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

Developmental contribution of c-maf in the kidney: distribution and developmental study of c-maf mRNA in normal mice kidney and histological study of c-maf knockout mice kidney and liver

Maf is a family of oncogenes which encodes a nuclear bZip transcription factor protein and has been originally identified from the avian oncogenic retrovirus, AS42. Maf genes have been reported to have critical roles in embryological development and cellular differentiation. In this study, in situ hybridization with (35)S-labeled antisense riboprobes was used to investigate the distribution of c-maf mRNA in balb/c mouse kidneys from 12 (E12) through 17 days (E17) of gestation and then 1 and 4 weeks after birth. Immunocytochemistry of 4-week-old mouse kidney using anti-c-maf antisera was also performed. Kidney and liver sections from c-maf knockout mice at 4 weeks were stained with hematoxylin-eosin, and their histological features were examined. Expression of c-maf mRNA was first detected on E16 in the renal proximal tubules, and it was expressed through 4 weeks after birth. In the c-maf knockout mice at 4 weeks the cytoplasmic volume of the proximal tubule and liver cell was smaller. These findings suggest that expression of the c-maf gene may be involved in the embryological development and/or cell differentiation of kidney and liver cells.

MafT, a new member of the small Maf protein family in zebrafish

Small Maf proteins play critical roles on morphogenesis and homeostasis through associating with CNC proteins. To date, three small Maf proteins, MafF, MafG, and MafK, have been reported in vertebrates, which share redundant functions. In this study, we tried to identify and characterize small Maf proteins in zebrafish to elucidate their conservation and diversity in the fish kingdom. We identified homolog genes of MafG and MafK but not MafF in zebrafish, indicating the former two are conserved among vertebrates. In addition, a novel type of small Maf protein MafT was identified. MafT protein bound MARE sequence as a homodimer or heterodimers with zebrafish Nrf2 or p45 Nfe2. Co-overexpression of MafT and Nrf2 synergistically activated MARE-mediated gene expression in zebrafish embryos. These results indicated that MafT is a new member of small Maf proteins and involved in the Nrf2-dependent gene regulation in cellular defense system.

Nrf2 signaling in coordinated activation of antioxidant gene expression

Antioxidant response element (ARE)-mediated expression and coordinated induction of antioxidant enzymes is a critical mechanism of protection against chemically induced oxidative/electrophilic stress. NF-E2-related nuclear factors (Nrf1 and Nrf2) bind to ARE and regulate ARE-mediated gene expression and induction. Nrf2 is more potent than Nrf1 in activation of ARE-regulated gene expression. Nrf2 is retained in the cytoplasm by an inhibitor INrf2. Nrf2 binding to INrf2 leads to proteasomal degradation of Nrf2. An increase in oxidative/electrophilic stress, due to chemical exposure, leads to the activation of protein kinase C (PKC) and other cytosolic factors. PKC phosphorylation of Nrf2 at serine 40 results in the escape or release of Nrf2 from INrf2. Nrf2 translocates to the nucleus, forms heterodimers with its unknown partner proteins, and binds to the ARE. This leads to the coordinated activation of ARE-regulated genes. Additional nuclear factor including small Mafs (MafG and MafK), large Maf (c-Maf), c-Fos, and Fra1, also bind to ARE and negatively regulate ARE-mediated gene expression. This is presumably to keep the expression of antioxidant enzymes "in check" to maintain the cellular defenses active and/or to rapidly restore induced enzymes to normal levels. Future investigations are expected to reveal that a balance between positive and negative factors regulates ARE-mediated gene expression and induction. The future studies should also reveal a complete mechanism of signal transduction from antioxidants and xenobiotics to the transcription factors, such as Nrf2, that bind to ARE.

Mouse MafA, homologue of zebrafish somite Maf 1, contributes to the specific transcriptional activity through the insulin promoter

Large Maf transcription factors, which are members of the basic leucine zipper (b-Zip) superfamily, have been reported to be involved in embryonic development and cell differentiation. Previously, we isolated a novel zebrafish large Maf cDNA, somite Maf1 (SMaf1), which possesses transactivational activity within its N-terminus domain. To elucidate SMaf1 function in mammals, we tried to isolate the mouse homologue of zebrafish SMaf1. We isolated the mouse homologue of zebrafish SMaf1, which is the same molecule as the recently reported MafA. MafA mRNA was detected in formed somites, head neural tube, and liver cells in the embryos. In the adult mouse, MafA transcript was amplified in the brain, lung, spleen, and kidney by RT-PCR. MafA mRNA was also detectable in beta-cell line. Next, we analyzed the transcriptional activity of MafA using rat insulin promoters I and II (RIPI and II), since a part of RIP sequence was similar to the Maf recognition element (MARE) and MafA was expressed in pancreatic beta cells. MafA was able to activate transcription from RIPII, but not RIPI, in a dose dependent manner and the activity was dependent on RIPE3b/C1 sequences. In addition, the amount of MafA protein was regulated by glucose concentration. These results indicate that MafA is the homologue of zebrafish SMaf1 and acts as a transcriptional activator of the insulin gene promoter through the RIPE3b element.

Regulation and differential expression of the c-maf gene in differentiating cultured cells

The Maf transcription factors are involved in a variety of developmental and cellular differentiation processes, but their role in the differentiation of mesenchymal cells has not been described. Here, we have analyzed c-maf expression during the differentiation of adipocytes and muscle cells in cultured systems. The expression of c-maf mRNA was down-regulated during adipogenesis and up-regulated during myogenesis. In adipogenesis, the c-maf mRNA was down-regulated 58h after switching to the differentiation medium and just after PPARgamma2 mRNA was induced. A transient transfection analysis of a reporter gene containing the 5(')-flanking region of the c-maf gene showed that PPARgamma2 represses c-maf gene expression. We previously found that c-Maf, c-Jun, and Pax6 bind to and stimulate the c-maf gene. The PPARgamma2 repression of c-maf expression seems to be due, at least in part, to inhibition of the transactivation functions of c-Maf, c-Jun, and Pax6. The repression of c-maf was partly reversed by CBP, suggesting that these transcription factors compete for CBP or related transcription co-factors. In myogenesis, there was a differentiation-dependent stimulation of c-maf mRNA expression. The increased expression correlated with myoD expression. A transient transfection analysis showed that myoD stimulated a c-maf reporter gene through binding to two typical E-box elements located between 160 and 180 nucleotides upstream of the cap site. Binding of MyoD to the E-boxes was confirmed by a gel mobility shift assay and DNaseI footprinting analysis. Combined, these results suggest that the c-maf gene plays an important role during the differentiation of adipocyte and muscle cells from mesenchymal fibroblast cells.

The large Maf factor Traffic Jam controls gonad morphogenesis in Drosophila

Interactions between somatic and germline cells are critical for the normal development of egg and sperm. Here we show that the gene traffic jam (tj) produces a soma-specific factor that controls gonad morphogenesis and is required for female and male fertility. tj encodes the only large Maf factor in Drosophila melanogaster, an orthologue of the atypical basic Leu zipper transcription factors c-Maf and MafB/Kreisler in vertebrates. Expression of tj occurs in somatic gonadal cells that are in direct contact with germline cells throughout development. In tj mutant gonads, somatic cells fail to inter-mingle and properly envelop germline cells, causing an early block in germ cell differentiation. In addition, tj mutant somatic cells show an increase in the level of expression for several adhesion molecules. We propose that tj is a critical modulator of the adhesive properties of somatic cells, facilitating germline-soma interactions that are essential for germ cell differentiation.

MafA has strong cell transforming ability but is a weak transactivator

The maf oncogene of the avian oncogenic retrovirus AS42 encodes a nuclear bZip protein, v-Maf, that recognizes sequences related to the AP-1 target site. The corresponding cellular protein, c-Maf belongs to a family of related bZip proteins together with MafA and MafB. In this paper, we compare the transactivation and cell transforming abilities of MafA and MafB along with two forms of the c-Maf protein. These proteins induce cellular transformation when expressed in chicken embryo fibroblasts. In reporter assays, MafA is a much less effective transactivator than the other Maf proteins, but unexpectedly shows the strongest activity in cell transformation. Chimeras of MafA and MafB correlate the strong cell transforming ability of MafA with its DNA-binding domain. The DNA-binding domain of MafA is also correlated with weak transactivation. Additional mutagenesis experiments show that transactivation and transformation by MafA are also controlled by phosphorylation of two conserved serine residues in the transactivation domain. Finally, we constructed MafA-estrogen receptor fusion molecules that show tightly hormone-dependent cell transforming ability. These regulatable constructs permit a kinetic characterization of target gene responses and facilitate discrimination between direct and indirect targets.

c-jun regulation and function in the developing hindbrain

Hindbrain development is a well-characterised segmentation process in vertebrates. The bZip transcription factor MafB/kreisler is specifically expressed in rhombomeres (r) 5 and 6 of the developing vertebrate hindbrain and is required for proper caudal hindbrain segmentation. Here, we provide evidence that the mouse protooncogene c-jun, which encodes a member of the bZip family, is coexpressed with MafB in prospective r5 and r6. Analysis of mouse mutants suggests that c-jun expression in these territories is dependent on MafB but independent of the zinc-finger transcription factor Krox20, another essential determinant of r5 development. Loss- and gain-of-function studies, performed in mouse and chick embryos, respectively, demonstrate that c-Jun participates, together with MafB and Krox20, in the transcriptional activation of the Hoxb3 gene in r5. The action of c-Jun is likely to be direct, since c-Jun homodimers and c-Jun/MafB heterodimers can bind to essential regulatory elements within the transcriptional enhancer responsible for Hoxb3 expression in r5. These data indicate that c-Jun acts both as a downstream effector and a cofactor of MafB and belongs to the complex network of factors governing hindbrain patterning.

Clinical and biologic implications of recurrent genomic aberrations in myeloma

Nonrandom recurrent chromosomal abnormalities are ubiquitous in multiple myeloma (MM) and include, among others, translocations of the immunoglobulin heavy chain locus (IgH). IgH translocations in MM result in the up-regulation of oncogenes, and include more commonly t(11;14)(q13;q32), t(4;14)(p16;q32), and t(14;16)(q32;q23). Based on the recurrent nature of these translocations and their finding since the early stages of the plasma cell (PC) disorders, we hypothesized that they would confer biologic and clinical variability. In addition, deletions of 13q14 and 17p13 have also been associated with a shortened survival. We used cytoplasmic Ig-enhanced interphase fluorescent in situ hybridization to detect deletions (13q14 and 17p13.1), and translocations involving IgH in 351 patients treated with conventional chemotherapy entered into the Eastern Cooperative Oncology Group clinical trial E9486/9487. Translocations were frequently unbalanced with loss of one of the derivative chromosomes. The presence of t(4; 14)(p16;q32) (n = 42; 26 vs 45 months, P <.001), t(14;16)(q32;q23) (n = 15; 16 vs 41 months, P =.003), - 17p13 (n = 37; 23 vs 44 months, P =.005), and - 13q14 (n = 176; 35 vs 51 months, P =.028) were associated with shorter survival. A stratification of patients into 3 distinct categories allowed for prognostication: poor prognosis group (t(4;14)(p16;q32), t(14; 16)(q32;q23), and - 17p13), intermediate prognosis (- 13q14), and good prognosis group (all others), with median survivals of 24.7, 42.3, and 50.5 months, respectively (P <.001). This molecular cytogenetic classification identifies patients into poor, intermediate, and good risk categories. More importantly it provides further compelling evidence that MM is composed of subgroups of patients categorized according to their underlying genomic aberrations.

Downregulation of mafB expression in T-helper cells during early differentiation in vitro

We have studied the expression of a human homologue of mafB (maf-1), a member of the family of large maf transcription factors. In support of the suggested key role that mafB expression plays in differentiating macrophages, we found mafB to be expressed at a very high level in monocytic U937 and THP-1 cell lines. However, we show here that mafB transcription is not restricted to myeloid cells but can also be detected in lymphoid cells, indicating transcriptional plasticity during haematopoiesis. In conclusion, strong proliferative signals mediated by T-cell activation and interleukins (IL-4 and IL-12) downregulate the mafB messenger RNA transcript level when resting naïve CD4+ T-helper cells enter the differentiation pathway in vitro.

Novel activities of Mafb underlie its dual role in hindbrain segmentation and regional specification

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.

MafA is a glucose-regulated and pancreatic beta-cell-specific transcriptional activator for the insulin gene

The insulin gene is specifically expressed in beta-cells of the Langerhans islets of the pancreas, and its transcription is regulated by the circulating glucose level. Previous reports have shown that an unidentified beta-cell-specific nuclear factor binds to a conserved cis-regulatory element called RIPE3b and is critical for its glucose-regulated expression. Based on the sequence similarity of the RIPE3b element and the consensus binding sequence of the Maf family of basic leucine zipper transcription factors, we here identified mammalian homologue of avian MafA/L-Maf, an eye-specific member of the Maf family, as the RIPE3b-binding transcriptional activator. Reverse transcription-PCR analysis showed that mafA mRNA is detected only in the eyes and in pancreatic beta-cells and not in alpha-cells. MafA protein as well as its mRNA is up-regulated by glucose, consistent with the glucose-regulated binding of MafA to the RIPE3b element in beta-cell nuclear extracts. In transient luciferase assays, we also showed that expression of MafA greatly enhanced insulin promoter activity and that a dominant-negative form of MafA inhibited it. Therefore, MafA is a beta-cell-specific and glucose-regulated transcriptional activator for insulin gene expression and thus may be involved in the function and development of beta-cells as well as in the pathogenesis of diabetes.

Molecular aspects of multiple myeloma

Multiple myeloma is a malignant tumour of plasma cells with a median survival of two to three years. Karyotypic instability is seen at the earliest stage of the disease and increases with disease progression, leading to extreme genetic abnormalities similar to solid tumours. Translocations involving the immunoglobulin heavy chain region on chromosome 14q32 are clearly important in the pathogenesis of most myelomas. This review focuses on the different genetic abnormalities found in myeloma and discusses possible pathogenetic mechanisms and the implications for biologically based treatments.

Chromosomal and genetic abnormalities in myeloma

Chromosomal translocations are a hallmark of lymphoid tumours. Multiple myeloma (MM) is a tumour of the plasma cell, the terminally differentiated B lymphoid cell. In recent years, a large number of chromosomal and genetic abnormalities have been detected in myeloma, the most prominent being chromosome 13q deletions and translocations affecting the immunoglobulin heavy chain (IgH) locus on chromosome 14q32. The latter involve a large array of chromosomal partners, from which multiple oncogenes have been proposed as candidates for dysregulation. In addition, a wide variety of changes including numerical aberrations, translocations involving loci other than the immunoglobulin genes, and aberrations of known oncogenes such as N-ras mutations, have been found. With the refinement of molecular cytogenetic techniques, the sensitivity of detecting these molecular abnormalities is continuing to increase. However, with the exception of 13q deletions which have been consistently associated with an adverse prognosis, the role of the other changes in the pathogenesis of MM, and their effect on disease behaviour and prognosis are still being clarified. In this review, we will discuss the most common molecular abnormalities found in primary MM and cell lines, and consider the available evidence for a pathogenic role in MM.

The biology and cytogenetics of multiple myeloma

Despite the advances in our knowledge of myeloma cell biology, our understanding of myeloma pathogenesis is still incomplete. In this review, we present a summary of the cellular and molecular aspects of B-cell development and immunoglobulin (lg) gene rearrangement which have been important in defining the characteristics of the myeloma plasma cell (MPC). The PMC has undergone variable gene recombination, somatic hypermutation and isotype switching, and is therefore at a postgerminal center stage of development. The finding of preswitch clonal cells and isotype variants have raised interesting questions about the cell of origin of myeloma, for which no conclusive data is as yet available. However much information has been obtained about the chromosomal and genetic aberrations in myeloma, including monosomy 13, Ig heavy chain (IgH) switch region translocations, numerical abnormalities and a multitude of heterogeneous changes. A variety of techniques have been developed to overcome the insensitivity of conventional karyotyping, utilizing molecular cytogenetic strategies ranging from the delineation of precise loci by fluorescent in situ hybridization, a more "global" assessment of the genome by multicolor spectral karyotyping, to the quantitation of chromosomal material of specific origin by comparative genomic hybridization. Whether the abnormalities detected represent oncogenic insults, are involved in disease progression or are simply "by-products" of genetic instability is still unclear. For IgH translocations, the role of candidate genes such as Cyclin D1 and FGFR3 has been studied extensively by quantitating their expression and assessment of their oncogenicity (e.g. for FGFR3) in animal models. The significance of other aberrations such as c-myc, ras and p53 has also been investigated. With the advent of oligonucleotide microarrays, the expression of thousands of genes can be efficiently examined. So far, this approach seems promising in defining subgroups of different disease behavior, and may highlight specific genes and molecular mechanisms which are important in myeloma pathogenesis.

Cloning MafF by recognition site screening with the NFE2 tandem repeat of HS2: analysis of its role in globin and GCSl genes regulation

The erythroid-specific enhancer within hypersensitivity site 2 (HS2) of the human beta-globin locus control region is required for high level globin gene expression. We used an oligonucleotide of the NF-E2 tandem repeat, within HS2, as recognition site probe to screen a K562 cDNA library for interacting transcription factors. A 2.3 kb full length cDNA encoding the b-zip transcription factor MafF was isolated. MafF can form both homodimers and high affinity heterodimers with Nrf1, Nrf2 and Nf-E2, three members of the CNC-bZip family. Despite obvious structural similarities with the other small Maf proteins, MafF differs in its tissue distribution and its inability to repress transcription when overexpressed as homodimer. In fact, in different cell lines and on different promoters (gamma-globin, beta-globin and glutamylcysteine synthetase genes) the MafF homodimers do not appreciably affect transcription of target promoters, whereas MafF/CNC member heterodimers act as weak transcriptional activators. Even though MafF was cloned using probes derived from the globin LCR, it is in the context of the GCSl promoter and in combination with Jun that MafF shows a rather distinct and specific regulatory role. These observations suggest that a complex network of small Maf and CNC-AP1 protein interactions might be involved in regulating transcription in diverse tissues or developmental stages.

c-Maf negatively regulates ARE-mediated detoxifying enzyme genes expression and anti-oxidant induction

Anti-oxidant response element (ARE) and nuclear factors including Nrf2 and small Maf (MafG and MafK) proteins are known to regulate expression and induction of detoxifying enzyme genes including quinone oxidoreductase1 (NQO1). Nrf2 upregulates and small Maf proteins lacking the transcriptional activation domain down regulates ARE-mediated expression and induction. In this report, we have investigated the role of c-Maf (large Maf) containing the transcriptional activation domain in the regulation of ARE-mediated genes expression. The overexpression of c-Maf in human hepatoblastoma (Hep-G2) cells led to the repression of ARE-mediated NQO1 and GST Ya genes expression and induction in response to tert-butyl hydroquinone (t-BHQ). This was in contrast to the role of c-Maf in the activation of Maf recognition element (MARE) mediated p53 gene expression. Deletion of transcriptional activation domain of c-Maf (ĉ-Maf) led to significant loss of MARE-mediated p53 gene expression but had no effect on the repression of ARE-mediated NQO1 gene expression. The overexpression of MafG in Hep-G2 cells repressed both ARE and MARE-mediated genes expression. The co-expression of c-Maf with MafG rescued the MafG repression of MARE but not ARE-mediated gene expression. Band and super shift assays showed the presence of c-Maf in the ARE-nuclear protein complex. Similar assays with in vitro translated proteins revealed that both c-Maf and ĉ-Maf bound to NQO1 gene ARE as homodimers and heterodimers with small Maf but not as heterodimers with Nrf2. Mutational analysis of the NQO1 gene ARE indicated that core ARE sequence is essential for binding of c-Maf leading to repression of NQO1 gene expression. Northern analysis revealed that c-Maf expression increases 2 h after t-BHQ treatment. It reached a plateau at 4 h after t-BHQ treatment. The results together led to the conclusion that c-Maf negatively regulates ARE-mediated detoxifying enzyme genes expression and induction in response to anti-oxidants.

Integration and diversity of the regulatory network composed of Maf and CNC families of transcription factors

Recent progress in the analysis of transcriptional regulation has revealed the presence of an exquisite functional network comprising the Maf and Cap 'n' collar (CNC) families of regulatory proteins, many of which have been isolated. Among Maf factors, large Maf proteins are important in the regulation of embryonic development and cell differentiation, whereas small Maf proteins serve as obligatory heterodimeric partner molecules for members of the CNC family. Both Maf homodimers and CNC-small Maf heterodimers bind to the Maf recognition element (MARE). Since the MARE contains a consensus TRE sequence recognized by AP-1, Jun and Fos family members may act to compete or interfere with the function of CNC-small Maf heterodimers. Overall then, the quantitative balance of transcription factors interacting with the MARE determines its transcriptional activity. Many putative MARE-dependent target genes such as those induced by antioxidants and oxidative stress are under concerted regulation by the CNC family member Nrf2, as clearly proven by mouse germline mutagenesis. Since these genes represent a vital aspect of the cellular defense mechanism against oxidative stress, Nrf2-null mutant mice are highly sensitive to xenobiotic and oxidative insults. Deciphering the molecular basis of the regulatory network composed of Maf and CNC families of transcription factors will undoubtedly lead to a new paradigm for the cooperative function of transcription factors.

Characterization of the chicken L-Maf, MafB and c-Maf in crystallin gene regulation and lens differentiation

BACKGROUND

Members of the Maf family, including L-Maf, MafB and c-Maf, are "basic region/leucine zipper" (bZIP) transcription factors. Maf proteins contain a highly conserved acidic transactivation domain (AD), and a bZIP region that mediates DNA-binding activity. The hinge region between AD and bZIP varies considerably in length between different proteins. Recent studies reveal that L-Maf, c-Maf and MafB play key roles in vertebrate lens development.

RESULTS

We investigated the transactivation activity of individual factors in culture cells to analyse their specific functions. In transient transfection assays with a reporter gene containing Maf responsive elements, MafB and c-Maf activated higher levels of the reporter gene than L-Maf. However, L-Maf transactivated the alphaA-crystallin promoter as effectively as MafB and c-Maf, and induced the expression of the endogenous delta-crystallin gene more efficiently than the other two proteins. Domain-swapping experiments reveal that the bZIP region of MafB takes part in strong transcriptional activity, while the acidic and hinge regions (AH) of c-Maf collectively serve as a strong transactivation domain. The AH region of L-Maf (but not c-Maf) conferred transactivation activity to induce delta-crystallin gene expression.

CONCLUSIONS

These results suggest that despite their similar DNA binding properties, L-Maf, MafB and c-Maf regulate different sets of target genes by complex interactions with multiple factors that recognize cis-elements in promoters. The AH region of L-Maf has a distinct role in inducing endogenous delta-crystallin gene.

Activation of Maf/AP-1 repressor Bach2 by oxidative stress promotes apoptosis and its interaction with promyelocytic leukemia nuclear bodies

The oxidative stress response operates by inducing the expression of genes that counteract the stress. We show here that the oxidative stress-responsive transcription factor Bach2 is a generic inhibitor of gene expression directed by the 12-O-tetradecanoylphorbol-13-acetate response element, the Maf recognition element, and the antioxidant-responsive element. The Bach2-enhanced green fluorescent protein bicistronic retrovirus was used to monitor the fate of Bach2-expressing cells at the single cell level. Bach2 exerted an inhibitory effect on NIH3T3 cell proliferation and caused massive apoptosis upon mild oxidative stress in both NIH3T3 and Raji B-lymphoid cells. Interestingly, Bach1, a highly homologous protein, could not induce cell death, demonstrating the specificity for the apoptosis induction. Although both oxidative stress and leptomycin B, an inhibitor of nuclear export, induce nuclear accumulation of Bach2, the leptomycin B-induced nuclear accumulation of Bach2 was not sufficient to elicit apoptosis. Upon oxidative stress, Bach2 formed nuclear foci that associated with promyelocytic leukemia nuclear bodies. Our results suggest that Bach2 constitutes a cell lineage-specific system that couples oxidative stress and cell death and that inhibition of 12-O-tetradecanoylphorbol-13-acetate response element, the Maf recognition element, and the antioxidant-responsive element upon oxidative stress may be critical determinants for apoptosis.

c-Maf, the gammaD-crystallin Maf-responsive element and growth factor regulation

The transcription factor c-Maf has been suggested to regulate the activity of gamma-crystallin promoters in lens fibre cells. We here show that the transactivation potential of c-Maf and MafB for the rat gammaD-crystallin Maf-responsive element (gammaD MARE) is dependent upon the cellular context and, using chimeric and single domain mutants, that c-Maf is most likely to be the cognate factor for the gammaD MARE in the lens. Transactivation of the gammaD MARE by c-Maf in lens cells was not enhanced by c-Fos or c-Jun and was not blocked by dominant negative c-Fos or c-Jun constructs. c-Maf can activate the gammaD MARE as a homodimer since activation of the gammaD-crystallin promoter in P19 embryonic carcinoma cells required only c-Maf, but none of a number of c-Fos and c-Jun family members tested. Transactivation by c-Maf was inhibited by activation of protein kinase A (PKA) (by signal transduction agonist forskolin) or of protein kinase C (PKC) (by signal transduction agonist tetradecanoyl phorbol acetate). Site-directed mutagenesis showed that this effect is not mediated by phosphorylation of the consensus PKA/PKC site in the extended DNA-binding domain, but likely involves activation of MAP kinase kinase, as inhibition by PD98059 increased transactivation by c-Maf.

Differential induction of mafF, mafG and mafK expression by electrophile-response-element activators

The three small Maf proteins, MafF, MafG and MafK, have been implicated in a number of physiological processes, including development, differentiation, haematopoiesis and stress response. Here we report the constitutive expression of mafF, mafG and mafK in six human cell lines derived from various tissues (HepG2, IMR-32, K-562, HEK-293, RD and A549). The expression patterns of mafF, mafG and mafK varied widely among cell lines. Because small Maf proteins have been implicated in electrophile response element (EpRE)-mediated stress response, the ability of three EpRE activators [pyrrolidinedithiocarbamate (PDTC), phenylethyl isothiocyanate (PEITC) and t-butylhydroquinone (tBHQ)] to induce small Maf expression was examined in detail in HepG2 cells. Both PDTC and PEITC induced mafF, mafG and mafK expression, whereas tBHQ failed to markedly induce any of the three small Mafs. Where a response was observed, mafF was induced to the greatest extent compared with mafG and mafK, and this response was transcriptionally mediated. PDTC also induced small Maf expression in the other cell lines examined, with patterns of induction varying among cell lines. The differences in expression among the cell lines examined, coupled with the induction patterns observed, indicate that the three small maf genes are stress-responsive, but may be regulated via differing mechanisms. Furthermore, the fact that tBHQ, PDTC and PEITC induce EpRE activity, but that tBHQ fails to markedly induce any of the small Mafs, suggests that up-regulation of small Mafs is not an absolute requirement for EpRE-mediated gene expression.

Transgenic over-expression of MafK suppresses T cell proliferation and function in vivo

BACKGROUND

The small Maf proteins regulate gene transcription from Maf recognition elements (MARE). These proteins do not contain a canonical transactivation domain. Depending upon the ratio of small Maf proteins to their partner proteins, which either possess a transactivation domain or not, transcription can be switched on or off.

RESULTS

Transgenic mice were generated which over-express the small Maf family member MafK, specifically in the T cell lineage. It was our expectation that the high level of MafK would shift the balance to the formation of MafK homodimer and thereby repress MARE-dependent transcription. The transgenic mice had a shortened life span because of Pneumocystis carinii pneumonia and displayed a decrease in thymocytes and lower IL-2 and IL-4 mRNA expression levels. Analyses by electrophoretic gel mobility shift assay revealed that over-expressed MafK could interact with the proximal AP-1 sequence of IL-2 and the MARE in the IL-4 promoter region.

CONCLUSION

These results indicate that when over-expressed, MafK binds to a MARE-like sequence and represses MARE-dependent transcription. Consequently, T cell proliferation and cytokine secretion are affected. The MafK homodimer serves as an important molecular probe for evaluating the role played by cis-acting MAREs in the proliferation and function of T cells.

MafG-2 is a novel Maf protein that is expressed by stimulation of extracellular H(+)

We cloned MafG-2, a novel splice variant of MafG, from rat brain by RT-PCR method. MafG-2 differs from the previously published MafG by an insertion of 27 amino acids. Sequence analysis of the cDNA-encoded MafG-2 showed that MafG-2 contains basic domain and basic leucine zipper (bZip) motif. Transient transfection studies with GFP-MafG-2 chimera protein indicate that MafG-2 is localized in the nuclei of transfected COS-7 cells. To determine whether gene expression of mafG-2 mRNA is induced by an increase in extracellular protons, we analyzed expression of the mRNA in PC12 cells after an increase in extracellular proton concentration. We found that the mafG-2 mRNA expression increased when extracellular pH was decreased gradually from 7.40 to 7.20 and that there was a significant correlation between extracellular pH value and the expression of mafG-2 mRNA. These results suggest that an increase in extracellular proton may induce the expression of mafG-2 mRNA and MafG-2 may be involved in signal transduction of extracellular of H(+).

Maf and Jun nuclear oncoproteins share downstream target genes for inducing cell transformation

The Maf oncoprotein is a basic leucine zipper (bZip)-bearing transcriptional activator that recognizes the Maf recognition element (MARE) DNA sequence. In this study, we investigated the role of Maf's transactivation function in cell transformation. Replacement of the conserved amino terminus transactivator domain of Maf by a heterologous and stronger transactivator domain (the acidic transactivator domain of VP16) resulted in enhanced transformation of chicken embryo fibroblast cells. In contrast, the fusing of a transcriptional repressor domain (Sin3 interaction domain of Mxi1) with the whole Maf protein masked the transactivator function of Maf, which in turn inhibited its transforming activity. Furthermore, the leucine zipper domain of Maf, which defines its dimer-forming specificity, was exchangeable with that of GCN4 yeast protein in terms of its transactivating and cell transforming activities. Thus, heterodimer formation with other bZip factors is not required for Maf's ability to transform. These results together suggest that transactivation through MARE is necessary for Maf-induced transformation and that there exist downstream target gene(s) for transformation. Since the MARE sequence overlaps with the recognition element of another bZip oncoprotein Jun, we assessed whether Jun and Maf induce cell transformation through activating the same genes. We thus constructed a mutated version of Jun that has a GCN4 leucine zipper and lacks the transactivator domain. This mutant repressed the cell transformation not only by Jun but also by Maf. Thus, Maf and Jun share downstream target gene(s) that are involved in cell transformation.

Interaction of Maf transcription factors with Pax-6 results in synergistic activation of the glucagon promoter

In the endocrine pancreas, alpha-cell-specific expression of the glucagon gene is mediated by DNA-binding proteins that interact with the G1 proximal promoter element. Among these proteins, the paired domain transcription factor Pax-6 has been shown to bind to G1 and to transactivate glucagon gene expression. Close to the Pax-6-binding site, we observed the presence of a binding site for a basic leucine zipper transcription factor of the Maf family. In the present study, we demonstrate the presence of Maf family members in the endocrine pancreas that bind to G1 and transactivate glucagon promoter expression. In transient transfection experiments, we found that the transactivating effect on the glucagon promoter was greatly enhanced by the simultaneous expression of Maf transcription factors and Pax-6. This enhancement on glucagon transactivation could be correlated with the ability of these proteins to interact together but does not require binding of Maf proteins to the G1 element. Furthermore, we found that Maf enhanced the Pax-6 DNA binding capacity. Our data indicate that Maf transcription factors may contribute to glucagon gene expression in the pancreas.

Functional activity of HERV-K-T47D-related long terminal repeats

The human genome contains a family of endogenous retroviruses, HERV-K(HML-4), that comprises the full-length provirus HERV-K-T47D, five related elements, and hundreds of solitary long terminal repeats (LTRs). We here show that HERV-K-T47D-related LTRs are dispersed over all human chromosomes and have arisen after the divergence of Old and New World monkeys. By screening a cDNA library derived from the human mammary carcinoma cell line T47D with a HERV-K-T47D LTR probe, we isolated several clones containing LTR/cellular gene chimeras and assessed the transcriptional activity of these LTRs in transient transfection experiments. All LTRs were able to drive the expression of a reporter gene, thereby displaying distinct activities in different cell lines. We found that sequences located downstream of the LTR-U3 region modulate the level of gene expression. Based on the impact of the R region we distinguished between three different LTR types; the activity of type I LTRs was enhanced in the presence of the LTR-R region in all cell lines tested, whereas a type II LTR was downregulated. Type III LTRs are characterized by lacking or having a varying influence of the R region that was dependent on the cell line used. Finally, our results attribute to LTR-U5-gag sequences a role in determining LTR activity.

Distinct roles of Jun : Fos and Jun : ATF dimers in oncogenesis

Jun : Fos and Jun : ATF complexes represent two classes of AP-1 dimers that (1) preferentially bind to either heptameric or octameric AP-1 binding sites, and (2) are differently regulated by cellular signaling pathways and oncogene products. To discriminate between the functions of Jun : Fos, Jun : ATF and Jun : Jun, mutants were developed that restrict the ability of Jun to dimerize either to itself, or to Fos(-like) or ATF(-like) partners. Introduction of these mutants in chicken embryo fibroblasts shows that Jun : Fra2 and Jun : ATF2 dimers play distinct, complementary roles in in vitro oncogenesis by inducing either anchorage independence or growth factor independence, respectively. v-Jun : ATF2 rather than v-Jun : Fra2 triggers the development of primary fibrosarcomas in the chicken wing. Genes encoding extracellular matrix components seem to constitute an important subset of v-Jun : ATF2-target genes. Repression of the matrix component SPARC by Jun is essential for the induction of fibrosarcomas. Avian primary cells transformed by either Jun : Fra2 or Jun : ATF2 thus provide powerful tools for the investigation of the downstream pathways involved in oncogenesis. Further genetic studies with Jun dimerization mutants will be required to be precise and extend the specific roles of the Jun : Fos and Jun : ATF dimers during cancer progression in avian and mammalian systems.

Mutational definition of RNA-binding and protein-protein interaction domains of heterogeneous nuclear RNP C1

The heterogeneous nuclear ribonucleoprotein (hn- RNP) C proteins, among the most abundant pre-mRNA-binding proteins in the eukaryotic nucleus, have a single RNP motif RNA-binding domain. The RNA-binding domain (RBD) is comprised of approximately 80-100 amino acids, and its structure has been determined. However, relatively little is known about the role of specific amino acids of the RBD in the binding to RNA. We have devised a phage display-based screening method for the rapid identification of amino acids in hnRNP C1 that are essential for its binding to RNA. The identified mutants were further tested for binding to poly(U)-Sepharose, a substrate to which wild type hnRNP C1 binds with high affinity. We found both previously predicted, highly conserved residues as well as additional residues in the RBD to be essential for C1 RNA binding. We also identified three mutations in the leucine-rich C1-C1 interaction domain near the carboxyl terminus of the protein that both abolished C1 oligomerization and reduced RNA binding. These results demonstrate that although the RBD is the primary determinant of C1 RNA binding, residues in the C1-C1 interaction domain also influence the RNA binding activity of the protein. The experimental approach we described should be generally applicable for the screening and identification of amino acids that play a role in the binding of proteins to nucleic acid substrates.

A set of Hox proteins interact with the Maf oncoprotein to inhibit its DNA binding, transactivation, and transforming activities

Maf oncoprotein is a basic-leucine zipper (bZip) type of transcriptional activator. Since many transcription factors are known to form functional complexes, we searched for proteins that interact with the DNA-binding domain of Maf using the phage display method and identified two homeodomain-containing proteins, Hoxd12 and MHox/Prx1/Phox1/Pmx1. Studies with mutants of Hox and Maf proteins showed that they associate through their DNA-binding domains; the homeodomain of Hox and the bZip domain of Maf, respectively. Reflecting the high similarity of the bZip domain, all other Maf family members tested (c-/v-Maf, MafB, MafK, MafF, and MafG) also associated with the Hox proteins. Pax6, whose homeodomain is relatively similar to MHox, also could interact with Maf. However, two other bZip oncoproteins, Fos and Jun, failed to associate with the Hox proteins, while a distantly related Hox family member, Meis1, could not interact with Maf. Through interactions with the bZip domain, the Hox proteins inhibited the DNA binding activity of Maf, whereas the binding of Hox proteins to their recognition sequences was not abrogated by Maf. We further showed that coexpression of the Hox proteins repressed transcriptional activation and transforming activity of Maf. These results suggested that the interaction of a set of Hox proteins with Maf family members may interfere not only with their oncogenicity but also with their physiological roles.

A role of the kinase mTOR in cellular transformation induced by the oncoproteins P3k and Akt

The oncoproteins P3k (homolog of the catalytic subunit of class IA phosphoinositide 3-kinase) and Akt (protein kinase B) induce oncogenic transformation of chicken embryo fibroblasts. The transformed cells show constitutive phosphorylation of the positive regulator of translation p70S6 kinase (S6K) and of the eukaryotic initiation factor 4E-BP1 binding protein (4E-BP1), a negative regulator of translation. Phosphorylation activates S6K and inactivates 4E-BP1. A mutant of Akt that retains kinase activity but does not induce phosphorylation of S6K or of 4E-BP1 fails to transform chicken embryo fibroblasts, suggesting a correlation between the oncogenicity of Akt and phosphorylation of S6K and 4E-BP1. The macrolide antibiotic rapamycin effectively blocks oncogenic transformation induced by either P3k or Akt but does not reduce the transforming activity of 11 other oncoproteins. Rapamycin inhibits the kinase mTOR, an important regulator of translation, and this inhibition requires binding of the antibiotic to the immunophilin FKBP12. Displacement of rapamycin from FKBP12 relieves the inhibition of mTOR and also restores P3k-induced transformation. These data are in accord with the hypothesis that transformation by P3k or Akt involves intervention in translational controls.

A role of the kinase mTOR in cellular transformation induced by the oncoproteins P3k and Akt

The oncoproteins P3k (homolog of the catalytic subunit of class IA phosphoinositide 3-kinase) and Akt (protein kinase B) induce oncogenic transformation of chicken embryo fibroblasts. The transformed cells show constitutive phosphorylation of the positive regulator of translation p70S6 kinase (S6K) and of the eukaryotic initiation factor 4E-BP1 binding protein (4E-BP1), a negative regulator of translation. Phosphorylation activates S6K and inactivates 4E-BP1. A mutant of Akt that retains kinase activity but does not induce phosphorylation of S6K or of 4E-BP1 fails to transform chicken embryo fibroblasts, suggesting a correlation between the oncogenicity of Akt and phosphorylation of S6K and 4E-BP1. The macrolide antibiotic rapamycin effectively blocks oncogenic transformation induced by either P3k or Akt but does not reduce the transforming activity of 11 other oncoproteins. Rapamycin inhibits the kinase mTOR, an important regulator of translation, and this inhibition requires binding of the antibiotic to the immunophilin FKBP12. Displacement of rapamycin from FKBP12 relieves the inhibition of mTOR and also restores P3k-induced transformation. These data are in accord with the hypothesis that transformation by P3k or Akt involves intervention in translational controls.

Small maf (MafG and MafK) proteins negatively regulate antioxidant response element-mediated expression and antioxidant induction of the NAD(P)H:Quinone oxidoreductase1 gene

The antioxidant response element (ARE) is known to regulate expression and induction of NQO1, GST Ya, and other detoxifying enzyme genes in response to antioxidants and xenobiotics. The nuclear transcription factor Nrf2 and Nrf1 bind to the ARE and positively regulate expression and induction of the NQO1 and GST Ya genes. In this study, we demonstrate that overexpression of small Maf (MafG and MafK) proteins negatively regulate ARE-mediated expression and tert-butyl hydroquinone induction of the NQO1 and GST Ya genes in transfected Hep-G2 cells. In similar experiments, overexpression of small Maf proteins also repressed Nrf2-mediated up-regulation of ARE-mediated NQO1 and GST Ya genes expression in Hep-G2 cells co-transfected with Nrf2 and small Maf proteins. Band and supershift assays with the NQO1 gene ARE and nuclear proteins demonstrate that small MafG and MafK bind to the ARE as Maf-Maf homodimers and Maf-Nrf2 heterodimers. Therefore, Maf-Maf homodimers and possibly Maf-Nrf2 heterodimers play a role in negative regulation of ARE-mediated transcription and antioxidant induction of NQO1 and other detoxifying enzyme genes. In contrast to Maf-Nrf2, the Maf-Nrf1 heterodimers failed to bind with the NQO1 gene ARE and did not demonstrate the repressive effect in transfection assays.

Positive or negative MARE-dependent transcriptional regulation is determined by the abundance of small Maf proteins

The small Maf transcription factor proteins bind to Maf Recognition Elements (MAREs) by dimerizing with CNC proteins or themselves. We undertook experiments to clarify the functional relationship between the small Mafs and their partners in vivo. Embryos expressing abundant transgene-derived MafK died of severe anemia, while lines expressing lower levels of small Maf lived to adulthood. Megakaryocytes from the latter overexpressing lines exhibited reduced proplatelet formation and MARE-dependent transcription, phenocopying mafG null mutant mice. When the mafG null mutants were bred to small Maf-overexpressing transgenic animals, both loss- and gain-of-function phenotypes were reversed. These results provide direct in vivo evidence that transcriptional regulation through MARE elements hinges on an exquisitely sensitive balance of activating CNC molecules and their small Maf partners.

Transcriptional control of SPARC by v-Jun and other members of the AP1 family of transcription factors

Transformation of chick embryo fibroblasts by the v-Jun oncoprotein correlates with a down-regulation of the extracellular matrix protein SPARC and repression of the corresponding mRNA. Alteration in SPARC expression has been repeatedly reported in human cancers of various origin, and is thought to contribute to the remodeling of the extracellular matrix during neoplastic progression. Transcriptional control of SPARC is poorly understood. We show here that (i) v-Jun-mediated repression of the endogenous SPARC gene is enhanced by Fra2 but alleviated by ATF2, Fra2 and ATF2 being the two major partners of v-Jun in the transformed cells; (ii) high basal activity as well as repression by v-Jun and modulation by Fra2 and ATF2 is restricted to a small proximal fragment (-124/+16) of the chicken SPARC promoter; (iii) the activity of this minimal promoter is modulated by all the AP1 family members known in chickens (c-Jun and JunD; c-Fos and Fra2; ATF2; c-Maf, MafA, and MafB). Taken together these data demonstrate that, at least in avian primary cells, SPARC expression is under the control of the AP1 transcription factor. Further studies with the minimal (-124/+16) promoter fragment are needed to understand how this control takes place at the molecular level.

Cloning and expression of human B cell-specific transcription factor BACH2 mapped to chromosome 6q15

The transcription factor Bach2, a member of the BTB-basic region leucine zipper (bZip) factor family, binds to a 12-O-tetradecanoylphorbol-13-acetate (TPA)-responsive element and the related Maf-recognition element (MARE) by forming homodimers or heterodimers with Maf-related transcription factors. Bach2 regulates transcription by binding to these elements. To understand the function in hematopoiesis, we isolated a cDNA clone for human Bach2 (BACH2) encoding a protein of 841 amino acid residues with a deduced amino acid sequence having 89.5% identity to mouse homolog. Among human hematopoietic cell lines, BACH2 is expressed abundantly only in some B-lymphocytic cell lines. RT-PCR analysis of hematopoietic cells revealed that BACH2 mRNA is expressed in primary B-cells. Enforced expression of BACH2 in a human Burkitt cell line, RAJI that does not express endogenous BACH2, resulted in marked reduction of clonogenic activity, indicating that BACH2 possesses an inhibitory effect on cell proliferation. By fluorescent in situ hybridization, the BACH2 gene was localized to chromosome 6q15. Because deletion of the long arm of chromosome 6 (6q) is one of the commonest chromosomal alterations in human B-cell lymphoma, we examined for the loss of heterozygosity (LOH) of the BACH2 gene in human B-cell non-Hodgkin's lymphomas (NHL). Among 25 informative cases, five (20%) showed LOH. These results indicate that BACH2 plays important roles in regulation of B cell development.

Identification and characterization of genes upregulated in cells transformed by v-Jun

The transcription factor Jun (c-Jun) functions as a recipient of extracellular growth signals and converts them into patterns of gene expression. An oncogenic variant of c-Jun was isolated from the acutely transforming retrovirus ASV17. Overexpression of this viral Jun (v-Jun) induces transformation of chicken embryo fibroblasts (CEF) in culture and fibrosarcomas in chickens. v-Jun is a constitutively active form of c-Jun and transforms cells presumably by deregulating the expression of specific target genes. In this report, we describe six genes whose transcripts are upregulated in v-Jun-transformed CEF. Three of these genes show homology to known mammalian genes, to MAP kinase phosphatase 2 (MKP-2), to reversion-induced LIM protein (RIL) and to cytokine-inducible SH2-containing protein (CIS). Northern blot analysis, using CEF infected with various Jun mutants or an estrogen-regulatable Jun chimera, revealed distinct induction patterns of individual targets by v-Jun. The chicken RIL homolog showed an expression pattern tightly correlated with the activity of v-Jun. Its expression is also transformation-dependent, suggesting a role for this gene in v-Jun transformation. The newly identified v-Jun targets can serve as molecular markers in the v-Jun transformation process. Oncogene (2000) 19, 3537 - 3545

Maf transcriptionally activates the mouse p53 promoter and causes a p53-dependent cell death

An increase in the level of the tumor suppressor protein p53 can induce cell cycle arrest or cell death. Although mechanisms for regulating the life span of p53 have been described, there is growing evidence that transcriptional regulation of the p53 gene contributes significantly to controlling p53 protein levels and therefore the fate of a cell. However, the signal transduction pathways that lead to transcriptional activation of the p53 gene are poorly understood. The oncoprotein v-Maf and its cellular counterparts belong to the large combinatorially complex basic leucine zipper family of transcription factors, which include the AP1 family. To date few cellular targets of c-Maf have been identified. It is demonstrated here that v-Maf can bind as a homodimer to a variant Maf recognition element located between -66 and -54 upstream in the mouse p53 promoter. V-Maf and its cellular counterparts are shown to activate p53 expression through this site. The ability of v-Maf to activate p53 expression is modulated by AP1 family members. In addition, overexpression of v-Maf in primary cells leads to a p53-dependent cell death. Thus, Maf and members of the AP1 family are able to regulate p53 expression through this site in the p53 promoter.

Developmental expression of maf-1 messenger ribonucleic acids in rat kidney by in situ hybridization histochemistry

maf is a family of oncogenes, originally identified from the avian oncogenic retrovirus AS42, which encodes a nuclear bZip transcription factor protein. It has been reported that maf family genes have critical roles in embryological development and cellular differentiation. In this study, the distribution of maf-1 genes, the rat homologues of mafB, was examined in rat kidneys at the embryonic stages from 13 days, gestation (E13) through E21 and then 1, 2, 4, and 8 weeks after birth by in situ hybridization with (35)S-labeled antisense riboprobes. The cellular localization was determined using double in situ hybridization. Expression of maf-1 mRNA appeared weakly on E15 and was restricted to glomerular visceral epithelial cells during the pre- and postnatal stages until 2 weeks after birth and then gradually diminished. Double in situ hybridization demonstrated that maf-1 mRNA-positive cells in glomerulus also expressed Pod-1 gene, suggesting that maf-1 mRNA was expressed in the podocyte. These findings suggest that the expression of maf-1 gene may be involved in embryological development and/or differentiation of the kidney.

Perinatal synthetic lethality and hematopoietic defects in compound mafG::mafK mutant mice

Prior studies exploring the mechanisms controlling erythroid gene regulation implicated MARE (Maf recognition element) cis-elements as crucial to the transcriptional activity of many erythroid genes. Numerous transcription factors can elicit responses through MAREs, including not only the AP-1 family proteins, but also a growing list of factors composed of Cap-N-Collar (CNC)-small Maf heterodimers. While these factors can activate transcription from MAREs in co-transfection assays, mouse germline mutations in cnc genes tested to date have failed to reveal primary erythroid phenotypes. Here we report that after combining the mafK and mafG targeted null alleles, mutant animals display several synthetic phenotypes, including erythroid deficiencies. First, compound homozygous small maf gene mutants survive embryogenesis, but die postnatally. Secondly, compound mutant animals develop severe neurological disorders. Thirdly, they exhibit an exacerbated mafG deficiency in megakaryopoiesis, specifically in proplatelet formation, resulting in profound thrombocytopenia. Finally, the compound mutant animals develop severe anemia accompanied by abnormal erythrocyte morphology and membrane protein composition. These data provide direct evidence that the small Maf transcription factors play an important regulatory role in erythropoiesis.

Sequence and insertion sites of murine melanoma-associated retrovirus

We previously showed that B16 melanoma cells produce ecotropic melanoma-associated retrovirus (MelARV) which encodes a melanoma-associated antigen recognized by MM2-9B6 monoclonal antibody. The biological significance of MelARV in melanoma formation remains unknown. We found that infection of normal melanocytes with MelARV resulted in malignant transformation. It is likely that MelARV emerged from the defective Emv-2 provirus, a single copy of ecotropic provirus existing in the genome of C57BL/6 mice. In the present study, we cloned and sequenced the full-length MelARV genome and its insertion sites and we completed sequencing of the Emv-2 provirus. Our data show that MelARV has a typical full-length retroviral genome with high homology (98.54%) to Emv-2, indicating a close relationship between both viruses. MelARV probably emerged as a result of recombination between Emv-2 and an endogenous nonecotropic provirus. Some observed differences in the gag and pol regions of MelARV might account for the restoration of productivity and infectivity of a novel retrovirus that somatically emerged during melanoma formation. MelARV does not contain any oncogene and therefore might induce transformation by insertional mutagenesis. We sequenced two insertion sites of MelARV. The first insertion site represents the 3' coding region of the c-maf proto-oncogene at 67.0 centimorgans (cM) on chromosome 8. The c-maf proto-oncogene encodes a basic leucine zipper protein homologous to c-fos and c-jun. Insertion of MelARV in BL6 melanoma cells resulted in the up-regulation of c-maf. It is noteworthy that the Emv-2 provirus is also inserted into a noncoding region at 61.0 cM on the same chromosome 8. The second insertion site is the 3' noncoding region of the DNA polymerase gamma (PolG) gene on chromosome 7. The expression of PolG was not affected by the MelARV insertion. Further investigation of the biological significance of MelARV in melanoma formation is being undertaken.

Regulatory mechanisms of cellular response to oxidative stress

An antioxidant responsive element (ARE) or electrophile responsive element (EpRE) mediates the transcriptional activation of genes encoding phase II drug metabolizing enzymes. The ARE consensus sequence shows high similarity to an erythroid gene regulatory element, and based on the observation, we have recently found that transcription factor Nrf2 is essential for the coordinate induction of phase II detoxifying enzymes. The expression of anti-oxidative stress enzyme genes is also regulated by Nrf2. Detailed analysis of the regulatory mechanisms of Nrf2 activity has ultimately led us to the identification of a new protein, which we have named Keap1, that suppresses Nrf2 activity by specific binding to its evolutionarily-conserved N-terminal Neh2 regulatory domain.

Characterization of the murine mafF gene

Small Maf proteins are obligatory heterodimeric partner molecules of mammalian Cap'n'Collar proteins that together control a wide variety of eukaryotic genes. Although both MafK and MafG are expressed in overlapping but distinct tissue distribution patterns during embryonic development, the physiological consequences of loss-of-function mutations in either gene are modest. This suggested that compensation by the third small Maf protein, MafF, might be a major reason for such mild phenotypes and that further analysis of MafF might therefore provide important insights for understanding small Maf regulatory function(s). We therefore cloned, mapped, transcriptionally and developmentally characterized, and finally disrupted the mafF gene. We show that murine mafF is transcriptionally regulated by three different promoters and is most abundantly expressed in the lung. The lacZ gene inserted into the mafF locus revealed prominent expression sites in the gut, lung, liver, outflow tract of the heart, cartilage, bone membrane, and skin but not in hematopoietic cells at any developmental stage. Homozygous mafF null mutant mice were born in a normal Mendelian ratio and displayed no obvious functional deficiencies, indicating that MafF activity may be dispensable even in tissues where the expression of other small Maf proteins is quite low.

Transcriptional regulation of T lymphocyte development and function

The development and function of T lymphocytes are regulated tightly by signal transduction pathways that include specific cell-surface receptors, intracellular signaling molecules, and nuclear transcription factors. Since 1988, several families of functionally important T cell transcription factors have been identified. These include the Ikaros, LKLF, and GATA3 zinc-finger proteins; the Ets, CREB/ATF, and NF-kappa B/Rel/NFAT transcription factors; the Stat proteins; and HMG box transcription factors such as LEF1, TCF1, and Sox4. In this review, we summarize our current understanding of the transcriptional regulation of T cell development and function with particular emphasis on the results of recent gene targeting and transgenic experiments. In addition to increasing our understanding of the molecular pathways that regulate T cell development and function, these results have suggested novel targets for genetic and pharmacological manipulation of T cell immunity.

Heparin-binding epidermal growth factor-like growth factor, a v-Jun target gene, induces oncogenic transformation

Jun is a transcription factor belonging to the activator protein 1 family. A mutated version of Jun (v-Jun) transduced by the avian retrovirus ASV17 induces oncogenic transformation in avian cell cultures and sarcomas in young galliform birds. The oncogenicity of Jun probably results from transcriptional deregulation of v-Jun-responsive target genes. Here we describe the identification and characterization of a growth-related v-Jun target, a homolog of heparin-binding epidermal growth factor-like growth factor (HB-EGF). HB-EGF is strongly expressed in chicken embryo fibroblasts (CEF) transformed by v-Jun. HB-EGF expression is not detectable or is marginal in nontransformed CEF. Using a hormone-inducible Jun-estrogen receptor chimera, we found that HB-EGF expression is correlated with v-Jun activity. In this system, induction of v-Jun is followed within 1 hr by elevated levels of HB-EGF. In CEF infected with various Jun mutants, HB-EGF expression is correlated with the oncogenic potency of the mutant. Constitutive expression of HB-EGF conveys to CEF the ability to grow in soft agar and to form multilayered foci of transformed cells on a solid substrate. These observations suggest that HB-EGF is an effector of Jun-induced oncogenic transformation.