Paradoxical attenuation of beta2-AR function in airway smooth muscle by Gi-mediated counterregulation in transgenic mice overexpressing type 5 adenylyl cyclase.

摘要

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 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 ( approximately 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(alphai) expression and ERK1/2 activation were markedly increased in AC5-TG (5- and 8-fold, respectively), and beta-AR expression was decreased by approximately 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.