Urinary growth hormone (GH), insulin-like growth factor I (IGF-I), and IGF-binding protein-3 measurements in the diagnosis of adult GH deficiency

The diagnosis of GH deficiency (GHD) in the elderly is based at present on the peak GH concentration during a stimulation test. We have now evaluated the performance of urinary GH (uGH), urinary insulin-like growth factor I (uIGF-I), and urinary IGF-binding protein-3 (uIGFBP-3) in the diagnosis of GHD in this group. Twenty GHD elderly patients with a history of pituitary disease and a peak GH response to arginine stimulation of less than 3 ng/mL (15 men and 5 women; age, 61.1-83.4 yr) and 19 controls (12 men and 7 women; age, 60.8-87.5 yr) were studied. GH secretion was assessed by 24-h profile and expressed as the area under the curve (AUCGH). Serum (s) IGF-I and sIGFBP-3 were measured in a single morning, fasted sample. Urinary GH, uIGF-I, and uIGFBP-3 were measured in a 24-h urine sample collected over the same interval as the GH profile, and results were expressed as total amount excreted in 24 h (tuGH24, nanograms; tuIGF-I24, nanograms; tuIGFBP-3(24), micrograms). Data are presented as the mean +/- SD, except for AUCGH, tuGH24, and tuIGFBP-3(24), which are presented as the geometric mean (-1, +1 tolerance factor). AUCGH, sIGF-I, and sIGFBP-3 were significantly lower in GHD subjects than in controls. Total uGH24 was lower in GHD subjects, but tuIGF-I24 and tuIGFBP-3(24) excretion were not different in the two groups. AUCGH provided the best separation between GHD and control subjects, whereas there was substantial overlap for sIGF-I, sIGFBP-3, and tuGH24. In both groups sIGF-I was correlated to sIGFBP-3 (GHD, r = 0.75; controls, r = 0.65; both P < 0.01), whereas tuIGF-I24 was not correlated to tuIGFBP-3(24) in either group. Moreover, tuIGF-I24 and tuIGFBP-3(24) were not related to their respective serum concentrations in either group. Total uGH24 was correlated with AUCGH only in controls (r = 0.54; P < 0.05). These data demonstrate that urinary GH and urinary and serum IGF-I and IGFBP-3 are not suitable diagnostic markers for GHD in elderly subjects.     

Use of insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 in the diagnosis of acromegaly and growth hormone deficiency in adults

The goal of our study was to compare the clinical usefulness of plasma insulin-like growth factor-I (IGF-I) (with and without binding protein extraction) and IGF binding protein-3 (IGFBP-3) measurements in the diagnosis of growth hormone (GH) disorders in adults. IGF-I and IGFBP-3 concentrations were measured in 25 acromegalic and 25 GH-deficient adult (GHDA) subjects (20-76 years) by comparison to a control population (n = 81) after age and sex stratification. In untreated acromegaly, IGF-I and IGFBP-3 were clearly increased (10 times the mean of controls for unextracted IGF-I, 4 times for extracted IGF-I and 2 times for IGFBP-3). Using the mean + 2SD of the control population as the cut-off point, the sensitivity of IGF-I for the diagnosis of acromegaly was higher than that of IGFBP-3 (unextracted IGF-I: 96% and extracted IGF-I: 100% vs IGFBP-3: 76%). In GHDAs, IGF-I and IGFBP-3 were decreased (34% of the mean of controls for unextracted IGF-I, 37% for extracted IGF-I and 70% for IGFBP-3). Using the mean - 2SD of the control population as the cut-off point, the sensitivity of IGF-I measurement for the diagnosis of GHDA was relatively low, but better for unextracted (68%) than for extracted IGF-I (52%). The sensitivity of IGFBP-3 was much lower (36%), thus invalidating this parameter for the diagnosis of GHDA. Our observations demonstrate that IGF-I measurement is a more powerful tool than IGFBP-3 measurement for the diagnosis of GH disorders in adults. Both IGF-I and IGFBP-3 are very useful for the diagnosis of acromegaly, but they are less reliable for diagnosing GHDA, as normal IGF-I or IGFBP-3 values do not rule out GH deficiency.     

How much insulin-like growth factor I (IGF-I) circulates? Impact of standardization on IGF-I assay accuracy

There is a significant systematic difference between the normal range obtained from ethylenediamine tetraacetate plasma samples using the Genentech total insulin-like growth factor I (IGF-I) RIA and normal ranges for other total IGF-I RIAs. To determine whether the quality of the assay standard was the cause of this systematic difference, we analyzed commercially available preparations of recombinant human IGF-I (rhIGF-I) typical of those used as IGF-I immunoassay standards along with our own well characterized rhIGF-I assay standard. For the commercial standards, high performance liquid chromatography-derived purities were low, and some vendor-assigned protein concentrations were inconsistent with values from quantitative amino acid analysis. The Genentech rhIGF-I assay standard was highly pure and quantitatively correct. However, the poor quality of some commercial rhIGF-I preparations was not the primary reason for the systematic discrepancy between the Genentech total IGF-I RIA normal range and most other normal ranges. Most assays for total IGF-I are calibrated against the WHO International Reference Reagent (IRR) for IGF-I Immunoassays (87/518). The Genentech total IGF-I RIA is not calibrated against WHO IRR 87/518. The protein content assigned to WHO IRR 87/518 was a consensus value from a multicenter collaborative study. Physicochemical analyses showed that WHO IRR 87/518 is Met(-1)-IGF-I of low purity (44%), and that the assigned protein content is higher than the value determined by quantitative amino acid analysis. Thus, assays that are calibrated against WHO IRR 87/518 will report total IGF-I concentrations in excess of actual values. We believe that calibration against WHO IRR 87/518 is the cause of the systematic discrepancy between the Genentech IGF-I assay normal range and most other normal ranges, and that much of the plasma IGF-I concentration data in the literature are of questionable accuracy.     

Filter paper blood spot assay of human insulin-like growth factor I (IGF-I) and IGF-binding protein-3 and preliminary application in the evaluation of growth hormone status

To facilitate broader applications of insulin-like growth factor I (IGF-I) and IGF-binding protein-3 (IGFBP-3) analysis, we developed procedures for their measurements in extracts of whole blood dried on filter paper. A single 8-mm diameter filter paper disc containing about 13 microL blood was used. IGFBP-3 was efficiently extracted in a buffer within 1 h of incubation. IGF-I extraction involved incubation in buffer followed by acidification and neutralization steps. Blood spot assays showed intra- and interassay coefficients of variation (including interspot variations) of 5.4-16.7% for IGF-I and 6.6-11.7% for IGFBP-3; recoveries were 97 +/- 7.1% and 101 +/- 8.7%, respectively. Recoveries of IGF-I and IGFBP-3 in response to 4- to 8-fold variations in extraction buffer volume were 97 +/- 8.2% and 107 +/- 6.1%, respectively. Dried blood spot IGF-I and IGFBP-3 showed greater than 1-month stability at -20 C, 4 C, and room temperature and retained more than 65% of the immunoreactivity after approximately 1 month at 37 C. Both IGF-I and IGFBP-3 were contained within the plasma fraction of whole blood, and variations (mean +/- SD) in IGF-I (204 +/- 29 micrograms/L) and IGFBP-3 (4.4 +/- 0.48 mg/L) measured in extracts of dried blood spot with adjusted hematocrit of 0.2-0.62 were acceptable. IGF-I and IGFBP-3 in paired plasma and dried blood spot extracts of random samples (n = 46) showed excellent correlation (r > 0.94) with slopes of near unity. Compared to conventional methods, the filter paper procedures were equally effective in distinguishing IGF-I and IGFBP-3 levels in untreated GH receptor-deficient (n = 11) and age-matched normal controls (n = 16). We conclude that blood collected on filter paper is ideal for IGF-I and IGFBP-3 analysis and may find applications in pediatric and large scale infant screening programs.     

Decrease in insulin and insulin-like growth factor I (IGF-I) binding to erythrocytes from patients with cystic fibrosis

Guidelines for a healthy diet often recommend limiting dietary sugars and fats. Some researchers have called these aims mutually incompatible, suggesting that fat and sugar intakes, when expressed as percent dietary energy, are inversely linked. Others have argued that sugar, more specifically sucrose, acts as a vehicle for dietary fat and serves to suppress the overall quality of the diet. This study examined the relationship between age, sucrose and fat intakes, body mass index (BMI), and measures of dietary diversity and variety in a community-based sample of 837 French adults. Consistent with other studies, high consumption of added sucrose (in g/day or g/1000 kcal per day) was associated with higher consumption of energy and fat and lower consumption of vegetables and fruit. However, eating patterns were strongly influenced by age. High-sucrose consumers were significantly younger and had lower BMI values than did low-sucrose consumers, who were both older and had higher BMIs. High-sucrose diets had minimal effect on the diet diversity score and were associated with more varied diets, as evidenced by a higher dietary variety score.     

Structural changes in insulin-like growth factor (IGF) I mutant proteins affecting binding kinetic rates to IGF binding protein 1 and IGF-I receptor

Ligand binding properties of five single amino acid substituted variants (V11A, D12A, Q15A, Q15E, and F16A) of human insulin-like growth factor I (IGF-I) were analyzed with respect to their binding affinities and binding kinetics to recombinant IGF binding protein 1 (IGFBP-1) and a soluble form of the IGF type I receptor (sIGF-I(R)), respectively. Side chains of the substituted residues are all predicted to be the most surface exposed in the alpha-helical portion of the B-region of the IGF-I molecule. The IGF-I variants were produced as fusion proteins to a IgG(Fc) binding protein domain, Z. Ligand binding kinetic rates were determined using BIAcore biosensor interaction analysis technology. All IGF-I variants showed altered binding affinities to both IGFBP- I and sIGF-I(R). Secondary structure content of the IGF-I variants was estimated using far-UV circular dichroism spectroscopy, followed by variable selection secondary structure calculations. The amount of calculated alpha-helicity is reduced for all the mutants, most predominantly for IGF-I(V11A) and IGF-I(F16A) proteins. Surprisingly, most of the effects of reduced binding affinities to both target proteins are attributed to lowered on-rates of binding, and these are correlated with the amount of alpha-helicity in each IGF-I variant. In addition, in some of the IGF-I variants, lowered off-rates of binding are observed. From the results, we propose that IGF-I is unusually sensitive to structural changes by surface amino acid substitutions in the B-region of the molecule. Therefore, biochemical or biological properties of amino acid substituted variants of IGF-I cannot be used in a straightforward way to dissect the direct involvement in binding of individual amino acid residues since structural changes may be involved.     

Autocrine-paracrine regulation of human trophoblast invasiveness by insulin-like growth factor (IGF)-II and IGF-binding protein (IGFBP)-1

As a consequence of their mechanism of action, DNA (cytosine-5) methyltransferases from both prokaryotes and eukaryotes necessarily recognize mispaired bases in unusual DNA structures as catalytic transition-state analogs. A review of the available data suggests that the enzymes are designed to stall at these sites because they are unable to release substrates or products that are fixed in a conformation resembling the transition state. The enzymes can operate by a two-step process in which they first methylate extrahelical cytosines satisfying their recognition requirements and subsequently stall at the site of methylation. On RNA and DNA RNA hybrids they may operate by a similar one-step process in which they stall at transition-state analogs without methylating cytosine moieties. These natural capacities suggest that the enzymes may physically participate in stable nucleoprotein assemblies formed as components of normal chromatin structure or as intermediates in the repair of unusual structures. The methyltransferases, themselves, may physically participate in chromosome remodelling as part of a mechanism of inactivation or imprinting by stabilizing RNA DNA hybrids or RNA RNA secondary structure involving cis-acting untranslated RNAs like the product of the Xist gene. Methyl-transferase may physically participate in the repair of certain unusual structures by serving as a nucleation point. The affinity for secondary structure in nucleic acids may account for the spreading of DNA methylation patterns. Titration of host methyltransferase by RNA DNA hybrids and RNA secondary structure formed during retroviral replication in certain tumorigenic retroviruses, like MMTV, may account for global hypomethylation observed in retrovirally transformed cells. In a similar fashion, titration of methyltransferase by secondary structures associated with chromosome instability may account for global hypomethylation observed in association with local hypermethylation in tumorigenesis.     

Long-term effects of insulin-like growth factor (IGF)-I on serum IGF-I, IGF-binding protein-3 and acid labile subunit in Laron syndrome patients with normal growth hormone binding protein

A minority of patients with Laron syndrome have normal serum GH binding protein (GHBP), indicating that the defect is elsewhere than in the extracellular domain of the GH receptor. We have evaluated the effect of long-term IGF-I treatment on serum IGF-binding protein (IGFBP)-3 and the acid-labile subunit (ALS) in three sibling with Laron syndrome caused by a GH post-receptor defect and with normal GHBP. The children (a boy aged 3 years, a girl aged 4 years and a boy aged 10 years) were treated by daily s.c. injection of IGF-I in a dose of 150 micrograms/kg. IGFBP-3 was measured by RIA and Western ligand blotting, ALS by RIA. Based values of IGFBP-3 and ALS were low. During IGF-I treatment, the IGFBP-3 concentrations in the girl gradually increased, whereas in the boys there was a 60% decrease during the first week, followed by gradual increase towards baseline. The ALS concentrations followed a similar pattern. We conclude that IGF-I treatment induces and initial suppression and then an increase in the IGFBP-3 and ALS concentrations, confirming data from animal experiments that IGFBP-3 synthesis is not solely under GH control. The differences in responsiveness between the female and male siblings may reflect genetic differences, or lower circulating concentrations of IGF-I in the boys compared with the girl.     

Src stimulates insulin-like growth factor I (IGF-I)-dependent cell proliferation by increasing IGF-I receptor number in human pancreatic carcinoma cells

We examined the potential function of Src in human pancreatic carcinoma. Overexpression of kinase-activated SrcY527F resulted in a significant increase of insulin-like growth factor I (IGF-I)-dependent cell proliferation in the cell line PANC-1. Western blotting and competition binding studies demonstrated 2.3 +/- 0.2-fold increase in IGF-I receptor expression and 2.8 +/- 0.4-fold increase in IGF-I-specific binding sites/cell. SrcY527F transfection alone did not change receptor affinity or basal receptor tyrosine phosphorylation, whereas IGF-I-stimulated receptor phosphorylation was increased by 2.1 +/- 0.5-fold. IGF-I mRNA expression and protein secretion did not change to exclude autocrine activation. We conclude that Src stimulates IGF-I-dependent proliferation of PANC-1 cells by increasing the number of IGF-I receptors/cell.     

Developmental changes in serum levels of free and total insulin-like growth factor I (IGF-I), IGF-binding protein-1 and -3, and the acid-labile subunit in rats

We have recently described a competitive binding assay for rat insulin-like growth factor-binding protein-3 (IGFBP-3) based on the ability of IGFBP-3 to form a ternary complex with the acid-labile subunit (ALS) in the presence of IGF-I. Using this assay we studied groups of male (n = 6) and female rats (n = 6) at 20, 30, 40, 50, 60, 80, and 130 days of age. Nonfasting serum levels of IGFBP-3 were compared with those of total (extractable) IGF-I (tIGF-I) and ALS as well as IGFBP-3 determined by ligand blotting. Additionally, we studied the relationship between ultrafiltered free IGF-I (fIGF-I) and immunoassayable IGFBP-1. IGFBP-3 was dependent on age only (P < 0.0001), but tended to be higher in males than in females (P = 0.06); between 20-130 days levels increased from 6.5 +/- 1.7 to 73.6 +/- 7.2 nmol/liter in males and from 5.4 +/- 1.6 to 51.3 +/- 8.0 nmol/liter in females. IGFBP-3 correlated positively with tIGF-I (r = 0.90; P < 0.0001), ALS (r = 0.92; P < 0.0001), and IGFBP-3, as determined by ligand blotting (r = 0.88; P < 0.0001). The molar ratio of IGFBP-3 to tIGF-I increased from 0.23 +/- 0.04 to 0.76 +/- 0.04 (P < 0.0001) without any sex dependence. An age- and sex-dependent decrease in IGFBP-1 was observed (P < 0.0001), from 10.9 +/- 2.5 to 1.2 +/- 0.2 nmol/liter in females and from 8.9 +/- 0.7 to 0.2 +/- 0.04 nmol/liter in males. Free IGF-I (fIGF-I) increased with age (from 0.7 +/- 0.2 to 7.1 +/- 0.5 nmol/liter; P < 0.0001), and levels were inversely correlated with IGFBP-1 (r = -0.80; P < 0.0001). In young rats, IGFBP-1 circulated in a 10-fold molar excess over the level of fIGF-I, whereas in older rats, fIGF-I exceeded IGFBP-1 by an average of 9-fold in females and by up to almost 60-fold in males. We conclude that in rats 1) IGFBP-3 and fIGF-I are strongly age dependent; 2) IGFBP-3 correlates positively with ALS and tIGF-I; and 3) fIGF-I and IGFBP-1 are inversely correlated. This is in accordance with clinical findings. However, in humans the adult level of fIGF-I rarely exceeds 0.3 nmol/liter, and IGFBP-1 usually circulates in excess of fIGF-I. Thus, our results also imply species differences in the IGF systems of humans and rats.     

Recovery of growth hormone release from suppression by exogenous insulin-like growth factor I (IGF-I): evidence for a suppressive action of free rather than bound IGF-I

To determine the time course of recovery of GH release from insulin-like growth factor I (IGF-I) suppression, 11 healthy adults (18-29 yr) received, in randomized order, 4-h i.v. infusions of recombinant human IGF-I (rhIGF-I; 3 microg/kg-h) or saline (control) from 25.5-29.5 h of a 47.5-h fast. Serum GH was maximally suppressed within 2 h and remained suppressed for 2 h after the rhIGF-I infusion; during this 4-h period, GH concentrations were approximately 25% of control day levels [median (interquartile range), 1.2 (0.4-4.0) vs. 4.8 (2.8-7.9) microg/L; P < 0.05]. A rebound increase in GH concentrations occurred 5-7 h after the end of rhIGF-I infusion [7.6 (4.6 -11.7) vs. 4.3 (2.5-6.0) microg/L; P < 0.05]. Thereafter, serum GH concentrations were similar on both days. Total IGF-I concentrations peaked at the end of the rhIGF-I infusion (432 +/- 43 vs. 263 +/- 44 microg/L; P < 0.0001) and remained elevated 18 h after the rhIGF-I infusion (360 +/- 36 vs. 202 +/- 23 microg/L; P = 0.001). Free IGF-I concentrations were approximately 140% above control day values at the end of the infusion (2.1 +/- 0.4 vs. 0.88 +/- 0.3 microg/L; P = 0.001), but declined to baseline within 2 h after the infusion. The close temporal association between the resolution of GH suppression and the fall of free IGF-I concentrations, and the lack of any association with total IGF-I concentrations suggest that unbound (free), not protein-bound, IGF-I is the major IGF-I component responsible for this suppression. The rebound increase in GH concentrations after the end of rhIGF-I infusion is consistent with cessation of an inhibitory effect of free IGF-I on GH release.     

Regulation of insulin-like growth factor I (IGF-I) gene expression in brain of transgenic mice expressing an IGF-I-luciferase fusion gene

Insulin-like growth factor I (IGF-I) plays an important role in the development and function of the central nervous system (CNS). Little is known, however, about the factors and mechanisms involved in regulation of CNS IGF-I gene expression. To facilitate our goal to define mechanisms of IGF-I gene regulation in the CNS, we generated several lines of transgenic (Tg) mice that express firefly luciferase (LUC) under control of a 11.3-kb fragment from the 5' region of the rat IGF-I gene. Consistent with expression of the native IGF-I gene in murine brain, expression of the transgene predominated in neurons and astrocytes and used promoter 1, the major IGF-I promoter in the CNS and in most tissues. Transgene messenger RNA and protein expression rapidly increased after birth and peaked at postnatal (P) day 4 in all brain regions studied. LUC activities in all regions then gradually decreased to 0.5-4% of their peak values at P31, except for the olfactory bulb, which maintained about one third of its maximal activity. Compared with littermate controls, administration of dexamethasone decreased LUC activity and transgenic IGF-I messenger RNA abundance, whereas GH significantly increased the expression of the transgene. Addition of GH to cultured fetal brain cells from Tg mice for 12 h also increased LUC activity in a dose-dependent manner (77-388%). These results show that this IGF-I promoter transgene is expressed in a fashion similar to the endogenous IGF-I gene, and thus indicates that the transgene contains cis-elements essential for developmental, GH, and glucocorticoid regulation of IGF-I gene expression in the CNS. These Tg mice should serve as an useful model to study mechanisms of IGF-I gene regulation in the brain.     

Developmental pattern of fetal growth hormone, insulin-like growth factor I, growth hormone binding protein and insulin-like growth factor binding protein-3

AIMS: To evaluate the developmental pattern of fetal growth hormone (GH), insulin-like growth factor I (IGF-I), GH binding protein (GHBP) and IGF binding protein-3 (IGF-3); to determine the implications for fetal growth. METHODS: Serum GH, IGF-I, GHBP and IGFBP-3 were measured in 53 fetuses, 41 aged 20-26 weeks (group A) and 12 aged 31-38 weeks (group B). Fetal blood samples were obtained by direct puncture of the umbilical vein in utero. Fetal blood samples were taken to rule out beta thalassaemia, chromosome alterations, mother to fetus transmissible infections, and for maternal rhesus factor. GHBP was determined by gel filtration chromatography of serum incubated overnight with 125I-GH. GH, IGF-I and IGFBP-3 were determined by radioimmunoassay. RESULTS: Fetal serum GH concentrations in group A (median 29 micrograms/l, range 11-92) were significantly higher (P < 0.01) than those of group B (median 16.7 micrograms/l, range 4.5-29). IGF-I in group A (median 20 micrograms/l, range 4.1-53.3) was significantly lower (P < 0.01) than in group B (median 75.2 micrograms/l, range 27.8-122.3). Similarly, IGFBP-3 concentrations in group A (median 950 micrograms/l, range 580-1260) were significantly lower than those of group B (median 1920 micrograms/l, range 1070-1770). There was no significant difference between GHBP values in group A (median 8.6%, range 6.6-12.6) and group B (median 8.3%, range 6-14.3). Gestational age correlated positively with IGF-I concentrations (P < 0.0001) and IGFBP-3 (P < 0.0001) and negatively with GH (P < 0.0001). GHBP values did not correlate with gestational age. Multiple regression analysis showed a negative correlation between GH:IGF-I ratio and fetal growth indices CONCLUSIONS: The simultaneous evaluation of fetal GH, IGF-I, IGFBP-3 and GHBP suggests that the GH-IGF-I axis might already be functional in utero. The progressive improvement in the efficiency of this axis in the last part of gestation does not seem to be due to an increase in GH receptors.     

Free and total insulin-like growth factor I (IGF-I), IGF-binding protein-1 (IGFBP-1), and IGFBP-3 and their relationships to the presence of diabetic retinopathy and glomerular hyperfiltration in insulin-dependent diabetes mellitus

The existing literature on serum insulin-like growth factor I (IGF-I) levels in insulin-dependent diabetes mellitus (IDDM) is conflicting. Free IGF-I may have greater physiological and clinical relevance than total IGF-I. Recently, a validated method has been developed to measure free IGF-I levels in the circulation. Serum free and total IGF-I, IGF-binding protein-1 (IGFBP-1), and IGFBP-3 levels were measured in 56 insulin-treated IDDM patients and 52 healthy sex- and age-matched controls. Diabetic retinopathy was established by direct fundoscopy. In 54 IDDM patients, the glomerular filtration rate (GFR) and effective renal plasma flow were calculated from the clearance rate of [125I]iothalamate and [131I]iodohippurate sodium. Fasting free IGF-I, total IGF-I, and IGFBP-3 levels were significantly lower in IDDM patients than in age- and sex-matched healthy controls (free IGF-I, P < 0.005; total IGF-I, P < 0.001; IGFBP-3, P = 0.001), whereas IGFBP-1 levels were higher (P < 0.001). In IDDM subjects, decreases in free IGF-I, total IGF-I, and IGFBP-3 levels with age were observed (free IGF-I, r = -0.27 and P = 0.05; total IGF-I, r = -0.52 and P < 0.001; IGFBP-3, r = -0.37 and P = 0.005). Free IGF-I was inversely related to fasting glucose in IDDM subjects (r = -0.35; P = 0.01), whereas the relationship between total IGF-I and fasting glucose did not reach significance (r = -0.27; P = 0.06). Age-adjusted free IGF-I levels were significantly higher (P < 0.05) in IDDM subjects with retinopathy than in subjects without retinopathy after adjustment for age. Total IGF-I and IGFBP-3 levels were positively related to GFR (total IGF-I, r = 0.35 and P < 0.05; IGFBP-3, r = 0.28 and P < 0.05). Both of these differences lost significance after adjustment for age. Free IGF-I, total IGF-I, and IGFBP-3 levels were lower and IGFBP-1 levels were higher in insulin-treated IDDM subjects compared to those in age- and sex-matched controls. Free IGF-I, total IGF-I, and IGFBP-3 levels decreased significantly with age in IDDM subjects. Age-adjusted free IGF-I levels in subjects with diabetic retinopathy were higher than those in subjects without diabetic retinopathy. Total IGF-I and IGFBP-3 levels were positively related to GFR in IDDM subjects, but these relations were lost after adjustment for age. Measurement of serum free IGF-I levels in IDDM subjects did not have clear advantages compared to that of total IGF-I, IGFBP-1, and IGFBP-3 levels. Serum IGF-I and IGFBPs reflect their tissue concentrations to a various degree. Consequently, extrapolations concerning the pathogenetic role of the IGF/IGFBP system in the development of diabetic complications at the tissue level remain speculative.     

Exposure of vascular allografts to insulin-like growth factor-I (IGF-I) increases vascular expression of IGF-I ligand and receptor protein and accelerates arteriosclerosis in rats

BACKGROUND: Accelerated arteriosclerosis limits the survival of transplanted hearts. We hypothesized that insulin-like growth factor-I (IGF-I) is crucial in accelerating transplant arteriosclerosis. Recently, we reported that exposure to IGF-I prior to transplantation accelerates transplant arteriosclerosis in the rat aorta allograft model. Here, we studied the mechanism whereby IGF-I exposure accelerates transplant arteriosclerosis. METHODS: The abdominal aorta was harvested from male Brown Norway rats and exposed to 0, 200, or 500 ng/ml of IGF-I at 37 degrees C for 30 min prior to transplantation to the abdominal position of male Lewis rats. The allografts were harvested 14 days later and processed for immunohistochemical staining for alpha-actin, growth factors (IGF-I, IGF-I receptor, platelet-derived growth factor-BB, and basic fibroblast growth factor), and immunological markers (major histocompatibility complex class II antigen, macrophage, and CD4- and CD8-positive T cells). RESULTS: By 14 days, the ex vivo IGF-I donor aorta treatment with IGF-I increased in a concentration-dependent manner the expression of IGF-I and IGF-I receptor in both the intima and the adventitia. In contrast, the expression of platelet-derived growth factor-BB was decreased in a concentration-dependent manner in the intima while basic fibroblast growth factor remained unchanged. The cell-mediated immune response was not affected by IGF-I at 14 days after transplantation, which suggests that the immune events associated with acceleration of transplant arteriosclerosis may occur at an earlier time. CONCLUSION: Acceleration of transplant arteriosclerosis by exposure to IGF-I is associated with increased IGF-I ligand and receptor expression in the allograft vascular wall. These data further suggest that IGF-I may be a major factor in mediating graft arteriosclerosis.     

Alcohol withdrawal-induced change in lipoprotein(a): association with the growth hormone/insulin-like growth factor-I (IGF-I)/IGF-binding protein-1 (IGFBP-1) axis

Lipoprotein(a) [Lp(a)] is an important risk factor for cardiovascular disease. Alcohol is one of the few nongenetic factors that lower Lp(a) levels, but the metabolic mechanisms of this action are unknown. Alcohol inhibits the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis. Alcohol might also affect IGF-binding protein-1 (IGFBP-1), which is an acute inhibitor of IGF-I. We studied how alcohol withdrawal affects Lp(a) levels and the GH/IGF-I/IGFBP-1 axis. Male alcohol abusers (n=27; 20 to 64 years old) were monitored immediately after alcohol withdrawal for 4 days. Twenty-six healthy men, mainly moderate drinkers, served as control subjects. Fasting blood samples were drawn to determine Lp(a), IGF-I, and IGFBP-1 (by ELISA, RIA, and immunoenzymometric assay, respectively). Nocturnal (12 hours) urine collection was performed in 9 alcoholics and 11 control subjects for GH analyses (RIA). The groups were similar in age and body mass index. Lp(a), GH, and IGF-I tended to be lower and IGFBP-1 higher in the alcoholics immediately after alcohol withdrawal than in the control subjects. During the 4-day observation in alcoholics, Lp(a) levels increased by 64% and IGF-I levels by 41%, whereas IGFBP-1 levels decreased by 59% (P<.001 after ANOVA for all comparisons). Urinary GH levels tended to decline. The increase in Lp(a) correlated inversely with the changes in IGFBP-1 (r= -.63, P<.001, n=27) and GH (r=-.70, P<.05, n=9), but not with IGF-I. In multiple regression analysis, the main predictors for the increase in Lp(a) were IGFBP-1 and urinary GH. In conclusion, alcohol withdrawal induces interrelated and potentially atherogenic changes in Lp(a) and IGFBP-1 levels.     

Structure of the IGF-binding domain of the insulin-like growth factor-binding protein-5 (IGFBP-5): implications for IGF and IGF-I receptor interactions

Binding proteins for insulin-like growth factors (IGFs) IGF-I and IGF-II, known as IGFBPs, control the distribution, function and activity of IGFs in various cell tissues and body fluids. Insulin-like growth factor-binding protein-5 (IGFBP-5) is known to modulate the stimulatory effects of IGFs and is the major IGF-binding protein in bone tissue. We have expressed two N-terminal fragments of IGFBP-5 in Escherichia coli; the first encodes the N-terminal domain of the protein (residues 1-104) and the second, mini-IGFBP-5, comprises residues Ala40 to Ile92. We show that the entire IGFBP-5 protein contains only one high-affinity binding site for IGFs, located in mini-IGFBP-5. The solution structure of mini-IGFBP-5, determined by nuclear magnetic resonance spectroscopy, discloses a rigid, globular structure that consists of a centrally located three-stranded anti-parallel beta-sheet. Its scaffold is stabilized further by two inside packed disulfide bridges. The binding to IGFs, which is in the nanomolar range, involves conserved Leu and Val residues localized in a hydrophobic patch on the surface of the IGFBP-5 protein. Remarkably, the IGF-I receptor binding assays of IGFBP-5 showed that IGFBP-5 inhibits the binding of IGFs to the IGF-I receptor, resulting in reduction of receptor stimulation and autophosphorylation. Compared with the full-length IGFBP-5, the smaller N-terminal fragments were less efficient inhibitors of the IGF-I receptor binding of IGFs.     

The effects of thyroxine on serum and tissue concentrations of insulin-like growth factors (IGF-I and -II) and IGF-binding proteins in the fetal pig

We have extensively studied the effect of hypophysectomy on the growth and development of tissues in the fetal pig. However, little is known about the effect of hypophysectomy on tissue levels of insulin-like growth factors I and II (IGF-I and -II) and how these growth factors are affected by T4 replacement. Fetal pigs were hypophysectomized (Hypox) at 70 days of gestation, and pellets containing 15 mg T4 were implanted into the lateral musculature of the hind limb at either 70 or 90 days of gestation. Fetuses were removed at either 90 or 105 days of gestation, respectively. Control (non-Hypox), Hypox, and T4 (Hypox-T4) fetal weights were similar at 90 days, but Hypox-T4 weighted less than control and Hypox fetuses at 105 days. Hypophysectomy decreased levels of serum T4, LH, cortisol, and IGF-I (105 days) when compared with controls. Heart and liver (105 days and 90 days) and fat, muscle, and kidney (90 days) IGF-I levels were lower in Hypox fetuses when compared with controls. Hypophysectomy decreased concentrations of IGF-II in only 105-day fetal kidneys. Hypophysectomy decreased serum levels of IGF binding protein 1 (IGFBP-1) (90 days) and IGFBP-2 (105 days) and increased IGFBP-4 (105 days) in comparison with control. T4 treatment of Hypox fetuses increased serum concentrations of T4 and IGF-I over Hypox levels at both 90 and 105 days gestation. Cortisol levels remained decreased in the T4-treated fetuses. Levels of IGF-I in the heart (90 and 105 days) and liver (90 days) of Hypox fetuses were increased by T4 treatment. T4 did not effect tissue IGF-II levels when compared with Hypox. T4 increased serum IGFBP-1, -2, and -4 levels over Hypox values. We suggest that T4 enhances production of IGF-I (as opposed to IGF-II), which in turn mediates some of T4's capability to enhance tissue development in the fetal pig.     

The insulin-like growth factor (IGF)binding protein 1 binding epitope on IGF-I probed by heteronuclear NMR spectroscopy and mutational analysis

NMR spectroscopy studies and biosensor interaction analysis of native and site-directed mutants of insulin-like growth factor I (IGF-I) was applied to identify the involvement of individual residues in IGF-I binding to IGF-binding protein 1 (IGFBP-1). Backbone NMR chemical shifts were found to be affected by IGFBP-1 binding in the following residues: Pro2, Glu3, Cys6, Gly7, Gly19, Pro28-Gly30, Gly32, Arg36, Arg37, Gln40-Gly42, Pro63, Lys65, Pro66, and Lys68-Ala70. Three IGF-I arginine side chains were identified by NMR to participate in IGFBP-1 binding. All IGF-I arginine residues were replaced by alanines, using site-directed mutagenesis, in four single substituted variants, IGF-I(R21A), IGF-I(R50A), IGF-I(R55A), and IGF-I(R56A), and one double replacement mutant, IGF-I(R36A/R37A). Biosensor interaction analysis binding studies demonstrate the involvement of Arg36-Arg37 and Arg50 in IGFBP-1 binding, while experiments with the IGF-I receptor implicate Arg21, Arg36-Arg37, and Arg56 as part of the receptor binding epitope. These overlapping binding surfaces explain why IGF-I receptor and IGFBP-1 binding to IGF-I is competitive. The C terminus of free, but not IGFBP-1-bound, IGF-I is found to exist in two distinct, NMR-detectable conformations at 30 degreesC. One possible explanation for this structural heterogeneity could be cis-trans isomerization of the Cys6-Cys48 disulfide bond.