Role of MKK3 and p38 MAPK in cytokine-induced death of insulin-producing cells

Posttranscriptional regulation of the prostaglandin E receptor spliced-isoform EP3-γ and its implication in pancreatic β-cell failure

In Type 2 diabetes (T2D), elevated lipid levels have been suggested to contribute to insulin resistance and β-cell dysfunction. We previously reported that the expression of the PGE2 receptor EP3 is elevated in islets of T2D individuals and is preferentially stimulated by palmitate, leading to β-cell failure. The mouse EP3 receptor generates three isoforms by alternative splicing which differ in their C-terminal domain and are referred to as mEP3α, mEP3β, and mEP3γ. We bring evidence that the expression of the mEP3γ isoform is elevated in islets of diabetic db/db mice and is selectively upregulated by palmitate. Specific knockdown of the mEP3γ isoform restores the expression of β-cell-specific genes and rescues MIN6 cells from palmitate-induced dysfunction and apoptosis. This study indicates that palmitate stimulates the expression of the mEP3γ by a posttranscriptional mechanism, compared to the other spliced isoforms, and that the de novo synthesized ceramide plays an important role in FFA-induced mEP3γ expression in β-cells. Moreover, induced levels of mEP3γ mRNA by palmitate or ceramide depend on p38 MAPK activation. Our findings suggest that mEP3γ gene expression is regulated at the posttranscriptional level and defines the EP3 signaling axis as an important pathway mediating β-cell-impaired function and demise.

RANKL inhibition: a new target of treating diabetes mellitus?

Accumulating evidence demonstrates the link between glucose and bone metabolism. The receptor activator of nuclear factor-kB ligand (RANKL)/the receptor activator of NF-κB (RANK)/osteoprotegerin (OPG) axis is an essential signaling axis maintaining the balance between bone resorption and bone formation. In recent years, it has been found that RANKL and RANK are distributed not only in bone but also in the liver, muscle, adipose tissue, pancreas, and other tissues that may influence glucose metabolism. Some scholars have suggested that the blockage of the RANKL signaling may protect islet β-cell function and prevent diabetes; simultaneously, there also exist different views that RANKL can improve insulin resistance through inducing the beige adipocyte differentiation and increase energy expenditure. Currently, the results of the regulatory effect on glucose metabolism of RANKL remain conflicting. Denosumab (Dmab), a fully human monoclonal antibody that can bind to RANKL and prevent osteoclast formation, is a commonly used antiosteoporosis drug. Recent basic studies have found that Dmab seems to regulate glucose homeostasis and β-cell function in humanized mice or in vitro human β-cell models. Besides, some clinical data have also reported the glucometabolic effects of Dmab, however, with limited and inconsistent results. This review mainly describes the impact of the RANKL signaling pathway on glucose metabolism and summarizes clinical evidence that links Dmab and DM to seek a new therapeutic strategy for diabetes.

The Prospective Application of Melatonin in Treating Epigenetic Dysfunctional Diseases

In the past, different human disorders were described by scientists from the perspective of either environmental factors or just by genetically related mechanisms. The rise in epigenetic studies and its modifications, i.e., heritable alterations in gene expression without changes in DNA sequences, have now been confirmed in diseases. Modifications namely, DNA methylation, posttranslational histone modifications, and non-coding RNAs have led to a better understanding of the coaction between epigenetic alterations and human pathologies. Melatonin is a widely-produced indoleamine regulator molecule that influences numerous biological functions within many cell types. Concerning its broad spectrum of actions, melatonin should be investigated much more for its contribution to the upstream and downstream mechanistic regulation of epigenetic modifications in diseases. It is, therefore, necessary to fill the existing gaps concerning corresponding processes associated with melatonin with the physiological abnormalities brought by epigenetic modifications. This review outlines the findings on melatonin’s action on epigenetic regulation in human diseases including neurodegenerative diseases, diabetes, cancer, and cardiovascular diseases. It summarizes the ability of melatonin to act on molecules such as proteins and RNAs which affect the development and progression of diseases.

Metformin inhibits MAPK signaling and rescues pancreatic aquaporin 7 expression to induce insulin secretion in type 2 diabetes mellitus

Metformin is the first-line antidiabetic agent for type 2 diabetes mellitus (T2DM) treatment. Although accumulated evidence has shed light on the consequences of metformin action, the precise mechanisms of its action, especially in the pancreas, are not fully understood. Aquaporin 7 (AQP7) acts as a critical regulator of intraislet glycerol content, which is necessary for insulin production and secretion. The aim of this study was to investigate the effects of different doses of metformin on AQP7 expression and explore the possible mechanism of its protective effects in the pancreatic islets. We used an in vivo model of high-fat diet in streptozocin-induced diabetic rats and an in vitro model of rat pancreatic β-cells (INS-1 cells) damaged by hyperglycemia and hyperlipidemia. Our data showed that AQP7 expression levels were decreased, whereas p38 and JNK mitogen-activated protein kinases (MAPKs) were activated in vivo and in vitro in response to hyperglycemia and hyperlipidemia. T2DM rats treated with metformin demonstrated a reduction in blood glucose levels and increased regeneration of pancreatic β-cells. In addition, metformin upregulated AQP7 expression as well as inhibited activation of p38 and JNK MAPKs both in vivo and in vitro. Overexpression of AQP7 increased glycerol influx into INS-1 cells, whereas inhibition of AQP7 reduced glycerol influx, thereby decreasing subsequent insulin secretion. Our findings demonstrate a new mechanism by which metformin suppresses the p38 and JNK pathways, thereby upregulating pancreatic AQP7 expression and promoting glycerol influx into pancreatic β-cells and subsequent insulin secretion in T2DM.

NLRP3 inflammasome is expressed and regulated in human islets

NRLP3 inflammasome is a protein complex involved in the maturation of IL1β. In the onset of type 1 diabetes as well as in islet transplantation, IL-1β is one of the cytokines involved in the recruitment of immune cells in islets and eventually in islet destruction. Whether IL-1β is produced by islet cells is still under debate and NLRP3 inflammasome-dependent IL-1β production has not been yet determined in human islets. The aim of this study was to determine the expression and the regulation of the NRLP3 inflammasome in human islets. Human islets were stimulated with LPS and successively with ATP (LPS + ATP) in the presence or absence of the inflammasome inhibitor glyburide. Islets were also incubated in hypoxic or normoxic conditions for 24 h in the presence or absence of glyburide. Then, IL1B and NLRP3 expression was studied by real time PCR, protein expression by western blot, protein localization by immunofluorescence and protein secretion by ELISA. LPS + ATP increased gene expression of NRLP3 and IL1B. Glyburide partially prevented this effect. IL-1β protein was localized in β and non-β cells. Moreover, LPS + ATP increased IL-1β protein expression and production, which were prevented by glyburide. Hypoxia increased gene expression of NRLP3 and IL1B and induced IL-1β and caspase-1 production. Finally, hypoxia-induced cell death which was not prevented by inhibition of NLRP3 inflammasome. NRLP3 inflammasome is expressed and plays a role in IL-1β production by human islets. By contrast, NRLP3 inflammasome activation is not involved in islet cell death induced by hypoxia.

Persistence of STAT-1 inhibition and induction of cytokine resistance in pancreatic β cells treated with St John's wort and its component hyperforin

OBJECTIVES

St John's wort extract (SJW) and its component hyperforin (HPF) were shown to potently inhibit cytokine-induced STAT-1 and NF-κB activation in pancreatic β cells and protect them against injury. This study aimed at exploring the time course of STAT-1 inhibition afforded by these natural compounds in the β-cell line INS-1E.

METHODS

INS-1E cells were pre-incubated with SJW extract (2-5 μg/ml) or HPF (0.5-2 μm) and then exposed to a cytokine mixture. In some experiments, these compounds were added after or removed before cytokine exposure. STAT-1 activation was assessed by electrophoretic mobility shift assay, apoptosis by caspase-3 activity assay, mRNA gene expression by RT-qPCR.

KEY FINDINGS

Pre-incubation with SJW/HPF for 1-2 h exerted a remarkable STAT-1 downregulation, which was maintained upon removal of the compounds before early or delayed cytokine addition. When the protective compounds were added after cell exposure to cytokines, between 15 and 90 min, STAT-1 inhibition also occurred at a progressively decreasing extent. Upon 24-h incubation, SJW and HPF counteracted cytokine-induced β-cell dysfunction, apoptosis and target gene expression.

CONCLUSIONS

SJW and HPF confer to β cells a state of 'cytokine resistance', which can be elicited both before and after cytokine exposure and safeguards these cells from deleterious cytokine effects.

Development of the Nonobese Diabetic Mouse and Contribution of Animal Models for Understanding Type 1 Diabetes

In 1974, the discovery of a mouse and a rat that spontaneously developed hyperglycemia led to the development of 2 autoimmune diabetes models: nonobese diabetic (NOD) mouse and Bio-Breeding rat. These models have contributed to our understanding of autoimmune diabetes, provided tools to dissect autoimmune islet damage, and facilitated development of early detection, prevention, and treatment of type 1 diabetes. The genetic characterization, monoclonal antibodies, and congenic strains have made NOD mice especially useful.Although the establishment of the inbred NOD mouse strain was documented by Makino et al (Jikken Dobutsu. 1980;29:1-13), this review will focus on the not-as-well-known history leading to the discovery of a glycosuric female mouse by Yoshihiro Tochino. This discovery was spearheaded by years of effort by Japanese scientists from different disciplines and dedicated animal care personnel and by the support of the Shionogi Pharmaceutical Company, Osaka, Japan. The history is based on the early literature, mostly written in Japanese, and personal communications especially with Dr Tochino, who was involved in diabetes animal model development and who contributed to the release of NOD mice to the international scientific community. This article also reviews the scientific contributions made by the Bio-Breeding rat to autoimmune diabetes.

Kinase Signaling in Apoptosis Induced by Saturated Fatty Acids in Pancreatic β-Cells

Pancreatic β-cell failure and death is considered to be one of the main factors responsible for type 2 diabetes. It is caused by, in addition to hyperglycemia, chronic exposure to increased concentrations of fatty acids, mainly saturated fatty acids. Molecular mechanisms of apoptosis induction by saturated fatty acids in β-cells are not completely clear. It has been proposed that kinase signaling could be involved, particularly, c-Jun N-terminal kinase (JNK), protein kinase C (PKC), p38 mitogen-activated protein kinase (p38 MAPK), extracellular signal-regulated kinase (ERK), and Akt kinases and their pathways. In this review, we discuss these kinases and their signaling pathways with respect to their possible role in apoptosis induction by saturated fatty acids in pancreatic β-cells.

Exendin-4 protects murine MIN6 pancreatic β-cells from interleukin-1β-induced apoptosis via the NF-κB pathway

BACKGROUND

Glucagon-like peptide-1 (GLP-1) and its potent analog, exendin-4, are well known to inhibit β- cell apoptosis and promote β-cell proliferation. Meanwhile, cytokines, such as interleukin-1β (IL-1β), stimulate inducible nitric oxide synthase (iNOS) expression and nitric oxide overproduction leading to β-cell damage. However, the protective mechanisms of GLP-1 in β-cells exposed to cytokines have not been fully elucidated.

AIMS

In this study, the protective effects of exendin-4 on IL-1β-induced apoptosis were investigated in murine MIN6 pancreatic β-cells. The role of nuclear factor-κB (NF-κB) signaling in this process was also explored.

METHODS

The effects of exendin-4 pre-treatment on IL-1β-induced apoptosis were investigated by Hoechst/PI and Annexin V/PI staining. Levels of iNOS and NF-κB proteins were investigated by Western blotting and cytoplasmic nitrite levels were determined using Griess reagent.

RESULTS

IL-1β treatment (range, 5-40 ng/ml) for 24 h was positively correlated with nitrite production (R2=0.9668, p<0.01), a significant increase in the percentage of apoptotic cells (p<0.01) and a concomitant dose-dependent increase in cytoplasmic levels of iNOS and NF-κB p65 activation. N-acetyl- L-cysteine (NAC), NG-nitro-L-arginine methyl ester (L-NAME) and pyrrolidine dithiocarbamate (PDTC), partially rescued apoptotic β-cells, suggesting involvement of NF-κB-iNOS-nitrite in this process. Exendin-4 (100 nM) treatment significantly decreased IL-1β-induced apoptosis (p<0.01), downregulated NF-κB activation and subsequently decreased iNOS and nitrite levels in IL-1β-induced β-cells (p<0.001), in a similar manner to L-NAME, PDTC and NAC.

CONCLUSIONS

These results suggest that exendin-4 protects against IL-1β- induced apoptosis in β-cells via downregulation of the NF- κB-iNOS-nitrite pathway.

Preventing p38 MAPK-mediated MafA degradation ameliorates β-cell dysfunction under oxidative stress

The reduction in the expression of glucose-responsive insulin gene transcription factor MafA accompanies the development of β-cell dysfunction under oxidative stress/diabetic milieu. Humans with type 2 diabetes have reduced MafA expression, and thus preventing this reduction could overcome β-cell dysfunction and diabetes. We previously showed that p38 MAPK, but not glycogen synthase kinase 3 (GSK3), is a major regulator of MafA degradation under oxidative stress. Here, we examined the mechanisms of this degradation and whether preventing MafA degradation under oxidative stress will overcome β-cell dysfunction. We show that under oxidative and nonoxidative conditions p38 MAPK directly binds to MafA and triggers MafA degradation via ubiquitin proteasomal pathway. However, unlike nonoxidative conditions, MafA degradation under oxidative stress depended on p38 MAPK-mediated phosphorylation at threonine (T) 134, and not T57. Furthermore the expression of alanine (A) 134-MafA, but not A57-MafA, reduced the oxidative stress-mediated loss of glucose-stimulated insulin secretion, which was independent of p38 MAPK action on protein kinase D, a regulator of insulin secretion. Interestingly, the expression of proteasomal activator PA28γ that degrades GSK3-phosphorylated (including T57) MafA was reduced under oxidative stress, explaining the dominance of p38 MAPK over the GSK3 pathway in regulating MafA stability under oxidative stress. These results identify two distinct pathways mediating p38 MAPK-dependent MafA degradation under oxidative and nonoxidative conditions and show that inhibiting MafA degradation under oxidative stress ameliorates β-cell dysfunction and could lead to novel therapies for diabetes.

Toll-like receptors, the NLRP3 inflammasome, and interleukin-1β in the development and progression of type 1 diabetes

Traditionally, type 1 diabetes (T1D) has been thought of as a disease of cellular immunity, but there is increasing evidence that components of the innate immune system, controlled largely by Toll-like receptors (TLRs), play a significant role in T1D development. TLRs are pattern-recognition molecules on immune cells that recognize pathogens, leading to the production of cytokines such as interleukin-1β (IL1β, encoded by the IL1B gene). IL1β is increased in patients with newly diagnosed T1D and likely acts as an early inflammatory signal in T1D development. Because hyperglycemia is a hallmark of T1D, the effects of hyperglycemia on IL1β expression in peripheral blood mononuclear cells (PBMCs) and islet cells have been examined, but with inconsistent results, and the mechanisms leading to this increase remain unknown. Fatty acids stimulate IL1β expression and may promote inflammation, causing hyperglycemia and insulin resistance. The mechanisms by which IL1β is involved in T1D pathogenesis are controversial. Overall, studies in pancreatic β-cells suggest that IL1β-mediated damage to islet cells involves multiple downstream targets. Potential therapies to decrease the progression of T1D based on IL1β biology include pioglitazone, glyburide, IL1 receptor antagonists, and agents that remove IL1β from the circulation.

Decrease in {beta}-cell proliferation precedes apoptosis during diabetes development in bio-breeding/worcester rat: beneficial role of Exendin-4

In autoimmune type 1 diabetes mellitus, proinflammatory cytokine-mediated apoptosis of beta-cells has been considered to be the first event directly responsible for beta-cell mass reduction. In the Bio-Breeding (BB) rat, an in vivo model used in the study of autoimmune diabetes, beta-cell apoptosis is observed from 9 wk of age and takes place after an insulitis period that begins at an earlier age. Previous studies by our group have shown an antiproliferative effect of proinflammatory cytokines on cultured beta-cells in Wistar rats, an effect that was partially reversed by Exendin-4, an analogue of glucagon-like peptide-1. In the current study, the changes in beta-cell apoptosis and proliferation during insulitis stage were also determined in pancreatic tissue sections in normal and thymectomized BB rats, as well as in Wistar rats of 5, 7, 9, and 11 wk of age. Although stable beta-cell proliferation in Wistar and thymectomized BB rats was observed along the course of the study, a decrease in beta-cell proliferation and beta-cell mass from the age of 5 wk, and prior to the commencement of apoptosis, was noted in BB rats. Exendin-4, in combination with anti-interferon-gamma antibody, induced a near-total recovery of beta-cell proliferation during the initial stages of insulitis. This highlights the importance of early intervention and, as well, the possibilities of new therapeutic approaches in preventing autoimmune diabetes by acting, initially, in the insulitis stage and, subsequently, on beta-cell regeneration and on beta-cell apoptosis.

P38alpha-selective mitogen-activated protein kinase inhibitor for improvement of cultured human islet recovery

OBJECTIVES

We investigated whether the recovery of cultured human islets is improved through the addition of a p38alpha-selective mitogen-activated protein kinase inhibitor, SD-282, to clinically used serum-free culture medium.

METHODS

Immediately after isolation, islets were cultured for 24 hours in medium alone (control) or medium containing dimethyl sulfoxide, 0.1 microM SD-282, or 0.3 microM SD-282. Cytokine expression, apoptotic beta-cell percentage, and islet function were assessed postculture.

RESULTS

Expression of p38 and phosphorylated p38 in islets increased during culture. Interleukin 6 mRNA expression in cultured islets, as well as IL-6, IL-8, and granulocyte-macrophage colony-stimulating factor released into the medium, was significantly reduced by adding SD-282. The apoptotic beta-cell percentage was significantly lower in islets cultured with 0.1 microM SD-282, but not 0.3 microM, as compared with the control. Stimulation indices measured in vitro were higher but without significance (P = 0.06); the function of transplanted islets in diabetic NOD-scid mice was also better in 0.1-microM SD-282 group as compared with control.

CONCLUSIONS

Better islet function was obtained by adding 0.1 microM SD-282 to the serum-free culture medium. This improvement was associated with suppression of cytokine production and prevention of beta-cell apoptosis. However, this beneficial effect was diminished at a higher concentration.

Blockade of sphingosine 1-phosphate receptor 2 signaling attenuates streptozotocin-induced apoptosis of pancreatic beta-cells

Sphingosine 1-phosphate (S1P) is a potent sphingolipid mediator that acts through five cognate G protein-coupled receptors (S1P(1)-S1P(5)) and regulates many critical biological processes. Recent studies indicated that S1P at nanomolar concentrations significantly reduces cytokine-induced apoptosis of pancreatic beta-cells in which genes for S1P(1)-S1P(4) are co-expressed. However, the S1P receptor subtype(s) involved in this effect remains to be clarified. In this study, we investigated the potential role of S1P(2) in streptozotocin (STZ)-induced apoptosis of pancreatic beta-cells and progression of diabetes. S1P(2)-deficient (S1P(2)(-/-)) mice displayed a greater survive ability, lower blood glucose levels, and smaller numbers of TUNEL-positive apoptotic beta-cells to administration of a high dose of STZ than wild-type (WT) mice. S1P(2)(-/-) mice showed higher insulin/glucose ratios (an index of relative insulin deficiency) and larger insulin-positive islet areas to administration of a low dose of STZ than WT mice. Moreover, administration of JTE-013, a S1P(2)-specific antagonist, to WT mice ameliorated STZ-induced blood glucose elevation and reduced the incidence of diabetes. Our findings indicate that blockade of S1P(2) signaling attenuates STZ-induced apoptosis of pancreatic beta-cells and decreases the incidence of diabetes.

T cells cooperate with palmitic acid in induction of beta cell apoptosis

Background

Diabetes is characterized by progressive failure of insulin producing beta cells. It is well known that both saturated fatty acids and various products of immune cells can contribute to the reduction of beta cell viability and functionality during diabetes pathogenesis. However, their joint action on beta cells has not been investigated, so far. Therefore, we explored the possibility that leukocytes and saturated fatty acids cooperate in beta cell destruction.

Results

Rat pancreatic islets or insulinoma cells (RIN) were co-cultivated with concanavalin A (ConA)-stimulated rat lymph node cells (LNC), or they were treated with cell-free supernatants (Sn) obtained from ConA-stimulated spleen cells or from activated CD3⁺ cells, in the absence or presence of palmitic acid (PA). ConA-stimulated LNC or Sn and PA cooperated in inducing caspase-3-dependent RIN cell apoptosis. The observed effect of PA and Sn on RIN cell viability was mediated by p38 mitogen-activated protein kinase (MAPK)-signaling and was achieved through auto-destructive nitric oxide (NO) production. The cooperative effect of Sn was mimicked with the combination of interleukin-1β, interleukin-2, interleukin-6, interleukin-17, interferon-γ and tumor necrosis factor-α.

Conclusion

These results imply that stimulated T cells produce cytokines that cooperate with saturated free fatty acids in beta cell destruction during diabetes pathogenesis.

Regulation of PKD by the MAPK p38delta in insulin secretion and glucose homeostasis

Dysfunction and loss of insulin-producing pancreatic beta cells represent hallmarks of diabetes mellitus. Here, we show that mice lacking the mitogen-activated protein kinase (MAPK) p38delta display improved glucose tolerance due to enhanced insulin secretion from pancreatic beta cells. Deletion of p38delta results in pronounced activation of protein kinase D (PKD), the latter of which we have identified as a pivotal regulator of stimulated insulin exocytosis. p38delta catalyzes an inhibitory phosphorylation of PKD1, thereby attenuating stimulated insulin secretion. In addition, p38delta null mice are protected against high-fat-feeding-induced insulin resistance and oxidative stress-mediated beta cell failure. Inhibition of PKD1 reverses enhanced insulin secretion from p38delta-deficient islets and glucose tolerance in p38delta null mice as well as their susceptibility to oxidative stress. In conclusion, the p38delta-PKD pathway integrates regulation of the insulin secretory capacity and survival of pancreatic beta cells, pointing to a pivotal role for this pathway in the development of overt diabetes mellitus.

Ionizing radiation-induced gene modulations, cytokine content changes and telomere shortening in mouse fetuses exhibiting forelimb defects

Several lines of evidence have linked limb teratogenesis to radiation-induced apoptosis and to the p53 status in murine fetuses. In previous reports, we studied the occurrence of various malformations after intrauterine irradiation and showed that these malformations were modulated by p53-deficiency as well as by the developmental stage at which embryos were irradiated. In this new study, we focused onto one particular phenotype namely forelimb defects to further unravel the cellular and molecular mechanisms underlying this malformation. We measured various parameters expected to be directly or indirectly influenced by irradiation damage. The mouse fetuses were irradiated at day 12 p.c. (post conception) and examined for forelimb defects on gestational days 15, 16, 17 and 19 of development. The release of inflammatory cytokines was determined in the amniotic fluid on day 16 p.c. and the mean telomere lengths assessed at days 12, 13 and 19 p.c. Differential gene expression within the forelimb bud tissues was determined using Real Time quantitative PCR (RTqPCR) 24 h following irradiation. Apoptosis was investigated in the normal and malformed fetuses using the TUNEL assay and RTqPCR. First, we found that irradiated fetuses with forelimb defects displayed excessive apoptosis in the predigital regions. Besides, overexpression of the pro-apoptotic Bax gene indicates a mitochondrial-mediated cell death. Secondly, our results showed overexpression of MKK3 and MKK7 (members of the stress-activated MAP kinase family) within the malformed fetuses. The latter could be involved in radiation-induced apoptosis through activation of the p38 and JNK pathways. Thirdly, we found that irradiated fetuses exhibiting forelimb defects showed a marked telomere shortening. Interestingly, telomere shortening was observed as the malformations became apparent. Fourthly, we measured cytokine levels in the amniotic fluid and detected a considerable inflammatory reaction among the irradiated fetuses as evidenced by the increase in pro-inflammatory cytokine levels. Altogether, our data suggest that transcriptional modulations of apoptotic, inflammation, stress, and DNA damage players are early events in radiation-induced forelimb defects. These changes resulted in harsh developmental conditions as indicated by a marked increase in cytokine levels in the amniotic fluid and telomere shortening, two features concomitant with the onset of the forelimb defect phenotype in our study.

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.

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

The aim of the present investigation was to characterize the role of the MAPK kinase kinase-1 (MEKK-1) in stress-induced cell death of insulin producing cells. We observed that transient overexpression of the wild type MEKK-1 protein in the insulin-producing cell lines RIN-5AH and betaTC-6 increased c-Jun N-terminal kinase (JNK) phosphorylation and augmented cell death induced by diethylenetriamine/nitroso-1-propylhydrazino)-1-propanamine (DETA/NO), streptozotocin (STZ), and hydrogen peroxide (H2O2). Furthermore, DETA/NO or STZ induced a rapid threonine phosphorylation of MEKK-1. Silencing of MEKK-1 gene expression in betaTC-6 and human dispersed islet cells, using in vitro-generated diced small interfering RNA, resulted in protection from DETA/NO, STZ, H2O2, and tunicamycin induced cell death. Moreover, in DETA/NO-treated cells diced small interfering RNA-mediated down-regulation of MEKK-1 resulted in decreased activation of JNK but not p38 and ERK. Inhibition of JNK by treatment with SP600125 partially protected against DETA/NO- or STZ-induced cell death. In summary, our results support an essential role for MEKK-1 in JNK activation and stress-induced beta-cell death. Increased understanding of the signaling pathways that augment or diminish beta-cell MEKK-1 activity may aid in the generation of novel therapeutic strategies in the treatment of type 1 diabetes.

Transforming growth factor-beta-activated protein kinase 1-binding protein (TAB)-1alpha, but not TAB1beta, mediates cytokine-induced p38 mitogen-activated protein kinase phosphorylation and cell death in insulin-producing cells

Previous studies have indicated that the p38 MAPK participates in signaling events that lead to the death of the insulin-producing beta-cell. The aim of the present study was to elucidate the role of the TGF-beta-activated protein kinase 1-binding protein 1 (TAB1) in the cytokine-induced activation of p38. Levels of TAB1 mRNA and protein were analyzed by real-time PCR and immunoblotting, and TAB1 expression in mouse and human islet cells was down-regulated using lipofection of diced-small interfering RNA. TAB1 overexpression in beta-TC6 cells was achieved by transient transfections followed by fluorescence activated cell sorting. Phosphorylation of p38, c-Jun N-terminal kinase, and ERK was assessed by immunoblotting, and viability was determined using vital staining with bisbenzimide and propidium iodide. We observed that TAB1 is expressed in insulin-producing cells. Cytokine (IL-1beta + interferon-gamma)-stimulated p38 phosphorylation was significantly increased by TAB1alpha overexpression, but not TAB1beta overexpression, in beta-TC6 cells. The TAB1alpha-augmented p38 phosphorylation was paralleled by an increased cell death rate. Treatment of islet cells with diced-small interfering RNA specific for TAB1, but not for TGF-beta-activated kinase 1, resulted in lowered cytokine-induced p38 phosphorylation and protection against cell death. The cytokine-induced phosphorylation of c-Jun N-terminal kinase and ERK was not affected by changes in TAB1 levels. Finally, TAB1 phosphorylation was decreased by the p38 inhibitor SB203580. We conclude that TAB1alpha, but not TAB1beta, plays an important role in the activation of p38 in insulin-producing cells and therefore also in cytokine-induced beta-cell death.

Role of TAB1 in nitric oxide-induced p38 activation in insulin-producing cells

The aim of present study was to elucidate the role of TAB1 in nitric oxide-induced activation of p38 MAPK. For this purpose we over-expressed TAB1 in insulin-producing β-TC6 cells. We observed in cells transiently over-expressing TAB1 that p38 activation was enhanced in response to DETA/NONOate. A lowering of TAB1 levels, using the siRNA technique, resulted in the opposite effect. The DETA/NONOate-induced cell death rate was increased in cells transiently overexpressing TAB1. In stable β-TC6 cell clones with very high TAB1 levels p38 phosphorylation was enhanced also at basal conditions. DETA/NONOate increased also the phosphorylation of JNK and ERK in β-TC6 cells, but these events were not affected by TAB1. Interestingly, the inhibitory effect of SB203580 on p38 phosphorylation was paralleled by a stimulatory effect on JNK phosphorylation and an inhibitory effect on ERK phosphorylation. In summary, we propose that TAB1 promotes nitric oxide-induced p38 autophosphorylation. In addition, nitric oxide-induced p38 activation seems to promote JNK inhibition and ERK activation, but this effect appears to not require TAB1. A better understanding of how the TAB1/p38 pathway promotes β-cell death in response to nitric oxide might help in the development of novel pharmacological approaches in the treatment of diabetes.