Lyn tyrosine kinase is essential for erythropoietin-induced differentiation of J2E erythroid cells

Tyrosine kinases compete for growth hormone receptor binding and regulate receptor mobility and degradation

Growth hormone (GH) acts via JAK2 and LYN to regulate growth, metabolism, and neural function. However, the relationship between these tyrosine kinases remains enigmatic. Through an interdisciplinary approach combining cell biology, structural biology, computation, and single-particle tracking on live cells, we find overlapping LYN and JAK2 Box1-Box2-binding regions in GH receptor (GHR). Our data implicate direct competition between JAK2 and LYN for GHR binding and imply divergent signaling profiles. We show that GHR exhibits distinct mobility states within the cell membrane and that activation of LYN by GH mediates GHR immobilization, thereby initiating its nanoclustering in the membrane. Importantly, we observe that LYN mediates cytokine receptor degradation, thereby controlling receptor turnover and activity, and this applies to related cytokine receptors. Our study offers insight into the molecular interactions of LYN with GHR and highlights important functions for LYN in regulating GHR nanoclustering, signaling, and degradation, traits broadly relevant to many cytokine receptors.

Loci Associated With Antibody Response in Feral Swine (Sus scrofa) Infected With Brucella suis

Feral swine (Sus scrofa) are a destructive invasive species widespread throughout the United States that disrupt ecosystems, damage crops, and carry pathogens of concern for the health of domestic stock and humans including Brucella suis-the causative organism for swine brucellosis. In domestic swine, brucellosis results in reproductive failure due to abortions and infertility. Contact with infected feral swine poses spillover risks to domestic pigs as well as humans, companion animals, wildlife, and other livestock. Genetic factors influence the outcome of infectious diseases; therefore, genome wide association studies (GWAS) of differential immune responses among feral swine can provide an understanding of disease dynamics and inform management to prevent the spillover of brucellosis from feral swine to domestic pigs. We sought to identify loci associated with differential antibody responses among feral swine naturally infected with B. suis using a case-control GWAS. Tissue, serum, and genotype data (68,516 bi-allelic single nucleotide polymorphisms) collected from 47 feral swine were analyzed in this study. The 47 feral swine were culture positive for Brucella spp. Of these 47, 16 were antibody positive (cases) whereas 31 were antibody negative (controls). Single-locus GWAS were performed using efficient mixed-model association eXpedited (EMMAX) methodology with three genetic models: additive, dominant, and recessive. Eight loci associated with seroconversion were identified on chromosome 4, 8, 9, 10, 12, and 18. Subsequent bioinformatic analyses revealed nine putative candidate genes related to immune function, most notably phagocytosis and induction of an inflammatory response. Identified loci and putative candidate genes may play an important role in host immune responses to B. suis infection, characterized by a detectable bacterial presence yet a differential antibody response. Given that antibody tests are used to evaluate brucellosis infection in domestic pigs and for disease surveillance in invasive feral swine, additional studies are needed to fully understand the genetic component of the response to B. suis infection and to more effectively translate estimates of Brucella spp. antibody prevalence among feral swine to disease control management action.

Focal Adhesion Genes Refine the Intermediate-Risk Cytogenetic Classification of Acute Myeloid Leukemia

In recent years, several attempts have been made to identify novel prognostic markers in patients with intermediate-risk acute myeloid leukemia (IR-AML), to implement risk-adapted strategies. The non-receptor tyrosine kinases are proteins involved in regulation of cell growth, adhesion, migration and apoptosis. They associate with metastatic dissemination in solid tumors and poor prognosis. However, their role in haematological malignancies has been scarcely studied. We hypothesized that PTK2/FAK, PTK2B/PYK2, LYN or SRC could be new prognostic markers in IR-AML. We assessed PTK2, PTK2B, LYN and SRC gene expression in a cohort of 324 patients, adults up to the age of 70, classified in the IR-AML cytogenetic group. Univariate and multivariate analyses showed that PTK2B, LYN and PTK2 gene expression are independent prognostic factors in IR-AML patients. PTK2B and LYN identify a patient subgroup with good prognosis within the cohort with non-favorable FLT3/NPM1 combined mutations. In contrast, PTK2 identifies a patient subgroup with poor prognosis within the worst prognosis cohort who display non-favorable FLT3/NPM1 combined mutations and underexpression of PTK2B or LYN. The combined use of these markers can refine the highly heterogeneous intermediate-risk subgroup of AML patients, and allow the development of risk-adapted post-remission chemotherapy protocols to improve their response to treatment.

The Growth Hormone Receptor: Mechanism of Receptor Activation, Cell Signaling, and Physiological Aspects

The growth hormone receptor (GHR), although most well known for regulating growth, has many other important biological functions including regulating metabolism and controlling physiological processes related to the hepatobiliary, cardiovascular, renal, gastrointestinal, and reproductive systems. In addition, growth hormone signaling is an important regulator of aging and plays a significant role in cancer development. Growth hormone activates the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway, and recent studies have provided a new understanding of the mechanism of JAK2 activation by growth hormone binding to its receptor. JAK2 activation is required for growth hormone-mediated activation of STAT1, STAT3, and STAT5, and the negative regulation of JAK-STAT signaling comprises an important step in the control of this signaling pathway. The GHR also activates the Src family kinase signaling pathway independent of JAK2. This review covers the molecular mechanisms of GHR activation and signal transduction as well as the physiological consequences of growth hormone signaling.

Control of nuclear-cytoplasmic shuttling of Ankrd54 by PKCδ

AIM

To identify and characterize the effect of phosphorylation on the subcellular localization of Ankrd54.

METHODS

HEK293T cells were treated with calyculin A, staurosporin or phorbol 12-myristate 13-acetate (PMA). Cells were transfected with eGFP-tagged Ankrd54 with or without Lyn tyrosine kinase (wild-type, Y397F mutant, or Y508F mutant). The subcellular localization was assessed by immunofluorescence imaging of cells, immunoblotting of subcellular fractionations. The phosphorylation of Ankrd54 was monitored using Phos-tagTM gel retardation. Phosphorylated peptides were analysed by multiple-reaction-monitoring (MRM) proteomic analysis.

RESULTS

Activation of PKC kinases using PMA promoted nuclear export of Ankrd54 and correlated with increased Ankrd54 phosphorylation, assayed using Phos-tagTM gel retardation. Co-expression of an active form of the PKCδ isoform specifically promoted both phosphorylation and cytoplasmic localization of Ankrd54, while PKCδ, Akt and PKA did not. Alanine mutation of several serine residues in the amino-terminal region of Ankrd54 (Ser14, Ser17, Ser18, Ser19) reduced both PMA induced cytoplasmic localization and phosphorylation of Ankrd54. Using MRM proteomic analysis, phosphorylation of the Ser18 residue of Ankrd54 was readily detectable in response to PMA stimulation. PMA stimulation of cells co-expressing Ankrd54 and Lyn tyrosine kinase displayed increased co-immunoprecipitation and enhanced co-localization in the cytoplasm.

CONCLUSION

We identify phosphorylation by PKCδ as a major regulator of nuclear-cytoplasmic shuttling of Ankrd54, and its interaction with the tyrosine kinase Lyn.

Csk-binding protein controls red blood cell development via regulation of Lyn tyrosine kinase activity

Erythropoiesis is controlled principally through erythropoietin (Epo) receptor signaling, which involves Janus kinase 2 (JAK2) and Lyn tyrosine kinase, both of which are important for regulating red blood cell (RBC) development. Negative regulation of Lyn involves C-Src kinase (Csk)-mediated phosphorylation of its C-terminal tyrosine, which is facilitated by the transmembrane adaptor Csk-binding protein (Cbp). Although Cbp has significant functions in controlling Lyn levels and activity in erythroid cells in vitro, its importance to primary erythroid cell development and signaling has remained unclear. To address this, we assessed the consequence of loss of Cbp on the erythroid compartment in vivo and whether Epo-responsive cells isolated from Cbp-knockout mice exhibited altered signaling. Our data show that male Cbp^-/- mice display a modest but significant alteration to late erythroid development in bone marrow with evidence of increased erythrocytes in the spleen, whereas female Cbp^-/- mice exhibit a moderate elevation in early erythroid progenitors (not seen in male mice) that does not influence the later steps in RBC development. In isolated primary erythroid cells and cell lines generated from Cbp^-/- mice, survival signaling through Lyn/Akt/FoxO3 was elevated, resulting in sustained viability during differentiation. The high Akt activity disrupted GAB2/SHP-2 feedback inhibition of Lyn; however, the elevated Lyn activity also increased inhibitory signaling via SHP-1 to restrict the Erk1/2 pathway. Interestingly, whereas loss of Cbp led to mild changes to late RBC development in male mice, this was not apparent in female Cbp^-/- mice, possibly due to their elevated estrogen, which is known to facilitate early progenitor self-renewal.

Src family kinases and their role in hematological malignancies

The Src family protein tyrosine kinases (SFKs) are non-receptor intracellular kinases that have important roles in both hematopoiesis and leukemogenesis. The derangement of their expression or activation has been demonstrated to contribute to hematological malignancies. This review first examines the mechanisms of SFK overexpression and hyperactivation, emphasizing the dysregulation of the upstream modulators. Subsequently, the role of SFK up-regulation in the initiation, progression and therapy resistance of many hematological malignancies is also analyzed. The presented evidence endeavors to highlight the influence of SFK up-regulation on an extensive number of hematological malignancies and the need to consider them as candidates in targeted anticancer therapy.

Lyn kinase plays important roles in erythroid expansion, maturation and erythropoietin receptor signalling by regulating inhibitory signalling pathways that control survival

In erythroid cells both positive viability signals and feedback inhibitory signalling require the Src family kinase Lyn, influencing cell survival and their ability to differentiate. This illustrates that Lyn is critical for normal erythropoiesis and erythroid cell development.

Lyn facilitates glioblastoma cell survival under conditions of nutrient deprivation by promoting autophagy

Members of the Src family kinases (SFK) can modulate diverse cellular processes, including division, death and survival, but their role in autophagy has been minimally explored. Here, we investigated the roles of Lyn, a SFK, in promoting the survival of human glioblastoma tumor (GBM) cells in vitro and in vivo using lentiviral vector-mediated expression of constitutively-active Lyn (CA-Lyn) or dominant-negative Lyn (DN-Lyn). Expression of either CA-Lyn or DN-Lyn had no effect on the survival of U87 GBM cells grown under nutrient-rich conditions. In contrast, under nutrient-deprived conditions (absence of supplementation with L-glutamine, which is essential for growth of GBM cells, and FBS) CA-Lyn expression enhanced survival and promoted autophagy as well as inhibiting cell death and promoting proliferation. Expression of DN-Lyn promoted cell death. In the nutrient-deprived GBM cells, CA-Lyn expression enhanced AMPK activity and reduced the levels of pS6 kinase whereas DN-Lyn enhanced the levels of pS6 kinase. Similar results were obtained in vitro using another cultured GBM cell line and primary glioma stem cells. On propagation of the transduced GBM cells in the brains of nude mice, the CA-Lyn xenografts formed larger tumors than control cells and autophagosomes were detectable in the tumor cells. The DN-Lyn xenografts formed smaller tumors and contained more apoptotic cells. Our findings suggest that on nutrient deprivation in vitro Lyn acts to enhance the survival of GBM cells by promoting autophagy and proliferation as well as inhibiting cell death, and Lyn promotes the same effects in vivo in xenograft tumors. As the levels of Lyn protein or its activity are elevated in several cancers these findings may be of broad relevance to cancer biology.

Gain-of-function Lyn induces anemia: appropriate Lyn activity is essential for normal erythropoiesis and Epo receptor signaling

Gain-of-function Lyn mice develop hemolytic anemia with acanthocyte red blood cells and display compensatory extramedullary erythropoiesis. Hyperactive Lyn notably alters Epo receptor signaling, particularly an Akt-FoxO3 pathway, enhancing viability and delaying differentiation.

A crucial role for Lyn in TIMP-1 erythroid cell survival signalling pathway

TIMP-1, a well-known MMP inhibitor, displays other biological activities such as cell survival, proliferation and differentiation in hematopoietic cells. In this report, we investigated the role of the Src-related kinase Lyn in TIMP-1 induced UT-7 erythroleukemic cell survival. We showed that (i) tyrosine 507 of Lyn was dephosphorylated and Lyn kinase activity enhanced by TIMP-1, (ii) Lyn silencing suppressed TIMP-1 anti-apoptotic activity and (iii) Lyn was activated upstream the JAK2/PI 3-kinase/Akt pathway. Our data suggest a novel role for Lyn in erythroid cell survival.

Functions of the Lyn tyrosine kinase in health and disease

Src family kinases such as Lyn are important signaling intermediaries, relaying and modulating different inputs to regulate various outputs, such as proliferation, differentiation, apoptosis, migration and metabolism. Intriguingly, Lyn can mediate both positive and negative signaling processes within the same or different cellular contexts. This duality is exemplified by the B-cell defect in Lyn-/- mice in which Lyn is essential for negative regulation of the B-cell receptor; conversely, B-cells expressing a dominant active mutant of Lyn (Lynup/up) have elevated activities of positive regulators of the B-cell receptor due to this hyperactive kinase. Lyn has well-established functions in most haematopoietic cells, viz. progenitors via influencing c-kit signaling, through to mature cell receptor/integrin signaling, e.g. erythrocytes, platelets, mast cells and macrophages. Consequently, there is an important role for this kinase in regulating hematopoietic abnormalities. Lyn is an important regulator of autoimmune diseases such as asthma and psoriasis, due to its profound ability to influence immune cell signaling. Lyn has also been found to be important for maintaining the leukemic phenotype of many different liquid cancers including acute myeloid leukaemia (AML), chronic myeloid leukaemia (CML) and B-cell lymphocytic leukaemia (BCLL). Lyn is also expressed in some solid tumors and here too it is establishing itself as a potential therapeutic target for prostate, glioblastoma, colon and more aggressive subtypes of breast cancer. LAY To relay information, a cell uses enzymes that put molecular markers on specific proteins so they interact with other proteins or move to specific parts of the cell to have particular functions. A protein called Lyn is one of these enzymes that regulate information transfer within cells to modulate cell growth, survival and movement. Depending on which type of cell and the source of the information input, Lyn can positively or negatively regulate the information output. This ability of Lyn to be able to both turn on and turn off the relay of information inside cells makes it difficult to fully understand its precise function in each specific circumstance. Lyn has important functions for cells involved in blood development, including different while blood cells as well as red blood cells, and in particular for the immune cells that produce antibodies (B-cells), as exemplified by the major B-cell abnormalities that mice with mutations in the Lyn gene display. Certain types of leukaemia and lymphoma appear to have too much Lyn activity that in part causes the characteristics of these diseases, suggesting it may be a good target to develop new anti-leukaemia drugs. Furthermore, some specific types, and even specific subtypes, of solid cancers, e.g. prostate, brain and breast cancer can also have abnormal regulation of Lyn. Consequently, targeting this protein in these cancers could also prove to be beneficial.

Erythropoietin receptor signaling is membrane raft dependent

Upon erythropoietin (Epo) engagement, Epo-receptor (R) homodimerizes to activate JAK2 and Lyn, which phosphorylate STAT5. Although recent investigations have identified key negative regulators of Epo-R signaling, little is known about the role of membrane localization in controlling receptor signal fidelity. Here we show a critical role for membrane raft (MR) microdomains in creation of discrete signaling platforms essential for Epo-R signaling. Treatment of UT7 cells with Epo induced MR assembly and coalescence. Confocal microscopy showed that raft aggregates significantly increased after Epo stimulation (mean, 4.3±1.4(SE) vs. 25.6±3.2 aggregates/cell; p≤0.001), accompanied by a >3-fold increase in cluster size (p≤0.001). Raft fraction immunoblotting showed Epo-R translocation to MR after Epo stimulation and was confirmed by fluorescence microscopy in Epo stimulated UT7 cells and primary erythroid bursts. Receptor recruitment into MR was accompanied by incorporation of JAK2, Lyn, and STAT5 and their activated forms. Raft disruption by cholesterol depletion extinguished Epo induced Jak2, STAT5, Akt and MAPK phosphorylation in UT7 cells and erythroid progenitors. Furthermore, inhibition of the Rho GTPases Rac1 or RhoA blocked receptor recruitment into raft fractions, indicating a role for these GTPases in receptor trafficking. These data establish a critical role for MR in recruitment and assembly of Epo-R and signal intermediates into discrete membrane signaling units.

Integrating novel signaling pathways involved in erythropoiesis

Many extrinsic and intrinsic factors control the development of red blood cells from committed progenitors, with the Erythropoietin-receptor (Epo-R) signaling network being the primary controlling molecular hub. Although much is understood about erythroid signaling pathways, new and intriguing factors that influence different aspects of erythroid cell development are still being uncovered. New extrinsic effectors include hypoxia and polymeric IgA1 (pIgA1), and new Epo-R signaling pathway components include Lyn/Cbp and Lyn/Liar. Hypoxia directly activates committed erythroid progenitors to expand, whereas pIgA1 activates the Akt and MAP-Kinase (MAPK) pathways through transferrin receptors on more mature erythroid cells. The Lyn/Cbp pathway controls the activity and protein levels of Lyn through recruitment of Csk and SOCS1, as well as feeding into the control of other pathways mediated by recruitment of ras-GAP, PI3-kinase, PLCγ, Fes, and EBP50. Nuclear/cytoplasmic shuttling of Lyn and other signaling molecules is influenced by Liar and results in regulation of their intersecting signaling pathways. The challenge of future research is to flesh out the details of these new signaling regulators/networks and integrate their influences during the different stages of erythropoiesis.

Peginesatide and erythropoietin stimulate similar erythropoietin receptor-mediated signal transduction and gene induction events

Peginesatide is a synthetic, PEGylated, peptide-based erythropoiesis-stimulating agent that is designed and engineered to stimulate specifically the erythropoietin receptor dimer that governs erythropoiesis. Peginesatide has a unique structure that consists of a synthetic peptide dimer (with no sequence similarity to erythropoietin) conjugated to a 40-kDa PEG moiety. Peginesatide is being developed for the treatment of anemia associated with chronic kidney disease in dialysis patients. To compare signaling effects of peginesatide to recombinant human erythropoietin (rHuEPO), dose-dependent effects on protein phosphorylation and gene expression were evaluated using phosphoproteomics, quantitative signal transduction analyses, and gene profiling. After stimulation with peginesatide or rHuEPO, cell lysates were prepared from UT-7/EPO cells. Liquid chromatography-tandem mass spectrometry and MesoScale arrays were used to quantify phosphorylation events. Transcriptional changes were analyzed using microarrays and quantitative reverse transcription polymerase chain reaction. Peginesatide and rHuEPO were found to regulate the tyrosine phosphorylation of an essentially equivalent set of protein substrates, and modulate the expression of a similar set of target genes. Consistent with their roles in stimulating erythropoiesis, peginesatide and rHuEPO regulate similar cellular pathways.

Targeting Lyn tyrosine kinase through protein fusions encompassing motifs of Cbp (Csk-binding protein) and the SOCS box of SOCS1

The tyrosine kinase Lyn is involved in oncogenic signalling in several leukaemias and solid tumours, and we have previously identified a pathway centred on Cbp [Csk (C-terminal Src kinase)-binding protein] that mediates both enzymatic inactivation, as well as proteasomal degradation of Lyn via phosphorylation-dependent recruitment of Csk (responsible for phosphorylating the inhibitory C-terminal tyrosine of Lyn) and SOCS1 (suppressor of cytokine signalling 1; an E3 ubiquitin ligase). In the present study we show that fusing specific functional motifs of Cbp and domains of SOCS1 together generates a novel molecule capable of directing the proteasomal degradation of Lyn. We have characterized the binding of pY (phospho-tyrosine) motifs of Cbp to SFK (Src-family kinase) SH2 (Src homology 2) domains, identifying those with high affinity and specificity for the SH2 domain of Lyn and that are preferred substrates of active Lyn. We then fused them to the SB (SOCS box) of SOCS1 to facilitate interaction with the ubiquitination-promoting elongin B/C complex. As an eGFP (enhanced green fluorescent protein) fusion, these proteins can direct the polyubiquitination and proteasomal degradation of active Lyn. Expressing this fusion protein in DU145 cancer cells (but not LNCaP or MCF-7 cells), that require Lyn signalling for survival, promotes loss of Lyn, loss of caspase 3, appearance of an apoptotic morphology and failure to survive/expand. These findings show how functional domains of Cbp and SOCS1 can be fused together to generate molecules capable of inhibiting the growth of cancer cells that express high levels of active Lyn.

Direct activation of STAT5 by ETV6-LYN fusion protein promotes induction of myeloproliferative neoplasm with myelofibrosis

Myeloproliferative neoplasms (MPN), a group of haematopoietic stem cell (HSC) disorders, are often accompanied by myelofibrosis. We previously identified the fusion of the ETV6 gene to the LYN gene (ETV6-LYN) in idiopathic myelofibrosis with ins(12;8)(p13;q11q21). The introduction of ETV6-LYN into HSCs resulted in fatal MPN with massive myelofibrosis in mice, implicating the rearranged LYN kinase in the pathogenesis of MPN with myelofibrosis. However, the signalling molecules directly downstream from and activated by ETV6-LYN remain unknown. In this study, we demonstrated that the direct activation of STAT5 by ETV6-LYN is crucial for the development of MPN. ETV6-LYN was constitutively active as a kinase through autophosphorylation. ETV6-LYN, but not its kinase-dead mutant, supported cytokine-free proliferation of haematopoietic cells. STAT5 was activated in a JAK2-independent manner in ETV6-LYN-expressing cells. ETV6-LYN interacted with STAT5 and directly activated STAT5 both in vitro and in vivo. Of note, ETV6-LYN did not support the formation of colonies by Stat5-deficient HSCs under cytokine-free conditions and the capacity of ETV6-LYN to induce MPN with myelofibrosis was profoundly attenuated in a Stat5-null background. These findings define STAT5 as a direct target of ETV6-LYN and unveil the LYN-STAT5 axis as a novel pathway to augment proliferative signals in MPN and leukaemia.

The growth hormone receptor: mechanism of activation and clinical implications

Growth hormone is widely used clinically to promote growth and anabolism and for other purposes. Its actions are mediated via the growth hormone receptor, both directly by tyrosine kinase activation and indirectly by induction of insulin-like growth factor 1 (IGF-1). Insensitivity to growth hormone (Laron syndrome) can result from mutations in the growth hormone receptor and can be treated with IGF-1. This treatment is, however, not fully effective owing to the loss of the direct actions of growth hormone and altered availability of exogenous IGF-1. Excessive activation of the growth hormone receptor by circulating growth hormone results in gigantism and acromegaly, whereas cell transformation and cancer can occur in response to autocrine activation of the receptor. Advances in understanding the mechanism of receptor activation have led to a model in which the growth hormone receptor exists as a constitutive dimer. Binding of the hormone realigns the subunits by rotation and closer apposition, resulting in juxtaposition of the catalytic domains of the associated tyrosine-protein kinase JAK2 below the cell membrane. This change results in activation of JAK2 by transphosphorylation, then phosphorylation of receptor tyrosines in the cytoplasmic domain, which enables binding of adaptor proteins, as well as direct phosphorylation of target proteins. This model is discussed in the light of salient information from closely related class 1 cytokine receptors, such as the erythropoietin, prolactin and thrombopoietin receptors.

Erythropoietin down-regulates stem cell factor receptor (Kit) expression in the leukemic proerythroblast: role of Lyn kinase

Overexpression of the transcription factor Spi-1/PU.1 by transgenesis in mice induces a maturation arrest at the proerythroblastic stage of differentiation. We have previously isolated a panel of spi-1 transgenic erythroleukemic cell lines that proliferated in the presence of either erythropoietin (Epo) or stem cell factor (SCF). Using these cell lines, we observed that EpoR stimulation by Epo down-regulated expression of the SCF receptor Kit and induced expression of the Src kinase Lyn. Furthermore, enforced expression of Lyn in the cell lines increased cell proliferation in response to Epo, but reduced cell growth in response to SCF in accordance with Lyn ability to down-regulate Kit expression. Together, the data suggest that Epo-R/Lyn signaling pathway is essential for extinction of SCF signaling leading the proerythroblast to strict Epo dependency. These results highlight a new role for Lyn as an effector of EpoR in controlling Kit expression. They suggest that Lyn may play a central role in during erythroid differentiation at the switch between proliferation and maturation.

Csk-binding protein can regulate Lyn signals controlling cell morphology

The Src family kinase Lyn is involved in differentiation signals emanating from activated erythropoietin (Epo) receptors, it interacts with COOH-terminal Src kinase-binding protein (Cbp), an adaptor protein that recruits negative regulators COOH-terminal Src kinase (Csk) and suppressor of cytokine signaling-1 (SOCS1). Lyn phosphorylates Cbp on several tyrosine residues, including Tyr314, which recruits Csk/SOCS1, as well as Tyr381 and Tyr409 that bind Lyns own SH2 domain. We show that Cbp alters not only the ability of erythroid cells to differentiate but also their colony morphology. Consequently, we detailed the ability of Cbp to interact with and influence Lyns ability to initiate changes in cellular architecture, which affect cell-cell and cell-substratum interactions. Over-expression of active Lyn promotes filopodia formation while inactive Lyn promotes lamellipodia formation. Conversely, Cbp over-expression, which inhibits Lyn activity, promotes lamellipodia formation, while Cbp mutants preventing its interaction/signaling consequently allow Lyn to promote filopodia formation. Thus, the Lyn-Cbp pathway and subsequent regulation of Lyn signaling and cell morphology involves a dynamic and complex series of interactions.

Liar, a novel Lyn-binding nuclear/cytoplasmic shuttling protein that influences erythropoietin-induced differentiation

Erythropoiesis is primarily controlled by erythropoietin (Epo), which stimulates proliferation, differentiation, and survival of erythroid precursors. We have previously shown that the tyrosine kinase Lyn is critical for transducing differentiation signals emanating from the activated Epo receptor. A yeast 2-hybrid screen for downstream effectors of Lyn identified a novel protein, Liar (Lyn-interacting ankyrin repeat), which forms a multiprotein complex with Lyn and HS1 in erythroid cells. Interestingly, 3 of the ankyrin repeats of Liar define a novel SH3 binding region for Lyn and HS1. Liar also contains functional nuclear localization and nuclear export sequences and shuttles rapidly between the nucleus and cytoplasm. Ectopic expression of Liar inhibited the differentiation of normal erythroid progenitors, as well as immortalized erythroid cells. Significantly, Liar affected Epo-activated signaling molecules including Erk2, STAT5, Akt, and Lyn. These results show that Liar is a novel Lyn-interacting molecule that plays an important role in regulating intracellular signaling events associated with erythroid terminal differentiation.

Crystal structures of the Lyn protein tyrosine kinase domain in its Apo- and inhibitor-bound state

The Src-family protein-tyrosine kinase (PTK) Lyn is the most important Src-family kinase in B cells, having both inhibitory and stimulatory activity that is dependent on the receptor, ligand, and developmental context of the B cell. An important role for Lyn has been reported in acute myeloid leukemia and chronic myeloid leukemia, as well as certain solid tumors. Although several Src-family inhibitors are available, the development of Lyn-specific inhibitors, or inhibitors with reduced off-target activity to Lyn, has been hampered by the lack of structural data on the Lyn kinase. Here we report the crystal structure of the non-liganded form of Lyn kinase domain, as well as in complex with three different inhibitors: the ATP analogue AMP-PNP; the pan Src kinase inhibitor PP2; and the BCR-Abl/Src-family inhibitor Dasatinib. The Lyn kinase domain was determined in its "active" conformation, but in the unphosphorylated state. All three inhibitors are bound at the ATP-binding site, with PP2 and Dasatinib extending into a hydrophobic pocket deep in the substrate cleft, thereby providing a basis for the Src-specific inhibition. Analysis of sequence and structural differences around the active site region of the Src-family PTKs were evident. Accordingly, our data provide valuable information for the further development of therapeutics targeting Lyn and the important Src-family of kinases.

Haem-regulated eIF2alpha kinase is necessary for adaptive gene expression in erythroid precursors under the stress of iron deficiency

Haem-regulated eIF2alpha kinase (HRI) is essential for the regulation of globin gene translation and the survival of erythroid precursors in iron/haem deficiency. This study found that that in iron deficiency, fetal definitive erythropoiesis is inhibited at the basophilic erythroblast stage with increased proliferation and elevated apoptosis. This hallmark of ineffective erythropoiesis is more severe in HRI deficiency. Microarray gene profiling analysis showed that HRI was required for adaptive gene expression in erythroid precursors during chronic iron deficiency. The number of genes with expression affected more than twofold increased, from 213 in iron deficiency and 73 in HRI deficiency, to 3135 in combined iron and HRI deficiencies. Many of these genes are regulated by Gata1 and Fog1. We demonstrate for the first time that Gata1 expression in developing erythroid precursors is decreased in iron deficiency, and is decreased further in combined iron and HRI deficiencies. Additionally, Fog1 expression is decreased in combined deficiencies, but not in iron or HRI deficiency alone. Our results indicate that HRI confers adaptive gene expression in developing erythroblasts during iron deficiency through maintaining Gata1/Fog1 expression.

The erythropoietin receptor in normal and cancer tissues

The hormone erythropoietin (EPO) is essential for the survival, proliferation and differentiation of the erythrocytic progenitors. The EPO receptor (EPO-R) of erythrocytic cells belongs to the cytokine class I receptor family and signals through various protein kinases and STAT transcription factors. The EPO-R is also expressed in many organs outside the bone marrow, suggesting that EPO is a pleiotropic anti-apoptotic factor. The controversial issue as to whether the EPO-R is functional in tumor tissue is critically reviewed. Importantly, most studies of EPO-R detection in tumor tissue have provided falsely positive results because of the lack of EPO-R specific antibodies. However, endogenous EPO appears to be necessary to maintain the viability of endothelial cells and to promote tumor angiogenesis. Although there is no clinical proof that the administration of erythropoiesis stimulating agents (ESAs) promotes tumor growth and mortality, present recommendations are that (i) ESAs should be administered at the lowest dose sufficient to avoid the need for red blood cell transfusions, (ii) ESAs should not be used in patients with active malignant disease not receiving chemotherapy or radiotherapy, (iii) ESAs should be discontinued following the completion of a chemotherapy course, (iv) the target Hb should be 12 g/dL and not higher and (v) the risks of shortened survival and tumor progression have not been excluded when ESAs are dosed to target Hb <12 g/dL.

Erythroid defects in TRα−/− mice

Thyroid hormone and its cognate receptor (TR) have been implicated in the production of red blood cells. Here, we show mice deficient for TRα have compromised fetal and adult erythropoiesis. Erythroid progenitor numbers were significantly reduced in TRα−/− fetal livers, and transit through the final stages of maturation was impeded. In addition, immortalized TRα−/− erythroblasts displayed increased apoptosis and reduced capacity for proliferation and differentiation. Adult TRα−/− mice had lower hematocrit levels, elevated glucocorticoid levels, and an altered stress erythropoiesis response to hemolytic anemia. Most TRα−/− animals contained markedly altered progenitor numbers in their spleens. Strikingly, 20% of TRα−/− mice failed to elicit a stress erythropoiesis response and recovered very poorly from hemolytic anemia. We conclude that an underlying erythroid defect exists in TRα−/− mice, demon-strating the importance of TRα to the erythroid compartment.

Src family kinases: regulation of their activities, levels and identification of new pathways

While the Src family of protein tyrosine kinases (SFK), and the main ancillary molecules involved in their regulation, have been studied for many years, the details of their interplay are not fully understood and thus remain under active investigation. Additionally, new players that coordinate their regulation and direct their signalling cascades are also being uncovered, shedding new light on the complexity of these signalling networks. Through the utilization of novel interaction assays, several new interconnecting mediators that are helping to show the elegance of Src family kinase regulation have been discovered. This review outlines SFK regulation, the discovery of the Csk binding protein (Phosphoprotein Associated with Glycosphingolipid-enriched microdomains, Cbp/PAG), and its role in regulating SFK kinase activity status, as well as protein levels. Further, details of the methods used to identify this dual mode of regulation can be applied to delineate the full gamut of SH2/SH3-directed SFK pathways and, indeed, those of any tyrosine kinase. Using Lyn as a model SFK, we and others have shown that Cbp recruits negative regulators of COOH-terminal Src kinase (Csk)/Csk-like protein-tyrosine kinase (Ctk) after Lyn is activated and bound to Cbp. Lyn phosphorylates Cbp on multiple tyrosine residues, including two that can bind Lyn's SH2 domain with high affinity. Lyn also phosphorylates Y314, which recruits Csk/Ctk to phosphorylate Lyn at its Y508 negative site, allowing an inactive conformation to form. However, the pY508 site has a low affinity for Lyn's SH2 domain, while the Cbp sites have high affinity. Thus, until these Cbp sites are dephosphorylated, Lyn can remain active. Intriguingly, phosphorylated Y314 also binds the suppressor of cytokine signalling 1 (SOCS1), resulting in elevated ubiquitination and degradation of Lyn. Thus, a single phosphotyrosine residue within Cbp co-ordinates a two-phase process involving distinct negative regulatory pathways that allow inactivation, followed by degradation, of SFKs.

Functional conservation of erythropoietin signaling in zebrafish

Erythropoietin (Epo) and its cognate receptor (EpoR) are required for maintaining adequate levels of circulating erythrocytes during embryogenesis and adulthood. Here, we report the functional characterization of the zebrafish epo and epor genes. The expression of epo and epor was evaluated by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization, revealing marked parallels between zebrafish and mammalian gene expression patterns. Examination of the hypochromic mutant, weissherbst, and adult hypoxia-treated hearts indicate that zebrafish epo expression is induced by anemia and hypoxia. Overexpression of epo mRNA resulted in severe polycythemia, characterized by a striking increase in the number of cells expressing scl, c-myb, gata1, ikaros, epor, and betae1-globin, suggesting that both the erythroid progenitor and mature erythrocyte compartments respond to epo. Morpholino-mediated knockdown of the epor caused a slight decrease in primitive and complete block of definitive erythropoiesis. Abrogation of STAT5 blocked the erythropoietic expansion by epo mRNA, consistent with a requirement for STAT5 in epo signaling. Together, the characterization of zebrafish epo and epor demonstrates the conservation of an ancient program that ensures proper red blood cell numbers during normal homeostasis and under hypoxic conditions.

Deciphering the cross talk between hnRNP K and c-Src: the c-Src activation domain in hnRNP K is distinct from a second interaction site

The protein tyrosine kinase c-Src is regulated by two intramolecular interactions. The repressed state is achieved through the interaction of the Src homology 2 (SH2) domain with the phosphorylated C-terminal tail and the association of the SH3 domain with a polyproline type II helix formed by the linker region between SH2 and the kinase domain. hnRNP K, the founding member of the KH domain protein family, is involved in chromatin remodeling, regulation of transcription, and translation of specific mRNAs and is a target in different signal transduction pathways. In particular, it functions as a specific activator and a substrate of the tyrosine kinase c-Src. Here we address the question how hnRNP K interacts with and activates c-Src. We define the proline residues in hnRNP K in the proline-rich motifs P2 (amino acids [aa] 285 to 297) and P3 (aa 303 to 318), which are necessary and sufficient for the specific activation of c-Src, and we dissect the amino acid sequence (aa 216 to 226) of hnRNP K that mediates a second interaction with c-Src. Our findings indicate that the interaction with c-Src and the activation of the kinase are separable functions of hnRNP K. hnRNP K acts as a scaffold protein that integrates signaling cascades by facilitating the cross talk between kinases and factors that mediate nucleic acid-directed processes.

Myeloid Leukemia Factor 1 Associates with a Novel Heterogeneous Nuclear Ribonucleoprotein U-like Molecule

Myeloid leukemia factor 1 (MLF1) is an oncoprotein associated with hemopoietic lineage commitment and acute myeloid leukemia. Here we show that Mlf1 associated with a novel binding partner, Mlf1-associated nuclear protein (Manp), a new heterogeneous nuclear ribonucleoprotein (hnRNP) family member, related to hnRNP-U. Manp localized exclusively in the nucleus and could redirect Mlf1 from the cytoplasm into the nucleus. The nuclear content of Mlf1 was also regulated by 14-3-3 binding to a canonical 14-3-3 binding motif within the N terminus of Mlf1. Significantly Mlf1 contains a functional nuclear export signal and localized primarily to the nuclei of hemopoietic cells. Mlf1 was capable of binding DNA, and microarray analysis revealed that it affected the expression of several genes, including transcription factors. In summary, this study reveals that Mlf1 translocates between nucleus and cytoplasm, associates with a novel hnRNP, and influences gene expression.

Erythropoietin induces sustained phosphorylation of STAT5 in primitive but not definitive erythrocytes generated from mouse embryonic stem cells

OBJECTIVE

During embryonic development murine erythropoiesis occurs in two waves by producing first primitive erythroid cells (EryPs) and then definitive erythroid cells (EryDs). Erythropoietin (EPO) signaling is compared between EryPs and EryDs.

METHODS

We studied the EPO signaling in EryPs and EryDs using an embryonic stem-derived culture system, which can recapitulate this in vivo development process and has thus been used as a convenient in vitro model system of erythropoiesis.

RESULTS

We found that EPO induced sustained phosphorylation and nuclear translocation of signal transducer and activator of transcription 5 (STAT5) in EryPs but not EryDs. EryPs expressed dramatically higher amounts of EPO receptor compared with EryDs, indicating there was excessive signaling from the receptor upon EPO stimulation. In addition, reduced expression of tyrosine phosphatase, Src homology region 2 domain-containing phosphatase-1, and decreased total phosphatase activity in EryPs partly explain the persistent activation of STAT5. Nevertheless, Janus kinase 2 (JAK2) phosphorylation, which is essential for transduction of EPO signaling from the EPO receptor to STAT5, was observed in a transient but not a persistent manner. Inhibition of JAK activity resulted in partial suppression of transient phosphorylation of STAT5 and no suppression of sustained phosphorylation of STAT5.

CONCLUSION

This study presents a unique feature of EryPs, as this is the first known example of sustained activation of STAT5 in normal cells. Our results also imply the existence of a JAK2-independent pathway of EPO signaling to induce STAT5 activation.

Mutation of the Lyn tyrosine kinase delays the progression of Friend virus induced erythroleukemia without affecting susceptibility

During the initial phase of Friend virus (FV) induced erythroleukemia, the interaction between the viral envelope glycoprotein gp55, the Erythropoietin receptor (EpoR) and the naturally occurring truncated version of the Mst1r receptor tyrosine kinase, called Sf-Stk, drives the polyclonal expansion of infected progenitors in an erythropoietin independent manner. Sf-Stk provides signals that cooperate with EpoR signals to effect expansion of erythroid progenitors. The latter phase of disease is characterized by a clonal expansion of transformed leukemic cells causing an acute erythroleukemia in mice. Signaling by Sf-Stk and EpoR mediated by gp55 renders erythroid progenitors Epo independent through the activation of the EpoR downstream pathways such as PI3K, MAPK and JAK/STAT. Previous work has shown that Src family kinases also play an important role in erythropoiesis. In particular, mutation of Src and Lyn can affect erythropoiesis. In this report we analyze the role of the Lyn tyrosine kinase in the pathogenesis of Friend virus. We demonstrate that during FV infection of primary erythroblasts, Lyn is not required for expansion of viral targets. Lyn deficient bone marrow and spleen cells are able to form Epo independent FV colonies in vitro. In vivo infection of Lyn deficient animals also results in a massive splenomegaly characteristic of the virus. However, we observe differences in the pathogenesis of Friend erythroleukemia in Lyn-/- mice. Lyn-/- mice infected with the polycythemia inducing strain of FV, FVP, do not develop polycythemia suggesting that Lyn-/- infected erythroblasts have a defect in terminal differentiation. Furthermore, the expansion of transformed cells in the spleen is reduced in Lyn-/- mice. Our data show that Lyn signals are not required for susceptibility to Friend erythroleukemia, but Lyn plays a role in later events, the terminal differentiation of infected cells and the expansion of transformed cells.

Csk-binding protein mediates sequential enzymatic down-regulation and degradation of Lyn in erythropoietin-stimulated cells

We have shown previously that the Src family kinase Lyn is involved in differentiation signals emanating from activated erythropoietin (Epo) receptors. The importance of Lyn to red cell maturation has been highlighted by Lyn-/- mice developing anemia. Here we show that Lyn interacts with C-terminal Src kinase-binding protein (Cbp), an adaptor protein that recruits negative regulators C-terminal Src kinase (Csk)/Csk-like protein-tyrosine kinase (Ctk). Lyn phosphorylated Cbp on several tyrosine residues, including Tyr314, which recruited Csk/Ctk to suppress Lyn kinase activity. Intriguingly, phosphorylated Tyr314 also bound suppressor of cytokine signaling 1 (SOCS1), another well characterized negative regulator of cell signaling, resulting in elevated ubiquitination, and degradation of Lyn. In Epo-responsive primary cells and cell lines, Lyn rapidly phosphorylated Cbp, suppressing Lyn kinase activity via Csk/Ctk within minutes of Epo stimulation; hours later, SOCS1 bound to Cbp and was involved in the ubiquitination and turnover of Lyn protein. Thus, a single phosphotyrosine residue on Cbp coordinates a two-phase process involving distinct negative regulatory pathways to inactivate, then degrade, Lyn.

The duplicitous nature of the Lyn tyrosine kinase in growth factor signaling

The Lyn tyrosine kinase is a unique member of the Src family of non-receptor protein tyrosine kinases whose principal role is to regulate signals through inhibitory receptors thereby promoting signal attenuation. Lyn is renowned for its role in B cell antigen receptor and FcepsilonRI signaling; however, it is becoming increasingly apparent that Lyn also functions in signal transduction from growth factor receptors including the receptors for GM-CSF, IL-3, IL-5, SCF, erythropoietin, CSF-1, G-CSF, thrombopoietin and Flt3 ligand. Numerous studies have implicated Lyn in growth factor receptor signal amplification, while a number also suggest that Lyn participates in negative regulation of growth factor signaling. Indeed Lyn-deficient mice are hyper-responsive to myeloid growth factors and develop a myeloproliferative disorder that predisposes the mice to macrophage tumours, with loss of negative regulation through SHP-1 and SHIP-1 thought to be the major contributing factor to this phenotype. Developing a clear understanding of Lyn's role in establishing signaling thresholds in growth factor receptor signal amplification and signal inhibition may have important implications in the management of leukemias that may depend on Lyn activity.

Lyn kinase promotes erythroblast expansion and late-stage development

Lyn kinase is known to modulate the formation and function of B cells, monocytes, and mast cells. However, Lyn-/- mice also develop erythrosplenomegaly, and cases for both negative and positive erythropoietic actions of Lyn recently have been outlined. In phenylhydrazine-treated Lyn-/- mice, extramedullary splenic erythropoiesis was hyperactivated, but this did not lead to accelerated recovery from anemia. Furthermore, ex vivo analyses of the development of bone marrow-derived Lyn-/- erythroblasts in unique primary culture systems indicated positive roles for Lyn at 2 stages. Late-stage Lyn-/- erythroblasts exhibited deficit Ter119(pos) cell formation, and this was paralleled by increased apoptosis (and decreased Bcl-xL expression). During early development, Lyn-/- erythroblasts accumulated at a Kit(pos)CD71(high) stage, possessed decreased proliferative capacity, and were attenuated in entering an apparent G1/S cell-cycle phase. In proposed compensatory responses, Lyn-/- erythroblasts expressed increased levels of activated Akt and p60-Src and decreased levels of death-associated protein kinase-2. Stat5 activation and Bcl-xL expression, in contrast, were significantly decreased in keeping with decreased survival and developmental potentials. Lyn, therefore, is proposed to function via erythroid cell-intrinsic mechanisms to promote progenitor cell expansion beyond a Kit(pos)CD71(high) stage and to support subsequent late-stage development.

Cross-regulation of JAK and Src kinases

Members of the Janus kinase (JAK) family, JAK1, JAK2, JAK3 and Tyk2 are intimately involved in the signalling events initiated by cytokines activating cell surface receptors. They are responsible for phosphorylating these receptors, which create docking sites for downstream molecules such as the signal transducer and activator of transcription family members. In addition, cytokine receptors associate with members of the Src family kinase (SFK). JAKs and SFK work in concert to activate many of the signalling molecules, with both kinase families required for optimal transmission of intracellular signals. JAKs and SFK are also required for the activation and recruitment of negative regulators of cytokine signalling, e.g., protein tyrosine phosphatases (PTPs) and suppressors of cytokine signalling. Aberrant activity of the JAK-Src kinase duet can result in hemopoietic abnormalities including leukaemia. Additionally, the recent identification of a somatic JAK2 mutation as the cause of polycythema vera, further highlights the clinical importance of these molecules.

Increased megakaryocytopoiesis in Lyn-deficient mice

Previous studies in cell lines have shown Lyn kinase to be a negative regulator of thrombopoietin (TPO)-induced proliferation. To further investigate the role of Lyn during megakaryocytopoiesis, Lyn-deficient mice (lyn(-/-)) were analyzed. We observed that lyn(-/-) mice have more bone marrow-derived GPIIB (CD41) and Mpl(+) cells when compared to their wild-type littermates. In addition, colony-forming unit-megakaryocytes (CFU-MK) are increased and TPO-induced expansion of primary marrow cells yielded a greater number of mature megakaryocytes (MKs) with increased nuclear ploidy. Histopathology of bone marrow and spleens from lyn(-/-) mice showed an increase in the number of MKs. Mechanistic studies revealed that TPO stimulation of MKs from lyn(-/-) mice did not affect phosphorylation of Janus kinase 2 (JAK2), signal transducer and activator of transcription (STAT) 3, STAT5, or MAP kinase kinase (MEK). Lyn-deficient MKs supported greater TPO-mediated phosphorylation and kinase activity of both Erk1/2 (mitogen-activated protein kinase, MAPK) and Akt. In contrast, there was a reduction of tyrosine phosphorylation of the inositol phosphatase, SHIP. This is the first direct evidence using primary MKs from Lyn-deficient mice that confirms our prior data from cell lines that Lyn kinase is a negative regulator of TPO signaling.

The regulation of the expression of inducible nitric oxide synthase by Src-family tyrosine kinases mediated through MyD88-independent signaling pathways of Toll-like receptor 4

Bacterial lipopolysaccharide (LPS) activates Toll-like receptor 4 (TLR4) leading to the expression of inflammatory gene products. Src-family tyrosine kinases (STKs) are known to be activated by LPS in monocytes/macrophages. Therefore, we determined the role of STKs in TLR4 signaling pathways and target gene expression in macrophages. The activation of NFkappaB, and p38 MAPK, and the expression of inducible nitric oxide synthase (iNOS) induced by LPS were not affected in macrophages deficient in three STKs (Lyn, Hck, and Fgr). These results suggest that the deletion of the three STKs among possibly nine STKs is not sufficient to abolish total activity of STKs possibly due to the functional redundancy of other STKs present in macrophages. However, two structurally unrelated pan-inhibitors of STKs, PP1 and SU6656, suppressed LPS-induced iNOS expression in MyD88-knockout as well as wild-type macrophages. The suppression of iNOS expression by the inhibitors was correlated with the downregulation of IFNbeta (a MyD88-independent gene) expression and subsequent decrease in STAT1 phosphorylation. Moreover, PP1 suppressed the expression of IFNbeta and iNOS induced by TRIF, a MyD88-independent adaptor of TLR4. PP1 suppressed STAT1 phosphorylation induced by LPS, but not by IFNbeta suggesting that STKs are involved in the primary downstream signaling pathways of TLR4, but not the secondary signaling pathways downstream of IFNbeta receptor. Together, these results demonstrate that STKs play a positive regulatory role in TLR4-mediated iNOS expression in a MyD88-independent (TRIF-dependent) manner. These results provide new insight in understanding the role of STKs in TLR4 signaling pathways and inflammatory target gene expression.

Turning cells red: signal transduction mediated by erythropoietin

Erythropoietin (EPO) is the crucial cytokine regulator of red blood-cell production. Since the discovery of EPO in 1985 and the isolation of its cognate receptor four years later, there has been significant interest in understanding the unique ability of this ligand-receptor pair to promote erythroid mitogenesis, survival and differentiation. The development of knockout mice has elucidated the precise role of the ligand, receptor and downstream players in murine erythroid development. In this review, we summarize EPO-mediated signaling pathways and examine their significance in vivo.

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.

Phosphoprotein profiling of erythropoietin receptor- dependent pathways using different proteomic strategies

Proteomic techniques provide new tools for the global analysis of protein profiles but also for the investigation of specific protein functions. The analysis of signaling cascades has traditionally been performed by the determination of enzymatic or transcription factor activities representing a certain pathway. Functional proteomics now allows more comprehensive approaches to study cellular responses induced during ligand/receptor interactions. In this study we evaluated proteomic strategies for the investigation of structure-function relationships in the erythropoietin receptor signalling complex. After expression of epidermal growth factor/erythropoietin receptor mutant molecules in an identical cellular background we characterized their potential to induce cellular activities. Using this system we focused our efforts on post-translational modifications of signalling proteins reflecting a substantial part of receptor-dependent signaling events. Although tyrosine phosphorylated proteins were enriched by immunoprecipitation the analysis using the classical approach combining two-dimensional gel electrophoresis and identification by matrix assisted laser desorption/ionization-time of flight-mass spectrometry revealed that low expressed signaling proteins cannot be detected by this technique. An alternative strategy using one-dimensional gel separation of phosphoproteins and liquid chromatography-tandem mass spectrometry, however, allowed us to identify multiple proteins involved in intracellular signalling representing already established pathways but also proteins which have not been linked to EPO-induced signaling so far. This approach offers the potential to extend functional proteomic studies to complex signaling processes.

Perturbed myelo/erythropoiesis in Lyn-deficient mice is similar to that in mice lacking the inhibitory phosphatases SHP-1 and SHIP-1

The Lyn tyrosine kinase plays essential inhibitory signaling roles within hematopoietic cells by recruiting inhibitory phosphatases such as SH2-domain containing phosphatase-1 (SHP-1), SHP-2, and SH2-domain containing 5'-inositol phosphatase (SHIP-1) to the plasma membrane in response to specific stimuli. Lyn-deficient mice display a collection of hematopoietic defects, including autoimmune disease as a result of autoantibody production, and perturbations in myelopoiesis that ultimately lead to splenomegaly and myeloid neoplasia. In this study, we demonstrate that loss of Lyn results in a stem/progenitor cell-intrinsic defect leading to an age-dependent increase in myeloid, erythroid, and primitive hematopoietic progenitor numbers that is independent of autoimmune disease. Despite possessing increased numbers of erythroid progenitors, and a more robust expansion of these cells following phenylhydrazine challenge, Lyn-deficient mice are more severely affected by the chemotherapeutic drug 5-fluorouracil, revealing a greater proportion of cycling progenitors. We also show that mice lacking SHIP-1 have defects in the erythroid and myeloid compartments similar to those in mice lacking Lyn or SHP-1, suggesting an intimate relationship between Lyn, SHP-1, and SHIP-1 in regulating hematopoiesis.

Myeloid Leukemia Factor 1 inhibits erythropoietin-induced differentiation, cell cycle exit and p27Kip1 accumulation

Myeloid leukemia factor 1 (MLF1) is a novel oncoprotein involved in translocations associated with acute myeloid leukemia (AML), especially erythroleukemias. In this study, we demonstrate that ectopic expression of Mlf1 prevented J2E erythroleukemic cells from undergoing biological and morphological maturation in response to erythropoietin (Epo). We show that Mlf1 inhibited Epo-induced cell cycle exit and suppressed a rise in the cell cycle inhibitor p27(Kip1). Unlike differentiating J2E cells, Mlf1-expressing cells did not downregulate Cul1 and Skp2, components of the ubiquitin E3 ligase complex SCF(Skp2) involved in the proteasomal degradation of p27(Kip1). In contrast, Mlf1 did not interfere with increases in p27(Kip1) and terminal differentiation initiated by thyroid hormone withdrawal from erythroid cells, or cytokine-stimulated maturation of myeloid cells. These data demonstrate that Mlf1 interferes with an Epo-responsive pathway involving p27(Kip1) accumulation, which inhibits cell cycle arrest essential for erythroid terminal differentiation.

Familial and congenital polycythemias: a diagnostic approach

The rare absolute polycythemias with an innate and hereditary character can be grouped together under the heading "familial and congenital polycythemias" (FCPs). Primary forms, due to an intrinsic defect in the erythroid progenitor cells, and secondary forms, resulting from extrinsic factors such as an elevated erythropoietin level, have both been reported. Despite the widely divergent characteristics of the different FCPs, the range of possible diagnoses is much more restricted and the distribution of disorders markedly different compared with polycythemias in general. Therefore, in FCP, one can argue against following the algorithm of the Polycythemia Vera Study Group for the evaluation of an elevated hematocrit level, following instead a more specific algorithm. In this article the authors describe a child with primary FCP, review the different FCPs, and propose an adapted work-up scheme.

Tyrosine kinase receptor RON functions downstream of the erythropoietin receptor to induce expansion of erythroid progenitors

Erythropoietin (EPO) is required for cell survival during differentiation and for progenitor expansion during stress erythropoiesis. Although signaling pathways may couple directly to docking sites on the EPO receptor (EpoR), additional docking molecules expand the signaling platform of the receptor. We studied the roles of the docking molecules Grb2-associated binder-1 (Gab1) and Gab2 in EPO-induced signal transduction and erythropoiesis. Inhibitors of phosphatidylinositide 3-kinase and Src kinases suppressed EPO-dependent phosphorylation of Gab2. In contrast, Gab1 activation depends on recruitment and phosphorylation by the tyrosine kinase receptor RON, with which it is constitutively associated. RON activation induces the phosphorylation of Gab1, mitogen-activated protein kinase (MAPK), and protein kinase B (PKB) but not of signal transducer and activator of transcription 5 (Stat5). RON activation was sufficient to replace EPO in progenitor expansion but not in differentiation. In conclusion, we elucidated a novel mechanism specifically involved in the expansion of erythroblasts involving RON as a downstream target of the EpoR.

Identification and activation of Src family kinases in primary megakaryocytes

OBJECTIVES

We have recently shown that the Src family of tyrosine kinases (SFKs) are activated by TPO stimulation in both primary megakaryocytic progenitors and a hematopoietic cells line (BaF3) expressing the TPO receptor (Mpl). In this study, we examine which of the eight Src family members are expressed in primary megakaryocytes (MKs) and determine which of these become activated in response to TPO.

MATERIALS AND METHODS

High-density oligonucleotide microarrays were used to compare the gene expression profiles of Src kinases from undifferentiated hematopoietic progenitors (CD34+/CD38(lo)) and after in vitro megakaryocytic differentiation. Western blot analysis of lysates from purified, mature murine MKs identified which of SFKs are present. Finally, in vitro kinase assays determined which of the SFKs in primary MKs are activated by TPO stimulation.

RESULTS

Array profiles demonstrate that Fyn, Lyn, Fgr, Hck, Src, and Yes are all expressed in cultured human MKs (Fyn, Lyn>Src, Yes, Fgr, Hck). Similarly, Western blots of murine MKs identified the same six SFKs (Fyn, Fgr, Hck, Lyn, Src, and Yes). Of these, only Fyn and Lyn demonstrate increased kinase activity after TPO stimulation. Interestingly, gene expression analysis indicates that, among the SFKs, Fyn expression is uniquely upregulated during MK development.

CONCLUSION

These results provide the first direct evidence that two Src kinases are activated in primary MKs, Fyn and Lyn. The fact that only Fyn expression is significantly upregulated during MK differentiation suggests variable gene regulation. Specificity of the TPO signaling cascade is demonstrated by the selective activation of Fyn and Lyn.