Arsenic trioxide (As(2)O(3)) is toxic to multidrug-resistant neuroblastoma cells in vivo and in vitro. In neuroblastoma, As(2)O(3) does not exert its cell death-promoting effects via a classical apoptotic pathway. A death mechanism involving proteolytic cleavage of Bax to a p18 form seems to be of importance, because inhibition of Bax cleavage coincides with diminished cell death. As existing models of cell death implicate Bax in the intrinsic apoptotic pathway, triggering death after Bax translocation to the mitochondria, we investigated the cellular localization of p18 Bax by subcellular fractionation. After As(2)O(3) treatment, p18 Bax was only present in nuclei-enriched, mitochondria-depleted fractions. Cytoplasmic p21 Bax levels decreased, whereas total (p21 and p18) nuclear Bax increased. Overexpressed p21 Bax localized to the cytoplasm and nuclei, whereas overexpressed p18 Bax localized to extra-nuclear structures only. The inability of overexpressed p18 Bax to locate to the nucleus, and the As(2)O(3)-induced reduction of p21 Bax in the cytosol, suggest an As(2)O(3)-induced mechanism where p18 Bax gets cleaved and 'trapped' in the nucleus. This model is strengthened by the observation that calpain, the protease responsible for p18 Bax generation, is present in the nuclei, and that nuclear calpain is induced by increasing As(2)O(3) and Ca(2+) levels.