Pleiotropic effects of HTLV type 1 Tax protein on cellular metabolism: mitotic checkpoint abrogation and NF-kappaB activation

Decreased TAX1BP1 participates in systemic lupus erythematosus by regulating monocyte/macrophage function

Systemic lupus erythematosus (SLE) involves disorders of innate and adaptive immune pathways. Tax1-binding protein 1 (TAX1BP1) modulates the production of antibodies in B cells and the T-cell cycle by regulating the NF-κB signaling pathway. However, the potential association of TAX1BP1 with SLE and its role in monocytes/macrophages have not been fully elucidated. In this study, we utilized whole-exome sequencing (WES) in combination with Sanger sequencing and identified 16 gene mutations, including in TAX1BP1, in an SLE family. TAX1BP1 protein expression with western blotting detection was reduced in SLE patients and correlated with disease activity negatively. Furthermore, RNA sequencing and 4D Label-Free Phosphoproteomic analysis were employed to characterize the transcriptome and phosphoproteome profiles in THP-1 and THP-1-differentiated M1 macrophages with TAX1BP1 knockdown. Silencing of TAX1BP1 in THP-1 and THP-1-differentiated M1 macrophages led to an increase in cluster of differentiation 80 (CD80) expression and differential changes in CD14 and CD16 expression, as assessed by flow cytometry. Additionally, western blot analysis showed that knockdown of TAX1BP1 led to a reduction in TRAF6 and p-p65 in THP-1-differentiated macrophages, with or without lipopolysaccharide (LPS) or tumor necrosis factor (TNF)-α stimulation. Taken together, our findings suggest that TAX1BP1 participates in SLE activity by regulating antigen presentation in monocytes and inflammatory responses in M1 macrophages.

Complexity on modulation of NF-κB pathways by hepatitis B and C: A double-edged sword in hepatocarcinogenesis

Nuclear factor-κB (NF-κB), a family of master regulated dimeric transcription factors, signaling transduction pathways are active players in the cell signaling that control vital cellular processes, including cell growth, proliferation, differentiation, apoptosis, morphogenesis, angiogenesis, and immune responses. Nevertheless, aberrant regulation of the NF-κB signaling pathways has been associated with a significant number of human cancers. In fact, NF-κB acts as a double-edged sword in the vital cellular processes and carcinogenesis. This review provides an overview on the modulation of the NF-κB signaling pathways by proteins of hepatitis B and C viruses. One of the major NF-κB events that are modulated by these viruses is the induction of hepatocellular carcinoma. Given the central function of NF-κB in carcinogenesis, it has turned out to be a considerable therapeutic target for cancer therapy.

TAX1BP1 downregulation by EBV-miR-BART15-3p enhances chemosensitivity of gastric cancer cells to 5-FU

MicroRNAs (miRNAs) are a class of noncoding RNA molecules approximately 19 to 25 nucleotides in length that downregulate the expression of target genes at the post-transcriptional level by binding to the 3'-untranslated region (3'-UTR). Epstein-Barr virus (EBV) generates at least 44 miRNAs, but the functions of most of these miRNAs have not yet been identified. Previously, we reported BRUCE as a target of miR-BART15-3p, a miRNA produced by EBV, but our data suggested that there might be other apoptosis-associated target genes of miR-BART15-3p. Thus, in this study, we searched for new target genes of miR-BART15-3p using in silico analyses. We found a possible seed match site in the 3'-UTR of Tax1-binding protein 1 (TAX1BP1). The luciferase activity of a reporter vector including the 3'-UTR of TAX1BP1 was decreased by miR-BART15-3p. MiR-BART15-3p downregulated the expression of TAX1BP1 mRNA and protein in AGS cells, while an inhibitor against miR-BART15-3p upregulated the expression of TAX1BP1 mRNA and protein in AGS-EBV cells. Mir-BART15-3p modulated NF-κB activity in gastric cancer cell lines. Moreover, miR-BART15-3p strongly promoted chemosensitivity to 5-fluorouracil (5-FU). Our results suggest that miR-BART15-3p targets the anti-apoptotic TAX1BP1 gene in cancer cells, causing increased apoptosis and chemosensitivity to 5-FU.

Crystallographic and modelling studies suggest that the SKICH domains from different protein families share a common Ig-like fold but harbour substantial structural variations

TAX1BP1 is a pleiotropic multi-domain protein involved in many important biological processes such as signal transduction, cell growth and apoptosis, transcriptional coactivation, membrane trafficking, neurotransmission and autophagy. The N-terminus of TAX1BP1 contains a SKICH domain implicated in autophagy. SKICH domains are also present in four other proteins including NDP52, CALCOCO1, SKIP and PIPP. The SKICH domains of SKIP and PIPP mediate plasma membrane localisation. The functions of the SKICH domains of NDP52 and CALCOCO1 are not known. Here we report the crystal structure of the TAX1BP1 SKICH domain, which has an Ig-like fold similar to the NDP52 SKICH domain. Extensive pairwise and clustered aromatic π-stacking interactions are present in the TAX1BP1 SKICH domain. The aromatic residues mediating these interactions can be classified into four groups with varying degrees of conservation among different protein families. The interactions mediated by highly conserved residues are found in the interior and one outward face of the Ig-like β-barrel, representing common structural features of the SKICH domains. Three TAX1BP1-specific pairwise interactions locate in the loop regions, each augmented by a proline-aromatic interaction. The three proline-aromatic clusters are linked together by more generic hydrophobic interactions, forming a unique hydrophobic surface at one end of the TAX1BP1 SKICH domain. The structures and homologous models of SKICH domains from different proteins reveal substantial differences in electrostatic surface properties of the domains. Together with existing biochemical data, results from the structural study suggest that an intact SKICH domain is required for the autophagy function of TAX1BP1.

Expression, purification and crystallization of the SKICH domain of human TAX1BP1

TAX1BP1 is a highly conserved, pleiotropic protein that plays many essential functions in human cells, including negative regulation of inflammatory and antimicrobial responses mediated by NF-κB and IRF3 signaling, inhibition of apoptosis, transcriptional coactivation and autophagy etc. TAX1BP1 contains a SKICH domain at the N-terminus, three coiled-coil domains in the middle and two ubiquitin-binding zinc-finger motifs at the C-terminus. The SKICH domain and the linker sequence between the SKICH domain and the coiled-coil region mediate interaction with ubiquitin-like proteins of the LC3/GABARAP family, which are autophagosome markers. For structure determination of the SKICH domain of TAX1BP1, a protein construct (amino acids 15-148) corresponding to the SKICH domain plus the linker region was expressed, purified and crystallized. A native diffraction data set has been collected to 1.9 Å resolution. A molecular-replacement solution has been found by using the structure of the SKICH domain of NDP52, a paralog of TAX1BP1.

Human T-cell leukemia virus type 1 (HTLV-1) bZIP factor requires cellular transcription factor JunD to upregulate HTLV-1 antisense transcription from the 3' long terminal repeat

Infection with the human T-cell leukemia virus type 1 (HTLV-1) results in a variety of diseases including adult T-cell leukemia (ATL), a fatal malignancy characterized by the uncontrolled proliferation of virally infected CD4(+) T cells. The HTLV-1 basic leucine zipper factor (HBZ) is believed to contribute to development and maintenance of ATL. Unlike the other HTLV-1 genes, the hbz gene is encoded on the complementary strand of the provirus and therefore is not under direct control of the promoter within the 5' long terminal repeat (LTR) of the provirus. This promoter can undergo inactivating genetic or epigenetic changes during the course of ATL that eliminates expression of all viral genes except that of hbz. In contrast, repressive modifications are not known to occur on the hbz promoter located in the 3' LTR, and hbz expression has been consistently detected in all ATL patient samples. Although Sp1 regulates basal transcription from the HBZ promoter, other factors that activate transcription remain undefined. In this study, we used a proviral reporter construct deleted of the 5' LTR to show that HBZ upregulates its own expression through cooperation with JunD. Activation of antisense transcription was apparent in serum-deprived cells in which the level of JunD was elevated, and elimination of JunD expression by gene knockout or shRNA-mediated knockdown abrogated this effect. Activation through HBZ and JunD additionally required Sp1 binding at the hbz promoter. These data favor a model in which JunD is recruited to the promoter through Sp1, where it heterodimerizes with HBZ thereby enhancing its activity. Separately, hbz gene expression led to an increase in JunD abundance, and this effect correlated with emergence of features of transformed cells in immortalized fibroblasts. Overall, our results suggest that JunD represents a novel therapeutic target for the treatment of ATL.

Ubiquitin-specific peptidase 20 targets TRAF6 and human T cell leukemia virus type 1 tax to negatively regulate NF-kappaB signaling

NF-κB plays a key role in innate and acquired immunity. Its activity is regulated through intricate signaling networks. Persistent or excessive activation of NF-κB induces diseases, such as autoimmune disorders and malignant neoplasms. Infection by human T cell leukemia virus type 1 (HTLV-1) causes a fatal hematopoietic malignancy termed adult T cell leukemia (ATL). The HTLV-1 viral oncoprotein Tax functions pivotally in leukemogenesis through its potent activation of NF-κB. Recent findings suggest that protein ubiquitination is crucial for proper regulation of NF-κB signaling and for Tax activity. Here, we report that ubiquitin-specific peptidase USP20 deubiquitinates TRAF6 and Tax and suppresses interleukin 1β (IL-1β)- and Tax-induced NF-κB activation. Our results point to USP20 as a key negative regulator of Tax-induced NF-κB signaling.

Human T-cell leukemia virus type 1 Tax and cellular transformation

Infection of T-cells by human T-cell leukemia virus type 1 (HTLV-1) causes a lymphoproliferative malignancy known as adult T-cell leukemia (ATL). ATL is characterized by abnormal lymphocytes, called flower cells, which have cleaved and convoluted nuclei. Tax, encoded by the HTLV-1 pX region, is a critical nonstructural protein that plays a central role in leukemogenesis; however, the mechanisms of HTLV-1 oncogenesis have not been clarified fully. In this review, we summarize current thinking on how Tax may affect ATL leukemogenesis.

Evidence for cooperative transforming activity of the human pituitary tumor transforming gene and human T-cell leukemia virus type 1 Tax

Aneuploidy is frequent in cancers. Recently it was found that pituitary tumor transforming gene (PTTG; also called Pds1p or securin) is overexpressed in many different tumors. Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that primarily infects CD4+ T lymphocytes and causes adult T-cell leukemia. Here, we report that overexpression of human PTTG cooperated with the HTLV-I Tax oncoprotein in cellular transformation. Coexpression of Tax and PTTG enhanced chromosomal instability and neoplastic changes to levels greater than overexpression of either factor singularly. Cells that overexpressed both PTTG and Tax induced tumors more robustly in nude mice than cells that expressed either PTTG alone or Tax alone.

Hepatitis C virus core protein induces expression of genes regulating immune evasion and anti-apoptosis in hepatocytes

Hepatitis C virus (HCV) Core protein is implicated in the development of hepatocellular carcinoma (HCC). We utilized a HepG2 human hepatocyte cell line with inducible expression of HCV Core protein (HCV-1b) to investigate the early effects of Core protein on hepatocyte gene expression and to identify molecular processes modulated by the Core protein. A significant change was observed in the expression of 407 genes, which included genes regulating apoptosis, immune response, and cell cycle. Some of these genes were previously known to be tumor markers. The decreased expression of chemo-attractants such as TNFSF10, CCL20, and osteopontin was observed, which suggested that HCV Core expression could lead to suppression of inflammatory response as well as trafficking of macrophages and neutrophils to the site of HCV infection. An increased expression of anti-apoptosis factors including PAK2, API5, BH1, Tax1BP1, DAXX, and TNFAIP3/A20 was observed. Some of these genes were also linked to the regulation of NFKB activation and that the alteration of their expression levels, by HCV Core, might lead to the suppression NFKB activation of inflammatory responses. Our data suggested that Core expression may contribute to the viral persistence by protecting infected hepatocytes from cell death by the suppressing apoptosis and inflammatory reaction to HCV viral infection.

The HTLV-I Tax oncoprotein targets the retinoblastoma protein for proteasomal degradation

Human T-cell leukemia virus type-I (HTLV-I), the etiologic agent of adult T-cell leukemia (ATL), is estimated to affect 10-20 million people worldwide. The transforming ability of HTLV-I has been largely attributed to the viral protein Tax, which modulates the activity of several well-known cell cycle regulators. An important cell cycle regulator, the retinoblastoma (Rb) protein, is often inactivated in many cancers including virally induced cancers. Upon examination of Rb status, we observed a decrease in Rb protein expression in HTLV-1-infected cell lines as well as in ex vivo ATL patient samples. Transient transfection assays indicated that decreased Rb protein levels were Tax dependent. Here, we demonstrate for the first time that Tax directly associates with Rb. This interaction was localized within the B pocket of Rb and the C-terminus of Tax (aa 245-353). Within the C-terminus of Tax, we have identified an LXCXE-like motif, that when mutated resulted in the loss of Tax/Rb interaction. Furthermore, through the use of proteasome inhibitors, such as MG-132, in vivo and proteasome degradation assays in vitro, we found that Tax destabilizes the hypo-phosphorylated (active) form of Rb via the proteasome pathway. Therefore, we propose a model whereby Tax targets Rb to the proteasome by acting as a molecular bridge bringing Rb into contact with the proteasome for degradation.

Ubiquitination of human T-cell leukemia virus type 1 tax modulates its activity

Human T-cell leukemia virus type 1 (HTLV-1) encodes a 40-kDa Tax phosphoprotein. Tax is a transcriptional activator which modulates expression of the viral long terminal repeat and transcription of many cellular genes. Because Tax is a critical HTLV-1 factor which mediates viral transformation of T cells during the genesis of adult T-cell leukemia, it is important to understand the processes which can activate or inactivate Tax function. Here, we report that ubiquitination of Tax is a posttranscriptional mechanism which regulates Tax function. We show that ubiquitination does not target Tax for degradation by the proteasome. Rather, ubiquitin addition modifies Tax in a proteasome-independent manner from an active to a less-active transcriptional form.

Role of Tax protein in human T-cell leukemia virus type-I leukemogenicity

HTLV-1 is the etiological agent of adult T-cell leukemia (ATL), the neurological syndrome TSP/HAM and certain other clinical disorders. The viral Tax protein is considered to play a central role in the process leading to ATL. Tax modulates the expression of many viral and cellular genes through the CREB/ATF-, SRF- and NF-κB-associated pathways. In addition, Tax employs the CBP/p300 and p/CAF co-activators for implementing the full transcriptional activation competence of each of these pathways. Tax also affects the function of various other regulatory proteins by direct protein-protein interaction. Through these activities Tax sets the infected T-cells into continuous uncontrolled replication and destabilizes their genome by interfering with the function of telomerase and topoisomerase-I and by inhibiting DNA repair. Furthermore, Tax prevents cell cycle arrest and apoptosis that would otherwise be induced by the unrepaired DNA damage and enables, thereby, accumulation of mutations that can contribute to the leukemogenic process. Together, these capacities render Tax highly oncogenic as reflected by its ability to transform rodent fibroblasts and primary human T-cells and to induce tumors in transgenic mice. In this article we discuss these effects of Tax and their apparent contribution to the HTLV-1 associated leukemogenic process. Notably, however, shortly after infection the virus enters into a latent state, in which viral gene expression is low in most of the HTLV-1 carriers' infected T-cells and so is the level of Tax protein, although rare infected cells may still display high viral RNA. This low Tax level is evidently insufficient for exerting its multiple oncogenic effects. Therefore, we propose that the latent virus must be activated, at least temporarily, in order to elevate Tax to its effective level and that during this transient activation state the infected cells may acquire some oncogenic mutations which can enable them to further progress towards ATL even if the activated virus is re-suppressed after a while. We conclude this review by outlining an hypothetical flow of events from the initial virus infection up to the ultimate ATL development and comment on the risk factors leading to ATL development in some people and to TSP/HAM in others.

Segregation of NF-kappaB activation through NEMO/IKKgamma by Tax and TNFalpha: implications for stimulus-specific interruption of oncogenic signaling

Nuclear factor-kappaB essential modulator (NEMO), also called IKKgamma, has been proposed as a 'universal' adaptor of the I-kappaB kinase (IKK) complex for stimuli such as proinflammatory cytokines, microbes, and the HTLV-I Tax oncoprotein. Currently, it remains unclear whether the many signals that activate NF-kappaB through NEMO converge identically or differently. We have adopted two approaches to answer this question. First, we generated and targeted intracellularly three NEMO-specific monoclonal antibodies (mAbs). These mAbs produced two distinct intracellular NF-kappaB inhibition profiles segregating TNFalpha from Tax activation. Second, using NEMO knockout mouse fibroblasts and 10 NEMO mutants, we found that different regions function in trans either to complement or to inhibit dominantly TNFalpha, IL-1beta, or Tax activation of NF-kappaB. For instance, NEMO (1-245 amino acids) supported Tax-mediated NF-kappaB activation, but did not serve TNFalpha- or IL-1beta signaling. Altogether, our findings indicate that while NEMO 'universally' adapts numerous NF-kappaB activators, it may do so through separable domains. We provide the first evidence that selective targeting of NEMO can abrogate oncogenic Tax signaling without affecting signals used for normal cellular metabolism.

Human T-lymphotropic virus type 1 oncoprotein tax promotes unscheduled degradation of Pds1p/securin and Clb2p/cyclin B1 and causes chromosomal instability

Human T-lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia. The HTLV-1 transactivator, Tax, is implicated as the viral oncoprotein. Naïve cells expressing Tax for the first time develop severe cell cycle abnormalities that include increased DNA synthesis, mitotic arrest, appearance of convoluted nuclei with decondensed DNA, and formation of multinucleated cells. Here we report that Tax causes a drastic reduction in Pds1p/securin and Clb2p/cyclin B levels in yeast, rodent, and human cells and a loss of cell viability. With a temperature-sensitive mutant of the CDC23 subunit of the anaphase-promoting complex (APC), cdc23(ts); a temperature-sensitive mutant of cdc20; and a cdh1-null mutant, we show that the diminution of Pds1p and Clb2p brought on by Tax is mediated via the Cdc20p-associated anaphase-promoting complex, APC(Cdc20p). This loss of Pds1p/securin and Clb2p/cyclin B1 occurred before cellular entry into mitosis, caused a G(2)/M cell cycle block, and was accompanied by severe chromosome aneuploidy in both Saccharomyces cerevisiae cells and human diploid fibroblasts. Our results support the notion that Tax aberrantly targets and activates APC(Cdc20p), leading to unscheduled degradation of Pds1p/securin and Clb2p/cyclin B1, a delay or failure in mitotic entry and progression, and faulty chromosome transmission. The chromosomal instability resulting from a Tax-induced deficiency in securin and cyclin B1 provides an explanation for the highly aneuploid nature of adult T-cell leukemia cells.

Human T-lymphotropic virus type 1 oncoprotein tax promotes S-phase entry but blocks mitosis

Human T-lymphotropic virus type 1 (HTLV-1) Tax exerts pleiotropic effects on multiple cellular regulatory processes to bring about NF-kappaB activation, aberrant cell cycle progression, and cell transformation. Here we report that Tax stimulates cellular G(1)/S entry but blocks mitosis. Tax expression in naive cells transduced with a retroviral vector, pBabe-Tax, leads to a significant increase in the number of cells in the S phase, with an accompanying rise in the population of cells with a DNA content of 4N or more. In all cell types tested, including BHK-21, mouse NIH 3T3, and human diploid fibroblast WI-38, Tax causes an uncoupling of DNA synthesis from cell division, resulting in the formation of multinucleated giant cells and cells with decondensed, highly convoluted and lobulated nuclei that are reminiscent of the large lymphocytes with cleaved or cerebriform nuclei seen in HTLV-1-positive individuals. This contrasts with the Tax-transformed cell lines, PX1 (fibroblast) and MT4 (lymphocyte), which produce Tax at high levels, but without the accompanying late-stage cell cycle abnormalities. PX1 and MT4 may have been selected to harbor somatic mutations that allow a bypass of the Tax-induced block in mitosis.

Activation of HTLV-I long terminal repeat by stress-inducing agents and protection of HTLV-I-infected T-cells from apoptosis by the viral tax protein

HTLV-I is etiologically implicated with tropical spastic paraparesis/HTLV-I associated myelopathy, adult T-cell leukemia and certain other diseases. However, after infection the virus enters into a dormant state, whereas the characteristics of the HTLV-I related diseases indicate that their genesis requires activation of the dormant virus by a Tax-independent mechanism. In the present study we demonstrate that a variety of stress-inducing agents (TPA, cisplatin, etoposide, taxol, and 3-methylcholanthrene) are capable of Tax-independent activation of HTLV-I LTR and that this activation is detected mainly in cells that are undergoing through the apoptotic process. Furthermore, it is demonstrated that both apoptosis induction and HTLV-I LTR activation are inhibited by Bcl-2 and by PKC, indicating that these two processes are mechanistically cross-linked. In addition, using an HTLV-I producing human T-cell line which permanently express the negatively transdominant tax mutant, Delta58tax, under the Tet-Off control system, we prove that the virally encoded Tax protein protects the host cells from apoptosis. Together, these data suggest that activation of the dormant virus in the carriers' infected T-cells by certain stress-inducing conditions and protecting these cells from the consequent apoptotic death by the viral Tax protein emerging after this activation, might be the basis for switching the virus from latency to a pathogenic phase.

Functional activities of the human T-cell leukemia virus type I Tax oncoprotein: cellular signaling through NF-kappa B

Human T-cell leukemia virus type I (HTLV-I) is the etiological agent for adult T-cell leukemia (ATL), as well as for tropical spastic paraparesis (TSP) and HTLV-I associate myelopathy (HAM). A biological understanding of the involvement of HTLV-I and in ATL has focused significantly on the workings of the virally-encoded 40 kDa phospho-oncoprotein, Tax. Tax is a transcriptional activator. Its ability to modulate the expression and function of many cellular genes has been reasoned to be a major contributory mechanism explaining HTLV-I-mediated transformation of cells. In activating cellular gene expression, Tax impinges upon several cellular signal-transduction pathways, including those for CREB/ATF and NF-kappa B. In this paper, we review aspects of Tax's transcriptional potential with particular focus on recent evidence linking Tax to IKK (I kappa B-kinase)-complex and MAP3Ks (mitogen-activated protein kinase kinase kinases).