Zebrafish anti-apoptotic gene Bcl-xL can prevent aquatic birnavirus-induced cell death in fish cells without affecting expression of viral proteins

The aquatic birnavirus induces mitochondria-mediated cell death in fish; however, the molecular mechanism remains unknown. In the present study, we demonstrated that aquatic birnavirus-induced mitochondria-mediated cell death is regulated by the anti-apoptotic Bcl-2 family member, zfBcl-xL, which is anti-apoptotic and enhances host cell viability. First, CHSE-214 cells carrying EGFP-zfBcl-xL fused genes were selected, established in culture, and used to examine the involvement of zfBcl-xL in host cell protection from the effects of viral infection. EGFP-zfBcl-xL was found to prevent infectious pancreatic necrosis virus (IPNV)-induced phosphatidylserine exposure up to 40% at 12 h and 24 h post-infection (p.i.), block IPNV-induced loss of mitochondrial membrane potential (ΔΨm), and enhance host viability at the middle and late replication stages. In addition, zfBcl-xL overexpression prevented IPNV-induced caspase-9 activation up to 25% and 85% at the middle (12 h p.i.) and late (24 h p.i.) replication stages without affecting expression of viral proteins such as VP3 (as a viral death protein) protein. In the present study, we demonstrated that aquatic birnavirus-induced cell death is prevented by the anti-apoptotic Bcl-2 family member, zfBcl-xL, which enhances host cell viability through blockage of mitochondrial disruption and caspase-9 activation.     

Aquatic birnavirus capsid protein,VP3, induces apoptosis via the Bad-mediated mitochondria pathway in fish and mouse cells

Aquatic birnavirus induces post-apoptotic necrotic cell death via a newly synthesized protein-dependent pathway. However, the involvement of viral genome-encoded protein(s) in this death process remains unknown. In the present study, we demonstrated that the submajor capsid protein, VP3, up-regulates the pro-apoptotic protein, Bad, in fish and mouse cells. Western blot analysis revealed that VP3 was expressed in CHSE-214 cells at 4 h post-infection (pi), indicating an early role during viral replication. We cloned the VP3 gene and tested its function in fish and mouse cells; VP3 overexpression induced apoptotic cell death by TUNEL assay. In addition, it up-regulated Bad gene expression in zebrafish ZLE cells by threefold at 12 h post-transfection (pt) and in mouse NIH3T3 cells by tenfold at 24 h pt. VP3 up-regulation of Bad expression altered mitochondria function, inducing mitochondrial membrane potential (MMP) loss and activating initiator caspase-9 and effector caspase-3. Furthermore, reduced Bad expression (65% reduction), MMP loss (up to 40%), and enhanced cell viability (up to 60%) upon expression of VP3 antisense RNA in CHSE-214 cells at 24 h post-IPNV infection was observed. Finally, overexpression of the anti-apoptotic gene, zfBcl-xL, reduced VP3-induced apoptotic cell death and caspase-3 activation at 24 h in fish cells. Taken together, these results suggest that aquatic birnavirus VP3 induces apoptosis via up-regulation of Bad expression and mitochondrial disruption, which activates a downstream caspase-3-mediated death pathway that is blocked by zfBcl-xL.     

Induction of apoptotic death in cells via Bad gene expression by infectious pancreatic necrosis virus infection

A Bcl-2 related family member, Bad, promotes cell death, and its function is regulated by phosphorylation. In this study, we show how the IPNV elicits the induction of Bad gene expression and promotes host apoptotic death. Anti-IPNV-E1S polyclonal and anti-VP3 monoclonal antibodies are used to neutralize the virus that blocks the prime death signal via the virus receptor. In the viability assay, each antibody could also enhance cell viability during IPNV infection. We tested tyrosine kinase inhibitors on IPNV-infected cells in order to assess their effect on blocking the death signal. With 100 microg/ml genistein treatment, Bad-like gene expression was blocked, either by rescuing the IPNV-infected CHSE-214 cells or by blocking internucleosomal DNA cleavage; but the tyrphostin group did not block Bad expression. For CHSE-214 cells, treatment with the protein synthesis-inhibitor, cycloheximide (1microg/ml), blocked new protein synthesis via activated tyrosine kinase during IPNV infection. We found that Bad protein expression could be blocked, and apoptotic death prevented. Together, these results demonstrate that the IPNV exerts up-regulation of a pro-apoptotic death gene (Bad), the expression of which serves to trigger apoptotic cell death. Our data also suggests that the IPNV induces apoptotic death via a viral receptor which triggers death effector Bad gene expression, possibly through a tyrosine kinase-dependent pathway.     

Aquatic birnavirus-induced ER stress-mediated death signaling contribute to downregulation of Bcl-2 family proteins in salmon embryo cells

Aquatic birnavirus induces mitochondria-mediated cell death, but whether connects to endoplasmic reticulum (ER) stress is still unknown. In this present, we characterized that IPNV infection triggers ER stress-mediated cell death via PKR/eIF2α phosphorylation signaling for regulating the Bcl-2 family protein expression in fish cells. The IPNV infection can induce ER stress as follows: (1) ER stress sensor ATF6 cleavaged; (2) ER stress marker GRP78 upregulation, and (3) PERK/eIF2α phosphorylation. Then, the IPNV-induced ER stress signals can induce the CHOP expression at early (6 h p.i.) and middle replication (12 h p.i.) stages. Moreover, IPNV-induced CHOP upregulation dramatically correlates to apparently downregulate the Bcl-2 family proteins, Bcl-2, Mcl-1 and Bcl-xL at middle replication stage (12 h p.i.) and produces mitochondria membrane potential (MMP) loss and cell death. Furthermore, with GRP78 synthesis inhibitor momitoxin (VT) and PKR inhibitor 2-aminopurine (2-AP) treatment for blocking GRP78 expression and eIF2α phosphorylation, PKR/PERK may involve in eIF2α phosphorylation/CHOP upregulation pathway that enhances the downstream regulators Bcl-2 family proteins expression and increased cell survival. Taken together, our results suggest that IPNV infection activates PKR/PERK/eIF2α ER stress signals for regulating downstream molecules CHOP upregulation and Bcl-2 family downregulation that led to induce mitochondria-mediated cell death in fish cells, which may provide new insight into RNA virus pathogenesis and disease.     

Antiviral function of tilapia hepcidin 1–5 and its modulation of immune-related gene expressions against infectious pancreatic necrosis virus (IPNV) in Chinook salmon embryo (CHSE)-214 cells

Antimicrobial peptides, small cysteine-rich molecules, play vital roles in host defense mechanisms against pathogen infection. Recently, tilapia hepcidin (TH)1–5, was characterized, and its antimicrobial functions against several pathogens were reported. Herein, we investigated the antiviral functions of TH1-5 against infectious pancreatic necrosis virus (IPNV) in Chinook salmon embryo (CHSE)-214 cells. The presence of TH1-5 enhanced the survival of CHSE-214 cells infected with IPNV. Additionally, the number of plaques formed by the cytopathic effect of IPNV in CHSE-214 cells decreased when IPNV was preincubated with TH1-5. This observation demonstrates the antiviral function of TH1-5. Real-time PCR studies showed the modulation of interleukin, annexin, and other viral-responsive gene expressions by TH1-5. When TH1-5 and IPNV were used to co-treat CHSE-214 cells, then cells were re-challenged with IPNV at 24 h, the cells did not survive the IPNV infection. This shows that in the absence of TH1-5, viral re-challenge killed CHSE-214 cells. In conclusion TH1-5 protected CHSE-214 cells against IPNV by direct antimicrobial and immunomodulatory functions.     

Dynamics of nontypical apoptotic morphological changes visualized by green fluorescent protein in living cells with infectious pancreatic necrosis virus infection

Morphologically, apoptotic cells are characterized by highly condensed membrane blebbing and formation of apoptotic bodies. Recently, we reported that apoptosis precedes necrosis in a fish cell line infected with infectious pancreatic necrosis virus (IPNV). In the present study, we tested the possibility that nontypical apoptosis is a component of IPNV-induced fish cell death. A variant type of green fluorescent protein (EGFP) was expressed in a fish cell line such that EGFP served as a protein marker for visualizing dynamic apoptotic cell morphological changes and for tracing membrane integrity changes during IPNV infection. Direct morphological changes were visualized by fluorescence microscopy by EGFP in living cells infected with IPNV. The nontypical apoptotic morphological change stage occurred during the pre-late stage (6 to 7 h postinfection). Nontypical apoptotic features, including highly condensed membrane blebbing, occurred during the middle apoptotic stage. At the pre-late apoptotic stage, membrane vesicles quickly formed, blebbed, and were finally pinched off from the cell membrane. At the same time, at this pre-late apoptotic stage, apoptotic cells formed unique small holes in their membranes that ranged from 0.39 to 0.78 micrometer according to examination by scanning electron microscopy and immunoelectron microscopy. Quantitation of the intra- and extracellular release of EGFP by CHSE-214-EGFP cells after IPNV infection was done by Western blotting and fluorometry. Membrane integrity was quickly lost during the late apoptotic stage (after 8 h postinfection), and morphological change and membrane integrity loss could be prevented and blocked by treatment with apoptosis inhibitors such as cycloheximide, genistein, and EDTA before IPNV infection. Together, these findings show the apoptotic features at the onset of pathology in host cells (early and middle apoptotic stages), followed secondarily by nontypical apoptosis (pre-late apoptotic stage) and then by postapoptotic necrosis (late apoptotic stage), of a fish cell line. Our results demonstrate that nontypical apoptosis is a component of IPNV-induced fish cell death.     

Zebrafish anti-apoptotic protein zfBcl-xL can block betanodavirus protein alpha-induced mitochondria-mediated secondary necrosis cell death

Betanodavirus protein alpha induces cell apoptosis or secondary necrosis by a poorly understood process. In the present work, red spotted grouper nervous necrosis virus (RGNNV) RNA 2 was cloned and transfected into tissue culture cells (GF-1) which then underwent apoptosis or post-apoptotic necrosis. In the early apoptotic stage, progressive phosphatidylserine externalization was evident at 24h post-transfection (p.t.) by Annexin V-FLUOS staining. TUNEL assay revealed apoptotic cells at 24-72 h p.t, after which post-apoptotic necrotic cells were identified by acridine orange/ethidium bromide dual dye staining from 48 to 72 h p.t. Protein alpha induced progressive loss of mitochondrial membrane potential (MMP) which was detected in RNA2-transfected GF-1 cells at 24, 48, and 72 h p.t., which correlated with cytochrome c release, especially at 72 h p.t. To assess the effect of zfBcl-xL on cell death, RNA2-transfected cells were co-transfected with zfBcl-x(L). Co-transfection of GF-1 cells prevented loss of MMP at 24 h and 48 h p.t. and blocked initiator caspase-8 and effector caspase-3 activation at 48 h p.t. We conclude that RGNNV protein alpha induces apoptosis followed by secondary necrotic cell death through a mitochondria-mediated death pathway and activation of caspases-8 and -3.     

Stage-specific expression of TNFα regulates bad/bid-mediated apoptosis and RIP1/ROS-mediated secondary necrosis in Birnavirus-infected fish cells

Infectious pancreatic necrosis virus (IPNV) can induce Bad-mediated apoptosis followed by secondary necrosis in fish cells, but it is not known how these two types of cell death are regulated by IPNV. We found that IPNV infection can regulate Bad/Bid-mediated apoptotic and Rip1/ROS-mediated necrotic death pathways via the up-regulation of TNFα in zebrafish ZF4 cells. Using a DNA microarray and quantitative RT-PCR analyses, two major subsets of differentially expressed genes were characterized, including the innate immune response gene TNFα and the pro-apoptotic genes Bad and Bid. In the early replication stage (0-6 h post-infection, or p.i.), we observed that the pro-inflammatory cytokine TNFα underwent a rapid six-fold induction. Then, during the early-middle replication stages (6-12 h p.i.), TNFα level was eight-fold induction and the pro-apoptotic Bcl-2 family members Bad and Bid were up-regulated. Furthermore, specific inhibitors of TNFα expression (AG-126 or TNFα-specific siRNA) were used to block apoptotic and necrotic death signaling during the early or early-middle stages of IPNV infection. Inhibition of TNFα expression dramatically reduced the Bad/Bid-mediated apoptotic and Rip1/ROS-mediated necrotic cell death pathways and rescued host cell viability. Moreover, we used Rip1-specific inhibitors (Nec-1 and Rip1-specific siRNA) to block Rip1 expression. The Rip1/ROS-mediated secondary necrotic pathway appeared to be reduced in IPNV-infected fish cells during the middle-late stage of infection (12-18 h p.i.). Taken together, our results indicate that IPNV triggers two death pathways via up-stream induction of the pro-inflammatory cytokine TNFα, and these results may provide new insights into the pathogenesis of RNA viruses.     

Arsenic trioxide induces human pulmonary fibroblast cell death via the regulation of Bcl-2 family and caspase-8

Arsenic trioxide (ATO; As₂O₃) can induce apoptotic cell death in various cancer cells including lung cancer cells. However, little is known about the toxicological effects of ATO on normal primary lung cells. In this study, we investigated the cellular effects of ATO on human pulmonary fibroblast (HPF) cells in relation to cell growth inhibition and death. ATO inhibited HPF cell growth with an IC₅₀ of approximately 30–40 μM at 24 h and induced cell death accompanied by the loss of mitochondrial membrane potential (MMP; ΔΨ_m). Thus, HPF cells were considered to be very resistant to ATO insults. ATO increased the expression of p53 protein and decreased that of Bcl-2 protein. This agent activated caspase-8 but not caspase-3 in HPF cells. Z-VAD (a pan-caspase inhibitor; 15 μM) did not significantly decrease cell growth inhibition, death and MMP (ΔΨ_m) loss by ATO. Moreover, administration of Bax or casase-8 siRNA attenuated HPF cell death by ATO whereas p53 or caspase-3 siRNAs did not affect cell death. In conclusion, HPF cells were resistant to ATO and higher doses of ATO induced the growth inhibition and death in HPF cells via the regulation of Bcl-2 family and caspase-8.     

Inter-laboratory comparison of cell lines for susceptibility to three viruses: VHSV, IHNV and IPNV.

Eleven European National Reference Laboratories participated in an inter-laboratory comparison of the susceptibility of 5 selected cell lines to 3 fish pathogenic viruses. The test included viral hemorrhagic septicaemia virus (VHSV); infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV), and the cell lines derived from bluegill fry (BF-2), chinook salmon embryo (CHSE-214), epithelioma papulosum cyprini (EPC), fathead minnow (FHM) and rainbow trout gonad (RTG-2). The results showed that for isolation of VHSV, BF-2 and RTG-2 cells performed equally well and had higher sensitivity compared to the other cell lines. For IHNV, EPC and FHM cells gave the best results, and for IPNV it was BF-2 and CHSE-214 cells. FHM cells showed the largest variability among laboratories, whereas EPC was the cell line showing the smallest variability.     

An antiviral state induced in Chinook salmon embryo cells (CHSE-214) by transfection with the double-stranded RNA poly I:C

Type I interferons (IFN alpha and beta) convert vertebrate cells into an antiviral state by inducing expression of proteins that inhibit virus replication. In humans and mice, Mx proteins constitute one family of interferon-induced antiviral proteins. Mx genes have recently been cloned from Atlantic salmon and rainbow trout. Moreover, double-stranded RNA (dsRNA) and type I IFN-like activity have been shown to induce Mx protein in salmonid cells. Chinook salmon embryo cells (CHSE-214 cells) have been suggested to have a defect in the IFN-system because the dsRNA polyinosinic polycytidylic acid (poly I:C) failed to induce an antiviral state in the cells. We have studied this phenomenon more closely in the present work. CHSE-214 cells were either transfected with poly I:C or incubated with poly I:C without transfection reagent. The cells were then studied for Mx protein expression and protection against infectious pancreatic necrosis virus (IPNV) infection. The results showed that cells transfected with poly I:C were protected from IPNV infection, whilst cells incubated with poly I:C were not protected. Cells transfected with the double-stranded DNA poly dI:dC were also not protected against IPNV. Mx protein was expressed in CHSE-214 cells upon transfection with poly I:C, but not after incubation with poly I:C alone. Stimulation of CHSE-214 cells with supernatants from cells transfected with poly I:C, induced protection against IPNV, indicating production of type I IFN-like activity. These results suggest that CHSE-214 cells in fact are able to produce type I IFN, but may have defects in the mechanisms mediating uptake of poly I:C or may degrade unprotected poly I:C.     

Involvement of caspase-2 activation in aurora kinase inhibitor-induced cell death in axin-expressing L929 cells

Axin is a multifunctional protein that participates in many cellular events including Wnt signaling and cell fate determination. Aurora kinase inhibitor (AKI)-induced cell death and cell membrane rupture is facilitated in L929 cells expressing axin (L-axin cells) through the activation of poly ADP-ribose polymerase (PARP). We observed that caspase-2 activity is required for AKI-induced cell death. Inhibition of caspase-2 activity suppressed AKI-induced PARP activation and mitochondrial dysfunction, resulting in a decrease in AKI-induced cell death. When an axin mutant deleted for the glycogen synthase kinase 3β (GSK3β)-binding domain was expressed in L929 cells (L-ΔGSK cells), AKI-induced caspase-2 activation and cell death decreased. AKI treatment reduced the expression of a 32-kDa caspase-2 splicing variant (caspase-2S) in most L-axin cells, but not in L-ΔGSK cells. These results suggest that AKI-induced caspase-2 activation in L-axin cells might be due to a decrease in the expression of caspase-2S, which inhibits caspase-2 activity. In addition, AKI treatment failed to activate caspase-8 and treatment with necrostatin inhibited AKI-induced cell death in L-axin cells, suggesting that the absence of caspase-8 activation might favor necrotic cell death. Axin expression may facilitate AKI-induced caspase-2 activation followed by activation of PARP and initiation of the necrotic cell death pathway.     

Mechanism of staurosporine-induced apoptosis in murine hepatocytes

Staurosporine (STS) induces apoptosis in various cell lines. We report in this study that primary cultured mouse hepatocytes are less sensitive to STS compared with Jurkat cells and Huh-7 cells. In contrast to the cell lines, no apparent release of cytochrome c or loss of mitochondrial transmembrane potential was detected in primary hepatocytes undergoing STS-induced apoptosis. Caspase-3 was activated in primary hepatocytes by STS treatment, but caspase-9 and -12 were not activated, and caspase-3 activation is not dependent on caspase-8. These findings point to a novel pathway for caspase-3 activation by STS in primary hepatocytes. Pretreatment with caspase inhibitor converted STS-induced apoptosis of hepatocytes to necrotic cell death without significantly changing total cell death. Thus STS causes hepatocytes to commit to death upstream of the activation of caspases. We also demonstrated that STS dramatically sensitized primary hepatocytes to tumor necrosis factor-alpha-induced apoptosis. STS activated I kappa B kinase and nuclear factor-kappa B (NF-kappa B) nuclear translocation and DNA binding but inhibited transactivation of I kappa B-alpha, inducible nitric oxide synthase, and inhibitor of apoptosis protein-1 in hepatocytes and NF-kappa B reporter in transfected Huh-7 cells.     

Tissue inhibitor of metalloproteinase-3 induces a Fas-associated death domain-dependent type II apoptotic pathway

Tissue inhibitors of metalloproteinases (TIMPs) are important regulators of matrix metalloproteinase (MMP) and adamalysin metalloproteinase activity. We previously reported that overexpression of TIMP-3 inhibits MMPs and induces apoptotic cell death in a variety of cell types and demonstrated that apoptosis is mediated through the N terminus of TIMP-3, which harbors the MMP inhibitory domain. However, little is known about the mechanisms underlying TIMP-3-induced apoptosis. Here we demonstrate that overexpression of TIMP-3 induced activation of initiator caspase-8 and -9 and promoted caspase-mediated cleavage of the death substrates poly(ADP-ribose) polymerase and focal adhesion kinase. Furthermore, TIMP-3 induced mitochondrial activation as demonstrated by loss of mitochondrial membrane potential and release of cytochrome c. Intervention studies demonstrated that overexpression of Bcl-2, the anti-apoptotic mitochondrial membrane protein, or CrmA, a viral serpin inhibitor of caspase-8, completely inhibited TIMP-3-induced apoptosis. Furthermore, a dominant-negative Fas-associated death domain mutant inhibited TIMP-3-induced death substrate cleavage and apoptotic death. Taken together, these results indicate that TIMP-3 overexpression induces a type II apoptotic pathway initiated via a Fas-associated death domain-dependent mechanism.     

Amyloid β-protein Induces Necrotic Cell Death Mediated by ICE Cascade in PC12 Cells

A major component of Alzheimer's disease plaque amyloid β protein (βAP) showed the cytolytic activity to rat pheochromocytoma PC12 cells. Nuclear morphological study revealed that βAP-induced cytolytic activity is due to necrotic cell death, rather than apoptotic cell death. To examine the molecular machinery of βAP-induced necrotic cell death in detail, I investigated the direct involvement of caspase. When nerve growth factor-treated and -untreated PC12 cells were incubated with the synthesized tetrapeptide inhibitors of caspase, YVAD-CHO (Ac-Tyr-Val-Ala-Asp-CHO) or DEVD-CHO (Ac-Asp-Glu-Val-Asp-CHO), βAP-induced necrotic cell death was prevented. In addition, the interleukin-1β converting enzyme (ICE) subfamily activation preceded CPP32 subfamily activation during βAP-induced necrotic cell death. On the basis of these findings, I suggest that βAP induces necrotic cell death mediated by the ICE cascade and that the ICE cascade may possibly be involved in Alzheimer's disease.     

Evidence for necrosis,but not apoptosis,in human hepatoma cells with knockdown of mitochondrial aquaporin-8

We previously found that mitochondrial aquaporin-8 (mtAQP8) channels facilitate mitochondrial H₂O₂ release in human hepatoma HepG2 cells and that their knockdown causes oxidant-induced mitochondrial dysfunction and loss of viability. Here, we studied whether apoptosis or necrosis is involved as the mode of cell death. We confirmed that siRNA-induced mtAQP8 knockdown significantly decreased HepG2 viability by MTT assay, LDH leakage, and trypan blue exclusion test. Analysis of mitochondrial proapoptotic Bax-to-antiapoptotic Bcl_XL ratio, mitochondrial cytochrome c release and caspase-3 activation showed no alterations in mtAQP8-knockdown cells. This indicates a primary mechanism of cell death other than the intrinsic mitochondrial apoptotic pathway. Thus, nuclear staining with DAPI did not reveal any increase of apoptotic features, i.e. chromatin condensation or nuclear fragmentation. Flow cytometry studies after double cell staining with annexin V and propidium iodide confirmed lack of apoptosis and suggested necrosis as the primary mechanism of death in mtAQP8-knockdown HepG2 cells. Necrosis was further supported by the increased nuclear delocalization and extracellular release of the High Mobility Group Box 1 protein. The knockdown of mtAQP8 in another human hepatoma-derived cell line, i.e. HuH-7 cells, also induced necrotic but not apoptotic death. Our data suggest that mtAQP8 knockdown induces necrotic cell death in human neoplastic hepatic cells, a finding that might be relevant to therapeutic strategies against hepatoma cells.     

The changes of reactive oxygen species and glutathione by MG132, a proteasome inhibitor affect As4.1 juxtaglomerular cell growth and death

The proteasome inhibitor MG132 has been shown to induce apoptotic cell death through the formation of reactive oxygen species (ROS). Here, we evaluated the effects of MG132 on the growth and death of As4.1 juxtaglomerular cells in relation to ROS and glutathione (GSH) levels. MG132 inhibited the growth of As4.1 cells with an IC₅₀ of approximately 0.3–0.4 μM at 48 h and induced cell death, which was accompanied by the loss of mitochondrial membrane potential (MMP; ΔΨ_m), Bcl-2 decrease, activation of caspase-3 and -8, and PARP cleavage. MG132 increased intracellular ROS levels including O₂^? and GSH depleted cell numbers. N-acetyl cysteine (NAC, a well-known antioxidant) significantly decreased ROS level and GSH depleted cell numbers in MG132-treated As4.1 cells, along with the prevention of cell growth inhibition, cell death and MMP (ΔΨ_m) loss. NAC also decreased the caspase-3 activity of MG132. l-Buthionine sulfoximine (BSO; an inhibitor of GSH synthesis) or diethyldithiocarbamate (DDC; an inhibitor of Cu/Zn-SOD) did not affect cell growth, death, ROS and GSH levels in MG132-treated As4.1 cells. Conclusively, MG132 reduced the growth of As4.1 cells via apoptosis. The changes of ROS and GSH by MG132 were involved in As4.1 cell growth and death.