Feline leukemia virus long terminal repeat activates collagenase IV gene expression through AP-1

Molecular detection, phylogenetic analysis, and identification of transcription motifs in feline leukemia virus from naturally infected cats in malaysia

A nested PCR assay was used to determine the viral RNA and proviral DNA status of naturally infected cats. Selected samples that were FeLV-positive by PCR were subjected to sequencing, phylogenetic analysis, and motifs search. Of the 39 samples that were positive for FeLV p27 antigen, 87.2% (34/39) were confirmed positive with nested PCR. FeLV proviral DNA was detected in 38 (97.3%) of p27-antigen negative samples. Malaysian FeLV isolates are found to be highly similar with a homology of 91% to 100%. Phylogenetic analysis revealed that Malaysian FeLV isolates divided into two clusters, with a majority (86.2%) sharing similarity with FeLV-K01803 and fewer isolates (13.8%) with FeLV-GM1 strain. Different enhancer motifs including NF-GMa, Krox-20/WT1I-del2, BAF1, AP-2, TBP, TFIIF-beta, TRF, and TFIID are found to occur either in single, duplicate, triplicate, or sets of 5 in different positions within the U3-LTR-gag region. The present result confirms the occurrence of FeLV viral RNA and provirus DNA in naturally infected cats. Malaysian FeLV isolates are highly similar, and a majority of them are closely related to a UK isolate. This study provides the first molecular based information on FeLV in Malaysia. Additionally, different enhancer motifs likely associated with FeLV related pathogenesis have been identified.

Antisense transcription in gammaretroviruses as a mechanism of insertional activation of host genes

Transcription of retroviruses is initiated at the U3-R region boundary in the integrated provirus and continues unidirectionally to produce genomic and mRNA products of positive polarity. Several studies have recently demonstrated the existence of naturally occurring protein-encoding transcripts of negative polarity in complex retroviruses. We report here on the identification of transcripts of negative polarity in simple murine leukemia virus (MLV). In T-cell and B-cell lymphomas induced by SL3-3 and Akv MLV, antisense transcripts initiated in the U3 region of the proviral 5' long terminal repeat (LTR) and continued into the cellular proto-oncogenes Jdp2 and Bach2 to create chimeric transcripts consisting of viral and host sequence. The phenomenon was validated in vivo using a knock-in mouse model homozygous for a single LTR at a position known to activate Nras in B-cell lymphomas. A 5' rapid amplification of cDNA ends (RACE) analysis indicated a broad spectrum of initiation sites within the U3 region of the 5' LTR. Our data show for the first time transcriptional activity of negative polarity initiating in the U3 region of simple retroviruses and suggest a novel mechanism of insertional activation of host genes. Elucidation of the nature and potential regulatory role of 5' LTR antisense transcription will be relevant to the design of therapeutic vectors and may contribute to the increasing recognition of pervasive eukaryotic transcription.

Identification of LTR-specific small non-coding RNA in FeLV infected cells

The U3-LTR region of leukemia viruses transactivates cancer-related signaling pathways through the production of a non-coding RNA transcript although the role of this transcript in virus infection remains unknown. In this study we demonstrate for the first time that an long terminal repeat (LTR)-specific small non-coding RNA is produced from a feline leukemia virus (FeLV)-infected feline cell line. RNA cloning identified this as a 104 base transcript that originates from the U3-LTR region. We also demonstrate that in in vitro assays this LTR-RNA transcript activates NF kappaB signaling. Taken together, our findings suggest a possible role for this LTR transcript in FeLV pathogenesis.

Leukemia virus long terminal repeat activates NFkappaB pathway by a TLR3-dependent mechanism

The long terminal repeat (LTR) region of leukemia viruses plays a critical role in tissue tropism and pathogenic potential of the viruses. We have previously reported that U3-LTR from Moloney murine and feline leukemia viruses (Mo-MuLV and FeLV) upregulates specific cellular genes in trans in an integration-independent way. The U3-LTR region necessary for this action does not encode a protein but instead makes a specific RNA transcript. Because several cellular genes transactivated by the U3-LTR can also be activated by NFkappaB, and because the antiapoptotic and growth promoting activities of NFkappaB have been implicated in leukemogenesis, we investigated whether FeLV U3-LTR can activate NFkappaB signaling. Here, we demonstrate that FeLV U3-LTR indeed upregulates the NFkappaB signaling pathway via activation of Ras-Raf-IkappaB kinase (IKK) and degradation of IkappaB. LTR-mediated transcriptional activation of genes did not require new protein synthesis suggesting an active role of the LTR transcript in the process. Using Toll-like receptor (TLR) deficient HEK293 cells and PKR(-/-) mouse embryo fibroblasts, we further demonstrate that although dsRNA-activated protein kinase R (PKR) is not necessary, TLR3 is required for the activation of NFkappaB by the LTR. Our study thus demonstrates involvement of a TLR3-dependent but PKR-independent dsRNA-mediated signaling pathway for NFkappaB activation and thus provides a new mechanistic explanation of LTR-mediated cellular gene transactivation.

Mutations that abrogate transactivational activity of the feline leukemia virus long terminal repeat do not affect virus replication

The U3 region of the LTR of oncogenic Moloney murine leukemia virus (Mo-MuLV) and feline leukemia viruses (FeLV) have been previously reported to activate expression of specific cellular genes in trans, such as MHC class I, collagenase IV, and MCP-1, in an integration-independent manner. It has been suggested that transactivation of these specific cellular genes by leukemia virus U3-LTR may contribute to the multistage process of leukemogenesis. The U3-LTR region, necessary for gene transactivational activity, also contains multiple transcription factor-binding sites that are essential for normal virus replication. To dissect the promoter activity and the gene transactivational activity of the U3-LTR, we conducted mutational analysis of the U3-LTR region of FeLV-A molecular clone 61E. We identified minimal nucleotide substitution mutants on the U3 LTR that did not disturb transcription factor-binding sites but abrogated its ability to transactivate the collagenase gene promoter. To determine if these mutations actually have altered any uncharacterized important transcription factor-binding site, we introduced these U3-LTR mutations into the full-length infectious molecular clone 61E. We demonstrate that the mutant virus was replication competent but could not transactivate cellular gene expression. These results thus suggest that the gene transactivational activity is a distinct property of the LTR and possibly not related to its promoter activity. The cellular gene transactivational activity-deficient mutant FeLV generated in this study may also serve as a valuable reagent for testing the biological significance of LTR-mediated cellular gene activation in the tumorigenesis caused by leukemia viruses.

Oncogene cooperativity in Friend erythroleukemia: erythropoietin receptor activation by the env gene of SFFV leads to transcriptional upregulation of PU.1, independent of SFFV proviral insertion

Cancer is a multi-step, multi-genetic event. Whether oncogenic mutations cooperate with one another to transform cells and how is not well understood. The Friend murine retroviral erythroleukemia model involves mitogenic activation of the erythropoietin receptor (EpoR) by the virus env gene (F-gp55), aberrant over-expression of the transcription factor PU.1, and inactivating mutations in p53. In this report we demonstrate that concurrent expression of F-gp55 and PU.1 in erythroid target cells, in vivo, cooperate to accelerate erythroleukemia induction. Early in the disease, prior to the detection of clonal leukemic cells, activation of the EpoR by F-gp55, but not erythropoietin, resulted in transcriptional upregulation of PU.1 through a trans regulatory mechanism. This could occur in the absence of an integrated provirus within the PU.1 gene locus. The regulation of PU.1 transcription in established erythroleukemia cell lines differed depending upon the level of PU.1 protein present. Our results suggest that the action of F-gp55 contributes to both early and late stages of Friend erythroleukemia and that persistence of F-gp55 expression may be required not only to initiate erythroleukemia but to also maintain erythroleukemia following Friend virus infection.

Analysis of the disease potential of a recombinant retrovirus containing Friend murine leukemia virus sequences and a unique long terminal repeat from feline leukemia virus

We have molecularly cloned a feline leukemia virus (FeLV) (clone 33) from a domestic cat with acute myeloid leukemia (AML). The long terminal repeat (LTR) of this virus, like the LTRs present in FeLV proviruses from other cats with AML, contains an unusual structure in its U3 region upstream of the enhancer (URE) consisting of three tandem direct repeats of 47 bp. To test the disease potential and specificity of this unique FeLV LTR, we replaced the U3 region of the LTR of the erythroleukemia-inducing Friend murine leukemia virus (F-MuLV) with that of FeLV clone 33. When the resulting virus, F33V, was injected into newborn mice, almost all of the mice eventually developed hematopoietic malignancies, with a significant percentage being in the myeloid lineage. This is in contrast to mice injected with an F-MuLV recombinant containing the U3 region of another FeLV that lacks repetitive URE sequences, none of which developed myeloid malignancies. Examination of tumor proviruses from F33V-infected mice failed to detect any changes in FeLV U3 sequences other than that in the URE. Like F-MuLV-infected mice, those infected with the F-MuLV/FeLV recombinants were able to generate and replicate mink cell focus-inducing viruses. Our studies are consistent with the idea that the presence of repetitive sequences upstream of the enhancer in the LTR of FeLV may favor the activation of this promoter in myeloid cells and contribute to the development of malignancies in this hematopoietic lineage.

Long terminal repeat regions from exogenous but not endogenous feline leukemia viruses transactivate cellular gene expression

We have previously reported that the long terminal repeat (LTR) region of feline leukemia viruses (FeLVs) can enhance expression of certain cellular genes such as the collagenase IV gene and MCP-1 in trans (S. K. Ghosh and D. V. Faller, J. Virol. 73:4931-4940, 1999). Genomic DNA of all healthy feline species also contains LTR-like sequences that are related to exogenous FeLV LTRs. In this study, we evaluated the cellular gene transactivational potential of these endogenous FeLV LTR sequences. Unlike their exogenous FeLV counterparts, neither nearly full-length endogenous FeLV molecular clones (CFE-6 and CFE-16) nor their isolated LTRs were able to activate collagenase IV gene or MCP-1 expression in transient transfection assays. We had also demonstrated previously that production of an RNA transcript from exogenous FeLV LTRs correlates with their transactivational activity. In the present study, we demonstrate that the endogenous FeLV LTRs do not generate LTR-specific RNA transcripts in the feline embryo fibroblast cell line AH927. Furthermore, infection of AH927 cells by an exogenous FeLV subgroup A virus did not induce production of such LTR-specific transcripts from the endogenous proviral genomes, although the LTR-specific transcripts from the exogenous virus were readily detected. Finally, LTR-specific transcripts were not generated in BALB/3T3 cells transiently transfected with isolated CFE-6 LTR, in contrast to transfections with LTRs from exogenous viruses. Our data thus suggest that the inability of endogenous FeLV LTRs in gene transactivation is not due to cell line specificity or presence of any upstream inhibitory cis-acting element. Endogenous, nonleukemogenic FeLV LTRs, therefore, do not transactivate cellular gene expression, and this property appears to be specific to exogenous, leukemogenic FeLVs.