The tyrosinase-related protein-1 gene has a structure and promoter sequence very different from tyrosinase

On the Metal Cofactor in the Tyrosinase Family

The production of pigment in mammalian melanocytes requires the contribution of at least three melanogenic enzymes, tyrosinase and two other accessory enzymes called the tyrosinase-related proteins (Trp1 and Trp2), which regulate the type and amount of melanin. The last two proteins are paralogues to tyrosinase, and they appeared late in evolution by triplication of the tyrosinase gene. Tyrosinase is a copper-enzyme, and Trp2 is a zinc-enzyme. Trp1 has been more elusive, and the direct identification of its metal cofactor has never been achieved. However, due to its enzymatic activity and similarities with tyrosinase, it has been assumed as a copper-enzyme. Recently, recombinant human tyrosinase and Trp1 have been expressed in enough amounts to achieve for the first time their crystallization. Unexpectedly, it has been found that Trp1 contains a couple of Zn(II) at the active site. This review discusses data about the metal cofactor of tyrosinase and Trps. It points out differences in the studied models, and it proposes some possible points accounting for the apparent discrepancies currently appearing. Moreover, some proposals about the possible flexibility of the tyrosinase family to uptake copper or zinc are discussed.

LEF-1 Regulates Tyrosinase Gene Transcription In Vitro

TYR, DCT and MITF are three important genes involved in maintaining the mature phenotype and producing melanin; they therefore participate in neural crest cell development into melanocytes. Previous studies have revealed that the Wnt signaling factor lymphoid enhancer-binding factor (LEF-1) can enhance DCT and MITF gene expression. However, whether LEF-1 also affects TYR gene expression remains unclear. In the present study, we found that LEF-1 regulated TYR transcription in vitro. LEF-1 overexpression increased TYR gene promoter activity, whereas LEF-1 knockdown by RNA interference significantly decreased TYR expression. Moreover, the core GTTTGAT sequence (-56 to -50) within the TYR promoter is essential for the effect of LEF-1 on TYR expression, and chromatin immunoprecipitation (ChIP) assay indicated that endogenous LEF-1 interacts with the TYR promoter. In addition, we observed a synergistic transactivation of the TYR promoter by LEF-1 and MITF. These data suggest that Wnt signaling plays an important role in regulating melanocyte development and differentiation.

Mechanistic studies of anti-hyperpigmentary compounds: elucidating their inhibitory and regulatory actions

Searching for depigmenting agents from natural sources has become a new direction in the cosmetic industry as natural products are generally perceived as relatively safer. In our previous study, selected Chinese medicines traditionally used to treat hyperpigmentation were tested for anti-hyperpigmentary effects using a melan-a cell culture model. Among the tested chemical compounds, 4-ethylresorcinol, 4-ethylphenol and 1-tetradecanol were found to possess hypopigmentary effects. Western blot analysis, reverse transcriptase polymerase chain reaction (RT-PCR), cyclic adenosine monophosphate (cAMP) assay, protein kinase A (PKA) activity assay, tyrosinase inhibition assay and lipid peroxidation inhibition assay were performed to reveal the underlying cellular and molecular mechanisms of the hypopigmentary effects. 4-Ethylresorcinol and 4-ethylphenol attenuated mRNA and protein expression of tyrosinase-related protein (TRP)-2, and possessed antioxidative effect by inhibiting lipid peroxidation. 1-Tetradecanol was able to attenuate protein expression of tyrosinase. The hypopigmentary actions of 4-ethylresorcinol, 4-ethylphenol and 1-tetradecanol were associated with regulating downstream proteins along the PKA pathway. 4-Ethylresorcinol was more effective in inhibiting melanin synthesis when compared to 4-ethylphenol and 1-tetradecanol.

Activation of MITF by Argan Oil Leads to the Inhibition of the Tyrosinase and Dopachrome Tautomerase Expressions in B16 Murine Melanoma Cells

Argan (Argania spinosa L.) oil has been used for centuries in Morocco as cosmetic oil to maintain a fair complexion and to cure skin pimples and chicken pox pustules scars. Although it is popular, the scientific basis for its effect on the skin has not yet been established. Here, the melanogenesis regulatory effect of argan oil was evaluated using B16 murine melanoma cells. Results of melanin assay using B16 cells treated with different concentrations of argan oil showed a dose-dependent decrease in melanin content. Western blot results showed that the expression levels of tyrosinase (TYR), tyrosinase-related protein 1 (TRP1), and dopachrome tautomerase (DCT) proteins were decreased. In addition, there was an increase in the activation of MITF and ERK1/2. Real-time PCR results revealed a downregulation of Tyr, Trp1, Dct, and Mitf mRNA expressions. Argan oil treatment causes MITF phosphorylation which subsequently inhibited the transcription of melanogenic enzymes, TYR and DCT. The inhibitory effect of argan oil on melanin biosynthesis may be attributed to tocopherols as well as the synergistic effect of its components. The results of this study provide the scientific basis for the traditionally established benefits of argan oil and present its therapeutic potential against hyperpigmentation disorders.

New insights into the active site structure and catalytic mechanism of tyrosinase and its related proteins

Tyrosinases are widely distributed in nature. They are copper-containing oxidases belonging to the type 3 copper protein family, together with catechol oxidases and haemocyanins. Tyrosinases are essential enzymes in melanin biosynthesis and therefore responsible for pigmentation of skin and hair in mammals, where two more enzymes, the tyrosinase-related proteins (Tyrps), participate in the pathway. The structure and catalytic mechanism of mammalian tyrosinases have been extensively studied but they are not completely understood because of the lack of information on the tertiary structure. The availability of crystallographic data of one plant catechol oxidase and one bacterial tyrosinase has improved the model of the three-dimensional structure of the active site of the enzyme. Furthermore, sequence comparison of tyrosinase and the Tyrps reveals that the three orthologue proteins share many key structural features, because of their common origin from an ancestral gene, although the specific residues responsible for their different catalytic capabilities have not been identified yet. This review summarizes our current knowledge of tyrosinase and Tyrps structure and function and describes the catalytic mechanism of tyrosinase and Dct/Tyrp2, which are better characterized.

The tyrosinase enhancer is activated by Sox10 and Mitf in mouse melanocytes

The terminal differentiation of melanocytes is associated with the transcriptional activation of genes responsible for pigment production such as tyrosinase. Pigment cell-specific transcription factors, such as Mitf, as well as specific proximal and distal regulatory elements (DRE) are implicated in the tight control of tyrosinase expression during development and adulthood. Proper tyrosinase expression in melanocytes depends upon the presence of a DRE that is located at -15 kb and provides enhancer activity via a central element termed core-enhancer. In this report, we show that the transcription factors Sox10, Mitf and USF-1 are able to activate the core-enhancer in luciferase reporter assays. Comparative sequence analysis identified evolutionarily motifs resembling Sox10 binding sites that were required for full enhancer activity in melanoma cells and in tyrosinase::lacZ transgenic mice. Sox10 was able to bind the DRE in vitro and mutation of the conserved motifs abolished the enhancer transactivation mediated by Sox10. In addition, two highly conserved CAGCTG E-box motifs were identified that were also required for enhancer activity and for transactivation by Mitf. The results suggest that Sox10 directly, and Mitf, most likely indirectly, activate the tyrosinase enhancer, underlining the contribution of Sox10 to tyrosinase gene regulation in melanocytes.

A conserved transcriptional enhancer that specifies Tyrp1 expression to melanocytes

Pigment cells of mammals originate from two different lineages: melanocytes arise from the neural crest, whereas cells of the retinal pigment epithelium (RPE) originate from the optic cup of the developing forebrain. Previous studies have suggested that pigmentation genes are controlled by different regulatory networks in melanocytes and RPE. The promoter of the tyrosinase-related family gene Tyrp1 has been shown to drive detectable transgene expression only to the RPE, even though the gene is also expressed in melanocytes as evident from Tyrp1-mutant mice. This indicates that the regulatory elements responsible for Tyrp1 gene expression in the RPE are not sufficient for expression in melanocytes. We thus searched for a putative melanocyte-specific regulatory sequence and demonstrate that a bacterial artificial chromosome (BAC) containing the Tyrp1 gene and surrounding sequences is able to target transgenic expression to melanocytes and to rescue the Tyrp1b (brown) phenotype. This BAC contains several highly conserved non-coding sequences that might represent novel regulatory elements. We further focused on a sequence located at -15 kb, which we identified as a melanocyte-specific enhancer as shown by cell culture and transgenic mice experiments. In addition, we show that the transcription factor Sox10 can activate this conserved enhancer. The presence of a distal Tyrp1 regulatory element, which specifies melanocyte-specific expression, supports the idea that separate regulatory sequences can mediate differential gene expression in melanocytes and RPE.

Genomic anatomy of the Tyrp1 (brown) deletion complex

Chromosome deletions in the mouse have proven invaluable in the dissection of gene function. The brown deletion complex comprises >28 independent genome rearrangements, which have been used to identify several functional loci on chromosome 4 required for normal embryonic and postnatal development. We have constructed a 172-bacterial artificial chromosome contig that spans this 22-megabase (Mb) interval and have produced a contiguous, finished, and manually annotated sequence from these clones. The deletion complex is strikingly gene-poor, containing only 52 protein-coding genes (of which only 39 are supported by human homologues) and has several further notable genomic features, including several segments of >1 Mb, apparently devoid of a coding sequence. We have used sequence polymorphisms to finely map the deletion breakpoints and identify strong candidate genes for the known phenotypes that map to this region, including three lethal loci (l4Rn1, l4Rn2, and l4Rn3) and the fitness mutant brown-associated fitness (baf). We have also characterized misexpression of the basonuclin homologue, Bnc2, associated with the inversion-mediated coat color mutant white-based brown (B(w)). This study provides a molecular insight into the basis of several characterized mouse mutants, which will allow further dissection of this region by targeted or chemical mutagenesis.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Pigment cell lineage-specific expression activity of the ascidian tyrosinase-related gene

Solitary ascidian tadpole larvae develop two types of black pigment cells in the major sensory organs of the brain. Such pigment cells have been demonstrated to express the melanogenic genes, tyrosinase and Tyrp/TRP (tyrosinase-related protein). To understand the genetic and developmental mechanisms underlying the differentiation of chordate pigment cells, we examined the function of the promoter region of Tyrp/TRP gene, an ascidian (Halocynthia roretzi) tyrosinase family gene. The expression of the gene in pigment cell lineage starts at the early-mid gastrula stages. To identify the transcriptional regulatory region of the gene allowing cell-type-specific expression, a deletion series of the HrTyrp 5' flanking region fused to a lacZ reporter gene was constructed and microinjected into ascidian fertilized eggs. The region of 73 bp in HrTyrp was identified as sufficient for expression in pigment cell-precursors of tailbud stage embryos. It is noteworthy that there is no M-box element highly conserved in the promoters for vertebrate tyrosinase family genes such as tyrosinase, Tyrp1/TRP-1 and Tyrp2/TRP-2 (Dct). Although the regulatory system of ascidian pigment-cell development is likely to contain most factors critical to vertebrate pigment-cell development, there might be critical differences in the mode of regulation, such as the developmental timing of interactions of factors, proteins and genes, involved in pigment cell differentiation and pigmentation.

Isolation and developmental expression of tyrosinase family genes in Xenopus laevis

The tyrosinase family of genes in vertebrates consists of three related members encoding melanogenic enzymes, tyrosinase (Tyr), tyrosinase-related protein-1 (TRP-1, Tyrp1) and tyrosinase-related protein-2 (Dct, TRP-2, Tyrp2). These proteins catalyze melanin production in pigment cells and play important roles in determining vertebrate coloration. This is the first report examining melanogenic gene expression in pigment cells during embryonic development of amphibians. Xenopus provides a useful experimental system for analyzing molecular mechanisms of pigment cells. However, in this animal little information is available not only about the developmental expression but also about the isolation of pigmentation genes. In this study, we isolated homologues of Tyr, Tyrp1 and Dct in Xenopus laevis (XlTyr, XlTyrp1, and XlDct). We studied their expression during development using in situ hybridization and found that all of them are expressed in neural crest-derived melanophores, most of which migrate through the medial pathway, and in the developing diencephalon-derived retinal pigment epithelium (RPE). Further, XlDct was expressed earlier than XlTyr and XlTyrp1, which suggests that XlDct is the most suitable marker gene for melanin-producing cells among them. XlDct expression was detected in migratory melanoblasts and in the unpigmented RPE. In addition, the expression of XlDct was detected in the pineal organ. The sum of these studies suggests that expression of the tyrosinase family of genes is conserved in pigment cells of amphibians and that using XlDct as a marker gene for pigment cells will allow further study of the developmental mechanisms of pigment cell differentiation using Xenopus.

Combination of overlapping bacterial artificial chromosomes by a two-step recombinogenic engineering method

Recombinogenic engineering or recombineering is a powerful new method to engineer DNA without the need for restriction enzymes or ligases. We report here a general method for using recombineering to combine overlapping bacterial artificial chromosomes (BACs) to build larger, unified BACs. In order to test the feasibility of using recombineering to combine two large DNA fragments (>20 kb), we constructed a unified BAC containing the full-length tyrosinase-related protein-1 (Tyrp-1) gene from two library-derived BACs, one containing the 5' regulatory elements and the other containing the 3' coding exons. This was achieved using a two-step homologous recombination method enabled by the bacteriophage lambda Red proteins. In the first step, retrieval, a large DNA fragment (approximately 22 kb) was retrieved from one of the original BACs. In the second step, recombination, the retrieved DNA fragment was inserted into the second original BAC to form the unified BAC containing all the desired Tyrp-1 sequence. To further demonstrate the general applicability of our approach, an additional DNA fragment (approximately 20 kb) was inserted into the unified BAC downstream of the coding region. This method should prove very useful for enabling BAC manipulation in a variety of scenarios.

TYRP1 is associated with dun coat colour in Dexter cattle or how now brown cow?

Tyrosinase related protein 1 (TYRP1), which is involved in the coat colour pathway, was mapped to BTA8 between microsatellites BL1080 and BM4006, using a microsatellite in intron 5 of TYRP1. The complete coding sequence of bovine TYRP1 was determined from cDNA derived from skin biopsies of cattle with various colours. Sequence data from exons 2-8 from cattle with diluted phenotypes was compared with that from non-diluted phenotypes. In addition, full-sib families of beef cattle generated by embryo transfer and half-sib families from traditional matings in which coat colour was segregating were used to correlate TYRP1 sequence variants with dilute coat colours. Two non-conservative amino acid changes were detected in Simmental, Charolais and Galloway cattle but these polymorphisms were not associated with diluted shades of black or red, nor with the dun coat colour of Galloway cattle or the taupe brown colour of Braunvieh and Brown Swiss cattle. However, in Dexter cattle all 25 cattle with a dun brown coat colour were homozygous for a H424Y change. One Dexter that was also homozygous Y434 was red because of an "E+/E+" genotype at MC1R which lead to the production of only phaeomelanin. None of the 70 remaining black or red Dexter cattle were homozygous for Y434. This tyrosine mutation was not found in any of the 121 cattle of other breeds that were examined.

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.

Genomic structure and evolutionary conservation of the tyrosinase gene family from Fugu

The tyrosinase gene family encompasses three members, tyrosinase, tyrosinase-related protein 1 (Tyrp1) and dopachrome tautomerase (Dct), which encode for proteins implicated in melanin synthesis. In human and mouse, genomic organization is known for all three genes, revealing common features of regulatory elements and of exon/intron structure. We have set out to identify the complete family from a more primitive vertebrate, the pufferfish Fugu (Takifugu rubripes), which is characterized by a compact genome. We had recently isolated and characterized the Fugu tyrosinase gene (Genesis 28 (2000) 99-105). We now report the isolation and characterization of the two other members of the family, Tyrp1 and Dct. Regulatory sequences from these genes function in mouse pigment cells and are able to mediate reporter gene expression. Our results demonstrate the existence of all three tyrosinase family members in teleosts and underline the evolutionary conservation of the pigmentary system.

Functional conservation of the promoter regions of vertebrate tyrosinase genes

Tyrosinase is the key enzyme for synthesizing melanin pigments, which primarily determine mammalian skin coloration. Considering the important roles of pigments in the evolution and the adaptation of vertebrates, phylogenetic changes in the coding and flanking regulatory sequences of the tyrosinase gene are particularly intriguing. We have now cloned cDNA encoding tyrosinase from Japanese quail and snapping turtle. These nonmammalian cDNA are highly homologous to those of the mouse and human tyrosinases, whereas the 5' flanking sequences are far less conserved except for a few short sequence motifs. Nevertheless, we demonstrate that the 5' flanking sequences from the quail or turtle tyrosinase genes are capable of directing the expression of a fused mouse tyrosinase cDNA when introduced into cultured mouse albino melanocytes. This experimental method, which reveals the functional conservation of regulatory sequences in one cell type (the melanocyte), may be utilized to evaluate phylogenetic differences in mechanisms controlling specific gene expression in many other types of cells. We also provide evidence that the 5' flanking sequences from these nonmammalian genes are functional in vivo by producing transgenic mice. Phylogenetic changes of vertebrate tyrosinase promoters and the possible involvement of conserved sequence motifs in melanocyte-specific expression of tyrosinase are discussed.

The Fugu rubripes tyrosinase gene promoter targets transgene expression to pigment cells in the mouse

The regulation of the mouse tyrosinase gene expression is controlled by a highly conserved element at -100 bp, the M-box, and an enhancer at -12 kb. In most vertebrates, the length of intergenic sequences makes it difficult to analyze the whole gene and the complete regulatory region. We took advantage of the compact Fugu genome to identify regulatory regions involved in pigment cell-specific expression. We isolated the Fugu tyrosinase gene, and identified putative cis-acting regulatory elements within the promoter. We then asked whether the Fugu promoter sequence functions in mouse pigment cells. We showed that E11.5 transgenic embryos bearing 6 kb or 3 kb of Fugu tyrosinase 5' sequence fused to the reporter gene lacZ revealed melanoblast and RPE-specific expression. This is the first evidence that the tyrosinase promoter is active at midgestation in melanoblasts, long before the onset of pigmentation.

Microphthalmic Mice Display a B Cell Deficiency Similar to that Seen for Mast and NK Cells

The microphthalmic mouse (mi) possesses a 3-bp deletion of the Mi gene that alters the DNA binding site of the transcription factor gene product. This animal has diminished numbers of NK and mast cells (MC) and is osteopetrotic due to a lack of the normal complement of functional osteoclasts. The reduction of MC has been proposed to be due to the lack of adequate c-Kit expression that is required for MC differentiation. However, data from other labs has questioned this interpretation. In this report, we present data suggesting bone marrow-derived deficiencies of the mi mouse are not due to a lack of c-Kit expression and function, but instead due to an inhospitable environment within the bone marrow itself. Specifically, we have found that such animals also lack virtually all B cell precursors within the marrow and rely upon other lymphatic sites, such as the spleen, for B cell development and maturation. Although the animal has depressed numbers of NK cells, B cells, and MC, it still possesses a normal thymus and peripheral T cells. Therefore, the block in cellular differentiation must be within the marrow environment, which is essential for maturing B cells, NK cells, and MC but not T cells.

Tyrosinase stabilization by Tyrp1 (the brown locus protein)

Mammalian melanogenesis is regulated directly or indirectly by over 85 distinct loci. The Tyr/albino locus, in which mutations cause a lack of pigmentation, encodes tyrosinase (Tyr), the critical and rate-limiting melanogenic enzyme. Other melanogenic enzymes include Tyrp1 (or TRP1) and 3,4-dihydroxyphenylalanine-chrome tautomerase (Dct or TRP2) encoded at the Tyrp1/brown and Dct/slaty loci, respectively. Murine Tyrp1 can oxidize 5, 6-dihydroxyindole-2-carboxylic acid (DHICA) produced by Dct, but mutations in Tyrp1 also affect the catalytic functions of Tyr. All three enzymes are membrane-bound melanosomal proteins with similar structural features and are thought to interact within and stabilize a melanogenic complex. We have now further investigated the effect of a Tyrp1(b) mutation on Tyr stability. Pulse/chase labeling experiments show that Tyr is degraded more quickly in Tyrp1(b) mutant melanocytes than in melanocytes wild type at that locus. This reduced stability of Tyr can be partly rescued by infection with the wild type Tyrp1 gene, and this is accompanied by phenotypic rescue of infected melanocytes. In sum, these results suggest that, in addition to its catalytic function in oxidizing DHICA, Tyrp1 may play an important role in stabilizing Tyr, a second potential role in the regulation of melanin formation.

The rodent alphaA-crystallin gene: mutagenesis of a non-consensus 5'-splice site to study alternative splicing in vivo

alphaA-Crystallin is a member of the small heat shock protein family that is abundantly expressed as a structural component in the vertebrate eye lens. In lenses of rodents and some other mammals, there occurs a minor variant of alphaA-crystallin, which has an insertion of 23 amino acid residues. This variant, alphaA(ins)-crystallin, results from differential integration of an optional exon into a small fraction of the mRNA. We have studied whether this alternative splicing is caused by a non-consensus cytosine in the 5' splice site adjacent to the optional exon. After replacement of the aberrant cytosine in the hamster alphaA-crystallin gene by a consensus thymine, and transient transfection of this gene in Chinese Hamster Ovary cells, the optional exon is indeed almost completely spliced into the mature mRNA. In contrast, replacement of the cytosine by adenine or guanine completely abolishes the splicing of the optional exon. Our results confirm that alternative splicing of the alphaA-crystallin primary transcript is mainly due to a non-consensus 5' splice site nucleotide. However, we conclude that the small size of the optional exon is probably an additional contributing factor and therefore it seems that the splicing mechanism is based on recognition of exons rather than introns.

Characterization of the murine fatty acid transport protein gene and its insulin response sequence

Fatty acid transport protein (FATP) was identified by expression cloning strategies (Schaffer, J. E., and Lodish, H. F. (1994) Cell 79, 427-436) and shown by transfection analysis to catalyze the transfer of long-chain fatty acids across the plasma membrane of cells. It is expressed highly in tissues exhibiting rapid fatty acid metabolism such as skeletal muscle, heart, and adipose. FATP mRNA levels are down-regulated by insulin in cultured 3T3-L1 adipocytes and up-regulated by nutrient depletion in murine adipose tissue (Man, M. Z., Hui, T. Y., Schaffer, J. E., Lodish, H. F., and Bernlohr, D. A. (1996) Mol. Endocrinol. 10, 1021-1028). To determine the molecular mechanism of insulin regulation of FATP transcription, we have isolated the murine FATP gene and its 5'-flanking sequences. The FATP gene spans approximately 16 kilobases and contains 13 exons, of which exon 2 is alternatively spliced. S1 nuclease and RNase protection assays revealed the presence of multiple transcription start sites; the DNA sequence upstream of the predominant transcription start sites lacks a typical TATA box. By transient transfection assays in 3T3-L1 adipocytes, the inhibitory action of insulin on FATP transcription was localized to a cis-acting element with the sequence 5'-TGTTTTC-3' from -1347 to -1353. This sequence is very similar to the insulin response sequence found in the regulatory region of other genes negatively regulated by insulin such as those encoding phosphoenolpyruvate carboxykinase, tyrosine aminotransferase, and insulin-like growth factor-binding protein 1. Fluorescence in situ hybridization analysis revealed that the murine FATP gene is localized to chromosome 8, band 8B3.3. Interestingly, this region of chromosome 8 contains a cluster of three other genes important for fatty acid homeostasis, lipoprotein lipase, the mitochondrial uncoupling protein 1 (UCP1) and sterol regulatory element-binding protein 1. These results characterize the murine FATP gene and its insulin responsiveness as well as present a framework for future studies of its role in lipid metabolism, obesity, and type II diabetes mellitus.

Brachyury-related transcription factor Tbx2 and repression of the melanocyte-specific TRP-1 promoter

Previous work has demonstrated that two key melanocyte-specific elements termed the MSEu and MSEi play critical roles in the expression of the melanocyte-specific tyrosinase-related protein 1 (TRP-1) promoter. Both the MSEu and MSEi, located at position -237 and at the initiator, respectively, bind a melanocyte-specific factor termed MSF but are also recognized by a previously uncharacterized repressor, since mutations affecting either of these elements result in strong up-regulation of TRP-1 promoter activity in melanoma cells. Here we demonstrate that repression mediated by the MSEu and MSEi also operates in melanocytes. We also report that both the MSEu and MSEi are recognized by the brachyury-related transcription factor Tbx2, a member of the recently described T-box family, and that Tbx2 is expressed in melanocyte and melanoblast cell lines but not in melanoblast precursor cells. Although Tbx2 and MSF each recognize the TRP-1 MSEu and MSEi motifs, it is binding by Tbx-2, not binding by MSF, that correlates with repression. Several lines of evidence tend to point to the brachyury-related transcription factor Tbx2 as being the repressor of TRP-1 expression: both the MSEu and MSEi bind Tbx2, and mutations in either element that result in derepression of the TRP-1 promoter diminish binding by Tbx2; the TRP-1 promoter, but not the tyrosinase, microphthalmia, or glyceraldehyde-3-phosphate dehydrogenase (G3PDH) promoter, is repressed by Tbx2 in cotransfection assays; a high-affinity consensus brachyury/Tbx2-binding site is able to constitutively repress expression of the heterologous IE110 promoter; and a low-affinity brachyury/Tbx2 binding site is able to mediate Tbx2-dependent repression of the G3PDH promoter. Although we cannot rule out the presence of an additional, as yet unidentified factor playing a role in the negative regulation of TRP-1 in vivo, the evidence presented here suggests that Tbx2 most likely is the previously unidentified repressor of TRP-1 expression and as such is likely to represent the first example of transcriptional repression by a T-box family member.

Expression of tyrosinase and the tyrosinase related proteins in the Mitfvit (vitiligo) mouse eye: implications for the function of the microphthalmia transcription factor

Mitf (Microphthalmia transcription factor), a basic-helix-loop-helix zipper protein, encoded at the microphthalmia (Mitf) locus, regulates the transcription of the gene encoding tyrosinase, the rate-limiting enzyme in melanin biosynthesis, by binding the DNA sequence CATGTG. This binding site is present also in the genes encoding two tyrosinase related proteins, TRP-1 and TRP-2. To gain insight into the function of Mitf in vivo, we determined whether there was a difference in the levels of these proteins in the RPE/choroid of the vitiligo (Mitfvit) mouse, in which there is a mutation of the Mitf gene. This mouse has alteration of RPE pigmentation and function that presumably leads to slow progressive loss of photoreceptor cells. The RPE/choroid was dissected from eyes of vitiligo and C57BL/6 wild-type mice at postnatal ages 2, 4, 7, 10, 14, 21 and 42 days. Extracts of pooled tissues were subjected to electrophoresis and immunoblotting. The levels of tyrosinase, TRP-1 and TRP-2 were determined densitometrically following immunodetection with rabbit antipeptide antisera. In addition, the tyrosine hydroxylase activity of tyrosinase as assayed radiometrically. Levels of TRP-1 were 3-7 fold greater in control RPE/choroid compared with mutants. This marked difference in protein level was observed at the earliest age examined (P2) and persisted throughout the first two weeks. Tyrosinase levels in mutants were similar to controls at P2 and P4, but were reduced at P10 and beyond. Tyrosinase activity was diminished also in mutants by P10. Levels of TRP-2 were similar between mutants and controls, although the typical decrease seen in controls after P14 was attenuated in the mutant mice. There is a significant reduction in the level of TRP-1 in the RPE/choroid of the Mitfvit mouse. The data suggests that transcription of the gene encoding TRP-1 is extremely dependent upon functional Mitf. It provides in vivo evidence that Mitf regulates the transcription of the gene encoding TRP-1 as well as tyrosinase.

Complete sequence and polymorphism study of the human TYRP1 gene encoding tyrosinase-related protein 1

The complete 24,667 nucleotide sequence spanning the human TYRP1 gene has been determined from the inserts of two overlapping lambda clones. A LINE-1 repeat element is immediately adjacent to and may demarcate the immediate 5' promoter region of the gene. A search for polymorphism within the seven TYRP1 coding exons has been performed by an RNase mismatch detection procedure. Analysis of the TYRP1 gene in 100 Caucasian individuals of varying hair color has found no amino acid sequence variation nor revealed any hemizygous mutant allele in the hypopigmented phenotype of two 9p- syndrome patients.

Acquisition and amplification of a testis-expressed autosomal gene, SSL, by the Drosophila Y chromosome

The acquisition of autosomal fertility genes has been proposed to be an important process in human Y chromosome evolution. For example, the Y-linked fertility factor DAZ (Deleted in Azoospermia) appears to have arisen after the transposition and tandem amplification of the autosomal DAZH gene. The Drosophila melanogaster Y chromosome contains tandemly repeated Su(Ste) units that are thought to affect male fertility as suppressors of the homologous X-linked Stellate repeats. Here we report the detection of a testis-expressed autosomal gene, SSL [Su(Ste)-like], that appears to be an ancestor of the Y-linked Su(Ste) units. SSL encodes a casein kinase 2 (CK2) beta-subunit-like protein. Its putative ORF shares extensive (45%) homology with the genuine beta-subunit of CK2 and retains the conserved C-terminal and Glu/Asp-rich domains that are essential for CK2 holoenzyme regulation. SSL maps within region 60D1-2 of D. melanogaster and D. simulans polytene chromosomes. We present evidence that SSL was derived from the genuine betaCK2 gene by reverse transcription. This event resulted in the loss of the first three introns in the coding region of the SSL ancestor gene. Evolutionary analysis indicates that SSL has evolved under selective pressure at the translational level. Its sequence, especially in the 3' region, is much closer to the Y-linked Su(Ste) tandem repeats than to the betaCK2 gene. These results suggest that the acquisition of testis-specific autosomal genes may be important for the evolution of Drosophila as well as human Y chromosomes.

Regulation of the tyrosinase promoter in transgenic mice: expression of a tyrosinase-lacZ fusion gene in embryonic and adult brain

The enzyme tyrosinase is indispensable for pigmentation and the gene is expressed mainly in pigment cells. Regulatory elements, at -12 to -15 kb (enhancer) and within the 270 bp directly upstream of the transcription start site, have been described recently and their importance demonstrated in transgenic experiments. We were interested in tyrosinase promoter activity during development and used beta-galactosidase as reporter gene. Transgenic mice were generated carrying a tyrosinase-lacZ fusion gene, containing 6.1 kb of tyrosinase 5' sequences. In transgenic embryos, beta-galactosidase activity was detected along the entire neural tube, with the most prominent expression in the developing telencephalon, and also in the adult brain. Equivalent expression was observed in the developing retina. Tyrosinase protein was identified in embryonic and adult brain, but no DOPAoxidase or tyrosine hydroxylase activity was detected. From our results we conclude that 1) tyrosinase protein is present in embryonic and adult mouse brain and 2) the tyrosinase promoter can direct expression of a reporter gene to pigment cells and neural tissues.

Ascidian tyrosinase gene: its unique structure and expression in the developing brain

Tadpole larvae of ascidians have two sensory pigment cells in the brain. One is the otolith cell that functions as a gravity receptor, the other pigment cell is part of a primitive photosensory structure termed the ocellus. These sensory cells, like vertebrate pigment cells, contain membrane-bounded melanin granules and are considered to reflect a crucial position in the evolutionary process of this cell type. To investigate the molecular changes accompanying the evolution of pigment cells, we have isolated from Halocynthia roretzi a gene encoding tyrosinase, a key enzyme in melanin biosynthesis. The cDNA has an open reading frame (ORF) of 596 amino acids, which is 36-39% identical in amino acid sequence to vertebrate tyrosinases. In addition, the sequence analysis of both cDNA and genomic clones reveals an unusual organization of the tyrosinase gene, an extraordinary 3' untranslated region of the transcripts with significant homology to the coding sequence, and a single short intron in the sequence encoding a cytoplasmic domain. Expression of the gene is detected first in two pigment precursor cells positioned in the neural plate of early neurulae, and later in two melanin-containing pigment cells within the brain of late tailbud embryos. Its expression pattern correlates well with the appearance of tyrosinase enzyme activity in the developing brain. These results provide the first description of pigment cell differentiation at the molecular level in the ascidian embryo, and also will contribute to a better understanding of the evolution of chordate pigment cells.

Tyrosinase and related proteins in mammalian pigmentation

Tyrosinase is the key enzyme in pigment synthesis, initiating a cascade of reactions which convert the amino acid tyrosine to the melanin biopolymer. Two other tyrosinase-related proteins (TRP) are known, TRP-1 (probably DHICAoxidase) and TRP-2 (DOPAchrome tautomerase). These proteins show about 40% homology, and recent results have indicated that the genes might be derived from a common ancestor. We will discuss recent findings on genomic organization, and on the proteins and their presumed function, which is important for eumelanin synthesis in mouse and man.

Incomplete expression of the tyrosinase gene family (tyrosinase, TRP-1, and TRP-2) in human malignant melanoma cells in vitro

Sequence analysis of two clones found repressed in melanoma cell lines in earlier studies showed 9F2 to be identical with the TRP-1 gene and 6F5 with TRP-2 containing a long untranslated 3' end. For further investigation of the expression of the tyrosinase gene family in normal and malignant melanocytic cells, a series of melanoma cell lines and of cultured melanocytes were analyzed by Northern blotting and by reverse transcriptase-polymerase chain reaction (RT-PCR). The Northern blots were probed with cDNA fragments specific for TRP-1, TRP-2, and tyrosinase, for nested tyrosinase-PCR the outer primers specified a 284 bp and the nested primers a 207 bp fragment. Investigations on 14 established melanoma cell lines grown in different media compared with seven normal human melanocyte (NHM) cultures revealed that all three pigment genes were expressed in NHM, whereas pigment gene expression was found repressed in nearly all melanoma cell lines and was completely absent in 4 of 14 specimen. In particular, tyrosinase and TRP-2 genes were found always to be expressed together, and TRP-1 mRNA alone was absent in four melanoma cell lines. Negativity of cultured melanoma cells for tyrosinase mRNA was confirmed by nested RT-PCR, and gene deletion was ruled out by genomic Southern blots. The gene expression seemed independent from the type of medium used for cultivation. These findings indicate repressed or lacking expression of pigment genes in melanoma cell lines, most likely due to regulatory mechanisms, and that differences may exist between tyrosinase and TRP-2 on one hand and TRP-1 on the other. Overall, it seemed that RT-PCR for tyrosinase has limited value for identifying melanoma cells in the peripheral blood of melanoma patients; TRP-1, TRP-2, and other, additional markers may be required.

The retinal pigmented epithelium is required for development and maintenance of the mouse neural retina

BACKGROUND

During development of the vertebrate eye, there is a series of reciprocal cellular interactions that determine the fate of the eye components. Although evidence from organ culture suggests that the retinal pigmented epithelium (RPE) organizes the laminar structure of the differentiated neural retina, no role has been identified for the RPE in early eye development, nor has the later function of RPE been demonstrated in vivo.

RESULTS

To investigate the role of RPE cells in eye development, we generated transgenic mice that carry the attenuated diphtheria toxin-A gene; this transgene was driven by the promoter of the gene encoding the tyrosinase-related protein-1, which is specifically expressed in pigment cells. Depending on the expression level of the transgene, the retinal epithelium was ablated before or after its differentiation into a pigmented cell layer. We show that an early ablation (embryonic day E10-11) resulted in disorganization of the retinal layer, immediate arrest of eye growth and subsequent eye resorption. A later ablation (E11.5-12.5) allowed the eye to be maintained during embryogenesis, but the laminar structure of the retina became disrupted by the end of gestation, the vitreous failed to accumulate the adults were anophthalmic or severely microphthalmic. In some microphthalmic eyes, a number of RPE cells escaped ablation and formed patches of pigmented cells; the laminar structure of the retina was maintained immediately adjacent to such pigmented areas but disrupted elsewhere. In both cases--early or late ablation of the RPE--the retina appears to be the primary affected tissue.

CONCLUSIONS

We conclude that presence of the RPE is required for the normal development of the eye in vivo. Its presence early in development is necessary for the correct morphogenesis of the neural retina. After the neural retina has started to differentiate, the RPE is still necessary, either directly or indirectly, to maintain the organization of the retinal lamina.

Changes in expression of putative antigens encoded by pigment genes in mouse melanomas at different stages of malignant progression

Cutaneous melanomas of Tyr-SV40E transgenic mice (mice whose transgene consists of the tyrosinase promoter fused to the coding regions of simian virus 40 early genes) strikingly resemble human melanomas in their development and progression. Unlike human melanomas, the mouse tumors all arise in genetically identical individuals, thereby better enabling expression of specific genes to be characterized in relation to advancing malignancy. The products of pigment genes are of particular interest because peptides derived from these proteins have been reported to function as autoantigens with immunotherapeutic potential in some melanoma patients. However, the diminished pigmentation characteristic of many advanced melanomas raises the possibility that some of the relevant products may no longer be expressed in the most malignant cells. We have therefore investigated the contributions of several pigment genes in melanotic vs. relatively amelanotic components of primary and metastatic mouse melanomas. The analyses reveal marked differences within and among tumors in levels of mRNAs and proteins encoded by the wild-type alleles at the albino, brown, slaty, and silver loci. Tyrosinase (the protein encoded by the albino locus) was most often either absent or undetectable as melanization declined. The protein encoded by the slaty locus (tyrosinase-related protein 2) was the only one of those tested that was clearly present in all the tumor samples. These results suggest that sole reliance on targeting tyrosinase-based antigens might selectively favor survival of more malignant cells, whereas targeting the ensemble of the antigens tested might contribute toward a more inclusive and effective antimelanoma strategy.

Melatonin antagonizes alpha-melanocyte-stimulating hormone enhancement of melanogenesis in mouse melanoma cells by blocking the hormone-induced accumulation of the c locus tyrosinase

Melatonin was found to have a small inhibitory effect on tyrosinase activity and a slight stimulatory action on dopachrome tautomerase activity in B16 mouse melanoma cells. These effects were time and dose dependent, with the maximal response being observed after 24-48 h treatment and at concentrations of melatonin higher than the physiologic levels of the circulating hormone. Although these effects on the melanogenic activities were modest, incubation of melanocytes with melatonin prior to the addition of the melanotropin mediated a dramatic inhibition of alpha-melanocyte-stimulating-hormone-(alpha-MSH)-induced melanogenesis. This inhibitory effect was evident at melatonin concentrations as low as 10 nM. Inhibition was nearly total at 0.1 mM melatonin, even at high concentrations of alpha-MSH (1 microM). The inhibitory effect of melatonin on alpha-MSH stimulation of melanogenesis was investigated. Melatonin appeared to act at least at two stages. Pharmacological concentrations of melatonin diminished the number of alpha-MSH receptors to about 75% of the control values without an apparent effect on receptor affinity, as determined by receptor-binding studies using 125I-[N-Leu4-D-Phe7]alpha-MSH as a probe. Physiological concentrations of melatonin also appeared to interfere with the intracellular events coupling increased cAMP levels and induction of the c locus tyrosinase, since it strongly inhibited the theophylline-mediated stimulation of melanogenesis. The inhibition of tyrosinase stimulation was higher in the microsomal than in the melanosomal fractions of cells which were treated with melatonin, then exposed to either alpha-MSH (1 microM) or theophylline (1 mM), suggesting that one of the main effects of melatonin might be inhibition of the induction of tyrosinase de novo synthesis.

Organization and sequence of the human P gene and identification of a new family of transport proteins

We have determined the structure, nucleotide sequence, and polymorphisms of the human P gene. Mutations of the P gene result in type II oculocutaneous albinism (OCA2) in humans and pink-eyed dilution (p) in mice. We find that the human P gene is quite large, consisting of 25 exons spanning 250 to 600 kb in chromosome segment 15q11-q13. The P polypeptide appears to define a novel family of small molecule transporters and may be involved in transport of tyrosine, the precursor to melanin synthesis, within the melanocyte. These results provide the basis for analyses of patients with OCA2 and may point toward eventual pharmacologic treatment of this and related disorders of pigmentation.

Regulation of melanotransferrin gene in melanoma cells. Analysis of the enhancer region

The human iron-binding protein melanotransferrin is up-regulated in most skin melanomas. With the goal to understand the mechanisms controlling the expression of the gene in these tumor cells, we previously reported the identification of an enhancer exhibiting melanoma specificity. We show here that, in the highly expressing SK-MEL-28 melanoma cell line, the chromatin structure in the enhancer region is in an open configuration and that the transcription factors governing its activity belong to the helix-loop-helix and the Jun/Fos leucine zipper families.

Melanocyte-specific expression of the human tyrosinase promoter: activation by the microphthalmia gene product and role of the initiator

The tyrosinase gene is expressed specifically in melanocytes and the cells of the retinal pigment epithelium, which together are responsible for skin, hair, and eye color. By using a combination of DNase I footprinting and band shift assays coupled with mutagenesis of specific DNA elements, we examined the requirements for melanocyte-specific expression of the human tyrosinase promoter. We found that as little as 115 bp of the upstream sequence was sufficient to direct tissue-specific expression. This 115-bp stretch contains three positive elements: the M box, a conserved element found in other melanocyte-specific promoters; an Sp1 site; and a highly evolutionarily conserved element located between -14 and +1 comprising an E-box motif and an overlapping octamer element. In addition, two further elements, one positive and one negative, are located between positions -185 and -150 and positions -150 and -115, respectively. We also found that the basic helix-loop-helix factor encoded by the microphthalmia gene, which is essential for melanocyte differentiation, can transactivate the tyrosinase promoter via the M box and the conserved E box located close to the initiator. Since in vitro assays failed to identify any melanocyte-specific DNA-binding activity, the possibility that the specific arrangement of elements within the basal tyrosinase promoter determines melanocyte-specific expression is discussed.

Melanocyte-specific expression of the human tyrosinase promoter: activation by the microphthalmia gene product and role of the initiator

The tyrosinase gene is expressed specifically in melanocytes and the cells of the retinal pigment epithelium, which together are responsible for skin, hair, and eye color. By using a combination of DNase I footprinting and band shift assays coupled with mutagenesis of specific DNA elements, we examined the requirements for melanocyte-specific expression of the human tyrosinase promoter. We found that as little as 115 bp of the upstream sequence was sufficient to direct tissue-specific expression. This 115-bp stretch contains three positive elements: the M box, a conserved element found in other melanocyte-specific promoters; an Sp1 site; and a highly evolutionarily conserved element located between -14 and +1 comprising an E-box motif and an overlapping octamer element. In addition, two further elements, one positive and one negative, are located between positions -185 and -150 and positions -150 and -115, respectively. We also found that the basic helix-loop-helix factor encoded by the microphthalmia gene, which is essential for melanocyte differentiation, can transactivate the tyrosinase promoter via the M box and the conserved E box located close to the initiator. Since in vitro assays failed to identify any melanocyte-specific DNA-binding activity, the possibility that the specific arrangement of elements within the basal tyrosinase promoter determines melanocyte-specific expression is discussed.

Molecular control of melanogenesis in malignant melanoma: functional assessment of tyrosinase and lamp gene families by UV exposure and gene co-transfection, and cloning of a cDNA encoding calnexin, a possible melanogenesis "chaperone"

Melanogenesis is a cascade of events significantly controlled by regulatory genes which are associated with the melanosomal membrane. This report introduces our current research efforts dealing with (a) the gene and protein expressions of tyrosinase and Lamp (lysosome-associated membrane protein) families by human melanoma cells after repeated exposures to UV light, (b) the coordinated alterations in the expression of the Lamp family gene and its encoding product after transfection of two genes of the tyrosinase family in human melanoma cells and (c) cloning and sequencing of a Ca(2+)-binding phosphoprotein, calnexin, which could be a candidate as a chaperone for sorting and maturation of tyrosinase and Lamp family glycoproteins in melanogenesis cascade. Our UV exposure study, as well as gene transfection and antisense hybridization experiments, has clearly indicated a marked and coordinated interaction of the Lamp-1 gene with the tyrosinase and TRP-1 genes in this process. We propose that melanogenesis is controlled at least by two major gene family products, i.e., (a) the tyrosinase family of tyrosinase, TRP-1 and TRP-2, and the Lamp family of Lamp-1, Lamp-2 and Lamp-3. These two gene families probably derived from primordial melanogenesis-associated genes which are common or closely related to each other.

Analysis of the mouse tyrosinase promoter in vitro and in vivo

The restricted expression of the tyrosinase gene in cells producing pigment suggests the presence of cis-regulatory elements and trans-acting tissue-specific factors. Since 270 bp upstream of the transcriptional start site contain sufficient information for tissue-specific and developmentally regulated expression, we confined our analyses to this region. In this article, we discuss the recent results we have obtained on the regulation of the mouse tyrosinase gene expression demonstrating the existence of one negative and two positive-acting elements in vitro. We have evidence that the positive elements do not determine pigment production in vivo but rather modulate transcription of the mouse tyrosinase gene.

Human TRP-1 has tyrosine hydroxylase but no dopa oxidase activity

Human TRP-1 has been immunopurified from normal human melanocytes cultured from black neonatal subjects and used to investigate the catalytic function of TRP-1 for the two substrates, L-tyrosine and L-DOPA. Immunopurified TRP-1 did not demonstrate DOPA staining on SDS/PAGE nor DOPA oxidase (DO) activity with either routine or modified assays. The purified TRP-1 also demonstrated no tyrosine hydroxylase (TH) activity using the routine Pomerantz assay. However, there was apparent TH activity exhibited by immunopurified TRP-1 under conditions with low tyrosine concentration (< or = 0.8 microCi/ml of 3H-tyrosine), prolonged incubation time (i.e., overnight) and in the absence of the cofactor L-DOPA. Using these latter specific conditions, TH activity was also detected in cell lysates from a tyrosinase-negative albino melanocyte line which exhibited no TH activity with the routine Pomerantz assay. In addition, TH activity under low substrate assay conditions was not exhibited in a melanocyte line derived from a TRP-1 deficient, Brown albino individual. However, the absence of TH in this Brown albino cell line could be compensated for by the addition of L-DOPA to the assay. These results suggested that TRP-1 has some tyrosine hydroxylase but no DOPA oxidase activity. We propose that one function of TRP-1 is to modulate tyrosinase activity by making DOPA available as a cofactor to perpetuate the initial steps in melanogenesis.

Melanocyte-specific gene expression: role of repression and identification of a melanocyte-specific factor, MSF

For a gene to be transcribed in a tissue-specific fashion, expression must be achieved in the appropriate cell type and also be prevented in other tissues. As an approach to understanding the regulation of tissue-specific gene expression, we have analyzed the requirements for melanocyte-specific expression of the tyrosinase-related protein 1 (TRP-1) promoter. Positive regulation of TRP-1 expression is mediated by both an octamer-binding motif and an 11-bp element, termed the M box, which is conserved between the TRP-1 and other melanocyte-specific promoters. We show here that, consistent with its ability to activate transcription in a non-tissue-specific fashion, the M box binds the basic-helix-loop-helix factor USF in vitro. With the use of a combination of site-directed mutagenesis and chimeric promoter constructs, additional elements involved in regulating TRP-1 expression were identified. These include the TATA region, which appears to contribute to the melanocyte specificity of the TRP-1 promoter. Mutational analysis also identified two repressor elements, one at the start site, the other located at -240, which function both in melanoma and nonmelanoma cells. In addition, a melanocyte-specific factor, MSF, binds to sites which overlap both repressor elements, with substitution mutations demonstrating that binding by MSF is not required for repression. Although a functional role for MSF has not been unequivocally determined, the location of its binding sites leads us to speculate that it may act as a melanocyte-specific antirepressor during transcription of the endogenous TRP-1 gene.

Melanocyte-specific gene expression: role of repression and identification of a melanocyte-specific factor, MSF

For a gene to be transcribed in a tissue-specific fashion, expression must be achieved in the appropriate cell type and also be prevented in other tissues. As an approach to understanding the regulation of tissue-specific gene expression, we have analyzed the requirements for melanocyte-specific expression of the tyrosinase-related protein 1 (TRP-1) promoter. Positive regulation of TRP-1 expression is mediated by both an octamer-binding motif and an 11-bp element, termed the M box, which is conserved between the TRP-1 and other melanocyte-specific promoters. We show here that, consistent with its ability to activate transcription in a non-tissue-specific fashion, the M box binds the basic-helix-loop-helix factor USF in vitro. With the use of a combination of site-directed mutagenesis and chimeric promoter constructs, additional elements involved in regulating TRP-1 expression were identified. These include the TATA region, which appears to contribute to the melanocyte specificity of the TRP-1 promoter. Mutational analysis also identified two repressor elements, one at the start site, the other located at -240, which function both in melanoma and nonmelanoma cells. In addition, a melanocyte-specific factor, MSF, binds to sites which overlap both repressor elements, with substitution mutations demonstrating that binding by MSF is not required for repression. Although a functional role for MSF has not been unequivocally determined, the location of its binding sites leads us to speculate that it may act as a melanocyte-specific antirepressor during transcription of the endogenous TRP-1 gene.

Genetics and molecular biology of mouse pigmentation

The formation of mouse coat color is a relatively complex developmental process that is affected by a large number of mutations, both naturally occurring and induced. The cloning of the genes in which these mutations occur and the elucidation of the mechanisms by which these mutations disrupt the normal pigmentation pattern is leading to an understanding of the way interactions between gene products lead to a final phenotype.

Analysis of the promoter of the human prostatic acid phosphatase gene

From the analysis of two overlapping cosmid clones prepared from human genomic DNA libraries, a contig of 44 kb containing a 5' portion of the PAP gene and 17 kb of the upstream region was established. It was characterized by restriction mapping and sequence analysis of 2.5 kb upstream of the initiation codon. Two major transcription initiation sites were found to be located around 56 and 91 bp upstream of the initiation codon, as determined by nuclease S1 and primer extension mapping. Expression of the PAP gene was measured by Northern blots in the androgen responsive LNCaP cell line. It was found to be induced 2-3-fold by the addition of the synthetic androgen mibolerone to the cells. The induced mRNA levels were approx. 10-times lower than those for the prostate-specific antigen (PSA) in LNCaP cells.