Constitutive expression of fibroblast growth factor-4 does not alter the growth or the differentiation of embryonal carcinoma cells

Previous studies have demonstrated that embryonal carcinoma (EC) cells express both fibroblast growth factor-4 (FGF-4) and FGF receptors. It has also been established that differentiation of EC cells represses the expression of the FGF-4 gene. Currently, the role of FGF-4 in the growth and differentiation of EC cells is unclear. In this study, we examined whether the differentiation of EC cells requires the repression of FGF-4 expression. To address this and related questions, F9 EC cells were transfected with an expression vector that uses the human β-actin promoter to drive the constitutive expression of recombinant FGF-4. Unlike their untransfected counterparts, F9 EC cells transfected with this plasmid continue to produce recombinant FGF-4 after they differentiate. However, constitutive expression of this growth factor does not block morphological differentiation of the cells, nor does it alter the expression of six genes regulated by the differentiation of EC cells. Constitutive expression of recombinant FGF-4 also did not noticeably alter the growth of the transfected F9 EC cells before or after differentiation. Furthermore, unlike immortalized fibroblasts, which are known to grow in soft agar after transfection with FGF-4 expression plasmids, continued expression of recombinant FGF-4 activity did not enhance the ability of the EC-derived differentiated cells to form colonies in soft agar. These findings argue that continuous expression of recombinant FGF-4 activity does not block the differentiation of EC cells and that repression of the FGF-4 gene after EC cells differentiate does not appear, on its own, to be responsible for the loss of tumorigenicity that accompanies the differentiation of EC cells.