Nontransformed 3T3 T mesenchymal/proadipocyte stem cells can be readily induced to differentiate, yet previous work has shown that 3T3 T cells that are spontaneously or virally transformed not only lose their normal growth control mechanisms but also lose the ability to differentiate. Loss of growth control can be due to autocrine mechanisms in some transformed cells, but the mechanisms involved in disrupting differentiation control are poorly understood. Our goal is to further define the growth and differentiation defects that arise in neoplastically transformed cells and the mechanisms underlying those defects. For example, exogenous transforming growth factor beta and tumor necrosis factor, both of which are secreted aberrantly by some tumor cells, are known inhibitors of different steps of the normal 3T3 T adipocyte differentiation process, suggesting a potential role as autocrine factors in disrupting differentiation of transformed 3T3 T cells. In the current study we transformed 3T3 T cells in vitro with chemical or UV irradiation treatment in order to determine if the acquisition of the transformed phenotype after these treatments is also associated with loss of differentiation control as it is with spontaneously or virally transformed cells. Four chemically and two UV-treated 3T3 T cell lines were isolated from type III foci and all have been found to be tumorigenic in syngeneic animals and to have lost the ability to differentiate. Relative to the parental cell line the differentiation abilities of the transformed clones ranged from 0 to less than 5%. In this regard, we also analyzed the normal and aberrant expression of three growth factors and differentiation inhibitors in transformed cells. Both transforming growth factor alpha and beta were found to be expressed in nontransformed 3T3 T cells as determined by Northern blot analyses. In addition. both were found (o be down-regulated during differentiation of 3T3 T cells. Transformed/differentiation-defective 3T3 T cells expressed varied levels of transforming growth factor alpha and beta. Three of the new transformed clones expressed particularly high levels of transforming growth factor alpha. Very low levels of tumor necrosis factor expression were found in the normal cells and the transformed cells appeared to express tumor necrosis factor at similar levels. In contrast, none of the transformed cells expressed any of the differentiation-specific genes tested (lipoprotein lipase, glycerol-3-phosphate dehydrogenase, etc.). Even a transformed clone which could undergo growth arrest but not morphological differentiation expressed no differentiation-specific genes. Together, these data suggest that neoplastic transformation in general disrupts differentiation control. The aberrant expression of growth or differentiation-inhibiting factors may be involved in the loss of differentiation control in some transformed cells, but other mechanisms appear to be involved as well.
