Abstract
We previously reported that EphA4, a member of the Eph family of receptor tyrosine kinases, is an important modulator of growth hormone (GH) signaling, leading to augmented synthesis of insulin-like growth factor 1 (IGF1) for postnatal body growth. In the present study, we report the molecular interactions of EphA4, GH receptor (GHR), Janus kinase 2 (JAK2), and signal transducer and activator of transcription 5B (STAT5B). EphA4 binds to GHR at both its extracellular and intracellular domains and phosphorylates GHR when stimulated with a ligand. The cytoplasmic domain of EphA4 binds to the carboxy-terminus of JAK2 in contrast to the known binding of GHR to the amino-terminus. STAT5B binds to the amino-terminal kinase domain of EphA4. Ligand-activated EphA4 and JAK2 phosphorylate each other and STAT5B, but JAK2 does not appear to phosphorylate EphA4-bound STAT5B. Ligand-activated EphA4 induces the nuclear translocation of STAT5B in a JAK2-independent manner. GHR expression is required for the activation of STAT5B signaling, even via the JAK2-independent pathway. Various ephrins that have affinity for EphA4 induce STAT5B phosphorylation. These findings suggest the molecular mechanisms by which ephrin/EphA4 signaling enhances the canonical GH-IGF1 axis.