Increased expression of matrix metalloproteinase in Clara cell-ablated mice inhaling crystalline silica.

We investigated the function of Clara cells in vivo during exposure to inhaled crystalline silica by examining pulmonary matrix metalloproteinase (MMP)-2 and MMP-9 mRNA levels in mice. The Clara cells of male FVB/n mice (8-12 weeks old) were ablated by intraperitoneal administration of naphthalene (300 mg/kg) in a corn oil vehicle. The mice were then exposed to crystalline silica (Min-U-Sil-5 silica, 97.1 +/- 9.5 mg/m(2), 6 hr/day, 5 days/week) for up to 2 weeks. Transcriptional levels of mRNA extracted from the lungs were assessed by reverse transcription-polymerase chain reaction. Gene expression of both MMP-2 and MMP-9 was significantly more marked in the Clara cell-ablated group than in the group with Clara cells, indicating that Clara cells inhibit MMP expression. Our findings suggest that Clara cells inhibit pulmonary inflammation induced by crystalline silica via MMPs in vivo.