Insulin-like Growth Factors IGF-I Research Articles

Low levels of low molecular weight insulin-like growth factor-binding protein in patients with polycystic ovarian disease

Insulin-like growth factors IGF-I and IGF-II are bound to specific binding proteins in serum. The lower mol. wt binding protein (IGF-BP) has been detected in various tissues, including secretory endometrium and preovulatory follicles of the ovary. This group studied the circulating levels of IGF-BP in the serum of 23 patients with polycystic ovarian disease (PCOD) and found that one-third of them have a subnormal level. In comparison with PCOD patients with a normal level, those with a subnormal level had a higher degree of obesity and a tendency to be more hirsute. They also had a higher serum insulin concentration and testosterone/sex hormone-binding globulin (SHBG) ratio, but lower serum SHBG concentration than those with a normal IGF-BP level. PCOD is the second abnormal clinical condition, after insulinoma, in which subnormal serum IGF-BP concentrations have been reported. The significance of low serum IGF-BP levels to pathophysiology of PCOD remains to be elucidated by studies on local interaction between IGF-BP and insulin in the polycystic ovary.

Binding and degradation of 125I-insulin by isolated rat renal brush border membranes: evidence for low affinity, high capacity insulin recognition sites.

The kidney plays a major role in the handling of circulating insulin in the blood, primarily via reuptake of filtered insulin at the luminal brush border membrane. 125I-insulin associated with rat renal brush border membrane vesicles (BBV) in a time- and temperature-dependent manner accompanied by degradation of the hormone to trichloroacetic acid (TCA)-soluble fragments. Both association and degradation of 125I-insulin were linearly proportional to membrane protein concentration with virtually all of the degradative activity being membrane associated. Insulin, proinsulin and desoctapeptide insulin all inhibited the association and degradation of 125I-insulin by BBV, but these processes were not appreciably affected by the insulin-like growth factors IGF-I and IGF-II or by cytochrome c and lysozyme, low molecular weight, filterable, proteins, which are known to be reabsorbed in the renal tubules by luminal endocytosis. When the interaction of 125I-insulin with BBV was studied at various medium osmolarities (300-1100 mosM) to alter intravesicular space, association of the ligand with the vesicles was unaffected, but degradation of the ligand by the vesicles decreased progressively with increasing medium osmolarity. Therefore, association of 125I-insulin to BBV represented binding of the ligand to the membrane surface and not uptake of the hormone or its degradation products into the vesicles. Attempts to crosslink 125I-insulin to a high-affinity insulin receptor using the bifunctional reagent disuccinimidyl suberate revealed only trace amounts of an 125I-insulin-receptor complex in brush border membrane vesicles in contrast to intact renal tubules where this complex was readily observed. Both binding and degradation of 125I-insulin by brush border membranes did not reach saturation even at concentrations of insulin approaching 10(-5) M. These results indicate the presence of low-affinity, high-capacity binding sites for 125I-insulin on renal brush border membranes which can clearly distinguish insulin from the insulin-like growth factors and other low molecular weight proteins and polypeptides, but which do not differentiate insulin from its analogues as do the biological receptors for the hormone. The properties and location of these binding sites make them attractive candidates for the sites at which insulin is reabsorbed in the renal tubule.

Insulin-like growth factors are mitogens for rat pheochromocytoma PC 12 cells

We have addressed the issue of a mitogenic effect of insulin-like growth factors IGF-I and IGF-II on the PC 12 line of rat pheochromocytoma cells. The proliferation of PC 12 cells cultured in serum-free medium is stimulated threefold by IGF-I and IGF-II with significantly higher potency than epidermal growth factor, whereas platelet-derived growth factor, nerve growth factor, growth hormone and bombesin are inactive. Two types of IGF receptor are present in PC 12 cells and the dose-response curves suggest that the mitogenic responses to IGF's are mediated by the IGF-I receptor. These results suggest that IGF-I and IGF-II act as mitogens on pluripotent chromaffin cells in the development of the peripheral nervous system and adrenal medulla as well as in promotion of in vivo growth of neural crest-derived tumors.

An insulin-like growth factor (IGF) binding protein enhances the biologic response to IGF-I.

The insulin-like growth factors IGF-I and IGF-II circulate in blood bound to carrier proteins. The higher molecular mass IGF-binding protein complex (150 kDa) is composed of subunits, and one subunit that forms this complex is growth hormone dependent. In addition, many cell types and tissues secrete another form of IGF binding protein that is not growth hormone dependent. Both forms of the IGF binding protein are believed to inactivate the IGFs and to function as delivery systems to tissues. This conclusion was based on studies that determined the effects of impure preparations of these binding proteins or that examined the effect of these proteins only on the insulin-like actions of the IGFs. We report here that a pure preparation of the extracellular form of the IGF binding protein (purified from human amniotic fluid) markedly potentiated replication of several cell types in response to human IGF-I. Secondary cultures of human, mouse, and chicken embryo fibroblasts as well as porcine aortic smooth muscle cells showed marked enhancement of their DNA synthesis response (2.8- to 4.4-fold increases) to IGF-I in the presence of this protein. These responses were synergistic since the sum of the responses to either IGF-I or to the binding protein alone was between 8 and 17% of the increase obtained in cultures exposed to both peptides. The binding protein not only potentiated the DNA synthesis response but also enhanced the increase in cell number in response to IGF-I. This stimulation is specific for growth factors that bind to the binding protein since incubation with insulin, which binds to the type I IGF receptor but not to the binding protein, did not result in potentiation of this response. We conclude that a form of IGF binding protein that is present in extracellular fluids and is secreted by many types of cells can markedly potentiate the cellular response to IGF-I.

Insulin-like growth factor (IGF)/somatomedin receptor subtypes: structure, function, and relationships to insulin receptors and IGF carrier proteins.

The insulin-like growth factors IGF-I and IGF-II are mitogenic polypeptides with a high degree of chemical homology. Two distinct subtypes of receptors for the IGFs have been identified on the basis of structure and binding specificity. Type I IGF receptors bind IGF-I with equal or greater affinity than IGF-II, and also bind insulin with a low but definite affinity. They are structurally homologous to insulin receptors, containing disulfide-linked a-subunits that bind the peptides and beta-subunits that have intrinsic tyrosine-specific kinase activity. Type II IGF receptors typically bind IGF-II with greater affinity than IGF-I, and do not interact with insulin. They consist of a single polypeptide and lack tyrosine kinase activity. Because of the extensive cross-reactivity of IGF-I and IGF-II with both type I and type II receptors, we believe that potentially either receptor may mediate the biological responses of either peptide. Type I IGF receptors have been shown to mediate the mitogenic effects of the IGFs in some cell types. Whether type II IGF receptors mediate the same or different functions remains to be elucidated.

Protein utilization in growth: effect of calorie deficiency on serum growth hormone, somatomedins, total thyroxine (T4) and triiodothyronine, free T4 index, and total corticosterone.

We have studied the effect of a calorie-restricted diet on the growth of young rats and on serum levels of GH, somatomedins (insulin-like growth factors I and II), total T4 and T3, free T4 index, and total corticosterone. Experimental rats consumed the same quantities of protein and carbohydrate as control animals, but less fat, so that their calorie intake was approximately 76% that of controls. The mean (+/- SEM) GH level in the experimental group (78 +/- 21 ng/ml) was not significantly different from that in the control group (89 +/- 31 ng/ml). In contrast, serum total insulin-like growth factor and insulin-like growth factor IGF-I, while not correlated with serum GH, were significantly correlated with age and body weight (r = 0.93 and r = 0.69, respectively; P less than 0.01). The levels of these somatomedins in the calorie-restricted rats were significantly lower than those in their age-matched, but not weight-matched, controls after approximately 2 weeks of study. Serum T4, T3, and free T4 index were all significantly reduced in the experimental animals and may represent an adaptive response to calorie restriction. Serum corticosterone levels in the experimental and control rats were essentially identical. In this study, by restricting calorie intake we have compromised the ability of growing rats to use dietary protein anabolically, creating a useful model to examine in some detail nutritional influences on growth and the growth-promoting endocrine system.

The measurement of arterial smooth muscle cell mitogens in the blood of oral contraceptive users

We have previously shown that serum from young women receiving the same combined mestranol-norethindrone containing oral contraceptive (OC) preparation accelerated the proliferation of arterial smooth muscle cells (SMC) in tissue culture, and this in vitro effect was not a direct action of either of its estrogenic or progestagenic constituents. To identify the substance(s) which might contribute to this potentially atherogenic action, blood was obtained from 20 OC users and control women for the measurement of growth hormone, insulin, somatomedins (insulin-like growth factor IGF-I and IGF-II), and the platelet α-granule constituents platelet-derived growth factor (PDGF), β-thromboglobulin, and platelet factor 4 (PF4). No difference was demonstrable between OC users and controls in the levels of any of these growth-promoting hormones, nor in plasma concentrations of any of the platelet α-granule proteins. These studies indicate that the enhanced mitogenicity found in OC sera is probably not attributable directly to these hormones or PDGF, and may instead result from an in vivo OC-induced alteration in other as yet unidentified mediators of cellular growth.

Localization of insulin-like growth factor genes to human chromosomes 11 and 12

The insulin-like growth factors IGF-I and IGF-II are required for growth and development. Both are single-chain proteins (of 70 and 67 amino acids respectively) derived from precursors by proteolytic processing. IGF-I may be particularly important in promoting normal stature and IGF-II may be a fetal growth hormone. The IGF proteins are probably synthesized by many normal tissues and by some tumours. The secretion of growth factors by tumours and tumour-derived cell lines suggests that they may act as autocrine regulators of cell proliferation. Because of the possible role of these proteins in growth disorders and cancer, and their sequence homology with insulin, we have determined their chromosomal localization. Using somatic cell hybrids and cloned cDNA probes for these proteins, we have assigned the genes for IGF-I and IGF-II to human chromosomes 12 and 11, respectively. We present evidence that the IGF-II gene is located on the short arm of chromosome 11 with a ras proto-oncogene and the insulin structural gene, and also suggest the existence of a fragment length polymorphism using the IGF-I probe.

Insulin regulation of insulin-like growth factor action in rat hepatoma cells.

We have reported previously that insulin causes a complete but reversible desensitization to insulin action in rat hepatoma HTC cells in tissue culture, and that this insulin resistance is mediated by postbinding mechanisms rather than receptor down-regulation (Heaton, J. H., and Gelehrter, T. D. (1981) J. Biol. Chem. 256, 12257-12262). We report here that insulin causes a similar desensitization to the induction of tyrosine aminotransferase by the insulin-like growth factors IGF-I and IGF-II isolated from human plasma, and by multiplication-stimulating activity, the rat homologue of IGF-II. The results of both competition-binding studies and affinity cross-linking experiments indicate that insulin-like growth factors (IGFs) bind primarily to IGF receptors rather than to insulin receptors. The low concentrations at which these factors induce transaminase is consistent with their acting primarily via IGF receptors. This is confirmed by experiments utilizing anti-insulin receptor antibody which both inhibits 125I-insulin binding and shifts the concentration dependence of insulin induction of tyrosine aminotransferase to the right. This same immunoglobulin does not inhibit 125I-multiplication-stimulating activity binding and only minimally inhibits 125I-IGF-I binding. Anti-insulin receptor antibody also does not significantly shift the concentration dependence for the IGFs, suggesting that IGFs induce transaminase by acting via IGF receptors. Although insulin down regulates insulin receptors, it does not decrease IGF-I or IGF-II binding. We conclude that insulin causes desensitization of HTC cells to IGFs by affecting a postbinding step in IGF action, which may be common to the actions of both insulin and insulin-like growth factors.

Effects of human growth hormone administration on serum somatomedins, somatomedin carrier proteins, and growth rates in children with growth hormone deficiency.

We have determined the growth rates and serum somatomedin (Sm) and Sm carrier protein (CP) levels in 30 children with GH deficiency and short stature before and up to 3 yr of GH therapy. Initially, the mean (±SEM) bone age (BA) of this group (6.9 ± 0.5 yr) was significantly (P<0.001) less than the chronological age (CA; 10.4 ± 0.7 yr), and their growth rate (3.45 ± 0.33 cm/yr) was markedly less than those of normal control groups matched either for CA (5.45 ± 0.22 cm/yr) or BA (6.46 ± 0.33 cm/yr). Compared to serum levels in a group of 40 age-matched controls, we found diminished concentrations of total Sm insulin-like growth factor (IGF) activity (0.31 ± 0.03 vs. 1.61 ± 0.12 μg/ml), IGF-I (0.09 ± 0.02 vs. 0.44 ± 0.04/ig/ml), and CP (123 ± 18.8 vs. 366 ± 28.2 μg/ml). Both total IGF and IGF-I were inversely related to the degree of growth retardation, expressed as the ratio of BA to CA. Serum Sm increased in 19 of 21 subjects after short term (up to 18 h) GH administration (0.1 U/kg), but the magnitude of ...

Demonstration of receptors for insulin and insulin-like growth factors on Swarm rat chondrosarcoma chondrocytes. Evidence that insulin stimulates proteoglycan synthesis through the insulin receptor.

The insulin-like growth factor, multiplication stimulating activity (MSA), and insulin were recently shown to stimulate proteoglycan synthesis in monolayer cultures of chondrocytes derived from the Swarm rat chondrosarcoma. Insulin produced significant stimulation at a concentration of less than 1 ng/ml, suggesting that insulin was acting through the insulin receptor rather than through a somatomedin receptor. In this paper we have shown that the insulin-like growth factors IGF-I and IGF-II also are potent stimulators of [35S]sulfate incorporation into macromolecules recovered from the medium and cell layer matrix of the chondrosarcoma chondrocytes. Proinsulin was 3% as potent as insulin in stimulating [35S]sulfate incorporation. We identified receptors for insulin and the insulin-like growth factors, MSA, IGF-I, and IGF-II. Insulin, at concentrations 1000 times the concentration required to produce the biologic response, did not compete for binding of 125I-MSA-II-1 or of 125I-IGF-II and only partially competed for 125I-IGF-I binding. Anti-insulin receptor IgG stimulated proteoglycan synthesis and competed for 125I-insulin binding. Fab fragment prepared from anti-insulin receptor IgG completely blocked the stimulation of [35S]sulfate incorporation into macromolecules by insulin while only partially inhibiting the biologic response to insulin-like growth factors, MSA, IGF-I, and IGF-II. Similarly, the anti-insulin receptor IgG only partially inhibited the binding of 125I-IGF-I and 125I-IGF-II while completely blocking the binding of 125I-insulin. We conclude that insulin stimulates proteoglycan synthesis in the chondrosarcoma chondrocytes by acting through the insulin receptor whereas the insulin-like growth factors probably act through their own receptors.