Transforming Growth Factor-β1 Decreases β2-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)-beta 1, induces HASM cell shortening and airway hyperresponsiveness. Here, we assessed whether TGF-beta 1 affects the ability of HASM cells to relax in response to beta(2)-agonists, a mainstay treatment for airway hyperresponsiveness in asthma. Overnight TGF-beta 1 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-beta 1-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-beta 1 selectively induced phosphodiesterase 4D gene expression in HASM cells in a Smad2/3-dependent manner. Together, these data suggest that TGF-beta 1 decreases HASM cell beta(2)-agonist relaxation responses by modulating intracellular cAMP levels via a Smad2/3-dependent mechanism. Our findings further define the mechanisms underlying beta(2)-agonist hyporesponsiveness in asthma, and suggest TGF-beta 1 as a potential therapeutic target to decrease asthma exacerbations in severe and treatment-resistant asthma.