Mechanism regulating proasthmatic effects of prolonged homologous [[beta].sub.2]-adrenergic receptor desensitization in airway smooth muscle

Use of long-acting [[beta].sub.2]-adrenergic receptor ([beta]2AR) agonists to treat asthma incurs an increased risk of asthma morbidity with impaired bronchodilation and heightened bronchoconstriction, reflecting the adverse effects of prolonged homologous [beta]2AR desensitization on airway smooth muscle (ASM) function. Since phosphodiesterase 4 (PDE4) regulates ASM relaxation and contractility, we examined whether the changes in ASM function induced by prolonged homologous [beta]2AR desensitization are attributed to altered expression and action of PDE4. Cultured human ASM cells and isolated rabbit ASM tissues exposed for 24 h to the long-acting [beta]2AR agonist salmetero! exhibited impaired acute [beta]2AR-mediated cAMP accumulation and relaxation, respectively, together with ASM constrictor hyperresponsiveness. These proasthmatic-like changes in ASM function were associated with upregulated PDE4 activity due to enhanced expression of the PDE4D5 isoform and were prevented by pretreating the ASM preparations with the PDE4 inhibitor rolipram or with inhibitors of either PKA or ERK1/2 signaling. Extended studies using gene silencing and pharmacological approaches demonstrated that: 1) the mechanism underlying upregulated PDE4D5 expression following prolonged [beta]2AR agonist exposure involves PKA-dependent activation of [G.sub.i] protein signaling via its [beta][gamma]-subunits, which elicits downstream activation of ERKI/2 and its induction of PDE4D5 transcription; and 2) the induction of PDE4 activity and consequent changes in ASM responsiveness are prevented by pretreating the [beta]2AR agonist-exposed ASM preparations with inhibitors of [G.sub.i]-[beta][gamma] signaling. Collectively, these findings identify that the proasthmatic changes in ASM function resulting from prolonged homologous [beta]2AR desensitization are attributed to upregulated PDE4 expression induced by [G.sub.i]-[beta][gamma]-mediated cross-talk between the PKA and ERK1/2 signaling pathways. asthma; long-acting [[beta].sub.2]-agonists; salmeterol; cAMP signaling; G proteins; phosphodiesterase-4 doi: 10.1152/ajplung.00079.2009.