Transcript Analysis of the Selenoproteome Indicates That Dietary Selenium Requirements of Rats Based on Selenium-Regulated Selenoprotein mRNA Levels Are Uniformly Less Than Those Based on Glutathione Peroxidase Activity

Abstract

Dietary selenium (Se) requirements in rats have been based largely upon glutathione peroxidase-1 (Gpx1) enzyme activity and Gpx1 mRNA levels can also be used to determine Se requirements. The identification of the complete selenoprotein proteome suggests that we might identify additional useful molecular biomarkers for assessment of Se status. To characterize Se regulation of the entire rat selenoproteome, weanling male rats were fed a Se-deficient diet (<0.01 microg Se/g) supplemented with graded levels of Se (0-0.8 microg/g diet) for 28 d, Se status was determined by tissue Se concentration and selenoenzyme activity, and selenoprotein mRNA abundance in liver, kidney, and muscle was determined by quantitative real-time-PCR. Tissue Se and selenoenzyme biomarkers indicated that minimal Se requirements were <or=0.1 microg Se/g diet for most biomarkers. Liver Gpx1 mRNA also decreased to <10% of Se-adequate levels, with a minimum Se requirement at 0.07 microg/g diet. Five selenoprotein mRNA in liver, 4 in kidney, and 2 in muscle decreased to <41% of Se-adequate levels, all with minimum Se requirements at <or=0.07 microg/g diet; the majority of selenoprotein mRNA in each tissue were not significantly regulated by Se status, and 1 selenoprotein, selenophosphate synthetase-2, was upregulated in Se-deficient kidney. Plateau breakpoints for all regulated selenoprotein mRNA were very similar, suggesting that 1 underlying mechanism is in play in Se regulation of selenoprotein mRNA. Lastly, we did not find any selenoprotein mRNA that could be used as biomarkers for super-nutritional/anticarcinogenic levels (up to 0.8 microg Se/g diet) of Se.

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

“…These additional selenoproteins expanded the array of both biochemical and transcript-based biomarkers of selenium status in our studies (14)(15)(16)(17)(18). The identification of the complete selenoproteome (19) further expanded the number of selenoprotein transcripts as "molecular biomarkers" for the assessment of selenium status and requirements in rodents (20)(21)(22)(23) and is helping researchers to better understand the hierarchy of selenium incorporation into selenoproteins.…”

“…Selenium biomarker data for rats are from the report of Barnes et al (20), which used the basal selenium-deficient 30% torula-yeast diet containing 0.007 mg Se/g diet supplemented with graded levels of dietary selenium from 0 to 0.8 mg Se/g as Na 2 SeO 3 . Vitamin E was provided at 100 mg/kg diet.…”

“…Vitamin E was provided at 100 mg/kg diet. Weanling 21-d-old male rats were housed in hanging wire cages and fed these diets for 28 d. All other aspects of the study have been previously described in detail (20).…”

“…To allow direct comparison of species, selenium-response curves for selenium biomarkers that reached well-defined plateaus [indicated by nonsignificant differences in at least the highest 4 (2 for lambs) dietary selenium supplementation levels] were recalculated and redrawn with plateaus of zero slope at a level equal to the mean of individual values in treatment groups on the plateau. The "plateau breakpoint" for each seleniumresponse curve, defined as the intersection of the line tangent to the point of steepest slope and the plateau, was calculated as described previously (14,18,20) to estimate the "minimum dietary Se requirement" necessary to obtain the plateau response.…”

“…The detailed significance of biomarker differences within an experiment is described in the original publications; for each biomarker, the resulting ANOVA P values and the levels of dietary selenium that were significantly different from plateau values are given in Table 1. An "Se-response curve" was constructed by using sigmoidal or hyperbolic regression analysis (Sigma Plot; Jandel Scientific) with the use of all individual values at each dietary selenium treatment, as described previously (14,18,20). To allow direct comparison of species, selenium-response curves for selenium biomarkers that reached well-defined plateaus [indicated by nonsignificant differences in at least the highest 4 (2 for lambs) dietary selenium supplementation levels] were recalculated and redrawn with plateaus of zero slope at a level equal to the mean of individual values in treatment groups on the plateau.…”