| Abstract |
We found that treatment of U937 cells with ZnCl(2) resulted in marked inhibition of ricin-induced DNA fragmentation and nuclear morphological change. Zn(2+) also completely inhibited the activation of caspase-3-, caspase-6-, and caspase-9-like proteases in ricin-treated cells, while no significant effect of Zn(2+) on these protease activities was observed when added directly to the lysate of ricin-treated cells, suggesting that Zn(2+) blocks the process of the activation of these caspases rather than the direct inhibition of the already activated enzymes. Fluorescence microscopic observation with Zn(2+) specific fluorescent probe dansylaminoethyl-cyclen suggested that there was a substantial increase in probe-detectable Zn(2+) in ricin-treated cells. Since the differences in the total Zn(2+) contents between ricin-treated and -untreated cells as measured with an atomic absorption spectrophotometer were too small to explain the increase in probe fluorescence in ricin-treated cells, it was suggested that release of Zn(2+) from intracellular stores or metalloproteins may occur rather than enhanced uptake from the medium. The Zn(2+) probe fluorescence change was observed prior to the depletion of intracellular glutathione. Carbobenzoxy-Asp-1-yl-[(2,6-dichlorobenzoyl)oxy]methane (Z-Asp-CH(2)-DCB), a caspase family protease inhibitor, prevented ricin-induced increase in Zn(2+) probe fluorescence. These results suggest that redistribution of intracellular Zn(2+) occurs during ricin-induced apoptosis as early apoptotic event, and exogenously added Zn(2+) may prevent such intracellular Zn(2+) redistribution resulting in the inhibition of apoptosis.
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