Blood glutathione peroxidase-1 mRNA levels can be used as molecular biomarkers to determine dietary selenium requirements in rats

Arsenic exposure and increased C-reactive protein are independently associated with lower erythrocyte glutathione peroxidase activity in Bangladeshi children

Toxic metal contaminants present in food and water have widespread effects on health and disease. Chalcophiles, such as arsenic, cadmium, and mercury, show a high affinity to selenium and exposure to these metals could have a modulating effect on enzymes dependent on selenocysteine in their active sites. The aim of this study was to assess the influence of these metals on the activity of the selenoprotein glutathione peroxidase 1 (GPX1) in erythrocytes of 100 children residing in rural Bangladesh, where drinking water often contains arsenic. GPX1 expression, as measured using high-throughput immunoblotting, showed little correlation with GPX activity (rs = 0.02, p = 0.87) in blood samples. Toxic metals and selenium measured in erythrocytes using inductively coupled plasma mass spectrometry (ICP-MS) and C-reactive protein (CRP) measured in plasma, were all considered as effectors of this divergence in GPX enzymatic activity. Arsenic concentrations in erythrocytes were most influential for GPX1 activity (rs = -0.395, p < 0.0001), and CRP levels also negatively impacted GPX1 activity (rs = -0.443, p < 0.0001). These effects appear independent of each other as arsenic concentrations and CRP showed no correlation (rs = 0.124, p = 0.2204). Erythrocyte selenium, cadmium, and mercury did not show any correlation with GPX1 activity, nor with CRP or arsenic. Our findings suggest that childhood exposure to inorganic arsenic, as well as inflammation triggering the release of CRP, may negatively affect GPX1 activity in erythrocytes.

Reduced Dietary Selenium Impairs Vascular Function by Increasing Oxidative Stress in Sprague-Dawley Rat Aortas

This study aimed to determine whether low dietary Se content affects the function and mechanisms mediating the vascular relaxation of rat aortas, and to test the role of oxidative stress in observed differences. Male Sprague Dawley (SD) rats were maintained for 10 weeks on low Se (low-Se group; N = 20) or normal Se content (norm-Se group; N = 20) rat chow. Dose responses to acetylcholine (ACh; 10-9-10-5M) and the response to reduced pO₂ were tested in noradrenaline-precontracted aortic rings in the absence/presence of the nitric oxide synthase (NOS) inhibitor nitro-l-arginine methyl ester (l-NAME), the cyclooxygenase 1 and 2 (COX-1, 2) inhibitor Indomethacin, and the antioxidative agent Tempol in tissue bath. mRNA expression of glutathione peroxidase 1 (GPx1), catalase (CAT), and Cu/Zn superoxide dismutase (SOD) was measured in rat aortas. Oxidative stress (Thiobarbituric Acid Reactive Substances; TBARS), antioxidative plasma capacity (ferric reducing ability of plasma assay; FRAP), and protein levels of GPx1 were measured in plasma and serum samples, respectively. Reduced ACh-induced relaxation (AChIR) (dominantly mediated by NO) in the low-Se group compared to the norm-Se group was restored by Tempol administration. Hypoxia-induced relaxation (HIR) (dominantly mediated by COX-1, 2), TBARS, and FRAP as well as GPx1 serum concentrations were similar between the groups. mRNA GPx1 expression in rat aortas was significantly decreased in the low-Se compared to the norm-Se group. These data suggest that low dietary Se content increases the local oxidative stress level, which subsequently affects the NO-mediated vascular response.

Insights for Setting of Nutrient Requirements, Gleaned by Comparison of Selenium Status Biomarkers in Turkeys and Chickens versus Rats, Mice, and Lambs

To gain insights into nutrient biomarkers and setting of dietary nutrient requirements, selenium biomarker levels and requirements in response to multiple graded levels of dietary selenium were compared between day-old turkeys and chickens versus weanling rats and mice and 2-d-old lambs supplemented with sodium selenite. In rodents, there was no significant effect of dietary selenium on growth, indicating that the minimum selenium requirement was <0.007 μg Se/g diet. In contrast, there was a significant effect in turkeys, chicks, and lambs, which showed selenium requirements for growth of 0.05, 0.025, and 0.05 μg Se/g diet, respectively. Liver glutathione peroxidase (GPX) 1 activity fell in all species to <4% of selenium-adequate levels, plasma GPX3 activity fell to <3% in all species except for mice, and liver GPX4 activity fell to <10% in avians but only to ∼50% of selenium-adequate levels in rodents. Selenium-response curves for these biomarkers reached well-defined plateaus with increasing selenium supplementation in all species, collectively indicating minimum selenium requirements of 0.06-0.10 μg Se/g for rats, mice, and lambs but 0.10-0.13 μg Se/g for chicks and 0.23-0.33 μg Se/g for turkeys. In contrast, increasing dietary selenium did not result in well-defined plateaus for erythrocyte GPX1 activity and liver selenium in most species. Selenium-response curves for GPX1 mRNA for rodents and avians had well-defined plateaus and similar breakpoints. GPX4 mRNA was not significantly regulated by dietary selenium in rodents, but GPX4 mRNA in avians decreased in selenium deficiency to ∼35% of selenium-adequate plateau levels. Notably, no selenoprotein activities or mRNA were effective biomarkers for supernutritional selenium status. Robust biomarkers, such as liver GPX1 and plasma GPX3 activity for selenium, should be specific for the nutrient, fall dramatically in deficiency, and reach well-defined plateaus. Differences in biomarker-response curves may help researchers better understand nutrient metabolism and targeting of tissues in deficiency, thus to better characterize requirements.

Selenium-enriched foods are more effective at increasing glutathione peroxidase (GPx) activity compared with selenomethionine: a meta-analysis

Selenium may play a beneficial role in multi-factorial illnesses with genetic and environmental linkages via epigenetic regulation in part via glutathione peroxidase (GPx) activity. A meta-analysis was undertaken to quantify the effects of dietary selenium supplementation on the activity of overall GPx activity in different tissues and animal species and to compare the effectiveness of different forms of dietary selenium. GPx activity response was affected by both the dose and form of selenium (p < 0.001). There were differences between tissues on the effects of selenium supplementation on GPx activity (p < 0.001); however, there was no evidence in the data of differences between animal species (p = 0.95). The interactions between dose and tissue, animal species and form were significant (p < 0.001). Tissues particularly sensitive to changes in selenium supply include red blood cells, kidney and muscle. The meta-analysis identified that for animal species selenium-enriched foods were more effective than selenomethionine at increasing GPx activity.

Adverse effects of excessive leucine intake depend on dietary protein intake: a transcriptomic analysis to identify useful biomarkers

The present study was conducted to identify reliable gene biomarkers for the adverse effects of excessive leucine (Leu) in Sprague-Dawley rats by DNA microarray. It has long been known that the adverse effects of excessive amino acid intake depend on dietary protein levels. Male rats were divided into 12 groups (n=6) and fed for 1 wk a diet containing low (6%), moderate (12%) or high (40%) protein. Different levels of Leu (0, 2, 4, and 8%) were added to the diets. Consumption of diets containing more than 4% Leu in 6% protein resulted in growth retardation and reduced liver weight, whereas the administration of the same dose of Leu with 12% or 40% protein did not affect them. By a process of systematic data extraction, 6 candidate gene markers were identified. The liver gene expression data obtained from another experiment with 0, 2, 3, 4, and 8% Leu in a low-protein diet was used to examine the validity of these biomarker candidates with receiver operating characteristic (ROC) curve analysis. All of AUC values of the biomarker candidates were more than 0.700, suggesting the effectiveness of the marker candidates as the indices of Leu excess. The cut-off value for the ROC curve of the gene-marker panel, which was obtained by multiple regression analysis of gene markers, indicated that Leu levels higher than 3% have adverse effects. In conclusion, the gene-marker panel suggested that for male rats dietary Leu supplementation of 2% is the NOAEL dose in low-protein (6%) diets.

Encapsulated nanoepigallocatechin-3-gallate and elemental selenium nanoparticles as paradigms for nanochemoprevention

Chemoprevention that impedes one or more steps in carcinogenesis, via long-term administration of naturally occurring or synthetic compounds, is widely considered to be a crucial strategy for cancer control. Selenium (Se) has chemopreventive effects, but its application is limited due to a low therapeutic index as shown in numerous animal experiments. In contrast to Se, which was known for its toxicity prior to the discovery of its beneficial effects, the natural compound epigallocatechin-3-gallate (EGCG) was originally considered to be nontoxic. Due to its preventive effects on many types of cancer in various animal models, EGCG has been regarded as a prime example of a promising chemopreventive agent without major toxicity concerns. However, very recently, evidence has accumulated showing that efficacious doses of EGCG used in health promotion may not be far from its toxic dose level. Therefore, both Se and EGCG need to be modified by novel pharmaceutical technologies to attain enhanced efficacy and/or reduced toxicity. Nanotechnology may be one of these technologies. In support of this hypothesis, the characteristics of polylactic acid and polyethylene glycol-encapsulated nano-EGCG and elemental Se nanoparticles dispersed by bovine serum albumin are reviewed in this article. Encapsulation of EGCG to form nano-EGCG leads to its enhanced stability in plasma and remarkably superior chemopreventive effects, with more than tenfold dose advantages in inducing apoptosis and inhibition of both angiogenesis and tumor growth. Se at nanoparticle size (“Nano-Se”), compared with Se compounds commonly used in dietary supplements, has significantly lower toxicity, without compromising its ability to upregulate selenoenzymes at nutritional levels and induce phase II enzymes at supranutritional levels.

Developmental expression of plasma glutathione peroxidase during mouse organogenesis

Plasma glutathione peroxidase (pGPx) is an extracellular antioxidative selenoenzyme which has been detected in various adult tissues, but little is known about the expression and distribution of pGPx during embryogenesis. To investigate the expression patterns of pGPx during embryogenesis, we performed quantitative real-time PCR, in situ hybridization, Western blot, and immunohistochemistry analyses in whole embryos or each developing organ of mice on embryonic days (E)7.5-18.5. In whole embryos of E7.5-8.5, pGPx mRNA was more typically expressed in extra-embryonic tissues including ectoplacental cone, trophectoderm, and decidual cells than in embryos. However, after E9.5, pGPx mRNA and protein levels were increased in the embryos with differentiation and growth, but trended to gradually decrease in the extra-embryonic tissues until E18.5. In sectioned embryonic tissues on E13.5-18.5, pGPx mRNA and protein were mainly expressed in the developing nervous tissues, the sensory organs, and the epithelia of lung, skin, and intestine, the heart and artery, and the kidney. In particular, pGPx immunoreactivity was very strong in the developing liver. These results indicate that pGPx is spatio-temporally expressed in various embryonic organs as well as extra-embryonic tissues, suggesting that pGPx may function to protect the embryos against endogenous and exogenous reactive oxygen species during organogenesis.

Selenium regulation of the selenoprotein and nonselenoprotein transcriptomes in rodents

This review discusses progress in understanding the hierarchy of selenoprotein expression at the transcriptome level from selenium (Se) deficiency to Se toxicity. Microarray studies of the full selenoproteome have found that 5 of 24 rodent selenoprotein mRNA decrease to <40% of Se adequate levels in Se deficient liver but that the majority of selenoprotein mRNA are not regulated by Se deficiency. These differences match with the hierarchy of selenoprotein expression, helping to explain these differences and also showing that selenoprotein transcripts can be used as molecular biomarkers for assessing Se status. The similarity of the response curves for regulated selenoproteins suggests one underlying mechanism is responsible for the downregulation of selenoprotein mRNA in Se deficiency, but the heterogeneity of the UGA position in regulated and nonregulated selenoprotein transcripts now indicates that current nonsense mediated decay models cannot explain which transcripts are susceptible to mRNA decay. Microarray studies on the full liver transcriptome in rats found only <10 transcripts/treatment were significantly down- or upregulated by Se deficiency or by supernutritional Se up to 2.0 μg Se/g diet (20× requirement), suggesting that cancer prevention associated with supernutritional Se may not be mediated by transcriptional changes. Toxic dietary Se at 50× requirement (5 μg Se/g diet), however, significantly altered ∼4% of the transcriptome, suggesting number of transcriptional changes itself as a biomarker of Se toxicity. Finally, panels of Se regulated selenoprotein plus nonselenoprotein transcripts predict Se status from deficient to toxic better than conventional biomarkers, illustrating potential roles for molecular biomarkers in nutrition.

Molecular biomarker panels for assessment of selenium status in rats

Molecular biomarkers are mRNA transcripts that indicate the (nutrient) status of an organism or tissue. Molecular biomarker panels have the potential to readily and more accurately determine nutrient status than individual traditional biomarkers. To study the efficacy of molecular biomarker panels for predicting selenium (Se) status, we examined 30 biomarkers from rats fed graded levels of Se from deficient to eight times the minimum Se requirement, including four liver and four kidney traditional biomarkers, and 13 liver and nine kidney selenoprotein mRNA levels. Multiple regression analysis against liver and kidney Se and glutathione peroxidase-1 (Gpx1) activity, with stepwise single elimination of biomarkers that did not significantly contribute, was used to identify biomarker panels with significant (P < 0.05) regression coefficients. Resulting regression equations were then used to predict Se status, and compared with traditional Se biomarkers panels. Over the full spectrum of Se status from 0 to 0.8 microg Se/g diet, the resulting 4-selenoprotein mRNA biomarker panel predicted liver Se concentration with a correlation of 0.948, which was nominally higher and statistically the same as the correlation of 0.909 for the panel based on Gpx1 activity. The molecular biomarker panels for predicting kidney Se and liver and kidney Gpx1 activity were all comparable to predictions based on traditional biomarkers. These analyses show that molecular biomarker panels can be used to predict accurately two traditional biomarkers of Se status. The resulting analyses also illustrate that additional orthogonal biomarkers reflecting higher Se intakes are needed to better predict supernutritional Se status and further strengthen this approach.

mRNA transcripts as molecular biomarkers in medicine and nutrition

In medicine, mRNA transcripts are being developed as molecular biomarkers for the diagnosis and treatment of a number of diseases. These biomarkers offer early and more accurate prediction and diagnosis of disease and disease progression, and ability to identify individuals at risk. Use of microarrays also offers opportunity to identify orthogonal (uncorrelated) biomarkers not known to be linked with conventional biomarkers. Investigators are increasingly using blood as a surrogate tissue for biopsy and analysis; total RNA isolated from whole blood is predominantly from erythroid cells, and whole blood mRNA shares more than 80% of the transcriptome with major tissues. Thus blood mRNA biomarkers for individualized disease prediction and diagnosis are an exciting area in medicine; mRNA biomarkers in nutrition have potential application that parallels these opportunities. Assessment of selenium (Se) status and requirements is one area where tissue mRNA levels have been used successfully. Selenoprotein-H and selenoprotein-W as well as glutathione peroxidase-1 (Gpx1) mRNAs are highly down-regulated in Se deficiency in rat liver, and the minimum dietary Se requirement is 0.06-0.07 microg Se/g based on these biomarkers, similar to requirements determined using conventional biomarkers. Blood Gpx1 mRNA can also be used to determine Se requirements in rats, showing that blood mRNA has potential for assessment of nutrient status. Future research is needed to develop mRNA biomarker panels for all nutrients that will discriminate between deficient, marginal, adequate and supernutritional individuals and populations, and differentiate between individuals who will benefit vs. be adversely affected by nutrient supplementation.