The International Journal of Biochemistry & Cell Biology
ReviewThe biology of stem cell factor and its receptor C-kit
Introduction
Three separate lines of investigation led to the discovery of c-Kit and its ligand, Stem Cell Factor (SCF), also known as Kit Ligand, Steel Factor and Mast Cell Growth Factor. Early experiments on single gene-induced anaemias in the mouse led to the identification of the W and Steel (Sl) loci. Mutations at either of these loci resulted in very similar phenotypes characterised by alterations of coat colour (‘white spotting’), anaemia and lack of mast cells in the tissues suggesting that the genes involved were important in haemopoiesis and melanogenesis. A number of mutations at each of these loci were identified, associated with phenotypic effects of severity varying from neonatal mortality to mild anaemia [1]. Interestingly, transplantation of bone marrow from Sl mice could cure the anaemia of W mice, but not vice versa, indicating that the W mutations affected haemopoietic stem and/or progenitor cells, while the Sl mutations impaired stromal cell function. These findings were confirmed in vitro in stroma-dependent haemopoietic cell cultures [2], and led to the proposal that Sl and W encode a receptor-ligand pair.
At the protein level, c-Kit was first identified as a cell-surface marker of human acute myeloid leukaemia (AML) cells using a monoclonal antibody (MAb), YB5.B8 [3]. The antibody was selected for study based on its binding to primary AML cells, but not to autologous EBV-transformed B cells, or to normal peripheral blood leucocytes. It was subsequently shown to identify a 145–150 kDa protein [4] expressed by normal haemopoietic progenitor cells (colony-forming units-culture, CFU-c) [5] and, strongly, by mast cells in human tissues [6]. The observations that MAb YB5.B8 bound to a lineage-restricted cell surface protein of low abundance which was apparently over-expressed by AML cells, and that the antibody was inhibitory in haemopoietic colony assays [5], [7] suggested that it identified a growth factor receptor. The mapping of c-kit to the W locus [8], [9] suggested that MAb YB5.B8 identified the human c-Kit protein and this was subsequently demonstrated [10].
At the genetic level, Kit was originally identified as the viral oncogene (v-kit) responsible for the transforming activity of the Hardy-Zuckerman IV feline sarcoma virus [11]. Subsequently, the corresponding human and murine proto-oncogenes were cloned and shown to encode a receptor tyrosine kinase related to the platelet-derived growth factor receptor (PDGFR) and the macrophage colony stimulating factor receptor (c-fms) [12], [13]. The mapping of c-kit to the W locus in the mouse, and the identification of human c-Kit as the target of the YB5.B8 MAb brought together a substantial body of information about the expression and function of this receptor in normal haemopoiesis and leukaemia.
In 1990 several groups cloned the cDNA encoding the Kit ligand, here referred to as SCF, using characteristics based on the properties of Sl mutant mice (reviewed [14]). Since that time a vast literature on SCF and c-Kit has accumulated and several extensive reviews have been published [15], [16], [17]. This review will focus on recent data concerning the biological functions of SCF and c-Kit in the haemopoietic system and the subtleties of their regulation. Signal transduction via c-Kit is the subject of another article in this issue.
Section snippets
Expression of c-Kit and SCF
In normal tissues early experiments demonstrated expression of c-Kit mRNA or protein by mast cells [6], [18], [19], melanocytes [19], testis [18], and in bone marrow [20], specifically in the progenitor (CFU-c) compartment [5], all of which are known targets of W mutations. In addition, c-kit mRNA was found in embryonic brain [13], [21]. Subsequently, and with more sensitive methodology, c-Kit protein expression has been found in a wide range of non-haemopoietic cell types including vascular
Regulation of c-Kit expression
Transcriptional regulation of c-Kit expression has been described in several studies. The human and murine c-kit genes consist of 21 exons spanning around 80 kb of DNA [42], [43], [44], [45] and display similar structure and organisation to genes encoding other type III receptor tyrosine kinases, PDGFR, c-fms and Flt-3. The c-kit gene promoter lacks CCAAT or TATA boxes but contains consensus binding sites for multiple transcription factors including Sp1, AP-2, Ets, Myb, SCL and GATA–1 [46], [47]
Isoforms of SCF
SCF exists in both soluble and membrane bound forms as a result of differential splicing and proteolytic cleavage [16] (see Fig. 1). The ratios of the two forms vary considerably in different tissues [72]. Their differential effects on survival and proliferation of haemopoietic cell lines [57], [73] and primary cells [74], [75] have been reported. Miyazawa et al. [73] showed more sustained signalling was mediated by membrane associated SCF and proposed that soluble SCF can act to down regulate
Role of SCF and c-Kit in haemopoietic cell survival, proliferation and differentiation
While the phenotypes of Sl and W mutant mice indicate a critical role for SCF in mast cell and erythroid cell production, in vitro experiments have shown that it is a potent growth factor for primitive haemopoietic cells and multiple differentiating lineages, acting in synergy with other cytokines (reviewed, [16], [17]). SCF was shown to be a survival factor for primitive haemopoietic cells although it did not promote their self-renewal [82] and this effect on survival was seen even when cell
Role of SCF and c-Kit in cell adhesion and migration
Adhesion and migration are important in determining the correct tissue localisation of cells, and in facilitating the action of juxtacrine growth factors. As for mast cells, SCF is a chemotactic agent for haemopoietic progenitor cells [98] and promotes cell adhesion by two distinct mechanisms. Firstly, binding of stromal membrane-bound SCF to haemopoietic cell c-Kit may directly mediate attachment [99] and, secondly, signalling through c-Kit has been shown to upregulate the avidity of mast cell
c-Kit in transformation
This subject has been reviewed elsewhere [109]. Briefly, as stated previously, at the genetic level, Kit was first identified as a retroviral oncogene of feline origin. The nature of the mutations in v-kit was examined by comparison with feline c-Kit [110]. The key change leading to constitutive activation appeared to be loss of the membrane proximal binding site for Src family kinases [110], [111]. Autocrine or juxtacrine cycles involving c-Kit and SCF may be important in some solid tumours,
Isoforms of c-Kit
Two isoforms of c-Kit in the mouse and four in humans have been identified and result from alternate mRNA splicing (Fig. 2). In addition, a soluble extracellular domain of c-Kit (KitS) is generated by proteolysis of the cell surface protein in vitro [71], [120], and relatively high levels (around 300 ng/ml) have been found in normal serum [121]. In both mouse and human alternate splicing results in isoforms characterised by the presence or absence of a tetrapeptide sequence, GNNK, in the
Conclusions
Although much is now known about the cell biology of SCF and c-Kit, several key issues remain to be addressed. Kit signalling has been associated with a large array of biological responses in several cell types. For example, in the mast cell lineage SCF has been associated with proliferation, differentiation, survival, adhesion, chemotaxis and functional activation. How these divergent responses are regulated remains unknown, but it is certain to be complex. Regulation of membrane-bound and
Acknowledgements
The author is supported by a Senior Research Fellowship of the National Health and Medical Research Council of Australia.
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