Phenotypic modulation during tumorigenesis by clones of transformed rat liver epithelial cells

From nine clonal subpopulations (strains) of chemically transformed cultured rat hepatic epithelial cells which were tumorigenic when implanted into 1-day-old isogeneic rats, a cell line was reestablished from each tumor and the cellular properties of the tumor-derived cell lines were compared to those of the corresponding progenitor cells that were implanted to produce the tumors. In seven of eight instances, the cellular DNA content of the tumor-derived cells was virtually identical to the DNA content of the respective progenitor cells, but in one case the tumor cells had twice as much DNA as did their progenitor cells. During the development of tumors in vivo, other cellular phenotypic properties often underwent considerable, but variable changes. These changes included the activity of gamma-glutamyl transpeptidase, the growth properties on plastic surfaces, and the expression of LDH isozymes. Although there was a relative enhancement in the ability of most of the tumor-derived cells to proliferate or to form colonies in calcium-poor medium, several tumor-derived cell lines had very low colony-forming efficiencies in media containing either normal or low levels of calcium. The most consistent association between phenotypes expressed in vitro and tumorigenicity was the ability of cells to form colonies in soft agar; all tumor-derived lines expressed this phenotype, and with some of them this phenotype was acquired only during the process of tumor formation in vivo. These results demonstrate that further phenotypic and genotypic alterations may occur in vivo during tumor formation by chemically transformed cultured cells following their implantation into isogeneic animals; and some of the alterations that occur in vivo may be necessary for the complete expression of tumorigenicity. Although anchorage-independent growth capacity cannot be used to predict the tumorigenicity of clones of rat liver epithelial cells chemically transformed in vitro, this growth property appears to be invariably induced prior to or during the formation of tumors in vivo by these cells.