Effect of Retinoic Acid on Mucin Gene Expression in Rat Airways in Vitro

Ultrastructural examination of rat tracheal explants at various times of culture in a serum-free and hormone-supplemented medium containing retinoic acid showed that the cytological characteristics of the epithelium were well preserved for at least 192 h. Hybridization analyses for mucin core protein mRNA in the explants were performed with a 30-base oligonucleotide probe, the design of which was based on the tandem repeat sequence of the rat intestine mucin core protein. The probe reacted with total RNA prepared from trachea, intestine and colon, but not with total RNA obtained from liver or alveolar region of the lung. Type-I keratin expression was observed in the explant grown at different periods of time in a medium with and without retinoic acid. The hybridization probe gave a prominent reaction with RNA preparations obtained from tracheal explants incubated for as long as 192 h in a medium containing retinoic acid. In the absence of retinoic acid, however, the mucin message was evident at the 24 h time point but thereafter decreased to barely detectable levels. When retinoic acid was added at 96 h to the latter cultures, the mucin mRNA was prominent again after additional incubation for 24 and 48 h. Northern-blot analyses of tracheal RNA showed a diffuse band at approx. 7.5 kb. Addition of a variety of chemical and pharmacological agents to explants cultured in the presence of retinoic acid had no dramatic induction or inhibitory effects on the mucin mRNA. Only the steroid prednisolone had a reproducible inhibitory effect.