The Activin A-Peroxisome Proliferator-Activated Receptor Gamma Axis Contributes to the Transcriptome of GM-CSF-Conditioned Human Macrophages

GM-CSF promotes the functional maturation of lung alveolar macrophages (A-MO), whose differentiation is dependent on the peroxisome proliferator-activated receptor gamma (PPAR gamma) transcription factor. In fact, blockade of GM-CSF-initiated signaling or deletion of the PPAR gamma-encoding gene PPARG leads to functionally defective A-MO and the onset of pulmonary alveolar proteinosis. In vitro, macrophages generated in the presence of GM-CSF display potent proinflammatory, immunogenic and tumor growth-limiting activities. Since GM-CSF upregulates PPAR gamma expression, we hypothesized that PPAR gamma might contribute to the gene signature and functional profile of human GM-CSF-conditioned macrophages. To verify this hypothesis, PPAR gamma expression and activity was assessed in human monocyte-derived macrophages generated in the presence of GM-CSF [proinflammatory GM-CSF-conditioned human monocyte-derived macrophages (GM-MO)] or M-CSF (anti-inflammatory M-MO), as well as in ex vivo isolated human A-MO. GM-MO showed higher PPAR. expression than M-MO, and the expression of PPAR gamma in GM-MO was found to largely depend on activin A. Ligand-induced activation of PPAR gamma also resulted in distinct transcriptional and functional outcomes in GM-MO and M-MO. Moreover, and in the absence of exogenous activating ligands, PPAR gamma knockdown significantly altered the GM-MO transcriptome, causing a global upregulation of proinflammatory genes and significantly modulating the expression of genes involved in cell proliferation and migration. Similar effects were observed in ex vivo isolated human A-MO, where PPAR gamma silencing led to enhanced expression of genes coding for growth factors and chemokines and downregulation of cell surface pathogen receptors. Therefore, PPAR gamma shapes the transcriptome of GM-CSF-dependent human macrophages (in vitro derived GM-MO and ex vivo isolated A-MO) in the absence of exogenous activating ligands, and its expression is primarily regulated by activin A. These results suggest that activin A, through enhancement of PPAR gamma expression, help macrophages to switch from a proinflammatory to an anti-inflammatory polarization state, thus contributing to limit tissue damage and restore homeostasis.