Selenium maintains Ca2+ homeostasis in sheep lymphocytes challenged by oxidative stress

Abstract

Selenium (Se) is an essential element in human and animal diets, based upon a widespread range of beneficial effects that are primarily due to its antioxidant properties. While Se can be associated to anti-cancer and anti-diabetic activities, reproductive efficiency, and enhancement of the immune system, the mechanistic details of the corresponding biological processes are still largely elusive. To avoid deficiencies and increase bioavailability, Se it is generally supplied to livestock through Se-supplemented feeds or forage plants fertilized with inorganic Se. While the relationship between Ca2+ and ROS (reactive oxygen species) is well known, only a few studies have addressed the possible involvement of Se in the control of cytosolic Ca2+ in oxidative stress. The results on Ca2+ homeostasis were obtained adding exogenous Se in the form of SeO42- to sheep lymphomonocytes cultured in vitro. In particular, Se strongly attenuated 1mM H2O2-induced alteration of intracellular [Ca2+]C as well as the entry of extracellular Ca2+ into the cells with comparable EC50 values for sodium selenate accounting to 1.72 and 2.28 mM, respectively. In an ex vivo trial, it was observed that Ca2+ homeostasis can effectively be rescued in sheep lymphomonocytes exposed in vivo to a Se concentration of approximately 1.9 mM, that was achieved by feeding sheep with olive leaves previously sprayed with 500 mg/plant Na-selenate. Thus the results obtained suggest that the mode of action of selenium markedly influenced Ca2+-related signaling events. Furthermore, results clearly reveal that the protective effect of Se on Ca2+ homeostasis under oxidative challenge can be clearly and effectively achieved through an appropriate dietary regimen obtained also in a circular economy logic using pruning of olive trees treated to reduce tree drought stress.

Fig 1. Oxidative stress by H₂O₂ in sheep lymphomonocytes.

Time-course of sheep lymphomonocytes (1x10⁶) after treatment with H₂O₂ (0.2, 0.5,1.0 mM) and, 200s later, 1mM CaCl₂. Shown are levels (nM) of cytosolic calcium, measured at different time points. Each value calculated and expressed as the means from 5 experiments (biological replicates) ± SEM.

Fig 2. SeO₄^2- effect on calcium homeostasis during H₂O₂-induced oxidative stress.

Increasing concentrations (0.01–10μM) of SeO₄^2- were employed to pre-treat sheep lymphomonocytes prior to inducing oxidative stress with 1mM H₂O₂. Panel A shows the impact of selenium on [Ca²⁺]_c, and Panel B on Ca²⁺-entry. In both instances, results are expressed in relation to samples that were not exposed to SeO₄^2-. Each value corresponds to the mean calculated from 5 biological replicates ± SEM.

Fig 3. Total Se content in sheep blood.

Tests for total Se content were performed in the blood of sheep that were exposed to diets featuring different: C1, C2, C3. Each value corresponds to the mean calculated from 5 biological replicates ± SEM.

Fig 4. Relevance of dietary Se intake to calcium homeostasis in sheep lymphomonocytes subject to H₂O₂-induced oxidative stress.

Panel A (Ca²⁺ free): variations of Ca²⁺ cytosolic, and Panel B on Ca²⁺-entry.Each point corresponds to the average of five biological replicates (± SEM) measured on five preparations of lymphocytes obtained from the blood of five different animals included in the project.