Endogenous Bcl-2 Research Articles

Role of Bcl-2 Family in Lymphoid Malignancies

Impaired apoptosis is considered one of the prerequisites for the development of most, if not all, cancers, but the mechanisms that guarantee the sustained survival of most cancer cells remain unknown. Members of the Bcl-2 family are key regulators of apoptosis and include proteins essential for cell survival and those required to initiate cell death. Studies with transgenic mice have shown that over-expression of Bcl-2 or related pro-survival family proteins, such as Bcl-xL or Mcl-1, can promote tumorigenesis, particularly in conjunction with mutations that deregulate cell cycle control, such as deregulated c-myc expression. It is, however, not known whether expression of pro-survival Bcl-2 family members under endogenous control is required to maintain the survival of cells undergoing neoplastic transformation. Using Eµ-myc transgenic mice, a well-characterized model of human Burkitt's lymphoma, we investigated the role of endogenous Bcl-2 in lymphoma development. Bcl-2 was found to be dispensable for the development of Eµ-myc pre-B/B lymphoma. In contrast, loss of Bcl-xL and even, more remarkable, loss of a single allele of Mcl1 greatly impaired lymphoma development. Experiments with inducible knockout mice demonstrated that Mcl-1 but not Bcl-xL is essential for the sustained survival and expansion of Myc-driven malignant pore-B/B lymphoma. Remarkably, even loss of one Mcl1allele greatly impaired lymphoma growth. These findings were translated into using lymphoid malignancies by using inducible expression of selective antagonists of distinct pro-survival Bcl-2 family members. Such studies showed that Mcl-1 is also critical for the sustained survival and expansion of Burkitt Lymphoma, a Myc-driven malignancy. These observations indicate that (even relatively weak) targeting of Mcl-1 may be an attractive strategy for the treatment of Myc-driven hematological malignancies. Disclosures:Strasser:Genentech Inc: Consultancy.

BAXgene over-expression via nucleofection to induce apoptosis in human lens epithelial cells

Despite significant advances in cataract surgery techniques, posterior capsule opacification (PCO) remains a common complication. In PCO, remaining epithelial cells cloud the lens capsule and impair postoperative vision. This in vitro study was designed to investigate the potential of a gene-based approach, specifically over-expression of the proapoptotic BAX gene, to prevent PCO. Human lens epithelial cells (HLECs) were transfected by nucleofection with a plasmid encoding a fusion protein of green fluorescent protein and human BAX. The expression levels of BAX and its antiapoptotic counterpart BCL2 were determined by realtime reverse transcription polymerase chain reaction, Western blotting and immunofluorescence. BAX over-expression-induced cell death was analyzed by fluorescence-activated cell sorting using the Annexin V antibody. Fluorescence microscopy and transmission electron microscopy were used to assess changes in morphology and ultrastructure. Differential expression of the downstream apoptosis-related factor, caspase 3, was detected by Western blotting. Nucleofection efficiency was high (nearly 80%). BAX-transfected HLECs showed remarkably enhanced BAX gene expression and BAX:BCL2 ratio, but relatively little change in endogenous BCL2 expression. BAX over-expression also led to significant cytotoxicity, induction of apoptosis-related characteristics and activation of caspase 3. In conclusion, our results indicate that BAX gene over-expression can trigger cell death in HLECs via an apoptotic pathway. Thus, BAX may be a promising candidate for human gene therapy to treat PCO.

Rapid Detection of an ABT-737-Sensitive Primed for Death State in Cells Using Microplate-Based Respirometry

Cells that exhibit an absolute dependence on the anti-apoptotic BCL-2 protein for survival are termed “primed for death” and are killed by the BCL-2 antagonist ABT-737. Many cancers exhibit a primed phenotype, including some that are resistant to conventional chemotherapy due to high BCL-2 expression. We show here that 1) stable BCL-2 overexpression alone can induce a primed for death state and 2) that an ABT-737-induced loss of functional cytochrome c from the electron transport chain causes a reduction in maximal respiration that is readily detectable by microplate-based respirometry. Stable BCL-2 overexpression sensitized non-tumorigenic MCF10A mammary epithelial cells to ABT-737-induced caspase-dependent apoptosis. Mitochondria within permeabilized BCL-2 overexpressing cells were selectively vulnerable to ABT-737-induced cytochrome c release compared to those from control-transfected cells, consistent with a primed state. ABT-737 treatment caused a dose-dependent impairment of maximal O2 consumption in MCF10A BCL-2 overexpressing cells but not in control-transfected cells or in immortalized mouse embryonic fibroblasts lacking both BAX and BAK. This impairment was rescued by delivering exogenous cytochrome c to mitochondria via saponin-mediated plasma membrane permeabilization. An ABT-737-induced reduction in maximal O2 consumption was also detectable in SP53, JeKo-1, and WEHI-231 B-cell lymphoma cell lines, with sensitivity correlating with BCL-2:MCL-1 ratio and with susceptibility (SP53 and JeKo-1) or resistance (WEHI-231) to ABT-737-induced apoptosis. Multiplexing respirometry assays to ELISA-based determination of cytochrome c redistribution confirmed that respiratory inhibition was associated with cytochrome c release. In summary, cell-based respiration assays were able to rapidly identify a primed for death state in cells with either artificially overexpressed or high endogenous BCL-2. Rapid detection of a primed for death state in individual cancers by “bioenergetics-based profiling” may eventually help identify the subset of patients with chemoresistant but primed tumors who can benefit from treatment that incorporates a BCL-2 antagonist.

µ-Calpain Conversion of Antiapoptotic Bfl-1 (BCL2A1) into a Prodeath Factor Reveals Two Distinct alpha-Helices Inducing Mitochondria-Mediated Apoptosis

Anti-apoptotic Bfl-1 and pro-apoptotic Bax, two members of the Bcl-2 family sharing a similar structural fold, are classically viewed as antagonist regulators of apoptosis. However, both proteins were reported to be death inducers following cleavage by the cysteine protease µ-calpain. Here we demonstrate that calpain-mediated cleavage of full-length Bfl-1 induces the release of C-terminal membrane active α-helices that are responsible for its conversion into a pro-apoptotic factor. A careful comparison of the different membrane-active regions present in the Bfl-1 truncated fragments with homologous domains of Bax show that helix α5, but not α6, of Bfl-1 induces cell death and cytochrome c release from purified mitochondria through a Bax/Bak-dependent mechanism. In contrast, both helices α5 and α6 of Bax permeabilize mitochondria regardless of the presence of Bax or Bak. Moreover, we provide evidence that the α9 helix of Bfl-1 promotes cytochrome c release and apoptosis through a unique membrane-destabilizing action whereas Bax-α9 does not display such activities. Hence, despite a common 3D-structure, C-terminal toxic domains present on Bfl-1 and Bax function in a dissimilar manner to permeabilize mitochondria and induce apoptosis. These findings provide insights for designing therapeutic approaches that could exploit the cleavage of endogenous Bcl-2 family proteins or the use of Bfl-1/Bax-derived peptides to promote tumor cell clearance.

The Bcl-2 proteins Noxa and Bcl-xL co-ordinately regulate oxidative stress-induced apoptosis

Because the detailed molecular mechanisms by which oxidative stress induces apoptosis are not completely known, we investigated how the complex Bcl-2 protein network might regulate oxidative stress-induced apoptosis. Using MEFs (mouse embryonic fibroblasts), we found that the endogenous anti-apoptotic Bcl-2 protein Bcl-xL prevented apoptosis initiated by H(2)O(2). The BH3 (Bcl-2 homology 3)-only Bcl-2 protein Noxa was required for H(2)O(2)-induced cell death and was the single BH3-only Bcl-2 protein whose pro-apoptotic activity was completely antagonized by endogenous Bcl-xL. Upon H(2)O(2) treatment, Noxa mRNA displayed the greatest increase among BH3-only Bcl-2 proteins. Expression levels of the anti-apoptotic Bcl-2 protein Mcl-1 (myeloid cell leukaemia sequence 1), the primary binding target of Noxa, were reduced in H(2)O(2)-treated cells in a Noxa-dependent manner, and Mcl-1 overexpression was able to prevent H(2)O(2)-induced cell death in Bcl-xL-deficient MEF cells. Importantly, reduction of the expression of both Mcl-1 and Bcl-xL caused spontaneous cell death. These studies reveal a signalling pathway in which H(2)O(2) activates Noxa, leading to a decrease in Mcl-1 and subsequent cell death in the absence of Bcl-xL expression. The results of the present study indicate that both anti- and pro-apoptotic Bcl-2 proteins co-operate to regulate oxidative stress-induced apoptosis.

Telomerase activity in response to mild oxidative stress

We have analysed telomerase activity to determine whether it can be modified when BCL-2 is endogenously overexpressed in response to a mild oxidative stress treatment as part of a survival mechanism, in contrast with an exogenous bcl-2 overexpression due to a retroviral infection. Endogenous bcl-2 overexpression was induced after a low oxidative insult of H2O2 in mice primary lung fibroblasts and L929 cell, whereas bcl-2 exogenous overexpression was performed using a retroviral infection in L929 cells. Telomerase activity was quantified in Bcl-2 overexpressing cells by the TRAP assay. When the cells were treated with different H2O2 concentrations, only those exposed to 50 μM showed increased telomerase activity. This correlates with BCL-2 expression as part of the endogenous response to mild oxidative stress. Oxidative stress generated during the toxic mechanism of chemotherapeutic drugs might induce BCL-2 increment, enhancing telomerase activity and reactivating the oncogenic process. Clinical trials should take into consideration the possibility of telomerase activation following increased BCL-2 expression when treating patients with ROS (reactive oxygen species) generation by anti-cancer drugs.

Endogenous Bcl-xL is essential for Myc-driven lymphomagenesis in mice

AbstractImpaired apoptosis is a cancer hallmark, and some types of lymphomas and other cancers harbor mutations that directly affect key cell death regulators, such as Bcl-2 family members. However, because the majority of tumors seem to lack such mutations, we are examining the hypothesis that tumorigenesis can be sustained at least initially by the normal expression of specific endogenous pro-survival Bcl-2 family members. We previously demonstrated that the lymphomagenesis in Εμ-myc transgenic mice, which constitutively overexpress the c-Myc oncoprotein in B-lymphoid cells and develop pre-B and B-cell lymphomas, does not require endogenous Bcl-2. In striking contrast, we report here that loss in these mice of its close relative Bcl-xL attenuated the pre-neoplastic expansion of pro-B and pre-B cells otherwise driven by c-Myc overexpression, sensitized these cells to apoptosis and ablated lymphoma formation. Remarkably, even loss of a single bcl-x allele delayed the lymphomagenesis. These findings identify Bcl-xL as a prerequisite for the emergence of c-Myc–driven pre-B/B lymphoma and suggest that BH3 mimetic drugs may provide a prophylactic strategy for c-Myc–driven tumors.

Odontoblast-targeted Bcl-2 overexpression promotes dentine damage repair

ObjectiveBcl-2 is widely expressed in a developing tooth organ and regulates tooth morphogenesis. However, whether Bcl-2 is related to tooth damage repair is unknown yet. Using an odontoblast-targeted Bcl-2 overexpression transgenic mouse (Col2.3Bcl-2) and artificial cavity preparation as a model system, the relationship between Bcl-2 and reparative dentinogenesis is investigated in this study. MethodsThe odontoblastic-like cell cultures derived from mouse molar pulps were established. The expression of transgenic human Bcl-2 (hBcl-2) and endogenous mouse Bcl-2 (mBcl-2) and mouse Bax (mBax, a Bcl-2 antagonist) was detected in vivo and in vitro by Western blot and immunocytochemistry, respectively. Basal level and artificial cavity-induced odontoblast apoptosis was detected by the Deoxynucleotidyl Transferase (TdT) dUTP Nick End labelling (TUNEL) technique. Reparative dentine formation induced by artificial cavity drilled to a half dentine thickness on mesial cervical region of mandibular first molars 2, 4, and 6 weeks post-op was evaluated histologically and via micro-CT. ResultsThe transgenic hBcl-2 was stably expressed in odontoblasts of the transgenic animals without interference with the expression of mBcl-2 and mBax. Basal level as well as artificial cavity- induced odontoblast apoptosis was prevented by the transgene. Compared to the wild type, the transgenic animals produced reparative dentine with significantly higher mineral density 6 weeks after the operation. ConclusionsBcl-2 overexpression prevents odontoblast apoptosis and promotes dentine damage repair, indicating that genetic manipulation of Bcl-2 may be a novel strategy to maintain the vitality and function of dentine-pulp complex under detrimental mechanical stimuli.

The matrix (M) protein of newcastle disease virus binds to human bax through its BH3 domain

The underlying mechanisms by which Newcastle disease virus (NDV) kills cancer cells are still unclear. Recent discoveries have shown that many viruses contain Bcl-2 homology-like domains which enabled their interaction with Bcl-2 family members, and thereby accounting for their virulence and pathogenicity. Alignment of the protein sequences of Malaysian strain of NDV, known as AF2240, with those from members of the human Bcl-2 family showed many similar regions; most notably we found that its matrix (AF2240-M) protein, large (AF2240-L) protein and fusion (AF2240-F) protein all contain BH3-like regions. In addition, there are BH1-like domains in these proteins, where AF2240-F and Mcl-1 share 55% identity within this region. To further investigate our hypothesis that the presence of the BH3-like domains in these proteins may convey cytotoxicity, AF2240-M and AF2240-F genes were cloned into pFLAG and pEGFP.N2 vectors and transfected into HeLa cells. The expression of these constructs promoted cell death. As shown by flow cytometry, AF2240-M protein with deleted BH3-like region showed five-fold decrease in apoptosis. Moreover, the construct containing the N-terminal of AF2240-M showed nearly the same cell death rate as to that of the full-length protein, strongly suggesting that the BH3-like domain within this protein participates in promoting cell death. Moreover, AF2240-M transfection promoted Bax redistribution to mitochondria. Therefore, to determine whether there is any direct interaction between NDV viral proteins with some members of the Bcl-2 family, various constructs were co-transfected into HeLa cells. Co-immunoprecipitation trials showed that the AF2240-M indeed directly interacted with Bax protein via its BH3-domain, as the mutant proteins failed to interact with Bax. AF2240-F failed to interact with any of the tested proteins, although Bcl-XL slowed down the rate of cell death caused by this construct by nearly five-fold. In a parallel experiment, the level of expression of endogenous Bax and Bcl-2 after infection of HeLa cells with NDV was assessed by qRT-PCR, but no statistically significant change was observed. Consequently, the Bax/Bcl-2 ratio at the mRNA level did not alter. Overall, our study has shed additional light into the mechanisms by which NDV induces apoptosis.

Physical Association of HDAC1 and HDAC2 with p63 Mediates Transcriptional Repression and Tumor Maintenance in Squamous Cell Carcinoma

Squamous cell carcinoma (SCC) is a treatment-refractory subtype of human cancer arising from stratified epithelium of the skin, lung, esophagus, oropharynx, and other tissues. A unifying feature of SCC is high-level expression of the p53-related protein p63 (TP63) in 80% of cases. The major protein isoform of p63 expressed in SCC is ΔNp63α, an N-terminally truncated form which functions as a key SCC cell survival factor by mechanisms that are unclear. In this study, we show that ΔNp63α associates with histone deacetylase 1 (HDAC1) and HDAC2 to form an active transcriptional repressor complex that can be targeted to therapeutic advantage. Repression of proapoptotic Bcl-2 family member genes including p53 upregulated modulator of apoptosis (PUMA) by p63/HDAC is required for survival of SCC cells. Cisplatin chemotherapy, a mainstay of SCC treatment, promotes dissociation of p63 and HDAC from the PUMA promoter, leading to increased histone acetylation, PUMA activation, and apoptosis. These effects are recapitulated upon targeting the p63/HDAC complex selectively with class I/II HDAC inhibitors using both in vitro and in vivo models. Sensitivity to HDAC inhibition is directly correlated with p63 expression and is abrogated in tumor cells that overexpress endogenous Bcl-2. Together, our results elucidate a mechanism of p63-mediated transcriptional repression and they identify the ΔNp63α/HDAC complex as an essential tumor maintenance factor in SCC. In addition, our findings offer a rationale to apply HDAC inhibitors for SCC treatment.

Noxa/Bcl-2 Protein Interactions Contribute to Bortezomib Resistance in Human Lymphoid Cells

Previous studies have suggested that the BH3 domain of the proapoptotic Bcl-2 family member Noxa only interacts with the anti-apoptotic proteins Mcl-1 and A1 but not Bcl-2. In view of the similarity of the BH3 binding domains of these anti-apoptotic proteins as well as recent evidence that studies of isolated BH3 domains can potentially underestimate the binding between full-length Bcl-2 family members, we examined the interaction of full-length human Noxa with anti-apoptotic human Bcl-2 family members. Surface plasmon resonance using bacterially expressed proteins demonstrated that Noxa binds with mean dissociation constants (K(D)) of 3.4 nm for Mcl-1, 70 nm for Bcl-x(L), and 250 nm for wild type human Bcl-2, demonstrating selectivity but not absolute specificity of Noxa for Mcl-1. Further analysis showed that the Noxa/Bcl-2 interaction reflected binding between the Noxa BH3 domain and the Bcl-2 BH3 binding groove. Analysis of proteins expressed in vivo demonstrated that Noxa and Bcl-2 can be pulled down together from a variety of cells. Moreover, when compared with wild type Bcl-2, certain lymphoma-derived Bcl-2 mutants bound Noxa up to 20-fold more tightly in vitro, pulled down more Noxa from cells, and protected cells against killing by transfected Noxa to a greater extent. When killing by bortezomib (an agent whose cytotoxicity in Jurkat T-cell leukemia cells is dependent on Noxa) was examined, apoptosis was enhanced by the Bcl-2/Bcl-x(L) antagonist ABT-737 or by Bcl-2 down-regulation and diminished by Bcl-2 overexpression. Collectively, these observations not only establish the ability of Noxa and Bcl-2 to interact but also identify Bcl-2 overexpression as a potential mechanism of bortezomib resistance.

BCL-2 Is a Downstream Target of ATF5 That Mediates the Prosurvival Function of ATF5 in a Cell Type-dependent Manner

ATF5 loss of function has been shown previously to cause apoptotic cell death in glioblastoma and breast cancer cells but not in non-transformed astrocytes and human breast epithelial cells. The mechanism for the cell type-dependent survival function of ATF5 is unknown. We report here that the anti-apoptotic factor BCL-2 is a downstream target of ATF5 that mediates the prosurvival function of ATF5 in C6 glioma cells and MCF-7 breast cancer cells. ATF5 binds to an ATF5-specific regulatory element that is downstream of and adjacent to the negative regulatory element in the BCL-2 P2 promoter, stimulating BCL-2 expression. Highlighting the critical role of BCL-2 in ATF5-dependent cancer cell survival, expression of BCL-2 blocks death of C6 and MCF-7 cells induced by dominant-negative ATF5, and depletion of BCL-2 impairs ATF5-promoted cell survival. Moreover, we found that BCL-2 expression is not regulated by ATF5 in non-transformed rat astrocytes, mouse embryonic fibroblasts, and human breast epithelial cells, where expression of BCL-2 but not ATF5 is required for cell survival. These findings identify BCL-2 as an essential mediator for the cancer-specific cell survival function of ATF5 in glioblastoma and breast cancer cells and provide direct evidence that the cell type-specific function of ATF5 derives from differential regulation of downstream targets by ATF5 in different types of cells.

Mitochondrial proteomic approach reveals galectin-7 as a novel BCL-2 binding protein in human cells

Although the anti-apoptotic activity of Bcl-2 has been extensively studied, its mode of action remains incompletely understood. Deciphering the network of Bcl-2 interacting factors is necessary to better understand the key function of Bcl-2 in apoptosis initiation. To identify novel Bcl-2 mitochondrial partners, we have combined a Bcl-2 immunocapture with a mass spectrometry analysis using highly pure mitochondrial fractions isolated from human cancer cells. We identified at high confidence 127 potential Bcl-2-interacting proteins. Gene ontology mining reveals enrichment for mitochondrial proteins, endoplasmic reticulum-associated proteins, and cytoskeleton-associated proteins. Importantly, we report the identification of galectin-7 (Gal7), a member of a family of β-galactoside-binding lectins that was already known to exhibit a pro-apoptotic function, as a new mitochondrial Bcl-2 interacting partner. Our data further show that endogenous Bcl-2 coimmunoprecipitates with Gal7 and that recombinant Gal7 directly interacts with recombinant Bcl-2. A fraction of Gal7 is constitutively localized at mitochondria in a Bcl-2-dependent manner and sensitizes the mitochondria to the apoptotic signal. In addition, we show that the Bcl-2/Gal7 interaction is abolished following genotoxic stress. Taken together, our findings suggest that the binding of Gal7 to Bcl-2 may constitute a new target for enhancing the intrinsic apoptosis pathway.

Bax-derived membrane-active peptides act as potent and direct inducers of apoptosis in cancer cells.

Although many cancer cells are primed for apoptosis, they usually develop resistance to cell death at several levels. Permeabilization of the outer mitochondrial membrane, which is mediated by proapoptotic Bcl-2 family members such as Bax, is considered as a point of no return for initiating apoptotic cell death. This crucial role has placed Bcl-2 family proteins as recurrent targets for anticancer drug development. Here, we propose and demonstrate a new concept based on minimal active versions of Bax to induce cell death independently of endogenous Bcl-2 proteins. We show that membrane-active segments of Bax can directly induce the release of mitochondria-residing apoptogenic factors and commit tumor cells promptly and irreversibly to caspase-dependent apoptosis. On this basis, we designed a peptide encompassing part of the Bax pore-forming domain, which can target mitochondria, induce cytochrome c release and trigger caspase-dependent apoptosis. Moreover, this Bax-derived 'poropeptide' produced effective tumor regression after peritumoral injection in a nude mouse xenograft model. Thus, peptides derived from proteins that form pores in the mitochondrial outer membrane represent novel templates for anticancer agents.

Targeted knockdown of Bcl2 in tumor cells using a synthetic TRAIL 3′‐UTR microRNA

Targeting tumor-related overexpression of anti-apoptotic Bcl2 protein by RNAi has been suggested as a potential treatment for cancer. However, the stability of RNAi and its delivery are still major obstacles to the clinical testing of Bcl2 RNAi. Here, we explore a novel strategy of expressing a synthetic Bcl2 microRNA (smRNA) in the 3' untranslated region (UTR) of the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), an apoptosis-inducing protein without apparent toxic effects in normal cells. TRAIL was specifically expressed from the human telomerase reverse transcriptase promoter (pTRT) that is active in many human tumors. Using this approach, we demonstrated that pTRT drove the tumor-specific expression of Bcl2 smRNA, which was processed by the host RNAi machinery and silenced endogenous Bcl2 expression in tumor cells. Bcl2 smRNA induced tumor cell apoptosis by activating caspase-3 and led to significant sensitization of tumor cells to TRAIL-induced apoptosis, while normal cells were spared. We also showed that the combined therapy of TRAIL-induced apoptosis and Bcl2 downregulation was superior to the mono-therapy of TRAIL or Bcl2 smRNA alone. This study proves a general paradigm for cancer therapy by using 3' UTR microRNA technology.

Extracellular BCL2 Proteins Are Danger-Associated Molecular Patterns That Reduce Tissue Damage in Murine Models of Ischemia-Reperfusion Injury

BackgroundIschemia-reperfusion (I/R) injury contributes to organ dysfunction in a variety of clinical disorders, including myocardial infarction, stroke, organ transplantation, and hemorrhagic shock. Recent investigations have demonstrated that apoptosis as an important mechanism of cell death leading to organ dysfunction following I/R. Intracellular danger-associated molecular patterns (DAMPs) released during cell death can activate cytoprotective responses by engaging receptors of the innate immune system.Methodology/Principal FindingsIschemia was induced in the mouse hind limb by tourniquet or in the heart by coronary artery ligation. Reperfusion injury of skeletal or cardiac muscle was markedly reduced by intraperitoneal or subcutaneous injection of recombinant human (rh)BCL2 protein or rhBCL2-related protein A1 (BCL2A1) (50 ng/g) given prior to ischemia or at the time of reperfusion. The cytoprotective activity of extracellular rhBCL2 or rhBCL2A1 protein was mapped to the BH4 domain, as treatment with a mutant BCL2 protein lacking the BH4 domain was not protective, whereas peptides derived from the BH4 domain of BCL2 or the BH4-like domain of BCL2A1 were. Protection by extracellular rhBCL2 or rhBCL2A1 was associated with a reduction in apoptosis in skeletal and cardiac muscle following I/R, concomitant with increased expression of endogenous mouse BCL2 (mBCL2) protein. Notably, treatment with rhBCL2A1 protein did not protect mice deficient in toll-like receptor-2 (TLR2) or the adaptor protein, myeloid differentiation factor-88 (MyD88).Conclusions/SignificanceTreatment with cytokine-like doses of rhBCL2 or rhBCL2A1 protein or BH4-domain peptides reduces apoptosis and tissue injury following I/R by a TLR2-MyD88-dependent mechanism. These findings establish a novel extracellular cytoprotective activity of BCL2 BH4-domain proteins as potent cytoprotective DAMPs.

Effect of Modulation of hnRNP L Levels on the Decay of bcl-2 mRNA in MCF-7 Cells

It has been shown that CA repeats in the 3'-untranslated region (UTR) of bcl-2 mRNA contribute the constitutive decay of bcl-2 mRNA and that hnRNP L (heterogenous nuclear ribonucleoprotein L) interacts with CA repeats in the 3'-UTR of bcl-2 mRNA, both in vitro and in vivo. The aim of this study was to determine whether the alteration of hnRNP L affects the stability of bcl-2 mRNA in vivo. Human breast carcinoma MCF-7 cells were transfected with hnRNP L-specific shRNA or hnRNP L-expressing vector to decrease or increase hnRNP L levels, respectively, followed by an actinomycin D chase. An RT-PCR analysis showed that the rate of degradation of endogenous bcl-2 mRNA was not affected by the decrease or increase in the hnRNP L levels. Furthermore, during apoptosis or autophagy, in which bcl-2 expression has been reported to decrease, no difference in the degradation of bcl-2 mRNA was observed between control and hnRNP L-knock down MCF-7 Cells. On the other hand, the levels of AUF-1 and nucleolin, transacting factors for ARE in the 3'UTR of bcl-2 mRNA, were not significantly affected by the decrease in hnRNP L, suggesting that a disturbance in the quantitative balance between these transacting factors is not likely to interfere with the effect of hnRNP L. Collectively, the findings indicate that the decay of bcl-2 mRNA does not appear to be directly controlled by hnRNP L in vivo.

C/EBPα or Its Leukemic Mutants Directly Activate nfkb1 Transcription and Displace HDAC1 or HDAC3 From Chromatin-Associated NF-κB p50 to Induce Anti-Apoptotic Genes.

Abstract 76Mutated variants of C/EBPα are expressed in approximately 15% of AML cases. We have previously shown that C/EBPα and its AML mutants interact directly with NF-κB p50 through the basic region. Through binding to a DNA κB site they activate anti-apoptotic genes including BCL2 and FLIP, inhibiting both the extrinsic and intrinsic apoptotic pathways. These effects do not depend on DNA binding but rather on the integrity of the basic region, which is rarely altered in C/EBPα AML oncoproteins. Interaction with p50 is necessary for the C/EBPα induction of bcl-2 and FLIP but not of myeloid differentiation genes. In addition, NF-κB p50 binds and activates the CEBPA gene in myeloid cells. We now report that C/EBPα or its AML mutants, including a leukemia-derived leucine zipper mutant that cannot bind DNA directly (C/EBPαLZ), bind in vivo to the nfkb1 (p50) promoter. Using Ba/F3 cell lines expressing an estrogen receptor fusion of C/EBPα or the C/EBPαLZ mutant we demonstrate induction of p50 mRNA within 7 hrs after the addition of estradiol. A basic region mutant (C/EBPαBR) that cannot bind the p50 protein did not activate nfkb1 transcription. Importantly, the translation inhibitor cycloheximide did not prevent nfkb1 induction by C/EBPα, suggesting direct regulation of nfkb1 transcription by C/EBPα. Remarkably, C/EBPα did not induce p65 RNA expression. A minimal 200 bp nfkb1 promoter, containing κB but not C/EBP binding sites, is trans-activated by C/EBPα 9 fold, or 33 fold by the C/EBPαLZ AML oncoprotein. The variant C/EBPαBR fails to trans-activate the nfkb1 promoter. Together, these data indicate that the gene encoding NF-κB p50 is a direct target of C/EBPα. NF-κB p65 is held in the cytoplasm IκBα and enters the nucleus to bind DNA predominantly as p65:p50 heterodimers upon cell stimulation. In contrast, p50 homodimers are present in the nucleus bound to chromatin in resting cells and inhibit transcription due to the lack of a trans-activation domain and the recruitment of co-repressors and histone deacetylases (HDACs). Therefore, the induction of p50 by C/EBPα would be expected to result in diminished expression of target genes. However, our data indicates that C/EBPα and p50 synergistically activate anti-apoptotic genes. Remarkably, ChIP analysis demonstrated that C/EBPα induction from the metallothionein promoter in Ba/F3 cells diminishes the occupation of the endogenous BCL2 and FLIP promoters by HDAC1 but not HDAC3, although induction of C/EBPα expression did not alter the cellular content of either HDAC. In contrast, occupancy of Bcl-3, another p50 transcriptional cofactor, on these promoters did not change. In transiently transfected 293T cells we confirmed that p50 binds to HDAC1 in a co-immunoprecipitation reaction. Remarkably, interaction between p50 and HDAC1 was disrupted in the presence of co-expressed C/EBPα, C/EBPβ, or the AML mutants C/EBPαLZ, or C/EBPαp30. In a striking correlation with our previous findings, the C/EBPαBR basic region mutant of C/EBPα that does not bind p50 did not interrupt this interaction. As an important control, we confirmed that HDAC1 does not interact directly with C/EBPα or its mutants, indicating that C/EBPα competes with HDAC1 for interaction with p50. Interestingly, the AML-derived C/EBPαLZ or C/EBPαp30 oncoproteins, but not wild-type C/EBPα, interrupt the interaction between HDAC3 and p50. This suggests a greater capacity for these leukemia mutants to de-repress NF-κB target genes compared with C/EBPα itself. In summary, C/EBPα cooperates with NF-κB p50 to induce several anti-apoptotic genes, thereby providing a survival advantage to the leukemic blasts. C/EBPα and p50 reciprocally regulate each other's expression at the transcriptional level, establishing a positive feedback relationship. Moreover, C/EBPα or its AML mutants displaces HDAC1, and the C/EBPα oncoproteins displace HDAC3, from NF-κB p50 homodimers bound to anti-apoptotic genes, contributing to NF-κB dysregulation in leukemia. Our data underscore the concept that the C/EBPα:NF-κB p50 complex is a potential novel therapeutic target in AML, and the C/EBPβ:p50 complex may functional similarly in lymphomas and in other malignancies where C/EBPβ rather than C/EBPα expression predominates. Disclosures:No relevant conflicts of interest to declare.

HnRNP L binds to CA repeats in the 3′UTR of bcl-2 mRNA

We previously reported that the CA-repeat sequence in the 3′-untranslated region (3′UTR) of bcl-2 mRNA is involved in the decay of bcl-2 mRNA. However, the trans-acting factor for the CA element in bcl-2 mRNA remains unidentified. The heterogeneous nuclear ribonucleoprotein L (hnRNP L), an intron splicing factor, has been reported to bind to CA repeats and CA clusters in the 3′UTR of several genes. We reported herein that the CA repeats of bcl-2 mRNA have the potential to form a distinct ribonuclear protein complex in cytoplasmic extracts of MCF-7 cells, as evidenced by RNA electrophoretic mobility shift assays (REMSA). A super-shift assay using the hnRNP L antibody completely shifted the complex. Immunoprecipitation with the hnRNP L antibody and MCF-7 cells followed by RT-PCR revealed that hnRNP L interacts with endogenous bcl-2 mRNA in vivo. Furthermore, the suppression of hnRNP L in MCF-7 cells by the transfection of siRNA for hnRNP L resulted in a delay in the degradation of RNA transcripts including CA repeats of bcl-2 mRNA in vitro, suggesting that the interaction between hnRNPL and CA repeats of bcl-2 mRNA participates in destabilizing bcl-2 mRNA.

Bcl-2 Blocks Cisplatin-Induced Apoptosis and Predicts Poor Outcome Following Chemoradiation Treatment in Advanced Oropharyngeal Squamous Cell Carcinoma

This study aimed to test the hypothesis that elevated expression of antiapoptotic Bcl-2 family proteins predicts a poor therapeutic response of oropharyngeal squamous cell carcinoma (OPSCC) to concurrent platinum-based chemoradiation therapy. Levels of Bcl-2, Bcl-XL, and Bcl-w were determined and correlated with resistance to cisplatin in a large panel of cell lines derived from squamous cell carcinoma of the head and neck (HNSCC). Univariate and multivariate analyses were used to evaluate the relationship between Bcl-2 and Bcl-XL expression and disease-free survival following chemoradiation therapy in a uniformly treated cohort of patients with OPSCC. In HNSCC cell lines, high endogenous Bcl-2 expression was associated with increased cisplatin resistance, and experimental overexpression of Bcl-2 promoted cisplatin resistance. In patients, tumors positive for Bcl-2 before treatment had greater risk of treatment failure (hazard ratio, 5.99; 95% confidence interval, 1.73-20.8; P=0.0014). In contrast, endogenous Bcl-XL showed no correlation either with cisplatin sensitivity in the cell line panel in vitro, or with risk of recurrence in vivo (hazard ratio, 1.28; 95% confidence interval, 0.39-4.19; P=0.68). Associations between Bcl-2 expression and other clinical characteristics did not account for the predictive value of Bcl-2. Immunohistochemical assessment of Bcl-2 in pretreatment biopsy specimens can predict response of advanced OPSCC to concurrent platinum-based chemoradiation. As treatments targeting Bcl-2 and its family members become available, this immunohistochemical assessment could help personalize therapy by identifying a subpopulation of patients with a poor prognosis who might benefit from such treatments.