Noxa controls Mule-dependent Mcl-1 ubiquitination through the regulation of the Mcl-1/USP9X interaction

Targeting Myeloid Leukemia-1 in Cancer Therapy: Advances and Directions

As a tripartite cell death switch, B-cell lymphoma protein 2 (Bcl-2) family members precisely regulate the endogenous apoptosis pathway in response to various cell signal stresses through protein-protein interactions. Myeloid leukemia-1 (Mcl-1), a key anti-apoptotic Bcl-2 family member, is positioned downstream in the endogenous apoptotic pathway and plays a central role in regulating mitochondrial function. Mcl-1 is highly expressed in a variety of hematological malignancies and solid tumors, contributing to tumorigenesis, poor prognosis, and chemoresistance, making it an attractive target for cancer treatment. This Perspective aims to discuss the mechanism by which Mcl-1 regulates apoptosis and non-apoptotic functions in cancer cells and to outline the discovery and optimization process of potent Mcl-1 modulators. In addition, we summarize the structural characteristics of potent inhibitors that bind to Mcl-1 through multiple co-crystal structures and analyze the cardiotoxicity caused by current Mcl-1 inhibitors, providing prospects for rational targeting of Mcl-1.

Friend or foe? Reciprocal regulation between E3 ubiquitin ligases and deubiquitinases

Protein ubiquitination is a post-translational modification that entails the covalent attachment of the small protein ubiquitin (Ub), which acts as a signal to direct protein stability, localization, or interactions. The Ub code is written by a family of enzymes called E3 Ub ligases (∼600 members in humans), which can catalyze the transfer of either a single ubiquitin or the formation of a diverse array of polyubiquitin chains. This code can be edited or erased by a different set of enzymes termed deubiquitinases (DUBs; ∼100 members in humans). While enzymes from these distinct families have seemingly opposing activities, certain E3-DUB pairings can also synergize to regulate vital cellular processes like gene expression, autophagy, innate immunity, and cell proliferation. In this review, we highlight recent studies describing Ub ligase-DUB interactions and focus on their relationships.

Integrated epigenetic and transcriptional single-cell analysis of t(11;14) multiple myeloma and its BCL2 dependency

ABSTRACT

The translocation t(11;14) occurs in 20% of patients with multiple myeloma (MM) and results in the upregulation of CCND1. Nearly two-thirds of t(11;14) MM cells are BCL2 primed and highly responsive to the oral BCL2 inhibitor venetoclax. Although it is evident that this unique sensitivity to venetoclax depends on the Bcl-2 homology domain 3- proapoptotic protein priming of BCL2, the biology underlying t(11;14) MM dependency on BCL2 is poorly defined. Importantly, the epigenetic regulation of t(11;14) transcriptomes and its impact on gene regulation and clinical response to venetoclax remain elusive. In this study, by integrating assay for transposase-accessible chromatin by sequencing (ATAC-seq) and RNA-seq at the single-cell level in primary MM samples, we have defined the epigenetic regulome and transcriptome associated with t(11;14) MM. A B-cell-like epigenetic signature was enriched in t(11;14) MM, confirming its phylogeny link to B-cell rather than plasma cell biology. Of note, a loss of a B-cell-like epigenetic signature with a gain of canonical plasma cell transcription factors was observed at the time of resistance to venetoclax. In addition, MCL1 and BCL2L1 copy number gains and structural rearrangements were linked to venetoclax resistance in patients with t(11;14) MM. To date, this is the first study in which both single-cell (sc) ATAC-seq and scRNA-seq analysis are integrated into primary MM cells to obtain a deeper resolution of the epigenetic regulome and transcriptome associated with t(11;14) MM biology and venetoclax resistance.

Marine Polyether Phycotoxin Palytoxin Induces Apoptotic Cell Death via Mcl-1 and Bcl-2 Downregulation

Palytoxin is considered one of the most potent biotoxins. As palytoxin-induced cancer cell death mechanisms remain to be elucidated, we investigated this effect on various leukemia and solid tumor cell lines at low picomolar concentrations. As palytoxin did not affect the viability of peripheral blood mononuclear cells (PBMC) from healthy donors and did not create systemic toxicity in zebrafish, we confirmed excellent differential toxicity. Cell death was characterized by a multi-parametric approach involving the detection of nuclear condensation and caspase activation assays. zVAD-sensitive apoptotic cell death was concomitant with a dose-dependent downregulation of antiapoptotic Bcl-2 family proteins Mcl-1 and Bcl-xL. Proteasome inhibitor MG-132 prevented the proteolysis of Mcl-1, whereas the three major proteasomal enzymatic activities were upregulated by palytoxin. Palytoxin-induced dephosphorylation of Bcl-2 further exacerbated the proapoptotic effect of Mcl-1 and Bcl-xL degradation in a range of leukemia cell lines. As okadaic acid rescued cell death triggered by palytoxin, protein phosphatase (PP)2A was involved in Bcl-2 dephosphorylation and induction of apoptosis by palytoxin. At a translational level, palytoxin abrogated the colony formation capacity of leukemia cell types. Moreover, palytoxin abrogated tumor formation in a zebrafish xenograft assay at concentrations between 10 and 30 pM. Altogether, we provide evidence of the role of palytoxin as a very potent and promising anti-leukemic agent, acting at low picomolar concentrations in cellulo and in vivo.

Mechanisms of MCL-1 Protein Stability Induced by MCL-1 Antagonists in B-Cell Malignancies

PURPOSE

Several MCL-1 inhibitors (MCL-1i), including AMG-176 and AZD5991, have shown promise in preclinical studies and are being tested for the treatment of hematologic malignancies. A unique feature of these agents is induction and stability of Mcl-1 protein; however, the precise mechanism is unknown. We aim to study the mechanism of MCL-1i-induced Mcl-1 protein stability.

EXPERIMENTAL DESIGN

Using several B-cell leukemia and lymphoma cell lines and primary chronic lymphocytic leukemia (CLL) lymphocytes, we evaluated molecular events associated with Mcl-1 protein stability including protein half-life, reverse-phase protein array, protein-protein interaction, phosphorylation, ubiquitination, and de-ubiquitination, followed by molecular simulation and modeling.

RESULTS

Using both in vivo and in vitro analysis, we demonstrate that MCL-1i-induced Mcl-1 protein stability is predominantly associated with defective Mcl-1 ubiquitination and concurrent apoptosis induction in both cell lines and primary CLL subjects. These MCL1i also induced ERK-mediated Mcl-1Thr163 phosphorylation, which partially contributed to Mcl-1 stability. Disruption of Mcl-1:Noxa interaction followed by Noxa degradation, enhanced Mcl-1 de-ubiquitination by USP9x, and Mule destabilization are the major effects of these inhibitors. However, unlike other BH3 proteins, Mule:Mcl-1 interaction was unaffected by MCL-1i. WP1130, a global deubiquitinase (DUB) inhibitor, abrogated Mcl-1 induction reaffirming a critical role of DUBs in the observed Mcl-1 protein stability. Further, in vitro ubiquitination studies of Mcl-1 showed distinct difference among these inhibitors.

CONCLUSIONS

We conclude that MCL-1i blocked Mcl-1 ubiquitination via enhanced de-ubiquitination and dissociation of Mcl-1 from Noxa, Bak and Bax, and Mule de-stabilization. These are critical events associated with increased Mcl-1 protein stability with AMG-176 and AZD5991.

Understanding MCL1: from cellular function and regulation to pharmacological inhibition

Myeloid cell leukemia-1 (MCL1), an antiapoptotic member of the BCL2 family characterized by a short half-life, functions as a rapid sensor that regulates cell death and other relevant processes that include cell cycle progression and mitochondrial homeostasis. In cancer, MCL1 overexpression contributes to cell survival and resistance to diverse chemotherapeutic agents; for this reason, several MCL1 inhibitors are currently under preclinical and clinical development for cancer treatment. However, the nonapoptotic functions of MCL1 may influence their therapeutic potential. Overall, the complexity of MCL1 regulation and function represent challenges to the clinical application of MCL1 inhibitors. We now summarize the current knowledge regarding MCL1 structure, regulation, and function that could impact the clinical success of MCL1 inhibitors.

Myeloid cell leukemia-1: a formidable barrier to anticancer therapeutics and the quest of targeting it

The antiapoptotic B cell lymphoma-2 (Bcl-2) family members are apical regulators of the intrinsic pathway of apoptosis that orchestrate mitochondrial outer membrane permeabilization (MOMP) through interactions with their proapoptotic counterparts. Overexpression of antiapoptotic Bcl-2 family proteins has been linked to therapy resistance and poor prognosis in diverse cancers. Among the antiapoptotic Bcl-2 family members, predominant overexpression of the prosurvival myeloid cell leukemia-1 (Mcl-1) has been reported in a myriad of hematological malignancies and solid tumors, contributing to therapy resistance and poor outcomes, thus making it a potential druggable target. The unique structure of Mcl-1 and its complex regulatory mechanism makes it an adaptive prosurvival switch that ensures tumor cell survival despite therapeutic intervention. This review focusses on diverse mechanisms adopted by tumor cells to maintain sustained elevated levels of Mcl-1 and how high Mcl-1 levels contribute to resistance in conventional as well as targeted therapies. Moreover, recent developments in the Mcl-1-targeted therapeutics and the underlying challenges and considerations in designing novel Mcl-1 inhibitors are also discussed.

Constitutive High Expression of NOXA Sensitizes Human Embryonic Stem Cells for Rapid Cell Death

Human embryonic stem (hES) cells are highly sensitive to apoptotic stimuli such as DNA damage, which allows for the rapid elimination of mutated cells during development. However, the mechanisms that maintain hES cells in the primed apoptotic state are not completely known. Key activators of apoptosis, the BH3-only proteins, are present at low levels in most cell types. In contrast, hES cells have constitutive high levels of the BH3-only protein, NOXA. We examined the importance of NOXA for enabling apoptosis in hES cells. hES cells deleted for NOXA showed remarkable protection against multiple apoptotic stimuli. NOXA was constitutively localized to the mitochondria, where it interacted with MCL1. Strikingly, inhibition of MCL1 in NOXA knockout cells was sufficient to sensitize these cells to DNA damage-induced cell death. Our study demonstrates that an essential function of constitutive high levels of NOXA in hES cells is to effectively antagonize MCL1 to permit rapid apoptosis.

BH3-Only Proteins Noxa and Puma Are Key Regulators of Induced Apoptosis

Apoptosis is an evolutionarily conserved and tightly regulated cell death pathway. Physiological cell death is important for maintaining homeostasis and optimal biological conditions by continuous elimination of undesired or superfluous cells. The BH3-only pro-apoptotic members are strong inducers of apoptosis. The pro-apoptotic BH3-only protein Noxa activates multiple death pathways by inhibiting the anti-apoptotic Bcl-2 family protein, Mcl-1, and other protein members leading to Bax and Bak activation and MOMP. On the other hand, Puma is induced by p53-dependent and p53-independent apoptotic stimuli in several cancer cell lines. Moreover, this protein is involved in several physiological and pathological processes, such as immunity, cancer, and neurodegenerative diseases. Future heat shock research could disclose the effect of hyperthermia on both Noxa and BH3-only proteins. This suggests post-transcriptional mechanisms controlling the translation of both Puma and Noxa mRNA in heat-shocked cells. This study was also the chance to recapitulate the different reactional mechanisms investigated for caspases.

Potent, p53-independent induction of NOXA sensitizes MLL-rearranged B-cell acute lymphoblastic leukemia cells to venetoclax

The prognosis for B-cell precursor acute lymphoblastic leukemia patients with Mixed-Lineage Leukemia (MLL) gene rearrangements (MLLr BCP-ALL) is still extremely poor. Inhibition of anti-apoptotic protein BCL-2 with venetoclax emerged as a promising strategy for this subtype of BCP-ALL, however, lack of sufficient responses in preclinical models and the possibility of developing resistance exclude using venetoclax as monotherapy. Herein, we aimed to uncover potential mechanisms responsible for limited venetoclax activity in MLLr BCP-ALL and to identify drugs that could be used in combination therapy. Using RNA-seq, we observed that long-term exposure to venetoclax in vivo in a patient-derived xenograft model leads to downregulation of several tumor protein 53 (TP53)-related genes. Interestingly, auranofin, a thioredoxin reductase inhibitor, sensitized MLLr BCP-ALL to venetoclax in various in vitro and in vivo models, independently of the p53 pathway functionality. Synergistic activity of these drugs resulted from auranofin-mediated upregulation of NOXA pro-apoptotic protein and potent induction of apoptotic cell death. More specifically, we observed that auranofin orchestrates upregulation of the NOXA-encoding gene Phorbol-12-Myristate-13-Acetate-Induced Protein 1 (PMAIP1) associated with chromatin remodeling and increased transcriptional accessibility. Altogether, these results present an efficacious drug combination that could be considered for the treatment of MLLr BCP-ALL patients, including those with TP53 mutations.

Anti-apoptotic MCL1 Protein Represents Critical Survival Molecule for Most Burkitt Lymphomas and BCL2-negative Diffuse Large B-cell Lymphomas

The pro-survival MCL1 protein is overexpressed in many cancers, including B-cell non-Hodgkin lymphomas (B-NHL). S63845 is a highly specific inhibitor of MCL1. We analyzed mechanisms of sensitivity/resistance to S63845 in preclinical models of diffuse large B-cell lymphoma (DLBCL) and Burkitt lymphoma. Annexin V-based cytotoxic assays, Western blot analysis, protein co-immunoprecipitation, and cell clones with manipulated expression of BCL2 family proteins were used to analyze mechanisms of sensitivity to S63845. Experimental in vivo therapy with S63845 and/or venetoclax was performed using patient-derived xenografts (PDX) of treatment-refractory B-NHL. A subset of DLBCL and majority of Burkitt lymphoma cell lines were sensitive to S63845. The level of BCL2 protein expression was the major determinant of resistance to S63845: BCL2 serves as a buffer for pro-apoptotic proteins released from MCL1 upon exposure to S63845. While BCL2-negative lymphomas were effectively eliminated by single-agent S63845, its combination with venetoclax was synthetically lethal in BCL2-positive PDX models. Concerning MCL1, both, the level of MCL1 protein expression, and its occupational status represent key factors mediating sensitivity to S63845. In contrast to MCL1-BIM/BAK1 complexes that prime lymphoma cells for S63845-mediated apoptosis, MCL1-NOXA complexes are associated with S63845 resistance. In conclusion, MCL1 represents a critical survival molecule for most Burkitt lymphomas and a subset of BCL2-negative DLBCLs. The level of BCL2 and MCL1 expression and occupational status of MCL1 belong to the key modulators of sensitivity/resistance to S63845. Co-treatment with venetoclax can overcome BCL2-mediated resistance to S63845, and enhance efficacy of MCL1 inhibitors in BCL2-positive aggressive B-NHL.

The role of Ubiquitination in Apoptosis and Necroptosis

Cell death pathways have evolved to maintain tissue homoeostasis and eliminate potentially harmful cells from within an organism, such as cells with damaged DNA that could lead to cancer. Apoptosis, known to eliminate cells in a predominantly non-inflammatory manner, is controlled by two main branches, the intrinsic and extrinsic apoptotic pathways. While the intrinsic pathway is regulated by the Bcl-2 family members, the extrinsic pathway is controlled by the Death receptors, members of the tumour necrosis factor (TNF) receptor superfamily. Death receptors can also activate a pro-inflammatory type of cell death, necroptosis, when Caspase-8 is inhibited. Apoptotic pathways are known to be tightly regulated by post-translational modifications, especially by ubiquitination. This review discusses research on ubiquitination-mediated regulation of apoptotic signalling. Additionally, the emerging importance of ubiquitination in regulating necroptosis is discussed.

BH3 Mimetics in Hematologic Malignancies

Hematologic malignancies (HM) comprise diverse cancers of lymphoid and myeloid origin, including lymphomas (approx. 40%), chronic lymphocytic leukemia (CLL, approx. 15%), multiple myeloma (MM, approx. 15%), acute myeloid leukemia (AML, approx. 10%), and many other diseases. Despite considerable improvement in treatment options and survival parameters in the new millennium, many patients with HM still develop chemotherapy-refractory diseases and require re-treatment. Because frontline therapies for the majority of HM (except for CLL) are still largely based on classical cytostatics, the relapses are often associated with defects in DNA damage response (DDR) pathways and anti-apoptotic blocks exemplified, respectively, by mutations or deletion of the TP53 tumor suppressor, and overexpression of anti-apoptotic proteins of the B-cell lymphoma 2 (BCL2) family. BCL2 homology 3 (BH3) mimetics represent a novel class of pro-apoptotic anti-cancer agents with a unique mode of action—direct targeting of mitochondria independently of TP53 gene aberrations. Consequently, BH3 mimetics can effectively eliminate even non-dividing malignant cells with adverse molecular cytogenetic alterations. Venetoclax, the nanomolar inhibitor of BCL2 anti-apoptotic protein has been approved for the therapy of CLL and AML. Numerous venetoclax-based combinatorial treatment regimens, next-generation BCL2 inhibitors, and myeloid cell leukemia 1 (MCL1) protein inhibitors, which are another class of BH3 mimetics with promising preclinical results, are currently being tested in several clinical trials in patients with diverse HM. These pivotal trials will soon answer critical questions and concerns about these innovative agents regarding not only their anti-tumor efficacy but also potential side effects, recommended dosages, and the optimal length of therapy as well as identification of reliable biomarkers of sensitivity or resistance. Effective harnessing of the full therapeutic potential of BH3 mimetics is a critical mission as it may directly translate into better management of the aggressive forms of HM and could lead to significantly improved survival parameters and quality of life in patients with urgent medical needs.

Elevated USP9X drives early-to-late-stage oral tumorigenesis via stabilisation of anti-apoptotic MCL-1 protein and impacts outcome in oral cancers

BACKGROUND

Overexpression of anti-apoptotic MCL-1 protein in oral squamous cell carcinoma (OSCC) is linked to disease progression, therapy resistance and poor outcome. Despite its characteristic short half-life owing to ubiquitin-proteasome-dependent degradation, oral tumours frequently show elevated MCL-1 protein expression. Hence, we investigated the role of deubiquitinase USP9X in stabilising MCL-1 protein and its contribution to oral tumorigenesis.

METHODS

Expression of MCL-1 and USP9X was assessed by immunoblotting and immunohistochemistry in oral cancer cell lines and tissues. The association between MCL-1 and USP9X was confirmed by coimmunoprecipitation and immunofluorescence. Cell death assessment was performed by MTT, flow cytometry and clonogenic assays.

RESULTS

Both USP9X and MCL-1 are significantly elevated in oral premalignant lesions and oral tumours versus normal mucosa. USP9X interacts with and deubiquitinates MCL-1, thereby stabilising it. Pharmacological inhibition of USP9X potently induced cell death in OSCC cells in vitro and in vivo. The elevated expression of USP9X and MCL-1 correlated with poor prognosis in OSCC patients.

CONCLUSION

We demonstrate the oncogenic role of USP9X in driving early-to-late stages of oral tumorigenesis via stabilisation of MCL-1, suggesting its potential as a prognostic biomarker and therapeutic target in oral cancers.

Impaired ribosome biogenesis checkpoint activation induces p53-dependent MCL-1 degradation and MYC-driven lymphoma death

MYC-driven B-cell lymphomas are addicted to increased levels of ribosome biogenesis (RiBi), offering the potential for therapeutic intervention. However, it is unclear whether inhibition of RiBi suppresses lymphomagenesis by decreasing translational capacity and/or by p53 activation mediated by the impaired RiBi checkpoint (IRBC). Here we generated Eμ-Myc lymphoma cells expressing inducible short hairpin RNAs to either ribosomal protein L7a (RPL7a) or RPL11, the latter an essential component of the IRBC. The loss of either protein reduced RiBi, protein synthesis, and cell proliferation to similar extents. However, only RPL7a depletion induced p53-mediated apoptosis through the selective proteasomal degradation of antiapoptotic MCL-1, indicating the critical role of the IRBC in this mechanism. Strikingly, low concentrations of the US Food and Drug Administration-approved anticancer RNA polymerase I inhibitor Actinomycin D (ActD) dramatically prolonged the survival of mice harboring Trp53+/+;Eμ-Myc but not Trp53-/-;Eμ-Myc lymphomas, which provides a rationale for treating MYC-driven B-cell lymphomas with ActD. Importantly, the molecular effects of ActD on Eμ-Myc cells were recapitulated in human B-cell lymphoma cell lines, highlighting the potential for ActD as a therapeutic avenue for p53 wild-type lymphoma.

The role of post-translational modifications in the regulation of MCL1

Apoptosis is an evolutionarily conserved form of programed cell death (PCD) that has a vital effect on early embryonic development, tissue homeostasis and clearance of damaged cells. Dysregulation of apoptosis can lead to many diseases, such as Alzheimer's disease, cancer, AIDS and heart disease. The anti-apoptotic protein MCL1, a member of the BCL2 family, plays important roles in these physiological and pathological processes. Its high expression is closely related to drug resistances in the treatment of tumor. This review summarizes the structure and function of MCL1, the types of post-translational modifications of MCL1 and their effects on the functions of MCL1, as well as the treatment strategies targeting MCL1 in cancer therapy. The research on the fine regulation of MCL1 will be favorable to the provision of a promising future for the design and screening of MCL1 inhibitors.

The future potential of Annona muricata L. extract and its bioactive compounds as radiation sensitizing agent: proposed mechanisms based on a systematic review

Despite technological advances in cancer treatment, especially in radiotherapy, many efforts are being made in improving cancer cell radio-sensitivity to increase therapeutic ratio and overcome cancer cell radio-resistance. In the present review, we evaluated the anticancer mechanism of Annona muricata L. (AM) leaves extract and its bioactive compounds such as annonaceous acetogenins, annomuricin, annonacin, or curcumin; and further correlated them with the potential of the mechanism to increase or to reduce cancer cells radio-sensitivity based on literature investigation. We see that AM has a promising future potential as a radio-sensitizer agent.

BH3 Mimetics for the Treatment of B-Cell Malignancies-Insights and Lessons from the Clinic

Simple Summary

B-cell malignancies, including chronic lymphocytic leukemia (CLL), non-Hodgkin lymphoma (NHL), and plasma cell dyscrasias, are significant contributors to cancer morbidity and mortality worldwide. The pathogenesis of many B-cell malignancies involves perturbations in the intrinsic pathway of apoptosis that allow cells to evade cell death. BH3 mimetics represent a class of anti-cancer agents that can restore the ability of cancer cells to undergo apoptosis. Venetoclax, a recently approved BH3 mimetic, has transformed the therapeutic landscape for CLL. Other BH3 mimetics are currently under development. This review summarizes the available data on existing BH3 mimetics and highlights both the rapidly expanding role of BH3 mimetics in the treatment of B-cell malignancies and the clinical challenges of their use.

Abstract

The discovery of the link between defective apoptotic regulation and cancer cell survival engendered the idea of targeting aberrant components of the apoptotic machinery for cancer therapy. The intrinsic pathway of apoptosis is tightly controlled by interactions amongst members of three distinct subgroups of the B-cell lymphoma 2 (BCL2) family of proteins. The pro-survival BCL2 proteins prevent apoptosis by keeping the pro-apoptotic effector proteins BCL2-associated X protein (BAX) and BCL2 homologous antagonist/killer (BAK) in check, while the BH3-only proteins initiate apoptosis by either neutralizing the pro-survival BCL2 proteins or directly activating the pro-apoptotic effector proteins. This tripartite regulatory mechanism is commonly perturbed in B-cell malignancies facilitating cell death evasion. Over the past two decades, structure-based drug discovery has resulted in the development of a series of small molecules that mimic the function of BH3-only proteins called the BH3 mimetics. The most clinically advanced of these is venetoclax, which is a highly selective inhibitor of BCL2 that has transformed the treatment landscape for chronic lymphocytic leukemia (CLL). Other BH3 mimetics, which selectively target myeloid cell leukemia 1 (MCL1) and B-cell lymphoma extra large (BCLxL), are currently under investigation for use in diverse malignancies. Here, we review the current role of BH3 mimetics in the treatment of CLL and other B-cell malignancies and address open questions in this rapidly evolving field.

Inhibition of proteasome reveals basal mitochondrial ubiquitination

Strict quality control for mitochondrial proteins is necessary to ensure cell homeostasis. Two cellular pathways-Ubiquitin Proteasome System (UPS) and autophagy-contribute to mitochondrial homeostasis under stressful conditions. Here, we investigate changes to the mitochondria proteome and to the ubiquitin landscape at mitochondria in response to proteasome inhibition. Treatment of HeLa cells devoid of Parkin, the primary E3 ligase responsible for mitophagy, with proteasome inhibitor MG132 for a few hours caused mitochondrial oxidative stress and fragmentation, reduced energy output, and increased mitochondrial ubiquitination without inducing mitophagy. Overexpression of Parkin did not show any induction of mitophagy in response to MG132 treatment. Analysis of ubiquitin chains on isolated mitochondria revealed predominance of K48, K29 and K63-linked polyubiquitin. Interestingly, of all ubiquitinated mitochondrial proteins detected in response to MG132 treatment, a majority (≥90%) were intramitochondrial irrespective of Parkin expression. However, overall levels of these ubiquitinated mitochondrial proteins did not change significantly upon proteasome inhibition when evaluated by quantitative proteomics (LFQ and SILAC), suggesting that only a small portion are ubiquitinated under basal conditions. Another aspect of proteasome inhibition is significant enrichment of UPS, lysosomal and phagosomal components, and other heat shock proteins associated with isolated mitochondria. Taken together, our study highlights a critical role of UPS for ubiquitinating and removing imported proteins as part of a basal mitochondrial quality control system independent of Parkin. SIGNIFICANCE: As centers of cellular bioenergetics, numerous metabolic pathways and signaling cascades, the health of mitochondria is of utmost importance for ensuring cell survival. Due to their unique physiology, mitochondria are constantly subjected to damaging oxidative radicals (ROS) and protein import-related stress due to buildup of unfolded aggregate-prone proteins. Thus, for quality control purposes, mitochondria are constantly under surveillance by Autophagy and the Ubiquitin Proteasome System (UPS), both of which share ubiquitin as a common signal. The ubiquitin landscape of mitochondria has been studied in detail under stressful conditions, however, little is known about basal mitochondrial ubiquitination. Our study reveals that the extent of ubiquitination at mitochondria greatly increases upon proteasome inhibition, pointing to a large number of potential substrates for proteasomal degradation. Interestingly, most of the ubiquitination occurs on intramitochondrial proteins, components of the electron transport chain (ETC) and matrix-resident metabolic enzymes in particular. Moreover, numerous cytosolic UPS components, chaperones and autophagy-lysosomal proteins were recruited to mitochondria upon proteasome inhibition. Taken together, this suggests that the levels and functions of mitochondrial proteins are constantly regulated through ubiquitin-dependent proteasomal degradation even under basal conditions. Unclogging mitochondrial import channels may provide a mechanism to alleviate stress associated with mitochondrial protein import or to adapt cells according to their metabolic needs. Therefore, targeting the mitochondrial ubiquitination/deubiquitination machinery, such as improving the therapeutic potency of proteasome inhibitors, may provide an additional therapeutic arsenal against tumors.

Ubiquitination and deubiquitination of MCL1 in cancer: deciphering chemoresistance mechanisms and providing potential therapeutic options

MCL1 is an important antiapoptotic member of the BCL-2 family that is distinguishable from other family members based on its relatively short half-life. Emerging studies have revealed the crucial role of MCL1 in the chemoresistance of cancer cells. The antiapoptotic function of MCL1 makes it a popular therapeutic target, although specific inhibitors have begun to emerge only recently. Notably, emerging studies have reported that several E3 ligases and deubiquitinases modulate MCL1 stability, providing an alternate means of targeting MCL1 activity. In addition, the emergence and development of proteolysis-targeting chimeras, the function of which is based on ubiquitination-mediated degradation, has shown great potential. In this review, we provide an overview of the studies investigating the ubiquitination and deubiquitination of MCL1, summarize the latest evidence regarding the development of therapeutic strategies targeting MCL1 in cancer treatment, and discuss the promising future of targeting MCL1 via the ubiquitin–proteasome system in clinical practice.

MARCH5 mediates NOXA-dependent MCL1 degradation driven by kinase inhibitors and integrated stress response activation

MCL1 has critical antiapoptotic functions and its levels are tightly regulated by ubiquitylation and degradation, but mechanisms that drive this degradation, particularly in solid tumors, remain to be established. We show here in prostate cancer cells that increased NOXA, mediated by kinase inhibitor activation of an integrated stress response, drives the degradation of MCL1, and identify the mitochondria-associated ubiquitin ligase MARCH5 as the primary mediator of this NOXA-dependent MCL1 degradation. Therapies that enhance MARCH5-mediated MCL1 degradation markedly enhance apoptosis in response to a BH3 mimetic agent targeting BCLXL, which may provide for a broadly effective therapy in solid tumors. Conversely, increased MCL1 in response to MARCH5 loss does not strongly sensitize to BH3 mimetic drugs targeting MCL1, but instead also sensitizes to BCLXL inhibition, revealing a codependence between MARCH5 and MCL1 that may also be exploited in tumors with MARCH5 genomic loss.

Mcl-1 as a "barrier" in cancer treatment: Can we target it now?

During the last two decades, the study of Mcl-1, an anti-apoptotic member of the Bcl-2 family, attracted researchers due to its important role in cancer cell survival and tumor development. The significance of Mcl-1 protein in resistance to chemotherapeutics makes it an attractive target in cancer therapy. Here, we discuss the diverse possibilities for indirect Mcl-1 inhibition through its downregulation, for example, via targeting for proteasomal degradation or blockage of translation and transcription. We also provide an overview of the direct blocking of protein-protein interactions with pro-apoptotic Bcl-2 family proteins, including examples of the most promising regulators of Mcl-1 and selective BH3-mimetics, which at present are under clinical evaluation. Moreover, several approaches for the co-targeting of Mcl-1 and other proteins (e.g., CDKs) are also presented. In addition, we highlight the broad spectrum of problems that accompanied the discovery and development of effective Mcl-1 inhibitors.

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.

Recent advances in the development of Mcl-1 inhibitors for cancer therapy

Dysregulation of the mitochondrial apoptotic pathway controlled by members of the Bcl-2 protein family plays a central role in cancer development and resistance to conventional cytotoxic as well as targeted therapies. Hence, selective inhibition of pro-survival Bcl-2 family of proteins to activate apoptosis in malignant cells represents an exciting anti-cancer strategy. The remarkable clinical performance of the selective Bcl-2 antagonist venetoclax has highlighted the potential for selective inhibitors of the other pro-survival members of the Bcl-2 family, particularly Mcl-1. Here we review the latest progress on the discovery and development of selective inhibitors of Mcl-1 that are undergoing clinical evaluation for cancer therapy.

Phosphorylation alters Bim-mediated Mcl-1 stabilization and priming

Mcl-1 is a highly labile protein, subject to extensive post-translational regulation. This distinguishes Mcl-1 from other antiapoptotic proteins and necessitates further study to better understand how interactions with proapoptotic Bcl-2 proteins affect its regulation. One such protein, Bim, is known to stabilize Mcl-1, and Bim phosphorylation has been associated with increased Mcl-1 binding. Consequently, we investigated the potential impact of Bim phosphorylation on Mcl-1 stability. We found that Bim stabilizes and primes Mcl-1 in RPCI-WM1 cells and is constitutively phosphorylated. Additionally, introduction of several phospho-mimetic and unphosphosphorylateable Bim mutations resulted in altered Mcl-1 stability and distinct Bim binding to antiapoptotic proteins. These findings suggest Bim phosphorylation not only regulates Mcl-1 stability but also is a potential mechanism for enforcing Mcl-1 dependence.

Delineating Crosstalk Mechanisms of the Ubiquitin Proteasome System That Regulate Apoptosis

Regulatory functions of the ubiquitin-proteasome system (UPS) are exercised mainly by the ubiquitin ligases and deubiquitinating enzymes. Degradation of apoptotic proteins by UPS is central to the maintenance of cell health, and deregulation of this process is associated with several diseases including tumors, neurodegenerative disorders, diabetes, and inflammation. Therefore, it is the view that interrogating protein turnover in cells can offer a strategy for delineating disease-causing mechanistic perturbations and facilitate identification of drug targets. In this review, we are summarizing an overview to elucidate the updated knowledge on the molecular interplay between the apoptosis and UPS pathways. We have condensed around 100 enzymes of UPS machinery from the literature that ubiquitinates or deubiquitinates the apoptotic proteins and regulates the cell fate. We have also provided a detailed insight into how the UPS proteins are able to fine-tune the intrinsic, extrinsic, and p53-mediated apoptotic pathways to regulate cell survival or cell death. This review provides a comprehensive overview of the potential of UPS players as a drug target for cancer and other human disorders.

Metformin enhances TRAIL-induced apoptosis by Mcl-1 degradation via Mule in colorectal cancer cells

Metformin is an anti-diabetic drug with a promising anti-cancer potential. In this study, we show that subtoxic doses of metformin effectively sensitize human colorectal cancer (CRC) cells to tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), which induces apoptosis. Metformin alone did not induce apoptosis, but significantly potentiated TRAIL-induced apoptosis in CRC cells. CRC cells treated with metformin and TRAIL showed activation of the intrinsic and extrinsic pathways of caspase activation. We attempted to elucidate the underlying mechanism, and found that metformin significantly reduced the protein levels of myeloid cell leukemia 1 (Mcl-1) in CRC cells and, the overexpression of Mcl-1 inhibited cell death induced by metformin and/or TRAIL. Further experiments revealed that metformin did not affect mRNA levels, but increased proteasomal degradation and protein stability of Mcl-1. Knockdown of Mule triggered a significant decrease of Mcl-1 polyubiquitination. Metformin caused the dissociation of Noxa from Mcl-1, which allowed the binding of the BH3-containing ubiquitin ligase Mule followed by Mcl-1ubiquitination and degradation. The metformin-induced degradation of Mcl-1 required E3 ligase Mule, which is responsible for the polyubiquitination of Mcl-1. Our study is the first report indicating that metformin enhances TRAIL-induced apoptosis through Noxa and favors the interaction between Mcl-1 and Mule, which consequently affects Mcl-1 ubiquitination.

Myeloid cell leukemia 1 (MCL-1), an unexpected modulator of protein kinase signaling during invasion

Myeloid cell leukemia-1 (MCL-1), closely related to B-cell lymphoma 2 (BCL-2), has a well-established role in cell survival and has emerged as an important target for cancer therapeutics. We have demonstrated that inhibiting MCL-1 is efficacious in suppressing tumour progression in pre-clinical models of breast cancer and revealed that in addition to its role in cell survival, MCL-1 modulated cellular invasion. Utilizing a MCL-1-specific genetic antagonist, we found two possible mechanisms; firstly MCL-1 directly binds to and alters the phosphorylation of the cytoskeletal remodeling protein, Cofilin, a protein important for cytoskeletal remodeling during invasion, and secondly MCL-1 modulates the levels SRC family kinases (SFKs) and their targets. These data provide evidence that MCL-1 activities are not limited to endpoints of extracellular and intracellular signaling culminating in cell survival as previously thought, but can directly modulate the output of SRC family kinases signaling during cellular invasion. Here we review the pleotropic roles of MCL-1 and discuss the implications of this newly discovered effect on protein kinase signaling for the development of cancer therapeutics.

High efficacy of the BCL-2 inhibitor ABT199 (venetoclax) in BCL-2 high-expressing neuroblastoma cell lines and xenografts and rational for combination with MCL-1 inhibition

The anti-apoptotic protein B cell lymphoma/leukaemia 2 (BCL-2) is highly expressed in neuroblastoma and plays an important role in oncogenesis. In this study, the selective BCL-2 inhibitor ABT199 was tested in a panel of neuroblastoma cell lines with diverse expression levels of BCL-2 and other BCL-2 family proteins. ABT199 caused apoptosis more potently in neuroblastoma cell lines expressing high BCL-2 and BIM/BCL-2 complex levels than low expressing cell lines. Effects on cell viability correlated with effects on BIM displacement from BCL-2 and cytochrome c release from the mitochondria. ABT199 treatment of mice with neuroblastoma tumors expressing high BCL-2 levels only resulted in growth inhibition, despite maximum BIM displacement from BCL-2 and the induction of a strong apoptotic response. We showed that neuroblastoma cells might survive ABT199 treatment due to its acute upregulation of the anti-apoptotic BCL-2 family protein myeloid cell leukaemia sequence 1 (MCL-1) and BIM sequestration by MCL-1. In vitro inhibition of MCL-1 sensitized neuroblastoma cell lines to ABT199, confirming the pivotal role of MCL-1 in ABT199 resistance. Our findings suggest that neuroblastoma patients with high BCL-2 and BIM/BCL-2 complex levels might benefit from combination treatment with ABT199 and compounds that inhibit MCL-1 expression.

Found in Translation: How Preclinical Research Is Guiding the Clinical Development of the BCL2-Selective Inhibitor Venetoclax

Since the discovery of apoptosis as a form of programmed cell death, targeting the apoptosis pathway to induce cancer cell death has been a high-priority goal for cancer therapy. After decades of effort, drug-discovery scientists have succeeded in generating small-molecule inhibitors of antiapoptotic BCL2 family proteins. Innovative medicinal chemistry and structure-based drug design, coupled with a strong fundamental understanding of BCL2 biology, were essential to the development of BH3 mimetics such as the BCL2-selective inhibitor venetoclax. We review a number of preclinical studies that have deepened our understanding of BCL2 biology and facilitated the clinical development of venetoclax.Significance: Basic research into the pathways governing programmed cell death have paved the way for the discovery of apoptosis-inducing agents such as venetoclax, a BCL2-selective inhibitor that was recently approved by the FDA and the European Medicines Agency. Preclinical studies aimed at identifying BCL2-dependent tumor types have translated well into the clinic thus far and will likely continue to inform the clinical development of venetoclax and other BCL2 family inhibitors. Cancer Discov; 7(12); 1376-93. ©2017 AACR.

The BET-Bromodomain Inhibitor JQ1 synergized ABT-263 against colorectal cancer cells through suppressing c-Myc-induced miR-1271-5p expression

Colorectal cancer (CRC) cells undergo apoptosis in the presence of the small-molecule inhibitor ABT-263 by up-regulating antiapoptotic Bcl-2 family members. However, the resistance to ABT-263 gradually developed in most solid tumors due to its low affinity to Mcl-1. Here, we found the BET-Bromodomain inhibitor JQ1, when combined with ABT-263, synergistically reduced Mcl-1 protein level, induced apoptosis, and decreased cell viability in the CRC HCT-15, HT-29 and SW620 cells. The subsequent mechanism study revealed that a pathway of c-Myc/miR-1271-5p/Noxa/Mcl-1 underlies the synergistic effect of such combination treatment. We discovered that miR-1271-5p, the key mediator for the synergistic effect, is transcriptionally activated by c-Myc, and binds to the 3'-UTR of noxa to inhibit its protein production. The combination treatment of JQ1 and ABT-263 inhibited c-Myc protein level and also c-Myc-driven expression of miR-1271-5p, subsequently increased the protein level of Noxa, and finally promotes the degradation of Mcl-1. Our findings provide an alternative strategy to resolve the resistance during treatment of CRC by JQ1, and also discovered a novel miR-1271-5p-dependent regulatory mechanism for gene expression of noxa.

Arenobufagin Induces Apoptotic Cell Death in Human Non-Small-Cell Lung Cancer Cells via the Noxa-Related Pathway

Arenobufagin, an active component isolated from the traditional Chinese medicine Chan Su, exhibits anticancer influences in several human malignancies. However, the effects and action mechanisms of arenobufagin on non-small-cell lung cancer (NSCLC) are still unknown. In this study, we reported that arenobufagin acted through activation of Noxa-related pathways and promoted apoptotic cell death in human NSCLC cells. Our results revealed that arenobufagin-induced apoptosis was caspase-dependent, as evidenced by the fact that caspase-9, caspase-3 and poly (ADP-ribose) polymerase (PARP) were cleaved, and pretreatment with a pan-caspase inhibitor Z-VAD-FMK inhibited the pro-apoptosis effect of arenobufagin. Mechanistically, we further found that arenobufagin rapidly upregulated the expression of the pro-apoptosis protein Noxa, and abrogated the anti-apoptosis protein Mcl-1, a major binding partner of Noxa in the cell. More importantly, the knockdown of Noxa greatly blocked arenobufagin-induced cell death, highlighting the contribution of this protein in the anti-NSCLC effects of arenobufagin. Interestingly, arenobufagin also increased the expression of p53, a direct transcriptional activator for the upregulation of the Noxa protein. Taken together, our results suggest that arenobufagin is a potential anti-NSCLC agent that triggers apoptotic cell death in NSCLC cells through interfering with the Noxa-related pathway.

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.

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

A common therapeutic strategy to combat human cancer is the use of combinations of drugs, each targeting different cellular processes or vulnerabilities. Recent studies suggest that addition of an MCL-1 inhibitor to such anticancer drug treatments could be an attractive therapeutic strategy. Thus, it is of great interest to understand whether combinations of conventional anticancer drugs with an MCL-1 inhibitor will be tolerable and efficacious. In order to mimic the combination of MCL-1 inhibition with other cancer therapeutics, we treated Mcl-1^+/- heterozygous mice, which have a ~50% reduction in MCL-1 protein in their cells, with a broad range of chemotherapeutic drugs. Careful monitoring of treated mice revealed that a wide range of chemotherapeutic drugs had no significant effect on the general well-being of Mcl-1^+/- mice with no overt damage to a broad range of tissues, including the haematopoietic compartment, heart, liver and kidney. These results indicate that MCL-1 inhibition may represent a tolerable strategy in cancer therapy, even when combined with select cytotoxic drugs.

HDAC1 Upregulation by NANOG Promotes Multidrug Resistance and a Stem-like Phenotype in Immune Edited Tumor Cells

Cancer immunoediting drives the adaptation of tumor cells to host immune surveillance. Immunoediting driven by antigen (Ag)-specific T cells enriches NANOG expression in tumor cells, resulting in a stem-like phenotype and immune resistance. Here, we identify HDAC1 as a key mediator of the NANOG-associated phenotype. NANOG upregulated HDAC1 through promoter occupancy, thereby decreasing histone H3 acetylation on K14 and K27. NANOG-dependent, HDAC1-driven epigenetic silencing of cell-cycle inhibitors CDKN2D and CDKN1B induced stem-like features. Silencing of TRIM17 and NOXA induced immune and drug resistance in tumor cells by increasing antiapoptotic MCL1. Importantly, HDAC inhibition synergized with Ag-specific adoptive T-cell therapy to control immune refractory cancers. Our results reveal that NANOG influences the epigenetic state of tumor cells via HDAC1, and they encourage a rational application of epigenetic modulators and immunotherapy in treatment of NANOG⁺ refractory cancer types. Cancer Res; 77(18); 5039-53. ©2017 AACR.

Histone Deacetylase Inhibitor Enhances the Efficacy of MEK Inhibitor through NOXA-Mediated MCL1 Degradation in Triple-Negative and Inflammatory Breast Cancer

Purpose: Inflammatory breast cancer (IBC), diagnosed clinically, and triple-negative breast cancer (TNBC), diagnosed by molecular receptor status, are the two most aggressive forms of breast cancer, and both lack effective targeted therapies. We previously demonstrated involvement of histone deacetylase (HDAC) inhibitor entinostat in regulating apoptosis in IBC and TNBC cells; here, we aimed to identify novel combination therapy candidates.Experimental Design: Potential therapeutic targets were identified by mRNA expression profiling of TNBC and IBC cells treated with entinostat. Drug action and synergism were assessed by in vitro proliferation assays, tumor growth in vivo, and proteomic analyses. Gain/loss-of-expression studies were utilized to functionally validate the role of identified targets in sensitivity of TNBC and IBC cells to combination therapy.Results: Entinostat induced activity of the oncogenic ERK pathway and expression of proapoptotic NOXA. These are known to stabilize and degrade, respectively, MCL1, an antiapoptotic Bcl-2 protein. In breast cancer patients, high-MCL1/low-NOXA tumor expression correlated significantly with poor survival outcomes. Combination treatment of entinostat with MEK inhibitor pimasertib reduced the growth of TNBC and IBC cells in vitro and inhibited tumor growth in vivo The synergistic action of combination therapy was observed in TNBC and IBC cell lines in which NOXA expression was induced following entinostat treatment. The therapeutic activity depended on induction of mitochondrial cell death pathways initiated by NOXA-mediated MCL1 degradation.Conclusions: Our preclinical findings provide a rationale for the clinical testing of combination HDAC and MEK pathway inhibition for TNBC and IBC that exhibit elevated baseline tumor MCL1 expression. Clin Cancer Res; 23(16); 4780-92. ©2017 AACR.

Inhibition of histone/lysine acetyltransferase activity kills CoCl2-treated and hypoxia-exposed gastric cancer cells and reduces their invasiveness

Hypoxia enhances immortality and metastatic properties of solid tumors. Deregulation of histone acetylation has been associated with several metastatic cancers but its effect on hypoxic responses of cancer cells is not known. This study aimed at understanding the effectiveness of the hydrazinocurcumin, CTK7A, an inhibitor of p300 lysine/histone acetyltransferase (KAT/HAT) activity, in inducing apoptosis of gastric cancer cells (GCCs) exposed to cobalt chloride (CoCl₂), a hypoxia-mimetic chemical, or 1% O₂. Here, we show that CTK7A-induced hydrogen peroxide (H₂O₂) generation in CoCl₂-exposed and invasive gastric cancer cells (GCCs) leads to p38 MAPK-mediated Noxa expression and thereafter, mitochondrial apoptotic events. Noxa induction in normal immortalized gastric epithelial cells after CTK7A and hypoxia-exposure is remarkably less in comparison to similarly-treated GCCs. Moreover, hypoxia-exposed GCCs, which have acquired invasive properties, become apoptotic after CTK7A treatment to a significantly higher extent than normoxic cells. Thus, we show the potential of CTK7A in sensitizing hypoxic and metastatic GCCs to apoptosis induction.

Akt inhibition improves irinotecan treatment and prevents cell emergence by switching the senescence response to apoptosis

Activated in response to chemotherapy, senescence is a tumor suppressive mechanism that induces a permanent loss of proliferation. However, in response to treatment, it is not really known how cells can escape senescence and how irreversible or incomplete this pathway is. We have recently described that cells that escape senescence are more transformed than non-treated parental cells, they resist anoikis and rely on Mcl-1. In this study, we further characterize this emergence in response to irinotecan, a first line treatment used in colorectal cancer. Our results indicate that Akt was activated as a feedback pathway during the early step of senescence. The inhibition of the kinase prevented cell emergence and improved treatment efficacy, both in vitro and in vivo. This improvement was correlated with senescence inhibition, p21waf1 downregulation and a concomitant activation of apoptosis due to Noxa upregulation and Mcl-1 inactivation. The inactivation of Noxa prevented apoptosis and increased the number of emergent cells. Using either RNA interference or p21waf1-deficient cells, we further confirmed that an intact p53-p21-senescence pathway favored cell emergence and that its downregulation improved treatment efficacy through apoptosis induction. Therefore, although senescence is an efficient suppressive mechanism, it also generates more aggressive cells as a consequence of apoptosis inhibition. We therefore propose that senescence-inducing therapies should be used sequentially with drugs favoring cell death such as Akt inhibitors. This should reduce cell emergence and tumor relapse through a combined induction of senescence and apoptosis.

Repression of Mcl-1 and disruption of the Mcl-1/Bak interaction in myeloma cells couple ER stress to mitochondrial apoptosis

As myeloma cells actively produce and secrete immunoglobulins, they are prone to ER stress, which if unresolved leads to apoptosis. We found that myeloma cell death induced by the ER stressor Thapsigargin was highly variable, ranging from 2 to 89%. Induction of ATF4 and CHOP was observed in myeloma cells under Thapsigargin independently of cell death. The decrease in Mcl-1 was associated with protein translation inhibition and identified as a crucial factor in Thapsigargin sensitivity, since it was the only Bcl-2 family protein differentially modified between sensitive and resistant myeloma cells. Bak but not Bax was found to contribute to Thapsigargin-induced apoptosis. Appropriately, a basal Mcl-1/Bak interaction was demonstrated in Thapsigargin-sensitive cells. Of note, the only pro-apoptotic protein freed from Mcl-1 under Thapsigargin was Bak, whereas Mcl-1/Noxa or Mcl-1/Bim complexes were simultaneously increased. Thus, the disruption of the basal Mcl-1/Bak complex in Thapsigargin-sensitive cells seemed to be an essential event in cell death induction, probably favored by the induced Noxa and Bim BH3-only proteins. These findings underscore the implication of the Mcl-1/Bak axis in myeloma cell death triggered by Thapsigargin.

Impact of loss of BH3-only proteins on the development and treatment of MLL-fusion gene-driven AML in mice

Inhibition of the apoptosis pathway controlled by opposing members of the Bcl-2 protein family plays a central role in cancer development and resistance to therapy. To investigate how pro-apoptotic Bcl-2 homology domain 3 (BH3)-only proteins impact on acute myeloid leukemia (AML), we generated mixed lineage leukemia (MLL)-AF9 and MLL-ENL AMLs from BH3-only gene knockout mice. Disease development was not accelerated by loss of Bim, Puma, Noxa, Bmf, or combinations thereof; hence these BH3-only proteins are apparently ineffectual as tumor suppressors in this model. We tested the sensitivity of MLL-AF9 AMLs of each genotype in vitro to standard chemotherapeutic drugs and to the proteasome inhibitor bortezomib, with or without the BH3 mimetic ABT-737. Loss of Puma and/or Noxa increased resistance to cytarabine, daunorubicin and etoposide, while loss of Bim protected against cytarabine and loss of Bmf had no impact. ABT-737 increased sensitivity to the genotoxic drugs but was not dependent on any BH3-only protein tested. The AML lines were very sensitive to bortezomib and loss of Noxa conveyed significant resistance. In vivo, several MLL-AF9 AMLs responded well to daunorubicin and this response was highly dependent on Puma and Noxa but not Bim. Combination therapy with ABT-737 provided little added benefit at the daunorubicin dose trialed. Bortezomib also extended survival of AML-bearing mice, albeit less than daunorubicin. In summary, our genetic studies reveal the importance of Puma and Noxa for the action of genotoxics currently used to treat MLL-driven AML and suggest that, while addition of ABT-737-like BH3 mimetics might enhance their efficacy, new Noxa-like BH3 mimetics targeting Mcl-1 might have greater potential.

BCL2 Inhibitors as Anticancer Drugs: A Plethora of Misleading BH3 Mimetics

Antiapoptotic BCL2 proteins play a major role in tumor cell survival. Hence, BCL2 inhibitors have been developed as direct inducers of apoptosis. ABT-199 (venetoclax) received breakthrough therapy designation from the FDA due to its apparent efficacy in CLL and AML. However, resistance to ABT-199 is mediated by other BCL2 proteins including BCLXL and MCL1. Considerable effort has been expended seeking novel "BH3 mimetics" that inhibit all of these BCL2 proteins. While many BH3 mimetics inhibit BCL2 proteins in vitro, they fail to directly inhibit them in intact cells. Many BH3 mimetics induce the unfolded protein response culminating in induction of the proapoptotic protein NOXA, which in turn inhibits MCL1. We propose simple experiments to validate BH3 mimetics in cells. A true BCL2 inhibitor will rapidly induce apoptosis in chronic lymphocytic leukemia cells ex vivo A BCLXL inhibitor will rapidly induce apoptosis in platelets. Finally, a BH3 mimetic targeting MCL1 will inhibit its degradation thereby inducing rapid MCL1 accumulation. Compounds that fail these tests should no longer be called BH3 mimetics. We now have a toolbox of selective inhibitors for most of the BCL2 proteins, and we hope these new tools will lead to effective treatment options for many cancers. Mol Cancer Ther; 15(9); 2011-7. ©2016 AACR.

PTBP1 modulation of MCL1 expression regulates cellular apoptosis induced by antitubulin chemotherapeutics

Myeloid cell leukemia sequence 1 (MCL1), an anti-apoptotic BCL2 family protein, is a key regulator of intrinsic apoptosis. Normal cells require strict control over MCL1 expression with aberrant MCL1 expression linked to the emergence of various diseases and chemoresistance. Previous studies have detailed how MCL1 expression is regulated by multiple mechanisms both transcriptionally and translationally. However, characterization of the post-transcriptional regulators of MCL1 mRNA is limited. Polypyrimidine tract binding protein 1 (PTBP1) is a known regulator of post-transcriptional gene expression that can control mRNA splicing, translation, stability and localization. Here we demonstrate that PTBP1 binds to MCL1 mRNA and that knockdown of PTBP1 upregulates MCL1 expression in cancer cells by stabilizing MCL1 mRNA and increasing MCL1 mRNA accumulation in cytoplasm. Further, we show that depletion of PTBP1 protects cancer cells from antitubulin agent-induced apoptosis in a MCL1-dependent manner. Taken together, our findings suggest that PTBP1 is a novel regulator of MCL1 mRNA by which it controls apoptotic response to antitubulin chemotherapeutics.

Regulation of Bim in Health and Disease

The BH3-only Bim protein is a major determinant for initiating the intrinsic apoptotic pathway under both physiological and pathophysiological conditions. Tight regulation of its expression and activity at the transcriptional, translational and post-translational levels together with the induction of alternatively spliced isoforms with different pro-apoptotic potential, ensure timely activation of Bim. Under physiological conditions, Bim is essential for shaping immune responses where its absence promotes autoimmunity, while too early Bim induction eliminates cytotoxic T cells prematurely, resulting in chronic inflammation and tumor progression. Enhanced Bim induction in neurons causes neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's diseases. Moreover, type I diabetes is promoted by genetically predisposed elevation of Bim in β-cells. On the contrary, cancer cells have developed mechanisms that suppress Bim expression necessary for tumor progression and metastasis. This review focuses on the intricate network regulating Bim activity and its involvement in physiological and pathophysiological processes.

Dichloroacetate affects proliferation but not survival of human colorectal cancer cells

Dichloroacetate (DCA) is a metabolic reprogramming agent that reverses the Warburg effect, causing cancer cells to couple glycolysis to oxidative phosphorylation. This has been shown to induce apoptosis and reduce the growth of various types of cancer but not normal cells. Colorectal cancer cells HCT116, HCT116 p53(-/-), and HCT116 Bax(-/-), were treated with DCA in vitro. Response to treatment was determined by measuring PDH phosphorylation, apoptosis, proliferation, and cell cycle. Molecular changes associated with these responses were determined using western immunoblotting and quantitative PCR. Treatment with 20 mM DCA did not increase apoptosis, despite decreasing levels of anti-apoptotic protein Mcl-1 after 6 h, in any of the cell lines observed. Mcl-1 expression was stabilized with MG-132, an inhibitor of proteasomal degradation. A decrease in Mcl-1 correlated with a decrease in proliferation, both of which showed dose-dependence in DCA treated cells. Cells showed nuclear localization of Mcl-1, however cell cycle was unaffected by DCA treatment. These data suggest that a reduction in the prosurvival Bcl-2 family member Mcl-1 due to increased proteasomal degradation is correlated with the ability of DCA to reduce proliferation of HCT116 human colorectal cancer cells without causing apoptosis.