The CREB, ATF-1, and ATF-2 transcription factors from bovine leukemia virus-infected B lymphocytes activate viral expression

Abstract

Efficient transcription and replication of the bovine leukemia virus (BLV) genome require both the viral long terminal repeat (LTR) and the virus-coded transcriptional activator Tax, which functions through a 21-bp sequence (Tax-responsive element [TxRE]) which is repeated three times within the LTR. Since Tax does not bind directly to DNA, host cell transcription factors play a central role in BLV expression. Electrophoretic mobility shift assays with nuclear extracts prepared with infected bovine B lymphocytes revealed fiveTxREspecific complexes (C1, C2, C3, C4, and C5). Here, by using a UV-induced indirect labeling technique (UV cross-linking) in conjunction with mobility shift assays, eight major polypeptides of 31, 33, 42, 46, 51, 57, 87, and 119 kDa were identified within these five complexes. Immunoprecipitation experiments identified the 57and 119-kDa proteins as cyclic AMP response element-binding (CREB) proteins, the 46-and 51-kDa proteins as activating transcription factor-1 (ATF-1), and the 87-kDa as protein ATF-2. All of these proteins (except the ATF-1 protein of 51 kDa) belong to the complex C1, which is the major complex identified in freshly isolated BLV-infected lymphocytes from cattle with persistent lymphocytosis. In transient-cotransfection experiments, these three transcription factors were able to activate LTR-directed gene expression in the presence of protein kinase A or Ca 2؉ /calmodulin-dependent protein kinase IV. CREB protein, ATF-1, and ATF-2 thus appear to be the major transcription factors involved in the early stages of viral expression.

Cellular pathways involved in the ex vivo expression of bovine leukemia virus

“…However, the results reported here are in opposition to our transient transfection data showing that cotransfection of PKA and CREB2 expression vectors activate LTR-directed expression in D17 cells (45). It should be mentioned that in these transient transfections, the PKA-expressing plasmid can be replaced by a CaM kinase IV expression vector (1). In contrast, CaM kinase II strongly inhibits LTR-directed gene expression.…”

Recent Advances in BLV Research

“…Among these, Tax and G4 are oncogenes able to promote transformation of primary rat embryo fibroblasts [ 13 , 14 ]. Tax activates transcription by acting on a triplicate 21 bp enhancer motif in the 5′ the LTR promoter via the CREB/ATF signaling pathway [ 15 , 16 ]. Although, the mechanisms of cell transformation remain to be further characterized, it is interesting to note that BLV and HTLV-1 Tax share cellular targets.…”

Characterization of new RNA polymerase III and RNA polymerase II transcriptional promoters in the Bovine Leukemia Virus genome

“…Transcription of BLV genes initiates at the U3/R junction in the 5'-long terminal repeat (LTR) and is regulated by cellular transcription factors for which several binding sites have been identified in the LTR (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19), by the viral transactivator TAX BLV (20) and by the chromatin status of the BLV provirus (21)(22)(23)(24)(25). Indeed, we have previously demonstrated that the 5'LTR RNA polymerase IIdriven transcriptional repression is due to the epigenetic state of the 5'LTR characterized by weak histone acetylation and DNA CpG hypermethylation associated to closed chromatin in a lymphoma-derived BLV-infected L267 ovine cell line harboring a fully competent provirus (14,19,21,25).…”