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

The effect of repeated umbilical cord occlusions on pulmonary surfactant protein mRNA levels in the ovine fetus

OBJECTIVES

In this study we sought to determine the effect of brief repeated umbilical cord occlusions (rUCO) on surfactant protein (SP) mRNA levels in the fetal sheep lung at two different gestational ages.

METHODS

Fourteen fetuses at 112 to 115 days' gestation (control n = 7, rUCO n = 7) and 15 fetuses at 130 to 133 days' gestation (control n = 7, rUCO n = 8) were studied over 4 successive days with rUCO of 90 seconds duration performed every 30 minutes for 3 to 5 hours each day in the rUCO animals. Blood samples were collected for corticotrophin (ACTH) and cortisol measurements. Animals were killed within 1 hour of the final cord occlusion. SP-A, -B, -C, and -D mRNA levels were determined in lung tissue using a ribonuclease protection assay.

RESULTS

Cord occlusions resulted in temporary increases in circulating ACTH on day 1 with both gestational ages, but the elevations were blunted by day 4. Plasma cortisol levels increased transiently with the larger effect being observed on day 4, in particular with the near-term group. With advancing gestational age there was a significant (P < .05) increase in the level of SP-A (control 112-115 days: 0.01 +/- 0.01 vs control 130-133 days: 0.07 +/- 0.02 fmol/mg RNA), SP -B (control 112-115 days: 0.02 +/- 0.01 vs control 130-133 days: 0.07 +/- 0.01 fmol/mg RNA) and SP-C (control 112-115 days: 0.13 +/- 0.09 vs control 130-133 days: 0.51 +/- 0.10 fmol/mg RNA), but not SP-D mRNA levels (control 112-115 days: 0.002 +/- 0.002 vs control 130-133 days: 0.01 +/- 0.002 fmol/mg RNA). At 112 to 115 days, there was no significant change in any of the SP mRNA levels following rUCO compared to controls. However, the same regime of rUCO at 130 to 133 days resulted in an 85% reduction in SP-A and SP-B mRNA content and a 66% reduction in SP-C mRNA levels compared to controls.

CONCLUSION

The surprising decrease in SP-A and SP-B mRNA levels, which contrasts with other studies, suggests intermittent asphyxial episodes impact differently on surfactant apoprotein mRNA expression than does prolonged hypoxia.

Inflammatory and anti-inflammatory responsiveness of surfactant proteins in fetal and neonatal rabbit lung

Spontaneous preterm birth due to intrauterine infection is associated with increased concentrations of cytokines in amniotic fluid and in the airways at birth. Intra-amniotic IL-1 induces fetal lung maturity, consistent with the decrease in the incidence of respiratory distress syndrome (RDS) in intrauterine inflammation. On the other hand, antenatal corticosteroid decreases the incidence of RDS in infants born prematurely. The aim of the present study was to investigate the interaction between IL-1 and glucocorticoid in the expression of the surfactant proteins SP-A, -B, and -C. Lung explants from rabbit fetuses at 22 (immature), 27 (transitional), and 30 (mature) d of gestation (term, 30-31 d) and on d 1 after term birth were cultured with dexamethasone (Dx), IL-1alpha, or vehicle in the presence or absence of actinomycin D. According to the present results, IL-1alpha and Dx additively increased the expression of SP-A and SP-B on d 22. Later in gestation, SP-B and SP-C were suppressed by IL-1, whereas glucocorticoid tended to increase the expression of SP-B and SP-C and prevented the IL-1-induced suppression of SP. IL-1alpha and steroid interactively increased the stability of SP mRNA compared with the single agonist, possibly explaining the additive effects on the SP mRNA levels. The present results reveal beneficial additive effects of glucocorticoid and cytokine on lung surfactant. They may explain some of the acute beneficial effects of glucocorticoid therapy in chorioamnionitis before premature birth and in inflammatory lung disease after birth.

Retinoblastoma protein complexes with C/EBP proteins and activates C/EBP-mediated transcription

The retinoblastoma protein (RB) recruits histone deacetylase (HDAC) to repress E2F-mediated transactivation that plays a critical role in cell cycle regulation. RB is also involved in activation of expression of a number of tissue specific- and differentiation-related genes. In this study, we examined the mechanism by which RB stimulated the expression of a differentiation-related gene, the surfactant protein D (SP-D), which plays important roles in innate host defense and the regulation of surfactant homeostasis. We demonstrated that RB specifically stimulated the activity of human SP-D gene promoter. The RB family member, p107 but not p130, also increased SP-D promoter activity. Activation by RB was mediated through a NF-IL6 (C/EBP beta) binding motif in the human SP-D promoter, and this sequence specifically bound to C/EBP alpha, C/EBP beta, and C/EBP delta. RB formed stable complexes with all three C/EBP family members. RB small pocket (amino acid residues 379-792), but not the C-pocket (amino acid residues 792-928), was necessary and sufficient for its interaction with C/EBP proteins. Furthermore, we demonstrated that the complexes containing RB and C/EBP proteins directly interacted with C-EBP binding site on DNA. These findings indicate that RB plays a positive, selective, and direct role in the C/EBP-dependent transcriptional regulation of human SP-D expression.

Changes in surfactant-associated protein mRNA profile in growth-restricted fetal sheep

To test the hypothesis that chronic placental insufficiency resulting in fetal growth restriction causes an increase in fetal lung surfactant-associated protein (SP) gene expression, we embolized chronically catheterized fetal sheep (n = 6) daily using nonradioactive microspheres in the abdominal aorta for 21 days (between 0.74 and 0.88 of gestation) until fetal arterial oxygen content was reduced by approximately 40-50%. Control animals (n = 7) received saline only. Basal fetal plasma cortisol concentration was monitored. At the end of the experiment, fetal lung tissues were collected, and ratios of tissue levels of SP-A, SP-B, and SP-C mRNA to 18S rRNA were determined by standard Northern blot analysis. Total DNA content of fetal lungs was reduced by 30% in the embolized group compared with control group (P = 0.01). There was a 2.7-fold increase in fetal lung SP-A mRNA (P < 0.05) and a 3.2-fold increase in SP-B mRNA (P < 0.01) in the chronically embolized group compared with those in the control group. SP-A and SP-B mRNA tissue levels were highly correlated with the mean fetal plasma cortisol levels on days 20-21 (r = 0.90, P < 0.01 for SP-A mRNA and r = 0.94, P < 0.01 for SP-B mRNA). SP-C mRNA tissue levels were not significantly affected by placental insufficiency. We conclude that fetal growth restriction due to placental insufficiency is associated with alterations in fetal lung SP, suggesting enhanced lung maturation that was highly dependent on the degree of increase in fetal plasma cortisol levels.

Glucocorticoid deficiency delays stratum corneum maturation in the fetal mouse

The stratum corneum (SC) matures during late gestation in man and other mammals. Using the fetal rat as an experimental model, we have previously shown that glucocorticoids given in pharmacologic doses accelerate fetal SC maturation and barrier formation. To determine whether glucocorticoids are required for normal SC maturation, we examined the epidermal morphology of glucocorticoid-deficient (C-) murine pups, derived from matings of mice homozygous for null mutations of the corticotropin-releasing hormone alleles. In control pups on day 17.5 of gestation (term is 19.5 d), a multilayered SC was present and neutral lipid deposition in a membrane pattern was observed using Nile red fluorescence histochemistry. Ultrastructurally, mature lamellar unit structures predominate in the SC intercellular domains. In contrast, in C-pups only a single layer of SC was evident on day 17.5, and secreted lamellar material was not organized into mature lamellar structures. Furthermore, the expression of structural proteins necessary for cornified envelope formation, involucrin, loricrin, and filaggrin, and the activity of the lipid synthetic enzymes beta-glucocerebrosidase and steroid sulfatase, markers of barrier maturation, were reduced in day 17.5 C-pups. C-pups derived from pregnancies supplemented with physiologic amounts of cortisone, however, display normal SC ultrastructure on day 17.5 of gestation. Furthermore, at birth, both control and C-pups exhibit a multilayered SC replete with mature lamellar membrane structures. These data demonstrate that fetal glucocorticoid deficiency delays SC maturation, and suggests that normal levels of glucocorticoids are not absolutely required for SC development.

Glucocorticoid-induced effects on pattern formation and epithelial cell differentiation in early embryonic rat lungs

In this study, we examined the effects of dexamethasone (DEX) on airway branching and subsequent lung maturation. DEX treatment of fetal rat lung explants was initiated during the early pseudoglandular stage of development. Day 14 fetal lung explants were cultured with and without DEX for 4 d. Explants treated with 10 nM or higher concentrations of DEX showed features of both distorted and accelerated maturation. DEX-treated lungs had growth retardation, distorted branching, dilated proximal tubules, and suppressed proliferation of epithelial cells of the distal tubules. Several biochemical and morphologic features of accelerated maturation were also observed: 1) the epithelial cells lining the distal tubules (prospective respiratory airways) were generally cuboidal or flattened; 2) the cuboidal cells often contained lamellar bodies and abundant glycogen; 3) rudimentary septa and large airspace were present; 4) mesenchymal tissue was attenuated and compressed between adjacent epithelial tubules; 5) the distribution of SP-C mRNA in distal tubules was more mature, with individual and clusters of cells expressing SP-C transcripts; and 6) the transcript levels of several genes related to epithelial growth [keratinocyte growth factor (KGF), KGF receptor, and hepatocyte growth factor receptor] and differentiation [surfactant proteins, SP-A, SP-B and SP-C and the Clara cell secretory protein, CC10] were precociously increased. These results show that DEX treatment of the lung during the early pseudoglandular stage accelerates the acquisition of several features of advanced maturation that normally accompany late stages of fetal development. We postulate that KGF mediates at least some effects of DEX on lung maturation and gene expression.