MafB Expression Research Articles

Cdx1 refines positional identity of the vertebrate hindbrain by directly repressingMafbexpression

An interplay of transcription factors interprets signalling pathways to define anteroposterior positions along the vertebrate axis. In the hindbrain, these transcription factors prompt the position-appropriate appearance of seven to eight segmental structures, known as rhombomeres (r1-r8). The evolutionarily conserved Cdx caudal-type homeodomain transcription factors help specify the vertebrate trunk and tail but have not been shown to directly regulate hindbrain patterning genes. Mafb (Kreisler, Krml1, valentino), a basic domain leucine zipper transcription factor, is required for development of r5 and r6 and is the first gene to show restricted expression within these two segments. The homeodomain protein vHnf1 (Hnf1b) directly activates Mafb expression. vHnf1 and Mafb share an anterior expression limit at the r4/r5 boundary but vHnf1 expression extends beyond the posterior limit of Mafb and, therefore, cannot establish the posterior Mafb expression boundary. Upon identifying regulatory sequences responsible for posterior Mafb repression, we have used in situ hybridization, immunofluorescence and chromatin immunoprecipitation (ChIP) analyses to determine that Cdx1 directly inhibits early Mafb expression in the neural tube posterior of the r6/r7 boundary, which is the anteriormost boundary of Cdx1 expression in the hindbrain. Cdx1 dependent repression of Mafb is transient. After the 10-somite stage, another mechanism acts to restrict Mafb expression in its normal r5 and r6 domain, even in the absence of Cdx1. Our findings identify Mafb as one of the earliest direct targets of Cdx1 and show that Cdx1 plays a direct role in early hindbrain patterning. Thus, just as Cdx2 and Cdx4 govern the trunk-to-tail transition, Cdx1 may regulate the hindbrain-to-spinal cord transition.

Pathogenic pathway related to head abnormalities induced by the fungicide Triadimefon in Xenopus laevis embryos

The bZip transcription factor Mafb is expressed in two segments of the developing vertebrate hindbrain: the rhombomeres 5 and 6. Loss of Mafb expression in the mouse mutant kreisler leads to elimination of r5 and to alterations of r6 regional identity. Here, we further investigated the role of Mafb in hindbrain patterning using gain-of-function experiments in the chick embryo. Our work has revealed novel functions for Mafb, including a positive autoregulatory activity, the capacity to repress Hoxb1 expression, and the capacity to synergise with or antagonise Krox20 activity. These different activities appear to be spatially restricted in the hindbrain, presumably due to interactions with other factors. Reinvestigation of the kreisler mutation indicated that it also results in an ectopic activation of Mafb in rhombomere 3, accounting for the previously described molecular alterations of this rhombomere in the mutant. Together, these data allow us to refine our view of the dual function of Mafb in both segmentation and specification of anteroposterior identity in the hindbrain.

The transcription factor MafB antagonizes antiviral responses by blocking recruitment of coactivators to the transcription factor IRF3

Viral infections induce Type I interferons (IFN-α and -β) that recruit unexposed cells in a self-amplifying response. We report that the transcription factor MAFB thwarts auto-amplification by a metastable switch behavior. MAFB acts as a weak positive basal regulator of transcription at the IFN-β promoter through activity at AP-1-like sites. Interferon elicitors recruit the transcription factor IRF3 to the promoter, whereupon MAFB acts as a transcriptional antagonist, impairing the interaction of CREB-binding protein (CBP) with IRF3. Mathematical modeling supports the view that prepositioning of MAFB on the promoter allows the system to respond rapidly to fluctuations in IRF3 activity. Elevated expression of MAFB in human pancreatic islet β-cells might increase cellular vulnerability to viral infections associated with the etiology of type I diabetes.

Transcription factor MafB is a mediator of retinoic acid‐induced monocytic cell differentiation

All‐trans‐retinoic acid (RA), the major active metabolite of vitamin A, regulates monocytic cell cycle and differentiation. We asked whether MafB, a factor involved in development and hematopoiesis, is a mediator of RA‐induced differentiation in THP‐1 human monocytic cells. MafB mRNA and protein levels were up‐regulated by RA dose‐ and time‐dependently. RA and TNFα synergistically regulated MafB expression, mainly at the transcriptional level. MafB protein was highly phosphorylated. Blocking the ERK and p38 pathways reduced phospho‐MafB protein without changing mRNA levels. Next, MafB target genes were screened. SPOCK1, CCL2 and Blimp1 were identified as potential targets, due to the presence of Maf recognition elements in their putative promoter regions. These genes are related to cell communication and differentiation. By RT‐PCR analysis, the regulation of SPOCK1, Blimp1 and CCL2 mRNA levels were highly correlated with MafB mRNA. Decreased expression of SPOCK1, Blimp1 and CCL2 was observed when MafB phosphorylation was blocked. Chromatin immunoprecipitation assays showed increased MafB protein binding to the elements on the promoter regions of SPOCK1, Blimp1 and CCL2 in cells treated with RA and TNFα. We conclude that RA and TNFα‐induced MafB may play a role in monocytic cell differentiation through transcriptional regulation of SPOCK1, CCL2 and Blimp1. Support: NIH DK41479.Grant Funding Source: NIH DK41479

C-Maf and MafB transcription factors are differentially expressed in Huxley's and Henle's layers of the inner root sheath of the hair follicle and regulate cuticle formation

The hair follicle of mammalian skin consists of a group of concentric epithelial cell layers. The inner root sheath (IRS), which surrounds the hardening hair shaft beneath the skin surface, is subdivided into three layers, termed the cuticle of the IRS, Huxley's layer, and Henle's layer. The IRS forms a follicular wall in the hair canal and helps guide the developing hair shaft. c-Maf and MafB, members of the Maf family of transcription factors, play important roles in the developmental processes of various tissues and in cell type-specific gene expression. The aim of this study is to reveal the pattern of expression and functional roles of c-Maf and MafB in the hair follicle. We determined the precise location of c-Maf and MafB expression using immunofluorescent staining of mouse skin sections with layer-specific markers. We also analyzed whiskers of c-maf- and mafB-null mice (c-maf(-/-) and mafB(-/-), respectively) using scanning electron microscopy. c-Maf and MafB were differentially expressed in the Huxley's and Henle's layers of the IRS. Scanning electron microscopic analysis showed irregular cuticle patterning of whiskers of c-maf(-/-) and mafB(-/-) mice. The cuticles of mafB(-/-) mice were also thinner than those of wild-type mice. c-Maf and MafB are expressed in the IRS layers in a lineage-restricted manner and are involved in hair morphogenesis.

Nucleophosmin Is Required for Macrophage Function and Maturation.

Abstract 238Nucleophosmin (NPM1) is a multifunctional phosphoprotein predominantly localized in the nucleolus that is involved in numerous cellular processes. The NPM1 gene is frequently a target of genetic alteration in hematological tumors and recent published reports have highlighted the importance of NPM1 in malignancies that affect predominantly the myeloid lineage. Complete inactivation of Npm1 in the mouse disrupts primitive hematopoiesis and results in embryonic lethality. Npm1 heterozygosity produces features similar to those of MDS patients, and can progress to overt leukemia. Here we generated a conditional mousemutant to inactivate Npm1 in the myeloid compartment crossing mice carrying a floxed conditional allele of Npm1 (Npm1fl/fl) with the MRP8-Cre transgenic mouse model. Specific deletion of the Npm1 allele in Npm1fl/fl;MRP8-Cre+ mice resulted in complete loss of Npm1 at the protein level in 40% of peritoneal macrophages analyzed by immunofluorescence. In vitro analysis demonstrates that Npm1 deletion altered peritoneal macrophage functions and influenced monocytic differentiation in Npm1fl/fl;MRP8-Cre+ mice. Specifically we demonstrate that (i) superoxide production was significantly attenuated over time in Npm1fl/fl;MRP8-Cre+ compared with Npm1fl/fl;MRP8-Cre- macrophages; (ii) The chemotactic response was decreased by up to 46% ± 9.6 in Npm1fl/fl;MRP8-Cre+ macrophages when compared to controls; and (iii) Phagocytic index of Npm1fl/fl;MRP8-Cre+ macrophages dropped to 40% of that for the control Npm1fl/fl;MRP8-Cre- macrophages. Analysis of monocyte transcriptional regulators revealed a significant reduction of Maf-b and Irf-8 mRNA expression in Npm1 deficient macrophages, demonstrating an altered maturation process in the absence of Npm1. Our analysis demonstrates that Npm1 loss in the myeloid compartment affects macrophage function and profoundly alters gene expression patterns required for proper monocyte/macrophage development and maturation. Disclosures:No relevant conflicts of interest to declare.

The Hexosamine Biosynthesis Pathway Is Essential for Pancreatic Beta Cell Development

Pancreatic exocrine and endocrine cells develop during embryonic life from endodermal progenitors. This process depends on activation of a hierarchy of transcription factors. Although information is available regarding the mesodermal signals controlling pancreas development, little is known about the role of environmental factors such as nutrients, including glucose, that also may impact development. Previously, we showed that glucose plays an important and specific role in beta cell development by activating the transition of Neurogenin3-positive endocrine progenitors into beta cells. Here, we examined the implication of glucose metabolism and more precisely the role of the hexosamine biosynthesis pathway (HBP) to understand the mechanisms by which glucose regulates beta cell development. We have established an in vitro model of endocrine and exocrine cells development from embryonic day 13.5 rat pancreases in a manner that replicates in vivo pancreas development perfectly. Using this model, we tested the effect of selective inhibitors and activators of the HBP and found that the HBP has a modest effect on cell proliferation and exocrine cell differentiation. On the other hand, beta cell development is tightly controlled by the HBP. Specifically, HBP activators increase beta cell development, whereas inhibitors repress such development. Importantly, both the HBP and glucose control the same steps in beta cell development.

Neither MafA/L‐Maf nor MafB is essential for lens development in mice

The importance of the large Maf transcription factor family has been investigated in lens development in the chick, Xenopus and mammals. Previously we reported that c-maf-deficient mice exhibit severe defects in lens fibre cells. Here, we report the roles of other large Mafs, MafA/L-Maf and MafB, during mouse lens development. MafA/L-Maf and MafB were expressed in lens epithelial cells and fibre cells at E12.5 but had largely disappeared from the lens at E18.5. The lens of mafA-, mafB-deficient and mafA::mafB double-deficient mice developed normally. In c-maf-deficient mice, the pattern of expression of MafA and MafB differed from their expression in wild-type mice. Moreover, the expression of crystallin genes was unchanged in mafA-, mafB- and mafA::mafB double-deficient lens. These results indicate that c-Maf alone is essential for lens development, and that MafA/L-Maf and MafB are dispensable in mice.

Transcription factor C/EBPβ isoform ratio regulates osteoclastogenesis through MafB

Disequilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts is central to many bone diseases. Here, we show that dysregulated expression of translationally controlled isoforms of CCAAT/enhancer-binding protein β (C/EBPβ) differentially affect bone mass. Alternative translation initiation that is controlled by the mammalian target of rapamycin (mTOR) pathway generates long transactivating (LAP*, LAP) and a short repressive (LIP) isoforms from a single C/EBPβ transcript. Rapamycin, an inhibitor of mTOR signalling increases the ratio of LAP over LIP and inhibits osteoclastogenesis in wild type (WT) but not in C/EBPβ null (c/ebpβ−/−) or in LIP knock-in (L/L) osteoclast precursors. C/EBPβ mutant mouse strains exhibit increased bone resorption and attenuated expression of MafB, a negative regulator of osteoclastogenesis. Ectopic expression of LAP and LIP in monocytes differentially affect the MafB promoter activity, MafB gene expression and dramatically affect osteoclastogenesis. These data show that mTOR regulates osteoclast formation by modulating the C/EBPβ isoform ratio, which in turn affects osteoclastogenesis by regulating MafB expression.

TFE3 transcription factor regulates the expression of MAFB during macrophage differentiation

Transcription Factor for Immunoglobulin Heavy-Chain Enhancer 3 (Tfe3) is a transactivator of metabolic genes that are regulated through an EBox located in their promoters. It is involved in physiological processes such as osteoclast and macrophage differentiation, as well as in pathological processes such as translocations underlying different cancer diseases. MAFB is a basic region/leucine zipper transcription factor that affects transcription by binding specific DNA regions known as MARE. It plays a pivotal role in regulating lineage-specific hematopoiesis by repressing transcription of erythroid specific genes in myeloid cells and enhancing expression of macrophage and megakaryocytic genes. Here we have shown MAFB to be highly induced in human hematopoietic cells undergoing macrophage differentiation following Tfe3 ectopic expression, and to be down regulated, compared to the controls, in the same cell population following Phorbol Esters (PMA) dependent differentiation coupled to Tfe3 gene silencing. Electrophoretic mobility shift assays identified a Tfe3-binding site (EBox) in the MAFB promoter region that is conserved in different mammalian species. MAFB promoter was transactivated by co-expression of Tfe3 in reporter gene assays while deletion or mutation of the MAFB EBox prevented transactivation by Tfe3. Both of these genes were previously included in the group of transcription factors able to drive macrophage differentiation. The observation that MAFB belongs to the Tfe3 regulon suggests the existence of a pathway where these two gene families act synergistically to determine differentiation.

Identification of primary MAFB target genes in multiple myeloma

In multiple myeloma (MM), seven primary recurrent translocations involving the immunoglobulin heavy chain locus have been identified. One of the partner loci maps to 20q12 and involves the MAFB gene resulting in its ectopic expression. We attempt here to identify MAFB target genes in MM. We used an inducible system to upregulate MAFB in MM cell lines not carrying the t(14;20). Microarray expression analysis was used to detect gene expression changes upon MAFB expression. These genes were further evaluated comparatively with gene expression profiles obtained from MM or plasma cell leukemia tumors carrying an activated MAFB gene. Functional implications of these upregulated genes were studied by testing their promoter activity in reporter assays. C-MAF was included comparatively as well. The inducible cell lines identified a total of 284 modulated transcripts. After further evaluation using ex vivo data 14 common upregulated genes were found, common to the C-MAF pathway as well. The promoter activity of some of these secondary genes proved a functional relationship with MAFB. In connection with one of these secondary genes (NOTCH2), even tertiary upregulated genes were found. Functional studies indicated that inducible MAFB expression conferred antiapoptotic effects. We identified 14 upregulated genes, and their downstream consequences in the combined MAFB/C-MAF pathway. Eleven of these genes are novel in the C-MAF pathway as well. These direct target genes may be responsible for the oncogenic transformation of MAF expressing myeloma cells.

Pax-6 and c-Maf Functionally Interact with the α-Cell-specific DNA Element G1 in Vivo to Promote Glucagon Gene Expression

Specific expression of the glucagon gene in the rat pancreas requires the presence of the G1 element localized at -100/-49 base pairs on the promoter. Although it is known that multiple transcription factors such as Pax-6, Cdx-2/3, c-Maf, Maf-B, and Brain-4 can activate the glucagon gene promoter through G1, their relative importance in vivo is unknown. We first studied the expression of Maf-B, c-Maf, and Cdx-2/3 in the developing and adult mouse pancreas. Although Maf-B was detectable in a progressively increasing number of alpha-cells throughout development and in adulthood, c-Maf and Cdx-2/3 were expressed at low and very low levels, respectively. However, c-Maf but not Cdx-2/3 was detectable in adult islets by Western blot analyses. We then demonstrated the in vivo interactions of Pax-6, Cdx-2/3, Maf-B, and c-Maf but not Brain-4 with the glucagon gene promoter in glucagon-producing cells. Although Pax-6, Cdx-2/3, Maf-B, and c-Maf were all able to bind G1 by themselves, we showed that Pax-6 could interact with Maf-B, c-Maf, and Cdx-2/3 and activate transcription of the glucagon gene promoter. Overexpression of dominant negative forms of Cdx-2/3 and Mafs in alpha-cell lines indicated that Cdx-2/3 and the Maf proteins interact on an overlapping site within G1 and that this binding site is critical in the activation of the glucagon gene promoter. Finally, we show that specific inhibition of Pax-6 and c-Maf but not Cdx-2/3 or Maf-B led to decreases in endogenous glucagon gene expression and that c-Maf binds the glucagon gene promoter in mouse islets. We conclude that Pax-6 and c-Maf interact with G1 to activate basal expression of the glucagon gene.

Expression, purification, crystallization and preliminary crystallographic analysis of the mouse transcription factor MafB in complex with its DNA-recognition motif Cmare.

The MafB transcription factor (residues 211-305) has been overexpressed in and purified from Escherichia coli. A protein-DNA complex between the MafB homodimer and the 21 bp Maf-recognition sequence known as Cmare has been successfully reconstituted in vitro and subsequently crystallized. The diffraction properties of the protein-DNA complex crystals were improved using a combination of protein-construct boundary optimization and targeted mutagenesis to promote crystal lattice stability. Both native and mercury-derivatized crystals have been prepared using these optimized conditions. The crystals belong to space group P4(1)2(1)2 or P4(3)2(1)2, with unit-cell parameters a = b = 94.8, c = 197.9 A. An anomalous difference Patterson map computed using data collected from crystals grown in the presence of HgCl(2) reveals four peaks. This corresponds to two copies of the protein-DNA complex in the asymmetric unit, with a solvent content of 62% and a Matthews coefficient of 3.22 A(3) Da(-1).

MafB negatively regulates RANKL-mediated osteoclast differentiation

AbstractReceptor activator of nuclear factor κB ligand (RANKL) induces osteoclast formation from hematopoietic cells via regulation of various transcription factors. Here, we show that MafB negatively regulates RANKL-induced osteoclast differentiation. Expression levels of MafB are significantly reduced by RANKL during osteoclastogenesis. Overexpression of MafB in bone marrow-derived monocyte/macrophage lineage cells (BMMs) inhibits the formation of TRAP+ multinuclear osteoclasts, but phagocytic activity of BMMs is retained. Furthermore, overexpression of MafB in BMMs attenuates the gene induction of NFATc1 and osteoclast-associated receptor (OSCAR) during RANKL-mediated osteoclastogenesis. In addition, MafB proteins interfere with the DNA-binding ability of c-Fos, Mitf, and NFATc1, inhibiting their transactivation of NFATc1 and OSCAR. Furthermore, reduced expression of MafB by RNAi enhances osteoclastogenesis and increases expression of NFATc1 and OSCAR. Taken together, our results suggest that MafB can act as an important modulator of RANKL-mediated osteoclastogenesis.

CS1 Is Expressed on Myeloma Cells from Early Stage, Late Stage, and Drug-Treated Multiple Myeloma Patients, and Is Selectively Targeted by the HuLuc63 Antibody.

Introduction: One-third of multiple myeloma (MM) patients exhibit high-risk features such as abnormal cytogenetics, high LDH, amplification of CKS1-B or spiked expression of MAF, MAF-B or FGFR3. While not affecting complete response rates, median durations of event-free and overall survival, even with high-dose melphalan-based tandem autotransplants of such patients, do not exceed 24 mo and 36 mo, compared to 60 mo and ≥ 90 mo for the remainder. Monoclonal antibody (mAb)-mediated therapy may target a chemotherapy-resistant myeloma cell pool. CS1 (CD2 subset 1, CRACC, SLAMF7), a member of the CD2 family of cell surface glycoproteins, exhibits high-level expression on primary myeloma cells, indicating that CS1 is a potential target for treatment in MM.Methods: Gene expression was assessed using an Affymetrix GeneChip array. Protein expression was measured by flow cytometry, and immunohistochemistry (IHC), using HuLuc63, a novel humanized anti-CS1 mAb. HuLuc63-mediated lysis of myeloma cells via antibody dependent cellular cytotoxicity (ADCC) was measured by 51Cr-release.Results: CS1 mRNA was detected in >95% of CD138+ purified plasma cells from >95% of healthy donors, newly diagnosed myeloma patients, and those with relapsed myeloma (Fig. 1a). CS1 remained highly expressed in patients following VDTPACE treatment, albeit at a reduced level. CS1 expression was also high following bortezomib (Velcade®) treatment, with a subset of patients showing increased expression post-treatment. CS1 protein expression on primary myeloma cells was confirmed by flow cytometry, while IHC analysis of normal tissues revealed anti-CS1 staining primarily on CD138+ tissue plasma cells. Finally, we determined that HuLuc63 could induce killing of myeloma cells using purified allogeneic NK cells (Fig. 1b). Blocking the Fc receptor greatly reduced this activity indicating an ADCC mechanism. Killing of myeloma targets was also observed in autologous systems suggesting that HuLuc63 can overcome KIR-mediated NK cell inhibition of autologous NK cells. In summary, we observed high mRNA and protein expression of CS1 in myeloma from early stage, late stage, and treated patients, and showed enhanced lysis of myeloma cells in vitro with HuLuc63. Our data support the potential clinical utility of CS1-targeted therapy. HuLuc63 will be entering a phase I clinical trial for advanced myeloma patients. [Display omitted] [Display omitted]

Enhanced Expression of MafB Inhibits Macrophage Apoptosis Induced by Cigarette Smoke Exposure

In the lungs of smokers, oxidative stress rises due to increase of free radicals and oxidants, including lipid peroxide (LPO). The functions of alveolar macrophages (AMs) are altered in such an environment, and their survival is prolonged against toxicities of cigarette smoke (CS) by an unknown mechanism. Whereas functions of AMs are potentially regulated by various transcriptional factors, their expressions and roles in smoking individuals have not been elucidated. Therefore, we investigated their expressions using murine model of CS exposure. Eight-week-old male B6C3F1 mice were whole-bodily exposed to CS (2 cigarettes/mouse/day, 5 d/wk) for 6 mo. Development of pulmonary emphysema in 6-mo CS-exposed mice was confirmed by a morphometric analysis. Among the transcriptional factors investigated, only MafB was upregulated in AMs from CS-exposed mice. DNA binding capacity of MafB for Maf recognition element was also increased in AMs from those mice. LPO was increased significantly in the lungs of CS-exposed mice. Because the end product of LPO, 4-hydroxy-2-nonenal, enhanced MafB expression and its transcriptional activity in a cultured macrophage cell line, LPO-related oxidative stress was suggested to be involved in the mechanism of MafB expression in CS-exposed lung. Furthermore, we established a macrophage cell line that can overexpress MafB and thereby clarify the role of MafB. Forced expression of MafB heightened cell viability and attenuated the occurrence of apoptosis in cells treated with CS-extract. These results suggest that enhanced MafB expression by oxidative stress inhibits AM cell death and prolongs their survival in the CS-exposed lung.

Expression of a Novel Gene, MafB, in Dupuytren’s Disease

Dupuytren's disease (DD) is characterized by fibroblastic proliferation of the palmar fascia, often leading to flexion contracture in the hand. Although there is a strong genetic component the genome-wide expression of novel genes is not known. The purpose of this study was to use DNA microarray technology to identify upregulated genes in DD. Human tissue samples were harvested from 3 patient sources: DD cord tissue (n = 20), normal-appearing adjacent control fascia (n = 15), and palmar fascia from patients having carpal tunnel release (n = 15). DNA microarray analysis was performed on amplified sample RNA. Novel genes were compared with known gene functions. A candidate gene of interest was studied further by using immunohistochemistry on DD tissue samples and controls. Several novel genes not described previously in the study of DD were upregulated significantly, including MafB, collagen type V, alpha-2 (COL5A2), collagen type VIII, alpha-1 (COL8A1), contactin I (CNTN1), and leucine-rich repeat containing 17 (LRRC17). These upregulated genes were compared with their known gene-expression profiles in other tissues and their purported functions. MafB was found to be of particular interest because of its prominent role in tissue development and cellular differentiation. MafB immunohistochemistry showed positive staining in 50% of the DD specimens but complete absence of MafB in all control tissues (adjacent control fascia, carpal tunnel fascia). Co-localization experiments with MafB and alpha-smooth muscle actin showed staining properties in similar regions but these 2 proteins were not confined solely to the same cells. Microarray analysis of DD tissue has identified significant upregulated gene expression of MafB. MafB protein also is found in Dupuytren's cords but not in control fascia. Co-localization data suggest that the association of MafB with DD is not related exclusively to myofibroblast proliferation. Because of its role in fibroblastic transformation in other models MafB and its relationship to the pathogenesis of DD deserves further study.

MafB

The large Maf family of basic leucine-zipper-containing transcription factors are known regulators of key developmental and functional processes in various cell types, including pancreatic islets. Here, we demonstrate that within the adult pancreas, MafB is only expressed in islet alpha-cells and contributes to cell type-specific expression of the glucagon gene through activation of a conserved control element found between nucleotides -77 to -51. MafB was also shown to be expressed in developing alpha- and beta-cells as well as in proliferating hormone-negative cells during pancreatogenesis. In addition, MafB expression is maintained in the insulin(+) and glucagon(+) cells remaining in mice lacking either the Pax4 or Pax6 developmental regulators, implicating a potentially early role for MafB in gene regulation during islet cell development. These results indicate that MafB is not only important to islet alpha-cell function but may also be involved in regulating genes required in both endocrine alpha- and beta-cell differentiation.

MafB Deficient Mouse Results in Impaired Macrophage Differentiation.

MafB is one of the large Maf (musculoaponeurotic fibrosarcoma) proteins sharing a structurally similar basic region/leucine zipper DNA binding motif and a putative activation domain at the N-terminus. MafB is known to be expressed in the myelomonocytic lineages of hematopoietic cells. The recent in vitro experiments showed that the forced MafB expression induced monocytic differentiation of myeloblast progenitors, suggesting MafB could be the critical determinant in monocyte/macrophage differentiation. Nevertheless, the precise expression pattern and the physiological function of MafB in hematopoietic cells have never been analyzed in vivo setting. To address these issues, we have generated mafB locus GFP knock-in mouse. In E14.5 mafB GFP knock-in heterozygous embryos, GFP expression was specifically observed in F4/80 (differentiated macrophage marker) positive fetal liver cells. In homozygous mutant, expression of F4/80+ was significantly decreased in the neonatal liver, spleen and peripheral blood. Interestingly, the colony assay using E14.5 fetal liver cells revealed that the number of macrophage colony was significantly increased in homozygous mutant under the M-CSF supplemented condition, whereas the colony size was dramatically reduced, suggesting MafB deficiency prevent the terminal macrophage differentiation of fetal liver cells, then evoked the abnormal accumulation of premature progenitors. In the macrophages of homozygous mutant obtained by in vitro fetal liver differentiation, F4/80 expression and phagocytotic activity was dramatically suppressed compared to the wild type control. These observations strongly suggest that MafB is indispensable for the differentiation and the functional maturation of macrophages.

ARK5 is transcriptionally regulated by the Large-MAF family and mediates IGF-1-induced cell invasion in multiple myeloma: ARK5 as a new molecular determinant of malignant multiple myeloma

ARK5, AMP-activated protein kinase (AMPK)-related protein kinase mediating Akt signals, is closely involved in tumor progression, and its stage-associated expression was observed in colorectal cancer. In this study, we found ARK5 expression in multiple myeloma cell lines expressing c-MAF and MAFB. In addition, gene expression profiling of 351 clinical specimens revealed ARK5 expression in primary myelomas expressing c-MAF and MAFB, suggesting that ARK5 may be a transcriptional target of the Large-MAF family. Sequence analysis of the ARK5 gene promoter revealed that it contains two putative MAF-recognition element (MARE) sequences. In support of this hypothesis, ARK5 was induced when an MAFB or c-MAF expression vector was introduced into non-ARK5-expressing colon cancer cells. Furthermore, ARK5 promoter activity was dramatically decreased by mutation or deletion of MARE sequences. Chromatin immunoprecipitation assays revealed an interaction between the Large-MAF family proteins and MARE sequences in the ARK5 promoter. Moreover, in ARK5 mRNA-expressing multiple myeloma lines, but not in ARK5-negative lines, insulin-like growth factor (IGF)-1 increased invasion activity. IGF-1-induced invasion was reproduced when ARK5 was overexpressed in Burkitt's lymphoma and plasmacytoma lines. Based on results, we conclude that ARK5 is a transcriptional target of the Large-MAF family through MARE sequence and that ARK5 may in part mediate the aggressive phenotype associated with c-MAF- and MAFB-expressing myelomas.