'Seed' analysis of off-target siRNAs reveals an essential role of Mcl-1 in resistance to the small-molecule Bcl-2/Bcl-XL inhibitor ABT-737

The Bcl-2 inhibitor ABT-263 enhances the response of multiple chemotherapeutic regimens in hematologic tumors in vivo

PURPOSE

This study was designed to test the ability of the Bcl-2 family inhibitor ABT-263 to potentiate commonly used chemotherapeutic agents and regimens in hematologic tumor models.

METHODS

Models of B-cell lymphoma and multiple myeloma were tested in vitro and in vivo with ABT-263 in combination with standard chemotherapeutic regimens, including VAP, CHOP and R-CHOP, as well as single cytotoxic agents including etoposide, rituximab, bortezomib and cyclophosphamide. Alterations in Bcl-2 family member expression patterns were analyzed to define mechanisms of potentiation.

RESULTS

ABT-263 was additive with etoposide, vincristine and VAP in vitro in the diffuse large B-cell lymphoma line (DLBCL) DoHH-2, while rituximab potentiated its activity in SuDHL-4. Bortezomib strongly synergized with ABT-263 in the mantle cell lymphoma line Granta 519. Treatment of DoHH-2 with etoposide was associated with an increase in Puma expression, while bortezomib upregulated Noxa expression in Granta 519. Combination of ABT-263 with cytotoxic agents demonstrated superior tumor growth inhibition and delay in multiple models versus cytotoxic therapy alone, along with significant improvements in tumor response rates.

CONCLUSIONS

Inhibition of the Bcl-2 family of proteins by ABT-263 enhances the cytotoxicity of multiple chemotherapeutics in hematologic tumors and represents a promising addition to the therapeutic arsenal for treatment of these diseases.

Small-molecule Bcl-2 antagonists as targeted therapy in oncology

Dynamic protein–protein interactions between proapoptotic and pro-survival Bcl-2 family members regulate outer-mitochondrial membrane permeabilization and cytochrome c release, key events in the path to apoptosis. Their relative levels often dictate the fate of a cell following an apoptotic stimulus. However, in cancer cells, the pro-survival Bcl-2 family members are frequently upregulated, thereby creating a constitutive block to apoptosis and resulting in continued cell survival under conditions that normally result in cell death. Because many chemotherapeutics used to treat cancer also trigger apoptosis, this upregulation of pro-survival members also contributes to resistance to conventional cancer therapies. Strategies that inactivate pro-survival Bcl-2 family members therefore suggest a means by which this downstream block in apoptosis can be alleviated, resulting in the selective killing of malignant cells. Here, we outline the progress of three small-molecule Bcl-2 antagonists that have advanced into clinical evaluation.

Synergistic interactions between sorafenib and bortezomib in hepatocellular carcinoma involve PP2A-dependent Akt inactivation

BACKGROUND & AIMS

Previously we reported that Akt inactivation determines the sensitivity of hepatocellular carcinoma (HCC) cells to bortezomib. Here we report that combined treatment with sorafenib and bortezomib shows synergistic effects in HCC.

METHODS

HCC cell lines (PLC/PRF/5, Huh-7, and Hep3B) were treated with sorafenib and/or bortezomib and analyzed in terms of apoptosis signal transduction. In vivo efficacy was determined in nude mice with PLC/PRF/5 xenografts.

RESULTS

Pretreatment with sorafenib enhanced bortezomib-induced apoptotic cell death by restoring bortezomib's ability to inactivate Akt in PLC/PRF/5 cells. Knocking down Akt1 by RNA-interference overcame apoptotic resistance to bortezomib in PLC/PRF/5 cells and ectopic expression of active Akt in HCC cells abolished the bortezomib sensitizing effect of sorafenib, indicating Akt inactivation plays a key role in mediating the combinational effects. Moreover, okadaic acid, a protein phosphatase 2A (PP2A) inhibitor, reversed down-regulation of phospho-Akt (P-Akt) expression induced by co-treatment with sorafenib and bortezomib, and 1, 9 di-deoxy-forskolin, a PP2A agonist, restored bortezomib's effect on P-Akt and apoptosis. Importantly, silencing of PP2A by RNA-interference reduced the apoptotic effect induced by sorafenib-bortezomib co-treatment, indicating that PP2A is indispensable for mediating the effects of these drugs. Notably, sorafenib with bortezomib increased PP2A activity in PLC/PRF/5 cells without altering protein levels of PP2A complex or the interaction between PP2A and Akt proteins. Finally, sorafenib plus bortezomib significantly suppressed PLC/PRF/5 xenograft tumor growth, down-regulated P-Akt expression, and up-regulated PP2A activity.

CONCLUSIONS

The combination of sorafenib and bortezomib shows synergy in HCC through PP2A-dependent Akt inactivation.

The Bcl-2 homology domain 3 mimetic ABT-737 targets the apoptotic machinery in acute lymphoblastic leukemia resulting in synergistic in vitro and in vivo interactions with established drugs

Antiapoptotic Bcl-2 proteins are overexpressed in a number of cancers, including leukemias, and are frequently associated with resistance to conventional chemotherapeutic drugs. ABT-737, a Bcl-2 homology domain 3 mimetic (for structure, see Nature 435:677-681, 2005) inhibits the prosurvival function of Bcl-2, Bcl-X(L), and Bcl-w. We show that ABT-737 was effective as a single agent against a panel of pediatric acute lymphoblastic leukemia (ALL) xenografts, previously established, from patient biopsies, in immunodeficient mice. Although in vitro resistance of leukemia cell lines correlated with expression of the prosurvival protein Mcl-1, there was no relationship between Mcl-1 expression and in vivo xenograft response to ABT-737. However, expression of the pro-apoptotic protein Bim, and the extent of its association with Bcl-2, significantly correlated with in vivo ABT-737 sensitivity. ABT-737 potentiated the antileukemic effects of L-asparaginase, topotecan, vincristine, and etoposide against drug-resistant xenografts in vitro and in vivo. Finally, we show that the combination of L-asparaginase (by specifically down-regulating Mcl-1 protein levels), topotecan (by activating p53 via DNA damage), and ABT-737 (by inhibiting antiapoptotic Bcl-2 family members) caused profound synergistic antileukemic efficacy both in vitro and in vivo. Rational targeting of specific components of the apoptotic pathway may be a useful approach to improve the treatment of refractory or relapsed pediatric ALL. Overall, this study supports the inclusion of the clinical derivative of ABT-737, ABT-263 (for structure, see Cancer Res 68:3421-3428, 2008), into clinical trials against relapsed/refractory pediatric ALL.

Bim upregulation by histone deacetylase inhibitors mediates interactions with the Bcl-2 antagonist ABT-737: evidence for distinct roles for Bcl-2, Bcl-xL, and Mcl-1

The Bcl-2 antagonist ABT-737 kills transformed cells in association with displacement of Bim from Bcl-2. The histone deactetylase (HDAC) inhibitor suberoyl bis-hydroxamic acid (SBHA) was employed to determine whether and by what mechanism ABT-737 might interact with agents that upregulate Bim. Expression profiling of BH3-only proteins indicated that SBHA increased Bim, Puma, and Noxa expression, while SBHA concentrations that upregulated Bim significantly potentiated ABT-737 lethality. Concordance between SBHA-mediated Bim upregulation and interactions with ABT-737 was observed in various human leukemia and myeloma cells. SBHA-induced Bim was largely sequestered by Bcl-2 and Bcl-x(L), rather than Mcl-1; ABT-737 attenuated these interactions, thereby triggering Bak/Bax activation and mitochondrial outer membrane permeabilization. Knockdown of Bim (but not Puma or Noxa) by shRNA or ectopic overexpression of Bcl-2, Bcl-x(L), or Mcl-1 diminished Bax/Bak activation and apoptosis. Notably, ectopic expression of these antiapoptotic proteins disabled death signaling by sequestering different proapoptotic proteins, i.e., Bim by Bcl-2, both Bim and Bak by Bcl-x(L), and Bak by Mcl-1. Together, these findings indicate that HDAC inhibitor-inducible Bim is primarily neutralized by Bcl-2 and Bcl-x(L), thus providing a mechanistic framework by which Bcl-2 antagonists potentiate the lethality of agents, such as HDAC inhibitors, which upregulate Bim.

Noxa: at the tip of the balance between life and death

Among all Bcl2 homology domain 3 (BH3)-only proteins known to date, APR/PMAIP1/Noxa, albeit showing weak proapoptotic potential on its own, appears to be crucial in fine-tuning cell death decisions by targeting the prosurvival molecule Mcl1 for proteasomal degradation. This event appears critical for cell death induction along the mitochondrial Bcl2-regulated apoptosis pathway in response to factor deprivation or DNA damage, presumably by sensitizing the cell toward the action of additional BH3-only protein family members. This review aims to summarize the function of Noxa in normal physiology, stress-induced cell death and tumorigenesis.

Mimicking the BH3 domain to kill cancer cells

Cancer cells show deviant behavior that induces apoptotic signaling. To survive, cancer cells typically acquire changes enabling evasion of death signals. One way they do this is by increasing the expression of anti-apoptotic BCL-2 proteins. Anti-apoptotic BCL-2 family proteins antagonize death signaling by forming heterodimers with pro-death proteins. Heterodimer formation occurs through binding of the pro-apoptotic protein's BH3 domain into the hydrophobic cleft of anti-apoptotic proteins. The BH3 mimetics are small molecule antagonists of the anti-apoptotic BCL-2 members that function as competitive inhibitors by binding to the hydrophobic cleft. Under certain conditions, antagonism of anti-apoptotic BCL-2 family proteins can unleash pro-death molecules in cancer cells. Thus, the BH3 mimetics are a new class of cancer drugs that specifically target a mechanism of cancer cell survival to selectively kill cancer cells.

Inhibitors of MyD88-dependent proinflammatory cytokine production identified utilizing a novel RNA interference screening approach

BACKGROUND

The events required to initiate host defenses against invading pathogens involve complex signaling cascades comprised of numerous adaptor molecules, kinases, and transcriptional elements, ultimately leading to the production of proinflammatory cytokines, such as tumor necrosis factor alpha (TNF-alpha). How these signaling cascades are regulated, and the proteins and regulatory elements participating are still poorly understood.

RESULTS

We report here the development a completely random short-hairpin RNA (shRNA) library coupled with a novel forward genetic screening strategy to identify inhibitors of Toll-like receptor (TLR) dependent proinflammatory responses. We developed a murine macrophage reporter cell line stably transfected with a construct expressing diphtheria toxin-A (DT-A) under the control of the TNF-alpha-promoter. Stimulation of the reporter cell line with the TLR ligand lipopolysaccharide (LPS) resulted in DT-A induced cell death, which could be prevented by the addition of an shRNA targeting the TLR adaptor molecule MyD88. Utilizing this cell line, we screened a completely random lentiviral short hairpin RNA (shRNA) library for sequences that inhibited TLR-mediated TNF-alpha production. Recovery of shRNA sequences from surviving cells led to the identification of unique shRNA sequences that significantly inhibited TLR4-dependent TNF-alpha gene expression. Furthermore, these shRNA sequences specifically blocked TLR2 but not TLR3-dependent TNF-alpha production.

CONCLUSIONS

Thus, we describe the generation of novel tools to facilitate large-scale forward genetic screens in mammalian cells and the identification of potent shRNA inhibitors of TLR2 and TLR4- dependent proinflammatory responses.

Conformational changes in Bcl-2 pro-survival proteins determine their capacity to bind ligands

Antagonists of anti-apoptotic Bcl-2 family members hold promise as cancer therapeutics. Apoptosis is triggered when a peptide containing a BH3 motif or a small molecule BH3 peptidomimetic, such as ABT 737, binds to the relevant Bcl-2 family members. ABT-737 is an antagonist of Bcl-2, Bcl-x(L), and Bcl-w but not of Mcl-1. Here we describe new structures of mutant BH3 peptides bound to Bcl-x(L) and Mcl-1. These structures suggested a rationale for the failure of ABT-737 to bind Mcl-1, but a designed variant of ABT-737 failed to acquire binding affinity for Mcl-1. Rather, it was selective for Bcl-x(L), a result attributable in part to significant backbone refolding and movements of helical segments in its ligand binding site. To date there are few reported crystal structures of organic ligands in complex with their pro-survival protein targets. Our structure of this new organic ligand provided insights into the structural transitions that occur within the BH3 binding groove, highlighting significant differences in the structural properties of members of the Bcl-2 pro-survival protein family. Such differences are likely to influence and be important in the quest for compounds capable of selectively antagonizing the different family members.

Mcl1 downregulation sensitizes neuroblastoma to cytotoxic chemotherapy and small molecule Bcl2-family antagonists

Neuroblastoma (NB) is a common, highly lethal pediatric cancer, with treatment failures largely attributable to the emergence of chemoresistance. The pro-survival Bcl2 homology (BH) proteins critically regulate apoptosis, and may represent important therapeutic targets for restoring drug sensitivity in NB. We used a human NB tumor tissue microarray to survey the expression of pro-survival BH proteins Mcl1 and Bcl2, and correlated expression to clinical prognostic factors and survival. Primary NB tumors heterogeneously expressed Mcl1 or Bcl2, with high expression correlating to high-risk phenotype. Co-expression is infrequent (11%), but correlates to reduced survival. Using RNA interference, we investigated the functional relevance of Mcl1 and Bcl2 in high-risk NB cell lines (SK-N-AS, IMR-5, NLF). Mcl1 knockdown induced apoptosis in all NB cell lines, while Bcl2 knockdown inhibited only NLF, suggesting functional heterogeneity. Finally, we determined the relevance of Mcl1 in resistance to conventional chemotherapy (etoposide, doxorubicin) and small molecule Bcl2-family antagonists (ABT-737 and AT-101). Mcl1 silencing augmented sensitivity to chemotherapeutics 2- to 300-fold, while Bcl2 silencing did not, even in Bcl2-sensitive NLF cells. Resistance to ABT-737, which targets Bcl2/-w/-x, was overcome by Mcl1 knockdown. AT-101, which also neutralizes Mcl1, had single-agent cytotoxicity, further augmented by Mcl1 knockdown. In conclusion, Mcl1 appears a predominant pro-survival protein contributing to chemoresistance in NB, and Mcl1 inactivation may represent a novel therapeutic strategy. Optimization of compounds with higher Mcl1 affinity, or combination with additional Mcl1 antagonists, may enhance the clinical utility of this approach.

Endocrine glands-derived vascular endothelial growth factor protects pancreatic cancer cells from apoptosis via upregulation of the myeloid cell leukemia-1 protein

Endocrine glands-derived vascular endothelial growth factor (EG-VEGF, also termed as Prok1)--a novel cytokine that selectively acts on the endothelial cells of endocrine glands--was recently reported to be involved in the regulation of tumor cell growth and survival. However, its roles in the regulation of pancreatic cancer progression remain unclear. In this report, we investigated the suppressive effects of EG-VEGF on pancreatic cancer cell apoptosis and the relevant mechanisms. By using reverse-transcriptase polymerase chain reaction (RT-PCR) we found that the Mia PaCa II cells of the pancreatic cancer cell line express the mRNAs of both EG-VEGF (Prok1) and its receptors. EG-VEGF protects pancreatic cancer cells from apoptosis through upregulation of myeloid cell leukemia-1 (Mcl-1), an anti-apoptotic protein of the bcl-2 family. Treatment of pancreatic cancer cells with EG-VEGF results in the rapid phosphorylation of mitogen-activated protein kinase (MAPK), STAT3, and AKT, which are involved in the upregulation of Mcl-1 expression. EG-VEGF (Prok1) protects Mia PaCa II cells from apoptosis through G protein-coupled receptor (GPR)-induced activation of multiple signal pathways, and hence can be a novel target for pancreatic cancer therapy.

Alterations in the Noxa/Mcl-1 axis determine sensitivity of small cell lung cancer to the BH3 mimetic ABT-737

To understand the molecular basis for variable sensitivity to the BH3 mimetic drug ABT-737, the abundance of Bcl-2 family members was assayed in a panel of small cell lung cancer cell lines whose sensitivity varied over a 2-log range. Elevated Noxa and Bcl-2 levels directly correlated with sensitivity to ABT-737, whereas Mcl-1 levels were similar in all cell lines tested regardless of sensitivity. Transgenically enforced expression of Noxa but not Bcl-2 resulted in increased sensitivity to ABT-737 in multiple cell lines. This increase was especially pronounced in the H209 cell line in which expression of Noxa resulted in a proportionate decline in Mcl-1 expression. Although overexpression of Noxa enhanced sensitivity of the H526 and H82 cell lines to ABT-737, it did not result in altered Mcl-1 levels. Similarly, small interfering RNA-mediated knockdown of Noxa expression in the H146 cell line, which increased resistance to ABT-737, did not result in altered Mcl-1 levels. Therefore, three of four cell lines studied failed to show Noxa-mediated regulation of Mcl-1 expression. However, despite failure to regulate Mcl-1 levels, Noxa blocked binding of Bim to Mcl-1 following its release from Bcl-2 by ABT-737. Finally, we observed that a 24-hour incubation of the H526 and WBA cell lines with ABT-737 resulted in increased Noxa expression, suggesting that Noxa may play a direct role in ABT-737-mediated apoptosis. These results indicate that Noxa expression is the critical determinant of ABT-737 sensitivity and loss of Noxa-mediated regulation of Mcl-1 expression may be an important feature of small cell lung cancer biology.

Unleashing the power of inhibitors of oncogenic kinases through BH3 mimetics

Therapeutic targeting of tumours on the basis of molecular analysis is a new paradigm for cancer treatment but has yet to fulfil expectations. For many solid tumours, targeted therapeutics, such as inhibitors of oncogenic kinase pathways, elicit predominantly disease-stabilizing, cytostatic responses, rather than tumour regression. Combining oncogenic kinase inhibitors with direct activators of the apoptosis machinery, such as the BH3 mimetic ABT-737, may unlock potent anti-tumour potential to produce durable clinical responses with less collateral damage.

Concurrent up-regulation of BCL-XL and BCL2A1 induces approximately 1000-fold resistance to ABT-737 in chronic lymphocytic leukemia

ABT-737 and its orally active analog, ABT-263, are rationally designed inhibitors of BCL2 and BCL-XL. ABT-263 shows promising activity in early phase 1 clinical trials in B-cell malignancies, particularly chronic lymphocytic leukemia (CLL). In vitro, peripheral blood CLL cells are extremely sensitive to ABT-737 (EC50 ∼7 nM), with rapid induction of apoptosis in all 60 patients tested, independent of parameters associated with disease progression and chemotherapy resistance. In contrast to data from cell lines, ABT-737–induced apoptosis in CLL cells was largely MCL1-independent. Because CLL cells within lymph nodes are more resistant to apoptosis than those in peripheral blood, CLL cells were cultured on CD154-expressing fibroblasts in the presence of interleukin-4 (IL-4) to mimic the lymph node microenvironment. CLL cells thus cultured developed an approximately 1000-fold resistance to ABT-737 within 24 hours. Investigations of the underlying mechanism revealed that this resistance occurred upstream of mitochondrial perturbation and involved de novo synthesis of the antiapoptotic proteins BCL-XL and BCL2A1, which were responsible for resistance to low and high ABT-737 concentrations, respectively. Our data indicate that after therapy with ABT-737–related inhibitors, resistant CLL cells might develop in lymph nodes in vivo and that treatment strategies targeting multiple BCL2 antiapoptotic members simultaneously may have synergistic activity.

Bcl-2 inhibitors: targeting mitochondrial apoptotic pathways in cancer therapy

Defects in apoptotic pathways can promote cancer cell survival and also confer resistance to antineoplastic drugs. One pathway being targeted for antineoplastic therapy is the anti-apoptotic B-cell lymphoma-2 (Bcl-2) family of proteins (Bcl-2, Bcl-XL, Bcl-w, Mcl-1, Bfl1/A-1, and Bcl-B) that bind to and inactivate BH3-domain pro-apoptotic proteins. Signals transmitted by cellular damage (including antineoplastic drugs) or cytokine deprivation can initiate apoptosis via the intrinsic apoptotic pathway. It is controversial whether some BH3-domain proteins (Bim or tBid) directly activate multidomain pro-apoptotic proteins (e.g., Bax and Bak) or act via inhibition of those anti-apoptotic Bcl-2 proteins (Bcl-2, Bcl-XL, Bcl-w, Mcl-1, Bfl1/A-1, and Bcl-B) that stabilize pro-apoptotic proteins. Overexpression of anti-apoptotic Bcl-2 family members has been associated with chemotherapy resistance in various human cancers, and preclinical studies have shown that agents targeting anti-apoptotic Bcl-2 family members have preclinical activity as single agents and in combination with other antineoplastic agents. Clinical trials of several investigational drugs targeting the Bcl-2 family (oblimersen sodium, AT-101, ABT-263, GX15-070) are ongoing. Here, we review the role of the Bcl-2 family in apoptotic pathways and those agents that are known and/or designed to inhibit the anti-apoptotic Bcl-2 family of proteins.

ABT-737 synergizes with chemotherapy to kill head and neck squamous cell carcinoma cells via a Noxa-mediated pathway

Overexpression of Bcl-X(L), an antiapoptotic Bcl-2 family member, occurs in a majority of head and neck squamous cell carcinomas (HNSCCs) and correlates with chemotherapy resistance in this disease. Overexpression of Bcl-2 is also observed in HNSCC, albeit less frequently. We have previously shown that peptides derived from the BH3 domains of proapoptotic proteins can be used to target Bcl-X(L) and Bcl-2 in HNSCC cells, promoting apoptosis. In this report, we examined the impact of ABT-737 (for structure, see Nature 435: 677-681, 2005 ), a potent small-molecule inhibitor of Bcl-X(L) and Bcl-2, on HNSCC cells. As a single agent, ABT-737 was largely ineffective at promoting HNSCC cell death. By contrast, ABT-737 strongly synergized with the chemotherapy drugs cisplatin and etoposide to promote HNSCC cell death and loss of clonogenic survival. Synergism between ABT-737 and chemotherapy was associated with synergistic activation of caspase-3 and cleavage of poly(ADP-ribose) polymerase. Treatment with ABT-737 plus chemotherapy resulted in dramatic up-regulation of proapoptotic Noxa protein, and small interfering RNA (siRNA)-mediated inhibition of Noxa up-regulation partially attenuated cell death by the synergistic combination. Treatment with cisplatin or etoposide, alone or in combination with ABT-737, resulted in substantial down-regulation of Mcl-1L, a known inhibitor of ABT-737 action. Further down-regulation of Mcl-1L using siRNA failed to enhance killing by the cisplatin/ABT-737 synergistic combination, indicating that chemotherapy treatment of HNSCC cells is sufficient to remove this impediment to ABT-737. Together, our results demonstrate potent synergy between ABT-737 and chemotherapy drugs in the killing of HNSCC cells and reveal an important role for Noxa in mediating synergism by these agents.

Genetic ablation of Bcl-x attenuates invasiveness without affecting apoptosis or tumor growth in a mouse model of pancreatic neuroendocrine cancer

Tumor cell death is modulated by an intrinsic cell death pathway controlled by the pro- and anti-apoptotic members of the Bcl-2 family. Up-regulation of anti-apoptotic Bcl-2 family members has been shown to suppress cell death in pre-clinical models of human cancer and is implicated in human tumor progression. Previous gain-of-function studies in the RIP1-Tag2 model of pancreatic islet carcinogenesis, involving uniform or focal/temporal over-expression of Bcl-x_L, demonstrated accelerated tumor formation and growth. To specifically assess the role of endogenous Bcl-x in regulating apoptosis and tumor progression in this model, we engineered a pancreatic β-cell-specific knockout of both alleles of Bcl-x using the Cre-LoxP system of homologous recombination. Surprisingly, there was no appreciable effect on tumor cell apoptosis rates or on tumor growth in the Bcl-x knockout mice. Other anti-apoptotic Bcl-2 family members were expressed but not substantively altered at the mRNA level in the Bcl-x-null tumors, suggestive of redundancy without compensatory transcriptional up-regulation. Interestingly, the incidence of invasive carcinomas was reduced, and tumor cells lacking Bcl-x were impaired in invasion in a two-chamber trans-well assay under conditions mimicking hypoxia. Thus, while the function of Bcl-x in suppressing apoptosis and thereby promoting tumor growth is evidently redundant, genetic ablation implicates Bcl-x in selectively facilitating invasion, consistent with a recent report documenting a pro-invasive capability of Bcl-x_L upon exogenous over-expression.

HSF1-mediated BAG3 expression attenuates apoptosis in 4-hydroxynonenal-treated colon cancer cells via stabilization of anti-apoptotic Bcl-2 proteins

4-Hydroxynonenal (HNE) is a pro-apoptotic electrophile generated during the spontaneous decomposition of oxidized lipids. We have previously shown that HNE activates the transcription factor, heat shock factor 1 (HSF1), and promotes cytoprotective heat shock gene expression and that silencing HSF1 sensitizes the colon cancer cell line RKO to HNE-induced apoptosis. Here we report a reduction in the anti-apoptotic proteins Bcl-X(L), Mcl-1, and Bcl-2 in HSF1-silenced RKO cells, and we examine the underlying mechanism. To investigate the regulation of the Bcl-2 family by HSF1, microarray analysis of gene expression was performed. We observed that the Hsp70 co-chaperone, BAG3 (Bcl-2-associated athanogene domain 3), is strongly induced by HNE in control but not in HSF1-silenced colon cancer cells. Silencing BAG3 expression with small interfering RNA caused a dramatic reduction in Bcl-X(L), Mcl-1, and Bcl-2 protein levels in colon cancer cells and increased apoptosis, similar to the effect of silencing HSF1. Also, immunoprecipitation experiments indicate specific interactions between BAG3, Hsp70, and the Bcl-2 family member, Bcl-X(L). Overall, our data reveal that BAG3 is HSF1-inducible and has a unique role facilitating cancer cell survival during pro-apoptotic stress by stabilizing the level of Bcl-2 family proteins.

Is the focus moving toward a combination of targeted drugs?

The concept of combining targeted agents for the treatment of acute myeloid leukemia (AML) is a relatively new but potentially promising area of investigation. A number of targeted agents may have limited single-agent activity but could show significant promise when used in conjunction with other types of similar compounds. Combinations of targeted agents may effectively interrupt multiple pathways in either a linear or parallel fashion. There are currently numerous combination regimens under investigation at either the preclinical or clinical levels, including histone deacetylase (HDAC) and CDK inhibitors; HDAC and proteasome inhibitors; HDAC and NF-kappaB (IKKbeta) inhibitors; CHK1 and MEK1/2 inhibitors; and BCL-2 antagonists and CDK inhibitors. Although combinations of targeted agents will not displace conventional cytotoxic regimens in AML or related disorders in the foreseeable future, these combinations clearly warrant further attention.

New drugs for the treatment of lymphoma

Historically, most drugs developed for treatment of leukemias, lymphomas, and myeloma had already been studied in the solid tumor setting. Nearly 10 years ago, chronic myelogenous leukemia (CML) forever changed this paradigm. Imatinib showed that it was possible to nullify the pathognomic genetic lesion in a hematologic malignancy. Since the approval of imatinib for CML, a host of new drugs active in blood cancers have emerged. This article highlights some areas of innovative drug development in lymphoma where possible; it emphasizes the biologic basis for the approach, linking this essential biology to the biochemical pharmacology. The article focuses on the many new targets including Syk, Bcl-2, CD-40, and the phosphoinositide-3 kinase/AKT/mammalian target of rapamycin pathway.

Defining the target specificity of ABT-737 and synergistic antitumor activities in combination with histone deacetylase inhibitors

The apoptotic and therapeutic activities of the histone deacetylase inhibitor (HDACi) vorinostat are blocked by overexpression of Bcl-2 or Bcl-X(L). Herein, we used the small molecule inhibitor ABT-737 to restore sensitivity of Emu-myc lymphomas overexpressing Bcl-2 or Bcl-X(L) to vorinostat and valproic acid (VPA). Combining low-dose ABT-737 with vorinostat or VPA resulted in synergistic apoptosis of these cells. ABT-737 was ineffective against Emu-myc/Mcl-1 and Emu-myc/A1 cells either as a single agent or in combination with HDACi. However, in contrast to the reported binding specificity data, Emu-myc/Bcl-w lymphomas were insensitive to ABT-737 used alone or in combination with HDACi, indicating that the regulatory activity of ABT-737 is restricted to Bcl-2 and Bcl-X(L). Emu-myc lymphomas that expressed Bcl-2 throughout the tumorigenesis process were especially sensitive to ABT-737, while those forced to overexpress Mcl-1 were not. This supports the notion that tumor cells "addicted" to ABT-737 target proteins (ie, Bcl-2 or Bcl-X(L)) are likely to be the most sensitive target cell population. Our studies provide important preclinical data on the binding specificity of ABT-737 and its usefulness against primary hematologic malignancies when used as a single agent and in combination with HDACi.

In vivo efficacy of the Bcl-2 antagonist ABT-737 against aggressive Myc-driven lymphomas

Deregulated Myc expression drives many human cancers, including Burkitt's lymphoma and a highly aggressive subset of diffuse large cell lymphomas. Myc-driven tumors often display resistance to chemotherapeutics because of acquisition of mutations that impair the apoptosis pathway regulated by the Bcl-2 protein family. Given the need to identify new therapies for such lymphomas, we have evaluated the efficacy of ABT-737, a small molecule that mimics the action of the BH3-only proteins, natural antagonists of the prosurvival Bcl-2 proteins. ABT-737 selectively targets certain prosurvival proteins (Bcl-2, Bcl-x(L), and Bcl-w) but not others (Mcl-1 and A1). We treated mice transplanted with lymphomas derived either from Emu-myc transgenic mice or Emu-myc mice that also expressed an Emu-bcl-2 transgene. As a single agent, ABT-737 significantly prolonged the survival of mice transplanted with the myc/bcl-2 lymphomas but was ineffective for the myc lymphomas, probably because of the relatively higher Mcl-1 levels found in the latter. Strikingly, when combined with low-dose cyclophosphamide, ABT-737 produced sustained disease-free survival of all animals transplanted with two of three myc/bcl-2 lymphomas tested. The combination therapy was also more effective against some myc lymphomas than treatment with either agent alone. Our data suggest that antagonism of Bcl-2 with small organic compounds is an attractive approach to enhance the efficacy of conventional therapy for the treatment of Myc-driven lymphomas that over-express this prosurvival molecule.

BH3 activation blocks Hdmx suppression of apoptosis and cooperates with Nutlin to induce cell death

The Hdmx protein restricts p53 activity in vivo and is overexpressed in a significant fraction of human tumors that retain the wild type p53 allele. An understanding of how Hdmx limits p53 activation and blocks apoptosis could therefore lead to development of novel therapeutic agents. We previously showed that Hdmx modulates tumor cell sensitivity to Nutlin-3a, a potent antagonist of the p53/Hdm2 interaction. In this report, we demonstrate that this also applies to MI-219, another Hdm2 antagonist. Thus, the inability to disrupt Hdmx/p53 complexes is a potential barrier to the efficacy of these compounds as single agents. We show that sensitivity to apoptosis in cells with high Hdmx levels is restored by combined treatment with Nutlin and a Bcl-2 family member antagonist to activate Bax. The data are consistent with a model in which Hdmx attenuates p53-dependent activation of the intrinsic apoptotic pathway, and that this occurs upstream of Bax activation. Thus, selectively inhibiting Hdm2 and activating Bax is one effective strategy to induce apoptosis in tumors with high Hdmx levels. Our findings also indicate that preferential induction of apoptosis in tumor versus normal cells occurs using appropriate drug doses.

Inhibition of proteasomal degradation of Mcl-1 by cobalt chloride suppresses cobalt chloride-induced apoptosis in HCT116 colorectal cancer cells

Cobalt promotes apoptosis in multiple cell systems, however, the molecular mechanisms that influence cobalt-induced apoptosis are not fully understood. We investigated mechanisms of cobalt chloride induced apoptosis in HCT116 colorectal cancer cells. Cobalt chloride induced dose dependent apoptosis in HCT116 cells (250-750 muM) which, at higher concentrations (500-750 muM), was associated with an increase in the expression of the Bcl-2-related Mcl-1 survival protein. Cobalt chloride caused the accumulation of higher molecular weight ubiquitin-conjugates of Mcl-1 in intact HCT116 cells and inhibited the activity of the trypsin-like site of the 20S proteasome in an in vitro assay. Although siRNA-mediated knockdown of Mcl-1 increased apoptosis in HCT116 cells, the combination of Mcl-1 siRNA and cobalt chloride induced very high levels of cell killing. Therefore, inhibition of the proteasome by cobalt chloride leads to the accumulation of Mcl-1 which acts to limit cobalt chloride induced apoptosis.

Regulation of antiapoptotic MCL-1 function by gossypol: mechanistic insights from in vitro reconstituted systems

Small-molecule drugs that induce apoptosis in tumor cells by activation of the BCL-2-regulated mitochondrial outer membrane permeabilization (MOMP) pathway hold promise for rational anticancer therapies. Accumulating evidence indicates that the natural product gossypol and its derivatives can kill tumor cells by targeting antiapoptotic BCL-2 family members in such a manner as to trigger MOMP. However, due to the inherent complexity of the cellular apoptotic network, the precise mechanisms by which interactions between gossypol and individual BCL-2 family members lead to MOMP remain poorly understood. Here, we used simplified systems bearing physiological relevance to examine the impact of gossypol on the function of MCL-1, a key determinant for survival of various human malignancies that has become a highly attractive target for anticancer drug design. First, using a reconstituted liposomal system that recapitulates basic aspects of the BCL-2-regulated MOMP pathway, we demonstrate that MCL-1 inhibits BAX permeabilizing function via a "dual-interaction" mechanism, while submicromolar concentrations of gossypol reverse MCL-1-mediated inhibition of functional BAX activation. Solution-based studies showed that gossypol competes with BAX/BID BH3 ligands for binding to MCL-1 hydrophobic groove, thereby providing with a mechanistic explanation for how gossypol restores BAX permeabilizing function in the presence of MCL-1. By contrast, no evidence was found indicating that gossypol transforms MCL-1 into a BAX-like pore-forming molecule. Altogether, our findings validate MCL-1 as a direct target of gossypol, and highlight that making this antiapoptotic protein unable to inhibit BAX-driven MOMP may represent one important mechanism by which gossypol exerts its cytotoxic effect in selected cancer cells.

ABT-737 induces expression of the death receptor 5 and sensitizes human cancer cells to TRAIL-induced apoptosis

Because Bcl-2 family members inhibit the ability of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) to induce apoptosis, we investigated whether ABT-737, a small molecule Bcl-2 inhibitor, enhances TRAIL killing. We demonstrate that a combination of ABT-737 and TRAIL induced significant cell death in multiple cancer types, including renal, prostate, and lung cancers, although each agent individually had little activity in these tumor cells. All of these cell lines expressed the Mcl-1 protein that is known to block the activity of ABT-737 and TRAIL but did not block the synergy between these agents. However, Bax-deficient cell lines, including DU145 and HCT116 cells and those cell lines expressing low levels of TRAIL receptor, were resistant to apoptosis induced by these agents. To understand how ABT-737 functions to markedly increase TRAIL sensitivity, the levels of specific death-inducing signaling complex components were evaluated. Treatment with ABT-737 did not change the levels of c-FLIP, FADD, and caspase-8 but up-regulated the levels of the TRAIL receptor DR5. DR5 up-regulation induced by ABT-737 treatment occurred through a transcriptional mechanism, and mutagenesis studies demonstrated that the NF-kappaB site found in the DR5 promoter was essential for the ability of ABT-737 to increase the levels of this mRNA. Using luciferase reporter plasmids, ABT-737 was shown to stimulate NF-kappaB activity. Together, these results demonstrate that the ability of ABT-737 and TRAIL to induce apoptosis is mediated through activation of both the extrinsic and intrinsic pathways. Combinations of ABT-737 and TRAIL can be exploited therapeutically where antiapoptotic Bcl-2 family members drive tumor cell resistance to current anticancer therapies.

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.

ABT-263: a potent and orally bioavailable Bcl-2 family inhibitor

Overexpression of the prosurvival Bcl-2 family members (Bcl-2, Bcl-xL, and Mcl-1) is commonly associated with tumor maintenance, progression, and chemoresistance. We previously reported the discovery of ABT-737, a potent, small-molecule Bcl-2 family protein inhibitor. A major limitation of ABT-737 is that it is not orally bioavailable, which would limit chronic single agent therapy and flexibility to dose in combination regimens. Here we report the biological properties of ABT-263, a potent, orally bioavailable Bad-like BH3 mimetic (K(i)'s of <1 nmol/L for Bcl-2, Bcl-xL, and Bcl-w). The oral bioavailability of ABT-263 in preclinical animal models is 20% to 50%, depending on formulation. ABT-263 disrupts Bcl-2/Bcl-xL interactions with pro-death proteins (e.g., Bim), leading to the initiation of apoptosis within 2 hours posttreatment. In human tumor cells, ABT-263 induces Bax translocation, cytochrome c release, and subsequent apoptosis. Oral administration of ABT-263 alone induces complete tumor regressions in xenograft models of small-cell lung cancer and acute lymphoblastic leukemia. In xenograft models of aggressive B-cell lymphoma and multiple myeloma where ABT-263 exhibits modest or no single agent activity, it significantly enhances the efficacy of clinically relevant therapeutic regimens. These data provide the rationale for clinical trials evaluating ABT-263 in small-cell lung cancer and B-cell malignancies. The oral efficacy of ABT-263 should provide dosing flexibility to maximize clinical utility both as a single agent and in combination regimens.

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.

Mechanism of synergy of N-(4-hydroxyphenyl)retinamide and ABT-737 in acute lymphoblastic leukemia cell lines: Mcl-1 inactivation

BACKGROUND

ABT-737 is a pan-Bcl-2 inhibitor that has a wide range of single-agent activity against acute lymphoblastic leukemia (ALL) cell lines and xenografts. A relationship between expression of myeloid cell leukemia 1 (Mcl-1), an antiapoptotic member of the Bcl-2 family of proteins, and resistance to ABT-737 has been reported for various cancers. The synthetic cytotoxic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) is known to generate reactive oxygen species (ROS), and ROS have been shown to activate c-Jun kinase (JNK), which in turn phosphorylates and inhibits Mcl-1. Thus, we investigated whether 4-HPR-mediated inactivation of Mcl-1 could act synergistically with ABT-737 to promote leukemia cell death.

METHODS

Cytotoxicity was determined using the fluorescence-based DIMSCAN assay. Synergy was defined as a combination index (CIN) less than 1. The expression of Bcl-2 family messenger RNAs was measured by real-time reverse transcription-polymerase chain reaction, and caspase activity was measured enzymatically. Changes in Bcl-2 family proteins and release of mitochondrial cytochrome c were detected by immunoblotting. ROS, apoptosis, mitochondrial membrane depolarization, and phospho-JNK were measured by flow cytometry. Gene silencing was by small interfering RNA (siRNA). All statistical tests were two-sided.

RESULTS

ABT-737 decreased Mcl-1 protein expression in ABT-737-sensitive ALL cell lines but not in ABT-737-resistant lines. Using the antioxidant ascorbic acid and siRNA-mediated knockdown of JNK, we showed that 4-HPR decreased Mcl-1 via ROS generation (that phosphorylates JNK) in ABT-737-resistant cell lines. Combining ABT-737 with 4-HPR enhanced the mitochondrial apoptotic cascade (percentage of cells with depolarized mitochondrial membrane at 6 hours, ABT-737 vs ABT-737 plus 4-HPR: 24.5% vs 45.5%, difference = 20.1%, 95% CI = 18.9% to 13.9%; P < .001) and caused caspase-dependent, synergistic multilog cytotoxicity in all seven ALL cell lines examined (mean CIN = 0.57, 95% CI = 0.37 to 0.87), with minimal cytotoxicity for normal lymphocytes.

CONCLUSIONS

An increase of Mcl-1 protein in response to ABT-737 is one mechanism of ABT-737 resistance that can be overcome by 4-HPR, resulting in synergistic cytotoxicity of ABT-737 combined with 4-HPR in ALL cell lines.

Targeting the Bcl-2-regulated apoptosis pathway by BH3 mimetics: a breakthrough in anticancer therapy?

Induction of apoptosis in tumor cells by direct activation of the Bcl-2-regulated apoptosis pathway by small molecule drugs carries high hopes to overcome the shortcomings of current anticancer therapies. This novel therapy concept builds on emerging insights into how Bcl-2-like molecules maintain mitochondrial integrity and how pro-apoptotic BH3-only proteins lead to its disruption. Means to unleash the pro-apoptotic potential of BH3-only proteins in tumor cells, or to bypass the need for BH3-only proteins by directly blocking possible interactions of Bcl-2-like pro-survival molecules with Bax and/or Bak, constitute interesting options for the design of novel anticancer therapies. For the optimization and clinical implementation of these novel anticancer strategies, a detailed understanding of the role of individual BH3-only proteins in cell death signaling in healthy cells and during tumor suppression is required. In this review, we will touch on the latest findings on BH3-only protein function and attempts to define the molecular properties of the so-called 'BH3 mimetics,' a novel class of anticancer agents, able to prompt apoptosis in tumor cells, regardless of their p53 or Bcl-2 status.

BCL-2 family proteins: critical checkpoints of apoptotic cell death

Apoptosis is a morphologically distinct form of programmed cell death essential for normal development and tissue homeostasis. Aberrant regulation of this pathway is linked to multiple human diseases, including cancer, autoimmunity, neurodegenerative disorders, and diabetes. The BCL-2 family of proteins constitutes a critical control point in apoptosis residing immediately upstream of irreversible cellular damage, where family members control the release of apoptogenic factors from mitochondria. The cardinal member of this family, BCL-2, was originally discovered as the defining oncogene in follicular lymphomas, located at one reciprocal breakpoint of the t(14;18) (q32;q21) chromosomal translocation. Since this original discovery, remarkable efforts marshaled by many investigators around the world have advanced our knowledge of the basic biology, molecular mechanisms, and therapeutic targets in the apoptotic pathway. This review highlights findings from many laboratories that have helped uncover some of the critical control points in apoptosis. The emerging picture is that of an intricate cellular machinery orchestrated by tightly regulated molecular interactions and conformational changes within BCL-2 family proteins that ultimately govern the cellular commitment to apoptotic death.

A novel paradigm for rapid ABT-737-induced apoptosis involving outer mitochondrial membrane rupture in primary leukemia and lymphoma cells

Primary chronic lymphocytic leukemia (CLL) cells are exquisitely sensitive to ABT-737, a small molecule BCL2-antagonist, which induces many of the classical biochemical and ultrastructural features of apoptosis, including BAX/BAK oligomerization, cytochrome c release, caspase activation and chromatin condensation. Surprisingly, ABT-737 also induces mitochondrial inner membrane permeabilization (MIMP) resulting in mitochondrial matrix swelling and rupture of the outer mitochondrial membrane (OMM), so permitting the rapid efflux of cytochrome c from mitochondrial cristae and facilitating rapid caspase activation and apoptosis. BAX and BAK appear to be involved in the OMM discontinuities as they localize to the OMM break points. Notably, ABT-737 induced mitochondrial matrix swelling and OMM discontinuities in other primary B-cell malignancies, including mantle cell, follicular and marginal zone lymphoma cells but not in several cell lines studied. Thus, we describe a new paradigm of apoptosis in primary B-cell malignancies, whereby targeting of BCL2 results in all the classical features of apoptosis together with OMM rupture independent of caspase activation. This mechanism may be far more prevalent than hitherto recognized due to the failure of most methods, used to measure apoptosis, to recognize such a mechanism.

Inducing apoptosis and enhancing chemosensitivity to gemcitabine via RNA interference targeting Mcl-1 gene in pancreatic carcinoma cell

PURPOSE

Resistance to chemotherapy is a major cause of treatment failure and poor prognosis in pancreatic carcinoma. Myeloid cell leukemia-1 (Mcl-1) is highly up-regulated in pancreatic carcinoma and is associated with the anti-apoptosis and the resistance to chemotherapy drugs. Suppression of Mcl-1 would be an approach to induce apoptosis and enhance the chemosensitivity.

METHODS

In this study, three pancreatic cancer cell lines (PANC-1, BxPC-3 and SW1900) stably expressing shRNAs targeting Mcl-1 gene were established and gene expression inhibition was assessed by Real-Time QPCR and Western blotting. The effects of Mcl-1 downregulation mediated by RNAi were explored in vitro and in vivo.

RESULTS

We showed that the specific downregulation of Mcl-1 strikingly inhibited cell growth, colony formation, cell cycle arrest and induced apoptosis in pancreatic cancer cells in vitro, and markedly decreased the tumorigenicity in a mouse xenograft model. Moreover, knockdown of Mcl-1 significantly increased the chemosensitivity to Gemcitabine in pancreatic carcinoma cells.

CONCLUSIONS

Our data suggests that the specific downregulation of Mcl-1 by RNAi is a promising approach to induce apoptosis and enhance the chemosensitivity for pancreatic carcinoma gene therapy.

A novel BH3 ligand that selectively targets Mcl-1 reveals that apoptosis can proceed without Mcl-1 degradation

Like Bcl-2, Mcl-1 is an important survival factor for many cancers, its expression contributing to chemoresistance and disease relapse. However, unlike other prosurvival Bcl-2–like proteins, Mcl-1 stability is acutely regulated. For example, the Bcl-2 homology 3 (BH3)–only protein Noxa, which preferentially binds to Mcl-1, also targets it for proteasomal degradation. In this paper, we describe the discovery and characterization of a novel BH3-like ligand derived from Bim, Bim_S2A, which is highly selective for Mcl-1. Unlike Noxa, Bim_S2A is unable to trigger Mcl-1 degradation, yet, like Noxa, Bim_S2A promotes cell killing only when Bcl-x_L is absent or neutralized. Furthermore, killing by endogenous Bim is not associated with Mcl-1 degradation. Thus, functional inactivation of Mcl-1 does not always require its elimination. Rather, it can be efficiently antagonized by a BH3-like ligand tightly engaging its binding groove, which is confirmed here with a structural study. Our data have important implications for the discovery of compounds that might kill cells whose survival depends on Mcl-1.

A chemical genomics screen highlights the essential role of mitochondria in HIF-1 regulation

Hypoxia-inducible factor-1 (HIF-1) plays an essential role in tumor development and progression by regulating genes that are vital for proliferation, glycolysis, angiogenesis, and metastasis. To identify strategies of targeting the HIF-1 pathway, we screened a siRNA library against the entire druggable genome and a small-molecule library consisting of 691,200 compounds using a HIF-1 reporter cell line. Although the siRNA library screen failed to reveal any druggable targets, the small-molecule library screen identified a class of alkyliminophenylacetate compounds that inhibit hypoxia-induced HIF-1 reporter activity at single-digit nanomolar concentrations. These compounds were found to inhibit hypoxia but not deferoxamine-induced HIF-1alpha protein stabilization. Further analysis indicated that the alkyliminophenylacetate compounds likely inhibit the HIF-1 pathway through blocking the hypoxia-induced mitochondrial reactive oxygen species (ROS) production. Strikingly, all of the nonalkyliminophenylacetate HIF-1 inhibitors identified from the small-molecule library screen were also found to target mitochondria like the alkyliminophenylacetate compounds. The exclusive enrichment of mitochondria inhibitors from a library of >600,000 diverse compounds by using the HIF-1 reporter assay highlights the essential role of mitochondria in HIF-1 regulation. These results also suggest that targeting mitochondrial ROS production might be a highly effective way of blocking HIF-1 activity in tumors.

BH3 mimetics to improve cancer therapy; mechanisms and examples

Tumor cell survival is highly dependent on the expression of certain pro-survival Bcl-2 family proteins. An attractive therapeutic approach is to inhibit these proteins using agents that mimic the Bcl-2 homology 3 (BH3) domains of the proapoptotic Bcl-2 family members, which neutralize these proteins by binding to their surface hydrophobic grooves. A number of BH3 mimetic peptides and small molecules have been described, a few of which have advanced into clinical trials. Recent studies have highlighted ABT-737, a bona fide BH3 mimetic and potent inhibitor of antiapoptotic Bcl-2 family members, as a promising anticancer agent. This review summarizes recent advances in understanding the mechanisms of action of BH3 domains and several classes of BH3 mimetics, as well as the prospects of using these agents to improve cancer therapy.

BCL-2 family regulation by the 20S proteasome inhibitor bortezomib

Bortezomib (Velcade, PS341) was licensed in 2003 as a first-in-class 20S proteasome inhibitor indicated for treatment of multiple myeloma, and is currently being evaluated clinically in a range of solid tumours. The mechanisms underlying its cancer cell toxicity are complex. A growing body of evidence suggests proteasome inhibition-dependent regulation of the BCL-2 family is a critical requirement. In particular, the stabilization of BH3-only proteins BIK, NOXA and BIM, appear to be essential for effecting BAX- and BAK-dependent cell death. These mechanisms are reviewed and the implications for favourable novel drug interactions are highlighted.

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.

Bcl-2-regulated apoptosis: mechanism and therapeutic potential

Apoptosis is essential for tissue homeostasis, particularly in the hematopoietic compartment, where its impairment can elicit neoplastic or autoimmune diseases. Whether stressed cells live or die is largely determined by interplay between opposing members of the Bcl-2 protein family. Bcl-2 and its closest homologs promote cell survival, but two other factions promote apoptosis. The BH3-only proteins sense and relay stress signals, but commitment to apoptosis requires Bax or Bak. The BH3-only proteins appear to activate Bax and Bak indirectly, by engaging and neutralizing their pro-survival relatives, which otherwise constrain Bax and Bak from permeabilizing mitochondria. The Bcl-2 family may also regulate autophagy and mitochondrial fission/fusion. Its pro-survival members are attractive therapeutic targets in cancer and perhaps autoimmunity and viral infections.

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.

Targeting multiple arms of the apoptotic regulatory machinery

ABT-737 targets Bcl-2/Bcl-xL but not Mcl-1, which confers resistance to this novel agent. Here, we summarize recent findings indicating that Mcl-1 represents a critical determinant of ABT-737 sensitivity and resistance, and that Mcl-1 down-regulation by various pharmacologic agents or genetic approaches dramatically increases ABT-737 lethality in diverse malignant cell types. These findings also show that the multidomain proapoptotic proteins Bax and Bak play important functional roles in ABT-737-mediated apoptosis, and that Bak activation is essential in potentiation of ABT-737 lethality by agents that down-regulate Mcl-1. Collectively, these findings suggest a novel therapeutic strategy targeting multiple arms of the apoptotic machinery.