EGF enhances alveolar type II cell differentiation. We have previously reported that EGF stimulates the development of surfactant phospholipid synthesis in fetal rabbit lung explant cultures. The effect of EGF on the development of surfactant protein mRNA expression in fetal rabbit lung is not known. We hypothesized that EGF would enhance the developmental expression of surfactant proteins SP-A, SP-B, and SP-C in fetal rabbit lung. Fetal lung explant cultures were prepared from New Zealand White rabbits on days 21 and 24 of gestation (term 31 days). Cultures were grown in Waymouths medium plus 10% stripped fetal calf serum (stripping removes steroid hormones and mutiple growth factors) as control or with added EGF (10 ng/ml) for the treatment group. The culture medium and EGF were changed daily. Total RNA was extracted from lung explant cultures after 3, 5, and 7 days in culture. Northern blots were probed with cDNA's for rabbit SP-A, SP-B, and SP-C to study the effect of EGF on surfactant protein mRNA expression. Blots were stripped and rehybridized with a cDNA probe for ribosomal protein (RPL 32) as a denominator. Blots were evaluated using a Phosphorimager. In day 21 and day 24 cultures, mRNA for SP-A, SP-B and SP-C was present after day 3 of culture in both control and EGF-treated groups. Expression was higher for each surfactant protein in day 24 cultures than day 21 cultures throughout the study. Expression of SP-A, SP-B, and SP-C did not change with time in culture for either day 21 or day 24 cultures. EGF treatment did not affect the development of surfactant protein mRNA expression in either day 21 or day 24 cultures. These data indicate that the development of the three major surfactant proteins' mRNA expression does not mature in fetal rabbit lung explant culture when steroid hormones and growth factors are deficient. In such a system, EGF does not stimulate development of SP-A, SP-B, or SP-C mRNA levels. Both of these results differ from the development of surfactant phospholipid synthesis in this model. We conclude that fetal rabbit lung surfactant protein mRNA expression does not mature in a system lacking steroid and other hormones. In fetal rabbit lung, epithelial differentiation leading to surfactant protein synthesis is regulated differently from epithelial differentiation leading to surfactant phospholipid synthesis.
