TNF-alpha increases ceramide without inducing apoptosis in alveolar type II epithelial cells

Ceramide is a bioactive lipid mediator that has been observed to induce apo ptosis in vitro. The purpose of this study was to determine whether endogen ous ceramide, generated in response to in vivo administration of tumor necr osis factor-alpha (TNF-alpha), increases apoptosis in primary rat alveolar type II epithelial cells. Intratracheal instillation of TNF-alpha (5 mu g) produced a decrease in sphingomyelin and activation of a neutral sphingomye linase. These changes were associated with a significant increase in lung c eramide content. TNF-alpha concomitantly activated the p42/44 extracellular signal-related kinases and induced nuclear factor-kappa B activation in th e lung. Hypodiploid nuclei studies revealed that intratracheal TNF-alpha di d not increase type II cell apoptosis compared with that in control cells a fter isolation. A novel observation from separate in vitro studies demonstr ated that type II cells undergo a gradual increase in apoptosis after time in culture, a process that was accelerated by exposure of cells to ultravio let light. However, culture of cells with a cell-permeable ceramide, TNF-al pha, or a related ligand, anti-CD95, did not increase apoptosis above the c ontrol level. The results suggest that ceramide resulting from TNF-alpha ac tivation of sphingomyelin hydrolysis might activate the mitogen-activated p rotein kinase and nuclear factor-kappa B pathways without increasing progra mmed cell death in type II cells.