Development of resistance to the growth inhibitory effects of transforming growth factor beta 1 during the spontaneous transformation of rat liver epithelial cells.

The temporary maintenance of a rat liver epithelial cell population at confluence before passaging followed by periods of rapid proliferation resulted in the generation of spontaneous transformants after about 108 population doublings. The appearance of morphologically aberrant transformants correlated directly with an increased resistance of the population to the growth inhibitory effects of transforming growth factor beta 1 (TGF-beta 1). Clonal cell lines derived from the transformants were resistant to TGF-beta 1 dependent inhibition of DNA synthesis. These cell lines were also highly tumorigenic and aneuploid, with characteristic gross chromosomal abnormalities, and they expressed a number of phenotypic markers common to rat liver epithelial cells transformed by oncogenes or chemicals. In contrast, apparently normal looking cell lines cloned from the same population were nontumorigenic and near diploid, with few chromosomal abnormalities, and they were as sensitive to TGF-beta 1 as early passage normal rat liver epithelial cells. Morphologically normal late passage rat liver epithelial cells were sensitive to transformation by the DNA hypomethylating agent 5-aza-2-deoxycytidine, in contrast to earlier passage cells, and this transformation was accompanied by the development of resistance to the growth inhibitory effects of TGF-beta 1. These findings suggest that acquisition of resistance to the effects of growth inhibitors such as TGF-beta 1 is an important and possibly essential stage in the spontaneous transformation of rat liver epithelial cells.