We investigated the effects of 2-methoxyestradiol (2-ME), a promising new antitumor agent, on viable cell number and nuclear morphology of malignant glioma cells (three human and one rat glioma cell lines) and analyzed the controversial role of death recepor 5 (DR5) upregulation in 2-ME induced apoptosis. Microtiter-tetrazolium (MTT) assays showed a significant reduction of viable cells after incubation with 2 microM and 20 microM 2-ME for 48 and 72 hours in all cultures. In the 20 microM concentration, there were even significant effects in the majority of shorter incubation periods. Hoechst 33258 stains showed a substantial amount of cells with nuclear fragmentation indicating a late stage of apoptosis after 20 microM 2-ME treatments of 24 hours and more. The role of the DR5-mediated extrinsic apoptotic pathway was further studied in the three human glioma cell lines; 50 ng/ml of the DR5 ligand TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) and 2 microM 2-ME showed no synergism, as determined by MTT assays. Real-time PCR revealed no significantly increased amount of DR5 mRNA, suggesting that receptor upregulation does not play a major role for 2-ME-induced apoptosis in glioma cells, in contrast to data for a breast cancer cell line in the literature.