The anti-apoptotic proteins of the BCL-2 family (BCL-2, BCL-X and MCL-1) have been variously implicated in the resistance of tumors to anti-cancer therapies. Numerous small molecules have been developed to mimic the binding of BH3-only proteins to the BH3 pocket of anti-apoptotic proteins. These “BH3 mimetics” thereby disrupt the ability of anti-apoptotic proteins to bind and inhibit pro-apoptotic proteins. ABT-737 is a BH3 mimetic that inhibits BCL-2 and BCL-X but not MCL-1 and kills cells in a BAX/BAK-dependent manner. The ability of other BH3 mimetics to inhibit anti-apoptotic proteins has been questioned because they kill cells in a BAX/BAK-independent manner. It has thus been assumed that the actions of these BH3 mimetics must be mediated by off-target effects. In the NB4 leukemia cell line, we show for the first time that most BH3 mimetics preferentially inhibit MCL-1. Six putative BH3 mimetics investigated (GX015-070, gossypol, apogossypol, methoxy-antimycin A, HA14-1 and S1) led to an increase in both NOXA and MCL-1. NOXA binds to MCL-1 and targets the NOXA/MCL-1 complex for degradation. Hence, their accumulation is consistent with these BH3 mimetics disrupting the formation of the complex and preventing its degradation. The rapid increase in NOXA observed within 6 h is associated with the induction of apoptosis within 24 h. Considering a recent report that NOXA can induce cell death through a calcium-dependent pathway, our findings suggest that the BAX/BAK-independent cell death induced by BH3 mimetics may in fact be the result of their on-target mechanism of action. ABT-737 did not increase the levels of NOXA or MCL1, which is consistent with its inability to inhibit the binding of MCL-1 to NOXA. Furthermore, ABT-737 was the only BH3 mimetic that disrupted the binding of BCL-2 to BAD, suggesting that the other BH3 mimetics did not inhibit BCL-2. Lastly, gossypol, S1, and HA14-1 at a concentration that increased NOXA but not apoptosis, sensitized NB4 cells to ABT-737, inducing apoptosis within 6 h. We thus conclude that most BH3 mimetics inhibit MCL-1 but not BCL-2 or BCL-X, while combinations can be used to inhibit all these anti-apoptotic proteins and rapidly induce apoptosis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1040.

©2010 American Association for Cancer Research
2010