The MAPK kinase kinase-1 is essential for stress-induced pancreatic islet cell death

Regulation and function of AP-1 in insulinoma cells and pancreatic β-cells

Ca_v1.2 L-type voltage-gated Ca²⁺ channels play a central role in pancreatic β-cells by integrating extracellular signals with intracellular signaling events leading to insulin secretion and altered gene transcription. Here, we investigated the intracellular signaling pathway following stimulation of Ca_v1.2 Ca²⁺ channels and addressed the function of the transcription factor activator protein-1 (AP-1) in pancreatic β-cells of transgenic mice. Stimulation of Ca_v1.2 Ca²⁺ channels activates AP-1 in insulinoma cells. Pharmacological and genetic experiments identified c-Jun N-terminal protein kinase as a signal transducer connecting Ca_v1.2 Ca²⁺ channel activation with gene transcription. Moreover, the basic region-leucine zipper proteins ATF2 and c-Jun or c-Jun-related proteins were involved in stimulus-transcription coupling. We addressed the functions of AP-1 in pancreatic β-cells analyzing a newly generated transgenic mouse model. These transgenic mice expressed A-Fos, a mutant of c-Fos that attenuates DNA binding of c-Fos dimerization partners. In insulinoma cells, A-Fos completely blocked AP-1 activation following stimulation of Ca_v1.2 Ca²⁺ channels. The analysis of transgenic A-Fos-expressing mice revealed that the animals displayed impaired glucose tolerance. Thus, we show here for the first time that AP-1 controls an important function of pancreatic β-cells in vivo, the regulation of glucose homeostasis.

Glucose homeostasis is regulated by pancreatic β-cell cilia via endosomal EphA-processing

Diabetes mellitus affects one in eleven adults worldwide. Most suffer from Type 2 Diabetes which features elevated blood glucose levels and an inability to adequately secrete or respond to insulin. Insulin producing β-cells have primary cilia which are implicated in the regulation of glucose metabolism, insulin signaling and secretion. To better understand how β-cell cilia affect glucose handling, we ablate cilia from mature β-cells by deleting key cilia component Ift88. Here we report that glucose homeostasis and insulin secretion deteriorate over 12 weeks post-induction. Cilia/basal body components are required to suppress spontaneous auto-activation of EphA3 and hyper-phosphorylation of EphA receptors inhibits insulin secretion. In β-cells, loss of cilia/basal body function leads to polarity defects and epithelial-to-mesenchymal transition. Defective insulin secretion from IFT88-depleted human islets and elevated pEPHA3 in islets from diabetic donors both point to a role for cilia/basal body proteins in human glucose homeostasis.

Primary cilia have been proposed to regulate glucose metabolism and insulin secretion in beta cells, but it is not known how. Here the authors show that primary cilia play a role in adult β-cell function via a mechanism involving endosomal EphA-processing.

PAHSAs attenuate immune responses and promote β cell survival in autoimmune diabetic mice

Palmitic acid esters of hydroxy stearic acids (PAHSAs) are endogenous antidiabetic and antiinflammatory lipids. Here, we show that PAHSAs protect against type 1 diabetes (T1D) and promote β cell survival and function. Daily oral PAHSA administration to nonobese diabetic (NOD) mice delayed the onset of T1D and markedly reduced the incidence of T1D, whether PAHSAs were started before or after insulitis was established. PAHSAs reduced T and B cell infiltration and CD4+ and CD8+ T cell activation, while increasing Treg activation in pancreata of NOD mice. PAHSAs promoted β cell proliferation in both NOD mice and MIN6 cells and increased the number of β cells in NOD mice. PAHSAs attenuated cytokine-induced apoptotic and necrotic β cell death and increased β cell viability. The mechanism appears to involve a reduction of ER stress and MAPK signaling, since PAHSAs lowered ER stress in NOD mice, suppressed thapsigargin-induced PARP cleavage in human islets, and attenuated ERK1/2 and JNK1/2 activation in MIN6 cells. This appeared to be mediated in part by glucagon-like peptide 1 receptor (GLP-1R) and not the G protein-coupled receptor GPR40. PAHSAs also prevented impairment of glucose-stimulated insulin secretion and improved glucose tolerance in NOD mice. Thus, PAHSAs delayed the onset of T1D and reduced its incidence by attenuating immune responses and exerting direct protective effects on β cell survival and function.

SRp55 Regulates a Splicing Network That Controls Human Pancreatic β-Cell Function and Survival

Progressive failure of insulin-producing β-cells is the central event leading to diabetes, but the signaling networks controlling β-cell fate remain poorly understood. Here we show that SRp55, a splicing factor regulated by the diabetes susceptibility gene GLIS3, has a major role in maintaining the function and survival of human β-cells. RNA sequencing analysis revealed that SRp55 regulates the splicing of genes involved in cell survival and death, insulin secretion, and c-Jun N-terminal kinase (JNK) signaling. In particular, SRp55-mediated splicing changes modulate the function of the proapoptotic proteins BIM and BAX, JNK signaling, and endoplasmic reticulum stress, explaining why SRp55 depletion triggers β-cell apoptosis. Furthermore, SRp55 depletion inhibits β-cell mitochondrial function, explaining the observed decrease in insulin release. These data unveil a novel layer of regulation of human β-cell function and survival, namely alternative splicing modulated by key splicing regulators such as SRp55, that may cross talk with candidate genes for diabetes.

Metformin prevents glucotoxicity by alleviating oxidative and ER stress-induced CD36 expression in pancreatic beta cells

AIM/HYPOTHESIS

Cluster determinant 36 (CD36), a fatty acid transporter, was reported to have a pivotal role in glucotoxicity-induced beta cell dysfunction. However, little is known about how glucotoxicity influences CD36 expression, and it is unknown whether this action can be counteracted by metformin. In the present study, we showed that metformin counteracts glucotoxicity by alleviating oxidative and endoplasmic reticulum (ER) stress-induced CD36 expression.

METHODS

We used primary rat islets as well as INS-1 cells for 72h to 24h with 30mM glucose, respectively. Thapsigargin was used as strong ER stressor, and Sulfo-N-succinimidyl oleate (SSO) and RNA interference were chosen for CD36 inhibition. Free fatty acid uptake was measured by radioisotope tracing technique.

RESULTS

Exposure of isolated rat islets to high glucose (HG) for 3days decreased insulin and pancreatic duodenal homeobox1 (Pdx1) mRNA expression, with the suppression of glucose-stimulated insulin secretion (GSIS) along with elevation of reactive oxygen species (ROS) levels. Incubation with metformin restored insulin and Pdx1 mRNA expression with significant improvements in GSIS and decrease in ROS production. HG exposure in INS-1 cells increased free fatty acid uptake via induction of CD36 along with impaired insulin and Pdx1 mRNA expression. Moreover, thapsigargin also increased the induction of CD36 expression. Metformin blocked HG- and thapsigargin-induced CD36 expression. In addition, the simultaneous inhibition of intracellular ROS production by metformin or CD36 activation by SSO or CD36 siRNA significantly decreased the apoptotic response in HG-treated INS-1 cells.

CONCLUSION/INTERPRETATION

In conclusion, metformin conferred protection against HG-induced apoptosis of pancreatic beta cells, largely by interfering with ROS production, and inhibited the CD36-mediated free fatty acid influx. This report provides evidence that the inhibition of CD36 may have potential therapeutic effects against hyperglycemia-induced beta cell damage in diabetes.

Rac1-NADPH oxidase signaling promotes CD36 activation under glucotoxic conditions in pancreatic beta cells

We recently reported that cluster determinant 36 (CD36), a fatty acid transporter, plays a pivotal role in glucotoxicity-induced β-cell dysfunction. However, little is known about how glucotoxicity influences CD36 expression. Emerging evidence suggests that the small GTPase Rac1 is involved in the pathogenesis of beta cell dysfunction in type 2 diabetes (T2D). The primary objective of the current study was to determine the role of Rac1 in CD36 activation and its impact on β-cell dysfunction in diabetes mellitus. To address this question, we subjected INS-1 cells and human beta cells (1.1B4) to high glucose conditions (30mM) in the presence or absence of Rac1 inhibition either by NSC23766 (Rac1 GTPase inhibitor) or small interfering RNA. High glucose exposure in INS-1 and human beta cells (1.1b4) resulted in the activation of Rac1 and induced cell apoptosis. Rac1 activation mediates NADPH oxidase (NOX) activation leading to elevated ROS production in both cells. Activation of the Rac1-NOX complex by high glucose levels enhanced CD36 expression in INS-1 and human 1.1b4 beta cell membrane fractions. The inhibition of Rac1 by NSC23766 inhibited NADPH oxidase activity and ROS generation induced by high glucose concentrations in INS-1 & human 1.1b4 beta cells. Inhibition of Rac1-NOX complex activation by NSC23766 significantly reduced CD36 expression in INS-1 and human 1.1b4 beta cell membrane fractions. In addition, Rac1 inhibition by NSC23766 significantly reduced high glucose-induced mitochondrial dysfunction. Furthermore, NADPH oxidase inhibition by VAS2870 also attenuated high glucose-induced ROS generation and cell apoptosis. These results suggest that Rac1-NADPH oxidase dependent CD36 expression contributes to high glucose-induced beta cell dysfunction and cell death.

JNK Signaling: Regulation and Functions Based on Complex Protein-Protein Partnerships

The c-Jun N-terminal kinases (JNKs), as members of the mitogen-activated protein kinase (MAPK) family, mediate eukaryotic cell responses to a wide range of abiotic and biotic stress insults. JNKs also regulate important physiological processes, including neuronal functions, immunological actions, and embryonic development, via their impact on gene expression, cytoskeletal protein dynamics, and cell death/survival pathways. Although the JNK pathway has been under study for >20 years, its complexity is still perplexing, with multiple protein partners of JNKs underlying the diversity of actions. Here we review the current knowledge of JNK structure and isoforms as well as the partnerships of JNKs with a range of intracellular proteins. Many of these proteins are direct substrates of the JNKs. We analyzed almost 100 of these target proteins in detail within a framework of their classification based on their regulation by JNKs. Examples of these JNK substrates include a diverse assortment of nuclear transcription factors (Jun, ATF2, Myc, Elk1), cytoplasmic proteins involved in cytoskeleton regulation (DCX, Tau, WDR62) or vesicular transport (JIP1, JIP3), cell membrane receptors (BMPR2), and mitochondrial proteins (Mcl1, Bim). In addition, because upstream signaling components impact JNK activity, we critically assessed the involvement of signaling scaffolds and the roles of feedback mechanisms in the JNK pathway. Despite a clarification of many regulatory events in JNK-dependent signaling during the past decade, many other structural and mechanistic insights are just beginning to be revealed. These advances open new opportunities to understand the role of JNK signaling in diverse physiological and pathophysiological states.

Nitric oxide is a mediator of antiproliferative effects induced by proinflammatory cytokines on pancreatic beta cells

Nitric oxide (NO) is involved in several biological processes. In type 1 diabetes mellitus (T1DM), proinflammatory cytokines activate an inducible isoform of NOS (iNOS) in β cells, thus increasing NO levels and inducing apoptosis. The aim of the current study is to determine the role of NO (1) in the antiproliferative effect of proinflammatory cytokines IL-1β, IFN-γ, and TNF-α on cultured islet β cells and (2) during the insulitis stage prior to diabetes onset using the Biobreeding (BB) rat strain as T1DM model. Our results indicate that NO donors exert an antiproliferative effect on β cell obtained from cultured pancreatic islets, similar to that induced by proinflammatory cytokines. This cytokine-induced antiproliferative effect can be reversed by L-NMMA, a general NOS inhibitor, and is independent of guanylate cyclase pathway. Assays using NOS isoform specific inhibitors suggest that the NO implicated in the antiproliferative effect of proinflammatory cytokines is produced by inducible NOS, although not in an exclusive way. In BB rats, early treatment with L-NMMA improves the initial stage of insulitis. We conclude that NO is an important mediator of antiproliferative effect induced by proinflammatory cytokines on cultured β cell and is implicated in β-cell proliferation impairment observed early from initial stage of insulitis.

Genome-wide association study for type 2 diabetes in Indians identifies a new susceptibility locus at 2q21

Indians undergoing socioeconomic and lifestyle transitions will be maximally affected by epidemic of type 2 diabetes (T2D). We conducted a two-stage genome-wide association study of T2D in 12,535 Indians, a less explored but high-risk group. We identified a new type 2 diabetes-associated locus at 2q21, with the lead signal being rs6723108 (odds ratio 1.31; P = 3.32 × 10⁻⁹). Imputation analysis refined the signal to rs998451 (odds ratio 1.56; P = 6.3 × 10⁻¹²) within TMEM163 that encodes a probable vesicular transporter in nerve terminals. TMEM163 variants also showed association with decreased fasting plasma insulin and homeostatic model assessment of insulin resistance, indicating a plausible effect through impaired insulin secretion. The 2q21 region also harbors RAB3GAP1 and ACMSD; those are involved in neurologic disorders. Forty-nine of 56 previously reported signals showed consistency in direction with similar effect sizes in Indians and previous studies, and 25 of them were also associated (P < 0.05). Known loci and the newly identified 2q21 locus altogether explained 7.65% variance in the risk of T2D in Indians. Our study suggests that common susceptibility variants for T2D are largely the same across populations, but also reveals a population-specific locus and provides further insights into genetic architecture and etiology of T2D.

Cadmium induces apoptosis in pancreatic β-cells through a mitochondria-dependent pathway: the role of oxidative stress-mediated c-Jun N-terminal kinase activation

Cadmium (Cd), one of well-known highly toxic environmental and industrial pollutants, causes a number of adverse health effects and diseases in humans. The growing epidemiological studies have suggested a possible link between Cd exposure and diabetes mellitus (DM). However, the toxicological effects and underlying mechanisms of Cd-induced pancreatic β-cell injury are still unknown. In this study, we found that Cd significantly decreased cell viability, and increased sub-G1 hypodiploid cells and annexin V-Cy3 binding in pancreatic β-cell-derived RIN-m5F cells. Cd also increased intracellular reactive oxygen species (ROS) generation and malondialdehyde (MDA) production and induced mitochondrial dysfunction (the loss of mitochondrial membrane potential (MMP) and the increase of cytosolic cytochrome c release), the decreased Bcl-2 expression, increased p53 expression, poly (ADP-ribose) polymerase (PARP) cleavage, and caspase cascades, which accompanied with intracellular Cd accumulation. Pretreatment with the antioxidant N-acetylcysteine (NAC) effectively reversed these Cd-induced events. Furthermore, exposure to Cd induced the phosphorylations of c-jun N-terminal kinases (JNK), extracellular signal-regulated kinases (ERK)1/2, and p38-mitogen-activated protein kinase (MAPK), which was prevented by NAC. Additionally, the specific JNK inhibitor SP600125 or JNK-specific small interference RNA (si-RNA) transfection suppressed Cd-induced β-cell apoptosis and related signals, but not ERK1/2 and p38-MAPK inhibitors (PD98059 and SB203580) did not. However, the JNK inhibitor or JNK-specific si-RNA did not suppress ROS generation in Cd-treated cells. These results indicate that Cd induces pancreatic β-cell death via an oxidative stress downstream-mediated JNK activation-triggered mitochondria-regulated apoptotic pathway.

Arsenic and the epigenome: interindividual differences in arsenic metabolism related to distinct patterns of DNA methylation

Biotransformation of inorganic arsenic (iAs) is one of the factors that determines the character and magnitude of the diverse detrimental health effects associated with chronic iAs exposure, but it is unknown how iAs biotransformation may impact the epigenome. Here, we integrated analyses of genome-wide, gene-specific promoter DNA methylation levels of peripheral blood leukocytes with urinary arsenical concentrations of subjects from a region of Mexico with high levels of iAs in drinking water. These analyses revealed dramatic differences in DNA methylation profiles associated with concentrations of specific urinary metabolites of arsenic (As). The majority of individuals in this study had positive indicators of As-related disease, namely pre-diabetes mellitus or diabetes mellitus (DM). Methylation patterns of genes with known associations with DM were associated with urinary concentrations of specific iAs metabolites. Future studies will determine whether these DNA methylation profiles provide mechanistic insight into the development of iAs-associated disease, predict disease risk, and/or serve as biomarkers of iAs exposure in humans.

Streptozotocin-nicotinamide-induced diabetes in the rat. Characteristics of the experimental model

Administration of both streptozotocin (STZ) and nicotinamide (NA) has been proposed to induce experimental diabetes in the rat. STZ is well known to cause pancreatic B-cell damage, whereas NA is administered to rats to partially protect insulin-secreting cells against STZ. STZ is transported into B-cells via the glucose transporter GLUT2 and causes DNA damage leading to increased activity of poly(ADP-ribose) polymerase (PARP-1) to repair DNA. However, exaggerated activity of this enzyme results in depletion of intracellular NAD(+) and ATP, and the insulin-secreting cells undergo necrosis. The protective action of NA is due to the inhibition of PARP-1 activity. NA inhibits this enzyme, preventing depletion of NAD(+) and ATP in cells exposed to STZ. Moreover, NA serves as a precursor of NAD(+) and thereby additionally increases intracellular NAD(+) levels. The severity of diabetes in experimental rats strongly depends on the doses of STZ and NA given to these animals. Therefore, in diabetic rats, blood glucose may be changed in a broad range--from slight hyperglycemia to substantial hyperglycemia compared with control animals. Similarly, blood insulin may be only slightly decreased or substantial hypoinsulinemia may be induced. In vitro studies demonstrated that the insulin-secretory response to glucose is attenuated in STZ-NA-induced diabetic rats compared with control animals. This is due to reduced B-cell mass as well as metabolic defects in the insulin-secreting cells. Results of numerous experiments have demonstrated that this model of diabetes is useful in studies of different aspects of diabetes.

Inositol 1,4,5-trisphosphate receptor-mediated initial Ca(2+) mobilization constitutes a triggering signal for hydrogen peroxide-induced apoptosis in INS-1 β-cells

Reactive oxygen species, including hydrogen peroxide (H(2)O(2)), are known to induce β-cell apoptosis. The present study investigated the role of Ca(2+) in H(2)O(2)-induced apoptosis of the β-cell line INS-1. Annexin V assay with flow cytometry and DNA ladder assay demonstrated that treatment of INS-1 cells with 100 µM H(2)O(2) for 18 h significantly increased apoptotic cells. A comparable level of apoptosis was also observed after 18 h when the cells were treated with 100 µM H(2)O(2) only for initial 30 min. The H(2)O(2)-induced apoptosis was abolished by 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)ester (BAPTA/AM), a chelator of intracellular Ca(2+), by 2-aminoethoxydiphenylborate (2-APB), a blocker of inositol 1,4,5-trisphosphate (IP(3)) receptors and cation channels, and by xestospongin D, a blocker of IP(3) receptors, and was partially blocked by SKF-96365, a non-selective cation channel blocker. However, nicardipine, an L-type voltage-dependent Ca(2+) channel blocker, or N-(p-amylcinnamoyl)anthranilic acid (ACA), a TRPM2 blocker, had little effect on the apoptosis. The inhibitory effect of BAPTA/AM or 2-APB on the H(2)O(2)-induced apoptosis was largely attenuated when the drug was added 30 min or 1 h after start of the treatment with H(2)O(2). These results suggest that the initial intracellular Ca(2+) elevation induced by H(2)O(2), which is mediated via IP(3) receptors and store-operated cation channels, plays an obligatory role in the induction of β-cell apoptosis.

Thrombospondin-1: an islet endothelial cell signal of importance for β-cell function

OBJECTIVE

Loss of thrombospondin (TSP)-1 in pancreatic islets has been shown to cause islet hyperplasia. This study tested the hypothesis that endothelial-derived TSP-1 is important for β-cell function.

RESEARCH DESIGN AND METHODS

Islet function was evaluated both in vivo and in vitro. Messenger RNA and protein expression were measured by real-time PCR and Western blot, respectively. The role of endothelial-derived TSP-1 for β-cell function was determined using a transplantation design in which recipient blood vessels either were allowed to grow or not into the transplanted islets.

RESULTS

TSP-1-deficient mice were glucose intolerant, despite having an increased β-cell mass. Moreover, their islets had decreased glucose-stimulated insulin release, (pro)insulin biosynthesis, and glucose oxidation rate, as well as increased expression of uncoupling protein-2 and lactate dehydrogenase-A when compared with control islets. Almost all TSP-1 in normal islets were found to be derived from the endothelium. Transplantation of free and encapsulated neonatal wild-type and TSP-1-deficient islets was performed in order to selectively reconstitute with TSP-1-positive or -negative blood vessels in the islets and supported that the β-cell defects occurring in TSP-1-deficient islets reflected postnatal loss of the glycoprotein in the islet endothelial cells. Treatment of neonatal TSP-1-deficient mice with the transforming growth factor (TGF)β-1-activating sequence of TSP-1 showed that reconstitution of TGFβ-1 activation prevented the development of decreased glucose tolerance in these mice. Thus, endothelial-derived TSP-1 activates islet TGFβ-1 of importance for β-cells.

CONCLUSIONS

Our study indicates a novel role for endothelial cells as functional paracrine support for pancreatic β-cells.

Interleukin-targeted therapy for metabolic syndrome and type 2 diabetes

Interleukin-1β Interleukin-1β (IL-1β) is a key regulator of the body's inflammatory response and is produced after infection, injury, and an antigenic challenge. Cloned in 1984, the single polypeptide IL-1β has been shown to exert numerous biological effects. It plays a role in various diseases, including autoimmune diseases such as rheumatoid arthritis, inflammatory bowel diseases, and Type 1 diabetes, as well as in diseases associated with metabolic syndrome such as atherosclerosis, chronic heart failure, and Type 2 diabetes. The macrophage is the primary source of IL-1β, but epidermal, epithelial, lymphoid, and vascular tissues also synthesize IL-1. Recently, IL-1β production and secretion have also been reported from pancreatic islets. Insulin-producing β-cells β-cells within the pancreatic islets are specifically prone to IL-β-induced destruction and loss of function. Macrophage-derived IL-1β production in insulin-sensitive organs leads to the progression of inflammation inflammation and induction of insulin resistance in obesity. This chapter explains the mechanisms involved in the inflammatory response during diabetes progression with specific attention to the IL-1β signal effects influencing insulin action and insulin secretion insulin secretion . We highlight recent clinical studies, rodent and in vitro experiments with isolated islets using IL-1β as a potential target for the therapy of Type 2 diabetes.

Role of JNK activation in pancreatic beta-cell death by streptozotocin

c-Jun N-terminal kinase (JNK) is activated by cellular stress and plays critical roles in diverse types of cell death. However, role of JNK in beta-cell injury is obscure. We investigated the role for JNK in streptozotocin (STZ)-induced beta-cell death. STZ induced JNK activation in insulinoma or islet cells. JNK inhibitors attenuated insulinoma or islet cell death by STZ. STZ-induced JNK activation was decreased by PARP inhibitors, suggesting that JNK activation is downstream of PARP-1. Phosphatase inhibitors induced activation of JNK and abrogated the suppression of STZ-induced JNK activation by PARP inhibitors, suggesting that the inhibition of phosphatases is involved in the activation of JNK by STZ. STZ induced production of reactive oxygen species (ROS) as potential inhibitors of phosphatases, which was suppressed by PARP inhibitors. PARP-1 siRNA attenuated insulinoma cell death and JNK activation after STZ treatment, which was reversed by MKP (MAP kinase phosphatase)-1 siRNA. These results suggest that JNK is activated by STZ downstream of PARP-1 through inactivation of phosphatases such as MKP, which plays important roles in STZ-induced beta-cell death.

JunB Inhibits ER Stress and Apoptosis in Pancreatic Beta Cells

Cytokines contribute to pancreatic β-cell apoptosis in type 1 diabetes (T1D) by modulation of β-cell gene expression networks. The transcription factor Activator Protein-1 (AP-1) is a key regulator of inflammation and apoptosis. We presently evaluated the function of the AP-1 subunit JunB in cytokine-mediated β-cell dysfunction and death. The cytokines IL-1β+IFN-γ induced an early and transitory upregulation of JunB by NF-κB activation. Knockdown of JunB by RNA interference increased cytokine-mediated expression of inducible nitric oxide synthase (iNOS) and endoplasmic reticulum (ER) stress markers, leading to increased apoptosis in an insulin-producing cell line (INS-1E) and in purified rat primary β-cells. JunB knockdown β-cells and junB^−/− fibroblasts were also more sensitive to the chemical ER stressor cyclopiazonic acid (CPA). Conversely, adenoviral-mediated overexpression of JunB diminished iNOS and ER markers expression and protected β-cells from cytokine-induced cell death. These findings demonstrate a novel and unexpected role for JunB as a regulator of defense mechanisms against cytokine- and ER stress-mediated apoptosis.

MAPK kinase kinase-1 is essential for cytokine-induced c-Jun NH2-terminal kinase and nuclear factor-kappaB activation in human pancreatic islet cells

OBJECTIVE

The transcription factor nuclear factor-kappaB (NF-kappaB) and the mitogen-activated protein kinases (MAPKs) c-Jun NH(2)-terminal kinase (JNK) 1/2 are known to play decisive roles in cytokine-induced damage of rodent beta-cells. The upstream events by which these factors are activated in response to cytokines are, however, uncharacterized. The aim of the present investigation was to elucidate a putative role of the MAPK kinase kinase-1 (MEKK-1) in cytokine-induced signaling.

RESEARCH DESIGN AND METHODS

To establish a functional role of MEKK-1, the effects of transient MEKK-1 overexpression in betaTC-6 cells, achieved by lipofection and cell sorting, and MEKK-1 downregulation in betaTC-6 cells and human islet cells, achieved by diced-small interfering RNA treatment, were studied.

RESULTS

We observed that overexpression of wild-type MEKK-1, but not of a kinase dead MEKK-1 mutant, resulted in potentiation of cytokine-induced JNK activation, inhibitor of kappaB (IkappaB) degradation, and cell death. Downregulation of MEKK-1 in human islet cells provoked opposite effects, i.e., attenuation of cytokine-induced JNK and MKK4 activation, IkappaB stability, and a less pronounced NF-kappaB translocation. betaTC-6 cells with a downregulated MEKK-1 expression displayed also a weaker cytokine-induced iNOS expression and lower cell death rates. Also primary mouse islet cells with downregulated MEKK-1 expression were protected against cytokine-induced cell death.

CONCLUSIONS

MEKK-1 mediates cytokine-induced JNK- and NF-kappaB activation, and this event is necessary for iNOS expression and cell death.