Erythropoietin Induces the Tyrosine Phosphorylation of GAB1 and Its Association With SHC, SHP2, SHIP, and Phosphatidylinositol 3-Kinase

GRB2-mediated recruitment of GAB2, but not GAB1, to SF-STK supports the expansion of Friend virus-infected erythroid progenitor cells

Friend virus induces the development of erythroleukemia in mice through the interaction of a viral glycoprotein, gp55, with a truncated form of the Stk receptor tyrosine kinase, short form-Stk (Sf-Stk), and the EpoR. We have shown previously that the ability of Sf-Stk to participate in the transformation of Friend virus-infected cells requires the kinase activity and Grb2-binding site of Sf-Stk. Here we show that Grb2 heterozygous mice exhibit decreased susceptibility to Friend erythroleukemia and that expansion of erythroid progenitors in response to infection requires the C-terminal SH3 domain of Grb2. A fusion protein in which the Grb2-binding site in Sf-Stk is replaced by Gab2, supports the growth of progenitors from mice lacking Sf-Stk, whereas a Sf-Stk/Gab1 fusion protein does not. Gab2 is expressed in spleens from Friend virus-infected mice, co-immunoprecipitates with Sf-Stk and is tyrosine phosphorylated in the presence of Sf-Stk. Mice with a targeted deletion in Gab2 are less susceptible to Friend erythroleukemia and the expansion of erythroid progenitor cells in response to infection can be rescued by expression of Gab2, but not Gab1. Taken together, these data indicate that a Sf-Stk/Grb2/Gab2 complex mediates the growth of primary erythroid progenitor cells in response to Friend virus.

Lyn deficiency reduces GATA-1, EKLF and STAT5, and induces extramedullary stress erythropoiesis

In vitro studies have implicated the Lyn tyrosine kinase in erythropoietin signaling. In this study, we show that J2E erythroid cells lacking Lyn have impaired signaling and reduced levels of transcription factors STAT5a, EKLF and GATA-1. Since mice lacking STAT5, EKLF or GATA-1 have red cell abnormalities, this study also examined the erythroid compartment of Lyn(-/-) mice. Significantly, STAT5, EKLF and GATA-1 levels were appreciably lower in Lyn(-/-) erythroblasts, and the phenotype of Lyn(-/-) animals was remarkably similar to GATA-1(low) animals. Although young adult Lyn-deficient mice had normal hematocrits, older mice developed anemia. Grossly enlarged erythroblasts and florid erythrophagocytosis were detected in the bone marrow of mice lacking Lyn. Markedly elevated erythroid progenitors and precursor levels were observed in the spleens, but not bone marrow, of Lyn(-/-) animals indicating that extramedullary erythropoiesis was occurring. These data indicate that Lyn(-/-) mice display extramedullary stress erythropoiesis to compensate for intrinsic and extrinsic erythroid defects.

Role of Gab proteins in phosphatidylinositol 3-kinase activation by thrombopoietin (Tpo)

In this study, we show that upon thrombopoietin (Tpo) stimulation the two adapter proteins Gab1 and Gab2 are strongly tyrosine phosphorylated and associated with Shc, SHP2, PI 3-kinase and Grb2 in mpl-expressing UT7 cells. Although Gab1 and Gab2 seem to mediate overlapping biological signals in many cells, only Gab1 is expressed and phosphorylated in response to Tpo in primary human megakaryocytic progenitors; furthermore, it associates with the same proteins. Although a low level of tyrosine phosphorylated IRS-2 protein is also detected in PI 3-kinase immunoprecipitates, Gab proteins are the essential proteins associated with PI 3-kinase after Tpo stimulation. We demonstrate that, albeit no association is detected between the Tpo receptor mpl and Gab proteins, Y112 located in the C-terminal cytoplasmic domain of mpl is required for Gab1/2 tyrosine phosphorylation. Gab proteins are not tyrosine phosphorylated after Tpo stimulation of UT-7 and Ba/F3 cells expressing a mpl mutant lacking Y112. Moreover, no activation of the PI 3-kinase/Akt pathway is observed in cells expressing this mpl mutant. Finally, we show that this mutant does not allow cell proliferation, thereby confirming that PI 3-kinase activation is required for Tpo-induced cell proliferation.

Constitutively active STAT5 variants induce growth and survival of hematopoietic cells through a PI 3-kinase/Akt dependent pathway

Signal Transducer and Activator of Transcription (STATs) are important mediators of cytokine and growth factor-induced signal transduction. STAT5A and STAT5B have been shown to play a role in survival and proliferation of hematopoietic cells both in vitro and in vivo and to contribute to the growth and viability of cells transformed by the TEL-JAK2 oncoprotein. In this study, we investigated the molecular mechanisms by which constitutively active STAT5 proteins induce cell proliferation and survival of Ba/F3 cell lines expressing either dominant positive STAT5A or STAT5B variants or TEL-JAK2 or TEL-ABL fusion proteins. Our results showed that active STAT5 constitutively interacted with p85, the regulatory subunit of the PI 3-kinase. A constitutive activity of the PI 3-kinase/Akt pathway was observed in these cells and required for their cell cycle progression. In contrast, while activity of the PI 3-kinase/Akt pathway was required for survival of Ba/F3 cells expressing the constitutively active forms of STAT5A or STAT5B, it was dispensable for cells transformed by TEL-JAK2 or TEL-ABL fusion proteins, suggesting that additional survival pathways take place in these transformed cells.

Gab1 phosphorylation: a novel mechanism for negative regulation of HGF receptor signaling

Signal transduction by HGF receptor, the tyrosine kinase encoded by the MET oncogene, switches on a genetic program called 'invasive growth' inducing epithelial cell dissociation, migration, growth, and ultimately leading to differentiation into branched tubular structures. Sustained tyrosine phosphorylation of the downstream adaptor protein Gab1 is required for the HGF response. Here we show that serine/threonine phosphorylation of Gab1 provides a control mechanism for negative regulation. Treatment with okadaic acid, a potent inhibitor of the serine/threonine protein phosphatases PP1 and PP2A, was followed by activation of a number of serine/threonine kinases, hyper-phosphorylation in serine and threonine of Gab1 and severe inhibition of the HGF-induced biological responses. Under these conditions, Gab1 was found to be concomitantly hypo-phosphorylated in tyrosine, and thus endowed with reduced ability to recruit SH2 containing signal transducers such as PI3 kinase. Among the serine-threonine kinases activated by PP1 and PP2A inhibition, we found that PKC-alpha and PKC-beta1 are required for negative regulation of Gab1. These data provide a novel negative mechanism for the HGF receptor signaling pathways and highlight a potentially useful target for inhibitors of invasive growth.