The BH3 domain of Bcl-x(S) is required for inhibition of the antiapoptotic function of Bcl-x(L)

Aberrant Bcl-x splicing in cancer: from molecular mechanism to therapeutic modulation

Bcl-x pre-mRNA splicing serves as a typical example to study the impact of alternative splicing in the modulation of cell death. Dysregulation of Bcl-x apoptotic isoforms caused by precarious equilibrium splicing is implicated in genesis and development of multiple human diseases, especially cancers. Exploring the mechanism of Bcl-x splicing and regulation has provided insight into the development of drugs that could contribute to sensitivity of cancer cells to death. On this basis, we review the multiple splicing patterns and structural characteristics of Bcl-x. Additionally, we outline the cis-regulatory elements, trans-acting factors as well as epigenetic modifications involved in the splicing regulation of Bcl-x. Furthermore, this review highlights aberrant splicing of Bcl-x involved in apoptosis evade, autophagy, metastasis, and therapy resistance of various cancer cells. Last, emphasis is given to the clinical role of targeting Bcl-x splicing correction in human cancer based on the splice-switching oligonucleotides, small molecular modulators and BH3 mimetics. Thus, it is highlighting significance of aberrant splicing isoforms of Bcl-x as targets for cancer therapy.

BCL-XL expression is essential for human erythropoiesis and engraftment of hematopoietic stem cells

The anti-apoptotic BCL-2 proteins (BCL-2, BCL-XL, MCL-1, A1, BCL-W) counteract apoptotic signals emerging during development and under stress conditions, and are thus essential for the survival of every cell. While the “BCL-2 addiction” of different cell types is well described in mouse models, there is only limited information available on the role of different anti-apoptotic BCL-2 proteins in a given human cell type. Here we characterize the role of BCL-XL for survival and function of human hematopoietic cells, with the aim to predict hematological side effects of novel BCL-XL-inhibiting BH3-mimetics and to identify hematological malignancies potentially responsive to such inhibitors. Earlier clinical studies have shown that the combined BCL-2/BCL-XL/BCL-W inhibitor, Navitoclax (ABT-263) induces severe thrombocytopenia caused by direct platelet demise and counteracted by increased megakaryopoiesis. In contrast, murine studies have reported important contribution of BCL-XL to survival of late erythroid cells and megakaryocytes. Using lentiviral knockdown, we show that the roles of BCL-XL for human hematopoietic cells are much more pronounced than expected from murine data and clinical trials. Efficient genetic or chemical BCL-XL inhibition resulted in significant loss of human erythroid cells beginning from very early stages of erythropoiesis, and in a reduction of megakaryocytes. Most importantly, BCL-XL deficient human hematopoietic stem cells and multipotent progenitors were reduced in numbers, and they showed a severely impaired capacity to engraft in mice during xenotransplantation. BCL-XL deficiency was fully compensated by BCL-2 overexpression, however, loss of its antagonist BIM did not result in any rescue of human erythroid or stem and progenitor cells. We thus conclude that novel and specific BCL-XL inhibitors might be efficient to treat malignancies of erythroid or megakaryocytic origin, such as polycythemia vera, acute erythroid leukemia, essential thrombocytosis or acute megakaryocytic leukemia. At the same time, it can be expected that they will have more severe hematological side effects than Navitoclax.

The Structural Biology of Bcl-xL

Interactions between the pro-survival and pro-apoptotic members of the Bcl-2 family of proteins dictate whether a cell lives or dies. Much of our knowledge of the molecular details of these interactions has come from biochemical and structural studies on the pro-survival protein Bcl-x_L. The first high-resolution structure of any Bcl-2 family member was of Bcl-x_L, which revealed the conserved topology amongst all family members. Subsequent structures of Bcl-x_L complexes with pro-apoptotic ligands demonstrated the general features of all pro-survival:pro-apoptotic complexes. Structural studies involving Bcl-x_L were also the basis for the discovery of the first small-molecule pro-survival protein inhibitors, leading ultimately to the development of a new class of drugs now successfully used for cancer treatment in the clinic. This article will review our current knowledge of the structural biology of Bcl-x_L and how this has impacted our understanding of the molecular details of the intrinsic apoptotic pathway.

BCL-2 family isoforms in apoptosis and cancer

The BCl-2 family has long been identified for its role in apoptosis. Following the initial discovery of BCL-2 in the context of B-cell lymphoma in the 1980s, a number of homologous proteins have since been identified. The members of the Bcl-2 family are designated as such due to their BCL-2 homology (BH) domains and involvement in apoptosis regulation. The BH domains facilitate the family members’ interactions with each other and can indicate pro- or anti-apoptotic function. Traditionally, these proteins are categorised into one of the three subfamilies; anti-apoptotic, BH3-only (pro-apoptotic), and pore-forming or ‘executioner’ (pro-apoptotic) proteins. Each of the BH3-only or anti-apoptotic proteins has a distinct pattern of activation, localisation and response to cell death or survival stimuli. All of these can vary across cell or stress types, or developmental stage, and this can cause the delineation of the roles of BCL-2 family members. Added to this complexity is the presence of relatively uncharacterised isoforms of many of the BCL-2 family members. There is a gap in our knowledge regarding the function of BCL-2 family isoforms. BH domain status is not always predictive or indicative of protein function, and several other important sequences, which can contribute to apoptotic activity have been identified. While therapeutic strategies targeting the BCL-2 family are constantly under development, it is imperative that we understand the molecules, which we are attempting to target. This review, discusses our current knowledge of anti-apoptotic BCL-2 family isoforms. With significant improvements in the potential for splicing therapies, it is important that we begin to understand the distinctions of the BCL-2 family, not limited to just the mechanisms of apoptosis control, but in their roles outside of apoptosis.

Deubiquitinase OTUD6B Isoforms Are Important Regulators of Growth and Proliferation

Deubiquitinases (DUB) are increasingly linked to the regulation of fundamental processes in normal and cancer cells, including DNA replication and repair, programmed cell death, and oncogenes and tumor suppressor signaling. Here, evidence is presented that the deubiquitinase OTUD6B regulates protein synthesis in non-small cell lung cancer (NSCLC) cells, operating downstream from mTORC1. OTUD6B associates with the protein synthesis initiation complex and modifies components of the 48S preinitiation complex. The two main OTUD6B splicing isoforms seem to regulate protein synthesis in opposing fashions: the long OTUD6B-1 isoform is inhibitory, while the short OTUD6B-2 isoform stimulates protein synthesis. These properties affect NSCLC cell proliferation, because OTUD6B-1 represses DNA synthesis while OTUD6B-2 promotes it. Mutational analysis and downstream mediators suggest that the two OTUD6B isoforms modify different cellular targets. OTUD6B-2 influences the expression of cyclin D1 by promoting its translation while regulating (directly or indirectly) c-Myc protein stability. This phenomenon appears to have clinical relevance as NSCLC cells and human tumor specimens have a reduced OTUD6B-1/OTUD6B-2 mRNA ratio compared with normal samples. The global OTUD6B expression level does not change significantly between nonneoplastic and malignant tissues, suggesting that modifications of splicing factors during the process of transformation are responsible for this isoform switch.

IMPLICATIONS

Because protein synthesis inhibition is a viable treatment strategy for NSCLC, these data indicate that OTUD6B isoform 2, being specifically linked to NSCLC growth, represents an attractive, novel therapeutic target and potential biomarker for early diagnosis of malignant NSCLC. Mol Cancer Res; 15(2); 117-27. ©2016 AACR.

Hydrogen peroxide-induced apoptosis in human gingival fibroblasts

In the process of bleaching vital, discolored teeth, low concentrations of hydrogen peroxide (H2O2) are effective alternatives to heat-activated 30% H2O2. However, interest has been expressed in the assessment of pathological effects of long-term exposure to bleaching agents such as irritation and ulceration of the gingival or other soft tissues. The aim of the present study was to determine the effect of hydrogen peroxide on apoptosis in human gingival fibroblasts (HGF). Cytochrome c, Bcl-2, Bax, Bid and caspase-3 protein expression were detected by Western blotting. HGF cell apoptosis induced by H2O2 was both dose and time dependent. The addition of H2O2 resulted in the release of cytochrome c to the cytosol, and an increase of Caspase-3 cleavage. Data suggest that oxidative stress-induced apoptosis in HGF is intrinsic pathway involved the release of apoptotic signal from mitochondria.

Bcl-xL is necessary for neurite outgrowth in hippocampal neurons

AIMS

B-cell lymphoma-extra large (Bcl-xL) protects survival in dividing cells and developing neurons, but was not known to regulate growth. Growth and synapse formation are indispensable for neuronal survival in development, inextricably linking these processes. We have previously shown that, during synaptic plasticity, Bcl-xL produces changes in synapse number, size, activity, and mitochondrial metabolism. In this study, we determine whether Bcl-xL is required for healthy neurite outgrowth and whether neurite outgrowth is necessary for survival in developing neurons in the presence or absence of stress.

RESULTS

Depletion of endogenous Bcl-xL impairs neurite outgrowth in hippocampal neurons followed by delayed cell death which is dependent on upregulation of death receptor 6 (DR6), a molecule that regulates axonal pruning. Under hypoxic conditions, Bcl-xL-depleted neurons demonstrate increased vulnerability to neuronal process loss and to death compared with hypoxic controls. Endogenous DR6 expression and upregulation during hypoxia are associated with worsened neurite damage; depletion of DR6 partially rescues neuronal process loss, placing DR6 downstream of the effects of Bcl-xL on neuronal process outgrowth and protection. In vivo ischemia produces early increases in DR6, suggesting a role for DR6 in brain injury.

INNOVATION

We suggest that DR6 levels are usually suppressed by Bcl-xL; Bcl-xL depletion leads to upregulation of DR6, failure of neuronal outgrowth in nonstressed cells, and exacerbation of hypoxia-induced neuronal injury.

CONCLUSION

Bcl-xL regulates neuronal outgrowth during development and protects neurites from hypoxic insult, as opposed by DR6. Factors that enhance neurite formation may protect neurons against hypoxic injury or neurodegenerative stimuli.

The functional differences between pro-survival and pro-apoptotic B cell lymphoma 2 (Bcl-2) proteins depend on structural differences in their Bcl-2 homology 3 (BH3) domains

Bcl-2 homology 3 (BH3) domains are short sequence motifs that mediate nearly all protein-protein interactions between B cell lymphoma 2 (Bcl-2) family proteins in the intrinsic apoptotic cell death pathway. These sequences are found on both pro-survival and pro-apoptotic members, although their primary function is believed to be associated with induction of cell death. Here, we identify critical features of the BH3 domains of pro-survival proteins that distinguish them functionally from their pro-apoptotic counterparts. Biochemical and x-ray crystallographic studies demonstrate that these differences reduce the capacity of most pro-survival proteins to form high affinity "BH3-in-groove" complexes that are critical for cell death induction. Switching these residues for the corresponding residues in Bcl-2 homologous antagonist/killer (Bak) increases the binding affinity of isolated BH3 domains for pro-survival proteins; however, their exchange in the context of the parental protein causes rapid proteasomal degradation due to protein destabilization. This is supported by further x-ray crystallographic studies that capture elements of this destabilization in one pro-survival protein, Bcl-w. In pro-apoptotic Bak, we demonstrate that the corresponding distinguishing residues are important for its cell-killing capacity and antagonism by pro-survival proteins.

Gelsius: a literature-based workflow for determining quantitative associations between genes and biological processes

An effective knowledge extraction and quantification methodology from biomedical literature would allow the researcher to organize and analyze the results of high-throughput experiments on microarrays and next-generation sequencing technologies. Despite the large amount of raw information available on the web, a tool able to extract a measure of the correlation between a list of genes and biological processes is not yet available. In this paper, we present Gelsius, a workflow that incorporates biomedical literature to quantify the correlation between genes and terms describing biological processes. To achieve this target, we build different modules focusing on query expansion and document cononicalization. In this way, we reached to improve the measurement of correlation, performed using a latent semantic analysis approach. To the best of our knowledge, this is the first complete tool able to extract a measure of genes-biological processes correlation from literature. We demonstrate the effectiveness of the proposed workflow on six biological processes and a set of genes, by showing that correlation results for known relationships are in accordance with definitions of gene functions provided by NCI Thesaurus. On the other side, the tool is able to propose new candidate relationships for later experimental validation. The tool is available at >http://bioeda1.polito.it:8080/medSearchServlet/.

Cancer-Associated Perturbations in Alternative Pre-messenger RNA Splicing

For most of our 25,000 genes, the removal of introns by pre-messenger RNA (pre-mRNA) splicing represents an essential step toward the production of functional messenger RNAs (mRNAs). Alternative splicing of a single pre-mRNA results in the production of different mRNAs. Although complex organisms use alternative splicing to expand protein function and phenotypic diversity, patterns of alternative splicing are often altered in cancer cells. Alternative splicing contributes to tumorigenesis by producing splice isoforms that can stimulate cell proliferation and cell migration or induce resistance to apoptosis and anticancer agents. Cancer-specific changes in splicing profiles can occur through mutations that are affecting splice sites and splicing control elements, and also by alterations in the expression of proteins that control splicing decisions. Recent progress in global approaches that interrogate splicing diversity should help to obtain specific splicing signatures for cancer types. The development of innovative approaches for annotating and reprogramming splicing events will more fully establish the essential contribution of alternative splicing to the biology of cancer and will hopefully provide novel targets and anticancer strategies. Metazoan genes are usually made up of several exons interrupted by introns. The introns are removed from the pre-mRNA by RNA splicing. In conjunction with other maturation steps, such as capping and polyadenylation, the spliced mRNA is then transported to the cytoplasm to be translated into a functional protein. The basic mechanism of splicing requires accurate recognition of each extremity of each intron by the spliceosome. Introns are identified by the binding of U1 snRNP to the 5' splice site and the U2AF65/U2AF35 complex to the 3' splice site. Following these interactions, other proteins and snRNPs are recruited to generate the complete spliceosomal complex needed to excise the intron. While many introns are constitutively removed by the spliceosome, other splice junctions are not used systematically, generating the phenomenon of alternative splicing. Alternative splicing is therefore the process by which a single species of pre-mRNA can be matured to produce different mRNA molecules (Fig. 1). Depending on the number and types of alternative splicing events, a pre-mRNA can generate from two to several thousands different mRNAs leading to the production of a corresponding number of proteins. It is now believed that the expression of at least 70 % of human genes is subjected to alternative splicing, implying an enormous contribution to proteomic diversity, and by extension, to the development and the evolution of complex animals. Defects in splicing have been associated with human diseases (Caceres and Kornblihtt, Trends Genet 18(4):186-93, 2002, Cartegni et al., Nat Rev Genet 3(4):285-98, 2002, Pagani and Baralle, Nat Rev Genet 5(5):389-96, 2004), including cancer (Brinkman, Clin Biochem 37(7):584-94, 2004, Venables, Bioessays 28(4):378-86, 2006, Srebrow and Kornblihtt, J Cell Sci 119(Pt 13):2635-2641, 2006, Revil et al., Bull Cancer 93(9):909-919, 2006, Venables, Transworld Res Network, 2006, Pajares et al., Lancet Oncol 8(4):349-57, 2007, Skotheim and Nees, Int J Biochem Cell Biol 39:1432-1449, 2007). Numerous studies have now confirmed the existence of specific differences in the alternative splicing profiles between normal and cancer tissues. Although there are a few cases where specific mutations are the primary cause for these changes, global alterations in alternative splicing in cancer cells may be primarily derived from changes in the expression of RNA-binding proteins that control splice site selection. Overall, these cancer-specific differences in alternative splicing offer an immense potential to improve the diagnosis and the prognosis of cancer. This review will focus on the functional impact of cancer-associated alternative splicing variants, the molecular determinants that alter the splicing decisions in cancer cells, and future therapeutic strategies.

Molecular cloning of novel alternatively spliced variants of BCL2L12, a new member of the BCL2 gene family, and their expression analysis in cancer cells

In the past, we identified and cloned the BCL2-like 12 (BCL2L12) gene, a novel member of the BCL2 family, which is implicated in various malignancies. The classical BCL2L12 protein isoform contains a highly conserved BH2 domain, a BH3-like motif, and a proline-rich region, and is involved in apoptosis. Most members of this apoptosis-related family are subjected to alternative splicing, thus generating multiple protein isoforms with distinct properties, and sometimes even with opposite function (pro- vs. anti-apoptotic). In the current study, we report the identification, molecular cloning, and expression pattern of novel splice variants of the human BCL2L12 gene in cancer cell lines. EST clones displaying high sequence identity (≥90%) with the classical BCL2L12 transcript were aligned, in order to identify those containing at least one novel splice junction. EST database mining led to the identification of three previously unknown splice variants of this apoptotic gene. In our effort to experimentally validate these novel transcripts, we also cloned seven more, previously unidentified, BCL2L12 alternatively spliced variants. Expression analysis of all BCL2L12 splice variants in human cancer cell lines and embryonic kidney cells revealed remarkable differences between their BCL2L12 expression profiles. Interestingly, 7 out of 10 novel splice variants of BCL2L12 are predicted to encode new protein isoforms, some of which are BH3-only proteins, in contrast to the classical BCL2L12 isoform, which also contains a functional BH2 domain. The remaining three novel splice variants of BCL2L12 are nonsense-mediated mRNA decay (NMD) candidates.

Gene expression in osteoblast cells treated with submicron to nanometer hydroxyapatite-mullite eluate particles

The present research focused on determining the effect of hydroxyapatite-20 wt% mullite (H20M) particle eluates on apoptosis and differentiation of human fetal osteoblast (hFOB) cells. The H20M particles (257 ± 37 nm) were prepared, starting with the production of a nanocomposite using a unique route of spark plasma sintering, followed by a repeated grinding-cryo treatment and elution process. Tetrazolium based cytotoxicity assay results showed a time- and dose-dependent effect of H20M particle eluates on hFOB cytotoxicity. In particular, the results revealed statistically reduced cell viability after hFOB were exposed to the above 10% H20M (257 ± 37 nm) eluates for 48 h. The apoptotic cell death triggered by H20M treatment was proven by the analysis of molecular markers of apoptosis, that is, the Bcl-2 family of genes. hFOB expression of Bcl-xL and Bcl-xS significantly increased 25.6- and 25.2-fold for 50% of H20M concentrations, respectively. The ratio of Bcl-xL/Bax (4.01) decreased 2-fold for hFOB exposed to 100% of H20M eluates than that for 10% H20M eluate (7.94) treated hFOB cells. On the other hand, the Bcl-xS/Bax ratio for the 10% H20M eluate was 4.15-fold, whereas for 100% H20M eluates, it was 11.55-fold. Specifically, the anti-apoptotic effect of the H20M particle eluates was corroborated by the up-regulation of bone cell differentiation marker genes such as, collagen type I, cbfa, and osteocalcin. In summary, the present work clearly demonstrated that H20M submicron to nanometer composite particle eluates have a minimal effect on hFOB apoptosis and can even up-regulate the expression of bone cell markers at the molecular level.

The BH4 domain of Bcl-X(L) rescues astrocyte degeneration in amyotrophic lateral sclerosis by modulating intracellular calcium signals

Collective evidence indicates that motor neuron degeneration in amyotrophic lateral sclerosis (ALS) is non-cell-autonomous and requires the interaction with the neighboring astrocytes. Recently, we reported that a subpopulation of spinal cord astrocytes degenerates in the microenvironment of motor neurons in the hSOD1(G93A) mouse model of ALS. Mechanistic studies in vitro identified a role for the excitatory amino acid glutamate in the gliodegenerative process via the activation of its inositol 1,4,5-triphosphate (IP(3))-generating metabotropic receptor 5 (mGluR5). Since non-physiological formation of IP(3) can prompt IP(3) receptor (IP(3)R)-mediated Ca(2+) release from the intracellular stores and trigger various forms of cell death, here we investigated the intracellular Ca(2+) signaling that occurs downstream of mGluR5 in hSOD1(G93A)-expressing astrocytes. Contrary to wild-type cells, stimulation of mGluR5 causes aberrant and persistent elevations of intracellular Ca(2+) concentrations ([Ca(2+)](i)) in the absence of spontaneous oscillations. The interaction of IP(3)Rs with the anti-apoptotic protein Bcl-X(L) was previously described to prevent cell death by modulating intracellular Ca(2+) signals. In mutant SOD1-expressing astrocytes, we found that the sole BH4 domain of Bcl-X(L), fused to the protein transduction domain of the HIV-1 TAT protein (TAT-BH4), is sufficient to restore sustained Ca(2+) oscillations and cell death resistance. Furthermore, chronic treatment of hSOD1(G93A) mice with the TAT-BH4 peptide reduces focal degeneration of astrocytes, slightly delays the onset of the disease and improves both motor performance and animal lifespan. Our results point at TAT-BH4 as a novel glioprotective agent with a therapeutic potential for ALS.

Bax activation by engagement with, then release from, the BH3 binding site of Bcl-xL

Bcl-2 homologues (such as Bcl-x(L)) promote survival in part through sequestration of "activator" BH3-only proteins (such as Puma), preventing them from directly activating Bax. It is thus assumed that inhibition of interactions between activators and Bcl-x(L) is a prerequisite for small molecules to antagonize Bcl-x(L) and induce cell death. The biological properties, described here of a terphenyl-based alpha-helical peptidomimetic inhibitor of Bcl-x(L) attest that displacement of Bax from Bcl-x(L) is also critical. Terphenyl 14 triggers Bax-dependent but Puma-independent cell death, disrupting Bax/Bcl-x(L) interactions without affecting Puma/Bcl-x(L) interactions. In cell-free assays, binding of inactive Bax to Bcl-x(L), followed by its displacement from Bcl-x(L) by terphenyl 14, produces mitochondrially permeabilizing Bax molecules. Moreover, the peptidomimetic kills yeast cells that express Bax and Bcl-x(L), and it uses Bax-binding Bcl-x(L) to induce mammalian cell death. Likewise, ectopic expression of Bax in yeast and mammalian cells enhances sensitivity to another Bcl-x(L) inhibitor, ABT-737, when Bcl-x(L) is present. Thus, the interaction of Bcl-x(L) with Bax paradoxically primes Bax at the same time it keeps Bax activity in check, and displacement of Bax from Bcl-x(L) triggers an apoptotic signal by itself. This mechanism might contribute to the clinical efficiency of Bcl-x(L) inhibitors.

Effect of endogenous androgens on 17beta-estradiol-mediated protection after spinal cord injury in male rats

Several groups have recently shown that 17beta-estradiol is protective in spinal cord injury (SCI). Testosterone can be aromatized to 17beta-estradiol and may increase estrogen-mediated protection. Alternatively, testosterone has been shown to increase excitotoxicity in models of central nervous system (CNS) injury. These experiments test the hypothesis that endogenous testosterone in male rats alters 17beta-estradiol-mediated protection by evaluating a delayed administration over a clinically relevant dose range and manipulating testicular-derived testosterone. Adult male Sprague Dawley rats were either gonadectomized or left gonad-intact prior to SCI. SCI was produced by a midthoracic crush injury. At 30 min post SCI, animals received a subcutaneous pellet of 0.0, 0.05, 0.5, or 5.0 mg of 17beta-estradiol, released over 21 days. Hindlimb locomotion was analyzed weekly in the open field. Spinal cords were collected and analyzed for cell death, expression of Bcl-family proteins, and white-matter sparing. Post-SCI administration of the 0.5- or 5.0-mg pellet improved hindlimb locomotion, reduced urinary bladder size, increased neuronal survival, reduced apoptosis, improved the Bax/Bcl-xL protein ratio, and increased white-matter sparing. In the absence of endogenous testicular-derived androgens, SCI induced greater apoptosis, yet 17beta-estradiol administration reduced apoptosis to the same extent in gonadectomized and gonad-intact male rats. These data suggest that delayed post-SCI administration of a clinically relevant dose of 17beta-estradiol is protective in male rats, and endogenous androgens do not alter estrogen-mediated protection. These data suggest that 17beta-estradiol is an effective therapeutic intervention for reducing secondary damage after SCI in males, which could be readily translated to clinical trials.

Protein kinase C-dependent control of Bcl-x alternative splicing

The alternative splicing of Bcl-x generates the proapoptotic Bcl-x(S) protein and the antiapoptotic isoform Bcl-x(L). Bcl-x splicing is coupled to signal transduction, since ceramide, hormones, and growth factors alter the ratio of the Bcl-x isoforms in different cell lines. Here we report that the protein kinase C (PKC) inhibitor and apoptotic inducer staurosporine switches the production of Bcl-x towards the x(S) mRNA isoform in 293 cells. The increase in Bcl-x(S) elicited by staurosporine likely involves signaling events that affect splicing decisions, because it requires active transcription and no new protein synthesis and is independent of caspase activation. Moreover, the increase in Bcl-x(S) is reproduced with more specific inhibitors of PKC. Alternative splicing of the receptor tyrosine kinase gene Axl is similarly affected by staurosporine in 293 cells. In contrast to the case for 293 cells, PKC inhibitors do not influence the alternative splicing of Bcl-x and Axl in cancer cell lines, suggesting that these cells have sustained alterations that uncouple splicing decisions from PKC-dependent signaling. Using minigenes, we show that an exonic region located upstream of the Bcl-x(S) 5' splice site is important to mediate the staurosporine shift in Bcl-x splicing. When transplanted to other alternative splicing units, portions of this region confer splicing modulation and responsiveness to staurosporine, suggesting the existence of factors that couple splicing decisions with PKC signaling.

Mitochondrial membrane permeabilization in cell death

Irrespective of the morphological features of end-stage cell death (that may be apoptotic, necrotic, autophagic, or mitotic), mitochondrial membrane permeabilization (MMP) is frequently the decisive event that delimits the frontier between survival and death. Thus mitochondrial membranes constitute the battleground on which opposing signals combat to seal the cell's fate. Local players that determine the propensity to MMP include the pro- and antiapoptotic members of the Bcl-2 family, proteins from the mitochondrialpermeability transition pore complex, as well as a plethora of interacting partners including mitochondrial lipids. Intermediate metabolites, redox processes, sphingolipids, ion gradients, transcription factors, as well as kinases and phosphatases link lethal and vital signals emanating from distinct subcellular compartments to mitochondria. Thus mitochondria integrate a variety of proapoptotic signals. Once MMP has been induced, it causes the release of catabolic hydrolases and activators of such enzymes (including those of caspases) from mitochondria. These catabolic enzymes as well as the cessation of the bioenergetic and redox functions of mitochondria finally lead to cell death, meaning that mitochondria coordinate the late stage of cellular demise. Pathological cell death induced by ischemia/reperfusion, intoxication with xenobiotics, neurodegenerative diseases, or viral infection also relies on MMP as a critical event. The inhibition of MMP constitutes an important strategy for the pharmaceutical prevention of unwarranted cell death. Conversely, induction of MMP in tumor cells constitutes the goal of anticancer chemotherapy.

Effect of sphingosine on Ca2+ entry and mitochondrial potential of Jurkat T cells--interaction with Bcl2

Triggers of Jurkat T cell apoptosis include sphingosine and ceramide. Sphingosine and ceramide further inhibit capacitative Ca2+ entry (ICRAC), an effect leading to inactivation but not death of Jurkat T cells. Mitochondria are key organelles in the machinery leading to apoptosis and on the other hand have been shown to participate in the regulation of Ca2+ entry. The present experiments were performed to explore whether treatment of Jurkat T cells with sphingosine leads to apoptosis and reduced Ca2+ entry and whether those effects are sensitive to expression of the antiapoptotic protein Bcl2, localized in the outer mitochondrial membrane. Exposure of Jurkat T cells to 10 microM spingosine was according to DiOC6 fluorescence followed by mitochondrial depolarization and according to Fura-red/Fluo-3 fluorescence followed by decreased capacitative Ca2+ entry. Mitochondrial depolarization was significantly delayed in cells overexpressing wild type Bcl2 or Bcl2 targeted to the mitochondrial membrane, whereas no significant influence on mitochondrial depolarization was observed in cells expressing Bcl2 lacking the membrane targeting motif or Bcl2 targeted to the endoplasmatic reticulum. In contrast to mitochondrial potential, the blunting of capacitative Ca2+ entry following sphingosine treatment was not sensitive to mitochondrial Bcl2 expression. In conclusion sphingosine exposure leads to both, mitochondrial depolarization and inhibition of capacitative Ca2+ entry. Mitochondrial Bcl2 reverses the effect on mitochondria but not on Ca2+ entry and thus leads to dissociation of those two sequelae of sphingosine treatment.

Bak but not Bax is essential for Bcl-xS-induced apoptosis

Bcl-x(S), a proapoptotic member of the Bcl-2 protein family, is localized in the mitochondria and induces apoptosis in a caspase- and BH3-dependent manner by a mechanism involving cytochrome c release. The way in which Bcl-x(S) induces caspase activation and cytochrome c release, as well as the relationship between Bcl-x(S) and other proapoptotic members of the Bcl-2 family, is not known. Here we used embryonic fibroblasts derived from mice deficient in the multidomain proapoptotic members of the Bcl-2 family (Bax and Bak) and the apoptotic components of the apoptosome (Apaf-1 and caspase-9) to unravel the cascade of events by which Bcl-x(S) promotes apoptosis. Our results show that Bak but not Bax is essential for Bcl-x(S)-induced apoptosis. Bcl-x(S) induced activation of Bak, which in turn promoted apoptosis by apoptosome-dependent and -independent pathways. These findings provide the first evidence that a proapoptotic Bcl-2 family protein induces apoptosis exclusively via Bak.

Bcl-xS induces an NGF-inhibitable cytochrome c release

Bcl-x(S), a pro-apoptotic member of the Bcl-2 protein family, is localized in the mitochondrial outer membrane and induces caspase-dependent and nerve growth factor (NGF)-inhibitable apoptosis in PC12 cells. The mechanism of action of Bcl-x(S) and how NGF inhibits this death are not fully understood. It is still unknown whether Bcl-x(S) induces mitochondrial cytochrome c release, and which apoptotic step NGF inhibits. We show that Bcl-x(S) induces cytochrome c release and caspase-3 activation in several cell types, and that in PC12 cells, these events are inhibited by NGF treatment. The survival effect of NGF was inhibited by inhibitors of protein kinase C (PKC), phosphatidylinositol-3-kinase (PI 3-kinase), and the mitogen-activated protein kinase kinase (MEK) inhibitors GF109203X, LY294002, and U0126. These findings show that cytochrome c release and caspase-3 activation participate in Bcl-x(S)-induced apoptosis, and that NGF inhibits Bcl-x(S)-induced apoptosis at the mitochondrial level via the PKC, PI 3-kinase, and MEK signaling pathways.

The role of Bcl-X(S) in radiation sensitivity

Bcl-X(S) is a pro-apoptosis member of the Bcl2 family that has been shown to induce cell death and enhance chemosensitivity. We have investigated the effect of Bcl-X(S) overexpression on radiation sensitivity. Using a tetracycline-repressible system, we found that removal of tetracycline for 16 h induced Bcl-X(S) and reduced the surviving fraction of NIH 3T3 cells to 25%. However, radiation sensitivity was not significantly affected by Bcl-X(S) expression; the mean inactivation doses for Bcl-X(S) repressed and Bcl-X(S) induced cells were 2.7 +/- 0.3 and 2.3 +/- 0.1 Gy, respectively. We conclude that Bcl-X(S) induces cell death without affecting radiation sensitivity. These results suggest that mitochondrial pathways to apoptosis may not have a significant role in survival after irradiation.

Mechanisms of cytochrome c release by proapoptotic BCL-2 family members

A crucial amplificatory event in several apoptotic cascades is the nearly complete release of cytochrome c from mitochondria. Proteins of the BCL-2 family which include both anti- and proapoptotic members control this step. Here, we review the proposed mechanisms by which proapoptotic BCL-2 family members induce cytochrome c release. Data support a model in which the apoptotic pathway bifurcates following activation of a "BH3 only" family member. BH3 only molecules induce the activation of the multidomain proapoptotics BAX and BAK, resulting in the permeabilization of the outer mitochondrial membrane and the efflux of cytochrome c. This is coordinated with the activation of a distinct pathway characterized by profound changes of the inner mitochondrial membrane morphology and organization. This mitochondrial remodelling insures complete release of cytochrome c and the onset of mitochondrial dysfunction that is a typical feature of many apoptotic deaths.

Down-regulation of Bcl-2 and Bcl-xL expression with bispecific antisense treatment in glioblastoma cell lines induce cell death

The functions of the antiapoptotic proteins Bcl-2 and Bcl-xL were examined in glioblastoma cells. Expression of both Bcl-2 and Bcl-xL were found to be elevated in protein lysates from seven early passage cell lines derived from human glioblastoma tumors compared with non-neoplastic glial cells. Down-regulation of both bcl-2 and bcl-xL expression in glioblastoma cell lines U87 and NS008 with bcl-2/bcl-xL bispecific antisense oligonucleotide resulted in spontaneous cell death. The mechanism of cell death was partially caspase-dependent. Executioner caspase 6 and caspase 7, but not caspase 3, were involved in apoptosis induced by bcl-2/bcl-xL antisense treatment. Interestingly, western blots failed to demonstrate expression of caspase 3 in two of the seven glioblastoma cell lines examined. The data support the hypothesis that Bcl-2 and Bcl-xL are important in preventing cell death in glioblastoma cells. It also suggests that there are functional pathways capable of successful completion of caspase-dependent cell death in gliomas. These findings support a potential role of bcl-2/bcl-xL bispecifc antisense oligonucleotide therapy as a treatment strategy to enhance caspase-dependent cell death in patients with glioblastoma.

Bcl-x(S) can form homodimers and heterodimers and its apoptotic activity requires localization of Bcl-x(S) to the mitochondria and its BH3 and loop domains

Proteins of the Bcl-2 family regulate apoptosis, some antagonizing cell death and others, such as Bcl-x(S), promoting it. We previously showed that expression of Bcl-x(S) in PC12 cells is a useful system for studying the mechanism of Bcl-x(S)-induced apoptosis. To further investigate this apoptotic effect and its prevention by anti-apoptotic agents, we assessed the role of distinct Bcl-x(S) domains, via the study of their mutations, on the ability of Bcl-x(S) to induce apoptosis and to localize to the mitochondria, as well as the ability of these domains to counteract the effects of anti-apoptotic agents on Bcl-x(S). Deletion of the transmembrane domain (DeltaTM) prevented the localization of Bcl-x(S) DeltaTM to the mitochondria and the ability of this mutant to induce apoptosis. Deletion of the amino acids GD 94-95 from the BH3 domain, or deletion of the loop region, impaired the ability of these mutants to induce apoptosis but not their localization to the mitochondria. Deletion of the BH4 domain or destruction of the caspase cleavage site in the loop region (by replacing amino acid D61 with A61) did not affect either the localization of these mutants to the mitochondria or their ability to induce cell death. It thus appears that Bcl-x(S)-induced apoptosis in PC12 cells is mediated by localization of Bcl-x(S) to the mitochondria by a process that requires the transmembrane domain. Furthermore, once localized to the mitochondria Bcl-x(S) requires the BH3 domain, and to a lesser extent the loop domain, for its subsequent activity. The anti-apoptotic agents Bcl-2 and Bcl-x(L), the caspase inhibitor Z-VAD-FMK, and nerve growth factor (NGF) did not prevent Bcl-x(S) localization to the mitochondria, and did not require the BH4 or the loop domains of Bcl-x(S) for their survival effect. Bcl-x(S) is capable of forming homodimers with itself and heterodimers with Bcl-x(L) or Bcl-2. Accordingly co-expression of Bcl-x(S) DeltaTM with Bcl-x(S), Bcl-2, or Bcl-x(L) leads to a change in the subcellular distribution of Bcl-x(S) DeltaTM, from a diffuse distribution throughout the cell to a more defined distribution. Moreover co-immunoprecipitation experiments directly demonstrated that Bcl-x(S) can associate with GFP-Bcl-x(S), Bcl-x(L), or Bcl-2. These results suggest that such Bcl-x(S) interactions may be important for the mechanism of action of this protein.

Killing of sarcoma cells by proapoptotic Bcl-X(S): role of the BH3 domain and regulation by Bcl-X(L)

Kaposi's sarcoma (KS) is the most common tumor affecting AIDS patients with over 20% of these patients afflicted by this disease. Previous studies have demonstrated that KS tumor cells predominantly express the prosurvival protein Bcl-X(L) compared with Bcl-2. In the current study, we have used an adenoviral vector that expresses Bcl-X(S), a functional inhibitor of Bcl-X(L), to study the significance of Bcl-X(L) expression in the KS cell line (SLK) or KS primary cultures. The results demonstrate that 75% to 80% of SLK or KS primary cells were killed by the Bcl-X(S) containing adenovirus whereas KS cells infected with control adenovirus showed no significant cell death or growth inhibition. Overexpression of Bcl-X(L), but not Bcl-2, in SLK cells attenuated apoptosis induced by adenovirus Bcl-X(S). Immunoprecipitation experiments revealed that adenoviral Bcl-X(S) associated with Bcl-X(L), but not with Bcl-2. Mutational analysis showed that the alpha 2 helical region of Bcl-X(S) containing the BH3 motif was critical for killing activity and interaction with Bcl-X(L). These results suggest that Bcl-X(S) is a direct killer and Bcl-X(L) may act by interacting with and sequestering Bcl-X(S.) These studies also suggest that targeting Bcl-X(L) may be of therapeutic benefit for the treatment of tumors that are characterized by inappropriate expression of Bcl-X(L).

Bcl-XL expression correlates with primary macrophage differentiation, activation of functional competence, and survival and results from synergistic transcriptional activation by Ets2 and PU.1

Depriving primary bone marrow-derived macrophages of colony-stimulating factor-1 (CSF-1) induces programmed cell death by apoptosis. We show that cell death is accompanied by decreases in the expression of anti-apoptotic Bcl-x(L) protein and the Ets2 and PU.1 proteins of the Ets transcription factor family. Macrophages require both priming and triggering signals independent of CSF-1 to kill neoplastic cells or microorganisms, and this activation of macrophage competence is accompanied by increased expression of bcl-x(L), ets2, and PU.1. Furthermore, we show that only Ets2 and PU.1, but not Ets1, function in a synergistic manner to transactivate the bcl-x promoter. The synergy observed between PU.1 and Ets2 is dependent on the transactivation domains of both proteins. Although other transcription factors like Fos, c-Jun, Myc, STAT3, and STAT5a are implicated in the activation of macrophage competence or in CSF-1 signaling, no synergy was observed between Ets2 and these transcription factors on the bcl-x promoter. We demonstrate that the exogenous expression of both Ets2 and PU.1 in macrophages increases the number of viable cells upon CSF-1 depletion and that Ets2 and PU.1 can functionally replace Bcl-x(L) in inhibiting Bax-induced apoptosis. Together, these results demonstrate that PU.1 and Ets2 dramatically increase bcl-x activation, which is necessary for the cytocidal function and survival of macrophages.

Neuron-specific Bcl-2 homology 3 domain-only splice variant of Bak is anti-apoptotic in neurons, but pro-apoptotic in non-neuronal cells

We have identified and characterized N-Bak, a neuron-specific isoform of the pro-apoptotic Bcl-2 family member Bak. N-Bak is generated by neuron-specific splicing of a novel 20-base pair exon, which changes the previously described Bak, containing Bcl-2 homology (BH) domains BH1, BH2, and BH3, into a shorter BH3-only protein. As demonstrated by reverse transcription-polymerase chain reaction and RNase protection assay, N-Bak transcripts are expressed only in central and peripheral neurons, but not in other cells, whereas the previously described Bak is expressed ubiquitously, but not in neurons. Neonatal sympathetic neurons microinjected with N-Bak resisted apoptotic death caused by nerve growth factor (NGF) removal, whereas microinjected Bak accelerated NGF deprivation-induced death. Overexpressed Bak killed sympathetic neurons in the presence of NGF, whereas N-Bak did not. N-Bak was, however, still death-promoting when overexpressed in non-neuronal cells. Thus, N-Bak is an anti-apoptotic BH3-only protein, but only in the appropriate cellular environment. This is the first example of a neuron-specific Bcl-2 family member.

Akt and Bcl-xL promote growth factor-independent survival through distinct effects on mitochondrial physiology

A comparison of Akt- and Bcl-x(L)-dependent cell survival was undertaken using interleukin-3-dependent FL5.12 cells. Expression of constitutively active Akt allows cells to survive for prolonged periods following growth factor withdrawal. This survival correlates with the expression level of activated Akt and is comparable in magnitude to the protection provided by the anti-apoptotic gene Bcl-x(L). Although both genes prevent cell death, Akt-protected cells can be distinguished from Bcl-x(L)-protected cells on the basis of increased glucose transporter expression, glycolytic activity, mitochondrial potential, and cell size. In addition, Akt-expressing cells require high levels of extracellular nutrients to support cell survival. In contrast, Bcl-x(L)-expressing cells deprived of interleukin-3 survive in a more vegetative state, in which the cells are smaller, have lower mitochondrial potential, reduced glycolytic activity, and are less dependent on extracellular nutrients. Thus, Akt and Bcl-x(L) suppress mitochondrion-initiated apoptosis by distinct mechanisms. Akt-mediated survival is dependent on promoting glycolysis and maintaining a physiologic mitochondrial potential. In contrast, Bcl-x(L) maintains mitochondrial integrity in the face of a reduced mitochondrial membrane potential, which develops as a result of the low glycolytic rate in growth factor-deprived cells.

Identification of small-molecule inhibitors of interaction between the BH3 domain and Bcl-xL

To study the role of the BH3 domain in mediating pro-apoptotic and anti-apoptotic activities of Bcl-2 family members, we identified a series of novel small molecules (BH3Is) that inhibit the binding of the Bak BH3 peptide to Bcl-xL. NMR analyses revealed that BH3Is target the BH3-binding pocket of Bcl-xL. Inhibitors specifically block the BH3-domain-mediated heterodimerization between Bcl-2 family members in vitro and in vivo and induce apoptosis. Our results indicate that BH3-dependent heterodimerization is the key function of anti-apoptotic Bcl-2 family members and is required for the maintenance of cellular homeostasis.

MCL-1S, a splicing variant of the antiapoptotic BCL-2 family member MCL-1, encodes a proapoptotic protein possessing only the BH3 domain

MCL-1 (myeloid cell leukemia-1) is an antiapoptotic BCL-2 family protein discovered as an early induction gene during myeloblastic leukemia cell differentiation. This survival protein has the BCL-2 homology (BH) domains 1, 2, and 3 and a C-terminal transmembrane region. We identified a short splicing variant of the MCL-1 mRNA in the human placenta encoding a protein, termed MCL-1 short (MCL-1S), with an altered C terminus as compared with the full-length MCL-1 long (MCL-1L), leading to the loss of BH1, BH2, and the transmembrane domains. Analysis of the human MCL-1 gene indicated that MCL-1S results from the splicing out of exon 2 during mRNA processing. MCL-1S, unlike MCL-1L, does not interact with diverse proapoptotic BCL-2-related proteins in the yeast two-hybrid system. In contrast, MCL-1S dimerizes with MCL-1L in the yeast assay and coprecipitates with MCL-1L in transfected mammalian cells. Overexpression of MCL-1S induces apoptosis in transfected Chinese hamster ovary cells, and the MCL-1S action was antagonized by the antiapoptotic MCL-1L. Thus, the naturally occurring MCL-1S variant represents a new proapoptotic BH3 domain-only protein capable of dimerizing with the antiapoptotic MCL-1L. The fate of MCL-1-expressing cells could be regulated through alternative splicing mechanisms and interactions of the resulting anti- and proapoptotic gene products.