Matthew E. Poynter

Recent advances torwards understanding redox mechanisms in the activation of nuclear factor κb

The transcription factor, nuclear factor-kappaB (NF-kappaB) has been studied extensively due to its prominent role in the regulation of immune and inflammatory genes, apoptosis, and cell proliferation. It has been known for more that a decade that NF-kappaB is a redox-sensitive transcription factor. The contribution of redox regulation and the location of potential redox-sensitive sites within the NF-kappaB activation pathway are subject to intense debate due to many conflicting reports. Redox regulation of NF-kappaB has been extensively addressed in this journal and the reader is referred to two comprehensive reviews on the subject [1,2]. With the identification of signaling intermediates proximal to the degradation of the inhibitor, IkappaB, the number of potential redox-sensitive sites is rapidly increasing. The purpose of this review is to address recent insights into the NF-kappaB signaling cascades that are triggered by proinflammatory cytokines such as TNF-alpha and IL-1beta. In addition, the role of nitrogen monoxide (.NO) in the regulation of NF-kappaB will be reviewed. Opportunities for redox regulation that occur upstream of IkappaB-alpha degradation, as well as the potential for redox control of phosphorylation of NF-kappaB subunits, will be discussed. Redox-sensitive steps are likely to depend on the nature of the NF-kappaB activator, the type of reactive oxygen or nitrogen species involved, the selectivity of signaling pathways activated, as well as the cell type under investigation. Lastly, it is discussed how redox regulation of NF-kappaB activation is likely to involve multiple subcellular compartments.

A Prominent Role for Airway Epithelial NF-κB Activation in Lipopolysaccharide-Induced Airway Inflammation

To reveal the causal role of airway epithelial NF-κB activation in evoking airway inflammation, a transgenic mouse was created expressing a mutant version of the inhibitory protein I-κBα. This I-κBα superrepressor (I-κBαSR) acts to repress NF-κB activation exclusively in airway epithelial cells, under the transcriptional control of the rat CC10 promoter (CC10-I-κBαSR). Compared with transgene-negative littermates, intranasal instillation of LPS did not induce nuclear translocation of NF-κB in airway epithelium of CC10-I-κBαSR transgenic mice. Consequently, the influx of neutrophils into the airways and secretion of the NF-κB-regulated neutrophilic chemokine, macrophage-inflammatory protein-2, and the inflammatory cytokine, TNF-α, were markedly reduced in CC10-I-κBαSR mice relative to the transgene-negative mice exposed to LPS. Despite an inability to activate NF-κB in airway epithelium, resident alveolar macrophages from transgene-positive mice were capable of activating NF-κB in a manner indistinguishable from transgene-negative mice. These findings demonstrate that airway epithelial cells play a prominent role in orchestrating the airway inflammatory response to LPS and suggest that NF-κB signaling in these cells is important for modulating innate immune responses to microbial products.

NF-κB Activation in Airways Modulates Allergic Inflammation but Not Hyperresponsiveness

Airways display robust NF-κB activation and represent targets for anti-inflammatory asthma therapies, but the functional importance of NF-κB activation in airway epithelium remains enigmatic. Therefore, transgenic mice were created in which NF-κB activation is repressed specifically in airways (CC10-IκBαSR mice). In response to inhaled Ag, transgenic mice demonstrated significantly ameliorated inflammation, reduced levels of chemokines, T cell cytokines, mucus cell metaplasia, and circulating IgE compared with littermate controls. Despite these findings, Ag-driven airways hyperresponsiveness was not attenuated in CC10-IκBαSR mice. This study clearly demonstrates that airway epithelial NF-κB activation orchestrates Ag-induced inflammation and subsequent adaptive immune responses, but does not contribute to airways hyperresponsiveness, the cardinal feature that underlies asthma.

Rapid Activation of Nuclear Factor-κB in Airway Epithelium in a Murine Model of Allergic Airway Inflammation

Bronchiolar epithelium is postulated to play a critical role in the orchestration of responses to inhaled allergens, and may contribute to the pathogenesis of asthma. Using a murine model of allergic airway inflammation and hyperresponsiveness, we demonstrate in mice sensitized with ovalbumin (OVA) that following a single challenge with nebulized OVA, a rapid and protracted activation of inhibitor of kappa B kinase (IKK) occurred in lung tissue. IKK activation was followed by nuclear localization of nuclear factor (NF)-kappaB within the bronchiolar epithelium and increased luciferase activity in lungs of mice containing a NF-kappaB-dependent reporter gene. Challenge of sensitized mice with OVA also induced mRNA expression of the chemokines, macrophage inflammatory protein-2 (MIP-2) and eotaxin in lung tissue, which corresponded temporally with the observed influx of neutrophils and eosinophils, respectively, into the airspaces. Using laser capture microdissection and quantitative polymerase chain reaction, we demonstrated that MIP-2 and eotaxin were predominantly expressed in bronchiolar epithelium, in contrast to distal regions of the lungs, which expressed lower or undetectable levels of these mRNAs. These studies strengthen the potential importance of the bronchiolar epithelial cell as a source of production of NF-kappaB-dependent mediators that play a role in asthma.

Serum Amyloid A Activates the NLRP3 Inflammasome and Promotes Th17 Allergic Asthma in Mice

IL-1β is a cytokine critical to several inflammatory diseases in which pathogenic Th17 responses are implicated. Activation of the NLRP3 inflammasome by microbial and environmental stimuli can enable the caspase-1–dependent processing and secretion of IL-1β. The acute-phase protein serum amyloid A (SAA) is highly induced during inflammatory responses, wherein it participates in systemic modulation of innate and adaptive immune responses. Elevated levels of IL-1β, SAA, and IL-17 are present in subjects with severe allergic asthma, yet the mechanistic relationship among these mediators has yet to be identified. In this study, we demonstrate that Saa3 is expressed in the lungs of mice exposed to several mixed Th2/Th17-polarizing allergic sensitization regimens. SAA instillation into the lungs elicits robust TLR2-, MyD88-, and IL-1–dependent pulmonary neutrophilic inflammation. Furthermore, SAA drives production of IL-1α, IL-1β, IL-6, IL-23, and PGE2, causes dendritic cell (DC) maturation, and requires TLR2, MyD88, and the NLRP3 inflammasome for secretion of IL-1β by DCs and macrophages. CD4+ T cells polyclonally stimulated in the presence of conditioned media from SAA-exposed DCs produced IL-17, and the capacity of polyclonally stimulated splenocytes to secrete IL-17 is dependent upon IL-1, TLR2, and the NLRP3 inflammasome. Additionally, in a model of allergic airway inflammation, administration of SAA to the lungs functions as an adjuvant to sensitize mice to inhaled OVA, resulting in leukocyte influx after Ag challenge and a predominance of IL-17 production from restimulated splenocytes that is dependent upon IL-1R signaling.

Pulmonary stromal-derived factor-1 expression and effect on neutrophil recruitment during acute lung injury

The severe and protracted inflammation that characterizes acute lung injury (ALI) is driven by the ongoing recruitment of neutrophils to the lung. Although much of the cytokine signaling responsible for the initial phase of ALI has been elaborated, relatively little is known about the mechanisms governing the recruitment of neutrophils from the bone marrow to the lung in the later period of this disease. Given its previously described chemoattractant effects on marrow neutrophils, we investigated whether stromal-derived factor-1 (SDF-1) (CXCL12) might participate in this later phase of recruitment. Using immunohistochemistry to examine both banked human lung specimens from patients with ALI and lungs from mice with LPS-induced pneumonitis, we found that pulmonary SDF-1 expression increases during ALI. We further determined that both lung SDF-1 protein expression and mRNA expression rise in a delayed but sustained pattern in this mouse model and that the major source of the increase in expression appears to be the lung epithelium. Lastly, we found that expression of the SDF-1 receptor CXCR4 rises in a similar temporal pattern on neutrophils in both the blood and airspace of LPS-injured mice and that Ab-mediated SDF-1 blockade significantly attenuates late but not early pulmonary neutrophilia in this model. These results implicate SDF-1 in neutrophil recruitment to the lung in the later period of acute lung injury and suggest a novel role for this cytokine in coordinating the transition from the inflammatory response to the initiation of tissue repair.

Bone Marrow-Derived Mesenchymal Stromal Cells Inhibit Th2-Mediated Allergic Airways Inflammation in Mice†‡§

Bone marrow‐derived mesenchymal stromal cells (BMSCs) mitigate inflammation in mouse models of acute lung injury. However, specific mechanisms of BMSC actions on CD4 T lymphocyte‐mediated inflammation in vivo remain poorly understood. Limited data suggests promotion of Th2 phenotype in models of Th1‐mediated diseases. However, whether this might alleviate or worsen Th2‐mediated diseases such as allergic asthma is unknown. To ascertain the effects of systemic administration of BMSCs in a mouse model of Th2‐mediated allergic airways inflammation, ovalbumin (OVA)‐induced allergic airways inflammation was induced in wild‐type C57BL/6 and BALB/c mice as well as in interferon‐γ (IFNγ) receptor null mice. Effects of systemic administration during antigen sensitization of either syngeneic or allogeneic BMSC on airways hyperreactivity, lung inflammation, antigen‐specific CD4 T lymphocytes, and serum immunoglobulins were assessed. Both syngeneic and allogeneic BMSCs inhibited airways hyperreactivity and lung inflammation through a mechanism partly dependent on IFNγ. However, contrary to existing data, BMSCs did not affect antigen‐specific CD4 T lymphocyte proliferation but rather promoted Th1 phenotype in vivo as assessed by both OVA‐specific CD4 T lymphocyte cytokine production and OVA‐specific circulating immunoglobulins. BMSCs treated to prevent release of soluble mediators and a control cell population of primary dermal skin fibroblasts only partly mimicked the BMSC effects and in some cases worsened inflammation. In conclusion, BMSCs inhibit Th2‐mediated allergic airways inflammation by influencing antigen‐specific CD4 T lymphocyte differentiation. Promotion of a Th1 phenotype in antigen‐specific CD4 T lymphocytes by BMSCs is sufficient to inhibit Th2‐mediated allergic airways inflammation through an IFNγ‐dependent process. STEM CELLS 2011;29:1137–1148

Nuclear Factor-κB Activation in Airway Epithelium Induces Inflammation and Hyperresponsiveness

RATIONALE Nuclear factor (NF)-kappaB is a prominent proinflammatory transcription factor that plays a critical role in allergic airway disease. Previous studies demonstrated that inhibition of NF-kappaB in airway epithelium causes attenuation of allergic inflammation. OBJECTIVES We sought to determine if selective activation of NF-kappaB within the airway epithelium in the absence of other agonists is sufficient to cause allergic airway disease. METHODS A transgenic mouse expressing a doxycycline (Dox)-inducible, constitutively active (CA) version of inhibitor of kappaB (IkappaB) kinase-beta (IKKbeta) under transcriptional control of the rat CC10 promoter, was generated. MEASUREMENTS AND MAIN RESULTS After administration of Dox, expression of the CA-IKKbeta transgene induced the nuclear translocation of RelA in airway epithelium. IKKbeta-triggered activation of NF-kappaB led to an increased content of neutrophils and lymphocytes, and concomitant production of proinflammatory mediators, responses that were not observed in transgenic mice not receiving Dox, or in transgene-negative littermate control animals fed Dox. Unexpectedly, expression of the IKKbeta transgene in airway epithelium was sufficient to cause airway hyperresponsiveness and smooth muscle thickening in absence of an antigen sensitization and challenge regimen, the presence of eosinophils, or the induction of mucus metaplasia. CONCLUSIONS These findings demonstrate that selective activation NF-kappaB in airway epithelium is sufficient to induce airway hyperresponsiveness and smooth muscle thickening, which are both critical features of allergic airway disease.

A Lipidomics Analysis of the Relationship Between Dietary Fatty Acid Composition and Insulin Sensitivity in Young Adults

Relative to diets enriched in palmitic acid (PA), diets rich in oleic acid (OA) are associated with reduced risk of type 2 diabetes. To gain insight into mechanisms underlying these observations, we applied comprehensive lipidomic profiling to specimens collected from healthy adults enrolled in a randomized, crossover trial comparing a high-PA diet to a low-PA/high-OA (HOA) diet. Effects on insulin sensitivity (SI) and disposition index (DI) were assessed by intravenous glucose tolerance testing. In women, but not men, SI and DI were higher during HOA. The effect of HOA on SI correlated positively with physical fitness upon enrollment. Principal components analysis of either fasted or fed-state metabolites identified one factor affected by diet and heavily weighted by the PA/OA ratio of serum and muscle lipids. In women, this factor correlated inversely with SI in the fasted and fed states. Medium-chain acylcarnitines emerged as strong negative correlates of SI, and the HOA diet was accompanied by lower serum and muscle ceramide concentrations and reductions in molecular biomarkers of inflammatory and oxidative stress. This study provides evidence that the dietary PA/OA ratio impacts diabetes risk in women.

Molecular mechanisms of nitrogen dioxide induced epithelial injury in the lung

The lung can be exposed to a variety of reactive nitrogen intermediates through the inhalation of environmental oxidants and those produced during inflammation. Reactive nitrogen species (RNS) include, nitrogen dioxide (·NO2) and peroxynitrite (ONOO–). Classically known as a major component of both indoor and outdoor air pollution, ·NO2 is a toxic free radical gas. ·NO2 can also be formed during inflammation by the decomposition of ONOO– or through peroxidase-catalyzed reactions. Due to their reactive nature, RNS may play an important role in disease pathology. Depending on the dose and the duration of administration, ·NO2 has been documented to cause pulmonary injury in both animal and human studies. Injury to the lung epithelial cells following exposure to ·NO2 is characterized by airway denudation followed by compensatory proliferation. The persistent injury and repair process may contribute to airway remodeling, including the development of fibrosis. To better understand the signaling pathways involved in epithelial cell death by ·NO2 or other RNS, we routinely expose cells in culture to continuous gas-phase ·NO2. Studies using the ·NO2 exposure system revealed that lung epithelial cell death occurs in a density dependent manner. In wound healing experiments, ·NO2 induced cell death is limited to cells localized in the leading edge of the wound. Importantly, ·NO2-induced death does not appear to be dependent on oxidative stress per se. Potential cell signaling mechanisms will be discussed, which include the mitogen activated protein kinase, c-Jun N-terminal Kinase and the Fas/Fas ligand pathways. During periods of epithelial loss and regeneration that occur in diseases such as asthma or during lung development, epithelial cells in the lung may be uniquely susceptible to death. Understanding the molecular mechanisms of epithelial cell death associated with the exposure to ·NO2 will be important in designing therapeutics aimed at protecting the lung from persistent injury and repair.