Identification of beta-cell-specific insulin gene transcription factor RIPE3b1 as mammalian MafA

Transcription factor cycling on the insulin promoter

Using MIN6 beta-cells and chromatin immunoprecipitation (ChIP) assays, the chronological sequence of binding of MafA, E47/beta2 and PDX-1 to the insulin promoter in living beta-cells were investigated. All four factors were shown to bind to the mouse insulin 2 promoter in a cyclical manner with a periodicity of approximately 10-15 min. The cyclical binding of MafA, E47 and beta2 was largely unaffected by the glucose or insulin concentration in the media. However, the binding and cycling of PDX-1 was markedly abolished in low glucose (1 mM), and this was reversed in the presence of low concentrations of insulin.

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.

Interactions between areas I and II direct pdx-1 expression specifically to islet cell types of the mature and developing pancreas

PDX-1 regulates transcription of genes involved in islet beta cell function and pancreas development. Islet-specific expression is controlled by 5'-flanking sequences from base pair (bp) -2917 to -1918 in transgenic experiments, which encompasses both conserved (i.e. Area I (bp -2761/-2457), Area II (bp -2153/-1923)) and non-conserved pdx-1 sequences. However, only an Area II-driven transgene is independently active in vivo, albeit in only a fraction of islet PDX-1-producing cells. Our objective was to identify the sequences within the -2917/-1918-bp region that act in conjunction with Area II to allow comprehensive expression in islet PDX-1(+) cells. In cell line-based transfection assays, only Area I effectively potentiated Area II activity. Both Area I and Area II functioned in an orientation-independent manner, whereas synergistic, enhancer-like activation was uniquely found with duplicated Area II. Chimeras of Area II and the generally active SV40 enhancer or the beta cell-specific insulin enhancer suggested that islet cell-enriched activators were necessary for Area I activation, because Area II-mediated stimulation was reduced by the SV40 enhancer and activated by the insulin enhancer. Several conserved sites within Area I were important in Area I/Area II activation, with binding at bp -2614/-2609 specifically controlled by Nkx2.2, an insulin gene regulator that is required for terminal beta cell differentiation. The ability of Area I to modulate Area II activation was also observed in vivo, as an Area I/Area II-driven transgene recapitulated the endogenous pdx-1 expression pattern in developing and adult islet cells. These results suggest that Area II is a central pdx-1 control region, whose islet cell activity is uniquely modified by Area I regulatory factors.

Relative contribution of PDX-1, MafA and E47/beta2 to the regulation of the human insulin promoter

The insulin promoter binds a number of tissue-specific and ubiquitous transcription factors. Of these, the homoeodomain protein PDX-1 (pancreatic duodenal homeobox factor-1), the basic leucine zipper protein MafA and the basic helix-loop-helix heterodimer E47/BETA2 (beta-cell E box transactivator 2; referred to here as beta2) bind to important regulatory sites. Previous studies have shown that PDX-1 can interact synergistically with E47 and beta2 to activate the rat insulin 1 promoter. The aim of the present study was to determine the relative contribution of PDX-1, MafA and E47/beta2 in regulating the human insulin promoter, and whether these factors could interact synergistically in the context of the human promoter. Mutagenesis of the PDX-1, MafA and E47/beta2 binding sites reduced promoter activity by 60, 74 and 94% respectively, in INS-1 beta-cells. In the islet glucagonoma cell line alphaTC1.6, overexpression of PDX-1 and MafA separately increased promoter activity approx. 2.5-3-fold, and in combination approx. 6-fold, indicating that their overall effect was additive. Overexpression of E47 and beta2 had no effect. In HeLa cells, PDX-1 stimulated the basal promoter by approx. 40-fold, whereas MafA, E47 and beta2 each increased activity by less than 2-fold. There was no indication of any synergistic effects on the human insulin promoter. On the other hand, the rat insulin 1 promoter and a mutated version of the human insulin promoter, in which the relevant regulatory elements were separated by the same distances as in the rat insulin 1 promoter, did exhibit synergy. PDX-1 was shown further to activate the endogenous insulin 1 gene in alphaTC1.6 cells, whereas MafA activated the insulin 2 gene. In combination, PDX-1 and MafA activated both insulin genes. Chromatin immunoprecipitation assays confirmed that PDX-1 increased the association of acetylated histones H3 and H4 with the insulin 1 gene and MafA increased the association of acetylated histone H3 with the insulin 2 gene.

Synergistic activation of the insulin gene promoter by the beta-cell enriched transcription factors MafA, Beta2, and Pdx1

Specific expression of the insulin gene in pancreatic islet beta-cells requires multiple cis-regulatory elements in its promoter. Pdx1, MafA, and Beta2 have been identified as beta-cell enriched transcription factors that bind to these elements. Pdx1 has been shown to bind to A1, A3, A5, and GG2, and Beta2 binds to E1 by forming a heterodimer with the ubiquitous factor E47. MafA was recently identified as a C1-element binding factor. However, interactions between these factors and the promoter have not been characterized in detail. In this report, we show that these transactivators synergistically stimulate insulin promoter activity. Among multiple binding sites for Pdx1, MafA, and Beta2, at least GG2, C1, and E1 elements located in the promoter region between -150 and -100 base pairs are necessary for the synergism. We also found that neither MafB nor c-Maf, close relatives of MafA, showed synergistic activation. These results suggest that co-expression and functional synergism of these beta-cell enriched transactivators, MafA, Pdx1, and Beta2, are critical for establishing the beta-cell-specific and efficient expression of the insulin gene.

Palmitate inhibits insulin gene expression by altering PDX-1 nuclear localization and reducing MafA expression in isolated rat islets of Langerhans

Abnormalities in lipid metabolism have been proposed as contributing factors to both defective insulin secretion from the pancreatic beta cell and peripheral insulin resistance in type 2 diabetes. Previously, we have shown that prolonged exposure of isolated rat islets of Langerhans to excessive fatty acid levels impairs insulin gene transcription. This study was designed to assess whether palmitate alters the expression and binding activity of the key regulatory factors pancreas-duodenum homeobox-1 (PDX-1), MafA, and Beta2, which respectively bind to the A3, C1, and E1 elements in the proximal region of the insulin promoter. Nuclear extracts of isolated rat islets cultured with 0.5 mm palmitate exhibited reduced binding activity to the A3 and C1 elements but not the E1 element. Palmitate did not affect the overall expression of PDX-1 but reduced its nuclear localization. In contrast, palmitate blocked the stimulation of MafA mRNA and protein expression by glucose. Combined adenovirus-mediated overexpression of PDX-1 and MafA in islets completely prevented the inhibition of insulin gene expression by palmitate. These results demonstrate that prolonged exposure of islets to palmitate inhibits insulin gene transcription by impairing nuclear localization of PDX-1 and cellular expression of MafA.

MafA transcription factor is phosphorylated by p38 MAP kinase

Basic-leucine zipper transcription factors of the Maf family are key regulators of various developmental and differentiation processes. We previously reported that the phosphorylation status of MafA is a critical determinant of its biological functions. Using Western blot and mass spectrometry analysis, we now show that MafA is phosphorylated by p38 MAP kinase and identify three phosphoacceptor sites: threonine 113 and threonine 57, evolutionarily conserved residues located in the transcription activating domain, and serine 272. Mutation of these residues severely impaired MafA biological activity. Furthermore, we show that p38 also phosphorylates MafB and c-Maf. Together, these findings suggest that the p38 MAP kinase pathway is a novel regulator of large Maf transcription factors.

Microarray analysis of the genes induced by tetracycline-regulated expression of NDRF/NeuroD2 in P19 cells

NeuroD-related factor (NDRF)/NeuroD2 is a basic helix-loop-helix (bHLH) protein that plays important roles in neuronal development. To elucidate the NDRF transcription network, we used mouse cDNA microarray analysis combined with a tetracycline-regulatable expression system in P19 embryonal carcinoma cells. Five genes were identified to be up-regulated in the presence of NDRF protein. RNA hybridization analysis confirmed that brain-lipid-binding protein (BLBP) and inhibitor of differentiation 1 (Id1) genes were among the five genes that were rapidly and significantly up-regulated after induction of NDRF. When a dominant negative form of NDRF protein was expressed during retinoic acid-induced neuronal differentiation of P19 cells, the BLBP gene, but not the Id1 gene, was potently repressed. Immunohistochemical analysis revealed that both NDRF and Id1 immunoreactivities were observed in some granule cells of the cerebellum in the postnatal period. These results suggest that NDRF or its related bHLH proteins may act upstream of these genes in a subset of developing neurons.

FoxO1 protects against pancreatic beta cell failure through NeuroD and MafA induction

Diabetes causes pancreatic beta cell failure through hyperglycemia-induced oxidative stress, or "glucose toxicity." We show that the forkhead protein FoxO1 protects beta cells against oxidative stress by forming a complex with the promyelocytic leukemia protein Pml and the NAD-dependent deacetylase Sirt1 to activate expression of NeuroD and MafA, two Insulin2 (Ins2) gene transcription factors. Using acetylation-defective and acetylation-mimicking mutants, we demonstrate that acetylation targets FoxO1 to Pml and prevents ubiquitin-dependent degradation. We show that hyperglycemia suppresses MafA expression in vivo and that MafA inhibition can be prevented by transgenic expression of constitutively nuclear FoxO1 in beta cells. The findings provide a mechanism linking glucose- and growth factor receptor-activated pathways to protect beta cells against oxidative damage via FoxO proteins.

Sterol regulatory element-binding proteins activate insulin gene promoter directly and indirectly through synergy with BETA2/E47

Insulin gene expression is regulated by pancreatic beta cell-specific factors, PDX-1 and BETA2/E47. Here we have demonstrated that the insulin promoter is a novel target for SREBPs established as lipid-synthetic transcription factors. Promoter analyses of rat insulin I gene in non-beta cells revealed that nuclear SREBP-1c activates the insulin promoter through three novel SREBP-binding sites (SREs), two of which overlap with E-boxes, binding sites for BETA2/E47. SREBP-1c activation of the insulin promoter was markedly enhanced by co-expression of BETA2/E47. This synergistic activation by SREBP-1c/BETA2/E47 was not mediated through SREs but through the E-boxes on which BETA2/E47 physically interacts with SREBP-1c, suggesting a novel function of SREBP as a co-activator. These two cis-DNA regions, E1 and E2, with an appropriate distance separating them, were mandatory for the synergism, which implicates formation of SREBP-1c.BETA2.E47 complex in a DNA looping structure for efficient recruitment of CREB-binding protein/p300. However, in the presence of PDX1, the synergistic action of SREBP-1c with BETA2/E47 was canceled. SREBP-1c-mediated activation of the insulin promoter and expression became overt in beta cell lines and isolated islets when endogenous PDX-1 expression was low. This cryptic SREBP-1c action might play a compensatory role in insulin expression in diabetes with beta cell lipotoxicity.

MafA is a key regulator of glucose-stimulated insulin secretion

MafA is a transcription factor that binds to the promoter in the insulin gene and has been postulated to regulate insulin transcription in response to serum glucose levels, but there is no current in vivo evidence to support this hypothesis. To analyze the role of MafA in insulin transcription and glucose homeostasis in vivo, we generated MafA-deficient mice. Here we report that MafA mutant mice display intolerance to glucose and develop diabetes mellitus. Detailed analyses revealed that glucose-, arginine-, or KCl-stimulated insulin secretion from pancreatic beta cells is severely impaired, although insulin content per se is not significantly affected. MafA-deficient mice also display age-dependent pancreatic islet abnormalities. Further analysis revealed that insulin 1, insulin 2, Pdx1, Beta2, and Glut-2 transcripts are diminished in MafA-deficient mice. These results show that MafA is a key regulator of glucose-stimulated insulin secretion in vivo.

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.

ERK1/2-dependent Activation of Transcription Factors Required for Acute and Chronic Effects of Glucose on the Insulin Gene Promoter

The insulin promoter is both positively and negatively regulated in response to conditions to which pancreatic beta-cells are exposed. Exposure of intact rat islets and INS-1 pancreatic beta-cells to 11 mm glucose for minutes to hours results in an enhancement in the rate of insulin gene transcription assessed with a reporter linked to the insulin gene promoter. In contrast, chronic exposure of rat islets or beta-cells to 11 mm glucose results in loss of the glucose responsiveness of the insulin gene promoter. By 48 h, glucose inhibits insulin gene promoter activity. Here we show that not only the acute effect of elevated glucose to stimulate the insulin gene promoter but also the chronic effect of elevated glucose to inhibit the insulin gene promoter depend on ERK1/2 mitogen-activated protein kinase activity. In examining the underlying mechanism, we found that acute exposure to 11 mm glucose resulted in the binding of the transcription factors NFAT and Maf to the glucose-responsive A2C1 element of the insulin gene promoter. An NFAT and C/EBP-beta complex was observed in cells chronically exposed to 11 mm glucose. Formation of NFAT-Maf and NFAT-C/EBP-beta complexes was sensitive to inhibitors of ERK1/2 and calcineurin, consistent with our previous finding that activation of ERK1/2 by glucose required calcineurin activity and the well documented regulation of NFAT by calcineurin. These results indicate that the ERK1/2 pathway modulates partners of NFAT, which may either stimulate or repress insulin gene transcription during stimulatory and chronic exposure to elevated glucose.

Regulation of gene expression by Pax6 in ocular cells: a case of tissue-preferred expression of crystallins in lens

Lens development is an excellent model for genetic and biochemical studies of embryonic induction, cell cycle regulation, cellular differentiation and signal transduction. Differentiation of lens is characterized by lens-preferred expression and accumulation of water-soluble proteins, crystallins. Crystallins are required for light transparency, refraction and maintenance of lens integrity. Here, we review mechanisms of lens-preferred expression of crystallin genes by employing synergism between developmentally regulated DNA-binding transcription factors: Pax6, c-Maf, MafA/L-Maf, MafB, NRL, Sox2, Sox1, RARbeta/RXRbeta, RORalpha, Prox1, Six3, gammaFBP-B and HSF2. These factors are differentially expressed in lens precursor cells, lens epithelium and primary and secondary lens fibers. They exert their function in combination with ubiquitously expressed factors (e.g. AP-1, CREB, pRb, TFIID and USF) and co-activators/chromatin remodeling proteins (e.g. ASC-2 and CBP/p300). A special function belongs to Pax6, a paired domain and homeodomain-containing protein, which is essential for lens formation. Pax6 is expressed in lens progenitor cells before the onset of crystallin expression and it serves as an important regulatory factor required for expression of c-Maf, MafA/L-Maf, Six3, Prox1 and retinoic acid signaling both in lens precursor cells and the developing lens. The roles of these factors are illustrated by promoter studies of mouse alphaA-, alphaB-, gammaF- and guinea pig zeta-crystallins. Pax6 forms functional complexes with a number of transcription factors including the retinoblastoma protein, pRb, MafA, Mitf and Sox2. We present novel data showing that pRb antagonizes Pax6-mediated activation of the alphaA-crystallin promoter likely by inhibiting binding of Pax6 to DNA.

Embryonic stem cells and islet replacement in diabetes mellitus

Transplantation of functional islets of Langerhans may emerge as a useful therapy for some patients with type 1 diabetes mellitus (DM), but donor islet shortages motivate the search for new sources of transplantable islets. Pluripotent embryonic stem (ES) cells are expandable in culture and have the potential to give rise to all cell types in the body. The recent isolation of pluripotent ES cells from humans has generated excitement over the possibility of engineering glucose-responsive islet replacement tissue from these cells in large quantities. In this study, we review the recent advances in generating insulin-producing cells (IPC) from mouse and human ES (hES) cells.

Minireview: transcriptional regulation in pancreatic development

Considerable progress has been made in the understanding of the sequential activation of signal transduction pathways and the expression of transcription factors during pancreas development. Much of this understanding has been obtained by analyses of the phenotypes of mice in which the expression of key genes has been disrupted (knockout mice). Knockout of the genes for Pdx1, Hlxb9, Isl1, or Hex results in an arrest of pancreas development at a very early stage (embryonic d 8-9). Disruption of genes encoding components of the Notch signaling pathway, e.g. Hes1 or neurogenin-3, abrogates development of the endocrine pancreas (islets of Langerhans). Disruption of transcription factor genes expressed more downstream in the developmental cascade (Beta2/NeuroD, Pax4, NKx2.2, and Nkx6.1) curtails the formation of insulin-producing beta-cells. An understanding of the importance of transcription factor genes during pancreas development has provided insights into the pathogenesis of diabetes, in which the mass of insulin-producing beta-cells is reduced.

A crucial role of MafA as a novel therapeutic target for diabetes

MafA, a recently isolated pancreatic beta-cell-specific transcription factor, is a potent activator of insulin gene transcription. In this study, we show that MafA overexpression, together with PDX-1 (pancreatic and duodenal homeobox factor-1) and NeuroD, markedly increases insulin gene expression in the liver. Consequently, substantial amounts of insulin protein were induced by such combination. Furthermore, in streptozotocin-induced diabetic mice, MafA overexpression in the liver, together with PDX-1 and NeuroD, dramatically ameliorated glucose tolerance, while combination of PDX-1 and NeuroD was much less effective. These results suggest a crucial role of MafA as a novel therapeutic target for diabetes.

Oxidative stress-mediated, post-translational loss of MafA protein as a contributing mechanism to loss of insulin gene expression in glucotoxic beta cells

Glucose toxicity in pancreatic islet beta cells causes loss of insulin gene expression, content, and secretion due to loss of binding of transcription factors, most notably PDX-1 and RIPE-3b1 activator, to the promoter region of the insulin gene. Recently, RIPE-3b1 activator was cloned and identified as the mammalian homologue of avian MafA/Maf-L (MafA). This enabled us to carry out more extensive studies of the role of MafA in glucotoxicity than were hitherto possible. Northern analysis of glucotoxic HIT-T15 cells revealed normal amounts of MafA mRNA, but Western analysis demonstrated a 97 +/- 1% reduction in MafA protein (p < 0.0001). The proteasome is a likely site for MafA degradation as lactacystin, an irreversible proteasome inhibitor, caused an accumulation of MafA protein. Antioxidants have previously been shown to prevent the adverse effects of glucose toxicity on beta cell function both in vivo and in vitro. In the current study, chronic culturing of HIT-T15 cells with the antioxidant N-acetylcysteine (NAC) prevented loss of MafA protein (late passage = 18.9 +/- 10.4% of early passage, p < 0.001; late passage with NAC = 68.7 +/- 19.7% of early passage, p = not significant) and loss of DNA binding (late passage = 63.7 +/- 9% of early passage, p < 0.02; late passage with NAC = 116 +/- 10% of early passage, p = not significant). Additionally, transient transfection of PDX-1 or MafA cDNA into glucotoxic cells increased PDX-1 and MafA protein levels and individually increased insulin promoter activity (untreated = 34%, PDX-1 = 70%, MafA = 78%; percentage of activity of early passage cells), whereas the combined transfection of MafA and PDX-1 completely restored insulin promoter activity. This recovery of promoter activity following transient transfection had no effect on endogenous insulin mRNA. However, adenoviral infection of MafA and PDX-1 significantly increased endogenous insulin mRNA levels by 93% (121 +/- 9 versus 233 +/- 18 density light units; n = 5, p < 0.001). We conclude that the absence of MafA protein from beta cells via chronic oxidative stress contributes importantly to the loss of endogenous insulin gene expression as glucose toxicity develops.

The islet beta cell-enriched MafA activator is a key regulator of insulin gene transcription

The islet-enriched MafA, PDX-1, and BETA2 activators contribute to both beta cell-specific and glucose-responsive insulin gene transcription. To investigate how these factors impart activation, their combined impact upon insulin enhancer-driven expression was first examined in non-beta cell line transfection assays. Individual expression of PDX-1 and BETA2 led to little or no activation, whereas MafA alone did so modestly. MafA together with PDX-1 or BETA2 produced synergistic activation, with even higher insulin promoter activity found when all three proteins were present. Stimulation was attenuated upon compromising either MafA transactivation or DNA-binding activity. MafA interacted with endogenous PDX-1 and BETA2 in coimmunoprecipitation and in vitro GST pull-down assays, suggesting that regulation involved direct binding. Dominant-negative acting and small interfering RNAs of MafA also profoundly reduced insulin promoter activity in beta cell lines. In addition, MafA was induced in parallel with insulin mRNA expression in glucose-stimulated rat islets. Insulin mRNA levels were also elevated in rat islets by adenoviral-mediated expression of MafA. Collectively, these results suggest that MafA plays a key role in coordinating and controlling the level of insulin gene expression in islet beta cells.

Transcriptional regulation of mouse alphaB- and gammaF-crystallin genes in lens: opposite promoter-specific interactions between Pax6 and large Maf transcription factors

Mammalian alphaB-crystallin is highly expressed both in lens epithelium and lens fibers. In contrast, gammaF-crystallin is highly expressed in the lens fiber cells. Crystallin gene expression in lens is regulated at the level of transcription by a sparse number of specific DNA-binding transcription factors. Here, we report studies on transcriptional regulation of mouse alphaB- and gammaF-crystallin promoters by specific combinations of Pax6/Pax6(5a), large Mafs (MafA, MafB, c-Maf, and NRL), Sox1, Sox2, Six3, and RARbeta/RXRbeta. Two sets of these factors, co-expressed both in lens epithelium and in lens fibers, were tested in co-transfection assays using cultured lens and non-lens cells. Regulation of alphaB-crystallin was studied in the presence of lens epithelial-factors Pax6, MafB, and RARbeta/RXRbeta, and lens fiber-factors Pax6, MafA, c-Maf, and NRL. Pax6 proteins activated the alphaB-crystallin promoter (-162 to +45) with any combination of Mafs. Addition of RARbeta/RXRbeta further increased its promoter activity. Gel shift assays using lens nuclear extracts demonstrated interactions of Pax6, Maf, and retinoic acid nuclear receptor proteins with two lens-specific regions, the distal LSR1 (-147/-118) and proximal LSR2 (-78/-40), of the alphaB-crystallin promoter. In contrast, Pax6 proteins acted as repressors of gammaF-crystallin promoter activity elicited by a combination of large Mafs, Sox, and RARbeta/RXRbeta proteins in transiently transfected lens and non-lens cells. The results show that Pax6 conversely regulates these two lens crystallin promoters. We propose that the opposite roles of Pax6 in crystallin gene regulation are results of different promoter architectures of the alphaB- and gammaF-crystallin genes, developmentally regulated association of transcription factors with the corresponding cis-regulatory sites, and specific recruitment of transcriptional co-activators and co-repressors by Pax6.

Regulation of ATP-sensitive potassium channel subunit Kir6.2 expression in rat intestinal insulin-producing progenitor cells

We have reported that the combined expression of Pdx-1 (pancreatic duodenal homeobox 1) and Isl-1 (islet 1) enables immature rat enterocytes (IEC-6) to produce and release insulin. A key component regulating the release of insulin is the ATP-sensitive potassium channel subunit Kir6.2. To investigate the regulation of Kir6.2 gene expression, we assessed Kir6.2 expression in IEC-6 cells expressing Pdx-1 and/or Isl-1. We observed that Kir6.2 protein was expressed de novo in IEC-6 cells expressing both Pdx-1 and Isl-1 but not in cells expressing Pdx-1 alone. Next, we analyzed the regions of the Kir6.2 promoter (-1677/-45) by performing a luciferase assay and electrophoretic mobility shift assay. The results have demonstrated that Kir6.2 promoter possesses two regions regulating the promoter activity: a Foxa2-binding site (-1364 to -1210) and an Sp1/Sp3-binding site (-1035 to -939). The additional expression of Isl-1 in IEC-6 cells expressing Pdx-1 attenuated overexpression of Foxa2 protein and enhanced Kir6.2 expression. Finally, knockdown of Isl-1 using the iRNA technique resulted in decreased expression of Kir6.2 protein in a rat pancreatic beta-cell line (RIN-5F cells). These results indicate that expression of Kir6.2 in the rat intestine is moderated by Isl-1.

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.

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.

Proendocrine genes coordinate the pancreatic islet differentiation program in vitro

In the developing pancreas, the basic helix-loop-helix (bHLH) protein Neurogenin3 (Ngn3) specifies which precursor cells ultimately will become endocrine cells and initiates the islet differentiation program. NeuroD1, a closely related bHLH protein and a downstream target of Ngn3, maintains the differentiation program initiated by Ngn3. We have developed an in vitro model of Ngn3-dependent differentiation by infecting pancreatic duct cell lines with an Ngn3-expressing adenovirus. We found that both Ngn3 and its downstream target NeuroD1 activated the islet differentiation program in these cells by inducing the expression of genes with early roles in the differentiation cascade, as well as genes characteristic of fully differentiated islet cells. Induction of these genes, as exemplified by the insulin1 gene, involved alteration of the local chromatin structure. Interestingly, the subsets of genes activated by Ngn3 and NeuroD1 were not completely overlapping, indicating that these two bHLH proteins serve specific functions in the development of the endocrine pancreas. In addition, microarray gene expression analysis identified a previously uncharacterized group of Ngn3-induced genes with potentially important roles in islet development and function. These studies demonstrate how Ngn3 initiates islet differentiation and provide us with a model for testing methods for producing islet cells for people with diabetes.

Transcriptional networks controlling pancreatic development and beta cell function

Transcription factors provide the genetic instructions that drive pancreatic development and enable mature beta cells to function properly. To understand fully how this is accomplished, it is necessary to unravel the regulatory networks formed by transcription factors acting on their genomic targets. This article discusses recent advances in our understanding of how transcriptional networks control early pancreas organogenesis, embryonic endocrine cell formation and the differentiated function of adult beta cells. We discuss how mutations in several transcription factor genes involved in such networks cause Maturity onset diabetes of the young (MODY). Finally, we propose that pancreatic gene programs might be manipulated to generate beta cells or to enhance the function of existing beta cells, thereby providing a possible treatment of different forms of diabetes.

Identification of a novel PDX-1 binding site in the human insulin gene enhancer

Islet beta cell type-specific transcription of the insulin gene is regulated by a number of cis-acting elements found within the proximal 5'-flanking region. The control sequences conserved between mammalian insulin genes are acted upon by transcription factors, like PDX-1 and BETA-2, that are also involved in islet beta cell function and formation. In the current study, we investigated the contribution to human insulin expression of the GG2 motif found between nucleotides -145 and -140 relative to the transcription start site. Site-specific mutants were generated within GG2 that displayed a parallel increase (i.e. -144 base pair) or decrease (i.e. -141 base pair) in insulin enhancer-driven reporter and gel shift binding activity in beta cells consistent with human GG2 being under positive regulatory control. In contrast, the corresponding site in the rodent insulin gene, which only differs from the human at nucleotides -144 and -141, is negatively regulated by the Nkx2.2 transcription factor (Cissell, M. A., Zhao, L., Sussel, L., Henderson, E., and Stein, R. (2003) J. Biol. Chem. 278, 751-756). Human GG2 activator binding activity was present in nuclear extracts prepared from human islets and enriched in those from rodent beta cell lines. The human GG2 activator binding factor(s) was shown to be approximately 38-40 kDa and distinct from other size-matched islet-enriched transcription factors, including Nkx2.2, Pax-4, Cdx2/3, and Isl-1. Combined DNA chromatographic purification and mass spectrometry analysis revealed that the GG2 activator was PDX-1. These results demonstrate that the GG2 element, despite its divergence from the core homeodomain consensus binding motif, is a site for PDX-1 activation in the human insulin gene.

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.

Functional interactions between alternatively spliced forms of Pax6 in crystallin gene regulation and in haploinsufficiency

Pax6 is essential for development of the eye, olfactory system, brain and pancreas. Haploinsufficiency of Pax6 causes abnormal eye development. Two forms of Pax6 protein, PAX6 and PAX6(5a), differ in a 14 amino acid insertion encoded by an alternatively spliced exon 5a in the N-terminal DNA-binding paired domain (PD), and they are simultaneously expressed. Here, we show that PAX6 and PAX6(5a) together synergistically activate transcription from promoters recognized by Pax6 PD and PD5a, but not by their homeodomain. This synergism promotes activation of transcription by c-Maf and MafA on the alphaB-crystallin promoter, and is required for transcriptional co-activation by RARbeta/RXRbeta and PAX6/PAX6(5a) on the gammaF-crystallin promoter. To determine the role of this synergism in haploinsufficiency, we tested four human missense (G18W, R26G, G64V and R128C) and one nonsense (R317X) mutants, with reporters driven by Pax6 PD consensus binding sites and the alphaB-crystallin promoter. The simultaneous activity of Pax6 proteins [PAX6, mutated PAX6, PAX6(5a) and mutated PAX6(5a)] modeling haploinsufficiency yielded results not predicted by properties of individual PAX6 or PAX6(5a). Taken together, these results indicate that complex ocular phenotypes due to Pax6 haploinsufficiency originate, at least partially, from functional interactions between alternatively spliced PAX6 and PAX6(5a) variants and other factors, e.g. MafA/c-Maf.

Gene regulatory factors in pancreatic development

The intensity of research on pancreatic development has increased markedly in the past 5 years, primarily for two reasons: we now know that the insulin-producing beta-cells normally arise from an endodermally derived, pancreas-specified precursor cell, and successful transplants of islet cells have been performed, relieving patients with type I diabetes of symptoms for extended periods after transplantation. Combining in vitro beta-cell formation from a pancreatic biopsy of a diabetic patient or from other stem-cell sources followed by endocrine cell transplantation may be the most beneficial route for a future diabetes therapy. However, to achieve this, a thorough understanding of the genetic components regulating the development of beta-cells is required. The following review discusses our current understanding of the transcription factor networks necessary for pancreatic development and how several genetic interactions coming into play at the earliest stages of endodermal development gradually help to build the pancreatic organ. Developmental Dynamics 229:176-200, 2004.

The islet β-cell

The beta-cell is one of four major types of cells present in the islets of Langerhans, which are islands of cells distributed throughout the endocrine pancreas in most mammals. The beta-cell synthesizes and secretes the hormone insulin mainly in response to glucose but also in response to several nutrients, hormones and nervous stimuli. In adult rodents the beta-cell has a slow mitotic rate. Recent studies provide novel insights into the functions of the beta-cell. The presence of functional insulin-like growth factor-1 and insulin receptors and components of their signaling pathway indicate an important role for insulin/IGF-1 signaling in the regulation of beta-cell function. Further, the recent discovery of glucokinase (GK) and the ATP-dependent potassium channels on insulin secretory granules, the detection of AMP-protein kinase in the beta-cell and the identification of a new beta-cell transcription factor, mMafA, are some exciting new areas of research currently underway to further understand the complex pathways that regulate the functions of beta-cells.

The MafA transcription factor appears to be responsible for tissue-specific expression of insulin

Insulin gene expression is regulated by several islet-enriched transcription factors. However, MafA is the only beta cell-specific activator. Here, we show that MafA selectively induces endogenous insulin transcription in non-beta cells. MafA was also first detected in the insulin-producing cells formed during the second and predominant phase of beta cell differentiation, and absent in the few insulin-positive cells found in Nkx6.1(-/-) pancreata, which lack the majority of second-phase beta cells. These results demonstrate that MafA is a potent insulin activator that is likely to function downstream of Nkx6.1 during islet insulin-producing cell development.

Cell-based therapies for diabetes: progress towards a transplantable human beta cell line

Achieving normoglycemia is the goal of diabetes therapy. Potentially, there are many ways to achieve this goal, including transplantation of cells exhibiting glucose-responsive insulin secretion. However, to be applicable to the large number of people who might benefit from beta cell replacement, an unlimited supply of beta cells must be found. To address this problem, we have been developing cell lines from the human endocrine pancreas. In one case, a cell line, betalox5, has been developed from human islets that can be induced under some circumstances to differentiate into functional beta cells exhibiting appropriate glucose-responsive insulin secretion. Inducing differentiation is complex, requiring the activation of multiple signaling pathways, including those downstream of those involved in cell-cell contact and the glucagon-like peptide-1 receptor. In addition, transfer of the PDX-1 gene is also necessary to render the cells competent for differentiation. However, it is clear that many other genes are involved in maintaining the commitment of betalox5 cells towards the beta cell lineage. Understanding the complement of genes required to establish and maintain a beta cell lineage commitment would be enormously helpful in efforts to develop a cell line that can be used for beta cell replacement therapies. Here, we provide further information on the characteristics of cell lines that we have developed from the human pancreas that are relevant to the development of a beta cell replacement therapy for diabetes.

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.

Dmaf, a novel member of Maf transcription factor family is expressed in somatic gonadal cells during embryonic development and gametogenesis in Drosophila

Members of the maf gene family encode basic/leucine zipper transcription factors and play important roles during cell differentiation and organogenesis in vertebrate development. In this study, we show that the maf family is evolutionarily conserved and that the Drosophila maf (Dmaf) gene is expressed in somatic gonadal cells. During embryonic development, Dmaf mRNA is detected in somatic gonadal precursor cells emerging from dorsolateral mesoderm. Relatively weak expression is also observed in subset of neuronal cells in the central nervous system. In adult flies, Dmaf is expressed in somatic gonadal cells surrounding developing oocytes and spermatocytes. These results suggest a specific function for Dmaf in gonadal development, including migration and differentiation of primordial germ cells.

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.

Members of the large Maf transcription family regulate insulin gene transcription in islet beta cells

The C1/RIPE3b1 (-118/-107 bp) binding factor regulates pancreatic-beta-cell-specific and glucose-regulated transcription of the insulin gene. In the present study, the C1/RIPE3b1 activator from mouse beta TC-3 cell nuclear extracts was purified by DNA affinity chromatography and two-dimensional gel electrophoresis. C1/RIPE3b1 binding activity was found in the roughly 46-kDa fraction at pH 7.0 and pH 4.5, and each contained N- and C-terminal peptides to mouse MafA as determined by peptide mass mapping and tandem spectrometry. MafA was detected in the C1/RIPE3b1 binding complex by using MafA peptide-specific antisera. In addition, MafA was shown to bind within the enhancer region (-340/-91 bp) of the endogenous insulin gene in beta TC-3 cells in the chromatin immunoprecipitation assay. These results strongly suggested that MafA was the beta-cell-enriched component of the RIPE3b1 activator. However, reverse transcription-PCR analysis demonstrated that mouse islets express not only MafA but also other members of the large Maf family, specifically c-Maf and MafB. Furthermore, immunohistochemical studies revealed that at least MafA and MafB were present within the nuclei of islet beta cells and not within pancreas acinar cells. Because MafA, MafB, and c-Maf were each capable of specifically binding to and activating insulin C1 element-mediated expression, our results suggest that all of these factors play a role in islet beta-cell function.

Glucose regulates insulin gene transcription by hyperacetylation of histone h4

Induction of insulin gene expression in response to high blood glucose levels is essential for maintaining glucose homeostasis. Although several transcription factors including Beta-2, Ribe3b1, and Pdx-1 have been shown to play a role in glucose stimulation of insulin gene expression, the exact molecular mechanism(s) by which this regulation occurs is unknown. Previous data demonstrate that the transcription factors Beta-2/NeuroD1 and Pdx-1, which are involved in glucose-stimulated insulin gene expression, interact with the histone acetylase p300, suggesting a role for histone acetylation in glucose regulation of the insulin gene expression. We report that exposure of mouse insulinoma 6 cells to high concentrations of glucose results in hyperacetylation of histone H4 at the insulin gene promoter, which correlates with the increased level of insulin gene transcription. In addition, we demonstrate that hyperacetylation of histone H4 in response to high concentrations of glucose also occurs at the glucose transporter-2 gene promoter. Using histone deacetylase inhibitors, we show that increases in histone H4 acetylation cause stimulation of insulin gene transcription even in the absence of high concentrations of glucose. Furthermore, we show that fibroblasts, which lack insulin gene expression, also lack histone acetylation at the insulin gene promoter. In summary, our data support the idea that high concentrations of glucose stimulate insulin gene expression by causing hyperacetylation of histone H4 at the insulin gene promoter.

The islet beta cell-enriched RIPE3b1/Maf transcription factor regulates pdx-1 expression

Pancreatic duodenal homeobox factor-1, PDX-1, is required for pancreas development, islet cell differentiation, and the maintenance of beta cell function. Selective expression in the pancreas appears to be principally regulated by Area II, one of four conserved regulatory sequence domains found within the 5'-flanking region of the pdx-1 gene. Detailed mutagenesis studies have identified potential sites of interaction for both positive- and negative-acting factors within the conserved sequence blocks of Area II. The islet beta cell-enriched RIPE3b1 transcription factor, the activator of insulin C1 element-driven expression, was shown here to also stimulate Area II by binding to sequence blocks 4 and 5 (termed B4/5). Accordingly, B4/5 DNA-binding protein's molecular mass (i.e. 46 kDa), binding specificity, and islet beta cell-enriched distribution were identical to RIPE3b1. Area II-mediated activation was also unaffected upon replacing B4/5 with the insulin C1/RIPE3b1 binding site. In addition, the chromatin immunoprecipitation assay showed that the Area II region of the endogenous pdx-1 gene was precipitated by an antiserum that recognizes the large Maf protein that comprises the RIPE3b1 transcription factor. These results strongly suggest that RIPE3b1/Maf has an important role in generating and maintaining physiologically functional beta cells.

Transcription factors direct the development and function of pancreatic beta cells

Transcription factors orchestrate intricate pathways of cellular growth and differentiation by regulating the rate of transcription of an array of genes. Genetic and biochemical studies have begun to unravel the complex cascade of factors that controls the proliferation and differentiation of cells in the developing pancreas. The specific pathway leading to the development of the insulin-secreting beta cell has been a focus of many of these studies because an understanding of the transcription factors governing this pathway will be crucial to the engineering of new beta cells to cure diabetes. In recent years, the number of transcription factors that has been implicated in beta-cell differentiation and function has grown considerably. Here, we outline the known role of transcription factors in beta-cell development, and describe how these factors form a network of gene activation signals that mediates insulin transcription.

Gene expression cascades in pancreatic development

The specialized endocrine and exocrine cells of the pancreas originally derive from a pool of apparently identical cells in the early gut endoderm. Serial changes in their gene expression program, controlled by a hierarchy of pancreatic transcription factors, direct this progression from multipotent progenitor cell to mature pancreatic cell. When the cells differentiate, this hierarchy of factors coalesces into a network of factors that maintain the differentiated phenotype of the cells. As we develop an understanding of the pancreatic transcription factors, we are also acquiring the tools with which we can ultimately control pancreatic cell differentiation.

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.