Bortezomib-resistant Cell Lines Research Articles

Pharmacological Perturbation of the Immunoproteasome in Hematologic Neoplasias: Therapeutic Implications

Pharmacological inhibition of the canonical proteasome by inhibitors targeting its beta5 (PSMB5) subunit represent a cornerstone of the therapeutic management of plasma cell neoplasias. However, proteasome inhibitors have limited clinical applications in other hematologic malignancies. Notably, a broad spectrum of cell lines and patient-derived samples from lymphoma, leukemia and myeloma tend to express high levels of transcripts for and exhibit substantial activity of the immunoproteasome, an alternative proteasome particle which is enriched in hematopoietic/immune tissues. Established proteasome inhibitors used in clinical practice (e.g. bortezomib, carfilzomib, ixazomib) target both the canonical 20S proteasome and the immunoproteasome. We reasoned though that targeting of the immunoproteasome with more selective small molecule inhibitors that largely spare the canonical proteasome may provide insights into the specific functional role of the immunoproteasome in hematologic neoplasias, and importantly may also uncover underappreciated therapeutic opportunities related to targeting of hematologic neoplasias beyond myeloma, that might be more dependent on the immunoproteasome vs. the canonical proteasome; as well as potential benefits related to the therapeutic index of these agents. To address these questions, we characterized the activity of the previously disclosed selective immunoproteasome inhibitor M3258, which targets the LMP7 (beta5i subunit). The bortezomib-resistant cell line MM1VDR, which has a "gatekeeper" PSMB5 mutation preventing bortezomib binding to the canonical proteasome, remained sensitive to in vitro treatment with M3258, in further support of the functional specificity of this inhibitor. To obtain a comprehensive assessment of the activity of M3258 across hematologic malignancies, we studied a pool of 74 "DNA-barcoded" cell lines (PRISM system) from hematologic malignancies, including multiple myeloma (MM), leukemias and lymphomas, which were treated for 48hr with M3258. MM cell lines (9/16) were strongly represented among the sensitive lines which displayed <50% relative cell viability (compared to DMSO control) after M3258 treatment. Notably, 15 of 58 non-MM lines also exhibited >50% reduction in their cell viability in this analysis; and their responsiveness did not differ significantly from the M3258-responsive MM lines. Importantly, we observed no correlation between the responses of leukemia, lymphoma or MM lines to in vitro treatment with M3258 vs. clinically-relevant pulse of bortezomib. Furthermore, 7 of the 15 lymphoma/leukemia lines that were M3258-responsive had limited, if any, response to a clinically-relevant pulse of bortezomib. Taken together, these observations suggest that immunoproteasome inhibition with M3258 can be active against subsets of hematologic neoplasias with resistance/limited responsiveness to canonical proteasome inhibitors. To further examine the in vivo relevance of these observations, we studied the in vivo activity of M3258 against the MM.1S myeloma cell line, which has only moderate in vitro response to M3258 compared to other blood cancer cell lines tested in our "DNA-barcoded" pooled analysis. M3258 significantly increased the overall survival of mice with diffuse MM-bone lesions established after tail vein injection of MM.1S cells; and of mice harboring MM.1S lesions within a subcutaneously implanted ceramic scaffold-based system engineered to establish a bone marrow-like histological and functional unit with a "humanized" stromal compartment. The anti-tumor activity of M3258 in this scaffold model exhibited a trend for increased efficacy compared to twice weekly subcutaneous treatments with bortezomib. These results highlight that in vivo administration of M3258 exhibits anti-tumor activity against clinically-relevant models of MM lesions, even from cell lines that have modest in vitro responsiveness to this immunoproteasome inhibitor. Furthermore, our studies with a large panel of "DNA-barcoded" cell lines indicates that the anti-tumor effects of M3258 may extend beyond MM and into other classes of hematologic malignancies, including subsets of leukemias and lymphomas, identifying previously underappreciated therapeutic implications for the class of selective immunoproteasome inhibitors. Disclosures Walter: Merck Healthcare KGaA: Employment. Friese-Hamim:Merck Healthcare KGaA: Employment. Goodstal:EMD Serono Research and Development Institute: Employment. Sanderson:Merck Healthcare KGaA: Employment. Mitsiades:Takeda: Other: employment of a relative ; Ionis Pharmaceuticals: Honoraria; Fate Therapeutics: Honoraria; Arch Oncology: Research Funding; Sanofi: Research Funding; Karyopharm: Research Funding; Abbvie: Research Funding; TEVA: Research Funding; EMD Serono: Research Funding; Janssen/Johnson & Johnson: Research Funding.

Syrbactin proteasome inhibitor TIR-199 overcomes bortezomib chemoresistance and inhibits multiple myeloma tumor growth in vivo

Multiple myeloma (MM) and mantle cell lymphoma (MCL) are blood cancers that respond to proteasome inhibitors. Three FDA-approved drugs that block the proteasome are currently on the market, bortezomib, carfilzomib, and ixazomib. While these proteasome inhibitors have demonstrated clinical efficacy against refractory and relapsed MM and MCL, they are also associated with considerable adverse effects including peripheral neuropathy and cardiotoxicity, and tumor cells often acquire drug resistance. TIR-199 belongs to the syrbactin class, which constitutes a novel family of irreversible proteasome inhibitors. In this study, we compare TIR-199 head-to-head with three FDA-approved proteasome inhibitors. We demonstrate that TIR-199 selectively inhibits to varying degrees the sub-catalytic proteasomal activities (C-L/β1, T-L/β2, and CT-L/β5) in three actively dividing MM cell lines, with Ki50 (CT-L/β5) values of 14.61 ± 2.68 nM (ARD), 54.59 ± 10.4 nM (U266), and 26.8 ± 5.2 nM (MM.1R). In most instances, this range was comparable with the activity of ixazomib. However, TIR-199 was more effective than bortezomib, carfilzomib, and ixazomib in killing bortezomib-resistant MM and MCL cell lines, as judged by a low resistance index (RI) between 1.7 and 2.2, which implies that TIR-199 indiscriminately inhibits both bortezomib-sensitive and bortezomib-resistant MM and MCL cells at similar concentrations. Importantly, TIR-199 reduced the tumor burden in a MM mouse model (p < 0.01) confirming its potency in vivo. Given the fact that there is still no cure for MM, the further development of TIR-199 or similar molecules that belong to the syrbactin class of proteasome inhibitors is warranted.

Proteasome inhibitor induced SIRT1 deacetylates GLI2 to enhance hedgehog signaling activity and drug resistance in multiple myeloma.

Multiple myeloma (MM) is still incurable despite the successful application of proteasome inhibitors in clinic. Bortezomib represents the most common chemotherapy for MM, whereas acquired drug resistance and eventually developed relapse remain the major obstruction. In the current study, we established bortezomib-resistant myeloma cell lines and screened gene expression profiles using single cell RNA-sequencing. Resistant MM cells exhibited increased clonogenic potential, specific metabolic, and epigenetic signatures, along with the self-renewal signaling characteristic of MM stem-like cells. Aberrant activation of hedgehog (Hh) signaling was correlated with drug resistance and stem cell-like transcriptional program. The key transcriptional factor GLI2 of the Hh pathway was restricted in the high acetylation and low ubiquitination states in bortezomib-resistant myeloma cells. Further investigation revealed that SIRT1 deacetylates and stabilizes GLI2 protein at lysine 757 and consequentially activates the Hh signaling, and itself serves as a direct target of Hh signaling to format a positive regulating loop. Using combination screening with an epigenetic compound library, we identified the SIRT1 specific inhibitor S1541 and S2804 had very obvious synergetic antimyeloma effect. Sirt1 inhibition could partially impeded the Hh pathway and conferred bortezomib sensitivity in vitro and in vivo. Notably, elevated SIRT1 level was also a prominent hallmark for the resistant myeloma cells, and this expression pattern was confirmed in myeloma patients, but independent of RAS/RAF mutations. Clinically, SIRT1 expression in patients with complete response was suppressed but elevated in relapsed patients, and retrospective analysis showed patients with higher SIRT1 expression had poorer outcomes. In conclusion, the cooperation of SIRT1 and Hh is an important mechanism of drug resistance in myeloma, and therapeutics combining SIRT1 inhibitors will sensitize myeloma cells to proteasome inhibitors.

INHIBITION OF SPHINGOSINE KINASE 2 RESENSITISES BORTEZOMIB-RESISTANT MULTIPLE MYELOMA

Multiple myeloma, a haematological malignancy of the plasma cell, is still largely incurable. The introduction of proteasome inhibitors such as bortezomib has significantly improved patient survival, however bortezomib resistance is still a major hurdle which requires new therapeutic options. One such option may be inhibitors to sphingolipid metabolism. Sphingolipids are a family of lipids which can act as signalling molecules, influencing cellular pathways such as apoptosis. The enzyme sphingosine kinase is an especially attractive target in cancer, as it converts pro-apoptotic ceramide and sphingosine to pro-survival sphingosine-1-phosphate. We have previously shown that inhibition of sphingosine kinase 2 with K145 causes synergistic cell death when combined with bortezomib in bortezomib-naive myeloma cells, and thus we decided to test this combination in the bortezomib-resistant setting. To do so, bortezomib-resistant myeloma cell lines were generated and analysed. A 5TGM1 cell line generated to be resistant to bortezomib was also found to be resistant to carfilzomib, demonstrating cross-resistance that has also been seen in the clinic. Notably, these cells were found to possess a clinically-relevant mutation in the bortezomib-binding site of proteasomal subunit PSMB5. The SK2 inhibitor K145, when combined with either bortezomib or carfilzomib, caused synergistic cell death in the bortezomib-resistant cell lines tested. This was preceded by a significant increase in ER stress markers, as measured by Western blot, suggesting that this cell death may be induced by enhanced ER stress. When tested in vivo in a highly aggressive, bortezomib-resistant myeloma model, the combination of bortezomib and K145 gave a significant decrease in disease burden and increase in survival, suggesting this combination may be effective in bortezomib-resistant patients.

MiR-520g and miR-520h overcome bortezomib resistance in multiple myeloma via suppressing APE1

ABSTRACT Background: Nowadays, microRNAs (miRNAs) attract much attention in regulating anticancer drug resistance in cancers including multiple myeloma (MM). Bortezomib is the first-line choice in MM treatment, and bortezomib resistance caused by aberrant DNA repair leads to the recurrence and therapeutic failure of MM. Objective: Our study aims to identify a miRNA that overcomes bortezomib resistance in MM. Methods: We established bortezomib-resistant MM cell lines, and screened several miRNAs that have aberrant expressions in MM cell lines. The expression of DNA-repair-related proteins were assessed by western blot, and cell viability was determined by the MTT assay in bortezomib-resistant cell lines. The binding between miRNAs and 3ʹ-UTR of APE1 mRNA was confirmed by luciferase reporter assay. The mouse bortezomib-resistant xenograft was established to verify the therapeutic effect of miRNA overexpression. Results: miR-520g and miR-520h were significantly downregulated in bortezomib-resistant MM cell lines, and overexpression of miR-520g and miR-520h together inhibited expression of homologous recombination-related protein Rad51 and cell viability of bortezomib-resistant MM cells in vitro by binding with 3ʹ-UTR of APE1 mRNA. Combined overexpression of miR-520g and miR-520h inhibited bortezomib-resistant MM tumor growth in vivo. Conclusion: Our findings demonstrated that combined overexpression of miR-520g and miR-520h overcomes bortezomib resistance in MM through inhibition of DNA repair, offering a promising therapeutic target for MM treatment.

PS1209 NKG2D AND BCMA‐CAR NK CELLS EFFICIENTLY ELIMINATE MULTIPLE MYELOMA CELLS. A COMPREHENSIVE COMPARISON BETWEEN TWO CLINICALLY RELEVANT CARS

Background:Chimeric antigen receptors (CARs) have been used in the past several years in cancer therapy to redirect immune effector cells. Despite impressive preliminary efficacy of CAR‐T cells in multiple myeloma (MM), NK cell engineering has emerged as a competitive and safer approach. NK‐92 is a universal, cheap and fast “off‐the‐shelf” cellular therapeutic previously used in clinical trials. Although modest responses with these cells have been reported in MM, their oncolytic potential can be enhanced by genetic modification. So far, two preclinical studies have been performed with CAR NK‐92 against MM, targeting CD138 or CS1 (SLAMF7). However, there are still reasonable doubts about its clinical outcomes due to on‐target off‐tumor effect or fratricide, respectively.Aims:Thus, the aim of our study is to generate and compare two novel CAR NK‐92 products for MM treatment.Methods:NK‐92MI cells were lentivirally transduced with the full‐length ectodomain sequence of the human native NKG2D receptor or with an anti‐BCMA scFv, both containing identical 4–1BB costimulatory and CD3‐ζ signaling domains (Figure 1A). To compare the efficacy between these 2nd generation NKG2D‐CAR NK‐92MI and BCMA‐CAR NK‐92MI cells, the same MOI 10 was used to transduce them; both populations were then purified by FACS sorting to obtain stable modified cell lines and vector copy number was measured by qPCR to ensure similar CAR expression. Cytotoxicity assays were performed by 3‐hours Calcein‐AM analysis. We used MM cell lines with different expression of target ligands: U266, BCMAhigh and NKG2DLhigh; XG‐1, a NK resistant cell line, BCMAhigh and NKG2DLlow; NCI H929 R20, a bortezomib resistant cell line with NKG2DLlow. K562, a leukemia cell line with BCMAnegative and NKG2DLhigh expression, was used as negative control.Results:NKG2D‐CAR NK‐92MI cells consistently showed much higher in vitro antitumor activity than the parental line NK‐92MI against U266 (84 ± 2% vs 40.7 ± 4% at a 1:1 E:T ratio), XG‐1 (67.9 ± 9% vs 18.5 ± 4% at a 16:1 E:T ratio) and NCI H929 #R20 (50.9 ± 6% vs 23.7 ± 2% at a 16:1 E:T ratio) cell lines (Figure 1B). NKG2D‐CAR NK‐92MI proved specificity in a blocking assay with an anti‐NKG2D antibody (82% decrease compares to baseline lysis). Furthermore, the oncolytic potential of NKG2D‐CAR NK‐92MI was not altered by physiological levels of soluble MICA, described in MM patients, or mandatory 10Gy irradiation prior to clinical use. Next, we compared cytotoxicity between NKG2D‐CAR NK‐92MI and BCMA‐CAR NK‐92MI against U‐266 (84 ± 2% vs 91.9 ± 3% at a 1:1 E:T ratio), XG‐1 (67.9 ± 9% vs 89.9 ± 2% at a 16:1 E:T ratio) and K562 (94 ± 3% vs 25.74 ± 4% at a 1:1 E:T ratio) cell lines (Figure 1C). Strikingly, there were no significant differences between NKG2D‐CAR NK and the gold standard BCMA‐CAR against MM cell lines with high and similar BCMA and NKG2DL expression. In addition, correlation between target ligands expression on the tumor and efficacy and specificity of both CARs was also shown. None of the CAR NK‐92MI studied populations showed toxicity against PBMCs from healthy donors and in vivo MM orthotopic xenograft mouse model experiments are ongoing.Summary/Conclusion:We have generated two novel and stable CAR NK‐92 immunoproducts that improve the oncolytic efficacy of the parental cell line. Indeed, NKG2D‐CAR NK‐92MI cells are as equally efficient as BCMA‐CAR NK‐92MI cells to eradicate diverse MM cells. To summarize, all these data show the feasibility to use this NK ‘off‐the‐shelf’ approach as immunotherapy for MM.image

Novel Combination Therapy Targeting rDNA Transcription and Histone Deacetylation Provides Effective Treatment for Multiple Myeloma, and Synergises in Bortezomib-Resistant MM

Background: Multiple myeloma (MM) requires combination drug therapies to delay acquired drug resistance and clinical relapse. We co-developed CX-5461, a highly-selective inhibitor of RNA polymerase I-mediated rDNA transcription(1), currently in phase I trials for relapsed haematological malignancies (Peter Mac). CX-5461 produces a targeted nucleolar DNA damage response (DDR), triggering both a p53-dependent and -independent nucleolar stress response and killing malignant cells while sparing normal cells(2,3). Single-agent CX-5461 provides an impressive survival benefit in mouse models of B-cell lymphoma, acute myeloid leukaemia and now MM(2,4,5). However, drug resistance eventually occurs, confirming the need for combination therapies.Aim: To test the efficacy of CX-5461 in combination with the histone deacetylase inhibitor panobinostat, (prioritised from a boutique high-throughput screen of anti-myeloma agents), with a focus on the setting of resistance to proteasome-inhibitors (PIs).Methods: We assessed the impact of CX-5461 and panobinostat on overall survival in mouse models of MM, then surveyed the effects on cellular response and molecular markers of DDR. We developed bortezomib-resistant cell lines and an in vivo model of bortezomib-resistance to test this combination in the setting of PI-resistance.Results: CX-5461 in combination with panobinostat provides a significant survival advantage in both the transplanted Vk*MYC and the 5T33/KaLwRij models, with minimal bone marrow toxicity.The combination showed increased anti-proliferative effects and cell death in vitro. Interestingly, experiments interrogating the downstream cellular response of this combination suggest that the mechanism(s) driving synergy are complex and cell context-dependent. Cell cycle analysis indicates that both CX-5461- and panobinostat-driven cell cycle effects, i.e. G2/M and G1/S arrest, respectively, are dominant in the combination setting in a cell line-dependent manner, suggesting that context-dependent factors such as p53 may influence the cellular response.Mechanistically, in both p53-wild type and -null cell lines we observe an increase in DDR signalling with single agent CX-5461, with only moderate further increase with the combination. Moreover, CX-5461-mediated MYC downregulation is not universally observed, with the combination promoting further downregulation only in some cell lines. Given the potential for affecting global transcription programs downstream of panobinostat, we are performing transcriptome analyses in the combination setting compared to single agent treatment.We have generated bortezomib-resistant cell lines, sequentially increasing drug exposure to establish populations growing at concentrations above the IC90 of the parental lines. The resistant 5T33 cells retain their resistance to bortezomib in vivo and we have demonstrated that CX-5461 remains effective in this model, significantly increasing survival. We are currently examining the combination of CX-5461 with panobinostat in this model of bortezomib-resistance, which will give critical information guiding patient selection for future clinical trials.Conclusion: The rDNA transcription inhibitor CX-5461 synergises in vitro and in vivo with panobinostat, and CX-5461 retains efficacy in the setting of bortezomib-resistant myeloma.ReferencesDrygin et al., Cancer Research 2011Bywater et al., Cancer Cell 2012Quin et al, Oncotarget, 2016Devlin et al., Cancer Discovery 2016Hein et al., Blood 2017 DisclosuresHarrison:Janssen-Cilag: Other: Scientific advisory board.

HRI-mediated translational repression reduces proteotoxicity and sensitivity to bortezomib in human pancreatic cancer cells.

Human cancer cells display extensive heterogeneity in their sensitivities to the proteasome inhibitor bortezomib (Velcade). The molecular mechanisms underlying this heterogeneity remain unclear, and strategies to overcome resistance are limited. Here, we discover that inherent differences in eIF2α phosphorylation among a panel of 10 human pancreatic cancer cell lines significantly impacts bortezomib sensitivity, and implicate the HRI (heme-regulated inhibitor) eIF2α kinase as a novel therapeutic target. Within our panel, we identified a subset of cell lines with defective induction of eIF2α phosphorylation, conferring a high degree of sensitivity to bortezomib. These bortezomib-sensitive cells exhibited impaired translation attenuation followed by toxic accumulation of protein aggregates and reactive oxygen species (ROS), whereas the bortezomib-resistant cell lines displayed increased phosphorylation of eIF2α, decreased translation, few protein aggregates, and minimal ROS production. Importantly, we identified HRI as the primary bortezomib-activated eIF2α kinase, and demonstrated that HRI knockdown promoted cell death in the bortezomib-resistant cells. Overall, our data implicate inducible HRI-mediated phosphorylation of eIF2α as a central cytoprotective mechanism following exposure to bortezomib and provide proof-of-concept for the development of HRI inhibitors to overcome proteasome inhibitor resistance.

Inhibition of thioredoxin activates mitophagy and overcomes adaptive bortezomib resistance in multiple myeloma

BackgroundAlthough current chemotherapy using bortezomib (Velcade) against multiple myeloma in adults has achieved significant responses and even remission, a majority of patients will develop acquired resistance to bortezomib. Increased thioredoxin level has been reported to be associated with carcinogenesis; however, the role of thioredoxin in bortezomib drug resistance of myeloma remains unclear.MethodsWe generated several bortezomib-resistant myeloma cell lines by serially passaging with increased concentrations of bortezomib over a period of 1.5 years. Thioredoxin expression was measured by real-time PCR and western blot.ResultsThe role of thioredoxin in the survival of bortezomib-resistant myeloma cells was determined by specific shRNA knockdown in vitro and in vivo. Thioredoxin inhibitor (PX12) was used to determine the effectiveness of thioredoxin inhibition in the treatment of bortezomib-resistant myeloma cells. The effect of thioredoxin inhibition on mitophagy induction was examined. The correlation of thioredoxin expression with patient overall survival was interrogated. Thioredoxin expression was significantly upregulated in bortezomib-resistant myeloma cells and the change correlated with the increase of bortezomib concentration. Thioredoxin gene knockdown using specific shRNA sensitized bortezomib-resistant myeloma cells to bortezomib efficiency in vitro and in vivo. Similarly, pharmacological inhibition with PX12 inhibited the growth of bortezomib-resistant myeloma cells and overcame bortezomib resistance in vitro and in vivo. Furthermore, inhibition of thioredoxin resulted in the activation of mitophagy and blockage of mitophagy prevented the effects of PX12 on bortezomib-resistant myeloma cells, indicating that mitophagy is the important molecular mechanism for the induction of cell death in bortezomib-resistant myeloma cells by PX12. Moreover, inhibition of thioredoxin resulted in downregulation of phosphorylated mTOR and ERK1/2. Finally, thioredoxin was overexpressed in primary myeloma cells isolated from bortezomib-resistant myeloma patients and overexpression of thioredoxin correlated with poor overall survival in patients with multiple myeloma.ConclusionsOur findings demonstrated that increased thioredoxin plays a critical role in bortezomib resistance in multiple myeloma through mitophagy inactivation and increased mTOR and ERK1/2 phosphorylation. Thioredoxin provides a potential target for clinical therapeutics against multiple myeloma, particularly for bortezomib-resistant/refractory myeloma patients.

Abstract A136: KDM5A activity inhibition by 2-PCPA: overcoming tolerance to bortezomib

Abstract Purpose: The proteasome inhibitor bortezomib has shown impressive clinical activity alone and in combination with conventional and other novel agents for myeloma treatment. Although bortezomib is known to be a selective proteasome inhibitor, the downstream mechanisms of cytotoxicity and drug resistance remain to be understood. However, resistance to bortezomib as a single agent develops in the majority of patients, and activity in other malignancies has been less impressive. To overcome bortezomib resistance, we compared differential gene expression profiles of bortezomib-resistant and bortezomib-sensitive IM-9 cell lines in response to bortezomib, and expressions of KDM5A genes were significantly increased. Methods: The differential gene expression profiles of bortezomib-resistant IM-9 and bortezomib-sensitive IM-9 cell lines in response to bortezomib were performed using Affymetrix GeneChip. To confirm the results, real-time PCR and Western blot analysis were performed. Bortezomib-resistant IM-9 cells were treated with or without bortezomib and&amp;lt;/&amp;gt;or histone demethylase inhibitor, trans-2-phenylcyclopropylamine hydrochloride (2-PCPA). Moreover, level of histone methylation was examined by Western blot analysis. Results: At concentrations that effectively inhibited proteasome activity (maximum dose with 100nM), bortezomib induced cell death in bortezomib-sensitive IM-9 cells, but not in bortezomib-resistant IM-9. In comparison of differential gene expression profiles between bortezomib-resistant IM-9 and bortezomib-sensitive IM-9 cells, we showed overexpression of KDM5A, which is associated with chromatin-mediated reversible drug-tolerant state, Moreover, 2-PCPA overcomes bortezomib resistance in a dose-dependent manner. The histone demethylation at H3K4, H3K9, and H3K27 was suppressed with 2-PCPA. Conclusion: Histone lysine-specific demethylase inhibitor, 2-PCPA, with bortezomib may be useful for overcoming bortezomib resistance in myeloma cells by suppression of KDM3A and KDM5A activities. Citation Format: Yasuhito Terui, Yuji Mishima, Kiyohiko Hatake. KDM5A activity inhibition by 2-PCPA: overcoming tolerance to bortezomib [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A136.

Truncated protein tyrosine phosphatase receptor type O suppresses AKT signaling through IQ motif containing GTPase activating protein 1 and confers sensitivity to bortezomib in multiple myeloma.

Proteasome inhibitors are an important part of our chemotherapeutic armamentarium against multiple myeloma, but the vast majority of patients eventually develop drug-resistant disease through incompletely understood mechanisms. Comparison of gene expression profiles (GEPs) of bortezomib-resistant (BR) myeloma cell lines with their drug-naïve counterparts revealed decreased expression of truncated Protein tyrosine phosphatase receptor-type O (PTPROt) in BR cells. Over-expression of wild-type PTPROt in drug-naïve and BR cells reduced myeloma cell proliferation, induced apoptosis, and sensitized cells to bortezomib and to alkylating agents. PTPROt expression reduced AKT phosphorylation and activity, and sensitized to pharmacologic AKT pathway inhibitors, but this was not the case for a substrate-trapping catalytic domain-inactivating mutant. Co-immunoprecipitation and mass spectrometry studies identified IQ motif containing GTPase activating protein 1 (IQGAP1) as a PTPROt binding partner, and PTPROt reduced tyrosine phosphorylation of IQGAP1, providing a link to AKT activity. Analysis of clinically annotated GEP databases identified high PTPROt expression as being related to an increased likelihood of achieving complete remission with bortezomib therapy, while low expression was linked to a greater likelihood of disease progression. Finally, high PTPROt expression associated with prolonged median overall survival in patients receiving bortezomib-based therapy in the front-line or relapsed and/or refractory settings. Taken together, these data identify PTPROt suppression as a novel mechanism of myeloma resistance to bortezomib in myeloma cell lines, and also support the possibility that PTPROt expression could be used as a biomarker to predict outcomes with bortezomib, and by which to select patients for therapy with AKT inhibitors.

YM155 exerts potent cytotoxic activity against quiescent (G0/G1) multiple myeloma and bortezomib resistant cells via inhibition of survivin and Mcl-1.

YM155, a novel small molecule inhibitor of survivin, shows broad anticancer activity. Here, we have focused on the cytotoxic activity of YM155 against multiple myeloma (MM) including cytokinetically quiescent (G0/G1) cells and bortezomib resistant cells. YM155 strongly inhibited the growth of MM cell lines with the IC50 value of below 10 nM. YM155 also showed potent anti-myeloma activity in mouse xenograft model. YM155 suppressed the expression of survivin and rapidly directed Mcl-1 protein for proteasome degradation. YM155 abrogated the interleukin-6-induced STAT3 phosphorylation, subsequently blocked Mcl-1 expression and induced apoptosis in MM cells. Triple-color flow cytometric analysis revealed that YM155 potently induced cell death of MM cells in G0 phase. Quiescent primary MM cells were also sensitive to YM155. We established bortezomib-resistant MM cell line, U266/BTZR1, which possess a point mutation G322A. YM155 exhibited similar cytotoxic potency against U266/BTZR1 compared with parental cells. Interestingly, survivin expression was markedly elevated in U266/BTZR1 cells. Treatment with YM155 significantly down-regulated this increased survivin and Mcl-1 expression in U266/BTZR1 cells. In conclusion, our data indicate that YM155 exhibits potent cytotoxicity against quiescent (G0/G1) MM cells and bortezomib-resistant cells. These unique features of YM155 may be beneficial for the development of new therapeutic strategies to eliminate quiescent MM cells and overcome bortezomib resistance.

MK2206 enhances the cytocidal effects of bufalin in multiple myeloma by inhibiting the AKT/mTOR pathway

Despite the development of promising cancer therapeutic drugs, multiple myeloma (MM) remains an incurable disease. Bufalin is a bufanolide steroid compound of the traditional Chinese medicine Chan Su that was previously shown to exert growth suppression effects on myeloma cell lines. Previous studies conducted by our group demonstrated that bufalin activated the AKT/mTOR pathway in myeloma cells, which is considered an essential pathway to disease progression and is related to drug resistance in MM. In view of the significant role of AKT in MM, the allosteric AKT inhibitor MK2206 was selected in order to enhance the antitumor effects of bufalin in different MM cell lines (NCI-H929, U266, LP-1 and RPMI8226). The data indicated that MK2206 enhanced the cytotoxicity of bufalin in MM cells, via the suppression of cellular proliferation and the induction of apoptosis, as demonstrated by cleavage of apoptosis-related proteins. This effect was further noted in the presence of exogenous interleukin-6 and/or following the co-culture of MM cells with bone marrow stromal cells (BMSC). This process was associated with the inhibition of the AKT/mTOR pathway. The combination of bufalin with MK2206 reduced the secretion of IL-6 in U266 cells. The combined treatment exhibited similar anti-MM effects in bortezomib-resistant cell lines (NCI-H929R, U266R). In addition to the in vitro cell line models, the synergistic effect was noted in primary MM cells and in MM xenografts of BALB-c and NOD-SCID mice. In conclusion, the data suggested that MK2206 significantly enhanced the cytocidal effects of bufalin in MM cells, regardless of the sensitivity to bortezomib, via the inhibition of the AKT/mTOR pathway. The study provided the basis of a promising treatment approach for MM.

The blueberry component pterostilbene has potent anti-myeloma activity in bortezomib-resistant cells.

Multiple myeloma (MM) is an incurable hematologic malignancy because of its drug resistance. Pterostilbene (Pter) is found mainly in blueberries and grapes. The effects of Pter and its exact pharmacologic mechanisms on chemoresistant myeloma are not known. Herein, we investigated the anti-myeloma activity of Pter in bortezomib-resistant cell line H929R and explored the related mechanism of action for the first time. We found that Pter inhibited proliferation of H929R cells and promoted apoptosis of the cells through a caspase-dependent pathway, loss of mitochondrial membrane potential, and activation of Akt and p38 mitogen-activated protein kinase (MAPK) signaling pathways. DNA damage and S-phase arrest might be involved in Pter-related toxicity in H929R cells. Pter and the histone deacetylase inhibitors panobinostat or vorinostat inhibited proliferation of H929R cells in a synergistic manner. These data supported that Pter might be a promising natural compound for relapsed/refractory myeloma therapy, especially against myeloma resistant to bortezomib chemotherapy.

The Metabolic Phenotype of Myeloma Plasma Cells Differs Between Active and Residual Disease States

BackgroundUnderstanding the basis for drug resistance and relapse from treatment is a key aim and studying cells left after treatment exposure can inform the mechanisms underlying these features.Tumor cells are characterized by high ROS levels, increased glycolytic capacity, and low mitochondrial content. Cancer stem cells, however are reported to utilize oxidative phosphorylation, not glycolysis, to produce ATP. This metabolic phenotype is associated with features of stemness and constitutes a platform with which to understand the biology of residual and drug resistant cancer cells to elucidate novel targets.While this phenotype has not been extensively studied in myeloma,Bortezomib resistant cell lines have increased mitochondrial function as measured by membrane potential, oxygen consumption rate, ATP level, and mitochondrial Ca2+ concentrations.We have shown that ROS levels in CD138-selected myeloma cells are variable at diagnosis and are lower in plasma cells (PCs) from focal lesions (FLs) than PCs from random bone marrow (BM) aspirates consistent with greater features of stemness in FLs. MM cases with low ROS levels have increased oxidative phosphorylation pathway activation as determined by pathway enrichment analysis.We have previously demonstratedin MM cell lines (MMCLs) that ROSLo cells have a significantly greater percentage of cells in G1 and in S phase and an increased clonogenic capacity than ROSHi cells. Based on these observations we hypothesized that quiescent ROSLo cells differ in their mitochondrial function from bulk tumor cells, possibly indicating their stemness. We tested this hypothesis in PCs from active and residual disease as well as in MMCLs.MethodsPCs from active and residual (< 10% PCs in BM) MM cases treated on our Total Therapy protocols were stained for ROS level (H2DCFDA) or mitochondrial content (Mitotracker) and detected by FACS. Mitochondrial content was also assessed by electron microscopy. We examined oxidative capacity, mitochondrial content, and ATP content in ROS-sorted MMCLs, H929 and U266. We cultured MMCLs with varying mitochondrial content and oxidative capacity in high glucose growth media or in media with galactose substituted for glucose to force cells to use oxidative phosphorylation. Mitochondrial content, ATP levels, and chemosensitivity (Cell Titer Glo and Annexin V/PI) were compared.ResultsResidual MMPCs contain more mitochondria/cell than normal PCs (NPCs, p=0.0183). PCs from active disease have an intermediate number of mitochondria/cell. Residual MMPCs and NPCs had higher mitochondrial content than MMPCs from active disease as assessed by mitotracker staining. To correlate mitochondrial content with ROS levels, H2DCFDA staining of residual MMPCs, NPCs, and PCs from the same cases used for mitotracker staining was performed. Residual MMPCs have lower ROS levels than MMPCs from active disease (p<0.0001) but were similar to NPCs.We linked these metabolic characteristics to subpopulations of MMCLs with differing ROS levels. From these analyses we found that ROSHi cells had increased ATP content and lower oxygen consumption rate than ROSLo cells. Interestingly ROSHi cells from lines had higher mitochondrial content by mitotracker staining (p<0.01), whereas electron microscopy showed fewer mitochondria/cell in ROSHi cells than bulk tumor cells from active MM cases, highlighting differences between highly proliferating MM cell lines grown in normoxic conditions and freshly obtained MMPCs.In an effort to recapitulate the metabolic characteristics of residual PCs, MMCLs were cultured in galactose and exhibited reduced ROS levels, increased mitochondrial content, and increased basal ATP levels compared to cells cultured in glucose. MMCLs grown in galactose were also more sensitive to metformin (targets OxPhos) and navitoclax (targets Bcl-2, Bcl-XL, and Bcl-w) and more resistant to 2-DG (blocks glycolysis) and bortezomib than cells grown in glucose.ConclusionsResidual PCs have higher mitochondrial content and lower ROS levels, markers of stemness, than normal PCs and PCs from active MM. Cell lines forced to use oxidative phosphorylation for energy production acquire ROSLo MitoHi phenotype and are associated with chemoresistance, and representative of residual PCs. These ROSLoMitoHi cells may be functionally similar to MMPCs held in a BM niche that are dormant and have been described as chemoresistant. DisclosuresDavies:Janssen: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Takeda: Consultancy, Honoraria. Morgan:Univ of AR for Medical Sciences: Employment; Janssen: Research Funding; Bristol Meyers: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria.

Bortezomib-resistance is associated with increased levels of proteasome subunits and apoptosis-avoidance.

Bortezomib (BTZ), a proteasome inhibitor, is the first proteasome inhibitor to be used in clinical practice. Here we investigated the mechanisms underlying acquired bortezomib resistance in hepatocellular carcinoma (HCC) cells. Using stepwise selection, we established two acquired bortezomib-resistant HCC cell lines, a bortezomib-resistant HepG2 cell line (HepG2/BTZ) and bortezomib-resistant HuH7 cell line (HuH7/BTZ). The 50% inhibitory concentration values of HepG2/BTZ and HuH7/BTZ were respectively 15- and 39-fold higher than those of parental cell lines. Sequence analysis of the bortezomib-binding pocket in the β5-subunit showed no mutation. However, bortezomib-resistant HCC cells had increased expression of β1 and β5 proteasome subunits. These alterations of proteasome expression were accompanied by a weak degree of proteasome inhibition in bortezomib-resistant cells than that in wild-type cells after bortezomib exposure. Furthermore, bortezomib-resistant HCC cells acquired resistance to apoptosis. Bortezomib up-regulated pro-apoptotic proteins of the Bcl-2 protein family, Bax and Noxa in wild-type HCC cells. However, in bortezomib-resistant HCC cells, resistance to apoptosis was accompanied by loss of the ability to stabilize and accumulate these proteins. Thus, increased expression and increased activity of proteasomes constitute an adaptive and auto regulatory feedback mechanism to allow cells to survive exposure bortezomib.

Abstract B02: Bortezomib-resistant pediatric acute myeloid leukemia cell lines derived from Down syndrome patients are sensitive to disulfiram

Abstract Background: Acute myeloid leukemia (AML) continues to have an overall poor prognosis of about 50% in children. Treatment options for relapsed or refractory AML are limited. Bortezomib, a well-established proteasome inhibitor, is used in AML therapy. It binds to the beta 5 subunit (PSMB5) of the 26S proteasome and inhibits the chymotrypsin (CT)-like activity. Although patients develop resistance to bortezomib, the mechanism is not fully elucidated. Disulfiram (DS) is an aldehyde dehydrogenase (ALDH) inhibitor used to treat alcoholism without major side effects. DS has anticancer cytotoxicity that is in part copper (Cu2+)-dependent (DS/Cu2+). One of the reported mechanisms of action of DS is through proteasome inhibition. We previously demonstrated that both DS/Cu2+ and bortezomib are cytotoxic in a panel of AML cell lines. In the present study, we focused on two acute megakaryoblastic leukemia cell lines derived from patients with Down syndrome (CMY and CMK), one of the cell lines is resistant to cytarabine (CMY), a key drug used in AML treatment. Down syndrome children with leukemia are more prone to suffer from significant toxicity to chemotherapy; therefore, the need for novel treatment options for this category of patients is warranted. Purpose: The purpose of the present study was to develop an in vitro model of bortezomib resistance in CMY and CMK cell lines, to understand the mechanism of resistance, and to identify drugs that are cytotoxic in the bortezomib-resistant cell lines. Methods and Results: The bortezomib-resistant cell lines CMY and CMK were generated by exposure to stepwise increasing concentrations of bortezomib over a period of 9 months up to a concentration of 200 nM in CMY, and 100 nM in CMK cells. Viability was determined using Cell Titer Glo assay. CMY B200nM was 70 fold resistant to bortezomib with decreased inhibition of CT-like activity compared to the parent CMY cell line. Similar results were observed in the CMK cell line. An A362C mutation in PSMB5 causing a change from glutamine to proline was found in the bortezomib resistant CMY cell line. In order to confirm that this mutation caused bortezomib resistance, the PSMB5 (A362C) from CMY B200nM was cloned into the pcDNA3.1 vector and transfected into HEK 293A cells. This caused a two-fold higher bortezomib resistance than the corresponding control. The bortezomib-resistant AML cell lines were cross-resistant to the proteasome inhibitors carfilzomib and MG132 compared to the parent cell line, but were sensitive to DS/Cu2+. Moreover, the PSMB5 mRNA levels and the beta 5-subunit protein levels increased with bortezomib resistance , as evaluated by quantitative real-time RT-PCR and western blot, respectively. Although DS/Cu2+ induced ubiquitination similar to bortezomib, no comparable inhibition of CT-like activity was observed, suggesting a different mechanism of proteasome inhibition. Conclusions: The present results show that (1) the A362C mutation in the PSMB5 may underlie bortezomib-resistance in the cytarabine-resistant CMY cell line. This may provide an explanation for resistance to bortezomib in AML patients, which we hope to assess in the ongoing COG AML trial in which patients are randomized to receive bortezomib plus chemotherapy versus chemotherapy alone, and (2) DS/Cu2+ sensitivity may provide an effective and less toxic treatment approach for AML to be tested in the Down syndrome group of patients with poor outcome following relapse. Citation Format: Ranjan Bista, Oliver Pepper, David W. Lee, David A. Azorsa, Robert J. Arceci, Eiman Aleem. Bortezomib-resistant pediatric acute myeloid leukemia cell lines derived from Down syndrome patients are sensitive to disulfiram. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr B02.

Long Non-Coding RNA MALAT1 Is Associated with the Progression of Multiple Myeloma and Induced By Cellular Stress

Background: Recent transcriptome-wide analyses have revealed an overwhelming amount of transcribed, but not translated, non-coding RNAs capable of influencing diverse cellular processes such as proliferation, apoptosis, and motility. Long non-coding RNA (lncRNAs), which are commonly defined as transcripts >200 nt in length, have emerged as a class of key regulatory RNA. LncRNAs are deregulated in diverse human cancers and associated with disease progression; however, little is known about its role in multiple myeloma (MM). To elucidate the role of lncRNAs in MM, we studied the expression patterns of several well-known lncRNAs in the plasma cells of MM, MGUS and plasmacytoma patients and the function in MM cell lines in vitro. Moreover, to reveal the distinct lncRNA signature comprehensively, we performed next-generation sequencing-based RNA sequencing.Methods: CD138+ plasma cells from bone marrow (BM) mononuclear cells were obtained from 110 MM patients, 48 MGUS patients, 19 control subjects and 1 patient with extramedullary plasmacytoma of the liver and analyzed after obtaining informed consent from all the patients. The expression levels of lncRNAs MALAT1, ANRIL, HOTAIR, HOTTIP, and XIST were determined by a RQ-PCR analysis. RNase H-activating LNA™ GapmeR antisense oligonucleotides were used to knockdown lncRNA in vitro in MM cell lines. The cell lines were then treated with bortezomib, MG132, doxorubicin and hypoxic conditions to evaluate the effects of cytotoxic stress on the lncRNA expression. This study was approved by the IRB of Gunma University Hospital in accordance with the Declaration of Helsinki.Results: A significant higher level of MALAT1 expression was observed in BM plasma cells of MM patients (4.49) compared to MGUS patients (1.51) and control subjects (0.55) (p<0.001). Strikingly, MALAT1 expression in extramedullary plasmacytoma of the liver was 140-fold higher compared with BM plasma cells obtained at the same time of sampling (433.7 vs 3.21). MALAT1 expression was higher in MM patients with t(4;14) and del 17p (10.05 vs 3.90, p=0.049; 5.22 vs 2.76, p=0.03, respectively), but no difference was observed between stages according to the International Staging System (ISS) (p=0.87). Neither the overall survival nor the progression-free survival differed between patients with high and low MALAT1 expression. ANRIL expression levels were diverse according to the patients (range, 0 to 294.3), however, the median expression was significantly higher in MM patients (p<0.001). HOTAIR and HOTTIP expression levels were not detected in most samples, and XIST expression was found only in female patient samples as expected. Interestingly, the MM cell lines KMS12PE, OPM2, KMS11 treated with bortezomib showed elevated MALAT1 expression by 4.3 -21.8 fold and ANRIL by 2.2-4.7 fold; however, this increase was not observed in bortezomib-resistant cell lines. Another proteasome inhibitor, MG132, and a low dose of the cytotoxic drug doxorubicin also elevated both lncRNAs in the cell lines. Hypoxic stress, which has been shown to induce MALAT1 in vascular cells, did not increase either lncRNA. MALAT1 knockdown by GapmeR did not affect cell proliferation. It has been shown that MALAT1 enhances cell motility of lung adenocarcinoma cells by influencing cell motility associated genes; however, the expression of previously reported affected genes, such as HMMR, CTHRC1 and ROD1, was not altered in the MALAT1 knockdown MM cell lines. Although t(4;14) was associated with a high MALAT1 expression in the patient samples, MMSET knockdown by siRNA did not change the MALAT1 expression in the cell lines, thus MMSET was not a regulator of MALAT1. RNA sequencing of MM and MGUS samples revealed a distinct lncRNA expression signature as well as protein coding genes.Conclusion: Significant upregulation of lncRNAs MALAT1 and ANRIL might be associated with MM progression. Given that MALAT1 is associated with lung cancer metastasis, MALAT1 might be strongly associated with extramedullary plasmacytoma formation due to its high expression in liver plasmacytoma. Genotoxic and ER stress induced by therapeutic drugs might upregulate MALAT1 expression, leading to extramedullary extension, which is a recent problem in MM treatment. Determining the distinct lncRNA signature of MM is a current important issue to clarify the molecular mechanisms underlying MM progression for the development of novel therapies. DisclosuresNo relevant conflicts of interest to declare.

Suppression of Cdc37 Induces Bortezomib Resistance through Autophagy Activation and Plasma Cell Immaturation in Multiple Myeloma

Introduction: Cell division cycle protein 37 (Cdc37) is considered as a key co-chaperone for heat shock protein 90 (HSP90) to guide the stabilization and activation of protein kinases, which is critical for cell signaling transduction, proliferation and survival. Our previous study suggested that the population of low expression of cdc37 was correlated with bortezomib resistance in clinical MM samples and in vitro study. The purpose of this study is to investigate how cdc37 contributes to drug resistance in MM.Material and method: Nineteen sequential MM samples with gene expression profiling (GSE19554) obtained from published studies were analyzed. The expression of cdc37 was investigated in MM cell lines and primary purified CD138+ tumor cells by qRT-PCR. Cdc37 was down-regulated by lentivirus in MM cell line NCI-H929. Sensitivity of bortezomib was evaluated by CCK-8 and apoptosis analysis. Autophagosome was detected by electron microscope. Bone marrow MM cells from MM clinical samples were enriched by negative selection for CD2 (T and NK cells) and CD14 (monocytes) using FACS. Tumor cells were gated into subpopulations by CD38 and CD138 status. CD38+ CD138+ plasma cells, CD38+ CD138- plasmablasts and CD38- CD138- B cells were examined for xbp1s and cdc37 protein expression by IF.Results: The GEP analysis showed that cdc37 was down-regulated as the progression of MM (base line, pre-1st transplantation, pre-2nd transplantation, post-2nd transplantation) in 13 out of 19 sequential samples (Data base GEP). Cdc37 was suppressed in bortezomib resistant cell line ANBL-6. BR compared with their wild type ANBL-6.WT. While there were no significant differences between doxorubicin-, dexamethasone- resistant cell line compared with their counterparts. Primary tumor cells from five relapsed MM patients after bortezomib treatment also showed low expression of cdc37 compared with five MM samples that were sensitive to bortezomib treatment. Indeed, suppression cdc37 in MM cell line NC-H929 by shRNA lentivirus (cdc37-shRNA) resulted in a higher IC50 of bortezomib, and lower apoptosis rate after bortezomib treatment compared with scramble. These results indicated that cdc37 may participate in bortezomib resistance in MM samples and in vitro.Autophagy has recently emerged as a new mechanism involved in bortezomib resistance. Our results also showed that autophagy associated proteins Beclin-1 and LC-3b were increased in ANBL-6.BR compared with ANBL-6.WT. Besides, Beclin-1 and LC-3b were up-regulated in NCI-H929- cdc37 shRNA cells. What's more, there were more autophagosomes in NCI-H929- cdc37 shRNA cells detected by electron microscope. Furthermore, combinatorial application of autophagy inhibitor chloroquine and bortezomib would partially rescue bortezomib resistance after cdc37 down-regulation.Moreover, cdc37 expression was also found to be positively correlated with xbp1s in 62 newly diagnosed, 25 relapsed and 18 remission MM patients. Xbp1s, CHOP and BLIMP1, which were essential for the plasma cell differentiation and maturation, were also lower expression in 5 bortezomib resistant primary MM patients compared with the sensitive ones detected by qRT-PCR. Then CD38+ CD138+ plasma cells, CD38+ CD138- plasmablasts and CD38- CD138- B cells were examined for xbp1s and cdc37 protein expression by IF and showed that cdc37 and xbp1s were gradually up-regulated during the maturation of plasma cells in 3/5 patients. The results indicated that cdc37, together with xbp1s, may induce bortezomib resistance through plasma cell immaturation.Conclusion: Our results demonstrated that cdc37 downregulation may result in bortezomib resistance through compensatory increasing of autophagy level and the immaturation of plasma cells. Further study will be focused on how cdc37 interacts with xbp1s to participate in autophagy and promotion plasma cell maturation in MM. DisclosuresNo relevant conflicts of interest to declare.

Small molecule inhibitor screen identifies synergistic activity of the bromodomain inhibitor CPI203 and bortezomib in drug resistant myeloma.

Despite significant therapeutic progress in multiple myeloma, drug resistance is uniformly inevitable and new treatments are needed. Our aim was to identify novel, efficacious small-molecule combinations for use in drug resistant multiple myeloma. A panel of 116 small molecule inhibitors was used to screen resistant myeloma cell lines for potential therapeutic targets. Agents found to have enhanced activity in the bortezomib or melphalan resistant myeloma cell lines were investigated further in combination. Synergistic combinations of interest were evaluated in primary patient cells. The overall single-agent drug sensitivity profiles were dramatically different between melphalan and bortezomib resistant cells, however, the bromodomain inhibitor, CPI203, was observed to have enhanced activity in both the bortezomib and melphalan resistant lines compared to their wild-type counterparts. The combination of bortezomib and CPI203 was found to be synergistic in both the bortezomib and melphalan resistant cell lines as well as in a primary multiple myeloma sample from a patient refractory to recent proteasome inhibitor treatment. The CPI203-bortezomib combination led to enhanced apoptosis and anti-proliferative effects. Finally, in contrast to prior reports of synergy between bortezomib and other epigenetic modifying agents, which implicated MYC downregulation or NOXA induction, our analyses suggest that CPI203-bortezomib synergy is independent of these events. Our preclinical data supports a role for the clinical investigation of the bromodomain inhibitor CPI203 combined with bortezomib or alkylating agents in resistant multiple myeloma.