Abstract P3-03-12: Gamma-tocotrienol induces autophagy in malignant mammary tumor cells

Abstract

Abstract γ-Tocotrienol, a member of the vitamin E family of compounds, displays potent antiproliferative and apoptotic activity in a variety of cancer cell types at treatment doses that have little or no effect on normal cell viability or growth. Autophagy is a tightly regulated lysosomal self-digestive process that can either promote cell survival or be involved in type II cell death. Autophagy is induced in several diseases such as liver disease, neurodegeneration, Crohn's disease, aging, cancer, and metabolic syndrome. Autophagy is accompanied by the progressive development of vesicle structures from autophagomsomes (not acidic) to amphisome and autolysosomes (acidic). The execution and regulation of the autophagic program relies on several highly conserved autophagy-related genes (the Atg genes) that regulate the cannibalism of intracellular cytoplasm, proteins and organelles. The potential role of autophagy in mediating γ-tocotrienol-induced cytotoxicity is not currently understood. Therefore, studies were conducted to determine the effects of γ-tocotrienol on the induction of autophagy in various breast cancer cell lines. Mouse (+SA) and human (MCF-7 and MDA-MD-231) mammary tumor cells lines were exposed for an acute 24 h period to 20 or 40 μM γ-tocotrienol. Results showed that γ-tocotrienol treatment caused a relatively large increase in the accumulation of monodansylcadaverine (MDC)-labeled vacuoles, an early marker of autophagic compartment formation, in all mammary tumor cell lines. These cells also displayed an increased conversion of microtubule-associated protein 1A/1B-light chain 3 (LC3-I, the cytosolic form) to LC3-II (lapitated form associated with autophagosome), which is associated with the increased formation of autophagic vacuoles, followed by upregulation of Beclin-1. Treatment with γ-tocotrienol also caused a significant increase in the LC3II/LC3I ratio, a characteristic marker of autophagy, as compared to vehicle-treated control groups. Additional flow cytometery analysis showed that γ-tocotrienol treatment caused an increase in acridine orange staining, an indicator of autophagy, in all mammary cancer cell lines. In contrast, similar treatment of γ-tocotrienol on immortalized mouse (CL-S1) and human (MCF10A) normal mammary epithelial cell lines did not show significant accumulation of MDC–labeled vacuoles or the conversion of LC3-I to LC3-II. In addition, γ-tocotrienol-induced autophagy was also associated with a suppression in PI3K/Akt/mTOR signaling and the initiation of apoptosis as indicated by caspase activation in these mammary cancer cells. In summary, these finding suggest that γ-tocotrienol selectively induces autophagy in mouse and human mammary cancer cell lines and support a novel function of autophagy in promoting breast cancer cells death. This study was supported by grants from First Tech International Ltd., and the Malaysian Palm Oil Council. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P3-03-12.