About IGF, Insulin Like Growth Factor:
Both types of IGF are synthesized in many fetal and adult tissues. IGF-1 is produced constitutively in large amounts in the liver (approximately 10 mg/day). It is produced also locally in many other tissues including kidney, heart, lung, fat tissues, and various glandular tissues. IGF-1 is produced also by chondroblasts, fibroblasts, and osteoclasts.
Normal serum levels in healthy human subjects are 150-250 microgram/L for IGF-1 and 400-900 microgram/L for IGF-2. These levels may vary with age, sex, hormonal status, and nutritional status. Lowered levels of IGF-1 are observed after removal of the pituitary. Human milk also contains these factors (see: MGF, milk growth factor).
Two different IGF activities have been described. They are called IGF-1 (abbr. also ILGF1; Insulin-like growth factor-1 = somatomedin C; see also: Somatomedins) and IGF-2 (abbr. also ILGF2; Insulin-like growth factor-2).
IGF-1 has a length of 70 amino acids (7.6 kDa). An N-terminally truncated form is Des[1-3]-IGF-1. IGF-1A and IGF-1B are splice variants of IGF-1 representing exons I, II, III, V, and exons I, II, III and IV, respectively. IGF-1 Ea is expressed in muscle of rabbits and humans and is the same as the circulating IGF-IEa produced by the liver (Yang et al, 1996). IGF-1 Eb, originally identified as an isoform of hepatic IGF-1 (Roberts et al, 1987), is identical with MGF [mechano growth factor].
IGF-2 has a length of 67 amino acids (7.4 kDa). A variant of IGF-2, probably encoded by a separate gene, has been described. This protein (10 kDa) has a substitution of cys-gly-asp for ser-33 and a carboxyterminal extension of 21 residues. A peptide derived from the proform of IGF-2 is Preptin.
The structure of both IGF proteins is homologous to human pro-insulin. Approximately 50 % sequence homology are observed in individual protein domains. For peptides with biological activities different from those of the parent growth factors see also: pro-IGF-1 E peptides, pro-IGF-2 E peptide.
Both IGF factors contain three intramolecular disulfide bonds. They display approximately 62 % sequence homology with each other and 47 % identity with insulin. IGF-1 and IGF-2 do not cross-react immunologically with each other. Rat and mouse IGF-1 differ from each other by a single amino acid and differ from human IGF-1 in three and four amino acids. respectively. Rat MSA differs from IGF-2 by 5 amino acids.
IGF-1 and IGF-2 are obtained by processing of longer precursor molecules at the amino- and carboxyterminal ends. Several shortened variants have been described which presumably arise by differential processing of the precursor protein.
Bryant et al (1996) have used site-directed mutagenesis to construct pepsin-resistant muteins of an IGF-1 analogue, long-R3-IGF-1. All five muteins retain growth-promoting activity equivalent to or higher than that of IGF-1.
IGF-1 and IGF-2 are encoded by two different genes that are expressed differentially in different tissues and at different times of development. The human IGF-1 gene contains five exons and has a length of approximately 90 kb. It maps to human chromosome 12q23 in close vicinity of the gene encoding phenylalanine hydroxylase. The murine IGF-1 gene maps to chromosome 10. The human IGF-2 gene contains five exons and has a length of approximately 30 kb. It maps to chromosome 11p15 and is flanked at the 5' end by the gene encoding tyrosine hydroxylase and at the 3' end by the insulin gene. The IGF-2 and insulin genes have the same polarity and are separated by 12.6 kb of intergenic DNA that includes a dispersed middle repetitive Alu sequence.
The IGF-2 gene has been shown to be subject to parental imprinting with the paternal IGF-2 gene being expressed while the maternal allele being silent in most tissues.
IGF-1 and IGF-2 are members of a protein family of related proteins with hormonal activities including nerve growth factor (see: NGF) and the female sex hormone relaxin.
The IGF-1 receptor (IGF1R, CD221) is a transmembrane glycoprotein of 350 kDa which is generated from a precursor of 1367 amino acids. The receptor is a heterotetrameric disulfide-linked protein. It consists of two alpha (135 kDa) subunits, which bind IGF-1, and two beta (90 kDa) subunits. The beta subunit possesses intrinsic tyrosine-specific protein kinase activity. This kinase domain shows approximately 84 % homology with the insulin receptor. IGF-1 also binds to the insulin receptor and vice versa although the heterologous ligand always binds approximately 100-fold less well. Binding of IGF-1 to its receptor leads to the autophosphorylation of the beta subunit and also to the phosphorylation of some cytoplasmic substrate proteins. The gene encoding the IGF-1 receptor maps to human chromosome 15q25-q26 in the vicinity of the fes oncogene.
Araki et al (1994) have described transgenic mice that carry a targeted disruption (see also: Knock-out, ES cells) of the gene encoding IRS-1 [insulin-receptor substrate-1]. This cytoplasmic protein, known also as pp185, is the principal substrate for the insulin and IGF-1 receptors. The protein undergoes tyrosine phosphorylation at several sites and then binds to and activates phosphatidylinositol-3'-OH kinase and several other proteins containing src homology domains domains. The animals display a 50 % reduction in intrauterine growth, impaired glucose tolerance, and a decrease in glucose uptake stimulated by insulin or IGF-1 in vivo and in vitro. An alternative substrate appears to be responsible for the residual insulin/IGF-1 action. This protein, designated IRS-2, is immunologically distinct from IRS-1. This demonstrates the existence of IRS-1 dependent and -independent pathways of insulin/IGF-1 signaling.
The IGF-2 receptor (CD222) is a single-chain protein of 250 kDa. It is a high affinity binding protein for IGF-2 and shows very low affinity for IGF-1 and none for insulin. The cytoplasmic domain of the receptor does not possess intrinsic tyrosine kinase activity. This receptor is identical with cation-independent mannose-6-phosphate receptor (see: MPR) involved in lysosomal enzyme targeting. The IGF-2R gene is also subject to parental imprinting with the maternal allele being expressed. A soluble truncated form of the IGF-2 receptor has been described also. The gene maps to human chromosome 6.
A number of IGF binding proteins lacking receptor signal transduction functions have been found in serum (see: IGFBP, IGF binding proteins). These factors modulate IGF activities. Some of them also possess stimulating effects in vitro or are growth inhibitory (see: IDF-45).
Soluble receptors and binding proteins have been found also for IL1 (see: IL1ra, IL1 receptor antagonist), IL2, IL4, IL6, IL7, TNF-alpha and IFN-gamma. They probably function as physiological regulators of cytokine activities (see also: Cytokine inhibitors) by inhibiting receptor binding or act as transport proteins.
The two known IGF factors were isolated initially as serum factors with insulin-like activities that could not be neutralized by antibodies directed against insulin.
In healthy subjects exogenous IGF-1, like insulin, causes hypoglycemia and a transient decrease in free serum fatty acids. Binding of IGF-1 to carrier proteins prevents the establishment of a permanent hypoglycemia in spite of high serum IGF concentrations. These carrier proteins also increase plasma half lives of IGF and prevent the release of IGF from the blood stream into interstitial spaces.
Cell types responding to the two IGF factors include adipocytes, chondrocytes, epithelial cells, fibroblasts, glial cells, hepatocytes, muscle cells, and osteoblasts. Both forms of IGF are mitogenic in vitro for a number of mesodermal cell types. IGF-1 is usually a stronger mitogen than IGF-2. IGF-1 is an autocrine growth modulator for astrocytes and a differentiation factor for oligodendrocytes and neurons. IGF-2 is an autocrine growth factor for human rhabdomyosarcomas and also influences the motility of these cells. It is also an autocrine modulator of growth for neuroblastoma cells.
IGF influences the cellular differentiation of ovary granulosa cells. In the ovary IGF-1 enhances the activities of estrogens and androgens and upregulates the expression of receptors for Luteinizing hormone. IGF-1 and IGF-2 also promote oocyte maturation. In the male reproductive tract IGF-1 (and also EGF) stimulate androgen production in Leydig cells.
IGF-1 stimulates collagen and matrix synthesis by bone cells in vitro. The synthesis of IGF-1 in bone cells is inhibited by cortisol. IGF-1 is considered to be one of the major anabolic factors regulating the metabolism of joint cartilage in vivo.
IGF-2 has long been considered to be the fetal form of IGF-1. Although this strict separation can no longer be made IGF-2 still plays an important role as embryonic and fetal growth factor during embryogenesis.
The main factor regulating plasma concentrations of IGF is Growth hormone which regulates the expression of the IGF-1 gene and also controls the expression of some IGF binding proteins. Growth hormone stimulates the synthesis of IGF-1 in hepatocytes, fibroblasts, chondrocytes. In fibroblasts the synthesis of IGF-1 is stimulated also by PDGF and FGF. IGF-1 is more dependent on growth hormone than IGF-2 (see also: Factor-dependent cell lines). IGF-1 promotes the growth of hypophysectomized or diabetic rats and also of Snell dwarf mice with congenital growth hormone deficiency. While growth hormone is ineffective in diabetic animals their growth is enhanced by IGF-1. IGF-1 establishes a negative feedback loop in inhibiting the secretion of growth hormone. This, in turn, reduces the secretion of GHRH (growth hormone releasing hormone) and stimulates the release of Somatostatin in the hypothalamus. Serum levels of IGF-1 are also modulated by Prolactin, placental lactogen, thyroid hormones and steroids, which act, at least in part, by influencing growth hormone levels. IGF-1 is therefore responsible for many, if not all, growth-promoting activities ascribed to Growth hormone.
IGF-1 induces skeletal growth and also differentiation of myoblasts during fetal development. IGF, which is probably produced also by stromal cells of the bone marrow (see also: BMC (bone marrow culture) also influences hematopoiesis, in particular the development of the red blood cell lineage (see: BFU-E, CFU-E). In cells cultured in a serum-free medium, IGF-1 functions as a colony stimulating factor for erythroid cells (see also: CSF). The Erythropoietic factor found in the serum of anephric patients has been identified as IGF-1. In neonatal or hypophysectomized experimental animals the injection of IGF-1 also enhances erythropoiesis.
TRANSGENIC ANIMALS, KNOCK-OUT, AND ANTISENSE STUDIES
Brem et al (1994) have described the use of transgenic rabbits to express a synthetic DNA coding for human IGF-1. IGF-1 is synthesized specifically in the mammary gland in a nearly homogenous active form.
The analysis of brain growth and myelination in a transgenic mouse line that overexpresses IGF-1 indicate that IGF-1 is a potent inducer of brain growth and myelination in vivo. At postnatal day 55, when brain growth and myelination are essentially complete in normal mice, the brains of transgenic mice are 55 % larger than those of controls owing to an increase in cell size and apparently in cell number. Total myelin content of the transgenic mice is also enhanced, and this is primarily the result of an increase in myelin production by oligodendrocytes.
van Buul-Offers et al (1995) have shown that overexpression of IGF-2 in transgenic normal and pituitary deficient Snell dwarf mice causes increased growth of the thymus, suggesting a role for IGF-2 in thymic development by paracrine or autocrine mechanisms.
Kooijman et al (1995) have studied the effects of IGF-2 on the development of T-cells in two transgenic mouse lines overexpressing the human IGF-2 gene under the control of the H2Kb promoter. Overexpression of IGF-2 increases thymic cellularity and stimulates the generation of phenotypically normal T-cells with a preference to CD4(+) cells.
Mice that carry a targeted disruption of the gene encoding IGF-1 have been constructed from ES cells. Heterozygous knock-out animals are approximately 10-20 % smaller than wild-type litter mates and have lower than normal levels of IGF-1. These mice are healthy and fertile and all tissues appear histologically normal. Homozygous mutant mice are < 60 % body weight of wild type animals. More than 95 % of these animals die perinatally. Histophathology is characterized by a gross underdevelopment of muscle tissues. Lung tissues of late embryonic and neonates are less organized with ill-defined alveolae. IGF-1 thus appears to be essential for correct embryonic development in mice.
Beck et al (1995) have described mice homozygous for a disruption of the IGF-1 gene. At 2 months of age have reduced brain weights and a normal gross morphology of the CNS. The size of white matter structures in brain and spinal cord is strongly reduced due to decreased numbers of axons and oligodendrocytes. Myelinated axons are more strongly reduced in number than unmyelinated axons. A selective reduction in the number of striatal parvalbumin-containing cells is observed.
Mice that carry a targeted disruption of the gene encoding IGF-2 have been constructed from ES cells. In knock-out animals, transmission of this mutation through the male germline results in heterozygous progeny that are growth deficient. In contrast, when the disrupted gene is transmitted maternally, the heterozygous offspring are phenotypically normal. Homozygous mutants are indistinguishable in appearance from growth deficient heterozygous siblings. The analysis of transcripts from the wild-type and mutated alleles indicates that only the paternal allele is expressed in embryos, while the maternal allele is silent. An exception is the choroid plexus and leptomeninges, where both alleles are transcriptionally active. These results demonstrate that IGF-2 is indispensable for normal embryonic growth and that the IGF-2 gene is subject to tissue-specific parental imprinting. Mice carrying a targeted disruption of the IGF-1 gene also exhibit growth deficiencies similar in severity to that observed in IGF-2 null mutants. Some of the IGF-1 deficient dwarf mice die shortly after birth, but some survive and reach adulthood.
Null mutants of the IGF-1 receptor gene die invariably at birth and exhibit a more severe growth deficiency (45 % normal size. These animals have a general organ hypoplasia. Double mutants deficient in IGF-1 and IGF-1 receptor expression do not differ in phenotype from single mutants of the IGF-1 receptor. Double mutants of IGF-2 and IGF-1 receptor and of IGF-1/IGF-2 are phenotypically identical, showing a dwarfism with 30 % of the normal size.
The use of antisense oligonucleotide inhibiting production and secretion of IGF-1 by human embryonic lung fibroblasts (WI-38) has shown that IGF-1 acts as an autocrine modulator of growth for these cells.
Valentinis have studied the effects of the IGF binding protein IGFBP3 in fibroblasts with a targeted disruption of the IGF type 1 receptor gene (see also: Knock-out). Overexpression of the binding protein was found to have an inhibitory effect on cell growth which was independent of IGF binding or signaling through the type 1 IGF receptor.
DETECTION AND ASSAY METHODS
IGF can be detected by sensitive immunoassays and radioreceptor assays. Mason et al (1996) have described improved extraction procedures for the determination of IGF-2 to avoid problems associated with the presence of IGF binding proteins. IGF is also detected by measurement of sulfate incorporation into porcine cartilage cells. IGF can be detected in bioassays involving the use of the MCF-7 breast cancer cell line. For further information see also subentry "Assays" in the reference section. For further information on assays for cytokines see also: bioassays, cytokine assays.
CLINICAL USE AND SIGNIFICANCE
IGF-1 levels are used in the diagnosis of pathological states affecting levels of Growth hormone since normal levels of IGF-1 practically exclude a growth hormone deficiency. In patients with acromegaly (enhanced growth hormone levels) IGF-1 levels are also increased and these levels correlate with the severity of the disease. The importance of IGF-1 as a laboratory parameter is based on the fact that the expression of IGF-1, in contrast to growth hormone, does not show daily fluctuations. IGF-1 levels can be measured without a provocation test, in contrast to growth hormone.
IGF-1 is not found in patients with Laron-type dwarfism, a congenital endocrinological disease characterized by lacking growth hormone receptors.
IGF-1 also binds to the insulin receptor and therefore induces hypoglycemia. Growth hormone induces an increase in blood glucose levels and stimulates the secretion of insulin and may, therefore, precipitate diabetes and insulin resistance. IGF-1, on the other hand, promotes insulin actions by inhibiting growth hormone. IGF-1 may be of interest, therefore, in improving relative insulin sensitivity in metabolic diseases associated with insulin resistance such as adiposity, diabetes mellitus and hypertriglyceridemia.
IGF-1 may be of therapeutical interest also since it lowers total cholesterol/HDL ratios. There are some indications that IGF-1, unlike insulin, may not affect lipogenetic activities of insulin and the down-r3egulation of insulin receptors and also atherogenetic processes to the same extent as insulin. IGF-1 may be of interest in the treatment of renal complications associated with diabetes mellitus since IGF-1 stimulates kidney functions. The infusion of IGF-1 increases creatinine clearance and renal plasma flow by approximately 30 % without causing proteinura and without altering glomerular filtration pressures.
In degenerative joint diseases local application of IGF-1 may be of interest due to its ability to stimulate osteoblast activity and the production of collagen. The factor may be valuable also in fracture healing and in the treatment of postmenopausal osteoporosis.
It has been suggested also that centrally administered IGF-1 may have therapeutic potential for brain injury since it reduces neuronal loss after unilateral hypoxic-ischemic injury in experimental animals (see also: inflammation, wound healing).
Rat hippocampal and human cortical neurons have been shown to be protected by both forms of IGF against induced damage induced by iron, which is believed to contribute to the process of cell damage and death resulting from ischemic and traumatic insults (see also: inflammation, wound healing) by catalyzing the oxidation of protein and lipids.
It has been demonstrated that the continuous subcutaneous infusion of IGF-1 reduces gut atrophy and bacterial translocation in severely burned experimental animals. Recombinant human IGF-1 (see also: Recombinant cytokines) may be useful, therefore, to improve gut mucosal function and reduce infectious morbidity in severely traumatized or septic patients.