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

Abstract

Efficient bovine leukemia virus (BLV) transcription requires the virus-encoded transactivatorBovine leukemia virus (BLV) 1 is a B-lymphotropic retrovirus associated with enzootic bovine leukosis, a disease characterized by an increased number of B-lymphocytes and, in some cases, after a long latency period, by the subsequent development of B-cell leukemia or lymphosarcoma (1). BLV is closely related structurally and biologically to the human T-lymphotropic viruses HTLV-I and HTLV-II (1). Expression of BLV is regulated at the transcriptional level by the virus-encoded transactivator Tax BLV (2, 3). The molecular mechanism by which Tax BLV activates viral transcription is not fully understood. Transactivation by Tax BLV requires three 21-bp imperfect repeats located in the U3 region of the 5Ј long terminal repeat (LTR) (2, 4). These Tax BLV -responsive elements (called TxREs) contain a core octanucleotide sequence with similarity to the cAMP-responsive element (CRE) consensus (TGACGTCA) (5). The BLV CRE-like motifs located in the middle of each TxRE have been shown to serve as binding sites for three members of the basic leucine zipper (bZIP) family of cellular transcription factors: the CRE-binding protein (CREB) and the activating transcription factors-1 and Ϫ2 (ATF-1 and ATF-2) (4, 6). Because there is no evidence for direct binding of Tax BLV to DNA, it has been proposed that Tax BLV transactivation of the BLV promoter could be mediated, as reported for the HTLV-I system, through protein-protein interactions with CREB/ATF (4, 6, 7). The formation of this promoter-bound Tax BLV -CREB/ATF complex could then serve for the recruitment of the multifunctional cellular coactivators CBP (CREB-binding protein) and p300.There is now strong evidence that both transcriptional activation and silencing are mediated through the recruitment of enzymes that control protein acetylation: the histone deacetylases (HDACs) and the histone acetyltransferases (HATs). Acetylation of specific lysine residues within the amino-terminal tails of nucleosomal histones is generally linked to chroma-

Transcriptional Regulation of the Bovine Leukemia Virus Promoter by the Cyclic AMP-response Element Modulator τ Isoform

“…3, lane 3). These data confirmed the binding of CREB to the BLV TxREs in vivo, as demonstrated previously by our laboratory (9), and demonstrated in vivo the binding of other CREB/CREM/ATF members, including CREM protein(s), to this LTR region. As a control, we used another set of primers amplifying a region of the gag gene of the BLV provirus located ϳ1.8 kb downstream of the TxRE region, which has not been reported so far as binding CREB/ATF family members.…”

“…Our laboratory has reported previously that a dominant negative inhibitor of CREB (A-CREB) efficiently inhibits Tax BLV transactivation of the BLV promoter, suggesting that recruitment of CREB to the BLV TxREs plays a critical role in the mechanism of Tax BLV transactivation (9). However, the effect of CREM expression on Tax BLV -mediated transactivation of the BLV 5Ј-LTR has not been examined so far.…”

“…Plasmid Constructs-The pLTRwt-luc, containing the luciferase gene under the control of the complete 5Ј-LTR of the 344 BLV provirus and its derivative pLTR-mutCRE-luc, mutated in the three viral CRE-like motifs, were previously described (9,10,21). The eukaryotic expression vectors pSG-Tax BLV and pSG-CREB2 were gifts from Drs.…”

“…Importantly, the 21-bp enhancers are also called Tax BLV -responsive elements (TxREs) because transactivation of the BLV LTR by Tax BLV requires these enhancers. It has been proposed that Tax BLV activation of transcription could be mediated, as reported for human T-cell leukemia virus-1, through increased binding of the cellular proteins CREB, ATF-1, and ATF-2 (and possibly other factors yet to be identified) to the TxREs (7,9,11,12).…”

“…In the early stages of BLV transcription, before Tax BLV expression and transactivation, the basal transcriptional promoter activity is ensured by several cis-acting elements located in the 5Ј-LTR. In the U3 region are present the promoter CAAT and TATA boxes (5,6), and three 21-bp enhancers, each containing an imperfectly conserved 8-bp cyclic AMP-responsive element (CRE), which binds at least three proteins: CRE-binding protein (CREB) and activating transcription factors 1 and 2 (ATF-1 and ATF-2) (7)(8)(9)(10). Importantly, the 21-bp enhancers are also called Tax BLV -responsive elements (TxREs) because transactivation of the BLV LTR by Tax BLV requires these enhancers.…”