Structural and functional complementation of an inactive Bcl-2 mutant by Bax truncation

The C-terminal sequences of Bcl-2 family proteins mediate interactions that regulate cell death

Programmed cell death via the both intrinsic and extrinsic pathways is regulated by interactions of the Bcl-2 family protein members that determine whether the cell commits to apoptosis via mitochondrial outer membrane permeabilization (MOMP). Recently the conserved C-terminal sequences (CTSs) that mediate localization of Bcl-2 family proteins to intracellular membranes, have been shown to have additional protein-protein binding functions that contribute to the functions of these proteins in regulating MOMP. Here we review the pivotal role of CTSs in Bcl-2 family interactions including: (1) homotypic interactions between the pro-apoptotic executioner proteins that cause MOMP, (2) heterotypic interactions between pro-apoptotic and anti-apoptotic proteins that prevent MOMP, and (3) heterotypic interactions between the pro-apoptotic executioner proteins and the pro-apoptotic direct activator proteins that promote MOMP.

Yeast Bax Inhibitor (Bxi1p/Ybh3p) Is Not Required for the Action of Bcl-2 Family Proteins on Cell Viability

Permeabilization of mitochondrial membrane by proteins of the BCL-2 family is a key decisive event in the induction of apoptosis in mammalian cells. Although yeast does not have homologs of the BCL-2 family, when these are expressed in yeast, they modulate the survival of cells in a way that corresponds to their activity in mammalian cells. The yeast gene, alternatively referred to as BXI1 or YBH3, encodes for membrane protein in the endoplasmic reticulum that was, contradictorily, shown to either inhibit Bax or to be required for Bax activity. We have tested the effect of the deletion of this gene on the pro-apoptotic activity of Bax and Bak and the anti-apoptotic activity of Bcl-XL and Bcl-2, as well on survival after treatment with inducers of regulated cell death in yeast, hydrogen peroxide and acetic acid. While deletion resulted in increased sensitivity to acetic acid, it did not affect the sensitivity to hydrogen peroxide nor to BCL-2 family members. Thus, our results do not support any model in which the activity of BCL-2 family members is directly affected by BXI1 but rather indicate that it may participate in modulating survival in response to some specific forms of stress.

Bax mitochondrial relocation is linked to its phosphorylation and its interaction with Bcl-xL

The heterologous expression of Bax, and other Bcl-2 family members, in the yeast Saccharomyces cerevisiae, has proved to be a valuable reporter system to investigate the molecular mechanisms underlying their interaction with mitochondria. By combining the co-expression of Bax and Bcl-xL mutants with analyzes of their localization and interaction in mitochondria and post-mitochondrial supernatants, we showed that the ability of Bax and Bcl-xL to interact is dependent both on Bax phosphorylation - mimicked by a substitution S184D - and by Bax and Bcl-xL localization. This, and previous data, provide the molecular basis for a model of dynamic equilibrium for Bax localization and activation, regulated both by phosphorylation and Bcl-xL.

A brewing understanding of the regulation of Bax function by Bcl-xL and Bcl-2

Bcl-2 family members form a network of protein-protein interactions that regulate apoptosis through permeabilization of the mitochondrial outer membrane. Deciphering this intricate network requires streamlined experimental models, including the heterologous expression in yeast. This approach had previously enabled researchers to identify domains and residues that underlie the conformational changes driving the translocation, the insertion and the oligomerization of the pro-apoptotic protein Bax at the level of the mitochondrial outer membrane. Recent studies that combine experiments in yeast and in mammalian cells have shown the unexpected effect of the anti-apoptotic protein Bcl-xL on the priming of Bax. As demonstrated with the BH3-mimetic molecule ABT-737, this property of Bcl-xL, and of Bcl-2, is crucial to elaborate about how apoptosis could be reactivated in tumoral cells.

After embedding in membranes antiapoptotic Bcl-XL protein binds both Bcl-2 homology region 3 and helix 1 of proapoptotic Bax protein to inhibit apoptotic mitochondrial permeabilization

Bcl-XL binds to Bax, inhibiting Bax oligomerization required for mitochondrial outer membrane permeabilization (MOMP) during apoptosis. How Bcl-XL binds to Bax in the membrane is not known. Here, we investigated the structural organization of Bcl-XL·Bax complexes formed in the MOM, including the binding interface and membrane topology, using site-specific cross-linking, compartment-specific labeling, and computational modeling. We found that one heterodimer interface is formed by a specific interaction between the Bcl-2 homology 1-3 (BH1-3) groove of Bcl-XL and the BH3 helix of Bax, as defined previously by the crystal structure of a truncated Bcl-XL protein and a Bax BH3 peptide (Protein Data Bank entry 3PL7). We also discovered a novel interface in the heterodimer formed by equivalent interactions between the helix 1 regions of Bcl-XL and Bax when their helical axes are oriented either in parallel or antiparallel. The two interfaces are located on the cytosolic side of the MOM, whereas helix 9 of Bcl-XL is embedded in the membrane together with helices 5, 6, and 9 of Bax. Formation of the helix 1·helix 1 interface partially depends on the formation of the groove·BH3 interface because point mutations in the latter interface and the addition of ABT-737, a groove-binding BH3 mimetic, blocked the formation of both interfaces. The mutations and ABT-737 also prevented Bcl-XL from inhibiting Bax oligomerization and subsequent MOMP, suggesting that the structural organization in which interactions at both interfaces contribute to the overall stability and functionality of the complex represents antiapoptotic Bcl-XL·Bax complexes in the MOM.

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

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

Bcl-2 and Bax interact via the BH1-3 groove-BH3 motif interface and a novel interface involving the BH4 motif

The interaction of Bcl-2 family proteins at the mitochondrial outer membrane controls membrane permeability and thereby the apoptotic program. The anti-apoptotic protein Bcl-2 binds to the pro-apoptotic protein Bax to prevent Bax homo-oligomerization required for membrane permeabilization. Here, we used site-specific photocross-linking to map the surfaces of Bax and Bcl-2 that interact in the hetero-complex formed in a Triton X-100 micelle as a membrane surrogate. Heterodimer-specific photoadducts were detected from multiple sites in Bax and Bcl-2. Many of the interaction sites are located in the Bcl-2 homology 3 (BH3) region of Bax and the BH1-3 groove of Bcl-2 that likely form the BH3-BH1-3 groove interface. However, other interaction sites form a second interface that includes helix 6 of Bax and the BH4 region of Bcl-2. Loss-of-function mutations in the BH3 region of Bax and the BH1 region of Bcl-2 disrupted the BH3-BH1-3 interface, as expected. Surprisingly the second interface was also disrupted by these mutations. Similarly, a loss-of-function mutation in the BH4 region of Bcl-2 that forms part of the second interface also disrupted both interfaces. As expected, both kinds of mutation abolished Bcl-2-mediated inhibition of Bax oligomerization in detergent micelles. Therefore, Bcl-2 binds Bax through two interdependent interfaces to inhibit the pro-apoptotic oligomerization of Bax.

A conserved hydrophobic core at Bcl-xL mediates its structural stability and binding affinity with BH3-domain peptide of pro-apoptotic protein

Bcl-2 family proteins regulate apoptosis through their homo- and heterodimerization. By protein sequence analysis and structural comparison, we have identified a conserved hydrophobic core at the BH1 and BH2 domains of Bcl-2 family proteins. The hydrophobic core is stabilized by hydrophobic interactions among the residues of Trp137, Ile140, Trp181, Ile182, Trp188 and Phe191 in Bcl-x(L). Destabilization of the hydrophobic core can promote the protein unfolding and pore formation in synthetic lipid vesicles. Interestingly, though the hydrophobic core does not participate in binding with BH3 domain of pro-apoptotic proteins, disruption of the hydrophobic core can reduce the affinity of Bcl-x(L) with BH3-domain peptide by changing the conformation of Bcl-x(L) C-terminal residues that are involved in the peptide interaction. The BH3-domain peptide binding affinity and pore forming propensity of Bcl-x(L) were correlated to its death-repressor activity, which provides new information to help study the regulatory mechanism of anti-apoptotic proteins. Meanwhile, as the tryptophans are conserved in the hydrophobic core, in vitro binding assay based on FRET of "Trp-->AEDANS" can be devised to screen for new modulators targeting anti-apoptotic proteins as well as "multi-BH domains" pro-apoptotic proteins.

Molecular modeling of human BAD and its interaction with PKAc or PP1c

To build up the structure of human BAD (Bcl-2 antagonist of cell death), subsequently combined with PKAc or PP1c (protein phosphatase 1), to investigate the interaction relationship between BAD and its kinase/PTPese at the molecular level. Additionally, it is concerned with the search for all optimal positions and orientations of a set of amino acid residues of BAD, while its binding sites include N-termini (Glu19, Ala27, and Ser34-Lys35), BH3-located helical domain (Arg98-Lys126), and C-termini (Trp154-Ser163 and Ser167-Gln168). The related sites of PKAc are mainly assembled in C-terminal alpha/beta-domain of PKAc, which comprises the KTL motif (47-49), Glu203 residue, a helical region (Asp241-Arg256), and the span from 328 to 333; while the interaction sites with BAD converge at C-terminal beta-domain of PP1c, which includes the DEK motif (166-168), the stretch from 179 to 197 including a helix (Glu184-Arg188), Glu230-Asp242 segment containing Val232-His237 helix, and Glu287-Leu289 loop. In conclusion, analysis of the complex between BAD and PKAc or PP1c provides a novel viewpoint on the structural origins of molecular recognition. And the complex models suggest that BH3 domain of BAD interact with PKAc or PP1c by electrostatic, van der Waals contacts, hydrogen bond and salt bridge. This is helpful for our development and research of some new drugs, especially mimetic BH3 peptides and inspires scientists with BAD complex and molecular mechanism of its integrating glycolysis and apoptosis.

To trigger apoptosis, Bak exposes its BH3 domain and homodimerizes via BH3:groove interactions

The Bcl-2 relative Bak is thought to drive apoptosis by forming homo-oligomers that permeabilize mitochondria, but how it is activated and oligomerizes is unclear. To clarify these pivotal steps toward apoptosis, we have characterized multiple random loss-of-function Bak mutants and explored the mechanism of Bak conformation change during apoptosis. Single missense mutations located to the alpha helix 2-5 region of Bak, with most altering the BH3 domain or hydrophobic groove (BH1 domain). Loss of function invariably corresponded to impaired ability to oligomerize. An essential early step in Bak activation was shown to be exposure of the BH3 domain, which became reburied in dimers. We demonstrate that oligomerization involves insertion of the BH3 domain of one Bak molecule into the groove of another and may produce symmetric Bak dimers. We conclude that this BH3:groove interaction is essential to nucleate Bak oligomerization, which in turn is required for its proapoptotic function.

Functional and physical interaction between Bcl-X(L) and a BH3-like domain in Beclin-1

The anti-apoptotic proteins Bcl-2 and Bcl-X(L) bind and inhibit Beclin-1, an essential mediator of autophagy. Here, we demonstrate that this interaction involves a BH3 domain within Beclin-1 (residues 114-123). The physical interaction between Beclin-1 and Bcl-X(L) is lost when the BH3 domain of Beclin-1 or the BH3 receptor domain of Bcl-X(L) is mutated. Mutation of the BH3 domain of Beclin-1 or of the BH3 receptor domain of Bcl-X(L) abolishes the Bcl-X(L)-mediated inhibition of autophagy triggered by Beclin-1. The pharmacological BH3 mimetic ABT737 competitively inhibits the interaction between Beclin-1 and Bcl-2/Bcl-X(L), antagonizes autophagy inhibition by Bcl-2/Bcl-X(L) and hence stimulates autophagy. Knockout or knockdown of the BH3-only protein Bad reduces starvation-induced autophagy, whereas Bad overexpression induces autophagy in human cells. Gain-of-function mutation of the sole BH3-only protein from Caenorhabditis elegans, EGL-1, induces autophagy, while deletion of EGL-1 compromises starvation-induced autophagy. These results reveal a novel autophagy-stimulatory function of BH3-only proteins beyond their established role as apoptosis inducers. BH3-only proteins and pharmacological BH3 mimetics induce autophagy by competitively disrupting the interaction between Beclin-1 and Bcl-2 or Bcl-X(L).

Bcl-2 changes conformation to inhibit Bax oligomerization

Bcl-2 inhibits apoptosis by regulating the release of cytochrome c and other proteins from mitochondria. Oligomerization of Bax promotes cell death by permeabilizing the outer mitochondrial membrane. In transfected cells and isolated mitochondria, Bcl-2, but not the inactive point mutants Bcl-2-G145A and Bcl-2-V159D, undergoes a conformation change in the mitochondrial membrane in response to apoptotic agonists such as tBid and Bax. A mutant Bcl-2 with two cysteines introduced at positions predicted to result in a disulfide bond that would inhibit the mobility of alpha5-alpha6 helices (Bcl-2-S105C/E152C) was only active in a reducing environment. Thus, Bcl-2 must change the conformation to inhibit tBid-induced oligomerization of integral membrane Bax monomers and small oligomers. The conformationally changed Bcl-2 sequesters the integral membrane form of Bax. If Bax is in excess, apoptosis resumes as Bcl-2 is consumed by the conformational change and in complexes with Bax. Thus, Bcl-2 functions as an inhibitor of mitochondrial permeabilization by changing conformation in the mitochondrial membrane to bind membrane-inserted Bax monomers and prevent productive oligomerization of Bax.

The small organic compound HA14-1 prevents Bcl-2 interaction with Bax to sensitize malignant glioma cells to induction of cell death

A functional imbalance between proapoptotic Bax and antiapoptotic Bcl-2 is likely to participate in the resistance of cancer cells to therapy. We show here that ethyl 2-amino-6-bromo-4-(1-cyano-2-ethoxy-2-oxoethyl)-4H-chromene-3-carboxylate (HA14-1), a small organic compound recently proposed to function as an inhibitor of Bcl-2, increases the sensitivity of human glioblastoma cells to radiotherapy and chemotherapy. This sensitizing effect is lost if Bcl-2 expression, but not Bcl-xL expression, is knocked down or if cells only express a mutant of Bax that does not interact with Bcl-2. This points to a specific Bcl-2 inhibitory function of HA14-1 and implies that it selectively involves hindrance of Bcl-2 binding to Bax, which HA14-1 inhibits in cell-free assays and in cells in receipt of an apoptotic stimulation. Moreover, HA14-1, in combination with a cytotoxic treatment, slows down the growth of glioblastoma in vivo. Thus, the inhibition of Bcl-2 achieved by HA14-1 might improve treatment outcome.

Inhibition of CWR22Rnu1 tumor growth and PSA secretion in athymic nude mice by green and black teas

Cancer of the prostate gland (CaP), the most common invasive malignancy and a major cause of cancer related deaths in male population in the USA, is an ideal candidate disease for chemoprevention because it is typically detected in elderly population with a relatively slower rate of growth and progression. Many dietary phytochemicals are showing promising chemopreventive effects, at-least in pre-clinical models of CaP. Our published data in cell culture and animal studies, supported by the work from other laboratories, as well as epidemiological observations and case-control studies, suggest that polyphenols present in green tea possess CaP chemopreventive and possibly therapeutic effects. This present study was designed to compare CaP cancer chemopreventive effects of green tea polyphenols (GTP), water extract of black tea, and their major constituents epigallocatechin-3-gallate and theaflavins, respectively, in athymic nude mice implanted with androgen-sensitive human CaP CWR22Rnu1 cells. Our data demonstrated that the treatment with all the tea ingredients resulted in (i) significant inhibition in growth of implanted prostate tumors, (ii) reduction in the level of serum prostate specific antigen, (iii) induction of apoptosis accompanied with upregulation in Bax and decrease in Bcl-2 proteins, and (iv) decrease in the levels of VEGF protein. Furthermore, we also found that GTP (0.01 or 0.05% w/v; given after establishment of CWR22Rnu1 tumor) causes a significant regression of tumors suggesting therapeutic effects of GTP at human achievable concentrations.

Bcl-2 and Bax proteins are increased in neocortical but not in thalamic apoptosis following devascularizing lesion of the cerebral cortex in the rat: an immunohistochemical study

The hypothesis that devascularization of somatosensory and motor cortex causes apoptosis in infarcted regions and in the linked thalamic nuclei was evaluated. To unravel whether Bcl-related proteins, known to regulate apoptosis, participate in neuronal and glial responses to devascularization, we analyzed immunohistochemically the distribution and intensity of staining of Bcl-2 and Bax proteins at different time points after lesion. Both early (up to 6 h) and late (1-7 days) responses were studied. Devascularization led to rapid (within hours) apoptosis in the cortex and to a delayed (within 3-7 days) apoptosis in thalamic nuclei. In control groups, Bcl-2 and Bax immunoreactivity (IR) was detected in neurons and oligodendrocytes but not in astrocytes or microglia. Following devascularization, Bcl-2 IR and Bax IR increased in neurons before the onset of the apoptosis. In the ischemic focus, the increase reached maximal values 3 h after the lesion. The increase was of slower onset in the penumbra zone (24 h and after), a region in which both proteins were induced in astrocytes also. The change of Bax IR intensity exceeded four times that of Bcl-2 at all time points investigated, indicating a diminution of Bcl-2/Bax ratio that may direct neurons to apoptotic pathway. In numerous neurons, an increase of IR in the cytoplasm was accompanied by induction of nuclear staining. No changes of Bcl-2 and Bax IR were found in thalamic nuclei. Our results point to different mechanisms underlying apoptosis of cortical and thalamic neurons. Nuclear appearance of Bcl-2 and Bax suggests they possess regulatory role of gene expression changes triggered by cortical infarct.

Role of PI3-kinase-dependent Bad phosphorylation and altered transcription in cytokine-mediated neutrophil survival

Phosphoinositide 3-kinase (PI3-kinase)-dependent phosphorylation of the proapoptotic Bcl-2 family member Bad has been proposed as an important regulator of apoptotic cell death. To understand the importance of this pathway in nontransformed hematopoietic cells, we have examined the effect of survival cytokines on PI3-kinase activity and Bad expression and phosphorylation status in human neutrophils. Granulocyte macrophage-colony-stimulating factor (GM-CSF) and tumor necrosis factor-alpha (TNF-alpha) both reduced the rate of apoptosis in neutrophils cultured in vitro for 20 hours. Coincubation with the PI3-kinase inhibitor LY294002, which in parallel experiments abolished GM-CSF-primed, fMLP-stimulated superoxide anion production and GM-CSF-stimulated PtdIns(3,4,5)P(3) accumulation, inhibited the GM-CSF and TNF-alpha survival effect. In contrast, the MAP kinase kinase (MEK1/2) inhibitor PD98059 and the protein kinase A inhibitor H-89 had only a marginal effect on GM-CSF-mediated neutrophil survival. GM-CSF substantially increased Bad phosphorylation at Ser112 and Ser136 and increased the cytosolic accumulation of Bad. GM-CSF also regulated Bad at a transcription level with a marked decrease in mRNA levels at 4 hours. TNF-alpha caused a biphasic effect on the rate of morphologic apoptosis, which corresponded to an early increase, and a late inhibition, of Bad mRNA levels. LY294002 inhibited GM-CSF- and TNF-alpha-mediated changes in Bad phosphorylation and mRNA levels. These data suggest that the survival effect of GM-CSF and TNF-alpha in neutrophils is caused by a PI3-kinase-dependent phosphorylation and cytosolic translocation of Bad, together with an inhibition of Bad mRNA levels. This has important implications for the regulation of neutrophil apoptosis in vivo.

Molecular cloning, physical mapping, and expression analysis of a novel gene, BCL2L12, encoding a proline-rich protein with a highly conserved BH2 domain of the Bcl-2 family

Members of the Bcl-2 family of apoptosis-regulating proteins contain at least one of the four evolutionarily conserved domains, termed BH1, BH2, BH3, or BH4. Here, we report the identification, cloning, physical mapping, and expression pattern of BCL2L12, a novel gene that encodes a BCL2-like proline-rich protein. Proline-rich sites have been shown to interact with Src homology region 3 (SH3) domains of several tyrosine kinases, mediating their oncogenic potential. This new gene maps to chromosome 19q13.3 and is located between the IRF3 and the PRMT1/HRMT1L2 genes, close to the RRAS gene. BCL2L12 is composed of seven coding exons and six intervening introns, spanning a genomic area of 8.8 kb. All of the exon-intron splice sites conform to the consensus sequence for eukaryotic splice sites. The BCL2L12 protein is composed of 334 amino acids, with a calculated molecular mass of 36.8 kDa and an isoelectric point of 9.45. The BCL2L12 protein contains one BH2 homology domain, one proline-rich region similar to the TC21 protein and, five consensus PXXP tetrapeptide sequences. BCL2L12 is expressed mainly in breast, thymus, prostate, fetal liver, colon, placenta, pancreas, small intestine, spinal cord, kidney, and bone marrow and to a lesser extent in many other tissues. We also identified one splice variant of BCL2L12 that is primarily expressed in skeletal muscle.

Escape from apoptosis after prolonged serum deprivation is associated with the regulation of the mitochondrial death pathway by Bcl-x(l)

Bcl-x(l) and Bax play important roles in the regulation of apoptosis. This study investigated the involvement of the mitochondrial death pathway and the role of Bcl-x(l) and Bax in the escape from apoptosis after prolonged serum deprivation in Madin-Darby canine kidney (MDCK) cells. Low level apoptosis and basal activity of the mitochondrial death pathway were detectable in normal cell growth. In serum deprivation, mitosis was partially suppressed, and the mitochondrial activity was stimulated. The level of apoptosis continuously rose over 48 h. This rise was concomitant with the increasing presence of cytochrome c in cytosol. However, both apoptosis and cytosolic cytochrome c fell dramatically at 72 h. Elevation of whole cell Bcl-x(l) and redistribution of Bcl-x(l) protein from cytosol to the membrane at 48 h and 72 h was observed. Redistribution of Bax protein from the membrane to cytosol occurred at 24 h, and remained steady to 72 h. Bax/Bcl-x(l) coimmunoprecipitation by anti-Bax antibody showed reduced Bax/Bcl-x(l) interaction at the membrane at 72 h, but not at 24 or 48 h. These results suggest that apoptosis upon serum withdrawal results from the leakage of cytochrome c to cytosol. Amelioration of the leakage of cytochrome c and apoptosis requires not only the increase of Bcl-x(l)/Bax ratio, but also the release of Bcl-x(l) from Bax at the membrane.

Antiapoptotic Bcl-2 family protein expression increases with progression of oligodendroglioma

BACKGROUND

Altered expression of Bcl-2 family proteins has been associated with tumorigenesis and tumor progression as well as resistance to radiotherapy and chemotherapy. In the current study, Bcl-2 family protein expression was examined in oligodendrogliomas and anaplastic oligodendrogliomas, and an attempt was made to determine whether these proteins accumulate during disease progression and to search for protein expression patterns predictive of time to progression and overall survival.

METHODS

A total of 42 oligodendroglioma tissue samples, 26 de novo World Health Organization (WHO) Grade 2 oligodendrogliomas, and 16 de novo WHO Grade 3 anaplastic oligodendrogliomas were studied. Nineteen Grade 2 tumors progressed: 10 again were Grade 2 oligodendrogliomas and 8 had progressed to higher grade lesions. Eight anaplastic oligodendrogliomas progressed: five again were WHO Grade 3 tumors and three were glioblastoma multiforme. Expression of Bcl-2, Bax, Bcl-X, and Mcl-1 proteins and of the proliferation marker Ki-67 was evaluated by immunohistochemistry. Apoptotic cells were quantified by in situ nick translation (ISNT).

RESULTS

De novo WHO Grade 2 oligodendrogliomas had higher Bcl-2 scores (P = 0.037), lower MIB-1 scores (P = 0.0012), and lower ISNT scores (P = 0.049) compared with de novo WHO Grade 3 anaplastic oligodendrogliomas. In de novo oligodendrogliomas, low numbers of Bax positive cells were associated with a short time to disease progression (P = 0.043). In de novo anaplastic oligodendrogliomas, low numbers of Bcl-2 positive cells correlated with short survival (P = 0.029). In tumors that had progressed from WHO Grade 3 anaplastic oligodendrogliomas, the authors found significantly more Bcl-X positive (P = 0.005), Mcl-1 positive (P = 0.002), and Bax positive (P = 0.03) cells.

CONCLUSIONS

The results of the current study show that progression of oligodendrogliomas and anaplastic oligodendrogliomas is associated with an enhanced expression of antiapoptotic Bcl-2 family proteins.

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.

Mutagenesis of the BH3 domain of BAX identifies residues critical for dimerization and killing

The BCL-2 family of proteins is comprised of proapoptotic as well as antiapoptotic members (S. N. Farrow and R. Brown, Curr. Opin. Genet. Dev. 6:45-49, 1996). A prominent death agonist, BAX, forms homodimers and heterodimerizes with multiple antiapoptotic members. Death agonists have an amphipathic alpha helix, called BH3; however, the initial assessment of BH3 in BAX has yielded conflicting results. Our BAX deletion constructs and minimal domain constructs indicated that the BH3 domain was required for BAX homodimerization and heterodimerization with BCL-2, BCL-XL, and MCL-1. An extensive site-directed mutagenesis of BH3 revealed that substitutions along the hydrophobic face of BH3, especially charged substitutions, had the greatest affects on dimerization patterns and death agonist activity. Particularly instructive was the BAX mutant mIII-1 (L63A, G67A, L70A, and M74A), which replaced the hydrophobic face of BH3 with alanines, preserving its amphipathic nature. BAXmIII-1 failed to form heterodimers or homodimers by yeast two-hybrid or immunoprecipitation analysis yet retained proapoptotic activity. This suggests that BAX's killing function reflects mechanisms beyond its binding to BCL-2 or BCL-XL to inhibit them or simply displace other protein partners. Notably, BAXmIII-1 was found predominantly in mitochondrial membranes, where it was homodimerized as assessed by homobifunctional cross-linkers. This characteristic of BAXmIII-1 correlates with its capacity to induce mitochondrial dysfunction, caspase activation, and apoptosis. These data are consistent with a model in which BAX death agonist activity may require an intramembranous conformation of this molecule that is not assessed accurately by classic binding assays.

Dimerization properties of human BAD. Identification of a BH-3 domain and analysis of its binding to mutant BCL-2 and BCL-XL proteins

Bad, an inducer of programmed cell death, was recently isolated from a mouse cDNA library by its ability to bind to the anti-apoptotic protein BCL-2. Sequence analysis suggested that Bad was a member of the BCL-2 gene family that encodes both inducers and inhibitors of programmed cell death. To further analyze the role of BAD in the network of homo- and heterodimers formed by the BCL-2 family, we have cloned the human homologue of BAD and assessed its biological activity and its interactions with wild type and mutant BCL-2 family proteins. Our results indicate that the human BAD protein, like its mouse homologue, is able to induce apoptosis when transfected into mammalian cells. Furthermore, in yeast two-hybrid assays as well as quantitative in vitro interaction assays, human Bad interacted with BCL-2 and BCL-XL. Sequence alignments of human BAD revealed the presence of a BH-3 homology domain as seen in other BCL-2 family proteins. Peptides derived from this domain were able to completely inhibit the dimerization of BAD with BCL-XL. Thus, as previously shown for BAX, BAK, BCL-2, and BCL-XL, the BH3 domain of BAD is required for its dimerization with other BCL-2 family proteins. BAD was further analyzed for its ability to bind to various mutants of BCL-2 and BCL-XL that have lost the ability to bind BAX and BAK, some of which retain biological activity and some of which do not. Surprisingly, all of the mutated BCL-2 and BCL-XL proteins analyzed strongly interacted with human BAD. Our data thus indicate that mutations in BCL-2 and BCL-XL can differentially affect the heterodimeric binding of different death-promoting proteins and have implications concerning the relationship between heterodimerization and biological activity.