Synergistic anticancer effects of combined γ-tocotrienol and oridonin treatment is associated with the induction of autophagy
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γ-Tocotrienol and oridonin are natural phytochemicals that display potent anticancer activity. Studies showed that combined treatment with subeffective doses of γ-tocotrienol with oridonin resulted in synergistic autophagic and apoptotic effects in malignant +SA, but not normal CL-S1 mouse mammary epithelial cells in vitro. Specifically, combined treatment with low doses of γ-tocotrienol (8 µM) and oridonin (2 µM) for 24 h resulted in synergistic inhibition of +SA mammary cancer cells viability. This combination significantly enhanced the expression of autophagy cellular markers including the conversion of LC3B-I to LC3B-II, beclin-1, Atg3, Atg7, Atg5–Atg12, LAMP-1 and cathepsin-D, and pretreatment with the autophagy inhibitors 3-methyladenine (3-MA) or bafilomycin A1 (Baf1) blocked these effects. Furthermore, blockade of γ-tocotrienol and oridonin-induced autophagy with 3-MA or Baf1 induced a modest, but significant reduction in cytotoxicity resulting from the combined treatment of these phytochemicals. The anticancer effects of combination treatment was also associated with a large suppression in Akt/mTOR mitogenic signaling and corresponding increase in the levels of apoptotic cellular marker including cleaved caspase-3 and PARP, and Bax/Bcl-2 ratio in these tumor cells. These effects were also found to be selective against cancer cells, since similar combined treatment with γ-tocotrienol and oridonin did not induce autophagy or reduce viability of normal mouse CL-S1 mammary epithelial cells. These findings indicate that combined γ-tocotrienol and oridonin-induced autophagy plays a role in mediating the synergistic anticancer effects of these phytochemicals.
Keywordsγ-Tocotrienol Autophagy Breast cancer Beclin-1 LAMP-1 Cathepsin-D
The authors would like to thank the First Tech International Ltd. for generously providing γ-tocotrienol for use in these studies. This work was performed at the School of Pharmacy, University of Louisiana at Monroe, Monroe, LA USA, and supported in part by grants from First Tec International Ltd. (Hong Kong), the Louisiana Cancer Foundation and the Louisiana Campuses Research Initiative (LACRI). The authors would also like to thank Dr. Karen P Briski for her assistance in studies with the confocal microscope.
Compliance with Ethical Standards
Conflict of interest
The authors declare that they have no personal, financial or competing interests. First Tech International Ltd. provided a grant that partially paid for the funding of these experiments and the purified γ-tocotrienol that was used in these experiments.
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