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

Emerging roles of MITF as a crucial regulator of immunity

Microphthalmia-associated transcription factor (MITF), a basic helix-loop-helix leucine zipper transcription factor (bHLH-Zip), has been identified as a melanocyte-specific transcription factor and plays a critical role in melanocyte survival, differentiation, function, proliferation and pigmentation. Although numerous studies have explained the roles of MITF in melanocytes and in melanoma development, the function of MITF in the hematopoietic or immune system-beyond its function in melanin-producing cells-is not yet fully understood. However, there is convincing and increasing evidence suggesting that MITF may play multiple important roles in immune-related cells. Therefore, this review is focused on recent advances in elucidating novel functions of MITF in cancer progression and immune responses to cancer. In particular, we highlight the role of MITF as a central modulator in the regulation of immune responses, as elucidated in recent studies.

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.

Development and maturation of natural killer cells

Natural killer (NK) cells are innate lymphocytes that are critical for host protection against pathogens and cancer due to their ability to rapidly release inflammatory cytokines and kill infected or transformed cells. In the 40 years since their initial discovery, much has been learned about how this important cellular lineage develops and functions. We now know that NK cells are the founding members of an expanded family of lymphocyte known as innate lymphoid cells (ILC). Furthermore, we have recently discovered that NK cells can possess features of adaptive immunity such as antigen specificity and long-lived memory responses. Here we will review our current understanding of the molecular mechanisms driving development of NK cells from the common lymphoid progenitor (CLP) to mature NK cells, and from activated effectors to long-lived memory NK cells.

Transcriptional and post-transcriptional regulation of NK cell development and function

Natural killer (NK) cells are specialized innate lymphoid cells that survey against viral infections and malignancy. Numerous advances have improved our understanding of the molecular mechanisms that control NK cell development and function over the past decade. These include both studies on the regulatory effects of transcription factors and translational repression via microRNAs. In this review, we summarize our current knowledge of DNA-binding transcription factors that regulate gene expression and thereby orchestrate NK cell development and activation, with an emphasis on recent discoveries. Additionally, we highlight our understanding of how RNA-binding microRNAs fine tune the NK cell molecular program. We also underscore the large number of open questions in the field that are now being addressed using new technological approaches and genetically engineered model organisms. Ultimately, a deeper understanding of the basic molecular biology of NK cells will facilitate new strategies to manipulate NK cells for the treatment of human disease.

Transcriptional Control of NK Cells

Natural killer (NK) cells are innate lymphocytes that survey the environment and protect the host from infected and cancerous cells. As their name implies, NK cells represent an early line of defense during pathogen invasion by directly killing infected cells and secreting inflammatory cytokines. Although the function of NK cells was first described more than four decades ago, the development of this cytotoxic lineage is not well understood. In recent years, we have begun to identify specific transcription factors that control each stage of development and maturation, from ontogeny of the NK cell progenitor to the effector functions of activated NK cells in peripheral organs. This chapter highlights the transcription factors that are unique to NK cells, or shared between NK cells and other hematopoietic cell lineages, but govern the biology of this cytolytic lymphocyte.

Mitf regulates osteoclastogenesis by modulating NFATc1 activity

Transcription factors Mitf and NFATc1 share many downstream targets that are critical for osteoclastogenesis. Since RANKL signals induce/activate both NFATc1 and Mitf isoform-E (Mitf-E), a tissue-restricted Mitf isoform in osteoclasts, it is plausible that the two factors work together to promote osteoclastogenesis. Although Mitf was shown to function upstream of NFATc1 previously, this study showed that expression of Mitf had little effects on NFATc1 and NFATc1 was critical for the induction of Mitf-E. In Mitf(mi/mi) mice, the semi-dominant mutation in Mitf gene leads to arrest of osteoclastogenesis in the early stages. However, when stimulated by RANKL, the Mitf(mi/mi) preosteoclasts responded with a significant induction of NFATc1, despite that the cells cannot differentiate into functional osteoclasts. In the absence of RANKL stimulation, very high levels of NFATc1 are required to drive osteoclast development. Our data indicate that Mitf functions downstream of NFATc1 in the RANKL pathway, and it plays an important role in amplifying NFATc1-dependent osteoclastogenic signals, which contributes to the significant synergy between the two factors during osteoclastogenesis. We propose that Mitf-E functions as a tissue-specific modulator for events downstream of NFATc1 activation during osteoclastogenesis.

Transcriptional control of natural killer cell differentiation and function

Gene expression can be modulated depending on physiological and developmental requirements. A multitude of regulatory genes, which are organized in interdependent networks, guide development and eventually generate specific phenotypes. Transcription factors (TF) are a key element in the regulatory cascade controlling cell fate and effector functions. In this review, we discuss recent data on the diversity of TF that determine natural killer (NK) cell fate and NK cell function.

Transcriptional control of natural killer cell development and function

Natural killer (NK) cells play an important role in host defense against tumors and viruses and other infectious diseases. NK cell development is regulated by mechanisms that are both shared with and separate from other hematopoietic cell lineages. Functionally, NK cells use activating and inhibitory receptors to recognize both healthy and altered cells such as transformed or infected cells. Upon activation, NK cells produce cytokines and cytotoxic granules using mechanisms similar to other hematopoietic cell lineages especially cytotoxic T cells. Here we review the transcription factors that control NK cell development and function. Although many of these transcription factors are shared with other hematopoietic cell lineages, they control unexpected and unique aspects of NK cell biology. We review the mechanisms and target genes by which these transcriptional regulators control NK cell development and functional activity.

The transcriptional control of the perforin locus

SUMMARY

Natural killer (NK) cells and cytotoxic T lymphocytes (CTLs) use cytotoxic granules containing perforin and granzymes to lyse infected or malignant host cells, thereby providing immunity to intracellular microbes and tumors. Perforin is essential for cytotoxic granule-mediated killing. Perforin expression is regulated transcriptionally and correlates tightly with the development of cells that can exhibit cytotoxic activity. Although a number of genes transcribed by T cells and NK cells have been studied, the cell-specificity of perforin gene expression makes it an ideal model system in which to clarify the transcriptional mechanisms that guide the development and activation of cytotoxic lymphocytes. In this review, we discuss what is known about perforin expression and its regulation, then elaborate on recent studies that utilized chromosome transfer and bacterial artificial chromosome transgenics to define a comprehensive set of cis-regulatory regions that control transcription of the human PRF1 gene in a near-physiologic context. In addition, we compare the human and murine Prf1 loci and discuss how transcription factors known to be important for driving CTL differentiation might also directly regulate the cis-acting domains that control Prf1. Our review emphasizes how studies of PRF1/Prf1 gene transcription can illuminate not only the mechanisms of cytotoxic lymphocyte differentiation but also some basic principles of transcriptional regulation.

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.

Stromal-cell regulation of natural killer cell differentiation

Natural killer (NK) cells are bone-marrow-derived lymphocytes that play a crucial role in host defense against some viral and bacterial infections, as well as against tumors. Their phenotypic and functional maturation requires intimate interactions between the bone marrow stroma and committed precursors. In parallel to the identification of several phenotypic and functional stages of NK cell development, recent studies have shed new light on the role of stromal cells in driving functional maturation of NK cells. In this review, we provide an overview of the role of bone marrow microenvironment in NK cell differentiation.

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.

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.

Involvement of connective tissue-type mast cells in Th1 immune responses via Stat4 expression

Mast cells are the sentinels of immune systems and, like other immuno-competent cells, they are produced by hematopoietic stem cells. We analyzed the expression of signal transducer and activator of transcription 4 (Stat4), and investigated its role in mast cells. Murine mast cells are usually divided into 2 distinct populations by their distribution and contents of their granules: mucosal mast cells (MMCs) and connective tissue-type mast cells (CTMCs). Stat4 protein was detected in CTMCs but not in MMCs. The absence of Stat4 expression in cultured mast cells was due to the presence of Stat6. In T-helper (Th) cells, Stat4 plays an important role in Th1 shift by inducing a set of genes, such as interferon gamma (IFN-gamma) and interleukin-18 receptor alpha subunit (IL-18Ralpha). As in Th1 shift, we found that Stat4 trans-activated these genes in the Stat4-expressing cultured mast cells, namely, microphthalmia transcription factor (MITF)-deficient cultured MMCs, Stat6-deficient cultured MMCs, and cultured CTMCs. Stat4 also enhanced expression of nitric oxide synthase 2 (NOS2) in CTMCs, which brought about increased levels of NO-dependent cytotoxic activity. These data indicate that expression of Stat4 in CTMCs plays an important role on Th1 immune responses.

The dynamic life of natural killer cells

Natural killer (NK) cells play important roles in immunological processes, including early defense against viral infections. This review provides an overview of the dynamic in vivo life of NK cells from their development in the bone marrow to their mature peripheral responses and their ultimate demise, with particular emphasis on mouse NK cells and viral infections.

Strain-dependent inhibitory effect of mutant mi-MITF on cytotoxic activities of cultured mast cells and natural killer cells of mice

MITF is a transcription factor encoded by the mi locus. MITF encoded by mi and Mi(or) mutant alleles (mi-MITF and Mi(or)-MITF, respectively) possessed an inhibitory effect, whereas the tg, mi(ew) and mi(ce) were null mutants. We examined the cytotoxic activities of cultured mast cells (CMCs) and natural killer (NK) cells of various MITF mutants in C57BL/6 (B6) background. Cytotoxic activities of CMCs and NK cells of B6-mi/mi and B6-Mi(or)/Mi(or) mice were remarkably reduced. In B6-tg/tg, B6-mi(ew)/mi(ew) and B6-mi(ce)/mi(ce) mice, however, the cytotoxic activity of CMCs was reduced only slightly and the NK activity was normal. The cytotoxic activity of CMCs paralleled with the expression level of granzyme B (Gr B) mRNA, and the NK activity with that of perforin (Pfn) mRNA. In contrast to the case of B6-mi/mi mice, cytotoxic activities of CMCs and NK cells were not impaired in WB-mi/mi mice. The expression of Gr B mRNA was not reduced in CMCs of WB-mi/mi mice, and that of Pfn mRNA was not reduced in NK cells of WB-mi/mi mice. WB-mi/mi mice appeared to have factor(s) compensating for the inhibitory effect of mi-MITF on the expression of Gr B and Pfn genes.

What does it take to make a natural killer?

We know how B and T cells develop, what they 'see' and the receptors they 'see with'. By contrast, and despite an unprecedented increase in the number of receptors and ligands known to regulate the activity of natural killer (NK) cells, we still have many questions regarding how these cells develop. Nevertheless, we are beginning to understand the transcriptional programmes of NK-cell maturation and the role of the effector functions of NK cells in the regulation of immune responses. An improved knowledge of NK-cell development in mice and humans might be useful to harness the power of these natural killers in the clinic to fight autoimmune diseases, infection and cancer.

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.

Bcl2 regulation by the melanocyte master regulator Mitf modulates lineage survival and melanoma cell viability

Kit/SCF signaling and Mitf-dependent transcription are both essential for melanocyte development and pigmentation. To identify Mitf-dependent Kit transcriptional targets in primary melanocytes, microarray studies were undertaken. Among identified targets was BCL2, whose germline deletion produces melanocyte loss and which exhibited phenotypic synergy with Mitf in mice. BCL2's regulation by Mitf was verified in melanocytes and melanoma cells and by chromatin immunoprecipitation of the BCL2 promoter. Mitf also regulates BCL2 in osteoclasts, and both Mitf(mi/mi) and Bcl2(-/-) mice exhibit severe osteopetrosis. Disruption of Mitf in melanocytes or melanoma triggered profound apoptosis susceptible to rescue by BCL2 overexpression. Clinically, primary human melanoma expression microarrays revealed tight nearest neighbor linkage for MITF and BCL2. This linkage helps explain the vital roles of both Mitf and Bcl2 in the melanocyte lineage and the well-known treatment resistance of melanoma.

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.