Insulinotropic hormone glucagon-like peptide 1 (GLP-1) activation of insulin gene promoter inhibited by p38 mitogen-activated protein kinase

Short-Term Protocols to Obtain Insulin-Producing Cells from Rat Adipose Tissue: Signaling Pathways and In Vivo Effect

Studies using mesenchymal stromal cells (MSCs) as a source of insulin-secreting cells (IPCs) are a promising path in the pursuit for diabetes therapy. Here, we investigate three short-term differentiation protocols in order to generate IPCs from autologous adipose-derived stromal cells (ADSCs) with an expressive insulin-secreting profile in vitro and in vivo, as well as the signaling pathways involved in the chosen differentiation protocols. We extracted and cultured ADSCs and differentiated them into IPCs, using three different protocols with different inductors. Afterwards, the secretory profile was analyzed and IPCs differentiated in exendin-4/activin A medium, which presented the best secretory profile, was implanted in the kidney subcapsular region of diabetic rats. All protocols induced the differentiation, but media supplemented with exendin-4/activin A or resveratrol induced the expression and secretion of insulin more efficiently, and only the exendin-4/activin-A-supplemented medium generated an insulin secretion profile more like β-cells, in response to glucose. The PI3K/Akt pathway seems to play a negative role in IPC differentiation; however, the differentiation of ADSCs with exendin-4/activin A positively modulated the p38/MAPK pathway. Resveratrol medium activated the Jak/STAT3 pathway and generated IPCs apparently less sensitive to insulin and insulin-like receptors. Finally, the implant of IPCs with the best secretory behavior caused a decrease in hyperglycemia after one-week implantation in diabetic rats. Our data provide further information regarding the generation of IPCs from ADSCs and strengthen evidence to support the use of MSCs in regenerative medicine, specially the use of exendin-4/activin A to produce rapid and effectively IPCs with significant in vivo effects.

A synopsis of factors regulating beta cell development and beta cell mass

The insulin-secreting beta cells in the endocrine pancreas regulate blood glucose levels, and loss of functional beta cells leads to insulin deficiency, hyperglycemia (high blood glucose) and diabetes mellitus. Current treatment strategies for type-1 (autoimmune) diabetes are islet transplantation, which has significant risks and limitations, or normalization of blood glucose with insulin injections, which is clearly not ideal. The type-1 patients can lack insulin counter-regulatory mechanism; therefore, hypoglycemia is a potential risk. Hence, a cell-based therapy offers a better alternative for the treatment of diabetes. Past research was focused on attempting to generate replacement beta cells from stem cells; however, recently there has been an increasing interest in identifying mechanisms that will lead to the conversion of pre-existing differentiated endocrine cells into beta cells. The goal of this review is to provide an overview of several of the key factors that regulate new beta cell formation (neogenesis) and beta cell proliferation.

Endogenous GLP-1 as a key self-defense molecule against lipotoxicity in pancreatic islets

The number of pro-α cells is known to increase in response to β cell injury and these cells then generate glucagon-like peptide-1 (GLP-1), thus attenuating the development of diabetes. The aim of the present study was to further examine the role and the mechanisms responsible for intra-islet GLP-1 production as a self-protective response against lipotoxicity. The levels of the key enzyme, prohormone convertase 1/3 (PC1/3), as well as the synthesis and release of GLP-1 in models of lipotoxicity were measured. Furthermore, islet viability, apoptosis, oxidative stress and inflammation, as well as islet structure were assessed after altering GLP-1 receptor signaling. Both prolonged exposure to palmitate and a high-fat diet facilitated PC1/3 expression, as well as the synthesis and release of GLP-1 induced by β cell injury and the generation of pro-α cells. Prolonged exposure to palmitate increased reactive oxygen species (ROS) production, and the antioxidant, N-acetylcysteine (NAC), partially prevented the detrimental effects induced by palmitate on β cells, resulting in decreased GLP-1 levels. Furthermore, the inhibition of GLP-1 receptor (GLP-1R) signaling by treatment with exendin‑(9-39) further decreased cell viability, increased cell apoptosis and caused a stronger inhibition of the β cell-specific transcription factor, pancreatic duodenal homeobox 1 (PDX1). Moreover, treatment with the GLP-1R agonist, liraglutide, normalized islet structure and function, resulting in a decrease in cell death and in the amelioration of β cell marker expression. Importantly, liraglutide maintained the oxidative balance and decreased inflammatory factor and p65 expression. Overall, our data demonstrate that an increase in the number of pro-α cells and the activation of the intra-islet GLP-1 system comprise a self-defense mechanism for enhancing β cell survival to combat lipid overload, which is in part mediated by oxidative stress and inflammation.

The effects of exendin-4 treatment on graft failure: an animal study using a novel re-vascularized minimal human islet transplant model

Islet transplantation has become a viable clinical treatment, but is still compromised by long-term graft failure. Exendin-4, a glucagon-like peptide 1 receptor agonist, has in clinical studies been shown to improve insulin secretion in islet transplanted patients. However, little is known about the effect of exendin-4 on other metabolic parameters. We therefore aimed to determine what influence exendin-4 would have on revascularized minimal human islet grafts in a state of graft failure in terms of glucose metabolism, body weight, lipid levels and graft survival. Introducing the bilateral, subcapsular islet transplantation model, we first transplanted diabetic mice with a murine graft under the left kidney capsule sufficient to restore normoglycemia. After a convalescent period, we performed a second transplantation under the right kidney capsule with a minimal human islet graft and allowed for a second recovery. We then performed a left-sided nephrectomy, and immediately started treatment with exendin-4 with a low (20μg/kg/day) or high (200μg/kg/day) dose, or saline subcutaneously twice daily for 15 days. Blood was sampled, blood glucose and body weight monitored. The transplanted human islet grafts were collected at study end point and analyzed. We found that exendin-4 exerts its effect on failing human islet grafts in a bell-shaped dose-response curve. Both doses of exendin-4 equally and significantly reduced blood glucose. Glucagon-like peptide 1 (GLP-1), C-peptide and pro-insulin were conversely increased. In the course of the treatment, body weight and cholesterol levels were not affected. However, immunohistochemistry revealed an increase in beta cell nuclei count and reduced TUNEL staining only in the group treated with a low dose of exendin-4 compared to the high dose and control. Collectively, these results suggest that exendin-4 has a potential rescue effect on failing, revascularized human islets in terms of lowering blood glucose, maintaining beta cell numbers, and improving metabolic parameters during hyperglycemic stress.

Molecular mechanisms underlying physiological and receptor pleiotropic effects mediated by GLP-1R activation

The incidence of type 2 diabetes in developed countries is increasing yearly with a significant negative impact on patient quality of life and an enormous burden on the healthcare system. Current biguanide and thiazolidinedione treatments for type 2 diabetes have a number of clinical limitations, the most serious long-term limitation being the eventual need for insulin replacement therapy (Table 1). Since 2007, drugs targeting the glucagon-like peptide-1 (GLP-1) receptor have been marketed for the treatment of type 2 diabetes. These drugs have enjoyed a great deal of success even though our underlying understanding of the mechanisms for their pleiotropic effects remain poorly characterized even while major pharmaceutical companies actively pursue small molecule alternatives. Coupling of the GLP-1 receptor to more than one signalling pathway (pleiotropic signalling) can result in ligand-dependent signalling bias and for a peptide receptor such as the GLP-1 receptor this can be exaggerated with the use of small molecule agonists. Better consideration of receptor signalling pleiotropy will be necessary for future drug development. This is particularly important given the recent failure of taspoglutide, the report of increased risk of pancreatitis associated with GLP-1 mimetics and the observed clinical differences between liraglutide, exenatide and the newly developed long-acting exenatide long acting release, albiglutide and dulaglutide.

Cyclic AMP sensor EPAC proteins and energy homeostasis

The pleiotropic second-messenger cAMP plays a crucial role in mediating the effects of various hormones on metabolism. The major intracellular functions of cAMP are transduced by protein kinase A (PKA) and by exchange proteins directly activated by cAMP (EPACs). The latter act as guanine-nucleotide exchange factors for the RAS-like small G proteins Rap1 and Rap2. Although the role of PKA in regulating energy balance has been extensively studied, the impact of EPACs remains relatively enigmatic. This review summarizes recent genetic and pharmacological studies concerning EPAC involvement in glucose homeostasis and energy balance via the regulation of leptin and insulin signaling pathways. In addition, the development of small-molecule EPAC-specific modulators and their therapeutic potential for the treatment of diabetes and obesity are discussed.

Identification of 3-aminomethyl-1,2-dihydro-4-phenyl-1-isoquinolones: a new class of potent, selective, and orally active non-peptide dipeptidyl peptidase IV inhibitors that form a unique interaction with Lys554

The design, synthesis, and structure-activity relationships of a new class of potent and orally active non-peptide dipeptidyl peptidase IV (DPP-4) inhibitors, 3-aminomethyl-1,2-dihydro-4-phenyl-1-isoquinolones, are described. We hypothesized that the 4-phenyl group of the isoquinolone occupies the S1 pocket of the enzyme, the 3-aminomethyl group forms an electrostatic interaction with the S2 pocket, and the introduction of a hydrogen bond donor onto the 6- or 7-substituent provides interaction with the hydrophilic region of the enzyme. Based on this hypothesis, intensive research focused on developing new non-peptide DPP-4 inhibitors has been carried out. Among the compounds designed in this study, we identified 2-[(3-aminomethyl-2-(2-methylpropyl)-1-oxo-4-phenyl-1,2-dihydro-6-isoquinolinyl)oxy]acetamide (35a) as a potent, selective, and orally bioavailable DPP-4 inhibitor, which exhibited in vivo efficacy in diabetic model rats. Finally, X-ray crystallography of 35a in a complex with the enzyme validated our hypothesized binding mode and identified Lys554 as a new target-binding site available for DPP-4 inhibitors.

ATF2 interacts with beta-cell-enriched transcription factors, MafA, Pdx1, and beta2, and activates insulin gene transcription

Pancreatic β-cell-restricted expression of insulin is established through several critical cis-regulatory elements located in the insulin gene promoter region. The principal cis elements are A-boxes, E1, and C1/RIPE3b. The β-cell-enriched transcription factors Pdx1 and Beta2 bind to the A-boxes and E1 element, respectively. A β-cell-specific trans-acting factor binding to C1/RIPE3b (termed RIPE3b1 activator) was detected by electrophoretic mobility shift assay and has been identified as MafA, a member of the Maf family of basic leucine zipper (bZip) proteins. Here, ATF2, a member of the ATF/CREB family of basic leucine zipper proteins, was identified as a component of the RIPE3b1 activator. ATF2 alone was unable to bind to the C1/RIPE3b element but acquired binding capacity upon complex formation with MafA. ATF2 also interacted with Pdx1 and Beta2, and co-expression of ATF2, MafA, Pdx1, and Beta2 resulted in a synergistic activation of the insulin promoter. Immunohistochemical analysis of mouse pancreas tissue sections showed that ATF2 is enriched in islet endocrine cells, including β-cells. RNAi-mediated knockdown of MafA or ATF2 in the MIN6 β-cell line resulted in a significant decrease in endogenous levels of insulin mRNA. These data indicate that ATF2 is an essential component of the positive regulators of the insulin gene expression.

R-spondin-1 is a novel beta-cell growth factor and insulin secretagogue

R-spondin-1 (Rspo1) is an intestinal growth factor known to exert its effects through activation of the canonical Wnt (cWnt) signaling pathway and subsequent expression of cWnt target genes. We have detected Rspo1 mRNA in murine islets and the murine MIN6 and betaTC beta-cell lines, and Rspo1 protein in MIN6 beta-cells. Rspo1 activated cWnt signaling in MIN6 beta-cells by increasing nuclear beta-catenin and c-myc, a cWnt target gene. Rspo1 also induced insulin mRNA expression in MIN6 cells. Analysis of MIN6 and mouse beta-cell proliferation by [(3)H]thymidine and BrdU incorporation, respectively, revealed that Rspo1 stimulated cell growth. Incubation of MIN6 and mouse beta-cells with cytokines (IL1beta/TNFalpha/interferon-gamma) significantly increased cellular apoptosis; this increase was abolished by pretreatment with Rspo1. Rspo1 also stimulated insulin secretion in a glucose-independent fashion. We further demonstrated that the glucagon-like peptide-1 receptor agonist, exendin4 (EX4), stimulated Rspo1 mRNA transcript levels in MIN6 cells in a glucose-, time-, dose-, and PI3-kinase-dependent fashion. This effect was not limited to this beta-cell line, as similar time-dependent increases in Rspo1 were also observed in the betaTC beta-cell line and mouse islets in response to EX4 treatment. Together, these studies demonstrate that Rspo1 is a novel beta-cell growth factor and insulin secretagogue that is regulated by EX4. These findings suggest that Rspo1 and the cWnt signaling pathway may serve as a novel target to enhance beta-cell growth and function in patients with type 2 diabetes.

Lack of preservation of insulin gene expression by a glucagon-like peptide 1 agonist or a dipeptidyl peptidase 4 inhibitor in an in vivo model of glucolipotoxicity

Prolonged exposure of pancreatic beta-cells to elevated levels of glucose and fatty acids adversely affects insulin secretion and gene expression.

AIM

To examine whether the GLP-1 agonist exenatide or the inhibitor of the GLP-1-degrading enzyme dipeptidyl peptidase 4 (DPP-4) sitagliptin rescue insulin gene expression in rats infused for 72h with glucose+Intralipid, independently from their glucose-lowering action.

METHODS

Wistar rats were infused alternatively with glucose or Intralipid for cycles of 4h each for a total of 72h. The animals received exenatide (5microg/kg/day IV) or sitagliptin (5mg/kg/day IV) continuously starting 4 days prior to and continuing throughout the 3-day infusion period.

RESULTS

Plasma glucose, fatty acids, insulin and C-peptide levels were unaffected by exenatide or sitagliptin treatment during the infusion period. Insulin mRNA levels increased in response to the glucose infusion, but this increase was abolished in islets from rats receiving glucose+Intralipid. Neither exenatide nor sitagliptin administration rescued insulin mRNA in glucose+Intralipid infused rats.

CONCLUSIONS

Neither a GLP-1 agonist nor a DPP-4 inhibitor, at doses that do not alter blood glucose levels, prevented the inhibition of insulin gene expression in this in vivo model of glucolipotoxicity.

Glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 modulate beta-cell chromatin structure

Chromatin can exert a regulatory effect on gene transcription by modulating the access of transcription factors to target genes. In the present study, we examined whether nuclear actions of the incretin hormones, glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1, involve modulation of beta-cell chromatin structure. Stimulation of INS-1(832/13) beta-cells or dispersed mouse islets with glucose-dependent insulinotropic polypeptide or glucagon-like peptide-1 resulted in the post-translational modification of core H3 histones, through acetylation and phosphorylation. Both increased histone H3 acetyltransferase and reduced histone deacetylase activities contributed. Subsequent studies demonstrated that incretin-mediated histone H3 modifications involved activation of protein kinase A, p42/44 mitogen-activated protein kinase (MAPK), and p38 MAPK signaling modules, resulting in the activation of mitogen- and stress-activated kinase-1. Additionally, modification of histone H3 increased its association with the transcription factor, phosphorylated cAMP-response element-binding protein (phospho-CREB) and with cAMP-responsive CREB coactivator 2. Incretin-activated CREB-related Bcl-2 transcription was greatly reduced by a histone acetyltransferase inhibitor, demonstrating the functional importance of histone H3 modification. This appears to be the first demonstration of beta-cell chromatin modification in response to the incretins and the studies indicate that their regulatory effects involve coordinated nuclear interactions between specific signaling modules, chromatin-modifying enzymes and transcription factors.

The physiology of glucagon-like peptide 1

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells by differential processing of proglucagon, the gene which is expressed in these cells. The current knowledge regarding regulation of proglucagon gene expression in the gut and in the brain and mechanisms responsible for the posttranslational processing are reviewed. GLP-1 is released in response to meal intake, and the stimuli and molecular mechanisms involved are discussed. GLP-1 is extremely rapidly metabolized and inactivated by the enzyme dipeptidyl peptidase IV even before the hormone has left the gut, raising the possibility that the actions of GLP-1 are transmitted via sensory neurons in the intestine and the liver expressing the GLP-1 receptor. Because of this, it is important to distinguish between measurements of the intact hormone (responsible for endocrine actions) or the sum of the intact hormone and its metabolites, reflecting the total L-cell secretion and therefore also the possible neural actions. The main actions of GLP-1 are to stimulate insulin secretion (i.e., to act as an incretin hormone) and to inhibit glucagon secretion, thereby contributing to limit postprandial glucose excursions. It also inhibits gastrointestinal motility and secretion and thus acts as an enterogastrone and part of the "ileal brake" mechanism. GLP-1 also appears to be a physiological regulator of appetite and food intake. Because of these actions, GLP-1 or GLP-1 receptor agonists are currently being evaluated for the therapy of type 2 diabetes. Decreased secretion of GLP-1 may contribute to the development of obesity, and exaggerated secretion may be responsible for postprandial reactive hypoglycemia.

Targeting glucagon receptor signalling in treating metabolic syndrome and renal injury in Type 2 diabetes: theory versus promise

Pancreatic bi-hormones insulin and glucagon are the Yin and Yang in the regulation of glucose metabolism and homoeostasis. Insulin is synthesized primarily by pancreatic beta-cells and is released in response to an increase in blood glucose levels (hyperglycaemia). By contrast, glucagon is synthesized by pancreatic alpha-cells and is released in response to a decrease in blood glucose (hypoglycaemia). The principal role of glucagon is to counter the actions of insulin on blood glucose homoeostasis, but it also has diverse non-hyperglycaemic actions. Although Type 1 diabetes is caused by insulin deficiency (insulin-dependent) and can be corrected by insulin replacement, Type 2 diabetes is a multifactorial disease and its treatment is not dependent on insulin therapy alone. Type 2 diabetes in humans is characterized by increased insulin resistance, increased fasting blood glucose, impaired glucose tolerance and the development of glomerular hyperfiltration and microalbuminuria, ultimately leading to diabetic nephropathy and end-stage renal disease. Clinical studies have suggested that an inappropriate increase in hyperglycaemic glucagon (hyperglucagonaemia) over hypoglycaemic insulin (not insulin deficiency until advanced stages) plays an important role in the pathogenesis of Type 2 diabetes. However, for decades, research efforts and resources have been devoted overwhelmingly to studying the role of insulin and insulin-replacement therapy. By contrast, the implication of glucagon and its receptor signalling in the development of Type 2 diabetic metabolic syndromes and end-organ injury has received little attention. The aim of this review is to examine the evidence as to whether glucagon and its receptor signalling play any role(s) in the pathogenesis of Type 2 diabetic renal injury, and to explore whether targeting glucagon receptor signalling remains only a theoretical antidiabetic strategy in Type 2 diabetes or may realize its promise in the future.

Mechanisms of action of glucagon-like peptide 1 in the pancreas

Glucagon-like peptide 1 (GLP-1) is a hormone that is encoded in the proglucagon gene. It is mainly produced in enteroendocrine L cells of the gut and is secreted into the blood stream when food containing fat, protein hydrolysate, and/or glucose enters the duodenum. Its particular effects on insulin and glucagon secretion have generated a flurry of research activity over the past 20 years culminating in a naturally occurring GLP-1 receptor (GLP-1R) agonist, exendin 4 (Ex-4), now being used to treat type 2 diabetes mellitus (T2DM). GLP-1 engages a specific guanine nucleotide-binding protein (G-protein) coupled receptor (GPCR) that is present in tissues other than the pancreas (brain, kidney, lung, heart, and major blood vessels). The most widely studied cell activated by GLP-1 is the insulin-secreting beta cell where its defining action is augmentation of glucose-induced insulin secretion. Upon GLP-1R activation, adenylyl cyclase (AC) is activated and cAMP is generated, leading, in turn, to cAMP-dependent activation of second messenger pathways, such as the protein kinase A (PKA) and Epac pathways. As well as short-term effects of enhancing glucose-induced insulin secretion, continuous GLP-1R activation also increases insulin synthesis, beta cell proliferation, and neogenesis. Although these latter effects cannot be currently monitored in humans, there are substantial improvements in glucose tolerance and increases in both first phase and plateau phase insulin secretory responses in T2DM patients treated with Ex-4. This review will focus on the effects resulting from GLP-1R activation in the pancreas.

Glucagon receptor-mediated extracellular signal-regulated kinase 1/2 phosphorylation in rat mesangial cells: role of protein kinase A and phospholipase C

Glucagon, a major insulin counterregulatory hormone, binds to specific Gs protein-coupled receptors to activate glycogenolytic and gluconeogenic pathways, causing blood glucose levels to increase. Inappropriate increases in serum glucagon play a critical role in the development of insulin resistance and target organ damage in type 2 diabetes. We tested the hypotheses that: (1) glucagon induces proliferation of rat glomerular mesangial cells through glucagon receptor-activated phosphorylation of mitogen-activated protein kinase extracellular signal-regulated kinase 1/2 (p-ERK 1/2); and (2) this phosphorylation involves activation of cAMP-dependent protein kinase A (PKA) and phospholipase C (PLC)/[Ca2+]i signaling pathways. In rat mesangial cells, glucagon (1 nM) stimulated [3H]-thymidine incorporation by 96% (P<0.01). This proliferative effect was blocked by the specific glucagon receptor antagonist [Des-His1-Glu9] glucagon (1 micromol/L; P<0.01), a mitogen-activated protein kinase/ERK kinase inhibitor PD98059 (10 micromol/L; P<0.01), a PLC inhibitor U73122 (1 micromol/L; P<0.01), or a PKA inhibitor H-89 (1 micromol/L; P<0.01). The proliferation was associated with a 2-fold increase in p-ERK 1/2 that peaked 5 minutes after glucagon stimulation (P<0.01) and also was blocked by [Des-His1-Glu9] glucagon. Total ERK 1/2 was not affected by glucagon. Pretreating of mesangial cells with U73122 or H89 significantly attenuated ERK 1/2 phosphorylation induced by glucagon. We believe that these are the first data showing that glucagon activates specific receptors to induce ERK 1/2 phosphorylation and thereby increase mesangial cell proliferation and that this effect of glucagon involves both PLC/[Ca2+]i- and cAMP-dependent PKA-activated signaling cascades.

Exendin-4, a glucagon-like protein-1 (GLP-1) receptor agonist, reverses hepatic steatosis in ob/ob mice

Nonalcoholic fatty liver disease (NAFLD) represents a burgeoning problem in hepatology, and is associated with insulin resistance. Exendin-4 is a peptide agonist of the glucagon-like peptide (GLP) receptor that promotes insulin secretion. The aim of this study was to determine whether administration of Exendin-4 would reverse hepatic steatosis in ob/ob mice. Ob/ob mice, or their lean littermates, were treated with Exendin-4 [10 microg/kg or 20 microg/kg] for 60 days. Serum was collected for measurement of insulin, adiponectin, fasting glucose, lipids, and aminotransferase concentrations. Liver tissue was procured for histological examination, real-time RT-PCR analysis and assay for oxidative stress. Rat hepatocytes were isolated and treated with GLP-1. Ob/ob mice sustained a reduction in the net weight gained during Exendin-4 treatment. Serum glucose and hepatic steatosis was significantly reduced in Exendin-4 treated ob/ob mice. Exendin-4 improved insulin sensitivity in ob/ob mice, as calculated by the homeostasis model assessment. The measurement of thiobarbituric reactive substances as a marker of oxidative stress was significantly reduced in ob/ob-treated mice with Exendin-4. Finally, GLP-1-treated hepatocytes resulted in a significant increase in cAMP production as well as reduction in mRNA expression of stearoyl-CoA desaturase 1 and genes associated with fatty acid synthesis; the converse was true for genes associated with fatty acid oxidation. In conclusion, Exendin-4 appears to effectively reverse hepatic steatosis in ob/ob mice by improving insulin sensitivity. Our data suggest that GLP-1 proteins in liver have a novel direct effect on hepatocyte fat metabolism.

Cross-talk between phosphatidylinositol 3-kinase/AKT and c-jun NH2-terminal kinase mediates survival of isolated human islets

Therapeutic strategies aimed at the inhibition of specific cell death mechanisms may increase islet yield and improve cell viability and function after routine isolation. The aim of the current study was to explore the possibility of AKT-JNK cross-talk in islets after isolation and the relevance of c-jun NH2-terminal kinases (JNK) suppression on islet survival. After routine isolation, increased AKT activity correlated with suppression of JNK activation, suggesting that they may be related events. Indeed, the increase in AKT activation after isolation correlated with suppression of apoptosis signal-regulating kinase 1 (ASK1), a kinase acting upstream of JNK, by phosphorylation at Ser83. We therefore examined whether modulators of phosphatidylinositol 3-kinase (PI3K)/AKT signaling affected JNK activation. PI3K inhibition led to increased JNK phosphorylation and islet cell death, which could be reversed by the specific JNK inhibitor SP600125. In addition, IGF-I suppressed cytokine-mediated JNK activation in a PI3K-dependent manner. We also demonstrate that inhibition of PI3K rendered islets more susceptible to cytokine-mediated cell death. SP600125 transiently protected islets from cytokine-mediated cell death, suggesting that JNK may not be necessary for cytokine-induced cell death. When administered immediately after isolation, SP600125 improved islet survival and function, even 48 h after removal of SP600125, suggesting that JNK inhibition by SP600125 may be a viable strategy for improving isolated islet survival. Taken together, these results demonstrate that PI3K/AKT suppresses the JNK pathway in islets, and this cross-talk represents an important antiapoptotic consequence of PI3K/AKT activation.

Parathyroid hormone-related protein induces insulin expression through activation of MAP kinase-specific phosphatase-1 that dephosphorylates c-Jun NH2-terminal kinase in pancreatic beta-cells

Parathyroid hormone-related protein (PTHrP) increases the content and mRNA level of insulin in a mouse beta-cell line, MIN6, and primary-cultured mouse islets. We examined the mechanism of PTHrP-induced insulin expression. The PTHrP effect was markedly augmented by SB203580, a mitogen-activated protein (MAP) kinase inhibitor, and SB203580 itself increased insulin expression extensively, even without PTHrP. Because SB203580 inhibits both p38 and c-jun NH(2)-terminal kinases (JNKs), we investigated the JNK-specific inhibitor SP600125. SP600125 also increased insulin content and its mRNA level. PTHrP induced dephosphorylation of JNK1/2, and PTHrP-induced insulin expression was blocked by a dominant-negative type JNK-APF. We suspected that dual specificity MAP kinase phosphatases (MKPs) may be involved in the PTHrP-induced insulin expression by inactivating JNK1/2. MIN6 cells contained at least five MKPs, among which only MKP-1 was inducible by PTHrP. PTHrP-induced insulin expression was blocked by the MKP-1 expression inhibitor Ro-31-8220, indicating that the PTHrP effect is mediated by MKP-1. Indeed, adenoviral MKP-1 expression increased insulin expression by decreasing a phosphorylation form of JNKs and a resulting phosphorylated form of c-jun in MIN6 cells. The phosphorylated form of c-jun is known to repress cAMP-dependent insulin gene promoter activity. Thus, MKP-1 controls the insulin expression by downregulating a JNK/c-jun pathway.

The transcriptional activator Mirk/Dyrk1B is sequestered by p38alpha/beta MAP kinase

Mirk/Dyrk1B protein kinase was shown in an earlier study to function as a transcriptional activator of HNF1alpha, which Mirk phosphorylates at Ser(249) within its CREB (cAMP-response element-binding protein)-binding protein (CBP) binding domain (). The MAPK kinase MKK3 was also shown to activate Mirk as a protein kinase, implicating Mirk in the biological response to certain stress agents. Another MKK3 substrate, p38MAPK, is now shown to inhibit the function of Mirk as a transcriptional activator in a kinase-independent manner. Co-immunoprecipitation experiments demonstrated that kinase-inactive p38AF, as well as wild-type p38, sequestered Mirk and prevented its association with MKK3. Only the p38alpha and p38beta isoforms, but not the gamma or delta isoforms, complexed with Mirk. p38alphaMAPK blocked Mirk activation of HNF1alpha in a dose-dependent manner, with high levels of kinase-inactive p38alphaAF completely suppressing the activity of Mirk. Size fractionation by fast protein liquid chromatography on Superdex 200 demonstrated that Mirk is not found as a monomer in vivo, but is found within 150-700 kDa subnuclear complexes, which co-migrate with the nuclear body scaffolding protein PML. Endogenous Mirk, p38, and MKK3 co-migrate within 500-700-kDa protein complexes, which accumulate when nuclear export is blocked by leptomycin B. Stable overexpression of Mirk increases the fraction of Mirk protein and p38 protein within these 500-700 kDa complexes, suggesting that the complexes act as nuclear depots for Mirk and p38. Sequestration of Mirk by p38 may occur within these subnuclear complexes. Synchronization experiments demonstrated that Mirk levels fluctuate about 10-fold within the cell cycle, while p38 levels do not, leading to the speculation that endogenous p38 could only block Mirk function when Mirk levels were low in S phase and not when Mirk levels were elevated in G(0)/G(1). These data suggest a novel cell cycle-dependent function for p38, suppression of the function of Mirk as a transcriptional activator only when cells are proliferating, and thus limiting Mirk function to growth-arrested cells.

Exendin-4 as a stimulator of rat insulin I gene promoter activity via bZIP/CRE interactions sensitive to serine/threonine protein kinase inhibitor Ro 31-8220

Signal transduction properties of exendin-4 (Ex-4) underlying its ability to stimulate rat insulin I gene promoter (RIP1) activity were assessed in the pancreatic beta-cell line INS-1. Ex-4 acted via glucagon-like peptide-1 receptors to stimulate RIP1 in a glucose-dependent manner, as measured in cells transfected with a -410-bp RIP1-luciferase construct (RIP1-Luc). The action of Ex-4 was independent of cAMP and PKA because it was not blocked by cotransfection with dominant-negative G alpha(s), was unaffected by pretreatment with the membrane-permeant cAMP antagonist 8-Br-Rp-cAMPS, and remained apparent after treatment with PKA inhibitors H-89 or KT 5720. Similarly, cotransfection with a dominant-negative isoform of the type-2 cAMP-regulated guanine nucleotide exchange factor (Epac2) failed to alter the response to Ex-4. Ro 31-8220, a serine/threonine protein kinase inhibitor that targets PKC as as well as the 90-kDa ribosomal S6 kinase (RSK) and mitogen- and stress-activated protein kinase (MSK) family of cAMP response element-binding protein (CREB) kinases, blocked the stimulatory action of Ex-4 at RIP1-Luc. However, selective inhibition of PKC using K-252c, prolonged exposure to phorbol 1,2-myristate-13-acetate, or cotransfection with dominant-negative atypical PKC-zeta, was without effect. A-CREB, a dominant-negative inhibitor of basic region-leucine zipper transcription factors (bZIPs) related in structure to CREB, inhibited the action of Ex-4 at RIP1-Luc, whereas A-ATF-2 was ineffective. Similarly, introduction of deletions at the RIP1 cAMP response element (CRE), or truncation of RIP1 to remove the CRE, nearly abolished the action of Ex-4. Inactivating mutations introduced at the A4/A3 elements, binding sites for the glucose-regulated homeodomain transcription factor PDX-1, did not diminish the response to Ex-4, although a marked reduction of basal promoter activity was observed. The glucose-dependent stimulation of RIP1-Luc by Ex-4 was reproduced using a synthetic reporter (RIP1-CRE-Luc) incorporating multimerized CREs of the RIP1 nonpalindromic sequence 5'-TGACGTCC-3'. It is concluded that the bZIP and CRE-mediated stimulation of RIP1 by Ex-4 explains, at least in part, how this insulinotropic hormone facilitates transcriptional activity of the rat insulin I gene.

NFAT regulates insulin gene promoter activity in response to synergistic pathways induced by glucose and glucagon-like peptide-1

Currently there is intense interest to define the mechanism of action of glucagon-like peptide-1 (GLP-1) in regulating beta-cell function, including insulin gene transcription. In this study, GLP-1 (100 nmol/l), in the presence of glucose (11 mmol/l), induced a similar71-fold increase in insulin gene promoter activity in INS-1 pancreatic beta-cells, an effect that was an order of magnitude larger than with either stimulant alone. The response to GLP-1 was mimicked by forskolin and largely inhibited by the protein kinase A (PKA) inhibitors, H89 and myristoylated PKI(14--22) amide, indicating partial mediation via a cAMP/PKA pathway. Significantly, the actions of both GLP-1 and forskolin were abolished by the selective Ca(2+)/calmodulin-dependent phosphatase 2B (calcineurin) inhibitor, FK506, as well as by the chelation of intracellular Ca(2+) by BAPTA (bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate). Glucose and GLP-1 also synergistically activated NFAT (nuclear factor of activated T-cells)-mediated transcription from a minimal promoter construct containing tandem NFAT consensus sequences. Furthermore, two-point base pair mutations in any of the three identified NFAT sites within the rat insulin I promoter resulted in a significant reduction in the combined effect of glucose and GLP-1. These data suggest that the synergistic action of glucose and GLP-1 to promote insulin gene transcription is mediated through NFAT via PKA- and calcineurin-dependent pathways in pancreatic beta-cells.

Cooperation of multiple signaling pathways in CD40-regulated gene expression in B lymphocytes

CD40/CD40L interaction is essential for multiple biological events in T dependent humoral immune responses, including B cell survival and proliferation, germinal center and memory B cell formation, and antibody isotype switching and affinity maturation. By using high-density microarrays, we examined gene expression in primary mouse B lymphocytes after multiple time points of CD40L stimulation. In addition to genes involved in cell survival and growth, which are also induced by other mitogens such as lipopolysaccharide, CD40L specifically activated genes involved in germinal center formation and T cell costimulatory molecules that facilitate T dependent humoral immunity. Next, by examining the roles of individual CD40-activated signal transduction pathways, we dissected the overall CD40-mediated response into genes independently regulated by the individual pathways or collectively by all pathways. We also found that gene down-regulation is a significant part of the overall response and that the p38 pathway plays an important role in this process, whereas the NF-kappa B pathway is important for the up-regulation of primary response genes. Our finding of overlapping independent control of gene expression modules by different pathways suggests, in principle, that distinct biological behaviors that depend on distinct gene expression subsets can be manipulated by targeting specific signaling pathways.