Interspecies variation in yolk selenium concentrations among eggs of free-living birds: The effect of phylogeny

Birds deposit the trace element selenium (Se) into their eggs because an adequate supply of this micronutrient is essential for embryonic development. Although there is considerable interest in egg Se with regard to topics as diverse as poultry nutrition and environmental pollution, data on the natural levels of Se in eggs of free-living avian species are currently very limited. To address this lack of information, we measured the yolk Se concentrations in eggs of 14 avian species collected in the wild. The concentrations (ng/g wet yolk) varied from 394 to 2238, with a mean value of 1040. Values (means+/-SD) for eggs from the UK, Canada and New Zealand were, respectively, 522+/-192 (3 species), 1194+/-584 (8 species) and 1147+/-200 (3 species). However, analysis by appropriate statistical models indicates that the effect of phylogenetic relatedness among these species is so significant that it removes any effect of geographical location. In particular, species belonging to the order Passeriformes displayed significantly higher yolk Se levels than Non-Passeriforme species. In marked contrast to the free-living species, our previously published data indicate that the Se concentration in egg yolk of the domestic chicken is only about 100 ng/g wet yolk when the birds are maintained on a basal commercial diet without supplementary Se. The results reveal an extensive interspecies variation in yolk Se (across a 6-fold range) for eggs collected from the wild. Nevertheless, the Se concentrations in the yolks of all the free-living species were far higher (4-21-fold) than that achieved in the yolk of the domestic chicken consuming a standard basal diet.     

Effects of selenium supplementation on selenium status of farmed fallow deer in outdoor pens

The study investigated the effects of selenium (Se) supplementation on Se status in farmed fallow deer. Fallow deer were housed on grass pasture and adapted to consume ∼200 g of pelleted grain daily. Animals were divided into two groups. One group received pelleted grain enriched with sodium selenate for 12 weeks (Se+ group, N = 10). Se intake for the first 7 weeks was 0.18 mg/kg dry matter (DM) and 0.32 mg/kg DM for the subsequent 5 weeks. The control group was fed pelleted grain without extra Se (Se− group, N = 9, 0.06-0.08 mg/kg DM). Blood samples were collected at the beginning and the end of the experiment. After the animals were slaughtered, tissue samples were collected for analysis of Se concentrations and Se-dependent glutathione peroxidase 1 (GPx1) activity. In addition, Se-independent α-glutathione-S-transferase (α-GST) activity was analyzed in liver tissue. Se supplementation significantly increased Se levels in plasma and in tissues as follows: liver > spleen > skeletal muscle > myocardium > kidney. Se supplementation also significantly increased GPx1 activity in tissues in the following order: liver > skeletal muscle > spleen = myocardium > kidneys. However, hepatic α-GST activity did not differ between Se+ and Se− groups. As expected, Se supplementation increased blood and tissue Se concentrations and GPx1 activity, which suggests a better antioxidant status. However, the activity of α-GST, an important Se-independent antioxidant enzyme, was not altered, presumably because GPx provided an adequate antioxidant capacity even though Se intake was low.     

Selenoproteins and heat shock proteins play important roles in immunosuppression in the bursa of Fabricius of chickens with selenium deficiency

Selenium (Se) is necessary for the immune system in chicken and mediates its physiological functions through selenoproteins. Heat shock proteins (Hsps) are indispensable for maintaining normal cell function and for directing the immune response. The aim of the present study was to investigate the effects of Se deficiency on the messenger ribonucleic acid (mRNA) expression levels of selenoproteins and Hsps as well as immune functions in the chicken bursa of Fabricius. Two groups of chickens, namely the control and Se-deficient (L group) groups, were reared for 55 days. The chickens were offered a basal diet, which contained 0.15 mg Se/kg in the diet fed to the control group and 0.033 mg Se/kg in the diet fed to the L group. We performed real-time quantitative polymerase chain reactionto detect the mRNA expression levels of selenoproteins and Hsps on days 15, 25, 35, 45 and 55. Western blotting was used to determine the protein expression levels of Hsps on days 35, 45 and 55, and immune functions were assessed through an enzyme-linked immunosorbent assay on days 15, 35, and 55. The data showed that the mRNA expression levels of selenoproteins, such as Txnrd1, Txnrd2, Txnrd3, Dio1, Dio2, Dio3, GPx1, GPx2, GPx3 GPx4, Sepp1, Selo, Sel-15, Sepx1, Sels, Seli, Selu, Selh, and SPS2, were significantly lower (P < 0.05) in the L group compared with the control group. Additionally, the mRNA and protein expression levels of Hsps (Hsp27, Hsp40, Hsp60, Hsp70, and Hsp90) were also significantly higher (P < 0.05) in the L group. The expression levels of IL-2, IL-6, IL-8, IL-10, IL-17, IL-1β, IFN-α, IFN-β, and IFN-γ were significantly lower (P < 0.05) and TNF-α was significantly higher (P < 0.05) in the L group compared with the control group. Our results show that immunosuppression was accompanied by a downregulation of mRNA expression levels of selenoproteins and an upregulation of the Hsp mRNA expression levels. Thus, Se deficiency causes defects in the chicken bursa of Fabricius, and selenoproteins and Hsps play important roles in immunosuppression in the bursa of Fabricius of chickens with Se deficiency.     

The effects of selenium and the GPx-1 selenoprotein on the phosphorylation of H2AX

Background

Significant data supports the health benefits of selenium although supplementation trials have yielded mixed results. GPx-1, whose levels are responsive to selenium availability, is implicated in cancer etiology by human genetic data. Selenium's ability to alter the phosphorylation of the H2AX, a histone protein that functions in the reduction of DNA damage by recruiting repair proteins to the damage site, following exposure to ionizing radiation and bleomycin was investigated.

Methods

Human cell lines that were either exposed to selenium or were transfected with a GPx-1 expression construct were exposed to ionizing radiation or bleomycin. Phosphorylation of histone H2AX was quantified by flow cytometry and survival by the MTT assay. Phosphorylation of the Chk1 and Chk2 checkpoint proteins was quantified by western blotting.

Results

In colon-derived cells, selenium increases GPx-1 and attenuated H2AX phosphorylation following genotoxic exposures while the viability of these cells was unaffected. MCF-7 cells and transfectants that express high GPx-1 levels were exposed to ionizing radiation and bleomycin, and H2AX phosphorylation and cell viability were assessed. GPx-1 increased H2AX phosphorylation and viability following the induction of DNA damage while enhancing the levels of activated Chk1 and Chk2.

Conclusions

Exposure of mammalian cells to selenium can alter the DNA damage response and do so by mechanisms that are dependent and independent of its effect on GPx-1.

General significance

Selenium and GPx-1 may stimulate the repair of genotoxic DNA damage and this may account for some of the benefits attributed to selenium intake and elevated GPx-1 activity.     

Effect of source of supplemental selenium on uterine health and embryo quality in high-producing dairy cows

Lactating Holstein cows (n = 135) were randomly assigned to one of the two sources of supplemental selenium (Se), sodium selenite (SS) or Se yeast (SY), fed at 0.3 mg/kg diet dry matter from 25 d before calving to 70 d in milk (DIM), in diets not suboptimal in basal Se concentrations. Cows were evaluated for health daily in the first 10 DIM, and uterine cytology of the previously gravid uterine horn was assessed at 30 DIM. The Ovsynch protocol was initiated at 42 DIM; ovarian responses to hormonal treatments were evaluated by ultrasonography. The uteri of cows were flushed 6 d after timed AI for collection of embryos and oocytes. Plasma concentrations of Se and progesterone were measured throughout the postpartum period and during the reproductive protocol, respectively, and plasma glutathione peroxidase activity was determined 6 d after AI. Concentrations of Se in pre- and postpartum diets ranged from 0.43 to 0.56 mg/kg of dry matter. Incidence of retained placenta, fever, ketosis, mastitis, acute puerperal metritis, clinical endometritis, and subclinical endometritis were not significantly different between treatments. There were no differences between groups in concentrations of Se and progesterone or glutathione peroxidase activity in plasma. Treatment did not influence ovarian responses to the synchronization protocol, fertilization rate, number of blastomeres and live blastomeres, or proportions of grades 1 and 2, degenerated, and degenerated-unfertilized embryos/oocytes. Odds of subclinical endometritis on Day 30 postpartum more than doubled in cows with fever of unknown origin or acute puerperal metritis in the first 10 DIM. Fertilization rate tended to be reduced in cows with subclinical endometritis. In summary, replacing SS with an organic source of Se in diets not suboptimal in basal Se concentrations did not improve Se status, uterine health, fertilization, or embryo quality in early lactation dairy cows.     

Effects of long-term selenium yeast supplementation on selenium status studied in the rat

To investigate the selenium status during long-term dietary supply of selenium yeast, 30-day-old male rats were fed for 379 days a methionine-adequate low-selenium diet supplemented with 0.2 mg Se/kg (selenium-adequate diet) or 1.5 mg Se/kg (high-selenium diet) in the form of selenium yeast that contained 60% of the element as l-selenomethionine. Their selenium load was determined at several intervals by neutron activation analysis of the selenium concentrations in the main selenium body pools, skeletal muscle and liver. After 64 days the tissue selenium concentrations plateaued in both groups and then stayed at that level. Compared with the selenium-adequate group, elevated tissue selenium concentrations were found in the high-selenium group, but the increase by a factor of 3.5 in the muscle and by a factor of 2.3 in the liver was smaller than the 7.5-fold increase in the selenium intake. In the selenium-adequate group about 50% of the muscle selenium and 30% of the liver selenium and in the high-selenium group about 85% of the muscle selenium and 70% of the liver selenium were estimated to be present in non-selenoprotein forms. During selenium depletion the liver glutathione peroxidase activity in the high-selenium group remained unaffected for 4 weeks and then decreased more slowly than that in the selenium-adequate group. From these results it can be concluded that selenium incorporated from the selenium yeast diet into non-selenoprotein forms can serve as an endogenous selenium source to maintain selenoprotein levels in periods of insufficient selenium supply.     

Post-translational activation of non-selenium glutathione peroxidase of Chlamydomonas reinhardtii by specific incorporation of selenium

Glutathione peroxidase (GPX) plays a pivotal role in the protection of cells against oxidative damage. The green alga Chlamydomonas reinhardtii expresses both selenocysteine-containing GPX and the non-selenium GPX homolog (GPXH). We previously reported that supplementation of selenium to algal culture induces GPXH to exhibit GPX activity. Here we investigated the incorporation of selenium into GPXH and its causal relationship with the upregulation of the enzymatic activity. GPXH was purified from algal cells grown with selenium and proteolytically digested into four fragments. Selenium content analysis for these proteolytic fragments confirmed that GPXH-incorporated selenium is predominantly enriched in a fragment that carries the putative catalytic residue Cys-38. We next constructed three kinds of engineered GPXH proteins by substituting Ser for one of three Cys residues in native GPXH, Cys-38, -66, and -84, using a bacterial overexpression system, resulting in Cys38Ser, Cys66Ser, and Cys84Ser derivatives, respectively. Of these, the Cys66Ser and Cys84Ser derivatives exhibited the same level of selenium-dependent GPX activity as the normal recombinant GPXH, whereas the Cys38Ser mutant GPXH not only lost its activity completely but also demonstrated severely impaired incorporation of selenium. These findings strongly suggest that selenium is post-translationally assimilated into the Cys-38 of the GPXH protein, thereby enhancing its enzymatic activity.     

Influence of selenium on heat shock protein 70 expression in heat stressed turkey embryos (Meleagris gallopavo)

Heat shock protein 70 (hsp70) family of proteins, which functions as molecular chaperones, has been associated with tolerance to stressors in avian species. Selenium (Se) is an essential trace mineral incorporated into the seleno-enzymes such as glutathione peroxidase (GSHpx). GSHpx reduces oxidized glutathione (GSSG) to reduced glutathione (GSH) in the GSH/GSSG antioxidant system and protects cells from oxidative damage. This study was conducted to examine if the relationship between dietary supplementation of selenium to turkey (Meleagris gallopavo) hens and the embryonic expression of hsp70 and GSHpx activity in heat stressed embryos. Livers of embryos developing in eggs from turkey hens fed diets with or without supplemental Se were analyzed for hsp70 concentration and GSHpx activity before and after recovery from a heating episode. Before heat stress, hsp70 concentrations were equivalent in each treatment, but GSHpx activity was maximized in the SE treatment group. After recovery from the heating episode, hsp70 concentrations were significantly higher (P<0.05) in the non-Se-supplemented groups, but in the Se-supplemented groups the hsp70 concentrations were not different from pre-stress concentrations. In the pre-stress Se-supplemented group, liver GSHpx activity was significantly higher than GSHpx activity in the non-Se-supplemented embryo livers, and in the livers from embryos recovering from heat stress, GSHpx activity in the non-Se-supplemented group was lower than the pre-stress activity and significantly lower than the GSHpx activity in liver from Se-supplemented embryos recovering from heat distress. Se supplementation to the dams resulted in a significant increase in their embryos and that condition would facilitate a decreased incidence of oxidative damage to cells. A more reduced redox status in embryos from Se-supplemented dams decreased the need for cellular protection attributed to stress induced hsp70 and presumably allows heat distressed embryos to resume normal growth and development than embryos from dams with inadequate selenium nutrition.     

Comparing functional metabolic effects of marginal and sufficient selenium supply in sheep

This study was performed to characterise key data of long-term ovine Se metabolism and to work out the best biomarker of Se status. An upgrade from marginal (<0.05 mg Se/kg diet, ‘Se?’) to sufficient (0.2 mg Se/kg diet, ‘Se+’) nutritional Se supply using sodium selenite was monitered biweekly by analysing Se concentration, glutathione peroxidase (Gpx) activity and routine biochemistry in blood/serum over 2 years. Se, Cu, Zn, cytosolic Gpx and thioredoxin reductase (TrxR) activity were measured in the liver (biopsies/post-mortem). Se, Gpx, TrxR, glutathione-S-transferase-alpha (aGST) and iodothyronine deiodinase (Dio1) were analysed in the kidney, heart muscle and thyroid. Relative mRNA expression of hepatic aGST1 and Gpx1 was determined.Improvement of Se supply strongly increased serum and liver Se concentration within 10 and 20 days, respectively followed by a plateau. Whereas the achievement of a maximum whole blood Gpx activity was reached after 3 months, serum Gpx3 activity increased with high variations. Hepatic Gpx activity reached a maximum during days 100–200, decreasing thereafter. Distinct group differences in Se and cytosolic Gpx activity were evident in all organs (except Se in kidney). TrxR and Dio1 activity was affected only in the liver. The Se? sheep showed an ongoing decrease in serum Se concentration within 2 years, whereas liver Se remained almost unaffected. High relative Gpx1 mRNA expression in the Se+ group was consensual to high hepatic Gpx activity. Relative mRNA expression of hepatic aGST1 was higher in the Se? sheep. Clinical signs and abnormalities in routine biochemistry were absent.In summary, the best biomarker of Se deprivation and nutritional Se upgrade, respectively was Se in serum. Moreover, hepatic Se concentrations reliably reflected the upgrade of Se supply within days. Whole blood Gpx reacts slowly depending on newly formed erythrocytes restricting its diagnostic use. Vital organs are affected by Se deficiency due to a decrease of cytosolic Gpx activity attenuating the antioxidative system. Cellular up-regulation of aGST1 mRNA expression in the Se? group is assumed to partially compensate for the decreased antioxidant defence due to a loss in Gpx activity. This sheep model appears advantageous for long-term studies on sub-clinical metabolic effects in experimental modifiable nutritional Se supply.     

Assessment of selenium bioavailability from naturally produced high-selenium soy foods in selenium-deficient rats

We assessed the bioavailability of selenium (Se) from a protein isolate and tofu (bean curd) prepared from naturally produced high-Se soybeans. The Se concentrations of the soybeans, the protein isolate and tofu were 5.2 ± 0.2, 11.4 ± 0.1 and 7.4 ± 0.1 mg/kg, respectively. Male weanling Sprague–Dawley rats were depleted of Se by feeding them a 30% Torula yeast-based diet (4.1 μg Se/kg) for 56 days, and then they were replenished with Se for an additional 50 days by feeding them the same diet containing 14, 24 or 30 μg Se/kg from the protein isolate or 13, 23 or 31 μg Se/kg from tofu, respectively. l-Selenomethionine (SeMet) was used as a reference. Selenium bioavailability was determined on the basis of the restoration of Se-dependent enzyme activities and tissue Se concentrations in Se-depleted rats, comparing those responses for the protein isolate and tofu to those for SeMet by using a slope-ratio method. Dietary supplementation with the protein isolate or tofu resulted in linear or log-linear, dose-dependent increases in glutathione peroxidase activities in blood and liver and in thioredoxin reductase activity in liver. Furthermore, supplementation with the protein isolate or tofu resulted in linear or log-linear, dose-dependent increases in the Se concentrations of plasma, liver, muscle and kidneys. These results indicated an overall bioavailability of approximately 101% for Se from the protein isolate and 94% from tofu, relative to SeMet. We conclude that Se from naturally produced high-Se soybeans is highly bioavailable in this model and that high-Se soybeans may be a good dietary source of Se.     

Effects of selenium status and supplementary seleno-chemical sources on mouse T-cell mitogenesis

Although selenium is thought to be essential for various immune responses, the excess supplementation may have an adverse effect on certain immunological functions. The present study was designed to determine the effective chemical forms of selenium and their optimal levels on T-cell mitogenesis with splenic cells from mice given a selenium-deficient diet for 8 weeks to avoid effects of cellular selenium sources. Although selenium in tissues, except for spleen and thymus, was almost depleted by feeding selenium-deficient diet, the lymphoid organs still contained low levels of selenium. Both activities of cellular glutathione peroxidase (cGPx) and thioredoxin reductase (TR) in liver and splenic cells showed a tendency to decrease by selenium deficiency. However, splenic cells were tolerant against decrease of the selenoenzyme activities, and TR was also more tolerant than cGPx. T-cell proliferation of the selenium-insufficient splenic cells induced by concanavalin A was increased by addition of Na₂SeO₃, Na₂SeO₄, Na₂Se, seleno-dl-cystine, seleno-l-methionine and selenocystamine. Their promoting action was observed at levels lower than 0.1 μmol/L and was completely suppressed at the highest concentration (1 μmol/L), except for selenocystamine. Na₂SeO₃ was one of the efficient selenocompounds for the mitogenesis, which was concomitant with the significant induction of cGPx and TR. However, recovery of cGPx activity in the selenium-insufficient cells by supplementary Na₂SeO₃ was only partial, while TR activity was readily recovered from selenium deficiency. These results therefore indicate that only low levels of selenium is essential for T-cell mitogenesis even in selenium-insufficient splenic cells, and TR, which is readily recovered by Na₂SeO₃, may be the critical enzyme.     

Generational differences in selenium status of women

In this cross-sectional study of three generations of women, daughters (19–26 yr), mothers (40–58 yr) and maternal grandmothers (67–84 yr) from the same 10 families in central Ohio were studied to determine the effect of life-cycle differences, including estrogen status, on selenium status. Plasma and red blood cell (RBC) selenium and glutathione peroxidase (GPx) activities were determined and typical dietary selenium intakes were calculated from food-frequency questionnaires. Selenium status was lowest in the oldest generation. Plasma selenium of daughters and grandmothers were significantly lower than those of mothers, and plasma GPx and RBC selenium of grandmothers were also lower than those of the mothers. A positive correlation (r=0.42, p<0.04) was found between plasma estrogen and plasma selenium concentrations. Selenium intakes of all groups were adequate and no differences in selenium intakes were found among groups. The results of this study indicate that selenium status fluctuates during the female life cycle and is related to estrogen status.     

Development of antioxidant capacity in tissues of the chick embryo

The concentrations of vitamin E, vitamin A, selenium, reduced glutathione and the activities of glutathione peroxidase and superoxide dismutase were determined in the yolk, yolk sac membrane, liver and brain of the developing chick embryo. The changes in the concentrations of vitamins E and A in the yolk and liver during development were consistent with the occurrence of a preferential transfer of vitamin A from the yolk to the embryo before day 14 of incubation, whereas the main period of vitamin E transfer occurred later, during the last week of incubation. The concentrations of reduced glutathione in the yolk sac membrane, liver and brain were similar at all developmental stages studied. However, the levels of the other measured antioxidant systems were very much higher in the liver than in the brain. Thus, in the newly hatched chick, the levels of vitamin E, vitamin A, selenium, glutathione peroxidase and superoxide dismutase were, respectively, 58.0-, 174.7-, 3.6-, 4.0- and 4.7- fold higher in the liver than in the brain when expressed on the basis of tissue fresh weight.     

Determination of the daily selenium intake in slovakia

Three models were used to determine the daily dietary Selenium intake in Slovakia. The Selenium content of food produced and consumed in the Slovak Republic was used to estimate and calculate the daily Selenium intake based on food consumption data per capita and seven days, (24 h) eating protocol models. In a duplicate portion model, Selenium was analyzed in a whole day hospital diet during an eight-day period. According to these models the daily dietary Selenium intake was 38.2 μg; 43.3 ±6.5 μg for men and 32.6 ±6.6 μg for women; 27.1 ±7.8 μg for normal and 32.3 ±4.8 μg for nourishing hospital diets. The main contributors of Selenium to daily intake were the following: eggs, pork, and poultry. The obtained results indicate that the daily dietary intake of Selenium of the Slovak people is below the recommended values.     

Levels of selenium in plasma and glutathione peroxidase in erythrocytes and the risk of breast cancer

Plasma selenium and glutathione peroxidase in erythrocytes were analyzed in a case-control study encompassing 441 cases with breast cancer and 191 controls with benign breast disease. No difference in mean serum selenium level between cases and controls on supplementary selenium intake was seen. If only individuals without supplementary intake, 278 cases and 135 controls, were considered a preventive effect was found increasing with selenium level. This finding was significant among women 50 years old or more with Mantel-Haenszel odds ratio=0.16 for individuals with serum selenium >1.21 μmol/L. Also for subjects with serum selenium in the range 1.00–1.21 μmol/L a significant preventive effect was seen with odds ratio=0.38. For women under 50 years of age a nonsignificant preventive effect was seen. Glutathione peroxidase in erythrocytes did not correlate well with serum selenium and was not a marker for the risk of breast cancer.     

The protective role of selenium in recalcitrant Acer saccharium L. seeds subjected to desiccation

Freshly harvested silver maple (Acer saccharinum L.) seeds were soaked in either sodium selenite (10 mg/L) or water for 6 h. After washing and air drying, seeds were desiccated at 22 °C at a RH of 45-50% to comparable water levels from 50 to 12%. Germination capacity was significantly higher in seeds treated with selenium and desiccated [from 50 to 40, 35 and 30% of water content (WC)] than in water-soaked seeds. At 20% WC, the seeds from both treatments had low viability (approximately 20%). The electrolyte leakage and the MDA content were significantly lower in the embryonic axes of seeds soaked in selenite than in seeds soaked in water. We also found that the activity of glutathione peroxidase (GPX) of embryonic axes from selenium-treated seeds that were not desiccated, or from seeds that were desiccated to 40 and 35% WC, was significantly higher than that of non-treated axes. No difference in GPX activity was detected in cotyledons. This was confirmed by activity staining of GPX after native PAGE of proteins extracted from embryonic axes and cotyledons. An increase in glutathione reductase (GR) activity was also observed in embryonic axes of seeds treated with selenium and dried to 35 and 30% WC compared to non-treated samples. Selenium appeared to have no such effect on cotyledons.     

Study of the incorporation of selenium into peroxidase isozyme of wheat seedling

The effects of selenium on the activity of peroxidase (POD) of wheat seedling and its isozyme pattern were studied using a greenhouse hydroponic experiment. The results show that the activity of POD is increased in response to higher Se concentration (approx 5.0 mg/L) in culture medium. The electrophoretic pattern of the POD isozyme was altered by growth of the wheat in a selenium medium. Se could incorporate into some POD isozymes during either seed germination or the seedling growth period. There is a dose-responsive incorporation of selenium in isozyme of POD and selenium content in the isozyme increase along with the increase of selenium concentration in the medium.