CLL cells are resistant to smac mimetics because of an inability to form a ripoptosome complex

C Maas,J A Elias,A Malara,A Krippner-Heidenreich,M Hj Van Oers,J Van Laar,J M Tromp,E Eldering,R Thijssen,J Silke

doi:10.1038/cddis.2013.305

Abstract

In the lymph node (LN) environment, chronic lymphocytic leukemia (CLL) cells display increased NF-κB activity compared with peripheral blood CLL cells, which contributes to chemoresistance. Antagonists of cellular inhibitor of apoptosis proteins (cIAPs) can induce apoptosis in various cancer cells in a tumor necrosis factor-α (TNFα)-dependent manner and are in preclinical development. Smac-mimetics promote degradation of cIAP1 and cIAP2, which results in TNFR-mediated apoptosis via formation of a ripoptosome complex, comprising RIPK1, Fas-associated protein with death domain, FLICE-like inhibitory protein and caspase-8. CD40 stimulation of CLL cells in vitro is used as a model to mimic the LN microenvironment and results in NF-κB activation and TNFα production. In this study, we investigated the response of CLL cells to smac-mimetics in the context of CD40 stimulation. We found that treatment with smac-mimetics results in cIAP1 and cIAP2 degradation, yet although TNFα is produced, this did not induce apoptosis. Despite the presence of all components, the ripoptosome complex did not form upon smac-mimetic treatment in CLL cells. Thus, CLL cells seem to possess aberrant upstream NF-κB regulation that prevents ripoptosome formation upon IAP degradation. Unraveling the exact molecular mechanisms of disturbed ripoptosome formation may offer novel targets for treatment in CLL.

Highlights

Is not yet available, as relapses eventually occur from chemoresistant sites such as the lymph nodes (LNs)
In agreement with our earlier report, we observed stabilization of NF-kB-inducing kinase (NIK), phosphorylation of IkBa and the conversion of the p100 pro-form into the active nuclear factor kappa B (NF-kB) subunit p52 in patient Chronic lymphocytic leukemia (CLL) cells stimulated with CD40 ligand (CD40L) (Figure 1a)
The presence of cIAPs and receptor-interacting protein kinase 1 (RIPK1) in target cells is essential for the induction of cell death by smacmimetics, because smac-mimetics target inhibitor of apoptosis proteins (IAPs) while RIPK1 knockout cells are resistant to smac-mimeticinduced cell death.[14,15,26]

Summary

Introduction

Is not yet available, as relapses eventually occur from chemoresistant sites such as the lymph nodes (LNs). Smac-mimetics appear to prime cells, as intracellular RIPK1-containing complexes accumulate in the absence of an exogenous stimulus This intracellular complex, comprising Fas-associated protein with death domain (FADD), caspase-8 and cFLIP, has been called the ripoptosome and is most likely related to the secondary cytosolic complex that assembles post-TNF treatment.[22,23,24] cIAP1 and 2 act as negative regulators of the alternative or non-canonical NF-kB signaling pathway, by K48 ubiquitylating the upstream regulator NF-kB-inducing kinase (NIK).[15,20] In the absence of cIAPs, NIK levels rise, promoting its auto-activation and downstream activation of IkB kinase 1, processing of NF-kB2 p100 and nuclear translocation of NFkB2 p52. Targeting cIAP1/2 with smac-mimetics could be an attractive way to overcome CLL chemoresistance

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