Background <p>Persistent injury and impaired regeneration of the alveolar epithelium are key contributors to the pathogenesis of pulmonary fibrosis. In idiopathic pulmonary fibrosis (IPF), type 2 alveolar epithelial (AT2) cells fail to fully differentiate into type 1 alveolar epithelial (AT1) cells, remaining instead in a transitional state. Histone deacetylase (HDAC) inhibitors are promising therapeutic agents for pulmonary fibrosis. Therefore, this study investigated whether MPT0E028, a pan-HDAC inhibitor, ameliorated bleomycin (BLM)-induced pulmonary fibrosis in a therapeutic model of mice by promoting AT2-to-AT1 cell differentiation.</p> Methods <p>The effects of MPT0E028 on pulmonary fibrosis were assessed by evaluating the expression of fibrogenic proteins and cell markers of AT1 (T1α and aquaporin 5 [AQP5]), AT2 (surfactant protein C [SPC]) and alveolar epithelial transitional cells (Keratin 8 [KRT8]) in a therapeutic model of BLM-induced pulmonary fibrosis in mice. The role of the ataxia-telangiectasia mutated (ATM)/AMP-activated protein kinase (AMPK)/forkhead box O1 (FoxO1) signaling pathway in MPT0E028-induced T1α expression was examined in murine AT2 cells (MLE-12 cells).</p> Results <p>Administration of MPT0E028 significantly reduced fibrosis scores; suppressed the expression of connective tissue growth factor, collagen I, fibronectin, and α-smooth muscle actin; and improved lung function in the therapeutic model of BLM-induced pulmonary fibrosis in mice. MPT0E028 enhanced the expression of T1α and AQP5 but reduced the expression of SPC and KRT8 in lung tissues from BLM-treated mice. In MLE-12 cells and primary human AT2 cells, MPT0E028 upregulated T1α and AQP5 expression in a time-dependent manner, with this accompanied by a decrease in SPC expression. AS1842856, an FoxO1 inhibitor, and FoxO1 siRNA transfection inhibited MPT0E028-stimulated T1α expression, whereas transfection with FoxO3 siRNA had no effect. FoxO1 siRNA transfection also inhibited MPT0E028-stimulated T1α-luciferase activity. MPT0E028 induced FoxO1 serine phosphorylation, increased FoxO1 recruitment to the T1α promoter, and enhanced FoxO1-luciferase activity. Compound C, an AMPK inhibitor, and AMPK siRNA transfection suppressed MPT0E028-stimulated T1α expression, and compound C also inhibited MPT0E028-promoted FoxO1 recruitment to the T1α promoter. MPT0E028 induced AMPK phosphorylation in a time-dependent manner and increased ATM acetylation and phosphorylation in MLE-12 cells. ATM siRNA transfection suppressed MPT0E028-induced T1α expression, AMPK and FoxO1 serine phosphorylation, FoxO1 recruitment to the T1α promoter, and FoxO1-luciferase activity. MPT0E028 induced FoxO1 phosphorylation in AT2 cells in the therapeutic model of BLM-induced pulmonary fibrosis in mice.</p> Conclusions <p>MPT0E028 is the first pan-HDAC inhibitor shown to activate ATM acetylation-mediated AMPK/FoxO1 signaling to induce AT2-to-AT1 differentiation in a therapeutic model of BLM-induced pulmonary fibrosis in mice. Administration of MPT0E028 after BLM challenge effectively ameliorated pulmonary fibrosis by suppressing fibrogenic protein expression and promoting AT2-to-AT1 cell differentiation. These results suggest that MPT0E028 holds potential as a therapeutic agent for IPF treatment.</p>