The apoptotic capability of coxsackievirus B3 is influenced by the efficient interaction between the capsid protein VP2 and the proapoptotic host protein Siva

Coxsackievirus group B3 regulates ASS1-mediated metabolic reprogramming and promotes macrophage inflammatory polarization in viral myocarditis

Coxsackievirus group B3 (CVB3) belongs to the genus Enteroviruses of the family Picornaviridae and is the main pathogen underlying viral myocarditis (VMC). No specific therapeutic is available for this condition. Argininosuccinate synthase 1 (ASS1) is a key enzyme in the urea cycle that converts citrulline and aspartic acid to argininosuccinate. Here, we found that CVB3 and its capsid protein VP2 inhibit the autophagic degradation of ASS1 and that CVB3 consumes citrulline to upregulate ASS1, triggers urea cycle metabolic reprogramming, and then activates macrophages to develop pro-inflammatory polarization, thereby promoting the occurrence and development of VMC. Conversely, citrulline supplementation to prevent depletion can downregulate ASS1, rescue macrophage polarization, and alleviate the pathogenicity of VMC. These findings provide a new perspective on the occurrence and development of VMC, revealing ASS1 as a potential new target for treating this disease.

IMPORTANCE

Viral myocarditis (VMC) is a common and potentially life-threatening myocardial inflammatory disease, most commonly caused by CVB3 infection. So far, the pathogenesis of VMC caused by CVB3 is mainly focused on two aspects: one is the direct myocardial injury caused by a large number of viral replication in the early stage of infection, and the other is the local immune cell infiltration and inflammatory damage of the myocardium in the adaptive immune response stage. There are few studies on the early innate immunity of CVB3 infection in myocardial tissue, but the appearance of macrophages in the early stage of CVB3 infection suggests that they can play a regulatory role as early innate immune response cells in myocardial tissue. Here, we discovered a possible new mechanism of VMC caused by CVB3, revealed new drug targets for anti-CVB3, and discovered the therapeutic potential of citrulline for VMC.

Molecular characterization, expression, and apoptosis regulation of siva1 in protogynous hermaphrodite fish ricefield eel (Monopterus albus)

Siva1, which induces extensive apoptosis, has been well characterized. To elucidate the molecular function of Siva1 in ricefield eel, molecular characterization and phylogenetic analysis were performed, and the mRNA expression in the ovary at different developmental stages and ovary tissues exposed to H₂O₂ and Z-VAD-FMK in vitro were also evaluated. The results indicated that ricefield eel Siva1 was highly conserved and contains three conserved motifs, despite 83 amino acid differences upstream of the initiation codon. Phylogenetic analysis demonstrated that ricefield eel Siva1 clusters together with the Siva1 protein of the other fish, with high sequence homology with that of Lates calcarifer. Quantitative real-time polymerase chain reaction analysis showed high siva1 expression levels in the ovary and low expression levels in the liver. The higher mRNA levels of siva1 were detected in the IE and IM, and the lower siva1 mRNA levels were found in the OM, IL, and TE during gonadal development. Additionally, siva1 expression levels in the ovarian tissues were significantly increased at 1 h post incubation (hpi) with H₂O₂ and then significantly decreased at 2 hpi; however, siva1 expression was upregulated significantly at 4 and 8 hpi, similar to the patterns observed with caspase3, which was used as a molecular marker of apoptosis. Moreover, the siva1 mRNAs were elevated significantly than that in control groups at 1 hpi, but the expression of siva1 was down-regulated dramatically at 2, 4, and 8 hpi, which were similar with that of caspase3 expression profiles after Z-VAD-FMK incubation. What's more, Pearson's correlation coefficients showed strongly positive relationships between siva1 and caspase3. These findings suggest that Siva1 plays an important apoptosis role in gonadal development of ricefield eel.

Type B coxsackieviruses and central nervous system disorders: critical review of reported associations

Type B coxsackieviruses (CV-B) frequently infect the central nervous system (CNS) causing neurological diseases notably meningitis and encephalitis. These infections occur principally among newborns and children. Epidemiological studies of patients with nervous system disorders demonstrate the presence of infectious virus, its components, or anti-CV-B antibodies. Some experimental studies conducted in vitro and in vivo support the potential association between CV-B and idiopathic neurodegenerative diseases such as amyotrophic lateral sclerosis and psychiatric illness such as schizophrenia. However, mechanisms explaining how CV-B infections may contribute to the genesis of CNS disorders remain unclear. The proposed mechanisms focus on the immune response following the viral infection as a contributor to pathogenesis. This review describes these epidemiological and experimental studies, the modes of transmission of CV-B with an emphasis on congenital transmission, the routes used by CV-B to reach the brain parenchyma, and plausible mechanisms by which CV-B may induce CNS diseases, with a focus on potential immunopathogenesis.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

Regulation of Apoptosis by Enteroviruses

Enterovirus infection can cause a variety of diseases and severely impair the health of humans, animals, poultry, and other organisms. To resist viral infection, host organisms clear infected cells and viruses via apoptosis. However, throughout their long-term competition with host cells, enteroviruses have evolved a series of mechanisms to regulate the balance of apoptosis in order to replicate and proliferate. In the early stage of infection, enteroviruses mainly inhibit apoptosis by regulating the PI3K/Akt pathway and the autophagy pathway and by impairing cell sensors, thereby delaying viral replication. In the late stage of infection, enteroviruses mainly regulate apoptotic pathways and the host translation process via various viral proteins, ultimately inducing apoptosis. This paper discusses the means by which these two phenomena are balanced in enteroviruses to produce virus-favoring conditions – in a temporal sequence or through competition with each other. This information is important for further elucidation of the relevant mechanisms of acute infection by enteroviruses and other members of the picornavirus family.

Interleukin-37 Ameliorates Coxsackievirus B3-induced Viral Myocarditis by Modulating the Th17/Regulatory T cell Immune Response

Myocarditis is a heterogeneous group of disorders defined by inflammation of the heart muscle with an excessively activated immune response. Numerous interventions have been investigated for the treatment of myocarditis while success is limited. Interleukin-37 (IL-37), a novel member of the IL-1 cytokine family, is a natural inhibitor of innate immunity associated with autoimmune diseases. However, the modulatory effect of IL-37 in myocarditis is unknown. In this study, we investigated the immunological regulation of IL-37 in the coxsackievirus B3-induced model of murine viral myocarditis. The results show that IL-37 significantly ameliorates the signs of myocarditis with increased survival rate and bodyweight, improved histological changes, reduced activities of MB isoenzyme of creatine kinase and cardiac troponin I, and a suppressed response of Th17 cells and enhanced response of regulatory T cells (Tregs) in the spleen. Moreover, IL-37 down-regulates the expression of Th17-related cytokines IL-6 and IL-17A, while promoting Treg-related cytokine IL-10 levels in the heart. Therefore, IL-37 may exhibit anti-inflammatory activity in the murine model of myocarditis by regulating the balance between Th17 and Treg cells, thereby providing a possible novel therapeutic target in myocarditis.

Recent advances from studies on the role of structural proteins in enterovirus infection

Enteroviruses are a large group of small nonenveloped viruses that cause common and debilitating illnesses affecting humans and animals worldwide. The capsid composed by viral structural proteins packs the RNA genome. It is becoming apparent that structural proteins of enteroviruses play versatile roles in the virus–host interaction in the viral life cycle, more than just a shell. Furthermore, structural proteins to some extent may be associated with viral virulence and pathogenesis. Better understanding the roles of structural proteins in enterovirus infection may lead to the development of potential antiviral strategies. Here, we discuss recent advances from studies on the role of structural proteins in enterovirus infection and antiviral therapeutics targeted structural proteins.

Picornaviruses and nuclear functions: targeting a cellular compartment distinct from the replication site of a positive-strand RNA virus

The compartmentalization of DNA replication and gene transcription in the nucleus and protein production in the cytoplasm is a defining feature of eukaryotic cells. The nucleus functions to maintain the integrity of the nuclear genome of the cell and to control gene expression based on intracellular and environmental signals received through the cytoplasm. The spatial separation of the major processes that lead to the expression of protein-coding genes establishes the necessity of a transport network to allow biomolecules to translocate between these two regions of the cell. The nucleocytoplasmic transport network is therefore essential for regulating normal cellular functioning. The Picornaviridae virus family is one of many viral families that disrupt the nucleocytoplasmic trafficking of cells to promote viral replication. Picornaviruses contain positive-sense, single-stranded RNA genomes and replicate in the cytoplasm of infected cells. As a result of the limited coding capacity of these viruses, cellular proteins are required by these intracellular parasites for both translation and genomic RNA replication. Being of messenger RNA polarity, a picornavirus genome can immediately be translated upon entering the cell cytoplasm. However, the replication of viral RNA requires the activity of RNA-binding proteins, many of which function in host gene expression, and are consequently localized to the nucleus. As a result, picornaviruses disrupt nucleocytoplasmic trafficking to exploit protein functions normally localized to a different cellular compartment from which they translate their genome to facilitate efficient replication. Furthermore, picornavirus proteins are also known to enter the nucleus of infected cells to limit host-cell transcription and down-regulate innate antiviral responses. The interactions of picornavirus proteins and host-cell nuclei are extensive, required for a productive infection, and are the focus of this review.

Death waits for no man--does it wait for a virus? How enteroviruses induce and control cell death

Enteroviruses (EVs) are the most common human viral pathogens. They cause a variety of pathologies, including myocarditis and meningoencephalopathies, and have been linked to the onset of type I diabetes. These pathologies result from the death of cells in the myocardium, central nervous system, and pancreas, respectively. Understanding the role of EVs in inducing cell death is crucial to understanding the etiologies of these diverse pathologies. EVs both induce and delay host cell death, and their exquisite control of this balance is crucial for their success as human viral pathogens. Thus, EVs are tightly involved with cell death signaling pathways and interact with host cell signaling at multiple points. Here, we review the literature detailing the mechanisms of EV-induced cell death. We discuss the mechanisms by which EVs induce cell death, the signaling pathways involved in these pathways, and the strategies by which EVs antagonize cell death pathways. We also discuss the role of cell death in both the resulting pathology in the host and in the facilitation of viral spread.

In vivo delivery of interleukin-35 relieves coxsackievirus-B3-induced viral myocarditis by inhibiting Th17 cells

Interleukin (IL)-35 is a new member of the IL-12 cytokine family. The suppressive role of IL-35 in the immune response to parasitic and bacterial infections and in autoimmunity has been demonstrated in terms of its anti-inflammatory properties. However, the functional role of IL-35 in viral myocarditis has not been investigated. In this study, IL-35 expression was measured in heart tissues with coxsackievirus B3 (CVB3)-induced myocarditis. It was significantly reduced in the late stage of viral infection and correlated negatively with disease severity. To examine the therapeutic role of IL-35 in viral myocarditis, an IL-35-expressing plasmid (pIL-35-FC) was packaged with polyethyleneimine and delivered intraperitoneally to BALB/c male mice before and after CVB3 infection. The severity of myocarditis was assessed 7 days after infection. The in vivo delivery of IL-35 significantly ameliorated the severity of viral myocarditis, reflected in an increased survival rate and increased bodyweights, and reduced serum creatine kinase (CK) and CK-MB activities, cardiac pathological scores, and viral replication. We also show that the overexpression of IL-35 reduced splenic Th17 cells and Th17-related proinflammatory cytokines in heart tissues. In conclusion, our data indicate that IL-35 effectively protects the myocardium from the pathogenesis of CVB3-induced viral myocarditis, which may be attributable to reduced Th17 production. This suggests that supplementation with IL-35 could be a novel therapeutic treatment for viral myocarditis.

RNA virus capsid proteins: more than just a shell

Capsid proteins are obligatory components of infectious virions. Their primary structural function is to protect viral genomes during entry and exit from host cells. Evidence suggests that these proteins can also modulate the activity and specificity of viral replication complexes. More recently, it has become apparent that they play critical roles at the virus–host interface. Here, we discuss how capsid proteins of RNA viruses interact with key host cell proteins and pathways to modulate cell physiology in order to benefit virus replication. Capsid–host cell interactions may also have implications for viral disease. Understanding how capsids regulate virus–host interactions may lead to the development of novel antiviral therapies based on targeting the activities of cellular proteins.

Role of the myristoylation site in expressing exogenous functional proteins in coxsackieviral vector

We generated a cardiotropic replication-competent chimeric coxsackievirus B3 (CVB3) to express alcohol dehydrogenase (ADH). Although exogenously expressed ADH was found by Western blot analysis, its enzyme function was repressed. To define the factor that inhibits the enzymatic function of ADH, we introduced a site-directed mutation at the second amino acid (MGAQEF···) of the CVB3 VP0 capsid protein, effectively changing glycine to alanine. This glycine is known to be a myristoylation site during viral capsid protein maturation in infected cells. In contrast to the unmodified virus, ADH expression and enzymatic function were readily detectable in the mutated rCVB3-ADH (G2A) virus. While expression of ADH required mutation of the CVB3 VP0 myristoylation site for proper function, another chimeric virus that expresses green fluorescent protein (rCVB3-GFP (G or A)) worked independently of the myristoylation site. Indeed, infected HeLa cells displayed GFP under a fluorescent microscope. These results indicate that the myristoylation site in the VP0 capsid protein inhibited the expression of enzymatically active ADH but not GFP. VP0 myristoylation is dispensable for chimeric CVB3 virus replication.

Multifaceted functions of Siva-1: more than an Indian God of Destruction

Siva-1, as a p53-inducible gene, has been shown to induce extensive apoptosis in a number of different cell lines. Recent evidence suggests that Siva-1 functions as a part of the auto-regulatory feedback loop that restrains p53 through facilitating Mdm2-mediated p53 degradation. Also, Siva-1 plays an important role in suppressing tumor metastasis. Here we review the current understanding of Siva-1-mediated apoptotic signaling pathway. We also add comments on the p53-Siva-1 feedback loop, the novel function of Siva-1 in suppressing tumor metastasis, and their potential implications.

Regulation of IL-2 gene expression by Siva and FOXP3 in human T cells

BACKGROUND

Severe autoinflammatory diseases are associated with mutations in the Foxp3 locus in both mice and humans. Foxp3 is required for the development, function, and maintenance of regulatory T cells (Tregs), a subset of CD4 cells that suppress T cell activation and inflammatory processes. Siva is a pro-apoptotic gene that is expressed across a range of tissues, including CD4 T cells. Siva interacts with three tumor necrosis factor receptor (TNFR) family members that are constitutively expressed on Treg cells: CD27, GITR, and OX40.

RESULTS

Here we report a biophysical interaction between FOXP3 and Siva. We mapped the interaction domains to Siva's C-terminus and to a central region of FOXP3. We showed that Siva repressed IL-2 induction by suppressing IL-2 promoter activity during T cell activation. Siva-1's repressive effect on IL-2 gene expression appears to be mediated by inhibition of NFkappaB, whereas FOXP3 repressed both NFkappaB and NFAT activity.

CONCLUSIONS

In summary, our data suggest that both FOXP3 and Siva function as negative regulators of IL-2 gene expression in Treg cells, via suppression of NFAT by FOXP3 and of NFkappaB by both FOXP3 and Siva. Our work contributes evidence for Siva's role as a T cell signalling mediator in addition to its known pro-apoptotic function. Though further investigations are needed, evidence for the biophysical interaction between FOXP3 and Siva invites the possibility that Siva may be important for proper Treg cell function.

Foreign gene transfer to cardiomyocyte using a replication-defective recombinant coxsackievirus B3 without cytotoxicity

BACKGROUND

Replication-competent coxsackievirus B3 (CVB3) has been used as a gene transfer vector for cultured cardiomyocytes and hearts in vivo. However, CVB3 induces cell lysis when it replicates in infected cells. In this study, we investigated whether a replication-defective rCVB3 vector could be generated and used as a noncytotoxic gene transfer vector for cardiomyocytes.

METHODS

We generated a replication-defective luciferase-expressing CVB3 plasmid. This recombinant cDNA and pCMV-P1 plasmids were amplified and cotransfected into Hek293 cells using transfection reagents. Replication-defective rLuCVB3 virus was recovered from the cells and cell culture supernatants for 3 days after transfection. The generated rLuCVB3 viruses were concentrated on a 30% sucrose cushion and semiquantified using a luciferase assay. In addition, foreign gene delivery by the rLuCVB3 was tested in cultured cardiomyocytes and intact mouse hearts after rLuCVB3 infection.

RESULTS

Luciferase was expressed in Hek293, HeLa cells and cardiomyocytes after rLuCVB3 infection. In addition, these cells did not show a significant cytopathic effect after 72 h. Luciferase protein expression or activity were detected for 3 days in the myocardium of rLuCVB3-infected mouse hearts without producing cytotoxicity or inflammation.

CONCLUSION

As a proof-of-concept, these data indicate that a replication-defective rCVB3 vector can be generated and used as a novel gene transfer system to transfect exogenous genes into cardiomyocytes without generating cytotoxicity.

Viral and host proteins involved in picornavirus life cycle

Picornaviruses cause several diseases, not only in humans but also in various animal hosts. For instance, human enteroviruses can cause hand-foot-and-mouth disease, herpangina, myocarditis, acute flaccid paralysis, acute hemorrhagic conjunctivitis, severe neurological complications, including brainstem encephalitis, meningitis and poliomyelitis, and even death. The interaction between the virus and the host is important for viral replication, virulence and pathogenicity. This article reviews studies of the functions of viral and host factors that are involved in the life cycle of picornavirus. The interactions of viral capsid proteins with host cell receptors is discussed first, and the mechanisms by which the viral and host cell factors are involved in viral replication, viral translation and the switch from translation to RNA replication are then addressed. Understanding how cellular proteins interact with viral RNA or viral proteins, as well as the roles of each in viral infection, will provide insights for the design of novel antiviral agents based on these interactions.

Theiler's murine encephalomyelitis virus leader protein is the only nonstructural protein tested that induces apoptosis when transfected into mammalian cells

Theiler's murine encephalomyelitis virus (TMEV) induces two distinct cell death programs, necrosis and apoptosis. The apoptotic pathway is of particular interest because TMEV persists in the central nervous system of mice, largely in infiltrating macrophages, which undergo apoptosis. Infection of murine macrophages in culture induces apoptosis that is Bax dependent through the intrinsic or mitochondrial pathway, restricting infectious-virus yields and raising the possibility that apoptosis represents a mechanism to attenuate TMEV yet promote macrophage-to-macrophage spread during persistent infection. To help define the cellular stressors and upstream signaling events leading to apoptosis during TMEV infection, we screened baby hamster kidney (BHK-21) cells transfected to express individual nonstructural genes (except 3B) of the low-neurovirulence BeAn virus strain for cell death. Only expression of the leader protein led to apoptosis, as assessed by fluorescence-activated cell sorting analysis of propidium iodide- and annexin V-stained transfected cells, immunoblot analysis of poly(ADP-ribose) polymerase and caspase cleavages, electron microscopy, and inhibition of apoptosis by the pancaspase inhibitor qVD-OPh. After transfection, Bak and not Bax expression increased, suggesting that the apical pathway leading to activation of these Bcl-2 multi-BH-domain proapoptotic proteins differs in BeAn virus infection versus L transfection. Mutation to remove the CHCC Zn finger motif from L, a motif required by L to mediate inhibition of nucleocytoplasmic trafficking, significantly reduced L-protein-induced apoptosis in both BHK-21 and M1-D macrophages.

Proapoptotic protein Siva binds to the muscle protein telethonin in cardiomyocytes during coxsackieviral infection

AIMS

Coxsackievirus B3 (CVB3) is known to cause a variety of human diseases including acute and chronic myocarditis as well as dilated cardiomyopathy (DCM). However, the mechanisms by which CVB3 causes diseases are not well understood.

METHODS AND RESULTS

Studies identifying protein-protein interactions during CVB3 infection are useful in delineating the pathogenesis of acute or chronic myocarditis. Screening a human heart cDNA library revealed a yet unknown interaction partner of the proapoptotic protein Siva. We demonstrate that Siva specifically interacts with the heart and skeletal muscle protein telethonin. The expression of Siva is increased in heart tissue of CVB3-infected mice and the proteins colocalize in cardiomyocytes.

CONCLUSION

telethonin might be involved in CVB3-mediated cell damage and in the resulting cardiac dysfunction due to the interaction with Siva. We suggest a molecular mechanism through which coxsackieviral infection contributes to the pathogenesis of chronic myocarditis and in particular of acquired forms of DCM.

Remission of CVB3-induced viral myocarditis by in vivo Th2 polarization via hydrodynamics-based interleukin-4 gene transfer

BACKGROUND

Regulation of Th polarization was critical for the prevention of Coxsackievirus B3 (CVB3) induced myocarditis. In the present study, interleukin (IL)-4 was over-expressed by hydrodynamics-based gene transfection (HGT) to induce the in vivo Th2 bias and evaluate the influence of Th polarization on the pathogenesis of CVB3-myocaditis.

METHODS

IL-4 expressing plasmid was delivered into BALB/c mice by HGT after CVB3 infection. In vivo Th polarization was evaluated by detecting expression of Th1/Th2 cytokine, antibody isotype and Th related transcription factor, as well as the proliferation of CD8(+) T cells. The severity of myocarditis was assessed by weight loss, serological index of myocarditis, pathological feature, as well as survival rate.

RESULTS

HGT of IL-4 plasmid resulted in high-level and long-lasting expression of IL-4 in different organs, which rescued mice from severe heart inflammation and death. This may due to the induction of a Th2-bised immune response specified with decreased expression of tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma but increased expression of IL-10 and IL-4 in serum and heart tissue, more IL-4 but less IFN-gamma secreting splenic CD4+ T cells, an immunoglobulin G1 isotype switch, increased expression of GATA-3 and low proliferation of CD8+ T cells, without significant change of virus titer in heart tissue.

CONCLUSIONS

CVB3-induced myocarditis could be remitted through in vivo Th2 polarization, which has implications for our understanding of the role of Th2 population in immunity to CVB3 infection and for the development of new therapies for CVB3-myocarditis.

Pyrin, product of the MEFV locus, interacts with the proapoptotic protein, Siva

Mutations in pyrin cause the autoinflammatory disorder familial Mediterranean fever (FMF), a syndrome characterized by sporadic and unpredictable attacks of fever and localized severe pain. Currently, it is not clear how attacks are triggered, nor why they spontaneously resolve after 2 or 3 days. In fact, the cellular function of the pyrin protein and the molecular underpinnings of its malfunction in FMF have so far eluded clear definition. The identification of pyrin-interacting proteins has the potential to increase our understanding of the cellular networks in which pyrin functions. Previous reports have established that pyrin interacts with the apoptotic protein ASC, the cytoskeletal adaptor protein PSTPIP1, the inflammatory caspase, Caspase-1 and certain forms of the cytosolic anchoring protein 14-3-3. Here, we report that pyrin also interacts with Siva, a pro-apoptotic protein first identified for its interaction with the cytosolic tail of CD27, a TNF family receptor. The interaction between pyrin and Siva involves the C-terminal B30.2/rfp/SRPY domain of pyrin and exon 1 of Siva. We show that Siva and pyrin are indeed co-expressed in human neutrophils, monocytes, and synovial cells. Furthermore, using a novel protein/protein interaction assay, we demonstrate that pyrin can recruit Siva to ASC specks, establishing a potential platform for intersection of ASC and Siva function. Finally, we show that pyrin modulates the apoptotic response to oxidative stress mediated by Siva. Thus, the Siva-pyrin interaction may be a potential target for future therapeutic strategies.

Adenine nucleotide translocase in the focus of cardiovascular diseases

Adenine nucleotide translocase (ANT) facilitates the exchange of extramitochondrial adenosine diphosphate and intramitochondrial adenosine triphosphate across the inner mitochondrial membrane and appears to be a member of the mitochondrial permeability transition pore whose opening induces apoptosis. Genetically or physiologically restricted ANT function associated with insufficient energy supply and induced apoptosis leads to severe cardiac disturbance. In contrast, to counter myocardial stress, heart tissue developed cell protecting gene programs including ANT1 up-regulation to stabilize energy supply and concurrently suppress apoptotic processes. This review describes characteristics of ANT function and expression in cardiovascular diseases and ANT's role in cardioprotection.

Influence of pan-caspase inhibitors on coxsackievirus B3-infected CD19+ B lymphocytes

Coxsackievirus B3 (CVB3), together with other enteroviruses of the picornavirus family, is associated with a wide variety of acute and chronic forms of human diseases. Using the murine model of CVB3-caused myocarditis, this pathogen can be detected not only in solid organs but also in different types of immune cells, preferentially in B lymphocytes. Therefore, these cells could represent a non-cardiac virus reservoir and may play an important role with regard to viral dissemination in the infected host. In addition, the infection of specific immune cells might modulate the severity of tissue injury and the pattern of virus-caused pathology in susceptible or resistant individuals. In the present study it could be demonstrated that CVB3 was capable to infect productively a certain percentage of murine CD19(+) B cells. In vivo studies revealed that CVB3 invaded murine CD19(+) B cells during an acute infection. Three days p. i. approximately 0.5-1.0% of these cells were productively infected. This proportion could be decreased up to 45%, if 3 days p. i. mice were intravenously treated with the pan-caspase inhibitors Z-VAD-FMK or Q-VD-OPH. These data were compared with results obtained from CVB3-infected human Raji cells.

The human fatty acid synthase: a new therapeutic target for coxsackievirus B3-induced diseases?

Coxsackievirus is linked to a large variety of severe human and animal diseases such as myocarditis. The interplay between host factors and virus components is crucial for the fate of the infected cells. However, host proteins which may play a role in coxsackievirus-induced diseases are ill-defined. Two-dimensional gel electrophoresis of protein extracts obtained from coxsackievirus B3 (CVB3)-infected and uninfected HeLa or HepG2 cells combined with spot analysis revealed several proteins which are exclusively up-regulated in infected cells. One of these proteins was identified as the fatty acid synthase (FAS). By using cerulenin and C75, two known inhibitors of FAS we were able to significantly block CVB3 replication. FAS appears to be directly involved in CVB3-caused pathology and is therefore suitable as a therapeutic target in CVB3-induced diseases.

Antiviral effects of pan-caspase inhibitors on the replication of coxsackievirus B3

The induction of apoptosis during coxsackievirus B3 (CVB3) infection is well documented. In order to study whether the inhibition of apoptosis has an impact on CVB3 replication, the pan-caspase inhibitor Z-VAD-FMK was used. The decreased CVB3 replication is based on reduced accumulation of both viral RNA and viral proteins. These effects are due to an inhibitory influence of Z-VAD-FMK on the proteolytic activity of the CVB3 proteases 2A and 3C, which was demonstrated by using the target protein poly(A)-binding protein (PABP). The antiviral effect of the structurally different pan-caspase inhibitor Q-VD-OPH was independently of the viral protease inhibition and resulted in suppression of virus progeny production and impaired release of newly produced CVB3 from infected cells. A delayed release of cytochrome c into the cytoplasm was detected in Q-VD-OPH-treated CVB3-infected cells pointing to an involvement of caspases in the initial steps of mitochondrial membrane-permeabilization.

Proteome alterations in human host cells infected with coxsackievirus B3

Coxsackievirus B3 (CVB3) is a common factor in human myocarditis. The interplay between host factors and virus components is crucial for the fate of the infected cells. Despite that, host protein responses, which characterize CVB3-induced diseases, have not yet been determined in detail. To investigate the nature of modified protein patterns in infected human cells compared with uninfected cells, two-dimensional gel electrophoresis in combination with matrix-assisted laser desorption/ionization-mass spectrometry were used. The regulated proteins, e.g. nucleophosmin (nucleolar protein B23), lamin, the RNA-binding protein UNR and the p38 mitogen-activated protein kinase, were sorted according to their functional groups and interpreted in the context of the myocarditis process.

Glutathione is required for efficient production of infectious picornavirus virions

Glutathione is an intracellular reducing agent that helps maintain the redox potential of the cell and is important for immune function. The drug L-buthionine sulfoximine (BSO) selectively inhibits glutathione synthesis. Glutathione has been reported to block replication of HIV, HSV-1, and influenza virus, whereas cells treated with BSO exhibit increased replication of Sendai virus. Pre-treatment of HeLa cell monolayers with BSO inhibited replication of CVB3, CVB4, and HRV14 with viral titers reduced by approximately 6, 5, and 3 log10, respectively. The addition of glutathione ethyl ester, but not dithiothreitol or 2-mercaptoethanol, to the culture medium reversed the inhibitory effect of BSO. Viral RNA and protein synthesis were not inhibited by BSO treatment. Fractionation of lysates from CVB3-infected BSO-treated cells on cesium chloride and sucrose gradients revealed that empty capsids but not mature virions were being produced. The levels of the 5S and 14S assembly intermediates, however, were not affected by BSO treatment. These results demonstrate that glutathione is important for production of mature infectious picornavirus virions.

The zinc containing pro-apoptotic protein siva interacts with the peroxisomal membrane protein pmp22

Host answers to pathogen attacks define the course of pathogenic events and decide about the fate of the host organism. Infection with coxsackievirus B3 (CVB3) can induce severe myocarditis and pancreatitis. The interplay between host factors and virus components is crucial for the fate of the infected host. As we have shown before, expression of the pro-apoptotic host protein Siva is significantly increased after CVB3 infection, and infected cells are removed by programmed cell death. Analysis of Siva expressed in Escherichia coli revealed that this protein binds three zinc ions, suggesting a rather complex three-dimensional structure. By screening a human heart cDNA library we found a new interaction partner of Siva. The peroxisomal membrane protein PMP22 may be involved in the host response against CVB3. Previous investigations showed that Siva interacts with the cytoplasmic C-terminus of CD27, a member of the tumor necrosis factor receptor group, and transmits an apoptotic signal. With the help of directed two-hybrid assays we determined the N-terminal part of Siva as the binding region for CD27.

Nucleocytoplasmic traffic disorder induced by cardioviruses

Some picornaviruses, for example, poliovirus, increase bidirectional permeability of the nuclear envelope and suppress active nucleocytoplasmic transport. These activities require the viral protease 2A(pro). Here, we studied nucleocytoplasmic traffic in cells infected with encephalomyocarditis virus (EMCV; a cardiovirus), which lacks the poliovirus 2A(pro)-related protein. EMCV similarly enhanced bidirectional nucleocytoplasmic traffic. By using the fluorescent "Timer" protein, which contains a nuclear localization signal, we showed that the cytoplasmic accumulation of nuclear proteins in infected cells was largely due to the nuclear efflux of "old" proteins rather than impaired active nuclear import of newly synthesized molecules. The nuclear envelope of digitonin-treated EMCV-infected cells permitted rapid efflux of a nuclear marker protein. Inhibitors of poliovirus 2A(pro) did not prevent the EMCV-induced efflux. Extracts from EMCV-infected cells and products of in vitro translation of viral RNAs contained an activity increasing permeability of the nuclear envelope of uninfected cells. This activity depended on the expression of the viral leader protein. Mutations disrupting the zinc finger motif of this protein abolished its efflux-inducing ability. Inactivation of the L protein phosphorylation site (Thr47-->Ala) resulted in a delayed efflux, while a phosphorylation-mimicking (Thr47-->Asp) replacement did not significantly impair the efflux-inducing ability. Such activity of extracts from EMCV-infected cells was suppressed by the protein kinase inhibitor staurosporine. As evidenced by electron microscopy, cardiovirus infection resulted in alteration of the nuclear pores, but it did not trigger degradation of the nucleoporins known to be degraded in the poliovirus-infected cells. Thus, two groups of picornaviruses, enteroviruses and cardioviruses, similarly alter the nucleocytoplasmic traffic but achieve this by strikingly different mechanisms.

Arrhythmogenic right ventricular cardiomyopathy/dysplasia: is there a role for viruses?

Arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) is a primary heart muscle disease characterized structurally by progressive fibrofatty replacement of the right ventricle and clinically by life-threatening ventricular arrhythmias with left bundle branch block morphology. Recently, there has been a great deal of interest on ARVC/D as a cause of sudden death in young people, and it has been reported as the most common cause of exercise-related sudden death among competitive athletes in Italy. An autosomic dominant familial occurrence has been recognized, and four disease-causing genes have been recently identified in the dominant forms: ryanodinic cardiac receptor 2, desmoplakin, plakophilin 2, and transforming growth factor (TGF)-beta3. Furthermore, plakoglobin has been identified as the first gene responsible for the recessive variant of ARVC/D associated with palmoplantar keratosis and woolly hair (Naxos disease). However, although much progress has been made in molecular genetics, up to today, the pathogenesis of the disease is still unclear. The occurrence of myocyte apoptosis has been documented, suggesting that recurrent bouts of apoptosis may account for progressive atrophy of the myocardium, which is then replaced by fibrofatty tissue. Considering the frequent finding of myocarditis at histology, an inflammatory theory has been advanced, and infective mechanisms have been postulated to contribute to the onset and the progression of the disease. Cardiotropic viruses have been detected in some ARVC/D cases, and they have been proposed as possible etiologic agents. Several etiopathogenetic theories are herein presented in detail with particular attention to the inflammatory/infective one and its possible links between this and the genetic/dystrophic theories are discussed.

A novel and rapid method to quantify cytolytic replication of picornaviruses in cell culture

Determining viral titers is a key issue in a wide variety of studies regarding different aspects of virology. The standard methods used for determining picornavirus titers are endpoint titration assay and plaque assay, both time consuming and laborious. The method described uses the tetrazolium salt MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-bromide) that is reduced to formazane by cellular dehydrogenase, genes shown to be down-regulated during picornavirus infection. The amount formazane produced correlates with the viral titers obtained and can easily be measured using an ELISA plate reader. The colorimetric method has been evaluated using virus types from different genera of the Picornaviridae family. The MTT method reduces the time spent on determining the viral titers and still maintains a reliable accuracy.

Expression of Siva-1 protein or its putative amphipathic helical region enhances cisplatin-induced apoptosis in breast cancer cells: effect of elevated levels of BCL-2

cis-Diaminedichloroplatinum (II) (cisplatin) is routinely used to treat various types of cancers; however, a significant number develop resistance. One of the underlying factors that contribute to cisplatin resistance is the elevated level of BCL-2 and/or BCL-XL, which promotes cell survival. A potential method of overcoming such resistance is to use a potentiator that is capable of neutralizing the antiapoptotic effects of BCL-2/BCL-XL, such as Siva-1. We previously cloned the proapoptotic protein Siva-1 and showed a possible role for it in both extrinsic and intrinsic apoptosis. Using an adenovirus-based expression system, we now show that Siva-1 can synergize with cisplatin in inducing apoptosis in MCF7 and MDA-MB-231 breast cancer cells. In an anchorage-independent clonogenicity assay, MCF7/caspase-3 cells stably expressing Siva-1, but not the control cells, showed a dramatic decrease in the number of colonies formed on one-time cisplatin treatment. Further, we show that the unique putative amphipathic helical region (SAH) in Siva-1 (amino acid residues 36-55) is necessary and sufficient for the observed enhancement in cisplatin-induced apoptosis by Siva-1. Although cisplatin treatment results in significant elevation in the expression of Fas ligand and intracellular p21 levels, expression of Siva-1 has no additional benefit. Instead, the enhancement in apoptosis seems to be due to activation of intrinsic pathway that involves caspase-9 activation. Moreover, Siva-1 augments cisplatin-mediated cell death in MCF7 cells stably expressing BCL-2. We therefore propose that Siva-1 or its SAH region can be used as a potentiator of cisplatin-based chemotherapy.

Dual functions of murine gammadelta cells in inflammation and autoimmunity in coxsackievirus B3-induced myocarditis: role of Vgamma1+ and Vgamma4+ cells

Coxsackieviruses are a cause of clinical myocarditis. Both virus replication and host defense mechanisms, including virus-induced autoimmunity, mediate heart injury and cardiac dysfunction. Vgamma4+ cells kill infected cardiocytes and virus-specific CD4+ Th2 cells through Fas-dependent apoptosis and CD1d. The CD4+ Th1 response is necessary for activation of the autoimmune CD8+ T cells, which kill uninfected cardiocytes through perforin-dependent mechanisms.

Variability in apoptotic response to poliovirus infection

In several cell types, poliovirus activates the apoptotic program, implementation of which is suppressed by viral antiapoptotic functions. In such cells, productive infection leads to a necrotic cytopathic effect (CPE), while abortive reproduction, associated with inadequate viral antiapoptotic functions, results in apoptosis. Here, we describe two other types of cell response to poliovirus infection. Murine L20B cells expressing human poliovirus receptor responded to the infection by both CPE and apoptosis concurrently. Interruption of productive infection decreased rather than increased the proportion of apoptotic cells. Productive infection was accompanied by the early efflux of cytochrome c from the mitochondria in a proportion of cells and by activation of DEVD-specific caspases. Inactivation of caspase-9 resulted in a marked, but incomplete, prevention of the apoptotic response of these cells to viral infection. Thus, the poliovirus-triggered apoptotic program in L20B cells was not completely suppressed by the viral antiapoptotic functions. In contrast, human rhabdomyosarcoma RD cells did not develop appreciable apoptosis during productive or abortive infection, exhibiting inefficient efflux of cytochrome c from mitochondria and no marked activation of DEVD-specific caspases. The cells were also refractory to several nonviral apoptosis inducers. Nevertheless, typical caspase-dependent signs of apoptosis in a proportion of RD cells were observed after cessation of viral reproduction. Such "late" apoptosis was also observed in productively infected HeLa cells. In addition, a tiny proportion of all studied cells were TUNEL positive even in the presence of a caspase inhibitor. Degradation of DNA in such cells appeared to be a postmortem phenomenon. Biological relevance of variable host responses to viral infection is discussed.

Coxsackievirus-induced pancreatitis

In humans, infections with the group B coxsackieviruses (CVBs) range from asymptomatic infections to chronic, debilitating diseases. The CVBs are associated with chronic inflammatory diseases of the pancreas, heart, and central nervous system. A major focus in CVB pathogenesis is to understand the mechanisms by which these viruses cause acute diseases that resolve or acute diseases that progress to chronic diseases. The present review explores CVB infections in the development of acute and chronic pancreatitis. Mouse models of CVB-induced pancreatitis share many features with the human diseases and are providing insight into the multi-faceted processes of pancreatic tissue repair and irreversible tissue destruction. The development and progression of CVB-induced pancreatic inflammatory disease is an extremely complex process, involving both viral and host factors. The review examines the roles of the virus and host in contributing to the disease process. Recent studies of global gene expression during CVB-induced pancreatitis have increased our understanding of host factors that influence the outcome of infection and have highlighted interrelationships among complex biological programs. As we unravel the complexity of the disease process, the information gained will lead to the design of therapeutics that not only prevent the progression of chronic inflammatory disease, but that also restore functionality of affected tissues and organs.

Bidirectional increase in permeability of nuclear envelope upon poliovirus infection and accompanying alterations of nuclear pores

Poliovirus and some other picornaviruses trigger relocation of certain nuclear proteins into the cytoplasm. Here, by using a protein changing its fluorescence color with time and containing a nuclear localization signal (NLS), we demonstrate that the poliovirus-triggered relocation is largely due to the exit of presynthesized nuclear protein into the cytoplasm. The leakiness of the nuclear envelope was also documented by the inability of nuclei from digitonin-permeabilized, virus-infected (but not mock-infected) cells to retain an NLS-containing derivative of green fluorescent protein (GFP). The cytoplasm-to-nucleus traffic was also facilitated during infection, as evidenced by experiments with GAPDH (glyceraldehyde-3-phosphate dehydrogenase), cyclin B1, and an NLS-lacking derivative of GFP, which are predominantly cytoplasmic in uninfected cells. Electron microscopy demonstrated that a bar-like barrier structure in the channel of the nuclear pores, seen in uninfected cells, was missing in the infected cells, giving the impression of fully open pores. Transient expression of poliovirus 2A protease also resulted in relocation of the nuclear proteins. Lysates from poliovirus-infected or 2A-expressing cells induced efflux of 3xEGFP-NLS from the nuclei of permeabilized uninfected cells. This activity was inhibited by the elastase inhibitors elastatinal and N-(methoxysuccinyl)-L-alanyl-L-alanyl-L-prolyl-L-valine chloromethylketone (drugs known also to be inhibitors of poliovirus protease 2A), a caspase inhibitor zVAD(OMe), fmk, and some other protease inhibitors. These data suggest that 2A elicited nuclear efflux, possibly in cooperation with a zVAD(OMe).fmk-sensitive protease. However, poliovirus infection facilitated nuclear protein efflux also in cells deficient in caspase-3 and caspase-9, suggesting that the efflux may occur without the involvement of these enzymes. The biological relevance of nucleocytoplasmic traffic alterations in infected cells is discussed.

Recruitment of Hsp70 chaperones: a crucial part of viral survival strategies

Virus proliferation depends on the successful recruitment of host cellular components for their own replication, protein synthesis, and virion assembly. In the course of virus particle production a large number of proteins are synthesized in a relatively short time, whereby protein folding can become a limiting step. Most viruses therefore need cellular chaperones during their life cycle. In addition to their own protein folding problems viruses need to interfere with cellular processes such as signal transduction, cell cycle regulation and induction of apoptosis in order to create a favorable environment for their proliferation and to avoid premature cell death. Chaperones are involved in the control of these cellular processes and some viruses reprogram their host cell by interacting with them. Hsp70 chaperones, as central components of the cellular chaperone network, are frequently recruited by viruses. This review focuses on the function of Hsp70 chaperones at the different stages of the viral life cycle emphasizing mechanistic aspects.

Antiviral activity of caspase inhibitors: effect on picornaviral 2A proteinase

Peptide-based fluoromethyl ketones have been considered for many years to be highly specific caspase inhibitors distinctly blocking the progress of apoptosis in a variety of systems. Here we demonstrate that these compounds can significantly reduce rhinovirus multiplication in cell culture. In their methylated forms they block eIF4GI cleavage in vivo and in vitro and inhibit the activity of picornaviral 2A proteinases.

Inhibition of translation and induction of apoptosis by Bunyaviral nonstructural proteins bearing sequence similarity to reaper

Members of the California serogroup of bunyaviruses (family Bunyaviridae) are the leading cause of pediatric viral encephalitis in North America. Significant cell death is observed as part of the infection pathology. We now report that a Bunyaviral nonstructural protein termed NSs shows sequence similarity to Reaper, a proapoptotic protein from Drosophila. Although NSs proteins lack the Reaper N-terminal motif critical for IAP inhibition, they do retain other functions of Reaper that map to conserved C-terminal regions. Like Reaper, NSs proteins induce mitochondrial cytochrome c release and caspase activation in cell-free extracts and promote neuronal apoptosis and mortality in a mouse model. Independent of caspase activation, Bunyavirus NSs proteins also share with Reaper the ability to directly inhibit cellular protein translation. We have found that the shared capacity to inhibit translation and induce apoptosis resides in common sequence motifs present in both Reaper and NSs proteins. Data presented here suggest that NSs induce apoptosis through a mechanism similar to that used by Reaper, as both proteins bind to an apoptotic regulator called Scythe and can relieve Scythe inhibition of Hsp70. Thus, bunyavirus NSs proteins have multiple Reaper-like functions that likely contribute to viral pathogenesis by promoting cell death and/or inhibiting cellular translation.

High-neurovirulence GDVII virus induces apoptosis in murine astrocytes through tumor necrosis factor (TNF)-receptor and TNF-related apoptosis-inducing ligand

We carried out a study to determine if the high-neurovirulence GDVII strain of Theiler's murine encephalomyelitis virus (TMEV) and the demyelinating, low-neurovirulence BeAn strain induced apoptosis in murine astrocytes. Astrocytes, the major glial cell population of the central nervous system, were semipermissive for GDVII virus replication. Programmed cell death, demonstrated by apoptosis-specific caspase-3 protease activity, was maximal 8 h after GDVII infection at an m.o.i. of 1. Purified TMEV capsid proteins VP1, VP2, and VP3 did not induce apoptosis but antibodies to VP1 and VP2 inhibited it. Antibody inhibition of caspase-3 activity as well as flow cytometry experiments implicated TNF-related apoptosis-inducing ligand (TRAIL) and TNF-alpha-receptor (TNF-R) in apoptosis signaling. Conversely, TNF-alpha and the TRAIL-receptor were not upregulated. Furthermore, the number of functional TNF-alpha receptors, but not their affinity, was increased in apoptotic GDVII virus-infected astrocytes, as confirmed in binding experiments with 125I-labeled recombinant murine TNF-alpha. In vivo studies showed that most of the cells loaded with the virus when injected in the brains of SJL mice were neurons but very few showed TUNEL costaining. Conversely, many of the apoptotic cells found were also positive for GFAP staining.

Toward testing the hypothesis that group B coxsackieviruses (CVB) trigger insulin-dependent diabetes: inoculating nonobese diabetic mice with CVB markedly lowers diabetes incidence

Insulin-dependent (type 1) diabetes mellitus (T1D) onset is mediated by individual human genetics as well as undefined environmental influences such as viral infections. The group B coxsackieviruses (CVB) are commonly named as putative T1D-inducing agents. We studied CVB replication in nonobese diabetic (NOD) mice to assess how infection by diverse CVB strains affected T1D incidence in a model of human T1D. Inoculation of 4- or 8-week-old NOD mice with any of nine different CVB strains significantly reduced the incidence of T1D by 2- to 10-fold over a 10-month period relative to T1D incidences in mock-infected control mice. Greater protection was conferred by more-pathogenic CVB strains relative to less-virulent or avirulent strains. Two CVB3 strains were employed to further explore the relationship of CVB virulence phenotypes to T1D onset and incidence: a pathogenic strain (CVB3/M) and a nonvirulent strain (CVB3/GA). CVB3/M replicated to four- to fivefold-higher titers than CVB3/GA in the pancreas and induced widespread pancreatitis, whereas CVB3/GA induced no pancreatitis. Apoptotic nuclei were detected by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) assay in CVB3/M-infected pancreata but not in CVB3/GA-infected pancreata. In situ hybridization detected CVB3 RNA in acinar tissue but not in pancreatic islets. Although islets demonstrated inflammatory infiltrates in CVB3-protected mice, insulin remained detectable by immunohistochemistry in these islets but not in those from diabetic mice. Enzyme-linked immunosorbent assay-based examination of murine sera for immunoglobulin G1 (IgG1) and IgG2a immunoreactivity against diabetic autoantigens insulin and HSP60 revealed no statistically significant relationship between CVB3-protected mice or diabetic mice and specific autoimmunity. However, when pooled sera from CVB3/M-protected mice were used to probe a Western blot of pancreatic proteins, numerous proteins were detected, whereas only one band was detected by sera from CVB3/GA-protected mice. No proteins were detected by sera from diabetic or normal mice. Cumulatively, these data do not support the hypothesis that CVB are causative agents of T1D. To the contrary, CVB infections provide significant protection from T1D onset in NOD mice. Possible mechanisms by which this virus-induced protection may occur are discussed.

Siva-1 binds to and inhibits BCL-X(L)-mediated protection against UV radiation-induced apoptosis

We previously cloned Siva-1 by using the cytoplasmic tail of CD27, a member of the tumor necrosis factor receptor family, as the bait in the yeast two-hybrid system. The Siva gene is organized into four exons that code for the predominant full-length Siva-1 transcript, whereas its alternate splice form, Siva-2, lacks exon 2 coding sequence. Various groups have demonstrated a role for Siva-1 in several apoptotic pathways. Interestingly, the proapoptotic properties of Siva-1 are lacking in Siva-2. The fact that Siva-1 is partly localized to mitochondria despite the absence of any mitochondrial targeting signal, it harbors a 20-aa-long putative amphipathic helical structure that is absent in Siva-2, and that its expression is restricted to double-positive (CD3(+), CD4(+), CD8(+)) thymocytes like BCL-X(L), prompted us to test for a potential interaction between Siva-1 and BCL-X(L). Here, we show that Siva-1 binds to and inhibits BCL-X(L)-mediated protection against UV radiation-induced apoptosis. Indeed, the unique amphipathic helical region (SAH) present in Siva-1 is required for its binding to BCL-X(L) and sensitizing cells to UV radiation. Natural complexes of Siva-1/BCL-X(L) are detected in HUT78 and murine thymocyte, suggesting a potential role for Siva-1 in regulating T cell homeostasis.