cDNA sequence and alternative mRNA splicing of surfactant-associated protein C (SP-C) in rabbit lung

Molecular characterization of the porcine surfactant, pulmonary-associated protein C gene

The surfactant, pulmonary-associated protein C (SFTPC) is a peptide secreted by the alveolar type II pneumocytes of the lung. We have characterized the porcine SFTPC gene at genomic, transcriptional, and protein levels. The porcine SFTPC is a single-copy gene on pig chromosome 14. Two transcripts were found in a newborn pig lung cDNA library: a full-length clone and a clone missing exon 5. cDNA sequence comparison revealed four synonymous and two nonsynonymous substitutions and in-frame insertions at the beginning of exon 5. Comparison of the SFTPC coding region between several mammals showed high levels of conservation. Northern blot studies showed lung-specific expression of the full-length SFTPC transcript, appearing in 50-day-old fetus and increasing during lung development. Both SFTPC transcripts were detected mainly in lung by real-time RT-PCR and they were significantly down-regulated in necrotic lungs of pigs infected with Actinobacillus pleuropneumoniae. Additionally, the protein levels were also decreased or absent in the necrotic tissue.

Ovine surfactant protein cDNAs: use in studies on fetal lung growth and maturation after prolonged hypoxemia

cDNAs for ovine surfactant-associated protein (SP) A, SP-B, and SP-C have been cloned and shown to possess strong similarity to cDNAs for surfactant apoproteins in other species. These reagents were employed to examine the effect of fetal hypoxia on the induction of surfactant apoprotein expression in the fetal lamb. Postnatal lung function is dependent on adequate growth and maturation during fetal development. Insulin-like growth factor (IGF) I and IGF-II, which are present in all fetal tissues studied, possess potent mitogenic and proliferative actions, and their effects can be modulated by IGF-specific binding proteins (IGFBPs). Hypoxia can lead to increases in circulating cortisol and catecholamines that can influence lung maturation. Therefore, the effects of mild hypoxia in chronically catheterized fetal lambs at gestational days 126-130 and 134-136 (term 145 days) on the expression of pulmonary surfactant apoproteins and IGFBPs were examined. Mild hypoxia for 48 h resulted in an increase in plasma cortisol that was more pronounced at later gestation, and in these animals, there was a twofold increase in SP-A mRNA. SP-B mRNA levels also increased twofold, but this was not significant. SP-C mRNA was not altered. No significant changes in apoprotein mRNA were observed with the younger fetuses. However, these younger animals selectively exhibited reduced IGFBP-5 mRNA levels. IGF-I mRNA was also reduced at 126-130 days, although this conclusion is tentative due to low abundance. IGF-II levels were not affected at either gestational age. We conclude that these data suggest that mild prolonged fetal hypoxia produces alterations that could affect fetal cellular differentiation early in gestation and can induce changes consistent with lung maturation closer to term.

Regulation of mRNA levels for pulmonary surfactant-associated proteins in developing rabbit lung

Gene transcriptional activities and steady-state mRNA levels have been examined for the surfactant-associated proteins SP-A, SP-B and SP-C in developing rabbit lung. It was observed SP-C mRNA levels increase early in gestation, while SP-A and SP-B mRNA levels increase rapidly between 26 and 30 days gestation. Transcriptional activities for all three surfactant apoproteins increase between 26 and 30 days. Studies conducted with fetal lung explants of 26 days gestation demonstrated exposure to low doses of dexamethasone increases SP-A and SP-C mRNA levels, while high doses stimulate transcription, although this only significant for SP-C. Time course studies revealed different temporal patterns and glucocorticoid responses for SP-A and SP-C mRNAs. SP-A and SP-C mRNA production and steady-state levels were reduced after treatment with cycloheximide. In contrast, SP-B gene transcription was selectively stimulated, suggesting involvement of a labile negative regulatory factory. It is concluded that expression of the three surfactant apoproteins is independently regulated. Early in gestation, SP-C mRNA levels may be regulated in vivo through message stabilization. Glucocorticoids can affect SP-A and SP-C mRNA levels in culture at both transcriptional and post-transcriptional levels. The ability of glucocorticoids to influence these processes declines during fetal development.

Human surfactant protein-C: genetic homogeneity and expression in RDS; comparison with other species

Human surfactant protein C (SP-C) mRNA is detected early during fetal lung development before the differentiation of the type II cell and the need for surfactant. Later in life SP-C contributes to the surface-lowering properties of surfactant, as shown by several investigators. In this study we sequenced both coding and noncoding regions of 12 genomic SP-C clones from several human groups including RDS, whites, and black Nigerians, and examined the expression of SP-C in tissues from RDS and from non-RDS. The data showed that all clones had identical DNA sequences, not only within coding regions, consistent with previous observations, but also within intervening, 5' flanking, and 3' untranslated regions. Some differences from the previously published sequence were noted. The expression of SP-C in tissues from RDS and non-RDS as determined by tissue in situ hybridization was comparable between the two groups, suggesting that altered SP-C expression, the result of pretranslational regulatory abnormalities, is an unlikely contributor to the pathogenesis of RDS. In addition the results show, using genomic blot analysis, that a remarkable conservation within coding and 5' flanking but not within 3' untranslated sequences exists in all mammalian species examined. These data taken together suggest that strong evolutionary pressures have been exerted on SP-C to maintain conservation, not only among humans but also among species, which may underscore important roles of SP-C in as yet unknown developmental/functional lung processes.