New findings on transcription regulation across different HIV-1 subtypes

Transcriptional activation of gene expression in HIV-1 is controlled by the interaction of sequence-specific transcription factors with the long terminal repeat (LTR) of the provirus. The identification and characterization of cellular proteins involved in the process has provided a basic understanding about both general eukaryotic and HIV-1 proviral transcription regulation. The HIV-1 epidemic is expanding worldwide with an increasing number of distinct viral subtypes as well as intersubtype recombinant viruses. LTR-specific sequence variability among different HIV-1 variants could affect LTR binding to cellular and/or viral factors, influencing the extent of transcription. In vitro assays have demonstrated subtype-specific functional differences between the LTR regions of distinct HIV-1 subtypes. This observation could have consequences on the biology of the different HIV-1 clades and influence HIV-1 disease progression. Finally, the knowledge of the molecular mechanisms of transcription regulation events could help in the search for new compounds targeting the critical steps of viral transcription.

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.

Herpes simplex virus type 1 glycoprotein D inhibits T-cell proliferation

Herpes simplex virus type 1 (HSV-1) glycoprotein D (gD) binds to its cellular receptor, herpesvirus entry mediator (HVEM), to enter into activated T cells. Since gD is expressed on the cell surface of activated T cells after infection and can interact with HVEM, a co-stimulatory molecule for T cells, we hypothesized that the membrane-bound gD can exert an immunomodulatory effect on activated T cells. In this report, we demonstrated the following: (1) The gD expression was detected on the cell surface of activated T cells after HSV-1 infection. (2) Recombinant soluble gD protein or gD-expressing mouse fibroblasts inhibited T-cell proliferation that was induced by OKT3 [anti-CD3 monoclonal antibody (mAb)]. (3) The co-expression of gD and HVEM resulted in the inhibition of the nuclear factor (NF)-kappaB activation that was induced by the HVEM overexpression. Taken together, our results suggest that the inhibitory effect of gD may be due to its ability to actively inhibit the signaling pathway that is mediated by HVEM on the cell surface level, which may be a novel immune evasion mechanism that is utilized by HSV-1.

[Mechanism of transcriptional activation by HTLV-1 Tax]

A trans-activator protein Tax of HTLV-1 activates the transcription of various cellular genes as well as viral promoter through the multiple enhancer elements and leads cells to the transformation. Tax protein interacts to enhancer DNA elements by associating with cellular enhancer binding proteins such as CREB, NF-kappa B and SRF. In addition, we found recently that Tax interacts with an I kappa B protein, the inhibitor of NF-kappa B, resulting in the enhancement of nuclear translocation of NF-kappa B molecules. Therefore, Tax seems to modulate the transcription by the interaction with multiple transcription factors in different modes of control.

Hexamethylene bisacetamide activates the human immunodeficiency virus type 1 provirus by an NF-kappa B-independent mechanism

Expression of the human immunodeficiency virus type 1 (HIV-1) provirus in T lymphocytic and monocytic cells can be induced by treatment with hexamethylene bisacetamide (HMBA). The induction occurs at the transcriptional level within 1 to 3 h after the addition of the drug, and is not associated with detectable changes in the binding of transcription factors to the enhancer, TATA box or other regulatory regions of the HIV-1 long terminal repeat (LTR). Using the 5' deletion mutants of HIV-1 LTR controlling the expression of the chloramphenicol acetyltransferase gene, we found that the deletion of the kappa B enhancer did not affect HIV-1 inducibility, whereas the deletion of the Sp1 binding sites abolished transcriptional activation. However, the presence of the HIV-1 LTR Sp1 binding sites in the context of the heterologous promoter did not induce responsiveness to HMBA. We conclude that HMBA increases transcription through the secondary modification of the basal transcription complex suggesting the existence of a regulatory pathway that circumvents the requirement for the induction of NF-kappa B or other DNA-specific binding proteins.

Human thioredoxin/adult T cell leukemia-derived factor activates the enhancer binding protein of human immunodeficiency virus type 1 by thiol redox control mechanism

Transcription from the human immunodeficiency virus type 1 (HIV-1) provirus is activated by a cellular factor, NF kappa B, recognizing the tandemly repeated 10-base-pair sequences, termed the kappa B sequence, present in the enhancer region within the viral long terminal repeat (LTR). Using electrophoretic mobility shift assay (EMSA), which demonstrates specific DNA-protein interaction in vitro, we could demonstrate that reducto-oxidative modulation of NF kappa B dramatically changes its DNA binding activity and that a cellular physiological reducing catalyst, thioredoxin (TRX) also known as adult T cell leukemia derived factor (ADF), fully restored the DNA-binding activity of the oxidized NF kappa B. We also observed that purified TRX/ADF protein could augment gene expression from HIV LTR as demonstrated by transient chloramphenicol acetyltransferase (CAT) assay. These observations confirmed the previous notion that ADF might be an inducing factor of cellular interleukin-2 receptor alpha subunit (IL-2R alpha) through the kappa B sequence that is a common central cis-regulatory element in both IL-2R alpha and HIV gene expression. These observations indicate that reducto-oxidative regulation (or redox regulation) of a cysteine residue(s) on the NF kappa B molecule might play an important role in its specific DNA interaction and that it might provide a clue to the understanding of a pathway of cellular signal transduction to NF kappa B that is independent from the known pathways involving protein phosphorylation.

Herpes simplex virus type 1-mediated induction of human immunodeficiency virus type 1 provirus correlates with binding of nuclear proteins to the NF-kappa B enhancer and leader sequence

Herpes simplex virus type 1 (HSV-1) infection induces expression of the human immunodeficiency virus type 1 (HIV-1) provirus in the chronically infected T-cell line ACH-2. The HSV-1-mediated induction correlates with the appearance of two NF-kappa B-specific proteins of 55 and 85 kDa in the nucleus and with the binding of 50-kDa nuclear protein to the LBP-1 binding site of the untranslated leader sequence of the HIV-1 long terminal repeat. The HSV-1-induced LBP-1 binding protein, designated HLP-1, is present exclusively in HSV-1-infected, but not in phorbol-12-myristate-13-acetate- or tumor necrosis factor alpha-treated ACH-2 cells. Both the NF-kappa B and LBP-1 target sequences, when inserted either alone or together 5' of a heterologous minimal promoter (thymidine kinase), confer inducibility by HSV-1 infection in a transient transfection assay. Thus, it appears that the HSV-1-mediated activation of HIV-1 provirus is brought about by the binding of both NF-kappa B and HLP-1 specific proteins to two distinct regions of HIV-1 long terminal repeat.

A comparison of regulatory features in primate lentiviruses

Historically, research into the regulation of gene expression in primate lentiviruses has focused on human immunodeficiency virus type 1 (HIV-1), the primary cause of acquired immunodeficiency syndrome (AIDS) in humans. The increasing emergence of HIV-2 as a human pathogen, and the importance of the various simian immunodeficiency viruses (SIV) as models for the treatment and prevention of HIV-1-induced disease, suggest that an understanding of gene regulation in these related viruses will become increasingly important. Here, the present state of knowledge in this latter field is reviewed. In general, while the data support the hypothesis that viral gene expression is regulated by very similar mechanisms in all primate lentiviruses, it also is clear that differences in detail do exist. These differences may influence the pathogenic potential of the different strains of primate lentiviruses and must be considered in evaluating SIV as an appropriate in vivo model for HIV-1.

Modulation of HIV-enhancer activity by heterologous agents: a minireview

The enhancer region of the human immunodeficiency virus (HIV) long terminal repeat (LTR) contains two 10-bp (5'-GGGACTTTCC) repeats (core enhancers) which constitute the binding sites for the ubiquitous inducible cellular transcription factor, NF-kappa B. The NF-kappa B motifs of the LTR play a central role in transcriptional activation of the LTR by several heterologous viral proteins and various external chemical and physical stimuli. Activation of the HIV enhancer by these agents may lead to the onset of HIV gene expression resulting in active viral replication. Viral genes and chemical agents, which interfere with the activity of the enhancers may be useful in inhibiting HIV gene expression, thereby suppressing HIV replication.