Involvement of growth hormone (GH) and insulin-like growth factor (IGF) system in ovarian folliculogenesis

During the last decade, involvement of growth hormone (GH), insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs) in ovarian folliculogenesis has been extensively studied. This review provides an update on the GH, IGF system and their role in ovarian follicular development. In vitro studies and knockout experiments demonstrated an important role of GH in preantral follicle growth and differentiation through their binding with GH receptors, which are located both in the oocyte and follicular somatic tissues. Furthermore, GH stimulates the development of small antral follicles to gonadotrophin-dependent stages, as well as maturation of oocytes. With regard to the IGF system, IGF-I has no effects on primordial follicle development, but both IGF-I and IGF-II stimulate growth of secondary follicles. Depending on the species studies and method used, these proteins have been detected in oocytes and/or somatic cells. In antral follicles, these IGFs stimulate granulosa cell proliferation and steroidogenesis in most mammals. The bioavailability of IGFs is regulated by a family of intrafollicular expressed IGF binding proteins (IGFBPs). Facilitation of IGF can be increased through the activity of specific IGFBP proteases, which degrade the IGF/IGFBP complex, resulting in the production of IGFBP fragments and release of attached IGF.     

The insulin-like growth factor (IGF) system and gonadotropin regulation: actions and interactions

Insulin-like growth factors (IGF) are polypeptides that regulate growth, differentiation and survival in a multitude of cells and tissues. The IGF system consists of ligands, receptors, binding proteins and binding protein proteases. The influence of the IGF system on reproductive parameters, specifically gonadotropin release and interactions between the IGF system and other effectors of gonadotropin release will be examined in this review.     

类胰岛素生长因子信号途径及其对脊椎动物生长和发育的调控作用:以斑马鱼为模式动物的新进展

类胰岛素生长因子(包括IGF-I和IGF-Ⅱ)是进化上保守性很强的多肽。IGFs对脊椎动物的生长和早期发育有极其重要的调控作用。IGF的生理作用是由IGF受体中介并受几个分泌性的IGF结合蛋白调节。本文主要介绍了以斑马鱼为模式动物，用基因敲除、转基因动物和培养细胞系等现代实验方法对IGF信号途径的最新研究进展，综述了IGF配体、受体和结合蛋白的结构特点、基因表达和调节和生物学功能。此外，也对斑马鱼作为模式动物的许多优点进行了探讨。     

IGF-1对细胞凋亡的抑制调控

胰岛素样生长因子-1（insulin—like growth factor,IGF—1）是胰岛素样生长因子家族中的一种,通过与IGF-1受体相结合产生生物学效应,是通过内分泌、自分泌和旁分泌的三种途径分泌的低分子多肽。近些年来,研究发现IGF-1不仅具有胰岛素类似的功能以及介导生长激素的作用,还是多种类型细胞凋亡的一个重要抑制因子。本文就IGF-1抑制细胞凋亡的信号转导途径和IGF-1对Bcl-2家族、caspases家族以及关键转录因子的调控机制作一综述。     

Structural analogs of human insulin-like growth factor (IGF) I with altered affinity for type 2 IGF receptors

We have used site-directed mutagenesis of a synthetic gene for insulin-like growth factor (IGF) I to prepare three analogs in which specific residues in the A region are replaced with the corresponding residues in the A chain of insulin. The analogs are [Ile41, Glu45, Gln46, Thr49, Ser50, Ile51, Ser53, Tyr55, Gln56]IGF I (A chain mutant), in which residue 41 is changed from threonine to isoleucine and residues 42 to 56 of the A region are replaced, [Thr49, Ser50, Ile51]IGF I, and [Tyr55, Gln56]IGF I. These analogs are all equipotent to IGF I at the type 1 IGF receptor in human placental membranes, and in stimulating the incorporation of [3H]thymidine into DNA in the rat vascular smooth muscle cell line A10. However, the A chain mutant and [Thr49, Ser50, Ile51]IGF I have greater than 20-fold lower relative affinity for the type 2 IGF receptor of rat liver membranes, respectively. In contrast, [Tyr55, Gln56]IGF I has 7-fold higher affinity than IGF I for the type 2 IGF receptor. Residues 49, 50, and 51 in IGF I are Phe-Arg-Ser and are strictly conserved in IGF II. Residues 55 and 56 of IGF I and the corresponding residues in IGF II are Arg-Arg and Ala-Leu, respectively. Thus, the presence of the charged residues at these positions in IGF I appears to be responsible, in part, for the lower affinity of IGF I for the type 2 IGF receptor. In addition to the alterations in affinity for the type 2 IGF receptor, the A chain mutant has a 7-fold increase in affinity for insulin receptors, and [Thr49, Ser50, Ile51]IGF I has a 4-fold lower affinity for acid-stable human serum binding protein. These data strongly suggest that specific determinants in the A region of IGF I are important for maintaining binding to the type 2 IGF receptor, and that these determinants are different from those required for maintaining high affinity for the type 1 IGF receptor.     

Switching maternal dietary intake at the end of the first trimester has profound effects on placental development and fetal growth in adolescent ewes carrying singleton fetuses.

The aim was to investigate whether placental growth and hence pregnancy outcome could be altered by switching adolescent dams from a high to a moderate nutrient intake, and vice-versa, at the end of the first trimester. Embryos recovered from adult ewes inseminated by a single sire were transferred in singleton to peripubertal adolescents. After transfer, adolescent ewes were offered a high (H, n = 33) or moderate (M, n = 32) level of a diet calculated to promote rapid or moderate maternal growth rates, respectively. At Day 50 of gestation, half the ewes had their dietary intakes switched, yielding 4 treatment groups: HH, MM, HM, and MH. A subset of ewes were killed at Day 104 of gestation to determine maternal body composition in relation to growth of the products of conception. Maternal body composition measurements revealed that the higher live weight in the high-intake dams was predominantly due to an increase in body fat deposition, with a less pronounced increase in body protein. At Day 104, HH and MH groups (high intake during second trimester) compared with MM and HM groups (moderate intake during second trimester) had a lower (p < 0.002) total fetal cotyledon weight; but fetal weight, conformation, and individual organ weights were not significantly influenced by maternal dietary intake. In ewes delivering live young at term, a high plane of nutrition from the end of the first trimester (HH and MH groups) compared with moderate levels (MM and HM groups) was associated with a reduction in gestation length (p < 0.009), total placental weight (p < 0.002), total fetal cotyledon weight (p < 0.001), and mean fetal cotyledon weight per placenta (p < 0.001). Fetal cotyledon number was dependent on maternal dietary intake during the first trimester only and was lower (p < 0.007) in HH and HM ewes compared to MM and MH ewes. The inhibition of fetal cotyledon growth in HH and MH groups was associated with a major decrease (p < 0.001) in lamb birth weight at term relative to the MM and HM groups. Thus, reducing maternal dietary intake from a high to a moderate level at the end of the first trimester stimulates placental growth and enhances pregnancy outcome, and increasing maternal dietary intake at this time point has a deleterious effect on placental development and fetal growth.     

Association of the insulin-like growth factor II (IGF2) gene with human cognitive functions

Active search for candidate genes whose polymorphisms are associated with human cognitive functions has been in progress in the past years. The study focused on the role that the insulin-like growth factor II (IGF2) gene may play in the variation of cognitive processes related to executive functions. The ApaI polymorphism of the IGF2 gene was tested for association with selective attention during visual search, working memory/mental control, and semantic verbal fluency in a group of 182 healthy individuals. The ApaI polymorphism was associated with the general cognitive index and selective attention measure. Carriers of genotype AA displayed higher values of the two parameters than carriers of genotype GG. It was assumed that the ApaI polymorphism of the IGF2 gene influences the human cognitive functions, acting possibly via modulation of the IGF-II level in the central nervous system.     

The C region of human insulin-like growth factor (IGF) I is required for high affinity binding to the type 1 IGF receptor

We have produced and characterized the binding properties of three structural analogs of human insulin-like growth factor I (hIGF-I). These analogs are [1-62]hIGF-I, an analog lacking the carboxyl-terminal 8-amino acid D region of hIGF-I; [1-27, Gly4, 38-70]hIGF-I, an analog in which residues 28-37 of the C region of hIGF-I are replaced by a 4-reside glycine bridge; and [1-27,Gly4,38-62]hIGF-I, an analog with the C region glycine replacement and a D region deletion. The removal of the D region of hIGF-I has little effect on binding to the type 1 and type 2 insulin-like growth factor (IGF) receptors. [1-62]hIGF-I has 2-fold higher affinity for the insulin receptor and 4-fold higher affinity for IGF serum-binding proteins. The replacement of the C region of hIGF-I with a four-glycine span results in a 30-fold loss of affinity for the type 1 IGF receptor. However this analog has near normal affinity for the type 2 IGF receptor, the insulin receptor, and IGF serum-binding proteins. Incorporating the C region glycine replacement and the D region deletion into one analog does not affect binding to either the type 2 receptor or to IGF serum-binding proteins. As predicted from the single deletion analogs [1-27,Gly4,38-62]hIGF-I has reduced affinity for the type 1 IGF receptor (approximately 40-fold) and increased affinity for the insulin receptor (5-fold). These data indicate that determinants in the C region of hIGF-I are involved in maintaining high affinity binding to the type 1 IGF receptor and that neither the C region nor the D region are required for high affinity binding to the type 2 IGF receptor or to IGF serum-binding proteins.     

Porcine insulin-like growth factor (IGF) binding protein blocks IGF-I action on porcine adipose tissue

A growth hormone-dependent insulin-like growth factor (IGF) binding protein (IGFBP) purified from porcine serum specifically blocked the acute insulin-like effects of IGF-I on lipogenesis and glucose oxidation in porcine adipose tissue. This inhibition was dose dependent with half-maximal effective concentrations of IGFBP of 530 ng/ml for lipogenesis and 590 ng/ml for glucose oxidation in the presence of 10(-8) M IGF-I. The IGFBP also caused decreased rates of lipogenesis following a 1-hr preincubation of tissue with IGF-I (10(-8) M). The IGFBP had no effect on insulin action on porcine adipose tissue. These findings demonstrate the inhibitory effects of a highly purified porcine serum IGFBP on the biologic effects of IGF-I in vitro, and provide evidence that the growth hormone-dependent IGFBP blocks the acute insulin-like actions of IGF-I in vivo.     

Porcine insulin-like growth factor 1 (IGF1) gene polymorphisms are associated with body size variation

Previous studies have confirmed that insulin growth factor-1 (IGF1) plays important roles in growth and body size in humans and animals. However, whether single nucleotide polymorphisms (SNPs) within the IGF1 gene affects body size and growth in pigs has been unclear. We identified IGF1 SNPs among 5 pig breeds (Berkshire, Duroc, Landrace, Yorkshire and Korea Native Pig) and found that the G allele of SNP (c.G189A) was associated with higher body weight and was more predominant in western pig breeds, while the Korean Native Pig is the breed with the highest frequency of the A allele. Four haplotypes (–GA–, –GG–, –AG–, and –AA–) were constructed using the 2 identified SNPs. The GA haplotype was most frequently observed, except in the Berkshire breed. In addition, these SNPs and haplotypes were significantly associated with body size (final weight), average daily gain, and backfat thickness (P < 0.05) in 2 intercrossed F₂ pig populations (KNP × YS F₂ and KNP × LR F₂). Furthermore, the major GA haplotype had a significant additive effect on body size and average daily gain. In conclusion, specific SNPs within the porcine IGF1 gene may contribute to the smaller body size and lower growth rate of Korea Native Pigs.     

Cooperativity of the N- and C-terminal domains of insulin-like growth factor (IGF) binding protein 2 in IGF binding

A family of six insulin-like growth factor (IGF) binding proteins (IGFBP-1-6) binds IGF-I and IGF-II with high affinity and thus regulates their bioavailability and biological functions. IGFBPs consist of N- and C-terminal domains, which are highly conserved and cysteine-rich, joined by a variable linker domain. The role of the C-domain in IGF binding is not completely understood in that C-domain fragments have very low or even undetectable IGF binding affinity, but loss of the C-domain dramatically disrupts IGF binding by IGFBPs. We recently reported the solution structure and backbone dynamics of the C-domain of IGFBP-2 (C-BP-2) and identified a pH-dependent heparin binding site [Kuang, Z., Yao, S., Keizer, D. W., Wang, C. C., Bach, L. A., Forbes, B. E., Wallace, J. C., and Norton, R. S. (2006) Structure, dynamics and heparin binding of the C-terminal domain of insulin-like growth factor-binding protein-2 (IGFBP-2), J. Mol. Biol. 364, 690-704]. Here, we have analyzed the molecular interactions among the N-domain of IGFBP-2 (N-BP-2), C-BP-2, and IGFs using cross-linking and nuclear magnetic resonance (NMR) spectroscopy. The binding of C-BP-2 to the IGF-I.N-BP-2 binary complex was significantly stronger than the binding of C-BP-2 to IGF-I alone, switching from intermediate exchange to slow exchange on the NMR time scale. A conformational change or stabilization of the IGF-I Phe49-Leu54 region and the Phe49 aromatic ring upon binding to the N-domains, as well as an interdomain interaction between N-BP-2 and C-BP-2 (which is also detectable in the absence of ligand), may contribute to this cooperativity in IGF binding. Glycosaminoglycan binding by IGFBPs can affect their IGF binding although the effects appear to differ among different IGFBPs; here, we found that heparin bound to the IGF-I.N-BP-2.C-BP-2 ternary complex, but did not cause it to dissociate.     

Expression of mRNAs encoding insulin-like growth factor (IGF) ligands, IGF receptors and IGF binding proteins during follicular growth and atresia in the ovine ovary throughout the oestrous cycle

The components of the insulin-like growth factor (IGF) system appear to be involved in the regulation of ovarian follicular growth and atresia in sheep. However, previous studies have only investigated a select few components of the system. The aim of the present study was to investigate the expression of mRNA encoding all of the components of the sheep IGF system among follicles of varying size and health status throughout the oestrous cycle using sheep-specific ribonucleotide probes and in situ hybridisation. For all IGF components, gene expression was unaffected by stage of oestrous cycle. IGF-I mRNA expression in all classes of follicle was generally low throughout the oestrous cycle, while IGFBP-1 mRNA expression could not be demonstrated at all. In contrast, there was relatively intense follicular expression of mRNAs encoding all remaining IGF system components. For IGF-II, both IGF receptors and IGFBP-2, -3, -4, -5, and -6, gene expression decreased as follicles increased in diameter (P < 0.01). IGF-II, type I IGF-R and IGFBP-2, -3, -4, and -6 mRNA expression significantly decreased as follicles progressed from healthy to atretic status (P < 0.01), whereas gene expression for type II IGF-R and IGFBP-5 was greater in atretic follicles (P < 0.01). This study demonstrates the spatial patterns of follicular gene expression for all of the IGF system components in cycling sheep for the first time. These results further highlight the potential functional role of IGF-II, in contrast to IGF-I, in the autocrine and/or paracrine regulation of follicle growth in sheep.     

Purification and characterization of insulin-like growth factor II (IGF II) and an IGF II variant from human placenta

In order to purify variant IGF II peptides from human placenta, we have developed a purification procedure combining heparin affinity chromatography and cation-exchange, reversed-phase and size-exclusion HPLC. Two peptides were purified, both having apparent M_r values of ca. 7300 Da as evaluated by SDS-PAGE. N-Terminal sequencing revealed IGF II and an IGF II variant in which Ser²⁹ was replaced by the tetrapeptide Arg-Leu-Pro-Gly. The final yield of variant IGF II was about eight-fold lower than that of IGF II. Both pure peptides were functionally active as they bound to type I and type II IGF receptors from ovine and human placental membranes, as determined by crosslinking experiments and displacement curve studies.     

C-terminal domain of insulin-like growth factor (IGF) binding protein-6: structure and interaction with IGF-II

IGFs are important mediators of growth. IGF binding proteins (IGFBPs) 1-6 regulate IGF actions and have IGF-independent actions. The C-terminal domains of IGFBPs contribute to high-affinity IGF binding and modulation of IGF actions and confer some IGF-independent properties, but understanding how they achieve this has been constrained by the lack of a three-dimensional structure. We therefore determined the solution structure of the C-domain of IGFBP-6 using nuclear magnetic resonance (NMR). The domain consists of a thyroglobulin type 1 fold comprising an alpha-helix followed by a loop, a three-stranded antiparallel beta-sheet incorporating a second loop, and finally a disulfide-bonded flexible third loop. The IGF-II binding site on the C-domain was identified by examining NMR spectral changes upon complex formation. It consists of a largely hydrophobic surface patch involving the alpha-helix, the first beta-strand, and the first and second loops. The site was confirmed by mutagenesis of several residues, which resulted in decreased IGF binding affinity. The IGF-II binding site lies adjacent to surfaces likely to be involved in glycosaminoglycan binding of IGFBPs, which might explain their decreased IGF affinity when bound to glycosaminoglycans, and nuclear localization. Our structure provides a framework for understanding the roles of IGFBP C-domains in modulating IGF actions and conferring IGF-independent actions, as well as ultimately for the development of therapeutic IGF inhibitors for diseases including cancer.     

Understanding the mechanism of insulin and insulin-like growth factor (IGF) receptor activation by IGF-II

Background

Insulin-like growth factor-II (IGF-II) promotes cell proliferation and survival and plays an important role in normal fetal development and placental function. IGF-II binds both the insulin-like growth factor receptor (IGF-1R) and insulin receptor isoform A (IR-A) with high affinity. Interestingly both IGF-II and the IR-A are often upregulated in cancer and IGF-II acts via both receptors to promote cancer proliferation. There is relatively little known about the mechanism of ligand induced activation of the insulin (IR) and IGF-1R. The recently solved IR structure reveals a folded over dimer with two potential ligand binding pockets arising from residues on each receptor half. Site-directed mutagenesis has mapped receptor residues important for ligand binding to two separate sites within the ligand binding pocket and we have recently shown that the IGFs have two separate binding surfaces which interact with the receptor sites 1 and 2.

Methodology/Principal Findings

In this study we describe a series of partial IGF-1R and IR agonists generated by mutating Glu12 of IGF-II. By comparing receptor binding affinities, abilities to induce negative cooperativity and potencies in receptor activation, we provide evidence that residue Glu12 bridges the two receptor halves leading to receptor activation.

Conclusions/Significance

This study provides novel insight into the mechanism of receptor binding and activation by IGF-II, which may be important for the future development of inhibitors of its action for the treatment of cancer.