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

Epithelial cells/progenitor cells in developing human lower respiratory tract: Characterization and transplantation to rat model of pulmonary injury

Introduction

For cell-based therapies of lung injury, several cell sources have been extensively studied. However, the potential of human fetal respiratory cells has not been systematically explored for this purpose. Here, we hypothesize that these cells could be one of the top sources and hence, we extensively updated the definition of their phenotype.

Methods

Human fetal lower respiratory tissues from pseudoglandular and canalicular stages and their isolated epithelial cells were evaluated by immunostaining, electron microscopy, flow cytometry, organoid assay, and gene expression studies. The regenerative potential of the isolated cells has been evaluated in a rat model of bleomycin-induced pulmonary injury by tracheal instillation on days 0 and 14 after injury and harvest of the lungs on day 28.

Results

We determined the relative and temporal, and spatial pattern of expression of markers of basal (KRT5, KRT14, TRP63), non-basal (AQP3 and pro-SFTPC), and early progenitor (NKX2.1, SOX2, SOX9) cells. Also, we showed the potential of respiratory-derived cells to contribute to in vitro formation of alveolar and airway-like structures in organoids. Cell therapy decreased fibrosis formation in rat lungs and improved the alveolar structures. It also upregulated the expression of IL-10 (up to 17.22 folds) and surfactant protein C (up to 2.71 folds) and downregulated the expression of TGF-β (up to 5.89 folds) and AQP5 (up to 3.28 folds).

Conclusion

We provide substantial evidence that human fetal respiratory tract cells can improve the regenerative process after lung injury. Also, our extensive characterization provides an updated phenotypic profile of these cells.

Cathepsin H: molecular characteristics and clues to function and mechanism

Cathepsin H (CatH) is a lysosomal cysteine protease with a unique aminopeptidase activity that is extensively expressed in the lung, pancreas, thymus, kidney, liver, skin, and brain. Owing to its specific enzymatic activity, CatH has critical effects on the regulation of biological behaviours of cancer cells and pathological processes in brain diseases. Moreover, a neutral pH level is optimal for CatH activity, so it is expected to be active in the extra-lysosomal and extracellular space. In the present review, we describe the expression, maturation, and enzymatic properties of CatH, and summarize the available experimental evidence that mechanistically links CatH to various physiological and pathological processes. Finally, we discuss the challenges and potentials of CatH inhibitors in CatH-induced disease therapy.

Methylprednisolone pulse treatment improves ProSP-C trafficking in twins with SFTPC mutation: An isoform story?

Mutations in the gene encoding surfactant protein C (SP-C) cause interstitial lung disease (ILD), and glucocorticosteroid (GC) treatment is the most recognized therapy in children. We aimed to decipher the mechanisms behind successful GC treatment in twins carrying a BRICHOS c.566G > A (p.Cys189Tyr) mutation in the SP-C gene (SFTPC). METHODS: The twins underwent bronchoscopy before and after GC treatment and immunoblotting analysis of SP-C proprotein (proSP-C) and SP-C mature in bronchoalveolar fluid (BALF). Total RNA was extracted and analysed using quantitative real-time PCR assays. In A549 cells, the processing of mutated protein C189Y was studied by immunofluorescence and immunoblotting after heterologous expression of eukaryotic vectors containing wild type or C189Y mutant cDNA. RESULTS: Before treatment, BALF analysis identified an alteration of the proSP-C maturation process. Functional study of C189Y mutation in alveolar A549 cells showed that pro-SP-C^C189Y was retained within the endoplasmic reticulum together with ABCA3. After 5 months of GC treatment with clinical benefit, the BALF analysis showed an improvement of proSP-C processing. SFTPC mRNA analysis in twins revealed a decrease in the expression of total SFTPC mRNA and a change in its splicing, leading to the expression of a second shorter proSP-C isoform. In A549 cells, the processing and the stability of this shorter wild-type proSP-C isoform was similar to that of the longer isoform, but the half-life of the mutated shorter isoform was decreased. These results suggest a direct effect of GC on proSP-C metabolism through reducing the SFTPC mRNA level and favouring the expression of a less stable protein isoform.

Altered expression of p63 isoforms and expansion of p63- and club cell secretory protein-positive epithelial cells in the lung as novel features of aging

Aging is a key contributor for subclinical progression of late-onset lung diseases. Basal, club, and type II alveolar epithelial cells (AECs) are lung epithelial progenitors whose capacities of differentiation are extensively studied. The timely transition of these cells in response to environmental changes helps maintain the intricate organization of lung structure. However, it remains unclear how aging affects their behavior. This paper demonstrates that the protein expression profiles of a type II AEC marker, prosurfactant protein C (pro-SPC), and a basal cell marker, p63, are altered in the lungs of 14-mo-old versus 7- to 9-wk-old mice. Expression of NH2-terminal-truncated forms of p63 (ΔNp63), a basal cell marker, and claudin-10, a club cell marker, in cytoplasmic extracts of lungs of 14-mo-old mice was upregulated. In contrast, nuclear expression of full-length forms of p63 (TAp63) decreases with age. These alterations in protein expression profiles coincide with dramatic changes in lung functions including compliance. Whole tissue lysates of middle-aged versus aged rhesus monkey lungs display similar age-associated alterations in pro-SPC expression. An age-associated decrease of TAp63 in nuclear lysates was observed in aged monkey group. Moreover, the lungs of 14-mo-old versus 7- to 9-wk-old mice display a wider spreading of ΔNp63-positive CCSP-positive bronchiolar epithelial cells. This expansion did not involve upregulation of Ki67, a representative proliferation marker. Collectively, it is postulated that 1) this expansion is secondary to a transition of progenitor cells committed to club cells from ΔNp63-negative to ΔNp63-positive status, and 2) high levels of cytoplasmic ΔNp63 expression trigger club cell migration.

Immunolocalization of Surfactant Proteins SP-A, SP-B, SP-C, and SP-D in Infantile Labial Glands and Mucosa

Surfactant proteins in different glandular structures of the oral cavity display antimicrobial activity for protection of invading microorganisms. Moreover, they are involved in lowering liquid tension in fluids and facilitate secretion flows. Numerous investigations for studying the occurrence of surfactant proteins in glandular tissues were performed using different methods. In the oral cavity, minor salivary glands secrete saliva continuously for the maintenance of a healthy oral environment. For the first time, we could show that infantile labial glands show expression of the surfactant proteins (SP) SP-A, SP-B, SP-C, and SP-D in acinar cells and the duct system in different intensities. The stratified squamous epithelium of the oral mucosa revealed positive staining for SPs in various cell layers.

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.

The surfactant protein C mutation A116D alters cellular processing, stress tolerance, surfactant lipid composition, and immune cell activation

BACKGROUND

Surfactant protein C (SP-C) is important for the function of pulmonary surfactant. Heterozygous mutations in SFTPC, the gene encoding SP-C, cause sporadic and familial interstitial lung disease (ILD) in children and adults. Mutations mapping to the BRICHOS domain located within the SP-C proprotein result in perinuclear aggregation of the proprotein. In this study, we investigated the effects of the mutation A116D in the BRICHOS domain of SP-C on cellular homeostasis. We also evaluated the ability of drugs currently used in ILD therapy to counteract these effects.

METHODS

SP-CA116D was expressed in MLE-12 alveolar epithelial cells. We assessed in vitro the consequences for cellular homeostasis, immune response and effects of azathioprine, hydroxychloroquine, methylprednisolone and cyclophosphamide.

RESULTS

Stable expression of SP-CA116D in MLE-12 alveolar epithelial cells resulted in increased intracellular accumulation of proSP-C processing intermediates. SP-CA116D expression further led to reduced cell viability and increased levels of the chaperones Hsp90, Hsp70, calreticulin and calnexin. Lipid analysis revealed decreased intracellular levels of phosphatidylcholine (PC) and increased lyso-PC levels. Treatment with methylprednisolone or hydroxychloroquine partially restored these lipid alterations. Furthermore, SP-CA116D cells secreted soluble factors into the medium that modulated surface expression of CCR2 or CXCR1 receptors on CD4+ lymphocytes and neutrophils, suggesting a direct paracrine effect of SP-CA116D on neighboring cells in the alveolar space.

CONCLUSIONS

We show that the A116D mutation leads to impaired processing of proSP-C in alveolar epithelial cells, alters cell viability and lipid composition, and also activates cells of the immune system. In addition, we show that some of the effects of the mutation on cellular homeostasis can be antagonized by application of pharmaceuticals commonly applied in ILD therapy. Our findings shed new light on the pathomechanisms underlying SP-C deficiency associated ILD and provide insight into the mechanisms by which drugs currently used in ILD therapy act.

A non-BRICHOS surfactant protein c mutation disrupts epithelial cell function and intercellular signaling

Background

Heterozygous mutations of SFTPC, the gene encoding surfactant protein C (SP-C), cause sporadic and familial interstitial lung disease (ILD) in children and adults. The most frequent SFTPC mutation in ILD patients leads to a threonine for isoleucine substitution at position 73 (I73T) of the SP-C preprotein (proSP-C), however little is known about the cellular consequences of SP-C^I73T expression.

Results

To address this, we stably expressed SP-C^I73T in cultured MLE-12 alveolar epithelial cells. This resulted in increased intracellular accumulation of proSP-C processing intermediates, which matched proSP-C species recovered in bronchial lavage fluid from patients with this mutation. Exposure of SP-C^I73T cells to drugs currently used empirically in ILD therapy, cyclophosphamide, azathioprine, hydroxychloroquine or methylprednisolone, enhanced expression of the chaperones HSP90, HSP70, calreticulin and calnexin. SP-C^I73T mutants had decreased intracellular phosphatidylcholine level (PC) and increased lyso-PC level without appreciable changes of other phospholipids. Treatment with methylprednisolone or hydroxychloroquine partially restored these lipid alterations. Furthermore, SP-C^I73T cells secreted into the medium soluble factors that modulated surface expression of CCR2 or CXCR1 receptors on CD4+ lymphocytes and neutrophils, suggesting a direct paracrine influence of SP-C^I73T on neighboring cells in the alveolar space.

Conclusion

We show that I73T mutation leads to impaired processing of proSP-C in alveolar type II cells, alters their stress tolerance and surfactant lipid composition, and activates cells of the immune system. In addition, we show that some of the mentioned cellular aspects behind the disease can be modulated by application of pharmaceutical drugs commonly applied in the ILD therapy.

The effect of interleukin-13 (IL-13) and interferon-γ (IFN-γ) on expression of surfactant proteins in adult human alveolar type II cells in vitro

Background

Surfactant proteins are produced predominantly by alveolar type II (ATII) cells, and the expression of these proteins can be altered by cytokines and growth factors. Th1/Th2 cytokine imbalance is suggested to be important in the pathogenesis of several adult lung diseases. Recently, we developed a culture system for maintaining differentiated adult human ATII cells. Therefore, we sought to determine the effects of IL-13 and IFN-γ on the expression of surfactant proteins in adult human ATII cells in vitro. Additional studies were done with rat ATII cells.

Methods

Adult human ATII cells were isolated from deidentified organ donors whose lungs were not suitable for transplantation and donated for medical research. The cells were cultured on a mixture of Matrigel and rat-tail collagen for 8 d with differentiation factors and human recombinant IL-13 or IFN-γ.

Results

IL-13 reduced the mRNA and protein levels of surfactant protein (SP)-C, whereas IFN-γ increased the mRNA level of SP-C and proSP-C protein but not mature SP-C. Neither cytokine changed the mRNA level of SP-B but IFN-γ slightly decreased mature SP-B. IFN-γ reduced the level of the active form of cathepsin H. IL-13 also reduced the mRNA and protein levels of SP-D, whereas IFN-γ increased both mRNA and protein levels of SP-D. IL-13 did not alter SP-A, but IFN-γ slightly increased the mRNA levels of SP-A.

Conclusions

We demonstrated that IL-13 and IFN-γ altered the expression of surfactant proteins in human adult ATII cells in vitro. IL-13 decreased SP-C and SP-D in human ATII cells, whereas IFN-γ had the opposite effect. The protein levels of mature SP-B were decreased by IFN-γ treatment, likely due to the reduction in active form cathpesin H. Similarly, the active form of cathepsin H was relatively insufficient to fully process proSP-C as IFN-γ increased the mRNA levels for SP-C and proSP-C protein, but there was no increase in mature SP-C. These observations suggest that in disease states with an overexpression of IL-13, there would be some deficiency in mature SP-C and SP-D. In disease states with an excess of IFN-γ or therapy with IFN-γ, these data suggest that there might be incomplete processing of SP-B and SP-C.

Surfactant protein-C promoter variants associated with neonatal respiratory distress syndrome reduce transcription

Dominant mutations in coding regions of the surfactant protein-C gene, SFTPC, cause respiratory distress syndrome (RDS) in infants. However, the contribution of variants in noncoding regions of SFTPC to pulmonary phenotypes is unknown. By using a case-control group of infants > or =34 weeks gestation (n = 538), we used complete resequencing of SFTPC and its promoter, genotyping, and logistic regression to identify 80 single nucleotide polymorphisms (SNPs). Three promoter SNPs were statistically associated with neonatal RDS among European descent infants. To assess the transcriptional effects of these three promoter SNPs, we selectively mutated the SFTPC promoter and performed transient transfection using MLE-15 cells and a firefly luciferase reporter vector. Each promoter SNP decreased SFTPC transcription. The combination of two variants in high linkage dysequilibrium also decreased SFTPC transcription. In silico evaluation of transcription factor binding demonstrated that the rare allele at g.-1167 disrupts a SOX (SRY-related high mobility group box) consensus motif and introduces a GATA-1 site, at g.-2385 removes a MZF-1 (myeloid zinc finger) binding site, and at g.-1647 removes a potential methylation site. This combined statistical, in vitro, and in silico approach suggests that reduced SFTPC transcription contributes to the genetic risk for neonatal RDS in developmentally susceptible infants.

Novel therapeutic strategies for acute lung injury induced by lung damaging agents: the potential role of growth factors as treatment options

The increasing threat from terrorism has brought attention to the possible use of toxic industrial compounds (TICs) and other lung-damaging agents as weapons against civilian populations. The way in which these agents could be used favours the development of generic countermeasures. Improved medical countermeasures would increase survivability and improve the quality of recovery of lung damaged casualties. It is evident that there is a dearth of therapeutic regimes available to treat those forms of lung damage that currently require intensive care management. It is quite possible that mass casualties from a terrorist incident or major industrial accident involving the release of large quantities of inhaled TICs would place a severe burden on already scarce intensive care facilities. The development of effective pharmacological approaches to assist the recovery of casualties suffering from acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) may improve the prognosis of such patients (which is currently poor) and would ideally be used as a means of preventing subjects from developing the pulmonary oedema characteristic of ALI/ARDS. Many promising candidate pharmacological treatments have been evaluated for the treatment of ALI/ARDS, but their clinical value is often debatable. Thus, despite improvements in ventilation strategies, pharmacological intervention for ALI/ARDS remains problematical. A new approach is clearly required for the treatment of patients with severely compromised lungs. Whilst the pathology of ALI/ARDS associated with exposure to a variety of agents is complex, numerous experimental studies suggest that generic therapeutic intervention directed at approaches that aim to upregulate repair of the damaged alveolar blood/air barrier of the lung may be of value, particularly with respect to chemical-induced injury. To this end, keratinocyte growth factor (KGF), epithelial growth factor (EGF) and basic fibroblast growth factor (bFGF) are emerging as the most important candidates. Hepatocyte growth factor (HGF) does not have epithelial specificity for lung tissue. However, the enhanced effects of combinations of growth factors, such as the synergistic effect of HGF upon vascular endothelial growth factor (VEGF)-mediated endothelial cell activity, and the combined effect of HGF and KGF in tissue repair should be investigated, particularly as the latter pair of growth factors are frequently implicated in processes associated with the repair of lung damage. Synergistic interactions also occur between trefoil factor family (TFF) peptides and growth factors such as EGF. TFF peptides are most likely to be of value as a short term therapeutic intervention strategy in stimulating epithelial spreading activities which allow damaged mucosal surfaces to be rapidly covered by epithelial cells.

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.

Lamellar body formation and dark multivesicular body pockets in fetal and postnatal normal rat alveolar type II cells: ultrastructural study

To investigate the osmiophilic lamellar body (LB) formation in the alveolar type II cells, the authors examined lung tissues from fetal (day 19 of gestation) and postnatal normal rat by electron microscopy. Lamellar body formation was identical in fetal and postnatal rat lungs. Immature dark multivesicular body (MVB) showed incomplete formation of the limiting membrane and contained a cluster of vesicles and an electron-dense granule probably originated from Golgi complex. Dark and light MVBs were seen in alveolar type II cells. However, only dark MVB was involved in the production of LB. First, a drumstick-shaped projection arising from the surface of MVB appeared, which partly trapped cytoplasmic materials such as glycogen granules and ribosomes. The projection was ultimately fused with the MVB surface to form an MVB pocket described here for the first time. In prelamellar body, lamellar structures appeared to provide a communicative MVB pocket and even found in a drumstick-shape. Mature LBs showed disappearance of multivesicles, dense matrix, and MVB pocket. Thus, lamellar body formation might not only need MVB materials, but also glycogen granules and ribosomes in the MVB pocket.

Anterograde transport of surfactant protein C proprotein to distal processing compartments requires PPDY-mediated association with Nedd4 ubiquitin ligases

Biosynthesis of surfactant protein C (SP-C) by alveolar type 2 cells requires proteolytic processing of a 21-kDa propeptide (proSP-C21) in post-Golgi compartments to yield a 3.7-kDa mature form. Scanning alanine mutagenesis, binding assays, and co-immunoprecipitation were used to characterize the proSP-C targeting domain. Delivery of proSP-C21 to distal processing organelles is dependent upon the NH2-terminal cytoplasmic SP-C propeptide, which contains a conserved PPDY motif. In A549 cells, transfection of EGFP/proSP-C21 constructs containing polyalanine substitution for Glu11-Thr18, 13PPDY16, or 14P,16Y produced endoplasmic reticulum retention of the fusion proteins. Protein-protein interactions of proSP-C with known WW domains were screened using a solid-phase array that revealed binding of the proSP-C NH2 terminus to several WW domains found in the Nedd4 family of E3 ligases. Specificity of the interaction was confirmed by co-immunoprecipitation of proSP-C and Nedd4 or Nedd4-2 in epithelial cell lines. By Western blotting and reverse transcription-PCR, both forms were detected in primary human type 2 cells. Knockdown of Nedd4-2 by small interference RNA transfection of cultured human type 2 cells blocked processing of 35S-labeled proSP-C21. Mutagenesis of potential acceptor sites for ubiquitination in the cytosolic domain of proSP-C (Lys6, Lys34, or both) failed to inhibit trafficking of EGFP/proSP-C21. These results indicate that PPDY-mediated interaction with Nedd4 E3-ligases is required for trafficking of proSP-C. We speculate that the Nedd4/proSP-C tandem is part of a larger protein complex containing a ubiquitinated component that further directs its transport.

Differentiation of xenografted human fetal lung parenchyma

The goal of this study was to characterize xenografted human fetal lung tissue with respect to developmental stage-specific cytodifferentiation. Human fetal lung tissue (pseudoglandular stage) was grafted either beneath the renal capsule or the skin of athymic mice (NCr-nu). Tissues were analyzed from 3 to 42 days post-engraftment for morphological alterations by light and electron microscopy (EM), and for surfactant protein mRNA and protein by reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry (ICC), respectively. The changes observed resemble those seen in human lung development in utero in many respects, including the differentiation of epithelium to the saccular stage. Each stage occurred over approximately one week in the graft in contrast to the eight weeks of normal in utero development. At all time points examined, all four surfactant proteins (SP-A, SP-B, SP-C, and SP-D) were detected in the epithelium by ICC. Lamellar bodies were first identified by EM in 14 day xenografts. By day 21, a significant increase in lamellar body expression was observed. Cellular proliferation, as marked by PCNA ICC and elastic fiber deposition resembled those of canalicular and saccular in utero development. This model in which xenografted lung tissue in different stages of development is available may facilitate the study of human fetal lung development and the impact of various pharmacological agents on this process.

Aberrant processing forms of lung surfactant proteins SP-B and SP-C revealed by high-resolution mass spectrometry

The mutation (g.1286T>C) of the pulmonary surfactant-associated protein C gene (SFTPC) leads to the I73T substitution in the precursor protein (pro-SP-C) and results in interstitial lung disease with the histological pattern of non-specific interstitial pneumonia and pulmonary alveolar proteinosis. Central for the disease is the abnormal processing of the SP-C pro-protein to mature SP-C; however little is known about the nature of intermediates and processing products. We report here the application of high resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry to the characterization of processing intermediates of hydrophobic pulmonary surfactant proteins SP-B and SP-C in intra- alveolar surfactant material of a patient with I73T mutation. SP-C and SP-B processing forms were separated from broncho-alveolar lavage fluid using chloroform/methanol extraction and sodium dodecyl sulfate poly acrylamide gel electrophoreis, detected by Western blot and identified by electrospray- and matrix-assisted laser desorption/ionization-FT-ICR mass spectrometry. The mass spectrometric and immuno-analytical results show the intra-alveolar accumulation of an aberrant C-terminal SP-C processing products in which the mature SP-C protein part is missing and aberrant processing intermediates of SP-B.

Perinatal increases in TGF-α disrupt the saccular phase of lung morphogenesis and cause remodeling: microarray analysis

Transforming growth factor-α (TGF-α) and its receptor, the epithelial growth factor receptor (EGFR), have been associated with lung remodeling in premature infants with bronchopulmonary dysplasia (BPD). The goal of this study was to target TGF-α overexpression to the saccular phase of lung morphogenesis and determine early alterations in gene expression. Conditional lung-specific TGF-α bitransgenic mice and single-transgene control mice were generated. TGF-α overexpression was induced by doxycycline (Dox) treatment from embryonic day 16.5 (E16.5) to E18.5. After birth, all bitransgenic pups died by postnatal day 7 (P7). Lung histology at E18.5 and P1 showed abnormal lung morphogenesis in bitransgenic mice, characterized by mesenchymal thickening, vascular remodeling, and poor apposition of capillaries to distal air spaces. Surfactant levels (saturated phosphatidylcholine) were not reduced in bitransgenic mice. Microarray analysis was performed after 1 or 2 days of Dox treatment during the saccular (E17.5, E18.5) and alveolar phases (P4, P5) to identify genes induced by EGFR signaling that were shared or unique to each phase. We found 196 genes to be altered (>1.5-fold change; P < 0.01 for at least 2 time points), with only 32% similarly altered in both saccular and alveolar phases. Western blot analysis and immunostaining showed that five genes selected from the microarrays (egr-1, SP-B, SP-D, S100A4, and pleiotrophin) were also increased at the protein level. Pathological changes in TGF-α-overexpressing mice bore similarities to premature infants born in the saccular phase who develop BPD, including remodeling of the distal lung septae and arteries.

Surfactant maturation is not delayed in human fetuses with diaphragmatic hernia

BACKGROUND

Pulmonary hypoplasia and persistent pulmonary hypertension account for significant mortality and morbidity in neonates with congenital diaphragmatic hernia (CDH). Global lung immaturity and studies in animal models suggest the presence of surfactant deficiency that may further complicate the pathophysiology of CDH. However, data about surfactant status in human fetuses with CDH at birth are contradictory. The lack of a chronological study of surfactant content in late pregnancy has been a significant limitation. The appropriateness of administering surfactant supplements to neonates with CDH is therefore a debated question.

METHODS AND FINDINGS

We investigated surfactant content in human fetuses with CDH compared to age-matched fetuses with nonpulmonary diseases used as controls. Concentrations of disaturated phosphatidylcholine and surfactant proteins were found to be similar at a given stage of pregnancy, with both components showing a similar pattern of increase with progressing pregnancy in fetuses with CDH and in control fetuses. Thyroid transcription factor 1, a critical regulator of surfactant protein transcription, similarly displayed no difference in abundance. Finally, we examined the expression of three glucocorticoid-regulated diffusible mediators involved in lung epithelial maturation, namely: keratinocyte growth factor (KGF), leptin, and neuregulin 1 beta 1 (NRG1-beta1). KGF expression decreased slightly with time in control fetuses, but remained unchanged in fetuses with CDH. Leptin and NRG1-beta1 similarly increased in late pregnancy in control and CDH lungs. These maturation factors were also determined in the sheep fetus with surgical diaphragmatic hernia, in which surfactant deficiency has been reported previously. In contrast to the findings in humans, surgical diaphragmatic hernia in the sheep fetus was associated with decreased KGF and neuregulin expression. Fetoscopic endoluminal tracheal occlusion performed in the sheep model to correct lung hypoplasia increased leptin expression, partially restored KGF expression, and fully restored neuregulin expression.

CONCLUSIONS

Our results indicate that CDH does not impair surfactant storage in human fetuses. CDH lungs exhibited no trend toward a decrease in contents, or a delay in developmental changes for any of the studied surfactant components and surfactant maturation factors. Surfactant amounts are likely to be appropriate to lung size. These findings therefore do not support the use of surfactant therapy for infants with CDH. Moreover, they raise the question of the relevance of CDH animal models to explore lung biochemical maturity.

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.

Surfactant protein C biosynthesis and its emerging role in conformational lung disease

Surfactant protein C (SP-C) is a hydrophobic 35-amino acid peptide that co-isolates with the phospholipid fraction of lung surfactant. SP-C represents a structurally and functionally challenging protein for the alveolar type 2 cell, which must synthesize, traffic, and process a 191-197-amino acid precursor protein through the regulated secretory pathway. The current understanding of SP-C biosynthesis considers the SP-C proprotein (proSP-C) as a hybrid molecule that incorporates structural and functional features of both bitopic integral membrane proteins and more classically recognized luminal propeptide hormones, which are subject to post-translational processing and regulated exocytosis. Adding to the importance of a detailed understanding of SP-C biosynthesis has been the recent association of mutations in the proSP-C sequence with chronic interstitial pneumonias in children and adults. Many of these mutations involve either missense or deletion mutations located in a region of the proSP-C molecule that has structural homology to the BRI family of proteins linked to inherited degenerative dementias. This review examines the current state of SP-C biosynthesis with a focus on recent developments related to molecular and cellular mechanisms implicated in the emerging role of SP-C mutations in the pathophysiology of diffuse parenchymal lung disease.

Surfactant protein B in type II pneumocytes and intra-alveolar surfactant forms of human lungs

Surfactant protein B (SP-B) is synthesized by type II pneumocytes as a proprotein (proSP-B) that is proteolytically processed to an 8-kD protein. In human type II pneumocytes, we identified not only proSP-B, processing intermediates of proSP-B, and mature SP-B, but also fragments of the N-terminal propeptide. By means of immunoelectron microscopy, proSP-B and processing intermediates were localized in the endoplasmic reticulum, Golgi vesicles, and few multivesicular bodies in type II pneumocytes in human lungs. A colocalization of fragments of the N-terminal propeptide and mature SP-B was found in multivesicular, composite, and some lamellar bodies. Mature SP-B was localized over the projection core of lamellar bodies and core-like structures in tubular myelin figures. In line with immunoelectron microscopy and Western blot analysis of human type II pneumocytes, a fragment of the N-terminal propeptide was also detected in isolated rat lamellar bodies. In conclusion, our data indicate that the processing of proSP-B occurs between the Golgi complex and multivesicular bodies and provide evidence that a fragment of the N-terminal propeptide and mature SP-B are transported together to the lamellar bodies. In human lungs, mature SP-B is involved in the structural organization of lamellar bodies and tubular myelin by the formation of core particles.

Processing of surfactant protein C requires a type II transmembrane topology directed by juxtamembrane positively charged residues

Surfactant protein C (SP-C) is a lung-specific protein that is synthesized as a 21-kDa integral membrane propeptide (pro-SP-C) and proteolytically processed to a 3.7-kDa secretory product. Previous studies have shown that palmitoylation of pro-SP-C is dependent on two N-terminal juxtamembrane positively charged residues. We hypothesized that these residues influence modification of pro-SP-C by directing transmembrane orientation. Double substitution mutation of these juxtaposed residues from positive to neutral charged species resulted in complete reversal of transmembrane orientation of pro-SP-C and total abrogation of post-translational processing. Mutation of a single residue resulted in mixed orientation. Protein trafficking studies in A549 cells showed that while the double mutant was retained in the endoplasmic reticulum, single mutants produced a mixed pattern of both endoplasmic reticulum (double mutant-like) and vesicular (wild type-like) expression. Our study demonstrates the crucial role juxtamembrane positively charged residues play in establishing membrane topology and their influence on the trafficking and processing of pro-SP-C. Moreover this study provides a likely precedent for a mechanism in disorders associated with mutations in the membrane-flanking region of integral membrane proteins.

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.

The role of extrinsic and intrinsic factors in the evolution of the control of pulmonary surfactant maturation during development in the amniotes

Pulmonary surfactant is a mixture of lipids and proteins that is secreted by alveolar Type II cells. It reduces alveolar surface tension and hence the work of breathing. Despite the tremendous diversity of lung structures amongst the vertebrates, the composition of surfactant is highly conserved. Conserved elements of the surfactant system amongst distantly related species are likely to be crucial factors for successful lung development. Understanding the mechanisms by which the surfactant system becomes operational in animals with dramatically different birthing strategies and in distantly related species will provide important information about the role of the surfactant system in the commencement of air breathing and the processes regulating surfactant maturation and secretion. In mammals, the embryonic maturation of the surfactant system is controlled by a host of factors, including glucocorticoids, thyroid hormones, and autonomic neurotransmitters. Here we review the mechanisms controlling the maturation of surfactant production, including birthing strategy, phylogeny, lung structure, and posthatching environment. Using four species of egg-laying amniote (chicken, dragon lizard, sea turtle, and crocodile) previously described in detail and the large amount of information available for mammals, we examine the hypothesis that the control of surfactant production is dependent on glucocorticoids (dexamethasone [Dex]), thyroid hormones (T3), and autonomic neurotransmitters (epinephrine and carbachol). We also examine whether the overall intrinsic pattern of the control of surfactant maturation is conserved throughout the vertebrate radiation and then how the environment (extrinsic factors) may account for the observed differences in the patterns of development. We also discuss the utility of a coculture system of embryonic Type II cells and fibroblasts to determine the evolutionary pattern behind the control of surfactant and to demonstrate that the surfactant system matures under multihormonal control. We demonstrate that Dex and T3 are stimulators of surfactant production during embryonic development, but they lose their efficacy closer to hatching or birth. Epinephrine stimulates surfactant secretion beyond 75% of development and also after hatching or birth. Carbachol stimulates surfactant secretion in the bearded dragon and saltwater crocodile but not in the sea turtle, chicken, or mammals. It is likely that the differences in control of surfactant development are likely to be primarily related to metabolic activity and the duration of incubation (i.e., the "speed" of development). Moreover, the hormones examined appear important in promoting development and therefore appear conserved within the amniotes. However, the autonomic neurotransmitters induced different responses in different species. Hence, some factors are crucial for the proper maturation of the surfactant system, whereas others vary throughout evolution without being detrimental to the overall function of the system.

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.

Involvement of cathepsin H in the processing of the hydrophobic surfactant-associated protein C in type II pneumocytes

Surfactant protein C (SP-C) is synthesized by type II pneumocytes as a 21-kD propeptide (proSP-C) which is proteolytically processed to a 4.2-kD dipalmitoylated protein. To characterize the processing of proSP-C and the role of the cysteine protease cathepsin H, we studied the localization of proSP-C and cathepsin H in human as well as proSP-C in rat lungs, the enzymatic cathepsin H activity in isolated rat lamellar bodies, and the cleavage of human proSP-C by purified cathepsin H. Using antisera directed against the N-terminal E(11)-R(23) (NPROSP-C(11-23)), the C-terminal G(162)-G(174) domain (CPROSP-C(162-174)) of proSP-C, and against cathepsin H, immunogold labeling identified all three in electron-dense multivesicular bodies, but only NPROSP-C(11-23) and cathepsin H in composite as well as lamellar bodies of type II pneumocytes. Immuno double-labeling further distinguished electron-dense vesicles containing cathepsin H or electron light vesicles/multivesicular bodies containing proSP-C. Isolated lamellar bodies contained enzymatically active cathepsin H, a 6-kD proSP-C processing intermediate detected only by NPROSP-C(11-23), and mature SP-C. Using enzyme activities comparable to those in isolated lamellar bodies, purified cathepsin H generated a partially N-terminal processed proSP-C intermediate in vitro. In conclusion, our results indicate that after the fusion of electron-dense vesicles containing cathepsin H and electron-light vesicles or multivesicular bodies containing proSP-C, cathepsin H is involved in the first N-terminal processing step of proSP-C in electron-dense multivesicular bodies of type II pneumocytes.

Biosynthesis of surfactant protein C (SP-C). Sorting of SP-C proprotein involves homomeric association via a signal anchor domain

Rat surfactant protein C (SP-C) is synthesized as a 194-amino acid propeptide (SP-C-(1-194)) that is directed to the distal secretory pathway and proteolytically processed as an integral membrane protein to yield its mature form. We had shown previously that trafficking of proSP-C is mediated both by a signal anchor domain contained within the mature SP-C sequence and by a targeting domain in the NH(2)-flanking propeptide. Based on evidence from other integral membrane proteins, we hypothesized that proSP-C targeting is effected by oligomerization of proSP-C monomers. To evaluate this in vitro, cDNA constructs encoding for either wild type proSP-C (pcDNA3/SP-C-(1-194)) or heterologous fusion proteins containing green fluorescent protein (EGFP) linked to SP-C-(1-194) (EGFP/SP-C-(1-194)), to mutant proSP-C lacking the NH(2) targeting domain (EGFP/SP-C-(24-194)), or to mature SP-C alone (EGFP/SP-C-(24-58)) were produced. In transfected A549 cells, fluorescence microscopy revealed that pcDNA3/SP-C-(1-194) and EGFP/SP-C-(1-194) were each expressed in CD63 (+), EEA1 (-) cytoplasmic vesicles. Expression of EGFP/SP-C-(24-194) or EGFP/SP-C-(24-58) resulted in translocation but retention in early compartments. When co-transfected with pcDNA3/SP-C-(1-194), both EGFP/SP-C-(24-194) and EGFP/SP-C-(24-58) were directed to CD63 (+) vesicles that also contained SP-C-(1-194). In contrast, trafficking of a folding mutant that forms juxtanuclear aggregates, EGFP/SP-C(C122/186G), was not corrected by cotransfection with pcDNA3/SP-C-(1-194). Chemical cross-linking studies of transfected cell lysates with bismaleimidohexane produced multimeric forms of both EGFP/SP-C-(1-194) and EGFP/SP-C-(24-58). These results indicate that sorting involves oligomeric association of proSP-C monomers mediated by the mature SP-C domain. Heteromeric assembly allows wild type proSP-C to facilitate trafficking of SP-C mutants with intact transmembrane domains but lacking targeting signals. We speculate that heterotypic oligomerization of wild type with SP-C folding mutants produces a dominant negative thus contributing to the pathology of chronic lung disease associated with patients heterozygous for mutant SP-C alleles.

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 differentiated function of the surfactant system in human fetal lung type II epithelial cells cultured on plastic

We report a simplified culture system for human fetal lung type II cells that maintains surfactant expression. Type II cells isolated from explant cultures of hormone-treated lungs (18-22 wk gestation) by collagenase + trypsin digestion were cultured on plastic for 4 days in serum-free medium containing dexamethasone (Dex, 10 nM) + 8-bromo-cAMP (0.1 mM + isobutylmethylxanthine (0.1 mM) or were untreated (control). Surfactant protein (SP) mRNAs decreased markedly in control cells between days 1 and 4 of culture, but mRNA levels were high in treated cells on day) 4 (SP-A, SP-B, SP-C, SP-D; 600%, 100%, 85%, 130% of day 0 content, respectively). Dex or cAMP alone increased SP-B, SP-C, and SP-D mRNAs and together had additive effects. The greatest increase in SP-A mRNA occurred with cAMP alone. Treated cells processed pro-SP-B and pro-SP-C proteins to mature forms and had a higher rate of phosphatidylcholine (PC) synthesis (2-fold) and higher saturation of PC (approximately 34% versus 27%) than controls. Only treated cells maintained secretagogue-responsive phospholipid synthesis. By electron microscopy, the treated cells retained lamellar bodies and extensive microvilli. We conclude that Dex and cAMP additively stimulate expression of surfactant components in isolated fetal type II cells, providing a simplified culture system for investigation of surfactant-related, and perhaps other, type II cell functions.

Post-translational processing of surfactant protein-C proprotein: targeting motifs in the NH(2)-terminal flanking domain are cleaved in late compartments

Rat surfactant protein (SP)-C is a 3.7-kD hydrophobic lung-specific protein generated from proteolytic processing of a 21-kD propeptide (SP-C(21)). We have demonstrated that initial post-translational processing of SP-C(21) involves two cleavages of the COOH-terminus (Beers and colleagues, J. Biol. Chem. 1994;269:20,318--20,328). The goal of the current study was to define processing and function of the NH(2)-terminal flanking domain. Epitope-specific antisera directed against spatially distinct regions of the NH(2) terminus, NPROSP-C(2-9) (epitope = D(2)-L(9)) and NPROSP-C(11-23) (= E(11)-Q(23)) were produced. By Western blotting, both antisera identified SP-C(21) in microsomes. A 6-kD form (SP-C(6)), enriched in lamellar bodies (LBs), was detected only by NPROSP-C(11-23) and not extractable with NaCO(3) treatment. Immunogold staining of ultrathin lung sections with NPROSP-C(11-23) identified proSP-C in both multivesicular bodies (mvb) and LBs whereas NPROSP-C(2-9) labeled only mvb. (35)S-pulse chase analysis demonstrated synthesis of SP-C(21) and three intermediate forms (SP-C(16), SP-C(7), and SP-C(6)). Complete processing involved four separate cleavages with a precursor- product relationship between the low molecular weight forms SP-C(7) and SP-C(6). Fluorescence microscopy of A549 cells expressing fusion proteins of enhanced green fluorescent protein (EGFP) and proSP-C NH(2)-terminal deletion mutants showed targeting of EGFP/SP-C(1-194) and EGFP/SP-C(10-194) to early endosomal antigen-1-negative, CD-63-positive cytoplasmic vesicles whereas EGFP/SP-C(19-194), EGFP/SP-C(Delta 10-18), and EGFP/SP-C(24-194) were restricted to the endoplasmic reticulum (ER). We conclude that synthetic processing includes a previously unrecognized cleavage of the proximal NH(2) terminus (M(1)-L(9)), which occurs after removal of COOH-flanking domains (H(59)-I(194)) but before packaging in LBs, and that the region M(10)-T(18) is required for targeting of proSP-C to post-ER vesicular compartments in the biosynthetic pathway.

Biosynthesis of surfactant protein C: characterization of aggresome formation by EGFP chimeras containing propeptide mutants lacking conserved cysteine residues

Surfactant protein C (SP-C) is a lung-specific secreted protein, which is synthesized as a 21-kDa propeptide (SP-C(21)) and then proteolytically processed as a bitopic transmembrane protein in subcellular compartments distal to the medial Golgi to produce a 3.7 kDa mature form. We have shown that initial processing of SP-C(21) involves two endoproteolytic cleavages of the C terminus and that truncation of nine amino acids from the C-flanking peptide resulted in retention of mutant protein in proximal compartments. Because these truncations involved removal of a conserved cysteine residue (Cys(186)), we hypothesized that intralumenal disulfide-mediated folding of the C terminus of SP-C(21) is required for intracellular trafficking. To test this, cDNA constructs encoding heterologous fusion proteins consisting of enhanced green fluorescent protein (EGFP) attached to the N terminus of wild-type rat proSP-C (EGFP/SP-C(1–194)), C-terminally deleted proSP-C (EGFP/SP-C(1–185); EGFP/SP-C(1–191)) or point mutations of conserved cysteine residues (EGFP/SP-C(C122G); EGFP/SP-C(C186G); or EGFP/SP-C(C122/186G)) were transfected into A549 cells. Fluorescence microscopy revealed that transfected EGFP/SP-C(1–194) and EGFP/SP-C(1–191)were expressed in a punctate pattern within CD-63 positive, EEA-1 negative cytoplasmic vesicles. In contrast, EGFP/SP-C(1–185), EGFP/SP-C(C122G), EGFP/SP-C(C186G) and EGFP/SP-C(C122/186G) were expressed but retained in a juxtanuclear compartment that stained for ubiquitin and that contained (γ)-tubulin and vimentin, consistent with expression in aggresomes. Treatment of cells transfected with mutant proSP-C with the proteasome inhibitor lactacysteine enhanced aggresome formation, which could be blocked by coincubation with nocodazole. Western blots using a GFP antibody detected a single form in lysates of cells transfected with EGFP/SP-C cysteine mutants, without evidence of smaller degradation fragments. We conclude that residues Cys(122) and Cys(186) of proSP-C are required for proper post-translational trafficking. Mutation or deletion of one or both of these residues results in misfolding with mistargeting of unprocessed mutant protein, leading to formation of stable aggregates within aggresomes.

BR22, a novel protein, interacts with thyroid transcription factor-1 and activates the human surfactant protein B promoter

Surfactant protein (SP)-B expression is restricted to type II pneumocytes and Clara cells in the lung. Previously, a promoter region of human SP-B gene from -64 to -118 has been identified as critical for the tissue-specific expression of this gene. Two cis-elements for thyroid transcription factor (TTF)-1 and hepatocyte nuclear factor (HNF)-3alpha binding were found within this area. Using an oligonucleotide fragment, we incorporated this region sequence into the promoter of a HIS3 reporter gene in yeast. With this modified yeast a human lung complementary DNA (cDNA) library was screened for DNA-binding proteins, other than TTF-1 and HNF-3alpha, that interacted with this promoter segment. A cDNA clone encoding a novel polypeptide, BR22, was identified that activated the reporter gene expression in yeast. This gene is expressed in many tissues and encodes a protein with bipartite nuclear localization signals. Studies using in vivo yeast two-hybrid analysis, in vitro protein-protein interactions, and coimmunoprecipitation analyses demonstrated that BR22 formed a protein complex with TTF-1. In vivo cotransfection studies further indicated that BR22 could act with TTF-1 to synergistically activate the SP-B promoter in mammalian cells. Our data suggest that BR22 is a TTF-1-associated protein. Through a protein-protein interaction with TTF-1, BR22 can form a complex and activate the human SP-B promoter in vivo.

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.

Surfactant protein-C in ventilated premature lamb lung

Although surfactants containing only lipids and surfactant protein C (SP-C) or SP-C analogs can be effective for the treatment of surfactant deficiency in animal models, there is no information concerning the alveolar or lung clearance of SP-C. Because the other lipid and protein components of surfactant are cleared very slowly from the preterm lung, we hypothesized that SP-C also would be cleared slowly. Therefore, we compared the losses of iodinated native SP-C (nSP-C) and a recombinant SP-C analog (rSP-C, phenylalanines in positions 4 and 5 and isoleucine in position 32 of the human sequence) to [14C]dipalmitoylphosphatidylcholine (DPPC) after airway administration at birth of trace or treatment doses of surfactant given to preterm lambs. In preterm lambs given trace doses at 134-136 d gestation, alveolar [14C]DPPC and [125I]rSP-C decreased to 14.7% recovery for DPPC and 8.3% recovery for rSP-C after 2 h ventilation. There was no loss of [14C]DPPC from the total lungs (alveolar wash + lung tissue), and approximately 20% of the [125I]rSP-C was lost from the lungs. For 128 d gestational age lambs treated with 100-mg/kg doses of surfactants containing nSP-C or 2% rSP-C, the alveolar and total lung recoveries for [125I]nSP-C or [125I]rSP-C were equivalent to that of [14C]DPPC after 5 h ventilation. These results demonstrate that nSP-C and rSP-C have alveolar clearances and accumulations into preterm lung tissue that are similar to those of DPPC.

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.

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.