Triptolide is a Chinese herb that has been shown to induce apoptosis in various tumor cells. We have previously demonstrated that triptolide induces lysosomal-mediated apoptosis in MCF-7 breast cancer cells. These findings are significant because MCF-7 cells lack caspase-3, a key executioner caspase, causing them to be resistant to chemotherapeutics. In the present study, we examine whether triptolide can induce apoptosis by targeting lysosomes and mitochondria. The effects of triptolide on lysosomal membrane integrity, subcellular localization of cathepsin B, mitochondrial localization, and mitochondrial membrane permeabilization in MCF-7 cells were assessed via fluorescence microscopy. Acridine orange staining demonstrated that triptolide caused rupture of lysosomal membranes. This effect on disruption of the lysosomal membrane was confirmed by immunofluorescent detection of cathepsin B in the cytosol. MitoTracker Green staining revealed mitochondria limited to the cytosol in control cells while mitochondria were observed in nuclear regions in experimental cells. Triptolide caused depolarization of the mitochondrial membrane, as assessed by JC-1 staining. Taken together, our results demonstrate for the first time in MCF-7 cells that triptolide induces apoptosis by lysosomal- and mitochondrial-dependent pathways. Our study provides a mechanism that may be used to develop novel breast cancer therapies wherein triptolide sensitizes resistant breast cancer cells to cell death
