IONIZING-RADIATION ACTS ON CELLULAR MEMBRANES TO GENERATE CERAMIDE AND INITIATE APOPTOSIS

Recent investigations provided evidence that the sphingomyelin signal transduction pathway mediates apoptosis for tumor necrosis factor alph a (TNF-alpha) in several hematopoietic and nonhematopoietic cells. In this pathway, TNF-receptor interaction initiates sphingomyelin hydroly sis to ceramide by a sphingomyelinase. Ceramide acts as a second messe nger stimulating a ceramide-activated serine/threonine protein kinase. The present studies show that ionizing radiation, like TNF, induces r apid sphingomyelin hydrolysis to ceramide and apoptosis in bovine aort ic endothelial cells. Elevation of ceramide with exogenous ceramide an alogues was sufficient for induction of apoptosis. Protein kinase C ac tivation blocked both radiation-induced sphingomyelin hydrolysis and a poptosis, and apoptosis was restored by ceramide analogues added exoge nously. Ionizing radiation acted directly on membrane preparations dev oid of nuclei, stimulating sphingomyelin hydrolysis enzymatically thro ugh a neutral sphingomyelinase. These studies provide the first conclu sive evidence that apoptotic signaling can be generated by interaction of ionizing radiation with cellular membranes and suggest an alternat ive to the hypothesis that direct DNA damage mediates radiation-induce d cell kill.