TNF-alpha Signaling in Silica-Induced Lung Fibrosis

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The pneumoconioses are a group of fibrotic lung diseases caused by the environmental exposure to a variety of inorganic dusts. Several of these dusts, including silica, are associated with lung inflammation and the subsequent development of lung fibrosis. The inflammatory response to silica is characterized by an increased expression of tumor necrosis factor-alpha (TNF). Inhaled silica triggers TNF receptor-mediated signal transduction pathways promoting NF-kappaB and AP-1 activation which in turn induce the expression of genes mediating the proinflammatory (TNF) and profibrotic (collagen, matrix metalloproteinases, TIMP- 1) effects of silica. These TNF-mediated signal transduction pathways, in response to silica, includes activation of the ERK1/2 family of mitogen-activated protein kinases (MAPK) that induces TNF receptor phosphorylation. The importance of TNF receptor phosphorylation in silicosis is not known. Here we propose that TNF receptor phosphorylation promotes cell survival signals that protect the lungs from silica-induced apoptosis. Our in vitro preliminary data show that differences in the silica-induced-TNF-mediated signal transduction correlate with macrophage cell survival. The macrophage cell line RAW 264.7 reacts to silica exposure with enhanced TNF expression. This enhanced TNF expression promotes NF-kappaB and ERK1/2 activation. Activated ERK1/2 kinases induce phosphorylation of the TNF receptors and protect RAW 264.7 cells from silica-induced apoptosis. In contrast, the IC-21 macrophage cell line does not upregulate TNF expression in response to silica. IC-21 cells do not activate NF-kappaB or ERK kinases in response to silica. IC- 21 cells do not phosphorylate TNF receptors and exhibit enhanced silica-induced apoptosis. Our in vivo data demonstrate that individual TNF receptor deficient mice are protected from the fibrogenic effects of silica. This protection correlates with a decreased AP-1 activation and decreased expression of the Tissue Inhibitor of Metalloproteinase 1 (TIMP-1) observed in the lungs of silica-sensitive (C57BL/6) mice in response to silica. Our working hypothesis to further understand the mechanisms of silica-induced lung fibrosis is that disruption of the silica- induced and TNF receptor-mediated activation of NFkappaB and ERK in the lung will enhance apoptosis in alveolar epithelial type II cells thus aggravating silica-induced lung injury and fibrosis. Specific Aims are: 1). To determine whether inhibition of NF- kappaB activation in alveolar epithelial type II cells will exacerbate silica-induced lung injury in mice. 2). To determine whether inhibition of ERK-mediated phosphorylation of TNF receptors will exacerbate silica-induced lung injury. 3). To determine whether overexpression of Tissue Inhibitor of Metalloproteinase 1 (TIMP-1) in mouse lung exacerbates silica- induced lung injury.