Cellular selenoproteins and the effects of selenite on cell proliferation

Sodium selenite inhibits the expression of VEGF, TGFbeta(1) and IL-6 induced by LPS in human PC3 cells via TLR4-NF-(K)B signaling blockage

Lipopolysaccharide (LPS)-induced TLR4-NF-(K)B signaling plays an important role in the development of prostatic tumors from chronic bacterial prostatic infection. Although many studies support the role of selenium in protecting against the development of prostate cancer secondary to chronic prostatitis, the mechanism of action remains unclear. The aim of our study was to investigate whether selenium inhibits the LPS-induced TLR4 signaling pathway in human prostate cancer PC3 cells. Using real-time quantitative PCR and ELISA analysis, we found that pretreatment with selenium (0.5-5uM) inhibited the LPS-induced expression of TGFbeta(1) and VEGF and production of these cytokines and IL-6 by PC3 cells, but did not alter the expression of TLR4 mRNA. Further experiments using Western blot showed that selenium at 3 and 5uM significantly inhibited the translocation of the NF-(K)B p65 subunit to the nucleus in LPS-stimulated PC3 cells. Our results suggest that low doses of selenium may protect the prostate from prostatitis-induced cancer by inhibiting nuclear translocation of the NF-(K)B and the subsequent production of the immunosuppressive cytokine TGFbeta(1), proangiogenic factor VEGF and pro-inflammatory factor IL-6.

Selenomethionine stimulates expression of glutathione peroxidase 1 and 3 and growth of bovine mammary epithelial cells in primary culture

This study examined the localization of cellular glutathione peroxidase (GPx1) and extracellular glutathione peroxidase (GPx3) in lactating mammary tissue and in primary cultures of bovine mammary epithelial cells (BMEC). The effect of selenium as selenomethionine (SeMet) on the growth and viability of BMEC and GPx protein expression and activity were also studied. Single mammary epithelial cells were recovered by serial collagenase/hyaluronidase digestion from lactating bovine mammary tissue and cultured in a low-serum collagen gel system enriched with lactogenic hormones and 0, 10, 20, or 50 nM SeMet. Positive immunostaining with anti-cytokeratin and bovine anti-casein confirmed the epithelial nature and differentiated state of BMEC. Addition of SeMet to media facilitated rapid confluence of BMEC and formation of dome structures. Immunohistochemical and immunocytochemical staining revealed that both GPx1 and GPx3 are synthesized by BMEC and localized in the cytoplasm and nucleus. Up to 50 nM SeMet linearly increased BMEC number and viability over 5 d of culture. Bovine mammary epithelial cells cultured in SeMet-supplemented medium also exhibited markedly elevated GPx activity and linear increases in abundance of GPx1 and GPx3 proteins. It is apparent that SeMet degradation to release Se for synthesis of selenoproteins is carried out by BMEC. Results indicate that bovine mammary epithelial cells express GPx1 and GPx3 in vivo and in vitro; SeMet enhances expression of these selenoproteins in vitro and the growth and viability of BMEC.

Dietary selenium variation-induced oxidative stress modulates CDC2/cyclin B1 expression and apoptosis of germ cells in mice testis

Oxidative stress has been linked with apoptosis in germ cells and with male infertility. However, the molecular mechanism of oxidative-stress-mediated apoptosis in germ cells has not been clearly defined so far. Because of the involvement of CDC2 and cyclin B1 in cell cycle regulation and their plausible role in apoptosis, the present study aimed to investigate the possibility that selenium (Se)-induced oxidative-stress-mediated modulations of these cell cycle regulators cause DNA damage and apoptosis in germ cells. To create different Se status (deficient, adequate and excess), male Balb/c mice were fed yeast-based Se-deficient diet (Group I) and a deficient diet supplemented with Se as sodium selenite (0.2 and 1 ppm Se in Groups II and III, respectively) for a period of 8 weeks. After the completion of the diet feeding schedule, a significant decrease in Se levels and glutathione peroxidase activity was observed in the Se-deficient group (Group I), whereas the Se-excess group (Group III) demonstrated an increase in Se levels. Increased levels of lipid peroxidation were seen in both Groups I and III when compared to Group II, indicating oxidative stress. The mRNA and protein expressions of both CDC2 and cyclin B1 were found to be significantly decreased in Groups I and III. A decrease in the immunohistochemical localization of these proteins was also observed in spermatogenic cells. The mRNA expressions of apoptotic factors such as Bcl-2, Bax, caspase-3 and caspase-9 were found to be increased in Groups I and III. A decrease in CDC2 kinase activity was also seen in these groups. Increased apoptosis was observed in Group I and Group III animals by terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labeling assay indicating oxidative-stress-mediated DNA damage. These findings suggest the effect of Se-induced oxidative stress on the cell cycle regulators and apoptotic activity of germ cells, thus providing new dimensions to molecular mechanisms underlying male infertility.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Selenium regulates transcription factor NF-kappaB activation during the acute phase reaction

BACKGROUND

We reported a reciprocal relationship between reduced serum selenium (Se) and elevated serum C-reactive protein (CRP) in various pathological conditions in comparison with the levels in 141 healthy subjects. To clarify the implications of these observations, the effect of Se on nuclear factor (NF)-kappaB, which upregulates the CRP synthesis in the liver, was examined.

METHODS

Human hepatoma cell line HuH-7 was cultured in medium with 2% fetal calf serum (FCS) for 3 days for the Se deprivation, followed by another 3 days in the same medium containing sodium selenite prior to stimulation of the cells with either monocyte-conditioned medium (MoCM) or tumor necrosis factor-alpha (TNF-alpha). NF-kappaB activation and the synthesis of CRP in hepatocytes were examined by a non-radioisotope (non-RI) gel shift assay for the nuclear extract from the cells and by a highly sensitive ELISA for the cellular extract, respectively.

RESULTS

The NF-kappaB activation induced by MoCM and TNF-alpha were inhibited by Se at the physiological levels. The maximum activation of NF-kappaB was induced by TNF-alpha or MoCM at a Se concentration (0.5 approximately 1 micromol/l) which was half the level of the serum Se in healthy subjects and was equivalent to level in subjects with pathological conditions together with high serum CRP values. Under the same conditions, the hepatocytes synthesized maximal amounts of CRP.

CONCLUSIONS

Selenium at physiological levels mediates inhibition of the activation of the transcription factor NF-kappaB which regulates genes that encode inflammatory cytokines, and that conversely, the reduction of selenium induces the synthesis of CRP by hepatocytes during the acute phase response.

Selenite and selenomethionine promote HL-60 cell cycle progression

The essential role of selenium (Se) in nutrition is well established. The elucidation of the mechanisms by which selenium regulates the cell cycle can lead to a better understanding of the nature of selenium's essentiality and its role in disease prevention. In this study, the effects of selenium deficiency or adequacy (0.25 micromol/L selenite or selenomethionine) on HL-60 cell cycle progression were examined in serum-free media. Selenium was critical for promotion of HL-60 cell growth. Cell-cycle analysis revealed that selenium deficiency caused a decrease in G1 phase cells that corresponded to an increase in G2 and sub-G1 phase cells. Gene array analysis suggested that c-Myc, cyclin C, proliferating cell nuclear antigen, cyclin-dependent kinase (cdk)1, cdk2, cdk4, cyclin B and cyclin D2 mRNA levels were lower in selenium-deficient cells than in the cells supplemented with 0.25 micromol/L selenomethionine. The decrease in the c-Myc mRNA level in selenium-deficient cells was confirmed by reverse transcription-polymerase chain reaction analysis. Furthermore, the phosphorylation state of total cellular protein was higher (57%) in selenium-supplemented cells than in selenium-deficient cells. Collectively, these results suggest a novel role for selenium at 0.25 micromol/L in up-regulation of the expression of numerous cell cycle-related genes and total cellular phosphorylated proteins in HL-60 cells in serum-free culture media. This leads to the promotion of cell cycle progression, particularly G2/M transition and/or the reduction of apoptosis, primarily in G1 cells. These observations may have additional implications for understanding the nature of selenium's essentiality.

Redox regulation of cytosolic glycerol-3-phosphate dehydrogenase: Cys(102) is the target of the redox control and essential for the catalytic activity

Cytosolic glycerol-3-phosphate dehydrogenase (cG3PDH) occupies the branch point between the glycolytic pathway and triglyceride biosynthesis. However, the regulatory mechanism of the cG3PDH activity has remained obscure. Here we report that cG3PDH is efficiently inhibited by modification of the thiol group through a redox mechanism. In this study, we found that sodium selenite and nitric oxide (NO) donors such as S-nitroso-N-acetylpenicillamine and 3-morpholinosydnonimine inhibited cG3PDH activity, and that similar effects could be achieved with selenium metabolites such as selenocysteine and selenomethionine. Furthermore, we found that reducing agents, such as dithiothreitol and beta-mercaptoethanol, restored the cG3PDH activity suppressed by selenite and NO both in vitro and in cultured cells. Buthionine sulfoximine depleted levels of both reduced glutathione and the oxidized form but had no effect on the suppression of cG3PDH activity by selenite in cultured cells. Moreover, thiol-reactive agents, such as N-ethylmaleimide and o-iodosobenzoic acid, blocked the enzyme activity of cG3PDH through the modification of redox-sensitive cysteine residues in cG3PDH. The inhibitor of NO synthase, L-N(G)-nitro-arginine, restored the cG3PDH activity inhibited by NO in cultured cells, whereas the inhibitor of guanylyl cyclase, 1H-[1,2,4] oxadiazole[4,3-alpha] quinoxalin-1-one (ODQ), has no effect. NO directly inhibits cG3PDH activity not via a cGMP-dependent mechanism. Finally, using site-directed mutagenesis, we found that Cys(102) of cG3PDH was sensitive to both selenite and NO. From the results, we suggest that cG3PDH is a target of cellular redox regulation.

Human Immunodeficiency Virus Type 1 Tat Protein Impairs Selenoglutathione Peroxidase Expression and Activity by a Mechanism Independent of Cellular Selenium Uptake: Consequences on Cellular Resistance to UV-A Radiation

The expression of the HIV-1 Tat protein in HeLa cells resulted in a 2.5-fold decrease in the activity of the antioxidant enzyme glutathione peroxidase (GPX). This decrease seemed not to be due to a disturbance in selenium (Se) uptake. Indeed, the intracellular level of Se was similar in parental and tat-transfected cells. A Se enrichment of the medium did not lead to an identical GPX activity in both cell lines, suggesting a disturbance in Se utilization. Total intracellular 75Se selenoproteins were analyzed. Several quantitative differences were observed between parental and tat-transfected cells. Mainly, cytoplasmic glutathione peroxidase and a 15-kDa selenoprotein were decreased in HeLa-tat cells, while phospholipid hydroperoxide glutathione peroxidase and low-molecular-mass selenocompounds were increased. Thioredoxin reductase activity and total levels of 75Se-labeled proteins were not different between the two cell types. The effect of Tat on GPX mRNA levels was also analyzed. Northern blots revealed a threefold decrease in the GPX/glyceraldehyde phosphate dehydrogenase mRNA ratio in HeLa-tat versus wild type cells. By deregulating the intracellular oxidant/antioxidant balance, the Tat protein amplified UV sensitivity. The LD50 for ultraviolet radiation A was 90 J/cm2 for HeLa cells and only 65 J/cm2 for HeLa-tat cells. The oxidative stress occurring in the Tat-expressing cells and demonstrated by the diminished ratio of reduced glutathione/oxidized glutathione was not correlated with the intracellular metal content. Cellular iron and copper levels were significantly decreased in HeLa-tat cells. All these disturbances, as well as the previously described decrease in Mn superoxide dismutase activity, are part of the viral strategy to modify the redox potential of cells and may have important consequences for patients.

Prevention of the development of melphalan resistance in vitro by selenite

Exposure of A2780 human ovarian tumor cells to a low concentration of melphalan in vitro for 7 d results in the development of melphalan resistance, which is dependent on elevated cellular levels of glutathione and glutathione S-transferase. The inclusion of selenite (at concentrations as low as 0.2 microM) during the exposure to melphalan completely prevented the development of resistance. Selenite did not prevent the melphalan-induced increase in glutathione, but it did prevent the increase in the activity of glutathione S-transferase. It also prevented the increase in the expression of the glutathione S-transferase gene, suggesting that this may be the mechanism by which it prevents the development of melphalan resistance. The results of this in vitro study suggest that selenite may prove to be useful in preventing the development of drug resistance in vivo.

Effects of mercuric chloride and sodium selenite on some immune responses of blue gourami, Trichogaster trichopterus (Pallus)

The immunotoxicological effects of mercuric chloride and sodium selenite on blue gourami were studied. Some immune responses ranging from non-specific to specific were investigated. These include tissue lysozyme activity, kidney lymphocyte proliferation and plasma agglutinating antibody titre against bacteria. After 2 weeks of chronic exposure, 0.09 mg/l of Hg2+ alone induced a significant increase of kidney lysozyme activity of 4196.3 +/- 1171.0 U/g, but it decreased to 1577.4 +/- 902.4 U/g when exposed simultaneously to equiconcentration of selenium. Plasma lysozyme activity was also increased by co-administration of Hg2+ and SeO3(2-). The level of plasma agglutinating antibody against Aeromonas hydrophila L37 was lowered in the chemical-treated fish. This indicates that the fish immunity was impaired by action of mercury and selenium. However, the in vitro lymphocyte proliferation test shows that mercury concentration lower than 0.045 mg/l Hg2+ enhanced the mitotic rate of kidney lymphocytes by approximately 30%. A high concentration of mercury caused irreversible damaging effects on con A-induced lymphoblastogenesis. In contrast, the inhibitory effect of low concentrations of mercury could be removed by washing. On the other hand, selenium showed a suppressive effect on the lymphocyte proliferation even at 0.5 mg/l.

In vitro effects of sodium selenite on nuclear 3,5,3'-triiodothyronine (T3) receptor gene expression in rat pituitary GH4C1 cells

The present study was undertaken in order to investigate the effects of sodium selenite on: 1. The growth of rat pituitary GH4C1 cells; 2. The nuclear T3 receptor gene expression; 3. The cytoplasmic protein phosphorylation; and 4. The prolactin secretion in rat pituitary GH4C1 cell line. Sodium selenite (up to 2.5 microM) has no inhibitory effect on GH4C1 cell proliferation as well as the prolactin secretion. On the other hand, 0.5 microM sodium selenite significantly decreases the rate of mRNA synthesis and/or degradation of both, the alpha 1 form of the T3 receptor (TR alpha 1) and the alpha 2 isoform of the T3 receptor. At 1 microM of sodium selenite, significant changes in the electrophoretic profile of low molecular mass cytoplasmic proteins were found, moreover, sodium selenite (1 microM) also considerably affects phosphorylation of a higher molecular mass proteins. The results based on the in vitro experiments suggest that sodium selenite may affect specific processes at the pretranslational level as well as it may also take part in processes of posttranslational modification of protein(s), the cell vitality and the cell growth remaining unchanged.

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.

Selenium. Mechanistic aspects of anticarcinogenic action

Selenium is increasingly recognized as a versatile anticarcinogenic agent. Its protective functions cannot be solely attributed to the action of glutathione peroxidase. Instead, selenium appears to operate by several mechanisms, depending on dosage and chemical form of selenium and the nature of the carcinogenic stress. In a major protective function, selenium is proposed to prevent the malignant transformation of cells by acting as a "redox switch" in the activation-inactivation of cellular growth factors and other functional proteins through the catalysis of oxidation-reduction reactions of critical SH groups of SS bonds. The growth-modulatory effects of selenium are dependent on the levels of intracellular GSH and the oxygen supply. In general, growth inhibition is achieved by the Se-mediated stimulation of cellular respiration. Selenium appears to inhibit the replication of tumor viruses and the activation of oncogenes by similar mechanisms. However, it may also alter carcinogen metabolism and protect DNA against carcinogen-induced damage. In additional functions of relevance to its anticarcinogenic activity, selenium acts as an acceptor of biogenic methyl groups, and is involved in the detoxification of metals and of certain xenobiotics. In its interactions with transformed cells at higher concentrations, it may induce effects ranging from metabolic and phenotypical changes, and partial renormalization to selective cytotoxicity owing to reversible or irreversible inhibition of protein and DNA synthesis. Selenium also has immunopotentiating properties. It is required for optimal macrophage and NK cell function. Its protective effects are influenced by synergistic and antagonistic dietary and environmental factors. The latter include a variety of toxic heavy metals and xenobiotic compounds, but they are also influenced by essential elements, such as zinc. The exposure to antagonistic factors must be minimized for the full expression of its anticarcinogenic potential.