Transcriptional activation of the rat glucagon gene by the cyclic AMP-responsive element in pancreatic islet cells

Intra-islet α-cell Gs signaling promotes glucagon release

Glucagon, a hormone released from pancreatic α-cells, is critical for maintaining euglycemia and plays a key role in the pathophysiology of diabetes. To stimulate the development of new classes of therapeutic agents targeting glucagon release, key α-cell signaling pathways that regulate glucagon secretion need to be identified. Here, we focused on the potential importance of α-cell Gs signaling on modulating α-cell function. Studies with α-cell-specific mouse models showed that activation of α-cell Gs signaling causes a marked increase in glucagon secretion. We also found that intra-islet adenosine plays an unexpected autocrine/paracrine role in promoting glucagon release via activation of α-cell Gs-coupled A2A adenosine receptors. Studies with α-cell-specific Gαs knockout mice showed that α-cell Gs also plays an essential role in stimulating the activity of the Gcg gene, thus ensuring proper islet glucagon content. Our data suggest that α-cell enriched Gs-coupled receptors represent potential targets for modulating α-cell function for therapeutic purposes.

Intestinal HIF-2α Regulates GLP-1 Secretion via Lipid Sensing in L-Cells

BACKGROUND & AIMS

Compelling evidence shows that glucagon-like peptide-1 (GLP-1) has a profound effect in restoring normoglycemia in type 2 diabetic patients by increasing pancreatic insulin secretion. Although L-cells are the primary source of circulating GLP-1, the current therapies do not target L-cells to increase GLP-1 levels. Our study aimed to determine the molecular underpinnings of GLP-1 secretion as an impetus to identify new interventions to target endogenous L-cells.

METHODS

We used genetic mouse models of intestine-specific overexpression of hypoxia-inducible factor (HIF)-1α and HIF-2α (Vhl^ΔIE), conditional overexpression of intestinal HIF-2α (Hif-2α^{LSL;Vilin-Cre/ERT2}), and intestine-specific HIF-2α knockout mice (Hif-2α^ΔIE) to show that HIF signaling, especially HIF-2α, regulates GLP-1 secretion.

RESULTS

Our data show that intestinal HIF signaling improved glucose homeostasis in a GLP-1-dependent manner. Intestinal HIF potentiated GLP-1 secretion via the lipid sensor G-protein-coupled receptor (GPR)40 enriched in L-cells. We show that HIF-2α regulates GPR40 in L-cells and potentiates fatty acid-induced GLP-1 secretion via extracellular regulated kinase (ERK). Using a genetic model of intestine-specific overexpression of HIF-2α, we show that HIF-2α is sufficient to increase GLP-1 levels and attenuate diet-induced metabolic perturbations such as visceral adiposity, glucose intolerance, and hepatic steatosis. Lastly, we show that intestinal HIF-2α signaling acts as a priming mechanism crucial for postprandial lipid-mediated GLP-1 secretion. Thus, disruption of intestinal HIF-2α decreases GLP-1 secretion.

CONCLUSIONS

In summary, we show that intestinal HIF signaling, particularly HIF-2α, regulates the lipid sensor GPR40, which is crucial for the lipid-mediated GLP-1 secretion, and suggest that HIF-2α is a potential target to induce endogenous GLP-1 secretion.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

Effects of mutation of the CREB binding site of the somatostatin promoter on cyclic AMP responsiveness in CV-1 cells

The transcription factors CREB (cAMP response element binding protein) and ATF (activating transcription factor) recognize DNA containing the consensus sequence TGACGTCA. We compared the neuropeptide somatostatin promoter, which binds CREB and is activated by cAMP, to the adenovirus E2A promoter, which binds ATF but is not activated by cAMP, to determine which specific nucleotides within a CREB/ATF recognition sequence confer cAMP responsiveness. Several mutant somatostatin promoters were generated containing part of all of the E2A ATF binding site. Some of the hybrid CREB/ATF binding sites competed for factor binding to a wild-type somatostatin promoter probe. However, only the wild-type CREB binding site promoter could confer cAMP activation on a linked CAT plasmid. Furthermore, this wild-type CREB binding site could confer cAMP activation on the CAT plasmid only if it was adjacent to a wild-type somatostatin TATA box and cap site. These results suggest that slight deviation from a wild-type CREB recognition sequence might be tolerated by factor(s) binding to cAMP response element-like sequences. However, transcription activation may require a particular CREB recognition sequence, as well as additional promoter elements that bind proteins that interact with CREB.

Vascular Biology of Glucagon Receptor Superfamily Peptides: Mechanistic and Clinical Relevance

Regulatory peptides produced in islet and gut endocrine cells, including glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, and glucose-dependent insulinotropic polypeptide, exert actions with considerable metabolic importance and translational relevance. Although the clinical development of GLP-1 receptor agonists and dipeptidyl peptidase-4 inhibitors has fostered research into how these hormones act on the normal and diseased heart, less is known about the actions of these peptides on blood vessels. Here we review the effects of these peptide hormones on normal blood vessels and highlight their vascular actions in the setting of experimental and clinical vascular injury. The cellular localization and signal transduction properties of the receptors for glucagon, GLP-1, GLP-2, and glucose-dependent insulinotropic polypeptide are discussed, with emphasis on endothelial cells and vascular smooth muscle cells. The actions of these peptides on the control of blood flow, blood pressure, angiogenesis, atherosclerosis, and vascular inflammation are reviewed with a focus on elucidating direct and indirect mechanisms of action. How these peptides traverse the blood-brain barrier is highlighted, with relevance to the use of GLP-1 receptor agonists to treat obesity and neurodegenerative disorders. Wherever possible, we compare actions identified in cell lines and primary cell culture with data from preclinical studies and, when available, results of human investigation, including studies in subjects with diabetes, obesity, and cardiovascular disease. Throughout the review, we discuss pitfalls, limitations, and challenges of the existing literature and highlight areas of controversy and uncertainty. The increasing use of peptide-based therapies for the treatment of diabetes and obesity underscores the importance of understanding the vascular biology of peptide hormone action.

Targeting cAMP/PKA pathway for glycemic control and type 2 diabetes therapy

In mammals, cyclic adenosine monophosphate (cAMP) is an intracellular second messenger that is usually elicited by binding of hormones and neurotransmitters to G protein-coupled receptors (GPCRs). cAMP exerts many of its physiological effects by activating cAMP-dependent protein kinase (PKA), which in turn phosphorylates and regulates the functions of downstream protein targets including ion channels, enzymes, and transcription factors. cAMP/PKA signaling pathway regulates glucose homeostasis at multiple levels including insulin and glucagon secretion, glucose uptake, glycogen synthesis and breakdown, gluconeogenesis, and neural control of glucose homeostasis. This review summarizes recent genetic and pharmacological studies concerning the regulation of glucose homeostasis by cAMP/PKA in pancreas, liver, skeletal muscle, adipose tissues, and brain. We also discuss the strategies for targeting cAMP/PKA pathway for research and potential therapeutic treatment of type 2 diabetes mellitus (T2D).

Delineating the regulation of energy homeostasis using hypothalamic cell models

Attesting to its intimate peripheral connections, hypothalamic neurons integrate nutritional and hormonal cues to effectively manage energy homeostasis according to the overall status of the system. Extensive progress in the identification of essential transcriptional and post-translational mechanisms regulating the controlled expression and actions of hypothalamic neuropeptides has been identified through the use of animal and cell models. This review will introduce the basic techniques of hypothalamic investigation both in vivo and in vitro and will briefly highlight the key advantages and challenges of their use. Further emphasis will be place on the use of immortalized models of hypothalamic neurons for in vitro study of feeding regulation, with a particular focus on cell lines proving themselves most fruitful in deciphering fundamental basics of NPY/AgRP, Proglucagon, and POMC neuropeptide function.

P21-activated protein kinase 1 (Pak1) mediates the cross talk between insulin and β-catenin on proglucagon gene expression and its ablation affects glucose homeostasis in male C57BL/6 mice

In gut endocrine L cells, the Wnt signaling pathway effector β-catenin (β-cat)/transcription factor 7-like 2 mediates the stimulatory effect of insulin on proglucagon (gcg) expression and glucagon-like peptide-1 (GLP-1) production. In several other cell lineages, insulin is able to stimulate p21-activated protein kinase 1 (Pak1). Here we determined the role of Pak1 in gcg expression and the effect of Pak1 deletion on glucose homeostasis. Insulin stimulated Pak1 activation through increasing its Thr423 phosphorylation in gut gcg-expressing cell lines, associated with increased gcg mRNA levels. This stimulation was attenuated by the Pak inhibitor 2,2'-dihydroxy-1,1'-dinaphthyldisulfide (IPA3) or dominant-negative Pak1. Both insulin and cAMP-promoting agents activated β-cat Ser675 phosphorylation, which was attenuated by IPA3 or protein kinase A inhibition, respectively. Gut gcg levels were reduced in male Pak1(-/-) mice, associated with impaired glucose tolerance after an ip or oral glucose challenge. These mice had lower circulating active GLP-1 levels after a glucose challenge as well as reduced distal ileum GLP-1 content after insulin treatment. Finally, the Pak1(-/-) mice exhibited reduced brainstem gcg level and abolished β-cat Ser675 phosphorylation in brain neurons after insulin treatment. We suggest that Pak1 mediates the cross talk between insulin and Wnt signaling pathways on gut and brain gcg expression, and its ablation impairs glucose homeostasis.

Direct regulation of the proglucagon gene by insulin, leptin, and cAMP in embryonic versus adult hypothalamic neurons

The proglucagon gene is expressed not only in the pancreas and intestine but also in the hypothalamus. Proglucagon-derived peptides have emerged as potential regulators of energy homeostasis. Whether leptin, insulin, or cAMP activation controls proglucagon gene expression in the hypothalamus is not known. A key reason for this has been the inaccessibility of hypothalamic proglucagon-expressing neurons and the lack of suitable neuronal cell lines. Herein we describe the mechanisms involved in the direct regulation of the proglucagon gene by insulin, leptin, and cAMP in hypothalamic cell models. Insulin, through an Akt-dependent manner, significantly induced proglucagon mRNA expression by 70% in adult-derived mHypoA-2/10 neurons and significantly suppressed it by 45% in embryonic-derived mHypoE-39 neurons. Leptin, via the Janus kinase-2/ signal transducer and activator of transcription-3 pathway, caused an initial increase by 66 and 43% at 1 h followed by a decrease by 45 and 34% at 12 h in mHypoA-2/10 and mHypoE-39 cells, respectively. Furthermore, cAMP activation by forskolin up-regulated proglucagon expression by 87% in mHypoE-39 neurons and increased proglucagon mRNA, through Epac activation, in the mHypoE-20/2 neurons. Specific regions of the proglucagon promoter were regulated by cAMP signaling, as determined by transient transfections, whereas mRNA stability assays demonstrate that insulin and leptin increase proglucagon mRNA stability in the adult cells. These findings suggest that insulin, leptin, and cAMP act directly, but differentially, on specific hypothalamic neurons to regulate proglucagon gene expression. Because proglucagon-derived peptides are potential regulators of energy homeostasis, an understanding of hypothalamic proglucagon neurons is important to further expand our knowledge of alternative feeding circuits.

Glucagon gene expression in the endocrine pancreas: the role of the transcription factor Pax6 in α-cell differentiation, glucagon biosynthesis and secretion

The glucagon gene is expressed in α-cells of the pancreas, L cells of the intestine and the hypothalamus. The determinants of the α-cell-specific expression of the glucagon gene are not fully characterized, although Arx, Pax6 and Foxa2 are critical for α-cell differentiation and glucagon gene expression; in addition, the absence of the β-cell-specific transcription factors Pdx1, Pax4 and Nkx6.1 may allow for the glucagon gene to be expressed. Pax6, along with cMaf and MafB, binds to the DNA control element G(1) which confers α-cell specificity to the promoter and to G(3) and potently activates glucagon gene transcription. In addition, to its direct role on the transcription of the glucagon gene, Pax6 controls several transcription factors involved in the activation of the glucagon gene such as cMaf, MafB and NeuroD1/Beta2 as well as different steps of glucagon biosynthesis and secretion. We conclude that Pax6 independently of Arx and Foxa2 is critical for α-cell function by coordinating glucagon gene expression as well as glucagon biosynthesis and secretion.

POU homeodomain protein Oct-1 functions as a sensor for cyclic AMP

Cyclic AMP is a fundamentally important second messenger for numerous peptide hormones and neurotransmitters that control gene expression, cell proliferation, and metabolic homeostasis. Here we show that cAMP works with the POU homeodomain protein Oct-1 to regulate gene expression in pancreatic and intestinal endocrine cells. This ubiquitously expressed transcription factor is known as a stress sensor. We found that it also functions as a repressor of Cdx-2, a proglucagon gene activator. Through a mechanism that involves the activation of exchange protein activated by cyclic AMP, elevation of cAMP leads to enhanced phosphorylation and nuclear exclusion of Oct-1 and reduced interactions between Oct-1 or nuclear co-repressors and the Cdx-2 gene promoter, detected by chromatin immunoprecipitation. In rat primary pancreatic islet cells, cAMP elevation also reduces nuclear Oct-1 content, which causes increased proglucagon and proinsulin mRNA expression. Our study therefore identifies a novel mechanism by which cAMP regulates hormone-gene expression and suggests that ubiquitously expressed Oct-1 may play a role in metabolic homeostasis by functioning as a sensor for cAMP.

New insights into the role of cAMP in the production and function of the incretin hormone glucagon-like peptide-1 (GLP-1)

The proglucagon gene (gcg) encodes both glucagon and glucagon-like peptide-1 (GLP-1), produced in pancreatic alpha cells and intestinal endocrine L cells, respectively. The incretin hormone GLP-1 stimulates insulin secretion and pro-insulin gene transcription. GLP-1 also enhances pancreatic beta-cell proliferation, inhibits cell apoptosis, and has been utilized in the trans-differentiation of insulin producing cells. A long-term effective GLP-1 receptor agonist, Byetta, has now been developed as the drug in treating type II diabetes and potentially other metabolic disorders. The expression of gcg and the production of GLP-1 can be activated by the elevation of the second messenger cyclic AMP (cAMP). Recent studies suggest that in addition to protein kinase A (PKA), exchange protein activated by cAMP (Epac), another effector of cAMP, and the crosstalk between PKA and the Wnt signaling pathway, are involved in cAMP-stimulated gcg transcription and GLP-1 production as well. Finally, functions of GLP-1 in pancreatic beta cells are also mediated by PKA, Epac, as well as the effector of the Wnt signaling pathway. Together, these novel findings bring us a new insight into the role of cAMP in the production and function of the incretin hormone GLP-1.

Epac is involved in cAMP-stimulated proglucagon expression and hormone production but not hormone secretion in pancreatic alpha- and intestinal L-cell lines

Both Epac and PKA are effectors of the second messenger cAMP. Utilizing an exchange protein directly activated by cAMP (Epac) pathway-specific cAMP analog (ESCA), we previously reported that Epac signaling regulates proglucagon gene (gcg) expression in the glucagon-like peptide-1 (GLP-1)-producing intestinal endocrine L-cell lines GLUTag and STC-1. We now show that Epac-2 is also expressed in glucagon-producing pancreatic alpha-cell lines, including PKA-deficient InR1-G9 cells, and that ESCA stimulates gcg promoter and mRNA expression in the InR1-G9 cells. Using a dominant-negative Epac-2 expression plasmid (Epac-2DN), we found that Epac inhibition attenuated forskolin-stimulated gcg promoter expression in the PKA-active STC-1 cell line and blocked forskolin-stimulated gcg promoter expression in the InR1-G9 cells. Consistently, ESCA was shown to stimulate glucagon and GLP-1 production in the InR1-G9 and GLUTag cell lines, respectively. Surprisingly, ESCA treatment did not show a notable stimulation of glucagon or GLP-1 secretion from these two cell lines. This is in contrast to its ability to stimulate insulin secretion from the pancreatic INS-1 beta-cell line. Our findings suggest that Epac is selectively involved in peptide hormone secretion in pancreatic and intestinal endocrine cells and that distinct signaling cascades are involved in stimulating production vs. secretion of glucagon and GLP-1 in response to cAMP elevation.

The role of incretins in glucose homeostasis and diabetes treatment

Incretins are gut hormones that are secreted from enteroendocrine cells into the blood within minutes after eating. One of their many physiological roles is to regulate the amount of insulin that is secreted after eating. In this manner, as well as others to be described in this review, their final common raison d'être is to aid in disposal of the products of digestion. There are two incretins, known as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1), that share many common actions in the pancreas but have distinct actions outside of the pancreas. Both incretins are rapidly deactivated by an enzyme called dipeptidyl peptidase 4 (DPP4). A lack of secretion of incretins or an increase in their clearance are not pathogenic factors in diabetes. However, in type 2 diabetes (T2DM), GIP no longer modulates glucose-dependent insulin secretion, even at supraphysiological (pharmacological) plasma levels, and therefore GIP incompetence is detrimental to beta-cell function, especially after eating. GLP-1, on the other hand, is still insulinotropic in T2DM, and this has led to the development of compounds that activate the GLP-1 receptor with a view to improving insulin secretion. Since 2005, two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in T2DM: an incretin mimetic (exenatide, which is a potent long-acting agonist of the GLP-1 receptor) and an incretin enhancer (sitagliptin, which is a DPP4 inhibitor). Exenatide is injected subcutaneously twice daily and its use leads to lower blood glucose and higher insulin levels, especially in the fed state. There is glucose-dependency to its insulin secretory capacity, making it unlikely to cause low blood sugars (hypoglycemia). DPP4 inhibitors are orally active and they increase endogenous blood levels of active incretins, thus leading to prolonged incretin action. The elevated levels of GLP-1 are thought to be the mechanism underlying their blood glucose-lowering effects.

Orexin-A inhibits glucagon secretion and gene expression through a Foxo1-dependent pathway

Orexin-A (OXA) regulates food intake and energy homeostasis. It increases insulin secretion in vivo and in vitro, although controversial effects of OXA on plasma glucagon are reported. We characterized the effects of OXA on glucagon secretion and identify intracellular target molecules in glucagon-producing cells. Glucagon secretion from in situ perfused rat pancreas, isolated rat pancreatic islets, and clonal pancreatic A-cells (InR1-G9) were measured by RIA. The expression of orexin receptor 1 (OXR1) was detected by Western blot and immunofluorescence. The effects of OXA on cAMP, adenylate-cyclase-kinase (AKT), phosphoinositide-dependent kinase (PDK)-1, forkhead box O-1 (Foxo1), and cAMP response element-binding protein were measured by ELISA and Western blot. Intracellular calcium (Ca(2+)(i)) concentration was detected by fura-2and glucagon expression by real-time PCR. Foxo1 was silenced in InR1-G9 cells by transfecting cells with short interfering RNA. OXR1 was expressed on pancreatic A and InR1-G9 cells. OXA reduced glucagon secretion from perfused rat pancreas, isolated rat pancreatic islets, and InR1-G9 cells. OXA inhibited proglucagon gene expression via the phosphatidylinositol 3-kinase-dependent pathway. OXA decreased cAMP and Ca(2+)(i) concentration and increased AKT, PDK-1, and Foxo1 phosphorylation. Silencing of Foxo1 caused a reversal of the inhibitory effect of OXA on proglucagon gene expression. Our study provides the first in vitro evidence for the interaction of OXA with pancreatic A cells. OXA inhibits glucagon secretion and reduces intracellular cAMP and Ca(2+)(i) concentration. OXA increases AKT/PDK-1 phosphorylation and inhibits proglucagon expression via phosphatidylinositol 3-kinase- and Foxo-1-dependent pathways. As a physiological inhibitor of glucagon secretion, OXA may have a therapeutic potential to reduce hyperglucagonemia in type 2 diabetes.

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.

Protein hydrolysates stimulate proglucagon gene transcription in intestinal endocrine cells via two elements related to cyclic AMP response element

AIMS/HYPOTHESIS

Protein hydrolysates (peptones) increase not only glucagon-like peptide-1 (GLP-1) secretion but also transcription of the proglucagon ( PG) gene in the intestine. The critical physiological roles of gut-derived GLPs raised hope for their therapeutic use in several disorders, especially GLP-1 in diabetes. We aimed to investigate the molecular mechanisms involved in this nutrient- PG gene interaction.

METHODS

Wild-type and mutated PG promoter fragments fused to the luciferase reporter gene were transfected into enteroendocrine STC-1 cells, which were then either treated or not with peptones. Co-transfection with expression vectors of dominant-negative forms of cAMP response element binding protein (CREB) and protein kinase A (PKA) proteins were performed, as well as electrophoresis mobility shift assays.

RESULTS

Deletion analysis showed that the promoter region spanning between -350 and -292 bp was crucial for the transcriptional stimulation induced by peptones. Site-directed mutagenesis of the canonical cAMP response element (CRE(PG)) and of the adjacent putative CRE site (CRE-like1) led to a dramatic inhibition of the promoter responsiveness to peptones. Over expression of a dominant-negative mutant of CREB or of PKA produced a comparable and selective inhibitory effect on the activity of transfected promoter fragment containing the -350/-292 sequence. EMSA showed that CREB and fra2 transcription factors bound to CRE(PG) and CRE-like1 elements respectively, independently of peptone treatment.

CONCLUSIONS/INTERPRETATION

Our report identified cis- and trans-regulatory elements implicated in the transcriptional control of PG gene by nutrients in enteroendocrine cells. It highlights the role of a previously unsuspected CRE-like1 element, and emphasises the importance of CRE-related sequences in the regulation of PG gene transcription in the intestine.

Regulation of proglucagon transcription by activated transcription factor (ATF) 3 and a novel isoform, ATF3b, through the cAMP-response element/ATF site of the proglucagon gene promoter

Glucagon, the second major glucose-regulated hormone in the control of glucose homeostasis, functions as a counter-regulator to insulin and is specifically produced by the pancreatic alpha cells. Its excessive biosynthesis and secretion is associated with diabetes mellitus. The expression of the proglucagon gene has been demonstrated to be regulated by a cAMP-dependent pathway through cAMP-response element-binding protein (CREB) and possibly other transcription factors bound to its cAMP-response element (CRE)/activated transcription factor (ATF) site. Elsewhere we have shown that ATF3, a member of the ATF/CREB subfamily of the basic leucine zipper domain proteins, is expressed predominantly in the alpha cells of the pancreatic islets. In our attempts to further dissect the role of ATF3 proteins in alpha cells, we have identified and characterized a novel alternatively spliced form, ATF3b, and have compared the specific binding ability of ATF3 and ATF3b on the CRE/ATF motif of the proglucagon promoter. Our findings indicate the existence of a novel mechanism by which the transcription of the proglucagon gene is regulated in response to cAMP signals, in addition to CREB and in relation to glucose fluctuations in pancreatic alpha cells.

Aberrant regulation of human intestinal proglucagon gene expression in the NCI-H716 cell line

Despite interest in understanding glucagon-like peptide-1 (GLP-1) production, the factors important for GLP-1 biosynthesis remain poorly understood. We examined control of human proglucagon gene expression in NCI-H716 cells, a cell line that secretes GLP-1 in a regulated manner. Insulin, phorbol myristate acetate, or forskolin, known regulators of rodent proglucagon gene expression, had no effect, whereas sodium butyrate decreased levels of NCI-H716 proglucagon mRNA transcripts. The inhibitory effect of sodium butyrate was mimicked by trichostatin A but was not detected with sodium acetate or isobutyrate. The actions of butyrate were not diminished by the ERK1/2 inhibitor PD98059, p38 inhibitor SB203580, or soluble guanylate cyclase inhibitor LY83583 or following treatment of cells with KT5823, a selective inhibitor of cGMP-dependent protein kinase. NCI-H716 cells expressed multiple proglucagon gene transcription factors including isl-1, pax-6, pax-2, cdx-2/3, pax-4, hepatocyte nuclear factor (HNF)-3 alpha, HNF-3beta, HNF-3 gamma, and Nkx2.2. Nevertheless, the butyrate-dependent inhibition of proglucagon gene expression was not associated with coordinate changes in transcription factor expression and both the human and rat transfected proglucagon promoters were transcriptionally inactive in NCI-H716 cells. Hence, NCI-H716 cells may not be a physiologically optimal model for studies of human enteroendocrine proglucagon gene transcription.

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.

Activation of serum response factor in the depolarization induction of Egr-1 transcription in pancreatic islet beta-cells

The results of the current studies define the major elements whereby glucose metabolism in islet beta-cells leads to transcriptional activation of an early response gene in insulinoma cell lines and in rat islets. Glucose stimulation (2-20 mm) resulted in a 4-fold increase in Egr-1 mRNA at 30 min, as did the depolarizing agents KCl and tolbutamide. This response was inhibited by diazoxide and EGTA, indicating that beta-cell depolarization and Ca(2+) influx, respectively, are essential. Pharmacological inhibition of the Egr-1 induction by H89 (48%) and calmidazolium (35%), but not by mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1 and 2 or phosphatidylinositol 3-kinase inhibitors, implied that protein kinase A and Ca(2+)/calmodulin pathways are involved. Deletion mapping of the Egr-1 promoter revealed that the proximal -198 base pairs containing two serum response elements (SREs) and one cAMP-response element retained the depolarization response. Depolarization resulted in phosphorylation of cAMP-response element-binding protein, yet partial inhibition by a dominant negative cAMP-response element-binding protein, along with a robust response of a cAMP-response element-mutated Egr-1 promoter suggested the presence of a second Ca(2+)-responsive element. Depolarization activation of 5XSRE-LUC and serum response factor (SRF)-GAL4 constructs, along with activation of SRF-GAL4 by co-transfection with constitutively active calmodulin kinase IV and protein kinase A, and binding of Ser(103)-phosphorylated SRF in nuclear extracts, indicated that the SRE.SRF complexes contribute to the Ca(2+)-mediated transcriptional regulation of Egr-1. The results of the current experiments demonstrate for the first time SRE-dependent transcription and the role of SRF, a transcription factor known to be a major component of growth responses, in glucose-mediated transcriptional regulation in insulinoma cells.

Divergent regulation of human and rat proglucagon gene promoters in vivo

A single mammalian proglucagon gene is expressed in the brain, islets, and intestinal enteroendocrine cells, which gives rise to a unique profile of proglucagon-derived peptides (PGDPs) in each tissue. The biological importance of glucagon, glucagon-like peptide (GLP)-1, and GLP-2 has engendered considerable interest in the factors regulating the synthesis and secretion of the PGDPs in vivo. Although rat proglucagon gene transcription has been extensively studied, the factors important for control of human proglucagon gene expression have not been examined. We now report that, despite conservation of proximal promoter G1-G4 enhancer-like elements, human proglucagon reporter plasmids containing these elements are transcriptionally inactive in islet cell lines. Remarkably, larger human proglucagon promoter fragments, such as the 1604 hGLU-Luc, are expressed in GLUTag enteroendocrine cells but not in islet cell lines. A total of 5775 bases of human proglucagon promoter were required for expression in islet cell lines. Analysis of human proglucagon promoter expression in transgenic mice demonstrated that approximately 1.6 kb of human proglucagon gene sequences directs expression of a human growth hormone reporter gene to the brain and intestinal enteroendocrine cells but not islet cells in vivo. These findings provide the first evidence demonstrating divergence in the mechanisms utilized for tissue-specific regulation of the human and rodent proglucagon genes.

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.

Inhibition of protein kinase A-induced glucagon synthesis and secretion by glucose

The control of glucagon biosynthesis and secretion in the pancreatic islet was examined in response to protein kinase A stimulation at various glucose concentrations. Forskolin plus 3-isobutyl 1-methylxanthine (IBMX) stimulated both glucagon synthesis and secretion at a glucose concentration equivalent to hypoglycemia (0.5 g/L, P<.001), but not at higher glucose concentrations (1.0, 2.0, and 4.0 g/L, P>.05). Destruction of B cells with streptozotocin or inhibition of glycolysis with mannoheptulose did not reverse the inhibitory action of high glucose (4.0 g/L) on the response of glucagon to forskolin plus IBMX. In contrast, citrate but not EGTA treatment permitted forskolin plus IBMX to stimulate glucagon synthesis and secretion (P<.05 and P<.001, respectively) in the presence of high glucose. We conclude that citrate can block the inhibitory action of glucose on the response of A cells to the protein kinase A pathway, possibly through its effects on an intracellular metabolic pathway.

Identification of a functional cAMP response element in the secretogranin II gene

Secretogranin II is an acidic secretory protein with a widespread distribution in secretory granules of neuronal and endocrine cells. The secretogranin II gene contains, like other members of the granin family, a cAMP response element (CRE) in its upstream region. To investigate the functional significance of this motif, intracellular cAMP levels were increased in a neuronal cell line derived from the septal region of the brain and the level of secretogranin II gene expression was analysed. It was found that increased cAMP levels did, in fact, induce secretogranin II gene expression. To analyse the cis-acting sequence responsible for this induction, a hybrid gene containing the upstream region of the mouse secretogranin II gene fused to beta-globin as a reporter was constructed. Transfection analysis revealed that cAMP-induced transcription of the secretogranin II promoter/beta-globin gene in septal and insulinoma cells. DNA-protein binding assays showed that recombinant CRE-binding protein (CREB), produced in bacteria or human cells, bound in a sequence-specific manner to the secretogranin II promoter CRE. Moreover, deletion mutagenesis revealed that the CRE motif is a bifunctional genetic regulatory element in that it mediates basal as well as cAMP-stimulated transcription. Interestingly, cAMP had no effect upon secretogranin II gene transcription in PC12 and neuroblastoma cells. An increase in the intracellular cAMP concentration activated a GAL4-CREB fusion protein upon transcription in neuroblastoma cells indicating the integrity of the cAMP signaling pathway to the nucleus. Basal as well as cAMP-stimulated transcription, directed from the secretogranin II promoter was, however, impaired in insulinoma cells by overexpression of CREB-2, a negative-acting CRE-binding protein. These results indicate that competitive effects are likely to occur between CRE-bound transcriptional activators and repressors. We conclude that cAMP-stimulated induction of secretogranin II gene transcription is mediated by the CRE motif in a cell-type-specific manner, and is likely to depend on the balance between positive and negative CRE-binding proteins in a particular cell type.

Transcriptional repression by the human homeobox protein EVX1 in transfected mammalian cells

The human homeobox protein EVX1 (EVX1) is thought to play an important role during embryogenesis. In this study, the effect of EVX1 on gene transcription has been investigated in transfected mammalian cells. EVX1 expression represses transcription of a reporter gene directed by either cell-specific or viral promoter/enhancer sequences in a variety of mammalian cell lines and in a concentration-dependent manner. Transcriptional repression is independent of the presence of DNA-binding sites for EVX1 in all the promoters we tested. Furthermore, repression by EVX1 is evident also using a TATA-less minimal promoter in the reporter construct. A carboxyl-terminal proline/alanine-rich region of EVX1 seems to be responsible for the transcriptional repression activity, as suggested by transfection of EVX1 mutants. We speculate that the repressor function of EVX1 contributes to its proposed role in embryogenesis.

Involvement of the Ca(2+)-dependent phosphatase calcineurin in gene transcription that is stimulated by cAMP through cAMP response elements

Gene transcription can be induced by cAMP and Ca2+ through distinct protein kinases phosphorylating the transcription factor CREB, which binds to cAMP response elements (CREs) in various genes. Induction of gene transcription by Ca2+ has been shown recently to depend on the Ca2+/calmodulin-dependent protein phosphatase calcineurin in pancreatic islet cells. This study investigates the role of calcineurin in CRE-directed gene transcription after stimulation by cAMP. Reporter fusion genes under the transcriptional control of CREs were transiently transfected into the cell line HIT. Pharmacological evidence suggests that cAMP stimulates CRE-mediated transcription through a Ca(2+)-dependent mechanism. The immunosuppressive drugs cyclosporin A and FK506 inhibited CRE-mediated transcription stimulated by cAMP. At the same concentrations they also inhibited calcineurin phosphatase activity. Reversal of calcineurin inhibition by rapamycin or overexpression of calcineurin led to disinhibition of CRE-mediated gene transcription. Immunoblots with a phosphoCREB-specific antibody showed that cyclosporin A and FK506 do not interfere with CREB phosphorylation at serine 119 stimulated with cAMP or membrane depolarization. These results indicate that in HIT cells stimulation of CRE-mediated transcription depends not only on the activity of protein kinases phosphorylating CREB but also on the Ca2+/calmodulin-dependent protein phosphatase calcineurin that is necessary for the transcriptional competence of phosphorylated CREB.

Functional characterization of somatostatin receptors expressed on hamster glucagonoma cells

We characterized somatostatin receptors expressed in hamster glucagonoma INR1G9 cells and the effects of somatostatin on glucagon secretion, proglucagon gene expression, and the adenosine 3',5'-cyclic monophosphate (cAMP)-dependent signal-transduction cascade. 125I-labeled somatostatin was displaced by somatostatin-14 and somatostatin-28 with a dissociation constant of 2 nmol/l. Stable GTP analogues decreased binding of 125I-somatostatin to its receptors, suggesting an interaction of somatostatin receptors with G proteins. Chemical cross-linking of 125I-somatostatin to its receptor revealed a molecular mass of the ligand-receptor complex of 47 kDa. Somatostatin inhibited forskolin-stimulated activation of adenylate cyclase [2.5 microM forskolin (161%) + 1 microM somatostatin (128%); P < 0.05] and protein kinase A [10 microM forskolin (143%) + 1 microM somatostatin (114%); P < 0.05] but did not influence basal activities of these enzymes. Forskolin-induced stimulation of cAMP generation was reduced by somatostatin [2.5 microM forskolin (306%) + 1 microM somatostatin (145%); P < 0.05]. Somatostatin inhibited forskolin, theophylline, and arginine stimulation of glucagon secretion. Basal as well as forskolin-, theophylline-, and isobutyl methylxanthine-induced proglucagon gene expression was significantly reduced by somatostatin. Our data show that, in INR1G9 cells, somatostatin receptors are at least in part coupled to the adenylate cyclase system. Somatostatin is a potent negative regulator of both basal and forskolin-stimulated proglucagon gene expression. The interaction with forskolin occurs at the level of adenylate cyclase. The effect of somatostatin on basal proglucagon gene transcription is most probably mediated by an unrelated second messenger system. Somatostatin may influence several functions of the pancreatic A cell.

Differential regulation of chromogranin B and synapsin I gene promoter activity by cAMP and cAMP-dependent protein kinase

cAMP has neutrotrophic effects in the nervous system. We have investigated whether there is a correlation between cAMP-induced neurite outgrowth and induction of chromogranin B and synapsin I gene expression. These genes encode marker proteins of distinct populations of vesicles in neurons, neuroendocrine and endocrine cells, and in addition, they contain a cAMP response element (CRE) in their upstream regions, making it likely that cAMP-induced neuronal differentiation might be accompanied by increased transcription of these genes. We increased intracellular cAMP levels in neuronal and neuroendocrine cells and analyzed the levels of chromogranin B and synapsin I mRNA. Our data revealed that, while chromogranin B mRNA was in fact induced following cAMP stimulation, synapsin I mRNA was not affected. To analyze the cis-acting sequences, we constructed hybrid genes containing the upstream region of the mouse chromogranin B gene fused to a reporter gene. Similar plasmids containing the synapsin I or the glucagon promoter were constructed. Transfections of neuronal and endocrine cells, together with deletion mutagenesis, revealed that the CRE of the chromogranin B gene mediated the effect of cAMP upon transcription. This effect was mimicked by overexpression of the catalytic subunit of the cAMP-dependent protein kinase. In addition, overexpression of the negative-acting CRE-binding protein CREB-2 revealed that the chromogranin B CRE functions as a bifunctional genetic regulatory element in that it mediates basal as well as cAMP-stimulated transcription. Synapsin I gene expression, however, was not induced by either elevated intracellular cAMP concentration or by overexpression of protein kinase A, although a similar pattern of proteins, including CREB, bound to the synapsin I and chromogranin B CRE in vitro. Thus while the CRE element in the chromogranin B gene promoter is responsive to cAMP, the same element, when present in the synapsin I promoter, does not confer cAMP inducibility.

Ras antagonizes cAMP stimulated glucagon gene transcription in pancreatic islet cell lines

Ras, a GTP-binding protein, converts membrane tyrosine kinase signalling to changes in gene expression patterns. Utilising a rat glucagon promoter-CAT construct (p[-1.1]GLU-CAT) we demonstrate in transient transfection experiments that the oncogenic Ras inhibits cAMP-dependent activation of p[-1.1]GLU-CAT in both glucagonoma InR1-G9 and insulinoma beta-TC1 cells. Conversely, the expression of a dominant negative mutant of Ras enhances the cAMP-induced activation of p[-1.1]GLU-CAT transcription in these cells. Our data suggests a functional interference of Ras with the cAMP-dependent transcription of the glucagon gene.

Humoral regulation of intestinal adaptation

After the loss of small bowel through disease or surgery the residual bowel adapts by increasing its functional capacity. This process of adaptation involves dilatation, hypertrophy and mucosal hyperplasia, particularly distal to the area of bowel loss or disease. The response of the residual bowel is mediated by a complex interplay of factors including luminal nutrition, pancreaticobiliary secretions, luminal or local growth factors and also humoral or endocrine factors. The experimental model commonly used to characterize the adaptive response, massive small bowel resection (MSBR), involves 80% resection of the small bowel in the rat. Of the various putative humoral factors, most work has focused on the products of the ileal L cells: enteroglucagon and peptide YY. Plasma levels of both hormones are increased after MSBR and indeed their mRNA levels are also increased as a result of an increase in the amount of message per L cell. Whilst PYY probably serves as an 'ileal brake' to slow the movement of the luminal contents and hence increase their mucosal contact time, the role of the enteroglucagon is unresolved. The molecular cloning of the proglucagon gene has revealed, firstly, that there are a number of biologically active peptides which derive from the propeptide and, secondly, that tissue-specific differential processing occurs. Most studies do not clearly define which of these products of proglucagon is being measured and is termed as glucagon-like or enteroglucagon immunoreactivity. The insulin-like growth factors (IGF) have a potent mitogenic action on the bowel. Their role after MSBR is likely to be largely paracrine. Though IGF-I mRNA levels do not increase after MSBR, the precipitous and early fall in ileal IGF-binding protein-3 (IGFBP-3) mRNA levels suggests a fall in IGFBP-3 levels may increase local IGF-I bioactivity. Polyamine synthesis is a critical component of the adaptive response, although the stimulus to their dramatic increase in synthesis after MSBR remains to be elucidated. Other humoral factors such as cholecystokinin, neurotensin and bombesin probably have minor indirect roles in the adaptive response. Components of the epidermal growth factor/transforming growth factor alpha response pathway family of growth factors may be involved as paracrine regulators. There is thus strong evidence that humoral factors play an important role in intestinal adaptation; characterization of the nature of the humoral factors and their relationship with other influences such as luminal nutrition and pancreatic biliary secretions may facilitate the development of new therapeutic strategies for the short bowel syndromes.