Transformation and amplification of the K-fgf proto-oncogene in NIH-3T3 cells, and induction of metastatic potential.

A plasmid containing the K-fgf proto-oncogene linked to the dihydrofolate reductase gene has been constructed, and used in transfection experiments to investigate the effects of K-fgf expression on the tumorigenic and metastatic properties of NIH-3T3 fibroblasts. Analysis of cells transfected with K-fgf revealed that expression of the K-fgf proto-oncogene can, in a single step, induce both tumorigenic and metastatic characteristics, as determined in soft agar cloning experiments, and in tumorigenicity and experimental lung metastasis assays with BALB/c nu/nu mice. Selection for resistance to increasing concentrations of methotrexate lead to the isolation of a series of cell lines containing amplifications of both the dihydrofolate reductase gene and the linked K-fgf gene, which synthesized elevated levels of growth factor message and protein. The most highly resistant and gene amplified cell lines exhibited lower than expected levels of K-fgf mRNA, and also appeared to have down-regulated cell surface growth factor receptors. Further support for the concept that altered K-fgf expression can induce fully malignant and metastatic cells was obtained in experimental metastasis assays, where K-fgf transfected and gene amplified cell lines were highly aggressive.