Eph receptors are the largest family of membrane bound receptor tyrosine kinases (RTKs) that have been implicated in various developmental processes as well as tumorigenesis. But the role of Eph kinases in tumor progression is controversial since on one hand it has been shown to inhibit or attenuate growth factors dependent MAPK activity and on the other hand, they are overexpressed and in some cases mutated in various types of tumors. However, in contrast to other RTKs, activation of Eph receptors fails to promote cell proliferation, or to transform rodent fibroblasts, indicating that Eph kinases may initiate signaling pathways that are distinct from those transmitted by other RTKs. We have previously reported that activation of EphA kinases by their ligand ephrinA1 inhibits the ras/Erk signaling cascade(Nature Cell Biology;3;p527). To characterize further regarding which EphA kinase/s are involved in this regulation, we established primary mouse embryonic fibroblasts (MEFs) from the EphA2 knock out (KO) mice. Here we show that disruption of EphA2 abolishes the inhibitory effect of ephrinA1 on Erk1/2 activity. More importantly, restoration of EphA2 expression rescued the inhibitory effect of ephrinA1 on Erk1/2 activity. In addition, deletion and site directed mutation analysis revealed that catalytic activity of kinase domain of EphA2 is required for Erk1/2 inhibition. Furthermore, we demonstrate that ephrinA1 is also capable of mediating EphA1 activation induced Erk1/2 inhibition. In summary, our data establishes that EphA1 and EphA2 are sufficient in mediating suppression of Erk1/2 activity in MEFs, and suggest that this activation of EphA kinases can be attractive targets for cancer therapy.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]