Identification of a new adapter protein that may link the common beta subunit of the receptor for granulocyte/macrophage colony-stimulating factor, interleukin (IL)-3, and IL-5 to phosphatidylinositol 3-kinase

PTEN mediates the cross talk between breast and glial cells in brain metastases leading to rapid disease progression

Despite improvement of therapeutic treatments for breast cancer, the development of brain metastases has become a major limitation to life expectancy for many patients. Brain metastases show very commonly alterations in EGFR and HER2 driven pathways, of which PTEN is an important regulator. Here, we analyzed PTEN expression in 111 tissue samples of breast cancer brain metastases (BCBM). Loss of PTEN was found in a substantial proportion of BCBM samples (48.6%) and was significantly associated with triple-negative breast cancer (67.5%, p = 0.001) and a shorter survival time after surgical resection of brain metastases (p = 0.048). Overexpression of PTEN in brain-seeking MDA-MB-231 BR cells in vitro reduced activation of the AKT pathway, notably by suppression of Akt1 kinase activity. Furthermore, the migration of MDA-MB-231 BR cells in vitro was promoted by co-culturing with both astrocytes and microglial cells. Interestingly, when PTEN was overexpressed the migration was significantly inhibited. Moreover, in an ex vivo organotypic brain slice model, PTEN overexpression reduced invasion of tumor cells. This was accompanied by reduced astrocyte activation that was mediated by autocrine and paracrine activation of GM-CSF/ CSF2RA and AKT/ PTEN pathways. In conclusion, loss of PTEN is frequently detected in triple-negative BCBM patients and associated with poor prognosis. The findings of our functional studies suggest that PTEN loss promotes a feedback loop between tumor cells and glial cells, which might contribute to disease progression.

Determination of Genes Related to Uveitis by Utilization of the Random Walk with Restart Algorithm on a Protein-Protein Interaction Network

Uveitis, defined as inflammation of the uveal tract, may cause blindness in both young and middle-aged people. Approximately 10–15% of blindness in the West is caused by uveitis. Therefore, a comprehensive investigation to determine the disease pathogenesis is urgent, as it will thus be possible to design effective treatments. Identification of the disease genes that cause uveitis is an important requirement to achieve this goal. To begin to answer this question, in this study, a computational method was proposed to identify novel uveitis-related genes. This method was executed on a large protein–protein interaction network and employed a popular ranking algorithm, the Random Walk with Restart (RWR) algorithm. To improve the utility of the method, a permutation test and a procedure for selecting core genes were added, which helped to exclude false discoveries and select the most important candidate genes. The five-fold cross-validation was adopted to evaluate the method, yielding the average F1-measure of 0.189. In addition, we compared our method with a classic GBA-based method to further indicate its utility. Based on our method, 56 putative genes were chosen for further assessment. We have determined that several of these genes (e.g., CCL4, Jun, and MMP9) are likely to be important for the pathogenesis of uveitis.

CSF2 Overexpression Is Associated with STAT5 Phosphorylation and Poor Prognosis in Patients with Urothelial Carcinoma

Background: Urothelial carcinoma (UC) commonly occurs in the urinary bladder (UB) and rarely in upper the upper urinary tract (UT). Its molecular pathogenesis, however, remains obscure. Though the constitutive phosphorylation of Signal Transducer and Activator of Transcription 5 (STAT5) is an important part of carcinogenesis generally, researchers have not systematically investigated this process specifically in relation to UC. The present study addresses this gap. Through data mining a published transcriptomic database of UBUCs (GSE32894), it identified Colony Stimulating Factor 2 (CSF2) as the stepwise upregulated gene of much significance among those related to the positive regulation of tyrosine phosphorylation of STAT5 (GO:0042523). Since the phosphorylation of STAT5, a key process in the development of UC, is closely associated with CSF2, we then examine CSF2 transcript and protein expression, justifying their association with clinicopathological features and survival in our well-established cohort of patients with UC.

Design: Laser capture microdissection in conjunction with real-time qRT-PCR are used to detect CSF2 transcript levels in 24 UBUCs and 6 non-tumor urothelium samples. We then used the H-score method to evaluate the immunohistochemistry in order to determine CSF2 protein expression in 296 UBUCs and 340 UTUCs, respectively. After correlating protein expression status with key clinicopathological features, the prognostic significance of CSF2 protein expression was determined for disease-specific survival (DSS) and metastasis-free survival (MeFS).

Results: We exclusively detected the CSF2 transcript, which was stepwise upregulated in tumor lesions (p=0.010). In both groups of UC we found overexpression of CSF2 significantly related to incremental pT status (UTUC, p=0.011; UBUC, p<0.001), as well as with perineural invasion (UTUC, p=0.002; UBUC, p=0.001). Univariate analysis found a close correlation between CSF2 overexpression and unfavorable prognosis for both DSS (UTUC, p=0.0001; UBUC, p<0.0001) and MeFS (UTUC, p=0.0001; UBUC, p=0.0002). High expression of CSF2 still remained prognostically for DSS (UTUC, p=0.015; UBUC, p=0.004) and MeFS (UTUC, p=0.008; UBUC, p=0.027) in multivariate comparison.

Conclusion: Our data showed that overexpression of CSF2 was inferred in advanced disease status and poor clinical outcomes for both UTUC and UBUC patients, suggesting that CSF2 may serve as an important prognosticator and a potential therapeutic target of UC.

Leukemia-associated mutations in SHIP1 inhibit its enzymatic activity, interaction with the GM-CSF receptor and Grb2, and its ability to inactivate PI3K/AKT signaling

The inositol 5-phosphatase SHIP1 is a negative regulator of the PI3K/AKT pathway, which is constitutively activated in 50-70% of acute myeloid leukemias (AML). Ten different missense mutations in SHIP1 have been described in 3% of AML patients suggesting a functional role of SHIP1 in AML. Here, we report the identification of two new SHIP1 mutations T162P and R225W that were detected in 2 and 1 out of 96 AML patients, respectively. The functional analysis of all 12 AML-associated SHIP1 mutations, one ALL-associated SHIP1 mutation (Q1076X) and a missense SNP (H1168Y) revealed that two mutations i.e. Y643H and P1039S abrogated the ability of SHIP1 to reduce constitutive PI3K/AKT signaling in Jurkat cells. The loss of function of SHIP1 mutant Y643H which is localized in the inositol phosphatase domain was due to a reduction of the specific activity by 84%. Because all other SHIP1 mutants had a normal enzymatic activity, we assumed that these SHIP1 mutants may be functionally impaired due to a loss of interaction with plasma membrane receptors or adapter proteins. In agreement with this model, we found that the SHIP1 mutant F28L located in the FLVR motif of the SH2 domain was incapable of binding tyrosine-phosphorylated proteins including the GM-CSF receptor and that the SHIP1 mutant Q1076X lost its ability to bind to the C-terminal SH3 domain of the adapter protein Grb2. In addition, SHIP1 mutant P1039S which does not reduce PI3K/AKT signaling anymore is located in a PXXP SH3 domain consensus binding motif suggesting that mutation of the conserved proline residue interferes with binding of SHIP1 to a so far unidentified SH3 domain containing protein. In summary, our data indicate that SHIP1 mutations detected in human leukemia patients impair the negative regulatory function of SHIP1 on PI3K/AKT signaling in leukemia cells either directly by reduced enzymatic activity or indirectly by disturbed protein interaction with tyrosine-phosphorylated membrane receptors or adapter proteins. These results further support a functional role of SHIP1 as tumor suppressor protein in the pathogenesis of AML.

The signaling mechanism of eosinophil activation

Eosinophils play an important role in certain aspects of asthma pathogenesis. This review focuses on the mechanism of activation of eosinophils by the growth factor interleukin-5 and the CC chemokine receptor-3. Interleukin-5 activates members of the Janus and Src family of kinases. The latter kinases are largely responsible for the generation of initial signaling events. CC chemokine receptor-3, in contrast, signals through heterotrimeric G-proteins. Subsequently, various signaling pathways are activated, which converge on four major pathways - the mitogen-activated protein kinase pathway, the phosphoinositide-3 kinase pathway, the calcium signaling pathway and the Janus-signal transducer and activator of transcription signaling pathway. The biologic consequences of many of these signaling pathways are also discussed.

The transcription factors STAT5A/B regulate GM-CSF-mediated granulopoiesis

Neutrophils play a vital role in the immune defense, which is evident by the severity of neutropenia causing life-threatening infections. Granulocyte macrophage-colony stimulating factor (GM-CSF) controls homeostatic and emergency development of granulocytes. However, little is known about the contribution of the downstream mediating transcription factors signal transducer and activator of transcription 5A and 5B (STAT5A/B). To elucidate the function of this pathway, we generated mice with complete deletion of both Stat5a/b genes in hematopoietic cells. In homeostasis, peripheral neutrophils were markedly decreased in these animals. Moreover, during emergency situations, such as myelosuppression, Stat5a/b-mutant mice failed to produce enhanced levels of neutrophils and were unable to respond to GM-CSF. Both the GM-CSF-permitted survival of mature neutrophils and the generation of granulocytes from granulocyte-macrophage progenitors (GMPs) were markedly reduced in Stat5a/b mutants. GMPs showed impaired colony-formation ability with reduced number and size of colonies on GM-CSF stimulation. Moreover, continuous cell fate analyses by time-lapse microscopy and single cell tracking revealed that Stat5a/b-null GMPs showed both delayed cell-cycle progression and increased cell death. Finally, transcriptome analysis indicated that STAT5A/B directs GM-CSF signaling through the regulation of proliferation and survival genes.

Downregulation of VEGF-A, STAT5 and AKT in acute myeloid leukemia blasts of patients treated with SU5416

In acute myeloid leukemia (AML), autocrine or paracrine activation of receptor tyrosine kinases such as c-kit and FLT3 contributes to proliferation and apoptosis resistance of leukemic blasts. This provided the rationale for a multicenter clinical trial in patients with refractory AML with SU5416, a small molecule kinase inhibitor which blocks phosphorylation of c-kit, FLT3, VEGFR-1, VEGFR-2 (KDR) and VEGFR-3. The levels of VEGF mRNA expression were investigated in peripheral blood leukemic blasts taken from AML patients before and during treatment with SU5416. Rapid down regulation of VEGF was observed in AML blasts from 72% (13 of 18) of patients analysed. Patients initially expressing high VEGF-levels had a stronger downregulation and a higher clinical response rate (mean 865-fold, n = 10, P = 0,01) than patients initially expressing low VEGF-levels (mean four-fold, n = 8). These results suggest that abnormal high VEGF expression is downregulated by SU5416 treatment, and furthermore that decreases in VEGF mRNA levels may provide an early marker of therapeutic response with anti-angiogenic therapy. Additionally, protein expression of STAT5 and AKT was assessed by western blotting in these patient samples, as well as in the leukemia cell line, M-07e, treated in vitro with SU5416 as a model system. In the AML patient samples, parallel downregulation of both STAT5 and AKT was observed in several cases (STAT5 in four of 15; AKT in three of six examined patients). These effects were confirmed with the cell line M-07e after incubation with SU5416 in vitro using concentrations that are achievable in patients. In summary, our data show suppression of the expression of VEGF and key signal transduction intermediates in AML blasts during treatment with SU5416.

IL-21 promotes T lymphocyte survival by activating the phosphatidylinositol-3 kinase signaling cascade

IL-21 is a Type I cytokine, which uses the common gamma chain (gamma(c)) in its receptor. As members of the gamma(c) cytokine/cytokine receptors family play crucial role in the differentiation, activation, and survival of lymphocytes, we have investigated if IL-21 could promote T cell survival and thus, contribute to T cell homeostasis and expansion. Unlike most gamma(c) cytokine receptors, we report that IL-21R is constitutively expressed by all mature T lymphocytes and that stromal cells of lymphoid organs are a constitutive source of IL-21. These observations are reminiscent of what is observed for IL-7/IL-7R, which control T cell survival and homeostasis and suggest a role for IL-21 in T cell homeostasis. Indeed, our results show that IL-21 is a survival factor for resting and activated T cells. Moreover, the ability of IL-21 to costimulate T cell proliferation is mediated by enhancing T cell viability. Further investigation of how IL-21R signaling induces T cell survival shows for the first time that IL-21 binding to its receptor activates the PI-3K signaling pathway and induces Bcl-2 expression. Moreover, the activation of the PI-3K signaling pathway is essential for IL-21-mediated T cell survival. Our data provide a new role for IL-21 in the immune system, which might be used to improve T cell homeostasis in immunocompromised patients.

Signal transduction pathways that contribute to myeloid differentiation

The production of mature, differentiated myeloid cells is regulated by the action of hematopoietic cytokines on progenitor cells in the bone marrow. Cytokines drive the process of myeloid differentiation by binding to specific cell-surface receptors in a stage- and lineage-specific manner. Following the binding of a cytokine to its cognate receptor, intracellular signal-transduction pathways become activated that facilitate the myeloid differentiation process. These intracellular signaling pathways may promote myelopoiesis by stimulating expansion of a progenitor pool, supporting cellular survival during the differentiation process, or by directly driving the phenotypic changes associated with differentiation. Ultimately, pathways that drive the differentiation process converge on myeloid transcription factors, including PU.1 and the C/EBP family, that are critical for differentiation to proceed. While much is known about the cytokines, cytokine receptors and transcription factors that regulate myeloid differentiation, less is known about the precise roles that specific signaling mediators play in promoting myeloid differentiation. Recently, however, the application of novel pharmacologic inhibitors, siRNA strategies, and transgenic and knockout models has begun to shed light on the involvement and function of signaling pathways in normal myeloid differentiation. This review will discuss the roles that key signaling pathways and mediators play in myeloid differentiation.

Detection of N-RAS and K-RAS in their active GTP-bound form in acute myeloid leukemia without activating RAS mutations

RAS genes, predominantly N-RAS and K-RAS, have been implicated in the pathogenesis of acute myeloid leukemia (AML), due to activating RAS mutations detectable in approximately 20% of AML patients. In the present study, RAS proteins were detected in their activated, GTP-bound form, in AML patients (n = 10) not expressing mutated forms of H-RAS, K-RAS and N-RAS. Further analysis revealed the simultaneous presence of N-RAS and K-RAS proteins in the GTP-bound state in seven out of 10 AML samples. In four out of 10 samples the levels of RAS-GTP were comparable to an AML cell line (TF-1) with an activating N-RAS mutation (Q61P). The detection of RAS-GTP in AML patients without RAS mutations further supports a functional role of RAS proteins in the pathogenesis of AML and may explain the observed effects of RAS inhibitors in some AML patients in the absence of activating RAS mutations.

Restoration of SHIP activity in a human leukemia cell line downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3beta signaling and leads to an increased transit time through the G1 phase of the cell cycle

The inositol 5-phosphatase SHIP (SHIP-1) is a negative regulator of signal transduction in hematopoietic cells and targeted disruption of SHIP in mice leads to a myeloproliferative disorder. We analyzed the effects of SHIP on the human leukemia cell line Jurkat in which expression of endogenous SHIP protein is not detectable. Restoration of SHIP expression in Jurkat cells with an inducible expression system caused a 69% reduction of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) and a 65% reduction of Akt kinase activity, which was associated with reduced phosphorylation of glycogen synthase kinase 3beta (GSK-3beta) (Ser-9) without changing the phosphorylation of Bad (Ser-136), FKHR (Ser-256) or MAPK (Thr-202/Tyr-204). SHIP-expressing Jurkat cells showed an increased transit time through the G1 phase of the cell cycle, but SHIP did not cause a complete cell cycle arrest or apoptosis. Extension of the G1 phase was associated with an increased stability of the cell cycle inhibitor p27(Kip1) and reduced phosphorylation of the retinoblastoma protein Rb at serine residue 780. Our data indicate that restoration of SHIP activity in a human leukemia cell line, which has lost expression of endogenous SHIP, downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3beta signaling and leads to an increased transit time through the G1 phase of the cell cycle.

Complex regulation of the Csk homologous kinase (Chk) by IL-4 family cytokines and IFN-γ in human peripheral blood monocytes

Csk homologous kinase (Chk) is a tyrosine kinase that shares homology with Csk and, like Csk, has the potential to inhibit src-family kinase function through phosphorylation. In myeloid lineage cells, Chk expression is dependent on monocytic differentiation. IL-4 and IL-13 are cytokines involved in monocytic differentiation that have recently been shown to induce Chk expression in peripheral blood monocytes (PBMs). In this study, we show that two other members of the IL-4 family, IL-3 and GM-CSF, can also induce Chk expression at RNA and protein levels. Interestingly, Chk induction is both blocked and reversed by IFN-gamma treatment. Additionally, a short pretreatment with IFN-gamma is sufficient to prevent Chk induction, and the effects of IFN-gamma are dependent on protein synthesis. Collectively, these results suggest that activation of Chk expression and signaling may have a role in the IL-4 family-mediated differentiation of myeloid cells, and inhibition of Chk activation may be one mechanism by which IFN-gamma alters IL-4-mediated affects.

An increase in the expression and total activity of endogenous p60(c-Src) in several factor-independent mutants of a human GM-CSF-dependent leukemia cell line (TF-1)

Growth factor independence of hematopoietic cells can be induced by ectopic expression of a variety of oncogenes encoding receptor or cytoplasmic tyrosine kinases. To examine whether the activation of tyrosine kinases occurs in factor-independent mutants in vivo, the tyrosine-phosphorylated proteins from 14 factor-independent mutants of a GM-CSF-dependent cell line (TF-1) were analysed. These mutants did not secrete any growth-stimulating activity for TF-1 cells, suggesting that activation of intracellular signaling rather than an autocrine stimulation by secreted growth factors is responsible for their factor-independent growth. In 11 out of 14 GM-CSF-independent mutants analysed, a constitutively tyrosine-phosphorylated protein of 60 kDa was detected, which was subsequently identified as p60(c-Src). The kinase activity of p60(c-Src) was increased up to 12-fold in these mutants, which was at least in part due to overexpression of the c-src gene on the RNA and protein level. The Src substrate Sam68 showed an increased phosphorylation in mutants with high Src activity, suggesting that p60(c-Src) triggers downstream signaling in these cells. Treatment of the factor-independent mutants with the Src kinase inhibitor PP2 resulted in a reduced proliferation, demonstrating that Src kinases are essential for these cells for maximal proliferation. Further analysis of factor-independent mutants with low or undetectable Src activity revealed a constitutive phosphorylation of the common beta chain of the GM-CSF receptor and STAT5. Our data indicate an increase in the expression and total activity of endogenous p60(c-Src) in several GM-CSF-independent TF-1 mutants, further underlining the role of Src in the process of autonomous growth of hematopoietic cells.

JAKs, STATs and Src kinases in hematopoiesis

Hematopoiesis is the cumulative result of intricately regulated signal transduction cascades that are mediated by cytokines and their cognate receptors. Proper culmination of these diverse signaling pathways forms the basis for an orderly generation of different cell types and aberrations in these pathways is an underlying cause for diseases such as leukemias and other myeloproliferative and lymphoproliferative disorders. Over the past decade, downstream signal transduction events initiated upon cytokine/growth factor stimulation have been a major focus of basic and applied biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of transcription factors, termed signal transducers and activators of transcription (STATs) appear to orchestrate the downstream events propagated by cytokine/growth factor interactions with their cognate receptors. Similarly, cytoplasmic Janus protein tyrosine kinases (JAKs) and Src family of kinases seem to play a critical role in diverse signal transduction pathways that govern cellular survival, proliferation, differentiation and apoptosis. Accumulating evidence suggests that STAT protein activation may be mediated by members of both JAK and Src family members following cytokine/growth factor stimulation. In addition, JAK kinases appear to be essential for the phosphorylation of the cytokine receptors which results in the creation of docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Cell and tissue-specificity of cytokine action appears to be determined by the nature of signal transduction pathways activated by cytokine/receptor interactions. The integration of these diverse signaling cues from active JAK kinases, members of the Src-family kinases and STAT proteins, leads to cell proliferation, cell survival and differentiation, the end-point of the cytokine/growth factor stimulus.

Expression of a mutated form of the p85alpha regulatory subunit of phosphatidylinositol 3-kinase in a Hodgkin's lymphoma-derived cell line (CO)

Phosphatidylinositol (PI) 3-kinase plays an important role in a variety of biological processes, including proliferation and apoptosis. PI3-kinase is a heterodimer consisting of an 85 kDa adapter protein (p85) containing one SH3 domain and two SH2 domains and a 110 kDa catalytic subunit (p110). Recently an oncogenic form of p85 named p65-PI3K lacking the C-terminal SH2 domain has been cloned from an irradiation-induced murine thymic lymphoma and transgenic mice expressing p65-PI3K in T lymphocytes develop a lymphoproliferative disorder. Here we describe the cloning of a C-terminal truncated form of p85 expressed in a human lymphoma cell line (CO) with a T cell phenotype derived from a patient with Hodgkin's disease. As a result of a frame-shift mutation at amino acid 636, p76 is lacking most of the C-terminal SH2 domain, but contains the inter-SH2 domain and is associated with an active form of PI3-kinase. A PI3-kinase-dependent constitutive activation of Akt was detected in CO cells which was only partially reduced after serum starvation. Treatment of CO cells with the PI3-kinase inhibitor wortmannin resulted in a concentration-dependent inhibition of cell proliferation associated with an increased number of apoptotic cells. This is the first detection of a mutated form of the p85 subunit of PI3-kinase in human hematopoietic cells further underlining a potential role of PI3-kinase/Akt signaling in human leukemogenesis.

Activated Fes protein tyrosine kinase induces terminal macrophage differentiation of myeloid progenitors (U937 cells) and activation of the transcription factor PU.1

The c-fps/fes proto-oncogene encodes a 92-kDa protein tyrosine kinase that is preferentially expressed in myeloid and endothelial cells. Fes is believed to play a role in vascular development and myelopoiesis and in the inflammatory responses of granulocytes and macrophages. To help define the biological role of this kinase and identify its downstream targets, we have developed a gain-of-function allele of Fes that has potent biological activity in myeloid cell progenitors. Introduction of constitutively active Fes into bipotential U937 cells induced the appearance of fully differentiated macrophages within 6 to 12 days. The Fes-expressing differentiated cells became adherent, had distinctive macrophage morphology, and exhibited increased expression of myelomonocytic differentiation markers, including CD11b, CD11c, CD18, CD14, and the macrophage colony-stimulating factor receptor. These cells acquired phagocytic properties and exhibited NADPH oxidase and nonspecific esterase activities, confirming that they were functionally active macrophages. Concomitantly, there was downregulation of the granulocytic marker granulocyte colony-stimulating factor receptor, indicating that the biological activity of Fes was coordinated in a lineage-specific manner. A constitutively active Src did not induce macrophage morphology or upregulation of myelomonocytic markers in U937 cells, suggesting that the biological activity we observed was not a general consequence of expression of an activated nonreceptor tyrosine kinase. Analysis of possible downstream targets of Fes revealed that this kinase activated the ets family transcription factor PU.1, which is essential for macrophage development. Our results strongly implicate Fes as a key regulator of terminal macrophage differentiation and identify PU.1 as a transcription factor that may mediate some of its biological activities in myeloid cells.

Role of Bax in apoptosis of IL-3-dependent cells

IL-3 removal was reported to induce membrane association of the apoptotic effector Bax. This report demonstrates that IL-3-dependent cells from Bax-null mice failed to activate caspases after IL-3 removal and survived in an 10-fold lower concentration of IL-3. As IL-3 removal also down-regulates expression of Bcl-X, we examined the relationship between Bcl-X decrease and Bax membrane association. IL-3 removal from BAF-3 cells, followed by sorting caspase-active and caspase-inactive populations, showed that both expressed similar levels of Bcl-X. Inhibition of IL-3 signalling via PI-3 kinase and MEK1/2 resulted in cells with minimal Bcl-X, which remained viable with soluble Bax. However BAF-3-derived cells, which maintained Bcl-X expression without IL-3, also remained viable with soluble Bax on IL-3 removal. Therefore a decrease in Bcl-X is necessary, though not sufficient, for Bax membrane association on IL-3 removal. In contrast, treatment of BAF-3 cells with hydroxyurea induced apoptosis in the absence of a Bcl-X decrease. Furthermore, IL-3-dependent cells from Bax-null mice activated caspases after hydroxyurea treatment and show the same sensitivity to a variety of cytotoxic drugs. Thus, apoptosis after IL-3 removal requires a decrease in Bcl-X and Bax membrane association, whereas that induced by cytotoxic drugs does not.

The inositol 5-phosphatase SHIP is expressed as 145 and 135 kDa proteins in blood and bone marrow cells in vivo, whereas carboxyl-truncated forms of SHIP are generated by proteolytic cleavage in vitro

The inositol polyphosphate 5-phosphatase SHIP plays an important role in negative signalling in B cells and mast cells and in the down-regulation of cytokine receptor-mediated signals in myeloid cells. SHIP is expressed as a 145 kDa full-length protein and an isoform of 135 kDa due to alternative splicing. Additional smaller forms of SHIP which are truncated at the carboxy terminus have been described in bone marrow and peripheral blood mononuclear cells (PBMC). Our data demonstrate that human bone marrow cells and PBMC from healthy donors and patients with acute myeloid leukemia express the 145 kDa form of SHIP and low amounts of a 135 kDa form of SHIP in vivo whereas C-terminal-truncated SHIP proteins are generated by a PMSF-sensitive protease during the preparation of cell lysates in vitro. We have further characterized this protease and identified a proteolytic cleavage site in the human SHIP protein C-terminal to tryptophan residue 941. These data support a physiological role for the 145 and 135 kDa forms of SHIP in bone marrow and peripheral blood cells from normal donors and patients with acute myeloid leukemia.

Factor VIIa/tissue factor-induced signaling via activation of Src-like kinases, phosphatidylinositol 3-kinase, and Rac

Tissue factor (TF), apart from activating the extrinsic pathway of the blood coagulation, is a principal regulator of embryonic angiogenesis and oncogenic neoangiogenesis, but also influences inflammation, leukocyte diapedesis and tumor progression. The intracellular domain of TF lacks homology to other classes of receptors and hence the signaling mechanism is poorly understood. Here we demonstrate that factor VIIa (the natural ligand for TF) induces the activation of the Src family members c-Src, Lyn, and Yes, and subsequently phosphatidylinositol 3-kinase (PI3K), followed by stimulation of c-Akt/protein kinase B as well as the small GTPases Rac and Cdc42. In turn Rac mediates p38 mitogen-activated protein (MAP) kinase activation and cytoskeletal reorganization, whereas factor VIIa-induced p42/p44 MAP kinase stimulation required PI3K enzymatic activity but was not inhibited by dominant negative Rac proteins. We propose that this Src family member/PI3K/Rac-dependent signaling pathway is a major mediator of factor VIIa/TF effects in pathophysiology.

Interleukin 10 modulation of tumour necrosis factor receptors requires tyrosine kinases but not the PI 3-kinase/p70 S6 kinase pathway

We have previously shown that interleukin (IL-)10-induced proliferation of the murine mast cell line D36, was dependent upon the activation of PI 3-kinase and p70 S6 kinase. Conversely, we were able to show that this pathway was not involved in the signal transduction pathway mediating IL-10 inhibition of pro-inflammatory cytokine release from monocytes. We have extended these studies to investigate the induction of p75 tumour necrosis factor receptor (TNF-R) shedding, another anti-inflammatory property of IL-10. Using the inhibitors of PI 3-kinase (LY294002 and wortmannin) and an inhibitor of p70 S6 kinase activation (rapamycin), we were able to show that this anti-inflammatory effect of IL-10 was not mediated by the PI 3-kinase/p70 S6 kinase pathway, indicating that another signalling cascade(s) was involved. Further studies also investigated the role of tyrosine kinases in the response to IL-10. Two distinct tyrosine kinase inhibitors, herbimycin and genistein affected the expression of TNF-R in response to IL-10 but, surprisingly, with opposite effects. However, both compounds inhibited the activation of both PI 3-kinase and p70 S6 kinase, with a concomitant inhibition of IL-10-induced proliferation. We observed that whilst tyrosine kinase activity was involved in the regulation of TNF-R expression, IL-10-induced activation of JAK kinases was not sensitive to inhibition by the tyrosine kinase inhibitors. These data suggest that multiple unknown tyrosine kinases are mediating the IL-10-induced signal transduction pathways leading to the regulation of TNF-R expression and IL-10-induced proliferation.

IL-3 signaling and the role of Src kinases, JAKs and STATs: a covert liaison unveiled

Hematopoiesis is the cumulative result of intricately regulated signal transduction cascades that are mediated by cytokines and their cognate receptors. Proper culmination of these diverse signaling pathways forms the basis for an orderly generation of different cell types and aberrations in these pathways is an underlying cause for diseases such as cancer. Over the past several years, downstream events initiated upon cytokine/growth factor stimulation have been a major focus of biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of novel transcription factors, termed signal transducers and activators of transcription (STATs) appear to orchestrate the downstream events propagated by cytokine/growth factor interactions with their cognate receptors. Until recently, the JAK proteins were considered to be the tyrosine kinases, which dictated the levels of phosphorylation and activation of STAT proteins, forming the basis of the JAK-STAT model. However, over the past few years, increasing evidence has accumulated which indicates that at least some of the STAT protein activation may be mediated by members of the Src gene family following cytokine/growth factor stimulation. Studies have demonstrated that the Src-family of tyrosine kinases can phosphorylate and activate certain STAT proteins, in lieu of JAK kinases. In such a scenario, JAK kinases may be more crucial to phosphorylation of the cytokine/growth factor receptors and in the process create docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Tyrosine phosphorylation and activation of STAT proteins can be achieved either by JAKs or Src-kinases depending on the nature of STAT that is being activated. This forms the basis for the JAK-Src-STAT model proposed in this review. The concerted action of JAK kinases, members of the Src-kinase family and STAT proteins, leads to cell proliferation and cell survival, the end-point of the cytokine/growth factor stimulus. Oncogene (2000).

Regulation and function of protein kinase B and MAP kinase activation by the IL-5/GM-CSF/IL-3 receptor

Interleukin (IL)-3, IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) regulate proliferation, differentiation and apoptosis of target cells. Receptors for these cytokines consist of a cytokine-specific alpha subunit and a common shared beta c subunit. Tyrosine phosphorylation of the beta c is thought to play a critical role in mediating signal transduction events. We have examined the effect of mutation of beta c tyrosines on the activation of multiple signal transduction pathways. Activation of protein kinase B (PKB) required JAK2 and was inhibited by dominant-negative phosphatidylinositol 3-kinase (P13K). Overexpression of JAK2 was sufficient to activate both protein kinase B (PKB) and extracellular regulated kinase-1 (ERK1). Tyrosine 577 and 612 were found to be critical for the activation of PKB and ERK1, but not activation of STAT transcription factors. Activation of both PKB and ERK have been implicated in the regulation of proliferation and apoptosis. We generated GM-CSFR stable cell lines expressing receptor mutants to evaluate their effect on these processes. Activation of both PKB and ERK was perturbed, while STAT activation remained unaffected. Tyrosines 577 and 612 were necessary for optimal proliferation, however, mutation of these tyrosine residues did not affect GM-CSF mediated rescue from apoptosis. These data demonstrate that while phosphorylation of beta c tyrosine residues 577 and 612 are important for optimal cell proliferation, rescue from apoptosis can be mediated by alternative signalling routes apparently independent of PKB or ERK activation.

Late signals from the PDGF receptors leading to the activation of the p70S6-kinase are necessary for the transition from G1 to S phase in AKR-2B cells

Platelet-derived growth factor AB (PDGF-AB) has to be permanently present in the culture medium to achieve full proliferation (>90%) of AKR-2B fibroblasts. Upon removal after 1 h incubation time, only a small number of cells (<20%) entered the cell cycle. Concomitantly there was no increase in RNA- and protein-synthesis. The PDGF-receptor autophosphorylation reached a maximum after 30 min incubation with PDGF-AB. Tyrosine phosphorylation was no longer detectable after 2-4 h. The clustering of receptors into coated pits, analyzed by indirect immunofluorescence using a specific antibody against PDGF-beta-receptor, showed in contrast to autophosphorylation a biphasic kinetic. A first maximum was reached after 30 min, followed by a complete disappearance of coated pits, which regenerated in a second phase after 3 h and were long lasting. If PDGF-AB was removed after 1 h, the second phase was obliterated. The involvement of two different signalling pathways in these two phases was investigated in detail: (1) The ras-raf-MAP-kinase pathway and (2) the PI-3-kinase/p70(S6)-kinase pathway. PDGF-AB addition caused a fast (10 min) activation of MAP-kinase, which returned to background level after 1 h without any further activation later on. In contrast PDGF-AB led to a rapid (15-30 min) activation of the p70(S6)-kinase that persisted for 8-12 h just prior to the entry of the cells into S-phase. If PDGF-AB was removed after 1 h, the activation of this kinase ceased 3 h later. PDGF-AA, which is unable to promote division of AKR-2B cells, induced only a shortlasting p70(S6)-kinase activation. These observations add further evidence for the involvement of the p70(S6)-kinase pathway in the proliferation control of AKR-2B fibroblasts in the late G1 phase (4-8 h after growth factor addition). On the other hand, if the p70(S6)-kinase activation was prevented by the addition of 10 nM rapamycin, the cell division was not inhibited but only delayed by 4 h. Similar kinetics were observed when the PI-3-kinase was inhibited by 400 nM wortmannin. It is suggested that a regulatory element exists upstream of the p70(S6)-kinase and the PI-3-kinase. This regulatory element should be responsible for the transmission of late signals required for the progression through the cell cycle. This element is not involved in the immediate responses after PDGF-AB addition but must be stimulated within a second later phase of PDGF activation.

Mechanism of activation of the GM-CSF, IL-3, and IL-5 family of receptors

The process of ligand binding leading to receptor activation is an ordered and sequential one. High-affinity binding of GM-CSF, interleukin 3 (IL-3), and IL-5 to their receptors induces a number of key events at the cell surface and within the cytoplasm that are necessary for receptor activation. These include receptor oligomerization, activation of tyrosine kinase activity, phosphorylation of the receptor, and the recruitment of SH2 (src-homology) and PTB (phosphotyrosine binding) domain proteins to the receptor. Such a sequence of events represents a recurrent theme among cytokine, growth factor, and hormone receptors; however, a number of very recent and interesting findings have identified unique features in this receptor system in terms of: A) how GM-CSF/IL-3/IL-5 bind, oligomerize, and activate their cognate receptors; B) how multiple biological responses such as proliferation, survival, and differentiation can be transduced from activated GM-CSF, IL-3, or IL-5 receptors, and C) how the presence of novel phosphotyrosine-independent signaling motifs within a specific cytoplasmic domain of betaC may be important for mediating survival and differentiation by these cytokines. This review does not attempt to be all-encompassing but rather to focus on the most recent and significant discoveries that distinguish the GM-CSF/IL-3/IL-5 receptor subfamily from other cytokine receptors.

Regulation of proliferation, differentiation and survival by the IL-3/IL-5/GM-CSF receptor family

The receptors for the I1-3/IL-5/GM-CSF cytokine family are composed of a heterodimeric complex of a cytokine-specific alpha chain and a common beta chain (betac). Binding of IL-3/IL-5/GM-CSF to their respective receptors rapidly induces activation of multiple intracellular signalling pathways, including the Ras-Raf-ERK, the JAK/STAT, the phosphatidylinositol 3-kinase PKB, and the JNK/SAPK and p38 signalling pathways. This review focuses on recent advancements in understanding how these different signalling pathways are activated by IL-3/IL-5/GM-CSF receptors, and how the individual pathways contribute to the pleiotropic effects of IL-3/IL-5/GM-CSF on their target cells, including proliferation, differentiation, survival, and effector functions.

The mechanism of IL-5 signal transduction

Cytokines are important regulators of hematopoiesis. They exert their actions by binding to specific receptors on the cell surface. Interleukin-5 (IL-5) is a critical cytokine that regulates the growth, activation, and survival of eosinophils. Because eosinophils play a seminal role in the pathogenesis of asthma and allergic diseases, an understanding of the signal transduction mechanism of IL-5 is of paramount importance. The IL-5 receptor is a heterodimer of alpha- and beta-subunits. The alpha-subunit is specific, whereas the beta-subunit is common to IL-3, IL-5, and granulocyte/macrophage colony-stimulating factor (GM-CSF) receptors and is crucial for signal transduction. It has been shown that there are two major signaling pathways of IL-5 in eosinophils. IL-5 activates Lyn, Syk, and JAK2 and propagates signals through the Ras-MAPK and JAK-STAT pathways. Studies suggest that Lyn, Syk, and JAK2 tyrosine kinases and SHP-2 tyrosine phosphatase are important for eosinophil survival. In contrast to their survival-promoting activity, Lyn and JAK2 appear to have no role in eosinophil degranulation or expression of surface adhesion molecules. Raf-1 kinase, on the other hand, is critical for eosinophil degranulation and adhesion molecule expression. Btk is involved in IL-5 stimulation of B cell function. However, it does not appear to be important for eosinophil function. Thus a clear segregation of signaling molecules based on their functional importance is emerging. This review describes the signal transduction mechanism of the IL-3/GM-CSF/IL-5 receptor system and compares and contrasts IL-5 signaling between eosinophils and B cells.

c-Fes tyrosine kinase binds to and activates STAT3 after granulocyte-macrophage colony-stimulating factor stimulation

Granulocyte-macrophage colony stimulating factor (GM-CSF) induces proliferation and maturation of myeloid progenitor cells and also activates neutrophils. In order to investigate the pleiotropic effects of GM-CSF stimulation, we examined the signaling pathways of protein tyrosine kinases (PTKs) and signal transducers and activators of transcription (STATs) in GM-CSF-dependent proliferation of leukemia cells. Using TF-1, a GM-CSF-dependent human erythroleukemia cell line, we found that GM-CSF enhanced DNA-binding and tyrosine phosphorylation of STAT3. GM-CSF receptor (GM-CSFR) and c-Fes tyrosine kinase were also activated upon GM-CSF stimulation. Furthermore, c-Fes formed a complex with STAT3. Experiments using a c-Fes mutant that lacked tyrosine kinase activity revealed that the activation of STAT3 is kinase-dependent, but that the c-Fes-STAT3 interaction is not affected by c-Fes tyrosine kinase activity. The results suggest that STAT3 is activated by c-Fes tyrosine kinase through direct interaction during hematopoietic cell proliferation induced by GM-CSF.

Lyn Physically Associates With the Erythropoietin Receptor and May Play a Role in Activation of the Stat5 Pathway

Protein tyrosine phosphorylation plays a crucial role in signaling from the receptor for erythropoietin (Epo), although the Epo receptor (EpoR) lacks the tyrosine kinase domain. We have previously shown that the Jak2 tyrosine kinase couples with the EpoR to transduce a growth signal. In the present study, we demonstrate that Lyn, a Src family tyrosine kinase, physically associates with the EpoR in Epo-dependent hematopoietic cell lines, 32D/EpoR-Wt and F36E. Coexpression experiments in COS7 cells further showed that Lyn induces tyrosine phosphorylation of the EpoR and that both LynA and LynB, alternatively spliced forms of Lyn, bind with the membrane-proximal 91-amino acid region of the EpoR cytoplasmic domain. In vitro binding studies using GST-Lyn fusion proteins further showed that the Src homology (SH)-2 domain of Lyn specifically binds with the tyrosine-phosphorylated EpoR in lysate from Epo-stimulated cells, whereas the tyrosine kinase domain of Lyn binds with the unphosphorylated EpoR. Far-Western blotting and synthetic phosphopeptide competition assays further indicated that the Lyn SH2 domain directly binds to the tyrosine-phosphorylated EpoR, most likely through its interaction with phosphorylated Y-464 or Y-479 in the carboxy-terminal region of the EpoR. In vitro binding studies also demonstrated that the Lyn SH2 domain directly binds to tyrosine-phosphorylated Jak2. In vitro reconstitution experiments in COS7 cells further showed that Lyn induces tyrosine phosphorylation of Stat5, mainly on Y-694, and activates the DNA-binding and transcription-activating abilities of Stat5. In agreement with this, Lyn enhanced the Stat5-dependent transcriptional activation when overexpressed in 32D/EpoR-Wt cells. In addition, Lyn was demonstrated to phosphorylate the EpoR and Stat5 on tyrosines in vitro. These results suggest that Lyn may play a role in activation of the Jak2/Stat5 and other signaling pathways by the EpoR.

Lyn Physically Associates With the Erythropoietin Receptor and May Play a Role in Activation of the Stat5 Pathway

Protein tyrosine phosphorylation plays a crucial role in signaling from the receptor for erythropoietin (Epo), although the Epo receptor (EpoR) lacks the tyrosine kinase domain. We have previously shown that the Jak2 tyrosine kinase couples with the EpoR to transduce a growth signal. In the present study, we demonstrate that Lyn, a Src family tyrosine kinase, physically associates with the EpoR in Epo-dependent hematopoietic cell lines, 32D/EpoR-Wt and F36E. Coexpression experiments in COS7 cells further showed that Lyn induces tyrosine phosphorylation of the EpoR and that both LynA and LynB, alternatively spliced forms of Lyn, bind with the membrane-proximal 91-amino acid region of the EpoR cytoplasmic domain. In vitro binding studies using GST-Lyn fusion proteins further showed that the Src homology (SH)-2 domain of Lyn specifically binds with the tyrosine-phosphorylated EpoR in lysate from Epo-stimulated cells, whereas the tyrosine kinase domain of Lyn binds with the unphosphorylated EpoR. Far-Western blotting and synthetic phosphopeptide competition assays further indicated that the Lyn SH2 domain directly binds to the tyrosine-phosphorylated EpoR, most likely through its interaction with phosphorylated Y-464 or Y-479 in the carboxy-terminal region of the EpoR. In vitro binding studies also demonstrated that the Lyn SH2 domain directly binds to tyrosine-phosphorylated Jak2. In vitro reconstitution experiments in COS7 cells further showed that Lyn induces tyrosine phosphorylation of Stat5, mainly on Y-694, and activates the DNA-binding and transcription-activating abilities of Stat5. In agreement with this, Lyn enhanced the Stat5-dependent transcriptional activation when overexpressed in 32D/EpoR-Wt cells. In addition, Lyn was demonstrated to phosphorylate the EpoR and Stat5 on tyrosines in vitro. These results suggest that Lyn may play a role in activation of the Jak2/Stat5 and other signaling pathways by the EpoR.

Analysis of Signal Transduction Pathways in Human Eosinophils Activated by Chemoattractants and the T-Helper 2–Derived Cytokines Interleukin-4 and Interleukin-5

Activation and recruitment of eosinophils in allergic inflammation is in part mediated by chemoattractants and T-helper 2 (Th2)-derived cytokines. However, little is known concerning the signal transduction mechanisms by which this activation occurs. We have investigated tyrosine kinase-mediated activation of phosphatidylinositol 3-kinase (PI3K) and compared this with the activation of the p21ras-ERK signaling pathway in human eosinophils. The related cytokines interleukin-3 (IL-3), IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF), all induced PI3K activity detected in antiphosphotyrosine immunoprecipitates. Furthermore, the chemoattractants platelet-activating factor (PAF), RANTES, and C5a were also able to induce phosphotyrosine-associated PI3K activity. Protein kinase B (PKB) is a downstream target of PI3K activation by growth factors. Induction of PKB phosphorylation in human eosinophils was transiently induced on activation with the cytokines IL-4 and IL-5, as well as the chemoattractants PAF, C5a, and RANTES showing a broad activation profile. Surprisingly, analysis of the activation of the mitogen-activated protein (MAP) kinases p44ERK1 and p42ERK2, showed that ERK2, but not ERK1, was transiently activated in human eosinophils after stimulation with IL-5 or PAF. Activation kinetics correlated with activation of p21ras by both cytokines and chemoattractants as measured by a novel assay for guanosine triphosphate (GTP)-loading. Finally, using specific inhibitors of both the p21ras-ERK and PI3K signaling pathways, a role was demonstrated for PI3K, but not p21ras-ERK, in activation of the serum-treated zymosan (STZ)-mediated respiratory burst in IL-5 and PAF-primed eosinophils. In summary, these data show that in human eosinophils, Th2-derived cytokines differentially activate both PI3K and MAP kinase signal transduction pathways with distinct functional consequences showing complex regulation of eosinophil effector functions.

Analysis of Signal Transduction Pathways in Human Eosinophils Activated by Chemoattractants and the T-Helper 2–Derived Cytokines Interleukin-4 and Interleukin-5

Activation and recruitment of eosinophils in allergic inflammation is in part mediated by chemoattractants and T-helper 2 (Th2)-derived cytokines. However, little is known concerning the signal transduction mechanisms by which this activation occurs. We have investigated tyrosine kinase-mediated activation of phosphatidylinositol 3-kinase (PI3K) and compared this with the activation of the p21ras-ERK signaling pathway in human eosinophils. The related cytokines interleukin-3 (IL-3), IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF), all induced PI3K activity detected in antiphosphotyrosine immunoprecipitates. Furthermore, the chemoattractants platelet-activating factor (PAF), RANTES, and C5a were also able to induce phosphotyrosine-associated PI3K activity. Protein kinase B (PKB) is a downstream target of PI3K activation by growth factors. Induction of PKB phosphorylation in human eosinophils was transiently induced on activation with the cytokines IL-4 and IL-5, as well as the chemoattractants PAF, C5a, and RANTES showing a broad activation profile. Surprisingly, analysis of the activation of the mitogen-activated protein (MAP) kinases p44ERK1 and p42ERK2, showed that ERK2, but not ERK1, was transiently activated in human eosinophils after stimulation with IL-5 or PAF. Activation kinetics correlated with activation of p21ras by both cytokines and chemoattractants as measured by a novel assay for guanosine triphosphate (GTP)-loading. Finally, using specific inhibitors of both the p21ras-ERK and PI3K signaling pathways, a role was demonstrated for PI3K, but not p21ras-ERK, in activation of the serum-treated zymosan (STZ)-mediated respiratory burst in IL-5 and PAF-primed eosinophils. In summary, these data show that in human eosinophils, Th2-derived cytokines differentially activate both PI3K and MAP kinase signal transduction pathways with distinct functional consequences showing complex regulation of eosinophil effector functions.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

Granulocyte-macrophage colony-stimulating factor and IL-5 activate mitogen-activated protein kinase through Jak2 kinase and phosphatidylinositol 3-kinase in human eosinophils

Mitogen-activated protein (MAP) kinases are activated by the sequential activation of Ras, Raf, and MEK (MAP kinase kinase) and regulate a wide variety of cell functions. To determine the kinase cascade for granulocyte-macrophage colony-stimulating factor (GM-CSF)- and IL-5-induced MAP kinase activation in eosinophils, we studied the effect of inhibitors of Jak2 kinase, tyrosine kinases, phosphatidylinositol 3-kinase, and protein kinase C on GM-CSF- and IL-5-induced MAP kinase activation in human eosinophils. GM-CSF and IL-5 activated 40, 42, and 44 kilodalton MAP kinase isoforms in eosinophils. This was indicated by the electrophoretic mobility shift of the three isoforms of MAP kinase in immunoblotting with anti-MAP kinase antibody and also by in-gel MAP kinase assay. MAP kinase activation was time- and dose-dependent, becoming maximal 3 to 15 minutes after stimulation. A Jak2 kinase inhibitor AG-490, a tyrosine kinase inhibitor genistein, and a phosphatidylinositol 3-kinase inhibitor wortmannin inhibited GM-CSF- and IL-5-induced MAP kinase activation in eosinophils, whereas a protein kinase C inhibitor staurosporine had a weak inhibitory effect. Furthermore, AG-490 and genistein prevented GM-CSF-induced tyrosine phosphorylation of Jak2 kinase in eosinophils. Taken together, these results indicate that GM-CSF and IL-5 activate MAP kinases through the signaling pathway of Jak2 kinase-tyrosine phosphorylated beta chain-phosphatidylinositol 3-kinase-Ras in eosinophils.

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.

A Tyrosine-Phosphorylated Protein of 140 kD Is Constitutively Associated With the Phosphotyrosine Binding Domain of Shc and the SH3 Domains of Grb2 in Acute Myeloid Leukemia Cells

The Shc gene encodes three proteins that have been implicated as mediators of signal transduction from growth factor receptors and nonreceptor tyrosine kinases to Ras. Overexpression of Shc in established murine fibroblasts results in oncogenic transformation, indicating that Shc has oncogenic potential. Shc proteins contain a carboxy terminal SH2 domain and a novel non-SH2 phosphotyrosine-binding (PTB) domain that specifically recognizes a phosphorylated NPXpY motif in target proteins such as the epidermal growth factor receptor. We show here that Shc is constitutively tyrosine-phosphorylated in all primary acute myeloid leukemias analyzed and that, in some of these leukemias, Shc is associated through its PTB domain with a tyrosinephosphorylated protein of 140 kD (p140) in vivo. In factor-dependent cells, this 140-kD protein can be tyrosine-phosphorylated in vitro in response to cytokines involved in myeloid proliferation and differentiation, ie, granulocyte-macrophage colony-stimulating factor and colony-stimulating factor-1. A similar or identical protein of 140 kD is constitutively bound to the C-terminal SH3 domain of Grb2 in the same acute myeloid leukemias. In addition to p140, other tyrosine-phosphorylated proteins of 61 and 200 kD are constitutively associated with Shc in some of the leukemias analyzed. Our results implicate Shc, Grb2, p140, and additional tyrosine-phosphorylated proteins of 61 and 200 kD in signalling of acute myeloid leukemia cells.

Activation of 12-O-tetradecanoylphorbol-13-acetate response element- and dyad symmetry element-dependent transcription by interleukin-5 is mediated by Jun N-terminal kinase/stress-activated protein kinase kinases

Interleukin-5 (IL-5) is one of the major regulators of eosinophilic granulocytes in vivo. IL-5 exerts its pleiotropic effects by binding to the IL-5 receptor, which is composed of an IL-5-specific alpha chain and a common betac chain shared with the receptors for IL-3 and granulocyte-macrophage colony-stimulating factor. Previous studies have shown that binding of IL-5 to its receptor triggers the activation of multiple signaling cascades, including the Ras/mitogen-activated protein kinase, the phosphatidyl -3'-kinase, and the Janus kinase/signal transducer and activator of transcription pathways. Here we describe that IL-5 activates the serine/threonine protein kinase Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) pathway. We show that IL-5 activates TPA response element (TRE)-dependent transcription in transfection experiments. TRE activation by IL-5 is mediated by a region of the betac (577-581) that is also responsible for activation of JNK/SAPK and for activation of dyad symmetry element (DSE)-dependent transcription. Dominant-negative SAPK or ERK kinase-1 was used to demonstrate that JNK/SAPK activation is necessary for induction of DSE- and TRE-dependent transcription by IL-5, whereas extracellular signal-regulated kinase 2 was not essential for TRE- and DSE-dependent transcription. By contrast, IL-5-induced activation of the tyrosine kinase Janus kinase 2 seems to be a prerequisite for TRE- and DSE-dependent transcription. Taken together, we show for the first time that IL-5 activates kinases of the JNK/SAPK family, and that this activation is linked to IL-5-induced TRE- and DSE-dependent transcription.

Phosphorylation of Cbl following stimulation with interleukin-3 and its association with Grb2, Fyn, and phosphatidylinositol 3-kinase

We have demonstrated that a 120-kDa protein, identified as Cbl, becomes rapidly phosphorylated on tyrosine residues following stimulation of factor-dependent cells with interleukin-3 (IL-3). Little or no phosphorylation of Cbl was observed in the absence of IL-3 stimulation and phosphorylation is maximal by 20-30 min after IL-3 stimulation. Association of Cbl with Grb2 was noted in unstimulated cells, and the amount of Cbl associated with Grb2 increased following IL-3 stimulation. The p85 subunit of phosphatidylinositol 3-kinase was constitutively associated with Cbl. Approximately 10% of the PI kinase activity present in anti-phosphotyrosine immunoprecipitates was present in anti-Cbl immunoprecipitates of IL-3-stimulated cells. The constitutive association of Cbl with Fyn was also observed. Cbl was observed to bind to bacterial fusion proteins encoding the unique, SH3, and SH2 domains of Fyn, Hck, and Lyn. The SH2 domain of Fyn alone was able to bind Cbl to nearly the same extent as did the fusion protein encoding the unique, SH3, and SH2 domains. This was not the case for the SH2 domain of Hck, however, as binding of the Hck fusion protein to Cbl appeared to require multiple domains. The binding of the fusion proteins to Cbl occurred regardless of whether Cbl was tyrosine-phosphorylated or not, and the binding could not be disrupted by the addition of 30 mM free phosphotyrosine. These data suggest the unexpected conclusion that the Fyn SH2 domain may bind to Cbl in a phosphotyrosine-independent manner.

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.

Emerging paradigms in granulocyte-macrophage colony-stimulating factor signaling

The myelomonocytic lineage of hematopoiesis is regulated by the growth factor granulocyte-macrophage colony-stimulating factor (GM-CSF). This cytokine has proven to be safe for use in coordination with the treatments for bone marrow transplants and acute myelogenous leukemia. GM-CSF and related cytokines operate through specific receptors in the membranes of target cells of the myelopoietic lineages (both immature and mature cells). The exact signal transduction mechanisms in the cell are only beginning to be clarified and involve a plethora of signaling molecules. With a wealth of new information from studies in GM-CSF-induced cell activation, three major experimental approaches are emerging as gold standards in the exploration of those signaling pathways initiated by hematopoietic growth factors. We consider here: (1) a protein-protein interaction, as exemplified by the association between the cytokine membrane receptor and JAK kinase; (2) a covalent modification of an enzyme, as studied in the phosphorylation of MAP kinase; and (3) a protein-DNA interaction, as demonstrated by the translocation of STAT from the cytosol to the nucleus where it can bind to the promoters of specific genes.

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.