Paradoxical attenuation of β2-AR function in airway smooth muscle by Gi-mediated counterregulation in transgenic mice overexpressing type 5 adenylyl cyclase.
The limiting component within the receptor-G protein-effector complex in airway smooth muscle (ASM) for β(2)-adrenergic receptor (β(2)-AR)-mediated relaxation is unknown. In cardiomyocytes, adenylyl cyclase (AC) is considered the "bottleneck" for β-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 β-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 (∼50%) in AC5-TG, as was cAMP production, suggesting compensatory regulatory events limiting β(2)-AR signaling when AC expression is increased. In contrast, acetylcholine-mediated contraction was preserved. G(αi) expression and ERK1/2 activation were markedly increased in AC5-TG (5- and 8-fold, respectively), and β-AR expression was decreased by ∼40%. Other G proteins, G protein-coupled receptor kinases, and β-arrestins were unaffected. β-agonist-mediated airway relaxation of AC5-TG was normalized to that of nontransgenic mice by pertussis toxin, implicating β(2)-AR coupling to the increased G(i) as a mechanism of depressed agonist-promoted relaxation in these mice. The decrease in β(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 β(2)-AR to AC in ASM by increasing AC fails to improve (and actually decreases) β-agonist efficacy due to counterregulatory events.
Dept. of Medicine, Univ. of Maryland School of Medicine, Baltimore, MD 21201, USA.
Wayne C H
WC
Wang
Rachel M
RM
Schillinger
Molly M
MM
Malone
Stephen B
SB
Liggett
D000262:Adenylyl Cyclases / Q000378:metabolism*; D058666:Adrenergic beta-2 Receptor Agonists / Q000494:pharmacology; D000818:Animals; D000242:Cyclic AMP / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism*; D066298:In Vitro Techniques; D051379:Mice; D008822:Mice, Transgenic; D009126:Muscle Relaxation / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000201:enzymology / Q000502:physiology*; D032389:Myocytes, Smooth Muscle / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism*; D014132:Trachea / Q000187:drug effects / Q000502:physiology*
D058666:Adrenergic beta-2 Receptor Agonists; D018343:Receptors, Adrenergic, beta-2; D000242:Cyclic AMP; D048049:Extracellular Signal-Regulated MAP Kinases; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D000262:Adenylyl Cyclases; C419510:adenylyl cyclase type V