Assessment of Brd4 inhibition in idiopathic pulmonary fibrosis lung fibroblasts and in vivo models of lung fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a chronic lung disease of high unmet medical need. Although bromodomain (Brd) and extra terminal domain isoforms have recently been implicated in mediating inflammatory and oncologic indications, their roles in lung fibrosis have not been comprehensively assessed. We investigated the role of Brd on the profibrotic responses of lung fibroblasts (LFs) in patients with rapidly progressing IPF and a mouse bleomycin model of lung fibrosis. The enhanced migration, proliferation, and IL-6 release observed in LFs from patients with rapidly progressing IPF are attenuated by pharmacologic inhibition of Brd4. These changes are accompanied by enhanced histone H4 lysine5 acetylation and association of Brd4 with genes involved in the profibrotic responses in IPF LFs as demonstrated using chromatin immunoprecipitation and quantitative PCR. Oral administration of 200 mg/kg per day Brd4 inhibitor JQ1 in a therapeutic dosing regimen substantially attenuated lung fibrosis induced by bleomycin in C57BL/6 mice. In conclusion, this study shows that the Brd4 inhibitor JQ1, administered in a therapeutic dosage, is capable of inhibiting the profibrotic effects of IPF LFs and attenuates bleomycin-induced lung fibrosis in mice. These results suggest that Brd4 inhibitors may represent a novel therapy for the treatment of rapidly progressing IPF.

BET bromodomain proteins mediate downstream signaling events following growth factor stimulation in human lung fibroblasts and are involved in bleomycin-induced pulmonary fibrosis.

Epigenetic alterations, such as histone acetylation, regulate the signaling outcomes and phenotypic responses of fibroblasts after growth factor stimulation. The bromodomain and extra-terminal domain-containing proteins (Brd) bind to acetylated histone residues, resulting in recruitment of components of the transcriptional machinery and subsequent gene transcription. Given the central importance of fibroblasts in tissue fibrosis, this study sought to determine the role of Brd proteins in human lung fibroblasts (LFs) after growth factor stimulation and in the murine bleomycin model of lung fibrosis. Using small interfering RNA against human Brd2 and Brd4 and pharmacologic Brd inhibitors, this study found that Brd2 and Brd4 are essential in mediating the phenotypic responses of LFs downstream of multiple growth factor pathways. Growth factor stimulation of LFs causes increased histone acetylation, association of Brd4 with growth factor-responsive genes, and enhanced transcription of these genes that could be attenuated with pharmacologic Brd inhibitors. Of note, lung fibrosis induced after intratracheal bleomycin challenge in mice could be prevented by pretreatment of animals with pharmacologic inhibitors of Brd proteins. This study is the first demonstration of a role for Brd2 and Brd4 proteins in mediating the responses of LFs after growth factor stimulation and in driving the induction of lung fibrosis in mice in response to bleomycin challenge.