The role of all-trans retinoic acid (ATRA) in pulmonary fibrosis is relatively unknown, although this metabolite modulates cell differentiation, proliferation, and development. We aimed to evaluate the role of ATRA in bleomycin-induced pulmonary fibrosis, and whether the mechanism involves EphA2-EphrinAl and PI3K-Akt signaling. We evaluated three groups of mice: a control group (intraperitoneal DMSO injection 3 times weekly after PBS instillation), bleomycin group (intraperitoneal DMSO injection 3 times weekly after bleomycin instillation), and bleomycin + ATRA group (intraperitoneal ATRA injection 3 times weekly after bleomycin instillation). The cell counts and protein concentration in the bronchoalveolar lavage fluid (BALF), changes in histopathology, Ashcroft score, hydroxyproline assay, expression of several signal pathway proteins including EphA2-EphrinAl, and PI3K-Akt, and cytokine levels were compared among the groups. We found that bleomycin significantly increased the protein concentration in the BALF, Ashcroft score in lung tissue, and hydroxyproline contents in lung lysates. Furthermore, bleomycin upregulated EphA2, EphrinAl, PI3K 110 gamma, Akt, IL-6 and THE-alpha. However, administration of ATRA attenuated the upregulation of EphA2-EphrinAl and PI3K-Akt after bleomycin instillation, and decreased pulmonary fibrosis. In addition, ATRA suppressed IL-6 and TNF-alpha production induced by bleomycin-induced injury. Collectively, these data suggest that ATRA attenuates bleomycin-induced pulmonary fibrosis by regulating EphA2-EphrinAl and PI3K-Akt signaling. (C) 2017 Elsevier Inc. All rights reserved.