cAMP increases synthesis of surfactant-associated protein A by perfused rat lung

Mutation of Serine 32 to Threonine in Peroxiredoxin 6 Preserves Its Structure and Enzymatic Function but Abolishes Its Trafficking to Lamellar Bodies

Peroxiredoxin 6 (Prdx6), a bifunctional protein with phospholipase A2 (aiPLA2) and GSH peroxidase activities, protects lungs from oxidative stress and participates in lung surfactant phospholipid turnover. Prdx6 has been localized to both cytosol and lamellar bodies (LB) in lung epithelium, and its organellar targeting sequence has been identified. We propose that Prdx6 LB targeting facilitates its role in the metabolism of lung surfactant phosphatidylcholine (PC). Ser-32 has been identified as the active site in Prdx6 for aiPLA2 activity, and this activity was abolished by the mutation of serine 32 to alanine (S32A). However, aiPLA2 activity was unaffected by mutation of serine 32 in Prdx6 to threonine (S32T). Prdx6 protein expression and aiPLA2 activity were normal in the whole lung of a "knock-in" mouse model carrying an S32T mutation in the Prdx6 gene but were absent from isolated LB. Analyses by proximity ligation assay in lung sections demonstrated the inability of S32T Prdx6 to bind to the chaperone protein, 14-3-3ϵ, that is required for LB targeting. The content of total phospholipid, PC, and disaturated PC in lung tissue homogenate, bronchoalveolar lavage fluid, and lung LB was increased significantly in Prdx6-S32T mutant lungs, whereas degradation of internalized [(3)H]dipalmitoyl-PC was significantly decreased. Thus, Thr can substitute for Ser for the enzymatic activities of Prdx6 but not for its targeting to LB. These results confirm an important role for LB Prdx6 in the degradation and remodeling of lung surfactant phosphatidylcholine.

Pathway to lamellar bodies for surfactant protein A

Alveolar surfactant protein A (SP-A) is endocytosed by type II epithelial cells through clathrin-dependent uptake and targeted to lamellar bodies for resecretion. However, the mechanism for secretion of newly synthesized SP-A, whether regulated exocytosis of lamellar bodies or constitutive secretion, is unresolved. If it is the latter, lamellar body SP-A would represent endocytosed protein. Amantadine, an inhibitor of clathrin-coated vesicle budding, was used to evaluate the role of endocytosis in accumulation of SP-A in lamellar bodies. In isolated rat lungs, amantadine (10 mM) inhibited uptake of endotracheally instilled (35)S-labeled biosynthesized surfactant proteins by >80%. To study trafficking of newly synthesized SP-A, lungs were perfused for up to 6 h with [(35)S]methionine, and surfactant was isolated from lung lavage fluid and lamellar bodies were isolated from lung homogenate. With control lungs, the mean specific activity of [(35)S]SP-A (disintegrations per minute per microgram of SP-A) increased linearly with time of perfusion: it was significantly higher in isolated lamellar bodies than in surfactant and was increased in both compartments by 50-60% in the presence of 0.1 mM 8-bromo-cAMP. These results suggest a precursor-product relationship between lamellar body and extracellular [(35)S]SP-A. Specific activities in both compartments were unaffected by addition of amantadine (10 mM) to the lung perfusate, indicating that uptake from the alveolar space was not responsible for the increase in lamellar body [(35)S]SP-A. Thus the pathway for secretion of newly synthesized SP-A is by transfer from the site of synthesis to the storage/secretory organelle prior to lamellar body exocytosis.

P63 (CKAP4) as an SP-A receptor: implications for surfactant turnover

Surfactant protein-A (SP-A) plays an important role in the clearance of surfactant from the lung alveolar space and in the regulation of surfactant secretion and uptake by type II pneumocytes in culture. Two pathways are important for the endocytosis of surfactant by type II cells and the intact lung, a receptor-mediated clathrin-dependent pathway and a non-clathrin, actin-mediated pathway. The critical role of the clathrin/receptor-mediated pathway in normal mice is supported by the finding that SP-A gene-targeted mice use the actin-dependent pathway to maintain normal clearance of surfactant. Addition of SP-A to the surfactant of the SP-A null mice "rescued" the phenotype, further emphasizing the essential role of the SP-A/receptor-mediated process in surfactant turnover. This review presents an overview of the structure of SP-A and its function in surfactant turnover. The evidence that the interaction of SP-A with type II cells is a receptor-mediated process is presented. A newly identified receptor for SP-A, P63/CKAP4, is described in detail, with elucidation of the specific structural features of this 63 kDa, nonglycosylated, highly coiled, transmembrane protein. The compelling evidence that P63 functions as a receptor for SP-A on type II cells is summarized. Regulation of P63 receptor density on the surface of pneumocytes may be a novel approach for the regulation of surfactant homeostasis by the lung.

Dietary flaxseed enhances antioxidant defenses and is protective in a mouse model of lung ischemia-reperfusion injury

Dietary flaxseed (FS) is a nutritional whole grain with high contents of omega-3 fatty acids and lignans with anti-inflammatory and antioxidant properties. We evaluated FS in a murine model of pulmonary ischemia-reperfusion injury (IRI) by dietary supplementation of 0% (control) or 10% (treatment) FS before IRI. Mice fed 0% FS undergoing IRI had a significant decrease in arterial oxygenation (Pa(O(2))) and a significant increase in bronchoalveolar lavage (BAL) protein compared with sham-operated mice. However, mice fed 10% FS undergoing IRI had a significant improvement in both Pa(O(2)) and BAL protein compared with mice fed 0% FS undergoing IRI. In addition, oxidative lung damage was decreased in 10% FS-supplemented mice undergoing IRI, as assessed by malondialdehyde levels. Immunohistochemical staining of lungs for iPF(2alpha)-III F(2) isoprostane, a measure of lipid oxidation, was diminished. FS-supplemented mice had less reactive oxygen species (ROS) release from the vascular endothelium in lungs in an ex vivo model of IRI, and alveolar macrophages isolated from FS-fed mice had significantly reduced ROS generation in response to oxidative burst. Pulmonary microvascular endothelial cells produced less ROS in a flow cessation model of ischemia when preincubated with purified FS lignan metabolites. Pharmacological inhibition of heme oxygenase-1 (HO-1) resulted in only a partial reduction of FS protection in the same model. We conclude that dietary FS is protective against IRI in an experimental murine model and that FS affects ROS generation and ROS detoxification via pathways not limited to upregulation of antioxidant enzymes such as HO-1.

Identification and characterization of p63 (CKAP4/ERGIC-63/CLIMP-63), a surfactant protein A binding protein, on type II pneumocytes

Surfactant protein A (SP-A) binds to alveolar type II cells through a specific high-affinity cell membrane receptor, although the molecular nature of this receptor is unclear. In the present study, we have identified and characterized an SP-A cell surface binding protein by utilizing two chemical cross-linkers: profound sulfo-SBED protein-protein interaction reagent and dithiobis(succinimidylpropionate) (DSP). Sulfo-SBED-biotinylated SP-A was cross-linked to the plasma membranes isolated from rat type II cells, and the biotin label was transferred from SP-A to its receptor by reduction. The biotinylated SP-A-binding protein was identified on blots by using streptavidin-labeled horseradish peroxidase. By using DSP, we cross-linked SP-A to intact mouse type II cells and immunoprecipitated the SP-A-receptor complex using anti-SP-A antibody. Both of the cross-linking approaches showed a major band of 63 kDa under reduced conditions that was identified as the rat homolog of the human type II transmembrane protein p63 (CKAP4/ERGIC-63/CLIMP-63) by matrix-assisted laser desorption ionization and nanoelectrospray tandem mass spectrometry of tryptic fragments. Thereafter, we confirmed the presence of p63 protein in the cross-linked SP-A-receptor complex by immunoprobing with p63 antibody. Coimmunoprecipitation experiments and functional assays confirmed specific interaction between SP-A and p63. Antibody to p63 could block SP-A-mediated inhibition of ATP-stimulated phospholipid secretion. Both intracellular and membrane localized pools of p63 were detected on type II cells by immunofluorescence and immunobloting. p63 colocalized with SP-A in early endosomes. Thus p63 closely interacts with SP-A and may play a role in the trafficking or the biological function of the surfactant protein.

Recovery of rat type II cell surfactant components during primary cell culture

A culture system designed to maintain the differentiated characteristics of rat type II cells based on protocols used for human fetal lung pneumocytes was investigated. Type II cells were isolated either from adult rats with elastase (adult type II cells) or from young rats (4-11 days postnatal) with collagenase and trypsin (young type II cells) and were incubated with dexamethasone (Dex, 10 nM) and cAMP (0.1 mM). By day 4 of culture with hormone treatment, the mRNA levels in adult type II cells were less than 3% of day 0 values, whereas surfactant protein (SP)-A protein content was 26%. However, young type II cells maintained lamellar bodies and microvilli and secreted phospholipid in response to ATP. SP-A, -B, and -C mRNA levels were elevated to 159, 350, and 39%, respectively, of day 0 values with a synergistic response to Dex and cAMP, whereas SP-A protein content rose to 119%. Surfactant mRNA and protein did not recover in cells cultured without hormones. This cell culture system restored surfactant components in rat type II cells.

Characterisation of the beta adrenergic response cascade in fetal guinea pig lung

BACKGROUND

Suitable models for the study of lung development are needed. The suitability of the guinea pig for studying the role of the beta adrenergic response cascade in fetal lung development has been evaluated.

METHODS

Radioligand binding assays with iodine-125 labelled iodopindolol were performed to identify and characterise the beta adrenergic receptors. To demonstrate that these receptors were functional, isoprenaline and forskolin stimulated generation of cyclic AMP (cAMP) in the lung tissue was quantitated by radioimmunoassay.

RESULTS

The concentration of beta receptors increased with gestational age from 23 fmol/mg at 35 days to 140 fmol/mg at 64 days. Competition binding studies were consistent with a predominance of beta 2 receptors. The ability of isoprenaline to stimulate cAMP generation was greater during the saccular phase than during the canalicular phase of lung development. Incorporation of tritium labelled choline into phosphatidylcholine increased significantly between the canalicular and saccular phases.

CONCLUSIONS

The beta adrenergic response cascade in fetal guinea pig lung exhibits similar characteristics to those previously described in fetal human lung and is therefore a good model in which to study the effects of beta agonists on fetal lung development.

Inhibition of Trimeresurus flavoviridis phospholipase A2 by lung surfactant protein A (SP-A)

A marked sequence homology has been noted between lung surfactant protein A (SP-A) and an inhibitor of phospholipase A2 (PLA2) isolated from the serum of Trimeresurus flavoviridis (Habu snake). This study evaluated the effect of SP-A on PLA2 activity from several sources. SP-A was isolated from bovine or rat lung surfactant by extraction with 1-butanol and octyl beta-D-glucopyranoside. The addition of SP-A produced a concentration-dependent inhibition of T. flavoviridis PLA2 that indicated non-competitive kinetics with Ki 5 micrograms/ml. Inhibition was reversed by heat inactivation, disulfide bond reduction or alkylation of SP-A, or by the presence of anti-SP-A antibody. Treatment of SP-A with endoglycosidase F or the presence of variation monosaccharides or lectins did not alter SP-A inhibition. Binding of PLA2 to SP-A was shown by ultrafiltration and was abolished by SP-A alkylation or the presence of SDS. The SP-A/PLA2 complex recovered from the ultrafilter had essentially no enzymatic activity, but activity was restored by treatment with mercaptoethanol. SP-A had no effect on activity of PLA2 from Naja naja, Crotalus atrox, or bovine pancreas. These results indicate that surfactant protein A selectively inhibits Trimeresurus phospholipase A2 activity and suggest that binding to the enzyme is the mechanism for inhibition.

Secretion of surfactant protein A and phosphatidylcholine from type II cells of human fetal lung

Surfactant protein A (SP-A) appears to have a role in lung immune defense as well as generation and metabolism of the alveolar surface-active film. Previous studies indicated that lamellar bodies isolated from lung tissue had a relatively low content of SP-A and that exogenous SP-A was needed for rapid formation of a surface-active film in vitro. We therefore tested the hypothesis that SP-A was secreted from type II cells primarily by a pathway separate from lamellar bodies. Cells were isolated from explants of human fetal lung that had been cultured with hormones to promote differentiation of type II cells, and secretion of surfactant lipid and SP-A were compared. Cultured cells secreted labeled phosphatidylcholine in a nearly linear fashion for 48 h. Basal secretion of SP-A, assayed by enzyme-linked immunosorbent assay, was linear for only 12 h after plating of cells; during this time, there was no accumulation of intracellular SP-A. Addition of secretagogues (phorbol ester, calcium ionophore, and beta-adrenergic agonist) stimulated phosphatidylcholine secretion approximately 4-fold. In contrast, the secretion rate of SP-A was not significantly affected by secretagogues. These findings indicate that a relatively small amount of secreted SP-A (approximately 10%) is released with lamellar bodies. Most SP-A is released by constitutive secretion and may be important for both surfactant- and nonsurfactant-related functions.