Bisindolylmaleimide IX is a potent inducer of apoptosis in chronic lymphocytic leukaemic cells and activates cleavage of Mcl-1

Identification of Bisindolylmaleimide IX as a potential agent to treat drug-resistant BCR-ABL positive leukemia

Chronic myeloid leukemia (CML) treatment with BCR-ABL inhibitors is often hampered by development of drug resistance. In a screen for novel chemotherapeutic drug candidates with genotoxic activity, we identified a bisindolylmaleimide derivative, IX, as a small molecule compound with therapeutic potential against CML including drug-resistant CML. We show that Bisindolylmaleimide IX inhibits DNA topoisomerase, generates DNA breaks, activates the Atm-p53 and Atm-Chk2 pathways, and induces cell cycle arrest and cell death. Interestingly, Bisindolylmaleimide IX is highly effective in targeting cells positive for BCR-ABL. BCR-ABL positive cells display enhanced DNA damage and increased cell cycle arrest in response to Bisindolylmaleimide IX due to decreased expression of topoisomerases. Cells positive for BCR-ABL or drug-resistant T315I BCR-ABL also display increased cytotoxicity since Bisindolylmaleimide IX inhibits B-Raf and the downstream oncogene addiction pathway. Mouse cancer model experiments showed that Bisindolylmaleimide IX, at doses that show little side effect, was effective in treating leukemia-like disorders induced by BCR-ABL or T315I BCR-ABL, and prolonged the lifespan of these model mice. Thus, Bisindolylmaleimide IX presents a novel drug candidate to treat drug-resistant CML via activating BCR-ABL-dependent genotoxic stress response and inhibiting the oncogene addiction pathway activated by BCR-ABL.

Jacarelhyperol A induced apoptosis in leukaemia cancer cell through inhibition the activity of Bcl-2 proteins

Background

Hypericum japonicum Thunb. ex Murray is widely used as an herbal medicine for the treatment of hepatitis and tumours in China. However, the molecular mechanisms of its effects are unclear. Our previous research showed that extracts of H. japonicum can induce apoptosis in leukaemia cells. We also previously systematically analysed and isolated the chemical composition of H. japonicum.

Methods

The fluorescence polarisation experiment was used to screen for inhibitors of Bcl-2 proteins which are proved as key proteins in apoptosis. The binding mode was modelled by molecular docking. We investigated the proliferation attenuating and apoptosis inducing effects of active compound on cancer cells by MTT assay and flow cytometry analysis. Activation of caspases were tested by Western blot. A broad-spectrum caspase inhibitor Z-VAD-FMK was used to investigate the caspases-dependence. In addition, co-immunoprecipitation was performed to analyse the inhibition of heterodimerization between anti-apoptotic Bcl-2 proteins with pro-apoptotic proteins. Moreover, in vivo activity was tested in a mouse xenograph tumour model.

Result

Jacarelhyperol A (Jac-A), a characteristic constituent of H. japonicum, was identified as a potential Bcl-2 inhibitor. Jac-A showed binding affinities to Bcl-x_L, Bcl-2, and Mcl-1 with K_i values of 0.46 μM, 0.43 μM, and 1.69 μM, respectively. This is consistent with computational modelling results, which show that Jac-A presents a favorable binding mode with Bcl-x_L in the BH3-binding pocket. In addition, Jac-A showed potential growth inhibitory activity in leukaemia cells with IC₅₀ values from 1.52 to 6.92 μM and significantly induced apoptosis of K562 cells by promoting release of cytochrome c and activating the caspases. Jac-A also been proved that its effect is partly caspases-dependent and can disrupt the heterodimerization between anti-apoptotic Bcl-2 proteins with pro-apoptotic proteins. Moreover, Jac-A dose-dependently inhibited human K562 cell growth in a mouse xenograph tumour model with low toxicity.

Conclusion

In this study, a characteristic constituent of H. japonicum, Jac-A, was shown to induce apoptosis in leukaemia cells by mediating the Bcl-2 proteins. Therefore, we propose a new lead compound for cancer therapy with a low toxicity, and have provided evidence for using H. japonicum as an anti-cancer herb.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2407-14-689) contains supplementary material, which is available to authorized users.

The VTLISFG motif in the BH1 domain plays a significant role in regulating the degradation of Mcl-1

Mcl-1 is a member of the Bcl-2 family protein; its degradation is required for the initiation of apoptosis. The mechanism, however, is not yet clearly known. Previously, it was reported that Mcl-1 is degraded through the ubiquitination-mediated pathway and the PEST domain is the motif responsible for promoting this degradation. We found evidence that this may not be true. We generated several Mcl-1 deletion mutants and examined their effects on protein stability. Deletion of the PEST domain did not prevent the degradation of Mcl-1 during apoptosis. The BH1 domain, but not the PEST, BH3 or BH2 domain, exhibited a short half-life. A peptide named “F3” (VTLISFG) in the C-terminus of the BH1 domain appears to be critical for the rapid turnover of Mcl-1. Deletion of F3 from GFP-Mcl-1-ΔPEST retarded the degradation of this mutant. F3 appeared to be the minimum functional sequence of the degradation motif, since deletion of a single residue was sufficient to abrogate its short half-life. Fusion of F3 with p32 resulted in the degradation of p32 during UV-induced apoptosis, while wild type p32 was not affected. Taken together, these findings suggest that F3 (VTLISFG), instead of PEST, is the major motif responsible for the degradation of Mcl-1 during apoptosis.

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The PEST domain may not be responsible for the short half-life of Mcl-1 during apoptosis.

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A short peptide (F3) inside the BH1 domain was found to have a short half-life.

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Fusion of F3 with p32 impairs the stability of p32 during apoptosis.

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Deletion of F3 increases the stability of GFP-Mcl-1-ΔPEST.

Regulation of protein translation initiation in response to ionizing radiation

Background

Proliferating tumor cells require continuous protein synthesis. De novo synthesis of most proteins is regulated through cap-dependent translation. Cellular stress such as ionizing radiation (IR) blocks cap-dependent translation resulting in shut-down of global protein translation which saves resources and energy needed for the stress response. At the same time, levels of proteins required for stress response are maintained or even increased. The study aimed to analyze the regulation of signaling pathways controlling protein translation in response to IR and the impact on Mcl-1, an anti-apoptotic and radioprotective protein, which levels rapidly decline upon IR.

Methods

Protein levels and processing were analyzed by Western blot. The assembly of the translational pre-initiation complex was examined by Immunoprecipitation and pull-down experiments with 7-methyl GTP agarose. To analyze IR-induced cell death, dissipation of the mitochondrial membrane potential and DNA fragmentation were determined by flow cytometry. Protein levels of the different initiation factors were down-regulated using RNA interference approach.

Results

IR induced caspase-dependent cleavage of the translational initiation factors eIF4G1, eIF3A, and eIF4B resulting in disassembly of the cap-dependent initiation complex. In addition, DAP5-dependent initiation complex that regulates IRES-dependent translation was disassembled in response to IR. Moreover, IR resulted in dephosphorylation of 4EBP1, an inhibitor of cap-dependent translation upstream of caspase activation. However, knock-down of eIF4G1, eIF4B, DAP5, or 4EBP1 did not affect IR-induced decline of the anti-apoptotic protein Mcl-1.

Conclusion

Our data shows that cap-dependent translation is regulated at several levels in response to IR. However, the experiments indicate that IR-induced Mcl-1 decline is not a consequence of translational inhibition in Jurkat cells.

Protein kinases: emerging therapeutic targets in chronic lymphocytic leukemia

INTRODUCTION

Although protein kinases are primary targets for inhibition in hematological malignancies, until recently their contribution to chronic lymphocytic leukemia (CLL) was poorly understood. Insights into B-cell receptor signaling and its role in regulating key cellular functions have shed light on candidate protein kinases that are aberrantly activated in CLL. In this regard, protein kinases are now considered as potential drug targets in CLL.

AREA COVERED

This review has covered signaling pathways and associated protein kinases in CLL and the kinase inhibitors currently available in preclinical and clinical investigations. Individual protein kinases that are abnormally active in CLL and the functional consequences of their inhibition are discussed.

EXPERT OPINION

A growing body of evidence suggests that protein kinases are druggable targets for patients with CLL. The emergence of novel and bio-available kinase inhibitors and their promising clinical activity in CLL underscore the oncogenic role of kinases in leukemogenesis. Further investigations directed towards their role as single agents or in combinations may provide insight into understanding the substantial role of kinase-mediated signal transduction pathways and their inhibition in B- CLL.

Simultaneous inhibition of DNA methyltransferase and histone deacetylase induces p53-independent apoptosis via down-regulation of Mcl-1 in acute myelogenous leukemia cells

We have recently established the MV4-11 acute myelogenous leukemia (AML) subline, designated as MV4-11 TP53 R248W, which possesses a missense mutation (CGG→TGG; R248W) in the TP53 gene, leading to inactivation of this transcription factor. DNA methyltransferase (DNMT) inhibitor 5-Aza-2'-deoxycytidine (5-AzadC) induced apoptosis in MV4-11, but not in MV4-11 TP53 R248W cells. Another class of anti-epigenetic agent histone deacetylase inhibitor (HDACI) inhibited the proliferation of both MV4-11 and MV4-11 TP53 R248W cells. Notably, when 5-AzadC was combined with HDACI MS-275, apoptosis in MV4-11 TP53 R248W cells was significantly enhanced in parallel with activation of the caspase cascade, up-regulation of p21waf1 and γ-H2AX, and down-regulation of Mcl-1. Interestingly, inhibition of caspase 3 by the pan-caspase inhibitor attenuated the combination of 5-AzadC and MS-275-mediated apoptosis and down-regulation of Mcl-1 in MV4-11 TP53 R248W cells. Moreover, down-regulation of p21waf1 in MV4-11 R248W cells by a small interfering RNA blunted activation of caspase 3 after exposure to the combination of 5-AzadC and MS-275, indicating the role of p21waf1 to activate caspase 3. Taken together, TP53-independent up-regulation of p21waf1 activates caspase 3 and down-regulates Mcl-1 in AML cells. Combination of 5-AzadC and MS-275 may be a promising treatment strategy for individuals with leukemia in which TP53 is inactivated.

Role of NOXA and its ubiquitination in proteasome inhibitor-induced apoptosis in chronic lymphocytic leukemia cells

BACKGROUND

Bortezomib has been successfully used in the treatment of multiple myeloma and has been proposed as a potential treatment for chronic lymphocytic leukemia. In this study we investigated the mechanism by which bortezomib induces apoptosis in chronic lymphocytic leukemia cells.

DESIGN AND METHODS

Using western blot analysis, we monitored the regulation of BCL2 family members, proteins of the unfolded protein response (endoplasmic reticulum stress response) and activation of caspases in relation to induction of apoptosis (measured by annexin-propidium iodide staining and loss of mitochondrial membrane potential) by bortezomib in chronic lymphocytic leukemia cells.

RESULTS

Bortezomib induced apoptosis through activation of the mitochondrial pathway independently of changes associated with endoplasmic reticulum stress. Perturbation of mitochondria was regulated by a rapid and transcription-independent increase of NOXA protein, which preceded release of cytochrome c, HtrA2, Smac and activation of caspase-9 and -3. NOXA had a short half life (approximately 1-2 h) and was ubiquitinated on at least three primary lysine residues, resulting in proteasomal-dependent degradation. Down-regulation of NOXA, using short interfering RNA in chronic lymphocytic leukemia cells, decreased bortezomib-induced apoptosis. Finally bortezomib when combined with seliciclib resulted in a stronger and earlier increase in NOXA protein, caspase-3 cleavage and induction of apoptosis in chronic lymphocytic leukemia cells.

CONCLUSIONS

These results highlight a critical role for NOXA in bortezomib-induced apoptosis in chronic lymphocytic leukemia cells and suggest that this drug may become more efficient for the treatment of chronic lymphocytic leukemia if combined with other agents able to interfere with the basal levels of MCL1.

The promyelocytic leukemia zinc-finger gene, PLZF, is frequently downregulated in malignant mesothelioma cells and contributes to cell survival

DNA copy number analysis was performed, using single-nucleotide polymorphism mapping arrays, to fine map genomic imbalances in human malignant mesothelioma (MM) cell lines derived from primary tumors. Chromosomal losses accounted for the majority of genomic imbalances. All 22 cell lines examined showed homozygous deletions of 9p21.3, centering at the CDKN2A/ARF and CDKN2B loci. Other commonly underrepresented segments included 1p36, 1p22, 3p21-22, 4q13, 4q34, 11q23, 13q12-13, 14q32, 15q15, 18q12, and 22q12, each observed in 55-90% of cell lines. Focal deletions of 11q23 encompassed the transcriptional repressor gene promyelocytic leukemia zinc finger (PLZF), which was validated by analysis of genomic DNA using real-time polymerase chain reaction (PCR). Semi-quantitative RT-PCR and immunoblot analysis revealed that PLZF is greatly downregulated in MM cell lines compared with non-malignant mesothelial cells. Ectopic expression of PLZF in PLZF-deficient MM cells resulted in decreased cell viability, reduced colony formation, as well as increased apoptosis, the latter based on results of various cell death assays and the observation of increased cleavage of caspase 3, PARP, and Mcl-1. These data indicate that deletions of PLZF are a common occurrence in MM and that downregulation of PLZF may contribute to MM pathogenesis by promoting cell survival.

Akt inhibitors induce apoptosis in chronic lymphocytic leukemia cells

BACKGROUND

The phosphatidylinositol-3-kinase/Akt pathway has been described to be critical in the survival of chronic lymphocytic leukemia cells. In this study we analyzed the effect of two selective chemical inhibitors of Akt (Akti-1/2 and A-443654) on the survival of chronic lymphocytic leukemia cells.

DESIGN AND METHODS

Using cytometry we studied the cytotoxic effects of Akt inhibitors on peripheral B and T lymphocytes from patients with chronic lymphocytic leukemia and from healthy donors. We studied the changes induced by Akti-1/2 and A-443654 at the mRNA level by performing reverse transcriptase multiplex ligation-dependent probe amplification. We also studied the changes induced by both Akt inhibitors in some BCL-2 protein family members on chronic lymphocytic leukemia cells by western blotting. Moreover, we analyzed the cytotoxic effect of Akt inhibitors in patients' cells with deleted/mutated TP53.

RESULTS

Both inhibitors induced apoptosis in chronic lymphocytic leukemia cells in a dose-dependent manner. Moreover, B cells from patients with chronic lymphocytic leukemia were more sensitive to Akt inhibitors than T cells from leukemic patients, and B or T cells from healthy donors. Survival factors for chronic lymphocytic leukemia cells, such as interleukin-4 and stromal cell-derived factor-1alpha, were not able to block the apoptosis induced by either Akt inhibitor. Akti-1/2 did not induce any change in the mRNA expression profile of genes involved in apoptosis, while A-443654 induced some changes, including an increase in NOXA and PUMA mRNA levels, suggesting the existence of additional targets for A-443654. Both inhibitors induced an increase in PUMA and NOXA protein levels, and a decrease in MCL-1 protein level. Moreover, Akti-1/2 and A-443654 induced apoptosis irrespective of TP53 status.

CONCLUSIONS

These results demonstrate that Akt inhibitors induce apoptosis of chronic lymphocytic leukemia cells and might be a new therapeutic option for the treatment of chronic lymphocytic leukemia.

Akt is activated in chronic lymphocytic leukemia cells and delivers a pro-survival signal: the therapeutic potential of Akt inhibition

BACKGROUND

The aims of the present study were to ascertain the activation status of Akt in the primary cells of chronic lymphocytic leukemia and to investigate the effects of specific Akt inhibition on chronic lymphocytic leukemia-cell survival.

DESIGN AND METHODS

Anti-phospho-Akt (Ser473 or Thr308) antibodies and western blotting were used to establish the activation status of Akt. The effects of two different, specific small-molecule inhibitors (A-443654 or Akti-1/2) or small interfering RNA on cell survival and downstream targets of Akt were assessed. Apoptosis was determined by fluorescence-activated cell sorting analysis of phosphatidylserine exposure and by measurement of PARP cleavage. The phosphorylation status of GSK-3 and MDM2, two immediate downstream substrates of Akt, levels of the anti-apoptotic proteins BCL2 and MCL1, and expression of p53 and p21 were all measured by western blotting.

RESULTS

Fully activated Akt was demonstrable in all chronic lymphocytic leukemia clones examined (n=26). These results were validated with extensive controls and it was shown that a harsh method of cell extraction is needed for detection of the active enzyme. Specific inhibition of Akt induced extensive apoptosis of chronic lymphocytic leukemia cells, which was associated with both a rapid loss of MCL1 through proteasomal degradation and increased expression of p53. Moreover, the Akt inhibitors, at concentrations that induced extensive apoptosis in chronic lymphocytic leukemia cells, had little or no effect on normal peripheral blood mononuclear cells.

CONCLUSIONS

Chronic lymphocytic leukemia clones consistently contain activated Akt which plays a pivotal role in maintaining cell survival. Inhibition of the Akt pathway may be of potential value as a novel therapeutic strategy in chronic lymphocytic leukemia.

The novel plant-derived agent silvestrol has B-cell selective activity in chronic lymphocytic leukemia and acute lymphoblastic leukemia in vitro and in vivo

Therapeutic options for advanced B-cell acute lymphoblastic leukemia (ALL) and chronic lymphocytic leukemia (CLL) are limited. Available treatments can also deplete T lymphocytes, leaving patients at risk of life-threatening infections. In the National Cancer Institute cell line screen, the structurally unique natural product silvestrol produces an unusual pattern of cytotoxicity that suggests activity in leukemia and selectivity for B cells. We investigated silvestrol efficacy using primary human B-leukemia cells, established B-leukemia cell lines, and animal models. In CLL cells, silvestrol LC(50) (concentration lethal to 50%) is 6.9 nM at 72 hours. At this concentration, there is no difference in sensitivity of cells from patients with or without the del(17p13.1) abnormality. In isolated cells and whole blood, silvestrol is more cytotoxic toward B cells than T cells. Silvestrol causes early reduction in Mcl-1 expression due to translational inhibition with subsequent mitochondrial damage, as evidenced by reactive oxygen species generation and membrane depolarization. In vivo, silvestrol causes significant B-cell reduction in Emu-Tcl-1 transgenic mice and significantly extends survival of 697 xenograft severe combined immunodeficient (SCID) mice without discernible toxicity. These data indicate silvestrol has efficacy against B cells in vitro and in vivo and identify translational inhibition as a potential therapeutic target in B-cell leukemias.

Bisindolylmaleimides in anti-cancer therapy - more than PKC inhibitors

Bisindolylmaleimide derivatives were originally described as protein kinase C inhibitors. However, several studies have shown that bisindolylmaleimides target several other signaling molecules. The review presents bisindolylmaleimide-mediated PKC-dependent and PKC-independent biological effects, such as reversal of MDR and modulation of Wnt signaling through GSK-3b and b-catenin. Importantly, the potent proapoptotic properties of bisindolylmaleimides are also described. Bis-IX appears as the most efficient activator of intrinsic apoptotic pathway and additionally, facilitates extrinsic apoptosis. Presented molecular mechanisms indicate that bisindolylmaleimides could be useful agents in anticancer therapy. They repress uncontrolled proliferation and restore the sensitivity to chemotherapy which allows eradication of cancer cells.

Noxa mediates bortezomib induced apoptosis in both sensitive and intrinsically resistant mantle cell lymphoma cells and this effect is independent of constitutive activity of the AKT and NF-kappaB pathways

Bortezomib is more active against mantle cell lymphoma (MCL) than against most other lymphoma subtypes. Nevertheless, up to half of patients with MCL have bortezomib resistant disease. Factors contributing to intrinsic resistance to bortezomib have not been determined. Here we used a panel of eight bortezomib sensitive (median IC(50) 5.9 nM) and three relatively bortezomib resistant cell lines (median IC(50) 12.9 nM) to investigate differences in tumor biology that could determine sensitivity to bortezomib. Bortezomib effectively inhibited high baseline proteasome activity and induced a comparable degree of proteasome inhibition in both sensitive and resistant cells. At 10 nM, bortezomib induced the proapoptotic BH3-only protein Noxa in sensitive but not resistant cells. At higher concentrations of bortezomib, however, Noxa was also upregulated in resistant cells and this effect was sufficient to induce apoptosis. Silencing of Noxa with siRNA rescued these cells from apoptosis, arguing against a defect in Noxa regulation or function as the basis of bortezomib resistance. Bortezomib was equally effective against cells with high and low constitutive NF-kappaB signaling. Also, sensitive and resistant MCL cell lines showed comparable activation of the AKT pathway. We conclude that bortezomib can overcome classic mechanisms of resistance to apoptosis and that determinants of bortezomib sensitivity in MCL are due to differences in signaling or stress pathways upstream of Noxa.

Bisindolylmaleimide IX facilitates extrinsic and initiates intrinsic apoptosis in TNF-alpha-resistant human colon adenocarcinoma COLO 205 cells

Human COLO 205 colon adenocarcinoma cells are immune to extrinsic apoptosis induced by immunomodulatory cytokines. Among the antiapoptotic mechanisms responsible for the immune escape, the overexpression of the cFLIP protein seems to be critical. cFLIP appears to inhibit the TNF-alpha-induced death receptor signal. The application of the metabolic inhibitor bisindolylmaleimide IX (Bis-IX), known as a potent PKC repressor, sensitized COLO 205 cells to TNF-alpha-mediated apoptosis. The Western-blot analysis revealed that the susceptibility of human COLO 205 cells to apoptogenic stimuli resulted from time-dependent reduction in cFLIP(L) and TRADD protein levels. At the same time, the level of FADD protein was up-regulated. Additionally, the combined TNF-alpha and Bis-IX treatment caused cleavages of Bid and procaspase-9, as well as cytochrome c release. Thus, the evidence of this study indicates that Bis-IX facilitates the death receptor signal mediated by TNF-R1. Moreover, Bis-IX alone initiated intrinsic apoptosis, which could be abolished by Bcl-2 delivery. It heralds the involvement of mitochondria in caspase-8-independent intrinsic apoptosis. In turn, the treatment with bisindolylmaleimide III (Bis-III) did not assist TNF-alpha-dependent apoptosis.

A novel paradigm for rapid ABT-737-induced apoptosis involving outer mitochondrial membrane rupture in primary leukemia and lymphoma cells

Primary chronic lymphocytic leukemia (CLL) cells are exquisitely sensitive to ABT-737, a small molecule BCL2-antagonist, which induces many of the classical biochemical and ultrastructural features of apoptosis, including BAX/BAK oligomerization, cytochrome c release, caspase activation and chromatin condensation. Surprisingly, ABT-737 also induces mitochondrial inner membrane permeabilization (MIMP) resulting in mitochondrial matrix swelling and rupture of the outer mitochondrial membrane (OMM), so permitting the rapid efflux of cytochrome c from mitochondrial cristae and facilitating rapid caspase activation and apoptosis. BAX and BAK appear to be involved in the OMM discontinuities as they localize to the OMM break points. Notably, ABT-737 induced mitochondrial matrix swelling and OMM discontinuities in other primary B-cell malignancies, including mantle cell, follicular and marginal zone lymphoma cells but not in several cell lines studied. Thus, we describe a new paradigm of apoptosis in primary B-cell malignancies, whereby targeting of BCL2 results in all the classical features of apoptosis together with OMM rupture independent of caspase activation. This mechanism may be far more prevalent than hitherto recognized due to the failure of most methods, used to measure apoptosis, to recognize such a mechanism.

Downregulation of Mcl-1 potentiates HDACi-mediated apoptosis in leukemic cells

Mcl-1 is an antiapoptotic Bcl-2 family member, whose degradation is supposedly required for the induction of apoptosis. However, histone deacetylase inhibitors (HDACi) induce apoptosis primarily through the Bak/Mcl-1/Noxa and Bim pathways without decreasing Mcl-1. To investigate this discrepancy, we examined the role of Mcl-1 on HDACi-mediated apoptosis. Inhibition of either class I or class II HDAC by selective HDACi caused an upregulation of Mcl-1 mRNA and protein. Downregulation of Mcl-1 by three structurally unrelated cyclin-dependent kinase inhibitors potentiated HDACi-mediated apoptosis in primary chronic lymphocytic leukemic (CLL) cells and K562 cells. Sensitivity to HDACi-induced apoptosis was increased approximately 10-fold by the cyclin-dependent kinase inhibitors. Nanomolar concentrations of HDACi, approximately 300-fold lower than that required to induce apoptosis alone, sensitized cells to TRAIL, emphasizing that the mechanism(s) whereby HDACi induce apoptosis is clearly distinct from those by which they sensitize to TRAIL. Furthermore, knockdown of Mcl-1-potentiated HDACi-mediated apoptosis in K562 cells. Thus, HDACi-mediated Mcl-1 upregulation plays an important antiapoptotic regulatory role in limiting the efficacy of HDACi-induced apoptosis, which can be overcome by combination with an agent that downregulates Mcl-1. Thus, a clinical trial in some cancers is warranted using a combination of an HDACi with agents that downregulate Mcl-1.

The dual effects of TNFalpha on neutrophil apoptosis are mediated via differential effects on expression of Mcl-1 and Bfl-1

Neutrophils have a very short half-life in the circulation, undergoing rapid death by apoptosis, but a number of agents can either delay or accelerate the rate at which these cells undergo death. TNFalpha can exert opposing, concentration-dependent effects on neutrophils to either accelerate their apoptosis or enhance their survival. We show that TNFalpha greatly increases the rate of turnover of Mcl-1, an antiapoptotic protein that plays a key role in neutrophil survival. In contrast to Mcl-1 turnover in control- or granulocyte-macrophage colony-stimulating factor (GM-CSF)-treated neutrophils that occurs via the proteasome, TNFalpha-accelerated Mcl-1 turnover occurs via activation of caspases. Mcl-1-depleted cells thus have accelerated rates of apoptosis. While TNFalpha had no effect on MCL-1 transcription, it induced expression of another antiapoptotic molecule, BFL-1. Low concentrations of TNFalpha (<or=1 ng/mL) stimulated BFL-1 expression, whereas higher concentrations (>or=10 ng/mL) triggered caspase-dependent acceleration of Mcl-1 turnover. These opposing effects on 2 separate antiapoptotic systems of neutrophils explain the divergent effects of TNFalpha on neutrophil apoptosis and have important implications for understanding how TNFalpha may affect immune function in inflammatory diseases.

Hypoxic switch in mitochondrial myeloid cell leukemia factor-1/Mtd apoptotic rheostat contributes to human trophoblast cell death in preeclampsia

Preeclampsia, a disorder of pregnancy, is characterized by increased trophoblast cell death and altered trophoblast-mediated remodeling of myometrial spiral arteries resulting in reduced uteroplacental perfusion. Mitochondria-associated Bcl-2 family members are important regulators of programed cell death. The mechanism whereby hypoxia alters the mitochondrial apoptotic rheostat is essential to our understanding of placental disease. Herein, myeloid cell leukemia factor-1 (Mcl-1) isoform expression was examined in physiological/pathological models of placental hypoxia. Preeclamptic placentae were characterized by caspase-dependent cleavage of death-suppressing Mcl-1L and switch toward cell death-inducing Mcl-1S. In vitro, Mcl-1L cleavage was induced by hypoxia-reoxygenation in villous explants, whereas Mcl-1L overexpression under hypoxia-reoxygenation rescued trophoblast cells from undergoing apoptosis. Cleavage was mediated by caspase-3/-7 because pharmacological caspase inhibition prevented this process. Altitude-induced chronic hypoxia was characterized by expression of Mcl-1L; resulting in a reduction of apoptotic markers (cleaved caspase-3/-8 and p85 poly-ADP-ribose polymerase). Moreover, in both physiological (explants and high altitude) and pathological (preeclampsia) placental hypoxia, decreased trophoblast syncytin expression was observed. Hence, although both pathological and physiological placental hypoxia are associated with slowed trophoblast differentiation, trophoblast apoptosis is only up-regulated in preeclampsia, because of a hypoxia-reoxygenation-induced switch in generation of proapoptotic Mcl-1 isoforms.

Myeloid cell leukemia-1 as a therapeutic target

Apoptosis, or programmed cell death, is essential for normal development and homeostasis. Insufficient apoptosis may contribute to the pathogenesis of malignancy and acute and chronic inflammation. Apoptosis may be induced by the death receptor or the mitochondrial pathways. Myeloid cell leukemia (Mcl)-1 is a member of the Bcl-2 family that contributes to the control of mitochondrial integrity, which is critical for maintaining cell viability. Mcl-1 has been shown to be essential for the development and survival of a variety of cell types. This review characterizes the role of Mcl-1 in the regulation of apoptosis and the promotion of disease, and defines novel strategies that have been identified to target this molecule.

The BH3-mimetic GX15-070 synergizes with bortezomib in mantle cell lymphoma by enhancing Noxa-mediated activation of Bak

Mantle cell lymphoma (MCL) is an aggressive B-cell lymphoma resistant to conventional chemotherapy. The Bcl-2 pathway is deregulated in these tumors and may represent an interesting target for new therapeutic strategies. The new small-molecule pan-Bcl-2 inhibitor GX15-070 mimics BH3-only proteins by binding to multiple antiapoptotic Bcl-2 members. Here we show that GX15-070 induced apoptosis in vitro in MCL cell lines and primary cells from patients with MCL by releasing Bak from Mcl-1 and Bcl-X(L) at short incubation times and low micromolar doses. GX15-070 was effective in cells bearing defective DNA damage-sensor genes or cell-cycle regulators, inducing Bax and Bak conformational changes, mitochondrial depolarization, phosphatidylserine exposure, and caspase-3 activation. Furthermore, GX15-070 synergized with bortezomib, sensitizing MCL cells to low doses of this proteasome inhibitor, by neutralizing bortezomib-induced Mcl-1 accumulation and cooperating with Noxa to induce Bak displacement from this protein. These events led to an increased activation of the mitochondrial apoptotic pathway. Importantly, GX15-070 alone or in combination with bortezomib showed no significant cytotoxic effect in peripheral blood mononuclear cells from healthy donors. All these findings suggest that GX15-070 alone or in combination with bortezomib represents a new attractive therapeutic approach for MCL treatment.

Elucidating the role of protein kinase C in chronic lymphocytic leukaemia

While advances have been made in the clinical treatment of chronic lymphocytic leukaemia (CLL) in recent years, it is still an incurable disease and therefore the identification of novel drug therapies is of paramount importance. Understanding the molecular mechanisms that govern the survival of CLL cells is fundamental in achieving this goal. A number of studies indicate that protein kinase C (PKC)- and phosphatidylinositol-3-kinase (PI3K)- mediated signalling pathways are central to CLL cell survival, and as such PKC has gained renewed interest as a potential drug target in CLL. This may be because it represents a closely-related family of ten protein kinases, which due to the redundancy that exists between isoforms offers an opportunity for the design of isoform specific inhibitors drugs that target leukaemic cells whilst showing reduced toxicity for normal cells. Indeed, PKC signalling pathways have already been considered as targets for specific anticancer drugs [1-3]. Therefore, this short review will focus on the effect of modulating PKC activity in CLL cells and explore whether targeting PKCs could represent a valid therapy for this leukaemia.

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.

A proteolytic fragment of Mcl-1 exhibits nuclear localization and regulates cell growth by interaction with Cdk1

Mcl-1 (myeloid cell leukaemia-1) is a Bcl-2 family member with short-term pro-survival functions but whose other functions, demonstrated by embryonic lethality of knockout mice, do not involve apoptosis. In the present study, we show a cell-cycle-regulatory role of Mcl-1 involving a shortened form of the Mcl-1 polypeptide, primarily localized to the nucleus, which we call snMcl-1. snMcl-1 interacts with the cell-cycle-regulatory protein Cdk1 (cyclin-dependent kinase 1; also known as cdc2) in the nucleus, and Cdk1 bound to snMcl-1 was found to have a lower kinase activity. The interaction with Cdk1 occurs in the absence of its cyclin partners and is enhanced on treatment of cells with G2/M blocking agents, but not by G1/S blocking. The snMcl-1 polypeptide is present during S and G2 phases and is negligible in G1. Overexpression of human Mcl-1 in a murine myeloid progenitor cell line resulted in a lower rate of proliferation. Furthermore, Mcl-1-overexpressing cells had lower total Cdk1 kinase activity compared with parental cells, in both anti-Cdk1 and anti-cyclin B1 immunoprecipitates. The latter results suggest that binding to snMcl-1 alters the ability of Cdk1 to bind its conventional partner, cyclin B1. Given the important role of Cdk1 in progression through G2 and M phases, it is probable that the inhibition of Cdk1 activity accounts for the inhibitory effect of Mcl-1 on cell growth.

Sodium Salicylate Promotes Neutrophil Apoptosis by Stimulating Caspase-Dependent Turnover of Mcl-1

Mcl-1 is an antiapoptotic member of the Bcl-2 family of proteins that plays a central role in cell survival of neutrophils and other cells. The protein is unusual among family members in that it has a very short half-life of 2-3 h. In this report, we show that sodium salicylate (at 10 mM) greatly enhances the rate at which neutrophils undergo apoptosis and, in parallel, greatly accelerates the turnover rate of Mcl-1, decreasing its half-life to only 90 min. Whereas constitutive and GM-CSF-modified Mcl-1 turnover is regulated by the proteasome, the accelerated sodium salicylate-induced Mcl-1 turnover is mediated largely via caspases. Sodium salicylate resulted in rapid activation of caspase-3, -8, -9, and -10, and salicylate-accelerated Mcl-1 turnover was partly blocked by caspase inhibitors. Sodium salicylate also induced dramatic changes in the activities of members of the MAPK family implicated in Mcl-1 turnover and apoptosis. For example, sodium salicylate blocked GM-CSF-stimulated Erk and Akt activation, but resulted in rapid and sustained activation of p38-MAPK, an event mimicked by okadaic acid that also accelerates Mcl-1 turnover and neutrophil apoptosis. These data thus shed important new insights into the dynamic and highly regulated control of neutrophil apoptosis that is effected by modification in the rate of Mcl-1 turnover.

Melphalan-induced apoptosis in multiple myeloma cells is associated with a cleavage of Mcl-1 and Bim and a decrease in the Mcl-1/Bim complex

Multiple myeloma (MM) is a rapidly fatal plasma-cell malignancy that evolves mainly in the bone marrow. Melphalan is widely used to treat patients with MM but as yet its mechanisms of action are poorly documented. In the current study, we demonstrate that melphalan induces a drastic downregulation of Mcl-1L, Bcl-x(L) and BimEL in human melphalan-sensitive myeloma cells while the most potent proapoptotic isoforms, BimL and S, are affected to a lesser extent. Moreover, Mcl-1L and BimEL disappearance is associated with the generation of proapoptotic cleaved forms generated by a caspase cleavage. In myeloma cells, we have previously shown that Mcl-1 neutralizes the proapoptotic function of Bim and therefore, prevents the activation of death effectors. In this study, we demonstrate that melphalan disrupts the Mcl-1/Bim complex whereas the Bcl-2/Bim complex is not modified. The disappearance of full length Mcl-1 allows the release of Bim isoforms, particularly L and S, which can exert their proapoptotic function and leads to Bax activation and cytochrome c release. Thus, we can hypothesize that the cleaved 26 kDa proapoptotic Mcl-1 and the 19 and 12 kDa of Bim, generated during melphalan treatment could contribute to the amplification loop of apoptosis.

A caspase-resistant mutant of PKC-delta protects keratinocytes from UV-induced apoptosis

Keratinocyte apoptosis induced by UV radiation is a major protective mechanism from skin photocarcinogenesis. The induction of apoptosis by UV radiation, as well as a variety of genotoxic stimuli, involves the activation of PKC-delta by caspase-3-mediated cleavage in its hinge region, thus generating a constitutively active catalytic fragment. To determine the role of PKC-delta cleavage in UV apoptosis signaling, we introduced a caspase-resistant PKC-delta mutant (D330A) into human keratinocytes by retrovirus transduction. Overexpression of PKC-delta(D330A) protected keratinocytes from UV-induced apoptosis and enhanced long-term survival. PKC-delta(D330A) partially prevented the release of cytochrome c from the mitochondria and the loss of Mcl-1, a key antiapoptotic protein downregulated during UV apoptosis. Thus, the cleavage and activation of PKC-delta are critical components of UV-induced apoptosis in human keratinocytes, and the inactivation of PKC-delta can promote the survival of keratinocytes exposed to UV radiation.

Differential responses of Mcl-1 in photosensitized epithelial vs lymphoid-derived human cancer cells

The antiapoptotic Bcl-2-family proteins, Bcl-2 and Bcl-xL, are recognized phototargets of photodynamic therapy (PDT) with the mitochondrion-targeting phthalocyanine photosensitizer Pc 4. In the present study, we found that myeloid cell leukemia 1 (Mcl-1), another antiapoptotic member of the Bcl-2 family, was not photodamaged in Pc 4-PDT-treated human carcinoma cells MCF-7c3, MDA-MB468, DU145, and A431, although Mcl-1 turnover was observed after exposure of HeLa or MCF-7c3 cells to a supralethal dose of UVC. In contrast, when human lymphoma U937 and Jurkat cells were treated with Pc 4-PDT, staurosporine (STS) or UVC, Mcl-1 was cleaved to generate a 28-kDa fragment over a 2-4 h period. The cleavage of Mcl-1 was accompanied by the activation of caspases-3, -9, and -8. The broad-specificity caspase inhibitor z-VAD-fmk completely blocked Mcl-1 cleavage induced by PDT, STS or UVC, providing evidence for Mcl-1 as a substrate for caspases. Western blot analysis localized Mcl-1 to mitochondria, ER, and cytosol of both MCF-7c3 and U937 cells, suggesting that Mcl-1 protein, unlike Bcl-2 and Bcl-xL, is not a target for Pc 4-PDT, probably due to its localization to sites removed from those of Pc 4 binding. The 28-kDa cleaved fragment of Mcl-1, which has proapoptotic activity, was produced in PDT-treated lymphoid-derived cells, but not in cells of epithelial origin, suggesting that PDT-induced rapid and extensive apoptosis in lymphoma cells may result in part from the sensitivity of their Mcl-1 to caspase cleavage, removing an important negative control on apoptosis.

Histone deacetylase inhibitors potentiate TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in lymphoid malignancies

New therapies are required for chronic lymphocytic leukemia (CLL), an incurable disease characterized by failure of mature lymphocytes to undergo apoptosis. Activation of cell surface death receptors, such as via TRAIL receptor ligation, may provide a novel therapeutic target for various malignancies. However, CLL and other lymphoid malignancies are resistant to TRAIL. We report that low concentrations of histone deacetylase (HDAC) inhibitors, such as depsipeptide, which alone failed to induce apoptosis, markedly sensitize CLL cells and other primary lymphoid malignancies to TRAIL-induced apoptosis. These combinations caused little or no toxicity to normal lymphocytes. HDAC inhibitors sensitized resistant cells to TRAIL-induced apoptosis by facilitating formation of an active death-inducing signalling complex (DISC), leading to the rapid activation of caspase-8. The facilitated DISC formation also occurred in the absence of TRAIL-R2 upregulation. Thus, the combination of HDAC inhibitors and TRAIL may be valuable in the treatment of various hemopoietic malignancies.

Bodyguards and assassins: Bcl-2 family proteins and apoptosis control in chronic lymphocytic leukaemia

Chronic lymphocytic leukaemia (CLL) is the most common B-cell malignancy in the Western world and exists as subtypes with very different clinical courses. CLL is generally described as a disease of failed apoptosis. Apoptosis resistance may stem from a combination of microenvironmental survival signals as well as from intrinsic alterations in the apoptotic machinery within the CLL cell. The molecular mechanism involved in controlling apoptosis in CLL is complex and is influenced by many factors, including Bcl-2 family proteins, which are critical regulators of cell death. Here we review the significance of apoptosis dysregulation in CLL, focusing on the role of Bcl-2 and related Bcl-2 family proteins, such as Bax and Mcl-1. The differential properties of the newly described subsets of CLL are also highlighted.

Mcl-1

Mcl-1 is a Bcl-2 family protein which can act as an apical molecule in apoptosis control, promoting cell survival by interfering at an early stage in a cascade of events leading to release of cytochrome c from mitochondria. Mcl-1 has a short half life and is a highly regulated protein, induced by a wide range of survival signals and also rapidly down regulated during apoptosis. Mcl-1 can also readily be cleaved by caspases during apoptosis to produce a cell death promoting molecule. The multiple levels of control of Mcl-1 expression suggest that Mcl-1 plays a critical role in controlling life and death decisions in response to rapidly changing environmental cues and Mcl-1 is required for embryonic development and the function of the immune system. Expression of Mcl-1 may be useful in informing decision making in the treatment of various cancers, and countering Mcl-1 function may be an attractive therapeutic strategy in malignancy, inflammatory conditions and infectious disease where Mcl-1 may play a major role in suppressing apoptosis.

Cleavage of Mcl-1 by caspases impaired its ability to counteract Bim-induced apoptosis

Mcl-1 is an antiapoptotic member of the Bcl-2 family that can promote cell viability. We report here that Mcl-1 is a new substrate for caspases during induction of apoptosis. Mcl-1 cleavage occurs after Asp127 and Asp157 and generates four fragments of 24, 19, 17 and 12 kDa in both intact cells and in vitro, an effect prevented by selective caspase inhibitors. As a consequence, the resulting protein that lacks the first 127 or 157 amino acids contains only the BH1-BH3 domains of Bcl-2 family members. Mutation of Asp127 and Asp157 abolishes the generation of the 24 and 12 kDa fragments and that of the 19 and 17 kDa fragments, respectively. Interestingly, when expressed in HeLa cells Mcl-1 wt and Mcl-1 Delta127 showed a markedly different intracellular distribution. Mcl-1 wt colocalized with alpha-Tubulin near the internal face of the plasma membrane, while Mcl-1 Delta127 coassociated with Bim-EL at the mitochondrial level. Coimmunoprecipitation experiments also demonstrated that Mcl1 Delta127 exhibited increased binding to Bim when compared to Mcl-1 wt. Finally, Mcl-1 wt unlike Mcl-1 Delta127 inhibited Bim-EL-induced caspase activation. Altogether, our findings demonstrate that cleavage of Mcl-1 by caspases modifies its subcellular localization, increases its association with Bim and inhibits its antiapoptotic function.

Downregulation of the antiapoptotic MCL-1 protein and apoptosis in MA-11 breast cancer cells induced by an anti-epidermal growth factor receptor-Pseudomonas exotoxin a immunotoxin

Pseudomonas exotoxin (PE)-containing immunotoxins (ITs) act by arresting protein synthesis and promoting apoptosis, but the mechanisms of the induced apoptosis and the relationship to protein synthesis inhibition is not well elucidated. We studied these effects in MA-11 human breast cancer cells treated with 425.3PE, an unmodified PE covalently linked to the 425.3 antibody, which targets the EGF receptor. This IT induced efficient inhibition of protein synthesis with simultaneous induction of apoptosis. Thus, treatment of cells with 10 ng/ml of IT for 5 hr caused 85% inhibition of protein synthesis in parallel with caspase-3, -8 and -9 activation and PARP inactivation. Even after 72 hr of IT treatment, preincubation with the broad-spectrum caspase inhibitor z-VAD-FMK caused a significant increase in cell survival without affecting IT-induced protein synthesis inhibition. Interestingly, a combination of z-VAD-FMK and the cathepsin B/L inhibitor z-FA-FMK prevented completely IT-induced cell death in MA-11 cells after 24 hr, indicating that cathepsin activation may be important for optimal induction of IT-induced cell death. IT treatment caused after 2.5 hr a significant decrease in the level of the antiapoptotic protein Mcl-1 but not of Bcl-2 and Bcl-XL. Furthermore, Mcl-1 expression was not sensitive to caspase inhibitors but was totally prevented by the lactacystin proteasome inhibitor, suggesting that IT-induced apoptosis may be triggered by a reduction in the Mcl-1 level. Mitochondrial membrane potential (DeltaPsi mito) decreased concurrently with caspase activation, showing the involvement of DeltaPsi mito as a regulator of IT-induced apoptosis. Our results demonstrate that 425.3PE-mediated cell death involves simultaneous induction of apoptosis and protein synthesis inhibition in MA-11 cells, thus contributing to an understanding of the mechanisms involved in IT-induced apoptosis.

Investigation of extrinsic and intrinsic apoptosis pathways of new clerodane diterpenoids in human prostate cancer PC-3 cells

In our continuing search to discover bioactive compounds from natural products, we isolated six new clerodane diterpenes, caseamembrins A to F, from Casearia membranacea and examined their antiproliferative activities in human hormone-resistant prostate cancer PC-3 cells. All of these compounds displayed effective antiproliferative activity using sulforhodamine B assays and induced cell apoptosis by a terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling (TUNEL)-reaction technique. The data demonstrated that caseamembrin C was the most effective compound among these clerodane diterpenoids. Caseamembrin C induced down-regulation of Bcl-2 and Bcl-xL expression, while up-regulation of proapoptotic protein Mcl-1S (short chain), suggesting that these Bcl-2 family member proteins may play a role on arbitrating the apoptotic cell death. Caseamembrin C also induced the up-regulation of Fas ligand (FasL) expression, cleavage and activation of caspase-8 and caspase-9, Bid cleavage and activation of executor caspase-3. However, z-IETD-FMK (Z-Ile-Glu-Thr-Asp-fluoromethyl ketone, a selective caspase-8 inhibitor) almost completely inhibited caseamembrin C-induced Bid cleavage without any modification of caspase-9 activation, indicating that the extrinsic pathway of FasL/caspase-8/Bid cascade only played a minor role in the apoptotic signaling. Taken together, it is suggested that caseamembrin C-induced apoptosis is predominantly through the activation of intrinsic apoptosis pathways by causing the down-regulation of Bcl-2 and Bcl-xL expression, up-regulation of Mcl-1S protein and activation of caspase-9 and caspase-3.

Mcl-1 is required for Akata6 B-lymphoma cell survival and is converted to a cell death molecule by efficient caspase-mediated cleavage

Enforced expression of the antiapoptotic Bcl-2 family protein Mcl-1 promotes lymphomagenesis in the mouse; however, the functional role of Mcl-1 in human B-cell lymphoma remains unclear. We demonstrate that Mcl-1 is widely expressed in malignant B-cells, and high-level expression of Mcl-1 is required for B-lymphoma cell survival, since transfection of Mcl-1-specific antisense oligodeoxynucleotides was sufficient to promote apoptosis in Akata6 lymphoma cells. Mcl-1 was efficiently cleaved by caspases at evolutionarily conserved aspartic acid residues in vitro, and during cisplatin-induced apoptosis in B-lymphoma cell lines and spontaneous apoptosis of primary malignant B-cells. Overexpression of the Mcl-1 cleavage product that accumulated during apoptosis was sufficient to kill cells. Therefore, Mcl-1 is an essential survival molecule for B-lymphoma cells and is cleaved by caspases to a death-promoting molecule during apoptosis. In contrast to Mcl-1, Bcl-2 and Bcl-XL were relatively resistant to caspase cleavage in vitro and in intact cells. Interfering with Mcl-1 function appears to be an effective means of inducing apoptosis in Mcl-1-positive B-cell lymphoma, and the unique sensitivity of Mcl-1 to caspase-mediated cleavage suggests an attractive strategy for converting it to a proapoptotic molecule.