Ceramide path in human lung cell death

Lung epithelium plays a significant role in modulating the inflammatory res ponse to lung injury. Airway epithelial cells are targeted by hydrogen pero xide (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) hy drolysis to generate ceramide, which serves as a second messenger in initia ting an apoptotic response. Here we show that exposure of human airway epit helial (HAE) cells to 50 to 100 mu M H2O2 induces within 5 to 10 min a grea ter than 2-fold activation of neutral sphingomyelinase activity with concom itant SM hydrolysis, ceramide generation, and apoptosis, On the other hand, activation of protein kinase C (PKC) by 12-O-tetradecanoylphorbol-13-aceta te inhibits both H2O2-induced ceramide production and apoptosis. The apopto tic response could be restored by the addition of 25 mu M cell-permeant C6- ceramide. These findings indicate that ceramide, the product of SM hydrolys is, plays an important role in H2O2-induced apoptosis in HAE cells, and tha t PKC counteracts ceramide-mediated apoptosis in these cells. We suggest th at the mediation of epithelial cell apoptosis by ceramide and its inhibitio n by PKC constitute a central mechanism by which inflammatory processes are modulated in the epithelium of the lung.