Airway smooth muscle in the pathophysiology and treatment of asthma

Address for reprint requests and other correspondence: D. Doeing, Univ. of Chicago, 5841 S. Maryland Ave, MC6076, Chicago, IL 60637 (e-mail: ude.slatipsohcu@gnieod.anaid).

Abstract

Airway smooth muscle (ASM) plays an integral part in the pathophysiology of asthma. It is responsible for acute bronchoconstriction, which is potentiated by constrictor hyperresponsiveness, impaired relaxation and length adaptation. ASM also contributes to airway remodeling and inflammation in asthma. In light of this, ASM is an important target in the treatment of asthma.

asthma is a chronic disease of the airways affecting over 24 million people in the United States (94). 1 It is characterized by intermittent airflow obstruction and airway inflammation, producing symptoms of chest tightness, wheezing, and cough. Structural and inflammatory changes throughout the airway wall lead to bronchial thickening and edema as well as increased mucus production and bronchoconstriction, all of which contribute to the episodic airflow obstruction typically found in asthma ( Fig. 1 ). In recent years, there has been much attention on inflammation in asthma, for example, whether the type of inflammatory cell predominately found in the airway denotes a specific phenotype in asthma or whether targeting antibodies, inflammatory cytokines, and inflammatory cells is helpful for the treatment of asthma (56, 96, 115). Although airway smooth muscle (ASM) has been implicated in constrictor hyperresponsiveness in asthma for decades, other important roles of ASM have recently been identified. Through impaired airway relaxation and mediation of structural changes and inflammatory signaling, the ASM cell plays multiple diverse roles in the pathophysiology of asthma. This review is meant to update practicing physicians with current knowledge about these less discussed direct effects of ASM on airway narrowing and indirect influences on airway remodeling and inflammation in asthma ( Fig. 2 ).

Multiple mechanisms of airflow obstruction in asthma, including bronchoconstriction by airway muscle, obstruction of airflow by intraluminal mucus, and inflammation and remodeling of the airway wall. [Adapted with permission from the National Heart, Lung, and Blood Institute.]

Airway smooth muscle (ASM) contributes in multiple ways to the pathogenesis of asthma, including direct causation of airflow obstruction through contraction and indirect promotion of airflow obstruction through its participation in airway remodeling and as a modulator of airway inflammation. Each of these processes interacts with the other, so that the net contribution of ASM to asthma is multifold and complex. IgE, immunoglobulin E.

FUNCTIONAL CHANGES IN ASTHMATIC ASM

ASM is most well known for its role in acute bronchoconstriction. Smooth muscle surrounds the airway in a circumferential pattern, reducing the airway luminal diameter as it contracts. It is this function of ASM that causes the acute airflow obstruction, shortness of breath, and wheezing most commonly associated with the clinical syndrome of asthma. In asthma, ASM is primed to contract, often excessively, in response to various stimuli, but in addition, it resists relaxation. These two phenomena are now discussed in turn.

Airway Hyperresponsiveness

The excessive contractile response of ASM in asthma results in inordinate bronchoconstriction and airflow obstruction in response to relatively little provocation; this phenomenon is denoted as airway hyperresponsiveness. A variety of chemical and physical stimuli can trigger bronchoconstriction ( Table 1 ). Contractile agonists, like methacholine, can directly activate receptors on ASM cells that initiate myocyte contraction and consequent bronchoconstriction, and this is the basis of the methacholine challenge test sometimes used to diagnose asthma. Substantial respiratory heat and water loss, as occur during exercise in temperate or cold climates, can also provoke bronchoconstriction, likely mediated by contractile agonists released from mast cells or nerves exposed to a hyperosmolar milieu (4, 44). Why individuals with asthma are hyperresponsive compared with healthy individuals is complex. Contributing mechanisms include both an increased availability in asthmatic airways of contractile mediators such as histamine from mast cells (12) and increased ASM mass. Mathematical models initially proposed that excessive ASM generates abnormally increased force (77), but increased dynamic muscle stiffness (due to impaired breathing-induced muscle softening) may actually be the more important mechanism (102). Other explanations for airway hyperresponsiveness in asthma include increased vagal tone (13, 26), cytokine-potentiated increases in intracellular free calcium that enhance ASM cell contractility (51) and activation of the procontractile Rho kinase pathway (19, 124). Increased RhoA protein levels have also been identified in animal models of allergic asthma (20, 21, 123), and inhibition of the RhoA-Rho kinase pathway can prevent or reverse airway hyperresponsiveness ( Table 2 ) (121, 122).

Table 1.

List of stimuli provoking bronchoconstriction in asthma

NSAID, nonsteroidal anti-inflammatory drug; ASM, airway smooth muscle.

Table 2.

Potential new therapeutic targets of ASM function in asthma

AHR, airway hyperresponsiveness; CPAP, continuous positive airway pressure; FDA, Food and Drug Administration.

Impaired Relaxation

Aberrant shortening of ASM in asthma can also partly be explained by inadequate relaxation. There is no direct sympathetic innervation in human ASM (17). Additionally, β₂-adrenergic receptors may become downregulated when bombarded frequently with β₂-agonist medications, in a fashion that depends in part on genetic polymorphisms (69). This phenomenon has been observed in a number of cell types, including ASM (50, 93), and may further potentiate the imbalance of autonomic neurotransmitter influences on the airway in asthma. In fact, patients with asthma can develop tolerance to β-agonist therapy (57). Inflammatory cytokines further potentiate this effect. Interleukin-1β (IL-1β), a cytokine produced by a variety of lung cells, reduces β-adrenergic responsiveness in cultured human ASM cells (79). Another important inflammatory cytokine, IL-13, has a similar effect on adrenergic receptors, albeit through a different mechanism (78). Prevention or reduction of β₂-adrenergic receptor desensitization with agents that preserve receptor expression and function, such as ascorbate (31) or alendronate (67), might conceivably be helpful in the treatment of asthma.

Breathing-Induced Reversal of Bronchoconstriction and Length Adaptation

Other properties of ASM function may also contribute to airway narrowing in asthma. It has long been observed that in normal people, deep breathing can reverse bronchoconstriction (3, 98). Within the lung, the airways are connected to the lung parenchyma, which is firmly attached to their adventitial surfaces. With each inspiration, the lung parenchyma surrounding each airway expands and pulls radially outward on the airway, stretching it partially during the breath. Isolated ASM that is forcibly lengthened while still contracting reduces its force of contraction (49), likely due to perturbation of actin-myosin interactions (41). Furthermore, fluctuations in the force applied to isolated contracting smooth muscle, simulating the tidal stretches that occur with breathing, cause it to relengthen, even when the mean force is held constant (33, 76, 80). As a result of these behaviors of ASM, stretching of bronchoconstricted airways by breathing in part reverses the lumenal narrowing that had been present (102). However, in individuals with asthma, the ability of deep breaths to reverse bronchoconstriction seems blunted; this might stem from increased muscle mass and increased muscle stiffness as noted above or might reflect other mechanisms that are not yet fully understood. Nonetheless, the role of airway distension during a deep breath seems certain, for pronounced bronchoconstrictor responses to methacholine, similar to those of patients with asthma, can be elicited in normal subjects when deep breathing is restricted (127). Recently, we and others have found pharmacological interventions that potentiate ASM relengthening in response to force fluctuations in vitro (32, 33, 75, 76). Pharmacologically potentiating the ability of deep inspirations to reverse bronchoconstriction might represent a novel therapeutic approach to relieve or prevent airflow obstruction in the future. Indeed, corticosteroids, a mainstay of asthma treatment, might exert some of their therapeutic effect through this mechanism (72).

Another inherent property of smooth muscle may in part explain why asthmatic airways are not as susceptible to deep breathing-induced stretch during an asthma attack. When a smooth muscle cell shortens, it adapts by reorganizing its contractile filaments in a way that will allow it to generate the same force at its new length (125). This can create a vicious cycle in asthma, because once ASM has shortened, it quickly becomes primed to shorten further (89). Now more severely shortened, the muscle may be stiffer and less influenced by the antibronchoconstrictor effects of deep breathing (102). In sheep trachealis, passive stiffness of ASM is also increased at shorter lengths (10). Indeed, chronic application of continuous positive airway pressure (CPAP; which presumably chronically holds airway muscle at greater length) reduced ASM contractility in ferrets (149), reduced airway hyperresponsiveness in rabbits with experimental asthma induced by ovalbumin sensitization and challenge (148), and most importantly reduced airway constrictor responsiveness in volunteers with stable asthma and normal spirometry (15). But if CPAP reduces airway hyperresponsiveness, why doesn't the pulmonary hyperinflation that accompanies acute asthma attacks simply reverse the bronchoconstriction? We speculate that the pulmonary hyperinflation that attends acute asthma attacks may be less effective in reversing bronchoconstriction because the patients' tidal volumes are likely reduced by the dynamic hyperinflation. If dynamic stretch is more important than mean stretch in determining overall airway smooth muscle shortening [as appears to be the case (40, 41)], then the adverse effect of pulmonary hyperinflation (reducing dynamic stretch) during acute bronchoconstriction might outweigh its potential beneficial effect (enhancing mean stretch). No studies have specifically addressed this balance in acute asthma to date, and the potential for CPAP treatment to reduce asthma symptoms or medication use is currently under study.

STRUCTURAL CHANGES IN ASM IN ASTHMA: REMODELING

Remodeling of the airway refers to pathologic changes such as increased ASM mass, basement membrane thickening, and mucus gland hyperplasia (1, 66) ( Fig. 3 ). These are common features of asthma that contribute to airflow obstruction both by luminal encroachment and by enhancing constrictor hyperresponsiveness. In addition, bronchoconstriction per se appears to promote airway remodeling (46). Thus airway inflammation and bronchoconstriction might participate in a vicious cycle that maintains the structural abnormalities characteristic of asthma. Here we will focus on the pathophysiology of increased ASM mass in asthma.

Photomicrograph of a normal airway (top) compared with the airway of an asthmatic (bottom) showing marked thickening of sub-basement membrane (SBM), submucosal eosinophils (E), smooth muscle (SM) hyperplasia, and mucus filling the airway lumen (AL). Note that contraction of the asthmatic airway may contribute in part to the increase in apparent thickness of airway wall compartments. [Images courtesy of Aliya N. Husain, MD.]

At the outset, we should note that assessing ASM mass in living subjects is complicated by the nonrandom nature of endobronchial biopsies and the inability to sample the entire airway (61). Also, ASM abundance varies along the length of the airway and with age (63). To help create an internal reference when measuring ASM in bronchial biopsies, some studies have quantified ASM mass as a percentage of airway wall subepithelial tissue (112). Confocal bronchoscopy has recently been used to assess airway wall structure without the need for biopsy and holds promise for allowing assessment of ASM mass (97, 135).

Increased ASM Mass in Asthma

Increased ASM mass has been identified as a hallmark of asthma, and its abundance is particularly great in fatal (64) or severe (6) asthma. There is much debate, however, about the mechanism driving its excess accumulation. Both increased ASM cell size (hypertrophy) and cell number (hyperplasia) have been described (37, 65), with hypertrophy predominant in some subjects and hyperplasia characteristic of others. One study found that ASM hypertrophy was significantly increased in individuals with severe asthma (6), and studies in cell culture suggest that the PI3K-Akt-mTOR-p70 S6 kinase pathway is involved (30, 53, 85). However, most research has focused on the mechanism of ASM hyperplasia (58). Stimuli that induce hyperplasia in cultured ASM cells include growth factors such as transforming growth factor (TGF-β₁), epidermal growth factor, platelet-derived growth factor (24, 59, 103, 131, 147), and contractile stimuli acting through G-protein-coupled receptors, including histamine (105) and leukotriene D4 (104). TGF-β₁ is a particularly important growth factor implicated in asthma. It is secreted by both infiltrating inflammatory cells and resident cells native to the airway (35) (including ASM; see below), and exposure to allergens increases TGF-β₁ in the bronchoalveolar lavage fluid of individuals with asthma (111). Furthermore, cultured ASM cells obtained from endobronchial biopsies of individuals with asthma demonstrate more rapid proliferation than do those from normal individuals (70). This might be explained by the absence of an antiproliferative transcription factor, C/EBPα (116). As C/EBPα also mediates the antiproliferative effect of corticosteroids in normal ASM, its reduction in asthmatic ASM may represent the underlying mechanism (116).

Another possible mechanism of increased ASM mass is the migration and differentiation of fibrocytes from bone marrow (120). Fibrocytes exposed in culture to TGF-β acquire characteristics of smooth muscle cells (120), and the number of circulating fibrocytes in the peripheral blood of individuals with asthma with chronic airflow obstruction correlates with their rate of decline in lung function over time (142). Therefore, interest has developed in TGF-β as a therapeutic target in asthma ( Table 3 ). Animal models of allergen-induced asthma have shown significant reductions in ASM proliferation following TGF-β neutralization (88) or inhibition (83).

Table 3.

Potential new therapeutic targets of ASM structural changes in asthma