Mcl-1 expression predicts progression-free survival in chronic lymphocytic leukemia patients treated with pentostatin, cyclophosphamide, and rituximab

MNDA, a PYHIN factor involved in transcriptional regulation and apoptosis control in leukocytes

Inflammation control is critical during the innate immune response. Such response is triggered by the detection of molecules originating from pathogens or damaged host cells by pattern-recognition receptors (PRRs). PRRs subsequently initiate intra-cellular signalling through different pathways, resulting in i) the production of inflammatory cytokines, including type I interferon (IFN), and ii) the initiation of a cascade of events that promote both immediate host responses as well as adaptive immune responses. All human PYRIN and HIN-200 domains (PYHIN) protein family members were initially proposed to be PRRs, although this view has been challenged by reports that revealed their impact on other cellular mechanisms. Of relevance here, the human PYHIN factor myeloid nuclear differentiation antigen (MNDA) has recently been shown to directly control the transcription of genes encoding factors that regulate programmed cell death and inflammation. While MNDA is mainly found in the nucleus of leukocytes of both myeloid (neutrophils and monocytes) and lymphoid (B-cell) origin, its subcellular localization has been shown to be modulated in response to genotoxic agents that induce apoptosis and by bacterial constituents, mediators of inflammation. Prior studies have noted the importance of MNDA as a marker for certain forms of lymphoma, and as a clinical prognostic factor for hematopoietic diseases characterized by defective regulation of apoptosis. Abnormal expression of MNDA has also been associated with altered levels of cytokines and other inflammatory mediators. Refining our comprehension of the regulatory mechanisms governing the expression of MNDA and other PYHIN proteins, as well as enhancing our definition of their molecular functions, could significantly influence the management and treatment strategies of numerous human diseases. Here, we review the current state of knowledge regarding PYHIN proteins and their role in innate and adaptive immune responses. Emphasis will be placed on the regulation, function, and relevance of MNDA expression in the control of gene transcription and RNA stability during cell death and inflammation.

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.

Difference in the binding mechanisms of ABT-263/43b with Bcl-xL/Bcl-2: computational perspective on the accurate binding free energy analysis

B-cell lymphoma/leukemia gene-2(Bcl-2) protein family known for regulating cell cycle arrest and subsequent cell death is highly expressed in a variety of cancers. Among them, the Bcl-xL and Bcl-2 are two essential proteins in the Bcl-2 family. In the present work, the differences in binding modes as between the two proteins and two ligands ABT-263/43b were investigated and compared. And the computational alanine scanning combined with the recently developed interaction entropy (AS-IE) method was employed for predicting their binding free energies and finding those amino acids that were more critical during the binding process. The result showed that the binding free energy calculated by the AS-IE method was more in line with experimental values than the molecular mechanics/Poisson-Boltzmann surface area (MM/PBSA) method. Besides, no significant difference was found between Bcl-xL and ABT-263/43b in the binding free energy, which Bcl-xL showed slightly weaker binding free energy to 43b because of the fewer number of key residues with interactions. Nonetheless, compared with the Bcl-2 and 43b complex, the Bcl-2 and ABT-263 system had greater number of key residues interacting with ABT-263, in particular, contribute favorably, resulting in a stronger binding ability for the Bcl-2 and ABT-263 systems. The van der Waals and hydrogen bond contributions were significant in the four protein-ligand complexes. Overall, Tyr108 was found to be the common key residues in the Bcl-xL-ligand complex, while Tyr105, Glu100, and Glu143 were established as the common key residue in the Bcl-2-ligand systems. We hope that the predicted hot spot residues and their energy distributions can guide the design of peptide and small-molecule drugs targeting Bcl-xL and Bcl-2.

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.

Extrinsic interactions in the microenvironment in vivo activate an antiapoptotic multidrug-resistant phenotype in CLL

The Bcl-2 inhibitor venetoclax has yielded exceptional clinical responses in chronic lymphocytic leukemia (CLL). However, de novo resistance can result in failure to achieve negative minimal residual disease and predicts poor treatment outcomes. Consequently, additional proapoptotic drugs, such as inhibitors of Mcl-1 and Bcl-xL, are in development. By profiling antiapoptotic proteins using flow cytometry, we find that leukemic B cells that recently emigrated from the lymph node (CD69+/CXCR4Low) in vivo are enriched for cell clusters simultaneously overexpressing multiple antiapoptotic proteins (Mcl-1High/Bcl-xLHigh/Bcl-2High) in both treated and treatment-naive CLL patients. These cells exhibited antiapoptotic resistance to multiple BH-domain antagonists, including inhibitors of Bcl-2, Mcl-1, and Bcl-xL, when tested as single agents in a flow cytometry-based functional assay. Antiapoptotic multidrug resistance declines ex vivo, consistent with resistance being generated in vivo by extrinsic microenvironmental interactions. Surviving "persister" cells in patients undergoing venetoclax treatment are enriched for CLL cells displaying the functional and molecular properties of microenvironmentally induced multidrug resistance. Overcoming this resistance required simultaneous inhibition of multiple antiapoptotic proteins, with potential for unwanted toxicities. Using a drug screen performed using patient peripheral blood mononuclear cells cultured in an ex vivo microenvironment model, we identify novel venetoclax drug combinations that induce selective cytotoxicity in multidrug-resistant CLL cells. Thus, we demonstrate that antiapoptotic multidrug-resistant CLL cells exist in patients de novo and show that these cells persist during proapoptotic treatment, such as venetoclax. We validate clinically actionable approaches to selectively deplete this reservoir in patients.

Pharmacologic Targeting of Mcl-1 Induces Mitochondrial Dysfunction and Apoptosis in B-Cell Lymphoma Cells in a TP53- and BAX-Dependent Manner

PURPOSE

Bcl-2 has been effectively targeted in lymphoid malignancies. However, resistance is inevitable, and novel approaches to target mitochondrial apoptosis are necessary. AZD5991, a selective BH3-mimetic in clinical trials, inhibits Mcl-1 with high potency.

EXPERIMENTAL DESIGN

We explored the preclinical activity of AZD5991 in diffuse large B-cell lymphoma (DLBCL) and ibrutinib-resistant mantle cell lymphoma (MCL) cell lines, MCL patient samples, and mice bearing DLBCL and MCL xenografts using flow cytometry, immunoblotting, and Seahorse respirometry assay. Cas9 gene editing and ex vivo functional drug screen assays helped identify mechanisms of resistance to Mcl-1 inhibition.

RESULTS

Mcl-1 was expressed in DLBCL and MCL cell lines and primary tumors. Treatment with AZD5991 restricted growth of DLBCL cells independent of cell of origin and overcame ibrutinib resistance in MCL cells. Mcl-1 inhibition led to mitochondrial dysfunction as manifested by mitochondrial membrane depolarization, decreased mitochondrial mass, and induction of mitophagy. This was accompanied by impairment of oxidative phosphorylation. TP53 and BAX were essential for sensitivity to Mcl-1, and oxidative phosphorylation was implicated in resistance to Mcl-1 inhibition. Induction of prosurvival proteins (e.g., Bcl-xL) in stromal conditions that mimic the tumor microenvironment rendered protection of primary MCL cells from Mcl-1 inhibition, while BH3-mimetics targeting Bcl-2/xL sensitized lymphoid cells to AZD5991. Treatment with AZD5991 reduced tumor growth in murine lymphoma models and prolonged survival of MCL PDX mice.

CONCLUSIONS

Selective targeting Mcl-1 is a promising therapeutic approach in lymphoid malignancies. TP53 apoptotic network and metabolic reprogramming underlie susceptibility to Mcl-1 inhibition.

Correlation Between BCL2 and Mcl1 Single Nucleotide Polymorphisms and Chemotherapy Response in Jordanian Patients with Colorectal Cancer

BACKGROUND

Colorectal cancer is one of the most common types of cancer worldwide and a leading cause of death in Jordan. BCL-2 and MCL-1 are anti-apoptotic proteins that inhibit programmed cell death and their over-expression has been shown to be associated with reduced sensitivity to chemotherapy and poor survival in cancer patients.

OBJECTIVES

In the present study, three SNPs in the promoter region of antiapoptotic genes were investigated in an effort to inspect the occurrences of SNPs (rs2279115, rs4987852) in the promoter region of BCL2 and SNP (rs9803935) in the promoter region of MCL1 in Jordanian patients with CRC, and investigate correlations between BCL2 and MCL1 SNPs and clinical outcomes.

METHODS

PCR-restriction fragment length polymorphism (RFLP)-based analysis was used for samples genotyping.

RESULTS

The BCL2 rs2279115 and MCL1 rs9803935 SNPs showed significant distribution where mutant and hetero genotypes are more prominent in CRC patients. Additionally, the rs2279115 genotypes and alleles were associated with stages of disease, its recurrence and metastasis. The MCL1 rs9803935 genotypes were associated disease metastasis. However, for BCL2 rs4987852 SNP, there was no association of genotypes or alleles with any of the disease variables.

CONCLUSION

The BCL2 SNPs (rs2279115) and MCL1 SNP (rs9803935) present as important determinants of the progress of CRC in Jordanian patients.

Activation of Interferon Signaling in Chronic Lymphocytic Leukemia Cells Contributes to Apoptosis Resistance via a JAK-Src/STAT3/Mcl-1 Signaling Pathway

Besides their antiviral and immunomodulatory functions, type I (α/β) and II (γ) interferons (IFNs) exhibit either beneficial or detrimental effects on tumor progression. Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of abnormal CD5⁺ B lymphocytes that escape death. Drug resistance and disease relapse still occur in CLL. The triggering of IFN receptors is believed to be involved in the survival of CLL cells, but the underlying molecular mechanisms are not yet characterized. We show here that both type I and II IFNs promote the survival of primary CLL cells by counteracting the mitochondrial (intrinsic) apoptosis pathway. The survival process was associated with the upregulation of signal transducer and activator of transcription-3 (STAT3) and its target anti-apoptotic Mcl-1. Furthermore, the blockade of the STAT3/Mcl-1 pathway by pharmacological inhibitors against STAT3, TYK2 (for type I IFN) or JAK2 (for type II IFN) markedly reduced IFN-mediated CLL cell survival. Similarly, the selective Src family kinase inhibitor PP2 notably blocked IFN-mediated CLL cell survival by downregulating the protein levels of STAT3 and Mcl-1. Our work reveals a novel mechanism of resistance to apoptosis promoted by IFNs in CLL cells, whereby JAKs (TYK2, JAK2) and Src kinases activate in concert a STAT3/Mcl-1 signaling pathway. In view of current clinical developments of potent STAT3 and Mcl-1 inhibitors, a combination of conventional treatments with these inhibitors might thus constitute a new therapeutic strategy in CLL.

Computational analysis of binding free energies, hotspots and the binding mechanism of Bcl-xL/Bcl-2 binding to Bad/Bax

Hierarchical clustering tree of residues providing contributions to system binding based on the binding free energy of specific residues for (A) Bcl-xL systems (B) Bcl-2 systems.

Relation of Neutrophil Gelatinase-Associated Lipocalin Overexpression to the Resistance to Apoptosis of Tumor B Cells in Chronic Lymphocytic Leukemia

The resistance to apoptosis of chronic lymphocytic leukemia (CLL) cells partly results from the deregulated production of survival signals from leukemic cells. Despite the development of new therapies in CLL, drug resistance and disease relapse still occur. Recently, neutrophil gelatinase-associated lipocalin (NGAL), a secreted glycoprotein, has been suggested to have a critical role in the biology of tumors. Thus, we investigated the relevance of NGAL in CLL pathogenesis, analyzed the expression of its cellular receptor (NGAL-R) on malignant B cells and tested whether CLL cells are resistant to apoptosis through an autocrine process involving NGAL and NGAL-R. We observed that NGAL concentrations were elevated in the serum of CLL patients at diagnosis. After treatment (and regardless of the therapeutic regimen), serum NGAL levels normalized in CLL patients in remission but not in relapsed patients. In parallel, NGAL and NGAL-R were upregulated in leukemic cells from untreated CLL patients when compared to normal peripheral blood mononuclear cells (PBMCs), and returned to basal levels in PBMCs from patients in remission. Cultured CLL cells released endogenous NGAL. Anti-NGAL-R antibodies enhanced NGAL-R⁺ leukemia cell death. Conversely, recombinant NGAL protected NGAL-R⁺ CLL cells against apoptosis by activating a STAT3/Mcl-1 signaling pathway. Our results suggest that NGAL and NGAL-R, overexpressed in untreated CLL, participate in the deregulation of the apoptotic machinery in CLL cells, and may be potential therapeutic clues for CLL treatment.

Antihistamines are synergistic with Bruton's tyrosine kinase inhibiter ibrutinib mediated by lysosome disruption in chronic lymphocytic leukemia (CLL) cells

Lysosomes in chronic lymphocytic leukemia (CLL) cells have previously been identified as a promising target for therapeutic intervention in combination with targeted therapies. Recent studies have shown that antihistamines can induce lysosomal membrane permeabilization (LMP) in a variety of cell lines. Furthermore, our previous data indicates that lysosomotropic agents can cause synergistic cell death in vitro when combined with some tyrosine kinase inhibitors (TKI). In the current study, we have shown that three over-the-counter antihistamines, clemastine, desloratadine, and loratadine, preferentially induce cell death via LMP in CLL cells, as compared to normal lymphocytes. We treated primary CLL cells with antihistamines and found clemastine was the most effective at inducing LMP and cell death. More importantly, the antihistamines induced synergistic cytotoxicity when combined with the tyrosine kinase inhibitor, ibrutinib, but not with chemotherapy. Moreover, the synergy between clemastine and ibrutinib was associated with the induction of reactive oxygen species (ROS), loss of mitochondrial membrane potential and decreased Mcl-1 expression leading to apoptosis. This study proposes a potential novel treatment strategy for CLL, repurposing FDA-approved allergy medications in combination with the targeted therapy ibrutinib to enhance drug efficacy.

Venetoclax in the Treatment of Chronic Lymphocytic Leukemia: Evidence, Expectations, and Future Prospects

Chronic lymphocytic leukemia (CLL) is the most common form of leukemia in the western adult population; it is also prevalent worldwide. The B cell lymphoma-2 (BCL-2) family proteins play a key role in regulating intrinsic apoptosis and, in many cancers, are the main culprits behind tumor survival and therapy resistance. Hence, the role of BCL-2 inhibitors is very beneficial in the treatment of CLL. Venetoclax is the first selective, orally bioavailable BCL-2 inhibitor. This review article discusses factors such as the pharmacokinetics, pharmacodynamics, acquired resistance to venetoclax, responders vs. non-responders in venetoclax monotherapy, and the synergistic role of venetoclax with other drugs in detail. Venetoclax is the first BH3 mimetic drug and selective BCL-2 inhibitor that has received FDA approval. This drug has proved to provide good therapeutic responses in CLL patients irrespective of the presence of adverse clinical or genetic features, including in patients with relapsed or refractory forms of CLL. We anticipate that novel combination therapies, including venetoclax and immunotherapy, will further alter the treatment landscape for patients with relapsed CLL, particularly for those with deletion 17p (del 17p) CLL, which carries a very poor prognosis.

Inhibition of Anti-Apoptotic Bcl-2 Proteins in Preclinical and Clinical Studies: Current Overview in Cancer

The dynamic interplay between pro-death and pro-survival Bcl-2 family proteins is responsible for a cell’s fate. Due to the recognized relevance of this family in cancer progression and response to therapy, different efforts have made in recent years in order to develop small molecules able to target anti-apoptotic proteins such as Bcl-2, Bcl-xL and Mcl-1. The limitations of the first Bcl-2 family targeted drugs, regarding on-target and off-target toxicities, have been overcome with the development of venetoclax (ABT-199), the first BH3 mimetic inhibitor approved by the FDA. The purpose of this review is to discuss the state-of-the-art in the development of drugs targeting Bcl-2 anti-apoptotic proteins and to highlight the potential of their application as single agents or in combination for improving anti-cancer therapy, focusing in particular on solid tumors.

Transcriptional Silencing of MCL-1 Through Cyclin-Dependent Kinase Inhibition in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most common adult acute leukemia. Survival remains poor, despite decades of scientific advances. Cytotoxic induction chemotherapy regimens are standard-of-care for most patients. Many investigations have highlighted the genomic heterogeneity of AML, and several new targeted therapeutic options have recently been approved. Additional novel therapies are showing promising clinical results and may rapidly transform the therapeutic landscape of AML. Despite the emerging clinical success of B-cell lymphoma (BCL)-2 targeting in AML and a large body of preclinical data supporting myeloid leukemia cell (MCL)-1 as an attractive therapeutic target for AML, MCL-1 targeting remains relatively unexplored, although novel MCL-1 inhibitors are under clinical investigation. Inhibitors of cyclin-dependent kinases (CDKs) involved in the regulation of transcription, CDK9 in particular, are being investigated in AML as a strategy to target MCL-1 indirectly. In this article, we review the basis for CDK inhibition in oncology with a focus on relevant preclinical mechanism-of-action studies of CDK9 inhibitors in the context of their therapeutic potential specifically in AML.

From Biology to Therapy: The CLL Success Story

Chemoimmunotherapy has been the standard of care for patients with chronic lymphocytic leukemia (CLL) over the last decade. Advances in monoclonal antibody technology have resulted in the development of newer generations of anti-CD20 antibodies with improved therapeutic effectiveness. In parallel, our knowledge about the distinctive biological characteristics of CLL has progressively deepened and has revealed the importance of B-cell receptor (BCR) signaling and upregulated antiapoptotic proteins for survival and expansion of malignant cell clones. This knowledge provided the basis for development of novel targeted agents that revolutionized treatment of CLL. Ibrutinib and idelalisib inhibit the Bruton tyrosine kinase (BTK) and phosphoinositide 3-kinase (PI3K) delta, respectively, thus interfering with supportive signals coming from the microenvironment via the BCR. These drugs induce egress of CLL cells from secondary lymphoid organs and remarkably improve clinical outcomes, especially for patients with unmutated immunoglobulin heavy-chain genes or with p53 abnormalities that do not benefit from classical treatment schemes. Latest clinical trial results have established ibrutinib with or without anti-CD20 antibodies as the preferred first-line treatment for most CLL patients, which will reduce the use of chemoimmunotherapy in the imminent future. Further advances are achieved with venetoclax, a BH3-mimetic that specifically inhibits the antiapoptotic B-cell lymphoma 2 protein and thus causes rapid apoptosis of CLL cells, which translates into deep and prolonged clinical responses including high rates of minimal residual disease negativity. This review summarizes recent advances in the development of targeted CLL therapies, including new combination schemes, novel BTK and PI3K inhibitors, spleen tyrosine kinase inhibitors, immunomodulatory drugs, and cellular immunotherapy.

Dose and drug changes in chronic lymphocytic leukemia cell response in vitro: A comparison of standard therapy regimens with two novel cyclin‑dependent kinase inhibitors

Chronic lymphocytic leukemia (CLL) treatment is improving; however, some patients do not respond to therapy. Due to the high heterogeneity in disease development, there is an urgent need for personalization of therapy. In the present study, the response of leukemic mononuclear cells to anticancer drugs used for CLL treatment (cladribine + mafosfamide; CM or CM combined with rituximab; RCM) was compared with the response to new cyclin-dependent kinase (CDK) inhibitors: BP14 and BP30. Viable apoptotic and necrotic cells were quantified by flow cytometry using propidium iodide and Yo-Pro stains. CDK inhibitors were studied in several doses to determine the reduction of necrosis and simultaneous increase of apoptosis in leukemic cell incubations with anticancer agents. The distinct cell response to applied doses/anticancer agents was observed. Results obtained in the current manuscript confirmed that modulation of doses is important. This was particularly indicated in results obtained at 24 h of cells incubation with anticancer agent. While an important time for analysis of anticancer response efficacy (monitoring of apoptosis induction potential) seems to be 48 h of cells exposition to anticancer agents. High variability in response to the drugs revealed that both the nature and the dose of the anticancer agents could be important in the final effect of the therapy. The present findings support the thesis that personalized medicine, before drug administration in the clinic, could be important to avoid the application of ineffective therapy.

SYK inhibition thwarts the BAFF - B-cell receptor crosstalk and thereby antagonizes Mcl-1 in chronic lymphocytic leukemia

Although small molecule inhibitors of B-cell receptor-associated kinases have revolutionized therapy in chronic lymphocytic leukemia (CLL), responses are incomplete. Pro-survival signaling emanating from the microenvironment may foster therapeutic resistance of the malignant B cells resident in the protective lymphoid niches. B-cell activating factor (BAFF) is critical to the survival of both healthy and neoplastic B cells. However, the pro-survival pathways triggered by BAFF have not been fully characterized. Here we show that BAFF elicited resistance to spontaneous and drug-induced apoptosis in stromal co-cultures, induced activation of both canonical and non-canonical NFκB signaling pathways, and triggered B-cell receptor signaling in CLL cells, independently of IGHV mutational status. SYK, a proximal kinase in the B-cell receptor signaling cascade, acted via STAT3 to bolster transcription of the anti-apoptotic protein Mcl-1, thereby contributing to apoptosis resistance in BAFF-stimulated cells. SYK inhibitor entospletinib downregulated Mcl-1, abrogating BAFF-mediated cell survival. BAFF-B-cell receptor crosstalk in neoplastic B cells was mediated by SYK interaction with TRAF2/TRAF3 complex. Thus, SYK inhibition is a promising therapeutic strategy uniquely poised to antagonize crosstalk between BAFF and B-cell receptor, thereby disrupting the pro-survival microenvironment signaling in chronic lymphocytic leukemia.

Apoptosis signaling and BCL-2 pathways provide opportunities for novel targeted therapeutic strategies in hematologic malignances

Apoptosis is an essential biological process involved in tissue homeostasis and immunity. Aberrations of the two main apoptotic pathways, extrinsic and intrinsic, have been identified in hematological malignancies; many of these aberrations are associated with pathogenesis, prognosis and resistance to standard chemotherapeutic agents. Targeting components of the apoptotic pathways, especially the chief regulatory BCL-2 family in the intrinsic pathway, has proved to be a promising therapeutic approach for patients with hematological malignances, with the expectation of enhanced efficacy and reduced adverse events. Continuous investigations regarding the biological importance of each of the BCL-2 family components and the clinical rationale to achieve optimal therapeutic outcomes, using either monotherapy or in combination with other targeted agents, have generated inspiring progress in the field. Genomic, epigenomic and biological analyses including BH3 profiling facilitate effective evaluation of treatment response, cancer recurrence and drug resistance. In this review, we summarize the biological features of each of the components in the BCL-2 apoptotic pathways, analyze the regulatory mechanisms and the pivotal roles of BCL-2 family members in the pathogenesis of major types of hematologic malignances, and evaluate the potential of apoptosis- and BCL-2-targeted strategies as effective approaches in anti-cancer therapies.

Macrophages confer survival signals via CCR1-dependent translational MCL-1 induction in chronic lymphocytic leukemia

Protective interactions with bystander cells in micro-environmental niches, such as lymph nodes (LNs), contribute to survival and therapy resistance of chronic lymphocytic leukemia (CLL) cells. This is caused by a shift in expression of B-cell lymphoma 2 (BCL-2) family members. Pro-survival proteins B-cell lymphoma-extra large (BCL-X_L), BCL-2-related protein A1 (BFL-1) and myeloid leukemia cell differentiation protein 1 (MCL-1) are upregulated by LN-residing T cells through CD40L interaction, presumably via nuclear factor (NF)-κB signaling. Macrophages (Mφs) also reside in the LN, and are assumed to provide important supportive functions for CLL cells. However, if and how Mφs are able to induce survival is incompletely known. We first established that Mφs induced survival because of an exclusive upregulation of MCL-1. Next, we investigated the mechanism underlying MCL-1 induction by Mφs in comparison with CD40L. Genome-wide expression profiling of in vitro Mφ- and CD40L-stimulated CLL cells indicated activation of the phosphoinositide 3-kinase (PI3K)-V-Akt murine thymoma viral oncogene homolog (AKT)-mammalian target of rapamycin (mTOR) pathway, which was confirmed in ex vivo CLL LN material. Inhibition of PI3K-AKT-mTOR signaling abrogated MCL-1 upregulation and survival by Mφs, as well as CD40 stimulation. MCL-1 can be regulated at multiple levels, and we established that AKT leads to increased MCL-1 translation, but does not affect MCL-1 transcription or protein stabilization. Furthermore, among Mφ-secreted factors that could activate AKT, we found that induction of MCL-1 and survival critically depended on C-C motif chemokine receptor-1 (CCR1). In conclusion, this study indicates that two distinct micro-environmental factors, CD40L and Mφs, signal via CCR1 to induce AKT activation resulting in translational stabilization of MCL-1, and hence can contribute to CLL cell survival.

Prognosis biomarkers evaluation in chronic lymphocytic leukemia

OBJECTIVE/BACKGROUND

From clinical and biological points of view, chronic lymphocytic leukemia (CLL) is a heterogeneous disease characterized by a progressive accumulation of lymphocytes in the peripheral blood, bone marrow, and lymphoid organs. New prognostic markers in CLL may be useful to clinicians for predicting outcome and in clinical decision-making. The aim of this study was to evaluate the potential prognostic value of the apoptotic/survival-controlling proteins and protein tyrosine kinase ZAP-70 gene expression in CLL patients and control individuals, correlating such findings with patients' clinical data.

METHODS

Fifty-three patients diagnosed with CLL attending the hematology service of a clinical hospital, and 24 healthy individuals with no history of leukemia (Control group) were enrolled in this study. Analyses of apoptotic/survival-controlling proteins were performed by western blot and ZAP-70 gene expression was evaluated by real-time polymerase chain reaction.

RESULTS

Significant differences were observed for the p-p38, Mcl-1 long, and Mcl-1 short proteins when patients were compared with CLL and controls. A positive correlation between the results for Mcl-1 short and Mcl-1 long and lymphocyte count was observed, corroborating the hypothesis of an imbalance between proteins of cell survival pathways/apoptosis in CLL.

CONCLUSION

ZAP-70 gene expression was not detected as a discriminant biomarker in these CLL patients. An imbalance between apoptosis-related proteins was observed in the present study, corroborating the hypothesis of increased survival of lymphocytes in CLL patients.

The potential of venetoclax (ABT-199) in chronic lymphocytic leukemia

Venetoclax (VEN, ABT-199/GDC-0199) is an orally bioavailable BH3-mimetic that specifically inhibits the anti-apoptotic B-cell lymphoma/leukemia 2 (BCL2) protein. Although BCL2 overexpression is not genetically driven in chronic lymphocytic leukemia (CLL), it is nearly universal and represents a highly important and prevalent mechanism of apoptosis evasion, making it an attractive therapeutic target. This review summarizes the role of BCL2 in CLL pathogenesis, the development path targeting its inhibition prior to VEN, and the preclinical and clinical data regarding the effectiveness and safety of VEN. We further strive to contextualize VEN in the current CLL treatment landscape and discuss potential mechanisms of resistance.

Key Molecular Drivers of Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is an adult neoplastic disease of B cells characterized by variable clinical outcomes. Although some patients have an aggressive form of the disease and often encounter treatment failure and short survival, others have more stable disease with long-term survival and little or no need for theraphy. In the past decade, significant advances have been made in our understanding of the molecular drivers that affect the natural pathology of CLL. The present review describes what is known about these key molecules in the context of their role in tumor pathogenicity, prognosis, and therapy.

Optimising B-cell depletion in autoimmune disease: is obinutuzumab the answer?

In Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE), B-cell depletion therapy using rituximab results in variable clinical responses between individuals, which likely relates to variable B-cell depletion in the presence of immune defects. Outcomes in clinical trials with other type I anti-CD20 mAbs, ocrelizumab and ofatumumab, are comparable to rituximab. A mechanistically different type II mAb, obinutuzumab (OBZ), with greater capacity for B-cell depletion, has recently entered clinical trials in SLE. Here we consider whether type II anti-CD20 mAbs will provide mechanistic advantages to overcome the disease-related immune defects in autoimmune diseases such as SLE.

Sorafenib and Quinacrine Target Anti-Apoptotic Protein MCL1: A Poor Prognostic Marker in Anaplastic Thyroid Cancer (ATC)

PURPOSE AND EXPERIMENTAL DESIGN

Anaplastic thyroid cancer (ATC) comprises approximately 2% of all thyroid cancers, and its median survival rate remains poor. It is responsible for more than one third of thyroid cancer-related deaths. ATC is frequently resistant to conventional therapy, and NFκB signaling has been proposed to be a feature of the disease. We aimed to assess the activity of the antimalaria drug quinacrine known to target NFκB signaling in combination with the clinically relevant kinase inhibitor sorafenib in ATC cells. The presence of NFκB-p65/RELA and its target MCL1 was demonstrated in ATC by meta-data gene set enrichment analysis and IHC. We assessed the responses of a panel of human ATC cell lines to quinacrine and sorafenib in vitro and in vivo RESULTS: We detected increased expression of NFκB-p65/RELA and MCL1 in the nucleus of a subset of ATC compared with non-neoplastic thyroid. ATC cells were found to respond with additive/synergistic tumor cell killing to the combination of sorafenib plus quinacrine in vitro, and the drug combination improves survival of immunodeficient mice injected orthotopically with ATC cells as compared with mice administered either compound alone or doxorubicin. We also demonstrate that the combination of sorafenib and quinacrine is well tolerated in mice. At the molecular level, quinacrine and sorafenib inhibited expression of prosurvival MCL1, pSTAT3, and dampened NFκB signaling.

CONCLUSIONS

The combination of quinacrine and sorafenib targets emerging molecular hallmarks of ATC and shows promising results in clinically relevant models for the disease. Further testing of sorafenib plus quinacrine can be conducted in ATC patients. Clin Cancer Res; 22(24); 6192-203. ©2016 AACR.

Notch signaling sustains the expression of Mcl-1 and the activity of eIF4E to promote cell survival in CLL

In chronic lymphocytic leukemia (CLL), Notch1 and Notch2 signaling is constitutively activated and contributes to apoptosis resistance. We show that genetic inhibition of either Notch1 or Notch2, through small-interfering RNA, increases apoptosis of CLL cells and is associated with decreased levels of the anti-apoptotic protein Mcl-1. Thus, Notch signaling promotes CLL cell survival at least in part by sustaining Mcl-1 expression. In CLL cells, an enhanced Notch activation also contributes to the increase in Mcl-1 expression and cell survival induced by IL-4.

Mcl-1 downregulation by Notch targeting is not due to reduced transcription or degradation by caspases, but in part, to increased degradation by the proteasome. Mcl-1 downregulation by Notch targeting is also accompanied by reduced phosphorylation of eukaryotic translation initiation factor 4E (eIF4E), suggesting that this protein is another target of Notch signaling in CLL cells.

Overall, we show that Notch signaling sustains CLL cell survival by promoting Mcl-1 expression and eIF4E activity, and given the oncogenic role of these factors, we underscore the therapeutic potential of Notch inhibition in CLL.

The first MCL-1-selective BH3 mimetics have therapeutic potential for chronic lymphocytic leukemia

Small-molecule BH3 mimetics are designed to mimic the BH3 domain of BH3-only BCL-2 family members which are antagonists of the prosurvival members (such as BCL-2, BCL-XL and MCL-1). The BH3 mimetics are intended to bind with high affinity to prosurvival proteins, in order to inhibit their functional activity and hence to induce apoptosis in cancer cells. Both navitoclax (BCL-2/BCL-XL antagonist) and ABT-199/venetoclax (BCL-2-selective inhibitor) have demonstrated therapeutic efficacy especially in chronic lymphocytic leukemia (CLL). However, these BH3 mimetics cannot antagonize the prosurvival protein MCL-1 that is overexpressed and involved in therapeutic resistance in CLL. Furthermore, until now, none of the reported small-molecule MCL-1 inhibitors bound to their target with high affinity. The first MCL-1-selective BH3 mimetics capable of high-affinity binding and inducing apoptosis in cancer cells through an on-target mechanism have just been identified. This discovery should advance the translational research to implement novel drugs in treating CLL.

Broad targeting of resistance to apoptosis in cancer

Apoptosis or programmed cell death is natural way of removing aged cells from the body. Most of the anti-cancer therapies trigger apoptosis induction and related cell death networks to eliminate malignant cells. However, in cancer, de-regulated apoptotic signaling, particularly the activation of an anti-apoptotic systems, allows cancer cells to escape this program leading to uncontrolled proliferation resulting in tumor survival, therapeutic resistance and recurrence of cancer. This resistance is a complicated phenomenon that emanates from the interactions of various molecules and signaling pathways. In this comprehensive review we discuss the various factors contributing to apoptosis resistance in cancers. The key resistance targets that are discussed include (1) Bcl-2 and Mcl-1 proteins; (2) autophagy processes; (3) necrosis and necroptosis; (4) heat shock protein signaling; (5) the proteasome pathway; (6) epigenetic mechanisms; and (7) aberrant nuclear export signaling. The shortcomings of current therapeutic modalities are highlighted and a broad spectrum strategy using approaches including (a) gossypol; (b) epigallocatechin-3-gallate; (c) UMI-77 (d) triptolide and (e) selinexor that can be used to overcome cell death resistance is presented. This review provides a roadmap for the design of successful anti-cancer strategies that overcome resistance to apoptosis for better therapeutic outcome in patients with cancer.

Apoptosis inducers in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is characterized by a typical defect in apoptosis and is still an incurable disease. Numerous apoptosis inducers have been described. These synthetic compounds and natural products (mainly derived from plants) display antileukemic properties in vitro and in vivo and some have even been tested in the clinic in CLL. They act through several different mechanisms. Most of them involve proteins of the Bcl-2 family, which are the key regulators in triggering the mitochondrial pathway of caspase-dependent apoptosis. Thus, the Mcl-1/Noxa axis appeared as a target. Here I overview natural and synthetic apoptosis inducers and their mechanisms of action in CLL cells. Opportunities for developing novel, apoptosis-based therapeutics are presented.

Update on obinutuzumab in the treatment of B-cell malignancies

INTRODUCTION

The anti-CD20 mAb rituximab has revolutionized the treatment of B-cell malignancies, improving outcome for patients. Despite these improvements, the majority of patients still relapse and become refractory to rituximab. Further efforts to improve anti-CD20 mAb efficacy have recently focused on obinutuzumab /GA101, a novel anti-CD20 mAb glycoengineered to display enhanced Fc-mediated effector mechanisms and induce direct cell death.

AREAS COVERED

We provide an overview of the current insights into the mechanisms of action of obinutuzumab focusing on how structural modifications and differences to rituximab led to designation of obinutuzumab as a type II antibody. We summarize data from preclinical studies and recent clinical trials including the Phase III trial in chronic lymphocytic leukemia (CLL), which led to FDA approval in November 2013.

EXPERT OPINION

Clinical data are now emerging confirming the promise of the initial preclinical data that demonstrated superior efficacy of obinutuzumab over rituximab at similar dosing. The emerging randomized Phase III data from older comorbid patients with previously untreated CLL demonstrated significant improvements in molecular remission rates and median progression-free survival of obinutuzumab plus chlorambucil versus rituximab plus chlorambucil. This emerging data provide reasons to be optimistic that outcomes for patients with B-cell malignancies can be further improved with obinutuzumab.

A novel Cdk9 inhibitor preferentially targets tumor cells and synergizes with fludarabine

Cdk9 is a key elongation factor for RNA transcription and functions by phosphorylating the C-terminal domain of RNA polymerase II. Here we present direct evidence that cdk9 is important for cancer cell survival and describe the characterization of the potent cdk9 inhibitor CDKI-73 in primary human leukemia cells. CDKI-73 induced caspase-dependent apoptosis that was preceded by dephosphorylation of cdk9 and serine 2 of RNA polymerase II. CDKI-73 was more potent than the pan-cdk inhibitor flavopiridol and showed >200-fold selectivity against primary leukemia cells when compared with normal CD34+ cells. Furthermore, CDKI-73 was equipotent in poor prognostic sub-groups of leukemia patients and showed cytotoxic synergy with the nucleoside analog fludarabine. The Mechanism of synergy was associated with CDKI-73-mediated transcriptional inhibition of MCL1 and XIAP that was maintained when used in combination with fludarabine. Our data present a strong rationale for the development of cdk9 inhibitors such as CDKI-73 as anticancer therapeutics.

Proteasomal degradation of Mcl-1 by maritoclax induces apoptosis and enhances the efficacy of ABT-737 in melanoma cells

Background and purpose

Metastatic melanoma remains one of the most invasive and highly drug resistant cancers. The over expression of anti-apoptotic protein Mcl-1 has been associated with inferior survival, poor prognosis and chemoresistance of malignant melanoma. A BH3 mimetic, ABT-737, has demonstrated efficacy in several forms of cancers. However, the efficacy of ABT-737 depends on Mcl-1. Because the over expression of Mcl-1 is frequently observed in melanoma, specifically targeting of Mcl-1 may overcome the resistance of ABT-737. In this study, we investigated the effects of Maritoclax, a novel Mcl-1-selective inhibitor, alone and in combination with ABT-737, on the survival of human melanoma cells.

Experimental approach

For cell viability assessment we performed MTT assay. Apoptosis was determined using western blot and flow cytometric analysis.

Key results

The treatment of Maritoclax reduced the cell viability of melanoma cells with an IC₅₀ of between 2.2–5.0 µM. Further, treatment of melanoma cells with Maritoclax showed significant decrease in Mcl-1 expression. We found that Maritoclax was able to induce apoptosis in melanoma cells in a caspase-dependent manner. Moreover, Maritoclax induced Mcl-1 degradation via the proteasome system, which was associated with its pro-apoptotic activity. We also found that Maritoclax treatment increased mitochondrial translocation of Bim and Bmf. Importantly, Maritoclax markedly enhanced the efficacy of ABT-737 against melanoma cells in both two- and three-dimensional spheroids.

Conclusions and implications

Taken together, these results suggest that targeting of Mcl-1 by Maritoclax may represent a new therapeutic strategy for melanoma treatment that warrants further investigation as a single therapy or in combination with other agents such as Bcl-2 inhibitors.

Inhibition of NF-κB-mediated signaling by the cyclin-dependent kinase inhibitor CR8 overcomes prosurvival stimuli to induce apoptosis in chronic lymphocytic leukemia cells

PURPOSE

Chronic lymphocytic leukemia (CLL) is currently incurable with standard chemotherapeutic agents, highlighting the need for novel therapies. Overcoming proliferative and cytoprotective signals generated within the microenvironment of lymphoid organs is essential for limiting CLL progression and ultimately developing a cure.

EXPERIMENTAL DESIGN

We assessed the potency of cyclin-dependent kinase (CDK) inhibitor CR8, a roscovitine analog, to induce apoptosis in primary CLL from distinct prognostic subsets using flow cytometry-based assays. CLL cells were cultured in in vitro prosurvival and proproliferative conditions to mimic microenvironmental signals in the lymphoid organs, to elucidate the mechanism of action of CR8 in quiescent and proliferating CLL cells using flow cytometry, Western blotting, and quantitative real-time PCR.

RESULTS

CR8 was 100-fold more potent at inducing apoptosis in primary CLL cells than roscovitine, both in isolated culture and stromal-coculture conditions. Importantly, CR8 induced apoptosis in CD40-ligated CLL cells and preferentially targeted actively proliferating cells within these cultures. CR8 treatment induced downregulation of the antiapoptotic proteins Mcl-1 and XIAP, through inhibition of RNA polymerase II, and inhibition of NF-κB signaling at the transcriptional level and through inhibition of the inhibitor of IκB kinase (IKK) complex, resulting in stabilization of IκBα expression.

CONCLUSIONS

CR8 is a potent CDK inhibitor that subverts pivotal prosurvival and proproliferative signals present in the tumor microenvironment of CLL patient lymphoid organs. Our data support the clinical development of selective CDK inhibitors as novel therapies for CLL.

Programming cancer cells for high expression levels of Mcl1

The Bcl2 pro-survival protein family has long been recognized for its important contributions to cancer. At elevated levels relative to pro-apoptotic effector members, the survival proteins prevent cancer cells from initiating apoptosis in the face of many intrinsic tumour-suppressing pathways and extrinsic therapeutic treatments aimed at controlling tumorigenesis. Recent studies, including genome-wide analyses, have begun to focus attention on a particularly enigmatic member of the family-myeloid cell leukaemia 1 (Mcl1). For reasons that are not clear, Mcl1 in cancer cells is turned over rapidly, eliminated primarily through the ubiquitin-proteasome pathway. Moreover, the mechanistic aspects of this constitutive membrane-associated protein have not been fully elucidated. As the pro-cancer activity of Mcl1 requires elevated expression levels of the protein, the cancer genome adapts to ensure either high levels of synthesis or evasion of degradation, or both. Here, we focus on the complex strategies at play and their therapeutic implications.

Substituted 4-(thiazol-5-yl)-2-(phenylamino)pyrimidines are highly active CDK9 inhibitors: synthesis, X-ray crystal structures, structure-activity relationship, and anticancer activities

Cancer cells often have a high demand for antiapoptotic proteins in order to resist programmed cell death. CDK9 inhibition selectively targets survival proteins and reinstates apoptosis in cancer cells. We designed a series of 4-thiazol-2-anilinopyrimidine derivatives with functional groups attached to the C5-position of the pyrimidine or to the C4-thiazol moiety and investigated their effects on CDK9 potency and selectivity. One of the most selective compounds, 12u inhibits CDK9 with IC(50) = 7 nM and shows over 80-fold selectivity for CDK9 versus CDK2. X-ray crystal structures of 12u bound to CDK9 and CDK2 provide insights into the binding modes. This work, together with crystal structures of selected inhibitors in complex with both enzymes described in a companion paper, (34) provides a rationale for the observed SAR. 12u demonstrates potent anticancer activity against primary chronic lymphocytic leukemia cells with a therapeutic window 31- and 107-fold over those of normal B- and T-cells.

ABT-737 resistance in B-cells isolated from chronic lymphocytic leukemia patients and leukemia cell lines is overcome by the pleiotropic kinase inhibitor quercetin through Mcl-1 down-regulation

Chronic lymphocytic leukemia (CLL) is the most frequent form of leukemia in adult population and despite numerous studies, it is considered an incurable disease. Since CLL is characterized by overexpression of pro-survival Bcl-2 family members, treatments with their antagonists, such as ABT-737, represent a promising new therapeutic strategy. ABT-737 is a BH3 mimetic agent which binds Bcl-2, Bcl-XL and Bcl-w with high affinity, while weakly interacts with Mcl-1 and Bfl-1. Previous studies demonstrated that quercetin, a flavonoid naturally present in food and beverages, was able to sensitize B-cells isolated from CLL patients to apoptosis when associated with death ligands or fludarabine, through a mechanism involving Mcl-1 down-regulation. Here, we report that the association between ABT-737 and quercetin synergistically induces apoptosis in B-cells and in five leukemic cell lines (Combination Index <1). Peripheral blood mononuclear cell from healthy donors were not affected by quercetin treatment. The molecular pathways triggered by quercetin have been investigated in HPB-ALL cells, characterized by the highest resistance to both ABT-737 and quercetin when applied as single molecules, but highly sensitivity to the co-treatment. In this cell line, quercetin down-regulated Mcl-1 through the inhibition of PI3K/Akt signaling pathway, leading to Mcl-1 instability. The same mechanism was confirmed in B-cells. These results may open new clinical perspectives based on a translational approach in CLL therapy.

The Hsp90 inhibitor NVP-AUY922-AG inhibits NF-κB signaling, overcomes microenvironmental cytoprotection and is highly synergistic with fludarabine in primary CLL cells

Heat shock protein 90 (Hsp90) is a molecular chaperone required for the stability and function of multiple over-expressed signaling proteins that promote growth and survival in cancer cells. Chronic lymphocytic leukaemia (CLL) is characterized by increased expression of several Hsp90 client proteins making it a potentially susceptible to Hsp90 inhibition. In this study we showed that the novel Hsp90 inhibitor NVP-AUY922-AG was cytotoxic to primary CLL cells in vitro (LD₅₀=0.18μM±0.20). Importantly, its toxicity was preserved under cytoprotective co-culture conditions that rendered fludarabine ineffective. At the molecular level, NVP-AUY922-AG depleted the expression of multiple Hsp90 client proteins including Akt and activators of NF-κB, IKKα and IKKβ. Consistent with this inhibition profile, NVP-AUY922-AG resulted in decreased transcription of the NF-B target genes MCL1, CFLAR, BIRC5. In contrast, fludarabine significantly induced the transcription of MCL1 and BIRC5. Given the anti-apoptotic nature of these genes and the role they play in fludarabine resistance, we considered that the combination of NVP-AUY922-AG with fludarabine might resensitize CLL cells to the effects of fludarabine. In keeping with this hypothesis, the combination of NVP-AUY922-AG and fludarabine was highly synergistic (mean CI=0.110.06) and this synergy was enhanced in co-culture (mean CI=0.06±0.08). Furthermore, the combination maintained the decrease in MCL1, CFLAR and BIRC5 transcription suggesting that the ability of NVP-AUY922-AG to modulate expression of these genes may contribute to the efficacy of this drug under cytoprotective co-culture conditions and for its remarkable synergy with fludarabine. Taken together these findings indicate that Hsp90 inhibition is an attractive therapeutic strategy in CLL.

Thr 163 phosphorylation causes Mcl-1 stabilization when degradation is independent of the adjacent GSK3-targeted phosphodegron, promoting drug resistance in cancer

The antiapoptotic Bcl-2 family member Mcl-1 is a PEST protein (containing sequences enriched in proline, glutamic acid, serine, and threonine) and is subject to rapid degradation via multiple pathways. Impaired degradation leading to the maintenance of Mcl-1 expression is an important determinant of drug resistance in cancer. Phosphorylation at Thr 163 in the PEST region, stimulated by 12-O-tetradecanoylphorbol acetic acid (TPA)-induced activation of extracellular signal-regulated kinase (ERK), is associated with Mcl-1 stabilization in BL41-3 Burkitt lymphoma cells. This contrasts with the observation that Thr 163 phosphorylation in normal fibroblasts primes glycogen synthase kinase (GSK3)-induced phosphorylation at Ser 159, producing a phosphodegron that targets Mcl-1 for degradation. In the present follow-up studies in BL41-3 cells, Mcl-1 degradation was found to be independent of the GSK3-mediated pathway, providing a parallel to emerging findings showing that Mcl-1 degradation through this pathway is lost in many different types of cancer. Findings in Mcl-1-transfected CHO cells corroborated those in BL41-3 cells in that the GSK3-targeted phosphodegron did not play a major role in Mcl-1 degradation, and a phosphomimetic T163E mutation resulted in marked Mcl-1 stabilization. TPA-treated BL41-3 cells, in addition to exhibiting Thr 163 phosphorylation and Mcl-1 stabilization, exhibited an ∼10-fold increase in resistance to multiple chemotherapeutic agents, including Ara-C, etoposide, vinblastine, or cisplatin. In these cancer cells in which Mcl-1 degradation is not dependent on the GSK3/phosphodegron-targeted pathway, ERK activation and Thr 163 phosphorylation are associated with pronounced Mcl-1 stabilization and drug resistance – effects that can be suppressed by inhibition of ERK activation.

Perturbing pro-survival proteins using quinoxaline derivatives: a structure-activity relationship study

In HeLa cells the combinatorial knockdown of Bcl-xL and Mcl-1 is sufficient to induce spontaneous apoptosis. Quinoxaline derivatives were screened for the induction of Mcl-1 dependent apoptosis using a cell line without functional Bcl-xL. Quinoxaline urea analog 1 h was able to specifically induce apoptosis in an Mcl-1 dependent manner. We demonstrate that even small changes to 1h results in dramatic loss of activity. In addition, 1 h and ABT-737 synergistically inhibit cell growth and induce apoptosis. Our results also suggest that 1h could have therapeutic potential against ABT-737 refractory cancer.

Development of Noxa-like BH3 mimetics for apoptosis-based therapeutic strategy in chronic lymphocytic leukemia

Despite real advances made in chemoimmunotherapy, chronic lymphocytic leukemia (CLL) is still an incurable disease. New therapeutic strategies based on the restoration of the cell death program seemed relevant. Some members of the Bcl-2 family are critical players in the defective apoptotic program in CLL cells and/or targets of apoptosis inducers in vitro. The concept of BH3 mimetics has led to the characterization of small molecules mimicking proapoptotic BH3-only members of the Bcl-2 family by their ability to bind and antagonize the prosurvival members. Some putative or actual BH3 mimetics are already being tested in clinical trials with somewhat promising results. However, none of them has a high enough interaction affinity with Mcl-1, a crucial antiapoptotic factor in CLL. It has been suggested that resistance to BH3 mimetics can be overcome by using inhibitors of Mcl-1 expression. An alternative and more direct strategy is to design mimetics of the Noxa BH3 domain, which is a specific antagonistic Mcl-1 ligand. The development of such Noxa-like BH3 mimetics, capable of directly interacting with Mcl-1 and efficiently neutralizing its antiapoptotic activity, is extremely important to evaluate their impact on the clinical outcome of patients with CLL.

Design of novel BH3 mimetics for the treatment of chronic lymphocytic leukemia

Impaired programmed cell death is an important factor in the pathogenesis of chronic lymphocytic leukemia (CLL) and in the development of resistance to chemoimmunotherapy. Hence, the reactivation of apoptotic processes is likely to be a pertinent strategy for circumventing this resistance. Proteins from the Bcl-2 family are critical elements in defective apoptosis. Some compounds induce the apoptosis of CLL cells ex vivo by downregulation of prosurvival members of this family (for example, Bcl-2 and Mcl-1), whereas others act by upregulation of proapoptotic Bcl-2 homology (BH) 3-only members (for example, Noxa and Bim). The concept of BH3 mimetics was prompted by the fact that BH3-only proteins are specific antagonistic ligands of prosurvival Bcl-2 family members. This led to the design of small molecules capable of inhibiting the activity of prosurvival Bcl-2 proteins and inducing apoptosis in leukemia cells in vitro and antileukemic effects in animal models. Several putative or actual BH3 mimetics are currently being trialed in the clinic. Two novel BH3 mimetics that can specifically bind to and antagonize Mcl-1 (a crucial antiapoptotic factor in CLL) have recently been discovered. The evaluation of this type of compound's clinical impact in CLL can now be considered.

Histone deacetylases mediate the silencing of miR-15a, miR-16, and miR-29b in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) demonstrates a global down-regulation of miR-15a and miR-16 and a selective silencing of the related miR-29b in aggressive disease. Deletions in chromosome 13 [del(13q14)] partially account for the loss of expression of miR-15a and miR-16, but the mechanisms by which miR-29b becomes silenced is unknown. In the present study, we show that the histone deacetylases (HDACs) are overexpressed in CLL and mediate the epigenetic silencing of miR-15a, miR-16, and miR-29b. HDAC inhibition triggered the accumulation of the transcriptionally activating chromatin modification H3K4me2 and restored the expression of miR-15a, miR-16, and miR-29b in approximately 35% of samples. Ectopic expression of miR-15a and miR-16 and HDAC inhibition-induced expression of miR-15a, miR-16, or miR-29b in primary CLL cells was associated with declines in the levels of Mcl-1, but not Bcl-2, mitochondrial dysfunction, and induction of cell death. Therefore, our results show that HDACs aberrantly silence the expression of the critical tumor suppressors miR-15a, miR-16, and miR-29b in CLL. Deacetylase inhibition may be a therapeutic strategy that restores the expression of these miRs to antagonize Mcl-1, an important survival protein in these cells. Consequently, CLL patients who exhibit such epigenetic silencing may benefit from HDAC inhibitor-based therapy.

Quercetin downregulates Mcl-1 by acting on mRNA stability and protein degradation

Background:

We recently demonstrated that quercetin, a flavonoid naturally present in food and beverages belonging to the large class of phytochemicals, was able to sensitise leukaemic cells isolated from patients with chronic lymphocytic leukaemia (CLL) when associated with recombinant tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) or anti-CD95. We also showed that quercetin potentiated the effect of fludarabine on resistant B cells from CLL patients. Resistance to therapy in CLL depends on the expression and activity of anti-apoptotic proteins of the Bcl-2 family. Among these, myeloid cell leukaemia-1 (Mcl-1) has been associated with apoptotic resistance in CLL. Therefore, we investigate here whether the sensitising activity of this flavonoid, which leads to increased apoptosis in both cell lines and CLL, could be related to Mcl-1 expression and stability.

Results:

B cells isolated from CLL patients showed different levels of Mcl-1 protein expression, resulting, in several cases, in increased sensitivity to fludarabine. Quercetin significantly enhanced the downregulation of Mcl-1 in B cells isolated from selected patients expressing detectable levels of Mcl-1. In U-937 cells, quercetin increased Mcl-1 mRNA instability in the presence of actinomycin D. When cells were treated with MG-132, a proteasome inhibitor, Mcl-1 protein level increased. However, quercetin, in the presence of Z-Vad-FMK, continued to lower Mcl-1 protein expression, indicating its independence from caspase-mediated degradation. In contrast, co-treatment of quercetin and MG-132 did not revert the effect of MG-132 mono-treatment, thus suggesting a possible interference of quercetin in regulating the proteasome-dependent degradation of Mcl-1. Gossypol, a small-molecule inhibitor of Bcl-2 family members, mimics the activity of quercetin by lowering Mcl-1 expression and sensitising U-937 cells to apoptosis induced by recombinant TRAIL and the Fas-ligand.

Conclusion:

This study demonstrates that in U-937 cells, quercetin downregulates Mcl-1 acting directly or indirectly on its mRNA stability and protein degradation, suggesting that the same mechanism may bypass resistance to apoptosis in leukaemic cells isolated from CLL patients and sensitise B cells to apoptosis induced by drugs and death receptor inducers.

Mechanisms of action of a dual Cdc7/Cdk9 kinase inhibitor against quiescent and proliferating CLL cells

In chronic lymphocytic leukemia (CLL) the proliferation rate and resistance to drug-induced apoptosis are recognized as important factors in the outcome of treatment. In this study, we assess the activity and the mechanism of action of the prototype cell division cycle kinase 7 (Cdc7) inhibitor, PHA-767491, which inhibits the initiation of DNA replication but also has cyclin-dependent kinase 9 (Cdk9) inhibitory activity. We have studied the effects of this dual Cdc7/Cdk9 inhibitor in both quiescent CLL cells and CLL cells that have been induced to proliferate using a cellular coculture system that mimics the lymph node microenvironment. We find that this compound, originally developed as a DNA replication inhibitor, is particularly active in promoting mitochondrial dependent apoptosis in quiescent CLL cells purified from peripheral blood of patients regardless of recognized risk factors. In this setting, apoptosis is preceded by a decrease in the levels of Mcl-1 protein and transcript possibly due to inhibition of Cdk9. Following stimulation by CD154 and interleukin-4, CLL cells become highly chemoresistant, reenter into the cell cycle, reexpress Cdc7 kinase, a key molecular switch for the initiation of DNA replication, replicate their DNA, and undergo cell division. In this context, treatment with PHA-767491 abolished DNA synthesis by inhibiting Cdc7 but is less effective in triggering cell death, although Mcl-1 protein is no longer detectable. Thus, dual Cdc7/Cdk9 inhibition has the potential to target both the quiescent and actively proliferating CLL populations through two distinct mechanisms and may be a new therapeutic strategy in CLL.

Novel type II anti-CD20 monoclonal antibody (GA101) evokes homotypic adhesion and actin-dependent, lysosome-mediated cell death in B-cell malignancies

The anti-CD20 mAb rituximab has substantially improved the clinical outcome of patients with a wide range of B-cell malignancies. However, many patients relapse or fail to respond to rituximab, and thus there is intense investigation into the development of novel anti-CD20 mAbs with improved therapeutic efficacy. Although Fc-FcγR interactions appear to underlie much of the therapeutic success with rituximab, certain type II anti-CD20 mAbs efficiently induce programmed cell death (PCD), whereas rituximab-like type I anti-CD20 mAbs do not. Here, we show that the humanized, glycoengineered anti-CD20 mAb GA101 and derivatives harboring non-glycoengineered Fc regions are type II mAb that trigger nonapoptotic PCD in a range of B-lymphoma cell lines and primary B-cell malignancies. We demonstrate that GA101-induced cell death is dependent on actin reorganization, can be abrogated by inhibitors of actin polymerization, and is independent of BCL-2 overexpression and caspase activation. GA101-induced PCD is executed by lysosomes which disperse their contents into the cytoplasm and surrounding environment. Taken together, these findings reveal that GA101 is able to potently elicit actin-dependent, lysosomal cell death, which may potentially lead to improved clearance of B-cell malignancies in vivo.

Sorafenib induces cell death in chronic lymphocytic leukemia by translational downregulation of Mcl-1

Chronic lymphocytic leukemia (CLL) has a high prevalence in western countries and remains incurable to date. Here, we provide evidence that the multikinase inhibitor sorafenib induces apoptosis in primary CLL cells. This strong pro-apoptotic effect is not restricted to any subgroup of patients, based on Binet stage and the expression of ZAP70 or CD38. Mechanistically, sorafenib-induced cell death is preceded by a rapid downregulation of Mcl-1 through the inhibition of protein translation. Subsequently, the cell intrinsic apoptotic pathway is activated, indicated by destabilization of the mitochondrial membrane potential and activation of caspase-3 and -9. In contrast to sorafenib, the monoclonal vascular epidermal growth factor (VEGF)-antibody bevacizumab failed to induce apoptosis in CLL cells, suggesting that sorafenib induces cell death irrespectively of VEGF signalling. Notably, although sorafenib inhibits phosphorylation of the Scr-kinase Lck, knock-down of Lck did not induce apoptosis in CLL cells. Of note, the pro-apoptotic effect of sorafenib is not restricted to cell-cycle arrested cells, but is also maintained in proliferating CLL cells. In addition, we provide evidence that sorafenib can overcome drug resistance in CLL cells protected by microenvironmental signals from stromal cells. Conclusively, sorafenib is highly active in CLL and may compose a new therapeutic option for patients who relapse after immunochemotherapy.

c-Abl regulates Mcl-1 gene expression in chronic lymphocytic leukemia cells

Chronic lymphocytic leukemia (CLL) is a malignancy characterized by clonal expansion of mature B cells that are resistant to apoptosis. This resistance to apoptosis partly results from Mcl-1 expression because high levels of this protein in CLL cells correlate with poor disease prognosis and resistance to chemotherapy. Thus, understanding the mechanism(s) regulating Mcl-1 expression in CLL cells may be useful in the development of new therapies for this incurable disease. In the present study, we show a strong relationship between c-Abl and Mcl-1 expression in CLL cells. We show that treatment of CLL cells with Abl-specific siRNA or with imatinib, to inhibit c-Abl activity, results in the down-regulation of Mcl-1 protein and mRNA. A major regulator of Mcl-1 gene expression is STAT3. Our data show that CLL cells expressing high levels of c-Abl also show elevated levels of phospho-STAT3, and that STAT3 phosphorylation in CLL cells is dependent on c-Abl activity. However, STAT3 phosphorylation by c-Abl requires activation of nuclear factor-κB, secretion of autocrine interleukin-6, and active protein kinase C. Taken together, our data demonstrate the mechanism involved in c-Abl regulation of Mcl-1 expression in CLL cells, and suggest that c-Abl inhibition has therapeutic application in the treatment of this disease.