Organic selenium synergizes with tamoxifen to induce apoptosis in breast cancer cells and inhibit tamoxifen-sensitive and -resistant breast cancer xenografts

4293 Tamoxifen is widely prescribed as hormonal therapy for breast cancer and as a chemopreventative. Tamoxifen binds to estrogen receptor (ER) and antagonizes estrogen-mediated breast cancer growth. Despite the benefits of tamoxifen, almost all tamoxifen-responsive patients develop resistance to therapy. Selenium is an essential micronutrient currently in clinical trials for cancer prevention. Recent evidence now demonstrates the potential of utilizing selenium in a new way: as a novel therapy for overt cancer through combination with well-established chemotherapeutic and hormonal agents. We previously showed that organic selenium disrupts ERα signaling via downregulation of ERα and that selenium potentiates tamoxifen efficacy in tamoxifen-sensitive breast cancer cells and restored tamoxifen sensitivity in tamoxifen-resistant cells. The present work extends these studies to examine the effects of selenium + tamoxifen on apoptosis and growth of human breast cancer xenografts. Methylseleninic acid (MSA, 10 μM), a nontoxic, monomethylated form of selenium, but not 4-hydroxytamoxifen (10-7M), induced apoptosis of tamoxifen-sensitive MCF-7 cells and -resistant MCF-7-LCC2 cells in vitro as measured by quantitation of histone-associated DNA fragments. Remarkably, addition of tamoxifen to MSA resulted in synergistic apoptosis in both cell lines. In both MCF7 and MCF7-LCC2 cells, MSA and MSA + tamoxifen activated Bim, cytochrome-C, PARP, and caspases 7, 8, 9 but not caspases 2, 6, 10, 12 as measured by Western blot. Inhibitors of caspase 8 (Z-IETD-FMK, 20 μM) and 9 (Z-LEHD-FMK, 20 μM) blocked MSA + tamoxifen induced apoptosis. These data support a model in which MSA or MSA + tamoxifen activate the intrinsic, mitochondrial apoptotic pathway with cross-talk to caspase 8. Methylselenocysteine (MSC) was used in in vivo studies for reasons of efficacy, lack of toxicity, and recent approval for human clinical trials. 5 x 106 MCF-7 cells were bilaterally injected into mammary fat pads of ovariectomized nu/nu mice and implanted with an estradiol pellet (0.72 mg) until palpable tumor (10 days). At day 10, administration of either MSC (100 μg/mouse/day IP) or tamoxifen (5 mg implant) for an additional 30 days did not significantly alter estrogen-stimulated tumor growth. However, co-administration of MSC with tamoxifen completely suppressed growth of MCF-7 tumors. These data indicate that elevated apoptosis may underlie synergistic growth inhibition of breast cancer by combination of organic selenium with tamoxifen. Combination of selenium with tamoxifen may delay the onset of tamoxifen resistance while simultaneously improving therapeutic efficacy.