Transcriptional (p40x) and post-transcriptional (p27x-III) regulators are required for the expression and replication of human T-cell leukemia virus type I genes

Comparisons of defective HTLV-I proviruses predict the mode of origin and coding potential of internally deleted genomes

Cell lines infected with a variety of HTLV-I isolates were examined for the presence of defective proviruses that contain deletions spanning the gag, pol, and env genes. Internally deleted proviruses were identified by Southern blotting and by PCR amplification with 5' and 3' primers complementary to gag and tax sequences, respectively. PCR products representing eight defective proviruses from seven different cell lines were subsequently cloned and sequenced. The objectives of this study were twofold: first, we sought to determine whether nucleotide sequences surrounding sites of deletion shared common features that might reveal the mechanisms by which the defective genomes originated. Second, we asked whether deleted proviruses encode Gag fusion proteins with related C-terminal residues derived from open reading frames in the pX region. While most of the defective proviruses had incurred a single, large deletion, two of them displayed a more complex pattern of multiple rearrangements. Alignments of bases flanking the 5' and 3' deletion endpoints within each provirus showed tracts of sequence identity consistent with a mechanism involving aberrant intramolecular strand-transfer events during replication. We suggest that the amount or activity of HTLV-I polymerase in virions may contribute both to the poor infectivity of the virus and to the high deletion frequency. Two of the eight proviruses that were examined encoded a gag gene joined to an extended open reading frame; the other six had very short open reading frames (one to six amino acids) derived from pX or env regions joined to gag that showed no apparent amino acid sequence similarity.

Influence of Rex and intronic sequences on expression of spliced mRNAs produced by human T cell leukemia virus type I

Expression of the incompletely spliced HTLV-I mRNAs relies on the viral posttranscriptional activator Rex, whose interaction with the Rex-responsive element (RXRE) overcomes effects of cis-acting repressive sequences (CRSs). Studies based on heterologous reporter plasmids identified an intronic CRS in the 5' LTR and a CRS that overlaps with the RXRE. The present study investigated the effects of these elements in the context of spliced viral mRNAs encoding p21Rex (mRNA 1-3), Tax/Rex (mRNA 1-2-3), and Tof (mRNA 1-2-B). All three mRNAs were inefficiently expressed when transcribed in their mature intronless form, with the p21Rex mRNA showing the weakest expression. In contrast, efficient expression of p21Rex was obtained from a plasmid containing the 5' LTR and 3' portion of the genome that encoded a spliceable RNA. The defective expression of the intronless mRNAs reflected the inhibitory activity of the RXRE and the lack of 5' intronic sequences. Insertion of an intronic 5' LTR segment located upstream of the 5' CRS overcame Rex dependence conferred by the RXRE. The activity of this segment was mapped to the major splice donor and sequences overlapping with, but functionally distinct from, a previously described transcriptional enhancer. The three mRNAs responded differently to Rex and to insertion of the constitutive transport element of simian retrovirus type 1. Taken together, these results suggest that expression of the spliced mRNAs is controlled by the relative influence of positive and negative sequences present on the primary transcript as well as the Rex-RXRE interaction.

Human T-cell leukemia virus type 2 Rex protein increases stability and promotes nuclear to cytoplasmic transport of gag/pol and env RNAs

The human T-cell leukemia virus (HTLV) Rex protein is essential for efficient expression of the viral structural and enzymatic gene products. In this study, we assessed the role of the HTLV-2 rex gene in viral RNA expression and Gag protein production. Following transfection of human JM4 T cells with wild-type and rex mutant full-length proviral constructs, PCR was used for semiquantitative analysis of specific viral RNA transcripts. In the presence of Rex, the total amount of steady-state viral RNA was increased fourfold. Rex significantly up-regulated the level of incompletely spliced RNAs by increasing RNA stability and was associated with a twofold down-regulation of the completely spliced tax/rex RNA. PCR analysis of subcellular RNA fractions, isolated from transfected cells, indicated that the level of gag/pol and env cytoplasmic RNAs were increased 7- to 9-fold in the presence of Rex, whereas Gag protein production was increased 130-fold. These data indicate that HTLV-2 Rex increases the stability and promotes nucleus-to-cytoplasm transport of the incompletely spliced viral RNAs, ultimately resulting in increased structural protein production. Moreover, this model system provides a sensitive approach to further characterize HTLV gene expression from full-length proviral clones following transfection of human T cells.

Defective and wild-type human T-cell leukemia virus type I proviruses: characterization of gene products and trans-interactions between proviruses

Defective provirus genomes of human T-cell leukemia virus type I are frequently detected in lymphocytes from infected individuals and in infected cell lines. One type of defective provirus contains internal deletions spanning gag, pol, and env genes but retains portions of open reading frames for trans-regulatory proteins. The deleted proviruses could potentially contribute to viral pathology by producing novel gene products that directly affect cell metabolism or that modulate expression of resident, wild-type proviruses. Virus gene products and the control of their expression were examined in cells transfected with defined molecular clones of wild-type and defective proviruses. Internally deleted provirus clones, which are unable to produce functional Tax and Rex proteins, were transcriptionally inactive in transfected cells. Ectopic expression of p40Tax activated transcription of the deleted provirus, resulting in the accumulation of a two-exon mRNA that yields a truncated form of Rex (p21Rex). Although this two-exon mRNA also has a potential initiation codon in the tax frame, a truncated form of Tax was not detected by immunoblotting or in transactivation assays. When complemented with p40Tax and p27Rex, cells transfected with deleted proviruses accumulated an unspliced mRNA that could potentially encode gag-pX fusion proteins. Although expression of deleted proviruses was dependent on trans-acting factors produced from intact proviruses, gene products from defective proviruses did not significantly affect expression of a cotransfected, full-length provirus.

Anti-Rex aptamers as mimics of the Rex-binding element

RNA molecules that bind tightly and specifically to a Rex fusion protein have been isolated from a conformationally constrained pool of random sequence RNAs. The anti-Rex aptamers effectively mimic several features of the wild-type Rex-binding element (XBE). The highest-affinity aptamers effectively compete with the wild-type XBE for binding to the RNA-binding domain of Rex, an arginine-rich motif (ARM), but do not bind to the functionally analogous Rev protein or its ARM. However, characteristic sequence and structural motifs found in some of the anti-Rex aptamers may provide insights into how the Rex protein can interact with other viral RNAs, such as the Rev-responsive element. The anti-Rex aptamers can functionally substitute for the XBE in vivo, a result which supports a previously proposed model for mRNA transport in which the viral genome serves as a platform for assembling a nucleoprotein complex that can co-opt the cellular transport apparatus. Overall, these studies suggest that anti-Rex aptamers may serve as RNA decoys of the Rex protein.

Immortalization of CD4(+) and CD8(+) T lymphocytes by human T-cell leukemia virus type 1 Tax mutants expressed in a functional molecular clone

The human T-cell leukemia virus type 1 (HTLV-1) transcriptional trans-activator Tax has been demonstrated to have transforming activity in multiple cell culture and transgenic-mouse models. In addition to activating transcription from the viral long terminal repeat (LTR) through the cyclic AMP response element binding protein/activating transcription factor (CREB/ATF) family of transcription factors, Tax activates the expression of multiple cellular promoters through the NF-kappaB pathway of transcriptional activation. The Tax mutants M22 and M47 have previously been demonstrated to selectively abrogate the ability of Tax to activate transcription through the NF-kappaB or CREB/ATF pathway, respectively. These mutations were introduced in the tax gene of the ACH functional molecular clone of HTLV-1, and virus produced from the mutant ACH clones was examined for the ability to replicate and immortalize primary human lymphocytes. While virus derived from the clone containing the M47 mutation retained the ability to immortalize T lymphocytes, the M22 mutant lost the ability to immortalize infected cells. These results indicate that activation of the CREB/ATF pathway by Tax is dispensable for the immortalization of T cells by HTLV-1, whereas activation of the NF-kappaB pathway may be critical.

Activation of the interferon-inducible (2'-5') oligoadenylate synthetase by the Epstein-Barr virus RNA, EBER-1

The 2'-5' oligoadenylate synthetases and the protein kinase PKR are both interferon-induced, double-stranded RNA-dependent proteins that play important roles in the antiviral effects of the interferons and in cellular growth control. Both enzymes are activated by natural or synthetic dsRNAs and by single-stranded RNAs that possess extensive secondary structure. This report describes the effects of the small Epstein-Barr virus-encoded RNA EBER-1 on the regulation of 2-5(A) synthetase activity. We demonstrate that EBER-1 RNA binds to and activates the human 40-kDa 2-5(A) synthetase in a dose-dependent manner. The efficiency of EBER-1 as an activator of 2-5(A) synthetase is approximately 25% of that of the synthetic double-stranded RNA poly(I)/poly(C), and poly(I)/poly(C) further stimulates enzyme activity even in the presence of a high concentration of EBER-1. Conversely, EBER-1 neither stimulates nor inhibits 2-5(A) synthetase that has been activated by a high concentration of poly(I)/poly(C). Competitive binding assays suggest that the relative affinity of the enzyme for poly(I)/poly(C) is considerably higher than that for EBER-1. Our data indicate that EBER-1, like VAI RNA of adenovirus, TAR RNA of HIV-1, and Rex-RE RNA of HTLV-1, is able to activate the 2-5(A) synthetases. The significance of why several viruses may activate the 2-5(A) synthetase/RNase L-mediated RNA degradation pathway is discussed.

Epstein-Barr virus involvement in T-cell malignancy: significance in adult T-cell leukemia

Epstein-Barr virus (EBV) was first reported as the causative virus of Burkitt's lymphoma in 1964. Since then, EBV has also been associated with infectious mononucleosis, AIDS and transplant-related B cell lymphomas, and nasopharyngeal cancer. The virus has further been linked with T cell lymphomas, Hodgkin disease, and NK leukemia or LGL leukemia, establishing a concept of a wide spectrum of EBV associated malignant disorders. EBV DNA encodes several proteins such as EBNA1-6, LMP 1, 2 and others. Recent studies have demonstrated that EBNA2, EBNA5, EBNA3A, EBNA 3C are essential for transformation, and that any gene product is not sufficient to transform cells by itself. Further there are different mechanisms of virus-associated transformation or carcinogenesis among EBV-associated malignant disorders. On the other hand, human T lymphotropic virus type I (HTLV-I) is known as a causative virus of adult T cell leukemia (ATL). However, precise molecular mechanisms of leukemogenesis in ATL still remains unclear. Some additional factors to HTLV-I infection are supposed to be involved in complete leukemogenesis. We demonstrated that HTLV-I infected T cells and primary ATL cells express EBV receptor/CD21 on the cell surface. Therefore, it is possible that EBV infection is one of the factors. We further investigated this possibility in 6 HTLV-I infected T cell lines and primary ATL cells from 18 patients with ATL. However, no EBV genome was detected in both T cell lines and primary ATL cells. EBV involved T-cell lymphoma has unique clinical manifestations as compared to non-EBV involved T-cell lymphoma. Therefore, it is still possible that a small group of ATL patients with unique clinical manifestations is associated with EBV.

R region sequences in the long terminal repeat of a murine retrovirus specifically increase expression of unspliced RNAs

A stem-loop structure at the 5' end of the R region of the long terminal repeat (LTR) of the murine leukemia virus SL3 and other type C mammalian retroviruses is important for maximum levels of expression of a reporter gene under the control of the viral LTR. This element, termed the R region stem-loop (RSL), has a small effect on transcriptional initiation and no effect on RNA polymerase processivity. Its major effect is on posttranscriptional processing of LTR-driven transcripts. Here we tested whether the RSL affected the production of RNAs from a full-length SL3 genome. Mutation of the RSL in the 5' LTR of SL3 reduced the cytoplasmic levels of full-length viral transcripts but not those of spliced, env mRNA transcripts. Thus, the RSL specifically affected the cytoplasmic levels of the unspliced viral RNA. To test further whether the effect was specific for unspliced transcripts, a system was devised in which the expression of a reporter gene under the control of the viral LTR was tested in the presence or absence of an intron. Mutation of the RSL resulted in only about a twofold decline in the level of reporter gene expression when the transcripts contained an intron. However, when the intron was removed, mutation of the RSL reduced expression of the reporter gene about 10- to 60-fold in various cell lines. The secondary structure of the RSL was essential for its activity on the intronless transcript. Thus, the RSL appears to be important for the cytoplasmic accumulation of unspliced viral RNA and unspliced RNA from chimeric transcription units under the control of the viral LTR.

Comparison of HTLV-I basal transcription and expression of CREB/ATF-1/CREM family members in peripheral blood mononuclear cells and Jurkat T cells

HTLV-I is the etiologic agent of adult T-cell leukemia/lymphoma and is associated with tropical spastic paraparesis/HTLV-I-associated myelopathy. Following integration into the host cell genome, HTLV-I replication is regulated by both host and viral mechanisms that control transcription. Low levels of viral transcription (basal transcription) occur before expression of the virally encoded Tax protein (Tax-mediated transcription). Members of the cyclic adenosine monophosphate (cAMP) response element binding (CREB)/activating transcription factor 1 (ATF-1) family of transcription factors bind three 21-bp repeats (Tax-responsive element-1, or TRE-1) within the viral promoter and are important for basal and Tax-mediated transcription. Using mitogen stimulated and quiescent peripheral blood mononuclear cells (PBMC) and Jurkat cells, we compared differences in basal transcription and amounts and binding of transcription factors with TRE-1. We demonstrate that amounts of transcriptionally active phosphorylated CREB protein (P-CREB) differ between activated PBMC and Jurkat cells. Following stimulation, P-CREB levels remain elevated in PBMC for up to 24 hours whereas CREB is dephosphorylated in Jurkat cells within 4 hours following stimulation. The differences in P-CREB levels between PBMC and Jurkat cells were directly correlated with basal transcription of HTLV-I in the two cell types. Using electrophoretic mobility shift assays, we determined that the pattern of band migration differed between the two cell types. These data demonstrate that PBMC differentially regulate basal HTLV-I transcription compared with Jurkat T cells, and this differential regulation is due, in part to differential phosphorylation and binding of CREB/ATF-1 to TRE-1 in the HTLV-I promoter. We demonstrate the utility of using primary lymphocyte models to study HTLV-I transcription in the context of cell signaling and suggest that activated PBMC maintain elevated levels of P-CREB, which promote basal HTLV-I transcription and enhance viral persistence in vivo.

Reduction in HTLV-I proviral load and spontaneous lymphoproliferation in HTLV-I-associated myelopathy/tropical spastic paraparesis patients treated with humanized anti-Tac

Human T-lymphotropic virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a neurological disease that results from an interaction of retroviral infection and immune activation. In this study, five doses (1 mg/kg) of humanized anti-Tac antibody were administered to 9 HAM/TSP patients at weeks 0, 2, 6, 10, and 14. Preliminary immunological studies on HAM/TSP patients treated with humanized anti-Tac indicate that there is a selective down-regulation of activated T cells and a decrease in the HTLV-I viral load in peripheral blood lymphocytes, most likely through the selective removal of HTLV-I-infected, activated CD4+ lymphocytes.

Extracellular human T cell leukemia virus type I tax protein stimulates the proliferation of human synovial cells

OBJECTIVE

The present study was designed to investigate whether the proliferation of normal synovial cells from patients with meniscus injury is stimulated by human T cell leukemia virus type I (HTLV-I) Tax protein.

METHODS

The effect of Tax protein on the proliferation of synovial cells was evaluated using a 3H-thymidine incorporation assay. Production of cytokines was determined by enzyme-linked immunosorbent assay. Nuclear factor kappaB (NF-kappaB) DNA binding activity and the transcription of several NF-kappaB-mediated genes was detected by electrophoretic mobility shift assay and reverse transcriptase-polymerase chain reaction.

RESULTS

The proliferation of synovial cells, as well as their expression of tumor necrosis factor alpha, granulocyte-macrophage colony-stimulating factor, and interleukin-6, was significantly enhanced by extracellular Tax at concentrations of 2.5 pM to 25 nM. In contrast, extracellular bacterial extract did not change the cytokine expression or the proliferation of these cells. Proliferation of synovial cells induced by Tax protein may be due to activated expression of several cytokines and protooncogenes that contain NF-kappaB regulatory sequences.

CONCLUSION

Our results suggest that extracellular Tax can regulate the expression of endogenous cellular genes in synovial cells and may contribute to the NF-kappaB-mediated synovial hyperplasia.

Multimer formation is not essential for nuclear export of human T-cell leukemia virus type 1 Rex trans-activator protein

The Rex trans-regulatory protein of human T-cell leukemia virus type 1 (HTLV-1) is required for the nuclear export of incompletely spliced and unspliced viral mRNAs and is therefore essential for virus replication. Rex is a nuclear phosphoprotein that directly binds to its cis-acting Rex response element RNA target sequence and constantly shuttles between the nucleus and cytoplasm. Moreover, Rex induces nuclear accumulation of unspliced viral RNA. Three protein domains which mediate nuclear import-RNA binding, nuclear export, and Rex oligomerization have been mapped within the 189-amino-acid Rex polypeptide. Here we identified a different region in the carboxy-terminal half of Rex which is also required for biological activity. In inactive mutants with mutations that map within this region, as well as in mutants that are deficient in Rex-specific multimerization, Rex trans activation could be reconstituted by fusion to a heterologous leucine zipper dimerization interface. The intracellular trafficking capabilities of wild-type and mutant Rex proteins reveal that biologically inactive and multimerization-deficient Rex mutants are still efficiently translocated from the nucleus to the cytoplasm. This observation indicates that multimerization is essential for Rex function but is not required for nuclear export. Finally, we are able to provide an improved model of the HTLV-1 Rex domain structure.

The secondary structure of the R region of a murine leukemia virus is important for stimulation of long terminal repeat-driven gene expression

In addition to their role in reverse transcription, the R-region sequences of some retroviruses affect viral transcription. The first 28 nucleotides of the R region within the long terminal repeat (LTR) of the murine type C retrovirus SL3 were predicted to form a stem-loop structure. We tested whether this structure affected the transcriptional activity of the viral LTR. Mutations that altered either side of the stem and thus disrupted base pairing were generated. These decreased the level of expression of a reporter gene under the control of viral LTR sequences about 5-fold in transient expression assays and 10-fold in cells stably transformed with the LTR-reporter plasmids. We also generated a compensatory mutant in which both the ascending and descending sides of the stem were mutated such that the nucleotide sequence was different but the predicted secondary structure was maintained. Most of the activity of the wild-type SL3 element was restored in this mutant. Thus, the stem-loop structure was important for the maximum activity of the SL3 LTR. Primer extension analysis indicated that the stem-loop structure affected the levels of cytoplasmic RNA. Nuclear run-on assays indicated that deletion of the R region had a small effect on transcriptional initiation and no effect on RNA polymerase processivity. Thus, the main effect of the R-region element was on one or more steps that occurred after the template was transcribed by RNA polymerase. This finding implied that the main function of the R-region element involved RNA processing. R-region sequences of human immunodeficiency virus type 1 or mouse mammary tumor virus could not replace the SL3 element. R-region sequences from an avian reticuloendotheliosis virus partially substituted for the SL3 sequences. R-region sequences from Moloney murine leukemia virus or feline leukemia virus did function in place of the SL3 element. Thus, the R region element appears to be a general feature of the mammalian type C genus of retroviruses.

Involvement of human CRM1 (exportin 1) in the export and multimerization of the Rex protein of human T-cell leukemia virus type 1

We investigated the role of human CRM1 (hCRM1) (exportin 1) in the function of Rex protein encoded by human T-cell leukemia virus type 1. hCRM1 promoted the export of Rex protein from the nucleus to the cytoplasm. A Rex protein with a mutation in the activation domain, RexM90, lost both the ability to bind to hCRM1 and the ability to multimerize. The overexpression of hCRM1 complemented the functional defects of RexM64, which contains a mutation in the multimerization domain of Rex. A dominant-negative mutant of Rex which sequesters cofactors of Rex abrogated multimerization as well as the activity of the wild-type Rex protein. These two functions were simultaneously restored by the overexpression of hCRM1. Taken together, these results suggest that hCRM1 plays important roles in the multimerization and export of Rex protein.

Mapping of multiple RNA binding sites of human T-cell lymphotropic virus type I rex protein within 5'- and 3'-Rex response elements

Interaction between the human T-cell lymphotropic virus type I Rex protein and viral transcripts in the nucleus is essential to the cytoplasmic appearance of unspliced and singly spliced viral RNA. Rex has been shown to mediate its function through direct interaction with a highly ordered secondary structure in the 3'-untranslated region of all human T-cell lymphotropic virus type I mRNAs termed the Rex response element (3'-RxRE). Part of the 3'-RxRE sequence is also present in the 5'-end of viral transcripts (5'-RxRE), and we demonstrate that Rex binds to this RNA with essentially the same affinity and specificity as to the 3'-RxRE. We have analyzed the secondary structures and binding sites of Rex within the 5'- and 3'-RxREs by enzymatic probing and chemical modification interference and show that multiple Rex molecules bind within a stem-loop, which is similarly structured in the two RxREs. Our experiments confirm the presence of a previously characterized Rex binding site but also identify a common motif within an extended region that comprises an additional Rex binding site. This suggests that Rex oligomerizes on the RxREs similarly to what has been observed for binding of the human immunodeficiency virus type 1 Rev protein to the Rev response element.

Human T-cell leukemia virus type 1 pX-I and pX-II open reading frames are dispensable for the immortalization of primary lymphocytes

Human T-cell leukemia virus type 1 (HTLV-1) infects and transforms CD4+ T-lymphocytes both in vivo and in vitro. Although the Tax protein of HTLV-1 has been strongly implicated as a transforming agent, other virally encoded proteins may also play a role in the transformation process. In addition to the rex and tax genes, the pX region of the HTLV-1 genome contains two open reading frames (pX-I and pX-II) which encode the putative viral accessory proteins known as p12I, p30II, and p13II. Mutations in the ACH molecular clone of HTLV-1 that are predicted to abrogate the expression of p12I, p13II and p30II were constructed. These mutations had no effect on viral replication or the immortalization of primary lymphocytes. Although these proteins are dispensable for viral replication and immortalization in vitro, it remains possible that they alter infection in vivo.

The origin and evolution of human T-cell lymphotropic virus types I and II

Studies on human T-cell lymphotropic virus types I (HTLV-I) and II (HTLV-II) are briefly reviewed from the viewpoint of molecular evolution, with special reference to the evolutionary rate and evolutionary relationships among these viruses. In particular, it appears that, in contrast to the low level of variability of HTLV-I among different isolates, individual isolates form quasispecies structures. Elucidating the mechanisms connecting these two phenomena will be one of the future problems in the study of the molecular evolution of HTLV-I and HTLV-II.

Functional conservation of HTLV-1 rex balances the immune pressure for sequence variation in the rex gene

Naturally occurring mutations in Human T-cell Leukemia Virus Type 1 (HTLV-1) Tax protein lead to loss of recognition by cytotoxic T-lymphocytes. Most of these mutations also abolish or severely impair the transactivation function of Tax. Ninety percent of the rex gene, which encodes the viral regulator of mRNA splicing (Rex), overlaps with the tax gene. In this paper, we report that four previously described point mutations in tax that abolished CTL recognition and activity did not alter either the dimerisation function or the ability to export viral mRNA of the corresponding Rex proteins. Rex proteins containing two other amino acid changes were likewise functional. However, five Rex deletion mutants, predominantly but not exclusively found in HAM/TSP patients, had all lost these functions. We conclude that, although the Tax protein is subject to strong CTL-mediated selection, there are stronger functional constraints on amino acid variation in Rex. This may limit the variation in the tax/rex nucleotide sequence which results in immune evasion.

X-I and X-II open reading frames of HTLV-I are not required for virus replication or for immortalization of primary T-cells in vitro

In contrast to other retroviruses of the oncovirinae subgroup, the primate and bovine leukemia viruses (HTLV, STLV, and BLV) encode genes in the X-region of the genome, between the env gene and the 3' long terminal repeat. In HTLV-I, two overlapping open reading frames (ORFs) in the distal half of the X-region encode tax and rex genes, while two ORFs (X-I and X-II) in the proximal half of this region potentially encode proteins designated p12(XI) (or rof) and p30(XII) (or tof). The biological functions and mechanisms of tax and rex have been studied extensively whereas the roles of the other ORFs have not yet been established. To identify possible functions for ORFs X-I and X-II, an infectious molecular clone of HTLV-I and a mutant provirus lacking these ORFs were compared with respect to virus replication, gene expression, and ability to immortalize primary T-cells. When transiently transfected into 293 cells, both intact and deleted proviruses directed the synthesis of virus mRNAs and proteins that were quantitatively and qualitatively identical. These viruses were also indistinguishable in their abilities to infect and replicate in DBS-FRhL cells, which are permissive for HTLV-I propagation. Immortalized T-cell lines were established after cell-free or coculture methods for infection of activated, human peripheral blood or cord blood lymphocytes with each of the cloned viruses. The growth kinetics, cytokine dependence, and cell surface markers of the infected T-cell cultures were similar for each provirus clone. Thus, ORFs X-I and X-II are not essential for virus infectivity, replication, gene expression, or T-cell immortalization in vitro.

Increased expression of interleukin-2 receptor alpha on peripheral blood mononuclear cells in HTLV-I tax/rex mRNA-positive asymptomatic carriers

Using the double-nested reverse transcription-polymerase chain reaction, we assayed human T-cell leukemia virus type I (HTLV-I) tax/rex-encoded mRNA in the peripheral blood mononuclear cells (PBMCs) of asymptomatic carriers as an index of the expression of HTLV-I in vivo in relation to the proviral DNA level. HTLV-I tax/rex mRNA was detected in only 1 (3.3%) of 30 samples with medium or lower proviral DNA levels, but it was detected in 11 (39.3%) of 28 samples with high HTLV-I proviral DNA levels, estimated as equal to or more than the proviral DNA of 10 ng of HUT102 (i.e., HUT102 cells were used as positive controls). The mean number of interleukin-2 receptor alpha (IL-2R alpha)-positive cells as a percentage of the total number of PBMCs was higher (13.2%) in the tax/rex mRNA-positive carriers with high proviral DNA levels than in the carriers who were mRNA negative (8.4%) (p = 0.004, Wilcoxon test). These results suggest that virus activation as indicated by the presence of tax/rex mRNA in asymptomatic carriers with high proviral DNA levels is associated with an elevation of the IL-2R alpha-positive cells in vivo.

Human T cell leukemia virus type I (HTLV-I) and human diseases

HTLV-I infection is causally associated with a variety of human diseases including leukemia/lymphoma, myelopathy, uveitis, and arthropathy. Tax protein of HTLV-I, which is considered oncogenic, binds to transcription factors or other cytoplasmic cellular molecules involved in the fundamental cell function and thereby induces cellular changes. The interaction between HTLV-I-infected cells with dysregulated function and different kinds of cells in the host, such as lymphocytes and vascular endothelial cells through viral peptides, antigen receptors cell adhesion molecules, and cytokines, appears to be one of the basic mechanisms underlying the development of HTLV-I-associated diseases. This interaction may play a major role in determining tumorigenicity and in forming clinical features of the diseases. The in vivo cell proliferation model of HTLV-I-infected cells using severe combined immunodeficient (SCID) mice can differentiate tumorigenicity from cell immortalization in vitro. The OX40 and its ligand gp34, which are induced by HTLV-I infection and directly mediate the adhesion between HTLV-I-infected T cells and vascular endothelial cells, may be critically involved in the localization and proliferation of HTLV-I-infected cells in vivo.

Identification of an RNA sequence within an intracisternal-A particle element able to replace Rev-mediated posttranscriptional regulation of human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) replication depends on the posttranscriptional regulation by the viral Rev protein and can be replaced with the posttranscriptional RNA control element (CTE) of the type D simian retroviruses. We have identified a sequence which shares only nucleotide sequences of the internal loops and secondary structure with the CTE and which is part of a novel murine intracisternal-A particle (IAP) retroelement, inserted within the transcribed mouse osteocalcin-related gene. This sequence, named CTE(IAP), can replace the Rev-mediated regulation of HIV-1, hence it is a posttranscriptional regulatory element. Related elements have been identified in other IAPs. These results suggest that insertional mutagenesis can affect gene expression by providing a functional posttranscriptional control element. The CTE(IAP) and CTEs of the type D simian retroviruses represent a novel class of RNA elements characterized by unique sequences within the internal loops which are predicted to represent the interaction site with cellular factor(s). These findings suggest that such elements may be involved in posttranscriptional regulation of cellular mRNAs.

Interaction of the human T-lymphotropic virus type 1 Tax dimer with CREB and the viral 21-base-pair repeat

Human T-lymphotropic virus type 1 Tax interacts specifically with the cellular transcription factor CREB and the viral 21-bp repeat element to form a Tax-CREB-DNA ternary complex which mediates activation of viral mRNA transcription. Analyses of Tax and Tax mutants indicate that, like CREB, Tax incorporates into the ternary complex as a dimer. The ability of Tax to form a dimer is necessary for its interaction with CREB and the 21-bp element. Analyses of several Tax mutants with amino acid substitutions spanning residues 123 to 204 indicate that intersubunit Tax dimerization correlates with its ability to assemble into the ternary complex and activate transcription. Tax also enhances the DNA binding activities of specific bZip domains in vitro. The ability of Tax to enhance DNA binding of bZip proteins can be explained in part by Tax dimerization. This activity alone is not sufficient for transactivation. A dual amino acid substitution mutant of Tax, M47 (L319R, L320S), completely abrogated for activation of the human T-lymphotropic virus type 1 long terminal repeat as a result of a defect in the transactivation domain, continues to stimulate binding of bZip proteins to DNA.

Enhancement of nucleocytoplasmic export of HTLV-1 Rex mRNA through cis and trans interactions of the mRNA with the complex of Rex protein and Rex-responsive element

p27 rex of HTLV-1 promotes nucleocytoplasmic export of viral mRNAs through binding of the Rex-response element (RexRE) present at the 3' end of the viral transcripts in cis with respect to the ORFs of the viral mRNAs. We have found that expression of the RexRE in trans, as a separate RNA, still allows Rex protein to promote export of viral mRNAs lacking the RexRE. The data suggest the formation of a ternary complex between Rex protein, RexRE and upstream elements of viral mRNA and hence the existence of secondary sites of interaction between Rex protein and viral RNAs.

The human T-cell leukemia virus type 1 posttranscriptional trans-activator Rex contains a nuclear export signal

The Rex protein of human T-cell leukemia virus type 1 is required for the nuclear export of unspliced viral mRNA and, therefore, for virus replication. In this manuscript, we demonstrate that Rex shuttles between the nucleus and the cytoplasm and that its activation domain constitutes a nuclear export signal that specifies efficient transport to the cytoplasm. These findings are consistent with a model for Rex-mediated trans-activation in which Rex-viral mRNA complexes are targeted for nuclear export by the direct action of the activation domain.

Protein sequence requirements for function of the human T-cell leukemia virus type 1 Rex nuclear export signal delineated by a novel in vivo randomization-selection assay

The Rex protein of human T-cell leukemia virus type 1, like the functionally equivalent Rev protein of human immunodeficiency virus type 1, contains a leucine-rich activation domain that specifically interacts with the human nucleoporin-like Rab/hRIP cofactor. Here, this Rex sequence is shown to function also as a protein nuclear export signal (NES). Rex sequence libraries containing randomized forms of the activation domain/NES were screened for retention of the ability to bind Rab/hRIP by using the yeast two-hybrid assay. While the selected sequences differed widely in primary sequence, all were functional as Rex activation domains. In contrast, randomized sequences that failed to bind Rab/hRIP lacked Rex activity. The selected sequences included one with homology to the Rev activation domain/NES and a second that was similar to the NES found in the cellular protein kinase inhibitor alpha. A highly variant, yet fully active, activation domain sequence selected on the basis of Rab/hRIP binding retained full NES function even though this sequence preserved only a single leucine residue. In contrast, nonfunctional activation domain mutants that were unable to bind Rab/hRIP had also lost NES function. These data demonstrate that NES activity is a defining characteristic of the activation domains found in the Rev/Rex class of retroviral regulatory proteins and strongly support the hypothesis that the Rab/hRIP cofactor plays a critical role in mediating the biological activity of these NESs. In addition, these data suggest a consensus sequence for NESs of the Rev/Rex class.

Requirements for interleukin 2 promoter transactivation by the Tax protein of human T-cell leukemia virus type 1

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) upregulates the expression of several cellular genes by activating members of both the NF-kappaB and bZIP families of transcription factors. Recent studies demonstrate that the CD28 response element (CD28RE) of the interleukin 2 (IL-2) promoter is the site upregulated by Tax in stimulated T cells. Although some reports suggest that this site is transactivated by NF-kappaB family members, others disagree, leaving the identity of the transcription factor(s) binding the CD28RE unclear. The studies presented here further characterize the response of the IL-2 promoter and CD28RE to the HTLV-1 Tax protein and demonstrate that the TATA-proximal AP-1 binding site of the IL-2 promoter is also necessary for Tax transactivation in stimulated Jurkat cells. In contrast to its upregulation of the IL-2 promoter which requires T-cell stimulation, Tax transactivates the isolated CD28RE-AP-1 element without stimulation but is greatly synergized by calcium ionophore and phorbol ester. Additionally, transactivation of the IL-2 promoter requires the Tax activation domain involved in upregulation of bZIP-enhanced transcription while the NF-kappaB-activating domain of Tax is dispensable. Interestingly, both domains appear to be necessary for the activation of the isolated CD28RE-AP-1 sequence in the context of a heterologous promoter construct. This strongly suggests that activation of NF-kappaB is insufficient to activate transcription via the CD28RE-AP-1 element of the IL-2 promoter and that a different transcription factor, upregulated via the activation domain of the HTLV-1 Tax protein, may be involved.

Examining the molecular genetics of HTLV-I with an infectious molecular clone of the virus and permissive cell culture systems

Infectious molecular clones of HTLV-I proviruses have only recently been reported. The long wait for such provirus clones reflects the difficulties inherent in propagating HTLV-I in vitro, and thus a rigorous demonstration of infectivity has awaited improved cell culture systems and sensitive detection techniques for HTLV-I. An intact HTLV-I provirus, originating from an American ATL patient, was subcloned into a plasmid vector and was designated pCS-HTLV. Transient transfections of mammalian cells with pCS-HTLV resulted in the synthesis of viral proteins and mRNAs which were assembled into virions that had physical and morphological characteristics typical of HTLV-I particles. The ability of these virus particles to infect cells, replicate, and produce infectious progeny was demonstrated initially in short term, cell-free infection assays by monitoring the expression of specific viral mRNAs. These studies have been extended in cell culture systems that support continuous virus production. Primary T-lymphocytes have been infected either with cell-free supernatant fluids from, or by coculture with, cells transiently transfected with pCS-HTLV, giving rise to continuous, IL-2-dependent cell lines that have been in culture for >1 year. Furthermore, fetal rhesus lung cells (FRhL) were shown to be permissive for HTLV-I replication and sustained virus expression after infection with pCS-HTLV. Continuous FRhL cell lines now have been established that express various HTLV-I proviruses and mutants. These provirus clones and cell lines provide us with the means to address long-standing questions dealing with the biology of HTLV-I.

Negative regulation of gene expression from the HTLV type II long terminal repeat by Rex: functional and structural dissociation from positive posttranscriptional regulation

Regulation of human T cell leukemia virus type II (HTLV-II) gene expression by Rex is mediated by cis-acting elements in the 5' viral long terminal repeat (LTR). Rex acts posttranscriptionally to enhance cytoplasmic accumulation of incompletely spliced viral mRNAs encoding structural proteins. We report a distinct negative regulatory function mediated by Rex affecting expression from the viral 5' LTR. Using both LTR-driven CAT reporters and a full-length HTLV-II proviral construct, we demonstrate that Rex decreases total cellular levels of LTR-containing mRNA in a dose-dependent manner. Negative regulation is an independent function as demonstrated by structural and functional dissociation from Rex positive posttranscriptional regulation. This negative regulatory action was dependent on nuclear localization sequences, but did not require the previously defined Rex-responsive element (RxRE). Negative regulation was observed in T cell lines but not in B cell lines, suggesting the involvement of cell type-specific factors distinct from those involved in posttranscriptional regulation. An internal deletion mutant of Rex removing aa 38-80 retained the ability to repress, but did not posttranscriptionally increase expression, while negative regulation requires a previously uncharacterized carboxy-terminal region (aa 154-170). These findings suggest that Rex may serve two simultaneous functions: to decrease overall levels of transcribed viral mRNA, and to facilitate nuclear to cytoplasmic export of mRNAs encoding structural proteins. The negative regulatory function of Rex may play a role in viral latency.

High-resolution mapping of the human T-cell leukemia virus type 1 Rex-binding element by in vitro selection

Interactions between the Rex protein of HTLV-1 and the genomic Rex-binding element (XBE) mediate the cytoplasmic transport of viral mRNAs. However, it is uncertain which RNA sequences and structures contribute to Rex recognition. A portion of the viral genome that spanned the XBE was partially randomized, and functional Rex-binding variants were selected. Alignment of selected Rex-binding sequences revealed positions that were functionally conserved between different molecules. A model is presented in which a subset of the selected residues are in direct contact with Rex. Positions that covaried with one another were also found. These covariations support a secondary-structural model in which a central paired stem is symmetrically flanked by two bulge loops. On the basis of this model, site-directed mutations of the XBE were constructed and each half molecule was found to bind independently to Rex. The functional residues and secondary structures in the XBE half molecules bear a remarkable resemblance to the transactivation response region element of HIV-1. Since the transactivation response region element is known to interact specifically with arginine residues in the Tat protein, these results suggest that the XBE binds to the arginine-rich RNA-binding domain of Rex in a similar manner. This model is supported by the selection data.

Human T-cell-leukemia virus type I in post-transfusional spastic paraparesis: complete proviral sequence from uncultured blood cells

Human T-cell-leukemia virus type I (HTLV-I) is the causative agent of adult T-cell leukemia/lymphoma (ATL) and tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). The different disease outcome may be attributable to subtle mutations leading to modification of viral tropism or infectivity. Initial attempts found a very high level of sequence conservation among all HTLV-I strains. However, only one complete proviral DNA sequence is reported from a TSP/HAM patient, with a provirus derived from immortalized lymphocytes, which might be expected to be a leukemogenic variant rather than a neurotropic one. We cloned and sequenced a complete HTLV-I provirus (HTLV-IBoi) derived from the uncultured lymphocytes of a sub-acute post-transfusional TSP/HAM patient with clonal integration of HTLV-I. HTLV-IBoi proviral genome is 9033 bp long, and its overall genetic organization is similar to that of the prototype HTLV-I(ATK), without major deletions or insertions. No premature termination codon was found in the 4 open reading frames of the pX region. Divergence at the nucleotide level of HTLV-IBoi from the reported full-length HTLV-I varies from 1 to 9.4%, and indicates that it corresponds to a cosmopolitan genotype. This study did not identify specific sequences associated with neurotropic strains.

Antimitotic and antiviral activities of Kelletinin A in HTLV-1 infected MT2 cells

Kelletinin A [ribityl-pentakis (p-hydroxybenzoate)] (KA), a natural compound isolated from the marine gastropod Buccinulum corneum, showed antiviral activity on the human T-cell leukemia virus type-1 (HTLV-1) and antimitotic activity on HTLV-1-infected MT2 cells. KA inhibited cellular DNA and RNA synthesis, without influencing protein synthesis, and interfered with viral transcription by reducing the levels of high molecular weight transcripts. Finally, the compound inhibited HTLV-1 reverse transcriptase in vitro.

Binding of human prothymosin alpha to the leucine-motif/activation domains of HTLV-I Rex and HIV-1 Rev

Rex of human T-cell leukemia virus type I (HTLV-I) and Rev of human immunodeficiency virus 1 (HIV-1) are post-transcriptional regulators of viral gene expression. By means of affinity chromatography, we purified an 18-kDa cellular protein that bound to the conserved leucine-motif/activation domain of HTLV-I Rex or HIV-1 Rev. The protein that was purified through a Rev-affinity column was found to bind to Rex immunoprecipitated with anti-Rex IgG from an HTLV-I-producing cell line. We analyzed the purified approximately 18-kDa protein biochemically and identified it as prothymosin alpha. The binding activity of prothymosin alpha to Rev or Rex was completely abolished when the epsilon-amino groups of its lysine residues were chemically modified by N-succinimidyl-3-(4-hydroxy-3,5-diodo- phenyl)propionate. The functional relationship between the nuclear protein prothymosin alpha and Rex-Rev is discussed.

The human immunodeficiency virus type 1 Rev protein and the Rev-responsive element counteract the effect of an inhibitory 5' splice site in a 3' untranslated region

A 5' splice site located in a 3' untranslated region (3'UTR) has been shown previously to inhibit gene expression. Natural examples of inhibitory 5' splice sites have been identified in the late 3'UTRs of papillomaviruses and are thought to inhibit viral late gene expression at early stages of the viral life cycle. In this study, we demonstrate that the interaction of the human immunodeficiency virus type 1 Rev protein with the Rev-responsive element (RRE) overcomes the inhibitory effects of a 5' splice site located within a 3'UTR. This was studied by using both a bovine papillomavirus type 1 L1 cDNA expression vector and a chloramphenicol acetyltransferase expression vector containing a 5' splice site in the 3'UTR. In both systems, coexpression of Rev enhanced cytoplasmic expression from vectors containing the RRE even when the RRE and the inhibitory 5' splice site were separated by up to 1,000 nucleotides. In addition, multiple copies of a 5' splice site in a 3'UTR were shown to act synergistically, and this effect could also be moderated by the interaction of Rev and the RRE. These studies provide additional evidence that at least one mechanism of Rev action is through interactions with the splicing machinery. We have previously shown that base pairing between the U1 small nuclear RNA and a 3'UTR 5' splice site is required for inhibition of gene expression. However, experiments by J. Kjems and P. A. Sharp (J. Virol. 67:4769-4776, 1993) have suggested that Rev acts on spliceosome assembly at a stage after binding of the U1 small nuclear ribonucleoprotein to the 5' splice site. This finding suggests that binding of additional small nuclear ribonucleoproteins, as well as other splicing factors, may be necessary for the inhibitory action of a 3'UTR 5' splice site. These data also suggest that expression of the papillomavirus late genes in terminally differentiated keratinocytes can be regulated by a viral or cellular Rev-like activity.

Effects of translation initiation factor eIF-5A on the functioning of human T-cell leukemia virus type I Rex and human immunodeficiency virus Rev inhibited trans dominantly by a Rex mutant deficient in RNA binding

The viral transactivator proteins Rex and Rev are necessary for the expression of structural proteins of human T-cell leukemia virus type I and human immunodeficiency virus type 1, respectively. Although the interaction of Rex/Rev with a cellular cofactor(s) has been thought to be required for Rex/Rev action, there is no suitable system to search for the cofactor(s) in mammalian cells. We found that a Rex mutant, TAgRex, which contains a simian virus 40 nuclear localization signal in place of the N-terminal 19 amino acids of Rex, could dominantly inhibit wild-type Rex/Rev functions. The inhibition did not require either Rev response element/Rex response element binding or the oligomerization ability of the mutant, but it did require a region around amino acid 90 of the Rex protein, suggesting that TAgRex sequestered the cellular cofactor. Complementation with the eukaryotic translation initiation factor 5A (eIF-5A) in this system could restore the impaired Rex function. These results indicate that eIF-5A is the cofactor indispensable for Rex function. Additionally, by using a two-hybrid system, the homo-oligomer formation of Rex was found to be mediated by the region around amino acid 90 in addition to Tyr-64 and Trp-65 of Rex protein. Thus, eIF-5A may play a part in the formation of the Rex homo-oligomer.

Differential Tax expression in HTLV type I-infected asymptomatic carriers

tax gene expression in a family cluster of three HTLV-I-infected asymptomatic individuals was investigated. Two carriers had normal tax mRNA, Tax-specific humoral antibody, and cell-mediated immune (CMI) response. In one carrier who had only weak Tax-specific humoral and no Tax-specific CMI response, an abnormal Tax-related mRNA product was detected. This product was sequenced and found to consist of two exons derived from the LTR gag and pX regions. The abnormal mRNA has an ORF predicting a 17-kDa protein, the translation of which is initiated in the first exon. The presence of this protein, of antibody to it, and of its function remain to be elucidated.

Distinct regions in human T-cell lymphotropic virus type I tax mediate interactions with activator protein CREB and basal transcription factors

Human T-cell lymphotropic virus type I (HTLV-I) transactivator Tax augments transcription from three (cyclic AMP response element (CRE)-containing 21-bp repeats in the viral long terminal repeat and several other cis regulatory elements, including the NF-kappa B binding sites and the serum response element. Tax does not bind DNA directly; rather, it acts via cellular sequence-specific DNA binding proteins to stimulate transcription. We have shown recently that Tax forms multiprotein complexes with the heterodimeric and homodimeric forms of a ubiquitous cellular transcription factor, CREB (CRE binding protein). In vitro selection for preferred Tax-CREB binding sites indicates that the Tax-CREB complex exhibits greatly increased DNA recognition specificity and assembles preferentially on CRE motifs, TGACGT/C, flanked by long runs of G (5') and/or C (3') residues, as found in the HTLV-I 21-bp repeats. The indirect tethering of Tax to the 21-bp repeats via CREB is crucial for Tax transactivation. We now report the domain organization of Tax by characterizing its mutants. Tax mutants with alterations in the NH2 terminus, including three deletion mutants, Tax(6-353), Tax(21-353), and Tax(89-353), and two amino acid substitution mutants, M1 (H3S) and M7 (C29A, P30S), all failed to interact with CREB in vitro. In contrast, a short COOH-terminal deletion, Tax(1-319), and a Tax mutant with amino acid substitutions near the COOH end, M47 (L319R, L320S), were able to interact with CREB and the 21-bp repeats to assemble ternary Tax-CREB-DNA complexes. As demonstrated earlier, M1, M7, and M47 all failed to transactivate the HTLV-I long terminal repeat. Our data indicate that the defects in M1 and M7 result from an inability to interact with CREB. In contrast, the COOH-terminal mutations in M47 most likely inactivated the transactivation domain of Tax. As anticipated, a Tax mutant, M22 (G137A, L138S) which activated transcription from the 21-bp repeats with reduced capacity and was defective in trans activating the NF-kappa B binding sites, continued to interact with CREB in vitro, albeit with a lower level of efficiency. Finally, a glutathione S-transferase (GST)-Tax fusion protein with the GST moiety fused to the NH2 terminus of Tax failed to interact with CREB. Removal of the GST domain from GST-Tax by thrombin restores Tax's ability to assemble a ternary Tax-CREB-21-bp-repeat complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Virions released from cells transfected with a molecular clone of human T-cell leukemia virus type I give rise to primary and secondary infections of T cells

The ability of molecular clones of human T-cell leukemia virus type I (HTLV-I) to direct the synthesis of infectious virions has not previously been demonstrated. An HTLV-I provirus originating from an adult T-cell leukemia patient was cloned into a plasmid vector and is designated pCS-HTLV. This molecular clone was shown to direct the synthesis of viral mRNA and proteins in transiently transfected cells; in addition, virus structural proteins were released into the culture medium. Viral proteins were assembled into virions that sedimented at a buoyant density characteristic of retrovirus particles and whose morphology was verified by electron microscopy. Virions concentrated from transiently transfected cell supernatants were incubated with primary cord blood lymphocytes or with transformed T-cell lines to establish that these particles were infectious. Expression of spliced, viral mRNAs in the T-cell cultures after both primary and secondary infections with cell-free virus revealed that pCS-HTLV encodes an infectious provirus.

Genetically simpler bovine leukemia virus derivatives can replicate independently of Tax and Rex

Retrovirus genomes have a conserved modular organization that consists of trans-acting gag, pol, and env genes that function through cis-acting sequences to replicate the RNA genome to the DNA provirus. Genetically more complex retroviruses also encode regulatory genes and cis-acting sequences that are essential for their replication. We sought to convert a more complex retrovirus into a simpler retrovirus derivative that can replicate. We constructed novel, hybrid retrovirus vectors to replicate the gag, pol, and env genes of the more complex bovine leukemia virus (BLV) in the absence of regulatory genes and cis-acting response sequences. Most of the cis-acting sequences involved in the replication and regulation of BLV were replaced by the cis-acting transcriptional control sequences of a simpler retrovirus, spleen necrosis virus. We found that the resulting hybrid BLV derivatives can replicate independently of BLV Tax and Rex and the Tax and Rex cis-acting response sequences, as measured by successive passages of virus on target cells, detection of provirus sequences, and analyses of provirus and encapsidated RNAs.

Transactivation of cellular genes by human retroviruses

We have focused this chapter on interactions with two of the best characterized transregulatory genes, tax for HTLV-I/II and Tat for HIV-1. Both genes illustrate the complex interplay between retroviral regulatory genes and cellular gene regulation. In both instances a viral gene of relatively straightforward function in the viral context appears to cause extensive dysregulation of cellular genes, either directly or as a consequence of altered cellular differentiation. Understanding this viral/cellular gene cross-talk may elucidate mechanisms leading to malignant transformation autoimmune disease and to neurologic and paraneoplastic complications such as hypercalcemia for HTLV-I/II, as well as the pathogenesis of immune dysfunction and opportunistic malignancy in HIV-I/II-infected individuals. An understanding of functional mechanisms of these transregulatory viral genes will undoubtedly afford better explanations for the myriad manifestations of retroviral infection.

Mechanisms of Tax Regulation of Human T Cell Leukemia Virus Type I Gene Expression

During the last several years, the human T cell leukemia virus type I (HTLV-I) has become recognized as an important cause for public health concern throughout the world. HTLV-I is the causative agent of a variety of clinical diseases, including an aggressive lymphoproliferative disorder named adult T cell leukemia. HTLV-I induces pathogenicity in the infected host cell through the synthesis of a virally encoded protein called Tax. Expression of Tax is critical to the life cycle of the virus, as the protein greatly increases the efficiency of HTLV-I gene transcription and replication. Furthermore, Tax has been shown to deregulate the transcription of many cellular genes, leading to the hypothesis that the presence of Tax promotes unchecked growth in the HTLV-I-infected cell. The mechanism of Tax trans-activation of HTLV-I gene expression is not known. Tax does not bind directly to the Tax-responsive promoter elements of the virus, but appears to function through interaction with certain cellular DNA binding proteins, including activating transcription factor 2 and cAMP response element binding protein that recognize these sequences. This review summarizes some of the recent work in the field aimed at elucidating the mechanism of Tax trans-activation of HTLV-I gene expression. Copyright 1995 S. Karger AG, Basel

Human T-cell leukemia virus type II nucleotide sequences between env and the last exon of tax/rex are not required for viral replication or cellular transformation

Human T-cell leukemia virus types I (HTLV-I) and II (HTLV-II) and bovine leukemia virus contain a region of approximately 600 nucleotides located 3' to the env gene and 5' to the last exon of the tax and rex regulatory genes. This region was originally termed nontranslated or untranslated (UT) since it did not appear to be expressed. Several studies have identified novel mRNAs in HTLV-I-, HTLV-II-, a bovine leukemia virus-infected cells that splice into open reading frames (ORFs) contained in the UT region and, thus, have the potential to produce proteins that might contribute to the biological properties of these viruses. The HTLV-II infectious molecular clone pH6neo has several ORFs in the UT region (nucleotides 6641 to 7213) and a large ORF which overlaps the third exon of tax/rex. To investigate the importance of these ORF-containing sequences on viral replication and transformation in cell culture, proviral clones containing deletions in UT (pH6neo delta UT) or a stop codon insertion mutation (pH6neoST) were constructed. Lymphoid cells were transfected with mutant proviral constructs, and stable cell clones, designated 729pH6neo delta UT and 729pH6neoST, were characterized. Viral protein production, reverse transcriptase activity, and the capacity to induce syncytia were indistinguishable from cells transfected with the wild-type clone. Finally, 729pH6neo delta UT- and 729pH6neoST-producer cells cocultured with primary blood T lymphocytes resulted in cellular transformation characteristic of HTLV. These results indicate that putative protein-coding sequences between env and the last exon of tax/rex are not required for viral replication or transformation in cell culture.

Transcription of the human T-cell lymphotropic virus type I promoter by an alpha-amanitin-resistant polymerase

The human T-lymphotropic virus type I (HTLV-I) promoter contains the structural features of a typical RNA polymerase II (pol II) template. The promoter contains a TATA box 30 bp upstream of the transcription initiation site and binding sites for several pol II transcription factors, and long poly(A)+ RNA is synthesized from the integrated HTLV-I proviral DNA in vivo. Consistent with these characteristics, HTLV-I transcription activity was reconstituted in vitro by using TATA-binding protein, TFIIA, recombinant TFIIB, TFIIE, and TFIIF, TFIIH, and pol II. Transcription of the HTLV-I promoter in the reconstituted system requires RNA pol II. In HeLa whole cell extracts, however, the HTLV-I long terminal repeat also contains an overlapping transcription unit (OTU). HTLV-I OTU transcription is initiated at the same nucleotide site as the RNA isolated from the HTLV-I-infected cell line MT-2 but was not inhibited by the presence of alpha-amanitin at concentrations which inhibited the adenovirus major late pol II promoter (6 micrograms/ml). HTLV-I transcription was inhibited when higher concentrations of alpha-amanitin (60 micrograms/ml) were used, in the range of a typical pol III promoter (VA-I). Neutralization and depletion experiments with three distinct pol II antibodies demonstrate that RNA pol II is not required for HTLV-I OTU transcription. Antibodies to basal transcription factors TATA-binding protein and TFIIB, but not TFIIIC, inhibited HTLV-I OTU transcription. These observations suggest that the HTLV-I long terminal repeat contains overlapping promoters, a typical pol II promoter and a unique pol III promoter which requires a distinct set of transcription factors.

Phylogenesis and genetic complexity of the nonhuman primate retroviridae

The three known groups of nonhuman primate retroviruses (simian immunodeficiency virus, simian T cell lymphotropic/leukemic virus type I, and simian foamy virus) are thought to have equivalent human counterparts. This is clearly the case with human immunodeficiency virus types 1 and 2, the causative agents of acquired immunodeficiency syndrome, and with human T cell lymphotropic/leukemia virus type I (HTLV-I), which causes T cell leukemia and a progressive form of myelopathy (tropical spastic paraparesis/HTLV-I-associated myelopathy), and HTLV-II. However, the presence of spumaviruses (foamy viruses) in humans remains uncertain. Data accumulated in the last 5 years suggest the possibility that the human retroviruses are indeed the result of transmission of simian retroviruses to humans. In this article we attempt to parallel the genetic features of the simian retroviridae with their human counterparts and argue for the possibility of horizontal transmission of these viruses from monkeys to humans.

Novel interactions between human T-cell leukemia virus type I Tax and activating transcription factor 3 at a cyclic AMP-responsive element

Human proenkephalin gene transcription is transactivated by human T-cell leukemia virus type I (HTLV-I) Tax in human Jurkat T lymphocytes. This transactivation was further enhanced in Jurkat cells treated with concanavalin A, cyclic AMP, or 12-O-tetradecanoylphorbol-13-acetate. Deletion and cis-element transfer analyses of the human proenkephalin promoter identified a cyclic AMP-responsive AP-1 element (-92 to -86) as both necessary and sufficient to confer Tax-dependent transactivation. Different AP-1 or cyclic AMP-responsive element-binding protein (CREB)/activating transcription factor (ATF) proteins which bind this element were expressed in murine teratocarcinoma F9 cells to identify those capable of mediating Tax-dependent transactivation of human proenkephalin gene transcription. Although CREB, c-Fos, c-Jun, and JunD did not have significant effects, JunB inhibited the Tax-dependent transactivation. In contrast, ATF3 dramatically induced Tax-dependent transactivation, which was further enhanced by protein kinase A. Electrophoretic mobility shift assays with recombinant fusion proteins expressed and purified from bacteria indicate that the DNA-binding activity of ATF3 is also dramatically enhanced by Tax. Chimeric fusion proteins consisting of the DNA-binding domain of the yeast transcription factor Gal4 and the amino-terminal domain (residues 1 to 66) of ATF3 were able to mediate Tax-dependent transactivation of a Gal4-responsive promoter, which suggests a direct involvement of this region of ATF3. Recombinant fusion proteins of glutathione S-transferase with either the amino- or carboxy-terminal (residues 139 to 181) domain of ATF3 were able to specifically interact with Tax. Furthermore, specific antisera directed against Tax coimmunoprecipitated ATF3 only in the presence of Tax.

Novel interactions between human T-cell leukemia virus type I Tax and activating transcription factor 3 at a cyclic AMP-responsive element

Human proenkephalin gene transcription is transactivated by human T-cell leukemia virus type I (HTLV-I) Tax in human Jurkat T lymphocytes. This transactivation was further enhanced in Jurkat cells treated with concanavalin A, cyclic AMP, or 12-O-tetradecanoylphorbol-13-acetate. Deletion and cis-element transfer analyses of the human proenkephalin promoter identified a cyclic AMP-responsive AP-1 element (-92 to -86) as both necessary and sufficient to confer Tax-dependent transactivation. Different AP-1 or cyclic AMP-responsive element-binding protein (CREB)/activating transcription factor (ATF) proteins which bind this element were expressed in murine teratocarcinoma F9 cells to identify those capable of mediating Tax-dependent transactivation of human proenkephalin gene transcription. Although CREB, c-Fos, c-Jun, and JunD did not have significant effects, JunB inhibited the Tax-dependent transactivation. In contrast, ATF3 dramatically induced Tax-dependent transactivation, which was further enhanced by protein kinase A. Electrophoretic mobility shift assays with recombinant fusion proteins expressed and purified from bacteria indicate that the DNA-binding activity of ATF3 is also dramatically enhanced by Tax. Chimeric fusion proteins consisting of the DNA-binding domain of the yeast transcription factor Gal4 and the amino-terminal domain (residues 1 to 66) of ATF3 were able to mediate Tax-dependent transactivation of a Gal4-responsive promoter, which suggests a direct involvement of this region of ATF3. Recombinant fusion proteins of glutathione S-transferase with either the amino- or carboxy-terminal (residues 139 to 181) domain of ATF3 were able to specifically interact with Tax. Furthermore, specific antisera directed against Tax coimmunoprecipitated ATF3 only in the presence of Tax.

A spontaneous point mutation in the human T-cell leukemia virus type 1 pX gene leads to expression of a novel doubly spliced pX-mRNA that encodes a 25-kD, amino-terminal deleted rex protein

Primary RNA transcripts of the human T-cell leukemia virus type 1 (HTLV-1) are processed into mature mRNA by a complex series of splicing events. In this paper, we report the finding of a novel doubly spliced pX mRNA in two out of eight HTLV-1-infected cell lines and in one out of 13 peripheral blood mononuclear cells from HTLV-1-infected individuals. The second splicing for this novel pX mRNA is different from that for the known doubly spliced pX mRNA. A novel acceptor site in this splicing was generated by a single point mutation (G to A) at nucleotide 7,337 of the pX gene. This mRNA contained a complete open reading frame that encodes an amino-terminal truncated p27rex protein with 189 amino acids. A new 25-kD protein was detected in the cell lines expressing the novel pX mRNA by an antibody against the carboxy-terminal peptide of p27rex and was termed p25rex. Although the function of p25rex is not clear, we clarified that p25rex is a cytoplasmic phosphoprotein and its function is different from the transcriptional regulator function of p27rex. The possibility that the mutated virus is replicable only in cells coinfected with the wild type HTLV-1 may explain why the incidence of the mutants observed here is low.