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Signaling by Kit protein-tyrosine kinase—The stem cell factor receptor

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Abstract

Signaling by stem cell factor and Kit, its receptor, plays important roles in gametogenesis, hematopoiesis, mast cell development and function, and melanogenesis. Moreover, human and mouse embryonic stem cells express Kit transcripts. Stem cell factor exists as both a soluble and a membrane-bound glycoprotein while Kit is a receptor protein-tyrosine kinase. The complete absence of stem cell factor or Kit is lethal. Deficiencies of either produce defects in red and white blood cell production, hypopigmentation, and sterility. Gain-of-function mutations of Kit are associated with several human neoplasms including acute myelogenous leukemia, gastrointestinal stromal tumors, and mastocytomas. Kit consists of an extracellular domain, a transmembrane segment, a juxtamembrane segment, and a protein kinase domain that contains an insert of about 80 amino acid residues. Binding of stem cell factor to Kit results in receptor dimerization and activation of protein kinase activity. The activated receptor becomes autophosphorylated at tyrosine residues that serve as docking sites for signal transduction molecules containing SH2 domains. The adaptor protein APS, Src family kinases, and Shp2 tyrosyl phosphatase bind to phosphotyrosine 568. Shp1 tyrosyl phosphatase and the adaptor protein Shc bind to phosphotyrosine 570. C-terminal Src kinase homologous kinase and the adaptor Shc bind to both phosphotyrosines 568 and 570. These residues occur in the juxtamembrane segment of Kit. Three residues in the kinase insert domain are phosphorylated and attract the adaptor protein Grb2 (Tyr703), phosphatidylinositol 3-kinase (Tyr721), and phospholipase Cγ (Tyr730). Phosphotyrosine 900 in the distal kinase domain binds phosphatidylinositol 3-kinase which in turn binds the adaptor protein Crk. Phosphotyrosine 936, also in the distal kinase domain, binds the adaptor proteins APS, Grb2, and Grb7. Kit has the potential to participate in multiple signal transduction pathways as a result of interaction with several enzymes and adaptor proteins.

Section snippets

Properties of stem cell factor

Stem cell factor is widely expressed during embryogenesis and can be detected in brain, endothelium, gametes, heart, kidney, lung, melanocytes, skin, and the stromal cells of the bone marrow, liver, and thymus [10]. Stem cell factor acts synergistically with hematopoietic colony-stimulating factors such as granulocyte-macrophage colony-stimulating factor, interleukin 3, and erythropoietin [18].

SCF exists as membrane-anchored and soluble isoforms that arise from alternative RNA splicing and

Structure of Kit, the stem cell factor receptor

Kit is a type III receptor protein-tyrosine kinase [2] (see [22] for a description of types I through IX receptor protein-tyrosine kinases). The type III class also includes the platelet-derived growth factor receptor (α- and β-chains), the macrophage colony-stimulating-factor receptor (CSF-1), and the Fl cytokine receptor (Flk2/Flt3). Receptor protein-tyrosine kinases all share the same topology: an extracellular ligand-binding domain, a single transmembrane segment, and a cytoplasmic kinase

Kit and Src family kinase activation

Src and Src family protein kinases play key roles in cell differentiation, motility, proliferation, protein trafficking, and survival. From the N- to C-terminus, Src family kinases contain an N-terminal 14-carbon myristoyl group, a unique segment, an SH3 domain, an SH2 domain, a protein-tyrosine kinase domain, and a C-terminal regulatory tail [25], [26]. Src family kinases are controlled by integrin, G-protein coupled, antigen- and Fc-coupled, cytokine, and steroid hormone receptors, and

Binding of APS to Kit

APS is an adaptor protein that contains a PH and an SH2 domain (Table 1). APS was originally cloned using a yeast two-hybrid system with the constitutively active oncogenic Asp816Val mutant of human Kit as bait. Using immobilized phosphopeptides corresponding to various autophosphorylated tyrosine residues in Kit, Wollberg et al. [32] demonstrated that APS preferentially associates with pTyr568 and pTyr936. pTyr568 is the binding site for Chk, Shc, Shp2, and Src family kinases (Table 2). These

Binding of Cbl to Kit

Cbl functions as a negative regulator of many signaling pathways that start from receptors at the cell surface. The c-Cbl proto-oncogene was first discovered as the cellular homologue of v-Cbl, a viral transforming gene from a Cas murine retrovirus that causes pre-B cell lymphomas in mice (Cas refers to the Lake Casitas region in California where the virus was isolated) [33]. Cbl family members are components of the ubiquitin ligation machinery involved in the targeting and degradation of

Binding of Chk to Kit

Csk (C-terminal Src kinase), a cytoplasmic protein-tyrosine kinase, catalyzes the phosphorylation of the C-terminal regulatory tyrosine of Src which is inhibitory [26]. Chk (Csk homology kinase) also catalyzes the phosphorylation of the inhibitory tyrosine of Src family kinases. Csk is expressed in all mammalian cells, whereas Chk is limited to breast, hematopoietic cells, neurons, and testes. Csk and Chk consist of an SH3, SH2, and kinase domain. Price et al. [36] demonstrated that Chk, but

Regulation of Kit signaling by Shp1 phosphatase

Shp1 is a cytosolic phosphotyrosyl phosphatase containing two tandem SH2 domains, a phosphatase domain and a C-terminal tail. Shp1 occurs primarily in hematopoietic and epithelial cells, and it is a negative regulator of growth factor signaling. Besides inhibiting Kit signaling, Shp1 diminishes the growth-promoting properties of the colony-stimulating factor 1, erythropoietin, and interleukin 3 receptors, an effect mediated either directly by receptor dephosphorylation or indirectly by

Regulation of Kit signaling by Shp2 phosphatase

Shp2, like Shp1, is a cytosolic phosphotyrosyl phosphatase containing two tandem SH2 domains, a phosphatase domain, and a C-terminal tail. Shp2, in contrast to Shp1, occurs in many types of cells. The SH2 domains of Shp2 target this enzyme to phosphotyrosines in a variety of growth factor receptors and other signaling molecules [26].

The ability of Shp2 to associate with activated Kit in Ba/F3-Kit transfectants is markedly reduced by the Tyr568Phe mutation but is unaffected by the Tyr570Phe

Kit and phosphatidylinositol 3-kinase activation

Phosphatidylinositol 3-kinase (PI 3-kinase) represents a class of lipid kinases that catalyze the phosphorylation of the 3-hydroxyl group of the inositol ring of phosphatidylinositol derivatives. The preferred substrate in vivo is phosphatidylinositol 4,5-bisphosphate, and the product is phosphatidylinositol 3,4,5-trisphosphate. PI 3-kinase was initially identified through its association with two viral oncoproteins: v-Src and the middle T antigen of polyoma virus [38]. The PI 3-kinases are

Kit and phospholipase Cγ activation

Phospholipase C (PLC) represents an enzyme family that catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate and diacylglycerol. Inositol trisphosphate promotes the elevation of intracellular Ca2+; diacylglycerol activates protein kinase C (a protein-serine/threonine kinase family of isozymes). A myriad of cell functions are regulated by phospholipase C and Ca2+ including cell differentiation, division, motility, survival, egg fertilization, and the

Binding of Grb2 and Grb7 to Kit

Grb2 is an adaptor protein that contains one SH2 domain between two SH3 domains. It links receptor tyrosyl kinases including the insulin and epidermal growth factor receptor to the Ras/mitogen-activated protein kinase pathway. Grb2 forms a complex with the Ras guanine nucleotide exchange factor (Ras-GEF, or Sos), and the complex binds to phosphotyrosine residues. Grb7 is an adaptor protein that contains a Ras-associating domain, a PH domain, and an SH2 domain. Grb7 binds to the phosphorylated

Binding of Dok1 to Kit

Dok represents a family of six human docking proteins that contains an N-terminal PH domain, a central phosphotyrosine-binding (PTB) domain, and a C-terminal tail containing many tyrosine phosphorylation sites. Phosphotyrosine-binding domains consist of about 100 residues that bind to phosphotyrosine and to phosphoinositides. Dok proteins function as scaffolds to organize signaling complexes. Asn-Pro-Xxx-pTyr represents a consensus Dok-binding sequence. Dok1 is phosphorylated in response to a

Epilogue

Mice with mutations in the White locus (discovered in 1927) and in the Steel locus (discovered in 1956) exhibit a deficiency of melanocytes, a macrocytic anemia, and sterility [7], [8]. No morphologic or hematologic characteristic can be used to definitely distinguish between an adult W/Wv mutant mouse and an adult Sl/Sld mutant mouse [9]. Physiological studies in vivo and in vitro suggested that deficits in W mutant mice are expressed by the affected cells and deficits in Sl mutant mice are

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      The absence of SCF or CD117 causes death in utero during the perinatal period due to severe anemia, emphasizing the importance of the SCF/CD117 signaling axis during development (Roskoski, 2005; Broudy, 1997). Mice with mutations causing the deletion of the membrane-bound SCF isoform (at the Sl) display severe anemia, reduced number of tissue mast cells, sterility, and are white (Ashman, 1999; Roskoski, 2005). This deletion demonstrates that soluble SCF is insufficient in maintaining the CD117 signaling axis, verifying that both forms of SCF are needed for normal hematopoiesis, and these isoforms contribute to different cell signaling pathways (Broudy, 1997).

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    Abbreviations: Flk, fetal liver kinase; Flt, FMS-like tyrosyl kinase; GST, glutathione S-transferase; KDR, kinase insert domain receptor; PDGF, platelet-derived growth factor, PH, pleckstrin homology; PI 3-kinase, phosphatidylinositol 3-kinase; PLC, phospholipase C; pTyr, phosphotyrosine; SCF, stem cell factor; SH2, Src homology 2; SH3, Src homology 3; VEGF, vascular endothelial growth factor.

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