Scheme for alveolar epithelial to endothelial cell signal propagation initiated by TNF-α. The spatial interrelationships of participating cells and receptors are shown on the left. The molecular events are summarized on the right. The crucial molecule mediating intercellular communication is free AA, which is hydrolyzed from epithelial cell nuclear membrane phospholipids (PL) by cPLA₂, and which induces endothelial cell Ca²⁺ flux.