Effects of sodium selenite and selenate on DNA and RNA synthesis in vitro

Mechanisms of inhibition of zinc-finger transcription factors by selenium compounds ebselen and selenite

The anti-inflammatory selenium compounds, ebselen (2-phenyl-1,2-benzisoselenazol-3[2H]-one) and selenite, were found to alter the DNA binding mechanisms and structures of cysteine-rich zinc-finger transcription factors. As assayed by DNase I protection, DNA binding by TFIIIA (transcription factor IIIA, prototypical Cys(2)His(2) zinc finger protein), was inhibited by micromolar amounts of ebselen. In a gel shift assay, ebselen inhibited the Cys(2)His(2) zinc finger-containing DNA binding domain (DBD) of the NF-kappaB mediated transcription factor Sp1. Ebselen also inhibited DNA binding by the p50 subunit of the pro-inflammatory Cys-containing NF-kappaB transcription factor. Electrospray ionization mass spectrometry (ESI-MS) was utilized to elucidate mechanisms of chemical interaction between ebselen and a zinc-bound Cys(2)His(2) zinc finger polypeptide modeled after the third finger of Sp1 (Sp1-3). Exposing Sp1-3 to micromolar amounts of ebselen resulted in Zn(2+) release from this peptide and the formation of a disulfide bond by oxidation of zinc finger SH groups, the likely mechanism for DNA binding inhibition. Selenite was shown by ESI-MS to also eject zinc from Sp1-3 as well as induce disulfide bond formation through SH oxidation. The selenite-dependent inhibition/oxidation mechanism differed from that of ebselen by inducing the formation of a stable selenotrisulfide bond. Selenite-induced selenotrisulfide formation was dependent upon the structure of the Cys(2)His(2) zinc finger as alteration in the finger structure enhanced this reaction as well as selenite-dependent zinc release. Ebselen and selenite-dependent inhibition/oxidation of Cys-rich zinc finger proteins, with concomitant release of zinc and finger structural changes, points to mechanisms at the atomic and protein level for selenium-induced alterations in Cys-rich proteins, and possible amelioration of certain inflammatory, neurodegenerative, and oncogenic responses.

Inhibition of zinc finger protein-DNA interactions by sodium selenite

Sodium selenite and sodium selenate were analyzed for their ability to alter the DNA binding mechanisms of the Cys(2)His(2) zinc finger proteins, transcription factor IIIA (TFIIIA) and Sp1. TFIIIA is a positive regulator of 5S ribosomal RNA synthesis, and Sp1 is involved in cell proliferation and invasiveness. As assayed by DNase I protection, the interaction of the DNA binding domain of TFIIIA with the 5S ribosomal gene was inhibited by 25 microM selenite ions but not by 250 microM selenate ions. Selenite inhibition kinetics of TFIIIA progressed to completion in about 5 min. Preincubation of free TFIIIA with selenite resulted in DNA binding inhibition, whereas preincubation of a TFIIIA/5S RNA complex with selenite did not. Since 5S RNA binds to the TFIIIA DNA binding domain, this result is consistent with an inhibition mechanism via selenite binding to that region of this protein. Inhibition was not readily reversible and occurred in the presence of an excess of beta-mercaptoethanol; elevated amounts of dithiothreitol mitigated the inhibitory effect. Significantly less selenite (2.5-5 microM) inhibited the specific DNA binding of transcription factor Sp1 to the simian virus 40 (SV40) early promoter/enhancer. The selenite inhibition kinetics of Sp1 were fast, going to completion in about 1 min. SV40 DNA binding by the non-zinc finger transcription factor AP-2 was not inhibited by selenite. Inhibition of Cys(2)His(2) zinc finger proteins by micromolar amounts of selenite points to additional mechanisms for selenite-induced diminution of cell growth and anticancer activity.

Protective effects of vitamins and selenium compounds in yeast

Antimutagens and anticarcinogens are known to play an important role in decreasing damages induced by oxidants. In this study, we investigated the genotoxic and antimutagenic potential of two selenium compounds (sodium selenite: Na(2)SeO(3); seleno-DL-methionine: C(5)H(11)NO(2)Se) and Vitamins A and E in yeast cells of Saccharomyces cerevisiae. An oxidative mutagen (hydrogen peroxide (H(2)O(2)), HP) was chosen as positive control. We determined the enzymatic activities involved in the protection against oxidative damages (catalase: CAT; superoxide dismutase: SOD; glutathione peroxidase: GPx) in the cytosolic extract of yeast cells. The results demonstrated that selenium compounds exerted both mutagenic and antimutagenic effect at different concentrations. Antimutagenesis was evident both in stationary and in logarithmic phase cells. Catalase, SOD, and GPx were significantly increased in the presence of all the compounds assayed. Vitamins A (retinol) and E (alpha-tocopherol) did not have toxic or mutagenic action.

Intracellular distribution of selenium and the growth of mammary cells in culture

Retention of Se in CMT-13 cells increased with an increase in the concentration of selenite in the incubation medium, the duration of exposure, and the density of the culture. The enhanced toxicity of selenite coincided with a proportional increase in Se in both the cytoplasm and nucleus. About 90% of the accumulated Se was isolated with cytoplasmic macromolecules. Increased nuclear Se retention correlated with increased cytoplasmic Se retention. Greater quantities of cytosolic Se-containing proteins (74, 55, 41, 34, and 28 kDa) and a nuclear Se-containing protein (56 kDa) were detected as the quantity of Se within CMT-13 cells increased. These findings suggest that cellular retention and distribution of Se are determinants of the degree of cellular growth inhibition caused by this trace element.

Effect of selenite on tumor cell invasiveness

Pre-exposure of HeLa or NIH:OVCAR-3 cells to selenite resulted in a dose-dependent decrease in the ability of the cells to invade a layer of Matrigel, a reconstituted basement membrane preparation. In contrast, selenate, selenomethionine and sulfite had no significant effect on cell invasiveness. Exposure of HeLa cells to selenite also resulted in a decrease in two of the necessary steps of the invasion process, attachment and mobility; in contrast, exposure of OVCAR cells decreased attachment but not mobility. There was an apparent correlation between the processes that are affected by selenite and those that involve the cellular fibronectin receptor (alpha 5 beta 1 integrin).

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.

Inhibition by selenium of DNA and RNA synthesis in normal and malignant human cells in vitro

Several studies have demonstrated differences between normal and malignant cells in their sensitivity to various effects of selenite. We have compared the effect of selenite on DNA and RNA synthesis in two pairs of normal and malignant human cell lines. One pair of cells, CCL-210 (normal lung fibroblasts) and A549 (lung adenocarcinoma cells), exhibited a large difference in their sensitivity to selenite but no significant difference in their sensitivity to selenodiglutathione. They also had a large difference in the level of intracellular sulfhydryl (SH) compounds. In contrast the other pair of cells, WI-38 (normal fetal lung fibroblasts) and WI-38VA (SV-40 transformed WI-38 cells) both had low levels of intracellular SH compounds and exhibited similar (low) sensitivity to selenite. Our results indicate that differences between normal and malignant cells in their sensitivity to selenite could be due to a difference in the reaction of selenite with intracellular sulfhydryl compounds to form selenotrisulfides.

Involvement of cellular sulfhydryl compounds in the inhibition of RNA synthesis by selenite

Selenite has been shown previously to inhibit cellular RNA synthesis. Based upon our previous observation that selenite inhibits purified RNA polymerase only in the presence of a sulfhydryl compound (Frenkel et al., Mol Pharmacol 31: 112-116, 1987), we hypothesized that the inhibition of cellular RNA synthesis by selenite involves endogenous sulfhydryl compounds. We found that depletion of cells of endogenous sulfhydryl compounds, by exposure to diethylmaleate (DEM), virtually eliminated the inhibitory effect of a 1-hr exposure of cells to selenite. This inhibition was restored to normal or higher levels when the selenite was reacted with glutathione or cysteamine prior to addition to the DEM-treated cells. RNA synthesis in DEM-treated cells was inhibited after a 4-hr exposure to higher concentrations of selenite. In contrast to the effect of DEM, specific depletion of the cells of glutathione, by exposure to buthionine sulfoximine, had no effect on the inhibition of RNA synthesis by selenite. These results demonstrate the involvement of endogenous cellular sulfhydryl compounds in the inhibition of RNA synthesis by selenite, but indicate that glutathione, in particular, is not involved in this inhibition.