The 14 members of Eph kinases constitute the largest family of receptor tyrosine kinases. Recently, altered expression and mutation in Eph kinases have been documented in many human malignancies including colorectal and prostate cancer. In addition, Eph kinases are subject to alternative splicing which gives rise to diverse variants, further adding to the complexity in deciphering the significance of Eph kinases in tumor development and progression. Genetic studies have shown that Eph receptor tyrosine kinases have both kinase-dependent and kinase-independent functions through incompletely understood mechanisms. We report here that ephrin-B1 stimulation of endogenous EphB kinases in LS174 colorectal cancer epithelial cells inhibited integrin-mediated adhesion and HGF/SF-induced directional cell migration. Using 293 cells stably transfected with wild type (WT)- or kinase-deficient (KD)-EphB3, we found that inhibition of integrin-mediated cell adhesion and induction of cell rounding was kinase-dependent. Unexpectedly, in two independent assays, both KD- and WT-EphB3 significantly inhibited directional cell migration. Upon ephrin-B1 stimulation, the activities of Rac1 and Cdc42 were reduced in both WT- and KD-EphB3-expressing cells that were induced to migrate. Pharmacological evidence demonstrates that a relative increase in RhoA signaling as a result of decreased Rac1/Cdc42 activities contributes to the inhibitory effects. Furthermore, EphB3-mediated inhibitory effect on cell adhesion but not migration was abolished by the integrin-activating antibodies, suggesting that the inhibition of cell migration is not due to down-regulation of integrin function. These results uncover a differential requirement for EphB3 catalytic activity in the regulation of cell adhesion and migration, and suggest that while catalytic activity of EphB3 is required for inhibition of integrin-mediated cell adhesion, a distinct signaling pathway to Rho GTPases shared by WT- and KD-EphB3 receptor mediates inhibition of directional cell migration. Harnessing the intrinsic ability of EphB receptors to inhibit cell motility can provide a new strategy in the treatment and prevention of metastatic progression of colorectal as well as other malignancies.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]