Lysophospatidic acid (LPA) is a bioactive lipid mediator implicated in tissue repair and wound healing. It mediates diverse functional effects in fibroblasts, including proliferation, migration and contraction, but less is known about its ability to evoke paracrine signaling to other cell types involved in wound healing. We hypothesized that human pulmonary fibroblasts stimulated by LPA would exhibit ectodomain shedding of epidermal growth factor receptor (EGFR) ligands that signal to lung epithelial cells. To test this hypothesis, we used alkaline phosphatase-tagged EGFR ligand plasmids transfected into lung fibroblasts, and enzyme-linked immunosorbent assays to detect shedding of native ligands. LPA induced shedding of alkaline phosphatase-tagged heparin-binding epidermal growth factor (HB-EGF), amphiregulin, and transforming growth factor-a; non-transfected fibroblasts shed amphiregulin and HBEGF under baseline conditions, and increased shedding of HB-EGF in response to LPA. Treatment of fibroblasts with LPA resulted in elevated phosphorylation of extracellular signal-regulated kinase 1/2, enhanced expression of mRNA for c-fos, HB-EGF and amphiregulin, and enhanced proliferation at 96 hours. However, none of these fibroblast responses to LPA required ectodomain shedding or EGFR activity. To test the ability of LPA to stimulate paracrine signaling from fibroblasts, we transferred conditioned medium from LPA-stimulated cells, and found enhanced EGFR and extracellular signal-regulated kinase 1/2 phosphorylation in reporter A549 cells in excess of what could be accounted for by transferred LPA alone. These data show that LPA mediates EGF-family ectodomain shedding, resulting in enhanced paracrine signaling from lung fibroblasts to epithelial cells.
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