Pinocembrin triggers Bax-dependent mitochondrial apoptosis in colon cancer cells

Bioflavanoids are the major pigments in plants with multitude of biological activities including inhibition of proliferation or induction of apoptosis in tumor cells. Even though the safety records of most flavanoids are exceptional, its therapeutic use is still in its infancy. We have isolated pinocembrin (5,7-dihydroxyflavanone) from Alpinia galanga that showed cytotoxicity against a variety of cancer cells including normal lung fibroblasts with relative nontoxicity to human umbilical cord endothelial cells. The compound induced loss of mitochondrial membrane potential with subsequent release of cytochrome c and processing of caspase-9 and -3 in colon cancer cell line HCT 116. Processing of caspase-8 was minimal. The initial trigger for mitochondrial apoptosis appears to be by the translocation of cytosolic Bax protein to mitochondria. Overexpression of proapoptotic Bax protein sensitized the colon cancer cells to pinocembrin-induced apoptosis and Bax knockout cells were resistant to pinocembrin-induced apoptosis. Antiapoptotic protein Bcl-X(L) only partially prevented apoptosis induced by this compound. The Bax-dependent cell death involving classical cytochrome c release and processing of caspase-9 and -3 suggests that pinocembrin is a classical mitochondrial apoptosis inducer. But the failure of Bcl-X(L) overexpression to completely prevent apoptosis induced by this compound suggests that pinocembrin is capable of triggering mitochondrial-independent cell death that needs to be clarified. The existence of cell death upon Bcl-X(L) overexpression is a promising feature of this compound that can be exploited against drug resistant forms of cancer cells either alone or in combination with other drugs.     

Tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis is inhibited by Bcl-2 but restored by the small molecule Bcl-2 inhibitor, HA 14-1, in human colon cancer cells.

PURPOSE: Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent that induces apoptosis in multiple tumor cell types while sparing most normal cells. We determined the effect of ectopic Bcl-2 expression on TRAIL-induced apoptosis and whether the small molecule Bcl-2 inhibitor, HA14-1, could increase TRAIL sensitivity. EXPERIMENTAL DESIGN: SW480 human colon cancer cells were stably transfected with the PC3-Bcl-2 plasmid or vector alone. Cells were incubated with recombinant human TRAIL +/- HA14-1 or caspase-9 inhibitor (Z-LEHD-FMK). Apoptosis was analyzed by Annexin V-fluorescein isothiocyanate labeling and DNA fragmentation factor 45 (DFF45) cleavage. Clonigenic survival was also studied. Caspase activation was determined by immunoblotting or colorimetric assay. The cytosolic expression of Bid, Bax, and XIAP and release of cytochrome c and Smac/DIABLO were determined by immunoblotting. RESULTS: Bcl-2 overexpression partially protected SW480 cells from a dose-dependent induction of apoptosis by TRAIL, as did a caspase-9 inhibitor, and increased their clonogenic survival. Bcl-2 overexpression attenuated TRAIL-induced cleavage of caspase-8, indicating its activation upstream and downstream of mitochondria, as well as cleavage of Bid and caspase-3. Bcl-2 inhibited TRAIL-induced Bax translocation, cytosolic release of cytochrome c and Smac/DIABLO, and the downstream cleavage of XIAP and DFF45. Coadministration of HA14-1 and TRAIL increased apoptosis in SW480/Bcl-2 cells by restoring Bax redistribution and cytochrome c release. CONCLUSIONS: Bcl-2 confers apoptosis resistance to TRAIL by inhibiting a mitochondrial amplification step and by inactivating downstream XIAP in SW480 cells. HA14-1 reversed Bcl-2-mediated TRAIL resistance, suggesting a novel strategy for increasing TRAIL sensitivity in Bcl-2-overexpressing colon cancers.     

Involvement of proapoptotic molecules Bax and Bak in tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced mitochondrial disruption and apoptosis: differential regulation of cytochrome c and Smac/DIABLO release

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/Apo2L induces apoptosis in a wide variety of cancer and transformed cells. Activation of BID, a "BH3-domain-only" Bcl-2 family member, triggers the oligomerization of proapoptotic family members Bak or Bax, resulting in the release of mitochondrial proteins to cytosol. In this study, we have shown the importance of Bax and Bak in TRAIL-induced apoptosis by studying in murine embryonic fibroblasts (MEFs) from Bax(-/-) and Bak(-/-) animals. TRAIL induced cytochrome c release and apoptosis in wild-type, Bid(-/-), Bax(-/-), or Bak(-/-) MEFs, but not in Bax(-/-) Bak(-/-) double knockout (DKO) MEFs. Bid, which functions upstream of cytochrome c release, was cleaved in all of the knockout cells except in Bid(-/-) MEFs. The release of cytochrome c was correlated with caspase-9 activity. TRAIL increased caspase-3 activity in all of the cells except in DKO cells. TRAIL-induced drop in mitochondrial membrane potential was not observed in DKO MEFs. Unlike cytochrome c release, TRAIL-induced Smac/DIABLO release was blocked in Bid(-/-), Bax(-/-), Bak(-/-), or DKO MEFs, suggesting the differential regulation of these mitochondrial proteins during apoptosis. The apoptotic events downstream of mitochondria were intact in DKO MEFs, because microinjection of cytochrome c, or ectopic expression of mature Smac/DIABLO or pretreatment of Smac N7 peptide completely restored TRAIL sensitivity. In conclusion, the data suggest that Bax and Bak differentially regulate the release of cytochrome c and Smac/DIABLO from mitochondria, and Smac/DIABLO can be used to sensitize cells that are deficient in Bax and Bak genes, or resistant to TRAIL.     

Bax and Bak genes are essential for maximum apoptotic response by curcumin, a polyphenolic compound and cancer chemopreventive agent derived from turmeric, Curcuma longa

Curcumin, an active ingredient of turmeric (Curcuma longa), inhibits proliferation and induces apoptosis in cancer cells, but the sequence of events leading to cell death is poorly defined. The objective of this study was to examine the molecular mechanisms by which multidomain pro-apoptotic Bcl-2 family members Bax and Bak regulate curcumin-induced apoptosis using mouse embryonic fibroblasts (MEFs) deficient in Bax, Bak or both genes. Curcumin treatment resulted an increase in the protein levels of both Bax and Bak, and mitochondrial translocation and activation of Bax in MEFs to trigger drop in mitochondrial membrane potential, cytosolic release of apoptogenic molecules [cytochrome c and second mitochondria-derived activator of caspases (Smac)/direct inhibitor of apoptosis protein-binding protein with low isoelectric point], activation of caspase-9 and caspase-3 and ultimately apoptosis. Furthermore, MEFs derived from Bax and Bak double-knockout (DKO) mice exhibited even greater protection against curcumin-induced release of cytochrome c and Smac, activation of caspase-3 and caspase-9 and induction of apoptosis compared with wild-type MEFs or single-knockout Bax(-/-) or Bak(-/-) MEFs. Interestingly, curcumin treatment also caused an increase in the protein level of apoptosis protease-activating factor-1 in wild-type MEFs. Smac N7 peptide enhanced curcumin-induced apoptosis, whereas Smac siRNA inhibited the effects of curcumin on apoptosis. Mature form of Smac sensitized Bax and Bak DKO MEFs to undergo apoptosis by acting downstream of mitochondria. The present study demonstrates the role of Bax and Bak as a critical regulator of curcumin-induced apoptosis and over-expression of Smac as interventional approaches to deal with Bax- and/or Bak-deficient chemoresistant cancers for curcumin-based therapy.     

Regulation of the mitochondrial checkpoint in p53-mediated apoptosis confers resistance to cell death

The p53 tumor suppressor protein inhibits tumor formation, in part by inducing apoptosis, which is inhibited by anti-apoptotic Bcl-2 family members Bcl-2 and adenovirus E1B 19K. We have identified p53-apoptotic signaling events which are targeted for inhibition by E1B 19K. Apoptotic signaling by p53 induced a Bid-independent conformational change in Bax, a Bax-Bak interaction, release of cytochrome c and Smac/DIABLO from mitochondria, caspase-9 and -3 activation, cleavage of known caspase substrates, and apoptosis. When p53-dependent apoptosis was blocked by E1B 19K expression, E1B 19K bound Bak, and the Bax-Bak interaction was inhibited. Cytochrome c and Smac/DIABLO release from mitochondria was also inhibited in E1B 19K expressing cells and cells remained viable. After a prolonged p53 death stimulus, the inhibition of the mitochondrial death checkpoint by E1B 19K failed, and cytochrome c and Smac/DIABLO were released from mitochondria, and became degraded. Despite this eventual failure to inhibit the mitochondrial checkpoint, caspase-9 and -3 were not activated, and cells remained viable even upon treatment with an exogenous death stimulus. Thus, p53 induces apoptosis in part through Bax and Bak, and even an incomplete inhibition of this mitochondrial checkpoint may be sufficient to confer resistance to cell death.     

Bax plays a pivotal role in thapsigargin-induced apoptosis of human colon cancer HCT116 cells by controlling Smac/Diablo and Omi/HtrA2 release from mitochondria

Bax is a crucial mediator of the mitochondrial pathway for apoptosis, and loss of this proapoptotic Bcl-2 family protein contributes to drug resistance in human cancers. We report here that the endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin (THG) induces apoptosis of human colon cancer HCT116 cells through a Bax-dependent signaling pathway controlling the cytosolic release of mitochondrial apoptogenic molecules. Treating HCT116 cells with THG results in caspase-8 activation; Bid cleavage; Bax conformational change and mitochondrial translocation; the release of cytochrome c, Smac/Diablo, and Omi/HtrA2 into the cytosol; caspase-3 activation; and apoptosis. In contrast, knockout of Bax completely abrogates the full processing/activation of caspase-3 but has no effect on the processing of caspase-8 and the initial cleavage of caspase-3 to p24 fragment after THG treatment. The caspase-8-specific inhibitor z-IETD-fmk, as well as pan-caspase inhibitor z-VAD-fmk, but not the calpain inhibitor E-64d, prevents Bid cleavage, Bax conformational change, and subsequent caspase-3 processing and apoptosis. Caspase-8 processing is dependent on de novo protein synthesis; DR5 expression is strongly up-regulated by THG treatment. Moreover, the absence of Bax blocks THG-induced Omi and Smac release from mitochondria, and expression of cytosolic Omi (GFP-IETD-Omi) or Smac (GFP-IETD-Smac) restores the sensitivity of Bax-knockout HCT116 cells to apoptosis in response to THG treatment. Taken together, our results indicate that Bax-dependent Smac and Omi release plays an essential role in caspase-3 activation and apoptosis induced by THG in human colon cancer HCT116 cells.     

pseudo-G-Rh2 induces mitochondrial-mediated apoptosis in SGC-7901 human gastric cancer cells

This study was designed to investigate the effect of pseudo-G-Rh2, a novel metabolite of ginsenoside Rh2, on the apoptosis of SGC-7901 human gastric cancer cells. Pseudo-G-Rh2 demonstrated antitumor activity and significantly inhibited the proliferation of SGC-7901 cells in a concentration-dependent manner. After treatment with pseudo-G-Rh2, SGC-7901 cells showed typical apoptotic morphological features, such as chromatin condensation and DNA fragmentation. Pseudo-G-Rh2 could induce mitochondrial membrane potential loss, which led to the release of cytochrome c (Cyt?c), Smac/Diablo and apoptosis-inducing factor (AIF) to the cell cytoplasm. Furthermore, pseudo-G-Rh2 exposure not only decreased the expression of the Bcl-2 protein but also increased the expression of the Bax protein and the activities of caspase-9 and caspase-3 in SGC-7901 cells. These results demonstrated that pseudo-G-Rh2 inhibited the proliferation of SGC-7901 cells by initiating apoptosis. Pseudo-G-Rh2-induced apoptosis was associated with a drop in the mitochondrial transmembrane potential, down-regulation of Bcl-2, up-regulation of Bax and activation of caspase-9 and caspase-3.     

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.     

Cephalostatin 1 selectively triggers the release of Smac/DIABLO and subsequent apoptosis that is characterized by an increased density of the mitochondrial matrix

Cephalostatin 1 is a bis-steroidal marine natural product with a unique cytotoxicity profile in the in vitro screen system of the National Cancer Institute, suggesting that it may affect novel molecular target(s). Here we show that cephalostatin 1 induces a novel pathway of receptor-independent apoptosis that selectively uses Smac/DIABLO (second mitochondria-derived activator of caspases/direct inhibitor of apoptosis-binding protein with a low isoelectric point) as a mitochondrial signaling molecule. At nanomolar concentrations, cephalostatin 1 triggers dose- and time-dependent DNA fragmentation in leukemia Jurkat T cells. Apoptosis was found to be dependent on caspase activity because the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone blocks cephalostatin 1-mediated DNA fragmentation. The CD95 death receptor as well as other caspase-8-requiring death receptors were not involved because Jurkat T cells lacking the CD95 receptor or caspase-8 and control cells responded equally to cephalostatin 1. Although cephalostatin 1 affects mitochondria by dissipating the mitochondrial membrane potential, neither cytochrome c nor apoptosis-inducing factor is released, as shown by Western blot analysis. Interestingly, cephalostatin 1 selectively triggers the mitochondrial release of the inhibitor of apoptosis antagonist Smac/DIABLO. Overexpression of the antiapoptotic protein Bcl-x(L) delayed both Smac/DIABLO release and onset of apoptosis, suggesting that Smac/DIABLO is required for cephalostatin 1-induced apoptosis. This new mitochondrial pathway is accompanied by marked structural changes of mitochondria as shown by transmission electron microscopy.     

Caspase-8/FLICE functions as an executioner caspase in anticancer drug-induced apoptosis

Caspase-8 plays an essential role in apoptosis triggered by death receptors. Through the cleavage of Bid, a proapoptotic Bcl-2 member, it further activates the mitochondrial cytochrome c/Apaf-1 pathway. Because caspase-8 can be processed also by anticancer drugs independently of death receptors, we investigated its exact role and order in the caspase cascade. We show that in Jurkat cells either deficient for caspase-8 or overexpressing its inhibitor c-FLIP apoptosis mediated by CD95, but not by anticancer drugs was inhibited. In the absence of active caspase-8, anticancer drugs still induced the processing of caspase-9, -3 and Bid, indicating that Bid cleavage does not require caspase-8. Overexpression of Bcl-x(L) prevented the processing of caspase-8 as well as caspase-9, -6 and Bid in response to drugs, but was less effective in CD95-induced apoptosis. Similar responses were observed by overexpression of a dominant-negative caspase-9 mutant. To further determine the order of caspase-8 activation, we employed MCF7 cells lacking caspase-3. In contrast to caspase-9 that was cleaved in these cells, anticancer drugs induced caspase-8 activation only in caspase-3 transfected MCF7 cells. Thus, our data indicate that, unlike its proximal role in receptor signaling, in the mitochondrial pathway caspase-8 rather functions as an amplifying executioner caspase.     

Enhancing TRAIL-induced apoptosis by Bcl-X(L) siRNA

We previously found that a change in the balance between mitochondrial pro- and anti-apoptotic proteins caused by ectopic expression of the Bax gene led to increased induction of apoptosis by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). To investigate whether a similar effect can be elicited by down-regulating Bcl-X(L), an anti-apoptotic protein, we tested the effects of a small interfering RNA (siRNA) specific for Bcl-X(L) in TRAIL-resistant cells. The down-regulation of Bcl-X(L) by siRNA inhibited cell proliferation and sensitized TRAIL-induced apoptosis in human cancer cells with both acquired and intrinsic TRAIL resistance. Combining the Bcl-X(L) siRNA with TRAIL protein treatment resulted in an increase in the percentage of apoptotic cells and increased cleavage of caspase-8, caspase-9, caspase-3 and PARP. Furthermore, the release of cytochrome c but not Smac from mitochondria was induced by Bcl-X(L) siRNA alone, and this release was dramatically amplified by combining the Bcl-X(L) siRNA and TRAIL protein treatment. Together, our data suggest that simultaneous triggering of the death receptor and mitochondrial apoptotic pathways leads to enhanced induction of apoptosis, which makes it potentially useful for the treatment of resistant cancers.     

Induction of apoptosis by enediyne antibiotic calicheamicin thetaII proceeds through a caspase-mediated mitochondrial amplification loop in an entirely Bax-dependent manner

Calicheamicin thetaII is a member of the enediyne class of antitumor antibiotics that bind to DNA and induce apoptosis. These compounds differ, however, from conventional anticancer drugs as they bind in a sequence-specific manner noncovalently to DNA and cause sequence-selective oxidation of deoxyriboses and bending of the DNA helix. Calicheamicin is clinically employed as immunoconjugate to antibodies directed against, for example, CD33 in the case of gemtuzumab ozogamicin. Here, we show by the use of the unconjugated drug that calicheamicin-induced apoptosis is independent from death-receptor/FADD-mediated signals. Moreover, calicheamicin triggers apoptosis in a p53-independent manner as shown by the use of p53 knockout cells. Cell death proceeds via activation of mitochondrial permeability transition, cytochrome c release and activation of caspase-9 and -3. The overexpression of Bcl-x(L) or Bcl-2 strongly inhibited calicheamicin-induced apoptosis. Knockout of Bax abrogated cell death after calicheamicin treatment. Thus, the activation of mitochondria and execution of cell death occur through a fully Bax-dependent mechanism. Interestingly, caspase inhibition by the pancaspase-inhibitor zVAD-fmk interfered with mitochondrial activation by calicheamicin. This places caspase activation upstream of the mitochondria and indicates that calicheamicin-triggered apoptosis is enhanced through death receptor-independent activation of the caspase cascade, that is, an amplification loop that is required for full activation of the mitochondrial pathway.     

Bcl-xL inhibits p53- but not apoptin-induced apoptosis in head and neck squamous cell carcinoma cell line

Nonfunctional p53 and especially upregulation of Bcl-x(L) result in advanced disease and poor prognosis of patients suffering head and neck squamous cell carcinoma (HNSCC). Aberrancies of Bcl-x(L) and/or p53 in HNSCC lead to inability of anticancer drugs to induce apoptosis. Bcl-x(L) and/or mutated p53 inhibit the apoptotic process by preventing the mitochondrial release of cytochrome c and/or activation of execution caspases. Here, we report that expression of the avian virus-derived apoptin protein resulted in induction of apoptosis in the HNSCC-derived cell line UMSSC-14B despite the presence of nonfunctional p53. Apoptin activated the execution caspase 3 and induced the release of mitochondrial cytochrome c. Upregulation of Bcl-x(L) in UMSCC-14B cells did not interfere with the apoptin-induced apoptosis, whereas it clearly negatively affected the p53-induced one. Bcl-x(L) significantly decreased the p53-induced cytochrome c release, but not the apoptin-triggered one. Our data demonstrate that apoptin induces apoptosis independent of Bcl-x(L) and p53 and may constitute a potential therapeutic agent for treatment of HNSCC.     

Gefitinib induces apoptosis in human glioma cells by targeting Bad phosphorylation

Gefitinib, a selective epidermal growth factor receptor tyrosine kinase inhibitor, is under clinical testing and use in cancer patients, including glioma. However, the molecular mechanisms involved in gefitinib-mediated anticancer effects against glioma remain largely uncharacterized. Gefitinib inhibits cell growth and induces apoptosis in human glioma cells. Gefitinib also induces death of H4 cells with characteristics of the intrinsic apoptotic pathway, including Bax mitochondrial translocation, mitochondrial outer membrane permeabilization, cytochrome c cytosolic release, and caspase-9/caspase-3 activation. The importance of Bax in mediating gefitinib-induced apoptosis was confirmed by the attenuation of apoptosis by Bax siRNA and Bax channel blocker. Gefitinib caused Bad dephosphorylation, particularly in serine-112, and increased its binding preference to Bcl-2 and Bcl-xL. The dephosphorylation of Bad in gefitinib-treated cells was accompanied by reduced intracellular cyclic AMP content and protein kinase A (PKA) activity. Adenylyl cyclase activator forskolin attenuated, but PKA inhibitor H89 augmented, gefitinib-induced Bad dephosphorylation, Bax mitochondrial translocation, caspase-9/caspase-3 activation, and viability loss. Intriguingly, a nonselective protein phosphatase inhibitor okadaic acid alleviated gefitinib-induced alterations, except Bad dephosphorylation. In parallel with the higher basal PKA activity, response of U87 cells to gefitinib treatment was delayed and relatively resistant compared with that of H4 and T98G cells. Inactivation of PKA sensitized H4, T98G, and U87 cells to gefitinib cytotoxicity, Bad dephosphorylation in serine-112, and caspase-9/caspase-3 activation. Our findings suggest the involvement of the Bad/Bax signaling pathway in gefitinib-induced glioma apoptosis. Furthermore, the inactivation of PKA was shown to play a role in triggering the proapoptotic function of Bad.     

Current status of the molecular mechanisms of anticancer drug-induced apoptosis. The contribution of molecular-level analysis to cancer chemotherapy

Apoptosis is an important phenomenon in cytotoxicity induced by anticancer drugs. Here, we review the current status of the molecular mechanisms of anticancer drug-induced apoptosis in order to assess the contribution of molecular-level analysis to cancer chemotherapy. It is apparent that the molecular mechanisms by which anticancer drugs induce apoptosis are mediated by death receptor-dependent and -independent pathways, which are related to the release of cytochrome c through voltage-dependent anion channels in the mitochondrial inner membrane. The release of cytochrome c is the central gate in turning on/off apoptosis, and is regulated by the interaction of proapoptotic proteins, including Bid, Bax and Bak, and antiapoptotic proteins including Bcl-2 and Bcl-X(L), and a specific class of inhibitors of apoptosis proteins (IAPs) including Akt, survivin, and heat-shock proteins. The caspase cascade is activated by the release of cytochrome c, which is initiated by the formation of apoptosomes consisting of procaspase-9, Apaf-1 and cytochrome c in the presence of dATP, and results in the activation of caspase-9 and caspase-3, thereby leading to apoptosis. Drug sensitivity can be enhanced by the introduction of proapoptotic genes and the inhibition of antiapoptotic proteins. The latter process is mediated by antisense oligonucleotides and is associated with apoptosis. The signal transduction pathways that are triggered by the central gate in mitochondria play a critical role in anticancer drug-induced apoptosis. The modulation of signal transduction pathways targeting the proteins involved in these signal transduction pathways using antisense IAPs, and growth factor antibodies may be a good strategy for enhancing therapeutic efficacy of anticancer drugs in cancer chemotherapy.     

Bak functionally complements for loss of Bax during p14ARF-induced mitochondrial apoptosis in human cancer cells

In contrast to the initial notion that the biological activity of p14(ARF) strictly depends on a functional mdm-2/p53 signaling axis, we recently demonstrated that p14(ARF) mediates apoptosis in a p53/Bax-independent manner. Here, we show that p14(ARF) induces breakdown of the mitochondrial membrane potential and cytochrome c release before triggering caspase-9- and caspase-3/7-like activities in p53/Bax-deficient DU145 prostate cancer cells expressing wild-type Bak. Re-expression of Bax in these cells failed to further enhance p14(ARF)-induced apoptosis, suggesting that p14(ARF)-induced apoptosis primarily depends on Bak but not Bax in these cells. To further define the role of Bak and Bax in p14(ARF)-induced mitochondrial apoptosis, we employed short interference RNA for the knockdown of bak in isogeneic, p53 wild-type HCT116 colon cancer cells either proficient or deficient for Bax. There, combined loss of Bax and Bak attenuated p14(ARF)-induced apoptosis whereas single loss of Bax or Bak was only marginally effective, as in the case of DU145. Notably, HCT116 cells deficient for Bax and Bak failed to release cytochrome c and showed attenuated activation of caspase-9 (LEHDase) and caspase-3/caspase-7 (DEVDase) upon p14(ARF) expression. These data indicate that p14(ARF) triggers apoptosis via a Bax/Bak-dependent pathway in p53-proficient HCT116, whereas Bax is dispensable in p53-deficient DU145 cells. Nevertheless, a substantial proportion of p14(ARF)-induced cell death proceeds in a Bax/Bak-independent manner. This is also the case for inhibition of clonogenic growth that occurs, at least in part, through an entirely Bax/Bak-independent mechanism.     

Bax-induction alone is sufficient to activate apoptosis cascade in wild-type Bax-bearing K562 cells, and the initiation of apoptosis requires simultaneous caspase activation

It has been reported that expression of Bax by Tet-On system induces apoptosis in Jurkat cells. The parental Jurkat cells have mutation of Bax gene and do not express Bax protein. Wild-type Bax-bearing cells express endogenous Bax protein and it is still unclear whether overexpression of Bax alone can sufficiently induce apoptosis in these cells in the absence of any cytotoxic stimulus. To investigate this, wild-type Bax-bearing K562 cells were transfected with Tet-On Bax-inducible system (pTet-On and pTRE-Bax plasmids), and Bax-inducible stable cell lines were established. Overexpression of Bax in wild-type Bax-bearing K562 cells without any cyctotoxic signal resulted in increase of apoptotic cells, caspase-3 activation, mitochondrial release of cytochrome c, and mitochondrial membrane potential change. Western blotting and confocal microscopy revealed that overexpression of Bax was detected in mitochondria. A pancaspase inhibitor, zVAD-fmk, which has no effect on mitochondrial cytochrome c release and mitochondrial membrane potential change inhibited the apoptotic events in the presence of overexpressed Bax in mitochondria. These findings suggest that Bax protein, when present above a threshold level, is sufficient to trigger an apoptosis cascade, and its initiation requires simultaneous caspase activation probably not mediated by mitochondrial cytochrome c release and mitochondrial membrane potential change in K562 cells.     

Bax and Bak are required for apoptosis induction by sulforaphane, a cruciferous vegetable-derived cancer chemopreventive agent

Sulforaphane, a constituent of many edible cruciferous vegetables, including broccoli, effectively suppresses proliferation of cancer cells in culture and in vivo by causing apoptosis induction, but the sequence of events leading to cell death is poorly defined. Here, we show that multidomain proapoptotic Bcl-2 family members Bax and Bak play a critical role in apoptosis induction by sulforaphane. This conclusion is based on the following observations: (a) sulforaphane treatment caused a dose- and time-dependent increase in the protein levels of both Bax and Bak and conformational change and mitochondrial translocation of Bax in SV40-transformed mouse embryonic fibroblasts (MEF) derived from wild-type mice to trigger cytosolic release of apoptogenic molecules (cytochrome c and Smac/DIABLO), activation of caspase-9 and caspase-3, and ultimately cell death; (b) MEFs derived from Bax or Bak knockout mice resisted cell death by sulforaphane, and (c) MEFs derived from Bax and Bak double knockout mice exhibited even greater protection against sulforaphane-induced cytochrome c release, caspase activation, and apoptosis compared with wild-type or single knockout cells. Interestingly, sulforaphane treatment also caused a dose- and time-dependent increase in the protein level of Apaf-1 in wild-type, Bax-/-, and Bak-/- MEFs but not in double knockout, suggesting that Bax and Bak might regulate sulforaphane-mediated induction of Apaf-1 protein. A marked decline in the protein level of X-linked inhibitor of apoptosis on treatment with sulforaphane was also observed. Thus, it is reasonable to postulate that sulforaphane-induced apoptosis is amplified by a decrease in X-linked inhibitor of apoptosis level, which functions to block cell death by inhibiting activities of caspases. In conclusion, the results of the present study indicate that Bax and Bak proteins play a critical role in initiation of cell death by sulforaphane.     

Anti-proliferative effect of clitocine from the mushroom Leucopaxillus giganteus on human cervical cancer HeLa cells by inducing apoptosis

Clitocine, a natural biologically active substance isolated from the mushroom Leucopaxillus giganteus, possesses several bioactivities including antitumor. Here, for the first time, we studied the molecular mechanism of clitocine-induced apoptosis in human cervical cancer cells (HeLa). Clitocine-induced cell death was characterized with the changes in cell morphology, DNA fragmentation, activation of caspase-3, -8, and -9 (like) activities, poly(ADP-ribose) polymerase (PARP) cleavage, release of cytochrome c (cyt c) into cytosol, and increase of Bax:Bcl-2 ratio. These results indicated that the induction of apoptosis by clitocine involved the multiple pathway including death receptor and mitochondrial pathways, and strongly suggested that the mitochondrial pathways were mediated by down-regulation of Bcl-2 and up-regulation of Bax, release of cytochrome c and subsequent activation of caspase-3 followed by down stream events leading to apoptotic mode of cell death.     

Paclitaxel-induced apoptosis in BJAB cells proceeds via a death receptor-independent,caspases-3/-8-driven mitochondrial amplification loop

Caspase-8 is a key effector of death-receptor-triggered apoptosis. In a previous study, we demonstrated, however, that caspase-8 can also be activated in a death receptor-independent manner via the mitochondrial apoptosis pathway, downstream of caspase-3. Here, we show that caspases-3 and -8 mediate a mitochondrial amplification loop that is required for the optimal release of cytochrome c, mitochondrial permeability shift transition, and cell death during apoptosis induced by treatment with the microtubule-damaging agent paclitaxel (Taxol). In contrast, Smac release from mitochondria followed a different pattern, and therefore seems to be regulated independently from cytochrome c release. Taxol-induced cell death was inhibited by the use of synthetic, cell-permeable caspase-3- (zDEVD-fmk) or caspase-8-specific (zIETD-fmk) inhibitors. Apoptosis signaling was not affected by a dominant-negative FADD mutant (FADD-DN), thereby excluding a role of death receptor signaling in the amplification loop and drug-induced apoptosis. The inhibitor experiments were corroborated by the use of BJAB cells overexpressing the natural serpin protease inhibitor, cytokine response modifier A. These data demonstrate that the complete activation of mitochondria, release of cytochrome c, and execution of drug-induced apoptosis require a mitochondrial amplification loop that depends on caspases-3 and -8 activation. In addition, this is the first report to demonstrate death receptor-independent caspase-8 autoprocessing in vivo.