NF-IL6-mediated transcriptional activation of the long terminal repeat of the human immunodeficiency virus type 1

IFNα and β Mediated JCPyV Suppression through C/EBPβ-LIP Isoform

Polyomavirus JC (JCPyV) causes the demyelinating disease progressive multifocal leukoencephalopathy (PML). JCPyV infection is very common in childhood and, under conditions of severe immunosuppression, JCPyV may reactivate to cause PML. JC viral proteins expression is regulated by the JCPyV non-coding control region (NCCR), which contains binding sites for cellular transcriptional factors which regulate JCPyV transcription. Our earlier studies suggest that JCPyV reactivation occurs within glial cells due to cytokines such as TNF-α which stimulate viral gene expression. In this study, we examined interferon-α (IFNα) or β (IFNβ) which have a negative effect on JCPyV transcriptional regulation. We also showed that these interferons induce the endogenous liver inhibitory protein (LIP), an isoform of CAAT/enhancer binding protein beta (C/EBPβ). Treatment of glial cell line with interferons increases the endogenous level of C/EBPβ-LIP. Furthermore, we showed that the negative regulatory role of the interferons in JCPyV early and late transcription and viral replication is more pronounced in the presence of C/EBPβ-LIP. Knockdown of C/EBPβ-LIP by shRNA reverse the inhibitory effect on JCPyV viral replication. Therefore, IFNα and IFNβ negatively regulate JCPyV through induction of C/EBPβ-LIP, which together with other cellular transcriptional factors may control the balance between JCPyV latency and activation.

The E3 Ubiquitin-Protein Ligase Cullin 3 Regulates HIV-1 Transcription

The infectious life cycle of the human immunodeficiency virus type 1 (HIV-1) is characterized by an ongoing battle between a compendium of cellular proteins that either promote or oppose viral replication. On the one hand, HIV-1 utilizes dependency factors to support and sustain infection and complete the viral life cycle. On the other hand, both inducible and constitutively expressed host factors mediate efficient and functionally diverse antiviral processes that counteract an infection. To shed light into the complex interplay between HIV-1 and cellular proteins, we previously performed a targeted siRNA screen to identify and characterize novel regulators of viral replication and identified Cullin 3 (Cul3) as a previously undescribed factor that negatively regulates HIV-1 replication. Cul3 is a component of E3-ubiquitin ligase complexes that target substrates for ubiquitin-dependent proteasomal degradation. In the present study, we show that Cul3 is expressed in HIV-1 target cells, such as CD4+ T cells, monocytes, and macrophages and depletion of Cul3 using siRNA or CRISPR/Cas9 increases HIV-1 infection in immortalized cells and primary CD4+ T cells. Conversely, overexpression of Cul3 reduces HIV-1 infection in single replication cycle assays. Importantly, the antiviral effect of Cul3 was mapped to the transcriptional stage of the viral life cycle, an effect which is independent of its role in regulating the G1/S cell cycle transition. Using isogenic viruses that only differ in their promotor region, we find that the NF-κB/NFAT transcription factor binding sites in the LTR are essential for Cul3-dependent regulation of viral gene expression. Although Cul3 effectively suppresses viral gene expression, HIV-1 does not appear to antagonize the antiviral function of Cul3 by targeting it for degradation. Taken together, these results indicate that Cul3 is a negative regulator of HIV-1 transcription which governs productive viral replication in infected cells.

Safe CRISPR-Cas9 Inhibition of HIV-1 with High Specificity and Broad-Spectrum Activity by Targeting LTR NF-κB Binding Sites

Viral latency of human immunodeficiency virus type 1 (HIV-1) has become a major hurdle to a cure in the highly effective antiretroviral therapy (ART) era. The clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 system has successfully been demonstrated to excise or inactivate integrated HIV-1 provirus from infected cells by targeting the long terminal repeat (LTR) region. However, the guide RNAs (gRNAs) have classically avoided transcription factor binding sites (TFBSs) that are readily observed and known to be important in human promoters. Although conventionally thought unfavorable due to potential impact on human promoters, our computational pipeline identified gRNA sequences that were predicted to inactivate HIV-1 transcription by targeting the nuclear factor κB (NF-κB) binding sites (gNFKB0, gNFKB1) with a high safety profile (lack of predicted or observed human edits) and broad-spectrum activity (predicted coverage of known viral sequences). Genome-wide, unbiased identification of double strand breaks (DSBs) enabled by sequencing (GUIDE-seq) showed that the gRNAs targeting NF-κB binding sites had no detectable CRISPR-induced off-target edits in HeLa cells. 5′ LTR-driven HIV-1 transcription was significantly reduced in three HIV-1 reporter cell lines. These results demonstrate a working model to specifically target well-known TFBSs in the HIV-1 LTR that are readily observed in human promoters to reduce HIV-1 transcription with a high-level safety profile and broad-spectrum activity.

Resistance to the Tat Inhibitor Didehydro-Cortistatin A Is Mediated by Heightened Basal HIV-1 Transcription

Human immunodeficiency virus type 1 (HIV-1) Tat binds the viral RNA structure transactivation-responsive element (TAR) and recruits transcriptional cofactors, amplifying viral mRNA expression. The Tat inhibitor didehydro-cortistatin A (dCA) promotes a state of persistent latency, refractory to viral reactivation. Here we investigated mechanisms of HIV-1 resistance to dCA in vitro Mutations in Tat and TAR were not identified, consistent with the high level of conservation of these elements. Instead, viruses resistant to dCA developed higher Tat-independent basal transcription. We identified a combination of mutations in the HIV-1 promoter that increased basal transcriptional activity and modifications in viral Nef and Vpr proteins that increased NF-κB activity. Importantly, these variants are unlikely to enter latency due to accrued transcriptional fitness and loss of sensitivity to Tat feedback loop regulation. Furthermore, cells infected with these variants become more susceptible to cytopathic effects and immune-mediated clearance. This is the first report of viral escape to a Tat inhibitor resulting in heightened Tat-independent activity, all while maintaining wild-type Tat and TAR.IMPORTANCE HIV-1 Tat enhances viral RNA transcription by binding to TAR and recruiting activating factors. Tat enhances its own transcription via a positive-feedback loop. Didehydro-cortistatin A (dCA) is a potent Tat inhibitor, reducing HIV-1 transcription and preventing viral rebound. dCA activity demonstrates the potential of the "block-and-lock" functional cure approaches. We investigated the viral genetic barrier to dCA resistance in vitro While mutations in Tat and TAR were not identified, mutations in the promoter and in the Nef and Vpr proteins promoted high Tat-independent activity. Promoter mutations increased the basal transcription, while Nef and Vpr mutations increased NF-κB nuclear translocation. This heightened transcriptional activity renders CD4⁺ T cells infected with these viruses more susceptible to cytotoxic T cell-mediated killing and to cell death by cytopathic effects. Results provide insights on drug resistance to a novel class of antiretrovirals and reveal novel aspects of viral transcriptional regulation.

Regulation of HIV-1 transcription

Human immunodeficiency virus type-1 (HIV-1) is a highly pathogenic lentivirus that requires transcription of its provirus genome for completion of the viral life cycle and the production of progeny virions. Since the first genetic analysis of HIV-1 in 1985, much has been learned about the transcriptional regulation of the HIV-1 genome in infected cells. It has been demonstrated that HIV-1 transcription depends on a varied and complex interaction of host cell transcription factors with the viral long terminal repeat (LTR) promoter. The regulatory elements within the LTR interact with constitutive and inducible transcription factors to direct the assembly of a stable transcription complex that stimulates multiple rounds of transcription by RNA polymerase II (RNAPII). However, the majority of these transcripts terminate prematurely in the absence of the virally encoded trans-activator protein Tat, which stimulates HIV-1 transcription elongation by interacting with a stem-loop RNA element (TAR) formed at the extreme 5' end of all viral transcripts. The Tat-TAR interaction recruits a cellular kinase into the initiation-elongation complex that alters the elongation properties of RNAPII during its transit through TAR. This review summarizes our current knowledge and understanding of the regulation of HIV-1 transcription in infected cells and highlights the important contributions human lentivirus gene regulation has made to our general understanding of the transcription process.

Distinctive variation in the U3R region of the 5' Long Terminal Repeat from diverse HIV-1 strains

Functional mapping of the 5’LTR has shown that the U3 and the R regions (U3R) contain a cluster of regulatory elements involved in the control of HIV-1 transcription and expression. As the HIV-1 genome is characterized by extensive variability, here we aimed to describe mutations in the U3R from various HIV-1 clades and CRFs in order to highlight strain specific differences that may impact the biological properties of diverse HIV-1 strains. To achieve our purpose, the U3R sequence of plasma derived virus belonging to different clades (A1, B, C, D, F2) and recombinants (CRF02_AG, CRF01_AE and CRF22_01A1) was obtained using Illumina technology. Overall, the R region was very well conserved among and across different strains, while in the U3 region the average inter-strains nucleotide dissimilarity was up to 25%. The TAR hairpin displayed a strain-distinctive cluster of mutations affecting the bulge and the loop, but mostly the stem. Like in previous studies we found a TATAA motif in U3 promoter region from the majority of HIV-1 strains and a TAAAA motif in CRF01_AE; but also in LTRs from CRF22_01A1 isolates. Although LTRs from CRF22_01A1 specimens were assigned CRF01_AE, they contained two NF-kB sites instead of the single TFBS described in CRF01_AE. Also, as previously describe in clade C isolates, we found no C/EBP binding site directly upstream of the enhancer region in CRF22_01A1 specimens. In our study, one-third of CRF02_AG LTRs displayed three NF-kB sites which have been mainly described in clade C isolates. Overall, the number, location and binding patterns of potential regulatory elements found along the U3R might be specific to some HIV-1 strains such as clade F2, CRF02_AG, CRF01_AE and CRF22_01A1. These features may be worth consideration as they may be involved in distinctive regulation of HIV-1 transcription and replication by different and diverse infecting strains.

Functional Studies of CCAAT/Enhancer Binding Protein Site Located Downstream of the Transcriptional Start Site

Previous studies have identified a CCAAT/enhancer binding protein (C/EBP) site located downstream of the transcriptional start site (DS3). The role of the DS3 element with respect to HIV-1 transactivation by Tat and viral replication has not been characterized. We have demonstrated that DS3 was a functional C/EBPβ binding site and mutation of this site to the C/EBP knockout DS3-9C variant showed lower HIV-1 long terminal repeat (LTR) transactivation by C/EBPβ. However, it was able to exhibit similar or even higher transcription levels by Tat compared to the parental LTR. C/EBPβ and Tat together further enhanced the transcription level of the parental LAI-LTR and DS3-9C LTR, with higher levels in the DS3-9C LTR. HIV molecular clone viruses carrying the DS3-9C variant LTR demonstrated a decreased replication capacity and delayed rate of replication. These results suggest that DS3 plays a role in virus transcriptional initiation and provides new insight into C/EBP regulation of HIV-1.

The Immune Interaction between HIV-1 Infection and Mycobacterium tuberculosis

The modulation of tuberculosis (TB)-induced immunopathology caused by human immunodeficiency virus (HIV)-1 coinfection remains incompletely understood but underlies the change seen in the natural history, presentation, and prognosis of TB in such patients. The deleterious combination of these two pathogens has been dubbed a "deadly syndemic," with each favoring the replication of the other and thereby contributing to accelerated disease morbidity and mortality. HIV-1 is the best-recognized risk factor for the development of active TB and accounts for 13% of cases globally. The advent of combination antiretroviral therapy (ART) has considerably mitigated this risk. Rapid roll-out of ART globally and the recent recommendation by the World Health Organization (WHO) to initiate ART for everyone living with HIV at any CD4 cell count should lead to further reductions in HIV-1-associated TB incidence because susceptibility to TB is inversely proportional to CD4 count. However, it is important to note that even after successful ART, patients with HIV-1 are still at increased risk for TB. Indeed, in settings of high TB incidence, the occurrence of TB often remains the first presentation of, and thereby the entry into, HIV care. As advantageous as ART-induced immune recovery is, it may also give rise to immunopathology, especially in the lower-CD4-count strata in the form of the immune reconstitution inflammatory syndrome. TB-immune reconstitution inflammatory syndrome will continue to impact the HIV-TB syndemic.

CCAAT enhancer binding protein and nuclear factor of activated T cells regulate HIV-1 LTR via a novel conserved downstream site in cells of the monocyte-macrophage lineage

Transcriptional control of the human immunodeficiency virus type 1 (HIV-1) promoter, the long terminal repeat (LTR), is achieved by interactions with cis-acting elements present both upstream and downstream of the start site. In silico transcription factor binding analysis of the HIV-1 subtype B LTR sequences revealed a potential downstream CCAAT enhancer binding protein (C/EBP) binding site. This binding site (+158 to+172), designated DS3, was found to be conserved in 67% of 3,858 unique subtype B LTR sequences analyzed in terms of nucleotide sequence as well as physical location in the LTR. DS3 was found to be well represented in other subtypes as well. Interestingly, DS3 overlaps with a previously identified region that bind members of the nuclear factor of activated T cells (NFAT) family of proteins. NFATc2 exhibited a higher relative affinity for DS3 as compared with members of the C/EBP family (C/EBP α and β). DS3 was able to compete efficiently with the low-affinity upstream C/EBP binding site I with respect to C/EBP binding, suggesting utilization of both NFAT and C/EBP. Moreover, cyclosporine A treatment, which has been shown to prevent dephosphorylation and nuclear translocation of NFAT isoforms, resulted in enhanced C/EBPα binding. The interactions at DS3 were also validated in an integrated HIV-1 LTR in chronically infected U1 cells. A binding knockout of DS3 demonstrated reduced HIV-1 LTR-directed transcription under both basal and interleukin-6-stimulated conditions only in cells of the monocyte-macrophage lineage cells and not in cells of T-cell origin. Thus, the events at DS3 positively regulate the HIV-1 promoter in cells of the monocyte-macrophage lineage.

Genotypic and phenotypic heterogeneity in the U3R region of HIV type 1 subtype C

Approximately 20% of all HIV-1 mother-to-child transmission (MTCT) occurs in utero (IU). In a chronic HIV infection, HIV-1 exists as a complex swarm of genetic variants, and following IU MTCT, viral genomic diversity is restricted through a mechanism that remains to be described. The 5' U3R region of the HIV-1 long terminal repeat (LTR) contains multiple transcription factor (TF) binding sites and regulates viral transcription. In this study, we tested the hypothesis that sequence polymorphisms in the U3R region of LTR are associated with IU MTCT. To this end, we used single template amplification to isolate 517 U3R sequences from maternal, placental, and infant plasma derived from 17 HIV-infected Malawian women: eight whose infants remained HIV uninfected (NT) and nine whose infants became HIV infected IU. U3R sequences show pairwise diversities ranging from 0.2% to 2.3%. U3R sequences from one participant contained two, three, or four putative NF-κB binding sites. Phylogenetic reconstructions indicated that U3R sequences from eight of nine IU participants were consistent with placental compartmentalization of HIV-1 while only one of eight NT cases was consistent with such compartmentalization. Specific TF sequence polymorphisms were not significantly associated with IU MTCT. To determine if replication efficiency of the U3R sequences was associated with IU MTCT, we cloned 90 U3R sequences and assayed promoter activity in multiple cell lines. Although we observed significant, yet highly variable promoter activity and TAT induction of promoter activity in the cell lines tested, there was no association between measured promoter activity and MTCT status. Thus, we were unable to detect a promoter genotype or phenotype associated with IU MTCT.

Genetic variation and HIV-associated neurologic disease

HIV-associated neurologic disease continues to be a significant complication in the era of highly active antiretroviral therapy. A substantial subset of the HIV-infected population shows impaired neuropsychological performance as a result of HIV-mediated neuroinflammation and eventual central nervous system (CNS) injury. CNS compartmentalization of HIV, coupled with the evolution of genetically isolated populations in the CNS, is responsible for poor prognosis in patients with AIDS, warranting further investigation and possible additions to the current therapeutic strategy. This chapter reviews key advances in the field of neuropathogenesis and studies that have highlighted how molecular diversity within the HIV genome may impact HIV-associated neurologic disease. We also discuss the possible functional implications of genetic variation within the viral promoter and possibly other regions of the viral genome, especially in the cells of monocyte-macrophage lineage, which are arguably key cellular players in HIV-associated CNS disease.

Mechanisms of HIV Transcriptional Regulation and Their Contribution to Latency

Long-lived latent HIV-infected cells lead to the rebound of virus replication following antiretroviral treatment interruption and present a major barrier to eliminating HIV infection. These latent reservoirs, which include quiescent memory T cells and tissue-resident macrophages, represent a subset of cells with decreased or inactive proviral transcription. HIV proviral transcription is regulated at multiple levels including transcription initiation, polymerase recruitment, transcription elongation, and chromatin organization. How these biochemical processes are coordinated and their potential role in repressing HIV transcription along with establishing and maintaining latency are reviewed.

Structural and functional studies of CCAAT/enhancer binding sites within the human immunodeficiency virus type 1 subtype C LTR

Human immunodeficiency virus type 1 (HIV-1) subtype C, which is most predominant in sub-Saharan Africa as well as in Asia and India, is the most prevalent subtype worldwide. A large number of transcription factor families have been shown to be involved in regulating HIV-1 gene expression in T lymphocytes and cells of the monocyte-macrophage lineage. Among these, proteins of the CCAAT/enhancer binding protein (C/EBP) family are of particular importance in regulating HIV-1 gene expression within cells of the monocytic lineage during the course of hematologic development and cellular activation. Few studies have examined the role of C/EBPs in long terminal repeat (LTR)-directed viral gene expression of HIV-1 subtypes other than subtype B. Within subtype B viruses, two functional C/EBP sites located upstream of the TATA box are required for efficient viral replication in cells of the monocyte-macrophage lineage. We report the identification of three putative subtype C C/EBP sites, upstream site 1 and 2 (C-US1 and C-US2) and downstream site 1 (C-DS1). C-US1 and C-DS1 were shown to form specific DNA-protein complexes with members of the C/EBP family (C/EBPα, β, and δ). Functionally, within the U-937 monocytic cell line, subtype B and C LTRs were shown to be equally responsive to C/EBPβ-2, although the basal activity of subtype C LTRs appeared to be higher. Furthermore, the synergistic interaction between C/EBPβ-2 and Tat with the subtype C LTR was also observed in U-937 cells as previously demonstrated with the subtype B LTR.

Innate and adaptive factors regulating human immunodeficiency virus type 1 genomic activation

Over the past decade, antiretroviral therapy targeting the viral entry process, reverse transcriptase, integrase, and protease, has prolonged the lives of people infected with human immunodeficiency virus type 1 (HIV-1). However, despite the development of more effective therapeutic strategies, reservoirs of viral infection remain. This review discusses molecular mechanisms surrounding the development of latency from the site of integration to pre- and post-integration maintenance of latency, including epigenetic factors. In addition, an overview of innate and adaptive cells important to HIV-1 infection are examined from the viewpoint of cytokines released and cytokines that act on these cells to explore an overall understanding of HIV-1 proviral genome activation. Finally, this review is discussed from the viewpoint of how an understanding of the interplay of all of these factors will help guide the next generation of therapies.

The macrophage in HIV-1 infection: from activation to deactivation?

Macrophages play a crucial role in innate and adaptative immunity in response to microorganisms and are an important cellular target during HIV-1 infection. Recently, the heterogeneity of the macrophage population has been highlighted. Classically activated or type 1 macrophages (M1) induced in particular by IFN-γ display a pro-inflammatory profile. The alternatively activated or type 2 macrophages (M2) induced by Th-2 cytokines, such as IL-4 and IL-13 express anti-inflammatory and tissue repair properties. Finally IL-10 has been described as the prototypic cytokine involved in the deactivation of macrophages (dM). Since the capacity of macrophages to support productive HIV-1 infection is known to be modulated by cytokines, this review shows how modulation of macrophage activation by cytokines impacts the capacity to support productive HIV-1 infection. Based on the activation status of macrophages we propose a model starting with M1 classically activated macrophages with accelerated formation of viral reservoirs in a context of Th1 and proinflammatory cytokines. Then IL-4/IL-13 alternatively activated M2 macrophages will enter into the game that will stop the expansion of the HIV-1 reservoir. Finally IL-10 deactivation of macrophages will lead to immune failure observed at the very late stages of the HIV-1 disease.

Host hindrance to HIV-1 replication in monocytes and macrophages

Monocytes and macrophages are targets of HIV-1 infection and play critical roles in multiple aspects of viral pathogenesis. HIV-1 can replicate in blood monocytes, although only a minor proportion of circulating monocytes harbor viral DNA. Resident macrophages in tissues can be infected and function as viral reservoirs. However, their susceptibility to infection, and their capacity to actively replicate the virus, varies greatly depending on the tissue localization and cytokine environment. The susceptibility of monocytes to HIV-1 infection in vitro depends on their differentiation status. Monocytes are refractory to infection and become permissive upon differentiation into macrophages. In addition, the capacity of monocyte-derived macrophages to sustain viral replication varies between individuals. Host determinants regulate HIV-1 replication in monocytes and macrophages, limiting several steps of the viral life-cycle, from viral entry to virus release. Some host factors responsible for HIV-1 restriction are shared with T lymphocytes, but several anti-viral mechanisms are specific to either monocytes or macrophages. Whilst a number of these mechanisms have been identified in monocytes or in monocyte-derived macrophages in vitro, some of them have also been implicated in the regulation of HIV-1 infection in vivo, in particular in the brain and the lung where macrophages are the main cell type infected by HIV-1. This review focuses on cellular factors that have been reported to interfere with HIV-1 infection in monocytes and macrophages, and examines the evidences supporting their role in vivo, highlighting unique aspects of HIV-1 restriction in these two cell types.

Cell-type-specific proteome and interactome: using HIV-1 Tat as a test case

HIV-1 is a small retrovirus that wreaks havoc on the human immune system. It is a puzzle to the scientific community how a virus that encodes only nine proteins can take complete control of its host and redirect the cell to complete replication or maintain latency when necessary. One way to explain the control elicited by HIV-1 is through numerous protein partners that exist between viral and host proteins, allowing HIV-1 to be intimately involved in virtually every aspect of cellular biology. In addition, we postulate that the complexity exerted by HIV-1 can not merely be explained by the large number of protein-protein interactions documented in the literature but, rather, cell-type-specific interactions and post-translational modifications of viral proteins must be taken into account. We use HIV-1 Tat and its influence on viral transcription as an example of cell-type-specific complexity. The influence of post-translational modifications (acetylation and methylation), as well as subcellular localization on Tat binding partners, is also discussed.

Regulation of HIV-1 transcription in cells of the monocyte-macrophage lineage

Human immunodeficiency virus type 1 (HIV-1) has been shown to replicate productively in cells of the monocyte-macrophage lineage, although replication occurs to a lesser extent than in infected T cells. As cells of the monocyte-macrophage lineage become differentiated and activated and subsequently travel to a variety of end organs, they become a source of infectious virus and secreted viral proteins and cellular products that likely initiate pathological consequences in a number of organ systems. During this process, alterations in a number of signaling pathways, including the level and functional properties of many cellular transcription factors, alter the course of HIV-1 long terminal repeat (LTR)-directed gene expression. This process ultimately results in events that contribute to the pathogenesis of HIV-1 infection. First, increased transcription leads to the upregulation of infectious virus production, and the increased production of viral proteins (gp120, Tat, Nef, and Vpr), which have additional activities as extracellular proteins. Increased viral production and the presence of toxic proteins lead to enhanced deregulation of cellular functions increasing the production of toxic cellular proteins and metabolites and the resulting organ-specific pathologic consequences such as neuroAIDS. This article reviews the structural and functional features of the cis-acting elements upstream and downstream of the transcriptional start site in the retroviral LTR. It also includes a discussion of the regulation of the retroviral LTR in the monocyte-macrophage lineage during virus infection of the bone marrow, the peripheral blood, the lymphoid tissues, and end organs such as the brain. The impact of genetic variation on LTR-directed transcription during the course of retrovirus disease is also reviewed.

Regulation of SIV mac 239 basal long terminal repeat activity and viral replication in macrophages: functional roles of two CCAAT/enhancer-binding protein beta sites in activation and interferon beta-mediated suppression

CCAAT/enhancer-binding protein (C/EBP) beta and C/EBP sites in the HIV-1 long terminal repeat (LTR) are crucial for HIV-1 replication in monocyte/macrophages and for the ability of interferon beta (IFN beta) to inhibit ongoing active HIV replication in these cells. This IFN beta-mediated down-regulation involves induction of the truncated, dominant-negative isoform of C/EBP beta referred to as liver-enriched transcriptional inhibitory protein (LIP). Although binding of the C/EBP beta isoform to C/EBP sites in the simian immunodeficiency virus (SIV) LTR has previously been examined, the importance of these sites in core promoter-mediated transcription, virus replication, IFN beta-mediated regulation, and the relative binding of the two isoforms (C/EBP beta and LIP) has not been investigated. Here, we specifically examine two C/EBP sites, JC1 (-100 bp) and DS1 (+134 bp), located within the minimal region of the SIV LTR, required for core promoter-mediated transcription and virus replication in macrophages. Our studies revealed that the JC1 but not DS1 C/EBP site is important for basal level transcription, whereas the DS1 C/EBP site is imperative for productive virus replication in primary macrophages. In contrast, either JC1 or DS1 C/EBP site is sufficient to mediate IFN beta-induced down-regulation of SIV LTR activity and virus replication in these cells. We also characterized the differential binding properties of C/EBP beta and LIP to the JC1 and DS1 sites. In conjunction with previous studies from our laboratory, we demonstrate the importance of these sites in virus gene expression, and we propose a model for their role in establishing latency and persistence in macrophages in the brain.

CCAAT/enhancer-binding proteins and the pathogenesis of retrovirus infection

Previous studies indicate that two upstream CCAAT/enhancer-binding protein (C/EBP) sites and C/EBPbeta are required for subtype B HIV-1 gene expression in cells of the monocyte-macrophage lineage. The mechanisms of C/EBP regulation of HIV-1 transcription and replication remain unclear. This review focuses on studies concerning the role of C/EBP factors in HIV-1, human T-cell leukemia virus type 1, and SIV transcription in various cell types and tissues cultured in vitro, animal models and during human infection. The structure and function of the C/EBPbeta gene and the related protein isoforms are discussed along with the transcription factors, coactivators, viral proteins, cytokines and chemokines that affect C/EBP function.

Oral infectious diseases: a potential risk factor for HIV virus recrudescence?

As the highly active antiretroviral therapy (HAART) has transitioned human immunodeficiency virus (HIV) infection into a 'chronic disease' management strategy, there is growing evidence that infection with non-HIV pathogens in HIV+ patients may have important public health implications in undermining HAART success and acquired immunodeficiency syndrome progression. Several bacterial and host cell products during infections with non-HIV pathogens have shown the capacity to regulate HIV replication in latently infected cells. A high prevalence of oral infections caused by bacteria, viruses and fungi has been described in HIV+ patients, including periodontal disease. The oral cavity appears to be a site of HIV pathogenesis and potential reservoir for the disease as HIV RNA and DNA forms are present in saliva as well as in gingival crevicular fluid, and oral epithelial cells are susceptible to either cell free or cell-associated HIV infection. The clinical and biological bases of potential associations between chronic oral inflammatory disorders, such as periodontal disease, and exacerbation of HIV viraemia have received little attention. This review attempts to evaluate the current understanding of HIV reactivation as a result of co-infection and/or inflammation induced by non-HIV pathogens in HIV-infected patients, and presents a hypothetic model about the potential role of periodontitis as a global oral infection that potentially contributes to HIV recrudescence.

Induction of HIV-1 latency and reactivation in primary memory CD4+ T cells

The use of antiretroviral therapy in HIV type 1 (HIV-1)-infected patients does not lead to virus eradication. This is due, to a significant degree, to the fact that HIV-1 can establish a highly stable reservoir of latently infected cells. In this work, we describe an ex vivo experimental system that generates high levels of HIV-1 latently infected memory cells using primary CD4+ T cells. Using this model, we were able to dissect the T cell-signaling pathways and to characterize the long terminal repeat (LTR) cis-acting elements involved in reactivation of HIV-1 in memory CD4+ T cells. We conclude that Lck and nuclear factor of activated T cells (NFAT), but not NF-kappaB, are required for optimal latent virus reactivation in memory T cells. We also found that the cis-acting elements which are critical toward HIV-1 reactivation are the Sp1 and kappaB/NFAT transcription factor binding sites.

NF-IL6 (C/EBPbeta) induces HIV-1 replication by inhibiting cytidine deaminase APOBEC3G

T cell activation is crucial for the productive HIV-1 infection of primary T cells; however, little is known about the host molecules involved in this process. We show that the host transcription factor NF-IL6 (also called C/EBPbeta) renders primary CD4(+) T cells highly permissive for HIV-1 replication. NF-IL6 facilitates reverse transcription of the virus by binding to and inhibiting the antiviral cytidine deaminase APOBEC3G. A mutation in NF-IL6 at Ser-288 weakened its binding to APOBEC3G and strongly inhibited HIV-1 replication. NF-IL6 also induced the replication of a Vif-deficient strain of HIV-1 in nonpermissive HUT78 cells. These data indicate that NF-IL6 is a natural inhibitor of APOBEC3G that facilitates HIV-1 replication. Host factors, such as NF-IL6, that are involved in early HIV-1 replication are potential targets for anti-HIV-1 therapy. Our findings shed light on the activation of HIV-1 replication by T cell host molecules and reveal a unique regulation of DNA deamination by APOBEC3G and NF-IL6.

LPS reduces HIV-1 replication in primary human macrophages partly through an endogenous production of type I interferons

It has been proposed that the systemic immune activation state seen in HIV-1-infected patients is caused by circulating microbial products such as lipopolysaccharide (LPS). Given that macrophages play a key role in HIV-1 pathogenesis, we investigated the LPS-mediated effect on HIV-1 replication in cells of the myeloid lineage. We demonstrate that LPS promotes virus gene expression in a monocytic cell line while it diminishes virus production in primary human monocyte-derived macrophages (MDM). The incapacity of LPS to drive HIV-1 production in MDM was not due to its inability to activate the ubiquitous transcription factor NF-kappaB even in virus-infected cells. Neutralization of type I interferons (IFN) with B18R, a soluble vaccinia virus-coded type I IFN receptor, significantly but not totally diminished the antiviral activity of LPS. Therefore, inhibition of HIV-1 replication in MDM treated with microbial-derived LPS resulted from the induction of type I interferons and a yet to be defined soluble factor.

Gene expression analysis from human immunodeficiency virus type 1 subtype C promoter and construction of bicistronic reporter vectors

We report the cloning and sequence analysis of the long terminal repeat (LTR) of several primary HIV-1 subtype C strains of India. Phylogenetically, all the LTRs and the paired env sequences clustered with subtype C reference strains. The LTRs demonstrated extensive polymorphism in the transcription factor binding sites (TFBS) within the enhancer and the modulator regions. We generated reporter vectors under the control of a select subset of the subtype C LTRs. The reporter vectors are distinguished by the simultaneous expression of two independent reporter genes, secreted alkaline phosphatase (SEAP) and enhanced green fluorescence protein (EGFP), in response to Tat. Expression of EGFP was facilitated by engineering an internal ribosome entry site (IRES) into the expression cassette. Although subtype C strains cause a large majority of the global infections, and important differences in the transcription factor binding sites have been identified in the subtype C promoter, few reporter vectors containing subtype C-LTR have been described. We analyzed gene expression from the C-LTR reporter vectors in different cell lines under diverse experimental conditions and compared it to the B-LTR reporter vector. The reporter vectors were responsive to Tat derived from diverse viral subtypes. Furthermore, a positive correlation was observed between the expression of the reporter genes and the viral structural protein p24 when the cells were infected with viral molecular clones. The LTR reporters we developed could be of significant use in the study of viral transactivation, in the evaluation of biological properties of viral subtypes, and in the screening for antiviral inhibitors.

Association of p65 and C/EBPbeta with HIV-1 LTR modulates transcription of the viral promoter

In human immunodeficiency virus type 1 (HIV-1) latently infected cells, NF-kappaB (NF-kappaB) plays a critical role in the transcriptional induction of the HIV-1 promoter. The trans-activating ability of NF-kappaB can be modified by another nuclear factor C/EBPbeta that can physically bind to NF-kappaB and regulate its activity. Because the HIV-1 promoter also contains a C/EBPbeta site adjacent to the NF-kappaB site, the present study examined cooperative functional in vivo interaction of the p65 subunit of NF-kappaB and C/EBPbeta, and the impact of Tat in this event. We demonstrated that ectopic expression of p65 along with Tat increases p65 binding to HIV-1 LTR, and that this increase correlates with enhanced HIV-1 promoter activity. Further, co-expression of C/EBPbeta and Tat leads to a decrease in p65 binding, which allows C/EBPbeta to bind more efficiently to the LTR. Inhibition of p65 expression by siRNA significantly decreases C/EBPbeta-binding and LTR expression. Using ChIP assay, we confirmed the existence of an interchange between p65 and C/EBPbeta and their abilities to bind to the LTR in vivo. These observations demonstrate that a delicate balance of interaction between p65, C/EBPbeta, and Tat can dictate the level of HIV-1 LTR transcription.

OTK18, a zinc-finger protein, regulates human immunodeficiency virus type 1 long terminal repeat through two distinct regulatory regions

It has previously been shown by our laboratory that OTK18, a human immunodeficiency virus (HIV)-inducible zinc-finger protein, reduces progeny-virion production in infected human macrophages. OTK18 antiviral activity is mediated through suppression of Tat-induced HIV-1 long terminal repeat (LTR) promoter activity. Through the use of LTR-scanning mutant vectors, the specific regions responsible for OTK18-mediated LTR suppression have been defined. Two different LTR regions were identified as potential OTK18-binding sites by an enhanced DNA-transcription factor ELISA system; the negative-regulatory element (NRE) at -255/-238 and the Ets-binding site (EBS) at -150/-139 in the LTR. In addition, deletion of the EBS in the LTR blocked OTK18-mediated LTR suppression. These data indicate that OTK18 suppresses LTR activity through two distinct regulatory elements. Spontaneous mutations in these regions might enable HIV-1 to escape from OTK18 antiretroviral activity in human macrophages.

p27(SJ), a novel protein in St John's Wort, that suppresses expression of HIV-1 genome

Transcription of the HIV-1 genome is controlled by the cooperation of viral regulatory proteins and several host factors which bind to specific DNA sequences within the viral promoter spanning the long terminal repeat, (LTR). Here, we describe the identification of a novel protein, p27(SJ), present in a laboratory callus culture of Hypericum perforatum (St John's Wort) that suppresses transcription of the HIV-1 genome in several human cell types including primary culture of microglia and astrocytes. p27(SJ) associates with C/EBPbeta, a transcription factor that regulates expression of the HIV-1 genome in macrophages and monocytic cells, and the viral transactivator, Tat. The association of p27(SJ) with C/EBPbeta and Tat alters their subcellular localization, causing their accumulation in the perinuclear cytoplasmic compartment of the cells. Fusion of a nuclear localization signal to p27(SJ) forces its entry into the nucleus and diminishes the capacity of p27(SJ) to suppress Tat activity, but does not alter its ability to suppress C/EBPbeta activation of the LTR. Results from binding assays showed the inhibitory effect of p27(SJ) on C/EBPbeta interaction with DNA. Finally, our results demonstrate that expression of p27(SJ) decreases the level of viral replication in HIV-1-infected cells. These observations suggest the potential for the development of a therapeutic advance based on p27(SJ) protein to control HIV-1 transcription and replication in cells associated with HIV-1 infection in the brain.

Exploring Transcription Factor Binding Properties of Several Non-coding DNA Sequence Elements in the Human NF-IL6 Gene

We examined several DNA segments upstream of the transcription start site of the human NF-IL6 gene to evaluate the predictions of two computational models developed to identify potential regulatory elements in the non-coding regions of genes. One model, comparative genomics, is based on the hypothesis that functional regulatory sequences can be localized in alignments of genomic DNA from several species. The other model is based on the hypothesis that protein-binding sites in genomic DNA may include sequence elements that occur frequently in proximal promoters of genes. The segments selected for DNA binding and functional evaluations included: (1) two conserved regions identified in multi-species sequence alignments; (2) a region containing several localized hits with 9-mers that ranked highly in studies of proximal promoters of human genes; and (3) two regions that were either GC-rich and/or contained tracts of G. The assays were done under nearly identical experimental conditions, using a cell line (U937) representing human monocytes/macrophages. The experiments also aimed at evaluating what effect, if any, cellular stimulation could have on the interactions of nuclear proteins with naturally occurring GC-rich elements in a human genomic DNA. In DNA binding assays, several complexes were formed with the conserved regions identified in multi-species sequence alignment. Furthermore, these regions were active in functional assays. The region containing several matches with 9-mers derived from proximal promoters of human genes was not conserved but formed several complexes with nuclear proteins including Sp1, Egr-1, and an unidentified protein. In addition, this region was active in functional assays and responded to cellular stimulations. Overall, the results of the assays suggest an important role for the sequence context of genomic DNA in protein binding and selection.

Mechanisms for HIV Tat upregulation of IL-10 and other cytokine expression: kinase signaling and PKR-mediated immune response

HIV Tat has been known to have multiple regulatory roles including replication of HIV and modulation of cellular kinases. We investigated whether signaling kinase PKR plays a critical role in mediating Tat-induced cytokine dysregulation. We showed Tat induction of IL-10 dysregulation is associated with PKR activation. To examine the mechanism involved, inhibition of PKR activity abrogated the Tat-induced cytokine induction. We next identified that the MAP kinases including ERK-1/2 and p38 are downstream of PKR in these Tat-induced pathways. Thus, PKR may play a critical role in mediating the subversive effects of HIV Tat resulting in IL-10 induction.

Interleukin-10 induces inhibitory C/EBPbeta through STAT-3 and represses HIV-1 transcription in macrophages

Pulmonary tuberculosis (TB) has been characterized by inflammation with increased pro- or anti-inflammatory cytokines produced by macrophages. We have reported that IFN produces inhibitory C/EBPbeta and represses transcription of the HIV-1 LTR in macrophages. STAT-1 and type I IFN receptor knockout mice have macrophages that are defective in IFN signaling, yet LPS stimulation induces inhibitory C/EBPbeta, demonstrating that other cytokines can induce this repressor. LPS or Mycobacterium tuberculosis-derived lipoarabinomannan induce the anti-inflammatory cytokine interleukin (IL)-10, which represses the HIV-1 LTR in differentiated THP-1 macrophages by inducing inhibitory C/EBPbeta. In contrast, in undifferentiated THP-1 monocytes, IL-10 did not inhibit HIV-1 replication or induce C/EBPbeta. IL-10 signal transduction uses STAT-3, and macrophages from STAT-3-/- mice fail to produce inhibitory C/EBPbeta after LPS or IL-10 stimulation. Transfection of STAT-3 into THP-1 cells enhances C/EBPbeta promoter activity. THP-1 differentiation also increases STAT-3 protein, but not STAT-3 gene transcription, and induces a translational regulator, CUG-binding protein, that was essential for production of C/EBPbeta. Differentiation induced post-transcriptional regulation is required to produce inhibitory C/EBPbeta in response to IL-10. Only macrophages are able to repress HIV-1 LTR promoter activity and inhibit viral replication in response to IL-10 or type I IFN.

Cooperative interaction of C/EBP beta and Tat modulates MCP-1 gene transcription in astrocytes

The chemoattractant protein 1 (MCP-1) is one of the most potent monocyte chemoattractants whose level is elevated during the course of AIDS dementia. Earlier studies showed that HIV-1 Tat protein is able to induce transcription of the MCP-1 promoter in astrocytic cells. Furthermore, the TGFbeta-1 signaling pathway through its regulatory proteins, Smads, modulates Tat activation of MCP-1. Here, we demonstrate that C/EBPbeta, whose activity is enhanced by a variety of cytokines during the course of viral infection, can stimulate basal- and Tat-mediated transcription of MCP-1 in human astrocytic cells. Results using promoter deletion mutants suggested the importance of multiple C/EBPbeta binding sites scattered within -200 to +1 of the MCP-1 promoter in the observed activity. Results from DNA binding studies have shown that the interaction of C/EBPbeta with its DNA motif is diminished by the C/EBPbeta homologous protein, CHOP, which possesses the ability to suppress the stimulatory effect of C/EBPbeta on MCP-1 transcription. Tat, which possesses the ability to interact with C/EBPbeta, alleviates the negative effect of CHOP and restores C/EBPbeta interaction with the DNA. Furthermore, Smad3 and its C-terminal regulatory motif, MH2, interact with C/EBPbeta and modulate its DNA binding and transcriptional activity on the MCP-1 promoter. Our results show that the physical and functional interactions of C/EBPbeta and Tat are severely affected by the presence of Smad3 and MH2. Altogether, these observations identify C/EBPbeta as a new partner for Tat in stimulating MCP-1 transcription in astrocytes and suggest that the delicate balance among the downstream regulatory proteins of several cytokines and immunomodulators can dictate the level of expression of chemoattractants, including MCP-1. Hence, inappropriate expression and function of regulatory proteins such as C/EBPbeta and Smads by Tat may induce MCP-1 production in astrocytes and contribute to the neuropathogenesis of AIDS through stimulation of inflammation in the CNS.

RON Receptor Tyrosine Kinase, a Negative Regulator of Inflammation, Inhibits HIV-1 Transcription in Monocytes/Macrophages and Is Decreased in Brain Tissue from Patients with AIDS

Activation of macrophages and microglia cells after HIV-1 infection and their production of inflammatory mediators contribute to HIV-associated CNS diseases. The mechanisms that initiate and maintain inflammation after HIV-1 infection in the brain have not been well studied. Furthermore, it is not understood why in HIV-associated CNS disease, macrophages and microglia are biased toward inflammation rather than production of mediators that control inflammation. We have focused on the receptor tyrosine kinase RON, a critical negative regulator of macrophage function and inflammation, to determine whether this receptor regulates HIV-1 expression. Overexpressing RON in monocytes/macrophages demonstrates that RON inhibits HIV-1 proviral transcription in part by decreasing the binding activity of NF-kappaB to the HIV-1 long terminal repeat. Because macrophages and microglia cells are a critical reservoir for HIV-1 in the CNS, we examined brain tissues for RON expression and detected RON in astrocytes, cortical neurons, and monocytoid cells. RON was detected in all control patients who were HIV seronegative (n = 7), whereas six of nine brain samples obtained from AIDS patients exhibited reduced RON protein. These data suggest that RON initiates signaling pathways that negatively regulate HIV-1 transcription in monocytes/macrophages and that HIV-1 suppresses RON function by decreasing protein levels in the brain to assure efficient replication. Furthermore, HIV-1 infection would compromise the ability of RON to protect against inflammation and consequent CNS damage.

Inhibition of human immunodeficiency virus type 1 replication by Z-100, an immunomodulator extracted from human-type tubercle bacilli, in macrophages

Z-100 is an arabinomannan extracted from Mycobacterium tuberculosis that has various immunomodulatory activities, such as the induction of interleukin 12, interferon gamma (IFN-gamma) and beta-chemokines. The effects of Z-100 on human immunodeficiency virus type 1 (HIV-1) replication in human monocyte-derived macrophages (MDMs) are investigated in this paper. In MDMs, Z-100 markedly suppressed the replication of not only macrophage-tropic (M-tropic) HIV-1 strain (HIV-1JR-CSF), but also HIV-1 pseudotypes that possessed amphotropic Moloney murine leukemia virus or vesicular stomatitis virus G envelopes. Z-100 was found to inhibit HIV-1 expression, even when added 24 h after infection. In addition, it substantially inhibited the expression of the pNL43lucDeltaenv vector (in which the env gene is defective and the nef gene is replaced with the firefly luciferase gene) when this vector was transfected directly into MDMs. These findings suggest that Z-100 inhibits virus replication, mainly at HIV-1 transcription. However, Z-100 also downregulated expression of the cell surface receptors CD4 and CCR5 in MDMs, suggesting some inhibitory effect on HIV-1 entry. Further experiments revealed that Z-100 induced IFN-beta production in these cells, resulting in induction of the 16-kDa CCAAT/enhancer binding protein (C/EBP) beta transcription factor that represses HIV-1 long terminal repeat transcription. These effects were alleviated by SB 203580, a specific inhibitor of p38 mitogen-activated protein kinases (MAPK), indicating that the p38 MAPK signalling pathway was involved in Z-100-induced repression of HIV-1 replication in MDMs. These findings suggest that Z-100 might be a useful immunomodulator for control of HIV-1 infection.

High-affinity interaction between HIV-1 Vpr and specific sequences that span the C/EBP and adjacent NF-kappaB sites within the HIV-1 LTR correlate with HIV-1-associated dementia

Numerous host and viral factors likely participate in the onset and progression of HIV-1-associated dementia (HIVD). Previous studies have suggested that viral gene expression in resident central nervous system (CNS) cells of monocyte/macrophage lineage play a central role in the production of neurotoxic viral proteins and infectious virus, deregulation of cellular gene expression, and/or dysfunction of glial and neuronal cell populations. HIV-1 replication is regulated, in part, by interactions between cellular transcription factors and the viral trans-activators, Tat and viral protein R (Vpr), with cis-acting promoter elements within the LTR. We have previously demonstrated that Vpr binds with high affinity to selected sequence configurations within CCAAT/enhancer binding protein (C/EBP) site I and downstream sequences immediately adjacent to this site. Studies reported herein establish a correlation between the diagnosis of HIVD and the increased prevalence of HIV-1 LTRs containing a C/EBP binding site I that exhibits high affinity for Vpr. To this end, the interaction of Vpr with C/EBP site I variants in 47 LTRs from three nondemented patients and 96 LTRs from seven demented patients was examined. Competition electrophoretic mobility shift (EMS) analyses were utilized to examine Vpr binding to oligonucleotide probes containing C/EBP site I variants. We demonstrated that 89% of LTRs derived from patients exhibiting clinical dementia contained C/EBP site I configurations that displayed a high relative affinity for Vpr, while only 11% of LTRs contained C/EBP site I configurations that exhibited a low relative affinity Vpr binding phenotype. In contrast, examination of LTRs derived from patients lacking clinically evident dementia revealed that only 53% of brain-derived LTRs contained C/EBP site I configurations that displayed a high relative affinity for Vpr, while 47% of LTRs contained C/EBP site I configurations that exhibited a low relative affinity Vpr binding phenotype. We propose that sequence-specific interactions between cis-acting elements in the LTR, members of the C/EBP family of transcription factors, and the virion-associated trans-activator protein Vpr play important roles in the pathogenesis of HIVD.

Retinoid-dependent restriction of human immunodeficiency virus type 1 replication in monocytes/macrophages

Vitamin A deficiency has been correlated with increased severity of human immunodeficiency virus type 1 (HIV-1)-associated disease. Moreover, vitamin A supplementation can reduce AIDS-associated morbidity and mortality. Our group and others have shown that retinoids, the bioactive metabolites of vitamin A, repress HIV-1 replication in monocytic cell lines and primary macrophages by blocking long-terminal-repeat (LTR)-directed transcription. Based on these studies, we hypothesize that retinoids are natural repressors of HIV-1 in vivo. We show here that all-trans-retinoic acid (RA)-mediated repression of HIV-1 activation requires pretreatment for at least 12 h and is blocked by the protein synthesis inhibitors cycloheximide and puromycin. Studies of the kinetics of RA-mediated repression in U1 cells and primary monocyte-derived macrophages (MDMs) reveal that the repressive effects of RA on HIV-1 expression are long-lasting but reversible. We demonstrate that HIV-1 expression is activated when U1 cells or MDMs are cultured in retinoid-free synthetic medium and show that physiological concentrations of RA repress this activation. In addition, the synthetic pan-retinoic acid receptor antagonist BMS-204 493 activates HIV-1 replication in U1 cells in a dose-dependent manner, suggesting that RA-induced transactivation of cellular gene expression is required for HIV-1 repression. Together, these data support the hypothesis that retinoids present in tissue culture media in vitro and serum in vivo maintain HIV-1 in a transcriptionally repressed state in monocytes/macrophages.

Molecular characterization of a putative antiretroviral transcriptional factor, OTK18

Elucidation of the factors involved in host defense against human immunodeficiency viral infection remains pivotal if viral control may be achieved. Toward these ends, we investigated the function of a putative antiretroviral factor, OTK18, isolated by differential display of mRNA from HIV type 1-infected primary human monocyte-derived macrophages. Molecular and immunohistochemical analyses showed that the OTK18 nucleotide sequence contains 13 adjacent C(2)H(2)-type zinc finger motifs, a Krüppel-associated box, and is localized to both cytosol and nucleus. Mutational analyses revealed that both the Krüppel-associated box and zinc finger regions of OTK18 are responsible for the transcriptional suppressive activities of this gene. OTK18 was copiously expressed in macrophages following HIV type I infection and diminished progeny virion production. A mechanism for this antiretroviral activity was by suppression of HIV type 1 Tat-induced viral long terminal repeat promoter activity. Our findings suggest that one possible function of OTK18 is as a HIV type 1-inducible transcriptional suppressor.

Interaction between C/EBPbeta and Tax down-regulates human T-cell leukemia virus type I transcription

The human T-cell leukemia virus type I (HTLV-I) Tax protein trans-activates viral transcription through three imperfect tandem repeats of a 21-bp sequence called Tax-responsive element (TxRE). Tax regulates transcription via direct interaction with some members of the activating transcription factor/CRE-binding protein (ATF/CREB) family including CREM, CREB, and CREB-2. By interacting with their ZIP domain, Tax stimulates the binding of these cellular factors to the CRE-like sequence present in the TxREs. Recent observations have shown that CCAAT/enhancer binding protein beta (C/EBPbeta) forms stable complexes on the CRE site in the presence of CREB-2. Given that C/EBPbeta has also been found to interact with Tax, we analyzed the effects of C/EBPbeta on viral Tax-dependent transcription. We show here that C/EBPbeta represses viral transcription and that Tax is no more able to form a stable complex with CREB-2 on the TxRE site in the presence of C/EBPbeta. We also analyzed the physical interactions between Tax and C/EBPbeta and found that the central region of C/EBPbeta, excluding its ZIP domain, is required for direct interaction with Tax. It is the first time that Tax is described to interact with a basic leucine-zipper (bZIP) factor without recognizing its ZIP domain. Although unexpected, this result explains why C/EBPbeta would be unable to form a stable complex with Tax on the TxRE site and could then down-regulate viral transcription. Lastly, we found that C/EBPbeta was able to inhibit Tax expression in vivo from an infectious HTLV-I molecular clone. In conclusion, we propose that during cell activation events, which stimulate the Tax synthesis, C/EBPbeta may down-regulate the level of HTLV-I expression to escape the cytotoxic-T-lymphocyte response.

Protein tyrosyl phosphatases in T cell activation: implication for human immunodeficiency virus transcriptional activity

The protein tyrosine phosphatases (PTPs) superfamily is a large group of enzymes showing a wide diversity of structure and biological functions. Their implication in the regulation of signal transduction processes is critical for homeostasis and efficient cellular activation. Disturbance of the delicate balance between protein tyrosine kinase and protein tyrosine phosphatase activities is at the heart of a large number of diseases. Control of cellular activation is especially important for human immunodeficiency virus type 1 (HIV-1) since this retrovirus requires activated T cells in order to replicate efficiently. Identification of PTPs implicated in signaling pathways leading to upregulation of HIV-1 gene transcription therefore contributes to the general understanding of cellular factors needed for strong HIV-1 replication and progression to AIDS. The use of bisperoxovanadium compounds as potent, specific, and highly purified PTP inhibitors releases HIV-1 from PTP control and strongly increases HIV-1 gene expression. These inhibitors can thus be used to study signal transduction mechanisms regulated by PTP activity that are important for HIV-1 replication and provide new and interesting therapeutic avenues for the efficient control of this debilitating retroviral infection.

Regulation of HIV-1 gene transcription: from lymphocytes to microglial cells

Transcription is a crucial step for human immunodeficiency virus type 1 (HIV-1) expression in all infected host cells, from T lymphocytes, thymocytes, monocytes, macrophages, and dendritic cells in the immune system up to microglial cells in the central nervous system. To maximize its replication, HIV-1 adapts transcription of its integrated proviral genome by ideally exploiting the specific cellular environment and by forcing cellular stimulatory events and impairing transcriptional inhibition. Multiple cell type-specific interplays between cellular and viral factors perform the challenge for the virus to leave latency and actively replicate in a great diversity of cells, despite the variability of its long terminal repeat region in different HIV strains. Knowledge about the molecular mechanisms underlying transcriptional regulatory events helps in the search for therapeutic agents that target the step of transcription in anti-HIV strategies.

Structural and functional evolution of human immunodeficiency virus type 1 long terminal repeat CCAAT/enhancer binding protein sites and their use as molecular markers for central nervous system disease progression

The appearance and progression of human immunodeficiency virus type 1 (HIV-1)-associated pathogenesis in the immune and central nervous systems is dependent on the ability of the virus to replicate in these compartments, which is, in turn, controlled by numerous factors, including viral binding and entry, receptor and coreceptor usage, and regulation of viral expression by the long terminal repeat (LTR). The LTR promotes viral expression in conjunction with viral and cellular regulatory proteins, including members of the CCAAT/enhancer binding protein (C/EBP) family, which modulate LTR activity through at least two cis-acting binding sites. Previous studies have shown that these sites are necessary for HIV-1 replication in cells of the monocyte/macrophage lineage, but dispensable in T lymphocytes. To establish potential links between this important family of transcription factors and HIV-1-associated pathogenesis, C/EBP site I and II sequence variation in peripheral blood mononuclear cell (PBMC)-derived LTRs from HIV-1-infected patients with varying degrees of disease severity was examined. A high prevalence of C/EBP site variants 3T (site I) and consensus B (site II) within PBMC-derived HIV-1 LTRs was shown to correlate with late stage disease in HIV-1-infected patients. These results suggest that the increased prevalence in the PBMCs of HIV-1 LTRs containing the 3T C/EBP site I variant and the consensus B site II variant may serve as a molecular marker for disease progression within the immune system. The relative low or high binding affinity of C/EBP beta to sites I and II in electrophoretic mobility shift (EMS) analyses correlated with low or high LTR activity, respectively, in transient expression analyses during both early and late disease stages. The 3T C/EBP site I was the only variant examined that was not found in LTRs derived from PBMCs of patients at early stages of HIV-1 disease, but was found at increasing frequencies in patients with late stage disease. Furthermore, the 3T C/EBP site I was not found in brain-derived LTRs of patients without HIV-1-associated dementia (HIVD), but was found in increasing numbers in brain-derived LTRs from patients diagnosed with HIVD. The C/EBP site I 3T variant appears to be exclusive to patients progressing to increasingly severe HIV-1-associated immunologic and neurologic disease.

Identification of binding sites for members of the CCAAT/enhancer binding protein transcription factor family in the simian immunodeficiency virus long terminal repeat

Members of the CCAAT/enhancer binding protein (C/EBP) transcription factor family are necessary for human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) activity and viral replication in cells of monocyte/macrophage lineage. The integral roles that HIV-1-infected monocytes and macrophages play in the development and progression of HIV-1-associated disease in the immune and central nervous systems underscore the importance of the C/EBP transcription factor family within the context of regulating HIV-1 gene expression. Although there are considerable similarities between HIV-1 and simian immunodeficiency virus (SIV), including viral-induced immunopathogenesis and neurologic dysfunction, infection of CD4(+) T cells and cells of monocyte/macrophage origin, and LTR structure/function, the involvement of C/EBP factors in regulating SIV transcription has not been previously demonstrated. Analyses of the SIV(mac)239 LTR sequence indicated the presence of five putative C/EBP binding sites within the LTR. Electrophoretic mobility shift (EMS) analyses demonstrated that four of the five sites within the SIV LTR were able to bind C/EBP factors (alpha and beta) and compete for DNA-protein complexes formed by the HIV-1 C/EBP site located adjacent to the promoter-distal NF-kappaB site. DNase I protection assays indicated that purified C/EBPbeta specifically was able to occupy each of the four binding sites. These studies suggest that C/EBP factors may also have important roles in the regulation of SIV gene expression and replication, and that these factors and signal transduction pathways that regulate their activity may impact SIV-associated pathogenesis.

Regulation of HIV-1 transcription by NF-IL6 in activated Jurkat T cells

To examine the mechanism of HIV-1 regulation by NF-IL6 in activated human cells, we selected a Jurkat cell line that did not contain endogenous NF-IL6. In this cellular environment, we evaluated the effect of exogenous NF-IL6 on transcription mediated by native and deleted LTR sequences. In Jurkat cells stimulated with LPS and PMA, LTR-mediated transcription was enhanced by NF-IL6. The results of deletion studies revealed a central role for the basal LTR region and the TATA element in the LTR, in upregulation of reporter gene expression by NF-IL6 in activated cells. In the selected cellular environment, regulation of transcription by NF-IL6 was not evident in studies of promoter regions of other genes. The results implied that the basal region of HIV-1 LTR includes molecular properties that support activation of HIV-1 by NF-IL6 in stimulated cells.

Organization of the promoter region of the human NF-IL6 gene

In monocyte/macrophages, the human NF-IL6 gene was activated by LPS or PMA. However, a robust response required stimulation of cells with both LPS and PMA. To examine the molecular basis of this response, we isolated human genomic DNA and determined the nucleotide sequence of a segment (6.4 kb) that included the transcription initiation site of the gene. The unique sequences in the 6.4-kb DNA include several potential transcription factor-binding elements that may explain the molecular basis of the activation of the human NF-IL6 gene by signaling molecules that control the immune and inflammatory responses. Deletion analysis localized an LPS+PMA responsive region downstream position -287, with respect to the transcription initiation site of the NF-IL6 gene. The responsive region includes a potential site for interactions with CREB and a region (-287 to -247) that interacts with SP1 and SP3. In functional assays, the potential CREB site responded to cellular stimulation. The region that interacted with SP1 and SP3 augmented the overall level of activity produced in response to LPS+PMA.

Regulation of HIV-1 transcription in activated monocyte macrophages

DNA-binding and functional assays examined the role played by NF-IL6 in regulation of HIV-1 transcription in human monocyte/macrophages (U937 cells), stimulated with LPS+PMA. When incubated with nuclear extracts from stimulated cells, a region (-189/-147), containing the major NF-IL6-binding sequence and the USF site, interacted selectively with USF1 and USF2. Anti-C/EBPbeta reacted poorly with the complexes produced with the wild-type probe. In contrast, complex formation with NF-IL6 was clearly evident in experiments analyzing a probe containing an insertion in the USF site. In functional assays, increasing concentrations of a decoy against NF-IL6 reduced gene expression from the LTR of the wild-type HIV-1 variant, supporting a critical role for NF-IL6 in regulation of HIV-1 transcription in stimulated monocyte/macrophages. The decoy also reduced gene expression from a deletion construct lacking NF-IL6-binding sequences. The results implied that in LPS+PMA-stimulated monocyte/macrophages, the endogenous NF-IL6 could act via a site-independent pathway in upregulation of HIV-1 transcription. Analysis of a short DNA segment, containing the -189/-147 region, suggested functional interactions of NF-IL6 and USF. In activated cells exogenous NF-IL6 enhanced dramatically gene expression through a short DNA segment containing the NF-kappaB sites, supporting functional interactions of NF-IL6 and NF-kappaB.

CCAAT/enhancer binding proteins are not required for HIV-1 entry but regulate proviral transcription by recruiting coactivators to the long-terminal repeat in monocytic cells

CCAAT/enhancer binding proteins (C/EBP) have been shown to be required for HIV-1 transcription and replication in macrophages. However, whether these transcription factors influence the ability of virus to establish infection by altering cytokine or receptor expression or primarily regulate HIV-1 transcription has not been determined. By inhibiting endogenous C/EBP activity with a dominant-negative protein, we demonstrate that functional C/EBPs are not required for HIV-1 infection and that these factors influence replication by a transcriptional mechanism. C/EBPbeta recruits coactivators to the HIV-1 long-terminal repeat (LTR) and physically interacts with histone acetyltransferase (HAT) complexes, suggesting that C/EBPs participate in remodeling the chromatin organization of the HIV-1 provirus. Furthermore, overexpression of a C/EBP dominant-negative inhibits displacement of nucleosomes located at the HIV-1 transcriptional start site. These results provide insight into the general mechanisms by which C/EBPs regulate macrophage-restricted HIV-1 transcription.