Surfactant peptides stimulate uptake of phosphatidylcholine by isolated cells

To determine whether small hydrophobic surfactant peptides (SP-B and SP-C) participate in recycling of pulmonary surfactant phospholipid, we determined the effect of these peptides on transfer of 3H- or 14C-labelled phosphatidylcholine from liposomes to isolated rat alveolar Type II cells and Chinese hamster lung fibroblasts. Both natural and synthetic SP-B and SP-C markedly stimulated phosphatidylcholine transfer to alveolar Type II cells and Chinese hamster lung fibroblasts in a dose- and time-dependent fashion. Effects of the peptides on phospholipid uptake were dose-dependent, but not saturable and occurred at both 4 and 37 degrees C. Uptake of labelled phospholipid into a lamellar body fraction prepared from Type II cells was augmented in the presence of SP-B. Neither SP-B nor SP-C augmented exchange of labelled plasma membrane phosphatidylcholine from isolated Type II cells or enhanced the release of surfactant phospholipid when compared to liposomes without SP-B or SP-C. Addition of native bovine SP-B and SP-C to the phospholipid vesicles perturbed the size and structure of the vesicles as determined by electron microscopy. To determine the structural elements responsible for the effect of the peptides on phospholipid uptake, fragments of SP-B were synthesized by solid-phase protein synthesis and their effects on phospholipid uptake assessed in Type II epithelial cells. SP-B (1-60) stimulated phospholipid uptake 7-fold. A smaller fragment of SP-B (15-60) was less active and the SP-B peptide (40-60) failed to augment phospholipid uptake significantly. Like SP-B and SP-C, surfactant-associated protein (SP-A) enhanced phospholipid uptake by Type II cells. However, SP-A failed to significantly stimulate phosphatidylcholine uptake by Chinese hamster lung fibroblasts. These studies demonstrate the independent activity of surfactant proteins SP-B and SP-C on the uptake of phospholipid by Type II epithelial cells and Chinese hamster lung fibroblasts in vitro.     

Mature Surfactant Protein-B Expression by Immunohistochemistry as a Marker for Surfactant System Development in the Fetal Sheep Lung

Evaluation of the number of type II alveolar epithelial cells (AECs) is an important measure of the lung’s ability to produce surfactant. Immunohistochemical staining of these cells in lung tissue commonly uses antibodies directed against mature surfactant protein (SP)-C, which is regarded as a reliable SP marker of type II AECs in rodents. There has been no study demonstrating reliable markers for surfactant system maturation by immunohistochemistry in the fetal sheep lung despite being widely used as a model to study lung development. Here we examine staining of a panel of surfactant pro-proteins (pro–SP-B and pro–SP-C) and mature proteins (SP-B and SP-C) in the fetal sheep lung during late gestation in the saccular/alveolar phase of development (120, 130, and 140 days), with term being 150 ± 3 days, to identify the most reliable marker of surfactant producing cells in this species. Results from this study indicate that during late gestation, use of anti-SP-B antibodies in the sheep lung yields significantly higher cell counts in the alveolar epithelium than SP-C antibodies. Furthermore, this study highlights that mature SP-B antibodies are more reliable markers than SP-C antibodies to evaluate surfactant maturation in the fetal sheep lung by immunohistochemistry.     

Respiratory distress after intratracheal bleomycin: selective deficiency of surfactant proteins B and C

Intratracheal bleomycin in rats is associated with respiratory distress of uncertain etiology. We investigated the expression of surfactant components in this model of lung injury. Maximum respiratory distress, determined by respiratory rate, occurred at 7 days, and surfactant dysfunction was confirmed by increased surface tension of the large-aggregate fraction of bronchoalveolar lavage (BAL). In injured animals, phospholipid content and composition were similar to those of controls, mature surfactant protein (SP) B was decreased 90%, and SP-A and SP-D contents were increased. In lung tissue, SP-B and SP-C mRNAs were decreased by 2 days and maximally at 4--7 days and recovered between 14 and 21 days after injury. Immunostaining of SP-B and proSP-C was decreased in type II epithelial cells but strong in macrophages. By electron microscopy, injured lungs had type II cells lacking lamellar bodies and macrophages with phagocytosed lamellar bodies. Surface activity of BAL phospholipids of injured animals was restored by addition of exogenous SP-B. We conclude that respiratory distress after bleomycin in rats results from surfactant dysfunction in part secondary to selective downregulation of SP-B and SP-C.     

Regulation of SP-B and SP-C secretion in rat type II cells in primary culture

Secretion of lung surfactant phospholipids is a highly regulated process. A variety of physiological and pharmacological agents stimulate surfactant phospholipid secretion in isolated type II cells. Although the lipid and hydrophobic protein components of surfactant are believed to be secreted together by exocytosis of lamellar body contents, regulation of surfactant protein (SP) B and SP-C secretion has not previously been examined. To address the question of whether secretion of SP-B and SP-C is stimulated by the same agonists that stimulate phospholipid secretion, we measured secretion of all four SPs under the same conditions used to measure phosphatidylcholine secretion. Freshly isolated rat type II cells were cultured overnight and exposed to known surfactant phospholipid secretagogues for 2.5 h, after which the amounts of SP-A, SP-B, SP-C, and SP-D in the medium were measured with immunoblotting. Secretion of SP-B and SP-C was stimulated three- to fivefold by terbutaline, 5'-(N-ethylcarboxyamido)adenosine, ATP, 12-O-tetradecanoylphorbol 13-acetate, and ionomycin. Similar to their effects on phospholipid secretion, the stimulatory effects of the agonists were abolished by Ro 31-8220. Secretion of SP-A and SP-D was not stimulated by the secretagogues tested. We conclude that secretion of the phospholipid and hydrophobic protein components of surfactant is similarly regulated, whereas secretion of the hydrophilic proteins is regulated differently.     

Surfactant lipid synthesis and lamellar body formation in glycogen-laden type II cells

Pulmonary surfactant is a lipoprotein complex that functions to reduce surface tension at the air liquid interface in the alveolus of the mature lung. In late gestation glycogen-laden type II cells shift their metabolic program toward the synthesis of surfactant, of which phosphatidylcholine (PC) is by far the most abundant lipid. To investigate the cellular site of surfactant PC synthesis in these cells we determined the subcellular localization of two key enzymes for PC biosynthesis, fatty acid synthase (FAS) and CTP:phosphocholine cytidylyltransferase-alpha (CCT-alpha), and compared their localization with that of surfactant storage organelles, the lamellar bodies (LBs), and surfactant proteins (SPs) in fetal mouse lung. Ultrastructural analysis showed that immature and mature LBs were present within the glycogen pools of fetal type II cells. Multivesicular bodies were noted only in the cytoplasm. Immunogold electron microscopy (EM) revealed that the glycogen pools were the prominent cellular sites for FAS and CCT-alpha. Energy-filtering EM demonstrated that CCT-alpha bound to phosphorus-rich (phospholipid) structures in the glycogen. SP-B and SP-C, but not SP-A, localized predominantly to the glycogen stores. Collectively, these data suggest that the glycogen stores in fetal type II cells are a cellular site for surfactant PC synthesis and LB formation/maturation consistent with the idea that the glycogen is a unique substrate for surfactant lipids.     

Synthesis of surfactant components by cultured type II cells from human lung

We examined the effect of monolayer culture on surfactant phospholipids and proteins of type II cells isolated from human adult and fetal lung. Type II cells were prepared from cultured explants of fetal lung (16-24 weeks gestation) and from adult surgical specimens. Cells were maintained for up to 6 days on plastic tissue culture dishes. Although incorporation of [methyl-3H]choline into phosphatidylcholine (PC) by fetal cells was similar on day 1 and day 5 of culture, saturation of PC fell from 35 to 26%. In addition, there was decreased distribution of labeled acetate into PC, whereas distribution into other phospholipids increased or did not change. The decrease in saturation of newly synthesized PC was not altered by triiodothyronine (T3) and dexamethasone treatment or by culture as mixed type II cell/fibroblast monolayers. The content of surfactant protein SP-A (28-36 kDa) in fetal cells, as measured by ELISA and immunofluorescence microscopy, rose during the first day and then fell to undetectable levels by the fifth. Synthesis of SP-A, as measured by [35S]methionine labeling and immunoprecipitation, was detectable on day 1 but not thereafter. Levels of mRNAs for SP-A and for the two lipophilic surfactant proteins SP-B (18 kDa) and SP-C (5 kDa) fell with half-times of maximally 24 h. In contrast, total protein synthesis measured by [35S]methionine incorporation increased and then plateaued. In adult cells, the content of SP-A and its mRNA decreased during culture, with time-courses similar to those for fetal cells. We conclude that in monolayer culture on plastic culture dishes, human type II cells lose their ability to synthesize both phospholipids and proteins of surfactant. The control of type II cell differentiation under these conditions appears to be at a pretranslational level.     

Regulation of human pulmonary surfactant protein gene expression by 1alpha,25-dihydroxyvitamin D3

1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] has been reported to stimulate lung maturity, alveolar type II cell differentiation, and pulmonary surfactant synthesis in rat lung. We hypothesized that 1,25(OH)(2)D(3) stimulates expression of surfactant protein-A (SP-A), SP-B, and SP-C in human fetal lung and type II cells. We found that immunoreactive vitamin D receptor was detectable in fetal lung tissue and type II cells only when incubated with 1,25(OH)(2)D(3). 1,25(OH)(2)D(3) significantly decreased SP-A mRNA in human fetal lung tissue but did not significantly decrease SP-A protein in the tissue. In type II cells, 1,25(OH)(2)D(3) alone had no significant effect on SP-A mRNA or protein levels but reduced SP-A mRNA and protein in a dose-dependent manner when the cells were incubated with cAMP. SP-A mRNA levels in NCI-H441 cells, a nonciliated bronchiolar epithelial (Clara) cell line, were decreased in a dose-dependent manner in the absence or presence of cAMP. 1,25(OH)(2)D(3) had no significant effect on SP-B mRNA levels in lung tissue but increased SP-B mRNA and protein levels in type II cells incubated in the absence or presence of cAMP. Expression of SP-C mRNA was unaffected by 1,25(OH)(2)D(3) in lung tissue incubated +/- cAMP. These results suggest that regulation of surfactant protein gene expression in human lung and type II cells by 1,25(OH)(2)D(3) is not coordinated; 1,25(OH)(2)D(3) decreases SP-A mRNA and protein levels in both fetal lung tissue and type II cells, increases SP-B mRNA and protein levels only in type II cells, and has no effect on SP-C mRNA levels.     

Fibrinolysis-inhibitory capacity of clot-embedded surfactant is enhanced by SP-B and SP-C

Incorporation of pulmonary surfactant into fibrin inhibits its plasmic degradation. In the present study we investigated the influence of surfactant proteins (SP)-A, SP-B, and SP-C on the fibrinolysis-inhibitory capacity of surfactant phospholipids. Plasmin-induced fibrinolysis was quantified by means of a (125)I-fibrin plate assay, and surfactant incorporation into polymerizing fibrin was analyzed by measuring the incorporation of (3)H-labeled L-alpha-dipalmitoylphosphatidylcholine into the insoluble clot material. Incorporation of a calf lung surfactant extract (Alveofact) and an organic extract of natural rabbit large surfactant aggregates (LSA) into a fibrin clot revealed a stronger inhibitory effect on plasmic cleavage of this clot than a synthetic phospholipid mixture (PLX) and unprocessed LSA. Reconstitution of PLX with SP-B and SP-C increased, whereas reconstitution with SP-A decreased, the fibrinolysis-inhibitory capacity of the phospholipids. The SP-B effect was paralleled by an increased incorporation of phospholipids into fibrin. We conclude that the inhibitory effect of surfactant incorporation into polymerizing fibrin on its susceptibility to plasmic cleavage is enhanced by SP-B and SP-C but reduced by SP-A. In the case of SP-B, increased phospholipid incorporation may underlie this finding.     

Developmental regulation of re-uptake of phosphatidylcholine by type II alveolar epithelium

Type II alveolar epithelia produce, store and secrete pulmonary surfactant, a phospholipid and protein mixture which stabilizes alveoli at low lung volumes and, thereby, prevents alveolar collapse. We determined the developmental changes in the uptake, metabolism and reutilization of surfactant-related phospholipid in primary cultures of type II cells derived from fetal rat lung. Primary cultures of fetal and neonatal type II cells were incubated in media containing labelled liposomes. After the incubation phospholipids were extracted from the cells and uptake of label was analyzed. Re-uptake of radiolabelled dipalmitoyl phosphatidylcholine (DPPC) was concentration-dependent in undifferentiated fetal cells, differentiated fetal cells and neonatal cells. Re-uptake of DPPC by undifferentiated fetal cells was lower than re-uptake by both differentiated fetal and neonatal cells at 15 and 75 μM PC. Binding of DPPC to the cell surface involved a protein interaction, since trypsin was able to dissociate this trypsin-releasable fraction from internalized label. Undifferentiated fetal, differentiated fetal and neonatal cells all exhibited approx. 50% metabolic degradation of internalized phospholipid. Degraded lipids were reutilized in the synthesis of phosphatidylglycerol, but neonatal cells resynthesized twice as much phosphatidylglycerol as did undifferentiated fetal cells. These are the first studies which show that morphologically undifferentiated fetal type II cells are capable of the uptake of surfactant phospholipid as well as the degradation and reutilization of internalized phospholipid. Re-uptake, degradation and reutilization of internalized phospholipid appear to be under developmental control.     

Leptin mediates the parathyroid hormone-related protein paracrine stimulation of fetal lung maturation

Developing rat lung lipofibroblasts express leptin beginning on embryonic day (E) 17, increasing 7- to 10-fold by E20. Leptin and its receptor are expressed mutually exclusively by fetal lung fibroblasts and type II cells, suggesting a paracrine signaling "loop." This hypothesized mechanism is supported by the following experimental data: 1) leptin stimulates the de novo synthesis of surfactant phospholipid by both fetal rat type II cells (400% x 100 ng(-1) x ml(-1) x 24 h(-1)) and adult human airway epithelial cells (85% x 100 ng(-1) x 24 h(-1)); 2) leptin is secreted by lipofibroblasts in amounts that stimulate type II cell surfactant phospholipid synthesis in vitro; 3) epithelial cell secretions such as parathyroid hormone-related protein (PTHrP), PGE(2), and dexamethasone stimulate leptin expression by fetal rat lung fibroblasts; 4) PTHrP or leptin stimulate the de novo synthesis of surfactant phospholipid (2- to 2.5-fold/24 h) and the expression of surfactant protein B (SP-B; >25-fold/24 h) by fetal rat lung explants, an effect that is blocked by a leptin antibody; and 5) a PTHrP receptor antagonist inhibits the expression of leptin mRNA by explants but does not inhibit leptin stimulation of surfactant phospholipid or SP-B expression, indicating that PTHrP paracrine stimulation of type II cell maturation requires leptin expression by lipofibroblasts. This is the first demonstration of a paracrine loop that functionally cooperates to induce alveolar acinar lung development.     

TGF-alpha perturbs surfactant homeostasis in vivo

To determine potential relationships between transforming growth factor (TGF)-alpha and surfactant homeostasis, the metabolism, function, and composition of surfactant phospholipid and proteins were assessed in transgenic mice in which TGF-alpha was expressed in respiratory epithelial cells. Secretion of saturated phosphatidylcholine was decreased 40-60% by expression of TGF-alpha. Although SP-A, SP-B, and SP-C mRNA levels were unchanged by expression of TGF-alpha, SP-A and SP-B content in bronchoalveolar lavage fluid was decreased. The minimum surface tension of surfactant isolated from the transgenic mice was significantly increased. Incubation of cultured normal mice type II cells with TGF-alpha in vitro did not change secretion of surfactant phosphatidylcholine and SP-B, indicating that TGF-alpha does not directly influence surfactant secretion. Expression of a dominant negative (mutant) EGF receptor in the respiratory epithelium blocked the TGF-alpha-induced changes in lung morphology and surfactant secretion, indicating that EGF receptor signaling in distal epithelial cells was required for TGF-alpha effects on surfactant homeostasis. Because many epithelial cells were embedded in fibrotic lesions caused by TGF-alpha, changes in surfactant homeostasis may at least in part be influenced by tissue remodeling that results in decreased surfactant secretion. The number of nonembedded type II cells was decreased 30% when TGF-alpha was expressed during development and was increased threefold by TGF-alpha expression in adulthood, suggesting possible alteration of type II cells on surfactant metabolism in the adult lung. Abnormalities in surfactant function and decreased surfactant level in the airways may contribute to the pathophysiology induced by TGF-alpha in both the developing and adult lung.     

Pneumonitis and emphysema in sp-C gene targeted mice

SP-C-deficient (SP-C -/-) mice developed a severe pulmonary disorder associated with emphysema, monocytic infiltrates, epithelial cell dysplasia, and atypical accumulations of intracellular lipids in type II epithelial cells and alveolar macrophages. Whereas alveolar and tissue surfactant phospholipid pools were increased, levels of other surfactant proteins were not altered (SP-B) or were modestly increased (SP-A and SP-D). Analysis of pressure-volume curves and forced oscillatory dynamics demonstrated abnormal respiratory mechanics typical of emphysema. Lung disease was progressive, causing weight loss and cardiomegaly. Extensive alveolar remodeling was accompanied by type II cell hyperplasia, obliteration of pulmonary capillaries, and widespread expression of alpha-smooth muscle actin, indicating myofibroblast transformation in the lung parenchyma. Dysplastic epithelial cells lining conducting airways stained intensely for the mucin, MUC5A/C. Tissue concentrations of proinflammatory cytokines were not substantially altered in the SP-C (-/-) mice. Production of matrix metalloproteinases (MMP-2 and MMP-9) was increased in alveolar macrophages from SP-C (-/-) mice. Absence of SP-C caused a severe progressive pulmonary disorder with histologic features consistent with interstitial pneumonitis.     

Mechanical stretch promotes alveolar epithelial type II cell differentiation.

Functional maturation of pulmonary alveolar epithelial cells is crucial for extrauterine survival. Mechanical distension and mesenchymal-epithelial interactions play important roles in this process. We hypothesized that mechanical stretch simulating fetal breathing movements is an important regulator of pulmonary epithelial cell differentiation. Using a Flexercell Strain Unit, we analyzed effects of stretch on primary cultures of type II cells and cocultures of epithelial and mesenchymal cells isolated from fetal rat lungs during late development. Cyclic stretch of isolated type II cells increased surfactant protein (SP) C mRNA expression by 150 +/- 30% over controls (P < 0.02) on gestational day 18 and by 130 +/- 30% on day 19 (P < 0.03). Stretch of cocultures with fibroblasts increased SP-C expression on days 18 and 19 by 170 +/- 40 and 270 +/- 40%, respectively, compared with unstretched cocultures. On day 19, stretch of isolated type II cells increased SP-B mRNA expression by 50% (P < 0.003). Unlike SP-C, addition of fibroblasts did not produce significant additional effects on SP-B mRNA levels. Under these conditions, we observed only modest increases in cellular immunoreactive SP-B, but secreted saturated phosphatidylcholine rose by 40% (P < 0.002). These results indicate that cyclic stretch promotes developmentally timed differentiation of fetal type II cells, as a direct effect on epithelial cell function and via mesenchymal-epithelial interactions. Expression of the SP-C gene appears to be highly responsive to mechanical stimulation.     

Surfactant proteins B and C are both necessary for alveolar stability at end expiration in premature rabbits with respiratory distress syndrome.

Modified natural surfactant preparations, used for treatment of respiratory distress syndrome in premature infants, contain phospholipids and the hydrophobic surfactant protein (SP)-B and SP-C. Herein, the individual and combined effects of SP-B and SP-C were evaluated in premature rabbit fetuses treated with airway instillation of surfactant and ventilated without positive end-expiratory pressure. Artificial surfactant preparations composed of synthetic phospholipids mixed with either 2% (wt/wt) of porcine SP-B, SP-C, or a synthetic poly-Leu analog of SP-C (SP-C33) did not stabilize the alveoli at the end of expiration, as measured by low lung gas volumes of approximately 5 ml/kg after 30 min of ventilation. However, treatment with phospholipids containing both SP-B and SP-C/SP-C33 approximately doubled lung gas volumes. Doubling the SP-C33 content did not affect lung gas volumes. The tidal volumes were similar in all groups receiving surfactant. This shows that SP-B and SP-C exert different physiological effects, since both proteins are needed to establish alveolar stability at end expiration in this animal model of respiratory distress syndrome, and that an optimal synthetic surfactant probably requires the presence of mimics of both SP-B and SP-C.     

Lipid mixing is mediated by the hydrophobic surfactant protein SP-B but not by 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 excimer/monomer ratio of pyrene-labeled PC fluorescence intensities was used to investigate the capacity of the hydrophobic surfactant proteins, SP-B and SP-C, to induce lipid mixing between protein-containing small unilamellar vesicles and pyrene-PC-labeled small unilamellar vesicles. At 37 degrees C SP-B induced lipid mixing between protein-containing vesicles and pyrene-PC-labeled vesicles. In the presence of negatively charged phospholipids (PG or PI) the SP-B-induced lipid mixing was enhanced, and dependent on the presence of (divalent) cations. The extent of lipid mixing was maximal at a protein concentration of 0.2 mol%. SP-C was not capable of inducing lipid mixing at 37 degrees C not even at protein concentrations of 1 mol%. The SP-B-induced lipid mixing may occur during the Ca(2+)-dependent transformation of lamellar bodies into tubular myelin and the subsequent formation of the phospholipid monolayer.     

Nucleotide and deduced amino acid sequence of the hydrophobic surfactant protein SP-C from rat: expression in alveolar type II cells and homology with SP-C from other species

Pulmonary surfactant lowers surface tension in the lung. Its deficiency leads to the severe physiologic abnormalities seen in the respiratory distress syndrome. The hydrophobic surfactant proteins, SP-B and SP-C, appear to be especially important in the surface-spreading characteristics of pulmonary surfactant. We report the nucleotide sequence of cDNA clones for rat SP-C and compare the deduced amino acid sequence for SP-C from several species. A highly conserved domain exists within the confines of mature human SP-C. An Eisenberg plot of this region predicts a membrane-associated helix. We also demonstrate by Northern analysis the tissue-specific expression of SP-C. A comparison of signal strength between total lung RNA and RNA derived from isolated type II cells supports the idea that most SP-C messenger RNA in total lung can be accounted for by that present in alveolar type II cells.     

A comparison of the molecular species compositions of mammalian lung surfactant phospholipids

While dipalmitoyl phosphatidylcholine (PC16:0/16:0) is essential for pulmonary surfactant function, roles for other individual molecular species of surfactant phospholipids have not been established. If any phospholipid species other than PC16:0/16:0 is important for surfactant function, then it may be conserved across animal species. Consequently, we have quantified, by electrospray ionisation mass spectrometry, molecular species compositions of phosphatidylcholine (PC), phosphatidylglycerol (PG) and phosphatidylinositol (PI) in surfactants from human, rabbit, rat and guinea pig lungs. While PC compositions displayed only relatively minor variations across the animal species studied, there were wide variations of PG and PI concentrations and compositions. Human surfactant PG and PI were enriched in the same three monounsaturated species (PG16:0/18:1, PG18:1/18:1 and PG18:0/18:1) with minimal amounts of PG16:0/16:0 or polyunsaturated species, while all animal surfactant PG contained increased concentrations of PG16:0/16:0 and PG16:0/18:2. Animal surfactant PIs were essentially monounsaturated except for a high content of PI18:0/20:4 (29%) in the rat. As these four surfactants all maintain appropriate lung function of the respective animal species, then all their varied compositions of acidic phospholipids must be adequate at promoting the processes of adsorption, film refinement, respreading and collapse characteristic of surfactant. We conclude that this effectively monounsaturated composition of anionic phospholipid molecular species is a common characteristic of mammalian surfactants.