Surfactant peptides stimulate uptake of phosphatidylcholine by isolated cells

Role of pulmonary surfactant protein Sp-C dimerization on membrane fragmentation: An emergent mechanism involved in lung defense and homeostasis

Surfactant protein C (SP-C) is a protein present in the pulmonary surfactant system that is involved in the biophysical properties of this lipoprotein complex, but it also has a role in lung defense and homeostasis. In this article, we propose that the link between both functions could rely on the ability of SP-C to induce fragmentation of phospholipid membranes and generate small vesicles that serve as support to present different ligands to cells in the lungs. Our results using bimolecular fluorescence complementation and tunable resistive pulse sensing setups suggest that SP-C oligomerization could be the triggering event that causes membrane budding and nanovesiculation. As shown by fluorescence microscopy and flow cytometry, these vesicles are differentially assimilated by alveolar macrophages and alveolar type II cells, indicating distinct roles of these alveoli-resident cells in the processing of the SP-C- induced vesicles and their cargo. These results depict a more accurate picture of the mechanisms of this protein, which could be relevant for the comprehension of pulmonary pathologies and the development of new therapeutic approaches.

Expression of a novel surfactant protein gene is associated with sites of extrapulmonary respiration in a lungless salamander

Numerous physiological and morphological adaptations were achieved during the transition to lungless respiration that accompanied evolutionary lung loss in plethodontid salamanders, including those that enable efficient gas exchange across extrapulmonary tissue. However, the molecular basis of these adaptations is unknown. Here, we show that lungless salamanders express in the larval integument and the adult buccopharynx—principal sites of respiratory gas exchange in these species—a novel paralogue of the gene surfactant-associated protein C (SFTPC), which is a critical component of pulmonary surfactant expressed exclusively in the lung in other vertebrates. The paralogous gene appears to be found only in salamanders, but, similar to SFTPC, in lunged salamanders it is expressed only in the lung. This heterotopic gene expression, combined with predictions from structural modelling and respiratory tissue ultrastructure, suggests that lungless salamanders may produce pulmonary surfactant-like secretions outside the lungs and that the novel paralogue of SFTPC might facilitate extrapulmonary respiration in the absence of lungs. Heterotopic expression of the SFTPC paralogue may have contributed to the remarkable evolutionary radiation of lungless salamanders, which account for more than two thirds of urodele species alive today.

Alveolar Proteinosis and Phospholipidoses of the Lungs

Three pulmonary disease conditions result from the accumulation of phospholipids in the lung. These conditions are the human lung disease known as pulmonary alveolar proteinosis, the lipoproteinosis that arises in the lungs of rats during acute silicosis, and the phospholipidoses induced by numerous cationic amphiphilic therapeutic agents. In this paper, the status of phospholipid metabolism in the lungs during the process of each of these lung conditions has been reviewed and possible mechanisms for their establishment are discussed. Pulmonary alveolar proteinosis is characterized by the accumulation of tubular myelin-like multilamellated structures in the alveoli and distal airways of patients. These structures appear to be formed by a process of spontaneous assembly involving surfactant protein A and surfactant phospholipids. Structures similar to tubular myelin-like multilamellated structures can be seen in the alveoli of rats during acute silicosis and, as with the human condition, both surfactant protein A and surfactant phospholipids accumulate in the alveoli. Excessive accumulation of surfactant protein A and surfactant phospholipids in the alveoli could arise from their overproduction and hypersecretion by a subpopulation of Type II cells that are activated by silica, and possibly other agents. Phospholipidoses caused by cationic amphiphilic therapeutic agents arise as a result of their inhibition of phospholipid catabolism. Inhibition of phospholipases results in the accumulation of phospholipids in the cytoplasm of alveolar macrophages and other cells. While inhibition of phospholipases by these agents undoubtedly occurs, there are many anomalous features, such as the accumulation of extracellular phospholipids and surfactant protein A, that cannot be accounted for by this simplistic hypothesis.

SP-B and SP-C containing new synthetic surfactant for treatment of extremely immature lamb lung

Although superiority of synthetic surfactant over animal-driven surfactant has been known, there is no synthetic surfactant commercially available at present. Many trials have been made to develop synthetic surfactant comparable in function to animal-driven surfactant. The efficacy of treatment with a new synthetic surfactant (CHF5633) containing dipalmitoylphosphatidylcholine, phosphatidylglycerol, SP-B analog, and SP-C analog was evaluated using immature newborn lamb model and compared with animal lung tissue-based surfactant Survanta. Lambs were treated with a clinical dose of 200 mg/kg CHF5633, 100 mg/kg Survanta, or air after 15 min initial ventilation. All the lambs treated with air died of respiratory distress within 90 min of age. During a 5 h study period, Pco(2) was maintained at 55 mmHg with 24 cmH(2)O peak inspiratory pressure for both groups. The preterm newborn lamb lung functions were dramatically improved by CHF5633 treatment. Slight, but significant superiority of CHF5633 over Survanta was demonstrated in tidal volume at 20 min and dynamic lung compliance at 20 and 300 min. The ultrastructure of CHF5633 was large with uniquely aggregated lipid particles. Increased uptake of CHF5633 by alveolar monocytes for catabolism was demonstrated by microphotograph, which might be associated with the higher treatment dose of CHF5633. The higher catabolism of CHF5633 was also suggested by the similar amount of surfactant lipid in bronchoalveolar lavage fluid (BALF) between CHF5633 and Survanta groups, despite the 2-fold higher treatment dose of CHF5633. Under the present ventilation protocol, lung inflammation was minimal for both groups, evaluated by inflammatory cell numbers in BALF and expression of IL-1β, IL-6, IL-8, and TNFα mRNA in the lung tissue. In conclusion, the new synthetic surfactant CHF5633 was effective in treating extremely immature newborn lambs with surfactant deficiency during the 5 h study period.

Transient in utero disruption of cystic fibrosis transmembrane conductance regulator causes phenotypic changes in alveolar type II cells in adult rats

Background

Mechanicosensory mechanisms regulate cell differentiation during lung organogenesis. We have previously demonstrated that cystic fibrosis transmembrane conductance regulator (CFTR) was integral to stretch-induced growth and development and that transient expression of antisense-CFTR (ASCFTR) had negative effects on lung structure and function. In this study, we examined adult alveolar type II (ATII) cell phenotype after transient knock down of CFTR by adenovirus-directed in utero expression of ASCFTR in the fetal lung.

Results

In comparison to (reporter gene-treated) Controls, ASCFTR-treated adult rat lungs showed elevated phosphatidylcholine (PC) levels in the large but not in the small aggregates of alveolar surfactant. The lung mRNA levels for SP-A and SP-B were lower in the ASCFTR rats. The basal PC secretion in ATII cells was similar in the two groups. However, compared to Control ATII cells, the cells in ASCFTR group showed higher PC secretion with ATP or phorbol myristate acetate. The cell PC pool was also larger in the ASCFTR group. Thus, the increased surfactant secretion in ATII cells could cause higher PC levels in large aggregates of surfactant. In freshly isolated ATII cells, the expression of surfactant proteins was unchanged, suggesting that the lungs of ASCFTR rats contained fewer ATII cells. Gene array analysis of RNA of freshly isolated ATII cells from these lungs showed altered expression of several genes including elevated expression of two calcium-related genes, Ca²⁺-ATPase and calcium-calmodulin kinase kinase1 (CaMkk1), which was confirmed by real-time PCR. Western blot analysis showed increased expression of calmodulin kinase I, which is activated following phosphorylation by CaMkk1. Although increased expression of calcium regulating genes would argue in favor of Ca²⁺-dependent mechanisms increasing surfactant secretion, we cannot exclude contribution of alternate mechanisms because of other phenotypic changes in ATII cells of the ASCFTR group.

Conclusion

Developmental changes due to transient disruption of CFTR in fetal lung reflect in altered ATII cell phenotype in the adult life.

Surfactant protein-A plays an important role in lung surfactant clearance: evidence using the surfactant protein-A gene-targeted mouse

Previous studies with the isolated perfused rat lung showed that both clathrin- and actin-mediated pathways are responsible for endocytosis of dipalmitoylphosphatidylcholine (DPPC)-labeled liposomes by granular pneumocytes in the intact lung. Using surfactant protein-A (SP-A) gene-targeted mice, we examined the uptake of [(3)H]DPPC liposomes by isolated mouse lungs under basal and secretagogue-stimulated conditions. Unilamellar liposomes composed of [(3)H]DPPC: phosphatidylcholine:cholesterol:egg phosphatidylglycerol (10:5:3:2 mol fraction) were instilled into the trachea of anesthetized mice, and the lungs were perfused (2 h). Uptake was calculated as percentage of instilled disintegrations per minute in the postlavaged lung. Amantadine, an inhibitor of clathrin and, thus, receptor-mediated endocytosis via clathrin-coated pits, decreased basal [(3)H]DPPC uptake by 70% in SP-A +/+ but only by 20% in SP-A -/- lung, data compatible with an SP-A/receptor-regulated lipid clearance pathway in the SP-A +/+ mice. The nonclathrin, actin-dependent process was low in the SP-A +/+ lung but accounted for 55% of liposome endocytosis in the SP-A -/- mouse. With secretagogue (8-bromoadenosine 3',5'-cyclic monophosphate) treatment, both clathrin- and actin-dependent lipid clearance were elevated in the SP-A +/+ lungs while neither pathway responded in the SP-A -/- lungs. Binding of iodinated SP-A to type II cells isolated from both genotypes of mice was similar indicating a normal SP-A receptor status in the SP-A -/- lung. Inclusion of SP-A with instilled liposomes served to "rescue" the SP-A -/- lungs by reestablishing secretagogue-dependent enhancement of liposome uptake. These data are compatible with a major role for receptor-mediated endocytosis of DPPC by granular pneumocytes, a process critically dependent on SP-A.

Effect of SP-B peptides on the uptake of liposomes by alveolar cells

BACKGROUND

Exogenous surfactant has been accepted worldwide as a therapy of RDS in premature and term infants. Exogenous surfactant is usually derived from lung extracts containing phospholipids and the surfactant proteins SP-B and SP-C. Synthetic peptides of SP-B and SP-C are being tested with the aim to develop a completely synthetic surfactant preparation. Nevertheless, the effects of these peptides on the endogenous surfactant metabolism remain unknown.

OBJECTIVES

The effect of synthetic SP-B peptides on uptake of surfactant-like liposomes was investigated in alveolar cells. Native SP-B and seven SP-B peptides were included: monomeric and dimeric SP-B(1-25) (Cys-11 --> Ala-11), SP-B(63-78)and Ala-SP-B(63-78) (Cys-71 --> Ala-71;Cys-77 --> Ala-77)and their serine mutants.

METHODS

In vitro, alveolar macrophages (AM) and alveolar type II cells (ATII) were incubated with liposomes containing SP-B or one of its peptides. In vivo, rats received intratracheally various SP-B peptides (SP-B/lipid ratio 1:33 w/w) incorporated in fluorescent surfactant-like liposomes. One hour after instillation, AM and ATII were isolated and cell-associated fluorescence was determined using flow cytometry. Confocal laser microscopy was performed to ensure internalization of the liposomes.

RESULTS

In vitro uptake by AM or ATII was not influenced by the SP-B peptides. In vivo, SP-B(1-25) and Ser-SP-B(1-25) increased the uptake by AM whereas dSP-B(1-25) decreased the uptake. Neither SP-B(1-25) nor dSP-B(1-25 )affected total uptake by ATII. The overall uptake by SP-B(63-78) variants was not changed.

CONCLUSIONS

Surface-active synthetic SP-B peptides do not interfere with the normal uptake of surfactant by ATII.

Distinct effects of SP-B and SP-C on the uptake of surfactant-like liposomes by alveolar cells in vivo and in vitro

The effects of surfactant protein B (SP-B) and SP-C on the uptake of surfactant-like liposomes by alveolar type II cells and alveolar macrophages were studied both in vivo and in vitro. In vivo, mechanically ventilated rats were intratracheally instilled with fluorescently labeled liposomes that had SP-B and/or SP-C incorporated in different concentrations. Consequently, the alveolar cells were isolated, and cell-associated fluorescence was determined using flow cytometry. The results show that the incorporation of SP-B does not influence the uptake, and it also does not in the presence of essential cofactors. The inclusion of SP-C in the liposomes enhanced the alveolar type II cells at a SP-C to lipid ratio of 2:100. If divalent cations (calcium and magnesium) were present at physiological concentrations in the liposome suspension, uptake of liposomes by alveolar macrophages was also enhanced. In vitro, the incorporation of SP-B affected uptake only at a protein-to-lipid ratio of 8:100, whereas the inclusion of SP-C in the liposomes leads to an increased uptake at a protein-to-lipid ratio of 1:100. From these results, it can be concluded that SP-B is unlikely to affect uptake of surfactant, whereas SP-C in combination with divalent cations and other solutes are capable of increasing the uptake.

Uptake of a natural surfactant and increased delivery of small organic anions into type II pneumocytes

The uptake of natural lung surfactant into differentiated type II cells may be used for the targeted delivery of other molecules. The fluorescent anion pyranine [hydroxypyren-1,3,6-trisulfonic acid, sodium salt (HPTS)] was incorporated into a bovine surfactant labeled with [3H]dipalmitoylphosphatidylcholine ([3H]DPPC). The uptake of [3H]DPPC and of HPTS increased with time of incubation and concentration, decreased with the size of the vesicles used, and was stimulated by 8-bromo-cAMP and partially inhibited by hypertonic sucrose. However, the amount of HPTS uptake was approximately 100 times smaller than that of [3H]DPPC. This large difference was due to a more rapid regurgitation of some of the HPTS from the cells but not to leakage from the surfactant before uptake. The acidification of the internalized surfactant increased linearly over 90 min to 7.13, and after 24 h, a pH of 6.83 was measured. In conclusion, after internalization of a double-labeled natural surfactant, the lipid moieties were accumulated in relation to the anions, which were targeted to a compartment not very acidic and in part rapidly expelled from the cells.

Alveolar macrophages, surfactant lipids, and surfactant protein B regulate the induction of immune responses via the airways

The influences of alveolar macrophages (AM) and pulmonary surfactant on the induction of immune responses via the airways were assessed. Mice were depleted of their AM by intratracheal instillation of multilamellar vesicles containing dichloromethylene-diphosphonate followed by intratracheal instillation of a T cell--dependent antigen, trinitrophenyl--keyhole limpet hemocyanin, in vesicles of various compositions. The primary immune response was determined in the spleen of these animals using an ELI-Spot assay. The secondary immune responses in the sera of the mice were assessed using enzyme-linked immunosorbent assays. An immune response was detected in animals depleted of their AM and intratracheally instilled with antigen in small unilamellar vesicles consisting of either phosphatidylcholine cholesterol or surfactant lipids. Incorporation of surfactant protein (SP)-B in the antigen vesicles enhanced the immune response, whereas SP-A or SP-C in the antigen vesicle did not have an effect. Strikingly, intratracheal instillation of SP-B containing antigen vesicles can induce an immunoglobulin M immune response in mice without depletion of AM. These results indicate that SP-B containing vesicles can enhance the induction of immune responses via the airways and further illustrate the important roles of both AM and pulmonary surfactant in the pulmonary immune system.

Role of pulmonary surfactant components in surface film formation and dynamics

Pulmonary surfactant is a mixture of lipids and proteins which is secreted by the epithelial type II cells into the alveolar space. Its main function is to reduce the surface tension at the air/liquid interface in the lung. This is achieved by forming a surface film that consists of a monolayer which is highly enriched in dipalmitoylphosphatidylcholine and bilayer lipid/protein structures closely attached to it. The molecular mechanisms of film formation and of film adaptation to surface changes during breathing in order to remain a low surface tension at the interface, are unknown. The results of several model systems give indications for the role of the surfactant proteins and lipids in these processes. In this review, we describe and compare the model systems that are used for this purpose and the progress that has been made. Despite some conflicting results using different techniques, we conclude that surfactant protein B (SP-B) plays the major role in adsorption of new material into the interface during inspiration. SP-C's main functions are to exclude non-DPPC lipids from the interface during expiration and to attach the bilayer structures to the lipid monolayer. Surfactant protein A (SP-A) appears to promote most of SP-B's functions. We describe a model proposing that SP-A and SP-B create DPPC enriched domains which can readily be adsorbed to create a DPPC-rich monolayer at the interface. Further enrichment in DPPC is achieved by selective desorption of non-DPPC lipids during repetitive breathing cycles.

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.

Pulmonary surfactant inhibits LPS-induced nitric oxide production by alveolar macrophages

The objectives of this investigation were 1) to report that pulmonary surfactant inhibits lipopolysaccharide (LPS)-induced nitric oxide (. NO) production by rat alveolar macrophages, 2) to study possible mechanisms for this effect, and 3) to determine which surfactant component(s) is responsible. NO produced by the cells in response to LPS is due to an inducible. NO synthase (iNOS). Surfactant inhibits LPS-induced. NO formation in a concentration-dependent manner;. NO production is inhibited by approximately 50 and approximately 75% at surfactant levels of 100 and 200 microg phospholipid/ml, respectively. The inhibition is not due to surfactant interference with the interaction of LPS with the cells or to disruption of the formation of iNOS mRNA. Also, surfactant does not seem to reduce. NO formation by directly affecting iNOS activity or by acting as an antioxidant or radical scavenger. However, in the presence of surfactant, there is an approximately 80% reduction in the amount of LPS-induced iNOS protein in the cells. LPS-induced. NO production is inhibited by Survanta, a surfactant preparation used in replacement therapy, as well as by natural surfactant. NO formation is not affected by the major lipid components of surfactant or by two surfactant-associated proteins, surfactant protein (SP) A or SP-C. However, the hydrophobic SP-B inhibits. NO formation in a concentration-dependent manner;. NO production is inhibited by approximately 50 and approximately 90% at SP-B levels of 1-2 and 10 microgram/ml, respectively. These results show that lung surfactant inhibits LPS-induced. NO production by alveolar macrophages, that the effect is due to a reduction in iNOS protein levels, and that the surfactant component responsible for the reduction is SP-B.

Interactions of surfactant protein A with epithelial cells and phagocytes

Surfactant protein A (SP-A) has been shown to bind to and regulate the functions of both alveolar type II cells and immune cells including alveolar macrophages. The interaction of SP-A with type II cells has been shown in vitro to inhibit lipid secretion and to promote the uptake of lipid by these cells and these observations led to the hypothesis that SP-A plays an important role in regulating surfactant turnover and metabolism. The finding that mice made deficient in SP-A by homologous recombination (SP-A -/- mice) have relatively normal surfactant pool sizes has raised the possibility that either redundant mechanisms function in vivo to keep pool sizes normal in the absence of SP-A or that the in vitro findings are not significant in the context of the whole, unstressed animal. The interaction of SP-A with immune cells has been shown to affect a variety of responses which, in general, function to promote host defense against infection. Although SP-A receptors have been identified, additional studies will be required to elucidate the mechanism of interaction of SP-A with these cells and the relative importance of the different receptors in SP-A mediated regulation of cell function.

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.

Regulation and function of pulmonary surfactant protein B

Pulmonary surfactant, a complex mixture of phospholipids and specific associated proteins, reduces the surface tension at the air-liquid interface of the distal conducting airways and gas exchanging alveoli of the lung. Lipids, primarily neutral and phospholipids, compose approximately 90% of the surfactant complex. The remaining 10% of surfactant is composed of at least three surfactant-specific proteins, designated surfactant protein A (SP-A), SP-B, and SP-C. These proteins contribute to the formation, stabilization, and function of organized surfactant structures. This article briefly reviews the normal composition and function of pulmonary surfactant and specifically reviews the structure, function, and regulation of surfactant protein B (SP-B). The recent identification of neonates with refractory respiratory failure due to a genetic absence of SP-B and the study of transgenic mice in which SP-B gene expression has been ablated highlight the importance of the protein to surfactant function, synthesis, and metabolism and to the maintenance of lung function. Gene reconstitution experiments in vitro and in SP-B-deficient transgenic mice suggest specific functions for the amino and carboxyl terminal domains of the protein. SP-B deficiency is a potential target for gene therapy in human patients.

Clearance of SP-C and recombinant SP-C in vivo and in vitro

Surfactant protein (SP) C metabolism was evaluated in vivo by measurements of the clearance of bovine native SP-C (nSP-C) and a recombinant SP-C (rSP-C) in rabbits and mice and in vitro by the uptake into MLE-12 cells. rSP-C is the 34-amino acid human sequence with phenylalanine instead of cysteine in positions 4 and 5 and isoleucine instead of methionine in position 32. Alveolar clearances of iodinated SP-C and rSP-C after tracheal instillation were similar and slower than those for dipalmitoyl phosphatidylcholine (DPC) in the rabbit. nSP-C and rSP-C were cleared from rabbit lungs similarly to DPC, each with a half-life (t1/2) of approximately 11 h. In mice, the clearance of rSP-C from the lungs was slower (t1/2 28 h) than the clearance of DPC (t1/2 12 h). Liposome-associated dinitrophenyl-labeled rSP-C was taken up by MLE-12 cells, and the uptake was inhibited by excess nSP-C. The pattern of inhibition of dinitrophenyl-rSP-C uptake by SP-B, but not by SP-A, was similar to that previously reported for nSP-C. Clearance kinetics of nSP-C were similar to previous measurements of pulmonary clearance of SP-B in rabbits and mice. rSP-C has clearance kinetics and uptake by cells similar to those of nSP-C.

Influence of blood constituents on uptake of a lipid-extracted natural surfactant by alveolar type II cells

During lung injury, blood constituents may leak into the alveolar space and impair surfactant function. This study investigated possible interferences with the alveolar surfactant life cycle, by assessing the effects of various blood constituents on size, state of aggregation, and uptake of a natural, lipid-extracted bovine surfactant preparation (Alveofact) into isolated rat type II cells in primary culture. The results showed that plasma, serum, albumin, immunoglobulin G, bilirubin, and galactose inhibited uptake according to concentration. Fibrinogen and transferrin enhanced uptake; hemoglobin, fibronectin, and vitronectin had no effect. Uptake was also impaired when the blood constituents were washed off and the surfactant was added afterward. For some of the blood constituents, a direct interaction with the surfactant liposomes was observed, resulting in changes of liposome size and state of aggregation. However, no correlation between these changes and effects on uptake were found. During lung injury with increased permeability edema, a direct inhibition of surfactant lipid uptake by blood constituents might lead to a reduced delivery of surfactant components into type II cells and disturb surfactant metabolism.

Pulmonary surfactant inhibits cationic liposome-mediated gene delivery to respiratory epithelial cells in vitro

Cationic lipid-mediated transfection of the alveolar epithelium in vivo will require exposure of plasmid DNA and cationic lipids to endogenous surfactant lipids and proteins in the alveolar space. Effects of pulmonary surfactant and of surfactant constituents on transfection in vitro of two respiratory epithelial cell lines (MLE-15 and H441) with a plasmid encoding the luciferase reporter gene were studied using two cationic lipid formulations: 1,2-dimyristyloxypropyl-3-dimethyl-hydroxyethyl ammonium bromide/cholesterol (DMRIE/C) and 1,2-dioleoyl-3-trimethylammonium propane/dioleoyl phosphatidylethanolamine (DOTAP/DOPE). Gene expression, as assessed by luciferase activity, decreased as increasing concentrations of natural surfactant were added to cationic lipid-DNA complexes. Incorporation of phospholipids DOPC/DOPG or surfactant proteins SP-B or SP-C in the cationic lipid formulation inhibited transfection. A fluorescent lipid mixing assay was used to determine the effects of surfactant proteins SP-B and SP-C on mixing between cationic lipid-DNA complexes and surfactant lipid vesicles. Mixing between DOPC/DOPG vesicles and cationic lipid-DNA complexes in the absence of added proteins amounted to 10-20%. Addition of SP-B or SP-C increased the mixing of DOPC/DOPG vesicles with DOTAP/DOPE-DNA complexes, but not DMRIEC-DNA complexes. These results demonstrate that pulmonary surfactant lipids and proteins inhibit transfection with cationic lipid-DNA complexes in vitro, and may therefore represent a barrier to gene transfer in the lung.

Hyaluronan production in vitro by fetal lung fibroblasts and epithelial cells exposed to surfactants of N-acetylcysteine

Fetal human lung fibroblasts and feline lung epithelial cells were exposed to either a surfactant or N-acetylcysteine in various concentrations for 24-48 hours, after which the hyaluronan concentration in the culture medium was determined. Most of the experiments showed no stimulatory effect of either artificial or natural surfactant on hyaluronan synthesis. N-acetylcysteine 5-100 mg/mL induced progressive stimulation of hyaluronan synthesis by human fetal lung fibroblasts, resulting in a maximum hyaluronan concentration six times that released by unexposed cells. A slight increase in hyaluronan synthesis was also observed after exposure of feline fetal lung epithelial cells to N-acetylcysteine 50-100 micrograms/mL.

Pulmonary surfactant: no mere paint on the alveolar wall

The gas-liquid interface within the alveolus is completely lined with a complex mixture of lipids and unique proteins termed pulmonary surfactant, which both reduces surface tension and permits it to vary directly with the radius of curvature. In this way it minimizes the work of breathing and permits alveoli of different sizes to exist in equilibrium. However, surfactant does far more in that it also controls fluid balance in the lung and appears to play a key role in host defence. Either a deficiency in surfactant or an aberrant surfactant results in atelectasis and oedema. The surfactant system is very dynamic: alveolar surfactant phosphatidylcholine, the principal component, having a half life of only a few hours, with as much as 85% being recycled. Although distortion of the alveolar type II cell is now accepted as the principal stimulus for release, much remains to be discovered of modulating factors and intracellular signalling in the control of surfactant homeostasis. Likewise, many questions remain concerning the control of synthesis of the surfactant phospholipids, neutral lipids and proteins and their assembly into the tubular myelin form of alveolar surfactant, the refining of the monolayer with breathing, the control of re-uptake of different components into the type II cells and the roles of the proteins.

Interaction of phospholipid liposomes with plasma membrane isolated from alveolar type II cells: effect of pulmonary surfactant protein A

Pulmonary surfactant protein A (SP-A) augments the uptake of phospholipid liposomes containing dipalmitoylphosphatidylcholine (DPPC) by alveolar type II cells. The SP-A-mediated uptake process of lipids by type II cells have not been well understood. In the present study we investigated the SP-A-mediated interaction of phospholipids with plasma membrane isolated from alveolar type II cells. SP-A increased the amount of liposomes containing radiolabeled DPPC associated with type II cell plasma membrane by 4-fold compared to the control without SP-A when analyzed by sucrose density gradient centrifugation. This effect is dependent upon the SP-A concentration. The enhancement was inhibited by anti-SP-A antibody and EGTA. When type II cell plasma membrane and liposomes containing [14C]DPPC and [3H]triolein were coincubated with or without SP-A, analysis on sucrose density gradients revealed that the profiles of [14C]DPPC and [3H]triolein in each fraction were almost identical with or without SP-A, indicating that SP-A mediates the binding of liposomes to plasma membrane but not transfer of DPPC. SP-A increased the association of liposomes containing DPPC with the membrane by 2-fold more than that containing 1-palmitoyl-2-linoleoyl-phosphatidylcholine (PLPC). SP-A induced aggregation of phospholipid liposomes containing PLPC as well as those containing DPPC, but the final turbidity of DPPC liposomes aggregated by SP-A was only by 15% greater than that of PLPC liposomes. The amount of DPPC liposomes associated with the plasma membrane derived from type II cells was 2-fold greater than that from liver. We speculate that the SP-A-mediated interaction of lipids with type II cell plasma membrane may contribute, in part, to the lipid uptake process by type II cells.

Association of surfactant protein C with isolated alveolar type II cells

Surfactant protein C (SP-C) is a small hydrophobic protein that is synthesized and secreted by alveolar type II cells. The mechanism of clearance of SP-C from the alveolar airspace is not well understood, although previous studies demonstrated that recombinant SP-C instilled into the lungs of spontaneously breathing anaesthetized rats was taken up by type II cells and incorporated into lamellar bodies. The current investigation was undertaken to characterize the interaction of a complex of SP-C and surfactant-like lipids with freshly isolated rat alveolar type II cells under conditions in which the extracellular milieu can be regulated. SP-C was isolated from alveolar proteinosis lavage fluid and radiolabeled with 125I-Bolton-Hunter reagent. The radiolabeled protein retained its ability to facilitate adsorption of phospholipids to an air/liquid interface. Labeled human SP-C associated with isolated type II cells in a concentration-dependent manner that was also dependent upon temperature and time. The association of labeled SP-C with isolated type II cells did not saturate up to 150 micrograms/ml. SP-A significantly enhanced the association of SP-C with isolated type II cells. Under the experimental conditions tested, SP-C was not degraded to TCA-soluble products. These results are consistent with the hypothesis that association or uptake of SP-C by type II cells may be enhanced by SP-A and that like SP-A, SP-C is recycled by type II cells.

The stimulation of cellular phospholipid uptake by surfactant apoproteins

Surfactant recycling by alveolar cells is influenced by the surfactant apoproteins SP-A, -B and -C (Wright, J.R. and Dobbs, L.G. (1991) Annu. Rev. Physiol. 53, 395-414). Alveolar macrophages and type II cells, but not lung fibroblasts, were reported to accumulate surfactant phospholipid in the presence of SP-A in low calcium medium, although high affinity binding of SP-A to alveolar macrophages and type II cells showed an absolute requirement for mM calcium. SP-B, one of two very hydrophobic surfactant proteins, stimulated phospholipid uptake by type II cells and Chinese hamster lung fibroblasts suspended in Dulbecco's minimum essential medium containing mM Ca2+. We postulated that calcium influences cellular phospholipid uptake stimulated by SP-A or SP-B. We used isolated rat alveolar and peritoneal macrophages and Vero cells, an African Green Monkey kidney fibroblast cell line, and studied the effect of calcium concentrations ranging from 2 microM to 2 mM on cellular uptake of liposomes containing 3H-labeled phosphatidylcholine. For alveolar and peritoneal macrophages, increasing calcium concentration enhanced SP-A stimulation of phospholipid uptake. SP-A did not stimulate phosphatidylcholine uptake by Vero cells. SP-B stimulated phosphatidylcholine uptake by alveolar and peritoneal macrophages and Vero cells independent of the calcium concentration. These studies demonstrate that the enhancement of phospholipid uptake in alveolar and peritoneal macrophages by SP-A, but not SP-B is augmented by calcium.

Utilization of modified surfactant-associated protein B for delivery of DNA to airway cells in culture

Pulmonary surfactant lines the airway epithelium and creates a potential barrier to successful transfection of the epithelium in vivo. Based on the functional properties of pulmonary surfactant protein B (SP-B) and the fact that this protein is neither toxic nor immunogenic in the airway, we hypothesized that SP-B could be modified to deliver DNA to airway cells. We have modified native bovine SP-B by the covalent linkage of poly(lysine) (average molecular mass of 3.3 or 10 kDa) to the N terminus of SP-B and formed complexes between a test plasmid and the modified SP-B. Transfection efficiency was determined by transfection of pulmonary adenocarcinoma cells (H441) in culture with the test plasmid pCPA-RSV followed by measurement of activity of the reporter gene encoding chloramphenicol acetyltransferase (CAT). Transfections were performed with DNA.protein complexes using poly(lysine)10kDa-SP-B ([Lys]10kDa-SP-B) or poly(lysine)3.3kDa-SP-B ([Lys]3.3kDa-SP-B), and results were compared with transfections using unmodified poly(lysine).DNA, unmodified SP-B.DNA, or DNA only. For [Lys]10kDa-SP-B.pCPA-RSV preparations, CAT activity was readily detectable above the background of [Lys]3.3kDa-SP-B or unmodified SP-B. The SP-B-poly(lysine) conjugates were effective over a broad range of protein-to-DNA molar ratios, although they were optimal at approximately 500:1-1000:1. Transfection efficiency varied with the tested cell line but was not specific to airway cells. Addition of replication-defective adenovirus to the [Lys]10kDa-SP-B.pCPA-RSV complex enhanced CAT activity about 30-fold with respect to that produced by the [Lys]10kDa-SP-B.pCPA-RSV complex alone. This increase suggests routing of the adenoviral.[Lys]10kDa-SP-B.pCPA-RSV complex through an endosomal pathway. Effects of covalent modification on the secondary structure of SP-B were examined by Fourier transform infrared spectrometry (FTIR). Results of FTIR indicated that the conformation of [Lys]10kDa-SP-B was comprised primarily of alpha-helical structure compared with a predominantly aggregated structure of unmodified poly(lysine). We conclude that poly(lysine) conjugates of SP-B effectively deliver DNA in vitro and may have utility as DNA delivery vehicles to the airway in vivo.

Lipid effects on aggregation of pulmonary surfactant protein SP-C studied by fluorescence energy transfer

The self-association of pulmonary surfactant protein SP-C in lipid vesicles was studied using fluorescence energy transfer. Bovine SP-C was labeled with two fluorescent probes, succinimidyl 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoate and eosin isothiocyanate, on the amino terminus of the protein, producing NBD-SP-C and EITC-SP-C, respectively. The N-terminus of SP-C was relatively immobile between 20 and 37 degrees C, as demonstrated by high fluorescence anisotropy of NBD-SP-C and EITC-SP-C. The mobility increased at the transition of the lipid to the fluid phase. Using fluorescence energy transfer, with NBD-SP-C as the donor and EITC-SP-C as the acceptor, a high degree of SP-C/SP-C association was found below 25 degrees C, decreasing to very little self-association above 42 degrees C in 7:1 1,2-dipalmitoylphosphatidylcholine-1,2-dipalmitoylphosphatidylglycerol (DPPC-DPPG) vesicles. The fraction of SP-C aggregated below 37 degrees C in 7:1 DPPC-DPPG was estimated from the observed energy transfer to be more than 70% of total SP-C. In various lipid mixtures, self-association of SP-C was dependent on the presence of at least some gel-phase lipids. In a lipid mixture resembling pulmonary surfactant, gradually increasing self-association was observed below 38 degrees C. The relation of the present data to the state of aggregation of SP-C in pulmonary surfactant is discussed.

Phosphatidylcholine synthesis, secretion, and reutilization during differentiation of the surfactant-producing type II alveolar cell from fetal rabbit lungs

Evidence indicates that pulmonary pool sizes of choline and related intermediates available for synthesis of phosphatidylcholine, the major component of the surfactant, change during gestation. Furthermore, recent data suggest that the type II lung cells that produce the surfactant potentially can reutilize components of this material. However, the relationship of the de novo synthetic mechanism to the secretion and reutilization of phosphatidylcholine has not been established. This is particularly true in the case of the fetal lung where, although alterations in precursor pool sizes, including choline, have been demonstrated, few or no data are available concerning how phosphatidylcholine synthesis affects or is affected by secretion and reutilization of this phospholipid by fetal type II cells. The present study was undertaken to determine the effect of availability of choline on de novo synthesis of phosphatidylcholine by isolated fetal rabbit type II cells during the differentiation process. In addition, differentiating type II alveolar cells were used to examine the hypothesis that these cells incorporate phospholipid from the extracellular environment and the quantity and/or composition of this phospholipid differently affects cellular secretion or de novo phosphatidylcholine synthesis. Assuming that the cells did not discriminate between radioactive and nonradioactive choline, elevation of extracellular choline increased the synthesis of cellular phosphatidylcholine and disaturated phosphatidylcholine in a dose-dependent manner to 0.08 mM choline. Cells induced to differentiated by exposure to fibroblast-conditioned medium synthesized more total and disaturated phosphatidylcholine at all extracellular choline concentrations. Incubation of the fetal type II cells with dipalmitoylphosphatidylcholine or 1-palmitoyl-2-oleoylphosphatidylcholine significantly depressed the incorporation of [3H]choline into cellular phosphatidylcholine after 24 or 48 h, but not necessarily at both times. Dipalmitoylphosphatidylcholine depressed the secretion of [3H]choline-labeled phosphatidylcholine after incubation for 24 h. 1-Palmitoyl-2-oleoylphosphatidylcholine stimulated the secretion of tritium-labeled phosphatidylcholine at a concentration of 25 micrograms/mL after 48 h. Comparison of the phospholipid effect by incubating the cells with 50 ng of 14C-labeled phospholipid for 24 h showed that 1-palmitoyl-2-oleoylphosphatidylcholine significantly reduced the synthesis of 3H-labeled phosphatidylcholine compared to dipalmitoylphosphatidylcholine. In contrast, 1-palmitoyl-2-oleoylphosphatidylcholine stimulated secretion of 3H-labeled phosphatidylcholine compared with the disaturated moiety. The differentiation state of the cells altered the magnitude of the cellular secretion response but not the character.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of lipid vesicles with monomolecular layers containing lung surfactant proteins SP-B or SP-C

Pulmonary surfactant contains two families of hydrophobic proteins, SP-B and SP-C. Both proteins are thought to promote the formation of the phospholipid monolayer at the air-fluid interface of the lung. The Wilhelmy plate method was used to study the involvement of SP-B and SP-C in the formation of phospholipid monolayers. The proteins were either present in the phospholipid vesicles which were injected into the subphase or included in a preformed phospholipid monolayer. In agreement with earlier investigators, we found that SP-B and SP-C, present in phospholipid vesicles, were able to induce the formation of a monolayer, as became apparent by an increase in surface pressure. However, when the proteins were present in a preformed phospholipid monolayer (20 mN/m) at similar lipid to protein ratios, the rate of surface pressure increase after injection of pure phospholipid vesicles into the subphase at similar vesicle concentrations was 10 times higher. The process of phospholipid insertion from phospholipid vesicles into the protein-containing monolayers was dependent on (1) the presence of (divalent) cations, (2) the phospholipid concentration in the subphase, (3) the size of the phospholipid vesicles, (4) the protein concentration in the preformed monolayer, and (5) the initial surface pressure at which the monolayers were formed. Both in vesicles and in preformed monolayers, SP-C was less active than SP-B in promoting the formation of a phospholipid monolayer. The use of preformed monolayers containing controlled protein concentrations may allow more detailed studies on the mechanism by which the proteins enhance phospholipid monolayer formation from vesicles.

Increased expression of pulmonary surfactant proteins in oxygen-exposed rats

Exposure of adult rats to 85% ambient oxygen increased the content of surfactant proteins SP-A, SP-B, and SP-C recovered from alveolar lavage. The surfactant proteins increased during 1 to 7 d of oxygen exposure. The increased surfactant protein was associated with increased relative abundance of mRNA encoding each of the proteins in lung tissue. Exposure to hyperoxia progressively increased the amounts of the surfactant proteins in alveolar lavage fluid as estimated by immunoblot analysis after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The mRNAs encoding SP-A (1.7 and 1.0 kb), SP-B (1.6 kb), and SP-C (0.9 kb) increased significantly after oxygen exposure for 5 d. The present findings support the concept that oxygen exposure mediates surfactant protein expression at a pretranslational level.

Tumor necrosis factor-alpha inhibits expression of pulmonary surfactant protein

Tumor necrosis factor-alpha (TNF-alpha) decreased the expression of pulmonary surfactant proteins SP-A and SP-B in human pulmonary adenocarcinoma cell lines. The effect of TNF alpha on SP-A content and mRNA in the pulmonary adenocarcinoma cell line, H441-4, was concentration and time dependent. TNF alpha decreased the cellular content of SP-A to less than 10% of control 48 h after addition. TNF alpha decreased de novo synthesis of SP-A and decreased the accumulation of SP-A in media. SP-A mRNA was decreased within 12 h of addition of TNF alpha, with nearly complete loss of SP-A mRNA observed after 24 h. Inhibitory effects of TNF alpha on SP-A mRNA were dose-related with nearly complete inhibition of SP-A mRNA caused by 25 ng/ml TNF alpha. The effects of TNF alpha on SP-A were distinct from the effects of interferon gamma which increased SP-A content approximately twofold in H441-4 cells. TNF alpha also decreased the content of SP-B mRNA. In contrast to the inhibitory effect of TNF alpha on SP-A and SP-B mRNA, TNF alpha increased mRNA encoding human manganese superoxide dismutase (Mn-SOD). TNF alpha did not inhibit growth, alter cell viability or beta-actin mRNA in either cell line. These in vitro studies demonstrate the marked pretranslational inhibitory effects of the cytokine, TNF alpha, on the expression of pulmonary surfactant proteins, SP-A and SP-B. The results support the concept that macrophage-derived cytokines may control surfactant protein expression.