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-γ (PPARγ) in pulmonary fibrosis.

Experimental approach: Expression of PPARγ and bioactive DNA methyltransferases (DNMTs) and PPARγ 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 PPARγ de-repression and anti-pulmonary fibrosis activities. PPARγ knockout mice were created to determine the critical role of PPARγ in this protection.

Key results: Lung PPARγ expression was markedly suppressed in IPF patients and Blm mice, accompanied by increased DNMT 1/DNMT3a and PPARγ promoter hypermethylation. Administration of 5-aza and GA similarly demethylated PPARγ promoter, restored PPARγ 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 PPARγ-mediated suppression of fibrosis in a gain of DNMT-sensitive manner, and in PPARγ knockout mice, the anti-fibrotic effects of 5aza and GA were significantly reduced, suggesting that PPARγ is a critical mediator of epigenetic pulmonary fibrogenesis.

Conclusion and implications: Aberrant DNMT1/3a elevations and the resultant PPARγ suppression contribute significantly to the development of pulmonary fibrosis, and strategies targeting DNMT/PPARγ axis with synthetic or natural compounds might benefit patients with pulmonary fibrotic disorders.