Membrane-bound ephrinB proteins bind to and activate EphB receptor tyrosine kinases on adjacent cells after cell-to-cell contact and regulate many processes, including cell adhesion. The activation of EphB receptors is considered a "forward" signal of ephrinB. In the cell expressing ephrinB, "reverse signaling" occurs and involves phosphorylation of the C terminus of ephrinB leading to the recruitment of the adaptor protein Grb4 (growth factor receptor-bound protein 4) and the activation of focal adhesion kinase (FAK). Sequence analysis of the ephrinB C terminus revealed the presence of the conserved YXXQ motif that is important for the binding and activation of signal transducer and activator of transcription 3 (STAT3), which led Bong et al. to investigate whether ephrinB might also activate STAT3. The authors performed a series of immunoprecipitation and Western blotting experiments in Xenopus oocytes (and mammalian cell lines) transfected with wild-type and mutant forms of ephrinB and STAT3. ephrinB was activated either by the addition of the EphB2 ectodomain fused to the Fc portion of human immunoglobulin or in response to fibroblast growth factor (FGF) receptor activation, which is known to lead to ephrinB phosphorylation. ephrinB that was phosphorylated at two specific tyrosine residues recruited STAT3, which resulted in STAT3 phosphorylation that was Janus kinase 2 (Jak2)-dependent. Reporter assays and gel-shift experiments showed that ephrinB activation also increased both the DNA-binding and transcriptional activity of STAT3. Immunofluorescence microscopy in mouse embryonic neuroepithelial cells showed that ephrinB activation resulted in the phosphorylation and nuclear localization of STAT3. Together, these data suggest that STAT3-mediated regulation of gene transcription may represent another "reverse" signaling pathway for ephrinB.
Y.-S. Bong, H.-S. Lee, L. Carim-Todd, K. Mood, T. G. Nishanian, L. Tessarollo, I. O. Daar, ephrinB1 signals from the cell surface to the nucleus by recruitment of STAT3. Proc. Natl. Acad. Sci. U.S.A. 104, 17305-17310 (2007). [Abstract] [Full Text]