Structural and functional characterization of mutants of the bovine leukemia virus transactivator protein p34

A complex network of transcription factors and epigenetic regulators involved in bovine leukemia virus transcriptional regulation

Bovine Leukemia Virus (BLV) is the etiological agent of enzootic bovine leukosis, a disease characterized by the neoplastic proliferation of B cells in cattle. While most European countries have introduced efficient eradication programs, BLV is still present worldwide and no treatment is available. A major feature of BLV infection is the viral latency, which enables the escape from the host immune system, the maintenance of a persistent infection and ultimately the tumoral development. BLV latency is a multifactorial phenomenon resulting in the silencing of viral genes due to genetic and epigenetic repressions of the viral promoter located in the 5' Long Terminal Repeat (5'LTR). However, viral miRNAs and antisense transcripts are expressed from two different proviral regions, respectively the miRNA cluster and the 3'LTR. These latter transcripts are expressed despite the viral latency affecting the 5'LTR and are increasingly considered to take part in tumoral development. In the present review, we provide a summary of the experimental evidence that has enabled to characterize the molecular mechanisms regulating each of the three BLV transcriptional units, either through cis-regulatory elements or through epigenetic modifications. Additionally, we describe the recently identified BLV miRNAs and antisense transcripts and their implications in BLV-induced tumorigenesis. Finally, we discuss the relevance of BLV as an experimental model for the closely related human T-lymphotropic virus HTLV-1.

Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human

In 1871, the observation of yellowish nodules in the enlarged spleen of a cow was considered to be the first reported case of bovine leukemia. The etiological agent of this lymphoproliferative disease, bovine leukemia virus (BLV), belongs to the deltaretrovirus genus which also includes the related human T-lymphotropic virus type 1 (HTLV-1). This review summarizes current knowledge of this viral system, which is important as a model for leukemogenesis. Recently, the BLV model has also cast light onto novel prospects for therapies of HTLV induced diseases, for which no satisfactory treatment exists so far.

Potential evolutionary influences on overlapping reading frames in the bovine leukemia virus pXBL region

Bovine leukemia virus contains a pXBL region encoding the 3' parts of four regulatory proteins (Tax, Rex, G4, R3) in overlapping reading frames. Here we report the pXBL polymorphisms of 30 isolates from four countries. Rates of overall and synonymous substitutions were consistently lower, and nucleotide/amino acid composition bias and codon bias higher, in more-overlapped than in less-overlapped regions. Ratios of nonsynonymous/synonymous substitutions were lowest in the tax gene and its subregions. The 5' parts of the four genes showed selection patterns corresponding to their genomic context outside of the pXBL region. Longer G4 variants due to a natural stop codon mutation had additional triple overlap with reduced sequence variability. These data support the concept that a higher level of overlapping in coding regions correlates with greater evolutionary constraint. Tax, the most conserved among the four regulatory proteins, showed purifying selection consistent with its importance in the viral life cycle.

Interaction of retroviral Tax oncoproteins with tristetraprolin and regulation of tumor necrosis factor-alpha expression

BACKGROUND

The Tax oncoproteins are transcriptional regulators of viral expression involved in pathogenesis induced by complex leukemogenic retroviruses (or delta-retroviruses, i.e., primate T-cell leukemia viruses and bovine leukemia virus). To better understand the molecular pathways leading to cell transformation, we aimed to identify cellular proteins interacting with Tax.

METHODS

We used a yeast two-hybrid system to identify interacting cellular proteins. Interactions between Tax and candidate interacting cellular proteins were confirmed by glutathione S-transferase (GST) pulldown assays, co-immunoprecipitation, and confocal microscopy. Functional interactions between Tax and one interacting protein, tristetraprolin (TTP), were assessed by analyzing the expression of tumor necrosis factor-alpha (TNF-alpha), which is regulated by TTP, in mammalian cells (HeLa, D17, HEK 293, and RAW 264.7) transiently transfected with combinations of intact and mutant Tax and TTP.

RESULTS

We obtained seven interacting cellular proteins, of which one, TTP, was further characterized. Tax and TTP were found to interact specifically through their respective carboxyl-terminal domains. The proteins colocalized in the cytoplasm in a region surrounding the nucleus of HeLa cells. Furthermore, coexpression of Tax was associated with nuclear accumulation of TTP. TTP is an immediate-early protein that inhibits expression of TNF-alpha at the post-transcriptional level. Expression of Tax reverted this inhibition, both in transient transfection experiments and in stably transfected macrophage cell lines.

CONCLUSION

Tax, through its interactions with the TTP repressor, indirectly increases TNF-alpha expression. This observation is of importance for the cell transformation process induced by leukemogenic retroviruses, because TNF-alpha overexpression plays a central role in pathogenesis.

The region between amino acids 245 and 265 of the bovine leukemia virus (BLV) tax protein restricts transactivation not only via the BLV enhancer but also via other retrovirus enhancers

Bovine leukemia virus (BLV) is associated with enzootic bovine leukosis and is closely related to human T-cell leukemia virus type 1 (HTLV-1). The Tax protein of BLV acts through the 5' long terminal repeat (LTR) of BLV and activates the transcription of BLV. In this study, we amplified tax genes from BLV-infected cattle using PCR. We cloned the genes and monitored the transcriptional activities of the products. Seven independent mutant Tax proteins, with at least one amino acid substitution between residues 240 and 265, exhibited a markedly stronger ability to stimulate the viral LTR-directed transcription than the wild-type Tax protein. Analysis of chimeric Tax proteins derived from wild-type and mutant Tax proteins clearly demonstrated that a single substitution between residue 240 and 265 might be critical for the higher activities of the Tax mutant proteins. Furthermore, it appeared that transient expression of a Tax mutant protein was better able to increase the production of viral proteins and particles from a defective recombinant proviral clone of BLV than was wild-type Tax. Analysis of mutations within the U3 region of the LTR revealed that a cyclic AMP-responsive element in Tax-responsive element 2 might be sufficient for the enhanced activation mediated by the mutant proteins. In addition to the LTR of BLV, other viral enhancers, such as the enhancers of HTLV-1 and of mouse mammary tumor virus, which cannot be activated by wild-type BLV Tax protein, were activated by a Tax mutant protein. Our observations suggest that the transactivation activity and target sequence specificity of BLV Tax might be limited or negatively regulated by the region of the protein between amino acids 240 and 265.

In vivo rescue of a silent tax-deficient bovine leukemia virus from a tumor-derived ovine B-cell line by recombination with a retrovirally transduced wild-type tax gene

The lack of bovine leukemia virus (BLV) expression is a consistent finding in freshly isolated ovine tumor cells and in the B-cell lines derived from these tumors. In order to gain further insight into the mechanisms of BLV silencing in these tumors, we have used the YR2 B-cell line, which was derived from the leukemic cells of a BLV-infected sheep. This cell line contains a single, monoclonally integrated, silent provirus, which cannot be reactivated either by stimulation in vitro or by in vivo injection of the tumor cells or cloned proviral DNA in sheep. Sequence analysis of the tax gene from the YR2 cell line identified two G-to-A transitions (G7924 to A7924 and G8149 to A8149) that result in E-to-K amino acid changes at positions 228 and 303 in the Tax protein. Following retroviral vector-mediated transfer of a wild-type tax gene into YR2 cells, we showed that BLV mRNA, viral proteins, and virions were produced, demonstrating that the cellular factors required for virus expression were present in the original YR2 cell line. Injection of this transduced YR2 cell line in sheep led to the rescue of replication-competent BLV proviruses. The integrated competent proviruses exhibited unique chimeric tax genes, which arose from homologous recombination between the transduced wild-type tax and the YR2-derived tax sequences. Furthermore, in one of these functional recombinant proviruses, only the A8149-to-G8149 reversion was present, providing clear evidence that the defect underlying the silent phenotype in YR2 cells results from a single C-terminal E303-to-K303 amino acid substitution in the BLV Tax protein. Our observations suggest that a single strategically located mutation in tax provides a mechanism for BLV inactivation in B-cell tumors.

Both wild-type and strongly attenuated bovine leukemia viruses protect peripheral blood mononuclear cells from apoptosis

Bovine leukemia virus (BLV) and the human T-cell leukemia viruses belong to the same subfamily of oncoviruses. Although much attention has focused on the mechanisms of cell proliferation and transformation by these viruses, experiments on the apoptotic process have yielded conflicting data in in vitro cell culture. Experimental infection of sheep with BLV proviruses offers the opportunity to analyze apoptosis in vivo. Here, we show that BLV-infected peripheral mononuclear cells, cultivated ex vivo, are protected from spontaneous programmed cell death. Moreover, the virus is able to specifically interfere with the apoptotic program of infected B lymphocytes. Strongly attenuated mutant proviruses that harbor deletions in the G4 and/or R3 genes also decrease the global susceptibility to apoptosis at levels similar to those obtained with the wild-type virus. In addition, cell culture supernatants from wild-type and mutant viruses can prevent uninfected cells from undergoing programmed cell death. These observations demonstrate that the R3 and G4 genes are not required to maintain both direct and indirect protection against apoptosis. They also imply that the level of programmed cell death observed ex vivo is independent of the amounts of proviruses in the animals. The failure of these cells to undergo apoptosis might be related to the pathogenesis induced by BLV.

Nucleotide sequence of the ovine P53 tumor-suppressor cDNA and its genomic organization

A 2155 bp cDNA clone corresponding to the ovine p53 tumor suppressor gene has been isolated from a cDNA library made from the BLV-transformed YR2 cell line RNA. After the sequencing analysis, it appeared that this clone contains the entire p53 coding region (including 126 bp upstream of the ATG initiation codon). The nucleotide sequence shows a high degree of homology with the human (73%), murine (74%) and rat (66%) cDNAs. The encoded ovine p53 protein is 382 amino acids long with an apparent molecular weight of 50 kDa and shares 79% and 72% amino acid homology with the human and the mouse p53 respectively. Furthermore, the homology is not equally distributed along the molecule but is mainly located within five highly conserved regions. As its mouse and human counterparts, the ovine p53 contains a high proportion of proline residues, an acidic N-terminal domain and a basic C-terminal domain. We also report the structure of the ovine p53 gene that is similar to those already defined from other species.

Serum response factor has functional roles both in indirect binding to the CArG box and in the transcriptional activation function of human T-cell leukemia virus type I Tax

We previously reported that Tax1 of human T-cell leukemia virus type I interacts directly with serum response factor (SRF) and that Tax1 activates the transcription of several cellular immediate-early genes through the SRF binding site (CArG box). This activation required the transcriptional activation function of Tax1, identified as an activity of GALTax (a chimeric Tax1 with the yeast transcription factor GAL4) at the GAL4-binding site. In this study, we examined whether SRF plays a role in the transcriptional activation function of Tax1. Expression of Tax1 suppressed the GALTax activity at the GAL4 site as a result of squelching, and the suppressed activity was recovered by the overexpression of SRF, suggesting that SRF is a factor that is required for GALTax activity and that is inhibited by competition with Tax1. The expression of antisense SRF RNA specifically inhibited GALTax activity to less than 20%. Deletion of the Tax1 interaction domain of SRF at its C terminus converted SRF from an activator of GALTax to an inhibitor. These results suggest that SRF is an essential component of the transcriptional activation of Tax1 in addition to a mediator of CArG box binding.

Interaction of HTLV-1 Tax1 with p67SRF causes the aberrant induction of cellular immediate early genes through CArG boxes

Tax1 of human T-cell leukemia virus type 1 (HTLV-1) is a transcriptional activator for viral gene expression and is also a transforming protein through inducing the expression of several cellular genes under the control of mitogenic signals. We identified the CArG boxes as a Tax1-responsive cis-acting element for the cellular immediate early genes c-fos, egr-1, and egr-2. Using a chimeric protein consisting of the CArG-binding factor p67SRF and the heterologous DNA-binding domain of a yeast transcription factor GAL4, we demonstrated that Tax1 activates the transcriptional activity of p67SRF through the GAL4-binding site. The carboxy-terminal half of p67SRF, which lacks domains for DNA-binding, dimerization, and ternary complex formation with p62TCF, was sufficient for the activation by Tax1. Tax1 produced in Escherichia coli bound p67SRF in vitro. The complex formation in vivo was also indicated by the finding that the acidic activation domain of VP16, by fusion to p67SRF, can complement the transcriptional activation function of a mutant Tax1 in trans. Thus, Tax1 activates CArG-mediated transcription without mitogenic signals through interaction with a CArG-binding factor, p67SRF. This must be one of the primary steps by which Tax1 causes aberration in growth control of the infected cells.

Mutations in the bovine leukemia virus Tax protein can abrogate the long terminal repeat-directed transactivating activity without concomitant loss of transforming potential

The bovine leukemia virus Tax protein transactivates gene expression directed by the viral long terminal repeat (LTR) and contributes to immortalization of primary cells. Theoretical analysis of the protein sequence revealed the presence of a putative zinc finger structure at its amino end. Selected mutations in that region completely abolished transactivation, demonstrating its importance for LTR-directed gene regulation. However, these mutations did not interfere with the ability of tax to bind zinc or to contribute to immortalization of primary cells. Thus, transactivation of bovine leukemia virus LTR and target cell transformation are independent functions of Tax and involve different functional domains of the protein.

Factors controlling the expression of mouse mammary tumour virus

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The bovine leukemia virus tax gene contains an enhancer sequence

Transactivator proteins of the bovine leukemia (BLV) and human T-lymphotropic (HTLV) viruses increase long terminal repeat (LTR)-directed viral gene expression and act as immortalizing oncogenes in tissue culture. We report here that the BLV tax gene sequence contains an enhancer-like activity. The X long open reading frame was cloned up-stream of the beta-globin promoter linked to the chloramphenicol acetyltransferase (CAT) gene. In the presence of tax sequences, up to sevenfold enhancement of CAT expression was observed. A computer-assisted homology search revealed the presence of a consensus enhancer core sequence (GTGTTGTTGGTTG) into the third exon of the 2.1-kb X mRNA. These studies demonstrate that the tax gene contains a transcriptional enhancer which could be involved in early viral gene expression in vivo.