A central role for G9a and EZH2 in the epigenetic silencing of cyclooxygenase-2 in idiopathic pulmonary fibrosis

Selective silencing of the cyclooxygenase-2 (COX-2) gene with the loss of the antifibrotic mediator prostaglandin E-2 contributes to the fibrotic process in idiopathic pulmonary fibrosis (IPF). This study explored the role of G9a- and enhancer of zeste homolog 2 (EZH2)-mediated methylation of histone H3 lysine 9 (H3K9me3) and histone H3 lysine 27 (H3K27me3) in COX-2 silencing in IPF. Chromatin immunoprecipitation (ChIP) and re-ChIP assays demonstrated marked increases in H3K9me3, H3K27me3, and DNA methylation, together with their respective modifying enzymes G9a, EZH2, and DNA methyltransferases (Dnmts) and respective binding proteins heterochromatin protein 1 (HP1), polycomb protein complex 1 (PRC1) and methyl CpG binding protein 2 (MeCP2), at the COX-2 promoter in lung fibroblasts from patients with IPF (F-IPFs) compared with fibroblasts from nonfibrotic lungs. HP1, EZH2, and MeCP2 in turn were associated with additional repressive chromatin modifiers in F-IPFs. G9a and EZH2 inhibitors and small interfering RNAs and the Dnmt1 inhibitor markedly reduced H3K9me3 (49-79%), H3K27me3 (44-81%), and DNA methylation (61-97%) at the COX-2 promoter. These reductions were correlated with increased histone H3 and H4 acetylation, resulting in COX-2 mRNA and protein reexpression in F-IPFs. Our results support a central role for G9a- and EZH2-mediated histone hypermethylation and a model of bidirectional, mutually reinforcing, and interdependent crosstalk between histone hypermethylation and DNA methylation in COX-2 epigenetic silencing in IPF.