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

Protein-containing synthetic surfactant versus protein-free synthetic surfactant for the prevention and treatment of respiratory distress syndrome

BACKGROUND

Respiratory distress syndrome (RDS) is a significant cause of morbidity and mortality in preterm infants. RDS is caused by a deficiency, dysfunction, or inactivation of pulmonary surfactant. Numerous surfactants of either animal extract or synthetic design have been shown to improve outcomes. New surfactant preparations that include peptides or whole proteins that mimic endogenous surfactant protein have recently been developed and tested.

OBJECTIVES

To assess the effect of administration of synthetic surfactant containing surfactant protein mimics compared to protein free synthetic surfactant on the risk of mortality, chronic lung disease, and other morbidities associated with prematurity in preterm infants at risk for or having RDS.

SEARCH STRATEGY

Standard search methods of the Cochrane Neonatal Review Group were used. The search included MEDLINE (1966 - March 2009) and the Cochrane Central Register of Controlled Trials (CENTRAL, The Cochrane Library) in all languages.

SELECTION CRITERIA

Randomized and quasi-randomized controlled clinical trials were considered for this review. Studies that enrolled preterm infants or low birth weight infants at risk for or having RDS who were treated with either a synthetic surfactant containing surfactant protein mimics or a protein free synthetic surfactant were included for this review. Studies of treatment or prevention of respiratory distress syndrome were included.

DATA COLLECTION AND ANALYSIS

Data regarding mortality, chronic lung disease and multiple secondary outcome measures were abstracted by the review authors. Statistical analysis was performed using Review Manager software. Categorical data were analyzed using relative risk, risk difference, and number needed to treat. 95% confidence intervals reported. A fixed effects model was used for the meta-analysis. Heterogeneity was assessed using the I(2) statistic.

MAIN RESULTS

One study was identified that compared protein containing synthetic surfactants (PCSS) to protein free synthetic surfactants. Infants who received protein containing synthetic surfactant compared to protein free synthetic surfactant did not demonstrate significantly different risks of prespecified primary outcomes: mortality at 36 weeks postmenstrual age (PMA) [RR 0.89 (95% CI 0.71, 1.11)], chronic lung disease at 36 weeks PMA [RR 0.89 (95% CI 0.78, 1.03)], or the combined outcome of mortality or chronic lung disease at 36 weeks PMA [RR 0.88 (95% CI 0.77, 1.01)]. Among the secondary outcomes, a decrease in the incidence of respiratory distress syndrome at 24 hours of age was demonstrated in the group that received PCSS [RR 0.83 (95% CI 0.72, 0.95).

AUTHORS' CONCLUSIONS

In the one trial comparing protein containing synthetic surfactants compared to protein free synthetic surfactant for the prevention of RDS, no statistically different clinical differences in death and chronic lung disease were noted. Clinical outcomes between the two groups were generally similar although the group receiving protein containing synthetic surfactants did have decreased incidence of respiratory distress syndrome. Further well designed studies comparing protein containing synthetic surfactant to the more widely used animal derived surfactant extracts are indicated.

Protein containing synthetic surfactant versus animal derived surfactant extract for the prevention and treatment of respiratory distress syndrome

BACKGROUND

Respiratory distress syndrome (RDS) is a significant cause of morbidity and mortality in preterm infants. RDS is caused by a deficiency, dysfunction, or inactivation of pulmonary surfactant. Numerous surfactants of either animal extract or synthetic design have been shown to improve outcomes. New surfactant preparations that include peptides or whole proteins that mimic endogenous surfactant protein have recently been developed and tested.

OBJECTIVES

To assess the effect of administration of synthetic surfactant containing surfactant protein mimics compared to animal derived surfactant extract on the risk of mortality, chronic lung disease, and other morbidities associated with prematurity in preterm infants at risk for or having RDS.

SEARCH STRATEGY

Standard search methods of the Cochrane Neonatal Review Group were used. The search included MEDLINE (1966 - May 2007) and the Cochrane Central Register of Controlled Trials (CENTRAL, The Cochrane Library) in all languages. In addition, published abstracts of the Society of Pediatric Research were searched electronically. For abstract books that did not include key words, the search was limited to the relevant sections on pulmonary and neonatology. The bibliography cited in each publication was obtained and searched in order to identify additional relevant articles.

SELECTION CRITERIA

Randomized and quasi-randomized controlled clinical trials were considered for this review. Studies that enrolled preterm infants or low birth weight infants at risk for or having RDS who were treated with either a synthetic surfactant containing surfactant protein mimics or an animal-derived surfactant preparation were included for this review. Studies that either attempted to treat or prevent respiratory distress syndrome were included.

DATA COLLECTION AND ANALYSIS

Primary outcome measures, including mortality, chronic lung disease and multiple secondary outcome measures were abstracted by the reviewers. Statistical analysis was performed using Review Manager software. Categorical data was analyzed using relative risk, risk difference, and number needed to treat. 95% confidence intervals reported. A fixed effects model was used for the meta-analysis. Heterogeneity was assessed using the I-squared statistic.

MAIN RESULTS

Two studies were identified that compared protein containing synthetic surfactants to animal derived surfactant preparations. In a meta-analysis of these two studies, infants who received protein containing synthetic surfactant compared to animal derived surfactant extract did not demonstrate significantly different risks of prespecified primary outcomes: mortality at 36 weeks [typical RR 0.81 (95% CI 0.64, 1.03)], chronic lung disease at 36 weeks [typical RR 0.99 (95% CI 0.84, 1.18)], or the combined outcome of mortality or chronic lung disease at 36 weeks [typical RR 0.96 (95% CI 0.82, 1.12)]. There were also no differences in any of the secondary outcomes regarding complications of prematurity between the two surfactant groups with the exception of necrotizing enterocolitis. A decrease in the risk of necrotizing enterocolitis was noted in infants who received protein containing synthetic surfactants compared to animal derived surfactant extract [typical RR 0.60 (95% CI 0.42, 0.86)]. However, this was a secondary outcome in both of the primary studies and there was moderate heterogeneity between the studies.

AUTHORS' CONCLUSIONS

In two trials of protein containing synthetic surfactants compared to animal derived surfactant extract, no statistically different clinical differences in death and chronic lung disease were noted. Further well designed studies of adequate size and power will be needed to confirm and refine these findings.

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.

Protein nitration in rat lungs during hyperoxia exposure: a possible role of myeloperoxidase

Several studies have suggested that exposure to hyperoxia causes lung injury through increased generation of reactive oxygen and nitrogen species. The present study was aimed to investigate the effects of hyperoxia exposure on protein nitration in lungs. Rats were exposed to hyperoxia (>95%) for 48, 60, and 72 h. Histopathological analysis showed a dramatic change in the severity of lung injury in terms of edema and hemorrhage between 48- and 60-h exposure times. Western blot for nitrotyrosine showed that several proteins with molecular masses of 29-66 kDa were nitrated in hyperoxic lung tissues. Immunohistochemical analyses indicate nitrotyrosine staining of alveolar epithelial and interstitial regions. Furthermore, immunoprecipitation followed by Western blot revealed the nitration of surfactant protein A and t1alpha, proteins specific for alveolar epithelial type II and type I cells, respectively. The increased myeloperoxidase (MPO) activity and total nitrite levels in bronchoalveolar lavage and lung tissue homogenates were observed in hyperoxic lungs. Neutrophils and macrophages isolated from the hyperoxia-exposed rats, when cocultured with a rat lung epithelial L2 cell line, caused a significant protein nitration in L2 cells. Inclusion of nitrite further increased the protein nitration. These studies suggest that protein nitration during hyperoxia may be mediated in part by MPO generated from activated phagocytic cells, and such protein modifications may contribute to hyperoxia-mediated lung injury.

Interaction of lung surfactant proteins with anionic phospholipids

Langmuir isotherms, fluorescence microscopy, and atomic force microscopy were used to study lung surfactant specific proteins SP-B and SP-C in monolayers of dipalmitoylphosphatidylglycerol (DPPG) and palmitoyloleoylphosphatidylglycerol (POPG), which are representative of the anionic lipids in native and replacement lung surfactants. Both SP-B and SP-C eliminate squeeze-out of POPG from mixed DPPG/POPG monolayers by inducing a two- to three-dimensional transformation of the fluid-phase fraction of the monolayer. SP-B induces a reversible folding transition at monolayer collapse, allowing all components of surfactant to remain at the interface during respreading. The folds remain attached to the monolayer, are identical in composition and morphology to the unfolded monolayer, and are reincorporated reversibly into the monolayer upon expansion. In the absence of SP-B or SP-C, the unsaturated lipids are irreversibly lost at high surface pressures. These morphological transitions are identical to those in other lipid mixtures and hence appear to be independent of the detailed lipid composition of the monolayer. Instead they depend on the more general phenomena of coexistence between a liquid-expanded and liquid-condensed phase. These three-dimensional monolayer transitions reconcile how lung surfactant can achieve both low surface tensions upon compression and rapid respreading upon expansion and may have important implications toward the optimal design of replacement surfactants. The overlap of function between SP-B and SP-C helps explain why replacement surfactants lacking in one or the other proteins often have beneficial effects.

Effects of lung surfactant proteins, SP-B and SP-C, and palmitic acid on monolayer stability

Langmuir isotherms and fluorescence and atomic force microscopy images of synthetic model lung surfactants were used to determine the influence of palmitic acid and synthetic peptides based on the surfactant-specific proteins SP-B and SP-C on the morphology and function of surfactant monolayers. Lung surfactant-specific protein SP-C and peptides based on SP-C eliminate the loss to the subphase of unsaturated lipids necessary for good adsorption and respreading by inducing a transition between monolayers and multilayers within the fluid phase domains of the monolayer. The morphology and thickness of the multilayer phase depends on the lipid composition of the monolayer and the concentration of SP-C or SP-C peptide. Lung surfactant protein SP-B and peptides based on SP-B induce a reversible folding transition at monolayer collapse that allows all components of surfactant to be retained at the interface during respreading. Supplementing Survanta, a clinically used replacement lung surfactant, with a peptide based on the first 25 amino acids of SP-B also induces a similar folding transition at monolayer collapse. Palmitic acid makes the monolayer rigid at low surface tension and fluid at high surface tension and modifies SP-C function. Identifying the function of lung surfactant proteins and lipids is essential to the rational design of replacement surfactants for treatment of respiratory distress syndrome.

Surfactant phospholipid catabolic rate is pool size dependent in mice

We increased surfactant pool size by surfactant treatment in mice to test if the catabolism of the major component of surfactant, saturated phosphatidylcholine (Sat PC), was rate limited. By intratracheal instillation, we gave mice trace doses, doses of 45 or 110 micromol/kg, or three doses of 110 micromol/kg of Sat PC in surfactant that contained radiolabeled dipalmitoylphosphatidylcholine (DPPC) and a radiolabeled phospholipase A-resistant ether analog of DPPC. Two strains of mice with 2-fold differences in alveolar and total Sat PC pool sizes were used; the mice with the higher pool sizes had a 2.3-fold higher steady-state catabolic rate. Acute increases in alveolar surfactant given by intratracheal instillation increased catabolic rates approximately 2-fold over the steady-state rates in both strains. There was minimal loss of the ether analog of DPPC from the lungs, and the alveolar macrophages did not accumulate more than 10% of the ether analog. In these two strains of mice, the catabolism of Sat PC was not rate limited because catabolic rate increased when alveolar pool sizes were increased.

Hoxa-5 in mouse developing lung: cell-specific expression and retinoic acid regulation

Hoxa-5 is a homeobox gene that is highly expressed in the developing mouse lung. However, little is known about the molecular mechanisms controlling expression. We characterized the ontogeny of Hoxa-5 gene and protein expressions during lung development and then studied the cell-specific effects of retinoic acid (RA) on Hoxa-5 mRNA in fetal lung fibroblasts and MLE-12 mouse lung epithelial cells. Strong but constant Hoxa-5 gene and protein expressions were detected from mouse lung on embryonic day 13.5 to postnatal day 2. At baseline, the gene was strongly expressed in the fibroblasts of day 17.5 fetal mouse lungs. A very weak but reproducible expression was present in the MLE-12 cells. RA stimulated gene expression in both cell types in a time- and dose-dependent manner. Peak expression occurred much later in the MLE-12 cells compared with that in fibroblasts. Cycloheximide and actinomycin D treatment studies suggested that the differences in RA effect on each cell type may involve the presence of a repressor that can be overcome by RA.

Distinct changes in pulmonary surfactant homeostasis in common beta-chain- and GM-CSF-deficient mice

Pulmonary alveolar proteinosis (PAP) is caused by inactivation of either granulocyte-macrophage colony-stimulating factor (GM-CSF) or GM receptor common beta-chain (beta(c)) genes in mice [GM(-/-), beta(c)(-/-)], demonstrating a critical role of GM-CSF signaling in surfactant homeostasis. To distinguish possible phenotypic differences in GM(-/-) and beta(c)(-/-) mice, surfactant metabolism was compared in beta(c)(-/-), GM(-/-), and wild-type mice. Although lung histology in beta(c)(-/-) and GM(-/-) mice was indistinguishable, distinct differences were observed in surfactant phospholipid and surfactant protein concentrations and clearance from lungs of beta(c)(-/-) and GM(-/-) mice. At 1-2 days of age, lung saturated phosphatidylcholine (Sat PC) pool sizes were higher in wild-type, beta(c)(-/-), and GM(-/-) mice compared with wild-type adult mice. In wild-type mice, Sat PC pool sizes decreased to adult levels by 7 days of age; however, Sat PC increased with advancing age in beta(c)(-/-) and GM(-/-) mice. Postnatal changes in Sat PC pool sizes were different in GM(-/-) compared with beta(c)(-/-) mice. After 7 days of age, the increased lung Sat PC pool sizes remained constant in beta(c)(-/-) mice but continued to increase in GM(-/-) mice, so that by 56 days of age, lung Sat PC pools were increased three- and sixfold, respectively, compared with wild-type controls. After intratracheal injection, the percent recovery of [(3)H]dipalmitoylphosphatidylcholine and (125)I-recombinant surfactant protein (SP) C was higher in beta(c)(-/-) compared with wild-type mice, reflecting decreased clearance in the receptor-deficient mice. The defect in clearance was significantly more severe in GM(-/-) than in beta(c)(-/-) mice. The ratio of SP Sat PC to SP-A, -B, and -C was similar in bronchoalveolar lavage fluid (BALF) from adult mice of all genotypes, but the ratio of SP-D to Sat PC was markedly increased in beta(c)(-/-) and GM(-/-) mice (10- and 5-fold, respectively) compared with wild-type mice. GM-CSF concentrations were increased in BALF but not in serum of beta(c)(-/-) mice, consistent with a pulmonary response to the lack of GM-CSF signaling. The observed differences in surfactant metabolism suggest the presence of alternative clearance mechanisms regulating surfactant homeostasis in beta(c)(-/-) and GM(-/-) mice and may provide a molecular basis for the range in severity of PAP symptoms. surfactant metabolism; alveolar macrophage; granulocyte-macrophage colony-stimulating factor

Surfactant protein metabolism in vivo

The surfactant components saturated phosphatidylcholine, SP-B and SP-C, are secreted together in lamellar bodies, and at least a part of the de novo synthesized SP-A is secreted independently. The surface film forms from tubular myelin and loose lipid arrays, and it generates unilamellar vesicles that lack surfactant proteins and are thought to represent catabolic forms. The half-life values for the clearance of surfactant proteins from lungs range from 6.5 to 28 h and vary with species. There is minimal information about the associations of the surfactant proteins with lipids or with each other after film formation, although all surfactant components seem to be recycled back into lamellar bodies in type II cells. The relative importance of type II cells or macrophages to the catabolism of the protein components of surfactant remains to be characterized, as do regulators of surfactant homeostasis.