Regulation of airway smooth muscle RhoA/ROCK activities by cholinergic and bronchodilator stimuli

Role of miR-185-5p as modulator of periostin synthesis and smooth muscle contraction in asthma

Asthma is a chronic respiratory disease produced by an aberrant immune response that originates with breathing difficulties and cough, through airway remodeling. The above pathophysiological events of asthma emerge the regulators of effectors, like epigenetics, which include microRNAs (miRNAs) who perform post‐transcriptional regulation, controlling diverse pathways in respiratory diseases. The objective of the study was to determine how miR‐185‐5p regulates the secretion of periostin by airway structural cells, and smooth muscle cells contraction, both related to airway remodeling in asthma. We used miR‐185‐5p mimic and inhibitors in bronchial smooth muscle cells (BSMCs) and small airway epithelial cells (SAECs) from healthy subjects. Gene expression and protein levels of periostin (POSTN), CDC42, and RHOA were analyzed by RT‐PCR and ELISA/Western blot, respectively. BSMC contractility was analyzed using cell‐embedded collagen gels and measurement of intracellular calcium was performed using Fura‐2. Additionally, miR‐185‐5p and periostin expression were evaluated in sputum from healthy and asthmatics. From these experiments, we observed that miR‐185‐5p modulation regulates periostin mRNA and protein in BSMCs and SAECs. A tendency for diminished miR‐185‐5p expression and higher periostin levels was seen in sputum cells from asthmatics compared to healthy, with an inverse correlation observed between POSTN and miR‐185‐5p. Inhibition of miR‐185‐5p produced higher BSMCs contraction induced by histamine. Calcium mobilization was not modified by miR‐185‐5p, showing that miR‐185‐5p role in BSMC contractility is performed by regulating CDC42 and RhoA pro‐contractile factors instead. In conclusion, miR‐185‐5p is a modulator of periostin secretion by airway structural cells and of smooth muscle contraction, which can be related to asthma pathophysiology, and thus, might be a promising therapeutic target.

Mechanopharmacology of Rho-kinase antagonism in airway smooth muscle and potential new therapy for asthma

The principle of mechanopharmacology of airway smooth muscle (ASM) is based on the premise that physical agitation, such as pressure oscillation applied to an airway, is able to induce bronchodilation by reducing contractility and softening the cytoskeleton of ASM. Although the underlying mechanism is not entirely clear, there is evidence to suggest that large-amplitude stretches are able to disrupt the actomyosin interaction in the crossbridge cycle and weaken the cytoskeleton in ASM cells. Rho-kinase is known to enhance force generation and strengthen structural integrity of the cytoskeleton during smooth muscle activation and plays a key role in the maintenance of force during prolonged muscle contractions. Synergy in relaxation has been observed when the muscle is subject to oscillatory length change while Rho-kinase is pharmacologically inhibited. In this review, inhibition of Rho-kinase coupled to therapeutic pressure oscillation applied to the airways is explored as a combination treatment for asthma.

Regulation of Airway Smooth Muscle Contraction in Health and Disease

Airway smooth muscle (ASM) extends from the trachea throughout the bronchial tree to the terminal bronchioles. In utero, spontaneous phasic contraction of fetal ASM is critical for normal lung development by regulating intraluminal fluid movement, ASM differentiation, and release of key growth factors. In contrast, phasic contraction appears to be absent in the adult lung, and regulation of tonic contraction and airflow is under neuronal and humoral control. Accumulating evidence suggests that changes in ASM responsiveness contribute to the pathophysiology of lung diseases with lifelong health impacts.Functional assessments of fetal and adult ASM and airways have defined pharmacological responses and signaling pathways that drive airway contraction and relaxation. Studies using precision-cut lung slices, in which contraction of intrapulmonary airways and ASM calcium signaling can be assessed simultaneously in situ, have been particularly informative. These combined approaches have defined the relative importance of calcium entry into ASM and calcium release from intracellular stores as drivers of spontaneous phasic contraction in utero and excitation-contraction coupling.Increased contractility of ASM in asthma contributes to airway hyperresponsiveness. Studies using animal models and human ASM and airways have characterized inflammatory and other mechanisms underlying increased reactivity to contractile agonists and reduced bronchodilator efficacy of β₂-adrenoceptor agonists in severe diseases. Novel bronchodilators and the application of bronchial thermoplasty to ablate increased ASM within asthmatic airways have the potential to overcome limitations of current therapies. These approaches may directly limit excessive airway contraction to improve outcomes for difficult-to-control asthma and other chronic lung diseases.

Rho kinase collaborates with p21-activated kinase to regulate actin polymerization and contraction in airway smooth muscle

KEY POINTS

The mechanisms by which Rho kinase (ROCK) regulates airway smooth muscle contraction were determined in tracheal smooth muscle tissues. ROCK may mediate smooth muscle contraction by inhibiting myosin regulatory light chain (RLC) phosphatase. ROCK can also regulate F-actin dynamics during cell migration, and actin polymerization is critical for airway smooth muscle contraction. Our results show that ROCK does not regulate airway smooth muscle contraction by inhibiting myosin RLC phosphatase or by stimulating myosin RLC phosphorylation. We find that ROCK regulates airway smooth muscle contraction by activating the serine-threonine kinase Pak, which mediates the activation of Cdc42 and neuronal Wiskott-Aldrich syndrome protein (N-WASp). N-WASP transmits signals from Cdc42 to the Arp2/3 complex for the nucleation of actin filaments. These results demonstrate a novel molecular function for ROCK in the regulation of Pak and Cdc42 activation that is critical for the processes of actin polymerization and contractility in airway smooth muscle.

ABSTRACT

Rho kinase (ROCK), a RhoA GTPase effector, can regulate the contraction of airway and other smooth muscle tissues. In some tissues, ROCK can inhibit myosin regulatory light chain (RLC) phosphatase, which increases the phosphorylation of myosin RLC and promotes smooth muscle contraction. ROCK can also regulate cell motility and migration by affecting F-actin dynamics. Actin polymerization is stimulated by contractile agonists in airway smooth muscle tissues and is required for contractile tension development in addition to myosin RLC phosphorylation. We investigated the mechanisms by which ROCK regulates the contractility of tracheal smooth muscle tissues by expressing a kinase-inactive mutant of ROCK, ROCK-K121G, in the tissues or by treating them with the ROCK inhibitor H-1152P. Our results show no role for ROCK in the regulation of non-muscle or smooth muscle myosin RLC phosphorylation during contractile stimulation in this tissue. We found that ROCK regulates airway smooth muscle contraction by mediating activation of p21-activated kinase (Pak), a serine-threonine kinase, to promote actin polymerization. Pak catalyses paxillin phosphorylation on Ser273 and coupling of the GIT1-βPIX-Pak signalling module to paxillin, which activates the guanine nucleotide exchange factor (GEF) activity of βPIX towards Cdc42. Cdc42 is required for the activation of neuronal Wiskott-Aldrich syndrome protein (N-WASp), which transmits signals from Cdc42 to the Arp2/3 complex for the nucleation of actin filaments. Our results demonstrate a novel molecular function for ROCK in the regulation of Pak and Cdc42 activation that is critical for the processes of actin polymerization and contractility in airway smooth muscle.

Mapping physiological G protein-coupled receptor signaling pathways reveals a role for receptor phosphorylation in airway contraction

G protein-coupled receptors (GPCRs) are known to initiate a plethora of signaling pathways in vitro. However, it is unclear which of these pathways are engaged to mediate physiological responses. Here, we examine the distinct roles of Gq/11-dependent signaling and receptor phosphorylation-dependent signaling in bronchial airway contraction and lung function regulated through the M3-muscarinic acetylcholine receptor (M3-mAChR). By using a genetically engineered mouse expressing a G protein-biased M3-mAChR mutant, we reveal the first evidence, to our knowledge, of a role for M3-mAChR phosphorylation in bronchial smooth muscle contraction in health and in a disease state with relevance to human asthma. Furthermore, this mouse model can be used to distinguish the physiological responses that are regulated by M3-mAChR phosphorylation (which include control of lung function) from those responses that are downstream of G protein signaling. In this way, we present an approach by which to predict the physiological/therapeutic outcome of M3-mAChR-biased ligands with important implications for drug discovery.

ROCK insufficiency attenuates ozone-induced airway hyperresponsiveness in mice

Ozone causes airway hyperresponsiveness (AHR) and pulmonary inflammation. Rho kinase (ROCK) is a key regulator of smooth muscle cell contraction and inflammatory cell migration. To determine the contribution of the two ROCK isoforms ROCK1 and ROCK2 to ozone-induced AHR, we exposed wild-type, ROCK1(+/-), and ROCK2(+/-) mice to air or ozone (2 ppm for 3 h) and evaluated mice 24 h later. ROCK1 or ROCK2 haploinsufficiency did not affect airway responsiveness in air-exposed mice but significantly reduced ozone-induced AHR, with a greater reduction in ROCK2(+/-) mice despite increased bronchoalveolar lavage (BAL) inflammatory cells in ROCK2(+/-) mice. Compared with wild-type mice, ozone-induced increases in BAL hyaluronan, a matrix protein implicated in ozone-induced AHR, were lower in ROCK1(+/-) but not ROCK2(+/-) mice. Ozone-induced increases in other inflammatory moieties reported to contribute to ozone-induced AHR (IL-17A, osteopontin, TNFα) were not different in wild-type vs. ROCK1(+/-) or ROCK2(+/-) mice. We also observed a dose-dependent reduction in ozone-induced AHR after treatment with the ROCK1/ROCK2 inhibitor fasudil, even though fasudil was administered after induction of inflammation. Ozone increased pulmonary expression of ROCK2 but not ROCK1 or RhoA. A ROCK2 inhibitor, SR3677, reduced contractile forces in primary human airway smooth muscle cells, confirming a role for ROCK2 in airway smooth muscle contraction. Our results demonstrate that ozone-induced AHR requires ROCK. Whereas ROCK1-dependent changes in hyaluronan may contribute to ROCK1's role in O3-induced AHR, the role of ROCK2 is downstream of inflammation, likely at the level of airway smooth muscle contraction.

IL-1β: a key modulator in asthmatic airway smooth muscle hyper-reactivity

Asthma is a chronic inflammatory disorder of the airway. It is characterized by airway hyper-reactivity, which can be attributed to the chronically inflamed airway. However, the molecular mechanism is still under investigation. In this article, we have shown that IL-1β is a key molecule that can orchestrate both Toll-like receptor and muscarinic receptor pathways, and that antagonizing the function of IL-1β has a promising future as a potential drug target for asthma treatment. IL-1β can activate NF-κB pathways via Toll-like receptors, and NF-κB will eventually transactivate the genes of cytokines, chemokines, proteins of the complement system, adhesion molecules and immune receptors involved in inflammation. IL-1β can activate eosinophils, which can release major basic protein (MBP) to antagonize the M2 receptors leading to excessive acetylcholine release. Acetylcholine has an effect on M3 receptors, which are related to airway smooth muscle contraction and mucus production. IL-1β is reported to activate COX-2 resulting in heterologous desensitization of adenylate cyclase and impairs relaxation of the ASM. IL-1β is involved in mediation of neutrophilic inflammation. Identification of the prominent role of IL-1β in asthma could lead to successful use of anti-IL1β agents.

The pharmacological rationale for combining muscarinic receptor antagonists and β-adrenoceptor agonists in the treatment of airway and bladder disease

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Muscarinic receptors increase smooth muscle tone in airways and urinary bladder.

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β-Adrenoceptors relax smooth muscle tone and oppose muscarinic contraction.

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Opposition involves transmitter release, signal transduction and receptor expression.

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This supports the combined use of muscarinic antagonists and β-adrenoceptor agonists.

Muscarinic receptor antagonists and β-adrenoceptor agonists are used in the treatment of obstructive airway disease and overactive bladder syndrome. Here we review the pharmacological rationale for their combination. Muscarinic receptors and β-adrenoceptors are physiological antagonists for smooth muscle tone in airways and bladder. Muscarinic agonism may attenuate β-adrenoceptor-mediated relaxation more than other contractile stimuli. Chronic treatment with one drug class may regulate expression of the target receptor but also that of the opposing receptor. Prejunctional β₂-adrenoceptors can enhance neuronal acetylcholine release. Moreover, at least in the airways, muscarinic receptors and β-adrenoceptors are expressed in different locations, indicating that only a combined modulation of both systems may cause dilatation along the entire bronchial tree. While all of these factors contribute to a rationale for a combination of muscarinic receptor antagonists and β-adrenoceptor agonists, the full value of such combination as compared to monotherapy can only be determined in clinical studies.

Targeting the mevalonate cascade as a new therapeutic approach in heart disease, cancer and pulmonary disease

The cholesterol biosynthesis pathway, also known as the mevalonate (MVA) pathway, is an essential cellular pathway that is involved in diverse cell functions. The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR) is the rate-limiting step in cholesterol biosynthesis and catalyzes the conversion of HMG-CoA to MVA. Given its role in cholesterol and isoprenoid biosynthesis, the regulation of HMGCR has been intensely investigated. Because all cells require a steady supply of MVA, both the sterol (i.e. cholesterol) and non-sterol (i.e. isoprenoid) products of MVA metabolism exert coordinated feedback regulation on HMGCR through different mechanisms. The proper functioning of HMGCR as the proximal enzyme in the MVA pathway is essential under both normal physiologic conditions and in many diseases given its role in cell cycle pathways and cell proliferation, cholesterol biosynthesis and metabolism, cell cytoskeletal dynamics and stability, cell membrane structure and fluidity, mitochondrial function, proliferation, and cell fate. The blockbuster statin drugs ('statins') directly bind to and inhibit HMGCR, and their use for the past thirty years has revolutionized the treatment of hypercholesterolemia and cardiovascular diseases, in particular coronary heart disease. Initially thought to exert their effects through cholesterol reduction, recent evidence indicates that statins also have pleiotropic immunomodulatory properties independent of cholesterol lowering. In this review we will focus on the therapeutic applications and mechanisms involved in the MVA cascade including Rho GTPase and Rho kinase (ROCK) signaling, statin inhibition of HMGCR, geranylgeranyltransferase (GGTase) inhibition, and farnesyltransferase (FTase) inhibition in cardiovascular disease, pulmonary diseases (e.g. asthma and chronic obstructive pulmonary disease (COPD)), and cancer.

Uridine adenosine tetraphosphate induces contraction of airway smooth muscle

Contraction of airway smooth muscle (ASM) plays an important role in the regulation of air flow and is potentially involved in the pathophysiology of certain respiratory diseases. Extracellular nucleotides regulate ASM contraction via purinergic receptors, but the signaling mechanisms involved are not fully understood. Uridine adenosine tetraphosphate (Up(4)A) contains both pyrimidine and purine moieties, which are known to potentially activate P2X and P2Y receptors. Both P2X and P2Y receptors have been identified in the lung, including airway epithelial cells and ASM. We report here a study of purinergic signaling in the respiratory system, with a focus on the effect of Up(4)A on ASM contraction. Up(4)A induced contraction of rat isolated trachea and extrapulmonary bronchi as well as human intrapulmonary bronchioles. Up(4)A-induced contraction was blocked by di-inosine pentaphosphate, a P2X antagonist, but not by suramin, a nonselective P2 antagonist. Up(4)A-induced contraction was also attenuated by α,β-methylene-ATP-mediated P2X receptor desensitization. Several P2X receptors were detected at the mRNA level: P2X1, P2X4, P2X6, and P2X7, and to a lesser extent P2X3. Furthermore, the Up(4)A response was inhibited by removal of extracellular Ca(2+) and by the presence of the L-type Ca(2+) channel blocker, nifedipine, or the Rho-associated kinase inhibitor, H1152. We conclude that Up(4)A stimulates ASM contraction, and the underlying signaling mechanism appears to involve P2X (most likely P2X1) receptors, extracellular Ca(2+) entry via L-type Ca(2+) channels, and Ca(2+) sensitization through the RhoA/Rho-associated kinase pathway. This study will add to our understanding of the pathophysiological roles of extracellular nucleotides in the lung.

Alcohol decreases RhoA activity through a nitric oxide (NO)/cyclic GMP(cGMP)/protein kinase G (PKG)-dependent pathway in the airway epithelium

BACKGROUND

Alcohol has been shown to have a number of harmful effects on the lung, including increasing the risk of pneumonia and bronchitis. How alcohol increases the risk of these diseases is poorly defined. RhoA is a small guanosine triphosphate (GTP)ase that plays an integral role in many basic functions of airway epithelial cells. It is not known how alcohol affects RhoA activity in the airway epithelium. We hypothesized that brief alcohol exposure modulates RhoA activity in the airway epithelium through a nitric oxide (NO)/cyclic GMP (cGMP)/protein kinase G (PKG)-dependent pathway.

METHODS

Primary airway epithelial cells were cultured and exposed to ethanol at various concentrations and times. The cell layers were harvested and RhoA activity was measured.

RESULTS

Alcohol induced a time- and concentration-dependent decrease in RhoA activity in airway epithelial cells. We were able to block this decrease in activity using Nω-nitro-l-arginine methyl ester (L-NAME) hydrochloride, a nitric oxide synthase (NOS) inhibitor. Likewise, we were able to demonstrate the same decrease in RhoA activation using 0.1 μM sodium nitroprusside, an NO donor. To determine the role of cGMP/PKG, we pretreated the cells with a cGMP antagonist analog, Rp-8Br-cGMPS. This blocked the decrease in RhoA activity caused by alcohol, suggesting that alcohol exerts its effect on RhoA activity through cGMP/PKG.

CONCLUSIONS

Alcohol decreases airway epithelial RhoA activity through an NO/cGMP/PKG-dependent pathway. RhoA activity controls many aspects of basic cellular function, including cell morphology, tight junction formation, and cell cycle progression and gene regulation. Dysregulation of RhoA activity can potentially have several consequences, including dysregulation of inflammation. This may partially explain how alcohol increases the risk of pneumonia and bronchitis.

Contribution of Rho kinase to the early phase of the calcium-contraction coupling in airway smooth muscle

We investigated theoretically and experimentally the role of Rho kinase (RhoK) in Ca(2+)-contraction coupling in rat airways. Isometric contraction was measured on tracheal, extrapulmonary and intrapulmonary bronchial rings. Intracellular [Ca(2+)] was recorded in freshly isolated tracheal myocytes. Stimulation by carbachol (0.3 and 10 μm) and 50 mm external KCl induced a short-time, Hill-shaped contraction obtained within 90 s, followed by a sustained or an additional delayed contraction. Responses of [Ca(2+)](i) to acetylcholine consisted in a fast peak followed by a plateau and, in 42% of the cells, superimposed Ca(2+) oscillations. The RhoK inhibitor Y27632 (10 μm) did not alter the [Ca(2+)](i) response. Whatever the agonist, Y27632 did not modify the basal tension but decreased the amplitude of the short-duration response, without altering the additional delayed contraction. The Myosin Light Chain Phosphatase (MLCP) inhibitor calyculin A increased the basal tension and abolished the effect of RhoK. KN93 (Ca(2+)-calmodulin-dependent protein kinase II inhibitor) and DIDS (inhibitor of Ca(2+)-activated Cl(-) channels) had no influence on the RhoK effect. We built a theoretical model of Ca(2+)-dependent active/inactive RhoK ratio and subsequent RhoK-dependent MLCP inactivation, which was further coupled with a four-state model of the contractile apparatus and Ca(2+)-dependent MLCK activation. The model explains the time course of the short-duration contraction and the role of RhoK by Ca(2+)-dependent activation of MLCK and RhoK, which inactivates MLCP. Oscillatory and non-oscillatory [Ca(2+)](i) responses result in a non-oscillatory contraction, the amplitude of which is encoded by the plateau value and oscillation frequency. In conclusion, Ca(2+)-dependent but CaMK II-independent RhoK activation contributes to the early phase of the contractile response via MLCP inhibition.

Should treatments for asthma be aimed at the airway smooth muscle?

The airway smooth muscle (ASM) cell is an important part of the airway wall of asthma patients because of its increased contractile properties, which appear to be enhanced in this condition and which contribute to airflow obstruction and bronchial hyper-responsiveness. ASM cells are also abnormal in asthma with increased expression of certain chemokines, with increased proliferation rate, numbers and size. beta-adrenergic agonists and corticosteroids are the two most important treatments for asthma; other drugs used are leukotriene receptor antagonists and theophylline. Combination therapy of beta-adrenergic agonists and corticosteroids has become the treatment of choice for moderate-to-severe asthma. beta-adrenergic agonists cause relaxation of ASM cells, leading to a decrease in airflow obstruction of asthma and acute relief of symptoms. Corticosteroids also have direct effects on ASM cells. It is postulated that the effect of anti-inflammatory agents on ASM cells is the most important determinant of the therapeutic effects of these agents. Targeting the ASM cell in asthma could be the focus of therapies for asthma. Specific delivery of active agents to ASM cells may also be part of this strategy.

The effects of the small GTPase RhoA on the muscarinic contraction of airway smooth muscle result from its role in regulating actin polymerization

The small GTPase RhoA increases the Ca(2+) sensitivity of smooth muscle contraction and myosin light chain (MLC) phosphorylation by inhibiting the activity of MLC phosphatase. RhoA is also a known regulator of cytoskeletal dynamics and actin polymerization in many cell types. In airway smooth muscle (ASM), contractile stimulation induces MLC phosphorylation and actin polymerization, which are both required for active tension generation. The objective of this study was to evaluate the primary mechanism by which RhoA regulates active tension generation in intact ASM during stimulation with acetylcholine (ACh). RhoA activity was inhibited in canine tracheal smooth muscle tissues by expressing the inactive RhoA mutant, RhoA T19N, in the intact tissues or by treating them with the cell-permeant RhoA inhibitor, exoenzyme C3 transferase. RhoA inactivation reduced ACh-induced contractile force by approximately 60% and completely inhibited ACh-induced actin polymerization but inhibited ACh-induced MLC phosphorylation by only approximately 20%. Inactivation of MLC phosphatase with calyculin A reversed the reduction in MLC phosphorylation caused by RhoA inactivation, but calyculin A did not reverse the depression of active tension and actin polymerization caused by RhoA inactivation. The MLC kinase inhibitor, ML-7, inhibited ACh-induced MLC phosphorylation by approximately 80% and depressed active force by approximately 70% but did not affect ACh-induced actin polymerization, demonstrating that ACh-stimulated actin polymerization occurs independently of MLC phosphorylation. We conclude that the RhoA-mediated regulation of ACh-induced contractile tension in ASM results from its role in mediating actin polymerization rather than from effects on MLC phosphatase or MLC phosphorylation.

Regulation of smooth muscle contraction by small GTPases

Next to changes in cytosolic [Ca(2+)], members of the Rho subfamily of small GTPases, in particular Rho and its effector Rho kinase, also known as ROK or ROCK, emerged as key regulators of smooth muscle function in health and disease. In this review, we will focus on the regulation of the contractile machinery by Rho/ROK signaling and its interaction with PKC and cyclic nucleotide signaling. We will briefly discuss the emerging evidence that remodeling of cortical actin is necessary for contraction.

Airway smooth muscle as a model for new investigative drugs in asthma

Bronchial asthma as such exists because airway smooth muscle (ASM) contracts excessively in response to various stimuli. After several decades during which research was mainly focused on airway inflammation, increasing attention is now being paid to a possible abnormal behaviour of ASM. Thus, ASM is regarded as a major target for anti-asthma treatments. This review first describes the mechanisms of ASM contraction and airway hyperresponsiveness, through cellular, animal and human models. The developments of new drugs targeting extra and/or intracellular pathway of ASM contraction are discussed.

Migration of airway smooth muscle cells

Migration of smooth muscle cells is a process fundamental to development of hollow organs, including blood vessels and the airways. Migration is also thought to be part of the response to tissue injury. It has also been suggested to contribute to airways remodeling triggered by chronic inflammation. In both nonmuscle and smooth muscle cells numerous external signaling molecules and internal signal transduction pathways contribute to cell migration. The review includes evidence for the functional significance of airway smooth muscle migration, a summary of promigratory and antimigratory agents, and summaries of important signaling pathways mediating migration. Important signaling pathways and effector proteins described include small G proteins, phosphatidylinositol 3-kinases (PI3-K), Rho activated protein kinase (ROCK), p21-activated protein kinases (PAK), Src family tyrosine kinases, and mitogen-activated protein kinases (MAPK). These signaling modules control multiple critical effector proteins including actin nucleating, capping and severing proteins, myosin motors, and proteins that remodel microtubules. Actin filament remodeling, focal contact remodeling and propulsive force of molecular motors are all coordinated to move cells along gradients of chemical cues, matrix adhesiveness, or matrix stiffness. Airway smooth muscle cell migration can be modulated in vitro by drugs commonly used in pulmonary medicine including beta-adrenergic agonists and corticosteroids. Future studies of airway smooth muscle cell migration may uncover novel targets for drugs aimed at modifying airway remodeling.

Role of tyrosine phosphorylation in U46619-induced vasoconstriction of pulmonary vasculature and its modulation by genistein, daidzein, and equol

We compared the effects of genistein with its structural derivatives daidzein and equol on excitation of pulmonary artery and vein. The concentration of genistein necessary to inhibit contractions evoked by U46619 (1nM-100 microM) ranged from 10 to 100 microM. Genistein (55 microM) reduced KCl-responses by approximately 50% and essentially abolished those evoked by U46619. Daidzein was much less effective against either agonist, and equol was ineffective against U46619. A23187-evoked contractions were markedly reduced by all 3 isoflavones, but caffeine-evoked contractions were not. Using the Western blot technique, we found many proteins were tyrosine phosphorylated within 30 seconds after stimulation with U46619, reaching a peak at 120 seconds and then falling at 300 seconds. One band at 110 kD was increased nearly 300% above baseline, while 3 others ranging from 60 to 80 kD were more than doubled in intensity. Genistein had little effect on baseline levels of phosphorylation but largely prevented the U46619-induced change; daidzein was much less effective in this respect, and equol did not significantly affect this phosphorylation. We conclude that these isoflavones provide powerful tools in the study of excitation-contraction coupling of pulmonary vasculature and that inhibition of tyrosine kinase activity may be useful clinically against pulmonary hypertension.

Roles of P2X receptors and Ca2+ sensitization in extracellular adenosine triphosphate-induced hyperresponsiveness in airway smooth muscle

BACKGROUND

The release of adenosine triphosphate (ATP) from the airway epithelial cells during the inflammatory process is considered to play an important role in the pathophysiology of asthma and chronic obstructive pulmonary disease.

OBJECTIVE

This study was designed to determine whether extracellular ATP is involved in the bronchial hyperresponsiveness as an interaction between epithelium and smooth muscle in the airways.

METHODS

We examined the contractile response to methacholine (MCh) before and after exposure to low concentrations (< or = 10 microm) of ATP in isolated, epithelium-denuded guinea-pig tracheal smooth muscle by measuring isometric tension. Intracellular Ca2+ concentrations ([Ca2+]i) were assessed by fluorescent intensities of fura-2.

RESULTS

MCh-induced contractile force was increased with no change in [Ca2+]i after exposure to 10 microm ATP for 15 min. The ability of ATP to enhance the MCh-induced contraction was markedly attenuated by suramin, a non-selective P2 receptor inhibitor. Pre-incubation with ATPgammaS, a non-hydrolysable analogue of ATP and alpha,beta-meATP, a P2X agonist, also enhanced the MCh-induced contraction. In contrast, uracil triphosphate, a P2Y agonist, did not affect the MCh-induced contraction. Y-27632, a Rho-kinase inhibitor, suppressed the ability of ATP to enhance the MCh-induced contraction. Moreover, PP1 and PP2, Src tyrosin kinase inhibitors, suppressed the enhancement of MCh-induced contraction by ATP.

CONCLUSION

Pre-treatment with ATP induces hyperresponsiveness to MCh mediated by Ca2+ sensitization via the P2X receptor in airway smooth muscle. The present findings suggest the possible involvement of both the Rho-kinase and Src pathways in the intracellular mechanism of this phenomenon.

Airway smooth muscle as a target of asthma therapy: history and new directions

Ultimately, asthma is a disease characterized by constriction of airway smooth muscle (ASM). The earliest approach to the treatment of asthma comprised the use of xanthines and anti-cholinergics with the later introduction of anti-histamines and anti-leukotrienes. Agents directed at ion channels on the smooth muscle membrane (Ca2+ channel blockers, K+ channel openers) have been tried and found to be ineffective. Functional antagonists, which modulate intracellular signalling pathways within the smooth muscle (beta-agonists and phosphodiesterase inhibitors), have been used for decades with success, but are not universally effective and patients continue to suffer with exacerbations of asthma using these drugs. During the past several decades, research energies have been directed into developing therapies to treat airway inflammation, but there have been no substantial advances in asthma therapies targeting the ASM. In this manuscript, excitation-contraction coupling in ASM is addressed, highlighting the current treatment of asthma while proposing several new directions that may prove helpful in the management of this disease.