Response of FR3T3 cells transformed by HA‐ras oncogene and epidermal growth factor gene to differentiation induction by N,N‐dimethylformamide

Abstract

Treatment of chemically transformed fibroblasts with N,N-dimethylformamide (DMF) results in the restoration of a nontransformed phenotype. In an attempt to identify more precisely the mechanisms by which DMF reverses the transformed phenotype, the effects of DMF on fibroblasts which were transformed by a single gene--specifically a synthetic epidermal growth factor (EGF) gene or the Ha-ras oncogene--were examined. The constitutive expression of either the Ha-ras oncogene or the EGF gene in FR3T3 fibroblasts resulted in cellular transformation. The effect of the differentiation-inducing agent DMF on several properties of these transformed cell lines was examined. The EGF-transfected cells were much more responsive to DMF than the ras-transfected cells. DMF treatment of the EGF-transfected cells resulted in the inhibition of anchorage-independent growth, the restoration of a normal cellular morphology and growth rate in monolayer culture, and the down-regulation of the proliferation-associated nucleolar protein B23. DMF treatment had a much slighter effect on growth of the ras-transfected cells in monolayer culture or under anchorage-independent growth conditions. The high proliferation rate of the ras-3 cells was associated with elevated expression of protein B23. The 19-3 cells, but not the ras-3 cells, expressed cell-surface fibronectin. Treatment of the ras-3 cells with DMF did not restore fibronectin expression. The binding of EGF was increased 3-fold in the EGF-transfected cells and decreased 20-fold in ras-transfected cells, but in neither case did DMF alter EGF binding. DMF treatment increased the secretion of EGF in the 2 transfected lines as well as in control cells. These results suggest that the aberrant-growth control in the EGF-transfected cells, but not in the ras-transfected cells, could be modulated by DMF and that the aberrant-growth control mechanisms were different in these 2 cell types.