Phosphorylation of tyrosine 503 in the erythropoietin receptor (EpR) is essential for binding the P85 subunit of phosphatidylinositol (PI) 3-kinase and for EpR-associated PI 3-kinase activity

Erythropoietin induces the tyrosine phosphorylation of insulin receptor substrate-2. An alternate pathway for erythropoietin-induced phosphatidylinositol 3-kinase activation

In this report, we demonstrate that insulin receptor substrate-2 (IRS-2) is phosphorylated on tyrosine following treatment of UT-7 cells with erythropoietin. We have investigated the expression of IRS-1 and IRS-2 in several cell lines with erythroid and/or megakaryocytic features, and we observed that IRS-2 was expressed in all cell lines tested. In contrast, we did not detect the expression of IRS-1 in these cells. In response to erythropoietin, IRS-2 was immediately phosphorylated on tyrosine, with maximal phosphorylation between 1 and 5 min. Tyrosine-phosphorylated IRS-2 was associated with phosphatidylinositol 3-kinase and with a 140-kDa protein that comigrated with the phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase, SHIP. Moreover, IRS-2 was constitutively associated with the erythropoietin receptor. We did not observe the association of IRS-2 with JAK2, Grb2, or PTP1D. Using BaF3 cells transfected with mutated erythropoietin receptors, we demonstrate that neither the tyrosine residues of the intracellular domain nor the last 109 amino acids of the erythropoietin receptor are required for erythropoietin-induced IRS-2 tyrosine phosphorylation. Altogether, our results indicate that erythropoietin-induced IRS-2 tyrosine phosphorylation could account for the previously reported activation of phosphatidylinositol 3-kinase mediated by erythropoietin receptors mutated in the phosphatidylinositol 3-kinase-binding site (Damen, J., Cutler, R. L., Jiao, H., Yi, T., and Krystal, G. (1995) J. Biol. Chem. 270, 23402-23406; Gobert, S., Porteu, F., Pallu, S., Muller, O., Sabbah, M., Dusanter-Fourt, I., Courtois, G., Lacombe, C., Gisselbrecht, S., and Mayeux, P. (1995) Blood 86, 598-606).

Role of the vav proto-oncogene product (Vav) in erythropoietin-mediated cell proliferation and phosphatidylinositol 3-kinase activity

The vav proto-oncogene product (Vav), which is specifically expressed in hematopoietic cells, contains multiple structural motifs commonly used by intracellular signaling molecules. Although a variety of stimuli including erythropoietin (Epo) have been shown to tyrosine phosphorylate Vav, little is known about the Vav signal transduction pathway. Here, we have investigated the role of Vav in the Epo signaling pathway by characterizing its interaction with other proteins, using the human Epo-responsive cell line, F-36P. Immunoprecipitation and immunoblot analyses have demonstrated that Vav was associated with the Epo receptor (EpoR) in an Epo-independent manner and was tyrosine-phosphorylated after Epo stimulation. Furthermore, two phosphotyrosine proteins (pp70 and pp100) co-immunoprecipitated with the regulatory subunit of phosphatidylinositol 3-kinase (PI3-kinase) (p85) were identified as EpoR and Vav, respectively. The interaction between Vav and p85 was shown to be mediated through the SH2 domains of p85 by an in vitro binding assay and confirmed by the presence of in vitro PI3-kinase activity associated with Vav. Treatment of the cells with antisense-vav and -p85 abrogated Epo-induced cell proliferation and PI3-kinase activity. Finally, we found that JAK2 was associated with Vav in vivo and that Vav could be tyrosine-phosphorylated by activated JAK2 in vitro. These results suggest the possible role of JAK2 for tyrosine phosphorylation of Vav and involvement of Vav and PI3-kinase in Epo-induced proliferative signals.

Identification of signalling components in tyrosine kinase cascades using phosphopeptide affinity chromatography

Various methods are now available to identify the molecular partners of the component of a signal transduction pathway. Some interactions, however, can be technically difficult to detect because they depend upon transient tyrosine phosphorylation. Here, we present a simple affinity chromatography approach based on synthetic phosphopeptides to purify potential partners of phosphotyrosine-containing proteins. With this approach, we confirm the previously characterized interaction between Grb2 and the EGF receptor, and we identify novel partners of the IGF-1 receptor and of the JAK proteins. Methenyltetrahydrofolate synthetase (MTHFS) was identified as a potential mediator of IGF-1R dependent transformation. P85alpha, the regulatory subunit of PI3 kinase, was identified as one of four proteins recruited by a phosphopeptide mimicking a motif conserved in all JAK family members.

Mitogenic signaling and inhibition of apoptosis via the erythropoietin receptor Box-1 domain

Studies of proliferative signaling via type 1 cytokine receptors have revealed a three-step activation mechanism. Cytokine-induced receptor dimerization mediates the trans-phosphorylation of Jak kinases, Jaks phosphorylate receptors at tyrosine sites, and SH2 domain-encoding effectors then are recruited to these sites. Signaling factors that associate with activated erythropoietin (Epo) receptor complexes include phospholipase C-gamma, phosphatidylinositol 3-kinase, SHIP, Shc, Grb2, Cbl, Crk-l, HCP, Syp, and STAT5. While at least certain of these factors modulate proliferative signaling, mutated Epo receptor forms lacking Tyr(P) sites retain substantial mitogenic activity. Presently we show that a highly truncated Epo receptor form that retains box-1, yet lacks the conserved box-2 domain (and all Tyr(P) sites) nonetheless effectively promotes mitogenesis, survival, and Myc and Pim-1 expression. In addition, mitogenesis and Myc expression are shown to be supported by a direct Epo receptor-Jak2 kinase domain chimera. Thus, Epo-dependent mitogenesis and inhibition of apoptosis each depend critically upon only the Epo receptor box-1 domain, with no essential role exerted in these response pathways by the box-2 domain.

Identification of a novel pathway important for proliferation and differentiation of primary erythroid progenitors

Homodimerization of the erythropoietin (EPO) receptor (EPO-R) in response to EPO binding transiently activates the receptor-associated protein tyrosine kinase JAK2. Tyrosine phosphorylation of the EPO-R creates "docking sites" for SH2 domain(s) in signaling molecules such as the protein tyrosine phosphatases SH-PTP1 and SH-PTP2, phosphoinositide 3-kinase (PI3 kinase), and STAT5. However, little is known about the specific intracellular signals essential for proliferation and differentiation of erythroid progenitors. Here we show that an EPO-R containing only one cytosolic (phospho)tyrosine residue, Y479, induces a signal transduction pathway sufficient for proliferation and differentiation of fetal liver progenitors of erythroid colony-forming units from EPO-R(-/-) mice as well as for proliferation of cultured hematopoietic cells. This cascade involves sequential EPO-induced recruitment of PI3 kinase to the EPO-R and activation of mitogen-activated protein kinase activity, independent of the Shc/Grb2-adapter pathway and of STAT5. Protein kinase C epsilon may be one of the mediators connecting PI3 kinase with the mitogen-activated protein kinase signaling cascade. Our results identify a signaling cascade important in vivo for erythroid cell proliferation and differentiation.

Functional interaction of erythropoietin and stem cell factor receptors is essential for erythroid colony formation

Production of mature erythrocytes requires multiple growth factors, but we do not know how their actions are coordinated. Here we show that erythroid progenitors from erythropoietin receptor (Epo-R)-/- fetal livers, infected in vitro with a retrovirus expressing the wild-type Epo-R, require addition of both Epo and stem cell factor (SCF) to form colony-forming unit erythroid (CFU-E) colonies. Thus, a functional interaction between KIT and the Epo-R, similar to what we reported in cultured cells, is essential for the function of CFU-E progenitors. In contrast, CFU-E colony formation in vitro by normal fetal liver progenitors requires only Epo; the essential interaction between activated KIT and the Epo-R must have occurred in vivo before or at the CFU-E progenitor stage. Using truncated dominant-negative mutant Epo-Rs, we show that KIT does not activate the Epo-R by inducing its dimerization, but presumably does so by phosphorylating tyrosine residue(s) in its cytosolic domain. By expressing mutant Epo-Rs containing only one of eight cytosolic tyrosines, we show that either tyrosine residue Y464 or Y479 suffices for Epo-dependent cell proliferation. However, only Epo-R F7Y479 is capable of supporting erythroid colony formation when expressed in (Epo-R)-/- fetal liver cells, indicating that Y464 either cannot send a differentiation signal or fails to respond to SCF/KIT activation. This work employs a novel experimental system to study the function of growth factors and their receptors in normal hematopoiesis.

Erythropoietin-Induced Activation of STAT5 Is Impaired in the Myelodysplastic Syndrome

Patients with myelodysplastic syndrome (MDS) have ineffective in vivo and in vitro erythropoiesis, characterized by an impaired response to erythropoietin (Epo). We examined proliferation and maturation of MDS marrow cells in response to Epo in more detail. Epo-dependent DNA synthesis as well as induction of GATA-1 binding activity in marrow cells from 15 MDS cases were severely reduced as compared with normal bone marrow (NBM). Additionally, the appearance of morphologically identifiable erythroid cells was decreased in MDS cell cultures. These data indicate that both the Epo-dependent proliferation as well as the differentiation induction by Epo is suppressed. To study more upstream events of the Epo signal transduction route we investigated activation of the signal transducer and activator of transcription (STAT) 5. In all 15 MDS samples tested, STAT5 activation was absent or greatly suppressed in response to Epo. In contrast, interleukin-3 induced a normal STAT5 response in MDS cells. Further, in MDS the subset of CD71+ BM cells that is phenotypically similar to Epo-responsive cells in normal marrow, was present. We conclude that the Epo response in MDS is disturbed at an early point in the Epo receptor (EpoR) signal transduction pathway.

Erythropoietin-Induced Activation of STAT5 Is Impaired in the Myelodysplastic Syndrome

Patients with myelodysplastic syndrome (MDS) have ineffective in vivo and in vitro erythropoiesis, characterized by an impaired response to erythropoietin (Epo). We examined proliferation and maturation of MDS marrow cells in response to Epo in more detail. Epo-dependent DNA synthesis as well as induction of GATA-1 binding activity in marrow cells from 15 MDS cases were severely reduced as compared with normal bone marrow (NBM). Additionally, the appearance of morphologically identifiable erythroid cells was decreased in MDS cell cultures. These data indicate that both the Epo-dependent proliferation as well as the differentiation induction by Epo is suppressed. To study more upstream events of the Epo signal transduction route we investigated activation of the signal transducer and activator of transcription (STAT) 5. In all 15 MDS samples tested, STAT5 activation was absent or greatly suppressed in response to Epo. In contrast, interleukin-3 induced a normal STAT5 response in MDS cells. Further, in MDS the subset of CD71+ BM cells that is phenotypically similar to Epo-responsive cells in normal marrow, was present. We conclude that the Epo response in MDS is disturbed at an early point in the Epo receptor (EpoR) signal transduction pathway.

Involvement of phosphatidylinositol 3-kinase in the mediation of erythropoietin-induced activation of p70S6k

We have previously shown that, in HCD-57 cells, erythropoietin (EPO) induces a biphasic activation of the ribosomal S6 kinase p70S6k, an enzyme playing a key role in the regulation of cell cycle progression. Here we present evidence that p70S6k is activated through both phosphatidylinositol (PI) 3-kinase-dependent and independent pathways: whereas the early phase of EPO-dependent stimulation of p70S6k activity was strongly suppressed by the potent PI 3-kinase inhibitor wortmannin, late phase was much less affected. The dose-dependent inhibition of cell growth by wortmannin indicates an important role of PI 3-kinase in the mediation of EPO-induced cell proliferation. Furthermore, our data suggest that the EPO-receptor-associated tyrosine kinase JAK2 is not essentially involved in the mediation of EPO-induced p70S6k activation.

Direct association with and dephosphorylation of Jak2 kinase by the SH2-domain-containing protein tyrosine phosphatase SHP-1

SHP-1 is an SH2-containing cytoplasmic tyrosine phosphatase that is widely distributed in cells of the hematopoietic system. SHP-1 plays an important role in the signal transduction of many cytokine receptors, including the receptor for erythropoietin, by associating via its SH2 domains to the receptors and dephosphorylating key substrates. Recent studies have suggested that SHP-1 regulates the function of Jak family tyrosine kinases, as shown by its constitutive association with the Tyk2 kinase and the hyperphosphorylation of Jak kinases in the motheaten cells that lack functional SHP-1. We have examined the interactions of SHP-1 with two tyrosine kinases activated during engagement of the erythropoietin receptor, the Janus family kinase Jak-2 and the c-fps/fes kinase. Immunoblotting studies with extracts from mouse hematopoietic cells demonstrated that Jak2, but not c-fes, was present in anti-SHP-1 immunoprecipitates, suggesting that SHP-1 selectively associates with Jak2 in vivo. Consistent with this, when SHP-1 was coexpressed with these kinases in Cos-7 cells, it associated with and dephosphorylated Jak2 but not c-fes. Transient cotransfection of truncated forms of SHP-1 with Jak2 demonstrated that the SHP-1-Jak2 interaction is direct and is mediated by a novel binding activity present in the N terminus of SHP-1, independently of SH2 domain-phosphotyrosine interaction. Such SHP-1-Jak2 interaction resulted in induction of the enzymatic activity of the phosphatase in in vitro protein tyrosine phosphatase assays. Interestingly, association of the SH2n domain of SHP-1 with the tyrosine phosphorylated erythropoietin receptor modestly potentiated but was not essential for SHP-1-mediated dephosphorylation of Jak2 and had no effect on c-fes phosphorylation. These data indicate that the main mechanism for regulation of Jak2 phosphorylation by SHP-1 involves a direct, SH2-independent interaction with Jak2 and suggest the existence of similar mechanisms for other members of the Jak family of kinases. They also suggest that such interactions may provide one of the mechanisms that control SHP-1 substrate specificity.

A novel phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase associates with the interleukin-3 receptor

To gain insight into the intracellular signaling cascades that are activated by the binding of interleukin-3 (IL-3) to its target cells, we have embarked on the identification of proteins that are associated with the IL-3 receptor (IL-3R). In a previous study we reported that a 110-kDa serine/threonine protein kinase is constitutively associated with the IL-3R and activated following IL-3 stimulation. We now report that a phosphatidylinositol-3,4, 5-trisphosphate (PtdIns-3,4,5-P3) 5-phosphatase (5-ptase) is also constitutively associated with the IL-3R. This 5-ptase is magnesium-dependent and removes the 5-position phosphate from PtdIns-3,4,5-P3 but does not metabolize PtdIns-4,5-P2, inositol (Ins)-1,3,4,5-P4, or Ins-1,4,5-P3. This substrate specificity distinguishes it from any previously characterized 5-ptase. Interestingly, it may be bound indirectly via phosphatidylinositol 3-kinase (PI 3-kinase), another enzyme that is constitutively bound to the IL-3R. However, unlike PI 3-kinase which becomes activated following IL-3 stimulation, this receptor-associated 5-ptase activity does not increase following IL-3 stimulation, and its primary function may be to keep the principal in vivo product of PI 3-kinase, PtdIns-3,4,5-P3, at low levels in unstimulated cells, to terminate the PI 3-kinase signal following IL-3 stimulation or to metabolize PtdIns-3,4,5-P3 to a metabolically active second messenger, i.e. PtdIns-3,4-P2.

Cytokine receptor signal transduction and the control of hematopoietic cell development

The cytokine receptor superfamily is characterized by structural motifs in the exoplasmic domain and by the absence of catalytic activity in the cytosolic segment. Activated by ligand-triggered multimerization, these receptors in turn activate a number of cytosolic signal transduction proteins, including protein tyrosine kinases and phosphatases, and affect an array of cellular functions that include proliferation and differentiation. Molecular study of these receptors is revealing the roles they play in the control of normal hematopoiesis and in the development of disease.

Phosphorylation of tyrosine 397 in focal adhesion kinase is required for binding phosphatidylinositol 3-kinase

We have shown previously that cell adhesion or platelet-derived growth factor (PDGF) promotes the in vivo association of focal adhesion kinase (FAK) with phosphatidylinositol (PI) 3-kinase. In vitro experiments indicated that this interaction was mediated by the p85 subunit of PI 3-kinase and dependent on the tyrosine phosphorylation of FAK. Here we report data suggesting that the major autophosphorylation site of FAK (Tyr-397) is the binding site for the SH2 domains of p85 in vitro and is also required for the association of FAK with PI 3-kinase in vivo. We also show that Tyr-397 is responsible for the increased FAK:PI 3-kinase association upon PDGF stimulation, implying that no additional site of FAK was involved in its binding to PI 3-kinase after PDGF stimulation. Finally, we present evidence that the interaction of PI 3-kinase with Tyr-397 of FAK stimulates its activity. Together, these results suggest that FAK activation and autophosphorylation at Tyr-397 may lead to its association with PI 3-kinase through the SH2 domains of p85, which can subsequently activate PI 3-kinase during cell adhesion.

Erythropoietin induces activation of Stat5 through association with specific tyrosines on the receptor that are not required for a mitogenic response

The cytoplasmic domain of the erythropoietin receptor (EpoR) contains a membrane-distal region that is dispensable for mitogenesis but is required for the recruitment and tyrosine phosphorylation of a variety of signaling proteins. The membrane-proximal region of 96 amino acids is necessary and sufficient for mitogenesis as well as Jak2 activation, induction of c-fos, c-myc, cis, the T-cell receptor gamma locus (TCR-gamma), and c-pim-1. The studies presented here demonstrate that this region is also necessary and sufficient for the activation of Stat5A and Stat5B. The membrane-proximal domain contains a single tyrosine, Y-343, which when mutated eliminates the ability of the receptor to couple Epo binding to the activation of Stat5. Furthermore, peptide competitions demonstrate that this site, when phosphorylated, can disrupt Stat5 DNA binding activity, consistent with a role of Y-343 as a site of recruitment to the receptor. Cells expressing the truncated, Y343F mutant (a mutant with a Y-to-F alteration at position 343) proliferate in response to Epo in a manner comparable to that of the controls. However, in these cells, Epo stimulation does not induce the appearance of transcripts for cis, TCR-gamma, or c-fos, suggesting a role for Stat5 in their regulation.

Janus kinases in cytokine signalling

Hematopoiesis is largely regulated by the binding of cytokines to receptors of the cytokine receptor superfamily. Although lacking catalytic domains, members of the cytokine receptor superfamily mediate ligand dependent activation of tyrosine phosphorylation which is critical for all receptor functions. Recent studies have demonstrated that this is mediated through the association and activation of members of the Janus kinase (Jak) family of protein tyrosine kinases. The activated Jaks phosphorylate the receptors, creating docking sites for SH2 containing signalling proteins which are tyrosine phosphorylated following their association with the receptor complex. Among the substrates of tyrosine phosphorylation are members of the signal transducers and activators of transcription family of proteins (Stats). Various cytokines induce the tyrosine phosphorylation and activation of one or more of the six family members. The pattern of Stat activation provides a level of cytokine individuality that is not observed in the activation of other signalling pathways. Although not required for mitogenic responses, it is speculated that the Stats may mediate many of the cytokine specific functional responses of hematopoietic cells.