Structural similarities between human receptors for somatomedin C and insulin: analysis by affinity labeling

Interaction of a viral insulin-like peptide with the IGF-1 receptor produces a natural antagonist

Lymphocystis disease virus-1 (LCDV-1) and several other Iridoviridae encode viral insulin/IGF-1 like peptides (VILPs) with high homology to human insulin and IGFs. Here we show that while single-chain (sc) and double-chain (dc) LCDV1-VILPs have very low affinity for the insulin receptor, scLCDV1-VILP has high affinity for IGF1R where it can antagonize human IGF-1 signaling, without altering insulin signaling. Consequently, scLCDV1-VILP inhibits IGF-1 induced cell proliferation and growth hormone/IGF-1 induced growth of mice in vivo. Cryo-electron microscopy reveals that scLCDV1-VILP engages IGF1R in a unique manner, inducing changes in IGF1R conformation that led to separation, rather than juxtaposition, of the transmembrane segments and hence inactivation of the receptor. Thus, scLCDV1-VILP is a natural peptide with specific antagonist properties on IGF1R signaling and may provide a new tool to guide development of hormonal analogues to treat cancers or metabolic disorders sensitive to IGF-1 without affecting glucose metabolism.

Pharmacophore modeling for identification of anti-IGF-1R drugs and in-vitro validation of fulvestrant as a potential inhibitor

Insulin-like growth factor 1 receptor (IGF-1R) is an important therapeutic target for breast cancer treatment. The alteration in the IGF-1R associated signaling network due to various genetic and environmental factors leads the system towards metastasis. The pharmacophore modeling and logical approaches have been applied to analyze the behaviour of complex regulatory network involved in breast cancer. A total of 23 inhibitors were selected to generate ligand based pharmacophore using the tool, Molecular Operating Environment (MOE). The best model consisted of three pharmacophore features: aromatic hydrophobic (HyD/Aro), hydrophobic (HyD) and hydrogen bond acceptor (HBA). This model was validated against World drug bank (WDB) database screening to identify 189 hits with the required pharmacophore features and was further screened by using Lipinski positive compounds. Finally, the most effective drug, fulvestrant, was selected. Fulvestrant is a selective estrogen receptor down regulator (SERD). This inhibitor was further studied by using both in-silico and in-vitro approaches that showed the targeted effect of fulvestrant in ER+ MCF-7 cells. Results suggested that fulvestrant has selective cytotoxic effect and a dose dependent response on IRS-1, IGF-1R, PDZK1 and ER-α in MCF-7 cells. PDZK1 can be an important inhibitory target using fulvestrant because it directly regulates IGF-1R.

The IGF system

The insulin-like growth factor (IGF) system plays essential role in the regulation of cell growth, proliferation and survival and affects nearly every organ system in the body. IGF-I, which has a high structural similarity to insulin, exerts growth-promoting effects, influences glucose metabolism and has neuroprotective and cardioprotective effects, partly because of its cell-proliferative and antiapoptotic properties. Aberrations in the IGF system may associate with various pathological conditions, including cancer. Insulin and its synthetic analogs are known to possess IGF-IR binding affinity, and concern has been raised about their mitogenic potential in humans. The present review summarizes the main aspects of the IGF system biology and the interactions among IGF-I, insulin, insulin analogs and their receptors.

Opposite effects of glucagon and insulin on compensation for spectacle lenses in chicks

PURPOSE

Chick eyes compensate for the defocus imposed by positive or negative spectacle lenses. Glucagon may signal the sign of defocus. Do insulin (or IGF-1) and glucagon act oppositely in controlling eye growth, as they do in metabolic pathways and in control of retinal neurogenesis?

METHODS

Chicks, wearing lenses or diffusers or neither over both eyes, were injected with glucagon, a glucagon antagonist, insulin, or IGF-1 in one eye (saline in the other eye). Alternatively, chicks without lenses received insulin plus glucagon in one eye, and either glucagon or insulin in the fellow eye. Ocular dimensions, refractive errors, and glycosaminoglycan synthesis were measured over 2 to 4 days.

RESULTS

Glucagon attenuated the myopic response to negative lenses or diffusers by slowing ocular elongation and thickening the choroid; in contrast, with positive lenses, it increased ocular elongation to normal levels and reduced choroidal thickening, as did a glucagon antagonist. Insulin prevented the hyperopic response to positive lenses by speeding ocular elongation and thinning the choroid. In eyes without lenses, both insulin and IGF-1 speeded, and glucagon slowed, ocular elongation, but glucagon and insulin each increased the rate of thickening of the crystalline lens. When injected together, insulin blocked choroidal thickening by glucagon, at a dose that did not, by itself, thin the choroid.

CONCLUSIONS

Glucagon and insulin (or IGF-1) cause generally opposite modulations of eye growth, with glucagon mostly increasing choroidal thickness and insulin mostly increasing ocular elongation. These effects are mutually inhibitory and depend on the visual input.

Polymorphisms in the IGF1 and IGF1R genes and children born small for gestational age: results of large population studies

Small for gestational age (SGA) is the term used to describe a group of children born with a birth weight and/or birth length below the normal range of a reference population, corrected for their gestational age. Although animal models have shown that insulin-like growth factor 1 (IGF1) and insulin-like growth factor 1 receptor (IGF1R) genes are important candidates for reduced pre- and postnatal growth, only limited case reports have been published describing mutations. This might suggest that IGF1 and IGF1R are such crucial growth factors that only common genetic polymorphisms are allowed to survive. Common IGF1 and IGF1R gene polymorphisms, such as single nucleotide polymorphisms and variable number of tandem repeats, have been investigated with conflicting results with respect to SGA-related outcomes. The exact contribution of these polymorphisms to clinical practice remains to be elucidated.

Characterisation of insulin-like growth factor receptors and insulin receptors in the human placenta using lectin affinity methods

Insulin and insulin-like growth factor receptors (IR, IGF-IR, IGF-IIR) from human placental cell membranes were solubilised and their glycoprotein properties were studied in terms of their interaction with five lectins: wheat germ agglutinin (WGA), banana lectin (BanLec), phytohaemagglutinin (PHA), concanavalin A (Con A), and Sambucus nigra agglutinin (SNA). The pattern of binding to the immobilised lectins indicated that the glycosylation of the IGF-IR, IGF-IIR and IR differed. We found several populations of receptors in placental cell membranes, differing with respect to their oligosaccharide moieties. IGF-IIR populations bore highly branched complex type N-glycans with a very high content of oligosaccharides terminating with Sia, high-mannose type N-glycans and hybrid type N-glycans. All these glycans seemed to be attached to the same IGF-II receptor molecules. Two major glycoforms of IR were detected, one having multiple highly branched N-glycans with a low content of terminal Sia and the other, having high-mannose type glycans attached to multiple N-glycosylation sites. As for the IGF-IR, multiple glycoforms were detected, bearing complex type N-glycans with various content of Sia-terminating branches, hybrid type N-glycans or high-mannose type N-glycans. The specific binding of (125)I-IGF-II to its receptor increased in the presence of immobilised WGA and SNA, which might imply the existence of a mammalian lectin counterpart whose potential physiological significance may lie in different targeting to various membrane compartments, thereby potentially modifying their cell signalling pathways.

Biomembrane-permeable and Ribonuclease-resistant siRNA with enhanced activity

Small interfering RNAs (siRNA) could be the ideal inhibitor of specific gene expression if they could be delivered efficiently to their targets. Although siRNAs cannot by themselves cross cell membranes, they can enter cells with the help of transfection reagents via facilitated transport. Inside cells, however, they can still be hydrolyzed by cytoplasmic RNases before reaching their target. Here we show that poly-2'- O-(2,4-dinitrophenyl)-siRNA (DNP-siRNA) can by itself diffuse into mammalian cells and stay in the cells for several days, with no detectable degradation. Using the DNP-RNA targeting to the insulin-like growth factor receptor (IGF-IR) as an example, we demonstrate that the efficacy of this DNP-siRNA for inhibiting the growth of three different types of tumor cells is several-fold higher than that of the corresponding native siRNA. Similar results were obtained from Western blotting assay. As expected, the control siRNAs with mismatched, scrambled, and reverse sequences are all inactive. These results illustrate that both the biomembrane permeability and intracellular stability of siRNA can be greatly improved by DNP derivatization without loss of its sequence specificity.

Molecular interactions of the IGF system

The insulin-like growth factor (IGF) system is a complex network of two soluble ligands; several cell surface transmembrane receptors and six soluble high-affinity binding-proteins. The IGF system is essential for normal embryonic and postnatal growth, and plays an important role in the function of a healthy immune system, lymphopoiesis, myogenesis and bone growth among other physiological functions. Deregulation of the IGF system leads to stimulation of cancer cell growth and survival. In order to manipulate the IGF system in the treatment of certain disorders, we must understand the protein-protein interactions at a molecular level. The complex molecular interactions of the ligands and receptors of the IGF system underlie all the biological actions mentioned above and will be the focus of this review.

Modulation of response to radiation of human lung cancer cells following insulin-like growth factor 1 receptor inactivation

Targeted disruption of the insulin-like growth factor 1 receptor (IGF-1R) restricts proliferation of tumor cells and enhances their in vitro radiosensitivity. However, there is little information regarding the effect of IGF-1R expression and function on the lung cancer response to radiotherapy. In this study, we evaluated the cell surface expression of IGF-1R and the antitumoral effect of IGF-1R blockade in combination with irradiation in 6 non-small cell lung cancer (NSCLC) cell lines. All cell lines showed specific IGF-1 binding with an affinity ranging from 0.95x10(-9) to 2.3x10(-9) M, which was evaluated by competitive binding assay. The amount of binding sites ranged from 118 to 377 fmol/mg protein. In one cell line (U1810), the combined treatment led to synergistic cell death and was associated with an accumulation of cells in the G2 phase. IGF-1R activation was able to obstruct serum starvation/radiation-induced cell death in U1810 cell line. Additive interactions were found for four cell lines (A549, H157, H23 and H125) whereas only subadditive effects were observed in U1752 cell line. Our results indicate that the IGF-1R is present on NSCLC cells and thereby its involvement in the modulation of radiosensitivity in lung cancer cells.

The three dimensional structure of the type I insulin-like growth factor receptor

Ever since the discovery of insulin and its role in the regulation of glucose metabolism, there has been great interest in the molecule itself, the insulin-like growth factors (IGFs), and their receptors (IR and IGF-R). These receptors form a subfamily of tyrosine kinase receptors which are large, transmembrane proteins consisting of several structural domains. Their ectodomains have a similar arrangement of two homologous domains (L1 and L2) separated by a Cys rich region. The C-terminal half of their ectodomains consists of three fibronectin type 3 repeats, and an insert domain that contains the alpha-beta cleavage site. This review summarises the key developments in the understanding of the structure of this family of receptors and their relation to other multidomain proteins. Data presented will include multiple sequence analyses, single molecule electron microscope images of the IGF-1R, insulin receptor (IR), and IR-Fab complexes, and the three dimensional structure of the first three domains of the IGF-1R determined to 2.6 A resolution by x ray crystallography. The L domains each adopt a compact shape consisting of a single stranded, right handed beta-helix. The Cys rich region is composed of eight disulphide bonded modules, seven of which form a rod shaped domain with modules associated in an unusual manner.

Structure and function of the insulin-like growth factor I receptor

Insulin-like growth factors I and II (IGF-I, IGF-II) were originally identified as potent mitogens and as the mediators of growth hormone action. Besides being mitogenic, however, these polypeptide growth factors play a crucial role in cell survival, and contribute to transformation and to maintenance of the malignant phenotype. Here we will discuss signaling by the IGFs, focusing specifically on the structure and function of the IGF-I receptor and the domains of this receptor responsible for distinct IGF functions: mitogenesis, transformation, and protection from apoptosis. We will also compare the structural domains of the related but functionally distinct receptor for insulin.

Insulin as a growth factor

Various clinical syndromes illustrate the essential role of insulin in modulating somatic growth both in utero and after birth. The effect of insulin on growth is a consequence of direct effects transduced via its homologous receptor and post-receptor signaling pathways and indirect effects on other modulators of growth, such as the growth hormone-IGF axis. Recent insights into the post-receptor mechanisms of insulin signaling provide a scientific framework for the distinction between the traditional role of insulin as a major modulator of metabolism and its role as a promoter of growth.

Insulin-like growth factor-I receptor increases in aortic endothelial cells from diabetic rats

Endothelial cells are likely to play an important role in the development of diabetic vascular diseases, since they are exposed directly to the abnormal circulating metabolites of diabetes and may be easily damaged early in the natural course of vascular complications. In this study, aortic endothelial cells were cultured from diabetic BB rats. Their binding and internalization of insulin-like growth factor-I (IGF-I) were measured. IGF-I binding was higher in cells of diabetic rats than of control rats at both 37 degrees C (4.5% +/- 1.6% v 2.74% +/- 0.9% per mg protein, P < .05) and 4 degrees C (20.6% +/- 5.6% v 13.7% +/- 4.6% per mg protein, P < .01). Internalization of IGF-I also increased (1.62% +/- 0.2% v 0.74% +/- 0.15% of total count at 37 degrees C after 60 minutes, P < .05). Cross-linking studies showed that in cells from diabetic rats, the major band of 140 kd corresponding to the alpha-subunit of the IGF-I receptor increased in density by 50% compared with those from control rats. The IGF-I-stimulated tyrosine kinase activity (TKA) of partially purified receptor from cells of diabetic rats, measured using poly-glu-tyr as substrate, was normal. Since the biological effects of IGF-I are initiated by its binding to the IGF-I receptor, which is able to transduce mitogenic and metabolic signals, our results support the hypothesis that the IGF-I receptor is involved in the development of diabetic vascular complications.

Three-dimensional structure of human insulin-like growth factor-I (IGF-I) determined by 1H-NMR and distance geometry

The three-dimensional structure of human insulin-like growth factor-I has been determined through a combination of NMR measurements and distance geometry calculations. A total of 320 interatomic distance constraints, including 12 related to the disulfide bridges, were used in these calculations. The resulting structure is characterized by the presence of three helical rods corresponding to the sequence regions, Ala8-Cys18, Gly42-Cys48 and Leu54-Cys61. Furthermore, a turn structure and an extended structure exist in the Gly19-Gly22 and Phe23-Asn26 regions, respectively. Neglecting the N- and C-termini, with their expectedly high degree of mobility as well as a fluctuating C-domain, the r.m.s.d. value is 1.9 A for backbone atoms. Those of the three alpha-helical regions are 1.0, 0.9 and 0.8 A, respectively, 1.8 A being that for the total backbone atoms participating in the formation of these three helices, showing the good convergence of their spatial arrangements. The overall structure obtained here shows that the human IGF-I molecule folds into a spatial structure very similar to that of insulin in an aqueous solution.

Tyrosine kinase activity of insulin-like growth factor I and insulin receptors in human endometrium during the menstrual cycle: cyclic variation of insulin receptor expression

OBJECTIVE

To determine whether expression and binding or signaling characteristics of endometrial insulin-like growth factor I (IGF-I) and insulin receptors are modulated throughout the menstrual cycle.

SETTING

Research laboratories of a university hospital and of the Institute for Diabetes Research.

DESIGN

In vitro receptor binding and phosphorylation studies of human proliferative and secretory endometrial tissue and of cultured endometrial stromal cells.

MAIN OUTCOME MEASURES

Binding and tyrosine kinase activation of IGF-I and insulin were studied in wheat germ agglutinin purified receptor protein. Binding data were analyzed by Scatchard plots. Autophosphorylation was measured by 32P incorporation into the 95 kd receptor beta-subunit; substrate phosphorylation was determined with poly(GluNa 4: Tyr 1).

RESULTS

Binding studies revealed no differences of the affinities between cycle phases. Half-maximal displacement of both receptors was approximately 1 nM in both phases. Insulin-like growth factor I receptor number appeared to be unaltered in both phases, whereas insulin receptor numbers and tyrosine kinase activity in the secretory phase were significantly increased. The increase of tyrosine kinase activity was entirely because of the increased receptor number as calculated from the binding data. In cultured endometrial stromal cells the increase of expression of insulin receptors could be induced by sexual steroids.

CONCLUSIONS

On the receptor level IGF-I signaling to human endometrium is not modulated during the menstrual cycle, whereas insulin binding and signaling are likely to be enhanced in the luteal phase. The increased insulin receptor level may be required for normal luteal function.

Puberty and polycystic ovarian syndrome: the insulin/insulin-like growth factor I hypothesis

OBJECTIVES

To provide an up-to-date review of studies that have examined the physiological effects of insulin and insulin-like growth factor I (IGF-I) on ovarian growth, maturation, and steroid synthesis, their physiological role in puberty, and their pathophysiological role in polycystic ovarian syndrome (PCOS). To deduce from these data a hypothesis, explaining the pathogenetic connections between puberty and PCOS.

DATA IDENTIFICATION

The most relevant studies related to this topic have been identified through a computerized bibliographic search (MEDLINE) and through manual scanning of what has been published during recent years in the most important journals in the field of reproductive endocrinology.

RESULTS

Insulin and IGF-I stimulate ovarian growth and potentiate the actions of gonadotropins on ovarian steroid synthesis. Insulin also augments the bioactive concentrations of IGF-I and androgens through regulation of the synthesis of their respective binding proteins insulin-like growth factor-1 binding protein (IGFBP-1) and sex hormone-binding globulin (SHBG) in the liver. Insulin and IGF-I might also be able to increase the adrenal sensitivity to adrenocorticotropic hormone (ACTH). Insulin resistance with compensating hyperinsulinism is a common feature of PCOS. It is also a normal phenomenon during puberty. Polycystic ovarian syndrome often develops during puberty. Ultrasonographic investigations suggest that it is much more common during adolescence than generally assumed. Actually, there is a striking resemblance between the endocrine characteristics of puberty and some forms of PCOS. Both conditions are characterized by insulin resistance, hyperpulsatile gonadotropin secretion, hyperactive ovarian and adrenal androgen synthesis, and decreased levels of IGFBP-1 and SHBG.

CONCLUSION

We propose the progressively increasing insulin levels and IGF-I activity during puberty as inducing factors in the development of PCOS in susceptible subjects.

The insulin-like growth factor family of ligands, receptors, and binding proteins

The insulin-like growth factors (IGFs) have important roles in normal cellular growth and development. The IGFs have also been implicated in regulation of tumor cell growth. Two ligands, IGF-I and IGF-II, have been identified that are expressed in both fetal and adult tissues. They interact with at least two specific cell surface receptors. The type I IGF receptor is homologous to the insulin receptor in structure and has tyrosine kinase activity. The type II receptor is identical to the mannose-6-phosphate receptor known to be important in the trafficking of lysosomal enzymes; its role in IGF signal transduction is not clear. Furthermore, a hybrid receptor composed of subunits from the insulin receptor and the type I IGF receptor have been identified. In addition to these receptors, six different IGF binding proteins have been identified, which modulate the activity of the IGFs in various ways. Thus, there is great potential for complex interactions between the family members that could ultimately regulate normal and neoplastic cell growth.

A monoclonal antibody to the T-cell receptor increases IGF-I receptor content in normal T-lymphocytes: comparison with phytohemagglutinin

The biological effects of the IGFs are mediated through interaction with specific cell surface receptors. It has been previously reported that mitogenic activation of T-lymphocytes by phytohemagglutinin (PHA) is associated with increased IGF-I receptor content. However, the mechanisms which regulate IGF-I receptor expression during T-lymphocyte activation are unknown. To explore further the regulation of IGF-I receptor expression in T-cells, we investigated IGF-I receptor content and mRNA abundance in T-lymphocytes after stimulation either by PHA or OKT-3, the latter being a monoclonal antibody directed against the CD-3 antigen of the T-cell receptor. IGF-I binding in T-cells demonstrated increased IGF-I receptor content after stimulation by both PHA and OKT-3. Peak binding was induced after 72 h of treatment with PHA and 48 h of treatment with OKT-3. Affinity cross-linking of 125I-IGF-I to T-cell membranes demonstrated a single approximately 130 kDa band which was increased after treatment with PHA or OKT-3. This band was inhibited by the addition of alpha-IR3, a monoclonal antibody to the IGF-I receptor. Both PHA and OKT-3 increased IGF-I receptor mRNA abundance with peak increases at 20 h and 60 h, respectively. Parallel increases in IGF-I receptor and beta-actin mRNA abundance were observed, consistent with previous studies demonstrating increased actin gene expression after T-cell activation. Thus, the increase in IGF-I receptor mRNA abundance markedly preceded the increase in IGF-I receptor content after PHA stimulation, but the increase in IGF-I receptor mRNA abundance followed the increase in IGF-I receptor content after OKT-3.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of insulin-like growth factor (IGF) I receptor expression during muscle cell differentiation. Potential autocrine role of IGF-II

Muscle is an important target tissue for insulin-like growth factor (IGF) action. The presence of specific, high affinity IGF receptors, as well as the expression of IGF peptides and binding proteins by muscle suggest that a significant component of IGF action in this tissue is mediated through autocrine and/or paracrine mechanisms. To explore autocrine/paracrine action of IGFs in muscle, we studied the regulation of the IGF-I receptor and the expression of IGF peptides during differentiation of the mouse BC3H-1 muscle cell line. Differentiation from myoblasts to myocytes was associated with a 60% decrease in IGF-I receptor sites determined by Scatchard analysis. Analysis of mRNA abundance and protein labeling studies indicated that the decrease in IGF-I receptor sites was associated with similar reductions in IGF-I receptor gene expression and receptor biosynthesis. IGF-II peptide gene expression was detected in myoblasts and increased 15-fold with differentiation; the increase in IGF-II gene expression preceded the decrease in IGF-I receptor gene expression. In contrast, IGF-I peptide gene expression was low in myoblasts and decreased slightly with differentiation. To explore the potential role of endogenous IGF-II in the differentiation-associated decrease in IGF-I receptor expression, we investigated the effects of IGF-II treatment in myoblasts. The addition of IGF-II to undifferentiated myoblasts resulted in downregulation of the IGF-I receptor which was associated with decreased IGF-I receptor biosynthesis and decreased IGF-I receptor mRNA abundance. These studies suggest, therefore, that IGF-I receptor expression during muscle cell differentiation may be regulated, at least in part, through autocrine production of IGF-II.

Distribution of insulin-like growth factor 1 (IGF-1) and 2 (IGF-2) receptors in the hippocampal formation of rats and mice

This study demonstrated species differences in IGF-1 and IGF-2 receptor binding and localization in the hippocampus of the rat and mouse. Competition binding studies indicated that there were no differences in the relative binding affinities for the type 1 or type 2 receptors between the brains of these animals. These results suggested that the observed species differences were not attributable to alterations in IGF receptor kinetics. Receptor autoradiographic analyses demonstrated that IGF-1 binding differed in both the localization and overall receptor densities observed, with the rat demonstrating more specific localization and greater receptor density in the hippocampus than the mouse. The rat also exhibited a greater density of IGF-2 receptors in the hippocampus than the mouse. Despite differences in IGF receptor populations, both species exhibit similar hippocampal structure and lamination. Therefore, these results demonstrate a disparity in the localization of IGF receptor binding in the rat and mouse, suggesting that IGFs in these species are differentially regulated, with distinct neuromodulatory, neurotrophic, and/or developmental roles in this region of the brain. Previous comparative anatomical studies of the hippocampal formation of rats and mice fail to offer an explanation for the absence or reduction of binding of IGF-1 in the mouse. Although the mouse has a greater cell density in the s. granulosum than the rat, and both species exhibit similar glia and synaptic contact densities in the s. moleculare of the dentate gyrus, the mouse exhibits a complete absence of IGF-1 binding in this region. The lack of anatomical differences in the hippocampal formation of these species suggests that the patterns observed in IGF binding result from alterations in either neurochemical modulation of these neurons or specific neurotrophic requirements of the cells in this region. Differences have been reported on the concentrations and binding of various neurotransmitters in the hippocampus of these species, however these differences do not easily account for the variations observed in IGF binding in this study. IGFs are known to influence acetylcholine neurotransmission in the hippocampus as well as other brain areas in the rat. Recently, a truncated form of IGF-1, in which a tripeptide is cleaved from the N-terminus of the peptide, has been reported in brain. The cleaved tripeptide has been shown to activate glutamate receptors, which may dramatically influence excitatory neurotransmission in this region. Therefore, in addition to the possible neurotrophic actions of the peptide itself, subsequent processing of IGF-1 may be an important aspect of IGF-1 activity in the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Canine tracheal epithelial cells express the type 1 insulin-like growth factor receptor and proliferate in response to insulin-like growth factor I

Disaggregated airway epithelial cells replicate in serum-free media containing supraphysiologic concentrations of insulin. To examine the hypothesis that the type 1 insulin-like growth factor (IGF) receptor mediates the mitogenic action of insulin on these cells, we studied the mitogenic effects of IGF-I and insulin, and the expression of type 1 IGF receptors in primary cultures of adult canine tracheal epithelial cells. Isolated tracheal epithelial cells were grown in varying concentrations of IGF-I or insulin in Ham's F12 medium supplemented with transferrin, cholera toxin, and endothelial cell growth supplement. Both IGF-I and insulin increased DNA synthesis (measured as [3H]thymidine incorporation into DNA) and cell number in a concentration-dependent fashion, but IGF-I was at least 20 to 60 times more potent than insulin in its mitogenic effects. No additive or synergistic effect was observed with the simultaneous addition of IGF-I and insulin in maximally effective doses. A monoclonal antibody directed against the type 1 IGF receptor (alpha IR3) blocked the mitogenic activity of both IGF-I and insulin. Affinity labeling of type 1 IGF receptors by covalent cross-linking with disuccinimidyl suberate demonstrated the tracheal epithelial cell IGF-I binding moiety to have a relative molecular weight of 130,000 D. Binding of [125I]IGF-I to this protein was inhibited by low concentrations of IGF-I, relative to insulin, and by alpha IR3. An 11-kb transcript characteristic of mRNA for the type 1 IGF receptor was recognized in poly(A+) RNA derived from cultured canine tracheal epithelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of insulin-like growth factor I with turkey satellite cells and satellite cell-derived myotubes

Satellite cells, isolated from the superficial pectoralis muscle of growing Nicholas tom turkeys, were cloned to obtain a pure population of myogenic cells. These cells proliferated rapidly and differentiated (fused) into myotubes typically containing 92-98% fused nuclei. Competitive binding assays were performed on near-confluent satellite cell or myotube cultures in 35 mm diameter wells by adding [125I]IGF-I along with increasing concentrations of unlabeled IGF-I, IGF-II, or insulin. Following incubation, the cultures were washed to remove the unbound hormones, solubilized with 0.5 N NaOH, and the radioactivity specifically bound was determined. Total and fused nuclei number as well as total protein were determined in parallel cultures. Our results indicate that turkey satellite cell and myotube cultures possess specific binding sites for IGF-I. Displacement of [125I]IGF-I was in the order of IGF-I greater than IGF-II greater than or equal to insulin. Although the [125I]IGF-I association constants were similar for turkey satellite cells and myotubes, a 2.8-fold decrease in the number of receptors per nuclei was observed as satellite cells differentiated into myotubes. The 50% inhibition constants for IGF-I, IGF-II, and insulin were 3.7 X 10(-9) M, 7.5 X 10(-8) M, and 8.7 X 10(-8) M for satellite cells and 3.1 X 10(-9) M, 7.5 X 10(-8) M, and 9.6 X 10(-8) M for myotubes, respectively. Receptor cross-linking analysis using disuccinimidyl suberate was performed on near-confluent satellite cell cultures incubated with [125I]IGF-I in the presence or absence of 1 X 10(-7) M IGF-I, IGF-II, or insulin. Receptor subunit species of Mr 130 kDa and 98 kDa were observed under reducing conditions (100 mM dithiothreitol) and at a Mr greater than 300 kDa (native receptor tetramer) under non-reduced conditions. Autoradiographic bands were displaced with IGF-I but not with equimolar levels of IGF-II or insulin. The results suggest that turkey satellite cells possess a type I IGF receptor.

The phosphorylation of p68, a calcium-binding protein associated with the human syncytiotrophoblast submembranous cytoskeleton, is modulated by growth factors, activators of protein kinase C and cyclic AMP

The phosphorylation of the lipocortin-related protein, p68, found in Ca2+-dependent association with the submembranous cytoskeleton has been studied using isolated human placental syncytiotrophoblast plasma membrane vesicles. p68 undergoes rapid, cation-independent phosphorylation in unstimulated membrane vesicles which was inhibited, in a dose-dependent manner, by insulin, platelet-derived growth factor, macrophage colony stimulating factor, protein kinase C-activating phorbol esters and phosphatidylinositol-specific phospholipase C. Epidermal growth factor had no effect on overall p68 phosphorylation. Transferrin induced an increase in p68 phosphorylation. However, phosphotyrosine was detected in p68 after treatment with epidermal growth factor, macrophage colony stimulating factor or transferrin, whereas a reduction in p68 phosphorylation appeared to be restricted to serine. cAMP and both cholera and pertussis toxins inhibited p68 phosphorylation. Both toxins were synergistic with the effects of insulin and platelet-derived growth factor whilst being antagonistic to the effect of transferrin. Epidermal growth factor and both human and equine immunoglobulin G, all of which alone did not affect overall p68 phosphorylation, reduced cholera or pertussis toxin-induced inhibition of p68 phosphorylation. Several phosphatase inhibitors failed to prevent macrophage colony stimulating factor-induced reduction of p68 phosphorylation. These results indicate that (i) p68 is a potential substrate of receptor tyrosyl kinases, (ii) p68 is not phosphorylated by protein kinase C or cAMP-dependent kinase and (iii) p68 phosphorylation is inhibited by activation of multiple pathways including those employing diacylglycerol or cAMP as second messengers.

Production of insulin-like growth factor binding proteins by small-cell lung cancer cell lines

Conditioned serum-free media (CM) from small-cell lung cancer (SCLC) cell lines were examined for the presence of insulin-like growth-factor-binding proteins (IGF-BP). 6/9 SCLC cell lines secreted binding proteins with high affinity for IGFs. When [125I]IGF-I or [125I]IGF-II was incubated with the CMs, complexes of tracer with proteins could be demonstrated by gel filtration, by precipitation with polyethylenglycol, and after adsorption of unbound tracer with activated charcoal. Analysis of binding data according to the method of Scatchard resulted in linear plots for IGF-I and IGF-II. The dissociation constants were determined to be 0.106 nM for IGF-I and 0.209 nM for IGF-II binding. Cross-linking of [125I]IGF-I or [125I]IGF-II to the CMs followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions revealed the presence of IGF-BPs with molecular masses in the range 24-32 kDa. The binding was competitively inhibited by addition of cold IGF-I and IGF-II but not by insulin. Northern blot hybridization with an IGF-BP cDNA probe encoding a low-molecular-weight IGF-BP from a human placenta cDNA library and Western blot analysis with a corresponding polyclonal antibody showed no expression of this gene. These data demonstrate that SCLC cell lines release IGF-BPs in culture supernatants, which differ from IGF-BPs detected in liver and placenta. These IGF-BPs might be important mediators in the autocrine/paracrine growth regulation of IGFs in SCLC.

Characterization of the receptor for insulin-like growth factor II in bone cells

We have previously shown that insulin-like growth factor II (IGF-II) is produced by bone cells and that IGF-II stimulates cell proliferation and collagen synthesis in bone cells. We now extend these in vitro findings by demonstrating specific IGF-II binding to bone cells derived from newborn mouse calvaria and embryonic chick calvaria. The kinetics of [125I] IGF-II binding in embryonic chick calvaria cells showed time and temperature dependence. Scatchard analysis of [125I]IGF-II binding to chick calvaria cells showed an apparent Kd of 1.4 x 10(-10) M, with a calculated receptor site concentration of 40,000/cell. The specificity characteristics showed that IGF-II was significantly more potent than IGF-I or insulin in displacing IGF-II tracer. Competition for binding of [125I]IGF-II by unlabeled IGF-II showed a dose-dependent displacement between 0.5 and 25 ng/ml. Fifty percent displacement of [125I]IGF-II binding to chick and mouse calvarial cells was achieved at 1-2 ng/ml; 90% of specific binding of [125I]IGF-II was displaceable in the presence of 125 ng/ml of unlabeled IGF-II. IGF-I showed less than 5% cross reactivity for displacement of [125I]IGF-II binding to chick and mouse bone cells. Type II receptor inhibitory antibodies, R-II-PAB1 inhibited the binding of [125I]IGF-II to mouse bone cells and H-35 rat hepatoma cells (which contain type II but not type I receptors) in a dose-dependent manner. R-II-PAB1 also inhibited basal cell proliferation as well as IGF-II-, IGF-I-, and fibroblast growth factor (FGF)-induced cell proliferation in mouse bone cells. In chick calvaria bone cells and TE89 human osteosarcoma cells, R-II-PABI inhibited neither binding of [125I]IGF-II nor IGF-II-induced cell proliferation. These results together with our findings that IGF-II increased chick bone cell proliferation in the presence of maximal doses of IGF-I suggest that at least part of the mitogenic action of IGF-II is mediated through type II rather than type I receptors in bone cells.

Dexamethasone effects on creatine kinase activity and insulin-like growth factor receptors in cultured muscle cells

We examined the effects of dexamethasone on creatine kinase (CK) activity and insulin-like growth factor I (IGF-I) binding in two skeletal muscle-derived cell lines (mouse, C2C12; rat, L6) and in one cardiac muscle-derived cell line (rat, H9c2). Dexamethasone treatment during differentiation of cultured cells caused a dose-dependent increase in CK activity as well as an increase in the degree of myotube formation in C2C12 and L6, whereas H9c2 cells did not exhibit significant CK activities during culture or dexamethasone treatment. Dexamethasone treatment of C2C12 did not stimulate proliferation in differentiating cultures, but a dose-dependent increase in the number of nuclei was observed for L6 concomitant with increased CK activity. In L6 the increased CK activity may therefore reflect a dose-dependent increase in proliferation. Short-term (48 hr) treatment of C2C12 with dexamethasone (20 nM) did not appear to alter myoblast fusion but reversibly increased CK activity. In C2C12 the observed increase in CK, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) activities with dexamethasone treatment suggest modulation of protein expression and/or turnover. Although the data for dexamethasone effects on CK activities varied in each of the cell lines, consistent behavior was observed in all three cell lines when IGF-I binding was examined. IGF-I binding to dexamethasone-treated cells (50 nM for 24 hr the day prior to confluence) resulted in an increased number of available binding sites, with no effect on the binding affinities. Affinity cross linking and autoradiography indicated that the increase in IGF-I binding was the result of dexamethasone up-regulation of type I IGF receptors. Our data for all three muscle cell lines suggest that similar heterologous hormone receptor modulation of type I IGF receptor sites occurs with dexamethasone treatment.

Actions of insulin-like growth factor-I on the B104 neuronal cell line: effects on cell replication, receptor characteristics, and influence of secreted binding protein on ligand binding

Several peptide growth factors influence the growth and differentiation of neural cells. To investigate further the growth-promoting effects of the somatomedins on cells of neural origin, the authors characterized the binding and mitogenic effects of insulin-like growth factor-I (IGF-I) on a functionally differentiated rat neuronal cell line (B104). Specific, high-affinity (Kd approximately equal to 10(-9) M) receptors for IGF-I were abundant (approximately 124,000 binding sites/B104 cell). These IGF-I receptors were similar to those of non-neural tissue in that they contained 135,000 dalton binding subunits (demonstrated by affinity labeling and autoradiography) and recognized insulin at high concentrations. IGF-I was more potent than insulin at stimulating B104 cell replication in serum-free medium and, at an initial concentration of 100 ng/ml, was the only exogenous growth factor needed to maintain growth through several cell divisions. Furthermore, cells of later passage were found to secrete specific IGF binding proteins that produced an unusual, biphasic binding curve in radioligand displacement studies. These binding proteins apparently sequester IGF-I, limiting its access to the cell. Experiments with B104 cells may provide useful information about the role of IGFs and their binding proteins as potential regulators of growth and differentiation of the primitive neuroblast.

Characterization of receptors for insulin and insulin-like growth factor-1 on FRTL-5 thyroid cells

FRTL-5 rat thyroid cells have receptors for both insulin and IGF-I which can be distinguished in binding studies. The ability of TSH to regulate each in an antiparallel manner is atypical. If these receptors are shown to have independent as well as coordinate activities, studies of the mechanisms of their receptor cross-talk in these cells will be relevant to understanding IGF-I and insulin receptors in other tissues.

Molecular aspects of insulin-like growth factors, their binding proteins and receptors

As we have tried to illustrate in the preceding brief review of some of the current research on the molecular biology of the IGF system, the physiological function of these important and pluripotent molecules will undoubtedly prove to be extraordinarily complex. This prediction is based upon the extensive heterogeneity of the IGF-I and IGF-II ligands themselves, the multiplicity of BPs which may influence IGF action either positively or negatively at numerous levels, and the ability of these hormones/growth factors (and possibly their BPs) to interact with disparate receptor moieties, both singly and in concert, in order to elicit their various effects.

IGF-I receptors in the bovine neural retina: structure, kinase activity and comparison with retinal insulin receptors

The retina contains specific high-affinity receptors for insulin-like growth factor-I (IGF-I). Although IGF-I binding was observed in photoreceptor outer segments, the level of this binding was only 10% of that found in whole retina or mixed preparations of rod outer (ROS) and inner (RIS) segments. The higher IGF-I binding activity in RIS and non-photoreceptor regions of the retina suggests these sites as candidates for putative IGF-I action. Data from crosslinking experiments with and without neuraminidase treatment indicate that the binding subunits of the retinal IGF-I receptor exist in two subpopulations (Mr = 121- and 131 kDa), and that the larger of the two subunits has either a greater number or more exposed sialic acid residues. In these characteristics, the retinal IGF-I receptor is similar to the retinal insulin receptor. Retinal IGF-I and insulin receptors possess kinase activity towards their own beta-subunits, a tyrosine containing copolymer, and various molecular forms and subunits of transducin (T alpha-GDP, T alpha-GTP, T beta). The transducin forms are phosphorylated with different efficiencies (e.g. T alpha-GDP is 10-15 times more effective than T alpha-GTP as substrate). These differences are also observed in basal conditions and may reflect differences in transducin subunit affinity for the IGF-I and insulin receptor. In all retinal areas examined, tracer IGF-I binding is 10 to 20-fold higher than insulin binding; however, autophosphorylation levels are approximately equal.

Structural and functional characterization of the human T lymphocyte receptor for insulin-like growth factor I in vitro

Growth factor receptors for T lymphocytes, such as interleukin 2 and insulin, are present on activated but not resting T lymphocytes. We sought to determine if insulin-like growth factor I (IGF-I) could act as a growth factor for human T cells and to characterize its receptor on resting and activated cells. Recombinant IGF-I induced two separate functions. It was chemotactic for and increased incorporation of tritiated thymidine into both unactivated (resting) and mitogen-activated T cells. High-affinity 125I-IGF-I binding to human T cells was saturable with an apparent Kd of 1.2 +/- .6 X 10(-10) M for binding to activated T cells and 1.2 +/- .9 X 10(-10) for unactivated T cells. The calculated binding for activated cells was 330 +/- 90 and for resting cells 45 +/- 9 high-affinity receptor sites per cell. Affinity cross-linking of 125I-IGF-I to resting or activated T cells revealed a radioligand-receptor complex of 360,000 mol wt when analyzed by SDS-PAGE without reduction and complexes of 270,000 and 135,000 mol wt upon reduction; prior incubation with excess unlabeled IGF-I prevented formation of the 125I-IGF-I receptor complex. Our data suggest that both resting and activated T lymphocytes bear functional IGF-I receptors similar to those found in other tissues. These receptors may mediate T cell growth and chemotaxis.

Insulin and insulin-like growth factor 1 receptors during postnatal development of rat brain

The binding of insulin and insulin-like growth factor 1 (IGF1) to high-affinity sites in the brain of rats aged 2-37 days was studied. Specific binding of insulin and IGF1 was assessed using tracer concentrations of 125I-insulin or 125I-IGF1. Sites for insulin and IGF1 were distinguished in these conditions as shown by competition experiments. The Kd were 3.6 nM (insulin) and 2.0 nM (IGF1). These values did not change significantly over the age range studied. The numbers of high-affinity binding sites for insulin and IGF1 were similar in adult animals. IGF1 binding was higher than the insulin binding in 2-day-old animals. The binding capacity for both insulin and IGF1 decreased from birth to age 15 and days remained stable thereafter. Tyrosine kinase activity, which is associated with these receptors, was measured using the artificial substrate poly (Glu, Tyr). It decreased over the first 15 days of life and remained stable thereafter. Autophosphorylation of the receptors confirmed this result. This decrease appears to be due to changes in the numbers of the two types of receptors, and is probably a reflection mainly of the variation in the number of IGF1 receptors. Similar results for insulin and IGF1 binding as well as tyrosine kinase activity were obtained with hypothyroid rats.

Production of insulin-like growth factor binding protein(s) (IGF-BPs) by human placenta: variation with gestational age

Preterm human placentae produce insulin-like growth factor- (IGF-)specific binding protein(s). Competitive binding experiments revealed at least two distinct, high-affinity binding sites for IGF-I and IGF-II. Gel permeation chromatography and sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis of the binding protein(s) suggested that they possess a molecular weight in the 45 to 50 kD range. In contrast, term placental tissue produced almost no detectable binding protein activity.