Human insulin-like growth factor-binding protein-1 (hIGFBP-1) in transgenic mice: characterization and insights into the regulation of IGFBP-1 expression

The insulin like growth factor and binding protein family: Novel therapeutic targets in obesity & diabetes

Background

Recent changes in nutrition and lifestyle have provoked an unprecedented increase in the prevalence of obesity and metabolic disorders. Recognition of the adverse effects on health has prompted intense efforts to understand the molecular determinants of insulin sensitivity and dysglycemia. In many respects, actions of insulin-like growth factors (IGFs) mirror those of insulin in metabolic regulation. Unlike insulin, however, the bioactivity of IGFs is regulated by a family of seven high-affinity binding proteins (IGFBPs) which confer temporospatial modulation with implications for metabolic homeostasis. In addition, evidence is accumulating that IGF-independent actions of certain of the IGFBPs can directly modulate insulin sensitivity.

Scope of review

In this review, we discuss the experimental data indicating a critical role for IGF/IGFBP axis in metabolic regulation. We highlight key discoveries through which IGFBPs have emerged as biomarkers or putative therapeutic targets in obesity and diabetes.

Major conclusions

Growing evidence suggests that several components of the IGF-IGFBP system could be explored for therapeutic potential in metabolic disorders. Both IGFBP-1 and IGFBP-2 have been favorably linked with insulin sensitivity in humans and preclinical data implicate direct involvement in the molecular regulation of insulin signaling and adiposity respectively. Further studies are warranted to evaluate clinical translation of these findings.

Regulation of skeletal growth and mineral acquisition by the GH/IGF-1 axis: Lessons from mouse models

The growth hormone (GH) and its downstream mediator, the insulin-like growth factor-1 (IGF-1), construct a pleotropic axis affecting growth, metabolism, and organ function. Serum levels of GH/IGF-1 rise during pubertal growth and associate with peak bone acquisition, while during aging their levels decline and associate with bone loss. The GH/IGF-1 axis was extensively studied in numerous biological systems including rodent models and cell cultures. Both hormones act in an endocrine and autocrine/paracrine fashion and understanding their distinct and overlapping contributions to skeletal acquisition is still a matter of debate. GH and IGF-1 exert their effects on osteogenic cells via binding to their cognate receptor, leading to activation of an array of genes that mediate cellular differentiation and function. Both hormones interact with other skeletal regulators, such as sex-steroids, thyroid hormone, and parathyroid hormone, to facilitate skeletal growth and metabolism. In this review we summarized several rodent models of the GH/IGF-1 axis and described key experiments that shed new light on the regulation of skeletal growth by the GH/IGF-1 axis.

Increasing circulating IGFBP1 levels improves insulin sensitivity, promotes nitric oxide production, lowers blood pressure, and protects against atherosclerosis

Low concentrations of insulin-like growth factor (IGF) binding protein-1 (IGFBP1) are associated with insulin resistance, diabetes, and cardiovascular disease. We investigated whether increasing IGFBP1 levels can prevent the development of these disorders. Metabolic and vascular phenotype were examined in response to human IGFBP1 overexpression in mice with diet-induced obesity, mice heterozygous for deletion of insulin receptors (IR(+/-)), and ApoE(-/-) mice. Direct effects of human (h)IGFBP1 on nitric oxide (NO) generation and cellular signaling were studied in isolated vessels and in human endothelial cells. IGFBP1 circulating levels were markedly suppressed in dietary-induced obese mice. Overexpression of hIGFBP1 in obese mice reduced blood pressure, improved insulin sensitivity, and increased insulin-stimulated NO generation. In nonobese IR(+/-) mice, overexpression of hIGFBP1 reduced blood pressure and improved insulin-stimulated NO generation. hIGFBP1 induced vasodilatation independently of IGF and increased endothelial NO synthase (eNOS) activity in arterial segments ex vivo, while in endothelial cells, hIGFBP1 increased eNOS Ser(1177) phosphorylation via phosphatidylinositol 3-kinase signaling. Finally, in ApoE(-/-) mice, overexpression of hIGFBP1 reduced atherosclerosis. These favorable effects of hIGFBP1 on insulin sensitivity, blood pressure, NO production, and atherosclerosis suggest that increasing IGFBP1 concentration may be a novel approach to prevent cardiovascular disease in the setting of insulin resistance and diabetes.

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.

Omeprazole stimulates the induction of human insulin-like growth factor binding protein-1 through aryl hydrocarbon receptor activation

5-Methoxy-2-{(4-methoxy-3,5-dimethyl-pyridin-2-yl)methylsulfinyl}-3H-benzoimidazole (omeprazole), a benzoimidazole-derived gastric H(+)/K(+)-ATPase proton pump inhibitor (PPI) extensively prescribed for the treatment of gastroesophageal acid reflux disease, can stimulate the expression of CYP1A1 via activation of the human aryl hydrocarbon receptor (hAhR) in an apparent nonligand-binding manner. Here, we have examined the effect of nonclassical, i.e., nonligand binding, AhR activation by omeprazole upon human insulin-like growth factor binding protein (hIGFBP)-1, a secreted phosphoprotein involved in regulation of insulin-like growth factor-I/II bioavailability and mitogenic activity. Analysis of the proximal promoter of the hIGFBP-1 gene reveals the presence of an aryl hydrocarbon binding/dioxin response element (DRE). Quantitative mRNA analysis revealed hIGFBP-1 expression to be responsive to both ligand (TCDD) and nonligand (omeprazole) modes of hAhR activation in the human hepatocarcinoma HepG2 cell line. Furthermore, mutagenesis of the DRE renders the hIGFBP-1 promoter unresponsive to both compounds in HepG2 cells. Likewise, small interfering RNA-mediated hAhR ablation inhibits TCDD and omeprazole-dependent hIGFBP-1 induction, as determined by quantitative mRNA analysis. Cotreatment with cycloheximide further suggests a direct transcriptional role for hAhR at the hIGFBP-1 promoter. Omeprazole exposure prompted a significant increase in both hIGFBP-1 mRNA and secreted protein from HepG2 cells. In addition, we present in vitro evidence indicating that omeprazole at a concentration comparable with that found circulating in subjects undergoing PPI therapy can stimulate the expression of hIGFBP-1. These data demonstrate that activation of hAhR by pharmaceuticals such as omeprazole can alter IGFBP-1 expression and thus may influence IGFBP-1-dependent physiological processes.

The role of the IGF system in cancer growth and metastasis: overview and recent insights

IGF-I receptor (IGF-IR) signaling and functions are mediated through the activities of a complex molecular network of positive (e.g., type I IGF) and negative (e.g., the type II IGF receptor, IGF-IIR) effectors. Under normal physiological conditions, the balance between the expression and activities of these molecules is tightly controlled. Changes in this delicate balance (e.g., overexpression of one effector) may trigger a cascade of molecular events that can ultimately lead to malignancy. In recent years, evidence has been mounting that the IGF axis may be involved in human cancer progression and can be targeted for therapeutic intervention. Here we review old and more recent evidence on the role the IGF system in malignancy and highlight experimental and clinical studies that provide novel insights into the complex mechanisms that contribute to its oncogenic potential. Controversies arising from conflicting evidence on the relevance of IGF-IR and its ligands to human cancer are discussed. Our review highlights the importance of viewing the IGF axis as a complex multifactorial system and shows that changes in the expression levels of any one component of the axis, in a given malignancy, should be interpreted with caution and viewed in a wider context that takes into account the expression levels, state of activation, accessibility, and functionality of other interacting components. Because IGF targeting for anticancer therapy is rapidly becoming a clinical reality, an understanding of this complexity is timely because it is likely to have an impact on the design, mode of action, and clinical outcomes of newly developed drugs.

Elevated circulating insulin-like growth factor binding protein-1 is sufficient to cause fetal growth restriction

IGF binding protein-1 (IGFBP-1) inhibits the mitogenic actions of the IGFs. Circulating IGFBP-1 is elevated in newborns and experimental animals with fetal growth restriction (FGR). To establish a causal relationship between high circulating IGFBP-1 and FGR, we have generated transgenic mice using the mouse alpha-fetoprotein gene promoter to target overexpression of human IGFBP-1 (hIGFBP-1) in the fetal liver. These transgenic mice (AFP-BP1) expressed hIGFBP-1 mainly in the fetal hepatocytes, starting at embryonic d 14.5 (E14.5), with lower levels in the gut. The expression peaked at 1 wk postnatally (plasma concentration, 474 +/- 34 ng/ml). At birth, AFP-BP1 pups were 18% smaller [weighed 1.34 +/- 0.02 g compared with 1.62 +/- 0.04 g for wild type (WT); P < 0.05], and they did not demonstrate any postnatal catch-up growth. The placentas of the AFP-BP1 mice were larger than WT from E16.5 onwards (150 +/- 12 for AFP-BP1 vs. 100 +/- 5 mg for WT at E16.5; P < 0.05). Thus, this model of FGR is associated with a larger placenta, but without postnatal catch-up growth. Overall, these data clearly demonstrate that high concentrations of circulating IGFBP-1 are sufficient to cause FGR.

Human insulin-like growth factor-IA expression in transgenic mice promotes adenomatous hyperplasia but not pulmonary fibrosis

Insulin-like growth factor-I (IGF-I) has been implicated in postnatal alveolar development, pulmonary fibrosis, and non-small cell lung cancer. To further investigate the role of IGF-I, we created a line of transgenic mice in which alveolar type II epithelial cells express human IGF-IA under the control of the surfactant protein C promoter. We determined the effect of pulmonary overexpression of human IGF-IA on 1) pulmonary inflammation and fibrosis in response to intratracheal instillation of bleomycin, 2) premalignant pulmonary adenomatous hyperplasia, and 3) adenoma formation. Transgenic expression of human IGF-IA had no effect on baseline gross lung pathology, cellularity of bronchoalveolar lavage, or total lung collagen content. In addition, there were no significant differences between transgenic mice and nontransgenic littermate controls in the development of pulmonary inflammation or pulmonary fibrosis in response to intratracheal bleomycin instillation. However, pulmonary expression of human IGF-IA in older mice (>12 mo) significantly increased the incidence of premalignant adenomatous hyperplastic lesions compared with littermate controls without affecting adenoma formation. These findings suggest that increased expression of human IGF-IA in alveolar air spaces does not affect the development of pulmonary fibrosis but promotes premalignant changes in the alveolar epithelium.

The circulating IGF system and its relationship with 24-h glucose regulation and insulin sensitivity in healthy subjects

OBJECTIVE AND DESIGN

It has been suggested that circulating free IGF-I participates in glucose homeostasis and that IGFBP-1 reflects changes in insulin sensitivity. To study this further, we examined 10 healthy, nonobese subjects under standardized conditions for 24 h with and without an intravenous infusion of glucose, the latter in order to augment insulin sensitivity. Serum was collected every 2 h for analysis of free and total IGFs, IGFBP-1, - 2 and - 3 and the acid labile subunit (ALS). Insulin sensitivity was estimated at the end of each 24-h study period by use of the hyperinsulinaemic euglycaemic clamp technique.

RESULTS

Glucose infusion resulted in mild hyperglycaemia (P < 0.0001), a reduction in IGFBP-1 by approximately 40% (P < 0.0003), and increased insulin and C-peptide levels (P < 0.0001). Glucose infusion also increased insulin sensitivity (P < 0.003). However, despite the reduction in IGFBP-1, glucose infusion did not increase free IGF-I over the control level, and free IGF-II was slightly reduced (P < 0.02). Irrespective of glucose infusion, free IGF-I and -II remained stable during daytime (i.e. they were unresponsive to meal-related changes in plasma glucose), but both free fractions decreased during the night, reaching nadir at 04.00 h. None of the other members of the IGF system showed any relationship with plasma glucose levels. Finally, we failed to observe any relationship between changes in insulin sensitivity and the circulating IGF system.

CONCLUSION

We found no evidence that the circulating IGF system is involved in meal-related blood glucose regulation or that it reflects short-term changes in insulin sensitivity in healthy, nonobese subjects. However, we cannot preclude that the observed changes in circulating IGFBP-1 may affect the glucose-lowering effect of IGF-I and -II at the local tissue level.

Comparative Biology of the IGF System in Endometrium, Decidua, and Placenta, and Clinical Implications for Foetal Growth and Implantation Disorders

The insulin like growth factors and their binding proteins appear to play a central role during implantation and establishment of pregnancy in all species studied. Although there are similarities among species in the cell types that express IGFs and IGFBPs and their regulation during implantation and pregnancy, there are also significant differences. Understanding of the role of the IGF system in placental function in the human is of immense clinical importance, because serious complications of pregnancy such as intrauterine growth restriction and pre-eclampsia are thought to be associated with alterations in IGF system during early pregnancy and later in gestation. Research in laboratory and domestic animals, including transgenic and gene targeting studies in mice, has significantly improved our understanding of the role of IGF system in placental and foetal development. This paper reviews the diversity in the expression and regulation of IGF system in the decidua and placenta at the foetal-maternal interface in the human and different animal species, which may benefit in directing future studies in understanding of various complications of human pregnancy.

Insulin-like growth factor binding protein-1 over-expression in transgenic mice inhibits hepatic preneoplasia

Insulin-like growth factor binding protein-1 (IGFBP-1) is synthesized in the liver and regulates the mitogenic effects of the insulin-like growth factors (IGFs). The evidence that IGFBP-1 plays a role in hepatocarcinogenesis, however, is equivocal. We have, therefore, investigated the development of preneoplastic hepatic lesions in transgenic mice in which the human IGFBP-1 gene is under the control of the mouse metallothionein promoter. The lesions were induced by treating 15-d-old male mice with a single intraperitoneal injection of 5 mg/kg diethylnitrosamine (DENA). Lesions were scored when the mice were 28 wk of age. Quantitative microscopy of liver sections revealed that significantly fewer transgenic mice treated with zinc to activate the transgene had focal lesions compared to either transgenic mice not treated with zinc or wild-type mice treated with zinc (36.4% versus 85.7% and 83.3%, respectively, P < 0.05 in each case). Zinc-treated transgenic mice also had significantly fewer lesions per liver (11.5 +/- 5.0 versus 74.7 +/- 18.4 and 59.4 +/- 15.6, respectively, P < 0.01 in each case) and a smaller percentage of liver volume occupied by lesions (0.2 +/- 0.1 versus 1.4 +/- 0.3 and 1.1 +/- 0.4 respectively, P < 0.05 in each case). Immunohistochemical staining showed that both IGF-I and IGF-II were overexpressed in most of the lesions. These results show that expression of the IGFBP-1 transgene leads to a marked inhibition of hepatic preneoplasia, possibly by decreasing the mitogenic activity of IGF-I and/or IGF-II. This study adds new evidence to the notion that the IGF axis plays an important role in liver cancer development.

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.

Insights from insulin-like growth factor binding protein transgenic mice

The existence of abundant high affinity binding proteins for the IGFs, the IGF binding proteins (IGFBPs), was first demonstrated more than 40 yr ago in the very early days of somatomedin research. With the development of molecular techniques and transgenic and knockout mouse models, the nature, complexity, and redundancy of the IGFBPs have now started to be elucidated. Indeed the functional role of the circulating IGFs and the originally proposed endocrine somatomedin hypothesis have recently been questioned. The limited reports to date indicate that IGFBP knockout mice have few phenotypic manifestations. In contrast, overexpression of IGFBPs in transgenic mice is associated with manifestations that provide some insight into the physiological role of the binding proteins. The predominant effect of generalized or tissue-specific overexpression of the IGFBPs has been growth inhibition as would be anticipated from inhibition of the actions of IGF-I and -II. In addition, impaired glucose homeostasis and reduced fecundity have been observed in both IGFBP-1- and IGFBP-3-overexpressing transgenic mice. This review examines the data reported to date for transgenic mouse models that overexpress IGFBPs. In addition, data from transgenic mice that overexpress the acid-labile subunit, an important component of the ternary complex, have also been reviewed.

Mutant mouse models of insulin-like growth factor actions in the central nervous system

Insulin-like growth factor-I (IGF-I) and its cognate receptor, the type 1 IGF receptor (IGF1R), as well as high-affinity IGF binding proteins (IGFBP) that modulate IGF-I actions, are expressed throughout the course of brain development. These observations, taken together with studies in cultured neural cells demonstrating a variety of IGF-I growth-promoting activities, provide a strong argument for IGF-I having a central role in the growth and development of the CNS. This report reviews studies of brain development in mutant mice with alterations of IGF-I expression or action. Transgenic (Tg) mice overexpressing IGF-I postnatally exhibit brain overgrowth characterized by increased neuron and oligodendrocyte number, as well as marked increases in myelination. Mutant mice with ablated IGF-I and IGF1R expression, as well as those with overexpression of IGFBPs capable of inhibiting IGF actions, exhibit brain growth retardation with a variety of growth deficits. These studies confirm a role for IGF-I in neural development, and indicate that IGF-I stimulates neurogenesis and synaptogenesis, facilitates oligodendrocyte development, promotes neuron and oligodendrocyte survival, and stimulates myelination. Evidence from experiments in these mouse models also indicates that IGF-I has a role in recovery from neural injury.

Physiological differences in insulin-like growth factor binding protein-1 (IGFBP-1) phosphorylation in IGFBP-1 transgenic mice

Insulin-like growth factor binding protein (IGFBP)-1 has been shown to alter cellular responses to insulin-like growth factor 1 (IGF-1). Human IGFBP-1 undergoes serine phosphorylation, and this enhances both its affinity for IGF-1 by six- to eightfold and its capacity to inhibit IGF-1 actions. To investigate the physiological role of IGFBP-1 in vivo, transgenic mice have been generated using either the human IGFBP-1 or rat IGFBP-1 transgene. Both lines of mice expressed high concentrations of IGFBP-1 in serum and tissues; however, human IGFBP-1 transgenic mice did not show glucose intolerance and exhibited no significant intrauterine growth retardation, whereas rat IGFBP-1 transgenic mice showed fasting hyperglycemia and intrauterine growth restriction. The aim of this study was to investigate the physiological differences in the phosphorylation state of human IGFBP-1 and rat IGFBP-1 in these transgenic mice. The phosphorylation status of IGFBP-1 in transgenic mouse serum was analyzed by nondenaturing PAGE. Almost all of the IGFBP-1 in serum from the human IGFBP-1 transgenic mice was present as a nonphosphorylated form. Most of the rat IGFBP-1 in the serum of the mice expressing the rat IGFBP-1 was phosphorylated. Immunoprecipitation showed that mouse hepatoma (Hepa 1-6) cells (exposed to [32P]H3PO4) secrete 32P-labeled IGFBP-1. When the human IGFBP-1 transgene was transfected into Hepa 1-6 cells, all of the IGFBP-1 was secreted in the nonphosphorylated form. However, when the rat IGFBP-1 transgene was transfected into these cells, phosphorylated forms of IGFBP-1 were secreted. To confirm this result, the mouse hepatoma cell protein kinase was partially purified. This kinase activity phosphorylated mouse and rat IGFBP-1 in vitro, but it did not phosphorylate human IGFBP-1. Scatchard analysis showed that the affinity of phosphorylated rat IGFBP-1 for IGF-1 was 3.9-fold higher than that of nonphosphorylated human IGFBP-1. We conclude that the mouse IGFBP-1 kinase activity cannot phosphorylate human IGFBP-1, whereas it can phosphorylate rat IGFBP-1. The phosphorylation state of human IGFBP-1 may account for part of the phenotypic differences noted in the two studies of transgenic mice, and it is an important determinant of the capacity of human IGFBP-1 to inhibit IGF-1 actions in vivo.

Inactivation of the acid labile subunit gene in mice results in mild retardation of postnatal growth despite profound disruptions in the circulating insulin-like growth factor system

Insulin-like growth factors (IGFs) I and II are important regulators of cell proliferation and differentiation. After birth, plasma IGFs, representing mostly liver-derived IGFs, circulate in ternary complexes of 150 kDa consisting of one molecule each of IGF, IGF-binding protein (IGFBP) 3, and an acid labile subunit (ALS). Onset of ALS synthesis after birth is the primary factor driving the formation of ternary complexes. Capture of IGFs by ALS is thought to allow the development of a plasma reservoir without negative effects such as hypoglycemia and cell proliferation. To evaluate the importance of ALS and ternary complexes, we have created mice in which the ALS gene has been inactivated. The mutation was inherited in a Mendelian manner, without any effects on survival rates and birth weights. A growth deficit was observed in null mice after 3 weeks of life and reached 13% by 10 weeks. This modest phenotype was observed despite reductions of 62 and 88% in the concentrations of plasma IGF-I and IGFBP-3, respectively. Increased turnover accounted for these reductions because indices of synthesis in liver and kidney were not decreased. Surprisingly, absence of ALS did not affect glucose and insulin homeostasis. Therefore, ALS is required for postnatal accumulation of IGF-I and IGFBP-3 but, consistent with findings supporting a predominant role for locally produced IGF-I, is not critical for growth. This model should be useful to determine whether presence of ALS is needed for other actions of liver-derived IGF-I and for maintenance of homeostasis in presence of high circulating levels of IGF-II.

Spatial and temporal patterns of expression of messenger RNA for insulin-like growth factors and their binding proteins in the placenta of man and laboratory animals

To better understand the role of the insulin-like growth factors (IGF-I and -II) and their binding proteins (IGFBPs 1-6) in placental development and function, it is important to review similarities and differences between species in expression of the respective mRNAs. In human placenta, IGF-II mRNA is expressed in chorionic mesoderm and first trimester villous cytotrophoblast, but not in syncytiotrophoblast. In contrast, in rhesus monkey placenta, IGF-II mRNA is expressed in syncytiotrophoblast but not in chorionic mesoderm. IGFBP-3 mRNA is present in the chorionic mesoderm of placental villi from both these species and may modulate IGF-II action through a paracrine mechanism. In rodent placentae, IGF-II mRNA is expressed both in fetal mesoderm and in the trophoblast of the placental labyrinth. In guinea pig, where IGFBP-5 mRNA is expressed in the marginal and interlobular syncytium and IGF-II mRNA in the labyrinth, interaction between IGF-II and IGFBP-5 mRNA may be involved in vascularization of the placenta by fetal vessels. In sheep placenta, IGF-II mRNA is expressed, not in the trophoblast layer, but in the fetal mesoderm immediately adjacent to it. In the basal plate of human, rhesus monkey and baboon placentae, extravillous trophoblasts express IGF-II mRNA and uterine decidual cells IGFBP 1-6 mRNAs. The inference is that there is interaction between IGF-II and IGFBPs at the maternal-fetal interface of the primate placenta during trophoblast invasion and decidualization. IGFBP-1 expressed by the decidua may also interact with alpha(5)beta(1)integrin expressed by the extravillous trophoblast. The placentae of rodents are also of the invasive type. Glycogen cells of the mouse placenta are analogous with human extravillous trophoblast and express IGF-II mRNA. However, expression of IGFBP mRNAs in the mouse, as in the guinea pig, is confined to non-decidualized endometrium and myometrium. IGF-II mRNA is strongly expressed by trophoblasts invading uterine vessels in human and guinea pig placentae. Interactions probably occur between IGF-II expressed by these trophoblasts and IGFBPs expressed in the vessel walls. However, it is possible that IGFBPs expressed by maternal vessels are associated with processes that are independent of trophoblast invasion. Thus, IGFBP-3 mRNA is highly expressed in the maternal blood vessels of the non-deciduate sheep placenta. Findings to date highlight the diversity in the expression of the IGF system among placentae of man and different laboratory animals, and even between closely related species. Comparative studies will continue to be required to understand the functional role of IGFs and IGFBPs in each species.

Human insulin-like growth factor (IGF) binding protein-1 inhibits IGF-I-stimulated body growth but stimulates growth of the kidney in snell dwarf mice

The actions of insulin-like growth factor-I (IGF-I) are modulated by IGF binding proteins (IGFBPs). The effects of IGFBP-1 in vivo are insufficiently known, with respect to inhibitory or stimulatory actions on IGF-induced growth of specific organs. Therefore, we studied the effects of IGFBP-1 on IGF-I-induced somatic and organ growth in pituitary-deficient Snell dwarf mice. Human GH, IGF-I, IGFBP-1, and a preequilibrated combination of equimolar amounts of IGF-I and IGFBP-1 were administered sc during 4 weeks. Treatment with IGF-I alone induced a significant increase in body length (108% of control) and weight (112%) as well as an increase in weight of the submandibular salivary glands (135%), kidneys (124%), femoral muscles (111%), testes (129%), and spleen (126%) compared with saline-treated controls. IGFBP-1 alone induced a significant increase in weight of the kidneys (152% of control). Coadministration of IGF-I with IGFBP-1 neutralized the stimulating effects of IGF-I on body length and weight as well as on the femoral muscles and testes. In contrast, the weights of the submandibular salivary glands (143%) were not significantly different from those of IGF-I-treated animals, whereas the weights of the kidneys (171%) and spleen (156%) were significantly increased compared with IGF-I-treated mice. The effect of IGFBP-1 plus IGF-I on kidney weight was not significantly greater than the effect of IGFBP-1 alone. Western ligand blotting showed induction of the IGFBP-3 doublet as well as IGFBPs with molecular masses of 24 kDa, most probably IGFBP-4, by human GH, IGF-I alone, and IGF-I in combination with IGFBP-1. Our data show that coadministration of IGFBP-1 inhibits IGF-I-induced body growth of GH-deficient mice but significantly stimulates the growth promoting effects of IGF-I on the kidneys and the spleen. These data warrant further investigation because differences in concentrations of IGFBP-1 occurring in vivo may influence IGF-I-induced anabolic processes.

AUUUA sequences compromise human insulin-like growth factor binding protein-1 mRNA stability

The instability of IGFBP-1 mRNA appears to play a role in regulating the expression of the IGFBP-1 gene, the 3' region of which contains five ATTTA sequences. We have studied the implication of these sequences for IGFBP-1 mRNA destabilization. Six plasmids were constructed, containing increasingly shorter lengths of IGFBP-1 cDNA, each with a successive ATTTA sequence deleted from the 3' end. These were stably transfected into two non-IGFBP-1-expressing (cervical carcinoma and neuroblastoma) cell lines. Kinetics studies following inhibition of transcription showed that (1) the half-life of the full-length messenger was 2.80 +/- 0.32 h; (2) deletion of each successive sequence (particularly the second and the fourth) yielded a transcript of increasing stability; and (3) the half-life of the AUUUA-free mRNA was 26.65 +/- 1.65 h. Although the primary source of IGFBP-1 is the liver, our results demonstrate that destabilization of its mRNA is not liver-specific. The ATTTA consensus sequences in the 3' untranslated region of the IGFBP-1 gene therefore provide a posttranscriptional regulation pathway that, combined with transcriptional regulation, may account for the variations in IGFBP-1 expression with developmental stage, nutritional status, and hormonal environment.

Insulin-like growth factor binding protein-1 induces insulin release in the rat

Injections of human insulin-like growth factor binding protein (hIGFBP-1) are reported to induce hyperglycemia in the rat, suggesting that IGFBP-1 acutely regulates glucose homeostasis. We now report the effects on glucose and insulin levels of administering recombinant (r) hIGFBP-1. In a series of studies, normal and streptozotocin (STZ) diabetic male Wistar rats (180-210 g), fasted for 6 or 16 h, were injected with rhIGFBP-1 (i.v., 80-500 microg/rat). rhIGFBP-1 did not affect blood glucose acutely but did stimulate insulin release in normal rats (5 min post injection; PBS, 103.5 +/- 8.5; rhIGFBP-1 (500 microg), 166.8 +/- 15.7; rhIGFBP-1 (100 microg); 151.4 +/- 14.1% initial). rhIGFBP-1 pretreatment, in normal and diabetic rats, reduced the hypoglycemic response to rhIGF-I (diabetic rats after 20 min: PBS, 103.4 +/- 11.4; BP-1 (500 microg) +/- rhIGF-I (50 microg), 97.6 +/- 3.6; rhIGF-I, 48.2 +/- 4.3% initial) but did not affect the hypoglycemic response to des(1-3)IGF-I or insulin (0.5 U/kg). These studies show that rhIGFBP-1 causes insulin release, has a minimal effect on blood glucose, and inhibits the hypoglycemic effect of rhIGF-I. These data suggest that endogenous IGF-I tonically suppresses insulin secretion and imply that aberrant IGFBP levels or reduced IGF-I bioactivity may lead to chronic hyperinsulinemia.

The role of the insulin-like growth factors in the central nervous system

Increasing evidence strongly supports a role for insulin-like growth factor-I (IGF-I) in central nervous system (CNS) development. IGF-I, IGF-II, the type IIGF receptor (the cell surface tyrosine kinase receptor that mediates IGF signals), and some IGF binding proteins (IGFBPs; secreted proteins that modulate IGF actions) are expressed in many regions of the CNS beginning in utero. The expression pattern of IGF system proteins during brain growth suggests highly regulated and developmentally timed IGF actions on specific neural cell populations. IGF-I expression is predominantly in neurons and, in many brain regions, peaks in a fashion temporally coincident with periods in development when neuron progenitor proliferation and/or neuritic outgrowth occurs. In contrast, IGF-II expression is confined mainly to cells of mesenchymal and neural crest origin. While expression of type I IGF receptors appears ubiquitous, that of IGFBPs is characterized by regional and developmental specificity, and often occurs coordinately with peaks of IGF expression. In vitro IGF-I has been shown to stimulate the proliferation of neuron progenitors and/or the survival of neurons and oligodendrocytes, and in some cultured neurons, to stimulate function. Transgenic (Tg) mice that overexpress IGF-I in the brain exhibit postnatal brain overgrowth without anatomic abnormality (20-85% increases in weight, depending on the magnitude of expression). In contrast, Tg mice that exhibit ectopic brain expression of IGFBP-1, an inhibitor of IGF action when present in molar excess, manifest postnatal brain growth retardation, and mice with ablated IGF-I gene expression, accomplished by homologous recombination, have brains that are 60% of normal size as adults. Taken together, these in vivo studies indicate that IGF-I can influence the development of most, if not all, brain regions, and suggest that the cerebral cortex and cerebellum are especially sensitive to IGF-I actions. IGF-I's growth-promoting in vivo actions result from its capacity to increase neuron number, at least in certain populations, and from its potent stimulation of myelination. These IGF-I actions, taken together with its neuroprotective effects following CNS and peripheral nerve injury, suggest that it may be of therapeutic benefit in a wide variety of disorders affecting the nervous system.

Saliva and growth factors: the fountain of youth resides in us all

The predominant focus of research dealing with saliva revolves around the role in the maintenance of oral health through a number of physiological and biological properties of constituent proteins. An ever-expanding literature exists indicating that the salivary glands additionally synthesize, and secrete into saliva, a wide range of growth factors. Animal studies with epidermal growth factor have provided evidence for a role in both oral and systemic health, through the promotion of wound healing rates. Thus, the ability to manipulate their rates of synthesis and absorption from saliva holds the potential to enhance tissue regeneration and homeostasis.

Gene-targeting and transgenic approaches to IGF and IGF binding protein function

The ability to manipulate genetic information in the germ line of mice has provided powerful approaches to study gene function in vivo. These approaches have included the establishment of mouse lines in which a specified gene or genes are overexpressed, ectopically expressed, or deleted. Transgenic and gene-targeted mouse lines have been used extensively to study the function of the insulin-like growth factors (IGF), IGF-I and IGF-II, and their receptors and binding proteins. In the IGF system, these technologies have elucidated the roles of the IGFs in fetal and somatic growth and have demonstrated a critical role for this system in transformation and tumorigenesis. Analysis of combinatorial crosses of gene-targeted mouse lines also has suggested the existence of an as yet unidentified IGF receptor that regulates fetal growth. Similar approaches using transgenic and gene-targeted mouse models have been initiated to study the in vivo functions of the IGF binding proteins. These mouse models provide important tools to test specific functional questions in vivo as well as to study the long-term physiological consequences of chronic gene alterations.

Brain growth retardation due to the expression of human insulin like growth factor binding protein-1 in transgenic mice: an in vivo model for the analysis of igf function in the brain

Three lines of transgenic (Tg) mice carrying a fusion gene linking the mouse metallothionein-I promoter to a cDNA encoding human insulin-like growth factor binding protein-1 (hIGFBP-1) were found to express the transgene in brain. As judged by comparing Tg brain weights to those of non-transgenic littermates, adult hemizygotic Tg mice of each line exhibited brain growth retardation (16.2%, 14.4% and 8.1% reductions in weight, respectively in each line). In two lines, total brain DNA and protein content were decreased. Further analysis indicated that the brain growth retardation was manifested in the second week of postnatal life. Given that the insulin-like growth factors (IGFs) stimulate cell proliferation and/or survival in neural cultures and that hIGFBP-1, when present in a molar excess, inhibits IGF interactions with their cell surface receptors, the brain growth retardation in hIGFBP-1 Tg mice likely results from hIGFBP-1 inhibition of IGF-stimulated growth-promoting actions. These hIGFBP-1 Tg mice should prove useful in defining IGF actions during postnatal brain maturation.