TNF-alpha upregulates Gialpha and Gqalpha protein expression and function in human airway smooth muscle cells

Ginger metabolites and metabolite-inspired synthetic products modulate intracellular calcium and relax airway smooth muscle

Asthma affects millions of people worldwide and its prevalence is increasing. It is characterized by chronic airway inflammation, airway remodeling, and pathologic bronchoconstriction, and it poses a continuous treatment challenge with very few new therapeutics available. Thus, many asthmatics turn to plant-based complementary products, including ginger, for better symptom control, indicating an unmet need for novel therapies. Previously, we demonstrated that 6-shogaol (6S), the primary bioactive component of ginger, relaxes human airway smooth muscle (hASM) likely by inhibition of phosphodiesterases (PDEs) in the β-adrenergic (cyclic nucleotide PDEs), and muscarinic (phospholipase C, PLC) receptor pathways. However, oral 6S is extensively metabolized and it is unknown if the resulting metabolites remain bioactive. Here, we screened all the known human metabolites of 6S and several metabolite-based synthetic derivatives to better understand their mechanism of action and structure-function relationships. We demonstrate that several metabolites and metabolite-based synthetic derivatives are able to prevent Gq-coupled stimulation of intracellular calcium [Ca2+]i and inositol trisphosphate (IP3) synthesis by inhibiting PLC, similar to the parent compound 6S. We also show that these compounds prevent recontraction of ASM after β-agonist relaxation likely by inhibiting PDEs. Furthermore, they potentiate isoproterenol-induced relaxation. Importantly, moving beyond cell-based assays, metabolites also retain the functional ability to relax Gq-coupled-contractions in upper (human) and lower (murine) airways. The current study indicates that, although oral ginger may be metabolized rapidly, it retains physiological activity through its metabolites. Moreover, we are able to use naturally occurring metabolites as inspiration to develop novel therapeutics for brochoconstrictive diseases.

Dexamethasone rescues TGF-β1-mediated β2-adrenergic receptor dysfunction and attenuates phosphodiesterase 4D expression in human airway smooth muscle cells

Glucocorticoids (GCs) and β2-adrenergic receptor (β2AR) agonists improve asthma outcomes in most patients. GCs also modulate gene expression in human airway smooth muscle (HASM), thereby attenuating airway inflammation and airway hyperresponsiveness that define asthma. Our previous studies showed that the pro-fibrotic cytokine, transforming growth factor- β1 (TGF-β1) increases phosphodiesterase 4D (PDE4D) expression that attenuates agonist-induced levels of intracellular cAMP. Decreased cAMP levels then diminishes β2 agonist-induced airway relaxation. In the current study, we investigated whether glucocorticoids reverse TGF-β1-effects on β2-agonist-induced bronchodilation and modulate pde4d gene expression in HASM. Dexamethasone (DEX) reversed TGF-β1 effects on cAMP levels induced by isoproterenol (ISO). TGF-β1 also attenuated G protein-dependent responses to cholera toxin (CTX), a Gαs stimulator downstream from the β2AR receptor. Previously, we demonstrated that TGF-β1 treatment increased β2AR phosphorylation to induce hyporesponsiveness to a β2 agonist. Our current data shows that expression of grk2/3, kinases associated with attenuation of β2AR function, are not altered with TGF-β1 stimulation. Interestingly, DEX also attenuated TGF-β1-induced pde4d gene expression. These data suggest that steroids may be an effective therapy for treatment of asthma patients whose disease is primarily driven by elevated TGF-β1 levels.

Bile acids inhibit cholinergic constriction in proximal and peripheral airways from humans and rodents

Duodenogastroesophageal reflux (DGER) is associated with chronic lung disease. Bile acids (BAs) are established markers of DGER aspiration and are important risk factors for reduced post-transplant lung allograft survival by disrupting the organ-specific innate immunity, facilitating airway infection and allograft failure. However, it is unknown whether BAs also affect airway reactivity. We investigated the acute effects of 13 BAs detected in post-lung-transplant surveillance bronchial washings (BW) on airway contraction. We exposed precision-cut slices from human and mouse lungs to BAs and monitored dynamic changes in the cross-sectional luminal area of peripheral airways using video phase-contrast microscopy. We also used guinea pig tracheal rings in organ baths to study BA effects in proximal airway contraction induced by electrical field stimulation. We found that most secondary BAs at low micromolar concentrations strongly and reversibly relaxed smooth muscle and inhibited peripheral airway constriction induced by acetylcholine but not by noncholinergic bronchoconstrictors. Similarly, secondary BAs strongly inhibited cholinergic constrictions in tracheal rings. In contrast, TC-G 1005, a specific agonist of the BA receptor Takeda G protein-coupled receptor 5 (TGR5), did not cause airway relaxation, and Tgr5 deletion in knockout mice did not affect BA-induced relaxation, suggesting that this receptor is not involved. BAs inhibited acetylcholine-induced inositol phosphate synthesis in human airway smooth muscle cells overexpressing the muscarinic M3 receptor. Our results demonstrate that select BAs found in BW of patients with lung transplantation can affect airway reactivity by inhibiting the cholinergic contractile responses of the proximal and peripheral airways, possibly by acting as antagonists of M3 muscarinic receptors.

Agonism of the TMEM16A calcium-activated chloride channel modulates airway smooth muscle tone

TMEM16A (anoctamin 1) is an important calcium-activated chloride channel in airway smooth muscle (ASM). We have previously shown that TMEM16A antagonists such as benzbromarone relax ASM and have proposed TMEM16A antagonists as novel therapies for asthma treatment. However, TMEM16A is also expressed on airway epithelium, and TMEM16A agonists are being investigated as novel therapies for cystic fibrosis. There are theoretical concerns that agonism of TMEM16A on ASM could lead to bronchospasm, making them detrimental as airway therapeutics. The TMEM16A agonist Eact induced a significant contraction of human ASM and guinea pig tracheal rings in an ex vivo organ bath model. Pretreatment with two different TMEM16A antagonists, benzbromarone or T16Ainh-A01, completely attenuated these Eact-induced contractions. Pretreatment with Eact alone augmented the maximum acetylcholine contraction. Pretreatment of A/J mice in vivo with nebulized Eact caused an augmentation of methacholine-induced increases in airway resistance measured by the forced oscillatory technique (flexiVent). Pretreatment with the TMEM16A antagonist benzbromarone significantly attenuated methacholine-induced increases in airway resistance. In in vitro cellular studies, TMEM16A was found to be expressed more abundantly in ASM compared with epithelial cells in culture (8-fold higher in ASM). Eact caused an increase in intracellular calcium in human ASM cells that was completely attenuated by pretreatment with benzbromarone. Eact acutely depolarized the plasma membrane potential of ASM cells, which was attenuated by benzbromarone or nifedipine. The TMEM16A agonist Eact modulates ASM contraction in both ex vivo and in vivo models, suggesting that agonism of TMEM16A may lead to clinically relevant bronchospasm.

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.

Attenuation of murine and human airway contraction by a peptide fragment of the cytoskeleton regulatory protein gelsolin

We have previously reported that mice genetically deficient in the actin binding protein gelsolin exhibit impaired airway smooth muscle (ASM) relaxation. Primary cultured ASM cells from these mice demonstrate enhanced inositol triphosphate (IP₃) synthesis and increased intracellular calcium in response to G_q-coupled agonists. We hypothesized that this was due to increased intracellular availability of unbound phosphatidylinositol 4,5-bisphosphate (PIP₂), based on the fact that gelsolin contains a short peptide region that binds PIP₂, presumably making it a less available substrate. We now questioned whether a peptide that corresponds to the PIP₂ binding region of gelsolin could modulate ASM signaling and contraction. The 10 amino acid sequence of the gelsolin peptide within the PIP₂-binding region was incubated with primary cultures of human ASM cells, and IP₃ synthesis was measured in response to a G_q-coupled agonist. Gelsolin peptide-treated cells generated less IP₃ under basal and bradykinin or acetylcholine (G_q-coupled) conditions. Acetylcholine-induced contractile force measured in isolated tracheal rings from mice and human tracheal muscle strips in organ baths was attenuated in the presence of the gelsolin peptide. The gelsolin peptide also attenuated methacholine-induced airway constriction in murine precision-cut lung slices. Furthermore, this peptide fragment delivered to the respiratory system of mice via nebulization attenuated subsequent methacholine-induced increases in airway resistance in vivo. The current study demonstrates that introduction of this small gelsolin peptide into the airway may be a novel therapeutic option in bronchoconstrictive diseases.

Modulation of airway hyperresponsiveness by rhinovirus exposure

Rhinovirus (RV) exposure has been implicated in childhood development of wheeze evoking asthma and exacerbations of underlying airways disease. Studies such as the Copenhagen Prospective Studies on Asthma in Childhood (COPSAC) and Childhood Origins of ASThma (COAST) have identified RV as a pathogen inducing severe respiratory disease. RVs also modulate airway hyperresponsiveness (AHR), a key characteristic of such diseases. Although potential factors underlying mechanisms by which RV induces AHR have been postulated, the precise mechanisms of AHR following RV exposure remain elusive.

A challenge to RV-related research stems from inadequate models for study. While human models raise ethical concerns and are relatively difficult in terms of subject recruitment, murine models are limited by susceptibility of infection to the relatively uncommon minor group (RV-B) serotypes, strains that are generally associated with infrequent clinical respiratory virus infections. Although a transgenic mouse strain that has been developed has enhanced susceptibility for infection with the common major group (RV-A) serotypes, few studies have focused on RV in the context of allergic airways disease rather than understanding RV-induced AHR. Recently, the receptor for the virulent RV-C CDHR3, was identified, but a dearth of studies have examined RV-C-induced effects in humans.

Currently, the mechanisms by which RV infections modulate airway smooth muscle (ASM) shortening or excitation-contraction coupling remain elusive. Further, only one study has investigated the effects of RV on bronchodilatory mechanisms, with only speculation as to mechanisms underlying RV-mediated modulation of bronchoconstriction.

Maintenance of intracellular Ca2+ basal concentration in airway smooth muscle (Review)

In airway smooth muscle, the intracellular basal Ca²⁺ concentration [_b(Ca²⁺)_i] must be tightly regulated by several mechanisms in order to maintain a proper airway patency. The _b[Ca²⁺]_i is efficiently regulated by sarcoplasmic reticulum Ca²⁺-ATPase 2b, plasma membrane Ca²⁺-ATPase 1 or 4 and by the Na⁺/Ca²⁺ exchanger. Membranal Ca²⁺ channels, including the L-type voltage dependent Ca²⁺ channel (L-VDCC), T-type voltage dependent Ca²⁺ channel (T-VDCC) and transient receptor potential canonical 3 (TRPC3), appear to be constitutively active under basal conditions via the action of different signaling pathways, and are responsible for Ca²⁺ influx to maintain _b[Ca²⁺]_i. The two types of voltage-dependent Ca²⁺ channels (L- and T-type) are modulated by phosphorylation processes mediated by mitogen-activated protein kinase kinase (MEK) and extracellular-signal-regulated kinase 1 and 2 (ERK1/2). The MEK/ERK signaling pathway can be activated by G-protein-coupled receptors through the α_q subunit when the endogenous ligand (i.e., acetylcholine, histamine, leukotrienes, etc.) is present under basal conditions. It may also be stimulated when receptor tyrosine kinases are occupied by the appropriate ligand (cytokines, growth factors, etc.). ERK1/2 phosphorylates L-VDCC on Ser⁴⁹⁶ of the β₂ subunit and Ser¹⁹²⁸ of the α₁ subunit, decreasing or increasing the channel activity, respectively, and enabling it to switch between an open and closed state. T-VDCC is also probably phosphorylated by ERK1/2, although further research is required to identify the phosphorylation sites. TRPC3 is directly activated by diacylglycerol produced by phospholipase C (PLC_β or _γ). Constitutive inositol 1,4,5-trisphosphate production induces the release of Ca²⁺ from the sarcoplasmic reticulum through inositol triphosphate receptor 1. This ion induces Ca²⁺-induced Ca²⁺ release through the ryanodine receptor 2 (designated as Ca²⁺ ‘sparks’). Therefore, several Ca²⁺ handling mechanisms are finely tuned to regulate basal intracellular Ca²⁺ concentrations. It is conceivable that alterations in any of these processes may render airway smooth muscle susceptible to develop hyperresponsiveness that is observed in ailments such as asthma.

Coupling of Airway Smooth Muscle Bitter Taste Receptors to Intracellular Signaling and Relaxation Is via Gαi1,2,3

Bitter taste receptors (TAS2Rs) are expressed on human airway smooth muscle (HASM) and evoke marked relaxation. Agonist interaction with TAS2Rs activates phospholipase C and increases compartmentalized intracellular Ca²⁺ ([Ca²⁺]_i) via inositol 1,4,5 triphosphate. In taste cells, the G protein gustducin couples TAS2R to phospholipase C; however, we find very low levels of G_αgust mRNA or protein in HASM. We hypothesized that another G protein in HASM transmits TAS2R function. TAS2R signaling to [Ca²⁺]_i, extracellular signal-regulated kinase (ERK) 1/2, and physiologic relaxation was sensitive to pertussis toxin, confirming a role for a member of the G_i family. α subunit expression in HASM was G_αi2 > G_αi1 = G_αi3 > G_αtrans1 ≈ G_αtrans2, with G_αgust and G_αo at the limits of detection (>100-fold lower than G_αi2). Small interfering RNA knockdowns in HASM showed losses of [Ca²⁺]_i and ERK1/2 signaling when G_αi1, G_αi2, or G_αi3 were reduced. G_αtrans1 and G_αtrans2 knockdowns had no effect on [Ca²⁺]_i and a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, G_αgust and G_αo knockdowns did not affect any TAS2R signaling. In overexpression experiments in human embryonic kidney-293T cells, we confirmed an agonist-dependent physical interaction between TAS2R14 and G_αi2. ASM cells from transgenic mice expressing a peptide inhibitor of G_αi2 had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, G_αi1, G_αi2, and G_αi3, with no evidence for functional coupling to G_αgust. This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of G_αgust, indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.

Impaired Relaxation of Airway Smooth Muscle in Mice Lacking the Actin-Binding Protein Gelsolin

Diverse classes of ligands have recently been discovered that relax airway smooth muscle (ASM) despite a transient increase in intracellular calcium concentrations ([Ca²⁺]_i). However, the cellular mechanisms are not well understood. Gelsolin is a calcium-activated actin-severing and -capping protein found in many cell types, including ASM cells. Gelsolin also binds to phosphatidylinositol 4,5-bisphosphate, making this substrate less available for phospholipase Cβ-mediated hydrolysis to inositol triphosphate and diacylglycerol. We hypothesized that gelsolin plays a critical role in ASM relaxation and mechanistically accounts for relaxation by ligands that transiently increase [Ca²⁺]_i. Isolated tracheal rings from gelsolin knockout (KO) mice showed impaired relaxation to both a β-agonist and chloroquine, a bitter taste receptor agonist, which relaxes ASM, despite inducing transiently increased [Ca²⁺]_i. A single inhalation of methacholine increased lung resistance to a similar extent in wild-type and gelsolin KO mice, but the subsequent spontaneous relaxation was less in gelsolin KO mice. In ASM cells derived from gelsolin KO mice, serotonin-induced Gq-coupled activation increased both [Ca²⁺]_i and inositol triphosphate synthesis to a greater extent compared to cells from wild-type mice, possibly due to the absence of gelsolin binding to phosphatidylinositol 4,5-bisphosphate. Single-cell analysis showed higher filamentous:globular actin ratio at baseline and slower cytoskeletal remodeling dynamics in gelsolin KO cells. Gelsolin KO ASM cells also showed an attenuated decrease in cell stiffness to chloroquine and flufenamic acid. These findings suggest that gelsolin plays a critical role in ASM relaxation and that activation of gelsolin may contribute to relaxation induced by ligands that relax ASM despite a transient increase in [Ca²⁺]_i.

Tumor Necrosis Factor Alpha Inhibits L-Type Ca(2+) Channels in Sensitized Guinea Pig Airway Smooth Muscle through ERK 1/2 Pathway

Tumor necrosis factor alpha (TNF-α) is a potent proinflammatory cytokine that plays a significant role in the pathogenesis of asthma by inducing hyperresponsiveness and airway remodeling. TNF-α diminishes the L-type voltage dependent Ca²⁺ channel (L-VDCC) current in cardiac myocytes, an observation that seems paradoxical. In guinea pig sensitized tracheas KCl responses were lower than in control tissues. Serum from sensitized animals (Ser-S) induced the same phenomenon. In tracheal myocytes from nonsensitized (NS) and sensitized (S) guinea pigs, an L-VDCC current (ICa) was observed and diminished by Ser-S. The same decrease was detected in NS myocytes incubated with TNF-α, pointing out that this cytokine might be present in Ser-S. We observed that a small-molecule inhibitor of TNF-α (SMI-TNF) and a TNF-α receptor 1 (TNFR1) antagonist (WP9QY) reversed ICa decrease induced by Ser-S in NS myocytes, confirming the former hypothesis. U0126 (a blocker of ERK 1/2 kinase) also reverted the decrease in ICa. Neither cycloheximide (a protein synthesis inhibitor) nor actinomycin D (a transcription inhibitor) showed any effect on the TNF-α-induced ICa reduction. We found that Ca_V1.2 and Ca_V1.3 mRNA and proteins were expressed in tracheal myocytes and that sensitization did not modify them. In cardiac myocytes, ERK 1/2 phosphorylates two sites of the L-VDCC, augmenting or decreasing ICa; we postulate that, in guinea pig tracheal smooth muscle, TNF-α diminishes ICa probably by phosphorylating the L-VDCC site that reduces its activity through the ERK1/2 MAP kinase pathway.

Attenuation of airway smooth muscle contractility via flavonol-mediated inhibition of phospholipase-Cβ

Enhanced contractility of airway smooth muscle (ASM) is a major pathophysiological characteristic of asthma. Expanding the therapeutic armamentarium beyond β-agonists that target ASM hypercontractility would substantially improve treatment options. Recent studies have identified naturally occurring phytochemicals as candidates for acute ASM relaxation. Several flavonoids were evaluated for their ability to acutely relax human and murine ASM ex vivo and murine airways in vivo and were evaluated for their ability to inhibit procontractile signaling pathways in human ASM (hASM) cells. Two members of the flavonol subfamily, galangin and fisetin, significantly relaxed acetylcholine-precontracted murine tracheal rings ex vivo (n = 4 and n = 5, respectively, P < 0.001). Galangin and fisetin also relaxed acetylcholine-precontracted hASM strips ex vivo (n = 6-8, P < 0.001). Functional respiratory in vivo murine studies demonstrated that inhaled galangin attenuated the increase in lung resistance induced by inhaled methacholine (n = 6, P < 0.01). Both flavonols, galangin and fisetin, significantly inhibited purified phosphodiesterase-4 (PDE4) (n = 7, P < 0.05; n = 7, P < 0.05, respectively), and PLCβ enzymes (n = 6, P < 0.001 and n = 6, P < 0.001, respectively) attenuated procontractile Gq agonists' increase in intracellular calcium (n = 11, P < 0.001), acetylcholine-induced increases in inositol phosphates, and CPI-17 phosphorylation (n = 9, P < 0.01) in hASM cells. The prorelaxant effect retained in these structurally similar flavonols provides a novel pharmacological method for dual inhibition of PLCβ and PDE4 and therefore may serve as a potential treatment option for acute ASM constriction.

Pharmacologic rationale underlying the therapeutic effects of tiotropium/olodaterol in COPD

Bronchodilators are the most important drugs used for the treatment of chronic obstructive pulmonary disease (COPD). In particular, these therapeutic agents are mostly long-acting compounds utilized via inhalation, and include LAMA (long-acting muscarinic receptor antagonists) and LABA (long-acting β₂-adrenoceptor agonists). Because LAMA and LABA induce bronchodilation by distinct mechanisms of action, LABA/LAMA combinations provide a reciprocal potentiation of the pharmacological effects caused by each component. Hence, many COPD patients who do not achieve a satisfactory control of their symptoms using a single, either LAMA or LABA bronchodilator, can experience relevant benefits with the use of LAMA/LABA fixed combinations. Many different LAMA/LABA combinations have been recently developed and evaluated in randomized clinical trials. In this context, our review focuses on the pharmacological mechanisms underpinning the bronchodilation elicited by the LAMA tiotropium bromide and the LABA olodaterol. We also discuss the results of the most important clinical studies carried out in COPD patients to assess the efficacy and safety of tiotropium/olodaterol combinations.

Novel identification of the free fatty acid receptor FFAR1 that promotes contraction in airway smooth muscle

Obesity is one of the major risk factors for asthma. Previous studies have demonstrated that free fatty acid levels are elevated in the plasma of obese individuals. Medium- and long-chain free fatty acids act as endogenous ligands for the free fatty acid receptors FFAR1/GPR40 and FFAR4/GPR120, which couple to Gq proteins. We investigated whether FFAR1 and FFAR4 are expressed on airway smooth muscle and whether they activate Gq-coupled signaling and modulate airway smooth muscle tone. We detected the protein expression of FFAR1 and FFAR4 in freshly dissected native human and guinea pig airway smooth muscle and cultured human airway smooth muscle (HASM) cells by immunoblotting and immunohistochemistry. The long-chain free fatty acids (oleic acid and linoleic acid) and GW9508 (FFAR1/FFAR4 dual agonist) dose-dependently stimulated transient intracellular Ca(2+) concentration ([Ca(2+)]i) increases and inositol phosphate synthesis in HASM cells. Downregulation of FFAR1 or FFAR4 in HASM cells by small interfering RNA led to a significant inhibition of the long-chain free fatty acids-induced transient [Ca(2+)]i increases. Oleic acid, linoleic acid, or GW9508 stimulated stress fiber formation in HASM cells, potentiated acetylcholine-contracted guinea pig tracheal rings, and attenuated the relaxant effect of isoproterenol after an acetylcholine-induced contraction. In contrast, TUG-891 (FFAR4 agonist) did not induce the stress fiber formation or potentiate acetylcholine-induced contraction. These results suggest that FFAR1 is the functionally dominant free fatty acid receptor in both human and guinea pig airway smooth muscle. The free fatty acid sensors expressed on airway smooth muscle could be an important modulator of airway smooth muscle tone.

The role of caveolae in the pathophysiology of lung diseases

Caveolae are flask-shaped plasma membrane invaginations formed by constitutive caveolin proteins and regulatory cavin proteins. Caveolae harbor a range of signaling components such as receptors, ion channels and regulatory molecules. There is now increasing evidence that caveolins and cavins play an important role in a variety of diseases. However, the mechanisms by which these caveolar proteins affect lung health and disease are still under investigation, with emerging data suggesting complex roles in disease pathophysiology. This review summarizes the current state of understanding of how caveolar proteins contribute to lung structure and function and how their altered expression and/or function can influence lung diseases.

Quercetin acutely relaxes airway smooth muscle and potentiates β-agonist-induced relaxation via dual phosphodiesterase inhibition of PLCβ and PDE4

Asthma is a disease of the airways with symptoms including exaggerated airway narrowing and airway inflammation. Early asthma therapies used methylxanthines to relieve symptoms, in part, by inhibiting cyclic nucleotide phosphodiesterases (PDEs), the enzyme responsible for degrading cAMP. The classification of tissue-specific PDE subtypes and the clinical introduction of PDE-selective inhibitors for chronic obstructive pulmonary disease (i.e., roflumilast) have reopened the possibility of using PDE inhibition in the treatment of asthma. Quercetin is a naturally derived PDE4-selective inhibitor found in fruits, vegetables, and tea. We hypothesized that quercetin relaxes airway smooth muscle via cAMP-mediated pathways and augments β-agonist relaxation. Tracheal rings from male A/J mice were mounted in myographs and contracted with acetylcholine (ACh). Addition of quercetin (100 nM-1 mM) acutely and concentration-dependently relaxed airway rings precontracted with ACh. In separate studies, pretreatment with quercetin (100 μM) prevented force generation upon exposure to ACh. In additional studies, quercetin (50 μM) significantly potentiated isoproterenol-induced relaxations. In in vitro assays, quercetin directly attenuated phospholipase C activity, decreased inositol phosphate synthesis, and decreased intracellular calcium responses to Gq-coupled agonists (histamine or bradykinin). Finally, nebulization of quercetin (100 μM) in an in vivo model of airway responsiveness significantly attenuated methacholine-induced increases in airway resistance. These novel data show that the natural PDE4-selective inhibitor quercetin may provide therapeutic relief of asthma symptoms and decrease reliance on short-acting β-agonists.

Effects of ginger and its constituents on airway smooth muscle relaxation and calcium regulation

The prevalence of asthma has increased in recent years, and is characterized by airway hyperresponsiveness and inflammation. Many patients report using alternative therapies to self-treat asthma symptoms as adjuncts to short-acting and long-acting β-agonists and inhaled corticosteroids (ICS). As many as 40% of patients with asthma use herbal therapies to manage asthma symptoms, often without proven efficacy or known mechanisms of action. Therefore, investigations of both the therapeutic and possible detrimental effects of isolated components of herbal treatments on the airway are important. We hypothesized that ginger and its active components induce bronchodilation by modulating intracellular calcium ([Ca(2+)](i)) in airway smooth muscle (ASM). In isolated human ASM, ginger caused significant and rapid relaxation. Four purified constituents of ginger were subsequently tested for ASM relaxant properties in both guinea pig and human tracheas: [6]-gingerol, [8]-gingerol, and [6]-shogaol induced rapid relaxation of precontracted ASM (100-300 μM), whereas [10]-gingerol failed to induce relaxation. In human ASM cells, exposure to [6]-gingerol, [8]-gingerol, and [6]-shogaol, but not [10]-gingerol (100 μM), blunted subsequent Ca(2+) responses to bradykinin (10 μM) and S-(-)-Bay K 8644 (10 μM). In A/J mice, the nebulization of [8]-gingerol (100 μM), 15 minutes before methacholine challenge, significantly attenuated airway resistance, compared with vehicle. Taken together, these novel data show that ginger and its isolated active components, [6]-gingerol, [8]-gingerol, and [6]-shogaol, relax ASM, and [8]-gingerol attenuates airway hyperresponsiveness, in part by altering [Ca(2+)](i) regulation. These purified compounds may provide a therapeutic option alone or in combination with accepted therapeutics, including β(2)-agonists, in airway diseases such as asthma.

Brain-derived neurotrophic factor induces proliferation of human airway smooth muscle cells

Airway diseases such as asthma involve increased airway smooth muscle (ASM) contractility and remodelling via enhanced proliferation. Neurotrophins (NTs) such as brain-derived neurotrophic factor (BDNF), well-known in the nervous system, can regulate Ca²⁺ signalling, and interact with cytokines in contributing to airway hyperreactivity. In this study, we determined whether and how BDNF regulates human ASM cell proliferation in the presence of inflammation, thus testing its potential role in airway remodelling. Cells were treated with 10 nM BDNF, 25 ng/ml tumour necrosis factor (TNF-α) or interleukin-13 (IL-13), or 10 ng/ml platelet-derived growth factor (PDGF). Proliferation was measured using CyQuant dye, with immunoblotting of cell cycle proteins predicted to change with proliferation. Forty-eight hours of BDNF enhanced ASM proliferation to ∼50% of that by PDGF or cytokines. Transfection with small interfering RNAs (siRNAs) targeting high-affinity tropomyosin-related kinase B receptor abolished BDNF effects on proliferation, whereas low-affinity 75 kD neurotrophin receptor (p75NTR) siRNA had no effect. Systematic pharmacologic inhibition of different components of ERK1/2 and PI3K/Akt1 pathways blunted BDNF or TNF-α–induced proliferation. BDNF also induced IκB phosphorylation and nuclear translocation of p50 and p65 NF-κB subunits, with electron mobility shift assay confirmation of NF-κB binding to consensus DNA sequence. These results demonstrate that NTs such as BDNF can enhance human ASM cell proliferation by activating proliferation-specific signalling pathways and a versatile transcription factor such as NF-κB, which are common to cytokines and growth factors involved in asthma.

Effects of ginger and its constituents on airway smooth muscle relaxation and calcium regulation

The prevalence of asthma has increased in recent years, and is characterized by airway hyperresponsiveness and inflammation. Many patients report using alternative therapies to self-treat asthma symptoms as adjuncts to short-acting and long-acting β-agonists and inhaled corticosteroids (ICS). As many as 40% of patients with asthma use herbal therapies to manage asthma symptoms, often without proven efficacy or known mechanisms of action. Therefore, investigations of both the therapeutic and possible detrimental effects of isolated components of herbal treatments on the airway are important. We hypothesized that ginger and its active components induce bronchodilation by modulating intracellular calcium ([Ca(2+)](i)) in airway smooth muscle (ASM). In isolated human ASM, ginger caused significant and rapid relaxation. Four purified constituents of ginger were subsequently tested for ASM relaxant properties in both guinea pig and human tracheas: [6]-gingerol, [8]-gingerol, and [6]-shogaol induced rapid relaxation of precontracted ASM (100-300 μM), whereas [10]-gingerol failed to induce relaxation. In human ASM cells, exposure to [6]-gingerol, [8]-gingerol, and [6]-shogaol, but not [10]-gingerol (100 μM), blunted subsequent Ca(2+) responses to bradykinin (10 μM) and S-(-)-Bay K 8644 (10 μM). In A/J mice, the nebulization of [8]-gingerol (100 μM), 15 minutes before methacholine challenge, significantly attenuated airway resistance, compared with vehicle. Taken together, these novel data show that ginger and its isolated active components, [6]-gingerol, [8]-gingerol, and [6]-shogaol, relax ASM, and [8]-gingerol attenuates airway hyperresponsiveness, in part by altering [Ca(2+)](i) regulation. These purified compounds may provide a therapeutic option alone or in combination with accepted therapeutics, including β(2)-agonists, in airway diseases such as asthma.

Brain-derived neurotrophic factor enhances calcium regulatory mechanisms in human airway smooth muscle

Neurotrophins (NTs), which play an integral role in neuronal development and function, have been found in non-neuronal tissue (including lung), but their role is still under investigation. Recent reports show that NTs such as brain-derived neurotrophic factor (BDNF) as well as NT receptors are expressed in human airway smooth muscle (ASM). However, their function is still under investigation. We hypothesized that NTs regulate ASM intracellular Ca²⁺ ([Ca²⁺]_i) by altered expression of Ca²⁺ regulatory proteins. Human ASM cells isolated from lung samples incidental to patient surgery were incubated for 24 h (overnight) in medium (control) or 1 nM BDNF in the presence vs. absence of inhibitors of signaling cascades (MAP kinases; PI3/Akt; NFκB). Measurement of [Ca²⁺]_i responses to acetylcholine (ACh) and histamine using the Ca²⁺ indicator fluo-4 showed significantly greater responses following BDNF exposure: effects that were blunted by pathway inhibitors. Western analysis of whole cell lysates showed significantly higher expression of CD38, Orai1, STIM1, IP₃ and RyR receptors, and SERCA following BDNF exposure, effects inhibited by inhibitors of the above cascades. The functional significance of BDNF effects were verified by siRNA or pharmacological inhibition of proteins that were altered by this NT. Overall, these data demonstrate that NTs activate signaling pathways in human ASM that lead to enhanced [Ca²⁺]_i responses via increased regulatory protein expression, thus enhancing airway contractility.

The dopamine D(2) receptor is expressed and sensitizes adenylyl cyclase activity in airway smooth muscle

Dopamine receptors are G protein-coupled receptors that are divided into two subgroups, "D(1)-like" receptors (D(1) and D(5)) that couple to the G(s) protein and "D(2)-like" receptors (D(2), D(3), and D(4)) that couple to G(i). Although inhaled dopamine has been reported to induce bronchodilation in patients with asthma, functional expression of dopamine receptor subtypes has never been described on airway smooth muscle (ASM) cells. Acute activation of G(i)-coupled receptors inhibits adenylyl cyclase activity and cAMP synthesis, which classically impairs ASM relaxation. In contrast, chronic activation of G(i)-coupled receptors produces a paradoxical enhancement of adenylyl cyclase activity referred to as heterologous sensitization. We questioned whether the dopamine D(2)-like receptor is expressed on ASM, whether it exhibits classical G(i)-coupling, and whether it modulates ASM function. We detected the mRNA encoding the dopamine D(2) receptor in total RNA isolated from native human ASM and from cultured human airway smooth muscle (HASM) cells. Immunoblots identified the dopamine D(2) receptor protein in both native human and guinea pig ASM and cultured HASM cells. The dopamine D(2) receptor protein was immunohistochemically localized to both human and guinea pig ASM. Acute activation of the dopamine D(2) receptor by quinpirole inhibited forskolin-stimulated adenylyl cyclase activity in HASM cells, which was blocked by the dopamine D(2) receptor antagonist L-741626. In contrast, the chronic pretreatment (1 h) with quinpirole potentiated forskolin-stimulated adenylyl cyclase activity, which was inhibited by L-741626, the phospholipase C inhibitor U73122, or the protein kinase C inhibitor GF109203X. Quinpirole also stimulated inositol phosphate synthesis, which was inhibited by L-741626 or U73122. Chronic pretreatment (1 h) of the guinea pig tracheal rings with quinpirole significantly potentiated forskolin-induced airway relaxation, which was inhibited by L-741626. These results demonstrate that functional dopamine D(2) receptors are expressed on ASM and could be a novel therapeutic target for the relaxation of ASM.

Caveolin-1 in cytokine-induced enhancement of intracellular Ca(2+) in human airway smooth muscle

Diseases such as asthma are characterized by airway hyperresponsiveness. Enhanced airway smooth muscle (ASM) intracellular Ca(2+) ([Ca(2+)](i)) response to agonist stimulation leading to increased airway constriction has been suggested to contribute to airway hyperresponsiveness. Caveolae are flask-shaped plasma membrane invaginations that express the scaffolding protein caveolin and contain multiple proteins important in [Ca(2+)](i) signaling (e.g., agonist receptors, ion channels). We recently demonstrated that caveolae and caveolin-1 are important in [Ca(2+)](i) regulation in human ASM. Proinflammatory cytokines such as tumor necrosis factor (TNF)-α and interleukin (IL)-13 modulate [Ca(2+)](i) in ASM. We hypothesized that cytokine upregulation of caveolar signaling in ASM contributes to enhanced agonist-induced [Ca(2+)](i) in inflammation. Enzymatically dissociated human ASM cells were exposed to medium (control), 20 ng/ml TNF-α, or 50 ng/ml IL-13 for 24 h. Caveolae-enriched membrane fractions displayed substantial increase in caveolin-1 and -2 expressions by TNF-α and IL-13. Transfection with caveolin-1-mRed DNA substantially accelerated and increased plasma membrane caveolin-1 expression by TNF-α and to a lesser extent by IL-13. Caveolin-1 enhancement was inhibited by nuclear factor-κB and mitogen-activated protein kinase inhibitors. In fura 2-loaded ASM cells, [Ca(2+)](i) responses to 1 μM ACh, 10 μM histamine, or 10 nM bradykinin were all exaggerated by TNF-α as well as IL-13 exposure. However, disruption of caveolae using caveolin-1 suppression via small-interfering RNA resulted in significant blunting of agonist-induced [Ca(2+)](i) responses of vehicle and TNF-α-exposed cells. These functional data were correlated to the presence of TNFR(1) receptor (but not the IL-4/IL-13 receptor) within caveolae. Overall, these results indicate that caveolin-1 plays an important role in airway inflammation by modulating the effect of specific cytokines on [Ca(2+)](i).

Gi-coupled γ-aminobutyric acid-B receptors cross-regulate phospholipase C and calcium in airway smooth muscle

γ-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system, and exerts its actions via both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. Although the functional expression of GABA(B) receptors coupled to the G(i) protein was reported for airway smooth muscle, the role of GABA(B) receptors in airway responsiveness remains unclear. We investigated whether G(i)-coupled GABA(B) receptors cross-regulate phospholipase C (PLC), an enzyme classically regulated by G(q)-coupled receptors in human airway smooth muscle cells. Both the GABA(B)-selective agonist baclofen and the endogenous ligand GABA significantly increased the synthesis of inositol phosphate, whereas GABA(A) receptor agonists, muscimol, and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol exerted no effect. The baclofen-induced synthesis of inositol phosphate and transient increases in [Ca(2+)](i) were blocked by CGP35348 and CGP55845 (selective GABA(B) antagonists), pertussis toxin (PTX, which inactivates the G(i) protein), gallein (a G(βγ) signaling inhibitor), U73122 (an inhibitor of PLC-β), and xestospongin C, an inositol 1,4,5-triphosphate receptor blocker. Baclofen also potentiated the bradykinin-induced synthesis of inositol phosphate and transient increases in [Ca(2+)](i), which were blocked by CGP35348 or PTX. Moreover, baclofen potentiated the substance P-induced contraction of airway smooth muscle in isolated guinea pig tracheal rings. In conclusion, the stimulation of GABA(B) receptors in human airway smooth muscle cells rapidly mobilizes intracellular Ca(2+) stores by the synthesis of inositol phosphate via the activation of PLC-β, which is stimulated by G(βγ) protein liberated from G(i) proteins coupled to GABA(B) receptors. Furthermore, crosstalk between GABA(B) receptors and G(q)-coupled receptors potentiates the synthesis of inositol phosphate, transient increases in [Ca(2+)](i), and smooth muscle contraction through G(i) proteins.

Expression and activation of the oxytocin receptor in airway smooth muscle cells: Regulation by TNFalpha and IL-13

Background

During pregnancy asthma may remain stable, improve or worsen. The factors underlying the deleterious effect of pregnancy on asthma remain unknown. Oxytocin is a neurohypophyseal protein that regulates a number of central and peripheral responses such as uterine contractions and milk ejection. Additional evidence suggests that oxytocin regulates inflammatory processes in other tissues given the ubiquitous expression of the oxytocin receptor. The purpose of this study was to define the role of oxytocin in modulating human airway smooth muscle (HASMCs) function in the presence and absence of IL-13 and TNFα, cytokines known to be important in asthma.

Method

Expression of oxytocin receptor in cultured HASMCs was performed by real time PCR and flow cytomery assays. Responses to oxytocin was assessed by fluorimetry to detect calcium signals while isolated tracheal rings and precision cut lung slices (PCLS) were used to measure contractile responses. Finally, ELISA was used to compare oxytocin levels in the bronchoalveloar lavage (BAL) samples from healthy subjects and those with asthma.

Results

PCR analysis demonstrates that OXTR is expressed in HASMCs under basal conditions and that both interleukin (IL)-13 and tumor necrosis factor (TNFα) stimulate a time-dependent increase in OXTR expression at 6 and 18 hr. Additionally, oxytocin increases cytosolic calcium levels in fura-2-loaded HASMCs that were enhanced in cells treated for 24 hr with IL-13. Interestingly, TNFα had little effect on oxytocin-induced calcium response despite increasing receptor expression. Using isolated murine tracheal rings and PCLS, oxytocin also promoted force generation and airway narrowing. Further, oxytocin levels are detectable in bronchoalveolar lavage (BAL) fluid derived from healthy subjects as well as from those with asthma.

Conclusion

Taken together, we show that cytokines modulate the expression of functional oxytocin receptors in HASMCs suggesting a potential role for inflammation-induced changes in oxytocin receptor signaling in the regulation of airway hyper-responsiveness in asthma.

Molecular mechanisms underlying airway smooth muscle contraction and proliferation: implications for asthma

Airway smooth muscle (ASM) plays a key role in bronchomotor tone, as well as in structural remodeling of the bronchial wall. Therefore, ASM contraction and proliferation significantly participate in the development and progression of asthma. Many contractile agonists also behave as mitogenic stimuli, thus contributing to frame a hyperresponsive and hyperplastic ASM phenotype. In this review, the molecular mechanisms and signaling pathways involved in excitation-contraction coupling and ASM cell growth will be outlined. Indeed, the recent advances in understanding the basic aspects of ASM biology are disclosing important cellular targets, currently explored for the implementation of new, more effective anti-asthma therapies.

Functional expression of GABAB receptors in airway epithelium

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system and exerts its actions via both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. The GABA(B) receptor is a dimer composed of R1 and R2 components and classically couples to the heterotrimeric G(i) protein. In addition to their location on neurons, GABA and functional GABA(B) receptors have been detected in peripheral tissue such as airway smooth muscle. We questioned whether airway epithelium expresses receptors that could respond to GABA. We detected the mRNA encoding multiple-splice variants of the GABA(B)R1 and GABA(B)R2 in total RNA isolated from native human and guinea pig airway epithelium and human airway epithelial cell lines (BEAS-2B and H441). Immunoblots identified the GABA(B)R1 and GABA(B)R2 proteins in both guinea pig airway epithelium and BEAS-2B cells. The expression of GABA(B)R1 protein was immunohistochemically localized to basal mucin-secreting and ciliated columnar epithelial cells in guinea pig trachea. Baclofen inhibited adenylyl cyclase activity, induced ERK phosphorylation and cross-regulated phospholipase C, leading to increased inositol phosphates in BEAS-2B cells in a pertussis toxin-sensitive manner, implicating G(i) protein coupling. Thus, these receptors couple to G(i) and cross-regulate the phospholipase C/inositol phosphate pathway. The second messengers of these pathways, cyclic AMP and calcium, play pivotal roles in airway epithelial cell primary functions of mucus clearance. Furthermore, the enzyme that synthesizes GABA, glutamic acid decarboxylase (GAD65/67), was also localized to airway epithelium. GABA may modulate an uncharacterized signaling cascade via GABA(B) receptors coupled to G(i) protein in airway epithelium.

Regulation of heterotrimeric G protein signaling in airway smooth muscle

Heterotrimeric G proteins transduce signals from G protein-coupled receptors to regulate numerous signaling events and functions in airway smooth muscle (ASM). In this article, we detail the function and regulation of heterotrimeric G protein signaling in ASM. We further discuss recent advances in the development of experimental tools in the study of G protein signaling, and speculate how these tools might be used in therapeutic strategies that seek to mitigate bronchospasm and airway remodeling that occur in obstructive airway disease.

Expression and coupling of neurokinin receptor subtypes to inositol phosphate and calcium signaling pathways in human airway smooth muscle cells

Neuropeptide tachykinins (substance P, neurokinin A, and neurokinin B) are present in peripheral terminals of sensory nerve fibers within the respiratory tract and cause airway contractile responses and hyperresponsiveness in humans and most mammalian species. Three subtypes of neurokinin receptors (NK1R, NK2R, and NK3R) classically couple to Gq protein-mediated inositol 1,4,5-trisphosphate (IP3) synthesis and liberation of intracellular Ca2+, which initiates contraction, but their expression and calcium signaling mechanisms are incompletely understood in airway smooth muscle. All three subtypes were identified in native and cultured human airway smooth muscle (HASM) and were subsequently overexpressed in HASM cells using a human immunodeficiency virus-1-based lentivirus transduction system. Specific NKR agonists {NK1R, [Sar9,Met(O2)11]-substance P; NK2R, [beta-Ala8]-neurokinin A(4-10); NK3R, senktide} stimulated inositol phosphate synthesis and increased intracellular Ca2+ concentration ([Ca2+]i) in native HASM cells and in HASM cells transfected with each NKR subtype. These effects were blocked by NKR-selective antagonists (NK1R, L-732138; NK2R, GR-159897; NK3R, SB-222200). The initial transient and sustained phases of increased [Ca2+]i were predominantly inhibited by the IP3 receptor antagonist 2-aminoethoxydiphenyl borate (2-APB) or the store-operated Ca2+ channel antagonist SKF-96365, respectively. These results show that all three subtypes of NKRs are expressed in native HASM cells and that IP3 levels are the primary mediators of NKR-stimulated initial [Ca2+]i increases, whereas store-operated Ca2+ channels mediate the sustained phase of the [Ca2+]i increase.

Role of CD38 in TNF-alpha-induced airway hyperresponsiveness

CD38 is involved in normal airway function, IL-13-induced airway hyperresponsiveness (AHR), and is also regulated by tumor necrosis factor (TNF)-alpha in airway smooth muscle (ASM) cells. This study aimed to determine whether TNF-alpha-induced CD38 upregulation in ASM cells contributes to AHR, a hallmark of asthma. We hypothesized that AHR would be attenuated in TNF-alpha-exposed CD38-deficient (CD38KO) mice compared with wild-type (WT) controls. Mice (n = 6-8/group) were intranasally challenged with vehicle control or TNF-alpha (50 ng) once and every other day during 1 or 4 wk. Lung inflammation and AHR, measured by changes in lung resistance after inhaled methacholine, were assessed 24 h following the last challenge. Tracheal rings were incubated with TNF-alpha (50 ng/ml) to assess contractile changes in the ASM. While a single TNF-alpha challenge caused no airway inflammation, both multiple-challenge protocols induced equally significant inflammation in CD38KO and WT mice. A single intranasal TNF-alpha challenge induced AHR in the WT but not in the CD38KO mice, whereas both mice developed AHR after 1 wk of challenges. The AHR was suppressed by extending the challenges for 4 wk in both mice, although to a larger magnitude in the WT than in the CD38KO mice. TNF-alpha increased ASM contractile properties in tracheal rings from WT but not from CD38KO mice. In conclusion, CD38 contributes to TNF-alpha-induced AHR after a brief airway exposure to the cytokine, likely by mediating changes in ASM contractile responses, and is associated with greater AHR remission following chronic airway exposure to TNF-alpha. The mechanisms involved in this remission remain to be determined.

Salmeterol and cytokines modulate inositol-phosphate signalling in human airway smooth muscle cells via regulation at the receptor locus

Background

Airway hyper-responsiveness (AHR) is a key feature of asthma and a causal relationship between airway inflammation and AHR has been identified. The aim of the current study was to clarify the effect of proinflammatory cytokines and asthma medication on primary human airway smooth muscle (ASM) inositol phosphate (IPx) signalling and define the regulatory loci involved.

Methods

Primary Human ASM cells were isolated from explants of trachealis muscle from individuals with no history of respiratory disease. The effect of cytokine or asthma medication on histamine or bradykinin induced IPx signalling was assessed by [³H] inositol incorporation. Quantitative Real Time PCR was used to measure mRNA levels of receptors and downstream signalling components. Transcriptional mechanisms were explored using a combination of 5'Rapid Amplification of cDNA Ends (5'RACE) and promoter-reporter techniques.

Results

Treatment of Human ASM cells with IL-13, IFNγ or salmeterol for 24 hours lead to a modest augmentation of histamine induced IPx responses (144.3 +/- 9.3, 126.4 +/- 7.5 and 117.7 +/- 5.2%, p < 0.05). Similarly, TNFα, IFNγ or salmeterol treatment augmented bradykinin induced IPx responses (127.4 +/- 8.3, 128.0 +/- 8.4 and 111.7 +/- 5.0%, P < 0.05). No treatment significantly influenced sodium fluoride induced IPx responses suggesting regulation occurs at the receptor locus. Analyses of mRNA expression of components of the IPx pathway i.e. H1 Histamine Receptor (HRH1), B2 Bradykinin Receptor (BDKRB2), Gαq/11 and PLC-β1 identified that a significant induction of receptor mRNA (>2 fold) was a feature of these responses explaining the cytokine and spasmogen specificity. The HRH1 and BDKRB2 promoter regions were mapped in ASM and promoter-reporter analyses identified that salmeterol can induce HRH1 (>2 fold) and BDKRB2 (2–5 fold) transcription. The effect of cytokines on HRH1 and BDKRB2 promoter-reporter expression suggested a more complex regulation of mRNA expression involving additional loci to the core promoter.

Conclusion

Our results indicate that the spasmogen specific receptor locus may be a key site of regulation determining the magnitude of spasmogen mediated ASM IPx responses during airway inflammation or following asthma medication. These data provide further insight into the molecular basis of AHR and extend our understanding of potentially detrimental effects associated with existing therapies used in the treatment of asthma.

Adenosine regulation of alveolar fluid clearance

Adenosine is a purine nucleoside that regulates cell function through G protein-coupled receptors that activate or inhibit adenylyl cyclase. Based on the understanding that cAMP regulates alveolar epithelial active Na(+) transport, we hypothesized that adenosine and its receptors have the potential to regulate alveolar ion transport and airspace fluid content. Herein, we report that type 1 (A(1)R), 2a (A(2a)R), 2b (A(2b)R), and 3 (A(3)R) adenosine receptors are present in rat and mouse lungs and alveolar type 1 and 2 epithelial cells (AT1 and AT2). Rat AT2 cells generated and produced cAMP in response to adenosine, and micromolar concentrations of adenosine were measured in bronchoalveolar lavage fluid from mice. Ussing chamber studies of rat AT2 cells indicated that adenosine affects ion transport through engagement of A(1)R, A(2a)R, and/or A(3)R through a mechanism that increases CFTR and amiloride-sensitive channel function. Intratracheal instillation of low concentrations of adenosine (< or =10(-8)M) or either A(2a)R- or A(3)R-specific agonists increased alveolar fluid clearance (AFC), whereas physiologic concentrations of adenosine (> or =10(-6)M) reduced AFC in mice and rats via an A(1)R-dependent pathway. Instillation of a CFTR inhibitor (CFTR(inh-172)) attenuated adenosine-mediated down-regulation of AFC, suggesting that adenosine causes Cl(-) efflux by means of CFTR. These studies report a role for adenosine in regulation of alveolar ion transport and fluid clearance. These findings suggest that physiologic concentrations of adenosine allow the alveolar epithelium to counterbalance active Na(+) absorption with Cl(-) efflux through engagement of the A(1)R and raise the possibility that adenosine receptor ligands can be used to treat pulmonary edema.

Raf-1 kinase mediates adenylyl cyclase sensitization by TNF-alpha in human airway smooth muscle cells

Tumor necrosis factor (TNF)-alpha is a potent inflammatory cytokine implicated in the exacerbation of asthma. Chronic exposure to TNF-alpha has been reported to induce G protein-coupled receptor desensitization, but adenylyl cyclase sensitization, in airway smooth muscle cells by an unknown mechanism. Cyclic AMP, which is synthesized by adenylyl cyclases in response to G protein-coupled receptor signals, is an important second messenger involved in the regulation of the airway muscle proliferation, migration, and tone. In other cell types, TNF-alpha receptors transactivate the EGF receptor, which activates raf-1 kinase. Further studies in transfected cells show that raf-1 kinase can phosphorylate and activate some isoforms of adenylyl cyclase. Cultured human airway smooth muscle cells were treated with TNF-alpha in the presence or absence of inhibitors of prostaglandin signaling, protein kinases, or G(i) proteins. TNF-alpha caused a significant dose- (1-10 ng/ml) and time-dependent (24 and 48 h) increase in forskolin-stimulated adenylyl cyclase activity, which was abrogated by pretreatment with GW5074 (a raf-1 kinase inhibitor), was partially inhibited by an EGF receptor inhibitor, but was unaffected by pertussis toxin. TNF-alpha also increased phosphorylation of Ser(338) on raf-1 kinase, indicative of activation. IL-1beta and EGF sensitization of adenylyl cyclase activity was also sensitive to raf-1 kinase inhibition by GW5074. Taken together, these studies link two signaling pathways not previously characterized in human airway smooth muscle cells: TNF-alpha transactivation of the EGF receptor, with subsequent raf-1 kinase-mediated activation of adenylyl cyclase.

Antinociception mediated by alpha(2)-adrenergic activation involves increasing tumor necrosis factor alpha (TNFalpha) expression and restoring TNFalpha and alpha(2)-adrenergic inhibition of norepinephrine release

The central component that establishes chronic pain from peripheral nerve injury is associated with increased tumor necrosis factor-alpha (TNFalpha) production in the brain. This study examined TNFalpha and its reciprocally permissive role with alpha(2)-adrenergic activation during peak and progressive decline of thermal hyperalgesia in sciatic nerve chronic constriction injury (CCI). Accumulation of TNFalpha mRNA (in situ hybridization) increases in the hippocampus and locus coeruleus during the onset of neuropathic pain and persists as hyperalgesia abates. Activation of alpha(2)-adrenergic receptors in control rats decreases TNFalpha mRNA accumulation in these brain regions. In contrast, during hyperalgesia, alpha(2)-adrenergic activation enhances TNFalpha mRNA accumulation. Whether this enhanced TNFalpha production is associated with changes in the regulation of norepinephrine (NE) release was tested. Hippocampal slices were electrically depolarized to evaluate alpha(2)-adrenergic and TNFalpha regulation of NE release. While inhibition of NE release by TNFalpha is maximal during peak hyperalgesia, it subsequently transforms to facilitate NE release. In addition, alpha(2)-adrenergic receptor activation with clonidine (0.2mg/kg, i.p.) in CCI rats experiencing hyperalgesia restores TNFalpha and alpha(2)-adrenergic inhibition of NE release. While TNFalpha directs the development of hyperalgesia, it also directs its resolution. Transformed sensitivity to alpha(2)-adrenergic agonists during hyperalgesia demonstrates a mechanism for therapy.

Bradykinin activates calcium-dependent potassium channels in cultured human airway smooth muscle cells

Bradykinin (BK) is an inflammatory mediator that can cause bronchoconstriction. In this study, we investigated the membrane currents induced by BK in cultured human airway smooth muscle (ASM) cells. Depolarization of the cells induced outward currents, which were inhibited by tetraethylammonium (TEA) in a concentration-dependent manner with an IC50 of 0.33 microM. The currents were increased by elevating intracellular free Ca2+ concentration, suggesting they are calcium-activated potassium channels [I(K(Ca))]. Preexposure to inhibitor of I(K(Ca)) of large conductance (BKCa), iberiotoxin, and small conductance (SKCa), apamin, inhibited the increase of outward current induced by BK. The relative contribution of BKCa was greatest in early passage cells. Both nickel and SKF-96365 (10 microM) inhibited the increase of the I(K(Ca)) induced by BK; however, the l-type Ca2+ channel blocker, nifedipine, had no effect. Activation of the BK-induced current was inhibited by heparin, indicating dependence on intact inositol 1,4,5-triphosphate (IP3)-sensitive intracellular Ca2+ stores. BK also increased inositol phosphate accumulation and induced a transient Ca2+-activated chloride current (CACC) and a sustained nonselective cation current (I(CAT)). In summary, BK activates BKCa, SKCa, CACC, and I(CAT) via IP3-sensitive stores in human ASM.

Functional expression of the GABAB receptor in human airway smooth muscle

gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system and exerts its actions via both ionotropic (GABA(A)/GABA(C)) and metabotropic (GABA(B)) receptors (R). In addition to their location on neurons, GABA and functional GABA(B) receptors have been detected in nonneuronal cells in peripheral tissue. Although the GABA(B)R has been shown to function as a prejunctional inhibitory receptor on parasympathetic nerves in the lung, the expression and functional coupling of GABA(B) receptors to G(i) in airway smooth muscle itself have never been described. We detected the mRNA encoding multiple-splice variants of the GABA(B)R1 and GABA(B)R2 in total RNA isolated from native human and guinea pig airway smooth muscle and from RNA isolated from cultured human airway smooth muscle (HASM) cells. Immunoblots identified the GABA(B)R1 and GABA(B)R2 proteins in human native and cultured airway smooth muscle. The GABA(B)R1 protein was immunohistochemically localized to airway smooth muscle in guinea pig tracheal rings. Baclofen, a GABA(B)R agonist, elicited a concentration-dependent stimulation of [(35)S]GTPgammaS binding in HASM homogenates that was abrogated by the GABA(B)R antagonist CGP-35348. Baclofen also inhibited adenylyl cyclase activity and induced ERK phosphorylation in HASM. Another GABA(B)R agonist, SKF-97541, mimicked while pertussis toxin blocked baclofen's effect on ERK phosphorylation, implicating G(i) protein coupling. Functional GABA(B) receptors are expressed in HASM. GABA may modulate an uncharacterized signaling cascade via GABA(B) receptors coupled to the G(i) protein in airway smooth muscle.

Muscarinic receptor signaling in the pathophysiology of asthma and COPD

Anticholinergics are widely used for the treatment of COPD, and to a lesser extent for asthma. Primarily used as bronchodilators, they reverse the action of vagally derived acetylcholine on airway smooth muscle contraction. Recent novel studies suggest that the effects of anticholinergics likely extend far beyond inducing bronchodilation, as the novel anticholinergic drug tiotropium bromide can effectively inhibit accelerated decline of lung function in COPD patients. Vagal tone is increased in airway inflammation associated with asthma and COPD; this results from exaggerated acetylcholine release and enhanced expression of downstream signaling components in airway smooth muscle. Vagally derived acetylcholine also regulates mucus production in the airways. A number of recent research papers also indicate that acetylcholine, acting through muscarinic receptors, may in part regulate pathological changes associated with airway remodeling. Muscarinic receptor signalling regulates airway smooth muscle thickening and differentiation, both in vitro and in vivo. Furthermore, acetylcholine and its synthesizing enzyme, choline acetyl transferase (ChAT), are ubiquitously expressed throughout the airways. Most notably epithelial cells and inflammatory cells generate acetylcholine, and express functional muscarinic receptors. Interestingly, recent work indicates the expression and function of muscarinic receptors on neutrophils is increased in COPD. Considering the potential broad role for endogenous acetylcholine in airway biology, this review summarizes established and novel aspects of muscarinic receptor signaling in relation to the pathophysiology and treatment of asthma and COPD.

Lipopolysaccharide-induced sensitization of adenylyl cyclase activity in murine macrophages

LPS is known to modulate macrophage responses during sepsis, including cytokine release, phagocytosis, and proliferation. Although agents that elevate cAMP reverse LPS-induced macrophage functions, whether LPS itself modulates cAMP and whether LPS-induced decreases in proliferation are modulated via a cAMP-dependent pathway are not known. Murine macrophages (RAW264.7 cells) were treated with LPS in the presence or absence of inhibitors of prostaglandin signaling, protein kinases, CaM, Giproteins, and NF-κB translocation or transcription/translation. LPS effects on CaMKII phosphorylation and the expression of relevant adenylyl cyclase (AC) isoforms were measured. LPS caused a significant dose (5–10,000 ng/ml)- and time (1–8 h)-dependent increase in forskolin-stimulated AC activity that was abrogated by pretreatment with SN50 (an NF-κB inhibitor), actinomycin D, or cycloheximide, indicating that the effect is mediated via NF-κB-dependent transcription and new protein synthesis. Furthermore, LPS decreased the phosphorylation state of CaMKII, and pretreatment with a CaM antagonist attenuated the LPS-induced sensitization of AC. LPS, cAMP, or PKA activation each independently decreased macrophage proliferation. However, inhibition of NF-κB had no effect on LPS-induced decreased proliferation, indicating that LPS-induced decreased macrophage proliferation can proceed via PKA-independent signaling pathways. Taken together, these findings indicate that LPS induces sensitization of AC activity by augmenting the stimulatory effect of CaM and attenuating the inhibitory effect of CaMKII on isoforms of AC that are CaMK sensitive.

Expression and muscarinic receptor coupling of Lyn kinase in cultured human airway smooth muscle cells

Src family tyrosine kinases are signaling intermediates in a diverse array of cellular events including cell differentiation, motility, proliferation, and survival. In nonairway smooth muscle cells, muscarinic receptors directly interact with Src family tyrosine kinases. As little is known about the expression and signaling of these Src family tyrosine kinases in human airway smooth muscle cells, we determined the expression of Src family members and characterized the muscarinic receptor-mediated activation of Lyn kinase in these cells. RT-PCR revealed mRNA transcripts for FYN, c-SRC, YES, FRK, and LYN. Fyn, c-Src, Yes, and Lyn were identified in cultured airway smooth muscle cells by immunoblot analysis. In both nontransformed human cultured airway smooth muscle cells and cells transduced with wild-type human Lyn kinase, carbachol increased Lyn kinase activity. Pertussis toxin pretreatment failed to block carbachol activation of Lyn kinase but did attenuate the carbachol-induced increase in ERK/MAPK phosphorylation. Moreover, carbachol inhibited adenylyl cyclase but failed to increase total inositol phosphate synthesis in these cells. The present study shows that Lyn kinase is expressed in human cultured airway smooth muscle cells at both the mRNA and protein levels and that carbachol, an M2 muscarinic receptor agonist in these cells, activates Lyn kinase by a pertussis toxin-insensitive signaling pathway.

Discovery of genetic difference between asthmatic children with high IgE level and normal IgE level by whole genome linkage disequilibrium mapping using 763 autosomal STR markers

The genome-wide linkage disequilibrium screen for loci associated with genetic difference between allergic and nonallergic asthma was conducted with 763 autosomal STR markers and included 190 asthmatic children. Evidence for association with differences between the two forms of asthma was observed for 36 STR markers. Marker-to-marker synergetic effect and by simulation resampling tests revealed D5S2011, D6S305, and D9S286 were important loci in allergic asthma while D6S1574, D8S1769, and D19S226 were important in nonallergic asthma. Our results show strong genetic evidence that these markers play an important role in defining allergic and nonallergic asthma and provides important candidates of susceptible genes in these two categories of asthma. This study further shows that asthma is, indeed, a heterogeneous group of underlying diseases and, although with similar clinical phenotypes, may have different clinical severities, outcomes, and need more tailor-made management.

CD38/cyclic ADP-ribose signaling: role in the regulation of calcium homeostasis in airway smooth muscle

The contractility of airway smooth muscle cells is dependent on dynamic changes in the concentration of intracellular calcium. Signaling molecules such as inositol 1,4,5-trisphosphate and cyclic ADP-ribose play pivotal roles in the control of intracellular calcium concentration. Alterations in the processes involved in the regulation of intracellular calcium concentration contribute to the pathogenesis of airway diseases such as asthma. Recent studies have identified cyclic ADP-ribose as a calcium-mobilizing second messenger in airway smooth muscle cells, and modulation of the pathway involved in its metabolism results in altered calcium homeostasis and may contribute to airway hyperresponsiveness. In this review, we describe the basic mechanisms underlying the dynamics of calcium regulation and the role of CD38/cADPR, a novel pathway, in the context of airway smooth muscle function and its contribution to airway diseases such as asthma.

Modulation of tracheal smooth muscle responses in inducible T-cell kinase knockout mice

It has been shown that the Tec family nonreceptor tyrosine kinase inducible T cell kinase (ITK) plays a role in the activation of naive T cells and in the differentiation of T helper (TH2)-type cells producing cytokines in a model of allergic inflammation, thereby possibly indirectly mediating hyperresponsivenes of airway smooth muscle tone. Using excised tracheae from wild type (WT) mice and those lacking ITK, we conducted a series of in vitro experiments in which isometric smooth muscle tones were assessed in response to several agonists to determine whether the absence of ITK would affect the responsiveness of tracheal smooth muscle cells. The resulting change in contractile responses was evaluated by measuring agonist cumulative concentration-response curves (CCRC). Our results indicate that the cholinergic agonist acetylcholine (ACh) and its analog carbachol (CCh) exhibited comparable CCRC profiles in contracting isolated tracheae from both WT and ITK-/- mice, with no alteration in their efficacies. However, the EC50 values for the two agonists were found to be significantly higher in ITK-/- tracheae than in those from WT mice, suggesting an alteration of the potencies of these cholinergic agonists in the trachea of ITK-/- mice. Moreover, we found that the depolarizing agent potassium chloride (KCl) had a significantly lower efficacy in contracting ITK-/- tracheae compared to those from WT mice. This difference in KCl efficacy was abolished in the presence of a calcium (Ca2+) voltage-dependent channel (VDC) agonist, Bay K8644, suggesting a modulation of the KCl induced permeability of VDC Ca2+ channels in the trachea of ITK-/- mice. Taken together, these results suggest that the presence of ITK may play a modulating role in the pharmacomechanical as well as in the electromechanical coupling of airway smooth muscle contraction.

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.

CD38/cyclic ADP-ribose-mediated Ca2+ signaling contributes to airway smooth muscle hyper-responsiveness

We previously demonstrated that cyclic ADP-ribose (cADPR) elicits Ca2+ release in airway smooth muscle (ASM) cells through ryanodine receptor channels. CD38 is a cell surface protein that catalyzes the synthesis and degradation of cADPR. In inflammatory diseases such as asthma, augmented Ca2+ responses and Ca2+ sensitivity contribute to increased ASM contractility in response to agonists. In this study, we investigated the regulation of CD38 expression and the role of cADPR-mediated Ca2+ release in airway inflammation. Human ASM cells in culture between the second and fifth passages were exposed to tumor necrosis factor alpha (TNF-alpha), interleukin 1beta, or interferon gamma, or bovine serum albumin (controls). CD38 expression was measured by reverse transcriptase-polymerase chain reaction (RT-PCR), real-time PCR, and Western blot analysis, and ADP-ribosyl cyclase activity was assayed with nicotinamide guanine dinucleotide as the substrate. Ca2+ responses to acetylcholine, bradykinin, and thrombin were measured in fura-2AM-loaded cells by fluorescence microscopy. Cytokines caused significant augmentation of CD38 expression, ADP-ribosyl cyclase activity, and Ca2+ responses to the agonists, compared with the control. TNF-alpha effects were greater than those of the other two cytokines. The cADPR antagonist 8-bromo-cADPR attenuated the Ca2+ responses to the agonists in control and cytokine-treated cells, with the magnitude of inhibition correlating with the level of CD38. This study provides the first demonstration of a role for CD38-cADPR signaling in a model of inflammatory airway disease.

Airway hyperresponsiveness: a story of mice and men and cytokines

Bronchial hyperresponsiveness (BHR) is an essential part of the definition of asthma. Although our understanding of the allergic inflammatory and immunologic mechanisms of asthma have markedly increased, the mechanism of BHR remains to be elucidated. Increased BHR is associated temporally with exposure to allergens, certain respiratory viruses, pollutants such as ozone, and certain occupational chemicals. An important research use of determining the degree of BHR to direct and indirect challenge is to determine the efficacy of pharmacologic and immunodulatory agents. Beta-adrenergic agents inhibit BHR and certain genetic polymorphisms of the beta-adrenergic receptor are associated with increased BHR. When beta-adrenergic receptors are blocked, sensitivity to allergens is markedly increased in patients with asthma and animal models of asthma. Allergen challenge and clinical asthma are associated with synthesis and release of pro-inflammatory cytokines such as IL-1 and TNF-alpha which have been shown to decrease the response to beta-agonists and increased the reactivity to methacholine and the airways neutrophils and alveolar macrophages. The Th2 cytokine IL-13 is increased in the airways of asthmatics and increases BHR in normal unsensitized animals. The mechanisms of this effect of IL-13 are being intensively investigated. Our group has shown that IL-13 induced BHR persisted for at least 7 days and the soluble receptor IL-13R2alpha protected against their BHR. Other investigators have demonstrated that IL-13 is necessary and sufficient for the induction of BHR and that eosinophilic airway inflammation in the absence of IL-13 fails to induce BHR. These studies indicate that treatment of human asthma with antagonists of IL-13 may be very effective.

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.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Adenosine attenuates oxidant injury in human proximal tubular cells via A(1) and A(2a) adenosine receptors

We have recently demonstrated protection against renal ischemic-reperfusion injury in vivo by A(1)- and A(2a)-adenosine receptor (AR) modulations. To further elucidate the signaling cascades of AR-induced cytoprotection against reperfusion/oxidant-mediated injury, immortalized human proximal tubule (HK-2) cells were treated with H(2)O(2). H(2)O(2) caused dose- and time-dependent HK-2 cell death that was measured by lactate dehydrogenase release and trypan blue dye uptake. Adenosine protected against H(2)O(2)-induced HK-2 cell death by means of A(1)- and A(2a)-AR activation. A(1)-AR-mediated protection involves pertussis toxin-sensitive G proteins and protein kinase C, whereas A(2a)-AR-mediated protection involves protein kinase A activation by means of cAMP and activation of the cAMP response element binding protein. Moreover, protein kinase A activators (forskolin and Sp-isomer cAMP) also protected HK-2 cells against H(2)O(2) injury. De novo gene transcription and protein synthesis are required for both A(1)- and A(2a)-AR-mediated cytoprotection as actinomycin D and cycloheximide, respectively, blocked cytoprotection. Chronic treatments with a nonselective AR agonist abolished the protection by adenosine. Moreover, chronic treatments with a nonselective AR antagonist increased the endogenous tolerance of HK-2 cells against H(2)O(2). We concluded that A(1)- and A(2a)-AR activation protects HK-2 cells against H(2)O(2)-induced injury by means of distinct signaling pathways that require new gene transcription and new protein synthesis.

Mechanisms of cytokine effects on G protein-coupled receptor-mediated signaling in airway smooth muscle

Numerous in vitro and in vivo studies have implicated the cytokines interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) as mediators of airway inflammation and therefore potentially important substances in the pathogenesis of asthma. In this study, we examined the mechanisms by which IL-1 beta and TNF-alpha affect inhibition of cell growth, G protein-coupled receptor (GPCR) desensitization, and the recently reported adenylyl cyclase sensitization in human airway smooth muscle (HASM) cultures. Our findings demonstrate that adenylyl cyclase sensitization is independent of cytokine-mediated cyclooxygenase type 2 (COX-2) and prostaglandin E(2) (PGE(2)) induction, whereas COX-2 induction appears to be required for both growth inhibition and GPCR desensitization. However, GPCR desensitization was highly dependent on the presence of EGF during chronic treatment with cytokines, which could be explained by a synergistic effect of EGF on cytokine-mediated COX-2 and PGE(2) induction. Interestingly, various agents (including inhibitors of p42/p44 and p38 mitogen-activated protein kinase signaling) were significantly more effective in inhibiting cytokine-mediated PGE(2) induction, GPCR desensitization, and cell growth inhibition than in inhibiting COX-2 induction. These data demonstrate disparity in the requirement and sufficiency of COX-2 induction in promoting different functional effects of IL-1 beta and TNF-alpha in HASM.

Association between IL-1beta/TNF-alpha-induced glucocorticoid-sensitive changes in multiple gene expression and altered responsiveness in airway smooth muscle

The pleiotropic cytokines interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha have been implicated in the pathophysiology of asthma. To elucidate the role of these cytokines in the pro-asthmatic state, the effects of IL-1beta and TNF-alpha on airway smooth muscle (ASM) responsiveness and ASM expression of multiple genes, assessed by high-density oligonucleotide array analysis, were examined in the absence and presence of the glucocorticoid dexamethasone (DEX). Administration of IL-1beta/TNF-alpha increased ASM contractility to acetylcholine and impaired ASM relaxation to isoproterenol. These pro-asthmatic- like changes in ASM responsiveness were associated with IL-1beta/ TNF-alpha-induced mRNA expression of a host of proinflammatory genes that regulate transcription, cytokines and chemokines, cellular adhesion molecules, and various signal transduction molecules that regulate ASM responsiveness. In the presence of DEX, the changes induced in ASM responsiveness were abrogated, and most of the IL-1beta/TNF-alpha-mediated changes in proinflammatory gene expression were repressed, although mRNA expression of a small number of genes was enhanced by DEX. Collectively, the observations support the concept that, together with its role as a regulator of airway tone, in response to IL-1beta/TNF-alpha, the ASM expresses a host of glucocorticoid-sensitive genes that contribute to the altered structure and function of the airways in the pro-asthmatic state. We speculate that glucocorticoid-sensitive, cytokine-induced pathways involved in ASM cell signaling represent important targets for new therapeutic interventions.

Human airway smooth muscle expresses 7 isoforms of adenylyl cyclase: a dominant role for isoform V

Adenylyl cyclases are a nine-member family of differentially regulated enzymes responsible for the synthesis of cAMP. cAMP is an important second messenger that contributes to the regulation of airway smooth muscle tone. However, little is known regarding the expression and regulation of adenylyl cyclase isoforms in airway smooth muscle cells. Nondegenerate specific primers were designed for all nine known isoforms of human adenylyl cyclase. RT-PCR experiments were performed using total RNA extracted from whole human brain (positive control), whole rat brain (negative control), whole human trachea, human airway smooth muscle, and primary cultures of human airway smooth muscle cells. Seven of the nine known isoforms of adenylyl cyclase (isoforms I, III-VII, and IX) were expressed at the mRNA level in both human airway smooth muscle and primary cultures of human airway smooth muscle cells. Immunoblot and adenylyl cyclase functional assay indicated that isoform V is likely among the functionally predominant isoforms of adenylyl cyclase in human airway smooth muscle. These results suggest that multiple isoforms of adenylyl cyclase enzymes are coexpressed in human airway smooth muscle cells and that isoform V is among the functionally important isoforms.