Tumor necrosis factor-alpha augments contraction and cytosolic Ca(2+) sensitivity through phospholipase A(2) in bovine tracheal smooth muscle

Mechanisms underlying TNFα-induced enhancement of force generation in airway smooth muscle

Airway diseases such as asthma are triggered by inflammation and mediated by proinflammatory cytokines such as tumor necrosis factor alpha (TNFα). Our goal was to systematically examine the potential mechanisms underlying the effect of TNFα on airway smooth muscle (ASM) contractility. Porcine ASM strips were incubated for 24 h with and without TNFα. Exposure to TNFα increased maximum ASM force in response to acetylcholine (Ach), with an increase in ACh sensitivity (hyperreactivity), as reflected by a leftward shift in the dose-response curve (EC50 ). At the EC50 , the [Ca2+ ]cyt response to ACh was similar between TNFα and control ASM, while force increased; thus, Ca2+ sensitivity appeared to increase. Exposure to TNFα increased the basal level of regulatory myosin light chain (rMLC) phosphorylation in ASM; however, the ACh-dependent increase in rMLC phosphorylation was blunted by TNFα with no difference in the extent of rMLC phosphorylation at the EC50 ACh concentration. In TNFα-treated ASM, total actin and myosin heavy chain concentrations increased. TNFα exposure also enhanced the ACh-dependent polymerization of G- to F-actin. The results of this study confirm TNFα-induced hyperreactivity to ACh in porcine ASM. We conclude that the TNFα-induced increase in ASM force, cannot be attributed to an enhanced [Ca2+ ]cyt response or to an increase in rMLC phosphorylation. Instead, TNFα increases Ca2+ sensitivity of ASM force generation due to increased contractile protein content (greater number of contractile units) and enhanced cytoskeletal remodeling (actin polymerization) resulting in increased tethering of contractile elements to the cortical cytoskeleton and force translation to the extracellular matrix.

TNF-α-induces airway hyperresponsiveness to cholinergic stimulation in guinea pig airways

BACKGROUND AND PURPOSE

TNF-α is an inflammatory cytokine implicated in the pathogenesis of asthma and it causes airway inflammation, bronchoconstriction and airway hyperresponsiveness to a number of spasmogens following inhalation.

EXPERIMENTAL APPROACH

We compared contractions of guinea pig isolated trachea incubated with saline or TNF-α for 1, 2 or 4 days to electrical field stimulation (EFS), 5-HT or methacholine. In addition, we compared bronchoconstriction in anaesthetized guinea pigs 6 h after intratracheal instillation of saline or TNF-α to vagal nerve stimulation, i.v. 5-HT or methacholine. Differential counts were performed on the bronchoalvelolar lavage fluid (BALF).

KEY RESULTS

Maximum contractions to methacholine, 5-HT and EFS were not different between freshly prepared and saline-incubated tissues. Exposure to TNF-α concentration-dependently potentiated contractions to 5-HT and EFS, but not methacholine. All contractions were atropine-sensitive, but not hexamethonium-sensitive. 5-HT-evoked contractions were inhibited by ketanserin or epithelial denudation. Only EFS-evoked contractions were tetrodotoxin-sensitive. Vagal stimulation, i.v. 5-HT or MCh caused a significant atropine-sensitive, frequency- and dose-dependent bronchoconstriction and decreased blood pressure similarly in both saline and TNF-α pre-treated animals. TNF-α potentiated the bronchoconstriction to vagal stimulation and 5-HT, but not MCh. The BALF from saline-treated animals contained predominantly macrophages, whereas that from TNF-α-treated animals contained neutrophils.

CONCLUSIONS AND IMPLICATIONS

TNF-α caused airway hyperresponsiveness to nerve stimulation in vivo and increased contractility in vitro. However, responsiveness to MCh was unchanged, suggesting a pre-synaptic action of TNF-α on parasympathetic nerves. TNF-α-induced airway hyperresponsiveness to 5-HT suggested an increased 5-HT(2A) receptor-mediated acetylcholine release from epithelial cells.

The pivotal role of airway smooth muscle in asthma pathophysiology

Asthma is characterized by the association of airway hyperresponsiveness (AHR), inflammation, and remodelling. The aim of the present article is to review the pivotal role of airway smooth muscle (ASM) in the pathophysiology of asthma. ASM is the main effector of AHR. The mechanisms of AHR in asthma may involve a larger release of contractile mediators and/or a lower release of relaxant mediators, an improved ASM cell excitation/contraction coupling, and/or an alteration in the contraction/load coupling. Beyond its contractile function, ASM is also involved in bronchial inflammation and remodelling. Whereas ASM is a target of the inflammatory process, it can also display proinflammatory and immunomodulatory functions, through its synthetic properties and the expression of a wide range of cell surface molecules. ASM remodelling represents a key feature of asthmatic bronchial remodelling. ASM also plays a role in promoting complementary airway structural alterations, in particular by its synthetic function.

Tumor necrosis factor-alpha: a promising therapeutic target for asthma?

Asthma is a disease that encompasses a variety of features including airway smooth muscle abnormalities, airway inflammation, and structural changes in the airway. Historically, it has been classified depending on the severity of the disease, the frequency of symptoms, and the level of treatment required to control them. Severe or refractory asthma accounts for approximately 10% of the patient population with asthma and for about 30% of the healthcare costs of this disease. It is often associated with conditions that might lead to activation of innate immunity in the lung, and it has been suggested that some of the features of severe asthma might be due to upregulation of the tumor necrosis factor-alpha (TNFalpha) pathway. In support of this, studies have shown that severe asthma is associated with an increased presence of TNFalpha within the airway and an increase in TNFalpha expression on peripheral blood mononuclear cells. Moreover, TNFalpha has the ability to induce several of the pro-inflammatory changes associated with severe asthma. Interest in the role of TNFalpha in severe asthma has increased following reports that antagonism with etanercept or infliximab is associated with improvement in asthma control in patients with severe asthma. In this article, we discuss the biology, function, and clinical effects of TNFalpha with particular reference to severe asthma.

In vitro tracheal hyperresponsiveness to muscarinic receptor stimulation by carbachol in a rat model of bleomycin-induced pulmonary fibrosis

1 Bleomycin-induced lung injury is widely used as an experimental model to investigate the pathophysiology of pulmonary fibrosis but the alterations in the pharmacological responsiveness of airways isolated from bleomycin-exposed animals has been scarcely investigated. The aim of this study was to examine the in vitro tracheal responses to muscarinic receptor stimulation with carbachol in a rat bleomycin model. 2 Concentration-response curves to carbachol (10 nm to 0.1 mm) were obtained in tracheal rings isolated from Sprague-Dawley rats 14 days after endotracheal bleomycin or saline. The intracellular calcium signal in response to carbachol (10 microm) was measured by epifluorescence microscopy using fura-2 in primary cultures of tracheal smooth muscle cells from bleomycin- and saline-exposed rats. Circulating plasma tumour necrosis factor (TNF)-alpha/interleukin (IL)-1beta levels were measured by enzyme-linked immunosorbent assay. 3 Maximal contraction in response to carbachol was significantly greater in tracheal rings from bleomycin-exposed rats compared with controls (15.8 +/- 1.3 mN vs. 11.8 +/- 1.4 mN; n = 19, P < 0.05). 4 Carbachol (10 microm) elicited a transient increase of intracellular calcium with greater increment in tracheal smooth muscle cells from bleomycin-exposed rats compared with controls (372 +/- 42 nmvs. 176 +/- 20 nm; n = 7, P < 0.01). 5 Circulating plasma levels of TNF-alpha/IL-1beta were augmented in bleomycin-exposed rats compared with controls. Tissue incubation with TNF-alpha (100 ng ml(-1))/IL-1beta (10 ng ml(-1)) increased in vitro tracheal responsiveness to carbachol. 6 In conclusion, tracheal contraction in response to muscarinic receptor stimulation with carbachol was increased in bleomycin-exposed rats. This in vitro cholinergic hyperresponsiveness may be related to the augmented levels of inflammatory cytokines in bleomycin-exposed rats.

TNF-alpha augments the expression of RhoA in the rat bronchus

While nonspecific airway hyperresponsiveness (AHR) is a central feature of allergic bronchial asthma, the mechanism underlying the development of AHR is not clearly understood. We have previously demonstrated in vitro hyperresponsiveness of bronchial smooth muscle to acetylcholine (ACh) in rats that were actively sensitized and repeatedly challenged with aerosolized antigen. It has also been demonstrated that the ACh-induced, RhoA-mediated Ca(2+) sensitization is markedly augmented concomitantly with an increased expression and activation of RhoA protein in the bronchial smooth muscle of the antigen-treated rats. In the present study, we have investigated whether TNF-alpha, a proinflammatory cytokine which is involved in bronchial asthma, causes upregulation of RhoA mRNA and protein in the rat bronchus. Treatment of rat bronchial smooth muscle preparations with TNF-alpha (300 ng/ml for 24 hr) significantly shifted the concentration-response curve to ACh upwards, but did not alter the response to high K(+), when compared to that of control tissues. Levels of RhoA mRNA and protein in the TNF-alpha-treated bronchus were significantly greater than those in the control group. In conclusion, it is suggested that the augmentation of the ACh-induced contractile response evoked by TNF-alpha might be mediated by an upregulation of RhoA in rat bronchial smooth muscle.

TNF-[alpha] modulates murine tracheal rings responsiveness to G-protein-coupled receptor agonists and KCl

Although the mechanisms that underlie airway hyperresponsiveness in asthma are complex and involve a variety of factors, evidence now suggests that intrinsic abnormalities in airway smooth muscle (ASM) may play an important role. We previously reported that TNF-alpha, a cytokine involved in asthma, augments G-protein-coupled receptor (GPCR) agonist-evoked calcium responses in cultured ASM cells. Here we have extended our previous studies by investigating whether TNF-alpha also modulates the contractile and relaxant responses to GPCR activation using cultured murine tracheal rings. We found that in tracheal rings treated with 50 ng/ml TNF-alpha, carbachol-induced isometric force was significantly increased by 30% compared with those treated with diluent alone (P < 0.05). TNF-alpha also augmented KCl-induced force generation by 70% compared with rings treated with diluent alone (P < 0.01). The enhancing effect of TNF-alpha on carbachol-induced isometric force generation was completely abrogated in the tracheal rings obtained from TNF-alpha receptor (TNFR)1-deficient mice and in control rings treated with a TNF-alpha mutant that solely activates TNFR2. TNF-alpha also attenuated relaxation responsiveness to isoproterenol but not to PGE2 or forskolin. TNF-alpha modulatory effects on GPCR-induced ASM responsiveness were completely abrogated by pertussis toxin, an inhibitor of Gialpha proteins. Taken together, these data suggest that TNF-alpha may participate in the development of airway hyperresponsiveness in asthma via the modulation of ASM responsiveness to both contractile and beta-adrenoceptor GPCR agonists.

An assay to evaluate the long-term effects of inflammatory mediators on murine airway smooth muscle: evidence that TNFalpha up-regulates 5-HT(2A)-mediated contraction

1. Asthma research is arguably limited by an absence of appropriate animal models to study the pharmacology of inflammatory mediators that affect airway hyperresponsiveness and remodelling. Here we assessed an assay based on mouse tracheal segments cultured for 1-32 days, and investigated contractile responses mediated by muscarinic and 5-hydroxytryptamine (5-HT) receptors following long-term exposure to tumour necrosis factor-alpha (TNFalpha). 2. Following culture, in the absence of TNFalpha, maximum contractile responses to KCl and carbachol were similar, with an increase in response up to day two and a decrease to a stable level after 8 days. Maximal relaxations to isoprenaline were not affected by the culture procedure. The potency of KCl and isoprenaline increased throughout the study. DNA microarray data revealed that global gene expression changes were greater when tissues were introduced to culture than when they were maintained in culture. The morphology of smooth muscle cells was maintained throughout the culture period. 3. 5-HT induced a weak contraction in both fresh and cultured (up to 8 days) segments. Culture with TNFalpha produced a time- and concentration-dependent increase in the maximal contraction to 5-HT, evidently mediated by 5-HT(2A) receptors, whereas, the potency for carbachol was reduced. 4. In conclusion, the phenotype of airway smooth muscle remained largely intact during the culture period, even though minor changes were obtained during the first days of culture. The time-dependent effect of TNFalpha indicates the importance of studying the long-term effect of cytokines on the smooth muscle cells in relation to airway hyperresponsiveness and remodelling.

Effects of cytokines on contractile and dilator responses of airway smooth muscle

1. Increased bronchoconstrictor responses to contractile agonists and decreased dilator responses to beta-adrenoceptor agonists are characteristics of human asthma. One explanation for these features of asthma is that cytokines released in the asthmatic airway have direct effects on airway smooth muscle cells that alter their phenotype. 2. The present review summarizes data indicating that inflammatory cytokines, such as interleukin (IL)-1 beta and tumour necrosis factor-alpha, T helper (h) 1 cytokines, such as interferon-gamma, and Th2 cytokines, such as IL-13 and IL-5, have the capacity to enhance contractile responses and/or decrease relaxant responses of airway smooth muscle. These effects are observed in smooth muscle from human airways and airway smooth muscle of other species. 3. Understanding the mechanistic basis for the effects of these cytokines may prove to be an important step in improving the efficacy of beta-adrenoceptor agonists for the treatment of asthma.

Tumor necrosis factor-alpha increases airway smooth muscle oxidants production through a NADPH oxidase-like system to enhance myosin light chain phosphorylation and contractility

Tumor necrosis factor plays a critical role in airway smooth muscle hyperresponsiveness observed in asthma. However, the mechanisms underlying this phenomenon are poorly understood. We investigated if tumor necrosis factor-stimulated airway smooth muscle produced reactive oxygen species, leading to muscular hyperresponsiveness. Tumor necrosis factor increased intracellular and extracellular oxidants production in guinea pig airway smooth muscle cells and tissue homogenates. This production was abolished by inhibitors of NADPH oxidase (diphenylene iodinium or apocynin) and was enhanced by NADPH, whereas inhibitors of mitochondrial respiratory chain, nitric-oxide synthase, cyclooxygenase, and xanthine oxidase had no effect. NADPH oxidase subunits p22(phox) and p47(phox) were detected in smooth muscle cells and tissue homogenates by Western blot, immunohistochemistry, and spectral analysis. Furthermore, oxidants production was significantly reduced by transient transfection of smooth muscle cells with p22(phox) antisense oligonucleotides. Intracellular antioxidants and diphenylene iodinium abolished tumor necrosis factor-induced muscular hyperresponsiveness and increased in phosphorylation of the myosin light chain. Finally, NADPH oxidase subunits p22(phox) and p47(phox) were also detected in human airway smooth muscle. Collectively, these results demonstrate that tumor necrosis factor-stimulated airway smooth muscle produces oxidants through a NADPH oxidase-like system, which plays a pivotal role in muscle hyperresponsiveness and myosin light chain phosphorylation.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

Role of Rho-associated protein kinase and histamine in lysophosphatidic acid-induced airway hyperresponsiveness in guinea pigs

Inhalation of oleoyl lysophosphatidic acid (LPA) induced airway hyperresponsiveness to acetylcholine (ACh). In contrast, palmitoyl and stearoyl LPA exerted minimal effects. Airway hyperresponsiveness was inhibited by inhalation of Y-27632, an inhibitor of Rho-associated protein kinase (ROCK). Mepyramine, an H1 histamine receptor antagonist and ketotifen, an inhibitor of histamine release and H1 histamine receptor antagonist, also inhibited airway hyperresponsiveness induced by LPA; however, aspirin failed to attenuate this response. The incubation of lung fragments with LPA gave rise to releases in histamine. On the other hand, LPA produced no significant changes on the smooth muscle contraction evoked by ACh. These findings suggest that LPA-induced airway hyperresponsiveness is attributable to activation of the Rho/ROCK-mediated pathway via endothelial cell differentiation gene (EDG) receptors, probably EDG 7. Moreover, histamine release may be involved.

Modulation of calcium homeostasis as a mechanism for altering smooth muscle responsiveness in asthma

Airway hyperresponsiveness remains a defining characteristic of asthma. Traditional views assert that airway smooth muscle is an important structural effector cell in the bronchi that modulates bronchomotor tone induced by contractile agonists. New evidence, however, suggests that abnormalities in airway smooth muscle functions, induced by variety of extracellular stimuli, may play an important role in the development of airway hyperresponsiveness. Studies using isolated bronchial preparations or cultured cells show that inflammatory mediators and cytokines may alter calcium homeostasis in airway smooth muscle and render the cells nonspecifically hyperreactive to agonists.

Activation of tumor necrosis factor receptor 1 in airway smooth muscle: a potential pathway that modulates bronchial hyper-responsiveness in asthma?

The cellular and molecular mechanisms that are involved in airway hyper-responsiveness are unclear. Current studies suggest that tumor necrosis factor (TNF)-alpha, a cytokine that is produced in considerable quantities in asthmatic airways, may potentially be involved in the development of bronchial hyper-responsiveness by directly altering the contractile properties of the airway smooth muscle (ASM). The underlying mechanisms are not known, but growing evidence now suggests that most of the biologic effects of TNF-alpha on ASM are mediated by the p55 receptor or tumor necrosis factor receptor (TNFR)1. In addition, activation of TNFR1 coupled to the tumor necrosis factor receptor-associated factor (TRAF)2-nuclear factor-kappaB (NF-kappaB) pathway alters calcium homeostasis in ASM, which appears to be a new potential mechanism underlying ASM hyper-responsiveness.