Bidirectional effect of ecdysterone on thymocyte volume regulation and proliferation

The development and maturation of T cells requires an efficient cell volume regulation (CVR) system in which the volume-sensitive outwardly rectifying anion channel (VSOR/VRAC) plays a pivotal role. Ecdysterone (20-hydroxyecdysone, 20HE) is known to exert multiple effects in mammals, but the response of the thymus and thymocytes to this hormone remains virtually unexplored. In the present study, we observed a bidirectional effect of 20HE on the thymus and its cellular contents. In the short term, thymocytes responded by blockage of the VSOR/VRAC with a half-maximal effective concentration of ∼33-37 μM and a maximum observed inhibition by 57-62 % at 100 μM. Suppression of the thymocytic RVD occurred to a less degree of ∼31 % but more efficiently with a half-maximal concentration at ∼8 μM. In contrast to the short-term effects, prolonged exposure of thymocytes to 20HE increased their proliferative activity under primary culture conditions (by ∼67 % after 24 h) without detectable change in the VSOR/VRAC activity. Consistent with this result, at the whole organism level, administration of 20HE per os for 5 days strongly stimulated thymic growth (by ∼61 %) and up-regulated the CVR efficiency of the cells isolated from the 20HE-treated animals (parameter RVD increase by ∼12 %). The results obtained suggest that systemic effects of 20HE, which become apparent only after long-term exposure in primary culture conditions or in the whole organism, may counteract the acute blockade of thymocyte VSOR/VRAC and RVD induced by the steroid in the buffered saline.