In vitro translation, post-translational processing and secretion of pulmonary surfactant protein B precursors

Stem Cell-derived Respiratory Epithelial Cell Cultures as Human Disease Models

Advances in stem cell biology and the understanding of factors that determine lung stem cell self-renewal have enabled long-term in vitro culture of human lung cells derived from airway basal and alveolar type II cells. Improved capability to expand and study primary cells long term, including in clonal cultures that are recently derived from a single cell, will allow experiments that address fundamental questions about lung homeostasis and repair, as well as translational questions in asthma, chronic obstructive pulmonary disease, pulmonary fibrosis, and lung cancer research. Here, we provide a brief history of postnatal lung epithelial cell culture and describe recent methodological advances. We further discuss the applications of primary cultures in defining "normal" epithelium, in modeling lung disease, and in future cell therapies.

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.

Immunohistochemical expression of Napsin A in normal human fetal lungs at different gestational ages and in acquired and congenital pathological pulmonary conditions

Surfactant protein B (SP-B) is a key component of pulmonary surfactant. SP-B is processed to a mature, surface-active protein from a pro-peptide by two distinct cleavage events in its N-terminal and C-terminal regions. Napsin A, a protease expressed in type II pneumocytes, is responsible for the N-terminal cleavage event. Here, for the first time, we have evaluated the expression of Napsin A in normal fetal lungs at different gestational ages and in lungs from fetuses and neonates with congenital and acquired pathological pulmonary conditions. Lung samples were collected from fetal and neonatal autopsies at the Department of Medicine and Surgery's Pathology Unit of Parma University (Italy). Immunohistochemical analysis was performed using a primary anti-Napsin A (clone IP64 clone) monoclonal antibody. A section of lung adenocarcinoma was used as an external positive control. Napsin A was expressed early in normal fetal lungs throughout the epithelium of the distal pseudoglandular tracts. In fetuses at 30 weeks of gestation and term newborns, Napsin A was already expressed only in isolated cells within the alveolar epithelium, similar to adult subjects. Furthermore, increased expression of Napsin A compared with a control group was observed in lung tissue from fetuses and a newborn with pathological conditions (inflammatory diseases and pulmonary hypoplasia). In conclusion, this study demonstrates that Napsin A is produced early in fetal life, and that its production is increased in many diseases, presumably in an effort to remedy functional pulmonary failure.

Featured Article: Electroporation-mediated gene delivery of surfactant protein B (SP-B) restores expression and improves survival in mouse model of SP-B deficiency

Surfactant Protein B Deficiency is a rare but lethal monogenetic, congenital lung disease of the neonate that is unresponsive to any treatment except lung transplantation. Based on the potential that gene therapy offers to treat such intractable diseases, our objective was to test whether an electroporation-based gene delivery approach could restore surfactant protein B expression and improve survival in a compound knockout mouse model of surfactant protein B deficiency. Surfactant protein B expression can be shut off in these mice upon withdrawl of doxycycline, resulting in decreased levels of surfactant protein B within four days and death due to lung dysfunction within four to seven days. Control or one of several different human surfactant protein B-expressing plasmids was delivered to the lung by aspiration and electroporation at the time of doxycycline removal or four days later. Plasmids expressing human surfactant protein B from either the UbC or CMV promoter expressed surfactant protein B in these transgenic mice at times when endogenous surfactant protein B expression was silenced. Mean survival was increased 2- to 5-fold following treatment with the UbC or CMV promoter-driven plasmids, respectively. Histology of all surfactant protein B treated groups exhibited fewer neutrophils and less alveolar wall thickening compared to the control groups, and electron microscopy revealed that gene transfer of surfactant protein B resulted in lamellar bodies that were similar in the presence of electron-dense, concentric material to those in surfactant protein B-expressing mice. Taken together, our results show that electroporation-mediated gene delivery of surfactant protein B-expressing plasmids improves survival, lung function, and lung histology in a mouse model of surfactant protein B deficiency and suggest that this may be a useful approach for the treatment of this otherwise deadly disease. Impact statement Surfactant protein B (SP-B) deficiency is a rare but lethal genetic disease of neonates that results in severe respiratory distress with no available treatments other than lung transplantation. The present study describes a novel treatment for this disease by transferring the SP-B gene to the lungs using electric fields in a mouse model. The procedure is safe and results in enough expression of exogenous SP-B to improve lung histology, lamellar body structure, and survival. If extended to humans, this approach could be used to bridge the time between diagnosis and lung transplantation and could greatly increase the likelihood of affected neonates surviving to transplantation and beyond.

Amiloride-sensitive fluid resorption in NCI-H441 lung epithelia depends on an apical Cl(-) conductance

Proper apical airway surface hydration is essential to maintain lung function. This hydration depends on well‐balanced water resorption and secretion. The mechanisms involved in resorption are still a matter of debate, especially as the measurement of transepithelial water transport remains challenging. In this study, we combined classical short circuit current (I_SC) measurements with a novel D₂O dilution method to correlate ion and water transport in order to reveal basic transport mechanisms in lung epithelia. D₂O dilution method enabled precise analysis of water resorption with an unprecedented resolution. NCI‐H441 cells cultured at an air–liquid interface resorbed water at a rate of 1.5 ± 0.4 μL/(h cm²). Water resorption and I_SC were reduced by almost 80% in the presence of the bulk Cl⁻ channel inhibitor 5‐nitro‐2‐(3‐phenylpropylamino)benzoic acid (NPPB) or amiloride, a specific inhibitor of epithelial sodium channel (ENaC). However, water resorption and I_SC were only moderately affected by forskolin or cystic fibrosis transmembrane regulator (CFTR) channel inhibitors (CFTR_inh‐172 and glybenclamide). In line with previous studies, we demonstrate that water resorption depends on ENaC, and CFTR channels have only a minor but probably modulating effect on water resorption. However, the major ENaC‐mediated water resorption depends on an apical non‐CFTR Cl⁻ conductance.

We investigated water transport across lung epithelia, using a novel D₂O dilution method in combination with Ussing chamber experiments. Our results revealed that CFTR channels have a minor modulating effect on water resorption. The major ENaC‐mediated resorption depends on non‐CFTR channels.

Functional expression of the transient receptor potential channel TRPA1, a sensor for toxic lung inhalants, in pulmonary epithelial cells

The cation channel TRPA1 functions as a chemosensory protein and is directly activated by a number of noxious inhalants. A pulmonary expression of TRPA1 has been described in sensory nerve endings and its stimulation leads to the acceleration of inflammatory responses in the lung. Whereas the function of TRPA1 in neuronal cells is well defined, only few reports exist suggesting a role in epithelial cells. The aim of the present study was therefore (1) to evaluate the expression of TRPA1 in pulmonary epithelial cell lines, (2) to characterize TRPA1-promoted signaling in these cells, and (3) to study the extra-neuronal expression of this channel in lung tissue sections. Our results revealed that the widely used alveolar type II cell line A549 expresses TRPA1 at the mRNA and protein level. Furthermore, stimulating A549 cells with known TRPA1 activators (i.e., allyl isothiocyanate) led to an increase in intracellular calcium levels, which was sensitive to the TRPA1 blocker ruthenium red. Investigating TRPA1 coupled downstream signaling cascades it was found that TRPA1 activation elicited a stimulation of ERK1/2 whereas other MAP kinases were not affected. Finally, using epithelial as well as neuronal markers in immunohistochemical approaches, a non-neuronal TRPA1 protein expression was detected in distal parts of the porcine lung epithelium, which was also found examining human lung sections. TRPA1-positive staining co-localized with both epithelial and neuronal markers underlining the observed epithelial expression pattern. Our findings of a functional expression of TRPA1 in pulmonary epithelial cells provide causal evidence for a non-neuronal TRPA1-mediated control of inflammatory responses elicited upon TRPA1-mediated registration of toxic inhalants in vivo.

Surfactant protein B detection and gene expression in chronic rhinosinusitis

INTRODUCTION

Surfactant protein (SP)-B is a hydrophobic protein secreted within pulmonary surfactant that facilitates the adsorption of surface-active lipids to the air-liquid interface of the alveoli and increases alveolar stability. SP-B may also have anti-inflammatory properties. It is implicated in decreasing the pulmonary inflammatory response to bacterial lipopolysaccharide. However, the expression and function of SP-B in the sinonasal cavities has not been elucidated. Our objective was to detect the presence of SP-B, measure alterations in several forms of chronic rhinosinusitis (CRS), and localize cellular protein expression.

MATERIALS/METHODS

Sinus mucosal biopsies were performed in patients with allergic fungal rhinosinusitis (AFRS), nonatopic CRS with nasal polyposis (NP), and cystic fibrosis (CF) and in healthy controls. SP-B mRNA was measured in CRS and control patients using quantitative polymerase chain reaction. Immunoblot analysis and immunolocalization of SP-B were also performed.

RESULTS

CF (n = 4) showed significantly increased levels of SP-B (169-fold) mRNA (P = .004) when compared with controls (n = 5). CRS with NP (n = 5) and AFRS (n = 7) also demonstrated elevated levels of SP- B (14-fold and 4-fold, respectively) when compared with the control group, although these were not statistically significant. Immunoblot analysis confirmed the presence of the translated product, and immunolocalization revealed expression in the epithelium and submucosal glandular elements.

CONCLUSION

This is the first study to detect and characterize SP-B in human sinus mucosa. Furthermore, SP-B is significantly up-regulated in CF CRS.

A major deletion in the surfactant protein-B gene causing lethal respiratory distress

BACKGROUND

Loss of function mutations in the surfactant protein-B gene (SFTPB) cause lethal neonatal respiratory distress due to reduced or absent expression of mature surfactant protein B (SP-B, encoded in exons 6 and 7). No large deletions in SFTPB have been previously identified.

AIM

Genomic, proteomic and immunohistochemical characterization of a 3 kb deletion in SFTPB.

METHODS

A full-term newborn presented with refractory respiratory failure. We amplified and sequenced SFTPB from the infant and both parents, determined SP-B protein expression in tracheal aspirate samples using Western-blot analysis, and performed immunohistochemical staining and electron microscopy of lung biopsy tissue.

RESULTS

The infant was homozygous for a 2958 bp deletion in SFTPB that included exons 7 and 8. Both asymptomatic parents were heterozygous for the deletion. A truncated mature SP-B peptide was detected on Western blotting of tracheal aspirate. Amino acid sequence specific to that encoded in exon 5 was present, but that encoded by exon 7 was absent. ProSP-B expression was robust within alveolar type II cells and lamellar body structure was disrupted.

CONCLUSIONS

This deletion in SFTPB resulted in SP-B deficiency due to absence of elements in mature SP-B that are critical for appropriate peptide folding, trafficking and processing.

Proteasome dysfunction inhibits surfactant protein gene expression in lung epithelial cells: mechanism of inhibition of SP-B gene expression

Surfactant proteins maintain lung function through their actions to reduce alveolar surface tension and control of innate immune responses in the lung. The ubiquitin proteasome pathway is responsible for the degradation of majority of intracellular proteins in eukaryotic cells, and proteasome dysfunction has been linked to the development of neurodegenerative, cardiac, and other diseases. Proteasome function is impaired in interstitial lung diseases associated with surfactant protein C (SP-C) mutation mapping to the BRICHOS domain located in the proSP-C protein. In this study we determined the effects of proteasome inhibition on surfactant protein expression in H441 and MLE-12 lung epithelial cells to understand the relationship between proteasome dysfunction and surfactant protein gene expression. Proteasome inhibitors lactacystin and MG132 reduced the levels of SP-A, SP-B, and SP-C mRNAs in a concentration-dependent manner in H441 and MLE-12 cells. In H441 cells, lactacystin and MG132 inhibition of SP-B mRNA was associated with similar decreases in SP-B protein, and the inhibition was due to inhibition of gene transcription. Proteasome inhibitors decreased thyroid transcription factor-1 (TTF-1)/Nkx2.1 DNA binding activity, and the reduced TTF-1 DNA binding activity was due to reduced expression levels of TTF-1 protein. These data indicated that the ubiquitin proteasome pathway is essential for the maintenance of surfactant protein gene expression and that disruption of this pathway inhibits surfactant protein gene expression via reduced expression of TTF-1 protein.

Production in Escherichia coli of a recombinant C-terminal truncated precursor of surfactant protein B (rproSP-BΔc). Structure and interaction with lipid interfaces

SP-B, a protein absolutely required to maintain the lungs open after birth, is synthesized in the pneumocytes as a precursor containing C-terminal and N-terminal domains flanking the mature sequence. These flanking-domains are cleaved to produce mature SP-B, coupled with its assembly into pulmonary surfactant lipid-protein complexes. In the present work we have optimized over-expression in Escherichia coli and purification of rproSP-B(DeltaC), a recombinant form of human proSP-B lacking the C-terminal flanking peptide, which is still competent to restore SP-B function in vivo. rProSP-B(DeltaC) has been solubilized, purified and refolded from bacterial inclusion bodies in amounts of about 4 mg per L of culture. Electrophoretic mobility, immunoreactivity, N-terminal sequencing and peptide fingerprinting all confirmed that the purified protein had the expected mass and sequence. Once refolded, the protein was soluble in aqueous buffers. Circular dichroism and fluorescence emission spectra of bacterial rproSP-B(DeltaC) indicated that the protein is properly folded, showing around 32% alpha-helix and a mainly hydrophobic environment of its tryptophan residues. Presence of zwitterionic or anionic phospholipids vesicles caused changes in the fluorescence emission properties of rproSP-B(DeltaC) that were indicative of lipid-protein interaction. The association of this SP-B precursor with membranes suggests an intrinsic amphipathic character of the protein, which spontaneously adsorbs at air-liquid interfaces either in the absence or in the presence of phospholipids. The analysis of the structure and properties of recombinant proSP-B(DeltaC) in surfactant-relevant environments will open new perspectives on the investigation of the mechanisms of lipid and protein assembly in surfactant complexes.

Adenosine-evoked Na+ transport in human airway epithelial cells

BACKGROUND AND PURPOSE

Absorptive epithelia express apical receptors that allow nucleotides to inhibit Na(+) transport but ATP unexpectedly stimulated this process in an absorptive cell line derived from human bronchiolar epithelium (H441 cells) whilst UTP consistently caused inhibition. We have therefore examined the pharmacological basis of this anomalous effect of ATP.

EXPERIMENTAL APPROACH

H441 cells were grown on membranes and the short circuit current (I(SC)) measured in Ussing chambers. In some experiments, [Ca(2+)](i) was measured fluorimetrically using Fura -2. mRNAs for adenosine receptors were determined by the polymerase chain reaction (PCR).

KEY RESULTS

Cross desensitization experiments showed that the inhibitory response to UTP was abolished by prior exposure to ATP whilst the stimulatory response to ATP persisted in UTP-pre-stimulated cells. Apical adenosine evoked an increase in I(SC) and this response resembled the stimulatory component of the response to ATP, and could be mimicked by adenosine receptor agonists. Pre-stimulation with adenosine abolished the stimulatory component of the response to ATP. mRNA encoding A(1), A(2A) and A(2B) receptor subtypes, but not the A(3) subtype, was detected in H441 cells and adenosine receptor antagonists could abolish the ATP-evoked stimulation of Na(+) absorption.

CONCLUSIONS AND IMPLICATIONS

The ATP-induced stimulation of Na(+) absorption seems to be mediated via A(2A/B) receptors activated by adenosine produced from the extracellular hydrolysis of ATP. The present data thus provide the first description of adenosine-evoked Na(+) transport in airway epithelial cells and reveal a previously undocumented aspect of the control of this physiologically important ion transport process.

Cell culture models of the respiratory tract relevant to pulmonary drug delivery

The respiratory tract holds promise as an alternative site of drug delivery due to fast absorption and rapid onset of drug action, with avoidance of hepatic and intestinal first-pass metabolism as an additional benefit compared to oral drug delivery. At present, the pharmaceutical industry increasingly relies on appropriate in vitro models for the faster evaluation of drug absorption and metabolism as an alternative to animal testing. This article reviews the various existing cell culture systems that may be applied as in vitro models of the human air-blood barrier, for instance, in order to enable the screening of large numbers of new drug candidates at low cost with high reliability and within a short time span. Apart from such screening, cell culture-based in vitro systems may also contribute to improve our understanding of the mechanisms of drug transport across such epithelial tissues, and the mechanisms of action how advanced drug carriers, such as nanoparticles or liposomes, can help to overcome these barriers. After all, the increasing use and acceptance of such in vitro models may lead to a significant acceleration of the drug development process by facilitating the progress into clinical studies and product registration.

Alterations in SP-B and SP-C expression in neonatal lung disease

The hydrophobic surfactant proteins, SP-B and SP-C, have important roles in surfactant function. The importance of these proteins in normal lung function is highlighted by the lung diseases associated with abnormalities in their expression. Mutations in the gene encoding SP-B result in severe, fatal neonatal lung disease, and mutations in the gene encoding SP-C are associated with chronic interstitial lung diseases in newborns, older children, and adults. This work reviews the current state of knowledge concerning the lung diseases associated with mutations in the SP-B and SP-C genes, and the potential roles of abnormal SP-B and SP-C expression and genetic variation in these genes in other lung diseases.

Processing of Pulmonary Surfactant Protein B by Napsin and Cathepsin H

Surfactant protein B (SP-B) is an essential constituent of pulmonary surfactant. SP-B is synthesized in alveolar type II cells as a preproprotein and processed to the mature peptide by the cleavage of NH2- and COOH-terminal peptides. An aspartyl protease has been suggested to cleave the NH2-terminal propeptide resulting in a 25-kDa intermediate. Napsin, an aspartyl protease expressed in alveolar type II cells, was detected in fetal lung homogenates as early as day 16 of gestation, 1 day before the onset of SP-B expression and processing. Napsin was localized to multivesicular bodies, the site of SP-B proprotein processing in type II cells. Incubation of SP-B proprotein from type II cells with a crude membrane extract from napsin-transfected cells resulted in enhanced levels of a 25-kDa intermediate. Purified napsin cleaved a recombinant SP-B/EGFP fusion protein within the NH2-terminal propeptide between Leu178 and Pro179, 22 amino acids upstream of the NH2 terminus of mature SP-B. Cathepsin H, a cysteine protease also implicated in pro-SP-B processing, cleaved SP-B/EGFP fusion protein 13 amino acids upstream of the NH2 terminus of mature SP-B. Napsin did not cleave the COOH-terminal peptide, whereas cathepsin H cleaved the boundary between mature SP-B and the COOH-terminal peptide and at several other sites within the COOH-terminal peptide. Knockdown of napsin by small interfering RNA resulted in decreased levels of mature SP-B and mature SP-C in type II cells. These results suggest that napsin, cathepsin H, and at least one other enzyme are involved in maturation of the biologically active SP-B peptide.

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.

Cell-specific involvement of HNF-1beta in alpha(1)-antitrypsin gene expression in human respiratory epithelial cells

The synergistic action of hepatocyte nuclear factor (HNF)-1alpha and HNF-4 plays an important role in expression of the alpha(1)-antitrypsin (alpha(1)-AT) gene in human hepatic and intestinal epithelial cells. Recent studies have indicated that the alpha(1)-AT gene is also expressed in human pulmonary alveolar epithelial cells, a potentially important local site of the lung antiprotease defense. In this study, we examined the possibility that alpha(1)-AT gene expression in a human pulmonary epithelial cell line H441 was also directed by the synergistic action of HNF-1alpha and HNF-4 and/or by the action of HNF-3, which has been shown to play a dominant role in gene expression in H441 cells. The results show that alpha(1)-AT gene expression in H441 cells is predominantly driven by HNF-1beta, even though HNF-1beta has no effect on alpha(1)-AT gene expression in human hepatic Hep G2 and human intestinal epithelial Caco-2 cell lines. Expression of alpha(1)-AT and HNF-1beta was also demonstrated in primary cultures of human respiratory epithelial cells. HNF-4 has no effect on alpha(1)-AT gene expression in H441 cells, even when it is cotransfected with HNF-1beta or HNF-1alpha. HNF-3 by itself has little effect on alpha(1)-AT gene expression in H441, Hep G2, or Caco-2 cells but tends to have an upregulating effect when cotransfected with HNF-1 in Hep G2 and Caco-2 cells. These results indicate the unique involvement of HNF-1beta in alpha(1)-AT gene expression in a cell line and primary cultures derived from human respiratory epithelium.

Expression and localization of a novel Rab small G protein (Rab38) in the rat lung

The Rab small G protein family participates in intracellular vesicle transport, including exocytosis and endocytosis. The cDNA encoding a novel Rab-related small G protein (Rab38) has been cloned from rat lung cDNA library and recorded in GenBank (accession no. M94043). However, the expression and localization of the protein in the lung remains primarily unknown. We produced polyhistidine-tagged recombinant Rab38 and a polyclonal antibody with a synthetic peptide. Immunohistochemistry demonstrated that the protein is specifically localized in alveolar type II cells and in bronchial epithelial cells. In situ hybridization using a digoxygenin-labeled RNA riboprobe clearly showed that the mRNA of the protein is localized in alveolar type II cells and bronchial epithelial cells, especially terminal airway epithelial cells. Western blot and reverse transcriptase-polymerase chain reaction showed distinct expression of the protein and mRNA in isolated alveolar type II cells, but not in alveolar macrophages. The native protein was predominantly hydrophobic and was enriched in a high-density vesicle fraction but was barely detectable in nuclear and lamellar body fractions in alveolar type II cells. Immunofluorescence cytochemistry performed on cultured alveolar type II cells showed that Rab38 distributed extensively in the cytoplasm with a distribution pattern similar to endoplasmic reticulum rather than other subcellular organelles. These results suggest that this novel rab small G protein (Rab38) mediates vesicular transport in terminal airway epithelium.

Expression and localization of lung surfactant protein B in Eustachian tube epithelium

Surfactant protein (SP) B is an essential component of the pulmonary surfactant complex, which participates in reducing the surface tension across the alveolar air-liquid interface. The Eustachian tube (ET) connects the upper respiratory tract to the middle ear, serving as an intermittent airway between the pharynx and the middle ear. Recently, we described the expression of SP-A and SP-D in the ET, suggesting their role in middle ear host defense. Our present aim was to detect whether the expression of SP-B is evident in the porcine ET. With Northern blot analysis, RT-PCR, and in situ hybridizations, SP-B mRNA was identified and localized in the ET epithelium. The cellular localization of SP-B was revealed with immunohistochemistry, electron microscopy, and immunoelectron microscopy. The protein was found in the secretory granules of epithelial cells and also attached to the microvilli at the luminal side of these cells. The SP-B immunoreactivity of aggregates isolated from ET lavage fluid was similar to that isolated from bronchoalveolar lavage fluid. We conclude that there are specialized cells in the ET epithelium expressing and secreting SP-B and propose that SP-B may facilitate normal opening of the tube and mucociliary transport.

Prolonged survival in hereditary surfactant protein B (SP-B) deficiency associated with a novel splicing mutation

Hereditary surfactant protein B (SP-B) deficiency has been lethal in the first year of life without lung transplantation. We tested the hypothesis that SP-B gene mutations may result in milder phenotypes by investigating the mechanisms for lung disease in two children with less severe symptoms than have been previously observed in SP-B deficiency. Immunostaining patterns for pulmonary surfactant proteins were consistent with SP-B deficiency in both children. DNA sequence analysis indicated that both children were homozygous for a mutation in exon 5 that created an alternative splice site. Reverse transcriptase PCR and sequence analysis confirmed use of this splice site, which resulted in a frameshift and a premature termination codon in exon 7. The predominant reverse transcriptase PCR product, however, lacked exon 7, which restored the reading frame but would not allow translation of the exons that encode mature SP-B. Western blot analysis detected reduced amounts of mature SP-B as well as an aberrant SP-B proprotein that corresponded to the size expected from translation of the abnormal transcript. We conclude that a novel splicing mutation was the cause of lung disease in these children and that hereditary SP-B deficiency can be the cause of lung disease in older children.

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.

Lamellar body formation in normal and surfactant protein B-deficient fetal mice

Surfactant protein B (SP-B) -/- mice die of lethal respiratory distress syndrome shortly after birth. Alveolar type II epithelial cells in SP-B-deficient mice are characterized by a complete absence of lamellar bodies, the intracellular storage form of pulmonary surfactant, and the presence of inclusions containing numerous small vesicles and electron-dense masses. The present study was undertaken to characterize the formation of these inclusions during fetal lung development and clarify their relationship to lamellar bodies. In wild-type and SP-B +/- mice, small lamellar bodies with loosely organized lamellae and distinct limiting membranes were first detected on day 16 to 16.5 of gestation. SP-B -/- mice were readily identified on day 16 by the absence of immature lamellar bodies, the appearance of vesicular inclusions similar to those previously described in late gestation SP-B -/- mice, and the accumulation of misprocessed SP-C protein. Vesicular inclusions were rarely detected in SP-B +/- mice and were never detected in wild-type littermates. Classical multivesicular bodies were observed fusing with lamellar bodies in wild-type mice, and with the vesicular inclusions in SP-B -/- mice that occasionally contained a few membrane lamellae. On day 18, the airways of SP-B -/- mice lacked tubular myelin and were filled with vesicles and electron-dense masses, suggesting that the contents of the vesicular inclusions were secreted. Taken together, these observations suggest that vesicular inclusions in SP-B -/- mice are disorganized lamellar bodies in which the absence of SP-B leads to failure to package surfactant phospholipids into concentric lamellae.

Hepatocyte growth factor is elevated in chronic lung injury and inhibits surfactant metabolism

Adult respiratory distress syndrome may incorporate in its pathogenesis the hyperplastic proliferation of alveolar epithelial type II cells and derangement in synthesis of pulmonary surfactant. Previous studies have demonstrated that hepatocyte growth factor (HGF) in the presence of serum is a potential mitogen for adult type II cells (R. J. Panos, J. S. Rubin, S. A. Aaronson, and R. J. Mason. J. Clin. Invest. 92: 969-977, 1993) and that it is produced by fetal mesenchymal lung cells (J. S. Rubin, A. M.-L. Chan, D. P. Botarro, W. H. Burgess, W. G. Taylor, A. C. Cech, D. W. Hirschfield, J. Wong, T. Miki, P. W. Finch, and S. A. Aaronson. Proc. Natl. Acad. Sci. USA 88: 415-419, 1991). In these studies, we expand on this possible involvement of HGF in chronic lung injury by showing the following. First, normal adult lung fibroblasts transcribe only small amounts of HGF mRNA, but the steady-state levels of this message rise substantially in lung fibroblasts obtained from animals exposed to oxidative stress. Second, inflammatory cytokines produced early in the injury stimulate the transcription of HGF in isolated fibroblasts, providing a plausible mechanism for the increased amounts of HGF seen in vivo. Third, HGF is capable of significantly inhibiting the synthesis and secretion of the phosphatidylcholines of pulmonary surfactant. Fourth, HGF inhibits the rate-limiting enzyme in de novo phosphatidylcholine synthesis, CTP:choline-phosphate cytidylyltransferase (EC 2.7.7.15). Our data indicate that fibroblast-derived HGF could be partially responsible for the changes in surfactant dysfunction seen in adult respiratory distress syndrome, including the decreases seen in surfactant phosphatidylcholines.

Surfactant protein-B-deficient mice are susceptible to hyperoxic lung injury

Surfactant protein-B (SP-B) is a small, hydrophobic peptide that plays a critical role in pulmonary function and surfactant homeostasis. To determine whether SP-B protects mice from oxygen-induced injury, heterozygous SP-B(+/-) gene-targeted mice and wild-type SP-B(+/+) littermates were exposed to hyperoxia (95% oxygen for 3 d) or room air. Although specific lung compliance in room air in SP-B(+/-) mice was slightly reduced as compared with that in SP-B(+/+) mice, it was reduced more markedly during hyperoxia (46% versus 25% decrease, respectively). The larger decrease in lung compliance in SP-B(+/-) mice was associated with increased severity of pulmonary edema, hemorrhage and inflammation, lung permeability and protein leakage into the alveolar space. Hyperoxia increased SP-B messenger RNA (mRNA) and total protein concentrations by 2-fold in SP-B(+/+) and SP-B(+/-) mice, but decreased the abundance of SP-B protein in lavage fluid relative to total protein only in SP-B(+/-) mice. Hyperoxia increased SP-B expression, but apparently not enough to maintain SP-B function and lung compliance in the presence of increased protein leakage in SP-B(+/-) mice. Increased alveolar-capillary leakage and relative deficiency of SP-B may therefore contribute to oxygen-induced pulmonary dysfunction in SP-B(+/-) mice. These data support the concept that SP-B plays an important protective role in the lung.

Surfactant protein B (SP-B) -/- mice are rescued by restoration of SP-B expression in alveolar type II cells but not Clara cells

Surfactant protein B (SP-B) mRNA and protein are restricted to alveolar Type II and Clara cells in the respiratory epithelium. In order to investigate the function of SP-B in these distinct cell types, transgenic mice were generated in which SP-B expression was selectively restored in Type II cells or Clara cells of SP-B -/- mice. The 4.8-kilobase murine SP-C promoter was used to generate 3 transgenic lines which expressed human SP-B in Type II cells (mSP-C/hSP-B). Likewise, the 2.3-kilobase murine CCSP promoter was used to generate two transgenic lines which expressed human SP-B in Clara cells (mCCSP/hSP-B). mSP-C/hSP-B and mCCSP/hSP-B transgenic mice were subsequently bred to SP-B +/- mice in order to selectively express SP-B in Type II cells or Clara cells of SP-B -/- mice. Selective restoration of SP-B expression in Type II cells completely rescued the neonatal lethal phenotype in SP-B -/- mice. Expression of SP-B in some, but not all Type II cells of SP-B -/- mice, allowed postnatal survival, but resulted in significantly altered lung architecture and function. Selective restoration of SP-B expression in Clara cells of SP-B -/- mice resulted in respiratory dysfunction and invariable neonatal death, related to the complete absence of mature SP-B peptide in these mice. These results indicate that expression and processing of the SP-B proprotein to the mature peptide in Type II cells is absolutely required for lung function in vivo and that SP-B expression in Clara cells cannot substitute for this function.

Synthesis, processing and secretion of surfactant proteins B and C

Two small, hydrophobic peptides, surfactant protein (SP)-B and SP-C, play important roles in the generation and maintenance of a surface active film in the alveolus. Isolation and characterization of the cDNAs encoding SP-B and SP-C indicate that both peptides are synthesized as larger proproteins which are proteolytically processed to peptides with Mr approx. 8000 and 4000, respectively. The biosynthetic pathway leading to generation and secretion of the biophysically active mature SP-B and SP-C peptides is reviewed.

Insulin inhibits surfactant protein A and B gene expression in the H441 cell line

Fetuses of mothers with uncontrolled gestational diabetes have an increased risk of developing neonatal respiratory distress syndrome and are frequently hyperinsulinemic, thus it has been proposed that high levels of insulin delay fetal lung maturation. We have shown previously that insulin inhibits the accumulation of mRNA for the surfactant-associated proteins A and B (SP-A and SP-B) in human fetal lung explants maintained in vitro. To test the hypothesis that the inhibitory effects of insulin on the surfactant proteins are the result of a direct action of insulin on the lung epithelial cell, we evaluated the effects of insulin in the H441 cell line, a human pulmonary adenocarcinoma cell line that expresses SP-A and SP-B mRNA. We observed that insulin treatment for 48 h decreased SP-A mRNA and protein levels in a concentration-dependent manner when compared to controls. The inhibitory effect of insulin on SP-A mRNA levels was apparent as early as after 4 h of exposure. SP-B mRNA levels were also significantly decreased by insulin in a concentration-dependent manner. Insulin, at 2.5 microg/ml, inhibited SP-A gene transcription by approx. 67%, and inhibited SP-B gene transcription by about 32%. There was no significant effect of insulin on SP-A or SP-B mRNA stability. Thus, we have observed a pattern of insulin inhibition of SP-A and SP-B gene expression in the H441 lung epithelial cell line similar to that previously observed in human fetal lung explants, which are comprised of both epithelial and mesenchymal cells. Our findings provide further evidence that insulin may delay fetal lung maturation by inhibiting SP-A and SP-B gene expression. Furthermore, our findings suggest that the inhibitory effects of insulin are, at least partially, the result of a direct action on the lung epithelial cell.

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.

Identification of a novel alternatively spliced mRNA of murine pulmonary surfactant protein B

An alternatively spliced mRNA of pulmonary surfactant protein B (SP-B) was identified in murine lung. Sequencing analysis revealed a 69 base-pair (bp) deletion at the beginning of exon 7 of SP-B, presumably the result of an alternative splicing event. Reverse transcription-polymerase chain reaction (RT-PCR) of mouse, rat, and rabbit lung RNA revealed the existence of full-length and the 69-bp deleted short form. Ribonuclease protection assay of the SP-B messenger RNA (mRNA) demonstrated expression of both isoforms in five strains of adult and fetal mice with different genetic backgrounds, as well as in rabbit, but not in human. Splice junction sequences in exon 6 and at the exon 7 splice boundary for the two isoforms are similar, including AG doublet identity, but sequence differences do not account for species variation in isoform abundance. The abundance of the short SP-B mRNA isoform was approximately 30% of total SP-B mRNA in mouse and rabbit. Analysis of precursor SP-B protein in mouse lung suggested that the two mRNA species are expressed as stable protein isoforms.

Regulation of CCSP (PCB-BP/uteroglobin) expression in primary cultures of lung cells: involvement of C/EBP

The Clara-cell secretory protein (CCSP) is a cell-specific differentiation marker for the bronchiolar Clara cell. Isolated rat Clara and alveolar type 2 cells kept in primary culture proliferate and dedifferentiate, providing the opportunity to study differentiation-dependent mechanisms. In freshly isolated Clara cells, high levels of CCSP and the corresponding mRNA were detected. During culture in vitro, these levels decreased. In the type 2 cell fraction, low levels of CCSP were detected, which decreased further during culture. A promoter fragment of the rat CCSP gene encompassing the sequence from -188 to +53 was able to drive high-level expression of reporter genes in transfected Clara cells. Reporter gene expression in transfected type 2 cells was markedly lower, and no expression could be detected in alveolar macrophages. Expression of transcription factors previously described to stimulate CCSP expression appeared not to parallel CCSP levels in the primary Clara cells. However, expression of the transcription factor C/EBP alpha correlated with the CCSP expression pattern. In electrophoretic mobility shift assays, we were able to demonstrate binding of C/EBP alpha from rat Clara cell nuclear extracts to an element located 85 bp upstream of the start site of transcription. Overexpression of C/EBP alpha increased expression from the CCSP -188 promoter fragment up to fivefold in NCI-H441-cells and 30-fold in A549-cells, establishing the functional importance of C/EBP alpha. Our results show that primary cultures of Clara cells constitute a useful model for investigating terminal airway differentiation and suggest a role for C/EBP-factor(s) in this process.

Secondary structure and limited proteolysis give experimental evidence that the precursor of pulmonary surfactant protein B contains three saposin-like domains

The 42 kDa precursor of surfactant protein B generates the 79 residue mature SP-B polypeptide, which belongs to the family of saposin-like proteins and has unique functional roles in pulmonary surfactant. From sequence comparisons it has been suggested that proSP-B, in addition to SP-B, contains two saposin-like domains, but their existence has until now not been experimentally verified. The 381 residue human proSP-B was now fused to an N-terminal poly-His tag, expressed in Escherichia coli, and purified from inclusion bodies by resolubilisation with 2.5% (w/v) SDS and, after removal of SDS, subsequent metal affinity chromatography. Recombinant proSP-B thus obtained exhibits about 35% alpha-helical structure in sodium phosphate buffer and is proteolytically cleaved preferentially between the three saposin-like domains. These results experimentally support that proSP contains, in addition to SP-B, two further saposin-like domains.

Targeting type II and Clara cells for adenovirus-mediated gene transfer using the surfactant protein B promoter

We examined the ability of the human surfactant protein B (SP-B) promoter to confer cell specificity of transgene expression in an adenoviral vector. Using similar replication-deficient adenoviruses (rAd), we compared lacZ reporter gene expression driven by the human SP-B promoter (rAd.SPBlacZ) with the ubiquitously expressed Rous sarcoma virus promoter (rAd.RSVlacZ). rAd.SPBlacZ expressed lacZ in H-441 and A549 lung epithelial cell lines and not in HeLa cells whereas rAd.RSVlacZ expressed in all three cell lines. In primary human fetal lung fibroblasts, beta-galactosidase activity from rAd.RSVlacZ transduction increased in a dose-dependent manner whereas activity from rAd.SPBlacZ remained low. In mixed cell cultures prepared from human fetal lung explants that contained fibroblasts and type II cells, X-Gal staining localized rAd.SPBlacZ expression to only type II cells whereas rAd.RSVlacZ expressed in both cell types. In 24-wk gestation human fetal tissue explants infected ex vivo, the RSV promoter directed lacZ expression in lung, trachea, heart, liver, and esophagus, whereas with the SP-B promoter lacZ was expressed only in lung, specifically in air space-lining cells. This specificity was maintained in vivo. lacZ expression was undetectable in lung and other tissues after intravenous administration of rAd.SPBlacZ whereas rAd.RSV-lacZ expressed primarily in liver. After intratracheal instillation of rAd.SPBlacZ into mice, X-Gal staining localized expression to type II and Clara cells. In contrast, rAd.RSVlacZ expressed in all pulmonary epithelial cell types. Our results indicate that the SP-B promoter may be useful in targeting type II and Clara cells for gene therapy of conditions such as inherited deficiency of SP-B.

Hepatocyte nuclear factor-3beta limits cellular diversity in the developing respiratory epithelium and alters lung morphogenesis in vivo

Hepatocyte nuclear factor-3beta (HNF-3beta), a nuclear protein of the winged helix family of transcription factors, is known to play a critical role in the formation of the embryonic node, notochord, and foregut endoderm. HNF-3beta influences the expression of a number of target genes in the respiratory epithelium, activating transcription of thyroid transcription factor-1, surfactant protein-B and clara cell secretory protein. In order to discern the role of HNF-3beta in differentiation and gene expression in the lung, HNF-3beta was expressed in developing respiratory epithelial cells of transgenic mice, under the control of the human surfactant protein C gene promoter. Pulmonary abnormalities were observed in the lungs of fetal mice bearing the HNF-3beta transgene. Differentiation of distal respiratory epithelial cells was arrested in the early pseudoglandular stage. Branching morphogenesis and vasculogenesis were markedly disrupted in association with decreased E-cadherin and vascular endothelial growth factor expression. HNF-3beta limits cellular diversity of developing respiratory epithelium and alters lung morphogenesis in vivo, suggesting that precise temporal-spatial regulation of HNF-3beta expression is critical for respiratory epithelial cell differentiation and lung morphogenesis.

Surfactant proteins-A and -B are elevated in plasma of patients with acute respiratory failure

Surfactant protein-A (SP-A) leaks into the circulation of patients with acute respiratory distress syndrome (ARDS) or acute cardiogenic pulmonary edema (APE) in a manner inversely related to lung function. Since surfactant protein-B (SP-B) is synthesized as a precursor considerably smaller than alveolar SP-A, we investigated whether it enters the circulation more readily. Reactivities consistent with SP-B proprotein (approximately 42 to approximately 45 kD) and the approximately 25 kD processing intermediate were detected in plasma. Plasma immunoreactive SP-B levels were significantly higher in ARDS (8,007+/-1,654 ng/ml [mean+/-SEM], n = 22) and APE (3,646+/-635 ng/ml, n = 10) patients compared with normal subjects (1,685+/-58 ng/ml, n = 33) and ventilated patients with no cardiorespiratory disease (1,829+/-184 ng/ml, n = 7). All groups had plasma SP-B/SP-A ratios approximately 6- to approximately 8-fold higher than in normal lavage or ARDS tracheal aspirate fluid, consistent with protein sieving. During admission, both plasma SP-B and the SP-B/SP-A ratio were inversely related to blood oxygenation (PaO2/FIO2) (p < 0.0001 and p < 0.025, n = 260 from 39 patients; Spearman) and static respiratory system compliance (deltaV/deltaP) (p < 0.0001 and p < 0.01, n = 168 from 25 patients). We describe in detail three patients and conclude that immunoreactive SP-B enters more readily than SP-A, is cleared acutely, and provides a better indicator of lung trauma.

Rescue of SP-B knockout mice with a truncated SP-B proprotein. Function of the C-terminal propeptide

The function of the 102-amino acid C-terminal propeptide of surfactant protein B (SP-B) was analyzed by characterizing the phenotype associated with loss of expression of this peptide domain in transgenic mice. A construct encoding the signal peptide, N-terminal propeptide, and mature peptide of human SP-B (hSP-BDeltac) was cloned under the control of the 3.7-kilobase human SP-C promoter and injected into fertilized eggs of the FVB/N mouse strain. Founder mice expressing the hSP-BDeltac transgene were bred with heterozygous SP-B knockout mice (SP-B +/-). Offspring containing the transgene and one allele of mouse SP-B were identified and subsequently crossed to generate a transgenic line that expressed SP-BDeltac in a null background (SP-B(-/-)/hSP-BDeltac(+/+)). Expression of hSP-BDeltac in SP-B(-/-) mice was restricted to type II cells and resulted in a 2-fold increase in mature SP-B relative to wild type littermates. These mice survived without any evidence of respiratory problems and had normal lung function, normal alveolar surfactant phospholipid pool sizes, and typical tubular myelin indicating that the 102-residue C-terminal propeptide of SP-B is not required for normal structure and function of extracellular surfactant. However, proteolytic processing of the SP-C proprotein was perturbed resulting in the accumulation of a processing intermediate, Mr = 11,000, similar to the phenotype detected in SP-B(-/-) mice; furthermore, lamellar bodies in type II cells of SP-B(-/-)/hSP-BDeltac(+/+) mice were much larger than in the wild type animal and saturated phosphatidylcholine content in lung tissue was significantly increased although the incorporation of choline into saturated phosphatidylcholine was normal. Collectively, these results demonstrate a role for the C-terminal propeptide of SP-B in SP-C proprotein processing and the maintenance of lamellar body size. The C-terminal propeptide may be an important determinant of intracellular surfactant pool size.

Molecular structures and interactions of pulmonary surfactant components

The dominating functional property of pulmonary surfactant is to reduce the surface tension at the alveolar air/liquid interface, and thereby prevent the lungs from collapsing at the end of expiration. In addition, the system exhibits host-defense properties. Insufficient amounts of pulmonary surfactant in premature infants causes respiratory distress syndrome, a serious threat which nowadays can be effectively treated by airway instillation of surfactant preparations. Surfactant is a mixture of many molecular species, mainly phospholipids and specific proteins, surfactant protein A (SP-A), SP-B, SP-C and SP-D. SP-A and SP-D are water-soluble and belong to the collectins, a family of large multimeric proteins which structurally exhibit collagenous/lectin hybrid properties and functionally are Ca2+-dependent carbohydrate binding proteins involved in innate host-defence functions. SP-A and SP-D also bind lipids and SP-A is involved in organization of alveolar surfactant phospholipids. SP-B belongs to another family of proteins, which includes also lipid-interacting polypeptides with antibacterial and lytic properties. SP-B is a 17.4-kDa homodimer and each subunit contains three intrachain disulphides and has been proposed to contain four amphipathic helices oriented pairwise in an antiparallel fashion. SP-A, SP-B and SP-D all have been detected also in the gastrointestinal tract. SP-C, in contrast, appears to be a unique protein with extreme structural and stability properties and to exist exclusively in the lungs. SP-C is a lipopeptide containing covalently linked palmitoyl chains and is folded into a 3.7-nm alpha-helix with a central 2.3-nm all-aliphatic part, making it perfectly suited to interact in a transmembranous way with a fluid bilayer composed of dipalmitoylglycerophosphocholine, the main component of surfactant. Homozygous genetic deficiency of proSP-B causes lethal respiratory distress soon after birth and is associated with aberrant processing of the precursor of SP-C. This review focuses on the chemical composition, structures and interactions of the pulmonary surfactant, in particular the associated proteins.

Inhibition of tyrosine kinase activity decreases expression of surfactant protein A in a human lung adenocarcinoma cell line independent of epidermal growth factor receptor

Epidermal growth factor (EGF) enhances fetal lung development in vivo and in vitro. Ligand binding to the EGF receptor stimulates an intrinsic receptor tyrosine kinase initiating a signal transduction cascade. We hypothesized that blocking EGF receptor function with tyrosine kinase inhibitors would decrease the expression of surfactant protein A in human pulmonary epithelial cells. Human pulmonary adenocarcinoma cells (NCI-H441) were exposed to genistein (a broad range inhibitor of tyrosine kinases) and tyrphostin AG1478 (a specific inhibitor of EGF receptor tyrosine kinase). Genistein significantly decreased surfactant protein A (SP-A) and SP-A mRNA levels in H441 cells without affecting cell viability. The inhibitory effect of genistein on SP-A content was reversible. In contrast, tyrphostin AG1478 had no effect on SP-A levels despite a greater inhibitory effect than genistein on EGF receptor tyrosine autophosphorylation. Furthermore, treatment of H441 cells with exogenous EGF did not increase SP-A content or mRNA levels beyond baseline. We conclude that inhibition of tyrosine kinase activity other than the EGF receptor decreases the expression of surfactant protein A at a pretranslational level in human pulmonary adenocarcinoma cells. These results suggest the importance of tyrosine kinases in modulating human SP-A synthesis.

Secretion of unprocessed human surfactant protein B in milk of transgenic mice

Because of the apparent clinical importance of human pulmonary surfactant B (SP-B), the expression of SP-B was directed to the mammary gland of transgenic mice using previously characterized rat whey acidic protein (WAP) regulatory sequences. rWAP/SP-B mRNA was expressed specifically in the mammary gland, and ranged from 1 to 5% of the endogenous WAP mRNA levels. SP-B was detected immunologically in both tissue and milk. The transgene product had an apparent molecular weight of 40-45 kDa, corresponding to the predicted size of the SP-B proprotein. Incubation of an SP-B-enriched fraction of milk with cathepsin D in vitro produced 20-25 kDa species, consistent with cleavage of the amino terminal domain by cathepsin D. This was confirmed using antibodies specific to the carboxy-terminal domain of SP-B. However, the appearance of only the SP-B proprotein in milk suggests that cathepsin D is not involved in the in vivo processing of SP-B. The SP-B proprotein in milk suggests that cathepsin D is not involved in the in vivo processing of SP-B. The SP-B proprotein can be expressed in milk of transgenic mice without any observed effects on mammary gland morphology or lactation.

Structural requirements for intracellular transport of pulmonary surfactant protein B (SP-B)

Human SP-B is synthesized by the alveolar Type II epithelial cell as a 381 amino acid preproprotein. The 79 residue mature SP-B peptide is extremely hydrophobic and flanked by propeptides of 200 and 102 amino acids at its NH2- and COOH-termini, respectively. The purpose of this study was to identify peptide domains of the SP-B proprotein necessary for trafficking of the mature peptide in the secretory pathway. To this end several constructs were generated, by subcloning the full length human SP-B (SP-B), COOH-terminally truncated SP-B (SP-B delta C, in which residues 201-381 were deleted), NH2-terminally deleted SP-B (SP-B delta N, in which residues 28-200 were deleted), NH2-terminal propeptide (SP-BN), mature SP-B (SP-BM) and COOH-terminal propeptide (SP-BC), into the mammalian expression vector pcDNA3. The resulting expression constructs were characterized by DNA sequencing and in vitro transcription/translation and subsequently transfected into Chinese hamster ovary cells. 48 h after transfection, cells were labeled with [35S]-met/cys and analyzed by immunoprecipitation, SDS-PAGE and autoradiography. Proteins encoded by SP-B, SP-B delta C, SP-BN and SP-BC constructs were secreted into media; in contrast, SP-B constructs lacking the NH2-terminal propeptide (SP-B delta N) remained in the endoplasmic reticulum (as assessed by endoglycosidase H sensitivity) and were rapidly degraded. We conclude that (1) 27 amino acids at the NH2-terminus of SP-B contain a functional signal peptide and (2) the NH2-terminal propeptide of the SP-B precursor is necessary and sufficient for intracellular trafficking of the mature peptide.

NCI series of cell lines: an historical perspective

The NCI series of cell lines represent a unique collection of permanent human tumor cell lines established by one laboratory over a period of approximately 16 years. More than 300 cell lines were established, mainly from human lung cancers (both small cell and non-small cell types). In addition, smaller numbers of lines were established from rare and unusual tumors such as cutaneous T cell lymphomas, myelomas and adrenal cortical carcinoma. The T cell lines played a pivotal role in the isolation of human retroviruses including HTLV-1 and HIV. The establishment of such a large panel of lines was aided by the development of defined media for culturing specific cell types. The lines are well characterized, and full clinical data are available for most of them. Many of the lines have been deposited with the American Type Culture Collection, Rockville, MD, where they are readily available for a modest handling fee. The lines have been widely distributed to investigators, and have had a major impact on biomedical research.

Marker proteins in the particulate fraction of third-trimester amniotic fluid

The present clinical evaluation of fetal lung maturity relies largely on the determination of the amniotic surfactant phospholipids phosphotidylglycerol, lecithin, and sphingomyelin, but there are many false negatives as well as false positives among diabetics. The use of other components of lung surfactant, namely, the hydrophobic surfactant proteins (SPs) has long been suggested as an alternative to the classical assay, but tests based on the detection of immunoreactive SP-A have not proved superior or supplanted phospholipid ratios as an index. This report investigates the proteins in a fraction of third-trimester human amniotic fluid (the particulate fraction) enriched in the SP complexes that form the surfactant monolayer. The proteins were analyzed by two-dimensional polyacrylamide gel electrophoresis and visualized by silver staining and immunoblotting. Eight proteins are of particular interest. Three novel proteins (termed AFPP-1, AFPP-4, and AFPP-8) and the alpha-fetoprotein/human serum albumin complex (AFPP-7) can be detected throughout the 28- to 38-week gestational window. The protein that is referred to as AFPP-2 could be identified as SP-A on the basis of immunologic cross-reactivity as well as size and charge characteristics. The time course of appearance of AFPP-2 was also followed in patients with Rh isoimmunization syndrome and was found to be the same as that seen for SP-A. The SP-A was detected as at least five major charged isoforms with multiple subisoforms of different molecular weight and can be distinguished from a related set of proteins (AFPP-5) that appear with a different time course but are possible precursors. Two other proteins (AFPP-3, AFPP-6), which are detectable inconsistently bear some similarity to others reported previously but not extensively characterized. These results define both constant and variable proteins of the particulate fraction of the amniotic fluid and indicate that certain protein isoforms are changing throughout the third trimester. These data enhance the possibility of the utilization of these proteins as markers of lung maturity in conditions such as maternal diabetes.

Production of immortalized distal respiratory epithelial cell lines from surfactant protein C/simian virus 40 large tumor antigen transgenic mice

Murine lung epithelial (MLE) cell lines representing the distal bronchiolar and alveolar epithelium were produced from lung tumors generated in transgenic mice harboring the viral oncogene simian virus 40 (SV40) large tumor antigen under transcriptional control of a promoter region from the human surfactant protein C (SP-C) gene. The cell lines exhibited rapid growth, lack of contact inhibition, and an epithelial cell morphology for 30-40 passages in culture. Microvilli, cytoplasmic multivesicular bodies, and multilamellar inclusion bodies (morphologic characteristics of alveolar type II cells) were detected in some of the MLE cell lines by electron microscopic analysis. The MLE cells also maintained functional characteristics of distal respiratory epithelial cells including the expression of surfactant proteins and mRNAs and the ability to secrete phospholipids. Expression of the exogenous SV40 large tumor antigen gene was detected in all of the generated cell lines. The SP-C/SV40 large tumor antigen transgenic mice and the MLE cell lines will be useful for the study of pulmonary surfactant production and regulation as well as lung development and tumorigenesis.

Biosynthetic routing of pulmonary surfactant proteins in alveolar type II cells

Surfactant proteins A, B, and C (SP-A, SP-B, and SP-C) are synthesized in alveolar type II cells. SP-B and SP-C are both synthesized as large precursor molecules that are proteolytically processed to their mature sizes. In a previous immunoelectron microscopic study, we showed that precursor SP-B is processed to its mature size in multivesicular bodies. In the present study, using a specific antibody against precursor SP-C, we demonstrate that precursor SP-C is present in the same intracellular compartments of the biosynthetic pathway, i.e., endoplasmic reticulum, Golgi complex, and multivesicular bodies, as precursor SP-B. Since mature SP-C is known to be present in multilamellar bodies, this suggests a biosynthetic routing and site of processing of this protein similar to those of SP-B. Double-labeling experiments using antibodies against SP-A, precursor SP-B, precursor SP-C, and an antibody against HA I, an adaptor protein involved in the budding of transport vesicles from the Golgi complex, showed that the different surfactant proteins traverse and exit the Golgi complex via the same route. The surfactant proteins do not exit the Golgi complex via HA I-positive coated buds or vesicles. These data are in accordance with the concept that SP-A, SP-B, and SP-C are transported together through the same biosynthetic pathway via multivesicular bodies to multilamellar bodies.

Transforming growth factor-beta inhibits surfactant protein A expression in vitro

Effects of members of the transforming growth factor-beta (TGF-beta) family on expression of surfactant protein A (SP-A) were determined in human pulmonary adenocarcinoma cells. TGF-beta decreased SP-A content in two distinct pulmonary adenocarcinoma cell lines with bronchiolar (NCI-H441-4) and alveolar (NCI-H820) cell characteristics. TGF-beta 1, beta 2 and beta 3 were equally effective in decreasing SP-A. Effects of the TGF-beta's on SP-A content were dose dependent, EC50 approximately 20-30 pg/ml for each form of TGF-beta. TGF-beta decreased cellular SP-A content in association with decreased levels of SP-A mRNA. Inhibitory effects of TGF-beta 1 on SP-A mRNA was time dependent, reaching maximal effects within 12-24 h, after which SP-A mRNA was approximately 10% of that present in untreated cells. Maximal inhibition of SP-A mRNA was observed at 250 pg/ml TGF-beta 1. TGF-beta-dependent inhibition of SP-A expression was not associated with altered cell morphology, growth, or viability. TGF-beta family members act directly on pulmonary adenocarcinoma cells to inhibit SP-A expression by mechanisms which are mediated, at least in part, at a pretranslational level.