Asthma and lower airway disease
Mechanism of glucocorticoid protection of airway smooth muscle from proasthmatic effects of long-acting β2-adrenoceptor agonist exposure

https://doi.org/10.1016/j.jaci.2010.02.007Get rights and content

Background

Chronic use of long-acting β2-adrenergic receptor agonists (LABAs), resulting in β2-adrenergic receptor desensitization, has been associated with increased asthma morbidity. When LABAs are used in combination with inhaled glucocorticoids, however, asthma control is improved, raising the following question: Do glucocorticoids inhibit the proasthmatic mechanism that mediates altered contractility in LABA-exposed airway smooth muscle (ASM)?

Objective

This study aimed to identify the potential protective role and mechanism of action of glucocorticoids in mitigating the effects of prolonged LABA exposure on ASM constrictor and relaxation responsiveness.

Methods

Cultured human ASM cells and isolated rabbit ASM tissues were examined for induced changes in agonist-mediated cyclic adenosine monophosphate accumulation, constrictor and relaxation responsiveness, and expression of specific glucocorticoid-regulated molecules after 24-hour exposure to the LABA salmeterol in the absence and presence of dexamethasone.

Results

Salmeterol-exposed ASM exhibited impaired cyclic adenosine monophosphate and relaxation responses to isoproterenol and increased acetylcholine-induced contractility. These proasthmatic effects of prolonged LABA exposure were attributed to upregulated phosphodiesterase 4 (PDE4) activity and were ablated by pretreatment with dexamethasone. Further studies demonstrated that (1) dexamethasone suppressed activation of the mitogen-activated protein kinases extracellular signal-regulated kinases 1 and 2 (ERK1/2), which upregulate PDE4 expression in salmeterol-exposed ASM; and (2) the inhibitory actions of dexamethasone on salmeterol-induced ERK1/2 activation and resultant PDE4-mediated changes in ASM responsiveness were prevented by gene silencing or pharmacologic inhibition of dexamethasone-induced expression of mitogen-activated protein kinase phosphatase 1, an endogenous deactivator of ERK1/2 signaling.

Conclusion

Glucocorticoids prevent the adverse proasthmatic effects of prolonged LABA exposure on airway responsiveness as a result of glucocorticoid-induced upregulation of mitogen-activated protein kinase phosphatase 1, which inhibits proasthmatic ERK1/2 signaling in the LABA-exposed ASM.

Section snippets

Materials

Human ASM cells were obtained from Bio Whittaker, Inc (Walkersville, Md), and all chemicals were purchased from Sigma-Aldrich (St Louis, Mo) unless otherwise indicated.

Animals

Sixteen adult New Zealand White rabbits were used in this study, which was approved by the Biosafety and Animal Research Committee of the Research Institute at Children's Hospital of Philadelphia.

Culture and treatment of cells

Human ASM cells were cultured in growth medium supplemented with 10% FBS (Bio Whittaker, Inc, Walkersville, Md) and maintained in an

Glucocorticoid prevention of homologous ß2AR desensitization in HASM cells

To evaluate the role of glucocorticoids in modulating ASM function in the ß2AR-desensitized state, we initially examined the effect of pretreatment with dexamethasone on isoproterenol-induced cAMP accumulation in HASM cells after their prolonged homologous ß2AR desensitization. Accordingly, acute changes in intracellular cAMP accumulation, detected at 5 minutes after administration of a near half-maximal effective concentration of isoproterenol (1.0 μmol/L),13, 14 were compared in HASM cells

Discussion

Chronic use of LABAs to treat asthma has been associated with impaired bronchodilation, heightened bronchoconstriction, and an increase in asthma morbidity,1, 2, 3 presumably reflecting the adverse effects of prolonged homologous β2AR desensitization of the airways.4 Although mechanisms implicated in mediating acute homologous desensitization of the β2AR in ASM are well described, including uncoupling of the β2AR from the stimulatory G protein, Gs, as a result of its phosphorylation by G

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      These differences might be explained simply by the different agonists used. In another series of reports, Grunstein and co-workers showed that chronic exposure of ASM to salmeterol induced transcriptional up-regulation of phosphodiesterase 4D5, which promotes cAMP degradation, through a Gαi-Erk-MKP1 dependent mechanism (20, 41–42). Although procontractile signaling elements such as IP3 and Ca2+ flux in ASM cells exposed to salmeterol were not examined in these studies, rabbit tracheas treated with salmeterol contracted more to acetylcholine, which was reversed by either glucocorticoid or phosphodiesterase inhibitor pretreatment.

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    Supported by NIH grants HL-031467, HL-061038, and HL097739.

    Disclosure of potential conflict of interest: The authors have declared that they have no conflict of interest.

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