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A₃ Adenosine Receptor Signaling Influences Pulmonary Inflammation and Fibrosis

¹ Department of Biochemistry and Molecular Biology, University of Texas Health Science Center at Houston, Medical School, Houston, Texas; ² Division of Pulmonary Medicine, Department of Biochemistry and Molecular Biology, Mayo Clinic Arizona, Scottsdale, Arizona; and ³ Department of Medicine, Pulmonary, Baylor College of Medicine, Houston, Texas

Correspondence and requests for reprints should be addressed to Michael R. Blackburn, Ph.D., Department of Biochemistry and Molecular Biology, University of Texas Health Science Center at Houston, Medical School, 6431 Fannin, Houston, TX 77030. E-mail: michael.r.blackburn{at}uth.tmc.edu

Adenosine is a signaling molecule produced during conditions that cause cellular stress or damage. This signaling pathway is implicated in the regulation of pulmonary disorders through the selective engagement of adenosine receptors. The goal of this study was to examine the involvement of the A₃ adenosine receptor (A₃R) in a bleomycin model of pulmonary inflammation and fibrosis. Results demonstrated that A₃R-deficient mice exhibit enhanced pulmonary inflammation that included an increase in eosinophils. Accordingly, there was a selective up-regulation of eosinophil-related chemokines and cytokines in the lungs of A₃R-deficient mice exposed to bleomycin. This increase in eosinophil numbers was accompanied by a decrease in the amount of extracellular eosinophil peroxidase activity in lavage fluid from A₃R-deficient mice exposed to bleomycin, an observation suggesting that the A₃R is necessary for eosinophil degranulation in this model. Despite an increase in inflammatory metrics associated with A₃R-deficient mice treated with bleomycin, there was little difference in the degree of pulmonary fibrosis. Examination of fibrotic mediators demonstrated enhanced transforming growth factor (TGF)-β1 expression, but not a concomitant increase in TGF-β1 activity. This was associated with the loss of expression of matrix metalloprotease 9, an activator of TGF-β1, in alveolar macrophages and airway mast cells in the lungs of A₃R-deficient mice. Together, these results suggest that the A₃R serves antiinflammatory functions in the bleomycin model, and is also involved in regulating the production of mediators that can impact fibrosis.

Key Words: pulmonary fibrosis • adenosine receptors • inflammation • eosinophil • extracellular matrix

CLINICAL RELEVANCE Pulmonary fibrosis is associated with many interstitial lung diseases. The current study suggests that A3 adenosine receptor signaling regulates key aspects of pulmonary inflammation and fibrosis.