Selectively targeting Mcl-1 for the treatment of acute myelogenous leukemia and solid tumors

Ubiquitin-proteasome pathway-mediated regulation of the Bcl-2 family: effects and therapeutic approaches

Proteasomal degradation of proteins represents an important regulatory mechanism in maintaining healthy homeostasis in cells. Deregulation of the ubiquitin-proteasome system is associated with various diseases as it controls protein abundance and turnover in cells. Furthermore, proteasomal regulation of protein turnover rate can determine a cell's response to external stimuli. The Bcl-2 family of proteins is an important group of proteins involved in mediating cell survival or cell death in response to external stimuli. Aberrant overexpression of anti-apoptotic proteins or deletion of pro-apoptotic proteins can lead to the development of cancer. Unsurprisingly, proteasomal degradation of Bcl-2 proteins also serves as an important factor regulating the level of Bcl-2 proteins and thereby affecting the functional outcome of cell death. This review aims to highlight the regulation of the Bcl-2 family of proteins with particular emphasis on proteasomal-mediated degradation pathways and the current literature on the therapeutic approaches targeting the proteasome system.

Comprehensive analysis of the RSK gene family in acute myeloid leukemia determines a prognostic signature for the prediction of clinical prognosis and treatment responses

OBJECTIVE

The prognosis of acute myeloid leukemia (AML) remains poor although the basic and translational research has been highly productive in understanding the genetics and pathopoiesis of AML and a plethora of targeted therapies have been developed. Consequently, it is crucial to deepen the knowledge of molecular pathogenesis underlying AML for the advancement of new treatment options.

METHOD

A RSK gene family-related signature was constructed to investigate whether RSK gene family members were useful in predicting the prognosis of AML patients. The relationship between the RSK gene family-related signature and the infiltration of immune cells was further assessed using the CIBERSORT algorithm. The 'oncoPredict' package was used to analyze relationships between the RSK gene family-related signature and the sensitivity to drugs or small molecules.

RESULTS

Patients were classified into two groups using the RSK gene family-related signature following the median risk score. Overall survival (OS) was significantly longer in patients with low-risk scores than that in patients with high-risk scores as showed by both training and validation datasets. Moreover, the signature was helpful in predicting 1-year, 3-year, and 5-year OS in training and validation datasets. In addition, it was identified that low-risk patients exhibited greater sensitivity to 20 drugs or small molecules and that high-risk patients had higher sensitivity to 38 drugs or small molecules.

CONCLUSION

RSK gene family members, particularly RPS6KA1 and RPS6KA4, may help to predict prognosis for AML patients. Furthermore, RPS6KA1 may serve as a novel drug target for AML.

Protease-activated receptor 2 attenuates doxorubicin-induced apoptosis in colon cancer cells

Drug resistance represents a major problem in cancer treatment. Doxorubicin (adriamycin) is an injectable DNA intercalating drug that halts cancer cell growth by inhibiting topoisomerase 2, but its long-term effectiveness is compromised by onset of resistance. This study demonstrates that expression of the PAR2 gene in human colon adenocarcinoma tissue samples was the highest among 32 different cancer types (n = 10,989), and higher in colon adenocarcinoma tissues (n = 331) than normal colon tissues (n = 308), revealing an association between PAR2 expression and human colon cancer. HT29 cells are a human colorectal adenocarcinoma cell line that is sensitive to the chemotherapeutic drug doxorubicin and also expresses PAR2. We find that PAR2 activation in HT29 cells, either by an endogenous protease agonist (trypsin) or an exogenous peptide agonist (2f-LIGRL-NH2), significantly reduces doxorubicin-induced cell death, reactive oxygen species production, caspase 3/7 activity and cleavage of caspase-8 and caspase-3. Moreover, PAR2-mediated MEK1/2-ERK1/2 pathway induced by 2f-LIGRL-NH2 leads to upregulated anti-apoptotic MCL-1 and Bcl-xL proteins that promote cellular survival. These findings suggest that activation of PAR2 compromises efficacy of doxorubicin in colon cancer. Further support for this conclusion came from experiments with human colon cancer HT29 cells, either with the PAR2 gene deleted or in the presence of a pharmacological antagonist of PAR2, which showed full restoration of all doxorubicin-mediated effects. Together, these findings reveal a strong link between PAR2 activation and signalling in human colon cancer cells and increased survival against doxorubicin-induced cell death. They support PAR2 antagonism as a possible new strategy for enhancing doxorubicin therapy.

Targeting MCL-1 protein to treat cancer: opportunities and challenges

Evading apoptosis has been linked to tumor development and chemoresistance. One mechanism for this evasion is the overexpression of prosurvival B-cell lymphoma-2 (BCL-2) family proteins, which gives cancer cells a survival advantage. Mcl-1, a member of the BCL-2 family, is among the most frequently amplified genes in cancer. Targeting myeloid cell leukemia-1 (MCL-1) protein is a successful strategy to induce apoptosis and overcome tumor resistance to chemotherapy and targeted therapy. Various strategies to inhibit the antiapoptotic activity of MCL-1 protein, including transcription, translation, and the degradation of MCL-1 protein, have been tested. Neutralizing MCL-1's function by targeting its interactions with other proteins via BCL-2 interacting mediator (BIM)S2A has been shown to be an equally effective approach. Encouraged by the design of venetoclax and its efficacy in chronic lymphocytic leukemia, scientists have developed other BCL-2 homology (BH3) mimetics-particularly MCL-1 inhibitors (MCL-1i)-that are currently in clinical trials for various cancers. While extensive reviews of MCL-1i are available, critical analyses focusing on the challenges of MCL-1i and their optimization are lacking. In this review, we discuss the current knowledge regarding clinically relevant MCL-1i and focus on predictive biomarkers of response, mechanisms of resistance, major issues associated with use of MCL-1i, and the future use of and maximization of the benefits from these agents.

Protein-protein interactions: developing small-molecule inhibitors/stabilizers through covalent strategies

The development of small-molecule inhibitors or stabilizers of selected protein-protein interactions (PPIs) of interest holds considerable promise for the development of research tools as well as candidate therapeutics. In this context, the covalent modification of selected residues within the target protein has emerged as a promising mechanism of action to obtain small-molecule modulators of PPIs with appropriate selectivity and duration of action. Different covalent labeling strategies are now available that can potentially allow for a rational, ground-up discovery and optimization of ligands as PPI inhibitors or stabilizers. This review article provides a synopsis of recent developments and applications of such tactics, with a particular focus on site-directed fragment tethering and proximity-enabled approaches.

Myeloid cell leukemia-1 as a candidate prognostic biomarker in cancers: a systematic review and meta-analysis

INTRODUCTION

Studies have shown that myeloma cell leukemia-1 (MCL-1) is associated with the prognosis of patients with cancer. To further validate the prognostic value of MCL-1 in cancer, a meta-analysis was conducted.

METHODS

Six databases were searched using Boolean logic search formulas. Data were extracted from the included literature, and pooled odds ratio, hazard ratio, and 95% confidence interval were calculated to determine the relationship between MCL-1 levels and clinicopathological characteristics and prognosis of patients with cancer. When heterogeneity was found to be significant, a random effects model was used, otherwise, a fixed effects model was used.

RESULTS

Twelve articles were included in this meta-analysis, totaling 2208 patients with cancer across 14 studies. A high MCL-1 expression level was associated with patients with high T stage, M stage, and TNM stage in some cancers. Additionally, high MCL-1 expression was likely to be observed in patients with poorly differentiated digestive system tumors and patients with lung adenocarcinoma. Notably, a higher expression of MCL-1 was found to be associated with shorter overall survival in patients with hematological tumors, digestive system tumors, and lung cancer.

CONCLUSION

MCL-1 may be a prognostic biomarker in patients with some types of cancer.

PD-L1 and MCL-1 markers and the relationship with prognostic characteristics of differentiated thyroid carcinoma

OBJECTIVES

MCL-1 and PD-L1 proteins are related to carcinogenesis mechanisms in differentiated thyroid carcinoma(DTC). Tumor antigens stimulate the expression of PD-1 in immune cells, which binds to PD-L1 of tumor cells, inducing immune escape from the tumor. MCL-1, an anti-apoptotic member of the BCL-2 family, is necessary for the survival of T and B lymphocytes and has a high oncogenic potential. We aim to evaluate the clinical utility and relevance of MCL-1 and PD-L1 in the long-term prognosis of DTC.

METHODS

120 DTC patients after total thyroidectomy and radioiodine therapy followed for a minimum of 2 years were included. Demographic features, tumor histopathology, persistence/recurrence risk, factors associated with outcome, initial response to therapy, persistence or disease-free at the follow-up were related to MCL-1 and PD-L1 immunohistochemical expression and BRAFV600E mutation.

RESULTS

100(83.3%) were women, 46.64 ± 16.73 years old at diagnosis; 37(30.8%) patients were at high, 45(37.5%) of intermediate and 38(31.7%) of low disease recurrence/persistence risk. At the end of follow-up of 124.86 ± 65.36 months, 48(42.5%) had persistent disease. 103(85.8%) patients had papillary thyroid carcinoma (PTC), 17(14.2%) follicular thyroid carcinoma (FTC). In PTC, moderate/strong PD-L1 and MCL-1 expressions were associated to BRAFV600E (p=0.0467; p=0.0044). PD-L1 was also associated with tall cell subtype (p=0.0274). In FTC, weak PD-L1 expression was associated to the largest nodule diameter (p=0.0100). Strong/moderate PD-L1 expression was associated to T2 and the weak expression with T3 in TNM classification (p=0.0490). Moderate MCL-1 expression was associated to smoking (p=0.0350).

CONCLUSIONS

PDL-1, marker of progression of tumor cells and MCL-1, anti-apoptotic marker, were associated with PTC carrying BRAFV600E mutation, while PDL-1 was associated with more aggressive PTC subtype. MCL-1 and PD-L1 could be useful in composing a panel to assess the prognosis of PTC patients. On the other hand, both markers seemed to have lower relevance to FTC patients.

Patent landscape of inhibitors and PROTACs of the anti-apoptotic BCL-2 family proteins

INTRODUCTION

The anti-apoptotic BCL-2 family proteins, such as BCL-2, BCL-XL, and MCL-1, are excellent cancer therapeutic targets. The FDA approval of BCL-2 selective inhibitor venetoclax in 2016 validated the strategy of targeting these proteins with BH3 mimetic small molecule inhibitors.

AREAS COVERED

This review provides an overview of the patent literature between 2016 and 2021 covering inhibitors and PROTACs of the anti-apoptotic BCL-2 proteins.

EXPERT OPINION

Since the FDA approval of venetoclax, tremendous efforts have been made to develop its analogues with improved drug properties. These activities will likely result in new drugs in coming years. Significant progress on MCL-1 inhibitors has also been made, with multiple compounds entering clinical trials. However, MCL-1 inhibition could cause on-target toxicity to normal tissues especially the heart. Similar issue exists with BCL-XL inhibitors, which cause on-target platelet toxicity. To overcome this issue, several strategies have been applied, including prodrug, dendrimer-based drug delivery, antibody-drug conjugate (ADC), and proteolysis targeting chimera (PROTAC); and amazingly, each of these approaches has resulted in a drug candidate entering clinical trials. We envision technologies like ADC and PROTAC could also be utilized to increase the therapeutic index of MCL-1 inhibitors.

c-Myc plays a critical role in the antileukemic activity of the Mcl-1-selective inhibitor AZD5991 in acute myeloid leukemia

Acute myeloid leukemia (AML) is an aggressive disease with a low 5-year overall survival rate of 29.5%. Thus, more effective therapies are in need to prolong survival of AML patients. Mcl-1 is overexpressed in AML and is associated with poor prognosis, representing a promising therapeutic target. The oncoprotein c-Myc is also overexpressed in AML and is a significant prognostic factor. In addition, Mcl-1 is required for c-Myc induced AML, indicating that c-Myc-driven AML harbors a Mcl-1 dependency and co-targeting of Mcl-1 and c-Myc represents a promising strategy to eradicate AML. In this study, we investigated the role of c-Myc in the antileukemic activity of Mcl-1 selective inhibitor AZD5991 and the antileukemic activity of co-targeting of Mcl-1 and c-Myc in preclinical models of AML. We found that c-Myc protein levels negatively correlated with AZD5991 EC₅₀s in AML cell lines and primary patient samples. AZD5991 combined with inhibition of c-Myc synergistically induced apoptosis in AML cell lines and primary patient samples, and cooperatively targeted leukemia progenitor cells. AML cells with acquired resistance to AZD5991 were resensitized to AZD5991 when c-Myc was inhibited. The combination also showed promising and synergistic antileukemic activity in vitro against AML cell lines with acquired resistance to the main chemotherapeutic drug AraC and primary AML cells derived from a patient at relapse post chemotherapy. The oncoprotein c-Myc represents a potential biomarker of AZD5991 sensitivity and inhibition of c-Myc synergistically enhances the antileukemic activity of AZD5991 against AML.

Homoharringtonine is synergistically lethal with BCL-2 inhibitor APG-2575 in acute myeloid leukemia

Background

Despite advances in targeted agent development, effective treatment of acute myeloid leukemia (AML) remains a major clinical challenge. The B-cell lymphoma-2 (BCL-2) inhibitor exhibited promising clinical activity in AML, acute lymphoblastic leukemia (ALL) and diffuse large B-cell lymphoma (DLBCL) treatment. APG-2575 is a novel BCL-2 selective inhibitor, which has demonstrated anti-tumor activity in hematologic malignancies. Homoharringtonine (HHT), an alkaloid, exhibited anti-AML activity.

Methods

The synergistic effects of APG-2575 and HHT were studied in AML cell lines and primary samples. MTS was used to measure the cell viability. Annexin V/propidium iodide staining was used to measure the apoptosis rate by flow cytometry. AML cell xenografted mouse models were established to evaluate the anti-leukemic effect of BCL-2 inhibitor, HHT and their combination in vivo. Western blot was used to determine the expression of related proteins.

Results

APG-2575 showed comparable anti-leukemic effect to the FDA-approved BCL-2 inhibitor ABT-199 in vitro and in vivo. Combined treatment of HHT with APG-2575 synergistically inhibited AML cell growth and engraftment. Mechanistically, HHT promoted degradation of myeloid cell leukemia-1 (MCL-1), which was reported to induce BCL-2 inhibitor resistant, through the PI3K/AKT/GSK3β signaling pathway.

Conclusion

Our results provide an effective AML treatment strategy through combination of APG-2575 and HHT, which is worthy of further clinical research.

Radiation Response of Human Leukemia/Lymphoma Cells was Improved by 7-Geranyloxycoumarin

Objectives

Adult T-cell leukemia/lymphoma (ATLL) is a blood neoplasm with specific geographic distribution. Although radiotherapy is a palliative treatment that provides long-term local control, single use of radiation leads to complications for patients. To introduce a novel multimodal approach against ATLL, we investigated combinatorial effects of 7-geranyloxycoumarin and radiation in vitro.

Methods

Viability of MT-2 cells was determined by resazurin assay upon administration of 7-geranyloxycoumarin alone and followed by radiation. Then, apoptosis was detected by annexin V and propidium iodide, and the expression of candidate genes was analyzed by qPCR.

Results

Findings revealed significant (P<.0001) improvement in radiation effects upon 7-geranyloxycoumarin pretreatment, most notably when cells were pretreated with 5 µg/ml 7-geranyloxycoumarin for 96 h, exposed to 6 Gy radiation and recovered for 48 h. These results were confirmed by flow cytometry, as the percentage of early and late apoptotic cells was increased after combinatorial treatment. In addition, significant (P< .0001) changes in CD44, c-MYC, cFLIPL, BMI-1, NF-κB (Rel A), and P53 expression was induced by 7-geranyloxycoumarin and radiation.

Conclusions

Current research indicated, for the first time, that combinatorial use of 7-geranyloxycoumarin and ionizing radiation could be considered as an effective therapeutic modality for ATLL.

Skp2 stabilizes Mcl-1 and confers radioresistance in colorectal cancer

Overexpression of Skp2 plays a critical role in tumorigenesis and correlates with poor prognosis in human malignancies. Thus, Skp2 has been proposed as an attractive target for anti-tumor interventions. The expression of Skp2 in human colorectal cancer (CRC) and the role of Skp2 in tumorigenic properties and irradiation sensitivities of CRC cells were examined by anchorage-dependent and -independent growth assays, immunoblot, flow cytometry, immunohistochemical staining, ubiquitination analysis, co-immunoprecipitation assay, CRISPR-Cas9-based gene knockout, and xenograft experiments. Skp2 is highly expressed in CRC patient tissues. Blocking Skp2 expression reduces the tumorigenic properties of CRC cells in vitro and in vivo. Depletion of Skp2 confers sensitivity to irradiation of CRC cells. Skp2 deficiency enhances irradiation-induced intrinsic apoptosis by facilitating E3 ligase FBW7-mediated Mcl-1 ubiquitination and degradation. Knockout of Skp2 sensitizes CRC cells to irradiation treatments in vivo. Our findings indicate that Skp2 stabilizes Mcl-1, and targeting Skp2 in combination with traditional radiotherapy might be efficacious in treating CRC.

Circular RNA ATAD1 is upregulated in acute myeloid leukemia and promotes cancer cell proliferation by downregulating miR-34b via promoter methylation

A previous study has reported the oncogenic role of circular RNA (circ)-ATAD1 in gastric cancer. The aim of the present study was to investigate the role of circ-ATAD1 in acute myeloid leukemia (AML). Bone marrow mononuclear cells were collected from 60 patients with AML and 60 healthy controls, followed by RNA isolation and reverse transcription-quantitative PCR to assess the expression of circ-ATAD1 and microRNA (miR)-34b. A subcellular fractionation assay was used to determine the subcellular location of circ-ATAD1 in AML cells. Furthermore, circ-ATAD1 and miR-34b were overexpressed in AML cells to study crosstalk between the two molecules. The effect of circ-ATAD1 overexpression on miR-34b gene methylation was also analyzed by methylation-specific PCR, and the roles of circ-ATAD1 and miR-34b in the regulation of AML cell proliferation were analyzed by BrdU assay. circ-ATAD1 expression was found to be elevated, and inversely correlated with that of miR-34b, in patients with AML. Subcellular fractionation assays showed that circ-ATAD1 was specifically expressed in the nucleus. In addition, circ-ATAD1 overexpression in AML cells decreased miR-34b expression and increased miR-34b gene methylation. Moreover, AML cell proliferation was increased by circ-ATAD1 overexpression, but decreased by miR-34b overexpression, and the effect of circ-ATAD1 overexpression on AML cell proliferation was reduced by miR-34b overexpression. Together, these results indicate circ-ATAD1 as a nucleus-specific circRNA in AML, which promotes AML cell proliferation by downregulating miR-34b via methylation.

Targeting MCL-1 in cancer: current status and perspectives

Myeloid leukemia 1 (MCL-1) is an antiapoptotic protein of the BCL-2 family that prevents apoptosis by binding to the pro-apoptotic BCL-2 proteins. Overexpression of MCL-1 is frequently observed in many tumor types and is closely associated with tumorigenesis, poor prognosis and drug resistance. The central role of MCL-1 in regulating the mitochondrial apoptotic pathway makes it an attractive target for cancer therapy. Significant progress has been made with regard to MCL-1 inhibitors, some of which have entered clinical trials. Here, we discuss the mechanism by which MCL-1 regulates cancer cell apoptosis and review the progress related to MCL-1 small molecule inhibitors and their role in cancer therapy.

Arsenic trioxide synergistically promotes the antileukaemic activity of venetoclax by downregulating Mcl-1 in acute myeloid leukaemia cells

BACKGROUND

The evasion of apoptosis through dysregulated Bcl-2 family members is a hallmark of leukaemia stem cells (LSCs) in acute myeloid leukaemia (AML). Therefore, targeting Bcl-2 with venetoclax has been suggested as an attractive strategy for inducing apoptosis in AML LSCs. However, the selective inhibition of Bcl-2 in AML often leads to upregulation of Mcl-1, another dominant anti-apoptotic Bcl-2 family protein conferring venetoclax resistance.

METHODS

We assessed the combined effect of venetoclax and arsenic trioxide (ATO) on leukaemic cell viability, apoptosis, combination index, and cell cycle in the human LSC-like KG1 and KG1a cells. The synergistic effect of venetoclax and ATO on apoptosis was also examined in primary CD34⁺ and CD34⁺CD38^- LSCs from the bone marrow (BM) of AML patients, and compared with those from healthy donors.

RESULTS

Venetoclax efficiently impaired cell viability and dose-dependently promoted apoptosis when combined with ATO; their synergism was aptly represented by the combination index. The combination of venetoclax and ATO impaired cell cycle progression by restricting cells within the sub-G1 phase and facilitating caspase-dependent apoptotic cell death associated with the loss of mitochondrial membrane potential, while sparing healthy BM haematopoietic stem cells. Mechanistically, ATO mitigated venetoclax-induced upregulation of Mcl-1 by the inhibition of AKT and ERK, along with activation of GSK-3β. This led to the Mcl-1 destabilisation, triggering Noxa and Bim to facilitate apoptosis and the consequent activation of the apoptosis executioner protein Bak. Moreover, the combination promoted phosphorylation of ATM, Chk2, p38, and H2AX, indicating an active DNA damage response.

CONCLUSIONS

Our findings demonstrate the synergistic, preferential antileukaemic effects of venetoclax and ATO on LSCs, providing a rationale for preclinical and clinical trials by combining these agents already being used in clinical practice to treat acute leukaemia.

TAM kinases as regulators of cell death

Receptor Tyrosine Kinases are critical regulators of signal transduction that support cell survival, proliferation, and differentiation. Dysregulation of normal Receptor Tyrosine Kinase function by mutation or other activity-altering event can be oncogenic or can impact the transformed malignant cell so it becomes particularly resistant to stress challenge, have increased proliferation, become evasive to immune surveillance, and may be more prone to metastasis of the tumor to other organ sites. The TAM family of Receptor Tyrosine Kinases (TYRO3, AXL, MERTK) is emerging as important components of malignant cell survival in many cancers. The TAM kinases are important regulators of cellular homeostasis and proper cell differentiation in normal cells as receptors for their ligands GAS6 and Protein S. They also are critical to immune and inflammatory processes. In malignant cells, the TAM kinases can act as ligand independent co-receptors to mutant Receptor Tyrosine Kinases and in some cases (e.g. FLT3-ITD mutant) are required for their function. They also have a role in immune checkpoint surveillance. At the time of this review, the Covid-19 pandemic poses a global threat to world health. TAM kinases play an important role in host response to many viruses and it is suggested the TAM kinases may be important in aspects of Covid-19 biology. This review will cover the TAM kinases and their role in these processes.

Discovery of AZD4573, a Potent and Selective Inhibitor of CDK9 That Enables Short Duration of Target Engagement for the Treatment of Hematological Malignancies

A CDK9 inhibitor having short target engagement would enable a reduction of Mcl-1 activity, resulting in apoptosis in cancer cells dependent on Mcl-1 for survival. We report the optimization of a series of amidopyridines (from compound 2), focusing on properties suitable for achieving short target engagement after intravenous administration. By increasing potency and human metabolic clearance, we identified compound 24, a potent and selective CDK9 inhibitor with suitable predicted human pharmacokinetic properties to deliver transient inhibition of CDK9. Furthermore, the solubility of 24 was considered adequate to allow i.v. formulation at the anticipated effective dose. Short-term treatment with compound 24 led to a rapid dose- and time-dependent decrease of pSer2-RNAP2 and Mcl-1, resulting in cell apoptosis in multiple hematological cancer cell lines. Intermittent dosing of compound 24 demonstrated efficacy in xenograft models derived from multiple hematological tumors. Compound 24 is currently in clinical trials for the treatment of hematological malignancies.

IRF4 Activity Is Required in Established Plasma Cells to Regulate Gene Transcription and Mitochondrial Homeostasis

The transcription factor interferon regulatory factor 4 (IRF4) is critical for the development, maintenance, and function of plasma cells. The mechanism by which IRF4 exerts its action in mature plasma cells has been elusive due to the death of all such cells upon IRF4 loss. While we identify apoptosis as a critical pathway for the death of plasma cells caused by IRF4 loss, we also determine that IRF4 did not regulate the intrinsic apoptotic pathway directly. By using an inducible IRF4 deletion system in the presence of the overexpression of anti-apoptotic BCL2, we identify genes whose expression is coordinated by IRF4 and that in turn specify plasma cell identity and mitochondrial homeostasis.

Mcl-1 targeting strategies unlock the proapoptotic potential of TRAIL in melanoma cells

TNF-related apoptosis-inducing ligand (TRAIL) induces apoptosis selectively in cancer cells. For melanoma, the targeting of TRAIL signaling appears highly attractive, due to pronounced TRAIL receptor expression in tumor tissue. However, mechanisms of TRAIL resistance observed in melanoma cells may limit its clinical use. The Bcl-2 family members are critical regulators of cell-intrinsic apoptotic pathways. Thus, the antiapoptotic Bcl-2 protein myeloid cell leukemia 1 (Mcl-1) is overexpressed in many tumor types and was linked to chemotherapy resistance in melanoma. In this study, we evaluated the involvement of antiapoptotic Bcl-2 proteins (Bcl-2, Bcl-x_L , Bcl-w, Mcl-1, Bcl-A1, and Bcl-B) in TRAIL resistance. They were targeted by small interfering RNA-mediated silencing in TRAIL-sensitive (A-375, Mel-HO) and in TRAIL-resistant melanoma cell lines (Mel-2a, MeWo). This highlighted Mcl-1 as the most efficient target to overcome TRAIL resistance. In this context, we investigated the effects of Mcl-1-targeting microRNAs as well as the Mcl-1-selective inhibitor S63845. Both miR-193b and S63845 resulted in significant enhancement of TRAIL-induced apoptosis, associated with decreased cell viability. Apoptosis induction was mediated by caspase-3 processing as well as by Bax and Bak activation, indicating the critical involvement of intrinsic apoptosis pathways. These data may indicate a high relevance of Mcl-1 targeting also in melanoma therapy. Furthermore, the data may suggest to consider the use of the tumor suppressor miR-193b as a strategy for countering TRAIL resistance in melanoma.

An expert overview of emerging therapies for acute myeloid leukemia: novel small molecules targeting apoptosis, p53, transcriptional regulation and metabolism

INTRODUCTION

Acute myeloid leukemia (AML) is an aggressive malignancy of clonal myeloid precursor cells. Curative therapy has classically involved the use of intensive induction chemotherapy followed by consolidation with additional chemotherapy or allogeneic hematopoietic stem cell transplant. For many patients, such an approach is prohibitive because of high treatment-related toxicities. Advancements in the molecular understanding of AML have led to the introduction of new targeted therapies that are changing the treatment landscape for AML.

AREAS COVERED

We review emerging small molecule inhibitors that have shown preclinical efficacy for the treatment of AML. The compounds discussed affect apoptosis, p53-mediated interactions, transcriptional regulation, and cellular metabolism. We performed a literature search of PubMed and primarily included relevant sources published from 2000 to the present, though earlier sources are also referenced.

EXPERT OPINION

Most clinical trials for AML currently employ novel targeted therapies that demonstrate promising activity in preclinical models. We anticipate that new small molecule inhibitors will continue to enter the clinical realm and alter the treatment paradigm for AML. In a field where clinical advancement was comparatively slow for many years, it appears that we are now starting to see the rapid growth borne out of the deepening molecular understanding of AML.

Shrimp miR-965 induced the human melanoma stem-like cell apoptosis and inhibited their stemness by disrupting the MCL-1-ER stress-XBP1 feedback loop in a cross-species manner

BACKGROUND

Melanoma is a type of aggressive skin cancer with a poor survival rate. The resistance to conventional therapy of this disease is, at least in part, attributed to its cancer stem cell population. However, the mechanism of survival and stemness maintenance of cancer stem cells remains to be investigated.

METHODS

Tumorsphere formation assay was used to study the stem-like property of melanoma stem-like cells (MSLC). Chromatin immunoprecipitation (ChIP), promoter luciferase reporter assay were included for exploring the role of MCL-1 in MSLC and electrophoretic mobility shift assay were used to evaluate the interaction between shrimp miR-965 and human Ago2 protein. Melanoma xenograft nude mice were used to study the inhibition of tumor development.

RESULTS

In the present study, our results showed that myeloid cell leukemia sequence 1 (MCL-1) knocking down induced ER stress and apoptosis, and the expression reduction of stemness associated genes in MSLC, which implied a significant role of MCL-1 in MSLC. Further study indicated that ER stress agonist (tunicamycin) treatment in MSLC results in the translocation of XBP1, an ER stress sensor, into the nucleus to induce MCL-1 expression through direct binding to the - 313- to - 308-bp region of MCL-1 promoter. In addition, we found that a shrimp-derived miRNA (shrimp miR-965) could interact with the human Ago2 protein and suppressed the human MCL-1 expression by binding to the 3' UTR of MCL-1 mRNA, thereby inhibiting the MSLC proliferation and stemness in vitro and in vivo in a cross-species manner.

CONCLUSION

In conclusion, we identified an important role of MCL-1-ER stress-XBP1 feedback loop in the stemness and survival maintenance of MSLC, and shrimp miR-965, a natural food derived miRNA, could regulate MSLC stemness and survival by targeting MCL-1 and disrupting the balance of MCL-1-ER stress-XBP1 feedback loop. In conclusion, this study indicated an important mechanism of the regulation of MSLC stemness and survival, otherwise it also demonstrated the significance of cross-species-derived miRNA as promising natural drugs in melanoma therapy.

DYRK1A suppression restrains Mcl-1 expression and sensitizes NSCLC cells to Bcl-2 inhibitors

Objective: Mcl-1 overexpression confers acquired resistance to Bcl-2 inhibitors in non-small cell lung cancer (NSCLC), but no direct Mcl-1 inhibitor is currently available for clinical use. Thus, novel therapeutic strategies are urgently needed to target Mcl-1 and sensitize the anti-NSCLC activity of Bcl-2 inhibitors. Methods: Cell proliferation was measured using sulforhodamine B and colony formation assays, and apoptosis was detected with Annexin V-FITC staining. Gene expression was manipulated using siRNAs and plasmids. Real-time PCR and Western blot were used to measure mRNA and protein levels. Immunoprecipitation and immunofluorescence were used to analyze co-localization of dual specificity tyrosine-phosphorylation-regulated kinase 1A (DYRK1A) and Mcl-1. Results: Suppression of DYRK1A resulted in reduced Mcl-1 expression in NSCLC cells, whereas overexpression of DYRK1A significantly increased Mcl-1 expression. Suppression of DYRK1A did not alter Mcl-1 mRNA levels, but did result in an accelerated degradation of Mcl-1 protein in NSCLC cells. Furthermore, DYRK1A mediated proteasome-dependent degradation of Mcl-1 in NSCLC cells, and DYRK1A co-localized with Mcl-1 in NSCLC cells and was co-expressed with Mcl-1 in tumor samples from lung cancer patients, suggesting that Mcl-1 may be a novel DYRK1A substrate. We showed that combined therapy with harmine and Bcl-2 antagonists significantly inhibited cell proliferation and induced apoptosis in NSCLC cell lines as well as primary NSCLC cells. Conclusions: Mcl-1 is a novel DYRK1A substrate, and the role of DYRK1A in promoting Mcl-1 stability makes it an attractive target for decreasing Bcl-2 inhibitor resistance.

Saga of Mcl-1: regulation from transcription to degradation

The members of the Bcl-2 family are the central regulators of various cell death modalities. Some of these proteins contribute to apoptosis, while others counteract this type of programmed cell death, thus balancing cell demise and survival. A disruption of this balance leads to the development of various diseases, including cancer. Therefore, understanding the mechanisms that underlie the regulation of proteins of the Bcl-2 family is of great importance for biomedical research. Among the members of the Bcl-2 family, antiapoptotic protein Mcl-1 is characterized by a short half-life, which renders this protein highly sensitive to changes in its synthesis or degradation. Hence, the regulation of Mcl-1 is of particular scientific interest, and the study of Mcl-1 modulators could aid in the understanding of the mechanisms of disease development and the ways of their treatment. Here, we summarize the present knowledge regarding the regulation of Mcl-1, from transcription to degradation, focusing on aspects that have not yet been described in detail.

FLT3-ITD Activates RSK1 to Enhance Proliferation and Survival of AML Cells by Activating mTORC1 and eIF4B Cooperatively with PIM or PI3K and by Inhibiting Bad and BIM

FLT3-ITD is the most frequent tyrosine kinase mutation in acute myeloid leukemia (AML) associated with poor prognosis. We previously found that FLT3-ITD activates the mTORC1/S6K/4EBP1 pathway cooperatively through the STAT5/PIM and PI3K/AKT pathways to promote proliferation and survival by enhancing the eIF4F complex formation required for cap-dependent translation. Here, we show that, in contrast to BCR/ABL causing Ph-positive leukemias, FLT3-ITD distinctively activates the serine/threonine kinases RSK1/2 through activation of the MEK/ERK pathway and PDK1 to transduce signals required for FLT3-ITD-dependent, but not BCR/ABL-dependent, proliferation and survival of various cells, including MV4-11. Activation of the MEK/ERK pathway by FLT3-ITD and its negative feedback regulation by RSK were mediated by Gab2/SHP2 interaction. RSK1 phosphorylated S6RP on S235/S236, TSC2 on S1798, and eIF4B on S422 and, in cooperation with PIM, on S406, thus activating the mTORC1/S6K/4EBP1 pathway and eIF4B cooperatively with PIM. RSK1 also phosphorylated Bad on S75 and downregulated BIM-EL in cooperation with ERK. Furthermore, inhibition of RSK1 increased sensitivities to BH3 mimetics inhibiting Mcl-1 or Bcl-2 and induced activation of Bax, leading to apoptosis, as well as inhibition of proliferation synergistically with inhibition of PIM or PI3K. Thus, RSK1 represents a promising target, particularly in combination with PIM or PI3K, as well as anti-apoptotic Bcl-2 family members, for novel therapeutic strategies against therapy-resistant FLT3-ITD-positive AML.

A-1210477, a selective MCL-1 inhibitor, overcomes ABT-737 resistance in AML

Acute myeloid leukemia (AML) is one of the most common hematological malignancies. It is difficult to treat since it easily develops resistance to therapeutic drugs. Myeloid cell leukemia 1 (MCL-1), BCL-2 and BCL-XL, which belong to the anti-apoptotic group of proteins in the BCL-2 family, are overexpressed in AML. The effects of inhibitors that target anti-apoptotic proteins of the BCL-2 family in AML were evaluated in the present study. MCL-1 protein levels of HL60, MOLM13, OCI-AML3 and MV4-11 cell lines were investigated. Furthermore, following treatment with MCL-1-selective antagonist A-1210477 and/or BCL-2/BCL-XL antagonist ABT-737, cell viability was detected. The chimera rate of human CD45(+) cells of bone marrow from mouse models was analyzed via flow cytometry and immunohistochemistry using murine tissues (lung, spleen and liver). The data revealed that the HL-60 cell line, which exhibited a low MCL-1 protein level, and MOLM-13 and MV4-11 cell lines, whose MCL level was intermediate, were sensitive to ABT-737, whereas OCI-AML3 cells, which exhibited a high MCL-1 level, were insensitive to ABT-737. However, multiple AML mouse models and AML cell lines were sensitive to the MCL-1-selective antagonist A-1210477. The results of the present study indicated that the MCL-1-selective antagonist could overcome the resistance to the BCL-2/BCL-XL antagonist (ABT-737) in vitro and in vivo.

MicroRNAs in cancer cell death pathways: Apoptosis and necroptosis

To protect tissues and the organism from disease, potentially harmful cells are removed through programmed cell death processes, including apoptosis and necroptosis. These types of cell death are critically controlled by microRNAs (miRNAs). MiRNAs are short RNA molecules that target and inhibit expression of many cellular regulators, including those controlling programmed cell death via the intrinsic (Bcl-2 and Mcl-1), extrinsic (TRAIL and Fas), p53-and endoplasmic reticulum (ER) stress-induced apoptotic pathways, as well as the necroptosis cell death pathway. In this review, we discuss the current knowledge of apoptosis and necroptosis pathways and how these are impaired in cancer cells. We focus on how miRNAs disrupt apoptosis and necroptosis, thereby critically contributing to malignancy. Understanding which and how miRNAs and their targets affect cell death pathways could open up novel therapeutic opportunities for cancer patients. Indeed, restoration of pro-apoptotic tumor suppressor miRNAs (apoptomiRs) or inhibition of oncogenic miRNAs (oncomiRs) represent strategies that are currently being trialed or are already applied as miRNA-based cancer therapies. Therefore, better understanding the cancer type-specific expression of apoptomiRs and oncomiRs and their underlying mechanisms in cell death pathways will not only advance our knowledge, but also continue to provide new opportunities to treat cancer.

Targeting intermediary metabolism enhances the efficacy of BH3 mimetic therapy in hematologic malignancies

BH3 mimetics are novel targeted drugs with remarkable specificity, potency and enormous potential to improve cancer therapy. However, acquired resistance is an emerging problem. We report the rapid development of resistance in chronic lymphocytic leukemia cells isolated from patients exposed to increasing doses of navitoclax (ABT-263), a BH3 mimetic. To mimic such rapid development of chemoresistance, we developed simple resistance models to three different BH3 mimetics, targeting BCL-2 (ABT-199), BCL-XL (A-1331852) or MCL-1 (A-1210477), in relevant hematologic cancer cell lines. In these models, resistance could not be attributed to either consistent changes in expression levels of the anti-apoptotic proteins or interactions among different pro- and anti-apoptotic BCL-2 family members. Using genetic silencing, pharmacological inhibition and metabolic supplementation, we found that targeting glutamine uptake and its downstream signaling pathways, namely glutaminolysis, reductive carboxylation, lipogenesis, cholesterogenesis and mammalian target of rapamycin signaling resulted in marked sensitization of the chemoresistant cells to BH3 mimetic-mediated apoptosis. Furthermore, our findings highlight the possibility of repurposing widely used drugs, such as statins, to target intermediary metabolism and improve the efficacy of BH3 mimetic therapy.

Histone deacetylase inhibition in combination with MEK or BCL-2 inhibition in multiple myeloma

Despite recent advances in the treatment of multiple myeloma, patients with this disease still inevitably relapse and become refractory to existing therapies. Mutations in K-RAS, N-RAS and B-RAF are common in multiple myeloma, affecting 50% of patients at diagnosis and >70% at relapse. However, targeting mutated RAS/RAF via MEK inhibition is merely cytostatic in myeloma and largely ineffective in the clinic. We examined mechanisms mediating this resistance and identified histone deacetylase inhibitors as potent synergistic partners. Combining the MEK inhibitor AZD6244 (selumetinib) with the pan-histone deacetylase inhibitor LBH589 (panobinostat) induced synergistic apoptosis in RAS/RAF mutated multiple myeloma cell lines. Interestingly, this synergy was dependent on the pro-apoptotic protein BIM. We determined that while single-agent MEK inhibition increased BIM levels, the protein remained sequestered by antiapoptotic BCL-2 family members. LBH589 dissociated BIM from MCL-1 and BCL-X_L, which allowed it to bind BAX/BAK and thereby initiate apoptosis. The AZD6244/LBH589 combination was specifically active in cell lines with more BIM:MCL-1 complexes at baseline; resistant cell lines had more BIM:BCL-2 complexes. Those resistant cell lines were synergistically killed by combining the BH3 mimetic ABT-199 (venetoclax) with LBH589. Using more specific histone deacetylase inhibitors, i.e. MS275 (entinostat) and FK228 (romidepsin), and genetic methods, we determined that concomitant inhibition of histone deacetylases 1 and 2 was sufficient to synergize with either MEK or BCL-2 inhibition. Furthermore, these drug combinations effectively killed plasma cells from myeloma patients ex vivo. Given the preponderance of RAS/RAF mutations, and the fact that ABT-199 has demonstrated clinical efficacy in relapsed/refractory multiple myeloma, these drug combinations hold prom ise as biomarker-driven therapies.

Targeting sphingosine kinase 2 suppresses cell growth and synergizes with BCL2/BCL-XL inhibitors through NOXA-mediated MCL1 degradation in cholangiocarcinoma

Sphingosine kinase 2 (SPHK2) is a key factor within sphingolipid metabolism, responsible for the conversion of pro-apoptotic sphingosine to the pro-survival sphingosine-1-phosphate. We have previously shown that ABC294640, a first-in-class SPHK2 inhibitor, inhibits growth of cholangiocarcinoma cells. In a Phase I study of ABC294640 in tumors, the best response was achieved in a cholangiocarcinoma patient. These data suggest SPHK2 as a novel therapeutic target of cholangiocarcinoma. However, the antitumor mechanism of ABC294640 in cholangiocarcinoma remains not clear. In the current study, we found that ABC294640 upregulated expression of pro-apoptotic NOXA. In cholangiocarcinoma patients, high NOXA mRNA expression was associated with better overall survival. Also, SPHK2 mRNA expression was negatively correlated with NOXA mRNA expression. NOXA is known to degrade MCL1, an anti-apoptotic BCL2 protein. We showed that ABC294640 directed MCL1 for proteasome degradation. Knockdown of NOXA prevented ABC294640-induced MCL1 degradation and apoptosis. In addition, ABC294640 had a synergistic effect with BCL2/BCL-XL inhibitors ABT-263 and Obatoclax in inhibiting cell growth. Combined treatment with ABC294640 and BCL2/BCL-XL inhibitors induced potent apoptosis. Silencing of MCL1 also potentiated ABT-263-induced cytotoxicity. Furthermore, we found that both SPHK2 and MCL1 protein expression were significantly higher in cholangiocarcinoma than that in nontumoral bile ducts. SPHK2 expression correlated significantly with MCL1 expression. Our study reveals that ABC294640 inhibits cholangiocarcinoma cell growth and sensitizes the antitumor effect of BCL2/BCL-XL inhibitors through NOXA-mediated MCL1 degradation. Combinations of ABC294640 with BCL2/BCL-XL inhibitors may provide novel strategies for the treatment of cholangiocarcinoma.

MCL-1 or BCL-xL-dependent resistance to the BCL-2 antagonist (ABT-199) can be overcome by specific inhibitor as single agents and in combination with ABT-199 in acute myeloid leukemia cells

Aberrant over-expression of BCL-2 family proteins (BCL-2, BCL-xL, MCL-1) are associated with hematological malignancies. Antagonists of BCL-2 family proteins include BCL-2-selective inhibitor ABT-199, MCL-1-selective inhibitor A-1210477, BCL-xL-selective inhibitor A-1155463. In this study, we evaluated their potential inhibitory effectiveness. Our data showed that OCI-AML3 cells and U937 cells were resistant to BCL-2-selective inhibitor ABT-199 in vitro and in vivo, however, while OCI-AML3 cells were sensitive to MCL-1-selective inhibitor A-1210477 in vitro and in vivo, indicating that A-1210477 could counteract the resistance of AML cells to ABT-199 as a single agent in MCL-1-dependent AML cells. U-937 cell line and mouse model were resistant to A-1210477 or ABT-199, and expressed high level of BCL-xL, indicating that BCL-xL might play an important role in the resistance of A-1210477 or ABT-199. Besides, this study also showed that ABT-199 could synergize with A-1210477 in vitro or in vivo.

Inhibition of the STAT5/Pim Kinase Axis Enhances Cytotoxic Effects of Proteasome Inhibitors on FLT3-ITD-Positive AML Cells by Cooperatively Inhibiting the mTORC1/4EBP1/S6K/Mcl-1 Pathway

FLT3-ITD and FLT3-TKD are the most frequent tyrosine kinase mutations in acute myeloid leukemia (AML), with the former conferring a poor prognosis. We have recently revealed that FLT3-ITD confers resistance to the PI3K/AKT pathway inhibitors by protecting the mTORC1/4EBP1/Mcl-1 pathway through Pim kinases induced by STAT5 activation in AML. The proteasome inhibitor bortezomib has recently been reported as a promising agent for treatment of AML. Here, we show that the proteasome inhibitor bortezomib as well as carfilzomib induces apoptosis through the intrinsic pathway more conspicuously in cells transformed by FLT3-TKD than FLT3-ITD. Mechanistically, bortezomib upregulated the stress-regulated protein REDD1 and induced downregulation of the mTORC1 pathway more distinctively in cells transformed by FLT3-TKD than FLT-ITD, while overexpression of Pim-1 partly prevented this downregulation and apoptosis in FLT3-TKD-transformed cells. Genetic enhancement of the REDD1 induction or pharmacological inhibition of STAT5, Pim kinases, mTORC1, or S6K by specific inhibitors, such as pimozide, AZD1208, PIM447, rapamycin, and PF-4708671, accelerated the downregulation of mTORC1/Mcl-1 pathway to enhance bortezomib-induced apoptosis in FLT3-ITD-expressing cells, including primary AML cells, while overexpression of Mcl-1 prevented induction of apoptosis. Thus, FLT3-ITD confers a resistance to the proteasome inhibitors on AML cells by protecting the mTORC1/Mcl-1 pathway through the STAT5/Pim axis, and inhibition of these signaling events remarkably enhances the therapeutic efficacy.

The suppressive effect of arsenic trioxide on TET2-FOXP3-Lyn-Akt axis-modulated MCL1 expression induces apoptosis in human leukemia cells

Arsenic trioxide (ATO) has been reported to inhibit the activity of Ten-eleven translocation methylcytosine dioxygenase (TET). TET modulates FOXP3 expression, while dysregulation of FOXP3 expression promotes the malignant progression of leukemia cells. We examined the role of TET-FOXP3 axis in the cytotoxic effects of ATO on the human acute myeloid leukemia cell line, U937. ATO-induced apoptosis in U937 cells was characterized by activation of caspase-3/-9, mitochondrial depolarization, and MCL1 downregulation. In addition, ATO-treated U937 cells showed ROS-mediated inhibition of TET2 transcription, leading to downregulation of FOXP3 expression and in turn, suppression of FOXP3-mediated activation of Lyn and Akt. Overexpression of FOXP3 or Lyn minimized the suppressive effect of ATO on Akt activation and MCL1 expression. Promoter luciferase activity and chromatin immunoprecipitation assays revealed the crucial role of Akt-mediated CREB phosphorylation in MCL1 transcription. Further, ATO-induced Akt inactivation promoted GSK3β-mediated degradation of MCL1. Transfection of constitutively active Akt expression abrogated ATO-induced MCL1 downregulation. MCL1 overexpression lessened the ATO-induced depolarization of mitochondrial membrane and increased the viability of ATO-treated cells. Thus, our data suggest that ATO induces mitochondria-mediated apoptosis in U937 cells through its suppressive effect on TET2-FOXP3-Lyn-Akt axis-modulated MCL1 transcription and protein stabilization. Our findings also indicate that the same pathway underlies ATO-induced death in human leukemia HL-60 cells.

Identifying the druggable interactome of EWS-FLI1 reveals MCL-1 dependent differential sensitivities of Ewing sarcoma cells to apoptosis inducers

Ewing sarcoma (EwS) is an aggressive pediatric bone cancer in need of more effective therapies than currently available. Most research into novel targeted therapeutic approaches is focused on the fusion oncogene EWSR1-FLI1, which is the genetic hallmark of this disease. In this study, a broad range of 3,325 experimental compounds, among them FDA approved drugs and natural products, were screened for their effect on EwS cell viability depending on EWS-FLI1 expression. In a network-based approach we integrated the results from drug perturbation screens and RNA sequencing, comparing EWS-FLI1-high (normal expression) with EWS-FLI1-low (knockdown) conditions, revealing novel interactions between compounds and EWS-FLI1 associated biological processes. The top candidate list of druggable EWS-FLI1 targets included genes involved in translation, histone modification, microtubule structure, topoisomerase activity as well as apoptosis regulation. We confirmed our in silico results using viability and apoptosis assays, underlining the applicability of our integrative and systemic approach. We identified differential sensitivities of Ewing sarcoma cells to BCL-2 family inhibitors dependent on the EWS-FLI1 regulome including altered MCL-1 expression and subcellular localization. This study facilitates the selection of effective targeted approaches for future combinatorial therapies of patients suffering from Ewing sarcoma.

Fibroblast growth factor receptor inhibition induces loss of matrix MCL1 and necrosis in cholangiocarcinoma

BACKGROUND & AIMS

Myeloid cell leukemia 1 (MCL1), a prosurvival member of the BCL2 protein family, has a pivotal role in human cholangiocarcinoma (CCA) cell survival. We previously reported that fibroblast growth factor receptor (FGFR) signalling mediates MCL1-dependent survival of CCA cells in vitro and in vivo. However, the mode and mechanisms of cell death in this model were not delineated.

METHODS

Human CCA cell lines were treated with the pan-FGFR inhibitor LY2874455 and the mode of cell death examined by several complementary assays. Mitochondrial oxidative metabolism was examined using a XF24 extracellular flux analyser. The efficiency of FGFR inhibition in patient-derived xenografts (PDX) was also assessed.

RESULTS

CCA cells expressed two species of MCL1, a full-length form localised to the outer mitochondrial membrane, and an N terminus-truncated species compartmentalised within the mitochondrial matrix. The pan-FGFR inhibitor LY2874455 induced non-apoptotic cell death in the CCA cell lines associated with cellular depletion of both MCL1 species. The cell death was accompanied by failure of mitochondrial oxidative metabolism and was most consistent with necrosis. Enforced expression of N terminus-truncated MCL1 targeted to the mitochondrial matrix, but not full-length MCL1 targeted to the outer mitochondrial membrane, rescued cell death and mitochondrial function. LY2874455 treatment of PDX-bearing mice was associated with tumour cell loss of MCL1 and cell necrosis.

CONCLUSIONS

FGFR inhibition induces loss of matrix MCL1, resulting in cell necrosis. These observations support a heretofore unidentified, alternative MCL1 survival function, namely prevention of cell necrosis, and have implications for treatment of human CCA.

LAY SUMMARY

Herein, we report that therapeutic inhibition of a cell receptor expressed by bile duct cancer cells resulted in the loss of a critical survival protein termed MCL1. Cellular depletion of MCL1 resulted in the death of the cancer cells by a process characterised by cell rupture. Cell death by this process can stimulate the immune system and has implications for combination therapy using receptor inhibition with immunotherapy.

Mcl-1 targeting could be an intriguing perspective to cure cancer

The Bcl-2 family, which plays important roles in controlling cancer development, is divided into antiapoptotic and proapoptotic members. The change in the balance between these members governs the life and death of the cells. Mcl-1 is an antiapoptotic member of this family and its distribution in normal and cancerous tissues strongly differs from that of Bcl-2. In human cancers, where upregulation of antiapoptotic proteins is common, Mcl-1 expression is regulated independent of Bcl-2 and its inhibition promotes senescence, a major barrier to tumorigenesis. Cancer chemotherapy determines various kinds of responses, such as senescence and autophagy; however, the ideal response to chemotherapy is apoptosis. Mcl-1 is a potent oncogene that is regulated at the transcriptional, posttranscriptional, and posttranslational levels. Mcl-1 is a short-lived protein that, in the NH2 terminal region, contains sites for posttranslational regulation that can lead to proteasomal degradation. The USP9X Mcl-1 deubiquitinase regulates Mcl-1 and the levels of these two proteins are strongly correlated. Mcl-1 has three splicing variants (the antiapoptotic protein Mcl-1L and the proapoptotic proteins Mcl-1S and Mcl-1ES), each contributing toward apoptosis regulation. In cancers responsible for the most deaths in the world, the presence of Mcl-1 is associated with malignant cell growth and evasion of apoptosis. Mcl-1 is also one of the key regulators of cancer stem cells' self-renewal that contributes to tumor survival. A great number of indirect and selective Mcl-1 inhibitors have been produced and some of these have shown efficacy in several clinical trials. Thus, therapeutic manipulation of Mcl-1 can be a useful strategy to combat cancer.

Measurement of BH3-only protein tolerance

The BCL2 family of proteins regulates cellular life and death decisions. Among BCL2 family members, BH3-only proteins have critical roles by neutralizing antiapoptotic family members, as well as directly activating BAX and BAK. Despite widespread occurrence of BH3-only protein upregulation in response to various stresses, this process is rarely quantified. Moreover, it is unclear whether all BH3-only proteins are equipotent at inducing cell death. Here we show that BH3-only proteins increase as much as 15- to 20-fold after various treatments and define a parameter, termed BH3-only tolerance, which measures how many copies of a particular BH3-only protein can be expressed before the majority of cells in a population undergo apoptosis. We not only assess the relative contributions of anti- and proapoptotic BCL2 family members to BH3-only tolerance, but also illustrate how the study of this parameter can be used to understand cellular sensitivity to anticancer drugs and new combinations. These observations provide a new quantitative framework for assessing apoptotic susceptibility under various conditions.

PIK3CA mutations enable targeting of a breast tumor dependency through mTOR-mediated MCL-1 translation

Therapies that efficiently induce apoptosis are likely to be required for durable clinical responses in patients with solid tumors. Using a pharmacological screening approach, we discovered that combined inhibition of B cell lymphoma-extra large (BCL-X_L) and the mammalian target of rapamycin (mTOR)/4E-BP axis results in selective and synergistic induction of apoptosis in cellular and animal models of PIK3CA mutant breast cancers, including triple-negative tumors. Mechanistically, inhibition of mTOR/4E-BP suppresses myeloid cell leukemia-1 (MCL-1) protein translation only in PIK3CA mutant tumors, creating a synthetic dependence on BCL-X_L This dual dependence on BCL-X_L and MCL-1, but not on BCL-2, appears to be a fundamental property of diverse breast cancer cell lines, xenografts, and patient-derived tumors that is independent of the molecular subtype or PIK3CA mutational status. Furthermore, this dependence distinguishes breast cancers from normal breast epithelial cells, which are neither primed for apoptosis nor dependent on BCL-X_L/MCL-1, suggesting a potential therapeutic window. By tilting the balance of pro- to antiapoptotic signals in the mitochondria, dual inhibition of MCL-1 and BCL-X_L also sensitizes breast cancer cells to standard-of-care cytotoxic and targeted chemotherapies. Together, these results suggest that patients with PIK3CA mutant breast cancers may benefit from combined treatment with inhibitors of BCL-X_L and the mTOR/4E-BP axis, whereas alternative methods of inhibiting MCL-1 and BCL-X_L may be effective in tumors lacking PIK3CA mutations.

Synthetic Lethality of Combined Bcl-2 Inhibition and p53 Activation in AML: Mechanisms and Superior Antileukemic Efficacy

Evasion of apoptosis is a hallmark of cancer. Bcl-2 and p53 represent two important nodes in apoptosis signaling pathways. We find that concomitant p53 activation and Bcl-2 inhibition overcome apoptosis resistance and markedly prolong survival in three mouse models of resistant acute myeloid leukemia (AML). Mechanistically, p53 activation negatively regulates the Ras/Raf/MEK/ERK pathway and activates GSK3 to modulate Mcl-1 phosphorylation and promote its degradation, thus overcoming AML resistance to Bcl-2 inhibition. Moreover, Bcl-2 inhibition reciprocally overcomes apoptosis resistance to p53 activation by switching cellular response from G₁ arrest to apoptosis. The efficacy, together with the mechanistic findings, reveals the potential of simultaneously targeting these two apoptosis regulators and provides a rational basis for clinical testing of this therapeutic approach.

FLT3-ITD induces expression of Pim kinases through STAT5 to confer resistance to the PI3K/Akt pathway inhibitors on leukemic cells by enhancing the mTORC1/Mcl-1 pathway

FLT3-ITD is the most frequent tyrosine kinase mutation in acute myeloid leukemia (AML) associated with poor prognosis. We previously reported that activation of STAT5 confers resistance to PI3K/Akt inhibitors on the FLT3-ITD-positive AML cell line MV4-11 and 32D cells driven by FLT3-ITD (32D/ITD) but not by FLT3 mutated in the tyrosine kinase domain (32D/TKD). Here, we report the involvement of Pim kinases expressed through STAT5 activation in acquisition of this resistance. The specific pan-Pim kinase inhibitor AZD1208 as well as PIM447 in combination with the PI3K inhibitor GDC-0941 or the Akt inhibitor MK-2206 cooperatively downregulated the mTORC1/4EBP1 pathway, formation of the eIF4E/eIF4G complex, and Mcl-1 expression leading to activation of Bak and Bax to induce caspase-dependent apoptosis synergistically in these cells. These cooperative effects were enhanced or inhibited by knock down of mTOR or expression of its activated mutant, respectively. Overexpression of Mcl-1 conferred the resistance on 32D/ITD cells to combined inhibition of the PI3K/Akt pathway and Pim kinases, while the Mcl-1-specific BH3 mimetic A-1210477 conquered the resistance of MV4-11 cells to GDC-0941. Furthermore, overexpression of Pim-1 in 32D/TKD enhanced the mTORC1/Mcl-1 pathway and partially protected it from the PI3K/Akt inhibitors or the FLT3 inhibitor gilteritinib to confer the resistance to PI3K/Akt inhibitors. Finally, AZD1208 and GDC-0941 cooperatively inhibited the mTORC1/Mcl-1 pathway and reduced viable cell numbers of primary AML cells from some FLT3-ITD positive cases. Thus, Pim kinases may protect the mTORC1/4EBP1/Mcl-1 pathway to confer the resistance to the PI3K/Akt inhibitors on FLT3-ITD cells and represent promising therapeutic targets.

Pathways and mechanisms of venetoclax resistance

The approval of venetoclax, a 'BH3-mimetic' antagonist of the BCL-2 anti-apoptotic protein, for chronic lymphocytic leukemia represents a major milestone in translational apoptosis research. Venetoclax has already received 'breakthrough' designation for acute myeloid leukemia, and is being studied in many other tumor types. However, resistance to BCL-2 inhibitor monotherapy may rapidly ensue. Several studies have shown that the other two major anti-apoptotic BCL-2 family proteins, BCL-XL and MCL-1, are the main determinants of resistance to venetoclax. This opens up possibilities for rationally combining venetoclax with other targeted agents to circumvent resistance. Here, we summarize the most promising combinations, and highlight those already in clinical trials. There is also increasing recognition that different tumors display different degrees of addiction to individual BCL-2 family proteins, and of the need to refine current 'BH3 profiling' techniques. Finally, the successful clinical development of potent and selective antagonists of BCL-XL and MCL-1 is eagerly awaited.

A Case of AML Characterized by a Novel t(4;15)(q31;q22) Translocation That Confers a Growth-Stimulatory Response to Retinoid-Based Therapy

Here we report the case of a 30-year-old woman with relapsed acute myeloid leukemia (AML) who was treated with all-trans retinoic acid (ATRA) as part of investigational therapy (NCT02273102). The patient died from rapid disease progression following eight days of continuous treatment with ATRA. Karyotype analysis and RNA-Seq revealed the presence of a novel t(4;15)(q31;q22) reciprocal translocation involving the TMEM154 and RASGRF1 genes. Analysis of primary cells from the patient revealed the expression of TMEM154-RASGRF1 mRNA and the resulting fusion protein, but no expression of the reciprocal RASGRF1-TMEM154 fusion. Consistent with the response of the patient to ATRA therapy, we observed a rapid proliferation of t(4;15) primary cells following ATRA treatment ex vivo. Preliminary characterization of the retinoid response of t(4;15) AML revealed that in stark contrast to non-t(4;15) AML, these cells proliferate in response to specific agonists of RARα and RARγ. Furthermore, we observed an increase in the levels of nuclear RARγ upon ATRA treatment. In summary, the identification of the novel t(4;15)(q31;q22) reciprocal translocation opens new avenues in the study of retinoid resistance and provides potential for a new biomarker for therapy of AML.

BIRC6 mediates imatinib resistance independently of Mcl-1

Baculoviral IAP repeat containing 6 (BIRC6) is a member of the inhibitors of apoptosis proteins (IAPs), a family of functionally and structurally related proteins that inhibit apoptosis. BIRC6 has been implicated in drug resistance in several different human cancers, however mechanisms regulating BIRC6 have not been extensively explored. Our phosphoproteomic analysis of an imatinib-resistant chronic myelogenous leukemia (CML) cell line (MYL-R) identified increased amounts of a BIRC6 peptide phosphorylated at S480, S482, and S486 compared to imatinib-sensitive CML cells (MYL). Thus we investigated the role of BIRC6 in mediating imatinib resistance and compared it to the well-characterized anti-apoptotic protein, Mcl-1. Both BIRC6 and Mcl-1 were elevated in MYL-R compared to MYL cells. Lentiviral shRNA knockdown of BIRC6 in MYL-R cells increased imatinib-stimulated caspase activation and resulted in a ~20-25-fold increase in imatinib sensitivity, without affecting Mcl-1. Treating MYL-R cells with CDK9 inhibitors decreased BIRC6 mRNA, but not BIRC6 protein levels. By contrast, while CDK9 inhibitors reduced Mcl-1 mRNA and protein, they did not affect imatinib sensitivity. Since the Src family kinase Lyn is highly expressed and active in MYL-R cells, we tested the effects of Lyn inhibition on BIRC6 and Mcl-1. RNAi-mediated knockdown or inhibition of Lyn (dasatinib/ponatinib) reduced BIRC6 protein stability and increased caspase activation. Inhibition of Lyn also increased formation of an N-terminal BIRC6 fragment in parallel with reduced amount of the BIRC6 phosphopeptide, suggesting that Lyn may regulate BIRC6 phosphorylation and stability. In summary, our data show that BIRC6 stability is dependent on Lyn, and that BIRC6 mediates imatinib sensitivity independently of Mcl-1 or CDK9. Hence, BIRC6 may be a novel target for the treatment of drug-resistant CML where Mcl-1 or CDK9 inhibitors have failed.

The emergence of acid ceramidase as a therapeutic target for acute myeloid leukemia

INTRODUCTION

Acute myeloid leukemia (AML) is the most common adult leukemia. Only a fraction of AML patients will survive with existing chemotherapy regimens. Hence, there is an urgent and unmet need to identify novel targets and develop better therapeutics in AML. In the past decade, the field of sphingolipid metabolism has emerged into the forefront of cancer biology due to its importance in cancer cell proliferation and survival. In particular, acid ceramidase (AC) has emerged as a promising therapeutic target due to its role in neutralizing the pro-death effects of ceramide. Areas covered: This review highlights key information about AML biology as well as current knowledge on dysregulated sphingolipid metabolism in cancer and AML. We describe AC function and dysregulation in cancer, followed by a review of studies that report elevated AC in AML and compounds known to inhibit the enzyme. Expert opinion: AML has a great need for new drug targets and better therapeutic agents. The finding of elevated AC in AML supports the concept that this enzyme represents a novel and realistic therapeutic target for this common leukemia. More effort is needed towards developing better AC inhibitors for clinical use and combination treatment with existing AML therapies.

Dual modulation of MCL-1 and mTOR determines the response to sunitinib

Most patients who initially respond to treatment with the multi-tyrosine kinase inhibitor sunitinib eventually relapse. Therefore, developing a deeper understanding of the contribution of sunitinib's numerous targets to the clinical response or to resistance is crucial. Here, we have shown that cancer cells respond to clinically relevant doses of sunitinib by enhancing the stability of the antiapoptotic protein MCL-1 and inducing mTORC1 signaling, thus evoking little cytotoxicity. Inhibition of MCL-1 or mTORC1 signaling sensitized cells to clinically relevant doses of sunitinib in vitro and was synergistic with sunitinib in impairing tumor growth in vivo, indicating that these responses are triggered as prosurvival mechanisms that enable cells to tolerate the cytotoxic effects of sunitinib. Furthermore, higher doses of sunitinib were cytotoxic, triggered a decline in MCL-1 levels, and inhibited mTORC1 signaling. Mechanistically, we determined that sunitinib modulates MCL-1 stability by affecting its proteasomal degradation. Dual modulation of MCL-1 stability at different dose ranges of sunitinib was due to differential effects on ERK and GSK3β activity, and the latter also accounted for dual modulation of mTORC1 activity. Finally, comparison of patient samples prior to and following sunitinib treatment suggested that increases in MCL-1 levels and mTORC1 activity correlate with resistance to sunitinib in patients.

Binding of Released Bim to Mcl-1 is a Mechanism of Intrinsic Resistance to ABT-199 which can be Overcome by Combination with Daunorubicin or Cytarabine in AML Cells

PURPOSE

To investigate the molecular mechanism underlying intrinsic resistance to ABT-199.

EXPERIMENTAL DESIGN

Western blots and real-time RT-PCR were used to determine levels of Mcl-1 after ABT-199 treatment alone or in combination with cytarabine or daunorubicin. Immunoprecipitation of Bim and Mcl-1 were used to determine the effect of ABT-199 treatment on their interactions with Bcl-2 family members. Lentiviral short hairpin RNA knockdown of Bim and CRISPR knockdown of Mcl-1 were used to confirm their role in resistance to ABT-199. JC-1 assays and flow cytometry were used to determine drug-induced apoptosis.

RESULTS

Immunoprecipitation of Bim from ABT-199-treated cell lines and a primary patient sample demonstrated decreased association with Bcl-2, but increased association with Mcl-1 without corresponding change in mitochondrial outer membrane potential. ABT-199 treatment resulted in increased levels of Mcl-1 protein, unchanged or decreased Mcl-1 transcript levels, and increased Mcl-1 protein half-life, suggesting that the association with Bim plays a role in stabilizing Mcl-1 protein. Combining conventional chemotherapeutic agent cytarabine or daunorubicin with ABT-199 resulted in increased DNA damage along with decreased Mcl-1 protein levels, compared with ABT-199 alone, and synergistic induction of cell death in both AML cell lines and primary patient samples obtained from AML patients at diagnosis.

CONCLUSIONS

Our results demonstrate that sequestration of Bim by Mcl-1 is a mechanism of intrinsic ABT-199 resistance and supports the clinical development of ABT-199 in combination with cytarabine or daunorubicin for the treatment of AML. Clin Cancer Res; 22(17); 4440-51. ©2016 AACR.

A Hippo and Fibroblast Growth Factor Receptor Autocrine Pathway in Cholangiocarcinoma

Herein, we have identified cross-talk between the Hippo and fibroblast growth factor receptor (FGFR) oncogenic signaling pathways in cholangiocarcinoma (CCA). Yes-associated protein (YAP) nuclear localization and up-regulation of canonical target genes was observed in CCA cell lines and a patient-derived xenograft (PDX). Expression of FGFR1, -2, and -4 was identified in human CCA cell lines, driven, in part, by YAP coactivation of TBX5. In turn, FGFR signaling in a cell line with minimal basal YAP expression induced its cellular protein expression and nuclear localization. Treatment of YAP-positive CCA cell lines with BGJ398, a pan-FGFR inhibitor, resulted in a decrease in YAP activation. FGFR activation of YAP appears to be driven largely by FGF5 activation of FGFR2, as siRNA silencing of this ligand or receptor, respectively, inhibited YAP nuclear localization. BGJ398 treatment of YAP-expressing cells induced cell death due to Mcl-1 depletion. In a YAP-associated mouse model of CCA, expression of FGFR 1, 2, and 4 was also significantly increased. Accordingly, BGJ398 treatment was tumor-suppressive in this model and in a YAP-positive PDX model. These preclinical data suggest not only that the YAP and Hippo signaling pathways culminate in an Mcl-1-regulated tumor survival pathway but also that nuclear YAP expression may be a biomarker to employ in FGFR-directed therapy.

Maritoclax and dinaciclib inhibit MCL-1 activity and induce apoptosis in both a MCL-1-dependent and -independent manner

The anti-apoptotic BCL-2 family proteins are important targets for cancer chemotherapy. Specific and potent inhibitors of the BCL-2 family, such as ABT-263 (navitoclax) and ABT-199, are only effective against some members of the BCL-2 family but do not target MCL-1, which is commonly amplified in tumors and associated with chemoresistance. In this report, the selectivity and potency of two putative MCL-1 inhibitors, dinaciclib and maritoclax, were assessed. Although both compounds induced Bax/Bak- and caspase-9-dependent apoptosis, dinaciclib was more potent than maritoclax in downregulating MCL-1 and also in inducing apoptosis. However, the compounds induced apoptosis, even in cells lacking MCL-1, suggesting multiple mechanisms of cell death. Furthermore, maritoclax induced extensive mitochondrial fragmentation, and a Bax/Bak- but MCL-1-independent accumulation of mitochondrial reactive oxygen species (ROS), with an accompanying loss of complexes I and III of the electron transport chain. ROS scavengers, such as MitoQ, could not salvage maritoclax-mediated effects on mitochondrial structure and function. Taken together, our data demonstrate that neither dinaciclib nor maritoclax exclusively target MCL-1. Although dinaciclib is clearly not a specific MCL-1 inhibitor, its ability to rapidly downregulate MCL-1 may be beneficial in many clinical settings, where it may reverse chemoresistance or sensitize to other chemotherapeutic agents.