Apoptosis is deregulated in most, if not all, cancers, including hematological malignancies. Smac mimetics that antagonize Inhibitor of Apoptosis (IAP) proteins have so far largely been investigated in acute myeloid leukemia (AML) cell lines; however, little is yet known on the therapeutic potential of Smac mimetics in primary AML samples. In this study, we therefore investigated the antileukemic activity of the Smac mimetic BV6 in diagnostic samples of 67 adult AML patients and correlated the response to clinical, cytogenetic and molecular markers and gene expression profiles. Treatment with cytarabine (ara-C) was used as a standard chemotherapeutic agent. Interestingly, about half (51%) of primary AML samples are sensitive to BV6 and 21% intermediate responsive, while 28% are resistant. Notably, 69% of ara-C-resistant samples show a good to fair response to BV6. Furthermore, combination treatment with ara-C and BV6 exerts additive effects in most samples. Whole-genome gene expression profiling identifies cell death, TNFR1 and NF-κB signaling among the top pathways that are activated by BV6 in BV6-sensitive, but not in BV6-resistant cases. Furthermore, sensitivity of primary AML blasts to BV6 correlates with significantly elevated expression levels of TNF and lower levels of XIAP in diagnostic samples, as well as with NPM1 mutation. In a large set of primary AML samples, these data provide novel insights into factors regulating Smac mimetic response in AML and have important implications for the development of Smac mimetic-based therapies and related diagnostics in AML.

Inflammatory responses mediated by NOD2 rely on RIP2 kinase and ubiquitin ligase XIAP for the activation of nuclear factor κB (NF-κB), mitogen-activated protein kinases (MAPKs), and cytokine production. Herein, we demonstrate that selective XIAP antagonism blocks NOD2-mediated inflammatory signaling and cytokine production by interfering with XIAP-RIP2 binding, which removes XIAP from its ubiquitination substrate RIP2. We also establish that the kinase activity of RIP2 is dispensable for NOD2 signaling. Rather, the conformation of the RIP2 kinase domain functions to regulate binding to the XIAP-BIR2 domain. Effective RIP2 kinase inhibitors block NOD2 signaling by disrupting RIP2-XIAP interaction. Finally, we identify NOD2 signaling and XIAP-dependent ubiquitination sites on RIP2 and show that mutating these lysine residues adversely affects NOD2 pathway signaling. Overall, these results reveal a critical role for the XIAP-RIP2 interaction in NOD2 inflammatory signaling and provide a molecular basis for the design of innovative therapeutic strategies based on XIAP antagonists and RIP2 kinase inhibitors.

Inhibitor of apoptosis (IAP) proteins are antiapoptotic regulators that block cell death in response to diverse stimuli. They are expressed at elevated levels in human malignancies and are attractive targets for the development of novel cancer therapeutics. Herein, we demonstrate that small-molecule IAP antagonists bind to select baculovirus IAP repeat (BIR) domains resulting in dramatic induction of auto-ubiquitination activity and rapid proteasomal degradation of c-IAPs. The IAP antagonists also induce cell death that is dependent on TNF signaling and de novo protein biosynthesis. Additionally, the c-IAP proteins were found to function as regulators of NF-kappaB signaling. Through their ubiquitin E3 ligase activities c-IAP1 and c-IAP2 promote proteasomal degradation of NIK, the central ser/thr kinase in the noncanonical NF-kappaB pathway.