Roles for an Epo receptor Tyr-343 Stat5 pathway in proliferative co-signaling with Kit

Erythroid progenitor cell expansion depends upon cosignaling by Epo receptor (EpoR) and Kit, but underlying mechanisms are incompletely understood. To quantitatively analyze EpoR contributions to co-signaling, phosphotyrosine (Tyr(P)) mutants were expressed as human epidermal growth factor ( hEGF) receptor-mEpoR EE chimeras at matched and physiological levels in FDCW2 hematopoietic progenitor cells and were assayed for proliferative activities in the absence or presence of endogenous Kit stimulation. Two Tyr( P)-null ( but Jak2-coupled) EpoR forms each retained less than or equal to 25% of the wild-type activity, whereas the add-back of single Tyr( P) sites in the EpoR forms EE-T-Y343 (Stat5 binding site), EE-Y479 (p85/phosphatidylinositol 3-kinase binding site), or EE-Y464 (Src kinase binding site) significantly enhanced activities ( to 100, 95, and 50% of EE-WT ( wild type) levels, respectively). EE-Y343&Y401 and EE-F343& F401 double add-back and deletion constructs were also prepared and were shown to possess 90 and less than or equal to 50% of wild-type activity. In contrast, efficient Kit cosignaling activity was retained only by EE-T-Y343 and EE-Y343&Y401 EpoR forms. EE-T-Y343 together with EE-T-Y343F and EE-WT EpoR forms were also analyzed in embryonic stem cell-derived erythroid G1E-2 cells with highly comparable outcomes, including the ability of EE-T-Y343 ( but not EE-T-Y-343F) to synergize with Kit. Despite specific connection of EE-T-Y343 to Stat5, the contributions of Kit to EpoR-dependent proliferation did not involve Kit effects on Stat5 activation ( but was limited by the mutation of Kit Tyr( P)-567 and Tyr( P)-569 Src kinase recruitment sites). Instead, cosignaling appears to depend upon the downstream integration of Kit signals with the targets of an EpoR/Jak2/ Y343/Stat 5 response axis.