1. ¹Department of Hematology, Oncology and Tumor Immunology, University Medical Center Charité, Campus Buch, Humboldt University, Berlin, Germany
2. ²Institute of Molecular Medicine, Heinrich Heine University, Düsseldorf, Germany
3. ³Clinical and Molecular Oncology, Max Delbrück Center for Molecular Medicine, 13125 Berlin, Germany

Correspondence: PT Daniel, Clinical and Molecular Oncology, Charité University Medical Center, Campus Virchow Klinikum, Augustenburger Platz 1, 13353 Berlin, Germany. E-mail: pdaniel@mdc-berlin.de

Received 23 February 2005; Revised 16 May 2005; Accepted 16 May 2005; Published online 11 July 2005.

Abstract

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 As₂O₃ 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 As₂O₃-induced cell death. In detail, we demonstrate that As₂O₃ 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, As₂O₃-induced cell death occurred in a regulated manner and was abrogated upon Bcl-2 overexpression. In contrast, As₂O₃-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.