Vitamin E succinate mediated apoptosis by juxtaposing endoplasmic reticulum and mitochondria

Vitamin E succinate (VES) is an esterified form of natural α-tocopherol, has turned out to be novel anticancer agent. However, its anticancer mechanisms have not been illustrated. Previously, we reported VES mediated Ca²⁺ release from the endoplasmic reticulum (ER) causes mitochondrial Ca²⁺ overload, leading to mitochondrial depolarization and apoptosis. Here, we elucidated the mechanism of VES-induced Ca²⁺ transfer from ER to mitochondria by investigating the role of VES in ER-mitochondria contact formation. Transmission electron microscopic observation confirms VES mediated ER-mitochondria contact while fluorescence microscopic analysis revealed that VES increased mitochondria-associated ER membrane (MAM) formation. Pre-treatment with the inositol 1,4,5-triphosphate receptor (IP₃R) antagonist 2-aminoethyl diphenylborinate (2-APB) decreased VES-induced MAM formation, suggesting the involvement of VES-induced Ca²⁺ efflux from ER in MAM formation. The ER IP₃R receptor is known to interact with voltage-dependent anion channels (VDAC) via the chaperone glucose-regulated protein 75 kDa (GRP75) to bring ER and mitochondria nearby. Although we revealed that VES treatment does not affect GRP75 protein level, it increases GRP75 localization in the MAM. In addition, the inhibition of Ca²⁺ release from ER by 2-APB decreases GRP75 localization in the MAM, suggesting the possibility of Ca²⁺-induced conformational change of GRP75 that promotes formation of the IP₃R-GRP75-VDAC complex and thereby encourages MAM formation. This study identifies the mechanism of VES-induced enhanced Ca²⁺ transfer from ER to mitochondria, which causes mitochondrial Ca²⁺ overload leading to apoptosis.