Combinatorial Targeting of BCL6 and Anti-Apoptotic Proteins in Diffuse Large B-Cell Lymphoma (DLBCL) and Follicular Lymphoma (FL)

Abstract 64

BCL6 represents a survival factor in DLBCL and FL since specific BCL6 inhibitors (i.e: the peptidomimetic RI-BPI and the small molecule 79-6) kill DLBCL and transformed FL (tFL) cell lines. Our group showed that BCL2 and other anti-apoptotic genes are transcriptionally repressed by BCL6 and could be reactivated upon treatment with RI-BPI or 79-6. We also showed that BCL6 and BCL2 control distinct and non-overlapping survival pathways in these lymphomas. This suggests that blocking the function of anti-apoptotic proteins might overcome any resistance that these proteins might mediate in response to BCL6 inhibition. In addition, constitutive expression of BCL2 has been observed in DLBCL and FL cases.

We therefore hypothesized that targeting both BCL6 and BCL2 would eliminate two of the most potent survival mechanisms and would translate in synergistic killing of these lymphomas. In order to test this hypothesis, we examined the proteome-wide consequence of BCL6 inhibition in a DLBCL cell line (SU-DHL6) transfected with siBCL6 by phospho-protein arrays. We found that 280 unique proteins changed their abundance after siBCL6, 40 of them related to (pro and anti) apoptosis signaling (p<0.001). By means of pathway analysis bioinformatic tools, we then identified anti-apoptotic proteins with increased abundance after siBCL6 that could be therapeutically targeted. Among these druggable proteins we found BCL2, BCL-XL, MCL-1, NEDD8, PARP1 and several ubiquitin ligases. We confirmed in independent experiments that BCL6 knockdown induced mRNA (by qPCR) and protein up-regulation (by immunobloting) of these genes in 2 additional DLBCL and 2 tFL cell lines. Treatment of these siBCL6-transfected cell lines with small molecules inhibitors of BCL2 family members (ABT-737 and oblatoclax), NEDD8 activating enzyme (MLN4924), PARP (olaparib) and proteasome (bortezomib) showed increased killing compared to each treatment alone. In order to identify rational combinatorial therapies that could be potentially translated for use in clinical trials, we performed additional studies with the BCL6 inhibitor RI-BPI that is being developed for clinical use. We first analyzed the impact of RI-BPI on the apoptosome in a panel of 6 DLBCL (SU-DHL6, Ly1, Ly7, Ly3, Ly10, SU-DHL4) and 4 tFL (DoHH2, WSU-DLCL2, Granta452, SC-1) cell lines. RI-BPI induced a profile of up-regulated pro and anti-apoptotic proteins similar to siBCL6. Because ABT-737 and obatoclax are active in DLBCL cells where apoptotic BH3 activators are neutralized by BCL2 or BCL-XL and RI-BPI treatment changes the stoichiometry of pro and anti-apoptotic proteins, we determined the post-RI-BPI BH3 profiling accordingly to the amount of BIM sequestration (by co-immunoprecipitation). Accordingly, sequential treatment of DLBCL and tFL cell lines with RI-BPI and ABT-737 or obatoclax synergistically killed BCL2/BCL-XL dependent cells (but not MCL-1 dependent cells). This effect was independent of the mutational status of BCL2, MCL1, BCL6, MYC and TP53. Olaparib was not tested in combination since most cell lines were resistant to clinically achievable concentrations of this drug. Bortezomib and MLN4924 were synergistic in most cell lines when combined with RI-BPI (as determined by isobologram analysis). The synergistic killing was associated with increase in caspase 7/3 activation (by a plate-based assay) and NFkB inhibition (by p65 DNA binding assay). This effect was independent of the cell of origin classification of the cell line (i.e. ABC vs. GCB). We then tested the combination of RI-BPI with ABT-737, MLN4924 or bortezomib in Ly1 xenograft models (n=10 mice per combination). Ly1 represents a DLBCL with 3q27 and t(14,18). We found that after 10 days of treatment, each combinatorial treatment was more effective than their individual components (p=0.02, p=0.01 and p<0.01 for RI-BPI with ABT-737, MLN4924 and bortezomib, respectively; T-test, day 10). Detailed toxicity studies revealed no toxicity excess with these combinations.

In sum, our work shows that pharmacologic targeting of anti-apoptotic pathways induced by inhibition of BCL6 activity successfully sensitized DLBCL and tFL cells to apoptosis. This effect was evident in cells in which the apoptosis resistant mechanism evolved as response to BCL6 inhibition and gene de-repression as well as those with constitutive overexpression of anti-apoptotic genes.