IBtkα Activates the β-Catenin-Dependent Transcription of MYC through Ubiquitylation and Proteasomal Degradation of GSK3β in Cancerous B Cells

The IBTK gene encodes the IBtkα protein that is a substrate receptor of E3 ubiquitin ligase, Cullin 3. We have previously reported the pro-tumorigenic activity of Ibtk in MYC-dependent B-lymphomagenesis observed in Eμ-myc transgenic mice. Here, we provide mechanistic evidence of the functional interplay between IBtkα and MYC. We show that IBtkα, albeit indirectly, activates the β-catenin-dependent transcription of the MYC gene. Of course, IBtkα associates with GSK3β and promotes its ubiquitylation, which is associated with proteasomal degradation. This event increases the protein level of β-catenin, a substrate of GSK3β, and results in the transcriptional activation of the MYC and CCND1 target genes of β-catenin, which are involved in the control of cell division and apoptosis. In particular, we found that in Burkitt’s lymphoma cells, IBtkα silencing triggered the downregulation of both MYC mRNA and protein expression, as well as a strong decrease of cell survival, mainly through the induction of apoptotic events, as assessed by using flow cytometry-based cell cycle and apoptosis analysis. Collectively, our results shed further light on the complex puzzle of IBtkα interactome and highlight IBtkα as a potential novel therapeutic target to be employed in the strategy for personalized therapy of B cell lymphoma.

DING Protein Inhibits Transcription of HIV-1 Gene through Suppression of Phosphorylation of NF-κB p65

Introduction:

Novel plant DING proteins (full-length 38 kDa p38SJ, and 27 kDa p27SJ) exhibit phosphatase activity and modulate HIV-1 gene transcription. Previously, we demonstrated that DING regulates HIV-1 gene transcription by dephosphorylation and inactivation of CTD RNA polymerase II, the major elongating factor of HIV-1 Long Terminal Repeats (LTR). Because the transcription of HIV-1 is controlled by several viral and cellular factors, including p65/p50 subunits of NF-κB, we hypothesized that DING phosphatase can also affect the phosphorylation and activity of p65 NF-κB, in addition to C-terminal Domain (CTD) of RNA Polymerase II (RNAPII), to suppress HIV-1 gene transcription and inhibit HIV-1 infection.

Methods:

Here, we describe the inhibition of HIV-1 infection and the p65/p50 NF-κB phosphorylation by DING protein, analyzed by ELISA and northern-blot assays, western-blot assays, cell fractionation, and promoter-reporter assays in DING-expressing cells, using a pTet-on inducible system.

Results:

Results from HIV-1 infection assays demonstrate a strong inhibition of HIV-1 and HIV-LTR RNA expression by DING protein, determined by p24 ELISA and by northern blot assay. Results from the western blot assays and cell fractionation assays show that there is an increase in the level of hypo-phosphorylated form of p65 NF-κB in DING-expressing cells. Both fractions of p65/p50, nuclear or cytoplasmic, are affected by DING phosphatase, but more cytoplasmic accumulation of p65 NF-κB was found in the presence of DING, suggesting that subsequent activation and nuclear import of active NF-κB is affected by DING. The major portion of nuclear p65 was dephosphorylated in DING-expressing cells. The promoter-reporter assay demonstrated that DING-mediated dephosphorylation and dysregulation of NF-κB p65 lead to the suppression of its binding to HIV-1 LTR, and resulted in the inhibition of p65-mediated activation of LTR transcription. Mapping of the region within LTR that was affected by DING revealed that both, NF-κB and CTD RNA Polymerase II binding sites were important, and cooperativity of these cellular factors was diminished by DING. In addition, mapping of the region within DING-p38SJ that affected LTR transcription, revealed that phosphate-binding domain is essential for this inhibitory activity.

Conclusion:

We have demonstrated the effect of DING phosphatases on HIV-1 infection, phosphorylation of p65 NF-κB, and transcription of HIV-1 LTR. Our studies suggest that one possible mechanism by which DING can regulate the expression of HIV-1 LTR can be through dysregulation of the transcription factor NF-κB p65 by preventing its phosphorylation and translocation to the nucleus and binding to the HIV-1 LTR, an action that could contribute to the utility of DING p38SJ as an antiviral agent. Importantly, DING not only inhibits HIV-1 LTR gene transcription in the presence of increased p65 NF-κB, but also suppresses HIV-1 infection. DING protein improved inhibitory effects of the known anti-retroviral drugs, Tenofovir (TFV) and Emtricitabine (FTS) on HIV-1, since in the combination with these drugs; the suppression of HIV-1 by DNG was significantly higher when it was in combination with these drugs, compared to controls or cases without DING. Thus, our data support the use of neuroprotective DING proteins as novel therapeutic antiviral drugs that suppress HIV-1 LTR transcription by interfering with the function of NF-κB.

Insights into Thymus Development and Viral Thymic Infections

T-cell development in the thymus is a complex and highly regulated process, involving a wide variety of cells and molecules which orchestrate thymocyte maturation into either CD4⁺ or CD8⁺ single-positive (SP) T cells. Here, we briefly review the process regulating T-cell differentiation, which includes the latest advances in this field. In particular, we highlight how, starting from a pool of hematopoietic stem cells in the bone marrow, the sequential action of transcriptional factors and cytokines dictates the proliferation, restriction of lineage potential, T-cell antigen receptors (TCR) gene rearrangements, and selection events on the T-cell progenitors, ultimately leading to the generation of mature T cells. Moreover, this review discusses paradigmatic examples of viral infections affecting the thymus that, by inducing functional changes within this lymphoid gland, consequently influence the behavior of peripheral mature T-lymphocytes.

Activation of NF-κB in B cell receptor signaling through Bruton's tyrosine kinase-dependent phosphorylation of IκB-α

The antigen-mediated triggering of B cell receptor (BCR) activates the transcription factor NF-κB that regulates the expression of genes involved in B cell differentiation, proliferation, and survival. The tyrosine kinase Btk is essentially required for the activation of NF-κB in BCR signaling through the canonical pathway of IKK-dependent phosphorylation and proteasomal degradation of IκB-α, the main repressor of NF-κB. Here, we provide the evidence of an additional mechanism of NF-κB activation in BCR signaling that is Btk-dependent and IKK-independent. In DeFew B lymphoma cells, the anti-IgM stimulation of BCR activated Btk and NF-κB p50/p65 within 0.5 min in absence of IKK activation and IκB-α degradation. IKK silencing did not affect the rapid activation of NF-κB. Within this short time, Btk associated and phosphorylated IκB-α at Y289 and Y305, and, concomitantly, p65 translocated from cytosol to nucleus. The mutant IκB-α Y289/305A inhibited the NF-κB activation after BCR triggering, suggesting that the phosphorylation of IκB-α at tyrosines 289 and 305 was required for NF-κB activation. In primary chronic lymphocytic leukemia cells, Btk was constitutively active and associated with IκB-α, which correlated with Y305-phosphorylation of IκB-α and increased NF-κB activity compared with healthy B cells. Altogether, these results describe a novel mechanism of NF-κB activation in BCR signaling that could be relevant for Btk-targeted therapy in B-lymphoproliferative disorders. KEY MESSAGES: Anti-IgM stimulation of BCR activates NF-κB p50/p65 within 30 s by a Btk-dependent and IKK-independent mechanism. Btk associates and phosphorylates IκB-α at Y289 and Y305, promoting NF-κB activation. In primary CLLs, the binding of Btk to IκB-α correlates with tyrosine phosphorylation of IκB-α and increased NF-κB activity.

HIV-1 Tat protein induces DNA damage in human peripheral blood B-lymphocytes via mitochondrial ROS production

Human immunodeficiency virus (HIV) infection is associated with B-cell malignancies in patients though HIV-1 is not able to infect B-cells. The rate of B-cell lymphomas in HIV-infected individuals remains high even under the combined antiretroviral therapy (cART) that reconstitutes the immune function. Thus, the contribution of HIV-1 to B-cell oncogenesis remains enigmatic. HIV-1 induces oxidative stress and DNA damage in infected cells via multiple mechanisms, including viral Tat protein. We have detected elevated levels of reactive oxygen species (ROS) and DNA damage in B-cells of HIV-infected individuals. As Tat is present in blood of infected individuals and is able to transduce cells, we hypothesized that it could induce oxidative DNA damage in B-cells promoting genetic instability and malignant transformation. Indeed, incubation of B-cells isolated from healthy donors with purified Tat protein led to oxidative stress, a decrease in the glutathione (GSH) levels, DNA damage and appearance of chromosomal aberrations. The effects of Tat relied on its transcriptional activity and were mediated by NF-κB activation. Tat stimulated oxidative stress in B-cells mostly via mitochondrial ROS production which depended on the reverse electron flow in Complex I of respiratory chain. We propose that Tat-induced oxidative stress, DNA damage and chromosomal aberrations are novel oncogenic factors favoring B-cell lymphomas in HIV-1 infected individuals.

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B-cells of HIV-infected individuals exhibit elevated levels of oxidative stress, DNA damage and chromosomal aberrations.

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Purified HIV-1 Tat protein reproduces this effect and induces oxidative stress and DNA damage in B-cells.

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HIV-1 Tat induces mitochondrial oxidative stress and activates NF-kB in B-cells.

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This condition increases the risk of developing chromosomal abnormalities and translocations.

HIV and HIV-Tat inhibit LPS-induced IL-27 production in human macrophages by distinct intracellular signaling pathways

Monocyte-derived Mϕs (MDMs) from HIV-infected patients and MDM infected in vitro with HIV exhibit a reduced ability to secrete various cytokines, including IL-12. Recently, IL-27, an IL-12 family cytokine, was shown to inhibit HIV replication in Mϕ. Whether HIV infection or HIV accessory protein(s) impact IL-27 production in Mϕs remains unknown. Herein, we show that in vitro HIV infection, as well as intracellular HIV-Tat (Tat) and Tat peptides, inhibit LPS-induced IL-27 production in human MDMs, suggesting impairment of the TLR4 signaling pathway. To understand the signaling pathways governing HIV or Tat-mediated inhibition of LPS-induced IL-27 production, we first demonstrated that p38 MAPK, PI3K, Src-homology region 2 domain-containing tyrosine phosphatase 1 (SHP-1), and Src kinases regulate LPS-induced IL-27 production in MDMs. Tat caused down-regulation of TNFR-associated factor (TRAF)-6 and inhibitor of apoptosis 1 (cIAP-1) and subsequently decreased phosphorylation of downstream PI3K and p38 MAPKs, which were implicated in LPS-induced IL-27 production. Whereas SHP-1 and Src kinases regulated LPS-induced IL-27 production, Tat did not inhibit these kinases, suggesting that they were not involved in Tat-mediated inhibition of LPS-induced IL-27 production. In contrast to Tat, in vitro HIV infection of MDM inhibited LPS-induced IL-27 production via inhibition of p38 MAPK activation. Overall, HIV and Tat inhibit LPS-induced IL-27 production in human macrophages via distinct mechanisms: Tat through the inhibition of cIAP-1-TRAF-6 and subsequent inhibition of PI3K and p38 MAPKs, whereas HIV through the inhibition of p38 MAPK activation.

HIV infection model of chronic obstructive pulmonary disease in mice

Cigarette smoke usage is prevalent in human immunodeficiency virus (HIV)-positive patients, and, despite highly active antiretroviral therapy, these individuals develop an accelerated form of chronic obstructive pulmonary disease (COPD). Studies investigating the mechanisms of COPD development in HIV have been limited by the lack of suitable mouse models. Here we describe a model of HIV-induced COPD in wild-type mice using EcoHIV, a chimeric HIV capable of establishing chronic infection in immunocompetent mice. A/J mice were infected with EcoHIV and subjected to whole body cigarette smoke exposure. EcoHIV was detected in alveolar macrophages of mice. Compared with uninfected mice, concomitant EcoHIV infection significantly reduced forced expiratory flow 50%/forced vital capacity and enhanced distal airspace enlargement following cigarette smoke exposure. Lung IL-6, granulocyte-macrophage colony-stimulating factor, neutrophil elastase, cathepsin G, and matrix metalloproteinase-9 expression was significantly enhanced in smoke-exposed EcoHIV-infected mice. These changes coincided with enhanced IκBα, ERK1/2, p38, and STAT3 phosphorylation and lung cell apoptosis. Thus, the EcoHIV smoke exposure mouse model reproduces several of the pathophysiological features of HIV-related COPD in humans, indicating that this murine model can be used to determine key parameters of HIV-related COPD and to test future therapies for this disorder.

Impairment of T cell development and acute inflammatory response in HIV-1 Tat transgenic mice

Immune activation and chronic inflammation are hallmark features of HIV infection causing T-cell depletion and cellular immune dysfunction in AIDS. Here, we addressed the issue whether HIV-1 Tat could affect T cell development and acute inflammatory response by generating a transgenic mouse expressing Tat in lymphoid tissue. Tat-Tg mice showed thymus atrophy and the maturation block from DN4 to DP thymic subpopulations, resulting in CD4⁺ and CD8⁺ T cells depletion in peripheral blood. In Tat-positive thymus, we observed the increased p65/NF-κB activity and deregulated expression of cytokines/chemokines and microRNA-181a-1, which are involved in T-lymphopoiesis. Upon LPS intraperitoneal injection, Tat-Tg mice developed an abnormal acute inflammatory response, which was characterized by enhanced lethality and production of inflammatory cytokines. Based on these findings, Tat-Tg mouse could represent an animal model for testing adjunctive therapies of HIV-1-associated inflammation and immune deregulation.

XIAP and cIAP1 amplifications induce Beclin 1-dependent autophagy through NFκB activation

Perturbations in autophagy and apoptosis are associated with cancer development. XIAP and cIAP1 are two members of the inhibitors of apoptosis protein family whose expression is elevated in different cancers. Here we report that XIAP and cIAP1 induce autophagy by upregulating the transcription of Beclin 1, an essential autophagy gene. The E3 ubiquitin ligase activity of both proteins activates NFκB signalling, leading to the direct binding of p65 to the promoter of Beclin 1 and to its transcriptional activation. This mechanism may be relevant in cancer cells, since we found increased levels of autophagy in different B-cell lymphoma-derived cell lines where XIAP is overexpressed and pharmacological inhibition of XIAP in these cell lines reduced autophagosome biogenesis. Thus, the chemotherapy resistance associated with XIAP and cIAP1 overexpression observed in several human cancers may be, at least in part, due to the Beclin 1-dependent autophagy activation by IAPs described in this study. In this context, the disruption of this increased autophagy might represent a valuable pharmacological tool to be included in combined anti-neoplastic therapies.

Cancer-associated CD43 glycoforms as target of immunotherapy

CD43 is a sialoglycosylated membrane protein that is involved in cell proliferation and differentiation. CD43 glycoforms that are recognized by the UN1 monoclonal antibody (mAb) were expressed in lymphoblastoid T-cell lines and solid tumors, such as breast, colon, gastric, and squamous cell lung carcinomas, while unexpressed in the normal counterparts. The cancer association of UN1/CD43 epitope suggested the possibility to use the UN1 mAb for tumor diagnosis and therapy. In this study, we show that the UN1 mAb was endowed with antitumor activity in vivo because its passive transfer inhibited the growth of UN1-positive HPB-ALL lymphoblastoid T cells in mice. Furthermore, we demonstrate that tumor inhibition was due to UN1 mAb-dependent natural killer-mediated cytotoxicity. By screening a phage-displayed random peptide library, we identified the phagotope 2/165 as a mimotope of the UN1 antigen, as it harbored a peptide sequence that was specifically recognized by the UN1 mAb and inhibited the binding of the UN1 mAb to UN1-positive tumor cells. On the basis of sequence homology with the extracellular region of CD43 (amino acids 64 to 83), the 2/165 peptide sequence was likely mimicking the protein core of the UN1/CD43 epitope. When used as vaccine in mice, the 2/165 phagotope raised antibodies against the UN1/CD43 antigen, indicating that the 2/165 phagotope mimicked the UN1 antigen structure, and could represent a novel immunogen for cancer immunotherapy. These findings support the feasibility of using monoclonal antibodies to identify cancer-associated mimotopes for immunotherapy.

Eukaryotic Initiation Factor 4H Is under Transcriptional Control of p65/NF-κB

Protein synthesis is mainly regulated at the initiation step, allowing the fast, reversible and spatial control of gene expression. Initiation of protein synthesis requires at least 13 translation initiation factors to assemble the 80S ribosomal initiation complex. Loss of translation control may result in cell malignant transformation. Here, we asked whether translational initiation factors could be regulated by NF-κB transcription factor, a major regulator of genes involved in cell proliferation, survival, and inflammatory response. We show that the p65 subunit of NF-κB activates the transcription of eIF4H gene, which is the regulatory subunit of eIF4A, the most relevant RNA helicase in translation initiation. The p65-dependent transcriptional activation of eIF4H increased the eIF4H protein content augmenting the rate of global protein synthesis. In this context, our results provide novel insights into protein synthesis regulation in response to NF-κB activation signalling, suggesting a transcription-translation coupled mechanism of control.

Design and characterization of a peptide mimotope of the HIV-1 gp120 bridging sheet

The Bridging Sheet domain of HIV-1 gp120 is highly conserved among the HIV-1 strains and allows HIV-1 binding to host cells via the HIV-1 coreceptors. Further, the bridging sheet domain is a major target to neutralize HIV-1 infection. We rationally designed four linear peptide epitopes that mimic the three-dimensional structure of bridging sheet by using molecular modeling. Chemically synthesized peptides BS3 and BS4 showed a fair degree of antigenicity when tested in ELISA with IgG purified from HIV(+) broadly neutralizing sera while the production of synthetic peptides BS1 and BS2 failed due to their high degree of hydrophobicity. To overcome this limitation, we linked all four BS peptides to the COOH-terminus of GST protein to test both their antigenicity and immunogenicity. Only the BS1 peptide showed good antigenicity; however, no envelope specific antibodies were elicited upon mice immunization. Therefore we performed further analyses by linking BS1 peptide to the NH2-terminus of the E2 scaffold from the Geobacillus Stearothermophylus PDH complex. The E2-BS1 fusion peptide showed good antigenic results, however only one immunized rabbit elicited good antibody titers towards both the monomeric and oligomeric viral envelope glycoprotein (Env). In addition, moderate neutralizing antibodies response was elicited against two HIV-1 clade B and one clade C primary isolates. These preliminary data validate the peptide mimotope approach as a promising tool to obtain an effective HIV-1 vaccine.

Human immunodeficiency virus-1 Tat activates NF-κB via physical interaction with IκB-α and p65

Nuclear factor (NF)-κB is a master regulator of pro-inflammatory genes and is upregulated in human immunodeficiency virus 1 (HIV-1) infection. Mechanisms underlying the NF-κB deregulation by HIV-1 are relevant for immune dysfunction in AIDS. We report that in single round HIV-1 infection, or single-pulse PMA stimulation, the HIV-1 Tat transactivator activated NF-κB by hijacking the inhibitor IκB-α and by preventing the repressor binding to the NF-κB complex. Moreover, Tat associated with the p65 subunit of NF-κB and increased the p65 DNA-binding affinity and transcriptional activity. The arginine- and cysteine-rich domains of Tat were required for IκB-α and p65 association, respectively, and for sustaining the NF-κB activity. Among an array of NF-κB-responsive genes, Tat mostly activated the MIP-1α expression in a p65-dependent manner, and bound to the MIP-1α NF-κB enhancer thus promoting the recruitment of p65 with displacement of IκB-α; similar findings were obtained for the NF-κB-responsive genes CSF3, LTA, NFKBIA and TLR2. Our results support a novel mechanism of NF-κB activation via physical interaction of Tat with IκB-α and p65, and may contribute to further insights into the deregulation of the inflammatory response by HIV-1.

The ins and outs of HIV-1 Tat

HIV-1 encodes for the small basic protein Tat (86-101 residues) that drastically enhances the efficiency of viral transcription. The mechanism enabling Tat nuclear import is not yet clear, but studies using reporter proteins fused to the Tat basic domain indicate that Tat could reach the nucleus by passive diffusion. Tat also uses an unusual transcellular transport pathway. The first step of this pathway involves high-affinity binding of Tat to phosphatidylinositol (4,5) bisphosphate (PI(4,5)P(2)), a phospholipid that is concentrated in the inner leaflet of the plasma membrane and enables Tat recruitment at this level. Tat then crosses the plasma membrane to reach the outside medium. Although unconventional, Tat secretion by infected cells is highly active, and export is the major destination for HIV-1 Tat. Secreted Tat can bind to a variety of cell types using several different receptors. Most of them will allow Tat endocytosis. Upon internalization, low endosomal pH triggers a conformational change in Tat that results in membrane insertion. Later steps of Tat translocation to the target-cell cytosol are assisted by Hsp90, a general cytosolic chaperone. Cytosolic Tat can trigger various cell responses. Indeed, accumulating evidence suggests that extracellular Tat acts as a viral toxin that affects the biological activity of different cell types and has a key role in acquired immune-deficiency syndrome development. This review focuses on some of the recently identified molecular details underlying the unusual transcellular transport pathway used by Tat, such as the role of the single Trp in Tat for its membrane insertion and translocation.

Structural and functional insights into IκB-α/HIV-1 Tat interaction

Protein-protein interactions play fundamental roles in physiological and pathological biological processes. The characterization of the structural determinants of protein-protein recognition represents an important step for the development of molecular entities able to modulate these interactions. We have recently found that IκB-α (nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha) blocks the HIV-1 expression and replication in a NF-κB-independent manner by directly binding to the virus-encoded Tat transactivator. Here, we report the evaluation of the entity of binding of IκB-α to Tat through in vitro Surface Plasmon Resonance assay. Moreover, by designing and characterizing a set of peptides of the C-terminus region of IκB-α, we show that the peptide corresponding to the IκB-α sequence 262-287 was able to bind to Tat with high affinity (300 nM). The characterization of a number of IκB-α-based peptides also provided insights into their intrinsic folding properties. These findings have been corroborated by mutagenesis studies on the full-length IκB-α, which unveil that different IκB-α residues are involved in NF-κB or Tat recognition.

Btk regulation in human and mouse B cells via protein kinase C phosphorylation of IBtkγ

The inhibitor of Bruton tyrosine kinase γ (IBtkγ) is a negative regulator of the Bruton tyrosine kinase (Btk), which plays a major role in B-cell differentiation; however, the mechanisms of IBtkγ-mediated regulation of Btk are unknown. Here we report that B-cell receptor (BCR) triggering caused serine-phosphorylation of IBtkγ at protein kinase C consensus sites and dissociation from Btk. By liquid chromatography and mass-mass spectrometry and functional analysis, we identified IBtkγ-S87 and -S90 as the critical amino acid residues that regulate the IBtkγ binding affinity to Btk. Consistently, the mutants IBtkγ carrying S87A and S90A mutations bound constitutively to Btk and down-regulated Ca(2+) fluxes and NF-κB activation on BCR triggering. Accordingly, spleen B cells from Ibtkγ(-/-) mice showed an increased activation of Btk, as evaluated by Y551-phosphorylation and sustained Ca(2+) mobilization on BCR engagement. These findings identify a novel pathway of Btk regulation via protein kinase C phosphorylation of IBtkγ.

Physical and functional characterization of the genetic locus of IBtk, an inhibitor of Bruton's tyrosine kinase: evidence for three protein isoforms of IBtk

Bruton's tyrosine kinase (Btk) is required for B-cell development. Btk deficiency causes X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Btk lacks a negative regulatory domain and may rely on cytoplasmic proteins to regulate its activity. Consistently, we identified an inhibitor of Btk, IBtk, which binds to the PH domain of Btk and down-regulates the Btk kinase activity. IBtk is an evolutionary conserved protein encoded by a single genomic sequence at 6q14.1 cytogenetic location, a region of recurrent chromosomal aberrations in lymphoproliferative disorders; however, the physical and functional organization of IBTK is unknown. Here, we report that the human IBTK locus includes three distinct mRNAs arising from complete intron splicing, an additional polyadenylation signal and a second transcription start site that utilizes a specific ATG for protein translation. By northern blot, 5′RACE and 3′RACE we identified three IBTKα, IBTKβ and IBTKγ mRNAs, whose transcription is driven by two distinct promoter regions; the corresponding IBtk proteins were detected in human cells and mouse tissues by specific antibodies. These results provide the first characterization of the human IBTK locus and may assist in understanding the in vivo function of IBtk.