Selenium compounds are substrates for glutaredoxins: a novel pathway for selenium metabolism and a potential mechanism for selenium-mediated cytotoxicity.

The Grx (glutaredoxin) proteins are oxidoreductases with a central function in maintaining the redox balance within the cell. In the present study, we have explored the reactions between selenium compounds and the glutaredoxin system. Selenite, GS-Se-SG (selenodiglutathione) and selenocystine were all shown to be substrates of human Grx1, implying a novel role for the glutaredoxins in selenium metabolism. During the past few years, selenium has further evolved as a potential therapeutic agent in cancer treatment, and a leading mechanism of cytotoxicity is the generation of ROS (reactive oxygen species). Both selenite and GS-Se-SG were reduced by Grx1 and Grx2 in a non-stoichiometric manner due to redox cycling with oxygen, which in turn generated ROS. The role of Grx in selenium toxicity was therefore explored. Cells were treated with the selenium compounds in combination with transient overexpression of, or small interfering RNA against, Grx1. The results demonstrated an increased viability of the cells during silencing of Grx1, indicating that Grx1 is contributing to selenium toxicity. This is in contrast with TrxR (thioredoxin reductase), which previously was shown to protect cells from selenium cytotoxicity, verifying a diverse role between Grx and TrxR in selenium-mediated cytotoxicity. Furthermore, selenium treatment led to a marked increase in protein glutathionylation and cysteinylation that potentially can influence the activity and function of several proteins within the cell.

Selenium and the selenoprotein thioredoxin reductase in the prevention, treatment and diagnostics of cancer.

Selenium is an essential element that is specifically incorporated as selenocystein into selenoproteins. It is a potent modulator of eukaryotic cell growth with strictly concentration-dependant effects. Lower concentrations are necessary for cell survival and growth, whereas higher concentrations inhibit growth and induce cell death. It is well established that selenium has cancer preventive effects, and several studies also have shown that it has strong anticancer effects with a selective cytotoxicity on malignant drug-resistant cells while only exerting marginal effects on normal and benign cells. This cancer-specific cytotoxicity is likely explained by high affinity selenium uptake dependent on proteins connected to multidrug resistance. One of the most studied selenoproteins in cancer is thioredoxin reductase (TrxR) that has important functions in neoplastic growth and is an important component of the resistant phenotype. Several reports have shown that TrxR is induced in tumor cells and pre-neoplastic cells, and several commonly used drugs interact with the protein. In this review, we summarize the current knowledge of selenium as a potent preventive and tumor selective anticancer drug, and we also discuss the potential of using the expression and modulation of the selenoprotein TrxR in the diagnostics and treatment of cancer.

Phenotype-dependent apoptosis signalling in mesothelioma cells after selenite exposure.

BACKGROUND: Selenite is a promising anticancer agent which has been shown to induce apoptosis in malignant mesothelioma cells in a phenotype-dependent manner, where cells of the chemoresistant sarcomatoid phenotype are more sensitive. METHODS: In this paper, we investigate the apoptosis signalling mechanisms in sarcomatoid and epithelioid mesothelioma cells after selenite treatment. Apoptosis was measured with the Annexin-PI assay. The mitochondrial membrane potential, the expression of Bax, Bcl-XL, and the activation of caspase-3 were assayed with flow cytometry and a cytokeratin 18 cleavage assay. Signalling through JNK, p38, p53, and cathepsins B, D, and E was investigated with chemical inhibitors. Furthermore, the expression, nuclear translocation and DNA-binding activity of p53 was investigated using ICC, EMSA and the monitoring of p21 expression as a downstream event. Levels of thioredoxin (Trx) were measured by ELISA. RESULTS: In both cell lines, 10 microM selenite caused apoptosis and a marked loss of mitochondrial membrane potential. Bax was up-regulated only in the sarcomatoid cell line, while the epithelioid cell line down-regulated Bcl-XL and showed greater caspase-3 activation. Nuclear translocation of p53 was seen in both cell lines, but very little p21 expression was induced. Chemical inhibition of p53 did not protect the cells from apoptosis. p53 lost its DNA binding ability after selenite treatment and was enriched in an inactive form. Levels of thioredoxin decreased after selenite treatment. Chemical inhibition of MAP kinases and cathepsins showed that p38 and cathepsin B had some mediatory effect while JNK had an anti-apoptotic role. CONCLUSION: We delineate pathways of apoptosis signalling in response to selenite, showing differences between epithelioid and sarcomatoid mesothelioma cells. These differences may partly explain why sarcomatoid cells are more sensitive to selenite.

Extracellular thiol-assisted selenium uptake dependent on the x(c)- cystine transporter explains the cancer-specific cytotoxicity of selenite.

The selenium salt selenite (SeO(3)(2-)) is cytotoxic in low to moderate concentrations, with a remarkable specificity for cancer cells resistant to conventional chemotherapy. Our data show that selenium uptake and accumulation, rather than intracellular events, are crucial to the specific selenite cytotoxicity observed in resistant cancer cells. We show that selenium uptake depends on extracellular reduction, and that the extracellular environment is a key factor specific to selenite cytotoxicity. The extracellular reduction is mediated by cysteine, and the efficacy is determined by the uptake of cystine by the x(c)(-) antiporter and secretion of cysteine by multidrug resistance proteins, both of which are frequently overexpressed by resistant cancer cells. This mechanism provides molecular evidence for the existence of an inverse relationship between resistance to conventional chemotherapy and sensitivity to selenite cytotoxicity, and highlights the great therapeutic potential in treating multidrug-resistant cancer.

Selenite is a potent cytotoxic agent for human primary AML cells.

Selenite is a potent inhibitor of malignant cell growth. Although the cytotoxic effects have been extensively investigated in vitro, there are only a limited number of studies using primary tumor cells with concomitant comparison to conventional drugs. An ex vivo model with primary cells from 39 consecutive patients with acute myeloid leukemia (AML) were exposed to a panel of conventional cytotoxic drugs, and the effects on viability were compared to those of clinically achievable concentrations of selenite. Selenite at 5 microM caused the lowest mean survival of primary tumor cells in the panel of all tested drugs (28.95% CI 18.60-39.30%). The cells showed a significant (p<0.05) correlation in the resistance to all tested conventional AML drugs whereas selenite did not, indicating sensitivity to selenite also in multi drug resistant cells. Exposure to selenite also resulted in an increased mRNA expression of the antioxidant proteins TrxR1 and Grx, while staining for TrxR1 showed decreased protein levels. The results strongly suggest a great potential for selenite in the treatment of multi drug resistant AML.