Smac mimetic and glucocorticoids synergize to induce apoptosis in childhood ALL by promoting ripoptosome assembly.

ABSTRACT

Apoptosis resistance contributes to poor outcome in pediatric acute lymphoblastic leukemia (ALL). Here, we identify a novel synergistic combination of Smac mimetic BV6 and glucocorticoids (GCs) (ie, dexamethasone, prednisolone) to trigger apoptosis in ALL cells. BV6 and GCs similarly cooperate to induce apoptosis in patient-derived leukemia samples, underlining the clinical relevance. Importantly, BV6/dexamethasone cotreatment is significantly more effective than monotherapy to delay leukemia growth in a patient-derived xenograft model of pediatric ALL without causing additional side effects. In contrast, BV6 does not increase cytotoxicity of dexamethasone against nonmalignant peripheral blood lymphocytes, mesenchymal stromal cells, and CD34-positive hematopoietic cells. We identify a novel mechanism by showing that BV6 and dexamethasone cooperate to deplete cIAP1, cIAP2, and XIAP, thereby promoting assembly of the ripoptosome, a RIP1/FADD/caspase-8-containing complex. This complex is critical and is required for BV6/dexamethasone-induced cell death, because RIP1 knockdown reduces caspase activation, reactive oxygen species production, and cell death. Ripoptosome formation occurs independently of autocrine/paracrine loops of death receptor ligands, because blocking antibodies for TNFα, tumor necrosis factor-related apoptosis-inducing ligand, or CD95 ligand or knockdown of death receptors fail to rescue BV6/dexamethasone-induced cell death. This is the first report showing that BV6 sensitizes for GC-triggered cell death by promoting ripoptosome formation with important implications for apoptosis-targeted therapies of ALL.