Vol. 280, Issue 3, 1219-1227, 1997

Up-regulation of Platelet-activating Factor Receptors in Lung and Alveolar Macrophages in the Bleomycin-Hamster Model of Pulmonary Fibrosis¹

Jin Chen² , Vincent Ziboh and Shri N. Giri

Department of Molecular Biosciences, School of Veterinary Medicine (J.C., S.N.G.), and Department of Dermatology (V.Z.), School of Medicine, University of California, Davis, California

The mechanisms of lung fibrosis caused by bleomycin (BL) and other fibrogenic agents are not clearly understood. Our previous studies demonstrated that the platelet-activating factor (PAF) antagonist WEB2086 reduced lung fibrosis induced by BL and amiodarone in hamsters, suggesting a critical role for PAF and/or PAF receptors in this pathogenic process. In the present study, the PAF receptors in the lung and the functional activity of PAF receptors in the alveolar macrophages from BL (7.5 U/kg, intratracheally)-treated hamsters were investigated. The PAF receptor binding, measured by a [³H]WEB2086 binding assay in lung homogenates, was significantly increased at all times after BL treatment, compared with saline-treated control hamsters. At 3 days after BL treatment, the PAF receptor density (B_max = 202.4 fmol/mg protein, with K_d = 41 nM) was increased over control (B_max = 116.9 fmol/mg protein, with K_d = 45.3 nM). Most importantly, the functional activities of PAF receptors in alveolar macrophages, as determined by PAF-induced elevation of cytosolic Ca⁺⁺ (both by mobilization of Ca⁺⁺ stores and by Ca⁺⁺ influx), were significantly higher in the BL-treated animals than in the saline control. The EC₅₀ of PAF to increase internal Ca⁺⁺ release was 5-fold less in BL-treated lungs than in control. The Ca⁺⁺ signaling could not be stimulated by lyso-PAF (inactive PAF) but was inhibited by the PAF antagonists WEB2086 (at 100 nM) and L659,989, in a dose-dependent fashion, suggesting the involvement of specific receptors for PAF. The cells from BL-treated hamster lung required much higher concentrations of the antagonists, with increases in the IC₅₀ values of 14-fold for WEB2086 and 63-fold for L659,989 over control. These results indicated that PAF receptors were functionally up-regulated in the lungs after BL treatment in vivo, and this may be an important mechanism, at least in part, for BL-induced lung injury. These findings also explain the antifibrotic effect of the PAF receptor antagonist WEB2086 in the BL-hamster model of lung fibrosis, as reported in our earlier paper.