Dose-dependent transformation of cells of human fibroblast cell strain MSU-1.1 by cobalt-60 gamma radiation and characterization of the transformed cells.

Cells from an infinite-life-span near-diploid human fibroblast cell strain, MSU-1.1, were transformed after a single exposure to 60Co gamma radiation. The frequency of transformation as measured by the number of induced foci per 10(6) cells was a linear function of dose. Cells from 13 independent foci from gamma-irradiated cell populations and one from a nonirradiated cell population were isolated, clonally expanded and assayed for characteristics of malignantly transformed cells. Eight of the 13 focus-derived cell strains from the irradiated populations formed tumors in athymic mice with latent periods (time required for the tumors to reach 1 cm in diameter) of 4-27 weeks. Of these 8 cell strains, 3 were fully growth factor-independent, formed large colonies (> 120 microm in diameter) in 0.33% agarose at a high frequency (50%), and produced malignant tumors with a mean latency of 6 weeks or less at all sites injected. Four others formed colonies in agarose at a slightly lower frequency, were only partially growth factor-independent, and produced malignant tumors with a longer mean latency (7-18 weeks). The tumor-derived cell lines from these latter 4 cell strains, when tested for growth in agarose, showed markedly enhanced anchorage independence. The eighth tumorigenic focus-derived cell strain was growth factor-independent but could not produce large colonies in agarose. It produced benign tumors (fibromas) with a mean latency of 27 weeks. All 8 tumorigenic focus-derived cell strains had lost the transactivating function of the TP53 (formerly known as p53) gene. However, loss of TP53 activity was not sufficient to cause tumorigenicity since 3 of the 6 nontumorigenic focus-derived cell strains had also lost all TP53 transactivation function. The other 3, which included a cell strain from the unirradiated control, had wild-type TP53 alleles and did not form tumors. These latter results support the hypothesis that loss of TP53 transactivating function plays a role in focus formation, but does not directly cause tumorigenicity. This is in agreement with studies that demonstrate that the loss of TP53 transactivation facilitates the other changes required for tumorigenicity.

Malignant transformation of human fibroblasts by ionizing radiation.

As one step in developing an assay for quantifying the induction of malignant transformation of human cells by ionizing radiation, we exposed cells from a non-tumorigenic, infinite life span, near-diploid fibroblast strain MSU-1.1 to 4.35 Gy 60Co radiation and assayed them for focus formation. The mean frequency of foci in the irradiated population was 6 x 10(-7) cells assayed. No foci were found in the control cells. Of four focus-derived cell strains studied in detail, two produced malignant tumours within 3-7 weeks. The other two did not produce tumours during the 12-month period of study. The tumours from one strain were classified as sarcomas composed exclusively of spindle-shaped cells. Tumours from the other strain were sarcomas consisting of a mixed population of round and spindle cells. Immunoprecipitation analysis of the status of the p53 gene in the focus-derived strains, using a mutant-specific anti-body (Pab240) and an antibody that recognizes both mutant and wild-type p53 protein (Pab421), showed that the tumorigenic strains were completely devoid of p53 protein. One non-tumorigenic strain expressed wild-type p53 protein, and the other expressed a lower molecular weight form of the protein. Karyotypic analysis showed that the tumour-derived cells from one tumorigenic strain had lost one copy of chromosome 6, 14, 16 and 17. The tumour-derived cells from the second strain had lost one copy of chromosome 7, 13, 14 and 17 and part of chromosome 6, as well as part of the other copy of chromosome 7 and 17. These results suggest that the common loss of one copy of chromosome 14, 17 and part of 6 plays a causal role in the malignant transformation of these cells. Furthermore, the results indicate that it will be possible to develop a system that uses near-diploid human fibroblasts to quantify radiation-induced malignant transformation.

v-sis oncogene-induced transformation of human fibroblasts into cells capable of forming benign tumors.

Normal human fibroblasts do not synthesize platelet-derived growth factor (PDGF), but they respond to its mitogenic action. In contrast, cells from several human fibrosarcomas have been shown to synthesize PDGF at significant levels. To investigate the possible role of PDGF expression in the development of tumors of mesenchymal origin in humans, we transfected a plasmid carrying the v-sis oncogene and a selectable marker into an infinite lifespan, non-tumorigenic human fibroblast cell strain, MSU-1.1. The v-sis gene codes for a protein homolog of PDGF-B. Of the six independent drug-resistant transfectants clonally isolated, three expressed a relatively low level of v-sis mRNA and protein, grew to a saturation density only 2-2.5 times higher than MSU-1.1 cells, did not form colonies in agar, were not growth factor independent and did not form tumors. The other three expressed v-sis mRNA and protein at a high level, grew to a very high saturation density (> 750,000 cells/cm2), replicated in medium lacking exogenous growth factors almost as rapidly as with 10% serum, formed large colonies in 0.33% agarose and, when injected into athymic mice, formed tumors that grew rapidly. Tumors that were removed after 6 weeks were classified as benign (fibromas). However, several of the tumors that were left in the animals for 6 months developed focal areas of cells showing features of poorly differentiated spindle cell or round cell sarcomas. Similarly, cells isolated from a very large sized agarose colony formed by one of the other v-sis strains expressed very high levels of v-sis mRNA and protein and formed large, very fast growing benign tumors. Re-injection of cells from the tumors yielded tumors composed of two distinct areas: spindle cell fibromas and high grade sarcomas. These results suggest that if mesenchymal cells in the body were induced to express their PDGF-B gene, this could lead to the formation of benign tumors of mesenchymal origin. The system described can serve as a model for studying the mechanisms of formation of such tumors in humans and their progression to the malignant state.

Malignant transformation of human fibroblast cell strain MSU-1.1 by (+-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo [a]pyrene.

Treatment of MSU-1.1 cells, a near-diploid, karyotypically stable, infinite life-span human fibroblast strain, with (+-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene induced focus formation. Eight independent foci were isolated and the cell strains developed from them were examined for characteristics of malignant cells. Each grew to a higher density in medium containing 1% serum than did the MSU-1.1 cells. Three of the eight grew rapidly in serum-free medium without added growth factors, formed colonies in agarose with diameters of greater than or equal to 120 microns at a frequency of 5-19%, exhibited loss of genetic material, and, when injected into athymic mice, formed sarcomas that reached 6 mm in diameter within 2-3 wk. One produced high-grade sarcomas (progressively growing, invasive tumors exhibiting high mitotic activity); the other two produced low-grade sarcomas (tumors with a lower degree of mitotic activity) that developed focal areas of high-grade malignant cells if left in the animals for greater than 4 wk. A fourth cell strain formed high-grade sarcomas only after 2.5-3 mo, but the tumor-derived cells analyzed showed the same growth properties as the three malignant cell strains described above, exhibited loss of genetic material, and, when reinjected into athymic mice, produced high-grade sarcomas with a short latency period.

Characteristics of an infinite life span diploid human fibroblast cell strain and a near-diploid strain arising from a clone of cells expressing a transfected v-myc oncogene.

Diploid human fibroblasts were transfected with a plasmid carrying a v-myc oncogene linked to the neo gene or with a vector control carrying a neo gene. Drug-resistant clones were isolated and subcultured as needed. All populations went into crisis and eventually senesced. But while they were senescing, viable-appearing clones were noted among the progeny of a transfected population that expressed the v-myc oncogene. After several months, these cells began replicating more rapidly. Karyotype analysis indicated that they were clonally derived since all of them had 45 chromosomes, including 2 marker chromosomes. This cell strain was designated MSU-1.1. Similar analysis showed that cells from an earlier passage were diploid. These cells were designated MSU-1.0. Both strains have undergone more than 200 population doublings since their siblings senesced, without any change in chromosome complement. Both strains express the v-myc protein and have the same integration site for the transfected v-myc and neo genes. The MSU-1.0 cells cannot grow without exogenously added growth factors. The MSU-1.1 cells grow moderately well under the same conditions and grow to a higher saturation density than MSU-1.0 cells. Since the chance of human cells acquiring an infinite life span in culture is very rare, the data suggest that MSU-1.1 cells are derived from MSU-1.0 cells. The expression of v-myc is probably required for acquisition of an infinite life span, since this phenotype did not develop in populations not expressing this oncogene. However, expression of v-myc is clearly not sufficient, since all of the progeny of the clone that gave rise to the MSU-1.0 cells expressed this oncogene, but the vast majority of them senesced.