Antiproliferative effects of γ-tocotrienol are associated with lipid raft disruption in HER2-positive human breast cancer cells

A large percentage of human breast cancers are characterized by excessive or aberrant HER2 activity. Lipid rafts are specialized microdomains within the plasma membrane that are required for HER2 activation and signal transduction. Since the anticancer activity of gamma-tocotrienol is associated with suppression in HER2 signaling, studies were conducted to examine the effects of gamma-tocotrienol on HER2 activation within the lipid raft microdomain in HER2-positive SKBR3 and BT474 human breast cancer cells. Treatment with 0-5 mu M gamma-tocotrienol induced a significant dose-dependent inhibition in cancer cell growth after a 5-day culture period, and these growth inhibitory effects were associated with a reduction in HER2 dimerization and phosphorylation (activation). Phosphorylated HER2 was found to be primarily located in the lipid raft microdomain of the plasma membrane in vehicle-treated control groups, whereas gamma-tocotrienol treatment significantly inhibited this effect. Assay of plasma membrane subcellular fractions showed that gamma-tocotrienol also accumulates exclusively within the lipid raft microdomain. Hydroxypropyl-beta-cyclodextrin (HP beta CD) is an agent that disrupts lipid raft integrity. Acute exposure to 3 mM HP beta CD alone had no effect, whereas an acute 24-h exposure to 20 mu M gamma-tocotrienol alone significantly decreased SKBR3 and BT474 cell viability. However, combined treatment with these agents greatly reduced gamma-tocotrienol accumulation in the lipid raft microdomain and cytotoxicity. In summary, these findings demonstrate that the anticancer effects of gamma-tocotrienol are associated with its accumulation in the lipid raft microdomain and subsequent interference with HER2 dimerization and activation in SKBR3 and BT474 human breast cancer cells. (C) 2015 Elsevier Inc. All rights reserved.