Evidence that ceramide mediates the ability of tumor necrosis factor to modulate primitive human hematopoietic cell fates

In this study, it is shown that short-term exposure of normal human marrow CD34(+)CD38(-) cells to low concentrations of tumor necrosis factor (TNF) i n the presence of 100 ng/mL Flt3 ligand and Steel factor and 20 ng/mL inter leukin-3 (IL-3), IL-6, and granulocyte colony-stimulating factor, in either bulk or single-cell serum-free cultures, markedly reduces their ability su bsequently to generate colony-forming cells (CFCs) in 6-week stromal cell-c ontaining long-term cultures without affecting their viability, mitogenic r esponse, or short-term ability to produce CFCs. A similar differential eff ect on the functional attributes of CD34(+)CD38(-) cells was seen when C2- or C6-ceramide, but not dihydro-C2-ceramide (an inactive analog of ceramide ), was substituted for TNF. The addition of D-erythro-MAPP (a specific inhi bitor of intracellular ceramide degradation) enhanced the ability of TNF to selectively eliminate long-term culture-initiating cell (LTC-IC) activity. These findings indicate that TNF can directly modulate the ability of CD34 (+)CD38(-) cells to maintain their LTC-IC function at doses below those req uired to initiate apoptosis, cell cycle arrest, or both, and they suggest t hat this may be mediated by the TNF-induced generation of intracellular cer amide, Identification of a signaling intermediate that can influence primit ive hematopoietic cell fate decisions offers a new approach to the investig ation of signaling mechanisms in normal stem cell populations and to how th ese may be altered in leukemic cells. (C) 2000 by The American Society of H ematology.