Differential responses of pancreatic β-cells to ROS and RNS

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) direct the activation of distinct signaling pathways that determine cell fate. In this study, the pathways activated and the mechanisms by which ROS and RNS control the viability of pancreatic β-cells were examined. Although both nitric oxide and hydrogen peroxide (H₂O₂) induce DNA damage, reduce cell viability, and activate AMPK, the mechanisms of AMPK activation and cell death induction differ between each reactive species. Nitric oxide activates the unfolded protein and heat shock responses and MAPK kinase signaling, whereas H₂O₂ stimulates p53 stabilization and poly(ADP-ribose) polymerase (PARP) activation but fails to induce the unfolded protein or heat shock responses or MAPK activation. The control of cell fate decisions is selective for the form of stress. H₂O₂-mediated reduction in β-cell viability is controlled by PARP, whereas cell death in response to nitric oxide is PARP independent but associated with the nuclear localization of GAPDH. These findings show that both ROS and RNS activate AMPK, induce DNA damage, and reduce cell viability; however, the pathways controlling the responses of β-cells are selective for the type of reactive species.

Ketamine enhances human neural stem cell proliferation and induces neuronal apoptosis via reactive oxygen species-mediated mitochondrial pathway

BACKGROUND

Growing evidence indicates that ketamine causes neurotoxicity in a variety of developing animal models, leading to a serious concern regarding the safety of pediatric anesthesia. However, if and how ketamine induces human neural cell toxicity is unknown. Recapitulation of neurogenesis from human embryonic stem cells (hESCs) in vitro allows investigation of the toxic effects of ketamine on neural stem cells (NSCs) and developing neurons, which is impossible to perform in humans. In the present study, we assessed the influence of ketamine on the hESC-derived NSCs and neurons.

METHODS

hESCs were directly differentiated into neurons via NSCs. NSCs and 2-week-old neurons were treated with varying doses of ketamine for different durations. NSC proliferation capacity was analyzed by Ki67 immunofluorescence staining and bromodeoxyuridine assay. Neuroapoptosis was analyzed by TUNEL staining and caspase 3 activity measurement. The mitochondria-related neuronal apoptosis pathway including mitochondrial membrane potential, cytochrome c distribution within cells, mitochondrial fission, and reactive oxygen species (ROS) production were also investigated.

RESULTS

Ketamine (100 µM) increased NSC proliferation after 6-hour exposure. However, significant neuronal apoptosis was only observed after 24 hours of ketamine treatment. In addition, ketamine decreased mitochondrial membrane potential and increased cytochrome c release from mitochondria into cytosol. Ketamine also enhanced mitochondrial fission as well as ROS production compared with no-treatment control. Importantly, Trolox, a ROS scavenger, significantly attenuated the increase of ketamine-induced ROS production and neuronal apoptosis.

CONCLUSIONS

These data for the first time demonstrate that (1) ketamine increases NSC proliferation and causes neuronal apoptosis; (2) mitochondria are involved in ketamine-induced neuronal toxicity, which can be prevented by Trolox; and (3) the stem cell-associated neurogenesis system may provide a simple and promising in vitro model for rapidly screening anesthetic neurotoxicity and studying the underlying mechanisms as well as prevention strategies to avoid this toxic effect.

Reduction in ATP levels triggers immunoproteasome activation by the 11S (PA28) regulator during early antiviral response mediated by IFNβ in mouse pancreatic β-cells

Autoimmune destruction of insulin producing pancreatic β-cells is the hallmark of type I diabetes. One of the key molecules implicated in the disease onset is the immunoproteasome, a protease with multiple proteolytic sites that collaborates with the constitutive 19S and the inducible 11S (PA28) activators to produce immunogenic peptides for presentation by MHC class I molecules. Despite its importance, little is known about the function and regulation of the immunoproteasome in pancreatic β-cells. Of special interest to immunoproteasome activation in β-cells are the effects of IFNβ, a type I IFN secreted by virus-infected cells and implicated in type I diabetes onset, compared to IFNγ, the classic immunoproteasome inducer secreted by cells of the immune system. By qPCR analysis, we show that mouse insulinoma MIN6 cells and mouse islets accumulate the immune proteolytic β1_i, β2_i and β5_i, and 11S mRNAs upon exposure to IFNβ or IFNγ. Higher concentrations of IFNβ than IFNγ are needed for similar expression, but in each case the expression is transient, with maximal mRNA accumulation in 12 hours, and depends primarily on Interferon Regulatory Factor 1. IFNs do not alter expression of regular proteasome genes, and in the time frame of IFNβ-mediated response, the immune and regular proteolytic subunits co-exist in the 20S particles. In cell extracts with ATP, these particles have normal peptidase activities and degrade polyubiquitinated proteins with rates typical of the regular proteasome, implicating normal regulation by the 19S activator. However, ATP depletion rapidly stimulates the catalytic rates in a manner consistent with levels of the 11S activator. These findings suggest that stochastic combination of regular and immune proteolytic subunits may increase the probability with which unique immunogenic peptides are produced in pancreatic β-cells exposed to IFNβ, but primarily in cells with reduced ATP levels that stimulate the 11S participation in immunoproteasome function.

The role of reactive oxygen species and proinflammatory cytokines in type 1 diabetes pathogenesis

Type 1 diabetes (T1D) is a T cell–mediated autoimmune disease characterized by the destruction of insulin-secreting pancreatic β cells. In humans with T1D and in nonobese diabetic (NOD) mice (a murine model for human T1D), autoreactive T cells cause β-cell destruction, as transfer or deletion of these cells induces or prevents disease, respectively. CD4⁺ and CD8⁺ T cells use distinct effector mechanisms and act at different stages throughout T1D to fuel pancreatic β-cell destruction and disease pathogenesis. While these adaptive immune cells employ distinct mechanisms for β-cell destruction, one central means for enhancing their autoreactivity is by the secretion of proinflammatory cytokines, such as IFN-γ, TNF-α, and IL-1. In addition to their production by diabetogenic T cells, proinflammatory cytokines are induced by reactive oxygen species (ROS) via redox-dependent signaling pathways. Highly reactive molecules, proinflammatory cytokines are produced upon lymphocyte infiltration into pancreatic islets and induce disease pathogenicity by directly killing β cells, which characteristically possess low levels of antioxidant defense enzymes. In addition to β-cell destruction, proinflammatory cytokines are necessary for efficient adaptive immune maturation, and in the context of T1D they exacerbate autoimmunity by intensifying adaptive immune responses. The first half of this review discusses the mechanisms by which autoreactive T cells induce T1D pathogenesis and the importance of ROS for efficient adaptive immune activation, which, in the context of T1D, exacerbates autoimmunity. The second half provides a comprehensive and detailed analysis of (1) the mechanisms by which cytokines such as IL-1 and IFN-γ influence islet insulin secretion and apoptosis and (2) the key free radicals and transcription factors that control these processes.

Immunoproteasome Activation During Early Antiviral Response in Mouse Pancreatic β-cells: New Insights into Auto-antigen Generation in Type I Diabetes?

Type 1 diabetes results from autoimmune destruction of the insulin producing pancreatic β-cells. The immunoproteasome, a version of the proteasome that collaborates with the 11S/PA28 activator to generate immunogenic peptides for presentation by MHC class I molecules, has long been implicated in the onset of the disease, but little is known about immunoproteasome function and regulation in pancreatic β-cells. Interesting insight into these issues comes from a recent analysis of the immunoproteasome expressed in pancreatic β-cells during early antiviral defenses mediated by interferon β (IFNβ), a type I IFN implicated in the induction of the diabetic state in human and animal models. Using mouse islets and the MIN6 insulinoma cell line, Freudenburg et al. found that IFNβ stimulates expression of the immunoproteasome and the 11S/PA28 activator in a manner fundamentally similar to the classic immuno-inducer IFNγ, with similar timing of mRNA accumulation and decline; similar transcriptional activation mediated primarily by the IRF1 and similar mRNA and protein levels. Furthermore, neither IFNβ nor IFNγ altered the expression of regular proteolytic subunits or prevented their incorporation into proteolytic cores. As a result, immunoproteasomes had stochastic combinations of immune and regular proteolytic sites, an arrangement that would likely increase the probability with which unique immunogenic peptides are produced. However, immunoproteasomes were activated by the 11S/PA28 only under conditions of ATP depletion. A mechanism that prevents the activation of immunoproteasome at high ATP levels has not been reported before and could have a major regulatory significance, as it could suppress the generation of immunogenic peptides as cell accumulate immunoproteasome and 11S/PA28, and activate antigen processing only when ATP levels drop. We discuss implications of these new findings on the link between early antiviral response and the onset of type 1 diabetes.

Cytokine-mediated β-cell damage in PARP-1-deficient islets

Poly(ADP)-ribose polymerase (PARP) is an abundant nuclear protein that is activated by DNA damage; once active, it modifies nuclear proteins through attachment of poly(ADP)-ribose units derived from β-nicotinamide adenine dinucleotide (NAD(+)). In mice, the deletion of PARP-1 attenuates tissue injury in a number of animal models of human disease, including streptozotocin-induced diabetes. Also, inflammatory cell signaling and inflammatory gene expression are attenuated in macrophages isolated from endotoxin-treated PARP-1-deficient mice. In this study, the effects of PARP-1 deletion on cytokine-mediated β-cell damage and macrophage activation were evaluated. There are no defects in inflammatory mediator signaling or inflammatory gene expression in macrophages and islets isolated from PARP-1-deficient mice. While PARP-1 deficiency protects islets against cytokine-induced islet cell death as measured by biochemical assays of membrane polarization, the genetic absence of PARP-1 does not effect cytokine-induced inhibition of insulin secretion or cytokine-induced DNA damage in islets. While PARP-1 deficiency appears to provide protection from cell death, it fails to provide protection against the inhibitory actions of cytokines on insulin secretion or the damaging actions on islet DNA integrity.

A role for aberrant protein palmitoylation in FFA-induced ER stress and β-cell death

Exposure of insulin-producing cells to elevated levels of the free fatty acid (FFA) palmitate results in the loss of β-cell function and induction of apoptosis. The induction of endoplasmic reticulum (ER) stress is one mechanism proposed to be responsible for the loss of β-cell viability in response to palmitate treatment; however, the pathways responsible for the induction of ER stress by palmitate have yet to be determined. Protein palmitoylation is a major posttranslational modification that regulates protein localization, stability, and activity. Defects in, or dysregulation of, protein palmitoylation could be one mechanism by which palmitate may induce ER stress in β-cells. The purpose of this study was to evaluate the hypothesis that palmitate-induced ER stress and β-cell toxicity are mediated by excess or aberrant protein palmitoylation. In a concentration-dependent fashion, palmitate treatment of RINm5F cells results in a loss of viability. Similar to palmitate, stearate also induces a concentration-related loss of RINm5F cell viability, while the monounsaturated fatty acids, such as palmoleate and oleate, are not toxic to RINm5F cells. 2-Bromopalmitate (2BrP), a classical inhibitor of protein palmitoylation that has been extensively used as an inhibitor of G protein-coupled receptor signaling, attenuates palmitate-induced RINm5F cell death in a concentration-dependent manner. The protective effects of 2BrP are associated with the inhibition of [(3)H]palmitate incorporation into RINm5F cell protein. Furthermore, 2BrP does not inhibit, but appears to enhance, the oxidation of palmitate. The induction of ER stress in response to palmitate treatment and the activation of caspase activity are attenuated by 2BrP. Consistent with protective effects on insulinoma cells, 2BrP also attenuates the inhibitory actions of prolonged palmitate treatment on insulin secretion by isolated rat islets. These studies support a role for aberrant protein palmitoylation as a mechanism by which palmitate enhances ER stress activation and causes the loss of insulinoma cell viability.

Ccr5 regulates inflammatory gene expression in response to encephalomyocarditis virus infection

Encephalomyocarditis virus (EMCV) is capable of stimulating inflammatory gene expression by macrophages as a result of interactions between EMCV capsid proteins and cell surface receptors. In this study, biochemical and genetic approaches identified a role for Ccr5, a chemokine receptor, in transducing the signals of EMCV infection that result in the expression of inflammatory genes in macrophages. Antibody neutralization and gene knockout strategies were used to show that the presence of Ccr5 is required for EMCV-stimulated mitogen-activated protein (MAP) kinase and nuclear factor-kappa B (NF-κB) activation, and the subsequent expression of the inflammatory gene-inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX-2). Ccr5 appears to participate in the early control of virus replication: EMCV mRNA accumulates to sevenfold higher levels in Ccr5-deficient mice when compared to wild-type controls. These findings support a regulatory role for Ccr5 in the antiviral response to EMCV in which this chemokine receptor participates in regulation of inflammatory gene expression in response to virus infection.

FoxO1 and SIRT1 regulate beta-cell responses to nitric oxide

For many cell types, including pancreatic β-cells, nitric oxide is a mediator of cell death; paradoxically, nitric oxide can also activate pathways that promote the repair of cellular damage. In this report, a role for FoxO1-dependent transcriptional activation and its regulation by SIRT1 in determining the cellular response to nitric oxide is provided. In response to nitric oxide, FoxO1 translocates from the cytoplasm to the nucleus and stimulates the expression of the DNA repair gene GADD45α, resulting in FoxO1-dependent DNA repair. FoxO1-dependent gene expression appears to be regulated by the NAD(+)-dependent deacetylase SIRT1. In response to SIRT1 inhibitors, the FoxO1-dependent protective actions of nitric oxide (GADD45α expression and DNA repair) are attenuated, and FoxO1 activates a proapoptotic program that includes PUMA (p53-up-regulated mediator of apoptosis) mRNA accumulation and caspase-3 cleavage. These findings support primary roles for FoxO1 and SIRT1 in regulating the cellular responses of β-cells to nitric oxide.

Enhanced rat islet function and survival in vitro using a biomimetic self-assembled nanomatrix gel

Peptide amphiphile (PA) is a peptide-based biomaterial that can self-assemble into a nanostructured gel-like scaffold, mimicking the chemical and biological complexity of natural extracellular matrix. To evaluate the capacity of the PA scaffold to improve islet function and survival in vitro, rat islets were cultured in three different groups--(1) bare group: isolated rat islets cultured in a 12-well nontissue culture-treated plate; (2) insert group: isolated rat islets cultured in modified insert chambers; (3) nanomatrix group: isolated rat islets encapsulated within the PA nanomatrix gel and cultured in modified insert chambers. Over 14 days, both the bare and insert groups showed a marked decrease in insulin secretion, whereas the nanomatrix group maintained glucose-stimulated insulin secretion. Moreover, entire islets in the nanomatrix gel stained positive for dithizone up to 14 days, indicating better maintained glucose-stimulated insulin production. Fluorescein diacetate/propidium iodide staining results also verified necrosis in the bare and insert groups after 7 days, whereas the PA nanomatrix gel maintained islet viability after 14 days. Thus, these results demonstrate the potential of PAs as an intermediary scaffold for increasing the efficacy of pancreatic islet transplantation.

Homogeneous insulin and C-Peptide sensors for rapid assessment of insulin and C-peptide secretion by the islets

OBJECTIVE

Glucose-stimulated islet insulin or C-peptide secretion experiments are a fundamental tool for studying and assessing islet function. The goal of this work was to develop Ab-based fluorescent homogenous sensors that would allow rapid, inexpensive, near-instantaneous determinations of insulin and C-peptide levels in biological samples.

RESEARCH DESIGN AND METHODS

Our approach was to use molecular pincer design (Heyduk et al., Anal Chem 2008;80:5152-5159) in which a pair of antibodies recognizing nonoverlapping epitopes of the target are modified with short fluorochrome-labeled complementary oligonucleotides and are used to generate a fluorescence energy transfer (FRET) signal in the presence of insulin or C-peptide.

RESULTS

The sensors were capable of detecting insulin and C-peptide with high specificity and with picomolar concentration detection limits in times as short as 20 min. Insulin and C-peptide levels determined with the FRET sensors showed outstanding correlation with determinations performed under the same conditions with enzyme-linked immunosorbent assay. Most importantly, the sensors were capable of rapid and accurate determinations of insulin and C-peptide secreted from human or rodent islets, verifying their applicability for rapid assessment of islet function.

CONCLUSIONS

The homogeneous, rapid, and uncomplicated nature of insulin and C-peptide FRET sensors allows rapid assessment of β-cell function and could enable point-of-care determinations of insulin and C-peptide.

Digital models vs plaster models using alginate and alginate substitute materials

OBJECTIVE

To compare the dimensional stability of four impression materials over time and to compare OraMetrix digital models vs traditional plaster models.

MATERIALS AND METHODS

Two traditional alginates (Identic and imprEssix) and two alginate substitutes (Alginot FS and Position PentaQuick) were used to take multiple impressions of a maxillary typodont. Fifteen impressions for each material were taken and poured with plaster at three time points: 72 hours, 120 hours, and 1 week. Five impressions for each material were taken and were sent to OrthoProof for digital model reproduction at 72 hours. Digital models were then integrated with OraMetrix software. Plaster and digital models were measured in the anterior-posterior, transverse, and vertical dimensions. The control typodont and plaster models were measured using a digital caliper, and digital models were measured using OraMetrix software.

RESULTS

Statistically significant changes were found for models replicated from Identic impression material in all three dimensions by 72 hours. Statistically significant changes were seen in imprEssix impressions in the vertical and intercanine dimensions. Digital models were significantly smaller in all dimensions compared with plaster models and the control.

CONCLUSIONS

Identic impression material showed a statistically and clinically significant change in all dimensions within 72 hours and therefore should not be used if impressions are not going to be poured immediately. Alginate substitutes were dimensionally stable over an extended period. Digital models produced by OraMetrix were not clinically acceptable compared with plaster models.

Src family kinases participate in the regulation of encephalomyocarditis virus-induced cyclooxygenase-2 expression by macrophages

Src family kinases (SFKs) are non-receptor tyrosine kinases that have been implicated as regulators of the inflammatory response. In this study, the role of SFK activation in the inflammatory response of macrophages to encephalomyocarditis virus (EMCV) infection was examined. Virus infection of macrophages stimulates the expression of cyclooxygenase-2 (COX-2), interleukin (IL)-1β and inducible nitric oxide synthase (iNOS). Inhibition of SFK attenuates EMCV-induced COX-2 expression and prostaglandin E₂ production, iNOS expression and subsequent nitric oxide production, and IL-1β expression. EMCV-induced COX-2 expression requires the activation of nuclear factor-κB and the mitogen-activated protein kinase p38. Consistent with these previous findings, inhibition of SFKs attenuated the phosphorylation of p38 in response to EMCV infection, suggesting that SFKs may act upstream of p38. These findings provide evidence that SFK activation plays an active role in the regulation of inflammatory gene expression by virus-infected macrophages.

Facilitative glucose transporter 9 expression affects glucose sensing in pancreatic beta-cells

Facilitative glucose transporters (GLUTs) including GLUT9, accelerate the facilitative diffusion of glucose across the plasma membrane. Studies in GLUT2-deficient mice suggested the existence of another GLUT in the mammalian beta-cell responsible for glucose sensing. The objective of this study was to determine the expression and function of GLUT9 in murine and human beta-cells. mRNA and protein expression levels were determined for both isoforms of GLUT9 in murine and human isolated islets as well as insulinoma cell lines (MIN6). Immunohistochemistry and subcellular localization were performed to localize the protein within the cell. Small interfering RNA knockdown of GLUT9 was used to determine the effect of this transporter, in the presence of GLUT2, on cell metabolism and insulin secretion in MIN6 and INS cells. In this report we demonstrate that GLUT9a and GLUT9b are expressed in pancreatic islets and that this expression localizes to insulin-containing beta-cells. Subcellular localization studies indicate that mGLUT9b is found associated with the plasma membrane as well as in the high-density microsome fraction and low-density microsome fraction, whereas mGLUT9a appears to be located only in the high-density microsome and low-density microsome under basal conditions. Functionally GLUT9 appears to participate in the regulation of glucose-stimulated insulin secretion in addition to GLUT2. small interfering RNA knockdown of GLUT9 results in reduced cellular ATP levels that correlate with reductions in glucose-stimulated insulin secretion in MIN6 and INS cells. These studies confirm the expression of GLUT9a and GLUT9b in murine and human beta-cells and suggest that GLUT9 may participate in glucose-sensing in beta-cells.

AMP-activated protein kinase attenuates nitric oxide-induced beta-cell death

During the initial autoimmune response in type 1 diabetes, islets are exposed to a damaging mix of pro-inflammatory molecules that stimulate the production of nitric oxide by beta-cells. Nitric oxide causes extensive but reversible cellular damage. In response to nitric oxide, the cell activates pathways for functional recovery and adaptation as well as pathways that direct beta-cell death. The molecular events that dictate cellular fate following nitric oxide-induced damage are currently unknown. In this study, we provide evidence that AMPK plays a primary role controlling the response of beta-cells to nitric oxide-induced damage. AMPK is transiently activated by nitric oxide in insulinoma cells and rat islets following IL-1 treatment or by the exogenous addition of nitric oxide. Active AMPK promotes the functional recovery of beta-cell oxidative metabolism and abrogates the induction of pathways that mediate cell death such as caspase-3 activation following exposure to nitric oxide. Overall, these data show that nitric oxide activates AMPK and that active AMPK suppresses apoptotic signaling allowing the beta-cell to recover from nitric oxide-mediated cellular stress.

Nitric oxides mediates a shift from early necrosis to late apoptosis in cytokine-treated β-cells that is associated with irreversible DNA damage

For many cell types, including pancreatic β-cells, nitric oxide is a mediator of cell death; however, it is paradoxical that for a given cell type nitric oxide can induce both necrosis and apoptosis. This report tests the hypothesis that cell death mediated by nitric oxide shifts from an early necrotic to a late apoptotic event. Central to this transition is the ability of β-cells to respond and repair nitric oxide-mediated damage. β-Cells have the ability to repair DNA that is damaged following 24-h incubation with IL-1; however, cytokine-induced DNA damage becomes irreversible following 36-h incubation. This irreversible DNA damage following 36-h incubation with IL-1 correlates with the activation of caspase-3 (cleavage and activity). The increase in caspase activity correlates with reductions in endogenous nitric oxide production, as nitric oxide is an inhibitor of caspase activity. In contrast, caspase cleavage or activation is not observed under conditions in which β-cells are capable of repairing damaged DNA (24-h incubation with cytokines). These findings provide evidence that β-cell death in response to cytokines shifts from an early necrotic process to apoptosis and that this shift is associated with irreversible DNA damage and caspase-3 activation.

Phosphatidylinositol 3-kinase regulates macrophage responses to double-stranded RNA and encephalomyocarditis virus

Virus infection of macrophages stimulates the expression of proinflammatory and antiviral genes interleukin-1 (IL-1), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). In this study, we show that phosphatidylinositol 3-kinase (PI3K) is required for the inflammatory response of macrophages to virus infection. When macrophages are infected with encephalomyocarditis virus (EMCV) there is a rapid and transient activation of PI3K and phosphorylation of its downstream target Akt. Inhibitors of PI3K attenuate EMCV- and double-stranded RNA-induced iNOS, COX-2 and IL-1 beta expression in RAW264.7 cells and mouse peritoneal macrophages. The attenuation of inflammatory gene expression in response to PI3K inhibition correlates with the induction of macrophage apoptosis. The morphology of macrophages shifts from activation in response to EMCV infection to apoptosis in the cells treated with PI3K inhibitors and EMCV. These morphological changes are accompanied by the activation of caspase-3. These findings suggest that PI3K plays a central role in the regulation of macrophage responses to EMCV infection. When PI3K is activated, it participates in the regulation of inflammatory gene expression; however, if PI3K is inhibited macrophages are unable to mount an inflammatory antiviral response and die by apoptosis.

Repair of nitric oxide-damaged DNA in beta-cells requires JNK-dependent GADD45alpha expression

Proinflammatory cytokines induce nitric oxide-dependent DNA damage and ultimately beta-cell death. Not only does nitric oxide cause beta-cell damage, it also activates a functional repair process. In this study, the mechanisms activated by nitric oxide that facilitate the repair of damaged beta-cell DNA are examined. JNK plays a central regulatory role because inhibition of this kinase attenuates the repair of nitric oxide-induced DNA damage. p53 is a logical target of JNK-dependent DNA repair; however, nitric oxide does not stimulate p53 activation or accumulation in beta-cells. Further, knockdown of basal p53 levels does not affect DNA repair. In contrast, expression of growth arrest and DNA damage (GADD) 45alpha, a DNA repair gene that can be regulated by p53-dependent and p53-independent pathways, is stimulated by nitric oxide in a JNK-dependent manner, and knockdown of GADD45alpha expression attenuates the repair of nitric oxide-induced beta-cell DNA damage. These findings show that beta-cells have the ability to repair nitric oxide-damaged DNA and that JNK and GADD45alpha mediate the p53-independent repair of this DNA damage.

Glucose-modulated tyrosine nitration in beta cells: targets and consequences

Hyperglycemia, key factor of the pre-diabetic and diabetic pathology, is associated with cellular oxidative stress that promotes oxidative protein modifications. We report that protein nitration is responsive to changes in glucose concentrations in islets of Langerhans and insulinoma beta cells. Alterations in the extent of tyrosine nitration as well as the cellular nitroproteome profile correlated tightly with changing glucose concentrations. The target proteins we identified function in protein folding, energy metabolism, antioxidant capacity, and membrane permeability. Nitration of heat shock protein 60 in vitro was found to decrease its ATP hydrolysis and interaction with proinsulin, suggesting a mechanism by which protein nitration could diminish insulin secretion. This was supported by our finding of a decrease in stimulated insulin secretion following glycolytic stress in cultured cells. Our results reveal that protein tyrosine nitration may be a previously unrecognized factor in beta-cell dysfunction and the pathogenesis of diabetes.

The CDK domain of p21 is a suppressor of IL-1beta-mediated inflammation in activated macrophages

Significant morbidity and mortality can be attributed to inflammatory diseases; therefore, a greater understanding of the mechanisms involved in the progression of inflammation is crucial. Here, we demonstrate that p21((WAF1/CIP1)), an established suppressor of cell cycle progression, is a inhibitor of IL-1beta synthesis in macrophages. Mice deficient in p21 (p21(-/-)) display increased susceptibility to endotoxic shock, which is associated with increased serum levels of IL-1beta. Administration of IL-1 receptor antagonist reduces LPS-induced lethality in p21(-/-) mice. Analysis of isolated macrophages, which are one of the central producers of IL-1beta, reveals that deficiency for p21 led to more IL-1beta mRNA and pro-protein synthesis following TLR ligation. The increase in IL-1beta pro-protein is associated with elevated secretion of active IL-1beta by p21(-/-) macrophages. siRNA-mediated knockdown of p21 in human macrophages results in increased IL-1beta secretion as well. A peptide mapping strategy shows that the cyclin-dependent-kinase (CDK)-binding domain of p21 is sufficient to reduce the secretion of IL-1beta by p21(-/-) macrophages. These data suggest a novel role for p21 and specifically for the CDK-binding domain of p21((WAF1/CIP1)) in inhibiting inflammation.

Selective mtDNA mutation accumulation results in beta-cell apoptosis and diabetes development

To test the hypothesis that somatic mitochondrial (mt)DNA mutation accumulation predisposes mice to beta-cell loss and diabetes development, transgenic mice expressing a proofreading-deficient mtDNA polymerase-gamma under the control of the rat insulin-1 promoter were generated. At 6 wk of age, mtDNA mutations reached 0.01% (1.05 mutations/10,000 bp) in islets isolated from transgenic mice. This mutational burden is associated with impaired glucose tolerance and a diabetes prevalence of 52% in male transgenic mice. Female transgenic mice maintain slightly elevated fasting glucose levels, mild glucose intolerance, and a diabetes prevalence of 14%. Diabetes in transgenic animals is associated with insulin insufficiency that results from a significant reduction in beta-cell mass. Importantly, apoptosis of beta-cells is increased 7-fold in female and 11-fold in male transgenic mice compared with littermate controls. These results are consistent with a causative role of somatic mtDNA mutation accumulation in the loss of beta-cell mass and diabetes development.

Primary naive and interleukin-2-activated natural killer cells do not support efficient ectromelia virus replication

Natural killer (NK) cells are known for their ability to lyse tumour cell targets. Studies of infections by a number of viruses, including poxviruses and herpesviruses, have demonstrated that NK cells are vital for recovery from these infections. Little is known of the ability of viruses to infect and complete a productive replication cycle within NK cells. Even less is known concerning the effect of infection on NK cell biology. This study investigated the ability of ectromelia virus (ECTV) to infect NK cells in vitro and in vivo. Following ECTV infection, NK cell gamma interferon (IFN-gamma) production was diminished and infected cells ceased proliferating and lost viability. ECTV infection of NK cells led to early and late virus gene expression and visualization of immature and mature virus particles, but no detectable increase in viable progeny virus. It was not unexpected that early gene expression occurred in infected NK cells, as the complete early transcription system is packaged within the virions. The detection of the secreted early virus-encoded immunomodulatory proteins IFN-gamma-binding protein and ectromelia inhibitor of complement enzymes (EMICE) in NK cell culture supernatants suggests that even semi-permissive infection may permit immunomodulation of the local environment.

The role of nitric oxide and the unfolded protein response in cytokine-induced beta-cell death

OBJECTIVE

The unfolded protein response (UPR) is a conserved cellular response designed to alleviate damage and promote survival of cells experiencing stress; however, prolonged UPR activation can result in apoptotic cell death. The UPR, activated by cytokine-induced nitric oxide (NO) production, has been proposed to mediate beta-cell death in response to cytokines. In this study, the role of UPR activation in cytokine-induced beta-cell death was examined.

RESEARCH DESIGN AND METHODS

The effects of cytokine treatment of rat and human islets and RINm5F cells on UPR activation, NO production, and cell viability were examined using molecular and biochemical methodologies.

RESULTS

UPR activation correlates with beta-cell death in interleukin (IL)-1-treated rat islets. NO mediates both cytokine-induced UPR activation and beta-cell death as NO synthase inhibitors attenuate each of these IL-1-stimulated events. Importantly, cytokines and tunicamycin, a classical UPR activator, induce beta-cell death by different mechanisms. Cell death in response to the classical UPR activator is associated with a 2.5-fold increase in caspase-3 activity, while IL-1 fails to stimulate caspase-3 activity. In addition, cell death is enhanced by approximately 35% in tunicamycin-treated cells expressing an S51A eIF2 alpha mutant that cannot be phosphorylated or in cells lacking PERK (protein kinase regulated by RNA/endoplasmic reticulum-like kinase). In contrast, neither the absence of PERK nor the expression of the S51A eIF2 alpha mutant affects the levels of cytokine-induced death.

CONCLUSIONS

While cytokine-induced beta-cell death temporally correlates with UPR activation, the lack of caspase activity and the ability of NO to attenuate caspase activity suggest that prolonged UPR activation does not mediate cytokine-induced beta-cell death.

A transgenic model to study the pathogenesis of somatic mtDNA mutation accumulation in beta-cells

Low levels of somatic mutations accumulate in mitochondrial DNA (mtDNA) as we age; however, the pathogenic nature of these mutations is unknown. In contrast, mutational loads of >30% of mtDNA are associated with electron transport chain defects that result in mitochondrial diseases such as mitochondrial encephalopathy lactic acidosis and stroke-like episodes. Pancreatic beta-cells may be extremely sensitive to the accumulation of mtDNA mutations, as insulin secretion requires the mitochondrial oxidation of glucose to CO(2). Type 2 diabetes arises when beta-cells fail to compensate for the increased demand for insulin, and many type 2 diabetics progress to insulin dependence because of a loss of beta-cell function or beta-cell death. This loss of beta-cell function/beta-cell death has been attributed to the toxic effects of elevated levels of lipids and glucose resulting in the enhanced production of free radicals in beta-cells. mtDNA, localized in close proximity to one of the major cellular sites of free radical production, comprises more than 95% coding sequences such that mutations result in changes in the coding sequence. It has long been known that mtDNA mutations accumulate with age; however, only recently have studies examined the influence of somatic mtDNA mutation accumulation on disease pathogenesis. This article will focus on the effects of low-level somatic mtDNA mutation accumulation on ageing, cardiovascular disease and diabetes.

Zinc induces ERK-dependent cell death through a specific Ras isoform

The effect of Zn on p53-independent cell death was examined in IIC9 embryonic fibroblasts. Despite the fact that these cells are p53-minus, Zn-mediated death occurs via an apoptotic mechanism. Death is facilitated by the presence of the Zn ionophore, pyrithione, indicating that intracellular Zn initiates the death response. Our investigations of the mechanism of Zn action demonstrate that Zn induces the death of IIC9 cells in a manner that is ERK-dependent. Expression of dn-(dominant negative)Ras attenuates ERK1/2 activation by Zn, and correspondingly reduces its cytotoxic effects. Raf-RBD pull-down experiments confirm that Zn treatment activates Ras and identified H-Ras as the specific isoform activated. This contrasts the activation of N-Ras that occurs when IIC9 cells are stimulated with thrombin. Thus, although the prolonged activation of the Ras/ERK pathway by Zn is similar to that seen when induced by mitogen, the distinguishing feature appears to be the isoform specificity of Ras activation.

The success of orthodontic satellite practices

OBJECTIVE

To evaluate the financial success and personal satisfaction of orthodontists operating satellite practices.

MATERIALS AND METHODS

A questionnaire was e-mailed to 4426 members of the American Association of Orthodontists (AAO), and 817 completed surveys were returned, yielding an 18% response rate. Only replies from solo practitioner orthodontists were analyzed to limit variability among respondents.

RESULTS

Solo practitioner orthodontists with satellite practices reported an increase in net profit and started significantly more cases than orthodontists with one office. Yet, only 27% of respondents with one satellite and 48% of practitioners operating multiple satellites recommend opening a satellite practice. Ninety-two percent of orthodontists stated they were satisfied with the clinical aspects of their practice, and 99% of orthodontists described themselves as happy and satisfied individuals.

CONCLUSION

Despite the ability of a satellite office to expand a patient base and increase net income, most practitioners would not encourage young orthodontists to establish a satellite office.

PGJ2-stimulated beta-cell apoptosis is associated with prolonged UPR activation

Peroxisome proliferator-activated receptor-gamma (PPARgamma) ligands have been shown to possess anti-inflammatory properties that include the inhibition of transcription factor activation and the expression of inflammatory genes. Using pancreatic beta-cells, we have shown that PPARgamma ligands such as 15-deoxy-Delta(12,14)-prostaglandin J(2) (PGJ(2)) attenuate interferon-gamma-induced signal transducer and activator of transcription 1 activation and interleukin (IL)-1beta-induced nuclear factor-kappaB activation by a pathway that correlates with endoplasmic reticulum stress and the induction of the unfolded protein response (UPR). The UPR is a conserved cellular response activated by a number of cell stressors and is believed to alleviate the stress and promote cell survival. However, prolonged activation of the UPR results in cellular death by apoptosis. In this report, we have examined the effects of PGJ(2) on UPR activation and the consequences of this activation on cell survival. Consistent with induction of a cell death pathway, treatment of rat islets and RINm5F cells for 24 h with PGJ(2) results in caspase-3 activation and caspase-dependent beta-cell death. The actions of these ligands do not appear to be selective for beta-cells, because PGJ(2) stimulates macrophage apoptosis in a similar fashion. Associated with cell death is the enhanced phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha), and in cells expressing a mutant of eIF2alpha that cannot be phosphorylated, the stimulatory actions of PGJ(2) on caspase-3 activation are augmented. These findings suggest that, whereas PGJ(2)-induced UPR activation is associated with an inhibition of cytokine signaling, prolonged UPR activation results in cell death, and that eIF2alpha phosphorylation may function in a protective manner to attenuate cell death.

Induction of natural killer cell responses by ectromelia virus controls infection

Natural killer (NK) cells play a pivotal role in the innate immune response to viral infections, particularly murine cytomegalovirus (MCMV) and human herpesviruses. In poxvirus infections, the role of NK cells is less clear. We examined disease progression in C57BL/6 mice after the removal of NK cells by both antibody depletion and genetic means. We found that NK cells were crucial for survival and the early control of virus replication in spleen and to a lesser extent in liver in C57BL/6 mice. Studies of various knockout mice suggested that gammadelta T cells and NKT cells are not important in the C57BL/6 mousepox model and CD4+ and CD8+ T cells do not exhibit antiviral activity at 6 days postinfection, when the absence of NK cells has a profound effect on virus titers in spleen and liver. NK cell cytotoxicity and/or gamma interferon (IFN-gamma) secretion likely mediated the antiviral effect needed to control virus infectivity in target organs. Studies of the effects of ectromelia virus (ECTV) infection on NK cells demonstrated that NK cells proliferate within target tissues (spleen and liver) and become activated following a low-dose footpad infection, although the mechanism of activation appears distinct from the ligand-dependent activation observed with MCMV. NK cell IFN-gamma secretion was detected by intracellular cytokine staining transiently at 32 to 72 h postinfection in the lymph node, suggesting a role in establishing a Th1 response. These results confirm a crucial role for NK cells in controlling an ECTV infection.

Chemically chaperoning the actions of insulin

The role of inflammation as a mediator of insulin resistance in type 2 diabetes and obesity has been a major focus of studies over the past ten years. In mouse models of obesity and type 2 diabetes, the development of insulin resistance correlates with elevated levels of endoplasmic reticulum stress and induction of the unfolded protein response. Activation of N-terminal C-Jun kinase is known to be associated with unfolded protein response activation, and has been shown to participate in the inhibition of insulin action by stimulating serine phosphorylation of the insulin receptor substrate 1, an event that attenuates insulin signaling. 'Chemical chaperones' are small molecules that have been shown to attenuate unfolded protein response activation. The exciting new findings of Ozcan et al. indicate that chemical chaperones improve glucose tolerance and insulin action in a mouse model of type 2 diabetes. These findings offer a potential new target for therapeutic strategies designed to improve insulin action and glucose tolerance in diabetic individuals.

Role of MAPK in the regulation of double-stranded RNA- and encephalomyocarditis virus-induced cyclooxygenase-2 expression by macrophages

In response to virus infection or treatment with dsRNA, macrophages express the inducible form of cyclooxygenase-2 (COX-2) and produce proinflammatory prostaglandins. Recently, we have shown that NF-kappaB is required for encephalomyocarditis virus (EMCV)- and dsRNA-stimulated COX-2 expression in mouse macrophages. The dsRNA-dependent protein kinase R is not required for EMCV-stimulated COX-2 expression, suggesting the presence of protein kinase R-independent pathways in the regulation of this antiviral gene. In this study, the role of MAPK in the regulation of macrophage expression of cyclooxygenase-2 (COX)-2 in response to EMCV infection was examined. Treatment of mouse macrophages or RAW-264.7 cells with dsRNA or infection with EMCV stimulates the rapid activation of the MAPKs p38, JNK, and ERK. Inhibition of p38 and JNK activity results in attenuation while ERK inhibition does not modulate dsRNA- and EMCV-induced COX-2 expression and PGE2 production by macrophages. JNK and p38 appear to selectively regulate COX-2 expression, as inhibition of either kinase fails to prevent dsRNA- or EMCV-stimulated inducible NO synthase expression by macrophages. Using macrophages isolated from TLR3-deficient mice, we show that p38 and JNK activation and COX-2 expression in response to EMCV or poly(IC) does not require the presence the dsRNA receptor TLR3. These findings support a role for p38 and JNK in the selective regulation of COX-2 expression by macrophages in response to virus infection.

Insulin biosynthesis: the IREny of it all

Recent studies have shown that the unfolded protein response (UPR) is essential for the survival of insulin-producing beta cells. Work in this issue of Cell Metabolism (Lipson et al., 2006) identifies a novel role for UPR activation in beta cell function by demonstrating that the UPR effector IRE1 is a positive regulator of glucose-stimulated proinsulin biosynthesis.

Posttranslational, translational, and transcriptional responses to nitric oxide stress in Cryptococcus neoformans: implications for virulence

The ability of the fungal pathogen Cryptococcus neoformans to evade the mammalian innate immune response and cause disease is partially due to its ability to respond to and survive nitrosative stress. In this study, we use proteomic and genomic approaches to elucidate the response of C. neoformans to nitric oxide stress. This nitrosative stress response involves both transcriptional, translational, and posttranslational regulation. Proteomic and genomic analyses reveal changes in expression of stress response genes. In addition, genes involved in cell wall organization, respiration, signal transduction, transport, transcriptional control, and metabolism show altered expression under nitrosative conditions. Posttranslational modifications of transaldolase (Tal1), aconitase (Aco1), and the thiol peroxidase, Tsa1, are regulated during nitrosative stress. One stress-related protein up-regulated in the presence of nitric oxide stress is glutathione reductase (Glr1). To further investigate its functional role during nitrosative stress, a deletion mutant was generated. We show that this glr1Delta mutant is sensitive to nitrosative stress and macrophage killing in addition to being avirulent in mice. These studies define the response to nitrosative stress in this important fungal pathogen.

Interleukin-1 stimulates beta-cell necrosis and release of the immunological adjuvant HMGB1

BACKGROUND

There are at least two phases of beta-cell death during the development of autoimmune diabetes: an initiation event that results in the release of beta-cell-specific antigens, and a second, antigen-driven event in which beta-cell death is mediated by the actions of T lymphocytes. In this report, the mechanisms by which the macrophage-derived cytokine interleukin (IL)-1 induces beta-cell death are examined. IL-1, known to inhibit glucose-induced insulin secretion by stimulating inducible nitric oxide synthase expression and increased production of nitric oxide by beta-cells, also induces beta-cell death.

METHODS AND FINDINGS

To ascertain the mechanisms of cell death, the effects of IL-1 and known activators of apoptosis on beta-cell viability were examined. While IL-1 stimulates beta-cell DNA damage, this cytokine fails to activate caspase-3 or to induce phosphatidylserine (PS) externalization; however, apoptosis inducers activate caspase-3 and the externalization of PS on beta-cells. In contrast, IL-1 stimulates the release of the immunological adjuvant high mobility group box 1 protein (HMGB1; a biochemical maker of necrosis) in a nitric oxide-dependent manner, while apoptosis inducers fail to stimulate HMGB1 release. The release of HMGB1 by beta-cells treated with IL-1 is not sensitive to caspase-3 inhibition, while inhibition of this caspase attenuates beta-cell death in response to known inducers of apoptosis.

CONCLUSIONS

These findings indicate that IL-1 induces beta-cell necrosis and support the hypothesis that macrophage-derived cytokines may participate in the initial stages of diabetes development by inducing beta-cell death by a mechanism that promotes antigen release (necrosis) and islet inflammation (HMGB1 release).

Encephalomyocarditis virus induces PKR-independent mitogen-activated protein kinase activation in macrophages

In this study, we provide evidence that the double-stranded RNA-dependent protein kinase (PKR) is not required for virus-induced expression of inducible nitric oxide synthase (iNOS) or the activation of specific signaling pathways in macrophages. The infection of RAW264.7 cells with encephalomyocarditis virus (EMCV) induces iNOS expression and nitric oxide production, which are unaffected by a dominant-negative mutant of PKR. EMCV infection also activates the mitogen-activated protein kinase, cyclic AMP response element binding protein, and nuclear factor kappaB (NF-kappaB) signaling cascades at 15 to 30 min postinfection in PKR+/+ and PKR-/- macrophages. Activation of these signaling cascades does not temporally correlate with PKR activity or the accumulation of EMCV RNA, suggesting that an interaction between a structural component of the virion and the cell surface may activate macrophages. Consistent with this hypothesis, empty EMCV capsids induced comparable levels of iNOS expression, nitrite production, and activation of these signaling cascades to those induced by intact virions. These findings support the hypothesis that virion-host cell interactions are primary mediators of the PKR-independent activation of signaling pathways that participate in the macrophage antiviral response of inflammatory gene expression.

Genetic and pharmacologic evidence that calcium-independent phospholipase A2beta regulates virus-induced inducible nitric-oxide synthase expression by macrophages

Recent evidence supports a regulatory role for the calcium-independent phospholipase A2 (iPLA2) in the antiviral response of inducible nitric-oxide synthase (iNOS) expression by macrophages. Because two mammalian isoforms of iPLA2 (iPLA2beta and iPLA2gamma) have been cloned and characterized, the aim of this study was to identify the specific isoform(s) in macrophages that regulates the expression of iNOS in response to virus infection. Bromoenol lactone (BEL), a suicide substrate inhibitor of iPLA2, inhibits the activity of both isoforms at low micromolar concentrations. However, the R- and S-enantiomers of BEL display approximately 10-fold greater potency for inhibition of the enzymatic activity of iPLA2gamma and iPLA2beta, respectively. In this study, we show that the iPLA2beta-selective (S)-BEL inhibits encephalomyocarditis virus (EMCV)-induced iNOS expression, nitric oxide production, and iPLA2 enzymatic activity in macrophages in a concentration-related manner that closely resembles the inhibitory properties of racemic BEL. cAMP response element-binding protein (CREB) is one downstream target of iPLA2 that is required for the transcriptional activation of iNOS in response to virus infection, and consistent with the effects of BEL enantiomers on iNOS expression, (S)-BEL more effectively inhibits EMCV-induced CREB phosphorylation than (R)-BEL in macrophages. Using macrophages isolated from iPLA2beta-null mice, virus infection fails to stimulate iNOS mRNA accumulation and protein expression, thus providing genetic evidence that iPLA2beta is required for EMCV-induced iNOS expression. These findings provide evidence for a signaling role for iPLA2beta in virus-induced iNOS expression by macrophages.

Distinct stress responses of two functional laccases in Cryptococcus neoformans are revealed in the absence of the thiol-specific antioxidant Tsa1

Laccases are thought to be important to the virulence of many fungal pathogens by producing melanin, a presumed oxygen radical scavenger. A laccase in Cryptococcus neoformans has been shown to synthesize melanin and contributes to the virulence and the survival in macrophages of this fungal pathogen. One C. neoformans laccase gene, LAC1, previously called CNLAC1, has been extensively studied, and we describe a homologous gene, LAC2, that is found 8 kb away from LAC1 in the genome. In this study we report a role for both laccases, in addition to the thiol peroxidase, Tsa1, in oxidative and nitrosative stress resistance mechanisms of C. neoformans. With use of real-time PCR, similar changes in expression of the two laccase genes occur in response to oxidative and nitrosative stresses, but only the regulation of the LAC2 gene during stress is influenced by Tsa1. Both laccases contribute to melanin production using L-dopa as a substrate and are differentially localized in the cell based on green fluorescent protein fusions. A single deletion of either LAC1 or LAC2 alone had no effect on sensitivity to H2O2 or nitric oxide. However, deletion of either LAC1 or LAC2 in combination with a TSA1 deletion resulted in a slight peroxide sensitivity, and a lac2Delta tsa1Delta deletion strain was sensitive to nitric oxide stress. In addition, the deletion of both laccases reduces survival of C. neoformans in primary macrophages. Based on our expression and functional analysis, we propose a novel model for the interaction of these two systems, which are both important for virulence.

Incretin hormones and insulin sensitivity

Incretin hormones such as glucagon-like peptide-1 (GLP-1) and the longer lasting analog exendin-4 show clinical promise for the treatment of diabetes because of glucoregulatory activities that enhance beta-cell function and growth, and actions in the central nervous system that induce satiety and decrease caloric intake. The actions of these peptides on insulin-responsive tissues is less clear, but recent advances indicate that chronic treatment with exendin-4 increases insulin sensitivity via two distinct mechanisms: one is attributable to changes in food intake and the subsequent improvements in glycemia; the second is largely independent of reductions in blood glucose. In addition, exendin-4 might also have direct effects on beta-cell neogenesis that are independent of insulin demand.

PPARgamma ligands induce ER stress in pancreatic beta-cells: ER stress activation results in attenuation of cytokine signaling

Peroxisome proliferator-activated receptor (PPAR)gamma ligands are known to have anti-inflammatory properties that include the inhibition of cytokine signaling, transcription factor activation, and inflammatory gene expression. We have recently observed that increased expression of heat shock protein (HSP)70 correlates with, but is not required for, the anti-inflammatory actions of PPARgamma ligands on cytokine signaling. In this study, we provide evidence that the inhibitory actions of PPARgamma ligands on cytokine signaling are associated with endoplasmic reticulum (ER) stress or unfolded protein response (UPR) activation in pancreatic beta-cells. 15-Deoxy-Delta(12,14)-prostaglandin J(2), at concentrations that inhibit cytokine signaling, stimulates phosphorylation of eukaryotic initiation factor-2alpha, and this event is followed by a rapid inhibition of protein translation. Under conditions of impaired translation, PPARgamma ligands stimulate the expression of a number of ER stress-responsive genes, such as GADD 153, BiP, and HSP70. Importantly, ER stress activation in response to PPARgamma ligands or known UPR activators results in the attenuation of IL-1 and IFN-gamma signaling. These findings indicate that PPARgamma ligands induce ER stress, that ER stress activation is associated with an attenuation of cytokine signaling in beta-cells, and that the attenuation of responsiveness to extracellular stimuli appears to be a novel protective action of the UPR in cells undergoing ER stress.

Role of cyclooxygenase-2 in cytokine-induced beta-cell dysfunction and damage by isolated rat and human islets

Type I diabetes mellitus is an autoimmune disease characterized by the selective destruction of the insulin-secreting beta-cell found in pancreatic islets of Langerhans. Cytokines such as interleukin-1 (IL-1), interferon-gamma (IFN-gamma), and tumor necrosis factor-alpha (TNF-alpha) mediate beta-cell dysfunction and islet degeneration, in part, through the induction of the inducible isoform of nitric-oxide synthase and the production of nitric oxide by beta-cells. Cytokines also stimulate the expression of the inducible isoform of cyclooxygenase, COX-2, and the production of prostaglandin E(2) (PGE(2)) by rat and human islets; however, the role of increased COX-2 expression and PGE(2) production in mediating cytokine-induced inhibition of islet metabolic function and viability has been incompletely characterized. In this study, we have shown that treatment of rat islets with IL-1beta or human islets with a cytokine mixture containing IL-1beta + IFN-gamma +/- TNF-alpha stimulates COX-2 expression and PGE(2) formation in a time-dependent manner. Co-incubation of rat and human islets with selective COX-2 inhibitors SC-58236 and Celecoxib, respectively, attenuated cytokine-induced PGE(2) formation. However, these inhibitors failed to prevent cytokine-mediated inhibition of insulin secretion or islet degeneration. These findings indicate that selective inhibition of COX-2 activity does not protect rat and human islets from cytokine-induced beta-cell dysfunction and islet degeneration and, furthermore, that islet production of PGE(2) does not mediate these inhibitory and destructive effects.

Glucose Regulation of Insulin Gene Expression Requires the Recruitment of p300 by the β-Cell-Specific Transcription Factor Pdx-1

Regulation of insulin gene expression in response to increases in blood glucose levels is essential for maintaining normal glucose homeostasis; however, the exact mechanisms by which glucose stimulates insulin gene transcription are not known. We have shown previously that glucose stimulates insulin gene expression by causing the hyperacetylation of histone H4 at the insulin promoter. We demonstrate that the histone acetyltransferase p300 is recruited to the insulin promoter only at high concentrations of glucose via its interaction with the beta-cell-specific transcription factor Pdx-1. Disruption of the function of the endogenous Pdx-1 abolishes the recruitment of p300 to the insulin gene promoter at high concentrations of glucose and results in decreased histone H4 acetylation and insulin gene expression. Furthermore, we demonstrate that the glucose-dependent interaction of Pdx-1 with p300 is regulated by a phosphorylation event that changes the localization of Pdx-1. Based on these data, we conclude that hyperacetylation of histone H4 at the insulin gene promoter in response to high concentrations of glucose depends on the beta-cell-specific transcription factor Pdx-1, which is required for the recruitment of the histone acetyltransferase p300 to the insulin gene promoter.

The role and regulation of COX-2 during viral infection

Prostaglandins are lipid mediators, generated by cyclooxygenase (COX), that have been shown to participate in the regulation of virus replication and the modulation of inflammatory responses following infection. A number of studies support a role for PGE2 in the modulation of virus replication and virulence in a cell type and virus selective manner. Virus infection also stimulates the expression of a number of proinflammatory gene products, including COX-2, inducible nitric oxide synthase (iNOS) as well as proinflammatory cytokines. This review will focus on the mechanisms by which proinflammatory prostaglandin production regulates virus replication and virulence. In addition, the signaling pathways that are activated during a virus infection, and that regulate proinflammatory gene expression in macrophages will be reviewed. Specific attention will be placed on the ability of virus infection to activate multiple signaling cascades (such as PKR, MAPK, iPLA2, NF-kappaB) and how these pathways are integrated in the regulation of individual target gene expression.

PPARgamma is not required for the inhibitory actions of PGJ2 on cytokine signaling in pancreatic beta-cells

Peroxisome proliferator-activated receptor (PPAR)gamma agonists, such as 15-deoxy-delta 12,14-prostaglandin J2 (PGJ2) and troglitazone, have been shown to elicit anti-inflammatory effects in pancreatic beta-cells that include inhibition of cytokine-stimulated inducible nitric oxide synthase (iNOS) gene expression and production of nitric oxide. In addition, these ligands impair IL-1-induced NF-kappaB and MAPK as well as IFN-gamma-stimulated signal transducer and activator of transcription (STAT)1 activation in beta-cells. The purpose of this study was to determine if PPARgamma activation participates in the anti-inflammatory actions of PGJ2 in beta-cells. Pretreatment of RINm5F cells for 6 h with PGJ2 results in inhibition of IL-1-stimulated IkappaB degradation and IFN-gamma-stimulated STAT1 phosphorylation. Overexpression of a dominant-negative (dn) PPARgamma mutant or treatment with the PPARgamma antagonist GW-9662 does not modulate the inhibitory actions of PGJ2 on cytokine signaling in RINm5F cells. Although these agents fail to attenuate the inhibitory actions of PGJ2 on cytokine signaling, they do inhibit PGJ2-stimulated PPARgamma response element reporter activity. Consistent with the inability to attenuate the inhibitory actions of PGJ2 on cytokine signaling, neither dnPPARgamma nor GW-9662 prevents the inhibitory actions of PGJ2 on IL-1-stimulated iNOS gene expression or nitric oxide production by RINm5F cells. These findings support a PPARgamma-independent mechanism by which PPARgamma ligands impair cytokine signaling and iNOS expression by islets.

Calcium-independent phospholipase A2 mediates CREB phosphorylation in double-stranded RNA-stimulated endothelial cells

One of the products of a calcium-independent phospholipase A2 (iPLA2) attack of plasmenylcholine, lysoplasmenylcholine, has previously been shown to activate cAMP-dependent protein kinase (PKA). Because endothelial cells respond to some agonists in part by the activation of iPLA2, the present study was designed to determine whether double-stranded RNA (dsRNA), the primary activator of the antiviral response in endothelial cells, elicits cAMP response element binding protein (CREB) phosphorylation through a mechanism mediated by iPLA2. dsRNA stimulated CREB phosphorylation in bovine pulmonary artery endothelial cells that was inhibited by the iPLA2 inhibitor, bromoenol lactone, and the PKA inhibitor, H-89. Additionally, the product of iPLA2 hydrolysis of plasmenylcholine and lysoplasmenylcholine elicited CREB phosphorylation in bovine pulmonary endothelial cells. Taken together, the present studies suggest that dsRNA as well as other agonists of endothelial cells elicit signaling mechanisms that include in part CREB phosphorylation mediated by iPLA2.

Role for c-Jun N-terminal kinase in beta-cell recovery from nitric oxide-mediated damage

Treatment of rat islets with the cytokine IL-1 results in the inhibition of mitochondrial function and insulin secretion, events that are mediated by beta-cell expression of iNOS [inducible nitric oxide (NO) synthase] and production of NO. beta-Cells recover from the inhibitory actions of NO, produced following 24 h incubation with IL-1, on islet oxidative metabolism and insulin secretion if iNOS enzymatic activity is inhibited and the islets are cultured (in the presence of IL-1 and iNOS inhibitors) for a brief period of 8 h. Islet recovery from cytokine- and NO-mediated damage is an active process that requires new gene expression, and NO itself is one activator of this recovery process. In this study, the mechanism by which NO stimulates islet recovery has been examined. Incubation of rat islets or RINm5F cells with the NO donor compound, sodium (Z)-1(N,N-diethylamino) diazen-1-ium-1,2-diolate (DEA-NO) for 1 h results in a 60% inhibition of mitochondrial aconitase activity. beta-Cells completely recover aconitase activity if the cells are washed to remove the NO donor compound and incubated for an additional 5 h in the absence of DEA-NO. The recovery of mitochondrial aconitase activity correlates with a 4-fold increase in cyclic GMP accumulation and is prevented by the inhibition of guanylate cyclase. The recovery of aconitase activity also correlates with the activation of members of the MAPKs, p38, c-Jun N-terminal kinase (JNK) and ERK, and the activation p38 and JNK is attenuated by inhibition of guanylate cyclase. ERK and p38 do not appear to participate in the recovery process as selective inhibition of these kinases fails to prevent recovery of aconitase activity; however, transduction of beta-cells with a dominant negative mutant JNK prevents beta-cell recovery from NO-mediated damage. These findings support a role for guanylate cyclase and JNK in the recovery of beta-cells from NO-mediated damage.

ERK activation is required for double-stranded RNA- and virus-induced interleukin-1 expression by macrophages

Double-stranded (ds) RNA, which accumulates during viral replication, activates the antiviral response of infected cells. In this study, we have identified a requirement for extracellular signal-regulated kinase (ERK) in the regulation of interleukin 1 (IL-1) expression by macrophages in response to dsRNA and viral infection. Treatment of RAW 264.7 cells or mouse macrophages with dsRNA stimulates ERK phosphorylation that is first apparent following a 15-min incubation and persists for up to 60 min, the accumulation of iNOS and IL-1 mRNA following a 6-h incubation, and the expression of iNOS and IL-1 at the protein level following a 24-h incubation. Inhibitors of ERK activation prevent dsRNA-induced ERK phosphorylation and IL-1 expression by macrophages. The regulation of macrophage activation by ERK appears to be selective for IL-1, as ERK inhibition does not attenuate dsRNA-induced iNOS expression by macrophages. dsRNA stimulates both ERK activation and IL-1 expression by macrophages isolated from dsRNA-dependent protein kinase (PKR)-deficient mice, indicating that PKR does not participate in this antiviral response. These findings support a novel PKR-independent role for ERK in the regulation of the antiviral response of IL-1 expression and release by macrophages.

Inhibition of IFN-gamma -induced STAT1 activation by 15- deoxy-Delta 12,14-prostaglandin J2

The inhibitory actions of 15-deoxy-Delta(12,14)-prostaglandin J(2) (PGJ(2)) on inflammatory gene expression have been attributed to the ability of this prostaglandin to inhibit the activation of NF-kappaB. In this study, we have identified an additional signaling pathway sensitive to inhibition by PGJ(2). We show that PGJ(2) inhibits interferon (IFN)-gamma-stimulated phosphorylation and DNA-binding activity of STAT1. The inhibitory actions on STAT1 phosphorylation are first apparent after a 1- to 2-h incubation and are maximal after a 6-h incubation with PGJ(2), and they correlate with the expression of heat shock protein (HSP)70 in islets. In previous studies, we have correlated the inhibitory actions of PGJ(2) on inducible nitric oxide synthase (iNOS) expression and NF-kappaB activation in response to IL-1 with the increased expression of HSP70. Using overexpression and antisense depletion, we provide evidence that HSP70 does not mediate the inhibitory actions of PGJ(2) on IL-1-induced NF-kappaB or IFN-gamma-induced STAT1 activation or cytokine-stimulated iNOS expression by beta-cells. Last, we show that the inhibitory actions of a short 6-h pulse with PGJ(2) on IL-1 plus IFN-gamma-stimulated iNOS expression and NO production by beta-cells are persistent for extended periods (< or =48 h). These findings suggest that PGJ(2) inhibits multiple cytokine-signaling pathways (IL-1 and IFN-gamma), that the inhibitory actions are persistent for extended periods, and that increased HSP70 expression correlates with, but does not appear to mediate, the inhibitory actions of PGJ(2) on IL-1 and IFN-gamma signaling in beta-cells.

Regulation of cyclooxygenase-2 expression by macrophages in response to double-stranded RNA and viral infection

In this study the regulation of macrophage expression of cyclooxygenase-2 (COX-2) in response to dsRNA and virus infection was examined. Treatment of RAW 264.7 macrophages with dsRNA results in COX-2 mRNA accumulation and protein expression and the production of PGE(2). Similar to dsRNA, encephalomyocarditis virus (EMCV) infection of RAW 264.7 cells stimulates COX-2 expression and PGE(2) accumulation. The dsRNA-dependent protein kinase (PKR), which has been shown to participate in the regulation of gene expression in response to dsRNA and virus infection, does not appear to participate in the regulation of COX-2 expression by macrophages. Expression of dominant negative mutants of PKR in RAW 264.7 cells fails to attenuate dsRNA- and EMCV-induced COX-2 expression or PGE(2) production. Furthermore, dsRNA and EMCV stimulate COX-2 expression and PGE(2) accumulation to similar levels in macrophages isolated from wild-type and PKR-deficient mice. Recently, a novel PKR-independent role for the calcium-independent phospholipase A(2) (iPLA(2)) in the regulation of inducible NO synthase expression by macrophages in response to virus infection has been identified. The selective iPLA(2) suicide substrate inhibitor bromoenol lactone prevents dsRNA- and EMCV-stimulated inducible NO synthase expression; however, bromoenol lactone does not attenuate dsRNA- or EMCV-induced COX-2 expression by macrophages. In contrast, inhibition of NF-kappaB activation prevents dsRNA-stimulated COX-2 expression and PGE(2) accumulation by macrophages. These findings indicate that virus infection and treatment with dsRNA stimulate COX-2 expression by a mechanism that requires the activation of NF-kappaB and that is independent of PKR or iPLA(2) activation.

Novel role for calcium-independent phospholipase A(2) in the macrophage antiviral response of inducible nitric-oxide synthase expression

The double-stranded (ds) RNA-dependent protein kinase (PKR) is a primary regulator of antiviral responses; however, the ability of dsRNA to activate nuclear factor-kappa B (NF-kappa B) and dsRNA + interferon gamma (IFN-gamma) to stimulate inducible nitric-oxide synthase (iNOS) expression by macrophages isolated from PKR(-/-) mice suggests that signaling pathways in addition to PKR participate in antiviral activities. We have identified a novel phospholipid-signaling cascade that mediates macrophage activation by dsRNA and viral infection. Bromoenol lactone (BEL), a selective inhibitor of the calcium-independent phospholipase A(2) (iPLA(2)), prevents dsRNA- and virus-induced iNOS expression by RAW 264.7 cells and mouse macrophages. BEL does not modulate dsRNA-induced interleukin 1 expression, nor does it affect dsRNA-induced NF-kappa B activation. Protein kinase A (PKA) and the cAMP response element binding protein (CREB) are downstream targets of iPLA(2), because selective PKA inhibition prevents dsRNA-induced iNOS expression, and the inhibitory actions of BEL on dsRNA-induced iNOS expression are overcome by the direct activation of PKA. In addition, BEL inhibits dsRNA-induced CREB phosphorylation and CRE reporter activation. PKR does not participate in iPLA(2) activation or iNOS expression, because dsRNA stimulates iPLA(2) activity and dsRNA + IFN-gamma induces iNOS expression and nitric oxide production to similar levels by macrophages isolated from PKR(+/+) and PKR(-/-) mice. These findings support a PKR-independent signaling role for iPLA(2) in the antiviral response of macrophages.