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RETINOIC ACID AND GLUCOCORTICOIDS REGULATE SURFACTANT PROTEIN B GENE EXPRESSION IN HUMAN LUNG EPITHELIAL CELLS. • 337

Surfactant protein B (SP-B) plays a vital role in the pulmonary function of neonates; infants with inherited SP-B deficiency follow a course of progressive and fatal pulmonary failure unless they undergo lung transplantation. In previous studies, we have shown that all-trans retinoic acid (RA), a ligand of the steroid hormone receptor family, increases SP-B mRNA levels in H441 human lung adenocarcinoma cells and in cultured human fetal lung explants. In order to determine the temporal nature of the RA effect on SP-B mRNA, H441 cells were incubated with 10-6M all-trans RA for 1, 2, 4, 8, 12 or 24 hours. The stimulatory effect of all-trans RA on SP-B mRNA levels was observed as early as after 4 hours of exposure to the hormone. We also found that other retinoids, including 9-cis retinoic acid, 13-cis retinoic acid and retinol, do not affect SP-B mRNA levels in the H441 cell line. Glucocorticoids have been shown to increase SP-B mRNA levels in H441 cells and in cultured human fetal lung tissue. We hypothesized that the combination of glucocorticoids and all-trans RA regulates SP-B gene expression in a manner different from the effect of either agent alone. In initial experiments, H441 cells were treated in serum-free medium for 24 hours with cortisol (10-10M to 10-6M) added alone. SP-B mRNA levels were increased compared to control levels in a dose-dependent manner from 1.8 times over control levels in 10-9M cortisol to 4.7 times over control levels in 10-6M cortisol; there was no effect of 10-10M cortisol. H441 cells were then exposed to varying concentrations of cortisol (10-10 M to 10-6M) in the presence of all-trans RA (10-6M). SP-B mRNA levels were further stimulated to 2.8 times control levels in 10-9M cortisol and to 7 times control levels in 10-6M cortisol. At each concentration of cortisol, 10-9M to 10-6M, there was an average 40% increase in the stimulatory effect of cortisol on the level of SP-B mRNA with the addition of 10-6M all-trans RA. These data are suggestive that there is an additive effect of all-trans retinoic acid on cortisol-stimulated surfactant protein B gene expression in human lung epithelial cells.