Bax/Bak-independent mitochondrial depolarization and reactive oxygen species induction by sorafenib overcome resistance to apoptosis in renal cell carcinoma.

Renal cell carcinoma (RCC) is polyresistant to chemo- and radiotherapy and biologicals, including TNF-related apoptosis-inducing ligand (TRAIL). Sorafenib, a multikinase inhibitor approved for the treatment of RCC, has been shown to sensitize cancer cells to TRAIL-induced apoptosis, in particular by down-regulation of the Bak-inhibitory Bcl-2 family protein Mcl-1. Here we demonstrate that sorafenib overcomes TRAIL resistance in RCC by a mechanism that does not rely on Mcl-1 down-regulation. Instead, sorafenib induces rapid dissipation of the mitochondrial membrane potential (ΔΨm) that is accompanied by the accumulation of reactive oxygen species (ROS). Loss of ΔΨm and ROS production induced by sorafenib are independent of caspase activities and do not depend on the presence of the proapoptotic Bcl-2 family proteins Bax or Bak, indicating that both events are functionally upstream of the mitochondrial apoptosis signaling cascade. More intriguingly, we find that it is sorafenib-induced ROS accumulation that enables TRAIL to activate caspase-8 in RCC. This leads to apoptosis that involves activation of an amplification loop via the mitochondrial apoptosis pathway. Thus, our mechanistic data indicate that sorafenib bypasses central resistance mechanisms through a direct induction of ΔΨm breakdown and ROS production. Activation of this pathway might represent a useful strategy to overcome the cell-inherent resistance to cancer therapeutics, including TRAIL, in multiresistant cancers such as RCC.

Bok is a genuine multi-BH-domain protein that triggers apoptosis in the absence of Bax and Bak.

The pro-apoptotic multidomain Bcl-2 proteins Bax and Bak (also known as BAK1) are considered the gatekeepers of the intrinsic pathway of apoptosis by triggering the mitochondrial release of cytochrome c The role of the third Bax- and Bak-homologous multidomain protein Bok, however, is still unresolved. As cells doubly deficient for Bax and Bak are largely resistant to various apoptotic stimuli, it has been proposed that Bok is either dispensable for apoptosis or that its role is dependent on Bax and Bak. Here, we demonstrate, in several cell systems, that Bok efficiently induces cytochrome c release and apoptosis even in the complete absence of both Bak and Bax. Moreover, modulation of endogenous Bok levels affects the apoptosis response. By RNA interference and targeted deletion of the Bok gene, we demonstrate that Bok can significantly influence the apoptotic response to chemotherapeutic drugs in ovarian carcinoma cells. Hence, our results not only establish Bok as a Bak- and Bax-independent apoptosis inducer, but also suggest a potential impact of Bok expression in ovarian cancer therapy.

2D and 3D similarity landscape analysis identifies PARP as a novel off-target for the drug Vatalanib.

BACKGROUND: Searching for two-dimensional (2D) structural similarities is a useful tool to identify new active compounds in drug-discovery programs. However, as 2D similarity measures neglect important structural and functional features, similarity by 2D might be underestimated. In the present study, we used combined 2D and three-dimensional (3D) similarity comparisons to reveal possible new functions and/or side-effects of known bioactive compounds. RESULTS: We utilised more than 10,000 compounds from the SuperTarget database with known inhibition values for twelve different anti-cancer targets. We performed all-against-all comparisons resulting in 2D similarity landscapes. Among the regions with low 2D similarity scores are inhibitors of vascular endothelial growth factor receptor (VEGFR) and inhibitors of poly ADP-ribose polymerase (PARP). To demonstrate that 3D landscape comparison can identify similarities, which are untraceable in 2D similarity comparisons, we analysed this region in more detail. This 3D analysis showed the unexpected structural similarity between inhibitors of VEGFR and inhibitors of PARP. Among the VEGFR inhibitors that show similarities to PARP inhibitors was Vatalanib, an oral "multi-targeted" small molecule protein kinase inhibitor being studied in phase-III clinical trials in cancer therapy. An in silico docking simulation and an in vitro HT universal colorimetric PARP assay confirmed that the VEGFR inhibitor Vatalanib exhibits off-target activity as a PARP inhibitor, broadening its mode of action. CONCLUSION: In contrast to the 2D-similarity search, the 3D-similarity landscape comparison identifies new functions and side effects of the known VEGFR inhibitor Vatalanib.

p14(ARF)-induced apoptosis in p53 protein-deficient cells is mediated by BH3-only protein-independent derepression of Bak protein through down-regulation of Mcl-1 and Bcl-xL proteins.

The p14(ARF) tumor suppressor plays a central role in regulating cell cycle arrest and apoptosis. We reported previously that p14(ARF) is capable of triggering apoptosis in a p53-independent manner. However, the mechanism remained unclear. Here we demonstrate that the p53-independent activation of the mitochondrial apoptosis pathway by p14(ARF) is primarily mediated by the pro-apoptotic Bax-homolog Bak. Expression of p14(ARF) exclusively triggers a N-terminal conformational switch of Bak, but not Bax, which allows for mitochondrial permeability shift, release of cytochrome c, activation of caspases, and subsequent fragmentation of genomic DNA. Although forced expression of Bak markedly sensitizes toward p14(ARF)-induced apoptosis, re-expression of Bax has no effect. Vice versa, knockdown of Bak by RNA interference attenuates p14(ARF)-induced apoptosis, whereas down-regulation of Bax has no effect. Bak activation coincides with a prominent, caspase-independent deprivation of the endogenous Bak inhibitors Mcl-1 and Bcl-x(L). In turn, mitochondrial apoptosis is fully blocked by overexpression of either Mcl-1 or Bcl-x(L). Taken together, these data indicate that in the absence of functional p53 and Bax, p14(ARF) triggers mitochondrial apoptosis signaling by activating Bak, which is facilitated by down-regulating anti-apoptotic Mcl-1 and Bcl-x(L). Moreover, our data suggest that the simultaneous inhibition of two central endogenous Bak inhibitors, i.e. Mcl-1 and Bcl-x(L), may be sufficient to activate mitochondrial apoptosis in the absence of BH3-only protein regulation.

Concurrent inhibition of PI3K and mTORC1/mTORC2 overcomes resistance to rapamycin induced apoptosis by down-regulation of Mcl-1 in mantle cell lymphoma.

Mantle cell lymphoma (MCL) is an aggressive form of Non-Hodgkin-lymphoma (NHL) with an ongoing need for novel treatments. Apart from the translocation t(11:14), which facilitates constitutive transcription of cyclin D1, additional aberrations are frequently observed in MCL, including a recurrent dysregulation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway. mTOR, a key component of this pathway, is pivotal for the assembly of mTOR complex (mTORC) 1 and 2. Temsirolimus, an analog of the mTOR inhibitor rapamycin, is approved for the treatment of relapsed MCL. Response rates, however, are low and response durations are short. We demonstrate that inhibition of mTORC1 by rapamycin or blocking of mTORC1 and mTORC2 in conjunction with PI3K by NVP-BEZ235 reduces proliferation of MCL cell lines to a similar extent. However, only NVP-BEZ235 is able to sufficiently inhibit the downstream pathway of mTOR and to mediate cell death through activation of the intrinsic apoptosis pathway. Further analysis demonstrated that the anti-apoptotic Bcl-2 family member Mcl-1 plays a central role in regulation of MCL survival. While Mcl-1 protein levels remained unchanged after coculture with rapamycin, they were down-regulated in NVP-BEZ235 treated cells. Furthermore, inhibition of Mcl-1 by the BH3-only mimetic obatoclax or down-regulation of constitutive Mcl-1, but not of Bcl-2 or Bcl-xL, by siRNA facilitated cell death of MCL cells and enhanced NVP-BEZ235's capacity to induce cell death. Our findings may help to lay the foundation for further improvements in the treatment of MCL.

p14ARF induces apoptosis via an entirely caspase-3-dependent mitochondrial amplification loop.

The p14(ARF) tumor suppressor triggers cell death or cell cycle arrest upon oncogenic stress. In MCF-7 breast carcinoma cells, expression of the tumor suppressor gene p14(ARF) fails to trigger apoptosis but induces an arrest in the G1 and, to a lesser extent, in the G2 phase in the cell division cycle. Here, inhibition of cell cycle arrest resulted in apoptosis induction in caspase-3 proficient MCF-7 cells upon expression of p14(ARF) . This occurred in the absence of S-phase progression or mitotic entry. In contrast, syngeneic, caspase-3-deficient MCF-7 cells remained entirely resistant to p14(ARF) -induced apoptosis. Thus, cell cycle checkpoint abrogation overcomes resistance to p14(ARF) -induced cell death and promotes cell death via a caspase-3-dependent pathway. Cell death coincided with dissipation of the mitochondrial membrane potential, release of cytochrome c, and was inhibitable by pan-caspase inhibitors and the caspase-3/7 inhibitor zDEVD-fmk. Of note, mitochondrial events of apoptosis execution depended entirely on caspase-3 proficiency indicating that caspase-3 either acts "up-stream" of the mitochondria in a "non-canonical" pathway or mediates a mitochondrial feedback loop to amplify the apoptotic caspase signal in p14(ARF) -induced stress signaling.

Synthetic glycosidated phospholipids induce apoptosis through activation of FADD, caspase-8 and the mitochondrial death pathway.

Apoptosis is modulated by extrinsic and intrinsic signaling pathways through the formation of the death receptor-mediated death-inducing signaling complex (DISC) and the mitochondrial-derived apoptosome, respectively. Ino-C2-PAF, a novel synthetic phospholipid shows impressive antiproliferative and apoptosis-inducing activity. Little is known about the signaling pathway through which it stimulates apoptosis. Here, we show that this drug induces apoptosis through proteins of the death receptor pathway, which leads to an activation of the intrinsic apoptotic pathway. Apoptosis induced by Ino-C2-PAF and its glucosidated derivate, Glc-PAF, was dependent on the DISC components FADD and caspase-8. This can be inhibited in FADD--/-- and caspase-8--/-- cells, in which the breakdown of the mitochondrial membrane potential, release of cytochrome c and activation of caspase-9, -8 and -3 do not occur. In addition, the overexpression of crmA, c-Flip or dominant negative FADD as well as treatment with the caspase-8 inhibitor z-IETD-fmk protected against Ino-C2-PAF-induced apoptosis. Apoptosis proceeds in the absence of CD95/Fas-ligand expression and is independent of blockade of a putative death-ligand/receptor interaction. Furthermore, apoptosis cannot be inhibited in CD95/Fas--/-- Jurkat cells. Expression of Bcl-2 in either the mitochondria or the endoplasmic reticulum (ER) strongly inhibited Ino-C2-PAF- and Glc-PAF-induced apoptosis. In conclusion, Ino-C2-PAF and Glc-PAF trigger a CD95/Fas ligand- and receptor-independent atypical DISC that relies on the intrinsic apoptotic pathway via the ER and the mitochondria.

Inhibition of the ER Ca2+ pump forces multidrug-resistant cells deficient in Bak and Bax into necrosis.

Tumor cells deficient in the proapoptotic proteins Bak and Bax are resistant to chemotherapeutic drugs. Here, we demonstrate that murine embryonic fibroblasts deficient for both Bak and Bax are, however, efficiently killed by thapsigargin, a specific inhibitor of ER Ca(2+) pumps that induces ER stress by depleting ER Ca(2+) stores. In the presence of Bak and Bax, thapsigargin eliminates cells by release of mitochondrial cytochrome c and subsequent caspase activation, which leads to the proteolytic inactivation of the molecular necrosis switch PARP-1 and results in apoptosis. By contrast, in the absence of Bak and Bax, a failure to activate caspases results in PARP-1-mediated ATP depletion. The subsequent necrosis is not prevented by autophagy as an alternative energy source. Moreover, in cells deficient for both Bak and Bax, thapsigargin induces permanent mitochondrial damage by Ca(2+) overload, permeability transition and membrane rupture. Thus, even though deficiency in Bak and Bax protects these cells against apoptosis, it does not compromise necrosis induced by SERCA inhibitors. Importantly, thapsigargin induces caspase-independent cell death also in colon and prostate carcinoma cells deficient in Bak and Bax expression. Therefore, targeted application of ER stressors such as thapsigargin might be a promising approach for the treatment of Bak- and Bax-deficient, drug-resistant tumors.

BCR-ABL positive cells and chronic myeloid leukemia in immune suppressed organ transplant recipients.

The constitutively activated tyrosine kinase activity of the p210(bcr-abl) fusion protein, generated by a t(9;22)(q34;q11) chromosomal translocation, is pathogenetically associated with chronic myeloid leukemia (CML). However, mechanisms contributing to the expansion of a BCR-ABL positive clone are largely obscure. In the presence of an impaired immune surveillance, cells carrying any of these alterations may become phenotypically relevant. Therefore, immunosuppressed solid organ recipients represent an optimal population to investigate the frequency of mRNA products of this translocation. Blood leukocytes were studied in 201 individuals (100 organ recipients and 101 control individuals) for the presence of BCR-ABL transcripts by a nested-reverse transcriptase-polymerase chain reaction assay, routinely used in our institution. In 5/100 immunosuppressed patients, at least one out of two RT-PCR products was bcr-abl positive while all controls were negative. These findings were extended by four CML cases of organ transplant recipients (three renal and one liver transplants). Three of these cases developed CML in a total of 2088 transplantations in 9 yr, suggesting a higher incidence of CML in these patients.

Combined action of celecoxib and ionizing radiation in prostate cancer cells is independent of pro-apoptotic Bax.

BACKGROUND AND PURPOSE: The cyclooxygenase-2-inhibitor celecoxib has been shown to inhibit cell growth and to reduce prostatic intraepithelial neoplasia in mice. The drug was suggested to increase efficacy of ionizing radiation. However, extent and mechanisms of the suggested benefit of celecoxib on the radiation response are still unclear. The aim of the present study was to analyze cytotoxic efficacy of celecoxib in combination with irradiation on human prostate cancer cell lines and to define the importance of pro-apoptotic Bax in this process. MATERIALS AND METHODS: Induction of apoptosis and global and clonogenic cell survival upon irradation- (2-10Gy), celecoxib- (10-75microM) or combined treatment were evaluated in prostate cancer cells by fluorescence microscopy, WST-1 assay and standard colony formation assays. RESULTS: Celecoxib <25microM caused morphological changes and growth inhibition without substantial apoptosis or radiosensitization in terms of decreased clonogenic cell survival. In contrast, celecoxib 25microM increased radiation-induced cell death and clonogenic kill. While radiation-induced clonogenic death was increased in the presence of Bax, effects of celecoxib or combined treatment were Bax independent. CONCLUSIONS: Our findings reveal Bax-independent beneficial effects of celecoxib on radiation-induced apoptosis and eradication of clonogenic prostate cancer cells in vitro providing a rationale for clinical evaluation of high-dose celecoxib in combination with irradiation in prostate cancer patients.

Efficacy of a triple treatment with irradiation, agonistic TRAIL receptor antibodies and EGFR blockade.

BACKGROUND AND PURPOSE: : Since the efficacy of a single targeted agent in combination with ionizing radiation is limited by putative treatment resistances, a rationally designed triple treatment consisting of an agonistic antibody targeting either TRAIL-R1 (mapatumumab) or TRAIL-R2 (lexatumumab), radiation and an epidermal growth factor receptor-(EGFR-)inhibiting antibody (cetuximab) was tested. MATERIAL AND METHODS: : Induction of apoptosis after triple treatment was determined in Colo205, HCT116 and FaDu cells by Hoechst 33342 stain. The degree of interaction was determined by isobologram analysis. A knockout variant of HCT116 was used to examine Bax dependence of the triple treatment. The role of Akt/PKB signaling was analyzed using the phosphatidylinositol 3-kinase inhibitor LY294002. Clonogenic assays were performed to examine the effect on clonogenic survival of tumor cells. RESULTS: : A synergistic effect of radiation, cetuximab and agonistic TRAIL-R antibodies was demonstrated in cell lines derived from colorectal tumors or head-and-neck cancers. The efficacy of this multimodal approach was dependent on Bax and inhibition of Akt/PKB in the cell systems used. The results also show a positive impact on clonogenic cell death in several cell lines. CONCLUSION: : These data suggest that rationally designed multimodal therapy approaches integrating radiation with more than one targeted agent will open new perspectives in radiation oncology.

Efficacy of triple therapies including ionising radiation, agonistic TRAIL antibodies and cisplatin.

The detection of molecular targeted agents is a milestone in cancer treatment. Despite the achievements, the efficacy of single targeted agents in combination with radiotherapy is limited by putative treatment resistance. We therefore tested a rationally designed triple therapy consisting of an agonistic antibody against either TRAIL-R1 (mapatumumab/HGS-ETR1) or TRAIL-R2 (lexatumumab/HGS-ETR2) in combination with the established chemotherapeutic drug cisplatin in a panel of solid tumour cell lines derived from head and neck as well as colorectal carcinomas. Induction of apoptosis after monotherapy, double or triple treatment was determined in FaDu (squamous cancer cell line of the head and neck), Colo205 and HCT116 cells (colorectal adenocarcinoma cell lines) by Hoechst 33342 stain. Double and triple therapies were compared using analysis of variance followed by post hoc tests. The degree of interaction was determined by 3D-isobologram analysis. A knockout variant of HCT116 was used to examine Bax-dependence of the triple therapy to gain insight into the underlying molecular signaling pathways possibly responsible for the observed effects. Dose-response relationships revealed different baseline activities of the modalities dependent on cell type. Triple therapy was more effective than double therapy in most cases according to the induction of apoptosis. Furthermore, a synergistic efficacy of the triple therapy was demonstrated in a subset of tumour cell lines. The efficacy of this multimodal approach was highly dependent on the presence of Bax. Our data suggest that targeted agents can be effectively added to existing multimodal therapy approaches which might open new perspectives in radiation oncology.

Loss of nuclear p27 expression and its prognostic role in relation to cyclin E and p53 mutation in gastroenteropancreatic neuroendocrine tumors.

PURPOSE: Gastroenteropancreatic neuroendocrine tumors (GEP-NET) are classified by the WHO, yet its prognostic value needs to be confirmed. Therefore, we aimed to determine the prognostic role of cell cycle key regulatory genes p53, p27kip1 (p27), and cyclin E in this tumor entity. EXPERIMENTAL DESIGN: Tumor specimen from 89 patients with a complete follow-up were studied immunohistochemically for p27 and cyclin E expression and for p53 mutations. The functional relevance of p27 was evaluated in the neuroendocrine cell lines BON1 (human) and INS1 (rat) by the use of small interfering RNA. RESULTS: Twenty-six of 29 benign, well-differentiated endocrine tumors (WHO class 1) showed a high expression (> 50%) of p27, whereas all 10 poorly differentiated endocrine carcinomas (WHO class 3) displayed a low expression of p27. Metastatic well-differentiated endocrine carcinomas (WHO class 2) showed a low p27 expression in 20 of 50 (40%) patients, which conferred a poor prognosis (median survival, 57 versus 140 months; P = 0.037). This prognostic dichotomy was improved by the use of a combination of p27 and cyclin E (high cyclin E/low p27 versus low cyclin E/high p27: median survival 53 months versus not reached; P = 0.0044). p53 mutations were rare (1 of 10 poorly differentiated endocrine carcinomas). CONCLUSIONS: Loss of p27 and overexpression of cyclin E play a critical role in the aggressiveness of gastroenteropancreatic neuroendocrine tumors. This coincides with increased cell cycle progression. We propose a discussion whether to incorporate the immunohistochemical expression of p27 into a revised classification to individualize therapeutic strategies in this tumor entity.

Pan-class I PI3-kinase inhibitor BKM120 induces MEK1/2-dependent mitotic catastrophe in non-Hodgkin lymphoma leading to apoptosis or polyploidy determined by Bax/Bak and p53.

Constitutive signaling of PI3K/Akt/mTOR plays a prominent role in malignant transformation and progression of B-cell non-Hodgkin lymphomas (B-NHL) underscoring the need for PI3K targeted therapies. The pan-class I PI3-kinase inhibitor BKM120 has shown preclinical activity in distinct malignancies and is currently tested in clinical trials. Intratumor heterogeneity is an intrinsic property of cancers that contributes to drug resistance and tumor recurrence. Here, we demonstrate that inhibition of PI3-kinases by BKM120 attenuates growth and survival of B-NHL cell lines by inducing mitotic arrest with subsequent induction of intrinsic apoptosis. BKM120-mediated downregulation of Cyclin A and activation of the CDK1/Cyclin B1 complex facilitates mitotic entry. In addition, concomitant BKM120-mediated upregulation of Cyclin B1 expression attenuates completion of mitosis, which results in mitotic catastrophe and apoptotic cell death. In Bax and Bak deficient B-NHL, which are resistant to BKM120-induced apoptosis, BKM120-induced mitotic catastrophe results in polyploidy. Upon re-expression of wt p53 in these p53 mutated cells, BKM120-induced polyploidy is strongly reduced demonstrating that the genetic status of the cells determines the outcome of a BKM120-mediated pathway inhibition. Mitotic catastrophe and unfavorable induction of polyploidy can be prevented in this setting by additional inhibition of MEK1/2 signaling. Combining MEK1/2 inhibitors with BKM120 enhances the anti-tumor effects of BKM120, prevents prognostic unfavorable polyploidy and might be a potential strategy for the treatment of B-NHL.

Inorganic selenium sensitizes prostate cancer cells to TRAIL-induced apoptosis through superoxide/p53/Bax-mediated activation of mitochondrial pathway.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to induce apoptosis in prostate cancer cells through DR4 and DR5 death receptors, but not in normal prostate cells, which do not express these receptors. Therefore, TRAIL has excellent potential to be a selective prostate cancer therapeutic agent with minimal toxic side effects. However, prostate cancer cells, as many other cancer types, develop resistance to TRAIL, and the underlying molecular mechanisms require further investigation. We hypothesize that selenium may sensitize TRAIL-resistant cells to undergo caspase-mediated apoptosis and increase therapeutic efficacy. Here, we report that TRAIL signaling in LNCaP prostate cancer cells stalled at downstream of caspase-8 and BID cleavage, as indicated by the lack of Bax translocation into mitochondria, and no subsequent activation of the caspase-9 cascade. Selenite induced a rapid generation of superoxide and p53 Ser(15) phosphorylation and increased Bax abundance and translocation into the mitochondria. Selenite and TRAIL combined treatment led to synergistic increases of Bax abundance and translocation into mitochondria, loss of mitochondrial membrane potential, cytochrome c release, and cleavage activation of caspase-9 and caspase-3. Inactivating p53 with a dominant-negative mutant abolished apoptosis without affecting superoxide generation, whereas a superoxide dismutase mimetic agent blocked p53 activation, Bax translocation to mitochondria, cytochrome c release, and apoptosis induced by selenite/TRAIL. In support of Bax as a crucial target for cross-talk between selenite and TRAIL pathways, introduction of Bax into p53 mutant DU145 cells enabled selenite to sensitize these cells for TRAIL-induced apoptosis. Taken together, the results indicate that selenite induces a rapid superoxide burst and p53 activation, leading to Bax up-regulation and translocation into mitochondria, which restores the cross-talk with stalled TRAIL signaling for a synergistic caspase-9/3 cascade-mediated apoptosis execution.

MACC1 regulates Fas mediated apoptosis through STAT1/3 - Mcl-1 signaling in solid cancers.

MACC1 was identified as a novel player in cancer progression and metastasis, but its role in death receptor-mediated apoptosis is still unexplored. We show that MACC1 knockdown sensitizes cancer cells to death receptor-mediated apoptosis. For the first time, we provide evidence for STAT signaling as a MACC1 target. MACC1 knockdown drastically reduced STAT1/3 activating phosphorylation, thereby regulating the expression of its apoptosis targets Mcl-1 and Fas. STAT signaling inhibition by the JAK1/2 inhibitor ruxolitinib mimicked MACC1 knockdown-mediated molecular signatures and apoptosis sensitization to Fas activation. Despite the increased Fas expression, the reduced Mcl-1 expression was instrumental in apoptosis sensitization. This reduced Mcl-1-mediated apoptosis sensitization was Bax and Bak dependent. MACC1 knockdown also increased TRAIL-induced apoptosis. MACC1 overexpression enhanced STAT1/3 phosphorylation and increased Mcl-1 expression, which was abrogated by ruxolitinib. The central role of Mcl-1 was strengthened by the resistance of Mcl-1 overexpressing cells to apoptosis induction. The clinical relevance of Mcl-1 regulation by MACC1 was supported by their positive expression correlation in patient-derived tumors. Altogether, we reveal a novel death receptor-mediated apoptosis regulatory mechanism by MACC1 in solid cancers through modulation of the STAT1/3-Mcl-1 axis.

TRAIL sensitizes for ionizing irradiation-induced apoptosis through an entirely Bax-dependent mitochondrial cell death pathway.

The death ligand TRAIL has been suggested as a suitable biological agent for the selective induction of cell death in cancer cells. Moreover, TRAIL synergizes with DNA-damaging therapies such as chemotherapeutic drugs or ionizing irradiation (IR). Here, we show that synergy of TRAIL and IR, that is, crosssensitization between TRAIL and IR for induction of apoptosis, entirely depends on Bax proficiency in human DU145 and HCT116 carcinoma cells. DU145 prostate carcinoma cells that have lost Bax protein expression due to mutation fail to activate caspase-3 and -9 when exposed to TRAIL and IR. In contrast, TRAIL sensitized for IR-induced apoptosis and vice versa upon reconstitution of Bax expression. Notably, both DU145 and HCT116 still express significant levels of the multidomain proapoptotic Bcl-2 homolog Bak. This indicates that Bak is not sufficient to mediate crosssensitization and synergism between IR and TRAIL. These data clearly establish distinct roles for Bax and Bak in linking the TRAIL death receptor pathway to the mitochondrial apoptosis signaling cascade upon DNA damage by IR.

Arsenic trioxide triggers a regulated form of caspase-independent necrotic cell death via the mitochondrial death pathway.

Cell death is generally believed to occur either by accidental, lytic necrosis or by programmed cell death, that is, apoptosis. The initiation and execution of cell death, however, is far more complex and includes pathways like caspase-independent apoptosis or actively triggered necrosis. In this study, we investigated the mechanisms of cell death induced by arsenic trioxide (arsenite, As2O3), a clinically efficient agent in anticancer therapy. As2O3-induced cell death coincides with cytochrome c release, facilitates mitochondrial permeability transition and is sensitive to inhibition by Bcl-x(L), indicating that cell demise is regulated through the mitochondrial apoptosis pathway. Nevertheless, only little caspase-3 activation was observed and As2O3-induced cell death was only weakly obstructed by the broad spectrum caspase inhibitor z-VAD-fmk. Moreover, disruption of caspase-9 or -2 failed to decrease the amount of As2O3-mediated cell death. Interestingly, As2O3-induced cell death had a predominantly necrosis-like phenotype as assessed by Annexin-V/propidium iodide staining and LDH release. Finally, blocking glutathione synthetase by buthionine sulfoximine enhanced the As2O3-mediated necrosis-like cell death without increasing caspase-3 cleavage. As2O3 does, however, not directly inhibit caspases, but appears to interfere with caspase activation. Altogether, our data clearly delineate a mode of As2O3-triggered cell death that differs considerably from that induced by conventional anticancer drugs. These findings may explain the capability of As2O3 to efficiently kill even chemoresistant tumor cells with disturbed apoptosis signaling and caspase activation, a frequent finding in malignancy.