OPPOSITE EFFECTS OF TUMOR-NECROSIS-FACTOR-ALPHA ON THE SPHINGOMYELIN-CERAMIDE PATHWAY IN 2 MYELOID-LEUKEMIA CELL-LINES - ROLE OF TRANSVERSESPHINGOMYELIN DISTRIBUTION IN THE PLASMA-MEMBRANE

Tumor necrosis factor alpha (TNF alpha) mediates proliferation, functi onal activation, and apoptotic cell death depending on the target cell type. Although sphingomyelin (SPM) hydrolysis and ceramide generation may function as an important mediator in TNF alpha signaling, the mol ecular mechanisms of the signaling pathway(s) are still not well under stood. The aim of the present study is to compare the effect of TNF al pha on SPM metabolism and cell growth in two myeloid leukemic cell lin es (U937 and KG1a) that differ in their sensitivity to TNF alpha. Our results show that TNF alpha induced apoptosis in U937 but not in KG1a cells. TNF alpha triggered in KG1a cells neither SPM hydrolysis nor ce ramide generation, but induced SPM synthesis and ceramide breakdown as well as dose-dependent cell proliferation. In contrast, TNF alpha ind uced in U937 SPM hydrolysis and ceramide generation as well as dose-de pendent cell death. Synthetic cell permeant ceramide, as well as natur al ceramide, generated by treatment with bacterial sphingomyelinase (S PMase), all induced apoptosis in both U937 and KG1a cells. These findi ngs indicate that the SPM-ceramide pathway is altered in KG1a cells up stream of the ceramide generation, Analysis of the transverse distribu tion of SPM in the plasma membrane showed that the SPM pool involved i n cell signaling (inner leaflet) was markedly reduced in KG1a cells; i t is 7-fold lower than that found in the inner leaflet of U937 cells. Therefore, our study strongly suggests that the different resp by TNF alpha in myeloid cells are dependent on the SPM plasma membrane transv erse asymmetry. (C) 1996 by The American Society of Hematology.