Arsenic Trioxide Induces Regulated, Death Receptor- and Caspase-Independent Cell Death through the Mitochondrial Death Pathway.

Arsenic trioxide (As₂O₃, arsenite) efficiently kills cells from various hematologic malignancies and has successfully been employed especially for the treatment of acute promyelocytic leukemia. There, and in lymphoid cells we demonstrated that arsenite induces cell death in a caspase-2 and -9-independent fashion. Here, we address a potential role of death receptor signaling through the FADD/caspase-8 death inducing signaling complex in arsenite-induced cell death. In detail, we demonstrate that arsenite induces cell death independently of caspase-8 or FADD and cannot be blocked by disruption of CD95/Fas receptor ligand interaction. Unlike in death receptor ligation-induced apoptosis, As₂O₃-induced cell death was not blocked by the broad spectrum caspase inhibitor z-VAD-fmk or the caspase-8-specific inhibitor z-IETD-fmk. Nevertheless, arsenite-induced cell death occurred in a regulated manner and was abrogated upon Bcl-2 overexpression. In contrast, arsenite-induced cell demise was neither blocked by the caspase-9 inhibitor z-LEHD-fmk nor substantially inhibited through the expression of a dominant negative caspase-9 mutant. Altogether our data demonstrate that As₂O₃-induced cell death occurs independently of the extrinsic death receptor pathway of apoptosis. Cell death proceeds entirely via an intrinsic, Bcl-2 controlled mitochondrial pathway that does, however, not rely on caspase-9.