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V.M. Smith et al.
proteins for A1331852-induced apoptosis in HCT-116 cells,34 highlighting important differences from DLBCL.
In terms of S63845-induced apoptosis, the MCL-1- dependent cell lines SUDHL10 and U2946 did not express especially high levels of MCL-1, but both cell lines expressed only small amounts of BCL-2 and BCL-XL. BIM and BAK were predominantly sequestered by MCL-1 in these cells. BAK has previously been identified as an essential mediator of S63845-induced cell death in breast cancer cells,35 but our data demonstrate that BAX may be more important for MCL-1 inhibition in DLBCL. Thereby, BAK and/or BH3-only proteins displaced from MCL-1 contributed to the activation of BAX and apopto- sis. Besides BIM, our data also indicate that NOXA is a potential mediator of S63845-induced apoptosis. NOXA is highly bound by MCL-1 and displaced by S63845, which may enable NOXA to act as a direct activator for BAX, as suggested previously.36,37
Taken together, our study demonstrates that the sensi- tivity to BH3-mimetics is underlined by sequestration of BIM, BAX and/or BAK by the anti-apoptotic BCL-2 pro- teins, a phenomenon that is disrupted by BH3-mimetics, leading to predominantly BAX-mediated apoptosis. Therefore, our data support a model in which the major
function of the anti-apoptotic BCL-2 proteins in DLBCL cells is to directly sequester or inhibit BAX. Dependent on the abundance of the different anti-apoptotic BCL-2 pro- teins, the pro-apoptotic proteins preferentially bind to either BCL-2, BCL-XL or MCL-1 which renders these cells highly sensitive to selective BH3-mimetics. However, our data also highlight that besides BCL-2, BCL-XL or MCL-1 additional anti-apoptotic BCL-2 proteins such as BCL2A138 and BCL-w39 may play important roles in DLBCL, as some cell lines, including Pfeiffer and OCI- LY3, display high priming but are nevertheless not responsive to inhibition of BCL-2, BCL-XL or MCL-1. A more detailed understanding of the molecular mecha- nisms of resistance in these cell is required to enable the best use of potent BCL-2 family inhibitors in clinical prac- tice.
Acknowledgments
The authors would like to thank C. Hugenberg for expert sec- retarial assistance and Sandeep Dave for providing us with OCI-LY10 cells. This work was partially supported by the Else Kröner-Fresenius-Stiftung (to MV), the Experimental Cancer Medicine Center Leicester and funding from the Scott Waudby Trust (to SJ and MJSD).
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