- Open Access
HIV-1 Vpr stimulates NF-κB and AP-1 signaling by activating TAK1
- Ruikang Liu†1,
- Yongquan Lin†1,
- Rui Jia1,
- Yunqi Geng1,
- Chen Liang2, 3, 4,
- Juan Tan1Email author and
- Wentao Qiao1Email author
© Liu et al.; licensee BioMed Central Ltd. 2014
- Received: 7 January 2014
- Accepted: 20 May 2014
- Published: 9 June 2014
The Vpr protein of human immunodeficiency virus type 1 (HIV-1) plays an important role in viral replication. It has been reported that Vpr stimulates the nuclear factor-κB (NF-κB) and activator protein 1 (AP-1) signaling pathways, and thereby regulates viral and host cell gene expression. However, the molecular mechanism behind this function of Vpr is not fully understood.
Here, we have identified transforming growth factor-β-activated kinase 1 (TAK1) as the important upstream signaling molecule that Vpr associates with in order to activate NF-κB and AP-1 signaling. HIV-1 virion-associated Vpr is able to stimulate phosphorylation of TAK1. This activity of Vpr depends on its association with TAK1, since the S79A Vpr mutant lost interaction with TAK1 and was unable to activate TAK1. This association allows Vpr to promote the interaction of TAB3 with TAK1 and increase the polyubiquitination of TAK1, which renders TAK1 phosphorylation. In further support of the key role of TAK1 in this function of Vpr, knockdown of endogenous TAK1 significantly attenuated the ability of Vpr to activate NF-κB and AP-1 as well as the ability to stimulate HIV-1 LTR promoter.
HIV-1 Vpr enhances the phosphorylation and polyubiquitination of TAK1, and as a result, activates NF-κB and AP-1 signaling pathways and stimulates HIV-1 LTR promoter.
Human immunodeficiency virus type 1 (HIV-1) codes an accessory protein called viral protein R (Vpr) that all primate lentiviruses have [1, 2]. Vpr is a 14-kDa protein, and is found in both the cytoplasm and the nucleus of infected cells . Notably, Vpr is specifically incorporated into progeny virions through interacting with viral Gag protein [4–6]. This presence in HIV-1 particles enables Vpr to play important roles at the early stage of viral infection, such as promoting the nuclear transport of the viral pre-integration complex (PIC) and enhancing the fidelity of the reverse transcription [7, 8]. In addition, Vpr has also been reported to induce cell cycle G2/M arrest, regulate apoptosis, transactivate HIV-1 LTR and affect the production of IL-6, IL-8, and CCL5 [9–18]. These diverse functions of Vpr partially result from its ability to modulate the activities of cellular factors such as nuclear factor-κB (NF-κB) and activator protein 1 (AP-1) that are not only essential for multiple important cellular processes but also for HIV-1 gene expression [16–19]. However, it is not fully understood how Vpr exerts its effect on NF-κB and AP-1.
The activities of NF-κB and AP-1 are regulated by a couple of signaling pathways in response to various stimuli such as cytokines, bacterial and viral infections. These signal transduction cascades share a key regulator called TAK1 (transforming growth factor-β-activated kinase 1), a member of MAPK kinase kinase (MAP3K) family, that responds to different stimuli and assembles into a complex including the receptor proximal complex containing TNF receptor associated factor (TRAF), TAK1 binding protein 1, 2 and 3 (TAB1, TAB2, and TAB3) [20–26]. Among these TAK1-binding proteins, TAB1 interacts constitutively with the NH2-terminal catalytic domain of TAK1 and regulates the oligomerization and auto-phosphorylation of TAK1 [27–31]. TAB2 or TAB3, binds to the COOH-terminal region of TAK1, and functions as an adaptor protein to recruit TAK1 to the K63-linked polyubiquitin chains formed by TRAF6 [24, 26, 32–35]. TAK1 then initiates the MKK cascade by activating c-Jun N-terminal kinase (JNK) and p38 MAPK, and stimulates the IκB kinase (IKK) pathway that leads to NF-κB activation [25, 36].
The activity of TAK1 is regulated by phosphorylation and polyubiquitination. Upon cytokine stimulation, TAK1 undergoes auto-phosphorylation as a result of association with TAB2/3-polyubiquitin chains  or TAB1-dependent oligomerization [30, 38]. Phosphorylation occurs at four conserved serine and threonine residues within TAK1 activation loop, including Thr-178, Thr-184, Thr-187, and Ser-192 [28, 30, 31, 39, 40], among which Thr-187 phosphorylation has a major role in regulating TAK1 activity [30, 31]. TNFα, IL-1β, and TGFβ cause K63-linked polyubiquitination of TAK1 by TRAF2 or TRAF6 [41, 42]. The ubiquitin acceptor sites have been mapped to Lys-34, Lys-158, or Lys-209 in a stimulus-specific manner [41, 43–47]. The possible interplay between phosphorylation and polyubiquitination of TAK1 is unclear .
In light of its key roles in different signaling pathways, it is not surprising that TAK1 has become the target of many viruses in order to modulate the production of pro-inflammatory cytokines expression for the benefit of virus replication. In the case of HIV-1, the viral gp41 protein has been reported to induce TAK1-dependent NF-κB signaling which enhances viral replication in CD4+ T cells . In this study, we further show that the HIV-1 virion-associated Vpr activates NF-κB and AP-1 via TAK1-dependent pathways. This activity of Vpr results from its association with the TAK1 and consequent stimulation of TAK1 phosphorylation and polyubiquitination. Our findings not only highlight the essential role of TAK1 in regulating the NF-κB and AP-1 signaling cascades, but also present one example in regard to how pathogens assimilate TAK1 to enhance their multiplication.
Vpr induces the phosphorylation of TAK1
Vpr associates with TAK1
TRAF6 is required for Vpr-induced TAK1 phosphorylation
Vpr increases the polyubiquitination of TAK1
Activation of TAK1 by Vpr stimulates the NF-κB and AP-1 signaling
We also tested the role of TAK1 on the HIV-1 replication in TAK1-knockdown THP-1 cells. We first mutated the translation start codon of Vpr in the context of the HIV-Luc proviral DNA and generated HIV-Luc ΔVpr. We then produced VSV-G pseudotyped HIV-1 virus particles by transfecting HEK293T cells. As expected, the HIV-Luc virus, but not HIV-Luc ΔVpr, expressed Vpr (Additional file 1: Figure S2A). We used the same amount of these two viruses (equivalent to 2 ng p24) to infect TAK1-knockdown THP-1 cells (Additional file 1: Figure S2B). The infection of HIV-1 was assessed by measuring the luciferase activities of infected cells. Depleting endogenous TAK1 diminished the replication of HIV-Luc virus by 2-fold, whereas the infection of HIV-Luc ΔVpr virus was not affected (Additional file 1: Figure S2C). Together, these data demonstrate that TAK1 is involved in the replication of HIV-1 in a Vpr-dependent manner.
HIV-1 Vpr plays an important role in viral replication and pathogenesis. It has been well established that Vpr can activate both NF-κB and AP-1 signaling [16–19, 50], but how Vpr regulates these cascades remains unclear. In this study, we have identified TAK1 as an important player in Vpr-mediated NF-κB and AP-1 signaling. We present data showing that virion-associated Vpr enhances the phosphorylation of TAK1 by increasing TAK1 polyubiquitination through strengthening the association of TAB3 with TAK1.
Many viruses have been reported to modulate the activity of TAK1 and thereby the NF-κB and AP-1 pathways in order to benefit viral replication. For example, the X protein of hepatitis B virus, LMP1 of Epstein-Barr virus, vGPCR of KHSV, ICP0 of herpes simplex virus type 1, and Tax of human T cell leukemia virus type 1 activate NF-κB or AP-1 signaling by targeting TAK1 [51–55]. TAK1 has also been identified as an important upstream signaling molecule that RSV assimilates to activate NF-κB and AP-1 . Recently, it was reported that HIV-1 gp41 activates NF-κB signaling via a direct interaction with TAK1, which leads to activation of CD4+ T lymphocyte cells and facilitate HIV-1 replication . For the first time, we report that HIV-1 virion-associated Vpr associates with TAK1 and enhances the phosphorylation of TAK1 (Figure 1). We noted that knockdown of TAK1 does not completely prevent Vpr from activating NF-κB or AP-1 (Figure 6D). This is likely because Vpr may also activate these two important transcription factors by modulating other signaling pathways. An incoming HIV-1 particle carries up to 250-700 copies of Vpr protein and only about 7-16 envelope trimmers [6, 57, 58]. Compared with the small number of gp41, the relatively abundant virion-associated Vpr protein is expected to be more effective in activating TAK1 and NF-κB. Indeed, using VSV-G pseudotyped HIV-1 to infect cells, we found that Vpr alone, in the absence of gp41, is able to activate TAK1 (Figure 1B-E).
Recent studies showed that two cellular restriction factors, TRIM5 and tetherin, act as innate immune sensors to detect retroviral infection. Both restriction factors activate TAK1, followed by stimulating the expression of NF-κB and AP-1-dependent genes [59, 60]. Given that HIV-1 benefits from activation of TAK1 by Vpr at the early stage of HIV-1 infection, it is possible that TAK1 is activated via different mechanisms by Vpr or TRIM5/tetherin. As a result, the downstream signaling and resulting gene expression profiles differ in regard to either facilitate HIV-1 replication (in the case of Vpr) or suppress HIV-1 replication (in the case of TRIM5/tetherin). It is also possible that Vpr competes with TRIM5/tetherin for binding to TAK1 and therefore counters innate immune response.
In addition to regulating the phosphorylation of TAK1, we also found that Vpr can enhance the polyubiquitination of TAK1 by recruiting TAB3 (Figure 5). It is possible that Vpr may facilitate the formation of TAK1-TAB2/3 complexes, and therefore enhances TRAF6-mediated K63-linked polyubiquitination of TAK1. We also found that polyubiquitination of TAK1 is crucial for its phosphorylation and activation (Figure 4B and C). Several lysine residues within TAK1, including Lys-34, Lys-158, and Lys-209, have been reported to be ubiquitinated in response to various stimuli [41, 43–47]. Of these three lysines, Lys-158 and Lys-209 are located in the kinase domain of TAK1 and are essential in the polyubiquitination and activation of TAK1, with Lys-158 playing a major role (Figure 4B). These data support the notion that polyubiquitination of TAK1 may cause conformational change of the TAK1 that exposes the critical phosphorylation site (Thr-187) in the activation loop of TAK1 for auto-phosphorylation.
Several viral proteins have been reported to co-opt cellular signaling molecules. For example, HTLV-1 Tax protein constitutively activates NF-κB by directly interacting with NEMO in viral infected T lymphoma cell lines . Recently, it was demonstrated that Tax interacts with TAK1 and recruits IKK complex to TAK1 [62, 63]. In a similar fashion, we previously showed that Vpr activates both canonical and noncanonical NF-κB signaling by enhancing the phosphorylation of IKK complex . Results in this study further show that TAK1 is the upstream kinase that is activated by Vpr and subsequently acts on IKK/NF-κB pathway. Activation of both NF-κB and AP-1 signaling suffice to enhance LTR-dependent viral gene expression and up-regulate the expression of several cytokines, such as IL-6, IL-8, and CCL5 [16–18]. This may facilitate the virus to establish a productive infection at the very early stage.
In summary, results of this study demonstrate the role of TAK1 in Vpr-induced NF-κB and AP-1 activation. Vpr itself is able to enhance the phosphorylation and polyubiquitination of TAK1, and as a result, activates HIV-1 LTR promoter. These results support an important role of Vpr in assisting HIV-1 to establish productive infection by elevating early viral gene expression through modifying the cellular environment.
The DNA constructs Myc-TAK1, HA-TAB1, HA-TRAF6, HA-IKKβ, 3 × NF-κB-Luc, 7 × AP-1-Luc, and HIV-1 LTR-Luc were described previously [50, 55, 64–66]. The DNA constructs T7-TAB3 were gifts from Giichi Takaesu (Center for Integrated Medicine Research, School of Medicine, Keio University, Japan) . The pCDNA-Flag Vpr (Flag-Vpr) construct was kindly provided by Kuan-Teh Jeang (Molecular Virology Section, Laboratory of Molecular Microbiology, National Institutes of Allergy and Infectious Diseases, USA) . Plasmid pcDNA-FLAG Vpr was constructed by PCR amplification of pNL4-3 Vpr and cloning of PCR products into pCDNA3.1 vector.
The DNA construct pcDNA3-Vpr was generated by overlap PCR using pCDNA-Flag Vpr as a template. The coding sequence of human full length MKK7 was amplified from HEK293T cDNA, then cloned into the pCMV-HA expression vector (Clontech).
Myc-TAK1 K34R, K158R, K209R and Flag-Vpr S79A were generated using PCR-based mutagenesis. The PCR primers are listed in Additional file 1: Table S1.
The pNLENY1-ES-IRES DNA construct was kindly provided by David Levy . This HIV-1 DNA was derived from the NL4-3 strain by inserting two-stop codons into the envelope gene. The enhanced yellow fluorescent protein (YFP) sequence was inserted between the env and nef ORFs. An internal ribosome entry sequence (IRES) was inserted upstream of nef to direct the expression of Nef. All accessory genes are left intact. The HIV-1 Luc was a gift from Johnny J He . This proviral DNA construct was derived from the NL4-3 strain with env gene inactivated and the firefly luciferase (Luc) gene in place of HIV-1 nef. The pNLENY1-ΔVpr and pHIV-Luc ΔVpr were derived from pNLENY1-ES-IRES or pHIV-Luc by mutating the start codon ATG of vpr gene . The pNLENY1-S79A was generated with PCR-based mutagenesis using primers listed in Additional file 1: Table S1.
All DNA constructs used in this study were verified by sequencing.
Cell culture and transfection
HeLa and HEK293T cells were maintained in Dulbecco’s modified Eagle’s medium (high glucose) supplemented with 10% FBS (Gibco), 100 U/ml penicillin/streptomycin (Invitrogen). Jurkat, THP-1 and human peripheral blood mononuclear cells (PBMCs) were maintained in RPMI 1640 supplemented with 10% FBS, 100 U/ml penicillin/streptomycin, 2.4 mM L-glutamine. Cells were maintained in a humidified atmosphere containing 5% CO2 at 37°C. HeLa and HEK293T were transfected by PEI (Polysciences) or Lipofectamine 2000 (Invitrogen) in accordance with the manufacturer’s instructions.
PBMCs were isolated from buffy coat from healthy blood donors by histopaque and percoll gradient centrifugation, and were activated with phytohemagglutinin (PHA; 5 mg/ml) (Sigma) and IL-2 (20 U/ml) (PeproTech) for 24 h before virus infection.
To differentiate THP-1 monocytes into macrophage-like cells, THP-1 cells were seeded at a concentration of 1 × 106 cells/ml in 1.5 ml fresh cell culture medium containing 200 ng/ml tetradecanoylphorbol acetate (PMA) (Sigma). After twenty-four hours, cell culture media were changed to remove PMA. Then cells were rested for an additional 48 hours before use.
Antibodies and reagents
Anti-phospho-TAK1 (Thr-187), rabbit anti-TAK1, anti-TRAF2, anti-TRAF6, anti-TAB1, anti-TAB2, anti-phospho-IKKα/β (Ser176/180), anti-phospho-MKK7 (Ser-271/Thr-275), and anti-MKK7 antibodies were purchased from Cell Signaling Technology. Anti-TAB3 antibody was from Abcam. Anti-Myc antibody, normal mouse IgG and normal rabbit IgG were from Millipore. Anti-Flag antibody (M2) was obtained from Sigma. Anti-HA, anti-α-Tubulin, and horseradish peroxidase-conjugated secondary antibodies were from Santa Cruz Biotechnology. Rabbit anti-Vpr antibody was provided by National Institutes of Health (NIH) AIDS Research and Reference Reagent Program. Mouse anti-p24 and anti-TAK1 were generated by immunizing mice with the corresponding full length proteins purified form E. coli BL21 (DE3). 4′, 6-diamidino-2-phenylindole (DAPI) and PMA were purchased from Sigma. Fluorescein-conjugated anti-mouse and anti-rabbit secondary antibodies were purchased from Jackson ImmunoResearch Laboratories. Serine/Threonine phosphatase inhibitor Calyculin A was purchased from Cell Signaling Technology.
Generation of stably transduced cell lines
First, shRNA oligos targeting TAK1, TAB3, and TRAF6 were designed by shRNA Sequence Designer (Clontech). Double-stranded oligonucleotides corresponding to the target sequences were cloned into the pSIREN-RetroQ(Clontech). The target sequences were as follows, shControl (without specific target in cells) (GACAGAACCAGAGGATAGA), TAK1 (AGGCAAAGCAACAGAGTGA), TAB3 (CCTTCACCCATCAGTAATC), TRAF6 (GCCTGGATTCTACACTGGCAAA), and TRAF2 (GGACCAAGACAAGATTGA A).
Then, retrovirus particles were prepared by transfecting HEK293T cells with 1 μg pMLV-Gag-Pol, 0.5 μg pVSV-G and 1 μg pSIREN-RetroQ DNA constructs. After forty-eight hours, supernatants were collected and centrifuged at 3,000 rpm to remove cell debris. HeLa cells were infected with the harvested virus particles in the presence of 5 μg/ml polybrene by spinoculation at 450 × g for 30 min at room temperature. Forty-eight hours after infection, cells were subcultured in selection medium containing 2 μg/ml puromycin (Sigma). Knockdown efficiency was assessed by Western blotting using specific antibodies.
Viruses and infections
Infection with VSV-G pseudotyped HIV-1 viruses were performed as we previously described . Briefly, the viruses were produced by transfecting HEK293T cells with plasmid DNA pVSV-G, and pNLENY1-ES-IRES or pNLENY1-ΔVpr. After forty-eight hours, the viral supernatants were collected, and the virus titer was determined by ELISA (Biomerieux) to quantify viral p24 amounts. The viruses (equivalent to 500 ng p24) were used to infect Jurkat, THP-1, THP-1 differentiated macrophage, and PBMC in the presence of 5 μg/ml polybrene by spinoculation in 1.6 ml fresh medium. After two hours, the infected cells were harvested to detect HIV-1-induced phosphorylation of endogenous TAK1.
Protein phosphorylation analysis
At 2 hours post-infection or 48 hours post-transfection, cells were harvested by centrifugation at 1000 × g for 3 min at 4°C, washed twice with ice-cold 1xphosphate-buffered saline, and suspended in 70 μl lysis buffer (20 mM Tris (pH 7.4), 150 mM NaCl, 2 mM EDTA, 1% Triton-X-100, 1 mM sodium orthovanadate, 50 mM sodium fluoride, phosphatase inhibitor mixture tablet (Roche), protease inhibitor cocktail tablets complete, EDTA-free (Roche)), and chilled on ice for 30 min with frequent agitation. Cell lysates were collected by centrifugation at 13,000 × g for 10 min at 4°C. The supernatants were separated by SDS-PAGE and subjected to Western blotting.
For protein-protein interaction analysis, HEK293T cells were washed twice with ice-cold 1xphosphate-buffered saline, and lysed in lysis buffer (50 mM Tris (pH 7.4), 150 mM NaCl, 2 mM EDTA, 3% Glycerol, 1% NP-40, protease inhibitor cocktail tablets complete, EDTA-free(Roche)). The cell lysates were sonicated and centrifuged at 13,000 × g for 10 min at 4°C. To detect the polyubiquitinated form of TAK1 and TRAF6, in vivo ubiquitination assay were performed as follows. Cells were lysed in 0.4 ml TNET buffer (20 mM Tris (pH 7.4), 150 mM NaCl, 5 mM EDTA, 1% Triton-X-100). Supernatants were denatured in 1% SDS by boiling for 15 min to remove noncovalently attached proteins, and then diluted to 0.1% SDS with regular TENT buffer. Then, supernatants were incubated with indicated antibody for 3 h at 4°C and rotated with Protein A-agarose (Millipore) for 3 h or overnight (for endogenous protein immunoprecipitation) at 4°C. After six times washes with lysis buffer, the immunoprecipitated materials were boiled in 40 μl 2 × SDS loading buffer and subjected to Western blotting.
Cell lysates or immunoprecipitated materials were resolved by SDS-PAGE and transferred to a PVDF membrane (GE Healthcare). The membranes were blocked in 5% non-fat milk-TBS for 45 min at room temperature, and probed with indicated primary antibodies overnight at 4°C. After hybridizing the membranes with either goat anti-rabbit or goat anti-mouse secondary antibody, enhanced chemiluminescence reagents (Millipore) were used for signal detection with X-ray film.
Immunofluorescence microscopy assay (IFA)
Indirect IFA was performed as we previously described . HeLa cells grown on poly-lysine-coated glass slides were first fixed with 4% paraformaldehyde in PBS for 10 min and then permeabilized with 0.1% Triton X-100 in PBS for 10 min. After incubation in the blocking buffer containing 3% BSA and 6% skim milk, cells were stained with anti-Flag or anti-Myc primary antibodies (1:100 dilution, 2 h at room temperature) followed by FITC- or TRITC-conjugated secondary antibodies (1:100 dilution, 45 min at room temperature). DAPI was utilized to stain nuclei. Images were captured using Leica TCS SP5 laser scanning confocal microscope.
HeLa RNAi cell lines were seeded at a concentration of 0.1 × 106 cells/well. The next day, cells were transfected with indicated reporter gene and Flag-Vpr DNA along with the Renilla luciferase DNA. After forty-eight hours, luciferase activity was measured with a Dual-Luciferase reporter assay system according to the manufacturer’s instructions (Promega). The relative luciferase activity was calculated by dividing the firefly luciferase activity by the Renilla luciferase activity. Three independent transfection experiments were performed.
Data were presented as mean values ± SD of at least three independent experiments. Statistical comparison of Vpr-transfected cells with matched control cells was performed using F-test and Student’s T-test. P values of <0.05 were considered to be statistically significant.
We thank Dr. Kuan-Teh Jeang (Molecular Virology Section, Laboratory of Molecular Microbiology, National Institutes of Allergy and Infectious Diseases, USA), Dr. Giichi Takaesu (Center for Integrated Medicine Research, School of Medicine, Keio University, Japan), Dr. David Levy (Departments of Medicine, University of Alabama at Birmingham, USA), Dr. Johnny J He (Department of Microbiology and Immunology, Center for AIDS Research, Indiana University School of Medicine, USA), Dr. Chen Wang (Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, China) and National Institutes of Health (NIH) AIDS Research and Reference Reagent Program for providing valuable regents.
The work was supported by grants from the Chinese Ministry of Health (2012ZX10001006), the National Natural Science Foundation of China (81071343 and 81271812) and 111 Project (B08011).
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