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

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 Zn²⁺ administration at concentrations higher than, corresponding to, and lower than the physiological concentration, in thymocytes from young mice. We demonstrate that Zn²⁺ has an opposite effect on apoptosis, inhibiting or increasing it depending on the Zn²⁺ concentration used. High Zn²⁺ concentrations (from 600 to 75 μM) inhibit both serum-free medium and DEX-induced thymocyte apoptosis. Low Zn²⁺ concentrations (from 15 to 7.5 μM) induce apoptosis or increase serum-free medium-induced apoptosis. The effect of low Zn²⁺ concentrations on DEX-induced apoptosis is dependent on the length of incubation, since Zn²⁺ 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 Zn²⁺-induced DNA fragmentation require protein synthesis, being blocked through cycloheximide. The inhibiting and inducing effects of Zn²⁺ on apoptosis are exerted on G₀/G₁ phase thymocytes. The inhibiting effect of Zn²⁺ on apoptosis is related to an increase in the number of CD4⁺CD8⁺ thymocytes. Concentrations of Zn²⁺ inducing apoptosis sometimes cause a decrease of CD4⁺CD8^- cells with a corresponding increase of CD4⁺CD8^-thymocytes. These data show that in vitro Zn²⁺ has a dose-dependent opposite effect on apoptosis, suggesting that Zn²⁺ not only acts as an inhibitor but also plays a more complex role in physiological intrathymic cell selection.