Expression of the protein product of the mouse mammary tumor virus long terminal repeat gene in phorbol ester-treated mouse T-cell-leukemia cells

Selection for c-myc integration sites in polyclonal T-cell lymphomas

Type B leukemogenic virus (TBLV) is highly related to mouse mammary tumor virus but induces rapidly appearing T-cell lymphomas in mice. Unlike other T-cell tumors induced by retroviruses, only 5 to 10% of TBLV-induced lymphomas have detectable viral integrations near c-myc by Southern blotting, whereas Northern blotting has shown that most tumors have two- to sixfold overexpression of c-myc RNA. In this report, PCR was used to demonstrate that at least 30% of these lymphomas have TBLV insertions near c-myc. Some tumors contained multiple TBLV proviruses in different locations and orientations, suggesting that the tumors are polyclonal. The integrated proviruses near c-myc had different numbers (two to four) of long terminal repeat (LTR) enhancer repeats, although LTRs with three-repeat enhancers dominated the proviral population. Passage of polyclonal tumors in immunocompetent mice and semiquantitative PCR revealed that only cells with particular integrations were selected for growth. In three of six tumors tested, proviruses containing four-repeat enhancers near c-myc were selected during tumor passage. Since tumor cell selection may be accomplished by overexpression of c-myc RNA due to proximity to the unique TBLV LTR enhancer, we inserted LTRs at various locations within a plasmid containing the entire c-myc locus and cellular flanking sequences. To quantitatively measure effects on transcription, the Renilla luciferase gene was substituted for most of c-myc exon 2, and transient transfections were performed with c-myc reporter constructs in two different T-cell lines. As expected, insertion of a TBLV LTR with three-repeat enhancers in either orientation, 5" and 3", of the myc gene elevated reporter activity from 2- to 160-fold, consistent with enhancer function, but four-repeat LTRs had lower levels of expression compared to three-repeat LTRs. Surprisingly, LTR insertions that gave maximal c-myc expression in transient-transfection assays declined in tumor cells selected for growth in vivo. Selection for clonal growth may occur in tumor cells that have modest c-myc overexpression after proviral insertion to prevent apoptosis.

Evidence for superantigen activity of the Bel 3 protein of the human foamy virus

The human foamy virus is a complex retrovirus that codes for several regulatory bel genes in addition to the conventional gag, pol, and env genes. The bel 3 gene is located in the 3' part of the viral genome comparable to that of the superantigen of the mouse mammary tumor virus. Superantigens bound to major histocompatibility complex (MHC) class II molecules have been shown to stimulate T cells in a V beta-specific manner. The recombinant Bel 3 protein purified to near homogeneity was assayed in vitro to determine whether or not it functions as a superantigen that stimulates human T lymphocytes expressing particular V beta T cell receptor (TCR) chains. Therefore, an analysis including all known human V beta elements was performed. The expression of different V beta chains of the TCR was analyzed by reverse transcription of the V beta RNAs and subsequent amplification of the corresponding V beta cDNA elements by polymerase chain reaction in unstimulated, phytohemaggluttinin (PHA)- and Bel 3-stimulated human T lymphocytes. In addition, eight monoclonal antibodies directed against particular V beta family members were used to determine any change in the expression of the remaining known V beta elements upon treatment with PHA and Bel 3. The comparative V beta-specific transcriptional analysis revealed that the in vitro expression of the V beta 18 chain was specifically and strongly expanded in Bel 3-stimulated human T cells, a property characteristic for a superantigen.

Polyclonal and monoclonal antibodies monospecific to MMTV LTR orf protein produced in E. coli

Monoclonal and polyclonal antibodies specific to an open reading frame of the mouse mammary tumor virus long terminal repeat were generated using an open reading frame-beta-galactosidase fusion protein produced in E. coli. Both antibodies reacted with the open reading frame-beta-galactosidase fusion protein but not with beta-galactosidase alone using an immunoblotting technique. It is concluded that these antibodies were specific for the protein encoded by the open reading frame of the mouse mammary tumor virus long terminal repeat.

An activation-dependent, T-lymphocyte-specific transcriptional activator in the mouse mammary tumor virus env gene

Transcription of the complete mouse mammary tumor virus (MMTV) proviral genome in mouse cells is controlled by a strong promoter in its long terminal repeat. In the mouse T lymphoma EL4, there is a second, activation-dependent transcriptional initiation site within the envelope (env) gene, from which a short mRNA is generated, encoding the open reading frame of the long terminal repeat. We now report the isolation of a segment of the MMTV env gene (called META, for MMTV env transcriptional activator) which has the expected transcription-activating properties seen in EL4.E1 cells. Namely, it induces activation-dependent, T-lymphocyte-specific transcription of a chloramphenicol acetyltransferase reporter gene. It is active in mouse or human T-helper lymphocyte lines when they are stimulated to transcribe lymphokine genes but is inactive in unstimulated T-helper cells, fibroblasts, a cytotoxic T-lymphocyte line, and a mastocytoma cell line. Its activity is inhibited by cyclosporin A, a specific inhibitor of lymphokine transcription. Several forms of the META have been isolated from EL4.E1 cells, a mouse T-helper cell hybridoma, and from BALB/c spleen cells. Linked to the heterologous thymidine kinase promoter, a 400-bp portion of it is an inducible, orientation-independent, and cyclosporin A-sensitive transcriptional activator in T-helper cells.

An activation-dependent, T-lymphocyte-specific transcriptional activator in the mouse mammary tumor virus env gene

Transcription of the complete mouse mammary tumor virus (MMTV) proviral genome in mouse cells is controlled by a strong promoter in its long terminal repeat. In the mouse T lymphoma EL4, there is a second, activation-dependent transcriptional initiation site within the envelope (env) gene, from which a short mRNA is generated, encoding the open reading frame of the long terminal repeat. We now report the isolation of a segment of the MMTV env gene (called META, for MMTV env transcriptional activator) which has the expected transcription-activating properties seen in EL4.E1 cells. Namely, it induces activation-dependent, T-lymphocyte-specific transcription of a chloramphenicol acetyltransferase reporter gene. It is active in mouse or human T-helper lymphocyte lines when they are stimulated to transcribe lymphokine genes but is inactive in unstimulated T-helper cells, fibroblasts, a cytotoxic T-lymphocyte line, and a mastocytoma cell line. Its activity is inhibited by cyclosporin A, a specific inhibitor of lymphokine transcription. Several forms of the META have been isolated from EL4.E1 cells, a mouse T-helper cell hybridoma, and from BALB/c spleen cells. Linked to the heterologous thymidine kinase promoter, a 400-bp portion of it is an inducible, orientation-independent, and cyclosporin A-sensitive transcriptional activator in T-helper cells.

Biochemical analysis of the mouse mammary tumor virus long terminal repeat product. Evidence for the molecular structure of an endogenous superantigen

Recent reports have shown that both exogenous and endogenous mouse mammary tumor viruses (MMTV) can encode superantigens. Transfection and transgenic studies have identified the open reading frame (ORF) present in the 3' long terminal repeat (LTR) as encoding superantigen function. In this study, we have used an in vitro translation system in an attempt to characterize the molecular nature of the protein encoded by the 3' ORF of Mtv-8. Using various constructs encoding full-length and truncated versions of the ORF product, we report that the hydrophobic region close to the amino terminus of the 36-kDa protein can function as a transmembrane domain. Protease digestion experiments also demonstrate that the protein has a type-II transmembrane conformation with an extra-cytoplasmic carboxy terminus. Since this hydrophobic region is conserved between all known MMTV, we speculate that LTR ORF, including those proposed to encode the minor lymphocyte stimulatory antigens, are also capable of encoding type-II transmembrane glycoproteins. The polymorphism between MMTV LTR ORF products, which correlates with deletion phenotypes, is predominantly in the carboxy-terminal extracellular region, consistent with a major role in interaction with the T cell receptor.

Expression of the mouse mammary tumor virus ORF gene in cultured cells

We have recently shown that expression vectors harboring the open reading frame of the long terminal repeat region of mouse mammary tumor virus direct the synthesis of a product which acts as a superantigen in transgenic mice. The detection of the ORF protein has been hampered by the extremely low levels of expression observed in these mice, as estimated from the low levels of specific mRNA. To study the properties of the ORF protein, we attempted its expression in different cell types in culture. The experiments performed in yeast show that the ORF gene product is a glycoprotein of approximately 45 kDA. As expected from the derived primary sequence, the unglycosylated product made in the presence of tunicamycin has a molecular weight of 36 kDA. No secretion of the glycosylated protein was observed. Curiously, the full-length molecule was made in lower amounts than a truncated version which contains only the C-terminal half of the protein. Transfection experiments in different mammalian cells suggest that high expression of the ORF protein might have an adverse effect on survival of cells in culture.

Detection and characterization of a glycoprotein encoded by the mouse mammary tumor virus long terminal repeat gene

Mouse mammary tumor virus (MMTV) is a retrovirus that causes mammary tumors in susceptible mice. MMTV contains a unique open reading frame (ORF) in the unique 3' region of the proviral long terminal repeat (LTR) with the potential to encode a 36-kDa protein. However, the ORF gene product has not been detected in murine mammary tissues or cell lines. We utilized the baculovirus expression vector system to generate large amounts of the ORF protein. Putative ORF gene products of 36 and 45 kDa were detected as unique proteins in extracts of insect cells infected with recombinant baculovirus (LTR-ORF BV), and the identities of these proteins as viral gene products were confirmed immunologically. Antipeptide antisera were generated in rabbits against peptides chosen from computer-predicted hydrophilic regions of the ORF coding sequence. These antisera reacted specifically by immunoprecipitation and by immunoblot with the proteins expressed in LTR-ORF BV-infected insect cells, as well as with MMTV LTR ORF in vitro translation products. Polyclonal antisera were raised against two putative ORF protein species partially purified from insect cells. These sera specifically immunoprecipitated viral protein products translated in vitro. In vitro translation of MMTV LTR ORF transcripts in the presence of canine pancreatic microsomal membranes generated a higher-molecular-weight ORF gene product, indicating that the ORF protein is modified by N-linked glycosylation. This glycosylated ORF product comigrated with the larger ORF protein species produced in infected insect cells. The gp45 product was metabolically labeled with [3H] mannose, [3H] galactose, and [3H] N-acetyl-D-glucosamine in insect cells, whereas this incorporation was inhibited in the presence of tunicamycin. Digestion of gp45 with endoglycosidase H yielded the lower-molecular-weight ORF protein p36. These observations suggest that the ORF glycoprotein contains hybrid N-linked oligosaccharides. Demonstration of the modified nature of the ORF gene product will facilitate characterization of ORF protein expression in murine tissues.

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.

Regulation of gene expression by tumor promoters

Tumor promoters change the program of genes expressed in cells in culture and in the multicellular organism. The growing list of genes that are induced or repressed includes protooncogenes, transcription factors, secreted proteases and viruses. Most of the regulation is at the level of transcription. Several of the cis-acting promoter elements mediating regulation, the transcription factors binding to these elements and their post-translational activation, as well as some of the initial steps of the interaction of cells with tumor promoters have been characterized. The components of the signal transduction chain to the nucleus are, however, still unknown. Mutant and inhibitor studies suggest that the activation or inactivation of certain genes constitute the basis for the development of the tumor promotion phenotype.

Phorbol ester-inducible T-cell-specific expression of variant mouse mammary tumor virus long terminal repeats

Acquired proviruses of mouse mammary tumor virus (MMTV) in T-cell leukemias of male GR mice have rearrangements in the U3 region of their long terminal repeats (LTR). In contrast to the endogenous nonrearranged MMTV proviruses, these mutated copies are highly expressed in leukemic T cells. To investigate whether the sequence alterations in the LTR are responsible for the high expression of rearranged MMTV proviruses, we made constructs in which normal and variant LTRs drive the bacterial reporter gene chloramphenicol acetyltransferase (CAT). Two different rearranged LTRs were used, one containing a 420-base-pair (bp) deletion (L13) and another carrying a 456-bp deletion plus an 82-bp insertion (L42). These constructs were transfected into murine (GRSL) and human (MOLT-4) T-cell lines that either had or had not been treated with phorbol ester (12-O-tetradecanoylphorbol-13-acetate [TPA]). In GRSL cells, the L13-LTR-CAT construct showed transcriptional activity that was further enhanced by TPA. In MOLT-4 cells, both variant LTRs were active, but only after stimulation with TPA. In contrast, normal(N)-LTR-CAT constructs were not expressed, irrespective of TPA addition. In XC rat fibrosarcoma cells, neither normal nor variant LTRs gave rise to detectable CAT activity, either in the presence or in the absence of TPA, but dexamethasone strongly stimulated CAT activity driven by N and L42 LTRs. The L13 LTR was considerably less active, probably caused by the deletion of the distal part of the glucocorticoid responsive element. We conclude that the LTR rearrangements generate TPA responsiveness and contribute to T-cell-specific expression of MMTV variants.

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

Extra proviral copies of mouse mammary tumor virus (MMTV) are known to be present in the genomes of certain T-cell lymphomas of mice. Analysis of additional non-mammary tumor cell types known to express MMTV transcripts and antigens revealed the presence of extra acquired MMTV proviruses in a pituitary tumor cell line, a macrophage line, and Leydig testicular tumor cells. The nature of the amplified MMTV proviruses in these various tumor cell types differed with regard to copy number and presence of alterations in the long terminal repeat region.

Phorbol diester-inducible, cyclosporine-suppressible transcription from a novel promoter within the mouse mammary tumor virus env gene

The mouse T-cell lymphoma cell line EL4.E1 constitutively synthesizes mouse mammary tumor virus (MMTV) transcripts encoding either the entire proviral genome or segments of it. In addition to these conventional mRNAs, however, an mRNA of about 1 kilobase accumulates after induction of these cells with phorbol myristate acetate (PMA). The accumulation of this transcript is strongly inhibited by the immunosuppressive agent cyclosporin A. Its pattern of induction by PMA and suppression by cyclosporin A is thus the same as seen for several lymphokine mRNAs in these cells, including interleukin-2 and granulocyte-macrophage colony-stimulating factor. The short MMTV transcript is the most abundant PMA-induced transcript in EL4.E1 cells, but was not found in a series of other leukocyte tumor cell lines. It is initiated from a novel promoter within the env gene, and a segment of 1,161 nucleotides is then spliced out. The major part of the transcript is a copy of the long terminal repeat (LTR) of MMTV. The MMTV proviral genomes in these cells, and the short transcript, contain a 491-nucleotide deletion in the LTR compared with the normal MMTV provirus. The resulting open reading frame could encode a protein of molecular weight 22,800, which is a likely candidate for an LTR-related protein with a similar molecular weight recently described in this system (J. Racevskis, J. Virol. 58:441-449, 1986).