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 Co-60 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 f oci from gamma-irradiated cell populations and one from a nonirradiated cel l population were isolated, clonally expanded and assayed for characteristi cs of malignantly transformed cells. Eight of the 13 focus-derived cell str ains from the irradiated populations formed tumors in athymic mice with lat ent periods (time required for the tumors to reach 1 cm in diameter) of 4-2 7 weeks. Of these 8 cell strains, 3 were fully growth factor-independent, f ormed large colonies (>120 mu m in diameter) in 0.338 agarose at a high fre quency (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, an d produced malignant tumors with a longer mean latency (7-18 weeks). The tu mor-derived cell lines from these latter 4 cell strains, when tested for gr owth in agarose, showed markedly enhanced anchorage independence. The eight h tumorigenic focus-derived cell strain was growth factor-independent but c ould not produce large colonies in agarose. It produced benign tumors (fibr omas) 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 tu morigenicity 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 d emonstrate that the loss of TP53 transactivation facilitates the other chan ges required for tumorigenicity. (C) 1998 by Radiation Research Society.