Alveolar Sphingolipids Generated in Response to Tnfα Modifies Surfactant Biophysical Activity

Sphingolipids represent a diverse group of bioactive lipid species that are generated intracellularly in response to tumor necrosis factor α (TNFα) and are implicated as potential mediators of acute lung injury. The purpose of these studies was to determine if there is an extracellular, TNFα–regulated pool of sphingolipids in the alveolus that modulates the surface-tension lowering capacity of pulmonary surfactant. Intratracheal instillation of TNFα in adult rats led to a 2-fold increase in the amount of surfactant-associated ceramide and tended to decrease levels of sphingomyelin without significantly altering sphingosine or sphinganine content.. TNFα –induction of alveolar ceramide was associated with nearly an 80% increase in acid sphingomyelinase activity recovered in cell-free alveolar lavage. Ceramide in a dose-dependent manner potently antagonized the surface-tension lowering effects of natural surfactant in vitro. Intratracheal TNFα and ceramide treatment of rats significantly increased lung permeability as evidenced by extravasation of Evan's Blue dye into alveolar lavage and lung tissue. Thus, these studies are the first demonstrating the existence of a cytokine-regulated alveolar pool of sphingomyelin hydrolysis products that impairs the biophysical properties of the alveolar surfactant film. The results also suggest the presence of a secretory alveolar sphingomylinase that is TNFα-responsive and mediates effects of the cytokine on alveolar sphingolipid metabolism.