Form of Supplemental Selenium Affects the Expression of mRNA Transcripts Encoding Selenoproteins, and Proteins Regulating Cholesterol Uptake, in the Corpus Luteum of Grazing Beef Cows

Simple Summary In regions with selenium (Se) deficient soils, producers should supplement this mineral to the diet of forage-grazing cattle. Commercial supplements are typically formulated using inorganic forms of Se, but the organic forms are those naturally available in forages. We previously reported that circulating concentrations of progesterone are affected by the form of Se supplemented to cows. In this report, we aimed to determine (1) how the form of Se affects the expression of mRNA encoding selenoproteins in the corpus luteum, and (2) whether form of Se-induced increases in progesterone are the result of direct increases in the expression of steroidogenic transcripts. Cows were supplemented with the industry standard, an inorganic form of Se, or a 1:1 mix of organic and inorganic forms (MIX), with corpora lutea recovered on Day 7 of the estrous cycle. Transcripts encoding selenoproteins, as well as those regulating the uptake of cholesterol were affected by form of Se, but not those directly regulating steroidogenesis. We demonstrated that the expression of multiple selenoprotein mRNAs are affected by the form of Se, some of which are associated with intracellular antioxidant activity, and that the previously reported form of Se-induced increase in progesterone appears to be due to increased cholesterol availability by the corpus luteum, not by an increase in the expression of key steroidogenic enzymes. Abstract Selenium (Se)-deficient soils necessitate supplementation of this mineral to the diet of forage-grazing cattle. Functionally, Se is incorporated into selenoproteins, some of which function as important antioxidants. We have previously shown that the source of supplemental Se; inorganic (sodium selenite or sodium selenate; ISe), organic (selenomethionine or selenocysteine; OSe) or 1:1 mix of ISe and OSe (MIX), provided to Angus-cross cows affects concentrations of progesterone (P4) during the early luteal phase of the estrous cycle. In this study, we sought to investigate (1) the effect of form of Se on the expression of mRNA encoding selenoproteins in the corpus luteum (CL), and (2) whether this previously reported MIX-induced increase in P4 is the result of increased luteal expression of key steroidogenic transcripts. Following a Se depletion and repletion regimen, 3-year-old, non-lactating, Angus- cross cows were supplemented with either ISe as the industry standard, or MIX for at least 90 days, with the CL then retrieved on Day 7 post-estrus. Half of each CL was used for analysis of targeted mRNA transcripts and the remainder was dissociated for culture with select agonists. The expression of three selenoprotein transcripts and one selenoprotein P receptor was increased (p < 0.05), with an additional five transcripts tending to be increased (p < 0.10), in cows supplemented with MIX versus ISe. In cultures of luteal cells, hCG-induced increases in P4 (p < 0.05) were observed in CL obtained from ISe-supplemented cows. The abundance of steroidogenic transcripts in the CL was not affected by the form of Se, however, the abundance of mRNA encoding 2 key transcripts regulating cholesterol availability (Ldlr and Hsl) was increased (p < 0.05) in MIX-supplemented cows. Overall, the form of Se provided to cows is reported to affect the expression of mRNA encoding several selenoproteins in the CL, and that the form of Se-induced effects on luteal production of P4 appears to be the result of changes in cholesterol availability rather than a direct effect on the expression of steroidogenic enzymes within the CL.