Enhanced expression of surfactant protein mRNAs has been observed in acute hyperoxic lung injury. We wondered whether similar changes in surfactant protein expression occurred during repair of injury leading to pulmonary fibrosis. We used the butylated hydroxytoluene (BHT) model of lung injury in the mouse because exposure to 70% oxygen during the period of alveolar reepithelialization results in chronic inflammation and pulmonary fibrosis, while BHT alone causes acute lung injury followed by full recovery. Since BHT is toxic primarily to type I cells, we postulated that surfactant protein-C(SP-C) expression would be altered during repair.

Male BALB/c mice, 6-8 weeks of age, were given BHT 400mg/kg IP or vehicle(corn oil) on d0. Half of each group also received 70% O2 for 5d. Mice were killed on d5, d10, d14, d21, and d42; there were 6 mice per time point per group. The right lung lobes were snap-frozen in liquid nitrogen for RNA analysis and the left lobes were inflated with 4% paraformaldehyde at 20cm H20 and fixed overnight. For histological analysis, specimens from 24h, 48h, and 6 months were also examined. Lung sections were stained with an anti-pro-SP-C antibody to identify type II cells. SP-C mRNA levels increased by d14, peaked by d21, and normalized by d42 after BHT/O2 (2.5-fold over air controls). SP-C mRNA levels were suppressed on d5 after BHT/air, then increased to 2-fold over air controls on d10, and normalized by d21. 70% O2 did not significantly change SP-C mRNA levels. Immunohistochemistry revealed altered type II cell morphology during the early repair phase after either BHT/O2 or BHT/air. After BHT/O2, there was subsequently a diffuse increase in type II cell density on d14 and d21; after BHT/air, foci of increased type II cell density were observed on d10 and d14. Type II cell density subsequently normalized in both groups.

We conclude that SP-C expression is increased after injury with either BHT/O2 or BHT/air, but that the increase is more pronounced and persistent when associated with pulmonary fibrosis. We speculate that this increased expression is due in part to increased numbers of type II cells resulting from the response to injury, and as remodeling ensues, the decrease in SP-C expression and type II cell number may be due to a type II to type I transition.