Evidence that HAX-1 is an interleukin-1 alpha N-terminal binding protein

HAX-1 interferes in assembly of NLRP3-ASC to block microglial pyroptosis in cerebral I/R injury

Acute cerebral ischemia has a high rate of disability and death. Although timely recanalization therapy may rescue the ischemic brain tissue, cerebral ischemia-reperfusion injury has been shown to limit the therapeutic effects of vascular recanalization. Protein HAX-1 has been reported as a pro-survival protein that plays an important role in various disorders, particularly in association with the nervous system. However, the effects and mechanisms of HAX-1 in cerebral IR injury have yet to be elucidated. So, we aimed to investigate the effect of HAX-1 on microglial pyroptosis and explore its potential neuroprotective effects in ischemia-reperfusion injury. Our results show that the expression of HAX-1 decreased after cerebral IR injury, accompanied by an increase in pyroptosis pathway activation. In addition, HAX-1 could inhibit microglial pyroptosis both in vivo and in vitro and reduce the release of inflammatory mediators. The above neuroprotective effects might be partially mediated by inhibiting of interaction of NLRP3 and ASC through competitive binding, followed by the attenuation of NLRP3 inflammasome formation. In conclusion, Our findings support that HAX-1 exhibits a protective role in cerebral I/R injury, and further study on HAX-1 expression regulation will contribute to cerebral infarction therapy.

Analysis of the effects of importin α1 on the nuclear translocation of IL-1α in HeLa cells

Interleukin-1α (IL-1α), a cytokine released by necrotic cells, causes sterile inflammation. On the other hand, IL-1α is present in the nucleus and also regulates the expression of many proteins. A protein substrate containing a classical nuclear localization signal (cNLS) typically forms a substrate/importin α/β complex, which is subsequently transported to the nucleus. To the best of our knowledge, no study has directly investigated whether IL-1α-which includes cNLS-is imported into the nucleus in an importin α/β-dependent manner. In this study, we noted that all detected importin α subtypes interacted with IL-1α. In HeLa cells, importin α1-mediated nuclear translocation of IL-1α occurred at steady state and was independent of importin β1. Importin α1 not only was engaged in IL-1α nuclear transport but also concurrently functioned as a molecule that regulated IL-1α protein level in the cell. Furthermore, we discussed the underlying mechanism of IL-1α nuclear translocation by importin α1 based on our findings.

Interleukin-1α: Novel functions in cell senescence and antiviral response

The interleukin-1 proteins are a hub of innate inflammatory signaling that activates diverse aspects of adaptive immunity. Until recently, the IL-1α isoform was relatively incompletely understood compared with IL-1β. This review briefly summarizes novel and surprising aspects of IL-1α biology. IL-1α localizes to the nucleus, cytoplasm, mitochondria, cell membrane or extracellular space in various contexts, with corresponding distinct functions. In particular, we focus on multiple pathways by which IL-1α promotes the senescent cell phenotype, unexpectedly involving signaling molecules including mTOR, GATA4, mitochondrial cardiolipin and caspases-4/5. Finally, I review a novel pathway by which IL-1α promotes antiviral immunity.

IL-1α Processing, Signaling and Its Role in Cancer Progression

Interleukin-1α (IL-1α) is a major alarmin cytokine which triggers and boosts the inflammatory responses. Since its discovery in the 1940s, the structure and bioactivity of IL-1α has been extensively studied and emerged as a vital regulator in inflammation and hematopoiesis. IL-1α is translated as a pro-form with minor bioactivity. The pro-IL-1α can be cleaved by several proteases to generate the N terminal and C terminal form of IL-1α. The C terminal form of IL-1α (mature form) has several folds higher bioactivity compared with its pro-form. IL-1α is a unique cytokine which could localize in the cytosol, membrane, nucleus, as well as being secreted out of the cell. However, the processing mechanism and physiological significance of these differentially localized IL-1α are still largely unknown. Accumulating evidence suggests IL-1α is involved in cancer pathogenesis. The role of IL-1α in cancer development is controversial as it exerts both pro- and anti-tumor roles in different cancer types. Here, we review the recent development in the processing and signaling of IL-1α and summarize the functions of IL-1α in cancer development.

Recruitment of pro-IL-1α to mitochondrial cardiolipin, via shared LC3 binding domain, inhibits mitophagy and drives maximal NLRP3 activation

The balance between NLRP3 inflammasome activation and mitophagy is essential for homeostasis and cellular health, but this relationship remains poorly understood. Here we found that interleukin-1α (IL-1α)-deficient macrophages have reduced caspase-1 activity and diminished IL-1β release, concurrent with reduced mitochondrial damage, suggesting a role for IL-1α in regulating this balance. LPS priming of macrophages induced pro-IL-1α translocation to mitochondria, where it directly interacted with mitochondrial cardiolipin (CL). Computational modeling revealed a likely CL binding motif in pro-IL-1α, similar to that found in LC3b. Thus, binding of pro-IL-1α to CL in activated macrophages may interrupt CL-LC3b-dependent mitophagy, leading to enhanced Nlrp3 inflammasome activation and more robust IL-1β production. Mutation of pro-IL-1α residues predicted to be involved in CL binding resulted in reduced pro-IL-1α-CL interaction, a reduction in NLRP3 inflammasome activity, and increased mitophagy. These data identify a function for pro-IL-1α in regulating mitophagy and the potency of NLRP3 inflammasome activation.

Interleukin-38 interacts with destrin/actin-depolymerizing factor in human keratinocytes

Interleukin (IL)-38 is a member of the IL-1 family of cytokines, which was proposed to exert anti-inflammatory effects. IL-38 is constitutively expressed in the skin, where keratinocytes are the main producing cells. Little information is currently available concerning IL-38 biology. Here, we investigated the subcellular localization and interaction partners of the IL-38 protein in human keratinocytes. IL-38 expression was reduced in primary keratinocytes grown in monolayer (2D) cultures. We thus used IL-38 overexpressing immortalized normal human keratinocytes (NHK/38) to study this cytokine in cell monolayers. In parallel, differentiation of primary human keratinocytes in an in vitro reconstructed human epidermis (RHE) 3D model allowed us to restore endogenous IL-38 expression. In NHK/38 cells and in RHE, IL-38 was mainly cell-associated, rather than released into culture supernatants. Intracellular IL-38 was preferentially, although not exclusively, cytoplasmic. Similarly, in normal human skin sections, IL-38 was predominantly cytoplasmic in the epidermis and essentially excluded from keratinocyte nuclei. A yeast two-hybrid screen identified destrin/actin-depolymerizing factor (DSTN) as a potential IL-38-interacting molecule. Co-immunoprecipitation and proximity ligation assay confirmed this interaction. We further observed partial co-localization of IL-38 and DSTN in NHK/38 cells. Endogenous IL-38 and DSTN were also co-expressed in all epidermal layers in RHE and in normal human skin. Finally, IL-38 partially co-localized with F-actin in NHK/38 cells, in particular along the cortical actin network and in filopodia. In conclusion, IL-38 is found predominantly in the cytoplasm of human keratinocytes, where it interacts with DSTN. The functional relevance of this interaction remains to be investigated.

Redefining the ancestral origins of the interleukin-1 superfamily

The interleukin-1 (IL-1) receptor and ligand families are components of the immune system. Knowledge of their evolutionary history is essential to understand their function. Using chromosomal anatomy and sequence similarity, we show that IL-1 receptor family members are related and nine members are likely formed from duplication and modification of a proto-IL-1R1 receptor. The IL-1 ligands have a different evolutionary history. The first proto-IL-1β gene coincided with proto-IL-1R1 and duplication events resulted in the majority of IL-1 ligand family members. However, large evolutionary distances are observed for IL-1α, IL-18 and IL-33 proteins. Further analysis show that IL-33 and IL-18 have poor sequence similarity and no chromosomal evidence of common ancestry with the IL-1β cluster and therefore should not be included in the IL-1 ligand ancestral family. IL-1α formed from the duplication of IL-1β, and moonlighting functions of pro-IL-1α acted as divergent selection pressures for the observed sequence dissimilarity.

Critical Role of HAX-1 in Promoting Avian Influenza Virus Replication in Lung Epithelial Cells

The PB1-F2 protein of influenza A virus has been considered a virulence factor, but its function in inducing apoptosis may be of disadvantage to viral replication. Host mechanisms to regulate PB1-F2-induced apoptosis remain unknown. We generated a PB1-F2-deficient avian influenza virus (AIV) H9N2 and found that the mutant virus replicated less efficiently in human lung epithelial cells. The PB1-F2-deficient virus produced less apoptotic cells, indicating that PB1-F2 of the H9N2 virus promotes apoptosis, occurring at the early stage of infection, in the lung epithelial cells. To understand how host cells regulate PB1-F2-induced apoptosis, we explored to identify cellular proteins interacting with PB1-F2 and found that HCLS1-associated protein X-1 (HAX-1), located mainly in the mitochondria as an apoptotic inhibitor, interacted with PB1-F2. Increased procaspase-9 activations, induced by PB1-F2, could be suppressed by HAX-1. In HAX-1 knockdown A549 cells, the replication of AIV H9N2 was suppressed in parallel to the activation of caspase-3 activation, which increased at the early stage of infection. We hypothesize that HAX-1 promotes AIV replication by interacting with PB1-F2, resulting in the suppression of apoptosis, prolonged cell survival, and enhancement of viral replication. Our data suggest that HAX-1 may be a promoting factor for AIV H9N2 replication through desensitizing PB1-F2 from its apoptotic induction in human lung epithelial cells.

HMGB1, IL-1α, IL-33 and S100 proteins: dual-function alarmins

Our immune system is based on the close collaboration of the innate and adaptive immune systems for the rapid detection of any threats to the host. Recognition of pathogen-derived molecules is entrusted to specific germline-encoded signaling receptors. The same receptors have now also emerged as efficient detectors of misplaced or altered self-molecules that signal tissue damage and cell death following, for example, disruption of the blood supply and subsequent hypoxia. Many types of endogenous molecules have been shown to provoke such sterile inflammatory states when released from dying cells. However, a group of proteins referred to as alarmins have both intracellular and extracellular functions which have been the subject of intense research. Indeed, alarmins can either exert beneficial cell housekeeping functions, leading to tissue repair, or provoke deleterious uncontrolled inflammation. This group of proteins includes the high-mobility group box 1 protein (HMGB1), interleukin (IL)-1α, IL-33 and the Ca²⁺-binding S100 proteins. These dual-function proteins share conserved regulatory mechanisms, such as secretory routes, post-translational modifications and enzymatic processing, that govern their extracellular functions in time and space. Release of alarmins from mesenchymal cells is a highly relevant mechanism by which immune cells can be alerted of tissue damage, and alarmins play a key role in the development of acute or chronic inflammatory diseases and in cancer development.

Cell membrane penetrating function of the nuclear localization sequence in human cytokine IL-1α

Cytokines are released from the cell, bind to their receptors, and affect cellular responses. The precursor form of interleukin 1 alpha (pIL-1α) has a nuclear localization sequence (NLS) that causes it to be localized to the nucleus and regulate specific gene expression. The amino acids of the NLS are basic amino acid-rich sequences, as is the cell penetrating peptide (CPP), which has been widely studied as a way to deliver macromolecules into cells. Here, we hypothesized that the NLS in pIL-1α (pIL-1αNLS) can penetrate the cell membrane and it could deliver macromolecules such as protein in vivo. We characterized cell membrane penetration ability of pIL-1αNLS or its tandem repeated form (2pIL-1αNLS) to enhance its intracellular delivery efficiency. 2pIL-1αNLS showed comparable protein delivery efficiency to TAT-CPP and it mediates endocytosis following heparan sulfate interaction. 2pIL-1αNLS conjugated enhanced green fluorescence protein was localized to the nucleus and the cytoplasm. Intra-peritoneal administration of 2pIL-1αNLS conjugated dTomato protein showed remarkable in vivo intracellular delivery efficiency in various tissues including spleen, liver, and intestine in mice. Moreover, cytotoxicity of 2pIL-1αNLS was not observed even at 100 μM. Our results demonstrate cell membrane-penetrating function of NLS in pIL-1α, which can be used as a safe therapeutic macromolecular delivery peptide.

New Insights in the Immunobiology of IL-1 Family Members

The interleukin-1 (IL 1) family of ligands is associated with acute and chronic inflammation, and plays an essential role in the non-specific innate response to infection. The biological properties of IL 1 family ligands are typically pro-inflammatory. The IL 1 family has 11 family members and can be categorized into subfamilies according to the length of their precursor and the length of the propiece for each precursor (Figure 1). The IL 1 subfamily consists of IL 1α, IL 1β, and IL 33, with the longest propieces of the IL 1 family. IL 18 and IL 37 belong to the IL 18 subfamily and contain smaller propieces than IL 1 and IL-33. Since IL 37 binds to the IL 18Rα chain it is part of the IL 18 subfamily, however it remains to be elucidated how the propiece of IL 37 is removed. IL 36α, β, and γ as well as IL 36 Ra belong to the IL 36 subfamily. In addition, IL 38 likely belongs to this family since it has the ability to bind to the IL 36R. The IL 36 subfamily has the shortest propiece. The one member of the IL 1 family that cannot be categorized in these subfamilies is IL 1 receptor antagonist (IL 1Ra), which has a signal peptide and is readily secreted. In the present review we will describe the biological functions of the IL-1F members and new insights in their biology.

Endogenous IL-1α is a chromatin-associated protein in mouse macrophages

The cytokine interleukin-1α (IL-1α) is synthesized as a 31kDa peptide that lacks a leader peptide and is not secreted by the conventional secretory pathway. A distinctive characteristic of pro-IL-1α is the presence of a nuclear localization sequence in its amino-terminal moiety that allows its translocation to the nucleus. However no nuclear function(s) of the endogenous pro-IL-1α has been reported to date. In the present study, we used murine macrophages that produce IL-1α in response to pro-inflammatory stimuli, to gain further insight into the biology of the endogenous IL-1α protein in innate immune cells. We show that endogenous IL-1α is essentially found as a chromatin-associated nuclear protein in LPS-stimulated macrophages. In contrast to IL-1β, IL-1α was not released upon inflammasome activation unless significant cell damage occurred. IL-1β mRNA and protein levels were specifically decreased in IL-1α deficient macrophages after LPS stimulation. However, overexpression of human pro-IL-1α did not rescue this defective IL-1β production, suggesting that this finding might be related to the insertion of the targeting construct into the IL-1 locus, rather than to a specific nuclear function of pro-IL-1α. Finally, by using both genomic and proteomic approaches, we could not identify a nuclear function of IL-1α. Taken together, these observations suggest that in macrophages IL-1α primarily acts as an alarmin that is rapidly released upon cell damage to activate early mechanisms of host defense.

N-terminal domain of nuclear IL-1α shows structural similarity to the C-terminal domain of Snf1 and binds to the HAT/core module of the SAGA complex

Interleukin-1α (IL-1α) is a proinflammatory cytokine and a key player in host immune responses in higher eukaryotes. IL-1α has pleiotropic effects on a wide range of cell types, and it has been extensively studied for its ability to contribute to various autoimmune and inflammation-linked disorders, including rheumatoid arthritis, Alzheimer’s disease, systemic sclerosis and cardiovascular disorders. Interestingly, a significant proportion of IL-1α is translocated to the cell nucleus, in which it interacts with histone acetyltransferase complexes. Despite the importance of IL-1α, little is known regarding its binding targets and functions in the nucleus. We took advantage of the histone acetyltransferase (HAT) complexes being evolutionarily conserved from yeast to humans and the yeast SAGA complex serving as an epitome of the eukaryotic HAT complexes. Using gene knock-out technique and co-immunoprecipitation of the IL-1α precursor with TAP-tagged subunits of the yeast HAT complexes, we mapped the IL-1α-binding site to the HAT/Core module of the SAGA complex. We also predicted the 3-D structure of the IL-1α N-terminal domain, and by employing structure similarity searches, we found a similar structure in the C-terminal regulatory region of the catalytic subunit of the AMP-activated/Snf1 protein kinases, which interact with HAT complexes both in mammals and yeast, respectively. This finding is further supported with the ability of the IL-1α precursor to partially rescue growth defects of snf1Δ yeast strains on media containing 3-Amino-1,2,4-triazole (3-AT), a competitive inhibitor of His3. Finally, the careful evaluation of our data together with other published data in the field allows us to hypothesize a new function for the ADA complex in SAGA complex assembly.

The biology of interleukin-1: emerging concepts in the regulation of the actin cytoskeleton and cell junction dynamics

Interleukin (IL)-1 is a proinflammatory cytokine with important roles in innate immunity, as well as in normal tissue homeostasis. Interestingly, recent studies have also shown IL-1 to function in the dynamics of the actin cytoskeleton and cell junctions. For example, treatment of different epithelia with IL-1α often results in the restructuring of the actin network and cell junctions, thereby leading to junction disassembly. In this review, we highlight new and interesting findings that show IL-1 to be a critical player of restructuring events in the seminiferous epithelium of the testis during spermatogenesis.

The classical swine fever virus N-terminal protease N(pro) binds to cellular HAX-1

The positive-stranded RNA genome of classical swine fever virus (CSFV) encodes 12 known proteins. The first protein to be translated is the N-terminal protease (N(pro)). N(pro) helps evade the innate interferon response by targeting interferon regulatory factor-3 for proteasomal degradation and also participates in the evasion of dsRNA-induced apoptosis. To elucidate the mechanisms by which N(pro) functions, we performed a yeast two-hybrid screen in which the anti-apoptotic protein HAX-1 was identified. The N(pro)-HAX-1 interaction was confirmed using co-precipitation assays. A dramatic redistribution of both N(pro) and HAX-1 was observed in co-transfected cells, as well as in transfected cells infected with wild-type CSFV, but not in cells infected with an N(pro)-deleted CSFV strain.

Dual functionality of interleukin-1 family cytokines: implications for anti-interleukin-1 therapy

Dysregulated inflammation contributes to disease pathogenesis in both the periphery and the brain. Cytokines are coordinators of inflammation and were originally defined as secreted mediators, released from expressing cells to activate plasma membrane receptors on responsive cells. However, a group of cytokines is now recognized as having dual functionality. In addition to their extracellular effects, these cytokines act inside the nuclei of cytokine-expressing or cytokine-responsive cells. Interleukin-1 (IL-1) family cytokines are key pro-inflammatory mediators, and blockade of the IL-1 system in inflammatory diseases is an attractive therapeutic goal. All current therapies target IL-1 extracellular actions. Here we review evidence that suggests IL-1 family members have dual functionality. Several IL-1 family members have been detected inside the nuclei of IL-1-expressing or IL-1-responsive cells, and intranuclear IL-1 is reported to regulate gene transcription and mRNA splicing. However, further work is required to determine the impact of IL-1 intranuclear actions on disease pathogenesis. The intranuclear actions of IL-1 family members represent a new and potentially important area of IL-1 biology and may have implications for the future development of anti-IL-1 therapies.

Immunological and inflammatory functions of the interleukin-1 family

More than any other cytokine family, the interleukin (IL)-1 family is closely linked to the innate immune response. This linkage became evident upon the discovery that the cytoplasmic domain of the IL-1 receptor type I is highly homologous to the cytoplasmic domains of all Toll-like receptors (TLRs). Thus, fundamental inflammatory responses such as the induction of cyclooxygenase type 2, increased expression of adhesion molecules, or synthesis of nitric oxide are indistinguishable responses of both IL-1 and TLR ligands. Both families nonspecifically affect antigen recognition and lymphocyte function. IL-1beta is the most studied member of the IL-1 family because of its role in mediating autoinflammatory diseases. Although the TLR and IL-1 families evolved to assist in host defense against infection, unlike the TLR family, the IL-1 family also includes members that suppress inflammation, both specifically within the IL-1 family but also nonspecifically for TLR ligands and the innate immune response.

Hairpin structure within the 3'UTR of DNA polymerase beta mRNA acts as a post-transcriptional regulatory element and interacts with Hax-1

Aberrant expression of DNA polymerase beta, a key enzyme involved in base excision repair, leads to genetic instability and carcinogenesis. Pol beta expression has been previously shown to be regulated at the level of transcription, but there is also evidence of post-transcriptional regulation, since rat transcripts undergo alternative polyadenylation, and the resulting 3'UTR contain at least one regulatory element. Data presented here indicate that RNA of the short 3'UTR folds to form a strong secondary structure (hairpin). Its regulatory role was established utilizing a luciferase-based reporter system. Further studies led to the identification of a protein factor, which binds to this element-the anti-apoptotic, cytoskeleton-related protein Hax-1. The results of in vitro binding analysis indicate that the formation of the RNA-protein complex is significantly impaired by disruption of the hairpin motif. We demonstrate that Hax-1 binds to Pol beta mRNA exclusively in the form of a dimer. Biochemical analysis revealed the presence of Hax-1 in mitochondria, but also in the nuclear matrix, which, along with its transcript-binding properties, suggests that Hax-1 plays a role in post-transcriptional regulation of expression of Pol beta.

Characterization of KIAA0513, a novel signaling molecule that interacts with modulators of neuroplasticity, apoptosis, and the cytoskeleton

KIAA0513 was previously identified as upregulated in the dorsolateral prefrontal cortex of subjects with schizophrenia by microarray analysis. In the present study, the differential expression in the schizophrenic subjects was confirmed by quantitative RT-PCR. The limited homology to proteins of known function and lack of functional domains in the encoded protein have made it difficult to predict a function for KIAA0513. We used in situ hybridization, RNA blots, western blots, and immunocytochemistry to examine KIAA0513 expression in normal brain and peripheral tissues. The gene is ubiquitously expressed but is enriched in the brain, particularly in the cerebellum. Finally, interacting proteins were identified using a yeast two-hybrid screen to functionally characterize the protein. KIAA0513 interacts with KIBRA, HAX-1, and INTS4, which also interact with proteins involved in neuroplasticity, apoptosis, and cytoskeletal regulation. Therefore, KIAA0513 is likely to be involved in signaling pathways related to these processes.

Intracellular IL-1alpha-binding proteins contribute to biological functions of endogenous IL-1alpha in systemic sclerosis fibroblasts

The aberrant production of precursor IL-1alpha (pre-IL-1alpha) in skin fibroblasts that are derived from systemic sclerosis (SSc) is associated with the induction of IL-6 and procollagen, which contributes to the fibrosis of SSc. However, little is understood about how intracellular pre-IL-1alpha regulates the expression of the other molecules in fibroblasts. We report here that pre-IL-1alpha can form a complex with IL-1alpha-binding proteins that is translocated into the nuclei of fibroblasts. Immunoprecipitation that used anti-human IL-1alpha Ab and (35)S-labeled nuclear extracts of fibroblasts showed three specific bands (approximately equal to 31, 35, and 65 kDa). The 31-kDa molecule was identified as pre-IL-1alpha, and the 35- and 65-kDa molecules might be pre-IL-1alpha-binding proteins. A partial sequencing for the 10 aa from the N-terminals of the molecules showed 100% homology for HAX-1 (HS1-associated protein X-1) and IL-1 receptor type II (IL-1RII). Suppression of the genes of HAX-1 or IL-1RII induced the inhibitory effects of IL-1 signal transduction, including production of IL-6 and procollagen, by fibroblasts. In particular, pre-IL-1alpha was not translocated into the nucleus by an inhibition of HAX-1. These findings reveal that nuclear localization of pre-IL-1alpha depends on the binding to HAX-1 and that biological activities might be elicited by the binding to both HAX-1 and IL-1RII in SSc fibroblasts.

Expression and tissue distribution of mouse Hax1

HAX1 is an ubiquitously expressed human gene. Though a number of cellular and viral proteins are known to interact with HAX1, its function is still not completely understood. On the basis of these identified interaction partners, HAX1 seems to play a role in apoptosis and the organization of the cytoskeleton. The cDNAs for human and mouse Hax1 share 86% identity and 80% identity at the protein level, suggesting a similar functional importance. To date, no conclusive data on the tissue specific expression of the murine Hax1 are available and only one interaction partner has been identified. Here, we show a detailed expression analysis for the murine ortholog by RT-PCR, Northern and Western blot. Furthermore, the distribution of Hax1 within different mouse tissues was studied by immunohistochemistry (IHC). In general, we found a good correlation between the results obtained from different detection techniques. Similar to its human counterpart, mouse Hax1 seems to be ubiquitously expressed. At the RNA level, we found the highest expression of Hax1 in liver, kidney and testis. In sharp contrast to the human HAX1 which is highly expressed in skeletal muscle, the mouse ortholog was detected only at very low levels. Using a specific antibody, we detected Hax1 in the majority of mouse tissues by IHC. Interestingly, the most prominent expression of Hax1 was found in epithelial, endothelial and muscle cells. Surprisingly, thymus, spleen and pancreas did not show detectable immunostaining. Furthermore, we have studied the subcellular localisation of Hax1 in a keratinocyte and a neuronal cell line by immunofluorescence. We found Hax1 to be localised mainly in the cytoplasm and detected a partial colocalisation with mitochondria. The results presented here summarize for the first time the expression of the murine Hax1 in different tissues and two cell lines. Further studies will elucidate the functional importance of this protein in individual cell types with respect to structural aspects, cell mobility and apoptosis.

Overexpression of HAX-1 Protects Cardiac Myocytes From Apoptosis Through Caspase-9 Inhibition

Caspase-9 is a critical regulator of mitochondria-mediated apoptosis. We found that adult cardiac myocytes, but not nonmyocytes, have high caspase-9 expression, and exhibit relative resistance to caspase-9–induced cell death. Thus, we hypothesized that cardiac myocytes possess factors that resist apoptosis. Through a yeast two-hybrid screening of adult human heart cDNA library, we identified HS-1 associated protein-1 (HAX-1), a 35-kD BH-domain containing protein localized to the mitochondria as one of the molecules that interacts with caspase-9. Recombinant HAX-1 protein inhibited caspase-9 processing in a dose-dependent manner in a cell-free caspase activation assay. Overexpression of HAX-1 in adult cardiac myocytes conferred 30% protection from apoptosis as compared with the control. Suppression of HAX-1 expression using siRNA-HAX-1 resulted in significant cell death in adult cardiac myocytes, suggesting the importance of HAX-1 in cardiac myocyte resistance to apoptotic stimulation. On apoptotic stimulation, some caspase-9 translocated to the mitochondria and co-localized with HAX-1, confirming the spatial proximity of caspase-9 and HAX-1. In summary, HAX-1 is a newly identified anti-apoptotic factor and its mechanism of action is through caspase-9 inhibition.

HS1-associated protein X-1 interacts with membrane-bound IgE: impact on receptor-mediated internalization

Engagement of the BCR triggers signals that control affinity maturation, memory induction, differentiation, and various other physiological processes in B cells. In previous work, we showed that truncation of the cytoplasmic tail of membrane-bound Ig (mIg)E in vivo resulted in lower serum IgE levels, decreased numbers of IgE-secreting plasma cells, and the abrogation of specific secondary responses correlating with a defect in the selection of high-affinity Abs during the germinal center reaction. We concluded that the Ag receptor is necessary at all times during Ab responses not only for the maturation process, but also for the expansion of Ag-specific B cells. Based on these results, we asked whether the cytoplasmic tail of mIgE, or specific proteins binding the cytoplasmic tail in vivo commit a signal transduction accompanying the B cell along its differentiation process. In this study, we present the identification of HS1-associated protein X-1 as a novel protein interacting with the cytoplasmic tail of mIgE. ELISA, surface plasmon resonance analysis, and coimmunoprecipitation experiments confirmed the specific interaction in vitro. In functional assays, we clearly showed that HS1-associated protein X-1 expression levels influence the efficiency of BCR-mediated Ag internalization.

Identification and expression analysis of alternative splice variants of the rat Hax-1 gene

Hax-1 protein, which has been studied in mice and humans, shows a potent anti-apoptotic activity and is involved in regulation of cell motility. Cloning of the rat Hax-1 cDNA has revealed seven alternative transcripts, which differ mostly in their 5' region. Alternative splicing concerns exon 1, skipped in 5 transcripts, intron 1 which is partially retained in these transcripts, exon 2, which can be partially skipped, and intron 2, retained in one variant. The existence of different splicing variants was confirmed by exon-junction-specific RT-PCR and RNase protection assay. Analysis of expression indicates that overall Hax-1 mRNA level is relatively low in most tissues and very high in testes, and that the expression pattern of the variants is similar in different tissues. Presence of different transcripts implies the existence of several protein isoforms, with three putative start codons. The existence of at least three protein isoforms was confirmed by Western blot. Interestingly, high mRNA level in testes does not translate into high protein level, suggesting the existence of tissue-specific translational regulation or regulated protein degradation.

Intracellular interleukin-1 receptor antagonist type 1 antagonizes the stimulatory effect of interleukin-1α precursor on cell motility

Interleukin (IL)-1alpha, a proinflammatory cytokine, is produced as a 33 kDa protein precursor (preIL-1alpha) which is cleaved to generate the 17 kDa C-terminal mature IL-1alpha (mIL-1alpha) and the 16kDa N-terminal IL-1alpha propiece (NIL-1alpha). The biological effect of IL-1alpha is regulated by the IL-1 receptor antagonist (IL-1Ra), its naturally occurring inhibitor. Four different isoforms of the IL-1Ra have been described, one secreted (sIL-1Ra) and three intracellular (icIL-1Ra1, 2, 3). Whether the icIL-1Ra1 isoform can antagonize some of the biological effects of intracellular IL-1alpha is still unknown. The aim of this study is to investigate effects of preIL-1alpha and icIL-1Ra1 on cell motility in stably transfected ECV304 cells. We show that expression of preIL-1alpha in ECV304 cells significantly increases cell motility. Furthermore, transfection with NIL-1alpha propiece also increases cell motility whereas this stimulatory effect was not observed by addition of exogenous mIL-1alpha, suggesting an intracellular effect of preIL-1alpha mediated by NIL-1alpha propiece. Co-transfection of ECV304 cells with icIL-1Ra1 completely antagonizes the stimulatory effect of preIL-1alpha and NIL-1alpha propiece on cell motility. In conclusion, NIL-1alpha propiece increases ECV304 cell motility and icIL-1Ra1 exerts intracellular functions regulating this stimulatory effect.

Identification of HAX-1 as a protein that binds bile salt export protein and regulates its abundance in the apical membrane of Madin-Darby canine kidney cells

ATP-binding cassette (ABC)-type proteins are essential for bile formation in vertebrate liver. BSEP, MDR1, MDR2, and MRP2 ABC transporters are targeted to the apical (canalicular) membrane of hepatocytes where they execute ATP-dependent transport of bile acids, drugs, amphipathic cations, phospholipids, and conjugated organic anions, respectively. Changes in activity and abundance of transporters in the canalicular membrane regulate bile flow; however, little is known regarding cellular proteins that bind ABC transporters and regulate their trafficking. A yeast two-hybrid screen identified HAX-1 as a binding partner for BSEP, MDR1, and MDR2. The interactions were validated biochemically by glutathione S-transferase pull-down and co-immunoprecipitation assays. BSEP and HAX-1 were over-represented in rat liver subcellular fractions enriched for canalicular membrane vesicles, microsomes, and clathrin-coated vesicles. HAX-1 was bound to BSEP, MDR1, and MDR2 in canalicular membrane vesicles and co-localized with BSEP and MDR1 in the apical membrane of Madin-Darby canine kidney (MDCK) cells. RNA interference of HAX-1 increased BSEP levels in the apical membrane of MDCK cells by 71%. Pulse-chase studies indicated that HAX-1 depletion did not affect BSEP translation, post-translational modification, delivery to the plasma membrane, or half-life. HAX-1 depletion resulted in an increased peak of metabolically labeled apical membrane BSEP at 4 h and enhanced retention at 6 and 9 h. HAX-1 also interacts with cortactin. Expression of dominant negative cortactin increased steady state levels of BSEP 2-fold in the apical membrane of MDCK cells, as did expression of dominant negative EPS15. These findings suggest that HAX-1 and cortactin participate in BSEP internalization from the apical membrane.

The precursor form of IL-1alpha is an intracrine proinflammatory activator of transcription

Although most cytokines are studied for biological effects after engagement of their specific cell surface membrane receptors, increasing evidence suggests that some function in the nucleus. In the present study, the precursor form of IL-1alpha was overexpressed in various cells and assessed for activity in the presence of saturating concentrations of IL-1 receptor antagonist to prevent receptor signaling. Initially diffusely present in the cytoplasm of resting cells, IL-1alpha translocated to the to nucleus after activation by endotoxin, a Toll-like receptor ligand. The IL-1alpha precursor, but not the C-terminal mature form, activated the transcriptional machinery in the GAL4 system by 90-fold; a 50-fold increase was observed using only the IL-1alpha propiece, suggesting that transcriptional activation was localized to the N terminus where the nuclear localization sequence resides. Under conditions of IL-1 receptor blockade, intracellular overexpression of the precursor and propiece forms of IL-1alpha were sufficient to activate NF-kappaB and AP-1. Stable transfectants overproducing precursor IL-1alpha released the cytokines IL-8 and IL-6 but also exhibited a significantly lower threshold of activation to subpicomolar concentrations of tumor necrosis factor alpha or IFN-gamma. Thus, intracellular functions of IL-1alpha might play an unforeseen role in the genesis of inflammation. During disease-driven events, the cytosolic precursor moves to the nucleus, where it augments transcription of proinflammatory genes. Because this mechanism of action is not affected by extracellular inhibitors, reducing intracellular functions of IL-1alpha might prove beneficial in some inflammatory conditions.

Intracellular interleukin-1alpha functionally interacts with histone acetyltransferase complexes

Interleukin-1alpha (IL-1alpha) is an inflammatory cytokine acting extracellularly via membrane receptors. Interestingly, a significant portion of synthesized IL-1alpha is not secreted; instead, it is actively translocated into the cell nucleus. IL-1alpha was indeed shown to be involved in certain intracellular processes, such as control of proliferation, apoptosis, or migration, however, the mechanisms of such actions are not known. Here we show that intracellular IL-1alpha fused to the Gal4p DNA-binding domain (Gal4BD) possesses strong transactivation potential that can be boosted by overexpression of the transcriptional coactivator p300. We demonstrate that the IL-1alpha precursor interacts via its N-terminal peptide (IL-1NTP) with histone acetyltransferases p300, PCAF, Gcn5 and with the adaptor component Ada3, and that it integrates into the PCAF.p300 complex in a non-destructive manner. In analogy with known acidic coactivators, yeast strains expressing Gal4BD/IL-1NTP display a toxic phenotype that can be relieved by depletion of various components of the SAGA complex. Our data provide the first solid evidence for the nuclear target of the IL-1alpha precursor and suggest its novel function in transcriptional control.

Subtractive hybridisation screen identifies sexually dimorphic gene expression in the embryonic mouse gonad

The sex of most mammals is determined by the action of SRY. Its presence initiates testis formation resulting in male differentiation, its absence results in ovary formation and female differentiation. We have used suppression subtraction hybridisation between 12.0-12.5 days postcoitum (dpc) mouse testes and ovaries to identify genes that potentially lie within the Sry pathway. Normalised urogenital ridge libraries comprising 8,352 clones were differentially screened with subtracted probes. A total of 272 candidate cDNAs were tested for qualitative differential expression and localisation by whole mount in situ hybridisation; germ cell-dependent or -independent expression was further resolved using busulfan. Fifty-four genes were identified that showed higher expression in the testis than the ovary. One novel gene may be a candidate for interactions with WT1, based on its localisation to Sertoli cells and map position (16q24.3).

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) forms complexes with a cellular anti-apoptosis protein Bcl-2 or its EBV counterpart BHRF1 through HS1-associated protein X-1

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) plays a critical role in EBV-induced transformation. An earlier report (Y. Kawaguchi et al., J. Virol. 74: 10104-10111, 2000) showed that EBNA-LP interacts with a cellular protein HS1-associated protein X-1 (HAX-1). The predicted amino acid sequence of HAX-1 exhibits similarity to that of another cellular protein Nip3 which has been shown to interact with cellular and viral anti-apoptotic proteins such as Bcl-2 and BHRF1, an EBV homolog of Bcl-2. Here we investigated whether HAX-1, like Nip3, interacts with Bcl-2 proteins and report the following. (i) A purified chimeric protein consisting of gluthathione S-transferase (GST) fused to BHRF1 (GST-BHRF1) or Bcl-2 (GST-Bcl-2) specifically pulled down HAX-1 transiently expressed in COS-7 cells. (ii) GST-BHRF1 or GST-Bcl-2 was not able to pull down EBNA-LP transiently expressed in COS-7 cells, whereas each of the GST fusion proteins formed complexes with EBNA-LP in the presence of RAX-1. These results indicated that EBNA-LP interacts with the viral and cellular Bcl-2 proteins through HAX-1, suggesting that EBNA-LP possesses a potential function in the regulation of apoptosis in EBV-infected cells.

K15 protein of Kaposi's sarcoma-associated herpesvirus is latently expressed and binds to HAX-1, a protein with antiapoptotic function

The Kaposi's sarcoma-associated herpesvirus (KSHV) (or human herpesvirus 8) open reading frame (ORF) K15 encodes a putative integral transmembrane protein in the same genomic location as latent membrane protein 2A of Epstein-Barr virus. Ectopic expression of K15 in cell lines revealed the presence of several different forms ranging in size from full length, approximately 50 kDa, to 17 kDa. Of these different species the 35- and 23-kDa forms were predominant. Mutational analysis of the initiator AUG indicated that translation initiation from this first AUG is required for K15 expression. Computational analysis indicates that the different forms detected may arise due to proteolytic cleavage at internal signal peptide sites. We show that K15 is latently expressed in KSHV-positive primary effusion lymphoma cell lines and in multicentric Castleman's disease. Using a yeast two-hybrid screen we identified HAX-1 (HS1 associated protein X-1) as a binding partner to the C terminus of K15 and show that K15 interacts with cellular HAX-1 in vitro and in vivo. Furthermore, HAX-1 colocalizes with K15 in the endoplasmic reticulum and mitochondria. The function of HAX-1 is unknown, although the similarity of its sequence to those of Nip3 and Bcl-2 infers a role in the regulation of apoptosis. We show here that HAX-1 can form homodimers in vivo and is a potent inhibitor of apoptosis and therefore represents a new apoptosis regulatory protein. The putative functions of K15 with respect to its interaction with HAX-1 are discussed.