Demonstration of infectious deoxyribonucleic acid in transformed cells. I. Recovery of simian virus 40 from yielder and nonyielder transformed cells

Viral regulatory region effects on vertical transmission of polyomavirus SV40 in hamsters

Viral strain differences influence the oncogenic potential of polyomavirus simian virus 40 (SV40). We hypothesized that viral strain differences might also affect vertical transmission of SV40 in susceptible hosts. Pregnant Syrian golden hamsters were inoculated intraperitoneally with 10(7) plaque-forming units of SV40 and offspring were sacrificed post-delivery (1-21 days, 6 months). Organ extracts were analyzed for SV40 DNA by polymerase chain reaction assay. Transmission of SV40 from mother to offspring was detected in over half of litters. Most placentas were virus-positive. Mothers inoculated with SV40 strains containing complex regulatory regions transmitted virus more frequently than those infected with simple enhancer viruses (p<0.001). Virus was detected more often in progeny brain than in spleen (p<0.05). Several progeny were virus-positive at 6 months of age, suggesting viral persistence. Maternal animals retained virus in several tissues through day 21 and developed T-antigen antibodies. These results indicate that SV40 replicates in hamsters, vertical transmission of SV40 can occur, and the viral regulatory region influences transmission.

Differential ability of two simian virus 40 strains to induce malignancies in weanling hamsters

Different strains of simian virus 40 (SV40) exist and are associated with some human malignancies, but it is not known if SV40 strains differ in biological potential in vivo. In two long-term experiments, Syrian golden hamsters 21 days of age were inoculated by the intraperitoneal route with two different strains of SV40 (10(7) plaque-forming units/animal) and were followed for 8 or 12 months. In vivo responses to strain VA45-54, isolated originally from monkey kidney cells, and to strain SVCPC, recovered from human cancers, were compared. Control animals of the same age were inoculated intraperitoneally with cell culture media. Malignancies developed only in animals infected with SV40 and not in controls. The rate of tumor development was more frequent among animals infected with strain SVCPC than with VA45-54, both in experiments held for 8 months (11/22, 50% vs. 4/20, 20%) and for 12 months (7/15, 47% vs. 3/13, 23%). Histologically, the tumors resembled mesotheliomas, osteosarcoma, and poorly differentiated sarcomas. Metastases to lung and lymph nodes occurred with both viral strains. T-antigen expression was detected in most tumor cells by immunohistochemistry. Anti-T-antigen antibodies were produced by almost all tumor-bearing animals and by about two-thirds of those that did not develop tumors after virus inoculation. SV40 viral neutralizing antibodies were detected in all tumor-bearing animals and in 92% and 38% of those inoculated with SVCPC and VA45-54, respectively, that failed to develop tumors. Antibody titers were usually higher in animals with tumors than in those without. Control animals did not develop viral antibodies. Infectious virus was recovered from 2 of 15 tumors tested. This study showed that there are biological differences between these two SV40 strains that influence the outcome of infections in normal hosts, including the development of malignancies and neutralizing antibody, and proved the principle that SV40 strains from different clades can vary in biological properties in vivo.

Simian virus 40 regulatory region structural diversity and the association of viral archetypal regulatory regions with human brain tumors

The regulatory region (RR) of simian virus 40 (SV40) contains enhancer/promoter elements and an origin of DNA replication. Natural SV40 isolates from simian brain or kidney tissues typically have an archetypal RR arrangement with a single 72-basepair enhancer element. A rare simpler, shorter SV40 RR exists that lacks a duplicated sequence in the G/C-rich region and is termed protoarchetypal. Occasionally, SV40 strain variants arise de novo that have complex RRs, which typically contain sequence reiterations, rearrangements, and/or deletions. These variants replicate faster and to higher titers in tissue culture; we speculate that such faster-growing variants were selected when laboratory strains of SV40 were initially recovered. SV40 strains with archetypal RRs have been found in some human brain tumors. The possible implications of these findings and a brief review of the SV40 RR structure are presented.

Phenotypic changes and gene expression in human colon mucosal epithelial cells upon transfection of a SV40 DNA-gpt recombinant

Phenotypic changes (increased longevity, decreased growth factor requirements, altered cell surface features, growth in semisolid agarose, and SV40 T antigen expression) suggesting in vitro transformation were displayed by human normal colon mucosal epithelial cells transfected with pSV3gpt, a pBR322 recombinant containing the SV40 "early" T antigen coding region and the dominant selectable marker bacterial gene, xanthine-guanine phosphoribosyltransferase. In contrast, control cultures which received neither DNA nor the recombinant pSV2gpt (which is identical to pSV3gpt but lacks the SV40 T antigen region) were not phenotypically altered.

Transformation of mouse mammary epithelial cells by papovavirus SV40

Primary and established murine mammary epithelial cells and wild-type SV40 were employed to study the phenomenon of epithelial cell transformation. Thirteen independent transformed cell lines were derived. All contained SV40 intranuclear T antigen. Eight transformed mammary cell lines were examined ultrastructurally and all were found to exhibit pronounced epithelial cell characteristics, including desmosomes and tight junctions. Growth studies revealed that while normal mammary cells were unable to grow in low serum (2% FBS), established Cl S1 mammary cells and SV40-transformed mammary epithelial cells replicated well. Cell densities achieved by the transformants were only slightly elevated in high serum (13% FBS) over normal cell values. All the transformants formed colonies on plastic and exhibited anchorage-independent growth in methylcellulose. Five of the transformed lines were tumorigenic in syngeneic animals, in marked contrast to the lack of transplantability usually observed with SV40-transformed mouse fibroblasts. Anchorage-independent growth was not a predictor of tumorigenic potential in this system. The transformants exhibited a spectrum of responsiveness to exogenous growth factors. This study establishes that the SV40-murine mammary cell system is a valid model for analyses of the process and consequences of epithelial cell transformation, in general, and mammary cell transformation in particular.

Homologous and nonhomologous recombination in monkey cells

Though recombinational events are important for the proper functioning of most cells, little is known about the frequency and mechanisms of recombination in mammalian cells. We have used simian virus 40 (SV40)-pBR322 hybrid plasmids constructed in vitro as substrates to detect and quantitate intramolecular homologous and nonhomologous recombination events in cultured monkey cells. Excision of wild-type or defective SV40 DNAs by recombination from these plasmids was scored by the viral plaque assay, in either the absence or the presence of DNA from a temperature-sensitive helper virus. Several independent products of homologous and nonhomologous recombination have been isolated and characterized at the DNA sequence level. We find that neither DNA replication of the recombination substrate nor SV40 large T antigen is essential for either homologous or nonhomologous recombination involving viral or pBR322 sequences.

Homologous and nonhomologous recombination in monkey cells

Though recombinational events are important for the proper functioning of most cells, little is known about the frequency and mechanisms of recombination in mammalian cells. We have used simian virus 40 (SV40)-pBR322 hybrid plasmids constructed in vitro as substrates to detect and quantitate intramolecular homologous and nonhomologous recombination events in cultured monkey cells. Excision of wild-type or defective SV40 DNAs by recombination from these plasmids was scored by the viral plaque assay, in either the absence or the presence of DNA from a temperature-sensitive helper virus. Several independent products of homologous and nonhomologous recombination have been isolated and characterized at the DNA sequence level. We find that neither DNA replication of the recombination substrate nor SV40 large T antigen is essential for either homologous or nonhomologous recombination involving viral or pBR322 sequences.

Isolation and characterization of defective simian virus 40 genomes which complement for infectivity

A new variant of simian virus 40 (EL SV40), containing the complete viral DNA separated into two molecules, was isolated. One DNA species contains nearly all of the early (E) SV40 sequences, and the other DNA contains nearly all of the late (L) viral sequences. Each genome was encircled by reiterated viral origins and termini and migrated in agarose gels as covalently closed supercoiled circles. EL SV40 or its progenitor appears to have been generated in human A172 glioblastoma cells, as defective interfering genomes during acute lytic infections, but was selected during the establishment of persistently infected (PI) green monkey cells (TC-7). PI TC-7/SV40 cells contained EL SV40 as the predominant SV40 species. EL SV40 propagated efficiently and rapidly in BSC-1, another line of green monkey cells, where it also formed plaques. EL SV40 stocks generated in BSC-1 cells were shown to be free of wild-type SV40 by a number of criteria. E and L SV40 genomes were also cloned in the bacterial plasmid pBR322. When transfected into BSC-1 cell monolayers, only the combination of E and L genomes produced a lytic infection, followed by the synthesis of EL SV40. However, transfection with E SV40 DNA alone did produce T-antigen, although at reduced frequency.

Rescue of SV40 following transfection of TC7 cells with cellular DNAs containing complete and partial SV40 genomes

Infectious SV40 virions could be rescued from permissive TC7 cells within one to three subcultures following cotransfection with two cellular DNAs, each containing a complementary portion of the SV40 genome. SV40 virions could also be rescued by transfection of TC7 cells with cellular DNAs from a variety of SV40 transformed cells containing complete genome equivalents but not from cells containing subgenomes alone or defective genomes. Infectious virus was not rescued if the transfecting DNA species was treated with DNAase or if the DEAE-dextran pretreatment of the recipient cells was omitted.

Integration and expression of viral DNA in cells transformed by host range mutants of adenovirus type 5

Group I host range (hr) mutants of adenovirus type 5 are unable to transform rat embryo or rat embryo brain cells but induce an abnormal transformation of baby rat kidney cells. We established several transformed rat kidney cell lines and characterized them with respect to the transformed phenotype and the structure of the integrated viral DNA. The hr mutant-transformed cells, unlike wild-type virus transformants, were fibroblastic rather than epithelial, failed to grow in soft agar, and were also less tumorigenic in nude mice. Studies on the structure of the integrated viral DNA sequences showed that hr-transformed cells always contained the left end of the adenovirus DNA, but the size of the integrated DNA fragment varied among different lines, and a high percentage of the lines contained the entire viral genome colinearly integrated. The patterns of integration were maintained after prolonged growth in culture and after subcloning. Attempts to rescue infectious virus from lines which contained the entire genome were unsuccessful. Using immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, we analyzed the viral proteins expressed in hr-transformed cells. Results of these studies indicated that, like wild type-transformed cells, hr transformants expressed E1B proteins of molecular weight 58,000 and 19,000.

Presence of free viral DNA in simian virus 40-transformed nonproducer cells

Extracts from several simian virus 40 (SV40)-transformed nonproducer cells were prepared by the hot-phenol procedure normally used to extract cellular RNA. These extracts contained SV40 infectious units. Part of the infectious units were identified as SV40 form I DNA molecules. The results of reconstruction experiments suggest that SV40 form I DNA is extractable by the hot-phenol procedure because of its fast renaturation rate. The significance of the presence of free viral DNA in nonproducer transformed cells is discussed.

Rescue of infectious virus from permissive monkey cells containing simian virus 40 DNA fragments

Permissive TC7 cells were separately transfected with simian virus 40 (SV40) EcoRI/Hap II A (74% genome) DNA fragments and EcoRI/Hap II B (26% genome) DNA fragments in the presence of DEAE-dextran. Fusion of the progeny of recipient cells receiving the A fragment, TC7 (SV40/74) cells, with TC7 (SV40/26) cells, which had received the B fragment, resulted in SV40 rescue. TC7 (SV40/74 + 26) cells, which had simultaneously received both complementary subgenomes, either spontaneously produced SV40 upon subculture or yielded virus upon treatment with iododeoxyuridine. In addition, fusion of rat cells containing the EcoRI/Hap II A fragment with TC7 (SV40/26) cells resulted in SV40 rescue. Cytopathology, V-antigen production, neutralization, and electron microscopy were parameters used to verify that the rescued virus was SV40. No infectious virus was produced when the combinations of cells fused did not total a complete SV40 genome equivalent.

Characterization of simian virus 40 transformed African green monkey cells (CV-1). I. Defective virion and viral genome

Several clones of SV40 transformed CV-1 cells have been characterized for the production of T- and V-antigens and for the state of viral genome. The transformed CV-1 cells failed to produce infectious virions as assayed after sonication or cocultivation and fusion with normal CV-1 cells, and were resistant to super-infection by SV40. Some clones of the transformed cells contained V-antigens. The population of V-antigen positive cells varied from 0 to 100% depending on the passage number while the T-antigen positive cells were always 100%. The virions isolated from the transformed cells were similar in morphology to complete SV40, but lighter in density than complete SV40. In one clone, a small amount of SV40 DNA was detectable in a free state while a large proportion of the DNA hybridizable with SV40 3H cRNA was linearly integrated into the cell DNA. The free SV40 DNA was noninfectious, closed circular DNA with a size smaller than infectious SV40 DNA component I. Since the cell extracts of the transformed cells contained an agent(s) which induced T- and V-antigens in normal CV-1 cells, it was suggested that the SV40 transformed CV-1 cells contained free as well as integrated defective SV40 genomes responsible for the synthesis of T- and V-antigens.

Characterization of simian cells tranformed by temperature-sensitive mutants of simian virus 40

Seven lines derived from primary African green monkey kidney cells, which had survived lytic infection by wild-type simian virus 40 (SV40) or temperature-sensitive mutants belonging to the A and B complementation groups, were established. These cultures synthesize SV40 tumor (T) antigen constitutively and have been passaged more than 60 times in vitro. The cells released small amounts of virus even at high passage levels but eventually became negative for the spontaneous release of virus. Virus rescued from such "nonproducer" cells by the transfection technique exhibited the growth properties of the original inoculum virus. Four of the cell lines were tested for the presence of altered growth patterns commonly associated with SV40-induced transformation. Although each of the cell lines was greater than 99% positive for T antigen, none of the cultures could be distinguished from primary or stable lines of normal simian cells on the basis of morphology, saturation density in high or low serum concentrations, colony formation on plastic or in soft agar, hexose transport, or concanavalin A agglutinability. However, the cells could be distinguished from the parental green monkey kidney cells by a prolonged life span, the presence of T antigen, a resistance to the replication of superinfecting SV40 virus or SV40 viral DNA, and, with three of the four lines, an ability to complement the growth of human adenovirus type 7. These properties were expressed independent of the temperature of incubation. These results indicate that the presence of an immunologically reactive SV40 T antigen is not sufficient to ensure induction of phenotypic transformation and suggest that a specific interaction between viral and cellular genes and/or gene products may be a necessary requirement.

Characterization of cell lines derived from hamster tumors induced with the BK virus

Some of the properties of three continuous cell lines derived from BK virus-induced hamster tumors were examined. The cell lines had in vitro growth characteristics of transformed cells. Morphologically most of the cells were fibroblastic, but multinucleated giant cells were also common. Ultrastructurally all three cell lines displayed the usual features of cells grown in vitro. Marked variation in the nuclear size and shape as well as prominent nucleoli were characteristic to these cells. No viruses or virus-like particles were found. Virus isolation attempts by fusing the cells with Vero cells were negative, and no virion antigen was detected in these cells by immunofluorescence. T antigen similar to that of other papovaviruses was found in the cells. This antigen stained with sera from a number of hamsters carrying transplanted BK virus-induced tumors, and also with SV 40 T antisera. The antigen disappeared after 30 minutes at 56 degrees C. Cytogenetic analyses showed that the three cell lines were heteroploid with subtetraploid numbers of chromosomes. Chromosome abnormalities were also seen. All three cell lines induced sarcomatous tumors in adult hamsters after subcutaneous inoculation.

Demonstration of infectious DNA in transformed cells. II. Characterization of uptake of SV40-transformed mouse cell DNA by simian cells

The initial steps in the DNA-transfer, or transfection, method of virus rescue were characterized using primary green monkey (GMK) cells exposed to SV40-transformed mouse (SV-3T3) cell DNA in the presence of 1 mg/ml DEAE-dextran. When large amounts (10-50mug) of high molecular weight DNA (greater than 10(7) daltons) were inoculated onto 10(6) GMK cells, usually less than 1 mug became cell-associated. DNA fragmented to a size of 1-3 X 10(6) daltons was bound more efficiently by the recipient cells, but generally only 5-10 per cent of the inoculum (representing 1-4 mug) was taken up. Approximately 50 per cent of the cell-associated DNA had penetrated to a DNase-resistant state by the end of the 30-minute incubation period. The effect of the size of thr transformed cell DNA molecule on the recovery of SV40 in transfection experiments was investigated. The trend appeared to be that rescue was more efficient with the larger molecular weight samples.

Transformation of mouse cells after infection with ultraviolet irradiation-inactivated herpes simplex virus type 2

A transformed mouse cell line (H238) was obtained following the infection of 238 mouse cells with ultraviolet (UV) irradiation-inactivated herpes simplex virus type 2 (HSV-2). The transformed cells produced tumors with a 100% incidence within 8 weeks in 6-week-old syngeneic BALB/c mice at an inoculum of 1 times 10(6). Indirect immunofluorescence (IF) tests revealed the presence of HSV antigens in the transformed cells. Antibodies to HSV-2 were found in the sera of tumor-bearing animals by neutralization and IF techniques. Neither HSV-2 infectious virus nor viral antigens could be detected by the transfer of transformed-cell DNA into permissive cells.

Optimal conditions for uptake of exogenous DNA by Chinese hamster lung cells deficient in hypoxanthine-guanine phosphoribosyltransferase

Conditions were characterized for maximizing the uptake of exogenous mammalian cell DNA by hypoxanthine-guanine phosphoribosyltransferase-deficient Chinese hamster lung cells. Recipient cell cultures in an exponential growth phase were found to be more competent in taking up DNA than stationary cultures. Polyornithine enhanced the uptake of exogenous DNA more reproducibly and to a greater extent than did any of the other facilitators tested (DEAE-dextran, CaCl2, latex spheres, spermine, polylysine and polyarginine). Maximal DNA incorporation occurred when polyornithine and DNA were mixed together prior to inoculation. About 25-30% of the DNA inoculum became deoxyribonuclease-resistant in a typical experiment utilizing polyornithine as the facilitator. Both homologous and heterologous exogenous DNAs rapidly became associated with recipient cell nuclei: approximately 95% of the deoxyribonuclease-resistant donor DNA was nuclear-associated 15 min after inoculation.

Properties of cells transformed by DNA tumor viruses

Transformation by the papovaviruses, SV40 and polyoma, is reviewed briefly, including factors that affect the frequency of transformation. Virus markers useful in the determination of the etiology of virus-free tumors are described, including viral DNA, viral mRNA, virus-induced antigens, and the rescue of infectious virus. Finally, the evidence that viral genes are involved in the maintenance of transformation is presented.

Role of simian virus 40 gene A function in maintenance of transformation

Mouse, hamster, and human cells were transformed at the permissive temperature by mutants from simian virus 40 (SV40) complementation group A in order to ascertain the role of the gene A function in transformation. The following parameters of transformation were monitored with the transformed cells under permissive and nonpermissive conditions: morphology; saturation density; colony formation on plastic, on cell monolayers, and in soft agar; uptake of hexose; and the expression of SV40 tumor (T) and surface (S) antigens. Cells transformed by the temperature-sensitive (ts) mutants exhibited the phenotype of transformed cells at the nonrestrictive temperature for all of the parameters studied. However, when grown at the restrictive temperature, they were phenotypically similar to normal, untransformed cells. Growth curves showed that the (ts) A mutant-transformed cells exhibited the growth characteristics of wild-type virus-transformed cells at the permissive temperature and resembled normal cells when placed under restrictive conditions. There were 3-to 51-fold reductions in the levels of saturation density, colony formation, and uptake of hexose when the mutant-transformed cells were the elevated temperature as compared to when they were grown at the permissive temperature. Mutant-transformed cells from the nonpermissive temperature were able to produce transformed foci when shifted down to permissive conditions, indicating that the phenotypically reverted cells were still viable and that the reversion was a reversible event. SV40 T antigen was present in the cells at both temperatures, but S antigen was not detected in cells maintained at the nonpremissive temperature. All of the wild-type virus-transformed cells exhbited a transformed cells exhibited a transformed phenotype when grown under either restrictive or nonrestrictive conditions. Thers results indicate that the SV40 group A mutant-transformed cells are temperature sensitive for the maintenance of growth properties characteristics of transformation. Virus rescued from the mutant-transformed cells by the transfection method was ts, suggesting that the SV40 gene A function, rather than a cellular one, is responsible for the ts behavior of the cells.

Attempts of Transmission of the Newt Melanoma by Nucleic Acids Preparations

In the present study attempts were made to obtain DNA extracts with tumorigenic activity from the melanoma of the newt (Triturus cristatus Laur.). The DNA, prepared by two different methods from melanomas that had been transferred by cells for several generations, was injected subcutaneously in adult animals. Control animals were not treated or received DNA isolated from normal homologous organs. In all 320 animals were used, 238 of which were given DNA. The DNA preparations obtained according to the procedure of Colter et al. failed to induce tumors in any of the 111 inoculated animals, while the treatment with DNA extracted according to Kirby's method (127 cases) from tumors as well from normal tissues resulted in the development of melanomas with an incidence significantly higher than that observed in the group of untreated animals; however, the activity of the extracts was not modified by DNase treatment. The influence of factors such as neoplasia spread in the donors, duration of observation, geographical origin of animals and season of treatment was considered. Altogether 934 animals were studied, including those of other experimental series reported elsewhere: 670 were given nucleic acids extracted from tumors or normal organs, 204 received tumor or normal organ transplants, 60 were untreated. Of interest is the fact that the overall incidence of melanoma was much high in animals coming from areas of high industrialization, i.e. in the newts from the environs of Pavia (54.1 %), than in those from Naples (11 %): the difference between the two groups was highly significant and independent of the treatment. Due to the high tumor incidence in the newts from Pavia, no correlation could be demonstrated between biological activity of nucleic acid extracts and the above mentioned factors in the 670 animals treated. However, in the case of the newts from the Naples area a higher frequency of tumors was observed after treatment with nucleic acids extracted from melanoma, than after treatment with nucleic acids from normal organs: the result is statistically significant only at the 5 per cent level with the χ2 test.