Abstract B154: Thymoquinone causes mitochondrial membrane disruption and potentiates chemotherapeutic drug- and radiation-induced cytotoxicity in breast cancer cells

Abstract

Abstract Thymoquinone, which is the active constituent of the volatile oil extracted from black caraway seeds (Nigella sativa), has been reported to have antiinflammatory, antioxidant, and antineoplastic activities in vitro and in vivo. In this study we explored the mechanism by which thymoquinone inhibits breast cancer cell growth, as well as the potential of using thymoquinone in combination with conventional chemotherapeutic drugs or radiation therapy. Cytofluorimetric analysis of Oregon Green 488-stained MDA-MB-231 and MDA-MB-468 breast cancer cells following thymoquinone treatment demonstrated inhibition of cell proliferation. Colorimetric 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays performed on MDA-MB-231, MDA-MB-468, and T-47D breast cancer cells after 24, 48, and 72 h treatment with thymoquinone (0.5–10 µM) showed a time- and dose-dependent cytotoxic effect, while untransformed human mammary epithelial cells were not adversely affected by thymoquinone. Longer exposure (144 h) to lower doses of thymoquinone (0.25–1 µM) was also cytotoxic for breast carcinoma cells. Importantly, staining of thymoquinone-treated breast cancer cells with annexin-V-FLUOS/propidium iodide indicated that cell death was by apoptosis. Western blot analysis of thymoquinone-treated breast cancer cells revealed increased cytosolic cytochrome c and PARP cleavage, which suggested induction of the mitochondrial pathway of apoptosis. However, broad spectrum caspase inhibitors (Bod-D and z-VAD-fmk) did not prevent thymoquinone-induced cytotoxicity, implying caspase-independent cell death. Thymoquinone treatment also sensitized breast cancer cells to γ-radiation and the conventional chemotherapeutic agents docetaxel and cisplatin. We conclude that thymoquinone caused caspase-independent apoptosis in breast cancer cells that involved the loss of mitochondrial membrane integrity. Furthermore, the ability of thymoquinone to potentiate the cytotoxic effects of chemotherapeutic drugs and -radiation may have application in a clinical setting. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B154.