Inhibition of DNA methylation de-represses peroxisome proliferator-activated receptor-γ and attenuates pulmonary fibrosis

Background and Purpose: Development of pulmonary fibrosis is associated with altered DNA methylation modifications of fibrogenic gene expression. However, their causal relationships and the underlying mechanisms remain unclear. This study investigates the critical role of DNA methylation aberration-associated suppression of peroxisome proliferator-activated receptor-gamma (PPAR gamma) in pulmonary fibrosis. Experimental Approach: Expression of PPAR gamma and bioactive DNA methyltransferases (DNMTs) and PPAR gamma promoter methylation status were examined in fibrotic lungs of idiopathic pulmonary fibrosis (IPF) patients and bleomycin (Blm)-treated mice. DNA demethylating agent 5-aza-2'-deoxycytidine (5aza) and glycyrrhizic acid (GA) derived from medicinal plant were assessed for their PPAR gamma de-repression and anti-pulmonary fibrosis activities. PPAR gamma knockout mice were created to determine the critical role of PPAR gamma in this protection. Key Results: Lung PPAR gamma expression was markedly suppressed in IPF patients and Blm mice, accompanied by increased DNMT 1/DNMT3a and PPAR gamma promoter hypermethylation. Administration of 5-aza and GA similarly demethylated PPAR gamma promoter, restored PPAR gamma loss and alleviated fibrotic lung pathologies, including structural alterations and adverse expression of fibrotic mediators and inflammatory cytokines. In cultured lung fibroblasts and alveolar epithelial cells, GA alleviated PPAR gamma-mediated suppression of fibrosis in a gain of DNMT-sensitive manner, and in PPAR gamma knockout mice, the anti-fibrotic effects of 5aza and GA were significantly reduced, suggesting that PPAR gamma is a critical mediator of epigenetic pulmonary fibrogenesis. Conclusion and Implications: Aberrant DNMT1/3a elevations and the resultant PPAR gamma suppression contribute significantly to the development of pulmonary fibrosis, and strategies targeting DNMT/PPAR gamma axis with synthetic or natural compounds might benefit patients with pulmonary fibrotic disorders.