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

Abstract

Bovine leukemia virus (BLV) expression is controlled at the transcriptional level through three Tax Bovine leukemia virus (BLV)7 infection is characterized by viral latency in the large majority of infected cells and by the absence of viremia. These features are thought to be due to the transcriptional repression of viral expression in vivo (1, 2). BLV transcription initiates at the unique promoter located in the 5Ј-long terminal repeat (5Ј-LTR) of the BLV genome. The 5Ј-LTR is composed of the U3, R, and U5 regions and transcription initiates at the U3-R junction. BLV exhibits two distinct functional transcriptional states as follows: a low basal level of transcription ensured by host cellular transcription factors, and a high level of transcription directed by the virus-encoded transcriptional activator Tax BLV (3,4). 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) (7-10). 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. It has been proposed that Tax BLV activation of transcription

DNA Cytosine Methylation in the Bovine Leukemia Virus Promoter Is Associated with Latency in a Lymphoma-derived B-cell Line

“…6, A and B, lanes 2 and 7) or not (lanes 1 and 6) with a combination of PMA ϩ ionomycin (PϩI), two known activators of BLV expression (26,52,53); from the defective cell line YR2 treated (lanes 4 and 9) or not (lanes 3 and 8) with PMA ϩ ionomycin; or from PBMCs derived from a BLV-infected sheep (BLV-infected sheep M298 presenting a persistent lymphocytosis) (lanes 5 and 10). When the EMSAs were carried out with the unmethylated oligonucleotide TxRE1 and TxRE2 probes, we observed, as reported previously by our laboratory (17), a minor and two major retarded bands (called C1, C2, and C3, respectively) (Fig. 6, A and B, lanes 1-5).…”

“…S1, A or B, respectively). As shown previously by our laboratory (17), formation of complex C2 was competed out by molar excesses of the TxRE1 or TxRE2 unlabeled homologous oligonucleotides but not by the same molar excesses of a heterologous oligonucleotide of unrelated sequence. Moreover, the C2 complex was not or was weakly competed out by the same molar excesses of methylated TxRE1-154me (supplemental Fig.…”

“…Our laboratory had previously reported that the C1 complex contains the ATF-2 transcription factor and the C2 complex contains ATF-1, CREB, and CREM proteins (14,15,17,27). Supershift experiments did not identify the nature of the proteins present in complex C3 (17). Treatment of YR2 or L267 cells with PMA ϩ ionomycin increased the intensity of the retarded DNA-protein complex C2 as compared with mock-treated cells (Fig.…”

“…A minor and two major protein-DNA complexes were observed and named C1, C2, and C3, respectively (indicated by arrows). The presence of ATF-2 in the minor protein-DNA complex C1 and the presence of ATF-1, CREB, and CREM in the major protein-DNA complex C2 were reported previously by our laboratory (17). B, EMSA analysis of nuclear factors interacting with the methylated or unmethylated TxRE2 probes.…”

“…This binding inhibition could also impair the recruitment of the histone acetyltransferase coactivators CREB-binding protein (CBP)/p300; their role in BLV transcriptional regulation, recently demonstrated by our laboratory (17), leads to a reduction in the level of histone acetylation, reinforcing the transcriptionally silenced state of integrated BLV proviruses.…”