ERYTHROPOIETIN INDUCES TYROSINE PHOSPHORYLATION OF THE INTERLEUKIN-3 RECEPTOR-BETA SUBUNIT (BETA(IL3)) AND RECRUITMENT OF STAT5 TO POSSIBLESTAT5-DOCKING SITES IN BETA(IL3)

The receptors for erythropoietin (Epo) and interleukin-3 (IL-3) both i nduce the ligand-dependent activation of the Jak2 tyrosine kinase. Act ivated Jak2 then phosphorylates these receptors and thereby recruits v arious signaling molecules containing the Src homology (SH)-2 domain, including Stat5, to the tyrosine phosphorylated receptors. In the pres ent study, we demonstrate that Epo stimulation induces unidirectional cross-phosphorylation of the IL-3 receptor beta subunit (beta(IL3)) on tyrosines and its rapid and transient association with Stat5 in murin e IL-3-dependent cell lines engineered to express the Epo receptor (Ep oR), Using cell lines expressing various EpoR mutants, it was demonstr ated that the Epo-induced tyrosine phosphorylation of beta(IL3) is dep endent on the membrane-proximal EpoR cytoplasmic region involved in th e activation of Jak2, hut not on the extracellular and transmembrane r egions or on the carboxy-terminal 145 amino acid region containing all the intracellular tyrosine residues. It was also shown that IL-3 indu ces rapid and dose-dependent association of Jak2 with beta(IL3). Howev er, Epo failed to induce any detectable association of beta(IL3) with Jak2 or the EpoR, The present study also demonstrates that in IL-3-sti mulated cells, an ovine Stat5 mutant harboring a substitution of Tyr69 4 to Phe, which abolishes the tyrosine phosphorylation required for ac tivation, fails to dimerize with endogenous Stat5, shows sustained bin ding with tyrosine-phosphorylated beta(IL3), and inhibits the tyrosine phosphorylation of endogenous Stat5. These results suggest that beta( IL3) may have Stat5 docking sites, similar to those found in the EpoR, that facilitate the activation of Stat5 by Jak2 and raise the possibi lity that Epo may cross-activate or transmodulate the IL-3 receptor si gnaling pathways. (C) 1997 by The American Society of Hematology.