Proliferation Signaling and Activation of Shc, p21Ras, and Myc Via Tyrosine 764 of Human Granulocyte Colony-Stimulating Factor Receptor

Alternatively spliced CSF3R isoforms in SRSF2 P95H mutated myeloid neoplasms

Alternatively spliced colony stimulating factor 3 receptor (CSF3R) isoforms Class III and Class IV are observed in myelodysplastic syndromes (MDS), but their roles in disease remain unclear. We report that the MDS-associated splicing factor SRSF2 affects the expression of Class III and Class IV isoforms and perturbs granulopoiesis. Add-back of the Class IV isoform in Csf3r-null mouse progenitor cells increased granulocyte progenitors with impaired neutrophil differentiation, while add-back of the Class III produced dysmorphic neutrophils in fewer numbers. These CSF3R isoforms were elevated in patients with myeloid neoplasms harboring SRSF2 mutations. Using in vitro splicing assays, we confirmed increased Class III and Class IV transcripts when SRSF2 P95 mutations were co-expressed with the CSF3R minigene in K562 cells. Since SRSF2 regulates splicing partly by recognizing exonic splicing enhancer (ESE) sequences on pre-mRNA, deletion of either ESE motifs within CSF3R exon 17 decreased Class IV transcript levels without affecting Class III. CD34+ cells expressing SRSF2 P95H showed impaired neutrophil differentiation in response to G-CSF and was accompanied by increased levels of Class IV. Our findings suggest that SRSF2 P95H promotes Class IV splicing by binding to key ESE sequences in CSF3R exon 17, and that SRSF2, when mutated, contributes to dysgranulopoiesis.

G-CSF, the guardian of granulopoiesis

A considerable amount of continuous proliferation and differentiation is required to produce daily a billion new neutrophils in an adult human. Of the few cytokines and factors known to control neutrophil production, G-CSF is the guardian of granulopoiesis. G-CSF/CSF3R signaling involves the recruitment of non-receptor protein tyrosine kinases and their dependent signaling pathways of serine/threonine kinases, tyrosine phosphatases, and lipid second messengers. These pathways converge to activate the families of STAT and C/EBP transcription factors. CSF3R mutations are associated with human disorders of neutrophil production, including severe congenital neutropenia, neutrophilia, and myeloid malignancies. More than three decades after their identification, cloning, and characterization of G-CSF and G-CSF receptor, fundamental questions remain about their physiology.

G-CSF-dependent neutrophil differentiation requires downregulation of MAPK activities through the Gab2 signaling pathway

Granulocyte colony-stimulating factor (G-CSF) stimulation of myeloid cells induced tyrosine-phosphorylation of cellular proteins. One of the tyrosine-phosphorylated proteins was found to be a scaffold protein, Grb2-associated binding protein 2 (Gab2). Another member of Gab family protein, Gab3, was exogenously overexpressed in neutrophil progenitor cells to make the Gab3 protein to compete with the endogenous Gab2 for the G-CSF-dependent signaling. In Gab3-overexpressed cells, the level of tyrosine phosphorylation of endogenous Gab2 by G-CSF stimulation was markedly downregulated, while the phosphorylation of Gab3 was significantly enhanced. The Gab3-overexpressed cells continuously proliferated in the medium containing G-CSF and lost the ability to differentiate to the mature neutrophil, characterized by the lobulated nucleus. The G-CSF stimulation-dependent tyrosine phosphorylation of Gab3, the association of SHP2 to Gab3 and the following mitogen-activated protein kinase (MAPK) activation were prolonged in the Gab3-overexpressed cells, compared to the parental cells, where the binding of SHP2 to Gab2 protein and thereby the activation of MAPK were not sustained after G-CSF stimulation. Inhibition of MAPK by pharmaceutical inhibitor restored the Gab3-overexpressed cells to the ability to differentiate to mature neutrophil. Therefore, G-CSF-dependent Gab2 phosphorylation and following its downregulation led the short-term MAPK activation. The downregulation of MAPK after transient Gab2 phosphorylation was necessary for the consequent neutrophil differentiation induced by G-CSF stimulation.

MiR-221-regulated KIT level by wild type or leukemia mutant RUNX1: a determinant of single myeloblast fate decisions that - collectively - drives or hinders granulopoiesis

RUNX1, a master transcription factor of hematopoiesis, was shown to orchestrate both cell proliferation and differentiation during granulopoiesis by regulating microRNAs (miRs). In this study, taking advantage of the miR-ON reporter system, we monitored first, how the granulocyte colony stimulation factor (GCSF) temporally modulates the concomitant level variation of miR-221 and one of its prototypic targets, the stem cell factor receptor KIT, in single 32D^miR-ON-221 myeloblasts expressing wild type RUNX1. Second, with the same reporter system we assessed how these temporal dynamics are affected by the t(8;21)(q22;q22) acute myelogenous leukemia mutant RUNX1-MTG8 (RM8) in single 32D-RM8^miR-ON-221 myeloblasts. Depending on either wild type, or mutant, RUNX1 transcriptional regulation, the cell-context specific miR-221-regulated KIT level translates into differential single cell fate decisions. Collectively, single cell fate choices translate into either initial expansion of undifferentiated myeloblasts followed by terminal granulocyte differentiation, as it happens in normal granulopoiesis, or aggressive growth of undifferentiated myeloblasts, as it happens in RUNX1-MTG8-positive acute myelogenous leukemia. Increasing knowledge of biological changes, due to altered miRNA dynamics, is expected to have relevant translational implications for leukemia detection and treatment.

Understanding chronic neutropenia: life is short

The pathophysiological mechanisms underlying chronic neutropenia are extensive, varying from haematopoietic stem cell disorders resulting in defective neutrophil production, to accelerated apoptosis of neutrophil progenitors or circulating mature neutrophils. While the knowledge concerning genetic defects associated with congenital neutropenia or bone marrow failure is increasing rapidly, the functional role and consequences of these genetic alterations is often not well understood. In addition, there is a large group of diseases, including primary immunodeficiencies and metabolic diseases, in which chronic neutropenia is one of the symptoms, while there is no clear bone marrow pathology or haematopoietic stem cell dysfunction. Altogether, these disease entities illustrate the complexity of normal neutrophil development, the functional role of the (bone marrow) microenvironment and the increased propensity to undergo apoptosis, which is typical for neutrophils. The large variety of disorders associated with chronic neutropenia makes classification almost impossible and possibly not desirable, based on the clinical phenotypes. However, a better understanding of the regulation of normal myeloid differentiation and neutrophil development is of great importance in the diagnostic evaluation of unexplained chronic neutropenia. In this review we propose insights in the pathophysiology of chronic neutropenia in the context of the functional role of key players during normal neutrophil development, neutrophil release and neutrophil survival.

miR-17 deregulates a core RUNX1-miRNA mechanism of CBF acute myeloid leukemia

Background

Core Binding Factor acute myeloid leukemia (CBF-AML) with t(8;21) RUNX1-MTG8 or inv(16) CBFB-MYH11 fusion proteins often show upregulation of wild type or mutated KIT receptor. However, also non-CBF-AML frequently displays upregulated KIT expression. In the first part of this study we show that KIT expression can be also upregulated by miR-17, a regulator of RUNX1, the gene encoding a CBF subunit. Interestingly, both CBF leukemia fusion proteins and miR-17, which targets RUNX1-3′UTR, negatively affect a common core RUNX1-miRNA mechanism that forces myeloid cells into an undifferentiated, KIT-induced, proliferating state. In the second part of this study we took advantage of the conservation of the core RUNX1-miRNA mechanism in mouse and human, to mechanistically demonstrate in a mouse myeloid cell model that increased KIT-induced proliferation is per se a mechanism sufficient to delay myeloid differentiation.

Methods

Human (U937) or mouse (32D) myeloid clonal lines were used, respectively, to test: 1) the effect of RUNX1-MTG8 and CBFB-MYH11 fusion proteins, or upregulation of miR-17, on KIT-induced proliferation and myeloid differentiation, and 2) the effect of upregulation of KIT-induced proliferation per se on myeloid cell differentiation.

Results

In the first part of this study we found that stable miR-17 upregulation affects, like the CBF-AML fusion proteins (RUNX1-MTG8 or CBFB-MYH11), a core RUNX1-miRNA mechanism leading to KIT-induced proliferation of differentiation-arrested U937 myeloid cells. In the second part of the study we harnessed the conservation of this core mechanism in human and mouse to demonstrate that the extent of KIT upregulation in 32D mouse myeloid cells with wild type RUNX1 can per se delay G-CSF-induced differentiation. The integrated information gathered from the two myeloid cell models shows that RUNX1 regulates myeloid differentiation not only by direct transcriptional regulation of coding and non-coding myeloid differentiation functions (e.g. miR-223), but also by modulating KIT-induced proliferation via non-coding miRNAs (e.g. miR-221).

Conclusions

The novelty of this study is dual. On the one hand, miRNAs (e.g. miR-17) can mimic the effects of CBF-AML fusion proteins by affecting a core RUNX1-miRNA mechanism of KIT-induced proliferation of undifferentiated myeloid cells. On the other hand, the extent of KIT-induced proliferation itself can modulate myeloid differentiation of cells with wild type RUNX1 function.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-014-0283-z) contains supplementary material, which is available to authorized users.

Systems approach to phagocyte production and activation: neutrophils and monocytes

Granulocyte differentiation and immune response function is a dynamic process governed by a highly coordinated transcriptional program that regulates cellular fate and function, often in a context-dependent manner. Advances in high-throughput technologies and bioinformatics have allowed us to better understand complex biological processes at the genomic and proteomic levels. Components of the environmental milieu, along with the molecular mechanisms that drive the development, activation, and regulation of granulocytes, have since been elucidated. In this chapter, we present the intricate network in which these elements come together and influence one another. In particular, we describe the critical roles of transcription factors like PU.1, CCAAT/enhancer-binding protein (C/EBPα; alpha), C/EBPε (epsilon), and growth factor independent-1 (Gfi-1). We also review granulocyte colony-stimulating factor (G-CSF) receptor-induced signal transduction pathways, their influence on proliferation and differentiation, and the cooperativity of cytokines and chemokines in this process.

Granulocyte colony-stimulating factor receptor signaling: implications for G-CSF responses and leukemic progression in severe congenital neutropenia

Following activation by their cognate ligands, cytokine receptors undergo intracellular routing toward lysosomes, where they are degraded. This review focuses on the signaling function of the G-CSFR in relation to the dynamics of endosomal routing of the G-CSFR. Mechanisms involving receptor lysine ubiquitination and redox-controlled phosphatase activities are discussed. Specific attention is paid to the consequences of G-CSFR mutations, acquired in patients with severe congenital neutropenias who receive G-CSF therapy, particularly in the context of leukemic transformation, a major clinical complication of the disease.

G-CSF-activated STAT3 enhances production of the chemokine MIP-2 in bone marrow neutrophils

Neutrophil mobilization from the bone marrow is a critical aspect of the innate immune response, enabling a rapid deployment of phagocytes to infected or inflamed tissue. The cytokine G-CSF, which is induced rapidly during infection, elicits a swift and potent mobilizing response, yet its mechanisms of action remain poorly understood. Here, we studied the role of G-CSF and its principal signal transducer STAT3 in regulating expression of the neutrophil chemoattractant MIP-2. Our studies revealed Gr-1(hi) mature neutrophils as major sources of Cxcl2 (MIP-2) mRNA in bone marrow and G-CSF-responsive MIP-2 protein production. Induction of Cxcl2 was regulated directly by G-CSF-activated STAT3 via interaction at a STAT consensus element in the Cxcl2 promoter. G-CSF coordinately stimulated the association of STAT3, induction of the transcriptionally active H3K4me3 modification, and recruitment of RNA Pol II at the Cxcl2 proximal promoter, as well as the promoter region of Il8rb, encoding the MIP-2 receptor. These results suggest that the G-CSF-STAT3 pathway directly regulates transcriptional events that induce neutrophil mobilization.

Zoledronate and pamidronate depress neutrophil functions and survival in mice

BACKGROUND AND PURPOSE

Bisphosphonate-related osteonecrosis of the jaw (BRONJ) has been identified as a severe complication of patients previously treated with i.v. bisphosphonates. It has been noted that necrotic bone from BRONJ sites display signs of bacterial infection suggesting that an immune defect may play a role in the pathophysiology of BRONJ. Here, we have examined the effect of two potent bisphosphonates, zoledronate and pamidronate, on neutrophil function, differentiation and survival.

EXPERIMENTAL APPROACH

The effect of bisphosphonates on chemotaxis, NADPH oxidase activity and neutrophil survival were assessed in vitro using bone marrow-derived primary neutrophils or in vitro differentiated haematopoetic progenitors from mice. The same parameters and the number of circulating neutrophils were quantified in neutrophils isolated from mice treated in vivo with zoledronate. In vivo recruitment of neutrophils was assessed by sodium periodate-induced peritonitis.

KEY RESULTS

Zoledronate and pamidronate inhibited in vitro neutrophil chemotaxis and NADPH oxidase activity in a dose-dependent manner. In vivo recruitment of neutrophils was also suppressed. Zoledronate did not affect in vitro differentiation of neutrophils but shortened their life span in a granulocyte-colony stimulating factor-dependent manner. fMLP-induced activation of RhoA activity was decreased by zoledronate treatment.

CONCLUSIONS AND IMPLICATIONS

Our results show that bisphosphonate exposure leads to impaired neutrophil chemotaxis, neutrophil NADPH oxidase activity and reduced circulating neutrophil counts. This work suggests that bisphosphonates have the potential to depress the innate immune system for a prolonged time, possibly contributing to the pathogenesis of BRONJ.

The deubiquitinating enzyme DUB2A enhances CSF3 signalling by attenuating lysosomal routing of the CSF3 receptor

Ubiquitination of the CSF3R [CSF3 (colony-stimulating factor 3) receptor] occurs after activated CSF3Rs are internalized and reside in early endosomes. CSF3R ubiquitination is crucial for lysosomal routing and degradation. The E3 ligase SOCS3 (suppressor of cytokine signalling 3) has been shown to play a major role in this process. Deubiquitinating enzymes remove ubiquitin moieties from target proteins by proteolytic cleavage. Two of these enzymes, AMSH [associated molecule with the SH3 domain of STAM (signal transducing adaptor molecule)] and UBPY (ubiquitin isopeptidase Y), interact with the general endosomal sorting machinery. Whether deubiquitinating enzymes control CSF3R trafficking from early towards late endosomes is unknown. In the present study, we asked whether AMSH, UBPY or a murine family of deubiquitinating enzymes could fulfil such a role. This DUB family (deubiquitin enzyme family) comprises four members (DUB1, DUB1A, DUB2 and DUB2A), which were originally described as being haematopoietic-specific and cytokine-inducible, but their function in cytokine receptor routing and signalling has remained largely unknown. We show that DUB2A expression is induced by CSF3 in myeloid 32D cells and that DUB2 decreases ubiquitination and lysosomal degradation of the CSF3R, leading to prolonged signalling. These results support a model in which CSF3R ubiquitination is dynamically controlled at the early endosome by feedback mechanisms involving CSF3-induced E3 ligase (SOCS3) and deubiquitinase (DUB2A) activities.

Down-regulation of microRNAs 222/221 in acute myelogenous leukemia with deranged core-binding factor subunits

Core-binding factor leukemia (CBFL) is a subgroup of acute myeloid leukemia (AML) characterized by genetic mutations involving the subunits of the core-binding factor (CBF). The leukemogenesis model for CBFL posits that one, or more, gene mutations inducing increased cell proliferation and/or inhibition of apoptosis cooperate with CBF mutations for leukemia development. One of the most common mutations associated with CBF mutations involves the KIT receptor. A high expression of KIT is a hallmark of a high proportion of CBFL. Previous studies indicate that microRNA (MIR) 222/221 targets the 3' untranslated region of the KIT messenger RNA and our observation that AML1 can bind the MIR-222/221 promoter, we hypothesized that MIR-222/221 represents the link between CBF and KIT. Here, we show that MIR-222/221 expression is upregulated after myeloid differentiation of normal bone marrow AC133(+) stem progenitor cells. CBFL blasts with either t(8;21) or inv(16) CBF rearrangements with high expression levels of KIT (CD117) display a significantly lower level of MIR-222/221 expression than non-CBFL blasts. Consistently, we found that the t(8;21) AML1-MTG8 fusion protein binds the MIR-222/221 promoter and induces transcriptional repression of a MIR-222/221-LUC reporter. Because of the highly conserved sequence homology, we demonstrated concomitant MIR-222/221 down-regulation and KIT up-regulation in the 32D/WT1 mouse cell model carrying the AML1-MTG16 fusion protein. This study provides the first hint that CBFL-associated fusion proteins may lead to up-regulation of the KIT receptor by down-regulating MIR-222/221, thus explaining the concomitant occurrence of CBF genetic rearrangements and overexpression of wild type or mutant KIT in AML.

The transcription factor Gfi1 regulates G-CSF signaling and neutrophil development through the Ras activator RasGRP1

The transcription factor growth factor independence 1 (Gfi1) and the growth factor granulocyte colony-stimulating factor (G-CSF) are individually essential for neutrophil differentiation from myeloid progenitors. Here, we provide evidence that the functions of Gfi1 and G-CSF are linked in the regulation of granulopoiesis. We report that Gfi1 promotes the expression of Ras guanine nucleotide releasing protein 1 (RasGRP1), an exchange factor that activates Ras, and that RasGRP1 is required for G-CSF signaling through the Ras/mitogen-activated protein/extracellular signal-regulated kinase (MEK/Erk) pathway. Gfi1-null mice have reduced levels of RasGRP1 mRNA and protein in thymus, spleen, and bone marrow, and Gfi1 transduction in myeloid cells promotes RasGRP1 expression. When stimulated with G-CSF, Gfi1-null myeloid cells are selectively defective at activating Erk1/2, but not signal transducer and activator of transcription 1 (STAT1) or STAT3, and fail to differentiate into neutrophils. Expression of RasGRP1 in Gfi1-deficient cells rescues Erk1/2 activation by G-CSF and allows neutrophil maturation by G-CSF. These results uncover a previously unknown function of Gfi1 as a regulator of RasGRP1 and link Gfi1 transcriptional control to G-CSF signaling and regulation of granulopoiesis.

Site-specific ubiquitination determines lysosomal sorting and signal attenuation of the granulocyte colony-stimulating factor receptor

Ubiquitination of cytokine receptors controls intracellular receptor routing and signal duration, but the underlying molecular determinants are unclear. The suppressor of cytokine signaling protein SOCS3 drives lysosomal degradation of the granulocyte colony-stimulating factor receptor (G-CSFR), depending on SOCS3-mediated ubiquitination of a specific lysine located in a conserved juxtamembrane motif. Here, we show that, despite ubiquitination of other lysines, positioning of a lysine within the membrane-proximal region is indispensable for this process. Neither reallocation of the motif nor fusion of ubiquitin to the C-terminus of the G-CSFR could drive lysosomal routing. However, within this region, the lysine could be shifted 12 amino acids toward the C-terminus without losing its function, arguing against the existence of a linear sorting motif and demonstrating that positioning of the lysine relative to the SOCS3 docking site is flexible. G-CSFR ubiquitination peaked after endocytosis, was inhibited by methyl-beta-cyclodextrin as well as hyperosmotic sucrose and severely reduced in internalization-defective G-CSFR mutants, indicating that ubiquitination mainly occurs at endosomes. Apart from elucidating structural and spatio-temporal aspects of SOCS3-mediated ubiquitination, these findings have implications for the abnormal signaling function of G-CSFR mutants found in severe congenital neutropenia, a hematopoietic disorder with a high leukemia risk.

Neutropenia with impaired host defense against microbial infection in mice lacking androgen receptor

Neutrophils, the major phagocytes that form the first line of cell-mediated defense against microbial infection, are produced in the bone marrow and released into the circulation in response to granulocyte-colony stimulating factor (G-CSF). Here, we report that androgen receptor knockout (ARKO) mice are neutropenic and susceptible to acute bacterial infection, whereas castration only results in moderate neutrophil reduction in mice and humans. Androgen supplement can restore neutrophil counts via stabilizing AR in castrated mice, but not in ARKO and testicular feminization mutant (Tfm) mice. Our results show that deletion of the AR gene does not influence myeloid lineage commitment, but significantly reduces the proliferative activity of neutrophil precursors and retards neutrophil maturation. CXCR2-dependent migration is also decreased in ARKO neutrophils as compared with wild-type controls. G-CSF is unable to delay apoptosis in ARKO neutrophils, and ARKO mice show a poor granulopoietic response to exogenous G-CSF injection. In addition, AR can restore G-CSF–dependent granulocytic differentiation upon transduction into ARKO progenitors. We further found that AR augments G-CSF signaling by activating extracellular signal-regulated kinase 1/2 and also by sustaining Stat3 activity via diminishing the inhibitory binding of PIAS3 to Stat3. Collectively, our findings demonstrate an essential role for AR in granulopoiesis and host defense against microbial infection.

Zebrafish granulocyte colony-stimulating factor receptor signaling promotes myelopoiesis and myeloid cell migration

Granulocyte colony-stimulating factor receptor (GCSFR) signaling participates in the production of neutrophilic granulocytes during normal hematopoietic development, with a particularly important role during emergency hematopoiesis. This study describes the characterization of the zebrafish gcsf and gcsfr genes, which showed broad conservation and similar regulation to their mammalian counterparts. Morpholino-mediated knockdown of gcsfr and overexpression of gcsf revealed the presence of an anterior population of myeloid cells during primitive hematopoiesis that was dependent on GCSF/GCSFR for development and migration. This contrasted with a posterior domain that was largely independent of this pathway. Definitive myelopoiesis was also partially dependent on a functional GCSF/GCSFR pathway. Injection of bacterial lipopolysaccharide elicited significant induction of gcsf expression and emergency production of myeloid cells, which was abrogated by gcsfr knockdown. Collectively, these data demonstrate GCSF/GCSFR to be a conserved signaling system for facilitating the production of multiple myeloid cell lineages in both homeostatic and emergency conditions, as well as for early myeloid cell migration, establishing a useful experimental platform for further dissection of this pathway.

Pharmacologic rationale for early G-CSF prophylaxis in cancer patients and role of pharmacogenetics in treatment optimization

The use of recombinant human granulocyte colony stimulating factors (G-CSF) has become an integral part of supportive care during cytotoxic chemotherapy. Current guidelines recommend the use of G-CSF in patients with substantial risk of febrile neutropenia. However, little consensus exists about optimal timing and tailoring of this therapy. Based on the known effects of chemotherapy and G-CSF on bone marrow compartments, we propose a model that supports the prophylactic rather than therapeutic use of G-CSF therapy. In addition, several genetic alterations in G-CSF signalling pathway have been described. These genetic variants may predict the risk of febrile neutropenia and response to G-CSF. Thus, future pharmacogenetic/omics studies in this field are warranted. Through the identification of patients at risk and the knowledge of biological basis for optimal timing, hopefully we should soon be able to improve the application of the existing guidelines for G-CSF therapy and patient's prognosis.

Granulocyte colony-stimulating factor: molecular mechanisms of action during steady state and 'emergency' hematopoiesis

Neutrophils are phagocytes whose principal function is to maintain anti-bacterial immunity. Neutrophils ingest and kill invading bacteria, releasing cytotoxic, chemotactic and inflammatory mediators at sites of infection. This serves to control the immediate host immune response and attract other cells, such as macrophages and dendritic cells, which are important for establishing long-term adaptive immunity. Neutrophils thus contribute to both the initiation and the maintenance of inflammation at sites of infection. Aberrant neutrophil activity is deleterious; suppressed responses can cause extreme susceptibility to infection while overactivation can lead to excessive inflammation and tissue damage. This review will focus on neutrophil regulation by granulocyte colony-stimulating factor (G-CSF), the principal cytokine controlling neutrophil development and function. The review will emphasize the molecular aspects of G-CSF-driven granulopoiesis in steady state (healthy) conditions and during demand-driven or 'emergency' conditions elicited by infection or clinical administration of G-CSF. Understanding the molecular control of granulopoiesis will aid in the development of new approaches designed to treat disorders of neutrophil production and function.

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.

Suppressor of cytokine signaling 3 controls lysosomal routing of G-CSF receptor

The hematopoietic system provides an attractive model for studying growth factor-controlled expansion and differentiation of cells in relation to receptor routing and its consequences for signal transduction. Suppressor of cytokine signaling (SOCS) proteins regulate receptor signaling partly via their ubiquitin ligase (E3)-recruiting SOCS box domain. Whether SOCS proteins affect signaling through modulating intracellular trafficking of receptors is unknown. Here, we show that a juxtamembrane lysine residue (K632) of the granulocyte colony-stimulating factor receptor (G-CSFR) plays a key role in receptor routing and demonstrate that the effects of SOCS3 on G-CSF signaling to a major extent depend on this lysine. Mutation of K632 causes accumulation of G-CSFR in early endosomes and leads to sustained activation of signal transducer and activator of transcription 5 and ERK, but not protein kinase B. Myeloid progenitors expressing G-CSFR mutants lacking K632 show a perturbed proliferation/differentiation balance in response to G-CSF. This is the first demonstration of SOCS-mediated ubiquitination and routing of a cytokine receptor and its impact on maintaining an appropriate signaling output.

Multiple pathways contribute to the hyperproliferative responses from truncated granulocyte colony-stimulating factor receptors

Mutations in the granulocyte colony-stimulating factor receptor (G-CSF-R) gene leading to a truncated protein have been identified in a cohort of neutropenia patients highly predisposed to acute myeloid leukemia. Such mutations act in a dominant manner resulting in hyperproliferation but impaired differentiation in response to G-CSF. This is due, at least in part, to defective internalization and loss of binding sites for several negative regulators, leading to sustained receptor activation. However, those signaling pathways responsible for mediating the hyperproliferative function have remained unclear. In this study, analysis of an additional G-CSF-R mutant confirmed the importance of residues downstream of Box 2 as important contributors to the sustained proliferation. However, maximal proliferation correlated with the ability to robustly activate signal transducer and activator of transcription (STAT) 5 in a sustained manner, whereas co-expression of dominant-negative STAT5, but not dominant-negative STAT3, was able to inhibit G-CSF-stimulated proliferation from a truncated receptor. Furthermore, a Janus kinase (JAK) inhibitor also strongly reduced the proliferative response, whereas inhibitors of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) or phosphatidylinositol (PI) 3-kinase reduced proliferation to a lesser degree. These data suggest that sustained JAK2/STAT5 activation is a major contributor to the hyperproliferative function of truncated G-CSF receptors, with pathways involving MEK and PI 3-kinase playing a reduced role.

Essential role for cyclin D3 in granulocyte colony-stimulating factor-driven expansion of neutrophil granulocytes

The proliferation of neutrophil granulocyte lineage is driven largely by granulocyte colony-stimulating factor (G-CSF) acting via the G-CSF receptors. In this study, we show that mice lacking cyclin D3, a component of the core cell cycle machinery, are refractory to stimulation by the G-CSF. Consequently, cyclin D3-null mice display deficient maturation of granulocytes in the bone marrow and have reduced levels of neutrophil granulocytes in their peripheral blood. The mutant mice are unable to mount a normal response to bacterial challenge and succumb to microbial infections. In contrast, the expansion of hematopoietic stem cells and lineage-committed myeloid progenitors proceeds relatively normally in mice lacking cyclin D3, revealing that the requirement for cyclin D3 function operates at later stages of neutrophil development. Importantly, we verified that this requirement is specific to cyclin D3, as mice lacking other G(1) cyclins (D1, D2, E1, or E2) display normal granulocyte counts. Our analyses revealed that in the bone marrow cells of wild-type mice, activation of the G-CSF receptor leads to upregulation of cyclin D3. Collectively, these results demonstrate that cyclin D3 is an essential cell cycle recipient of G-CSF signaling, and they provide a molecular link of how G-CSF-dependent signaling triggers cell proliferation.

Ral is both necessary and sufficient for the inhibition of myeloid differentiation mediated by Ras

Hyperactivation of Ras is one of the most common abnormalities in acute myeloid leukemia. In experimental models, Ras inhibits myeloid differentiation, which is characteristic of leukemia; however, the mechanism through which it disrupts hematopoiesis is poorly understood. In multipotent FDCP-mix cells, Ras inhibits terminal neutrophil differentiation, thereby indefinitely extending their proliferative potential. Ras also strongly promotes the sensitivity of these cells to granulocyte-macrophage colony-stimulating factor (GM-CSF). Using this model, we have dissected the signaling elements downstream of Ras to determine their relative contribution to the dysregulation of hematopoiesis. Cells expressing Ras mutants selectively activating Raf (Ras*T35S) or phosphatidylinositol 3-kinase (Ras*Y40C) did not significantly affect differentiation or proliferative capacity, whereas Ras*E37G (which selectively activates RalGEFs) perpetuated proliferation and blocked neutrophil development in a manner similar to that of Ras. Correspondingly, expression of constitutively active versions of these effectors confirmed the overriding importance of Ral guanine nucleotide exchange factors. Cells expressing Ras demonstrated hyperactivation of Ral, which itself was able to exactly mimic the phenotype of Ras, including hypersensitivity to GM-CSF. Conversely, dominant negative Ral promoted spontaneous neutrophil development. Ral, in turn, appears to influence differentiation through multiple effectors. These data show, for the first time, the importance of Ral in regulating differentiation and self-renewal in hematopoietic cells.

Myeloid maturation block by AML1-MTG16 is associated with Csf1r epigenetic downregulation

De novo epigenetic changes at histone and DNA level that affect gene transcription in cancer may be less random than we originally thought. Leukemia fusion proteins associated with specific chromosome translocations could mechanistically determine the epigenetic fate of specific target genes critical for normal hematopoiesis. This seems to be the case with AML1-MTG16, a fusion protein resulting from the t(16;21) translocation, a hallmark of therapy-related leukemia and myelodysplastic syndrome. Here we show that AML1-MTG16 blocks both myeloid differentiation and proliferation in the 32D/WT1-mouse myeloid cell line. These biological effects can be traced to the AML1 and MTG16 moieties of the fusion protein, respectively. Further, we show that AML1-MTG16 can induce epigenetic repressive changes at the histone and DNA level of the AML1 target gene Csf1r (c-fms), encoding the macrophage colony stimulating factor receptor. We observed that, concomitant with Csf1r downregulation, 32D/WT1 cells lost the ability to undergo myeloid differentiation in response to the granulocyte macrophage colony-stimulating factor (GM-CSF). Thus, there seems to be an association between AML1-MTG16-induced myeloid maturation block and epigenetic changes of a myeloid master gene.

Tyrosine 729 of the G-CSF receptor controls the duration of receptor signaling: involvement of SOCS3 and SOCS1

Mutations in the granulocyte-colony stimulating factor receptor (G-CSF-R) gene resulting in carboxy terminal truncation have been associated with acute myeloid leukemia (AML). The truncated G-CSF-R from AML patients mediate enhanced and prolonged activation of signal transducer and activator of transcription 5 (Stat5). It has been shown that Src homology-2 (SH2)-containing tyrosine phosphatase-1 attenuates the intensity of G-CSF-induced Stat5 activation through interacting with the carboxy terminus of the G-CSF-R. Using a series of tyrosine-to-phenylalanine substitution mutants, we show here that tyrosine (Tyr) 729, located in the carboxy terminus of the G-CSF-R, controls the duration of G-CSF-stimulated activation of Stat5, Akt, and extracellular signal-regulated kinase 1/2. It is interesting that activation of these signaling molecules by G-CSF was prolonged by pretreating cells with actinomycin D or cyclohexamide, suggesting that de novo protein synthesis is required for appropriate termination of G-CSF-R signaling. The transcripts for suppressor of cytokine signaling 3 (SOCS3) and SOCS1 were up-regulated rapidly upon G-CSF stimulation. Expression of SOCS3 or SOCS1, but not SOCS2 and cytokine-inducible SH2 domain-containing protein, completely suppressed G-CSF-induced Stat5 activation but had only a weak effect on Stat5 activation mediated by the receptor mutant lacking Tyr 729. SOCS1 and SOCS3 also inhibited G-CSF-dependent cell proliferation, but the inhibitory effect of the two SOCS proteins on cell proliferation was diminished when Tyr 729 of the G-CSF-R was mutated. These data indicate that Tyr 729 of the G-CSF-R is required for SOCS1- and SOCS3-mediated negative regulation of G-CSF-R signaling and that the duration and intensity of G-CSF-induced Stat5 activation are regulated by two distinct mechanisms.

PRAM-1 is required for optimal integrin-dependent neutrophil function

PML-retinoic acid receptor alpha (RARalpha) regulated adaptor molecule 1 (PRAM-1) is an intracellular adaptor molecule that is upregulated during the induced granulocytic differentiation of promyelocytic leukemic cells and during normal human myelopoiesis. This report describes the generation of PRAM-1-deficient mice and an analysis of the function of this adaptor in neutrophil differentiation and mature neutrophil function. We demonstrate here that neutrophil differentiation is not impaired in PRAM-1-deficient mice and that PRAM-1-deficient neutrophils function normally following engagement of Fcgamma receptors. In contrast, mature PRAM-1-null neutrophils exhibit significant defects in adhesion-dependent reactive oxygen intermediate production and degranulation. Surprisingly, other integrin-dependent responses, such as cell spreading and activation of several signaling pathways, are normal. Together, these findings demonstrate the uncoupling of key integrin-dependent responses in the absence of PRAM-1 and show this adaptor to be critical for select integrin functions in neutrophils.

G-CSF receptor truncations found in SCN/AML relieve SOCS3-controlled inhibition of STAT5 but leave suppression of STAT3 intact

Truncated granulocyte colony-stimulating factor receptors (G-CSF-Rs) are implicated in severe congenital neutropenia (SCN) and the consecutive development of acute myeloid leukemia (AML). Mice expressing G-CSF-R truncation mutants (gcsfr-d715) show defective receptor internalization, an increased signal transducer and activator of transcription 5 (STAT5)/STAT3 activation ratio, and hyperproliferative responses to G-CSF treatment. We determined whether a lack of negative feedback by suppressor of cytokine signaling (SOCS) proteins contributes to the signaling abnormalities of G-CSF-R-d715. Expression of SOCS3 transcripts in bone marrow cells from G-CSF-treated gcsfr-d715 mice was approximately 60% lower than in wild-type (WT) littermates. SOCS3 efficiently suppressed STAT3 and STAT5 activation by WT G-CSF-R in luciferase reporter assays. In contrast, while SOCS3 still inhibited STAT3 activation by G-CSF-R-d715, STAT5 activation was no longer affected. This was due mainly to loss of the SOCS3 recruitment site Tyr729, with an additional contribution of the internalization defects of G-CSF-R-d715. Because Tyr729 is also a docking site for the Src homology 2-containing protein tyrosine phosphatase-2 (SHP-2), which binds to and inactivates STAT5, we suggest a model in which reduced SOCS3 expression, combined with the loss of recruitment of both SOCS3 and SHP-2 to the activated receptor complex, determine the increased STAT5/STAT3 activation ratio and the resulting signaling abnormalities projected by truncated G-CSF-R mutants.

Relation between CXCR-4 expression, Flt3 mutations, and unfavorable prognosis of adult acute myeloid leukemia

Recently it was shown that, analogous to normal hematopoietic cells, the level of CXC chemokine receptor 4 (CXCR-4) expression on acute myeloid leukemia (AML) cells correlates with stromal cell derived factor-1 alpha (SDF-1)-induced chemotaxis. As we speculated that an anomalous organ distribution of AML cells could affect cell survival and thus result in an altered fraction surviving chemotherapy, we examined a possible correlation between patient prognosis and CXCR-4 expression in AML patients. We found that patients with a high CXCR-4 expression in the CD34(+) subset had a significantly reduced survival and a higher probability of relapse, resulting in a median relapse-free survival (RFS) of only 8.3 months. CXCR-4 expression was significantly higher in fetal liver tyrosine kinase-3 (Flt3)/internal tandem duplication (ITD) AML than in Flt3/wild-type (wt) AML. Covariate analysis indicated that the prognostic significance of Flt3/ITDs with respect to RFS was no more apparent when analyzed in conjunction with the expression of CXCR-4 in the CD34(+) subset, suggesting that the poor prognosis of Flt3/ITD AML might be subordinate to the increased CXCR-4 expression. Using a granulocyte colony-stimulating factor receptor (G-CSF-R)-expressing 32D cell line, we observed that SDF-1/CXCR-4 interaction is required for the survival of myeloid differentiating cells, and it also induces a block in G-CSF-induced myeloid differentiation. These data suggest that the SDF-1/CXCR-4 axis may influence therapy responsiveness and defines unfavorable prognosis in AML.

The peripheral cannabinoid receptor Cb2, frequently expressed on AML blasts, either induces a neutrophilic differentiation block or confers abnormal migration properties in a ligand-dependent manner

Cb2, the gene encoding the peripheral cannabinoid receptor, is located in a common virus integration site and is overex-pressed in retrovirally induced murine myeloid leukemias. Here we show that this G protein-coupled receptor (GPCR) is also aberrantly expressed in a high percentage of human acute myeloid leukemias. We investigated the mechanism of transformation by Cb2 and demonstrate that aberrant expression of this receptor on hematopoietic precursor cells results in distinct effects depending on the ligand used. Cb2-expressing myeloid precursors migrate upon stimulation by the endocannabinoid 2-arachidonoylglycerol and are blocked in neutrophilic differentiation upon exposure to another ligand, CP55940. Both effects depend on the activation of G(alphai) proteins and require the mitogen-induced extracellular kinase/extracellular signal-regulated kinase (MEK/ERK) pathway. Down-regulation of cyclic adenosine monophosphate (cAMP) levels upon G(alphai) activation is important for migration induction but is irrelevant for the maturation arrest. Moreover, the highly conserved G protein-interacting DRY motif, present in the second intracellular loop of GPCRs, is critical for migration but unimportant for the differentiation block. This suggests that the Cb2-mediated differentiation block requires interaction of G(alphai) proteins with other currently unknown motifs. This indicates a unique mechanism by which a transforming GPCR, in a ligand-dependent manner, causes 2 distinct oncogenic effects: altered migration and block of neutrophilic development.

Granulocyte colony-stimulating factor and its receptor in normal hematopoietic cell development and myeloid disease

Hematopoiesis, the process of blood cell formation, is orchestrated by cytokines and growth factors that stimulate the expansion of different progenitor cell subsets and regulate their survival and differentiation into mature blood cells. Granulocyte colony-stimulating factor (G-CSF) is the major hematopoietic growth factor involved in the control of neutrophil development. G-CSF is now applied on a routine basis in the clinic for treatment of congenital and acquired neutropenias. G-CSF activates a receptor of the hematopoietin receptor superfamily, the G-CSF receptor (G-CSF-R), which subsequently triggers multiple signaling mechanisms. Here we review how these mechanisms contribute to the specific responses of hematopoietic cells to G-CSF and how perturbations in the function of the G-CSF-R are implicated in various types of myeloid disease.

Contribution of the Membrane-distal Tyrosine in Intracellular Signaling by the Granulocyte Colony-stimulating Factor Receptor

We have evaluated the contribution of intracellular tyrosine residues of the granulocyte colony-stimulating factor receptor (GCSF-R) to its signaling and cellular outcomes. We began with stable BaF3 cell lines overexpressing wild-type or mutant GCSF-Rs. When all four intracellular tyrosines of the GCSF-R were replaced with phenylalanine (FFFF GCSF-R), cell proliferation and survival were compromised. Replacement of only the membrane-distal tyrosine (YYYF GCSF-R) also showed reduced survival following a GCSF withdrawal/replacement protocol, suggesting a role for this tyrosine. Proliferation by FFFY GCSF-R cells was attenuated by approximately 70%. In evaluating the biochemical steps involved in signaling, we then showed that the membrane-distal tyrosine was necessary and sufficient for c-Jun N-terminal kinase (JNK) activation. With the use of a cell-permeable JNK-inhibitory peptide, JNK was implicated in the proliferation of the FFFY GCSF-R mutant. To further define the events linking the membrane-distal tyrosine and JNK activation, the Src homology 2 domains of Shc, Grb2, and 3BP2 were shown to bind the full-length GCSF-R and a phosphopeptide encompassing the membrane-distal tyrosine. When binding to variant phosphopeptides based on this membrane-distal tyrosine was tested, altering the amino acids immediately following the phosphotyrosine could selectively abolish the interaction with Shc or Grb2, or the binding to both Grb2 and 3BP2. When these changes were introduced into the full-length GCSF-R and new cell lines created, only the mutant that did not interact with Grb2 and 3BP2 did not activate JNK. Our results suggest that direct binding of Shc by the GCSF-R is not essential for JNK activation.

SLP-76 regulates Fcgamma receptor and integrin signaling in neutrophils

While the contribution of intracellular adaptor proteins to lymphocyte activation has been well studied, the function of these molecules in innate immune effector cells such as neutrophils has not been extensively addressed. Here we demonstrate a critical role for the adaptor molecule SH2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76) in FcgammaR and integrin signaling. Stimulation of these receptors induces tyrosine phosphorylation and cytoplasmic relocalization of SLP-76 in freshly isolated murine neutrophils. Neutrophils lacking SLP-76 demonstrate decreased FcgammaR-induced calcium flux and reactive oxygen intermediate (ROI) production in response to immune complex stimulation. More dramatically, SLP-76-/- neutrophils fail to produce ROI, spread, or activate critical downstream regulators in response to integrin ligation. These results provide genetic evidence for a critical role of SLP-76 in the regulation of neutrophil function.

The role of c-Myc on granulocyte colony-stimulating factor-dependent neutrophilic proliferation and differentiation of HL-60 cells

We have previously suggested that phosphatidylinositol 3-kinase (PI3K)/p70 S6 kinase (p70 S6K) plays an important role in the regulation of neutrophilic differentiation of HL-60 cells on the basis of analysis of transferrin receptor (Trf-R)-positive (Trf-R(+)) and -negative (Trf-R(-)) cells that appear after treatment with dimethyl sulfoxide (DMSO). In the present study, we analyzed the downstream events of p70 S6K in differentiation and proliferation of both cell types, with a particular focus on c-Myc. Similar to p70 S6K, we found that the expression of c-Myc in Trf-R(+) cells is also higher than that in Trf-R(-) cells. Wortmannin, a specific inhibitor of PI3K, partially inhibited G-CSF-induced p70 S6K activity, c-Myc expression, and G-CSF-dependent proliferation, whereas rapamycin, an inhibitor of p70 S6K, completely inhibited p70 S6K activity, c-Myc expression, and G-CSF-dependent proliferation, indicating that the extent of c-Myc inhibition by these inhibitors correlates with a reduction in proliferation, and that c-Myc is downstream from PI3K/p70 S6K. We also determined phosphorylation of the 4E-binding protein 1 (4E-BP1), which is regulated downstream of the mammalian target of rapamycin. The addition of G-CSF failed to enhance the phosphorylation state of 4E-BP1 of HL-60 cells 2 days after DMSO differentiation. An antisense oligonucleotide for c-myc inhibited both G-CSF-dependent enhancement of c-Myc expression and proliferation in Trf-R(+) cells, but did not enhance the differentiation in terms of O(2)(-)-generating ability or fMLP-R expression. In contrast, antisense oligonucleotide for c-myc promoted fMLP-R on non-treated HL-60 cells. We therefore conclude that the PI3K/p70 S6K/c-Myc cascade plays an important role in neutrophilic proliferation in HL-60 cells. Unlike that of rapamycin, however, the antisense oligonucleotide for c-myc could not promote differentiation of Trf-R(+) cells cultured with G-CSF, indicating that another target downstream of p70 S6K may control the differentiation of HL-60 cells in terms of the signal transduction of G-CSF.

The carboxy-terminal region of the granulocyte colony-stimulating factor receptor transduces a phagocytic signal

Granulocyte colony-stimulating factor (G-CSF) induces proliferation, maturation, and functional activities of myeloid progenitors and mature neutrophils through a specific receptor, the G-CSF-R. Different signals are mediated by distinct regions of the cytoplasmic domain of G-CSF-R, but the precise role of each region has not yet been fully clarified. We evaluated the involvement of Syk kinase, essential in mediating phagocytic signals by Fcgamma receptors, in G-CSF-induced phagocytosis, using murine myeloid 32D cells transfected with wild-type (WT) human G-CSF-R (hG-CSF-R) or with a G-CSF-R mutant truncated at cytoplasmic amino acid 715. The G-CSF-R mutant lacks the immunoreceptor tyrosine-based activation motif (ITAM), putative binding site for Syk. Following treatment of WT hG-CSF-R transfectants with IgG-coated particles, there was a significant increase in phagocytosis in G-CSF-stimulated cells, in which Syk tyrosine phosphorylation occurred, paralleled by enhancement of its tyrosine kinase activity. In the mutant transfectants, no significant increase in phagocytosis or Syk tyrosine phosphorylation occurred after stimulation with G-CSF. We also demonstrated that tyrosine phosphorylation of the Src kinases Hck and Lyn occurs following G-CSF stimulation of cells expressing WT G-CSF-R, but that Hck is not phosphorylated in mutant G-CSF-R transfectants. The increase in phagocytosis following G-CSF stimulation cannot be attributed to a rapid de novo increase in expression of Fcgamma receptors. G-CSF induced expression of Fcgamma receptors only after prolonged stimulation. Our data provide evidence that the carboxy-terminal region of G-CSF-R plays a role in the phagocytosis of IgG-coated particles and that Syk and Hck kinase tyrosine phosphorylation is involved.

Signaling mechanisms coupled to tyrosines in the granulocyte colony-stimulating factor receptor orchestrate G-CSF-induced expansion of myeloid progenitor cells

Granulocyte colony-stimulating factor (G-CSF) is the major regulator of neutrophil production. Studies in cell lines have established that conserved tyrosines Tyr704, Tyr729, Tyr744, Tyr764 within the cytoplasmic domain of G-CSF receptor (G-CSF-R) contribute significantly to G-CSF-induced proliferation, differentiation, and cell survival. However, it is unclear whether these tyrosines are equally important under more physiologic conditions. Here, we investigated how individual G-CSF-R tyrosines affect G-CSF responses of primary myeloid progenitors. We generated G-CSF-R-deficient mice and transduced their bone marrow cells with tyrosine "null" mutant (m0), single tyrosine "add-back" mutants, or wild-type (WT) receptors. G-CSF-induced responses were determined in primary colony assays, serial replatings, and suspension cultures. We show that removal of all tyrosines had no major influence on primary colony growth. However, adding back Tyr764 strongly enhanced proliferative responses, which was reverted by inhibition of ERK activity. Tyr729, which we found to be associated with the suppressor of cytokine signaling, SOCS3, had a negative effect on colony formation. After repetitive replatings, the clonogenic capacities of cells expressing m0 gradually dropped compared with WT. The presence of Tyr729, but also Tyr704 and Tyr744, both involved in activation of signal transducer and activator of transcription 3 (STAT3), further reduced replating efficiencies. Conversely, Tyr764 greatly elevated the clonogenic abilities of myeloid progenitors, resulting in a more than 10(4)-fold increase of colony-forming cells over m0 after the fifth replating. These findings suggest that tyrosines in the cytoplasmic domain of G-CSF-R, although dispensable for G-CSF-induced colony growth, recruit signaling mechanisms that regulate the maintenance and outgrowth of myeloid progenitor cells.

The peripheral cannabinoid receptor Cb2, a novel oncoprotein, induces a reversible block in neutrophilic differentiation

We previously identified a novel common virus integration site, Evi11, by means of retroviral insertional mutagenesis. We demonstrated that the gene encoding the peripheral cannabinoid receptor (Cb2) is the potential target, suggesting that Cb2 is a proto-oncogene. To elucidate a role for this G protein-coupled receptor (GPCR) in leukemic transformation we generated a Cb2-EGFP cDNA construct that was introduced into 32D/G-CSF-R cells. These cells require interleukin 3 (IL-3) to proliferate in vitro, whereas in the presence of granulocyte-colony-stimulating factor (G-CSF) they differentiate toward mature neutrophils. We demonstrate that 32D/G-CSF-R/Cb2-EGFP cells migrate in a transwell assay in reponse to the Cb2 ligand 2-arachidonoylglycerol (2-AG), indicating that the fusion protein was functional. When cultured in the presence of G-CSF neutrophilic differentiation of Cb2-EGFP-expressing 32D/G-CSF-R cells was completely blocked. Moreover, a Cb2-specific antagonist fully recovered the G-CSF-induced neutrophilic differentiation of 32D/G-CSF-R/Cb2-EGFP cells. To investigate which signal transduction pathway(s) may be involved in the block of neutrophilic maturation, differentiation experiments were carried out using specific inhibitors of signaling routes. Interestingly, full rescue of G-CSF-induced neutrophilic differentiation was observed when cells were cultured with the mitogen-induced extracellular kinase (MEK) inhibitors, PD98059 or U0126, and partial recovery was detected with the phosphoinositide 3-kinase (PI3-K) inhibitor LY-294002. These studies demonstrate that the Cb2 receptor is an oncoprotein that blocks neutrophilic differentiation when overexpressed in myeloid precursor cells. Cb2 appears to mediate its activity through MEK/extracellular signal-related kinase (ERK) and PI3-K pathways.

Granulocyte colony-stimulating factor administration upregulates telomerase activity in CD34+ haematopoietic cells and may prevent telomere attrition after chemotherapy

Hematopoietic reconstitution could be associated with premature ageing of the transplanted cells and a high frequency of myelodysplastic syndrome and secondary leukaemia. Telomere length decreases with cell divisions and age, and at a crucial length it is associated with chromosomal instability and cell senescence. Telomerase is a reverse transcriptase enzyme that adds nucleotides to chromosomal ends. Most somatic cells lack telomerase activity yet haematopoietic stem cells retain low levels of telomerase. Some studies have found that chemotherapy and stem cell transplantation lead to the accelerated shortening of telomere length. As granulocyte colony-stimulating factor (G-CSF) is routinely used in the mobilization of stem cells for transplantation, we evaluated its effects on telomerase activity and regulation, and on telomere dynamics, in normal donors and selected lymphoma patients. Administration of G-CSF increased telomerase activity in CD34+ haematopoietic cells compared with controls. In marrow-derived CD34+ cells, telomerase activity increased sevenfold, compared with a 14-fold increase in peripheral-blood-mobilized CD34+ cells. A parallel increase in the expression of human telomerase enzyme reverse transcriptase RNA and protein kinase C alpha occurred. In addition, G-CSF administration to five lymphoma patients after consecutive courses of CHOP (cyclophosphamide, doxorubicin, vincristine, prednisone) chemotherapy, resulted in telomere length preservation or elongation, as opposed to marked attrition in patients who did not receive growth factors. We conclude that the in vivo administration of G-CSF prevents or attenuates telomere attrition associated with chemotherapy administration. This attenuation may contribute to the preservation of telomere integrity inG-CSF-primed transplanted stem cells.

Control of myeloid-specific integrin alpha Mbeta 2 (CD11b/CD18) expression by cytokines is regulated by Stat3-dependent activation of PU.1

Granulocyte colony-stimulating factor (G-CSF) plays an essential role in regulating multiple aspects of hematopoiesis. To elucidate the role of G-CSF in controlling hematopoietic cell migration capabilities, we studied inducible expression of the myeloid-specific marker, integrin alpha(M)beta(2) (CD11b/CD18, Mac-1), in the myeloid cell line, 32D. We found that G-CSF stimulates the synthesis and cell surface expression of alpha(M) and beta(2) integrin subunits. Induction of both alpha(M) and beta(2) is dependent on Stat3, a major G-CSF-responsive signaling protein. However, the kinetics of expression suggested the involvement of an intermediate protein regulated by Stat3. Our results demonstrate that Stat3 signaling stimulates the expression of PU.1, a critical regulator of myelopoiesis. Furthermore, we show that PU.1 is an essential intermediate for the inducible expression of alpha(M)beta(2) integrin. Thus, Stat3 promotes alpha(M)beta(2) integrin expression through its activation of PU.1. These findings indicate that G-CSF-dependent Stat3 signals stimulate the changes in cell adhesion and migration capabilities that occur during myeloid cell development. These data also demonstrate a link between Stat3 and PU.1, suggesting that Stat3 may play an instructive role in hematopoiesis.

Dissection of the cytoplasmic domains of cytokine receptors involved in STAT and Ras dependent proliferation

Cytokine receptors have different signaling requirements which ultimately lead to various physiological responses. In an effort to precisely characterize the molecular determinants involved in the proliferative response mediated by cytokines, we examine dose-dependent proliferation of the betac (GM-CSF, IL-3, IL-5) and homodimeric (G-CSF, TPO) cytokine receptors. Here we report that all cytokine receptors tested activate mostly STAT3 and STAT5. While STAT3 had a positive effect on betac cytokine receptor dependent proliferation, STAT5 was strongly inhibitory. Similarly, G-CSF and TPO lead to activation of STAT3 and STAT5 but, unlike the betac cytokine receptors, both stimulated cellular growth. On the other hand, Ras activation was necessary for all receptor mediated proliferation with the exception of G-CSF R. Truncated mutants of the receptors intracellular domains were used to delineate the functional domains involved in JAK/STAT and Ras activation linked to cellular growth. For instance, we revealed a critical role for the specific alpha subunit of the betac receptors in triggering receptor activation, STAT3 stimulation and proliferation, while Ras activation originates from the distal intracellular portion of the betac subunit. Finally, we showed that proximal STAT activation is the triggering event of G-CSF and TPO receptor function.

Tyrosine residues of the granulocyte colony-stimulating factor receptor transmit proliferation and differentiation signals in murine bone marrow cells

Granulocyte colony-stimulating factor (G-CSF) is the major regulator of granulopoiesis and acts through binding to its specific receptor (G-CSF-R) on neutrophilic granulocytes. Previous studies of signaling from the 4 G-CSF-R cytoplasmic tyrosine residues used model cell lines that may have idiosyncratic, nonphysiological responses. This study aimed to identify specific signals transmitted by the receptor tyrosine residues in primary myeloid cells. To bypass the presence of endogenous G-CSF-R, a chimeric receptor containing the extracellular domain of the epidermal growth factor receptor in place of the entire extracellular domain of the G-CSF-R was used. A series of chimeric receptors containing tyrosine mutations to phenylalanine, either individually or collectively, was constructed and expressed in primary bone marrow cells from G-CSF-deficient mice. Proliferation and differentiation responses of receptor-expressing bone marrow cells stimulated by epidermal growth factor were measured. An increased 50% effective concentration to stimulus of the receptor Y(null) mutant indicated that specific signals from tyrosine residues were required for cell proliferation, particularly at low concentrations of stimulus. Impaired responses by mutant receptors implicated G-CSF-R Y(764) in cell proliferation and Y(729) in granulocyte differentiation signaling. In addition, different sensitivities to ligand stimulation between mutant receptors indicated that G-CSF-R Y(744) and possibly Y(729) have an inhibitory role in cell proliferation. STAT activation was not affected by tyrosine mutations, whereas ERK activation appeared to depend, at least in part, on Y(764). These observations have suggested novel roles for the G-CSF-R tyrosine residues in primary cells that were not observed previously in studies in cell lines.

Modulation of MEK activity during G-CSF signaling alters proliferative versus differentiative balancing

Previous studies of the granulocyte colony stimulating factor (G-CSF) receptor have demonstrated that discrete signals direct proliferative and maturation signaling. Receptor deletion/mutant studies have shown that although activation of the ras-mitogen activated protein (MAP) kinase pathway is necessary for G-CSF directed proliferation, it is not necessary for maturation induced by this cytokine. We have assessed the effects of selective inhibition or overexpression of MAP kinase kinase (MEK) in a cell line model of G-CSF-induced neutrophil progenitor growth. Using the human G-CSF responsive MPD cell line, we specifically inhibited MEK using PD 98059 and also transfected MPD cells with a constitutively active MEK construct. We then exposed the cells to G-CSF and assessed the effects of MEK inhibition and forced expression on proliferation and differentiation. Inhibition of MEK followed by G-CSF stimulation consistently resulted in an early 2.5-fold increase in morphologically differentiated neutrophils expressing CD11b and CD16 and containing lactoferrin over that produced by G-CSF alone. MEK inhibition alone had little effect on the differentiation stage of these cells, although proliferation was impaired. Forced expression of activated MEK resulted in a three- to five-fold decrease in differentiated, lactoferrin containing neutrophilic cells resultant from G-CSF induction, and a commensurate increase in cell proliferation. These observations suggest that modulation of MAPK activation may be a control point for altering the balance between proliferation and differentiation in response to G-CSF. Physiologically, this control is likely exerted by costimulatory cytokines.

Granulocyte colony-stimulating factor induces ERK5 activation, which is differentially regulated by protein-tyrosine kinases and protein kinase C. Regulation of cell proliferation and survival

Granulocyte colony-stimulating factor (G-CSF) plays a major role in the regulation of granulopoiesis. Treatment of cells with G-CSF has been shown to activate multiple signal transduction pathways. We show here that Erk5, a novel member of the MAPK family, and its specific upstream activator MEK5 were activated in response to incubation of cells with G-CSF. Different from other members of the MAPK family including Erk1/2, JNK, and p38, maximal activation of Erk5 by G-CSF required the C-terminal region of the G-CSF receptor. Genistein, a specific inhibitor of protein-tyrosine kinases, blocked G-CSF-induced Erk5 activation. In contrast, inhibition of protein kinase C activity increased G-CSF-mediated activation of Erk5 and MEK5, whereas stimulation of protein kinase C activity inhibited activation of the two kinases by G-CSF. The proliferation of BAF3 cells in response to G-CSF was inhibited by expression of a dominant-negative MEK5 but potentiated by expression of a constitutively active MEK5. Expression of the constitutively active MEK5 also increased the survival of BAF3 cells cultured in the absence of or in low concentrations of G-CSF. Together, these data implicate Erk5 as an important signaling component in the biological actions of G-CSF.

STAT3-mediated differentiation and survival and of myeloid cells in response to granulocyte colony-stimulating factor: role for the cyclin-dependent kinase inhibitor p27(Kip1)

The signal transducer and activator of transcription (STAT) proteins have been implicated in cytokine-regulated proliferation, differentiation and cell survival. Granulocyte colony-stimulating factor (G-CSF), a regulator of granulocytic differentiation, induces a robust and sustained activation of STAT3. Here, we show that introduction of dominant negative (DN) forms of STAT3 interferes with G-CSF-induced differentiation and survival in murine 32D cells. G-CSF induces expression of the cyclin-dependent kinase (cdk) inhibitor p27(KiP1) (but not p21(CiP1)), which is completely blocked by DN-STAT3. The ability of tyrosine-to-phenylalanine substitution mutants of the G-CSF receptor to activate STAT3 strongly correlated with their capacity to induce p27 expression and their ability to mediate differentiation and survival, suggesting a causal relationship between STAT3 activation, p27 expression and the observed cellular responses. We identified a putative STAT binding site in the promoter region of p27 that showed both STAT3 binding in electrophoretic mobility shift assays and functional activity in luciferase reporter assays. Finally, we studied G-CSF-induced responses in primary bone marrow and spleen cells of p27-deficient mice. Compared with wild-type, myeloid progenitors from p27-deficient mice showed significantly increased proliferation and reduced differentiation in response to G-CSF. These findings indicate that STAT3 controls myeloid differentiation, at least partly, via upregulation of p27(Kip1).

Regulation of granulopoiesis by transcription factors and cytokine signals

The development of mature granulocytes from hematopoietic precursor cells is controlled by a myriad of transcription factors which regulate the expression of essential genes, including those encoding growth factors and their receptors, enzymes, adhesion molecules, and transcription factors themselves. In particular, C/EBPalpha, PU.1, CBF, and c-Myb have emerged as critical players during early granulopoiesis. These transcription factors interact with one another as well as other factors to regulate the expression of a variety of genes important in granulocytic lineage commitment. An important goal remains to understand in greater detail how these various factors act in concert with signals emanating from cytokine receptors to influence the various steps of maturation, from the pluripotent hematopoietic stem cell, to a committed myeloid progenitor, to myeloid precursors, and ultimately to mature granulocytes.

Mutant RAS inhibits neutrophil but not macrophage differentiation and allows continued growth of neutrophil precursors

Mutational activation of RAS is the most common molecular abnormality in myeloid leukemias. In order to better understand its role in leukemogenesis, we have devised a model based on the multipotent cell line, FDCP-mix. We show that expression of mutant RAS in FDCP-mix strongly inhibits terminal neutrophil differentiation under the influence of G-CSF plus GM-CSF at the metamyelocyte stage, whereas macrophage differentiation was unaffected. In addition, whereas control cultures differentiated and became postmitotic under these conditions, FDCP-mix cells expressing mutant RAS continued to proliferate indefinitely while maintaining a metamyelocytic phenotype. Labeling of these cultures with the fluorescent tracking dye, PKH26, showed that this extended proliferative capacity resulted from continued division of metamyelocytes in the culture. Dissection of the growth factor response of these cells demonstrated that GM-CSF was critical in maintaining proliferation and inhibiting the differentiation of these cells. We further show the block in neutrophil differentiation could be partially overcome by treatment with low-dose Ara C, suggesting that maintenance of cell cycle progression may be partly responsible for the anti-differentiation effect of this oncogene. These findings suggest that activation of RAS is able to specifically inhibit terminal neutrophil differentiation and in so doing promotes continued division of metamyelocyte cells.

Development of a modified selective amplifier gene for hematopoietic stem cell gene therapy

We have proposed a novel concept, ie selective expansion of transduced cells, to overcome the low efficiency of gene transfer into hematopoietic stem cells. Previously, a fusion gene encoding a chimeric receptor (DeltaGCRER) between the mouse granulocyte colony-stimulating factor receptor (G-CSFR) and the hormone-binding domain of rat estrogen receptor was constructed as a 'selective amplifier gene'. Although the chimeric gene conferred estrogen-inducible proliferation on the transduced Ba/F3 cells, it also mediated differentiation of the retrovirally transduced 32D cells upon estrogen treatment. Since only a growth signal is required for our purpose, we further modified the DeltaGCRER gene to attenuate its differentiation signal. Based on the observation that tyrosine-703 in wild-type G-CSFR plays a pivotal role in transmitting the differentiation signal, phenylalanine was substituted for this residue in DeltaGCRER. When the resultant selective amplifier gene (DeltaY703F-GCRER gene) was expressed in 32D cells, sustained growth was supported by estrogen, while differentiation was suppressed. These cells ceased to grow upon estrogen withdrawal and differentiated with G-CSF treatment. The present findings suggested that DeltaY703F-GCRER may have desirable properties as a selective amplifier for hematopoietic stem cell expansion and gene therapy.

Multiple signals mediate proliferation, differentiation, and survival from the granulocyte colony-stimulating factor receptor in myeloid 32D cells

Granulocyte colony-stimulating factor (G-CSF) regulates neutrophil production through activation of its cognate receptor, the G-CSF-R. Previous studies with deletion mutants have shown that the membrane-proximal cytoplasmic domain of the receptor is sufficient for mitogenic signaling, whereas the membrane-distal domain is required for differentiation signaling. However, the function of the four cytoplasmic tyrosines of the G-CSF-R in the control of proliferation, differentiation, and survival has remained unclear. Here we investigated the role of these tyrosines by expressing a tyrosine "null" mutant and single tyrosine "add back" mutants in maturation-competent myeloid 32D cells. Clones expressing the null mutant showed only minimal proliferation and differentiation, with survival also reduced at low G-CSF concentrations. Analysis of clones expressing the add-back mutants revealed that multiple tyrosines contribute to proliferation, differentiation, and survival signals from the G-CSF-R. Analysis of signaling pathways downstream of these tyrosines suggested a positive role for STAT3 activation in both differentiation and survival signaling, whereas SHP-2, Grb2 and Shc appear important for proliferation signaling. In addition, we show that a tyrosine-independent "differentiation domain" in the membrane-distal region of the G-CSF-R appears necessary but not sufficient for mediating neutrophilic differentiation in these cells.

All-trans retinoic acid and hematopoietic growth factors regulating the growth and differentiation of blast progenitors in acute promyelocytic leukemia

Although acute leukemia is generally thought to be characterized by maturation arrest, it has been shown that differentiation occurs in blast cells of acute myelogenous leukemia (AML) in vitro as well as in vivo, and that morphologically abnormal mature polymorphonuclear neutrophils (PMNs) often seen in patients with AML are possibly derived from spontaneously differentiating leukemic cells. Acute promyelocytic leukemia (APL) is an unique example in which these features of AML are evident in an almost complete form; administration of all-trans retinoic acid (ATRA) induces differentiation of neoplastic cells into mature neutrophils and successfully induce complete remission in most patients. However, PMNs appearing during ATRA treatment are morphologically abnormal, as indicated not only by the presence of Auer rods but also by neutrophil secondary-granule deficiency that is commonly seen in AML. Moreover, ATRA has heterogeneous effects on the growth of blast progenitors in APL in different patients, being inhibitory, stimulatory or ineffective, which might account in part for the leukemia relapse in patients treated with ATRA alone. Hematopoietic growth factors regulate the growth of blast progenitors in APL. Among them, granulocyte colony-stimulating factor (G-CSF) is unique in that it preferentially stimulates clonal growth, but not self-renewal, in many APL cases, and synergistically enhances the differentiation-inducing effect of ATRA when used in combination. Many other compounds also exert such synergistic effects with ATRA, for which a variety of mechanisms have been suggested. It is crucial to precisely elucidate the functions of these molecules governing the growth/differentiation balance of AML blast progenitors and the mechanisms underlying their deregulated differentiation program in order to achieve effective differentiation therapy for patients with AML, not restricted to APL.