Functions of selenium are diverse as antioxidant, anti-inflammation, increased immunity, reduced cancer incidence, blocking tumor invasion and metastasis, and further clinical application as treatment with radiation and chemotherapy. These functions of selenium are mostly related to oxidation and reduction mechanisms of selenium metabolites. Hydrogen selenide from selenite, and methylselenol (MSeH) from Se-methylselenocyteine (MSeC) and methylseleninicacid (MSeA) are the most reactive metabolites produced reactive oxygen species (ROS); furthermore, these metabolites may involve in oxidizing sulfhydryl groups, including glutathione. Selenite also reacted with glutathione and produces hydrogen selenide via selenodiglutathione (SeDG), which induces cytotoxicity as cell apoptosis, ROS production, DNA damage, and adenosine-methionine methylation in the cellular nucleus. However, a more pronounced effect was shown in the subsequent treatment of sodium selenite with chemotherapy and radiation therapy. High doses of sodium selenite were effective to increase radiation therapy and chemotherapy, and further to reduce radiation side effects and drug resistance. In our study, advanced cancer patients can tolerate until 5000 μg of sodium selenite in combination with radiation and chemotherapy since the half-life of sodium selenite may be relatively short, and, further, selenium may accumulates more in cancer cells than that of normal cells, which may be toxic to the cancer cells. Further clinical studies of high amount sodium selenite are required to treat advanced cancer patients.

Selenium, an essential biological trace element, reduces the incidence of cancer. Our previous studies show that selenite inhibits tumor invasion by suppressing the expression of matrix metalloproteinases (MMP) -2 and -9.Methylseleninic acid (MSeA), an immediate precursor of methylselenol, inhibits tumor cell growth in vitro and mammary carcinogenesis in vivo. In this study, we demonstrate that MSeA suppresses pro-MMP-2 activation in a dose-dependent manner induced by 12-O-tetradecanoylphorbol-13-acetate (PMA), and further decreases the invasiveness of HT1080 tumor cells. Membrane type-1-MMP (MT1-MMP) is a crucial element in the process of pro-MMP-2 activation. Pro-MMP-2 binds MT1-MMP, using tissue inhibitor of metalloproteinase-2 (TIMP-2) as an adaptor, by forming a trimolecular complex on the cell surface. MSeA blocked MT1-MMP in a dose-dependent manner, but not TIMP-2 expression. MMP-9 and TIMP-1 levels were not affected by MSeA. Selenite induced a decrease in protein levels of both pro-MMPs -9 and -2, but not active forms of pro-MMP-2. MT1-MMP expression is regulated by NF-κB. Our data show that the effect of MSeA on MT1-MMP expression is mediated through suppression of NF-κB activity. Methylselenol generated by selenomethionine (SeMet) and methioninase (METase) inhibited pro-MMP-2 activation induced by PMA, confirming the effect of MSeA on pro-MMP-2 activity. Moreover, ROS production induced by PMA was partly decreased in the presence of MSeA. This suppression of ROS production may be related to diminished NF-κB activity. Thus, our results suggest that MSeA blocks tumor invasion in vitro via inhibiting pro-MMP-2 activation mediated by suppression of MT1-MMP expression, which is regulated by the NF-κB signal pathway.

To assess the inhibitory effects of methylselenol on the invasion of murine B16F10 melanoma cells, we carried out in vivo and in vitro experiments using Se-methylselenocysteine (Se-MSC) and selenomethionine (SeMet), respectively. In an animal experiment, the supplementation of drinking water with Se-MSC (4 ppm Se) led to a significant increase in Se levels in the lung, liver and serum in mice. Mice given a mash diet or water supplemented with Se-MSC (2, 4 and 6 ppm Se in the mash diet, and 2 and 4 ppm Se in the drinking water) displayed an almost completely diminished pulmonary metastasis of B16F10 melanoma cells and an enhanced survival, compared to the control mice which were given a basal diet. Treatment with non-cytotoxic concentrations of SeMet (2.5, 5 and 10 µM plus 0.02 U/ml METase, methioninase) induced a substantial decrease in the expression of integrin αvβ3, the FN receptor and adhesion ability to vitronectin (VN) and fibronectin (FN) in B16F10 melanoma cells. Moreover, these compounds suppressed gelatinase activity, invasive ability and wound migration in the culture system. SeMet-METase prevented the conversion of pro-MMP-9 to its active form and decreased pro-MMP-2 activities in a zymogram. The pre-treatment of B16F10 melanoma cells with SeMet-METase led to a decrease in pulmonary metastasis and extended survival in mice injected with tumor cells. Collectively, our results indicate that integrin expression is crucial in promoting the metastatic phenotype in murine B16F10 melanoma cells by supporting specific adhesive and invasive properties, suggesting that Se-MSC effectively reduces the metastasis of B16F10 melanoma cells as a nutritional adjuvant. Methylselenol may also contribute to the suppression of integrin expression.