The protein bcl-2 alpha does not require membrane attachment, but two conserved domains to suppress apoptosis

Identification and function analysis of BCL2 in immune response of Pteria penguin

B-cell lymphoma/leukemia-2 (BCL2), an anti-apoptotic factor in the mitochondrial regulatory pathway of apoptosis, is critically important in immune defenses. In this study, a novel BCL2 gene was characterized from Pteria penguin (P. penguin). The PpBCL2 was 1482 bp long, containing an open reading frame (ORF) of 588 bp encoding 195 amino acids. Four highly conserved BCL-2 homology (BH) domains were found in PpBCL2. Amino acid alignment and phylogenetic tree showed that PpBCL2 had the highest similarity with BCL2 of Crassostrea gigas at 65.24 %. Tissue expression analysis showed that PpBCL2 had high constitutive expression in gill, digestive diverticulum and mantle, and was significantly increased 72 h of Vibrio parahaemolyticus (V. parahaemolyticus) challenge in these immune tissues. Furthermore, PpBCL2 silencing significantly inhibited antimicrobial activity of hemolymph supernatant by 1.4-fold, and significantly reduced the survival rate by 51.7 % at 72 h post infection in P. penguin. These data indicated that PpBCL2 played an important role in immune response of P. penguin against V. parahaemolyticus infection.

Markedly divergent effects of Ouabain on a Temozolomide-resistant (T98G) vs. a Temozolomide-sensitive (LN229) Glioblastoma cell line

BACKGROUND

Glioblastoma multiforme (GBM) is the most aggressive primary brain tumor with poor prognosis. GMB are highly recurrent mainly because of radio- and chemoresistance. Radiotherapy with Temozolomide (TMZ) is until today the golden standard adjuvant therapy, however, the optimal treatment of recurrent glioblastoma remains controversial. Ouabain belongs to the Cardiotonic Steroids (CTS) the natural ligands of the Na/K-ATPase (NKA). It is established that the NKA represents a signal transducer with either stimulating or inhibiting cell growth, apoptosis, migration and angiogenesis. Over the last decade evidence grew that CTS have anti-tumor properties especially in GBM.

AIM

Proceeding from recent studies we wanted to further demonstrate a divergent effect of Ouabain on a TMZ-resistant (T98G) as compared to a TMZ-sensitive (LN229) GBM cell line.

METHODS

We analyzed the effect of Ouabain on cell migration and plasma cell membrane potential (PCMP) in the LN229 and T98G GBM cell line as well as underlying mechanisms (Bcl-2 and p-Akt/pan-Akt expression). Moreover, we analyzed the anti-angiogenic effect of Ouabain on human umbilical vein endothelial cells (HUVECs).

RESULTS

T98G cells showed a significant inhibition of cell migration and a significant depolarization of the PCMP at similar Ouabain concentrations (IC50 = 1.67 × 10^-7 M) resp. (IC50 = 2.72 × 10^-7 M) with a strong inverse correlation (R² = 0.95). In contrast, LN229 cells did not respond to Ouabain in these assays at all. Similarly, only T98G but not LN229 cells revealed Bcl-2 down-regulation at nanomolar Ouabain concentrations. This unique response to Ouabain is associated with a down-regulation of pan-Akt in T98G cells 24 h after Ouabain (1.0 × 10^-6 M) treatment. For the first time, the anti-angiogenic effect of Ouabain on HUVEC cells (IC50 = 5.49 × 10^-8 M) was demonstrated which correlated strongly with the anti-migratory effect (R² = 0.85).

CONCLUSION

The TMZ-resistant T98G cell line as compared to the TMZ-sensitive LN229 cell line shows a high sensitivity towards Ouabain. We consider it as a promising new compound especially in recurrent GBM to overcome the resistance to TMZ and irradiation.

Apoptotic priming is defined by the dynamic exchange of Bcl-2 proteins between mitochondria and cytosol

Apoptosis is regulated by interactions between the BH3-only and multi-domain Bcl-2 family proteins. These interactions are integrated on the outer mitochondrial membrane (OMM) where they set the threshold for apoptosis, known as mitochondrial priming. However, how mitochondrial priming is controlled at the level of single cells remains unclear. Retrotranslocation of Bcl-XL has been proposed as one mechanism, removing pro-apoptotic Bcl-2 proteins from the OMM, thus reducing priming. Contrary to this view, we now show that Bcl-XL retrotranslocation is inhibited by binding to its BH3-only partners, resulting in accumulation of these protein complexes on mitochondria. We find that Bcl-XL retrotranslocation dynamics are tightly coupled to mitochondrial priming. Quantifying these dynamics indicates the heterogeneity in priming between cells within a population and predicts how they subsequently respond to a pro-apoptotic signal.

BCL-2 isoform β promotes angiogenesis by TRiC-mediated upregulation of VEGF-A in lymphoma

Bcl-2 (B-cell lymphoma 2), the first identified anti-apoptosis factor, encodes two transcripts, the long isoform α and the short isoform β. The current understanding of the Bcl-2 function mainly focuses on Bcl-2α, while little is known about the function of Bcl-2β, which lacks the transmembrane domain and contains 10 unique amino acids at the C-terminus instead. Here, we analyzed the expressions of BCL-2 two isoforms in diffused large B-cell lymphoma (DLBCL) and found a significant positive correlation between them. Then, with the CRISPR/Cas9-based transcriptional activator (CRISPRa), we generated mouse B-cell lymphomas with Bcl-2 upregulation from the endogenous locus, in which both Bcl-2α and Bcl-2β levels were increased. Bcl-2β itself promoted angiogenesis both in vitro and in vivo through increased vascular endothelial growth factor A (VEGF-A). Inhibiting VEGF receptors with Axitinib reduced angiogenesis induced by Bcl-2β overexpression. Co-immunoprecipitation and mass spectrometry analysis revealed that Bcl-2β interacted with the T-complex protein ring complex (TRiC). Disruption of TRiC significantly impaired the angiogenesis-promoting activity of Bcl-2β, indicated by reduced VEGF-A protein level and HUVEC tube formation. Thus, our study suggests that Bcl-2 isoform β plays a role in promoting tumor angiogenesis through the Bcl-2β-TRiC-VEGF-A axis.

Identification and functional comparison of Bcl2 splicing isoforms in mouse embryonic stem cells

Embryonic stem cells (ESCs) provide an ideal model for investigating developmental processes and are great sources for developing regenerative medicine. Harnessing apoptosis facilitates accurate recapitulation of signalling events during embryogenesis and allows efficient expansion of the ESCs during differentiation. Bcl2, a key regulator of intrinsic anti-apoptotic pathway, encodes two splicing isoforms. However, the identification and functional comparison of Bcl2 splicing isoforms in mouse ESCs (mESCs) remains to be elucidated. Here, we provide the evidence that both Bcl2 splicing variants are expressed in mESCs. Despite the structural difference, they have similar subcellular localisation. Both Bcl2α and Bcl2β enhance differentiation efficiency of the ESCs and effectively improve the survival and growth of ESCs under serum-free conditions. However, the functional effect of Bcl2α was more potent than that of Bcl2β. Moreover, only Bcl2α could maintain the long-term expansion and pluripotency of ESCs cultured in serum-free medium. Taken together, our results demonstrate previously unknown functional differences in Bcl2 alternative splicing isoforms in ESCs, and lay the foundation for future efforts to engineer ESCs for regenerative medicine.

Exploring the Conformational Space of Bcl-2 Protein Variants: Dynamic Contributions of the Flexible Loop Domain and Transmembrane Region

Members of the Bcl-2 protein family regulate apoptosis through interactions with several proteins. A critical intrinsically disordered region (IDR) present in some members of the Bcl-2 family is essential for their function. Also, the structural and conformational plasticity of disordered regions is essential for the regulation of the Bcl-2 protein’s activity. Further, some proteins of the family contain transmembrane-helical regions, which anchor them into organelle membranes. Bcl-2, the archetypical member of the family, is characterized by an IDR labeled as a flexible loop domain (FLD) and a transmembrane domain (TMD). Another member of this family is the Bcl-2A1 protein, containing a TMD but lacking the FLD. To our knowledge, this is the first report which characterizes the individual and simultaneous dynamical contributions of FLD and TMD in Bcl-2 and Bcl-2A1 using molecular dynamics simulations (MDS). We examined the conformational spaces of Bcl-2, Bcl-2A1, and two artificial constructs lacking the TMD (Bcl-2ΔTM and Bcl-2A1ΔTM). As the results show, FLD and TMD stabilized each protein independently when they are present. When they coincided, such as in Bcl-2, an additive stabilizing effect is observed. This information is crucial for understanding the structural mechanisms of interaction in the Bcl-2 family.

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.

Identification of a Bcl-xL homolog from orange-spotted grouper (Epinephelus coioides) involved in SGIV-induced nonapoptotic cell death

Bcl-2 family proteins play essential roles in modulating immune response and controlling cells' fate. Bcl-xL is one of anti-apoptotic protein in this family. In this study, a new Bcl-xL homolog (EcBcl-xL) was identified and characterized from orange-spotted grouper, Epinephelus coioides. EcBcl-xL encoded a 221 amino acid peptides that shared 86% identity to Larimichthys crocea Bcl-xL protein, contained four conserved BH domains and one transmembrane region. The predicted three-dimensional structure of EcBcl-xL was similar with Homo sapiens Bcl-xL. EcBcl-xL widely expressed in all tested tissues with highest expression in head kidney. Its expression level was significantly up-regulated after SGIV infection in vivo. Furthermore, overexpression of EcBcl-xL could inhibit SGIV-induced nonapoptotic cell death and suppressed viral genes transcriptions in GS cells. Our findings suggested that EcBcl-xL might play a role during virus infection through modulating SGIV-induced nonapoptotic cell death.

Extra-mitochondrial prosurvival BCL-2 proteins regulate gene transcription by inhibiting the SUFU tumour suppressor

Direct interactions between pro- and anti-apoptotic BCL-2 family members form the basis of cell death decision-making at the outer mitochondrial membrane (OMM). Here we report that three antiapoptotic BCL-2 proteins (MCL-1, BCL-2, and BCL-XL) found untethered from the OMM function as transcriptional regulators of a prosurvival and growth program. Antiapoptotic BCL-2 proteins engage a BCL-2 homology (BH) domain sequence found in Suppressor of Fused (SUFU), a tumor suppressor and antagonist of the GLI DNA binding proteins. BCL-2 proteins directly promote SUFU turnover, inhibit SUFU-GLI interaction, and induce the expression of the GLI target genes BCL-2, MCL-1, and BCL-XL. Antiapoptotic BCL-2 protein/SUFU feedforward signaling promotes cancer cell survival and growth and can be disabled with BH3 mimetics – small molecules that target antiapoptotic BCL-2 proteins. Our findings delineate a chemical strategy for countering drug resistance in GLI-associated tumors and reveal unanticipated functions for BCL-2 proteins as transcriptional regulators.

Identification of the Bcl-2 family protein gene BOK from orange-spotted grouper (Epinephelus coioides) involved in SGIV infection

Apoptosis plays vital roles in many physiological process and immune response. BOK is one of the central regulators in apoptosis. In this study, a new BOK homolog (Ec-BOK) was cloned and characterized from Orange-spotted grouper, Epinephelus coioides. Ec-BOK encoded a 210 amino acid peptides which shared 97% identity to Stegastes partitus BOK protein, contained four BH domains and one transmembrane region. Ec-BOK widely expressed in all analyzed tissues with the higher expressions in kidney and spleen. Its expression level was up-regulated after SGIV infection in vitro. Further analysis revealed that overexpression of Ec-BOK inhibited viral genes transcriptions and virus replication in fish cell. Our findings suggested that Ec-BOK might play a role in the immune response against virus.

Evolution of the BCL-2-Regulated Apoptotic Pathway

The mitochondrion descends from a bacterium that, about two billion years ago, became endosymbiotic. This organelle represents a Pandora’s box whose opening triggers cytochrome-c release and apoptosis of cells from multicellular animals, which evolved much later, about six hundred million years ago. BCL-2 proteins, which are critical apoptosis regulators, were recruited at a certain time point in evolution to either lock or unlock this mitochondrial Pandora’s box. Hence, particularly intriguing is the issue of when and how the “BCL-2 proteins–mitochondria–apoptosis” triptych emerged. This chapter explains what it takes from an evolutionary perspective to evolve a BCL-2-regulated apoptotic pathway, by focusing on the events occurring upstream of mitochondria.

Identification and functional characterization of two Bcl-2 family protein genes in Zhikong scallop Chlamys farreri

Apoptosis plays significant roles in maintenance of homeostasis, immune defense and development. The Bcl-2 family proteins are important regulators of the intrinsic apoptosis. In the study, we have characterized a Bcl-2-like gene (named CfBcl-2) and a Bax-like gene (named CfBax) from the Zhikong scallop Chlamys farreri. The full-length of the CfBcl-2 cDNA is 944 nucleotides (nt) encoding a putative protein of 225 amino acid residues (aa) that contains four Bcl-2 homology (BH) domains, and the CfBax cDNA is 505 nt encoding a putative protein of 115 aa that contains three Bcl-2 BH domains. Sequence and phylogenetic analysis demonstrate that CfBcl-2 and CfBax present typical domain organization of the corresponding Bcl-2 related proteins and are more similar and clustered with their homologues of other molluscs. The two genes are ubiquitously expressed in six tissues of C. farreri, with the highest expression level of CfBcl-2 in adductor muscle and highest expression level of CfBax in gill. The expressions of CfBcl-2 and CfBax in hemocytes were both significantly up-regulated after an in vivo exposure of scallops to air, injection with lipopolysaccharide and infection with acute viral necrobiotic disease virus, and the expression patterns of the two genes after the three treatments vary in different change magnitude and up-regulation timespan. Yeast two-hybrid assay reveals a direct interaction between the CfBcl-2 and CfBax proteins. These results indicate that the CfBcl-2 and CfBax may participate in the apoptosis-based stress and immune responses against noxious stimulation.

Differential expression of the Bcl-2 and Bax isoforms in CD19 positive B-lymphocytes isolated from patients diagnosed with chronic lymphocytic leukaemia

AIM

To examine the relative gene expression levels of the anti-apoptotic Bcl-2α and β isoforms and the pro-apoptotic Baxα and β isoforms in patients with chronic lymphocytic leukaemia (CLL) and healthy controls (HC).

METHODS

Peripheral blood was obtained from 36 patients diagnosed with CLL and 10 HC. CD19 B-lymphocytes were isolated using an antibody coupled magnetic bead isolation system; from these cells the total RNA was isolated and purified. The relative levels of gene expression were examined by quantitative real-time polymerase chain reaction (qReTi-PCR) using primers specific for each isoform.

RESULTS

Bcl-2α and Baxα are expressed at higher levels than their β-isoforms in CLL and HC. Bcl-2α, Bcl-2β and Baxβ expression is increased in CLL while Bax-α is expressed at similar levels to HC. The Bcl-2α/Bcl-2β ratio is similar in CLL and HC. The Bcl-2α/Baxα ratio is increased in CLL when compared with HC.

CONCLUSION

Bcl-2α and Baxα appear to be the dominant anti- and pro-apoptotic isoforms in CLL. The Bcl-2α/Baxα ratio is increased in CLL while the Bcl-2α/Bcl-2β ratio is similar to HC.

Functions of the C-terminal domains of apoptosis-related proteins of the Bcl-2 family

Bcl-2 family proteins are involved in cell homeostasis, where they regulate cell death. Some of these proteins are pro-apoptotic and others pro-survival. Moreover, many of them share a similar domain composition with several of the so-called BH domains, although some only have a BH3 domain. A C-terminal domain is present in all the multi-BH domain proteins and in some of the BH3-only ones. This C-terminal domain is hydrophobic or amphipathic, for which reason it was thought when they were discovered that they were membrane anchors. Although this is indeed one of their functions, it has since been observed that they may also serve as regulators of the function of some members of this family, such as Bax. They may also serve to recognize the target membrane of some of these proteins, which only after an apoptotic signal, are incorporated into a membrane. It has been shown that peptides that imitate the sequence of C-terminal domains can form pores and may serve as a model to design cytotoxic molecules.

A study of the structural properties and thermal stability of human Bcl-2 by molecular dynamics simulations

The anti-apoptotic B-cell lymphoma 2 (Bcl-2) protein interacts with several proteins that regulate the apoptotic properties of cells. In this research, we conduct several all-atom molecular dynamics (MD) simulations under high-temperature unfolding conditions, from 400 to 800 K, for 25 ns. These simulations were performed using a model of an engineered Bcl-2 human protein (Bcl-2-Δ22Σ3), which lacks 22 C-terminal residues of the transmembrane domain. The aim of this study is to gain insight into the structural behavior of Bcl-2-Δ22Σ3 by mapping the conformational movements involved in Bcl-2 stability and its biological function. To build a Bcl-2-Δ22Σ3 three-dimensional model, the protein core was built by homology modeling and the flexible loop domain (FLD, residues 33-91) by ab initio methods. Further, the entire protein model was refined by MD simulations. Afterwards, the production MD simulations showed that the FLD at 400 and 500 K has several conformations reaching into the protein core, whereas at 600 K some of the alpha-helices were lost. At 800 K, the Bcl-2 core is destabilized suggesting a possible mechanism for protein unfolding, where the alpha helices 1 and 6 were the most stable, and a reduction in the number of hydrogen bonds initially occurs. In conclusion, the structural changes and the internal protein interactions suggest that the core and the FLD are crucial components of Bcl-2 in its function of regulate ng access to the recognition sites of kinases and caspases.

The mitochondrial pathways of apoptosis

Apoptosis is a process of programmed cell death that serves as a major mechanism for the precise regulation of cell numbers, and as a defense mechanism to remove unwanted and potentially dangerous cells. Studies in nematode, Drosophila and mammals have shown that, although regulation of the cell death machinery is somehow different from one species to another, it is controlled by homologous proteins and involves mitochondria. In mammals, activation of caspases (cysteine proteases that are the main executioners of apoptosis) is under the tight control of the Bcl-2 family proteins, named in reference to the first discovered mammalian cell death regulator. These proteins mainly act by regulating the release of caspases activators from mitochondria. Although for a long time the absence of mitochondrial changes was considered as a hallmark of apoptosis, mitochondria appear today as the central executioner of apoptosis. In this chapter, we present the current view on the mitochondrial pathway of apoptosis with a particular attention to new aspects of the regulation of the Bcl-2 proteins family control of mitochondrial membrane permeabilization: the mechanisms implicated in their mitochondrial targeting and activation during apoptosis, the function(s) of the oncosuppressive protein p53 at the mitochondria and the role of the processes of mitochondrial fusion and fission.

Deletion mutational analysis of BMRP, a pro-apoptotic protein that binds to Bcl-2

Bcl-2 is an anti-apoptotic member of the Bcl-2 family of proteins that protects cells from apoptosis induced by a large variety of stimuli. The protein BMRP (MRPL41) was identified as a Bcl-2 binding partner and shown to have pro-apoptotic activity. We have performed deletion mutational analyses to identify the domain(s) of Bcl-2 and BMRP that are involved in the Bcl-2/BMRP interaction, and the region(s) of BMRP that mediate its pro-apoptotic activity. The results of these studies indicate that both the BH4 domain of Bcl-2 and its central region encompassing its BH1, BH2, and BH3 domains are required for its interaction with BMRP. The loop region and the transmembrane domain of Bcl-2 were found to be dispensable for this interaction. The Bcl-2 deletion mutants that do not interact with BMRP were previously shown to be functionally inactive. Deletion analyses of the BMRP protein delimited the region of BMRP needed for its interaction with Bcl-2 to the amino-terminal two-thirds of the protein (amino acid residues 1-92). Further deletions at either end of the BMRP(1-92) truncated protein resulted in lack of binding to Bcl-2. Functional studies performed with BMRP deletion mutants suggest that the cell death-inducing domains of the protein reside mainly within its amino-terminal two-thirds. The region of BMRP required for the interaction with Bcl-2 is very relevant for the cell death-inducing activity of the protein, suggesting that one possible mechanism by which BMRP induces cell death is by binding to and blocking the anti-apoptotic activity of Bcl-2.

BCL2 protein expression in follicular lymphomas with t(14;18) chromosomal translocations

The t(14;18)(q32;q21) chromosomal translocation induces BCL2 protein overexpression in most follicular lymphomas. However the expression of BCL2 is not always homogeneous and may demonstrate a variable degree of heterogeneity. This study analysed BCL2 protein expression pattern in 33 cases of t(14;18)-positive follicular lymphomas using antibodies against two different epitopes (i.e. the widely used antibody BCL2/124 and an alternative antibody E17). 16/33 (49%) cases demonstrated strong BCL2 expression. In 10/33 (30%) cases, BCL2 expression was heterogeneous and in some of these, its loss appeared to be correlated with cell proliferation, as indicated by Ki67 expression. Double immunofluorescence labelling confirmed an inverse BCL2/Ki67 relationship, where in 24/28 (86%) cases cellular expression of BCL2 and Ki67 was mutually exclusive. In addition, seven BCL2 'pseudo-negative' cases were identified in which immunostaining was negative with antibody BCL2/124, but positive with antibody E17. Genomic DNA sequencing of these 'pseudo-negative' cases demonstrated eleven mutations in four cases and nine of these were missense mutations. It can be concluded that in follicular lymphomas, despite carrying the t(14;18) translocations, BCL2 protein expression may be heterogeneous and loss of BCL2 could be related to cell proliferation. Secondly, mutations in translocated BCL2 genes appear to be common and may cause BCL2 pseudo-negative immunostaining.

BCL-2 family proteins: critical checkpoints of apoptotic cell death

Apoptosis is a morphologically distinct form of programmed cell death essential for normal development and tissue homeostasis. Aberrant regulation of this pathway is linked to multiple human diseases, including cancer, autoimmunity, neurodegenerative disorders, and diabetes. The BCL-2 family of proteins constitutes a critical control point in apoptosis residing immediately upstream of irreversible cellular damage, where family members control the release of apoptogenic factors from mitochondria. The cardinal member of this family, BCL-2, was originally discovered as the defining oncogene in follicular lymphomas, located at one reciprocal breakpoint of the t(14;18) (q32;q21) chromosomal translocation. Since this original discovery, remarkable efforts marshaled by many investigators around the world have advanced our knowledge of the basic biology, molecular mechanisms, and therapeutic targets in the apoptotic pathway. This review highlights findings from many laboratories that have helped uncover some of the critical control points in apoptosis. The emerging picture is that of an intricate cellular machinery orchestrated by tightly regulated molecular interactions and conformational changes within BCL-2 family proteins that ultimately govern the cellular commitment to apoptotic death.

The C-terminal transmembrane domain of Bcl-xL mediates changes in mitochondrial morphology

We investigate the effect of mitochondrial localization and the Bcl-x(L) C-terminal transmembrane (TM) domain on mitochondrial morphology and subcellular light scattering. CSM 14.1 cell lines stably expressed yellow fluorescent protein (YFP), YFP-Bcl-x(L,) YFP-Bcl-x(L)-DeltaTM, containing the remainder of Bcl-x(L) after deletion of the last 21 amino acids corresponding to the TM domain, or YFP-TM, consisting of YFP fused at its C-terminal to the last 21 amino acids of Bcl-x(L). YFP-Bcl-x(L) and YFP-TM localized to the mitochondria. Their expression decreased the intensity ratio of wide-to-narrow angle forward scatter by subcellular organelles, and correlated with an increase in the proportion of mitochondria with an expanded matrix having greatly reduced intracristal spaces as observed by electron microscopy. Cells expressing YFP-TM also exhibited significant autophagy. In contrast, YFP-Bcl-x(L)-DeltaTM was diffusely distributed in the cells, and its expression did not alter light scattering or mitochondrial morphology compared with parental cells. Expression of YFP-Bcl-x(L) or YFP-Bcl-x(L)-DeltaTM provided significant resistance to staurosporine-induced apoptosis. Surprisingly however, YFP-TM expression also conferred a moderate level of cell death resistance in response to staurosporine. Taken together, our results suggest the existence of a secondary Bcl-x(L) function that is mediated by the transmembrane domain, alters mitochondrial morphology, and is distinct from BH3 domain sequestration.

Regulation of apoptosis by C. elegans CED-9 in the absence of the C-terminal transmembrane domain

Bcl-2 proteins regulate apoptosis in organisms as diverse as mammals and nematodes. These proteins are often localized at mitochondria by a C-terminal transmembrane domain. Although the transmembrane domain and mitochondrial localization are centrally involved in specific cases of vertebrate Bcl-2 activity, the significance of this localization is not clear for all species. Studying the Caenorhabditis elegans Bcl-2 homolog CED-9, we found that the transmembrane domain was both necessary and sufficient for localization at mitochondrial outer membranes. Furthermore, we found that in our assays, ced-9 transgenes lacking the transmembrane domain, although somewhat less active than equivalent transgenes derived from wild-type ced-9, rescued embryonic lethality of ced-9(lf) animals and responded properly to upstream signals in controlling the fate of Pn.aap neurons. Both of these apoptotic activities were retained in a construct where CED-9 lacking the transmembrane domain was targeted to the cytosolic surface of the endoplasmic reticulum and derived organelles, suggesting that in wild-type animals, accumulation at mitochondria is not essential for CED-9 to either inhibit or promote apoptosis in C. elegans. Taken together, these data are consistent with a multimodal character of CED-9 action, with an ability to regulate apoptosis through interactions in the cytosol coexisting with additional evolutionarily conserved role(s) at the membrane.

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.

Auto-activation of the apoptosis protein Bax increases mitochondrial membrane permeability and is inhibited by Bcl-2

Interactions among Bcl-2 family proteins mediated by Bcl-2 homology (BH) regions transform apoptosis signals into actions. The interactions between BH3 region-only proteins and multi-BH region proteins such as Bax and Bcl-2 have been proposed to be the dominant interactions required for initiating apoptosis. Experimental evidence also suggests that both homo- and hetero-interactions are mediated primarily by the BH3 regions in all Bcl-2 family proteins and contribute to commitment to or inhibition of apoptosis. We found that a peptide containing the BH3 helix of Bax was not sufficient to activate recombinant Bax to permeabilize mitochondria. However, an extended peptide containing the BH3 helix and additional downstream sequences activated Bax to permeabilize mitochondria and liposomes. Bcl-2 inhibited the membrane-permeabilizing activity of peptide-activated Bax. This activity of Bcl-2 was inhibited by the extended but not the BH3-only peptide despite both peptides binding to Bcl-2 with similar affinity. Further, membrane-bound Bax activation intermediates directly activated soluble Bax further permeabilizing the membrane. Bcl-2 inhibited Bax auto-activation. We therefore propose that Bax auto-activation amplifies the initial death signal produced by BH3-only proteins and that Bcl-2 functions as an inhibitor of Bax auto-activation.

Cell death regulation by B-cell lymphoma protein

Bcl-2 (B Cell Lymphoma) protein is an anti-apoptotic member of Bcl-2 family, which is comprised of pro- and anti-apoptotic members. It regulates cellular proliferation and death by inter- and intra-family interactions. It has a potential to suppress apoptotic cell death under variety of stress conditions by modulating mitochondrial transmembrane potential. However, prevalence of constitutively activated Bcl-2 cellular activity is not always required in cells; a mechanism likely exists in cells, which controls its activity. When expression of Bcl-2 is unregulated, it generates lymphoma like, follicular B-cell lymphoma. This article reviews the structural and functional regulation of Bcl-2 activity at transcriptional, translational, domain, structural and post-translational level, which also accounts for the effects of its deletion and site-directed mutants in the regulation of cellular proliferation and differentiation in vitro and in vivo. This concisely reviewed information on Bcl-2 helps us to update our understanding of cell death and its modulation by Bcl-2 and its mutant's interaction, which has gained therapeutic benefits in cell growth and proliferation, particularly for sensitive human hematopoietic stem cells.

Nuclear translocation of the pro-apoptotic Bcl-2 family member Bok induces apoptosis

The anti-apoptotic members of the Bcl-2 family, such as Bcl-2 and Bcl-XL, play a central role in preventing the induction of apoptosis via the intrinsic apoptotic pathway. It has been previously shown that induction of apoptosis by the pro-apoptotic Bcl-2 family member Bok is not antagonized by either Bcl-2 or Bcl-xL, suggesting that Bok might have a unique role in the apoptotic cascade. We showed here that human Bok is the only member of the Bcl-2 family to have a leucine-rich sequence indicative of a nuclear export signal within its BH3 domain. Western blot analysis of nuclear and cytoplasmic fractions identified Bok in both the nucleus and the cytoplasm of HEK 293T cells, HeLa cells, and breast cancer cells, and its nuclear concentration increased after treatment of those cells with leptomycin B, an inhibitor of the exportin Crm1. Immunocytochemistry of flag-tagged Bok confirmed its nuclear localization. Mutating the nuclear export signal of Bok by site-directed mutagenesis resulted in an increase in its nuclear localization and apoptotic activity. We also found that Crm1 interacted with wild-type Bok but not with the mutated form. These results suggest that nuclear export of Bok is a regulated process mediated by Crm1, and constitutes the first report of a link between the apoptotic activity and nuclear localization of a pro-apoptotic member of the Bcl-2 family.

Bcl-2 decreases cell proliferation and promotes accumulation of cells in S phase without affecting the rate of apoptosis in human ovarian carcinoma cells

OBJECTIVES

The Bcl-2 protein is an important regulator of the apoptotic cascade and promotes cell survival. Bcl-2 can also delay entry into the cell cycle from quiescence. In the present study, we used two isogenic human ovarian carcinoma cell lines, which expressed differential levels of Bcl-2 proteins, to demonstrate that Bcl-2 may regulate the growth rates of adenocarcinoma cells.

METHODS

The growth rates of two isogenic ovarian cancer cell lines were determined by XTT assays and flow cytometry combined with PI staining. Bcl-2-overexpressing SKOV3 cells were modified to express a doxycycline-inducible anti-Bcl-2 single-chain antibody and the effects of Bcl-2 protein inhibition on cell proliferation and apoptosis were assessed.

RESULTS

We demonstrate that Bcl-2 promotes the accumulation of proliferating carcinoma cells in S phase. The Bcl-2-overexpressing SKOV3 cell line proliferates markedly faster and shows delayed progression to G2M phase compared to its low Bcl-2-expressing counterpart SKOV3.ip1 cell line. Single-chain antibody-mediated inhibition of Bcl-2 in SKOV3 cells was associated with increased growth rates and more rapid cell cycle progression. Treatment with cisplatin resulted in more cells accumulating in S phase in Bcl-2-overexpressing SKOV3 cells, while the inhibition of Bcl-2 abolished delayed entry into G2M phase without affecting cisplatin-induced apoptosis.

CONCLUSIONS

Our results suggest that, in ovarian cancer cells, Bcl-2 delays cell cycle progression by promoting accumulation of cells in S phase without affecting the rate of apoptosis. Thus, in addition to its known role at the G0/G1 checkpoint, we demonstrate for the first time that Bcl-2 also regulates the S phase.

Distinct Domains Control the Addressing and the Insertion of Bax into Mitochondria

The translocation of Bax from the cytosol into the mitochondrial outer membrane is a central event during apoptosis. We report that beyond the addressing step, which involves its first alpha-helix (halpha1), the helices alpha5 and alpha6 (halpha5alpha6) are responsible for the insertion of Bax into mitochondrial outer membrane bilayer. The translocation of Bax to mitochondria is associated with specific changes in the conformation of the protein that are under the control of two prolines: Pro-13, which controls the unfolding of halpha1, and Pro-168, a proline located immediately before the hydrophobic carboxyl-terminal end (i.e. helix alpha9, halpha9), which controls the disclosure of halpha5alpha6. An additional step, the disruption of an electrostatic bond formed between Asp-33 (halpha1) and Lys-64 (BH3), allows the mitochondria addressing of Bax. We conclude that, although the intramolecular interactions of halpha1 with the BH3 region control the addressing of Bax to mitochondria, the Pro-168 is involved in the control of its membrane insertion through halpha5alpha6.

Programmed cell death via mitochondria: Different modes of dying

Programmed cell death (PCD) is a major component of normal development, preservation of tissue homeostasis, and elimination of damaged cells. Many studies have subdivided PCD into the three categories of apoptosis, autophagy, and necrosis based on criteria such as morphological alterations, initiating death signal, or the implication of caspases. However, these classifications fail to address the interplay between the three types of PCD. In this review, we will discuss the central role of the mitochondrion in the integration of the cell death pathways. Mitochondrial alterations such as the release of sequestered apoptogenic proteins, loss of transmembrane potential, production of reactive oxygen species (ROS), disruption of the electron transport chain, and decreases in ATP synthesis have been shown to be involved in, and possibly responsible for, the different manifestations of cell death. Thus, the mitochondria can be viewed as a central regulator of the decision between cellular survival and demise.

Molecular Mechanism of Apoptosis Induced by Mechanical Forces

In all biological systems, a balance between cell proliferation/growth and death is required for normal development as well as for adaptation to a changing environment. To affect their fate, it is essential for cells to integrate signals from the environment. Recently, it has been recognized that physical forces such as stretch, strain, and tension play a critical role in regulating this process. Despite intensive investigation, the pathways by which mechanical signals are converted to biochemical responses is yet to be completely understood. In this review, we will examine our current understanding of how mechanical forces induce apoptosis in a variety of biological systems. Rather than being a degenerative event, physical forces act through specific receptor-like molecules such as integrins, focal adhesion proteins, and the cytoskeleton. These molecules in turn activate a limited number of protein kinase pathways (p38 MAPK and JNK/SAPK), which amplify the signal and activate enzymes (caspases) that promote apoptosis. Physical forces concurrently activate other signaling pathways such as PIK-3 and Erk 1/2 MAPK, which modulate the apoptotic response. The cell phenotype and the character of the physical stimuli determine which pathways are activated and, consequently, allow for variability in response to a specific stimulus in different cell types.

Men are but worms: neuronal cell death in C elegans and vertebrates

Awarding the 2002 Nobel Prize in Physiology or Medicine to Sydney Brenner, H Robert Horvitz, and John E Sulston for 'their discoveries concerning the genetic regulation of organ development and programmed cell death (PCD)' highlights the significant contribution that the study of experimental organisms, such as the nematode Caenorhabditis elegans, has made to our understanding of human physiology and pathophysiology. Their studies of lineage determination in worms established the 'central dogma' of apoptosis: The BH3-only protein EGL-1 is induced in cells destined to die, interacts with the BCL-2-like inhibitor CED-9, displacing the adaptor CED-4, which then promotes activation of the caspase CED-3. The vast majority of cells undergoing PCD during development in C. elegans, as in vertebrates, are neurons. Accordingly, the genetic regulation of apoptosis is strikingly similar in nematode and vertebrate neurons. This review summarizes these similarities - and the important differences - in the molecular mechanisms responsible for neuronal PCD in C. elegans and vertebrates, and examines the implications that our understanding of physiological neuronal apoptosis may have for the diagnosis and treatment of acute and chronic human neurodegenerative disorders.

The phosphorylation status and anti-apoptotic activity of Bcl-2 are regulated by ERK and protein phosphatase 2A on the mitochondria

Bcl-2 protein play important roles in the regulation of apoptosis. We previously reported that the phosphorylation of Bcl-2 was augmented by treatment with protein phosphatase 2A (PP2A) inhibitor; however, the kinase responsible for Bcl-2 phosphorylation had not yet been identified. In this study, we identified extracellular-signal-regulated kinase (ERK) as the responsible kinase for the phosphorylation of Bcl-2. We also found that the transmembrane region (TM) deleted form of Bcl-2 (Bcl-2DeltaTM), which was unable to localize on the mitochondria was constitutively phosphorylated, whereas wild-type Bcl-2 that localized on the mitochondria, was present in its hypophosphorylated form. The phosphorylation of Bcl-2DeltaTM was retarded by treatment with MAP kinase ERK kinase (MEK) inhibitor and PP2A did not bind to Bcl-2DeltaTM. These observations suggest that Bcl-2DeltaTM is constitutively phosphorylated by ERK, but is not dephosphorylated by PP2A in human tumor cell lines. The phosphorylation of Bcl-2 resulted in a reduction in anti-apoptotic function, implying that dephosphorylation promoted the anti-apoptotic activity of Bcl-2 protein in human tumor cell lines. Thus, the present findings suggest that ERK and PP2A are physiological regulators of Bcl-2 phosphorylation, and these enzymes exert an influence on the anti-apoptotic function of Bcl-2.

Bcl-xES, a BH4- and BH2-containing antiapoptotic protein, delays Bax oligomer formation and binds Apaf-1, blocking procaspase-9 activation

Bcl-2 family members either negatively or positively regulate the apoptotic threshold of cells. Bcl-xES (extra short), a novel Bcl-x member, possesses a unique combination of BH4 and BH2 domains as well as a COOH-terminal hydrophobic transmembrane anchor domain. Bcl-xES contains sequences of hydrophobic alpha-6 helices but lacks sequences of alpha-5 helices, suggesting that it does not have pore channel-forming activity but functions uniquely as a trapping protein. mRNA expression analysis by reverse transcriptase-polymerase chain reaction and RNase protection assay reveal that Bcl-xES is expressed in a variety of human cancer cell lines and human tumors, including bone marrow from patients with acute lymphoblastic leukemia. Bcl-xES expression is much less pronounced in some specimens of normal human tissues, including the breast, ovary, testis and lung. Stable, transfected human B lymphoma Namalwa variant cells expressing Bcl-xES were derived to investigate its role in apoptosis. Bcl-xES had a preventive effect on cell death induced by tumor necrosis factor-alpha and various concentrations of anticancer drugs, including camptothecin, etoposide and cisplatin. Its protective action on cell death was correlated with the inhibition of mitochondrial cytochrome c release and caspase activation. In a yeast two-hybrid system, Bcl-xES interacted with most Bcl-2 family members, including those containing only a BH3 domain, and with the Ced-4 homolog Apaf-1. Co-immunoprecipitation and gel filtration chromatography experiments suggest that Bcl-xES delays drug-induced apoptosis by disturbing the formation of Bax oligomers and preventing cytochrome c release, but also by interacting with Apaf-1 and inhibiting procaspase-9 activation, thus averting the apoptogenic proteolytic caspase cascade and cell death.

Bcl-xL/Bcl-2 coordinately regulates apoptosis, cell cycle arrest and cell cycle entry

Bcl-x(L) and Bcl-2 inhibit both apoptosis and proliferation. In investigating the relationship between these two functions of Bcl-x(L) and Bcl-2, an analysis of 24 Bcl-x(L) and Bcl-2 mutant alleles, including substitutions at residue Y28 previously reported to selectively abolish the cell cycle activity, showed that cell cycle delay and anti-apoptosis co-segregated in all cases. In determining whether Bcl-2 and Bcl-x(L) act in G(0) or G(1), forward scatter and pyronin Y fluorescence measurements indicated that Bcl-2 and Bcl-x(L) cells arrested more effectively in G(0) than controls, and were delayed in G(0)-G(1) transition. The cell cycle effects of Bcl-2 and Bcl-x(L) were reversed by Bad, a molecule that counters the survival function of Bcl-2 and Bcl-x(L). When control and Bcl-x(L) cells of equivalent size and pyronin Y fluorescence were compared, the kinetics of cell cycle entry were similar, demonstrating that the ability of Bcl-x(L) and Bcl-2 cells to enhance G(0) arrest contributes significantly to cell cycle delay. Our data suggest that cell cycle effects and increased survival both result from intrinsic functions of Bcl-2 and Bcl-x(L).

BH4-domain peptide from Bcl-xL exerts anti-apoptotic activity in vivo

The Bcl-2 family of proteins regulates apoptosis chiefly by controlling mitochondrial membrane permeability. It has previously been shown that the BH4 domain of Bcl-2/Bcl-xL is essential for the prevention of apoptotic mitochondrial changes, including the release of cytochrome c and apoptotic cell death. We have previously reported that BH4 peptide fused to the protein transduction domain of HIV-1 TAT protein (TAT-BH4) significantly inhibits etoposide-induced apoptosis in a cell line. This time, we investigated whether TAT-BH4 peptide was cytoprotective in ex vivo and in vivo rodent models. Intraperitoneal injection of TAT-BH4 peptide greatly inhibited X-ray-induced apoptosis in the small intestine of mice and partially suppressed Fas-induced fulminant hepatitis. In addition, this peptide markedly suppressed heart failure after ischemia-reperfusion injury in isolated rat heart, probably by preventing mitochondrial dysfunction. These findings demonstrate that TAT-BH4 peptide exerts anti-apoptotic activity both in vivo and ex vivo, and imply that it may be a useful therapeutic agent for diseases involving mitochondrial dysfunction and apoptosis.

Deletion of the BH1 domain of Bcl-2 accelerates apoptosis by acting in a dominant negative fashion

To investigate the exact biochemical functions by which Bcl-2 regulates apoptosis, we established a stable human small cell lung carcinoma cell line, Ms-1, overexpressing wild-type human Bcl-2 or various deletion and point mutants thereof, and examined the effect of these Bcl-2 mutants on apoptosis induced by antitumor drugs such as camptothecin. Cytochrome c release, caspase-3-(-like) protease activation, and apoptosis induced by antitumor drugs were accelerated by overexpression of Bcl-2 lacking a Bcl-2 homology (BH) 1 domain (Bcl-2/ DeltaBH1), but not by that of BH2, BH3, or BH4 domain-deleted Bcl-2. A similar result was obtained upon the substitution of glycine 145 with alanine in the BH1 domain (Bcl-2/G145A), which failed to interact with either Bax or Bak. Pro-apoptotic Bax and Bak have been known to be activated in response to antitumor drugs, and Bcl-2/G145A as well as Bcl-2/DeltaBH1 also accelerated Bax- or Bak-induced apoptosis in HEK293T cells. These two mutants still retained the ability to interact with wild-type Bcl-2 and Bcl-xL, and abrogated the inhibitory effect of wild-type Bcl-2 or Bcl-xL on Bax- or Bak-induced apoptosis. In addition, immunoprecipitation studies revealed that Bcl-2/DeltaBH1 and Bcl-2/G145A interrupted the association between wild-type Bcl-2 and Bax/Bak. Taken together, our results demonstrate that Bcl-2/DeltaBH1 or Bcl-2/G145A acts as a dominant negative of endogenous anti-apoptotic proteins such as Bcl-2 and Bcl-xL, thereby enhancing antitumor drug-induced apoptosis, and that this dominant negative activity requires both a failure of interaction with Bax and Bak through the BH1 domain of Bcl-2 and retention of the ability to interact with Bcl-2 and Bcl-xL.

Transmembrane domain of Bcl-2 is required for inhibition of ceramide synthesis, but not cytochrome c release in the pathway of inostamycin-induced apoptosis

Bcl-2 protein plays important roles in the regulation of apoptosis. However, the exact mechanism by which Bcl-2 blocks apoptosis is still unclear. In the present study, we found that overexpression of Bcl-2 in human small cell lung carcinoma Ms-1 cells inhibited not only the release of cytochrome c from mitochondria into cytosol but also de novo ceramide synthesis induced by inostamycin, a phosphatidylinositol turnover inhibitor. To investigate the correlation between the structure of Bcl-2 and its inhibitory function in inostamycin-induced apoptosis, Ms-1 cells that stably overexpress domain-deletional mutants of Bcl-2 were established. Transmembrane domain-deleted Bcl-2 failed to inhibit inostamycin-induced de novo ceramide synthesis, whereas it inhibited inostamycin-induced cytochrome c release, indicating that anchoring of Bcl-2 to membrane was a requirement for its inhibitory effect on inostamycin-induced ceramide synthesis, but not cytochrome c release. Thus, the deletion mutant of tarnsmembrane domain of Bcl-2 can suppress inostamycin-induced apoptosis by inhibiting cytochrome c release, a downstream event of ceramide synthesis in the pathway of inostamycin-induced apoptosis. We also found that the BH3 and BH4 domains of Bcl-2 were necessary for inhibition of inostamycin-induced apoptosis, and deletion of BH1 or BH2 did not affect the inhibitory effect of Bcl-2 to inostamycin-induced apoptotic events.

The structure of Bcl-w reveals a role for the C-terminal residues in modulating biological activity

Pro-survival Bcl-2-related proteins, critical regulators of apoptosis, contain a hydrophobic groove targeted for binding by the BH3 domain of the pro-apoptotic BH3-only proteins. The solution structure of the pro-survival protein Bcl-w, presented here, reveals that the binding groove is not freely accessible as predicted by previous structures of pro-survival Bcl-2-like molecules. Unexpectedly, the groove appears to be occluded by the C-terminal residues. Binding and kinetic data suggest that the C-terminal residues of Bcl-w and Bcl-x(L) modulate pro-survival activity by regulating ligand access to the groove. Binding of the BH3-only proteins, critical for cell death initiation, is likely to displace the hydrophobic C-terminal region of Bcl-w and Bcl-x(L). Moreover, Bcl-w does not act only by sequestering the BH3-only proteins. There fore, pro-survival Bcl-2-like molecules probably control the activation of downstream effectors by a mechanism that remains to be elucidated.

Tauroursodeoxycholic acid prevents Bax-induced membrane perturbation and cytochrome C release in isolated mitochondria

Bax is a potent pro-apoptotic member of the Bcl-2 protein family that localizes to the mitochondrial membrane during apoptosis. Tauroursodeoxycholic acid (TUDCA) modulates the apoptotic threshold, in part, by preventing Bax translocation both in vitro and in vivo. The mechanisms by which Bax induces and TUDCA inhibits release of cytochrome c are unclear. We show here that recombinant Bax protein induced cytochrome c release in isolated mitochondria without detectable swelling. Co-incubation with TUDCA prevented efflux of mitochondrial factors and proteolytic processing of caspases in cytosolic extracts. Spectroscopic analyses of mitochondria exposed to Bax revealed increased polarity and fluidity of the membrane lipid core as well as altered protein order, indicative of Bax binding, together with loss of spin-label paramagnetism, characteristic of oxidative damage. TUDCA markedly abrogated the Bax-induced membrane perturbation. In conclusion, our results indicate that Bax protein directly induces cytochrome c release from mitochondria through a mechanism that does not require the permeability transition. Rather, it is accompanied by changes in the organization of membrane lipids and proteins. TUDCA is a potent inhibitor of Bax association with mitochondria. Thus, TUDCA modulates apoptosis by suppressing mitochondrial membrane perturbation through pathways that are also independent of the mitochondrial permeability transition.

Domain-dependent photodamage to Bcl-2. A membrane anchorage region is needed to form the target of phthalocyanine photosensitization

Photodynamic therapy using the photosensitizer Pc 4 and red light photochemically destroys the antiapoptotic protein Bcl-2 and induces apoptosis. To characterize the requirements for photodamage, we transiently transfected epitope-tagged Bcl-2 deletion mutants into DU-145 cells. Using confocal microscopy and Western blots, wild-type Bcl-2 and mutants with deletions near the N terminus were found in mitochondria, endoplasmic reticulum, and nuclear membranes and were photodamaged. A mutant missing the C terminus, including the transmembrane domain, spread diffusely in cells and was not photodamaged. Bcl-2 missing alpha-helices 5/6 was also not photodamaged. Bcl-2 missing only one of those alpha-helices, with or without substitutions of the singlet oxygen-targeted amino acids, behaved like wild-type Bcl-2 with respect to localization and photodamage. Using green fluorescent protein (GFP)-tagged Bcl-2 or mutants in live cells, no change in either the localization or the intensity of GFP fluorescence was observed in response to Pc 4 photodynamic therapy. Western blot analysis of either GFP- or Xpress-tagged Bcl-2 revealed that the photodynamic therapy-induced disappearance of the Bcl-2 band was accompanied by the appearance of bands indicative of heavily cross-linked Bcl-2 protein. Therefore, the alpha(5)/alpha(6) region of Bcl-2 is required for photodamage and cross-linking, and domain-dependent photodamage to Bcl-2 offers a unique mechanism for activation of apoptosis.

Characterization of the signal that directs Bcl-x(L), but not Bcl-2, to the mitochondrial outer membrane

It is assumed that the survival factors Bcl-2 and Bcl-x(L) are mainly functional on mitochondria and therefore must contain mitochondrial targeting sequences. Here we show, however, that only Bcl-x(L) is specifically targeted to the mitochondrial outer membrane (MOM) whereas Bcl-2 distributes on several intracellular membranes. Mitochondrial targeting of Bcl-x(L) requires the COOH-terminal transmembrane (TM) domain flanked at both ends by at least two basic amino acids. This sequence is a bona fide targeting signal for the MOM as it confers specific mitochondrial localization to soluble EGFP. The signal is present in numerous proteins known to be directed to the MOM. Bcl-2 lacks the signal and therefore localizes to several intracellular membranes. The COOH-terminal region of Bcl-2 can be converted into a targeting signal for the MOM by increasing the basicity surrounding its TM. These data define a new targeting sequence for the MOM and propose that Bcl-2 acts on several intracellular membranes whereas Bcl-x(L) specifically functions on the MOM.

The Bcl-2 protein family: sensors and checkpoints for life-or-death decisions

Members of the Bcl-2 family are crucial integrators of survival and death signals in higher eukaryotes. Although recent studies have provided novel and quite unexpected insights into the mechanisms by which these proteins might issue life permits or death sentences in cells, we are still on the way to fully understand their modes of action. This review provides a snapshot on where we are on this journey and how we may exploit our knowledge on this family of proteins to unveil the mysteries of immune regulation.

Induction of apoptosis of human B-CLL and ALL cells by a novel retinoid and its nonretinoidal analog

We have recently described a novel retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (CD437/AHPN) that induces apoptosis in a number of malignant cell types. We now describe our studies examining the effects of CD437 and a nonretinoidal analog (MM002) on the in vitro proliferation of the ALL-REH cell line, the in vitro and in vivo growth of a novel Epstein-Barr virus-negative (EBV(-)) B-cell chronic lymphocytic leukemia (B-CLL) cell line (WSU-CLL), and primary cultures of human B-CLL and acute lymphoblastic leukemia (ALL) cells. CD437 and MM002 induce apoptosis in both cell lines, as indicated by the activation of caspase-2 and caspase-3, cleavage of poly(adenosine diphosphate-ribose) (poly(ADP-ribose)) polymerase, increase in annexin V binding, and subsequent nuclear fragmentation. CD437-mediated apoptosis was not associated with the modulation of Bcl-2, Bax, or Mcl-1 levels, but was associated with the cleavage of the antiapoptotic protein Bcl-X(L) to a proapoptotic 18-kD form. This cleavage of Bcl-X(L) was dependent on caspase-3 activation since Bcl-X(L) cleavage and apoptosis were inhibited by the caspase-3 inhibitor Z-DVED-fmk. CD437 markedly inhibited the growth of WSU-CLL cells in severe combined immunodeficiency (SCID) mice. Tumor growth inhibition, growth delay, and log cell kill were 85.7%, 21 days, and 2.1, respectively, in the treated mice. Moreover, 1 of the 5 treated mice was tumor-free longer than 150 days and thus was considered cured. Exposure of primary cultures of both B-CLL and ALL cells obtained from patients to CD437 and MM002 resulted in their apoptosis. These results suggest that CD437 and MM002 analogs may have a potential role in the treatment of B-CLL and ALL.

Cellular mechanisms for the repression of apoptosis

Apoptosis, also known as programmed cell death, is a ubiquitous mode of cell death known to play an important role during embryogenesis, development, and adult cellular homeostasis. Disruption of this normal physiological cell death process can result in either excessive or insufficient apoptosis, which can lead to various disease states and pathology. Since most cells contain the machinery that brings about apoptosis, it is clear that living cells must contain inherent repressive mechanisms to keep the death process in check. In this review, we examine several modes of repression of apoptosis that exist in cells.

Inhibition of axotomy-induced neuronal apoptosis by extracellular delivery of a Bcl-XL fusion protein

Bcl-2 and Bcl-XL prevent neuronal apoptosis during development, neurodegenerative disease, and trauma. To test a new anti-apoptosis strategy for neuroprotection, we engineered nontoxic components of anthrax toxin into a Bcl-XL delivery system. Delivery of Bcl-XL by this system prevented apoptosis of cultured rat cerebellar granule cells and macrophages, and the prevention depended on both the Bcl-XL and the anthrax toxin receptor binding/translocation moieties. Furthermore, neuronal death in vivo in a retinal ganglion cell model of axotomy-induced apoptosis was inhibited by administration of this fusion protein. Thus, Bcl-XL protein can be delivered into cells from the medium or interstitial space, offering a new way to block apoptosis upstream of many caspases and the mitochondria dysfunction phase of apoptosis.

Early TNF-alpha levels correlate with ischaemic stroke severity

OBJECTIVES

The study aimed to evaluate the levels of an important proinflammatory cytokine tumour necrosis factor-alpha (TNF-alpha) in cerebrospinal fluid (CSF) and serum in acute stroke and to study the relation between those and the neurological stroke severity and functional disability.

MATERIAL AND METHODS

The investigations comprised 23 ischaemic stroke patients. CSF and blood samples were obtained 24 h after the onset of stroke, and stored until analysis. Patients were examined according to Scandinavian Stroke Scale (SSS) and to Barthel Index (BI).

RESULTS

The patients displayed statistically significant high levels of TNF-alpha in CSF and sera within the first 24 h of stroke. These correlated significantly with SSS and BI scores calculated within the same interval, and 1 and 2 weeks later.

CONCLUSION

Our results suggest the involvement of TNF-alpha in mechanisms of early stroke-induced inflammation and a predictive value of the initial TNF-alpha levels for the outcome of stroke.

Pathways of apoptosis and the modulation of cell death in cancer

Although cell death once was viewed exclusively as the disordered, chaotic outcome of metabolic catastrophe, apoptosis now is recognized as a highly ordered, evolutionarily conserved, and genetically selected program that is essential for normal development. The death receptor pathway of apoptosis, cytotoxic T cells, prolife survival signals, Bcl-2 family of regulators, p53 and regulated cell death in cancer, and oncogenes are reviewed. Future prospects in this arena also are discussed.

The Cellular Protein PRA1 Modulates the Anti-apoptotic Activity of Epstein-Barr Virus BHRF1, a Homologue of Bcl-2, through Direct Interaction

The Epstein-Barr virus-encoded early protein, BHRF1, is a structural and functional homologue of the anti-apoptotic protein, Bcl-2. There is accumulating evidence that BHRF1 protects a variety of cell types from apoptosis induced by various external stimuli. To identify specific proteins from normal epithelial cells that interact with BHRF1 and that might promote or inhibit its anti-apoptotic activity, we screened a yeast two-hybrid cDNA library derived from human normal foreskin keratinocytes and identified a cellular gene encoding human prenylated rab acceptor 1 (hPRA1). The interaction of hPRA1 with BHRF1 was confirmed using glutathione S-transferase pull-down assays, confocal laser scanning microscopy, and co-immunoprecipitation. Two regions of PRA1, amino acids 30-53 and the carboxyl-terminal 21 residues, are important for BHRF1 interactions and two regions of BHRF1, amino acids 1-18 and 89-142, including the Bcl-2 homology domains BH4 and BH1, respectively, are crucial for PRA1 interactions. PRA1 expression interferes with the anti-apoptotic activity of BHRF1, although not of Bcl-2. These results indicate that the PRA1 interacts selectively with BHRF1 to reduce its anti-apoptotic activity and might play a role in the impeding completion of virus maturation.

BH3-only proteins that bind pro-survival Bcl-2 family members fail to induce apoptosis in the absence of Bax and Bak

The BH3-only proteins Bim and Bad bind to the antiapoptotic Bcl-2 proteins and induce apoptosis in wild-type cells and cells from either bax(-/-) or bak(-/-) animals. In contrast, constitutively active forms of Bim and Bad failed to induce apoptosis in bax(-/-)bak(-/-) cells. Expression of Bax restored susceptibility of the cells to Bim and Bad. In addition, Bax but not Bim or Bad sensitized the bax(-/-)bak(-/-) cells to a wide variety of cell death stimuli including UV irradiation, chemotherapeutic agents, and ER stress. These results suggest that neither activation of BH3-only proteins nor suppression of pro-survival Bcl-2 proteins is sufficient to kill cells in the absence of both Bax and Bak. Furthermore, whereas mouse embryo fibroblasts (MEF) expressing only Bax or Bak displayed resistance to transformation, bax(-/-)bak(-/-) MEF were nearly as prone to oncogenic transformation as p53(-/-) MEF. Thus, the function of either Bax or Bak appears required to initiate most forms of apoptosis and to suppress oncogenic transformation.

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.