Structure of Bcl-xL-Bak peptide complex: recognition between regulators of apoptosis

Bcl-2 family: life-or-death switch

The Bcl-2 family of proteins that consists of anti-apoptotic and pro-apoptotic members determines life-or-death of a cell by controlling the release of mitochondrial apoptogenic factors, cytochrome c and apoptosis-inducing factor (AIF), that activate downstream executional phases, including the activation of death proteases called caspases. Cytochrome c release is, thus, central to apoptotic signal transduction in mammals, making study of the mechanism for cytochrome c release a major issue. Several models for cytochrome c release have been proposed, including rupture of mitochondrial outer membrane and involvement of a specific channel. Here, we provide an overview of recent findings on the role of Bcl-2 family members in the life-or-death decision of a cell.

BNIP3 heterodimerizes with Bcl-2/Bcl-X(L) and induces cell death independent of a Bcl-2 homology 3 (BH3) domain at both mitochondrial and nonmitochondrial sites

BNIP3 (formerly NIP3) is a pro-apoptotic, mitochondrial protein classified in the Bcl-2 family based on limited sequence homology to the Bcl-2 homology 3 (BH3) domain and COOH-terminal transmembrane (TM) domain. BNIP3 expressed in yeast and mammalian cells interacts with survival promoting proteins Bcl-2, Bcl-X(L), and CED-9. Typically, the BH3 domain of pro-apoptotic Bcl-2 homologues mediates Bcl-2/Bcl-X(L) heterodimerization and confers pro-apoptotic activity. Deletion mapping of BNIP3 excluded its BH3-like domain and identified the NH(2) terminus (residues 1-49) and TM domain as critical for Bcl-2 heterodimerization, and either region was sufficient for Bcl-X(L) interaction. Additionally, the removal of the BH3-like domain in BNIP3 did not diminish its killing activity. The TM domain of BNIP3 is critical for homodimerization, pro-apoptotic function, and mitochondrial targeting. Several TM domain mutants were found to disrupt SDS-resistant BNIP3 homodimerization but did not interfere with its killing activity or mitochondrial localization. Substitution of the BNIP3 TM domain with that of cytochrome b(5) directed protein expression to nonmitochondrial sites and still promoted apoptosis and heterodimerization with Bcl-2 and Bcl-X(L). We propose that BNIP3 represents a subfamily of Bcl-2-related proteins that functions without a typical BH3 domain to regulate apoptosis from both mitochondrial and nonmitochondrial sites by selective Bcl-2/Bcl-X(L) interactions.

Human homologue of S. pombe Rad9 interacts with BCL-2/BCL-xL and promotes apoptosis

DNA damage induces apoptosis through a signalling pathway that can be suppressed by the BCL-2 protein, but the mechanism by which DNA damage does this is unknown. Here, using yeast two-hybrid and co-immunoprecipitation studies, we show that RAD9, a human protein involved in the control of a cell-cycle checkpoint, interacts with the anti-apoptotic Bcl-2-family proteins BCL-2 and BCL-x L, but not with the pro-apoptotic BAX and BAD. When overexpressed in mammalian cells, RAD9 induces apoptosis that can be blocked by BCL-2 or BCL-x L. Conversely, antisense RAD9 RNA suppresses cell death induced by methyl methanesulphonate. These findings indicate that RAD9 may have a new role in regulating apoptosis after DNA damage, in addition to its previously described checkpoint-control and other radioresistance-promoting functions.

Characterization of the antiapoptotic Bcl-2 family member myeloid cell leukemia-1 (Mcl-1) and the stimulation of its message by gonadotropins in the rat ovary

The majority of ovarian follicles undergo atresia mediated by apoptosis. Bcl-2-related proteins act as regulators of apoptosis via the formation of dimers with proteins inside and outside the Bcl-2 family. Previous studies have identified BAD as a proapoptotic Bcl-2 family member expressed in the ovary. It is known that BAD phosphorylation induced by survival factors leads to its preferential binding to 14-3-3 and suppression of the death-inducing function of BAD. To identify ovarian binding partners for hypophosphorylated BAD, we performed a yeast two-hybrid screening of a rat ovary complementary DNA library using as bait a mutant BAD incapable of binding to 14-3-3. Screening of yeast transformants yielded positive clones encoding the rat ortholog of Mcl-1 (myeloid cell leukemia-1), an antiapoptotic Bcl-2 protein. Amino acid sequence analysis revealed that rat and human Mcl-1 showed a complete conservation of the Bcl-2 homology domains BH1, BH2, and BH3. In the yeast two-hybrid system, Mcl-1 binds to the hypophosphorylated mutant of BAD and interacts preferentially with different proapoptotic (Bax, Bak, Bok, Bik, and BOD) compared with antiapoptotic Bcl-2 family members (Bcl-2, Bcl-xL, Bcl-w, Bfl-1, CED-9, and BHRF-1). Northern blot hybridization demonstrated expression of Mcl-1 transcripts of 2.3 and 3.7 kb in the ovary and diverse other rat tissues. In immature rats, PMSG treatment led to a transient increase in the 2.3-kb Mcl-1 transcript, peaking at 6 h after injection and returning to baseline levels after 24 h. Moreover, the same transcript was induced in the PMSG-primed preovulatory rat ovary 6 h after the administration of ovulatory doses of either hCG or FSH. In situ hybridization studies revealed that the gonadotropin stimulation of ovarian Mcl-1 message occurs in both granulosa and thecal cells. In conclusion, rat Mcl-1 was identified as an ovarian BAD-interacting protein and the message for the antiapoptotic Mcl-1 protein was induced after treatment with gonadotropins in granulosa and thecal cells of growing follicles.

The pro-apoptotic proteins, Bid and Bax, cause a limited permeabilization of the mitochondrial outer membrane that is enhanced by cytosol

During apoptosis, an important pathway leading to caspase activation involves the release of cytochrome c from the intermembrane space of mitochondria. Using a cell-free system based on Xenopus egg extracts, we examined changes in the outer mitochondrial membrane accompanying cytochrome c efflux. The pro-apoptotic proteins, Bid and Bax, as well as factors present in Xenopus egg cytosol, each induced cytochrome c release when incubated with isolated mitochondria. These factors caused a permeabilization of the outer membrane that allowed the corelease of multiple intermembrane space proteins: cytochrome c, adenylate kinase and sulfite oxidase. The efflux process is thus nonspecific. None of the cytochrome c-releasing factors caused detectable mitochondrial swelling, arguing that matrix swelling is not required for outer membrane permeability in this system. Bid and Bax caused complete release of cytochrome c but only a limited permeabilization of the outer membrane, as measured by the accessibility of inner membrane-associated respiratory complexes III and IV to exogenously added cytochrome c. However, outer membrane permeability was strikingly increased by a macromolecular cytosolic factor, termed PEF (permeability enhancing factor). We hypothesize that PEF activity could help determine whether cells can recover from mitochondrial cytochrome c release.

The induction of apoptosis by bacterial pathogens

Apoptosis is a highly regulated process of cell death that is required for the development and homeostasis of multicellular organisms. In contrast to necrosis, apoptosis eliminates individual cells without inducing an inflammatory response. Activation or prevention of cell death could be a critical factor in the outcome of an infection. Programmed cell death has been observed as a response to infection by a wide range of animal and plant pathogens and is mediated by an array of pathogen-encoded virulence determinants. Pathogen-induced modulation of the host cell-death pathway may serve to eliminate key immune cells or evade host defenses that can act to limit the infection. Alternatively, suppression of the death pathway may facilitate the proliferation of intracellular pathogens.

Single amino acids determine specificity of binding of protein kinase A regulatory subunits by protein kinase A anchoring proteins

Cyclic AMP-dependent protein kinase is tethered to protein kinase A anchoring proteins (AKAPs) through regulatory subunits (R) by RIalpha-specific, RIIalpha-specific, or RIalpha/RIIalpha dual-specific binding. Ala- and Val-scanning mutagenesis determined that hydrophobic amino acids at three homologous positions are required for binding of RIalpha to FSC1/AKAP82 domain B and RIIalpha to AKAP Ht31. A mutation at the middle position reversed the binding specificity of both AKAPs, and mutations at this same position of the dual-specific domain A of FSC1/AKAP82 converted it into either an RIalpha or RIIalpha binding domain. This suggests that hydrophobic amino acids at three conserved positions within the primary sequence and an amphipathic helix of AKAPs are required for cyclic AMP-dependent protein kinase binding, with the size of the aliphatic side chain at the middle position determining RIalpha or RIIalpha binding specificity.

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

bcl-x is a member of the bcl-2 family of genes. The major protein product, Bcl-x(L), is a 233-amino-acid protein which has antiapoptotic properties. In contrast, one of the alternatively spliced transcripts of the bcl-x gene codes for the protein Bcl-x(S), which lacks 63 amino acids present in Bcl-x(L) and has proapoptotic activity. Unlike other proapoptotic Bcl-2 family members, such as Bax and Bak, Bcl-x(S) does not seem to induce cell death in the absence of an additional death signal. However, Bcl-x(S) does interfere with the ability of Bcl-x(L) to antagonize Bax-induced death in transiently transfected 293 cells. Mutational analysis of Bcl-x(S) was conducted to identify the domains necessary to mediate its proapoptotic phenotype. Deletion mutants of Bcl-x(S) which still contained an intact BH3 domain retained the ability to inhibit survival through antagonism of Bcl-x(L). Bcl-x(S) was able to form heterodimers with Bcl-x(L) in mammalian cells, and its ability to inhibit survival correlated with the ability to heterodimerize with Bcl-x(L). Deletion mutants of Bax and Bcl-2, which lacked BH1 and BH2 domains but contained a BH3 domain, were able to antagonize the survival effect conferred by Bcl-x(L). The results suggest that BH3 domains from both pro- and antiapoptotic Bcl-2 family members, while lacking an intrinsic ability to promote programmed cell death, can be potent inhibitors of Bcl-x(L) survival function.

Paradoxical regulation of Bcl-2 family proteins by 17beta-oestradiol in human breast cancer cells MCF-7

Tumorigenesis is related to the dysregulation of cell growth or cell death pathways. Hence, elucidation of the mechanisms involved in the modulation of pro- or anti-apoptotic proteins is important in furthering understanding of breast cancer aetiology and may aid in designing prevention and treatment strategies. In the present study, we examined the role of 17beta-oestradiol on the regulation of apoptosis in the breast cancer cell line MCF-7. Using multi-probe RNAase protection assays, we found changes in the mRNA levels of several Bcl-2 family proteins upon treatment of MCF-7 cells with 17beta-oestradiol. Unexpectedly, we found a paradoxical effects of 17beta-oestradiol on two anti-apoptotic proteins Bcl-2 and Bcl-x. Treatment with 17beta-oestradiol resulted in up-regulation of Bcl-2 mRNA and protein, but down-regulated Bcl-x(L) mRNA and protein. The effect of 17beta-oestradiol on Bcl-x(L) occurred at concentration-dependent fashion. The effect was specific to 17beta-oestradiol since other steroid hormones exert no effect on Bcl-x(L). Tamoxifen, an anti-oestrogen, blocked the down-regulation of Bcl-x(L) by 17beta-oestradiol demonstrating this effect is oestrogen receptor-dependent. We speculate that different members of the Bcl-2 family proteins may be regulated through different pathway and these pathways may be modulated by 17beta-oestradiol.

Bcl-X(L) inhibits apoptosis and necrosis produced by Alzheimer's beta-amyloid1-40 peptide in PC12 cells

Recent studies have shown that neuronal apoptosis induced by the Alzheimer's disease (AD) beta-amyloid peptide (Abeta) is related to alteration of the Bax/Bcl-2 ratio. It has been demonstrated that Bcl-X(L) (Bcl-X(L) = protein, bcl-X(L) = gene), a Bcl-2-related protein, prevents apoptosis in mammalian cells. Additionally, TGF-beta1 is able to protect cultured neuronal cells from Abeta-induced apoptosis via upregulation of bcl-X(L) and bcl-2 gene expression. We show that Abeta treatment (500 nM, freshly solubilized) results in apoptosis and necrosis in differentiated PC12 cells maintained with a low dose of NGF-beta (1 ng/ml). To investigate whether transfection of PC12 cells with bcl-X(L) could block Abeta-induced apoptosis, we transfected these cells with a bcl-X(L) construct (pcDNA-bcl-X(L)). Data show that bcl-X(L) significantly inhibits both early-stage apoptosis and late-stage apoptosis/necrosis produced by Abeta treatment (1000 nM) in pcDNA3-bcl-X(L)-transfected PC12 cells as compared with pcDNA3 vector-transfected PC12 cells. These results suggest that Bcl-X(L) exhibits both anti-necrotic as well as anti-apoptotic roles in Abeta-challenged PC12 cells.

Recent advances on neuronal caspases in development and neurodegeneration

In view of a large and growing literature, this overview emphasizes recent advances in neuronal caspases and their role in cell death. To provide historical perspective, morphology and methods are surveyed with emphasis on early studies on interleukin converting enzyme (ICE) as a prototype for identifying zymogen subunits. The unexpected homology of ICE (caspase-1) to Caenorhabditis elegans death gene CED-3 provided early clues linking caspases to programmed cell death, and led later to discovery of bcl-2 proteins (CED-9 homologs) and 'apoptosis associated factors' (Apafs). Availability of substrates, inhibitors, and cDNAs led to identification of up to 16 caspases as a new superfamily of unique cysteine proteinases targeting Asp groups. Those acting as putative death effectors dismantle neurons by catabolism of proteins essential for survival. Caspases degrade amyloid precursor protein (APP), presenilins (PS1, PS2), tau, and huntingtin, raising questions on their role in neurodegeneration. Brain contains 'inhibitors of apoptosis proteins' (IAPs) survivin and NAIP associated also with some neuronal disorders. Apoptotic stress in neurons initiates a chain of events leading to activation of distal caspases by pathways that remain to be fully mapped. Neuronal caspases play multiple roles for initiation and execution of cell death, for morphogenesis, and in non-mitotic neurons for homeostasis. Recent studies focus on cytochrome c as pivotal in mediating conversion of procaspase-9 as a major initiator for apoptosis. Identifying signaling pathways and related events paves the way to design useful therapeutic remedies to prevent neuronal loss in disease or aging.

Dysregulation of apoptosis in cancer

Each day, approximately 50 to 70 billion cells perish in the average adult because of programmed cell death (PCD). Cell death in self-renewing tissues, such as the skin, gut, and bone marrow, is necessary to make room for the billions of new cells produced daily. So massive is the flux of cells through our bodies that, in a typical year, each of us will produce and, in parallel, eradicate a mass of cells equal to almost our entire body weight. The morphologic ritual cells go through when experiencing PCD has been termed apoptosis and is executed by a family of intracellular proteases, called caspases. Unlike accidental cell deaths caused by infarction and trauma, these physiologic deaths culminate in fragmentation of cells into membrane-encased bodies which are cleared through phagocytosis by neighboring cells without inciting inflammatory reactions or tissue scarring. Defects in the processes controlling PCD can extend cell life span, contributing to neoplastic cell expansion independently of cell division. Moreover, failures in normal apoptosis pathways contribute to carcinogenesis by creating a permissive environment for genetic instability and accumulation of gene mutations, promoting resistance to immune-based destruction, allowing disobeyance of cell cycle checkpoints that would normally induce apoptosis, facilitating growth factor/hormone-independent cell survival, supporting anchorage-independent survival during metastasis, reducing dependence on oxygen and nutrients, and conferring resistance to cytotoxic anticancer drugs and radiation. Elucidation of the genes that constitute the core machinery of the cell death pathway has provided new insights into tumor biology, revealing novel strategies for combating cancer.

A novel BH3-like domain in BID is required for intramolecular interaction and autoinhibition of pro-apoptotic activity

Upon activation of the Fas apoptotic signaling pathway, Bid, a "BH3 domain-only" pro-apoptotic molecule, is cleaved by caspase-8 into a 6.5-kDa N-terminal and a 15-kDa BH3 domain-containing C-terminal fragment, referred to as t(n)-Bid and t(c)-Bid, respectively. t(c)-Bid is a more potent inducer of apoptosis than full-length Bid, suggesting that the N-terminal region of Bid has an inhibitory effect on its pro-apoptotic activity. Here, we report the identification of a novel BH3-like motif (amino acid residues 35-43) in t(n)-Bid. Although Bid does not homodimerize, t(n)-Bid is able to associate avidly with t(c)-Bid. Site-directed mutagenesis revealed that both the novel BH3-like and BH3 domains are necessary for direct binding between t(n)-Bid and t(c)-Bid. While full-length Bid does not associate with t(n)-Bid, substitution of Leu(35), a critical residue in mediating t(n)-Bid/t(c)-Bid interaction, with Ala in full-length Bid is sufficient to establish Bid/t(n)-Bid interaction. Interestingly, the L35A Bid mutant is as effective as t(c)-Bid in inducing apoptosis and binding Bcl-X(L). We propose that the intramolecular interaction involving the BH3-like and BH3 domains serves to regulate the pro-apoptotic potential of Bid.

Bcl-2/E1B 19 kDa-interacting protein 3-like protein (Bnip3L) interacts with bcl-2/Bcl-xL and induces apoptosis by altering mitochondrial membrane permeability

We have previously reported on cloning of the human gene encoding Bcl-2/adenovirus E1B 19 kDa-interacting protein 3-like protein (Bnip3L) and its growth inhibitory effect on cancer cells. Here we show that Bnip3L contains a motif similar to the BH3 domain which is conserved in Bcl-2 family proteins as well as containing a membrane-anchoring domain, and that Bnip3L interacts with Bcl-2 and Bcl-xL. Immunofluorescence microscopy revealed that Bnip3L was localized in the mitochondria, when in the presence of the membrane-anchoring domain. Transient expression of Bnip3L induced apoptosis of Rat-1 and HeLa cells and mutational analysis revealed that the BH3 domain and the membrane-anchoring domain were required for Bnip3L to induce cell death. Addition of recombinant Bnip3L to isolated mitochondria induced membrane potential loss and cytochrome c release both of which have been suggested to be prerequisite for apoptotic cell death. These results suggest that Bnip3L is one of the BH3-containing pro-apoptotic proteins and that it targets the mitochondria when inducing apoptosis.

Ion channel activity of the BH3 only Bcl-2 family member, BID

BID is a member of the BH3-only subgroup of Bcl-2 family proteins that displays pro-apoptotic activity. The NH(2)-terminal region of BID contains a caspase-8 (Casp-8) cleavage site and the cleaved form of BID translocates to mitochondrial membranes where it is a potent inducer of cytochrome c release. Secondary structure and fold predictions suggest that BID has a high degree of alpha-helical content and structural similarity to Bcl-X(L), which itself is highly similar to bacterial pore-forming toxins. Moreover, circular dichroism analysis confirmed a high alpha-helical content of BID. Amino-terminal truncated BIDDelta1-55, mimicking the Casp-8-cleaved molecule, formed channels in planar bilayers at neutral pH and in liposomes at acidic pH. In contrast, full-length BID displayed channel activity only at nonphysiological pH 4.0 (but not at neutral pH) in planar bilayers and failed to form channels in liposomes even under acidic conditions. On a single channel level, BIDDelta1-55 channels were voltage-gated and exhibited multiconductance behavior at neutral pH. When full-length BID was cleaved by Casp-8, it too demonstrated channel activity similar to that seen with BIDDelta1-55. Thus, BID appears to share structural and functional similarity with other Bcl-2 family proteins known to have channel-forming activity, but its activity exhibits a novel form of activation: proteolytic cleavage.

NH2-terminal BH4 domain of Bcl-2 is functional for heterodimerization with Bax and inhibition of apoptosis

The Bcl-2 family proteins comprise pro-apoptotic as well as anti-apoptotic members. Heterodimerization between members of the Bcl-2 family proteins is a key event in the regulation of apoptosis. We report here that Bcl-2 protein was selectively cleaved by active caspase-3-like proteases in CTLL-2 cell apoptosis in response to interleukin-2 deprivation. Structural and functional analyses of the cleaved fragment revealed that the NH2-terminal region of Bcl-2 (1-34 amid acids) was required for its anti-apoptotic activity and heterodimerization with pro-apoptotic Bax protein. Site-directed mutagenesis of the NH2-terminal region showed that substitutions of hydrophobic residues of BH4 domain resulted in the loss of ability to form a heterodimer with Bax. Particularly instructive was that the V15E mutant of Bcl-2, which completely lost the ability to form a heterodimer with Bax, failed to inhibit Bax- and staurosporine-induced apoptosis. Our results suggest that the BH4 domain of Bcl-2 is critical for its heterodimerization with Bax and for exhibiting anti-apoptotic activity. Therefore, agents interferring with the critical residues of the BH4 domain may provide a new strategy in cancer therapy by impairing Bcl-2 function.

A novel apoptotic cascade mediated by CDK4 in rat pheochromocytoma PC12 cells

Apoptosis induced by serum withdrawal in pheochromocytoma PC12 cells is promoted by overexpression of cyclin-dependent kinase 4 (CDK4). We compared CDK4-promoted apoptosis with that induced by serum withdrawal alone in PC12 cells. Protein synthesis inhibitors did not prevent apoptosis in parental cells, but prevented the promotion of apoptosis by CDK4 overexpression. Nerve growth factor, basic-fibroblast growth factor, and Bcl-2 proteins protected both parental and CDK4-overexpressing cells from apoptosis. However, insulin-like growth factor-I and Bcl-X(L) protein only partially inhibited apoptosis in the CDK4-overexpressing cells. Bcl-2 or Bcl-X(L) had no significant effect on CDK4 kinase activity in both cell lines. These results suggest a novel CDK4-mediated apoptotic cascade which is normally restrained, but which is activated by CDK4 overexpression. This apoptotic cascade should eventually converge with the cascade induced by serum withdrawal in normal PC12 cells. We discuss the interactions among these apoptotic cascades and the points where anti-apoptotic agents act.

The central effectors of cell death in the immune system

The immune system relies on cell death to maintain lymphoid homeostasis and avoid disease. Recent evidence has indicated that the caspase family of cysteine proteases is a central effector in apoptotic cell death and is absolutely responsible for many of the morphological features of apoptosis. Cell death, however, can occur through caspase-independent and caspase-dependent pathways. In the case of cells that are irreversibly neglected or damaged, death occurs even in the absence of caspase activity. In contrast, healthy cells require caspase activation to undergo cell death induced by surface receptors. This review summarizes the current understanding of these two pathways of cell death in the immune system.

Analysis of the Bak protein expression in human polymorphonuclear neutrophils

In this study, we investigated the expression of Bak, a member of the Bcl-2 protein family, in human polymorphonuclear neutrophils. Northern blot and Western blot analyses revealed that Bak messenger RNA and protein were constitutively expressed in peripheral polymorphonuclear neutrophils and mononuclear cells, as well as in several hematopoietic cell lines. Remarkably, culturing neutrophils for 24 h in the presence or absence of interferon-gamma or tumor necrosis factor-alpha, which have been described to modulate the survival rate of these cells, did not influence the expression of antigenic Bak. Taken together, our data indicate that the expression of the pro-apoptotic protein Bak in polymorphonuclear neutrophils is constitutive, is not subject to modulation, and does not correlate with the neutrophil life span in culture.

Btf, a novel death-promoting transcriptional repressor that interacts with Bcl-2-related proteins

The adenovirus E1B 19,000-molecular-weight (19K) protein is a potent inhibitor of apoptosis and cooperates with E1A to transform primary rodent cells. E1B 19K shows sequence and functional homology to the mammalian antiapoptotic gene product, Bcl-2. Like Bcl-2, the biochemical mechanism of E1B 19K function includes binding to and antagonization of cellular proapoptotic proteins such as Bax, Bak, and Nbk/Bik. In addition, there is evidence that E1B 19K can affect gene expression, but whether this contributes to its antiapoptotic function has not been determined. In an effort to further understand the functions of E1B 19K, we screened for 19K-associated proteins by the yeast two-hybrid system. A novel protein, Btf (Bcl-2-associated transcription factor), that interacts with E1B 19K as well as with the antiapoptotic family members Bcl-2 and Bcl-xL but not with the proapoptotic protein Bax was identified. btf is a widely expressed gene that encodes a protein with homology to the basic zipper (bZip) and Myb DNA binding domains. Btf binds DNA in vitro and represses transcription in reporter assays. E1B 19K, Bcl-2, and Bcl-xL sequester Btf in the cytoplasm and block its transcriptional repression activity. Expression of Btf also inhibited transformation by E1A with either E1B 19K or mutant p53, suggesting a role in either promotion of apoptosis or cell cycle arrest. Indeed, the sustained overexpression of Btf in HeLa cells induced apoptosis, which was inhibited by E1B 19K. Furthermore, the chromosomal localization of btf (6q22-23) maps to a region that is deleted in some cancers, consistent with a role for Btf in tumor suppression. Thus, btf may represent a novel tumor suppressor gene residing in a unique pathway by which the Bcl-2 family can regulate apoptosis.

Bak BH3 peptides antagonize Bcl-xL function and induce apoptosis through cytochrome c-independent activation of caspases

The Bcl-2 homology 3 (BH3) domain is crucial for the death-inducing and dimerization properties of pro-apoptotic members of the Bcl-2 protein family, including Bak, Bax, and Bad. Here we report that synthetic peptides corresponding to the BH3 domain of Bak bind to Bcl-xL, antagonize its anti-apoptotic function, and rapidly induce apoptosis when delivered into intact cells via fusion to the Antennapedia homeoprotein internalization domain. Treatment of HeLa cells with the Antennapedia-BH3 fusion peptide resulted in peptide internalization and induction of apoptosis within 2-3 h, as indicated by caspase activation and subsequent poly(ADP-ribose) polymerase cleavage, as well as morphological characteristics of apoptosis. A point mutation within the BH3 peptide that blocks its ability to bind to Bcl-xL abolished its apoptotic activity, suggesting that interaction of the BH3 peptide with Bcl-2-related death suppressors, such as Bcl-xL, may be critical for its activity in cells. While overexpression of Bcl-xL can block BH3-induced apoptosis, treatment with BH3 peptides resensitized Bcl-xL-expressing cells to Fas-mediated apoptosis. BH3-induced apoptosis was blocked by caspase inhibitors, demonstrating a dependence on caspase activation, but was not accompanied by a dramatic early loss of mitochondrial membrane potential or detectable translocation of cytochrome c from mitochondria to cytosol. These findings demonstrate that the BH3 domain itself is capable of inducing apoptosis in whole cells, possibly by antagonizing the function of Bcl-2-related death suppressors.

Expression of Bcl-2 and its homologues in human eosinophils. Modulation by interleukin-5

The Bcl-2 family has been shown to be vital regulators of programmed cell death in numerous systems. To investigate the role of such proteins in the regulation of apoptosis of eosinophils, the expression of Bcl-2 and homologues Bcl-xL (death antagonists), Bax, and Bcl-xS (death agonists) were examined by immunoblot, flow cytometry, and reverse transcriptase-polymerase chain reaction analysis. Potential modulation of apoptosis-associated molecules during spontaneous apoptosis and in the presence of interleukin (IL)-5 was also investigated. Peripheral blood eosinophils were found to express constitutively Bax and Bcl-x, but Bcl-2 was absent. Analysis of mRNA revealed that the bcl-xL isoform predominated, although bcl-xS was also detectable. Spontaneous apoptosis due to culturing in the absence of cytokines for 24 h did not result in modulation of any of the Bcl-2 homologues examined. Culturing eosinophils in the presence of 100 pg/ml IL-5 for 24 h significantly reduced apoptosis (P < 0.01) to 10.7 +/- 2.6% compared with 46.8 +/- 7.4% in the absence of IL-5, and induced Bcl-2 mRNA and protein expression, with no detectable change in Bax, Bcl-x, or beta-actin as a control. This investigation indicates a specific profile of apoptotic molecules in eosinophils distinct from that of neutrophils, and indicates that survival-enhancing IL-5 modulates the expression of Bcl-2 in vitro.

Phosphorylation and inactivation of BAD by mitochondria-anchored protein kinase A

Signaling pathways between cell surface receptors and the BCL-2 family of proteins regulate cell death. Survival factors induce the phosphorylation and inactivation of BAD, a proapoptotic member. Purification of BAD kinase(s) identified membrane-based cAMP-dependent protein kinase (PKA) as a BAD Ser-112 (S112) site-specific kinase. PKA-specific inhibitors blocked the IL-3-induced phosphorylation on S112 of endogenous BAD as well as mitochondria-based BAD S112 kinase activity. A blocking peptide that disrupts type II PKA holoenzyme association with A-kinase-anchoring proteins (AKAPs) also inhibited BAD phosphorylation and eliminated the BAD S112 kinase activity at mitochondria. Thus, the anchoring of PKA to mitochondria represents a focused subcellular kinase/substrate interaction that inactivates BAD at its target organelle in response to a survival factor.

Bid-induced conformational change of Bax is responsible for mitochondrial cytochrome c release during apoptosis

Here we report that in staurosporine-induced apoptosis of HeLa cells, Bid, a BH3 domain containing protein, translocates from the cytosol to mitochondria. This event is associated with a change in conformation of Bax which leads to the unmasking of its NH2-terminal domain and is accompanied by the release of cytochrome c from mitochondria. A similar finding is reported for cerebellar granule cells undergoing apoptosis induced by serum and potassium deprivation. The Bax-conformational change is prevented by Bcl-2 and Bcl-xL but not by caspase inhibitors. Using isolated mitochondria and various BH3 mutants of Bid, we demonstrate that direct binding of Bid to Bax is a prerequisite for Bax structural change and cytochrome c release. Bcl-xL can inhibit the effect of Bid by interacting directly with Bax. Moreover, using mitochondria from Bax-deficient tumor cell lines, we show that Bid- induced release of cytochrome c is negligible when Bid is added alone, but dramatically increased when Bid and Bax are added together. Taken together, our results suggest that, during certain types of apoptosis, Bid translocates to mitochondria and binds to Bax, leading to a change in conformation of Bax and to cytochrome c release from mitochondria.

Cell damage-induced conformational changes of the pro-apoptotic protein Bak in vivo precede the onset of apoptosis

Investigation of events committing cells to death revealed that a concealed NH2-terminal epitope of the pro-apoptotic protein Bak became exposed in vivo before apoptosis. This occurred after treatment of human Jurkat or CEM-C7A T-lymphoma cells with the mechanistically disparate agents staurosporine, etoposide or dexamethasone. The rapid, up to 10-fold increase in Bak-associated immunofluorescence was measured with epitope-specific monoclonal antibodies using flow cytometry and microscopy. In contrast, using a polyclonal antibody to Bak, immunofluorescence was detected both before and after treatment. There were no differences in Bak protein content nor in subcellular location before or after treatment. Immunofluorescence showed Bcl-xL and Bak were largely associated with mitochondria and in untreated cells they coimmunoprecipitated in the presence of nonioinic detergent. This association was significantly decreased after cell perturbation suggesting that Bcl-xL dissociation from Bak occurred on exposure of Bak's NH2 terminus. Multiple forms of Bak protein were observed by two dimensional electrophoresis but these were unchanged by inducers of apoptosis. This indicated that integration of cellular damage signals did not take place directly on the Bak protein. Release of proteins, including Bcl-xL, from Bak is suggested to be an important event in commitment to death.

Solution structure of the proapoptotic molecule BID: a structural basis for apoptotic agonists and antagonists

Members of the BCL2 family of proteins are key regulators of programmed cell death, acting either as apoptotic agonists or antagonists. Here we describe the solution structure of BID, presenting the structure of a proapoptotic BCL2 family member. An analysis of sequence/structure of BCL2 family members allows us to define a structural superfamily, which has implications for general mechanisms for regulating proapoptotic activity. It appears two criteria must be met for proapoptotic function within the BCL2 family: targeting of molecules to intracellular membranes, and exposure of the BH3 death domain. BID's activity is regulated by a Caspase 8-mediated cleavage event, exposing the BH3 domain and significantly changing the surface charge and hydrophobicity, resulting in a change of cellular localization.

Solution structure of BID, an intracellular amplifier of apoptotic signaling

We report the solution structure of BID, an intracellular cross-talk agent that can amplify FAS/TNF apoptotic signal through the mitochondria death pathway after Caspase 8 cleavage. BID contains eight alpha helices where two central hydrophobic helices are surrounded by six amphipathic ones. The fold resembles poreforming bacterial toxins and shows similarity to BCL-XL although sequence homology to BCL-XL is limited to the 16-residue BH3 domain. Furthermore, we modeled a complex of BCL-XL and BID by aligning the BID and BAK BH3 motifs in the known BCL-XL-BAK BH3 complex. Additionally, we show that the overall structure of BID is preserved after cleavage by Caspase 8. We propose that BID has both BH3 domain-dependent and -independent modes of action in inducing mitochondrial damage.

Bcl-xL regulates apoptosis by heterodimerization-dependent and -independent mechanisms

A hydrophobic cleft formed by the BH1, BH2 and BH3 domains of Bcl-xL is responsible for interactions between Bcl-xL and BH3-containing death agonists. Mutants were constructed which did not bind to Bax but retained anti-apoptotic activity. Since Bcl-xL can form an ion channel in synthetic lipid membranes, the possibility that this property has a role in heterodimerization-independent cell survival was tested by replacing amino acids within the predicted channel-forming domain with the corresponding amino acids from Bax. The resulting chimera showed a reduced ability to adopt an open conductance state over a wide range of membrane potentials. Although this construct retained the ability to heterodimerize with Bax and to inhibit apoptosis, when a mutation was introduced that rendered the chimera incapable of heterodimerization, the resulting protein failed to prevent both apoptosis in mammalian cells and Bax-mediated growth defect in yeast. Similar to mammalian cells undergoing apoptosis, yeast cells expressing Bax exhibited changes in mitochondrial properties that were inhibited by Bcl-xL through heterodimerization-dependent and -independent mechanisms. These data suggest that Bcl-xL regulates cell survival by at least two distinct mechanisms; one is associated with heterodimerization and the other with the ability to form a sustained ion channel.

Identification and characterization of baxepsilon, a novel bax variant missing the BH2 and the transmembrane domains

The Bax gene is a member of the Bcl2 family that functions to regulate the programmed cell death process. A number of Bax isoforms have been previously identified: alpha, beta, gamma, delta, and omega. Here we report the identification and characterization of an additional Bax variant, termed Baxepsilon. The newly identified Bax variant contains a 97-base insertion generated by alternative splicing which includes a previously unidentified exon between exons 4 and 5. The insertion causes the production of a truncated Bax protein, termed Baxepsilon, which encodes a protein of 164 residues with a calculated molecular weight of 18 kDa. The last 69 amino acids of Baxalpha that encompass the BH2 and the TM domains are missing in Baxepsilon. The Baxepsilon protein, when expressed as a GST fusion protein, associated efficiently with Baxalpha, Baxepsilon, Bcl2, and Bcl-xL. In addition, Baxepsilon was active in inducing apoptosis when tested in a transient transfection assay. Furthermore, the presence of antiapoptotic genes including Bcl2, Bcl-xL, and baculovirus p35 abrogated Baxepsilon and Baxalpha function. Although the newly identified Bax variant was detectable by RT-PCR in several normal mouse tissues, the role of this variant in controlling programmed cell death is currently unknown.

The structure of vitellogenin provides a molecular model for the assembly and secretion of atherogenic lipoproteins

The assembly of atherogenic lipoproteins requires the formation in the endoplasmic reticulum of a complex between apolipoprotein (apo)B, a microsomal triglyceride transfer protein (MTP) and protein disulphide isomerase (PDI). Here we show by molecular modelling and mutagenesis that the globular amino-terminal regions of apoB and MTP are closely related in structure to the ancient egg yolk storage protein, vitellogenin (VTG). In the MTP complex, conserved structural motifs that form the reciprocal homodimerization interfaces in VTG are re-utilized by MTP to form a stable heterodimer with PDI, which anchors MTP at the site of apoB translocation, and to associate with apoB and initiate lipid transfer. The structural and functional evolution of the VTGs provides a unifying scheme for the invertebrate origins of the major vertebrate lipid transport system.

Caspase cleaved BID targets mitochondria and is required for cytochrome c release, while BCL-XL prevents this release but not tumor necrosis factor-R1/Fas death

"BH3 domain only" members of the BCL-2 family including the pro-apoptotic molecule BID represent candidates to connect with proximal signal transduction. Tumor necrosis factor alpha (TNFalpha) treatment induced a caspase-mediated cleavage of cytosolic, inactive p22 BID at internal Asp sites to yield a major p15 and minor p13 and p11 fragments. p15 BID translocates to mitochondria as an integral membrane protein. p15 BID within cytosol targeted normal mitochondria and released cytochrome c. Immunodepletion of p15 BID prevents cytochrome c release. In vivo, anti-Fas Ab results in the appearance of p15 BID in the cytosol of hepatocytes which translocates to mitochondria where it releases cytochrome c. Addition of activated caspase-8 to normal cytosol generates p15 BID which is also required in this system for release of cytochrome c. In the presence of BCL-XL/BCL-2, TNFalpha still induced BID cleavage and p15 BID became an integral mitochondrial membrane protein. However, BCL-XL/BCL-2 prevented the release of cytochrome c, yet other aspects of mitochondrial dysfunction still transpired and cells died nonetheless. Thus, while BID appears to be required for the release of cytochrome c in the TNF death pathway, the release of cytochrome c may not be required for cell death.

Interleukin-3 induces the phosphorylation of a distinct fraction of bcl-2

Bcl-2-related proteins (i.e. Bcl-2 and Bax) regulate the effector stage of apoptosis and can modulate the entry of quiescent cells into the cell cycle. Phosphorylation of Bcl-2 is presumed to modify its apoptosis-inhibitory function. By utilizing an interleukin-3 (IL-3)-dependent hematopoietic cell line, we examined the structural requirements of Bcl-2 phosphorylation and the correlation of this post-translational modification with its function. In the presence of IL-3, constitutively expressed Bcl-2 was phosphorylated on serine residue(s), and phosphorylated Bcl-2 lost its capacity to heterodimerize with Bax. Whereas the majority of Bcl-2 resided in mitochondria, phosphorylation only affected a minor pool of total Bcl-2 that selectively partitioned into a soluble fraction. Cytosolic targeting of Bcl-2 greatly increased its ratio of phosphorylation. Bcl-2 phosphorylation was reduced during IL-3 deprivation, and its phosphorylation was also delayed after transient cytokine deprivation. This pattern of phosphorylation temporally correlated with the accelerated exit and delayed reentry of Bcl-2-expressing cells into the cell cycle upon transient IL-3 deprivation and subsequent cytokine restimulation. Thus, IL-3-induced phosphorylation of a distinct pool of Bcl-2 may contribute to the inactivation of its antiproliferative function.

Boo, a novel negative regulator of cell death, interacts with Apaf-1

In this report, we describe the cloning and characterization of Boo, a novel anti-apoptotic member of the Bcl-2 family. The expression of Boo was highly restricted to the ovary and epididymis implicating it in the control of ovarian atresia and sperm maturation. Boo contains the conserved BH1 and BH2 domains, but lacks the BH3 motif. Like Bcl-2, Boo possesses a hydrophobic C-terminus and localizes to intracellular membranes. Boo also has an N-terminal region with strong homology to the BH4 domain found to be important for the function of some anti-apoptotic Bcl-2 homologues. Chromosomal localization analysis assigned Boo to murine chromosome 9 at band d9. Boo inhibits apoptosis, homodimerizes or heterodimerizes with some death-promoting and -suppressing Bcl-2 family members. More importantly, Boo interacts with Apaf-1 and forms a multimeric protein complex with Apaf-1 and caspase-9. Bak and Bik, two pro-apoptotic homologues disrupt the association of Boo and Apaf-1. Furthermore, Boo binds to three distinct regions of Apaf-1. These results demonstrate the evolutionarily conserved nature of the mechanisms of apoptosis. Like Ced-9, the mammalian homologues Boo and Bcl-xL interact with the human counterpart of Ced-4, Apaf-1, and thereby regulate apoptosis.

Apoptosis-associated proteins and the development of oral squamous cell carcinoma

Expression of apoptosis-associated proteins was evaluated in premalignant and malignant oral epithelial lesions, to test the hypothesis that protein regulation of apoptosis may be altered in the development of oral squamous cell carcinoma. Ninety archived paraffin-embedded specimens from 25 patients (two or more sequential biopsies each) and eight control specimens were evaluated in immunohistochemically stained sections for tumor suppressor protein p53, p53 binding protein mdm-2, and apoptosis regulatory proteins Bcl-2, Bcl-X, Bax, and Bak. The initial histologic diagnosis for 17/25 patients was either focal keratosis, mild dysplasia, or moderate dysplasia; the initial diagnosis for the remaining eight patients ranged from severe dysplasia to moderately differentiated squamous cell carcinoma. Thirty of 90 specimens showed positive p53 expression, nine of which were dysplasias. In patients with one or more lesions displaying p53 expression, there was increased intensity of staining with disease progression. Bak was expressed in 57/90 specimens, including 27 dysplasias of various grades. There was also a significantly increased intensity of Bak staining with disease progression, which did not appear to be dependent upon p53 status. Bcl-X was expressed in 73/90 specimens, with staining displayed earlier in premalignant lesions than either p53 or Bak. Ten of 90 specimens were positive for Bcl-2 (all were dysplasias or carcinomas), and only 2/90 specimens were positive for Bax. Eleven of 90 specimens were positive for mdm-2; six of which were also positive for p53. These data show that apoptosis-associated proteins are altered in variable patterns in both premalignant and malignant oral epithelial lesions. p53 and especially Bak and Bcl-X are expressed early; Bax is largely absent; and Bcl-2 and mdm-2 show sporadic expression in the development of oral premalignant and malignant disease.

Caenorhabditis elegans EGL-1 disrupts the interaction of CED-9 with CED-4 and promotes CED-3 activation

In the nematode Caenorhabditis elegans, programmed cell death is implemented by the protease CED-3 whose activity is inhibited by CED-9 through physical associations with the regulator CED-4. The product of a recently described gene, egl-1, binds to and inhibits CED-9. In the present studies, we have addressed the molecular mechanism by which EGL-1 regulates CED-9 function and promotes cell death. Expression of CED-4 and CED-3 resulted in decreased survival and apoptosis of mammalian cells, activities that could be inhibited by CED-9. Importantly, this protective effect of CED-9 was antagonized by EGL-1. Immunoprecipitation analysis showed that EGL-1 binding to CED-9 disrupts the association between CED-4 and CED-9, an activity that required the BH3 motif of EGL-1. Consistent with these results, expression of EGL-1 promoted CED-4-dependent processing of CED-3, and this activity of EGL-1 was mediated through inhibition of CED-9. In mammalian cells, CED-9 is known to target the subcellular localization of CED-4 from the cytosol to intracellular membranes. Expression of EGL-1 resulted in redistribution of CED-4 from intracellular membranes, where it co-localized with CED-9, to the cytoplasm, providing further evidence that EGL-1 regulates CED-4 through CED-9. Finally, the levels of EGL-1 were greatly enhanced by co-expression of CED-9 in both mammalian cells and in a cell-free system, suggesting a role for CED-9 in the expression and/or stabilization of EGL-1. These studies provide a mechanism for how EGL-1 functions to antagonize pro-survival of CED-9 and to promote CED-3 activation and programmed cell death.

Bax interacts with the permeability transition pore to induce permeability transition and cytochrome c release in isolated mitochondria

Cytochrome c release and the mitochondrial permeability transition (PT), including loss of the transmembrane potential (Deltapsi), play an important role in apoptosis. Using isolated mitochondria, we found that recombinant Bax and Bak, proapoptotic members of the Bcl-2 family, induced mitochondrial Deltapsi loss, swelling, and cytochrome c release. All of these changes were dependent on Ca2+ and were prevented by cyclosporin A (CsA) and bongkrekic acid, both of which close the PT pores (megachannels), indicating that Bax- and Bak-induced mitochondrial changes were mediated through the opening of these pores. Bax-induced mitochondrial changes were inhibited by recombinant Bcl-xL and transgene-derived Bcl-2, antiapoptotic members of the Bcl-2 family, as well as by oligomycin, suggesting a possible regulatory effect of F0F1-ATPase on Bax-induced mitochondrial changes. Proapoptotic Bax- and Bak-BH3 (Bcl-2 homology) peptides, but not a mutant BH3 peptide nor a mutant Bak lacking BH3, induced the mitochondrial changes, indicating an essential role of the BH3 region. A coimmunoprecipitation study revealed that Bax and Bak interacted with the voltage-dependent anion channel, which is a component of PT pores. Taken together, these findings suggest that proapoptotic Bcl-2 family proteins, including Bax and Bak, induce the mitochondrial PT and cytochrome c release by interacting with the PT pores.

Diva, a Bcl-2 homologue that binds directly to Apaf-1 and induces BH3-independent cell death

We have identified and characterized Diva, which is a novel regulator of apoptosis. Sequence analysis revealed that Diva is a member of the Bcl-2 family of proteins containing Bcl-2 homology domain 1, 2, 3, and 4 (BH1, BH2, BH3, and BH4) regions and a carboxyl-terminal hydrophobic domain. The expression of Diva mRNA was detected in multiple embryonic tissues but was restricted to the ovary and testis in adult mice. The expression of Diva promoted the death of 293T, Ramsey, and T47D cells as well as that of primary sensory neurons, indicating that Diva is a proapoptotic protein. Significantly, Diva lacks critical residues in the conserved BH3 region that mediate the interaction between BH3-containing proapoptotic Bcl-2 homologues and their prosurvival binding partners. Consistent with this, Diva did not bind to cellular Bcl-2 family members including Bcl-2, Bcl-XL, Bcl-w, Mcl-1, and A1/Bfl-1. Furthermore, mutants of Diva lacking the BH3 region fully retained their proapoptotic activity, confirming that Diva promotes apoptosis in a BH3-independent manner. Significantly, Diva interacted with a viral Bcl-2 homologue (vBcl-2) encoded by the Kaposi's sarcoma-associated herpesvirus. Consistent with these associations, apoptosis induced by Diva was inhibited by vBcl-2 but not by Bcl-XL. Importantly, Diva interacted with Apaf-1, an adapter molecule that activates caspase-9, a central death protease of the apoptotic pathway. The expression of Diva inhibited the binding of Bcl-XL to Apaf-1, as determined by immunoprecipitation assays. Thus, Diva represents a novel type of proapoptotic Bcl-2 homologue that promotes apoptosis independently of the BH3 region through direct binding to Apaf-1, thus preventing Bcl-XL from binding to the caspase-9 regulator Apaf-1.

Cytoprotection by Bcl-2 requires the pore-forming alpha5 and alpha6 helices

We explored whether the putative channel-forming fifth and sixth alpha-helices of Bcl-2 and Bax account for Bcl-2-mediated cell survival and Bax-induced cell death in mammalian cells and in the yeast Saccharomyces cerevisiae. When alpha5-alpha6 were either deleted or swapped with each other, the Bcl-2Deltaalpha5alpha6 deletion mutant and Bcl-2-Bax(alpha5alpha6) chimeric protein failed to block apoptosis induced by either Bax or staurosporine in human cells and were unable to prevent Bax-induced cell death in yeast, implying that the alpha5-alpha6 region of Bcl-2 is essential for its cytoprotective function. Additional experiments indicated that, although alpha5-alpha6 is necessary, it is also insufficient for the anti-apoptotic activity of Bcl-2. In contrast, deletion or substitution of alpha5-alpha6 in Bax reduced but did not abrogate apoptosis induction in human cells, whereas it did completely nullify cytotoxic activity in yeast, implying that the pore-forming segments of Bax are critical for conferring a lethal phenotype in yeast but not necessarily in human cells. BaxDeltaalpha5alpha6 and Bax-Bcl-2(alpha5alpha6) also retained the ability to dimerize with Bcl-2. Bax therefore may have redundant mechanisms for inducing apoptosis in mammalian cells, based on its ability to form alpha5-alpha6-dependent channels in membranes and to dimerize with and antagonize anti-apoptotic proteins such as Bcl-2.

A splicing variant of the Bcl-2 member Bok with a truncated BH3 domain induces apoptosis but does not dimerize with antiapoptotic Bcl-2 proteins in vitro

Bok (Bcl-2-related ovarian killer) is a proapoptotic Bcl-2 family protein identified in the ovary based on its dimerization with the antiapoptotic protein Mcl-1. In addition to the Bcl-2 homology (BH) domains 1 and 2 and the transmembrane sequence, Bok also has a BH3 domain believed to be important for dimerization with selective antiapoptotic Bcl-2 proteins and cell killing. We identified a splicing variant of Bok mRNA with a deletion of 43 residues from the full-length protein (Bok-L), leading to the fusion of the N-terminal-half of its BH3 domain to the C-terminal-half of the BH1 domain. Genomic analysis indicated that the Bok has five exons, and the short form of Bok (Bok-S) represents the splicing out of exon three during transcription. Although Bok-S retains the apoptosis-inducing activity in transfected cells, it has lost the ability to dimerize with antiapoptotic proteins in vitro. Additional BH3 domain mutations of Bok-L also led to defective heterodimerization without affecting its proapoptotic action. Furthermore, similar deletions for the related channel-forming proapoptotic Bax and Bak did not impair their cell killing ability. Thus, the naturally occurring Bok-S variant represents a new form of proapoptotic protein that induces cell killing without heterodimerization with antiapoptotic Bcl-2 proteins. This variant appears to contain the minimal module spanning BH1 and BH2 domains and the transmembrane sequence for apoptosis induction by channel-forming Bcl-2 proteins.

Role of Bcl-2 family proteins in apoptosis: apoptosomes or mitochondria?

Apoptosis is an essential physiological process for the selective elimination of cells, which is involved in a variety of biological events. The Bcl-2 family is the best characterized protein family involved in the regulation of apoptotic cell death, consisting of anti-apoptotic and pro-apoptotic members. The anti-apoptotic members of this family, such as Bcl-2 and Bcl-XL, prevent apoptosis either by sequestering proforms of death-driving cysteine proteases called caspases (a complex called the apoptosome) or by preventing the release of mitochondrial apoptogenic factors such as cytochrome c and AIF (apoptosis-inducing factor) into the cytoplasm. After entering the cytoplasm, cytochrome c and AIF directly activate caspases that cleave a set of cellular proteins to cause apoptotic changes. In contrast, pro-apoptotic members of this family, such as Bax and Bak, trigger the release of caspases from death antagonists via heterodimerization and also by inducing the release of mitochondrial apoptogenic factors into the cytoplasm via acting on mitochondrial permeability transition pore, thereby leading to caspase activation. Thus, the Bcl-2 family of proteins acts as a critical life-death decision point within the common pathway of apoptosis.

The Bcl-2 gene family and apoptosis

Apoptosis, or programmed cell death, is an essential process for normal embryonic development, maintaining homeostasis in adult tissues, and suppressing carcinogenesis. The bcl-2 protein, discovered in association with follicular lymphoma, plays a prominent role in controlling apoptosis and enhancing cell survival in response to diverse apoptotic stimuli. The evolutionarily conserved bcl-2 protein is now recognized as being a member of a family of related proteins which can be categorized as death agonists or death antagonists. Progress in defining the role of bcl-2 and its family members in regulating apoptosis is rapidly advancing. This review describes, in detail, current bcl-2 family members and the possible mechanisms of function which allow the bcl-2 family of proteins to either promote or suppress cell death.

E1B 19,000-molecular-weight protein interacts with and inhibits CED-4-dependent, FLICE-mediated apoptosis

Genetic studies of the nematode Caenorhabditis elegans (C. elegans) have identified several important components of the cell death pathway, most notably CED-3, CED-4, and CED-9. CED-4 directly interacts with the Bcl-2 homologue CED-9 (or the mammalian Bcl-2 family member Bcl-xL) and the caspase CED-3 (or the mammalian caspases ICE and FLICE). This trimolecular complex of CED-4, CED-3, and CED-9 is functional in that CED-9 inhibits CED-4 from activating CED-3 and thereby inhibits apoptosis in heterologous systems. The E1B 19,000-molecular weight protein (E1B 19K) is a potent apoptosis inhibitor and the adenovirus homologue of Bcl-2-related apoptosis inhibitors. Since E1B 19K and Bcl-xL have functional similarity, we determined if E1B 19K interacts with CED-4 and regulates CED-4-dependent caspase activation. Binding analysis indicated that E1B 19K interacts with CED-4 in a Saccharomyces cerevisiae two-hybrid assay, in vitro, and in mammalian cell lysates. The subcellular localization pattern of CED-4 was dramatically changed by E1B 19K, supporting the theory of a functional interaction between CED-4 and E1B 19K. Whereas expression of CED-4 alone could not induce cell death, coexpression of CED-4 and FLICE augmented cell death induction by FLICE, which was blocked by expression of E1B 19K. Even though E1B 19K did not prevent FLICE-induced apoptosis, it did inhibit CED-4-dependent, FLICE-mediated apoptosis, which suggested that CED-4 was required for E1B 19K to block FLICE activation. Thus, E1B 19K functions through interacting with CED-4, and presumably a mammalian homologue of CED-4, to inhibit caspase activation and apoptosis.