A series of studies using farnesyltransferase (FTase) inhibitors that the inhibition of FTase function suppresses the growth of ras-transformed cells in vitro and in vivo. However, whether FTase is directly involved in the regulation of cell proliferation remains to be demonstrated. To investigate whether overexpression of FTase results in altered cell growth and transformation, we thus used NIH3T3 cells transfected with cDNA constructs of both β£ and β€ subunits of human FTase. FTase-overexpressing cells resulted in a 3-to 13-fold increase in the expression of the β£ and β€ subunit protein of FTase and a 1.5-to 3-fold increase in the level of the enzyme activity compared with untransfected NIH3T3 cells or vector-transfected cells. Further investigations using metabolic labeling indicated that farnesylation of Ras was enhanced in FTase-overexpressing cells. Insulin-like growth factor-I, platelet-derived growth factor (PDGF), and basic fibroblast growth factor (bFGF) more potently enhanced DNA synthesis and anchorage-dependent growth in FTaseoverexpressing cells than in control cells, in a dose-dependent manner. In particular, PDGF and bFGF also induced dosedependently enhanced colony formation in soft agar in FTaseoverexpressing cells. Furthermore, in FTase-transfectants, bFGF stimulated high activation of mitogen-activated protein kinase. Interestingly, FTase-transfectants developed progressive tumors in nude mice. Light and electron microscopy showed that the tumors were characteristic of fibrosarcoma, which were distinct from v-ras-induced tumors. Overexpression of FTase in NIH3T3 cells thus amplifies growth-factormediated cell growth and transformation, and FTaseoverexpressing cells form tumors in nude mice.