The RIPE3b1 activator of the insulin gene is composed of a protein(s) of approximately 43 kDa, whose DNA binding activity is inhibited by protein phosphatase treatment

MafA Regulation in β-Cells: From Transcriptional to Post-Translational Mechanisms

β-cells are insulin-producing cells in the pancreas that maintain euglycemic conditions. Pancreatic β-cell maturity and function are regulated by a variety of transcription factors that enable the adequate expression of the cellular machinery involved in nutrient sensing and commensurate insulin secretion. One of the key factors in this regulation is MAF bZIP transcription factor A (MafA). MafA expression is decreased in type 2 diabetes, contributing to β-cell dysfunction and disease progression. The molecular biology underlying MafA is complex, with numerous transcriptional and post-translational regulatory nodes. Understanding these complexities may uncover potential therapeutic targets to ameliorate β-cell dysfunction. This article will summarize the role of MafA in normal β-cell function and disease, with a special focus on known transcriptional and post-translational regulators of MafA expression.

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

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

Inactivation of specific β cell transcription factors in type 2 diabetes

Type 2 diabetes (T2DM) commonly arises from islet β cell failure and insulin resistance. Here, we examined the sensitivity of key islet-enriched transcription factors to oxidative stress, a condition associated with β cell dysfunction in both type 1 diabetes (T1DM) and T2DM. Hydrogen peroxide treatment of β cell lines induced cytoplasmic translocation of MAFA and NKX6.1. In parallel, the ability of nuclear PDX1 to bind endogenous target gene promoters was also dramatically reduced, whereas the activity of other key β cell transcriptional regulators was unaffected. MAFA levels were reduced, followed by a reduction in NKX6.1 upon development of hyperglycemia in db/db mice, a T2DM model. Transgenic expression of the glutathione peroxidase-1 antioxidant enzyme (GPX1) in db/db islet β cells restored nuclear MAFA, nuclear NKX6.1, and β cell function in vivo. Notably, the selective decrease in MAFA, NKX6.1, and PDX1 expression was found in human T2DM islets. MAFB, a MAFA-related transcription factor expressed in human β cells, was also severely compromised. We propose that MAFA, MAFB, NKX6.1, and PDX1 activity provides a gauge of islet β cell function, with loss of MAFA (and/or MAFB) representing an early indicator of β cell inactivity and the subsequent deficit of more impactful NKX6.1 (and/or PDX1) resulting in overt dysfunction associated with T2DM.

MafA and MafB activity in pancreatic β cells

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

Histone deacetylase (HDAC) inhibition as a novel treatment for diabetes mellitus

Both common forms of diabetes have an inflammatory pathogenesis in which immune and metabolic factors converge on interleukin-1β as a key mediator of insulin resistance and β-cell failure. In addition to improving insulin resistance and preventing β-cell inflammatory damage, there is evidence of genetic association between diabetes and histone deacetylases (HDACs); and HDAC inhibitors (HDACi) promote β-cell development, proliferation, differentiation and function and positively affect late diabetic microvascular complications. Here we review this evidence and propose that there is a strong rationale for preclinical studies and clinical trials with the aim of testing the utility of HDACi as a novel therapy for diabetes.

Regulation of MafA expression in pancreatic beta-cells in db/db mice with diabetes

OBJECTIVE

Islet beta-cells loose their ability to synthesize insulin under diabetic conditions, which is at least partially due to the decreased activity of insulin transcription factors such as MafA. Although an in vitro study showed that reactive oxygen species (ROS) decrease MafA expression, the underlying mechanism still remains unclear. In this study, we examined the effects of c-Jun, which is known to be upregulated by ROS, on the expression of MafA under diabetic conditions.

RESEARCH DESIGN AND METHODS

To examine the protein levels of MafA and c-Jun, we performed histological analysis and Western blotting using diabetic db/db mice. In addition, to evaluate the possible effects of c-Jun on MafA expression, we performed adenoviral overexpression of c-Jun in the MIN6 beta-cell line and freshly isolated islets. RESULTS MafA expression was markedly decreased in the islets of db/db mice, while in contrast c-Jun expression was increased. Costaining of these factors in the islets of db/db mice clearly showed that MafA and insulin levels are decreased in c-Jun-positive cells. Consistent with these results, overexpression of c-Jun significantly decreased MafA expression, accompanied by suppression of insulin expression. Importantly, MafA overexpression restored the insulin promoter activity and protein levels that were suppressed by c-Jun. These results indicate that the decreased insulin biosynthesis induced by c-Jun is principally mediated by the suppression of MafA activity.

CONCLUSIONS

It is likely that the augmented expression of c-Jun in diabetic islets decreases MafA expression and thereby reduces insulin biosynthesis, which is often observed in type 2 diabetes.

Phosphorylation within the MafA N terminus regulates C-terminal dimerization and DNA binding

Phosphorylation regulates transcription factor activity by influencing dimerization, cellular localization, activation potential, and/or DNA binding. Nevertheless, precisely how this post-translation modification mediates these processes is poorly understood. Here, we examined the role of phosphorylation on the DNA-binding properties of MafA and MafB, closely related transcriptional activators of the basic-leucine zipper (b-Zip) family associated with cell differentiation and oncogenesis. Many common phosphorylation sites were identified by mass spectrometry. However, dephosphorylation only precluded the detection of MafA dimers and consequently dramatically reduced DNA-binding ability. Analysis of MafA/B chimeras revealed that sensitivity to the phosphorylation status of MafA was imparted by sequences spanning the C-terminal dimerization region (amino acids (aa) 279-359), whereas the homologous MafB region (aa 257-323) conveyed phosphorylation-independent DNA binding. Mutational analysis showed that formation of MafA dimers capable of DNA binding required phosphorylation within the distinct N-terminal transactivation domain (aa 1-72) and not the C-terminal b-Zip region. These results demonstrate a novel relationship between the phosphoamino acid-rich transactivation and b-Zip domains in controlling MafA DNA-binding activity.

The stability and transactivation potential of the mammalian MafA transcription factor are regulated by serine 65 phosphorylation

The level of the MafA transcription factor is regulated by a variety of effectors of beta cell function, including glucose, fatty acids, and insulin. Here, we show that phosphorylation at Ser(65) of mammalian MafA influences both protein stability and transactivation potential. Replacement of Ser(65) with Glu to mimic phosphorylation produced a protein that was as unstable as the wild type, whereas Asp or Ala mutation blocked degradation. Analysis of MafA chimeric and deletion constructs suggests that protein phosphorylation at Ser(65) alone represents the initial degradation signal, with ubiquitinylation occurring within the C terminus (amino acids 234-359). Although only wild type MafA and S65E were polyubiquitinylated, both S65D and S65E potently stimulated transactivation compared with S65A. Phosphorylation at Ser(14) also enhanced activation, although it had no impact on protein turnover. The mobility of MafA S65A was profoundly affected upon SDS-PAGE, with the S65E and S65D mutants influenced less due to their ability to serve as substrates for glycogen synthase kinase 3, which acts at neighboring N-terminal residues after Ser(65) phosphorylation. Our observations not only illustrate the sensitivity of the cellular transcriptional and degradation machinery to phosphomimetic mutants at Ser(65), but also demonstrate the singular importance of phosphorylation at this amino acid in regulating MafA activity.

Glucose regulation of insulin gene expression in pancreatic β-cells

Production and secretion of insulin from the β-cells of the pancreas is very crucial in maintaining normoglycaemia. This is achieved by tight regulation of insulin synthesis and exocytosis from the β-cells in response to changes in blood glucose levels. The synthesis of insulin is regulated by blood glucose levels at the transcriptional and post-transcriptional levels. Although many transcription factors have been implicated in the regulation of insulin gene transcription, three β-cell-specific transcriptional regulators, Pdx-1 (pancreatic and duodenal homeobox-1), NeuroD1 (neurogenic differentiation 1) and MafA (V-maf musculoaponeurotic fibrosarcoma oncogene homologue A), have been demonstrated to play a crucial role in glucose induction of insulin gene transcription and pancreatic β-cell function. These three transcription factors activate insulin gene expression in a co-ordinated and synergistic manner in response to increasing glucose levels. It has been shown that changes in glucose concentrations modulate the function of these β-cell transcription factors at multiple levels. These include changes in expression levels, subcellular localization, DNA-binding activity, transactivation capability and interaction with other proteins. Furthermore, all three transcription factors are able to induce insulin gene expression when expressed in non-β-cells, including liver and intestinal cells. The present review summarizes the recent findings on how glucose modulates the function of the β-cell transcription factors Pdx-1, NeuroD1 and MafA, and thereby tightly regulates insulin synthesis in accordance with blood glucose levels.

MafA is a dedicated activator of the insulin gene in vivo

As successful generation of insulin-producing cells could be used for diabetes treatment, a concerted effort is being made to understand the molecular programs underlying islet beta-cell formation and function. The closely related MafA and MafB transcription factors are both key mammalian beta-cell regulators. MafA and MafB are co-expressed in insulin+beta-cells during embryogenesis, while in the adult pancreas only MafA is produced in beta-cells and MafB in glucagon+alpha-cells. MafB-/- animals are also deficient in insulin+ and glucagon+ cell production during embryogenesis. However, only MafA over-expression selectively induced endogenous Insulin mRNA production in cell line-based assays, while MafB specifically promoted Glucagon expression. Here, we analyzed whether these factors were sufficient to induce insulin+ and/or glucagon+ cell formation within embryonic endoderm using the chick in ovo electroporation assay. Ectopic expression of MafA, but not MafB, promoted Insulin production; however, neither MafA nor MafB were capable of inducing Glucagon. Co-electroporation of MafA with the Ngn3 transcription factor resulted in the development of more organized cell clusters containing both insulin- and glucagon-producing cells. Analysis of chimeric proteins of MafA and MafB demonstrated that chick Insulin activation depended on sequences within the MafA C-terminal DNA-binding domain. MafA was also bound to Insulin and Glucagon transcriptional control sequences in mouse embryonic pancreas and beta-cell lines. Collectively, these results demonstrate a unique ability for MafA to independently activate Insulin transcription.

MafA and MafB regulate Pdx1 transcription through the Area II control region in pancreatic beta cells

Pancreatic-duodenal homeobox factor-1 (Pdx1) is highly enriched in islet β cells and integral to proper cell development and adult function. Of the four conserved 5′-flanking sequence blocks that contribute to transcription in vivo, Area II (mouse base pairs -2153/-1923) represents the only mammalian specific control domain. Here we demonstrate that regulation of β-cell-enriched Pdx1 expression by the MafA and MafB transcription factors is exclusively through Area II. Thus, these factors were found to specifically activate through Area II in cell line transfection-based assays, and MafA, which is uniquely expressed in adult islet β cells was only bound to this region in quantitative chromatin immunoprecipitation studies. MafA and MafB are produced in β cells during development and were both bound to Area II at embryonic day 18.5. Expression of a transgene driven by Pdx1 Areas I and II was also severely compromised during insulin⁺ cell formation in MafB^-/- mice, consistent with the importance of this large Maf in β-cell production and Pdx1 expression. These findings illustrate the significance of large Maf proteins to Pdx1 expression in β cells, and in particular MafB during pancreatic development.

Regulation of insulin gene expression by overlapping DNA-binding elements

The transcription factor MafA/RIPE3b1 is an important regulator of insulin gene expression. MafA binds to the insulin enhancer element RIPE3b (C1-A2), now designated as insulin MARE (Maf response element). The insulin MARE element shares an overlapping DNA-binding region with another insulin enhancer element A2. A2.2, a beta-cell-specific activator, like the MARE-binding factor MafA, binds to the overlapping A2 element. Our previous results demonstrated that two nucleotides in the overlapping region are required for the binding of both factors. Surprisingly, instead of interfering with each other's binding activity, the MafA and the A2-binding factors co-operatively activated insulin gene expression. To understand the molecular mechanisms responsible for this functional co-operation, we have determined the nucleotides essential for the binding of the A2.2 factor. Using this information, we have constructed non-overlapping DNA-binding elements and their derivatives, and subsequently analysed the effect of these modifications on insulin gene expression. Our results demonstrate that the overlapping binding site is essential for maximal insulin gene expression. Furthermore, the overlapping organization is critical for MafA-mediated transcriptional activation, but has a minor effect on the activity of A2-binding factors. Interestingly, the binding affinities of both MafA and A2.2 to the overlapping or non-overlapping binding sites were not significantly different, implying that the overlapping binding organization may increase the activation potential of MafA by physical/functional interactions with A2-binding factors. Thus our results demonstrate a novel mechanism for the regulation of MafA activity, and in turn beta-cell function, by altering expression and/or binding of the A2.2 factor. Our results further suggest that the major downstream targets of MafA will in addition to the MARE element have a binding site for the A2.2 factor.

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.

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.

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.

Nuclear translocation of an ICA512 cytosolic fragment couples granule exocytosis and insulin expression in {beta}-cells

Islet cell autoantigen 512 (ICA512)/IA-2 is a receptor tyrosine phosphatase-like protein associated with the insulin secretory granules (SGs) of pancreatic β-cells. Here, we show that exocytosis of SGs and insertion of ICA512 in the plasma membrane promotes the Ca²⁺-dependent cleavage of ICA512 cytoplasmic domain by μ-calpain. This cleavage occurs at the plasma membrane and generates an ICA512 cytosolic fragment that is targeted to the nucleus, where it binds the E3-SUMO ligase protein inhibitor of activated signal transducer and activator of transcription-y (PIASy) and up-regulates insulin expression. Accordingly, this novel pathway directly links regulated exocytosis of SGs and control of gene expression in β-cells, whose impaired insulin production and secretion causes 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.

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.

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 factor occupancy of the insulin gene in vivo. Evidence for direct regulation by Nkx2.2

Consensus-binding sites for many transcription factors are relatively non-selective and found at high frequency within the genome. This raises the possibility that factors that are capable of binding to a cis-acting element in vitro and regulating transcription from a transiently transfected plasmid, which would not have higher order chromatin structure, may not occupy this site within the endogenous gene. Closed chromatin structure and competition from another DNA-binding protein with similar nucleotide specificity are two possible mechanisms by which a transcription factor may be excluded from a potential binding site in vivo. Multiple transcription factors, including Pdx-1, BETA-2, and Pax6, have been implicated in expression of the insulin gene in pancreatic beta cells. In this study, the chromatin immunoprecipitation assay has been used to show that these factors do, in fact, bind to insulin control region sequences in intact beta cells. In addition, another key islet-enriched transcription factor, Nkx2.2, was found to occupy this region using the chromatin immunoprecipitation assay. In vitro DNA-binding and transient transfection assays defined how Nkx2.2 affected insulin gene expression. Pdx-1 was also shown to bind within a region of the endogenous islet amyloid polypeptide, pax-4, and glucokinase genes that were associated with control in vitro. Because Pdx-1 does not regulate gene transcription in isolation, these sequences were examined for occupancy by the other insulin transcriptional regulators. BETA-2, Pax6, and Nkx2.2 were also found to bind to amyloid polypeptide, glucokinase, and pax-4 control sequences in vivo. These studies reveal the broad application of the Pdx-1, BETA-2, Pax6, and Nkx2.2 transcription factors in regulating expression of genes selectively expressed in islet beta cells.

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.

Conserved sequences in a tissue-specific regulatory region of the pdx-1 gene mediate transcription in Pancreatic beta cells: role for hepatocyte nuclear factor 3 beta and Pax6

Pancreas duodenum homeobox 1 (PDX-1) is absolutely required for pancreas development and the maintenance of islet beta-cell function. Temporal and cell-type-specific transcription of the pdx-1 gene is controlled by factors acting upon sequences found within its 5'-flanking region. Critical cis-acting transcriptional control elements are located within a nuclease hypersensitive site that contains three conserved subdomains, termed areas I, II, and III. We show that area II acts as a tissue-specific regulatory region of the pdx-1 gene, directing transgene expression to a subpopulation of islet cells. Mutation of the area II hepatocyte nuclear factor 3 (HNF3) binding element in the larger area I- and area II- containing PstBst fragment also decreases PB(hsplacZ) transgene penetrance. These two results indicate possible ontogenetic and/or functional heterogeneity of the beta-cell population. Several other potential positive- and negative-acting control elements were identified in area II after mutation of the highly conserved sequence blocks within this subdomain. Pax6, a factor essential for islet alpha-cell development and islet hormone gene expression, was shown to bind in area II in vitro. Pax6 and HNF3 beta were also found to bind to this region in vivo by using the chromatin immunoprecipitation assay. Collectively, these data suggest an important role for both HNF3 beta and Pax6 in regulating pdx-1 expression in beta cells.

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

Of the three critical enhancer elements that mediate beta-cell-specific and glucose-responsive expression of the insulin gene, only the identity of the transcription factor binding to the RIPE3b element (RIPE3b1) has remained elusive. Using a biochemical purification approach, we have identified the RIPE3b1 factor as a mammalian homologue of avian MafA/L-Maf (mMafA). The avian MafA is a cell-type determination factor that expressed ectopically can trigger lens differentiation program, but no mammalian homologue of avian MafA has previously been identified. Here, we report cloning of the human mafA (hMafA) and demonstrate that it can specifically bind the insulin enhancer element RIPE3b and activate insulin-gene expression. In addition, mMafA has a very restrictive cellular distribution and is selectively expressed in pancreatic beta but not in alpha cells. We suggest that mMafA has an essential role in the function and differentiation of beta-cells and thus may be associated with the pathophysiological origins of diabetes.

Insulin gene transcription is mediated by interactions between the p300 coactivator and PDX-1, BETA2, and E47

Pancreatic beta-cell-type-specific expression of the insulin gene requires both ubiquitous and cell-enriched activators, which are organized within the enhancer region into a network of protein-protein and protein-DNA interactions to promote transcriptional synergy. Protein-protein-mediated communication between DNA-bound activators and the RNA polymerase II transcriptional machinery is inhibited by the adenovirus E1A protein as a result of E1A's binding to the p300 coactivator. E1A disrupts signaling between the non-DNA-binding p300 protein and the basic helix-loop-helix DNA-binding factors of insulin's E-element activator (i.e., the islet-enriched BETA2 and generally distributed E47 proteins), as well as a distinct but unidentified enhancer factor. In the present report, we show that E1A binding to p300 prevents activation by insulin's beta-cell-enriched PDX-1 activator. p300 interacts directly with the N-terminal region of the PDX-1 homeodomain protein, which contains conserved amino acid sequences essential for activation. The unique combination of PDX-1, BETA2, E47, and p300 was shown to promote synergistic activation from a transfected insulin enhancer-driven reporter construct in non-beta cells, a process inhibited by E1A. In addition, E1A inhibited the level of PDX-1 and BETA2 complex formation in beta cells. These results indicate that E1A inhibits insulin gene transcription by preventing communication between the p300 coactivator and key DNA-bound activators, like PDX-1 and BETA2:E47.

The DNA binding activity of the RIPE3b1 transcription factor of insulin appears to be influenced by tyrosine phosphorylation

The RIPE3b1 DNA binding factor plays a critical role in pancreatic islet beta cell-specific and glucose-regulated transcription of the insulin gene. Recently it was shown that RIPE3b1 binding activity in beta cell nuclear extracts is reduced by treatment with either calf intestinal alkaline phosphatase (CIAP) or a brain-enriched phosphatase preparation (BPP) (Zhao, L., Cissell, M. A., Henderson, E., Colbran, R., and Stein, R. (2000) J. Biol. Chem. 275, 10532-10537). Evidence is presented here suggesting that a tyrosine phosphatase(s) influences the ability of RIPE3b1 to bind to the insulin C1 element in beta cells. We found that RIPE3b1 binding was inhibited upon incubating beta cell nuclear extracts at 30 degrees C. In contrast, PDX-1 and MLTF-1 transcription factor binding activity was unaffected under these conditions. The loss in RIPE3b1 binding activity was prevented by inhibitors of tyrosine phosphatases (sodium orthovanadate and sodium molybdate) but not by inhibitors of serine/threonine phosphatases (sodium fluoride, okadaic acid, and microcystin LR). CIAP- and BPP-catalyzed inhibition of RIPE3b1 binding was also blocked by these tyrosine phosphatase inhibitors. Collectively, the data suggested that removal of a tyrosine(s) within RIPE3b1 prevented activator binding to insulin C1 control element sequences. The presence of a key phosphorylated tyrosine(s) within this transcription factor was further supported by the ability of the 4G10 anti-phosphotyrosine monoclonal antibody to immunoprecipitate RIPE3b1 DNA binding activity. We discuss how tyrosine phosphorylation, a very rare and highly significant regulatory modification, may control RIPE3b1 activator function.

Transcription factors recognizing overlapping C1-A2 binding sites positively regulate insulin gene expression

Transcription factors binding the insulin enhancer region, RIPE3b, mediate beta-cell type-specific and glucose-responsive expression of the insulin gene. Earlier studies demonstrate that activator present in the beta-cell-specific RIPE3b1-binding complex is critical for these actions. The DNA binding activity of the RIPE3b1 activator is induced in response to glucose stimulation and is inhibited under glucotoxic conditions. The C1 element within the RIPE3b region has been implicated as the binding site for RIPE3b1 activator. The RIPE3b region also contains an additional element, A2, which shares homology with the A elements in the insulin enhancer. Transcription factors (PDX-1 and HNF-1 alpha) binding to A elements are critical regulators of insulin gene expression and/or pancreatic development. Hence, to understand the roles of C1 and A2 elements in regulating insulin gene expression, we have systematically mutated the RIPE3b region and analyzed the effect of these mutations on gene expression. Our results demonstrate that both C1 and A2 elements together constitute the binding site for the RIPE3b1 activator. In addition to C1-A2 (RIPE3b) binding complexes, three binding complexes that specifically recognize A2 elements are found in nuclear extracts from insulinoma cell lines; the A2.2 complex is detected only in insulin-producing cell lines. Furthermore, two base pairs in the A2 element were critical for binding of both RIPE3b1 and A2.2 activators. Transient transfection results indicate that both C1-A2 and A2-specific binding activators cooperatively activate insulin gene expression. In addition, RIPE3b1- and A2-specific activators respond differently to glucose, suggesting that their overlapping binding specificity and functional cooperation may play an important role in regulating insulin gene expression.