John R. Arthur

Selenium in the Immune System

Selenium as an essential component of selenocysteine-containing protein is involved in most aspects of cell biochemistry and function. As such, there is much potential for selenium to influence the immune system. For example, the antioxidant glutathione peroxidases are likely to protect neutrophils from oxygen-derived radicals that are produced to kill ingested foreign organisms. When the functions of all selenoproteins are described, only then will it be possible to fully understand their role in maintaining optimal immune function.

Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)

Selenium (Se), a micronutrient essential for human health, is incorporated into at least 25 selenoproteins including selenoprotein P (SePP)′ which transports Se within the body. This research identified two single nucleotide polymorphisms (SNPs) in the SePP gene, one in the coding region (position 24731, causing an Ala to Thr change) and one in the 3′untranslated region (position 25191). Their frequency was similar in Caucasian, Chinese, and South Asian populations. Prospectively geno‐typed volunteers were supplemented for 6 wk with 100 μg sodium selenite/day. Blood samples were analyzed for plasma Se and selenoprotein biomark‐ers at baseline, after supplementation, and during a washout period. Plasma Se, SePP, and glutathione peroxidase 3 (GPx3) levels increased with supplementation. Baseline plasma Se content depended on both SePP genotypes and body mass index. Pre‐supplementation SePP concentration was associated with gender and genotype at SNP 24731 and post‐supplementation concentration with SNP 25191. Both SNPs and gender were associated with differences in GPx3 activity, plasma, and erythrocyte thioredoxin reductase 1 concentrations and lymphocyte glutathione peroxidase 1 and 4 activities and concentrations. In conclusion, the data reveal two common functional SNPs within the human SePP gene that may predict behavior of biomarkers of Se status and response to supplementation and thus susceptibility to disease.—Méplan, C., Crosley, L. K., Nicol, F., Beckett, G. J., Howie, A. F., Hill, K. E., Horgan, G., Mathers, J. C., Arthur, J. R., Hesketh, J. E. Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender‐specific manner (the SELGEN study) FASEB J. 21, 3063–3074 (2007)

Effects of smoking and vitamin E on blood antioxidant status.

Despite similar vitamin E contents, erythrocytes of smokers have an increased tendency (P less than 0.01) to peroxidize in vitro compared with those of nonsmokers. This difference is abolished by vitamin E supplementation (1000 mg alpha-tocopherol acetate/d for 14 d). The increased susceptibility to erythrocyte peroxidation in the smokers may reflect lower glucose-6-phosphate dehydrogenase (P less than 0.02) and glutathione peroxidase (P less than 0.05) activities. Smokers seem to be under a sustained oxidant stress with increased plasma-conjugated dienes (P less than 0.01) and dehydroascorbate (P less than 0.05) and decreased ascorbate (P less than 0.06) concentrations. Additionally, plasma ceruloplasmin in smokers is elevated (P less than 0.01), consistent with an acute-stress response. Plasma total cholesterol is similar in smokers and nonsmokers and is unaffected by vitamin E supplementation. Indices of sustained oxidant stress in smokers are partially ameliorated by vitamin E supplementation.

Selenium and the regulation of cell signaling, growth, and survival: molecular and mechanistic aspects.

In the past 30 years, it has been recognized that dietary selenium (Se) is essential for the normal function of many of the systems of the body. Furthermore, low Se intake can have deleterious effects on several aspects of human and animal health. The importance of Se is characterized in its role as a constituent of several key antioxidant and redox enzyme families. Most of the effects of Se are probably mediated by selenoproteins, which have the micronutrient covalently incorporated into the protein. The purpose of this review is to examine basic mechanisms by which Se regulates cell growth, gene transcription, cell signaling, and cell death. We start with the historical background to Se. The synthesis and function of selenoproteins are described, followed by details of the dietary sources of Se and Se status in different parts of the world, together with the clinical effects of Se deficiency and toxicity. We consider some aspects of the molecular mechanisms by which Se modulates cell growth, intracellular signaling, and gene transcription.

Selective control of cytosolic glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase mRNA stability by selenium supply

Selenium depletion of H4 hepatoma cells reduced cytosolic glutathione peroxidase (cGSH‐Px) mRNA abundance but had no effect on phospholipid hydroperoxide glutathione peroxidase (PHGSH‐Px) mRNA abundance. Actinomycin D chase experiments showed that selenium depletion had no effect on the stability of PHGSH‐Px mRNA but decreased the stability of cGSH‐Px mRNA. In Se‐replete cells puromycin decreased the stability of both cGSH‐Px and PHGSH‐Px mRNAs. The results suggest that when selenium supply is limiting PHGSH‐Px mRNA translation is maintained more than that of cGSH‐Px mRNA, and thus more cGSH‐Px mRNA is released from polysomes and degraded.

Effects of combined iodine and selenium deficiency on thyroid hormone metabolism in rats.

This paper compares the effects of combined iodine and selenium deficiency, of single deficiencies of these trace elements, and of no deficiency on thyroid hormone metabolism in rats. In rats deficient in both trace elements, thyroidal triiodothyronine (T3), thyroidal thyroxin (T4), thyroidal total iodine, hepatic T4, and plasma T4 were significantly lower, and plasma thyroid-stimulating hormone (TSH) and thyroid weight were significantly higher than in rats deficient in iodine alone. Plasma and hepatic T3 concentrations were similar in the dietary groups. Hepatic type I iodothyronine deiodinase (ID-I) activity was inhibited by selenium deficiency irrespective of the iodine status. Type II deiodinase (ID-II) activity in the brain was significantly higher and in pituitary, significantly lower in combined deficiency than in iodine deficiency alone. These data show that selenium can play an important role in determining the severity of the hypothyroidism associated with iodine deficiency.

The role of selenium in thyroid hormone metabolism and effects of selenium deficiency on thyroid hormone and iodine metabolism

Selenium deficiency impairs thyroid hormone metabolism by inhibiting the synthesis and activity of the iodothyronine deiodinases, which convert thyroxine (T4) to the more metabolically active 3,3′–5 triiodothyronine (T3). Hepatic type I iodothyronine deiodinase, identified in partially purified cell fractions using affinity labeling with [125I]N-bromoacetyl reverse triiodothyronine, is also labeled with75Se by in vivo treatment of rats with75Se−Na2SeO3. Thus, the type I iodothyronine 5′-deiodinase is a selenoenzyme. In rats, concurrent selenium and iodine deficiency produces greater increases in thyroid weight and plasma thyrotrophin than iodine deficiency alone. These results indicate that a concurrent selenium deficiency could be a major determinant of the severity of iodine deficiency.

Effects of Se‐depletion on glutathione peroxidase and selenoprotein W gene expression in the colon

Selenium (Se)‐containing proteins have important roles in protecting cells from oxidative damage. This work investigated the effects of Se‐depletion on the expression of the genes encoding selenoproteins in colonic mucosa from rats fed diets of different Se content and in human intestinal Caco‐2 cells grown in Se‐adequate or Se‐depleted culture medium. Se‐depletion produced statistically significant (P < 0.05) falls in glutathione peroxidase (GPX) 1 mRNA (60–83%) and selenoprotein W mRNA (73%) levels, a small but significant fall in GPX4 mRNA (17–25%) but no significant change in GPX2. The data show that SelW expression in the colon is highly sensitive to Se‐depletion.

Thioredoxin reductase is the major selenoprotein expressed in human umbilical-vein endothelial cells and is regulated by protein kinase C.

Damage to the endothelium by reactive oxygen species favours atherogenesis. Such damage can be prevented by selenium, which is thought to exert its actions through the expression of selenoproteins. The family of glutathione peroxidases (GPXs) may have antioxidant roles in the endothelium but other intracellular and extracellular selenoproteins with antioxidant actions may also be important. The selenoproteins expressed by cultured human umbilical-vein endothelial cells (HUVECs) were labelled with [(75)Se]selenite and separated using SDS/PAGE. HUVECs secreted no extracellular selenoproteins. There were distinct differences between the intracellular selenoprotein profile of (75)Se-labelled HUVECs and those of other tissues. A single selenoprotein with a molecular mass of 58 kDa accounted for approx. 43% of the intracellular (75)Se-labelled proteins in HUVECs. This protein was identified by Western blotting as the redox-active lipid-hydroperoxide-detoxifying selenoprotein, thioredoxin reductase (TR). TR expression in HUVECs was down-regulated by transiently exposing cells to the phorbol ester PMA for periods as short as 1 min. However, there was a delay of 48 h after PMA exposure before maximal down-regulation of TR was observed. The protein kinase C (PKC) inhibitor bisindolylmaleimide I hydrochloride had no effect on TR expression when added alone, but the agent prevented the down-regulation of TR expression seen with PMA. The calcium ionophore A23187 increased TR expression in HUVECs after a 12-h exposure, but the maximal effect was only observed after a 35-h exposure. These findings suggest that TR may be an important factor in the known ability of Se to protect HUVECs from peroxidative damage. Furthermore, the results also suggest that TR expression can be negatively regulated through PKC. It is possible that TR expression may be positively regulated by the calcium-signalling cascade, although TR induction by A23187 may be due to toxicity.

Selenium supplementation acting through the induction of thioredoxin reductase and glutathione peroxidase protects the human endothelial cell line EAhy926 from damage by lipid hydroperoxides

The human endothelial cell line EAhy926 was used to determine the importance of selenium in preventing oxidative damage induced by tert-butyl hydroperoxide (tert-BuOOH) or oxidised low density lipoprotein (LDLox). In cells grown in a low selenium medium, tert-BuOOH and LDLox killed cells in a dose-dependent manner. At 555 mg/l LDLox or 300 microM tert-BuOOH, >80% of cells were killed after 20 h. No significant cell kill was achieved by these agents if cells were pre-incubated for 48 h with 40 nM sodium selenite, a concentration that maximally induced the activities of cytoplasmic glutathione peroxidase (cyGPX; 5.1-fold), phospholipid hydroperoxide glutathione peroxidase (PHGPX;1.9-fold) and thioredoxin reductase (TR; 3.1-fold). Selenium-deficient cells pre-treated with 1 microM gold thioglucose (GTG) (a concentration that inhibited 25% of TR activity but had no inhibitory effect on cyGPX or PHGPX activity) were significantly (P<0.05) more susceptible to tert-BuOOH toxicity (LC(50) 110 microM) than selenium-deficient cells (LC(50) 175 microM). This was also the case for LDLox. In contrast, cells pre-treated with 40 nM selenite prior to exposure to GTG were significantly more resistant to damage from tert-BuOOH and LDLox than Se-deficient cells. Treatment with GTG or selenite had no significant effect on intracellular total glutathione concentrations. These results suggest that selenium supplementation, acting through induction of TR and GPX, has the potential to protect the human endothelium from oxidative damage.