Characterization of a novel protein kinase C response element in the glucagon gene

A method for the generation of human stem cell-derived alpha cells

The generation of pancreatic cell types from renewable cell sources holds promise for cell replacement therapies for diabetes. Although most effort has focused on generating pancreatic beta cells, considerable evidence indicates that glucagon secreting alpha cells are critically involved in disease progression and proper glucose control. Here we report on the generation of stem cell-derived human pancreatic alpha (SC-alpha) cells from pluripotent stem cells via a transient pre-alpha cell intermediate. These pre-alpha cells exhibit a transcriptional profile similar to mature alpha cells and although they produce proinsulin protein, they do not secrete significant amounts of processed insulin. Compound screening identified a protein kinase c activator that promotes maturation of pre-alpha cells into SC-alpha cells. The resulting SC-alpha cells do not express insulin, share an ultrastructure similar to cadaveric alpha cells, express and secrete glucagon in response to glucose and some glucagon secretagogues, and elevate blood glucose upon transplantation in mice.

Glucagon regulates its own synthesis by autocrine signaling

Peptide hormones are powerful regulators of various biological processes. To guarantee continuous availability and function, peptide hormone secretion must be tightly coupled to its biosynthesis. A simple but efficient way to provide such regulation is through an autocrine feedback mechanism in which the secreted hormone is "sensed" by its respective receptor and initiates synthesis at the level of transcription and/or translation. Such a secretion-biosynthesis coupling has been demonstrated for insulin; however, because of insulin's unique role as the sole blood glucose-decreasing peptide hormone, this coupling is considered an exception rather than a more generally used mechanism. Here we provide evidence of a secretion-biosynthesis coupling for glucagon, one of several peptide hormones that increase blood glucose levels. We show that glucagon, secreted by the pancreatic α cell, up-regulates the expression of its own gene by signaling through the glucagon receptor, PKC, and PKA, supporting the more general applicability of an autocrine feedback mechanism in regulation of peptide hormone synthesis.

A peroxisome proliferator-activated receptor gamma-retinoid X receptor heterodimer physically interacts with the transcriptional activator PAX6 to inhibit glucagon gene transcription

The peptide hormone glucagon stimulates hepatic glucose output, and its levels in the blood are elevated in type 2 diabetes mellitus. The nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma) has essential roles in glucose homeostasis, and thiazolidinedione PPARgamma agonists are clinically important antidiabetic drugs. As part of their antidiabetic effect, thiazolidinediones such as rosiglitazone have been shown to inhibit glucagon gene transcription through binding to PPARgamma and inhibition of the transcriptional activity of PAX6 that is required for cell-specific activation of the glucagon gene. However, how thiazolidinediones and PPARgamma inhibit PAX6 activity at the glucagon promoter remained unknown. After transient transfection of a glucagon promoter-reporter fusion gene into a glucagon-producing pancreatic islet alpha-cell line, ligand-bound PPARgamma was found in the present study to inhibit glucagon gene transcription also after deletion of its DNA-binding domain. Like PPARgamma ligands, also retinoid X receptor (RXR) agonists inhibited glucagon gene transcription in a PPARgamma-dependent manner. In glutathione transferase pull-down assays, the ligand-bound PPARgamma-RXR heterodimer bound to the transactivation domain of PAX6. This interaction depended on the presence of the ligand and RXR, but it was independent of the PPARgamma DNA-binding domain. Chromatin immunoprecipitation experiments showed that PPARgamma is recruited to the PAX6-binding proximal glucagon promoter. Taken together, the results of the present study support a model in which a ligand-bound PPARgamma-RXR heterodimer physically interacts with promoter-bound PAX6 to inhibit glucagon gene transcription. These data define PAX6 as a novel physical target of PPARgamma-RXR.

Characterization of a novel Foxa (hepatocyte nuclear factor-3) site in the glucagon promoter that is conserved between rodents and humans

The pancreatic islet hormone glucagon stimulates hepatic glucose production and thus maintains blood glucose levels in the fasting state. Transcription factors of the Foxa [Fox (forkhead box) subclass A; also known as HNF-3 (hepatocyte nuclear factor-3)] family are required for cell-specific activation of the glucagon gene in pancreatic islet alpha-cells. However, their action on the glucagon gene is poorly understood. In the present study, comparative sequence analysis and molecular characterization using protein-DNA binding and transient transfection assays revealed that the well-characterized Foxa-binding site in the G2 enhancer element of the rat glucagon gene is not conserved in humans and that the human G2 sequence lacks basal enhancer activity. A novel Foxa site was identified that is conserved in rats, mice and humans. It mediates activation of the glucagon gene by Foxa proteins and confers cell-specific promoter activity in glucagon-producing pancreatic islet alpha-cell lines. In contrast with previously identified Foxa-binding sites in the glucagon promoter, which bind nuclear Foxa2, the novel Foxa site was found to bind preferentially Foxa1 in nuclear extracts of a glucagon-producing pancreatic islet alpha-cell line, offering a mechanism that explains the decrease in glucagon gene expression in Foxa1-deficient mice. This site is located just upstream of the TATA box (between -30 and -50), suggesting a role for Foxa proteins in addition to direct transcriptional activation, such as a role in opening the chromatin at the start site of transcription of the glucagon gene.

Protein Kinase B Activity Is Sufficient to Mimic the Effect of Insulin on Glucagon Gene Transcription

Insulin inhibits glucagon gene transcription, and insulin deficiency is associated with hyperglucagonemia that contributes to hyperglycemia in diabetes mellitus. However, the insulin signaling pathway to the glucagon gene is unknown. Protein kinase B (PKB) is a key regulator of insulin signaling and glucose homeostasis. Impaired PKB function leads to insulin resistance and diabetes mellitus. Therefore, the role of PKB in the regulation of glucagon gene transcription was investigated. After transient transfections of glucagon promoter-reporter genes into a glucagon-producing islet cell line, the use of kinase inhibitors indicated that the inhibition of glucagon gene transcription by insulin depends on phosphatidylinositol (PI) 3-kinase. Furthermore, insulin caused a PI 3-kinase-dependent phosphorylation and activation of PKB in this cell line as revealed by phospho-immunoblotting and kinase assays. Overexpression of constitutively active PKB mimicked the effect of insulin on glucagon gene transcription. Both insulin and PKB responsiveness of the glucagon promoter were abolished when the binding sites for the transcription factor Pax6 within the G1 and G3 promoter elements were mutated. Recruitment of Pax6 or its potential coactivator, the CREB-binding protein (CBP), to G1 and G3 by using the GAL4 system restored both insulin and PKB responsiveness. These data suggest that insulin inhibits glucagon gene transcription by signaling via PI 3-kinase and PKB, with the transcription factor Pax6 and its potential coactivator CBP being critical components of the targeted promoter-specific nucleoprotein complex. The present data emphasize the importance of PKB in insulin signaling and glucose homeostasis by defining the glucagon gene as a novel target gene for PKB.

Repression of glucagon gene transcription by peroxisome proliferator-activated receptor gamma through inhibition of Pax6 transcriptional activity

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is involved in glucose homeostasis and synthetic PPARgamma ligands, the thiazolidinediones, a new class of antidiabetic agents that reduce insulin resistance and, as a secondary effect, reduce hepatic glucose output. PPARgamma is highly expressed in normal human pancreatic islet alpha-cells that produce glucagon. This peptide hormone is a functional antagonist of insulin stimulating hepatic glucose output. Therefore, the effect of PPARgamma and thiazolidinediones on glucagon gene transcription was investigated. After transient transfection of a glucagon-reporter fusion gene into a glucagon-producing pancreatic islet cell line, thiazolidinediones inhibited glucagon gene transcription when PPARgamma was coexpressed. They also reduced glucagon secretion and glucagon tissue levels in primary pancreatic islets. A 5'/3'-deletion and internal mutation analysis indicated that a pancreatic islet cell-specific enhancer sequence (PISCES) motif within the proximal glucagon promoter element G1 was required for PPARgamma responsiveness. This sequence motif binds the paired domain transcription factor Pax6. When the PISCES motif within G1 was mutated into a GAL4 binding site, the expression of GAL4-Pax6 restored glucagon promoter activity and PPARgamma responsiveness. GAL4-Pax6 transcriptional activity was inhibited by PPARgamma in response to thiazolidinedione treatment also at a minimal viral promoter. These results suggest that PPARgamma in a ligand-dependent but DNA binding-independent manner inhibits Pax6 transcriptional activity, resulting in inhibition of glucagon gene transcription. These data thereby define Pax6 as a novel functional target of PPARgamma and suggest that inhibition of glucagon gene expression may be among the multiple mechanisms through which thiazolidinediones improve glycemic control in diabetic subjects.

Insulin responsiveness of the glucagon gene conferred by interactions between proximal promoter and more distal enhancer-like elements involving the paired-domain transcription factor Pax6

Regulation of gene transcription is an important aspect of insulin's action. However, the mechanisms involved are poorly understood. Insulin inhibits glucagon gene transcription, and insulin deficiency is associated with hyperglucagonemia that contributes to hyperglycemia in diabetes mellitus. Transfecting glucagon-reporter fusion genes into a glucagon-producing pancreatic islet cell line, a 5'-, 3'-, and internal deletion analysis, and oligonucleotide cassette insertions failed in the present study to identify a single insulin-responsive element in the glucagon gene. They rather indicate that insulin responsiveness depends on the presence of both proximal promoter elements and more distal enhancer-like elements. When the paired domain transcription factor Pax6 binding sites within the proximal promoter element G1 and the enhancer-like element G3 were mutated into GAL4 binding sites, the expression of GAL4-Pax6 and GAL4-VP16 restored basal activity, whereas only GAL4-Pax6 restored also insulin responsiveness. Likewise, GAL4-CBP activity was inhibited by insulin within the glucagon promoter context. The results suggest that insulin responsiveness is conferred to the glucagon gene by the synergistic interaction of proximal promoter and more distal enhancer-like elements, with Pax6 and its potential coactivator the CREB-binding protein being critical components. These data thereby support concepts of insulin-responsive element-independent mechanisms of insulin action to inhibit gene transcription.

Heterodimeric Pbx-Prep1 homeodomain protein binding to the glucagon gene restricting transcription in a cell type-dependent manner

Homeodomain proteins specify developmental pathways and cell-specific gene transcription whereby proteins of the PBC subclass can direct target gene specificity of Hox proteins. Proteins encoded by nonclustered homeobox genes have been shown to be essential for cell lineage differentiation and gene expression in pancreatic islets. Using specific antiserum in an electrophoretic mobility shift assay and in vitro transcribed/translated proteins, the nuclear proteins binding domain B of the G3 enhancer-like element of the glucagon gene were identified in the present study as heterodimers consisting of the ubiquitously expressed homeodomain protein Prep1 and the also widely expressed PBC homeoprotein Pbx (isoform 1a, 1b, or 2). These heterodimeric complexes were found to bind also to the glucagon cAMP response element and to a newly identified element termed G5 (from -169 to -140). Whereas the expression of Prep1 or Pbx forms alone had no effect, coexpression of Pbx1a/1b-Prep1 inhibited the glucagon promoter when activated by cotransfected Pax6 or another transcription factor in non-glucagon-producing cells. In contrast, in glucagon-producing pancreatic islet cells, Pbx-Prep1 had no effect on GAL4-Pax6-induced mutant glucagon promoter activity or on Pax6-dependent wild-type glucagon promoter activity. Furthermore, 5'-deletion of G5 enhanced glucagon promoter activity in a non-glucagon-producing cell line but not in glucagon-producing islet cells. This study thus identifies a novel target and Hox-independent function of Pbx-Prep1 heterodimers that, through repression of glucagon gene transcription in non-glucagon-producing cells, may help to establish islet cell-specific expression of the glucagon gene.

Characterization of a novel calcium response element in the glucagon gene

To maintain blood glucose levels within narrow limits, the synthesis and secretion of pancreatic islet hormones is controlled by a variety of extracellular signals. Depolarization-induced calcium influx into islet cells has been shown to stimulate glucagon gene transcription through the transcription factor cAMP response element-binding protein that binds to the glucagon cAMP response element. By transient transfection of glucagon-reporter fusion genes into islet cell lines, this study identified a second calcium response element in the glucagon gene (G2 element, from -165 to -200). Membrane depolarization was found to induce the binding of a nuclear complex with NFATp-like immunoreactivity to the G2 element. Consistent with nuclear translocation, a comigrating complex was found in cytosolic extracts of unstimulated cells, and the induction of nuclear protein binding was blocked by inhibition of calcineurin phosphatase activity by FK506. A mutational analysis of G2 function and nuclear protein binding as well as the effect of FK506 indicate that calcium responsiveness is conferred to the G2 element by NFATp functionally interacting with HNF-3beta binding to a closely associated site. Transcription factors of the NFAT family are known to cooperate with AP-1 proteins in T cells for calcium-dependent activation of cytokine genes. This study shows a novel pairing of NFATp with the cell lineage-specific transcription factor HNF-3beta in islet cells to form a novel calcium response element in the glucagon gene.

Inactivation of the winged helix transcription factor HNF3alpha affects glucose homeostasis and islet glucagon gene expression in vivo

Mice homozygous for a null mutation in the winged helix transcription factor HNF3alpha showed severe postnatal growth retardation followed by death between P2 and P12. Homozygous mutant mice were hypoglycemic despite unchanged expression of HNF3 target genes involved in hepatic gluconeogenesis. Whereas insulin and corticosteroid levels were altered as expected, plasma glucagon was reduced markedly in the mutant animals despite the hypoglycemia that should be expected to increase glucagon levels. This correlated with a 70% reduction in pancreatic proglucagon gene expression. We also showed that HNF3alpha could bind to and transactivate the proglucagon gene promoter. These observations invoke a central role for HNF3alpha in the regulatory control of islet genes essential for glucose homeostasis in vivo.

Gene-specific transcriptional activity of the insulin cAMP-responsive element is conferred by NF-Y in combination with cAMP response element-binding protein

Cyclic AMP stimulates insulin gene transcription through a cAMP response element (CRE). In the present study the insulin CRE-binding proteins and their functions were investigated. A mutational analysis of nuclear protein binding in electrophoretic mobility shift assays in combination with specific antisera showed that in the CRE of the rat insulin I gene the imperfect CRE octamer-like sequence TGACGTCC interacts weakly with CREB and overlaps with two sequence motifs (TTGTTGAC and CCAAT) that bind winged helix-like proteins and the transcription factor NF-Y, respectively. Transient transfection of wild-type and mutant insulin CRE-reporter fusion genes and the inactivation of cellular CREB or NF-Y by overexpression of the dominant negative mutants KCREB or NF-YA29, respectively, indicate that cAMP inducibility of the insulin CRE is mediated by CREB or closely related proteins; however, NF-Y binding to the insulin CRE confers constitutive, basal activity and decreases the ability of CREB to mediate cAMP-stimulated transcription and calcium responsiveness. Results from these studies demonstrate that NF-Y binds to the insulin CRE and modulates the function of CREB. Together with the nonpalindromic sequence of the CRE octamer motif, the interaction of NF-Y with CREB may be responsible for the gene-specific transcriptional activity of the insulin CRE and explain why it has considerable basal activity but is less responsive to cAMP stimulation than others.

Transcriptional activity of RBCK1 protein (RBCC protein interacting with PKC 1): requirement of RING-finger and B-Box motifs and regulation by protein kinases

The RBCK1 protein was recently identified as a protein kinase C-interacting protein with a new type of RBCC (RING-B-Box-Coiled-coil) region, possessing both DNA-binding and transcriptional activities unlike other proteins in the RBCC protein family (Tokunaga et al. Biochem. Biophys. Res. Commun. 244, 353-359, 1998). To identify protein motifs in the RBCC region of RBCK1 essential for the transcriptional activity, RBCK1 mutant proteins have been constructed and analyzed by using the GAL4 chimeric transcription regulator system. We have found that both of the RING-finger and the B-Box motifs are indispensable for the transcriptional activity of RBCK1. This is the first observation that these protein motifs of the RBCC protein family play a crucial role in transcriptional activation. In addition, we have examined the effect of co-expression of several protein kinases on the transcriptional activity of RBCK1. Protein kinase A (PKA) was found to enhance the activity by about eightfold, whereas both ERK (extracellular signal-regulated kinase) activator kinase 1 (MEK1) and MEK kinase 1 (MEKK1) significantly repressed the activity. Because RBCC proteins are presumed to act as a proto-oncoprotein, these results suggest that the RBCK1 protein is involved in the intracellular signaling cascades along with PKA, MEK1, and MEKK1 and mediates cell growth and differentiation.

Molecular cloning and characterization of a novel protein kinase C-interacting protein with structural motifs related to RBCC family proteins

A novel protein kinase C (PKC)-interacting protein was identified by the yeast two-hybrid screening using the regulatory domain of PKC beta I as a bait. The protein contained several structural motifs such as two putative coiled-coil regions, a RING-finger, a B-box, and a B-box-like motif in the order from NH2- to COOH-terminals. The molecular organization of the protein resembles the structure of the RBCC protein family proteins which usually have a RING-finger, a B-box, and a coiled-coil region. Therefore, the protein identified was designated as RBCK1 (RBCC protein interacting with PKC 1). Northern blot analysis showed that RBCK1 gene is expressed ubiquitously among rat tissues. RBCK1 protein associated with PKC beta I and PKC zeta when coexpressed in cultured mammalian cells. By the polymerase chain reaction-assisted DNA-binding site selection and the electrophoretic mobility shift assay, RBCK1 protein was shown to bind to several DNA fragments containing TGG-rich sequences. When the yeast GAL4 DNA-binding domain fused RBCK1 protein was expressed in COS-7 cells harboring the luciferase gene placed under a synthetic promoter containing GAL4-binding sites, the fusion protein showed enhanced transcriptional activity comparing with the GAL4 DNA-binding domain. These results suggest that RBCK1 protein might be a transcription factor that has a role in the signaling pathway through PKC.

The stress inducer arsenite activates mitogen-activated protein kinases extracellular signal-regulated kinases 1 and 2 via a MAPK kinase 6/p38-dependent pathway

Cell response to a wide variety of extracellular signals is mediated by either mitogenic activation of the Raf/MEK/ERK kinase cascade or stress-induced activation of the mitogen-activated protein kinase (MAPK) family members c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) or p38. We have examined communications between these stress- and mitogen-induced signaling pathways. We show here that the stress cascade activator arsenite activates extracellular signal-regulated kinase (ERK) in addition to p38 albeit with different kinetics. Whereas p38 is an early response kinase, ERK activation occurs with delayed time kinetics at 2-4 h. We observed activation of ERK upon arsenite treatment in many different cell lines. ERK activation is strongly enhanced by overexpression of p38 and mitogen-activated protein kinase kinase 6 (MKK6) but is blocked by dominant negative kinase versions of p38 and MKK6 or the specific p38 inhibitor SB203580. Arsenite-induced ERK activation is mediated by Ras, Raf, and MEK but appears to be independent of de novo protein synthesis. These data provide the first evidence for a p38 dependent activation of the mitogenic kinase cascade in stress-stimulated cells.

POU domain transcription factor brain 4 confers pancreatic alpha-cell-specific expression of the proglucagon gene through interaction with a novel proximal promoter G1 element

The proglucagon gene is expressed in a highly restricted tissue-specific manner in the alpha cells of the pancreatic islet, the hypothalamus, and the small and large intestines. Proglucagon is processed to glucagon and glucagon-like peptides GLP-1 and -2. Glucagon is expressed in alpha cells and regulates glucose homeostasis. GLP-1 is implicated in the control of insulin secretion, food intake, and satiety signaling, and GLP-2 is implicated in regulating small-bowel growth. Cell-specific expression of the proglucagon gene is mediated by proteins that interact with the proximal G1 promoter element which contains several AT-rich domains with binding sites for homeodomain transcription factors. In an attempt to identify major homeodomain proteins involved in pancreatic alpha-cell-specific proglucagon expression, we found that the POU domain transcription factor brain 4 is abundantly expressed in proglucagon-producing islet cell lines and rat pancreatic islets. In the latter, brain 4 and glucagon immunoreactivity colocalize in the outer mantle of islets. Electrophoretic mobility shift assays with specific antisera identify brain 4 as a major constituent of nuclear proteins of glucagon-producing cells that bind to the G1 element of the proglucagon gene proximal promoter. Transcriptional transactivation experiments reveal that brain 4 is a major regulator of proglucagon gene expression by its interaction with the G1 element. The finding that a neuronal transcription factor is involved in glucagon gene transcription may explain the presence of proglucagon in certain areas of the brain as well as in pancreatic alpha cells. Further, this finding supports the idea that the neuronal properties of endodermis-derived endocrine pancreatic cells may find their basis in regulation of gene expression by neuronal transcription factors.

Transient expression of a winged-helix protein, MNF-beta, during myogenesis

A novel winged-helix transcription factor, MNF-beta, is expressed coincidentally with cell cycle withdrawal and differentiation of skeletal myogenic cells. MNF-beta is closely related to the myocyte nuclear factor (MNF) protein previously described (now termed MNF-alpha), but expression of the two isoforms is differentially regulated, and they exhibit distinctive functional properties with respect to DNA binding in vitro and transcriptional regulatory activity in transient-transfection assays. A DNA sequence motif binding MNF-beta with high affinity was selected from a library of random oligonucleotides and was found to be similar to but distinct from the cognate binding site for HNF-3beta, a more distantly related winged-helix protein. The temporal pattern of MNF-beta expression and the presence of MNF binding motifs within conserved promoter elements of several genes that modulate cell cycle progression support a working hypothesis that MNF proteins may modulate proliferation of myogenic precursor cells during development and muscle regeneration.