Tocotrienol-Induced Caspase-8 Activation is Unrelated to Death Receptor Apoptotic Signaling in Neoplastic Mammary Epithelial Cells

Abstract

Tocotrienols, a subclass in the vitamin E family of compounds, have been shown to induce apoptosis by activating caspase-8 and caspase-3 in neoplastic mammary epithelial cells. Since caspase-8 activation is associated with death receptor apoptotic signaling, studies were conducted to determine the exact death receptor/ligand involved in tocotrienol-induced apoptosis. Highly malignant +SA mouse mammary epithelial cells were grown in culture and maintained in serum-free media. Treatment with 20 μM γ-tocotrienol decreased +SA cell viability by inducing apoptosis, as determined by positive terminal dUTP nick end labeling (TUNEL) immunocytochemical staining. Western blot analysis showed that γ-tocotrienol treatment increased the levels of cleaved (active) caspase-8 and caspase-3. Combined treatment with caspase inhibitors completely blocked tocotrienol-induced apoptosis. Additional studies showed that treatment with 100 ng/ml tumor necrosis factor-α (TNF-α), 100 ng/ml FasL, 100 ng/ml TNF-related apoptosis-inducing ligand (TRAIL), or 1 μg/ml apoptosis-inducing Fas antibody failed to induce death in +SA cells, indicating that this mammary tumor cell line is resistant to death receptor-induced apoptosis. Furthermore, treatment with 20 μM γ-tocotrienol had no effect on total, membrane, or cytosolic levels of Fas, Fas ligand (FasL), or Fas-associated via death domain (FADD) and did not induce translocation of Fas, FasL, or FADD from the cytosolic to the membrane fraction, providing additional evidence that tocotrienol-induced caspase-8 activation is not associated with death receptor apoptotic signaling. Other studies showed that treatment with 20 μM γ-tocotrienol induced a large decrease in the relative intracellular levels of phospho–phosphatidylinositol 3-kinase (PI3K)-dependent kinase 1 (phospho-PDK-1 active), phospho-Akt (active), and phospho-glycogen synthase kinase-3, as well as decreasing intracellular levels of FLICE-inhibitory protein (FLIP), an antiapoptotic protein that inhibits caspase-8 activation, in these cells. Since stimulation of the PI3K/PDK/Akt mitogenic pathway is associated with increased FLIP expression, enhanced cellular proliferation, and survival, these results indicate that tocotrienol-induced caspase-8 activation and apoptosis in malignant +SA mammary epithelial cells is associated with a suppression in PI3K/PDK-1/Akt mitogenic signaling and subsequent reduction in intracellular FLIP levels.