Secretory phospholipase A2-mediated depletion of phosphatidylglycerol in early acute respiratory distress syndrome

Abstract

Introduction: Secretory phospholipases A2 (sPLA2) hydrolyze phospholipids in cell membranes and extracellular structures such as pulmonary surfactant. This study tests the hypothesis that sPLA2 are elevated in human lungs during acute respiratory distress syndrome (ARDS) and that sPLA2 levels are associated with surfactant injury by hydrolysis of surfactant phospholipids.

Methods: Bronchoalveolar lavage (BAL) fluid was obtained from 18 patients with early ARDS (<72 hours) and compared with samples from 10 healthy volunteers. Secreted phospholipase A2 levels were measured (enzyme activity and enzyme immunoassay) in conjunction with ARDS subjects' surfactant abnormalities including surfactant phospholipid composition, large and small aggregates distribution and surface tension function.

Results: BAL sPLA2 enzyme activity was markedly elevated in ARDS samples relative to healthy subjects when measured by ex vivo hydrolysis of both phosphatidylglycerol (PG) and phosphatidylcholine (PC). Enzyme immunoassay identified increased PLA2G2A protein in the ARDS BAL fluid, which was strongly correlated with the sPLA2 enzyme activity against PG. Of particular interest, the authors demonstrated an average depletion of 69% of the PG in the ARDS sample large aggregates relative to the normal controls. Furthermore, the sPLA2 enzyme activity against PG and PC ex vivo correlated with the BAL recovery of in vivo PG and PC, respectively, and also correlated with the altered distribution of the large and small surfactant aggregates.

Conclusions: These results support the hypothesis that sPLA2-mediated hydrolysis of surfactant phospholipid, especially PG by PLA2G2A, contributes to surfactant injury during early ARDS.

Figure 1. sPLA₂ enzymatic activity vs PLA2G2A protein content from BAL

PLA2G2A protein was measured by EIA (x-axis) and compared in ARDS BAL samples to healthy normal controls. Data are shown relative to the amount of sPLA₂ in vitro hydrolysis of phosphatidylglycerol (PG) (A) or phosphotidylcholine (PC) (B) in a surfactant substrate (y-axis). PLA2G2A correlated with PG hydrolytic activity (p < 0.0001) and with PC hydrolytic activity (p < 0.05) from the ARDS BAL samples (Pearson correlations, n=18).

Figure 1. sPLA₂ enzymatic activity vs PLA2G2A protein content from BAL

PLA2G2A protein was measured by EIA (x-axis) and compared in ARDS BAL samples to healthy normal controls. Data are shown relative to the amount of sPLA₂ in vitro hydrolysis of phosphatidylglycerol (PG) (A) or phosphotidylcholine (PC) (B) in a surfactant substrate (y-axis). PLA2G2A correlated with PG hydrolytic activity (p < 0.0001) and with PC hydrolytic activity (p < 0.05) from the ARDS BAL samples (Pearson correlations, n=18).

Figure 2. Phospholipid composition of LA fractions shows depletion of PG in ARDS

The LA fractions from early ARDS BALs were analyzed for phospholipid composition by HPLC. Depletion of PG was noted in the ARDS samples relative to healthy control BALs, whereas PI and sphingomyelin increased (median ± standard deviation, * p < 0.001, Mann Whitney rank sum, n=16).

Figure 3. Change in surfactant LA/SA ratio is correlated with PG hydrolytic activity of sPLA₂ in the BAL sample

Phospholipid recovery was measured in the LA and SA fractions of ARDS and healthy control BAL samples and compared to sPLA₂ enzymatic activity ex vivo against PG. LA/SA ratios decreased with increasing sPLA₂ activity against PG in the ARDS BAL samples (p < 0.01, Pearson correlation, n=17).