Opposite effects of tumor necrosis factor alpha on the sphingomyelin- ceramide pathway in two myeloid leukemia cell lines: role of transverse sphingomyelin distribution in the plasma membrane

Tumor necrosis factor alpha (TNF alpha) mediates proliferation, functional 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 molecular mechanisms of the signaling pathway(s) are still not well understood. The aim of the present study is to compare the effect of TNF alpha on SPM metabolism and cell growth in two myeloid leukemic cell lines (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 ceramide generation, but induced SPM synthesis and ceramide breakdown as well as dose-dependent cell proliferation. In contrast, TNF alpha induced in U937 SPM hydrolysis and ceramide generation as well as dose-dependent cell death. Synthetic cell permeant ceramide, as well as natural ceramide, generated by treatment with bacterial sphingomyelinase (SPMase), all induced apoptosis in both U937 and KG1a cells. These findings indicate that the SPM-ceramide pathway is altered in KG1a cells upstream of the ceramide generation. Analysis of the transverse distribution of SPM in the plasma membrane showed that the SPM pool involved in cell signaling (inner leaflet) was markedly reduced in KG1a cells; it is 7-fold lower than that found in the inner leaflet of U937 cells. Therefore, our study strongly suggests that the different responses induced by TNF alpha in myeloid cells are dependent on the SPM plasma membrane transverse asymmetry.