Surfactant protein B in human fetal lung: developmental and glucocorticoid regulation

Trafficking of newly synthesized surfactant protein B to the lamellar body in alveolar type II cells

Lung surfactant accumulates in the lamellar body (LB) via not only the secretory (anterograde) pathway but also the endocytic (retrograde) pathway. Our previous studies suggested that the major surfactant components, phosphatidylcholine and surfactant protein A take independent trafficking routes in alveolar type II cells. Thus, trafficking of surfactant protein B (SP-B), a major hydrophobic surfactant apoprotein, should be re-evaluated by a straightforward method. Radiolabeling of cells and subsequent cell fractionation were employed to pursue the sequential trafficking of newly synthesized SP-B in rabbit alveolar type II cells. The LB fraction was prepared by gradient ultracentrifugation. Immunoprecipitation from the culture medium, total cells, and LB fraction was carried out with anti-SP-B antibody. Newly synthesized [³⁵S]-pro-SP-B (~ 42 kDa) was detected in the cells after 1 h. An ~ 8-kDa mature form of [³⁵S]-SP-B was detected in the cells after 3 h and in the LB after 6 h. Mature [³⁵S]-SP-B was predominant in the cells after 24 h, and the dominant portion was present in the LB. In contrast, only a small amount of mature [³⁵S]-SP-B was present in the culture medium. Molecular processing of ~ 42 kDa [³⁵S]-pro-SP-B and transport to the LB was inhibited by brefeldin A, which disassembles the Golgi apparatus. These results suggest that newly synthesized SP-B is sorted to the LB via the Golgi and stored until exocytosis. This pathway is distinct from the pathways reported for phosphatidylcholine and surfactant protein A.

Developmental Decline in the MicroRNA 199a (miR-199a)/miR-214 Cluster in Human Fetal Lung Promotes Type II Cell Differentiation by Upregulating Key Transcription Factors

The major surfactant protein, SP-A (a product of the SFTPA gene), serves as a marker of type II pneumocyte differentiation and surfactant synthesis. SFTPA expression in cultured human fetal lung (HFL) epithelial cells is upregulated by hormones that increase cyclic AMP (cAMP) and activate TTF-1/NKX2.1 and NF-κB. To further define mechanisms for type II cell differentiation and induction of SP-A, we investigated roles of microRNAs (miRNAs). Using microarray to identify differentially expressed miRNAs in HFL epithelial cells during type II cell differentiation in culture, we observed that members of the miRNA 199a (miR-199a)/miR-214 cluster were significantly downregulated during differentiation. Validated and predicted targets of miR-199a-3p/miR-199a-5p and miR-214, which serve roles in type II cell differentiation (COX-2, NF-κB p50/p65, and CREB1), and the CREB1 target, C/EBPβ, were coordinately upregulated. Accordingly, overexpression of miR-199a-5p, miR-199a-3p, or miR-214 mimics in cultured HFL epithelial cells decreased COX-2, NF-κB p50/p65, CREB1, and C/EBPβ proteins, with an associated inhibition of SP-A expression. Interestingly, overexpression of the EMT factor, ZEB1, which declines during cAMP-induced type II cell differentiation, increased pri-miR-199a and reduced the expression of the targets NF-κB/p50 and COX-2. Collectively, these findings suggest that the developmental decline in miR-199a/miR-214 in HFL causes increased expression of critical targets that enhance type II cell differentiation and SP-A expression.

Sonic Hedgehog Agonist Protects Against Complex Neonatal Cerebellar Injury

The cerebellum undergoes rapid growth during the third trimester and is vulnerable to injury and deficient growth in infants born prematurely. Factors associated with preterm cerebellar hypoplasia include chronic lung disease and postnatal glucocorticoid administration. We modeled chronic hypoxemia and glucocorticoid administration in neonatal mice to study whole cerebellar and cell type-specific effects of dual exposure. Chronic neonatal hypoxia resulted in permanent cerebellar hypoplasia. This was compounded by administration of prednisolone as shown by greater volume loss and Purkinje cell death. In the setting of hypoxia and prednisolone, administration of a small molecule Smoothened-Hedgehog agonist (SAG) preserved cerebellar volume and protected against Purkinje cell death. Such protective effects were observed even when SAG was given as a one-time dose after dual insult. To model complex injury and determine cell type-specific roles for the hypoxia inducible factor (HIF) pathway, we performed conditional knockout of von Hippel Lindau (VHL) to hyperactivate HIF1α in cerebellar granule neuron precursors (CGNP) or Purkinje cells. Surprisingly, HIF activation in either cell type resulted in no cerebellar deficit. However, in mice administered prednisolone, HIF overactivation in CGNPs resulted in significant cerebellar hypoplasia, whereas HIF overactivation in Purkinje cells caused cell death. Together, these findings indicate that HIF primes both cell types for injury via glucocorticoids, and that hypoxia/HIF + postnatal glucocorticoid administration act on distinct cellular pathways to cause cerebellar injury. They further suggest that SAG is neuroprotective in the setting of complex neonatal cerebellar injury.

Wnt5a reverses the inhibitory effect of hyperoxia on transdifferentiation of alveolar epithelial type II cells to type I cells

Transdifferentiation of alveolar epithelial type II cells (AECIIs) to type I cells (AECIs) is critical for reestablishment and maintenance of an intact alveolar epithelium. However, this process is frequently destroyed by hyperoxia treatment, which is commonly used in respiratory distress syndrome therapy in preterm infants. Wnt5a is considered to participate in this physiopathologic process, but the clear mechanisms still need to be further investigated. In this study, preterm rats and primary rat AECIIs were exposed to hyperoxia. Hematoxylin and eosin staining was used to examine the histological changes of the lungs. Real-time PCR and western blotting were used to examine Wnt5a expression and biomarkers of AECII and AECI expression. Immunohistochemistry and immunofluorescence were also used to determine the expression and location of selected biomarkers. Furthermore, AECIIs transfected with Wnt5a gene and exogenous Wnt5a were used to examine whether Wnt5a contributes to the transdifferentiation of AECIIs to AECIs. Results showed that hyperoxia inhibited the transdifferentiation of AECIIs to AECIs in vitro, which is represented by biomarkers of two types of cell that remained unchanged. In addition, Wnt5a protein expression was found to be decreased after hyperoxia exposure in vitro and in vivo. Furthermore, both the overexpression of Wnt5a and exogenous Wnt5a addition blocked the inhibitory effect of hyperoxia in vitro. In conclusion, our results suggest that the transdifferentiation of AECIIs to AECIs is impaired by hyperoxia, and this process may be associated with Wnt5a downregulation. Targeting Wnt5a may have the potential for the therapy of lung injury in preterm infants induced by hyperoxia.

Glucocorticoid genes and the developmental origins of asthma susceptibility and treatment response

Antenatal corticosteroids enhance lung maturation. However, the importance of glucocorticoid genes on early lung development, asthma susceptibility, and treatment response remains unknown. We investigated whether glucocorticoid genes are important during lung development and their role in asthma susceptibility and treatment response. We identified genes that were differentially expressed by corticosteroids in two of three genomic datasets: lymphoblastoid cell lines of participants in the Childhood Asthma Management Program, a glucocorticoid chromatin immunoprecipitation/RNA sequencing experiment, or a murine model; these genes made up the glucocorticoid gene set (GCGS). Using gene expression profiles from 38 human fetal lungs and C57BL/6J murine fetal lungs, we identified developmental genes that were in the top 5% of genes contributing to the top three principal components (PCs) most highly associated with post-conceptional age. Glucocorticoid genes that were enriched in this set of developmental genes were then included in the developmental glucocorticoid gene set (DGGS). We then investigated whether glucocorticoid genes are important during lung development, and their role in asthma susceptibility and treatment response. A total of 232 genes were included in the GCGS. Analysis of gene expression demonstrated that glucocorticoid genes were enriched in lung development (P = 7.02 × 10(-26)). The developmental GCGS was enriched for genes that were differentially expressed between subjects with asthma and control subjects (P = 4.26 × 10(-3)) and were enriched after treatment of subjects with asthma with inhaled corticosteroids (P < 2.72 × 10(-4)). Our results show that glucocorticoid genes are overrepresented among genes implicated in fetal lung development. These genes influence asthma susceptibility and treatment response, suggesting their involvement in the early ontogeny of asthma.

Association of vitamin D deficiency with acute lower respiratory tract infections in newborns

OBJECTIVE

To determine the association between serum 25-hydroxy vitamin D [25(OH)D] levels and acute respiratory tract infections (ALRTI) in newborns.

STUDY DESIGN

The study group consisted of 30 term newborns with ALRTI who were admitted to our neonatal intensive care unit. Controls were 30 healthy newborns with the same age as the study group. Newborns and their mothers were tested for serum 25(OH)D levels, with a low level defined as ≤15 ng/mL.

RESULTS

The groups were similar in gestational week, birthweight, postnatal age and gender. Forty-three of the 60 infants (including study and control) had low 25(OH)D levels. The median 25(OH)D levels were lower [9.5 ng/mL (IQR = 7.9-12.2)] in the study group than those of the control group [15.5 ng/mL (IQR: 12-18)] (p = 0.0001). The median serum 25(OH)D levels in the mothers of the study group were also lower than those in the mothers of the control group [11.6 ng/mL (IQR = 9.4-15.8) and 17.3 ng/mL (IQR = 13.7-20.6), respectively] (p = 0.0001).

CONCLUSION

Lower blood 25(OH)D levels might be associated with increased risk of ALRTI in term newborn babies. Appropriate vitamin D supplementation during pregnancy and early childhood may enhance newborns' respiratory health.

Allele-specific N-glycosylation delays human surfactant protein B secretion in vitro and associates with decreased protein levels in vivo

BACKGROUND

Surfactant protein B (SP-B) is essential for normal lung function, and decreased concentrations of SP-B have a deleterious effect on pulmonary outcome. SP-B levels may correlate with variations in the encoding gene (SFTPB). SFTPB single-nucleotide polymorphism Ile131Thr affects proSP-B N-glycosylation in humans and the glycosylated Thr variant associates with pulmonary diseases.

METHODS

We analyzed SP-B levels in amniotic fluid samples for associations with SFTPB polymorphisms and generated cell lines expressing either proSP-B/131Ile or proSP-B/131Thr for examining the effect of glycosylation on proSP-B secretion kinetics. To determine any transcription preference between Ile131Thr allelic variants, we used heterozygous human lungs for allelic expression imbalance assays.

RESULTS

Protein levels correlated with Ile131Thr genotype and the lowest SP-B levels were observed in Thr/Thr homozygotes. Our results suggest that Ile131Thr variation-dependent N-glycosylation associates with decreased levels of SP-B, which is secreted from fetal lung to amniotic fluid. Glycosylated proSP-B/131Thr was secreted from transfected cells at a lower rate than nonglycosylated proSP-B/131Ile. Expression levels of the mRNA variants were equal. Secretion of the glycosylated variant was thus delayed in vitro by a posttranscriptional mechanism.

CONCLUSION

These data support the hypothesis that proSP-B glycosylation due to Ile131Thr variation may have a causal role in genetic susceptibility to acute respiratory distress.

A small-molecule smoothened agonist prevents glucocorticoid-induced neonatal cerebellar injury

Glucocorticoids are used for treating preterm neonatal infants suffering from life-threatening lung, airway, and cardiovascular conditions. However, several studies have raised concerns about detrimental effects of postnatal glucocorticoid administration on the developing brain leading to cognitive impairment, cerebral palsy, and hypoplasia of the cerebellum, a brain region critical for coordination of movement and higher-order neurological functions. Previously, we showed that glucocorticoids inhibit Sonic hedgehog-Smoothened (Shh-Smo) signaling, the major mitogenic pathway for cerebellar granule neuron precursors. Conversely, activation of Shh-Smo in transgenic mice protects against glucocorticoid-induced neurotoxic effects through induction of the 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) pathway. Here, we show that systemic administration of a small-molecule agonist of the Shh-Smo pathway (SAG) prevented the neurotoxic effects of glucocorticoids. SAG did not interfere with the beneficial effects of glucocorticoids on lung maturation, and despite the known associations of the Shh pathway with neoplasia, we found that transient (1-week-long) SAG treatment of neonatal animals was well tolerated and did not promote tumor formation. These findings suggest that a small-molecule agonist of Smo has potential as a neuroprotective agent in neonates at risk for glucocorticoid-induced neonatal cerebellar injury.

Opposing regulation of human alveolar type II cell differentiation by nitric oxide and hyperoxia

Clinical trials demonstrated decreasing rates of bronchopulmonary dysplasia in preterm infants with hypoxic respiratory failure treated with inhaled nitric oxide (iNO). However, the molecular and biochemical effects of iNO on developing human fetal lungs remain vastly unknown. By using a well-characterized model of human fetal alveolar type II cells, we assessed the effects of iNO and hyperoxia, independently and concurrently, on NO-cGMP signaling pathway and differentiation. Exposure to iNO increased cGMP levels by 40-fold after 3 d and by 8-fold after 5 d despite constant expression of phosphodiesterase-5 (PDE5). The levels of cGMP declined significantly on exposure to iNO and hyperoxia at 3 and 5 d, although expression of soluble guanylyl cyclase (sGC) was sustained. Surfactant proteins B and C (SP-B, SP-C) and thyroid transcription factor (TTF)-1 mRNA levels increased in cells exposed to iNO in normoxia but not on exposure to iNO plus hyperoxia. Collectively, these data indicate an increase in type II cell markers when undifferentiated lung epithelial cells are exposed to iNO in room air. However, hyperoxia overrides these potentially beneficial effects of iNO despite sustained expression of sGC.

In vitroquantification of dexamethasone-induced surfactant protein B expression in human lung cells

OBJECTIVE

To determine whether the effect of a single 48-h exposure to dexamethasone in human lung cells is limited to 7-8 days.

STUDY DESIGN

We used the NCI-H441 cell line, in which stability can be maintained beyond 7 days. The outcome was the stimulatory effect of dexamethasone on surfactant protein B (SP-B) gene transcription as expressed by SP-B mRNA accumulation. The experiment was conducted five times, in parallel with control. SP-B mRNA was determined at baseline, 48 h after dexamethasone exposure, and at 48-h intervals thereafter, up to 14 days, by quantitative reverse transcription polymerase chain reaction. Comparisons were made by the Mann-Whitney test.

RESULTS

In conditions of our experiment, the inductive profile of SP-B mRNA after exposure to dexamethasone demonstrated maximal stimulation at 48 h (13-fold over control). Subsequently, there was a decline in mRNA, with return to near control levels by day 8, suggesting reversibility of dexamethasone action.

CONCLUSION

Our data support the view that the surfactant-inducing properties of corticosteroids are limited to 7-8 days.

Posttranslational regulation of surfactant protein B expression

Although a minor constituent by weight, surfactant protein B (SP-B) plays a major role in surfactant function. It is the unique structure of SP-B that promotes permeabilization, cross-linking, mixing, and fusion of phospholipids, facilitating the proper structure and function of pulmonary surfactant as well as contributing to the formation of lamellar bodies. SP-B production is a complex process within alveolar type 2 cells and is under hormonal and developmental control. Understanding the posttranslational events in the maturation of SP-B may provide new insight into the process of lamellar body formation and into the pathophysiology of pulmonary disorders associated with surfactant abnormalities.

Mechanical ventilation down-regulates surfactant protein A and keratinocyte growth factor expression in premature rabbits

Surfactant-associated proteins (SP-A, SP-B, and SP-C) are critical for the endogenous function of surfactant. Keratinocyte growth factor (KGF) and vascular endothelial growth factor (VEGF) are key regulators of lung development. The objective of this study was to evaluate the effects of early mechanical ventilation on the expression of these important regulatory proteins in a preterm rabbit model. Premature fetuses were delivered at 29 d of gestation and randomized to necropsy at birth, i.e. no ventilation (NV), spontaneous breathing (SB), or mechanical ventilation (MV) for 16 h. MV animals were further randomized to treatment with dexamethasone (dex). Our findings showed that SB rabbits increased their expression of SP-A mRNA and protein after birth compared with NV controls. MV significantly attenuated this response in the absence of dex. Exposure to dex elevated SP-B mRNA expression in both SB and MV rabbits. KGF protein levels were markedly increased in SB animals compared with MV counterparts. VEGF levels were similar in SB and MV animals, but were significantly increased compared with NV controls. These data suggest that MV alters surfactant-associated protein and growth factor expression, which may contribute to injury in the developing lung.

Surfactant function and composition in premature infants treated with inhaled nitric oxide

OBJECTIVES

We hypothesized that inhaled nitric oxide treatment of premature infants at risk for bronchopulmonary dysplasia would not adversely affect endogenous surfactant function or composition.

METHODS

As part of the Nitric Oxide Chronic Lung Disease Trial of inhaled nitric oxide, we examined surfactant in a subpopulation of enrolled infants. Tracheal aspirate fluid was collected at specified intervals from 99 infants with birth weights <1250 g who received inhaled nitric oxide (20 ppm, weaned to 2 ppm) or placebo gas for 24 days. Large-aggregate surfactant was analyzed for surface activity with a pulsating bubble surfactometer and for surfactant protein contents with an immunoassay.

RESULTS

At baseline, before administration of study gas, surfactant function and composition were comparable in the 2 groups, and there was a positive correlation between minimum surface tension and severity of lung disease for all infants. Over the first 4 days of treatment, minimum surface tension increased in placebo-treated infants and decreased in inhaled nitric oxide-treated infants. There were no significant differences between groups in recovery of large-aggregate surfactant or contents of surfactant protein A, surfactant protein B, surfactant protein C, or total protein, normalized to phospholipid.

CONCLUSIONS

We conclude that inhaled nitric oxide treatment for premature infants at risk of bronchopulmonary dysplasia does not alter surfactant recovery or protein composition and may improve surfactant function transiently.

ABCA3 is critical for lamellar body biogenesis in vivo

Mutations in ATP-binding cassette transporter A3 (human ABCA3) protein are associated with fatal respiratory distress syndrome in newborns. We therefore characterized mice with targeted disruption of the ABCA3 gene. Homozygous Abca3-/- knock-out mice died soon after birth, whereas most of the wild type, Abca3+/+, and heterozygous, Abca3+/-, neonates survived. The lungs from E18.5 and E19.5 Abca3-/- mice were less mature than wild type. Alveolar type 2 cells from Abca3-/- embryos contained no lamellar bodies, and expression of mature SP-B protein was disrupted when compared with the normal lung surfactant system of wild type embryos. Small structural and functional differences in the surfactant system were seen in adult Abca3+/- compared with Abca3+/+ mice. The heterozygotes had fewer lamellar bodies, and the incorporation of radiolabeled substrates into newly synthesized disaturated phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, and phosphatidylserine in both lamellar bodies and surfactant was lower than in Abca3+/+ mouse lungs. In addition, since the fraction of near term Abca3-/- embryos was significantly lower than expected from Mendelian inheritance ABCA3 probably plays roles in development unrelated to surfactant. Collectively, these findings strongly suggest that ABCA3 is necessary for lamellar body biogenesis, surfactant protein-B processing, and lung development late in gestation.

Lipidomics of cellular and secreted phospholipids from differentiated human fetal type II alveolar epithelial cells

Maturation of fetal alveolar type II epithelial cells in utero is characterized by specific changes to lung surfactant phospholipids. Here, we quantified the effects of hormonal differentiation in vitro on the molecular specificity of cellular and secreted phospholipids from human fetal type II epithelial cells using electrospray ionization mass spectrometry. Differentiation, assessed by morphology and changes in gene expression, was accompanied by restricted and specific modifications to cell phospholipids, principally enrichments of shorter chain species of phosphatidylcholine (PC) and phosphatidylinositol, that were not observed in fetal lung fibroblasts. Treatment of differentiated epithelial cells with secretagogues stimulated the secretion of functional surfactant-containing surfactant proteins B and C (SP-B and SP-C). Secreted material was further enriched in this same set of phospholipid species but was characterized by increased contents of short-chain monounsaturated and disaturated species other than dipalmitoyl PC (PC16:0/16:0), principally palmitoylmyristoyl PC (PC16:0/14:0) and palmitoylpalmitoleoyl PC (PC16:0/16:1). Mixtures of these PC molecular species, phosphatidylglycerol, and SP-B and SP-C were functionally active and rapidly generated low surface tension on compression in a pulsating bubble surfactometer. These results suggest that hormonally differentiated human fetal type II cells do not select the molecular composition of surfactant phospholipid on the basis of saturation but, more likely, on the basis of acyl chain length.

Regulation of human pulmonary surfactant protein gene expression by 1α,25-dihydroxyvitamin D3

1α,25-Dihydroxyvitamin D3[1,25(OH)2D3] has been reported to stimulate lung maturity, alveolar type II cell differentiation, and pulmonary surfactant synthesis in rat lung. We hypothesized that 1,25(OH)2D3stimulates expression of surfactant protein-A (SP-A), SP-B, and SP-C in human fetal lung and type II cells. We found that immunoreactive vitamin D receptor was detectable in fetal lung tissue and type II cells only when incubated with 1,25(OH)2D3. 1,25(OH)2D3significantly decreased SP-A mRNA in human fetal lung tissue but did not significantly decrease SP-A protein in the tissue. In type II cells, 1,25(OH)2D3alone had no significant effect on SP-A mRNA or protein levels but reduced SP-A mRNA and protein in a dose-dependent manner when the cells were incubated with cAMP. SP-A mRNA levels in NCI-H441 cells, a nonciliated bronchiolar epithelial (Clara) cell line, were decreased in a dose-dependent manner in the absence or presence of cAMP. 1,25(OH)2D3had no significant effect on SP-B mRNA levels in lung tissue but increased SP-B mRNA and protein levels in type II cells incubated in the absence or presence of cAMP. Expression of SP-C mRNA was unaffected by 1,25(OH)2D3in lung tissue incubated ± cAMP. These results suggest that regulation of surfactant protein gene expression in human lung and type II cells by 1,25(OH)2D3is not coordinated; 1,25(OH)2D3decreases SP-A mRNA and protein levels in both fetal lung tissue and type II cells, increases SP-B mRNA and protein levels only in type II cells, and has no effect on SP-C mRNA levels.

Surfactant metabolism in the neonate

With the use of stable isotope-labeled intravenous precursors for surfactant phosphatidylcholine (PC) synthesis, it has been shown that the de novo synthesis rates in preterm infants with respiratory distress syndrome (RDS) are very low as are turnover rates. This is consistent with animal data. Surfactant therapy does not inhibit endogenous surfactant synthesis, and prenatal corticosteroids stimulate it. With the use of stable isotope-labeled PC given endotracheally, surfactant pool size was estimated. It turned out to be low in RDS, as expected. Similar studies were performed in term neonates with severe lung diseases. In general, patients with lung injury show a lower surfactant synthesis. The controversy around surfactant in congenital diaphragmatic hernia (CDH) persists: studies on CDH with and without extracorporeal membrane oxygenation yielded different results. In severe meconium aspiration syndrome surfactant synthesis was found to be decreased but surfactant pool size was maintained. It is possible and safe to study surfactant metabolism in human neonates with the use of stable isotopes. This can help in answering clinical questions and has the potential to bring new in vitro and animal findings about surfactant metabolism to the patient.

Interstitial lung disease in children -- genetic background and associated phenotypes

Interstitial lung disease in children represents a group of rare chronic respiratory disorders. There is growing evidence that mutations in the surfactant protein C gene play a role in the pathogenesis of certain forms of pediatric interstitial lung disease. Recently, mutations in the ABCA3 transporter were found as an underlying cause of fatal respiratory failure in neonates without surfactant protein B deficiency. Especially in familiar cases or in children of consanguineous parents, genetic diagnosis provides an useful tool to identify the underlying etiology of interstitial lung disease. The aim of this review is to summarize and to describe in detail the clinical features of hereditary interstitial lung disease in children. The knowledge of gene variants and associated phenotypes is crucial to identify relevant patients in clinical practice.

Progressive Lung Disease and Surfactant Dysfunction with a Deletion in Surfactant Protein C Gene

Mutations in the surfactant protein (SP)-C gene are responsible for familial and sporadic interstitial lung disease (ILD). The consequences of such mutations on pulmonary surfactant composition and function are poorly understood. To determine the effects of a mutation in the SP-C gene on surfactant, we obtained lung tissue at the time of transplantation from a 14-mo-old infant with progressive ILD. An in-frame 9-bp deletion spanning codons 91-93 in Exon 3 of the SP-C gene was present on one allele; neither parent carried this deletion. SP-C mRNA was present in normal size and amount. By immunofluorescence, proSP-C was aggregated within alveolar Type II cells in a compartment separate from SP-B. In airway surfactant, there was little or no mature SP-B or SP-C; SP-A content was increased. Minimum surface tension was increased (20 mN/m, normal < 5 mN/m). Type II cells contained normal and disorganized appearing lamellar bodies by electron microscopy. This spontaneous deletion on one allele of the SP-C gene was associated with sporadic ILD and abnormalities in surfactant composition and function. We propose that a dominant negative effect on surfactant protein metabolism and function results from aggregation of misfolded proSP-C and subsequent cell injury and inflammation.

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.

Surfactant protein profile of pulmonary surfactant in premature infants

Although premature infants are known to be deficient in pulmonary surfactant, there is limited information regarding surfactant protein (SP) composition. To assess the postnatal profile of SPs, tracheal aspirate samples were collected from 35 intubated infants of 23-31 weeks of gestation between 8 and 80 days of age. In 71 large aggregate surfactant samples that had normal in vitro function (minimum surface tension of less than 1 mN/m by pulsating bubble surfactometry), mean +/- SEM contents of SP-A, SP-B, and SP-C (3.7 kD) were 7.1 +/- 1.4%, 1.8 +/- 0.2%, and 4.6 +/- 0.6%, respectively, of phospholipid. To assess SPs in the 1st week of life, we analyzed samples from additional infants receiving only synthetic replacement surfactant. On the 2nd day of life, contents of SP-A, SP-B, and SP-C were 13.4%, 8.4%, and 0.1%, respectively, of the mean levels for Day 8-80 samples. The major postnatal increases for SP-A, SP-B, and SP-C occurred during the 1st, 2nd, and 3rd weeks, respectively. We conclude that surfactant of newborn premature infants is markedly deficient in SPs, in particular SP-C. Despite continuing lung disease, some infants who are more than 1 week of age have surfactant with normal in vitro function that contains SPs at levels comparable to adult surfactant.

Pepsinogen C: a type 2 cell-specific protease

Pepsinogen C, also known as progastricsin or pepsinogen II, is an aspartic protease expressed primarily in gastric chief cells. Prior microarray studies of an in vitro model of type 2 cell differentiation indicated that pepsinogen C RNA was highly induced, comparable to surfactant protein RNA induction. Using second-trimester human fetal lung, third-trimester postnatal and adult lung, and a model of type 2 cell differentiation, we examined the specificity of pepsinogen C expression in lung. Pepsinogen C RNA and protein were only detected in >22 wk gestation samples of neonatal lung or in adult lung tissue. By immunohistochemistry and in situ hybridization, pepsinogen C expression was restricted to type 2 cells. Pepsinogen C expression was rapidly induced during type 2 cell differentiation and rapidly quenched with dedifferentiation of type 2 cells after withdrawal of hormones. In all samples, pepsinogen C expression occurred concomitantly with or in advance of processing of surfactant protein-B to its mature 8-kDa form. Our results indicate that pepsinogen C is a type 2 cell-specific marker that exhibits tight developmental regulation in vivo during human lung development, as well as during in vitro differentiation and dedifferentiation of type 2 cells.

In vitro surfactant protein B deficiency inhibits lamellar body formation

Surfactant protein (SP) B is essential for normal pulmonary surfactant activity and lamellar body genesis in type 2 cells. However, the role of SP-B in lamellar body genesis is poorly understood. We developed an adenovirus vector expressing antisense SP-B as an alternative in vitro model of SP-B deficiency to begin to explore the role of SP-B in lamellar body genesis. RT-PCR analysis revealed that antisense SP-B expression interfered with translation of endogenous SP-B mRNA. Antisense SP-B expression resulted in reliable in vitro reproduction of many features of SP-B deficiency, including absent mature SP-B and decreased lamellar bodies and SP-C. Light and electron microscopy demonstrated significant reductions in lamellar body number. Western blotting revealed a significant reduction in mature 8-kD SP-B protein and decreased mature SP-C. Our data indicate that antisense SP-B can be effectively used to replicate the SP-B-deficient type 2 cell phenotype in vitro, and provides an attractive alternative to transgenic models for the further study of the role of SP-B in lamellar body genesis.

Cysteine protease activity is required for surfactant protein B processing and lamellar body genesis

Surfactant protein (SP)-B is essential for lamellar body genesis and for the final steps in proSP-C post-translational processing. The mature SP-B protein is derived from multistep processing of the primary translation product proSP-B; however, the enzymes required for these events are currently unknown. Recent ultrastructural colocalization studies have suggested that the cysteine protease Cathepsin H may be involved in proSP-B processing. Using models of isolated human type 2 cells in culture, we describe the effects of cysteine protease inhibition by E-64 on SP-B processing and type 2 cell differentiation. Pulse-chase labeling and Western immunoblotting studies showed that the final step of SP-B processing, specifically cleavage of SP-B(9) to SP-B(8), was significantly inhibited by E-64, resulting in delayed accumulation of SP-B(8) without adverse effects on SP-A or glyceraldehyde phosphate dehydrogenase expression. E-64 treatment during type 2 cell differentiation mimicked features of inherited SP-B deficiency in humans and mice, specifically disrupted lamellar body genesis, and aberrant processing of proSP-C. Reverse transcriptase-polymerase chain reaction and Western immunoblotting studies showed that Cathepsin H is induced during in vitro differentiation of type 2 cells and localizes with SP-B in multivesicular bodies, composite bodies, and lamellar bodies by immunoelectron microscopy. Furthermore, Cathepsin H activity was specifically inhibited in a dose-dependent fashion by E-64. Our data show that a cysteine protease is involved in SP-B processing, lamellar body genesis, and SP-C processing, and suggest that Cathepsin H is the most likely candidate protease.

Differentiation of human pulmonary type II cells in vitro by glucocorticoid plus cAMP

Mature alveolar type II cells that produce pulmonary surfactant are essential for adaptation to extrauterine life and prevention of infant respiratory distress syndrome. We have developed a new in vitro model to further investigate regulation of type II cell differentiation. Epithelial cells isolated from human fetal lung were cultured in serum-free medium on plastic. Cells treated with dexamethasone + cAMP analog and isobutylmethylxanthine for 4 days exhibited increased phosphatidylcholine synthesis and content of disaturated phosphatidylcholine species, manyfold increases in all surfactant proteins with processing to mature forms, and abundant lamellar bodies. DNA microarray analysis identified approximately 3,100 expressed genes, including subsets of genes induced 2- to >100-fold (approximately 2.5%) or repressed 2- to 18-fold (approximately 1.2%) by hormone treatment. Of the highly regulated genes, most were coregulated in an additive or synergistic manner by dexamethasone and cAMP agents. Approximately 90% of the regulated genes identified by this initial microarray analysis have not been previously recognized as hormone responsive. One newly identified hormone-induced gene is Nkx2.1 (thyroid transcription factor-1), which has a critical role in surfactant protein gene expression. Our findings indicate that glucocorticoid + cAMP is sufficient and necessary for precocious induction of functional type II cells in this in vitro system and that these hormones act primarily in combination to regulate expression of a subset of specific genes.

Identification of LBM180, a lamellar body limiting membrane protein of alveolar type II cells, as the ABC transporter protein ABCA3

Lamellar bodies are the specialized secretory organelles of alveolar type II (ATII) epithelial cells through which the cell packages pulmonary surfactant and regulates its secretion. Surfactant within lamellar bodies is densely packed as circular arrays of lipid membranes and appears to be the product of several trafficking and biosynthetic processes. To elucidate these processes, we reported previously on the generation of a monoclonal antibody (3C9) that recognizes a unique protein of the lamellar body membrane of 180 kDa, which we named LBM180. We report that mass spectrometry of the protein precipitated by this antibody generated a partial sequence that is identical to the ATP-binding cassette protein, ABCA3. Homology analysis of partial sequences suggests that this protein is highly conserved among species. The ABCA3 gene transcript was found in cell lines of human lung origin, in ATII cells of human, rat, and mouse, as well as different tissues of rat, but the highest expression of ABCA3 was observed in ATII cells. Expression of this transcript was at its maximum prior to birth, and hormonal induction of ABCA3 transcript was observed in human fetal lung at the same time as other surfactant protein transcripts were induced, suggesting that ABCA3 is developmentally regulated. Molecular and biochemical studies show that ABCA3 is targeted to vesicle membranes and is found in the limiting membrane of lamellar bodies. Because ABCA3 is a member of a subfamily of ABC transporters that are predominantly known to be involved in the regulation of lipid transport and membrane trafficking, we speculate that this protein may play a key role in lipid organization during the formation of lamellar bodies.

Human surfactant protein B promoter in transgenic mice: temporal, spatial, and stimulus-responsive regulation

Surfactant protein B (SP-B) is a developmentally and hormonally regulated lung protein that is required for normal surfactant function. We generated transgenic mice carrying the human SP-B promoter (-1,039/+431 bp) linked to chloramphenicol acetyltransferase (CAT). CAT activity was high in lung and immunoreactive protein localized to alveolar type II and bronchiolar epithelial cells. In addition, thyroid, trachea, and intestine demonstrated CAT activity, and each of these tissues also expressed low levels of SP-B mRNA. Developmental expression of CAT activity and SP-B mRNA in fetal lung were similar and both increased during explant culture. SP-B mRNA but not CAT activity decreased during culture of adult lung, and both were reduced by transforming growth factor (TGF)-beta(1). Treatment of adult mice with intratracheal bleomycin caused similar time-dependent decreases in lung SP-B mRNA and CAT activity. These findings indicate that the human SP-B promoter fragment directs tissue- and lung cell-specific transgene expression and contains cis-acting elements involved in regulated expression during development, fetal lung explant culture, and responsiveness to TGF-beta and bleomycin-induced lung injury.

Maintenance of surfactant protein A and D secretion by rat alveolar type II cells in vitro

Secretion of surfactant proteins A and D (SP-A and SP-D) has been difficult to study in vitro because a culture system for maintaining surfactant secretion has been difficult to establish. We evaluated several growth factors, corticosteroids, rat serum, and a fibroblast feeder layer for the ability to produce and maintain a polarized epithelium of type II cells that secretes SP-A and SP-D into the apical medium. Type II cells were plated on a filter insert coated with an extracellular matrix and were cultured at an air-liquid interface. Keratinocyte growth factor (KGF) stimulated type II cell proliferation and secretion of SP-A and SP-D more than fibroblast growth factor-10 (FGF-10), hepatocyte growth factor (HGF), or heparin-binding epidermal-like growth factor (HB-EGF). Cells cultured in the presence of KGF and rat serum with or without fibroblasts had high surfactant protein mRNA levels and exhibited a high level of SP-A and SP-D secretion. Dexamethasone inhibited type II cell proliferation but increased expression of SP-B. In the presence of KGF, rat serum, and dexamethasone, the mRNAs for the surfactant proteins were maintained at high levels. Secretion of SP-A and SP-D was found to be independent of phospholipid secretion.

Intracellular localization of processing events in human surfactant protein B biosynthesis

Surfactant protein B (SP-B) is essential to the function of pulmonary surfactant and to alveolar type 2 cell phenotype. Human SP-B is the 79-amino acid product of extensive post-translational processing of a 381-amino acid preproprotein. Processing involves modification of the primary translation product from 39 to 42 kDa and at least 3 subsequent proteolytic cleavages to produce the mature 8-kDa SP-B. To examine the intracellular sites of SP-B processing, we carried out immunofluorescence cytochemistry and inhibitor studies on human fetal lung in explant culture and isolated type 2 cells in monolayer culture using polyclonal antibodies to human SP-B(8) (Phe(201)-Met(279)) and specific epitopes within the N- (NFProx, Ser(145)-Leu(160); NFlank Gln(186)-Gln(200)) and C-terminal (CFlank, Gly(284)-Ser(304)) propeptides of pro-SP-B. Fluorescence immunocytochemistry using epitope-specific antisera showed colocalization of pro-SP-B with the endoplasmic reticulum resident protein BiP. The 25-kDa intermediate was partially endo H-sensitive, colocalized with the medial Golgi resident protein MG160, and shifted into the endoplasmic reticulum in the presence of brefeldin A, which interferes with anterograde transport from endoplasmic reticulum to Golgi. The 9-kDa intermediate colocalized in part with MG160 but not with Lamp-1, a transmembrane protein resident in late endosomes and lamellar bodies. Brefeldin A induced a loss of colocalization between MG160 and NFlank, shifting NFlank immunostaining to a juxtanuclear tubular array. In pulse-chase studies, brefeldin A blocked all processing of 42-kDa pro-SP-B whereas similar studies using monensin blocked the final N-terminal processing event of 9 to 8 kDa SP-B. We conclude that: 1) the first enzymatic cleavage of pro-SP-B to the 25-kDa intermediate is in the brefeldin A-sensitive, medial Golgi; 2) cleavage of the 25-kDa intermediate to a 9-kDa form is a trans-Golgi event that is slowed but not blocked by monensin; 3) the final cleavage of 9 to 8 kDa SP-B is a monensin-sensitive, post-Golgi event occurring prior to transfer of SP-B to lamellar bodies.

Pulmonary surfactant metabolism in infants lacking surfactant protein B

Infants with inherited deficiency of pulmonary surfactant protein (SP) B develop respiratory failure at birth and die without lung transplantation. We examined aspects of surfactant metabolism in lung tissue and lavage fluid acquired at transplantation or postmortem from ten infants born at term with inherited deficiency of SP-B; comparison groups were infants with other forms of chronic lung disease (CLD) and normal infants. In pulse/chase labeling studies with cultured deficient tissue, no immunoprecipitable SP-B was observed and an approximately 6-kD form of SP-C accumulated that was only transiently present in CLD tissue. SP-B messenger RNA (mRNA) was approximately 8% of normal in deficient specimens, and some intact message was observed after, but not before, explant culture. Transcription rates for SP-B, assessed by nuclear run-on assay using probes for sequences both 5' and 3' of the common nonsense mutation (121ins2), were comparable in all lungs examined. The minimal surface tension achieved with lavage surfactant was similarly elevated in both deficient and CLD infants (26-31 mN/m) compared with normal infants (6 mN/m). Both SP-B-deficient and CLD infants had markedly decreased phosphatidylglycerol content of lavage and tissue compared with normal lung, whereas synthetic rates for phospholipids, including phosphatidylglycerol, were normal. We conclude that the mutated SP-B gene is transcribed normally but produces an unstable mRNA and that absence of SP-B protein blocks processing of SP-C. Chronic infant lung disease, of various etiologies, reduces surfactant function and apparently alters phosphatidylglycerol degradation.

P. carinii induces selective alterations in component expression and biophysical activity of lung surfactant

Studies of Pneumocystis carinii pneumonia (PCP) suggest an important role for the surfactant system in the pathogenesis of the hypoxemic respiratory insufficiency associated with this infection. We hypothesized that PCP induces selective alterations in alveolar surfactant component expression and resultant biophysical properties. PCP was induced by intratracheal inoculation of 2 x 10(5) P. carinii organisms into C.B-17 scid/scid mice. Six weeks after inoculation, large (LA)- and small (SA)-aggregate surfactant fractions were prepared from bronchoalveolar lavage fluids and analyzed for expression of surfactant components and for biophysical activity. Total phospholipid content was significantly reduced in LA surfactant fractions from mice infected with PCP (53 +/- 15% of uninfected mice; P < 0.05). Quantitation of hydrophobic surfactant protein (SP) content demonstrated significant reductions of alveolar SP-B and SP-C protein levels in mice with PCP compared with those in uninfected mice (46 +/- 7 and 19 +/- 6%, respectively; P < 0.05 for both). The reductions in phospholipid, SP-B, and SP-C in LA fractions measured during PCP were associated with an increase in the minimum surface tension of LAs as measured by pulsating bubble surfactometer (13.1 +/- 1.1 vs. 5.4 +/- 1.8 mN/m; P < 0.05). In contrast to decreases in the hydrophobic SPs, SP-D content in the SA fraction was markedly increased (343 +/- 30% of control value; P < 0. 05) and SP-A levels in LA surfactant were maintained (93 +/- 26% of control value) during P. carinii infection. In all cases, the changes in SP content were reflected by commensurate changes in the levels of mRNA. We conclude that PCP induces selective alterations in surfactant component expression, including profound decreases in hydrophobic protein contents and resultant increases in surface tension. These changes, demonstrated in an immunologically relevant animal model, suggest that alterations in surfactant could contribute to the hypoxemic respiratory insufficiency observed in PCP.

Developmental and glucocorticoid regulation of surfactant protein mRNAs in preterm lambs

Glucocorticoid treatment increases content of surfactant protein (SP) A and SP-B in lung tissue and lavage fluid of preterm lambs. To investigate this process, we determined the ontogeny and glucocorticoid induction of SP mRNAs. In separate treatment protocols, each with its own controls, sheep were injected with betamethasone 15 h, 48 h, or weekly for 1-4 doses before preterm delivery. Using ovine SP cDNAs, we found an increase equal to or more than threefold in basal levels of all three SP mRNAs between 125 days and term. After betamethasone treatment, SP-B and SP-C mRNA levels increased by 15 h and all SP mRNAs were elevated after 24 h (>/=2-fold); mRNA levels in fetuses delivered 1-3 wk after betamethasone were not different from control. We conclude that in vivo betamethasone rapidly induces a coordinated increase in SP mRNAs, which is fully reversible within 7 days despite repetitive doses of betamethasone. Similar increases in mRNA and protein contents for SP-A and SP-B suggest that glucocorticoid regulation of these SPs in vivo is primarily pretranslational.

Effects of dexamethasone on meconium aspiration syndrome in newborn piglets

We examined the effects of dexamethasone on lung function in a piglet model of meconium aspiration syndrome. We induced lung injury in 10 newborn piglets (age 5 +/- 0.2 d) with 4 mL/kg body weight of 20% sterile human meconium in normal saline given via tracheostomy. Ventilator management was aimed at maintaining comparable values of end tidal carbon dioxide, Hb saturation, and arterial blood gases. Lung function was assessed using a BICORE CP100 neonatal monitor. Five piglets received 0.5 mg/kg of dexamethasone 2 and 8 h after meconium administration, whereas control piglets received normal saline at similar times. Ventilator settings, oxygen requirements, and lung compliance were similar between groups at the start of the study. Two hours after the instillation of meconium, there was marked lung dysfunction in both groups as evidenced by increased oxygen requirements [fraction of inspired oxygen (FiO2) 0.98 +/- 0.01 versus FiO2 0.21 +/- 0, p < 0.0001] and reduced lung compliance (0.35 +/- 0.03 versus 0.8 +/- 0.03 mL x kg(-1) x cm(-1) H2O, p < 0.0001). Administration of dexamethasone resulted in lower oxygen requirements (FiO2 0.27 +/- 0.01 versus FiO2 1.0 +/- 0.0, p < 0.00001), lower oxygenation index (2.17 +/- 0.17 versus 22.64 +/- 3.39, p < 0.0001), ventilatory efficiency index (0.30 +/- 0.01 versus 0.07 +/- 0.01, p < 0.0001), and improved lung compliance (0.68 +/- 0.04 versus 0.34 +/- 0.05 mL x kg(-1) x cm(-1) H2O, p < 0.001) compared with the control group. In summary, a two-dose course of 0.5 mg/kg of dexamethasone improved blood gases and lung function in a piglet model of meconium aspiration syndrome.

Hormonal regulation and cellular localization of fatty acid synthase in human fetal lung

Fatty acid synthase (FAS; EC 2.3.1.85) supplies de novo fatty acids for pulmonary surfactant synthesis, and FAS gene expression is both developmentally and hormonally regulated in the fetal lung. To further examine hormonal regulation of FAS mRNA and to determine the cellular localization of FAS gene expression, we cultured human fetal lungs (18-22 wk gestation) as explants for 1-4 days in the absence (control) or presence of glucocorticoid [dexamethasone (Dex), 10 nM] and/or cAMP agents (8-bromo-cAMP, 0.1 mM and IBMX, 0.1 mM). FAS protein content and activity increased similarly in the presence of Dex (109 and 83%, respectively) or cAMP (87 and 111%, respectively), and responses were additive in the presence of both hormones (230 and 203%, respectively). With a rabbit anti-rat FAS antibody, FAS immunoreactivity was not detected in preculture lung specimens but appeared in epithelial cells lining the tubules with time in culture. Dex and/or cAMP markedly increased staining of epithelial cells, identified as type II cells, whereas staining of mesenchymal fibroblasts was very low under all conditions. With in situ hybridization, FAS mRNA was found to be enriched in epithelial cells lining the alveolar spaces, and the reaction product increased in these cells when the explants were cultured with the hormones. The increased FAS mRNA content in the presence of Dex and/or cAMP is primarily due to increased stabilization of mRNA, although Dex alone increased the transcription rate by approximately 30%. We conclude that hormonal treatment of cultured human fetal lungs increases FAS gene expression primarily by increasing stability of the message. The induction of FAS during explant culture and by hormones occurs selectively in type II epithelial cells, consistent with the regulatory role of this enzyme in de novo synthesis of fatty acid substrate for surfactant synthesis in perinatal lungs.

Inhibition of lung surfactant protein B expression during Pneumocystis carinii pneumonia in mice

The pathogenesis of Pneumocystis carinii pneumonia (PCP) suggests an important role for dysfunction of the pulmonary surfactant system in the hypoxemic respiratory insufficiency associated with this infection. Surfactant protein B (SP-B) is a hydrophobic protein shown to be essential for normal surfactant function in vivo. Therefore, we hypothesized that the inhibition of SP-B expression occurs during PCP, and we tested this hypothesis in two immunodeficient animal models. PCP was induced in C.B-17 scid/scid mice by intratracheal inoculation of P. carinii organisms. Infected lung homogenates, obtained at time points up to 6 weeks after inoculation, were analyzed for SP-B and mRNA content. When a comparison was made with uninfected scid controls, the densitometric quantitation of Western blots of lung homogenates demonstrated significant reductions in 8 kd SP-B in mice infected with P. carinii 4 weeks after inoculation (16% of the control value). Northern blot analysis showed a concomitant decrease in SP-B mRNA to 24% of the control level. The decrease in SP-B and mRNA levels in lung homogenates of infected mice was reflected in lower SP-B levels in the surfactant. An enzyme-linked immunosorbent assay for the SP-B level in surfactant prepared from bronchoalveolar lavage samples of infected scid mice demonstrated a significant reduction in alveolar SP-B content (45% of the control value). In contrast to the results with SP-B, neither the SP-A protein content nor the mRNA level was significantly altered by PCP infection. To confirm these observations, SP-B expression was studied in an additional animal model of PCP. The SP-B content of lung homogenates from BALB/c mice depleted of CD4+ T cells and infected with P. carinii was also reduced (51% of the control value). We conclude that P. carinii induces selective inhibition of the expression of SP-B in two mouse models of PCP and that this down-regulation is mediated at the level of mRNA expression. Therefore, an acquired deficiency of SP-B is likely to be an important contributor to the pathogenesis of hypoxemic respiratory failure that is observed in patients with PCP.

Inhibition of hSP-B promoter in respiratory epithelial cells by a dominant negative retinoic acid receptor

Retinoic acid (RA) receptors (RARs) belong to the nuclear hormone receptor superfamily and play important roles in lung differentiation, growth, and gene regulation. Surfactant protein (SP) B is a small hydrophobic protein synthesized and secreted by respiratory epithelial cells in the lung. Expression of the SP-B gene is modulated at the transcriptional and posttranscriptional levels. In the present work, immunohistochemical staining revealed that RAR-alpha is present on day 14.5 of gestation in the fetal mouse lung. To assess whether RAR is required for SP-B gene transcription, a dominant negative mutant human (h) RAR-alpha403 was generated. The hRAR-alpha403 mutant was transcribed and translated into the truncated protein product by reticulocyte lysate in vitro. The mutant retained DNA binding activity in the presence of retinoid X receptor-gamma to an RA response element in the hSP-B promoter. When transiently transfected into pulmonary adenocarcinoma epithelial cells (H441 cells), the mutant hRAR-alpha403 was readily detected in the cell nucleus. Cotransfection of the mutant hRAR-alpha403 repressed activity of the hSP-B promoter and inhibited RA-induced surfactant proprotein B production in H441 cells, supporting the concept that RAR is required for hSP-B gene transcription in vitro.

TGF-beta1 inhibits surfactant component expression and epithelial cell maturation in cultured human fetal lung

Transforming growth factor-beta1 (TGF-beta1) is a multifunctional cytokine shown to play a critical role in organ morphogenesis, development, growth regulation, cellular differentiation, gene expression, and tissue remodeling after injury. We examined the effect of exogenously administered TGF-beta1 on the expression of surfactant proteins (SPs) and lipids, fatty acid synthetase, and ultrastructural morphology in human fetal lung cultured for 5 days with and without dexamethasone (10 nM). Expression of the type II cell-specific marker surfactant proprotein C (proSP-C), studied by [35S]Met incorporation and immunoprecipitation, increased sevenfold with dexamethasone treatment. TGF-beta1 (0.1-100 ng/ml) in the presence of dexamethasone inhibited 21-kDa proSP-C expression in a dose-dependent manner (maximal inhibition 31% of control level at 100 ng/ml). There was no change in [35S]Met incorporation into total protein in any of the treatment groups vs. the control group. In immunoblotting experiments, TGF-beta1 blocked culture-induced accumulation of SP-A and SP-B. Under the same conditions, TGF-beta1 reduced mRNA content for SP-A, SP-B, and SP-C to 20, 38, and 41%, respectively, of matched control groups but did not affect levels of beta-actin mRNA. SP transcription rates after 24 h of exposure to TGF-beta1 were reduced to a similar extent (20-50% of control level). In both control and dexamethasone-treated explants, TGF-beta1 (10 ng/ml) also decreased fatty acid synthetase mRNA, protein, and enzyme activity and the rate of [3H]choline incorporation into phosphatidylcholine. By electron microscopy, well-differentiated type II cells lining potential air spaces were present in explants cultured with dexamethasone, whereas exposure to TGF-beta1 with or without dexamethasone resulted in epithelial cells lacking lamellar bodies. We conclude that exogenous TGF-beta1 disrupts culture-induced maturation of fetal lung epithelial cells and inhibits expression of surfactant components through effects on gene transcription.

Surfactant protein B processing in human fetal lung

Surfactant protein B (SP-B8), an 8-kDa hydrophobic protein essential for surfactant and normal lung function, is produced from the intracellular processing of preproSP-B. To characterize SP-B processing in human type 2 cells, we used human fetal lung in explant culture and polyclonal antibodies to human SP-B8 (Phe201-Met279) and to specific epitopes within the NH2- and COOH-terminal propeptide domains (Ser145-Leu160, Gln186-Gln200, and Gly284-Ser304). Western blot analysis revealed a novel intermediate at approximately 9 kDa, representing mature SP-B8, with a residual NH2-terminal peptide of approximately 10 amino acids. Pulse-chase studies showed a precursor-product relationship between the 9- and 8-kDa forms. During differentiation of type 2 cells in explant culture, the rate of proSP-B conversion to 25-kDa intermediate remained constant, whereas the rate of 25-kDa intermediate conversion to SP-B8 increased, resulting in a net increase in tissue SP-B8. Dexamethasone did not affect the rate of proSP-B processing but markedly enhanced the rate of SP-B8 accumulation. We conclude that NH2-terminal propeptide cleavage of proSP-B is a multistep process and that more distal processing events are rate limiting and both developmentally and hormonally regulated.

Retinoic acid-receptor activation of SP-B gene transcription in respiratory epithelial cells

Retinoids are known to play important roles in organ development of the lung. Retinoids exert their activity by modulating the expression of numerous genes, generally influencing gene transcription, in target cells. In the present work, the mechanism by which retinoic acid (RA) regulates surfactant protein (SP) B expression was assessed in vitro. RA (9-cis-RA) enhanced SP-B mRNA in pulmonary adenocarcinoma cells (H441 cells) and increased transcriptional activity of the SP-B promoter in both H441 and mouse lung epithelial cells (MLE-15). Cotransfection of H441 cells with retinoid nuclear receptor (RAR)-alpha, -beta, and -gamma and retinoid X receptor (RXR)-gamma further increased the response of the SP-B promoter to RA. Treatment of H441 cells with RA increased immunostaining for the SP-B proprotein and increased the number of cells in which the SP-B proprotein was detected. An RA responsive element mediating RA stimulation of the human SP-B promoter was identified. RAR-alpha and -gamma and RXR-alpha but not RAR-beta or RXR-beta and -gamma were detected by immunohistochemical analysis of H441 cells. RA, by activating RAR activity, stimulated the transcription and synthesis of SP-B in pulmonary adenocarcinoma cells.

Generation and characterization of monoclonal antibodies to alveolar type II cell lamellar body membrane

Monoclonal antibodies against the limiting membrane of alveolar type II cell lamellar bodies were obtained after immunization of mice with a membrane fraction prepared from lamellar bodies isolated from rat lungs. The specificity of the antibodies was investigated with Western blot analysis, indirect immunofluorescence, and electron-microscopic immunogold studies of freshly isolated or cultured alveolar type II cells, alveolar macrophages, and rat lung tissue. One of the monoclonal antibodies identified, MAb 3C9, recognized a 180-kDa lamellar body membrane (lbm180) protein. Immunogold labeling of rat lung tissue with MAb 3C9 demonstrated that lbm180 protein is primarily localized at the lamellar body limiting membrane and is not found in the lamellar body contents. Most multivesicular bodies of type II cells were also labeled, as were some small cytoplasmic vesicles. Golgi complex labeling and plasma membrane labeling were weak. The appearance of lbm180 protein by immunofluorescence in fetal rat lung cryosections correlated with the biogenesis of lamellar bodies. The lbm180 protein decreased with time in type II cells cultured on plastic. The lbm180 protein is an integral membrane protein of lamellar bodies and was also found in the pancreas and the pancreatic betaHC9 cell line but not in the rat brain, liver, kidney, stomach, or intestine. The present study provides evidence that the lbm180 protein is a lung lamellar body and/or multivesicular body membrane protein and that its antibody, MAb 3C9, will be a valuable reagent in further investigations of the biogenesis and trafficking of type II cell organelles.

Glucocorticoid receptor mRNA and protein in fetal rat lung in vivo: modulation by glucocorticoid and androgen

Pulmonary glucocorticoid receptor (GR) is essential to timely preparation for the onset of breathing air at birth. We have previously used primary culture of late-gestation fetal rat lung cells to demonstrate differential regulation of GR by glucocorticoid depending on cell type. In this study, we hypothesized that the action of glucocorticoid on GR mRNA expression and protein elaboration in lung cells might be modulated by interactions present in vivo but not in primary culture. Given that male sex hormone (androgen) has an inhibitory effect on antenatal lung development, we also postulated that androgen would decrease antenatal lung GR. We report that antenatal maternal injection of the glucocorticoid dexamethasone (1 mg/kg) enhanced fetal lung cellular levels of GR mRNA and protein as assessed by in situ hybridization and immunocytochemistry (ICC), respectively. ICC was performed using polyclonal rabbit anti-human antibody that reacts with rat GR whether bound to ligand or not and does not interfere with GR binding to DNA. Levels of GR mRNA and protein were enhanced in cells throughout all areas of the lung tissue, suggesting that interactions occurring in intact tissue may override the previously reported direct inhibition by glucocorticoid of GR protein elaboration in isolated fetal rat lung epithelial cells. Furthermore, antenatal administration of the androgen 5alpha-dihydrotestosterone (0.2 mg/kg) reduced tissue levels of GR mRNA and protein, consistent with androgenic inhibition of antenatal lung development by decreasing GR. We conclude that glucocorticoids and androgens exert opposite effects on fetal lung GR.

Glucocorticoid regulation of surfactant components in immature lambs

To assess effects of dose and duration of glucocorticoid exposure on maturation of the fetal lung, we administered single or multiple doses of betamethasone (0.5 mg/kg im) to pregnant sheep for 2 or 21 days before preterm delivery at 125 days of gestation. Lung function (compliance, lung volume at 40 cmH2O pressure, and ventilatory efficiency index) was increased after two to four weekly doses of glucocorticoid (2.5- to 4-fold increase) and after 48 h of exposure (1.4- to 2.3-fold). Total protein of lavage fluid decreased similarly with three doses, four doses, and 48 h of treatment. In lambs with long-term exposure to betamethasone, there was a similar, dose-dependent increase in concentrations of saturated phosphatidylcholine and surfactant proteins A (SP-A) and B (SP-B) (maximal 2- to 3-fold in tissue and 10- to 15-fold in lavage fluid). Levels of SP-A and SP-B were closely correlated in lavage fluid. In animals treated for 48 h, only tissue SP-B was increased (2.7-fold). We conclude that 48 h of glucocorticoid treatment improves lung function in the premature lamb without a detectable increase in lavage surfactant components and that longer exposure to antenatal glucocorticoid increases surfactant lipid and proteins in a coordinated fashion. The enhanced response with repetitive dosing indicates that the process of glucocorticoid-induced lung maturation is either reversible and/or gestational age dependent.

Expression and glucocorticoid regulation of surfactant protein C in human fetal lung

The hydrophobic surfactant protein C (SP-C) is known to modulate the biophysical properties of surfactant phospholipid. Although SP-C mRNA has been demonstrated in human fetal lung, there is limited information regarding developmental expression and processing of proSP-C protein. Two epitope-specific human proSP-C antisera, anti-hCPROSP-C (His59-Ser72) and anti-hCTERMSP-C (Gly162-Gly175), were generated to complement previously produced anti-NPROSP-C (Met10-Gln23) for the study of proSP-C expression in human fetal lung. Western blotting and immunocytochemistry detected expression of proSP-C protein by 12-16 wk of gestation. ProSP-C immunoreactivity of preculture lung, limited to expression of proSP-C21 in airway epithelial cells, was markedly enhanced by culture of lung explants in dexamethasone. To examine synthesis of proSP-C, homogenates from explants were labeled with 35S-Met/Cys for 0.5-4 h. Immunoprecipitation with anti-NPROSP-C detected 35S-proSP-C21 by 30 min and, after 2 h of labeling, there was a 15-fold increase in 35S-proSP-C21 in dexamethasone-treated lungs versus controls. Synthesis of proSP-C21 was followed by the appearance of a 24-kD form and smaller processing intermediates including 6-10-kD forms. Posttranslational processing of proSP-C21 was not observed in control explants. SP-C(6-10) were not recognized by either anti-CPROSP-C or anti-hCTERMSP-C. These results indicate that low level expression of proSP-C protein first occurs in epithelial cells early in the second trimester and that expression can be enhanced by dexamethasone. Initial posttranslational processing of human proSP-C involves modification of proSP-C21 to SP-C24 and subsequent proteolysis of C-terminal propeptide domains. We speculate that absence of low Mr intermediates in unstimulated second trimester fetal lung tissue reflects developmental and glucocorticoid dependent regulation of proSP-C21 synthesis and posttranslational processing.

Phorbol ester down-regulation of lung surfactant protein B gene expression by cytoplasmic trapping of thyroid transcription factor-1 and hepatocyte nuclear factor 3

The lung-specific surfactant protein B (SP-B) is essential for surfactant function and normal respiration. We investigated the role of thyroid transcription factor-1 (TTF-1) and hepatocyte nuclear factor 3 (HNF3) in the down-regulation of SP-B gene expression by phorbol ester in pulmonary adenocarcinoma H441 cells. Responsiveness to 12-O-tetradecanoylphorbol-13-acetate (TPA) localized to the SP-B proximal promoter (-140/-65 bp) and specifically to binding sites for TTF-1 and HNF3, which act as cell-specific enhancers of SP-B expression. Treatment of cells with TPA (10 nM) caused a time-dependent decrease in both TTF-1 and HNF3 in nuclear extracts and accumulation of both factors in the cytoplasm as assessed by electromobility shift, Western, Southwestern, and immunofluorescence assays. Treatment did not alter the mRNA content or DNA binding activity for either transcription factor. We conclude that down-regulation of SP-B gene expression by phorbol ester involves cytoplasmic trapping and loss of TTF-1 and HNF3 from the nucleus. This mechanism of action is independent of AP-1 and other transcription factors known to be influenced by phorbol ester.

Transcriptional regulation of surfactant proteins SP-A and SP-B by phorbol ester

Phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) activate protein kinase C and have been previously shown to down-regulate surfactant proteins SP-A and SP-B in H441 adenocarcinoma cells. We used H441 cells and human fetal lung to further study the mechanism of TPA action and to examine physiologic relevance. In H441 cells, TPA (10 nM) treatment for 24 h decreased SP-A mRNA content to approximately 5% of control cells, with half-maximal effect at approximately 0.5 nM, and reduced SP-A gene transcription rate to 28% of control after 8 h exposure. In cells cultured in the presence of dexamethasone, which increases the low basal level of SP-B expression, TPA decreased both SP-B mRNA content (approximately 8% of control) and rate of transcription (7% of control). In cultured human fetal lung explants, TPA decreased SP-A and SP-B protein and mRNA in a time- and dose-dependent fashion, with half-maximal effect on mRNAs at approximately 3 nM and approximately 50% inhibition after 24 h of exposure, and similarly reduced SP-A and SP-B gene transcription (approximately 55% of control at 8-24 h). We conclude that TPA acts primarily at the level of gene transcription to down-regulate both SP-A and SP-B in H441 and fetal lung cells, and we speculate that inflammatory and other agents that act through PKC may modulate expression of the surfactant proteins and alter surfactant function in vivo.