Endothelial cell prostaglandin I₂ and platelet-activating factor production are markedly attenuated in the calcium-independent phospholipase A₂β knockout mouse

Damage and activation of lung endothelium can lead to interstitial edema, infiltration of inflammatory cells into the interstitium and airways, and production of inflammatory metabolites, all of which propagate airway inflammation in a variety of diseases. We have previously determined that stimulation of human microvascular endothelial cells from lung (HMVEC-L) results in activation of a calcium-independent phospholipase A₂ (iPLA ₂), and this leads to arachidonic acid release and production of prostaglandin I₂ (PGI₂) and platelet-activating factor (PAF). We stimulated lung endothelial cells isolated from iPLA ₂β-knockout (KO) and wild type (WT) mice with thrombin and tryptase to determine the role of iPLA₂β in endothelial cell membrane phospholipid hydrolysis. Thrombin or tryptase stimulation of WT lung endothelial cells resulted in increased arachidonic acid release and production of PGI₂ and PAF. Arachidonic acid release and PGI₂ production by stimulated iPLA₂β-KO endothelial cells were significantly reduced compared to WT. Measured PLA₂ activity and PGI₂ production by iPLA₂β-KO cells were suppressed by pretreatment with (R)-bromoenol lactone (R-BEL), which is a selective inhibitor of iPLA2γ. In contrast to the increase in PAF production induced by stimulation of WT endothelial cells, none was observed for KO cells, and this suggests that endothelial PAF production is entirely dependent on iPLA ₂β activity. Because inflammatory cell recruitment involves the interaction of endothelial cell PAF with PAF receptors on circulating cells, these data suggest that iPLA₂β may be a suitable therapeutic target for the treatment of inflammatory lung diseases.