Ceramide path in human lung cell death

Lung epithelium plays a significant role in modulating the inflammatory response to lung injury. Airway epithelial cells are targeted by hydrogen peroxide (H2O2) and oxygen radicals, which are agents commonly produced during inflammatory processes. The mechanisms and molecular sites affected by H2O2 are largely unknown but may involve the induction of sphingomyelin (SM) hydrolysis to generate ceramide, which serves as a second messenger in initiating an apoptotic response. Here we show that exposure of human airway epithelial (HAE) cells to 50 to 100 mu M H2O2 induces within 5 to 10 min a greater than 2-fold activation of neutral sphingomyelinase activity with concomitant SM hydrolysis, ceramide generation, and apoptosis, On the other hand, activation of protein kinase C (PKC) by 12-O-tetradecanoylphorbol-13-acetate inhibits both H2O2-induced ceramide production and apoptosis. The apoptotic response could be restored by the addition of 25 mu M cell-permeant C6-ceramide. These findings indicate that ceramide, the product of SM hydrolysis, plays an important role in H2O2-induced apoptosis in HAE cells, and that PKC counteracts ceramide-mediated apoptosis in these cells. We suggest that the mediation of epithelial cell apoptosis by ceramide and its inhibition by PKC constitute a central mechanism by which inflammatory processes are modulated in the epithelium of the lung.