Targeted PPARγ deficiency in alveolar macrophages disrupts surfactant catabolism

Abstract

Surfactant accumulates in alveolar macrophages of granulocyte-macrophage colony-stimulating factor (GM-CSF) knockout (KO) mice and pulmonary alveolar proteinosis (PAP) patients with a functional loss of GM-CSF resulting from neutralizing anti–GM-CSF antibody. Alveolar macrophages from PAP patients and GM-CSF KO mice are de­ficient in peroxisome proliferator-activated receptor-γ (PPARγ) and ATP-binding cassette (ABC) lipid transporter ABCG1. Previous studies have demonstrated that GM-CSF induces PPARγ. We therefore hypothesized that PPARγ promotes surfactant catabolism through regulation of ABCG1. To address this hypothesis, macrophage-specific PPARγ (MacPPARγ) knockout mice were utilized. MacPPARγ KO mice develop foamy, lipid-engorged Oil Red O positive alveolar macrophages. Lipid analyses revealed significant increases in the cholesterol and phospholipid contents of MacPPARγ KO alveolar macrophages and extracellular bronchoalveolar lavage (BAL)–derived fluids. MacPPARγ KO alveolar macrophages showed decreased expression of ABCG1 and a deficiency in ABCG1-mediated cholesterol efflux to HDL. Lipid metabolism may also be regulated by liver X receptor (LXR)–ABCA1 pathways. Interestingly, ABCA1 and LXRβ expression were elevated, indicating that this pathway is not sufficient to prevent surfactant accumulation in alveolar macrophages. These results suggest that PPARγ mediates a critical role in surfactant homeostasis through the regulation of ABCG1.

Keywords

• pulmonary alveolar proteinosis
• granulocyte-macrophage colony-stimulating factor
• peroxisome proliferator-activated receptor-γ