Paradoxical attenuation of β2-AR function in airway smooth muscle by Gi-mediated counterregulation in transgenic mice overexpressing type 5 adenylyl cyclase.

Wang WC, Schillinger RM, Malone MM, Liggett SB. Paradoxical attenuation of beta(2)-AR function in airway smooth muscle by G(i)-mediated counterregulation in transgenic mice overexpressing type 5 adenylyl cyclase. Am J Physiol Lung Cell Mol Physiol 300: L472-L478, 2011. First published December 3, 2010; doi:10.1152/ajplung.00273.2010.-The limiting component within the receptor-G protein-effector complex in airway smooth muscle (ASM) for beta(2)-adrenergic receptor (beta(2)-AR)-mediated relaxation is unknown. In cardiomyocytes, adenylyl cyclase (AC) is considered the "bottleneck" for beta-AR signaling, and gene therapy trials are underway to increase inotropy by increasing cardiac AC expression. We hypothesized that increasing AC in ASM would increase relaxation from beta-agonists, thereby providing a strategy for asthma therapy. Transgenic (TG) mice were generated with approximately two-to threefold overexpression of type 5 AC (AC5) in ASM. cAMP and airway relaxation in response to direct activation of AC by forskolin were increased in AC5-TG. Counter to our hypothesis, isoproterenol-mediated airway relaxation was significantly attenuated (similar to 50%) in AC5-TG, as was cAMP production, suggesting compensatory regulatory events limiting beta(2)-AR signaling when AC expression is increased. In contrast, acetylcholine-mediated contraction was preserved. G(alpha i) expression and ERK1/2 activation were markedly increased in AC5-TG (5- and 8-fold, respectively), and beta-AR expression was decreased by similar to 40%. Other G proteins, G protein-coupled receptor kinases, and beta-arrestins were unaffected. beta-agonist-mediated airway relaxation of AC5-TG was normalized to that of nontransgenic mice by pertussis toxin, implicating beta(2)-AR coupling to the increased G(i) as a mechanism of depressed agonist-promoted relaxation in these mice. The decrease in beta(2)-AR may account for additional relaxation impairment, given that there is no enhancement over nontransgenic after pertussis toxin, despite AC5 overexpression. ERK1/2 inhibition had no effect on the phenotype. Thus perturbing the ratio of beta(2)-AR to AC in ASM by increasing AC fails to improve (and actually decreases) beta-agonist efficacy due to counterregulatory events.