Engagement of Gab1 and Gab2 in erythropoietin signaling

Several signaling cascades are activated during engagement of the erythropo ietin receptor to mediate the biological effects of erythropoietin, The mem bers of the insulin receptor substrate (IRS) family of proteins play a cent ral role in signaling for various growth factor receptors and cytokines by acting as docking proteins for the SH2 domains of signaling elements, linki ng cytokine receptors to diverse downstream pathways. In the present study we provide evidence that the recently cloned IRS-related proteins, Gab1 and Gab2, of the Gab family of proteins, are rapidly phosphorylated on tyrosin e during erythropoietin treatment of erythropoietin-responsive cells and pr ovide docking sites for the engagement of the SHP2 phosphatase and the p85 subunit of the phosphatidylinositol 3'-kinase. Furthermore, our data show t hat Gab1 is the primary IRS-related protein activated by erythropoietin in primary erythroid progenitor cells, In studies to identify the erythropoiet in receptor domains required for activation of Gab proteins, we found that tyrosines 425 and 367 in the cytoplasmic domain of the erythropoietin recep tor are required for the phosphorylation of Gab2. Taken together, our data demonstrate that Gab proteins are engaged in erythropoietin signaling to me diate downstream activation of the SHP2 and phosphatidylinositol 3'-kinase pathways and possibly participate in the generation of the erythropoietin-i nduced mitogenic responses.