Fusarium secondary metabolites are structurally diverse, have a variety of activities and are generally poorly understood biosynthetically. The F. fujikuroi polyketide synthase gene bikbikaverin. Here we present the characterization of five genes adjacent to bikbikbikbikbikbikbikaverin synthesis. Expression studies revealed that all bik genes are repressed by high amounts of nitrogen in an AreA-independent manner and are subject to a time- and pH-dependent regulation. Deletion of the pH regulatory gene pacC resulted in partial derepression while complementation with a dominant active allele resulted in repression of bik genes at acidic ambient pH. Transcription of all bik genes in strains lacking bikbikbikbik genes was detected in strains lacking bikbikbikbikaverin synthesis is regulated by a complex regulatory network. Understanding how different factors influence the synthesis of this model secondary metabolite will aid understanding secondary metabolism in general.

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

During Plasmodium falciparum infection leading to cerebral malaria, cytokine production and cytoadherence of parasitized erythrocytes (PRBCs) to postcapillary venules are involved. We demonstrate that PRBC adhesion induces apoptosis in human endothelial cells (HLECs). PRBC adhesion modulated HLEC gene expression in tumor necrosis factor-alpha superfamily genes (Fas, Fas L, and DR-6) and apoptosis-related genes (Bad, Bax, caspase-3,SARP 2, DFF45/ICAD, IFN-gamma receptor 2, Bcl-w, Bik, and iNOS). Apoptosis was confirmed by (1) morphological modifications by electron microscopy, (2) annexin V binding, (3) DNA degradation, by measuring intracytoplasmic nucleosomes, and (4) caspase activity. The apoptotic stimulus was physical contact between HLECs and PRBCs and not parasite-secreted molecules. In addition, it was found that cytoplasmic (caspase 8) and mitochondrial (caspase 9) pathways were involved in this process. These data not only describe the direct apoptotic effect of PRBC adhesion on endothelial cells but also provide new useful tools that allow an evaluation of potential pharmaceuticals.

Through direct interaction with the voltage-dependent anion channel (VDAC), proapoptotic Bcl-2 family members such as Bax and Bak induce apoptogenic mitochondrial cytochrome c release and membrane potential (Deltapsi) loss in isolated mitochondria. Using isolated mitochondria, we showed that Bid and Bik, BH3-only proteins from the Bcl-2 family, induced cytochrome c release but not Deltapsi loss. Unlike Bax/Bak, the cytochrome c release induced by Bid/Bik was Ca(2+)-independent, cyclosporin A-insensitive, and respiration-independent. Furthermore, in contrast to Bax/Bak, Bid/Bik neither interacted with VDAC nor directly affected the VDAC activity in liposomes. Consistently, Bid/Bik induced apoptosis without Deltapsi loss, whereas Bax induced apoptosis with Deltapsi loss. These findings indicated the involvement of a different mechanism in BH3-only, protein-induced apoptogenic cytochrome c release.

High-resolution genome-wide mapping of exact boundaries of chromosomal alterations should facilitate the localization and identification of genes involved in gliomagenesis and may characterize genetic subgroups of glial brain tumors. We have done such mapping using cDNA microarray-based comparative genomic hybridization technology to profile copy number alterations across 42,000 mapped human cDNA clones, in a series of 54 gliomas of varying histogenesis and tumor grade. This gene-by-gene approach permitted the precise sizing of critical amplicons and deletions and the detection of multiple new genetic aberrations. It has also revealed recurrent patterns of occurrence of distinct chromosomal aberrations as well as their interrelationships and showed that gliomas can be clustered into distinct genetic subgroups. A subset of detected alterations was shown predominantly associated with either astrocytic or oligodendrocytic tumor phenotype. Finally, five novel minimally deleted regions were identified in a subset of tumors, containing putative candidate tumor suppressor genes (TOPORS, FANCG, RAD51, TP53BP1, and BIK) that could have a role in gliomagenesis.

Previously, we showed that the proteasome inhibitor bortezomib/Velcade (formerly PS-341) synergizes with the protein tumor necrosis factor alpha-related apoptosis-inducing ligand (TRAIL), a ligand for certain death receptors, to induce apoptosis in cell lines derived from prostate and colon cancers. Because apoptosis is often triggered by BH3-only proteins of the Bcl-2 family, we have explored the hypothesis that bortezomib contributes to the apoptosis by up-regulating their levels. Indeed, bortezomib induced increases of Bik and/or Bim in multiple cell lines but not notably of two other BH3-only proteins (Puma and Bid) nor other family members (Bax, Bak, Bcl-2, and Bcl-xL). The increase in Bik levels seems to reflect inhibition by bortezomib of its proteasome-mediated degradation. Importantly, both Bik and Bim seem central to the proapoptotic function of bortezomib, because mouse embryo fibroblasts in which the genes for both Bik and Bim had been disrupted were refractory to its cytotoxic action. Similarly, the synergy between bortezomib and TRAIL in killing human prostate cancer cells was impaired in cells in which both Bik and Bim were down-regulated by RNA interference. Further evidence that bortezomib acts through the mitochondrial pathway regulated by the Bcl-2 family is that deficiency for APAF-1, which acts downstream of Bcl-2, also blocked its apoptotic effect. These results implicate BH3-only proteins, in particular both Bik and Bim, as important mediators of the antitumor action of bortezomib and establish their role in its enhancement of TRAIL-induced apoptosis.

Bcl-2 and close homologues such as Bcl-xL promote cell survival, while other relatives such as Bax antagonize this function. Since only the pro-survival family members possess a conserved N-terminal region denoted BH4, we have explored the role of this amphipathic helix for their survival function and for interactions with several agonists of apoptosis, including Bax and CED-4, an essential regulator in the nematode Caenorhabditis elegans. BH4 of Bcl-2 could be replaced by that of Bcl-x without perturbing function but not by a somewhat similar region near the N-terminus of Bax. Bcl-2 cell survival activity was reduced by substitutions in two of ten conserved BH4 residues. Deletion of BH4 rendered Bcl-2 (and Bcl-xL) inactive but did not impair either Bcl-2 homodimerization or ability to bind to Bax or five other pro-apoptotic relatives (Bak, Bad, Bik, Bid or Bim). Hence, association with these death agonists is not sufficient to promote cell survival. Significantly, however, Bcl-xL lacking BH4 lost the ability both to bind CED-4 and antagonize its pro-apoptotic activity. These results favour the hypothesis that the BH4 domain of pro-survival Bcl-2 family members allows them to sequester CED-4 relatives and thereby prevent apoptosis.

Bortezomib (Velcade, PS341) was licensed in 2003 as a first-in-class 20S proteasome inhibitor indicated for treatment of multiple myeloma, and is currently being evaluated clinically in a range of solid tumours. The mechanisms underlying its cancer cell toxicity are complex. A growing body of evidence suggests proteasome inhibition-dependent regulation of the BCL-2 family is a critical requirement. In particular, the stabilization of BH3-only proteins BIK, NOXA and BIM, appear to be essential for effecting BAX- and BAK-dependent cell death. These mechanisms are reviewed and the implications for favourable novel drug interactions are highlighted.

The BH3-only members of the Bcl-2 protein family are essential for initiation of programmed cell death and stress-induced apoptosis. We have determined the expression pattern in mice of the BH3-only protein Bik, also called Blk or Nbk, and examined its physiological function by gene targeting. We found that Bik is expressed widely in the hematopoietic compartment and in endothelial cells of the venous but not arterial lineages. Nevertheless, its loss did not increase the numbers of such cells in mice or protect hematopoietic cells in vitro from apoptosis induced by cytokine withdrawal or diverse other cytotoxic stimuli. Moreover, whereas loss of the BH3-only protein Bim rescued mice lacking the prosurvival protein Bcl-2 from fatal polycystic kidney disease and lymphopenia, loss of Bik did not. These results indicate that any function of Bik in programmed cell death and stress-induced apoptosis must overlap that of other BH3-only proteins.

The E1B 19-kilodalton protein (19K protein) is a potent apoptosis inhibitor and the adenovirus homolog of Bcl-2 (E. White, Genes Dev. 10:1-15, 1996). To obtain a better understanding of the biochemical mechanism by which the E1B 19K protein regulates apoptosis, proteins that interact with 19K have been identified; one of these is Bax (J. Han, P. Sabbatini, D. Perez, L. Rao, D. Mohda, and E. White, Genes Dev. 10:461-477, 1996), and another is Bak (S. N. Farrow, J. H. M. White, I. Martinou, T. Raven, K.-T. Pun, C. J. Grinham, J.-C. Martinou, and R. Brown, Nature (London) 374:731-733, 1995). Bax and Bak are Bcl-2 family members which contain Bcl-2 homology regions 1, 2, and 3 (BH1, BH2, and BH3), which interact with E1B 19K and Bcl-2 and promote apoptosis. Like Bax and Bak, Nbk was cloned from a yeast two-hybrid screen for proteins that interact with E1B 19K. Nbk contained BH3 but not BH1 or BH2. It also interacted with Bcl-2 but not with Bax. Both Bcl-2 and E1B 19K interacted with Nbk in vitro, and this interaction was highly specific. In vivo, the Nbk and E1B 19K proteins may colocalize with cytoplasmic and nuclear membranes. Nbk expression functionally antagonized 19K-mediated inhibition of apoptotic cell death and completely prevented transformation by E1A and E1B 19K. Nbk was sufficient for induction of apoptosis in the presence of mutant p53 and thus low levels of Bax, suggesting that Nbk functions independently of Bax to induce apoptosis. Nbk may therefore represent a novel death regulator which contains only a BH3 that interacts with and antagonizes apoptosis inhibitors such as the E1B 19K protein.

Activation of the intrinsic apoptotic pathway represents a major mechanism for breast cancer regression resulting from anti-estrogen therapy. The BH3-only protein BIK is inducible by estrogen-starvation and anti-estrogen treatment and plays an important role in anti-estrogen induced apoptosis of breast cancer cells. BIK is predominantly localized to the endoplasmic reticulum where it regulates BAX/BAK-dependent release of Ca(2+) from the endoplasmic reticulum stores and cooperates with other BH3-only proteins such as NOXA to cause rapid release of cytochrome c from mitochondria and activate apoptosis. BIK is also known to inactivate BCL-2 through complex formation. Previously, we demonstrated that apoptosis triggered by BIK in estrogen-starved human breast cancer cells is suppressed by GRP78, a major endoplasmic reticulum chaperone. Here we described the isolation of a novel clonal human breast cancer cell line (MCF-7/BUS-10) resistant to long-term estrogen deprivation. These cells exhibit elevated level of GRP78, which protects them from estrogen starvation-induced apoptosis. Our studies revealed that overexpression of GRP78 suppresses apoptosis induced by BIK and NOXA, either alone or in combination. Surprisingly, the interaction of GRP78 with BIK does not require its BH3 domain, which has been implicated in all previous BIK protein interactions. We further showed GRP78 and BCL-2 form independent complex with BIK and that increased expression of GRP78 decreases BIK binding to BCL-2. Our findings provide the first evidence that GRP78 can decrease BCL-2 sequestration by BIK at the endoplasmic reticulum, thus uncovering a potential new mechanism whereby GRP78 confers endocrine resistance in breast cancer.

We have previously correlated Chlamydia trachomatis antiapoptotic activity with the blockade of mitochondrial cytochrome c release and the inhibition of Bax and Bak activation. We now report that C. trachomatis infection leads to degradation of Bik, Puma, and Bim, three upstream proapoptotic BH3-only proteins of the Bcl-2 family that can transmit death signals to mitochondria by inhibiting the Bcl-2 antiapoptotic proteins and/or activating the Bcl-2 proapoptotic members, such as Bax and Bak. This observation has provided new information on the chlamydial antiapoptosis mechanisms.

Nbk/Bik (natural born killer/Bcl-2-interacting killer) is a tissue-specific BH3-only protein whose molecular function is still largely unknown. To investigate the mechanism of Nbk action, we established a single- vector adenoviral system based on the Tet-off conditional expression of Nbk. Upon Nbk expression, only Bax-positive, but not Bax-deficient cells were found to undergo apoptosis. Interestingly, Nbk failed to induce apoptosis in the absence of Bax, even despite expression of the related molecule Bak. Re-expression of Bax restored the sensitivity to Nbk. Similarly, Bax wild-type HCT116 cells were highly susceptible, whereas HCT116 Bax knock-out cells remained resistant to Nbk-induced apoptosis. In Bax-positive cells, Nbk induced a conformational switch in the Bax N-terminus coinciding with cytochrome c release, mitochondrial permeability transition and caspase-9 processing. Immunoprecipitation studies revealed that Nbk interacts with Bcl-x(L) and Bcl-2 but not with Bax. Since, in addition, Nbk did not localize to the mitochondria, our data suggest a model in which Nbk acts as an indirect killer to trigger Bax-dependent apoptosis, whereas Bak is not sufficient to confer sensitivity to Nbk.

beta-Amyloid protein (Abeta) has been implicated as a key molecule in the neurodegenerative cascades of Alzheimer's disease (AD). Abeta directly induces neuronal apoptosis, suggesting an important role of Abeta neurotoxicity in AD neurodegeneration. However, the mechanism(s) of Abeta-induced neuronal apoptosis remain incompletely defined. In this study, we report that Abeta-induced neuronal death is preceded by selective alterations in expression of the Bcl-2 family of apoptosis-related genes. Specifically, we observe that Abeta significantly reduces expression of antiapoptotic Bcl-w and Bcl-x(L), mildly affects expression of bim, Bcl-2, and bax, but does not alter expression of bak, bad, bik, bid, or BNIP3.Abeta-induced downregulation of Bcl-w appears to contribute to the mechanism of apoptosis, because Abeta-induced neuronal death was significantly increased by Bcl-w suppression but significantly reduced by Bcl-w overexpression. Downstream of Bcl-w, Abeta-induced neuronal apoptosis is characterized by mitochondrial release of second mitochondrion-derived activator of caspase (Smac), an important precursor event to cell death. We observed that Smac release was potentiated by suppression of Bcl-w and reduced by overexpression of Bcl-w. Next, we investigated the upstream mediator of Abeta-induced Bcl-w downregulation and Smac release. We observed that Abeta rapidly activates c-Jun N-terminal kinase (JNK). Pharmacological inhibition of JNK effectively inhibited all measures of Abeta apoptosis: Bcl-w downregulation, Smac release, and neuronal death. Together, these results suggest that the mechanism of Abeta-induced neuronal apoptosis sequentially involves JNK activation, Bcl-w downregulation, and release of mitochondrial Smac, followed by cell death. Complete elucidation of the mechanism of Abeta-induced apoptosis promises to accelerate development of neuroprotective interventions for the treatment of AD.

Stimulation of apoptosis by p53 is accompanied by induction of the BH-3-only proapoptotic member of the BCL-2 family, BIK, and ectopic expression of BIK in p53-null cells caused the release of cytochrome c from mitochondria and activation of caspases, dependent on a functional BH-3 domain. A significant fraction of BIK, which contains a predicted transmembrane segment at its COOH terminus, was found inserted in the endoplasmic reticulum (ER) membrane, with the bulk of the protein facing the cytosol. Restriction of BIK to this membrane by replacing its transmembrane segment with the ER-selective membrane anchor of cytochrome b(5) also retained the cytochrome c release and cell death-inducing activity of BIK. Whereas induction of cell death by BIK was strongly inhibited by the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, the inhibitor was without effect on the ability of BIK to stimulate egress of cytochrome c from mitochondria. This benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone-insensitive pathway for stimulating cytochrome c release from mitochondria by ER BIK was successfully reconstituted in vitro and identified the requirement for components present in the light membrane (ER) and cytosol as necessary for this activity. Collectively, the results identify BIK as an initiator of cytochrome c release from mitochondria operating from a location at the ER.

The BH3-only proteins of the Bcl-2 family initiate apoptosis through the activation of Bax-like relatives. Loss of individual BH3-only proteins can lead either to no phenotype, as in mice lacking Bik, or to marked cell excess, as in the hematopoietic compartment of animals lacking Bim. To investigate whether functional redundancy with Bim might obscure a significant role for Bik, we generated mice lacking both genes. The hematopoietic compartments of bik-/-bim-/- and bim-/- mice were indistinguishable. However, although testes develop normally in mice lacking either Bik or Bim, adult bik-/-bim-/- males were infertile, with reduced testicular cellularity and no spermatozoa. The testis of young bik-/-bim-/- males, like those lacking Bax, exhibited increased numbers of spermatogonia and spermatocytes, although loss of Bik plus Bim blocked spermatogenesis somewhat later than Bax deficiency. The initial excess of early germ cells suggests that spermatogenesis fails because supporting Sertoli cells are overwhelmed. Thus, Bik and Bim share, upstream of Bax, the role of eliminating supernumerary germ cells during the first wave of spermatogenesis, a process vital for normal testicular development.

BIK is the founding member of the BH3-only family pro-apoptotic proteins. BIK is predominantly localized in the ER and induces apoptosis through the mitochondrial pathway by mobilizing calcium from the ER to the mitochondria and remodeling the mitochondrial cristae. BIK-mediated apoptosis is mediated by selective activation of BAX. BIK also induces non-apoptotic cell death in certain cell types by unknown mechanisms. BIK is non-essential for animal development, but appears to be functionally redundant for certain developmental functions with BIM. BIK is implicated in the selection of mature B cells in humans. BIK is a pro-apoptotic tumor suppressor in several human tissues and its expression in cancers is prevented by chromosomal deletions encompassing the Bik locus or by epigenetic silencing. BIK appears to be a critical effector in apoptosis induced by toxins, cytokines and virus infection. Several anti-cancer drugs transcriptionally activate Bik gene expression through transcriptional pathways dependent on factors such as E2F and p53 or by removal of epigenetic marks on the chromatin. BIK appears to be a prominent target for anti-cancer drugs that inhibit proteasomal functions. BIK has also been used as a therapeutic molecule in gene therapy-based approaches to treat difficult cancers.

The S334ter rhodopsin (Rho) rat (line 4) bears the rhodopsin gene with an early termination codon at residue 334 that is a model for several such mutations found in human patients with autosomal dominant retinitis pigmentosa (ADRP). The Unfolded Protein Response (UPR) is implicated in the pathophysiology of several retinal disorders including ADRP in P23H Rho rats. The aim of this study was to examine the onset of UPR gene expression in S334ter Rho retinas to determine if UPR is activated in ADRP animal models and to investigate how the activation of UPR molecules leads to the final demise of S334ter Rho photoreceptors. RT-PCR was performed to evaluate the gene expression profiles for the P10, P12, P15, and P21 stages of the development and progression of ADRP in S334ter Rho photoreceptors. We determined that during the P12-P15 period, ER stress-related genes are strongly upregulated in transgenic retinas, resulting in the activation of the UPR that was confirmed using western blot analysis and RT-PCR. The activation of UPR was associated with the increased expression of JNK, Bik, Bim, Bid, Noxa, and Puma genes and cleavage of caspase-12 that together with activated calpains presumably compromise the integrity of the mitochondrial MPTP, leading to the release of pro-apoptotic AIF1 into the cytosol of S334ter Rho photoreceptor cells. Therefore, two major cross-talking pathways, the UPR and mitochondrial MPTP occur in S334ter-4 Rho retina concomitantly and eventually promote the death of the photoreceptor cells.

OBJECTIVE

The human rhodopsin (Rho) mutation T17M leads to autosomal dominant retinitis pigmentosa (adRP). The goal of our study was to elucidate the role of endoplasmic reticulum (ER) stress in retinal degeneration in hT17M Rho mice and identify potential candidates for adRP gene therapy.

METHODS

We used transgenic mice expressing the ER stress-activated indicator (ERAI) and hT17M Rho to evaluate the activation of ER stress responses. Quantitative reverse transcription PCR (qRT-PCR) was used to analyze changes in the expression of 30 unfolded protein response (UPR)-associated genes at P12, 15, 18, 21, and 25. The cytosolic fraction of hT17M Rho retinal cells was used to measure the release of cytochrome C and apoptotic inducing factor-1 (AIF1) by Western blotting. Optical coherence tomography (OCT) analysis was performed for 1-month-old hT17M Rho mice.

RESULTS

hT17M Rho was localized in the outer nuclear layer (ONL) of T17M(+/-)ERAI(+/-) photoreceptors as well as C57BL/6 retinas injected with AAV-hT17M Rho-GFP. In P15 hT17M Rho retinas, we observed an up-regulation of UPR genes (Atf4, Eif2α, Xbp1, Bip, Canx, and Hsp90), autophagy genes and proapoptotic Bcl2 genes. OCT, and the downregulation of Nrl and Crx gene expression confirmed that cell death occurs in 55% of photoreceptors via the up-regulation of caspase-3 and caspase-12, and the release of AIF1 from the mitochondria.

CONCLUSIONS

The ER stress response is involved in retinal degeneration in hT17M Rho mice. The final demise of photoreceptors occurs via apoptosis involving ER stress-associated and mitochondria-induced caspase activation. We identified Atg5, Atg7, Bax, Bid, Bik, and Noxa as potential therapeutic targets for adRP treatment.

A DNA microarray analysis identified the BH3-only BCL-2 family member, BIK/NBK, as a transcript that is upregulated during induction of apoptosis by oncogenic E1A. E1A depended on wild-type p53 to induce BIK and activate the death program. Further, p53 independently induced BIK RNA and protein, and BIK alone stimulated cell death in p53-null cells, dependent on the activation of caspases. BIK function, however, was abrogated by a disabling point mutation within the BH3 domain. Collectively, these results argue that BIK is a downstream apoptotic effector of p53 in response to a physiological p53-mediated death stimulus provided by E1A. Elevated BCL-2 functioned downstream of p53 and BIK induction to inhibit the E1A death pathway, with the ratio of anti-apoptotic BCL-2 and pro-apoptotic BIK determining cell death or survival in E1A-expressing cells. Cells expressing BCL-2 or treated with the pan caspase inhibitor, zVAD-fmk, allowed accumulation of high levels of cytotoxic BIK compared to control cells. Of note, a significant fraction of either ectopic or endogenous BIK was found associated with the endoplasmic reticulum, suggesting that this organelle, in addition to mitochondria, may be a target of BIK function.

Proteasome inhibitors have emerged as promising anticancer therapeutic agents. Bortezomib (PS-341), a specific proteasome inhibitor, exhibits antitumor activity against a wide range of malignancies and has been approved by the US Food and Drug Administration for the treatment of relapsed or refractory multiple myeloma. However, the molecular mechanisms of bortezomib-mediated apoptosis remain unclear. To characterize the mechanisms of apoptosis induction by proteasome inhibitors, we examined levels of Bcl-2 protein family members (Bik/NBK, Bax, Bak, Bcl-2, and Bcl-XL), release of cytochrome c, and activation of caspase-9 and -3 in human colon cancer cell lines DLD1, LOVO, SW620, and HCT116; human lung cancer cell line H1299; and human ovarian cancer cell line SKOV3 after they were treated with bortezomib. The result showed that bortezomib induced rapid accumulation of Bik/NBK but not other Bcl-2 family members in all six cell lines. Bortezomib-mediated Bik/NBK accumulation and apoptosis were also observed in human embryonic kidney cells 293 and normal human bronchial epithelial cells. Moreover, dramatic Bik/NBK accumulation and apoptosis induction were observed when cells were treated with proteasome inhibitor MG132 and calpain inhibitor I (ALLN). Furthermore, no detectable changes in IkappaBalpha levels or in NFkappaB functionality were found after treatment with bortezomib. Finally, Bik/NBK accumulation was caused by stabilization of the protein from degradation and was associated with bortezomib cytotoxicity and apoptosis induction. Pretreatment of DLD1 cells with Bik/NBK siRNA reduced bortezomib-mediated Bik/NBK accumulation and cell death. Our results suggested that Bik/NBK is one of the mediators of proteasome inhibitor-induced apoptosis.

We report for the first time inactivation of a tissue-specific Bcl-2 homology domain 3 (BH3)-only protein as a common aspect in human cancer. In detail, we show that loss of the BH3-only protein natural born killer (Nbk)/Bcl-2-interacting killer (Bik) is a common feature of clear-cell renal cell carcinoma (RCC). While strong Nbk expression is found in the renal tubuli and the epithelial lining of the glomerula, a consistent loss of Nbk expression was observed in primary RCC tissue and RCC cell lines. Mutation of Nbk is, however, rare, whereas deletion of the Nbk gene at 22q13.2 is frequent. In addition to loss of heterozygosity (LOH), DNA methylation mediates transcriptional silencing of the Nbk gene. The conditional restoration of Nbk/Bik expression led to apoptotic death of RCC but not of nonmalignant renal epithelia. A broader expression analysis of RCC cell lines for BH3-only proteins revealed that loss of Nbk coincides with failure to express Bim, whereas Puma, Bid and BNIP3 are readily detectable and, in case of Puma, inducible by p53. These data delineate a role for defects in BH3-only proteins as tumor suppressors in RCC and may explain at the same time the impressive clinical apoptosis resistance of RCC.

Multiple myeloma is a plasma cell malignancy that is heterogeneous with respect to its causative molecular abnormalities and the treatment response of patients. The Bcl-2 protein family is critical for myeloma cell survival. ABT-737 is a cell-permeant compound that binds to Bcl-2 and Bcl-x(L) but not to Mcl-1. Using a myeloma cell line collection (n = 25) representative of different molecular translocations, we showed that ABT-737 effectively kills a subset of cell lines (n = 6), with a median lethal dose ranging from 7 ± 0.4 nM to 150 ± 7.5 nM. Of interest, all sensitive cell lines harbored a t(11;14). We demonstrated that ABT-737-sensitive and ABT-737-resistant cell lines could be differentiated by the BCL2/MCL1 expression ratio. A screen of a public expression database of myeloma patients indicates that the BCL2/MCL1 ratio of t(11;14) and hyperdiploid patients was significantly higher than in all other groups (P < .001). ABT-737 first induced the disruption of Bcl-2/Bax, Bcl-2/Bik, or Bcl-2/Puma complexes, followed by the disruption of Bcl-2 heterodimers with Bak and Bim. Altogether, the identification of a subset of cell lines and primary cells effectively killed by ABT-737 alone supported the evaluation of ABT-263, an orally active counterpart to ABT-737, for the treatment of t(11;14) and hyperdiploid groups of myeloma harboring a Bcl-2(high)/Mcl-1(low) profile.

The anthracyclin doxorubicin (DXR) is a major antitumor agent known to cause cellular damage via a number of mechanisms including free radical formation and inhibition of topoisomerase II. It is not clear, however, how the subsequent lesions may lead to the apoptotic death of the cell. We have here examined the effects of DXR on activation of pro-apoptotic members of the Bcl-2 family, all of which are connected to the mitochondrial events of apoptosis. In two human cell lines (lymphoma and myeloma), clinically relevant concentrations of DXR were found to induce apoptosis, first observed after 24 h of treatment. Apoptosis correlated with modulation of Bak and Bax to their active conformations. bax- as well as bak-deficient mouse embryo fibroblasts were resistant to DXR compared with wild-type mouse embryo fibroblasts further supporting a role for these proteins as main DXR-induced apoptosis regulators. Furthermore, using immunocytochemistry as well as chemical blocking of putative apical pathways we could demonstrate that Bak is activated prior to Bax. In the human cell lines, DXR was furthermore found to induce high protein levels of Bik, another BH3-only protein. DXR-induced apoptosis was completely blocked in Bcl-2-overexpressing U266 cells. Interestingly, in Bcl-2-transfected cells Bak activation was also blocked, while Bax was still partially active in agreement with differential regulation of these two proteins. Furthermore, co-incubation of the phosphatidylinositol 3-kinase (PI3K)-inhibitor LY294002 potentiated the apoptotic response to DXR. This enhanced apoptosis was preceded by enhanced Bak and Bax activation, and both responses as well as apoptosis were blocked in transfectants overexpressing Bcl-2. In summary, several pieces of evidence suggest that DXR induces apoptosis through a sequential and differential activation of Bak and Bax.