The combination of reduced MCL-1 and standard chemotherapeutics is tolerable in mice

Targeting MCL1 with Sanggenon C overcomes MCL1-driven adaptive chemoresistance via dysregulation of autophagy and endoplasmic reticulum stress in cervical cancer

BACKGROUND

Cervical cancer ranks as one of the most prevalent malignancies among women worldwide and poses a significant threat to health and quality of life. MCL1 is an antiapoptotic protein closely linked to tumorigenesis, drug-resistance and poor prognosis in various cancers. Sanggenon C, a natural flavonoid derived from Morus albal., exhibits multiple activities, including anti-oxidant, anti-inflammatory, antivirus, and antitumor properties. However, the molecular mechanisms by which Sanggenon C exerts antitumor effects on in cervical cancer remain unclear.

PURPOSE

To investigate the oncogenic role of MCL1 and elucidate the antitumor activity of Sanggenon C, along with its molecular mechanisms, in cervical cancer.

METHODS

In vitro, the effects of Sanggenon C on proliferation, the cell cycle, apoptosis, and autophagy were explored. Transcriptome sequencing was employed to analyze critical genes and pathways. The expression of genes or proteins was evaluated via immunofluorescence, qRT-PCR, immunohistochemistry, and Western blotting. To identify targets of Sanggenon C, various techniques such as clinical database analysis, molecular docking, cellular thermal shift assays, co-immunoprecipitation, and ubiquitination assays were utilized. Additionally, Xenograft mouse models were established to further investigate Sanggenon C as a novel MCL1 inhibitor and its anti-tumor activity in vivo.

RESULTS

Our investigation reveals that Sanggenon C effectively inhibits cervical cancer cell proliferation both in vitro and in vivo. Furthermore, Sanggenon C induces endoplasmic reticulum stress and triggers protective autophagy via activation of the ATF4-DDIT3-TRIB3-AKT-MTOR signaling axis. Furthermore, Sanggenon C specifically targets MCL1 to exert its antitumor effects by modulating MCL1 protein stability through SYVN1-mediated ubiquitination. Notably, MCL1 overexpression attenuates the Sanggenon C-induced decrease in cell viability and apoptosis. Our study further characterizes the role of MCL1 in cisplatin resistance and identifies MCL1 as a promising target for Sanggenon C, which effectively inhibits proliferation and induces apoptosis in cisplatin-resistant cervical cancer cells. Importantly, combining Sanggenon C with an autophagy inhibitor represents a promising strategy to enhance therapeutic outcomes in cisplatin-resistant cervical cancer cells.

CONCLUSION

Our findings demonstrates that Sanggenon C induces endoplasmic reticulum stress and highlights the potential of targeting MCL1 to exploit vulnerabilities in drug-resistant cervical cancer cells. Sanggenon C emerges as a promising therapeutic agent against MCL1-driven adaptive chemoresistance through disruption of autophagy and endoplasmic reticulum stress in cervical cancer.

Venetoclax therapy and emerging resistance mechanisms in acute myeloid leukaemia

Acute myeloid leukaemia (AML) is a highly aggressive and devastating malignancy of the bone marrow and blood. For decades, intensive chemotherapy has been the frontline treatment for AML but has yielded only poor patient outcomes as exemplified by a 5-year survival rate of < 30%, even in younger adults. As knowledge of the molecular underpinnings of AML has advanced, so too has the development new strategies with potential to improve the treatment of AML patients. To date the most promising of these targeted agents is the BH3-mimetic venetoclax which in combination with standard of care therapies, has manageable non-haematological toxicity and exhibits impressive efficacy. However, approximately 30% of AML patients fail to respond to venetoclax-based regimens and almost all treatment responders eventually relapse. Here, we review the emerging mechanisms of intrinsic and acquired venetoclax resistance in AML and highlight recent efforts to identify novel strategies to overcome resistance to venetoclax.

A CRISPR-drug perturbational map for identifying compounds to combine with commonly used chemotherapeutics

Combination chemotherapy is crucial for successfully treating cancer. However, the enormous number of possible drug combinations means discovering safe and effective combinations remains a significant challenge. To improve this process, we conduct large-scale targeted CRISPR knockout screens in drug-treated cells, creating a genetic map of druggable genes that sensitize cells to commonly used chemotherapeutics. We prioritize neuroblastoma, the most common extracranial pediatric solid tumor, where ~50% of high-risk patients do not survive. Our screen examines all druggable gene knockouts in 18 cell lines (10 neuroblastoma, 8 others) treated with 8 widely used drugs, resulting in 94,320 unique combination-cell line perturbations, which is comparable to the largest existing drug combination screens. Using dense drug-drug rescreening, we find that the top CRISPR-nominated drug combinations are more synergistic than standard-of-care combinations, suggesting existing combinations could be improved. As proof of principle, we discover that inhibition of PRKDC, a component of the non-homologous end-joining pathway, sensitizes high-risk neuroblastoma cells to the standard-of-care drug doxorubicin in vitro and in vivo using patient-derived xenograft (PDX) models. Our findings provide a valuable resource and demonstrate the feasibility of using targeted CRISPR knockout to discover combinations with common chemotherapeutics, a methodology with application across all cancers.

A1 is induced by pathogen ligands to limit myeloid cell death and NLRP3 inflammasome activation

Programmed cell death pathways play an important role in innate immune responses to infection. Activation of intrinsic apoptosis promotes infected cell clearance; however, comparatively little is known about how this mode of cell death is regulated during infections and whether it can induce inflammation. Here, we identify that the pro-survival BCL-2 family member, A1, controls activation of the essential intrinsic apoptotic effectors BAX/BAK in macrophages and monocytes following bacterial lipopolysaccharide (LPS) sensing. We show that, due to its tight transcriptional and post-translational regulation, A1 acts as a molecular rheostat to regulate BAX/BAK-dependent apoptosis and the subsequent NLRP3 inflammasome-dependent and inflammasome-independent maturation of the inflammatory cytokine IL-1β. Furthermore, induction of A1 expression in inflammatory monocytes limits cell death modalities and IL-1β activation triggered by Neisseria gonorrhoeae-derived outer membrane vesicles (NOMVs). Consequently, A1-deficient mice exhibit heightened IL-1β production in response to NOMV injection. These findings reveal that bacteria can induce A1 expression to delay myeloid cell death and inflammatory responses, which has implications for the development of host-directed antimicrobial therapeutics.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Increased apoptotic sensitivity of glioblastoma enables therapeutic targeting by BH3-mimetics

Glioblastoma (GBM) is the most prevalent malignant primary brain tumour in adults. GBM typically has a poor prognosis, mainly due to a lack of effective treatment options leading to tumour persistence or recurrence. We investigated the therapeutic potential of targeting anti-apoptotic BCL-2 proteins in GBM. Levels of anti-apoptotic BCL-xL and MCL-1 were consistently increased in GBM compared with non-malignant cells and tissue. Moreover, we found that relative to their differentiated counterparts, patient-derived GBM stem-like cells also displayed higher expression of anti-apoptotic BCL-2 family members. High anti-apoptotic BCL-xL and MCL-1 expression correlated with heightened susceptibility of GBM to BCL-2 family protein-targeting BH3-mimetics. This is indicative of increased apoptotic priming. Indeed, GBM displayed an obligate requirement for MCL-1 expression in both tumour development and maintenance. Investigating this apoptotic sensitivity, we found that sequential inhibition of BCL-xL and MCL-1 led to robust anti-tumour responses in vivo, in the absence of overt toxicity. These data demonstrate that BCL-xL and MCL-1 pro-survival function is a fundamental prerequisite for GBM survival that can be therapeutically exploited by BH3-mimetics.

What can we learn from mice lacking pro-survival BCL-2 proteins to advance BH3 mimetic drugs for cancer therapy?

In many human cancers the control of apoptosis is dysregulated, for instance as a result of the overexpression of pro-survival BCL-2 proteins. This promotes tumorigenesis by protecting nascent neoplastic cells from stress and renders malignant cells resistant to anti-cancer agents. Therefore, several BH3 mimetic drugs targeting distinct pro-survival proteins have been developed. The BCL-2 inhibitor Venetoclax/ABT-199, has been approved for treatment of certain blood cancers and tens of thousands of patients have already been treated effectively with this drug. To advance the clinical development of MCL-1 and BCL-XL inhibitors, a more detailed understanding of their distinct and overlapping roles in the survival of malignant as well as non-transformed cells in healthy tissues is required. Here, we discuss similarities and differences in pro-survival BCL-2 protein structure, subcellular localisation and binding affinities to the pro-apoptotic BCL-2 family members. We summarise the findings from gene-targeting studies in mice to discuss the specific roles of distinct pro-survival BCL-2 family members during embryogenesis and the survival of non-transformed cells in healthy tissues in adults. Finally, we elaborate how these findings align with or differ from the observations from the clinical development and use of BH3 mimetic drugs targeting different pro-survival BCL-2 proteins.

The manipulation of apoptosis for cancer therapy using BH3-mimetic drugs

Apoptosis is a form of programmed cell death that is regulated by the balance between prosurvival and proapoptotic BCL-2 protein family members. Evasion of apoptosis is a hallmark of cancer that arises when this balance is tipped in favour of survival. One form of anticancer therapeutic, termed 'BH3-mimetic drugs', has been developed to directly activate the apoptosis machinery in malignant cells. These drugs bind to and inhibit specific prosurvival BCL-2 family proteins, thereby mimicking their interaction with the BH3 domains of proapoptotic BCL-2 family proteins. The BCL-2-specific inhibitor venetoclax is approved by the US Food and Drug Administration and many regulatory authorities worldwide for the treatment of chronic lymphocytic leukaemia and acute myeloid leukaemia. BH3-mimetic drugs targeting other BCL-2 prosurvival proteins have been tested in preclinical models of cancer, and drugs targeting MCL-1 or BCL-X_L have advanced into phase I clinical trials for certain cancers. As with all therapeutics, efficacy and tolerability need to be carefully balanced to achieve a therapeutic window whereby there is significant anticancer activity with an acceptable safety profile. In this Review, we outline the current state of BH3-mimetic drugs targeting various prosurvival BCL-2 family proteins and discuss emerging data regarding primary and acquired resistance to these agents and approaches that may overcome this. We highlight issues that need to be addressed to further advance the clinical application of BH3-mimetic drugs, both alone and in combination with additional anticancer agents (for example, standard chemotherapeutic drugs or inhibitors of oncogenic kinases), for improved responses in patients with cancer.

Physiological Functions of Mcl-1: Insights From Genetic Mouse Models

The ability to regulate the survival and death of a cell is paramount throughout the lifespan of a multicellular organism. Apoptosis, a main physiological form of programmed cell death, is regulated by the Bcl-2 family proteins that are either pro-apoptotic or pro-survival. The in vivo functions of distinct Bcl-2 family members are largely unmasked by genetically engineered murine models. Mcl-1 is one of the two Bcl-2 like pro-survival genes whose germline deletion causes embryonic lethality in mice. Its requisite for the survival of a broad range of cell types has been further unraveled by using conditional and inducible deletion murine model systems in different tissues or cell lineages and at distinct developmental stages. Moreover, genetic mouse cancer models have also demonstrated that Mcl-1 is essential for the survival of multiple tumor types. The MCL-1 locus is commonly amplified across various cancer types in humans. Small molecule inhibitors with high affinity and specificity to human MCL-1 have been developed and explored for the treatment of certain cancers. To facilitate the pre-clinical studies of MCL-1 in cancer and other diseases, transgenic mouse models over-expressing human MCL-1 as well as humanized MCL-1 mouse models have been recently engineered. This review discusses the current advances in understanding the physiological roles of Mcl-1 based on studies using genetic murine models and its critical implications in pathology and treatment of human diseases.

The interplay between apoptosis and cellular senescence: Bcl-2 family proteins as targets for cancer therapy

Cell death by apoptosis and permanent cell cycle arrest by senescence serve as barriers to the development of cancer. Chemotherapeutic agents not only induce apoptosis, they can also induce senescence known as therapy-induced senescence (TIS). There are, however, controversies whether TIS improves or worsens therapeutic outcome. Unlike apoptosis, which permanently removes cancer cells, senescent cells are metabolically active, and can contribute to tumor progression and relapse. If senescent cells are not cleared by the immune system or if cancer cells escape senescence, they may acquire resistance to apoptotic stimuli and become highly aggressive. Thus, there have been significant efforts in developing senolytics, drugs that target these pro-survival molecules to eliminate senescent cells. The anti-apoptotic Bcl-2 family proteins not only protect against cell death by apoptosis, but they also allow senescent cells to survive. While combining senolytics with chemotherapeutic drugs is an attractive approach, there are also limitations. Moreover, members of the Bcl-2 family have distinct effects on apoptosis and senescence. The purpose of this review article is to discuss recent literatures on how members of the Bcl-2 family orchestrate the interplay between apoptosis and senescence, and the challenges and progress in targeting these Bcl-2 family proteins for cancer therapy.

Cytokine-like protein 1-induced survival of monocytes suggests a combined strategy targeting MCL1 and MAPK in CMML

Mouse models of chronic myeloid malignancies suggest that targeting mature cells of the malignant clone disrupts feedback loops that promote disease expansion. Here, we show that in chronic myelomonocytic leukemia (CMML), monocytes that accumulate in the peripheral blood show a decreased propensity to die by apoptosis. BH3 profiling demonstrates their addiction to myeloid cell leukemia-1 (MCL1), which can be targeted with the small molecule inhibitor S63845. RNA sequencing and DNA methylation pattern analysis both point to the implication of the mitogen-activated protein kinase (MAPK) pathway in the resistance of CMML monocytes to death and reveal an autocrine pathway in which the secreted cytokine-like protein 1 (CYTL1) promotes extracellular signal-regulated kinase (ERK) activation through C-C chemokine receptor type 2 (CCR2). Combined MAPK and MCL1 inhibition restores apoptosis of monocytes from patients with CMML and reduces the expansion of patient-derived xenografts in mice. These results show that the combined inhibition of MCL1 and MAPK is a promising approach to slow down CMML progression by inducing leukemic monocyte apoptosis.

Unmasking BCL-2 Addiction in Synovial Sarcoma by Overcoming Low NOXA

Simple Summary

Synovial sarcoma is a soft-tissue sarcoma that lacks effective systemic therapy and carries poor prognosis due to frequent late local recurrence and metastases. The cancer is known to be driven in part by increased expression of the pro-survival protein BCL-2. Surprisingly, synovial sarcoma proved resistant to BCL-2 inhibitors in pre-clinical trials. We identified increased activity of a second pro-survival protein, MCL-1, as responsible for this resistance. We showed that co-targeting both BCL-2 and MCL-1 proves to be an effective therapeutic approach both in cell culture and animal models of synovial sarcoma, supporting translation into clinical trials.

Abstract

Synovial sarcoma (SS) is frequently diagnosed in teenagers and young adults and continues to be treated with polychemotherapy with variable success. The SS18-SSX gene fusion is pathognomonic for the disease, and high expression of the anti-apoptotic BCL-2 pathologically supports the diagnosis. As the oncogenic SS18-SSX fusion gene itself is not druggable, BCL-2 inhibitor-based therapies are an appealing therapeutic opportunity. Venetoclax, an FDA-approved BCL-2 inhibitor that is revolutionizing care in some BCL-2-expressing hematological cancers, affords an intriguing therapeutic possibility to treat SS. In addition, there are now dozens of venetoclax-based combination therapies in clinical trials in hematological cancers, attributing to the limited toxicity of venetoclax. However, preclinical studies of venetoclax in SS have demonstrated an unexpected ineffectiveness. In this study, we analyzed the response of SS to venetoclax and the underlying BCL-2 family biology in an effort to understand venetoclax treatment failure and find a therapeutic strategy to sensitize SS to venetoclax. We found remarkably depressed levels of the endogenous MCL-1 inhibitor, NOXA, in SS compared to other sarcomas. Expressing NOXA led to sensitization to venetoclax, as did the addition of the MCL-1 BH3 mimetic, S63845. Importantly, the venetoclax/S63845 combination induced tumor regressions in SS patient-derived xenograft (PDX) models. As a very close analog of S63845 (S64315) is now in clinical trials with venetoclax in AML (NCT03672695), the combination of MCL-1 BH3 mimetics and venetoclax should be considered for SS patients as a new therapy.

BCL-XL exerts a protective role against anemia caused by radiation-induced kidney damage

Studies of gene-targeted mice identified the roles of the different pro-survival BCL-2 proteins during embryogenesis. However, little is known about the role(s) of these proteins in adults in response to cytotoxic stresses, such as treatment with anti-cancer agents. We investigated the role of BCL-XL in adult mice using a strategy where prior bone marrow transplantation allowed for loss of BCL-XL exclusively in non-hematopoietic tissues to prevent anemia caused by BCL-XL deficiency in erythroid cells. Unexpectedly, the combination of total body γ-irradiation (TBI) and genetic loss of Bcl-x caused secondary anemia resulting from chronic renal failure due to apoptosis of renal tubular epithelium with secondary obstructive nephropathy. These findings identify a critical protective role of BCL-XL in the adult kidney and inform on the use of BCL-XL inhibitors in combination with DNA damage-inducing drugs for cancer therapy. Encouragingly, the combination of DNA damage-inducing anti-cancer therapy plus a BCL-XL inhibitor could be tolerated in mice, at least when applied sequentially.

Targeting MCL-1 in hematologic malignancies: Rationale and progress

Myeloid cell leukemia sequence 1 (MCL-1) is an antiapoptotic protein that plays a key role in promoting cell survival in multiple myeloma (MM), acute myeloid leukemia (AML), and non-Hodgkin lymphoma (NHL). Overexpression of MCL-1 is associated with treatment resistance and poor prognosis; thus, MCL-1 inhibitors are rational therapeutic options for malignancies depending on MCL-1. Several MCL-1 inhibitors have entered clinical trials, including AZD5991, S64315, AMG 176, and AMG 397. A key area of investigation is whether MCL-1 inhibitors will complement the activity of BCL-2 inhibitors, such as venetoclax, and synergistically enhance anti-tumor efficacy when given in combination with other anti-cancer drugs. Another important question is whether a safe therapeutic window can be found for this new class of inhibitors. In summary, inhibition of MCL-1 shows potential as a treatment for hematologic malignancies and clinical evaluation of MCL-1 inhibitors is currently underway.

MCL-1 gains occur with high frequency in lung adenocarcinoma and can be targeted therapeutically

Evasion of programmed cell death represents a critical form of oncogene addiction in cancer cells. Understanding the molecular mechanisms underpinning cancer cell survival despite the oncogenic stress could provide a molecular basis for potential therapeutic interventions. Here we explore the role of pro-survival genes in cancer cell integrity during clonal evolution in non-small cell lung cancer (NSCLC). We identify gains of MCL-1 at high frequency in multiple independent NSCLC cohorts, occurring both clonally and subclonally. Clonal loss of functional TP53 is significantly associated with subclonal gains of MCL-1. In mice, tumour progression is delayed upon pharmacologic or genetic inhibition of MCL-1. These findings reveal that MCL-1 gains occur with high frequency in lung adenocarcinoma and can be targeted therapeutically.

Combined reduction in the expression of MCL-1 and BCL-2 reduces organismal size in mice

The intrinsic apoptotic pathway is controlled by the BCL-2 family of proteins, which exhibit either a pro-death or pro-survival function. Gene knockout studies revealed that different pro-survival BCL-2 proteins are critical for the survival of distinct cell types, although overlapping functions amongst such proteins have also been identified. In the process of studying mice lacking single alleles of Mcl-1 (Mcl-1^+/−), Bcl-2 (Bcl-2^+/−), or both in combination (Mcl-1^+/−Bcl-2^+/−), we observed that Mcl-1^+/−Bcl-2^+/− mice weighed less when compared with their wild-type littermates as they aged. Body composition analysis demonstrated that while fat mass was similar to wild-type controls, lean mass was significantly reduced in Mcl-1^+/−, Bcl-2^+/−, and, most strikingly in Mcl-1^+/−Bcl-2^+/− mice. The weights of several tissues including the heart, tibialis anterior, and kidney were likewise reduced in Mcl-1^+/−Bcl-2^+/− mice. When lean mass and specific tissue weights were expressed relative to body weight, these differences were no longer significant, indicating that that Mcl-1^+/−Bcl-2^+/− mice, and to a lesser extent Mcl-1^+/− and Bcl-2^+/− mice, are smaller than their wild-type counterparts. Consistently, the anal-naso length was reduced in Mcl-1^+/−Bcl-2^+/− mice. While minor reductions in size were observed in female Mcl-1^+/−Bcl-2^+/− mice, these effects were most prominent in males. Notably, Mcl-1^+/−Bcl-2^+/− males had markedly smaller testes even after accounting for differences in body weight. Collectively, these data reveal that combined loss of a single allele of Mcl-1 and Bcl-2, while not overtly impairing organismal development, leads to a reduction in animal size.

Toward Targeting Antiapoptotic MCL-1 for Cancer Therapy

Apoptosis is critical for embryonic development, tissue homeostasis, and the removal of infected or otherwise dangerous cells. It is controlled by three subgroups of the BCL-2 protein family—the BH3-only proteins that initiate cell death; the effectors of cell killing, BAX and BAK; and the antiapoptotic guardians, including MCL-1 and BCL-2. Defects in apoptosis can promote tumorigenesis and render malignant cells refractory to anticancer therapeutics. Activation of cell death by inhibiting antiapoptotic BCL-2 family members has emerged as an attractive strategy for cancer therapy, with the BCL-2 inhibitor venetoclax leading the way. Large-scale cancer genome analyses have revealed frequent amplification of the locus encoding antiapoptotic MCL-1 in human cancers, and functional studies have shown that MCL-1 is essential for the sustained survival and expansion of many types of tumor cells. Structural analysis and medicinal chemistry have led to the development of three distinct small-molecule inhibitors of MCL-1 that are currently undergoing clinical testing.

Subtle Changes in the Levels of BCL-2 Proteins Cause Severe Craniofacial Abnormalities

Apoptotic cell death removes unwanted cells and is regulated by interactions between pro-survival and pro-apoptotic members of the BCL-2 protein family. The regulation of apoptosis is thought to be crucial for normal embryonic development. Accordingly, complete loss of pro-survival MCL-1 or BCL-XL (BCL2L1) causes embryonic lethality. However, it is not known whether minor reductions in pro-survival proteins could cause developmental abnormalities. We explored the rate-limiting roles of MCL-1 and BCL-XL in development and show that combined loss of single alleles of Mcl-1 and Bcl-x causes neonatal lethality. Mcl-1^+/-;Bcl-x^+/- mice display craniofacial anomalies, but additional loss of a single allele of pro-apoptotic Bim (Bcl2l11) restores normal development. These findings demonstrate that the control of cell survival during embryogenesis is finely balanced and suggest that some human craniofacial defects, for which causes are currently unknown, may be due to subtle imbalances between pro-survival and pro-apoptotic BCL-2 family members.

miR17~92 restrains pro-apoptotic BIM to ensure survival of haematopoietic stem and progenitor cells

The miR17~92 cluster plays important roles in haematopoiesis. However, it is not clear at what stage of differentiation and through which targets miR17~92 exerts this function. Therefore, we generated miR17~92^fl/fl; RosaCreERT2 mice for inducible deletion of miR17~92 in haematopoietic cells. Bone marrow reconstitution experiments revealed that miR17~92-deleted cells were not capable to contribute to mature haematopoietic lineages, which was due to defects in haematopoietic stem/progenitor cells (HSPCs). To identify the critical factor targeted by miR17~92 we performed gene expression analysis in HSPCs, demonstrating that mRNA levels of pro-apoptotic Bim inversely correlated with the expression of the miR17~92 cluster. Strikingly, loss of pro-apoptotic BIM completely prevented the loss of HSPCs caused by deletion of miR17~92. The BIM/miR17~92 interaction is conserved in human CD34⁺ HSPCs, as miR17~92 inhibition or blockade of its binding to the BIM 3'UTR reduced the survival and growth of these cells. Despite the prediction that miR17~92 functions by impacting a plethora of different targets, the absence of BIM alone is sufficient to prevent all defects caused by deletion of miR17~92 in haematopoietic cells.

BH3-Mimetic Drugs: Blazing the Trail for New Cancer Medicines

Defects in apoptotic cell death can promote cancer and impair responses of malignant cells to anti-cancer therapy. Pro-survival BCL-2 proteins prevent apoptosis by keeping the cell death effectors, BAX and BAK, in check. The BH3-only proteins initiate apoptosis by neutralizing the pro-survival BCL-2 proteins. Structural analysis and medicinal chemistry led to the development of small-molecule drugs that mimic the function of the BH3-only proteins to kill cancer cells. The BCL-2 inhibitor venetoclax has been approved for treatment of refractory chronic lymphocytic leukemia and this drug and inhibitors of pro-survival MCL-1 and BCL-XL are being tested in diverse malignancies.

A Novel MCL1 Inhibitor Combined with Venetoclax Rescues Venetoclax-Resistant Acute Myelogenous Leukemia

Suppression of apoptosis by expression of antiapoptotic BCL2 family members is a hallmark of acute myeloblastic leukemia (AML). Induced myeloid leukemia cell differentiation protein (MCL1), an antiapoptotic BCL2 family member, is commonly upregulated in AML cells and is often a primary mode of resistance to treatment with the BCL2 inhibitor venetoclax. Here, we describe VU661013, a novel, potent, selective MCL1 inhibitor that destabilizes BIM/MCL1 association, leads to apoptosis in AML, and is active in venetoclax-resistant cells and patient-derived xenografts. In addition, VU661013 was safely combined with venetoclax for synergy in murine models of AML. Importantly, BH3 profiling of patient samples and drug-sensitivity testing ex vivo accurately predicted cellular responses to selective inhibitors of MCL1 or BCL2 and showed benefit of the combination. Taken together, these data suggest a strategy of rationally using BCL2 and MCL1 inhibitors in sequence or in combination in AML clinical trials. SIGNIFICANCE: Targeting antiapoptotic proteins in AML is a key therapeutic strategy, and MCL1 is a critical antiapoptotic oncoprotein. Armed with novel MCL1 inhibitors and the potent BCL2 inhibitor venetoclax, it may be possible to selectively induce apoptosis by combining or thoughtfully sequencing these inhibitors based on a rational evaluation of AML.See related commentary by Leber et al., p. 1511.This article is highlighted in the In This Issue feature, p. 1494.

MCL-1 inhibition in cancer treatment

Myeloid cell leukemia-1 (MCL-1), a member of antiapoptotic BCL-2 family proteins, is a key regulator of mitochondrial homeostasis. Frequent overexpression of MCL-1 in human primary and drug-resistant cancer cells makes it an attractive cancer therapeutic target. Significant progress has been made in the development of small-molecule MCL-1 inhibitors in recent years, and three MCL-1 selective inhibitors have advanced to clinical trials. This review briefly discusses recent advances in the development of small molecules targeting MCL-1 for cancer therapy.