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.In attempting to understand how the precise temporal and spatial pattern of gene expression necessary for the development of an organism is achieved, consideration should be given not only to the question of why a specific gene is expressed in a particular cell type at any given time, but also to why it is not expressed elsewhere or at other times. Analysis of tissue-specific promoters in many cell types has revealed that they often contain binding sites for widely expressed transcription factors which may act together with factors with a more restricted tissue distribution. However, while these tissue-specific factors may be present in only a very limited number of cell types, they frequently fall into transcription factor families, members of which have identical or highly similar DNA-binding properties. For example, the CANNTG E-box motif is recognized by members of the basic helix-loop-helix (bHLH) class of transcription factors, and w...
Melanocytes are specialized cells residing in the hair follicles, the eye, and the basal layer of the human epidermis whose primary function is the production of the pigment melanin, giving rise to skin, hair, and eye color. Melanogenesis, a process unique to melanocytes that involves the processing of tyrosine by a number of melanocyte-specific enzymes, including tyrosinase and tyrosinase-related protein 1 (TRP-1), occurs only after differentiation from the melanocyte precursor, the melanoblast. In humans, melanogenesis is inducible by UV irradiation, with melanin being transferred from the melanocyte in the epidermis to the surrounding keratinocytes as protection from UV-induced damage. Excessive exposure to UV, however, is the primary cause of malignant melanoma, an increasingly common and highly aggressive disease. As an initial approach to understanding the regulation of melanocyte differentiation and melanocyte-specific transcription, we have isolated the gene encoding TRP-1 and examined the cis-and trans-acting factors required for cell-type-specific expression. We find that the TRP-1 promoter comprises both positive and negative regulatory elements which confer efficient expression in a TRP-1-expressing, pigmented melanoma cell line but not in NIH 3T3 or JEG3 cells and that a minimal promoter extending between -44 and +107 is sufficient for cell-type-specific expression. Assays for DNA-protein interactions coupled with extensive mutagenesis identified three factors, whose binding correlated with the function of two positive and one negative regulatory element. One of these factors, termed M-box-binding factor 1, binds to an 11-bp motif, the M box, which acts as a positive regulatory element both in TRP-l-expressing and -nonexpressing cell lines, despite being entirely conserved between the melanocyte-specific tyrosinase and TRP-1 promoters. The possible mechanisms underlying melanocyte-specific gene expression are discussed.
Previous work has established that the melanocytespecific tyrosinase-related protein-1 (TRP-1) promoter is regulated positively by the microphthalmia-associated transcription factor Mitf, acting through the conserved M box and negatively by the T-box factor Tbx2, which can bind two "melanocyte-specific elements" termed the MSEu and MSEi. Both the MSEu and MSEi, which share a 6-base pair GTGTGA consensus, are also recognized by a previously unidentified melanocytespecific factor, MSF. Here we show using a combination of DNA binding assays, proteolytic clipping, and antiPax3 antibodies that MSF is indistinguishable from Pax3, a paired homeodomain transcription factor implicated genetically in melanocyte development and the regulation of the Mitf promoter. Consistent with Pax3 being able to bind the TRP-1 promoter, Pax3 is expressed in melanocytes and melanomas, and TRP-1 promoter activity is up-regulated by Pax3. The results identify a novel role for Pax3 in the expression of TRP-1, and the potential role of Pax3 in the melanocyte lineage is discussed.The development of the melanocyte lineage presents a fascinating opportunity to analyze the complex interplay between signal transduction pathways and transcription factors, which underlies development. Because melanocytes are not essential for viability and variations in pigmentation are obvious (1), over 70 independent genetic loci have been implicated in the development or function of these melanin-producing cells. Of the 20 or so that have been cloned to date, some, such as the genes encoding tyrosinase or tyrosinase-related protein-1 (TRP-1), 1 have a clearly defined function in the genesis of pigment. On the other hand, genes such as the endothelin B (2-4) and c-Kit receptors (5), and the microphthalmia (6 -8), Sox10 (9, 10), and Pax3 (11) transcription factors have been implicated in the developmental pathway leading to the genesis of the mature pigment-producing melanocyte from a nonpigmented melanoblast precursor cell originating in the trunk neural crest.Particularly interesting are mutations affecting the Pax3-paired homeodomain transcription factor, exemplified by the splotch allele (11), which expresses a truncated Pax3 protein.Although splotch homozygotes die in utero, heterozygous splotch mice exhibit pigmentation defects resulting from the loss of a proportion of the melanoblasts migrating away from the neural crest. The loss of melanocytes in splotch mice may be explained by the fact that Pax3 has recently been shown to activate expression from the promoter for the gene encoding the microphthalmia-associated basic helix-loop-helix-leucine zipper transcription factor (Mitf) (12); mice devoid of functional Mitf lack all pigment cells, and a decrease in Mitf levels resulting from monoallelic loss of Pax3 would account for the pigmentation defect exhibited by splotch mice. The ability of Pax3 to regulate expression of Mitf is paralleled by the role of Pax3 in skeletal muscle formation where it is required for expression of the basic helix-loop-helix tran...
DNA-dependent protein kinase (DNA-PK), which is involved in DNA double-strand break repair and V(D)J recombination, is comprised of a DNA-targeting component termed Ku and an ∼465-kD catalytic subunit, DNA-PK cs . Although DNA-PK phosphorylates proteins in the presence of DSBs or other discontinuities in the DNA double helix in vitro, the possibility exists that it is also activated in other circumstances via its association with additional proteins. Here, through use of the yeast two-hybrid screen, we discover that the recently identified high affinity DNA binding protein C1D interacts with the putative leucine zipper region of DNA-PK cs . Furthermore, we show that C1D can interact with DNA-PK in mammalian cells and that C1D is a very effective DNA-PK substrate in vitro. Finally, we establish that C1D directs the activation of DNA-PK in a manner that does not require DNA termini. Therefore, these studies provide a function for C1D and suggest novel mechanisms for DNA-PK activation in vivo.
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 melanocytespecific 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.
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