectively and completely inhibited while steady state levels of tyrosinase, TRP-2, MITF mRNA and melanin content showed a modest increase. HMBA caused no detectable change in cellular MITF or its nuclear localization. This MITF-independent regulation of TRP-1 required continued synthesis of RNA and protein. Selective down-regulation of TRP-1 by HMBA occurred even in the presence of cholera toxin which up-regulates TRP-1 by cAMP-mediated pathways. These data show that TRP-1 gene can be down-regulated independently of MITF by de novo activation of negative regulatory factors. Thus, both activation of positive factors such as MITF and inactivation of negative regulatory factors may be required for TRP-1 gene expression during melanocytic differentiation.

Dedifferentiation of the retinal pigment epithelium compared to the proliferative membranes of proliferative vitreoretinopathy

PURPOSE

To examine the gene expression for melanogenesis of retinal pigment epithelial cells during dedifferentiation and to compare the condition to that of eyes obtaining anatomical success after surgery for proliferative vitreoretinopathy.

METHODS

Gene expression for melanogenesis was determined by reverse transcriptase-polymerase chain reaction of tyrosinase and tyrosinase-related protein-1 genes in normal and cultured retinal pigment epithelial cells and in proliferative membranes in patients with proliferative vitreoretinopathy.

RESULTS

Gene expression for melanogenesis was classified into three types during dedifferentiation of retinal pigment epithelial cells: (1) tyrosinase-related protein-1 gene expression, (2) tyrosinase and tyrosinase-related protein-1 gene expression and (3) no expression of these genes. The expression of these genes were maintained better in mediums with basic fibroblast growth factor than in medium without basic fibroblast growth factor. Of the anatomically unsuccessful patients with proliferative vitreoretinopathy treated by surgery, 76.9% showed both tyrosinase and tyrosinase-related protein-1 gene expression; only 20% of the anatomically successful patients showed the gene expression.

CONCLUSIONS

We reported three different conditions of retinal pigment epithelial cells based on gene expression for melanogenesis during dedifferentiation. The different condition of the retinal pigment epithelial cells may have some relationship to the anatomical results for proliferative vitreoretinopathy surgery.

Pigmented human skin equivalent--as a model of the mechanisms of control of cell-cell and cell-matrix interactions

The melanin pigment system in human skin is extraordinarly well developed and assures the photoprotection of the skin against harmful solar radiation. Specific cell-cell interactions between one melanocytes and keratinocytes play a fundamental role in the regulation of melanogenesis and melanin pigementation, the two key elements of this system, giving rise to the concept of a structural, functional collaborative 'epidermal melanin unit,' Early experiments strongly suggested that melanocyte growth and differentiation are regulated by paracrine factors from keratinocytes and other skin cells. In addition, co-culture studies with keratinocytes has shown that the extracellular matrix acts as a local environmental signal for dendrite formation and melanogenesis. Attempts to reconstruct pigmented human skin in vitro have made great progress over the last decade. The behavior of cells in these pigmented human skin equivalents closely resembles that in vivo, and the cells can still respond to appropriate extrinsic regulatory stimuli such as ultraviolet radiation. Keratinocytes and fibroblasts have been shown to be active partners in the regulation of melanocyte distribution, viability and other differentiation functions, presumably by direct contact and the effects of various soluble paracrine factors. By reproducing cell-cell and cell-matrix interactions, these culture systems provide a promising experimental model for investigating regulation of the skin pigmentary system and the role of photoprotection against harmful solar radiation.

Sorting and secretion of a melanosome membrane protein, gp75/TRP1

The melanosome is an organelle specialized for melanin synthesis that is derived from the endocytic pathway. Several melanosome membrane proteins have been identified, forming a family of proteins known as tyrosinase-related proteins. Two members of this family, tyrosinase and gp75, are well-characterized melanocyte differentiation antigens. Our previous studies have shown that gp75, the mouse brown locus protein, is sorted to melanosomes along the endocytic pathway, directed by a hexapeptide sorting signal located in the cytoplasmic tail. In this study, we report the unexpected finding that a portion of gp75 is secreted. Substantial levels of secretory gp75 were detected in melanocytic cells. Cell surface expression of gp75 was also detected, representing 2% of cellular gp75. Characterization of secretory gp75 cells showed that it is: (i) a truncated form that lacks the transmembrane region, the cytoplasmic tail where the endosomal sorting signal is located, and a small portion of the lumenal domain; (ii) more extensively glycosylated than endocytic/melanosomal gp75, containing trans-Golgi processed sugar residues; and (iii) generated post-translationally in an acid sensitive compartment after processing in the trans-Golgi, and secreted rapidly after generation. Thus, these endocytic/melanosomal membrane proteins can be processed to abundant secretory forms, probably in an endocytic compartment through a potentially novel secretory pathway.

Pigmented human skin equivalent: new method of reconstitution by grafting an epithelial sheet onto a non-contractile dermal equivalent

We have established a new protocol for reconstituting a pigmented human skin equivalent (PSE) and have evaluated its functional responses to environmental stimulus, UVB. The PSE is reconstituted by grafting an epithelial sheet consisting of keratinocytes and melanocytes onto a porous non-contractile dermal equivalent populated with mitotically and metabolically active fibroblasts. i) The PSE has a multilayered, well-differentiated epidermis with cuboidal basal cells and highly organised dermis with newly synthesised extracellular matrix components. ii) Ki67-positive proliferating keratinocytes (18.1 +/- 7.4%) were detected on the basal layer of the epidermis. iii) Melanocytes located exclusively within the basal layer were detected by monoclonal antibody against tyrosinase-related protein (TRP-1). iv) After exposure to UVB (100 mJ/cm2 per day) for 7 consecutive days, the intensity of TRP-1 staining was increased in the PSE, showing their functional state, whereas the number of melanocytes was not changed. This non-contractile and functioning new PSE is potentially useful as a model for studying the role of melanocyte-keratinocyte-fibroblast interactions in photoprotection of the skin in more complex cutaneous microenvironment than monolayer culture, and for developing in vitro disease models and therapeutic protocols with genetically altered cells both in epidermis and dermis.

Tumor selectivity and transcriptional activation by azelaic bishydroxamic acid in human melanocytic cells

Azelaic bishydroxamic acid (ABHA), a potent differentiating agent for lymphoid cells, was selectively toxic for 5 human tumor cell lines and transformed human melanocytes and keratinocytes (dose for 37% survival, D37, 30-100 microg/mL) compared with normal cells (melanocytes, fibroblasts; D37 > 300 microg/mL). Dendritic morphology was the only indicator found for increased differentiation, markers for the pigmentation pathway being unchanged or inhibited by ABHA. In contrast to hexamethylene bisacetamide and azelaic acid, ABHA significantly increased the HIV LTR, SV40 and c-fos promoter activities during a 24 hr treatment. Metallothionein promoter activity was enhanced by 5 hr treatment with ABHA in a sensitive melanoma cell line (MM96L) but was inhibited in a more resistant line (HeLa); c-fos promoter activity was inhibited in HeLa during this time. Transcription from a p53 binding response element was inhibited in MM96L by a 24 hr ABHA treatment but enhanced in HeLa. ABHA may represent a structural prototype for designing more potent and selective anti-melanoma agents.

Use of a cDNA microarray to analyse gene expression patterns in human cancer

The development and progression of cancer and the experimental reversal of tumorigenicity are accompanied by complex changes in patterns of gene expression. Microarrays of cDNA provide a powerful tool for studying these complex phenomena. The tumorigenic properties of a human melanoma cell line, UACC-903, can be suppressed by introduction of a normal human chromosome 6, resulting in a reduction of growth rate, restoration of contact inhibition, and suppression of both soft agar clonogenicity and tumorigenicity in nude mice. We used a high density microarray of 1,161 DNA elements to search for differences in gene expression associated with tumour suppression in this system. Fluorescent probes for hybridization were derived from two sources of cellular mRNA [UACC-903 and UACC-903(+6)] which were labelled with different fluors to provide a direct and internally controlled comparison of the mRNA levels corresponding to each arrayed gene. The fluorescence signals representing hybridization to each arrayed gene were analysed to determine the relative abundance in the two samples of mRNAs corresponding to each gene. Previously unrecognized alterations in the expression of specific genes provide leads for further investigation of the genetic basis of the tumorigenic phenotype of these cells.