FGF-8 isoforms differ in NIH3T3 cell transforming potential.

We previously identified Fgf-8 as a frequently activated gene in tumors from mouse mammary tumor virus-infected Wnt-1 transgenic mice, suggesting that Fgf-8 is a proto-oncogene. We further determined that multiple, secreted protein isoforms that differ at their mature amino termini are encoded by alternatively spliced mRNAs transcribed from the gene. We now present evidence that there are differences in the potency of NIH3T3 cell transformation displayed by three of the FGF (fibroblast growth factor)-8 isoforms. We find that stable transfection of a cDNA for the FGF-8b isoform leads to marked morphological transformation of NIH3T3 cells and rapid tumorigenicity of the transfected cells in nude mice. In contrast, transfection of a cDNA for the FGF-8a or FGF-8c isoform results in moderate morphological changes in the NIH3T3 cells, and the transfected cells are weakly tumorigenic in nude mice. All three transfections result in cells that express comparable amounts of Fgf-8 mRNA and that produce the FGF-8 protein isoforms. The morphological changes observed in NIH3T3 cells can be reproduced by the addition of recombinant FGF-8 protein isoforms to the culture medium. Therefore, these results indicate that there are differences in the potency of transformation of NIH3T3 cells by FGF-8 protein isoforms and suggest that these FGF-8 isoforms may have different in vivo functions.