Mechanisms Mediating the Effects of γ-Tocotrienol When Used in Combination with PPARγ Agonists or Antagonists on MCF-7 and MDA-MB-231 Breast Cancer Cells
γ-Tocotrienol is a natural vitamin E that displays potent anticancer activity, and previous studies suggest that these effects involve alterations in PPARγ activity. Treatment with 0.5–6？μM？？γ-tocotrienol, 0.4–50？μM PPARγ agonists (rosiglitazone or troglitazone), or 0.4–25？μM PPARγ antagonists (GW9662 or T0070907) alone resulted in a dose-responsive inhibition of MCF-7 and MDA-MB-231 breast cancer proliferation. However, combined treatment of 1–4？μM？？γ-tocotrienol with PPARγ agonists reversed the growth inhibitory effects of γ-tocotrienol, whereas combined treatment of 1–4？μM？？γ-tocotrienol with PPARγ antagonists synergistically inhibited MCF-7 and MDA-MB-231 cell growth. Combined treatment of γ-tocotrienol and PPARγ agonists caused an increase in transcription activity of PPARγ along with increased expression of PPARγ and RXR, and decrease in PPARγ coactivators, CBP p/300, CBP C-20, and SRC-1, in both breast cancer cell lines. In contrast, combined treatment of γ-tocotrienol with PPARγ antagonists resulted in a decrease in transcription activity of PPARγ, along with decreased expression of PPARγ and RXR, increase in PPARγ coactivators, and corresponding decrease in PI3K/Akt mitogenic signaling in these cells. These findings suggest that elevations in PPARγ are correlated with increased breast cancer growth and survival, and treatment that decreases PPARγ expression may provide benefit in the treatment of breast cancer. 1. Introduction Peroxisome proliferator activated receptor γ (PPARγ) belongs to the nuclear receptor superfamily and functions as a ligand-activated transcription factor that forms a heterodimer complex with retinoid X receptor (RXR). This complex then binds to a specific DNA sequence called the peroxisome proliferator response element and initiates the recruitment of coactivator proteins such as CBP p/300, SRC-1, and CBP C-20, which further modulate gene transcription [1–3]. Studies have shown that PPARγ is overexpressed in many types of breast cancer cells [4–7]. Experimental evidence in rodents has shown that overexpression of PPARγ is associated with an increased incidence and growth in mammary tumors, whereas knockdown of PPARγ expression was found to significantly inhibit spontaneous mammary tumor development [8, 9]. Taken together these results suggest that inhibition of PPARγ expression and/or activity may be beneficial in the treatment of breast cancer. However, other studies have shown that treatment with the PPARγ agonist rosiglitazone and troglitazone, or conversely with PPARγ antagonists GW9662 and T0070907, were both found
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