Multidomain Proapoptotic Proteins Research Articles

Decoding the concealed transcriptional signature of the apoptosis-related BCL2 antagonist/killer 1 (BAK1) gene in human malignancies.

BCL2 antagonist/killer (BAK) is a multidomain pro-apoptotic effector protein, encoded by the human BAK1 gene, which has emerged as a key checkpoint in the apoptotic machinery. Disassembly of BAK's tertiary structure, such as the truncation of the α1 helix, leads to deregulation of the pro-apoptotic functions and reduction of the protein's stability, thus being implicated in human malignancies. Although many studies have already clarified the vital role of BAK in cellular mechanisms, its pre-mRNA maturation process under cancerous and physiological human cells is neglected. In the present work, we developed and employed a custom multiplexed nanopore sequencing approach that enabled the identification and structural characterization of previously undescribed BAK1 mRNA transcripts (BAK1 v.2-v.11). The described novel mRNAs are derived from multiple types of alternative splicing events, including exon skipping and intron retentions. The implemented multiplexed long-read sequencing approach provided the detailed expression profile of the novel mRNAs in a wide panel of human malignancies and at the same time allowed their relative quantification as compared to the annotated BAK1 v.1. The validation of each novel transcript was carried out with qPCR-based assays. Our results strongly support that most of the novel BAK1 mRNAs harbor open reading frames with conserved BH domains that provide new insights into the correlated mechanisms of apoptosis suppression and cancer. The current study highlights for the first time the hidden aspects of BAK1's transcriptional landscape in both physiological and cancerous human cells and distinguishes the amino acid sequence of the putative BAK isoforms that may possess key apoptosis-related functions not only in diseases, but also under normal cellular conditions.

Paradoxical implication of BAX/BAK in the persistence of tetraploid cells

Pro-apoptotic multi-domain proteins of the BCL2 family such as BAX and BAK are well known for their important role in the induction of mitochondrial outer membrane permeabilization (MOMP), which is the rate-limiting step of the intrinsic pathway of apoptosis. Human or mouse cells lacking both BAX and BAK (due to a double knockout, DKO) are notoriously resistant to MOMP and cell death induction. Here we report the surprising finding that BAX/BAK DKO cells proliferate less than control cells expressing both BAX and BAK (or either BAX or BAK) when they are driven into tetraploidy by transient exposure to the microtubule inhibitor nocodazole. Mechanistically, in contrast to their BAX/BAK-sufficient controls, tetraploid DKO cells activate a senescent program, as indicated by the overexpression of several cyclin-dependent kinase inhibitors and the activation of β-galactosidase. Moreover, DKO cells manifest alterations in ionomycin-mobilizable endoplasmic reticulum (ER) Ca2+ stores and store-operated Ca2+ entry that are affected by tetraploidization. DKO cells manifested reduced expression of endogenous sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 2a (Serca2a) and transfection-enforced reintroduction of Serca2a, or reintroduction of an ER-targeted variant of BAK into DKO cells reestablished the same pattern of Ca2+ fluxes as observed in BAX/BAK-sufficient control cells. Serca2a reexpression and ER-targeted BAK also abolished the tetraploidy-induced senescence of DKO cells, placing ER Ca2+ fluxes downstream of the regulation of senescence by BAX/BAK. In conclusion, it appears that BAX/BAK prevent the induction of a tetraploidization-associated senescence program. Speculatively, this may contribute to the low incidence of cancers in BAX/BAK DKO mice and explain why human cancers rarely lose the expression of both BAX and BAK.

Interactions of multidomain pro-apoptotic and anti-apoptotic proteins in cancer cell death.

Cancer is a global public health concern that is characterized by the uncontrolled growth of tumor cells. It is regarded as the subsequent cause of death after cardiovascular disease. The most common types of cancer include breast, colorectal, lung, and prostate. The risk factors attributed to the development of common types of cancer are tobacco smoking, excessive alcohol consumption, dietary factors, ultraviolet radiation (UV), and lack of physical activities. Two major cellular apoptotic pathways targeted in cancer therapies are intrinsic and extrinsic. These two pathways are regulated by different types of proteins, the multidomain pro-apoptotic proteins (Bak, Bax, and Bok), BH3-only pro-apoptotic proteins (Bid, Bim, Bad, Noxa, and Puma), and the anti-apoptotic proteins (Mcl-1, Bfl-1, Bcl-XL, Bcl-2, Bcl-w, and Bcl-B). Other significant molecules/factors that are known to execute cellular apoptotic pathways include bioactive compounds, and reactive oxygen species (ROS). Proteolytic caspases are known to play a vital role in the initiation of apoptotic activities in cancerous cells. Based on their functions, they are categorized into initiators and executioners. Nanotechnology has produced novel outcomes in modern medicine. The green synthesis of nanoparticles has demonstrated prospective improvements in cancer therapies in combination with the existing therapies including photodynamic therapy. This review aims at highlighting the association between pro-apoptotic and anti-apoptotic proteins, and their significance in cancer therapy.

Apoptotic stress induces Bax-dependent, caspase-independent redistribution of LINC complex nesprins

The canonical function of Bcl-2 family proteins is to regulate mitochondrial membrane integrity. In response to apoptotic signals the multi-domain pro-apoptotic proteins Bax and Bak are activated and perforate the mitochondrial outer membrane by a mechanism which is inhibited by their interaction with pro-survival members of the family. However, other studies have shown that Bax and Bak may have additional, non-canonical functions, which include stress-induced nuclear envelope rupture and discharge of nuclear proteins into the cytosol. We show here that the apoptotic stimuli cisplatin and staurosporine induce a Bax/Bak-dependent degradation and subcellular redistribution of nesprin-1 and nesprin-2 but not nesprin-3, of the linker of nucleoskeleton and cytoskeleton (LINC) complex. The degradation and redistribution were caspase-independent and did not occur in Bax/Bak double knockout (DKO) mouse embryo fibroblasts (MEFs). Re-expression of Bax in Bax/Bak DKO MEFs restored stress-induced redistribution of nesprin-2 by a mechanism which requires Bax membrane localization and integrity of the α helices 5/6, and the Bcl-2 homology 3 (BH3) domain. We found that nesprin-2 interacts with Bax in close proximity to perinuclear mitochondria in mouse and human cells. This interaction requires the mitochondrial targeting and N-terminal region but not the BH3 domain of Bax. Our results identify nesprin-2 as a Bax binding partner and also a new function of Bax in impairing the integrity of the LINC complex.

Bcl-2 family proteins, beyond the veil.

Apoptosis is an important part of both health and disease and is often regulated by the BCL-2 family of proteins. These proteins are either pro- or anti-apoptotic, existing in a delicate balance during homeostasis. They are best known for their role in regulating the activation of caspases and the execution of a cell in response to a variety of stimuli. However, it is often forgotten that these BCL-2 family proteins also have important roles to play in cell maintenance that are not associated with apoptosis. These include roles in regulating processes such as cell cycle progression, mitochondrial function, autophagy, intracellular calcium concentration, glucose and lipid metabolism, and the unfolded protein response. In addition to these established alternate functions, further discoveries are being made that have potential therapeutic benefits in diseases such as cancer. BOK, a BCL-2 family protein thought comparable to multidomain pro-apoptotic proteins BAX and BAK, has recently been identified as a key player in metabolism of and resistance to the commonly used chemotherapeutic 5-FU. As a result of such findings, which could see the potential use of BOK as a biomarker for 5-FU sensitivity or mimetic molecules as a resensitization strategy, new targets and mechanisms of pathology may arise from further investigation into the realm of alternate functions of BCL-2 family proteins.

Photodynamic therapy with redaporfin targets the endoplasmic reticulum and Golgi apparatus.

Preclinical evidence depicts the capacity of redaporfin (Redp) to act as potent photosensitizer, causing direct antineoplastic effects as well as indirect immune-dependent destruction of malignant lesions. Here, we investigated the mechanisms through which photodynamic therapy (PDT) with redaporfin kills cancer cells. Subcellular localization and fractionation studies based on the physicochemical properties of redaporfin revealed its selective tropism for the endoplasmic reticulum (ER) and the Golgi apparatus (GA). When activated, redaporfin caused rapid reactive oxygen species-dependent perturbation of ER/GA compartments, coupled to ER stress and an inhibition of the GA-dependent secretory pathway. This led to a general inhibition of protein secretion by PDT-treated cancer cells. The ER/GA play a role upstream of mitochondria in the lethal signaling pathway triggered by redaporfin-based PDT Pharmacological perturbation of GA function or homeostasis reduces mitochondrial permeabilization. In contrast, removal of the pro-apoptotic multidomain proteins BAX and BAK or pretreatment with protease inhibitors reduced cell killing, yet left the GA perturbation unaffected. Altogether, these results point to the capacity of redaporfin to kill tumor cells via destroying ER/GA function.

CoptidisRhizoma induces intrinsic apoptosis through BAX and BAK activation in human melanoma.

Malignant melanoma has exhibited a rising incidence in recent years worldwide. Although various molecular targeted drugs are being researched and developed for melanoma patients, their efficacy appears to be unsatisfactory. Over the past few years, several reports have demonstrated that CoptidisRhizoma water extracts (CR) or its major active chemical component, berberine, has anticancer activities in various types of cancer, including melanoma. However, their underlying mechanisms have not been well understood. In the present study, we determined that CR suppressed melanoma cell viability, which was mainly mediated through apoptosis. In addition, the expression levels of anti-apoptotic proteins, BCL2A1, MCL1 and BCL-w, were strongly suppressed by CR treatment. Furthermore, multi-domain pro-apoptotic proteins BAX and BAK were activated by CR treatment and were also required for the CR-induced apoptosis. Collectively, CR or some formulations containing CR, may be effective safe treatment strategies for human melanoma.

Cancer-selective death of human breast cancer cells by leelamine is mediated by bax and bak activation.

The present study is the first to report inhibition of breast cancer cell growth in vitro and in vivo and suppression of self-renewal of breast cancer stem cells (bCSC) by a pine bark component (leelamine). Except for a few recent publications in melanoma, anticancer pharmacology of this interesting phytochemical is largely elusive. Leelamine (LLM) dose-dependently inhibited viability of MDA-MB-231 (triple-negative), MCF-7 (estrogen receptor-positive), and SUM159 (triple-negative) human breast cancer cells in association with apoptotic cell death induction. To the contrary, a normal mammary epithelial cell line derived from fibrocystic breast disease and spontaneously immortalized (MCF-10A) was fully resistant to LLM-mediated cell growth inhibition and apoptosis induction. LLM also inhibited self-renewal of breast cancer stem cells. Apoptosis induction by LLM in breast cancer cells was accompanied by a modest increase in reactive oxygen species production, which was not due to inhibition of mitochondrial electron transport chain complexes. Nevertheless, ectopic expression of manganese superoxide dismutase conferred partial protection against LLM-induced cell death but only at a lower yet pharmacologically relevant concentration. Exposure of breast cancer cells to LLM resulted in (a) induction and/or activation of multidomain proapoptotic proteins Bax and Bak, (b) caspase-9 activation, and (c) cytosolic release of cytochrome c. Bax and Bak deficiency in immortalized fibroblasts conferred significant protection against cell death by LLM. Intraperitoneal administration of LLM (7.5 mg/kg; 5 times/wk) suppressed the growth of orthotopic SUM159 xenografts in mice without any toxicity. In conclusion, the present study provides critical preclinical data to warrant further investigation of LLM. © 2016 Wiley Periodicals, Inc.

Cathepsin-Cleavable BIM BH3 Peptide Amphiphiles Are Potent Inducers of Cellular Apoptosis

The BCL-2 family of proteins forms a complex interaction network that regulates cellular life and death decisions and contributes to cancer development, maintenance, and chemoresistance. BH3 only member proteins (e.g. BIM) serve as cellular stress sentinels and, when triggered, signal irreversible activation of apoptosis through their α-helical BH3 death domains. These pro-apoptotic signals are normally held in check by the multidomain anti-apoptotic proteins (e.g. BCL-XL, MCL-1) but when they are unable to do so the multidomain pro-apoptotic proteins BAX and BAK induce cell death through pore formation in the mitochondrial outer membrane. Therapeutic manipulation of the BCL-2 family with BH3 mimetics (including small molecules and synthetic peptides) is an emerging paradigm in cancer treatment and immune modulation. The design of next-generation therapeutics based on the BIM BH3 helix offers the unique advantage of recapitulating BIM's natural capacity to directly target the full complement of anti- and pro-apoptotic BCL-2 proteins. Here, we utilize the highly active BH3 domain of BIM as part of a peptide amphiphile nanostructure designed to overcome malignant cell death blockade. Peptide amphiphiles consist of bioactive peptides linked to hydrophobic lipid-like tail groups. In aqueous solutions, amphiphiles spontaneously assemble into micelles. Micelle-based peptide delivery provides several advantages: single micelles deliver high concentration of peptides into cells, they stabilize peptide secondary structure(s), and they have the potential for combinatorial synthesis using multiple bioactive moieties targeting non-redundant cell death escape pathways. While the exact mechanism behind cellular uptake of peptide amphiphiles remains controversial, recent work has shown that peptide amphiphiles intracellularly traffic through lysosomes and endosomes. In order to prevent the bioactive peptides from being sequestered within these structures, a system of escape is needed. Lysosomes contain many well-characterized proteases, and cathepsin B has previously been utilized to release chemotherapeutics in the context of targetable antibody-based treatments. Here, we generate peptide amphiphiles with BIM BH3 peptides and show that these nanostructures are able to specifically bind recombinant BCL-2 proteins, are stable at physiologic temperatures and pH, quickly enter into cells, and induce dose-responsive apoptosis in malignant hematologic cancers as measured by viability and caspase 3/7 activation. We further demonstrate that incorporating a cathepsin B-cleavable linker between the BIM BH3 peptide and the hydrophobic tail within individual amphiphiles results in increased binding to recombinant BCL-2 proteins while also allowing for increased cellular uptake and mitochondrial localization leading to faster and more potent dose-dependent cytotoxicity and caspase activation in malignant cells. Thus, we have developed a modular and potentially targetable nanostructure that represents a new promising strategy for BCL-2 family modulation and apoptosis induction in cancer. DisclosuresNo relevant conflicts of interest to declare.

The oncolytic peptide LTX-315 kills cancer cells through Bax/Bak-regulated mitochondrial membrane permeabilization.

LTX-315 has been developed as an amphipathic cationic peptide that kills cancer cells. Here, we investigated the putative involvement of mitochondria in the cytotoxic action of LTX-315. Subcellular fractionation of LTX-315-treated cells, followed by mass spectrometric quantification, revealed that the agent was enriched in mitochondria. LTX-315 caused an immediate arrest of mitochondrial respiration without any major uncoupling effect. Accordingly, LTX-315 disrupted the mitochondrial network, dissipated the mitochondrial inner transmembrane potential, and caused the release of mitochondrial intermembrane proteins into the cytosol. LTX-315 was relatively inefficient in stimulating mitophagy. Cells lacking the two pro-apoptotic multidomain proteins from the BCL-2 family, BAX and BAK, were less susceptible to LTX-315-mediated killing. Moreover, cells engineered to lose their mitochondria (by transfection with Parkin combined with treatment with a protonophore causing mitophagy) were relatively resistant against LTX-315, underscoring the importance of this organelle for LTX-315-mediated cytotoxicity. Altogether, these results support the notion that LTX-315 kills cancer cells by virtue of its capacity to permeabilize mitochondrial membranes.

BCL-2 family member BOK promotes apoptosis in response to endoplasmic reticulum stress

B-cell lymphoma 2 (BCL-2) ovarian killer (BOK) is a BCL-2 family protein with high homology to the multidomain proapoptotic proteins BAX and BAK, yet Bok(-/-) and even Bax(-/-)Bok(-/-) and Bak(-/-)Bok(-/-) mice were reported to have no overt phenotype or apoptotic defects in response to a host of classical stress stimuli. These surprising findings were interpreted to reflect functional compensation among the BAX, BAK, and BOK proteins. However, BOK cannot compensate for the severe apoptotic defects of Bax(-/-)Bak(-/-) mice despite its widespread expression. Here, we independently developed Bok(-/-) mice and found that Bok(-/-) cells are selectively defective in their response to endoplasmic reticulum (ER) stress stimuli, consistent with the predominant subcellular localization of BOK at the ER. Whereas Bok(-/-) mouse embryonic fibroblasts exposed to thapsigargin, A23187, brefeldin A, DTT, geldanamycin, or bortezomib manifested reduced activation of the mitochondrial apoptotic pathway, the death response to other stimuli such as etoposide, staurosporine, or UV remained fully intact. Multiple organs in Bok(-/-) mice exhibited resistance to thapsigargin-induced apoptosis in vivo. Although the ER stress agents activated the unfolded protein response, both ATF4 and CHOP activation were diminished in Bok(-/-) cells and mice. Importantly, BAX and BAK were unable to compensate for the defective apoptotic response to ER stress observed in SV40-transformed and primary Bok(-/-) cells, and in vivo. These findings support a selective and distinguishing role for BOK in regulating the apoptotic response to ER stress, revealing--to our knowledge--the first bona fide apoptotic defect linked to Bok deletion.

P38δ MAPK phenotype: an indicator of chemotherapeutic response in oesophageal squamous cell carcinoma.

We recently documented p38δ differential expression and function in oesophageal squamous cell carcinoma (OESCC). This study expands upon these findings and investigates whether p38δ status in OESCC can influence response(s) to cytotoxic drugs. The antiproliferative effect of conventional cisplatin and 5-fluorouracil (CF) treatment was compared with the recently reviewed triple regime of cisplatin, 5-fluorouracil and doxorubicin (ACF). p38δ-positive and p38δ-negative cell lines were employed using cell-growth and clonogenic assays. Key regulators of intrinsic and extrinsic apoptotic pathways were measured. Wound-healing assays and a Boyden chamber were used to investigate the effect of drug treatments on cell migration. Functional networks were analysed in terms of changes in MAPK expression. p38δ-negative OESCC is less sensitive to standard CF chemotherapy compared with p38δ-positive cells. However, following ACF treatment p38δ-negative cells showed markedly decreased proliferation and cell migration, and increased apoptosis. ACF induced apoptosis through the extrinsic pathway involving Fas activation, caspase-8 and caspase-3 cleavage and degradation of PARP. Loss of mitochondrial membrane potential (ΔΨm) was observed but downregulation of multidomain proapoptotic proteins, as well as BH3-only proteins, suggests involvement of pathways other than the mitochondrial pathway. Interestingly, induction of p38 and ERK1/2, but not JNK1/2, was observed following ACF treatment. p38δ-negative OESCC is more resistant to traditional CF treatment compared with p38δ-positive OESCC. In light of these results, p38δ phenotyping of tumour tissue may be of considerable value in deciding on an optimal therapeutic strategy for patients with p38δ-negative OESCC.

Abstract 225: Overexpression of truncated Ron augments benzyl isothiocyanate-induced apoptosis in human breast cancer cells

Abstract Phytochemicals from edible plants remain attractive for prevention of breast cancer, which is a leading cause of cancer-related mortality in women worldwide. We have shown previously that benzyl isothiocyanate (BITC), a small-molecule found in edible cruciferous vegetables such as garden cress, confers significant protection against mammary cancer development in MMTV-neu transgenic mice. BITC-mediated prevention of mammary carcinogenesis in MMTV-neu mice in vivo is associated with tumor cell apoptosis induction and elimination of breast cancer stem cells (bCSC). In cultured human breast cancer cells, BITC-induced apoptosis is accompanied by activation of multidomain proapoptotic protein Bax whereas inhibition of bCSC self-renewal is significantly attenuated by overexpression of truncated Ron (recepteur d’origine nantais) tyrosine kinase. Ron expression is very low in normal human breast epithelium but its overexpression has been reported in about 50% of primary breast cancers. In addition, mammary-specific overexpression of wild-type Ron as well as its constitutively active form induces highly metastatic breast tumors in mice. The present study was undertaken to study the role of truncated Ron (sfRon) in BITC-induced apoptosis using MCF-7 and MDA-MB-361 human breast cancer cells. Overexpression of sfRon resulted in statistically significant augmentation of BITC-induced apoptotic cell death in both cell lines as judged by flow cytometry after staining with Annexin V and propidium iodide, analysis of histone associated DNA fragment release into the cytosol, and/or cell proliferation assay. On the other hand, sfRon overexpressing MCF-7 cells were resistant to BITC-mediated G2/M phase cell cycle arrest when compared with parental cells. Increased apoptosis following BITC treatment in sfRon overexpressing cells was due to enhanced activation of Bax and/or Bak. Taken together, these findings indicate that overexpression of sfRon sensitizes human breast cancer cells to BITC-induced apoptosis. This investigation was supported by the NCI grant CA129347-06. Citation Format: Anuradha Sehrawat, Shivendra V. Singh. Overexpression of truncated Ron augments benzyl isothiocyanate-induced apoptosis in human breast cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 225. doi:10.1158/1538-7445.AM2014-225

Beclin 1, an autophagy-related gene, augments apoptosis in U87 glioblastoma cells

Beclin 1 acts as a tumor suppressor and is an essential mediator of autophagy. Beclin 1 also interacts with Bcl-2 and can induce apoptosis by activating the mitochondrion permeabilizing function of proapoptotic multidomain proteins from the Bcl-2 family. Moreover, these Bcl-2 family members can activate autophagy by liberating Beclin 1 from its inhibition by Bcl-2/Bcl-XL at the level of the endoplasmic reticulum. We found that overexpression of Beclin 1 in U87 glioblastoma cells enhanced the capacity for cellular autophagy and induced apoptosis. Silencing of Beclin 1 decreased autophagic capacity but had little effect on apoptosis and cell proliferation. Beclin 1-Bcl-2 and Beclin 1-Bcl-xL complexes were detected by immunoprecipitation in cells that overexpressed Beclin 1. Furthermore, the levels of cytochrome c in the cytosol and the activity of caspases-3/-9 in the cytosol increased after overexpression of Beclin 1. Our results suggest that Beclin 1 induces apoptosis via binding to Bcl-2 and Bcl-xL, followed by the release of cytochrome c into the cytosol and activation of caspases-3/-9.

BH3-only proteins Noxa, Bik, Bmf, and Bid activate Bax and Bak indirectly when studied in yeast model

BH3-only proteins of the Bcl-2 family regulate programmed cell death in mammals through activation of multidomain proapoptotic proteins Bax and Bak in response to various proapoptotic stimuli by mechanism that remains under dispute. Here, we report that the cell death-promoting activity of BH3-only proteins Bik, Bmf, Noxa, and tBid can only be reconstituted in yeast when both multidomain proapoptotic and antiapoptotic Bcl-2 family proteins are present. Inability of these proteins to induce cell death in the absence of antiapoptotic proteins suggests that all tested BH3-only proteins likely activate Bax and Bak indirectly by inhibiting antiapoptotic proteins.

BAK and NOXA Are Critical Determinants of Mitochondrial Apoptosis Induced by Bortezomib in Mesothelioma

Based on promising preclinical efficacy associated with the 20S proteasome inhibitor bortezomib in malignant pleural mesothelioma (MPM), two phase II clinical trials have been initiated (EORTC 08052 and ICORG 05–10). However, the potential mechanisms underlying resistance to this targeted drug in MPM are still unknown. Functional genetic analyses were conducted to determine the key mitochondrial apoptotic regulators required for bortezomib sensitivity and to establish how their dysregulation may confer resistance. The multidomain proapoptotic protein BAK, but not its orthologue BAX, was found to be essential for bortezomib-induced apoptosis in MPM cell lines. Immunohistochemistry was performed on tissues from the ICORG-05 phase II trial and a TMA of archived mesotheliomas. Loss of BAK was found in 39% of specimens and loss of both BAX/BAK in 37% of samples. However, MPM tissues from patients who failed to respond to bortezomib and MPM cell lines selected for resistance to bortezomib conserved BAK expression. In contrast, c-Myc dependent transactivation of NOXA was abrogated in the resistant cell lines. In summary, the block of mitochondrial apoptosis is a limiting factor for achieving efficacy of bortezomib in MPM, and the observed loss of BAK expression or NOXA transactivation may be relevant mechanisms of resistance in the clinic.

Bax induces activation of the unfolded protein response by inducing HAC1 mRNA splicing in Saccharomyces cerevisiae

Bax, a multidomain pro-apoptotic Bcl-2 protein, localizes to the endoplasmic reticulum (ER), where it regulates ER stress-induced apoptosis. Adaptation to ER stress depends on the activation of an integrated signal transduction pathway known as the unfolded protein response (UPR). This study examined the death-inducing activity of Bax and its ability to induce UPR signalling pathways in yeast. We observed that inhibition of global translation in yeast cells expressing Bax correlated with Bax-induced cell death. Using a lacZ reporter containing several UPR cis-activating regulatory elements, we also found that Bax directly activated the UPR. Furthermore, this correlated with the splicing of HAC1 mRNA, a gene involved in UPR activation. Bax induced expression of representative UPR target genes such as KAR2, DER1 and GCN4. Finally, we found that Ire1p function is critical for Bax-induced cell death.

Apigeninidin induces apoptosis through activation of Bak and Bax and subsequent mediation of mitochondrial damage in human promyelocytic leukemia HL-60 cells

Treatment of human promyelocytic leukemia HL-60 cells with apigeninidin could induce cytotoxicity (IC50=∼80μM), along with apoptotic sub-G1 cells, TUNEL-positive apoptotic DNA fragmentation, activation of the multidomain pro-apoptotic Bcl-2 proteins (Bak and Bax), mitochondrial membrane potential (Δψm) loss, release of mitochondrial cytochrome c and AIF into the cytoplasm, activation of caspase-9, -3, -8, and -7, and cleavage of PARP and lamin B. These induced apoptotic events were accompanied by decrease of Bcl-2 level and increase of Bak and Bax levels. Apigeninidin-induced sub-G1 cells and activation of Bak and Bax were also detected in human acute leukemia Jurkat T cells, but not in Jurkat T cells overexpressing Bcl-2. Pretreatment of HL-60 cells with the pan-caspase inhibitor z-VAD-fmk reduced significantly apigeninidin-induced sub-G1 cells and caspase cascade activation, whereas it failed to suppress Bak and Bax activations, Δψm loss, and release of mitochondrial cytochrome c and AIF. None of FADD and caspase-8 deficiencies affected the sensitivity of Jurkat T cells to apigeninidin-induced cytotoxicity. These results demonstrated that apigeninidin-induced apoptosis was mediated by activation of Bak and Bax, mitochondrial damage and resultant release of not only cytochrome c, causing caspase cascade activation, but also caspase-independent death effector AIF in HL-60 cells.

Discovery of Marinopyrrole A (Maritoclax) as a Selective Mcl-1 Antagonist that Overcomes ABT-737 Resistance by Binding to and Targeting Mcl-1 for Proteasomal Degradation

The anti-apoptotic Bcl-2 family of proteins, including Bcl-2, Bcl-X(L) and Mcl-1, are well-validated drug targets for cancer treatment. Several small molecules have been designed to interfere with Bcl-2 and its fellow pro-survival family members. While ABT-737 and its orally active analog ABT-263 are the most potent and specific inhibitors to date that bind Bcl-2 and Bcl-X(L) with high affinity but have a much lower affinity for Mcl-1, they are not very effective as single agents in certain cancer types because of elevated levels of Mcl-1. Accordingly, compounds that specifically target Mcl-1 may overcome this resistance. In this study, we identified and characterized the natural product marinopyrrole A as a novel Mcl-1-specific inhibitor and named it maritoclax. We found that maritoclax binds to Mcl-1, but not Bcl-X(L), and is able to disrupt the interaction between Bim and Mcl-1. Moreover, maritoclax induces Mcl-1 degradation via the proteasome system, which is associated with the pro-apoptotic activity of maritoclax. Importantly, maritoclax selectively kills Mcl-1-dependent, but not Bcl-2- or Bcl-X(L)-dependent, leukemia cells and markedly enhances the efficacy of ABT-737 against hematologic malignancies, including K562, Raji, and multidrug-resistant HL60/VCR, by ∼60- to 2000-fold at 1-2 μM. Taken together, these results suggest that maritoclax represents a new class of Mcl-1 inhibitors, which antagonizes Mcl-1 and overcomes ABT-737 resistance by targeting Mcl-1 for degradation.

From Mitochondrial Biology to Magic Bullet: Navitoclax Disarms BCL-2 in Chronic Lymphocytic Leukemia

From Mitochondrial Biology to Magic Bullet: Navitoclax Disarms BCL-2 in Chronic Lymphocytic Leukemia