Upregulation of c-Jun Induces Cell Death Via Caspase-Triggered c-Abl Cleavage in Human Multiple Myeloma.

The tyrophostin adaphostin (NSC680410) achieves remarkeable responses in patients with chronic myelocytic leukemia (CML), including Bcr/Abl-positive, Bcr/Abl-negative, and Bcr-Abl T315I mutant tumor cells resistant to both imatinib mesylate and second- generation BMS354825 and AMN107. In addition it also demonstrates cytotoxicity against chronic lymphcytic leukemia (CLL) and acute myelocytic leukemia (AML) cells. Several mechanisms have been proposed as a basis for its robust anti-tumor activity including generation and release of reactive oxygen species (ROS), cytochrome-c and apoptosis- inhibiting factor (AIF), caspase- cleavage, JNK activation, as well as inactivation of Raf-1, Stat3, and Stat5. As these adaphostin- targeted pathways are relevant to MM pathogenesis, we here sought to determine the potential molecular sequelae and therapeutic promise of adaphostin in MM. After demonstrating the anti- myeloma cytotoxicity of adaphostin, we carried out expression profiling of adaphostin- treated MM cells to identify its molecular targets. Surprisingly, c-Jun was the most upregulated gene even at the earliest point of analysis (2 hours). We also observed adaphostin- induced c-Abl cleavage in immunoblot analysis. Proteasome inhibitor bortezomib, but not melphalan or dexamethasone, induced similar effects, indicating unique agent- dependent mechanisms. Using caspase inhibitors as well as caspase- resistant mutants of c-Abl (TM-c-Abl and D565A-Abl), we then showed that c-Abl cleavage in MM cells requires caspase activity. Importantly, knockdown of c-Jun and c-Abl expression by siRNA confirms that adaphostin- induced c-Jun upregulation triggers downstream caspase- mediated c-Abl cleavage, inhibition of MM cell growth, and induction of apoptosis. Finally, our data suggest that this mechanism may not be restricted to MM, but may also be important in a broad range of malignancies including erythroleukemia and solid tumors.