Overexpression of acid ceramidase protects from tumor necrosis factor-induced cell death

Tumor necrosis factor (TNF) signals cell death and simultaneously induces g eneration of ceramide. To evaluate the contribution of ceramide to TNF-depe ndent cell death, we generated clones of the TNF-sensitive cell line L929 t hat constitutively overexpress human acid ceramidase (AC). Ceramidase, in c oncert with sphingosine kinase, metabolizes ceramide to sphingosine-l-phosp hate (SPP), an inducer of proliferation. In response to TNF, parental L929 cells display a significant increase in intracellular ceramide correlated w ith an "atypical apoptosis" characterized by membrane blebbing, DNA fragmen tation and degradation of poly(ADP-ribose) polymerase despite a lack of cas pase activity. These features are strongly reduced or absent in AC-overexpr essing cells. Pharmacological suppression of AC with N-oleoylethanolamine r estored the accumulation of intracellular ceramide as well as the sensitivi ty of the transfectants to TNF, implying that an enhanced metabolization of intracellular ceramide by AC shifts the balance between intracellular cera mide and SPP levels towards cell survival. Correspondingly, inhibition of c eramide production by acid sphingomyelinase also increased survival of TNF- treated L929 cells.