Rap1-suppressed tumorigenesis is concomitant with the interference in ras effector signaling

Expression of Rap1 blocks epithelial growth factor-induced extracellular signal-regulated kinases (ERKs) activation. However, recent studies demonstrated that Rap1 mediates ERKs activation induced by nerve growth factor. The anti-oncogenic effect of Rap1 has been reported but its mechanism remains unclear. To evaluate the correlation between the anti-transforming effect and the activation of ERKs, we transfected rap1 cDNA into Hep3B cells and selected stable transfectants. The Rap1 transfectants completely lost their intrinsic tumorigenicity in Balb/c nude mice. Both insulin and 12-O-tetradecanoyl phorbol-13-acetate (TPA)-stimulated ERK activations were also blocked. Our findings suggest that Rap1-suppressed tumorigenicity is concomitant with ERKs inhibition.