Adenosine A2B receptors are highly expressed on murine type II alveolar epithelial cells

Abstract

The adenosine A2B receptor (A2BR) has a wide tissue distribution that includes fibroblasts and endothelial and epithelial cells. The recent generation of an A2BR−/− mouse constructed with a β-galactosidase (β-gal) reporter gene under control of the endogenous promoter has provided a valuable tool to quantify A2BR promoter activity ( 29 ). To determine the sites of expression of the A2B receptor in the mouse lung, histological and flow cytometric analysis of β-gal reporter gene expression in various lung cell populations was performed. The major site of A2BR promoter activity was found to be the type II alveolar epithelial cells (AECs), identified by coexpression of prosurfactant protein C, with relatively less expression in alveolar macrophages, bronchial epithelial cells, and cells of the vasculature. Highly purified type II AECs were prepared by fluorescence-activated sorting of enhanced green fluorescent protein (eGFP)-positive cells from transgenic mice expressing eGFP under control of the surfactant protein C promoter ( 21 ). The type II cells expressed 89-fold higher A2BR mRNA than pulmonary leukocytes, and the A2BR was shown to be functional, as treatment of purified type II AECs with the nonspecific adenosine receptor agonist 5′- N-ethylcarboxamidoadenosine (NECA) induced an increase in intracellular cAMP greater that the β-adrenergic agonist isoproterenol that was inhibited completely following treatment by ATL-802, a novel, highly potent ( Ki = 8.6 nM), and selective (>900 fold over other adenosine receptor subtypes) antagonist of the mouse A2BR.