ABSTRACT
OBJECTIVES: This study was conducted in order to understand the cellular events associated with silica-induced pathogenesis of the rat lung. METHODS: Silicosis was induced by an intratracheal instillation of 50 mg of silica (SiO2, 0.15 - 10 micrometer) suspended in 500 microliter of a sterile saline solution in Sprague-Dawley rats weighing 200g. Silicotic nodules were excised from the rat lungs 4 weeks after silica instillation, then boiled for 4 days at 110 degrees in solution containing 2% SDS, 10 M urea and 40 mM DTT. The insoluble cellular encapsulates were electrophoresed on 4-12 % gradient SDS-PAGE, and the amino acid composition was analyzed. Affinity chromatographies of the homogenate supernatants of the control lung, silicotic nodule, and normal rat plasma were performed using rabbit IgG, anti-rat, cross-linked protein from the silicotic nodule. The amounts of N epsilon-(gamma-glutamyl) lysine cross-linked in the control lungs and silicotic nodules were determined using HPLC analysis. RESULTS: The remaining cross-linked protein was insoluble in the 10 M urea and 40 mM sulfhydryl reagents even under prolonged boiling conditions. The encapsulate revealed the retention of silica particles within the protein whose amino acid composition showed a high percentage of alanine, leucine and glycine. A 46 KDa protein was identified as a cross-linked protein in the silicotic nodule by affinity chromatography. The level of N epsilon-(gamma-glutamyl) lysine dipeptide in the nodule digest was prominently increased compared with that in the control lung. CONCLUSIONS: Transglutaminase (TGase)-catalyzed cross-linking appears to be involved in the silicotic nodule formation, and the 46 KDa protein may be cross-linked to itself and other extracellular matrix proteins during fibrosis and the formation of eventually insoluble nodule.