Transforming Growth Factor-b1 Decreases b₂-Agonist–induced Relaxation in Human Airway Smooth Muscle

Helper T effector cytokines implicated in asthma modulate the contractility of human airway smooth muscle (HASM) cells. We have reported recently that a profibrotic cytokine, transforming growth factor (TGF)-b1, induces HASM cell shortening and airway hyperresponsiveness. Here, we assessed whether TGF-b1 affects the ability of HASM cells to relax in response to b₂-agonists, a mainstay treatment for airway hyperresponsiveness in asthma. Overnight TGF-b1 treatment significantly impaired isoproterenol (ISO)induced relaxation of carbachol-stimulated, isolated HASM cells. This single-cell mechanical hyporesponsiveness to ISO was corroborated by sustained increases in myosin light chain phosphorylation. In TGF-b1–treated HASM cells, ISO evoked markedly lower levels of intracellular cAMP. These attenuated cAMP levels were, in turn, restored with pharmacological and siRNA inhibition of phosphodiesterase 4 and Smad3, respectively. Most strikingly, TGF-b1 selectively induced phosphodiesterase 4D gene expression in HASM cells in a Smad2/3-dependent manner. Together, these data suggest that TGF-b1 decreases HASM cell b₂-agonist relaxation responses by modulating intracellular cAMP levels via a Smad2/3-dependent mechanism. Our findings further define the mechanisms underlying b₂-agonist hyporesponsiveness in asthma, and suggest TGF-b1 as a potential therapeutic target to decrease asthma exacerbations in severe and treatment-resistant asthma.