Matrix metalloproteinase-9 (MMP-9) is an important angiogenic and prognostic factor in malignant tumors. Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is known as the death ligand, which induces preferential apoptosis of transformed tumor cells. In this study, we investigated the biological functions of TRAIL, other than its role in induction of apoptosis. We demonstrated that TRAIL induces MMP-9 expression in human astrocytoma cells, which is preceded by activation of extracellular signal-regulated protein kinase (ERK). In addition, TRAIL induces the DNA-binding activity of NF-kappaB, an important transcription factor for MMP-9 induction. The specific MEK inhibitor, U0126, significantly blocks TRAIL-mediated NF-kappaB activation and subsequent MMP-9 induction. These findings indicate that TRAIL treatment in human astrocytoma cells leads to the activation of NF-kappaB and subsequent expression of MMP-9, which are dependent on ERK activation. Collectively, these results suggest that TRAIL has alternative biological functions in addition to its role in inducing apoptosis in human malignant astrocytoma cells.