Structure of the Mr 140,000 growth hormone-dependent insulin-like growth factor binding protein complex: determination by reconstitution and affinity-labeling

Endocrine and cellular physiology and pathology of the insulin-like growth factor acid-labile subunit

The acid-labile subunit (ALS) of the insulin-like growth factor (IGF) binding protein (IGFBP) complex, encoded in humans by IGFALS, has a vital role in regulating the endocrine transport and bioavailability of IGF-1 and IGF-2. Accordingly, ALS has a considerable influence on postnatal growth and metabolism. ALS is a leucine-rich glycoprotein that forms high-affinity ternary complexes with IGFBP-3 or IGFBP-5 when they are occupied by either IGF-1 or IGF-2. These complexes constitute a stable reservoir of circulating IGFs, blocking the potentially hypoglycaemic activity of unbound IGFs. ALS is primarily synthesized by hepatocytes and its expression is lower in non-hepatic tissues. ALS synthesis is strongly induced by growth hormone and suppressed by IL-1β, thus potentially serving as a marker of growth hormone secretion and/or activity and of inflammation. IGFALS mutations in humans and Igfals deletion in mice cause modest growth retardation and pubertal delay, accompanied by decreased osteogenesis and enhanced adipogenesis. In hepatocellular carcinoma, IGFALS is described as a tumour suppressor; however, its contribution to other cancers is not well delineated. This Review addresses the endocrine physiology and pathology of ALS, discusses the latest cell and proteomic studies that suggest emerging cellular roles for ALS and outlines its involvement in other disease states.

The role of acid-labile subunit (ALS) in the modulation of GH-IGF-I action

Acid-labile subunit (ALS) deficiency (ACLSD) constitutes the first monogenic defect involving a member of the Insulin-like Growth Factor (IGF) binding protein system. The lack of ALS completely disrupts the circulating IGF system. Autocrine/paracrine action of local produced IGF-I could explain the mild effect on growth. In the present work we have revised the more relevant clinical and biochemical consequences of complete ACLSD in 61 reported subjects from 31 families. Low birth weight and/or length, reduced head circumference, height between -2 and -3 SD, pubertal delay and insulin resistance are commonly observed. Partial ACLSD could be present in children initially labeled as idiopathic short stature, presenting low IGF-I levels, suggesting that one functional IGFALS allele is insufficient to stabilize ternary complexes. Dysfunction of the GH-IGF axis observed in ACLSD may eventually result in increased risk for type-2 diabetes and tumor progression. Consequently, long term surveillance is recommended in these patients.

Effects of growth hormone and cortisol administration on plasma insulin-like growth factor binding proteins in juveniles of three subspecies of masu salmon (Oncorhynchus masou)

In this study, we examined the effects of porcine growth hormone (GH) and cortisol on plasma insulin-like growth factor binding proteins (IGFBPs) in juveniles of three subspecies of Oncorhynchus masou (masu, amago, and Biwa salmon). Ligand blotting using digoxigenin-labeled human IGF-I was used to detect and semi-quantify three major circulating IGFBP bands at 41, 28, and 22 kDa, corresponding to IGFBP-2b, -1a, and -1b, respectively. GH increased plasma IGFBP-2b concentration in masu and Biwa salmon but suppressed it in amago salmon. Plasma IGFBP-2b levels were increased by cortisol in the three subspecies. Cortisol induced plasma IGFBP-1a in the three subspecies, whereas GH had a suppressive effect in masu and Biwa salmon. Sham and cortisol injections increased plasma IGFBP-1b levels after 1 day in masu and amago salmon, suggesting that IGFBP-1b is induced following exposure to stressors via cortisol. Increased IGFBP-1b levels were restored to basal levels when co-injected with GH in Biwa salmon, and the same trend was seen in masu and amago salmon. However, the suppressive effect of GH disappeared 2 days after injection in the three subspecies. Despite some differences among subspecies, the findings suggest that cortisol is a primary inducer of plasma IGFBP-1b; however, GH counteracts it in the short term. Therefore, GH has the potential to modulate the degree of increase in circulating IGFBP-1b levels during acute stress.

Biochemical determinants of the IGFBP-3-hyaluronan interaction

IGFBP‐3, the most abundant IGFBP and the main carrier of insulin‐like growth factor I (IGF‐I) in the circulation, can bind IGF‐1 with high affinity, which attenuates IGF/IGF‐IR interactions, thereby resulting in antiproliferative effects. The C‐terminal domain of insulin‐like growth factor‐binding protein‐3 (IGFBP‐3) is known to contain an 18‐basic amino acid motif capable of interacting with either humanin (HN) or hyaluronan (HA). We previously showed that the 18‐amino acid IGFBP‐3 peptide is capable of binding either HA or HN with comparable affinities to the full‐length IGFBP‐3 protein and that IGFBP‐3 can compete with the HA receptor, CD44, for binding HA. Blocking the interaction between HA and CD44 reduced viability of A549 human lung cancer cells. In this study, we set out to better characterize IGFBP‐3‐HA interactions. We show that both stereochemistry and amino acid identity are important determinants of the interaction between the IGFBP‐3 peptide and HA and for the peptide's ability to exert its cytotoxic effects. Binding of IGFBP‐3 to either HA or HN was unaffected by glycosylation or reduction of IGFBP‐3, suggesting that the basic 18‐amino acid residue sequence of IGFBP‐3 remains accessible for interaction with either HN or HA upon glycosylation or reduction of the full‐length protein. Removing N‐linked oligosaccharides from CD44 increased its ability to compete with IGFBP‐3 for binding HA, while reduction of CD44 rendered the protein relatively ineffective at blocking IGFBP‐3‐HA interactions. We conclude that both deglycosylation and disulfide bond formation are important for CD44 to compete with IGFBP‐3 for binding HA.

IGFBP-3/IGFBP-3 Receptor System as an Anti-Tumor and Anti-Metastatic Signaling in Cancer

Insulin-like growth factor binding protein-3 (IGFBP-3) is a p53 tumor suppressor-regulated protein and a major carrier for IGFs in circulation. Among six high-affinity IGFBPs, which are IGFBP-1 through 6, IGFBP-3 is the most extensively investigated IGFBP species with respect to its IGF/IGF-I receptor (IGF-IR)-independent biological actions beyond its endocrine/paracrine/autocrine role in modulating IGF action in cancer. Disruption of IGFBP-3 at transcriptional and post-translational levels has been implicated in the pathophysiology of many different types of cancer including breast, prostate, and lung cancer. Over the past two decades, a wealth of evidence has revealed both tumor suppressing and tumor promoting effects of IGF/IGF-IR-independent actions of IGFBP-3 depending upon cell types, post-translational modifications, and assay methods. However, IGFBP-3′s anti-tumor function has been well accepted due to identification of functional IGFBP-3-interacting proteins, putative receptors, or crosstalk with other signaling cascades. This review mainly focuses on transmembrane protein 219 (TMEM219), which represents a novel IGFBP-3 receptor mediating antitumor effect of IGFBP-3. Furthermore, this review delineates the potential underlying mechanisms involved and the subsequent biological significance, emphasizing the clinical significance of the IGFBP-3/TMEM219 axis in assessing both the diagnosis and the prognosis of cancer as well as the therapeutic potential of TMEM219 agonists for cancer treatment.

Intact rather than total circulating insulin-like growth factor binding protein-1a is a negative indicator of growth in masu salmon

Insulin-like growth factor binding protein (IGFBP)-1a is one of three major circulating forms in salmon and induced under catabolic conditions. However, there is currently no immunoassay available for this form because of a lack of standard and specific antibodies. We developed a time-resolved fluoroimmunoassay (TR-FIA) for salmon IGFBP-1a using recombinant protein for labeling, an assay standard, and production of antiserum. The TR-FIA had a low cross-reactivity (3.6%) with IGFBP-1b, another major form in the circulation. Fasting for 4 wk had no effect on serum immunoreactive (total) IGFBP-1a levels in yearling masu salmon, whereas 6-wk fasting significantly increased it. There was a significant, but weak, negative relationship between serum total IGFBP-1a level and individual growth rate (r² = 0.12, P = 0.01). We next developed a ligand immuno-functional assay (LIFA) using europium-labeled IGF-I to quantify intact IGFBP-1a. In contrast to total IGFBP-1a, serum intact IGFBP-1a levels increased after 4 wk of fasting, and refeeding for 2 wk restored it to levels similar to those of the fed control. Serum intact IGFBP-1a levels showed a significant negative correlation with individual growth rate (r² = 0.52, P < 0.001), which was as good as that of IGFBP-1b. Our findings using newly developed TR-FIA and LIFA suggest that regulation of intact IGFBP-1a levels has an important effect on growth in salmon and that intact IGFBP-1a is a negative index of salmon growth.

Molecular identification of grouper Igfbp1 and its mRNA expression in primary hepatocytes under Gh and insulin

The insulin-like growth factor (IGF) system plays a pivotal role in the regulation of growth, and IGF binding proteins (IGFBPs) are important regulatory factors in the IGF system. Generally, IGFBPs inhibit IGF actions by preventing its binding to receptors. Under some conditions, the IGFBPs can also enhance IGF actions. IGFBP1 is generally inhibitory to IGFI. In this study, the grouper (Epinephelus coioides) igfbp1 (MK621003) gene was cloned from the liver. The sequence of igfbp1 cDNA was 1055 bp and contained a 5'UTR of 127 bp and a 3'UTR of 247 bp, and the ORF of grouper igfbp1 was 741 bp, encoding 246 amino acids. The tissue distribution results showed that igfbp1 has a higher expression in the liver. In the nutritional status experiment, igfbp1 expression was significantly increased in the liver after 7 days of fasting and was markedly decreased after refeeding. In in vitro experiments, igfbp1 expression in grouper primary hepatocytes was significantly inhibited by recombinant grouper Gh (growth hormone) in a dose-dependent manner. Additionally, igfbp1 expression decreased in grouper primary hepatocytes upon incubation with insulin. This is the first report describing grouper igfbp1, and these findings contribute to understanding the roles of IGFBP1 in metabolism and growth in grouper.

Interaction of Insulin-Like Growth Factor-Binding Protein 3 With Hyaluronan and Its Regulation by Humanin and CD44

Insulin-like growth factor-binding protein-3 (IGFBP-3) belongs to a family of IGF-binding proteins. Humanin is a peptide known to bind residues 215-232 of mature IGFBP-3 in the C-terminal region of the protein. This region of IGFBP-3 was shown earlier to bind certain glycosaminoglycans including hyaluronan (HA). Here, we characterized the binding affinities of the IGFBP-3 protein and peptide (²¹⁵-KKGFYKKKQCRPSKGRKR-²³²) to HA and to humanin and found that HA binds with a weaker affinity to this region than does humanin. Either HA or humanin could bind to this IGFBP-3 segment, but not simultaneously. The HA receptor, CD44, blocked HA binding to IGFBP-3 but had no effect on binding of humanin to either IGFBP-3 or its peptide. Upon incubation of HA with CD44 and either IGFBP-3 protein or peptide, humanin was effective at binding and sequestering IGFBP-3 or peptide, thereby enabling access of CD44 to HA. We show that IGFBP-3 and humanin in the medium of A549 lung cancer cells can immunoprecipitate in a complex. However, the fraction of IGFBP-3 in the medium that is able to bind HA was not complexed with humanin suggesting that HA binding to the 215-232 segment renders it inaccessible for binding to humanin. Moreover, while the cytotoxic effects of IGFBP-3 on cell viability were reversed by humanin, blocking HA-CD44 interaction with an anti-CD44 antibody in combination with IGFBP-3 did not have an additive negative effect on cell viability suggesting that IGFBP-3 exerts its cytotoxic effects on cell survival through a mechanism that depends on HA-CD44 interactions.

Insulin-Like Growth Factor Binding Proteins in Autoimmune Diseases

Insulin-like growth factor binding proteins (IGFBPs) are a family of proteins binding to Insulin-like growth factors (IGFs), generally including IGFBP1, IGFBP2, IGFBP3, IGFBP4, IGFBP5, and IGFBP6. The biological functions of IGFBPs can be classified as IGFs-dependent actions and IGFs-independent effects. In this review, we will discuss the structure and function of various IGFBPs, particularly IGFBPs as potential emerging biomarkers and therapeutic targets in various autoimmune diseases, and the possible mechanisms by which IGFBPs act on the pathogenesis of autoimmune diseases.

IGF-Binding Proteins: Why Do They Exist and Why Are There So Many?

Insulin-like growth factors (IGFs) are key growth-promoting peptides that act as both endocrine hormones and autocrine/paracrine growth factors. In the bloodstream and in local tissues, most IGF molecules are bound by one of the members of the IGF-binding protein (IGFBP) family, of which six distinct types exist. These proteins bind to IGF with an equal or greater affinity than the IGF1 receptor and are thus in a key position to regulate IGF signaling globally and locally. Binding to an IGFBP increases the half-life of IGF in the circulation and blocks its potential binding to the insulin receptor. In addition to these classical roles, IGFBPs have been shown to modulate IGF signaling locally under various conditions. Although members of the IGFBP family share significant sequence homology, they each have unique structural features and play distinct roles. These IGFBP genes also have different modes of regulation and distinct expression patterns. Some IGFBPs have been found to bind to their own receptors or to translocate into the interior compartments of cells where they may execute IGF-independent actions. In spite of this functional and regulatory diversity, it has been puzzling that loss-of-function studies have yielded relatively little information about the physiological functions of IGFBPs. In this review, we suggest that evolution has tended to retain an array of IGFBPs in order to facilitate fine-tuning of IGF signaling. We explore the emerging explanation that many IGFBP functions have evolved to allow the targeted adjustment of IGF signaling under stressful or irregular conditions, which would likely not be revealed in a standard laboratory setting.

Circulating insulin-like growth factor binding proteins in fish: Their identities and physiological regulation

Insulin-like growth factor binding proteins (IGFBPs) play crucial roles in regulating the availability of IGFs to receptors and prolong the half-lives of IGFs. There are six IGFBPs present in the mammalian circulation with IGFBP-3 being most abundant. In mammals IGFBP-3 is the major carrier of circulating IGFs, facilitated by forming a ternary complex with IGF and an acid-labile subunit (ALS). IGFBP-1 is generally inhibitory to IGF action by preventing it from interacting with its receptors. In teleosts, the third-round of vertebrate whole genome duplication created paralogs of each IGFBP, except IGFBP-4. In the fish circulation, three major IGFBPs are typically detected at molecular ranges of 20-25, 28-32 and 40-50kDa. However, their identities are not well established. Three major circulating IGFBPs in Chinook salmon have been identified through protein purification and cDNA cloning. Salmon 28- and 22-kDa IGFBPs are co-orthologs of IGFBP-1, termed IGFBP-1a and -1b, respectively. They are induced under catabolic conditions such as stress and fasting but their responses are somewhat different, with IGFBP-1b being the most sensitive of the two. Cortisol stimulates production and secretion of these IGFBP-1 subtypes while, unlike in mammals, insulin may not be a primary suppressor. Salmon 41-kDa IGFBP, a major carrier of IGF-I, is not IGFBP-3, as might be expected extrapolating from mammals, but is in fact IGFBP-2b. Salmon IGFBP-2b levels in plasma are high when fish are fed, and GH treatment increases its circulating levels similar to mammalian IGFBP-3. These findings suggest that salmon IGFBP-2b acquired the role and regulation similar to mammalian IGFBP-3. Multiple replications of fish IGFBPs offer a unique opportunity to investigate molecular evolution of IGFBPs.

Understanding the Key to Targeting the IGF Axis in Cancer: A Biomarker Assessment

Type 1 insulin like growth factor receptor (IGF-1R) targeted therapies showed compelling pre-clinical evidence; however, to date, this has failed to translate into patient benefit in Phase 2/3 trials in unselected patients. This was further complicated by the toxicity, including hyperglycemia, which largely results from the overlap between IGF and insulin signaling systems and associated feedback mechanisms. This has halted the clinical development of inhibitors targeting IGF signaling, which has limited the availability of biopsy samples for correlative studies to understand biomarkers of response. Indeed, a major factor contributing to lack of clinical benefit of IGF targeting agents has been difficulty in identifying patients with tumors driven by IGF signaling due to the lack of predictive biomarkers. In this review, we will describe the IGF system, rationale for targeting IGF signaling, the potential liabilities of targeting strategies, and potential biomarkers that may improve success.

Clinical features of a new acid-labile subunit (IGFALS) heterozygous mutation: anthropometric and biochemical characterization and response to growth hormone administration

BACKGROUND

Homozygous mutations in acid-labile subunit (IGFALS) gene result in short stature, very low circulating levels of acid-labile subunit (ALS), insulin growth factor 1 (IGF1) and insulin growth factor binding protein 3 (IGFBP3) and a poor response to growth hormone (GH). The impact of IGFALS mutations heterozygosity on growth is unknown.

PATIENT AND METHODS

We describe a 10-year-old girl with severe short stature (height -3.2 SDS), heterozygous for a new IGFALS mutation.

RESULTS

The girl showed low circulating IGF1, IGFBP3 and ALS levels and normal GH secretion. We found a novel heterozygous frameshift IGFALS mutation (c.1283delA, p.Gln428Argfs*14). Size-exclusion chromatography showed a reduction of the IGF1, IGFBP3 and ALS 150-kDa ternary complex (by about 55%) compared to a control. An IGF-1 generation test, with two different GH dosages, showed a good response in term of increase in IGF1 and in formation of the ternary complex at size-exclusion chromatography. Clinical response after 6 months of therapy with GH was satisfactory (height velocity increased from 3 to 8 cm/year).

CONCLUSION

We suggest that (1) heterozygous IGFALS mutations can be responsible for a subset of patients with severe short stature (below -2.5 SDS), low IGF1 (below -2 SDS) and normal GH secretion, and (2) the identification by IGFALS molecular screening of this subset of patients could help in the administration of the appropriate therapy.

Non-Islet Cell Tumor Hypoglycemia Is Caused by Big IGF-II in a Patient with a Carcinosarcoma of the Uterus

We report a 72-year-old Japanese woman with severe hypoglycemia. The laboratory data, which revealed the suppression of serum insulin, suggested the existence of non-islet cell tumor hypoglycemia (NICTH). Abdominal computed tomography demonstrated the presence of a huge uterine tumor. The patient was treated with a continuous infusion of glucose, but died of sepsis on day 46. An autopsy revealed the pathological diagnosis to be a carcinosarcoma of the uterus. Interestingly, an immunohistochemical study discovered the expression of insulin-like growth factor (IGF)-II in both the carcinoma and sarcoma cells. In addition, an immunoblot analysis of blood samples revealed the presence of circulating big IGF-II. Therefore, this is a novel case of NICTH that was caused by a uterine carcinosarcoma.

Modulation of the somatotropic axis in periparturient dairy cows

This review focuses on modulation of growth hormone (GH) and its downstream actions on periparturient dairy cows undergoing physiological and metabolic adaptations. During the periparturient period, cows experience a negative energy balance implicating that the feed intake does not meet the total energy demand for the onset of lactation. To regulate this metabolic condition, key hormones of somatotropic axis such as GH, IGF-I and insulin must coordinate adaptations required for the preservation of metabolic homeostasis. The hepatic GHR1A transcript and GHR protein are reduced at parturition, but recovers on postpartum. However, plasma IGF-I concentration remains low even though hepatic abundance of the GHR and IGF-I mRNA return to pre-calving value. This might be caused by alternation in IGFBPs and ALS genes, which consequently affect the plasma IGF-I stability. Plasma insulin level declines in a parallel manner with the decrease in plasma IGF-I after parturition. Increased GH stimulates the lipolytic effects and hepatic glucose synthesis to meet the energy requirement for mammary lactose synthesis, suggesting that GH antagonizes insulin-dependent glucose uptake and attenuates insulin action to decrease gluconeogenesis.

Correlation of Circulating Acid-Labile Subunit Levels with Insulin Sensitivity and Serum LDL Cholesterol in Patients with Type 2 Diabetes: Findings from a Prospective Study with Rosiglitazone

Silencing of acid-labile subunit (ALS) improved glucose metabolism in animal models. The aim of this study is to evaluate the effects of rosiglitazone (RSG) on ALS levels in individuals with type 2 diabetes. A randomized, double-blind, placebo-controlled trial was conducted. Subjects with type 2 diabetes mellitus were randomly distributed to an RSG-treated (n = 30) or a placebo (n = 31) group. Patients were evaluated prior to treatment at baseline and at 12 and 24 weeks after treatment. At baseline, ALS levels were negatively associated with low-density lipoprotein cholesterol (LDLc) levels and homeostatic model assessment version 2 insulin sensitivity (HOMA2-%S). Over 24 weeks, there was a significantly greater reduction in ALS levels in the nonobese RSG-treated individuals than placebo-treated group. The effect of RSG on ALS was not significant in obese individuals. Fasting plasma glucose and hemoglobin A1c were reduced, but total cholesterol and LDLc were increased, in patients on RSG. Change in ALS levels predicted changes in total cholesterol and HOMA2-%S over time. This study suggested a BMI-dependent effect of RSG treatment on ALS levels. Reduction of ALS by RSG increases the risk of atherosclerosis in individuals with type 2 diabetes.

The complexity of the IGF1 gene splicing, posttranslational modification and bioactivity

The insulinlike growth factor-I (IGF-I) is an important factor which regulates a variety of cellular responses in multiple biological systems. The IGF1 gene comprises a highly conserved sequence and contains six exons, which give rise to heterogeneous mRNA transcripts by a combination of multiple transcription initiation sites and alternative splicing. These multiple transcripts code for different precursor IGF-I polypeptides, namely the IGF-IEa, IGF-IEb and IGF-IEc isoforms in humans, which also undergo posttranslational modifications, such as proteolytic processing and glycosylation. IGF-I actions are mediated through its binding to several cell-membrane receptors and the IGF-I domain responsible for the receptor binding is the bioactive mature IGF-I peptide, which is derived after the posttranslational cleavage of the pro-IGF-I isoforms and the removal of their carboxy-terminal E-peptides (that is, the Ea, Eb and Ec). Interestingly, differential biological activities have been reported for the different IGF-I isoforms, or for their E-peptides, implying that IGF-I peptides other than the IGF-I ligand also possess bioactivity and, thus, both common and unique or complementary pathways exist for the IGF-I isoforms to promote biological effects. The multiple peptides derived from IGF-I and the differential expression of its various transcripts in different conditions and pathologies appear to be compatible with the distinct cellular responses observed to the different IGF-I peptides and with the concept of a complex and possibly isoform-specific IGF-I bioactivity. This concept is discussed in the present review, in the context of the broad range of modifications that this growth factor undergoes which might regulate its mechanism(s) of action.

IGF binding proteins in cancer: mechanistic and clinical insights

The six members of the family of insulin-like growth factor (IGF) binding proteins (IGFBPs) were originally characterized as passive reservoirs of circulating IGFs, but they are now understood to have many actions beyond their endocrine role in IGF transport. IGFBPs also function in the pericellular and intracellular compartments to regulate cell growth and survival - they interact with many proteins, in addition to their canonical ligands IGF-I and IGF-II. Intranuclear roles of IGFBPs in transcriptional regulation, induction of apoptosis and DNA damage repair point to their intimate involvement in tumour development, progression and resistance to treatment. Tissue or circulating IGFBPs might also be useful as prognostic biomarkers.

Hypothalamic-pituitary hormones during critical illness: a dynamic neuroendocrine response

Independent of the underlying condition, critical illness is characterized by a uniform dysregulation of the hypothalamic-pituitary-peripheral axes. In most axes a clear biphasic pattern can be distinguished. The acute phase of critical illness is characterized by low peripheral effector hormone levels such as T3, IGF-1 and testosterone, despite an actively secreting pituitary. The adrenal axis with high cortisol levels in the presence of low ACTH levels is a noteworthy exception. In the prolonged phase of critical illness, low peripheral effector hormone levels coincide with a uniform suppression of the neuroendocrine axes, predominantly of hypothalamic origin. The severity of the alterations in the different neuroendocrine axes is associated with a high risk of morbidity and mortality, but it remains unknown whether the observed changes are cause or consequence of adverse outcome. Several studies have identified therapeutic potential of hypothalamic releasing factors, but clinical outcome remains to be investigated with sufficiently powered randomized controlled trials.

Serum IGF-1 is insufficient to restore skeletal size in the total absence of the growth hormone receptor

States of growth hormone (GH) resistance, such those observed in Laron dwarf patients, are characterized by mutations in the GH receptor (GHR), decreased serum and tissue IGF-1 levels, impaired glucose tolerance, and impaired skeletal acquisition. IGF-1 replacement therapy in such patients increases growth velocity but does not normalize growth. Herein we combined the GH-resistant (GHR knockout [GHRKO]) mouse model with mice expressing the hepatic Igf-1 transgene (HIT) to generate the GHRKO-HIT mouse model. In GHRKO-HIT mice, serum IGF-1 levels were restored via transgenic expression of Igf-1, allowing us to study how endocrine IGF-1 affects growth, metabolic homeostasis, and skeletal integrity. We show that in a GH-resistant state, normalization of serum IGF-1 improved body adiposity and restored glucose tolerance but was insufficient to support normal skeletal growth, resulting in an osteopenic skeletal phenotype. The inability of serum IGF-1 to restore skeletal integrity in the total absence of GHR likely resulted from reduced skeletal Igf-1 gene expression, blunted GH-mediated effects on the skeleton that are independent of serum or tissue IGF-1, and poor delivery of IGF-1 to the tissues. These findings are consistent with clinical data showing that IGF-I replacement therapy in patients with Laron syndrome does not achieve full skeletal growth.

Unbound (bioavailable) IGF1 enhances somatic growth

Understanding insulin-like growth factor-1 (IGF1) biology is of particular importance because, apart from its role in mediating growth, it plays key roles in cellular transformation, organ regeneration, immune function, development of the musculoskeletal system and aging. IGF1 bioactivity is modulated by its binding to IGF-binding proteins (IGFBPs) and the acid labile subunit (ALS), which are present in serum and tissues. To determine whether IGF1 binding to IGFBPs is necessary to facilitate normal growth and development, we used a gene-targeting approach and generated two novel knock-in mouse models of mutated IGF1, in which the native Igf1 gene was replaced by Des-Igf1 (KID mice) or R3-Igf1 (KIR mice). The KID and KIR mutant proteins have reduced affinity for the IGFBPs, and therefore present as unbound IGF1, or 'free IGF1'. We found that both KID and KIR mice have reduced serum IGF1 levels and a concomitant increase in serum growth hormone levels. Ternary complex formation of IGF1 with the IGFBPs and the ALS was markedly reduced in sera from KID and KIR mice compared with wild type. Both mutant mice showed increased body weight, body and bone lengths, and relative lean mass. We found selective organomegaly of the spleen, kidneys and uterus, enhanced mammary gland complexity, and increased skeletal acquisition. The KID and KIR models show unequivocally that IGF1-complex formation with the IGFBPs is fundamental for establishing normal body and organ size, and that uncontrolled IGF bioactivity could lead to pathological conditions.

D440N mutation in the acid-labile subunit of insulin-like growth factor complexes inhibits secretion and complex formation

The acid-labile subunit (ALS) regulates IGF bioavailability by forming heterotrimeric complexes with IGFs and IGF-binding protein-3 (IGFBP-3). A homozygous missense mutation (D440N) resulting in undetectable circulating levels of ALS with a concomitant reduction in IGF-I and IGFBP-3 has been reported to cause mild growth retardation. To understand how this particular mutation affects ALS circulating levels and IGF-transport function, we expressed recombinant ALS and its variants, D440N-ALS, T442A-ALS, and D440N/T442A-ALS, using adenovirus vectors. Compared with wild-type ALS, the secretion of D440N-ALS was 80% lower. The D440N mutation was proposed to generate an N-glycosylation site additional to the seven existing motifs in ALS. D440N-ALS appeared larger than ALS, attributable to N-linked glycans because deglycosylation with N-glycosidase F reduced both proteins to the same molecular mass. When ALS was incubated with IGF-I and IGFBP-3, 70-80% of IGF-I was detected by gel-filtration chromatography in forms corresponding to the 150-kDa ternary complex. In contrast, when D440N-ALS was tested, less than 30% of IGF-I was found in high molecular mass complexes. Two other ALS variants mutated in the same putative glycosylation site, D440N/T442A-ALS and T442A-ALS, showed similar chromatographic profiles to wild-type ALS. The D440N mutation in ALS generates a hyperglycosylated form with impaired secretion and complex formation, potentially leading to dysregulation of endocrine IGF, thus contributing to the growth retardation observed in the affected patient. This is the first study to explain how a natural mutation, D440N, in ALS impairs its function.

Acid-labile subunit deficiency and growth failure: description of two novel cases

BACKGROUND/AIMS

Mutations in the acid-labile subunit (ALS) gene (IGFALS) have been associated with circulating insulin-like growth factor I (IGF-I) deficiency and short stature. Whether severe pubertal delay is also part of the phenotype remains controversial due to the small number of cases reported. We report 2 children with a history of growth failure due to novel IGFALS mutations.

METHODS

The growth hormone receptor gene (GHR) and IGFALS were analyzed by direct sequencing. Ternary complex formation was studied by size exclusion chromatography.

RESULTS

Two boys of 13.3 and 10.6 years, with pubertal stages 2 and 1, had mild short stature (-3.2 and -2.8 SDS, respectively) and a biochemical profile suggestive of growth hormone resistance. No defects were identified in the GHR. Patient 1 was homozygous for the IGFALS missense mutation P73L. Patient 2 was a compound heterozygote for the missense mutation L134Q and a novel GGC to AG substitution at position 546-548 (546-548delGGCinsAG). The latter causes a frameshift and the appearance of a premature stop codon. Size exclusion chromatography showed no peaks corresponding to ternary and binary complexes in either patient.

CONCLUSION

Screening of the IGFALS is important in children with short stature associated with low serum IGF-I, IGFBP-3 and ALS.

Effect of maternal asthma, inhaled glucocorticoids and cigarette use during pregnancy on the newborn insulin-like growth factor axis

BACKGROUND

Fetal growth varies in a sex-specific manner in response to maternal asthma during pregnancy, but the mechanisms are unclear.

OBJECTIVE

We examined the influence of maternal asthma severity and associated exposures, inhaled glucocorticoid treatment, maternal cigarette use, and fetal sex on fetal growth and placental function during pregnancy and on the newborn insulin-like growth factor (IGF) axis. STUDY SUBJECTS AND DESIGN: Fetal growth was assessed in a prospective cohort of asthmatic and non-asthmatic women (n=145). At delivery, umbilical vein plasma was collected from male (n=61, controls n=16 and asthmatic n=45) or female (n=84, controls n=22 and asthmatic n=62) fetuses. Cord plasma insulin-like growth factor (IGF) binding protein (BP)-1, IGFBP-3, IGF-1 and IGF-2 were measured by radioimmunoassay and ELISA.

RESULTS

Cord plasma IGF-1 was the main component of the neonatal IGF axis altered by asthma and cigarette use. IGF-1 was increased in the presence of mild asthma and a male fetus and decreased in the presence of a female fetus and maternal asthma with cigarette use. IGFBP-3 was also decreased in the female fetuses of pregnancies complicated by asthma and cigarette use. Inhaled glucocorticoid use for the treatment of asthma did not affect the IGF axis. The strongest overall predictor of female birth weight after accounting for asthma severity, inhaled glucocorticoid treatment and cigarette use was IGF-1. For males, the strongest predictor of birth weight was IGFBP-3.

CONCLUSION

The data suggest male and female fetuses institute different strategies in response to adverse pregnancy conditions such as asthma and cigarette use.

[Physiology of the GH-IGF axis]

Growth, the main characteristic of childhood and adolescence, has a similar pattern in the majority of the individuals. Genetic background and GH-IGF axis are the factors that directly influence this process. Pituitary GH acts on growth mainly through the regulation of IGF system. The IGFs (IGF-1 and IGF-2) are growth factors produced in the majority of the organs and body tissues. They have autocrine, paracrine and endocrine actions on metabolism and cell proliferation, growth and differentiation. The IGFs bind with high specificity and affinity to a family of 6 binding proteins, called IGFBPs (1 to 6) that modulate their bioactivity. Most of the known IGF actions are mediated via IGF type 1 receptor (IGF1R). In this article we are going to review the composition and regulation of the GH-IGF axis and the role of each component in the regulation of the growth process.

Serum complexes of insulin-like growth factor-1 modulate skeletal integrity and carbohydrate metabolism

Serum insulin-like growth factor (IGF) -1 is secreted mainly by the liver and circulates bound to IGF-binding proteins (IGFBPs), either as binary complexes or ternary complexes with IGFBP-3 or IGFBP-5 and an acid-labile subunit (ALS). The purpose of this study was to genetically dissect the role of IGF-1 circulatory complexes in somatic growth, skeletal integrity, and metabolism. Phenotypic comparisons of controls and four mouse lines with genetic IGF-1 deficits-liver-specific IGF-1 deficiency (LID), ALS knockout (ALSKO), IGFBP-3 (BP3) knockout, and a triply deficient LID/ALSKO/BP3 line-produced several novel findings. 1) All deficient strains had decreased serum IGF-1 levels, but this neither predicted growth potential or skeletal integrity nor defined growth hormone secretion or metabolic abnormalities. 2) IGF-1 deficiency affected development of both cortical and trabecular bone differently, effects apparently dependent on the presence of different circulating IGF-1 complexes. 3) IGFBP-3 deficiency resulted in increased linear growth. In summary, each IGF-1 complex constituent appears to play a distinct role in determining skeletal phenotype, with different effects on cortical and trabecular bone compartments.

Mecasermin rinfabate: rhIGF-I/rhIGFBP-3 complex: iPLEX

BACKGROUND

Mecasermin rinfabate (iPLEX), comprising rhIGF-I complexed to rhIGFBP-3, was developed in an attempt to prolong the half-life of IGF-I and potentially reduce side effects. It is administered as a once-daily subcutaneous injection. Treatment with rhIGF-I has been explored in a number of growth and endocrine disorders.

OBJECTIVE

To review the published literature regarding the pharmacokinetics, safety profile and clinical efficacy of Mecasermin rinfabate.

METHODS

A comprehensive search via the NCBI PubMed portal was performed using the search terms rhIGF-I/rhIGFBP-3 complex, iPLEX and Somatokine.

RESULTS

The effects of Mecasermin rinfabate have been explored in a number of clinical situations including diabetes, severe insulin resistance, osteopaenia, burns and growth hormone insensitivity syndrome, with outcomes similar to those of rhIGF-I alone.

CONCLUSIONS

The biological effects of Mecasermin rinfabate are largely similar to those previously reported with rhIGF-I. There are little published data pertaining to pharmacokinetic properties in human subjects, and the side effect profile appears similar to that of rhIGF-I alone.

Efficacy and safety of mecasermin rinfabate

There has been interest in using recombinant human (rh) insulin-like growth factor (IGF)-I (rhIGF-I) to treat short stature, either alone or in combination with its binding protein (insulin-like growth factor binding protein [IGFBP]-3). IGF-I has been shown to increase growth velocity in children with IGF deficiency, either as a result of growth hormone insensitivity syndrome (GHIS) or IGF gene deletion. However, there have been adverse events, particularly hypoglycaemia, reported with administration of unbound rhIGF-I. In addition, the serum half-life of unbound rhIGF-I is shorter when administered to patients with GHIS, who have low serum concentrations of its binding proteins IGFBP-3 and acid-labile subunit (ALS), than when administered to normal volunteers or to the patient with an IGF-I gene deletion (who had normal levels of IGFBP-3). iPlex (mecasermin rinfabate), an equimolar mixture of IGF-I and its binding protein IGFBP-3, was developed to prolong the half-life and to counteract acute adverse events (particularly hypoglycaemia) associated with administration of IGF-I. Although there are no published data on the efficacy of mecasermin rinfabate in treating growth disorders, it does appear that mecasermin rinfabate has a longer half-life in patients with GHIS than unbound IGF-I, and fewer reports of adverse events (including hypoglycaemia) when administered to patients with diabetes.

Characterization of the porcine insulin-like growth factor-binding protein, acid-labile subunit gene: full-length cDNA and DNA sequence, polymorphisms and expression profile

Insulin-like growth factor-binding protein acid-labile subunit (IGFALS) is an important glycoprotein in the circulation complex for insulin-like growth factors-I and -II (IGFs). So far only a partial porcine IGFALS sequence is available. We herein report the isolation, characterization, polymorphism identification and expression analysis of the porcine IGFALS gene. A 2000-bp full-length cDNA sequence was determined with 5'- and 3'-Rapid Amplification of cDNA End (RACE) assay. It contains a 1821-bp open reading frame encoding a protein of 606 amino acids with a calculated molecular mass of 66 kDa and a theoretical isoelectric point of 6.89. The deduced IGFALS protein shares high identity (70-80%) with other mammalian IGFALS. We also obtained a full-length genomic DNA sequence of the gene. Similar to the other mammalian orthologues, particularly in terms of exon size and exon/intron boundaries, the porcine IGFALS gene spans a transcription unit of 2990 bp, consisting of two exons and one intron. Three single nucleotide polymorphisms (synonymous mutations) were identified in the DNA sequence. RT-PCR assays indicate that IGFALS is expressed ubiquitously in pigs and transcripts are particularly abundant in the liver, lung, white adipose tissue, prostate, epididymis, thyroid and bladder.

Insulin-like Growth Factor Binding Protein-3 expression in the human corneal epithelium

Insulin-like Growth Factor Binding Protein-3 (IGFBP3) is a high-affinity binding protein shown to regulate cell growth, differentiation, and apoptosis in a variety of cellular systems. The primary aim of this study was to characterize IGFBP3 expression in the human corneal epithelium and in a corneal epithelial cell line and to establish a potential role for IGFBP3-mediated apoptotic signaling in corneal epithelial cells. Using a telomerase-immortalized human corneal epithelial (hTCEpi) cell line cultured in serum-free media and fresh human eye bank donor tissue, expression and localization of IGFBP3 were established in situ and in vitro by indirect immunofluorescence and western blotting. Real-time PCR was used to measure IGFBP3 mRNA levels following Trichostatin A (TSA) treatment and as a function of confluence. IGFBP3 protein levels were assessed in resting human tears and in conditioned media by western blotting as was the ability of recombinant human IGFBP3 protein to associate with the cell surface. Apoptotic signaling was assessed in vitro using TSA and recombinant human (rh)IGFBP3. Apoptosis was measured by Viability/Cytotoxicity, Annexin V, and TUNEL assays. IGFBP3 was localized to the plasma membrane of human corneal epithelial cells in situ and was upregulated in surface cells in the central cornea. IGFBP3 was secreted in conditioned media of growing cells, with a robust upregulation following confluence (P=0.014) and differentiation. IGFBP3 was undetectable in human tears. Addition of TSA to the culture media resulted in an upregulation of IGFBP3 mRNA (P<0.001) and protein. In addition, TSA treatment led to a significant increase in Annexin V positive cells at 18 and 24h (P<0.001) and TUNEL positive cells at 24 and 48 h (P<0.001). The addition of rhIGFBP3 to the cell culture media appeared to induce occasional membrane blebbing, but cells failed to become positive with Annexin V or TUNEL. Taken together, these results demonstrate that cell membrane-associated IGFBP3 is produced by corneal epithelial cells and associates with the plasma membrane of superficial cells in situ and in cultured cells, but not present in human tears. The differential localization and effect(s) on apoptosis suggest that the effects of IGFBP3 are likely tissue compartment and receptor specific and may be regulated by glycosylation.

First-trimester ADAM12 and PAPP-A as markers for intrauterine fetal growth restriction through their roles in the insulin-like growth factor system

BACKGROUND

PAPP-A is a marker used as part of the most effective method of screening for chromosomal anomalies in the first trimester. ADAM12 is a recently discovered pregnancy associated member of the ADAM (a multidomain glycoprotein metalloprotease) family. Recently, ADAM12 has been shown as a potential marker for early screening for chromosomal anomalies. Both PAPP-A and ADAM12 have been identified as proteases to insulin-like growth factor binding proteins. In this role, they may have a regulatory function in controlling the amount of free bioactive insulin-like growth factor (IGF). We therefore wish to examine if the levels of either of these proteases are related to various growth related adverse pregnancy outcomes.

MATERIALS AND METHODS

PAPP-A and ADAM12 were measured in a subset of samples collected at 11 to 14 weeks as part of an OSCAR clinic screening for chromosomal anomalies. Follow-up of pregnancies screened between September 1999 and August 2003 identified 1705 pregnancies with an outcome of intrauterine fetal demise on or after 24 weeks, preterm delivery at 24-34 weeks or 35-36 weeks, very low birthweight (<1.5 kg), low birthweight (<2.5 kg), large birthweight (>4.5 kg), and birth weight below the 3rd or 5th or 10th centile for gestation. A series of 414 normal outcome pregnancies constituted the control group. Marker levels were adjusted for gestation and maternal weight and the log MoM of the markers were compared using t-test of unequal variance between the control group and the various adverse outcome groups.

RESULTS

ADAM12 and PAPP-A concentrations were reduced in low for gestational age birth weights and in all births with weights below 2.5 kg. There was a linear relationship between the severity of the IUGR and the decrease in PAPP-A and ADAM12. In the larger babies, only ADAM12 was found to be significantly increased in babies above the 90th centile of weight for gestation.

CONCLUSIONS

The results of our study are compatible with the proposed role of ADAM12 and PAPP-A in promoting growth and development by breaking down IGF binding proteins and causing the release of free IGF for uptake into cells to promote growth. In those cases that eventually result in poor fetal growth, levels of PAPP-A and ADAM12 at 11-14 weeks are significantly lower than normal-in this instance, lowered PAPP-A and ADAM12 would result in less free IGF being available for cell uptake and growth stimulation. Further studies may elucidate if screening using such modalities can lead to new potential treatments for poorly growing fetuses.

Effect of thyroxine replacement on serum IGF-I, IGFBP-3 and the acid-labile subunit in patients with hypothyroidism and hypopituitarism

OBJECTIVE

To describe the effect of T4 replacement in patients with primary and central hypothyroidism on components of the IGF binding protein complex: IGF-I, the acid-labile subunit (ALS) and IGFBP-3.

PATIENTS AND METHODS

We determined IGF-I, ALS and IGFBP-3 (by 125I-IGF-II ligand blots and immunoblots) in serum of 19 patients with primary and 11 patients with central hypothyroidism.

RESULTS

Mean (+/- SD) free T4 (fT4) increased from 4.4 +/- 2.4 pmol/l at baseline to 18.6 +/- 5.2 pmol/l following T4 therapy. In patients with primary hypothyroidism, IGF-I concentrations increased from 101 +/- 57 to 158 +/- 60 microg/l (P < 0.001) and ALS from 12.6 +/- 4.7 to 15.6 +/- 5.2 mg/l (P = 0.001). IGFBP-3 levels (in arbitrary units, AU), assessed by 125I-IGF-II ligand blot and by Western blot (the intensity of the 45/42-kDa doublet following T4 replacement defined as 1 AU) increased from 0.74 +/- 0.47 to 1 (P = 0.029) and from 0.76 +/- 0.42 to 1 (P = 0.018), respectively. In patients with hypopituitarism, IGF-I and ALS concentrations increased on T4 therapy from 49 +/- 23 to 97 +/- 36 microg/l (P < 0.001) and from 7.8 +/- 4.1 to 11.0 +/- 2.7 mg/l (P = 0.010), respectively. IGFBP-3 remained unchanged during T4 replacement.

CONCLUSIONS

T4 replacement increases the serum levels of IGF-I and ALS in patients with primary as well as central hypothyroidism. IGFBP-3 levels increase in response to T4 replacement in patients with primary hypothyroidism but not in those with central hypothyroidism, suggesting that thyroid hormones increase IGF-I and ALS but not IGFBP-3 in patients with GH deficiency.

Endocrine regulation of human fetal growth: the role of the mother, placenta, and fetus

The environment in which the fetus develops is critical for its survival and long-term health. The regulation of normal human fetal growth involves many multidirectional interactions between the mother, placenta, and fetus. The mother supplies nutrients and oxygen to the fetus via the placenta. The fetus influences the provision of maternal nutrients via the placental production of hormones that regulate maternal metabolism. The placenta is the site of exchange between mother and fetus and regulates fetal growth via the production and metabolism of growth-regulating hormones such as IGFs and glucocorticoids. Adequate trophoblast invasion in early pregnancy and increased uteroplacental blood flow ensure sufficient growth of the uterus, placenta, and fetus. The placenta may respond to fetal endocrine signals to increase transport of maternal nutrients by growth of the placenta, by activation of transport systems, and by production of placental hormones to influence maternal physiology and even behavior. There are consequences of poor fetal growth both in the short term and long term, in the form of increased mortality and morbidity. Endocrine regulation of fetal growth involves interactions between the mother, placenta, and fetus, and these effects may program long-term physiology.

Investigational agents for the treatment of growth hormone-insensitivity syndrome

Growth hormone-insensitivity syndrome (GHIS) is usually caused by mutations in the growth hormone receptor. Recombinant IGF-I has been used to treat GHIS either alone (mecasermin) or in combination with IGF-binding protein (IGFBP)-3 (mecasermin rinfabate). IGF-I increases the growth velocity of children with IGF deficiency, which is either as a result of GHIS or an IGF-I gene deletion. Hypoglycaemia has been reported with administration of unbound IGF-I and, in addition, the serum half-life of unbound IGF-I is shorter when administered to patients with GHIS (who have low serum concentrations of its binding protein IGFBP-3) than when administered to normal volunteers or to patients with an IGF-I gene deletion (but normal IGFBP-3 levels). The IGF-I-IGFBP-3 combination was developed to prolong the half-life and counteract the acute adverse events (particularly hypoglycaemia) that are associated with administration of IGF-I. It seems that the IGF-I-IGFBP-3 combination has a longer half-life in patients with GHIS than unbound IGF-I, with fewer reports of adverse events (including hypoglycaemia) when administered to patients with diabetes. There are no studies comparing the efficacy of mecasermin with mecasermin rinfabate; both drugs have been shown to be effective but cannot be differentiated in terms of efficacy.

Insulin-like growth factor signaling in fish

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

Role of N- and C-terminal residues of insulin-like growth factor (IGF)-binding protein-3 in regulating IGF complex formation and receptor activation

Insulin-like growth factor-binding protein-3 (IGFBP-3), the major IGFBP in the circulation, sequesters IGF in a stable ternary complex with the acid-labile subunit. The high affinity IGF-binding site is proposed to reside within an N-terminal hydrophobic domain in IGFBP-3, but C-terminal residues have also been implicated in the homologous protein IGFBP-5. We have mutated in various combinations Leu(77), Leu(80), and Leu(81) in the N terminus and Gly(217) and Gln(223) in the C terminus of IGF-BP-3. All mutants retained immunoreactivity toward a polyclonal IGFBP-3 antibody, whereas IGF ligand blotting showed that all of the mutants had reduced binding to IGFs. Both solution IGF binding assays and BIAcore analysis indicated that mutations to the N-terminal region caused greater reduction in IGF binding activity than C-terminal mutations. The combined N- and C-terminal mutants showed undetectable binding to IGF-I but retained <10% IGF-II binding activity. Reduced ternary complex formation was seen only in mutants that had considerably reduced IGF-I binding, consistent with previous studies indicating that the binary IGF.IGFBP-3 complex is required for acid-labile subunit binding. Decreased IGF binding was also reflected in the inability of the mutants to inhibit IGF-I signaling in IGF receptor overexpressing cells. However, when present in excess, IGFBP-3 analogs defined as non-IGF-binding by biochemical assays could still inhibit IGF signaling. This suggests that residual binding activity of IGFBP-3 mutants may still be sufficient to inhibit IGF biological activity and questions the use of such analogs to study IGF-independent effects of IGFBP-3.

Dwarfism and impaired gut development in insulin-like growth factor II mRNA-binding protein 1-deficient mice

Insulin-like growth factor II mRNA-binding protein 1 (IMP1) belongs to a family of RNA-binding proteins implicated in mRNA localization, turnover, and translational control. Mouse IMP1 is expressed during early development, and an increase in expression occurs around embryonic day 12.5 (E12.5). To characterize the physiological role of IMP1, we generated IMP1-deficient mice carrying a gene trap insertion in the Imp1 gene. Imp1(-/-) mice were on average 40% smaller than wild-type and heterozygous sex-matched littermates. Growth retardation was apparent from E17.5 and remained permanent into adult life. Moreover, Imp1(-/-) mice exhibited high perinatal mortality, and only 50% were alive 3 days after birth. In contrast to most other organs, intestinal epithelial cells continue to express IMP1 postnatally, and Imp1(-/-) mice exhibited impaired development of the intestine, with small and misshapen villi and twisted colon crypts. Analysis of target mRNAs and global expression profiling at E12.5 indicated that Igf2 translation was downregulated, whereas the postnatal intestine showed reduced expression of transcripts encoding extracellular matrix components, such as galectin- 1, lumican, tenascin-C, procollagen transcripts, and the Hsp47 procollagen chaperone. Taken together, the results demonstrate that IMP1 is essential for normal growth and development. Moreover, IMP1 may facilitate intestinal morphogenesis via regulation of extracellular matrix formation.

Identification of a Functional Vitamin D Response Element in the Human Insulin-Like Growth Factor Binding Protein-3 Promoter

1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] plays a critical role in maintaining calcium and phosphate homeostasis and bone formation but also exhibits antiproliferative activity on many cancer cells, including prostate cancer. We have shown that the antiproliferative actions of 1,25-(OH)2D3 in the LNCaP human prostate cancer cell line are mediated in part by induction of IGF binding protein-3 (IGFBP-3). The purpose of this study was to determine the molecular mechanism involved in 1,25-(OH)2D3 regulation of IGFBP-3 expression and to identify the putative vitamin D response element (VDRE) in the IGFBP-3 promoter. We cloned approximately 6 kb of the IGFBP-3 promoter sequence and demonstrated its responsiveness to 1,25-(OH)2D3 in transactivation assays. Computer analysis identified a putative VDRE between -3296/-3282 containing the direct repeat motif GGTTCA ccg GGTGCA that is 92% identical with the rat 24-hydroxylase distal VDRE. In EMSAs, the vitamin D receptor (VDR) showed strong binding to the putative IGFBP-3 VDRE in the presence of 1,25-(OH)2D3. Supershift assays confirmed the presence of VDR in the IGFBP-3 VDRE complex. Chromatin immunoprecipitation assay demonstrated that 1,25-(OH)2D3 recruited the VDR/retinoid X receptor heterodimer to the VDRE site in the natural IGFBP-3 promoter in intact cells. In transactivation assays, the putative VDRE coupled to a heterologous simian virus 40 promoter construct was induced 2-fold by 1,25-(OH)2D3. Mutations in the VDRE resulted in a loss of inducibility confirming the critical hexameric sequence. In conclusion, we have identified a functional VDRE in the distal region of the human IGFBP-3 promoter. The induction of IGFBP-3 by 1,25-(OH)2D3 appears to be directly mediated via VDR interaction with this VDRE.

Differential impact of simple childhood obesity on the components of the growth hormone-insulin-like growth factor (IGF)-IGF binding proteins axis

Simple childhood obesity is characterized by normal or even accelerated growth in spite of reduced growth hormone (GH) secretion. There are conflicting reports on the effects of obesity upon components of the GH-insulin-like growth factor-I (IGF-I)-IGF binding proteins (IGFBPs) system. In the present study we aimed to determine GH, IGF-I, IGFBP-3 and IGFBP-2 as well as some of the less explored components of this axis (IGFBP-3 proteolytic activity, IGFBP-3 plasma fragments, and total acid labile subunit [ALS]) in 22 obese and 17 age-matched control children. We also evaluated not only total GH binding protein (GHBP) serum levels but also GHBP bound to GH (complexed) in both groups. Obese and control groups strongly differed in BMI (obese: 4.7 +/- 0.36 vs control: 0.37 +/- 0.25 SDS, p <0.0001). In the obese group, we found lower GH serum levels, but normal serum levels of GH-GHBP complex, IGF-I, IGFBP-3, IGF-I/IGFBP-3 molar ratio, IGFBP-3 proteolytic activity, IGFBP-3 plasma fragments and total ALS. Obese children presented higher total circulating GHBP (6.0 +/- 0.44 vs 2.9 +/- 0.29 nmol/l, p <0.001) and insulin levels (10.5 +/- 1.5 vs 5.1 +/- 0.8 mU/l, p <0.001), while IGFBP-2 (4.6 +/- 0.5 vs 6.6 +/- 0.7%, p <0.05) and the ratio IGFBP-2/IGF-I (0.032 +/- 0.019 vs 0.095 +/- 0.01, p = 0.013) were lower than in controls. BMI and insulin were directly, and IGFBP-2 serum levels inversely, correlated to total GHBP serum levels when multiple regression analysis was performed (r = 0.74, p <0.001). By stepwise regression analysis, insulin (r = -0.37, p <0.05) and BMI (r = -0.52, p <0.01) inversely determined IGFBP-2. In summary, obese children present normal growth in spite of reduced GH secretion, probably because the combination of increased total GHBP and normal GH-GHBP complex serum levels (suggesting increased GH receptor [GHR] number and a normal serum GH reservoir, respectively) allow for the achievement of normal levels of IGF-I, IGFBP-3, IGFBP-3 proteolytic activity, IGFBP-3 plasma fragments and total ALS. Reduced IGFBP-2 serum levels and a lower ratio of IGFBP-2/IGF-I in obese children may suggest an increase of tissue IGF-I bioavailability, thus promoting its action. Normal IGF-I and GH availability may be contributing to maintain normal growth in obese children.

Insulin-like growth factor-binding protein 5 (Igfbp5) compromises survival, growth, muscle development, and fertility in mice

The insulin-like growth factors (IGFs) are essential for development; bioavailable IGF is tightly regulated by six related IGF-binding proteins (IGFBPs). Igfbp5 is the most conserved and is developmentally up-regulated in key lineages and pathologies; in vitro studies suggest that IGFBP-5 functions independently of IGF interaction. Genetic ablation of individual Igfbps has yielded limited phenotypes because of substantial compensation by remaining family members. Therefore, to reveal Igfbp5 actions in vivo, we generated lines of transgenic mice that ubiquitously overexpressed Igfbp5 from early development. Significantly increased neonatal mortality, reduced female fertility, whole-body growth inhibition, and retarded muscle development were observed in Igfbp5-overexpressing mice. The magnitude of the response in individual transgenic lines was positively correlated with Igfbp5 expression. Circulating IGFBP-5 concentrations increased a maximum of only 4-fold, total and free IGF-I concentrations increased up to 2-fold, and IGFBP-5 was detected in high M(r) complexes; however, no detectable decrease in the proportion of free IGF-I was observed. Thus, despite only modest changes in IGF and IGFBP concentrations, the Igfbp5-overexpressing mice displayed a phenotype more extreme than that observed for other Igfbp genetic models. Although growth retardation was obvious prenatally, maximal inhibition occurred postnatally before the onset of growth hormone-dependent growth, regardless of Igfbp5 expression level, revealing a period of sensitivity to IGFBP-5 during this important stage of tissue programming.

Purification of a 41-kDa insulin-like growth factor binding protein from serum of chinook salmon, Oncorhynchus tshawytscha

In salmon, at least three insulin-like growth factor binding proteins (IGFBPs) with molecular masses of 41, 28, and 22kDa exist in serum. The 41-kDa IGFBP is up-regulated by growth hormone treatment and down-regulated by fasting, suggesting that it is a homolog of IGFBP-3. We purified the 41-kDa IGFBP from chinook salmon serum by IGF-I affinity chromatography followed by reversed-phase high pressure liquid chromatography. Purified IGFBP appeared as doublet bands on electrophoresis and was N-glycosylated. Analysis of partial N-terminal amino acid sequence revealed that salmon 41-kDa IGFBP has the cysteine rich domain conserved among IGFBP family. In a binding assay using 125I-salmon IGF-I, purified 41-kDa IGFBP specifically bound salmon IGF-I, human IGF-I and human IGF-II, but neither Long R(3)IGF-I nor salmon insulin, showing that binding characteristics of the salmon IGFBP are similar to those of mammalian IGFBPs. Although the partial amino acid sequence of 41-kDa IGFBP showed highest homologies with zebrafish and seabream IGFBP-2, the highly conserved nature of the N-terminus makes it impossible to identify the type of IGFBP from partial sequence data. However, based on physiological responses, molecular weight and type of glycosylation, the 41-kDa IGFBP is most similar to mammalian IGFBP-3.

Effects of recombinant human insulin-like growth factor (IGF)-I and estrogen administration on IGF-I, IGF binding protein (IGFBP)-2, and IGFBP-3 in anorexia nervosa: a randomized-controlled study

Administration of recombinant human (rh) IGF-I has been shown to have positive effects on bone density in anorexia nervosa, but the effects of rhIGF-I and estrogen on IGF binding protein (IGFBP)-2 and IGFBP-3 in anorexia nervosa are not known. Sixty-five osteopenic women with anorexia nervosa were randomized to rhIGF-I (30 micro g/kg sc twice daily) alone (n = 15), daily ethinyl estradiol (Ovcon 35) with rhIGF-I (n = 15), estradiol and placebo (n = 15), or placebo (n = 14) for 9 months. Subjects were 25.6 +/- 0.8 yr of age, low weight (body mass index 16.6 +/- 0.2 kg/m(2)) and osteopenic (T scores -2.06 +/- 0.09 for spine and -1.76 +/- 0.13 for hip). IGFBP-3 correlated with total hip bone density (r = 0.47, P = 0.0002) and was a significant predictor of hip bone density (P = 0.010) independent of IGF-I and body mass index in a multivariate regression model. During therapy, IGFBP-2 increased by 48 +/- 19 ng/ml in response to rhIGF-I and decreased by -38 +/- 22 ng/ml in response to placebo (P = 0.011). IGFBP-3 decreased (-895 +/- 120 ng/ml) in response to rhIGF-I but showed a minimal change (-53 +/- 99 ng/ml) in response to placebo (P < 0.0001). In contrast, no significant effect of estrogen was seen on IGF-I, IGFBP-2 or IGFBP-3. Among patients receiving rhIGF-I, the change in IGFBP-2 was inversely associated with the change in total hip bone density (R = -0.47, P = 0.013). In conclusion, our data suggest that chronic rhIGF-I administration increases IGF-I and IGFBP-2 and decreases IGFBP-3 in women with anorexia nervosa. IGFBP-2 and IGFBP-3 may be important determinants of bone density in this population.

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.

Age-dependency of insulin-like growth factors, insulin-like growth factor-binding proteins, and acid labile subunit in plasma and wounds of surgical patients

Wound problems are common in the elderly. We hypothesized that age-related decrements in blood levels of components of the insulin-like growth factor (IGF) system are reflected in the wound environment. In this prospective, observational study IGF-I, IGF-II, IGF-binding protein-2, IGF-binding protein-3, and acid labile subunit were measured by immunoassays in the wound fluid and plasma of young (23.5 +/- 3.3 years) and elderly (78.9 +/- 6.2 years) patients before and daily for 4 days after elective surgery. IGFs, IGFBP-3, and acid labile subunit in plasma were significantly lower in the elderly group (p < 0.0001). The decrements of these proteins in plasma were reflected in corresponding decrements of 25-70% in the wound fluid of elderly patients (p < 0.0001). Additionally, bioavailability of IGF-I was less in the aged. The IGF parameters in the wound displayed a constant ratio with those of blood, suggesting that blood contributes a major share of the IGF that enters the wound during the initial phase of healing. The current data adds to accumulating evidence that a decline in the IGF system in aged patients contributes to the healing deficits observed in the elderly.

Regulation of the acid-labile subunit in sustained endotoxemia

The effect of sustained endotoxemia on expression of the acid-labile subunit (ALS) in relation to hepatic markers of altered GH and insulin sensitivity was examined. Juvenile rats were injected with endotoxin twice daily for 48 h, causing reduced food intake and attenuated growth. In pair-fed controls, food restriction caused marked suppression of ALS gene expression and circulating levels within 12 h, and endotoxemia augmented this effect. This acute effect of endotoxin corresponded temporally with transient induction of suppressor of cytokine signaling (SOCS)-3, cytokine-inducible SH2-containing protein (CIS), phosphoenolpyruvate carboxykinase (PEPCK), and insulin-like growth factor-binding protein (IGFBP)-1 and suppression of GH receptor (GHR). During the subsequent 36 h of sustained endotoxin treatment, expression of ALS recovered to, and then rose above, that of their pair-fed controls. This effect was paralleled by other ternary complex components. The inductive effect of sustained endotoxemia relative to pair-fed controls could not be explained by differences in expression of GHR, SOCS-3, or CIS but coincided with normalized PEPCK and IGFBP-1 levels, suggesting better hepatic insulin sensitivity in these animals. These data may indicate that, in sustained endotoxemia, ALS levels are regulated through modulation of hepatic insulin sensitivity.

Trichostatin A, a histone deacetylase inhibitor, activates the IGFBP-3 promoter by upregulating Sp1 activity in hepatoma cells: alteration of the Sp1/Sp3/HDAC1 multiprotein complex

We determined the molecular mechanisms by which trichostatin A (TSA) induced insulin-like growth factor-binding protein 3 (IGFBP-3) gene expression in Hep3B cells, a p53-mutant human hepatocellular carcinoma (HCC) cell line. TSA induced the expressions of the IGFBP-3 mRNA and protein and the activation of its promoter. Using IGFBP-3 promoter deletion constructs, the TSA-responsive element was mapped to a region between -115 and -30, relative to the transcription start site. Promoter mutation analysis confirmed that the TSA-responsive element coincides with the Sp1/GC-rich region on the IGFBP-3 promoter. This transcriptional activation appears to be mediated by both the Sp1 and Sp3 transcription factors and, in particular, by the phosphorylation of Sp1, because treatment of Hep3B cells and Schneider (SL2) cells with TSA significantly activated phosphorylation of Sp1 in a dose-dependent manner. Consistent with the transcriptional activation of the IGFBP-3 promoter by TSA, TSA treatment led to the release of HDAC1 and Sp3 from the Sp1 transcriptional factor complex, indicating the involvement of multiprotein complexes containing Sp1, Sp3, p300, and HDAC-1 in IGFBP-3 activation by TSA. Taken together, these results show that Sp1 phosphorylation and the modulation of the Sp1/Sp3/HDAC1 multiprotein complex play a pivotal role in the transcriptional activation of the IGFBP-3 promoter through the Sp1/GC-rich site by TSA.