Systematic Method to Obtain Novel Genes That Are Regulated bymi Transcription Factor: Impaired Expression of Granzyme B and Tryptophan Hydroxylase in mi/mi Cultured Mast Cells

MRGPRX2 signaling involves the Lysyl-tRNA synthetase and MITF pathway

MRGPRX2, a G-protein-coupled-seven transmembrane domain receptor, is mainly expressed in mast cells and neurons and is involved in skin immunity and pain. It is implicated in the pathophysiology of non-IgE-mediated immediate hypersensitivity and has been related to adverse drug reactions. Moreover, a role has been proposed in asthma, atopic dermatitis, contact dermatitis, and chronic spontaneous urticaria. Although it has a prominent role in disease, its signaling transduction is poorly understood. This study shows that MRGPRX2 activation with substance P increased Lysyl t-RNA synthetase (LysRS) translocation to the nucleus. LysRS is a moonlighting protein with a dual role in protein translation and IgE signaling in mast cells. Upon allergen- IgE-FcεRI crosslinking, LysRS is translocated to the nucleus and activates microphthalmia-associated transcription factor (MITF) activity. In this study, we found that MRGPRX2 triggering led to MITF phosphorylation and increased MITF activity. Therefore, overexpression of LysRS increased MITF activity after MRGPRX2 activation. MITF silencing reduced MRGPRX2-dependent calcium influx and mast cell degranulation. Furthermore, a MITF pathway inhibitor, ML329, impaired MITF expression, calcium influx, and mast cell degranulation. Moreover, drugs such as atracurium, vancomycin, and morphine, reported to induce MRGPRX2-dependent degranulation, increased MITF activity. Altogether, our data show that MRGPRX2 signaling enhances MITF activity, and its abrogation by silencing or inhibition resulted in defective MRGPRX2 degranulation. We conclude that MRGPRX2 signaling involves the LysRS and MITF pathway. Thus, MITF and MITF-dependent targets may be considered therapeutic approaches to treat pathologies where MRGPRX2 is implicated.

Transcription Factors in the Development and Pro-Allergic Function of Mast Cells

Mast cells (MCs) are innate immune cells of hematopoietic origin localized in the mucosal tissues of the body and are broadly implicated in the pathogenesis of allergic inflammation. Transcription factors have a pivotal role in the development and differentiation of mast cells in response to various microenvironmental signals encountered in the resident tissues. Understanding the regulation of mast cells by transcription factors is therefore vital for mechanistic insights into allergic diseases. In this review we summarize advances in defining the transcription factors that impact the development of mast cells throughout the body and in specific tissues, and factors that are involved in responding to the extracellular milieu. We will further describe the complex networks of transcription factors that impact mast cell physiology and expansion during allergic inflammation and functions from degranulation to cytokine secretion. As our understanding of the heterogeneity of mast cells becomes more detailed, the contribution of specific transcription factors in mast cell-dependent functions will potentially offer new pathways for therapeutic targeting.

The underestimated role of the microphthalmia-associated transcription factor (MiTF) in normal and pathological haematopoiesis

Haematopoiesis, the process by which a restrained population of stem cells terminally differentiates into specific types of blood cells, depends on the tightly regulated temporospatial activity of several transcription factors (TFs). The deregulation of their activity or expression is a main cause of pathological haematopoiesis, leading to bone marrow failure (BMF), anaemia and leukaemia. TFs can be induced and/or activated by different stimuli, to which they respond by regulating the expression of genes and gene networks. Most TFs are highly pleiotropic; i.e., they are capable of influencing two or more apparently unrelated phenotypic traits, and the action of a single TF in a specific setting often depends on its interaction with other TFs and signalling pathway components. The microphthalmia-associated TF (MiTF) is a prototype TF in multiple situations. MiTF has been described extensively as a key regulator of melanocyte and melanoma development because it acts mainly as an oncogene. Mitf-mutated mice show a plethora of pleiotropic phenotypes, such as microphthalmia, deafness, abnormal pigmentation, retinal degeneration, reduced mast cell numbers and osteopetrosis, revealing a greater requirement for MiTF activity in cells and tissue. A growing amount of evidence has led to the delineation of key roles for MiTF in haematopoiesis and/or in cells of haematopoietic origin, including haematopoietic stem cells, mast cells, NK cells, basophiles, B cells and osteoclasts. This review summarizes several roles of MiTF in cells of the haematopoietic system and how MiTFs can impact BM development.

A novel role for antizyme inhibitor 2 as a regulator of serotonin and histamine biosynthesis and content in mouse mast cells

Antizymes and antizyme inhibitors are key regulatory proteins of polyamine levels by affecting ornithine decarboxylase and polyamine uptake. Our previous studies indicated a metabolic interplay among polyamines, histamine and serotonin in mast cells, and demonstrated that polyamines are present in mast cell secretory granules, being important for histamine storage and serotonin levels. Recently, the novel antizyme inhibitor-2 (AZIN2) was proposed as a local regulator of polyamine biosynthesis in association with mast cell serotonin-containing granules. To gain insight into the role of AZIN2 in the biosynthesis and storage of serotonin and histamine, we have generated bone marrow derived mast cells (BMMCs) from both wild-type and transgenic Azin2 hypomorphic mice, and have analyzed polyamines, serotonin and histamine contents, and some elements of their metabolisms. Azin2 hypomorphic BMMCs did not show major mast cell phenotypic alterations as judged by morphology and specific mast cell proteases. However, compared to wild-type controls, these cells showed reduced spermidine and spermine levels, and diminished growth rate. Serotonin levels were also reduced, whereas histamine levels tended to increase. Accordingly, tryptophan hydroxylase-1 (TPH1; the key enzyme for serotonin biosynthesis) mRNA expression and protein levels were reduced, whereas histidine decarboxylase (the enzyme responsible for histamine biosynthesis) enzymatic activity was increased. Furthermore, microphtalmia-associated transcription factor, an element involved in the regulation of Tph1 expression, was reduced. Taken together, our results show, for the first time, an element of polyamine metabolism -AZIN2-, so far described as exclusively devoted to the control of polyamine concentrations, involved in regulating the biosynthesis and content of other amines like serotonin and histamine.

Cardiac hypertrophy is negatively regulated by miR-541

Heart failure is a leading cause of death in aging population. Cardiac hypertrophy is an adaptive reaction of the heart against cardiac overloading, but continuous cardiac hypertrophy is able to induce heart failure. We found that the level of miR-541 was decreased in angiotensin II (Ang-II) treated cardiomyocytes. Enforced expression of miR-541 resulted in a reduced hypertrophic phenotype upon Ang-II treatment in cellular models. In addition, we generated miR-541 transgenic mice that exhibited a reduced hypertrophic response upon Ang-II treatment. Furthermore, we found miR-541 is the target of microphthalmia-associated transcription factor (MITF) in the hypertrophic pathway and MITF can negatively regulate the expression of miR-541 at the transcriptional levels. MITF(ce/ce) mice exhibited a reduced hypertrophic phenotype upon Ang-II treatment. Knockdown of MITF also results in a reduction of hypertrophic responses after Ang-II treatment. Knockdown of miR-541 can block the antihypertrophic effect of MITF knockdown in cardiomyocytes upon Ang-II treatment. This indicates that the effect of MITF on cardiac hypertrophy relies on the regulation of miR-541. Our present study reveals a novel cardiac hypertrophy regulating pathway that was composed of miR-541 and MITF. Modulation of their levels may provide a new approach for tackling cardiac hypertrophy.

Genetic evidence for critical roles of P38α protein in regulating mast cell differentiation and chemotaxis through distinct mechanisms

Mast cells mediate a range of immune responses. However, the mechanisms that contribute to their development remain poorly understood. Here, using a P38α conditional knockout system, we provide evidence to suggest that P38α plays critical roles in regulating mast cell differentiation and migration via distinct mechanisms. Induced deletion of P38α in bone marrow cells retards the maturation of mast cells in part by inhibiting the activation of cAMP response element-binding protein and expression of microphthalmia-associated transcription factor, which encourages the generation of basophils over mast cells. In fully differentiated mast cells, absence of P38α inhibits stem cell factor-induced activation of Akt and ERK, which is associated with reduced chemotaxis. In vivo, conditional deletion of P38α results in reduced numbers of mast cells in certain tissues and a failure to reconstitute these cells in W(sh) mice transplanted with P38α(-/-) Lin(-)c-kit(+)Sca-1(+) (LKS(+)) cells. Our findings suggest that P38α plays a dual role in mast cell development by regulating IL-3-induced differentiation of mast cell progenitor cells as well as by regulating stem cell factor-induced migration of fully differentiated mast cells.

Importin beta plays an essential role in the regulation of the LysRS-Ap(4)A pathway in immunologically activated mast cells

We recently reported that diadenosine tetraphosphate hydrolase (Ap(4)A hydrolase) plays a critical role in gene expression via regulation of intracellular Ap(4)A levels. This enzyme serves as a component of our newly described lysyl tRNA synthetase (LysRS)-Ap(4)A biochemical pathway that is triggered upon immunological challenge. Here we explored the mechanism of this enzyme's translocation into the nucleus and found its immunologically dependent association with importin beta. Silencing of importin beta prevented Ap(4)A hydrolase nuclear translocation and affected the local concentration of Ap(4)A, which led to an increase in microphthalmia transcription factor (MITF) transcriptional activity. Furthermore, immunological activation of mast cells resulted in dephosphorylation of Ap(4)A hydrolase, which changed the hydrolytic activity of the enzyme.

Involvement of MITF-A, an alternative isoform of mi transcription factor, on the expression of tryptase gene in human mast cells

Mast cells play a central role in the initiation and development of allergic diseases through release of various mediators. Tryptase has been known to be a key mediator in mast cell-mediated inflammatory reactions. In the present study, we investigated whether the transcription of tryptase gene in human mast cells was induced by microphthalmia (mi)-associated transcription factor (MITF). We observed that the human CD34+ progenitor-derived cultured mast cells and human mast cell line HMC-1 expressed strongly the transcripts of tryptase-beta1 and MITF-A, which is a MITF alterative splicing isoform. The transcriptional activity of tryptase gene was specifically higher in HMC-1 cells compared to the tryptase-negative cells. Using mutant constructs of tryptase promoter, we observed that two E-box (CANNTG) motifs including between -817 to -715 and -421 to -202 are able to involve in the transactivation of tryptase gene by MITF-A. In addition, the binding of these motifs-containing oligonucleotides to MITF proteins was detectable by EMGA using the nuclear extracts of HMC-1 cells and anti-MITF mAb. The overexpression of MITF-A elevated tryptase production by HMC-1 cells, while the introduction of specific siRNA against MITF attenuated the expression and enzymatic activity of tryptase. These data suggest that MITF might play a role in regulating the transcription of tryptase gene in human mast cells.

LysRS serves as a key signaling molecule in the immune response by regulating gene expression

Lysyl-tRNA synthetase (LysRS) was found to produce diadenosine tetraphosphate (Ap(4)A) in vitro more than two decades ago. Here, we used LysRS silencing in mast cells in combination with transfected normal and mutated LysRS to demonstrate in vivo the critical role played by LysRS in the production of Ap(4)A in response to immunological challenge. Upon such challenge, LysRS was phosphorylated on serine 207 in a MAPK-dependent manner, released from the multisynthetase complex, and translocated into the nucleus. We previously demonstrated that LysRS forms a complex with MITF and its repressor Hint-1, which is released from the complex by its binding to Ap(4)A, enabling MITF to transcribe its target genes. Here, silencing LysRS led to reduced Ap(4)A production in immunologically activated cells, which resulted in a lower level of MITF inducible genes. Our data demonstrate that specific LysRS serine 207 phosphorylation regulates Ap(4)A production in immunologically stimulated mast cells, thus implying that LysRS is a key mediator in gene regulation.

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.

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.

Human mast cells produce and release the cytotoxic lymphocyte associated protease granzyme B upon activation

Mast cells are widely distributed throughout the body and express effector functions in allergic reactions, inflammatory diseases, and host defense. Activation of mast cells results in exocytosis of preformed chemical mediators and leads to novel synthesis and secretion of lipid mediators and cytokines. Here, we show that human mast cells also express and release the cytotoxic lymphocyte-associated protease, granzyme B. Granzyme B was active and localized in cytoplasmic granules, morphologically resembling those present in cytotoxic lymphocytes. Expression and release of granzyme B by mast cell-lines HMC-1 and LAD 2 and by cord blood- and mature skin-derived human mast cells depended on the mode of activation of these cells. In mast cell lines and cord blood-derived mast cells, granzyme B expression was mainly induced by non-physiological stimuli (A23187/PMA, Compound 48/80) and substance P. In contrast, mature skin-derived mast cells only produced granzyme B upon IgE-dependent stimulation. We conclude that granzyme B is expressed and released by human mast cells upon physiologic stimulation. This suggests a role for granzyme B as a novel mediator in mast cell biology.

Microphthalmia transcription factor isoforms in mast cells and the heart

The microphthalmia transcription factor (Mitf) is critical for the survival and differentiation of a variety of cell types. While on the transcript level it has been noted that melanocytes and cardiomyocytes express specific Mitf isoforms, mast cells express several isoforms, mainly Mitf-H and Mitf-MC, whose function has not been thoroughly investigated. We found that in mast cells the expression of the specific Mitf isoforms is dependent on physiological stimuli that cause a major shifting of promoter usage and internal splicing. For example, activation of the c-kit signaling pathway almost totally abolished one of the main splice isoforms. Since cardiomyocytes express only the Mitf-H isoform, they were an ideal system to determine this isoform's physiological role. We identified that the expression of myosin light-chain 1a (MLC-1a) is regulated by Mitf-H. Interestingly, the transactivation of MLC-1a by Mitf-H in cardiomyocytes is decreased by overexpression of the splice form with exon 6a. In conclusion, we found that there is physiological switching of Mitf isoforms and that the promoter context and the cell context have a combined influence on gene expression programs.

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.

Involvement of the SgIGSF/Necl-2 adhesion molecule in degranulation of mesenteric mast cells

Spermatogenic immunoglobulin superfamily (SgIGSF) expressed on nerve and mast cells, binds homophilically between both in culture. In the steady-state mesentery of mice, the proportion of morphologically degranulating mast cells was approximately 20%, and it increased nearly two-fold when the mesenteric nerve root was stimulated electrically. In contrast, there was no significant increase detectable in the mesentery of MITF-mutants, from which bone marrow-derived cultured mast cells (BMMCs) lack SgIGSF. BMMCs from SgIGSF-knockout mice transplanted to the mesentery of mast cell-deficient W/W(v) mice did not degranulate in response to the mesenteric nerve stimulation, whereas transfection with SgIGSF cDNA restored those responses. SgIGSF appeared to promote communication between nerves and mast cells in the murine mesentery.

The Expression of Clcn7 and Ostm1 in Osteoclasts Is Coregulated by Microphthalmia Transcription Factor

Microphthalmia transcription factor (MITF) regulates osteoclast function by controling the expression of genes, including tartrate-resistant acid phosphatase (TRAP) and cathepsin K in response to receptor activator of nuclear factor-kappaB ligand (RANKL)-induced signaling. To identify novel MITF target genes, we have overexpressed MITF in the murine macrophage cell line RAW264.7 subclone 4 (RAW/C4) and examined the gene expression profile after sRANKL-stimulated osteoclastogenesis. Microarray analysis identified a set of genes superinduced by MITF overexpression, including Clcn7 (chloride channel 7) and Ostm1 (osteopetrosis-associated transmembrane protein 1). Using electrophoretic mobility shift assays, we identified two MITF-binding sites (M-boxes) in the Clcn7 promoter and a single M-box in the Ostm1 promoter. An anti-MITF antibody supershifted DNA-protein complexes for promoter sites in both genes, whereas MITF binding was abolished by mutation of these sites. The Clcn7 promoter was transactivated by coexpression of MITF in reporter gene assays. Mutation of one Clcn7 M-box prevented MITF transactivation, but mutation of the second MITF-binding site only reduced basal activity. Chromatin immunoprecipitation assays confirmed that the two Clcn7 MITF binding and responsive regions in vitro bind MITF in genomic DNA. The expression of Clcn7 is repressed in the dominant negative mutant Mitf mouse, mi/mi, indicating that the dysregulated bone resorption seen in these mice can be attributed in part to transcriptional repression of Clcn7. MITF regulation of the TRAP, cathepsin K, Clcn7, and Ostm1 genes, which are critical for osteoclast resorption, suggests that the role of MITF is more significant than previously perceived and that MITF may be a master regulator of osteoclast function and bone resorption.

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.

Recent developments in the transcriptional regulation of cytolytic effector cells

Transcription factors have a profound influence on both the differentiation and effector function of cells of the immune system. T-bet controls the cytotoxicity of CD8(+) T cells and the production of interferon-gamma, and it also affects the development and function of natural killer cells and natural killer T cells. Other factors such as eomesodermin, MEF, ETS1 and members of the interferon-regulatory factor family also contribute to the effector function of immune cells. In this review, we focus on recent studies that have shed light on the transcriptional mechanisms that regulate cellular effector function in the immune system.

Requirement of c-jun transcription factor on the mouse mast cell protease-6 expression in the mast cells

Mast cell tryptases may be a key mediator in mast cell-mediated inflammatory reactions, and these expressions can be regulated by microenvironmental factors of tissues, particularly stem cell factor. In the present study, we investigated whether the transcription of mouse mast cell protease-6 (mMCP-6) gene was caused by SCF-mediated c-jun. We observed that mMCP-6 mRNA was expressed by overexpression of c-jun in the immature mast cell line in which both mMCP-6 and c-kit receptor are negative. The c-jun increased synergistically the luciferase activity of mMCP-6 promoter through the direct interaction with mi transcription factor (MITF). The synergic effect of c-jun with MITF was abolished by deletion of sequence between nt -171 and -151 in the mMCP-6 promoter. Furthermore, the level of mMCP-6 mRNA in mast cells was attenuated by the introduction of dominant negative c-jun (TAM-67) and the treatment of Jun N-terminal kinase inhibitor, SP600125. These results show that c-jun might play a role in regulating the transcription of mMCP-6 gene in mast cells stimulated by SCF.

MITF is necessary for generation of prostaglandin D2 in mouse mast cells

Mast cells generate eicosanoids that are linked to asthma and other inflammatory diseases. A basic-helix-loop-helix leucine zipper transcription factor termed MITF is essential for the development of mast cells. Although other substances also linked to inflammatory reactions (such as various proteases and serotonin) require MITF for their expression, the role of MITF in eicosanoid generation has not been studied. We examined eicosanoid generation in bone marrow-derived mast cells (BMMCs) of tg/tg mice that lack MITF. Most eicosanoids generated by BMMCs are either prostaglandin (PG) D2 or leukotriene C4. The former is synthesized via the cyclooxygenase pathway, whereas the latter is synthesized via the 5-lipoxygenase pathway. In response to stimulation with IgE and antigens, BMMCs of tg/tg mice synthesized leukotriene C4 normally. However, neither immediate nor delayed PGD2 production was detected in these BMMCs. This indicates that MITF is a transcription factor that specifically activates the cyclooxygenase pathway, but not the 5-lipoxygenase pathway. Significant decreases in expression of hematopoietic PGD2 synthase (hPGDS, a terminal synthase for PGD2) were observed at both mRNA and protein levels in tg/tg BMMCs. MITF transactivated the hPGDS gene via a CACCTG motif located in the promoter region. MITF appeared to be essential for generation of PGD2 by enhancing expression of the hPGDS gene in BMMCs.

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.

Cloning of a soluble isoform of the SgIGSF adhesion molecule that binds the extracellular domain of the membrane-bound isoform

SgIGSF (spermatogenic immunoglobulin superfamily) is a recently identified intercellular adhesion molecule of the immunoglobulin superfamily. In a mast-cell cDNA library, we found a clone that resulted from the retention of intron 7 within the mature SgIGSF message. This clone was predicted to encode a soluble isoform of SgIGSF (sSgIGSF) with 336 amino-acid residues because its open reading frame ended just before the transmembrane domain. We constructed a plasmid expressing sSgIGSF fused to the human IgG Fc fragment at its C-terminus (sSgIGSF-Fc), and transfected it into COS-7 cells. The fusion protein was readily detectable in the culture supernatant. Solid-phase binding assay showed that sSgIGSF interacted directly the extracellular domain of membrane-bound SgIGSF (mSgIGSF). We next examined whether this interaction inhibited homophilic binding of mSgIGSF by aggregation assays using L cells that did not express mSgIGSF. A stable L-cell clone that overexpressed mSgIGSF aggregated with each other but not with mock-transfected L cells, indicating that a homophilic interaction of mSgIGSF mediated the aggregation. Addition of sSgIGSF-Fc inhibited the aggregation of L cells overexpressing mSgIGSF in a dose-dependent manner. Moreover, FACScan analyses revealed the specific binding of sSgIGSF-Fc to mSgIGSF expressed in L cells. Binding of sSgIGSF-Fc to mSgIGSF appeared to inhibit homophilic interactions of mSgIGSF.

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.

The function of lysyl-tRNA synthetase and Ap4A as signaling regulators of MITF activity in FcepsilonRI-activated mast cells

The involvement of microphthalmia transcription factor (MITF) in the function of mast cells, melanocytes, and osteoclasts has recently started to be investigated in depth. In a previous study, we found Hint to be associated with MITF in mast cells and showed that it suppresses MITF's transcriptional activity. Here, we have found that lysyl-tRNA synthetase (LysRS) is also associated with MITF and forms a multicomplex with MITF and Hint. We have also shown that Ap4A, an endogenous molecule consisting of two adenosine linked by four phosphate which is known to be synthesized by LysRS, is accumulated intracellularily above 700 microM in IgE-Ag-activated mast cells, binds to Hint, liberates MITF, and thus leads to the activation of MITF-dependent gene expression. This implies that LysRS plays a key role via Ap4A as an important signaling molecule in MITF transcriptional activity.

Role played by microphthalmia transcription factor phosphorylation and its Zip domain in its transcriptional inhibition by PIAS3

Mutation of microphthalmia transcription factor (MITF) results in deafness, bone loss, small eyes, and poorly pigmented eyes and skin. A search for MITF-associated proteins, using a mast cell library that was screened with a construct that encodes the basic helix-loop-helix leucine zipper (Zip) domain of MITF, resulted in the isolation of the STAT3 inhibitor, PIAS3. PIAS3 functions in vivo as a key molecule in suppressing the transcriptional activity of MITF. Here, we report that the Zip domain is the region of MITF that is involved in the direct interaction between MITF and PIAS3. Additionally, we investigated the effect of phosphorylation of MITF on its interaction with PIAS3. We found that phosphorylation of MITF on serines in positions 73 and 409 plays an important role in its association with PIAS3. This effect was profound with phosphorylation on Ser409, which significantly reduced the inhibitory effect of PIAS3 on MITF and also modulated the transcriptional activity of MITF. Thus, phosphorylation of MITF could be considered a fine, and alternative, tuning of its transcriptional machinery.

Identification of NDRG1 as an early inducible gene during in vitro maturation of cultured mast cells

Coculture of mouse bone marrow-derived mast cells (BMMC) with fibroblasts in the presence of stem cell factor (SCF) facilitates morphological and functional maturation toward a connective tissue mast cell (CTMC)-like phenotype. By means of cDNA subtraction, we identified several inducible genes during this mast cell maturation process. Of approximately 100 sequenced clones induced, nearly 50% were chromosome 14-associated serine proteases. Approximately 14% encoded NDRG1, a 43-kDa cytosolic protein that has been implicated in cell differentiation. NDRG1 was distributed in the cytosol of cultured mast cells and CTMC in rat skin. Overexpression of NDRG1 in RBL-2H3 cells resulted in enhanced degranulation in response to various stimuli. Thus, NDRG1 may be a mast cell maturation-associated inducible protein that allows the cells to be susceptible to extracellular stimuli leading to degranulation. Additionally, several unique maturation-associated inducible genes were identified, molecular and functional characterization of which will provide new insights into mast cell biology.

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.

Additive effect of mouse genetic background and mutation of MITF gene on decrease of skin mast cells

The mi transcription factor (MITF) is a basic-helix-loop-helix leucine zipper transcription factor and is encoded by mi locus. The mi/mi mutant mice showed a significant decrease of skin mast cells in C57BL/6 (B6) genetic background but not in WB genetic background. Kit ligand (KitL) is the most important growth factor for development of mast cells, and the decrease of skin mast cells in B6-mi/mi mice was attributable to the reduced expression of c-kit receptor tyrosine kinase (KIT) that is a receptor for KitL. However, the expression level of KIT in WB-mi/mi mast cells was comparable with that of B6-mi/mi mast cells, suggesting that a factor compensating the reduced expression of KIT was present in WB-mi/mi mice. By linkage analysis, such a factor was mapped on chromosome 10. The mapped position was closely located to the KitL locus. Two alternative spliced forms are known in KitL mRNA: KL-1 and KL-2. Soluble KitL, which is important for development of skin mast cells, is produced more efficiently from KL-1 mRNA than from KL-2 mRNA. The KL-1/KL-2 ratio was higher in WB-mi/mi than in B6-mi/mi mice, suggesting that the larger amount of soluble KitL may compensate for the reduced expression of KIT in WB-mi/mi mice.

A truncated isoform of the protein phosphatase 2A B56gamma regulatory subunit may promote genetic instability and cause tumor progression

F10, a subline of the B16 mouse melanoma cell line, is itself the parent of the more metastatic BL6 line. BL6 cells differ from F10 cells by an alteration of the gene encoding the B56gamma regulatory subunit of protein phosphatase 2A (PP2A), which results in the expression of a truncated variant of the subunit (Deltagamma1). PP2A is involved in regulating the cell-cycle checkpoint and we found that the checkpoint in BL6 cells is aberrant when the Deltagamma1 protein is expressed. That is, although Deltagamma1 protein levels in cultured BL6 cells are low and these cells do not show an altered checkpoint on gamma-irradiation, irradiated footpad BL6 tumor cells show both a marked increase in Deltagamma1 levels and more extensive polyploidy and less apoptosis than F10 cells. These observations were reproduced with Deltagamma1 gene-transfected F10 cells (F10(Deltagamma1)). Deltagamma1 expression and an aberrant checkpoint are also associated with a higher metastatic ability because irradiated F10(Deltagamma1) tumors metastasized much more frequently than F10 tumors, which rarely metastasized whether irradiated or not. Nonirradiated F10(Deltagamma1) tumors, which do not express Deltagamma1 protein, had similarly low rates of metastasis. The greater metastatic ability of irradiated F10(Deltagamma1) tumors also correlated with the acquisition of many more genomic alterations. Thus, it seems that Deltagamma1 expression may damage the checkpoint, which may then allow the acquisition of genetic alterations that promote metastasis. These observations support the notion that mechanisms promoting the genetic instability of tumors could also aid tumor progression from the nonmetastatic to the metastatic state.

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.

The function of MITF and associated proteins in mast cells

Mutation of microphthalmia transcription factor (MITF) results in deafness, bone loss, small eyes, and poorly pigmented eyes and skin. The primary cell types affected in MITF-deficient mice are melanocytes, osteoclasts and mast cells. A search for MITF-associated proteins, using a mast cell library that was screened with a construct that encodes the basic helix-loop-helix leucine zipper (bHLH-Zip) domain of MITF, resulted in the isolation of the protein kinase C interacting (PKCI) protein 1 and protein inhibitor of activated STAT3 (PIAS3). We have accumulated clear evidence of a function for these two proteins as repressors of MITF-induced transcriptional activity. Here, we describe this evidence and ideas that give some insight into the cellular network of interactions between various transcription factors and MITF.

Effect of MITF on mast cell differentiation

The mi transcription factor (MITF) is a basic helix-loop-helix leucine zipper (bHLH-Zip) transcription factor and encoded by the mi locus of mice. Double gene dose of mutant allele at the mi locus results the decrease of mast cells and phenotypic abnormalities of mast cells. Various mutations have been reported at the mi locus. We divided them to null and inhibitory mutations. The tg is a typical null mutation due to the insertion of a transgene into the promoter region of MITF gene. Adult mice of tg/tg genotype can easily obtained and are a potentially useful tool for studying development and function of mast cells.

The identification and functional characterization of a novel mast cell isoform of the microphthalmia-associated transcription factor

The microphthalmia-associated transcription factor (Mitf) is critical for mast cell development based on the severe mast cell deficiency seen in Mitf mutant mice. Mitf also is important for the development of melanocytes, osteoclasts, and retinal pigment epithelium. The lineage-restricted phenotypes of Mitf mutations correlate with tissue-restricted expression of Mitf, a feature due in part to the presence of several distinct Mitf isoforms. We report the identification and characterization of a novel mast cell isoform, Mitf-mc. This isoform arises from alternative splicing of a novel 5'-exon onto the common body of the gene and is predicted to encode a unique 43-amino acid sequence at its amino terminus. It is specifically expressed in mast cells. The mast cell isoform functions differently from the melanocyte isoform in its ability to activate cell type-specific Mitf gene targets. Mitf-mc functions only on a mast cell target promoter and fails to activate a melanocyte target promoter despite binding to its E-box element. Moreover, Mitf-mc heterodimerizes with a closely related transcription factor, Tfe3, and dominantly inhibits the ability of Tfe3 to transactivate a melanocyte-specific promoter. These studies identify a new isoform of Mitf with tissue-specific features that may underlie key aspects of the mast cell phenotype of Mitf mutations.

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.

Meu10 is required for spore wall maturation in Schizosaccharomyces pombe

BACKGROUND

Many genes are meiosis and/or sporulation-specifically transcribed during this process. Isolation and analysis of these genes might help us to understand how meiosis and sporulation are regulated. For this purpose, we have isolated a large number of cDNA clones from Schizosaccharomyces pombe whose expression is up-regulated during meiosis.

RESULTS

We have isolated meu10+ gene, which encodes 416 amino acids and bears homology to SPS2 of Saccharomyces cerevisiae. A strain whose meu10+ gene has been deleted forms no viable spores. Thin-section electron micrographs showed that the meu10Delta strain has abnormally formed spore walls, and then they disrupt, allowing cytoplasmic material to escape. The Meu10-GFP fusion protein is localized to the spore periphery, thereafter returned to the cytoplasm after sporulation. Meu10-GFP localization to the spore wall was almost normal in the bgs2Delta or chs1Delta mutants that lack 1,3-beta-glucan or chitin, respectively. In contrast, 1,3-beta-glucan is abnormally localized in meu10Delta cells. Meu10 has an N-terminal domain with homology to the mammalian insulin receptor and a C-terminal domain with a transmembrane motif. Mutants whose N-terminal or C-terminal domain was truncated were severely defective for sporulation.

CONCLUSIONS

Meu10 is a spore wall component and plays a pivotal role in the formation of the mature spore wall structure.

Isoforms of mi transcription factor preferentially expressed in cultured mast cells of mice

MITF is a basic helix-loop-helix leucine zipper transcription factor, which is important for normal phenotypic expression of mast cells. Three isoforms of MITF have been known in mice, MITF-A, -H, and -M. Since cultured mast cells (CMCs) are useful for studying the function of MITF, we examined isoforms of MITF expressed in CMCs using 5'-RACE, and found a new isoform of MITF, MITF-E. We assessed the relative mRNA amount of various MITF isoforms with reverse transcription-PCR. When the mRNA amount of MITF-E was used as a standard, that of MITF-M was approximately 10%, that of MITF-H was approximately 1%, and that of MITF-A was approximately 0.1%. Although MITF-E was the preferential isoform in CMCs, peritoneal mast cells expressed only MITF-M. The expression profile of MITF isoforms appeared to be influenced by the developing process of mast cells.

A new role for the STAT3 inhibitor, PIAS3: a repressor of microphthalmia transcription factor

In vitro and in vivo evidence suggest that microphthalmia transcription factor (MITF) plays a key regulatory role in tissue-specific gene regulation in several cell types, including melanocytes, osteoclasts, and mast cells. A yeast two-hybrid search, using a portion of a nonmutated MITF gene as the bait in the screening of a mast cell library, resulted in the isolation of the STAT3 inhibitor, PIAS3. PIAS3 is a transcriptional inhibitor that acts by specifically inhibiting STAT3's DNA binding activity. We found that it can directly associate with MITF using an in vitro pull-down assay. Immunoprecipitation of MITF from rat basophilic leukemic cells or mouse melanocytes resulted in the specific co-immunoprecipitation of PIAS3. Co-transfection of MITF with PIAS3 in NIH 3T3 fibroblasts containing an mMCP-6 promoter-luciferase reporter demonstrated up to 94% inhibition of MITF-mediated transcriptional activation. Using a gel-shift assay, it was shown that PIAS3 can block DNA binding activity. It was also found that STAT3 does not interfere, either in vitro or in vivo, with the interaction between PIAS3 and MITF. These data suggest that PIAS3 functions in vivo as a key molecule in supressing the transcriptional activity of MITF, a role of considerable importance in mast cell and melanocyte development.

Effect of MITF on transcription of transmembrane tryptase gene in cultured mast cells of mice

Mouse mast cell protease (mMCP)-6, mMCP-7 and transmembrane tryptase (TMT) are all tryptases. The normal mi transcription factor (+-MITF) transactivated mMCP-6 gene by binding three consensus motifs in the promoter region, but no MITF-binding motifs were found in the mMCP-7 promoter. Instead, c-Jun transactivated mMCP-7 gene, and +-MITF cooperated with it. The mi-MITF encoded by mutant mi allele inhibited the transactivation by c-Jun and reduced the mMCP-7 promoter activity. Here, the effect of MITF on the TMT gene expression was examined. The +-MITF enhanced the TMT promoter activity by binding two consensus motifs. The mi-MITF showed the inhibitory effect on TMT gene expression. The effect of +-MITF on TMT gene was similar to the effect on mMCP-6 gene, and that of mi-MITF was similar to the effect on mMCP-7 gene. The effects of MITF on TMT gene appeared distinct from its effects on either mMCP-6 or mMCP-7 gene.

Effect of a large deletion of the basic domain of mi transcription factor on differentiation of mast cells

The mi transcription factor (MITF) is a basic-helix-loop-helix-leucine zipper transcription factor that is important for the development of mast cells. Cultured mast cells (CMCs) of mi/mi genotype express abnormal MITF (mi-MITF), but CMCs of tg/tg genotype do not express any MITFs. It was previously reported that mi/mi CMCs showed more severe abnormalities than tg/tg CMCs, indicating that mi-MITF had inhibitory function. Whereas mi-MITF contains a single amino acid deletion in the basic domain, MITF encoded by mi(ew) allele (ew-MITF) deletes 16 of 21 amino acids of the basic domain. Here the effect of a large deletion of the basic domain was examined. In mi(ew)/mi(ew) CMCs, the expression pattern of genes whose transcription was affected by MITF was comparable to that of tg/tg CMCs rather than to that of mi/mi CMCs. This suggested that ew-MITF lacked any functions. The part of the basic domain deleted in ew-MITF appeared necessary for either transactivation or inhibition of transactivation.

Increased expression of a nucleolar Nop5/Sik family member in metastatic melanoma cells: evidence for its role in nucleolar sizing and function

F10 and BL6 cells of B16 mouse melanoma cells are metastatic after intravenous injection, but only BL6 cells can metastasize to lungs after subcutaneous injection. Differences in gene expression between the two cell lines were examined, and a greater expression of the Sik-similar protein (Sik-SP) gene was found in BL6 cells. Structurally, Sik-SP belongs to the nucleolar Nop5/Sik family whose members play central roles in ribosome biogenesis; however, the function of Sik-SP has not been examined. Cytology with green fluorescent protein-fused proteins showed that Sik-SP was localized to the nucleolus. To examine whether Sik-SP is involved in ribosome biogenesis, two parameters were measured: magnitude of ribosomal RNA synthesis per nucleus and magnitude of protein production from the same amount of mRNA of an exogenous luciferase gene. Both values and, in addition, nucleolar size were larger in COS-7 monkey kidney cells overexpressing Sik-SP and BL6 cells than in mock-transfected COS-7 and F10 cells, respectively. Sik-SP seemed to promote ribosome biogenesis in the nucleolus. Furthermore, the expression of Sik-SP seemed to confer a greater cell growth response to serum, because such a response was greater in BL6 cells and F10 cells overexpressing Sik-SP than in untreated and mock-transfected F10 cells. Sik-SP may render melanoma cells more competent to survive through augmenting the activity of nucleolus.

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

The mi transcription factor (MITF) is a basic-helix-loop-helix leucine zipper (bHLH-Zip) transcription factor that is important for the development of mast cells. Mast cells of mi/mi genotype express normal amounts of abnormal MITF (mi-MITF), whereas mast cells of tg/tg genotype do not express any MITFs. The synthesis of heparin is abnormal in the skin mast cells of mi/mi mice. Because N-deacetylase/N-sulfotransferase 2 (NDST-2) is essential for the synthesis of heparin, the amount of NDST-2 messenger RNA (mRNA) was compared among cultured mast cells (CMCs) of +/+, mi/mi, and tg/tg genotypes. The NDST-2 mRNA was detected by in situ hybridization in the skin mast cells of +/+ and tg/tg mice, but not in the skin mast cells of mi/mi mice. The amount of NDST-2 mRNA decreased significantly in CMCs derived from mi/mi mice when compared to the values of +/+ and tg/tg mice, suggesting that the defective form of MITF inhibited the expression of the NDST-2 transcript. The expression of NDST-2 transcript was mediated by the GGAA motif located in the 5'-untranslated region. GA binding protein (GABP) bound the GGAA motif and increased the amount of NDST-2 transcript. The mi-MITF appeared to inhibit the ability of GABP to express NDST-2 transcript by disturbing its nuclear localization. This is the first study to show that expression of an abnormal form of a bHLH-Zip transcription factor can dramatically alter the intracellular location of another DNA/RNA binding factor, which in turn brings about profound and unexpected consequences on transcript expression.

Inhibitory effect on natural killer activity of microphthalmia transcription factor encoded by the mutant mi allele of mice

The mouse mi locus encodes a basic-helix-loop-helix-leucine zipper-type transcription factor, microphthalmia transcription factor (MITF). Mice of mi/mi genotype express a mutant form of MITF (mi-MITF), whereas mice of tg/tg genotype have a transgene in the 5' flanking region of the mi gene and do not express MITF. Although the mi/mi mouse is deficient in natural killer (NK) activity, it was found that the tg/tg mouse was normal in this respect. To know the cause, spleen cells of both genotypes were compared. Although the proportion of spleen cells expressing an NK cell marker, NK1.1, was comparable in both mice, the proportion of large granular lymphocytes decreased only in mi/mi mice. The difference between mi/mi and tg/tg mice was reproducible in the culture supplemented with interleukin-2. Moreover, the perforin gene expression was reduced in mi/mi-cultured spleen cells. Wild-type (+) MITF transactivated, but mi-MITF suppressed, the perforin gene promoter through the NF-P motif, a strong cis-acting element. However, neither +-MITF nor mi-MITF bound the NF-P motif. Instead, 2 nuclear factors that bound the NF-P motif were retained in the cytoplasm of mi/mi-cultured spleen cells. In addition, overexpression of mi-MITF resulted in cytoplasmic retention of the 2 NF-P motif-binding factors in cytotoxic T lymphocytes. The presence of mi-MITF rather than the absence of +-MITF appeared to lead to poor transactivation of the NF-P motif by intercepting NF-P motif-binding factors. This inhibitory effect of mi-MITF may cause the deficient cytotoxicity of NK cells in mi/mi mice. (Blood. 2001;97:2075-2083)

Importance of leucine zipper domain of mi transcription factor (MITF) for differentiation of mast cells demonstrated using mi(ce)/mi(ce) mutant mice of which MITF lacks the zipper domain

The mi transcription factor (MITF) is a basic helix-loop-helix leucine zipper (bHLH-Zip) transcription factor that is important for the development of mast cells. Mast cells of mi/mi genotype express normal amount of abnormal MITF (mi-MITF), whereas mast cells of tg/tg genotype do not express any MITFs. Mast cells of mi/mi mice show more severe abnormalities than those of tg/tg mice, indicating that the mi-MITF possesses the inhibitory function. The MITF encoded by the mi(ce) mutant allele (ce-MITF) lacks the Zip domain. We examined the importance of the Zip domain using mi(ce)/mi(ce) mice. The amounts of c-kit, granzyme B (Gr B), and tryptophan hydroxylase (TPH) messenger RNAs decreased in mast cells of mi(ce)/mi(ce) mice to levels comparable to those of tg/tg mice, and the amounts were intermediate between those of +/+ mice and those of mi/mi mice. Gr B mediates the cytotoxic activity of mast cells, and TPH is a rate-limiting enzyme for the synthesis of serotonin. The cytotoxic activity and serotonin content of mi(ce)/mi(ce) mast cells were comparable to those of tg/tg mast cells and were significantly higher than those of mi/mi mast cells. The phenotype of mi(ce)/mi(ce) mast cells was similar to that of tg/tg mast cells rather than to that of mi/mi mast cells, suggesting that the ce-MITF had no functions. The Zip domain of MITF appeared to be important for the development of mast cells. (Blood. 2001;97:2038-2044)

Inhibitory effect of the transcription factor encoded by the mutant mi microphthalmia allele on transactivation of mouse mast cell protease 7 gene

The transcription factor encoded by the mi locus (MITF) is a transcription factor of the basic-helix-loop-helix zipper protein family. Mice of mi/mi genotype express a normal amount of abnormal MITF, whereas mice of tg/tg genotype do not express any MITFs due to the transgene insertional mutation. The effect of normal (+) and mutant (mi) MITFs on the expression of mouse mast cell protease (MMCP) 6 and 7 was examined. Both MMCP-6 and MMCP-7 are tryptases, and their coding regions with high homology are closely located on chromosome 17. Both MMCP-6 and MMCP-7 genes are expressed in normal cultured mast cells (+/+ CMCs). Although the transcription of MMCP-6 gene was severely suppressed in both mi/mi and tg/tg CMCs, that of MMCP-7 gene was severely suppressed only in mi/mi CMCs. The study identified the most significant segment for the transcription in the 5' flanking region of MMCP-7 gene. Unexpectedly, no CANNTG motifs were found that are recognized and bound by +-MITF in this segment. Instead, there was an AP-1 binding motif, and binding of c-Jun to the AP-1 motif significantly enhanced the transcription of MMCP-7 gene. The complex formation of c-Jun with either +-MITF or mi-MITF was demonstrated. The binding of +-MITF to c-Jun enhanced the transactivation of MMCP-7 gene, and that of mi-MITF suppressed the transactivation. Although the former complex was located only in the nucleus, the latter complex was predominantly found in the cytoplasm. The negative effect of mi-MITF on the transcription of MMCP-7 gene appeared to be executed through the interaction with c-Jun.

A role for heterologous gap junctions between melanoma and endothelial cells in metastasis

F10 and BL6 sublines of B16 mouse melanoma cells are metastatic after intravenous injection, but only BL6 cells are metastatic after subcutaneous injection. We found that connexin (Cx) 26 is upregulated in BL6 cells. To examine gap junction formation, we devised a coculture system, in which an opened vein segment was placed at the bottom of a culture dish and then dye-labeled melanoma cells were seeded onto it. Immunohistochemistry indicated that the vein segment preserved the integrity of the endothelial monolayer. In this system, BL6 cells could transfer dye into endothelial cells but F10 cells could not. Transfection with wild-type Cx26 rendered F10 cells competent for coupling with endothelial cells and as spontaneously metastatic as BL6 cells. Conversely, transfection with a dominant-negative form of Cx26 rendered BL6 cells deficient in coupling and less metastatic. In human melanoma lesions, the level of Cx26 expression was low in melanoma cells residing in the basal layer, but significantly upregulated in melanoma cells invading the dermis. The results suggested that Cx26 plays a role in intravasation and extravasation of tumor cells through heterologous gap junction formation with endothelial cells.

A truncated isoform of the PP2A B56 subunit promotes cell motility through paxillin phosphorylation

Both F10 and BL6 sublines of B16 mouse melanoma cells are metastatic after intravenous injection, but only BL6 cells are metastatic after subcutaneous injection. Retrotransposon insertion was found to produce an N-terminally truncated form (Deltagamma1) of the B56gamma1 regulatory subunit isoform of protein phosphatase (PP) 2A in BL6 cells, but not in F10 cells. We found an interaction of paxillin with PP2A C and B56gamma subunits by co-immunoprecipitation. B56gamma1 co-localized with paxillin at focal adhesions, suggesting a role for this isoform in targeting PP2A to paxillin. In this regard, Deltagamma1 behaved similarly to B56gamma1. However, the Deltagamma1-containing PP2A heterotrimer was insufficient for the dephosphorylation of paxillin. Transfection with Deltagamma1 enhanced paxillin phosphorylation on serine residues and recruitment into focal adhesions, and cell spreading with an actin network. In addition, Deltagamma1 rendered F10 cells as highly metastatic as BL6 cells. These results suggest that mutations in PP2A regulatory subunits may cause malignant progression.