A STAT6 Inhibitor AS1517499 Reduces Preventive Effects of Apoptotic Cell Instillation on Bleomycin-Induced Lung Fibrosis by Suppressing PPARγ

Background/Aims: The signal transducer and activator of transcription 6 (STAT6) transcription factor mediates PPARγ-regulated gene expression in macrophages. However, it remains largely unknown how proximal membrane signaling events initiated by apoptotic cell recognition upregulate PPARγ expression and activate the lung homeostatic program. Methods: The STAT6 inhibitor AS1517499 was used to determine the role of STAT6 in mediating PPARγ activity, anti-inflammatory effects, and anti-fibrotic effects induced by apoptotic cell instillation after bleomycin treatment into C57BL/6 mice. Bronchoalveolar lavage fluid, alveolar macrophages and lungs were harvested at days 2, 7, and 14 and then analyzed by real-time PCR, immunoblotting, ELISA, immunocytochemistry and immunohistochemistry assays. Results: Our data demonstrate that apoptotic cell instillation after bleomycin results in prolonged enhancement of STAT6 phosphorylation in alveolar macrophages and lung. Co-administration of the STAT6 inhibitor, AS1517499, reversed the enhanced PPARγ expression and activity induced by apoptotic cell instillation after bleomycin treatment. By reducing the expression of PPARγ target genes, including CD36, macrophage mannose receptor, and arginase 1, AS1517499 inhibited efferocytosis and restored pro-inflammatory cytokine expression, neutrophil recruitment, protein levels, hydroxyproline content, and expression of fibrosis markers, including type 1 collagen α2, fibronectin, and α-smooth muscle actin. STAT6 inhibition reversed the expression profile of hepatocyte growth factor and interleukin-10. Conclusion: These results indicate that prolonged STAT6 activation following one-time apoptotic cell instillation facilitates continuous PPARγ activation, resulting in the resolution of bleomycin-induced lung inflammation and fibrosis.