Selenate inhibits adipogenesis through induction of transforming growth factor-β1 (TGF-β1) signaling

Selenium is essential for many aspects of human health. While selenium is known to protect against cancer and cardiovascular diseases, the role of selenium in adipose development is unknown. Here we show that selenate at non-toxic concentration exhibits an anti-adipogenic function in vitro and ex vivo. In addition, selenate induced a morphological change of these cells from fibroblast-like to spindle cell shape. However, other forms of selenium, including selenite and methylseleninic acid, showed either toxic or no effect on adipogenesis and morphology change of preadipocytes. The effects of selenate on adipogenesis and cell morphology change were blunted by the treatment with SB431542, a specific inhibitor of transforming growth factor-β1 (TGF-β1) receptor, neutralization TGF-β1 by its antibody, and knockdown of TGF-β1 in preadipocytes, suggesting a requirement of TGF-β signaling for the anti-adipogenic function of selenate. Among tested forms of selenium, selenate appears to be an effective activator of TGF-β1 expression in preadipocytes. These results indicate that selenate is a novel dietary micromineral that activates TGF-β1 signaling in preadipocytes and modulates adipogenesis.     

Effect of selenium compounds and thiols on human mammary tumor cells

The effect on cell viability and growth rate of sodium selenite, selenocystine, sodium selenate, and selenomethionine at selenium concentrations of 6.25 and 12.5 uM was studied in vitro on cells of the human mammary tumor cell line HTB123/DU4475. Selenite and selenocystine affected both cell viability and growth rate of the tumor cells at these selenium concentrations. Selenite and selenocystine decreased intracellular glutathione concentrations, but did not affect tumor cell glutathione peroxidase activity. After six days of exposure to either selenate or selenomethionine, the viability of tumor cells remained stable, but cell growth, as measured by numbers of cells, was retarded. Neither selenate nor selenomethionine produced changes in concentrations of intracellular glutathione. The toxic effect of selenite on tumor cells was enhanced by addition of 0.25 mM glutathione to the growth medium. Preincubation of the tumor cells with 62.5 uM buthionine sulfoximine decreased cellular glutathione to 15% of controls at 24 h and enhanced the toxicity of selenite toward the tumor cells. Glutathione, 2-mercaptoethanol, and L-cysteine were all toxic to the tumor cells in a dose-dependent manner.     

Selenoprotein expression is regulated at multiple levels in prostate cells

Selenium supplementation in a population with low basal blood selenium levels has been reported to decrease the incidence of several cancers including prostate cancer. Based on the clinical findings, it is likely that the antioxidant function of one or more selenoproteins is responsible for the chemopreventive effect, although low molecular weight seleno-compounds have also been posited to selectively induce apoptosis in transformed cells. To address the effects of selenium supplementation on selenoprotein expression in prostate cells, we have undertaken an analysis of antioxidant selenoprotein expression as well as selenium toxicity in non-tumorigenic prostate epithelial cells （RWPE- 1 ） and prostate cancer cells （LNCaP and PC-3）. Our results show that two of the glutathione peroxidase family members （GPX1 and GPX4） are highly induced by supplemental selenium in prostate cancer cells but only slightly induced in RWPE-1 cells. In addition, GPX 1 levels are dramatically lower in PC-3 cells as compared to RWPE- 1 or LNCaP cells. GPX2 protein and mRNA, however, are only detectable in RWPE-1 cells. Of the three selenium compounds tested （sodium selenite, sodium selenate and selenomethionine）, only sodium selenite shows toxicity in a physiological range of selenium concentrations. Notably and in contrast to previous studies, RWPE-1 cells were significantly more sensitive to selenite than either of the prostate cancer cell lines. These results demonstrate that selenoproteins and selenium metabolism are regulated at multiple levels in prostate cells.     

Antioxidant and Anticancer Properties and Mechanisms of Inorganic Selenium,Oxo-Sulfur,and Oxo-Selenium Compounds

Inorganic selenium and oxo-sulfur compounds are widely available in dietary supplements and have been extensively studied for their antioxidant and anticancer properties. Although many in vivo and clinical trials have been conducted using these compounds, their biochemical and chemical mechanisms of efficacy are the focus of much current research. This review discusses the ability of inorganic selenium compounds, such as selenite, and selenate, to prevent damage from reactive oxygen species as well as their ability to promote cell death by reactive oxygen species generation. Oxo-sulfur and selenium compounds, such as allicin, dimethyl sulfone, methionine sulfoxide, and methylselenenic acid also have similar abilities to act as both antioxidants and pro-oxidants, but the mechanisms for these behaviors are distinctly different from those of the inorganic selenium compounds. The antioxidant and pro-oxidant properties of these small-molecule sulfur and selenium compounds are extremely complex and often greatly depend on experimental conditions, which may explain contradictory literature reports of their efficacy.     

Comparison of whole blood selenium values and erythrocyte glutathione peroxidase activities of normal individuals on supplementation with selenate,selenite,l-selenomethionine,and high selenium yeast

The selenium levels and the glutathione peroxidase activity GSH-PX of whole blood and of erythrocytes, respectively, were determined in 139 normal Danes and related to sex and smoking habits. No differences were found in relation to sex apart from a higher GSH-PX activity of females when assayed with tertiary butyl hydroperoxide. Smokers showed significantly lower selenium values than non-smokers (p<0.05), but the two groups had identical GSH-PX activities. Individuals from the above-mentioned group were divided into four groups, receiving daily oral doses of 200 μg of selenium in the form of selenite, selenate, L-selenomethionine, and selenium as contained in yeast. Whole blood selenium values and the erythrocyte glutathione peroxidase activities were determined during three months of supplementation followed by a withdrawal period of four months. Both the inorganic selenium compounds and the organic derivatives gave rise to steady state levels of GSH-PX after one month of supplementation. However, the selenium levels in the groups receiving organic selenium showed a steady rise during the whole period, whereas those supplemented with inorganic selenium leveled off after a period of one to three months. The data for smokers and non-smokers revealed identical results when organic selenium was supplemented. However, selenite gave rise to significantly higher selenium levels and GSH-PX activities in smokers than in non-smokers. Less significant (p<0.08) elevations of both parameters were also observed among the smokers in the selenate group. By taking both the selenium level and the GSH-PX activity into consideration, organic selenium (i.e.,l-(+) selenomethionine) was judged to be more bioavailable than selenite and selenate.     

Tissue distribution of S-adenosylmethionine and S-adenosylhomocysteine in the rat. Effect of age, sex and methionine administration on the metabolism of S-adenosylmethionine, S-adenosylhomocysteine and polyamines

In the presence of ATP and Mg2+, ATP sulphurylase from Saccharomyces cerevisiae catalysed the conversion of selenate into a compound with the electrophoretic and acid-lability properties of adenosine 5'-sulphatophosphate. Structural characterization, involving extensive purification of adenosine 5'-selenophosphate, proved impossible. However, we showed ATP-, Mg2+- and ATP sulphurylase-dependent, and inorganic pyrophosphatase-stimulated, production of elemental selenium from selenate in the presence of GSH (reduced glutathione). Since selenate was not reduced by GSH, this reaction proved that ATP sulphurylase had formed an active selenate. The enzyme catalysed formation of elemental selenium had the same kinetics and GSH-dependency as the non-enzymic reduction of selenite to elemental selenium by GSH. In the presence of inorganic pyrophosphatase, 2 mol of Pi was released for each mol of 'active selenate' formed. This was shown by a spectrophotometric assay for elemental selenium. The observed reactivity with thiols and the instability of the enzymic product were those predicted for selenium anhydrides. By analogy with the chemistry of sulphur, the product of the thiolytic cleavage of a selenium anhydride would be converted into selenite. The selenite would then be reduced by the thiol to elemental selenium. We conclude that ATP sulphurylase can catalyse the formation of adenosine 5'-selenophosphate. The anhydride can be reduced by thiols in a manner similar to the reduction of selenite. These results probably explain the ability of mammals, lacking a sulphate reductase system, to incorporate selenium from selenate into seleno-amino acids.     

Laboratory-scale continuous reactor for soluble selenium removal using selenate-reducing bacterium,Bacillus sp. SF-1

A model continuous flow bioreactor (volume 0.5 L) was constructed for removing toxic soluble selenium (selenate/selenite) of high concentrations using a selenate-reducing bacterium, Bacillus sp. SF-1, which transforms selenate into elemental selenium via selenite for anaerobic respiration. Model wastewater contained 41.8 mg-Se/L selenate and excess lactate as the carbon and energy source; the bioreactor was operated as an anoxic, completely mixed chemostat with cell retention time between 2.2-95.2 h. At short cell retention times selenate was removed by the bioreactor, but accumulation of selenite was observed. At long cell retention times soluble selenium, both selenate and selenite, was successfully reduced into nontoxic elemental selenium. A simple mathematical model is proposed to evaluate Se reduction ability of strain SF-1. First-order kinetic constants for selenate and selenite reduction were estimated to be 2.9 x 10(-11) L/cells/h and 5.5 x 10(-13) L/cells/h, respectively. The yield of the bacterial cells by selenate reduction was estimated to be 2.2 x 10(9) cells/mg-Se.     

Enhancement of epidermal growth factor (EGF) and insulin-stimulated tyrosine phosphorylation of endogenous substrates by sodium selenate.

Sodium selenate stimulated tyrosine phosphorylation of the epidermal growth factor (EGF) receptor in A431 cells and enhanced the tyrosine phosphorylation of endogenous proteins in response to EGF in A431 cells and insulin in NIH 3T3 HIR3.5 cells. These effects occurred without changes in ligand binding, were not abolished by mercaptoethanol in the case of the EGF receptor, and appeared distinct from the effects of vanadate. These results support a role for selenium or selenoproteins in regulating EGF and insulin receptor tyrosine kinase activity and suggest a mechanism whereby selenium-containing compounds contribute to cell growth.     

Effects of selenium oxyanions on the white-rot fungus <Emphasis Type="Italic">Phanerochaete chrysosporium</Emphasis>

The ability of Phanerochaete chrysosporium to reduce the oxidized forms of selenium, selenate and selenite, and their effects on the growth, substrate consumption rate, and pellet morphology of the fungus were assessed. The effect of different operational parameters (pH, glucose, and selenium concentration) on the response of P. chrysosporium to selenium oxyanions was explored as well. This fungal species showed a high sensitivity to selenium, particularly selenite, which inhibited the fungal growth and substrate consumption when supplied at 10 mg L^?1 in the growth medium, whereas selenate did not have such a strong influence on the fungus. Biological removal of selenite was achieved under semi-acidic conditions (pH 4.5) with about 40 % removal efficiency, whereas less than 10 % selenium removal was achieved for incubations with selenate. P. chrysosporium was found to be a selenium-reducing organism, capable of synthesizing elemental selenium from selenite but not from selenate. Analysis with transmission electron microscopy, electron energy loss spectroscopy, and a 3D reconstruction showed that elemental selenium was produced intracellularly as nanoparticles in the range of 30–400 nm. Furthermore, selenite influenced the pellet morphology of P. chrysosporium by reducing the size of the fungal pellets and inducing their compaction and smoothness.     

Fluctuations in Fe,Cu, Zn,Br, As,Se, and Rb concentrations in C57L/J mice bearing BW7756 murine hepatoma using radioisotope-induced X-ray fluorescence

The effects of selenite or selenate supplementation on the concentration and distribution of Fe, Cu, Zn, As, Br, and Rb are investigated using the radioisotope-induced X-ray fluorescence, RIXRF. These effects are studied in the animals bearing BW7756 murine hepatoma and healthy animals for both of the oxidation states. Selenite and selenate induce different effects on the distribution of selenium, zinc, copper, bromine, and rubidium. The differences may be attributed to the differences in the inter element interaction after absorption into the bloodstream as well as to the mode of their intestinal absorption. Simultaneous supplementation of copper with selenite or selenate at the described levels has a profound influence on the concentration levels of other elements in the normal as well as in the diseased mice. The administration of selenium (0.67 μg/g body wt sodium selenite or sodium selenate, daily) and selenium and copper (0.67 and 1.35 μ/g body wt, respectively) has no effect on the incidence rate of hepatoma development.     

Inhibition of RNA synthesis in animal cells by sodium selenite

The effect of selenium-containing compounds on RNA synthesis in rat liver cells was studied in vivo. All the selenium derivatives under study inhibited the rRNA synthesis in liver cells. The most potent inhibiting effect was exerted by sodium selenite. It was shown that in a cell-free system of RNA biosynthesis sodium selenite selectively inhibited the activity of RNA-polymerase I in isolated nuclei and purified enzyme preparations of normal and tumour cells. A feasible mechanism of inhibition of the RNA-polymerase I activity by selenium and a hypothesis on the anticarcinogenic effect of selenium are postulated.     

Estimating Intestinal Absorption of Inorganic and Organic Selenium Compounds by in Vitro Flux and Biotransformation Studies in Caco-2 Cells and ICP-MS Detection

The aim of the present work was to compare and estimate absorption and biotransformation of selected selenium compounds by studying their fluxes across Caco-2 cells. Five different selenium compounds, selenomethionine (SeMet), Se-methylselenocysteine (MeSeCys), selenate, selenite, and methylseleninic acid (MeSeA), were applied to Caco-2 cells in a concentration of 10 μM, and fluxes in both directions were studied for 2 h. Fluxes of selenite and MeSeA in the presence of excess reduced glutathione (selenite + GSH and MeSeA + GSH) and flux of MeSeA in the presence of excess cysteine (MeSeA + Cys) were also studied. Selenium absorptive and exsorptive fluxes and accumulation in cell cytosol were analyzed by means of flow injection inductively coupled plasma mass spectrometry (ICP-MS). Absorptive flux of SeMet, MeSeCys, and selenate showed values correlating to complete in vivo absorption, while selenite and MeSeA fluxes correlated to poor in vivo absorption. Speciation analysis of cell lysate and donor and receptor solutions by LC-ICP-MS showed limited transformation of all selenium compounds. Extensive transformation as well as significantly increased absorptive flux was observed when co-administering selenite with glutathione compared to administering selenite alone. These observations are possibly due to formation of selenodiglutathione (GS-Se-SG) which may be absorbed differently than selenite. Concomitant application of GSH or cysteine with MeSeA resulted in extensive transformation of MeSeA, including volatile species, whereas no significant increases in fluxes were observed. In summary, the absorption of selenite selenate and the selenoamino acids is considered complete under physiological conditions, but the absorption mechanisms and metabolism of the compounds are different.     

Isolation, Growth, and Metabolism of an Obligately Anaerobic, Selenate-Respiring Bacterium, Strain SES-3

A gram-negative, strictly anaerobic, motile vibrio was isolated from a selenate-respiring enrichment culture. The isolate, designated strain SES-3, grew by coupling the oxidation of lactate to acetate plus CO₂ with the concomitant reduction of selenate to selenite or of nitrate to ammonium. No growth was observed on sulfate or selenite, but cell suspensions readily reduced selenite to elemental selenium (Se⁰). Hence, SES-3 can carry out a complete reduction of selenate to Se⁰. Washed cell suspensions of selenate-grown cells did not reduce nitrate, and nitrate-grown cells did not reduce selenate, indicating that these reductions are achieved by separate inducible enzyme systems. However, both nitrate-grown and selenate-grown cells have a constitutive ability to reduce selenite or nitrite. The oxidation of [¹⁴C]lactate to ¹⁴CO₂ coupled to the reduction of selenate or nitrate by cell suspensions was inhibited by CCCP (carbonyl cyanide m-chlorophenylhydrazone), cyanide, and azide. High concentrations of selenite (5 mM) were readily reduced to Se⁰ by selenate-grown cells, but selenite appeared to block the synthesis of pyruvate dehydrogenase. Tracer experiments with [⁷⁵Se]selenite indicated that cell suspensions could achieve a rapid and quantitative reduction of selenite to Se⁰. This reduction was totally inhibited by sulfite, partially inhibited by selenate or nitrite, but unaffected by sulfate or nitrate. Cell suspensions could reduce thiosulfate, but not sulfite, to sulfide. These results suggest that reduction of selenite to Se⁰ may proceed, in part, by some of the components of a dissimilatory system for sulfur oxyanions.     

Comparative effects of selenite and selenite on the glutathione-related enzymes activity in pig blood platelets

The effects of inorganic selenium (Se) compounds (sodium selenite and selenate) on the activities of glutathione-related enzymes (glutathione peroxidase, glutathione-S-transferase [GST] and glutathione reductase [GR]) in pig blood platelets were investigated in vitro. GST activity in blood platelets treated with 10⁻⁴ M of selenite was reduced to 50%, whereas no decrease GST activity was observed after the treatment of platelets with the same dose of selenate. In platelets incubated with physiological doses (10⁻⁷, and 10⁻⁶ M) of Se compounds, the activity of glutathione peroxidase (GSH-Px) was enhanced (about 20%). GR activity after the exposure of platelets to tested Se compounds was unaffected.     

Dimethylselenide and Dimethyltelluride Formation by a Strain of Penicillium

A strain of Penicillium which produced dimethylselenide from inorganic selenium compounds was isolated from raw sewage. Sulfate and methionine enhanced growth of the fungus and its production of dimethylselenide in media containing selenite. In solutions containing selenate, methionine inhibited dimethylselenide formation while stimulating proliferation of the fungus. Dimethylselenide was also generated from inorganic selenide. Alkylation did not appear to be a significant mechanism of selenium detoxication by this organism. Dimethyltelluride was also produced by the organism from several tellurium compounds, but this product was synthesized only in the presence of both tellurium and selenium. The yields of dimethylselenide and dimethyltelluride varied with the relative concentrations of selenium and tellurium in the medium.     

Selenium compounds in the fathead minnow (Pimephales promelas)--I. Uptake, distribution, and elimination of orally administered selenate, selenite and l-selenomethionine

Treatment of fathead minnows (Pimephales promelas) with either [75Se]selenate, -selenite or -l-selenomethionine by gavage at 20 ng Se/g resulted in organ uptake and early distribution patterns which differed significantly between compounds. The greatest differences in uptake between compounds was observed in liver tissue which accumulated much less [75Se]selenate than either selenite or l-selenomethionine. The 75Se burdens and relative distribution among the various organs were nearly identical during the elimination phase for [75Se]selenate and -selenite. This suggests that selenium derived from these compounds converge to a common metabolic pool. The whole body T1/2, rate of 75Se uptake and magnitude of 75Se accumulation were generally greater for [75Se]selenomethionine than the inorganic forms. Selenium-75 was present in the bile following the oral administration of each compound. The partitioning of selenate and selenite into the plasma and cellular fraction of blood differs with both the compound and time following exposure.     

Effects of growth conditions on production of methyl selenides in cultures of Rhodobacter sphaeroides

Rhodobacter sphaeroides 2.4.1 exposed to selenate or selenite produced volatile selenium compounds. Total amounts of dimethyl selenide, dimethyl diselenide, dimethyl sulfide and dimethyl disulfide in culture medium and headspace were determined. The highest selenate volatilization occurred in the late stationary phase of growth. However, cultures deprived of light in the stationary phase of growth produced much less of the volatile organo-selenium compounds. Lower culture pHs increased the rate of selenium volatilization. Low sulfate concentration limited biomass production and selenium volatilization; high sulfate concentrations had an enhancing effect on the release of organo-selenium compounds. Cultures of R. sphaeroides reacted very differently to amendments with increasing amounts of selenate and selenite. Only small amounts of selenite were volatilized; meanwhile high amounts of methylated selenides were found in selenate-poisoned cultures. Received 03 February 1997/ Accepted in revised form 16 May 1997     

Transformation of selenate and selenite to elemental selenium byDesulfovibrio desulfuricans

Summary Desulfovibrio desulfuricans (DSM 1924) can be adapted to grow in the presence of 10 mM selenate or 0.1 mM selenite. This growth occurred in media containing formate as the electron donor and either fumarate or sulfate as the electron acceptor. As determined by electron microscopy with energy-dispersive X-ray analysis, selenate and selenite were reduced to elemental selenium which accumulated inside the cells. Selenium granules resulting from selenite metabolism were cytoplasmic while granules of selenium resulting from selenate reduction appeared to be in the periplasmic region. The accumulation of red elemental selenium in the media following stationary phase resulted from cell lysis with the liberation of selenium granules. Growth did not occur with either selenate or selenite as the electron acceptor and¹³C nuclear magnetic resonance indicated that neither selenium oxyanion interfered with fumarate respiration. At 1 M selenate and 100 M selenite, reduction byD. desulfuricans was 95% and 97%, respectively. The high level of total selenate and selenite reduced indicated the suitability ofD. desulfuricans for selenium detoxification.     

Responses of an American eel brain endothelial-like cell line to selenium deprivation and to selenite,selenate, and selenomethionine additions in different exposure media

The effect of selenium deprivation and addition on the American eel brain endothelial cell line (eelB) was studied in three exposure media: complete growth medium (L15/FBS), serum-free medium (L15), and minimal medium (L15/ex). L15/ex contains only galactose and pyruvate and allowed the deprivation of selenium on cells to be studied. In L15/ex, without any obvious source of selenium, eelB cells survived for at least 7 d, formed capillary-like structures (CLS) on Matrigel, and migrated to heal wounds. Three selenium compounds were added to cultures: selenite, selenate, and selenomethionine (SeMet). Adding selenite or selenate to eelB cell cultures for 24 h caused dose-dependent declines in cell viability, regardless of the exposure media. Although varying with exposure media and viability end point, selenite was approximately 70-fold more cytotoxic than selenate. By contrast, 24 h exposures to either dl- or l-SeMet in the three media caused little or no cytotoxicity. However for 7 d exposures in L15/ex, dl- and l-SeMet were very cytotoxic, even at the lowest tested concentration of 31 μM. By contrast in L15 and L15/FBS, cytotoxicity was only observed with 500 and 1000 μM l-SeMet. In L15/FBS, eelB continued to migrate and form CLS in the presence of SeMet but at 500 μM, cell migration appeared stimulated. As judged from a colony-forming assay over 14 d in L15/FBS, 500 and 1000 μM dl- and l-SeMet inhibited cell proliferation. Overall, the responses of eel cells to selenium depended on the selenium form, concentration, and exposure media, with responses to SeMet being most dependent on exposure media.