Figure 5. Aminochrome cytotoxicity in the presence of protease inhibitors. (A) 1.-U373MG (WT), U373MGpSR (pSR), and U373MGsiGST6 (siRNA) cells were incubated with cell culture medium (C), 50 µM leupeptin (LP), 75 µM aminochrome (AM), 50 µM leupeptin, and 75 µM aminochrome (AM + LP) for 24 h. Leupeptin treatment increased aminochrome-induced cell death in U373MG and U373MGpSR cells, but not in U373MGsiGST6 cells. (B) The cells were incubated with cell culture medium (C), 100 µM pepstatin A (PE), 75 µM aminochrome (AM), 100 µM pepstatin A, and 75 µM aminochrome (AM + PE) for 24 h. Pepstatin A treatment increased aminochrome-induced cell death in U373MG WT, U373MGpSR, and U373MGsiGST6 cells. The statistical significance was assessed using analysis of variance (ANOVA) for multiple comparisons and Student t test (*P < 0.05; **P < 0.01; ***P < 0.001).