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The liver represents the major site of synthesis of the IGFs, from where they enter the blood stream, thereby acting in a classical endocrine fashion. A wide variety of body cells express IGF receptors, of which there are two types. Furthermore, IGFs are also synthesized in smaller quantities at numerous sites in the body and function in an autocrine or paracrine manner at these specific locations. IGF activity is also modulated by a family of IGF binding proteins (IGFBPs), of which there are at least six.
280 BIOPHARMACEUTICALS IGF biochemistry
IGF-1 and -2 were first isolated from adult human plasma, although recombinant versions of both are now available. The human IGF-1 gene is present on chromosome 12 and contains six exons. The IGF-2 gene is located on chromosome 11, adjacent to the insulin gene. It consists of nine exons. Organization and regulation of both genes is complex, with transcription potentially regulated by one of several promoters (which may facilitate tissue-specific control of expression). Differential transcripts are also observed, allowing the possible production of several species of each factor, which may differ slightly from one another. The common form of IGF-1 is a 70 amino acid polypeptide displaying three intra-chain disulphide linkages and a molecular mass of 7.6 kDa. IGF-2 (67 amino acids; 7.5 kDa) also has three disulphide linkages.
IGF-1 and -2 display identical amino acid residues at 45 positions, and exhibit in excess of 60% sequence homology. Both display A and B domains, connected by a short C domain — similar to proinsulin. However, unlike in the case of proinsulin, the IGF’s C domain is not subsequently removed. The predicted tertiary structure of both IGFs closely resemble that of proinsulin. The overall amino acid homology displayed between insulin and the two IGFs is in excess of 40%.
Hepatic transcription of IGF-1, in particular, is initiated upon binding of growth hormone (GH) to its hepatic receptors and, indeed, most of the growth-promoting actions of GH are directly mediated by IGF-1.
IGFs induce their characteristic effects by binding to specific receptors present on the surface of sensitive cells. At least three receptor types have been identified: the IGF-1 receptor, the IGF-2 receptor and the insulin receptor. As is evident from Table 7.4 (with one important exception), IGF-1, IGF-2 and insulin can bind the three receptor types, but with varying affinities. This renders delineation of which factor is inducing any one characteristic effect quite difficult.
The IGF-1 receptor is structurally similar to the insulin receptor, both in its primary and tertiary structure (Figure 7.1). As is the case for the insulin receptor, the IGF-1 receptor is encoded by a single large gene (displaying 22 exons) whose primary product is a 1367 amino acid receptor precursor. This is proteolytically processed, yielding mature a- and b-subunits, with the biologically active form being an a2b2 tetramer (Figure 7.1). The intracellular tyrosine kinase domains of the human IGF and insulin receptors display 84% amino acid homology, while the extracellular cysteine-rich domains (a-subunits) exhibit 40% homology. As is the case
Table 7.4. The various IGF receptors and the ligands capable of binding to them. AA denotes binding with high affinity (Kd = approximately 10~10M), A=weaker binding (Kd = approximately 10~8-10~9m). -=failure to bind
Receptor (R) IGF-I IGF-II Insulin
IGF-I R AA AA A
IGF-II R A AA -
Insulin R A A AA
GROWTH FACTORS 281
Tyrosine kinase domain
IGF-1 receptor Insulin receptor IGF-2 receptor
Figure 7.1. Comparison of the structure of the IGF-1, IGF-2 and the insulin receptors. Refer to text for specific details
for the insulin receptor, binding of ligand to the IGF-1 receptor a-subunit triggers activation of the b-subunit tyrosine kinase activity, resulting in autophosphorylation of several b-tyrosine residues. This, in turn, facilitates phosphorylation of several additional cytoplasmic polypeptide substrates. This triggers further intracellular events, culminating in an appropriate cellular response to ligand binding. This (IGF-1) receptor is expressed on virtually all cell types and is thus widely distributed throughout the body. It appears that it mediates the mitogenic effects of IGF-1, IGF-2 and insulin.
The IGF-2 receptor is structurally and functionally distinct from the IGF-1/insulin receptor family. The bulk of this 250 kDa receptor resides extracellularly (Figure 7.1), and its short cytoplasmic domain displays no known enzymatic activity. IGF-2 binds to this receptor with greatest affinity, whereas insulin fails to bind. The mechanism of signal transduction remains to be elucidated, although G proteins appear to play a prominent role. Its physiological significance is also less clear than that of the IGF-1 receptor. It is expressed at highest levels during fetal development, with receptor numbers declining rapidly after birth (this pattern is also paralleled by IGF-2 expression). Thus IGF-2 and its receptor may be most relevant during fetal tissue development.