β-Agonists and asthma: too much of a good thing?

Abstract

In an unusual paradox, asthmatics who are chronically treated with bronchodilating β-agonists sometimes experience a worsening of their condition. A new study describes one possible mechanism and reveals a potential new therapeutic target in the treatment of asthma.

Inhaled selective β₂-agonists are the most widely used treatment for the acute relief of asthma symptoms. In patients with asthma, these agents cause bronchodilation (improvement in lung mechanics) and bronchoprotection (reduced responsiveness to nonspecific contractile stimuli). These actions result from binding to the β₂-adrenergic receptor (β₂AR), a heptahelical receptor that couples predominantly to the stimulatory G protein, G_s (Figure 1a). Once activated by receptor-ligand binding, the α subunit of G_s activates adenylyl cyclase (AC), resulting in cAMP formation. cAMP phosphorylates protein kinase A (PKA), which phosphorylates multiple proteins, leading to reductions in intracellular calcium, smooth muscle relaxation, and bronchodilation.

Figure 1.

(a) Mechanism of β-agonist–induced airway smooth muscle relaxation. Ligand binding to the β₂AR activates G_s, leading to adenylyl cyclase (AC) activation, cAMP formation, and subsequent protein kinase A (PKA) activation. PKA phosphorylation of target proteins leads to smooth muscle relaxation and may also inhibit ERK activation. PKA also phosphorylates the β₂AR, leading to increased G_i coupling. In addition, ligand binding causes G protein receptor kinase (GRK) phosphorylation of the β₂AR, recruiting β-arrestin (ARR). (b) Inflammatory events in the asthmatic airway or regular β-agonist use may augment G_i coupling and/or increase β-arrestin binding. Under these circumstances, β-agonists may result in ERK activation, potentially amplifying production of inflammatory cytokines and leading to airway remodeling. Persistent activation of the β₂AR may also lead to phospholipase C-β (PLCβ) expression and consequent airway hyperresponsiveness (8).

It is possible to have too much of a good thing. Despite the ability of β-agonists to effect immediate reversal of airway obstruction, there has been continuing concern that regular use of these drugs may be associated with adverse outcomes. In some, but not all, studies, regularly scheduled use (e.g., multiple times per day, every day) of inhaled β-agonists has resulted in loss of asthma control, declines in morning peak flow, longer durations of asthma exacerbations, and rebound airway hyperresponsiveness (1–6). These adverse effects appear to be particularly important with β-agonists of high intrinsic efficacy, like fenoterol, and in patients with certain β₂AR genotypes.

The common explanation for these observations is that the adverse effects of regular β-agonist therapy are related to desensitization of the β₂AR (2, 3, 5, 7). The data reported by McGraw et al. in this issue of the JCI (8) suggest an alternative explanation, namely that persistent high-level activation of the β₂AR leads to increased expression of phospholipase C-β (PLCβ) in airway smooth muscle. Since agonists such as acetylcholine, histamine, and leukotrienes that cause airway smooth muscle contraction do so by acting on receptors that couple to G_q and activate PLCβ, chronic β-agonists might augment the effects of these bronchoconstrictors. How could this occur? PLCβ hydrolyzes phosphatidylinositol 4,5 biphosphate, resulting in inositol 1,4,5 trisphosphate (IP₃) production. IP₃ increases intracellular calcium, leading to activation of myosin light chain kinase, myosin phosphorylation, and consequent muscle contraction. If the observations of McGraw et al. are borne out, this mechanism could explain the adverse effects of chronic stimulation of the β₂AR. However, before we embrace this hypothesis, which is based on data from mouse airways, it will be important to confirm the observations of McGraw et al. in human airway smooth muscle stimulated with β-agonists rather than by β₂AR overexpression.

We think that there may be additional explanations for the bronchoconstrictor effects of chronic β₂AR stimulation. For example, in some cell types the β₂AR can couple to G_i as well as G_s (9–12). The switch from G_s to G_i coupling appears to be regulated by PKA phosphorylation of the β₂AR (13). β₂AR-induced G_i activation leads to activation of the extracellular signal–regulated kinase (ERK) MAPKs (Figure 1b) through a pathway dependent on the G_i βγ subunits and activation of Ras (12). β₂AR activation also leads to G protein receptor kinase–induced (GRK-induced) phosphorylation of the β₂AR, resulting in its interaction with β-arrestin. β-arrestin can serve as a scaffolding protein linking the β₂AR to both the ERK and JNK MAPK pathways (9, 14). To complicate the matter, G_s-dependent formation of cAMP by β₂AR activation can also inhibit ERK activation (10), apparently as a result of Raf-1 phosphorylation by PKA (15). The ultimate effect of β₂AR activation on ERK phosphorylation is likely to reflect a balance of these various events.

Although these pathways have been described in other cell types, none of these events has been examined in airway smooth muscle cells. Thus it is interesting to consider the potential functional implications of β-agonist–induced ERK activation in these cells. Persistent ERK activation is known to be required for mitogenesis in airway smooth muscle cells (16). Activation of ERK is also required for the full expression by airway smooth muscle of the eosinophil chemotactic factors eotaxin and RANTES, as well as other chemokines (17). β-agonists normally inhibit mitogenesis (18) and inhibit expression of eotaxin (19) in airway smooth muscle. However, there may be conditions in which this is not the case. For example, G_s-to-G_i switching could be exaggerated by inflammatory cytokines, which have been shown to increase G_i expression (20). Inflammatory cytokines have also been shown to increase GRK expression in these cells (20). Increased GRK activation could be expected to enhance ERK activation through effects on β-arrestin binding to the β₂AR. It is also possible that the balance of ERK-activating and ERK-inhibiting effects of β₂AR activation may be affected by β₂AR desensitization under conditions of regular β-agonist use.

It is interesting to note that the adverse effects of regular β-agonist use can be observed even weeks after their withdrawal, at a time when β₂AR desensitization should be resolved (2). If regular use of β-agonists negatively impacts the balance of factors contributing to airway smooth muscle mitogenesis or airway remodeling in asthma, it could have long-term consequences. Importantly, studies of regular use of β-agonist have reported adverse effects only in subjects homozygous for the Arg16 variant of the β₂AR (2, 3, 5). The impact of β₂AR polymorphisms on non–G_s-mediated events such as ERK phosphorylation has not been examined in any cell type.

β-agonists are currently one of the most important forms of therapy for asthma. Little is known about non–G_s-mediated effects of β₂AR activation and other events that could negatively impact the function of airway smooth muscle in asthma. In this issue of the JCI, McGraw et al. (8) report one such event, induction of PLCβ, but it is possible that this is just the tip of the iceberg. Understanding the panoply of β₂AR-mediated events in airway smooth muscle might lead to the design of new agonists that avoid negative effects of β₂AR activation while enhancing events that lead to relaxation.