RNA silencing of Mcl-1 enhances ABT-737-mediated apoptosis in melanoma: role for a caspase-8-dependent pathway

Towards new therapeutic approaches for malignant melanoma

Recent progress in understanding the molecular mechanisms of the initiation and progression of melanoma has created new opportunities for developing novel therapeutic modalities to manage this potentially lethal disease. Although at first glance, melanoma carcinogenesis appears to be a chaotic system, it is indeed, arguably, a deterministic multistep process involving sequential alterations of proto-oncogenes, tumour suppressors and miRNA genes. The scope of this article is to discuss the most recent and significant advances in melanoma molecular therapeutics. It is apparent that using single agents targeting solely individual melanoma pathways might be insufficient for long-term survival. However, the outstanding results on melanoma survival observed with novel selective inhibitors of B-RAF, such as PLX4032 give hope that melanoma can be cured. The fact that melanoma develops acquired resistance to PLX4032 emphasises the importance of simultaneously targeting several pathways. Because the most striking feature of melanoma is its unsurpassed ability to metastasise, it is important to implement newer systems for drug delivery adapted from research on stem cells and nanotechnology.

miR-193b Regulates Mcl-1 in Melanoma

MicroRNAs play important roles in gene regulation, and their expression is frequently dysregulated in cancer cells. In a previous study, we reported that miR-193b represses cell proliferation and regulates cyclin D1 in melanoma cells, suggesting that miR-193b could act as a tumor suppressor. Herein, we demonstrate that miR-193b also down-regulates myeloid cell leukemia sequence 1 (Mcl-1) in melanoma cells. MicroRNA microarray profiling revealed that miR-193b is expressed at a significantly lower level in malignant melanoma than in benign nevi. Consistent with this, Mcl-1 is detected at a higher level in malignant melanoma than in benign nevi. In a survey of melanoma samples, the level of Mcl-1 is inversely correlated with the level of miR-193b. Overexpression of miR-193b in melanoma cells represses Mcl-1 expression. Previous studies showed that Mcl-1 knockdown cells are hypersensitive to ABT-737, a small-molecule inhibitor of Bcl-2, Bcl-X(L), and Bcl-w. Similarly, overexpression of miR-193b restores ABT-737 sensitivity to ABT-737-resistant cells. Furthermore, the effect of miR-193b on the expression of Mcl-1 seems to be mediated by direct interaction between miR-193b and seed and seedless pairing sequences in the 3' untranslated region of Mcl-1 mRNA. Thus, this study provides evidence that miR-193b directly regulates Mcl-1 and that down-regulation of miR-193b in vivo could be an early event in melanoma progression.

The combination of BH3-mimetic ABT-737 with the alkylating agent temozolomide induces strong synergistic killing of melanoma cells independent of p53

Metastatic melanoma has poor prognosis and is refractory to most conventional chemotherapies. The alkylating agent temozolomide (TMZ) is commonly used in treating melanoma but has a disappointing response rate. Agents that can act cooperatively with TMZ and improve its efficacy are thus highly sought after. The BH3 mimetic ABT-737, which can induce apoptosis by targeting pro-survival Bcl-2 family members, has been found to enhance the efficacy of many conventional chemotherapeutic agents in multiple cancers. We found that combining TMZ and ABT-737 induced strong synergistic apoptosis in multiple human melanoma cell lines. When the drugs were used in combination in a mouse xenograft model, they drastically reduced tumor growth at concentrations where each individual drug had no significant effect. We found that TMZ treatment elevated p53 levels, and that the pro-apoptotic protein Noxa was elevated in TMZ/ABT-737 treated cells. Experiments with shRNA demonstrated that the synergistic effect of TMZ and ABT-737 was largely dependent on Noxa. Experiments with nutlin-3, a p53 inducer, demonstrated that p53 induction was sufficient for synergistic cell death with ABT-737 in a Noxa-dependent fashion. However, p53 was not necessary for TMZ/ABT-737 synergy as demonstrated by a p53-null line, indicating that TMZ and ABT-737 together induce Noxa in a p53-independent fashion. These results demonstrate that targeting anti-apoptotic Bcl-2 members is a promising method for treating metastatic melanoma, and that clinical trials with TMZ and Bcl-2 inhibitors are warranted.

Targeting Mcl-1 for the therapy of cancer

INTRODUCTION

Human cancers are genetically and epigenetically heterogeneous and have the capacity to commandeer a variety of cellular processes to aid in their survival, growth and resistance to therapy. One strategy is to overexpress proteins that suppress apoptosis, such as the Bcl-2 family protein Mcl-1. The Mcl-1 protein plays a pivotal role in protecting cells from apoptosis and is overexpressed in a variety of human cancers.

AREAS COVERED

Targeting Mcl-1 for extinction in these cancers, using genetic and pharmacological approaches, represents a potentially effectual means of developing new efficacious cancer therapeutics. Here we review the multiple strategies that have been employed in targeting this fundamental protein, as well as the significant potential these targeting agents provide in not only suppressing cancer growth, but also in reversing resistance to conventional cancer treatments.

EXPERT OPINION

We discuss the potential issues that arise in targeting Mcl-1 and other Bcl-2 anti-apoptotic proteins, as well problems with acquired resistance. The application of combinatorial approaches that involve inhibiting Mcl-1 and manipulation of additional signaling pathways to enhance therapeutic outcomes is also highlighted. The ability to specifically inhibit key genetic/epigenetic elements and biochemical pathways that maintain the tumor state represent a viable approach for developing rationally based, effective cancer therapies.

Distribution of Bim determines Mcl-1 dependence or codependence with Bcl-xL/Bcl-2 in Mcl-1-expressing myeloma cells

Dependence on Bcl-2 proteins is a common feature of cancer cells and provides a therapeutic opportunity. ABT-737 is an antagonist of antiapoptotic Bcl-2 proteins and therefore is a good predictor of Bcl-x(L)/Bcl-2 dependence. Surprisingly, analysis of Mcl-1-dependent multiple myeloma cell lines revealed codependence on Bcl-2/Bcl-x(L) in half the cells tested. Codependence is not predicted by the expression level of antiapoptotic proteins, rather through interactions with Bim. Consistent with these findings, acquired resistance to ABT-737 results in loss of codependence through redistribution of Bim to Mcl-1. Overall, these results suggest that complex interactions, and not simply expression patterns of Bcl-2 proteins, need to be investigated to understand Bcl-2 dependence and how to better use agents, such as ABT-737.

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.

Born to be alive: a role for the BCL-2 family in melanoma tumor cell survival, apoptosis, and treatment

The global incidence of melanoma has dramatically increased during the recent decades, yet the advancement of primary and adjuvant therapies has not kept a similar pace. The development of melanoma is often centered on cellular signaling that hyper-activates survival pathways, while inducing a concomitant blockade to cell death. Aberrations in cell death signaling not only promote tumor survival and enhanced metastatic potential, but also create resistance to anti-tumor strategies. Chemotherapeutic agents target melanoma tumor cells by inducing a form of cell death called apoptosis, which is governed by the BCL-2 family of proteins. The BCL-2 family is comprised of anti-apoptotic proteins (e.g., BCL-2, BCL-xL, and MCL-1) and pro-apoptotic proteins (e.g., BAK, BAX, and BIM), and their coordinated regulation and function are essential for optimal responses to chemotherapeutics. Here we will discuss what is currently known about the mechanisms of BCL-2 family function with a focus on the signaling pathways that maintain melanoma tumor cell survival. Importantly, we will critically evaluate the literature regarding how chemotherapeutic strategies directly impact on BCL-2 family function and offer several suggestions for future regimens to target melanoma and enhance patient survival.

Rb-Raf-1 interaction disruptor RRD-251 induces apoptosis in metastatic melanoma cells and synergizes with dacarbazine

Metastatic melanoma is an aggressive cancer with very low response rate against conventional chemotherapeutic agents such as dacarbazine (DTIC). Inhibitor of Rb-Raf-1 interaction RRD-251 was tested against the melanoma cell lines SK-MEL-28, SK-MEL-5, and SK-MEL-2. RRD-251 was found to be a potent inhibitor of melanoma cell proliferation, irrespective of V600E B-Raf mutation status of the cell lines. In a SK-MEL-28 xenograft experiment, RRD-251 exerted a significant suppression of tumor growth compared with vehicle (P = 0.003). Similar to in vitro effects, tumors from RRD-251-treated animals showed decreased Rb-Raf-1 interaction in vivo. Growth suppressive effects of RRD-251 were associated with induction of apoptosis as well as a G(1) arrest, with an accompanying decrease in S-phase cells. RRD-251 inhibited Rb phosphorylation and downregulated E2F1 protein levels in these cells. Real-time PCR analysis showed that RRD-251 caused downregulation of cell-cycle regulatory genes thymidylate synthase (TS) and cdc6 as well as the antiapoptotic gene Mcl-1. Combinatorial treatment of RRD-251 and DTIC resulted in a significantly higher apoptosis in DTIC resistant cell lines SK-MEL-28 and SK-MEL-5, as revealed by increased caspase-3 activity and PARP cleavage. Because aberrant Rb/E2F pathway is associated with melanoma progression and resistance to apoptosis, these results suggest that the Rb-Raf-1 inhibitor could be an effective agent for melanoma treatment, either alone or in combination with DTIC.

Mcl-1 is critical for survival in a subgroup of non-small-cell lung cancer cell lines

Non-small-cell lung cancer (NSCLC) is the most deadly type of cancer in the United States and worldwide. Although new therapy is available, the survival rate of NSCLC patients remains low. One hallmark of cancer cells is defects in the apoptotic cell death program. In this study, we investigate the role of B-cell lymphoma 2 (Bcl-2) family members Bcl-2, Bcl-x(L) and Mcl-1, known to regulate cell survival and death, in a panel of fourteen NSCLC cell lines. NSCLC cell lines express high levels of Mcl-1 and Bcl-x(L), but not Bcl-2. Silencing the expression of Mcl-1 with small interfering RNA (siRNA) oligonucleotides potently killed a subgroup of NSCLC cell lines. In contrast, Bcl-x(L) siRNA had no effect in these lines unless Mcl-1 siRNA was also introduced. Interestingly, high MCL1 to BCL-xl messenger RNA determines whether the cells depend on Mcl-1 for survival. We further investigated the role of Mcl-1 in NSCLC cells using a Mcl-1-dependent cell line, H23. The expression of a complementary DNA containing only the coding region of MCL1 rescued H23 cells from the toxicity of a 3' untranslated region (UTR) targeting Mcl-1 siRNA but not a siRNA targeting the coding region of MCL1. Furthermore, we show that Mcl-1 sequesters the BH3-only protein Noxa and Bim and the apoptotic effector Bak. Not surprisingly, Noxa, Bim, or Bak knockdown partially rescued H23 cells from toxicity mediated by Mcl-1 siRNA to different degrees. Collectively, our results indicate that targeting Mcl-1 may improve therapy for a subset of NSCLC patients.

The Bcl-xL inhibitor, ABT-737, efficiently induces apoptosis and suppresses growth of hepatoma cells in combination with sorafenib

Tumor cells are characterized by uncontrolled proliferation, often driven by activation of oncogenes, and apoptosis resistance. The oncogenic kinase inhibitor sorafenib can significantly prolong median survival of patients with advanced hepatocellular carcinoma (HCC), although the response is disease-stabilizing and cytostatic rather than one of tumor regression. Bcl-xL (B cell lymphoma extra large), an antiapoptotic member of the B cell lymphoma-2 (Bcl-2) family, is frequently overexpressed in HCC. Here, we present in vivo evidence that Bcl-xL overexpression is directly linked to the rapid growth of solid tumors. We also examined whether ABT-737, a small molecule that specifically inhibits Bcl-xL but not myeloid cell leukemia-1 (Mcl-1), could control HCC progression, especially when used with sorafenib. Administration of ABT-737, even at an in vivo effective dose, failed to suppress Huh7 xenograft tumors in mice. ABT-737 caused the levels of Mcl-1 expression to rapidly increase by protein stabilization. This appeared to be related to resistance to ABT-737, because decreasing Mcl-1 expression levels to the baseline by a small interfering RNA-mediated strategy made hepatoma cells sensitive to this agent. Importantly, administration of ABT-737 to Mcl-1 knockout mice induced severe liver apoptosis, suggesting that tumor-specific inhibition of Mcl-1 is required for therapeutic purposes. Sorafenib transcriptionally down-regulated Mcl-1 expression specifically in tumor cells and abolished Mcl-1 up-regulation induced by ABT-737. Sorafenib, not alone but in combination with ABT-737, efficiently induced apoptosis in hepatoma cells. This combination also led to stronger suppression of xenograft tumors than sorafenib alone.

CONCLUSION

Bcl-xL inactivation by ABT-737 in combination with sorafenib was found to be safe and effective for anti-HCC therapy in preclinical models. Direct activation of the apoptosis machinery seems to unlock the antitumor potential of oncogenic kinase inhibitors and may produce durable clinical responses against HCC.

A microRNA screen to identify modulators of sensitivity to BCL2 inhibitor ABT-263 (navitoclax)

Evasion of apoptosis is a known feature of cancer cells. One mechanism of deregulating the apoptotic pathway is through overexpression of antiapoptotic BCL2 family members. ABT-263 (navitoclax) is a first-in-class BCL2 family inhibitor that restores the ability of cancer cells to undergo apoptosis. However, many cancer cells are resistant to ABT-263 due to high levels of a BCL2 family member, MCL1, which is not targeted by the drug. MCL1 expression is regulated transcriptionally, translationally, and through proteasome-mediated degradation. Recently, MCL1 expression was shown to be affected by microRNAs (miRNA). To identify miRNAs that modulate the sensitivity of cancer cells to ABT-263, we screened a library of 810 human miRNA mimics in HCT-116 cells in the presence of ABT-263. The screen revealed 19 miRNAs that sensitize HCT-116 cells to ABT-263. Fifteen of these miRNAs were also shown to sensitize CHL1 melanoma cells to the same agent. We further evaluated 12 of the strongest sensitizers in these cell lines. We found that these sensitizers induced apoptosis only in the presence of ABT-263. In addition, whereas all 12 of these miRNAs reduced MCL1 protein expression, only 10 of them targeted MCL1 through direct binding to the 3'-untranslated region of the gene, raising the possibility that other resistance regulators of MCL1 expression may be identified using our method. Finally, because sensitizing miRNA expression is lower in tumors compared with normal tissues, our data can facilitate the design of miRNA replacement therapies to increase sensitivity to BCL2 antagonists.

ARC synergizes with ABT-737 to induce apoptosis in human cancer cells

Previously, we reported that the nucleoside analogue/transcriptional inhibitor ARC (4-amino-6-hydrazino-7-beta-D-ribofuranosyl-7H-pyrrolo(2,3-d)-pyrimidine-5-carboxamide) was able to induce p53-independent apoptosis in multiple cancer cell lines of different origins. This occurred, at least in part, by the suppression of short-lived, prosurvival member of the Bcl-2 family, Mcl-1. In contrast, we show here that treatment of human cancer cells with the pan-Bcl-2 inhibitor ABT-737 alone led to upregulation of Mcl-1 protein expression. Combination of subapoptotic concentrations of ABT-737 and ARC induced mitochondrial injury and potent caspase-3/caspase-9-dependent apoptosis in a wide variety of human cancer cell lines. These data suggest that the ABT-737/ARC combination, which simultaneously targets Bcl-2 and Mcl-1, may be efficient against human cancer.

Inhibition of p38 MAPK enhances ABT-737-induced cell death in melanoma cell lines: novel regulation of PUMA

The mitogen-activated protein kinase (MAPK) pathway is constitutively activated in the majority of melanomas, promoting cell survival, proliferation and migration. In addition, anti-apoptotic Bcl-2 family proteins Mcl-1, Bcl-xL and Bcl-2 are frequently overexpressed, contributing to melanoma's well-documented chemoresistance. Recently, it was reported that the combination of MAPK pathway inhibition by specific MEK inhibitors and Bcl-2 family inhibition by BH3-mimetic ABT-737 synergistically induces apoptotic cell death in melanoma cell lines. Here we provide the first evidence that inhibition of another key MAPK, p38, synergistically induces apoptosis in melanoma cells in combination with ABT-737. We also provide novel mechanistic data demonstrating that inhibition of p38 increases expression of pro-apoptotic Bcl-2 protein PUMA. Furthermore, we demonstrate that PUMA can be cleaved by a caspase-dependent mechanism during apoptosis and identify what appears to be the PUMA cleavage product. Thus, our findings suggest that the combination of ABT-737 and inhibition of p38 is a promising, new treatment strategy that acts through a novel PUMA-dependent mechanism.

MicroRNA-193b represses cell proliferation and regulates cyclin D1 in melanoma

Cutaneous melanoma is an aggressive form of human skin cancer characterized by high metastatic potential and poor prognosis. To better understand the role of microRNAs (miRNAs) in melanoma, the expression of 470 miRNAs was profiled in tissue samples from benign nevi and metastatic melanomas. We identified 31 miRNAs that were differentially expressed (13 up-regulated and 18 down-regulated) in metastatic melanomas relative to benign nevi. Notably, miR-193b was significantly down-regulated in the melanoma tissues examined. To understand the role of miR-193b in melanoma, functional studies were undertaken. Overexpression of miR-193b in melanoma cell lines repressed cell proliferation. Gene expression profiling identified 314 genes down-regulated by overexpression of miR-193b in Malme-3M cells. Eighteen of these down-regulated genes, including cyclin D1 (CCND1), were also identified as putative miR-193b targets by TargetScan. Overexpression of miR-193b in Malme-3M cells down-regulated CCND1 mRNA and protein by > or = 50%. A luciferase reporter assay confirmed that miR-193b directly regulates CCND1 by binding to the 3'untranslated region of CCND1 mRNA. These studies indicate that miR-193b represses cell proliferation and regulates CCND1 expression and suggest that dysregulation of miR-193b may play an important role in melanoma development.

Acquired activation of the Akt/cyclooxygenase-2/Mcl-1 pathway renders lung cancer cells resistant to apoptosis

Acquired apoptosis resistance plays an important role in acquired chemoresistance in cancer cells during chemotherapy. Our previous observations demonstrated that acquired tumor necrosis factor-related apoptosis-inducing ligand resistance in lung cancer cells was associated with Akt-mediated stabilization of cellular FLICE-like inhibitory protein (c-FLIP) and Mcl-1. In this report, we determined that these cells also have acquired resistance to apoptosis induced by chemotherapeutics such as cisplatin and doxorubicin (Adriamycin), which was detected in vitro in cell cultures and in vivo in xenografted tumors. We further found that cyclooxygenase-2 (COX-2) is dramatically overexpressed in cells with acquired apoptosis resistance. COX-2 seems to be a crucial mediator in acquired apoptosis resistance because suppressing COX-2 activity with a chemical inhibitor or reducing COX-2 protein expression level with COX-2 small interfering RNA dramatically alleviated resistance to therapeutic-induced apoptosis. Inhibiting Akt markedly suppressed COX-2 expression, suggesting COX-2 is a downstream effector of this cell survival kinase-mediated apoptosis resistance. Furthermore, the expression of Mcl-1 but not c-FLIP was significantly reduced when COX-2 was suppressed, and knockdown of Mcl-1 substantially sensitized the cells to apoptosis. Our results establish a novel pathway that consists of Akt, COX-2, and Mcl-1 for acquired apoptosis resistance, which could be a molecular target for circumventing acquired chemoresistance in lung cancer.