Inhibitory effect of the mi transcription factor encoded by the mutant mi allele on GA binding protein-mediated transcript expression in mouse mast cells

TFCP2 is a transcriptional regulator of heparan sulfate assembly and melanoma cell growth

Heparan sulfate (HS) is a long, linear polysaccharide that is ubiquitously expressed in all animal cells and plays a key role in many cellular processes, including cell signaling and development. Dysregulation of HS assembly has been implicated in pathophysiological conditions, such as tumorigenesis and rare genetic disorders. HS biosynthesis occurs in a non-template-driven manner in the endoplasmic reticulum and Golgi through the activity of a large group of biosynthetic enzymes. While much is known about its biosynthesis, little is understood about the regulation of HS assembly across diverse tissue types and disease states. To address this gap in knowledge, we recently performed genome-wide CRISPR/Cas9 screens to identify novel regulatory factors of HS biosynthesis. From these screens, we identified the alpha globin transcription factor, TFCP2, as a top hit. To investigate the role of TFCP2 in HS assembly, we targeted TFCP2 expression in human melanoma cells using the CRISPR/Cas9 system. TFCP2 knockout cells exhibited decreased fibroblast growth factor binding to cell surface HS, alterations in HS composition, and slowed cell growth compared to wild-type cells. Additionally, RNA sequencing revealed that TFCP2 regulates the expression of multiple enzymes involved in HS assembly, including the secreted endosulfatase, SULF1. Pharmacological targeting of TFCP2 activity similarly reduced growth factor binding and increased SULF1 expression, and the knockdown of SULF1 expression in TFCP2 mutant cells restored melanoma cell growth. Overall, these studies identify TFCP2 as a novel transcriptional regulator of HS and highlight HS-protein interactions as a possible target to slow melanoma growth.

Spatiotemporal diversity and regulation of glycosaminoglycans in cell homeostasis and human disease

Glycosaminoglycans (GAGs) are long, linear polysaccharides that are ubiquitously expressed on the cell surface and in the extracellular matrix of all animal cells. These complex carbohydrates play important roles in many cellular processes and have been implicated in many disease states, including cancer, inflammation, and genetic disorders. GAGs are among the most complex molecules in biology with enormous information content and extensive structural and functional heterogeneity. GAG biosynthesis is a nontemplate-driven process facilitated by a large group of biosynthetic enzymes that have been extensively characterized over the past few decades. Interestingly, the expression of the enzymes and the consequent structure and function of the polysaccharide chains can vary temporally and spatially during development and under certain pathophysiological conditions, suggesting their assembly is tightly regulated in cells. Due to their many key roles in cell homeostasis and disease, there is much interest in targeting the assembly and function of GAGs as a therapeutic approach. Recent advances in genomics and GAG analytical techniques have pushed the field and generated new perspectives on the regulation of mammalian glycosylation. This review highlights the spatiotemporal diversity of GAGs and the mechanisms guiding their assembly and function in human biology and disease.

MITF Regulates Downstream Genes in Response to Vibrio parahaemolyticus Infection in the Clam Meretrix Petechialis

The microphthalmia-associated transcription factor (MITF) is a basic helix-loop-helix-leucine zipper protein that plays a key role in cell proliferation, survival and immune defense through the direct transcriptional control of downstream genes. We have found that MITF participates in the immune response to Vibrio parahaemolyticus infection in the clam Meretrix petechialis. In this study, we focused on how MITF functions in immunity. First, PO, CTSK, and BCL-2 were identified as the target genes of MpMITF in the clam by RNAi. EMSAs showed direct binding between the MpMITF protein and the E-box of the MpPO, MpCTSK, and MpBCL-2 promoters. Yeast one-hybrid assays also suggested that MpMITF could activate the expression of these three downstream genes. These results demonstrated that the transcriptional expression of MpPO, MpCTSK, and MpBCL-2 is directly regulated by MpMITF. Second, we analyzed the roles of MpPO, MpCTSK, and MpBCL-2 in clam immunity. The mRNA expression of MpPO, MpCTSK, and MpBCL-2 increased significantly after V. parahaemolyticus challenge, which implied that these genes might take part in the immune defense against V. parahaemolyticus challenge in clams. The purified recombinant proteins, MpPO and MpCTSK, inhibited the growth of V. parahaemolyticus. Additionally, the apoptosis rate of clam haemocytes rose significantly when the activity of MpBCL-2 was suppressed. These results revealed that MpPO, MpCTSK, and MpBCL-2 are involved in the immune defense against V. parahaemolyticus. This study supports the idea that the MpMITF pathway plays a key role in immune defense through the direct regulation of the downstream genes MpPO, MpCTSK, and MpBCL-2 in the clam, M. petechialis.

Metabolic engineering of mammalian cells to produce heparan sulfates

Heparan sulfate (HS) is a glycosaminoglycan produced by all mammalian cells that plays important roles in physiology and various pathologies. Heparin is a highly sulfated form of HS that is used clinically as an anticoagulant. Heparin and HSs may also have therapeutic benefits for a wide variety of other indications. Cultured mammalian cells produce HS and, through genetic modification, have been used to elucidate the biosynthetic pathway. Recently, metabolic engineering has been used to produce HS from cultured mammalian cells for clinical purposes. This review describes the HS biosynthetic pathway and its manipulation through metabolic engineering to produce bioengineered HSs. We also discuss current challenges and opportunities to advance the field of HS metabolic engineering.

Pathophysiology of heparan sulphate: many diseases, few drugs

Heparan sulphate (HS) polysaccharides are covalently attached to the core proteins of various proteoglycans at cell surfaces and in the extracellular matrix. They are composed of alternating units of hexuronic acid and glucosamine, with sulphate substituents in complex and variable yet cell-specific patterns. Whereas HS is produced by virtually all cells in the body, heparin, a highly sulphated HS variant, is confined to connective-tissue-type mast cells. The polysaccharides interact with a multitude of proteins, mainly through ionic binding, and thereby control key processes in development and homoeostasis. Similar interactions also implicate HS in various pathophysiological settings, including cancer, amyloid diseases, infectious diseases, inflammatory conditions and some developmental disorders. Prospects for the development of HS-based drugs, which are still largely unrealized, are discussed.

Heparan sulfate biosynthesis: regulation and variability

Nearly all vertebrate cells have been shown to express heparan sulfate proteoglycans (HSPGs) at the cell surface. The HSPGs bind to many secreted signaling proteins, including numerous growth factors, cytokines, and morphogens, to affect their tissue distribution and signaling. The heparan sulfate (HS) chains may have variable length and may differ with regard to both degree and pattern of sulfation. As the sulfation pattern of HS chains in most cases will determine if an interaction with a potential ligand will take place, as well as the affinity of the interaction, a key to understanding the function of HSPGs is to clarify how HS biosynthesis is regulated in different biological contexts. This review provides an introduction to the current understanding of HS biosynthesis and its regulation, and identifies research areas where more knowledge is needed to better understand how the HS biosynthetic machinery works.

Heparin biosynthesis

Heparin and heparan sulfate share the same polysaccharide backbone structure but differ in sulfation degree and expression pattern. Whereas heparan sulfate is found in virtually all cells of the human body, heparin expression is restricted to mast cells, where it has a function in storage of granular components such as histamine and mast cell specific proteases. Although differing in charge and sulfation pattern, current knowledge indicates that the same pathway is used for synthesis of heparin and heparan sulfate, with a large number of different enzymes taking part in the process. At present, little is known about how the individual enzymes are coordinated and how biosynthesis is regulated. These questions are addressed in this chapter together with a review of the basic enzymatic steps involved in initiation, elongation, and modification of the polysaccharides.

Novel MITF targets identified using a two-step DNA microarray strategy

Malignant melanoma is a chemotherapy-resistant cancer with high mortality. Recent advances in our understanding of the disease at the molecular level have indicated that it shares many characteristics with developmental precursors to melanocytes, the mature pigment-producing cells of the skin and hair follicles. The development of melanocytes absolutely depends on the action of the microphthalmia-associated transcription factor (MITF). MITF has been shown to regulate a broad variety of genes, whose functions range from pigment production to cell-cycle regulation, migration and survival. However, the existing list of targets is not sufficient to explain the role of MITF in melanocyte development and melanoma progression. DNA microarray analysis of gene expression offers a straightforward approach to identify new target genes, but standard analytical procedures are susceptible to the generation of false positives and require additional experimental steps for validation. Here, we introduce a new strategy where two DNA microarray-based approaches for identifying transcription factor targets are combined in a cross-validation protocol designed to help control false-positive generation. We use this two-step approach to successfully re-identify thirteen previously recorded targets of MITF-mediated upregulation, as well as 71 novel targets. Many of these new targets have known relevance to pigmentation and melanoma biology, and further emphasize the critical role of MITF in these processes.

Diadenosine tetraphosphate hydrolase is part of the transcriptional regulation network in immunologically activated mast cells

We previously discovered that microphthalmia transcription factor (MITF) and upstream stimulatory factor 2 (USF2) each forms a complex with its inhibitor histidine triad nucleotide-binding 1 (Hint-1) and with lysyl-tRNA synthetase (LysRS). Moreover, we showed that the dinucleotide diadenosine tetraphosphate (Ap(4)A), previously shown to be synthesized by LysRS, binds to Hint-1, and as a result the transcription factors are released from their suppression. Thus, transcriptional activity is regulated by Ap(4)A, suggesting that Ap(4)A is a second messenger in this context. For Ap(4)A to be unambiguously established as a second messenger, several criteria have to be fulfilled, including the presence of a metabolizing enzyme. Since several enzymes are able to hydrolyze Ap(4)A, we provided here evidence that the "Nudix" type 2 gene product, Ap(4)A hydrolase, is responsible for Ap(4)A degradation following the immunological activation of mast cells. The knockdown of Ap(4)A hydrolase modulated Ap(4)A accumulation, resulting in changes in the expression of MITF and USF2 target genes. Moreover, our observations demonstrated that the involvement of Ap(4)A hydrolase in gene regulation is not a phenomenon exclusive to mast cells but can also be found in cardiac cells activated with the beta-agonist isoproterenol. Thus, we have provided concrete evidence establishing Ap(4)A as a second messenger in the regulation of gene expression.

Heparan sulfate biosynthesis enzymes EXT1 and EXT2 affect NDST1 expression and heparan sulfate sulfation

Heparan sulfate (HS) proteoglycans influence embryonic development and adult physiology through interactions with protein ligands. The interactions depend on HS structure, which is determined largely during biosynthesis by Golgi enzymes. How biosynthesis is regulated is more or less unknown. During polymerization of the HS chain, carried out by a complex of the exostosin proteins EXT1 and EXT2, the first modification enzyme, glucosaminyl N-deacetylase/N-sulfotransferase (NDST), introduces N-sulfate groups into the growing polymer. Unexpectedly, we found that the level of expression of EXT1 and EXT2 affected the amount of NDST1 present in the cell, which, in turn, greatly influenced HS structure. Whereas overexpression of EXT2 in HEK 293 cells enhanced NDST1 expression, increased NDST1 N-glycosylation, and resulted in elevated HS sulfation, overexpression of EXT1 had opposite effects. Accordingly, heart tissue from transgenic mice overexpressing EXT2 showed increased NDST activity. Immunoprecipitaion experiments suggested an interaction between EXT2 and NDST1. We speculate that NDST1 competes with EXT1 for binding to EXT2. Increased NDST activity in fibroblasts with a gene trap mutation in EXT1 supports this notion. These results support a model in which the enzymes of HS biosynthesis form a complex, or a GAGosome.

The role played by key transcription factors in activated mast cells

The network of transcription factors in mast cells has not been investigated as widely as it has been in other differentiated hematopoietic cells. There are still many mechanisms of transcriptional regulation that need to be fully elucidated to understand how mast cell external stimuli lead to the appropriate physiological responses. Such information could be used to determine potential therapeutic targets for the control of mast cell activation in inflammatory diseases, allergy, and asthma. The aim of this article is to review hallmark studies in the field of transcription factor regulation in mast cells. We elaborate especially on several transcription factors studied in our laboratory in the past decade, including activator protein-1, microphthalmia-associated transcription factor, upstream stimulating factor-2, and signal transducer and activator of transcription 3.

Distinct and shared transcriptomes are regulated by microphthalmia-associated transcription factor isoforms in mast cells

The Microphthalmia-associated transcription factor (Mitf) is an essential basic helix-loop-helix leucine zipper transcription factor for mast cell development. Mice deficient in Mitf harbor a severe mast cell deficiency, and Mitf-mutant mast cells cultured ex vivo display a number of functional defects. Therefore, an understanding of the genetic program regulated by Mitf may provide important insights into mast cell differentiation. Multiple, distinct isoforms of Mitf have been identified in a variety of cell types; we found that Mitf-a, Mitf-e, and Mitf-mc were the major isoforms expressed in mast cells. To determine the physiologic function of Mitf in mast cells, we restored expression of these isoforms in primary mast cells from Mitf(-/-) mice. We found that these isoforms restored granular morphology and integrin-mediated migration. By microarray analysis, proteases, signaling molecules, cell surface receptor, and transporters comprised the largest groups of genes up-regulated by all isoforms. Furthermore, we found that isoforms also regulated distinct genes sets, suggesting separable biological activities. This work defines the transcriptome regulated by Mitf in mast cells and supports its role as master regulator of mast cell differentiation. Expression of multiple isoforms of this transcription factor may provide for redundancy of biological activities while also allowing diversity of function.

Molecular Mechanisms of Mast Cell Development

Mast cells are progeny of multipotential hematopoietic stem cells (MHSCs). MHSCs commit to the mast cell lineage in the bone marrow, and the mast cell-committed progenitors leave the bone marrow, migrate in blood, invade connective or mucosal tissue, and then proliferate and differentiate to connective tissue-type or mucosal mast cell. GATA-1, GATA-2, and PU.1 transcription factors seem to be involved i the commitment to mast cells, and MITF, a basic helix-loop-helix leucine zipper-type transcription factor, seems to be involved in the migration, phenotypic expression, and survival of mast cells. KIT ligand (KITL) is the most important cytoline for development of mast cells, and KIT is the receptor of KITL. Tissues of loss-of-function mutants of KIT, KITL, or MITF are deficient in mast cells.

Transcriptional regulation of ATP2C1 gene by Sp1 and YY1 and reduced function of its promoter in Hailey-Hailey disease keratinocytes

Hailey-Hailey disease (HHD) is a blistering skin disease caused by malfunction of the Ca2+-dependent ATPase, ATP2C1. In this study, key regulatory regions necessary for the expression of the gene encoding human ATP2C1 were investigated. The transient reporter assay demonstrated that region +21/+57 was necessary for activation of the ATP2C1 promoter, and the electrophoretic mobility shift assay demonstrated that the region was recognized by the transcription factors, Sp1 and YY1. In accordance with this result, when Sp1 or YY1 was overexpressed in keratinocytes, an obvious increase in ATP2C1 promoter activity was observed, which was in contrast with the case where a mutant promoter lacking the binding sites for Sp1 and YY1 was used as the reporter. Ca2+-stimulation signal increased nuclear Sp1 proteins and ATP2C1 mRNA levels in normal keratinocytes. In contrast, both these increases were suppressed in keratinocytes from HHD patients. These results indicate that Sp1 and YY1 transactivate the human ATP2C1 promoter via cis-enhancing elements and that incomplete upregulation of ATP2C1 transcription contributes to the keratinocyte-specific pathogenesis of HHD. This is a report describing the regulation of the expression of ATP2C1.

Melanocytes and the microphthalmia transcription factor network

The first mouse microphthalmia transcription factor (Mitf ) mutation was discovered over 60 years ago, and since then over 24 spontaneous and induced mutations have been identified at the locus. Mitf encodes a member of the Myc supergene family of basic helix-loop-helix zipper (bHLH-Zip) transcription factors. Like Myc, Mitf regulates gene expression by binding to DNA as a homodimer or as a heterodimer with another related family member, in the case of Mitf the Tfe3, Tfeb, and Tfec proteins. The study of Mitf has provided many insights into the biology of melanocytes and helped to explain how melanocyte-specific gene expression and signaling is regulated. The human homologue of MITF is mutated in patients with the pigmentary and deafness disorder Waardenburg Syndrome Type 2A (WS2A). The mouse Mitf mutations therefore serve as a model for the study of this human disease. Mutations and/or aberrant expression of several MITF family member genes have also been reported in human cancer, including melanoma (MITF), papillary renal cell carcinoma (TFE3, TFEB), and alveolar soft part sarcoma (TFE3). Genes in the MITF/TFE pathway may therefore also represent valuable therapeutic targets for the treatment of human cancer. Here we review recent developments in the analysis of Mitf function in vivo and in vitro and show how traditional genetics, modern forward genetics and in vitro biochemical analyses have combined to produce an intriguing story on the role and actions of a gene family in a living organism.

Reduced expression of IL-12 receptor beta2 and IL-18 receptor alpha genes in natural killer cells and macrophages derived from B6-mi/mi mice

The mi transcriptional factor (MITF) is a basic helix-loop-helix leucine zipper-type transcriptional factor. The mi mutant allele encodes an abnormal MITF, in which one out of four consecutive arginines is deleted in the basic domain. The VGA-9-tg (tg) allele is another mutant allele and considered to be a null mutant allele. C57BL/6 (B6)-mi/mi mice showed abnormal phenotypes of natural killer (NK) cells and macrophages, whereas B6-tg/tg mice did not. The expression levels of the genes for the interleukin-12 receptor (IL-12R) beta2 and IL-18Ralpha were reduced in both the NK cells and macrophages of B6-mi/mi mice, while the expression levels of the corresponding genes in B6-tg/tg mice were unaffected. The B6-mi/mi NK cells and B6-mi/mi macrophages showed impaired responses to stimulation with IL-12, IL-18, and IL-12 plus IL-18 stimulation. The abnormal NK cell and macrophage of B6-mi/mi mice appear to be due to decreased expression of the IL-12Rbeta2 and IL-18Ralpha genes.

Number of mast cells in the peritoneal cavity of mice: influence of microphthalmia transcription factor through transcription of newly found mast cell adhesion molecule, spermatogenic immunoglobulin superfamily

The mi (microphthalmia) locus of mice encodes a transcription factor, MITF. B6-tg/tg mice that do not express any MITF have white coats and small eyes. Moreover, the number of mast cells decreased to one-third that of normal control (+/+) mice in the skin of B6-tg/tg mice. No mast cells were detectable in the stomach, mesentery, and peritoneal cavity of B6-tg/tg mice. Cultured mast cells derived from B6-tg/tg mice do not express a mast cell adhesion molecule, spermatogenic immunoglobulin superfamily (SgIGSF). To obtain in vivo evidence for the correlation of nonexpression of SgIGSF with decrease in mast cell number, we used another MITF mutant, B6-mi(vit)/mi(vit) mice that have a mild phenotype, ie, black coat with white patches and eyes of normal size. B6-mi(vit)/mi(vit) mice had a normal number of mast cells in the skin, stomach, and mesentery, but the number of peritoneal mast cells decreased to one-sixth that of +/+ mice. Cultured mast cells and peritoneal mast cells of B6-mi(vit)/mi(vit) mice showed a reduced but apparently detectable level of SgIGSF expression, demonstrating the parallelism between mast cell number and expression level of SgIGSF. The number of peritoneal mast cells appeared to be influenced by MITF through transcription of SgIGSF.

Roles of MITF for development of mast cells in mice: effects on both precursors and tissue environments

The mutant tg/tg mice, which do not express mi transcription factor (MITF), lack mast cells in most tissues. Since MITF is expressed in both mast cells and tissues where mast cells develop, there is a possibility that the tg/tg mice may show abnormalities in both mast cell precursors and tissue environments. We examined this possibility by bone marrow and skin transplantation. When bone marrow cells of tg/tg mice were transplanted to W/W(v) mice that possess normal tissue environment, mast cells did not develop in all tissues examined. The number of developing mast cells in the skin of W/W(v) mice was much lower when grafted to tg/tg recipients than when grafted to normal (+/+) recipients. These results indicated that mast cell precursors of tg/tg mice were defective. When bone marrow cells of +/+ mice were transplanted, the number of developing mast cells was significantly lower in examined tissues of tg/tg recipients than in those of W/W(v) recipients, suggesting that the tissue environment for mast cell development was defective in tg/tg mice. MITF appeared essential for the function of both mast cell precursors and tissue environments for their development.

Effect of anatomical distribution of mast cells on their defense function against bacterial infections: demonstration using partially mast cell-deficient tg/tg mice

Mast cells were depleted in the peritoneal cavity of WBB6F1-tg/tg mice that did not express a transcription factor, MITF. When acute bacterial peritonitis was induced in WBB6F1-+/+, WBB6F1-W/Wv, and WBB6F1-tg/tg mice, the proportion of surviving WBB6F1-+/+ mice was significantly higher than that of surviving WBB6F1-W/Wv or WBB6F1-tg/tg mice. The poor survival of WBB6F1-W/Wv and WBB6F1-tg/tg mice was attributed to the deficient influx of neutrophils into the peritoneal cavity. The injection of cultured mast cells (CMCs) derived from WBB6F1-+/+ mice normalized the neutrophil influx and reduced survival rate in WBB6F1-W/Wv mice, but not in WBB6F1-tg/tg mice. This was not attributable to a defect of neutrophils because injection of TNF-alpha increased the neutrophil influx and survival rate in both WBB6F1-W/Wv and WBB6F1-tg/tg mice. Although WBB6F1-+/+ CMCs injection normalized the number of mast cells in both the peritoneal cavity and mesentery of WBB6F1-W/Wv mice, it normalized the number of mast cells only in the peritoneal cavity of WBB6F1-tg/tg mice. Mast cells within the mesentery or mast cells in the vicinity of blood vessels appeared to play an important role against the acute bacterial peritonitis. WBB6F1-tg/tg mice may be useful for studying the effect of anatomical distribution of mast cells on their antiseptic function.

SgIGSF: a new mast-cell adhesion molecule used for attachment to fibroblasts and transcriptionally regulated by MITF

Microphthalmia transcription factor (MITF) is a basic-helix-loop-helix-leucine zipper-type transcription factor. The mutant mi and Mi(wh) alleles encode MITFs with deletion and alteration of a single amino acid, respectively, whereas the tg is a null mutation. In coculture with NIH/3T3 fibroblasts, the numbers of cultured mast cells (CMCs) derived from C57BL/6 (B6)(mi/mi), B6(Miwh/Miwh), and B6(tg/tg) mice that adhered to NIH/3T3 fibroblasts were one third as large as the number of B6(+/+) CMCs that adhered to NIH/3T3 fibroblasts. From a cDNA library of B6(+/+) CMCs, we subtracted messenger RNAs expressed by B6(mi/mi) CMCs and found a clone encoding SgIGSF, a recently identified member of the immunoglobulin superfamily. Northern and Western blot analyses revealed that SgIGSF was expressed in B6(+/+) CMCs but not in CMCs derived from MITF mutants. Immunocytochemical analysis showed that SgIGSF localized to the cell-to-cell contact areas between B6(+/+) CMCs and NIH/3T3 fibroblasts. Transfection of B6(mi/mi) and B6(tg/tg) CMCs with SgIGSF cDNA normalized their adhesion to NIH/3T3 fibroblasts. NIH/3T3 fibroblasts did not express SgIGSF, indicating that SgIGSF acts as a heterophilic adhesion molecule. Transfection of B6(tg/tg) CMCs with normal MITF cDNA elevated their SgIGSF expression to normal levels. These results indicated that SgIGSF mediated the adhesion of CMCs to fibroblasts and that the transcription of SgIGSF was critically regulated by MITF.

Heparan sulfate and development: differential roles of the N-acetylglucosamine N-deacetylase/N-sulfotransferase isozymes

Heparan sulfates (HSs) are N- and O-sulfated polysaccharide components of proteoglycans, which are important constituents of the cell surface as well as the extracellular matrix. Heparin, with extensive clinical application as an anticoagulant, is a highly sulfated form of HS present within the granules of connective tissue type mast cells. The diverse functions of HS, which include the modulation of growth factor/cytokine activity, interaction with matrix proteins and binding of enzymes to cell surfaces, depend greatly on the presence of specific, high affinity regions on the chains. N-acetylglucosamine N-deacetylase/N-sulfotransferases, NDSTs, are an important group of enzymes in HS biosynthesis, initiating the sulfation of the polysaccharide chains and thus determining the generation of the high affinity sites. Here, we review the role of the four vertebrate NDSTs in HS biosynthesis as well as their regulated expression. The main emphasis is the phenotypes of mice lacking one or more of the NDSTs.

Dual abnormal effects of mutant MITF encoded by Mi(wh) allele on mouse mast cells: decreased but recognizable transactivation and inhibition of transactivation

MITF is a basic helix-loop-helix leucine zipper-type transcription factor and is important for development of mast cells. MITF encoded by Mi(wh) allele (Mi(wh)-MITF) was mutated at a single amino acid of basic domain, and possessed a deficient but apparent DNA-binding ability. Here, we characterized the unique effects of Mi(wh)-MITF on the expression of mast cell-related genes. The expression level of mouse mast cell protease (mMCP)-4, -5, and -6 genes in Mi(wh)/Mi(wh) cultured mast cells (CMCs) was intermediate between levels of normal (+/+) CMCs and tg/tg CMCs, which did not express any MITFs. Mi(wh)-MITF appeared to show the positive transactivation effect through the remaining DNA-binding ability. On the other hand, the expression level of tryptophan hydroxylase gene was lower in Mi(wh)/Mi(wh) CMCs than in tg/tg CMCs, suggesting the inhibitory effect of Mi(wh)-MITF on the transactivation. Mi(wh)-MITF possessed dual abnormal effects on transactivation of mast cell-related genes.

Regulated translation of heparan sulfate N-acetylglucosamine N-deacetylase/n-sulfotransferase isozymes by structured 5'-untranslated regions and internal ribosome entry sites

We report the full-length 5'-untranslated region (5'-UTR) sequences of the four vertebrate heparan sulfate/heparin GlcNAc N-deacetylase/N-sulfotransferases (NDSTs) and their role in translational regulation in vivo and in vitro. All four NDST 5'-UTR sequences are unusually long, have a high degree of predicted secondary structure, and contain multiple upstream AUG codons, which together impose a major barrier to conventional, cap-dependent ribosomal scanning. At least two alternatively spliced forms of NDST2 differing in their 5'-UTRs exist, and two forms of NDST4 arise from alternative transcriptional start sites. The 5'-UTRs do not show any significant sequence similarity between isozymes, but possess highly conserved regions between mouse and human orthologs, pointing toward evolutionarily conserved functions. Expression of bicistronic vector constructs showed that the 5'-UTRs of NDST1-4 are capable of regulating translation differentially in vivo dependent on cell type and culture conditions. In vitro translation of a reporter gene located downstream of the UTRs demonstrated the presence of internal ribosome entry sites, providing an additional, cap-independent step in fine-tuning NDST expression. Comparative studies of NDST1-3 mRNAs and protein expression in brain and embryonic extracts revealed striking differences in translational efficiency. Other genes necessary for glycosaminoglycan synthesis in addition to the NDST isozymes have long, structured 5'-UTRs. Because several growth factors and morphogens that bind heparan sulfate also contain structured 5'-UTRs, translational regulation may coordinate the action of these factors and their heparan sulfate co-receptors.

YEAF1/RYBP and YAF-2 are functionally distinct members of a cofactor family for the YY1 and E4TF1/hGABP transcription factors

The transcription factor hGABP/E4TF1 is a heterotetrameric complex composed of two DNA-binding subunits (hGABP alpha/E4TF1-60) and two transactivating subunits (hGABP beta/E4TF1-53). In order to understand the molecular mechanism of transcriptional regulation by hGABP, we searched for proteins that interact with the non-DNA-binding subunit, hGABP beta, using yeast two-hybrid screening. We identified a human cDNA encoding a protein related to YAF-2 (YY1-associated factor 2), which was previously isolated as an interacting partner of the Ying-Yang-1 (YY1) transcription factor. Reflecting this similarity, both YAF-2 and this novel protein (named YEAF1 for YY1- and E4TF1/hGABP-associated factor-1) interacted with hGABP beta and YY1 in vitro and in vivo, indicating that YEAF1 and YAF-2 constitute a cofactor family for these two structurally distinct transcription factors. By using yeast three-hybrid assay, we demonstrated that hGABP beta and YY1 formed a complex only in the presence of YEAF1, indicating that YEAF1 is a bridging factor of these two transcription factors. These cofactors are functionally different in that YAF-2 positively regulates the transcriptional activity of hGABP but YEAF1 negatively regulates this activity. Also, YAF-2 mRNA is highly expressed in skeletal muscle, whereas YEAF1 mRNA is highly expressed in placenta. We speculate that the transcriptional activity of hGABP is in part regulated by the expression levels of these tissue-specific cofactors. These results provide a novel mechanism of transcriptional regulation by functionally distinct cofactor family members.

Interaction and cooperation of mi transcription factor (MITF) and myc-associated zinc-finger protein-related factor (MAZR) for transcription of mouse mast cell protease 6 gene

The mi transcription factor (MITF) is a basic-helix-loop-helix leucine zipper (bHLH-Zip) transcription factor that is important for the normal phenotypic expression of mast cells. Most transcription factors function in cooperation with other factors by protein-protein interactions. To search proteins interacting with MITF, we carried out a yeast two-hybrid screen and isolated Myc-associated zinc-finger protein related factor (MAZR) as a partner of MITF. When expressed with MITF in NIH/3T3 cells, MAZR was colocalized with MITF. The association of MAZR with MITF was further confirmed by a co-immunoprecipitation study and in vitro binding assay. The zinc-finger domain of MAZR and the Zip domain of MITF were essential for the interaction. MAZR was expressed in cultured mast cells and MST mastocytoma cells containing mouse mast cell protease (mMCP)-6 transcript abundantly. The overexpression of dominant negative MAZR in MST mastocytoma cells reduced the amount of mMCP-6 mRNA. The simultaneous transfection of MAZR and MITF significantly increased the promoter activity of the mMCP-6 gene, indicating that the MAZR and MITF synergistically transactivated the mMCP-6 gene. MAZR appeared to play important roles in the normal phenotypic expression of mast cells in association with MITF.