Constitutive Nuclear Expression of the IκB Kinase Complex and Its Activation in Human Neutrophils

Phosphoinositol 3-kinase-driven NET formation involves different isoforms and signaling partners depending on the stimulus

Neutrophil extracellular traps (NETs) serve to immobilize and kill pathogens, but also can contribute to the progression of several inflammatory and auto-immune diseases, as well as cancer. Whence the importance of elucidating the mechanisms underlying NET formation. In this regard, the PI3K signaling pathway has been shown to be crucial; yet little is known about which of its components are involved. Here, we identified the PI3K isoforms and associated signaling partners that are mobilized in response to different classes of physiological NET inducers (inflammatory cytokines, growth factors, chemoattractants). NET generation was assessed by microscopy and signalling molecule activation by immunoblot using phospho-antibodies. Across the various stimuli, PI3Kα and PI3Kγ isoforms clearly contributed to NET induction, while the participation of other isoforms was stimulus-dependent. Some PI3K isoforms were also found to signal through Akt, the canonical downstream effector of PI3K, while others did not. Downstream of PI3K, mTOR and PLCγ2 were used by all stimuli to control NET generation. Conversely, the involvement of other kinases depended on the stimulus - both TNFα and GM-CSF relied on PDK1 and Akt; and both TNFα and fMLP additionally used S6K. We further established that all PI3K isoforms and downstream effectors act belatedly in NET generation, as reported previously for PI3K. Finally, we revisited the PI3K-PDK1-Akt signaling hierarchy in human neutrophils and again found stimulus-dependent differences. Our data uncover unsuspected complexity and redundancy in the signaling machinery controlling NET formation through the all-important PI3K pathway. Conserved signaling molecules represent therapeutic targets for pathologies involving NETs and in this regard, the existence of drugs currently used in the clinic or undergoing clinical trials (which target PI3K isoforms, mTOR or Akt), underscores the translational potential of our findings.

Human Neutrophils Generate Extracellular Vesicles That Modulate Their Functional Responses

Neutrophils influence innate and adaptive immunity by releasing various cytokines and chemokines, by generating neutrophil extracellular traps (NETs), and by modulating their own survival. Neutrophils also produce extracellular vesicles (EVs) termed ectosomes, which influence the function of other immune cells. Here, we studied neutrophil-derived ectosomes (NDEs) and whether they can modulate autologous neutrophil responses. We first characterized EV production by neutrophils, following MISEV 2018 guidelines to facilitate comparisons with other studies. We found that such EVs are principally NDEs, that they are rapidly released in response to several (but not all) physiological stimuli, and that a number of signaling pathways are involved in the induction of this response. When co-incubated with autologous neutrophils, NDE constituents were rapidly incorporated into recipient cells and this triggered and/or modulated neutrophil responses. The pro-survival effect of GM-CSF, G-CSF, IFNγ, and dexamethasone was reversed; CXCL8 and NET formation were induced in otherwise unstimulated neutrophils; the induction of inflammatory chemokines by TNFα was modulated depending on the activation state of the NDEs' parent cells; and inducible NET generation was attenuated. Our data show that NDE generation modulates neutrophil responses in an autocrine and paracrine manner, and indicate that this probably represents an important aspect of how neutrophils shape their environment and cellular interactions.

Early and Late Processes Driving NET Formation, and the Autocrine/Paracrine Role of Endogenous RAGE Ligands

Neutrophil extracellular trap (NET) formation has emerged as an important response against various pathogens; it also plays a role in chronic inflammation, autoimmunity, and cancer. Despite a growing understanding of the mechanisms underlying NET formation, much remains to be elucidated. We previously showed that in human neutrophils activated with different classes of physiological stimuli, NET formation features both early and late events that are controlled by discrete signaling pathways. However, the nature of these events has remained elusive. We now report that PAD4 inhibition only affects the early phase of NET generation, as do distinct signaling intermediates (TAK1, MEK, p38 MAPK). Accordingly, the inducible citrullination of residue R2 on histone H3 is an early neutrophil response that is regulated by these kinases; other arginine residues on histones H3 and H4 do not seem to be citrullinated. Conversely, elastase blockade did not affect NET formation by several physiological stimuli, though it did so in PMA-activated cells. Among belated events in NET formation, we found that chromatin decondensation is impaired by the inhibition of signaling pathways controlling both early and late stages of the phenomenon. In addition to chromatin decondensation, other late processes were uncovered. For instance, unstimulated neutrophils can condition themselves to be poised for rapid NET induction. Similarly, activated neutrophils release endogenous proteic factors that promote and largely mediate NET generation. Several such factors are known RAGE ligands and accordingly, RAGE inbibition largely prevents both NET formation and the conditioning of neutrophils to rapidly generate NETs upon stimulation. Our data shed new light on the cellular processes underlying NET formation, and unveil unsuspected facets of the phenomenon that could serve as therapeutic targets. In view of the involvement of NETs in both homeostasis and several pathologies, our findings are of broad relevance.

Human Inflammatory Neutrophils Express Genes Encoding Peptidase Inhibitors: Production of Elafin Mediated by NF-κB and CCAAT/Enhancer-Binding Protein β

The concept of plasticity of neutrophils is highlighted by studies showing their ability to transdifferentiate into APCs. In this regard, transdifferentiated neutrophils were found at inflammatory sites of autoimmune arthritis (AIA). Exposure of neutrophils to inflammatory stimuli prolongs their survival, thereby favoring the acquisition of pathophysiologically relevant phenotypes and functions. By using microarrays, quantitative RT-PCR, and ELISAs, we showed that long-lived (LL) neutrophils obtained after 48 h of culture in the presence of GM-CSF, TNF, and IL-4 differentially expressed genes related to apoptosis, MHC class II, immune response, and inflammation. The expression of anti-inflammatory genes mainly of peptidase inhibitor families is upregulated in LL neutrophils. Among these, the PI3 gene encoding elafin was the most highly expressed. The de novo production of elafin by LL neutrophils depended on a synergism between GM-CSF and TNF via the activation and cooperativity of C/EBPβ and NF-κB pathways, respectively. Elafin concentrations were higher in synovial fluids (SF) of patients with AIA than in SF of osteoarthritis. SF neutrophils produced more elafin than blood counterparts. These results are discussed with respect to implications of neutrophils in chronic inflammation and the potential influence of elafin in AIA.

Detection of Intact Transcription Factors in Human Neutrophils

The crucial contribution of neutrophils to innate immunity extends well beyond their traditional role as professional phagocytes. Indeed, it is now well established that neutrophils generate a plethora of inflammatory cytokines and chemokines that are profoundly involved in the onset and evolution of the inflammatory reaction. Several recent studies have shown that neutrophils can represent an important source of inflammatory cytokines in pathophysiological settings. The inflammatory and immunomodulatory cytokines produced by neutrophils are generally encoded by immediate-early response genes, which in turn depend on the activation of transcription factors such as those belonging to the nuclear factor κB (NF-κB) and signal transducers and activators of transcription (STAT) families. We have shown in the past that the expression of such factors is induced in neutrophils stimulated by physiological agonists. However, the detection of intact (i.e., undegraded) transcription factors in neutrophils requires special precautions and a specially designed protocol, due to the huge amounts of endogenous proteases present in these cells. This protocol is the focus of this chapter.

Airway Mucins Inhibit Oxidative and Non-Oxidative Bacterial Killing by Human Neutrophils

Neutrophil killing of bacteria is mediated by oxidative and non-oxidative mechanisms. Oxidants are generated through the NADPH oxidase complex, whereas antimicrobial proteins and peptides rank amongst non-oxidative host defenses. Mucus hypersecretion, deficient hydration and poor clearance from the airways are prominent features of cystic fibrosis (CF) lung disease. CF airways are commonly infected by Pseudomonas aeruginosa and Burkholderia cepacia complex bacteria. Whereas the former bacterium is highly sensitive to non-oxidative killing, the latter is only killed if the oxidative burst is intact. Despite an abundance of neutrophils, both pathogens thrive in CF airway secretions. In this study, we report that secreted mucins protect these CF pathogens against host defenses. Mucins were purified from CF sputum and from the saliva of healthy volunteers. Whereas mucins did not alter the phagocytosis of Pseudomonas aeruginosa and Burkholderia cenocepacia by neutrophils, they completely suppressed bacterial killing. Accordingly, mucins markedly inhibited non-oxidative bacterial killing by neutrophil granule extracts, or by lysozyme and the cationic peptide, human β defensin-2 (HBD2). Mucins also suppressed the neutrophil oxidative burst through a charge-dependent mechanism that could be reversed by the cationic aminoglycoside, tobramycin. Our data indicate that airway mucins protect Gram-negative bacteria against neutrophil killing by suppressing the oxidative burst and inhibiting the bactericidal capacity of cationic proteins and peptides. Mucin hypersecretion, dehydration, stasis and anionic charge represent key therapeutic targets for improving host defenses and airway inflammation in CF and other muco-secretory airway diseases.

TAK1 signaling activity links the mast cell cytokine response and degranulation in allergic inflammation

Mast cells drive the inappropriate immune response characteristic of allergic inflammatory disorders via release of pro-inflammatory mediators in response to environmental cues detected by the IgE-FcεRI complex. The role of TGF-β-activated kinase 1 (TAK1), a participant in related signaling in other contexts, remains unknown in allergy. We detect novel activation of TAK1 at Ser412 in response to IgE-mediated activation under SCF-c-kit potentiation in a mast cell-driven response characteristic of allergic inflammation, which is potently blocked by TAK1 inhibitor 5Z-7-oxozeaenol (OZ). We, therefore, interrogated the role of TAK1 in a series of mast cell-mediated responses using IgE-sensitized murine bone marrow-derived mast cells, stimulated with allergen under several TAK1 inhibition strategies. TAK1 inhibition by OZ resulted in significant impairment in the phosphorylation of MAPKs p38, ERK, and JNK; and mediation of the NF-κB pathway via IκBα. Impaired gene expression and near abrogation in release of pro-inflammatory cytokines TNF, IL-6, IL-13, and chemokines CCL1, and CCL2 was detected. Finally, a significant inhibition of mast cell degranulation, accompanied by an impairment in calcium mobilization, was observed in TAK1-inhibited cells. These results suggest that TAK1 acts as a signaling node, not only linking the MAPK and NF-κB pathways in driving the late-phase response, but also initiation of the degranulation mechanism of the mast cell early-phase response following allergen recognition and may warrant consideration in future therapeutic development.

Transcriptional regulation of neutrophil differentiation and function during inflammation

Neutrophils are the most abundant leukocytes in innate immunity where they elicit powerful effector functions to eliminate invading pathogens and modulate the adaptive as well as the innate immune response. Neutrophil function must be tightly regulated during inflammation and infection to avoid additional tissue damage. Increasing evidence suggests that transcription factors (TFs) function as key regulators to modulate transcriptional output, thereby controlling cell fate decision and the inflammatory responses. However, the molecular mechanisms underlying neutrophil differentiation and function during inflammation remain largely uncharacterized. Here, we provide a comprehensive overview of TFs known to be crucial for neutrophil maturation and in the signaling pathways that control neutrophil differentiation and activation. We also outline how emerging genomic and single-cell technologies may facilitate further discovery of neutrophil transcriptional regulators.

Cytokine Production and NET Formation by Monosodium Urate-Activated Human Neutrophils Involves Early and Late Events, and Requires Upstream TAK1 and Syk

Gout is a prevalent and incapacitating disease triggered by the deposition of monosodium urate (MSU) crystals in joints, which are also massively infiltrated by neutrophils. The interaction of the latter with MSU crystals triggers several responses, including the generation of inflammatory mediators and of neutrophil extracellular traps (NETs). Though some of the signaling events mobilized by MSU in neutrophils have been described (e.g., Src family kinases, Syk, PKC, PI3K), the picture remains fragmentary. Likewise, the impact of these signaling events on cellular responses is incompletely understood. In this study, we examined transcriptomic changes triggered by MSU in neutrophils and their impact on the corresponding proteins, as well as the role of various signaling pathways in prominent functional responses. We report for the first time that neutrophils can secrete the monocyte chemoattractant, CCL4, in response to MSU. Accordingly, we found that transcription factors NF-κB, CREB, and C/EBP are belatedly activated by MSU crystals, and at least the former is involved in chemokine generation. Moreover, we show that MAPKs and Akt are activated by MSU in neutrophils, that they are under the control of TAK1 and Syk, and that they participate in cytokine generation and NETosis. In the latter instance, we found the phenomenon to be independent of endogenous ROS, but under the control of PAD4. We finally provide evidence that endogenous factors contribute to the belated phosphorylation of kinases and transcription factors in response to MSU. Collectively, our findings unveil potentially important therapeutic targets for gouty arthritis.

Enabling precision medicine by unravelling disease pathophysiology: quantifying signal transduction pathway activity across cell and tissue types

Signal transduction pathways are important in physiology and pathophysiology. Targeted drugs aim at modifying pathogenic pathway activity, e.g., in cancer. Optimal treatment choice requires assays to measure pathway activity in individual patient tissue or cell samples. We developed a method enabling quantitative measurement of functional pathway activity based on Bayesian computational model inference of pathway activity from measurements of mRNA levels of target genes of the pathway-associated transcription factor. Oestrogen receptor, Wnt, and PI3K-FOXO pathway assays have been described previously. Here, we report model development for androgen receptor, Hedgehog, TGFβ, and NFκB pathway assays, biological validation on multiple cell types, and analysis of data from published clinical studies (multiple sclerosis, amyotrophic lateral sclerosis, contact dermatitis, Ewing sarcoma, lymphoma, medulloblastoma, ependymoma, skin and prostate cancer). Multiple pathway analysis of clinical prostate cancer (PCa) studies showed increased AR activity in hyperplasia and primary PCa but variable AR activity in castrate resistant (CR) PCa, loss of TGFβ activity in PCa, increased Wnt activity in TMPRSS2:ERG fusion protein-positive PCa, active PI3K pathway in advanced PCa, and active PI3K and NFκB as potential hormonal resistance pathways. Potential value for future clinical practice includes disease subtyping and prediction and targeted therapy response prediction and monitoring.

Cell Type-Specific Roles of NF-κB Linking Inflammation and Thrombosis

The transcription factor NF-κB is a central mediator of inflammation with multiple links to thrombotic processes. In this review, we focus on the role of NF-κB signaling in cell types within the vasculature and the circulation that are involved in thrombo-inflammatory processes. All these cells express NF-κB, which mediates important functions in cellular interactions, cell survival and differentiation, as well as expression of cytokines, chemokines, and coagulation factors. Even platelets, as anucleated cells, contain NF-κB family members and their corresponding signaling molecules, which are involved in platelet activation, as well as secondary feedback circuits. The response of endothelial cells to inflammation and NF-κB activation is characterized by the induction of adhesion molecules promoting binding and transmigration of leukocytes, while simultaneously increasing their thrombogenic potential. Paracrine signaling from endothelial cells activates NF-κB in vascular smooth muscle cells and causes a phenotypic switch to a “synthetic” state associated with a decrease in contractile proteins. Monocytes react to inflammatory situations with enforced expression of tissue factor and after differentiation to macrophages with altered polarization. Neutrophils respond with an extension of their life span—and upon full activation they can expel their DNA thereby forming so-called neutrophil extracellular traps (NETs), which exert antibacterial functions, but also induce a strong coagulatory response. This may cause formation of microthrombi that are important for the immobilization of pathogens, a process designated as immunothrombosis. However, deregulation of the complex cellular links between inflammation and thrombosis by unrestrained NET formation or the loss of the endothelial layer due to mechanical rupture or erosion can result in rapid activation and aggregation of platelets and the manifestation of thrombo-inflammatory diseases. Sepsis is an important example of such a disorder caused by a dysregulated host response to infection finally leading to severe coagulopathies. NF-κB is critically involved in these pathophysiological processes as it induces both inflammatory and thrombotic responses.

Physiological Stimuli Induce PAD4-Dependent, ROS-Independent NETosis, With Early and Late Events Controlled by Discrete Signaling Pathways

Neutrophils are known to extrude decondensed chromatin, thus forming NETs (neutrophil extracellular traps). These structures immobilize pathogens, thereby preventing their spreading, and are also adorned with antimicrobial molecules. NETs can also influence pathogenesis in chronic inflammation, autoimmunity, and cancer. Despite the importance of NETs, the molecular mechanisms underlying their formation, as well as the upstream signaling pathways involved, are only partially understood. Likewise, current methodological approaches to quantify NETs suffer from significant drawbacks, not the least being the inclusion of a significant non-specific signal. In this study, we used novel, fluorescent polymers that only bind extruded chromatin, allowing a specific and standardized quantification of NETosis. This allowed us to reliably rank the relative potency of various physiologic NET inducers. In neutrophils activated with such stimuli, inhibition of the Syk or PI3K pathways blocked NETosis by acting upon late events in NET formation. Inhibition of the TAK1, p38 MAPK, or MEK pathways also hindered NETosis, but by acting on early events. By contrast, inhibiting PKC, Src family kinases, or JNK failed to prevent NETosis; cycloheximide or actinomycin D were also ineffective. Expectedly, NET formation was deeply compromised following inhibition of the NADPH oxidase in PMA-activated neutrophils, but was found to be ROS-independent in response to physiological agonists. Conversely, we show for the first time in human neutrophils that selective inhibition of PAD4 potently prevents NETosis by all stimuli tested. Our data substantially extends current knowledge of the signaling pathways controlling NETosis, and reveals how they affect early or late stages of the phenomenon. In view of the involvement of NETs in several pathologies, our findings also identify molecular targets that could be exploited for therapeutic intervention.

TNFα-YAP/p65-HK2 axis mediates breast cancer cell migration

Clinical and experimental evidence indicates that macrophages could promote solid-tumor progression and metastasis. However, the mechanisms underlying this process remain unclear. Here we show that yes-associated protein 1 (YAP1), a transcriptional regulator that controls tissue growth and regeneration, has an important role in tumor necrosis factor α (TNF α)-induced breast cancer migration. Mechanistically, macrophage conditioned medium (CM) or TNFα triggers IκB kinases (IKKs)-mediated YAP phosphorylation and activation in breast cancer cells. We further found that TNFα or macrophage CM treatment increases the interaction between p65 and YAP. Chromatin immunoprecipitation (ChIP) assay shows that YAP/TEAD (TEA domain family member) and p65 proteins synergistically regulate the transcription of hexokinase 2 (HK2), a speed-limiting enzyme in glycolysis, and promotes TNFα-induced or macrophage CM-induced cell migration. Together, our findings indicate an important role of TNFα-IKK-YAP/p65-HK2 signaling axis in the process of inflammation-driven migration in breast cancer cells, which reveals a new molecular link between inflammation and breast cancer metastasis.

Regulation of Discrete Functional Responses by Syk and Src Family Tyrosine Kinases in Human Neutrophils

Neutrophils play a critical role in innate immunity and also influence adaptive immune responses. This occurs in good part through their production of inflammatory and immunomodulatory cytokines, in conjunction with their prolonged survival at inflamed foci. While a picture of the signaling machinery underlying these neutrophil responses is now emerging, much remains to be uncovered. In this study, we report that neutrophils constitutively express various Src family isoforms (STKs), as well as Syk, and that inhibition of these protein tyrosine kinases selectively hinders inflammatory cytokine generation by acting posttranscriptionally. Accordingly, STK or Syk inhibition decreases the phosphorylation of signaling intermediates (e.g., eIF-4E, S6K, and MNK1) involved in translational control. By contrast, delayed apoptosis appears to be independent of either STKs or Syk. Our data therefore significantly extend our understanding of which neutrophil responses are governed by STKs and Syk and pinpoint some signaling intermediates that are likely involved. In view of the foremost role of neutrophils in several chronic inflammatory conditions, our findings identify potential molecular targets that could be exploited for future therapeutic intervention.

Transforming Growth Factor-β-Activated Kinase 1 Is Required for Human FcγRIIIb-Induced Neutrophil Extracellular Trap Formation

Neutrophils (PMNs) are the most abundant leukocytes in the blood. PMN migrates from the circulation to sites of infection where they are responsible for antimicrobial functions. PMN uses phagocytosis, degranulation, and formation of neutrophil extracellular traps (NETs) to kill microbes. Several stimuli, including bacteria, fungi, and parasites, and some pharmacological compounds, such as Phorbol 12-myristate 13-acetate (PMA), are efficient inducers of NETs. Antigen-antibody complexes are also capable of inducing NET formation. Recently, it was reported that FcγRIIIb cross-linking induced NET formation similarly to PMA stimulation. Direct cross-linking of FcγRIIA or integrins did not promote NET formation. FcγRIIIb-induced NET formation presented different kinetics from PMA-induced NET formation, suggesting differences in signaling. Because FcγRIIIb also induces a strong activation of extracellular signal-regulated kinase (ERK) and nuclear factor Elk-1, and the transforming growth factor-β-activated kinase 1 (TAK1) has recently been implicated in ERK signaling, in the present report, we explored the role of TAK1 in the signaling pathway activated by FcγRIIIb leading to NET formation. FcγRIIIb was stimulated by specific monoclonal antibodies, and NET formation was evaluated in the presence or absence of pharmacological inhibitors. The antibiotic LL Z1640-2, a selective inhibitor of TAK1 prevented FcγRIIIb-induced, but not PMA-induced NET formation. Both PMA and FcγRIIIb cross-linking induced phosphorylation of ERK. But, LL Z1640-2 only inhibited the FcγRIIIb-mediated activation of ERK. Also, only FcγRIIIb, similarly to transforming growth factor-β-induced TAK1 phosphorylation. A MEK (ERK kinase)-specific inhibitor was able to prevent ERK phosphorylation induced by both PMA and FcγRIIIb. These data show for the first time that FcγRIIIb cross-linking activates TAK1, and that this kinase is required for triggering the MEK/ERK signaling pathway to NETosis.

Clathrin complexes with the inhibitor kappa B kinase signalosome: imaging the interactome

Many receptors involved with innate immunity activate the inhibitor kappa B kinase signalosome (IKK). The active complex appears to be assembled from the two kinase units, IKKα and IKKβ with the regulatory protein NEMO. Because we previously found that RNA silencing of clathrin heavy chains (CHC), in transformed human lung pneumocytes (A549), decreased TNFα‐induced signaling and phosphorylation of inhibitor kappa B (IκB), we hypothesized that CHC forms cytoplasmic complexes with members of the IKK signalosome. Widely available antibodies were used to immunoprecipitate IKKα and NEMO interactomes. Analysis of the affinity interactomes by mass spectrometry detected clathrin with both baits with high confidence. Using the same antibodies for indirect digital immunofluorescence microscopy and FRET, the CHC–IKK complexes were visualized together with NEMO or HSP90. The natural variability of protein amounts in unsynchronized A549 cells was used to obtain statistical correlation for several complexes, at natural levels and without invasive labeling. Analyses of voxel numbers indicated that: (i) CHC–IKK complexes are not part of the IKK signalosome itself but, likely, precursors of IKK–NEMO complexes. (ii) CHC–IKKβ complexes may arise from IKKβ–HSP90 complexes.

Clathrin forms complexes with IKKa, IKKb, and NEMO, but apparently not the canonical signalosome. These complexes are identified, for the first time, by affinity proteomics and triple FRET without altering molecular structure.

Detection of intact transcription factors in human neutrophils

The crucial contribution of neutrophils to innate immunity extends well beyond their traditional role as professional phagocytes. Indeed, it is now well established that neutrophils generate a plethora of inflammatory cytokines and chemokines that are profoundly involved in the onset and evolution of the inflammatory reaction. Several recent studies have also shown that neutrophils can represent an important source of inflammatory cytokines in a number of pathophysiological settings. The inflammatory cytokines produced by neutrophils are generally encoded by immediate-early response genes, which in turn depend on the activation of transcription factors such as those belonging to the nuclear factor-kappa B and signal transducers and activators of transcription families. We have shown in the past that such factors are expressed and inducible in neutrophils stimulated by physiological agonists. However, the detection of intact (i.e., undegraded) transcription factors in neutrophils requires special precautions and an alternative protocol due to the huge amounts of endogenous proteases present in these cells. This protocol is the focus of this chapter.

Proteasome inhibition increases recruitment of IκB kinase β (IKKβ), S536P-p65, and transcription factor EGR1 to interleukin-8 (IL-8) promoter, resulting in increased IL-8 production in ovarian cancer cells

Proinflammatory and pro-angiogenic chemokine interleukin-8 (IL-8, CXCL8) contributes to ovarian cancer progression through its induction of tumor cell proliferation, survival, angiogenesis, and metastasis. Proteasome inhibition by bortezomib, which has been used as a frontline therapy in multiple myeloma, has shown only limited effectiveness in ovarian cancer and other solid tumors. However, the responsible mechanisms remain elusive. Here, we show that proteasome inhibition dramatically increases the IL-8 expression and release in ovarian cancer cells. The responsible mechanism involves an increased nuclear accumulation of IκB kinase β (IKKβ) and an increased recruitment of the nuclear IKKβ, p65-phosphorylated at Ser-536, and the transcription factor early growth response-1 (EGR-1) to the endogenous IL-8 promoter. Coimmunoprecipitation studies identified the nuclear EGR-1 associated with IKKβ and with p65, with preferential binding to S536P-p65. Both IKKβ activity and EGR-1 expression are required for the increased IL-8 expression induced by proteasome inhibition in ovarian cancer cells. Interestingly, in multiple myeloma cells the IL-8 release is not increased by bortezomib. Together, these data indicate that the increased IL-8 release may represent one of the underlying mechanisms responsible for the decreased effectiveness of proteasome inhibition in ovarian cancer treatment and identify IKKβ and EGR-1 as potential new targets in ovarian cancer combination therapies.

The p38-MSK1 signaling cascade influences cytokine production through CREB and C/EBP factors in human neutrophils

Neutrophils influence innate and adaptative immunity by generating numerous cytokines and chemokines whose regulation largely depends on transcriptional activators such as NF-κB and C/EBP factors. In this study, we describe the critical involvement of CREB transcription factors (CREB1 and activating transcription factor-1) in this functional response as well as relevant upstream signaling components. Neutrophil stimulation with LPS or TNF led to the phosphorylation, DNA binding activity, and chemokine promoter association of CREB1 and activating transcription factor-1. These responses occurred downstream of the p38-MSK1 signaling axis, as did the phosphorylation and promoter association of another bZIP factor, C/EBPβ. Conversely, inhibition of RSK1 failed to alter the phosphorylation of either CREB1 or C/EBPβ in neutrophils. From a more functional standpoint, the inhibition of p38 MAPK or MSK1 interfered with cytokine generation in neutrophils. Likewise, overexpression of a dominant-negative CREB1 mutant (K-CREB) or of a point mutant (S133A) resulted in a decreased ability of human neutrophil-like PLB-985 cells to generate inflammatory cytokines (CXCL8, CCL3, CCL4, and TNF-α). Collectively, our data show the involvement of CREB1 in neutrophil cytokine production, the key role of its S133 residue, important upstream signaling events, and the parallel activation of another bZIP factor. These are all potential molecular targets that could be exploited in the context of several chronic inflammatory diseases that prominently feature neutrophils and their products.

Sauchinone, a lignan from Saururus chinensis, attenuates neutrophil pro-inflammatory activity and acute lung injury

Previous studies have shown that sauchinone modulates the expression of inflammatory mediators through mitogen-activated protein kinase (MAPK) pathways in various cell types. However, little information exists about the effect of sauchinone on neutrophils, which play a crucial role in inflammatory process such as acute lung injury (ALI). We found that sauchinone decreased the phosphorylation of p38 MAPK in lipopolysaccharide (LPS)-stimulated murine bone marrow neutrophils, but not ERK1/2 and JNK. Exposure of LPS-stimulated neutrophils to sauchinone or SB203580, a p38 inhibitor, diminished production of tumor necrosis factor (TNF)-α and macrophage inflammatory protein (MIP)-2 compared to neutrophils cultured with LPS. Treatment with sauchinone decreased the level of phosphorylated ribosomal protein S6 (rpS6) in LPS-stimulated neutrophils. Systemic administration of sauchinone to mice led to reduced levels of phosphorylation of p38 and rpS6 in mice lungs given LPS, decreased TNF-α and MIP-2 production in bronchoalveolar lavage fluid, and also diminished the severity of LPS-induced lung injury, as determined by reduced neutrophil accumulation in the lungs, wet/dry weight ratio, and histological analysis. These results suggest that sauchinone diminishes LPS-induced neutrophil activation and ALI.

Translational control of human neutrophil responses by MNK1

A growing number of inflammatory and immune processes in vivo have been shown to be influenced by neutrophil-derived cytokines. Whereas the underlying transcriptional mechanisms are increasingly well understood, the translational regulation of this neutrophil response remains largely unexplored. Here, we show that the MNK1, which participates in translational control in several cell types, is activated in response to physiological neutrophil agonists (LPS, TNF-α) in the cytoplasmic and nuclear compartments. With the use of various pharmacological inhibitors, we found that MNK1 activation takes place downstream of the TAK1-p38 MAPK axis in neutrophils, whereas the MEK/ERK, JNK, PI3K, and PKC pathways are not involved. Pharmacological blockade of MNK1, as well as overexpression experiments, established that cytokine protein synthesis (but not gene expression) is under the control of MNK1 in neutrophils. Likewise, MNK1 inhibition reversed the antiapoptotic effect of LPS and TNF-α in neutrophils, and this was accompanied by a decreased expression of the antiapoptotic protein Mcl-1. Thus, MNK1 appears to be an important regulator of neutrophil responses. Although MNK1 inhibition did not affect protein recruitment to mRNA caps, it decreased the phosphorylation of molecules implicated in translation initiation control, such as S6K, S6, and hyperphosphorylated 4E-BP1. These molecular targets of MNK1 are shared with those of PI3K in neutrophils, and accordingly, MNK1 inhibition partially impaired the belated PI3K/Akt activation elicited by LPS or TNF in these cells. Given the importance of neutrophils and their products in numerous chronic inflammatory disorders, MNK1 could represent an attractive therapeutic target.

Beyond NF-κB activation: nuclear functions of IκB kinase α

IκB kinase (IKK) complex, the master kinase for NF-κB activation, contains two kinase subunits, IKKα and IKKβ. In addition to mediating NF-κB signaling by phosphorylating IκB proteins during inflammatory and immune responses, the activation of the IKK complex also responds to various stimuli to regulate diverse functions independently of NF-κB. Although these two kinases share structural and biochemical similarities, different sub-cellular localization and phosphorylation targets between IKKα and IKKβ account for their distinct physiological and pathological roles. While IKKβ is predominantly cytoplasmic, IKKα has been found to shuttle between the cytoplasm and the nucleus. The nuclear-specific roles of IKKα have brought increasing complexity to its biological function. This review highlights major advances in the studies of the nuclear functions of IKKα and the mechanisms of IKKα nuclear translocation. Understanding the nuclear activity is essential for targeting IKKα for therapeutics.

Role of IKK/NF-κB signaling in extinction of conditioned place aversion memory in rats

The inhibitor κB protein kinase/nuclear factor κB (IKK/NF-κB) signaling pathway is critical for synaptic plasticity. However, the role of IKK/NF-κB in drug withdrawal-associated conditioned place aversion (CPA) memory is unknown. Here, we showed that inhibition of IKK/NF-κB by sulphasalazine (SSZ; 10 mM, i.c.v.) selectively blocked the extinction but not acquisition or expression of morphine-induced CPA in rats. The blockade of CPA extinction induced by SSZ was abolished by sodium butyrate, an inhibitor of histone deacetylase. Thus, the IKK/NF-κB signaling pathway might play a critical role in the extinction of morphine-induced CPA in rats and might be a potential pharmacotherapy target for opiate addiction.

FOXO3 as a new IKK-ε-controlled check-point of regulation of IFN-β expression

Cell survival transcription factor FOXO3 has been recently implicated in moderating pro-inflammatory cytokine production by dendritic cells (DCs), but the molecular mechanisms are unclear. It was suggested that FOXO3 could antagonize NF-κB activity, while IKK-β was demonstrated to inactivate FOXO3, suggesting a cross-talk between the two pathways. Therefore, FOXO3 activity must be tightly regulated to allow for an appropriate inflammatory response. Here, we show that in human monocyte-derived DCs (MDDCs), FOXO3 is able to antagonize signaling intermediates downstream of the Toll-like receptor (TLR) 4, such as NF-κB and interferon regulatory factors (IRFs), resulting in inhibition of interferon (IFN)-β expression. We also demonstrate that the activity of FOXO3 itself is regulated by IKK-ε, a kinase involved in IFN-β production, which phosphorylates and inactivates FOXO3 in response to TLR4 agonists. Thus, we identify FOXO3 as a new IKK-ε-controlled check-point of IRF activation and regulation of IFN-β expression, providing new insight into the role of FOXO3 in immune response control.

Flice inhibitory protein is associated with the survival of neonatal neutrophils

Neonatal polymorphonuclear leukocytes (PMN) exhibit delayed apoptosis both constitutively and under inflammatory conditions, and evidence has linked PMN longevity to the presence of antiapoptotic proteins. Activation of the survival-associated transcription factor, nuclear factor kappa B (NF-κB), promotes the synthesis of several antiapoptotic proteins including Flice inhibitory protein (FLIP). Neonatal and adult PMN were compared in this study to test the hypothesis that FLIP modulates age-related apoptosis. Expression of the short isoform, FLIP-S, was prominent at baseline and persisted during spontaneous apoptosis in neonatal PMN, whereas basal expression was lower and decreased under the same conditions in adult PMN. Stable FLIP-S expression in neonatal PMN was associated with a relative resistance to apoptosis in response to the protein synthesis inhibitor, cycloheximide (CHX), or the NF-κB inhibitor, gliotoxin. In contrast, similar treatment of adult PMN promoted greater overall apoptosis accompanied by FLIP degradation. Nuclear levels of phosphorylated p65, a critical NF-κB dimer, were relatively robust in neonatal PMN under basal conditions or after stimulation with TNF-α, a cytokine that induces FLIP. In conclusion, persistent FLIP-S expression is involved in the longevity of neonatal PMN, and our data suggest a contribution of NF-κB signaling and related survival mechanisms.

MT6-MMP is present in lipid rafts and faces inward in living human PMNs but translocates to the cell surface during neutrophil apoptosis

Polymorphonuclear neutrophils (PMNs) are the first line of defense against invading organisms in humans; in addition, PMNs contribute to the linking of innate and adaptive immunity. To fulfill their biological behavior, PMNs utilize an arsenal of proteolytic enzymes, including members of the matrix metalloproteinase family of zinc-dependent endopeptidases. PMNs express high levels of MT6-MMP (MMP-25), a glycosyl-phosphatidylinositol-anchored MMP, that belongs to the subfamily of membrane-anchored matrix metalloproteinases. Due to the paucity of information on MT6-MMP in primary cells, we set to investigate the localization and potential function of MT6-MMP in human PMNs. We found that MT6-MMP is present in the membrane, granules and nuclear/endoplasmic reticulum/Golgi fractions of PMNs where it is displayed as a disulfide-linked homodimer of 120 kDa. Stimulation of PMNs resulted in secretion of active MT6-MMP into the supernatants. Membrane-bound MT6-MMP, conversely, is located in the lipid rafts of resting PMNs and stimulation does not alter this location. In addition, TIMP-2, a natural inhibitor of MT6-MMP, does not co-localize with it in the lipid rafts. Interestingly, living PMNs do not display MT6-MMP on the cell surface. However, induction of apoptosis induces MT6-MMP relocation on PMNs' cell surface. Our studies suggest that metalloproteinases may play a role in respiratory burst and IL-8 secretion, but not chemotaxis or granulocyte macrophage colony-stimulating factor-induced survival. Collectively, these results provide new insights on the role of MT6-MMP in the physiology of human PMNs.

Mature human neutrophils constitutively express the transcription factor EGR-1

The immediate early response gene, Early Growth Response 1 (EGR-1) has emerged as a central regulator of early inflammatory and immune processes by rapidly regulating the transcription of a wide array of downstream effector genes. Neutrophils, which are among the first circulating leukocytes to respond to inflammatory signals, exhibit a broad set of transcriptional changes immediately upon exposure to inflammatory and pathogenic stimuli. Such transcriptional changes are likely to be controlled by early gene transcription factors such as EGR-1. We therefore examined the regulation and role of EGR-1 in mature human neutrophils exposed to the inflammatory stimuli fMLP and IL-8. We report that human neutrophils rapidly and transiently up-regulate EGR-1 mRNA upon stimulation with fMLP or IL-8. However in contrast to that seen in other cells, EGR-1 mRNA expression profiles were not predictive of protein expression. Instead, we show that human neutrophils constitutively express EGR-1 protein. The cellular content of EGR-1 did not change over time or upon neutrophil activation. Confocal microscopy revealed that EGR-1 was present in both the cytoplasm and nuclei of un-stimulated neutrophils and that activation did not change this subcellular localization or promote nuclear translocation. Using chromatin immunoprecipitation, we demonstrate that EGR-1 is associated with the promoter regions of the immune regulatory genes IL-1 beta, TGFbeta-1 and MIF in both resting and activated neutrophils with increased promoter association observed upon cell activation. This novel pattern of EGR-1 protein expression may underlie the ability of the neutrophil to respond rapidly to inflammatory stimuli.

Over-expression of IkappaB-kinase-epsilon (IKKepsilon/IKKi) induces secretion of inflammatory cytokines in prostate cancer cell lines

BACKGROUND

Elevated inflammatory cytokine levels in serum have been associated with advanced stage metastasis-related morbidity in prostate cancer. Several studies have shown that IL-6 and IL-8 can accelerate the growth of human prostate cancer cell lines. Previous studies, in murine embryonic fibroblasts, have shown that Ikappa-B kinase-epsilon (IKKepsilon/IKKi)-deficiency results in the reduction of lipopolysaccharide-mediated expression of IL-6.

RESULTS

In this study, we report that over-expression of IKKepsilon in hormone-sensitive 22Rv1 and LNCaP prostate cancer cells induces the secretion of several inflammatory cytokines including IL-6 and IL-8. Both of these cytokines are secreted by hormone-refractory PC-3 prostate cancer cells and IKKepsilon knock-down in these cells correlates with a strong decrease in IL-6 secretion. Furthermore, we demonstrate that IKKepsilon over-expression does not induce the activation of the IKKepsilon classical targets NF-kappaB and IRF-3, two transcription factors involved in the regulation of several cytokines. Finally, we observe that high IKKepsilon expression results in its nuclear translocation, a phenomena that is TBK1-independent.

CONCLUSIONS

This study identifies IKKepsilon as a potential prostate cancer gene that may favor chronic inflammation and create a tumor-supporting microenvironment that promotes prostate cancer progression, particularly by the induction of IL-6 secretion that may act as a positive growth factor in prostate cancer.

Autocrine role of endogenous interleukin-18 on inflammatory cytokine generation by human neutrophils

Neutrophils are key players of innate immunity and influence inflammatory and immune reactions through the production of numerous cytokines. Interleukin-18 (IL-18) is known to stimulate several neutrophil responses, and recent evidence suggests that neutrophils might represent a source of IL-18. Here, we show that neutrophils constitutively produce both IL-18 and its antagonist, IL-18BP. Cell activation does not affect IL-18BP release but leads to an increased gene expression and secretion of IL-18, a process that depends on NF-kappaB activation. Moreover, endogenous IL-18 feeds back on the neutrophils to augment cytokine generation in lipopolysaccharide-treated cells. Accordingly, exogenous IL-18 can induce the gene expression and release of several inflammatory cytokines in neutrophils, including its own expression. We finally report that IL-18 activates the p38 MAPK, MEK/ERK, and PI3K/Akt pathways in neutrophils. The IKK cascade is also activated by IL-18, resulting in IkappaB-alpha degradation, NF-kappaB activation, and RelA phosphorylation. Accordingly, these pathways contribute to the generation of inflammatory cytokines in IL-18-stimulated neutrophils. By contrast, the phosphorylation and DNA-binding activity of various STAT proteins were not induced by IL-18. Collectively, our results unveil new interactions between IL-18 and neutrophils and further support a role for these cells in influencing both innate and adaptive immunity.

A nuclear role for Kaposi's sarcoma-associated herpesvirus-encoded K13 protein in gene regulation

Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded viral FLICE inhibitory protein K13 interacts with a cytosolic IkappaB kinase (IKK) complex to activate nuclear factor-kappaB (NF-kappaB). We recently reported that K13 antagonizes KSHV lytic regulator RTA (replication and transcription activator) and blocks lytic replication, but spares RTA-induced viral interleukin-6 (vIL6). Here we report that K13 is also present in the nuclear compartment, a property not shared by its structural homologs. K13 interacts with and activates the nuclear IKK complex, and binds to the IkappaBalpha promoter. K13 mutants that are retained in the cytosol lack NF-kappaB activity. However, neither the IKKs nor NF-kappaB activation is required for nuclear localization of K13. Instead, this ability is dependent on a nuclear localization signal located in its N-terminal 40 amino acids. Finally, K13, along with p65/RelA, binds to the promoters of a number of KSHV lytic genes, including RTA, ORF57 and vGPCR, but not to the promoter of the vIL6 gene. Thus, K13 has an unexpected nuclear role in viral and cellular gene regulation and its differential binding to the promoters of lytic genes may not only contribute to the inhibition of KSHV lytic replication, but may also account for the escape of vIL6 from K13-induced transcriptional suppression.

Cytokine generation, promoter activation, and oxidant-independent NF-kappaB activation in a transfectable human neutrophilic cellular model

Background

Human neutrophils are key players of innate immunity, and influence inflammatory and immune reactions through the production of numerous cytokines and chemokines. Despite major advances in our understanding of this important functional response of neutrophils, the short lifespan of these cells and their resistance to transfection have always been an obstacle to the detailed dissection of signaling pathways and effector responses that is often possible in other cell types.

Results

Here, we report that granulocytic differentiation of human PLB-985 cells with DMSO yields cells that are neutrophil-like with respect to surface markers, acquisition of responsiveness to physiological neutrophil stimuli (fMLP, LPS), cytokine expression and production profile, and transcription factor activation profile (NF-κB, C/EBP, AP-1, STAT). We also show that granulocytic PLB-985 cells can be reliably tranfected by nucleofection in a rapid and efficient manner. Indeed, we overexpressed several proteins and luciferase constructs into these cells. In particular, overexpression of a dominant negative IκB-α confirmed the central role of NF-κB in the production of cytokines by granulocytes. Moreover, the use of PLB-985 granulocytes in which the NADPH oxidase is inactive due to the targeted disruption of a key component (gp91phox) revealed that NF-κB activation and κB-dependent responses are independent of endogenous reactive oxygen intermediates in these cells. Antioxidant studies performed in primary human neutrophils support this conclusion.

Conclusion

Our results unveil a new facet of the NF-κB system of human granulocytes, and pave the way for deciphering signal transduction pathways and promoter activation in these cells.

The IkappaB kinase regulates chromatin structure during reconsolidation of conditioned fear memories

Previously formed memories are susceptible to disruption immediately after recall due to a necessity to be reconsolidated after retrieval. Protein translation mechanisms have been widely implicated as being necessary for memory reconsolidation, but gene transcription mechanisms have been much less extensively studied in this context. We found that retrieval of contextual conditioned fear memories activates the NF-kappaB pathway to regulate histone H3 phosphorylation and acetylation at specific gene promoters in hippocampus, specifically via IKKalpha and not the NF-kappaB DNA-binding complex. Behaviorally, we found that inhibition of IKKalpha regulation of either chromatin structure or NF-kappaB DNA-binding complex activity leads to impairments in fear memory reconsolidation, and that elevating histone acetylation rescues this memory deficit in the face of IKK blockade. These data provide insights into IKK-regulated transcriptional mechanisms in hippocampus that are necessary for memory reconsolidation.

NFkappaB is persistently activated in continuously stimulated human neutrophils

Increased activation of the transcription factor NFkappaB in the neutrophils has been associated with the pathogenesis of sepsis, acute lung injury (ALI), bronchopulmonary dysplasia (BPD), and other neutrophil-mediated inflammatory disorders. Despite recent progress in analyzing early NFkappaB activation in human neutrophils, activation of NFkappaB in persistently stimulated neutrophils has not been previously studied. Because it is the persistent NFkappaB activation that is thought to be involved in the host response to sepsis and the pathogenesis of ALI and BPD, we hypothesized that continuously stimulated human neutrophils may exhibit a late phase of NFkappaB activity. The goal of this study was to analyze the NFkappaB activation and expression of IkappaB and NFkappaB proteins during neutrophil stimulation with inflammatory signals for prolonged times. We demonstrate that neutrophil stimulation with lipopolysaccharide (LPS) and tumor necrosis factor-alpha (TNFalpha) induces, in addition to the early activation at 30-60 min, a previously unrecognized late phase of NFkappaB activation. In LPS-stimulated neutrophils, this NFkappaB activity typically had a biphasic character, whereas TNFalpha-stimulated neutrophils exhibited a continuous NFkappaB activity peaking around 9 h after stimulation. In contrast to the early NFkappaB activation that inversely correlates to the nuclear levels of IkappaBalpha, however, in continuously stimulated neutrophils, NFkappaB is persistently activated despite considerable levels of IkappaBalpha present in the nucleus. Our data suggest that NFkappaB is persistently activated in human neutrophils during neutrophil-mediated inflammatory disorders, and this persistent NFkappaB activity may represent one of the underlying mechanisms for the continuous production of proinflammatory mediators.

Acute alcohol exposure exerts anti-inflammatory effects by inhibiting IkappaB kinase activity and p65 phosphorylation in human monocytes

Acute alcohol use is associated with impaired immune responses and decreased proinflammatory cytokine production. Our earlier studies have shown that acute alcohol intake inhibits NF-kappaB DNA binding in an IkappaBalpha-independent manner. We report using human peripheral blood monocytes and Chinese hamster ovary cells transfected with CD14 cells that acute alcohol treatment in vitro exerts NF-kappaB inhibition by disrupting phosphorylation of p65. Immunoprecipitation of p65 and IkappaBalpha revealed that acute alcohol exposure for 1 h decreased NF-kappaB-IkappaBalpha complexes in the cytoplasm. Phosphorylation of p65 at Ser(536) is mediated by IkappaB kinase (IKK)beta and is required for NF-kappaB-dependent cellular responses. We show that acute alcohol treatment decreased LPS-induced IKKalpha and IKKbeta activity resulting in decreased phosphorylation of p65 at Ser(536). Furthermore, nuclear expression of IKKalpha increased after alcohol treatment, which may contribute to inhibition of NF-kappaB. Decreased phosphorylation of nuclear p65 at Ser(276) was likely not due to alcohol-induced inhibition of protein kinase A and mitogen- and stress-activated protein kinase-1 activity. Although decreased IkappaBalpha phosphorylation after acute alcohol treatment was attributable to reduced IKKbeta activity, degradation of IkappaBalpha during alcohol exposure was IKKbeta-independent. Alcohol-induced degradation of IkappaBalpha in the presence of a 26S proteasome inhibitor suggested proteasome-independent IkappaBalpha degradation. Collectively, our studies suggest that acute alcohol exposure modulates IkappaBalpha-independent NF-kappaB activity primarily by affecting phosphorylation of p65. These findings further implicate an important role for IKKbeta in the acute effects of alcohol in immune cells.

The MyD88-independent pathway is not mobilized in human neutrophils stimulated via TLR4

LPS activates both MyD88-dependent and -independent signaling via TLR4, but the extent to which each cascade is operative in different cell types remains unclear. This prompted us to revisit the intriguing issue of CXCL10 production, which we previously showed to be inducible in neutrophils stimulated with LPS and IFN-gamma but not with either stimulus alone, contrary to other myeloid cells. We now report that in neutrophils the MyD88-independent pathway is not activated by LPS. Indeed, microarray and real-time PCR experiments showed that neither IFNbeta nor IFNbeta-dependent genes (including CXCL10) are inducible in LPS-treated neutrophils, in contrast to monocytes. Further investigation into the inability of LPS to promote IFNbeta expression in neutrophils revealed that the transcription factors regulating the IFNbeta enhanceosome, such as IFN-regulatory factor-3 and AP-1, are not activated in LPS-treated neutrophils as revealed by lack of dimerization, nuclear translocation, confocal microscopy, and inducible binding to DNA. Moreover, we show that the upstream TANK-binding kinase-1 is not activated by LPS in neutrophils. A lack of IFNbeta/CXCL10 mRNA expression and IFN-regulatory factor 3 activation was also observed in myeloid leukemia HL60 cells differentiated to granulocytes and then stimulated with LPS, indicating that the inability of neutrophils to activate the MyD88-independent pathway represents a feature of their terminal maturation. These results identify a disconnected activation of the two signaling pathways downstream of TLR4 in key cellular components of the inflammatory and immune responses and help us to better understand the primordial role of neutrophils in host defense against nonviral infections.

NFkappaB is persistently activated in continuously stimulated human neutrophils

Increased activation of the transcription factor NFkappaB in the neutrophils has been associated with the pathogenesis of sepsis, acute lung injury (ALI), bronchopulmonary dysplasia (BPD), and other neutrophil-mediated inflammatory disorders. Despite recent progress in analyzing early NFkappaB activation in human neutrophils, activation of NFkappaB in persistently stimulated neutrophils has not been previously studied. Because it is the persistent NFkappaB activation that is thought to be involved in the host response to sepsis and the pathogenesis of ALI and BPD, we hypothesized that continuously stimulated human neutrophils may exhibit a late phase of NFkappaB activity. The goal of this study was to analyze the NFkappaB activation and expression of IkappaB and NFkappaB proteins during neutrophil stimulation with inflammatory signals for prolonged times. We demonstrate that neutrophil stimulation with lipopolysaccharide (LPS) and tumor necrosis factor-alpha (TNFalpha) induces, in addition to the early activation at 30-60 min, a previously unrecognized late phase of NFkappaB activation. In LPS-stimulated neutrophils, this NFkappaB activity typically had a biphasic character, whereas TNFalpha-stimulated neutrophils exhibited a continuous NFkappaB activity peaking around 9 h after stimulation. In contrast to the early NFkappaB activation that inversely correlates to the nuclear levels of IkappaBalpha, however, in continuously stimulated neutrophils, NFkappaB is persistently activated despite considerable levels of IkappaBalpha present in the nucleus. Our data suggest that NFkappaB is persistently activated in human neutrophils during neutrophil-mediated inflammatory disorders, and this persistent NFkappaB activity may represent one of the underlying mechanisms for the continuous production of proinflammatory mediators.

Effects of aging on triggering receptor expressed on myeloid cells (TREM)-1-induced PMN functions

Triggering receptor expressed on myeloid cell-1 (TREM-1) is a recently described receptor that has many effects on polymorphonuclear neutrophil (PMN), as the engagement of this receptor on PMN can induce phagocytosis, respiratory burst and degranulation. We studied the effects of aging on TREM-1 engagement in human PMN. PMN from elderly were found to have impaired response following TREM-1 engagement. Notably they were not able to modulate the TREM-1-induced respiratory burst as PMN from young did. TREM-1 engagement could not reverse PMN survival following incubation with LPS or GM-CSF in the elderly whereas it did in the young. The phosphorylation of TREM-1 signal transduction molecules was altered with aging. Finally, TREM-1 engagement could not drive the recruitment of TREM-1 in the lipid-rafts of the elderly explaining in part the altered response. The observed alterations in TREM-1 response are possibly an important contributing factor in the higher incidence of sepsis-related deaths in the elderly population.

Detection of intact transcription factors in human neutrophils

The crucial contribution of neutrophils to innate immunity extends well beyond their traditional role as professional phagocytes. Indeed, it is now well established that neutrophils generate a plethora of inflammatory cytokines and chemokines that are profoundly involved in the onset and evolution of the inflammatory reaction. Several recent studies have also shown that neutrophils can represent an important source of inflammatory cytokines in a number of pathophysiological settings. The inflammatory cytokines produced by neutrophils are generally encoded by immediate-early response genes, which in turn depend on the activation of transcription factors such as those belonging to the nuclear factor (NF)-kappaB and signal transducers and activators of transcription (STAT) families. We have shown in the past that such factors are expressed and inducible in neutrophils stimulated by physiological agonists. However, the detection of intact (i.e., undegraded) transcription factors in neutrophils requires special precautions and an alternative protocol, as a result of the huge amounts of endogenous proteases present in these cells. This protocol is the focus of this chapter.

Differential involvement of NF-κB and MAP kinase pathways in the generation of inflammatory cytokines by human neutrophils

The ability of human neutrophils to express a variety of genes encoding inflammatory mediators is well documented, and mounting evidence suggests that neutrophil-derived cytokines and chemokines contribute to the recruitment of discrete leukocyte populations at inflammatory sites. Despite this, our understanding of the signaling intermediates governing the generation of inflammatory cytokines by neutrophils remains fragmentary. Here, we report that inhibitors of the p38 MAPK and MEK pathways substantially diminish the release of (and in the case of p38 inhibitors, the gene expression of) several inflammatory cytokines in neutrophils stimulated with LPS or TNF. In addition, various NF-kappaB inhibitors were found to profoundly impede the inducible gene expression and release of inflammatory cytokines in these cells. The MAPK inhibitors did not affect NF-kappaB activation; instead, the transcriptional effects of the p38 MAPK inhibitor appear to involve transcriptional factor IID. Conversely, the NF-kappaB inhibitors failed to affect the activation of MAPKs. Finally, the MAPK inhibitors were found to prevent the activation a key component of the translational machinery, S6 ribosomal protein, in keeping with their post-transcriptional impact on cytokine generation. To our knowledge, this constitutes the first demonstration that in neutrophils, the inducible expression of proinflammatory cytokines by physiological stimuli largely reflects the ability of the latter to activate NF-kappaB and selected MAPK pathways. Our data also raise the possibility that NF-kappaB or MAPK inhibitors could be useful in the treatment of inflammatory disorders in which neutrophils predominate.

Dual role for RhoA in suppression and induction of cytokines in the human neutrophil

Production of tumor necrosis factor-alpha (TNFalpha) by the neutrophil (PMN) is a pivotal event in innate immunity, but the signals regulating TNFalpha induction in this primary cell are poorly understood. Herein, we use protein transduction to identify novel, opposing anti- and pro-cytokine-inducing roles for RhoA in the resting and lipopolysaccharide (LPS)-stimulated human PMN, respectively. In the resting cell, RhoA suppresses Cdc42 activation, IkappaBalpha degradation, nuclear factor-kappaB (NF-kappaB) activation, and induction of TNFalpha and NF-kappaB-dependent chemokines. Suppression of TNFalpha induction by RhoA is Rho kinase alpha (ROCKalpha) independent, but Cdc42 dependent, because TNFalpha induction by C3 transferase is attenuated by inhibition of Cdc42, and constitutively active Cdc42 suffices to activate NF-kappaB and induce TNFalpha. By contrast, we also place RhoA downstream of p38 mitogen-activated protein kinase and Cdc42 in a novel LPS-activated pathway in which p38, Cdc42, and ROCKalpha all promote TNFalpha protein expression. The p65 subunit of NF-kappaB coprecipitates with RhoA in a manner sensitive to the RhoA activation state. Our findings suggest a new, 2-faced role for RhoA as a checkpoint in innate immunity.

Nuclear accumulation of cRel following C-terminal phosphorylation by TBK1/IKK epsilon

The NF-kappaB transcription factors are key regulators of immunomodulatory, cell cycle, and developmental gene regulation. NF-kappaB activity is mainly regulated through the phosphorylation of IkappaB by the IkappaB kinase (IKK) complex IKKalphabetagamma, leading to proteasome-mediated degradation of IkappaB, nuclear translocation of NF-kappaB dimers, DNA binding, and gene induction. Additionally, direct posttranslational modifications of NF-kappaB p65 and cRel subunits involving C-terminal phosphorylation has been demonstrated. The noncanonical IKK-related homologs, TNFR-associated factor family member-associated NF-kappaB activator (TANK)-binding kinase (TBK)1 and IKKepsilon, are also thought to play a role in NF-kappaB regulation, but their functions remain unclear. TBK1 and IKKepsilon were recently described as essential regulators of IFN gene activation through direct phosphorylation of the IFN regulatory factor-3 and -7 transcription factors. In the present study, we sought to determine whether IKKepsilon and TBK1 could modulate cRel activity via phosphorylation. TBK1 and IKKepsilon directly phosphorylate the C-terminal domain of cRel in vitro and in vivo and regulate nuclear accumulation of cRel, independently of the classical IkappaB/IKK pathway. IkappaBalpha degradation is not affected, but rather IKKepsilon-mediated phosphorylation of cRel leads to dissociation of the IkappaBalpha-cRel complex. These results illustrate a previously unrecognized aspect of cRel regulation, controlled by direct IKKepsilon/TBK1 phosphorylation.

Extending the nuclear roles of IkappaB kinase subunits

The transcription factor NF-kappaB plays a key role in a wide variety of cellular processes such as innate and adaptive immunity, cellular proliferation, apoptosis and development. In unstimulated cells, NF-kappaB is sequestered in the cytoplasm through its tight association with inhibitory proteins called IkappaBs, comprising notably IkappaBalpha. A key step in NF-kappaB activation is the phosphorylation of IkappaBalpha by the so-called IkappaB kinase (IKK) complex, which targets the inhibitory protein for proteasomal degradation and allows the freed NF-kappaB to enter the nucleus where it can transactivate its target genes. The IKK complex is composed of two catalytic subunits called IKKalpha and IKKbeta, and a regulatory subunit called NEMO/IKKgamma. Despite their key role in mediating IkappaBalpha phosphorylation in the cytoplasm, recent works have provided evidence that IKK subunits also translocate into the nucleus to regulate NF-kappaB-dependent and -independent gene expression, paving the way of a novel and exciting field of research. In this review, we will describe the current knowledge in that research area.