Amplification of mouse mammary tumor virus genomes in non-mammary tumor cells

A redundant nuclear protein binding site contributes to negative regulation of the mouse mammary tumor virus long terminal repeat

The tissue specificity of mouse mammary tumor virus (MMTV) expression is controlled by regulatory elements in the MMTV long terminal repeat (LTR). These regulatory elements include the hormone response element, located approximately between -200 and -75, as well as binding sites for NF-1, Oct-1 (OTF-1), and mammary gland enhancer factors. Naturally occurring MMTV deletion variants isolated from T-cell and kidney tumors, transgenic-mouse experiments with MMTV LTR deletions, and transient transfection assays with LTR constructs indicate that there are additional transcription regulatory elements, including a negative regulatory element (NRE), located upstream of the hormone response element. To further define this regulatory region, we have constructed a series of BAL 31 deletion mutants in the MMTV LTR for use in transient transfection assays. These assays indicated that deletion of two regions (referred to as promoter-distal and -proximal NREs) between -637 and -201 elevated basal MMTV promoter activity in the absence of glucocorticoids. The region between -637 and -264 was surveyed for the presence of nuclear protein binding sites by gel retardation assays. Only one type of protein complex (referred to as NRE-binding protein or NBP) bound exclusively to sites that mapped to the promoter-distal and -proximal NREs identified by BAL 31 mutations. The promoter-proximal binding site was mapped further by linker substitution mutations and transfection assays. Mutations that mapped to a region containing an inverted repeat beginning at -287 relative to the start of transcription elevated basal expression of a reporter gene driven by the MMTV LTR. A 59-bp DNA fragment from the distal NRE also bound the NBP complex. Gel retardation assays showed that mutations within both inverted repeats of the proximal NRE eliminated NBP binding and mutations within single repeats altered NBP binding. Intriguingly, the NBP complex was detected in extracts from T cells and lung cells but was absent from mammary gland cells. These results suggest that a factor contributing to high-level expression of MMTV in the mammary gland is the lack of negative regulation by NBP.

Mouse mammary tumor virus with rearranged long terminal repeats causes murine lymphomas

Mouse mammary tumor virus (MMTV) is a slowly transforming retrovirus associated primarily with the induction of mammary tumors. It is widely accepted that T-cell lymphomas of various mouse strains are associated with extra proviruses of MMTV. These extra proviruses showed site-specific rearrangements in the U3 region of long terminal repeats (LTRs), consisting of about 400 nucleotide deletions and occasional substitution resulting in unique tandem repeats. However, the question of whether these mutant MMTVs cause lymphomas has not been experimentally resolved. Here we present distinct evidence that they do. We constructed chimeric MMTVs by replacing the LTR of the recently constructed pathogenic MMTV provirus clone with rearranged LTRs of MMTV proviruses obtained from two DBA/2 mouse lymphoma cell lines, MLA and DL-8, and inoculated them into BALB/c mice. These mice developed lymphomas, but no mammary tumors, 4 to 11 months postinoculation, whereas the original pathogenic MMTV clone alone induced mammary tumors. These results showed that the tissue specificity of MMTV tumorigenesis is determined by the LTR structures.

Regulated expression of mouse mammary tumor proviral genes in cells of the B lineage

We evaluated the expression of mouse mammary tumor proviral (MMTV) transcripts during B cell ontogeny and compared levels of RNA in B lymphocytes and B cell lines with levels in other cells of the hematopoietic lineage and in a mammary cell line. We demonstrate that MMTV transcripts are expressed as early as the pro-B cell stage in ontogeny and are expressed at basal constitutive levels throughout most of the B cell developmental pathway. The level of MMTV expression in B cells is similar to constitutive levels in mammary tissues and two to three orders of magnitude greater than in activated T cells. Levels of MMTV transcripts in B cells are not solely due to positional effects. Transient transfection assays showed that MMTV upregulation resulted from transcriptional activation of the viral LTR, indicating that there are specific and inducible transcription factors that regulate MMTV expression in B cells. MMTV transcripts could not be upregulated in pre-B cell lines but could be induced in some mature B cell lines. There was a correlation between the ability to stimulate B cells to secrete antibody and the ability to induce upregulated MMTV expression. Evidence is presented that suggests that the principal transcription factors involved in MMTV expression do not include the B cell factors OTF-2 or NF-kappa B, but rather are likely to be novel factors that are induced during differentiation to antibody secretion. A hypothesis for why mammary tumor viruses are well adapted for expression in cells of the B lineage is proposed, and the implications of this for the documented influence of MMTV gene products on the T cell repertoire are discussed.

Altered mouse mammary tumor virus transcript synthesis in T-cell lymphoma cells

Proviral copies of mouse mammary tumor virus (MMTV) are known to be amplified in certain T-cell lymphomas. Transcription of the amplified MMTV proviruses was studied in detail in two T-cell lymphoma lines and showed the production of deletions and premature termination of env mRNAs and the premature termination of gag transcripts. EL-4 cells produce three env mRNAs, and sequence analysis of cDNAs of the two smaller transcripts revealed large deletions encompassing the 3' half of the env gene. The deletion in at least one of the altered transcripts appeared to be produced by a splicing mechanism. T-cell lymphoma line ML of DBA/2 mice also synthesizes two smaller env transcripts, both of which result from premature termination of transcription. Both lines transcribe high levels of gag mRNAs of about 0.8 kilobases in length, terminating at the end of the region encoding MMTV phosphoprotein pp21. Restriction enzyme BamHI analysis of the amplified proviruses of EL-4 and ML cells as well as of additional non-mammary tumor cell types containing amplified MMTV proviruses suggested that the amplified proviruses were derived from exogenous viruses, or activated endogenous provirus MTV-1 in the case of DBA/2 strain tumor cells.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Extra mouse mammary tumor proviruses in DBA/2 mouse lymphomas acquire a selective advantage in lymphocytes by alteration in the U3 region of the long terminal repeat

We determined the nucleotide sequences of the long terminal repeats (LTRs) from mouse mammary tumor virus (MMTV) proviruses acquired in two DBA/2 mouse lymphoma cell lines, MLA and DL-8. Proviruses from MLA contained a 352-base-pair deletion from nucleotides 669 to 1020 in the U3 region of the LTR, whereas the LTR alteration of the DL-8 provirus involved both a similar 360-base-pair deletion and generation of a tandem repeat region consisting of sequences of flanking deletions. To assess the function of the rearranged LTRs, we constructed plasmids in which normal and rearranged LTRs drove the reporter chloramphenicol acetyltransferase gene and transfected them into T-cell lines (Jurkat, Molt-3, and DL-8) and the mammary tumor cell line T47D. Both rearranged LTRs were transcriptionally active, but normal LTRs were not active in either the presence or absence of glucocorticoids in all T-cell lines. In T47D cells, however, the MLA provirus LTR showed the same glucocorticoid- or progestin-dependent transcriptional activity as did normal LTRs. The DL-8 provirus LTR acquired a novel enhancer(s) by rearrangement and thus had a high basal transcriptional activity in T47D cells. The results of chloramphenicol acetyltransferase assays using plasmids with various chimeric MMTV LTRs revealed that the rearranged LTRs had lost their negative regulatory element and contained an enhancer element that was highly homologous to the enhancer A element of polyomavirus (from nucleotides 525 to 558). GR but not C3H mouse MMTV contained this enhancer. These results elucidate some of the molecular mechanisms involved in the selection of mutant MMTVs with rearranged LTRs in lymphoma cells.

Absence of mouse mammary tumor virus proviral amplification in chemically induced lymphomas of RF/J mice

RF/J mice are susceptible to the induction of thymic lymphomas by the carcinogens 3-methylcholanthrene and N-methyl-N-nitrosourea. Given the association of mouse mammary tumor virus (MMTV) with certain thymomas, we examined genomic DNA from chemically induced lymphomas of RF/J mice for new MMTV proviruses. Of 13 tissue culture lines derived from 3-methylcholanthrene-induced tumors, 5 had acquired new proviruses. MMTV amplification coincided with the appearance of viral mRNAs and proteins. However, no primary tumors or animal-passaged tumors contained new proviruses. These observations indicate that MMTV does not have a role in the tumor induction process, although it may become activated and amplified in tissue culture lines derived from tumors.