The oligomerization status of soluble tumor necrosis factor-related apoptosis inducing ligand (TRAIL) trimers has an overwhelming impact on cell death induction in a cell-type dependent fashion. Thus, we evaluated the ability of single and oligomerized TRAIL trimers to induce cell death in human myeloma cells. In all myeloma cell lines analyzed, oligomerized TRAIL trimers induced caspase activation and complete cell death, whereas non-oligomerized TRAIL trimers showed no or only a modest effect. Caspase activation induced by oligomerized TRAIL was blocked in all cell lines by the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone (z-VAD-fmk). Cell death induction was largely blocked in two cell lines by z-VAD-fmk, but was only marginally attenuated in three other cell lines, indicating that TRAIL induces caspase-dependent and caspase-independent cell death in myeloma cells. Preceding cell death, TRAIL activated nuclear factor kappaB, c-Jun N-terminal kinase, p38 and p42/44. Although TRAIL-induced stimulation of c-Jun N-terminal kinase and p38 was caspase-dependent in a cell type-specific fashion, activation of nuclear factor kappaB and p42/44 was caspase-independent in all cases. In accordance with activation of the nuclear factor kappaB pathway, we observed transcriptional up-regulation of several well established nuclear factor kappaB target genes. Furthermore, we found that TRAIL activates proinflammatory pathways in approximately 50% of primary myeloma samples. Taken together, our data suggest (a) that oligomerized TRAIL variants are necessary to ensure maximal cell death induction in myeloma cells and (b) TRAIL should be used in combination with anti-inflammatory drugs for treatment of myeloma to avoid and/or minimize any potential side-effects arising from the proinflammatory properties of the molecule.