3-Methylcholanthrene triggers the differentiation of alveolar tumor cells from canine bronchial basal cells and an altered p53 gene promotes their clonal expansion

Alveolar type II cells arising in canine bronchial autografts following exposure to 3-methylcholanthrene (MCA) give rise to carcinomas of varying glandular and squamous growth patterns. To study the role of the tumor suppressor gene p53 in this process, sections from progressive lesions were immunostained for p53 protein; microdissected regions were screened for p53 mutations. Adjacent sections were examined for type II cell expression [cuboid cell shape, large roundish nucleus, cytoplasmic staining for surfactant protein A (SP-A)] and proliferating cell nuclear antigen expression. Evidence for an altered p53 function (nuclear staining, missense mutations) was found in most carcinomas of all histologic types and in all grades of bronchial dysplasia, but not in hyperplastic or normal bronchial epithelium. It was primarily associated with the hyperplastic type II cell populations present in the basal zone of the lesions. In addition, we found SP-A staining in hyperplastic (but not in normal) bronchial basal cells. These data suggest that MCA initiates type II cell differentiation through phenotypic selection (basal cells). Inactivation of the p53 gene promotes the clonal expansion of the type II cells into discernible populations of (squamous or glandular) alveolar tumor cells. This in vivo study is the first to show that p53 is involved in a specific pathway leading to bronchogenic carcinoma.

Surfactant protein A enhances mycobacterial killing by rat macrophages through a nitric oxide-dependent pathway

Surfactant-associated protein A (SP-A) is involved in surfactant homeostasis and host defense in the lung. We have previously demonstrated that SP-A specifically binds to and enhances the ingestion of bacillus Calmette-Guerin (BCG) organisms by macrophages. In the current study, we investigated the effect of SP-A on the generation of inflammatory mediators induced by BCG and the subsequent fate of ingested BCG organisms. Rat macrophages were incubated with BCG in the presence and absence of SP-A. Noningested BCG organisms were removed, and the release of tumor necrosis factor-alpha (TNF-alpha) and nitric oxide were measured at varying times. TNF-alpha and nitric oxide production induced by BCG were enhanced by SP-A. In addition, SP-A enhanced the BCG-induced increase in the level of inducible nitric oxide synthase protein. Addition of antibodies directed against SPR210, a specific macrophage SP-A receptor, inhibited the SP-A-enhanced mediator production. BCG in the absence of SP-A showed increased growth over a 5-day period, whereas inclusion of SP-A dramatically inhibited BCG growth. Inhibition of nitric oxide production blocked BCG killing in the presence and absence of SP-A. These results demonstrate that ingestion of SP-A-BCG complexes by rat macrophages leads to production of inflammatory mediators and increased mycobacterial killing.

Tolerance of SP-A-deficient mice to hyperoxia or exercise

Mice carrying a null mutation of the surfactant-associated protein A (SP-A) gene have normal respiratory function, but their surfactant lacks tubular myelin, is sensitive to protein inactivation in vitro, and contains decreased pool sizes of the biophysically active large-aggregate surfactant. We hypothesized that SP-A-deficient mice would be more susceptible to exercise-induced stress and O(2)-induced lung injury. SP-A-(-/-) and SP-A-(+/+) mice tolerated 1 h of swimming or 45 min of running on a treadmill at 15 m/min equivalently, without alterations of the amount of alveolar saturated phosphatidylcholine. After 3 days of hyperoxia, SP-A-(-/-) mice had increased alveolar protein, but pressure-volume curves were not different between groups. Alveolar protein concentration was similarly increased in SP-A-(-/-) and SP-A-(+/+) mice after 4 days of exposure to hyperoxia. Survival rates were similar after 4 days of hyperoxia. SP-A-(-/-) mice were equally tolerant to exercise and 4 days of hyperoxia, indicating that the SP-A-dependent alterations in surfactant did not result in functional deficits.

Determinants of surfactant function in acute lung injury and early recovery

Relationships between lung function and surfactant function and composition were examined during the evolution of acute lung injury in guinea pigs. Lung mechanics and gas exchange were assessed 12, 24, or 48 h after exposure to nebulized lipopolysaccharide (LPS). Bronchoalveolar lavage (BAL) fluid was processed for phospholipid and protein contents and surfactant protein (SP) A and SP-B levels; surfactant function was measured by pulsating bubble surfactometry. Lung elastance, tissue resistance, and arterial-alveolar gradient were moderately elevated by 12 h after LPS exposure and continued to increase over the first 24 h but began to recover between 24 and 48 h. Similarly, the absolute amount of 30,000 g pelleted SP-A and SP-B, the phospholipid content of BAL fluid, and surfactant function declined over the first 24 h after exposure, with recovery between 24 and 48 h. BAL fluid total protein content increased steadily over the first 48 h after LPS nebulization. In this model of acute lung injury, the intra-alveolar repletion of surfactant components in early recovery led to improved surfactant function despite the presence of potentially inhibitory plasma proteins.

Surfactant proteins A and D bind CD14 by different mechanisms

Surfactant proteins A (SP-A) and D (SP-D) are lung collectins that are constituents of the innate immune system of the lung. Recent evidence (Sano, H., Sohma, H., Muta, T., Nomura, S., Voelker, D. R., and Kuroki, Y. (1999) J. Immunol. 163, 387-395) demonstrates that SP-A modulates lipopolysaccharide (LPS)-induced cellular responses by direct interaction with CD14. In this report we examined the structural elements of the lung collectins involved in CD14 recognition and the consequences for CD14/LPS interaction. Rat SP-A and SP-D bound CD14 in a concentration-dependent manner. Mannose and EDTA inhibited SP-D binding to CD14 but did not decrease SP-A binding. The SP-A binding to CD14 was completely blocked by a monoclonal antibody that binds to the SP-A neck domain but only partially blocked by an antibody that binds to the SP-A lectin domain. SP-A but not SP-D bound to deglycosylated CD14. SP-D decreased CD14 binding to both smooth and rough LPS, whereas SP-A enhanced CD14 binding to rough LPS and inhibited binding to smooth LPS. SP-A also altered the migration profile of LPS on a sucrose density gradient in the presence of CD14. From these results, we conclude that 1) lung collectins bind CD14, 2) the SP-A neck domain and SP-D lectin domain participate in CD14 binding, 3) SP-A recognizes a peptide component and SP-D recognizes a carbohydrate moiety of CD14, and 4) lung collectins alter LPS/CD14 interactions.

Cell-specific expression of CCAAT/enhancer-binding protein delta (C/EBP delta) in epithelial lung cells

CCAAT/enhancer-binding proteins (C/EBP) constitute a family of transcription factors that are involved in regulation of proliferation and differentiation in several cell types. In epithelial lung cells the C/EBP alpha isoform seems to play a role in the regulation of surfactant proteins (SP) and Clara cell specific protein (CCSP), whereas the roles of C/EBP beta and C/EBP delta are unclear. We have examined the protein levels of C/EBP delta in bronchiolar Clara cells and alveolar type 2 cells, and its relation to the expression of lung specific proteins and cell proliferation. The protein expression of C/EBP delta was high in freshly isolated Clara cells compared to type 2 cells. In both cell types C/EBP delta levels increased during culture. Alterations of the levels of C/EBP delta did not correspond with the proliferation levels of Clara cells, but seemed to correspond in type 2 cells. Clara cell secretory protein (CCSP) was highly expressed in freshly isolated Clara cells, in contrast to type 2 cells. SP-D and CYP2B1 were expressed at somewhat higher levels in Clara cells than in type 2 cells, whereas SP-A exhibited highest expression in type 2 cells. During culture the levels of all these lung proteins were strongly reduced. However, compared to with serum we found an increase in CCSP in Clara cell cultures without serum, and this correlated with an increase in C/EBP delta. Overall our in vitro data suggest that C/EBP delta alone is not related to the maintenance of proteins involved in differentiation.

A clinicopathologic study of 100 cases of pulmonary sclerosing hemangioma with immunohistochemical studies: TTF-1 is expressed in both round and surface cells, suggesting an origin from primitive respiratory epithelium

Pulmonary sclerosing hemangioma (SH) is a lung neoplasm of uncertain histogenesis that is composed of two major cell types: surface and round cells. The authors studied 100 cases of pulmonary SH that presented as a peripheral (95%), solitary (96%) mass of less than 3 cm in diameter (74%) in asymptomatic patients who were mostly women (83%) with a mean age of 46.2 years. Immunohistochemistry of multiple epithelial, mesothelial, pneumocyte, neuroendocrine, and mesenchymal markers was performed on 47 cases to investigate the histogenesis of this neoplasm. Both surface and round cells stained with epithelial membrane antigen (EMA) and thyroid transcription factor-1 (TTF-1) in more than 90% of cases; however, the round cells were uniformly negative for pancytokeratin and positive for cytokeratin-7 and CAM 5.2 in only 31% and 17% of cases, respectively. Surfactant proteins A and B as well as Clara cell antigen were positive in varying numbers of surface cells but they were negative in the round cells. Neuroendocrine cells either as isolated scattered cells or as a tumorlet within the center of SH were detected (chromogranin, Leu-7, synaptophysin positive) in three cases. The expression of TTF-1 in the absence of surfactant proteins A and B and Clara cell antigens in the round cells of SH suggests that they are derived from primitive respiratory epithelium. The alveolar pneumocytes and neuroendocrine cells may either represent phenotypic differentiation of a primitive respiratory epithelial component or they may correspond to non-neoplastic entrapped or hyperplastic elements. The concomitant positivity of both cell types in SH for TTF-1 and EMA, and the negativity of round cells for pancytokeratin and neuroendocrine markers, provide useful clues not only for histogenesis but also for the diagnosis of this lung neoplasm.

Polymorphisms of surfactant protein A genes and the risk of bronchopulmonary dysplasia in preterm infants

The pathophysiology of bronchopulmonary dysplasia (BPD) as an inflammatory disorder secondary to neonatal respiratory distress syndrome (RDS) is not yet fully understood and still represents a major complication of prematurity. The main pathophysiologic feature of RDS is a primary surfactant deficiency in a structurally immature lung. Pulmonary surfactant contains 90 percent phospholipids and 10 percent proteins (surfactant proteins A, B, C, and D). As surfactant protein A (SP-A) has several major immunological and metabolic intrapulmonary functions, we aimed at investigating an association of polymorphisms of SP-A1 and SP-A2 encoding genes and the risk of BPD. We performed a case-control study exclusively including Caucasian preterm infants below 32 weeks of gestation matched for the degree of immaturity and the year of birth. Venous cord blood was taken prospectively and analyzed by polymerase chain reaction (PCR), single-strand conformation polymorphism (SSCP), cloning and sequencing. BPD was defined as oxygen dependency or need for mechanical ventilation at day 28. Twenty-three infants with BPD were enrolled (mean gestational age 26.2 weeks; mean birth weight 760.4 g) and compared with 23 infants matched on the basis of gestational age (mean gestational age 27.9 weeks; mean birthweight 1015 g). We observed a significantly increased frequency of the SP-A1 polymorphism 6A6 in infants with BPD compared with controls. In addition to previously established risk factors for BPD, 6A6 polymorphism for SP-A1 gene is an independent co-factor. We believe treatment of neonatal RDS should also include stratification according to genetic risk factors.

Serum levels of surfactant proteins A and D are useful biomarkers for interstitial lung disease in patients with progressive systemic sclerosis

To find a less-invasive and lung-specific clinical biomarker, we measured serum levels of surfactant proteins A and D (SP-A and SP-D) by sandwich enzyme-linked immunosorbent assays in 42 patients with progressive systemic sclerosis (PSS) to evaluate their significance in relation to the presence of interstitial lung disease (ILD) and to assess their diagnostic merits. The patients were divided into two groups based on findings by chest computed tomography (CT): 30 patients with ILD (CT-positive ILD group), and 12 patients without any lung abnormalities (CT-negative ILD group). The CT-positive ILD group was further divided into two groups: 24 patients with ILD detectable by chest plain radiography (X-ray-positive ILD group) and six patients with ILD showing no abnormality (X-ray-negative ILD group). The levels of SP-A and SP-D in sera were significantly higher in the CT-positive ILD group than in the CT-negative ILD group. They were also significantly higher in the X-ray-positive ILD group than in the CT-negative ILD group. In the X-ray-negative ILD group, their levels were higher than those of the CT-negative ILD group. We next estimated sensitivity and specificity of SP-A, SP-D, and X-ray for detecting ILD on CT. Sensitivity of SP-D was high (77%) as well as that of X-ray (80%), whereas SP-A showed a low sensitivity (33%). Remarkably, five of six patients in the X-ray-negative ILD group showed SP-D concentrations over its cut-off level, thereby demonstrating that an SP-D assay contributes to the detection of ILD overlooked by X-ray. Moreover, a combination of X-ray and SP-D dramatically increases sensitivity to 97%. Specificity of SP-A, SP-D, and X-ray to the CT-negative ILD group was 100%, 83%, and 100%, respectively. In conclusion, this study indicates that elevated levels of serum SP-A and SP-D reflect well the presence of ILD and that the combination of SP-D and X-ray contributes to reduce the risk of clinicians overlooking ILD complicated by PSS, although a repetition in another set of subjects is needed to confirm these indications.

Comparison of SP-A and LPS effects on the THP-1 monocytic cell line

Surfactant protein A (SP-A) increases production of proinflammatory cytokines by monocytic cells, including THP-1 cells, as does lipopolysaccharide (LPS). Herein we report differences in responses to these agents. First, polymyxin B inhibits the LPS response but not the SP-A response. Second, SP-A-induced increases in tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and IL-8 are reduced by >60% if SP-A is preincubated with Survanta (200 microgram/ml) for 15 min before addition to THP-1 cells. However, the LPS effects on TNF-alpha and IL-8 are inhibited by <20% and the effect on IL-1beta by <50%. Third, at Survanta levels of 1 mg/ml, SP-A-induced responses are reduced by >90%, and although the inhibitory effects on LPS action increase, they still do not reach those seen with SP-A. Finally, we tested whether SP-A could induce tolerance as LPS does. Pretreatment of THP-1 cells with LPS inhibits their response to subsequent LPS treatment 24 h later, including TNF-alpha, IL-1beta, and IL-8. Similar treatment with SP-A reduces TNF-alpha, but IL-1beta and IL-8 are further increased by the second treatment with SP-A rather than inhibited as with LPS. Thus, whereas both SP-A and LPS stimulate cytokine production, their mechanisms differ with respect to inhibition by surfactant lipids and in ability to induce tolerance.

Induction of AP-1 binding to intron 1 of SP-A1 and SP-A2 is implicated in the phorbol ester inhibition of human SP-A promoter activity

A deletional analysis of the SP-A1 promoter in NCI-H441 cells was performed to identify potential cis-acting elements involved in phorbol ester-mediated repression of human SP-A transcription. The phorbol ester TPA reduced SP-A1 and SP-A2 promoter activity to approximately 35% to 45% compared to that of control cells. The inhibitory effect of TPA was significantly reduced upon removal of the region +64/+394 relative to the SP-A1 transcription start site. Using NCI-H441 nuclear proteins, electromobility shift assay analysis showed that the intron region +309/+329 of SP-A1 and the corresponding region of SP-A2 formed sequence-specific DNA/protein complexes that were induced by TPA exposure. The region +318/+324 of SP-A1 contains sequences similar to a consensus AP-1 binding site, TGACTGA (TCACTGA for SP-A2), which when mutated (TGAGAGT) prevented the formation of the TPA-induced DNA/protein complex. The TPA-induced complex was supershifted in the presence of antibody against the Jun family of proteins, but not the Fos family of proteins. These results suggest that the binding of AP-1 or an AP-1--like factor to the first intron of SP-A1 and SP-A2 may be involved in the phorbol ester inhibition of human SP-A gene expression.

Different pathways of degradation of SP-A and saturated phosphatidylcholine by alveolar macrophages

Alveolar macrophages degrade surfactant protein (SP) A and saturated phosphatidycholine [dipalmitoylphosphatidylcholine (DPPC)]. To clarify this process, using rabbit alveolar macrophages, we analyzed the effect of drugs known to affect phagocytosis, pinocytosis, clathrin-mediated uptake, caveolae, the cytoskeleton, lysosomal pH, protein kinase C, and phosphatidylinositol 3-kinase (PI3K) on the degradation of SP-A and DPPC. We found the following: 1) SP-A binds to the plasma membrane, is rapidly internalized, and then moves toward degradative compartments. Uptake could be clathrin mediated, whereas phagocytosis, pinocytosis, or the use of caveolae are less likely. An intact cytoskeleton and an acidic milieu are necessary for the degradation of SP-A. 2) Stimulation of protein kinase C increases the degradation of SP-A. 3) PI3K influences the degradation of SP-A by regulating both the speed of internalization and subsequent intracellular steps, but its inhibition does not prevent SP-A from reaching the lysosomal compartment. 4) The degradation of DPPC is unaffected by most of the treatments able to influence the degradation of SP-A. Thus it appears that DPPC is degraded by alveolar macrophages through mechanisms very different from those utilized for the degradation of SP-A.

Human bronchoalveolar lavage fluid protein two-dimensional database: study of interstitial lung diseases

Recently, we published an analytical two-dimensional electrophoresis (2-DE) protein map of human bronchoalveolar lavage fluid (BALF) using a pool of BALFs from various patients. In this report, the effect of lung disorders on the protein composition of the lung epithelial lining fluid was investigated by 2-DE of BALFs from individual patients with well-defined interstitial lung diseases: sarcoidosis, idiopathic pulmonary fibrosis (IPF) and hypersensitivity pneumonitis (HP), using improved experimental conditions. On these gels, about 600-1000 stained protein spots could be identified in a BALF sample containing 25 microg of protein, and our original human BALF protein database has, therefore, been considerably extended. Altogether, 429 protein spots corresponding to 66 different proteins (including isoforms, protein subunits and fragments) were identified by microsequence analysis and by matching with a human blood plasma 2-DE protein map available in the SWISS-2DPAGE database. A human 2-DE BALF database was established and is available on the World Wide Web (http://www.umh.ac.be/-biochim/proteomic.htm+ ++). The significance of the modifications observed between the different lung pathologies is discussed with the aim of understanding the mechanistic bases of lung disease pathogenesis and finding new potential lung markers of disorders.

Incorporation of biotinylated SP-A into rat lung surfactant layer, type II cells, and clara cells

The goal of this study was to compare the functions of Clara and type II cells during alveolar clearance and recycling of surfactant protein (SP) A, a secretory product of both cell types. We examined the incorporation of instilled biotinylated SP-A (bSP-A) into rat lung type II and Clara cells as a measure of clearance and recycling of the protein. Ultrastructural localization of bSP-A was accomplished by an electron-microscopic immunogold technique at 7, 30, and 120 min after intratracheal instillation. Localization of bSP-A was quantitatively evaluated within extracellular surfactant components (lipid-rich forms: myelin figures, vesicles, and tubular myelin; and lipid-poor hypophase) and in compartments of type II and Clara cells. bSP-A was incorporated into myelinic and vesicular forms of extracellular surfactant, but tubular myelin and hypophase had little bSP-A. Lamellar bodies of type II cells demonstrated a significant time-dependent increase in their incorporation of bSP-A. There was a concentration of bSP-A in the secretory granules and mitochondria of Clara cells, but no Clara cell compartment showed a pattern of time-dependent change in immunolabeling. Our immunolabeling data demonstrated a time-dependent movement of exogenous SP-A from extracellular components into type II cells and their secretory granules. Clara cells did not demonstrate a time-dependent incorporation of bSP-A into their secretory granules during the period of this study. If Clara cells recycle SP-A, they must reach a steady state very quickly or very slowly.

Surfactant protein A binding to cytomegalovirus proteins enhances virus entry into rat lung cells

The role of surfactant protein (SP)-A in cytomegalovirus (CMV) infection of the lung was investigated. We found that SP-A binds to various immobilized human CMV proteins and those exposed on the surface of infected embryonal lung fibroblasts. The interaction between SP-A and immobilized CMV proteins was found to be calcium-dependent and inhibited by mannan, suggesting involvement of the carbohydrate recognition domain of SP-A and high-mannose carbohydrate residues of viral envelope glycoproteins. Using flow cytometry and confocal laser fluorescence microscopy in the rat model we showed that preincubation of rat CMV with SP-A stimulates its binding and internalization by rat type II pneumocytes and alveolar tissue macrophages. This effect was concentration- and Ca(2+)-dependent but was not inhibited by mannan. Therefore, the domains of SP-A involved in SP-A CMV interaction and in interaction of the SP-A/virus complex with rat lung cells are distinct. Additionally, in the human CMV model, sheep as well as human proteinosis SP-A did not significantly affect human CMV replication in embryonal lung fibroblasts. Thus, SP-A may contribute to CMV-associated pathology of the lung by increasing the efficiency of target cell infection.

Lung surfactant protein A provides a route of entry for respiratory syncytial virus into host cells

Lung surfactant protein A (SP-A) has a central role in host defense mediated by the interaction of surface carbohydrates of inhaled pathogens with the lectin domains of SP-A. Respiratory syncytial virus (RSV), the most important viral pathogen of neonates and infants, encodes a highly glycosylated attachment protein, G. Binding studies were performed with G-protein from RSV (human, A2 strain) and human SP-A. The effect of SP-A on the interaction between RSV and host cells was determined by two methods: an infectivity study with monolayers of Hep-2C cells and by interleukin-8 (IL-8) release from buffy coat (BC) cells. SP-A binds to RSV G-protein in a concentration-dependent manner that is inhibitable by both ethylenediamine tetraacetic acid (EDTA) and mannan, indicating that binding is through the carbohydrate recognition domain of the SP-A and a carbohydrate moiety of the G-protein. The level of RSV infection of Hep-2C cells increases with increasing concentrations of SP-A. The amount of IL-8 released by BC cells in the presence of RSV is increased with SP-A concentrations of 2.9 microg/mL or greater. Our results show that SP-A enhances the attachment of RSV and subsequent entry into host cells. The effect of SP-A on viral uptake by epithelial cells and macrophage may determine both innate and adaptive immune responses to RSV.

[The effects of NAC on the expression and activity of SPA in rats inflicted by smoke inhalation injury]

OBJECTIVE

To investigate the effects of NAC (N-acetyl-L-cysteine) on the expression and activity of SPA (surfactant-associated protein A) in rats inflicted by smoke inhalation injury.

METHODS

Wistar rats inflicted with smoke inhalation injury were employed as the model. The expression of SPA mRNA, the static pulmonary compliance, the surface tension of the alveolar lavage and the morphology of lamellae bodies (LB) of type II alveolar cells were examined.

RESULTS

After the application of NAC, there exhibited an increase in SPA mRNA expression and static pulmonary compliance, and the restoration of the BAL surface tension to normal.

CONCLUSION

The application of NAC could promote the expression of SPA mRNA after smoke inhalation injury and improve the SPA-related function.

Antisense inhibition of epidermal growth factor receptor decreases expression of human surfactant protein A

Epidermal growth factor (EGF) stimulates surfactant protein A (SP-A) synthesis in fetal lung tissue through ligand binding to the EGF receptor. We hypothesized that inhibition of EGF receptor messenger RNA (mRNA) would block SP-A expression in human fetal lung tissue during alveolar type II cell differentiation in vitro. Midtrimester human fetal lung explants were maintained in serum-free Waymouth's medium for 3 to 5 d in the presence or absence of an antisense 18-mer phosphorothioate oligonucleotide (ON) complementary to the initiation codon region of EGF receptor mRNA. Sense and scrambled ONs similarly modified were used as additional controls. The concentration of EGF receptor mRNA was semiquantitatively determined by reverse transcriptase/polymerase chain reaction (RT-PCR). We found a significant 3-fold decrease in EGF receptor mRNA levels in the antisense-treated groups compared with the control group with no effect in the sense condition. Immunohistochemical staining revealed a decrease in the amount of staining for EGF receptor protein in distal pulmonary epithelial cells in the antisense-treated groups compared with either control or sense conditions. Treatment with antisense EGF receptor ON decreased both SP-A mRNA and protein compared with controls with no effect in the sense condition. The ONs did not affect tissue viability as measured by the release of lactate dehydrogenase. We conclude that selective degradation of EGF receptor mRNA with antisense ON treatment results in a decrease in SP-A expression in human fetal lung. These findings support the critical importance of the EGF receptor for the regulation of SP-A gene expression during human alveolar type II cell differentiation.

Self-aggregation of surfactant protein A

Environmental factors of physiological relevance such as pH, calcium, ionic strength, and temperature can affect the state of self-aggregation of surfactant protein A (SP-A). We have studied the secondary structure of different SP-A aggregates and analyzed their fluorescence characteristics. (a) We found that self-aggregation of SP-A can be Ca(2+)-dependent. The concentration of Ca(2+) needed for half-maximal self-association (K(a)(Ca)()2+) depended on the presence of salts. Thus, at low ionic strength, K(a)(Ca)()2+ was 2.3 mM, whereas at physiological ionic strength, K(a)(Ca)()2+ was 2.35 microM. Circular dichroism and fluorescence measurements of Ca(2+)-dependent SP-A aggregates indicated that those protein aggregates formed in the absence of NaCl are structurally different from those formed in its presence. (b) We found that self-aggregation of SP-A can be pH-dependent. Self-aggregation of SP-A induced by H(+) was highly influenced by the presence of salts, which reduced the extent of self-association of the protein. The presence of both salts and Ca(2+) attenuated even more the effects of acidic media on SP-A self-aggregation. (c) We found that self-aggregation of SP-A can be temperature-dependent. At 20 degrees C, SP-A underwent self-aggregation at physiological but not at low ionic strength, in the presence of EDTA. All of these aggregates were dissociated by either adding EDTA (a), increasing the pH to neutral pH (b), or increasing the temperature to 37 degrees C (c). Dissociation of Ca(2+)-induced protein aggregates at low ionic strength was accompanied by an irreversible loss of both SP-A secondary structure and SP-A-dependent lipid aggregation properties. On the other hand, temperature-dependent experiments indicated that a structurally intact collagen-like domain was required for either Ca(2+)- or Ca(2+)/Na(+)-induced SP-A self-aggregation but not for H(+)-induced protein aggregation.

Three-dimensional structure of rat surfactant protein A trimers in association with phospholipid monolayers

Surfactant protein A (SP-A) is a C-type lectin found primarily in the lung and plays a role in innate immunity and the maintenance of surfactant integrity. To determine the three-dimensional (3D) structure of SP-A in association with a lipid ligand, we have used single particle electron crystallography and computational 3D reconstruction in combination with molecular modeling. Recombinant rat SP-A, containing a deletion of the collagen-like domain, was incubated with dipalmitoylphosphatidylcholine:egg phosphatidylcholine (1:1, wt/wt) lipid monolayers in the presence of calcium, negatively stained, and examined by transmission electron microscopy. Images of SP-A-lipid complexes with different angular orientations were used to reconstruct the 3D structure of the protein. These results showed that SP-A subunits readily formed trimers and interacted with lipid monolayers exclusively via the globular domains. A homology-based molecular model of SP-A was generated and fitted into the electron density map of the protein. The plane of the putative lipid-protein interface was relatively flat and perpendicular to the hydrophobic neck region, and the cleft region in the middle of the trimer had no apparent charge clusters. Amino acid residues that are known to affect lipid interactions, Glu(195) and Arg(197), were located at the protein-lipid interface. The molecular model indicated that the hydrophobic neck region of the SP-A did not interact with lipid monolayers but was instead involved in intratrimeric subunit interactions. The glycosylation site of SP-A was located at the side of each subunit, suggesting that the covalently linked carbohydrate moiety probably occupies the spaces between the adjacent globular domains, a location that would not sterically interfere with ligand binding.

Adsorption of pulmonary surfactant protein SP-A to monolayers of phospholipids containing hydrophobic surfactant protein SP-B or SP-C: potential differential role for tertiary interaction of lipids, hydrophobic proteins, and SP-A

Surface balance techniques were used to study the interactions of surfactant protein SP-A with monolayers of surfactant components preformed at the air-water interface. SP-A adsorption into the monolayers was followed by monitoring the increase in the surface pressure Deltapi after injection of SP-A beneath the films. Monolayers of dipalmitoylphosphatidylcholine (DPPC):egg phosphatidylglycerol (PG) (8:2, mol/mol) spread at initial surface pressure pi(i) = 5 mN/m did not promote the adsorption of SP-A at a subphase concentration of 0.68 microg/mL as compared to its adsorption to the monolayer-free surface. Surfactant proteins, SP-B or SP-C, when present in the films of DPPC:PG spread at pi(i) = 5 mN/m, enhanced the incorporation of SP-A in the monolayers to a similar extent; the Deltapi values being dependent on the levels of SP-B or SP-C, 3-17 wt %, in the lipid films. Calcium in the subphase did not affect the intrinsic surface activity of SP-A but reduced the Deltapi values produced by the adsorption of the protein to all the preformed films independently of their compositions and charges. The divalent ions likely modified the interaction of SP-A with the monolayers through their effects on the conformation, self-association, and charge state of SP-A. Values of Deltapi produced by adsorption of SP-A to the films of DPPC:PG with or without SP-B or SP-C were a function of the initial surface pressure of the films, pi(i). In the range of pressures 5 </= pi(i) </= 35 mN/m, where the monolayers existed in the liquid expanded (LE)/liquid condensed (LC) coexistence region, both the composition and the proportion of the LC phase in the films controlled the magnitude of Deltapi. Monolayers of DPPC:PG plus 17 wt % SP-B or SP-C, which had similar phase properties with LC phase occupying a maximum 25% of the total monolayer area, displayed different abilities to enhance the adsorption of SP-A to the surface. Results revealed that SP-B and SP-C in their pure monolayers had similar abilities in promoting the adsorption of SP-A, whereas SP-B, when present into the lipid films in the LE/LC coexistence state, displayed a higher capacity than SP-C to attract SP-A from the subphase. Lipid-induced changes in the conformations of the proteins might have modulated the interactions of SP-A with SP-B and SP-C incorporated into the phospholipid monolayers.

Human SP-A protein variants derived from one or both genes stimulate TNF-alpha production in the THP-1 cell line

In humans, two functional genes of surfactant protein (SP) A, SP-A1 and SP-A2, and several alleles of each functional gene have been characterized. SP-A is a multimeric molecule consisting of six trimers. Each trimer contains two SP-A1 molecules and one SP-A2 molecule. Until now, it has been unclear whether a single SP-A gene product is functional or whether there are functional differences either among alleles or between single-gene SP-A products and SP-A products derived from both genes. We tested the ability of in vitro expressed SP-A variants to stimulate tumor necrosis factor (TNF)-alpha production by THP-1 cells. We observed that 1) single-gene products and products derived from both genes stimulate TNF-alpha production, 2) there are differences among SP-A1 and SP-A2 alleles in their ability to stimulate TNF-alpha production, and 3) the increases in TNF-alpha production are lower after treatment with the SP-A1 alleles than after treatment with the SP-A2 alleles. Furthermore, coexpressed SP-As from SP-A1 and SP-A2 genes have a higher activity compared with SP-As from individual alleles or mixed SP-As from SP-A1 and SP-A2 genes. These data suggest that the SP-A-induced increases in TNF-alpha levels differ among SP-A variants and appear to be affected by SP-A genotype and whether SP-A is derived from one or both genes.

Mechanical strain and dexamethasone selectively increase surfactant protein C and tropoelastin gene expression

Physical forces derived from fetal breathing movements and hormones such as glucocorticoids are implicated in regulating fetal lung development. To elucidate whether the different signaling pathways activated by physical and hormonal factors are integrated and coordinated at the cellular and transcriptional levels, organotypic cultures of mixed fetal rat lung cells were subjected to static culture or mechanical strain in the presence and absence of dexamethasone. Tropoelastin and collagen type I were used as marker genes for fibroblasts, whereas surfactant protein (SP) A and SP-C were used as marker genes for distal epithelial cells. Mechanical strain, but not dexamethasone, significantly increased SP-C mRNA expression. Tropoelastin mRNA expression was upregulated by both mechanical strain and dexamethasone. No additive or synergistic effect was observed when cells were subjected to mechanical stretch in the presence of dexamethasone. Neither mechanical strain nor dexamethasone alone or in combination had any significant effect on the expression of SP-A mRNA. Dexamethasone decreased collagen type I mRNA expression, whereas mechanical strain had no effect. The increases in tropoelastin and SP-C mRNA levels induced by mechanical strain and/or dexamethasone were accompanied by increases in their heterogeneous nuclear RNA. In addition, the stretch- and glucocorticoid-induced alterations in tropoelastin and SP-C mRNA expression were abrogated with 10 microg/ml actinomycin D. These findings suggest that tropoelastin and SP-C genes are selectively stimulated by physical and/or hormonal factors at the transcriptional level in fetal lung fibroblasts and distal epithelial cells, respectively.

Lung surfactant proteins A and D in innate immune defense

The lung surfactant proteins (SP) A and D are large multimeric proteins and belong to a family of collagenous C-type lectins designated collectins. Both SP-A and SP-D are believed to play a role in the innate immunity of the lung. SP-A and SP-D bind to a broad spectrum of pathogens, including bacteria, viruses, fungi and yeasts but also lipopolysaccharides and allergens. Furthermore, SP-A and SP-D enhance the clearing of various pathogens by neutrophils and macrophages in vitro. Recent in vivo studies on SP-A deficient mice also support a role of SP-A in host defense.

Association between the surfactant protein A (SP-A) gene locus and respiratory-distress syndrome in the Finnish population

Respiratory-distress syndrome (RDS) in the newborn is a major cause of neonatal mortality and morbidity. Although prematurity is the most-important risk factor for RDS, the syndrome does not develop in many premature infants. The main cause of RDS is a deficiency of pulmonary surfactant, which consists of phospholipids and specific proteins. The genes underlying susceptibility to RDS are insufficiently known. The candidate-gene approach was used to study the association between the surfactant protein A (SP-A) gene locus and RDS in the genetically homogeneous Finnish population. In the present study, 88 infants with RDS and 88 control infants that were matched for degree of prematurity, prenatal glucocorticoid therapy, and sex were analyzed for SP-A genotypes. We show that certain SP-A1 alleles (6A2 and 6A3) and an SP-A1/SP-A2 haplotype (6A2/1A0) were associated with RDS. The 6A2 allele was overrepresented and the 6A3 allele was underrepresented in infants with RDS. These associations were particularly strong among small premature infants born at gestational age <32 wk. In infants protected from RDS (those that had no RDS, despite extreme prematurity and lack of glucocorticoid therapy), compared with infants that had RDS develop despite having received glucocorticoid therapy, the frequencies of 6A2 (.22 vs.71), 6A3 (.72 vs.17), 6A2/1A0 (.17 vs.68), 6A3/1A1 (.39 vs.10), and 6A3/1A2 (.28 vs.06) in the two groups, respectively, were strikingly different. According to the results of conditional logistic-regression analysis, diseases associated with premature birth did not explain the association between the odds of a particular homozygous SP-A1 genotype (6A2/6A2 and 6A3/6A3) and RDS. In the population evaluated in the present study, SP-B intron 4 variant frequencies were low and had no detectable association with RDS. We conclude that the SP-A gene locus is an important determinant for predisposition to RDS in premature infants.

Carbon dioxide enhances nitration of surfactant protein A by activated alveolar macrophages

We assessed whether reactive oxygen-nitrogen intermediates generated by alveolar macrophages (AMs) oxidized and nitrated human surfactant protein (SP) A. SP-A was exposed to lipopolysaccharide (100 ng/ml)-activated AMs in 15 mM HEPES (pH 7.4) for 30 min in the presence and absence of 1.2 mM CO(2). In the presence of CO(2), lipopolysaccharide-stimulated AMs had significantly higher nitric oxide synthase (NOS) activity (as quantified by the conversion of L-[U-(14)C]arginine to L-[U-(14)C]citrulline) and secreted threefold higher levels of nitrate plus nitrite in the medium [28 +/- 3 vs. 6 +/- 1 (SE) nmol. 6.5 h(-1). 10(6) AMs(-1)]. Western blotting studies of immunoprecipitated SP-A indicated that CO(2) enhanced SP-A nitration by AMs and decreased carbonyl formation. CO(2) (0-1.2 mM) also augmented peroxynitrite (0.5 mM)-induced SP-A nitration in a dose-dependent fashion. Peroxynitrite decreased the ability of SP-A to aggregate lipids, and this inhibition was augmented by 1.2 mM CO(2). Mass spectrometry analysis of chymotryptic fragments of peroxynitrite-exposed SP-A showed nitration of two tyrosines (Tyr(164) and Tyr(166)) in the absence of CO(2) and three tyrosines (Tyr(164), Tyr(166), and Tyr(161)) in the presence of 1.2 mM CO(2). These findings indicate that physiological levels of peroxynitrite, produced by activated AMs, nitrate SP-A and that CO(2) increased nitration, at least partially, by enhancing enzymatic nitric oxide production.

Surfactant protein A exhibits inhibitory effect on eosinophils IL-8 production

Eosinophils are believed to be one of the important sources of cytokines such as IL-8 at the site of allergic inflammation. It has been demonstrated that pulmonary surfactant protein A (SP-A) plays a potential role in modifying inflammation and the immune function. To verify the regulating effect of SP-A on eosinophil cytokine generation, we studied the effect of SP-A by determining of IL-8 production and expression stimulated with sIgA or PMA. SP-A purified from surfactant recovered from patients with alveolar proteinosis was added to eosinophils isolated by the negative selection method with immunomagnetic beads, and cultured for 24 h. The concentrations of IL-8 in the cell-free supernatants and cell lysates were then measured by ELISA. We also used a semiquantitative reverse transcriptase-polymerase chain reaction assay to detect the effect of SP-A on IL-8 mRNA expression. SP-A inhibited the secretion of IL-8 in a dose-dependent fashion. Suppression of IL-8 production by SP-A was significantly inhibited by SP-A antibody (PE10). SP-A also attenuated expression of IL-8 mRNA in eosinophils. These results indicate that SP-A might have the potential role to modify allergic inflammation by inhibiting IL-8 expression and production from eosinophils.

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

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

Neuregulin-1 and human epidermal growth factor receptors 2 and 3 play a role in human lung development in vitro

The human epidermal growth factor receptor (HER) family consists of four distinct receptors: HER1 (epidermal growth factor receptor), HER2, HER3, and HER4. Their specific activating ligands are collectively known as neuregulins (NRG). We hypothesized that one member of the NRG family, NRG-1, and the HER family would play a role in fetal lung development. To test this hypothesis, we defined NRG-1 and HER gene expression in mid-trimester human fetal lung tissue. HER2 and HER3 messenger RNA and protein were detected in the fetal lung, but HER4 expression was not detected. Immunohistochemical staining of fetal lung tissue localized HER2 and HER3 protein to the developing lung epithelium. NRG-1 expression was not found in freshly isolated human fetal lung, but it was observed in fetal lung explants after 2 d of explant culture. Immunohistochemistry of cultured human fetal lung explants revealed that NRG-1 protein was also expressed in pulmonary epithelial cells. Exposing human fetal lung to recombinant NRG-1 activated the HER receptor complex as measured by approximately 4-fold increases in receptor phosphotyrosine content. In addition, NRG-1 increased explant epithelial cell volume density approximately 2-fold (P < 0. 03); increased epithelial cell proliferation approximately 2-fold, as determined by bromodeoxyuridine labeling (P = 0.002); and reduced surfactant protein-A (SP-A) levels by 53% (P < 0.05). These data are consistent with an autocrine regulatory process mediated by NRG-1 activation of HER2/HER3 heterodimers expressed on developing human fetal lung epithelial cells. Receptor activation results in increased lung epithelial cell proliferation and volume density, and decreased SP-A production, a marker of type II pneumocyte differentiation.

Surfactant-associated protein A inhibits LPS-induced cytokine and nitric oxide production in vivo

The role of surfactant-associated protein (SP) A in the mediation of pulmonary responses to bacterial lipopolysaccharide (LPS) was assessed in vivo with SP-A gene-targeted [SP-deficient; SP-A(-/-)] and wild-type [SP-A(+/+)] mice. Concentrations of tumor necrosis factor (TNF)-alpha, macrophage inflammatory protein-2, and nitric oxide were determined in recovered bronchoalveolar lavage fluid after intratracheal administration of LPS. SP-A(-/-) mice produced significantly more TNF-alpha and nitric oxide than SP-A(+/+) mice after LPS treatment. Intratracheal administration of human SP-A (1 mg/kg) to SP-A(-/-) mice restored regulation of TNF-alpha, macrophage inflammatory protein-2, and nitric oxide production to that of SP-A(+/+) mice. Other markers of lung injury including bronchoalveolar fluid protein, phospholipid content, and neutrophil numbers were not influenced by SP-A. Data from experiments designed to test possible mechanisms of SP-A-mediated suppression suggest that neither binding of LPS by SP-A nor enhanced LPS clearance are the primary means of inhibition. Our data and others suggest that SP-A acts directly on immune cells to suppress LPS-induced inflammation. These results demonstrate that endogenous or exogenous SP-A inhibits pulmonary LPS-induced cytokine and nitric oxide production in vivo.

Surfactant protein A prevents silica-mediated toxicity to rat alveolar macrophages

Silicosis is a serious occupational lung disease associated with irreversible pulmonary fibrosis. The interaction between inhaled crystalline silica and the alveolar macrophage (AM) is thought to be a key event in the development of silicosis and fibrosis. Silica can cause direct injury to AMs and can induce AMs to release various inflammatory mediators. Acute silicosis is also characterized by a marked elevation in surfactant apoprotein A (SP-A); however, the role of SP-A in silicosis is unknown. We investigated whether SP-A directly affects the response of AMs to silica. In this study, the degree of silica toxicity to cultured rat AMs as assessed by a (51)Cr cytotoxicity assay was shown to be dependent on the time of exposure and the concentration and size of the silica particles. Silica directly injured rat AMs as evidenced by a cytotoxic index of 32.9 +/- 2.5, whereas the addition of rat SP-A (5 microg/ml) significantly reduced the cytotoxic index to 16.6 +/- 1.2 (P < 0. 001). This effect was reversed when SP-A was incubated with either polyclonal rabbit anti-rat SP-A antibody or D-mannose. These data indicate that SP-A mitigates the effect of silica on AM viability, and this effect may involve the carbohydrate recognition domain of SP-A. The elevation of SP-A in acute silicosis may serve as a normal host response to prevent lung cell injury after exposure to silica.

cDNA cloning of ovine pulmonary SP-A, SP-B, and SP-C: isolation of two different sequences for SP-B

Pulmonary surfactant promotes alveolar stability by lowering the surface tension at the air-liquid interface in the peripheral air spaces. The three surfactant proteins SP-A, SP-B, and SP-C contribute to dynamic surface properties involved during respiration. We have cloned and sequenced the complete cDNAs for ovine SP-A and SP-C and two distinct forms of ovine SP-B cDNAs. The nucleotide sequence of ovine SP-A cDNA consists of 1,901 bp and encodes a protein of 248 amino acids. Ovine SP-C cDNA contains 809 bp, predicting a protein of 190 amino acids. Ovine SP-B is encoded by two mRNA species, which differ by a 69-bp in-frame deletion in the region coding for the active airway protein. The larger SP-B cDNA comprises 1,660 bp, encoding a putative protein of 374 amino acids. With the sequences reported, a more complete analysis of surfactant regulation and the determination of their physiological function in vivo will be enabled.

Humoral autoreactivity directed against surfactant protein-A (SP-A) in rheumatoid arthritis synovial fluids

SP-A is found principally in the lung, and has been associated with lamellar bodies also found in the synovial joint. Both SP-A and C1q contain collagen-like regions, and SP-A and C1q have some structural similarities, both having a globular head region and a collagen-like tail. Here we are able to show that (i) autoreactivity to SP-A, as expressed by IgG and IgM autoantibodies, is present in synovial fluid (SF) isolated from patients with rheumatoid arthritis (RA); (ii) in absorption experiments only a limited degree of cross-reactivity between autoantibodies reactive with C1q and SP-A is observed; (iii) there is no cross-reactivity between autoantibodies reactive with type II collagen (CII) and those reactive with SP-A or C1q; (iv) autoantibodies react with polymeric (dimers and larger) SP-A, but not with monomeric SP-A subunits, indicating that a degree of quaternary structure is required for antibody binding. Unlike CII, which not accessible in the normal joint, both SP-A and C1q are available within the SF in patients with RA and may therefore provide antigens driving an autoimmune response directed against collagen-like structures.

Pulmonary surfactant-associated protein A levels in cadaveric sera with reference to the cause of death

Pulmonary surfactant-associated protein A (SP-A) is an exclusively lung specific protein, and is considered to leak into the blood stream in alveolar septal damage. In this study we examined the serum SP-A level in forensic autopsy materials using an enzyme immunoassay with monoclonal antibodies to assess the postmortem level in relation to the cause and mode of death. Although a gradual postmortem degradation should be taken into consideration, topological relationship of serum level seemed to be fairly stable (arterial> or =venous blood in most cases), indicating no evident influence of postmortem diffusion. Significant elevation of serum SP-A (76.7-250 ng/ml in left heart blood) was observed in hyaline membrane diseases from various causes independent of the postmortem intervals (<30 h). However, mean SP-A levels in postmortem heart blood were usually low in asphyxia including hanging, strangulation and choking (left, 25.5 ng/ml; right, 22.3 ng/ml), polytrauma (left, 13.1 ng/ml; right, 9.0 ng/ml) and stab wound to the neck (left, 34.1 ng/ml; right, 29.4 ng/ml). Prominent elevation was noted in a case of fatal strangulation with complication of idiopathic interstitial pneumonia, and also in some deaths due to drowning, burns in fires, blunt and gunshot chest injuries. These findings indicated that postmortem elevation of serum SP-A levels was associated with alveolar septal damage due to inflammation, mechanical and physical stresses, which caused leakage of SP-A into the bloodstream.

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.

Internalized SP-A and lipid are differentially resecreted by type II pneumocytes

Biochemical and morphological assays were developed to study surfactant protein A (SP-A) and lipid resecretion kinetics by isolated type II cells in vitro. After a 10-min uptake period with SP-A (3 microg/10(6) cells) in combination with liposomes (60 microg/10(6) cells), the cells were allowed to resecrete. After 5 min of resecretion, only 21.7 +/- 4.6% of the internalized SP-A remained intracellularly compared with 54 +/- 2.9% of the lipids. Extracellular SP-A present during the resecretion period partially inhibited resecretion (SP-A, 36% at 5 min; lipid, approximately 16% at 5 min). Lipid resecretion was also dependent on the SP-A concentration present during the uptake period. Although, as shown by confocal laser scanning microscopy, after a 10-min uptake period at 37 degrees C, most of the fluorescein isothiocyanate-labeled SP-A and rhodamine-phosphatidylethanolamine-labeled lipids colocalized within the cells, after an additional 10 min of resecretion, both the strength of the fluorescence signals and the extent of colocalization had markedly decreased. These data indicate that internalized lipid and SP-A can be resecreted rapidly by type II cells, likely via different pathways.

[Surfactant proteins A and D as biomarkers of disease activity in diffuse interstitial pneumonia]

We evaluated the clinical significance of surfactant proteins A (SP-A) and D (SP-D) as useful markers of disease activity in patients with diffuse interstitial pneumonia. Serum concentrations of SP-A and SP-D were measured by the sandwich ELISA method. The serum levels of SP-A and SP-D in patients with diffuse interstitial pneumonia (IIP, CVD-IP, HP, Ra-IP) were significantly higher than the levels in healthy controls, and showed high positive rates. IIP patients characterized by a predominantly ground-glass opacity (GGO) pattern on high-resolution computed tomograms had significantly higher concentrations of serum SP-A. Elevated SP-D levels reflected the extent not only of GGO but also of parenchymal collapse opacity (PCO). It is likely that the mechanisms behind the elevation of SP-A and SP-D do not correlate with pathologic changes in IIP. Serum SP-A and SP-D levels obtained at the time of initial evaluation from 9 patients who died after less than 3 years of follow-up were significantly higher than in patients with survival rates of more than 3 years. Serum SP-A and SP-D may be useful biomarkers of disease activity in patients with IIP.

Aquaporin-5 expression, but not other peripheral lung marker genes, is reduced in PTH/PTHrP receptor null mutant fetal mice

Parathyroid hormone-related peptide (PTHrP) and the parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor are important developmental regulators of cell growth and differentiation in some organs. In lung, both the peptide and the receptor are expressed early in development and in alveolar cells in adults. In adult alveolar cells, PTHrP appears to promote the alveolar type II cell phenotype in vitro. Mice carrying null mutations in genes for either receptor or ligand die at birth of respiratory failure. To determine if absence of the PTH/PTHrP receptor alters morphogenesis or cellular differentiation of the distal lung, we analyzed the morphology and gene expression patterns in PTH/PTHrP receptor null mutant mice right before birth and compared them with wild-type and heterozygous null littermates. Using semiquantitative Northern blots, we observed that messenger RNA (mRNA) for aquaporin-5, the type I cell-specific water channel, was markedly decreased. The abundance of other marker mRNAs for type I and type II cell phenotypes, including T1alpha, surfactant proteins, and others, was unaltered. Gross morphology and lung pattern, assessed by in situ hybridization for surfactant protein C, were normal. We conclude therefore that, although signaling through this receptor may influence expression of specific lung genes, it does not play a major role in the general regulation of lung development and growth.

Degree of lung maturity determines the direction of the interleukin-1- induced effect on the expression of surfactant proteins

Intra-amniotic interleukin (IL)-1 increases surfactant components in immature fetal lung, whereas high IL-1 after birth is associated with surfactant dysfunction. Our aim was to investigate whether the fetal age influences the responsiveness of surfactant proteins (SPs) to IL-1. Rabbit lung explants from fetuses at 19, 22, 27, and 30 d of gestation and 1-d-old newborns were cultured in serum-free medium in the presence of recombinant human (rh) IL-1alpha or vehicle. The influence of IL-1alpha on SP-A, -B, and -C messenger RNA (mRNA) content was dependent on the conceptional age. In very immature lung on Day 19, rhIL-1alpha (570 ng/ml for 20 h) increased SP-A, -B, and -C mRNA by 860+/-15%, 314+/-108%, and 64+/-17%, respectively. The increase in SP-A mRNA was evident within 4 to 6 h. IL-1alpha increased the SP-A concentration in alveolar epithelial cells and in the culture medium within 20 h. In contrast, at 27 to 30 d of gestation and in newborns, IL-1alpha decreased SP-C, -B, and -A mRNA by means of 64 to 67%, 48 to 59%, and 12 to 15%, respectively. SP-B protein decreased by 45 to 60%. The decrease in mRNA became evident within 8 to 12 h and was dependent on IL-1 concentration. On Day 27, IL-1alpha accelerated the degradation of SP-B mRNA in the presence of actinomycin D. IL-1 did not increase the degradation rate of SP-A mRNA unless both actinomycin D and cycloheximide were added to the explants. The present findings may explain some of the contrasting associations between inflammatory cytokines and lung diseases during the perinatal period. The determinants of the direction of the IL-1 effect on the expression of SPs remain to be identified.

Pulmonary hypoplasia in the myogenin null mouse embryo

Although fetal breathing movements are required for normal lung development, there is uncertainty concerning the specific effect of absent fetal breathing movements on pulmonary cell maturation. We set out to evaluate pulmonary development in a genetically defined mouse model, the myogenin null mouse, in which there is a lack of normal skeletal muscle fibers and thus skeletal muscle movements are absent in utero. Significant decreases were observed in lung:body weight ratio and lung total DNA at embryonic days (E)14, E17, and E20. Reverse transcriptase/polymerase chain reaction, in situ immunofluorescence, and electron microscopy revealed early lung cell differentiation in both null and wild-type lungs as early as E14. However at E14, myogenin null lungs had decreased 5'-bromo-2-deoxyuridine incorporation compared with that of wild-type littermates, whereas at E17 and E20, increased Bax immunolabeling and terminal deoxyribonucleotidyl transferase-mediated dUTP-biotin nick-end labeling staining were detected in the myogenin null mice but not in the wild-type littermates. These observations highlight the importance of skeletal muscle contractile activity in utero for normal lung organogenesis. Null mice lacking the muscle-specific transcription factor myogenin exhibit a secondary effect on lung development such that decreased lung cell proliferation and increased programmed cell death are associated with lung hypoplasia.

Surfactant protein A regulates pulmonary surfactant secretion via activation of phosphatidylinositol 3-kinase in type II alveolar cells

Pulmonary surfactant is secreted by the type II alveolar cells of the lung, and this secretion is induced by secretagogues of several types (e.g., ionomycin, phorbol esters, and terbutaline). Secretagogue-induced secretion is inhibited by surfactant-associated protein A (SP-A), which binds to a specific receptor (SPAR) on the surface of type II cells. The mechanism of SP-A-activated SPAR signaling is completely unknown. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 rescued surfactant secretion from inhibition by SP-A. In order to directly demonstrate a role for PI3K in SPAR signaling, PI3K activity was immunoprecipitated from type II cell extracts. PI3K activity increased rapidly after SP-A addition to type II cells. Since many receptors that activate PI3K do so through tyrosine-specific protein phosphorylation, antisera to phosphotyrosine, insulin-receptor substrate-1 (IRS-1), or SPAR were also examined. These antisera coimmunoprecipitated PI3K activity that was stimulated by SP-A. In addition, the tyrosine-specific protein kinase inhibitors genistein and herbimycin A blocked the action of SP-A on surfactant secretion. We conclude that SP-A signals to regulate surfactant secretion through SPAR, via pathways that involve tyrosine phosphorylation, include IRS-1, and entail activation of PI3K. This activation leads to inhibition of secretagogue-induced secretion of pulmonary surfactant.

Importance of the carboxy-terminal 25 amino acid residues of lung collectins in interactions with lipids and alveolar type II cells

Surfactant proteins A and D (SP-A and SP-D) are structurally related members of the collectin family found in the alveolar compartment of the lung. SP-A binds dipalmitoylphosphatidylcholine (DPPC) and galactosylceramide (GalCer), induces liposome aggregation, and regulates the uptake and secretion of surfactant lipids by alveolar type II cells in vitro. SP-D binds phosphatidylinositol (PI) and glucosylceramide. The purpose of this study was to identify a critical stretch of primary sequence in the SP-A region Cys(204)-Phe(228) and the SP-D region Cys(331)-Phe(355) that is involved in protein-specific lipid and type II cell interactions. Chimeras ad1 and ad2 were constructed with rat SP-A/SP-D splice junctions at Cys(218)/Gly(346) and Lys(203)/Cys(331), respectively. Chimera ad1 but not ad2 retained DPPC liposome binding activity. Both chimeras retained significant binding to GalCer liposomes. Chimera ad1 did not bind to PI, whereas chimera ad2 acquired a significant PI binding. Both chimeras failed to induce liposome aggregation and to interact with alveolar type II cells. In addition, monoclonal antibody 1D6 that blocks specific SP-A functions did not recognize either chimera. From these results, we conclude that (1) the SP-A region Leu(219)-Phe(228) is required for liposome aggregation and interaction with alveolar type II cells, (2) the SP-A region Cys(204)-Cys(218) is required for DPPC binding, (3) the SP-D region Cys(331)-Phe(355) is essential for minimal PI binding, and (4) the epitope for mAb 1D6 is located at the region contiguous to the SP-A region Leu(219)-Phe(228).

The role of homodimers in surfactant protein B function in vivo

Surfactant protein B (SP-B) is detected in the airways as a sulfhydryl-dependent dimer (M(r) approximately 16,000). To test the hypothesis that formation of homodimers is critical for SP-B function, the cysteine residue reported to be involved in SP-B dimerization was mutated to serine (Cys(248) --> Ser) and the mutated protein was targeted to the distal respiratory epithelium of transgenic mice. Transgenic lines which demonstrated appropriate processing, sorting, and secretion of human SP-B monomer were crossed with SP-B +/- mice to achieve expression of human monomer in the absence of endogenous SP-B dimer (hSP-B(mon), mSP-B-/-). In two of three transgenic lines, hSP-B(mon), mSP-B-/- mice had normal lung structure, complete processing of SP-C proprotein, well formed lamellar bodies, and normal longevity. Pulmonary function studies revealed an altered hysteresis curve for hSP-B(mon), mSP-B-/- mice relative to wild type mice. Large aggregate surfactant fractions from hSP-B(mon), mSP-B-/- mice resulted in higher minimum surface tension in vitro compared with surfactant from wild type mice. Surfactant lipids supplemented with 2% hSP-B monomer resulted in slower adsorption and higher surface tension than surfactant with 2% hSP-B dimer. Taken together, these data indicate a role for SP-B dimer in surface tension reduction in the alveolus.

Ablation of a critical surfactant protein B intramolecular disulfide bond in transgenic mice

The 79-amino acid, mature SP-B peptide contains three intramolecular disulfide bonds shared by all saposin-like proteins. This study tested the hypothesis that the disulfide bond formed between cysteine residues 35 and 46 (residues 235 and 246 of the SP-B proprotein) is essential for proper function of SP-B. To test the role of this bridge in SP-B function in vivo, a construct was generated in which cysteine residues 235 and 246 of the human SP-B proprotein were mutated to serine and cloned under the control of the 3.7-kilobase hSP-C promoter (hSP-B(C235S/C246S)). In two transgenic mouse lines, expression of the mutant peptide in the wild-type murine SP-B background was invariably lethal in the neonatal period. In four additional lines, survival was inversely related to the level of transgene expression. To test the ability of the mutant peptide to functionally replace the wild-type protein, transgenic mice were crossed into the SP-B null background. No animals that expressed hSP-B(C235S/C246S) in the murine SP-B-/- background survived the neonatal period. hSP-B(C235S/C246S) proprotein accumulated in the endoplasmic reticulum and was not processed to the mature, biologically active peptide. The results of these studies demonstrate that the intramolecular bridge between residues 235 and 246 is critical for intracellular trafficking of SP-B and suggest that overexpression of mutant SP-B in the wild-type background may be lethal.

Epidermal growth factor and lung development in the offspring of the diabetic rat

Fetuses of diabetic mothers who were exposed to excessive glucose show delayed maturation. Under these conditions, altered growth factor expression or signaling may have important regulatory influences. We examined the role of epidermal growth factor (EGF) in lung development and maternal diabetes in the rat. In order to evaluate the possible role of glucose for the expression of EGF and the growth of lung tissue, we performed in vitro studies with organotypic cultures of fetal alveolar cells obtained from control rats. Compared to pups of normal rats, the newborn rats of untreated diabetic rats had reduced body weight, but normal lung weight relative to body weight. The air:mesenchyme ratio and the average size of alveoli per mm(2) lung tissue were reduced. The immunoreactivity (IR) of EGF, which was quantified using a computerized image analysis system, appeared with increased intensity and was associated with a reduced intensity of surfactant protein A-IR. The only difference observed between pups of treated diabetic rats and controls was a decrease in the lung weight:body weight ratio. In organotypic cultures, the presence of 13 mmol/L glucose in the cell media increased immunoreactive staining against EGF, but decreased the incorporation of thymidine as compared to the results obtained with alveolar cells grown in a normophysiological concentration of glucose (3 mmol/L). Addition of EGF increased the thymidine incorporation only in cells grown in 3 mM glucose. These findings may indicate immaturity of the lungs of pups of untreated diabetic rats, and subtle alterations in the lungs of pups from treated diabetic rats. The results also suggest that glucose plays a role in the expression of EGF, and that cells exposed to high concentrations of glucose are less responsive to EGF.

Porcine lung surfactant protein D: complementary DNA cloning, chromosomal localization, and tissue distribution

Porcine organs and lung surfactant have medically important applications in both xenotransplantation and therapy. We have started to characterize porcine lung surfactant by cloning the cDNA of porcine surfactant protein D (SP-D). SP-D and SP-A are important mediators in innate immune defense for the lung and possibly other mucosal surfaces. Porcine SP-D will also be an important reagent for use in existing porcine animal models for human lung infections. The complete cDNA sequence of porcine SP-D, including the 5' and 3' untranslated regions, was determined from two overlapping bacteriophage clones and by PCR cloning. Three unique features were revealed from the porcine sequence in comparison to SP-D from other previously characterized species, making porcine SP-D an intriguing species addition to the SP-D/collectin family. The collagen region contains an extra cysteine residue, which may have important structural consequences. The other two differences, a potential glycosylation site and an insertion of three amino acids, lie in the loop regions of the carbohydrate recognition domain, close to the carbohydrate binding region and thus may have functional implications. These variations were ruled out as polymorphisms or mutations by confirming the sequence at the genomic level in four different pig breeds. Porcine SP-D was shown to localize primarily to the lung and with less abundance to the duodenum, jejunum, and ileum. The genes for SP-D and SP-A were also shown to colocalize to a region of porcine chromosome 14 that is syntenic with the human and murine collectin loci.

Effects of antenatal endotoxin and glucocorticoids on the lungs of preterm lambs

OBJECTIVE

We hypothesized that the proinflammatory response to intra-amniotic endotoxin would induce lung maturation in preterm lambs.

STUDY DESIGN

Ewes were randomly assigned to receive 20 mg Escherichia coli endotoxin by intra-amniotic injection, maternal betamethasone (0.5 mg/kg), or sodium chloride solution. Preterm lambs were delivered at 125 days' gestation and underwent ventilation to assess lung function. Lung gas volume, surfactant concentrations, and inflammation were subsequently evaluated, with data analyzed by analysis of variance.

RESULTS

Fetal endotoxin exposure 6 days before delivery increased compliance by 59%, increased lung gas volume 2.3-fold, increased concentrations of surfactant lipids, increased surfactant A and B protein levels, and increased messenger ribonucleic acid expressions for surfactant proteins (all P <.01, vs control group). Betamethasone exposure resulted in less consistent effects. White blood cell counts were increased in fetal membranes and lungs after endotoxin exposure, but there was no severe inflammation.

CONCLUSION

A single fetal exposure to endotoxin resulted in large improvements in postnatal lung function and increases in surfactant concentrations after preterm delivery. These effects were qualitatively larger than those achieved with betamethasone.

Collectins and pulmonary innate immunity

The surfactant-associated proteins SP-A and SP-D are members of a family of host defense lectins, designated collectins. There is increasing evidence that these pulmonary, epithelial-derived proteins are important components of the innate immune response to microbial challenge and participate in other aspects of immune and inflammatory regulation within the lung. Both proteins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms, and to certain organic particles, such as pollens. SP-A and SP-D have the capacity to modulate leukocyte function and, in some circumstances; to opsonize and enhance the killing of microorganisms. The biologic activity of cell wall components, such as Gram-negative bacterial polysaccharides, or viral glycoproteins, such as the hemagglutinin of influenza viruses, may be altered by interactions with collectins. In addition, complementary or cooperative interactions between SP-A, SP-D and other host defense lectins could contribute to the efficiency of this defense system. Collectins could play particularly important roles in settings of inadequate or impaired specific immunity, and acquired alterations in the levels of active collectins within the airspaces and distal airways may increase susceptibility to infection.

Prediction of respiratory distress syndrome by the level of pulmonary surfactant protein A in cord blood sera

The purpose of this study was to determine the utility of measuring the level of pulmonary surfactant protein A (SP-A) in cord blood sera to predict for respiratory distress syndrome (RDS). SP-A levels in cord blood sera from 48 infants born at gestational ages < 32 weeks were measured by a sandwich ELISA system. Mean value of SP-A in cord blood was 5.8 ng/ml in cases with RDS and 15.1 ng/ml in those without RDS (p = 0.002). The best cut-off point of cord blood SP-A to predict RDS was determined as 10 ng/ml. The sensitivity and the specificity of the cut-off point for predicting RDS were 81 and 76%, respectively. Multivariate regression analysis showed that high SP-A level in cord blood, premature rupture of the membranes longer than 24 h and heavy birth weight were all significantly related to the non-RDS outcome.