Modification of mucin gene expression in the airways of rats exposed to sulfur dioxide

The molecular mechanisms mediating mucous cell metaplasia and hypersecretion in the respiratory tract are unknown. Previous work suggests that mucous metaplasia requires the induction of mucin gene expression. We are investigating this possibility by monitoring steady state levels of mucin mRNA in a model of mucous cell metaplasia induced by SO₂ exposure. Male Sprague Dawley rats were exposed to 400 ppm SO₂ gas in air for 3h per day, 5 days per week for 0,1,2, or 3 weeks. Sham controls were exposed to air under similar conditions. After 3 weeks, morphological changes were apparent in the epithelium of SO₂ exposed rats at all levels from the trachea to the distal airways. The epithelial thickness increased, as well as the number and size of glands in the trachea. Epithelial mucous (goblet) cells increased from 0 to 4.5 per rom in the trachea, 0.2 to 6.2 per rom in the main stem bronchi, and 0.2 to 22.7 per rom in the distal airways (mean values obtained for 3-6 tissue blocks per airway level per condition). In parallel experiments, we used SMUC 41, a 950 bp human intestinal cDNA to isolate a human airway cDNA, HAM-1 from a cDNA library constructed in bacteriophage from human bronchial poly A+ RNA. HAM-1 is a 90 bp cDNA encoding a threonine- and proline-rich peptide with 96% homology to the human intestinal cDNA SMUC-41. Next we probed total and poly A+ airway RNA from rats in each exposure condition with SMUC-41 or HAM-1. Blots were then stripped and reprobed with a cDNA encoding beta actin. Densitometry values normalized for the amount of RNA loaded per lane (as determined by actin hybridization intensity) showed that mucin mRNA increased 8-9 fold as a function of SO₂ exposure. This is consistent with the possibility that mucin gene transcription is induced by SO₂ exposure, and may represent a primary event in the development of mucous metaplasia and hypersecretion.