RETINOIC ACID DIFFERENTIALLY REGULATES EXPRESSION OF SURFACTANT-ASSOCIATED PROTEINS IN HUMAN FETAL LUNG

Retinoic acid is known to play an essential role in maintaining the di fferentiation of a wide variety of epithelial cell types. However, its effects on the differentiation of lung alveolar epithelium have not b een described. In the present study, we examined the effects of retino ic acid on the differentiation of human fetal lung tissue maintained i n vitro. Human fetal lung explants were cultured in serum-free medium for 6 days in the absence or presence of all-trans retinoic acid at co ncentrations from 0.3 nm to 3 mum. Explant content of the surfactant-a ssociated protein SP-A was measured using a specific enzyme-linked imm unosorbent assay. Retinoic acid reduced SP-A protein levels in a conce ntration-dependent manner [analysis of variance (ANOVA), P < 0.01]. To evaluate possible cytotoxic effects of retinoic acid, culture media w ere assayed for lactate dehydrogenase (LDH), a cytoplasmic enzyme. LDH levels in media from retinoic acid-treated explants were not signific antly different than LDH levels in media from control explants, indica ting that retinoic acid is not cytotoxic in human fetal lung explants. Changes in messenger RNA (MRNA) levels for surfactant-associated prot eins SP-A, SP-B, and SP-C were measured by Northern blot analysis. Ret inoic acid reduced SP-A MRNA levels in a concentration-dependent manne r (ANOVA, P < 0.02) and reduced SP-C MRNA levels at 3 mum. In contrast , retinoic acid increased SP-B MRNA levels in a concentration-dependen t manner (ANOVA, P < 0.03). Morphometric analysis showed that retinoic acid decreased epithelial volume density in the explants by approxima tely 17% and increased connective tissue volume density by approximate ly 20% when compared to dimethyl sulfoxide vehicle controls. These dat a indicate that retinoic acid regulates type II cell surfactant protei n gene expression in human fetal lung tissue.