An albumin-associated PLA₂-like activity inactivates surfactant phosphatidylcholine secreted from fetal type II pneumocytes

Abstract

Damas JE, Cake MH. An albumin-associated PLA2-like activity inactivates surfactant phosphatidylcholine secreted from fetal type II pneumocytes. Am J Physiol Lung Cell Mol Physiol 301: L966-L974, 2011. First published September 9, 2011 doi:10.1152/ajplung.00103.2011.-Type II pneumocytes are responsible for the synthesis and secretion of pulmonary surfactant, which reduces surface tension in lung alveoli, thus decreasing their tendency to collapse during expiration. For this effect to be sustained, the integrity of the surface-active components of surfactant must be maintained. This study has shown that, when cultured type II pneumocytes are exposed to lipoprotein-free serum (LFS), the level of lyso-phosphatidylcholine (lyso-PC) in the secreted surfactant phospholipids is markedly elevated with a concomitant decline in the level of phosphatidylcholine (PC). This effect is the result of hydrolysis of surfactant PC by a phospholipase A2 (PLA2)-like activity present within serum. Anion-exchange chromatography, gel filtration chromatography and preparative electrophoresis of human LFS have shown that this PLA2-like activity coelutes with albumin and is biochemically distinct from the secretory form of PLA2. Furthermore, specific inhibitors of PLA2 such as pbromophenacyl bromide, aristolochic acid, and palmitoyl trifluoromethyl ketone do not inhibit this activity of serum. Commercially purified human serum albumin fraction V and recombinant human serum albumin (rHSA) are almost as effective as LFS in enhancing the level of lyso-PC in the media. The latter finding implies that rHSA directly generates lyso-PC from secreted PC and suggests that this PLA2-like activity may be an intrinsic attribute of albumin. surfactant phospholipids; phosphatidylcholine deacylation; lyso-phosphatidylcholine PULMONARY SURFACTANT IS ESSENTIAL for normal lung function. Through its ability to adsorb rapidly to the alveolar surface and reduce surface tension, surfactant increases lung compliance, thereby reducing the work of breathing (19,50,52). The ability of surfactant films to attain very low surface tension during compression stabilizes lung alveoli at end expiration by preventing alveolar collapse. The essential role of pulmonary surfactant in normal lung function is illustrated by the proven efficacy of exogenous surfactant to act as a replacement in the treatment of neonatal respiratory distress syndrome (NRDS) (8,20,33,62). Acute respiratory distress syndrome (ARDS), which can affect both adults and children, has a more complicated pathology than the simple absence of surfactant but shares many of the symptoms of NRDS, such as diminished lung compliance, a marked reduction in effective lung volume and profound hypoxemia (23). However, clinical trials with the most effective formulations of synthetic surfactant, which had been used successfully in the treatment of NRDS, showed that these formulations had only a modest and transient beneficial effect when administered to ARDS patients (45). Extracted bronchial fluid (lavage) from ARDS...

Long-chain Acyl-CoA Dehydrogenase Deficiency as a Cause of Pulmonary Surfactant Dysfunction

“…With regard to degradation, we found no observable increase in PLA 2 , which actively degrades phospholipids and is known to be increased in lavage fluid of children with RDS (66). Albumin, however, was consistently increased in LCAD Ϫ/Ϫ lavage fluid, and albumin has been shown to inactivate surfactant and to possess a phospholipase-like activity (67,68). Loss of FAO as an energy source in ATI and ATII cells may compromise the integrity of the alveolar epithelium, leading to increased albumin and other proteins in the epithelial lining fluid, which may then inactivate or disrupt pulmonary surfactant.…”

Influence of glucocorticoids, neuregulin-1β, and sex on surfactant phospholipid secretion from type II cells

“…Nineteen-day pregnant rats were killed by asphyxiation with CO 2, and fetuses were delivered by caesarean section. Fetal lungs were removed, minced, and incubated with collagenase (0.05 IU/ml) for 20 min in a 37°C shaking water bath, as previously described (9,34). After filtration through two layers of sterile gauze, the cells were centrifuged at 20 g for 2 min, and the pellet was resuspended in serum-free medium.…”

“…Type II pneumocyte cultures were given an initial media change 24 h after plating and thereafter at 2-day intervals. After three days in culture the plates were nearly confluent and consisted of predominantly differentiated type II cells, each containing numerous lamellar bodies and being capable of both synthesis (34) and secretion (2,9) of surfactant phosphatidylcholine.…”

“…Following incubation for a further 24 h, the cultures were washed three times with 3 ml of balanced salts solution to remove unincorporated choline and any detached cells before adding 1.7 ml of serum-free MEM. The cells were allowed to equilibrate at 37°C in a CO 2 incubator for 1 h before removing two plates to determine the media and cellular content of radiolabeled phospholipids as previously described (4,9). To the remaining plates (two for each treatment) was added 17 l of MilliQ water as vehicle (control) or the indicated concentrations of NRG1␤ or leptin.…”

Meconium-induced inflammation and surfactant inactivation: specifics of molecular mechanisms

“…Plasma proteins also include several enzymes able to hydrolyze surfactant, and surprisingly, not sPLA 2 , but rather albumin, had been found to exert the most significant PLA 2 -like activity in hydrolyzing surfactant phosphatidylcholine to the inactive lysophosphatidylcholine form (93). Both albumin and fibrinogen are able to decrease exogenous surfactant surface properties significantly (94,95).…”