ABT-737, a BH3 mimetic, induces glutathione depletion and oxidative stress

Purpose

This study assessed the role of oxidative stress and loss of glutathione in ABT-737-induced apoptosis.

Methods

Jurkat human acute lymphocytic leukemia cells and HeLa cells transfected with a tet-regulated Bcl-2 expression system were treated with ABT-737 or its less active stereoisomer. GSH concentrations, intracellular reactive oxygen species (ROS), caspase activation and apoptotic DNA fragmentation were measured.

Results

ABT-737 induced oxidative stress through decreased GSH and increased intracellular hydrogen peroxide and superoxide levels. Apoptotic DNA fragmentation and caspase activation were the consequences of this oxidative stress. Combining ABT-737 with ROS-inducing agents such as adaphostin or etoposide enhanced cell death.

Conclusions

These results demonstrate that inhibition of Bcl-2 causes a loss of GSH, an increase in ROS, caspase activation and subsequent apoptosis. Clinically, redox alterations as a consequence of Bcl-2 inhibition by ABT-737 should be considered in devising combination therapies with this novel agent or its derivatives.     

RASSF1A and the BH3-only mimetic ABT-737 promote apoptosis in pediatric medulloblastoma cell lines

The RASSF1A tumor suppressor is potentially the most important candidate gene identified in medulloblastoma to date, being epigenetically silenced in >79% of primary tumors. However, its functional role has not been previously addressed in this tumor type. Here, we demonstrate that expression of RASSF1A promotes the induction of cell death after activation of both the extrinsic and intrinsic apoptotic pathways in medulloblastoma cells. Treatment of UW228-3 cells stably expressing RASSF1A with an anti-CD95 antibody to induce extrinsic apoptosis and etoposide or cisplatin to activate intrinsic apoptosis augmented tumor cell killing in a caspase-dependent manner. This led to increased activation of the pro-apoptotic BCL-2 family member BAX. On the basis of this knowledge, we demonstrate how the loss of RASSF1A function in medulloblastoma cells might be overcome using the novel BH3-only mimetic ABT-737 in combination with chemotherapeutic agents to target the BCL-2 anti-apoptotic members. We show that ABT-737 increased susceptibility to apoptosis induced by DNA damage regardless of RASSF1A expression status through increased activation of BAX. Our findings identify the RASSF1A tumor suppressor as a promoter of apoptotic signaling pathways. Investigation of its mechanism of action has revealed that these pathways can still be promoted in its absence and how these potentially represent novel therapeutic targets for medulloblastoma.     

BH3 mimetic ABT-737 potentiates TRAIL-mediated apoptotic signaling by unsequestering Bim and Bak in human pancreatic cancer cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to induce mitochondrial apoptotic signaling that can be negatively regulated by prosurvival Bcl-2 proteins. ABT-737 is a small-molecule BH3 mimetic that binds to and antagonizes Bcl-2/Bcl-x(L) but not Mcl-1. We show that ABT-737 can synergistically enhance TRAIL-mediated cytotoxicity in human pancreatic cancer cell lines. ABT-737 was shown to enhance TRAIL-induced apoptosis as shown by DNA fragmentation, activation of caspase-8 and Bid, and cleavage of caspase-3 and poly(ADP-ribose) polymerase. A Bax conformational change induced by TRAIL was enhanced by ABT-737. ABT-737 disrupted the interaction of Bak with Bcl-x(L) in both cell lines. Furthermore, ABT-737 untethered the proapoptotic BH3-only protein Bim from its sequestration by Bcl-x(L) or Bcl-2. Bim small hairpin RNA (shRNA) was shown to attenuate caspase-3 cleavage and to reduce the cytotoxic effects of TRAIL plus ABT-737 compared with shRNA control cells. Finally, Mcl-1 shRNA potentiated caspase-3 cleavage by ABT-737 and enhanced its cytotoxic effects. Taken together, ABT-737 augments TRAIL-induced cell killing by unsequestering Bim and Bak and enhancing a Bax conformational change induced by TRAIL. These findings suggest a novel strategy to enhance cross-talk between the extrinsic and intrinsic apoptotic pathways to improve therapeutic efficacy against pancreatic cancer.     

Mechanisms of apoptosis sensitivity and resistance to the BH3 mimetic ABT-737 in acute myeloid leukemia

BCL-2 proteins are critical for cell survival and are overexpressed in many tumors. ABT-737 is a small-molecule BH3 mimetic that exhibits single-agent activity against lymphoma and small-cell lung cancer in preclinical studies. We here report that ABT-737 effectively kills acute myeloid leukemia blast, progenitor, and stem cells without affecting normal hematopoietic cells. ABT-737 induced the disruption of the BCL-2/BAX complex and BAK-dependent but BIM-independent activation of the intrinsic apoptotic pathway. In cells with phosphorylated BCL-2 or increased MCL-1, ABT-737 was inactive. Inhibition of BCL-2 phosphorylation and reduction of MCL-1 expression restored sensitivity to ABT-737. These data suggest that ABT-737 could be a highly effective antileukemia agent when the mechanisms of resistance identified here are considered.     

The BH3 mimetic ABT-737 targets selective Bcl-2 proteins and efficiently induces apoptosis via Bak/Bax if Mcl-1 is neutralized

Since apoptosis is impaired in malignant cells overexpressing prosurvival Bcl-2 proteins, drugs mimicking their natural antagonists, BH3-only proteins, might overcome chemoresistance. Of seven putative BH3 mimetics tested, only ABT-737 triggered Bax/Bak-mediated apoptosis. Despite its high affinity for Bcl-2, Bcl-x(L), and Bcl-w, many cell types proved refractory to ABT-737. We show that this resistance reflects ABT-737's inability to target another prosurvival relative, Mcl-1. Downregulation of Mcl-1 by several strategies conferred sensitivity to ABT-737. Furthermore, enforced Mcl-1 expression in a mouse lymphoma model conferred resistance. In contrast, cells overexpressing Bcl-2 remained highly sensitive to ABT-737. Hence, ABT-737 should prove efficacious in tumors with low Mcl-1 levels, or when combined with agents that inactivate Mcl-1, even to treat those tumors that overexpress Bcl-2.     

Actinomycin D synergistically enhances the efficacy of the BH3 mimetic ABT-737 by downregulating Mcl-1 expression

Many types of cancer cells possess the ability to evade apoptosis, leading to their rapid and uncontrolled proliferation. As major regulators of apoptosis, Bcl-2 proteins serve as emerging targets for novel chemotherapeutic strategies. In this study, we examined the involvement of Bcl-2 proteins in apoptosis induced by the chemotherapeutic agent actinomycin D. A dramatic decrease in anti-apoptotic myeloid leukemia cell differentiation protein (Mcl-1) mRNA and protein expression was detected upon actinomycin D treatment. Further, Mcl-l overexpression caused resistance to cell death upon treatment with actinomycin D, implicating a role for the downregulation of Mcl-1 in actinomycin D-induced apoptosis. We also explored the therapeutic potential of actinomycin D in combination with ABT-737, an experimental agent that inhibits anti-apoptotic Bcl-2 proteins. Actinomycin D sensitized cells to ABT-737 treatment in a Bak- or Bax-dependent manner. Importantly, low concentrations of actinomycin D and ABT-737 were more effective in inducing cell death in transformed cells than their untransformed counterparts. A synergistic effect of actinomycin D and ABT-737 on cell death was observed in several human tumor cell lines. Like actinomycin D treatment, knocking down Mcl-1 expression greatly sensitized tumor cells to ABT-737 and Mcl-1 overexpression abrogated the cytotoxic effect induced by ABT-737 and actinomycin D. These results suggest that the downregulation of Mcl-1 by actinomycin D is likely responsible for the observed synergistic effect between the two drugs. Overall, our studies provide compelling evidence that the combination of actinomycin D and ABT-737 may lead to an effective cancer treatment strategy.Key words: apoptosis, Bcl-2 proteins, Mcl-1, actinomycin D, ABT-737, NSCLC, pancreatic carcinoma     

Deciphering the molecular events necessary for synergistic tumor cell apoptosis mediated by the histone deacetylase inhibitor vorinostat and the BH3 mimetic ABT-737

The concept of personalized anticancer therapy is based on the use of targeted therapeutics through in-depth knowledge of the molecular mechanisms of action of these agents when used alone and in combination. We have identified the apoptotic proteins and pathways necessary for synergistic tumor cell apoptosis and in vivo antitumor responses seen when the HDAC inhibitor vorinostat is combined with the BH3-mimetic ABT-737 in lymphomas overexpressing Bcl-2. Vorinostat "primes" tumors overexpressing Bcl-2 for rapid ABT-737-mediated apoptosis by inducing expression of the BH3-only gene bmf. Moreover, these synergistic effects of vorinostat/ABT-737 were blunted in cells with an inactive p53 pathway or in cells lacking expression of the p53 target gene, noxa. These studies show the important and complex functional interaction between specific proapoptotic BH3-only proteins and the BH3-mimetic compound ABT-737 and provide the most comprehensive functional link between tumor genotype and the apoptotic and therapeutic effects of HDACi combined with ABT-737.     

MLN2238 synergizes BH3 mimetic ABT-263 in castration-resistant prostate cancer cells by induction of NOXA

Patients undergoing androgen blockade therapy develop castration-resistant prostate cancer (CRPC), which is associated with Bcl-2 upregulation and results in disease progression and death. In recent years, promising therapeutic agents, such as the BH3-only mimetic ABT-263 and proteasome inhibitors, have been developed and widely evaluated against a broad spectrum of cancer types, including prostate cancer, alone or in combination with other chemotherapeutic agents. In this study, the antitumor efficacy of ABT-263 and MLN2238 were evaluated as single agents and in combination in four CRPC cell lines: PC3, C4-2B, C4-2, and DU145. The viability of the treated cells and markers of apoptosis were assayed. Protein-protein interactions were analyzed by co-immunoprecipitation in drug-treated cells. Lentivirus-mediated short hairpin RNA was used to knockdown Bax, Mcl-1, and NOXA expressions. We found that ABT-263 and MLN2238 alone exhibited a mild cytotoxicity, and in combination, they elicited a synergistic cytotoxic effect in CRPC cells. The cell apoptosis induced by the combination drug treatment was evidenced by enhanced caspase-3 and Poly (ADP-ribose) polymerase (PARP) cleavage, and annexin-V-positive staining was significantly depleted by Bax knockdown. MLN2238 treatment upregulated NOXA and Mcl-1 expression, leading NOXA/Mcl-1 complexes to disassociate Bak from its complexes with Mcl-1 and enhancing ABT263-triggered Bax activation. NOXA knockdown by short hairpin RNA significantly attenuated the cytotoxicity of ABT-263 and MLN2238 co-administration. In conclusion, MLN2238 and ABT-263 synergistically triggered apoptosis in CRPC cells by upregulating NOXA and activating Bax, indicating a promising therapeutic strategy for the treatment of CRPC.     

BH3 mimetic ABT-737 neutralizes resistance to FLT3 inhibitor treatment mediated by FLT3-independent expression of BCL2 in primary AML blasts.

FLT3 defines a promising target for the treatment of acute myeloid leukemia (AML). In contrast to their efficacy in cell lines, FLT3-specific inhibitors as single agents have only modest clinical activity in patients with AML. As demonstrated here, overexpression of anti-apoptotic proteins of the BCL2 family leads to resistance against FLT3 inhibitors in a hematopoietic cell line model with activating FLT3 mutations. The susceptibility to FLT3 inhibition could be restored by treatment with the novel BH3 mimetic ABT-737. Primary AML samples tested in our study showed a high expression of BCL2 protein, but not of BCL-xL or MCL1. BCL2 protein levels were not reduced after dephosphorylation of FLT3 and its downstream target STAT5 in patient samples with FLT3 internal tandem duplications. Interestingly, treatment with ABT-737 caused apoptotic cell death in all primary AML samples at submicromolar level and synergized efficiently with FLT3 inhibition in AML samples with activating FLT3 mutations. In contrast to AML cell lines, BCR-ABL transformed human cells showed resistance to ABT-737, which might be due to the induction of MCL1 by BCR-ABL. Inhibition of BCL2 family members might define a novel highly efficient and specific strategy in the combined or monotreatment of AML.     

The BH3-mimetic ABT-737 targets the apoptotic machinery in cholangiocarcinoma cell lines resulting in synergistic interactions with zoledronic acid

Purpose

In TFK-1 and EGI-1 cholangiocarcinoma cell lines, zoledronic acid (ZOL) determines an S-phase block without apoptosis. Here, we investigated the occurrence of apoptosis stigmata when ZOL is associated to the BH3-mimetic ABT-737.

Methods

In EGI-1 and TFK-1 cholangiocarcinoma cell lines untreated or treated with ABT-737 alone or in combination with ZOL, the pro-survival protein??s pattern (BCL-2, BCL-XL, MCL-1, HSP72, HSP27) was investigated by biochemical criteria along with the occurrence of mitochondrial damage evaluated by cytofluorimetric analysis using a cationic dye.

Results

ABT-737 induced growth inhibition and significantly affected the colony-forming ability of both EGI-1 and TFK-1 cells. However, activated PARP-1 or/and caspase-3 cleavage (apoptosis markers) were detected only at the highest ABT-737 concentrations used. Combined treatment showed synergistic effect by converting the predominant cytostatic effect of ZOL into a cytotoxic one as shown by striking increment of mitochondrial harmed cells along with PARP-1 activation and caspase-3 cleavage.

Conclusion

The lack of apoptosis following ZOL treatment in these cholangiocarcinoma cell lines appears to be multifactorial and could be ascribed to the large constitutive expression of pro-survival proteins. The efficacy of ZOL treatment requires a concomitant unleashing of apoptosis using a selective BH3-mimetic as ABT-737. The rational targeting of specific components of the apoptotic pathway may appear a useful approach to improve the treatment of refractory or relapsed cholangiocarcinoma. Combined treatment could be further explored in in vivo tumor model of cholangiocarcinoma.     

BH3 profiling identifies three distinct classes of apoptotic blocks to predict response to ABT-737 and conventional chemotherapeutic agents

Cancer cells exhibit many abnormal phenotypes that induce apoptotic signaling via the intrinsic, or mitochondrial, pathway. That cancer cells nonetheless survive implies that they select for blocks in apoptosis. Identifying cancer-specific apoptotic blocks is necessary to rationally target them. Using a panel of 18 lymphoma cell lines, we show that a strategy we have developed, BH3 profiling, can identify apoptotic defects in cancer cells and separate them into three main classes based on position in the apoptotic pathway. BH3 profiling identifies cells that require BCL-2 for survival and predicts sensitivity to the BCL-2 antagonist ABT-737. BCL-2 dependence correlates with high levels of proapoptotic BIM sequestered by BCL-2. Strikingly, BH3 profiling can also predict sensitivity to conventional chemotherapeutic agents like etoposide, vincristine, and adriamycin.     

Synergistic killing of colorectal cancer cells by oxaliplatin and ABT-737

Background

Oxaliplatin is frequently used in the treatment of metastatic colorectal cancer (CRC). Our previous work shows that oxaliplatin induces the pro-apoptotic protein Noxa in CRC cells. The Bcl2-inhibitor ABT-737 is particularly effective in cells with high Noxa levels. Therefore, we tested whether oxaliplatin and ABT-737 display synergy in killing CRC cells.

Methods

A panel of CRC cell lines was treated with oxaliplatin and ABT-737, either alone or in combination. Apoptosis was measured by FACS analysis of sub-G1 DNA content and by Western blot analysis of caspase-3 processing. Noxa expression was suppressed by lentiviral RNA interference.

Results

Oxaliplatin and ABT-737 displayed a strong synergistic apoptotic response, which was dependent on wildtype TP53 and oncogenic KRAS. TP53 and KRAS were required for drug-induced Noxa expression and this was essential for tumor cell apoptosis. Oxaliplatin, but not ABT-737, induced p53 accumulation, but both drugs stimulated Noxa expression. Combination treatment of mice with subcutaneous tumor xenografts drastically reduced tumor volume, while single drug treatment had no effect.

Conclusion

ABT-737 synergizes with oxaliplatin to kill colorectal cancer cells. This requires induction of Noxa by wildtype TP53 and oncogenic KRAS. Future studies should explore the anti-tumor efficacy of this drug combination in mouse models for spontaneous CRC development and in patient-derived tumor cell cultures and xenografts.     

BMX Acts Downstream of PI3K to Promote Colorectal Cancer Cell Survival and Pathway Inhibition Sensitizes to the BH3 Mimetic ABT-737

Evasion of apoptosis is a hallmark of cancer, and reversing this process by inhibition of survival signaling pathways is a potential therapeutic strategy. Phosphoinositide 3-kinase (PI3K) signaling can promote cell survival and is upregulated in solid tumor types, including colorectal cancer (CRC), although these effects are context dependent. The role of PI3K in tumorigenesis combined with their amenability to specific inhibition makes them attractive drug targets. However, we observed that inhibition of PI3K in HCT116, DLD-1, and SW620 CRC cells did not induce apoptotic cell death. Moreover, these cells were relatively resistant to the Bcl-2 homology domain 3 (BH3) mimetic ABT-737, which directly targets the Bcl-2 family of apoptosis regulators. To test the hypothesis that PI3K inhibition lowers the apoptotic threshold without causing apoptosis per se, PI3K inhibitors were combined with ABT-737. PI3K inhibition enhanced ABT-737-induced apoptosis by 2.3- to 4.5-fold and reduced expression levels of MCL-1, the resistance biomarker for ABT-737. PI3K inhibition enhanced ABT-737-induced apoptosis a further 1.4- to 2.4-fold in CRC cells with small interfering RNA-depleted MCL-1, indicative of additional sensitizing mechanisms. The observation that ABT-737-induced apoptosis was unaffected by inhibition of PI3K downstream effectors AKT and mTOR, implicated a novel PI3K-dependant pathway. To elucidate this, an RNA interference (RNAi) screen of potential downstream effectors of PI3K signaling was conducted, which demonstrated that knockdown of the TEC kinase BMX sensitized to ABT-737. This suggests that BMX is an antiapoptotic downstream effector of PI3K, independent of AKT.     

The BH3 mimetic HA14-1 enhances 5-fluorouracil-induced autophagy and type II cell death in oesophageal cancer cells

Background:

Resistance to chemotherapeutic agents has been associated with a failure of cancer cells to induce apoptosis. Strategies to restore apoptosis have led to the development of BH3 mimetics, which inhibit anti-apoptotic Bcl-2 family members. We examined the sensitivity of three oesophageal cancer cell lines to 5-fluorouracil (5-FU) alone and in combination with the BH3 mimetic HA14-1.

Methods:

Clonogenic assays, morphology, markers of autophagy and apoptosis were used to assess the involved death mechanisms.

Results:

In response to 5-FU treatment, OE21 cells induce apoptosis, KYSE450 and KYSE70 cells are more resistant and induce autophagy accompanied by type II cell death. Autophagy induction results in ineffective treatment as substantial numbers of cells survive and re-populate. HA14-1 did not improve 5-FU treatment or reduce colony re-growth in the apoptosis deficient KYSE70 cells. However, the sensitivity of OE21 (apoptotic) and KYSE450 cells (apoptosis deficient/type II cell death) was significantly improved. In OE21 cells, treatment with 5-FU and HA14-1 resulted in augmentation of apoptosis. In KYSE450 cells, the reduction in recovering colonies following combination treatment was due to the enhancement of type II cell death.

Conclusion:

The efficacy of HA14-1 is cell line dependent and is not reliant on apoptosis induction.     

Therapeutic efficacy of ABT-737, a selective inhibitor of BCL-2, in small cell lung cancer

Bcl-2 is a central regulator of cell survival that is overexpressed in the majority of small cell lung cancers (SCLC) and contributes to both malignant transformation and therapeutic resistance. We compared primary SCLC xenografts prepared from de novo human tumors with standard cell line-based xenografts in the evaluation of a novel and highly potent small molecule inhibitor of Bcl-2, ABT-737. ABT-737 induced dramatic regressions in tumors derived from some SCLC cell lines. In contrast, only one of three primary xenograft SCLC tumors showed significant growth inhibition with ABT-737. Explanations for this apparent dichotomy may include relatively low expression of Bcl-2 in the primary xenografts or inherent differences in the model systems. The addition of etoposide to ABT-737 in the primary xenografts resulted in significant decreases in tumor growth, underscoring the clinical potential of ABT-737 in combination therapy. To identify factors that may contribute to resistance to ABT-737 and related inhibitors, we isolated resistant derivatives of an initially sensitive cell line-based xenograft. Acquired resistance in this model was associated with decreases in the expression of the primary target Bcl-2, of proapoptotic partners of Bcl-2 (Bax and Bim), and of Bcl-2:Bim heterodimers. Expression profiling reveals 85 candidate genes demonstrating consistent changes in gene expression with acquired resistance. Taken together, these data have specific implications for the clinical development of Bcl-2 inhibitors for SCLC and broader implications for the testing of novel anticancer strategies in relevant preclinical models.     

The Bcl-2/Bcl-XL family inhibitor ABT-737 sensitizes ovarian cancer cells to carboplatin.

PURPOSE: The effective treatment of ovarian cancer is hampered by the development of drug resistance, which may be mediated by members of the Bcl-2 family of apoptosis regulators. ABT-737 is a recently described inhibitor of members of this family. We investigated whether this compound could sensitize ovarian cancer cells to chemotherapeutic agents. EXPERIMENTAL DESIGN: The sensitivity of ovarian cancer cell lines to ABT-737 in combination with either carboplatin or paclitaxel was tested either in vitro by assessing cell growth/survival and apoptosis or in xenograft studies. RESULTS: As a single agent, ABT-737 inhibited the growth of eight ovarian cancer cell lines, although with relatively poor potency. However, ABT-737, but not a less active enantiomer, increased the sensitivity of several cell lines to carboplatin. The increased sensitivity to carboplatin was accompanied by a decrease in time at which apoptosis was observed when assessed according to the number of attached cells, PARP cleavage, and nucleosome formation. ABT-737 was more effective at sensitizing IGROV-1 cells when ABT-737 was administered after carboplatin. In addition, ABT-737 significantly enhanced the activity of carboplatin in one of three primary cultures derived directly from ascitic tumor cells in patients recently treated with chemotherapy. Small interfering RNA directed to Bcl-X(L) also increased the sensitivity of ovarian cancer cell lines to carboplatin. ABT-737 was also able to augment the inhibition of IGROV-1 tumor xenograft growth beyond that obtained with carboplatin alone. CONCLUSIONS: These data suggest that ABT-737, in combination with carboplatin, may find utility in the treatment of patients with ovarian cancer.