Glucocorticoid regulation of the Ha-MuSV p21 gene conferred by sequences from mouse mammary tumor virus

Evidence for a prokaryotic promoter in the murine mammary tumor virus long terminal repeat

The long terminal repeat (LTR) of C3H murine mammary tumor virus (MuMTV) is approx. 1.3 kb long. HaeIII digestion of a cloned PstI fragment containing the left-end LTR generated four fragments of sizes 0.56, 0.41, 0.34 and 0.14 kb, one of which (0.41 kb) had a promoter activity in Escherichia coli. This was demonstrated by replacing the bacterial promoter for the neomycin-resistance (NmR) gene in the plasmid pKC56 with the HaeIII fragments. Only the 0.41-kb fragment that contains sequences from the U3 region of the LTR was found to contain a promoter, as shown by the expression of the drug-resistance phenotype in the recombinant plasmid. The strength of this promoter was comparable to or greater than that found with the parental NmR gene promoter. S1 nuclease mapping of the NmR gene transcript indicated that the initiation of this transcript occurred within the 0.41-kb LTR fragment from a site approx. 10 bp upstream from the 3' end. A comparison of the known DNA sequences in the MuMTV LTR with those found in bacterial promoters revealed that a 'Pribnow box', the initiation signal for the prokaryotic promoters, is present in the 0.41-kb LTR fragment upstream from the initiation site. Furthermore, in a recombinant plasmid that contained the complete LTR the same promoter sequences appeared to be involved in the initiation of RNA transcription. The 0.34-kb LTR fragment, which contains sequences derived from the U3 and U5 regions of the LTR, did not possess promoter activity in E. coli. However, it was found to induce deletions of adjacent plasmid DNA sequences. The deletions were specifically initiated from the downstream end of the LTR-fragment insert. The presence of a prokaryotic promoter in the MuMTV LTR, together with the observation that certain LTR sequences can induce deletions, analogous to those caused by transposable elements, in recombinant plasmids suggest that the MuMTV LTR may have evolved from such elements.

Transcription initiation of transfected mouse mammary tumor virus LTR DNA is regulated by glucocorticoid hormones

A chimeric gene, recombined in vitro, containing a long terminal repeat (LTR) sequence from the proviral DNA of mouse mammary tumor virus and the thymidine kinase (tk) gene of Herpes Simplex Virus was introduced into L tk- cells. No transcription of LTR RNA was observed in transfected cells when glucocorticoid hormones were absent from the growth medium. Accumulation of LTR initiated RNA was measured upon hormone addition by the single strand specific nuclease RNA mapping procedure. The accumulation was rapid (detectable after 7.5 minutes), independent of simultaneous protein synthesis and mediated by a functional glucocorticoid receptor complex. Glucocorticoid hormones affect LTR transcription at the level of initiation. The rate of initiation (1.8 X 10(-2) molecules/cell/sec) and a half life of about 30 minutes could be calculated for LTR RNA. The half life of LTR RNA is independent of the presence of hormone.

Glucocorticoids and chromosomal position modulate murine mammary tumor virus transcription by affecting efficiency of promoter utilization

The rate of transcription of murine mammary tumor virus (MTV) sequences in MTV-infected rat hepatoma tissue culture cells is strongly affected by both glucocorticoid hormones and the chromosomal position of provirus integration. We have characterized MTV RNAs produced in J2.17 and M1.54, independent isolates containing, respectively, 1 and 10 proviruses integrated at distinct chromosomal loci. M1.54, but not J2.17, synthesized MTV RNA in the absence of glucocorticoids; the rate of hormone-stimulated viral gene transcription in M1.54 was 50- to 100-fold higher than in J2.17. In each case in which MTV genes were expressed (J2.17 induced, M1.54 basal and induced), the viral RNAs produced were indistinguishable. RNA blotting revealed accumulation of two transcripts, 7.8 and 3.8 kilobases; the latter was likely produced from the former by RNA splicing. Sites used for transcription initiation, polyadenylation, and splicing have been identified from the sizes of end-labeled hybridization probes protected from digestion with mung bean nuclease; the unique initiation and polyadenylation sites were both encoded within the MTV long-terminal-repeat sequence. The efficiencies of splicing and of utilization of the polyadenylation signal did not appear to vary as functions of chromosomal position or hormonal stimulation. Differences in rates of viral gene transcription were reflected in the differential accumulation of the 5'-terminal 136 nucleotides of MTV RNA. Thus, glucocorticoids and chromosomal position appeared to affect solely the efficiency of utilization of the MTV promoter, leaving unchanged the sites of initiation, splicing, and polyadenylation, as well as the efficiencies of the latter two processes.

DNA sequences bound specifically by glucocorticoid receptor in vitro render a heterologous promoter hormone responsive in vivo

Glucocorticoids stimulate transcriptional initiation within integrated mammary tumor virus (MTV) DNA sequences in infected cells. We report here that production of herpes simplex virus thymidine kinase (tk) RNA is stimulated as much as 50-fold by the glucocorticoid, dexamethasone, when sequences from a particular region of MTV DNA are fused upstream of the normally constitutive tk promoter region. Three cloned fragments of MTV DNA were tested, each itself lacking the sequences required for transcription initiation. We monitored the effects of dexamethasone on the efficiency with which these recombinants transfect a tk- rat cell line to a tk+ phenotype, and measured tk enzymatic activity, the size and abundance of tk mRNA, and the 5' termini of tk transcripts in the transfectants. We conclude that the MTV promoter region contains a "glucocorticoid response element" that can be separated from a second element essential for MTV transcription initiation. The hormone response element maps within a 340-base pair MTV DNA fragment that contains specific binding sites for purified glucocorticoid receptor protein in vitro, implying that receptor binding at these sites in vivo may mediate hormone responsiveness. Comparison of several different constructions indicates that the location and orientation of the glucocorticoid response element relative to the transcription start site is not rigidly constrained.

Subfragments of the large terminal repeat cause glucocorticoid-responsive expression of mouse mammary tumor virus and of an adjacent gene

After transfection of mouse mammary tumor virus (MMTV) proviral DNA into cultured cells, the DNA is transcribed in a glucocorticoid-sensitive fashion. The large terminal repeat (LTR) region of MMTV is 1,328 nucleotides long and contains the regulatory information necessary for the hormonal response. We have constructed a MMTV LTR-thymidine kinase (tk) chimeric gene and have tested the biological activity of molecules containing various deletions in the LTR after transformation of LTK- APRT- mouse cells. In the TK+ transformants, both a LTR- tk chimeric RNA and an authentic tk RNA are correctly initiated and transcribed. The synthesis of the chimeric RNA as well as that of the tk RNA is hormonally regulated. A plasmid containing 202 nucleotides of LTR DNA 5' to the RNA initiation site is fully sensitive to glucocorticoids; 50 nucleotides still cause a residual inducibility.

Regulation of endogenous murine mammary tumor virus expression in C57BL mouse lactating mammary glands: transcription of functional mRNA with a block at the translational level

Expression of endogenous murine mammary tumor viruses (MuMTVs) in various mouse strains is regulated in different ways, and in the absence of exogenous MuMTV, this regulation influences the incidence of spontaneous mammary tumors. Two mouse strains with low mammary tumor incidence, BALB/c and C57BL, control endogenous MuMTV expression at different stages. Neither of the strains had any detectable MuMTV polypeptides in its lactating mammary glands (LMG). However, in C57BL LMG, substantial amounts of MuMTV RNA were present, whereas very little viral RNA was detected in BALB/c LMG. By determining MuMTV RNA levels in LMG of hybrids and backcrosses of BALB/c and C57BL mice, we found that there are three unlinked, independently segregating genetic loci in C57BL mice that are responsible for the presence of moderately high amounts of MuMTV RNA in LMG. The viral RNA in C57BL LMG was processed and transported to the cytoplasm where it was found to cosediment with EDTA-sensitive polysomes. No viral proteins were detected in run-off reactions that permit completion of nascent polypeptide synthesis with polysomes from C57BL LMG, and sensitive radioimmunoassays failed to detect any MuMTV proteins in these tissues. In contrast, MuMTV mRNA purified from C57BL LMG did direct the synthesis of both gag and env MuMTV polypeptides when added to a heterologous rabbit reticulocyte lysate cell-free translation system. We propose that MuMTV mRNA in C57BL LMG, for unknown reasons, is blocked at the translational level.

Glucocorticoids and chromosomal position modulate murine mammary tumor virus transcription by affecting efficiency of promoter utilization

The rate of transcription of murine mammary tumor virus (MTV) sequences in MTV-infected rat hepatoma tissue culture cells is strongly affected by both glucocorticoid hormones and the chromosomal position of provirus integration. We have characterized MTV RNAs produced in J2.17 and M1.54, independent isolates containing, respectively, 1 and 10 proviruses integrated at distinct chromosomal loci. M1.54, but not J2.17, synthesized MTV RNA in the absence of glucocorticoids; the rate of hormone-stimulated viral gene transcription in M1.54 was 50- to 100-fold higher than in J2.17. In each case in which MTV genes were expressed (J2.17 induced, M1.54 basal and induced), the viral RNAs produced were indistinguishable. RNA blotting revealed accumulation of two transcripts, 7.8 and 3.8 kilobases; the latter was likely produced from the former by RNA splicing. Sites used for transcription initiation, polyadenylation, and splicing have been identified from the sizes of end-labeled hybridization probes protected from digestion with mung bean nuclease; the unique initiation and polyadenylation sites were both encoded within the MTV long-terminal-repeat sequence. The efficiencies of splicing and of utilization of the polyadenylation signal did not appear to vary as functions of chromosomal position or hormonal stimulation. Differences in rates of viral gene transcription were reflected in the differential accumulation of the 5'-terminal 136 nucleotides of MTV RNA. Thus, glucocorticoids and chromosomal position appeared to affect solely the efficiency of utilization of the MTV promoter, leaving unchanged the sites of initiation, splicing, and polyadenylation, as well as the efficiencies of the latter two processes.

Transcription of mouse mammary tumor virus: identification of a candidate mRNA for the long terminal repeat gene product

We have examined an assortment of preneoplastic and neoplastic mouse mammary tissues for the presence of an mRNA which could encode the putative long terminal repeat gene product of mouse mammary tumor virus. We report here the detection of a novel mouse mammary tumor virus-specific, polyadenylic acid-containing transcript in certain preneoplastic and neoplastic mammary tissue of BALB/c mice. The molecule is 1.6 kilobases in length and contains sequences from the transcriptional leader and the U3 region of the proviral DNA. The upstream terminus of the 3' information lies 75 to 80 nucleotides from the beginning of the long terminal repeat open reading frame, in close proximity to a consensus splice acceptor in the DNA. The transcript was detected in hormonally or chemically induced neoplastic, preneoplastic, and lactating mammary tissue of BALB/c mice, but not in preneoplastic or tumor tissue induced by exogenous viruses in any strain of mice examined. This implies that the RNA we observed is transcribed from an endogenous provirus template.

Androgen-regulated expression of a cloned rat prostatic c3 gene transfected into mouse mammary tumor cells

The expression of the rat prostatic steroid binding protein gene C3 is androgen-responsive in vivo in the rat prostate. Recombinant transfection vectors were constructed containing a cloned C3 rat gene ligated directly into the dominant acting selection vector pSV2-gpt, and were used to transfect androgen-responsive mouse mammary tumor cells (S115 cells). Transformants containing intact copies of the C3 gene were isolated and shown by primer extension analysis to produce C3 transcripts that had authentic 5' termini. The steady state level of the transcripts was under androgen control in three cell lines tested which had low copy numbers of the intact C3 gene, but was not regulated in a cell line which had a very high C3 gene copy number. Despite this difference in the androgenic regulation of integrated C3 rat genes, both type of cell lines still demonstrated androgen-regulated growth characteristics.

Cloned mouse mammary tumor virus DNA exhibits glucocorticoid-dependent expression in simian virus 40-transformed mink cells

Mink lung epithelial cells were transfected with two cloned mouse mammary tumor virus (MMTV) DNAs, a 9-kilobase clone derived from an unintegrated exogenous viral genome and a 14-kilobase clone containing an integrated endogenous provirus along with cellular flanking sequences. Mink lung cells were chosen because they do not contain endogenous MMTV sequences. On the basis of our observation that simian virus 40 DNA efficiently transforms these cells, we isolated cell clones containing MMTV DNA by using transformation with simian virus 40 DNA as a selective marker in cotransfection experiments. Levels of the 9-kilobase MMTV mRNA representing the entire viral genome and of the spliced 4.4-kilobase mRNA which codes for the viral envelope proteins were glucocorticoid dependent in transformed cells. Expression of low levels of Pr77gag, the precursor of the group-specific viral core proteins, and of gPr73env, the precursor of the viral envelope proteins, was also hormone dependent. We conclude that these cloned MMTV DNAs contain all the information necessary for the synthesis of normal viral RNAs and proteins. These findings also provide further evidence that the DNA sequences involved in the hormone responsiveness of MMTV expression are contained within the viral genome.

A comparison of nucleoside (beta-S)triphosphates and nucleoside (gamma-S)triphosphates as suitable substrates for measuring transcription initiation in preparations of cell nuclei

RNA chains initiated with nucleoside (beta-S)triphosphates and (gamma-S)triphosphates retain the thiol groups and can be separated from thiol-free RNA by chromatography on mercury-Sepharose. Thiol-containing mouse mammary tumor virus (MMTV) RNA synthesized by preparations of nuclei from virus-infected cells was quantitated by nucleic acid filter hybridization. With ATP beta S and GTP beta S, region-specific initiation of MMTV RNA chains was detected in the cell free system. However, with ATP gamma S and GTP gamma S, region-specific initiation was not clearly demonstrable. The nuclear preparations can also transfer thiol groups, presumably in the form of thiophosphate, from ATP gamma S or GTP gamma S onto preexisting RNA molecules; little or no thiol-transfer occurs with the two (beta-S)-analogues. The thiophosphate transfer activity apparently interferes with the measurement of RNA chain initiation with ATP gamma S and GTP gamma S.

Specific binding of the glucocorticoid-receptor complex to the mouse mammary tumor proviral promoter region

To elucidate the molecular mechanism by which steroid hormones exert their regulatory function, we investigated the interaction of a glucocorticoid-receptor complex with purified DNA fragments from cloned mouse mammary tumor (MMTV) proviral DNA. With a DNA-cellulose binding assay, rat and mouse glucocorticoid receptors were found to interact with a high affinity site or sites in or near the promoter region of the MMTV proviral DNA. The assay allowed the use of unpurified as well as purified receptor, and therefore made it possible to investigate the binding properties of mutant receptors. Two nuclear-transfer-deficient receptors have a decreased affinity for specific as well as unspecific DNA, but are still capable of distinguishing between the two types of DNA. The existence of specific DNA binding sites for steroid-receptor complexes is discussed in the context of a general model of steroid hormone action.

Glucocorticoid and thyroid hormones transcriptionally regulate growth hormone gene expression

In order to define the molecular mechanisms by which glucocorticoids and thyroid hormone act to regulate growth hormone gene expression, the sites at which they exert their effects on growth hormone biosynthesis were examined in vivo and in a pituitary cell line. Glucocorticoids were shown to rapidly increase accumulation of growth hormone mRNA and nuclear RNA precursors. Glucocorticoids and thyroid hormone were shown to rapidly and independently increase growth hormone gene transcription. These events are shown to occur physiologically in animals and further establish the importance of growth hormone gene expression as a model for steroid regulation.

Polyadenylylated RNA complementary to a mouse retrovirus-like multigene family is rapidly and specifically induced by epidermal growth factor stimulation of quiescent cells

Complementary DNA probes prepared from total polysomal poly(A)+RNA populations were used to identify clones of mouse DNA containing sequences whose expression is specifically enhanced after epidermal growth factor (EGF) stimulation of quiescent mouse embryo cells in culture. Three such clones were isolated and used to study changes in the levels of clone-specific poly(A)+RNA in the polysomes of cells after mitogenic stimulation by EGF. RNA complementary to sequences present in these clones increased approximately equal to 10-fold as a fraction of the total poly(A)+RNA by 6 hr after stimulation. All three clones were found by hybridization criteria to contain sequences related to the class of mouse retrovirus or transposon-like elements termed VL30. These VL30-related sequences were further found to be complementary to EGF-inducible poly(A)+RNAs and enhanced expression was detectable as early as 1 hr after EGF stimulation. In contrast, nine additional clones, including an AKR-type murine leukemia provirus DNA clone, contained no detectable VL30 sequence elements and were complementary to poly(A)+RNA species whose relative concentration was essentially constant in quiescent and EGF-stimulated cells. Therefore, VL30 sequence elements appear distinct in that they encompass members whose expression is specifically regulated in response to a defined peptide growth factor.

Construction of a mammalian transducing vector from the genome of Moloney murine leukemia virus

A 0.9-kilobase DNA fragment from the genome of Moloney murine leukemia virus, including the viral long terminal repeat, was covalently linked to the herpes simplex virus I thymidine kinase (tk) gene whose promoter was previously removed. The hybrid DNA structure was introduced into the chromosome of tk- mouse cells at single copy numbers, via transfection procedures. Cells expressing the newly introduced tk gene were identified by the HAT selection procedure and analyzed for tk- and moloney murine leukemia virus-specific DNA and RNA sequences by blot hybridization procedures. Expression of the tk gene is dependent on function(s) provided in cis by the viral DNA fragment. Vectors derived from this region are termed rGag (rG) vectors.

Host-specific activation of transcription by tandem repeats from simian virus 40 and Moloney murine sarcoma virus

The simian virus (SV40) 72-base pair (bp) tandem repeated sequences have recently been shown to function as activators or enhancers of early viral transcription. A recombinant viral genome was recently constructed by inserting 72-bp tandem repeats from the Moloney murine sarcoma virus (MSV) in place of the 72-bp repeats of SV40. Although this genome replicates in monkey kidney cells, its rate of large tumor antigen expression and replication is considerably slower than that of wild-type SV40. In mouse cells, however, equivalent levels of large tumor antigen appear to be expressed from both wild-type and recombinant genomes, suggesting a relationship between the level of enhancer activity and the host cell. To confirm this observation, we have applied a sensitive quantitative assay for gene expression based on the conversion of chloramphenicol to its acetylated forms. The gene encoding the enzymatic function chloramphenicol acetyltransferase was inserted into two vectors in which the enhancer sequences from SV40 or MSV were placed adjacent to the early SV40 promoter. The SV40 tandem repeats appear to activate gene expression to significantly higher levels in monkey kidney cells, but the MSV repeats are more active in two lines of mouse cells. These findings suggest that the tandem repeat elements may interact with host-specific molecules and, furthermore, may constitute one of the elements determining the host range of these eukaryotic viruses.

Monoclonal antibodies to the p21 products of the transforming gene of Harvey murine sarcoma virus and of the cellular ras gene family

We have isolated eight rat lymphocyte-myeloma hybrid cell lines producing monoclonal antibodies that react with the 21,000-dalton transforming protein (p21) encoded by the v-ras gene of Harvey murine sarcoma virus (Ha-MuSV). These antibodies specifically immunoprecipitate both phosphorylated and non-phosphorylated forms of p21 from lysates of cells transformed by Ha-MuSV. All eight react with the products of closely related ras genes expressed in cells transformed by two additional sarcoma viruses (rat sarcoma virus and BALB sarcoma virus) or by a cellular Harvey-ras gene placed under the control of a viral promoter. Three of the antibodies also react strongly with the p21 encoded by the v-ras gene of Kirsten MuSV. These same three antibodies immunoprecipitate the predominant p21 species synthesized normally in a variety of rodent cell lines, including the p21 produced at high levels in 416B murine hemopoietic cells. This suggests that an endogenous gene closely related to Kirsten-ras is expressed in these cells. The monoclonal antibodies have been used to confirm two properties associated with p21; localization at the inner surface of the membrane of Ha-MuSV-transformed cells, assayed by immunofluorescence microscopy, and binding of guanine nucleotides.

Regulated expression of human growth hormone genes in mouse cells

We have asked whether there are sequences around the human growth hormone gene that render this gene responsive to induction by glucocorticoid hormones. Recombinant clones encoding human growth hormone were introduced into the chromosome of murine fibroblasts by cotransformation. Exposure of cotransformants to glucocorticoids results in a three to five fold induction of human growth hormone mRNA and a similar induction in secreted human growth hormone protein. The DNA sequences required for induction reside within 500 nucleotides of 5'-flanking DNA. Fusion of this segment of 5'-flanking DNA to the structural gene sequences of a hormone-insensitive gene, such as thymidine kinase, now renders this gene responsive to glucocorticoid induction.

Construction and expression in vivo of an internally deleted mouse alpha-fetoprotein gene: presence of a transcribed Alu-like repeat within the first intervening sequence

An alpha-fetoprotein (AFP) 'minigene' was constructed by joining the first two exons along with 0.9 kilobases of 5' flanking sequence of the mouse AFP gene to its last exon and 0.4 kilobases of 3' flanking sequence. This 'minigene' was tested for activity by inserting it into an SV40 vector and infecting African green monkey kidney cells. Correct initiation, termination, polyadenylation and splicing were observed. Furthermore a 220 nucleotide transcript was detected and mapped to a mouse Alu-like or B1 repeat on the opposite strand to that encoding the AFP gene in the first intervening sequence.

Form and function of retroviral proviruses

Retroviruses have proved to be useful reagents for studying genetic and epigenetic (such as regulatory) changes in eukaryotic cells, for assessing functional and structural relationships between transposable genetic elements, for inducing insertional mutations, including some important in oncogenesis, and for transporting genes into eukaryotic cells, either after natural transduction of putative cellular oncogenes or after experimental construction of recombinant viruses. Many of these properties of retroviruses depend on their capacity to establish a DNA (proviral) form of their RNA genomes as a stable component of host chromosomes, in either somatic or germinal cells.

Multiple specific binding sites for purified glucocorticoid receptors on mammary tumor virus DNA

Glucocorticoid hormones selectively stimulate the rate of transcription of integrated mammary tumor virus (MTV) sequences in infected rat hepatoma cells. Using two independent assays, we find that purified rat liver glucocorticoid receptor protein binds specifically to at least four widely separated regions on pure MTV proviral DNA. One of these specific binding domains, which itself contains at least two distinct receptor binding sites, resides within a fragment of viral DNA that maps 110-449 bp upstream of the promoter for MTV RNA synthesis. Three other binding domains lie downstream of the promoter and within the MTV primary transcription unit. Restriction fragments bearing separate binding domains have been introduced into cultured cells; transformants have been recovered in which the introduced fragments are expressed under glucocorticoid control. Thus, it appears that this assay will be useful for assessing the biological significance of the receptor binding sites detected in vitro.

Delimitation of a DNA sequence which confers inducibility by glucocorticoid hormones

A chimeric long terminal repeat-thymidine kinase (LTR-tk) gene has been used to define the sequence requirements for glucocorticoid induction of gene expression. The original LTR-tk gene contains an entire mouse mammary tumor virus (MMTV) LTR preceding the tk gene. This gene can be expressed in a hormone-responsive fashion upon transfection into L tk--cells to produce a chimeric LTR-tk mRNA. Stepwise deletion of nucleotide sequences 5' of the viral RNA initiation site revealed that 202 nucleotides upstream of the viral cap site are sufficient for the hormonal regulation. Deletion of 5' sequences up to 59 nucleotides upstream of the viral cap site abolished RNA initiation in the LTR and hormonal induction.

Genetic variation in rates of antipyrine metabolite formation: a study in uninduced twins

Adult, male, unmedicated twins received antipyrine orally under carefully controlled environmental conditions. Relative contributions of genetic and environmental factors to 2-fold interindividual variations in rate constants for formation of the three main antipyrine metabolites were compared. Heritabilities for rate constants for formation of 4-hydroxyantipyrine, N-demethylantipyrine, and 3-hydroxymethylantipyrine were 0.88, 0.85, and 0.70, respectively. These results suggest that each molecular form of cytochrome P-450 that converts antipyrine to a different metabolite exhibits genetically controlled interindividual variations in activity. Unrelated adult male subjects whose environments were also carefully controlled exhibited highly reproducible rate constants for formation of antipyrine metabolites. Because the rate constant for metabolite formation sensitively detects certain variations in the gene product, it should be used in future pharmacogenetic studies on rates of production of multiple metabolites from a single parent drug.