Silica and its antagonistic effects on transforming growth factor-beta in lung fibroblast extracellular matrix production

Background: Silicosis, a pneumoconiosis marked by interstitial pulmonary fi brosis, is caused by inhalation of free crystalline silica particles. When silica particles are injected into the lower lung, they are translocated ac ross the epithelium into the interstitial space, where macrophage-derived g rowth factors affect lung fibroblast proliferation and collagen deposition, We hypothesized that silica may act directly on pulmonary fibroblasts modi fying extracellular matrix (ECM) synthesis and that the effects of silica m ay be mediated by transforming growth factor-beta (TGF beta) overproduction . Methods: To test this hypothesis, we studied a human lung fibroblast cell l ine (WI-1003) exposed to silica in vitro, We investigated cell morphology b y electron microscopic procedure, cell growth, collagen production, and gly cosaminoglycans (GAG) composition by radiolabeled precursors. Cytokine and growth factor synthesis were evaluated by specific enzyme-linked immunoadso rbent assay kits and Northern blotting analysis, Results: Pulmonary fibroblasts internalized silica particles without detect able cell damage. Silica directly stimulated collagen synthesis and decreas ed the amount of H-3-glucosamine-labeled GAG. Silica-treated fibroblasts se creted less TGF beta than untreated controls, antagonized the stimulatory e ffect of TGF beta on ECM synthesis, and reversed TGF beta -induced inhibiti on of cell proliferation. Northern blotting analysis showed increased inter leukin-1 alpha (IL-1 alpha) mRNA after silica treatment. IL-la! had no infl uence on collagen synthesis but increased the number of WI-1003 fibroblasts . Conclusions: These results support our hypothesis that lung fibroblasts are direct silica targets. However, contradicting our hypothesis, silica antag onized TGF beta activities through a TGF beta downregulation and an IL-1 al pha upregulation, The complex pattern of TGF beta and IL-1 alpha regulation in pulmonary fibroblasts is imbalanced by silica exposure and might play a key role in silica-mediated pulmonary fibrosis.