Review
Early signaling pathways activated by c-Kit in hematopoietic cells

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Abstract

c-Kit is a receptor tyrosine kinase that binds stem cell factor (SCF). Structurally, c-Kit contains five immunoglobulin-like domains extracellularly and a catalytic domain divided into two regions by a 77 amino acid insert intracellularly. Studies in white spotting and steel mice have shown that functional SCF and c-Kit are critical in the survival and development of stem cells involved in hematopoiesis, pigmentation and reproduction. Mutations in c-Kit are associated with a variety of human diseases. Interaction of SCF with c-Kit rapidly induces receptor dimerization and increases in autophosphorylation activity. Downstream of c-Kit, multiple signal transduction components are activated, including phosphatidylinositol-3-kinase, Src family members, the JAK/STAT pathway and the Ras–Raf–MAP kinase cascade. Structure-function studies have begun to address the role of these signaling components in SCF-mediated responses. This review will focus on the biochemical mechanism of action of SCF in hematopoietic cells.

Introduction

Stem cell factor (SCF) is a growth factor critical in hematopoiesis as well as in the generation of melanocytes and germ cells, reviewed in references [1], [2], [3], [4], [5], [6], [7], [8]. SCF also plays a role in development of the interstitial cells of Cajal in the intestine and in learning functions in the hippocampal region of the brain [9], [10], [11]. Occurring physiologically in either membrane-bound or soluble form, SCF promotes viability as well as proliferation and differentiation of hematopoietic progenitor cells. In addition, SCF is potently synergistic in combination with other growth factors such as Epo, IL-3 and GM–CSF. There are several excellent reviews of SCF biology in relation to hematopoiesis [2], [4], [5], [6]. A comprehensive overview of SCF in relation to its various target tissues can be found in the review by Galli et al. [1]. The purpose of this review is to examine the early signaling pathways activated in response to SCF in hematopoietic cells.

SCF was cloned and characterized in 1990 [12], [13], [14], [15]. The receptor for SCF is the product of the Kit proto-oncogene [16], [17]. The c-Kit gene maps to the White spotting (W) locus in mice, while SCF is encoded by the Steel locus (Sl) [16], [17], [18], [19]. The first descriptions of mutant W and Sl alleles were in 1927 and 1956, respectively [20]. Since that time, multiple alleles of both genes have been discovered and characterized [7], [8], [20]. While the absence of either SCF or c-Kit is lethal in utero, reductions in functional receptor, or ligand, results in aberrations in hematopoiesis, pigmentation and reproduction.

The Kit gene product has been associated with several forms of cancer. The v-Kit oncogene was originally identified as a component of Hardy–Zuckerman strain of feline sarcoma virus [21]. In humans, a series of gain-of-function mutations in the c-Kit juxtamembrane region have been found in gastrointestinal stromal cell tumors [22]. c-Kit is also aberrantly expressed in approximately 70% of all small cell carcinomas of the lung (SCCL), as well as in breast, cervical and ovarian tumors [23], [24], [25], [26], [27]. Coexpression of c-Kit and SCF in SCCL cells generates an autocrine loop that may play a role in the etiology of these cancers. A constitutively active form of human c-Kit (D816 V) has been found with high frequency in patients with mastocytosis and associated hematological disorders [28]. Hematological disorders in these patients range from myelodysplasia to myeloproliferative disease. In addition, these individuals develop leukemia at high frequencies.

The important role of SCF in development of stem cells involved in hematopoiesis, pigmentation, intestinal function and reproduction, as well as its association with some forms of human disease, has lead to interest in understanding its mechanism of action. This review will focus on signaling mechanisms of SCF, specifically in hematopoietic cells.

Section snippets

SCF induces receptor dimerization

c-Kit is a receptor tyrosine kinase (RTK) closely related to the receptors for platelet-derived growth factor and colony-stimulating factor-1. Structurally, the c-Kit extracellular domain can be divided into five immunoglobulin-like regions (Fig. 1). The studies of Yarden and coworkers have shown that the first three immunoglobulin-like regions bind SCF, inducing homodimerization of the receptor [29], [30], [31]. Two models for ligand-induced dimerization of c-Kit have been suggested [32], [33]

c-Kit autophosphorylation

The cDNA sequence of the c-Kit proto-oncogene predicted that the protein was a RTK [41], [42]. Indeed, in vitro kinase assays of c-Kit immunoprecipitates found intrinsic tyrosine kinase activity [41], [43]. The organization of the cytoplasmic domain of c-Kit is similar to that of the receptors for colony-stimulating factor-1 and platelet-derived growth factor. The catalytic domain is divided by a 77 amino acid insert (Fig. 1). The first catalytic domain contains the ATP binding region while the

Phosphatidylinositol-3-kinase and c-Kit signaling

SCF activates multiple signaling components, however, the best characterized of these with regards to structure-function relationships is phosphatidylinositol-3-kinase (PI3 K). PI3 K is a heterodimer composed of an 85 kDa regulatory subunit and a 110 kDa catalytic subunit. The 85 kDa subunit (p85) contains several motifs implicated in protein–protein interactions. These motifs include two SH2 domains, an SH3 domain and a proline-rich domain. Increases in autophosphorylation activity of RTKs

The JAK/STAT pathway and c-Kit signaling

Members of the Janus family of protein tyrosine kinases (JAKs) are activated by ligands interacting with a variety of receptors lacking intrinsic kinase activity. Among these are hematopoietic growth factors that bind receptors in the cytokine receptor superfamily. These include the erythropoietin (Epo) receptor, the granulocyte–macrophage colony-stimulating factor (GM–CSF) receptor, the interleukin 3 (IL-3) receptor as well as numerous others. Reviews relating to activation of Janus kinases by

Src family members and c-Kit signaling

Src family members are involved in a wide range of cellular functions including cell adhesion, cell motility, cell cycle progression, survival, differentiation, protein trafficking and cellular architecture, reviewed in [102], [103], [104]. They interact with one or more components of most known signaling pathways. With regard to signaling through RTKs, Src family members are activated in response to numerous RTK ligands including platelet-derived growth factor (PDGF), epidermal growth factor

The Ras–Raf–MAP kinase cascade and c-Kit signaling

One signaling pathway activated in response to many growth factors is the Ras–Raf–MAP kinase cascade, reviewed in [116], [117], [118]. In brief, phosphorylated tyrosine residues on ligand-activated receptors recruit multiple SH2-containing proteins to the receptor complex and these proteins couple RTKs to activation of Ras. Included among these are Grb2, Shc, SHP2 and Grap. A Grb family member is constitutively associated with Sos, a guanine nucleotide exchange factor. Recruitment of Grb2 to

Negative regulators of SCF signaling

Several lines of evidence suggest that SHP1, an SH2-containing protein tyrosine phosphatase specific for hematopoietic cells, is a negative regulator of c-Kit signaling. SCF induces association of the SHP1 SH2 domain with phosphorylated tyrosine 569 of murine c-Kit [89], [90]. Transfection of Y569F c-Kit into BaF3 cells resulted in increases in SCF-induced proliferation [90]. Genetic evidence also suggests that SHP1 is a negative regulator of c-Kit. Motheaten mice (me) and motheaten viable mice

Other signaling components activated by SCF

A variety of other signaling components are activated in response to SCF, although the signaling pathways these proteins are involved with, and their role in SCF-mediated responses, remain to be defined. For example, PLCγ hydrolyzes phosphatidylinositol-4,5-bisphosphate (PIP2) into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). SCF induces weak association between the SH2 domain of PLCγ and tyrosine 936 of human c-Kit as well as small increases in tyrosine phosphorylation of PLCγ

C-Kit signaling and cellular matrix components

Hematopoietic progenitor cells interact with both stromal cells and components of the extracellular matrix. These events, in conjunction with signals mediated by soluble and membrane-bound growth factors, are important in the regulation of hematopoiesis. Interaction of differentiated progeny, such as mast cells, with components of the extracellular matrix is also important in their functional responses. SCF-induced adhesion occurs through multiple mechanisms. One mechanism involves interaction

Integrated signaling mechanisms

Although delineation of linear signaling pathways is conceptually attractive, many of the signaling components activated by SCF play roles in multiple pathways. In most cases, SCF-mediated responses result from the input of multiple, interconnected signaling pathways (summarized in Fig. 3). For example, SHP2 is likely involved in activation of the Ras–Raf–MAP kinase cascade but may also negatively regulate other signaling pathways [90], [123]. Phosphorylation of c-Kit by PKC isoforms decreases

C-Kit structure-function summary

SCF activates multiple signal transduction components including PI3 K, Src family members, the Ras–Raf–MAP kinase cascade and the JAK/STAT pathway. Activation of many of these pathways is dependent on interaction of one or more upstream signaling components with specific sites on c-Kit. Table 1 is a summary of tyrosine residues on c-Kit that interact with components of these pathways. In addition, if known, the biological consequence of mutation of each site is also shown. Fig. 4 summarizes

SCF-mediated synergy

A remarkable feature of SCF is its capacity to synergize with other hematopoietic growth factors. This is a critical because stem cells and multipotential progenitor cells respond optimally to growth factors in combination. The mechanisms mediating synergy are poorly understood. Studies from the laboratory of Keller and coworkers [160] demonstrated that synergistic responses of hematopoietic progenitor cells to combinations of ligands binding members of the cytokine receptor superfamily

Conclusions

In the nine years since SCF was identified as the c-Kit ligand, a remarkable amount has been learned about its mechanism of action. SCF activates multiple signaling pathways and these pathways lead to a variety of biological responses. While many of the early studies focused on signaling pathways activated by SCF in fibroblasts transfected with c-Kit, subsequent work in hematopoietic cell lines generally supports these findings. Understanding the role of these signaling pathways in SCF-mediated

Acknowledgements

The author would like to thank Dr Doug Lowy, Dr Virginia Broudy and Ms Bridget O’Laughlin for their critical review of this manuscript. The efforts of Ms Karen Canon in preparation of the references were also greatly appreciated.

References (171)

  • E. Huang et al.

    The hematopoietic growth factor KL is encoded by the SI locus and is the ligand of the c-kit receptor, the gene product of the W locus

    Cell

    (1990)
  • E.N. Geissler et al.

    The dominant-white spotting (W) locus of the mouse encodes the c-kit proto-oncogene

    Cell

    (1988)
  • E.S. Russell

    Hereditary anemias of the mouse: a review for geneticists

    Advances in Genetics

    (1979)
  • J.M. Blechman et al.

    Soluble c-kit proteins and antireceptor monoclonal antibodies confine the binding site of the stem cell factor

    Journal of Biological Chemistry

    (1993)
  • Y-R. Hsu et al.

    The majority of stem cell factor exists as monomer under physiological conditions

    Journal of Biological Chemistry

    (1997)
  • J.S. Philo et al.

    Human stem cell factor dimer forms a complex with two molecules of the extracellular domain of its receptor, kit

    Journal of Biological Chemistry

    (1996)
  • M.A. Lemmon et al.

    Kit receptor dimerization is driven by bivalent binding of stem cell factor

    Journal of Biological Chemistry

    (1997)
  • J.M. Blechman et al.

    The fourth immunoglobulin domain of the stem cell factor receptor couples ligand binding to signal transduction

    Cell

    (1995)
  • V.C. Broudy et al.

    Analysis of c-kit receptor dimerization by fluorescence resonance energy transfer

    Blood

    (1998)
  • A. Kuriu et al.

    Proliferation of human myeloid leukemia cell line associated with the tyrosine-phosphorylation and activation of the proto-oncogene c-kit product

    Blood

    (1991)
  • K. Miyazawa et al.

    Membrane-bound steel factor induces more persistent tyrosine kinase activation and longer life span of c-kit gene-encoded protein than its soluble form

    Blood

    (1995)
  • R. Kapur et al.

    Signaling through the interaction of membrane-restricted stem cell factor and c-kit receptor tyrosine kinase: genetic evidence for a differential role in erythropoiesis

    Blood

    (1998)
  • T. Tsujimura et al.

    Ligand-independent activation of c-kit receptor tyrosine kinase in a murine mastocytoma cell line P-815 generated by a point mutation

    Blood

    (1994)
  • H. Kitayama et al.

    Constitutively activating mutations of c-kit receptor tyrosine kinase confer factor-independent growth and tumorigenicity of factor-dependent hematopoietic cell lines

    Blood

    (1995)
  • X. Piao et al.

    A point mutation in the catalytic domain of c-kit induces growth factor independence, tumorigenicity, and differentiation of mast cells

    Blood

    (1996)
  • H. Kitayama et al.

    Neoplastic transformation of normal hematopoietic cells by constitutively activating mutations of c-kit receptor tyrosine kinase

    Blood

    (1996)
  • P. Ferrao et al.

    Expression of constitutively activated human c-kit in myb transformed early myeloid cells leads to factor independence, histiocytic differentiation, and tumorigenicity

    Blood

    (1997)
  • T. Tsujimura et al.

    Constitutive activation of c-kit in FMA3 murine mastocytoma cells caused by deletion of seven amino acids at the juxtamembrane domain

    Blood

    (1996)
  • V. Duronio et al.

    Downstream signalling events regulated by phosphatidylinositol 3-kinase activity

    Cell Signalling

    (1998)
  • H. Serve et al.

    Tyrosine residue 719 of the c-kit receptor is essential for binding of the P85 subunit of phosphatidylinositol (PI) 3-kinase and for c-kit-associated PI 3-kinase activity in COS-1 cells

    Journal of Biological Chemistry

    (1994)
  • J.N. Ihle

    The Janus protein tyrosine kinases in hematopoietic cytokine signaling

    Seminars in Immunology

    (1995)
  • S.J. Rodig et al.

    Disruption of the Jak1 gene demonstrates obligatory and nonredundant roles of the Jaks in cytokine-induced biologic responses

    Cell

    (1998)
  • H. Neubauer et al.

    Jak2 deficiency defines an essential developmental checkpoint in definitive hematopoiesis

    Cell

    (1998)
  • E. Parganas et al.

    Jak2 is essential for signaling through a variety of cytokine receptors

    Cell

    (1998)
  • S.R. Weiler et al.

    JAK2 is associated with the c-kit proto-oncogene product and is phosphorylated in response to stem cell factor

    Blood

    (1996)
  • U. Klingmuller et al.

    Specific recruitment of SH-PTP1 to the erythropoietin receptor causes inactivation of JAK2 and termination of proliferative signals

    Cell

    (1995)
  • J.N. Ihle

    STATs: signal transducers and activators of transcription

    Cell

    (1996)
  • A. Gotoh et al.

    Steel factor induces serine phosphorylation of STAT3 in human growth factor-dependent myeloid cell lines

    Blood

    (1996)
  • B. Joneja et al.

    Mechanisms of stem cell factor and erythropoietin proliferative co-signaling in FDC2-ER cells

    Blood

    (1997)
  • S.M. Jacobs-Helber et al.

    Distinct signaling from stem cell factor and erythropoietin in HCD57 cells

    Journal of Biological Chemistry

    (1997)
  • S.J. Galli et al.

    The kit ligand, stem cell factor

    Advances in Immunology

    (1994)
  • I.K. McNiece et al.

    Stem cell factor

    Journal of Leukocyte Biology

    (1995)
  • I.K. McNiece et al.

    The role of stem cell factor in the hematopoietic system

    Cancer Investigation

    (1993)
  • K. Morrison-Graham et al.

    Steel factor and c-Kit receptor: from mutants to a growth factor system

    BioEssays

    (1993)
  • J.D. Huizinga et al.

    W/kit gene required for interstitial cells of Cajal and for intestinal pacemaker activity

    Nature

    (1995)
  • B. Motro et al.

    Steel mutant mice are deficient in hippocampal learning but not long-term potentiation

    Proceedings of the National Academy of Sciences USA

    (1996)
  • B. Chabot et al.

    The proto-oncogene c-kit encoding a transmembrane tyrosine kinase receptor maps to the mouse W locus

    Nature

    (1988)
  • P. Besmer et al.

    A new acute transforming feline retrovirus and relationship of its oncogene v-kit with the protein kinase gene family

    Nature

    (1986)
  • S. Hirota et al.

    Gain-of-function mutations of c-kit in human gastrointestinal stromal tumors

    Science

    (1998)
  • A.M. Turner et al.

    Nonhematopoietic tumor cell lines express stem cell factor and display c-kit receptors

    Blood

    (1992)
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