Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) up-regulated the expression of constitutive cyclooxygenase (COX)-1 protein in HL-60 cells without affecting COX-2. The TRAIL-mediated COX-1 up-regulation was accompanied by a significant increase of the PGE(2) synthesis and release, which was suppressed by the COX-1 inhibitor valeryl salicylate but not by the COX-2 inhibitor NS-398. Experiments carried out by adding exogenous PGE(2) to HL-60 cells indicated that PGE(2) was not involved in TRAIL cytotoxicity and rather showed a dose-dependent protection against TRAIL-induced apoptosis. Importantly, the ability of TRAIL to increase PGE(2) production was also observed in normal, human CD34-derived myeloid cells and in freshly isolated peripheral blood CD14(+) monocytes. Moreover, in contrast to HL-60 cells, primary, normal cells were not susceptible to TRAIL cytotoxicity. These data indicate that the ability of TRAIL to up-regulate eicosanoid production and release is not confined to malignant leukemic cells, but it may also play a role in normal hematopoiesis.