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Pathogenesis of idiopathic pulmonary fibrosis (IPF) remains unclear. Secreted protein acidic and rich in cysteine (SPARC) is a matricellular protein that participates in the assembly and turnover of the extracellular matrix, whose expression is regulated by transforming growth factor (TGF)-β1 through activation of mammalian target of rapamycin complex 2 (mTORC2). Exchange factor found in platelets, leukemic, and neuronal tissues (XPLN) is an endogenous inhibitor of mTORC2. However, whether XPLN modulates SPARC expression remains unknown. Herein, we investigated the regulatory mechanisms of XPLN in human lung fibroblasts. Effect of XPLN on mTORC2 activity was evaluated by silencing XPLN in human foetal lung fibroblasts (HFL-1 cells), using small interfering RNA. SPARC expression was quantified by quantitative real-time RT-PCR and western blotting. Fibroblasts were treated with TGF-β1, histone deacetylase (HDAC) inhibitors, entinostat, or vorinostat, to assess their effects on XPLN expression. Moreover, the effect of mTORC1 inhibition on SPARC and XPLN was examined. XPLN depletion stimulated SPARC expression and Akt phosphorylation on Ser473. TGF-β1 treatment down-regulated XPLN via Smad 2/3. XPLN mRNA expression was up-regulated upon treatment with HDAC inhibitors in a concentration-dependent manner, and TGF-β1-induced SPARC expression was reversed by entinostat treatment. mTORC1 inhibition by rapamycin and Raptor depletion stimulated SPARC expression. In conclusion, this is the first study describing the involvement of XPLN in the regulation of SPARC. These findings may help uncover the regulatory mechanisms of the mTORC2-SPARC axis. The up-regulation of XPLN by HDAC inhibitors may be a novel therapeutic approach in patients with IPF. [ABSTRACT FROM AUTHOR]

Idiopathic pulmonary fibrosis (IPF) is a progressive disease with a high mortality rate. Signalling pathways activated by several tyrosine kinase receptors are known to be involved in lung fibrosis, and this knowledge has led to the development of the triple tyrosine kinase inhibitor nintedanib, an inhibitor of vascular endothelial growth factor receptor (VEGFR), platelet-derived growth factor receptor (PDGFR), and fibroblast growth factor receptor (FGFR), for the treatment of IPF. Pulmonary surfactant protein D (SP-D), an important biomarker of IPF, reportedly attenuates bleomycin-induced pulmonary fibrosis in mice. In this study, we investigated whether nintedanib modulates SP-D expression in human lung epithelial (A549) cells using quantitative real-time reverse transcriptase polymerase chain reaction and western blotting. To investigate the mechanisms underlying the effects of nintedanib, we evaluated the phosphorylation of c-Jun N-terminal kinase (JNK) and its downstream target c-Jun. The effect of the JNK inhibitor SP600125 on c-Jun phosphorylation was also tested. Activation of activator protein-1 (AP-1) was examined using an enzyme-linked immunosorbent assay-based test, and cell proliferation assays were performed to estimate the effect of nintedanib on cell proliferation. Furthermore, we treated mice with nintedanib to examine its in vivo effect on SP-D levels in lungs. These experiments showed that nintedanib up-regulated SP-D messenger RNA expression in a dose-dependent manner at concentrations up to 5 μM, with significant SP-D induction observed at concentrations of 3 μM and 5 μM, in comparison with that observed in vehicle controls. Nintedanib stimulated a rapid increase in phosphorylated JNK in A549 cells within 30 min of treatment and stimulated c-Jun phosphorylation, which was inhibited by the JNK inhibitor SP600125. Additionally, nintedanib was found to activate AP-1. A549 cell proliferation was not affected by nintedanib at any of the tested concentrations. Moreover, blocking FGFR, PDGFR, and VEGFR function did not affect nintedanib-induced SP-D expression, suggesting that nintedanib mediates its effects through a mechanism that is distinct from its known role as a tyrosine kinase inhibitor. Nintedanib is also reported to inhibit Src kinase although pre-treatment of cells with a Src kinase inhibitor had no effect on nintedanib-induced SP-D expression. Increased expression of SFTPD mRNA and SP-D protein in the lungs of nintedanib-treated mice was also observed. In this work, we demonstrated that nintedanib up-regulated SP-D expression in A549 cells via the JNK-AP-1 pathway and did not affect cell proliferation. This is the first report describing SP-D induction by nintedanib. [ABSTRACT FROM AUTHOR]