Surfactant lipids inhibit PI3K-dependent signaling pathways induced by IL-4 in alveolar macrophages

Abstract Alveolar macrophages (AMs) are less able to respond to IL-4 in vivo than macrophages from the peritoneal cavity, due to a still-unknown factor of the lung environment. The aim of this study is to investigate whether surfactant lipids, which are continuously endocytosed by AMs, could influence IL-4-mediated alternative activation and proliferation of AMs. To that end, AMs were preincubated with surfactant lipids and stimulated with IL-4 in the presence or absence of surfactant protein SP-A, an amplifier of IL-4 actions. We found that alveolar lipids reduced IL-4- and IL-4+SP-A-dependent arginase activity, the expression of genes associated with alternative activation, and proliferation of AMs. Mechanistically, endocytosed lipids decreased IL-4- and IL-4+SP-A-induced activation of the PI3K-Akt-mTORC1 signaling axis, but not the IL-4-dependent STAT6 axis. Lipid-dependent inhibition of the Akt/mTORC1 signaling axis is consistent with reduced IL-4+SP-A-driven glycolysis and mitochondrial respiration as well as decreased ATP citrate lyase expression and histone acetylation stimulated by IL-4. We conclude that surfactant lipids inhibit PI3K-dependent signaling pathways and suggest that decrease of surfactant lipids in chronic lung diseases might augment IL-4-dependent fibrotic responses. This research is funded by the Spanish Ministry of Science and Innovation through Grant PID2021-123044OB-I00 to C. Casals