A new locus (Mtv-4) for endogenous mammary tumor virus expression and early mammary tumor development in the SHN mouse strain

The SHN mouse strain, which has a high incidence of mammary cancer, developed by inbreeding and selection from Swiss stock mice by Dr. H. Nagasawa and co-workers (Meiji University, Tokyo, Japan), harbored an endogenous mammary tumor virus (MTV) responsible for a high frequency of mammary tumors early in life. The locus was called "Mtv-4" and was only comparable with Mtv-2 of the GR mouse strain in its inducing capacity of mammary cancer. Molecular hybridization with 32P-labeled MTV complementary DNA showed that the characteristic Mtv-2 bands of the GR strain were absent in the SHN strain.

Acquisition of proviral DNA of mouse mammary tumor virus in thymic leukemia cells from GR mice

Male mice of strain GR develop T-cell leukemia at a low frequency late in life. These leukemia cells contain large amounts of mouse mammary tumor virus (MMTV) RNA and MMTV proteins in a precursor form (Nusse et al., J. Virol. 32:251-258, 1979). We used restriction enzyme analysis and molecular hybridization to identify MMTV proviruses in the DNA of these leukemia cells. GR leukemia cells contained additional integrated MMTV proviruses at various sites in the genome. This amplification of MMTV proviruses in GR leukemia cells is not restricted to one particular endogenous MMTV provirus of strain GR. The number and location of the extra MMTV proviruses present in transplants of GR leukemia cells did not change upon serial transplantation of the leukemia cells. Acquisition of MMTV proviruses was also found in a similar leukemia, L1210 of the DBA/2 mouse strain, but not in three other leukemias, SL2 of DBA/2, BW5147 of AKR, and a spontaneous thymoma of BALB/c. The two main classes of MMTV RNA, 35S and 24S, were present in the cytoplasmic RNA of GR leukemia cells, indicating that the aberrant processing of MMTV precursor proteins is not due to anomolously sized RNAs. We could not detect extra RNAs in GR leukemia cells which would represent read-through transcripts of cellular genes adjacent to the extra MMTV proviruses, initiated by a promoter signal in the right MMTV long terminal repeat sequence. These data suggest that acquisition of MMTV proviruses may coincide with the onset of leukemogenesis in GR male mice.

Mammary tumor virus DNA as a marker for genotypic variance within hormone-responsive GR mouse mammary tumors

Mammary tumors of GR mice acquire extra mammary tumor virus (MMTV) DNA information within their DNA during tumor growth and development. These extra MMTV genes have been used by us as genotypic markers to investigate the heterogeneity of GR mammary tumors and their loss of hormone dependence during serial transplantation. Our studies reveal that the various subpopulations of cells within individual GR mammary tumors are characterized by differences in number and location of acquired extra MMTV DNA fragments. Losses of certain of these extra MMTV DNA fragments occur when mammary tumors become hormone independent, indicating a loss of hormone-dependent cells. The study of MMTV DNA markers also reveals that low levels of autonomous cells are already present in some hormone-dependent mammary tumors at an early stage of their development. The genotypic analysis strongly indicates that mammary tumor progression is not due to phenotypic adaptation but to clonal selection of the more aggressive sublines.

Structure, stability, methylation, expression and glucocorticoid induction of endogenous and transfected proviral genes of mouse mammary tumor virus in mouse fibroblasts

A sequence of mouse genomic DNA containing an endogenous mouse mammary tumor virus (MMTV) provirus, which was isolated by molecular cloning in lambda vector phage, has been reintroduced into cultured mouse L cells, using the thymidine-kinase cotransfection technique. Individual cell clones that acquired the transfected MMTV proviral DNA have been isolated. The transfected DNA remains stable in these cells and does not become methylated. It is transcribed into MMTV RNA, And the transcription is stimulated by glucocorticoid hormones.

Mammary tumor virus proviral DNA in normal murine tissue and non-virally induced mammary tumors

The Southern DNA filter transfer technique was used to study the involvement of the endogenous mouse mammary tumor virus (MMTV) in the development of mammary tumors of nonviral etiology. The presence of extra MMTV proviruses in the genomes of these non-virally induced mammary tumors would indicate an integration of the provirus of an activated endogenous MMTV. Acquisition of MMTV proviruses did not seem to be an absolute requirement for the development of hormone or carcinogenically induced mammary tumors in strain BALB/c nor for hormone-induced mammary tumors in mouse strains 020, C57BL, and C3Hf. In some hormone-induced mammary tumors we did observe extra MMTV proviruses in submolar quantities, indicating that reintegration may occasionally occur and that only a part of the tumor cells acquired new MMTV DNA information. Hormone-dependent and -independent primary mammary tumors of the mouse strain GR, which are controlled by the Mtv-2 mammary tumor induction gene, all acquired extra MMTV proviruses. Most of these extra MMTV proviral-DNA-containing fragments appeared present in submolar quantities, suggesting that only part of the tumor cells acquired extra MMTV proviral information. These findings indicate that the initially transformed mammary gland cells of non-virally induced mammary tumors do not necessarily acquire extra MMTV proviral DNA information, in contrast to the MMTV-induced mammary tumors, in which all tumor cells contain extra MMTV DNA information.

Mammary tumor induction loci in GR and DBAf mice contain one provirus of the mouse mammary tumor virus

The mammary tumor induction genes Mtv-1 in mouse strain DBAf and Mtv-2 in strain GR control the complete expression of the endogenous mouse mammary tumor virus (MMTV). We have used a combination of genetic, biochemical and molecular biological methods to identify and correlate specific copies of the endogenous MMTV proviral genes with the biological properties of the tumor induction genes Mtv-1 and Mtv-2. These Mtv induction genes contain specific MMTV proviral information, as was concluded from restriction enzyme analysis and molecular hybridization of DNAs of congenic mouse strains and of progenitors of backcross populations. The congenic strains differed from the parental strains GR and 020 only in the Mtv-2 gene, one lacking the Mtv-2 gene (GR/Mtv-2-) and one having obtained this gene (020/Mtv-2+). The gain or loss coincided with two Eco RI cellular DNA fragments containing MMTV DNA information. Since Eco RI cuts the exogenous proviral variant of MMTV DNA once, we assume that these two cellular DNA fragments contain one MMTV provirus. The same cellular DNA fragments containing MMTV DNA information segregated together with MMTV expression in the offspring population of the backcross. In a similar backcross analysis of the induction gene Mtv-1 it was also demonstrated that the Mtv-1 gene comprises one MMTV provirus. These data indicate that Mtv induction genes contain specific but different MMTV proviral genes and that nly a limited number of the MMTV proviruses present in the cellular DNA is associated with the control of proviral expression.

Analysis of secondary modifications of mouse mammary tumor virus proteins by two-dimensional gel electrophoresis

The structural proteins of mouse mammary tumor virus (MMTV) were analyzed by two-dimensional electrophoresis on isoelectric focusing and sodium dodecyl sulfate gels. Many of the viral proteins displayed heterogeneity in charge due to variable contents of carbohydrates (in particular, sialic acid) and phosphate residues. Neuraminidase treatment of the virions influenced the isoelectric pattern of the envelope glycoproteins. The glycoproteins of an MMTV variant which was attenuated by replication in feline kidney cells had different isoelectric points. This suggested that the acquisition of an altered carbohydrate configuration had changed the host range of the virus. The major MMTV structural core protein, p27, consisted of two species, which had identical iodinated tryptic peptide compositions but differed in phosphate contents. Another MMTV phosphoprotein, p21, was separated into four different phosphorylated species. Phosphorylation of p21 could be performed in vitro by the MMTV virion-associated protein kinase. This enzyme also has a high affinity for MMTV p30 as a substrate. Possible functions of this enzyme are discussed.

Quantification of mouse mammary tumor virus structural proteins in hormone-induced mammary tumors of low mammary tumor mouse strains

The expression of the mouse mammary tumor virus (MMTV) in hormone-induced mammary tumors was investigated by means of a radioimmunoassay for two major MMTV proteins, gp52 and p27. MMTV proteins were isolated on lectin affinity- and ion-exchange chromatography columns. The purified viral proteins were electrophoretically homogeneous and retained immunoreactivity after labelling with 125iodine. Standard competition assays showed that group-specific antigenic determinants were reacting. Mammary tumors were induced in three strains of mice with a low natural incidence of mammary tumors, C57BL, O20 and C3Hf, by a combined hormone treatment, consisting of hypophyseal isografts and administration of progesterone and estrone. Mammary tumors and mammary glands of hormone-treated animals were extracted and used for competition radioimmunoassays. In general, the tumorigenic hormone treatment resulted in enhanced amounts of MMTV proteins in the mammary glands, compared to the amounts found in lactating mammary glands of untreated animals. The levels of MMTV proteins in the mammary tumors were lower than in the mammary glands.

Impaired maturation of mouse mammary tumor virus precursor polypeptides in lymphoid leukemia cells, producing intracytoplasmic A particles and no extracellular B-type virions

Processing of polypeptides of the mouse mammary tumor virus, a type B retrovirus, was investigated in a transplanted thymic lymphoma cell line of the GR strain (GRSL). This cell line was maintained in vivo in ascites form and in vitro as a suspension culture. GRSL cells produce clusters of intracytoplasmic A particles and are virtually deficient in the production of mature extracellular B-type particles. As control, a mammary tumor cell line of the same mouse strain capable of complete virion synthesis was used. The kinetics of viral polypeptide synthesis were studied by pulse labeling with various isotopes (including (35)S and (32)P), followed by immunoprecipitation of cell lysates with monospecific antisera to the major mouse mammary tumor virus gag and env proteins, p27 and gp52, respectively. Both the primary gag and env precursor polypeptides were synthesized in the GRSL cells, but their conversion into viral proteins was impaired. The major gag precursor, Pr73(gag), was stable over a period of 8 h, and mature viral core polypeptides could not be detected. Also, the highly phosphorylated intermediates in the proteolytic processing of Pr73(gag) in virus-producing cells were absent in GRSL cells. By immunoprecipitation, Pr73(gag) was detected in a GRSL particle fraction with the density of intracytoplasmic A particles. The precursor for envelope proteins, Pr73(env), was turned over without the generation of mature viral envelope components gp52 and gp36. The in vivo-transplanted ascites GRSL cells, however, were shown to express gp52 on the cell surface together with a 73,000-dalton polypeptide, as indicated by cell surface iodination and immunoprecipitation.

Induction of mouse mammary tumor virus RNA in mammary tumors of BALB/c mice treated with urethane, X-irradiation, and hormones

The involvement of mouse mammary tumor virus (MTV) in the development of mammary tumors of nonviral etiology in BALB/c mice was studied by measuring the levels of MTV RNA, MTV DNA, and MTV proteins in spontaneously arising and hormonally, chemically, and/or physically induced mammary tumors of BALB/c females. The following results were obtained. (i) Spontaneous mammary tumors contained very low levels of MTV RNA; 4 X 10(-6)% of the the cytoplasmic RNA was MTV RNA. No MTV proteins could be demonstrated by using sensitive radioimmunoassays for MTV proteins p27 and gp52. (ii) Mammary tumors induced by treatments with urethane or X-irradiation alone contained higher levels of MTV RNA; these tumors contained 3- and 19-fold more MTV RNA, respectively, compared with spontaneous mammary tumors. (iii) Mammary tumors induced by combined treatment with urethane and X-irradiation expressed high levels of MTV RNA in the mammary tumors; a 1,724-fold increase in MTV RNA content compared with spontaneous mammary tumors was observed. However, very low levels of MTV proteins gp52 and p27 were detected, suggesting some kind of impairment at the translation of the MTV RNA. MTV RNA was also induced by this treatment in mammary glands and spleens, but not in the livers of tumor-bearing animals. (iv) Balb/c females continuously exposed to prolactin contained high levels of MTV RNA and MTV proteins in stimulated mammary glands and in the hormonally induced mammary tumors. These findings suggest that MTV is not responsible for the maintenance and probably also not for the development of all murine mammary cancers.

Autogenous antibodies against the murine mammary tumor virus in strains of mice with low incidences of mammary tumors

The radioimmunoprecipitation assay for the murine mammary tumor virus (MuMTV) was used to detect naturally occurring antibodies against MuMTV in 3 groups of highly inbred mouse strains. 1) Some strains had high incidences of mammary tumors, such as strains GR and C3H. Antibodies against MuMTV were detected in the sera of females of these strains at early ages. 2) Some mouse strains had low incidences of mammary tumors with an intermediate MuMTV expression, such as strains C3Hf, RIIIf, and BALB/c. Some females of these strains developed antibodies against MuMTV. Hormone treatment of these mice resulted in an increase in the proportion of mice carrying antibodies against MuMTV. 3) Some mouse strains were MuMTV-free, such as strains O20, C57BL, and Gr-Mtv2-. No antibodies against MuMTV were detected in the sera of these mice. However, antibodies against MuMTV appeared in the sera of these animals after hormone treatment. The presence of a natural humoral immunity toward MuMTV appeared to be related to the expression of MuMTV in the animals.

Involvement of mouse mammary tumor virus in spontaneous and hormone-induced mammary tumors in low-mammary-tumor mouse strains

The involvement of the mouse mammary tumor virus (MTV) in spontaneous and hormone-induced mammary tumors in low-mammary-tumor mouse strains was studied by comparing the amounts of MTV RNA and MTV DNA sequences in mammary tumors and other tissues of mice with an without hormonal treatments. The following results were obtained. (i) Mammary tumors which appeared in C3H mice as a result of an infection with MTV contained more MTV DNA compared with noninfected organs; these mammary tumors also contained more MTV RNA than was present in lactating mammary gland cells. (ii) Hormonal stimulation by administration of excessive amounts of prolactin via hypophyseal isografts in C3Hf and O20 mice resulted in an increased expression of MTV RNA in the mammary glands. This elevated level of MTV RNA expression was, however, not maintained in the hormone-induced mammary tumors. (iii) Spontaneous mammary tumors in BALB/c mice contained similar levels of MTV DNA and MTV RNA sequences as were found in other cells of these animals.

Identification of the Mtv-2 gene responsible for the early appearance of mammary tumors in the GR mouse by nucleic acid hybridization

In the mouse strain GR, the Mtv-2 gene controls the expression of large amounts of mammary tumor virus (MTV) antigens in the milk at first lactation. It also controls the early appearance of mammary tumors. We have investigated the number of MTV proviral sequences associated with this Mtv-2 gene by nucleic acid hybridization between MTV [(3)H]cDNA and DNA from GR, B10, and GR-Mtv-2(-) mice. B10 and GR-Mtv-2(-) mice lack Mtv-2 gene expression. The molecular hybridizations revealed that the DNA of GR mice contains 12 copies of MTV proviral sequences, whereas only 4 copies are present in the DNA of B10 and GR-Mtv-2(-) mice. We therefore conclude that the Mtv-2 gene in the GR mouse strain is associated with eight additional MTV proviral sequences. The four Mtv proviral sequences in the GR-Mtv-2(-) DNA might represent another Mtv gene in the GR mouse. Different amounts of MTV RNA are detected in mammary glands at first lactation of B10 and GR-Mtv-2(-) mice, even though both contain four copies of MTV proviral sequences. This indicates a difference between these two mouse strains either in the regulation of expression of these MTV proviral sequences or in the location of these sequences in the murine genome.

Differences in mouse mammary tumor viruses. Relationship to early and late occurring mammary tumors

The murine model has been used extensively to study the various factors involved in the etiology of mammary carcinoma. Inbred mouse strains have been classically categorized into (i) high incidence stains with tumors occurring relatively early in the life of the animal, or (ii) low or moderate incidence strains with tumors occurring later on in life. We have radioactively labeled the RNA genome of the mouse mammary tumor virus (MMTV) from each of several mouse strains. We report here, using the technique of molecular hybridization, that the class of MMTVs responsible for the early occurring mammary tumors in high incidence strains can be clearly distinguished from the MMTVs associated with late occurring mammary tumors in low or moderate incidence strains; we also demonstrate that minor differences in MMTV genomes can also exist within these classes. Our findings show that MMTVs are transmitted via the germ line (as a germinal provirus) in some mouse strains, whereas in other strains, a non-germ line transmission is clearly demonstrated. Biochemical techniques can thus be used to track the mode of transmission of oncogenic viruses. The relationship of these findings to an understanding of the etiology of mammary carcinoma is discussed.

A biochemical approach to the study of the transmission of mouse mammary tumor viruses in mouse strains RIII and C3H

Mouse mammary tumor virus (MMTV) proviral sequences were detected in the cellular DNA of mammary tumors and livers of RIII and C3H mice by molecular hybridization with radioactively labelled MMTV 60-70S RNA or tritiated MMTV complementary DNA (cDNA). By means of DNA:DNA reassociation kinetics, the DNA of the mammary tumor cells of these two mouse strains were found to contain more MMTV proviral sequences than the DNA of liver cells of these same tumor-bearing mice. Evidence is also presented that the DNA of the liver cells lacks a part (approximately 25%) of the MMTV proviral sequences found in the mammary tumor cells of these mouse strains. The relationship of the extra MMTV proviral sequences found in mammary tumor cells to the early mammary tumor-igenesis seen in these mouse strains is discussed.

Characterization of the oncornavirus particles in the plasma of guinea pigs with L2C leukemia

The inoculation of L2C guinea pig leukemia cells into strain 2 guinea pigs results in the death of the animals within 12 to 15 days. Death is preceded by the simultaneous appearance in the plasma of (i) elevated leukocyte levels, (ii) extracellular virus particles, and (iii) a particle-associated RNA-directed DNA polymerase. This enzyme activity has a cation preference identical to that of the type B bromodeoxyuridine-induced guinea pig virus, i.e., an Mg2+ optimum at 20 mM and no activity using Mn2+. Competitive molecular hybridization studies also revealed that the plasma of leukemic guinea pigs contained approximately 2 X 10(9) genome equivalents per ml of an RNA that is homologous to the RNA of the bromodeoxyuridine-induced guinea pig virus. Morphological observations indicate that most, but not all, of the extracellular particles observed in leukemia plasma are derived from the intracisternal particles seen in the L2C tumor cells. The possibilities that either two viral populations are present or that the in vivo morphogenesis of the type B bromodexoyuridine-inducible guinea pig virus is markedly different from its in vitro morphogenesis are discussed.

Characterization of mouse mammary tumor viruses from primary tumor cell cultures. II. Biochemical and biophysical studies

Primary mammary tumor cultures of RIII, GR, DD, BALB/c, and BALB/cfC3H mice were examined for mouse mammary tumor virus (MuMTV) production. Levels of production of 12-32 mug virus protein/day/75-cm2 culture flask could be maintained for 30-50 days with daily virus harvests. The viruses from tumor cell cultures of these mouse strains contained DNA polymerase with a strong preference for Mg++ over Mn++ as the divalent cation, a characteristic of DNA polymerase of MuMTV from mouse milk. These viruses from tumor cell cultures were excellent sources of MuMTV 3H-complementary DNA (complexed to 60-70S RNA) and radioactive 60-70S RNA, sufficiently free of contaminating murine leukemia virus nucleic acids, that can be used in molecular hybridization experiments. The effects of several culture parameters on MuMTV production were also studied.

Relationship in nucleic acid sequences between mouse mammary tumor virus variants

Primary cultures of mouse mammary carcinomas were used as a source of both radioactively labeled and unlabeled 60-70S RNA of mouse mammary tumor virus (MMTV) obtained from various mouse strains. Competition molecular hybridization experiments revealed that, within the limits of the assay, the RNAs of the MMTVs synthesized in culture by the tumors of the mouse strains RIII, GR, A, and C3H, are identical. A comparison of the genomes of the milk-transmitted MMTV(C3H) and the vertically transmitted MMTV(C3Hf) revealed that these two viruses are approximately 75% similar. No nucleic acid sequence homology was observed between MMTV(C3H) 60-70S RNA and the RNAs of murine leukemia virus, Mason-Pfizer virus, or the BrdUrd-induced type-B quinea pig virus.

Biochemical properties of the bromodeoxyuridine-induced guinea pig virus

The biophysical and biochemical properties of the virus particles released by guinea pig embryo cells treated with 5-bromo-2'-deoxyuridine (BUdR) have been compared to those of the B-type mouse mammary tumor virus (MMTV) and the C-type Rauscher murine leukemia virus. The high-molecular-weight (60 to 70S) RNA of the BUdR-induced guinea pig virus (GPV) has a molecular weight of 8 X 106 when measred by mixed agarose polyacylamide gel electrophoresis. The virus particles isolated from the tissue culture medium of BUdR-induced guniea pig cells have the following properties in common with MMTV: (i) a buoyant density of 1.18 g/ml in sucrose and 1.21 g/ml in CsCl, and (ii) a DNA polymerase that prefers Mg2+ over Mn2+ in an assay using the synthetic template poly(rC):oligo(dG). No nucleic acid sequence homology between GPV RNA and the viral RNAs of the MMTV, murine leukemia virus, hamster sarcoma virus, or Mason-Pfizer monkey virus could be observed in a competition hybridization assay using the radioactive-labeled GPV 60 to 70S RNA. By this same competition by hybridization assay the frequency of GPV proviral sequences was estimated to be at least 83 per haploid cellular genome of guniea pig cells. No nucleic acid sequences related to be GPV RNA were detected in the DNA of normal tissues of mice, rats, cats, dogs, baboons, or humans by direct RNA-DNA hybridization using radioactive GPV60 to 70S RNA.