Activation of the TGF-beta/activin-Smad2 pathway during allergic airway inflammation

Changes in the levels of transforming growth factor (TGF)-beta cytokines or receptors observed during the progression of several inflammatory and fibr otic disorders have been used to implicate these cytokines in the pathophys iology of these diseases. Although correlative, these studies were inconclu sive because they were unable to demonstrate actual continuous TGF-beta -me diated signaling in the involved tissues. We reasoned that the phosphorylat ion state and subcellular localization of Smad2, the intracellular effector of TGF-beta /activin-mediated signaling, could be used as a marker of acti ve signaling mediated by these cytokines in situ. We therefore used an expe rimental model of ovalbumin-induced allergic airway inflammation and were a ble to demonstrate a dramatic increase in the numbers of bronchial epitheli al, alveolar, and infiltrating inflammatory cells expressing nuclear phosph orylated Smad2 within the allergen-challenged lungs. This was accompanied b y strong upregulation of the activin receptor ALK-4/ActR-1B and redistribut ion of the TGF-beta responsive ALK-5/T betaR-1. Although levels of TGF-beta 1, TGF-beta2, and TGF-beta3 messenger RNA (mRNA) were marginally altered, t he level of activin mRNA was strongly upregulated during the inflammatory r esponse. Our data illustrate the usefulness of antiphosphorylated Smad anti bodies in demonstrating active TGF-beta /activin-mediated signaling in vivo and strongly suggest that activin/Smad-mediated signaling could be a criti cal contributor in the pathophysiology of allergic pulmonary diseases.