The interaction of HTLV-1 Tax with HDAC1 negatively regulates the viral gene expression

Transcription regulatory complexes bind the human T-cell leukemia virus 5' and 3' long terminal repeats to control gene expression

The human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that integrates randomly into the T-cell genome. Two long terminal repeats (LTRs) flank the integrated provirus. The upstream and downstream LTRs carry identical promoter sequences. Studies with other retroviruses suggest that the downstream promoter is silent and that RNA polymerases initiating at the upstream promoter proceed through the 3' LTR. In this study, we used the chromatin immunoprecipitation assay to compare the binding of transcription regulatory proteins at both the upstream and downstream promoters in HTLV-1-infected cell lines and adult T-cell leukemia-lymphoma cells. Unexpectedly, we detected a nearly equal distribution of activator (Tax, CREB, ATF-1, ATF-2, c-Fos, and c-Jun) and regulatory protein (CBP, p300, TAF(II)250, and polymerase II) binding at both the upstream and downstream promoters. Consistent with this observation, we found that the downstream promoter was transcriptionally active, suggesting that the two promoters are functionally equivalent. We also detected asymmetrical binding of histone deacetylases (HDAC-1, -2, and -3) at both promoters. All three HDACs strongly repressed Tax transactivation, and this repression correlated with displacement of Tax from the HTLV-1 promoter. These effects were reciprocal, as Tax expression reversed HDAC repression and displaced HDACs from the HTLV-1 promoter. These data suggest that HTLV-1 transcriptional regulation at both the 5' and 3' LTRs is mediated, in part, through the mutually exclusive binding of Tax and HDACs at the proviral promoters.

Deacetylase inhibitors and the viral transactivator TaxBLV synergistically activate bovine leukemia virus gene expression via a cAMP-responsive element- and cAMP-responsive element-binding protein-dependent mechanism

Efficient bovine leukemia virus (BLV) transcription requires the virus-encoded transactivator Tax(BLV), which acts through three Tax(BLV)-responsive elements located in the 5' long terminal repeat. It has been proposed that the binding of the CRE-binding protein (CREB) and the activating transcription factor (ATF) to the three imperfect cAMP-responsive elements (CREs) located in each Tax(BLV)-responsive element mediates Tax(BLV) transactivation. Here we demonstrated that deacetylase inhibitors (HDACis) synergistically enhanced the transcriptional activation of the BLV promoter by Tax(BLV) in a CRE-dependent manner. Tax(BLV) was acetylated in vivo at its N(alpha) terminus but not at internal lysine residues. Rather, HDACi potentiation of Tax(BLV) transactivation was mediated by an HDACi indirect action that requires new protein synthesis. Mechanistically, using a dominant-negative form of CREB, we showed that Tax(BLV) and HDACi synergistically activated BLV gene expression via a CREB-dependent mechanism. Moreover, electrophoretic mobility shift assay and Western blot experiments revealed that HDACi increased the in vitro DNA binding activity of CREB/ATF but did not alter CREB/ATF intranuclear presence. Remarkably, chromatin immunoprecipitation assays demonstrated that HDACi treatment increased the level of CREB bound to the BLV promoter in vivo. Our results together suggest that an increase in CREB/ATF occupancy of the viral CREs in response to HDACi potentiates Tax(BLV) transactivation of the BLV promoter.

T-Cell Control by Human T-Cell Leukemia/Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Collective evidence from in vitro studies indicates that several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function, such as antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation may therefore be of importance, as also suggested by epidemiological data. Thus genetic and environmental factors together with the virus contribute to disease development. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells. The relevance of these laboratory findings is related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

p21(Waf1/Cip1/Sdi1) prevents apoptosis as well as stimulates growth in cells transformed or immortalized by human T-cell leukemia virus type 1-encoded tax

Human T-cell leukemia virus type 1 (HTLV-1) Tax regulates the expression of virally encoded genes, as well as various endogenous host genes in trans. Tax-mediated regulation of gene expression is important for the immortalization of normal human T lymphocytes and the transformation of fibroblast cells, such as Rat-1 cells. Tax has the ability to transactivate p21(Waf1/Cip1/Sdi1), resulting in high expression levels in HTLV-1-immortalized cells. Since p21 expression is suppressed due to methylation of the promoter region in Rat-l cell line, p21 may not be critical for the transformation of this cell line by Tax. To further understand the role of p21 for the proliferation of Tax-transformed Rat-1 cells, we examined the effect of ectopic expression of p21 in these cells. Here, we observed that p21 expression enhanced the transformation of this cell line via at least two mechanisms: (i) the enhancement of NF-kappaB activation and/or CREB signaling and (ii) the excitation of antiapoptotic machinery. To analyze the role of p21 that is overexpressed in HTLV-1-immortalized lymphocytes, p21 expression was suppressed by using an antisense oligonucleotide specific for p21 mRNA; these cells then became sensitive to apoptotic induction. These results suggest that p21 plays an important role in the proliferation of Tax-expressing cells through the regulation of at least two independent mechanisms.

Seizing of T Cells by Human T-Cell Leukemia⧸Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function. Viral proteins modulate the downstream effects of antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation are therefore important, as also suggested by epidemiological data. The ability of a given individual to respond to specific antigens is determined genetically. Thus, genetic and environmental factors, together with the virus, contribute to disease development. As in the case of other virus-associated cancers, HTLV-1-induced leukemia/lymphoma can be prevented by avoiding viral infection or by intervention during the asymptomatic phase with approaches able to interrupt the vicious cycle of virus-induced proliferation of a subset of T-cells. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells in vitro. The relevance of these laboratory findings will be related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Transcription factor binding and histone modifications on the integrated proviral promoter in human T-cell leukemia virus-I-infected T-cells

The human T-cell leukemia virus (HTLV-I)-encoded Tax protein is a potent transcriptional activator that stimulates expression of the integrated provirus. Biochemical studies indicate that Tax, together with cellular transcription factors, interacts with viral cAMP-response element enhancer elements to recruit the pleiotropic coactivators CREB-binding protein and p300. Histone acetylation by these coactivators has been shown to play a major role in activating HTLV-I transcription from chromatin templates in vitro. However, the extent of histone modification and the precise identity of the cellular regulatory proteins bound at the HTLV-I promoter in vivo is not known. Chromatin immunoprecipitation analysis was used to investigate factor binding and histone modification at the integrated HTLV-I provirus in infected T-cells (SLB-1). These studies reveal the presence of Tax, a variety of ATF/CREB and AP-1 family members (CREB, CREB-2, ATF-1, ATF-2, c-Fos, and c-Jun), and both p300 and CREB-binding protein at the HTLV-I promoter. Consistent with the binding of these coactivators, we observed histone H3 and H4 acetylation at three regions within the proviral genome. Histone deacetylases were also present at the viral promoter and, following their inhibition, we observe an increase in histone H4 acetylation on the HTLV-I promoter and a concomitant increase in viral RNA. Together, these results suggest that a variety of transcriptional activators, coactivators, and histone deacetylases participate in the regulation of HTLV-I transcription in infected T-cells.