Synergistic Inhibition of β2-adrenergic Receptor-mediated Alveolar Epithelial Fluid Transport by Interleukin-8 and Transforming Growth Factor-β

Background: Patients with acute respiratory distress syndrome who retain maximal alveolar fluid clearance (AFC) have better clinical outcomes. The release of endogenous catecholamines associated with shock or the administration of beta(2)-adrenergic receptor (beta(2)AR) agonists enhances AFC via a 3'-5'-cyclic adenosine monophosphate-dependent mechanism. The authors have previously reported that transforming growth factor-beta 1 (TGF-beta 1) and interleukin-8 (IL-8), two major mediators of alveolar inflammation associated with the early phase of acute respiratory distress syndrome, inhibit AFC upregulation by beta(2)AR agonists via a phosphoinositol-3-kinase (PI3K)-dependent mechanism. However, whether TGF-beta 1 and IL-8 cause an additive or synergistic inhibition of AFC is unclear. Thus, the central hypothesis of the study was to determine whether they synergistically inhibit the beta(2)AR-stimulated AFC by activating two different isoforms of PI3K. Methods: The effects of TGF-beta 1 or IL-8 on beta(2)AR agonist-stimulated net alveolar fluid transport were studied using short-circuit current studies. Molecular pathways of inhibition were confirmed by pharmacologic inhibitors and Western blotting of p-Akt, G-protein-coupled receptor kinase 2, protein kinase C-zeta, and phospho-beta(2)AR. Finally, our observations were confirmed by an in vivo model of AFC. Results: Combined exposure to TGF-beta 1 and IL-8/cytokine-induced neutrophil chemoattractant-1 caused synergistic inhibition of beta(2)AR agonist-stimulated vectorial Cl-across alveolar epithelial type II cells (n = 12 in each group). This effect was explained by activation of different isoforms of PI3K by TGF-beta 1 and IL-8/cytokine-induced neutrophil chemoattractant-1 (n = 12 in each group). Furthermore, the inhibitory effect of TGF-beta 1 on 3'-5'-cyclic adenosine monophosphate-stimulated alveolar epithelial fluid transport required the presence of IL-8/cytokine-induced neutrophil chemoattractant-1 (n = 12 in each group). Inhibition of cytokine-induced neutrophil chemoattractant-1 prevented TGF-beta 1-mediated heterologous beta(2)AR downregulation and restored physiologic beta(2)AR agonist-stimulated AFC in rats (n = 6 in each group). Conclusions: TGF-beta 1 and IL-8 have a synergistic inhibitory effect on beta(2)AR-mediated stimulation of pulmonary edema removal by the alveolar epithelium. This result may, in part, explain why a large proportion of the patients with acute respiratory distress syndrome have impaired AFC.