Hijacking of the O-GlcNAcZYME complex by the HTLV-1 Tax oncoprotein facilitates viral transcription.

The viral Tax oncoprotein plays a key role in both Human T-cell lymphotropic virus type 1 (HTLV-1)-replication and HTLV-1-associated pathologies, notably adult T-cell leukemia. Tax governs the transcription from the viral 5'LTR, enhancing thereby its own expression, via the recruitment of dimers of phosphorylated CREB to cAMP-response elements located within the U3 region (vCRE). In addition to phosphorylation, CREB is also the target of O-GlcNAcylation, another reversible post-translational modification involved in a wide range of diseases, including cancers. O-GlcNAcylation consists in the addition of O-linked-N-acetylglucosamine (O-GlcNAc) on Serine or Threonine residues, a process controlled by two enzymes: O-GlcNAc transferase (OGT), which transfers O-GlcNAc on proteins, and O-GlcNAcase (OGA), which removes it. In this study, we investigated the status of O-GlcNAcylation enzymes in HTLV-1-transformed T cells. We found that OGA mRNA and protein expression levels are increased in HTLV-1-transformed T cells as compared to control T cell lines while OGT expression is unchanged. However, higher OGA production coincides with a reduction in OGA specific activity, showing that HTLV-1-transformed T cells produce high level of a less active form of OGA. Introducing Tax into HEK-293T cells or Tax-negative HTLV-1-transformed TL-om1 T cells is sufficient to inhibit OGA activity and increase total O-GlcNAcylation, without any change in OGT activity. Furthermore, Tax interacts with the OGT/OGA complex and inhibits the activity of OGT-bound OGA. Pharmacological inhibition of OGA increases CREB O-GlcNAcylation as well as HTLV-1-LTR transactivation by Tax and CREB recruitment to the LTR. Moreover, overexpression of wild-type CREB but not a CREB protein mutated on a previously described O-GlcNAcylation site enhances Tax-mediated LTR transactivation. Finally, both OGT and OGA are recruited to the LTR. These findings reveal the interplay between Tax and the O-GlcNAcylation pathway and identify new key molecular actors involved in the assembly of the Tax-dependent transactivation complex.

A non-SUMOylated tax protein is still functional for NF-κB pathway activation.

UNLABELLED: Whether NF-κB promoter transactivation by the human T-cell leukemia virus type 1 (HTLV-1) Tax protein requires Tax SUMOylation is still a matter of debate. In this study, we revisited the role of Tax SUMOylation using a strategy based on the targeting of Ubc9, the unique E2 SUMO-conjugating enzyme. We show that either a catalytically inactive form of Ubc9 (Ubc9-C93S) or Ubc9 small interfering RNA (siRNA) dramatically reduces Tax conjugation to endogenous SUMO-1 or SUMO-2/3, demonstrating that as expected, Tax SUMOylation is under the control of the catalytic activity of Ubc9. We further report that a non-SUMOylated Tax protein produced in 293T cells is still able to activate either a transfected or an integrated NF-κB reporter promoter and to induce expression of an NF-κB-regulated endogenous gene. Importantly, blocking Ubc9 activity in T cells also results in the production of a non-SUMOylated Tax that is still fully functional for the activation of a NF-κB promoter. These results provide the definitive evidence that Tax SUMOylation is not required for NF-κB-driven gene induction. IMPORTANCE: Human T-cell leukemia virus type 1 is able to transform CD4(+) T lymphocytes. The viral oncoprotein Tax plays a key role in this process by promoting cell proliferation and survival, mainly through permanent activation of the NF-κB pathway. Elucidating the molecular mechanisms involved in NF-κB pathway activation by Tax is therefore a key issue to understand HTLV-1-mediated transformation. Tax SUMOylation was initially proposed to be critical for Tax-induced NF-κB promoter activation, which was challenged by our later observation that a low-level-SUMOylated Tax mutant was still functional for activation of NF-κB promoters. To clarify the role of Tax SUMOylation, we set up a new approach based on the inhibition of the SUMOylation machinery in Tax-expressing cells. We show that blocking the SUMO-conjugating enzyme Ubc9 abolishes Tax SUMOylation and that a non-SUMOylated Tax still activates NF-κB promoters in either adherent cells or T cells.

Low nuclear body formation and tax SUMOylation do not prevent NF-kappaB promoter activation.

BACKGROUND: The Tax protein encoded by Human T-lymphotropic virus type 1 (HTLV-1) is a powerful activator of the NF-κB pathway, a property critical for HTLV-1-induced immortalization of CD4⁺ T lymphocytes. Tax permanently stimulates this pathway at a cytoplasmic level by activating the IκB kinase (IKK) complex and at a nuclear level by enhancing the binding of the NF-κB factor RelA to its cognate promoters and by forming nuclear bodies, believed to represent transcriptionally active structures. In previous studies, we reported that Tax ubiquitination and SUMOylation play a critical role in Tax localization and NF-κB activation. Indeed, analysis of lysine Tax mutants fused or not to ubiquitin or SUMO led us to propose a two-step model in which Tax ubiquitination first intervenes to activate IKK while Tax SUMOylation is subsequently required for promoter activation within Tax nuclear bodies. However, recent studies showing that ubiquitin or SUMO can modulate Tax activities in either the nucleus or the cytoplasm and that SUMOylated Tax can serve as substrate for ubiquitination suggested that Tax ubiquitination and SUMOylation may mediate redundant rather than successive functions. RESULTS: In this study, we analyzed the properties of a new Tax mutant that is properly ubiquitinated, but defective for both nuclear body formation and SUMOylation. We report that reducing Tax SUMOylation and nuclear body formation do not alter the ability of Tax to activate IKK, induce RelA nuclear translocation, and trigger gene expression from a NF-κB promoter. Importantly, potent NF-κB promoter activation by Tax despite low SUMOylation and nuclear body formation is also observed in T cells, including CD4⁺ primary T lymphocytes. Moreover, we show that Tax nuclear bodies are hardly observed in HTLV-1-infected T cells. Finally, we provide direct evidence that the degree of NF-κB activation by Tax correlates with the level of Tax ubiquitination, but not SUMOylation. CONCLUSIONS: These data reveal that the formation of Tax nuclear bodies, previously associated to transcriptional activities in Tax-transfected cells, is dispensable for NF-κB promoter activation, notably in CD4⁺ T cells. They also provide the first evidence that Tax SUMOylation is not a key determinant for Tax-induced NF-κB activation.

Molecular aspects of HTLV-1 entry: functional domains of the HTLV-1 surface subunit (SU) and their relationships to the entry receptors.

The initial step in retroviral infection involves specific interactions between viral envelope proteins (Env) and specific receptors on the surface of target cells. For many years, little was known about the entry receptors for HTLV-1. During this time, however, functional domains of the HTLV-1 Env were identified by analyzing the effects of neutralizing antibodies and specific mutations in Env on HTLV-1 infectivity. More recent studies have revealed that HTLV-1 infectivity involves interactions with three different molecules: heparan sulfate proteoglycans (HSPG), the VEGF-165 receptor Neuropilin 1 (NRP-1) and glucose transporter type 1 (GLUT1). Here, we revisit previously published data on the functional domains of Env in regard to the recent knowledge acquired about this multi-receptor complex. We also discuss the similarities and differences between HTLV-1 and other deltaretroviruses in regards to receptor usage.

Oncogenic Kit controls neoplastic mast cell growth through a Stat5/PI3-kinase signaling cascade.

The D816V-mutated variant of Kit triggers multiple signaling pathways and is considered essential for malignant transformation in mast cell (MC) neoplasms. We here describe that constitutive activation of the Stat5-PI3K-Akt-cascade controls neoplastic MC development. Retrovirally transduced active Stat5 (cS5(F)) was found to trigger PI3K and Akt activation, and to transform murine bone marrow progenitors into tissue-infiltrating MCs. Primary neoplastic Kit D816V(+) MCs in patients with mastocytosis also displayed activated Stat5, which was found to localize to the cytoplasm and to form a signaling complex with PI3K, with consecutive Akt activation. Finally, the knock-down of either Stat5 or Akt activity resulted in growth inhibition of neoplastic Kit D816V(+) MCs. These data suggest that a downstream Stat5-PI3K-Akt signaling cascade is essential for Kit D816V-mediated growth and survival of neoplastic MCs.

Syncytin-A and syncytin-B, two fusogenic placenta-specific murine envelope genes of retroviral origin conserved in Muridae.

Recently, we and others have identified two human endogenous retroviruses that entered the primate lineage 25-40 million years ago and that encode highly fusogenic retroviral envelope proteins (syncytin-1 and -2), possibly involved in the formation of the placenta syncytiotrophoblast layer generated by trophoblast cell fusion at the materno-fetal interface. A systematic in silico search throughout mouse genome databases presently identifies two fully coding envelope genes, present as unique copies and unrelated to any known murine endogenous retrovirus, that we named syncytin-A and -B. Quantitative RT-PCR demonstrates placenta-specific expression for both genes, with increasing transcript levels in this organ from 9.5 to 14.5 days postcoitum. In situ hybridization of placenta cryosections further localizes these transcripts in the syncytiotrophoblast-containing labyrinthine zona. Consistently, we show that both genes can trigger cell-cell fusion in ex vivo transfection assays, with distinct cell type specificities suggesting different receptor usage. Genes orthologous to syncytin-A and -B and disclosing a striking conservation of their coding status are found in all Muridae tested (mouse, rat, gerbil, vole, and hamster), dating their entry into the rodent lineage approximately 20 million years ago. Together, these data strongly argue for a critical role of syncytin-A and -B in murine syncytiotrophoblast formation, thus unraveling a rather unique situation where two pairs of endogenous retroviruses, independently acquired by the primate and rodent lineages, would have been positively selected for a convergent physiological role.

Genomewide screening for fusogenic human endogenous retrovirus envelopes identifies syncytin 2, a gene conserved on primate evolution.

Screening human sequence databases for endogenous retroviral elements with coding envelope genes has revealed 16 candidate genes that we assayed for their fusogenic properties. All 16 genes were cloned in a eukaryotic expression vector and assayed for cell-cell fusion by using a large panel of mammalian cells in transient transfection assays. Fusion was observed for two human endogenous retrovirus (HERV) envelopes, the previously characterized HERV-W envelope, also called syncytin, and a previously uncharacterized gene from the HERV-FRD family. Cells prone to env-mediated fusion were different for the two envelopes, indicating different receptor usage. A search for the FRDenv gene in primates indicated that the corresponding proviral element is present in all simians, from New World monkeys to humans, being absent only in prosimians. Cloning of the corresponding env genes in simians disclosed conservation of the fully coding status of the gene, and most remarkably, conservation of its fusogenic property. Finally, a Northern blot analysis for the expression of the FRD family among a series of human tissues demonstrated specific expression in the placenta, as previously demonstrated for the other fusogenic human envelope of the HERV-W family. Altogether, the present data have identified a previously uncharacterized envelope (that we propose to name syncytin 2 after renaming syncytin as syncytin 1) with a potential role in placenta formation, and the identification of the complete set of retroviral envelopes with fusogenic properties now allows a definite analysis of the possible role of HERV in this physiological process, via classical genetic approaches.

Survey of human genes of retroviral origin: identification and transcriptome of the genes with coding capacity for complete envelope proteins.

Sequences of retroviral origin occupy approximately 8% of the human genome. Most of these "retroviral" genes have lost their coding capacities since their entry into our ancestral genome millions of years ago, but some reading frames have remained open, suggesting positive selection. The complete sequencing of the human genome allowed a systematic search for retroviral envelope genes containing an open reading frame and resulted in the identification of 16 genes that we have characterized. We further showed, by quantitative reverse transcriptase PCR using specifically devised primers which discriminate between coding and noncoding elements, that all 16 genes are expressed in at least some healthy human tissues, albeit at highly different levels. All envelope genes disclose significant expression in the testis, three of them have a very high level of expression in the placenta, and a fourth is expressed in the thyroid. Besides their primary role as key molecules for viral entry, the envelope genes of retroviruses can induce cell-cell fusion, elicit immunosuppressive effects, and even protect against infection, and as such, endogenous retroviral envelope proteins have been tentatively identified in several reports as being involved in both normal and pathological processes. The present study provides a comprehensive survey of candidate genes and tools for a precise evaluation of their involvement in these processes.

Characterization of the low-copy HERV-Fc family: evidence for recent integrations in primates of elements with coding envelope genes.

In a previous search based on the envelope gene, we had identified two related proviral elements that could not be included in identified ERV families. An in silico database screening associated with an in vivo polymerase chain reaction search using primers in the reverse transcriptase domain, now allowed identification of a series of related elements, found at a limited number in simians. A phylogenetic analysis led to their inclusion in a new family of endogenous retroviruses with limited expansion, which we named ERV-Fc, and which is part of the enlarged ERV-F/H family. The human genome comprises only six HERV-Fc, among which two possess full-length coding envelope genes. A complete provirus was identified in the baboon, also disclosing a fully open envelope gene. Cloning of the sites orthologous to the envelope-coding human proviruses demonstrated presence of the integrated proviruses in chimpanzee and gorilla, but not in orangutan. For the baboon element, the orthologous locus was found empty even in the phylogenetically most closely related macaque, again suggesting, together with the complete identity of its LTRs, "recent" integration. The data presented are compatible with an evolutionary scheme in which the ERV-Fc proviruses would be the endogenous traces of an active retroviral element, possibly acting as an infectious retrovirus with low endogeneization potency, with evidence for integrations at two distinct periods of primate evolution.