Abstract 5417: Suppression in mevalonate synthesis mediates the antiproliferative effects of combined statin and γ-tocotrienol treatment in mammary tumor cells

Abstract

Abstract Statins directly inhibit 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) activity, while γ-tocotrienol, an isoform of vitamin E, enhances the degradation and reduces the cellular levels of HMGR in various tumor cell lines. Since treatment with statins or γ-tocotrienol alone induced a dose-responsive inhibition, whereas combined treatment with subeffective doses of these agents resulted in a synergistic inhibition in +SA mammary tumor cell growth, studies were conducted to investigate the role of the HMGR pathway in mediating the antiproliferative effects of combined low-dose statin and γ-tocotrienol treatment in +SA cells. Cells were maintained in serum-free defined media containing EGF (10 ng/mL) and insulin (10 µg/mL) as co-mitogens. Treatment with 8 μM simvastatin alone inhibited +SA cell growth and isoprenylation of Rap1A and Rab6, and supplementation with 2 µM mevalonate reversed these effects, indicating that simvastatin inhibits +SA cell growth by suppressing mevalonate synthesis. Also, treatment with 4 μM γ-tocotrienol alone inhibited +SA cell growth but had no effect on the isoprenylation of Rap1A and Rab6, and supplementation with 2 µM mevalonate had no effect on cell growth or isoprenylation, suggesting that γ-tocotrienol inhibits +SA cell growth through mechanisms independent of mevalonate suppression. The confocal immunofluorescence imaging revealed that treatment with subeffective doses of simvastatin (0.25 μM), lovastatin (0.25 μM), mevastatin (0.25 μM), pravastatin (10 μM), or γ-tocotrienol (2 μM) alone had no effect on the positive Ki-67 staining, while combined low-dose statin and γ-tocotrienol treatments markedly suppressed the positive Ki-67 staining in +SA cells as compared with control. Treatment with same subeffective doses of these statins or γ-tocotrienol alone had no effect on protein prenylation or mitogenic signaling, whereas combined treatment with these agents resulted in a significant inhibition in +SA cell growth, and a corresponding decrease in the relative levels of total HMGR, Rap1A and Rab6 prenylation, phosphorylated (active) MAPK, and cyclin D1, as compared with respective controls. However, these inhibitory effects on +SA cell growth, prenylation and mitogenic signaling were completely reversed by mevalonate supplementation. These findings demonstrate that the synergistic antiproliferative effects of combined low-dose statin and γ-tocotrienol treatment are directly related to an inhibition in HMGR activity and subsequent suppression in mevalonate synthesis. Since HMGR activity is characteristically elevated in cancer, these results strongly suggest that combination therapy with statins and γ-tocotrienol may be effective in suppressing the growth of breast cancer in women. Supported by grants from NIH (CA 86833) and First Tech International Ltd. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5417.