Siva1 is a XIAP-interacting protein that balances NFkappaB and JNK signalling to promote apoptosis

Exploring the dual role of SIVA1 in cancer biology

The intricate molecular mechanisms associated with cancer development continue to engage researchers due to the significant impact of the disease on global mortality. This review delves into the role of the apoptosis regulatory protein SIVA1, which has emerged as a significant player in cellular homeostasis. SIVA1, initially characterized as a pro-apoptotic protein interacting with the TNF receptor CD27, has since been implicated in various cellular contexts, revealing its complex functional dynamics. The SIVA1 gene, located on chromosome 14, encodes a protein containing distinctive structural features, including an amphipathic helix and a death domain homology region. Localization studies show that SIVA1 is present in both the cytoplasm and nucleus, with its expression linked to tumor differentiation. Investigations into SIVA1's interactions have uncovered its pro-apoptotic mechanisms, such as binding to anti-apoptotic proteins from the BCL2 family, thus promoting apoptosis under stress conditions. Interestingly, SIVA1 also exhibits tumor-promoting properties in specific cancer types, suggesting a dual role in apoptosis induction and tumor progression. As research progresses, understanding the regulatory mechanisms governing SIVA1's multifaceted functions could pave the way for novel therapeutic strategies aimed at manipulating its activity for improved cancer treatment outcomes. Future studies are warranted to clarify SIVA1's contextual roles and explore its potential clinical implications.

Protection of Quiescence and Longevity of IgG Memory B Cells by Mitochondrial Autophagy

The development of long-lived immune memory cells against pathogens is critical for the success of vaccines to establish protection against future infections. However, the mechanisms governing the long-term survival of immune memory cells remain to be elucidated. In this article, we show that the maintenance mitochondrial homeostasis by autophagy is critical for restricting metabolic functions to protect IgG memory B cell survival. Knockout of mitochondrial autophagy genes, Nix and Bnip3, leads to mitochondrial accumulation and increases in oxidative phosphorylation and fatty acid synthesis, resulting in the loss of IgG⁺ memory B cells in mice. Inhibiting fatty acid synthesis or silencing necroptosis gene Ripk3 rescued Nix^-/-Bnip3^-/- IgG memory B cells, indicating that mitochondrial autophagy is important for limiting metabolic functions to prevent cell death. Our results suggest a critical role for mitochondrial autophagy in the maintenance of immunological memory by protecting the metabolic quiescence and longevity of memory B cells.

FAIM-L - SIVA-1: Two Modulators of XIAP in Non-Apoptotic Caspase Function

Apoptosis is crucial for the correct development of the nervous system. In adulthood, the same protein machinery involved in programmed cell death can control neuronal adaptiveness through modulation of synaptic pruning and synaptic plasticity processes. Caspases are the main executioners in these molecular pathways, and their strict regulation is essential to perform neuronal remodeling preserving cell survival. FAIM-L and SIVA-1 are regulators of caspase activation. In this review we will focus on FAIM-L and SIVA-1 as two functional antagonists that modulate non-apoptotic caspase activity in neurons. Their participation in long-term depression and neurite pruning will be described in base of the latest studies performed. In addition, the association of FAIM-L non-apoptotic functions with the neurodegeneration process will be reviewed.

Puma- and Caspase9-mediated apoptosis is dispensable for p53-driven neural crest-based developmental defects

Inappropriate activation of the p53 transcription factor is thought to contribute to the developmental phenotypes in a range of genetic syndromes. Whether p53 activation drives these developmental phenotypes by triggering apoptosis, cell cycle arrest, or other p53 cellular responses, however, has remained elusive. As p53 hyperactivation in embryonic neural crest cells (NCCs) drives a number of phenotypes, including abnormal craniofacial and neuronal development, we investigate the basis for p53 action in this context. We show that p53-driven developmental defects are associated with the induction of a robust pro-apoptotic transcriptional signature. Intriguingly, however, deleting Puma or Caspase9, which encode key components of the intrinsic apoptotic pathway, does not rescue craniofacial, neuronal or pigmentation defects triggered by p53 hyperactivation in NCCs. Immunostaining analyses for two key apoptosis markers confirm that deleting Puma or Caspase9 does indeed impair p53-hyperactivation-induced apoptosis in NCCs. Furthermore, we demonstrate that p53 hyperactivation does not trigger a compensatory dampening of cell cycle progression in NCCs upon inactivation of apoptotic pathways. Together, our results indicate that p53-driven craniofacial, neuronal and pigmentation defects can arise in the absence of apoptosis and cell cycle arrest, suggesting that p53 hyperactivation can act via alternative pathways to trigger developmental phenotypes.

Bacterial CagA protein compromises tumor suppressor mechanisms in gastric epithelial cells

Approximately half of the world's population is infected with the stomach pathogen Helicobacter pylori. Infection with H. pylori is the main risk factor for distal gastric cancer. Bacterial virulence factors, such as the oncoprotein CagA, augment cancer risk. Yet despite high infection rates, only a fraction of H. pylori-infected individuals develop gastric cancer. This raises the question of defining the specific host and bacterial factors responsible for gastric tumorigenesis. To investigate the tumorigenic determinants, we analyzed gastric tissues from human subjects and animals infected with H. pylori bacteria harboring different CagA status. For laboratory studies, well-defined H. pylori strain B128 and its cancerogenic derivative strain 7.13, as well as various bacterial isogenic mutants were employed. We found that H. pylori compromises key tumor suppressor mechanisms: the host stress and apoptotic responses. Our studies showed that CagA induces phosphorylation of XIAP E3 ubiquitin ligase, which enhances ubiquitination and proteasomal degradation of the host proapoptotic factor Siva1. This process is mediated by the PI3K/Akt pathway. Inhibition of Siva1 by H. pylori increases survival of human cells with damaged DNA. It occurs in a strain-specific manner and is associated with the ability to induce gastric tumor.

SIVA-1 regulates apoptosis and synaptic function by modulating XIAP interaction with the death receptor antagonist FAIM-L

The long isoform of Fas apoptosis inhibitory molecule (FAIM-L) is a neuron-specific death receptor antagonist that modulates apoptotic cell death and mechanisms of neuronal plasticity. FAIM-L exerts its antiapoptotic action by binding to X-linked inhibitor of apoptosis protein (XIAP), an inhibitor of caspases, which are the main effectors of apoptosis. XIAP levels are regulated by the ubiquitin-proteasome pathway. FAIM-L interaction with XIAP prevents the ubiquitination and degradation of the latter, thereby allowing it to inhibit caspase activation. This interaction also modulates non-apoptotic functions of caspases, such as the endocytosis of AMPA receptor (AMPAR) in hippocampal long-term depression (LTD). The molecular mechanism of action exerted by FAIM-L is unclear since the consensus binding motifs are still unknown. Here, we performed a two-hybrid screening to discover novel FAIM-L-interacting proteins. We found a functional interaction of SIVA-1 with FAIM-L. SIVA-1 is a proapoptotic protein that has the capacity to interact with XIAP. We describe how SIVA-1 regulates FAIM-L function by disrupting the interaction of FAIM-L with XIAP, thereby promoting XIAP ubiquitination, caspase-3 activation and neuronal death. Furthermore, we report that SIVA-1 plays a role in receptor internalization in synapses. SIVA-1 is upregulated upon chemical LTD induction, and it modulates AMPAR internalization via non-apoptotic activation of caspases. In summary, our findings uncover SIVA-1 as new functional partner of FAIM-L and demonstrate its role as a regulator of caspase activity in synaptic function.

Siva 1 Inhibits Cervical Cancer Progression and Its Clinical Prognosis Significance

Background

Cervical cancer is the second most common female malignancies. But the exact etiology of cervical cancer is still under investigation. Recent observations revealed that the loss expression of Siva 1 was related to several different types of tumors. It could play an indispensable role in both exogenous and endogenous apoptotic signaling pathways. Nevertheless, the relationship between Siva 1 expression and cervical cancer progression has not yet been fully clarified. This study aimed to explore the functional role of Siva1 in cervical cancer.

Materials and Methods

In this present experiment, expression of Siva 1 was detected in 87 cervical cancer, 34 CIN and 20 normal samples by immunohistochemistry. The correlation of Siva 1 expression and overall survival times (OS) was analyzed by Kaplan–Meier analysis. We up-regulated the expression of Siva 1 by plasmid pCMV3-Siva 1 in C33A cells. CCK8, flow cytometry, wound-healing, and transwell assays were performed to examine the influences of Siva 1 expression on cell proliferation, apoptosis, migration and invasion.

Results

The expression of Siva 1 was decreased in cervical cancer tissues compared with CIN and normal tissues. In addition, the Siva 1 immunoreactivity was significantly associated with tumor differentiation. Patients with Siva 1 negative staining exhibited a significantly decreased overall survival. Then, we established stable Siva 1 ectopic expression cells, and we found that elevated expression of Siva 1 promoted apoptosis, inhibited proliferation, and suppressed migration and invasion of cervical cancer cells.

Conclusion

The present study revealed a crucial role of Siva 1 in tumor progression and it may be a valuable prognostic indicator of cervical cancer.

Selenium Treatment Enhanced Clearance of Salmonella in Chicken Macrophages (HD11)

As an important micronutrient, selenium (Se) plays many essential roles in immune response and protection against pathogens in humans and animals, but underlying mechanisms of Se-based control of salmonella growth within macrophages remain poorly elucidated. In this study, using RNA-seq analyses, we demonstrate that Se treatment (at an appropriate concentration) can modulate the global transcriptome of chicken macrophages HD11. The bioinformatic analyses (KEGG pathway analysis) revealed that the differentially expressed genes (DEGs) were mainly enriched in retinol and glutathione metabolism, revealing that Se may be associated with retinol and glutathione metabolism. Meanwhile, Se treatment increased the number of salmonella invading the HD11 cells, but reduced the number of salmonella within HD11 cells, suggesting that enhanced clearance of salmonella within HD11 cells was potentially modulated by Se treatment. Furthermore, RNA-seq analyses also revealed that nine genes including SIVA1, FAS, and HMOX1 were differentially expressed in HD11 cells infected with salmonella following Se treatment, and GO enrichment analysis showed that these DEGs were mainly enriched in an extrinsic apoptotic signaling pathway. In summary, these results indicate that Se treatment may not only affect retinol and glutathione metabolism in macrophages, but could also inhibit salmonella-induced macrophage apoptosis via an extrinsic apoptotic signaling pathway involving SIVA1.

Siva plays a critical role in mouse embryonic development

The Siva protein, named after the Hindu God of Destruction, plays important roles in apoptosis in various contexts, including downstream of death receptor activation or p53 tumor suppressor engagement. The function of Siva in organismal development and homeostasis, however, has remained uncharacterized. Here, we generate Siva knockout mice to characterize the physiological function of Siva in vivo. Interestingly, we find that Siva deficiency causes early embryonic lethality accompanied by multiple phenotypes, including developmental delay, abnormal neural tube closure, and defective placenta and yolk sac formation. Examination of Siva expression during embryogenesis shows that Siva is expressed in both embryonic and extra-embryonic tissues, including within the mesoderm, which may explain the vascular defects observed in the placenta and yolk sac. The embryonic phenotypes caused by Siva loss are not rescued by p53 deficiency, nor do they resemble those of p53 null embryos, suggesting that the embryonic function of Siva is not related to the p53 pathway. Moreover, loss of the Ripk3 necroptosis protein does not rescue the observed lethality or developmental defects, suggesting that Siva may play a non-apoptotic role in development. Collectively, these studies reveal a key role for Siva in proper embryonic development.

Combined PPARγ Activation and XIAP Inhibition as a Potential Therapeutic Strategy for Ovarian Granulosa Cell Tumors

Ovarian granulosa cell tumors (GCT) are characterized by indolent growth and late relapse. No therapeutic modalities aside from surgery have proven effective. We previously reported overexpression of the nuclear receptor, peroxisome proliferator-activated receptor-gamma (PPARγ), and constitutive activity of the NFκB and AP1 signaling pathways in GCT. PPARγ presents as a potential therapeutic target as it impedes proliferation and promotes terminal differentiation of granulosa cells. However, resistance to the actions of PPARγ is caused by NFκB transrepression in GCT-derived cell lines, KGN and COV434. We showed that abrogation of NFκB signaling in GCT cells enables PPARγ agonists to initiate apoptosis. In addition, we observed overexpression of an NFκB-induced gene, X-linked inhibitor of apoptosis protein (XIAP), in GCT and GCT-derived cells. XIAP is an attractive therapeutic target due to its role in inhibiting the apoptotic pathway. We investigated the antitumor effects of combined XIAP inhibition using Smac-mimetics and PPARγ activation using thiazolidinediones (TZD) in the GCT-derived cells. Transactivation assays revealed that NFκB transrepression of PPARγ can be relieved by NFκB or XIAP inhibition. Combined Smac-mimetic and TZD significantly induced apoptosis, reduced cell viability and proliferation in KGN cells in monolayer and 3D spheroid culture, and in GCT explant models. The Smac-mimetic and TZD cotreatment also delayed cell invasion, upregulated proapoptotic genes, and compromised cell metabolism in KGN cells. This study provides evidence that PPARγ and XIAP cotreatment has antineoplastic effects in GCT. As therapeutics that target these proteins are already in clinical or preclinical use, expedient translation to the clinic is possible.

Siva-1 emerges as a tissue-specific oncogene beyond its classic role of a proapoptotic gene

Siva-1 is a typical apoptotic protein commonly activated by the p53 tumor suppressor protein and should therefore participate in a barrier against the development of cancer. It has proapoptotic activities in various cell systems. Recent findings suggest that Siva-1 possesses several other apoptosis-independent functions and interacts with many other proteins not directly involved in apoptosis. It harbors the ARF E3 ubiquitin protein ligase activity, a property that is clearly prooncogenic and leads to p53 degradation through the upregulation of the Hdm2 protein level. Surprisingly, recent evidence shows that Siva-1 absence prevents the development of non-small cell lung carcinomas in a mouse model and reveals the oncogenic roles in the same types of human cells, indicating its unique function as an oncogene in the cell context-dependent manner. Herein, we review reported activities of Siva-1 in various experimental settings and comment on its ambiguous function in tumor biology.

XIAP Interacts with and Regulates the Activity of FAF1

Cell death depends on the balance between the activities of pro- and anti-apoptotic factors. X-linked inhibitor of apoptosis protein (XIAP) plays an important role in the cytoprotective process by inhibiting the caspase cascade and regulating pro-survival signaling pathways. While searching for novel interacting partners of XIAP, we identified Fas-associated factor 1 (FAF1). Contrary to XIAP, FAF1 is a pro-apoptotic factor that also regulates several signaling pathways in which XIAP is involved. However, the functional relationship between FAF1 and XIAP is unknown. Here, we describe a new interaction between XIAP and FAF1 and describe the functional implications of their opposing roles in cell death and NF-κB signaling. Our results clearly demonstrate the interaction of XIAP with FAF1 and define the specific region of the interaction. We observed that XIAP is able to block FAF1-mediated cell death by interfering with the caspase cascade and directly interferes in NF-κB pathway inhibition by FAF1. Furthermore, we show that XIAP promotes ubiquitination of FAF1. Conversely, FAF1 does not interfere with the anti-apoptotic activity of XIAP, despite binding to the BIR domains of XIAP; however, FAF1 does attenuate XIAP-mediated NF-κB activation. Altered expression of both factors has been implicated in degenerative and cancerous processes; therefore, studying the balance between XIAP and FAF1 in these pathologies will aid in the development of novel therapies.

Expression profiling reveals genes involved in the regulation of wool follicle bulb regression and regeneration in sheep

Wool is an important material in textile manufacturing. In order to investigate the intrinsic factors that regulate wool follicle cycling and wool fiber properties, Illumina sequencing was performed on wool follicle bulb samples from the middle anagen, catagen and late telogen/early anagen phases. In total, 13,898 genes were identified. KRTs and KRTAPs are the most highly expressed gene families in wool follicle bulb. In addition, 438 and 203 genes were identified to be differentially expressed in wool follicle bulb samples from the middle anagen phase compared to the catagen phase and the samples from the catagen phase compared to the late telogen/early anagen phase, respectively. Finally, our data revealed that two groups of genes presenting distinct expression patterns during the phase transformation may have important roles for wool follicle bulb regression and regeneration. In conclusion, our results demonstrated the gene expression patterns in the wool follicle bulb and add new data towards an understanding of the mechanisms involved in wool fiber growth in sheep.

The p53 Target Gene SIVA Enables Non-Small Cell Lung Cancer Development

Although p53 transcriptional activation potential is critical for its ability to suppress cancer, the specific target genes involved in tumor suppression remain unclear. SIVA is a p53 target gene essential for p53-dependent apoptosis, although it can also promote proliferation through inhibition of p53 in some settings. Thus, the role of SIVA in tumorigenesis remains unclear. Here, we seek to define the contribution of SIVA to tumorigenesis by generating Siva conditional knockout mice. Surprisingly, we find that SIVA loss inhibits non-small cell lung cancer (NSCLC) development, suggesting that SIVA facilitates tumorigenesis. Similarly, SIVA knockdown in mouse and human NSCLC cell lines decreases proliferation and transformation. Consistent with this protumorigenic role for SIVA, high-level SIVA expression correlates with reduced NSCLC patient survival. SIVA acts independently of p53 and, instead, stimulates mTOR signaling and metabolism in NSCLC cells. Thus, SIVA enables tumorigenesis in a p53-independent manner, revealing a potential new cancer therapy target.

SIGNIFICANCE

These findings collectively reveal a novel role for the p53 target gene SIVA both in regulating metabolism and in enabling tumorigenesis, independently of p53. Importantly, these studies further identify SIVA as a new prognostic marker and as a potential target for NSCLC cancer therapy.

ANKHD1 silencing inhibits Stathmin 1 activity, cell proliferation and migration of leukemia cells

ANKHD1 is highly expressed in human acute leukemia cells and potentially regulates multiple cellular functions through its ankyrin-repeat domains. In order to identify interaction partners of the ANKHD1 protein and its role in leukemia cells, we performed a yeast two-hybrid system screen and identified SIVA, a cellular protein known to be involved in proapoptotic signaling pathways. The interaction between ANKHD1 and SIVA was confirmed by co-imunoprecipitation assays. Using human leukemia cell models and lentivirus-mediated shRNA approaches, we showed that ANKHD1 and SIVA proteins have opposing effects. While it is known that SIVA silencing promotes Stathmin 1 activation, increased cell migration and xenograft tumor growth, we showed that ANKHD1 silencing leads to Stathmin 1 inactivation, reduced cell migration and xenograft tumor growth, likely through the inhibition of SIVA/Stathmin 1 association. In addition, we observed that ANKHD1 knockdown decreases cell proliferation, without modulating apoptosis of leukemia cells, while SIVA has a proapoptotic function in U937 cells, but does not modulate proliferation in vitro. Results indicate that ANKHD1 binds to SIVA and has an important role in inducing leukemia cell proliferation and migration via the Stathmin 1 pathway. ANKHD1 may be an oncogene and participate in the leukemia cell phenotype.

Electrical stimulation enhances epidermal proliferation in human cutaneous wounds by modulating p53-SIVA1 interaction

Cutaneous wounds establish endogenous "wound current" upon injury until re-epithelialization is complete. Keratinocyte proliferation, regulated partly by p53, is required for epidermal closure. SIVA1 promotes human double minute 2 homolog (HDM2)-mediated p53 regulation. However, the role of SIVA1 in wound healing is obscure. Here, we report that electrical stimulation (ES) accelerates wound healing by upregulating SIVA1 and its subsequent ability to modulate p53 activities. Cultured donut-shaped human skin explants, subjected to ES, exhibited better epidermal stratification, increased proliferation, and upregulation of gene and protein expression of HDM2/SIVA1, compared with non-ES-treated explants. ES significantly increased in vitro keratinocyte proliferation and phospho-p53-SIVA1 interaction; however, this showed stable expression of phospho-p53, which increased significantly in the absence of SIVA1. Here, HDM2 alone was unable to downregulate nuclear-accumulated phospho-p53, which was evident from decreased proliferation and increased sub-G1 population seen by flow cytometry. Further examination of the epidermis of human cutaneous wounds showed higher p53-SIVA1 coexpression and proliferation 7 days after injury in ES-treated wounds compared with control wounds. In summary, ES-inducible SIVA1 modulates p53 activities in proliferating keratinocytes, and exogenous ES affects p53/HDM2/SIVA1 axis leading to increased proliferation during re-epithelialization. This highlights ES as a potential strategy for enhancing cutaneous repair.

SIVA1 directs the E3 ubiquitin ligase RAD18 for PCNA monoubiquitination

Translesion DNA synthesis (TLS) is a universal DNA damage tolerance mechanism conserved from yeast to mammals. A key event in the regulation of TLS is the monoubiquitination of proliferating cell nuclear antigen (PCNA). Extensive evidence indicates that the RAD6-RAD18 ubiquitin-conjugating/ligase complex specifically monoubiquitinates PCNA and regulates TLS repair. However, the mechanism by which the RAD6-RAD18 complex is targeted to PCNA has remained elusive. In this study, we used an affinity purification approach to isolate the PCNA-containing complex and have identified SIVA1 as a critical regulator of PCNA monoubiquitination. We show that SIVA1 constitutively interacts with PCNA via a highly conserved PCNA-interacting peptide motif. Knockdown of SIVA1 compromised RAD18-dependent PCNA monoubiquitination and Polη focus formation, leading to elevated ultraviolet sensitivity and mutation. Furthermore, we demonstrate that SIVA1 interacts with RAD18 and serves as a molecular bridge between RAD18 and PCNA, thus targeting the E3 ligase activity of RAD18 onto PCNA. Collectively, our results provide evidence that the RAD18 E3 ligase requires an accessory protein for binding to its substrate PCNA.

The N-terminal 33 amino acid domain of Siva-1 is sufficient for nuclear localization

Siva-1 induces apoptosis in multiple pathological processes and plays an important role in the suppression of tumor metastasis, protein degradation, and other functions. Although many studies have demonstrated that Siva-1 functions in the cytoplasm, a few have found that Siva-1 can relocate to the nucleus. In this study, we found that the first 33 amino acid residues of Siva-1 are required for its nuclear localization. Further study demonstrated that the green fluorescent protein can be imported into the nucleus after fusion with these 33 amino acid residues. Other Siva-1 regions and domains showed less effect on Siva-1 nuclear localization. By site-mutagenesis of all of these 33 amino acid residues, we found that mutants of the first 1-18 amino acids affected Siva-1 nuclear compartmentalization but could not complete this localization independently. In summary, we demonstrated that the N-terminal 33 amino acid residues were sufficient for Siva-1 nuclear localization, but the mechanism of this translocation needs additional investigation.

Relevance of signaling molecules for apoptosis induction on influenza A virus replication

Apoptosis is an important mechanism to maintain homeostasis in mammals, and disruption of the apoptosis regulation mechanism triggers a range of diseases, such as cancer, autoimmune diseases, and developmental disorders. The severity of influenza A virus (IAV) infection is also closely related to dysfunction of apoptosis regulation. In the virus infected cells, the functions of various host cellular molecules involved in regulation of induction of apoptosis are modulated by IAV proteins to enable effective virus replication. The modulation of the intracellular signaling pathway inducing apoptosis by the IAV infection also affects extracellular mechanisms controlling apoptosis, and triggers abnormal host responses related to the disease severity of IAV infections. This review focuses on apoptosis related molecules involved in IAV replication and pathogenicity, the strategy of the virus propagation through the regulation of apoptosis is also discussed.

Protection from cerebral ischemia by inhibition of TGFβ-activated kinase

OBJECTIVE

Transforming growth factor-β-activated kinase (TAK1) is a member of the mitogen-activated protein kinase family that plays important roles in apoptosis and inflammatory signaling, both of which are critical components of stroke pathology. TAK1 has recently been identified as a major upstream kinase that phosphorylates and activates adenosine monophosphate-activated protein kinase (AMPK), a major mediator of neuronal injury after experimental cerebral ischemia. We studied the functional role of TAK1 and its mechanistic link with AMPK after stroke.

METHODS

Male mice were subjected to transient middle cerebral artery occlusion (MCAO). The TAK1 inhibitor 5Z-7-oxozeaenol was injected either intracerebroventricularly or intraperitoneally at various doses and infarct size and functional outcome after long term survival was assessed. Mice with deletion of the AMPK α2 isoform were utilized to assess the contribution of downstream AMPK signaling to stroke outcomes. Levels of pTAK1, pAMPK, and other TAK1 targets including the pro-apoptotic molecule c-Jun-N-terminal kinase (JNK)/c-Jun and the pro-inflammatory protein cyclooxygenase-2 were also examined.

RESULTS

TAK1 is critical in stroke pathology. Delayed treatment with a TAK1 inhibitor reduced infarct size and improved behavioral outcome even when given several hours after stroke onset. This protective effect may be independent of AMPK activation but was associated with a reduction in JNK and c-Jun signaling.

CONCLUSIONS

Enhanced TAK1 signaling, via activation of JNK, contributes to cell death in ischemic stroke. TAK1 inhibition is a novel therapeutic approach for stroke as it is neuroprotective with systemic administration, has a delayed therapeutic window, and demonstrates sustained neuroprotective effects.

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.

Thromboxane A2 modulates cisplatin-induced apoptosis through a Siva1-dependent mechanism

Thromboxane A(2) (TXA(2)) is an important lipid mediator whose function in apoptosis is the subject of conflicting reports. Here, a yeast two-hybrid screen for proteins that interact with the C-terminus of the TXA(2) receptor (TP) identified Siva1 as a new TP-interacting protein. Contradictory evidence suggests pro- and anti-apoptotic roles for Siva1. We show that a cisplatin treatment induces TXA(2) synthesis in HeLa cells. We demonstrate that endogenous TP stimulation promotes cisplatin-induced apoptosis of HeLa cells and that such modulation requires the expression of Siva1, as evidenced by inhibiting its endogenous expression using siRNAs. We reveal that, upon stimulation of TP, degradation of Siva1 is impeded, resulting in an accumulation of the protein, which translocates from the nucleus to the cytosol. Translocation of Siva1 correlates with its reduced interaction with Mdm2 (an inhibitor of p53 signalling), as well as with its increased interaction with TRAF2 and XIAP (known to enhance pro-apoptotic signalling). Our data provide a model that reconciles the pro- and anti-apoptotic roles that were reported for Siva1 and identify a new mechanism for promoting apoptosis by G protein-coupled receptors. Our findings may have implications in the use of cyclo-oxygenase inhibitors during cisplatin chemotherapy and might provide a target to reduce cisplatin toxicity on non-cancerous tissues.

TTRAP is a novel component of the non-canonical TRAF6-TAK1 TGF-β signaling pathway

Transforming growth factor-β (TGF-β) principally relays its effects through the Smad pathway however, accumulating evidence indicate that alternative signaling routes are also employed by this pleiotropic cytokine. For instance recently, we have demonstrated that ligand occupied TGF-β receptors can directly trigger the TRAF6-TAK1 signaling module, resulting in MAP kinase activation. Here we report identification of the adaptor molecule TTRAP as a novel component of this non-canonical TGF-β pathway. We show that the protein associates with TGF-β receptors and components of the TRAF6-TAK1 signaling module, resulting in differential regulation of TGF-β activated p38 and NF-κB responses. Modulation of cellular TTRAP level affects cell viability in the presence of TGF-β, suggesting that the protein is an important component of the TGF-β induced apoptotic process.

A small peptide modeled after the NRAGE repeat domain inhibits XIAP-TAB1-TAK1 signaling for NF-κB activation and apoptosis in P19 cells

In normal growth and development, apoptosis is necessary to shape the central nervous system and to eliminate excess neurons which are not required for innervation. In some diseases, however, apoptosis can be either overactive as in some neurodegenerative disorders or severely attenuated as in the spread of certain cancers. Bone morphogenetic proteins (BMPs) transmit signals for regulating cell growth, differentiation, and apoptosis. Responding to BMP receptors stimulated from BMP ligands, neurotrophin receptor-mediated MAGE homolog (NRAGE) binds and functions with the XIAP-TAK1-TAB1 complex to activate p38^MAPK and induces apoptosis in cortical neural progenitors. NRAGE contains a unique repeat domain that is only found in human, mouse, and rat homologs that we theorize is pivotal in its BMP MAPK role. Previously, we showed that deletion of the repeat domain inhibits apoptosis, p38^MAPK phosphorylation, and caspase-3 cleavage in P19 neural progenitor cells. We also showed that the XIAP-TAB1-TAK1 complex is dependent on NRAGE for IKK-α/β phosphorylation and NF-κB activation. XIAP is a major inhibitor of caspases, the main executioners of apoptosis. Although it has been shown previously that NRAGE binds to the RING domain of XIAP, it has not been determined which NRAGE domain binds to XIAP. Here, we used fluorescence resonance energy transfer (FRET) to determine that there is a strong likelihood of a direct interaction between NRAGE and XIAP occurring at NRAGE's unique repeat domain which we also attribute to be the domain responsible for downstream signaling of NF-κB and activating IKK subunits. From these results, we designed a small peptide modeled after the NRAGE repeat domain which we have determined inhibits NF-κB activation and apoptosis in P19 cells. These intriguing results illustrate that the paradigm of the NRAGE repeat domain may hold promising therapeutic strategies in developing pharmaceutical solutions for combating harmful diseases involving excessive downstream BMP signaling, including apoptosis.

T-cell receptor-induced JNK activation requires proteolytic inactivation of CYLD by MALT1

The paracaspase mucosa-associated lymphoid tissue 1 (MALT1) is central to lymphocyte activation and lymphomagenesis. MALT1 mediates antigen receptor signalling to NF-κB by acting as a scaffold protein. Furthermore, MALT1 has proteolytic activity that contributes to optimal NF-κB activation by cleaving the NF-κB inhibitor A20. Whether MALT1 protease activity is involved in other signalling pathways, and the identity of the relevant substrates, is unknown. Here, we show that T-cell receptors (TCR) activation, as well as overexpression of the oncogenic API2-MALT1 fusion protein, results in proteolytic inactivation of CYLD by MALT1, which is specifically required for c-jun N-terminal kinase (JNK) activation and the inducible expression of a subset of genes. These results indicate a novel role for MALT1 proteolytic activity in TCR-induced JNK activation and reveal CYLD cleavage as the underlying mechanism.

Requirement for Siva-1 for replication of influenza A virus through apoptosis induction

Infection with influenza A virus causes acute respiratory tract infections in humans and may lead to lethal diseases including pneumonia. Identifying host factors that are involved in the severity of infectious diseases caused by influenza A virus is considered important for the prevention and treatment of these viral infections. This report demonstrated that Siva-1 is crucial for the induction of apoptosis caused by infection with influenza A virus and is involved in virus replication. Susceptibility to apoptosis induced by influenza A virus infection was increased in human lung-derived A549 cells, which stably express Siva-1. In addition, induction of apoptosis after influenza A virus infection was strongly inhibited by knockdown of Siva-1 expression. Furthermore, the replication of influenza A virus was significantly suppressed in A549 cells in which Siva-1 expression was inhibited and the effect of Siva-1 knockdown was eliminated by treatment with Z-VAD-FMK. These findings suggest that the caspase-dependent pathway for induction of apoptosis is involved in Siva-1-mediated influenza A virus replication.

IAP regulation of metastasis

Inhibitor-of-Apoptosis (IAP) proteins contribute to tumor progression, but the requirements of this pathway are not understood. Here, we show that intermolecular cooperation between XIAP and survivin stimulates tumor cell invasion and promotes metastasis. This pathway is independent of IAP inhibition of cell death. Instead, a survivin-XIAP complex activates NF-kappaB, which in turn leads to increased fibronectin gene expression, signaling by beta1 integrins, and activation of cell motility kinases FAK and Src. Therefore, IAPs are direct metastasis genes, and their antagonists could provide antimetastatic therapies in patients with cancer.