Zinc is a potent inhibitor of apoptosis, whereas zinc depletion induces apoptosis in many cell lines. To investigate the mechanisms of zinc depletion-induced apoptosis, HeLa cells were treated with the membrane permeable metal ion chelator, N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN). TPEN decreased the intracellular level of zinc and induced apoptosis with a characteristic cellular pattern, i.e. cell shrinkage and formation of apoptotic bodies, with DNA fragmentation and formation of a typical DNA ladder pattern. Following TPEN treatment, caspases-3, -8, and -9 were activated and caspase target proteins, poly(ADP-ribose) polymerase, and Sp transcription factors were cleaved. These effects were inhibited by adding zinc to the medium. To assess the role of zinc in the activation of the caspase cascade, we compared zinc inhibition during tumor necrosis factor alpha/cycloheximide- and etoposide-induced apoptosis with that induced by TPEN. Zinc addition partially inhibited caspase-3 activation, but not caspase-8 and -9 cleavage in HeLa cells treated with tumor necrosis factor alpha or etoposide. These results suggest that caspase-3 is rapidly and directly activated by zinc chelation, without a requirement for an upstream event. Caspase-3 activation is therefore the main event leading to apoptosis after intracellular zinc chelation. Finally, we conclude that cellular zinc inhibits apoptosis by maintaining caspase-3 inactive.