Inhibitory Effect Of Selenite Research Articles

Inhibitory Effect of Selenite on Invasion of HT1080 Tumor Cells

Selenium, an essential biological trace element, has been shown to reduce and prevent the incidence of cancer. Our previous studies have shown that selenite is involved in the chemoprevention of cancer and induction of apoptosis of cancer cells. In this study, we demonstrate that selenite also inhibits the invasion of tumor cells. Cancer cell invasion requires coordinated processes, such as changes in cell-cell and cell-matrix adhesion, degradation of the extracellular matrix, and cell migration. We found that selenite inhibited invasion of HT1080 human fibrosarcoma cells. Adhesion of HT1080 cells to the collagen matrix was also inhibited by treatment with selenite, but cell-cell interaction and cell motility were not affected by selenite. Moreover, selenite reduced expression of matrix metalloproteinase-2 and -9 and urokinase-type plasminogen activator, which are involved in matrix degradation, but increased a tissue inhibitor of metalloproteinase-1. This inhibitory effect of selenite on the protease expressions was mediated by the suppression of transcription factors, NF-kappaB and AP-1. However, selenate showed no remarkable effect on all the steps of cancer cell invasion.

Selenocompounds Induce a Redox Modulation of Protein Kinase C in the Cell, Compartmentally Independent from Cytosolic Glutathione: Its Role in Inhibition of Tumor Promotion

Since selenite and other redox-active selenocompounds can modify protein kinase C (PKC) in the test tube, we have determined whether or not this redox regulation occurs inside the cell despite having high concentrations of GSH and the role of this regulation in the inhibition of tumor promotion. By using phorbol ester-promoted JB6 epidermal cell transformation assay, the concentrations of selenite, selenocystine, and selenodiglutathione which are optimal for chemopreventive activity were determined. At such concentrations (0.5 to 2 μm) in the cells treated with these agents, only a slight but transient decrease in PKC activity was observed when measured with a low (5 μm), but not with a high (100 μm) concentration of ATP. However, when the cells were serum starved or pretreated with 2-deoxyglucose, there was a pronounced but transient inactivation of PKC when assayed with both low and high concentrations of ATP. The inactivation was reversed in the cell by an endogenous mechanism or by treatment with thiol agents in the test tube. In spite of a substantial (90%) depletion of GSH in the cells by pretreatment with buthionine sulfoximine, there was no further increase in the redox modification of PKC by selenite as well as no change in the inhibitory effect of selenite on the phorbol ester-stimulated induction of ornithine decarboxylase, which is an intermediate marker related to cell transformation. While GSH is known to influence certain actions of selenium, it may not be required to mediate the effects of selenite tested in this study. The water-soluble cytosolic GSH did not interfere with the redox modification of PKC probably due to the shielding of the cysteine-rich region of the enzyme by a weak hydrophobic association with the membrane. Due to the presence of cofactors in the crude cell extracts, PKC was more sensitive to selenite than in the purified form and was inactivated by low concentrations of selenite (IC50= 0.05 μm). This modification was reversed by thiol agents as well as by NADPH. A protein disulfide reductase, which can regenerate PKC, was present in the homogenate. Conceivably, selenite and other selenocompounds induce a redox modification of cellular PKC, compartmentally independent from the cytosolic GSH, but intimately connected to a NADPH-dependent reductase system, to mediate, at least in part, some of the cancer-preventive actions.

Inhibitory effect of selenite and other antioxidants on complement-mediated tissue injury in patients with epidemic hemorrhagic fever.

Results on the inhibitory effect on complement activation by sodium selenite, sodium glycyrrhizin, and a selenium-glycyrrhizin compound in hemorrhagic fever patients are presented.

The presence of thiols in the hepatic nuclear binding site for L-tryptophan: Studies with selenite

This study investigated the effects in vitro of selenite and selenate on nuclear L-tryptophan receptors of rat liver. Sodium selenite at 10 −6M inhibited the in vitro 3H-tryptophan binding to rat liver nuclear receptors. No inhibitory effect of selenite on 3H-tryptophan binding to nuclear receptor was found in the presence of 10 −4M dithiothreitol, a protective agent for sulfhydryl groups. Selenate as well as sulfite or sulfate did not exert an inhibitory effect on the tryptophan receptor. The results based upon in vitro studies suggest that selenium in the form of selenite may reversibly affect the L-tryptophan binding at the sulfhydryl sites in the nuclear receptor protein. In vivo administration of high (toxic) levels of selenite before or along with L-tryptophan inhibited the L-tryptophan-induced stimulation of hepatic protein synthesis.

Protein synthesis is not required for the inhibitory effect of selenite on cell colony formation and RNA synthesis

Selenite has been shown to undergo intracellular metabolism that results in its conversion to other low molecular weight Se-containing species and also to its incorporation into a selenocysteine residue in selenoprotein. In order to investigate whether the incorporation into protein is required for the cytotoxic effects of selenite, we have examined whether inhibition of protein synthesis prevents the inhibitory effect of selenite on the ability of cells to form colonies or to synthesize RNA. We have found that treatment of HeLa cells with cycloheximide inhibited protein synthesis by > 90% but had no effect on the inhibitory effect of selenite on cell colony formation or RNA synthesis. Since protein synthesis is not necessary for these cytotoxic effects of selenite they are unlikely to result from an increase in the synthesis of selenoproteins.

Effect of selenite and selenate on rat liver nuclear 3,5,3'-triiodothyronine (T3) receptor.

The present study was undertaken to investigate the effects of selenite (SeIV) or selenate (SeVI) on nuclear T3 receptors of rat liver. Selenite at 0.1 microM (p < 0.01) inhibited the T3 specific binding to rat liver nuclear receptors. The specific binding of the T3 receptor was fully restored when even 1.0 microM selenite was separated from the T3 receptor by gel filtration. No inhibitory effect of selenite (up to 100 microM) on the T3 binding to nuclear receptor was found in the presence of 1.0 mM dithiothreitol. The rate of dissociation of the T3-nuclear receptor complex was effectively increased by 0.1 microM selenite. Selenate up to 1 mM as well as sulfite or sulfate up to 0.1 mM did not exert an inhibitory effect on T3 receptors. The results based on the in vitro experiments suggest that the selenium in the form of selenite may reversibly affect the T3 binding on the receptor molecule.

Effect of cell density on the inhibition of tumor cell attachment and nucleic acid synthesis by selenite

The effect of cell density on the sensitivity of tumor cells to selenite has been examined. The inhibitory effect of selenite on cellular DNA and RNA synthesis was significantly greater in higher density cultures of HeLa cells and A2780 ovarian tumor cells. High-density cells were also more sensitive to the inhibitory effect of selenite on cell attachment. This difference could not be accounted for by a higher intracellular level of glutathione, since there was no significant difference between the cells at high or low density. The high-density cells were found to take up more selenium per cell during the exposure period; the resulting higher level of intracellular Se could explain their greater sensitivity to selenite. This hypothesis is supported by the observation that DNA synthesis in nuclei isolated from high-density cells did not exhibit higher sensitivity to inhibition by selenite than synthesis in nuclei isolated from low-density cells.

Physiology of selenite reduction by enterococci. II. Characterization of selenite inhibition and its reversal by ascorbate.

Cell-free extracts of Streptococcus faecalis N83 and Streptococcus faecium K6A were shown to possess comparable enzyme systems for the reduction of selenite to metallic selenium. The enzymatic complement of each of the organisms was oxygen sensitive, heat stable, and required flavin adenine dinucleotide (FAD) for maximum stimulation. Potent reduced nicotinamide adenine dinucleotide (NADH2) oxidase activity was demonstrated in extracts of both organisms. Although the NADH2 oxidase of S. faecium was sensitive to sulfhydryl inhibitors, the enzyme in S. faecalis was unaffected. The addition of ascorbate to the medium reversed the inhibitory effects of selenite and enhanced selenite reduction by S. faecium. Radioisotope studies, utilizing selenium75, demonstrated that selenite was taken up by both test organisms. The addition of ascorbate increased the rate and extent of selenium75 incorporation. It was concluded that the inhibitory effect of selenite and the inability of S. faecium to reduce the ion is a reflection of the oxidation by selenite of sulfhydryl groups on the active site.