Differential targeting of prosurvival Bcl-2 proteins by their BH3-only ligands allows complementary apoptotic function

Modeling backbone flexibility to achieve sequence diversity: the design of novel alpha-helical ligands for Bcl-xL

Computational protein design can be used to select sequences that are compatible with a fixed-backbone template. This strategy has been used in numerous instances to engineer novel proteins. However, the fixed-backbone assumption severely restricts the sequence space that is accessible via design. For challenging problems, such as the design of functional proteins, this may not be acceptable. Here, we present a method for introducing backbone flexibility into protein design calculations and apply it to the design of diverse helical BH3 ligands that bind to the anti-apoptotic protein Bcl-xL, a member of the Bcl-2 protein family. We demonstrate how normal mode analysis can be used to sample different BH3 backbones, and show that this leads to a larger and more diverse set of low-energy solutions than can be achieved using a native high-resolution Bcl-xL complex crystal structure as a template. We tested several of the designed solutions experimentally and found that this approach worked well when normal mode calculations were used to deform a native BH3 helix structure, but less well when they were used to deform an idealized helix. A subsequent round of design and testing identified a likely source of the problem as inadequate sampling of the helix pitch. In all, we tested 17 designed BH3 peptide sequences, including several point mutants. Of these, eight bound well to Bcl-xL and four others showed weak but detectable binding. The successful designs showed a diversity of sequences that would have been difficult or impossible to achieve using only a fixed backbone. Thus, introducing backbone flexibility via normal mode analysis effectively broadened the set of sequences identified by computational design, and provided insight into positions important for binding Bcl-xL.

Proapoptotic BH3-only protein Bim is essential for developmentally programmed death of germinal center-derived memory B cells and antibody-forming cells

T cell-dependent B-cell immune responses induce germinal centers that are sites for expansion, diversification, and selection of antigen-specific B cells. During the immune response, antigen-specific B cells are removed in a process that favors the retention of cells with improved affinity for antigen, a cell death process inhibited by excess Bcl-2. In this study, we examined the role of the BH3-only protein Bim, an initiator of apoptosis in the Bcl-2-regulated pathway, in the programmed cell death accompanying an immune response. After immunization, Bim-deficient mice showed persistence of both memory B cells lacking affinity-enhancing mutations in their immunoglobulin genes and antibody-forming cells secreting low-affinity antibodies. This was accompanied by enhanced survival of both cell types in culture. We have identified for the first time the physiologic mechanisms for killing low-affinity antibody-expressing B cells in an immune response and have shown this to be dependent on the BH3-only protein Bim.

c-Cbl is not required for ERK1/2-dependent degradation of BimEL

Bim(EL) the most abundant Bim splice variant, is subject to ERK1/2-catalysed phosphorylation, which targets it for ubiquitination and proteasome-dependent destruction. In contrast, inactivation of ERK1/2, following withdrawal of survival factors, promotes stabilization of Bim(EL). It has been proposed that the RING finger protein Cbl binds to Bim(EL) and serves as its E3 ubiquitin ligase. However, this is controversial since most Cbl substrates are tyrosine phosphoproteins and yet Bim(EL) is targeted for destruction by ERK1/2-catalysed serine phosphorylation. Consequently, a role for Cbl could suggest a second pathway for Bim(EL) turnover, regulated by direct tyrosine phosphorylation, or could suggest that Bim(EL) is a coincidence detector, requiring phosphorylation by ERK1/2 and a tyrosine kinase. Here we show that degradation of Bim(EL) does not involve its tyrosine phosphorylation; indeed, Bim(EL) is not a tyrosine phosphoprotein. Furthermore, Bim(EL) fails to interact with Cbl and growth factor-stimulated, ERK1/2-dependent Bim(EL) turnover proceeds normally in Cbl-containing or Cbl-/- fibroblasts. These results indicate that Cbl is not required for ERK1/2-dependent Bim(EL) turnover in fibroblasts and epithelial cells and any role it has in other cell types is likely to be indirect.

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.

Bcl-w protects hippocampus during experimental status epilepticus

Experimentally evoked seizures can activate the intrinsic mitochondrial cell death pathway, components of which are modulated in the hippocampus of patients with temporal lobe epilepsy. Bcl-2 family proteins are critical regulators of mitochondrial dysfunction, but their significance in this setting remains primarily untested. Presently, we investigated the mitochondrial pathway and role of anti-apoptotic Bcl-2 proteins using a mouse model of seizure-induced neuronal death. Status epilepticus was evoked in mice by intra-amygdala kainic acid, causing cytochrome c release, processing of caspases 9 and 7, and death of ipsilateral hippocampal pyramidal neurons. Seizures caused a rapid decline in hippocampal Bcl-w levels not seen for either Bcl-2 or Bcl-xl. To test whether endogenous Bcl-w was functionally significant for neuronal survival, we investigated hippocampal injury after seizures in Bcl-w-deficient mice. Seizures induced significantly more hippocampal CA3 neuronal loss and DNA fragmentation in Bcl-w-deficient mice compared with wild-type mice. Quantitative electroencephalography analysis also revealed that Bcl-w-deficient mice display a neurophysiological phenotype whereby there was earlier polyspike seizure onset. Finally, we detected higher levels of Bcl-w in hippocampus from temporal lobe epilepsy patients compared with autopsy controls. These data identify Bcl-w as an endogenous neuroprotectant that may have seizure-suppressive functions.

Apoptosis induced by direct triggering of mitochondrial apoptosis proceeds in the near-absence of some apoptotic markers

Apoptotic cell death is characterized by the activation of the apoptotic signal transduction pathway on one hand and a number of regularly found morphological and biochemical features, such as nuclear condensation and mitochondrial depolarisation. Although much of our knowledge of apoptosis was obtained using noxious stimuli in cell culture, these apoptotic stimuli are likely to have numerous off-target effects that may contribute to or obscure the immediate effects of the apoptotic pathway. We have developed a cellular model where mitochondrial apoptosis is directly triggered by the tetracycline-regulated expression of the pro-apoptotic BH3-only protein Bim(S). We report the comparison of Bim(S)-induced apoptosis with the commonly used apoptotic stimuli staurosporine and UV-light. While the release of mitochondrial cytochrome c and Smac/DIABLO, activation of caspases and nuclear morphological changes occurred with very similar kinetics, striking differences were found in other apoptotic assays. In particular, drop in mitochondrial membrane potential, loss of plasma membrane integrity and the appearance of sub-G1 nuclei were strongly reduced in cells dying upon Bim(S)-induction, compared to staurosporine- or UV-induced apoptosis. The results thus indicate that the link between the apoptotic pathway and commonly used indicators of apoptosis is less tight than it appears from experiments with cytotoxic stimuli.

Tumor necrosis factor-related apoptosis-inducing ligand activates a lysosomal pathway of apoptosis that is regulated by Bcl-2 proteins

The present studies were performed to determine whether lysosomal permeabilization contributes to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) cytotoxicity and to reconcile a role for lysosomes with prior observations that Bcl-2 family members regulate TRAIL-induced apoptosis. In KMCH cholangiocarcinoma cells stably expressing Mcl-1 small interference RNA (siRNA), treatment with TRAIL induced a redistribution of the cathepsin B from lysosomes to the cytosol. Pharmacological and small hairpin RNA-targeted inhibition of cathepsin B attenuated TRAIL-mediated apoptosis as assessed by morphological, biochemical, and clonogenic assays. Neither Bid siRNA nor Bak siRNA prevented cathepsin B release. In contrast, treatment of the cells with Bim siRNA or the JNK inhibitor SP600125 attenuated lysosomal permeabilization and cell death. Moreover, Bim and active Bax co-localized to lysosomes in TRAIL-treated cells in a JNK-dependent manner, and Bax siRNA reduced TRAIL-induced lysosomal permeabilization and cell death. Finally, BH3 domain peptides permeabilized isolated lysosomes in the presence of Bax. Collectively, these data suggest that TRAIL can trigger an apoptotic pathway that involves JNK-dependent activation of Bim, which in turn induces Bax-mediated permeabilization of lysosomes.

In vitro analysis of Bcl-2 proteins in mitochondria and endoplasmic reticulum: similarities in anti-apoptotic functions and differences in regulation

Anti-apoptotic Bcl-2 localizes in the membranes of mitochondria and endoplasmic reticulum (ER) and resists a broad range of apoptotic stimuli. However, the precise function of Bcl-2 in ER is still unclear. We herein examined the anti-apoptotic potencies of Bcl-2 in mitochondria and ER in vitro. The mitochondria isolated from HeLa cells, which have little or practically no Bcl-2, were apoptosis-competent. That is, membrane-bound Bax was activated and cytochrome c was released when the isolated mitochondria were incubated at 35 degrees C. Cytochrome c release from the apoptosis-competent mitochondria was suppressed by co-incubation with the mitochondria with overexpressed Bcl-2 (Bcl-2 mitochondria), suggesting that Bcl-2 anchored in one mitochondrion can suppress cytochrome c release from another mitochondrion. Similar results were obtained when microsomes with overexpressed Bcl-2 (Bcl-2 microsomes) were co-incubated with apoptosis-competent mitochondria. A quantitative titration analysis showed that Bcl-2 in the ER suppresses cytochrome c release as efficiently as that in the mitochondria. An immunoprecipitation assay showed that Bcl-2 in both mitochondria and ER binds to Bax at almost the same degree. However, in the presence of tBid, co-incubation of apoptosis-competent mitochondria with Bcl-2 microsomes, but not with Bcl-2 mitochondria, diminished the Bax-binding to Bcl-2 significantly, suggesting that Bcl-2 in ER is readily inactivated by tBid. Co-incubation assay further confirmed that Bcl-2 in the ER, but not Bcl-2 in the mitochondria, is potentially inactivated by tBid. Our quantitative in vitro studies indicate that Bcl-2 in mitochondria and ER are similarly potent in inhibiting Bax-associated apoptosis of other mitochondria, but are regulated by tBid differently.

Apoptosis induced by histone deacetylase inhibitors in leukemic cells is mediated by Bim and Noxa

Several histone deacetylase inhibitors (HDACi), which have recently entered early clinical trials, exert their anticancer activity in part through the induction of apoptosis although the precise mechanism of this induction is not known. Induction of apoptosis by structurally diverse HDACi in primary cells from patients with chronic lymphocytic leukemia (CLL) and different leukemic cell lines was mediated by the Bcl-2 regulated intrinsic pathway and demonstrated a requirement for de novo protein synthesis. A marked time-dependent induction of the pro-apoptotic BH3-only proteins, Bim, Noxa and Bmf was observed, which preceded the induction of apoptosis. A key role for both Bim and Noxa was proposed in HDACi-mediated apoptosis based on our findings that siRNA for Bim and Noxa but not Bmf largely prevented the HDACi-induced loss in mitochondrial membrane potential, caspase processing and phosphatidylserine externalization. Noxa, induced by HDACi, in CLL cells and tumor cell lines, bound extensively to Mcl-1, a major anti-apoptotic Bcl-2 family member present in CLL cells. Our data strongly suggests that HDACi induce apoptosis primarily through inactivation of anti-apoptotic Bcl-2 family members by increases in Bim and Noxa and highlights these increases as a potential clinical target for CLL/lymphoma therapy.

BH3 profiling identifies three distinct classes of apoptotic blocks to predict response to ABT-737 and conventional chemotherapeutic agents

Cancer cells exhibit many abnormal phenotypes that induce apoptotic signaling via the intrinsic, or mitochondrial, pathway. That cancer cells nonetheless survive implies that they select for blocks in apoptosis. Identifying cancer-specific apoptotic blocks is necessary to rationally target them. Using a panel of 18 lymphoma cell lines, we show that a strategy we have developed, BH3 profiling, can identify apoptotic defects in cancer cells and separate them into three main classes based on position in the apoptotic pathway. BH3 profiling identifies cells that require BCL-2 for survival and predicts sensitivity to the BCL-2 antagonist ABT-737. BCL-2 dependence correlates with high levels of proapoptotic BIM sequestered by BCL-2. Strikingly, BH3 profiling can also predict sensitivity to conventional chemotherapeutic agents like etoposide, vincristine, and adriamycin.

Life, death, BH3 profiles, and the salmon mousse

New drugs that neutralize the antiapoptotic members of the Bcl-2 family hold promise for rational cancer therapies, both alone and in combination with other agents. An understanding of how and why such agents may trigger apoptosis on their own, and how resistance to these drugs can occur, depends on the complexity of the Bcl-2 family interactions that control mitochondrial outer membrane permeabilization (MOMP). By extracting mitochondria from tumor cells and exposing them to peptides corresponding to the regulatory BH3-only proteins, MOMP predicts not only which cells will undergo apoptosis in response to Bcl-2 antagonists, but also why other cells may be resistant.

Apoptosis induction in human melanoma cells by inhibition of MEK is caspase-independent and mediated by the Bcl-2 family members PUMA, Bim, and Mcl-1

PURPOSE

Given that inhibitors of mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase (MEK) are being introduced into treatment for melanoma, the present study was carried out to better understand the mechanism by which they may induce apoptosis of melanoma cells.

EXPERIMENTAL DESIGN

A panel of human melanoma cell lines and fresh melanoma isolates was assessed for their sensitivity to apoptosis induced by the MEK inhibitor U0126. The apoptotic pathways and regulatory mechanisms involved were examined by use of the inhibitor and small interfering RNA (siRNA) techniques.

RESULTS

Inhibition of MEK induced apoptosis in the majority of melanoma cell lines through a mitochondrial pathway that was associated with the activation of Bax and Bak, release of mitochondrial apoptogenic proteins, and activation of caspase-3. However, apoptosis was independent of caspases and instead was associated with mitochondrial release of AIF as shown by the inhibition of apoptosis when AIF was knocked down by siRNA. Inhibition of MEK resulted in the up-regulation of the BH3-only proteins PUMA and Bim and down-regulation of the antiapoptotic protein Mcl-1. These changes were critical for the induction of apoptosis by U0126 as siRNA knockdown of PUMA or Bim inhibited apoptosis, whereas siRNA knockdown of Mcl-1 increased apoptosis particularly in the apoptosis-resistant cell lines.

CONCLUSIONS

Apoptosis of melanoma cells induced by the inhibition of the MEK/ERK pathway is mediated by the up-regulation/activation of PUMA and Bim and down-regulation of Mcl-1. Release of AIF rather than the activation of caspases seems to be the mediator of apoptosis. Our results suggest that cotargeting Mcl-1 and the MEK/ERK pathway may further improve treatment results in melanoma.

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.

A pivotal role for Mcl-1 in Bortezomib-induced apoptosis

Bortezomib is a proteasome inhibitor for the treatment of relapsed/refractory multiple myeloma (MM). Mechanisms of resistance to Bortezomib are undefined. Myeloid cell leukemia-1 (Mcl-1) is an antiapoptotic protein, which protects tumor cells against spontaneous and chemotherapy-induced apoptosis. In MM, specific downregulation of Mcl-1 induces apoptosis. Here, we examined the role of Mcl-1 in Bortezomib- and doxorubicin-induced apoptosis. We demonstrate that Bortezomib, but not doxorubicin, triggers caspase-dependent generation of a 28 kDa Mcl-1-fragment, in several MM cell lines, including MM.1S cells. Conversely, transient transfection of MM.1S cells with a previously reported 28 kDa Mcl-1(128-350) fragment, but not with the Mcl-1(1-127) fragment, induces apoptosis. Therefore, both downregulation of full-length antiapoptotic Mcl-1, as well as Bortezomib-induced generation of Mcl-1(128-350) cleaved protein, contribute to MM cell apoptosis. To verify further these findings, we next compared effects triggered by Bortezomib, doxorubicin and melphalan in Mcl-1(wt/wt) and Mcl-1(Delta/null) murine embryonic fibroblasts (MEFs). Our results show that Bortezomib, but not doxorubicin or melphalan, triggers Mcl-1 cleavage in Mcl-1(wt/wt), but not Mcl-1(Delta/null) MEFs and induces sub-G(1) phase cells; caspase-3 and -9, and PARP cleavage as well as morphological signs of apoptosis. Taken together, these results support an important role of Mcl-1 and a Mcl-1 fragment in Bortezomib-induced cell death in general, and in MM in particular. To prevent relapse of MM in patients treated with Bortezomib, we therefore recommend the combination of Bortezomib with agents that induce MM cell death independent of Mcl-1.

DNA-damage response network at the crossroads of cell-cycle checkpoints, cellular senescence and apoptosis

Tissue homeostasis requires a carefully-orchestrated balance between cell proliferation, cellular senescence and cell death. Cells proliferate through a cell cycle that is tightly regulated by cyclin-dependent kinase activities. Cellular senescence is a safeguard program limiting the proliferative competence of cells in living organisms. Apoptosis eliminates unwanted cells by the coordinated activity of gene products that regulate and effect cell death. The intimate link between the cell cycle, cellular senescence, apoptosis regulation, cancer development and tumor responses to cancer treatment has become eminently apparent. Extensive research on tumor suppressor genes, oncogenes, the cell cycle and apoptosis regulatory genes has revealed how the DNA damage-sensing and -signaling pathways, referred to as the DNA-damage response network, are tied to cell proliferation, cell-cycle arrest, cellular senescence and apoptosis. DNA-damage responses are complex, involving "sensor" proteins that sense the damage, and transmit signals to "transducer" proteins, which, in turn, convey the signals to numerous "effector" proteins implicated in specific cellular pathways, including DNA repair mechanisms, cell-cycle checkpoints, cellular senescence and apoptosis. The Bcl-2 family of proteins stands among the most crucial regulators of apoptosis and performs vital functions in deciding whether a cell will live or die after cancer chemotherapy and irradiation. In addition, several studies have now revealed that members of the Bcl-2 family also interface with the cell cycle, DNA repair/recombination and cellular senescence, effects that are generally distinct from their function in apoptosis. In this review, we report progress in understanding the molecular networks that regulate cell-cycle checkpoints, cellular senescence and apoptosis after DNA damage, and discuss the influence of some Bcl-2 family members on cell-cycle checkpoint regulation.

The BH3-only protein Puma plays an essential role in cytokine deprivation induced apoptosis of mast cells

Mast cells play critical roles in the regulation of inflammation. One characteristic feature of mast cells is their relatively long lifespan in vivo. Members of the Bcl-2 protein family are regulators of cell survival and apoptosis, where the BH3-only proteins are critical proapoptotic proteins. In this study we investigated the role of the BH3-only proteins Noxa, Bad, Bim, Bmf, Bid, and Puma in apoptosis of mucosal-like mast cells (MLMCs) and connective tissue-like mast cells (CTLMCs). We demonstrate that Puma is critical for the induction of mast-cell death following cytokine deprivation and treatment with the DNA-damaging agent etoposide in MLMCs and CTLMCs. Using p53-/- mast cells, we found that cytokine deprivation-induced apoptosis, in contrast to that elicited by etoposide, is p53-independent. Interestingly, mast cells deficient in FOXO3a, previously proposed as a transcription factor for Puma induction in response to growth factor deprivation, were markedly resistant to cytokine withdrawal compared with wild-type cells. Moreover, overexpression of phosphorylation-deficient, constitutively active FOXO3a caused an up-regulation of Puma. In conclusion, our data demonstrate a pivotal role for Puma in the regulation of cytokine deprivation-induced mast-cell apoptosis and suggest a plausible role for Puma in the regulation of mast cell numbers in vivo.

E2F1 death pathways as targets for cancer therapy

Defects in apoptotic programs contribute to a number of human diseases, ranging from neurodegenerative disorders to malignancy, and treatment failure. The genetic basis for apoptosis implies that cell death can be disrupted by mutations, raising the intriguing possibility that cell numbers can be regulated by factors that influence cell survival. It is well documented that the E2F1 transcription factor is a key regulator of apoptotic programs. E2F1-induced cell death occurs via multiple pathways, some of which involve the tumour suppressor p53, and autonomous of p53. This has led to the opinion that E2F1 functions as a tumour surveillance factor, detecting aberrant proliferation and engaging apoptotic pathways to protect the organism from developing tumours. Frequently, novel players are discovered that expand the interpretation of apoptosis control by E2F1. This information will help to produce new strategies to exploit E2F1-induced apoptosis for therapeutic benefit.

Regulating B‐cell activation and survival in response to TLR signals

Following encounters with microbes, cellular activation programs that involve the control of proliferation and survival are initiated in follicular B cells either via the B-cell receptor in a specific antigen-defined manner, or through Toll-like receptors (TLRs) that recognize specific microbial products. This review summarizes and discusses recent findings that shed light on how the nuclear factor kappaB pathway controls and coordinates B-cell division and survival following TLR4 engagement.

Bcl-2-regulated apoptosis: mechanism and therapeutic potential

Apoptosis is essential for tissue homeostasis, particularly in the hematopoietic compartment, where its impairment can elicit neoplastic or autoimmune diseases. Whether stressed cells live or die is largely determined by interplay between opposing members of the Bcl-2 protein family. Bcl-2 and its closest homologs promote cell survival, but two other factions promote apoptosis. The BH3-only proteins sense and relay stress signals, but commitment to apoptosis requires Bax or Bak. The BH3-only proteins appear to activate Bax and Bak indirectly, by engaging and neutralizing their pro-survival relatives, which otherwise constrain Bax and Bak from permeabilizing mitochondria. The Bcl-2 family may also regulate autophagy and mitochondrial fission/fusion. Its pro-survival members are attractive therapeutic targets in cancer and perhaps autoimmunity and viral infections.

Interleukin 15-mediated survival of natural killer cells is determined by interactions among Bim, Noxa and Mcl-1

Interleukin 15 (IL-15) promotes the survival of natural killer (NK) cells by preventing apoptosis through mechanisms unknown at present. Here we identify Bim, Noxa and Mcl-1 as key regulators of IL-15-dependent survival of NK cells. IL-15 suppressed apoptosis by limiting Bim expression through the kinases Erk1 and Erk2 and mechanisms dependent on the transcription factor Foxo3a, while promoting expression of Mcl-1, which was necessary and sufficient for the survival of NK cells. Withdrawal of IL-15 led to upregulation of Bim and, accordingly, both Bim-deficient and Foxo3a-/- NK cells were resistant to cytokine deprivation. Finally, IL-15-mediated inactivation of Foxo3a and cell survival were dependent on phosphotidylinositol-3-OH kinase. Thus, IL-15 regulates the survival of NK cells at multiple steps, with Bim and Noxa being key antagonists of Mcl-1, the critical survivor factor in this process.

Molecular basis of Bcl-xL's target recognition versatility revealed by the structure of Bcl-xL in complex with the BH3 domain of Beclin-1

Beclin-1, originally identified as a Bcl-2 binding protein, is an evolutionarily conserved protein required for autophagy. The direct interaction between Beclin-1 and Bcl-2 or Bcl-xL provides a potential convergence point for apoptosis and autophagy, two programmed cell death processes. Given the functional significance of the interaction between Beclin-1 and Bcl-2/Bcl-xL, we performed detailed biochemical and structural characterizations of this interaction. We demonstrated that the Bcl-xL-binding domain of Beclin-1 contains a BH3 domain. Therefore, Beclin-1 is a new member of the BH3-only family proteins. The structure of Bcl-xL in complex with the Beclin-1 BH3 domain was determined at high resolution by NMR spectroscopy. Although similar to other known BH3 domains, the Beclin-1 BH3 domain displays its own distinct features in the complex with Bcl-xL. Systematic analysis of all known Bcl-xL/BH3 domain complexes helped us to identify the molecular basis underlying the capacity of Bcl-xL to recognize diverse target sequences.

BimS-induced apoptosis requires mitochondrial localization but not interaction with anti-apoptotic Bcl-2 proteins

Release of apoptogenic proteins such as cytochrome c from mitochondria is regulated by pro- and anti-apoptotic Bcl-2 family proteins, with pro-apoptotic BH3-only proteins activating Bax and Bak. Current models assume that apoptosis induction occurs via the binding and inactivation of anti-apoptotic Bcl-2 proteins by BH3-only proteins or by direct binding to Bax. Here, we analyze apoptosis induction by the BH3-only protein Bim(S). Regulated expression of Bim(S) in epithelial cells was followed by its rapid mitochondrial translocation and mitochondrial membrane insertion in the absence of detectable binding to anti-apoptotic Bcl-2 proteins. This caused mitochondrial recruitment and activation of Bax and apoptosis. Mutational analysis of Bim(S) showed that mitochondrial targeting, but not binding to Bcl-2 or Mcl-1, was required for apoptosis induction. In yeast, Bim(S) enhanced the killing activity of Bax in the absence of anti-apoptotic Bcl-2 proteins. Thus, cell death induction by a BH3-only protein can occur through a process that is independent of anti-apoptotic Bcl-2 proteins but requires mitochondrial targeting.

Preclinical studies of TW-37, a new nonpeptidic small-molecule inhibitor of Bcl-2, in diffuse large cell lymphoma xenograft model reveal drug action on both Bcl-2 and Mcl-1

PURPOSE

Overexpression of Bcl-2 protein has been observed in more than 80% of B-cell lymphomas, including diffuse large cell lymphoma (DLCL), the most common subtype of non-Hodgkin's lymphoma. We have previously employed the natural product (-)-gossypol to test its therapeutic potential as a small-molecule inhibitor of Bcl-2 for the treatment of B-cell lymphomas.

EXPERIMENTAL DESIGN

Recently, we have used a structure-based strategy to design a new class of potent small-molecule inhibitor acting on Bcl-2. One such lead compound is the benzenesulfonyl derivative TW-37, which was designed to target the BH3-binding groove in Bcl-2 where proapoptotic Bcl-2 proteins, such as Bak, Bax, Bid, and Bim bind.

RESULTS

In our fluorescence polarization-based binding assays using recombinant Bcl-2, Bcl-X(L), and Mcl-1 proteins, TW-37 binds to Bcl-2, Bcl-X(L), and Mcl-1 with K(i) values of 290, 1,110 and 260 nmol/L, respectively. Hence, TW-37 is a potent inhibitor of Bcl-2 and has >3-fold selectivity over Bcl-X(L). In vitro, TW-37 showed significant antiproliferative effect in a de novo chemoresistant WSU-DLCL(2) lymphoma cell line and primary cells obtained from a lymphoma patient with no effect on normal peripheral blood lymphocytes. Coimmunoprecipitation experiments showed that TW-37 disrupted heterodimer formation between Bax or truncated-Bid and antiapoptotic proteins in the order Mcl-1 > Bcl-2 >> Bcl-X(L). As expected, TW-37 caused apoptotic death. Pre-exposure of lymphoma cells to TW-37 significantly enhanced the killing effect of cyclophosphamide-doxorubicin-vincristine-prednisone (CHOP) regimen. The maximum tolerated dose of TW-37 in severe combined immunodeficient (SCID) mice was 40 mg/kg for three i.v. injections when given alone and 20 mg/kg, x3 when given in combination with CHOP. Using WSU-DLCL(2)-SCID mouse xenograft model, the addition of TW-37 to CHOP resulted in more complete tumor inhibition compared with either CHOP or TW-37 alone.

CONCLUSIONS

We conclude that the administration of TW-37, as a potent Bcl-2 and Mcl-1 inhibitor, to standard chemotherapy may prove an effective strategy in the treatment of B-cell lymphoma.

Hrk/DP5 contributes to the apoptosis of select neuronal populations but is dispensable for haematopoietic cell apoptosis

The pro-apoptotic BH3-only members of the Bcl2 family, crucial initiators of cell death, are activated by a diverse array of developmental cues or experimentally applied stress stimuli. We have investigated, through gene targeting in mice, the biological roles for the BH3-only family member HRK (also known as DP5) in apoptosis regulation. Hrk gene expression was found to be restricted to cells and tissues of the central and peripheral nervous systems. Sensory neurons from mice lacking Hrk were less sensitive to apoptosis induced by nerve growth factor (NGF) withdrawal, consistent with the induction of Hrk following NGF deprivation. By contrast, cerebellar granule neurons that upregulate Hrk upon transfer to low-K+ medium underwent apoptosis normally under these conditions in the absence of Hrk. Furthermore, loss of Hrk was not sufficient to rescue the neuronal degeneration in lurcher mutant mice. Despite previous reports, no evidence was found for Hrk expression or induction in growth-factor-dependent haematopoietic cell lines following withdrawal of their requisite cytokine, and haematopoietic progenitors lacking HRK died normally in response to cytokine deprivation. These results demonstrate that HRK contributes to apoptosis signalling elicited by trophic factor withdrawal in certain neuronal populations but is dispensable for apoptosis of haematopoietic cells.

BH3-only proteins trigger cytochrome c release, but how?

The mitochondrial apoptosis pathway has been neatly ordered. Mitochondrial apoptosis is governed by Bcl-2 family proteins, and their respective contributions determine the release of cytochrome c. It is clear that, among the Bcl-2 family, BH3-only proteins are the triggers: activation of BH3-only proteins by apoptotic stimuli initiates the process. BH3-only proteins cause cytochrome c release by activating Bax and/or Bak, and the anti-apoptotic group of Bcl-2-like proteins prevents this. However, it is curiously uncertain how BH3-only proteins activate Bax/Bak. Current models suggest that this is either through direct interaction--although this interaction is not detectable experimentally--or by the neutralisation of Bcl-2-like proteins. Here we discuss the context in which these models are placed and attempt to weigh the evidence.

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.

Therapeutic window for melanoma treatment provided by selective effects of the proteasome on Bcl-2 proteins

Melanoma cells depend on sustained proteasomal function for survival. However, bortezomib, the first proteasome inhibitor in clinical use, is not sufficient to improve the poor prognosis of metastatic melanoma patients. Since the proteasome is also expressed in all normal cell compartments, it is unclear how to enhance the efficacy of bortezomib without exacerbating secondary toxicities. Here, we present pharmacological and genetic analyses of mechanisms of resistance to proteasome inhibition. We focused on Bcl-2, Bcl-x(L) and Mcl-1 as main antiapoptotic factors associated with melanoma progression. Despite an efficient blockage of the proteasome, bortezomib could not counteract the intrinsically high levels of Bcl-2 and Bcl-x(L) in melanoma cells. Moreover, Mcl-1 was only downregulated at late time points after treatment. Based on these results, a combination treatment including (-)-gossypol, an inhibitor of Mcl-1/Bcl-2/Bcl-x(L), was designed and proven effective in vivo. Using a specific RNA interference approach, the survival of bortezomib-treated melanoma cells was found to rely primarily on Mcl-1, and to a lesser extent on Bcl-x(L) (but not on Bcl-2). Importantly, neither Mcl-1 nor Bcl-x(L) inactivation affected the viability of normal melanocytes. This hierarchical requirement of Bcl-2 family members for the maintenance of normal and malignant cells offers a therapeutic window to overcome melanoma chemoresistance in a tumor cell-selective manner.

Overexpression of Bcl-2 or Bcl-xL prevents spiral ganglion neuron death and inhibits neurite growth

Spiral ganglion neurons (SGNs) provide afferent innervation to the cochlea and rely on contact with hair cells (HCs) for their survival. Following deafferentation due to hair cell loss, SGNs gradually die. In a rat culture model, we explored the ability of prosurvival members of the Bcl-2 family of proteins to support the survival and neurite outgrowth of SGNs. We found that overexpression of either Bcl-2 or Bcl-xL significantly increases SGN survival in the absence of neurotrophic factors, establishing that the Bcl-2 pathway is sufficient for SGN cell survival and that SGN deprived of trophic support die by an apoptotic mechanism. However, in contrast to observations in central neurons and PC12 cells where Bcl-2 appears to promote neurite growth, both Bcl-2 and Bcl-xL overexpression dramatically inhibit neurite outgrowth in SGNs. This inhibition of neurite growth by Bcl-2 occurs in nearly all SGNs even in the presence of multiple neurotrophic factors implying that Bcl-2 directly inhibits neurite growth rather than simply rescuing a subpopulation of neurons incapable of extending neurites without additional stimuli. Thus, although overexpression of prosurvival members of the Bcl-2 family prevents SGN loss following trophic factor deprivation, the inhibition of neurite growth by these molecules may limit their efficacy for support of auditory nerve maintenance or regeneration following hair cell loss.

Exploration of Backbone Space in Foldamers Containing α- and β-Amino Acid Residues: Developing Protease-Resistant Oligomers that Bind Tightly to the BH3-Recognition Cleft of Bcl-xL

Protein-protein interactions play crucial roles in cell-signaling events and are often implicated in human disease. Molecules that bind tightly to functional protein-surface sites and show high stability to degradative enzymes could be valuable pharmacological tools for dissection of cell-signaling networks and might ultimately lead to therapeutic agents. We recently described oligomers containing both alpha- and beta-amino acid residues that bind tightly to the BH3 recognition site of the anti-apoptotic protein Bcl-x(L). The oligomers with highest affinity had a nine-residue N-terminal segment with a 1:1 alpha:beta residue repeat and a six-residue C-terminal segment containing exclusively proteinogenic alpha-residues. The N-terminal portions of such (alpha/beta+alpha)-peptides are highly resistant to proteolysis, but the C-terminal alpha-segments are susceptible. This study emerged from efforts to modify the alpha-segment in an (alpha/beta+alpha)-peptide in a way that would diminish proteolytic degradation but retain high affinity for Bcl-x(L). Some of the oligomers reported here could prove useful in certain biological applications, particularly those for which extended incubation in a biological milieu is required.

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.

The N-terminus and alpha-5, alpha-6 helices of the pro-apoptotic protein Bax, modulate functional interactions with the anti-apoptotic protein Bcl-xL

Background

Bcl-2 family proteins are key regulators of mitochondrial integrity and comprise both pro- and anti-apoptotic proteins. Bax a pro-apoptotic member localizes as monomers in the cytosol of healthy cells and accumulates as oligomers in mitochondria of apoptotic cells. The Bcl-2 homology-3 (BH3) domain regulates interactions within the family, but regions other than BH3 are also critical for Bax function. Thus, the N-terminus has been variously implicated in targeting to mitochondria, interactions with BH3-only proteins as well as conformational changes linked to Bax activation. The transmembrane (TM) domains (α5-α6 helices in the core and α9 helix in the C-terminus) in Bax are implicated in localization to mitochondria and triggering cytotoxicity. Here we have investigated N-terminus modulation of TM function in the context of regulation by the anti-apoptotic protein Bcl-x_L.

Results

Deletion of 29 amino acids in the Bax N-terminus (Bax 30–192) caused constitutive accumulation at mitochondria and triggered high levels of cytotoxicity, not inhibited by Bcl-x_L. Removal of the TM domains (Bax 30–105) abrogated mitochondrial localization but resulted in Bcl-x_L regulated activation of endogenous Bax and Bax-Bak dependent apoptosis. Inclusion of the α5-α6 helices/TMI domain (Bax 30–146) phenocopied Bax 30–192 as it restored mitochondrial localization, Bcl-x_L independent cytotoxicity and was not dependent on endogenous Bax-Bak. Inhibition of function and localization by Bcl-x_L was restored in Bax 1–146, which included the TM1 domain. Regardless of regulation by Bcl-x_L, all N-terminal deleted constructs immunoprecipitated Bcl-x_Land converged on caspase-9 dependent apoptosis consistent with mitochondrial involvement in the apoptotic cascade. Sub-optimal sequence alignments of Bax and Bcl-x_L indicated a sequence similarity between the α5–α6 helices of Bax and Bcl-x_L. Alanine substitutions of three residues (T14A-S15A-S16A) in the N-terminus (Bax-Ala3) attenuated regulation by the serine-threonine kinase Akt/PKB but not by Bcl-x_L indicative of distinct regulatory mechanisms.

Conclusion

Collectively, the analysis of Bax deletion constructs indicates that the N-terminus drives conformational changes facilitating inhibition of cytotoxicity by Bcl-x_L. We speculate that the TM1 helices may serve as 'structural antagonists' for BH3-Bcl-x_L interactions, with this function being regulated by the N-terminus in the intact protein.

Mitochondrial permeabilization relies on BH3 ligands engaging multiple prosurvival Bcl-2 relatives, not Bak

The Bcl-2 family regulates apoptosis by controlling mitochondrial integrity. To clarify whether its prosurvival members function by sequestering their Bcl-2 homology 3 (BH3)–only ligands or their multidomain relatives Bak and Bax, we analyzed whether four prosurvival proteins differing in their ability to bind specific BH3 peptides or Bak could protect isolated mitochondria. Most BH3 peptides could induce temperature-dependent cytochrome c release, but permeabilization was prevented by Bcl-x_l, Bcl-w, Mcl-1, or BHRF1. However, their protection correlated with the ability to bind Bak rather than the added BH3 peptide and could be overcome only by BH3 peptides that bind directly to the appropriate prosurvival member. Mitochondria protected by both Bcl-x_l–like and Mcl-1 proteins were disrupted only by BH3 peptides that engage both. BH3-only reagents freed Bak from Bcl-x_l and Mcl-1 in mitochondrial and cell lysates. The findings support a model for the control of apoptosis in which certain prosurvival proteins sequester Bak/Bax, and BH3-only proteins must neutralize all protective prosurvival proteins to allow Bak/Bax to induce mitochondrial disruption.

Two molecular pathways initiate mitochondria-dependent dopaminergic neurodegeneration in experimental Parkinson's disease

Dysfunction of mitochondrial complex I is associated with a wide spectrum of neurodegenerative disorders, including Parkinson's disease (PD). In rodents, inhibition of complex I leads to degeneration of dopaminergic neurons of the substantia nigra pars compacta (SNpc), as seen in PD, through activation of mitochondria-dependent apoptotic molecular pathways. In this scenario, complex I blockade increases the soluble pool of cytochrome c in the mitochondrial intermembrane space through oxidative mechanisms, whereas activation of pro-cell death protein Bax is actually necessary to trigger neuronal death by permeabilizing the outer mitochondrial membrane and releasing cytochrome c into the cytosol. Activation of Bax after complex I inhibition relies on its transcriptional induction and translocation to the mitochondria. How complex I deficiency leads to Bax activation is currently unknown. Using gene-targeted mice, we show that the tumor suppressor p53 mediates Bax transcriptional induction after PD-related complex I blockade in vivo, but it does not participate in Bax mitochondrial translocation in this model, either by a transcription-independent mechanism or through the induction of BH3-only proteins Puma or Noxa. Instead, Bax mitochondrial translocation in this model relies mainly on the JNK-dependent activation of the BH3-only protein Bim. Targeting either Bax transcriptional induction or Bax mitochondrial translocation results in a marked attenuation of SNpc dopaminergic cell death caused by complex I inhibition. These results provide further insight into the pathogenesis of PD neurodegeneration and identify molecular targets of potential therapeutic significance for this disabling neurological illness.

Serine 64 phosphorylation enhances the antiapoptotic function of Mcl-1

Mcl-1 is an antiapoptotic Bcl-2 family member that is highly regulated and when dysregulated contributes to cancer. The Mcl-1 protein is phosphorylated at multiple sites in response to different signaling events. Phosphorylations at Thr163 (by ERK) and Ser159 (by glycogen-synthase kinase 3beta) have recently been shown to slow and enhance, respectively, Mcl-1 protein turnover. Phosphorylation is also known to be stimulated at other, as-yet uncharacterized sites in the G2/M phase of the cell cycle. Using an S peptide-tagged Mcl-1 T163A mutant, Ser64 was identified as a novel Mcl-1 phosphorylation site by mass spectrometry. Immunoblotting demonstrated that phosphorylation at this site was maximal in cells in G2/M phase, was enhanced by tumor necrosis factor-alpha-related apoptosis-inducing ligand (TRAIL) treatment, was blocked by inhibitors of CDK (but not ERK or glycogen-synthase kinase 3beta), and was stimulated in vitro by CDK 1, CDK2, and JNK1. The half-life of a nonphosphorylatable S64A Mcl-1 mutant was indistinguishable from that of the wild type polypeptide. In contrast, this mutant failed to protect cells from TRAIL-mediated apoptosis, whereas reconstitution with the phosphomimetic S64E Mcl-1 mutant rendered cells TRAIL-resistant. This anti-apoptotic phenotype of the S64E Mcl-1 mutant was also associated with enhanced binding to the proapoptotic proteins Bim, Noxa, and Bak. A pharmacological CDK inhibitor that reduced Ser64 phosphorylation also sensitized cells to TRAIL cytotoxicity. Collectively, these observations not only identify G2/M-associated phosphorylation at Ser64 as a critical determinant of the antiapoptotic activity of Mcl-1 but also elucidate a novel mechanism by which CDK1/2 inhibitors can enhance the effectiveness of the cytotoxic cytokine TRAIL.

The promise of TRAIL—potential and risks of a novel anticancer therapy

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising new anticancer biotherapeutic. As shown by many preclinical studies, TRAIL efficiently induces apoptosis in numerous tumor cell lines but not in the majority of normal cells. However, an increasing number of publications report on a predominance of TRAIL resistance in primary human tumor cells, which require sensitization for TRAIL-induced apoptosis. Sensitization of cancer cells by treatment with chemotherapeutic drugs and irradiation has been shown to restore TRAIL sensitivity in many TRAIL-resistant tumor cells. Accordingly TRAIL treatment has been successfully used in different in vivo models for the treatment of tumors also in combination with chemotherapeutics without significant toxicity. However, some reports demonstrated toxicity of TRAIL alone or in combination with chemotherapeutic drugs in normal cells. This review summarizes data concerning the apoptosis-inducing pathways and efficacy of TRAIL, alone or in combination with chemotherapeutic drugs, in primary cancer cells compared to the unwanted effects of TRAIL treatment on normal tissue. We discuss the different in vitro tumor cell models and the potential of different recombinant forms of TRAIL or agonistic antibodies to TRAIL death receptors. Most preclinical studies show a high efficiency of a combinatorial TRAIL-based therapy in animal models and in primary human ex vivo tumor cells with a low toxicity in normal cells. Accordingly clinical phase I/II studies have begun and will be developed further with caution.

NBK/BIK antagonizes MCL-1 and BCL-XL and activates BAK-mediated apoptosis in response to protein synthesis inhibition

Ribonucleases, antibiotics, bacterial toxins, and viruses inhibit protein synthesis, which results in apoptosis in mammalian cells. How the BCL-2 family of proteins regulates apoptosis in response to the shutoff of protein synthesis is not known. Here we demonstrate that an Escherichia coli toxin, MazF, inhibited protein synthesis by cleavage of cellular mRNA and induced apoptosis in mammalian cells. MazF-induced apoptosis required proapoptotic BAK and its upstream regulator, the proapoptotic BH3-only protein NBK/BIK, but not BIM, PUMA, or NOXA. Interestingly, in response to MazF induction, NBK/BIK activated BAK by displacing it from anti-apoptotic proteins MCL-1 and BCL-X(L) that sequester BAK. Furthermore, NBK/BIK- or BAK-deficient cells were resistant to cell death induced by pharmacologic inhibition of translation and by virus-mediated shutoff of protein synthesis. Thus, the BH3-only protein NBK/BIK is the apical regulator of a BAK-dependent apoptotic pathway in response to shutoff of protein synthesis that functions to displace BAK from sequestration by MCL1 and BCL-X(L). Although NBK/BIK is dispensable for development, it is the BH3-only protein targeted for inactivation by viruses, suggesting that it plays a role in pathogen/toxin response through apoptosis activation.

mir-29 regulates Mcl-1 protein expression and apoptosis

Cellular expression of Mcl-1, an anti-apoptotic Bcl-2 family member, is tightly regulated. Recently, Bcl-2 expression was shown to be regulated by microRNAs, small endogenous RNA molecules that regulate protein expression through sequence-specific interaction with messenger RNA. By analogy, we reasoned that Mcl-1 expression may also be regulated by microRNAs. We chose human immortalized, but non-malignant, H69 cholangiocyte and malignant KMCH cholangiocarcinoma cell lines for these studies, because Mcl-1 is dysregulated in cells with the malignant phenotype. By in silico analysis, we identified a putative target site in the Mcl-1 mRNA for the mir-29 family, and found that mir-29b was highly expressed in cholangiocytes. Interestingly, mir-29b was downregulated in malignant cells, consistent with Mcl-1 protein upregulation. Enforced mir-29b expression reduced Mcl-1 protein expression in KMCH cells. This effect was direct, as mir-29b negatively regulated the expression of an Mcl-1 3' untranslated region (UTR)-based reporter construct. Enforced mir-29b expression reduced Mcl-1 cellular protein levels and sensitized the cancer cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) cytotoxicity. Transfection of non-malignant cells (that express high levels of mir-29) with a locked-nucleic acid antagonist of mir-29b increased Mcl-1 levels and reduced TRAIL-mediated apoptosis. Thus mir-29 is an endogenous regulator of Mcl-1 protein expression, and thereby, apoptosis.

Mcl-1, Bcl-XL and Stat3 expression are associated with progression of melanoma whereas Bcl-2, AP-2 and MITF levels decrease during progression of melanoma

Members of the Bcl-2 family of antiapoptotic proteins (Bcl-2, Bcl-XL and Mcl-1) are key regulators of apoptosis. The purpose of the present study was to examine and better define the role of Bcl-2, Bcl-XL and Mcl-1 in the progression of melanoma. Immunohistochemical staining for Bcl-2, Bcl-XL and Mcl-1 was performed on paraffin sections of 100 cases of benign nevi, primary melanoma and metastatic melanoma. Expression was correlated with histopathologic features, clinical progress and expression of transcription factors (AP-2, MITF and p-Stat3). Bcl-2 was expressed in 100% of benign nevi and thin melanoma (<or=1.0 mm) but was less in thick melanoma (>1.0 mm) (88%), subcutaneous (62%) and lymph node metastases (35%). In contrast, Bcl-XL and Mcl-1 were expressed at lower levels in nevi and thin melanoma compared to Bcl-2 but their expression was much higher in thick melanoma and in subcutaneous and lymph node metastases (P<0.0001). Bcl-2 expression was negatively associated with tumor thickness (P<0.05) but Bcl-XL expression increased with increasing tumor thickness (P<0.05) and dermal tumor mitotic rate (P<0.05). Similarly Mcl-1 expression increased with increasing tumor thickness (P<0.09) and dermal tumor mitotic rate (P<0.17). Bcl-2 expression was positively correlated with expression of the transcription factors microphthalmia transcription factor (MITF) and nuclear AP-2 whereas Bcl-XL (and Mcl-1) expression were positively correlated with p-Stat3. This study is the first to show a clear dissociation between changes in Bcl-2 expression (downregulation) and Bcl-XL, Mcl-1 expression (upregulation) during progression of melanoma. The results were also consistent with a role for AP-2 and MITF in regulation of Bcl-2 and pStat3 in regulation of Bcl-XL. These findings have important implications for the development of treatments targeting antiapoptotic proteins in patients with melanoma.

The BH3-Only Protein Bid Is Dispensable for DNA Damage- and Replicative Stress-Induced Apoptosis or Cell-Cycle Arrest

Bid, a caspase-activated proapoptotic BH3-only protein, is essential for Fas-induced hepatocyte destruction. Recent studies published in Cell produced conflicting results, indicating that loss of Bid either protects or enhances apoptosis induced by DNA damage or replicative stress. To resolve this controversy, we generated novel Bid-deficient mice on an inbred C57BL/6 background and removed the drug-selection cassette from the targeted locus. Nine distinct cell types from these Bid-deficient mice underwent cell-cycle arrest and apoptosis in a manner indistinguishable from control WT cells in response to DNA damage or replicative stress. Moreover, we found that even cells from the original Bid-deficient mice responded normally to these stimuli, indicating that differences in genetic background or the presence of a strong promoter within the targeted locus are unlikely to explain the differences between our results and those reported previously. We conclude that Bid has no role in DNA damage- or replicative stress-induced apoptosis or cell-cycle arrest.

Structural insights into the degradation of Mcl-1 induced by BH3 domains

Apoptosis is held in check by prosurvival proteins of the Bcl-2 family. The distantly related BH3-only proteins bind to and antagonize them, thereby promoting apoptosis. Whereas binding of the BH3-only protein Noxa to prosurvival Mcl-1 induces Mcl-1 degradation by the proteasome, binding of another BH3-only ligand, Bim, elevates Mcl-1 protein levels. We compared the three-dimensional structures of the complexes formed between BH3 peptides of both Bim and Noxa, and we show that a discrete C-terminal sequence of the Noxa BH3 is necessary to instigate Mcl-1 degradation.

The Bcl-2 apoptotic switch in cancer development and therapy

Impaired apoptosis is both critical in cancer development and a major barrier to effective treatment. In response to diverse intracellular damage signals, including those evoked by cancer therapy, the cell's decision to undergo apoptosis is determined by interactions between three factions of the Bcl-2 protein family. The damage signals are transduced by the diverse 'BH3-only' proteins, distinguished by the BH3 domain used to engage their pro-survival relatives: Bcl-2, Bcl-x(L), Bcl-w, Mcl-1 and A1. This interaction ablates pro-survival function and allows activation of Bax and Bak, which commit the cell to apoptosis by permeabilizing the outer membrane of the mitochondrion. Certain BH3-only proteins (e.g. Bim, Puma) can engage all the pro-survival proteins, but others (e.g. Bad, Noxa) engage only subsets. Activation of Bax and Bak appears to require that the BH3-only proteins engage the multiple pro-survival proteins guarding Bax and Bak, rather than binding to the latter. The balance between the pro-survival proteins and their BH3 ligands regulates tissue homeostasis, and either overexpression of a pro-survival family member or loss of a proapoptotic relative can be oncogenic. Better understanding of the Bcl-2 family is clarifying its role in cancer development, revealing how conventional therapy works and stimulating the search for "BH3 mimetics" as a novel class of anticancer drugs.

Therapeutic targets in the mitochondrial apoptotic pathway

Every cell in the human body has most of the components of the apoptotic apparatus and is thus principally equipped to die by apoptosis. Situations of increased or decreased apoptosis contribute to many forms of human disease, making this pathway an attractive target of therapeutic intervention. The past few years have seen an enormous refinement in the understanding how apoptosis works on a molecular level and the role of mitochondria as a central element in apoptotic signal transduction has become obvious. Here, the authors consider the events that are critical in this mitochondrial pathway, in particular at mitochondria but also upstream and downstream. The authors' opinion is presented on the merits and feasibility of approaches that aim at treating disease by interfering with the mitochondrial apoptotic pathway.

Functional linkage between NOXA and Bim in mitochondrial apoptotic events

NOXA is a BH3-only protein whose expression is induced by certain p53-depenent or independent apoptotic stimuli. Both NOXA and Bim are avid binders of Mcl-1, but a functional linkage between these BH3-only proteins has not yet been reported. In this study, we demonstrate that Mcl-1 binding of endogenously induced NOXA interferes with the ability of Mcl-1 to efficiently sequester endogenous Bim, as Bim is displaced from its complex with Mcl-1. Induced NOXA significantly enhances the UV sensitivity of cells, and the ensuing mitochondrial depolarization is entirely abrogated by Bim knockdown. These results demonstrate a Mcl-1-mediated cross-talk between endogenous NOXA and Bim that occurs upstream of the Bak/Bax-dependent execution of UV-induced mitochondrial depolarization. The current findings demonstrate that the mitochondrial response to an induced expression of NOXA is executed by endogenous Bim and suggest a plausible mechanism for the observed NOXA-Bim linkage.

FcεRI Aggregation Promotes Survival of Connective Tissue-Like Mast Cells but Not Mucosal-Like Mast Cells

Mast cells play a critical role in IgE-dependent immediate hypersensitivity reactions. This is facilitated by their capacity to release inflammatory mediators and to undergo activation-induced survival upon cross-linking of the high-affinity IgE-receptor (FcepsilonRI). Due to their heterogeneity, mast cells can be divided into two major groups: the connective tissue mast cells and the mucosal mast cells. We have previously shown that IL-3-dependent bone marrow-derived mast cells can undergo activation-induced survival that is dependent on the prosurvival gene A1. In this study, we have used two different protocols to develop murine connective tissue-like mast cells (CTLMC) and mucosal-like mast cells (MLMC) to investigate their capacity to survive an allergic reaction in vitro. In this study, we demonstrate that FcepsilonRI stimulation promotes survival of CTLMC but not MLMC. Similarly, a prominent induction of A1 is observed only in CTLMC but not MLMC. MLMC have a higher basal level of the proapoptotic protein Bim compared with CTLMC. These findings demonstrate a difference among mast cell populations in their ability to undergo activation-induced survival after FcepsilonRI stimulation, which might explain the slower turnover of CTMC in IgE-dependent reactions.

Bim plays a crucial role in synergistic induction of apoptosis by the histone deacetylase inhibitor SBHA and TRAIL in melanoma cells

The wide variation in sensitivity of cancer cells to TRAIL- or histone deacetylase (HDAC) inhibitor - induced apoptosis precludes successful treatment of cancer with these agents. We report here that TRAIL and SBHA synergistically induce apoptosis of melanoma cells as revealed by quantitative analysis using the normalized isobologram method. This is supported by enhanced activation of caspase-3 and cleavage of its substrates, PARP and ICAD. Co-treatment with SBHA and TRAIL did not enhance formation of the death-inducing signaling complex (DISC) and processing of caspase-8 and Bid, but potentiated activation of Bax and release of Cytochrome C and Smac/DIABLO from mitochondria into the cytosol. SBHA down-regulated Bcl-X(L), Mcl-1 and XIAP, but up-regulated Bax, Bak, and the BH3-only protein Bim(EL). Up-regulation of the latter by SBHA was attenuated by the presence of TRAIL, which was inhibitable by the pan-caspase inhibitor z-VAD-fmk. Inhibition of Bim by siRNA attenuated conformational changes of Bax, mitochondrial apoptotic events, and activation of caspase-3, leading to marked inhibition of the synergy between SBHA and TRAIL. Thus, Bim plays an essential role in synergistic induction of apoptosis by SBHA and TRAIL in melanoma.

A Critical role for Bim in retinal ganglion cell death

Optic nerve transection results in the death of retinal ganglion cells (RGCs) by apoptosis. Apoptosis is regulated by the Bcl-2 family of proteins, of which the Bcl-2 homology (BH3) -only proteins forms a subset. As BH3-only proteins have been shown to play a significant role in regulating cell death in the central nervous system, we wished to investigate the role of Bcl-2 interacting mediator of cell death (Bim), a prominent member of this protein family in the regulation of cell death in the RGC layer using in vitro retinal explants. In this study, we use an innovative retinal shaving procedure to isolate the cells of the ganglion cell layer to use for western blotting. Members of the BH3-only protein family are down-regulated during retinal development and are not normally expressed in the adult retina. Using this procedure, we demonstrate that Bim is re-expressed and its expression is increased over time following axotomy. Expression of Bad and Bik decreases over the same time course, whereas there is no indication that Bid and Puma are re-expressed. We show that explants from Bim knockout mice are resistant to axotomy-induced death when compared with their wild-type counterparts. Genetic deletion of Bim also prevents caspase 3 cleavage. The activity of Bim can be negatively regulated by phosphorylation. We show that the decrease of Bim phosphorylation correlates with a decrease in expression of survival kinases such as pAkt and pERK over the same time course. These results implicate Bim re-expression as being essential for axotomy-induced death of RGCs and that phosphorylation of Bim negatively regulates its activity in RGCs.