Transcapillary permeability and subendothelial distribution of endothelial and amniotic fluid insulin-like growth factor binding proteins in the rat heart

Insulin-like growth factors: Ligands, binding proteins, and receptors

BACKGROUND

The insulin-like growth factor family of ligands (IGF-I, IGF-II, and insulin), receptors (IGF-IR, M6P/IGF-IIR, and insulin receptor [IR]), and IGF-binding proteins (IGFBP-1-6) play critical roles in normal human physiology and disease states.

SCOPE OF REVIEW

Insulin and insulin receptors are the focus of other chapters in this series and will therefore not be discussed further. Here we review the basic components of the IGF system, their role in normal physiology and in critical pathology's. While this review concentrates on the role of IGFs in human physiology, animal models have been essential in providing understanding of the IGF system, and its regulation, and are briefly described.

MAJOR CONCLUSIONS

IGF-I has effects via the circulation and locally within tissues to regulate cellular growth, differentiation, and survival, thereby controlling overall body growth. IGF-II levels are highest prenatally when it has important effects on growth. In adults, IGF-II plays important tissue-specific roles, including the maintenance of stem cell populations. Although the IGF-IR is closely related to the IR it has distinct physiological roles both on the cell surface and in the nucleus. The M6P/IGF-IIR, in contrast, is distinct and acts as a scavenger by mediating internalization and degradation of IGF-II. The IGFBPs bind IGF-I and IGF-II in the circulation to prolong their half-lives and modulate tissue access, thereby controlling IGF function. IGFBPs also have IGF ligand-independent cell effects.

The Neglected Insulin: IGF-II, a Metabolic Regulator with Implications for Diabetes, Obesity, and Cancer

When originally discovered, one of the initial observations was that, when all of the insulin peptide was depleted from serum, the vast majority of the insulin activity remained and this was due to a single additional peptide, IGF-II. The IGF-II gene is adjacent to the insulin gene, which is a result of gene duplication, but has evolved to be considerably more complicated. It was one of the first genes recognised to be imprinted and expressed in a parent-of-origin specific manner. The gene codes for IGF-II mRNA, but, in addition, also codes for antisense RNA, long non-coding RNA, and several micro RNA. Recent evidence suggests that each of these have important independent roles in metabolic regulation. It has also become clear that an alternatively spliced form of the insulin receptor may be the principle IGF-II receptor. These recent discoveries have important implications for metabolic disorders and also for cancer, for which there is renewed acknowledgement of the importance of metabolic reprogramming.

Endothelial cells and the IGF system

Endothelial cells line blood vessels and modulate vascular tone, thrombosis, inflammatory responses and new vessel formation. They are implicated in many disease processes including atherosclerosis and cancer. IGFs play a significant role in the physiology of endothelial cells by promoting migration, tube formation and production of the vasodilator nitric oxide. These actions are mediated by the IGF1 and IGF2/mannose 6-phosphate receptors and are modulated by a family of high-affinity IGF binding proteins. IGFs also increase the number and function of endothelial progenitor cells, which may contribute to protection from atherosclerosis. IGFs promote angiogenesis, and dysregulation of the IGF system may contribute to this process in cancer and eye diseases including retinopathy of prematurity and diabetic retinopathy. In some situations, IGF deficiency appears to contribute to endothelial dysfunction, whereas IGF may be deleterious in others. These differences may be due to tissue-specific endothelial cell phenotypes or IGFs having distinct roles in different phases of vascular disease. Further studies are therefore required to delineate the therapeutic potential of IGF system modulation in pathogenic processes.

Insulin-like growth factor physiology: what we have learned from human studies

Although very similar to insulin and its receptor; the modus operandi of the insulin-like growth factors (IGFs) within the body is very different from that of the traditional peptide hormone. The IGF-binding proteins bind the IGFs with greater affinity than the cell surface receptors, enabling them to tightly control tissue activity. In addition to their role in fetal and childhood growth, IGFs play an important role in metabolic regulation. This article describes the basic underlying human physiology of IGFs, how this differs from that of experimental models, and why some information can only be learned from human clinical studies.

Skeletal growth and IGF levels in rats after HT042 treatment

HT042, a new herbal prescription consisting of Astragalus membranaceus, Phlomis umbrosa and Eleutherococcus senticosus, is used in traditional Korean medicine to stimulate growth in children. This study was conducted to investigate the effects of HT042 on skeletal growth, insulin-like growth factor-1 (IGF-1) and insulin-like growth factor binding protein-3 (IGFBP-3) levels, and oestrogenic activity in female rats. Female Sprague-Dawley rats were divided into control, recombinant human growth hormone (rhGH; 20 µg/kg/day), and HT042 (100 mg/kg/day) groups and treated for 3 weeks. Axial skeletal growth, femur length, and growth plate length were measured every 3 weeks. The serum IGF-1 and IGFBP-3 levels were analysed. Moreover, the oestrogenic activity of the herbal extracts in the immature and ovariectomized rats was tested. The nose-anus, nose-tail, femur and growth-plate lengths were increased significantly in the HT042 group. Both IGF-1 and IGFBP-3 were highly expressed in the hypertrophic zone of the growth plate. The serum IGF-1 levels were increased. Moreover, HT042 had no uterotrophic effects in the rats. Consequently, HT042 promoted longitudinal bone growth by stimulating cell proliferation in the epiphyseal plate and inducing the expression of IGF-1 without an oestrogenic response. HT042 may be helpful in stimulating growth in children with short stature.

Impact of inhaled and intranasal corticosteroids on the growth of children

Since inhaled and intranasal corticosteroids may be systemically bioavailable, risk of growth suppression cannot be ruled out in children treated with these compounds. The mechanisms by which exogenous corticosteroids can cause growth suppression may be multifactorial, involving influences on growth hormone secretory profiles and insulin-like growth factor-I activity, direct effects on the epiphyseal growth plate, and effects on bone and collagen turnover. When studies on growth in children treated with inhaled and intranasal corticosteroids are interpreted, it is important to discriminate between data on the final outcome of growth (adult height) and data on growth rate. No firm conclusions can be drawn on adult height from the available data. While the data on children treated with inhaled corticosteroids appear reassuring, there are no peer-reviewed studies on the final height of children treated with intranasal corticosteroids. The possibility of additive effects on the final height or growth rate of children receiving intranasal plus inhaled corticosteroids has also not been studied. When assessing the risk of growth rate suppression, specific corticosteroids, doses and inhaler systems must be evaluated separately. Standard paediatric doses of inhaled corticosteroids (budesonide 200 to 400 microg/day delivered from a metered dose inhaler with a spacer, dry powder budesonide 200 microg/day, or dry powder fluticasone propionate 200 microg/day) do not affect growth rate when a twice daily administration regimen is used. The risk of growth rate suppression in children treated with inhaled budesonide depends on the dosage and may become significant with 800 microg/day administered with a spacer, or with 400 microg/day administered with a dry powder device. When high doses of inhaled corticosteroids are used, the risk of adverse effects on growth rate can be reduced by once daily dosage in the morning. In fact, intranasal mometasone furoate 100 and 200microg from an aqueous pump spray and dry powder budesonide 200 and 400microg once daily in the morning have been found not to affect growth rate. Sensitivity to adverse effects on growth rate may vary between individuals. If growth suppression is detected, 'catch-up growth' may be expected when the dose of the inhaled or intranasal corticosteroid is reduced or other treatment modalities are introduced. Inhaled or intranasal corticosteroids should not be withheld from children with asthma or rhinitis. Topical corticosteroids should be given in doses that control disease symptoms; however, height measurements should be performed regularly in children receiving corticosteroids.

Lack of sex differences in the IGF-IGFBP response to ultra endurance exercise

The insulin-like growth factor (IGF)-IGF binding proteins (BP) and the pituitary-gonadal axes were investigated during ultra endurance exercise in 16 endurance-trained athletes (seven women). Median duration of the race was 6.3 days. Although food and drink were ad libitum, energy balance was negative. Blood samples were drawn before (PRE), at the end of (END) and 24 h after (POST24h) the race. Serum concentrations of total IGF-I (t-IGF-I) and free IGF-I (f-IGF-I) decreased by 33 (SD 38)% and 54 (19)%, respectively. The decrease in t-IGF-I appeared to be associated to the total energy deficit during the race. At END, the IGFBP-3 fragmentation and IGFBP-1 were increased but these changes did not predict changes in f-IGF-I. An increase in POST24h IGFBP-2 levels in women was the only sex difference. Testosterone was decreased by 67 (12)% in the men and estradiol became undetectable in the women without any detectable increase in LH and/or FSH. In conclusion ultra endurance exercise results in similar IGF-IGFBP responses in men and women reflecting a catabolic state. IGFBP-2 was the only exception, with increased levels in women after exercise. A concomitant decrease in gonadal hormones was not related to endocrine changes in the IGF-IGFBP axis but may be related to local changes in IGF-I expression.

Effect of negative energy balance on the insulin-like growth factor system in pre-recruitment ovarian follicles of post partum dairy cows

Post partumnegative energy balance (NEB) in dairy cattle is associated with a delayed return to ovarian cyclicity and reduced fertility. This study compared the IGF system of pre-recruitment ovarian follicles between cows in mild (n= 6) or severe (n= 6) NEB during early lactation. Ovaries were collected in the second weekpost partum, when circulating concentrations of IGF-I and glucose were lower (P< 0.01) in severe NEB cows. mRNA expression for IGF-II, type 1 IGF receptor (IGF-1R) and IGF-binding proteins (IGFBP)-1 to IGFBP-6 was determined byin situhybridisation in individual follicles using radiolabelled oligonucleotide probes. Follicles were classified as very small (1–2.5 mm) or small (2.5–5 mm) and healthy or atretic. Relative mRNA concentrations were measured as optical density (OD) units using image analysis. Thecal IGF-II mRNA expression was highest in very small, healthy follicles (P< 0.05). Granulosa cell IGFBP-2 was the only component to change with EB status, with higher mRNA expression in mild compared with severe NEB cows (P< 0.05). IGFBP-1 and IGFBP-3 mRNA expression were undetectable. IGF-1R, IGFBP-4 and IGFBP-5 mRNA expression were not significantly altered by follicle size or health, but IGFBP-5 tended to increase in atretic follicles. The pattern of IGFBP-6 mRNA expression in theca paralleled that of IGF-II mRNA, with higher (P< 0.05) levels in healthy, very small follicles. In conclusion, the reduced expression of IGFBP-2 mRNA in severe NEB cows may alter the bioavailability of circulating IGF-I and locally produced IGF-II to modulate the pre-recruitment stages of follicles required to maintain normalpost partumovarian cyclicity.

Local changes in the insulin-like growth factor system in human skeletal muscle assessed by microdialysis and arterio-venous differences technique

IGF-I plays a direct role in whole body glucose homeostasis primarily by stimulating skeletal muscle glucose uptake. IGF-I is also involved in exercise induced muscle hypertrophy. Knowledge regarding local changes in muscle IGF-I bioavailability and its regulation by IGFBPs at rest and during exercise is limited. We have therefore explored changes in total IGF-I levels as well as circulating IGFBP levels and their post-translational modifications over an exercising leg. For the first time we have determined IGF-I levels in exercising skeletal muscle microdialysate in an attempt to assess local IGF-I bioavailability. Eighteen healthy young men performed one legged knee-extension exercise during 45min. Blood samples were taken from the femoral artery and vein of the exercising leg. No significant differences between arterial and venous concentrations of total IGF-I or IGFBP-1 were detected over the leg at any time. IGF-I concentrations increased significantly during exercise in the artery but not in the vein. Total IGFBP-1 increased after exercise in both artery and vein. The increase in non-plus less phosphorylated forms of IGFBP-1 was less pronounced and did not reach statistical significance. The proportion of fragmented IGFBP-3 (IGFBP-3 proteolysis) assessed by Western immunoblotting did not change significantly during or after exercise. Although optimization and validation of IGF-I determinations in muscle microdialysate (md) will be required, our first results using this technique demonstrate a significant 2-fold increase in mdIGF-I collected during and after exercise. We conclude that determination of A-V-differences appears to be of limited value in the assessments of local muscle change in the IGF-system. A substantial release of IGF-I during short time is required to detect significant change in the large circulating store of IGF-I. We suggest that an optimized and validated microdialysis technique for determination of local IGF-I may be advantageous in future studies.

Gender differences in the insulin-like growth factor axis response to a glucose load

AIMS

The insulin-like growth factors (IGFs) are thought to contribute to glucose homeostasis. The aim of our study was to examine the response of the IGFs and their binding proteins to an intravenous load of glucose in a cohort of young men and women with normal glucose tolerance.

METHODS

The intravenous glucose tolerance test (IVGTT) was used to quantify insulin sensitivity and insulin secretion in 160 adults aged 20-21 years in Adelaide, Australia. Serum IGF-I, IGF-II, IGF-binding protein (IGFBP)-1 and IGFBP-3 were measured during the IVGTT.

RESULTS

Women were less insulin sensitive than men with higher fasting insulin (women 55.6 +/- 4.4, men 44.1 +/- 3.6 pmol L(-1), P = 0.001) and first phase insulin secretion (women 3490 +/- 286, men 3038 +/- 271 pmol L(-1) min, P = 0.042). Women showed lower fasting free IGF-I (women 0.29 +/- 0.02, men 0.36 +/- 0.02 mug L(-1), P = 0.004) but higher IGFBP-3 (women 46.3 +/- 0.53, men 43.3 +/- 0.58 mg dL(-1), P = 0.001) and higher IGFBP-1 concentrations (women 37.0 +/- 2.9, men 24.8 +/- 2.3 mug L(-1), P = 0.012). IGFBP-1 fell by 5 min and remained suppressed. IGFBP-3 and total IGF-I fell until 60 min rising again by 2 h. IGF and IGFBP values were all higher in women. IGFBP-1 showed a negative association with fasting and stimulated insulin concentrations in both genders. First phase insulin secretion however showed positive correlations with IGFBP-3 (r = 0.321, P = 0.004) and IGF-I (r = 0.339 P = 0.002) in men but not women.

CONCLUSION

Our data show that IGFBP-1, IGFBP-3 and IGF-I show acute changes following a glucose load and there are marked gender differences in these responses.

Regulation of insulin-like growth factor-I (IGF-I) delivery by IGF binding proteins and receptors

Delivery of growth factors via the bloodstream for the treatment of various diseases is regulated in part by interactions with cell surface binding elements. Understanding the kinetics of growth factor binding and transport by cells would, therefore, be advantageous. This report quantifies the binding, internalization, and transport of insulin-like growth factor-I (IGF-I) across bovine aortic endothelial cells (BAEC) cultured in vitro. Binding analysis indicated that IGF binding proteins (IGFBPs), primarily localized with the extracellular matrix, were the primary IGF-I binding elements in our system, with twice as many binding sites (8.0 +/- 1.9 x 10(4) per cell) as IGF-I receptors (IGF-IR) (3.9 +/- 0.6 x 10(4) per cell). Internalization of IGF-I by IGF-IR, but not IGFBPs, was detected, however both receptor and IGFBP binding were shown to inhibit rather than enhance the transport of intact IGF-I, albeit in different ways. IGFBPs retained IGF-I in the apical region while IGF-IR binding led to protein degradation. Based on our computational modeling and experimental data, we hypothesize that IGFBPs could function as a reservoir for IGF-I, sequestering it for later release and transport, and that this reservoir function of the IGFBPs could be used to promote controlled localized delivery of IGF-I.

Insulin-like growth factor-1 increases endothelin receptor A levels and action in cultured rat aortic smooth muscle cells

Insulin is known to cause an increase in endothelin-1 (ET-1) receptors in vascular smooth muscle cells (SMCs), but the effect of insulin-like growth factor 1 (IGF-1) on ET-1 receptor expression is not known. We therefore carried out the present study to determine the effect of IGF-1 on the binding of ET-1 to, and ET type A receptor (ETAR) expression and ET-1-induced 3H-thymidine incorporation in, vascular SMCs. In serum-free medium, IGF-1 treatment increased the binding of 125I-ET-1 to SMC cell surface ET receptors from a specific binding of 20.1%+/-3.1% per mg of protein in control cells to 45.1%+/-8.6% per mg of protein in cells treated with IGF-1 (10 nM). The effect of IGF-1 was dose-related, with a significant effect (1.4-fold) being seen at 1 nM. The minimal time for IGF-1 treatment to be effective was 30 min and the maximal effect was reached at 6 h. Immunoblotting analysis showed that ETAR expression in IGF-1-treated cells was increased by 1.7-fold compared to controls. Levels of ETAR mRNA measured by the RT-PCR method and Northern blotting were also increased by 2-fold in IGF-1-treated SMCs. These effects of IGF-1 were abolished by cycloheximide or genistein. Finally, ET-1-stimulated thymidine uptake and cell proliferation were enhanced by IGF-1 treatment, with a maximal increase of 3.2-fold compared to controls. In conclusion, in vascular SMCs, IGF-1 increases the expression of the ET-1 receptor in a dose- and time-related manner. This effect is associated with increased thymidine uptake and involves tyrosine kinase activation and new protein synthesis. These findings support the role of IGF-1 in the development of atherosclerotic, hypertensive, and diabetic vascular complications.

Gender differences in the relation of insulin-like growth factor binding protein-1 to cardiovascular risk factors: a population-based study

OBJECTIVE

A possible involvement of insulin-like growth factor-I (IGF-I) and its binding protein IGFBP-1 in the pathogenesis of cardiovascular disorder has been suggested. However, few publications have addressed the gender differences in cardiovascular risk factors in relation to the IGF/IGFBP system. The aim of the present study was to study gender differences in the relationship between fasting serum levels of IGFBP-1 and cardiovascular risk factors in a normal population of men and women.

DESIGN

Cross-sectional study. Patients A normal population of 273 men and women aged 20-74 years.

MEASUREMENTS

A medical examination was performed and blood drawn in the morning after subjects had been fasting overnight. Before the examination, they were asked to fill out a questionnaire concerning lifestyle and psychosocial factors.

RESULTS

Fasting IGFBP-1 was lower in men than in women and was positively correlated to age in men but not in women. The men had in general a more disadvantageous cardiovascular risk profile than women, with several indicators of the metabolic syndrome: higher blood pressure and higher serum levels of total cholesterol, triglycerides, low density lipoprotein cholesterol (LDL-C), plasma-glucose and insulin, as well as lower IGFBP-1. Women had lower physical activity, lower consumption of alcohol, and lower values on indicators of psychosocial and mental health but had a healthier diet. Our findings indicate that low circulating levels of IGFBP-1 are associated with the well-known risk factors of cardiovascular disease; however, the association showed a different pattern for men and women. In men we found a negative association with body mass index (BMI), insulin resistance and diastolic blood pressure, and a positive association with SHBG, cortisol and testosterone. For women low IGFBP-1 appears in negative associations with BMI, waist-hip ratio (WHR), insulin resistance and testosterone, and in positive associations with SHBG and cortisol. Significant gender differences in the correlation with IGFBP-1 are seen for testosterone, cortisol, SHBG, WHR and oestradiol. For HDL-C and diastolic blood pressure the gender difference in correlation was at the limit of significance (P < 0.10).

CONCLUSION

Low circulating levels of IGFBP-1 are associated with the well-known risk factors of cardiovascular disease; however, the association showed a different pattern for men and women. The most marked gender differences in the correlation with IGFBP-1 are seen for testosterone, cortisol, SHBG, WHR, oestradiol, HDL-C and diastolic blood pressure. Our study emphasizes the importance of separate analyses for men and women. The results presented are a step towards gaining a better understanding of the gender differences in cardiovascular disease and in the regulation of IGFBP-1, though further prospective studies are needed.

Effects of dietary protein and growth hormone-releasing peptide (GHRP-2) on plasma IGF-1 and IGFBPs in Holstein steers

This study was conduct to determine the influence of dietary protein on the response of plasma insulin-like growth factor-1 (IGF-1) and insulin-like growth factor binding proteins (IGFBPs) to exogenous growth hormone releasing peptide-2 (GHRP-2 or KP 102) in Holstein steers. Eight 16-month-old Holstein steers were grouped by liveweight to two feeding treatments; high protein (HP; CP 1.38 kg/day and TDN 4.5 kg/day DM intake, n=4) or low protein (LP; CP 0.66 kg/day and TDN 4.42 kg/day DM intake, n=4). The experiment was a single reverse design whereby each group was injected twice daily with GHRP-2 (12.5 microg/kg body weight (BW)/day) or saline solution into the jugular vein for a 6-day period. Plasma IGF-1 in the HP group were higher than in the LP group (P<0.05), but plasma 34 kDa IGFBP-2 was lower in the HP than the LP group (P<0.05). The amplitude of the maximum growth hormone (GH) peaks responding to GHRP-2 injection were higher at day 1 than at day 6 of saline or GHRP-2 treatment in both LP and HP steers (P<0.05). The area under the GH response curve for 180 min after the GHRP-2 injection was not significantly different between the LP and the HP groups at days 1 and 6. A response in plasma IGF-1 concentration to GHRP-2 treatment in the HP group was observed at day 1 (198.9+/-18.1 ng/ml), day 2 (195.2+/-21.1 ng/ml) and day 6 (201.3+/-14.8 ng/ml) (P<0.05). No increase in plasma IGF-1 was observed from GHRP-2 administration in the LP group. Although the response of plasma IGF-1 concentration to GHRP-2 administration was increased in the HP group (P<0.05), there was no apparent effect of GHRP-2 treatment on plasma 38-43 kDa IGFBP-3 and 34 kDa IGFBP-2 at days 2 and 6 of treatment. In conclusion, it is proposed that the 34 kDa IGFBP-2 is sensitive to dietary protein level and may play an important role in the regulation of circulating IGF-1 in ruminant. In addition, increased plasma IGF-1 concentration observed in the HP group in response to the GHRP-2 treatment did not appear to affect plasma IGFBPs.

Insulin resistance in the liver-specific IGF-1 gene-deleted mouse is abrogated by deletion of the acid-labile subunit of the IGF-binding protein-3 complex: relative roles of growth hormone and IGF-1 in insulin resistance

Liver IGF-1 deficient (LID) mice demonstrate a 75% reduction in circulating IGF-1 levels and a corresponding fourfold increase in growth hormone (GH) levels. At 16 weeks of age, LID mice demonstrate, using the hyperinsulinemic-euglycemic clamp, insulin insensitivity in muscle, liver, and fat tissues. In contrast, mice with a gene deletion of the acid-labile subunit (ALSKO) demonstrate a 65% reduction in circulating IGF-1 levels, with normal GH levels and no signs of insulin resistance. To further clarify the relative roles of increased GH and decreased IGF-1 levels in the development of insulin resistance, we crossed the two mouse lines and created a double knockout mouse (LID+ALSKO). LID+ALSKO mice demonstrate a further reduction in circulating IGF-1 levels (85%) and a concomitant 10-fold increase in GH levels. Insulin tolerance tests showed an improvement in insulin responsiveness in the LID+ALSKO mice compared with controls; LID mice were very insulin insensitive. Surprisingly, insulin sensitivity, while improved in white adipose tissue and in muscle, was unchanged in the liver. The lack of improvement in liver insulin sensitivity may reflect the absence of IGF-1 receptors or increased triglyceride levels in the liver. The present study suggests that whereas GH plays a major role in inducing insulin resistance, IGF-1 may have a direct modulatory role.

Serum levels of insulin-like growth factor (IGF)-I and IGF binding protein (IGFBP)-1 to -6 and their relationship to bone metabolism in osteoporosis patients

BACKGROUND: Insulin-like growth factor (IGF) system components are important regulators of bone formation. Alterations of individual IGF system components have been described in osteoporosis (OP) patients; however, no study has addressed changes in free IGF-I and in all six IGF binding proteins (IGFBPs). METHODS: A cross-sectional study was performed in 45 OP patients and 100 healthy matched controls. Serum levels of free and total insulin-like growth factor I (IGF-I), IGFBP-1 through -6, intact parathyroid hormone (PTH), 25-OH-vitamin D(3) (25OHD(3)), 1,25-(OH)(2)-vitamin D(3) (1,25-(OH)(2)D(3)), osteocalcin (OSC), bone alkaline phosphatase (B-ALP), and carboxyterminal propeptide of type-I procollagen (PICP) were measured with specific assays. Bone mineral density (BMD) of the lumbar spine was determined by dual-energy X-ray absorptiometry (DEXA). RESULTS: Compared with age- and sex-matched control subjects, OP patients showed a 73% decrease in free IGF-I, a 29% decrease in total IGF-I, a 10% decrease in IGFBP-3, and a 52% decrease in IGFBP-5 levels; they had higher levels of IGFBP-1 (4.1-fold), IGFBP-2 (1.8-fold), IGFBP-4 (1.3-fold), and IGFBP-6 (2.1-fold). Alterations in IGF system components were most evident in 13 OP patients with vertebral fractures in the past 4 years compared to patients without fractures. In OP patients with fractures, the ratio between IGFBP-4 and IGFBP-5 was increased whereas levels of OSC were decreased. CONCLUSIONS: Our data provide strong indirect evidence for a functional connection between circulating IGF system components and bone metabolism and the susceptibility to fractures in OP patients.

Circulating levels of IGF-1 directly regulate bone growth and density

IGF-1 is a growth-promoting polypeptide that is essential for normal growth and development. In serum, the majority of the IGFs exist in a 150-kDa complex including the IGF molecule, IGF binding protein 3 (IGFBP-3), and the acid labile subunit (ALS). This complex prolongs the half-life of serum IGFs and facilitates their endocrine actions. Liver IGF-1-deficient (LID) mice and ALS knockout (ALSKO) mice exhibited relatively normal growth and development, despite having 75% and 65% reductions in serum IGF-1 levels, respectively. Double gene disrupted mice were generated by crossing LID+ALSKO mice. These mice exhibited further reductions in serum IGF-1 levels and a significant reduction in linear growth. The proximal growth plates of the tibiae of LID+ALSKO mice were smaller in total height as well as in the height of the proliferative and hypertrophic zones of chondrocytes. There was also a 10% decrease in bone mineral density and a greater than 35% decrease in periosteal circumference and cortical thickness in these mice. IGF-1 treatment for 4 weeks restored the total height of the proximal growth plate of the tibia. Thus, the double gene disruption LID+ALSKO mouse model demonstrates that a threshold concentration of circulating IGF-1 is necessary for normal bone growth and suggests that IGF-1, IGFBP-3, and ALS play a prominent role in the pathophysiology of osteoporosis.

Intestinal transport of insulin-like growth factor-I (igf-I) in the suckling rat

OBJECTIVES

Insulin-Like Growth Factor-1 is a potent growth-promoting peptide that is present in mammalian milk. Previous studies have suggested that milk-borne IGF-1 may be absorbed intact from the gastrointestinal tract of the suckling but the mechanism responsible for such transport is not well documented. The present study was designed to investigate in an in vivo suckling rat model whether or not intestinal absorption of IGF-1 is a saturable phenomenon.

METHODS

Suckling rats (10-12 days postnatal age) were studied under anesthesia. A jejunal loop from each rat pup was isolated and injected intraluminally with 1-2 x 10 cpm of rh I-IGF-I. Injections were performed in paired littermates either with or without a preceding injection of unlabeled IGF-I of 20, 500, or 1000 ng/ml concentration. After flushing, the loops and livers were homogenized and counted in a gamma counter. In addition, homogenates of jejunum and liver were precipitated with trichloroacetic acid (TCA) and the precipitates also counted. In selected instances (jejunum), acid gel chromatography of homogenates was also performed.

RESULTS

Retention of radioactivity was observed in all jejunal specimens, but the pre-incubation of jejunal loops with unlabeled IGF-1 was associated with a biphasic response, i.e. at low dose (20 ng/ml) pre-incubation limited retention of radioactivity, but at a high dose (1000 ng/ml), retention was enhanced (P < 0.05). Linear regression analysis confirmed this inverse relationship. Liver radioactivity followed a similar pattern. Between 40 and 49% of the radioactivity in jejunal and liver homogenates was TCA precipitable. Chromatography of jejunal homogenates showed that approximately 40% of cpm migrated in a position identical with that of intact IGF-1.

CONCLUSIONS

The intestinal uptake of IGF-1 in the suckling is nonsaturable, confirming previous in vitro studies and suggesting that a nonreceptor-dependent method of transepithelial transport is important in this process.

Circulating levels of IGF-1 directly regulate bone growth and density

IGF-1 is a growth-promoting polypeptide that is essential for normal growth and development. In serum, the majority of the IGFs exist in a 150-kDa complex including the IGF molecule, IGF binding protein 3 (IGFBP-3), and the acid labile subunit (ALS). This complex prolongs the half-life of serum IGFs and facilitates their endocrine actions. Liver IGF-1-deficient (LID) mice and ALS knockout (ALSKO) mice exhibited relatively normal growth and development, despite having 75% and 65% reductions in serum IGF-1 levels, respectively. Double gene disrupted mice were generated by crossing LID+ALSKO mice. These mice exhibited further reductions in serum IGF-1 levels and a significant reduction in linear growth. The proximal growth plates of the tibiae of LID+ALSKO mice were smaller in total height as well as in the height of the proliferative and hypertrophic zones of chondrocytes. There was also a 10% decrease in bone mineral density and a greater than 35% decrease in periosteal circumference and cortical thickness in these mice. IGF-1 treatment for 4 weeks restored the total height of the proximal growth plate of the tibia. Thus, the double gene disruption LID+ALSKO mouse model demonstrates that a threshold concentration of circulating IGF-1 is necessary for normal bone growth and suggests that IGF-1, IGFBP-3, and ALS play a prominent role in the pathophysiology of osteoporosis.

Effects of an endurance cycling competition on resting serum insulin-like growth factor I (IGF-I) and its binding proteins IGFBP-1 and IGFBP-3

OBJECTIVES

To determine whether consecutive bouts of intense endurance exercise over a three week period alters serum concentrations of insulin-like growth factor I (IGF-I) and/or its binding proteins.

METHODS

Seventeen professional cyclists (mean (SEM) VO(2)MAX, 74.7 (2.1) ml/kg/min; age, 27 (1) years) competing in a three week tour race were selected as subjects. Blood samples were collected at each of the following time points: t(0) (control, before the start of competition), t(1) (end of first week), and t(3) (end of third week). Serum levels of both total and free IGF-I and IGF binding proteins 1 and 3 (IGFBP-1 and IGFBP-3) were measured in each of the samples. Cortisol levels were measured in nine subjects.

RESULTS

A significant (p<0.01) increase was found in total IGF-I and IGFBP-1 at both t(1) and t(3) compared with t(o) (IGF-I: 110.9 (17.7), 186.8 (12.0), 196.9 (14.7) ng/ml at t(0), t(1), and t(3) respectively; IGFBP-1: 54.6 (6.6), 80.6 (8.0), and 89.2 (7.9) ng/ml at t(0), t(1), and t(3) respectively). A significant (p<0.01) decrease was noted in free IGF-I at t(3) compared with both t(o) and t(1) (t(0): 0.9 (0.1) ng/ml; t(1): 0.9 (0.1) ng/ml; t(3): 0.7 (0.1) ng/ml); in contrast, IGFBP-3 levels remained stable throughout the race.

CONCLUSIONS

It would appear that the increase in circulating levels of both IGF-I and its binding protein IGFBP-1 is a short term (one week) endocrine adaptation to endurance exercise. After three weeks of training, total IGF-I and IGFBP-1 remained stable, whereas free IGF-I fell below starting levels.

Distribution of chimeric IGF binding protein (IGFBP)-3 and IGFBP-4 in the rat heart: importance of C-terminal basic region

IGF binding proteins-3 and -4, whether given in the perfused rat heart or given iv in the intact animal, cross the microvascular endothelium of the heart and distribute in subendothelial tissues. IGF binding protein-3, like IGF-I/II, localizes in cardiac muscle, with lesser concentrations in CT elements. In contrast, IGFBP-4 preferentially localizes in CT. In this study, chimeric IGF binding proteins were prepared in which a basic 20-amino-acid C-terminal region of IGF binding protein-3 was switched with the homologous region of IGF binding protein-4, and vice-versa, to create IGF binding protein-3(4) and IGF binding protein-4(3). Perfused IGF binding protein-3(4) behaved like IGF binding protein-4, localizing in connective tissue elements, whereas IGF binding protein-4(3) now localized in cardiac muscle at concentrations identical to perfused IGF binding protein-3. To determine whether these small mutations altered the affinity of the chimera for cells, the ability of (125)I-IGF binding protein-3(4) and (125)I-IGF binding protein-4(3) to bind to microvascular endothelial cells was determined and compared with IGF binding protein-3. IGF binding protein-3(4) retained 15% of the binding capacity of IGF binding protein-3, whereas IGF binding protein-4(3) bound to microvessel endothelial cells with higher affinity and greater total binding than that of IGF binding protein-3. We conclude that small changes in the C-terminal basic domain of IGF binding protein-3 and the corresponding region of IGF binding protein-4 can alter their affinity for cultured cells and influence their tissue distribution in the rat heart.

Differential changes in insulin-like growth factors and their binding proteins following asphyxia in the preterm fetal sheep

1. The purpose of this study was to examine the changes in circulating concentrations of insulin-like growth factor (IGF)-I, IGF-II, IGF-binding protein (IGFBP)-1, IGFBP-2 and insulin following asphyxia in utero. 2. Fetal sheep at 90-93 days gestation underwent either sham occlusion (n = 7) or asphyxia (n = 6) induced by complete umbilical cord occlusion for 30 min. Fetal blood samples were taken before occlusion and 4, 6, 24, 48 and 72 h post-occlusion. 3. During the early phase of recovery there was a substantial fall (80 %) in circulating plasma IGF-I concentrations by 6 h post-asphyxia (P < 0.001). This was associated with a rapid rise in IGFBP-1 (P < 0.001), but no change in IGF-II or IGFBP-2. Insulin was significantly reduced at 4 h (P < 0.001) and glucose slightly elevated (P < 0.05), but insulin values returned to baseline by 6 h. Between 24 and 72 h of recovery, IGF-I gradually increased, IGFBP-1 returned to control values, and there was an increase in IGFBP-2 after 24 h (P < 0.05) and in IGF-II by 72 h (P < 0.05) after asphyxia. 4. These data demonstrate a differential effect of asphyxia on the IGF axis of the premature fetal sheep. A key finding was the large fall in circulating IGF-I, but not IGF-II, during the early phase of recovery. IGF-I bioavailability was, in part, regulated by IGFBP-1, but maximal changes in IGF-I and IGFBP-1 were independent of plasma insulin and glucose.5. The impact of this substantial change in circulating IGF-I on the fetus is unknown. It may facilitate metabolic requirements by promoting catabolism. Alternatively, as IGFs play a role in wound repair, the acute changes in IGF-I and IGFBP-1 may reflect transport of IGF-I from the circulatory pool to injured tissues to promote wound repair.

Acid-labile subunit regulation during the early stages of liver regeneration: implications for glucoregulation

The initiation of liver regeneration is regulated by endogenously produced growth factors and cytokines and is accompanied by suppression of growth hormone (GH) binding to hepatocytes. We have demonstrated some of these factors, particularly GH, which modulate acid-labile subunit (ALS) expression in vitro. Consequently, we investigated ALS hepatic mRNA and serum levels in rats for 24 h after partial hepatectomy (PHx). There was a significant suppression of ALS gene expression (approximately 50%, P < 0.005) and serum levels (approximately 30%, P < 0.02) by 12 h in PHx rats relative to controls. Relative to intact animals, hepatic mRNA and serum levels of ALS were suppressed by approximately 60% at 24 h. Similarly, hepatic GH receptor mRNA levels were significantly reduced in PHx animals. Moreover, hepatocytes isolated from PHx animals were less responsive to GH than those from controls. Overall, our results demonstrate that suppression of ALS gene expression and serum levels during liver regeneration relates to lowered hepatic GH sensitivity. Suppressed circulating ALS may alter insulin-like growth factor bioavailability and constitute a mechanism to maintain relatively normal glucoregulation after loss of liver mass.

A homologous radioimmunoassay for quantification of insulin-like growth factor-binding protein-2 in blood from cattle

Insulin-like growth factor-I and -II (IGF-I, IGF-II) circulate in biological fluids bound to six different IGF-binding proteins that regulate IGF bioactivity. The IGF-binding protein-2 is regulated by growth hormones, and its concentration depends on nutrition and physiological state. Specific antibodies directed against bovine IGF-binding protein-2 were produced, and IGF-binding protein-2 levels in bovine blood samples were quantified by radioimmunoassay. Parallel displacement curves showed strong cross-reactivity with bovine and ovine plasma, were low with porcine plasma, and no cross-reactivity with rat or chicken plasma. Addition of IGF-I or -II to a control pool of bovine plasma did not significantly alter control IGF-binding protein-2 values in a radioimmunoassay. Six nycthemeral periods, determined for three young bulls bled on two occasions, showed that IGF-binding protein-2 plasma levels were stable throughout the day; two or three samples were sufficient to characterize the animal. Cows treated with recombinant bovine somatotropin (bST) had significantly lower serum levels of IGF-binding protein-2 than did control cows. Furthermore, IGF-binding protein-2 levels were dramatically increased at the onset of lactation. This radioimmunoassay for bovine IGF-binding protein-2, which enables quantitative assessment of IGF-binding protein-2 concentration in cattle, confirmed that IGF-binding protein-2 concentrations are depressed by administration of bST, enhanced after calving, and showed absence of diurnal variation.

Insulin-like growth factor binding protein-1 (IGFBP-1) and IGF-I during oral and transdermal estrogen replacement therapy: relation to lipoprotein(a) levels

Low levels of insulin-like growth factor binding protein-1 (IGFBP-1) have recently been associated with several risk factors for cardiovascular disease. The effects of estrogen replacement therapy (ERT) on plasma IGFBP-1 levels are, however, unclear. A double-blind, placebo-controlled study for 6 months was conducted in 73 hysterectomized postmenopausal women randomized into two groups: oral estradiol (E2) valerate, 2 mg/day (n = 35) and transdermal E2 gel, 1 mg/day (n=38). Plasma IGFBP-1, insulin-like growth factor-I (IGF-I) and lipoprotein(a) (Lp(a)) were determined at baseline, 3 and 6 months. The groups were similar for age and BMI. The baseline levels of estrone (E1), E2, IGFBP-1, IGF-I and Lp(a) did not differ between the groups. During treatment, serum estradiol concentrations increased in both groups. During oral ERT, IGFBP-1 levels increased by 104% (P<0.001), whereas IGF-I levels decreased by 13% (mean, P<0.05). IGF-I and IGFBP-1 levels remained unchanged in the transdermal group. Lp(a) levels decreased by 23% (median, P<0.001) in the oral group, but were unaffected by transdermal therapy. The change in IGFBP-1 concentrations during oral ERT showed an inverse correlation to that in Lp(a) (r = -0.40, P<0.05, Spearman correlation). In conclusion, oral ERT seems to enhance plasma levels of IGFBP-1, which may be one reason for the reduced Lp(a) levels.

Insulin-like growth factor system components in hyperparathyroidism and renal osteodystrophy

BACKGROUND

The insulin-like growth factor (IGF) system plays a key role in regulation of bone formation. In patients with renal osteodystrophy, an elevation of some IGF binding proteins (IGFBPs) has been described, but there is no study measuring serum levels of both IGF-I and IGF-II as well as IGFBP-1 to -6 in different forms of renal osteodystrophy and hyperparathyroidism.

METHODS

In a cross-sectional study, we investigated 319 patients with mild (N = 29), moderate (N = 48), preuremic (N = 37), and end-stage renal failure (ESRF; N = 205). The ESRF group was treated by hemodialysis (HD; N = 148), peritoneal dialysis (PD; N = 27), or renal transplantation (RTX; N = 30). As controls without renal failure, we recruited age-matched healthy subjects (N = 87) and patients with primary hyperparathyroidism (pHPT; N = 25). Serum levels of total and free IGF-I, IGF-II, IGFBP-1 to -6, and biochemical bone markers including intact parathyroid hormone (PTH), bone alkaline phosphatase (B-ALP), and osteocalcin (OSC) were measured by specific immunometric assays. IGF system components and bone markers were correlated with clinical and bone histologic findings. Mean values +/- SEM are given.

RESULTS

With declining renal function a significant increase was measured for IGFBP-1 (range 7- to 14-fold), IGFBP-2 (3- to 8-fold), IGFBP-3 (1.5- to 3-fold), IGFBP-4 (3- to 19-fold), and IGFBP-6 (8- to 25-fold), whereas IGFBP-5 levels tended to decrease (1.3- to 1. 6-fold). In contrast, serum levels of IGF-I, free IGF-I, and IGF-II remained constant in most patients. Compared with renal failure patients, pHPT patients showed a similar decline in IGFBP-5 levels and less elevated levels of IGFBP-1 (3.5-fold), IGFBP-2 (2-fold), IGFBP-3 (1.2-fold), and IGFBP-6 (4-fold) but no elevation of IGFBP-4 levels. In all subjects, free and total IGF-I levels showed significant negative correlations with IGFBP-1, IGFBP-2, and IGFBP-4 (that is, inhibitory IGF system components) and significant positive correlations with IGFBP-3 and IGFBP-5 (that is, stimulatory IGF system components). A positive correlation was observed between IGF-II and IGFBP-6. ESRF patients with mixed uremic bone disease and histologic evidence for osteopenia revealed significantly (P < 0.05) higher levels of IGFBP-2 and IGFBP-4 but lower IGFBP-5 levels. Histologic parameters of bone formation showed significant positive correlations with serum levels of IGF-I, IGF-II, and IGFBP-5. In contrast, IGFBP-2 and IGFBP-4 correlated positively with indices of bone loss. Moreover, dialysis patients with low bone turnover (N = 24) showed significantly (P < 0.05) lower levels of IGFBP-5, PTH, B-ALP, and OSC than patients with high bone turnover.

CONCLUSION

Patients with primary and secondary hyperparathyroidism showed lower levels of the putative stimulatory IGFBP-5 but higher levels of IGFBP-1, -2, -3, and -6, whereas total IGF-I and IGF-II levels were not or only moderately increased. The marked increase in serum levels of IGFBP-4 appeared to be characteristic for chronic renal failure. IGFBP-5 correlated with biochemical markers and histologic indices of bone formation in renal osteodystrophy patients and was not influenced by renal function. Therefore, IGFBP-5 may gain significance as a serological marker for osteopenia and low bone turnover in long-term dialysis patients.

IGF binding protein-1 (IGFBP-1) is preferentially associated with the fetal-facing basal surface of the syncytiotrophoblast in the human placenta

IGF receptors are expressed in a spatially polarized manner on the syncytiotrophoblast cell membrane. We therefore examined the hypothesis that IGFBPs expressed at the maternal-fetal interface interact with distinct surfaces of the syncytiotrophoblast membrane to modulate IGF function. Membrane vesicles were prepared specifically from the maternal-facing, microvillous membrane (MVM) and the fetal-facing, basal membrane (BM) surfaces of the syncytiotrophoblast. The association of IGFBPs with each membrane preparation was determined by ligand blot analysis. A doublet migrating at 38/42 kD was detected in both MVM and BM preparations. Selective immunoprecipitation followed by ligand blot analysis identified this IGF binding species as IGFBP-3. Additionally, a protein migrating at approximately 29 kD was associated primarily with the BM. This protein was identified as IGFBP-1 by both immunoprecipitation and ligandblotting techniques. Non-denaturing PAGE revealed five distinct bands corresponding to different degrees of phosphorylation. The phosphorylation pattern of BM-associated IGFBP-1 was identical to that of native IGFBP-1 in amniotic fluid. Immunohistological analysis of term placenta revealed IGFBP-1-specific staining of the syncytiotrophoblast and the fetal capillary/pericapillary bed. The localization of IGFBP-1 to a distinct compartment within the fetal placenta, not in proximity to the syncytiotrophoblast type I IGF receptor, suggests it may play a role in regulating/targeting IGF activity within the stromal compartment or by exerting IGF-independent effects on the basal surface of the syncytiotrophoblast. The nature of its binding to the BM has not been determined.

A quantitative PCR measurement of messenger RNA expression of IGF-I, IGF-II and IGFBP-5 in human skeletal muscle

Insulin-like growth factor-I and -II (IGF-I and IGF-II) and their binding proteins are important components in growth promotion and tissue maintenance. We determined the presence of IGF-I, -II, and binding protein 5 (IGFBP-5) gene expression in human skeletal muscle and that mRNA abundance is not altered by nutrients and insulin. In the first protocol, (control) subjects were given water. In the second protocol, half of these subjects drank Polycose (carbohydrate) and the remaining subjects drank equal calories as a mixed meal. Quadriceps muscle biopsies were taken at 10 h. A semi-quantitative polymerase chain reaction was designed to measure gene expression. IGF-I, IGF-II and IGFBP-5 mRNA are present in adult human skeletal muscle, but no significant changes between meal groups were observed for IGF-I, IGF-II or IGFBP-5 mRNA levels, indicating that the expression of these genes are not altered acutely by nutrients and insulin.

Influence of physical activity on plasma insulin-like growth factor-1 and insulin-like growth factor binding proteins in healthy older women

It was examined whether physical activity could alter the bioavailability of insulin-like growth factor-1 (IGF-1) which is dependent upon plasma IGF-1 and insulin-like growth factor binding protein (IGFBPs) levels. The potential role that growth hormone (GH) and insulin play in this process was also examined. Seven healthy 62-69-year-old women performed four bouts of physical activity on separate occasions at either a low (L; heart rate = 100 bpm) or moderate intensity (M; heart rate = 120 bpm) for either 25 (S) or 50 (L) min (LS, low intensity/short duration; LL, low intensity/long duration; MS, moderate intensity/short duration; ML, moderate intensity/long duration). GH levels were elevated immediately following the physical activity from 1.3 to 2.6-fold (P < 0.05) whereas IGF-1 levels were not affected by any activity condition. Plasma insulin levels decreased about 35% under all activity conditions (P < 0.05). Plasma levels of IGFBP-1 (BP-1) were decreased immediately following the ML (-47%; P < 0.05) and the LL (-21%) activity bouts and remained lower than initial values 1 h after these activity bouts (-25 and 34%, respectively, P < 0.05). The ML exercise bout resulted in significant (P < 0.05) increases in IGFBP-2 (BP-2) and IGFBP-3 (BP-3) immediately following activity (+31, and +30%, respectively) and these binding proteins remained elevated following the activity (+28, and +48%, respectively). No relationship was found between any changes in plasma GH or insulin, and changes in plasma IGFBPs. Thus, moderate intensity physical activity of long duration may modulate the bioavailability of IGF-1 in the elderly via alterations in BP-1, -2 and -3. However, changes in circulating levels of GH, insulin or IGF-1 do not appear to be regulating IGF-1 bioavailability in response to physical activity.

Evidence supporting a direct suppressive effect of growth hormone on serum IGFBP-1 levels. Experimental studies in normal, obese and GH-deficient adults

It has occasionally been suggested that GH directly suppresses circulating IGFBP-1 levels, although it is generally believed that such an effect is secondary to a GH-induced increase in insulin levels. We present data from several experiments in which the effects of GH on IGFBP-1 could be studied more extensively. In normal subjects (n = 36), an i.v. GH bolus caused a small but significant decrease in plasma IGFBP-1 concentrations without changes in insulin [IGFBP-1 (microgram/l): 2.6 +/- 0.3 (GH) vs 3.2 +/- 0.4 (placebo), P < 0.05]. Conversely, a 28-h somatostatin infusion with and without GH administration during fasting in normal subjects yielded higher IGFBP-1 levels in the non-GH substituted study [50.5 +/- 5.3 (GH-suppression) vs 22.6 +/- 5.6 (GH-substitution), P < 0.01], comparable with an increased concentration of IGFBP-1 during fasting in GH-deficient patients without usual GH substitution [23.4 +/- 7.6 (GH pause) vs 14.1 +/- 4.9 (GH substitution), P < 0.01]. In both fasting studies insulin levels remained stable. During a hypocaloric diet, long-term GH treatment in obesity lead to a significant decline in IGFBP-1 level (2.3 +/- 0.6 vs 1.2 +/- 0.2, P < 0.01), while no changes were found in the placebo group. Again, insulin levels remained equally low in both studies. Finally, a significant rebound increase in IGFBP-1 level in response to insulin induced hypoglycemia was only observed among GH-deficient patients, but not in control subjects, the latter of whom responded to hypoglycemia with a significant increase in serum GH levels [23.2 +/- 7.2 (GHDA) vs 2.5 +/- 0.3 (controls), P < 0.01]. In conclusion, a suppressive effect of GH on IGFBP-1 appears to be unmasked in the presence of low or suppressed insulin levels, making GH a potential regulator of IGF-1 bioactivity in a hitherto unrecognized way.

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

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

Role of the insulin-like growth factor system in uterine function and placental development in ruminants

Maternal nutrition during pregnancy influences fetal and placental weights. The insulin-like growth factors (IGF) are also important determinants of fetal size. Furthermore, the expression of several components of the IGF system is regulated by nutrition. Effects of nutrition on fetal growth could therefore be mediated by the IGF system in the uterus and placenta. The oviductal mucosa produces IGF-I, which may influence oviductal secretions or act directly on embryonic type 1 IGF receptors. In the uterus, IGF-I mRNA is localized to the stroma surrounding the endometrial glands, which contain high concentrations of IGF type 1 receptors. Uterine IGF-I concentrations fall during pregnancy; therefore, glandular activity is more likely influenced by systemic than local IGF-I production. The IGF-II mRNA is present in both caruncles and fetal placental mesoderm, but concentrations are much higher in the latter. The actions of IGF-I and IGF-II on the endometrium and placenta are influenced by IGF-binding proteins. In the ewe, mRNAs for IGF binding protein-1 and -5 are located in the luminal and glandular epithelia, IGF binding proteins-2 and -4 are produced in the subepithelial stroma, and IGF binding protein-4 is also in the placentome capsule; IGF binding protein-3 is more widely expressed in both maternal and fetal tissues. The IGF binding proteins, therefore, form a major barrier to the passage of IGF between the fetal and maternal circulatory systems.

Serum total IGF-I, free IGF-I, and IGFB-1 levels in an elderly population: relation to cardiovascular risk factors and disease

Recently, a method to measure free insulin-like growth factor-I (IGF-I) levels has been developed. Free IGF-I levels may have greater physiological and clinical relevance than total (bound and free) IGF-I. The associations between the circulating IGF-I/IGF binding protein (IGFBP) system and cardiovascular disorders was studied. In a cross-sectional study of 218 healthy persons (103 men, 115 women) aged 55 to 80 years, fasting serum (total and free) IGF-I and IGFBP-1 levels, lipid profile, insulin, and glucose were measured. In addition, blood pressure, body mass index (BMI), and waist-hip ratio (WHR) were measured. Ultrasonography of both carotid arteries was performed to investigate the presence of atherosclerotic lesions. A history of angina pectoris, the presence of a possible or definite myocardial infarction on the ECG, and plaques in the carotid arteries were used as indicators of presence of cardiovascular signs and symptoms. Free IGF-I was inversely related to serum triglycerides (P=.04, adjusted for age and sex). Mean free IGF-I levels in subjects without signs or symptoms of cardiovascular diseases were significantly higher than in those with at least one cardiovascular symptom or sign (P=.002, adjusted for age and sex). Free IGF-I levels were also higher in subjects who had no atherosclerotic plaques in the carotid arteries (P=.02, adjusted for age and sex) and who had never smoked (P=.02, adjusted for age and sex). IGFBP-1 showed an inverse relation with insulin, BMI, and WHR and a positive relation with HDL cholesterol. The associations between IGFBP-1 levels and HDL cholesterol, WHR, and BMI remained significant after adjustment for fasting insulin levels. High fasting serum free IGF-I levels are associated with a decreased presence of atherosclerotic plaques and coronary artery disease and lower serum triglycerides, whereas high fasting IGFBP-1 levels are associated with a more favorable cardiovascular risk profile. The findings suggest that the IGF-I/IGFBP system is related to cardiovascular risk factors and atherosclerosis.

Simulation of IGF-I pharmacokinetics after infusion of recombinant IGF-I in human subjects

The pharmacokinetics of recombinant human insulin-like growth factor I (IGF-I) were studied in four healthy volunteers by a 3-h infusion at a rate of 20 micrograms.kg-1.h-1. A compartmental model was used to simulate the plasma "free" IGF-I and IGF-I associated with the 50- and 150-kDa plasma protein fractions. The model is based on the concept that free IGF-I and IGF binding proteins (IGFBPs) are the substrates for their own degradation and that they act as reservoirs for retention of IGF-I in the vascular compartment or inhibiting IGF-I action. The metabolic clearance rate (MCR) of free IGF-I is estimated as 2.62 +/- 0.94 ml.min-1.min-1 with a production rate of 4.75 +/- 1.74 mg/day (621.0 +/- 227.34 nmol/day). The simulation shows that higher concentrations of IGFBP-3 would increase the estimate of MCR for free IGF-I by reducing free IGF-I concentration. The model will be of value for simulation of dynamic profiles of free IGF-I and receptor-bound IGF-I in a variety of pathophysiological conditions.

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

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

Paracellular transport of insulin-like growth factor-I (IGF-I) across human umbilical vein endothelial cell monolayers

Insulin-like growth factors (IGFs) are well defined mitogens and growth promoters, which are found in blood associated with high affinity IGF binding proteins (IGFBPs). In vivo, the endothelium is potentially the primary site of uptake of IGFs or IGF-IGFBP complexes from blood for transport to the extravascular space. However, the pathway and mechanisms by which IGFs cross the endothelial cell barrier are not known. The presence of high affinity receptors for IGF-I and IGF-II on human umbilical vein endothelial (HUVE) cells was demonstrated by (i) radio-receptor assays using both IGF-I and IGF-II and (ii) affinity label cross-linking studies. In addition, Western ligand blotting and immunoblotting revealed that IGFBP-2, -3, and -4 are secreted into serum-free media conditioned by confluent HUVE cell monolayers. To study transendothelial migration of IGF-I, HUVE cells were grown on microporous membranes in a bichamber system. When compared with membranes without cells, HUVE monolayers restricted the passage of 125I-IGF-I and [3H]inulin, whereas the control Madin Darby canine kidney (MDCK) cell line virtually excluded all passage of these molecules. Transport of 125I-IGF-I across HUVE cell monolayers was not significantly different to that of [3H]inulin, a paracellular probe. Moreover, 125I-IGF-I transport was not inhibited by either excess unlabelled IGF-I or a monoclonal antibody to the type I IGF receptor at a concentration shown to inhibit 125I-IGF-I binding to HUVE cell monolayers. Our findings show that the movement of free IGF-I across HUVE cell monolayers occurs via a paracellular route and not by a receptor-mediated, transcellular pathway.

Functional epitope mapping of insulin-like growth factor I (IGF-I) by anti-IGF-I monoclonal antibodies

Based on a collection of monoclonal antibodies (mAb) against insulin-like growth factor I (IGF-I), we have defined the IGF-I epitopes involved in the interaction with IGF-binding proteins (IGFBP) and IGF-I receptors. We have also characterized the ability of these antibodies to block IGF-I-induced survival of the IL-3-dependent Ba/F3 cell line. More than 140 hybridomas secreting IGF-I-specific mAb were characterized, of which 28 were studied in detail. They display apparent affinity constants ranging from less than 10(6) to 10(10) M-1 and varying crossreactivity with IGF-II, including 2 mAb with higher affinity for IGF-II than for IGF-I. None crossreact with insulin or any other growth factor tested. Using both enzyme immunoassays and real-time biospecific interaction analysis, we have identified 8 epitopic clusters related to the primary structure of IGF-I, according to mAb reactivity to synthetic peptides, proteolytic fragments of IGF-I, and various IGF-I mutants. The mAb panel also was used to map the IGF domains implicated in the interaction with IGFBP and IGF-I receptors. An IGF-I domain has been identified that remains exposed after IGF-I binding to IGFBP-1 or to IGFBP-3, which is recognized by 6 different mAb. The mAb in this group also bind IGF-I, when complexed to the type-1 IGF receptor on the murine pro-B cell line Ba/F3, and BALB/c 3T3 fibroblasts overexpressing the human receptor. Finally, IGF-I-promoted survival can be blocked with mAb specific for target epitopes, and their potential use in tumor cell growth control is discussed.

Insulin-like growth factor-I in man enhances lipid mobilization and oxidation induced by a growth hormone pulse

Growth hormone (GH) secretion is suppressed during insulin-like growth factor-I (IGF-I) administration. The aim of the study was to examine whether IGF-I alters the metabolic response to a GH pulse. Seven healthy male subjects (age 27 +/- 4 years, BMI 21.8 +/- 1.7 kg/m2) were treated with NaCl 0.9% (saline) or IGF-I (8 micrograms.kg-1.h-1) for 5 days by continuous subcutaneous infusion in a randomized, crossover fashion while receiving an isocaloric diet (30 kcal.kg-1.day-1). On the third treatment day an intravenous bolus of 0.5 U GH was administered. Forearm muscle metabolism was examined by measuring arterialized and deep venous blood samples, forearm blood flow by occlusion plethysmography and substrate oxidation by indirect calorimetry. IGF-I treatment significantly reduced insulin concentrations by 80% (p < 0.02) and C-peptide levels by 78% (p < 0.02), as assessed by area under the curve. Non-esterified fatty acid (NEFA), glycerol and 3-OH-butyrate levels were elevated and alanine concentration decreased. Forearm blood flow rose from 2.10 +/- 0.43 (saline) to 2.79 +/- 0.37 ml.100ml-1. min-1 (IGF-I) (p < 0.02). GH-pulse: 10 h after i.v. GH injection serum GH peaked at 40.9 +/- 7.4 ng/ml. GH did not influence circulating levels of total IGF-I, C-peptide, insulin or glucose, but caused a further increase in NEFA, glycerol and 3-OH-butyrate levels, indicating enhanced lipolysis and ketogenesis. This effect of GH was much more pronounced during IGF-I: NEFA rose from 702 +/- 267 (saline) and 885 +/- 236 (IGF-I) to 963 +/- 215 (saline) (p < 0.05) and 1815 +/- 586 mumol/l (IGF-I) (p < 0.02), respectively; after 5 h, 3-OH-butyrate rose from 242 +/- 234 (saline) and 340 +/- 280 (IGF-I) to 678 +/- 638 (saline) (p < 0.02) and 1115 +/- 578 mumol/l (IGF-I) (p < 0.02) respectively. After injection of GH, forearm uptake of 3-OH-butyrate was markedly elevated only in the subjects treated with IGF-I: from 44 +/- 195 to 300 +/- 370 after 20 min (p < 0.03) and to 287 +/- 91 nmol.100 ml-1. min-1 after 120 min (p < 0.02). In conclusion, the lipolytic and ketogenic response to GH was grossly enhanced during IGF-I treatment, and utilization of ketone bodies by skeletal muscle was increased.

Insulin-like growth factor-I (IGF-I) colocalizes with IGF-binding proteins (IGFBPs) in mouse and rat ovary

The cellular localization of insulin-like growth factor (IGF)-I mRNA, IGF-I peptide, and IGF-binding proteins (IGFBPs) was examined in mouse and rat ovaries through use ofin situ hybridization and immunohistochemical methods. IGF-I mRNA was found to be most abundant in granulosa cells, although lower levels were also detected in cells of the theca interna, stroma, and corpus luteum. In contrast, IGF-I immunoreactivity was undetectable or low in granulosa cells, weak and variable in oocytes, high in theca interna and the corpus luteum, and highest in the stroma. Antibodies directed against IGFBP-2, 3, and 5 yielded similar patterns of immunoreactivity to that observed for IGF-I peptide. The results indicate that IGF-I is synthesized in ovarian follicles, and that IGF-I of ovarian or systemic origin becomes localized to sites containing IGFBPs in the ovary.

Insulin-like growth factor (IGF) binding proteins and their mRNAs in connective tissues of fasted guinea-pigs

Fasting (with vitamin C-supplementation) and vitamin C-deficiency in guinea-pigs are associated with decreased collagen gene expression in connective tissues. Recently we presented evidence that circulating insulin-like growth factor binding protein (IGFBP)-1 and-2 that are induced during both nutritional deficiencies may be responsible for this inhibition by interfering with IGF-I action. The present objective was to determine whether circulating IGFBPs are accumulated in bone, skin and cartilage during fasting, which would support an endocrine role for them. IGFBP-1 mRNA was not detected in any of the connective tissues. The protein, as measured by ligand blotting, was not present in tissues of normal animals but accumulated early during fasting in all of the tissues. Bone and cartilage from normal animals contained IGFBP-2 and its mRNA, but only in bone did their levels increase during fasting. IGFBP-3 mRNA was not detected in connective tissues from normal or fasted guinea-pigs. Little or no IGFBP-3 was detected in normal tissue extracts, but protein accumulated during fasting and presumably was derived from the circulation. IGF-I and-II mRNAs were expressed in bone and cartilage but in skin, only IGF-II mRNA was detected. Affinity cross-linking revealed that in skin, IGFBP-3 contained relatively few unoccupied IGF-I binding sites compared to IGFBP-1 while in bone and cartilage, only IGFBP-1 contained unoccupied binding sites. IGFBP-1, acting by endocrine action, is probably the major factor responsible for inhibition of IGF-I-dependent collagen gene expression during fasting.

Synthesis and secretion of insulin-like growth factor (IGF)-II and IGF binding protein-2 by cultivated brain meningeal cells

The meninges (the pia mater and the arachnoid) covering the surface of brain parenchyma are known to participate in the regulation of blood circulation and the blood-cerebrospinal fluid barrier. In the present study, we isolated and characterized some of the major proteins secreted into the conditioned medium of the meningeal cell cultures prepared from 1-2-day-old rats. Five protein molecules (7 kDa, 16 kDa, 18 kDa, 27 kDa and 32 kDa) were recognized as major proteins in the conditioned medium by SDS-PAGE. The major proteins were isolated and purified to homogeneity, respectively. Their N-terminal sequences, except that of 27 kDa protein, were successfully determined. Homology search has revealed that the N-terminal sequences of the 7 kDa protein and the 32 kDa protein were identical with those of insulin-like growth factor-II (IGF-II) and IGF-binding protein-2 (IGFBP-2), respectively. The N-terminal sequences of the 16 kDa and 18 kDa molecules were identical with those of the corresponding fragments of IGFBP-2. The present study demonstrates that cultured meningeal cells produce and secrete large amounts of IGF-II and IGFBP-2. The meninges may, therefore, be one of the main sources of these proteins in the cerebrospinal fluid and involved in the modulation of neuronal and/or glial cell survival or functioning.

Characterization of insulin-like growth factor binding proteins (IGFBP) and regulation of IGFBP-4 in bone marrow stromal cells

Bone marrow stromal cells synthesize and secrete insulin-like growth factor (IGF)-I and IGF-binding proteins (IGFBP). IGFBPs may modulate the action of IGF-I or IGF-II on haemopoiesis. However, the specific IGFBPs produced by various stromal cell types have not been identified. We examined six different stromal phenotypes for IGFBP protein and IGFBP-1 to -6 mRNA expression. [125I]IGF-I ligand blot analysis of conditioned medium demonstrate different patterns of IGFBP secretion by each cell type. The most prominent IGFBPs were 24 and 29 kD species, consistent with IGFBP4 and IGFBP5, respectively. RNase protection assays demonstrate that, overall, stromal cells express IGFBP-2 to -6 mRNAs, with IGFBP4, IGFBP5 and IGFBP6 mRNAs predominating. Since agents that modulate cAMP levels may influence haemopoiesis via the release of stromal-derived cytokines, we determined the effect of forskolin, a cAMP agonist, on IGFBP4 expression in TC-1 cells. Forskolin (10(-5) M) up-regulated IGFBP4 mRNA and protein secretion in a time-dependent manner. These findings suggest that IGFBP-4, -5 and -6 released by stromal cells may be key modulators of the haemopoietic response to IGFs. Release of IGFBP4 by agents that increase cAMP may be an important mechanism involved in regulating IGF bioavailability in the marrow microenvironment.