Abstract
BackgroundRecent clinical studies show that tyrosine kinase inhibitors slow the rate of lung function decline and decrease the number of acute exacerbations in patients with Idiopathic Pulmonary Fibrosis (IPF). However, in the murine bleomycin model of fibrosis, not all tyrosine kinase signaling is detrimental. Exogenous ligands Fibroblast Growth Factor (FGF) 7 and 10 improve murine lung repair and increase survival after injury via tyrosine kinase FGF receptor 2b-signaling. Therefore, the level and location of FGF/FGFR expression as well as the exogenous effect of the most highly expressed FGFR2b ligand, FGF1, was analyzed on human lung fibroblasts.MethodsFGF ligand and receptor expression was evaluated in donor and IPF whole lung homogenates using western blotting and qPCR. Immunohistochemistry for FGF1 and FGFR1/2/3/4 were performed on human lung tissue. Lastly, the effects of FGF1, a potent, multi-FGFR ligand, were studied on primary cultures of IPF and non-IPF donor fibroblasts. Western blots for pro-fibrotic markers, proliferation, FACS for apoptosis, transwell assays and MetaMorph analyses on cell cultures were performed.ResultsWhole lung homogenate analyses revealed decreased FGFR b-isoform expression, and an increase in FGFR c-isoform expression. Of the FGFR2b-ligands, FGF1 was the most significantly increased in IPF patients; downstream targets of FGF-signaling, p-ERK1/2 and p-AKT were also increased. Immunohistochemistry revealed FGF1 co-localization within basal cell sheets, myofibroblast foci, and Surfactant protein-C positive alveolar epithelial type-II cells as well as co-localization with FGFR1, FGFR2, FGFR3, FGFR4 and myofibroblasts expressing the migratory marker Fascin. Both alone and in the presence of heparin, FGF1 led to increased MAPK-signaling in primary lung fibroblasts. While smooth muscle actin was unchanged, heparin + FGF1 decreased collagen production in IPF fibroblasts. In addition, FGF1 + heparin increased apoptosis and cell migration. The FGFR inhibitor (PD173074) attenuated these effects.ConclusionsStrong expression of FGF1/FGFRs in pathogenic regions of IPF suggest that aberrant FGF1-FGFR signaling is increased in IPF patients and may contribute to the pathogenesis of lung fibrosis by supporting fibroblast migration and increased MAPK-signaling.Electronic supplementary materialThe online version of this article (doi:10.1186/s12931-015-0242-2) contains supplementary material, which is available to authorized users.
Highlights
Idiopathic pulmonary fibrosis (IPF) is a rare interstitial lung disease of unknown origin, with prevalence rates ranging from 2-4/10000 [1]
Transwell migration assays as well as MetaMorph analyses of cell motility indicated increased migration in fibroblasts exposed to FGF1 + heparin. These effects were attenuated in the presence of an FGFR-signaling inhibitor; PD173074. These results suggest that FGF1-FGFR-signaling in the presence of heparin may contribute to lung remodeling by enhancing invasive capabilities of fibroblasts
A significant increase in FGF1 was observed in Idiopathic Pulmonary Fibrosis (IPF) patients (Fig. 1a,b)
Summary
Idiopathic pulmonary fibrosis (IPF) is a rare interstitial lung disease of unknown origin, with prevalence rates ranging from 2-4/10000 [1]. Both Fgf and Fgf are mesenchymal-derived growth factors that signal in a paracrine manner to bind with high affinity to epithelial expressed Fgfr b-isoform [5]. Overexpression or administration of exogenous fibroblast growth factors (Fgf)-7/10 [6, 7] diminishes the extent of epithelial injury and apoptosis thereby attenuating bleomycin-induced lung fibrosis in rodents. Known as heparin binding growth factor, or acidic Fgf, is expressed by both mesenchymal and epithelial cell types in the lung [9] and binds with high affinity to all Fgfrs [10]. Exogenous ligands Fibroblast Growth Factor (FGF) 7 and 10 improve murine lung repair and increase survival after injury via tyrosine kinase FGF receptor 2b-signaling. The level and location of FGF/FGFR expression as well as the exogenous effect of the most highly expressed FGFR2b ligand, FGF1, was analyzed on human lung fibroblasts
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