Temperature-dependent properties of cells transformed by a thermosensitive mutant of polyoma virus

Immunization of T-cell deficient mice against polyomavirus infection using viral pseudocapsids or temperature sensitive mutants

A murine experimental model system aimed at developing potential vaccines to papovavirus infection in immunosuppressed individuals was explored. A VP1-pseudocapsid based on the major capsid protein of the murine polyomavirus A2 strain and a mutant, M17-pseudocapsid as well as four temperature sensitive (ts)-mutants were used as immunogens. T-cells deficient CD4-/-8-/- mice were immunized four times with each immunogen and then together with non-immunized control mice challenged with polyomavirus. In contrast to all control mice, only half of the immunized mice exhibited presence of polyoma DNA when assayed by PCR. The results indicate that pseudocapsids and ts-mutant immunization may potentially protect mice with an impaired T-cell function from polyomavirus infection.

High-level recombination specific to polyomavirus genomes targeted to the integration-transformation pathway

An unusually high incidence of interviral recombination was found in the process of integration of the polyomavirus genome concomitant with neoplastic transformation of nonpermissive cells. Transformants were isolated after mixed infections of Fischer rat cells with two mutants lacking restriction endonuclease sites and were analyzed for the presence of unselected integrated recombinant restriction fragments. A large fraction of the transformants isolated (38% of the 64 transformed cell lines studied) contained recombinant viral genomes that had undergone recombination in a 1.3-, 1.7-, or 3.6-kilobase-pair interval. More than 90% of these recombinant transformants showed evidence of crossovers in multiple intervals. To our knowledge, the recombination frequencies observed in these experiments represent the highest frequencies of homologous recombination reported for a mitotic mammalian system that does not involve transfection. In contrast to the elevated level of recombination in the integrated viral genomes, no evidence of recombination was obtained among the replicated unintegrated pool of viral genomes isolated from the same population of infected cells from which the recombinant transformants were derived. Either of two hypotheses can provide an explanation for the segregated recombination: either recombination occurs at elevated levels in a small, recombination-prone fraction of the population destined to become transformed, or recombination occurs only among those viral genomes which are engaged in the process of integration and thus interact with a recombinogenic host machinery (for example, the host scaffold). We favor the latter hypothesis.

High-level recombination specific to polyomavirus genomes targeted to the integration-transformation pathway

An unusually high incidence of interviral recombination was found in the process of integration of the polyomavirus genome concomitant with neoplastic transformation of nonpermissive cells. Transformants were isolated after mixed infections of Fischer rat cells with two mutants lacking restriction endonuclease sites and were analyzed for the presence of unselected integrated recombinant restriction fragments. A large fraction of the transformants isolated (38% of the 64 transformed cell lines studied) contained recombinant viral genomes that had undergone recombination in a 1.3-, 1.7-, or 3.6-kilobase-pair interval. More than 90% of these recombinant transformants showed evidence of crossovers in multiple intervals. To our knowledge, the recombination frequencies observed in these experiments represent the highest frequencies of homologous recombination reported for a mitotic mammalian system that does not involve transfection. In contrast to the elevated level of recombination in the integrated viral genomes, no evidence of recombination was obtained among the replicated unintegrated pool of viral genomes isolated from the same population of infected cells from which the recombinant transformants were derived. Either of two hypotheses can provide an explanation for the segregated recombination: either recombination occurs at elevated levels in a small, recombination-prone fraction of the population destined to become transformed, or recombination occurs only among those viral genomes which are engaged in the process of integration and thus interact with a recombinogenic host machinery (for example, the host scaffold). We favor the latter hypothesis.

Studies on the nature of protease-induced growth stimulation in normal and transformed BHK cells

In the presence of growth-limiting serum concentrations trypsin displays mitogenic activity on actively-growing but not quiescent BHK cells. These results suggest that BHK cells arrested in G1 (G0) are not sensitive to protease-induced growth stimulation. Previous work strongly suggested that the trypsin active-site is not directly involved in its mitogenic activity on BHK cells. Additional studies on denatured trypsin fragments further indicate that the molecular conformation and size of native trypsin may not be absolutely required for mitogenic activity. Cellular multiplication induced by the addition of fresh serum to quiescent BHK cultures is not inhibited by high concentrations of soybean trypsin inhibitor. Similar to our previous findings with trypsin, it has been further observed that plasmin is not sufficient to initiate the growth of BHK cells in soft agar. Trypsin also fails to enhance the growth of a thermosensitive polyoma-transformed BHK line in soft agar at the restrictive temperature. Finally, the growth of transformed BHK cells in soft agar does not display a requirement for plasminogen and is not inhibited by soybean trypsin inhibitor. These studies argue against the involvement of plasmin or other exogenous trypsin-like enzymes in the growth and transformation of BHK cells.

Temperature-sensitive growth regulation in one type of transformed rat cells induced by the tsa mutant of polyoma virus

A fibroblast line of the 3T3 type with a low saturation density was established from Fisher rat embryo cells. After infection with either wild-type or tsa mutant polyoma virus, transformants were isolated and cloned at 33 degrees C on the basis of their ability either to grow as dense foci on plastic in liquid medium (type N) or to form colonies in soft agar (type A). Polyoma T antigen was detected in all of the transformed lines. The following growth characteristics were studied for both types at 33 and 41 degrees C: saturation density, growth in soft agar and at a low serum concentration, colony-forming ability, and generation time. tsa-N transformants behaved at 33 degrees C similarly to transformed cells, but reverted at 41 degrees C to the nontransformed phenotype for all of these characters. tsa-A transformants and all of the wild-type transformants exhibited the transformed phenotype at both low and high temperatures. These results led us to distinguish at least two types of virus-induced transformants. In one of them, the activity of the protein affected by the tsa mutation appears to be necessary for the expression of several of the characters defining the transformed state.

Thymidine kinase, DNA synthesis and cancer

A resume has been presented of some recent investigations which show that DNA synthesis can be initiated in many types of quiescent animal cells by external stimuli, by introducing a quiescent nucleus into the cytoplasm of a proliferating cell, or by a virus infection. The components of the DNA replication apparatus are described. It is shown that deoxyribonucleoside triphosphate pools increase substantially in animal cells at the time DNA synthesis is initiated due to the enhanced activities of enzymes functioning in nucleotide synthesis. Especially striking is the increase of thymidine kinase activity, indicating that this enzyme may be a useful marker of the shift from the quiescent to the replicative state. The thymidine kinase isozymes of vertebrate cells have been characterized. Thymidine kinase F, which is found principally in the cytosol, is the isozyme that increases when G1 (Go) phase cells are stimulated or infected with oncogenic viruses. Chick cytosol thymidine kinase F can also be reactivated by introducing differentiated chick erythrocyte nuclei into the cytoplasm of enzyme-deficient LM (TK-) mouse cells. Furthermore, herpesviruses code for distinctive, virus-specific thymidine kinase isozymes, so that another way to transform thymidine kinase-deficient LM TK-) cells to kinase-positive cells is by infecting them with UV-irradiated herpes simplex viruses. The experiments on the activation of DNA synthesis and thymidine kinase F activity have been discussed in the context of the proliferative activity in vivo and the immortalization in culture of neoplastic cells. These experiments suggest that genes determining cell cycle proteins are readily accessible to transcription and translation in essentially all nucleated cells. The tendency of transformed cells to become multinucleated after cytochaliasin B treatment also suggests that one important difference between malignant cells and most normal cells may be the ability of malignant cells to 'stockpile' the proteins (and/or their messenger RNAs) of the DNA replicative apparatus and to maintain the 'stockpiles' in progeny cells.

Deficiency in histone acetylation in nontransforming host range mutants of polyoma virus

Histones H3 and H4 derived from transforming wild-type polyoma and simian virus 40 particles show extensive acetylation compared to the histones of the host cells. The same histone fractions derived from nontransforming host range mutants of polyoma virus fail to show this high degree of acetylation.

Independent regulation of the nuclear and mitochondrial DNA synthesis induced by either Simian virus 40 or serum in mouse fibroblast 3T3 cells

Resting cultures of 3T3 cells (an established line of mouse fibroblasts) were released from density inhibition by either infection with Simian virus 40 or addition of serum. The increased rate of thymidine incorporation into DNA, induced by these two agents, was measured in the presence and in the absence of three inhibitory conditions (cycloheximide or dibutyryladenosine 3':5'-monophosphate added to the medium, or lack of anchorage). The inhibition was found to be quite similar in cultures stimulated by virus or serum; under the same conditions, however, the incorporation into mitochondrial DNA was much less inhibited than that into nuclear DNA. The experiments also suggest that new protein synthesis may not be necessary, for either virus or serum, to start the inductive mechanism.

Temperature-dependent properties of cells transformed by a thermosensitive mutant (TS-121) of polyoma virus. II. Characterization of 121-6 cells

The properties of 121-6 cells were characterized. This cell line is transformed by ts-121, a temperature-sensitive mutant of polyoma virus. Both the morphology and saturation density of the 121-6 cells were affected by temperature; i.e. at 39 degrees C, the cells grew to monolayer sheets and remained contact inhibitied for a long time (at least 25 days), while at 35 degrees C, they grew beyond the monolayer density, like cells transformed by wild-type polyoma virus. These phenotypic changes were reversible, but increased agglutinability by Concanavalin A, which was another phenotype of these transformed cells, was not temperature-sensitive, and even at high temperature, 121-6 cells, like transformed cells, showed high agglutinability.

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.

Use of epithelial cell cultures for studies on the mechanism of transformation by chemical carcinogens

Evidence is reviewed for and against four major theories of chemical carcinogenesis. The development of several normal and transformed epithelial cell lines which should be useful for the analysis of this problem is described. The detection of RNA viral particles in cells transformed with chemical carcinogens is a recurrent finding in studies from our own and other laboratories, but the significance of these particles in terms of the mechanism of chemical carcinogenesis remains to be determined. Finally, we have described the first mutants of chemically transformed epithelial cells which are temperature sensitive in the maintenance of the transformed phenotype. These mutants should be particularly useful for detecting the critical biochemical changes that distinguish a chemically induced tumor cell from its normal counterpart.

Differential accumulation of virus-specific RNA during the cell cycle of adenovirus-transformed rat embyro cells

In adenovirus type 2-transformed rat embryo cells there is a threefold greater incorporation of [3-H]uridine into virus-specific RNA early in S phase than in late S or G2. This heightened accumulation of labeled RNA is true for both nuclear and cytoplasmic virus-specific labeling. Inhibition of DNA synthesis decreases the virus-specific RNA labeling, whereas reversal of inhibition again allows the elevated level of virus-specific RNA labeling.

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.

Initiation and maintenance of cell transformation by simian virus 40: a viral genetic property

The transforming ability in 10% serum medium of the temperature-sensitive mutants of simian virus 40 in the complementation group III (ts640 type mutants) was greatly reduced when the infected rat 3Y1 cells were incubated at the restrictive temperature of 40 degrees or incubated first at 40 degrees for 3 days and then shifted to the permissive temperature of 33 degrees. Transformation did occur efficiently after incubation at 33 degrees or after an initial incubation at 33 degrees for 5 days followed by a shift to 40 degrees. When growth properties of 3Y1 cells transformed at 33 degrees by the group III mutants were examined at 40 degrees, several aspects of the transformed state were rendered temperature-sensitive. These aspects were the ability of cells to grow in low serum (1.5%) medium and to make colonies, in 10% serum medium, on monolayers of untransformed 3Y1 cells and in soft agar. It is concluded that a simian virus 40 gene (cistron III) controls the initiation, as well as at least some aspects of the maintenance, of transformation and that the initiation reaction is a more heat-labile event than the maintenance reaction(s) under the experimental conditions.

Temperature-sensitive mutants of chemically transformed epithelial cells

The first temperature-sensitive mutants of epithelial cells transformed with chemical carcinogens have been isolated. Like the wild-type transformed parental cells, the mutants readily grow in agar suspension at 36 degrees, but in contrast to the wild type, they do not do so at 40 degrees. Detailed studies of one of these mutants, TS-223, indicate that at high temperature it also has reduced cloning efficiency in monolayer culture and a lower saturation density. Scanning electron microscopy revealed that at 40 degrees confluent cultures of TS-223 consist of a monolayer of generally flat polygonal cells, whereas 36 degrees cultures contain many patches of piled-up cells that are spherical and have rougher surface membranes. All of these cellular changes are reversible with upward or downward temperature shifts. The temperature-sensitive lesion appears to reside in a host cell gene which modulates expression of the transformed cell phenotype. These mutants may provide a useful system for elucidating the minimal biochemical changes required for expression of the transformed phenotype in epithelial cells.

Mutant of polyoma virus with impaired adsorption to BHK cells

A mutant of polyoma virus PY235 has an impaired adsorption to guinea pig red blood cells and BHK-21 hamster cells. Adsorption to 3T3 mouse cells is much less inhibited. These altered adsorption properties are responsible for the apparent inability of PY235 to cause cell transformation or hemagglutination.

A general theory of carcinogenesis

A GENERAL HYPOTHESIS OF CARCINOGENESIS IS PROPOSED CONSISTING OF THE FOLLOWING FEATURES: (1) It is suggested that all cells possess multiple structural genes (Tr) capable of coding for transforming factors which can release the cell from its normal constraints on growth. (2) In adult cells they are suppressed by diploid pairs of regulatory genes and some of the transforming genes are tissue specific. (3) The Tr loci are temporarily activated at some stage of embryogenesis and possibly during some stage of the cell cycle in adult cells. (4) Spontaneous tumors, or tumors induced by chemicals or radiation, arise as the result of a double mutation of any set of regulatory genes releasing the suppression of the corresponding Tr genes and leading to transformation of the cell. (5) Autosomal dominant hereditary tumors, such as retinoblastoma, are the result of germ-line inheritance of one inactive regulatory gene. Subsequent somatic mutation of the other regulatory gene leads to tumor formation. (6) The Philadelphia chromosome produces inactivation of one regulatory gene by position effect. A somatic mutation of the other leads to chronic myelogenous leukemia. (7) Oncogenic viruses evolved by the extraction of host Tr genes with their conversion to viral transforming genes. As a result, in addition to the above mechanisms, tumors may also be produced by the reintroduction of these genes into susceptible host cells.

Simian virus 40-host cell interactions. I. Temperature-sensitive regulation of SV40 T antigen in 3T3 mouse cells transformed by the ts*101 temperature-sensitive early mutant of SV40

BALB/3T3 and Swiss/3T3 mouse cells transformed at permissive temperature (33 C) by the early temperature-sensitive mutant of simian virus 40 (SV40), ts(*)101, exhibited a temperature-dependent modulation of SV40 tumor (T) antigen as assayed by immunofluorescence. The percentage of T antigen-positive nuclei in ts(*)101 transformed cells was reduced at restrictive temperature (39 C) when compared to 33 C and to wild-type SV40 transformed cells at either 33 C or 39 C. The percentage of T antigen-positive nuclei in ts(*)101 transformed cells returned to the 33 C control level when the cells were shifted from 39 to 33 C. The ts(*)101 transformed cells could be superinfected with wild-type, but not ts(*)101, virions at 39 C as assayed by an increase in T antigen-positive nuclei.

Mutation causing temperature-sensitive expression of cell transformation by a tumor virus (SV40-3T3 mouse cells-growth control)

A procedure has been devised to isolate 3T3 mouse fibroblasts transformed by simian virus 40 (SV40) that express their transformed phenotype at low (32 degrees C) but not at high (39 degrees C) temperature. Three parameters typical of malignant growth in vitro: (a) high saturation density in culture, (b) ability to form colonies on monolayers of normal 3T3 cells, and (c) lack of contact inhibition of DNA synthesis, are temperature sensitive. These phenotypic changes are fully reversible. The serum requirement for growth appears to be largely unchanged by temperature. These cells seem to owe their behavior to a cellular, rather than to a viral, alteration since after fusion of the temperature-sensitive transformed cells with permissive monkey cells, a procedure that leads to rescue (i.e., multiplication of the virus), wild-type SV40 virus is produced.

Integration of simian virus 40 deoxyribonucleic acid into the deoxyribonucleic acid of primary infected Chinese hamster cells

Simian virus 40 deoxyribonucleic acid (DNA) became associated in an alkaline-stable form with the DNA of Chinese hamster embryo cells at 15 to 20 hr post-infection, at the time when cell DNA synthesis and T antigen were induced. The integration process was not inhibited by d-arabinosyl cytosine and was only partially inhibited by cycloheximide.

Cell cycle-dependent immune lysis of Moloney virus-transformed lymphocytes: presence of viral antigen, accessibility to antibody, and complement activation

The expression of Moloney leukemia virus on the surface of a viral-induced lymphoma cell, availability of the virus to anti-viral antibody, and the nature and extent of activation of the complement system during the cell cycle were studied in vitro. Viral antigen was present on the cell surface, accessible to antibody, and was able to activate complement in the presence of antibody throughout all cellular growth phases, while cytotoxicity was confined to the G(1) phase of cell growth. In addition, when cells were arrested in metaphase, viral antigen could be demonstrated on the cell surface by immunofluorescence, and budding virus was seen by electron microscopy. All nine components of complement were activated on the addition of antibody throughout the cell cycle. Additional experiments indicated that in the presence of antibody, C3 and/or C4 were immunospecifically bound to viral-induced lymphoma cells throughout the cell cycle as a result of complement activation. These results indicate that the inability to lyse the cells in the presence of specific anti-viral antibody and complement during the logarithmic phase of cell growth is not due to the lack of expression of Moloney virus antigen(s) on the cell surface, inaccessibility of this surface antigen(s) to antibody, or failure to activate the complement effector system.

Temperature-dependent surface changes in cells infected or transformed by a thermosensitive mutant of polyoma virus

Infection of BALB/3T3 cells by polyoma virus causes an alteration in the cell surface, characterized by enhanced agglutination of the cells by wheat germ agglutinin or concanavalin A. Infection by the thermosensitive mutant of polyoma, ts-3, causes the cell surface alteration at the permissive temperature, but not at the nonpermissive temperature. The cell surface alteration requires cellular DNA synthesis, but not viral DNA synthesis. BHK cells transformed by ts-3 show the surface alteration when grown at the permissive temperature, but not when grown at the nonpermissive temperature. It is concluded that the surface alteration in transformed cells is under the control of a viral gene.