Expression of IkappaBalpha in the nucleus of human peripheral blood T lymphocytes

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.

The mutation of Transportin 3 gene that causes limb girdle muscular dystrophy 1F induces protection against HIV-1 infection

The causative mutation responsible for limb girdle muscular dystrophy 1F (LGMD1F) is one heterozygous single nucleotide deletion in the stop codon of the nuclear import factor Transportin 3 gene (TNPO3). This mutation causes a carboxy-terminal extension of 15 amino acids, producing a protein of unknown function (TNPO3_mut) that is co-expressed with wild-type TNPO3 (TNPO3_wt). TNPO3 has been involved in the nuclear transport of serine/arginine-rich proteins such as splicing factors and also in HIV-1 infection through interaction with the viral integrase and capsid. We analyzed the effect of TNPO3_mut on HIV-1 infection using PBMCs from patients with LGMD1F infected ex vivo. HIV-1 infection was drastically impaired in these cells and viral integration was reduced 16-fold. No significant effects on viral reverse transcription and episomal 2-LTR circles were observed suggesting that the integration of HIV-1 genome was restricted. This is the second genetic defect described after CCR5Δ32 that shows strong resistance against HIV-1 infection.

Different Expression of Interferon-Stimulated Genes in Response to HIV-1 Infection in Dendritic Cells Based on Their Maturation State

Dendritic cells (DCs) are professional antigen-presenting cells whose functions are dependent on their degree of differentiation. In their immature state, DCs capture pathogens and migrate to the lymph nodes. During this process, DCs become resident mature cells specialized in antigen presentation. DCs are characterized by a highly limiting environment for human immunodeficiency virus type 1 (HIV-1) replication due to the expression of restriction factors such as SAMHD1 and APOBEC3G. However, uninfected DCs capture and transfer viral particles to CD4 lymphocytes through a trans-enhancement mechanism in which chemokines are involved. We analyzed changes in gene expression with whole-genome microarrays when immature DCs (IDCs) or mature DCs (MDCs) were productively infected using Vpx-loaded HIV-1 particles. Whereas productive HIV infection of IDCs induced expression of interferon-stimulated genes (ISGs), such induction was not produced in MDCs, in which a sharp decrease in ISG- and CXCR3-binding chemokines was observed, lessening trans-infection of CD4 lymphocytes. Similar patterns of gene expression were found when DCs were infected with HIV-2 that naturally expresses Vpx. Differences were also observed under conditions of restrictive HIV-1 infection, in the absence of Vpx. ISG expression was not modified in IDCs, whereas an increase of ISG- and CXCR3-binding chemokines was observed in MDCs. Overall these results suggest that sensing and restriction of HIV-1 infection are different in IDCs and MDCs. We propose that restrictive infection results in increased virulence through different mechanisms. In IDCs avoidance of sensing and induction of ISGs, whereas in MDCs increased production of CXCR3-binding chemokines, would result in lymphocyte attraction and enhanced infection at the immune synapse.IMPORTANCE In this work we describe for the first time the activation of a different genetic program during HIV-1 infection depending on the state of maturation of DCs. This represents a breakthrough in the understanding of the restriction to HIV-1 infection of DCs. The results show that infection of DCs by HIV-1 reprograms their gene expression pattern. In immature cells, productive HIV-1 infection activates interferon-related genes involved in the control of viral replication, thus inducing an antiviral state in surrounding cells. Paradoxically, restriction of HIV-1 by SAMHD1 would result in lack of sensing and IFN activation, thus favoring initial HIV-1 escape from the innate immune response. In mature DCs, restrictive infection results in HIV-1 sensing and induction of ISGs, in particular CXCR3-binding chemokines, which could favor the transmission of HIV to lymphocytes. Our data support the hypothesis that genetic DC reprograming by HIV-1 infection favors viral escape and dissemination, thus increasing HIV-1 virulence.

Underlying mechanisms of HIV-1 latency

Similarly to other retroviruses, HIV-1 integrates its genome into the cellular chromosome. Expression of viral genes from the integrated viral DNA could then be regulated by the host genome. If the infected cell suppresses viral gene expression, the virus will undergo latency. The latently infected cells cannot be detected or cleared by the immune system since they do not express viral antigens. These cells remain undetected for several years, even under antiretroviral treatments. The silenced HIV-1 DNA could be reactivated under certain conditions. Despite the efficient use of antiretroviral drugs, HIV-1 latently infected cells remain the major obstacles to a permanent cure. In this review, we discuss the cellular and molecular mechanisms through which HIV-1 establishes latency.

PKCθ and HIV-1 Transcriptional Regulator Tat Co-exist at the LTR Promoter in CD4(+) T Cells

PKCθ is essential for the activation of CD4⁺ T cells. Upon TCR/CD28 stimulation, PKCθ is phosphorylated and migrates to the immunological synapse, inducing the activation of cellular transcription factors such as NF-κB and kinases as ERK that are critical for HIV-1 replication. We previously demonstrated that PKCθ is also necessary for HIV-1 replication but the precise mechanism is unknown. Efficient HIV-1 transcription and elongation are absolutely dependent on the synergy between NF-κB and the viral regulator Tat. Tat exerts its function by binding a RNA stem-loop structure proximal to the viral mRNA cap site termed TAR. Besides, due to its effect on cellular metabolic pathways, Tat causes profound changes in infected CD4⁺ T cells such as the activation of NF-κB and ERK. We hypothesized that the aberrant upregulation of Tat-mediated activation of NF-κB and ERK occurred through PKCθ signaling. In fact, Jurkat TetOff cells with stable and doxycycline-repressible expression of Tat (Jurkat-Tat) expressed high levels of mRNA for PKCθ. In these cells, PKCθ located at the plasma membrane was phosphorylated at T⁵³⁸ residue in undivided cells, in the absence of stimulation. Treatment with doxycycline inhibited PKCθ phosphorylation in Jurkat-Tat, suggesting that Tat expression was directly related to the activation of PKCθ. Both NF-κB and Ras/Raf/MEK/ERK signaling pathway were significantly activated in Jurkat-Tat cells, and this correlated with high transactivation of HIV-1 LTR promoter. RNA interference for PKCθ inhibited NF-κB and ERK activity, as well as LTR-mediated transactivation even in the presence of Tat. In addition to Tat-mediated activation of PKCθ in the cytosol, we demonstrated by sequential ChIP that Tat and PKCθ coexisted in the same complex bound at the HIV-1 LTR promoter, specifically at the region containing TAR loop. In conclusion, PKCθ-Tat interaction seemed to be essential for HIV-1 replication in CD4⁺ T cells and could be used as a therapeutic target.

Dasatinib inhibits HIV-1 replication through the interference of SAMHD1 phosphorylation in CD4+ T cells

Massive activation of infected CD4+ T cells during acute HIV-1 infection leads to reservoir seeding and T-cell destruction. During T-cell activation, the antiviral effect of the innate factor SAMHD1 is neutralized through phosphorylation at T592, allowing HIV-1 infection. Dasatinib, a tyrosine kinase inhibitor currently used for treating chronic myeloid leukemia, has been described to control HIV-1 replication through its negative effect on T-cell proliferation and viral entry. We demonstrate that Dasatinib can actually interfere with SAMHD1 phosphorylation in human peripheral blood lymphocytes, preserving its antiviral activity against HIV-1. Dasatinib prevented SAMHD1 phosphorylation in vitro and ex vivo, impairing HIV-1 reverse transcription and proviral integration. This was the major mechanism of action because the presence of Vpx, which degrades SAMHD1, in HIV-1 virions impeded the inhibitory effect of Dasatinib on HIV-1 replication. In fact, infection with VSV-pseudotyped HIV-1 virions and fusion of BlaM-Vpr-containing HIV-1 viruses with activated PBMCs in the presence of Dasatinib suggested that Dasatinib was not acting at fusion level. Finally, PBMCs from patients on chronic treatment with Dasatinib showed a lower level of SAMHD1 phosphorylation in response to activating stimuli and low susceptibility to HIV-1 infection ex vivo. Consequently, Dasatinib is a compound currently used in clinic that preserves the antiviral function of SAMHD1. Using Dasatinib as adjuvant of antiretroviral therapy during early primary HIV-1 infection would contribute to reduce viral replication and spread, prevent reservoir seeding, and preserve CD4 counts and CTL responses. These events would create a more favorable virologic and immunologic environment for future interventional studies aiming at HIV-1 eradication.

Analysis of protein kinase C theta inhibitors for the control of HIV-1 replication in human CD4+ T cells reveals an effect on retrotranscription in addition to viral transcription

HIV-1 infection cannot be cured due to reservoirs formed early after infection. Decreasing the massive CD4+ T cell activation that occurs at the beginning of the disease would delay reservoir seeding, providing a better prognosis for patients. CD4+ T cell activation is mediated by protein kinase C (PKC) theta (θ), which is involved in T-cell proliferation, as well as NF-κB, NF-AT, and AP-1 activation. We found that PKCθ activity increased viral replication, but also that HIV-1 induced higher activation of PKCθ in infected CD4+ T cells, creating a feedback loop. Therefore, specific inhibition of PKCθ activity could contribute to control HIV-1 replication. We tested the efficacy of seven PKCθ specific inhibitors to control HIV-1 replication in CD4+ T cells and selected two of the more potent and safer: CGX1079 and CGX0471. They reduced PKCθ phosphorylation at T538 and its translocation to the plasma membrane, which correlated with decreased HIV-1 retrotranscription through partial inhibition of SAMHD1 antiviral activity, rendering lower proviral integration. CGX1079 and CGX0471 also interfered with viral transcription, which would reduce the production of new virions, as well as the subsequent spread and infection of new targets that would increase the reservoir size. CGX1079 and CGX0471 did not completely abrogate T-cell functions such as proliferation and CD8-mediated release of IFN-γ in PBMCs from HIV-infected patients, thereby avoiding general immunosuppresion. Consequently, using PKCθ inhibitors as adjuvant of antiretroviral therapy in recently infected patients would decrease the pool of activated CD4+ T cells, thwarting proviral integration and reducing the reservoir size.

Transcription elongation regulator 1 (TCERG1) regulates competent RNA polymerase II-mediated elongation of HIV-1 transcription and facilitates efficient viral replication

Background

Control of RNA polymerase II (RNAPII) release from pausing has been proposed as a checkpoint mechanism to ensure optimal RNAPII activity, especially in large, highly regulated genes. HIV-1 gene expression is highly regulated at the level of elongation, which includes transcriptional pausing that is mediated by both viral and cellular factors. Here, we present evidence for a specific role of the elongation-related factor TCERG1 in regulating the extent of HIV-1 elongation and viral replication in vivo.

Results

We show that TCERG1 depletion diminishes the basal and viral Tat-activated transcription from the HIV-1 LTR. In support of a role for an elongation mechanism in the transcriptional control of HIV-1, we found that TCERG1 modifies the levels of pre-mRNAs generated at distal regions of HIV-1. Most importantly, TCERG1 directly affects the elongation rate of RNAPII transcription in vivo. Furthermore, our data demonstrate that TCERG1 regulates HIV-1 transcription by increasing the rate of RNAPII elongation through the phosphorylation of serine 2 within the carboxyl-terminal domain (CTD) of RNAPII and suggest a mechanism for the involvement of TCERG1 in relieving pausing. Finally, we show that TCERG1 is required for HIV-1 replication.

Conclusions

Our study reveals that TCERG1 regulates HIV-1 transcriptional elongation by increasing the elongation rate of RNAPII and phosphorylation of Ser 2 within the CTD. Based on our data, we propose a general mechanism for TCERG1 acting on genes that are regulated at the level of elongation by increasing the rate of RNAPII transcription through the phosphorylation of Ser2. In the case of HIV-1, our evidence provides the basis for further investigation of TCERG1 as a potential therapeutic target for the inhibition of HIV-1 replication

The presence of HIV-1 Tat protein second exon delays fas protein-mediated apoptosis in CD4+ T lymphocytes: a potential mechanism for persistent viral production

HIV-1 replication is efficiently controlled by the regulator protein Tat (101 amino acids) and codified by two exons, although the first exon (1-72 amino acids) is sufficient for this process. Tat can be released to the extracellular medium, acting as a soluble pro-apoptotic factor in neighboring cells. However, HIV-1-infected CD4(+) T lymphocytes show a higher resistance to apoptosis. We observed that the intracellular expression of Tat delayed FasL-mediated apoptosis in both peripheral blood lymphocytes and Jurkat cells, as it is an essential pathway to control T cell homeostasis during immune activation. Jurkat-Tat cells showed impairment in the activation of caspase-8, deficient release of mitochondrial cytochrome c, and delayed activation of both caspase-9 and -3. This protection was due to a profound deregulation of proteins that stabilized the mitochondrial membrane integrity, such as heat shock proteins, prohibitin, or nucleophosmin, as well as to the up-regulation of NF-κB-dependent anti-apoptotic proteins, such as BCL2, c-FLIPS, XIAP, and C-IAP2. These effects were observed in Jurkat expressing full-length Tat (Jurkat-Tat101) but not in Jurkat expressing the first exon of Tat (Jurkat-Tat72), proving that the second exon, and particularly the NF-κB-related motif ESKKKVE, was necessary for Tat-mediated protection against FasL apoptosis. Accordingly, the protection exerted by Tat was independent of its function as a regulator of both viral transcription and elongation. Moreover, these data proved that HIV-1 could have developed strategies to delay FasL-mediated apoptosis in infected CD4(+) T lymphocytes through the expression of Tat, thus favoring the persistent replication of HIV-1 in infected T cells.

Protein kinase Ctheta is a specific target for inhibition of the HIV type 1 replication in CD4+ T lymphocytes

Integration of HIV-1 genome in CD4(+) T cells produces latent reservoirs with long half-life that impedes the eradication of the infection. Control of viral replication is essential to reduce the size of latent reservoirs, mainly during primary infection when HIV-1 infects CD4(+) T cells massively. The addition of immunosuppressive agents to highly active antiretroviral therapy during primary infection would suppress HIV-1 replication by limiting T cell activation, but these agents show potential risk for causing lymphoproliferative disorders. Selective inhibition of PKC, crucial for T cell function, would limit T cell activation and HIV-1 replication without causing general immunosuppression due to PKC being mostly expressed in T cells. Accordingly, the effect of rottlerin, a dose-dependent PKC inhibitor, on HIV-1 replication was analyzed in T cells. Rottlerin was able to reduce HIV-1 replication more than 20-fold in MT-2 (IC(50) = 5.2 μM) and Jurkat (IC(50) = 2.2 μM) cells and more than 4-fold in peripheral blood lymphocytes (IC(50) = 4.4 μM). Selective inhibition of PKC, but not PKCδ or -ζ, was observed at <6.0 μM, decreasing the phosphorylation at residue Thr(538) on the kinase catalytic domain activation loop and avoiding PKC translocation to the lipid rafts. Consequently, the main effector at the end of PKC pathway, NF-κB, was repressed. Rottlerin also caused a significant inhibition of HIV-1 integration. Recently, several specific PKC inhibitors have been designed for the treatment of autoimmune diseases. Using these inhibitors in combination with highly active antiretroviral therapy during primary infection could be helpful to avoid massive viral infection and replication from infected CD4(+) T cells, reducing the reservoir size at early stages of the infection.

Modifications in host cell cytoskeleton structure and function mediated by intracellular HIV-1 Tat protein are greatly dependent on the second coding exon

The human immunodeficiency virus type 1 (HIV-1) regulator Tat is essential for viral replication because it achieves complete elongation of viral transcripts. Tat can be released to the extracellular space and taken up by adjacent cells, exerting profound cytoskeleton rearrangements that lead to apoptosis. In contrast, intracellular Tat has been described as protector from apoptosis. Tat gene is composed by two coding exons that yield a protein of 101 amino acids (aa). First exon (1–72aa) is sufficient for viral transcript elongation and second exon (73–101 aa) appears to contribute to non-transcriptional functions. We observed that Jurkat cells stably expressing intracellular Tat101 showed gene expression deregulation 4-fold higher than cells expressing Tat72. Functional experiments were performed to evaluate the effect of this deregulation. First, NF-κB-, NF-AT- and Sp1-dependent transcriptional activities were greatly enhanced in Jurkat-Tat101, whereas Tat72 induced milder but efficient activation. Second, cytoskeleton-related functions as cell morphology, proliferation, chemotaxis, polarization and actin polymerization were deeply altered in Jurkat-Tat101, but not in Jurkat-Tat72. Finally, expression of several cell surface receptors was dramatically impaired by intracellular Tat101 but not by Tat72. Consequently, these modifications were greatly dependent on Tat second exon and they could be related to the anergy observed in HIV-1-infected T cells.

Beta2-adrenergic receptor regulate Toll-like receptor 4-induced late-phase NF-kappaB activation

Stimulation of Toll-like receptor 4 (TLR4) by lipopolysaccharide (LPS) triggers myeloid differentiation factor 88 (MyD88)-dependent early-phase NF-kappaB activation and Toll/IL-1 receptor domain-containing adaptor-inducing IFN-beta (TRIF)-dependent late-phase NF-kappaB activation. In a previous study, we have shown that beta(2)-adrenergic receptor (beta(2)AR) functions as a negative regulator of NF-kappaB activation through beta-arrestin 2 in the macrophage cell line RAW264 and that down-regulation of beta(2)AR expression in response to LPS is essential for NF-kappaB activation and expression of its target gene, inducible nitric oxide synthase (NOS II). Here, we demonstrate that beta(2)AR plays an important role in TRIF-dependent late-phase NF-kappaB activation. LPS-stimulated down-regulation was induced in MyD88-knockdown cells, but not in TRIF-knockdown cells, suggesting that beta(2)AR expression was down-regulated by the TRIF-dependent pathway. On the other hand, depletion of beta(2)AR or beta-arrestin 2 expression by siRNA decreased cytoplasmic IkappaB alpha and abrogated late-phase IkappaB alpha degradation and NF-kappaB activation in response to LPS. Inducible nitric oxide synthase (NOS II) expression was increased continuously during 24 h of LPS stimulation in control cells, but decreased in beta(2)AR or beta-arrestin 2-knockdown cells after 6 h of LPS stimulation. These findings suggest that beta(2)AR functions not only as a negative regulator of NF-kappaB activation, but also as a stabilizing factor of the NF-kappaB/IkappaB alpha complex through cytoplasmic beta-arrestin 2, and that TRIF-dependent down-regulation of beta(2)AR expression increases the level of cytoplasmic NF-kappaB/IkappaB alpha complex free from beta-arrestin 2, leading to continuous late-phase NF-kappaB activation.

Caspase-3-mediated cleavage of p65/RelA results in a carboxy-terminal fragment that inhibits IkappaBalpha and enhances HIV-1 replication in human T lymphocytes

Background

Degradation of p65/RelA has been involved in both the inhibition of NF-κB-dependent activity and the onset of apoptosis. However, the mechanisms of NF-κB degradation are unclear and can vary depending on the cell type. Cleavage of p65/RelA can produce an amino-terminal fragment that was shown to act as a dominant-negative inhibitor of NF-κB, thereby promoting apoptosis. However, the opposite situation has also been described and the production of a carboxy-terminal fragment that contains two potent transactivation domains has also been related to the onset of apoptosis. In this context, a carboxy-terminal fragment of p65/RelA (ΔNH₂p65), detected in non-apoptotic human T lymphocytes upon activation, has been studied. T cells constitute one of the long-lived cellular reservoirs of the human immunodeficiency virus type 1 (HIV-1). Because NF-κB is the most important inducible element involved in initiation of HIV-1 transcription, an adequate control of NF-κB response is of paramount importance for both T cell survival and viral spread. Its major inhibitor IκBα constitutes a master terminator of NF-κB response that is complemented by degradation of p65/RelA.

Results and conclusions

In this study, the function of a caspase-3-mediated carboxy-terminal fragment of p65/RelA, which was detected in activated human peripheral blood lymphocytes (PBLs), was analyzed. Cells producing this truncated p65/RelA did not undergo apoptosis but showed a high viability, in spite of caspase-3 activation. ΔNH₂p65 lacked most of DNA-binding domain but retained the dimerization domain, NLS and transactivation domains. Consequently, it could translocate to the nucleus, associate with NF-κB1/p50 and IκBα, but could not bind -κB consensus sites. However, although ΔNH₂p65 lacked transcriptional activity by itself, it could increase NF-κB activity in a dose-dependent manner by hijacking IκBα. Thus, its expression resulted in a persistent transactivation activity of wild-type p65/RelA, as well as an improvement of HIV-1 replication in PBLs. Moreover, ΔNH₂p65 was increased in the nuclei of PMA-, PHA-, and TNFα-activated T cells, proving this phenomenon was related to cell activation. These data suggest the existence of a novel mechanism for maintaining NF-κB activity in human T cells through the binding of the carboxy-terminal fragment of p65/RelA to IκBα in order to protect wild-type p65/RelA from IκBα inhibition.

Modifications in the human T cell proteome induced by intracellular HIV-1 Tat protein expression

The effects of the human immunodeficiency virus type 1 (HIV-1) Tat protein on cellular gene expression were analysed using a Jurkat cell line that was stably transfected with tat gene in a doxycycline-repressible expression system. Expressed Tat protein (aa 1-101) was proved to present basically a nuclear localisation, and to be fully functional to induce HIV LTR transactivation. Tat expression also resulted in protection from Tunicamycin-induced apoptosis as determined by DNA staining and TUNEL assays. We applied proteomics methods to investigate changes in differential protein expression in the transfected Jurkat-Tat cells. Protein identification was performed using 2-D DIGE followed by MS analysis. We identified the down-regulation of several cytoskeletal proteins such as actin, beta-tubulin, annexin II, as well as gelsolin, cofilin and the Rac/Rho-GDI complex. Down-expression of these proteins could be involved in the survival of long-term reservoirs of HIV-infected CD4+ T cells responsible for continuous viral production. In conclusion, in addition to its role in viral mRNA elongation, the proteomic approach has provided insight into the way that Tat modifies host cell gene expression.

Beta2-adrenergic receptor regulates Toll-like receptor-4-induced nuclear factor-kappaB activation through beta-arrestin 2

Toll-like receptors (TLRs) play an important role in innate immunity while, beta(2)-adrenergic receptors (beta(2)AR) provide the key linkages for the sympathetic nervous system to regulate the immune system. However, their role in macrophages remains uncertain. Here, we demonstrate the cross-talk between beta(2)AR and TLR signalling pathways. Expression of beta(2)AR was down-regulated by TLR4 ligand lipopolysaccharide (LPS) stimulation. To investigate the physiological consequence of this down-regulation RAW264 cells, a macrophage cell line, were transfected with a beta(2)AR expression vector (RAWar). Both LPS-stimulated inducible nitric oxide synthase (NOS II) expression and NO production were markedly suppressed in the RAWar cells. The activation of nuclear factor-kappaB (NF-kappaB) and degradation of the inhibitor of NF-kappaB (IkappaBalpha) in response to LPS were markedly decreased in these cells. The level of beta-arrestin 2, which regulates beta(2)AR signalling, was also reduced in RAW264 cells after stimulation with LPS, but not in RAWar cells. Overexpression of beta-arrestin 2 (RAWarr2) also inhibited NO production and NOS II expression. Furthermore, we demonstrated that beta-arrestin 2 interacted with cytosolic IkappaBalpha and that the level of IkappaBalpha coimmunoprecipitated by anti-beta-arrestin 2 antibodies was decreased in the RAW264 cells but not in RAWar or RAWarr2 cells. These findings suggest that LPS-stimulated signals suppress beta(2)AR expression, leading to down-regulation of beta-arrestin 2 expression, which stabilizes cytosolic IkappaBalpha and inhibits the NF-kappaB activation essential for NOS II expression, probably to ensure rapid and sufficient production of NO in response to microbial attack.

Basal shuttle of NF-kappaB/I kappaB alpha in resting T lymphocytes regulates HIV-1 LTR dependent expression

Background

In HIV-infected T lymphocytes, NF-κB/Rel transcription factors are major elements involved in the activation of LTR-dependent transcription from latency. Most NF-κB heterodimer p65/p50 is sequestered as an inactive form in the cytoplasm of resting T lymphocytes via its interaction with IκB inhibitors. In these cells, both absolute HIV latency and low level ongoing HIV replication have been described. These situations could be related to differences in the balance between NF-κB and IκBα ratio. Actually, control of IκBα by cellular factors such as Murr-1 plays a critical role in maintaining HIV latency in unstimulated T lymphocytes. Formerly, our group demonstrated the presence of nuclear IκBα in T cells after PMA activation. Now we attempt to determine the dynamics of NF-κB/IκBα nucleocytosolic transport in absence of activation as a mechanism to explain both the maintenance of latency and the existence of low level ongoing HIV replication in resting CD₄⁺ T lymphocytes.

Results and conclusion

We show that the inhibition of the nuclear export by leptomycin B in resting CD₄⁺ T cells resulted in nuclear accumulation of both IκBα and p65/RelA, as well as formation of NF-κB/IκBα complexes. This proves the existence of a rapid shuttling of IκBα between nucleus and cytosol even in absence of cellular activation. The nuclear accumulation of IκBα in resting CD₄⁺ T lymphocytes results in inhibition of HIV-LTR dependent transcription as well as restrains HIV replication in CD₄⁺ T lymphocytes. On the other hand, basal NF-κB activity detected in resting CD₄⁺ T lymphocytes was related to low level HIV replication in these cells.

Characterization of glucocorticoid receptor translocation, cytoplasmic IkappaB, nuclear NFkappaB, and activation of NFkappaB in T lymphocytes exposed to stress-inducible concentrations of corticosterone in vivo

The present study was conducted to determine if selected events in glucocorticoid receptor (GR) signaling that have been identified using mostly in vitro approaches with synthetic glucocorticoids also occur in mature T cells exposed to relevant levels of corticosterone in vivo. In contrast to effects reported in vitro, corticosterone did not cause significant translocation of GR to the nucleus in splenic T cells, though it did increase the amount of nuclear GR in these cells capable of binding to a glucocorticoid response element. At most time points and dosages, corticosterone caused little or no change in cytoplasmic IkappaB or nuclear NFkappaB levels. Activation of T cells by anti-CD3 increased the amount of NFkappaB in the nucleus and decreased the amount of IkappaB in the cytoplasm. Corticosterone did not significantly inhibit the decrease in cytoplasmic IkappaB, but it did slightly diminish the increase in nuclear NFkappaB. The same dosages of corticosterone substantially suppressed anti-CD3-induced cytokine gene expression, indicating that a meaningful amount of glucocorticoid-mediated signaling (of some type) occurred in this experimental system. Thus, GR translocation per se seems not to be critical for GR-mediated signaling in vivo. Other considerations, such as the make-up of GR-containing complexes in the nucleus or unexpected sensitivity to small changes in total nuclear GR, may be important. Additionally, upregulation of IkappaB and consequent inhibition of NFkappaB activation are not prominent in vivo, in contrast to results obtained in vitro or with a synthetic glucocorticoid.

Transcriptional and post-transcriptional alterations of IkappaBalpha in active minimal-change nephrotic syndrome

Minimal-change nephrotic syndrome (MCNS) is a renal disease characterized by heavy glomerular proteinuria and increased production of cytokines by immune cells. Because of the central role of nuclear factor-kappaB (NF-kappaB) in the regulation of cytokine expression, its activity during the relapse and remission phases of steroid-sensitive MCNS was analyzed. During relapse, nuclear extracts from peripheral blood mononuclear cells displayed high levels of NF-kappaB DNA-binding activity, consisting primarily of p50/RelA (p65) complexes. NF-kappaB p65 and IkappaBalpha proteins were barely detected or not detected in cytosolic fractions during relapse, in contrast to remission. The lack of expression of IkappaBalpha protein was associated with downregulation of IkappaBalpha mRNA and increases in the levels of the mRNA encoding the proteasome alpha2 subunit proteolytic pathway. In addition, inhibition of proteasome activity induced cytosolic accumulation of phosphorylated IkappaBalpha and significant reductions in the NF-kappaB binding activity in nuclear extracts from peripheral blood mononuclear cells from patients experiencing relapses. These results suggest that alterations in the NF-kappaB/IkappaBalpha regulatory feedback loop may contribute to the immunologic abnormalities that occur in steroidsensitive MCNS.

NF-kappa B activation in tumor necrosis factor alpha-stimulated neutrophils is mediated by protein kinase Cdelta. Correlation to nuclear Ikappa Balpha

The transcription factor NF-kappaB is critical for the expression of multiple genes involved in inflammatory responses and apoptosis. However, the signal transduction pathways regulating NF-kappaB activation in human neutrophils in response to stimulation with tumor necrosis factor-alpha (TNFalpha) are undefined. Since recent studies implicated activation of NF-kappaB as well as protein kinase C-delta (PKCdelta) in neutrophil apoptosis, we investigated involvement of PKCdelta in the activation of NF-kappaB in TNFalpha-stimulated neutrophils. Specific inhibition of PKCdelta by rottlerin prevented IkappaBalpha degradation and NF-kappaB activation in TNFalpha-stimulated neutrophils. This regulation of NF-kappaB activation by PKCdelta was specific only for TNFalpha signaling, since lipopolysaccharide- or interleukin-1beta-induced NF-kappaB activation and IkappaBalpha degradation were not inhibited by rottlerin. In addition, we show that in human neutrophils, but not monocytes, IkappaBalpha localizes in significant amounts in the nucleus of unstimulated cells, and the amount of IkappaBalpha in the nucleus, as well as in the cytoplasm, correlates with the NF-kappaB DNA binding. These results suggest that in human neutrophils, the presence of IkappaBalpha in the nucleus may function as a safeguard against initiation of NF-kappaB dependent transcription of pro-inflammatory and anti-apoptotic genes, and represents a distinct and novel mechanism of NF-kappaB regulation.

Chemokine expression by systemic sclerosis fibroblasts: abnormal regulation of monocyte chemoattractant protein 1 expression

OBJECTIVE

Chemokines are important mediators in the chemoattraction of leukocytes to sites of inflammation. This study investigated the potential contribution of systemic sclerosis (SSc) fibroblasts to chemokine production and its potential relevance to the pathogenesis of SSc.

METHODS

The expression of messenger RNA (mRNA) for different C-C and C-X-C chemokines by SSc and normal fibroblasts was studied by RNase protection assay. Monocyte chemoattractant protein 1 (MCP-1) protein production was analyzed by enzyme-linked immunosorbent assay. The chemotactic effect of fibroblast-derived MCP-1 on monocytic cells was analyzed in a transmigration assay. Nuclear factor kappaB (NF-kappaB) and activator protein 1 (AP-1) activation in fibroblasts was studied by electromobility shift analysis. MCP-1 expression in SSc skin sections was studied by immunohistochemistry.

RESULTS

Among all chemokine genes studied, only MCP-1 and interleukin-8 mRNA were expressed by nonstimulated normal and SSc fibroblasts. SSc fibroblasts displayed increased constitutive expression of MCP-1 mRNA and protein and showed a blunted response to oxidative stress. Increased MCP-1 production was associated with higher chemotactic activity for monocytic cells. Increased NF-kappaB or AP-1 activation was not responsible for the constitutive overexpression of MCP-1 by SSc fibroblasts. In SSc skin sections, MCP-1 expression was detected in fibroblasts, keratinocytes, and mononuclear cells, whereas it was undetectable in normal skin.

CONCLUSION

SSc fibroblasts display a specific pattern of chemokine gene expression that is characterized by constitutively increased and abnormally regulated expression of MCP-1 in vitro. MCP-1 is also expressed in lesional skin and can participate in the pathogenesis of SSc.

Hypoxia induces expression of the chemokines monocyte chemoattractant protein-1 (MCP-1) and IL-8 in human dermal fibroblasts

Hypoxia is an important factor in the pathophysiology of vascular and inflammatory diseases. Leucocyte infiltration, as a consequence of adhesion molecule up-regulation and chemokine release, is a prominent feature of these diseases. The objective of our study was to investigate the potential role of resident fibroblasts in hypoxia-induced chemotactic responses. We show that MCP-1 and IL-8 mRNA are specifically induced by hypoxia in dermal fibroblasts. This response is paralleled by increased NF-kappaB p65/p50 binding activity, and it is inhibited by pretreatment with N-acetyl-L-cysteine. MCP-1 secreted by fibroblasts is chemotactic for monocytic cells and this activity is significantly increased by hypoxia. Chemotactic index correlates with MCP-1 protein levels and is significantly decreased by neutralizing anti-MCP-1 MoAb. These findings demonstrate the ability of resident fibroblasts to mediate chemotaxis of leucocytes through the release of chemokines in response to hypoxia. Our data point to MCP-1 as an important component in this response, and therefore it may be a potential target in inflammatory responses associated with hypoxia.

IkappaBalpha and IkappaBalpha /NF-kappa B complexes are retained in the cytoplasm through interaction with a novel partner, RasGAP SH3-binding protein 2

IkappaBalpha inhibits the transcriptional activity of NF-kappaB both in the cytoplasm by preventing the nuclear translocation of NF-kappaB and in the nucleus where it dissociates NF-kappaB from DNA and transports it back to the cytoplasm. Cytoplasmic localization of inactive NF-kappaB/IkappaBalpha complexes is controlled by mutual masking of nuclear import sequences of NF-kappaB p65 and IkappaBalpha and active CRM1-mediated nuclear export. Here, we describe an additional mechanism accounting for the cytoplasmic anchoring of IkappaBalpha or NF-kappaB/IkappaBalpha complexes. The N-terminal domain of IkappaBalpha contains a sequence responsible for the cytoplasmic retention of IkappaBalpha that is specifically recognized by G3BP2, a cytoplasmic protein that interacts with both IkappaBalpha and IkappaBalpha/NF-kappaB complexes. G3BP2 is composed of an N-terminal domain homologous to the NTF2 protein, followed by an acidic domain sufficient for the interaction with the IkappaBalpha cytoplasmic retention sequence, a region containing five PXXP motifs and a C-terminal domain containing RNA-binding motifs. Overexpression of G3BP2 directly promotes retention of IkappaBalpha in the cytoplasm, indicating that subcellular distribution of IkappaBalpha and NF-kappaB/IkappaBalpha complexes likely results from a equilibrium between nuclear import, nuclear export, and cytoplasmic retention. The molecular organization of G3BP2 suggests that this putative scaffold protein might connect the NF-kappaB signal transduction cascade with cellular functions such as nuclear transport or RNA metabolism.