A Back-Up Death Plan

Abstract

The ligand for the Fas receptor, FasL, stimulates cell death and is an important mediator controlling T and B cell populations. Hetz et al. analyzed A20 B lymphoma cells in culture and found that both apoptotic cell death and necrotic cell death occurred in response to FasL. FasL-induced apoptotic cell death was prevented by pharmacological inhibition of caspase 3, and all FasL-induced cell death was prevented by pretreatment of the cells with a nonselective caspase inhibitor. Apoptotic and necrotic cells were sorted into two populations on the basis of cell size (apoptosis is associated with cell shrinkage, whereas necrosis is associated with cell swelling), and caspase activities were determined in both populations. FasL treatment resulted in caspase 3 activation only in the apoptotic cells, whereas caspase 8 was activated in cells undergoing both types of cell death. Similar results were obtained in the Jurkat T cell line. However, FasL only stimulated apoptotic cell death in Raji B cells, which are different from Jurkat cells and A20 cells in that they do not respond to FasL with an increase in ceramide. Annexin V staining occurs when phosphatidylserine is exposed to the extracellular side of the plasma membrane and is an indication of phospholipid hydrolysis and "lipid scrambling," which is also associated with ceramide production. Annexin V staining occurred in response to Fas activation of A20 and Jurkat cells undergoing both apoptotic and necrotic cell death, but did not occur in Raji cells. FasL stimulated increased ceramide production in A20 cells. Also, adding cell-permeable ceramide analogs or increasing ceramide concentrations by addition of bacterial sphingomyelinase stimulated necrotic cell death. The kinetics of ceramide production suggest that necrosis may only be triggered in cells that are not committed to apoptotic cell death. Thus, it may serve as a back-up death mechanism for those remaining cells that do not achieve sufficient caspase 3 activation to undergo apoptosis. C. A Hetz, M. Hunn, P. Rojas, V. Torres, L. Leyton, A. F. G. Quest, Caspase-dependent initation of apoptosis and necrosis by the Fas receptor in lymphoid cells: Onset of necrosis is associated with delayed ceramide increase. J. Cell Sci. 115 , 4671-4683 (2002). [Abstract] [Full Text]