Anticancer effects of combined gamma-tocotrienol and EGF receptor inhibitor treatment and effects of gamma-tocotrienol on Met receptor activation in +SA mouse mammary tumor cells

ErbB receptor-dependent mitogenic signaling is important in regulating various cellular processes like growth, proliferation and survival. However, aberrant ErbB receptor signaling is associated with various types of malignancies. gamma-Tocotrienol, a vitamin E isoform, displays potent anticancer activity that is associated with suppression in ErbB3 receptor phosphorylation and mitogenic signaling. Erlotinib and gefitinib are tyrosine kinase inhibitors that block ErbB1 receptor activation, whereas trastuzumab is a monoclonal antibody that has been designed to specifically inhibit ErbB2 receptor activation. However, the clinical effectiveness of these agents has been disappointing because of cooperation between different ErbB family members. Therefore, studies were conducted to determine low-dose combination treatment of gamma-tocotrienol with ErbB receptor inhibitors that target multiple ErbB receptor subtypes, would provide greater anticancer effects than monotherapy targeting a single ErbB receptor subtype. Results showed that treatment with 3.5 muM gamma-tocotrienol, 0.5 muM erlotinib or 1.0 muM gefitinib alone, significantly inhibited the +SA tumor cell growth. Combined treatment with subeffective doses of erlotinib (0.25 muM) or gefitinib (0.5 muM) significantly inhibited the growth and induced apoptosis in +SA tumor cells. In contrast, trastuzumab treatment alone or in combination had no effect on the +SA cells growth and viability. Combined treatment of gamma-tocotrienol with erlotinib or gefitinib also caused a large reduction in ErbB3 and ErbB4 receptor activity associated with a suppression in Stat and Akt mitogenic signaling in +SA tumor cells. These results strongly suggest that targeting multiple ErbB receptors with combined gamma-tocotrienol and specific ErbB receptor inhibitors may significantly improve the therapeutic response in breast cancer patients. In addition, since ErbB and Met receptors share common mitogenic signaling pathways, studies were conducted to determine the effect of gamma-tocotrienol on epidermal growth factor (EGF) or hepatocyte growth factor (HGF) dependent activation of Met receptors in +SA tumor cells. Stimulation of +SA cells with EGF (10 ng/ml) or HGF (10 ng/ml) resulted in the activation of Met receptors and treatment with Met tyrosine kinase inhibitor, SU11274 (5.5 muM) or gamma-tocotrienol (4 muM) inhibited the EGF or HGF-dependent growth and activation of Met receptors in these cells. Also, knockdown of Met receptors significantly inhibited the EGF or HGF-dependent growth of +SA cells. Additional studies showed that simultaneous exposure to EGF (10 ng/ml) and HGF (10 ng/ml) reversed the growth inhibitory effects of gamma-tocotrienol (4 muM) and SU11274 (5.5 muM) associated with the activation of Met receptors. However, combined treatment with low-doses of gamma-tocotrienol (2 muM) and SU11274 (3muM) inhibited the EGF and/or HGF-dependent growth of neoplastic +SA mammary epithelial cells. These results strongly suggest that gamma-tocotrienol treatment may provide significant health benefit in the prevention and/or treatment of breast cancer in women with deregulated HGF/Met signaling.