Evidence against a role for NLRP3-driven islet inflammation in db/db mice

Objectives

Type 2 diabetes (T2D) is associated with chronic, low grade inflammation. Activation of the NLRP3 inflammasome and secretion of its target interleukin-1β (IL-1β) have been implicated in pancreatic β cell failure in T2D. Specific targeting of the NLRP3 inflammasome to prevent pancreatic β cell death could allow for selective T2D treatment without compromising all IL-1β-associated immune responses. We hypothesized that treating a mouse model of T2D with MCC950, a compound that specifically inhibits NLRP3, would prevent pancreatic β cell death, thereby preventing the onset of T2D.

Methods

Diabetic db/db mice were treated with MCC950 via drinking water for 8 weeks from 6 to 14 weeks of age, a period over which they developed pancreatic β cell failure. We assessed metabolic parameters such as body composition, glucose tolerance, or insulin secretion over the course of the intervention.

Results

MCC950 was a potent inhibitor of NLRP3-induced IL-1β in vitro and was detected at high levels in the plasma of treated db/db mice. Treatment of pre-diabetic db/db mice with MCC950, however, did not prevent pancreatic dysfunction and full onset of the T2D pathology. When examining the NLRP3 pathway in the pancreas of db/db mice, we could not detect an activation of this pathway nor increased levels of its target IL-1β.

Conclusions

NLRP3 driven-pancreatic IL-1β inflammation does not play a key role in the pathogenesis of the db/db murine model of T2D.

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Inhibition of NLRP3 via MCC950 in db/db mice did not improve glucose tolerance.

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MCC950 treatment did not prevent beta cell loss of function.

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Expression of IL1beta and NLRP3 does not appear increased in db/db islets.

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We conclude against a role for NLRP3 in db/db pancreatic dysfunction.

The anti-inflammatory cytokine, interleukin (IL)-10, blocks the inhibitory effect of IL-1 beta on long term potentiation. A role for JNK

Several effects of the proinflammatory cytokine, interleukin-1 beta (IL-1 beta), have been described in the central nervous system, and one area of the brain where marked changes have been reported is the hippocampus. Among these changes are an IL-1 beta-induced inhibition of long term potentiation (LTP) in perforant path-granule cell synapses and an attenuation of glutamate release in synaptosomes prepared from the hippocampus. Evidence suggests that, at least in circulating cells, the anti-inflammatory cytokine, IL-10, antagonizes certain effects of IL-1. We investigated the effect of IL-10 on IL-1 beta-induced inhibition of LTP and glutamate release. The evidence presented indicates that IL-1 beta stimulates the stress-activated protein kinase, c-Jun-activated protein kinase (JNK), and IL-1 receptor-associated kinase, which may explain its inhibitory effect on release and LTP, and that IL-10 reversed the IL-1 beta-induced stimulation of JNK activity and inhibition of release and LTP. We observed that IL-10 abrogated the stimulatory effect of IL-1 beta on superoxide dismutase activity and reactive oxygen species production, whereas the H(2)O(2)-induced inhibition of LTP was also blocked by IL-10. We present evidence that suggests that the action of IL-10 may be mediated by its ability to induce shedding of the IL-1 type I receptor.

Internalin B activates nuclear factor-kappa B via Ras, phosphoinositide 3-kinase, and Akt

Internalin B (InlB), a 630-amino acid protein loosely attached to the surface of Listeria monocytogenes, participates in the entry of the bacterium into mammalian cells. This process requires the activation of phosphoinositide (PI) 3-kinase by InlB. Previously, we demonstrated that InlB activates the transcription factor Nuclear Factor-kappaB in murine J774 macrophage-like cells, an event that also requires PI 3-kinase. Here we have further investigated this phenomenon. InlB activated the small G-protein Ras in J774 cells. Inhibition of Ras with the farnesyltransferase inhibitor manumycin A inhibited NF-kappaB activation and the recruitment of the p85 subunit of PI 3-kinase, implying that Ras is required for PI 3-kinase activation. InlB also activated the PI 3-kinase downstream effector, Akt, as assessed by increased phosphorylation of Akt on serine 473. Transfection of Hep2 cells with dominant negative Ras N17 or dominant negative Akt inhibited the induction of a reporter gene linked to the interleukin-8 promoter by InlB. Furthermore, the Ras inhibitor manumycin A, the PI 3-kinase inhibitor LY294002, and an Akt inhibitor all blocked the induction of interleukin-8 by InlB. Our study is the first report of a bacterial product activating a pathway involving Ras, PI 3-kinase, and Akt, which leads to NF-kappaB activation. This process could be involved in host defense or the inhibition of apoptosis during infection.

The serine protease inhibitor antithrombin III inhibits LPS-mediated NF-kappaB activation by TLR-4

In Drosophila, the Toll family of proteins mediates the innate immune response. Toll is activated by Spaetzle, which is generated in response to pathogens via a serine protease cascade. We wished to investigate if lipopolysaccharides (LPS) might activate Toll-like receptor (TLR) 4 via a serine protease in humans. The serpin antithrombin III (ATIII) and the thrombin inhibitor hirudin both inhibited nuclear factor (NF)-kappaB activation by LPS and Lipid A. ATIII and hirudin were also able to inhibit LPS-induced NF-kappaB activation in cells stably transfected with TLR4. These results suggest that LPS may activate a mammalian serine protease, which generates a product required for TLR4 signalling.

Mal (MyD88-adapter-like) is required for Toll-like receptor-4 signal transduction

The recognition of microbial pathogens by the innate immune system involves Toll-like receptors (TLRs), which recognize pathogen-associated molecular patterns. Different TLRs recognize different pathogen-associated molecular patterns, with TLR-4 mediating the response to lipopolysaccharide from Gram-negative bacteria. All TLRs have a Toll/IL-1 receptor (TIR) domain, which is responsible for signal transduction. MyD88 is one such protein that contains a TIR domain. It acts as an adapter, being involved in TLR-2, TLR-4 and TLR-9 signalling; however, our understanding of how TLR-4 signals is incomplete. Here we describe a protein, Mal (MyD88-adapter-like), which joins MyD88 as a cytoplasmic TIR-domain-containing protein in the human genome. Mal activates NF-kappaB, Jun amino-terminal kinase and extracellular signal-regulated kinase-1 and -2. Mal can form homodimers and can also form heterodimers with MyD88. Activation of NF-kappaB by Mal requires IRAK-2, but not IRAK, whereas MyD88 requires both IRAKs. Mal associates with IRAK-2 by means of its TIR domain. A dominant negative form of Mal inhibits NF-kappaB, which is activated by TLR-4 or lipopolysaccharide, but it does not inhibit NF-kappaB activation by IL-1RI or IL-18R. Mal associates with TLR-4. Mal is therefore an adapter in TLR-4 signal transduction.

Transactivation by the p65 subunit of NF-kappaB in response to interleukin-1 (IL-1) involves MyD88, IL-1 receptor-associated kinase 1, TRAF-6, and Rac1

We have examined the involvement of components of the interleukin-1 (IL-1) signaling pathway in the transactivation of gene expression by the p65 subunit of NF-kappaB. Transient transfection of cells with plasmids encoding wild-type MyD88, IL-1 receptor-associated kinase 1 (IRAK-1), and TRAF-6 drove p65-mediated transactivation. In addition, dominant negative forms of MyD88, IRAK-1, and TRAF-6 inhibited the IL-1-induced response. In cells lacking MyD88 or IRAK-1, no effect of IL-1 was observed. Together, these results indicate that MyD88, IRAK-1, and TRAF-6 are important downstream regulators of IL-1-mediated p65 transactivation. We have previously shown that the low-molecular-weight G protein Rac1 is involved in this response. Constitutively active RacV12-mediated transactivation was not inhibited by dominant negative MyD88, while dominant negative RacN17 inhibited the MyD88-driven response, placing Rac1 downstream of MyD88 on this pathway. Dominant negative RacN17 inhibited wild-type IRAK-1- and TRAF-6-induced transactivation, and in turn, dominant negative IRAK-1 and TRAF-6 inhibited the RacV12-driven response, suggesting a mutual codependence of Rac1, IRAK-1, and TRAF-6 in regulating this pathway. Finally, Rac1 was found to associate with the receptor complex via interactions with both MyD88 and the IL-1 receptor accessory protein. A pathway emanating from MyD88 and involving IRAK-1, TRAF-6, and Rac1 is therefore involved in transactivation of gene expression by the p65 subunit of NF-kappaB in response to IL-1.

The effects of the bacterial endotoxin lipopolysaccharide on synaptic transmission and plasticity in the CA1-subiculum pathway in vivo

Lipopolysaccharide is derived from the cell wall of gram-negative bacteria and is a potent endotoxin which causes the release of cytokines in the CNS. We examined the effect of lipopolysaccharide on synaptic transmission and synaptic plasticity in the hippocampal area CA1-subicular pathway in vivo. We found that lipopolysaccharide did not affect baseline synaptic transmission in this pathway; it did, however, reduce the magnitude of paired-pulse facilitation, a form of short-term plasticity thought to be primarily presynaptic in origin. We then examined the interaction between lipopolysaccharide and two common models for the biological basis of memory: high-frequency stimulation induced long-term potentiation and low-frequency stimulation induced long-term depression of synaptic transmission. We found that lipopolysaccharide blocked long-term potentiation following high-frequency stimulation and also induced potentiation of synaptic transmission after low-frequency stimulation. Lipolysaccharide blocked paired-pulse facilitation selectively at short rather than longer interstimulus intervals. Thus, lipopolysaccharide has different effects on synaptic transmission in this pathway depending on the frequency and length of stimulation. These results provide new insights into the action of lipopolysaccharide on various forms of plasticity in the hippocampus, an area known to play a vital role in learning and memory.

Vitamin C inhibits NF-kappa B activation by TNF via the activation of p38 mitogen-activated protein kinase

The transcription factor NF-kappaB is a central mediator of altered gene expression during inflammation, and is implicated in a number of pathologies, including cancer, atherosclerosis, and viral infection. We report in this study that vitamin C inhibits the activation of NF-kappaB by multiple stimuli, including IL-1 and TNF in the endothelial cell line ECV304 and in primary HUVECs. The induction of a NF-kappaB-dependent gene, IL-8, by TNF was also inhibited. The effect requires millimolar concentrations of vitamin C, which occur intracellularly in vivo, particularly during inflammation. Vitamin C was not toxic to cells, did not inhibit another inducible transcription factor, STAT1, and had no effect on the DNA binding of NF-kappaB. Inhibition by vitamin C was not simply an antioxidant effect, because redox-insensitive pathways to NF-kappaB were also blocked. Vitamin C was shown to block IL-1- and TNF-mediated degradation and phosphorylation of I-kappaBalpha (inhibitory protein that dissociates from NF-kappaB), due to inhibition of I-kappaB kinase (IKK) activation. Inhibition of TNF-driven IKK activation was mediated by p38 mitogen-activated protein kinase, because treatment of cells with vitamin C led to a rapid and sustained activation of p38, and the specific p38 inhibitor SB203580 reversed the inhibitory effect of vitamin C on IKK activity, I-kappaBalpha phosphorylation, and NF-kappaB activation. The results identify p38 as an intracellular target for high dose vitamin C.

A46R and A52R from vaccinia virus are antagonists of host IL-1 and toll-like receptor signaling

Poxviruses employ many strategies to evade and neutralize the host immune response. In this study, we have identified two vaccinia virus ORFs, termed A46R and A52R, that share amino acid sequence similarity with the Toll/IL-1 receptor (TIR) domain, a motif that defines the IL-1/Toll-like receptor (TLR) superfamily of receptors, which have a key role in innate immunity and inflammation. When expressed in mammalian cells, the protein products of both ORFs were shown to interfere specifically with IL-1 signal transduction. A46R partially inhibited IL-1-mediated activation of the transcription factor NFkappaB, and A52R potently blocked both IL-1- and TLR4-mediated NFkappaB activation. MyD88 is a TIR domain-containing adapter molecule known to have a central role in both IL-1 and TLR4 signaling. A52R mimicked the dominant-negative effect of a truncated version of MyD88 on IL-1, TLR4, and IL-18 signaling but had no effect on MyD88-independent signaling pathways. Therefore, A46R and A52R are likely to represent a mechanism used by vaccinia virus of suppressing TIR domain-dependent intracellular signaling.

Topoisomerase II is required for mitoxantrone to signal nuclear factor kappa B activation in HL60 cells

Topoisomerase II is a target for a number of chemotherapeutic agents used in the treatment of cancer. Its essential physiological role in modifying the topology of DNA involves the generation of transient double-strand breaks. Anti-cancer drugs, such as mitoxantrone, that target this enzyme interrupt its catalytic cycle and give rise to persistent double strand breaks, which may be lethal to a cell. We investigated the role of such lesions in signaling the activation of the transcription factor nuclear factor kappaB (NFkappaB) by this drug. Mitoxantrone activated NFkappaB and stimulated IkappaBalpha degradation in the promyelocytic leukemia cell line HL60 but not in the variant cells, HL60/MX2 cells, which lack the beta isoform of topoisomerase II and express a truncated alpha isoform that results in an altered subcellular distribution. Treatment of sensitive HL60 cells with mitoxantrone led to a depletion of both isoforms, suggesting the stabilization of transient DNA-topoisomerase II complexes. This depletion was absent in the variant cells, HL60/MX2. Activation of caspase 3 by mitoxantrone was also impaired in the HL60/MX2 cells. NFkappaB activation in response to tumor necrosis factor and bleomycin, the latter causing topoisomerase II-independent DNA damage, was intact in both cell lines. An inhibitor rather than a poison of topoisomerase II, Imperial Cancer Research Fund 187 (ICRF 187) the mechanism of which does not involve the generation of double strand breaks, did not activate NFkappaB, nor did it induce apoptosis in parental HL60 cells. However, ICRF 187 protected against IkappaB degradation in parental HL60 cells in response to mitoxantrone. This protection was also shown with another topoisomerase II inhibitor, merbarone, which is structurally and functionally distinct from ICRF 187. Their effects were specific, as neither protected against tumor necrosis factor-stimulated IkappaB degradation. The poisoning of topoiso- merase II with resultant DNA damage is therefore a critical signal for NFkappaB activation.

The interleukin-1 receptor/Toll-like receptor superfamily: signal transduction during inflammation and host defense

The signal transduction pathways activated by the proinflammatory cytokine interleukin-1 (IL-1) have been the focus of much attention because of the important role that IL-1 plays in inflammatory diseases. A number of proteins have been described that participate in the post-receptor activation of the transcription factor NF-kappaB and stress-activated protein kinases such as p38 mitogen-activated protein kinase (MAPK). It has also emerged that the type I IL-1 receptor (termed IL-1RI) is a member of an expanding receptor superfamily. These related receptors all have sequence similarity in their cytosolic regions. The family includes the Drosophila melanogaster protein Toll, the IL-18 receptor (IL-18R), and the Toll-like receptors TLR-2 and TLR-4, which bind molecules from Gram-positive and Gram-negative bacteria, respectively. Because of the similarity of IL-1RI to Toll, the conserved sequence in the cytosolic region of these proteins has been termed the Toll-IL-1 receptor (TIR) domain. The same proteins activated during signaling by IL-1RI also participate in signaling by IL-18R and TLR-4. The receptor superfamily is evolutionarily conserved; members occur in plants and insects and also function in host defense. The signaling proteins activated are also conserved across species. This receptor superfamily therefore represents an ancient signaling system that is a critical determinant of the innate immune and inflammatory responses.

The interleukin-1 receptor/Toll-like receptor superfamily: signal transduction during inflammation and host defense

The signal transduction pathways activated by the proinflammatory cytokine interleukin-1 (IL-1) have been the focus of much attention because of the important role that IL-1 plays in inflammatory diseases. A number of proteins have been described that participate in the post-receptor activation of the transcription factor NF-kappaB and stress-activated protein kinases such as p38 mitogen-activated protein kinase (MAPK). It has also emerged that the type I IL-1 receptor (termed IL-1RI) is a member of an expanding receptor superfamily. These related receptors all have sequence similarity in their cytosolic regions. The family includes the Drosophila melanogaster protein Toll, the IL-18 receptor (IL-18R), and the Toll-like receptors TLR-2 and TLR-4, which bind molecules from Gram-positive and Gram-negative bacteria, respectively. Because of the similarity of IL-1RI to Toll, the conserved sequence in the cytosolic region of these proteins has been termed the Toll-IL-1 receptor (TIR) domain. The same proteins activated during signaling by IL-1RI also participate in signaling by IL-18R and TLR-4. The receptor superfamily is evolutionarily conserved; members occur in plants and insects and also function in host defense. The signaling proteins activated are also conserved across species. This receptor superfamily therefore represents an ancient signaling system that is a critical determinant of the innate immune and inflammatory responses.

The role of the interleukin-1/Toll-like receptor superfamily in inflammation and host defence

The IL-1 receptor/Toll-like receptor superfamily comprises a diverse family of cell surface receptors defined by a characteristic conserved sequence in their cytosolic regions, termed the Toll/IL-1 receptor domain, which function in inflammation and host defence against microbial pathogens. Members include receptors for the proinflammatory cytokines IL-1 and IL-18 and Toll-like receptors 2 and 4, which are involved in host responses to Gram-positive and Gram-negative bacteria, respectively. Signalling pathways activated by these receptors are conserved and the superfamily represents a pan-genomic system involved in the host response to infection and injury.

A novel function of InIB from Listeria monocytogenes: activation of NF-kappaB in J774 macrophages

Listeria monocytogenes causes a pro-inflammatory response on adhesion to macrophages. Upregulation of inflammation genes involves the transcription factor NF-kappaB. Several components of L. monocytogenes, including lipoteichoic acid (LTA), phospholipases and listeriolysin O (LLO), have since been shown to mediate NF-kappaB activation. Here, we report that purified recombinant InlB, but not internalin (InlA), is a potent activator of NF-kappaB in the mouse macrophage-like cell line J774. Expression of InlB in Listeria innocua enhances its ability to activate NF-kappaB, while deletion of InlB from L. monocytogenes marginally decreases its effect on NF-kappaB, possibly because of the presence of NF-kappaB activators such as LTA and LLO. The effect correlates with the rapid degradation of IkappaBalpha, a sustained degradation of IkappaBbeta and increases in tumour necrosis factor alpha (TNF-alpha) and interleukin (IL) 6 production, two cytokines controlled by NF-kappaB. Using a series of anti-InlB monoclonal antibodies and domains of InlB, NF-kappaB activation was shown to be dependent upon the N-terminal 213-amino-acid leucine-rich repeat (LRR) domain of InlB, recently demonstrated to be responsible for InlB-mediated L. monocytogenes invasion and phosphoinositide-3 (PI-3) kinase activation. The effect of InlB was blocked by PI-3 kinase inhibitors, indicating the involvement of PI-3 kinase in this response. This report thus illustrates that InlB not only promotes invasion, but also contributes to the macrophage pro-inflammatory response.

A method to predict residues conferring functional differences between related proteins: application to MAP kinase pathways

Physicochemical properties are potentially useful in predicting functional differences between aligned protein subfamilies. We present a method that considers physicochemical properties from ancestral sequences predicted to have given rise to the subfamilies of interest by gene duplication. Comparison between two map kinases subfamilies, p38 and ERK, revealed a region that had an excess of change in properties after gene duplication followed by conservation within the two subfamilies. This region corresponded to that experimentally defined as important for substrate and pathway specificity. The derived scores for the region of interest were found to differ significantly in their distribution compared to the rest of the protein when the Kolmogorov-Smirnov test was applied (p = 0.005). Thus, the incorporation of ancestral physicochemical properties is useful in predicting functional differences between protein subfamilies. In addition, the method was applied to the MKK and MAPK components of the p38 and JNK pathways. These proteins showed a similar pattern in their evolution and regions predicted to confer functional differences are discussed.

The interleukin-1 receptor/Toll-like receptor superfamily: signal generators for pro-inflammatory interleukins and microbial products

The interleukin-1 (IL-1) receptor/Toll-like receptor (TLR) superfamily is a recently defined and expanding group of receptors that participate in host responses to injury and infection. The superfamily is defined by the Toll/IL-1 receptor (TIR) domain, which occurs in the cytosolic region of family members, and is further subdivided into two groups based on homology to either the Type I IL-1 receptor or Drosophila Toll receptor extracellular domain. The former group includes the receptor for the important Th1 cytokine IL-18, and T1/ST2, which may have a role in Th2 cell function. The latter group includes six mammalian TLRs, including TLR2 and TLR4, that largely mediate the host response to gram-positive and gram-negative bacteria, respectively. Whether bacterial products are actual ligands for TLRs, or whether they generate ligands via as yet unidentified pattern recognition receptors, has yet to be determined. Signaling pathways activated via the TIR domain trigger the activation of downstream kinases, and transcription factors such as NF-kappaB, and involve the adaptor protein MyD88, which itself contains a TIR domain.

Divergent roles for Ras and Rap in the activation of p38 mitogen-activated protein kinase by interleukin-1

We have found that lethal toxin from Clostridium sordellii, which specifically inactivates the low molecular weight G proteins Ras, Rap, and Rac, inhibits the activation of p38 mitogen-activated protein kinase (MAPK) by interleukin-1 (IL-1) in EL4.NOB-1 cells and primary fibroblasts. The target protein involved appeared to be Ras, because transient transfections with dominant negative RasN17 inhibited p38 MAPK activation by IL-1. Furthermore, transfections of cells with constitutively active RasVHa-activated p38 MAPK. Further evidence for Ras involvement came from the observation that IL-1 caused a rapid activation of Ras in the cells and from the inhibitory effects of the Ras inhibitors manumycin A and damnacanthal. Toxin B from Clostridium difficile, which inactivates Rac, Cdc42, and Rho, was without effect. Dominant negative versions of Rac (RacN17) or Rap (Rap1AN17) did not inhibit the response. Intriguingly, transfection of cells with dominant negative Rap1AN17 activated p38 MAPK. Furthermore, constitutively active Rap1AV12 inhibited p38 MAPK activation by IL-1, consistent with Rap antagonizing Ras function. IL-1 also activated Rap in the cells, but with slower kinetics than Ras. Our studies therefore provide clear evidence using multiple approaches for Ras as a signaling component in the activation of p38 MAPK by IL-1, with Rap having an inhibitory effect.

Ras, protein kinase C zeta, and I kappa B kinases 1 and 2 are downstream effectors of CD44 during the activation of NF-kappa B by hyaluronic acid fragments in T-24 carcinoma cells

We have investigated the ability of hyaluronic acid (HA) fragments to activate the transcription factor NF-kappa B. HA fragments activated NF-kappa B in the cell lines T-24, HeLa, MCF7, and J774. Further studies in T-24 cells demonstrated that HA fragments also induced I kappa B alpha phosphorylation and degradation, kappa B-linked reporter gene expression, and ICAM-1 promoter activity in an NF-kappa B-dependent manner. The effect of HA was size dependent as neither disaccharide nor native HA were active. CD44, the principal cellular receptor for HA, was critical for the response because the anti-CD44 Ab IM7.8.1 blocked the effect on NF-kappa B. HA fragments activated the I kappa B kinase complex, and the effect on a kappa B-linked reporter gene was blocked in T-24 cells expressing dominant negative I kappa B kinases 1 or 2. Activation of protein kinase C (PKC) was required because calphostin C inhibited NF-kappa B activation and I kappa B alpha phosphorylation. In particular, PKC zeta was required because transfection of cells with dominant negative PKC zeta blocked the effect of HA fragments on kappa B-linked gene expression and HA fragments increased PKC zeta activity. Furthermore, damnacanthal and manumycin A, two mechanistically distinct inhibitors of Ras, blocked NF-kappa B activation. Transfection of T-24 cells with dominant negative Ras (RasN17) blocked HA fragment-induced kappa B-linked reporter gene expression, and HA fragments activated Ras activity within 5 min. Taken together, these studies establish a novel signal transduction cascade emanating from CD44 to Ras, PKC zeta, and I kappa B kinase 1 and 2.

Rac1 regulates interleukin 1-induced nuclear factor kappaB activation in an inhibitory protein kappaBalpha-independent manner by enhancing the ability of the p65 subunit to transactivate gene expression

We have examined the involvement of Rac1 in nuclear factor kappaB (NFkappaB) activation by interleukin 1 (IL1). IL1 induced a rapid and sustained activation of Rac1 in the thymoma cell line EL4.NOB-1. Transient transfection with dominant negative RacN17 inhibited IL1-induced kappaB-dependent reporter gene expression but not IkappaBalpha degradation, whereas constitutively active RacV12 potentiated kappaB-dependent reporter gene expression in response to IL1 but had no effects on its own. Using porcine aortic endothelial cells stably transfected with RacV12 or RacN17 under the control of an inducible promoter, we confirmed that RacV12 did not affect IkappaBalpha degradation, nor did RacN17 inhibit the IL1-induced response. RacV12 was also unable to induce nuclear translocation of NFkappaB. These effects suggested a role for Rac1 in p65-mediated transactivation of NFkappaB, independent of IkappaBalpha regulation. In support of this we found that IL1 activated a pathway leading to increased p65 transactivation activity and that RacV12 alone could drive this response in both cell systems. Additionally, RacN17 inhibited IL1-driven p65-mediated transactivation. From data using specific inhibitors of p38 and p42/p44 kinases we propose that both p38 and p42/p44 lie downstream of Rac1 on the IL1 pathway leading to enhanced transactivation by p65.

Oxidative stress and nuclear factor-kappaB activation: a reassessment of the evidence in the light of recent discoveries

Nuclear factor-kappaB (NFKB) is a transcription factor with a pivotal role in inducing genes involved in physiological processes as well as in the response to injury and infection. A model has been proposed whereby the diverse agents that activate NFkappaB do so by increasing oxidative stress within the cell. Activation of NFkappaB involves the phosphorylation and subsequent degradation of an inhibitory protein, IKB, and recently many of the proximal kinases and adaptor molecules involved in this process have been elucidated. Additionally, we now understand in detail the NFkappaB activation pathway from cell membrane to nucleus for interleukin-1 (IL-1) and tumour necrosis factor (TNF). This review revisits the evidence for the oxidative stress model in light of these recent findings, and finds little in the new information to rationalise or justify a central role for oxidative stress in NF-kappaB activation. We demonstrate that much of the evidence for the involvement of oxidative stress is either specific to a stimulus in a particular cell line or open to reinterpretation. In particular, the activation of NFkappaB by hydrogen peroxide is cell-specific and distinct from physiological activators such as IL-1 and TNF, while inhibition by antioxidants, also found to be cell- and stimulus-specific, can involve diverse and unexpected targets which may be distinct from redox modulation. We conclude that in most cases the role of oxidative stress in NF-kappaB activation is at best facilitatory rather than causal, if a role exists at all. In addition, other evidence suggests a role for lipid peroxides in pathways where such a role exists. In future, when a role for oxidative stress in a pathway is postulated, the challenge will be to show which particular kinases or adaptor molecules, if any, are redox-modulated.

The evolution of the MAP kinase pathways: coduplication of interacting proteins leads to new signaling cascades

The MAP-kinase pathways are intracellular signaling modules that are likely to exist in all eukaryotes. We provide an evolutionary model for these signaling pathways by focusing on the gene duplications that have occurred since the divergence of animals from yeast. Construction of evolutionary trees with confidence assessed by bootstrap clearly shows that the mammalian JNK and p38 pathways arose from an ancestral hyperosmolarity pathway after the split from yeast and before the split from C. elegans. These coduplications of interacting proteins at the MAPK and MEK levels have since evolved toward substrate specificity, thus giving distinct pathways. Mammalian duplications since the split from C. elegans are often associated with divergent tissue distribution but do not appear to confer detectable substrate specificity. The yeast kinase cascades have undergone similar fundamental functional changes since the split from mammals, with duplications giving rise to central signaling components of the filamentous and hypoosmolarity pathways. Experimentally defined cross-talk between yeast pheromone and hyperosmolarity pathways is mirrored with corresponding cross-talk in mammalian pathways, suggesting the existence of ancient orthologous cross-talk; our analysis of gene duplications at all levels of the cascade is consistent with this model but does not always provide significant bootstrap support. Our data also provide insights at different levels of the cascade where conflicting experimental evidence exists.

Distinct roles for p42/p44 and p38 mitogen-activated protein kinases in the induction of IL-2 by IL-1

Interleukin 1 (IL-1) activates p42/p44 and p38 mitogen-activated protein kinases (MAP kinases) in target cells. Here we have used two specific inhibitors, PD98059 which inhibits MAP kinase kinase (MEK), and SB203580 which inhibits p38 MAP kinase to explore the involvement of these kinases in the induction of IL-2 by IL-1 in the murine thymoma cell line EL4.NOB-1. Both kinase inhibitors suppressed IL-1-stimulated IL-2 production. PD98059 blocked IL-2 mRNA accumulation and the induction of a reporter gene linked to the IL-2 promoter. In contrast, SB203580 only marginally inhibited IL-2 promoter-linked reporter gene expression and had no inhibitory effect on IL-2 mRNA levels. Neither PD98059 nor SB203580 had an inhibitory effect on NFkappaB-driven reporter gene expression in response to IL-1. Surprisingly, higher concentrations of SB203580 (30 microM) potentiated the IL-1 responses. PD98059 also inhibited induction of IL-2 by phorbol 12-myristate 13-acetate (PMA), and AP1-linked reporter gene expression in response to PMA but not IL-1. These results indicate that p42/p44 MAP kinase is involved in the regulation of IL-2 gene transcription by IL-1, whilst p38 MAP kinase has a post-transcriptional target. Additional IL-1 signalling pathways can clearly compensate for the lack of p38 MAP kinase which result in potentiation of the IL-1 responses observed at high-dose SB203580.

The P38 mitogen-activated protein kinase inhibitor SB203580 antagonizes the inhibitory effects of interleukin-1beta on long-term potentiation in the rat dentate gyrus in vitro

Levels of the pro-inflammatory cytokine interleukin-1beta are known to be elevated in patients with chronic disorders such as Alzheimer's disease. We have investigated the effects of interleukin-1beta on long-term potentiation and N-methyl-D-aspartate receptor-mediated field potentials in the rat dentate gyrus in vitro utilizing field extracellular recordings obtained from the middle third of the molecular layer of the dentate gyrus. Presynaptic stimulation was applied to the commissural/association pathway at a frequency of 0.05 Hz and at a distance of 50 microm from the granule cell body layer. As previously reported, interleukin-1beta (1 ng/ml) caused an inhibition of long-term potentiation (108+/-2% of baseline 1 h following application of tetanic stimulation compared with 145+/-5% in vehicle control slices). This action of interleukin-1beta on long-term potentiation, as well as an inhibition of N-methyl-D aspartate receptor-mediated field potentials, was attenuated by pre-treatment of slices with the p38 mitogen-associated protein kinase inhibitor SB203580 (1 microM). SB203580 alone had no significant affect on long-term potentiation, but did cause an increase in baseline synaptic transmission [107+/-2% of baseline, 1 h after SB203580 (1 microM) treatment]. The p42/44 mitogen-activated protein kinase cascade inhibitor PD98059 (50 microM) did not inhibit the interleukin-1beta-induced inhibition of N-methyl-D-aspartate receptor-mediated field potentials. The cyclooxygenase inhibitor indomethacin (50 microM) was found to attenuate the interleukin-1beta-induced effects on both long-term potentiation and N-methyl-D-aspartate receptor-mediated field potentials. The lipid second messenger analogue C2 ceramide (20 microM) was found to attenuate the expression of long-term potentiation (108+/-3% of baseline 1 h following tetanic stimulation), and this effect was not blocked by pre-treatment with SB203580. To investigate a possible role for interleukin-1beta in the normal expression of long-term potentiation, the interleukin-1 receptor antagonist (25 ng/ml) was applied during the maintenance phase of long-term potentiation. This was found to depress the sustained expression of long-term potentiation (116+/-6% of baseline 1 h following tetanic stimulation). Our results indicate possible signalling mechanisms by which interleukin-1beta at pathophysiological concentrations may serve to inhibit long-term potentiation, and also suggests a role for IL-1beta in the physiological expression of synaptic plasticity in the rat dentate gyrus in vitro.

Characterization of CD44 induction by IL-1: a critical role for Egr-1

The adhesion molecule CD44 is a multifunctional, ubiquitously expressed glycoprotein that participates in the process of leukocyte recruitment to sites of inflammation and to their migration through lymphatic tissues. In this study, we have investigated the effect of the proinflammatory cytokine IL-1alpha on CD44 gene expression in the human immortalized endothelial cell line ECV304. Immunoblotting of cell extracts showed constitutive expression of a 85-kDa protein corresponding to the standard form of CD44, which was potently up-regulated following IL-1alpha treatment. Furthermore, IL-1alpha induced expression of v3- and v6-containing isoforms of CD44, which migrated at 110 and 140-180 kDa, respectively. The effect of IL-1alpha on CD44 standard, v3- and v6-containing isoforms was dose and time dependent and was inhibited in the presence of IL-1 receptor antagonist. To elucidate the molecular mechanisms regulating CD44 expression in response to IL-1alpha, we investigated the effect of IL-1alpha on CD44 mRNA expression. Reverse-transcriptase PCR and Northern analysis demonstrated an increase in CD44 mRNA expression indicating a transcriptional mechanism of control by IL-1alpha. Furthermore, IL-1alpha increased expression of a reporter gene under the control of the CD44 promoter (up to -1.75 kb). The effect of IL-1alpha was critically dependent on the site spanning -151 to -701 of the promoter. This effect required the presence of an Egr-1 motif at position -301 within the CD44 promoter since mutation of this site abolished responsiveness. IL-1alpha also induced Egr-1 expression in these cells. These studies therefore identify Egr-1 as a critical transcription factor involved in CD44 induction by IL-1alpha.

Ceramide activates NFkappaB by inducing the processing of p105

The role of ceramide as a second messenger in tumor necrosis factor (TNF)-mediated signal transduction has been much debated. It is supported by recent reports describing an expanding number of potential targets for this lipid, but is opposed by those describing how ceramide is not necessary for many TNF-mediated cellular events. In this paper, we directly compare the effects of the cell-permeable ceramide analogue, N-acetylsphingosine (C2-ceramide), with TNF, on NFkappaB function, a transcription factor whose activation is central to many TNF-mediated effects. We describe how C2-ceramide failed to drive kappaB-linked chloramphenicol acetyltransferase gene expression in either HL60 promyelocytic or Jurkat T lymphoma cells. Furthermore, it had no effect on TNF-mediated transcription of this reporter gene. However, electrophoretic mobility shift analysis following cell stimulation with this ceramide analogue revealed a dose-responsive activation of NFkappaB, which was not apparent following cell treatment with the inactive dihydro form. Activated complexes from treated cells were shown to contain predominantly the p50 subunit, in contrast to complexes from TNF-treated cells, where both p50 and p65/RelA subunits were present. The specific activation of p50 homodimeric complexes by C2-ceramide, which are known to lack trans-activating activity, was strongly suggested from these data. Further investigations revealed that C2-ceramide had only a marginal effect on IkappaBalpha degradation but strongly promoted the processing of p105 to its p50 product as revealed by immunoblot analysis. The increase in p50 arising from the processing of its p105 precursor was further established from p105/p50 ratios obtained by scanning densitometric analysis of bands from immunoblots. TNF, on the other hand, stimulated both IkappaBalpha degradation and p105 processing, in accordance with previous findings. Furthermore, the effect of TNF on NFkappaB activation was rapid, whereas C2-ceramide required an optimal treatment time of 1 h. Interestingly, TNF was found to increase ceramide in cells but only after a 1-h contact time. Our data therefore suggest that ceramide promotes the activation of NFkappaB complexes that lack transactivating activity by enhanced processing of p105.

Signal transduction pathways activated by the IL-1 receptor family: ancient signaling machinery in mammals, insects, and plants

Interleukin-1 (IL-1) is a central regulator of the immune and inflammatory responses. Recently, significant advances have been made in the area of IL-1 receptors and IL-1 signal transduction. A family of proteins has been described that share significant homology in their signaling domains with the Type I IL-1 receptor (IL-1RI). These include the IL-1 receptor accessory protein (IL-1AcP), which does not bind IL-1 but is essential for IL-1 signaling; a Drosophila protein Toll; a number of human Toll-like receptors (hTLRs); the putative IL-18/IL-1-gamma receptor IL-1Rrp (IL-1 receptor-related protein); and a number of plant proteins. All appear to be involved in host responses to injury and infection. These homologies also extend to novel signaling proteins implicated in IL-1 action. Two IL-1 receptor-associated kinases, IRAK-1 and IRAK-2, which have homologs in Drosophila (Pelle) and plants (Pto), have been implicated in the activation of the transcription factor, nuclear factor kappaB (NF-kappaB). IRAK-1 has also been implicated in AP1 induction, Jun amino-terminal kinase (JNK) activation, and IL-2 induction. It recruits the adapter protein TRAF6 to the IL-1 receptor complex via an interaction with IL-1AcP. TRAF6 then relays the signal via NF-kappaB-inducing kinase (NIK) to two I-kappaB kinases (IKK-1 and -2), leading to NF-kappaB activation. Progress has also been made on other IL-1-responsive kinases, including JNK and p38 MAP kinase, with the latter having a role in multiple responses to IL-1. The remarkable conservation between diverse species indicates that the IL-1 system represents an ancient signaling machine critical for responses to environmental stresses and attack by pathogens.

Inhibition of nuclear factor kappaB by direct modification in whole cells--mechanism of action of nordihydroguaiaritic acid, curcumin and thiol modifiers

This study was set up to investigate the mechanism of four inhibitors of interleukin-1(IL-1)-alpha and tumor necrosis factor-(TNF)alpha activated nuclear factor kappaB (NFkappaB) in whole cells. The compounds fall into two classes: the first comprised two chain-breaking antioxidants, curcumin (diferulolylmethane) and nordihydroguaiaritic acid. The second class were two thiol-modifying agents, N-ethylmaleimide (NEM) and 2-chloro-1,3dinitrobenzene (CDNB). Both sets of compounds were found to inhibit NFkappaB in tumour necrosis factor-activated Jurkat T lymphoma cells and interleukin 1-activated EL4.NOB-1 thymoma cells as determined by electrophoretic mobility shift assay using a specific NFkappaB DNA probe. In unstimulated cells the compounds were found to modify NFkappaB prior to chemical dissociation with sodium deoxycholate. They also inhibited DNA binding by NFkappaB when added to nuclear extracts from stimulated cells. Both of these effects occurred over a concentration range comparable to that which inhibited cytokine-activated NFkappaB in intact cells. All four agents were found to react directly with the p50 subunit of NFkappaB. However, only the antioxidants, curcumin and nordihydroguaiaritic acid (NDGA) were found to inhibit IkappaBalpha degradation activated by tumour necrosis factor-alpha. These results suggest that NFkappaB itself is susceptible to direct inhibition by a range of pharmacological agents. Furthermore, curcumin and nordihydroguaiaritic acid inhibit NFkappaB by interfering with IkappaBalpha degradation and reacting with p50 in the NFkappaB complex. These findings are likely to be useful in the attempt to develop agents which inhibit NFkappaB-dependent gene transcription.

The production of a reactive oxygen intermediate during the induction of apoptosis by cytotoxic insult

We report the protective effects of two novel antioxidant compounds, 3 beta-doxyl-5 alpha cholestane and 2,2,6,6-tetramethyl-piperdinoxyl, in HL-60 and U937 leukemic cells subjected to a number of cytotoxic insults. In addition, the rapid production of peroxide is demonstrated as a general response to cytotoxic agents in these leukemic cell lines, indicating that changes in the redox status of a leukemic cell may contribute to the ultimate death of these cells. Closer examination of this peroxide production has identified enzymic production and/or disruption of resident antioxidants as possible sources. However, in contrast to recent reports from other model systems, mitochondrial transmembrane depolarization did not appear to be required for the production of peroxide in these cells. Finally, we demonstrated the activation of the redox-sensitive transcription factor, NF-kappa B, in response to these cytotoxic insults.

Lipid peroxidation is involved in the activation of NF-kappaB by tumor necrosis factor but not interleukin-1 in the human endothelial cell line ECV304. Lack of involvement of H2O2 in NF-kappaB activation by either cytokine in both primary and transformed endothelial cells

It has been proposed that reactive oxygen species, and in particular H2O2, may be involved in the activation of NF-kappaB by diverse stimuli in different cell types. Here we have investigated the effect of a range of putative antioxidants on NF-kappaB activation by interleukin-1 and tumor necrosis factor as well as the ability of H2O2 to activate NF-kappaB in primary human umbilical vein endothelial cells and the transformed human endothelial cell line ECV304. Activation of NF-kappaB and stimulation of IkappaBalpha degradation by H2O2 was only evident in the transformed cells and required much longer contact times than that observed with interleukin-1 or tumor necrosis factor. Furthermore, only H2O2 was sensitive to N-acetyl-L-cysteine, and no increase in H2O2 was detected in response to either cytokine. Pyrrolidine dithiocarbamate has been purported to be a specific antioxidant inhibitor of NF-kappaB that acts independently of activating agent or cell type. However, we found that tumor necrosis factor- but not interleukin-1-driven NF-kappaB activation and IkappaBalpha degradation were sensitive to pyrrolidine dithiocarbamate in transformed cells, while neither pathway was inhibited in primary cells. Phorbol ester-mediated activation was sensitive in both transformed and primary cells. Other antioxidants failed to inhibit either cytokine, while the iron chelators desferrioxamine and 2,2,6, 6-tetramethylpiperidine-1-oxyl mimicked the pattern of inhibition seen for the dithiocarbamate. This suggested that pyrrolidine dithiocarbamate was inhibiting NF-kappaB activation in endothelial cells primarily through its iron-chelating properties. Tumor necrosis factor, but not interleukin-1, was found to induce lipid peroxidation in ECV304 cells. This was inhibited by pyrrolidine dithiocarbamate and desferrioxamine. t-Butyl hydroperoxide, which induces lipid peroxidation, activated NF-kappaB. Finally, butylated hydroxyanisole, which inhibits lipid peroxidation but has no iron-chelating properties, inhibited NF-kappaB activation by tumor necrosis factor but not interleukin-1. Taken together, the results argue against a role for H2O2 in NF-kappaB activation by cytokines in endothelial cells. Furthermore, tumor necrosis factor and interleukin-1 activate NF-kappaB through different mechanisms in ECV304 cells, with the tumor necrosis factor pathway involving iron-catalyzed lipid peroxidation.

Staurosporine, but not Ro 31-8220, induces interleukin 2 production and synergizes with interleukin 1alpha in EL4 thymoma cells

Protein kinase C (PKC) has been implicated in interleukin 1 (IL1) signal transduction in a number of cellular systems, either as a key event in IL1 action or as a negative regulator. Here we have examined the effects of two PKC inhibitors, staurosporine and the more selective agent Ro 31-8220, on IL1 responses in the murine thymoma line EL4.NOB-1. A 1 h pulse of staurosporine was found to strongly potentiate the induction of IL2 by IL1alpha in these cells. In contrast, neither a pulse nor prolonged incubation with Ro 31-8220 affected the response to IL1alpha. Both agents blocked the response to PMA, however. A 1 h pulse of staurosporine was also found to induce IL2 production on its own, activate the transcription factor nuclear factor kappaB (NFkappaB) and increase the expression of a NFkappaB-linked reporter gene. It synergized with IL1alpha in all of these responses. Ro 31-8220 was again without effect, although both staurosporine and Ro 31-8220 blocked the activation of NFkappaB by PMA. Finally, staurosporine caused the translocation of PKC-alpha and -epsilon, and to a lesser extent PKC-beta, but not PKC-θ or -zeta, from the cytosol to the membrane, although a similar effect was observed with Ro 31-8220. The results suggest that PKC is not involved in IL1alpha signalling in EL4 cells. Furthermore, the potentiating effect of staurosporine on IL1alpha action does not involve PKC inhibition, and is likely to be at the level of NFkappaB activation.

NF-kappa B: a crucial transcription factor for glial and neuronal cell function

Transcription factors provide the link between early membrane-proximal signalling events and changes in gene expression. NF-kappa B is one of the best-characterized transcription factors. It is expressed ubiquitously and regulates the expression of many genes, most of which encode proteins that play an important and often determining role in the processes of immunity and inflammation. Apart from its role in these events, evidence has begun to accumulate that NF-kappa B is involved in brain function, particularly following injury and in neurodegenerative conditions such as Alzheimer's disease. NF-kappa B might also be important for viral replication in the CNS. An involvement of NF-kappa B in neuronal development is suggested from studies that demonstrate its activation in neurones in certain regions of the brain during neurogenesis. Brain-specific activators of NF-kappa B include glutamate (via both AMPA/KA and NMDA receptors) and neurotrophins, pointing to an involvement in synaptic plasticity. NF-kappa B can therefore be considered as one of the most important transcription factors characterized in brain to date and it might be as crucial for neuronal and glial cell function as it is for immune cells.

Daunorubicin activates NFkappaB and induces kappaB-dependent gene expression in HL-60 promyelocytic and Jurkat T lymphoma cells

The anthracycline antibiotic, daunorubicin, can induce programmed cell death (apoptosis) in cells. Recent work suggests that this event is mediated by ceramide via enhanced ceramide synthase activity. Since the generation of ceramide has been directly linked with the activation of the transcription factor, NFkappaB, this was investigated as a novel target for the action of daunorubicin. Here we describe how treatment of HL-60 promyelocytes and Jurkat T lymphoma cells with daunorubicin results in the activation of the transcription factor NFkappaB. The effect of daunorubicin was evident following 1-2 h treatment, which was in contrast to the time course of activation obtained with the cytokine, tumor necrosis factor, where NFkappaB activation was detected within minutes of cellular stimulation. Activated complexes were shown to contain predominantly p50 and p65/RelA subunit components. Daunorubicin also induced IkappaB degradation and increased the expression of an NFkappaB-linked reporter gene. In addition, the drug was found to strongly potentiate the ability of tumor necrosis factor to induce an NFkappaB-linked reporter gene, suggesting a synergy between these two agents in this response. These events were sensitive to the iron chelator, deferoxamine mesylate (desferal), and the anti-oxidant and metal chelator pyrrolidine dithiocarbamate. A structurally related compound, mitoxantrone, which, unlike daunorubicin, is unable to undergo redox cycling in cells, also activated NFkappaB in a pyrrolidine dithiocarbamate-sensitive manner. A specific inhibitor of ceramide synthase, fumonisin B1, had no effect on daunorubicin induced NFkappaB activation at a range of concentrations previously reported to block apoptosis induced by this drug. However, this agent could inhibit increases in ceramide induced by daunorubicin, in addition to blocking ceramide synthase activity from HL-60 cells which was activated in response to daunorubicin treatment. These data therefore suggest that the effect of daunorubicin on NFkappaB is unlikely to involve ceramide, but may involve reactive oxygen species generated as a result of endogenous cellular processes rather than reductive metabolism of the drug. As NFkappaB may be involved in apoptosis, this effect may be an important aspect of the cellular responses to this agent.

TNF-mediated cytotoxicity of L929 cells: role of staurosporine in enhancement of cytotoxicity and translocation of protein kinase C isozymes

The role of protein kinase C (PKC) in tumour necrosis factor (TNF)-mediated cytotoxicity was investigated, using the L929 cell line. The PKC activator phorbol-12-myristate 13-acetate (PMA) increased proliferation and inhibited TNF-induced cytotoxicity of L929 cells. A range of specific PKC inhibitors had no effect on TNF-mediated killing, suggesting that PKC does not play a direct role in this response. However, staurosporine enhanced cytotoxicity by TNF in the presence of actinomycin D, a protein synthesis inhibitor. In view of this finding, the authors investigated the role of specific PKC isozymes in both TNF-mediated cytotoxicity and staurosporine-induced sensitization to killing. PKC-alpha, beta, epsilon and zeta were detected in L929 cells. PKC-beta was only weakly detected in the cytoplasm, PKC alpha, epsilon and zeta were all found in resting cytoplasm and membrane. Stimulation with PMA caused a strong translocation of PKC-alpha but not zeta to the membrane. TNF had no effect on these isozymes but interestingly caused a translocation of PKC-epsilon, which also occurred in response to PMA. Staurosporine caused a translocation of PKC-zeta to the plasma membrane and a loss of PKC-epsilon from the cytosol. Although TNF induced PKC-epsilon translocation, this is unlikely to be involved in cytotoxicity as this effect was also induced by PMA which protected against TNF-mediated cytotoxicity. Staurosporine also induced translocation of PKC-zeta, an isozyme whose activity was previously found to be resistant to inhibition by staurosporine. These findings suggest the possibility that the mechanism by which staurosporine potentiates TNF action does not involve inhibition of PKC, but in contrast may involve modulation of PKC-zeta.

2-mercaptoethanol restores the ability of nuclear factor kappa B (NF kappa B) to bind DNA in nuclear extracts from interleukin 1-treated cells incubated with pyrollidine dithiocarbamate (PDTC). Evidence for oxidation of glutathione in the mechanism of inhibition of NF kappa B by PDTC

The metal chelator and anti-oxidant pyrollidine dithiocarbamate (PDTC) has been used extensively in studies implicating reactive oxygen intermediates in the activation of nuclear factor kappa B (NF kappa B). In agreement with other studies, we have shown that PDTC inhibits NF kappa B activation in response to the pro-inflammatory cytokines interleukin 1 (IL1) and tumour necrosis factor (TNF). However, we have found that the inhibition was reversed by treatment of inhibited nuclear extracts with the reducing agent 2-mercaptoethanol. This was observed in extracts prepared from IL1-treated EL4.NOB-1 thymoma cells and TNF-treated Jurkat E6.1 lymphoma cells. These results suggested that the inhibition was caused by oxidation of NF kappa B on a sensitive thiol, possibly on the p50 subunit (which was detected in NF kappa B complexes in both cell types), and not by inhibition of the activation pathway. The possibility that PDTC was acting as a pro-oxidant was therefore investigated. PDTC caused an increase in oxidized glutathione, suggesting that it acts as an oxidizing agent in the cells tested rather than as an anti-oxidant. Similar results were obtained with diamide, a compound designed to oxidize glutathione. Finally, an increase in the ratio of oxidized to reduced glutathione was shown to inhibit NF kappa B-DNA binding in vitro. On the basis of these results we suggest that, while NF kappa B activation is unaffected by PDTC, DNA binding is inhibited through a mechanism involving a shift towards oxidizing conditions, and that this is the mechanism of action of both PDTC and diamide in the cells tested here.

Endothelin-1 increases arachidonic acid release in C6 glioma cells through a potassium-modulated influx of calcium

Endothelin-1 (Et-1) but not a range of other receptor agonists stimulated the release of arachidonic acid (AA) in C6 glioma. Et-1 activation was concentration dependent and was inhibited by chelation of extracellular calcium. The calcium ionophores A23187 and ionomycin could also stimulate release of AA. Et-1 caused an early increase in intracellular Ca2+ concentration ([Ca2+]i) followed by a sustained but lower plateau level. The sensitivity of the response to quinacrine, its dependence on Ca2+, and the demonstration of an increase in phospholipase A2 (PLA2) activity that was insensitive to dithiothreitol suggested that the release of AA was due to activation of cytosolic PLA2 in the cells. Staurosporine, a protein kinase C (PKC) inhibitor, had no effect on Et-1-induced AA release but abolished that by phorbol 12-myristate 13-acetate, demonstrating that the Et-1 response was PKC independent. Raised levels of extracellular KCl inhibited both AA release and the increase in [Ca2+]i triggered by Et-1, whereas valinomycin, which causes K+ efflux, not only caused a rapid rise in [Ca2+]i but also caused AA mobilisation. The results therefore suggest that Et-1 activation of PLA2 in this cell type requires calcium influx dependent on K+ efflux.