Sensitization of BCL-2-expressing breast tumors to chemotherapy by the BH3 mimetic ABT-737

The Walter and Eliza Hall Institute of Medical Research, Parkville, Victoria 3052, Australia.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 07/2011; 109(8):2766-71. DOI: 10.1073/pnas.1104778108
Source: PubMed


Overexpression of the prosurvival protein BCL-2 is common in breast cancer. Here we have explored its role as a potential therapeutic target in this disease. BCL-2, its anti-apoptotic relatives MCL-1 and BCL-XL, and the proapoptotic BH3-only ligand BIM were found to be coexpressed at relatively high levels in a substantial proportion of heterogeneous breast tumors, including clinically aggressive basal-like cancers. To determine whether the BH3 mimetic ABT-737 that neutralizes BCL-2, BCL-XL, and BCL-W had potential efficacy in targeting BCL-2-expressing basal-like triple-negative tumors, we generated a panel of primary breast tumor xenografts in immunocompromised mice and treated recipients with either ABT-737, docetaxel, or a combination. Tumor response and overall survival were significantly improved by combination therapy, but only for tumor xenografts that expressed elevated levels of BCL-2. Treatment with ABT-737 alone was ineffective, suggesting that ABT-737 sensitizes the tumor cells to docetaxel. Combination therapy was accompanied by a marked increase in apoptosis and dissociation of BIM from BCL-2. Notably, BH3 mimetics also appeared effective in BCL-2-expressing xenograft lines that harbored p53 mutations. Our findings provide in vivo evidence that BH3 mimetics can be used to sensitize primary breast tumors to chemotherapy and further suggest that elevated BCL-2 expression constitutes a predictive response marker in breast cancer.

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Available from: Samantha Richelle Oakes
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    • "Evidence for the benefits of targeting Bcl-2 family proteins is accumulating as BH3-only mimicking agents can promote apoptosis and enhance apoptosis with chemotherapy [40]–[42]. Specifically, ABT-737 was recently observed to sensitize primary breast tumors overexpressing Bcl-2 to chemotherapy [42]. Also, tumor-specific PUMA gene transfer enhanced radiosensitivity of breast cancer cells, although this result was in cells that do not overexpress HER2 [43]. "
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    ABSTRACT: HER2 is overexpressed in 15-20% of breast cancers. HER2 overexpression is known to reduce apoptosis but the underlying mechanisms for this association remain unclear. To elucidate the mechanisms for HER2-mediated survival, we investigated the relationship between HER2 and p53 upregulated modulator of apoptosis (PUMA), a potent apoptosis inducer. Our results showed that HER2 interacts with PUMA, which was independent of HER2 activation. In addition, we observed that HER2 interacted with PUMA in both mitochondrial and non-mitochondrial compartments. We next examined whether HER2 phosphorylates PUMA. Notably, PUMA tyrosine phosphorylation has never been reported. Using an intracellular assay, we found PUMA to be phosphorylated in breast cancer cells with activated HER2. Via cell-free HER2 kinase assay, we observed that PUMA was directly phosphorylated by HER2. Activation of HER2 decreased PUMA protein half-life. To identify which of the three tyrosines within PUMA are targeted by HER2, we generated three PUMA non-phosphorylation mutants each with a single Tyr→Phe substitution. Results indicated that each PUMA single mutant had lost some, but not all phosphorylation by HER2 indicating that HER2 targets all three tyrosines. Consequently, we created an additional PUMA mutant with all three tyrosines mutated (TM-PUMA) that could not be phosphorylated by HER2. Importantly, TM-PUMA was found to have a longer half-life than PUMA. An inverse association was observed between HER2 and PUMA in 93 invasive breast carcinoma samples. We further found that TM-PUMA suppressed growth of breast cancer cells to a greater degree than PUMA. Also, TM-PUMA had a stronger propensity to induce apoptosis than PUMA. Together, our results demonstrate, for the first time, that PUMA can be tyrosine phosphorylated and that HER2-mediated phosphorylation destabilizes PUMA protein. The HER2-PUMA interplay represents a novel mechanism by which PUMA is regulated and a new molecular basis for HER2-mediated growth and survival of cancer cells.
    Full-text · Article · Nov 2013 · PLoS ONE
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    • "Vaillant and colleagues, using breast primary tumor xenografts in a nonobese diabetic/severe combined immunodeficient (IL2Rγc−/−) mouse model, demonstrate that the combination of tamoxifen and ABT-737 or ABT-199 produced the largest reduction of tumor volumes and much longer survival outcome than tamoxifen or ABT drug alone. This is consistent with the finding in BCL-2+ basal-like breast cancer xenografts, in which the tumor burden was reduced only in the combination of ABT-737 and cytotoxic docetaxel [13]. A mechanism of the activity of ABT-199 is that the BCL-2 protein was primed with pro-apoptotic protein BIM, suggesting that ABT-199 was provoking release of BIM and initiation of apoptosis. "
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    ABSTRACT: The B-cell lymphoma/leukemia 2 protein (BCL-2) may help many types of cancers to evade cell death. However, identifying exactly where this is the case is a challenge. ABT-199 is a small molecule that selectively inhibits BCL-2, which is currently in clinical trials in lymphoid malignancies. While inhibiting BCL-2 by itself can cause cell death in hematopoietic tumors, single-agent activity is harder to observe in solid tumors. Combining ABT-199 with tamoxifen, the standard endocrine therapy for estrogen receptor-positive breast cancers, 85% of which have BCL-2 expression, represents a new strategy to prime cancer cells for apoptosis and elicit better cancer cell death responses.
    Preview · Article · Oct 2013 · Breast cancer research: BCR
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    • "Overall, our results suggest that the cyclin B1/Cdk1-mediated hyperphosphorylation of Bcl-2, Bcl-xL, and Mcl-1 is a major mechanism linking mitotic arrest to the induction of apoptosis in PCa cells. Although our results do not address whether the Doc/1198 + ABT-737 combination will be effective in vivo, there is evidence in leukemia, and lung, prostate and breast cancer to indicate that the antimitotic + ABT-737 combination should prove to be effective in animal models of PCa (Oakes et al., 2012; Shoemaker et al., 2006; Kang et al., 2007; Bray et al., 2009). Navitoclax, previously known as ABT-263, is an orally bioavailable analog of ABT-737 with identical function that is currently in Phase II trials for refractory lymphoid malignancies and solid tumors and appears to be a promising agent for use in combination with Doc (Tse et al., 2008; Shi et al., 2011). "
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    ABSTRACT: Castration-resistant prostate cancer (CRPC) expresses high levels of the anti-apoptotic proteins Bcl-2, Bcl-xL and Mcl-1, resulting in resistance to apoptosis and association with poor prognosis. Docetaxel, an antimitotic drug that is the first-line treatment strategy for CRPC, is known to provide a small survival benefit. However, docetaxel chemotherapy alone is not enough to counteract the high levels of Bcl-2/Bcl-xL/Mcl-1 present in CRPC. ABT-737 is a small molecule that binds to Bcl-2/Bcl-xL (but not Mcl-1) with high affinity and disrupts their interaction with pro-apoptotic Bax/Bak, thus enhancing apoptosis. Our results indicate that ABT-737 can sensitize androgen-dependent LNCaP and CRPC PC3 cells to docetaxel- and to the novel antimitotic ENMD-1198-mediated caspase-dependent apoptosis. CRPC DU145 cells, however, are more resistant to ABT-737 because they are Bax null and not because they express the highest levels of anti-apoptotic Mcl-1 (associated with ABT-737 resistance). Knockdown of Bax or Bak in LNCaP indicates that ABT-737-induced antimitotic enhancement of apoptosis is more dependent on the levels of Bax than Bak. Furthermore, we find that the ability of docetaxel to increase cyclin B1/Cdk1-mediated phosphorylation of Bcl-2/Bcl-xL and decrease Mcl-1 is required for ABT-737 to enhance apoptosis in PC3 cells, as determined by addition of Cdk1 inhibitor purvalanol A and expression of shRNA specific for cyclin B1. Overall, our data suggests that the high levels of anti-apoptotic proteins in Bax-expressing CRPC cells can be overcome by targeting Bcl-2/Bcl-xL with ABT-737 and Mcl-1 with antimitotics.
    Full-text · Article · Sep 2013 · PeerJ
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