Dose-dependent opposite effect of zinc on apoptosis in mouse thymocytes

Abstract

Zinc is a crucial nutritional component required for the normal development and maintenance of immune functions. It has been reported that zinc is a potent inhibitor of DNA fragmentation, the specific marker of apoptosis. The effect of zinc on apoptotic cell death has been previously studied in a narrow range of high zinc concentrations, and the role of physiological zinc doses has not yet been elucidated. In this paper we evaluate the effect of in vitro Zn 2+ administration at concentrations higher than, corresponding to, and lower than the physiological concentration, in thymocytes from young mice. We demonstrate that Zn 2+ has an opposite effect on apoptosis, inhibiting or increasing it depending on the Zn 2+ concentration used. High Zn 2+ concentrations (from 600 to 75 μM) inhibit both serum-free medium and DEX-induced thymocyte apoptosis. Low Zn 2+ concentrations (from 15 to 7.5 μM) induce apoptosis or increase serum-free medium-induced apoptosis. The effect of low Zn 2+ concentrations on DEX-induced apoptosis is dependent on the length of incubation, since Zn 2+ has an additive effect with DEX in inducing DNA fragmentation at 8 h of culture, whereas it blocks DEX-induced apoptosis after 20 h incubation. Both DEX and 15 μM Zn 2+-induced DNA fragmentation require protein synthesis, being blocked through cycloheximide. The inhibiting and inducing effects of Zn 2+ on apoptosis are exerted on G 0/G 1 phase thymocytes. The inhibiting effect of Zn 2+ on apoptosis is related to an increase in the number of CD4 +CD8 + thymocytes. Concentrations of Zn 2+ inducing apoptosis sometimes cause a decrease of CD4 +CD8 − cells with a corresponding increase of CD4 +CD8 −thymocytes. These data show that in vitro Zn 2+ has a dose-dependent opposite effect on apoptosis, suggesting that Zn 2+ not only acts as an inhibitor but also plays a more complex role in physiological intrathymic cell selection.