TRAIL receptor and CD95 signal to mitochondria via FADD,caspase-8/10, Bid,and Bax but differentially regulate events downstream from truncated Bid

The death receptor ligand TRAIL arouses much interest for clinical application. We found that TRAIL receptor could induce cytochrome c (Cyt c) release from mitochondria in cells that failed to respond to CD95. Therefore, we examined whether these two closely related death receptors use different intermediates to convey the apoptotic signal to mitochondria. Dominant negative FADD, FLIP(L), or a Bid mutant lacking cleavage sites for caspase-8/10 completely inhibited Cyt c release in response to either receptor. Depletion of Bid from TRAIL- or CD95-activated cytosols blocked their capacity to mediate Cyt c release from mitochondria in vitro, whereas Bax depletion reduced it. We conclude that FADD, caspase-8/10, and caspase-cleaved Bid are required for TRAIL receptor and CD95 signaling to mitochondria, whereas Bax is a common accessory. In vitro, caspase-8 treatment of cytosol from CD95-resistant cells permitted generation of truncated Bid and its association with mitochondria. However, this cytosol impaired the ability of truncated Bid to liberate Cyt c from exogenous mitochondria. We conclude that the TRAIL receptor can bypass or neutralize the activity of cytosolic factor that blocks truncated Bid function. This may benefit the capacity of TRAIL to break apoptosis resistance in tumor cells.     

Apoptosis-inducing factor (AIF) is targeted in IFN-α2a-induced Bid-mediated apoptosis through Bak activation in ovarian cancer cells

Previously we have shown that interferon (IFN)-α induced apoptosis is predominantly mediated by the upregulation of tumor necrosis factor related apoptosis-inducing ligand (TRAIL) via the caspase-8 pathway. It was also shown that recruitment of mitochondria in IFN-α induced apoptosis involves the cleavage of BH3 interacting domain death agonist (Bid) to truncated Bid (tBid). In the present study, we demonstrate that tBid induced by IFN-α2a activates mitochondrial Bak to trigger the loss of mitochondrial membrane integrity, consequently causing release of apoptosis-inducing factor (AIF) in ovarian cancer cells, OVCAR3. AIF translocates from the mitochondria to the nucleus and induces nuclear fragmentation and cell death. Both a small molecule Bid inhibitor (BI-6C9) or Bid-RNA interference (RNAi) preserved mitochondrial membrane potential, prevented nuclear translocation of AIF, and abrogated IFN-α2a-induced cell death. Cell death induced by tBid was inhibited by AIF-RNAi, indicating that caspase-independent AIF signaling is the main pathway through which Bid mediates cell death. This was further supported by experiments showing that BI-6C9 did not prevent the release of cytochrome c from mitochondria to cytosol, while the release of AIF was prevented. In conclusion, IFN-α2a-induced apoptosis is mediated via the mitochondria-associated pathway involving the cleavage of Bid followed by AIF release that involves Bak activation and translocation of AIF from the mitochondria to the nucleus in OVCAR3 cells.     

Mitochondria from TRAIL-resistant prostate cancer cells are capable of responding to apoptotic stimuli

TNFalpha-related apoptosis inducing ligand (TRAIL) has been shown to induce apoptosis in prostate cancer cells. However, some prostate cancer cells, such as LNCaP are resistant to TRAIL. In addition to the involvement of several pathways in the TRAIL-resistance of LNCaP, it has been shown that mitochondrial response to TRIAL is low in these cells. Therefore, in this study, using in vitro cell free and reconstitution models, we have demonstrated that mitochondria from these cells are capable of responding to apoptotic stimuli. Furthermore, experiments to determine the influence of cytochrome c on apoptotic response noted that incubation of cytosol with exogenous cytochrome c induced truncation of Bid. We have demonstrated that truncation of Bid by exogenous cytochrome c is mediated through the activation of caspases-9 and -3. Incubation of cytosol with recombinant caspases-9 and -3 in the absence or presence of inhibitors showed that activation of caspase-9, leading to the activation of caspase-3 was necessary for the truncation of Bid. Published results indicate that in apoptotic cells cytochrome c is released from the mitochondria in two installments, an early small amount and a late larger amount. Our results suggest that the initial release of cytochrome generates tBid that is capable of translocation into the mitochondria causing further release of cytochrome c. Thus, in addition to providing functional explanation for the biphasic release of cytochrome c from mitochondria, we demonstrate the presence of a feedback amplification of mitochondrial apoptotic signal.     

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-induced apoptosis is dependent on activation of cysteine and serine proteases

We examined the role of caspases and serine protease(s) in cell death induced by tumour necrosis factor-related apoptosis-inducing ligand (TRAIL). After incubation of adenocarcinoma cells with TRAIL, caspase-3, -8 were activated and the cleavage of Bid induced the release of cytochrome c, from the mitochondria to the cytosol. Tetrapeptide inhibitors of caspase-1, -2, -3, and -8 suppressed DNA fragmentation and attenuated the release of cytochrome c, whereas inhibitors of caspase-5 did not. Interestingly, the general serine protease(s) inhibitor 4-(2-aminoethyl)benzylsulfonyl fluoride (AEBSF) resulted in the arrest of apoptosis. However, the AEBSF did not prevent the release of mitochondrial cytochrome c during TRAIL-induced apoptosis. From these results, we postulate that serine protease(s) may be involved in post-mitochondrial apoptotic events, that lead to the activation of the initiator, caspase-9.     

Hepatitis C virus core protein modulates TRAIL-mediated apoptosis by enhancing Bid cleavage and activation of mitochondria apoptosis signaling pathway

Hepatitis C virus (HCV) is a major human pathogen causing chronic liver disease, which leads to cirrhosis of liver and hepatocellular carcinoma. The HCV core protein, a viral nucleocapsid, has been shown to affect various intracellular events, including cell proliferation and apoptosis. However, the precise mechanisms of the effects are not fully understood. In this study, we show that HCV core protein sensitizes human hepatocellular carcinoma cell line, Huh7, conferred sensitivity to TRAIL-, but not Fas ligand-mediated apoptosis. Huh7 cells are resistant to TRAIL, despite the induction of caspase-8 after TRAIL engagement. However, HCV core protein induces TRAIL apoptosis signaling via sequential induction of caspase-8, Bid cleavage, activation of mitochondrial pathway, and effector caspase-3. HCV core protein also induces activation of caspase-9 after TRAIL engagement, and the induction of TRAIL sensitivity by HCV core protein could be reversed by caspase-9 inhibitor. Therefore, the HCV core protein-induced TRAIL-mediated apoptosis is dependent upon activation of caspase-8 downstream pathway to convey the death signal to mitochondria, leading to activation of mitochondrial signaling pathway and breaking the apoptosis resistance. These results combined indicate that the HCV core protein enhances TRAIL-, but not Fas ligand-mediated apoptotic cell death in Huh7 cells via a mechanism dependent on the activation of mitochondria apoptosis signaling pathway. These results suggest that HCV core protein may have a role in immune-mediated liver cell injury by modulation of TRAIL-induced apoptosis.     

Epstein-Barr virus BHRF1 functions downstream of Bid cleavage and upstream of mitochondrial dysfunction to inhibit TRAIL-induced apoptosis in BJAB cells

We have previously reported that TNF-related apoptosis inducing ligand (TRAIL) causes cleavage of Bid via activation of caspase-8 and the loss of mitochondrial membrane potential (DeltaPsim), resulting in apoptosis. Experiments with BJAB clones expressing Epstein-Barr virus (EBV) anti-apoptotic protein BHRF1 showed that BHRF1 drastically inhibited TRAIL-mediated apoptosis. Although Western blot analysis demonstrated that TRAIL-induced Bid cleavage was not inhibited by BHRF1, the decrease in DeltaPsim caused by TRAIL was effectively blocked by BHRF1. These findings suggest that in BJAB cells, BHRF1 acts downstream of Bid cleavage and upstream of mitochondrial damage, resulting in inhibition of TRAIL-induced apoptosis.     

A caspase-8-independent signaling pathway activated by Fas ligation leads to exposure of the Bak N terminus

Bak is a pro-apoptotic member of the Bcl-2 family that is activated by apoptotic stimulation: its activation is characterized by conformational changes such as exposure of the N terminus and oligomerization. In death receptor-mediated apoptosis, the activation of Bak depends on activation of caspase-8. However, we found that exposure of the N terminus of Bak (but not oligomerization) can occur in the absence of active caspase-8. Although exposure of the N terminus of Bak without oligomerization is not sufficient to release cytochrome c from the mitochondria and commit cells to apoptosis, this change sensitizes the mitochondria to apoptotic signals (including Bid) and thus sensitizes cells to apoptotic death. Fas-induced, caspase-8-independent exposure of the N terminus of Bak is blocked by staurosporine, a pan protein kinase inhibitor. These results suggest that Fas stimulation not only activates caspase-8, but also a distinct signaling pathway involving protein kinase(s) to induce exposure of the N terminus of Bak.     

Essential roles of the Bcl-2 family of proteins in caspase-2-induced apoptosis

Caspase-2 is an initiating caspase required for stress-induced apoptosis in various human cancer cells. Recent studies suggest that it can mediate the death function of tumor suppressor p53 and is activated by a multimeric protein complex, PIDDosome. However, it is not clear how caspase-2 exerts its apoptotic function in cells and whether its enzymatic activity is required for the apoptotic function. In this study, we used both in vitro mitochondrial cytochrome c release assays and cell culture apoptosis analyses to investigate the mechanism by which caspase-2 induces apoptosis. We show that active caspase-2, but neither a catalytically mutated caspase-2 nor active caspase-2 with its inhibitor, can cause cytochrome c release. Caspase-2 failed to induce cytochrome c release from mitochondria with Bid(-/-) background, and the release could be restored by addition of the wild-type Bid protein, but not by Bid with the caspase-2 cleavage site mutated. Caspase-2 was not able to induce cytochrome c release from Bax(-/-)Bak(-/-) mitochondria either. In cultured cells, gene deletion of Bax/Bak or Bid abrogated apoptosis induced by overexpression of caspase-2. Collectively, these results indicate that proteolytic activation of Bid and the subsequent induction of the mitochondrial apoptotic pathway through Bax/Bak is essential for apoptosis triggered by caspase-2.     

Bid is cleaved upstream of caspase-8 activation during TRAIL-mediated apoptosis in human osteosarcoma cells

TRAIL induces apoptosis in many malignant cell types. In this study, we used the human papilloma virus (HPV) 16 E6 protein as a molecular tool to probe the TRAIL pathway in HCT116 colon carcinoma cells and U2OS osteosarcoma cells. Intriguingly, we found that while E6 protected HCT116 cells from TRAIL, U2OS cells expressing E6 remained sensitive to TRAIL. Furthermore, silencing FADD and procaspase-8 expression with siRNA did not prevent TRAIL-induced apoptosis in U2OS cells. However, siBid provided significant protection from TRAIL, and the cleavage kinetics of Bid and caspase-8 revealed that Bid was cleaved prior to the activation of caspase-8. Cathepsin B activity in U2OS cells was significantly activated shortly after exposure to TRAIL, and the cathepsin B inhibitor, CA074Me, inhibited both TRAIL- and anti-DR5-mediated apoptosis and delayed the cleavage of Bid. These findings suggest that TRAIL activates a pathway dependent on Bid, but largely independent of FADD and caspase-8, in U2OS cells.     

Role of the mitochondrial signaling pathway in murine coronavirus-induced oligodendrocyte apoptosis

A previous study demonstrated that infection of rat oligodendrocytes by mouse hepatitis virus (MHV) resulted in apoptosis, which is caspase dependent (Y. Liu, Y. Cai, and X. Zhang, J. Virol. 77:11952-11963, 2003). Here we determined the involvement of the mitochondrial pathway in MHV-induced oligodendrocyte apoptosis. We found that caspase-9 activity was 12-fold higher in virus-infected cells than in mock-infected cells at 24 h postinfection (p.i.). Pretreatment of cells with a caspase-9 inhibitor completely blocked caspase-9 activation and partially inhibited the apoptosis mediated by MHV infection. Analyses of cytochrome c release further revealed an activation of the mitochondrial apoptotic pathway. Stable overexpression of the two antiapoptotic proteins Bcl-2 and Bcl-xL significantly, though only partially, blocked apoptosis, suggesting that activation of the mitochondrial pathway is partially responsible for the apoptosis. To identify upstream signals, we determined caspase-8 activity, cleavage of Bid, and expression of Bax and Bad by Western blotting. We found a drastic increase in caspase-8 activity and cleavage of Bid at 24 h p.i. in virus-infected cells, suggesting that Bid may serve as a messenger to relay the signals from caspase-8 to mitochondria. However, treatment with a caspase-8 inhibitor only slightly blocked cytochrome c release from the mitochondria. Furthermore, we found that Bax but not Bad was significantly increased at 12 h p.i. in cells infected with both live and UV-inactivated viruses and that Bax activation was partially blocked by treatment with the caspase-8 inhibitor. These results thus establish the involvement of the mitochondrial pathway in MHV-induced oligodendrocyte apoptosis.     

A caspase-8-independent component in TRAIL/Apo-2L-induced cell death in human rhabdomyosarcoma cells

Tumor necrosis factor related apoptosis inducing ligand (TRAIL) belongs to the Tumor necrosis factor (TNF) family of death-inducing ligands, and signaling downstream of TRAIL ligation to its receptor(s) remains to be fully elucidated. Components of the death-inducing signaling complex (DISC) and TRAIL signaling downstream of receptor activation were examined in TRAIL - sensitive and -resistant models of human rhabdomyosarcoma (RMS). TRAIL ligation induced DISC formation in TRAIL-sensitive (RD, Rh18, Rh30) and TRAIL-resistant RMS (Rh28, Rh36, Rh41), with recruitment of FADD and procaspase-8. In RD cells, overexpression of dominant-negative FADD (DNFADD) completely abolished TRAIL-induced cell death in contrast to dominant-negative caspase- 8 (DNC8), which only partially inhibited TRAIL-induced apoptosis, growth inhibition, or loss in clonogenic survival. DNC8 did not inhibit the cleavage of Bid or the activation of Bax. Overexpression of Bcl-2 or Bcl-xL inhibited TRAIL-induced apoptosis, growth inhibition, and loss in clonogenic survival. Bcl-2 and Bcl-xL, but not DNC8, inhibited TRAIL-induced Bax activation. Bcl-xL did not inhibit the early activation of caspase-8 (<4 h) but inhibited cleavage of Bid, suggesting that Bid is cleaved downstream of the mitochondria, independent of caspase-8. Exogenous addition of sphingosine also induced activation of Bax via a caspase-8-and Bid-independent mechanism. Further, inhibition of sphingosine kinase completely protected cells from TRAIL-induced apoptosis. Data demonstrate that in RMS cells, the TRAIL signaling pathway circumvents caspase-8 activation of Bid upstream of the mitochondria and that TRAIL acts at the level of the mitochondria via a mechanism that may involve components of the sphingomyelin cycle.     

Resveratrol induces mitochondria-mediated AIF and to a lesser extent caspase-9-dependent apoptosis in human lung adenocarcinoma ASTC-a-1 cells

Resveratrol (RV), a natural plant polyphenol widely present in foods such as grapes, wine, and peanuts, has an ability to inhibit various stages of carcinogenesis in vitro and in vivo. In this report, we explored the roles of intrinsic and extrinsic apoptotic pathways during RV-induced apoptosis in human lung adenocarcinoma (ASTC-a-1) cells. After exposure of cells to different concentrations of RV, we found that RV induced concentration-dependent apoptosis. Fluorometric substrates assay and western blotting (WB) analysis showed that caspase-8 was not activated, which was further verified by monitoring the cleavage of Bid to tBid using fluorescence resonance energy transfer (FRET) microscopy imaging inside single living cells, indicating that extrinsic apoptotic pathway was not involved in RV-induced apoptosis. In addition, inhibition of caspases-3 or -9 but not caspase-8 using the specific inhibitors of caspases modestly but significantly attenuated RV-induced apoptosis. Moreover, flow cytometry (FCM) analysis showed that RV treatment induced time-dependent loss of mitochondrial membrane potential (?ψ(m)), in combination with the activation of caspases-3 and -9; we therefore concluded that RV-induced apoptosis involved the intrinsic apoptotic pathway. It is noteworthy that RV treatment induced translocation of AIF from mitochondria to nucleus in a time dependent manner, and that knockdown of AIF remarkably attenuated RV-induced apoptosis. Collectively, our findings demonstrate that RV induces caspase-8-independent apoptosis via AIF and to a lesser extent caspase-9-dependent mitochondrial pathway in ASTC-a-1 cells.     

A20 inhibits oxidized low-density lipoprotein-induced apoptosis through negative Fas/Fas ligand-dependent activation of caspase-8 and mitochondrial pathways in murine RAW264.7 macrophages

A20 was originally characterized as a TNF-inducible gene in human umbilical vein endothelial cells. As an NF-kappaB target gene, A20 is also induced in many other cell types by a wide range of stimuli. Expression of A20 has been shown to protect from TNF-induced apoptosis and also functions via a negative-feedback loop to block NF-kappaB activation induced by TNF and other stimuli. To date, there are no reports on whether A20 can protect OxLDL-induced apoptosis in macrophages. For the first time we report that A20 expression blocks OxLDL-mediated cell toxicity and apoptosis. OxLDL induced the expression of Fas and FasL, and the subsequent caspase-8 cleavage and treatment with a neutralizing ZB4 anti-Fas antibody blocked apoptosis induced by OxLDL. Expression of dominant negative FADD efficiently prevented OxLDL-induced apoptosis and caspase-8 activation. A20 expression significantly attenuated the increased expression of Fas and FasL, and Fas-mediated apoptosis. These findings suggest that A20-mediated protection from OxLDL may occur at the level of Fas/FADD-caspase-8 and be FasL dependent. Treatment of RAW264.7 cells with OxLDL induces a series of time-dependent events, including the release of cytochrome c, Smac and Omi from the mitochondria to the cytosol, activation of caspase-9, -6, -2, and -3, which are blocked by A20 expression. No cleaved form of Bid was detected, even treatment with OxLDL for 48 h. Expression of dominant negative FADD also efficiently prevented OxLDL-induced the above apoptotic events. The release of cyto c, Smac and Omi from mitochondria to cytosol, activated by OxLDL treatment, and the activation of caspase-9 may not be a downstream event of caspase-8-mediated Bid cleavage. Therefore, the protective effect of A20 on mitochondrial apoptotic pathway activated by OxLDL may be dependent on FADD. A20 expression reversed OxLDL-mediated G(0)/G(1) stage arrest by maintaining the expression of cyclin B1, cyclin D1, and cyclin E, and p21 and p73. Thus, A20 expression blocks OxLDL-mediated apoptosis in murine RAW264.7 macrophages through disrupting Fas/FasL-dependent activation of caspase-8 and the mitochondria pathway.     

Caspase-2 can function upstream of bid cleavage in the TRAIL apoptosis pathway

In many mammalian cell types, engagement of the TRAIL/Apo2L death receptors DR4 and DR5 alters mitochondrial physiology, thereby promoting the release of pro-apoptotic proteins normally contained within this organelle. A contemporary view of this process is that in so-called type II cells death receptor-activated caspase-8 cleaves the Bcl-2 family member Bid, which generates a truncated Bid fragment that collaborates with Bax, another Bcl-2 relative, to promote the release of mitochondrial factors necessary for activation of executioner caspases and apoptosis. Here we show that in some type II cells caspase-2 is necessary for optimal TRAIL-mediated cleavage of Bid. Down-regulation of caspase-2 using RNA interference significantly inhibited TRAIL-induced apoptosis. Analysis of the TRAIL proteolytic cascade following gene silencing of specific pathway components revealed that caspase-2 is necessary for efficient cleavage of Bid; however, caspase-2 proteolytic processing, which occurs downstream of Bax, is not necessary for its role in Bid cleavage.     

Specific cleavage of Mcl-1 by caspase-3 in tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in Jurkat leukemia T cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces programmed cell death through the caspase activation cascade and translocation of cleaved Bid (tBid) by the apical caspase-8 to mitochondria to induce oligomerization of multidomain Bax and Bak. However, the roles of prosurvival Bcl-2 family proteins in TRAIL apoptosis remain elusive. Here we showed that, besides the specific cleavage and activation of Bid by caspase-8 and caspase-3, TRAIL-induced apoptosis in Jurkat T cells required the specific cleavage of Mcl-1 at Asp-127 and Asp-157 by caspase-3, while other prototypic antiapoptotic factors such as Bcl-2 or Bcl-X(L) seemed not to be affected. Mutation at Asp-127 and Asp-157 of Mcl-1 led to cellular resistance to TRAIL-induced apoptosis. In sharp contrast to cycloheximide-induced Mcl-1 dilapidation, TRAIL did not activate proteasomal degradation of Mcl-1 in Jurkat cells. We further established for the first time that the C-terminal domain of Mcl-1 became proapoptotic as a result of caspase-3 cleavage, and its physical interaction and cooperation with tBid, Bak, and voltage-dependent anion-selective channel 1 promoted mitochondrial apoptosis. These results suggested that removal of N-terminal domains of Bid by caspase-8 and Mcl-1 by caspase-3 enabled the maximal mitochondrial perturbation that potentiated TRAIL-induced apoptosis.     

Cotreatment with apicidin overcomes TRAIL resistance via inhibition of Bcr-Abl signaling pathway in K562 leukemia cells

TNF-related apoptosis-inducing ligand (TRAIL) is a pro-apoptotic cytokine that is capable of inducing apoptosis in a wide variety of cancer cells but not in normal cells. Although many cancer cells are sensitive to TRAIL-induced apoptosis, chronic myeloid leukemia (CML) develops resistance to TRAIL. In this study, we investigated whether apicidin, a novel histone deacetylase inhibitor, could overcome the TRAIL resistance in CML-derived K562 cells. Compared to treatment with apicidin or TRAIL alone, cotreatment with apicidin and TRAIL-induced apoptosis synergistically in K562 cells. This combination led to activation of caspase-8 and Bcl-2 interacting domain (Bid), resulting in the cytosolic accumulation of cytochrome c from mitochondria as well as an activation of caspase-3. Treatment with apicidin resulted in down-regulation of Bcr-Abl and inhibition of its downstream target, PI3K/AKT-NF-κB pathway. In addition, apicidin decreased the level of NF-κB-dependent Bcl-x_L, leading to caspase activation and Bid cleavage. These results suggest that apicidin may sensitize K562 cells to TRAIL-induced apoptosis through caspase-dependent mitochondrial pathway by regulating expression of Bcr-Abl and its related anti-apoptotic proteins. Therefore, the present study suggests that combination of apicidin and TRAIL may be an effective strategy for treating TRAIL-resistant Bcr-Abl expressing CML cells.     

Trail-induced apoptosis in Type I leukemic cells is not enhanced by overexpression of bax

We have previously shown that Bax translocation was crucial in TNFalpha or etoposide-induced apoptosis. Overexpression of Bax sensitized chronic myeloid leukemic K562 cells to etoposide-induced apoptosis. Treatment with TNF-related apoptosis-inducing ligand (TRAIL) induces a loss of mitochondrial membrane potential (DeltaPsim), cytochrome c release from mitochondria, activation of caspases-8, -9, and -3, and cleavage of Bid in the K562 cell line. Bax failed to sensitize K562 cells to TRAIL-induced apoptosis. TRAIL did not induce Bax expression and/or translocation from cytosol to mitochondria in the K562 cell line. However, 100 microM Z-VAD.fmk, a pan caspase inhibitor, completely blocked TRAIL-initiated mitochondrial alterations and cleavages of caspases and Bid. We propose that TRAIL-induced apoptosis in K562 cells is via Type I apoptotic signal pathway. Bax translocation is not essential for TRAIL-induced cytochrome c release and DeltaPsim collapse in the Type I cells.     

Caspase-9 Activation by the Apoptosome Is Not Required for Fas-mediated Apoptosis in Type II Jurkat Cells

Activation of executioner caspases during receptor-mediated apoptosis in type II cells requires the engagement of the mitochondrial apoptotic pathway. Although it is well established that recruitment of mitochondria in this context involves the cleavage of Bid to truncated Bid (tBid), the precise post-mitochondrial signaling responsible for executioner caspase activation is controversial. Here, we used distinct clones of type II Jurkat T-lymphocytes in which the mitochondrial apoptotic pathway had been inhibited to investigate the molecular requirements necessary for Fas-induced apoptosis. Cells overexpressing either Bcl-2 or Bcl-x_L were protected from apoptosis induced by agonistic anti-Fas antibody. By comparison, Apaf-1-deficient Jurkat cells were sensitive to anti-Fas, exhibiting Bid cleavage, Bak activation, the release of cytochrome c and Smac, and activation of executioner caspase-3. Inhibiting downstream caspase activation with the pharmacological inhibitor Z-DEVD-fmk or by expressing the BIR1/BIR2 domains of X-linked inhibitor of apoptosis protein (XIAP) decreased all anti-Fas-induced apoptotic changes. Additionally, pretreatment of Bcl-x_L-overexpressing cells with a Smac mimetic sensitized these cells to Fas-induced apoptosis. Combined, our findings strongly suggest that Fas-mediated activation of executioner caspases and induction of apoptosis do not depend on apoptosome-mediated caspase-9 activation in prototypical type II cells.     

Avian influenza virus A/HK/483/97(H5N1) NS1 protein induces apoptosis in human airway epithelial cells

Avian H5N1 influenza virus causes a remarkably severe disease in humans, with an overall case fatality rate of greater than 50%. Human influenza A viruses induce apoptosis in infected cells, which can lead to organ dysfunction. To verify the role of H5N1-encoded NS1 in inducing apoptosis, the NS1 gene was cloned and expressed in human airway epithelial cells (NCI-H292 cells). The apoptotic events posttransfection were examined by a terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick-end-labeling assay, flow cytometric measurement of propidium iodide, annexin V staining, and Western blot analyses with antibodies specific for proapoptotic and antiapoptotic proteins. We demonstrated that the expression of H5N1 NS1 protein in NCI-H292 cells was sufficient to induce apoptotic cell death. Western blot analyses also showed that there was prominent cleavage of poly(ADP-ribose) polymerase and activation of caspase-3, caspase-7, and caspase-8 during the NS1-induced apoptosis. The results of caspase inhibitor assays further confirmed the involvement of caspase-dependent pathways in the NS1-induced apoptosis. Interestingly, the ability of H5N1 NS1 protein to induce apoptosis was much enhanced in cells pretreated with Fas ligand (the time posttransfection required to reach >30% apoptosis was reduced from 24 to 6 h). Furthermore, 24 h posttransfection, an increase in Fas ligand mRNA expression of about 5.6-fold was detected in cells transfected with H5N1 NS1. In conclusion, we demonstrated that the NS1 protein encoded by avian influenza A virus H5N1 induced apoptosis in human lung epithelial cells, mainly via the caspase-dependent pathway, which encourages further investigation into the potential for the NS1 protein to be a novel therapeutic target.