The stable classes of transformed cells induced by SV40 infection of established 3T3 cells and primary rat embryonic cells

Growth suppression of hybrids between transformed cells and normal fibroblasts in serum-free medium: correlation with retention of human chromosomes

Somatic cell hybrids formed by crossing PG19 mouse melanoma cells with mouse embryo fibroblasts have a reduced ability to proliferate in growth factor-unsupplemented serum-free medium relative to the parental melanoma cells. The suppression of growth of the hybrid cells in serum-free medium is attributable to a strict requirement of these cells for polypeptide growth factors (insulin plus platelet-derived growth factor, fibroblast growth factor, or epidermal growth factor). In contrast, the parental melanoma cells are able to grow without exogenously added growth factors. Fifteen hybrids derived from crosses between mouse L cells and normal human skin fibroblasts also have been tested for ability to grow in growth factor-unsupplemented serum-free medium. Depending on which human chromosomes are retained, growth of these hybrids in serum-free medium is also suppressed relative to growth of the L cell parent. There appear to be several genes on different chromosomes that are involved in suppression of serum-free growth of the fibroblast x L cell hybrids. One weak suppressor gene appears to be on the human X chromosome.

Anchorage-independent growth of primary rat embryo cells is induced by platelet-derived growth factor and inhibited by type-beta transforming growth factor

Several growth factors implicated in the process of cellular transformation were tested for their ability to induce anchorage-independent (AI) growth of primary rat embryo (RE) cells. Our results show that in the presence of 10% calf serum, platelet-derived growth factor (PDGF), 1-30 ng/ml, has the strongest effect of all growth factors tested on AI growth. Type-beta transforming growth factor (TGF-beta), by itself, does not stimulate AI growth, and it inhibits the PDGF-induced colony formation in a dose-dependent manner (ED50 approximately 0.03 ng/ml). Qualitatively similar responses are obtained by using an established line of fibroblasts, NIH 3T3 cells; the principal difference between the response of the primary cells and the established cell line is in colony-forming efficiency in soft agar culture (15% and 90%, respectively, for growth of colonies greater than 1,500 micron2 diameter in the presence of 10 ng/ml PDGF). Since AI growth has been shown to correlate well with tumorigenicity in vivo, our results suggest that the transforming potential of PDGF in an appropriate responsive cell can be controlled not only through its interaction with its own receptor, but also by the presence of inhibitory factors such as TGF-beta.

SV40 transformation of Swiss 3T3 cells can cause a stable reduction in the calcium requirement for growth

A well-characterized SV40-transformed Swiss 3T3 line, SV101, and its revertants were tested for the ability to grow in reduced Ca++ (0.01 mM). Transformants and revertants did not differ from the parent 3T3 line in their Ca++ requirements. All three classes of cells grew less well in low Ca++ than in regular Ca++ (2.0 mM). SV40 transformants were then selected for the ability to grow in reduced Ca++. This new class of transformants was found to grow in 1% serum, grow in soft agarose, have a reorganized actin cytoskeleton, and express viral T antigens, as well as grow well in low Ca++. One of the selected clones was found to be T antigen-negative, yet was transformed in the serum, anchorage, actin, and Ca++ assays. It is possible that this clone was a spontaneous transformant. However, Southern blot analysis revealed the presence of integrated SV40 DNA. In addition, this analysis revealed the absence of an intact early region fragment, which codes for the viral T antigens. One explanation of this result may be that the mechanism of viral transformation for growth in low Ca++ involves viral-host DNA interactions that may not require a fully functional T antigen. In this case SV40 integration may be acting as a nonspecific cellular mutagen.

Growth control variant cell line having increased serum requirement and decreased response to platelet-derived growth factor: reversion by 5-azacytidine

Variants of the mouse embryo fibroblast X melanoma hybrid clone 100A have been isolated by a procedure that selects against cells that are able to grow in medium containing low concentrations of serum plus insulin. Three variant clones derived from this selection were found to have a much higher serum requirement than the parental clone 100A cells, as evidenced by a very low rate of DNA synthesis and growth in medium containing low concentrations of serum. Two of the variants had approximately double the number of chromosomes as the parental cell line, while one had approximately the same number of chromosomes as the parental cells. One of the variants was very strongly reverted by 5-azacytidine but not by ethyl methanesulfonate, suggesting that it reverted by a nonmutational mechanism such as a stable change in DNA methylation. Analysis of the growth requirements in hormone-supplemented serum-free media of the 100A parent, the INS 471 variant, and revertants of the variant indicated that the variant had a specific deficiency in its growth response to platelet-derived growth factor (PDGF). PDGF dose-response curves obtained with the variant cells were shifted approximately an order of magnitude toward higher PDGF concentrations relative to PDGF dose-response curves obtained with the parental 100A cells. This quantitative increase in PDGF requirement of the INS 471 variant appears to explain the increased serum requirement of this variant. Equilibrium binding experiments performed with 125I-PDGF suggest that the variant does not have a decreased number of PDGF receptors.

Polyoma virus mutant with normal transforming ability but impaired tumorigenic potential

Cloned DNA from the P155 mutant of polyoma virus transforms cells in culture as efficiently as wild-type DNA, but has a much lower tumorigenic potential when injected into newborn rats. Like cells transformed by wild-type DNA, cells transformed by the mutant DNA grow in low serum concentrations, form colonies in agar suspension, and grow to high saturation densities compared with untransformed cells. They are, however, much less tumorigenic since they transplant 100- to 2,000-fold less efficiently than cells transformed by wild-type DNA. Substitution of the region between 89.7 and 1.8 map units by the corresponding region of P155 DNA decreased the tumorigenicity of wild-type DNA. When this region was isolated from wild-type DNA and substituted in P155 DNA, the tumorigenicity of the latter increased to values comparable to those of wild-type DNA. This showed that the lesion affecting tumorigenicity occurred between 89.7 and 1.8 map units on the polyoma virus genome. Sequence analysis in this region revealed a 12-base-pair deletion between nucleotides 1,347 and 1,360. This identified P155 as an mlt mutant, i.e., a mutant with a deletion from a region which encodes parts of the large and middle T antigens.

Fragments of the simian virus 40 transforming gene facilitate transformation of rat embryo cells

Segments of the simian virus 40 (SV40) genome that encode only fragments of large tumor antigen can facilitate immortalization of secondary rat embryo cells. The phenotypes of the immortalized cells range from nearly "normal" to fully transformed. All of the cell lines contain SV40 sequences and express unstable NH2-terminal fragments of large tumor antigen. SV40 small tumor antigen does not appear to be essential for either immortalization or transformation.

Transforming genes and gene products of polyoma and SV40

The small DNA-containing viruses, SV40 and polyoma, transform cells in vitro and induce tumors in vivo. For both viruses two genes required for transformation have been found. The genes required for transformation are also involved in productive infection. Although the two viruses are similar in their effects on cells, the organization of the transforming genes and gene products is different. The purpose of this review is to compare what is known about the biology and the biochemistry of the early regions of the two viruses. The genetic and biochemical studies defining the sequences important for transformation will be reviewed. Then, the products of the transforming genes, called T antigens, will be discussed in detail. There is a substantial body of descriptive information on those products, and studies on the function of the T antigens have also begun.

Phenotype of polyoma-induced hamster tumor cells lines

Cell lines from polyoma-induced hamster tumors exhibit a fully transformed phenotype despite the absence of the 105K (105,000-dalton) form of polyoma T-antigen.

Transformation of rat embryo fibroblasts by cloned polyoma virus DNA fragments containing only part of the early region

Recombinant plasmids containing either the entire polyoma viral genome or one or the other of the two HindIII fragments of polyoma virus DNA were constructed and cloned in Escherichia coli X1776, and their DNAs were individually tested for the capacity to transform an established line of rat cells. The recombinant plasmids containing the entire polyoma genome and those containing the HindIII-1 fragment of polyoma DNA (45-1.4 map units) efficiently transform rat cells, whereas the plasmids containing the HindIII-2 fragment (1.4-45.0 map units) do not. The properties of many independent transformed cell lines established by infection with the cloned HindIII-1 fragment were determined. In contrast to the parent cell line, rat cells transformed with the cloned HindIII-1 fragment grow to high saturation densities, form colonies with high efficiency in dilute agar suspension, produce high levels of plasminogen activator, and display a disorganized arrangement of actin cables. By all criteria examined, these cells transformed by fragments are indistinguishable from cells transformed by whole polyoma viral DNA. Cellular DNA prepared from many HindIII-1 fragment-transformed cell lines was analyzed for the presence and arrangement of polyoma viral sequences by Southern blot-hybridization. In all cases examined, only those viral sequences contained within the HindIII-1 fragment of polyoma DNA were detected. These data establish a strong correlation between polyoma DNA sequences mapping within a restricted portion of the early region and the induction and maintenance of the transformed phenotype.

Noncoordinate expression of SV40-induced transformation and tumorigenicity in mouse cell hybrids

Somatic mouse cells hybrids formed by fusion of nontumorigenic 3T3 closely related SV40-transformed SVT2 cells were analyzed in a study designed to probe the genetic basis of the multiple phenotypic changes induced by SV40 transformation. These hybrids showed noncoordinate expression of the transformation phenotype. Although they cloned at high efficiency in medium with low serum and expressed the SV40 T-antigen of the SVT2 parent, hybrid cells grew poorly without anchorage and exhibited a cell and colony morphology intermediate between that of the parents. Tumorigenicity was assayed quantitatively by subcutaneous coinjection into athymic nude mice of serial dilutions of 10(2) to 10(5) hybrid cells with 10(7) lethally irradiated 3T3 cells. The results showed that 100--1000 times more hybrid cells had to be injected for tumor formation than were required with SVT2. These and other observations show that most 3T3/SVT2 hybrid cells are not tumorigenic but that each population contains a rare subset of tumorigenic cells.

Growth characteristics of human skin fibroblasts in vitro: a simple experimental approach for the identification of hereditary adenomatosis of the colon and rectum

Kinetic parameters of cell growth have been studied in cultured skin fibroblasts derived from individuals with hereditary adenomatosis of the colon and rectum. The growth parameters consisted of saturation density, plating density, cloning efficiency and colony-forming ability. The results suggest deployment of these selective assays to distinguish skin fibroblasts of presumptive ACR positive individuals from those of normal subjects in vitro.

Expression of transformation in cell hybrids. I. Isolation and application of density-inhibited Balb/3T3 cells deficient in hypoxanthine phosphoribosyltransferase and resistant to ouabain

A cell line, THO2, was isolated from Balb/3T3 clone A31 after sequential nitrosoguanidine treatments and selection for resistance to 6-thioguanine and ouabain. THO2 retains the properties of density-dependent inhibition of growth and serum dependence of DNA synthesis characteristic of 3T3. The codominant expression of ouabain resistance and inability of THO2 to utilize exogenous hypoxanthine in the presence of aminopterin allows isolation of somatic cell hybrids involving THO2 and any ouabain-sensitive, hypoxanthine phosphoribosyltransferase-positive cell line. Hybrid clones derived from THO2 and SV40-transformed cells show dominant expression of the transformed phenotype with respect to multilayered arrangement of cells and ability to synthesize DNA in 1% calf-serum medium.