Geranylgeraniol (GGO) at 50 microM induces apoptosis in HL-60 cells. We examined the effects of Zn2+ ions on this process. Treatment of HL-60 cells with Zn2+ ions inhibited subsequent GGO-induced fragmentation of DNA. In a cell-free system that consisted of a specific substrate for caspase-3 and a lysate of HL-60 cells that had been treated with 50 microM GGO, Zn2+ ions at concentrations above 0.1 mM inhibited the activity of caspase-3. The effect of Zn2+ ions on the processing of caspase-3 during GGO-induced apoptosis was investigated by Western blotting, which revealed that an inactive 32-kDa precursor of caspase-3 was cleaved, in response to GGO, to yield an activated 17-kDa enzyme. Treatment of HL-60 cells with Zn2+ ions inhibited the cleavage of the precursor by a protease that was induced by treatment with GGO, and inhibition of this processing was well correlated with the inhibition by Zn2+ ions of caspase-3 activity in the cell-free system. In cell-extracted cytosols, Zn2+ ions inhibited the cleavage of the 32-kDa precursor by caspase-9 (Aapf-3) that was activated by addition of cytochrome c and dATP. These results indicate that inhibition of GGO-induced apoptosis in HL-60 cells by Zn2+ ions might be due to inhibition by Zn2+ ions of the processing of a precursor to caspase-3.