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During normal haemopoiesis, only a small fraction of stem cells undergo differentiation at any given time. The remainder continue to self-renew. The molecular detail underpinning selfrenewal is poorly understood. However, it has been shown that certain transformed stem cells can be induced to undergo continuous proliferation in vitro under the influence of IL-3. The concentration of IL-3 is critical, with differentiation occurring below certain threshold concentrations of this cytokine. The delicate balance between stem cell renewal and differentiation is probably affected not only by the range of growth factors experienced but also by the concentration of each growth factor.
THE INTERLEUKINS AS HAEMOPOIETIC GROWTH FACTORS
The IL family of cytokines have been discussed in detail in Chapter 5. IL-3 is perhaps the IL that figures most prominently in the haemopoietic process, and appears to stimulate not only
CFU-GEMM, but also the precursor cells of basophils, eosinophils and platelets. The role of IL-11 has been discussed in Chapter 5.
GRANULOCYTE-COLONY STIMULATING FACTOR (G-CSF)
G-CSF is also known as pluripoietin and CSF-8. Two slight variants are known, one consisting of 174 amino acids, the other of 177. These are products of alternative splicing. The smaller polypeptide predominates, and also displays significantly greater biological activity than the larger variant.
G-CSF is a glycoprotein, displaying a single O-linked glycosylation site and an apparent molecular mass in the region of 21 kDa. The molecule is stabilized by the presence of two disulphide linkages. It displays a compact 3-D structure, featuring four a-helical bundles similar to growth hormone and IL-2. It is synthesized by various cell types (Table 6.3), and functions as a growth and differentiation factor for neutrophils and their precursor cells. It also appears to activate mature neutrophils (which are leukocytes capables of ingesting and killing bacteria). G-CSF also appears to act in synergy with additional haemopoietic growth factors to stimulate growth/differentiation of various other haemopoietic progenitor cells. In addition, this cytokine promotes the proliferation and migration of endothelial cells. The G-CSF receptor has been well characterized. It is a single transmembrane polypeptide, found on the surface of neutrophils, as well as various haemopoietic precursor cells, platelets, endothelial cells and, notably, various myeloid leukaemias (myeloid means derived from bone marrow; leukaemia refers to a cancerous condition in which there is uncontrolled overproduction of white blood cells in the bone marrow or other blood-forming organs. The white cells produced are generally immature/abnormal and the condition results in the suppression of production of healthy white blood cells).
The extracellular region of the G-CSF receptor is heavily glycosylated (containing nine potential glycosylation sites) and displays a molecular mass in the region of 150 kDa. Two variants of the receptor in humans have been noted which display differences in the intracellular
Table 6.3. Summary of some of the properties of G-CSF, M-CSF and GM-CSF
G-CSF M-CSF GM-CSF
Molecular mass (kDa) 21 45-90 22
Main producer cells Bone marrow Lymphocytes Macrophages
stromal cells Myoblasts T lymphocytes
Macrophages Osteoblasts Fibroblasts
Fibroblasts Monocytes Endothelial cells
Main target cells Neutrophils Macrophages and Haematopoietic progenitor
Also other their precursor cells
haemopoietic cells Granulocytes
progenitors and Monocytes
endothelial cells Endothelial cells
HAEMOPOIETIC GROWTH FACTORS 259
domain. The exact mode of signal transduction remains to be fully detailed. G-CSF binding does appear to prompt the phosphorylation of several cytoplasmic proteins, and an associated JAK 2 kinase has been implicated.
MACROPHAGE COLONY-STIMULATING FACTOR (M-CSF)
M-CSF serves as a growth, differentiation and activation factor for macrophages and their precursor cells. It is also known as CSF-1. This cytokine is produced by various cell types (Table 6.3). The mature form is a glycoprotein (containing three potential N-linked glycosylation sites), and its 3-D structure is stabilized by multiple disulphide linkages. Three related forms of human M-CSF have been characterized. All are ultimately derived from the same gene and share common C- and N-termini. The largest consists of 522 amino acids with the 406 and 224 amino acid forms lacking different lengths of the internal sequence of the 522 form. The molecular masses of these mature M-CSFs are in the range 45-90 kDa.