Insulin-like growth factor (IGF) binding to human fibroblast and glioblastoma cells: the modulating effect of cell released IGF binding proteins (IGFBPs)

Obesity-Associated Breast Cancer: Analysis of Risk Factors and Current Clinical Evaluation

Several studies show that a significantly stronger association is obvious between increased body mass index (BMI) and higher breast cancer incidence. Additionally, obese and postmenopausal women are at higher risk of all-cause and breast cancer-specific mortality compared with non-obese women with breast cancer. In this context, increased levels of estrogens, excessive aromatization activity of the adipose tissue, overexpression of pro-inflammatory cytokines, insulin resistance, adipocyte-derived adipokines, hypercholesterolemia, and excessive oxidative stress contribute to the development of breast cancer in obese women. Genetic evaluation is an integral part of diagnosis and treatment for patients with breast cancer. Despite trimodality therapy, the four-year cumulative incidence of regional recurrence is significantly higher. Axillary lymph nodes as well as primary lesions have diagnostic, prognostic, and therapeutic significance for the management of breast cancer. In clinical setting, because of the obese population primary lesions and enlarged lymph nodes could be less palpable, the diagnosis may be challenging due to misinterpretation of physical findings. Thereby, a nomogram has been created as the "Breast Imaging Reporting and Data System" (BI-RADS) to increase agreement and decision-making consistency between mammography and ultrasonography (USG) experts. Additionally, the "breast density classification system," "artificial intelligence risk scores," ligand-targeted receptor probes," "digital breast tomosynthesis," "diffusion-weighted imaging," "18F-fluoro-2-deoxy-D-glucose positron emission tomography," and "dynamic contrast-enhanced magnetic resonance imaging (MRI)" are important techniques for the earlier detection of breast cancers and to reduce false-positive results. A high concordance between estrogen receptor (ER) and progesterone receptor (PR) status evaluated in preoperative percutaneous core needle biopsy and surgical specimens is demonstrated. Breast cancer surgery has become increasingly conservative; however, mastectomy may be combined with any axillary procedures, such as sentinel lymph node biopsy (SLNB) and/or axillary lymph node dissection whenever is required. As a rule, SLNB-guided axillary dissection in breast cancer patients who have clinically axillary lymph node-positive to node-negative conversion following neoadjuvant chemotherapy is recommended, because lymphedema is the most debilitating complication after any axillary surgery. There is no clear consensus on the optimal treatment of occult breast cancer, which is much discussed today. Similarly, the current trend in metastatic breast cancer is that the main palliative treatment option is systemic therapy.

Insulin-Like Growth Factor Binding Protein-3 (IGFBP-3): Unraveling the Role in Mediating IGF-Independent Effects Within the Cell

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3), one of the six members of the IGFBP family, is a key protein in the IGF pathway. IGFBP-3 can function in an IGF-dependent as well as in an IGF-independent manner. The IGF-dependent roles of IGFBP-3 include its endocrine role in the delivery of IGFs from the site of synthesis to the target cells that possess IGF receptors and the activation of associated downstream signaling. IGF-independent role of IGFBP-3 include its interactions with the proteins of the extracellular matrix and the proteins of the plasma membrane, its translocation through the plasma membrane into the cytoplasm and into the nucleus. The C-terminal domain of IGFBP-3 has the ability to undergo cell penetration therefore, generating a short 8-22-mer C-terminal domain peptides that can be conjugated to drugs or genes for effective intracellular delivery. This has opened doors for biotechnological applications of the molecule in molecular medicine. The aim of this this review is to summarize the complex roles of IGFBP-3 within the cell, including its mechanisms of cellular uptake and its translocation into the nucleus, various molecules with which it is capable of interacting, and its ability to regulate IGF-independent cell growth, survival and apoptosis. This would pave way into understanding the modus operandi of IGFBP-3 in regulating IGF-independent processes and its pleiotropic ability to bind with potential partners thus regulating several cellular functions implicated in metabolic diseases, including cancer.

Obesity-associated Breast Cancer: Analysis of risk factors

Several studies show that a significantly stronger association is obvious between increased body mass index (BMI) and higher breast cancer incidence. Furthermore, obese women are at higher risk of all-cause and breast cancer specific mortality when compared to non-obese women with breast cancer. In this context, increased levels of estrogens due to excessive aromatization activity of the adipose tissue, overexpression of pro-inflammatory cytokines, insulin resistance, hyperactivation of insulin-like growth factors (IGFs) pathways, adipocyte-derived adipokines, hypercholesterolemia and excessive oxidative stress contribute to the development of breast cancer in obese women. While higher breast cancer risk with hormone replacement therapy is particularly evident among lean women, in postmenopausal women who are not taking exogenous hormones, general obesity is a significant predictor for breast cancer. Moreover, increased plasma cholesterol leads to accelerated tumor formation and exacerbates their aggressiveness. In contrast to postmenopausal women, premenopausal women with high BMI are inversely associated with breast cancer risk. Nevertheless, life-style of women for breast cancer risk is regulated by avoiding the overweight and a high-fat diet. Estrogen-plus-progestin hormone therapy users for more than 5 years have elevated risks of both invasive ductal and lobular breast cancer. Additionally, these cases are more commonly node-positive and have a higher cancer-related mortality. Collectively, in this chapter, the impacts of obesity-related estrogen, cholesterol, saturated fatty acid, leptin and adiponectin concentrations, aromatase activity, leptin and insulin resistance on breast cancer patients are evaluated. Obesity-related prognostic factors of breast cancer also are discussed at molecular basis.

Effects of the Insulin-like Growth Factor Pathway on the Regulation of Mammary Gland Development

The insulin-like growth factor (IGF) pathway is a key signal transduction pathway involved in cell proliferation, migration, and apoptosis. In dairy cows, IGF family proteins and binding receptors, including their intracellular binding partners, regulate mammary gland development. IGFs and IGF receptor interactions in mammary glands influence the early stages of mammogenesis, i.e., mammary ductal genesis until puberty. The IGF pathway includes three major components, IGFs (such as IGF-I, IGF-II, and insulin), their specific receptors, and their high-affinity binding partners (IGF binding proteins [IGFBPs]; i.e., IGFBP1–6), including specific proteases for each IGFBP. Additionally, IGFs and IGFBP interactions are critical for the bioactivities of various intracellular mechanisms, including cell proliferation, migration, and apoptosis. Notably, the interactions between IGFs and IGFBPs in the IGF pathway have been difficult to characterize during specific stages of bovine mammary gland development. In this review, we aim to describe the role of the interaction between IGFs and IGFBPs in overall mammary gland development in dairy cows.

The IGFBP7 homolog Imp-L2 promotes insulin signaling in distinct neurons of the Drosophila brain

In Drosophila, Insulin-like peptide 2 (Dilp-2) is expressed by insulin-producing cells in the brain, and is secreted into the hemolymph to activate insulin signaling systemically. Within the brain, however, a more local activation of insulin signaling may be required to couple behavioral and physiological traits to nutritional inputs. We show that a small subset of neurons in the larval brain has high Dilp-2-mediated insulin signaling activity. This local insulin signaling activation is accompanied by selective Dilp-2 uptake and depends on the expression of the Imaginal morphogenesis protein-late 2 (Imp-L2) in the target neurons. We suggest that Imp-L2 acts as a licensing factor for neuronal IIS activation through Dilp-2 to further increase the precision of insulin activity in the brain.

Insulin-like growth factors and their binding proteins in domestic animals

Insulin-like growth factors (IGFs) and their binding proteins play an essential role in regulating animal growth and metabolism. The initial portion of the current review focuses on the physiological effects of the IGFs and delineates their role as regulators of animal growth and metabolism. The role of IGFs as mediators of growth hormone effects, as insulin-like metabolic regulators and as foetal growth regulators is discussed. The remainder of the review is devoted to the IGF binding proteins, their modulation of IGF action and their role in foetal and postnatal regulation of growth.

Perspectives in mammalian IGFBP-3 biology: local vs. systemic action

Insulin-like growth factor (IGF) binding protein (IGFBP)-3 has traditionally been defined by its role as a binding protein and its association with IGF delivery and availability. Development of non-IGF binding IGFBP-3 analogs and the use of cell lines devoid of type 1 IGF receptors (IGF-R) have led to critical advances in the field of IGFBP-3 biology. These studies show that IGFBP-3 has IGF-independent roles in inhibiting cell proliferation in cancer cell lines. Nuclear transcription factor, retinoid X receptor (RXR)-alpha, and IGFBP-3 functionally interact to reduce prostate tumor growth and prostate-specific antigen in vivo. Moreover, IGFBP-3 inhibits insulin-stimulated glucose uptake into adipocytes independent of IGF. The purpose of this review is to highlight IGFBP-3 as a novel effector molecule and not just another "binding protein" by discussing its IGF-independent actions on metabolism and cell growth. Although this review presents studies that assume the role of IGFBP-3 as either an endocrine or autocrine/paracrine molecule, these systems may not exist as distinct entities, justifying the examination of IGFBP-3 in an integrated model. Also, we provide an overview of factors that regulate IGFBP-3 availability, including its production, methylation, and ubiquitination. We conclude with the role of IGFBP-3 in whole body systems and possible future applications of IGFBP-3 in physiology.

Insulin-like Growth Factor-binding Protein-3 Plays an Important Role in Regulating Pharyngeal Skeleton and Inner Ear Formation and Differentiation

Insulin-like growth factor-binding protein (IGFBP)-3 is the major insulin-like growth factor (IGF) carrier protein in the bloodstream. IGFBP-3 prolongs the half-life of circulating IGFs and prevents their potential hypoglycemic effect. IGFBP-3 is also expressed in many peripheral tissues in fetal and adult stages. In vitro, IGFBP-3 can inhibit or potentiate IGF actions and even possesses IGF-independent activities, suggesting that local IGFBP-3 may also have paracrine/autocrine function(s). The in vivo function of IGFBP-3, however, is unclear. In this study, we elucidate the developmental role of IGFBP-3 using the zebrafish model. IGFBP-3 mRNA expression is first detected in the migrating cranial neural crest cells and subsequently in pharyngeal arches in zebrafish embryos. IGFBP-3 mRNA is also persistently expressed in the developing inner ears. To determine the role of IGFBP-3 in these tissues, we ablated the IGFBP-3 gene product using morpholino-modified antisense oligonucleotides (MOs). The IGFBP-3 knocked down embryos had delayed pharyngeal skeleton morphogenesis and greatly reduced pharyngeal cartilage differentiation. Knockdown of IGFBP-3 also significantly decreased inner ear size and disrupted hair cell differentiation and semicircular canal formation. Furthermore, reintroduction of a MO-resistant form of IGFBP-3 "rescued" the MO-induced defects. These findings suggest that IGFBP-3 plays an important role in regulating pharyngeal cartilage and inner ear development and growth in zebrafish.

IGF-I receptor mutations resulting in intrauterine and postnatal growth retardation

BACKGROUND

Approximately 10 percent of infants with intrauterine growth retardation remain small, and the causes of their growth deficits are often unclear. We postulated that mutations in the gene for the insulin-like growth factor I receptor (IGF-IR) might underlie some cases of prenatal and postnatal growth failure.

METHODS

We screened two groups of children for abnormalities in the IGF-IR gene. In one group of 42 patients with unexplained intrauterine growth retardation and subsequent short stature, we used single-strand conformation polymorphism analysis, followed by direct DNA sequencing of any abnormalities found. A second cohort consisted of 50 children with short stature who had elevated circulating IGF-I concentrations. Complete sequencing of the IGF-IR gene was performed with DNA from nine children. We also studied a control group of 43 children with normal birth weights.

RESULTS

In the first cohort, we identified one girl who was a compound heterozygote for point mutations in exon 2 of the IGF-IR gene that altered the amino acid sequence to Arg108Gln in one allele and Lys115Asn in the other. Fibroblasts cultured from the patient had decreased IGF-I-receptor function, as compared with that in control fibroblasts. No such mutations were found in the 43 controls. In the second group, we identified one boy with a nonsense mutation (Arg59stop) that reduced the number of IGF-I receptors on fibroblasts. Both children had intrauterine growth retardation and poor postnatal growth.

CONCLUSIONS

Mutations in the IGF-IR gene that lead to abnormalities in the function or number of IGF-I receptors may also retard intrauterine and subsequent growth in humans.

Zinc partitions insulin-like growth factors (IGFs) from soluble IGF binding protein (IGFBP)-5 to the cell surface receptors of BC3H-1 muscle cells

Zinc (Zn(2+)) is a multifunctional micronutrient. The list of functions for this micronutrient expanded with the recent discovery that Zn(2+) retains insulin-like growth factors binding proteins (IGFBPs) on the surface of cultured cells, lowers the affinity of cell-associated IGFBPs, and increases the affinity of the cell surface insulin-like growth factor (IGF)-type 1 receptor (IGF-1R). However, currently there is no information concerning the effect of Zn(2+) on soluble IGFBPs. In the current study, the soluble IGFBP-5 secreted by BC(3)H-1 cells is shown to bind approximately 50% more [(125)I]-IGF-II than [(125)I]-IGF-I at pH 7.4. Zn(2+) is shown to depress the binding of both IGF-I and IGF-II to soluble secreted IGFBP-5; [(125)I]-IGF-I binding is affected more so than [(125)I]-IGF-II binding. Zn(2+) acts by lowering the affinity (K(a)) of IGFBP-5 for the IGFs. Scatchard plots are non-linear indicating the presence of high and low affinity binding sites; Zn(2+) affects only binding to the high affinity site. In contrast, Zn(2+) increases the affinity by which either [(125)I]-IGF-I or [(125)I]-R(3)-IGF-I binds to the IGF-1R, but depresses [(125)I]-IGF-II binding to the IGF-type 2 receptor (IGF-2R) on BC(3)H-1 cells. By depressing the association of the IGFs with soluble IGFBPs, Zn(2+) is shown to repartition either [(125)I]-IGF-I or [(125)I]-IGF-II from soluble IGFBP-5 onto cell surface IGF receptors. Zn(2+) was active at physiological doses depressing IGF binding to IGFBP-5 and the IGF-2R at 15-20 microM. Hence, a novel mechanism is further characterized by which the trace micronutrient Zn(2+) could regulate IGF activity.

Zinc alters the kinetics of IGF-II binding to cell surface receptors and binding proteins

The growth of most tissues is markedly depressed as a result of zinc deficiency by uncharacterized mechanisms that clearly involve the insulin-like growth factor (IGF) system. Herein, we describe the mechanism by which zinc (Zn2+) maintains IGF-II in an active form by directly regulating IGF-II binding to IGF-binding proteins (IGFBPs) and the type 1 IGF receptor (IGF-1R). The specificity of Zn2+ effects was confirmed by using other cations that can (Cd2+ and Au3+) or cannot (La3+) mimic Zn2+ actions. Human fibroblasts, glioblastoma cells, and murine myoblasts were used to determine the kinetics of IGF-II binding to cell surface IGFBP-3, IGFBP-5, and the IGF-1R, respectively. Zn2+, Cd2+, and Au3+, but not La3+, decreased total binding and the affinity for [125I]IGF-II association with IGFBP-3 and IGFBP-5. These effects were a result of lowered rate of ligand association without affecting rate of dissociation. In contrast, Zn2+ enhanced [125I]IGF-II binding to the IGF-1R by enhancing the rate of ligand association and decreasing the rate of dissociation. Our previous work had shown that Zn2+ acts at physiological levels to alter IGF binding. Together with the current work, these findings imply that Zn2+ acts in vivo to prevent secreted IGF-II from binding to IGFBP-3 and IGFBP- 5, thus maintaining IGF-II in an "active state," i.e., readily available for IGF-1R association.

Cellular actions of the insulin-like growth factor binding proteins

In addition to their roles in IGF transport, the six IGF-binding proteins (IGFBPs) regulate cell activity in various ways. By sequestering IGFs away from the type I IGF receptor, they may inhibit mitogenesis, differentiation, survival, and other IGF-stimulated events. IGFBP proteolysis can reverse this inhibition or generate IGFBP fragments with novel bioactivity. Alternatively, IGFBP interaction with cell or matrix components may concentrate IGFs near their receptor, enhancing IGF activity. IGF receptor-independent IGFBP actions are also increasingly recognized. IGFBP-1 interacts with alpha(5)beta(1) integrin, influencing cell adhesion and migration. IGFBP-2, -3, -5, and -6 have heparin-binding domains and can bind glycosaminoglycans. IGFBP-3 and -5 have carboxyl-terminal basic motifs incorporating heparin-binding and additional basic residues that interact with the cell surface and matrix, the nuclear transporter importin-beta, and other proteins. Serine/threonine kinase receptors are proposed for IGFBP-3 and -5, but their signaling functions are poorly understood. Other cell surface IGFBP-interacting proteins are uncharacterized as functional receptors. However, IGFBP-3 binds and modulates the retinoid X receptor-alpha, interacts with TGFbeta signaling through Smad proteins, and influences other signaling pathways. These interactions can modulate cell cycle and apoptosis. Because IGFBPs regulate cell functions by diverse mechanisms, manipulation of IGFBP-regulated pathways is speculated to offer therapeutic opportunities in cancer and other diseases.

Insulin-like growth factor (IGF) binding protein-3 regulation of IGF-I is altered in an acidic extracellular environment

While extracellular acidification within solid tumors is well-documented, how reduced pH impacts regulation of insulin-like growth factor-I (IGF-I) has not been studied extensively. Because IGF-I receptor binding is affected by IGF binding proteins (IGFBPs), we examined how pH impacted IGFBP-3 regulation of IGF-I. IGF-I binding in the absence of IGFBP-3 was diminished at reduced pH. Addition of IGFBP-3 reduced IGF-I cell binding at pH 7.4 but increased surface association at pH 5.8. This increase in IGF-I binding at pH 5.8 corresponded with an increase in IGFBP-3 cell association. This, however, was not due to an increase in affinity of IGFBP-3 for heparin at reduced pH although both heparinase III treatment and heparin addition reduced IGFBP-3 enhancement of IGF-I binding. An increase in IGF-I binding to IGFBP-3, though, was seen at reduced pH using a cell-free assay. We hypothesize that the enhanced binding of IGF-I at pH 5.8 is facilitated by increased association of IGFBP-3 at this pH and that the resulting cell associated IGF-I is IGFBP-3 and not IGF-IR bound. Increased internalization and nuclear association of IGF-I at pH 5.8 in the presence of IGFBP-3 was evident, yet cell proliferation was reduced by IGFBP-3 at both pH 5.8 and 7.4 indicating that IGFBP-3-cell associated IGF-I does not signal the cell to proliferate and that the resulting transfer of bound IGF-I from IGF-IR to IGFBP-3 results in diminished proliferation. Solution binding of IGF-I by IGFBP-3 is one means by which IGF-I-induced proliferation is inhibited. Our work suggests that an alternative pathway exists by which IGF-I and IGFBP-3 both associate with the cell surface and that this association inhibits IGF-I-induced proliferation.

Expression of insulin-like growth factor binding protein (IGFBP)-3, and the effects of IGFBP-2 and -3 in the bovine corpus luteum

The present study was conducted to gain insight into the insulin-like growth factor (IGF) system in the bovine corpus luteum (CL). Specific aims were to measure the levels of IGF binding protein-3 (IGFBP-3) and RNA encoding IGFBP-3 in the CL throughout diestrus, and to investigate the effects of IGFBP-2 and -3 on IGF-I-stimulated progesterone (P4) production and IGF-I-receptor binding. Bovine CL were collected from a local abattoir and classified according to stage of diestrus based on anatomical characteristics. Corpora lutea from early, mid and late diestrus were each analyzed for the presence of IGFBP-3 by ligand blot analysis, and for RNA encoding IGFBP-3 by Northern blot analysis. Dissociated cells from mid-cycle CL were treated with IGF-I, IGFBP-2 or -3, or a combination of IGF-I and IGFBP-2 or -3. The effect of IGFBP-2 and IGFBP-3 on [(125)I] IGF-I binding to its receptor on CL plasma membranes also was investigated. IGFBP-3 protein and RNA expression were higher in early CL, compared to mid or late CL (p < 0.05). IGF-I stimulated P4 production in a dose-dependant manner (p < 0.05). IGFBP-2 and -3 blocked the stimulatory effect of IGF-I on P4 production (p < 0.05). Both IGFBP-2 and -3 inhibited [(125)I]-IGF-I binding to its receptor in a dose-dependant manner. These results demonstrate that IGFBP-3 protein and RNA are expressed predominantly during early diestrus in the bovine CL. Moreover, both IGFBP-2 and -3 can modulate IGF-I actions in the CL by interfering with binding of IGF-I to its receptor.

Tissue transglutaminase facilitates the polymerization of insulin-like growth factor-binding protein-1 (IGFBP-1) and leads to loss of IGFBP-1's ability to inhibit insulin-like growth factor-I-stimulated protein synthesis

Insulin-like growth factor-binding protein-1 (IGFBP-1) binds to insulin-like growth factors (IGFs) and has been shown to inhibit or stimulate cellular responses to IGF-I in vitro. This capacity of IGFBP-1 to inhibit or stimulate IGF-I actions correlates with its ability to form stable high molecular weight multimers. Since the ability of some proteins to polymerize is dependent upon transglutamination, we determined if tissue transglutaminase could catalyze this reaction and the effect of polymerization of IGFBP-1 upon IGF-I action. Following incubation with pure tissue transglutaminase (Tg), IGFBP-1 formed covalently linked multimers that were stable during SDS-polyacrylamide gel electrophoresis using reducing conditions. Dephosphorylated IGFBP-1 polymerized more rapidly and to a greater extent compared with native (phosphorylated) IGFBP-1. Exposure to IGF-I stimulated transglutamination of IGFBP-1 in vitro. An IGFBP-1 mutant in which Gln(66)-Gln(67) had been altered to Ala(66)-Ala(67) (Q66A/Q67A) was relatively resistant to polymerization by Tg compared with native IGFBP-1. Tg localized in fibroblast membranes was also shown to catalyze the formation of native IGFBP-1 multimers, however, Q66A/Q67A IGFBP-1 failed to polymerize. Although the mutant IGFBP-1 potently inhibited IGF-I stimulated protein synthesis in pSMC cultures, the same concentration of native IGFBP-1 had no inhibitory effect. The addition of higher concentrations of native IGFBP-1 did inhibit the protein synthesis response, and this degree of inhibition correlated with the amount of monomeric IGFBP-1 that was present. In conclusion, IGFBP-1 is a substrate for tissue transglutaminase and Tg leads to the formation of high molecular weight covalently linked multimers. Polymerization is an important post-translational modification of IGFBP-1 that regulates cellular responses to IGF-I.

Substitutions for hydrophobic amino acids in the N-terminal domains of IGFBP-3 and -5 markedly reduce IGF-I binding and alter their biologic actions

Insulin-like growth factor-binding protein-3 and -5 (IGFBP-3 and -5) have been shown to bind insulin-like growth factor-I and -II (IGF-I and -II) with high affinity. Previous studies have proposed that the N-terminal region of IGFBP-5 contains a hydrophobic patch between residues 49 and 74 that is required for high affinity binding. These studies were undertaken to determine if mutagenesis of several of these residues resulted in a reduction of the affinity of IGFBP-3 and -5 for IGF-I. Substitutions for residues 68, 69, 70, 73, and 74 in IGFBP-5 (changing one charged residue, Lys(68), to a neutral one and the four hydrophobic residues to nonhydrophobic residues) resulted in an approximately 1000-fold reduction in the affinity of IGFBP-5 for IGF-I. Substitutions for homologous residues in IGFBP-3 also resulted in a >1000-fold reduction in affinity. The physiologic consequence of this reduction was that IGFBP-3 and -5 became very weak inhibitors of IGF-I-stimulated cell migration and DNA synthesis. Likewise, the ability of IGFBP-5 to inhibit IGF-I-stimulated receptor phosphorylation was attenuated. These changes did not appear to be because of alterations in protein folding induced by mutagenesis, because the IGFBP-5 mutant was fully susceptible to proteolytic cleavage by a specific IGFBP-5 protease. In summary, residues 68, 69, 70, 73, and 74 in IGFBP-5 appear to be critical for high affinity binding to IGF-I. Homologous residues in IGFBP-3 are also required, suggesting that they form a similar binding pocket and that for both proteins these residues form an important component of the core binding site. The availability of these mutants will make it possible to determine if there are direct, non-IGF-I-dependent effects of IGFBP-3 and -5 on cellular physiologic processes in cell types that secrete IGF-I.

Horse conceptuses secrete insulin-like growth factor-binding protein 3

Insulin-like growth factor-I (IGF-I) promotes early embryonic development in several species. In the rabbit, IGF-I binds to the embryonic coats from Day 3 of development onward by a 38-kDa protein that is probably insulin-like growth factor-binding protein 3 (IGFBP3). In the present study, ligand, Western, and Northern blot analyses were used to demonstrate the presence of IGF-I-binding activity, several immunoreactive IGFBP3 proteins, and IGFBP3 mRNA in horse conceptuses with particularly large amounts of immunoreactive IGFBP3 in the conceptus capsule. In addition, immunoprecipitation of radiolabeled proteins showed that cultured horse conceptuses secreted IGFBP3 into the culture medium. Endometrial samples from mares also contained IGFBP3 mRNA and protein; but there was no evidence of secretion of IGFBP3 into the uterine lumen by ligand blot analysis, and there was evidence of only very small amounts by Western blot analysis. These results indicate that the horse conceptus secretes significant quantities of IGFBP3 toward the conceptus capsule from as early as Day 10 after ovulation. Thus, most of the IGFBP3 contained within the capsule, which binds IGF-I to this special extracellular matrix of the preimplantation horse conceptus, is likely to be embryonic in origin. IGFBP3 in the horse conceptus capsule may enhance or modulate the action of IGFs on the developing conceptus.

Insulin-like growth factor binding proteins (IGFBPs) in serum and urine and IGFBP-2 protease activity in patients with insulin-dependent diabetes mellitus

Diabetes mellitus and glucose dysregulation have significant effects on the circulating level of insulin-like growth factor-I (IGF-I) and IGF binding proteins (IGFBPs). In the present study, serum and urine IGFBP (IGFBP-1, -2, and -3) and serum IGF-I and -II levels were measured by radioimmunoassay (RIA) in 27 patients with type 1 diabetes aged 9 to 48 years compared with 9 healthy subjects aged 10 to 28 years. The patients were divided into 3 groups according to the amount of albumin excreted in 24 hours. The macroalbuminuria group (>500 mg/24 h) had elevated serum IGFBP-1 and -2 and decreased IGF-I levels (P < .01 v normal controls). Serum IGFBP-3 and IGF-II were not different among the patient groups and controls (P > .05). The mean urinary IGFBP-1 was decreased in all 3 patient groups compared with the controls (P < .05). Urinary IGFBP-2 and IGFBP-3 were increased in patients with macroalbuminuria. Immunoblot analysis showed increased low-molecular-weight fragments of urinary IGFBP-2 in the poorly controlled diabetics, and direct evidence for increased urinary IGFBP-2 proteolytic activity could be demonstrated in both the microalbuminuric and macroalbuminuric groups. Low-molecular-weight fragments of urinary IGFBP-3 were also increased in both the microalbuminuric and macroalbuminuric groups. In conclusion, alterations of IGFBPs in urine and serum are related to metabolic control in diabetic patients, and there is an increase of urinary IGFBP-2 protease activity in poorly controlled diabetics. The changes in serum IGFBP concentrations (eg, increases in IGFBP-1 and IGFBP-2) may lead to alterations in the availability of IGF-I to peripheral tissues.

Thrombospondin and osteopontin bind to insulin-like growth factor (IGF)-binding protein-5 leading to an alteration in IGF-I-stimulated cell growth

Insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) has been shown to bind to extracellular matrix (ECM) with relatively high affinity, but the ECM components that mediate this interaction have not been identified. These studies show that radiolabeled IGFBP-5 specifically coprecipitates with two ECM proteins, thrombospondin-1 (TSP-1) and osteopontin (OPN). As TSP-1 binds avidly to heparin, as does IGFBP-5, the effect of glycosaminoglycans on the TSP-1/IGFBP-5 interaction was analyzed. Heparan and dermatan sulfate inhibited binding, whereas heparin increased binding. Chondroitin sulfate A and B had no effect. In contrast, both heparin and heparan sulfate significantly inhibited the OPN-IGFBP-5 interaction and chondroitin sulfate A, B, and C had no effect. To determine the region of IGFBP-5 that was involved in each interaction, synthetic peptides that spanned several regions of IGFBP-5 were tested for their capacity to competitively inhibit coprecipitation. A peptide that contained the amino acids between positions 201 and 218 resulted in 76% and 86% inhibition of binding to TSP-1 and OPN, respectively. Three other synthetic peptides that spanned regions ofIGFBP-5 with several charged residues had no effect. IGFBP-5 mutants that contained substitutions for basic residues in the 201-218 region were tested for their ability to bind to TSP-1 or OPN. A mutant with substitutions for amino acids at positions R201 and K202 and a mutant with substitutions for K211, R214, K217, and R218 had the greatest reduction in binding to TSP-1. Mutants containing substitutions for R214 alone and the combined K217A, R218A mutant had the greatest reductions in OPN binding. When the smooth muscle cell growth response to these components was assessed, IGF-I plus IGFBP-5 or the combination of TSP-1 or OPN with IGF-I potentiated the IGF-I effect. The addition of IGFBP-5 to these combinations resulted in further significant growth stimulation. Both OPN and TSP-1 specifically bind to IGFBP-5 with high affinity. These interactions may be important for concentrating intact IGFBP-5 in extracellular matrix and for modulating the cooperative interaction between the IGF-I receptor and integrin receptor signaling pathways.

Insulin-like growth factor (IGF) system in the bovine mammary gland and milk

Primary bovine mammary cells express the two IGF receptors (IGF-IR, IGF-IIR), insulin receptor, and four IGFBPs (IGFBP-2, -3, -4, and -5). Examination of the IGF-IR during the mammary gland lactation cycle shows that IGF-IR number declines at parturition, a change that coincides with decreases in the blood level of its ligand, IGF-I. IGF-II and IGF-IIR are largely unchanged. IGFBP-3 is the predominant mammary IGFBP and its concentration also declines in blood and milk during lactation compared to prepartum and involution periods. Time of lactation and pregnancy were the main determinants of milk but not blood IGFBP-3 levels. IGFBP-3 binds to membrane proteins of bovine mammary tissue; an IGFBP-3 binding protein has been identified as bovine lactoferrin. Lactoferrin has the capacity to compete with IGF binding to IGFBP-3. Appearance of both IGFBP-3 and lactoferrin in conditioned media of primary cultures of bovine mammary cells was stimulated by all trans retinoic acid (atRA). Furthermore, atRA was necessary for the entry of exogenously added lactoferrin into the mammary cell nucleus, while IGFBP-3 entry into the nuclei of atRA treated cells required the presence of lactoferrin. These findings reveal a novel role for lactoferrin, suggesting that lactoferrin is critically involved in the regulation of the IGF system during the involution period.

Effect of recombinant human growth hormone and insulin-like growth factor-1 administration on IGF-1 and IGF-binding protein-3 levels in brain injury

STUDY OBJECTIVE

To assess the effect of recombinant human growth hormone (rhGH) on the insulin-like growth factor-1 (IGF-1) plasma concentration versus time profile during continuous infusion of recombinant human (rh)IGF-1 to patients with traumatic brain injury (TBI).

SETTING

University of Kentucky Chandler Medical Center.

PATIENTS

Twenty-three patients with TBI (aged 18-59 yrs) with Glascow Coma Scale scores of 4-10.

INTERVENTION

Patients were randomized to receive rhIGF-1 0.01 mg/kg/hour and daily subcutaneous doses of rhGH 0.05 mg/kg/day or saline for 14 days.

MEASUREMENTS AND MAIN RESULTS

Plasma concentrations of IGF-1 and IGF-binding protein (BP)-3 were quantified by radioimmunoassay. Patients receiving rhIGF-1/rhGH reached a peak IGF-1 concentration (1199.3+/-84.0 microg/L) at 72 hours and maintained it throughout the study. Levels of IGF-1 in the control group did not change significantly above baseline throughout the study. Concentrations of IGFBP-3 were significantly higher after 48 hours in the treated group (5.1+/-0.4 mg/L) than in controls (2.9+/-0.5 mg/L) and continued until the end of the study (p<0.05).

CONCLUSION

Infusion of rhIGF-1 in conjunction with rhGH effectively achieved and maintained supraphysiologic IGF-1 plasma concentrations throughout the dosing period in patients with TBI. It appears that rhGH alters the IGF-1 plasma concentration versus time profile during continuous administration. Although speculative, changes in protein binding of IGF-1 are the most likely mechanism.

Monoclonal antibodies to growth hormone (GH) prolong liver GH binding and GH-induced IGF-I/IGFBP-3 synthesis

This time-course study further explored the mechanisms whereby monoclonal antibodies (MAbs) may enhance growth hormone (GH) effects. Hypophysectomized rats were killed 0, 1, 3, 6, 12, 24, and 48 h after a single injection of bovine (b) GH alone or complexed with an anti-bGH MAb. Serum insulin-like growth factor I (IGF-I) concentrations were increased more and for a longer period after MAb-GH complexes (peak at 24 h: 295 +/- 24 ng/ml) than after bGH alone (peak at 12 h: 219 +/- 37 ng/ml; P < 0.01), whereas liver IGF-I mRNA was similar at 12 h in both groups but remained higher at 24 h (by 65%, P < 0.001) and 48 h (by 64%, P < 0.001) in the presence of the MAb. Induction of serum insulin-like growth factor-binding protein (IGFBP)-3 and liver IGFBP-3 mRNA by bGH also was markedly amplified by the MAb (3.6- and 2-fold at 24 h, respectively; P < 0.01). GH receptors (GHR) remained occupied for a longer period after MAb-GH injection (36 +/- 16 and 35 +/- 8% at 6 and 12 h, respectively) compared with bGH alone (0 +/- 28 and -15 +/- 11%), whereas total liver GH-binding sites and GHR mRNA levels were not affected by the MAb. We conclude that MAbs against GH amplify and prolong the serum IGF-I response to GH, which may result from both a prolongation of liver IGF-I synthesis and an enhanced induction of IGFBP-3. These two effects may in turn be the consequences of sustained GH binding to its liver receptors in the presence of MAb.

IGFBP-3 and IGFBP-5 production by human intestinal muscle: reciprocal regulation by endogenous TGF-beta1

Insulin-like growth factor-I (IGF-I)-mediated growth of cells can be modulated by specific IGF binding proteins (IGFBPs) that inhibit or augment IGF-I ligand-receptor interaction. IGFBP expression and production by human intestinal muscle cells in culture was characterized in rapidly growing cells (day 3 of culture), in confluent cells (day 7), and in postconfluent cells (day 14). RT-PCR analysis identified IGFBP-3, IGFBP-4, and IGFBP-5 mRNA during all three phases of growth. The production of IGFBP-3 and IGFBP-5 was regulated in reciprocal fashion. IGFBP-5 production was high on day 3 and decreased two- to fivefold by day 14, and IGFBP-3 production was low on day 3 and increased five- to eightfold by day 14. IGFBP-4 production remained constant. IGFBP-3 inhibited and IGFBP-5 augmented IGF-I-induced proliferation. IGFBP-3 and IGFBP-5 production was regulated in reciprocal fashion by transforming growth factor-beta1 (TGF-beta1). Immunoneutralization of endogenous TGF-beta1 decreased the production of IGFBP-3 and increased the production of IGFBP-5. Addition of exogenous recombinant human TGF-beta1 had the opposite effect. We conclude that the expression and time-dependent production of IGFBP-3, IGFBP-4, and IGFBP-5 and their regulation by endogenous TGF-beta1 represent mechanisms by which human intestinal muscle cells regulate autocrine IGF-I-mediated growth.

Multivalent cations and ligand affinity of the type 1 insulin-like growth factor receptor on P2A2-LISN muscle cells

Mouse P2A2-LISN myoblasts are transfected cells that overexpress the human type 1 insulin-like growth factor (IGF) receptor. Because the type 1 IGF receptor is the major binding site for both IGF-I and IGF-II, this cell line is an excellent model to determine the effect of multivalent cations on ligand binding specifically to this type of receptor. Competitive binding assays were performed to characterize IGF binding and Scatchard analysis to quantify affinity (Ka). 125I-IGF-I, 125I-IGF-II, and 125I-R3-IGF-I bind only to the type 1 IGF receptor on these cells. Zn2+ increased binding of the three ligands to the type 1 IGF receptor by 17 to 35%. Cd2+ significantly increased binding of 125I-IGF-I, although by only 8%. La3+ and Cr3+ did not effect binding. Au3+ decreased IGF binding by approximately 56%. Scatchard analysis produced nonlinear concave-down plots yielding binding constants for high and low affinity sites. Zn2+ increased the strength of only the high affinity sites. Au3+ decreased the affinity of both high and low affinity sites. Zn2+ increased binding with a half-maximal effect between 40 microM and 60 microM. Half-maximal dose of Au3+ was >130 microM. Zinc, gold, and cadmium bind to similar regions within proteins (a zinc-binding motif) and only these cations were found to affect receptor binding indicating similar mechanisms of action. Thus, multivalent cations may alter IGF binding to cell surface receptors ultimately controlling growth. Physiologically this may be especially important for the growth promoting effects of Zn2+.

Regulatory Actions of Insulin-like Growth Factor-binding Proteins

The six insulin-like growth factor-binding proteins (IGFBPs) are important regulators of insulin-like growth factor (IGF) action. Circulating high molecular weight complexes that contain IGF and IGFBP-3 restrict IGF bioavailability, and excess IGFBPs inhibit IGF action by forming biologically inactive complexes. IGFs can be released from these complexes by proteolysis. Potentiation of IGF activity might occur under specific circumstances, and involves the slow dissociation of IGFs from IGFBP complexes localized in the pericellular space, whose affinity has been reduced by dephosphorylation or association with the cell surface or extracellular matrix. Several IGFBPs or IGFBP fragments also have activities that do not involve IGFs or IGF receptors. The mechanisms by which IGFBPs regulate IGF action and exert their independent actions will be examined.

Role of the insulin-like growth factors and their binding proteins in lactation

Insulin-like growth factor (IGF)-I is thought to mediate a portion of the effects of bST on lactation in dairy cows. Serum concentrations of IGF-I are increased in lactating cows that were treated with bST, and IGF-I receptors are present in bovine mammary tissue. In addition, close arterial infusion of IGF-I into the mammary gland of goats increases milk yield. Little evidence exists to support a direct galactopoietic effect of IGF-I in ruminants. However, IGF-I is a potent mitogen for mammary epithelial cells and may also influence the inhibition of apoptosis of this cell type. The IGF are found in association with a family of individual binding proteins. The high affinity of the IGF for these proteins relative to the IGF receptor allows them to modulate IGF-I bioactivity in the mammary gland at the cellular level. Mammary epithelial cells synthesize multiple forms of IGF binding proteins, and one of these, IGF binding protein-3, is specifically regulated by the IGF. Stimulation of DNA synthesis by IGF-I is enhanced in bovine mammary epithelial cells that overexpress the IGF binding protein-3. These data indicate that IGF-I can stimulate the synthesis of an IGF binding protein, which enhances its own mitogenic activity. However, whether this mechanism is operative in the lactating mammary gland in vivo is unknown. Given the complexity of the interactions between the IGF and their binding proteins, more information is needed before the role of these growth factors in regulating growth, differentiation, and apoptosis of mammary epithelial cells is delineated.

Controlling insulin-like growth factor activity and the modulation of insulin-like growth factor binding protein and receptor binding

The insulin-like growth factors (IGF) and insulin perform seemingly unique roles by causing the same metabolic effect: cellular hypertrophy. Although overlapping, there are different consequences to cellular hypertrophy induced by IGF and that induced by insulin. The IGF enhance the cell hypertrophy that is requisite for cell survival, hyperplasia, and differentiation, and insulin enhances cell hypertrophy primarily as a means to increase nutrient stores. The effects of IGF and insulin are controlled by the segregation of their receptors between different cell types. A model is discussed that describes the need for three hormones (IGF-I, IGF-II, and insulin) to control nutrient partitioning. Insulin receptor localization, as well as an episodic mode of secretion, evolved to perform the short-term action of clearing excess nutrients from the circulation. In contrast, a complex and interactive set of factors ensure that maximal IGF activity occurs only when conditions are optimal for growth. A relatively invariant rate of secretion and the IGF binding proteins serve to maintain a large mutable pool of IGF. This pool exists to ensure a constant supply of IGF to maintain the basal metabolic rate and to ensure that, once a cell begins to proliferate or differentiate, adequate exposure is available to complete the process even after severe short-term physiological insults. The IGF concentrations only change in response to prolonged differences in protein and energy availabilities, environmental and body temperatures, and external stress. Also, evidence is now emerging that describes a discrete role for trace nutrients in the regulation of IGF activity. In this latter regard, zinc has the notable role of targeting IGF binding proteins to the cell surface. New data are presented showing that zinc also changes the affinity of the type 1 IGF receptor and cell-associated IGF binding proteins to optimize IGF activity.

Molecular regulation of insulin-like growth factor-I and its principal binding protein, IGFBP-3

The insulin-like growth factors (IGFs) have diverse anabolic cellular functions, and structure similar to that of proinsulin. The distribution of IGFs and their receptors in a wide variety of organs and tissues enables the IGFs to exert endocrine, paracrine, and autocrine effects on cell proliferation and differentiation, caloric storage, and skeletal elongation. IGF-I exhibits particular metabolic responsiveness, and circulating IGF-I originates predominantly in the liver. Hepatic IGF-I production is controlled at the level of gene transcription, and transcripts are initiated largely in exon 1. Hepatic IGF-I gene transcription is reduced in conditions of protein malnutrition and diabetes mellitus, and our laboratory has used in vitro transcription to study mechanisms related to diabetes. We find that the presence of sequences downstream from the major transcription initiation sites in exon 1 is necessary for the diabetes-induced decrease in IGF-I transcription. Six nuclear factor binding sites have been identified within the exon 1 downstream region, and footprint sites III and V appear to be necessary for metabolic regulation; region V probes exhibit a decrease in nuclear factor binding with hepatic nuclear extracts from diabetic animals. IGFs in biological fluids are associated with IGF binding proteins, and IGFs circulate as a 150-kDa complex that consists of an IGF, an IGFBP-3, and an acid-labile subunit. Circulating IGFBP-3 originates mainly in hepatic nonparenchymal cells, where IGF-I increases IGFBP-3 mRNA stability, but insulin increases IGFBP-3 gene transcription. Regulation of IGFBP-3 gene transcription by insulin appears to be mediated by an insulin-responsive element, which recognizes insulin-responsive nuclear factors in both gel mobility shift assays and southwestern blots. Studies of mechanisms underlying the modulation of IGF-I and IGFBP-3 gene transcription, and identification of critical nuclear proteins involved, should lead to new understanding of the role and regulation of these important growth factors in diabetes mellitus and other metabolic disorders.

Overexpression of insulin-like growth factor-binding protein-4 (IGFBP-4) in smooth muscle cells of transgenic mice through a smooth muscle alpha-actin-IGFBP-4 fusion gene induces smooth muscle hypoplasia

Insulin-like growth factor I (IGF-I) has been postulated to function as a smooth muscle cell (SMC) mitogen and to play a role in the pathogenesis of bladder hypertrophy, estrogen-induced uterine growth, and restenosis after arterial angioplasty. IGF-binding protein-4 (IGFBP-4) inhibits IGF-I action in vitro and is the most abundant IGFBP in the rodent arterial wall. To explore the function of this binding protein in vivo, transgenic mouse lines were developed harboring fusion genes consisting of a rat IGFBP-4 complementary DNA cloned downstream of either a -724 bp fragment of the mouse smooth muscle alpha-actin 5'-flanking region (SMP2-BP-4) or -1074 bp, 63 bp of 5'-untranslated region, and 2.5 kb of intron 1 of smooth muscle alpha-actin (SMP8-BP-4). SMP2-BP-4 mice expressed low levels of the exogenous IGFBP-4 messenger RNA (mRNA), which was not specifically targeted to SMC-rich tissue environments, and were therefore not analyzed further. Six SMP8-BP-4 transgenic lines derived from separate founders were characterized. Mating of hemizygous SMP8-BP-4 mice with controls produced about 50% transgenic offspring, with equal sex distribution. Expression of IGFBP-4 mRNA in nontransgenic littermates was maximal in liver and kidney. By contrast, transgenic IGFBP-4 mRNA expression, distinguished because of a smaller transcript size, was confined to SMC-containing tissues, with the following hierarchy: bladder > aorta > stomach = uterus. There was no transgene expression in skeletal muscle, brain, or cardiac myocytes. The abundance of IGFBP-4 measured by Western ligand blotting or by immunoblotting, was 8- to 10-fold higher in aorta and bladder of SMP8-BP-4 mice than in their nontransgenic littermates, with no change in plasma IGFBP-4 levels. Transgenic mice exhibited a significant reduction in wet weight of SMC-rich tissues, including bladder, intestine, aorta, uterus, and stomach, with no change in total body or carcass weight. In situ hybridization showed that transgene expression was targeted exclusively to the muscular layers of the arteries, veins, bladder, ureter, stomach, intestine, and uterus. Overexpression of IGFBP-4 was associated with SMC hypoplasia, a reciprocal phenotype to that of transgenic mice overexpressing IGF-I under control of the same promoter (SMP8-IGF-I). Double transgenic mice derived from mating SMP8-BP-4 with SMP8-IGF-I animals showed a modest decrease in wet weight at selected SMC tissues. Although we cannot exclude that the effects of IGFBP-4 may be IGF independent, these data suggest that IGFBP-4 is a functional antagonist of IGF-I action on SMC in vivo.

Multivalent cations depress ligand binding to cell-associated insulin-like growth factor binding protein-5 on human glioblastoma cells

The current studies quantified the effect of the multivalent cations zinc, cadmium, lanthanum, chromium, and gold (Zn2+, Cd2+, La3+, Cr3+, and Au3+) on [125I]-insulin-like growth factor ([125I]-IGF) binding to T98G human glioblastoma cells. The major binding site for the IGFs on T98G cells is IGF binding protein-5 (IGFBP-5), as determined by affinity labeling. Competitive binding studies, using either [125I]-IGF-I or [125I]-IGF-II, indicated that La3+ and Cr3+ did not affect [125I]-IGF-I or [125I]-IGF-II binding to cell-associated IGFBP-5. Zn2+, Au3+, and Cd2+ depressed binding of both [125I]-IGF-I and [125I]-IGF-II. [125I]-IGF-I and [125I]-IGF-II binding resulted in nonlinear concave-down Scatchard plots, indicating the presence of high- and low-affinity equilibrium constant of association (Ka) sites. Assuming a preexisting asymmetric model with independent high (KaHi) and low (KaLo) sites; Zn2+, Cd2+, and Au3+ eliminated KaHi and Zn2+, and Au3+ lowered KaLo, compared with control values. The same results were found, independent of whether [125I]-IGF-I or [125I]-IGF-II was used. Similarly, assuming a ligand-induced model of negative cooperativity, all three cations eliminated the initial affinity for the high affinity sites (Ka), whereas Zn2+ and Au2+ reduced the final affinity for the low affinity sites (Kf). Dose-response studies indicated that Zn2+, Au3+, and Cd2+ depressed binding with half-maximal activities of approximately 20 microM, 14-60 microM, and 50-65 microM, respectively. Zn2+, Au3+, and Cd2+ bind to similar sites on proteins (a zinc-binding motif), indicating similar mechanisms of action. A zinc-binding motif is present within the IGFBPs but not the IGFs. We demonstrate, for the first time, that multivalent cations have the potential to modulate IGF activity by decreasing the amount of IGF bound to cell-associated IGFBP-5.

Expression of insulin-like growth factor-binding protein messenger RNAs in developing rat oligodendrocytes and astrocytes

Insulin-like growth factors, IGF-I and IGF-II, are potent regulators of oligodendrocyte development. Most of the IGF present in vivo is bound to members of a family of six high-affinity IGF-binding proteins (IGFBPs), which can either potentiate or inhibit IGF action, depending on other conditions. Additionally, serum contains a structurally unrelated protein, acid-labile sub-unit (ALS), which forms a ternary complex with IGF and IGFBP3. In this study, we used reverse-transcriptase polymerase chain reaction (RT-PCR) to examine the expression of mRNAs for IGFBP 1-6 and ALS in purified populations of oligodendroglial cells and astrocytes. We found that astrocytes express all six IGFBPs. A2B5+/O4- oligodendrocyte precursors, O4+/O1- intermediate precursors, and O1+ oligodendrocytes express IGFBP3, 5, and 6, while IGFBP4 is expressed in oligodendrocyte precursors but not at more mature stages. We were unable to detect ALS mRNA in whole brain or in cultured oligodendroglial cells. The presence of differentially expressed IGFBPs in developing oligodendrocytes and astrocytes could significantly affect the biological activity of IGF-I and IGF-II in the central nervous system and the IGF-responsiveness of the IGFBP-expressing cells.

Insulin-like growth factor binding protein production in bovine retinal endothelial cells

Retinopathy is the most frequent microangiopathic complication in diabetes. Many circulating hormones and locally produced mitogenic factors have been involved. Bovine retinal endothelial cells (BRECs) were cultured to investigate if insulin, insulin-like growth factors (IGFs), IGF binding proteins (IGFBPs), and a chronic high-glucose condition could control endothelial cell growth. Specific IGF-I receptors with two binding sites with high (Kd 0.03 nmol/L) and low (Kd 1.3 nmol/L) affinity were found when analyzing families of displacement curves between IGF-I versus IGF-I and IGF-I versus insulin. However, IGFs failed to be mitogenic factors in these cells. This could be explained by an inhibitory effect due to the presence of specific IGFBPs with a molecular weight between 24 and 43 kd. Using Western blot and immunoblot analysis, Northern blot study, and specific radioimmunoassay (RIA), these IGFBPs have been identified as IGFBP-3, -2, -5, and -4. Insulin, which does not bind to IGFBPs, was a potent mitogenic factor in these cells at a high concentration (10 nmol/L), suggesting a cross-reaction to IGF-I receptor. These IGFBPs, except the 24-kd form (IGFBP-4), were modulated by both IGF-I and IGF-II, with a maximum effect at 100 and 10 nmol/L, respectively. This regulation on IGFBPs was IGF-I receptor-independent. In fact, (1) IGFBP mRNA levels were not modified after stimulation with 100 nmol/L IGF-I, (2) 100 nmol/L IGF plus an equimolar concentration of alpha IR3 did not affect IGFBP production, (3) Des(1-3)IGF-I had no effect on IGFBP modulation, whereas at 10 nmol/L it enhanced BREC thymidine cell incorporation, and (4) 100 nmol/L insulin, which at this concentration can cross-react with the IGF-I receptor, did not modify the IGFBP pattern. Chronic exposure (4 weeks) of BRECs to 25 mmol/L glucose had no effect on cell growth. However, after 3 weeks, we observed a decreased IGFBP detection, and addition of 100 nmol/L IGF-I did not change IGFBP levels and did not modify cell growth. Conversely, BRECs grown in regular medium for 4 weeks showed increased IGFBP production. In conclusion, we showed that conditions mimicking hyperinsulinemia, rather than high levels of IGFs, could regulate BREC growth and that the IGF-I analog, Des(1-3), even with reduced affinity for IGFBPs but in part capable of binding to IGFBP-3, significantly stimulated BRECs growth only at 10 nmol/L. IGF actions are modulated by locally produced endothelial IGFBPs, and in turn, these endothelial IGFBPs are regulated, via in IGF-I receptor-independent mechanism, by the presence of IGFs. The autoregulatory IGF system together with the direct glucose modulation of IGFBPs could contribute in diabetic subjects to the retinal endothelial cell growth and metabolism through local changes in IGF bioavailability.

Insulin-like growth factor binding proteins-3 and -5 form sodium dodecyl sulfate-stable multimers

Insulin-like growth factor binding proteins (IGFBPs) are important modulators of IGF actions. IGFBP-3 and IGFBP-5 can bind to the extracellular matrix of a number of cell types. We now describe a new posttranslational structural modification of IGFBP-3 and IGFBP-5, which could play a role in determining their localization. We incubated radioiodinated forms of all six IGFBPs in the presence of a redox buffer consisting of 10 mM reduced glutathione and 0.2 mM oxidized glutathione. Under these conditions IGFBP-3 and IGFBP-5, but not the other IGFBPs, formed high molecular weight disulfide-linked multimers. Heparin and a peptide encompassing the high-affinity heparin-binding site in the C-terminal portion of IGFBP-3 were capable of blocking the multimerization of IGFBP-3. IGFBP-3, but not IGFBP-1, was shown to be able to self-associate non-covalently, which could be a requisite first step in the formation of covalent multimers. The self-association of IGFBP-3 required the high-affinity heparin-binding site in the C-terminal portion of the molecule.

Insulin-like growth factor binding protein-2 binds to cell surface proteoglycans in the rat brain olfactory bulb

A family of six insulin-like growth factor binding proteins (IGFBPs) bind IGF-I and modulate its biological activity. IGFBPs may bind to macromolecules on the cell surface or pericellular extracellular matrix, and this interaction may modulate their effect on IGF activity. To date, little is known about the specificity of IGFBPs in the regulation of IGF action in the brain. We therefore explored whether IGFBPs were associated with cell membrane or extracellular matrix components in the rat brain. IGF-I binding sites with the characteristics of an IGFBP were found in the olfactory bulb mitral cell layer. This IGFBP was identified as IGFBP-2 by immunoprecipitation of both solubilized membrane preparations and cross-linked 125I-IGF: IGFBP complexes. While binding of IGFBP-2 to cell membranes was unaffected by RGD-containing peptide, it was inhibited by high salt concentration, suggesting interaction with proteoglycans. IGFBP-2 bound in vitro to the glycosaminoglycans chondroitin-4 and -6-sulfate, keratan sulfate, and heparin. IGFBP-2 also bound the proteoglycan aggrecan, an effect reduced by digestion of its glycosaminoglycans. Binding of IGFBP-2 to chondroitin-6-sulfate decreased the binding affinity of IGFBP-2 for IGF-I approximately 3-fold. Finally, an IGFBP-2 antibody coimmunoprecipitated IGFBP-2 and an approximately 200 kDa proteoglycan containing chondroitin-sulfate side chains from the rat olfactory bulb, providing definitive evidence for IGFBP-2 binding to olfactory bulb proteoglycans. These findings indicate that IGFBP-2 binds to proteoglycans in cell membranes of the rat olfactory bulb. Because we have previously shown that IGFs are highly expressed in the rat olfactory bulb, cell associated IGFBP-2 may have an important role in directing IGFs to specific sites in this brain region.

Insulin-like growth factor binding protein gene expression in the pregnant rat uterus and placenta

While the insulin-like growth factor (IGF) system plays a fundamental role in regulating embryonic and placental growth, the specific contributions of the six IGF binding proteins (IGFBPs 1-6) to these processes are not well understood. We here focus on IGFBP expression in the extraembryonic environment, which both supports and constrains embryonic growth, and have used in situ hybridization to determine sites of IGFBP mRNA synthesis in the pregnant rat uterus and placenta. We find that all IGFBPs are expressed in distinct, changing patterns in the uterine endometrium, at the decidual boundary, in the decidual vasculature, and in the myometrium during pregnancy. Within the endometrium, the most prominent change is that expression of IGFBP-1 begins in some, but not all, endometrial glands prior to implantation and then expands to include all secretory epithelia shortly after implantation. During the period of rapid decidual proliferation that follows implantation, IGFBP-3, -4, and -5 transcripts are all detected in a laminar array at the boundary between the decidua and the nondecidualized endometrium. In the decidual vasculature at Day (d) 8.0, both IGFBP-3 and IGFBP-4 mRNAs are detected in dilating blood vessels, with BP-3 most prominent in the antimesometrial plexus and BP-4 primarily at the mesometrial pole. Later (d11.5), all decidual vessels express high levels of IGFBP-3 and lower levels of IGFBP-4 mRNAs. Finally, changes in expression of several IGFBPs also occur within the myometrium during pregnancy. For example, IGFBP-2 is expressed in the inner circular layer shortly after implantation, and expression increases through late gestation. In contrast, IGFBP-5 hybridization occurs over both myometrial layers before implantation, but decreases in intensity and spatial distribution as pregnancy proceeds. Finally, and most strikingly, IGFBP-6 expression, barely detectable in the d7.0 myometrium, gradually increases until it is very strongly transcribed during the placental stages. Taken together, these observations suggest multiple roles for IGFBPs in supporting implantation, regulating the extent of decidualization, modulating local levels of vascular IGFs, and regulating uterine muscular growth.

Purification and characterization of the insulin-like growth factor-binding protein-1 phosphoform found in normal plasma

Our previous work has shown that, in the normal circulation, insulin-like growth factor-binding protein-1 (IGFBP-1) is present as a single highly phosphorylated species. In this study, we have purified this previously uncharacterized isoform of IGFBP-1 to determine its ligand-binding affinity and the potential significance of highly phosphorylated IGFBP-I. Immunoaffinity chromatography was used to isolate IGFBP-1 from normal human plasma and from human hepatoma (Hep G2) cell medium as an alternative source of the IGFBP-1 phosphoform in the circulation. The affinity of this highly phosphorylated IGFBP-1 was compared with that of nonphosphorylated IGFBP-1 and recombinant human (rh) IGFBP-3 by equilibrium binding to IGF-II and IGF-II. Anion exchange (IEX) HPLC, nondenaturing electrophoresis, alkaline phosphatase treatment, and ligand-binding studies indicated that the highly phosphorylated IGFBP-1 from HepG2 cells was comparable with IGFBP-1 from plasma. In binding to IGF-I, the plasma phosphoform of IGFBP-1 was found to have a higher affinity (2.3 +/- 1.1 x 10(10) M-1) than nonphosphorylated IGFBP-1 (2.5 +/- 1.7 x 10(9) M-1, P < 0.002). However, when binding to IGF-II, phosphorylation had no affect on the affinity of IGFBP-1 (3.6 +/- 2 x 10(9) M-1 vs. 1.8 +/- 3 x 10(9) M-1, P not significant). Therefore, in the circulation, IGF-I has a considerably higher affinity than IGF-II for IGFBP-1 (P < 0.02). The affinity of phosphorylated IGFBP-1 from plasma (2.3 +/- 1.1 x 10(10) M-1) also was significantly higher than the affinity of IGFBP-3 for IGF-I (5.6 +/- 4.2 x 10(9) M-1, P < 0.005). These data suggest that the highly phosphorylated IGFBP-1 in the normal circulation will preferentially bind IGF-I rather than IGF-II, whereas in pregnancy, the affinity of IGFBP-1 for IGF-I will be reduced because of the appearance of non- and lesser-phosphorylated forms. This lends support to the theory that changes in IGFBP-1 phosphorylation may influence the modulatory effects of IGFBP-1 on IGF bioavailability.

Insulin-like growth factor binding proteins and their role in controlling IGF actions

The insulin-like growth factor binding proteins (IGFBPs) are a family of six proteins that bind to insulin-like growth factor-I and -II with very high affinity. Because their affinity constants are between two- and 50-fold greater than the IGF-I receptor, they control the distribution of the IGFs among soluble IGFBPs in interstitial fluids, IGFBPs bound to cell surfaces or extracellular matrix (ECM) and cell surface receptors. Although there are six forms of insulin-like growth factor binding proteins, most interstitial fluids contain only three or four forms, and usually only one or two predominate. The proteins differ significantly in their biochemical characteristics, and this accounts for many of the differences that have been observed in their biological actions. Several different types of protease cleave these binding proteins. Proteolytic cleavage generally inactivates the binding proteins or reduces their ability to bind to IGF-I or -II substantially. Several cell types have been shown to secrete these proteases; therefore, the factors that regulate protease activity can control binding protein actions indirectly. Other post-translational modifications, such as glycosylation and phosphorylation, have been shown to alter IGF binding protein activity. While binding protein actions have been studied extensively in vitro, many of the in vivo activities of these proteins remain to be defined.

Histamine-stimulated expression of insulin-like growth factors in human glioma cells

Glioma tumour growth is associated with the expression of insulin-like growth factors I and II (IGFs) and of both type I and type II IGF receptors. It has also been shown that IGFs can stimulate proliferation of cultured glioma cells. We previously reported that histamine too can stimulate the growth of glioma cells in vitro. In this report, we study whether the histamine-induced growth of G47 glioma cells is mediated by the IGFs. We found that histamine stimulates the expression of both IGF-I and IGF-II mRNAs, as determined by a semiquantitative in situ hybridization analysis. Furthermore, incubation of G47 cells with histamine also induced cellular immunostaining for IGF-II. It could be shown that IGF-I-stimulated proliferation is inhibited by IGFBP-3, which decreases the availability of IGFs for binding to the IGF receptors, and by beta-galactosidase, which may decrease IGF binding to the type II IGF receptor, but is not inhibited by the anti-type I IGF receptor monoclonal antibody alphaIR3. However, neither IGFBP-3 nor beta-galactosidase nor alphaIR3 inhibited the histamine-induced proliferation. These results show that the growth-stimulatory effect of histamine is accompanied by the induction of IGFs. This histamine-induced growth stimulation is not mediated by activation of cell surface IGF receptors, although intracrine activation of type II IGF receptors may be involved.

Interaction between the insulin-like growth factor family and the integrin receptor family in tissue repair processes. Evidence in a rabbit ear dermal ulcer model

We have determined previously that IGF-I is dependent on the presence of IGF binding protein-1 (IGFBP-1) to act as a wound healing agent. We sought to determine the mechanism whereby IGFBP-1 is able to enhance IGF-I bioactivity. As IGFBP-1 binds both the alpha5beta1 integrin as well as IGF-I in vitro, we asked which of the following interactions were important: (a) the ability of IGFBP-1 to interact with an integrin receptor, and/or (b) the binding of IGF-I by IGFBP-1. We used an IGF-1 analogue (des(1-3)IGF-I) with a > 100-fold reduction in affinity for IGFBP-1 as well as an IGFBP-1 mutant (WGD-IGFBP-1) which does not associate with the alpha5beta1 integrin to selectively abrogate each of these interactions. We also tested the ability of IGFBP-2, a related binding protein which has an arginine-glycine-aspartate sequence but does not associate with integrin family members, to enhance IGF-I bioactivity. Full-thickness dermal wounds were created on rabbit ears; various combinations of native IGF-I, native IGFBP-1, native IGFBP-2, and their respective analogues/mutants were applied to each wound. Wounds were harvested 7 d later for analysis. Only native IGF-I in combination with native IGFBP-1 was effective as a wound healing agent, enhancing reepithelialization and granulation tissue deposition by 64+/-5 and 83+/-12% over controls (P = 0.008 and 0.016, respectively). The same doses of IGF-I/WGD-IGFBP-1, des(1-3)IGF-I/IGFBP-1, and IGF-I/IGFBP-2 were ineffective. We propose that IGF-I physically interacts with IGFBP-1 and that IGFBP-1 also binds to an integrin receptor, most likely the alpha5beta1 integrin. This interaction is unique to IGFBP-1 as the closely related IGFBP-2 had no effect, a finding consistent with its inability to bind to integrin receptors. Our results suggest that activation of both the IGF-I receptor and the alpha5beta1 integrin is required for IGF-I to stimulate wound healing.

Characterization of glycosaminoglycan-binding domains present in insulin-like growth factor-binding protein-3

Matrix metalloproteinase 3 cleaves insulin-like growth factor-binding protein-3 (IGFBP-3) into six fragments, four of which bind heparin-Sepharose (Fowlkes, J. L., Enghild, J. J., Suzuki, K., and Nagase, H. (1994) J. Biol. Chem. 269, 25742-25746). Sequence analysis of IGFBP-3 heparin-binding fragments shows that all fragments contain at least one of two highly basic, putative heparin-binding consensus sequences present in IGFBP-3. Epitope-specific antibodies generated against synthetic peptides containing these domains recognized IGFBP-3, yet were significantly inhibited from binding in the presence of heparin, demonstrating that these regions of IGFBP-3 contain functional heparin-binding domains. IGFBP-3 peptides containing one of the two heparin-binding consensus sequences bound heparin in a solid phase binding assay in a dose-dependent and saturable manner. However, the IGFBP-3 peptide containing the heparin-binding consensus sequence 149KKGHA153 bound heparin with approximately 4-fold less affinity than the IGFBP-3 peptide containing the longer heparin-binding consensus sequence 219YKKKQCRP226. Examination of several well characterized glycosaminoglycans to inhibit the binding of heparin to both heparin-binding IGFBP-3 peptides revealed that the most potent inhibitors were heparin, heparan sulfate, and dermatan sulfate; chondroitin sulfate A and hyaluronic acid were intermediate in their inhibitory activities; and chondroitin sulfate C caused no inhibition. These studies identify and characterize the glycosaminoglycan-binding domains in IGFBP-3, providing a basis for the better understanding of IGFBP-3-glycosaminoglycan interactions at the cellular and extracellular interface.

Insulin-like growth factor-binding proteins (IGFBPs) and their regulatory dynamics

The IGFBPs are a family of homologous proteins that have co-evolved with the IGFs and that confer upon the IGF regulatory system both functional and tissue specificity. IGFBPs are not merely carrier proteins for IGFs, but hold a central position in IGF ligand-receptor interactions through influences on both the bioavailability and distribution of IGFs in the extracellular environment. In addition, IGFBPs appear to have intrinsic biological activity independent of IGFs. The current status of research on IGFBPs is reviewed herein. Following a brief introduction to the entire IGF/IGFBP system, separate sections for each of the six cloned mammalian IGFBPs, the most extensive for IGFBP3, cover selected topics that emphasize the dynamics of IGFBPs--that is, their regulation in cells, their functionally important post-translational modifications, and their interactions in the cellular microenvironment--and how these dynamics influence physiological function.

Substitution of specific amino acids in insulin-like growth factor (IGF) binding protein 5 alters heparin binding and its change in affinity for IGF-I response to heparin

Heparin binding to insulin-like growth factor (IGF)-binding protein 5 (IGFBP-5) leads to a 17-fold decrease in its affinity for IGF-I, and a region that contains several basic amino acids (Arg201-Arg218) may be involved in this affinity shift. In the present study, mutagenesis was used to analyze the effect of substitutions for basic amino acids in the Arg201-Arg218 region of IGFBP-5 on heparin-binding and the heparin-induced affinity shift. Nine mutant forms were prepared. Their association constants (Ka) for IGF-I were similar to native IGFBP-5. When 10 microg/ml of heparin was added, the Ka of native IGFBP-5 decreased 17-fold, and the Ka of the K134A/R136A mutant decreased 16-fold. In contrast, substitutions for specific basic amino acids in the Arg2O1-Arg218 region decrease the affinity shift to 1.1-3.2-fold. Lys 211 was especially important. When a mutant containing that single substitution was tested, heparin caused only a 2.5-fold reduction in IGF-I affinity. Affinity cross-linking studies showed that heparin was equipotent in inhibiting the formation of 125I-IGF-I-K134A/Rl36A mutant complexes compared to native IGFBP-5. In contrast, heparin had minimal effects on the formation of complexes between 125I-IGF-I and the other mutants. The heparin-binding activity of each mutant was determined. Four mutants, R201A/K202N, K202A/K206A/R207A, R201A/K202N/K206N/K208N, and K211N/R214A/K217A/R218A, had reduced heparin binding compared to native IGFBP-5. The other five mutants, including the K21IN mutant, showed no change in heparin binding. The four mutants with reduced heparin binding could be dissociated from heparin-Sepharose with much lower NaCl concentrations, indicating that they had reduced affinity. These findings suggest that Arg201 Lys202, LysS206, and Arg214 are important for heparin binding. In contrast, LyS211 is not important for the binding of IGFBP-5 to heparin, but substitution for it reduced the heparin-induced affinity shift.

Insulin-like growth factors and inflammatory bowel disease

Hallmarks of IGF-I action include synergy with other hormones and growth factors and the ability to stimulate proliferation or differentiated cell function dependent on physiological or pathophysiologial context. A complete understanding of IGF action in IBD will require analyses of mechanisms of IGF interaction with other growth factors, hormones and cytokines. GH and IGF-I may be administered to children over prolonged periods to correct growth disorders. The definition of the benefits and problems of GH/IGF-I therapy in IBD needs to distinguish between long-term and short-term effects. Short-term administration of GH and IGF-I to animal models of IBD such as the PG-PS and TNBS models, which share features of Crohn's disease (Sartor, 1992), and a recently developed murine model of ulcerative colitis induced by ingestion of dextran sulphate (Okayasu et al, 1990; Sartor, 1992; Cooper et al, 1993) could address the beneficial or detrimental consequences of short-term GH/IGF-I therapy. Adaptation of the PG-PS, TNBS and dextran sulphate models of inflammation to available transgenic mouse lines that over-express GH and IGF-I (Behringer et al, 1990; Ulshen et al, 1993), especially if over-expression is inducible, could help to define the potential benefits and problems of long-term GH/IGF-I therapy or the effects of GH/IGF-I on immune cell function and cytokine production during intestinal inflammation. It will be useful to study intestinal inflammation and complication in animal models of GH or IGF-I deficiency. In this regard, mice with targeted ablation of the IGF-I gene could be useful (Liu et al, 1993) although neonatal mortality in these models currently poses problems for in vivo studies. Development of mesenchymal cell lines from such animals could, however, provide a useful in vitro system to study the role of IGF-I in altered cell function in response to pro-inflammatory cytokines.

Serum factors inhibit melanoma cell surface expression of type I and type II IGF receptors

We have identified one class of IGF-I-binding sites and two classes of IGF-II-binding sites at the surface of the melanoma cell line IGR39. By means of affinity labeling with 125I-IGF-I, 290-300 kDa form was characterized. Using 125I-IGF-II, a 270 kDa polypeptide was labeled, corresponding to the type II IGF receptor. In the two serials of experiments, the order of potency in inhibiting 125I-IGF-I or 125I-IGF-II labeling of IGF-related peptides and alpha IR3, an antibody directed against type I receptor alpha subunit, was the same as in competition experiments. When IGR39 cells were cultured in a serum-free medium, the number of both high affinity IGF-II and IGF-I binding sites was increased, by 8- and 5-fold respectively, without any significant change in Kd values. In both culture conditions, we found IGFBP-2, -3 -4 and a 30 kDa form which Mr was consistent with IGFBP-5 or -6. Except for IGFBP-2, the amount of secreted IGFBPs was modified depending on culture conditions: in conditioned medium from cells cultured with 10% FCS, the amount of IGFBP-3 or -4 was higher, and the amount of the 30 kDa IGFBP was lower when compared to conditioned medium from cells cultured in serum-free medium.