Structure-function analysis of the maf oncogene product, a member of the b-Zip protein family

Broadening the genotypic and phenotypic spectrum of MAF in three Chinese Han congenital cataracts families

Pathogenic variants in the v-maf avian musculoaponeurotic fibrosarcoma oncogene homologue (MAF) encoding a transcription factor (from a unique subclass of basic leucine zipper transcription factors) are associated with isolated congenital cataracts (CCs) and Aymé-Gripp syndrome (AYGRPS). We collected detailed disease histories from, and performed comprehensive ophthalmic and systemic examinations in 269 patients with CCs; we then performed whole-exome sequencing. Pathogenicity assessments were evaluated using multiple predictive tools. The clinical validities of the reported gene-disease relationships for MAF genes (MAF-CCs and MAF-AYGRPS) were assessed using the ClinGen gene curation framework. We identified two novel (c.173C>A, p.Thr58Asn and c.947T>C, p. Leu316Pro) variants and one known (c.173C>T, p.Thr58Ile) MAF missense variant in three patients. We described novel phenotypes including cleft palate, macular hypoplasia, and retinal neovascularization in the peripheral avascular area and analyzed the genotype-phenotype correlations. We demonstrated associations of variants in the MAF C-terminal DNA-binding domain with CCs and associations of variants in the N-terminal transactivation domain of MAF with AYGRPS. We thus expand the genotypic and phenotypic spectrum of the MAF gene. The ClinGen gene curation framework results suggested that variants in different domains of MAF are associated with different diseases.

MafF Is an Antiviral Host Factor That Suppresses Transcription from Hepatitis B Virus Core Promoter

Hepatitis B virus (HBV) is a stealth virus that exhibits only minimal induction of the interferon system, which is required for both innate and adaptive immune responses. However, 90% of acutely infected adults can clear the virus, suggesting the presence of additional mechanisms that facilitate viral clearance. Here, we report that Maf bZIP transcription factor F (MafF) promotes host defense against infection with HBV. Using a small interfering RNA (siRNA) library and an HBV/NanoLuc (NL) reporter virus, we screened to identify anti-HBV host factors. Our data showed that silencing of MafF led to a 6-fold increase in luciferase activity after HBV/NL infection. Overexpression of MafF reduced HBV core promoter transcriptional activity, which was relieved upon mutation of the putative MafF binding region. Loss of MafF expression through CRISPR/Cas9 editing (in HepG2-hNTCP-C4 cells) or siRNA silencing (in primary hepatocytes [PXB cells]) induced HBV core RNA and HBV pregenomic RNA (pgRNA) levels, respectively, after HBV infection. MafF physically binds to the HBV core promoter and competitively inhibits HNF-4α binding to an overlapping sequence in the HBV enhancer II sequence (EnhII), as seen by chromatin immunoprecipitation (ChIP) analysis. MafF expression was induced by interleukin-1β (IL-1β) or tumor necrosis factor alpha (TNF-α) treatment in both HepG2 and PXB cells, in an NF-κB-dependent manner. Consistently, MafF expression levels were significantly enhanced and positively correlated with the levels of these cytokines in patients with chronic HBV infection, especially in the immune clearance phase. IMPORTANCE HBV is a leading cause of chronic liver diseases, infecting about 250 million people worldwide. HBV has developed strategies to escape interferon-dependent innate immune responses. Therefore, the identification of other anti-HBV mechanisms is important for understanding HBV pathogenesis and developing anti-HBV strategies. MafF was shown to suppress transcription from the HBV core promoter, leading to significant suppression of the HBV life cycle. Furthermore, MafF expression was induced in chronic HBV patients and in primary human hepatocytes (PXB cells). This induction correlated with the levels of inflammatory cytokines (IL-1β and TNF-α). These data suggest that the induction of MafF contributes to the host's antiviral defense by suppressing transcription from selected viral promoters. Our data shed light on a novel role for MafF as an anti-HBV host restriction factor.

A Comprehensive Survey of the Roles of Highly Disordered Proteins in Type 2 Diabetes

Type 2 diabetes mellitus (T2DM) is a chronic and progressive disease that is strongly associated with hyperglycemia (high blood sugar) related to either insulin resistance or insufficient insulin production. Among the various molecular events and players implicated in the manifestation and development of diabetes mellitus, proteins play several important roles. The Kyoto Encyclopedia of Genes and Genomes (KEGG) database has information on 34 human proteins experimentally shown to be related to the T2DM pathogenesis. It is known that many proteins associated with different human maladies are intrinsically disordered as a whole, or contain intrinsically disordered regions. The presented study shows that T2DM is not an exception to this rule, and many proteins known to be associated with pathogenesis of this malady are intrinsically disordered. The multiparametric bioinformatics analysis utilizing several computational tools for the intrinsic disorder characterization revealed that IRS1, IRS2, IRS4, MAFA, PDX1, ADIPO, PIK3R2, PIK3R5, SoCS1, and SoCS3 are expected to be highly disordered, whereas VDCC, SoCS2, SoCS4, JNK9, PRKCZ, PRKCE, insulin, GCK, JNK8, JNK10, PYK, INSR, TNF-α, MAPK3, and Kir6.2 are classified as moderately disordered proteins, and GLUT2, GLUT4, mTOR, SUR1, MAPK1, IKKA, PRKCD, PIK3CB, and PIK3CA are predicted as mostly ordered. More focused computational analyses and intensive literature mining were conducted for a set of highly disordered proteins related to T2DM. The resulting work represents a comprehensive survey describing the major biological functions of these proteins and functional roles of their intrinsically disordered regions, which are frequently engaged in protein–protein interactions, and contain sites of various posttranslational modifications (PTMs). It is also shown that intrinsic disorder-associated PTMs may play important roles in controlling the functions of these proteins. Consideration of the T2DM proteins from the perspective of intrinsic disorder provides useful information that can potentially lead to future experimental studies that may uncover latent and novel pathways associated with the disease.

Large Maf Transcription Factors: Cousins of AP-1 Proteins and Important Regulators of Cellular Differentiation

A large number of mammalian transcription factors possess the evolutionary conserved basic and leucine zipper domain (bZIP). The basic domain interacts with DNA while the leucine zipper facilitates homo- and hetero-dimerization. These factors can be grouped into at least seven families: AP-1, ATF/CREB, CNC, C/EBP, Maf, PAR, and virus-encoded bZIPs. Here, we focus on a group of four large Maf proteins: MafA, MafB, c-Maf, and NRL. They act as key regulators of terminal differentiation in many tissues such as bone, brain, kidney, lens, pancreas, and retina, as well as in blood. The DNA-binding mechanism of large Mafs involves cooperation between the basic domain and an adjacent ancillary DNA-binding domain. Many genes regulated by Mafs during cellular differentiation use functional interactions between the Pax/Maf, Sox/Maf, and Ets/Maf promoter and enhancer modules. The prime examples are crystallin genes in lens and glucagon and insulin in pancreas. Novel roles for large Mafs emerged from studying generations of MafA and MafB knockouts and analysis of combined phenotypes in double or triple null mice. In addition, studies of this group of factors in invertebrates revealed the evolutionarily conserved function of these genes in the development of multicellular organisms.

Somatic loss of function mutations in neurofibromin 1 and MYC associated factor X genes identified by exome-wide sequencing in a wild-type GIST case

Background

Approximately 10–15 % of gastrointestinal stromal tumors (GISTs) lack gain of function mutations in the KIT and platelet-derived growth factor receptor alpha (PDGFRA) genes. An alternate mechanism of oncogenesis through loss of function of the succinate-dehydrogenase (SDH) enzyme complex has been identified for a subset of these “wild type” GISTs.

Methods

Paired tumor and normal DNA from an SDH-intact wild-type GIST case was subjected to whole exome sequencing to identify the pathogenic mechanism(s) in this tumor. Selected findings were further investigated in panels of GIST tumors through Sanger DNA sequencing, quantitative real-time PCR, and immunohistochemical approaches.

Results

A hemizygous frameshift mutation (p.His2261Leufs*4), in the neurofibromin 1 (NF1) gene was identified in the patient’s GIST; however, no germline NF1 mutation was found. A somatic frameshift mutation (p.Lys54Argfs*31) in the MYC associated factor X (MAX) gene was also identified. Immunohistochemical analysis for MAX on a large panel of GISTs identified loss of MAX expression in the MAX-mutated GIST and in a subset of mainly KIT-mutated tumors.

Conclusion

This study suggests that inactivating NF1 mutations outside the context of neurofibromatosis may be the oncogenic mechanism for a subset of sporadic GIST. In addition, loss of function mutation of the MAX gene was identified for the first time in GIST, and a broader role for MAX in GIST progression was suggested.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1872-y) contains supplementary material, which is available to authorized users.

Multiple functions of Maf in the regulation of cellular development and differentiation

Cellular muscular aponeurotic fibrosarcoma (c-Maf) is a member of the large macrophage-activating factor family. C-Maf plays important roles in the morphogenetic processes and cellular differentiation of the lens, kidneys, liver, T cells and nervous system, and it is particularly important in pancreatic islet and erythroblastic island formation. However, the exact role of c-Maf remains to be elucidated. In this review, we summarize the research to clarify the functions of c-Maf in the cellular development and differentiation. The expression of c-Maf is higher in pancreatic duct cells than in pancreatic islet cells. Therefore, we suggest that pancreatic duct cells may be converted to the functional insulin-secreting cells by regulating c-Maf.

The mRNA expression of immune-related genes in crossbred and Tharparkar cattle in response to in vitro infection with Theileria annulata

Tropical theileriosis is a major protozoan disease of cattle and is associated with high rates of morbidity and mortality. Indigenous cattle (Bos indicus) are less affected by this disease than exotic and crossbred cattle. Genetic basis of resistance to tropical theileriosis in indigenous cattle is not well studied. Recent reports suggest that number of immune response genes expressed differentially in exotic and indigenous breeds play an important role in breed specific resistance to tropical theileriosis. Such studies comparing expression of these genes in crossbred cattle and indigenous cattle are lacking. The present study compares the mRNA expression of immune-related genes in response to Theileria annulata infection in indigenous and crossbred cattle. Peripheral blood mononuclear cells (PBMCs) were isolated from blood samples of indigenous (Tharparkar) and crossbred (HF/BS/Jersey × Hariana) cattle and challenged with prepared ground-up tick supernatant carrying Theileria annulata sporozoites in vitro. qPCR was employed to measure relative mRNA expression of toll-like receptor 10 (TLR10), signal-regulatory protein alpha (SIRPA), MHC class II DQα (BoLA-DQA), musculoaponeurotic fibrosarcoma (MAF) and prion protein (PRNP) genes in infected and control PBMCs from crossbred and indigenous cattle. On the basis of comparative fold change analysis, significant up-regulation in SIRPA, PRNP and MHC DQα genes and significant down-regulation in TLR10, cMAF and MAFB genes in crossbreds as compared to indigenous cattle was observed. Results of the present study suggest that breed specific differential expression of the genes under study may contribute to the breed specific resistance to Theileria annulata infection in indigenous cattle compared to crossbred cattle.

A novel DNA binding mechanism for maf basic region-leucine zipper factors inferred from a MafA-DNA complex structure and binding specificities

MafA is a proto-oncoprotein and is critical for insulin gene expression in pancreatic β-cells. Maf proteins belong to the AP1 superfamily of basic region-leucine zipper (bZIP) transcription factors. Residues in the basic helix and an ancillary N-terminal domain, the Extended Homology Region (EHR), endow maf proteins with unique DNA binding properties: binding a 13 bp consensus site consisting of a core AP1 site (TGACTCA) flanked by TGC sequences and binding DNA stably as monomers. To further characterize maf DNA binding, we determined the structure of a MafA-DNA complex. MafA forms base-specific hydrogen bonds with the flanking G(-5)C(-4) and central C(0)/G(0) bases, but not with the core-TGA bases. However, in vitro binding studies utilizing a pulse-chase electrophoretic mobility shift assay protocol revealed that mutating either the core-TGA or flanking-TGC bases dramatically increases the binding off rate. Comparing the known maf structures, we propose that DNA binding specificity results from positioning the basic helix through unique phosphate contacts. The EHR does not contact DNA directly but stabilizes DNA binding by contacting the basic helix. Collectively, these results suggest a novel multistep DNA binding process involving a conformational change from contacting the core-TGA to contacting the flanking-TGC bases.

The transcription factor c-Maf in sensory neuron development

The proto-oncogene c-Maf has been shown to be an important transcriptional regulator in the differentiation of a number of cellular contexts, like the eye and hematopoietic system. Here we discuss the recent progress made in understanding c-Maf function in the nervous system.

Tyrosine phosphorylation of c-Maf enhances the expression of IL-4 gene

Maf proteins are involved in a variety of biological processes, such as oncogenesis, lens development, and differentiation. In immune system, c-Maf transactivates IL-4 promoter, and ectopic expression of c-Maf skews primary T cell response toward the Th2 pathway. Numerous transcription factors are subjected to posttranslational modification. In this study, to our knowledge, we show for the first time that c-Maf is subjective to tyrosine phosphorylation in Th cells and that the level of its tyrosine phosphorylation positively correlates with IL-4 expression by peripheral Th cells, but is negatively associated with the severity of disease in NOD mice. c-Maf undergoes tyrosine phosphorylation at Tyr(21), Tyr(92), and Tyr(131) residues in Th2 cells. Furthermore, tyrosine phosphorylation at these three residues is critical for the recruitment of c-Maf to IL-4 promoter and IL-4 production in Th cells. Taken together, this study sheds new light on the role of posttranslational modification of c-Maf in IL-4 production and Th cell-mediated autoimmune diseases.

MafA and MafB activity in pancreatic β cells

Analyses in mouse models have revealed crucial roles for MafA (musculoaponeurotic fibrosarcoma oncogene family A) and MafB in islet β cells, with MafB being required during development and MafA in adults. These two closely related transcription factors regulate many genes essential for glucose sensing and insulin secretion in a cooperative and sequential manner. Significantly, the switch from MafB to MafA expression also appears to be vital for functional maturation of β cells produced by human embryonic stem (hES) cell differentiation. This review summarizes the discovery, distribution, and function of MafA and MafB in rodent pancreatic β cells, and describes some key questions regarding their importance to β cells.

MafA is a Key Molecule in Glucose and Energy Balance in the Central Nervous System and Peripheral Organs

MafA is a strong transactivator of insulin in pancreatic β cells. Elucidating the profile of MafA action in organs other than the pancreas is essential. We established an mRNA interference technique that modifies the level of target mRNAs in mice in vivo. After rapidly injecting MafA-siRNA, the resulting changes in the gene profile were analyzed using a microarray system. Significant suppression of the MafA mRNA levels was observed in the pancreas, liver, adipose tissue, and brain of siRNA-injected mice. As we reported previously, the down-regulation of insulin mRNA and adipocytokines was observed in the pancreas, and MafA siRNA caused alterations in the expressions of genes related to lipid metabolism and cell growth in the liver, and the attenuation of cell differentiation in cultured adipocytes. In addition to the effects on these organs, MafA expression was immunohistochemically detected in the brain in our preliminary data, and the expression level in siRNA-treated mice was significantly suppressed. The expressions of the affected genes were distinct, including growth hormone, vasopressin, hypocretin, and pro-melanin-concentrating hormone, were almost completely down-regulated (to ~1/100). These results suggested that MafA is likely involved in the regulation of hormonal systems related to glucose metabolism, and MafA is likely positioned near the beginning of the cascade or may influence the expressions of the above-mentioned genes in coordination with other factors in brain tissue. Taken together, the findings in this study suggested that MafA functions as a transcription factor with distinct activities in each organ and is cross-linked in several organs.

HTLV-1 basic leucine-zipper factor, HBZ, interacts with MafB and suppresses transcription through a Maf recognition element

HTLV-1 infection causes adult T-cell leukemia (ATL). The development of ATL is thought to be associated with disruption of transcriptional control of cellular genes. HTLV-1 basic leucine-zipper (bZIP) factor, HBZ, is encoded by the complementary strand of the provirus. We previously reported that HBZ interacts with c-Jun and suppresses its transcriptional activity. To identify the cellular factor(s) that interact with HBZ, we conducted a yeast two-hybrid screen using full-length HBZ as bait and identified MafB. HBZ heterodimerizes with MafB via each bZIP domain. Luciferase analysis revealed a significant decrease in transcription through Maf recognition element (MARE) in a manner dependent on the bZIP domain of HBZ. Indeed, production of full-length HBZ in cells decreased the MARE-bound MafB protein, indicating that HBZ abrogates the DNA-binding activity of MafB. In addition, HBZ reduced the steady-state levels of MafB, and the levels were restored by treatment with a proteasome inhibitor. These results suggest a suppressive effect of HBZ on Maf function, which may have a significant role in HTLV-1 related pathogenesis.

Clinical impact of chromosomal aberrations in multiple myeloma

Chromosomal aberrations are frequently found in multiple myeloma cells and play a major role in patient outcome and management of the disease. The most important chromosomal aberrations associated with poor outcome are del(17p), t(4;14), t(14;16) and t(14;20). Others that may be associated with adverse prognosis include amp(1)(q21), del(1p32), del(13), del(8p21) and hypodiploidy. Many chromosomal aberrations have no or uncertain impact; for example, t(11;14), t(8;14) and hyperdiploidy. Attempts have been made to overcome the negative prognostic impact of chromosomal aberrations using autologous or allogeneic transplantation or new immunomodulatory drugs such as thalidomide, lenalidomide and the proteasome inhibitor bortezomib, but the results are controversial. Data suggest that allogeneic transplantation and treatment with bortezomib or lenalidomide may help to overcome the negative effect of del(13) on prognosis, whereas bortezomib may have some influence on reducing the impact of del(17p), t(4;14) and t(14;16). Chromosome analysis should always be performed at diagnosis of multiple myeloma to improve the prediction of outcome and to aid treatment decision-making.

A Novel Transgenic Mouse Model of the Human Multiple Myeloma Chromosomal Translocation t(14;16)(q32;q23)

Multiple myeloma (MM) is a currently incurable neoplasm of terminally differentiated B cells. The translocation and/or overexpression of c-MAF have been observed in human MM. Although c-MAF might function as an oncogene in human MM, there has been no report thus far describing the direct induction of MM by c-MAF overexpression in vivo. In this study, we have generated transgenic (TG) mice that express c-Maf specifically in the B-cell compartment. Aged c-Maf TG mice developed B-cell lymphomas with some clinical features that resembled those of MM, namely, plasma cell expansion and hyperglobulinemia. Quantitative RT-PCR analysis demonstrated that Ccnd2 and Itgb7, which are known target genes of c-Maf, were highly expressed in the lymphoma cells. This novel TG mouse model of the human MM t(14;16)(q32;q23) chromosomal translocation should serve to provide new insight into the role of c-MAF in tumorigenesis. Cancer Res; 71(2); 339–48. ©2011 AACR.

SUMOylation attenuates c-Maf-dependent IL-4 expression

The function of transcription factors can be critically regulated by SUMOylation. c-Maf, the cellular counterpart of v-maf oncogene, is a potent transactivator of the IL-4 gene in Th2 cells. We found in a yeast two-hybrid screen that c-Maf can interact with Ubc9 and PIAS1, two key enzymes of the SUMOylation pathway. In this study, we report that c-Maf co-localized with these two SUMO (small ubiquitin-like modifier) ligases in the nucleus and that c-Maf can be SUMOylated in vitro and also in primary Th2 cells. We also demonstrated that lysine-33 is the dominant, if not the only, SUMO acceptor site of c-Maf. SUMOylation of c-Maf attenuated its transcriptional activity. Reciprocally, a SUMOylation resistant c-Maf was more potent than WT-c-Maf in driving IL-4 production in c-Maf-deficient Th2 cells. Furthermore, we showed that ablation of the SUMO site did not alter the subcellular localization or the stability of c-Maf protein but instead enhanced its recruitment to the Il4-promoter. We conclude that SUMOylation at lysine-33 is a functionally critical post-translational modification event of c-Maf in Th cells.

Identification of bZIP interaction partners of viral proteins HBZ, MEQ, BZLF1, and K-bZIP using coiled-coil arrays

Basic-region leucine-zipper transcription factors (bZIPs) contain a segment rich in basic amino acids that can bind DNA, followed by a leucine zipper that can interact with other leucine zippers to form coiled-coil homo- or heterodimers. Several viruses encode proteins containing bZIP domains, including four that encode bZIPs lacking significant homology to any human protein. We investigated the interaction specificity of these four viral bZIPs by using coiled-coil arrays to assess self-associations as well as heterointeractions with 33 representative human bZIPs. The arrays recapitulated reported viral-human interactions and also uncovered new associations. MEQ and HBZ interacted with multiple human partners and had unique interaction profiles compared to any human bZIPs, whereas K-bZIP and BZLF1 displayed homospecificity. New interactions detected included HBZ with MAFB, MAFG, ATF2, CEBPG, and CREBZF and MEQ with NFIL3. These were confirmed in solution using circular dichroism. HBZ can heteroassociate with MAFB and MAFG in the presence of MARE-site DNA, and this interaction is dependent on the basic region of HBZ. NFIL3 and MEQ have different yet overlapping DNA-binding specificities and can form a heterocomplex with DNA. Computational design considering both affinity for MEQ and specificity with respect to other undesired bZIP-type interactions was used to generate a MEQ dimerization inhibitor. This peptide, anti-MEQ, bound MEQ both stably and specifically, as assayed using coiled-coil arrays and circular dichroism in solution. Anti-MEQ also inhibited MEQ binding to DNA. These studies can guide further investigation of the function of viral and human bZIP complexes.

Low-threshold mechanoreceptor subtypes selectively express MafA and are specified by Ret signaling

Low-threshold mechanoreceptor neurons (LTMs) of the dorsal root ganglia (DRG) are essential for touch sensation. They form highly specialized terminations in the skin and display stereotyped projections in the spinal cord. Functionally defined LTMs depend on neurotrophin signaling for their postnatal survival and functioning, but how these neurons arise during development is unknown. Here, we show that specific types of LTMs can be identified shortly after DRG genesis by unique expression of the MafA transcription factor, the Ret receptor and coreceptor GFRalpha2, and find that their specification is Ngn2 dependent. In mice lacking Ret, these LTMs display early differentiation defects, as revealed by reduced MafA expression, and at later stages their central and peripheral projections are compromised. Moreover, in MafA mutants, a discrete subset of LTMs display altered expression of neurotrophic factor receptors. Our results provide evidence that genetic interactions involving Ret and MafA progressively promote the differentiation and diversification of LTMs.

Transcriptional activation of human MMP-13 gene expression by c-Maf in osteoarthritic chondrocyte

Matrix metalloproteinase (MMP)-13 has pivotal roles in the pathogenesis of Osteoarthritis (OA) and it is necessary to understand the regulatory mechanisms of MMP-13 expression. MMP-13 gene expression is regulated primarily at the transcriptional level. In this study, we investigated the role of c-maf in regulating MMP-13 transcription. Using transient transfection system with an c-maf construct, and MMP-13 promoter-luciferase constructs with specific mutations in transcription factor binding sites, we found that c-maf can significantly enhance MMP-13 promoter activity via the AP-1 site, By gene suppression with RNAi technology, we could show that c-maf downregulation leads to a reduced expression of MMP13. Chromatin immunoprecipitation assays reveal that c-maf binds to the MMP-13 gene promoter to a region of the MMP-13 promoter containing the AP-1 site. Taken together, these studies demonstrate a new level of transcriptional regulation of MMP-13 expression by the c-maf.

c-Maf activates the promoter and enhancer of the IL-21 gene, and TGF-beta inhibits c-Maf-induced IL-21 production in CD4+ T cells

Previous studies have shown that IL-6 potently induces IL-21 production in CD4(+) T cells, whereas TGF-beta inhibits IL-6-induced IL-21 production in CD4(+) T cells. In this study, we addressed the mechanisms underlying the transcriptional regulation of IL-21 production in CD4(+) T cells. We found that IL-6 induced c-Maf expression in CD4(+) T cells and that the enforced expression of c-Maf induced IL-21 production in CD4(+) T cells without IL-6, IL-4/STAT6 signaling, or an autocrine effect of IL-21. Moreover, we found that c-Maf directly bound to and activated IL-21P and the CNS-2 enhancer through MARE sites. On the other hand, we also found that although TGF-beta up-regulated IL-6-induced c-Maf expression in CD4(+) T cells, TGF-beta inhibited c-Maf-induced IL-21 production in CD4(+) T cells. Finally, we found that Foxp3 bound to IL-21P and the CNS-2 enhancer and inhibited c-Maf-induced IL-21 production modestly but significantly in CD4(+) T cells. Taken together, these results suggest that c-Maf induces IL-21 production directly in CD4(+) T cells by activating IL-21P and the CNS-2 enhancer and that TGF-beta suppresses c-Maf-induced IL-21 production in CD4(+) T cells.

c-Maf increases apoptosis in peripheral CD8 cells by transactivating Caspase 6

In addition to transactivation of interleukin-4 (IL-4), cellular muscular aponeurotic fibrosarcoma (c-Maf) enhances CD4 cell apoptosis by limiting Bcl-2 expression. The CD8 cells also express c-Maf and peripheral CD8 cell numbers are reduced in c-Maf transgenic mice, suggesting that c-Maf may influence CD8 cell survival in a manner similar to CD4 cells. Here we confirm that, similar to CD4 cells, c-Maf enhances CD8 cell susceptibility to apoptosis induced by multiple stimuli, independent of IL-4. However, unlike CD4 cells, c-Maf enhancement of apoptosis is independent of Bcl-2, suggesting that c-Maf uses other mechanisms to regulate CD8 cell apoptosis. Real-time reverse transcription-polymerase chain reaction reveals that the pro-apoptotic gene Caspase 6 is upregulated in c-Maf transgenic CD8 cells, suggesting that Caspase 6 is a novel c-Maf target gene. Luciferase reporter assays and site-directed mutagenesis reveal a functional c-Maf recognition element (MARE) within the first intron of Caspase 6. Binding of c-Maf to the MARE site is detectable by chromatin immunoprecipitation using non-transgenic T-cell lysates, so c-Maf can interact with the Caspase 6 MARE site in normal T cells. Furthermore, caspase 6 activity is increased among CD8 cells from c-Maf transgenic mice following T-cell receptor engagement. As expected, activity of the downstream caspases 3 and 7 is also increased. Consistent with the ability of caspase 6 to participate in positive feedback loops, cytochrome c release and caspase 8 activation are also increased. Together these results indicated that c-Maf increases CD8 cell sensitivity to apoptotic stimuli, at least in part, by direct transactivation of Caspase 6, providing increased substrate for Caspase 6-dependent apoptosis pathways.

Over-expression of c-maf by chondrocytes in osteoarthritis

The c-maf gene expression profile was investigated in normal and osteoarthritic articular cartilage using in situ hybridization, qualitative reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time RT-PCR. Osteoarthritic samples were obtained from 10 patients undergoing total knee replacement for severe osteoarthritis of the knee joints, and control samples from 10 trauma patients undergoing amputation. Expression of c-maf was significantly up-regulated in osteoarthritic cartilage compared with normal cartilage. Using in situ hybridization, distribution of a specific c-maf mRNA signal was found in the top zone and a decreased signal was found in the lower middle zone and the deep zone in osteoarthritic cartilage. A prominent c-maf mRNA signal was seen particularly in proliferating 'chondrocyte clusters'. In contrast, in normal cartilage almost no c-maf-positive cells were found. These findings suggest that c-maf may be important in chondrocyte hypertrophy and terminal differentiation, and may be involved in the pathogenesis of osteoarthritis.

The protozoan parasite Theileria annulata alters the differentiation state of the infected macrophage and suppresses musculoaponeurotic fibrosarcoma oncogene (MAF) transcription factors

The tick-borne protozoan parasite Theileria annulata causes a debilitating disease of cattle called Tropical Theileriosis. The parasite predominantly invades bovine macrophages (m phi) and induces host cell transformation by a mechanism that has not been fully elucidated. Infection is associated with loss of characteristic m phi functions and phenotypic markers, indicative of host cell de-differentiation. We have investigated the effect of T. annulata infection on the expression of the m phi differentiation marker c-maf. The up-regulation of c-maf mRNA levels observed during bovine monocyte differentiation to m phi was suppressed by T. annulata infection. Furthermore, mRNA levels for c-maf and the closely related transcription factor mafB were significantly lower in established T. annulata-infected cell-lines than in bovine monocyte-derived m phi. Treatment of T. annulata-infected cells with the theileriacidal drug buparvaquone induced up-regulation of c-maf and mafB, which correlated with altered expression of down-stream target genes, e.g. up-regulation of integrin B7 and down-regulation of IL12A. Furthermore, T. annulata infection is associated with the suppression of the transcription factors, Pu.1 and RUNX1, and colony stimulating factor 1 receptor (CSF1R) which are also involved in the regulation of monocyte/m phi differentiation. We believe these results provide the first direct evidence that T. annulata modulates the host m phi differentiation state, which may diminish the defence capabilities of the infected cell and/or promote cell proliferation. Musculoaponeurotic fibrosarcoma oncogene (MAF) transcription factors play an important role in cell proliferation, differentiation and survival; therefore, regulation of these genes may be a major mechanism employed by T. annulata to survive within the infected m phi.

Transcribe to survive: transcriptional control of antioxidant defense programs for neuroprotection in Parkinson's disease

Parkinson's disease (PD) is a progressive, primarily motor disorder that is characterized by loss of dopaminergic (DA) neurons within the substantia nigra (SN). Cell death in PD has been associated with impaired mitochondrial function and increased oxidative stress. Strategies to reduce the oxidative load in DA cells may be beneficial in slowing the progression of PD. The transcription factor nuclear factor-erythroid 2 (NF-E2) related factor 2 (NRF2) is emerging as a master regulator of antioxidant defense systems, which makes it an attractive target for manipulations that aim to increase cellular resistance to oxidative stress. Peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator-1 alpha (PGC1alpha) is a regulator of mitochondrial biogenesis genes that simultaneously upregulates many genes known to protect against oxidative stress. Pgc-1alpha knockout mice show enhanced susceptibility to SN neuronal loss following MPTP exposure, whilst overexpression of Pgc-1alpha appears to protect against oxidative stress in vitro. This makes PGC-1alpha a highly attractive target for neuroprotective therapies in PD. This review will explore the mechanisms behind the induction of NRF2 and PGC-1alpha in response to oxidative stress and identify common pathways that may provide targets for upregulating antioxidant defense programs.

MCRS2 represses the transactivation activities of Nrf1

Background

Nrf1 [p45 nuclear factor-erythroid 2 (p45 NF-E2)-related factor 1], a member of the CNC-bZIP (CNC basic region leucine zipper) family, is known to be a transcriptional activator by dimerization with distinct partners, such as Maf, FosB, c-Jun, JunD, etc. The transcriptional roles of CNC-bZIP family are demonstrated to be involved in globin gene expression as well as the antioxidant response. For example, CNC-bZIP factors can regulate the expression of detoxification proteins through AREs, such as expression of human gamma-glutamylcysteine synthetases (GCS), glutathione S-transferases (GST), UDP-glucuronosyl transferase (UDP-GT), NADP (H) quinone oxidoreductase (NQOs), etc. To further explore other factor(s) in cells related to the function of Nrf1, we performed a yeast two-hybrid screening assay to identify any Nrf1-interacting proteins. In this study, we isolated a cDNA encoding residues 126–475 of MCRS2 from the HeLa cell cDNA library. Some functions of MCRS1 and its splice variant-MSP58 and MCRS2 have been previously identified, such as transforming, nucleolar sequestration, ribosomal gene regulation, telomerase inhibition activities, etc. Here, we demonstrated MCRS2 can function as a repressor on the Nrf1-mediated transactivation using both in vitro and in vivo systems.

Results

To find other proteins interacting with the CNC bZIP domain of Nrf1, the CNC-bZIP region of Nrf1 was used as a bait in a yeast two-hybrid screening assay. MCRS2, a splicing variant of p78/MCRS1, was isolated as the Nrf1-interacting partner from the screenings. The interaction between Nrf1 and MCRS2 was confirmed in vitro by GST pull-down assays and in vivo by co-immunoprecipitation. Further, the Nrf1-MCRS2 interaction domains were mapped to the residues 354–447 of Nrf1 as well as the residues 314–475 of MCRS2 respectively, by yeast two-hybrid and GST pull-down assays. By immunofluorescence, MCRS2-FLAG was shown to colocalize with HA-Nrf1 in the nucleus and didn't result in the redistribution of Nrf1. This suggested the existence of Nrf1-MCRS2 complex in vivo. To further confirm the biological function, a reporter driven by CNC-bZIP protein binding sites was also shown to be repressed by MCRS2 in a transient transfection assay. An artificial reporter gene activated by LexA-Nrf1 was also specifically repressed by MCRS2.

Conclusion

From the results, we showed MCRS2, a new Nrf1-interacting protein, has a repression effect on Nrf1-mediated transcriptional activation. This was the first ever identified repressor protein related to Nrf1 transactivation.

A new MAFia in cancer

Like JUN and FOS, the Maf transcription factors belong to the AP1 family. Besides their established role in human cancer--overexpression of the large Maf genes promotes the development of multiple myeloma--they can display tumour suppressor-like activity in specific cellular contexts, which is compatible with their physiological role in terminal differentiation. However, their oncogenic activity relies mostly on the acquisition of new biological functions relevant to cell transformation, the most striking characteristic of Maf oncoproteins being their ability to enhance pathological interactions between tumour cells and the stroma.

c-Maf expression in angioimmunoblastic T-cell lymphoma

The oncogene c-Maf was recently found to be overexpressed in approximately 50% of multiple myeloma cases, and a role for c-Maf in promoting cyclin D2 expression has been postulated. We previously examined c-Maf expression in various T-cell lymphomas by reverse-transcription polymerase chain reaction and found extremely elevated c-Maf levels in angioimmunoblastic T-cell lymphoma (AILT). In this study, we examined T-cell lymphomas for c-Maf and cyclin expression immunohistochemically. Of 93 cases of T-cell lymphomas we investigated in the current study, c-Maf expression was seen in 23 out of 31 cases of AILT, 3 out of 11 of adult T-cell leukemia/lymphoma, 4 out of 19 of peripheral T-cell lymphoma, unspecified [PTCL(U)], and 0 out of 11 cases of mycosis fungoides, 0 out of 11 of anaplastic large cell lymphoma, and 1 out of 10 of extranodal NK/T-cell lymphoma, nasal type. Double immunostaining in AILT revealed that the majority of c-Maf-positive cells were also positive for CD43 (MT1), CD45RO (UCHL-1), and CD4 but were negative for CD20 (L26). Additionally, cyclins D1 and D2, which stimulate cell cycle progression, were overexpressed in a large number of the c-Maf-positive AILT samples. Quantitative reverse-transcription polymerase chain reaction analysis also showed that c-Maf was overexpressed in 8/31 cases of AILT, 0/19 cases of PTCL(U), 0/11 cases of anaplastic large cell lymphoma, 0/10 cases of extranodal NK/T-cell lymphoma, nasal type, and 2/8 cases of multiple myeloma, presenting significant difference between AILT and PTCL(U) (P=0.016, chi test). These findings strongly suggest that CD4-positive neoplastic T cells in AILT show c-Maf expression and provide new insight into the pathogenesis of AILT suggesting c-Maf to be a useful diagnostic marker for AILT.

Small Maf proteins in mammalian gene control: mere dimerization partners or dynamic transcriptional regulators?

The small Maf basic leucine zipper (bZIP) proteins MafF, MafG and MafK, while modest in size, have emerged as crucial regulators of mammalian gene expression. Intriguingly, small Mafs do not contain an obvious transcriptional activation domain. However, previously perceived as "mere" partner molecules conferring DNA binding specificity to complexes with larger bZIP proteins, such as the CNC family member Nrf2, it has become clear that small Maf proteins are essential and dynamically regulated transcription factors. Current data suggest stringent control of small Maf protein function through transcriptional and post-translational mechanisms. Initial gene targeting experiments revealed considerable functional redundancy among small Maf proteins in vivo. This was not unexpected, due to the high level of homology among the three small Mafs. Nevertheless, further studies showed that these transcription factors have critical roles in various cellular processes, including stress signaling, hematopoiesis, CNS function and oncogenesis. Recent data provide a possible link between small Maf-mediated transcription and the inflammatory response.

A chemical biology screen identifies glucocorticoids that regulate c-maf expression by increasing its proteasomal degradation through up-regulation of ubiquitin

The oncogene c-maf is frequently overexpressed in multiple myeloma cell lines and patient samples and contributes to increased cellular proliferation in part by inducing cyclin D2 expression. To identify regulators of c-maf, we developed a chemical screen in NIH3T3 cells stably overexpressing c-maf and the cyclin D2 promoter driving luciferase. From a screen of 2400 off-patent drugs and chemicals, we identified glucocorticoids as c-maf–dependent inhibitors of cyclin D2 transactivation. In multiple myeloma cell lines, glucocorticoids reduced levels of c-maf protein without influencing corresponding mRNA levels. Subsequent studies demonstrated that glucocorticoids increased ubiquitination-dependent degradation of c-maf and up-regulated ubiquitin C mRNA. Moreover, ectopic expression of ubiquitin C recapitulated the effects of glucocorticoids, demonstrating regulation of c-maf protein through the abundance of the ubiquitin substrate. Thus, using a chemical biology approach, we identified a novel mechanism of action of glucocorticoids and a novel mechanism by which levels of c-maf protein are regulated by the abundance of the ubiquitin substrate.

Molecular basis distinguishing the DNA binding profile of Nrf2-Maf heterodimer from that of Maf homodimer

Nrf2-small Maf heterodimer activates the transcription of many cytoprotective genes through the antioxidant response element and serves as a key factor in xenobiotic and oxidative stress responses. Our surface plasmon resonance-microarray binding analysis revealed that both Nrf2-MafG heterodimer and MafG homodimer bind to the consensus Maf recognition element with high affinity but bind differentially to the suboptimal binding sequences degenerated from the consensus. We examined the molecular basis distinguishing the binding profile of Nrf2-MafG heterodimer from that of MafG homodimer and found that the Ala-502 residue in the basic region of Nrf2 is a critical determinant of its binding specificity. In Maf proteins, a tyrosine resides in the position corresponding to Ala-502 in Nrf2. We prepared a mutant Nrf2 molecule in which Ala-502 was replaced with tyrosine. In surface plasmon resonance-microarray analysis, heterodimer of Nrf2(A502Y) and MafG displayed a binding specificity similar to that of MafG homodimer. The target genes activated by mutant Nrf2(A502Y)-small Maf heterodimer were largely different, albeit with some overlap, from those activated by wild-type Nrf2-small Maf, indicating that the array of target genes regulated by Nrf2-small Maf heterodimer differs substantially from that regulated by Maf homodimer in vivo. These results suggest that the distinct DNA binding profile of Nrf2-Maf heterodimer is biologically significant for Nrf2 to function as a key regulator of cytoprotective genes. Our contention is supported that the differential DNA binding specificity between Maf homodimers and Nrf2-Maf heterodimers establishes the differential gene regulation by these dimer-forming transcription factors.

CD13/APN transcription is regulated by the proto-oncogene c-Maf via an atypical response element

Angiogenic growth factors induce the transcription of the cell surface peptidase CD13/APN in activated endothelial cells of the tumor vasculature. Inhibition of CD13/APN abrogates endothelial invasion and morphogenesis in vitro and tumor growth in vivo suggesting a critical functional role for CD13 in angiogenesis. Experiments to identify the transcription factors responsible for this regulation demonstrated that exogenous expression of the proto-oncogene c-Maf, but not other bZip family members tested, potently activates transcription from a critical regulatory region of the CD13 proximal promoter between -115 and -70 bp which is highly conserved among mammalian species. Using promoter mutation, EMSA and ChIP analyses we established that both endogenous and recombinant c-Maf directly interact with an atypical Maf response element contained within this active promoter region via its basic DNA/leucine zipper domain. However full activity of c-Maf requires the amino-terminal transactivation domain, and site-directed mutation of putative phosphorylation sites within the transactivation domain (serines 15 and 70) shows that these sites behave in a dramatic cell type-specific manner. Therefore, this atypical response element predicts a broader range of c-Maf target genes than previously appreciated and thus impacts its regulation of multiple myeloma as well as endothelial cell function and angiogenesis.

MafA stability in pancreatic beta cells is regulated by glucose and is dependent on its constitutive phosphorylation at multiple sites by glycogen synthase kinase 3

Regulation of insulin gene expression by glucose in pancreatic beta cells is largely dependent on a cis-regulatory element, termed RIPE3b/C1, in the insulin gene promoter. MafA, a member of the Maf family of basic leucine zipper (bZip) proteins, is a beta-cell-specific transcriptional activator that binds to the C1 element. Based on increased C1-binding activity, MafA protein levels appear to be up-regulated in response to glucose, but the underlying molecular mechanism for this is not well understood. In this study, we show evidence supporting that the amino-terminal region of MafA is phosphorylated at multiple sites by glycogen synthase kinase 3 (GSK3) in beta cells. Mutational analysis of MafA and pharmacological inhibition of GSK3 in MIN6 beta cells strongly suggest that the rate of MafA protein degradation is regulated by glucose, that MafA is constitutively phosphorylated by GSK3, and that phosphorylation is a prerequisite for rapid degradation of MafA under low-glucose conditions. Our data suggest a new glucose-sensing signaling pathway in islet beta cells that regulates insulin gene expression through the regulation of MafA protein stability.

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).

In vivo suppression of mafA mRNA with siRNA and analysis of the resulting alteration of the gene expression profile in mouse pancreas by the microarray method

Maf is a family of transcription factor proteins that is characterized by a typical bZip structure, and one of the large mafs, mafA is a strong transactivator of insulin. To explore the role of mafA in the pancreas, we modified the mafA mRNA level in vivo in mice by the RNA interference (siRNA) technique and analyzed the resulting alteration of the expressed gene profile with a microarray system. The mafA expression level in siRNA-treated mice was reduced approximately 60% compared with control-siRNA-treated animals. Microarray analysis revealed changes in the expression level of several genes in the siRNA-treated mice, with prominent down-regulated expression of the genes encoding insulin, glucagon, and adipocytokines, suggesting possible role of mafA in the pathophysiological states of impaired metabolic responses or inflammatory reactions.

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.

A heterozygous c-Maf transactivation domain mutation causes congenital cataract and enhances target gene activation

MAF, one of a family of large Maf bZIP transcription factors, is mutated in human developmental ocular disorders that include congenital cataract, microcornea, coloboma and anterior segment dysgenesis. Expressed early in the developing lens vesicle, it is central to regulation of lens crystallin gene expression. We report a semi-dominant mouse c-Maf mutation recovered after ENU mutatgenesis which results in the substitution, D90V, at a highly conserved residue within the N-terminal 35 amino-acid minimal transactivation domain (MTD). Unlike null and loss-of-function c-Maf mutations, which cause severe runting and renal abnormalities, the phenotype caused by the D90V mutation is isolated cataract. In reporter assays, D90V results in increased promoter activation, a situation similar to MTD mutations of NRL that also cause human disease. In contrast to wild-type protein, the c-Maf D90V mutant protein is not inhibited by protein kinase A-dependent pathways. The MTD of large Maf proteins has been shown to interact with the transcriptional co-activator p300 and we demonstrate that c-Maf D90V enhances p300 recruitment in a cell-type dependent manner. We observed the same for the pathogenic human NRL MTD mutation S50T, which suggests a common mechanism of action.

Hodgkin's lymphoma associated T-cells exhibit a transcription factor profile consistent with distinct lymphoid compartments

BACKGROUND

Hodgkin's lymphoma (HL) is characterised by an ineffective immune response that is predominantly mediated by CD4+ T-cells.

AIMS

To analyse the expression of the key regulatory T-cell transcription factors (TFs) in the T-cells of HL involved tissues in order to assess the nature of the T(H) immune response in HL.

METHODS AND RESULTS

By immunohistochemistry, GATA3 was strongly and T-bet exclusively expressed in a subset of interfollicular lymphocytes in the reactive lymphoid tissues. In classical HL (CHL), which is generally located in the interfollicular zones, a predominance of T-bet+ T-cells and lesser amounts of GATA3+ and c-Maf+ T-cells was found, concordant with the pattern of the normal interfollicular compartment. In reactive lymphoid tissues, c-Maf was observed mostly in T-lymphocytes within the germinal centres (GCs). Nodular lymphocyte predominance type of Hodgkin's lymphoma (NLPHL) and progressively transformed germinal centres cases, showed a majority of c-Maf+ T-cells, consistent with the pattern in normal GCs. NLPHL cases uniformly showed c-Maf+/CD57+ T-cell rosettes around the neoplastic cells; these rosettes were absent in "paragranuloma-type" T-cell/histiocyte rich B-cell lymphoma.

CONCLUSIONS

T-cell TF expression profiles of the reactive T-cells in both subtypes of HL are in accordance with the expression profile observed in the distinct lymphoid compartments.

MEKK1 mediates the ubiquitination and degradation of c-Jun in response to osmotic stress

c-Jun, a major transcription factor in the activating protein 1 family of regulatory proteins, is activated by many physiologic and pathological stimuli. We show here that c-Jun was downregulated in response to osmotic stress via ubiquitination-dependent degradation by the PHD/RING finger domain of MEKK1, which exhibited E3 ubiquitin ligase activity toward c-Jun in vitro and in vivo. The reduced c-Jun protein level resulting from exogenous expression of wild-type MEKK1 and the opposite effect induced by expression of a MEKK1 PHD/RING finger domain mutant were consistent with a higher level of c-Jun protein in MEKK1(-/-) cells than in corresponding wild-type cells. The deficiency of MEKK1 blocked posttranslational downregulation of c-Jun in response to osmotic stress. Furthermore, apoptosis induced by osmotic stress was suppressed by overexpression of c-Jun, indicating that the downregulation of c-Jun promotes apoptosis.

Correlation between the expression level of c-maf and glutathione peroxidase-3 in c-maf -/- mice kidney and c-maf overexpressed renal tubular cells

Large mafs are transcriptional factors and members of the basic leucine zipper (b-Zip) superfamily. Since we previously identified expression of c-maf in mouse kidney, we presently investigated the mRNA expression profile in the kidney of c-maf gene knockout mice by using DNA microarray, and plasma glutathione peroxidase-3 (GPx3) was predominantly downregulated. We focused on the relation between the expression level of c-maf and GPx3 in vivo and in vitro. Since GPx3 is an antioxidant enzyme, oxidative stress was induced by exposing a culture cell derived from mouse renal tubules (mIMCD3) to hydrogen peroxide. Real-time PCR demonstrated that mRNA expression of both c-maf and GPx3 increased in parallel during exposure to oxidative stress in a time- and dose-dependent manner. Then, the mIMCD3 cells were transfected with c-maf-cDNA containing plasmid, which resulted in an increase in mRNA and protein expression of GPx3 compared with the control cells. Thus, c-maf may be transcriptional regulator of GPx3 expression and modulate the antioxidative pathway in the kidney.

MafK overexpression in pancreatic beta-cells caused impairment of glucose-stimulated insulin secretion

MafA is a member of large Maf transcription factors and activates the insulin gene in pancreatic beta-cells. Other large Maf transcription factors, MafB and c-Maf, also express and activate insulin transcription in beta-cells. However, the functional relationship between MafA and other Maf proteins in beta-cells has not been established. In order to suppress the function of large Maf proteins, we generated transgenic mice overexpressing MafK, which act as dominant negative protein in pancreatic beta-cells. These mice showed hyperglycemia at a young age due to impairment of glucose-stimulated insulin secretion. Although the transgenic mice showed an abnormal response in the glucose tolerance test, hyperglycemia was restored in adulthood. Histological analysis revealed islet hypertrophy in adult transgenic mice. Finally, an electrophoretic gel mobility shift assay showed that the DNA-binding activity of endogenous MafA was significantly increased in the MafK transgenic mice. These results indicated that MafA may have relevance to compensatory response.

Overexpression of c-Maf Contributes to T-Cell Lymphoma in Both Mice and Human

c-Maf translocation or overexpression has been observed in human multiple myeloma. Although c-maf might function as an oncogene in multiple myeloma, a role for this gene in other cancers has not been shown. In this study, we have found that mice transgenic for c-Maf whose expression was direct to the T-cell compartment developed T-cell lymphoma. Moreover, we showed that cyclin D2, integrin beta(7), and ARK5 were up-regulated in c-Maf transgenic lymphoma cells. Furthermore, 60% of human T-cell lymphomas (11 of 18 cases), classified as angioimmunoblastic T-cell lymphoma, were found to express c-Maf. These results suggest that c-Maf might cause a type of T-cell lymphoma in both mice and humans and that ARK5, in addition to cyclin D2 and integrin beta(7), might be downstream target genes of c-Maf leading to malignant transformation.

A novel mutation in the DNA-binding domain ofMAF at 16q23.1 associated with autosomal dominant “cerulean cataract” in an Indian family

Congenital cataract, a clinically and genetically highly heterogeneous eye disorder, is one of the significant causes of visual impairment or blindness in children. It is frequently inherited as an autosomal dominant trait. We investigated a three-generation family of Indian origin with 12 members affected with cerulean cataract. Linkage analysis was carried out in this family using more than 100 microsatellite markers for the known cataract candidate gene loci. A positive two-point lod score of 3.9 at theta = 0.000, indicative of linkage, was obtained with three microsatellite markers for chromosome 16. Multipoint and haplotype analysis narrowed the cataract locus to a 15.3 cM region between markers D16S518 and D16S511 that corresponds to the region 16q23.1. Direct sequencing of the candidate gene MAF, which lies in the critical linked region, revealed a novel heterozygous missense mutation in the basic region (BR) of the DNA-binding domain. This sequence change was considered pathogenic as it segregated in all affected family members, neither seen in unaffected family members nor in 106 unrelated controls. The mutation also results in substitution of highly conserved lysine 297 by arginine (K297R) that affects a residue that forms a part of a predicted DNA-interaction region of the protein. The association of microcornea with congenital cataract in some affected individuals further underlines the role of the MAF transcription factor in lens and anterior ocular development. Our findings expand the mutation spectrum of MAF in association with congenital cataract and highlight the genetic and phenotypic heterogeneity of congenital cataract.

The heme-Bach1 pathway in the regulation of oxidative stress response and erythroid differentiation

Heme--as a prosthetic group of proteins required for oxygen transport and storage, respiration, and biosynthetic pathways--is essential for practically all forms of life. Additionally, the degradation products of heme (i.e., carbon monoxide, biliverdin, and bilirubin) produced by the enzymatic actions of heme oxygenase (HO) and biliverdin reductase, possess various biological activities in vivo. In mammalian cells, heme also functions as an intracellular regulator of gene expression by virtue of its ability to bind to Bach1, a transcription factor that functions in association with small Maf proteins. Normally, such complexes function as repressors by binding to specific target sequences, the Maf recognition element (MARE), within enhancers of genes encoding proteins such as HO-1 and beta-globin. By binding to Bach1, heme induces selective removal of the repressor from the gene enhancers permitting subsequent occupancy of the MAREs by activators that, interestingly, also contain small Maf proteins. Thus small Maf proteins play dual functions in gene expression: complexes with Bach1 repress MARE-dependent gene expression, whereas heterodimers with NF-E2 p45 or related factors (Nrf1, Nrf2, and Nrf3) activate MARE-driven genes. By modulating the equilibrium of the small Maf heterodimer network, heme regulates expression of the cytoprotective enzyme HO-1 during the stress response and of beta-globin during erythroid differentiation. Implications of such heme-regulated gene expression in human diseases including atherosclerosis are discussed.

Activation of connective tissue growth factor gene by the c-Maf and Lc-Maf transcription factors

The Maf family of transcription factors is expressed during development of various organs and tissues, and is involved in a variety of developmental and cellular differentiation processes. We previously found that c-maf and mafB are strongly expressed in hypertrophic chondrocytes during cartilage development. Connective tissue growth factor (CTGF) is also expressed in hypertrophic chondrocytes. Adenovirus mediated introduction of c-maf gene into the mouse fibroblast cell line C3H10T1/2 strongly induced CTGF expression. CTGF can be induced by TGF-beta via the SMAD pathway; however, the c-Maf could not induce TGF-beta, nor could TGF-beta induce the c-Maf, suggesting that activation of CTGF by Maf is TGF-beta independent. Reporter transfection analysis using C3H10T1/2 cells shows that c-Maf stimulates a CTGF reporter gene. Lc-Maf, a splice variant of c-Maf containing an extra 10 amino acids in the carboxyl terminus, was a stronger inducer of the CTGF reporter gene than c-Maf. Chromatin immunoprecipitation analysis showed that c-Maf binds to the promoter region of the CTGF gene, indicating that Maf directly activates the CTGF gene. Taken together, these data indicate that the CTGF gene is a target of c-Maf and Lc-Maf in cartilage development.

The 5'-AT-rich half-site of Maf recognition element: a functional target for bZIP transcription factor Maf

The Maf family of proteins are a subgroup of basic region-leucine zipper (bZIP) transcription factors, which recognize a long palindromic DNA sequence [TGCTGAC(G)TCAGCA] known as the Maf recognition element (MARE). Interestingly, the functional target enhancer sequences present in the alphaA-crystallin gene contain a well-conserved half-site of MARE rather than the entire palindromic sequence. To resolve how Maf proteins bind to target sequences containing only MARE half-sites, we examined their binding activities using electrophoretic gel mobility shift assays as well as in vitro and in vivo reporter assays. Our results indicate that the 5'-flanking region of the MARE half-site is required for Maf proteins to bind both in vitro and in vivo. The critical 5'-flanking sequences for c-Maf were determined by a selection and amplification binding assay and show a preference for AT-rich nucleotides. Furthermore, sequence analysis of the regulatory regions of several target genes also suggests that AT-rich sequences are important. We conclude that Maf can bind to at least two types of target sequences, the classical MARE (palindrome type) and a 5'-AT-rich MARE half-site (half-site type). Our results provide important new insights into the DNA binding and site selection by bZIP transcription factors.

The protooncogene c-Maf is an essential transcription factor for IL-10 gene expression in macrophages

IL-10 is an important immunoregulatory factor. However, our understanding of IL-10 gene regulation remains very limited. In this study, following up on our previous novel finding that the protooncogene c-Maf of the basic leucine zipper family of transcription factors is expressed in monocytes and macrophages, we investigate the role of c-Maf in the transcriptional regulation of IL-10 and the underlying molecular mechanism in macrophages. c-Maf-null macrophages exhibit strongly impaired IL-10 protein production and mRNA expression upon LPS stimulation. Ectopic expression of c-Maf stimulates not only exogenously transfected IL-10 promoter-driven luciferase activity in a dose-dependent manner but also enhances endogenous IL-10 gene expression stimulated by LPS. Both in vitro and in vivo experiments identify a c-Maf response element localized to nucleotides -196/-184 relative to the transcription initiation site in the IL-10 promoter. This site represents an atypical 12-O-tetradecanoate-13-acetate-responsive element for musculoaponeurotic fibrosarcoma recognition and functions as an enhancer element in a heterologous and orientation-independent manner. Furthermore, c-Maf is expressed constitutively in resting monocytes/macrophages. IL-4 can up-regulate c-Maf expression, its binding to IL-10 promoter, and dose dependently enhance IL-10 production induced by LPS; moreover, IL-4 failed to enhance LPS-induced IL-10 production in c-Maf-null macrophages. Taken together, these data demonstrate that c-Maf is an indispensable yet constitutive transcription factor for IL-10 gene expression in LPS-activated macrophages, and IL-4 modulates IL-10 production in inflammatory macrophages likely via its ability to induce c-Maf expression. Thus, this study uncovers a novel and important function of c-Maf in macrophages and elucidates its transcriptional mechanism in the regulation of IL-10 gene expression.

Phylogenomic analysis and expression patterns of large Maf genes in Xenopus tropicalis provide new insights into the functional evolution of the gene family in osteichthyans

We have performed an exhaustive characterization of the large Maf family of basic leucine zipper transcription factors in vertebrates using the genome data available, and studied the embryonic expression patterns of the four paralogous genes thus identified in Xenopus tropicalis. Our phylogenetic analysis shows that, in osteichthyans, the large Maf family contains four orthology classes, MafA, MafB, c-Maf and Nrl, which have emerged in vertebrates prior to the split between actinopterygians and sarcopterygians. It leads to the unambiguous assignment of the Xenopus laevis XLmaf gene, previously considered a MafA orthologue, to the Nrl class, the identification of the amphibian MafA and c-Maf orthologues and the identification of the zebrafish Nrl gene. The four X. tropicalis paralogues display partially redundant but nevertheless distinct expression patterns in the somites, developing hindbrain, pronephros, ventral blood island and lens. Comparisons with the data available in the mouse, chick and zebrafish show that these large Maf expression territories are highly conserved among osteichthyans but also highlight a number of differences in the timing of large Maf gene expression, the precise extent of some labelled territories and the combinations of paralogues transcribed in some organs. In particular, the availability of robust phylogenies leads to a reinterpretation of previous expression pattern comparisons, suggesting an important part for function shuffling within the gene family in the developing lens. These data highlight the importance of exhaustive characterizations of gene families for comparative analyses of the genetic mechanisms, which control developmental processes in vertebrates.

A stage-specific functional role of the leucine zipper transcription factor c-Maf in lung Th2 cell differentiation

The transcription factor c-Maf controls IL-4 gene expression in CD4(+) T cells, and its expression is up-regulated in human asthmatic airways after allergen challenge. In the present study, we addressed the role of c-Maf in asthma by studying transgenic (Tg) mice overexpressing c-Maf in CD4(+) T cells under the control of the CD2 promoter. As shown, lung CD4(+) T cells of c-maf-Tg mice produced more IL-5 at the early stage (day 2) of culture in the presence of IL-4 than wild-type control cells. Consistently, c-maf-Tg mice spontaneously showed increased IL-5 expression and eosinophils in the bronchial alveolar lavage fluid (BALF) and activated IL-5 signal transduction via Raf-1 and Ras in lung eosinophils. Finally, IL-13 was suppressed in the BALF of c-maf-Tg mice and in supernatants of Tg lung CD4(+) T cells cultured in the presence of IL-2. Consistently, retroviral overexpression of c-Maf suppressed IL-13 production in developing lung Th2 cells. In summary, c-Maf induces IL-5 production in lung CD4(+) T cells at an early stage, but along with IL-2 suppresses IL-13 production in differentiating lung Th2 cells, thereby explaining the finding that overexpression of c-Maf does not cause airway hyperresponsiveness, a hallmark feature of asthma.

Comparison of maf gene expression patterns during chick embryo development

Maf proteins are basic-leucine zipper transcription factors belonging to the AP1 superfamily. Several developmental processes require Maf proteins yet, the redundancy or complementarity of their respective roles in common processes has been only partially investigated. We present for the first time a complete comparative analysis of maf gene expression patterns in vertebrates. Expression of c-maf, mafB/kreisler, mafA/L-maf, mafF, mafG and mafK was analyzed by whole-mount in situ hybridization within chick embryos and their extraembryonic tissues ranging from embryonic day (E) 1 to 7. We carefully examined the extent of overlap between distinct maf genes and report that the developing lens, kidney, pancreas and apoptotic zones of limb buds show sustained co-expression of large maf genes. Small maf genes also exhibit overlap, for example in the dermomyotome. We also describe so far unidentified sites of maf gene expression. mafA is found in the developing neural tube and dorsal root ganglia. c-maf hybridization is detected in the neuroretina, the notochord and the endothelium of extraembryonic blood vessels.