Cell Cycle, Cell Death and Senescence Phosphoinositide 3-kinase/AKT/mTORC1/2 Signaling Determines Sensitivity of Burkitt's Lymphoma Cells to BH3 mimetics

Burkitt's lymphoma (BL), driven by translocation and overexpression of the c-MYC gene, is an aggressive, highly proliferative lymphoma, and novel therapeutic strategies are required to overcome drug resistance following conventional treatments. The importance of the prosurvival BCL-2 family member BCL-XL in BL cell survival suggests that antagonistic BH3-mimetic compoundsmay have therapeutic potential. Here, we show that treatment of BL cell lines with ABT-737 induces caspase-3/7 activation and apoptosis with varying potency. Using selective inhibitors, we identify phosphoinositide 3-kinase (PI3K) as a proproliferative/survival pathway in BL cells and investigate the potential of combined pharmacologic inhibition of both the BCL-2 family and PI3K signaling pathway. PI3K/AKT inhibition and ABT-737 treatment induced synergistic caspase activation, augmented BL cell apoptosis, and rendered chemoresistant cells sensitive. TargetingmTORC1/2 with PP242 was also effective, either as a monotherapy or, more generally, in combination with ABT-737. The combined use of a dual specificity PI3K/mTOR inhibitor (PI 103) with ABT-737 proved highly efficacious. PI 103 treatment of BL cells was associated with an increase in BIM/MCL-1 expression ratios and loss of c-MYC expression. Furthermore, blocking c-MYC function using the inhibitor 10058-F4 also induced apoptosis synergistically with ABT-737, suggesting thatmaintenance of expression of BCL-2 familymembers and/or c-MYCby the PI3K/AKT/mTORpathway could contribute to BL cell survival and resistance to ABT-737. The combined use of BH3 mimetics and selective mTORC1/2 inhibitorsmay therefore be a useful novel therapeutic approach for the treatment of B-cellmalignancy, including chemoresistant lymphomas. Mol Cancer Res; 10(3); 347–59. 2012 AACR. Introduction Burkitt's lymphoma (BL) is an aggressive malignancy derived from germinal center (GC) B cells. Conventional intensive chemotherapeutic regimes are effective in eliminating disease in the majority of cases (1) however, novel treatment strategies are still required to treat patients who relapse with chemoresistant tumors. BLs are characterized by chromosomal translocation events which deregulate the c-MYC proto-oncogene (2), resulting in rapid proliferation of the malignant cells. Although BL cells proliferate rapidly, they are also sensitive to apoptotic stimuli and still maintain many of the proapoptotic and prosurvival signaling pathways that normally regulate their cell of origin. For example, BL cells are acutely sensitive to TGF-b–induced intrinsic apoptosis (3) which occurs via transcriptional control of members of the BCL-2 family of apoptosis regulators (4). BCL-2 family members are frequently deregulated in malignancies of GC origin (reviewed in ref. 5) and can be considered as valid targets for therapeutic intervention in B-cell lymphomas. The prosurvival members of the BCL-2 family include BCL-2 itself, BCL-XL, BCL-w, MCL-1, and BOO. These proteins are regulated by other BH3-only family members that share only one region of homology with BCL-2 (the BH3 domain). These proapoptotic proteins (BIK, BIM, PUMA, NOXA, BAD, HRK, BID, and BMF) function as naturally occurring inhibitors of the prosurvival family members. Several BCL-2 antagonists, designed to mimic the BH3 domain of the BH3-only proteins (reviewed in ref. 6), have been developed as potential therapeutics and have undergone testing against B-cell malignancies, either as monotherapies, or in combination with other drugs (7–12). BCL-2 antagonists have shown activity in mouse models of Myc-driven lymphomas (12–14) which suggests that BH3 mimetics may be useful in treating human disease.However, it is currently unknown whether such strategies would be effective against BL. The main targets of the BH3-mimetic ABT-737 (15) are BCL-2, BCL-XL, and potentially BCL-w Authors' Affiliation: Division of Cancer Research, Medical Research Institute, University of Dundee, Ninewells Hospital and Medical School, Dundee, United Kingdom Note: Supplementary data for this article are available at Molecular Cancer Research Online (http://mcr.aacrjournals.org/). Corresponding Author: Gareth J Inman, Division of Cancer Research, Medical Research Institute, University of Dundee, Level 5, Ninewells Hospital and Medical School, Dundee, DD1 9SY, United Kingdom. Phone: 44-1382-496696; Fax: 44-1382-669993; E-mail: [email protected] doi: 10.1158/1541-7786.MCR-11-0394 2012 American Association for Cancer Research. Molecular Cancer Research www.aacrjournals.org 347 on August 30, 2017. © 2012 American Association for Cancer Research. mcr.aacrjournals.org Downloaded from Published OnlineFirst January 12, 2012; DOI: 10.1158/1541-7786.MCR-11-0394