Activation of the mitochondrial death pathway is commonly mediated by a preferential engagement of Bak

Platycodin D induces apoptosis through JNK1/AP-1/PUMA pathway in non-small cell lung cancer cells: A new mechanism for an old compound

Platycodin D, a triterpenoid monomer, has been shown to possess an anti-tumor effect on various types of cancer. Although Platycodin D has been reported to suppress tumorigenesis, the detailed underlying mechanism remains elusive. Platycodin D treatment significantly reduced the cell viability, decreased the number of colonies, impaired the mitochondrial function, and induced apoptosis in non-small cell lung cancer (NSCLC) cells. To understand the mechanism by which platycodin D induces apoptosis, the expression levels of apoptosis-related proteins were examined, and we found that the expression of PUMA (p53 upregulated modulator of apoptosis) was upregulated upon platycodin D treatment. Knockdown of PUMA resulted in attenuation of platycodin D-induced apoptosis, indicating that PUMA up-regulation is essential for platycodin D to induce apoptosis. The induction of PUMA expression by platycodin D treatment was through activation of AP-1 since mutation of AP-1 binding site in the PUMA promoter abolished the PUMA promoter activity. In addition, the chromatin immunoprecipitation further demonstrated that platycodin D promoted AP-1 binding to PUMA promoter. Moreover, knockdown of JNK1, but not JNK2, significantly abolished the phosphorylation of c-Jun at Ser63 (a component of AP-1), decreased the platycodin D-induced expression of PUMA and cleaved caspase 3, indicating that platycodin D inhibits JNK1/AP-1 signaling pathway. Furthermore, immunohistochemical staining studies showed that tumors from the mice treated with platycodin D activated JNK by translocation of JNK into nuclei, increased phosphorylation of JNK and c-Jun at Ser63 in nuclei, and boosted the PUMA expression. Taken together, our in vitro and in vivo data revealed a novel mechanism by which platycodin D up-regulates PUMA to induce apoptosis through JNK1/AP-1 axis in NSCLC.

Bak instead of Bax plays a key role in metformin-induced apoptosis s in HCT116 cells

Metformin (Met) exhibits anticancer ability in various cancer cell lines. This report aims to explore the exact molecular mechanism of Met-induced apoptosis in HCT116 cells, a human colorectal cancer cell line. Met-induced reactive oxygen species (ROS) increase and ROS-dependent cell death accompanied by plasma membrane blistering, mitochondrial swelling, loss of mitochondrial membrane potential, and release of cytochrome c. Western blotting analysis showed that Met upregulated Bak expression but downregulated Bax expression. Most importantly, silencing Bak instead of Bax inhibited Met-induced loss of mitochondrial membrane potential, indicating the key role of Bak in Met-induced apoptosis. Live-cell fluorescence resonance energy transfer (FRET) analysis showed that Met unlocked the binding of Mcl-1 to Bak, and enhanced the binding of Bim to Bak and subsequent Bak homo-oligomerization. Western blotting analysis showed that Met enhanced AMPK phosphorylation and Bim expression, and compound C, an inhibitor of AMPK, inhibited Met-induced Bim upregulation. Although Met increased the expression of Bcl-xL, overexpression of Bcl-xL did not prevent Met-induced apoptosis. In summary, our data demonstrate for the first time that Met promotes ROS-dependent apoptosis by regulating the Mcl-1-Bim-Bak axis.

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.

Notch4 Signaling Confers Susceptibility to TRAIL-Induced Apoptosis in Breast Cancer Cells

Notch signaling has been established as a key regulator of cell fate in development, differentiation, and homeostasis. In breast cancers, increased Notch1 and Notch4 activity have been implicated in tumor progression and, accumulation of the intracellular domain of Notch4 (ICN4), reported in basal breast cancer cells. While, TNF-related apoptosis-inducing ligand (TRAIL) receptor agonists have demonstrated selectively in targeting tumor cells, the majority of primary tumors are resistant to TRAIL. This necessitates the identification of factors that might regulate TRAIL sensitivity. Here we investigate TRAIL sensitivity in tumor cells following the modulation of Notch (1 and 4) activity using siRNA-mediated depletions or ectopic expression of GFP-tagged constructs of the intracellular domains of Notch1 (ICN1) or Notch4 (ICN4). Our findings suggest that Notch4, but not Notch1 signaling, sensitizes breast tumor cells to TRAIL-induced apoptosis. ICN4-induced sensitization to TRAIL is characterized by CBF1-dependence. Apoptosis was mediated via caspase-8 activation and regulated by the Bcl-2 family pro-apoptotic proteins Bak and Bid. Finally, we present evidence that endogenous Notch4 activity regulates susceptibility to TRAIL in basal-like breast cancer cells but not in cell lines of luminal origin. These experiments reveal a hitherto unexplored Notch4-TRAIL signaling axis in breast cancer cells.

Dihydroartemisinin induces apoptosis preferentially via a Bim-mediated intrinsic pathway in hepatocarcinoma cells

This report is designed to dissect the detail molecular mechanism by which dihydroartemisinin (DHA), a derivative of artemisinin, induces apoptosis in human hepatocellular carcinoma (HCC) cells. DHA induced a loss of the mitochondrial transmemberane potential (ΔΨm), release of cytochrome c, activation of caspases, and externalization of phosphatidylserine indicative of apoptosis induction. Compared with the modest inhibitory effects of silencing Bax, silencing Bak largely prevented DHA-induced ΔΨm collapse and apoptosis though DHA induced a commensurable activation of Bax and Bak, demonstrating a key role of the Bak-mediated intrinsic apoptosis pathway. DHA did not induce Bid cleavage and translocation from cytoplasm to mitochondria and had little effects on the expressions of Puma and Noxa, but did increase Bim and Bak expressions and decrease Mcl-1 expression. Furthermore, the cytotoxicity of DHA was remarkably reduced by silencing Bim, and modestly but significantly reduced by silencing Puma or Noxa. Silencing Bim or Noxa preferentially reduced DHA-induced Bak activation, while silencing Puma preferentially reduced DHA-induced Bax activation, demonstrating that Bim and to a lesser extent Noxa act as upstream mediators to trigger the Bak-mediated intrinsic apoptosis pathway. In addition, silencing Mcl-1 enhanced DHA-induced Bak activation and apoptosis. Taken together, our data demonstrate a crucial role of Bim in preferentially regulating the Bak/Mcl-1 rheostat to mediate DHA-induced apoptosis in HCC cells.

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.

Pan-BH3 mimetic S1 exhibits broad-spectrum antitumour effects by cooperation between Bax and Bak

Small molecule S1 is a pan-BH3 mimetic that can bind antiapoptotic Bcl-2, Bcl-xL and Mcl-1 proteins. Herein, different Bcl-2 member expression cancer cell lines (NCI-H345, MCF-7, SMMC-7721 and Hela) and cells deficient in Bax and/or Bak by shRNA were used to unravel the cascade of events by which S1 promotes apoptosis compared with Bcl-2/Bcl-xL inhibitor ABT-737. We identified that S1 exhibited broader antitumour spectrum than ABT-737 through disruption of more Bcl-2 interactions including Mcl-1/Bak interaction. Moreover, the individual and combined roles of Bax and Bak in S1-induced apoptosis were revealed. Our results showed that S1 induced a Bak-mediated apoptosis. Bak played a predominant role in either S1 or ABT-737-induced apoptosis through the cooperation with Bax on the formation of large oligomers on mitochondrial membrane.

Blockade of the BAK hydrophobic groove by inhibitory phosphorylation regulates commitment to apoptosis

The BCL-2 family protein BAK is a key regulator of mitochondrial apoptosis. BAK activation first involves N-terminal conformational changes that lead to the transient exposure of the BAK BH3 domain that then inserts into a hydrophobic groove on another BAK molecule to form symmetric dimers. We showed recently that post-translational modifications are important in the regulation of BAK conformational change and multimerization, with dephosphorylation at tyrosine 108 constituting an initial step in the BAK activation process. We now show that dephosphorylation of serine 117 (S117), located in the BAK hydrophobic groove, is also critical for BAK activation to proceed to completion. Phosphorylation of BAK at S117 has two important regulatory functions: first, it occludes the binding of BH3-containing peptides that bind to BAK causing activation and cytochrome c release from mitochondria; second, it prevents BAK-BH3:BAK-Groove interactions that nucleate dimer formation for subsequent multimerization. Hence, BH3-mediated BAK conformational change and subsequent BAK multimerization for cytochrome c release and cell death is intimately linked to, and dependent on, dephosphorylation at S117. Our study reveals important novel mechanistic and structural insights into the temporal sequence of events governing the process of BAK activation in commitment to cell death and how they are regulated.

Artesunate induces apoptosis via a Bak-mediated caspase-independent intrinsic pathway in human lung adenocarcinoma cells

This report is designed to explore the exact molecular mechanism by which artesunate (ART), a semisynthetic derivative of the herbal antimalaria drug artemisinin, induces apoptosis in human lung adenocarcinoma (ASTC-a-1 and A549) cell lines. ART treatment induced ROS-mediated apoptosis in a concentration- and time-dependent fashion accompanying the loss of mitochondrial potential and subsequent release of Smac and AIF indicative of intrinsic apoptosis pathway. Blockage of casapse-8 and -9 did not show any inhibitory effect on the ART-induced apoptosis, but which was remarkably prevented by silencing AIF. Of the utmost importance, ART treatment induced the activation of Bak but not Bax, and silencing Bak but not Bax remarkably inhibited ART-induced apoptosis and AIF release. Furthermore, although ART treatment did not induced a significant down-regulation of voltage-dependent anion channel 2 (VDAC2) expression and up-regulation of Bim expression, silencing VDAC2 potently enhanced the ART-induced Bak activation and apoptosis which were significantly prevented by silencing Bim. Collectively, our data firstly demonstrate that ART induces Bak-mediated caspase-independent intrinsic apoptosis in which Bim and VDAC2 as well as AIF play important roles in both ASTC-a-1 and A549 cell lines, indicating a potential therapeutic effect of ART for lung cancer.

Sodium butyrate increases the effect of the photodynamic therapy: a mechanism that involves modulation of gene expression and differentiation in astrocytoma cells

OBJECTIVES

In order to evaluate the improvement of the photodynamic therapy (PDT) due to sodium butyrate (NaBu), its effectiveness in U373-MG and D54-MG astrocytoma cell lines was evaluated.

METHODS

Cells were exposed to delta-aminolevulinic acid (δ-ALA) as a precursor to endogenous photosensitizer protoporphyrin IX (PpIX). In both astrocytoma cells, an important increase by ALA was observed in uroporphyrinogen synthetase gene expression: 1.8- and 52-fold for D54-MG and U373-MG cells, respectively. After irradiation, they showed 16.67 and 28.9% of mortality in U373-MG and D54-MG, respectively. These mortalities increased to 70.62 and 96.7% when U373-MG and D54-MG cells, respectively, were exposed 24 h to 8 mM NaBu, before to PpIX induction. NaBu induced expression of caspase-3, caspase-9, and Bcl-2 and increased Bax in U373-MG cells. ALA-induced morphological changes are compatible to differentiation.

CONCLUSIONS

Genes and differentiation induced mainly by NaBu improve cell death performed by PDT in astrocytoma cells. These facts prove the synergistic effect of NaBu on cytotoxic damage induced by PDT.

Live or let die: manipulation of cellular suicide programs by murine cytomegalovirus

Cytomegaloviruses (CMVs) are large double-stranded DNA viruses that replicate slowly and cause life-long persisting infections in their hosts. To achieve this, the CMVs had to evolve numerous countermeasures against innate and adaptive immune responses. Induction of programmed cell death is one important host defense mechanism against intracellular pathogens such as viruses. For a multicellular organism, it is advantageous to let infected cells die in order to thwart viral replication and dissemination. For a virus, by contrast, it is better to inhibit cell death and keep infected cells alive until the viral replication cycle has been completed. As a matter of fact, the CMVs encode a number of proteins devoted to interfering with different forms of programmed cell death: apoptosis and necroptosis. In this review, we summarize the known functions of the four best characterized cell death inhibitors of murine cytomegalovirus (MCMV), which are encoded by open reading frames, M36, m38.5, m41.1, and M45. The viral proteins interact with key molecules within different cell death pathways, namely caspase-8, Bax, Bak, and RIP1/RIP3. In addition, we discuss which events during MCMV infection might trigger apoptosis or necrosis and how MCMV's countermeasures compare to those of other herpesviruses. Since both, MCMV and its natural host, are amenable to genetic manipulation, the mouse model for CMV infection provides a particularly suitable system to study mechanisms of cell death induction and inhibition.

Bak is a key molecule in apoptosis induced by methanol extracts of Codonopsis lanceolata and Tricholoma matsutake in HSC-2 human oral cancer cells

Since the 5-year survival rate of oral cancer remains low, more effective and non-toxic therapeutic and preventive strategies are required. Certain natural products possess anti-cancer properties. The present study investigated the effects of the methanol extracts of Codonopsis lanceolata (MECI) and Tricholoma matsutake (METM) and identified the molecular target in HSC-2 human oral cancer cells. The results revealed that MECI and METM inhibited growth and induced apoptosis, as demonstrated by poly (ADP-ribose) polymerase (PARP) cleavage and nuclear condensation and fragmentation. The compounds also increased Bak protein expression, while Bax, Bcl-XL and Mcl-1 were not affected. The results of the present study show that MECI and METM induce apoptosis to inhibit tumor growth of HSC-2 cells by modulating the Bak protein and suggest that Codonopsis lanceolata and Tricholoma matsutake are potential anticancer drug candidates for oral cancer.

Resveratrol induces apoptosis via a Bak-mediated intrinsic pathway in human lung adenocarcinoma cells

Our recent study have shown that resveratrol (RV), a natural plant polyphenol found in red grape skins as well as other food product, induced apoptosis via the downstream factors, caspase-independent AIF and to lesser extent caspase-9, of intrinsic apoptosis pathway in human lung adenocarcinoma (ASTC-a-1) cells. This report is designed to explore the roles of the upstream mediators of the intrinsic pathway, such as Bak/Bax, Bim, Puma and Noxa, during RV-induced apoptosis in human lung adenocarcinoma (ASTC-a-1 and A549) cell lines. RV treatment remarkably induced the activation of Bak but not Bax, and silencing Bak but not Bax by shRNA almost completely prevented RV-induced cell death, mitochondrial dysfunction and also largely prevented RV-induced AIF release, demonstrating the preferential engagement of Bak but not Bax during RV-induced apoptosis. In addition, although RV treatment induced a significant degradation of Mcl-1, knockdown of Mcl-1 by shRNA only modestly increased RV-induced Bak activation. Interestingly, silencing Bim but not Puma and Noxa remarkably attenuated RV-induced cell death, loss of mitochondrial membrane potential, and Bak activation, suggesting the important roles of Bim. Collectively, our findings for the first time demonstrate that RV induces apoptosis dominantly via a Bak- but not Bax-mediated AIF-dependent mitochondrial apoptotic signaling pathway in which Bim but not Puma and Noxa may supply the force to trigger Bak activation and subsequent apoptosis in both ASTC-a-1 and A549 cell lines.

The vaccinia virus-encoded Bcl-2 homologues do not act as direct Bax inhibitors

Many viruses, including members of several poxvirus genera, encode inhibitors that block apoptosis by simultaneously binding the proapoptotic Bcl-2 proteins Bak and Bax. The Orthopoxvirus vaccinia virus encodes the Bcl-2-like F1 protein, which sequesters Bak but not Bax. However, N1, a potent virulence factor, is reported to be antiapoptotic and to interact with Bax. Here we investigated whether vaccinia virus inhibits Bak/Bax-dependent apoptosis via the cooperative action of F1 and N1. We found that Western Reserve (WR) and ΔN1L viruses inhibited drug- and infection-induced apoptosis equally. Meanwhile, infections with ΔF1L or ΔN1L/F1L virus resulted in similar levels of Bax activation and apoptosis. Outside the context of infection, N1 did not block drug- or Bax-induced cell death or interact with Bax. In addition to F1 and N1, vaccinia virus encodes further structural homologs of Bcl-2 proteins that are conserved in orthopoxviruses, including A46, A52, B14, C1, C6, C16/B22, K7, and N2. However, we found that these do not associate with Bax or inhibit drug-induced cell death. Based on our findings that N1 is not an antiapoptotic protein, we propose that the F1 orthologs represent the only orthopoxvirus Bcl-2 homolog to directly inhibit the Bak/Bax checkpoint.

Predominant requirement of Bax for apoptosis in HCT116 cells is determined by Mcl-1's inhibitory effect on Bak

The intrinsic mitochondrial apoptotic pathway acts through two core pro-apoptotic proteins Bax (Bcl2-associated X protein) and Bak (Bcl2-antagonist/killer 1). Although Bax and Bak seem to have redundant roles in apoptosis, accumulating evidence also suggests that they might not be interchangeable under certain conditions, at least in some human cell lines. Here we report the generation of Bak knockout as well as BaxBak double knockout HCT116 human colon carcinoma cells. We show that Bak is dispensable for apoptosis induced by a variety of stimuli including ABT-737 but not for fluorouracil-induced apoptosis. In addition, Bax deficiency only provides partial protection against camptothecin and cisplatin-induced apoptosis and no protection against killing by Puma or ABT-737 plus Noxa overexpression. Moreover, Bak is activated normally in response to many chemotherapeutic drugs in the presence of Bax, but remains kept in check by Mcl-1 in the absence of Bax. Our data suggest that Bax and Bak are functionally redundant, but they are counteracted by distinct anti-apoptotic Bcl-2 family proteins in different species.

Inhibition of programmed cell death by cytomegaloviruses

The elimination of infected cells by programmed cell death (PCD) is one of the most ancestral defense mechanisms against infectious agents. This mechanism should be most effective against intracellular parasites, such as viruses, which depend on the host cell for their replication. However, even large and slowly replicating viruses like the cytomegaloviruses (CMVs) can prevail and persist in face of cellular suicide programs and other innate defense mechanisms. During evolution, these viruses have developed an impressive set of countermeasures against premature demise of the host cell. In the last decade, several genes encoding suppressors of apoptosis and necrosis have been identified in the genomes of human and murine CMV (HCMV and MCMV). Curiously, most of the gene products are not homologous to cellular antiapoptotic proteins, suggesting that the CMVs did not capture the genes from the host cell genome. This review summarizes our current understanding of how the CMVs suppress PCD and which signaling pathways they target.

Regulation of ceramide channels by Bcl-2 family proteins

Mitochondrial outer membrane permeabilization to proteins, an irreversible step in apoptosis by which critical proteins are released, is tightly regulated by Bcl-2 family proteins. The exact nature of the release pathway is still undefined. Ceramide is an important sphingolipid, involved in various cellular processes including apoptosis. Here we describe the structural properties of ceramide channels and their regulation by the anti-apoptotic and pro-apoptotic proteins of the Bcl-2 family. The evolutionarily conserved regulation of ceramide channels by Bcl-2 family proteins, consistent with their role in apoptosis, lends credibility to the notion that ceramide channels constitute the protein release pathway.

Cytomegaloviruses inhibit Bak- and Bax-mediated apoptosis with two separate viral proteins

Apoptosis of infected cells can limit virus replication and serves as an innate defense mechanism against viral infections. Consequently, viruses delay apoptosis by expressing antiapoptotic proteins, many of which structurally resemble the cellular antiapoptotic protein Bcl-2. Like Bcl-2, the viral analogs inhibit apoptosis by preventing activation and/or oligomerization of the proapoptotic mitochondrial proteins Bax and Bak. Here we show that cytomegaloviruses (CMVs) have adopted a different strategy. They encode two separate mitochondrial proteins that lack obvious sequence similarities to Bcl-2-family proteins and specifically counteract either Bax or Bak. We identified a small mitochondrion-localized protein encoded by the murine CMV open reading frame (ORF) m41.1, which functions as a viral inhibitor of Bak oligomerization (vIBO). It blocks Bak-mediated cytochrome c release and Bak-dependent induction of apoptosis. It protects cells from cell death-inducing stimuli together with the previously identified Bax-specific inhibitor viral mitochondria-localized inhibitor of apoptosis (vMIA) (encoded by ORF m38.5). Similar vIBO proteins are encoded by CMVs of rats, and possibly by other CMVs as well. These results suggest a non-redundant function of Bax and Bak during viral infection, and a benefit for CMVs derived from the ability to inhibit Bak and Bax separately with two viral proteins.

JNK1-dependent PUMA expression contributes to hepatocyte lipoapoptosis

Free fatty acids (FFA) induce hepatocyte lipoapoptosis by a c-Jun N-terminal kinase (JNK)-dependent mechanism. However, the cellular processes by which JNK engages the core apoptotic machinery during lipotoxicity, especially activation of BH3-only proteins, remain incompletely understood. Thus, our aim was to determine whether JNK mediates induction of BH3-only proteins during hepatocyte lipoapoptosis. The saturated FFA palmitate, but not the monounsaturated FFA oleate, induces an increase in PUMA mRNA and protein levels. Palmitate induction of PUMA was JNK1-dependent in primary murine hepatocytes. Palmitate-mediated PUMA expression was inhibited by a dominant negative c-Jun, and direct binding of a phosphorylated c-Jun containing the activator protein 1 complex to the PUMA promoter was identified by electrophoretic mobility shift assay and a chromatin immunoprecipitation assay. Short hairpin RNA-targeted knockdown of PUMA attenuated Bax activation, caspase 3/7 activity, and cell death. Similarly, the genetic deficiency of Puma rendered murine hepatocytes resistant to lipoapoptosis. PUMA expression was also increased in liver biopsy specimens from patients with non-alcoholic steatohepatitis as compared with patients with simple steatosis or controls. Collectively, the data implicate JNK1-dependent PUMA expression as a mechanism contributing to hepatocyte lipoapoptosis.

Requirement of voltage-dependent anion channel 2 for pro-apoptotic activity of Bax

Mitochondrial membrane permeabilization is central to apoptotic signaling and is directly regulated by the Bcl-2 family of proteins, consisting of anti-apoptotic members and pro-apoptotic members, although the precise mechanisms involved remain elusive. When cells are deficient in both pro-apoptotic multidomain members of this family (Bax and Bak), mitochondrial membrane permeabilization does not occur in response to various apoptotic stimuli. We have previously reported that the voltage-dependent anion channel (VDAC or porin) plays a role in apoptotic mitochondrial membrane permeabilization by interacting with Bcl-2 family members. Here, we have provided additional evidence that VDAC2 is required for pro-apoptotic activity of Bax in the absence of Bak. In the absence of Bak, VDAC2-deficient cells showed strong resistance to various apoptotic stimuli, whereas re-introduction of the Vdac2 gene restored their apoptotic response. Consistently, silencing of VDAC2 in Bak-deficient cells, but not Bax-deficient cells, also conferred resistance to various apoptotic stimuli. In the absence of VDAC2 and Bak, the activation of Bax (assessed by mitochondrial membrane integration, conformational changes and oligomerization) was markedly impaired. Taken together, these findings indicate that VDAC2 is required for pro-apoptotic activity of Bax in the absence of Bak.

Vesicular stomatitis virus induces apoptosis primarily through Bak rather than Bax by inactivating Mcl-1 and Bcl-XL

Vesicular stomatitis virus (VSV) induces apoptosis via the mitochondrial pathway. The mitochondrial pathway is regulated by the Bcl-2 family of proteins, which consists of both pro- and antiapoptotic members. To determine the relative importance of the multidomain proapoptotic Bcl-2 family members Bak and Bax, HeLa cells were transfected with Bak and/or Bax small interfering RNA (siRNA) and subsequently infected with recombinant wild-type VSV. Our results showed that Bak is more important than Bax for the induction of apoptosis in this system. Bak is regulated by two antiapoptotic Bcl-2 proteins, Mcl-1, which is rapidly turned over, and Bcl-X(L), which is relatively stable. Inhibition of host gene expression by the VSV M protein resulted in the degradation of Mcl-1 but not Bcl-X(L). However, inactivation of both Mcl-1 and Bcl-X(L) was required for cells to undergo apoptosis. While inactivation of Mcl-1 was due to inhibition of its expression, inactivation of Bcl-X(L) indicates a role for one or more BH3-only Bcl-2 family members. VSV-induced apoptosis was inhibited by transfection with siRNA against Bid, a BH3-only protein that is normally activated by the cleavage of caspase-8, the initiator caspase associated with the death receptor pathway. Similarly, treatment with an inhibitor of caspase-8 inhibited VSV-induced apoptosis. These results indicate a role for cross talk from the death receptor pathway in the activation of the mitochondrial pathway by VSV.

Cisplatin overcomes Bcl-2-mediated resistance to apoptosis via preferential engagement of Bak: critical role of Noxa-mediated lipid peroxidation

Increased expression of antiapoptotic Bcl-2 proteins confers therapeutic resistance in various cancer types. Targeting Bcl-2 proteins by small molecules or activating alternative pathways to bypass Bcl-2-mediated protection to promote apoptosis are two approaches to overcoming therapeutic resistance. Here, we show that cisplatin triggers a Bak-dependent pathway to induce apoptosis in Bcl-2-overexpressing MCF-7 cells. p53-mediated induction of Noxa expression, generation of lipid peroxidation end products and induction of Noxa-Mcl-1 interaction are necessary for this pathway to function. Although Puma is also induced by cisplatin treatment, it is not required for apoptosis. Similarly, reactive oxygen species production by cisplatin did not have any effect on cisplatin-induced apoptosis in MCF-7 Bcl-2 cells. Furthermore, p53 promotes cisplatin-induced apoptosis by directly binding and counteracting Bcl-x(L) antiapoptotic function. In conclusion, our findings suggest a novel mode of action for cisplatin to overcome Bcl-2-mediated protection against apoptosis, which requires preferential activation of Bak and p53-mediated upregulation of Noxa protein levels and lipid peroxidation.

Effects of sodium butyrate on cell death induced by photodynamic therapy in U373-MG and D54-MG astrocytoma cell lines

The damage induced by end products of photodynamic therapy (PDT) in astrocytoma tumors leads to cytotoxicity and cell death. Chromatin modifiers such as sodium butyrate (NaB) induce several genes involved in apoptosis, among others. The PDT improvement was evaluated by the measurement of its effectiveness in the treatment of U373-MG and D54-MG astrocytoma cell lines exposed to NaB. Cells exposed to 80 microg mL(-1) of delta-aminolevulinic acid (ALA) as precursor of endogenous photosensitizer (PS), protoporphyrin IX (PpIX), induced 16.67% and 28.9% of mortality in U373-MG and D54-MG, respectively. The mortality increased to 70.62% and 96.7%, respectively, when U373-MG and D54-MG cells were exposed for 24 h to 8 mm NaB prior to ALA-induction. In this condition, re-expression of some genes related to apoptosis in U373-MG, and differentiation in D54-MG were induced. PpIX accumulation was higher than ALA-induction and the acetylation of histone H4 induced by NaB was verified by immunocytochemistry in both cells. It can be concluded that modified chromatin and genes induced by NaB increment the cellular death induced by PDT in astrocytoma cells using PpIX as endogenous PS.

Suppression of cFLIP is sufficient to sensitize human melanoma cells to TRAIL- and CD95L-mediated apoptosis

Death ligands such as tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and certain forms of CD95L are attractive therapeutic options for metastatic melanoma. Since knowledge about the regulation of death receptor sensitivity in melanoma is sparse, we have analysed these signaling pathways in detail. The loss of CD95 or TRAIL-R1, but not of TRAIL-R2, surface expression correlated with apoptosis sensitivity in a panel of melanoma cell lines. In contrast, the expression of proteins of the apical apoptosis signaling cascade (FADD, initiator caspases-8 and cFLIP) did not predict apoptosis sensitivity. Since both TRAIL-R1 and -R2 transmit apoptotic signals, we asked whether cFLIP, highly expressed in several of the cell lines tested, is sufficient to maintain resistance to TRAIL-R2-mediated apoptosis. Downregulation of cFLIP in TRAIL-R2-positive, TRAIL-resistant IGR cells dramatically increased TRAIL sensitivity. Conversely ectopic expression of cFLIP in TRAIL-sensitive, TRAIL-R2-expressing RPM-EP melanoma cells inhibited TRAIL- and CD95L-mediated cell death. Thus, modulation of cFLIP is sufficient to sensitize TRAIL-R2-expressing cells for TRAIL. Taken together, albeit expressing all proteins necessary for death receptor-mediated apoptosis, TRAIL-R1 negative melanoma cells cannot undergo TRAIL- or CD95L-induced apoptosis due to expression of cFLIP. Hence, cFLIP represents an attractive therapeutic target for melanoma treatment, especially in combination with TRAIL receptor agonists.