Mouse mammary tumor virus superantigen expression in B cells is regulated by a central enhancer within the pol gene

Intestinal Immune System and Amplification of Mouse Mammary Tumor Virus

Mouse mammary tumor virus (MMTV) is a virus that induces breast cancer in mice. During lactation, MMTV can transmit from mother to offspring through milk, and Peyer's patches (PPs) in mouse intestine are the first and specific target organ. MMTV can be transported into PPs by microfold cells and then activate antigen-presenting cells (APCs) by directly binding with Toll-like receptors (TLRs) whereas infect them through mouse transferrin receptor 1 (mTfR1). After being endocytosed, MMTV is reversely transcribed and the cDNA inserts into the host genome. Superantigen (SAg) expressed by provirus is presented by APCs to cognate CD4+ T cells via MHCII molecules to induce SAg response, which leads to substantial proliferation and recruitment of related immune cells. Both APCs and T cells can be infected by MMTV and these extensively proliferated lymphocytes and recruited dendritic cells act as hotbeds for viral replication and amplification. In this case, intestinal lymphatic tissues can actually become the source of infection for the transmission of MMTV in vivo, which results in mammary gland infection by MMTV and eventually lead to the occurrence of breast cancer.

Embryonic expression of endogenous retroviral RNAs in somatic tissues adjacent to the Oikopleura germline

Selective pressure to maintain small genome size implies control of transposable elements, and most old classes of retrotransposons are indeed absent from the very compact genome of the tunicate Oikopleura dioica. Nonetheless, two families of retrotransposons are present, including the Tor elements. The gene organization within Tor elements is similar to that of LTR retrotransposons and retroviruses. In addition to gag and pol, many Tor elements carry a third gene encoding viral envelope-like proteins (Env) that may mediate infection. We show that the Tor family contains distinct classes of elements. In some classes, env mRNA is transcribed from the 5′LTR as in retroviruses. In others, env is transcribed from an additional promoter located downstream of the 5′LTR. Tor Env proteins are membrane-associated glycoproteins which exhibit some features of viral membrane fusion proteins. Whereas some elements are expressed in the adult testis, many others are specifically expressed in embryonic somatic cells adjacent to primordial germ cells. Such embryonic expression depends on determinants present in the Tor elements and not on their surrounding genomic environment. Our study shows that unusual modes of transcription and expression close to the germline may contribute to the proliferation of Tor elements.

Deletions within the U3 long terminal repeat alter the tumorigenic potential of myeloblastosis associated virus type 1(N)

The molecularly cloned myeloblastosis-associated virus type-1(N) (MAV-1(N)) strain induces specifically nephroblastomas in chicken. MAV-induced nephroblastoma constitutes a unique animal model of the human Wilms' tumor. We have previously shown that the MAV-1(N) long terminal repeats (LTR) were necessary and sufficient for nephroblastoma induction. Since major determinants for oncogenesis have been mapped in the U3 region of several other retroviruses, we have analyzed the tumorigenic potential of five recombinant viruses partially deleted in their U3 region. The results obtained indicated that deletions of the LTRs resulted in a modification of the pathogenic spectrum of MAV-1(N) and a decreased efficiency for nephroblastoma induction.

Pushing the limits of the scanning mechanism for initiation of translation

Selection of the translational initiation site in most eukaryotic mRNAs appears to occur via a scanning mechanism which predicts that proximity to the 5' end plays a dominant role in identifying the start codon. This "position effect" is seen in cases where a mutation creates an AUG codon upstream from the normal start site and translation shifts to the upstream site. The position effect is evident also in cases where a silent internal AUG codon is activated upon being relocated closer to the 5' end. Two mechanisms for escaping the first-AUG rule--reinitiation and context-dependent leaky scanning--enable downstream AUG codons to be accessed in some mRNAs. Although these mechanisms are not new, many new examples of their use have emerged. Via these escape pathways, the scanning mechanism operates even in extreme cases, such as a plant virus mRNA in which translation initiates from three start sites over a distance of 900 nt. This depends on careful structural arrangements, however, which are rarely present in cellular mRNAs. Understanding the rules for initiation of translation enables understanding of human diseases in which the expression of a critical gene is reduced by mutations that add upstream AUG codons or change the context around the AUG(START) codon. The opposite problem occurs in the case of hereditary thrombocythemia: translational efficiency is increased by mutations that remove or restructure a small upstream open reading frame in thrombopoietin mRNA, and the resulting overproduction of the cytokine causes the disease. This and other examples support the idea that 5' leader sequences are sometimes structured deliberately in a way that constrains scanning in order to prevent harmful overproduction of potent regulatory proteins. The accumulated evidence reveals how the scanning mechanism dictates the pattern of transcription--forcing production of monocistronic mRNAs--and the pattern of translation of eukaryotic cellular and viral genes.

Spliced and prematurely polyadenylated Jaagsiekte sheep retrovirus-specific RNAs from infected or transfected cells

Jaagsiekte sheep retrovirus (JSRV) is the etiologic agent of a contagious lung cancer of sheep, ovine pulmonary adenocarcinoma (OPA). In this study, we characterized the virus-specific RNAs in 293T cells transiently transfected with a human cytomegalovirus promoter-driven JSRV expression plasmid, in productively infected OHH1.LU deer lung cells, and in OPA tumors from field isolates. Typical unspliced (presumably for gag, pro, and pol) and singly spliced env mRNAs were detected. In addition, six other virus-specific RNAs were detected that resulted from the use of alternate splice acceptor sites and two premature polyadenylation sites (located in gag and in env). The orf-x gene of the virus appears to be expressed from two singly spliced subgenomic mRNAs of 3.2 kb that would encode an independent orf-x protein of 179 amino acids. In addition, the results suggested that there may also be an internal promoter for orf-x.

C3H mouse mammary tumor virus superantigen function requires a splice donor site in the envelope gene

Mouse mammary tumor virus (MMTV) encodes a superantigen (Sag) that is required for efficient milk-borne transmission of virus from mothers to offspring. The mRNA used for Sag expression is controversial, and at least four different promoters (two in the long terminal repeat and two in the envelope gene) for sag mRNA have been reported. To determine which RNA is responsible for Sag function during milk-borne MMTV transmission, we mutated a splice donor site unique to a spliced sag RNA from the 5' envelope promoter. The splice donor mutation in an infectious provirus was transfected into XC cells and injected into BALB/c mice. Mice injected with wild-type provirus showed Sag activity by the deletion of Sag-specific T cells and induction of mammary tumors in 100% of injected animals. However, mice injected with the splice donor mutant gave sporadic and delayed T-cell deletion and a low percentage of mammary tumors with a long latency, suggesting that the resulting tumors were due to the generation of recombinants with endogenous MMTVs. Third-litter offspring of mice injected with wild-type provirus showed Sag-specific T-cell deletion and developed mammary tumors with kinetics similar to those for mice infected by nursing on MMTV-infected mothers, whereas the third-litter offspring of the splice donor mutant-injected mice did not. One of the fifth-litter progeny of splice donor mutant-injected mice showed C3H Sag activity and had recombinants that repaired the splice donor mutation, thus confirming the necessity for the splice donor site for Sag function. These experiments are the first to show that the spliced sag mRNA from the 5' envelope promoter is required for efficient milk-borne transmission of C3H MMTV.

Mouse mammary tumor virus superantigen expression is reduced by glucocorticoid treatment

Expression of mouse mammary tumor virus (MMTV)-encoded superantigens in B lymphocytes are required for viral transmission and pathogenesis. Due to problems with detection and quantification of the superantigen protein, most reports about the mechanism of superantigen expression from the viral sag gene rely on the quantitative analysis of putative sag mRNAs. The description of multiple promoters as a source of putative sag mRNA has complicated the situation even further. All conclusions about the level of superantigen protein expression based on these data remain circumstantial. To test the effect of the glucocorticoid hormone dexamethasone on the total superantigen expression from an infectious MMTV provirus we used a quantitative assay that is based on a superantigen-luciferase fusion protein. MMTV gene expression from the major promoter in the 5' long terminal repeat (LTR) is strongly induced in the presence of glucocorticoid hormones. We now demonstrate that, in the presence of dexamethasone, sag gene expression is reduced despite increased transcription from the MMTV 5' LTR and increased amounts of putative sag mRNA initiated at the LTR promoter. These data show that the expression of the MMTV sag gene does not correlate with the activity of the major LTR promoter and thus differs from all other MMTV genes.

The mouse mammary tumor virus transcription enhancers for hematopoietic progenitor and mammary gland cells share functional elements

Expression of mouse mammary tumor virus (MMTV)-encoded superantigens in B lymphocytes is required for viral transmission and pathogenesis. We have previously established a critical role of an enhancer element within the long terminal repeat (LTR) for MMTV sag gene expression in B-lymphoid progenitor cells. We now demonstrate enhancer activity of this element in a promyelocytic progenitor cell line. We also map the position of the enhancer within the U3 region of the MMTV LTR and show that the progenitor cell enhancer shares functional elements with a previously described mammary gland-specific enhancer.

Synergistic action of GA-binding protein and glucocorticoid receptor in transcription from the mouse mammary tumor virus promoter

B lymphocytes are among the first cells to be infected by mouse mammary tumor virus (MMTV), and they play a crucial role in its life cycle. To study transcriptional regulation of MMTV in B cells, we have analyzed two areas of the long terminal repeat (LTR) next to the glucocorticoid receptor binding site, fp1 (at position -139 to -146 from the cap site) and fp2 (at -157 to -164). Both showed B-cell-specific protection in DNase I in vitro footprinting assays and contain binding sites for Ets transcription factors, a large family of proteins involved in cell proliferation and differentiation and oncogenic transformation. In gel retardation assays, fp1 and fp2 bound the heterodimeric Ets factor GA-binding protein (GABP) present in B-cell nuclear extracts, which was identified by various criteria: formation of dimers and tetramers, sensitivity to pro-oxidant conditions, inhibition of binding by specific antisera, and comigration of complexes with those formed by recombinant GABP. Mutations which prevented complex formation in vitro abolished glucocorticoid-stimulated transcription from an MMTV LTR linked to a reporter gene in transiently transfected B-cell lines, whereas they did not affect the basal level. Exogenously expressed GABP resulted in an increased level of hormone response of the LTR reporter plasmid and produced a synergistic effect with the coexpressed glucocorticoid receptor, indicating cooperation between the two. This is the first example of GABP cooperation with a steroid receptor, providing the opportunity for studying the integration of their intracellular signaling pathways.

Superantigen expression is driven by both mouse mammary tumor virus long terminal repeat-associated promoters in transgenic mice

In addition to the usual retroviral promoter, the mouse mammary tumor virus (MMTV) long terminal repeat carries a second promoter located in the U3 region. Here we show that both of these promoters are independently able to give rise to superantigen activity in transgenic mice. The ability of multiple MMTV promoters to drive superantigen expression underscores its importance in the virus life cycle.

Mouse mammary tumor virus carrying a bacterial supF gene has wild-type pathogenicity and enables rapid isolation of proviral integration sites

Mouse mammary tumor virus (MMTV) has frequently been used as an insertional mutagen to identify provirally activated mammary proto-oncogenes. To expedite and facilitate the process of cloning MMTV insertion sites, we have introduced a bacterial supF suppressor tRNA gene into the long terminal repeat (LTR) of MMTV, thus allowing selection of clones containing it in lambda vectors bearing amber mutations. The presence of supF in the LTR should circumvent the screening process for proviral insertion sites, since only those lambda clones with supF-containing proviral-cellular junction fragments should be able to form plaques on a lawn of wild-type Escherichia coli (i.e., lacking supF). The resulting virus (MMTVsupF) induced mammary tumors at the expected rate in infected mice, deleted the appropriate T-cell population by virtue of its superantigen gene, and stably retained the supF gene after passage via the milk to female offspring. To test the selective function of the system, size-selected DNA containing two proviral-cellular junction fragments from an MMTV supF-induced mammary tumor was ligated into lambdagtWES.lambdaB, packaged, and plated on a supF-deficient bacterial host for selection of supF-containing clones. All plaques tested contained the desired cloned fragments, thus demonstrating the utility of this modified provirus for the rapid cloning of MMTV insertion sites.