LOSS OF DIFFERENTIATION CONTROL IN TRANSFORMED 3T3 T-PROADIPOCYTES

Nontransformed 3T3 T mesenchymal/proadipocyte stem cells can be readil y induced to differentiate, yet previous work has shown that 3T3 T cel ls that are spontaneously or virally transformed not only lose their n ormal growth control mechanisms but also lose the ability to different iate. Loss of growth control can be due to autocrine mechanisms in som e transformed cells, but the mechanisms involved in disrupting differe ntiation control are poorly understood. Our goal is to further define the growth and differentiation defects that arise in neoplastically tr ansformed cells and the mechanisms underlying those defects. For examp le, exogenous transforming growth factor beta and tumor necrosis facto r, both of which are secreted aberrantly by some tumor cells, are know n inhibitors of different steps of the normal 3T3 T adipocyte differen tiation process, suggesting a potential role as autocrine factors in d isrupting differentiation of transformed 3T3 T cells. In the current s tudy we transformed 3T3 T cells in vitro with chemical or UV irradiati on treatment in order to determine if the acquisition of the transform ed phenotype after these treatments is also associated with loss of di fferentiation control as it is with spontaneously or virally transform ed cells. Four chemically and two UV-treated 3T3 T cell lines were iso lated from type III foci and all have been found to be tumorigenic in syngeneic animals and to have lost the ability to differentiate. Relat ive to the parental cell line the differentiation abilities of the tra nsformed clones ranged from 0 to less than 5%. In this regard, we also analyzed the normal and aberrant expression of three growth factors a nd differentiation inhibitors in transformed cells. Both transforming growth factor alpha and beta were found to be expressed in nontransfor med 3T3 T cells as determined by Northern blot analyses. In addition. both were found (o be down-regulated during differentiation of 3T3 T c ells. Transformed/differentiation-defective 3T3 T cells expressed vari ed levels of transforming growth factor alpha and beta. Three of the n ew transformed clones expressed particularly high levels of transformi ng growth factor alpha. Very low levels of tumor necrosis factor expre ssion were found in the normal cells and the transformed cells appeare d to express tumor necrosis factor at similar levels. In contrast, non e of the transformed cells expressed any of the differentiation-specif ic genes tested (lipoprotein lipase, glycerol-3-phosphate dehydrogenas e, etc.). Even a transformed clone which could undergo growth arrest b ut not morphological differentiation expressed no differentiation-spec ific genes. Together, these data suggest that neoplastic transformatio n in general disrupts differentiation control. The aberrant expression of growth or differentiation-inhibiting factors may be involved in th e loss of differentiation control in some transformed cells, but other mechanisms appear to be involved as well.