Chronic lymphocytic leukemia requires BCL2 to sequester prodeath BIM, explaining sensitivity to BCL2 antagonist ABT-737

Abstract

Antiapoptotic B cell leukemia/lymphoma 2 (BCL2) family proteins are expressed in many cancers, but the circumstances under which these proteins are necessary for tumor maintenance are poorly understood. We exploited a novel functional assay that uses BCL2 homology domain 3 (BH3) peptides to predict dependence on antiapoptotic proteins, a strategy we call BH3 profiling. BH3 profiling accurately predicts sensitivity to BCL2 antagonist ABT-737 in primary chronic lymphocytic leukemia (CLL) cells. BH3 profiling also accurately distinguishes myeloid cell leukemia sequence 1 (MCL1) from BCL2 dependence in myeloma cell lines. We show that the special sensitivity of CLL cells to BCL2 antagonism arises from the requirement that BCL2 tonically sequester proapoptotic BIM in CLL. ABT-737 displaced BIM from BCL2's BH3-binding pocket, allowing BIM to activate BAX, induce mitochondrial permeabilization, and rapidly commit the CLL cell to death. Our experiments demonstrate that BCL2 expression alone does not dictate sensitivity to ABT-737. Instead, BCL2 complexed to BIM is the critical target for ABT-737 in CLL. An important implication is that in cancer, BCL2 may not effectively buffer chemotherapy death signals if it is already sequestering proapoptotic BH3-only proteins. Indeed, activator BH3-only occupation of BCL2 may prime cancer cells for death, offering a potential explanation for the marked chemosensitivity of certain cancers that express abundant BCL2, such as CLL and follicular lymphoma.

Evaluation and critical assessment of putative MCL-1 inhibitors

“…CLL cells depend on BCL-2 for survival and can be considered BCL-2-addicted. 27,28 Both ABT-263 and ABT-199 induced a rapid concentration-dependent apoptosis in primary CLL cells, which was evident both at early times (4 h) and at low nanomolar concentrations in agreement with previous studies. 10,29 The only other inhibitor to induce a similar rapid concentration-dependent induction of apoptosis in primary CLL cells was UCB-1350883, although it was much less potent than either ABT-263 or ABT-199 (Figure 4b).…”

“…27,28 However, a dose-limiting thrombocytopenia is observed owing to inhibition of BCL-X L , the primary survival factor for platelet viability. 33 This has resulted in the synthesis of ABT-199, a BCL-2-specific inhibitor, which appears to act in vivo to specifically inhibit BCL-2 without affecting BCL-X L , and thus platelets are spared.…”

pRb/E2F-1-mediated caspase-dependent induction of Noxa amplifies the apoptotic effects of the Bcl-2/Bcl-xL inhibitor ABT-737

“…5 It potently inhibits the BH3-binding activity of Bcl-2, Bcl-xL and Bcl-w but not that of Mcl-1 and Bfl-1. 9 ABT-737 promotes cell death by displacing, from its targets, 'BH3 activators' such as Bim or Puma (BH3-only proteins that can directly activate multi-domain proteins when free from anti-apoptotic proteins) 10,11 and/or active Bax. 12 Efficient induction of apoptosis by ABT-737 requires that proapoptotic proteins are not sequestered by an excess of empty Mcl-1 or Bfl-1 that are not efficiently inhibited by ABT-737.…”

ABT-737 Induces Apoptosis in Mantle Cell Lymphoma Cells with a Bcl-2high/Mcl-1low Profile and Synergizes with Other Antineoplastic Agents

“…ABT-737 displaces proapoptotic BH3-only from Bcl-2 or Bcl-xL, leading to activation of Bax and Bak and downstream caspases (9). Because of the low affinity of ABT-737 for Mcl-1, high basal levels of Mcl-1 have been associated with ABT-737 resistance (11)(12)(13)(14). Previous studies have also shown that ABT-737 is effective as a single agent against some leukemia/ lymphoma cell lines both in vitro and in vivo (11,(15)(16)(17).…”