Reutilization of surfactant phosphatidylcholine in adult rabbits

Metabolism of a synthetic compared with a natural therapeutic pulmonary surfactant in adult mice

Secreted pulmonary surfactant phosphatidylcholine (PC) has a complex intra-alveolar metabolism that involves uptake and recycling by alveolar type II epithelial cells, catabolism by alveolar macrophages, and loss up the bronchial tree. We compared the in vivo metabolism of animal-derived poractant alfa (Curosurf) and a synthetic surfactant (CHF5633) in adult male C57BL/6 mice. The mice were dosed intranasally with either surfactant (80 mg/kg body weight) containing universally ¹³C-labeled dipalmitoyl PC (DPPC) as a tracer. The loss of [U¹³C]DPPC from bronchoalveolar lavage and lung parenchyma, together with the incorporation of ¹³C-hydrolysis fragments into new PC molecular species, was monitored by electrospray ionization tandem mass spectrometry. The catabolism of CHF5633 was considerably delayed compared with poractant alfa, the hydrolysis products of which were cleared more rapidly. There was no selective resynthesis of DPPC and, strikingly, acyl remodeling resulted in preferential synthesis of polyunsaturated PC species. In conclusion, both surfactants were metabolized by similar pathways, but the slower catabolism of CHF5633 resulted in longer residence time in the airways and enhanced recycling of its hydrolysis products into new PC species.

Estimating the contribution of surfactant replacement therapy to the alveolar pool: An in vivo study based on 13 C natural abundance in rabbits

Variation of the isotopic abundance of selected nutrients and molecules has been used for pharmacological and kinetics studies under the premise that the administered molecule has a different isotopic enrichment from the isotopic background of the recipient subject. The aim of this study is to test the feasibility of assessing the contribution of exogenous surfactant phospholipids to the endogenous alveolar pool in vivo after exogenous surfactant replacement therapy in rabbits. The study consisted in measuring the consistency of ¹³ C/¹² C ratio of disaturated-phosphatidylcholine palmitate (DSPC-PA) in 7 lots of poractant alfa, produced over a year, and among bronchoalveolar lavages of 20 rabbits fed with a standard chow. A pilot study was performed in a rabbit model of lavage-induced surfactant deficiency: 7 control rabbits and 4 treated with exogenous surfactant. The contribution of exogenous surfactant to the alveolar pool was assessed after intra-tracheal administration of 200 mg/kg of poractant alfa. The ¹³ C content of DSPC-PA was measured by isotope ratio mass spectrometry. The mean DSPC-PA ¹³ C/¹² C ratio of the 7 lots of poractant alfa was -18.8‰ with a SD of 0.1‰ (range: -18.9‰; -18.6‰). The mean ¹³ C/¹² C ratio of surfactant DSPC recovered from the lung lavage of 20 rabbits was -28.8 ± 1.2‰ (range: -31.7‰; -25.7‰). The contribution of exogenous surfactant to the total alveolar surfactant could be calculated in the treated rabbits, and it ranged from 83.9% to 89.6%. This pilot study describes a novel method to measure the contribution of the exogenous surfactant to the alveolar pool. This method is based on the natural variation of ¹³ C, and therefore it does not require the use of chemically synthetized tracers. This method could be useful in human research and especially in surfactant replacement studies in preterm infants.

Neonatal Respiratory Diseases in the Newborn Infant: Novel Insights from Stable Isotope Tracer Studies

Respiratory distress syndrome is a common problem in preterm infants and the etiology is multifactorial. Lung underdevelopment, lung hypoplasia, abnormal lung water metabolism, inflammation, and pulmonary surfactant deficiency or disfunction play a variable role in the pathogenesis of respiratory distress syndrome. High-quality exogenous surfactant replacement studies and studies on surfactant metabolism are available; however, the contribution of surfactant deficiency, alteration or dysfunction in selected neonatal lung conditions is not fully understood. In this article, we describe a series of studies made by applying stable isotope tracers to the study of surfactant metabolism and lung water. In a first set of studies, which we call 'endogenous studies', using stable isotope-labelled intravenous surfactant precursors, we showed the feasibility of measuring surfactant synthesis and kinetics in infants using several metabolic precursors including plasma glucose, plasma fatty acids and body water. In a second set of studies, named 'exogenous studies', using stable isotope-labelled phosphatidylcholine tracer given endotracheally, we could estimate surfactant disaturated phosphatidylcholine pool size and half-life. Very recent studies are focusing on lung water and on the endogenous biosynthesis of the surfactant-specific proteins. Information obtained from these studies in infants will help to better tailor exogenous surfactant treatment in neonatal lung diseases.

Surfactant phospholipid metabolism

Pulmonary surfactant is essential for life and is composed of a complex lipoprotein-like mixture that lines the inner surface of the lung to prevent alveolar collapse at the end of expiration. The molecular composition of surfactant depends on highly integrated and regulated processes involving its biosynthesis, remodeling, degradation, and intracellular trafficking. Despite its multicomponent composition, the study of surfactant phospholipid metabolism has focused on two predominant components, disaturated phosphatidylcholine that confers surface-tension lowering activities, and phosphatidylglycerol, recently implicated in innate immune defense. Future studies providing a better understanding of the molecular control and physiological relevance of minor surfactant lipid components are needed. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Pulmonary surfactant kinetics of the newborn infant: novel insights from studies with stable isotopes

Deficiency or dysfunction of the pulmonary surfactant plays a critical role in the pathogenesis of respiratory diseases of the newborn. After a short review of the pulmonary surfactant, including its role in selected neonatal respiratory conditions, we describe a series of studies conducted by applying two recently developed methods to measure surfactant kinetics. In the first set of studies, namely 'endogenous studies', which used stable isotope-labeled intravenous surfactant precursors, we have shown the feasibility of measuring surfactant synthesis and kinetics in infants using several metabolic precursors, including plasma glucose, plasma fatty acids and body water. In the second set of studies, namely 'exogenous studies', which used a stable isotope-labeled phosphatidylcholine (PC) tracer given endotracheally, we estimated the surfactant disaturated phosphatidylcholine (DSPC) pool size and half-life. The major findings of our studies are presented here and can be summarized as follows: (a) the de novo synthesis and turnover rates of the surfactant (DSPC) in preterm infants with respiratory distress syndrome (RDS) are very low with either precursor; (b) in preterm infants with RDS, pool size is very small and half-life much longer than what has been reported in animal studies; (c) patients recovering from RDS who required higher continuous positive airway pressure pressure after extubation or reintubation have a lower level of intrapulmonary surfactant than those who did well after extubation; (d) term newborn infants with pneumonia have greatly accelerated surfactant catabolism; and (e) infants with uncomplicated congenital diaphragmatic hernia (CDH) and on conventional mechanical ventilation have normal surfactant synthesis, but those requiring extracorporeal membrane oxygenated (ECMO) do not. Information obtained from these studies in infants will help to better tailor exogenous surfactant treatment in neonatal lung diseases.

Inherited surfactant protein-B deficiency and surfactant protein-C associated disease: clinical features and evaluation

The pulmonary surfactant is a mixture of phospholipids and proteins synthesized, packaged, and secreted by alveolar type II cells that lowers surface tension and prevents atelectasis at end-expiration. A tightly regulated, complex metabolic cycle involves all components of the pulmonary surfactant. Disorders of surfactant metabolism that have a genetic basis are rare, but causes of respiratory dysfunction in infants and children emerge. Recessive loss of function mutations in surfactant protein-B (SP-B) gene lead to respiratory failure that is lethal in the newborn period while single allelic mutations in the surfactant protein-C (SP-C) gene cause interstitial lung disease of varying severity and age of onset. The genetic basis, mechanisms, clinical presentation and outcome, diagnostic approach and limited therapeutic options for disease due to mutations the SP-B and SP-C genes will be described in detail in this article. These disorders provide insights into some of the distinct mechanisms that disrupt the surfactant metabolic cycle and cause respiratory disease in infants and children.

Surfactant metabolism in the neonate

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

Role of clathrin- and actin-dependent endocytotic pathways in lung phospholipid uptake

We evaluated the contribution of endocytotic pathways to pulmonary uptake of surfactant lipids from the alveolar space. Resting and stimulated 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) uptake of unilamellar liposomes labeled with either [(3)H]dipalmitoylphosphatidylcholine ([(3)H]DPPC) or 1-palmitoyl-2-[12-(7-nitro-2-1,3-benzoxadiazol-4-yl) amino] dodecanoyl-phosphatidylcholine (NBD-PC) was studied in isolated perfused rat lungs and isolated type II cells. Amantadine and phenylarsine oxide, inhibitors of clathrin-mediated endocytosis, each decreased [(3)H]DPPC uptake under resting conditions by approximately 40%; their combination had no additional effect. Cytochalasin D, an inhibitor of actin-dependent processes, reduced liposome uptake by 55% and potentiated the effect of either clathrin inhibitor alone. Relative inhibition for all agents was higher in the presence of 8-Br-cAMP. The effect of inhibitors was similar for liposomes labeled with [(3)H]DPPC or NBD-PC. By fluorescence microscopy, NBD-PC taken up by lungs was localized primarily to alveolar type II cells and was localized to lamellar bodies in both lungs and isolated cells. These studies indicate that both clathrin-mediated and actin-mediated pathways are responsible for endocytosis of DPPC-labeled liposomes by alveolar type II cells in the intact lung.

Effect of exogenous surfactant on the development of surfactant synthesis in premature rabbit lung

Surfactant replacement is an effective therapy for neonatal respiratory distress syndrome. Full recovery from respiratory distress syndrome requires development of endogenous surfactant synthesis and metabolism. The influence of exogenous surfactant on the development of surfactant synthesis in premature lungs is not known. We hypothesized that different exogenous surfactants have different effects on the development of endogenous surfactant production in the premature lung. We treated organ cultures of d 25 fetal rabbit lung for 3 d with 100 mg/kg body weight of natural rabbit surfactant, Survanta, and Exosurf and measured their effects on the development of surfactant synthesis. Additional experiments tested how these surfactants and Curosurf affected surfactant protein (SP) SP-A, SP-B, and SP-C mRNA expression. Surfactant synthesis was measured as the incorporation of 3H-choline and 14C-glycerol into disaturated phosphatidylcholine recovered from lamellar bodies. Randomized-block ANOVA showed significant differences among treatments for incorporation of both labels (p < 0.01), with natural rabbit surfactant less than control, Survanta greater than control, and Exosurf unchanged. Additional experiments with natural rabbit surfactant alone showed no significant effects in doses up to 1000 mg/kg. Survanta stimulated disaturated phosphatidylcholine synthesis (173 +/- 41% of control; p = 0.01), increased total lamellar body disaturated phosphatidylcholine by 22% (p < 0.05), and increased 14C-disat-PC specific activity by 35% (p < 0.05). The response to Survanta was dose-dependent up to 1000 mg/kg. Survanta did not affect surfactant release. No surfactant altered the expression of mRNA for SP-A, SP-B, or SP-C. We conclude that surfactant replacement therapy can enhance the maturation of surfactant synthesis, but this potential benefit differs with different surfactant preparations.

Biology of surfactant

Surfactant is a metabolically active assembly of phospholipids and surfactant-specific proteins that is essential for normal lung mechanics. The surfactant proteins SP-A and SP-D also have important innate host defense functions. Surfactant metabolism in the developing lung differs from the adult lung by having slower kinetics of secretion with a longer half-life and more efficient recycling. Ventilation styles that injure the lung also result in altered surfactant function.

The pulmonary collectins and surfactant metabolism

Lung surfactant covers and stabilizes a large, delicate surface at the interface between the host and the environment. The surfactant system is placed at risk by a number of environmental challenges such as inflammation, infection, or oxidant stress, and perhaps not surprisingly, it demonstrates adaptive changes in metabolism in response to alterations in the alveolar microenvironment. Recent experiments have shown that certain components of the surfactant system are active participants in the regulation of the alveolar response to a wide variety of environmental challenges. These components are capable not only of maintaining a low interfacial surface tension but also of amplifying or dampening inflammatory responses. These observations suggest that regulatory molecules are capable of both sensing the environment of the alveolus and providing feedback to the cells regulating surfactant synthesis, secretion, alveolar conversion, and clearance. In this review we examine the evidence from in vitro systems and gene-targeted mice that two surfactant-associated collectins (SP-A and SP-D) may serve in these roles and help modify surfactant homeostasis as part of a coordinated host response to environmental challenges.

Surfactant metabolism in SP-D gene-targeted mice

Mice with surfactant protein (SP)-D deficiency have three to four times more surfactant lipids in air spaces and lung tissue than control mice. We measured multiple aspects of surfactant metabolism and function to identify abnormalities resulting from SP-D deficiency. Relative to saturated phosphatidylcholine (Sat PC), SP-A and SP-C were decreased in the alveolar surfactant and the large-aggregate surfactant fraction. Although large-aggregate surfactant from SP-D gene-targeted [(-/-)] mice converted to small-aggregate surfactant more rapidly, surface tension values were comparable to values for surfactant from SP-D wild-type [(+/+)] mice. (125)I-SP-D was cleared with a half-life of 7 h from SP-D(-/-) mice vs. 13 h in SP-D(+/+) mice. Although initial incorporation and secretion rates for [(3)H]palmitic acid and [(14)C]choline into Sat PC were similar, the labeled Sat PC was lost from the lungs of SP-D(+/+) mice more rapidly than from SP-D(-/-) mice. Clearance rates of intratracheal [(3)H]dipalmitoylphosphatidylcholine were used to estimate net clearances of Sat PC, which were approximately threefold higher for alveolar and total lung Sat PC in SP-D(-/-) mice than in SP-D(+/+) mice. SP-D deficiency results in multiple abnormalities in surfactant forms and metabolism that cannot be attributed to a single mechanism.

Use of stable isotope labeling technique and mass isotopomer distribution analysis of [(13)C]palmitate isolated from surfactant disaturated phospholipids to study surfactant in vivo kinetics in a premature infant

Pulmonary surfactant is a complex mixture of phospholipids and proteins which lowers surface tension and maintains alveolar expansion at end expiration. Developmental and genetic disruption of pulmonary surfactant metabolism leads to respiratory distress in newborns. Stable isotope labeling of metabolic precursors of disaturated phospholipids, the most abundant and specific component of pulmonary surfactant, permits the measurement of the kinetics of surfactant metabolism in vivo. We measured [U-(13)C(6)]glucose incorporation into palmitic acid derived from disaturated surfactant phospholipids. A 24 h infusion of [U-(13)C(6)]glucose (140 mg kg(-1)) was administered to a premature infant who required mechanical ventilation for respiratory distress syndrome; tracheal aspirate samples were obtained at the start of the infusion and at regular intervals for the next 70 h. Each tracheal aspirate sample was incubated with osmium tetroxide to isolate disaturated surfactant phospholipids. Methyl esters of the fatty acids in the disaturated phospholipids were prepared and the enrichment of [(13)C]methyl palmitate was measured by gas chromatography/mass spectrometry (GC/MS) and gas chromatography/combination/isotope ratio mass spectrometry (GC/C/IRMS). Mass isotopomer distribution analysis (MIDA) was used to calculate the fractional synthetic rate (FSR) of palmitate synthesized from acetate. With both GC/MS and GC/C/IRMS, palmitate (13)C enrichment was first detected 12.3 h after the start of the tracer infusion. The enrichment increased in a linear fashion, reached a peak at 47 h and remained constant in the remainder of the samples. The FSR of palmitate from acetate was 5.2% per day. Stable isotope techniques and MIDA will provide insights into the kinetics of surfactant metabolism in newborns with respiratory dysfunction.

Site-directed mutagenesis of surfactant protein A reveals dissociation of lipid aggregation and lipid uptake by alveolar type II cells

Surfactant protein A (SP-A) binds to dipalmitoylphosphatidylcholine (DPPC) and induces phospholipid vesicle aggregation. It also regulates the uptake and secretion of surfactant lipids by alveolar type II cells. We introduced the single mutations Glu195-->Gln (rE195Q), Lys201-->Ala (rK201A) and Lys203-->Ala (rK203A) for rat SP-A, Arg199-->Ala (hR199A) and Lys201-->Ala (hK201A) for human SP-A, and the triple mutations Arg197, Lys201 and Lys203-->Ala (rR197A/K201A/K203A) for rat SP-A, into cDNAs for SP-A, and expressed the recombinant proteins using baculovirus vectors. All recombinant proteins avidly bound to DPPC liposomes. rE195Q, rK201A, rK203A, hR199A and hK201A function with activity comparable to wild type SP-A. Although rR197A/K201A/K203A was a potent inducer of phospholipid vesicle aggregation, it failed to stimulate lipid uptake. rR197A/K201A/K203A was a weak inhibitor for lipid secretion and did not competed with rat [125I]SP-A for receptor occupancy. From these results, we conclude that Lys201 and Lys203 of rat SP-A, and Arg199 and Lys201 of human SP-A are not individually critical for the interaction with lipids and type II cells, and that Glu195 of rat SP-A can be replaced with Gln without loss of SP-A functions. This study also demonstrates that the SP-A-mediated lipid uptake is not directly correlated with phospholipid vesicle aggregation, and that specific interactions of SP-A with type II cells are involved in the lipid uptake process.

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.

Intracellular distribution of lysozyme in rat alveolar type II epithelial cells

This study investigated the intracellular distribution of lysozyme, a protein that is synthesized and secreted by rat alveolar type II epithelial (ATII) cells and alveolar macrophages, using a polyclonal antibody generated against purified rat lysozyme. Lysozyme was immunoprecipitated with this antibody from Triton X-100 lysates of ATII cells cultured on a basement membrane derived from Englebreth-Holme-Swarm mouse sarcoma (EHS) and radiolabeled with 35S-methionine. ATII cells cultured on EHS basement membrane for several days were fixed and labeled with antibodies to surfactant apoprotein A (SP-A) and lgp-120 (a lysosomal glycoprotein), or lysozyme and lgp-120, and studied by confocal microscopy. Organelles were identified that stained positively for either anti-lysozyme or anti-lgp-120; a second population of organelles contained both markers. Similarly, two populations of SP-A-containing organelles were identified; one contained the lysosomal glycoprotein lgp-120. In addition, confocal images demonstrated that both SP-A and lysozyme were secreted by ATII cells, as evidenced by the accumulation of secretory products within the lumen of the cyst-like aggregates. When the subcellular localization of SP-A and lysozyme was studied by analytical cell fractionation, two populations of organelles were identified that contained SP-A or lysozyme. The lighter population accounted for approximately 32% of SP-A and 33% of total intracellular lysozyme and was recovered in the same region of the gradient as secretory lamellar bodies. The more dense population co-localized with lysosomes and accounted for approximately 67% of both SP-A and lysozyme recovered. Western blots of cell fractions revealed intact lysozyme in all the cell fractions. The results of these experiments suggest that lysozyme has a similar intracellular distribution as surfactant apoprotein A in ATII cells. Lysozyme is found in fractions containing lamellar bodies where it is packaged for secretion, and in lysosomal fractions where it may undergo degradation.

A competitive inhibitor of phospholipase A2 decreases surfactant phosphatidylcholine degradation by the rat lung

We have shown previously that radiolabelled phosphatidylcholine (PC) in liposomes or natural surfactant is removed from the alveolar space and metabolically recycled in a process that is stimulated by cyclic AMP (cAMP). In this study, we evaluated the effect of a transition-state phospholipid analogue (MJ33; 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol) that competitively inhibited acidic phospholipase A2 (PLA2) activity (pH 4.0) of lung homogenate by more than 97%, but had no effect on PLA2 activity at pH 8.5. MJ33 incorporated into unilamellar liposomes (dipalmitoyl PC/egg PC/cholesterol/phosphatidylglycerol, molar proportions 10:5:3:2) or co-sonicated with biosynthesized natural surfactant was instilled into the trachea of the anaesthetized rat; lungs were then removed for 2 h perfusion in the absence or presence of 0.1 mM-8-bromo cAMP. Total uptake for phospholipid was unchanged in the presence of the inhibitor MJ33. Degradation of labelled PC during 2 h perfusion in the absence of MJ33 was approx. 26% of that instilled for choline-labelled liposomal PC, 16% for liposomal PC labelled in the second fatty-acyl position, and 33% for choline-labelled natural surfactant. Degradation of PC was decreased by approx. 25-40% for each substrate in the presence of MJ33. Inhibition of lipid degradation depended on the mole fraction of MJ33 in the liposomes and was maximal at 1 mol%. These studies demonstrate a significant role for acidic Ca(2+)-independent PLA2 in the degradation of internalized alveolar PC, but further indicate that this enzyme accounts for a minor fraction of total lung PC metabolism.

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)

Temporal segregation of surfactant secretion and lamellar body biogenesis in primary cultures of rat alveolar type II cells

Pulmonary alveolar type II cells synthesize and secrete the phospholipids of surfactant. However, type II cells isolated from adult rat lungs rapidly lose their characteristic morphology and differentiated functions (such as surfactant-specific phospholipid and protein biosynthesis) when maintained on tissue culture plastic. In this study, phospholipid secretion and its regulation by type II cells grown on tissue culture plastic were examined up to 8 days after isolation. Type II cells were preincubated with [3H]choline for varying 24-h periods during culture prior to examining phosphatidylcholine ([3H]PtdCho) secretion. Type II cells cultured for 4 days and incubated with [3H]choline 24 h before the secretion experiment failed to show significant basal and tetradecanoyl phorbol acetate (TPA, 100 nM)-stimulated [3H]PtdCho secretion (basal, 0.29 +/- 0.01%; TPA, 0.48 +/- 0.04%). In contrast, type II cells incubated with [3H]choline for the first 24 h during culture and then cultured for 3 more days showed significant [3H]PtdCho secretion (basal, 1.27 +/- 0.19%; TPA, 6.24 +/- 0.82%). Subcellular fractionation of type II cells revealed that [3H]choline was incorporated into phosphatidylcholines in a lamellar body-enriched fraction during the first 24 h of culture but that the assimilation of phosphatidylcholine into the lamellar body fraction progressively declined with increasing time in culture. Radiolabel incorporated into the lamellar body fraction labeled during the first 24 h of culture was detectable for up to 8 days in culture. The [3H]PtdCho incorporated into the lamellar body during the first 24 h of culture was lost gradually over 8 days, suggesting the continuous secretion or turnover of the lamellar bodies during culture.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between lamellar bodies and lysosomes in type II pneumocytes

We have studied the relationship between lysosomes and lamellar bodies in alveolar type II (ATII) pneumocytes using a monoclonal antibody (anti-lgp-120) directed against a 120-kD rat lysosomal membrane glycoprotein and a polyclonal antibody (anti-SP-A) directed against rat surfactant protein A. The anti-lgp-120 precipitated a protein molecular mass of 120 kD from Triton cell lysates radiolabeled with [35S]methionine, and the anti-SP-A precipitated surfactant apoprotein A from the medium when analyzed under similar conditions. When ATII cells were cultured on Engelbreth-Holm-Swarm tumor basement membrane, and studied by indirect immunofluorescence, some structures seem to react with both antibodies, and others with only one. ATII cells cultured on plastic showed a major population of large vesicles that were labeled intensely with both antibodies, and a second population of vesicles that were labeled weakly and only with anti-SP-A. Analytical cell fractionation of freshly isolated ATII cells confirmed that lgp-120 was only present in structures containing the lysosomal matrix enzyme N-acetyl-beta-glucosaminidase. In contrast, SP-A was identified in two populations of vesicles with high phospholipid-to-protein ratios: one lacked N-acetyl-beta-glucosaminidase and lgp-120 and contained lamellar bodies; the other contained both lysosomal markers and a heterogeneous population of organelles that included multivesicular bodies, lamellar bodies, and lysosomes. Western blots of trichloroacetic acid precipitates of cell fractions identified proteins within the lysosomal compartment that reacted with anti-SP-A, but whose molecular mass was less than 28 kD. The results indicate that, in ATII cells, surfactant is located in two functionally distinct structures, one of which is probably involved in surfactant secretion, and the other, surfactant degradation. The techniques developed in this study should allow the role of these structures in the secretion and recycling of surfactant to be determined.

Alveolar type II cells isolated after silica-induced lung injury in rats have increased surfactant protein A (SP-A) receptor activity

We examined surfactant secretion and its regulation by surfactant protein A (SP-A) in alveolar type II cells isolated from silica-treated rats to determine the role of SP-A-mediated regulatory control of phospholipid secretion in the pathogenesis of silica-induced alveolar proteinosis. Type II cells were isolated at weekly intervals for 28 d after silica or saline instillation. The maximum total binding of [125I]SP-A (internalized and surface-bound SP-A) to type II cells increased with time after silica instillation and, at 21 d, was 4-fold greater than that of type II cells isolated from saline-treated rats (272.8 +/- 42.5 and 65.4 +/- 9.8 ng/10(5) cells, respectively; P less than 0.05). Type II cells isolated from silica-treated rats showed a 2-fold increased surface binding and a 3-fold increased internalization compared to control cells. The receptor affinity for SP-A was the same for type II cells isolated from silica- and saline-treated animals. Type II cells isolated 14 d after silica instillation were separated into normotrophic and hypertrophic populations by centrifugal elutriation. Hypertrophic cells showed significantly elevated maximum total binding compared to normotrophic cells. The secretion of [3H]phosphatidylcholine [( 3H]PC) by type II cells from silica- and saline-treated animals was also compared. Type II cells from silica-treated animals showed lower basal and tetradecanoyl phorbol acetate (TPA)-stimulated [3H]PC secretion than cells from saline-treated animals at each time point after instillation. SP-A inhibited TPA-stimulated [3H]PC secretion similarly in type II cells isolated after either silica or saline instillation.(ABSTRACT TRUNCATED AT 250 WORDS)

Corticosteroids and intratracheal surfactant both alter the distribution between the airways and lung tissue of intratracheally administered radiolabeled phosphatidylcholine in the preterm rabbit

Developmental differences exist regarding quantitative aspects of surfactant phosphatidylcholine clearance from the alveolar space and its subsequent reutilization. We wished to further extend observations of this nature to prematurely delivered rabbits undergoing mechanical ventilation. In addition we tested the hypothesis that prenatal corticosteroid exposure and/or intratracheal surfactant at birth would produce alterations in the lung's clearance of phosphatidylcholine from the airways. Pregnant does were injected with either Ringer's lactate or betamethasone on days 25 and 26 of gestation. Fetuses were delivered at 27 days and given by intratracheal injection either surfactant or one-half strength Ringer's lactate, both of which were trace labeled with [3H]phosphatidylcholine. Fetuses then underwent mechanical ventilation for periods of time ranging from 10 to 120 min. Following ventilation, alveolar lavage and lung tissue were examined to determine the distribution of [3H]phosphatidylcholine between these two compartments. Antenatal corticosteroid exposure was associated with decreased recovery of the radiolabel from the alveolar space and increased recovery of the label from the lung tissue in comparison to control fetuses. Intratracheal surfactant was associated with persistence of the radiolabel within the alveolar space. Therapy with both of these modalities produced a radiolabel distribution that resembled that seen in fetuses receiving intratracheal surfactant alone.

Cumulative effects of repeated surfactant treatments in the rabbit

The responses of the adult rabbit lung to multiple doses of surfactant after intratracheal injections of either natural calf surfactant or Surfactant-TA were evaluated. For each surfactant, four groups of 1.4-kg rabbits were studied: group 1 received 100 mg of surfactant containing isotopically-labeled dipalmitoylphosphatidylcholine; group 2 received the same labeled surfactant and then three tracheal injections of vehicle; group 3 received labeled surfactant and then three doses (100 mg) of unlabeled surfactant; group 4 was treated in the same way as group 3 except that the final dose was of the labeled surfactant. All rabbits were killed, and alveolar washes were recovered 24 h after the labeled surfactant dose had been given. The amount of labeled palmitate-saturated phosphatidylcholine (Sat PC) in alveolar washes did not change after multiple doses of calf surfactant, indicating that subsequent doses did not alter the clearance of previous doses. The four doses of calf surfactant increased the alveolar Sat PC pool size by a factor of 2.5 only when measured 6 h after the last dose, but the total lung Sat PC pool size doubled, indicating a loss of most of the surfactant Sat PC to the lung tissue. In contrast, Surfactant-TA increased the alveolar pool size by a factor of 4 after the single dose and by a factor of 11 after the multiple doses, and the percentage clearance of labeled Sat PC from the lungs decreased with multiple doses, indicating an effect of subsequent doses on the initial dose. The quantity of Sat PC cleared from the lungs increased by about a factor of 2 after the multiple doses of Surfactant-TA. Although repetitive surfactant doses changed alveolar and lung Sat PC pool sizes the quantity of Sat PC cleared from the lungs increased, and the lungs accommodated the large amount of surfactant without short-term adverse effects.

Lung surfactant apoprotein SP-A (26-36 kDa) binds with high affinity to isolated alveolar type II cells

Pulmonary surfactant is synthesized and secreted by alveolar type II cells. These cells recycle surfactant lipids by an internalization process that is enhanced in vitro by the surfactant proteins with molecular masses of 26-36 kDa (SP-A). SP-A also inhibits the secretion of lipid by type II cells. These results suggest that SP-A may play a role in feedback regulation of surfactant pool size and are consistent with the hypothesis that the type II cell surface has receptors for SP-A. The goal of this study is to characterize the binding of radioiodinated SP-A to isolated rat type II cells. Binding of SP-A to type II cells at 4 degrees C has a K1/2 of approximately 5 X 10(-10) M, is saturable, and is inhibited by excess unlabeled SP-A. Binding is dependent on calcium and is reduced by heat treatment of SP-A. The binding of a proteolytic fragment of SP-A that is produced by collagenase treatment is reduced by excess unlabeled SP-A. The binding of the fragment to macrophages and lung fibroblasts is not inhibited by excess unlabeled SP-A. Trypsinization of the type II cell surface reduces the binding of both intact SP-A and the collagenase-resistant fragment. These results show that SP-A binds to type II cells with high affinity and suggest that these cells have receptors that recognize the carboxyl-terminal domain of SP-A.

Reutilization of surfactant phosphatidylglycerol and lysophosphatidylcholine by adult rabbits

Adult rabbits reutilize the phosphatidylcholine (PC) of surfactant much less efficiently than developing rabbits (22% vs. 95%). Comparisons of reutilization efficiency of other components of surfactant in adult rabbits have not been determined. We injected adult rabbits intratracheally with [3H]dipalmitoylphosphatidylcholine (DPPG) mixed with [14C]lysophosphatidylcholine (lysoPC) and natural surfactant or [14C]DPPC mixed with [3H]dipalmitoylphosphatidylglycerol (DPPG) and natural surfactant. Recovery in the alveolar wash and lamellar bodies of labelled DPPC, lysoPC and DPPG was determined at different times after injection. By plotting the ratio of [3H]DPPG to [14C]DPPC in the alveolar wash versus time after injection we found that phosphatidylglycerol was reutilized with an efficiency of only 0-7% which was much less than the reutilization of PC in these animals. At early times after injection, adult rabbits injected with [14C]lysoPC had a ratio of [14C]PC in their alveolar wash to lamellar bodies that was larger than 1.0. By comparison, 3-day old rabbits injected intratracheally with [14C]lysoPC had a ratio of [14C]PC in alveolar wash to lamellar bodies less than 1.0 at the earliest times measurable. Thus adult rabbits demonstrate a pathway for accumulation of PC in their alveolar space prior to its appearance in lamellar bodies. This was not detected in developing rabbits. As in developing rabbits, adult rabbits reutilize the phosphatidylglycerol of surfactant less efficiently than the PC of surfactant.

Lysophosphatidylcholine uptake and metabolism in the adult rabbit lung

Tracer quantities of 3H-labeled lysoPC and 32P-labeled natural rabbit surfactant were given intratracheally via a bronchoscope and [14C]palmitate was given intravenously to 25 rabbits with labeled PC and lysoPC measured in the alveolar wash, lung homogenate, lamellar bodies and microsomes at five times from 10 min to 6 h after tracheal injection. Surprisingly, only 31% of the administered lysoPC remained in its original form in the total lungs (alveolar wash + lung homogenate) by 10 min, of which 77% was in the alveolar wash. Meanwhile, by 10 min an additional 37% was already converted to PC, of which more than 98% was in the lung homogenate. LysoPC continued to be rapidly and efficiently converted to PC, with 62% conversion measured at 3 h. The converted lysoPC initially appeared with high specific activity in microsomes, then in lamellar bodies, and finally in the alveolar wash. The intravascular palmitate labeled lung PC had similar specific activity-time profiles in the subcellular fractions, while intratracheally administered natural rabbit surfactant had a constantly low specific activity in microsomes and much higher specific activities in lamellar bodies and alveolar wash. Another 25 rabbits received intratracheal lysoPC labeled in both the choline and palmitate moieties and then were studied from 1 to 24 h after tracheal injection. The ratio of the palmitate to choline labels indicated uptake and conversion to PC primarily by direct acylation rather than transacylation and by intact reuptake and conversion rather than breakdown and resynthesis. LysoPC is an attractive 'metabolic probe' of surfactant metabolism which undergoes very rapid and efficient intracellular conversion to PC via a subcellular pathway that parallels the remodeling and de novo synthetic pathways.

Alveolar type II cells express a high-affinity receptor for pulmonary surfactant protein A

Primary cultures of rat alveolar type II cells bind radiolabeled pulmonary surfactant protein A (SP-A) with high affinity. The binding of 125I-labeled SP-A is time- and temperature-dependent and is not accompanied by significant degradation. The binding process is saturable at low concentrations of SP-A (5 micrograms/ml), and unlabeled SP-A readily competes with labeled SP-A for cellular binding sites. Subsequent to binding, two pools of cell-associated 125I-labeled SP-A can be identified based upon sensitivity to trypsin at 0 degrees C. It is likely that the trypsin-sensitive pool comprises 125I-labeled SP-A bound to the cell surface and the trypsin-insensitive pool comprises the internalized protein. Scatchard analysis of cell surface binding of SP-A at 0.1-10 micrograms/ml shows positive cooperativity at concentrations between 0.1 and 1 micrograms/ml. Hill plots give nH = 1.34 +/- 0.08 with an apparent dissociation constant K'd = 1.02 +/- 0.32 micrograms/ml (which is 0.64 +/- 0.19 nM if the native molecular mass of oligomeric SP-A is assumed to be 1.6 MDa). The binding of SP-A to type II cells shows an absolute requirement for Ca2+. The putative receptor for SP-A is unaffected by treatment of type II cells with a variety of proteases and N-Glycanase (EC 3.5.1.52). Alveolar macrophages also exhibit high-affinity binding of SP-A, but rat lung fibroblasts and the alveolar epithelial cell line L2 exhibit only nonspecific binding.

Prolonged ventilation of the premature newborn rabbit after treatment with natural or apoprotein-based artificial surfactant

Premature rabbit neonates (gestational age 27 days) were treated at birth with natural surfactant purified from chloroform extracts of porcine lung lipids either by acetone precipitation (Surfactant CK, n = 10) or liquid gel chromatography (Curosurf, n = 22). Another group of animals received artificial surfactant "reconstituted" from isolated low molecular weight (less than or equal to 15 K) apoproteins and synthetic dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) (Aposurf, n = 10). The phospholipid concentrations of the preparations were adjusted to provide the same individual dose of DPPC for each group of treated animals (3 or 4 mg). In comparison with untreated controls from the same litters, there was a 4-7-fold enhancement of lung-thorax compliance in all groups of surfactant-treated animals during a 3-h period of artificial ventilation. The average initial (20 min) compliance value was lower in the Aposurf-treated group than in animals receiving natural surfactant preparations, but the difference between the groups gradually diminished and was no longer statistically significant during the 2nd and 3rd h of artificial ventilation. Judged from the fall in tidal volume during ventilation with a short expiration phase (0.17 instead of 0.75 s), the apoprotein-based artificial surfactant was also less effective in stabilizing the lungs. A similar conclusion could be drawn from data on alveolar expansion in histological sections, evaluated by automated image analysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of acid phosphatase in lamellar bodies of tannic acid treated alveolar type II cells

Acid phosphatase was demonstrated in well preserved lamellar bodies of rats' alveolar type II cells. The highly ordered lamellar organization was preserved by using tannic acid in the tissue procession protocol. Acid phosphatase reaction products were observed in the amorphous regions of the lamellar bodies adjacent to the limiting membranes and in the central core regions. No reaction product was observed in the lamellar areas. 85% +/- 5% of the lamellar bodies were positively reactive, unrelated to their size. Multivesicular bodies were only partially reactive (approx. 50%), except for those attached to lamellar bodies which all had reaction product.

Evidence for a high activity of sphingomyelin biosynthesis by phosphocholine transfer from phosphatidylcholine to ceramides in lung lamellar bodies

Biosynthesis of sphingomyelin from ceramides was investigated in lung subcellular fractions by incubating a lyophilized mixture of albumin and subcellular fraction (0.1-0.2 mg of protein) coated with [acyl-14C]-ceramide and phosphatidyl[methyl-3H]choline in Tris-buffer. The lamellar-body-rich fraction exhibited the highest specific activity for sphingomyelin biosynthesis measured by 14C incorporation into sphingomyelins or by [3H]phosphocholine transfer from phosphatidylcholines. Plasma membranes formed the next most active fraction, followed by the 'smooth' and, then, the 'rough' endoplasmic reticulum. Sphingomyelin biosynthesis by lamellar bodies was optimum at pH 7.4 and was inhibited by sphingomyelins formed. Slight inhibitory effects were also observed with Mn2+, Ca2+ and lysophosphatidylcholine. Phosphocholine transfer from CDPcholine was not observed under the reaction conditions employed. Ceramide conversion and phosphocholine transfer increased with ceramide concentration to reach a maximum at about 0.06 mM. The highest conversion rate was observed when 18:1 ceramide was used as an acceptor. When 1-palmitoyl-2-oleoylphosphatidylcholine was the phosphocholine donor, the overall biosynthesis of sphingomyelin was much higher than when using dipalmitoylphosphatidylcholine. These results suggest the possible involvement of the studied reaction in the control of the degree of saturation of the surfactant phosphatidylcholine.

De novo sn-glycerol-3-phosphorylcholine synthetase activity in lung and muscle and its subcellular location

The activity of glycerophosphorylcholine synthetase, a newly discovered enzyme involved in the synthesis of acyl-specific phosphatidylcholines, is reported in rat lung and muscle. Its subcellular location appears to be mitochondrial. The implication of these findings in the synthesis of lung surfactant and the pathology of muscular dystrophy are discussed.