Endogenous association of Bim BH3-only protein with Mcl-1, Bcl-xL and Bcl-2 on mitochondria in human B cells

Non-canonical BIM-regulated energy metabolism determines drug-induced liver necrosis

Paracetamol (acetaminophen, APAP) overdose severely damages mitochondria and triggers several apoptotic processes in hepatocytes, but the final outcome is fulminant necrotic cell death, resulting in acute liver failure and mortality. Here, we studied this switch of cell death modes and demonstrate a non-canonical role of the apoptosis-regulating BCL-2 homolog BIM/Bcl2l11 in promoting necrosis by regulating cellular bioenergetics. BIM deficiency enhanced total ATP production and shifted the bioenergetic profile towards glycolysis, resulting in persistent protection from APAP-induced liver injury. Modulation of glucose levels and deletion of Mitofusins confirmed that severe APAP toxicity occurs only in cells dependent on oxidative phosphorylation. Glycolytic hepatocytes maintained elevated ATP levels and reduced ROS, which enabled lysosomal recycling of damaged mitochondria by mitophagy. The present study highlights how metabolism and bioenergetics affect drug-induced liver toxicity, and identifies BIM as important regulator of glycolysis, mitochondrial respiration, and oxidative stress signaling.

MAPK-ERK is a central pathway in T-cell acute lymphoblastic leukemia that drives steroid resistance

(Patho-)physiological activation of the IL7-receptor (IL7R) signaling contributes to steroid resistance in pediatric T-cell acute lymphoblastic leukemia (T-ALL). Here, we show that activating IL7R pathway mutations and physiological IL7R signaling activate MAPK-ERK signaling, which provokes steroid resistance by phosphorylation of BIM. By mass spectrometry, we demonstrate that phosphorylated BIM is impaired in binding to BCL2, BCLXL and MCL1, shifting the apoptotic balance toward survival. Treatment with MEK inhibitors abolishes this inactivating phosphorylation of BIM and restores its interaction with anti-apoptotic BCL2-protein family members. Importantly, the MEK inhibitor selumetinib synergizes with steroids in both IL7-dependent and IL7-independent steroid resistant pediatric T-ALL PDX samples. Despite the anti-MAPK-ERK activity of ruxolitinib in IL7-induced signaling and JAK1 mutant cells, ruxolitinib only synergizes with steroid treatment in IL7-dependent steroid resistant PDX samples but not in IL7-independent steroid resistant PDX samples. Our study highlights the central role for MAPK-ERK signaling in steroid resistance in T-ALL patients, and demonstrates the broader application of MEK inhibitors over ruxolitinib to resensitize steroid-resistant T-ALL cells. These findings strongly support the enrollment of T-ALL patients in the current phase I/II SeluDex trial (NCT03705507) and contributes to the optimization and stratification of newly designed T-ALL treatment regimens.

Molecular mechanisms of chemo- and radiotherapy resistance and the potential implications for cancer treatment

Cancer is a leading cause of death worldwide. Surgery is the primary treatment approach for cancer, but the survival rate is very low due to the rapid progression of the disease and presence of local and distant metastasis at diagnosis. Adjuvant chemotherapy and radiotherapy are important components of the multidisciplinary approaches for cancer treatment. However, resistance to radiotherapy and chemotherapy may result in treatment failure or even cancer recurrence. Radioresistance in cancer is often caused by the repair response to radiation-induced DNA damage, cell cycle dysregulation, cancer stem cells (CSCs) resilience, and epithelial-mesenchymal transition (EMT). Understanding the molecular alterations that lead to radioresistance may provide new diagnostic markers and therapeutic targets to improve radiotherapy efficacy. Patients who develop resistance to chemotherapy drugs cannot benefit from the cytotoxicity induced by the prescribed drug and will likely have a poor outcome with these treatments. Chemotherapy often shows a low response rate due to various drug resistance mechanisms. This review focuses on the molecular mechanisms of radioresistance and chemoresistance in cancer and discusses recent developments in therapeutic strategies targeting chemoradiotherapy resistance to improve treatment outcomes.

Dynein light chain binding determines complex formation and posttranslational stability of the Bcl-2 family members Bmf and Bim

The BH3-only class of Bcl-2 family proteins triggers mitochondrial apoptosis. Several mechanisms are used to restrain the pro-apoptotic activity of these proteins. Dynein light chain (DYNLL) 1 and 2 has been proposed to negatively regulate the activity of Bim and Bmf, respectively, and the Bim-DYNLL1 interaction leads to the formation of large protein complexes on mitochondria. Here we found that Bim and Bmf interact with both isoforms of DYNLL (DYNLL1 and DYNLL2). DYNLL1/2 not only induced homo-dimerization of Bim and Bmf but also led to the formation of ternary complexes (Bim-DYNLL-Bmf), both in cell-free and in cellular systems. DYNLL-induced oligomerization stabilized Bmf in cultured cells and inhibited its degradation by the ubiquitin-independent 20S proteasome in a cell-free system. Surprisingly, overexpression of wild-type Bmf but not of a DYNLL-binding-deficient mutant induced degradation of endogenous Bim in different cell lines, but both variants sensitized to apoptosis. Mutant Bmf incapable of interacting with anti-apoptotic Bcl-2 proteins and of inducing apoptosis still caused Bim degradation. These results suggest that Bmf overexpression-induced Bim degradation is not due to the displacement of Bim from anti-apoptotic Bcl-2 proteins but a direct consequence of the modulation of Bim-DYNLL association. A peptide derived from the DYNLL-binding domain of Bim also led to the degradation of Bim as well as of its preferred binding partner Mcl-1. Thus DYNLL regulates the mitochondrial pathway of apoptosis by determining the stability of Bmf, Bim, and Mcl-1 proteins.

New piperidine derivative DTPEP acts as dual-acting anti-breast cancer agent by targeting ERα and downregulating PI3K/Akt-PKCα leading to caspase-dependent apoptosis

OBJECTIVES

In our ongoing studies to develop ER targeting agents, we screened for dual-acting molecules with a hypothesis that a single molecule can also target both ER positive and negative groups of breast cancer.

MATERIALS AND METHODS

1-(2-(4-(Dibenzo[b,f]thiepin-10-yl)phenoxy)ethyl)piperidine (DTPEP) was synthesized and screened in both MCF-7 (ER+ve) and MDA-MB-231 (ER-ve) cells. Assays for analysis of cell cycle, ROS, apoptosis and MMP loss were carried out using flow cytometry. Its target was investigated using western blot, transactivation assay and RT-PCR. In vivo efficacy of DTPEP was validated in LA-7 syngeneic rat mammary tumour model.

RESULTS

Here, we report identification of dual-acting molecule DTPEP that downregualtes PI3K/Akt and PKCα expression, induces ROS and ROS-dependent apoptosis, loss of mitochondrial membrane potential, induces expression of caspase indicative of both intrinsic and extrinsic apoptosis in MCF-7 and MDA-MB-231 cells. In MCF-7 cells, DTPEP downregulates ERα expression and activation. In MDA-MB-231 cells, primary cellular target of DTPEP is not clearly known, but it downregualtes PI3K/Akt and PKCα expression. In vivo study showed regression of LA-7 syngeneic mammary tumour in SD rat.

CONCLUSIONS

We identified a new dual-acting anti-breast cancer molecules as a proof of concept which is capable of targeting both ER-positive and ER-negative breast cancer.

Erythropoietin drives breast cancer progression by activation of its receptor EPOR

Breast cancer is a leading cause of cancer-related deaths. Anemia is common in breast cancer patients and can be treated with blood transfusions or with recombinant erythropoietin (EPO) to stimulate red blood cell production. Clinical studies have indicated decreased survival in some groups of cancer patients treated with EPO. Numerous tumor cells express the EPO receptor (EPOR), posing a risk that EPO treatment would enhance tumor growth, but the mechanisms involved in breast tumor progression are poorly understood.

Here, we have examined the functional role of the EPO-EPOR axis in pre-clinical models of breast cancer. EPO induced the activation of PI3K/AKT and MAPK pathways in human breast cancer cell lines. EPOR knockdown abrogated human tumor cell growth, induced apoptosis through Bim, reduced invasiveness, and caused downregulation of MYC expression. EPO-induced MYC expression is mediated through the PI3K/AKT and MAPK pathways, and overexpression of MYC partially rescued loss of cell proliferation caused by EPOR downregulation. In a xenotransplantation model, designed to simulate recombinant EPO therapy in breast cancer patients, knockdown of EPOR markedly reduced tumor growth.

Thus, our experiments in vitro and in vivo demonstrate that functional EPOR signaling is essential for the tumor-promoting effects of EPO and underline the importance of the EPO-EPOR axis in breast tumor progression.

RACKI induces chemotherapy resistance in esophageal carcinoma by upregulating the PI3K/AKT pathway and Bcl-2 expression

Introduction

Accumulating evidence indicates that RACK1 is involved in the progression of tumors. We aimed to evaluate the function of RACK1 in esophageal squamous cell carcinoma (ESCC) and its role in the mechanism of chemotherapy resistance.

Materials and methods

Transfected ESCC cell lines with plasmids expressed shRACK1 or open reading frame (ORF) targeting RACK1 and established stable cell lines. We then examined the effects of RACK1 on cell proliferation and chemotherapy resistance in ESCC cell lines, and the expression of AKT, pAKT, ERK1/2, Bcl-2, and Bim was introduced to further detect the association between RACK1 and chemotherapy resistance.

Results

The proliferation ability of ESCC cells was improved in the overexpression RACK1 groups (P<0.001) and decreased in the transfected shRACK1 groups (P<0.001) compared with the control ones. Meanwhile, upregulation of RACK1 significantly suppressed cisplatin-induced apoptosis in Eca109 and EC9706 cells, while downregulation of RACK1 promoted the sensitivity compared to the control group (Eca109: P<0.001 for shRACK1, P<0.01 for shNC, and P<0.001 for overexpression group; EC9706: P<0.001 for shRACK1, P<0.001 for shNC, and P<0.05 for overexpression group). Furthermore, we found that RACK1 could activate the PI3K/AKT pathway and increase the expression level of Bcl-2 in ESCC, which leads to the enhancement of chemoresistance in ESCC.

Conclusion

RACK1 promotes proliferation and chemotherapy resistance in ESCC by activating the PI3K/AKT pathway and upregulating the Bcl-2 expression.

Dynein light chain 1 induces assembly of large Bim complexes on mitochondria that stabilize Mcl-1 and regulate apoptosis

In this study, Singh et al. investigated Bim structure and activity and show that Bim is regulated by the formation of large protein complexes containing dynein light chain 1 (DLC1). Their findings demonstrate that control of apoptosis at mitochondria extends beyond the interaction of monomers of proapoptotic and anti-apoptotic Bcl-2 family members and involves more complex structures of proteins at the mitochondrial outer membrane.

The Bcl-2 family protein Bim triggers mitochondrial apoptosis. Bim is expressed in nonapoptotic cells at the mitochondrial outer membrane, where it is activated by largely unknown mechanisms. We found that Bim is regulated by formation of large protein complexes containing dynein light chain 1 (DLC1). Bim rapidly inserted into cardiolipin-containing membranes in vitro and recruited DLC1 to the membrane. Bim binding to DLC1 induced the formation of large Bim complexes on lipid vesicles, on isolated mitochondria, and in intact cells. Native gel electrophoresis and gel filtration showed Bim-containing mitochondrial complexes of several hundred kilodaltons in all cells tested. Bim unable to form complexes was consistently more active than complexed Bim, which correlated with its substantially reduced binding to anti-apoptotic Bcl-2 proteins. At endogenous levels, Bim surprisingly bound only anti-apoptotic Mcl-1 but not Bcl-2 or Bcl-X_L, recruiting only Mcl-1 into large complexes. Targeting of DLC1 by RNAi in human cell lines induced disassembly of Bim–Mcl-1 complexes and the proteasomal degradation of Mcl-1 and sensitized the cells to the Bcl-2/Bcl-X_L inhibitor ABT-737. Regulation of apoptosis at mitochondria thus extends beyond the interaction of monomers of proapoptotic and anti-apoptotic Bcl-2 family members but involves more complex structures of proteins at the mitochondrial outer membrane, and targeting complexes may be a novel therapeutic strategy.

RACK1 promotes radiation resistance in esophageal cancer via regulating AKT pathway and Bcl-2 expression

RACK1 is a seven Trp-Asp 40 repeat protein, which interacts with a wide range of kinases and proteins. RACK1 plays an important role in the proliferation and progression of various cancers. The aim of this study is to detect the role of RACK1 in the radioresistance in esophageal cancer. The results indicated that downregulation of RACK1 reduced the colony formation ability, proliferation ability and resistance of cells to radiation effection through regulating the radiation-related proteins including pAKT, Bcl-2 and Bim; whereas upregulation of RACK1 promoted the ability and radioresistance of ESCC cells. Our findings suggest that RACK1 promotes proliferation and radioresistance in ESCC cells by activating the AKT pathway, upregulating Bcl-2 expression and downregulating protein levels of Bim. Our study fills in gaps in the field of RACK1 and radiation resistance and may provide new possibilities for improving strategies of radiotherapy in esophageal cancer.

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.

PUMA is invovled in ischemia/reperfusion-induced apoptosis of mouse cerebral astrocytes

PUMA (p53-upregulated modulator of apoptosis), a BH3-only member of the Bcl-2 protein family, is required for p53-dependent and p53-independent forms of apoptosis. PUMA has been invovled in the onset and progress of several diseases, including cancer, acquired immunodeficiency syndrome, and ischemic brain disease. Although many studies have shown that ischemia and reperfusion (I/R) can induce the apoptosis of astrocytes, the role of PUMA in I/R-mediated apoptosis of cerebral astrocyte apoptosis remains unclear. To mimic in vivo I/R conditions, primary mouse cerebral astrocytes were incubated in a combinational cultural condition of oxygen, glucose, and serum deprivation (OSGD) for 1 h followed by reperfusion (OSGD/R). Cell death determination assays and cell viability assays indicated that OSGD and OSGD/R induce the apoptosis of primary cerebral astrocytes. The expression of PUMA was significantly elevated in primary cerebral astrocytes during OSGD/R. Moreover, targeted down-regulation of PUMA by siRNA transfection significantly decreased the OSGD/R-induced apoptosis of primary cerebral astrocytes. We also found that OSGD and OSGD/R triggered the release of cytochrome c in astrocytes, indicating the dependence on a mitochondrial apoptotic pathway. Reactive oxygen species (ROS) was extremely generated during OSGD and OSGD/R, and the elimination of ROS by treated with N-acetyl-L-cysteine (NAC) remarkably inhibited the expression of PUMA and the apoptosis of primary cerebral astrocytes. The activation of Caspase 3 and Caspase 9 was extremely elevated in primary cerebral astrocytes during OSGD. In addition, we found that knockdown of PUMA led to the depressed expression of Bax, cleaved caspase-9 and caspase-3 during OSGD/R. These results indicate that PUMA is invovled in the apoptosis of cerebral astrocytes upon I/R injury.

RACK1, a versatile hub in cancer

RACK1 is a highly conserved intracellular adaptor protein with significant homology to Gβ and was originally identified as the anchoring protein for activated protein kinase C. In the past 20 years, the number of binding partners and validated cellular functions for RACK1 has increased, which facilitates clarification of its involvement in different biological events. In this review, we will focus on its role in cancer, summarizing its aberrant expression, pro- or anti-oncogenic effects and the underlying mechanisms in various cancers.

Rational combination of dual PI3K/mTOR blockade and Bcl-2/-xL inhibition in AML

Acute myeloid leukemia (AML) continues to represent an area of critical unmet need with respect to new and effective targeted therapies. The Bcl-2 family of pro- and antiapoptotic proteins stands at the crossroads of cellular survival and death, and the expression of and interactions between these proteins determine tumor cell fate. Malignant cells, which are often primed for apoptosis, are particularly vulnerable to the simultaneous disruption of cooperative survival signaling pathways. Indeed, the single agent activity of agents such as mammalian target of rapamycin (mTOR) and mitogen-activated protein kinase kinase (MEK) inhibitors in AML has been modest. Much work in recent years has focused on strategies to enhance the therapeutic potential of the bona fide BH3-mimetic, ABT-737, which inhibits B-cell lymphoma 2 (Bcl-2) and Bcl-xL. Most of these strategies target Mcl-1, an antiapoptotic protein not inhibited by ABT-737. The phosphatidylinositol-3-kinase (PI3K)/Akt/mTOR and Ras/Raf/MEK/ERK signaling pathways are central to the growth, proliferation, and survival of AML cells, and there is much interest currently in pharmacologically interrupting these pathways. Dual inhibitors of PI3K and mTOR overcome some intrinsic disadvantages of rapamycin and its derivatives, which selectively inhibit mTOR. In this review, we discuss why combining dual PI3K/mTOR blockade with inhibition of Bcl-2 and Bcl-xL, by virtue of allowing coordinate inhibition of three mutually synergistic pathways in AML cells, may be a particularly attractive therapeutic strategy in AML, the success of which may be predicted for by basal Akt activation.

The structural biology of BH3-only proteins

B-cell lymphoma-2 (Bcl-2) homology-3 (BH3)-only proteins are considered members of the Bcl-2 family, though they bear little sequence or structural identity with the multi-BH motif prosurvival or proapoptotic Bcl-2 proteins like Bcl-2 or Bax. They are better considered a separate phylogenetic entity. In combination, results from biophysical, biochemical, cell biological, and animal studies in conjunction with structural investigations have elucidated the function and mechanism of action of these proteins. Either by antagonizing prosurvival Bcl-2 proteins or directly activating proapoptotic Bcl-2 proteins (Bax or Bak) they initiate apoptosis. BH3-only proteins are intrinsically disordered and fold and bind into a groove provided by their cognate receptor Bcl-2 family proteins. Our detailed molecular understanding of BH3-only protein action has aided the development of novel chemical entities that initiate cell death by mimicking the properties of BH3-only proteins.

Dual inhibition of Bcl-2 and Bcl-xL strikingly enhances PI3K inhibition-induced apoptosis in human myeloid leukemia cells through a GSK3- and Bim-dependent mechanism

Effects of concomitant inhibition of the PI3K/AKT/mTOR pathway and Bcl-2/Bcl-xL (BCL2L1) were examined in human myeloid leukemia cells. Tetracycline-inducible Bcl-2 and Bcl-xL dual knockdown sharply increased PI3K/AKT/mTOR inhibitor lethality. Conversely, inducible knockdown or dominant-negative AKT increased, whereas constitutively active AKT reduced lethality of the Bcl-2/Bcl-xL inhibitor ABT-737. Furthermore, PI3K/mTOR inhibitors (e.g., BEZ235 and PI-103) synergistically increased ABT-737-mediated cell death in multiple leukemia cell lines and reduced colony formation in leukemic, but not normal, CD34+ cells. Notably, increased lethality was observed in four of six primary acute myelogenous leukemia (AML) specimens. Responding, but not nonresponding, samples exhibited basal AKT phosphorylation. PI3K/mTOR inhibitors markedly downregulated Mcl-1 but increased Bim binding to Bcl-2/Bcl-xL; the latter effect was abrogated by ABT-737. Combined treatment also markedly diminished Bax/Bak binding to Mcl-1, Bcl-2, or Bcl-xL. Bax, Bak, or Bim (BCL2L11) knockdown or Mcl-1 overexpression significantly diminished regimen-induced apoptosis. Interestingly, pharmacologic inhibition or short hairpin RNA knockdown of GSK3α/β significantly attenuated Mcl-1 downregulation and decreased apoptosis. In a systemic AML xenograft model, dual tetracycline-inducible knockdown of Bcl-2/Bcl-xL sharply increased BEZ235 antileukemic effects. In a subcutaneous xenograft model, BEZ235 and ABT-737 coadministration significantly diminished tumor growth, downregulated Mcl-1, activated caspases, and prolonged survival. Together, these findings suggest that antileukemic synergism between PI3K/AKT/mTOR inhibitors and BH3 mimetics involves multiple mechanisms, including Mcl-1 downregulation, release of Bim from Bcl-2/Bcl-xL as well as Bak and Bax from Mcl-1/Bcl-2/Bcl-xL, and GSK3α/β, culminating in Bax/Bak activation and apoptosis. They also argue that combining PI3K/AKT/mTOR inhibitors with BH3 mimetics warrants attention in AML, particularly in the setting of basal AKT activation and/or addiction.

Prolonged early G(1) arrest by selective CDK4/CDK6 inhibition sensitizes myeloma cells to cytotoxic killing through cell cycle-coupled loss of IRF4

Dysregulation of cyclin-dependent kinase 4 (CDK4) and CDK6 by gain of function or loss of inhibition is common in human cancer, including multiple myeloma, but success in targeting CDK with broad-spectrum inhibitors has been modest. By selective and reversible inhibition of CDK4/CDK6, we have developed a strategy to both inhibit proliferation and enhance cytotoxic killing of cancer cells. We show that induction of prolonged early-G(1) arrest (pG1) by CDK4/CDK6 inhibition halts gene expression in early-G(1) and prevents expression of genes programmed for other cell-cycle phases. Removal of the early-G(1) block leads to S-phase synchronization (pG1-S) but fails to completely restore scheduled gene expression. Consequently, the IRF4 protein required to protect myeloma cells from apoptosis is markedly reduced in pG1 and further in pG1-S in response to cytotoxic agents, such as the proteasome inhibitor bortezomib. The coordinated loss of IRF4 and gain of Bim sensitize myeloma tumor cells to bortezomib-induced apoptosis in pG1 in the absence of Noxa and more profoundly in pG1-S in cooperation with Noxa in vitro. Induction of pG1 and pG1-S by reversible CDK4/CDK6 inhibition further augments tumor-specific bortezomib killing in myeloma xenografts. Reversible inhibition of CDK4/CDK6 in sequential combination therapy thus represents a novel mechanism-based cancer therapy.

Inhibition of Bcl-2 antiapoptotic members by obatoclax potently enhances sorafenib-induced apoptosis in human myeloid leukemia cells through a Bim-dependent process

Interactions between the multikinase inhibitor sorafenib and the BH3-mimetic obatoclax (GX15-070) were examined in human acute myeloid leukemia (AML) cells. Treatment with sorafenib/obatoclax induced pronounced apoptosis in and reduced the clonogenic growth of multiple AML lines and primary AML cells but not normal CD34(+) cells. Sorafenib triggered rapid and pronounced Mcl-1 down-regulation accompanied by enhanced binding of Bim to Bcl-2 and Bcl-xL, effects that were abolished by obatoclax coadministration. Notably, shRNA knockdown of Bim, Bak, or Bax, but not Noxa, significantly attenuated obatoclax/sorafenib lethality, whereas ectopic expression of Mcl-1 exerted a protective effect. Furthermore, exposure of leukemia cells to sorafenib and obatoclax markedly induced autophagy, reflected by rapid and pronounced LC3 processing and LC3-green fluorescent protein (GFP) punctate formation. Multiple autophagy inhibitors or VPS34 knockdown, significantly potentiated sorafenib/obatoclax lethality, indicating a cytoprotective role for autophagy in this setting. Finally, studies in a xenograft mouse model revealed that combined sorafenib/obatoclax treatment markedly reduced tumor growth and significantly prolonged survival in association with Mcl-1 down-regulation and apoptosis induction, whereas agents administered individually had only modest effects. These findings suggest that combining sorafenib with agents that inhibit Mcl-1 and Bcl-2/Bcl-xL such as obatoclax may represent a novel and potentially effective strategy in AML.

Quercetin induces tumor-selective apoptosis through downregulation of Mcl-1 and activation of Bax

PURPOSE

To investigate the in vivo antitumor efficacy of quercetin in U937 xenografts and the functional roles of Mcl-1 and Bax in quercetin-induced apoptosis in human leukemia.

EXPERIMENTAL DESIGN

Leukemia cells were treated with quercetin, after which apoptosis, Mcl-1 expression, and Bax activation and translocation were evaluated. The efficacy of quercetin as well as Mcl-1 expression and Bax activation were investigated in xenografts of U937 cells.

RESULTS

Administration of quercetin caused pronounced apoptosis in both transformed and primary leukemia cells but not in normal blood peripheral mononuclear cells. Quercetin-induced apoptosis was accompanied by Mcl-1 downregulation and Bax conformational change and mitochondrial translocation that triggered cytochrome c release. Knockdown of Bax by siRNA reversed quercetin-induced apoptosis and abrogated the activation of caspase and apoptosis. Ectopic expression of Mcl-1 attenuated quercetin-mediated Bax activation, translocation, and cell death. Conversely, interruption of Mcl-1 by siRNA enhanced Bax activation and translocation, as well as lethality induced by quercetin. However, the absence of Bax had no effect on quercetin-mediated Mcl-1 downregulation. Furthermore, in vivo administration of quercetin attenuated tumor growth in U937 xenografts. The TUNEL-positive apoptotic cells in tumor sections increased in quercetin-treated mice as compared with controls. Mcl-1 downregulation and Bax activation were also observed in xenografts.

CONCLUSIONS

These data suggest that quercetin may be useful for the treatment of leukemia by preferentially inducing apoptosis in leukemia versus normal hematopoietic cells through a process involving Mcl-1 downregulation, which, in turn, potentiates Bax activation and mitochondrial translocation, culminating in apoptosis.

Human chorionic gonadotropin up-regulates expression of myeloid cell leukemia-1 protein in human granulosa-lutein cells: implication of corpus luteum rescue and ovarian hyperstimulation syndrome

CONTEXT

The corpus luteum is a dynamic endocrine structure with periodic development and regression during menstrual cycles. Its lifespan can be prolonged by human chorionic gonadotropin (hCG). However, the signal mechanisms of this phenomenon remain unclear.

OBJECTIVE

Our objective was to investigate the molecular mechanisms of hCG in the maintenance of the viability of granulosa-lutein cells.

DESIGN

Granulosa-lutein cells were obtained from women undergoing in vitro fertilization. We examined the effects of hCG on the survival of cultured granulosa-lutein cells. The signal pathway inducing antiapoptotic protein was investigated.

RESULTS

hCG enhanced viability of granulosa-lutein cells through antiapoptosis but not proliferation, because the apoptotic marker of annexin V was decreased, but the proliferative markers of Ki67 and proliferating cell nuclear antigen were not increased. Myeloid cell leukemia-1 (Mcl-1) protein, but not B-cell lymphoma protein-2 or B-cell lymphoma protein-xL, was significantly induced by hCG and LH. The granulosa-lutein cells secreted vascular endothelial growth factor that induced endothelial permeability. Mcl-1 small interfering RNA increased DNA fragmentation and diminished the antiapoptotic effect of hCG. hCG induced Mcl-1 expression through the LH/hCG receptor, adenylate cyclase, protein kinase A, and cAMP response element-binding protein signal pathway. Flavopiridol inhibited Mcl-1 production, released cytochrome c, and induced apoptosis of granulosa-lutein cells.

CONCLUSIONS

We first demonstrate that hCG prevents apoptosis of granulosa-lutein cells through the induction of Mcl-1 protein via the LH/hCG receptor and a cAMP response element-binding protein-dependent pathway. We may have found the molecular mechanism for luteal rescue during early pregnancy. Mcl-1 prevents apoptosis and increases cell viability but not proliferation as mechanisms for luteal rescue. Mcl-1 is a key molecule of hCG signaling.

IGF-1 suppresses Bim expression in multiple myeloma via epigenetic and posttranslational mechanisms

Insulin-like growth factor-1 (IGF-1) is an important growth and survival factor in multiple myeloma (MM). Here, we demonstrate that IGF-1 induces significant down-regulation of the proapoptotic BH3-only protein Bim in MM cells. Reduced Bim levels by RNA interference (RNAi) protected cells from drug-induced cell death. The IGF-1-mediated down-regulation of Bim was the result of (1) reduced transcription by activation of the Akt pathway and inactivation of the transcription factor FoxO3a, (2) increased proteasome-mediated degradation of the Bim extra-long protein by activation of the mitogen-activated protein kinase pathway, and (3) epigenetic regulation of both the Bim and the FoxO3a promoter. Treatment of cells with the histone deacetylase inhibitor LBH589 resulted in a clear up-regulation in the expression of Bim. Furthermore, the methylation inhibitor 5-aza-2'deoxycytidine (decitabine) significantly increased the effects of LBH589. On IGF-1 treatment, the Bim promoter region was found to be unmethylated, whereas chromatin immunoprecipitation analysis of the IGF-1-treated cells showed both a reduced histone H3 tail Lys9 (H3K9) acetylation and an increased H3K9 dimethylation, which contributed actively to its silencing. These data identify a new mechanism in the IGF-1-dependent survival of MM cells and emphasize the need for IGF-1-targeted drug therapy.

Targeting the BCL-2 family in malignancies of germinal centre origin

The germinal centre is a dynamic microenvironment where B-cell responses are honed. Antigen-specific cells undergo clonal expansion followed by antibody affinity maturation and class switching through somatic hypermutation and recombination of immunoglobulin genes respectively. The huge proliferative capacity of the B-cells and the potential for generating non-functional or autoreactive immunoglobulins, necessitate strict control measures. Pro-apoptotic signalling pathways via B-cell receptors, FAS and the TGF-beta receptor, ALK5, ensure that apoptosis of germinal centre B-cells is the norm and cells only survive to differentiate fully if they receive sufficient pro-survival signals to overcome their 'primed for death' status. Several of the B-cell signalling pathways converge on the intrinsic apoptotic machinery to control expression of the BCL-2 family of apoptosis regulators including BIM, the pro-survival factor BCL-X(L) and the BH3-only protein, BIK (recently identified as a mediator of a TGF-beta-induced default apoptosis pathway in human B-cells). It is a foreseeable hazard that cells undergoing genetic mutation and recombination events might unintentionally target some of these factors, resulting in defective programmed cell death. Here we discuss the function of BCL-2 family proteins in germinal centre reactions, their deregulation in malignancies of germinal centre origin, and the potential for targeting BCL-2-related proteins therapeutically in lymphomas.

Recruitment of PKC-betaII to lipid rafts mediates apoptosis-resistance in chronic lymphocytic leukemia expressing ZAP-70

ZAP-70 is a key signaling molecule in T cells. It couples the antigen-activated T-cell receptor to downstream signaling pathways. Its expression in leukemic B-cells derived from a subgroup of patients with chronic lymphocytic leukemia (CLL) is associated with an aggressive course of the disease. However, its implication for the pathogenesis of aggressive CLL is still unclear. In this study, we show that the expression of ZAP-70 enhances the signals associated with the B-cell receptor, recruiting protein kinase C-betaII (PKC-betaII) into lipid raft domains. Subsequently, PKC-betaII is activated and shuttles from the plasma membrane to the mitochondria. We unravel that the antiapoptotic protein Bcl-2 and its antagonistic BH3-protein Bim(EL) are putative substrates for PKC-betaII. PKC-betaII-mediated phosphorylation of Bcl-2 augments its antiapoptotic function by increasing its ability to sequester more pro-apoptotic Bim(EL.) In addition, the phosphorylation of Bim(EL) by PKC-betaII leads to its proteasomal degradation. These changes confer leukemic cells to a more antiapoptotic state with aggressiveness of the disease. Most importantly, these molecular changes can be therapeutically targeted with the small molecule inhibitor Enzastaurin. We provide evidence that this compound is highly active in leukemic cells and augments the cytotoxic effects of standard chemotherapeutic drugs.

BH3-only proteins Mcl-1 and Bim as well as endonuclease G are targeted in spongistatin 1-induced apoptosis in breast cancer cells

Spongistatin 1, a marine experimental substance with chemotherapeutic potential, induces apoptosis and inhibits clonogenic survival of MCF-7 cells. Regarding the apoptotic signaling pathways of spongistatin 1, we present two major facts. Firstly, spongistatin 1-induced cell death, mainly caspase-independent, involves the proapoptotic proteins apoptosis-inducing factor and endonuclease G. Both proteins translocate from mitochondria to the nucleus and contribute to spongistatin 1-mediated apoptosis as shown via gene silencing. Secondly, spongistatin 1 acts as a tubulin depolymerizing agent and is able to free the proapoptotic Bcl-2 family member Bim from its sequestration both by the microtubular complex and by the antiapoptotic protein Mcl-1. Silencing of Bim by small interfering RNA leads to a diminished translocation of apoptosis-inducing factor and endonuclease G to the nucleus and subsequently reduces apoptosis rate. Thus, we identified Bim as an important factor upstream of mitochondria executing a central role in the caspase-independent apoptotic signaling pathway induced by spongistatin 1. Taken together, spongistatin 1 is both a valuable tool for the characterization of apoptotic pathways and a promising experimental anticancer drug.

The BH3-only protein Bim plays a critical role in leukemia cell death triggered by concomitant inhibition of the PI3K/Akt and MEK/ERK1/2 pathways

Mechanisms underlying apoptosis induced by concomitant interruption of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase 1/2 (MEK/ERK1/2) and phosphatidylinositol 3-kinase (PI3K)/Akt pathways were investigated in human leukemia cells. Inhibition of these pathways using the MEK inhibitor PD184352 or U0126 and the PI3K/Akt inhibitor perifosine strikingly induced apoptosis in multiple malignant human hematopoietic cells, and substantially reduced the colony-forming capacity of primary acute myeloblastic leukemia, but not normal CD34+ cells. These events were associated with pronounced Bim up-regulation, Mcl-1 down-regulation, marked Bak/Bax conformational change accompanied by Bax membrane translocation, and a pronounced increase in Bax/Bak association. Molecular studies using tet-inducible Akt, constitutively active MEK1, dominant-negative Akt, and MEK1 small interfering RNA revealed that inhibition of both MEK/ERK1/2 and Akt pathways plays a critical functional role in perifosine/PD184352-mediated lethality. Ectopic Mcl-1 expression potently inhibited perifosine/PD184352-induced apoptosis, as did Bak or Bax knockdown. Notably, knockdown of Bim, but not Bad, blocked Bak and Bax conformational change, inhibited Bax membrane translocation, diminished Bax/Bak binding, and sharply attenuated perifosine/PD184352-induced apoptosis. Finally, enforced expression of Bim significantly enhanced apoptosis induced by PI3K/Akt inhibitors, analogous to the effects of MEK1/2 inhibitors. Collectively, these findings suggest that Bim, and Mcl-1, but not Bad, integrate death signaling triggered by concomitant disruption of the PI3K/Akt and MEK1/2/ERK1/2 pathways in human leukemia cells.

Mcl-1 functions as major epidermal survival protein required for proper keratinocyte differentiation

Rapid downregulation of the antiapoptotic Bcl-2 family protein myeloid cell leukemia 1 (Mcl-1) is required for UV-induced apoptosis, underlining an important role for Mcl-1 in epidermal pathology. To determine if Mcl-1 has a specific role in normal keratinocyte (KC) biology, Mcl-1 was downregulated in human KCs by RNAi and these KCs were induced to differentiate in organotypic raft cultures. Mcl-1 shRNA organotypic cultures showed increased levels of spontaneous premature apoptosis, implicating Mcl-1 as an essential KC survival protein. Mcl-1-downregulated cultures also had reduced granular and cornified layers, and produced lower levels of cross-linked protein and cornified envelopes. Cornification could only partially be rescued with the general caspase inhibitor z-VAD, suggesting that reduced cornification was not entirely because of premature apoptosis. Differentiation markers (K1, K10, filaggrin, loricrin, cleaved caspase-14) were normally expressed in control organotypic cultures, but were expressed at reduced levels in organotypic cultures with downregulated Mcl-1. The defect in differentiation marker expression was independent of apoptosis as it could not be rescued by z-VAD. Thus, Mcl-1 serves two important, independent functions in epidermal KCs: acting as a major survival protein by inhibiting premature apoptosis in the spinous and granular layers to promote conification, and promoting the robust induction of KC differentiation markers.

Novel indoloquinoline derivative, IQDMA, inhibits STAT5 signaling associated with apoptosis in K562 cells

N'-(11H-indolo[3,2-c]quinolin-6-yl)-N,N-dimethylethane-1,2-diamine (IQDMA), an indoloquinoline derivative, synthesized in our laboratory, has been demonstrated to be an effective antitumor agent in human leukemia cells. In the present study, treatment with IQDMA inhibited phosphorylation of epidermal growth factor receptor (EGFR), Src, Bcr-Abl, and Janus-activated kinase (JAK2) in a time-dependent manner. IQDMA also degraded JAK2 protein. Moreover, signal transducer and activator of transcription 5 (STAT5) signaling were also blocked by IQDMA. However, IQDMA did not inhibit other oncogenic and tumor survival pathways such as those mediated by Akt and extracellular signal-regulated kinase 1/2. Furthermore, IQDMA upregulated the expression of p21 and p27 and downregulated the expression of cyclin D1, myeloid cell leukemia-1(Mcl-1), Bcl-X(L), and vascular endothelial growth factor (VEGF). Taken together, these results indicate that IQDMA causes significant induction of apoptosis in K562 cells via downregulation of EGFR, Src, Bcr-Abl, JAK2, and STAT5 signaling and modulation of p21, p27, cyclin D1, Mcl-1, Bcl-X(L), and VEGF proteins. Thus, IQDMA appears to be a potential therapeutic agent for treating leukemia K562 cells.

T cell leukemia/lymphoma 1 and galectin-1 regulate survival/cell death pathways in human naive and IgM+ memory B cells through altering balances in Bcl-2 family proteins

BCR signaling plays a critical role in purging the self-reactive repertoire, or in rendering it anergic to establish self-tolerance in the periphery. Differences in self-reactivity between human naive and IgM(+) memory B cells may reflect distinct mechanisms by which BCR signaling dictates their survival and death. Here we demonstrate that BCR stimulation protected naive B cells from apoptosis with induction of prosurvival Bcl-2 family proteins, Bcl-x(L) and Mcl-1, whereas it rather accelerated apoptosis of IgM(+) memory B cells by inducing proapoptotic BH3-only protein Bim. We found that BCR-mediated PI3K activation induced the expression of Mcl-1, whereas it inhibited Bim expression in B cells. Phosphorylation of Akt, a downstream molecule of PI3K, was more sustained in naive than IgM(+) memory B cells. Abundant expression of T cell leukemia/lymphoma 1 (Tcl1), an Akt coactivator, was found in naive B cells, and enforced expression of Tcl1 induced a high level of Mcl-1 expression, resulting in prolonged B cell survival. In contrast, Galectin-1 (Gal-1) was abundantly expressed in IgM(+) memory B cells, and inhibited Akt phosphorylation, leading to Bim up-regulation. Enforced expression of Gal-1 induced accelerated apoptosis in B cells. These results suggest that a unique set of molecules, Tcl1 and Gal-1, defines distinct BCR signaling cascades, dictating survival and death of human naive and IgM(+) memory B cells.

Postnatal developmental regulation of Bcl-2 family proteins in brain mitochondria

Although it has been long recognized that the relative balance of pro- and antiapoptotic Bcl-2 proteins is critical in determining the susceptibility to apoptotic death, only a few studies have examined the level of these proteins specifically at mitochondria during postnatal brain development. In this study, we examined the age-dependent regulation of Bcl-2 family proteins using rat brain mitochondria isolated at various postnatal ages and from the adult. The results indicate that a general down-regulation of most of the proapoptotic Bcl-2 proteins present in mitochondria occurs during postnatal brain development. The multidomain proapoptotic Bax, Bak, and Bok are all expressed at high levels in mitochondria early postnatally but decline in the adult. Multiple BH3-only proteins, including direct activators (Bid, Bim, and Puma) and the derepressor BH3-only protein Bad, are also present in immature brain mitochondria and are down-regulated in the adult brain. Antiapoptotic Bcl-2 family members are differentially regulated, with a shift from high Bcl-2 expression in immature mitochondria to predominant Bcl-x(L) expression in the adult. These results support the concept that developmental differences in upstream regulators of the mitochondrial apoptotic pathway are responsible for the increased susceptibility of cells in the immature brain to apoptosis following injury.

Novel indoloquinoline derivative, IQDMA, suppresses STAT5 phosphorylation and induces apoptosis in HL-60 cells

Signal transducers and activators of transcription (STATs) are a family proteins that mediate cytokine and growth factor-induced signals playing a role in cell differentiation, proliferation, angiogenesis, and apoptosis. One STAT family member, STAT5, is often constitutively active in myeloid leukaemia. Agents that can suppress STAT5 activation have potential for prevention and treatment of cancer. N'-(11H-indolo[3,2-c]quinolin-6-yl)-N,N-dimethylethane-1,2-dia-mine (IQDMA), an indoloquinoline derivative, synthesized in our laboratory, has been demonstrated to be an effective anti-tumor agent in human leukemia cells. In the present report, we tested IQDMA for its ability to suppress STAT5 activation. We found that IQDMA inhibited constitutive activation of STAT5 in HL-60 cells in a dose- and time-dependent manner. The activation of Src and interleukin-6 (IL-6), implicated in STAT5 activation, was also inhibited by the IQDMA. Furthermore, IQDMA up-regulated Bax, and down-regulated Bcl-2, Bcl-X(L), cyclin D1, and vascular endothelial growth factor (VEGF) as followed by growth arrest of HL-60 cells, but the expression of survivin did not change in the presence of IQDMA. Taken together, these results indicate that IQDMA causes significant induction of apoptosis in HL-60 cells via down-regulation of Src, IL-6, and STAT5 signaling and modulation of Bcl-2 family, cyclin D1 and VEGF proteins. Thus, IQDMA appears to be a potential therapeutic agent for treating leukaemia HL-60 cells.

BH3 mimetic ABT-737 potentiates TRAIL-mediated apoptotic signaling by unsequestering Bim and Bak in human pancreatic cancer cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to induce mitochondrial apoptotic signaling that can be negatively regulated by prosurvival Bcl-2 proteins. ABT-737 is a small-molecule BH3 mimetic that binds to and antagonizes Bcl-2/Bcl-x(L) but not Mcl-1. We show that ABT-737 can synergistically enhance TRAIL-mediated cytotoxicity in human pancreatic cancer cell lines. ABT-737 was shown to enhance TRAIL-induced apoptosis as shown by DNA fragmentation, activation of caspase-8 and Bid, and cleavage of caspase-3 and poly(ADP-ribose) polymerase. A Bax conformational change induced by TRAIL was enhanced by ABT-737. ABT-737 disrupted the interaction of Bak with Bcl-x(L) in both cell lines. Furthermore, ABT-737 untethered the proapoptotic BH3-only protein Bim from its sequestration by Bcl-x(L) or Bcl-2. Bim small hairpin RNA (shRNA) was shown to attenuate caspase-3 cleavage and to reduce the cytotoxic effects of TRAIL plus ABT-737 compared with shRNA control cells. Finally, Mcl-1 shRNA potentiated caspase-3 cleavage by ABT-737 and enhanced its cytotoxic effects. Taken together, ABT-737 augments TRAIL-induced cell killing by unsequestering Bim and Bak and enhancing a Bax conformational change induced by TRAIL. These findings suggest a novel strategy to enhance cross-talk between the extrinsic and intrinsic apoptotic pathways to improve therapeutic efficacy against pancreatic cancer.

RACK1 and CIS mediate the degradation of BimEL in cancer cells

RACK1 is a 7-WD motif-containing protein with numerous downstream effectors regulating various cellular functions. Using a yeast two-hybrid screen, we identified dynein light chain 1 as a novel interacting partner of RACK1. Additionally, we demonstrated that RACK1 formed a complex with DLC1 and Bim, specifically BimEL, in the presence of apoptotic agents. Upon paclitaxel treatment, RACK1, DLC1, and CIS mediated the degradation of BimEL through the ElonginB/C-Cullin2-CIS ubiquitin-protein isopeptide ligase complex. We further showed that RACK1 conferred paclitaxel resistance to breast cancer cells in vitro and in vivo. Finally, we observed an inverse correlation between CIS and BimEL levels in both ovarian and breast cancer cell lines and specimens. Our study suggests a role of RACK1 in protecting cancer cells from apoptosis by regulating the degradation of BimEL, which together with CIS could play an important role of drug resistance in chemotherapy.

BCL-2 family regulation by the 20S proteasome inhibitor bortezomib

Bortezomib (Velcade, PS341) was licensed in 2003 as a first-in-class 20S proteasome inhibitor indicated for treatment of multiple myeloma, and is currently being evaluated clinically in a range of solid tumours. The mechanisms underlying its cancer cell toxicity are complex. A growing body of evidence suggests proteasome inhibition-dependent regulation of the BCL-2 family is a critical requirement. In particular, the stabilization of BH3-only proteins BIK, NOXA and BIM, appear to be essential for effecting BAX- and BAK-dependent cell death. These mechanisms are reviewed and the implications for favourable novel drug interactions are highlighted.

Crosstalk among Bcl-2 family members in B-CLL: seliciclib acts via the Mcl-1/Noxa axis and gradual exhaustion of Bcl-2 protection

Seliciclib (R-roscovitine) is a cyclin-dependent kinase inhibitor in clinical development. It triggers apoptosis by inhibiting de novo transcription of the short-lived Mcl-1 protein, but it is unknown how this leads to Bax/Bak activation that is required for most forms of cell death. Here, we studied the effects of seliciclib in B-cell chronic lymphocytic leukemia (B-CLL), a malignancy with aberrant expression of apoptosis regulators. Although seliciclib-induced Mcl-1 degradation within 4 h, Bax/Bak activation occurred between 16 and 20 h. During this period, no transcriptional changes in apoptosis-related genes occurred. In untreated cells, prosurvival Mcl-1 was engaged by the proapoptotic proteins Noxa and Bim. Upon drug treatment, Bim was quickly released. The contribution of Noxa and Bim as a specific mediator of seliciclib-induced apoptosis was demonstrated via RNAi. Significantly, 16 h after seliciclib treatment, there was accumulation of Bcl-2, Bim and Bax in the 'mitochondria-rich' insoluble fraction of the cell. This suggests that after Mcl-1 degradation, the remaining apoptosis neutralizing capacity of Bcl-2 is gradually overwhelmed, until Bax forms large multimeric pores in the mitochondria. These data demonstrate in primary leukemic cells hierarchical binding and crosstalk among Bcl-2 members, and suggest that their functional interdependence can be exploited therapeutically.

Reciprocal protection of Mcl-1 and Bim from ubiquitin-proteasome degradation

Survival of multiple myeloma cells is essentially dependent on Mcl-1 protein that neutralizes the pro-apoptotic function of Bim and prevents activation of death effectors. To clarify the relationship between Mcl-1 and Bim, we generated cell lines silenced for Mcl-1 (shMcl-1) or Bim (shBim). We demonstrate that Mcl-1 and Bim proteins are concomitantly down-regulated in either shBim or shMcl-1 cells. We show that the down-regulation of either Mcl-1 in shBim or Bim in shMcl-1 cells is not due to a transcriptional event, but results from post-translational regulation. Indeed, the multi-ubiquitinated forms of Mcl-1 or Bim are increased in shBim and shMcl-1 cells, respectively, indicating proteasome degradation. Since Mcl-1/Bim complexes are predominant in myeloma cells the down-regulation of Mcl-1 by shRNA leads to unliganded Bim sensitive to degradation and reciprocally for unliganded Mcl-1 in shBim cells. Finally, our results support that the interaction between Mcl-1 and Bim confers to themselves mutual protection.

The Multikinase Inhibitor Sorafenib Induces Apoptosis in Highly Imatinib Mesylate-Resistant Bcr/Abl+Human Leukemia Cells in Association with Signal Transducer and Activator of Transcription 5 Inhibition and Myeloid Cell Leukemia-1 Down-Regulation

The effects of the multikinase inhibitor sorafenib (BAY 43-9006), an agent shown previously to induce apoptosis in human leukemia cells through inhibition of myeloid cell leukemia-1 (Mcl-1) translation, have been examined in Bcr/Abl(+) leukemia cells resistant to imatinib mesylate (IM). When administered at pharmacologically relevant concentrations (10-15 microM), sorafenib potently induced apoptosis in imatinib mesylate-resistant cells expressing high levels of Bcr/Abl, cells exhibiting a Bcr/Abl-independent, Lyn-dependent form of resistance, and CD34(+) cells obtained from imatinib-resistant patients. In addition, Ba/F3 cells expressing mutations rendering them resistant to IM (e.g., E255K, M351T) or to IM, dasatinib, and nilotinib (T315I) remained fully sensitive to sorafenib. Induction of apoptosis by sorafenib was associated with rapid and pronounced down-regulation of Mcl-1 and diminished signal transducer and activator of transcription (STAT) 5 phosphorylation and reporter activity but only very modest and delayed inactivation of the Bcr/Abl downstream target Crkl. Moreover, transfection with a constitutively active STAT5 construct partially but significantly protected cells from sorafenib lethality. Ba/F3 cells expressing Bcr/Abl mutations were as sensitive to sorafenib-induced Mcl-1 down-regulation and dephosphorylation of STAT5 and eukaryotic initiation factor 4E as wild-type cells. Finally, stable knockdown of Bcl-2-interacting mediator of cell death (Bim) with short hairpin RNA in K562 cells significantly diminished sorafenib lethality, arguing strongly for a functional role of this proapoptotic Bcl-2 family member in the lethality of this agent. Together, these findings suggest that sorafenib effectively induces apoptosis in highly imatinib-resistant chronic myelogenous leukemia cells, most likely by inhibiting or down-regulating targets (i.e., STAT5 and Mcl-1) downstream or independent of Bcr/Abl.

Mcl-1 is a relevant therapeutic target in acute and chronic lymphoid malignancies: down-regulation enhances rituximab-mediated apoptosis and complement-dependent cytotoxicity

PURPOSE

The antiapoptotic Bcl-2 family member protein Mcl-1 is dynamically regulated in transformed B-cells, has a short mRNA and protein half-life, and is rapidly processed during apoptosis. Multiple therapies cause down-regulation of Mcl-1 in chronic and acute lymphoid leukemia (CLL and ALL) cells. Mcl-1 has also been reported to mediate resistance to rituximab in CLL. We therefore investigated whether direct reduction of Mcl-1 was sufficient to induce apoptosis and increase sensitivity to rituximab.

EXPERIMENTAL DESIGN

We used Mcl-1-specific small interfering RNA in ALL cell lines and tumor cells from CLL patients to block transcription of Mcl-1.

RESULTS

We show that Mcl-1 down-regulation alone is sufficient to promote mitochondrial membrane depolarization and apoptosis in ALL and CLL cells. Given the importance of rituximab in B-cell malignancies, we next assessed the influence of Mcl-1 down-regulation on antibody-mediated killing. Mcl-1 down-regulation by small interfering RNA increased sensitivity to rituximab-mediated killing both by direct apoptosis and complement-dependent cytotoxicity, but did not enhance antibody-dependent cellular cytotoxicity.

CONCLUSIONS

These results show that Mcl-1 is a relevant therapeutic target for ALL and CLL, and its down-regulation has the potential to enhance the therapeutic effect of rituximab in CD20-bearing lymphoid cells.

Preclinical studies of the pan-Bcl inhibitor obatoclax (GX015-070) in multiple myeloma

Bcl family members Bcl-2, Bcl-xL, and Mcl-1, are frequently expressed and implicated in the survival of myeloma cells. Obatoclax (GX015-070) is a novel, small-molecule antagonist of the BH3-binding groove of the Bcl family of proteins. We show that GX015-070 inhibits the binding of Bak to Mcl-1, up-regulates Bim, induces cytochrome c release, and activates capase-3 in human myeloma cell lines (HMCLs), confirming the predicted mechanism of action. Consequently, GX015-070 potently inhibited the viability of 15 of 16 HMCLs (mean IC50 of 246 nM), including those resistant to melphalan and dexamethasone. In combination studies, GX015-070 enhanced the antimyeloma activity induced by melphalan, dexamethasone, or bortezomib. Sensitivity to GX015-070 correlated with the absence or near absence of Bcl-xL. Coculture with interleukin-6 or adherence to bone marrow stroma conferred modest resistance; however, it did not overcome GX015-070–induced cytotoxicity. Of importance, GX015-070 as a single agent induced potent cytotoxic responses against patient-derived tumor cells. GX015-070 inhibited normal bone marrow–derived colony formation; however, cytotoxicity to human blood lymphocytes was not observed. Taken together, these studies describe a novel BH3 mimic with selectivity for Mcl-1, and support the therapeutic application of GX015-070 for diverse neoplasias including multiple myeloma.

Noxa Up-regulation and Mcl-1 Cleavage Are Associated to Apoptosis Induction by Bortezomib in Multiple Myeloma

Targeting the ubiquitin-proteasome pathway has emerged as a potent anticancer strategy. Bortezomib, a specific proteasome inhibitor, has been approved for the treatment of relapsed or refractory multiple myeloma. Multiple myeloma cell survival is highly dependent on Mcl-1 antiapoptotic molecules. In a recent study, proteasome inhibitors induced Mcl-1 accumulation that slowed down their proapoptotic effects. Consequently, we investigated the role of Bcl-2 family members in bortezomib-induced apoptosis. We found that bortezomib induced apoptosis in five of seven human myeloma cell lines (HMCL). Bortezomib-induced apoptosis was associated with Mcl-1 cleavage regardless of Mcl-1L accumulation. Furthermore, RNA interference mediated Mcl-1 decrease and sensitized RPMI-8226 HMCL to bortezomib, highlighting the contribution of Mcl-1 in bortezomib-induced apoptosis. Interestingly, an important induction of Noxa was found in all sensitive HMCL both at protein and mRNA level. Concomitant to Mcl-1 cleavage and Noxa induction, we also found caspase-3, caspase-8, and caspase-9 activation. Under bortezomib treatment, Mcl-1L/Noxa complexes were highly increased, Mcl-1/Bak complexes were disrupted, and there was an accumulation of free Noxa. Finally, we observed a dissociation of Mcl-1/Bim complexes that may be due to a displacement of Bim induced by Noxa. Thus, in myeloma cells, the mechanistic basis for bortezomib sensitivity can be explained mainly by the model in which the sensitizer Noxa can displace Bim, a BH3-only activator, from Mcl-1, thus leading to Bax/Bak activation.

ERK1/2-dependent phosphorylation of BimEL promotes its rapid dissociation from Mcl-1 and Bcl-xL

The proapoptotic protein Bim is expressed de novo following withdrawal of serum survival factors. Here, we show that Bim-/- fibroblasts and epithelial cells exhibit reduced cell death following serum withdrawal in comparison with their wild-type counterparts. In viable cells, Bax associates with Bcl-2, Bcl-x(L) and Mcl-1. Upon serum withdrawal, newly expressed Bim(EL) associates with Bcl-x(L) and Mcl-1, coinciding with the dissociation of Bax from these proteins. Survival factors can prevent association of Bim with pro-survival proteins by preventing Bim expression. However, we now show that even preformed Bim(EL)/Mcl-1 and Bim(EL)/Bcl-x(L) complexes can be rapidly dissociated following activation of ERK1/2 by survival factors. The dissociation of Bim from Mcl-1 is specific for Bim(EL) and requires ERK1/2-dependent phosphorylation of Bim(EL) at Ser(65). Finally, ERK1/2-dependent dissociation of Bim(EL) from Mcl-1 and Bcl-x(L) may play a role in regulating Bim(EL) degradation, since mutations in the Bim(EL) BH3 domain that disrupt binding to Mcl-1 cause increased turnover of Bim(EL). These results provide new insights into the role of Bim in cell death and its regulation by the ERK1/2 survival pathway.

Cutting Edge: Bim is required for superantigen-mediated B cell death

To impair B cell clonal regulation, the microbial virulence factor, protein A of Staphylococcus aureus, can interact with evolutionarily conserved BCR-binding sites to induce a form of Fas-independent activation-associated B cell death that results in selective immune tolerance. We now show that this in vivo death pathway is associated with induction of increased transcript and protein levels of Bim, a BH3-only proapoptotic Bcl-2 family protein, which is inhibited by excess B cell-activating factor. An absolute requirement for Bim was documented, since Bim-deficient B cells were protected from in vivo superantigen-induced death and instead underwent persistent massive supraclonal expansion without functional impairment. These studies characterize a BCR-dependent negative clonal selection pathway that has been co-opted by a common bacterial pathogen to induce selective defects in host immune defenses.

Mcl-1 down-regulation potentiates ABT-737 lethality by cooperatively inducing Bak activation and Bax translocation

The Bcl-2 antagonist ABT-737 targets Bcl-2/Bcl-xL but not Mcl-1, which may confer resistance to this novel agent. Here, we show that Mcl-1 down-regulation by the cyclin-dependent kinase (CDK) inhibitor roscovitine or Mcl-1-shRNA dramatically increases ABT-737 lethality in human leukemia cells. ABT-737 induces Bax conformational change but fails to activate Bak or trigger Bax translocation. Coadministration of roscovitine and ABT-737 untethers Bak from Mcl-1 and Bcl-xL, respectively, triggering Bak activation and Bax translocation. Studies employing Bax and/or Bak knockout mouse embryonic fibroblasts (MEFs) confirm that Bax is required for ABT-737+/-roscovitine lethality, whereas Bak is primarily involved in potentiation of ABT-737-induced apoptosis by Mcl-1 down-regulation. Ectopic Mcl-1 expression attenuates Bak activation and apoptosis by ABT-737+roscovitine, whereas cells overexpressing Bcl-2 or Bcl-xL remain fully sensitive. Finally, Mcl-1 knockout MEFs are extremely sensitive to Bak conformational change and apoptosis induced by ABT-737, effects that are not potentiated by roscovitine. Collectively, these findings suggest down-regulation of Mcl-1 by either CDK inhibitors or genetic approaches dramatically potentiate ABT-737 lethality through cooperative interactions at two distinct levels: unleashing of Bak from both Bcl-xL and Mcl-1 and simultaneous induction of Bak activation and Bax translocation. These findings provide a mechanistic basis for simultaneously targeting Mcl-1 and Bcl-2/Bcl-xL in leukemia.

Host-Cell Survival and Death During Chlamydia Infection

Different Chlamydia trachomatis strains are responsible for prevalent bacterial sexually-transmitted disease and represent the leading cause of preventable blindness worldwide. Factors that predispose individuals to disease and mechanisms by which chlamydiae cause inflammation and tissue damage remain unclear. Results from recent studies indicate that prolonged survival and subsequent death of infected cells and their effect on immune effector cells during chlamydial infection may be important in determining the outcome. Survival of infected cells is favored at early times of infection through inhibition of the mitochondrial pathway of apoptosis. Death at later times displays features of both apoptosis and necrosis, but pro-apoptotic caspases are not involved. Most studies on chlamydial modulation of host-cell death until now have been performed in cell lines. The consequences for pathogenesis and the immune response will require animal models of chlamydial infection, preferably mice with targeted deletions of genes that play a role in cell survival and death.

Regulation of Mcl-1 expression in rheumatoid arthritis synovial macrophages

OBJECTIVE

Resistance to apoptosis may be an important mechanism contributing to the persistence of rheumatoid arthritis (RA). This study was undertaken to characterize the expression, regulation, and function of the antiapoptotic Bcl-2 family member Mcl-1 in macrophages isolated from the joints of patients with RA.

METHODS

Mononuclear cells were isolated from the synovial fluid (SF) of patients with RA. Mcl-1 expression was documented by intracellular staining of CD14+ cells using flow cytometry, and by real-time polymerase chain reaction or immunoblot analysis of isolated macrophages. The expression of Mcl-1 was suppressed with small interfering RNA (siRNA) or chemical inhibitors of the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt-1 and signal transducer and activator of transcription 3 (STAT-3) pathways. Apoptosis was defined by the loss of mitochondrial transmembrane potential and by DNA fragmentation.

RESULTS

The expression of Mcl-1 was increased in CD14+ macrophages from the SF of patients with RA compared with normal in vitro-differentiated macrophages. Inhibition of the PI 3-kinase/Akt-1 or STAT-3 pathways significantly reduced the percentage of CD14+ cells within the SF and resulted in the reduction of Mcl-1 and the induction of apoptosis of synovial macrophages. Transfection of RA synovial macrophages with Mcl-1 siRNA resulted in apoptotic cell death.

CONCLUSION

Mcl-1 is critical for the survival of macrophages in the joints of patients with RA, and is therefore a potential therapeutic target in this disease.

B-cell receptor signaling in chronic lymphocytic leukemia cells is regulated by overexpressed active protein kinase CβII

Signals through the B-cell antigen receptor (BCR) are important for the survival of chronic lymphocytic leukemia (CLL) cells. Therefore, factors that influence these signals have important pathophysiological roles in this disease. One key mediator of BCR signaling is protein kinase C β (PKCβ), which regulates the activation of I-κB kinases and the deactivation of Bruton tyrosine kinase within the signaling pathways initiated by BCR engagement. The present study demonstrates that overexpression of the PKCβII isoform is a feature of CLL cells and that activity of this enzyme strongly correlates with CLL cell response to BCR engagement. Thus, intracellular Ca2+ release and increases in cell survival after BCR cross-linking were significantly greater in CLL patients with low levels than in CLL patients with high levels of active PKCβII. Furthermore, BCR-induced Ca2+ fluxes could be restored in CLL patients with high levels of active PKCβII by pretreating the cells with the PKCβ-specific inhibitor LY379196. Conversely, BCR-mediated intracellular Ca2+ release could be inhibited in CLL cells with low levels of active PKCβII by pretreatment with the PKC agonist bryostatin. Taken together, these results demonstrate that overexpressed active PKCβII plays a role in the regulation and outcome of BCR signals that can be important for the progression of CLL.

Mitochondria in hematopoiesis and hematological diseases

Mitochondria are involved in hematopoietic cell homeostasis through multiple ways such as oxidative phosphorylation, various metabolic processes and the release of cytochrome c in the cytosol to trigger caspase activation and cell death. In erythroid cells, the mitochondrial steps in heme synthesis, iron (Fe) metabolism and Fe-sulfur (Fe-S) cluster biogenesis are of particular importance. Mutations in the specific delta-aminolevulinic acid synthase (ALAS) 2 isoform that catalyses the first and rate-limiting step in heme synthesis pathway in the mitochondrial matrix, lead to ineffective erythropoiesis that characterizes X-linked sideroblastic anemia (XLSA), the most common inherited sideroblastic anemia. Mutations in the adenosine triphosphate-binding cassette protein ABCB7, identified in XLSA with ataxia (XLSA-A), disrupt the maturation of cytosolic (Fe-S) clusters, leading to mitochondrial Fe accumulation. In addition, large deletions in mitochondrial DNA, whose integrity depends on a specific DNA polymerase, are the hallmark of Pearson's syndrome, a rare congenital disorder with sideroblastic anemia. In acquired myelodysplastic syndromes at early stage, exacerbation of physiological pathways involving caspases and the mitochondria in erythroid differentiation leads to abnormal activation of a mitochondria-mediated apoptotic cell death pathway. In contrast, oncogenesis-associated changes at the mitochondrial level can alter the apoptotic response of transformed hematopoietic cells to chemotherapeutic agents. Recent findings in mitochondria metabolism and functions open new perspectives in treating hematopoietic cell diseases, for example various compounds currently developed to trigger tumor cell death by directly targeting the mitochondria could prove efficient as either cytotoxic drugs or chemosensitizing agents in treating hematological malignancies.

Requirement for JNK-dependent upregulation of BimL in anti-IgM-induced apoptosis in murine B lymphoma cell lines WEHI-231 and CH31

The cross-linking of B cell receptor (BCR) undergoes growth arrest, accompanied by apoptosis, in the CH31 and WEHI-231 B lymphoma cells, a model representing primary immature B cells. We have previously demonstrated that sustained activation of c-Jun N-terminal kinase (JNK) is required for BCR-mediated apoptosis. In the present study, we examined how the anti-IgM-induced prolonged activation of JNK results in apoptosis. Anti-IgM upregulated the expression levels of three isoforms of Bim protein, especially BimL, which appeared to be dependent on JNK activation. In contrast to protein expression, BimL mRNA levels were down-regulated upon anti-IgM stimulation, suggesting that anti-IgM-induced upregulation of BimL is regulated through post-transcriptional control. Upon JNK activation, phosphorylated form of JNK, together with Bax migrated from cytosol to mitochondria. In unstimulated cells, BimL protein was complexed with Bcl-x(L) and changed the partner to associate with Bax on the mitochondrial membrane after ligation of BCR, leading to initiation of apoptotic processes. Retroviral transduction of BimL into WEHI-231 cells overexpressing dominant-negative form of JNK1 (dnJNK1) resulted in a comparable level of apoptotic cells to control cells, whereas the BimL-mediated apoptosis was partially prevented by Bcl-x(L). Taken together, engagement of BCR with anti-IgM results in association of Bax-alpha with BimL in the mitochondria, at least in part, through a sustained activation of JNK.

Caloric restriction suppresses apoptotic cell death in the mammalian cochlea and leads to prevention of presbycusis

Presbycusis is characterized by an age-related progressive decline of auditory function, and arises mainly from the degeneration of hair cells or spiral ganglion (SG) cells in the cochlea. Here we show that caloric restriction suppresses apoptotic cell death in the mouse cochlea and prevents late onset of presbycusis. Calorie restricted (CR) mice, which maintained body weight at the same level as that of young control (YC) mice, retained normal hearing and showed no cochlear degeneration. CR mice also showed a significant reduction in the number of TUNEL-positive cells and cleaved caspase-3-positive cells relative to middle-age control (MC) mice. Microarray analysis revealed that CR down-regulated the expression of 24 apoptotic genes, including Bak and Bim. Taken together, our findings suggest that loss of critical cells through apoptosis is an important mechanism of presbycusis in mammals, and that CR can retard this process by suppressing apoptosis in the inner ear tissue.