Insulin-like growth factor binding proteins: a review of methodological aspects of their purification, analysis and regulation

The reason why there are at least 6 distinct high affinity IGFBPs is obviously a great challenge for investigators to unravel. In order to undertake this challenge a wide range of techniques have now been developed for investigating IGFBPs. As the IGFBPs have been characterized it has become clear that they are complex multi-faceted molecules; the more tools that can be applied to examine them, the better are the chances for getting a complete picture. The return for the investment of all this technology is the hope that elucidation of the sophisticated system of IGFBPs will provide a much better understanding of how the pluripotential actions of the ubiquitous IGFs can be most appropriately regulated in a manner specific to each tissue and according to developmental and environmental conditions.

The development and characterization of an eluted stain assay (ESTA) for the insulin-like growth factors

An Eluted Stain Assay System (ESTA) has been adapted for the bioassay of the insulin-like growth factors, IGF-I and IGF-II. This ESTA is based on the Fischer Rat Thyroid cell line FRTL-5 which was grown as uniform, adherent microcultures on 96-well microtitre plates. The cells were stimulated with the growth factors for 48 h in hormone and serum free conditions. Responses were determined by the addition of the tetrazolium salt MTT which was reduced to a purple formazan product in a dose related manner. This was directly eluted from the cells and measured with a microtitre plate reader. The signal generated was solely dependent on metabolic activation of the cells, since no increase in cell numbers was detected during the bioassay. The advantages of using this method are its sensitivity, precision, specificity, rapidity and high sample throughout. This bioassay, which is based on a colorimetric method, is technically convenient compared with other systems including the earlier cytochemical bioassays and the radioisotopic methods. We have demonstrated that this MTT ESTA provides a useful method for the study of complex interactions between IGF-I and IGF-II and their related binding proteins and that IGF bioactivity in serum may also be investigated using this ESTA bioassay.

Relationship between follicular fluid levels of insulin-like growth factor binding protein-1 and sex steroids from normal human ovarian follicles

The aim of this study was to examine the possible correlations between follicular fluid insulin-like growth factor binding protein 1 (IGFBP-1) and follicular fluid and serum parameters of granulosa cell function. Twenty-six subjects undergoing diagnostic laparoscopy for infertility had follicular fluid samples collected. Subjects were aged 20-42 years, were having regular ovulatory cycles and either had tubal disease or a partner with male factor infertility. Sex steroids, insulin-like growth factor 1 (IGF-1) and IGFBP-1 were measured by radioimmunoassay. Follicular fluid levels of IGFBP-1 were significantly correlated with levels of estradiol, progesterone in follicular fluid and with serum IGFBP-1 levels, and negatively correlated with follicular fluid androstenedione levels. These results suggests that IGFBP-1 may have a role in the regulation of human ovarian follicles.

Proteolytic modification of insulin-like growth factor-binding proteins: comparison of conditioned media from human cell lines, circulating proteases and characterized enzymes

Proteolytic modification of circulating insulin-like growth factor binding protein-3 (IGFBP-3) has been described in a number of conditions. Using Western ligand blotting and SDS-PAGE analysis of fragmentation patterns of 125I-labelled IGFBP-3 and 125-labelled IGFBP-1, we have examined conditioned media from cultured human cell lines for the presence of proteolytic activity and compared this with the action of circulating proteases and with characterized enzymes including cathepsin D, kallikrein, plasmin and tissue plasminogen activator. 125I-Labelled IGFBP-3 was incubated with serum from pregnant women, patients following heart surgery and patients with cancer of the breast, lung or head/neck. Following separation of the preincubated samples by SDS-PAGE, a distinct pattern of degradation fragments was observed which was similar in all cases. This proteolytic activity was inhibited in the presence of EDTA, phenanthroline and 4(-2-aminoethyl)-benzenesulphonylfluoride, HCl. These proteases had no detectable effect on IGFBP-1. Serum-free conditioned medium from a human dermal fibroblast cell line, a rabdomyosarcoma, a cervical, a bladder, a chorio- and two-tongue squamous cell carcinoma cell lines all contained proteolytic activity which fragmented IGFBP-3. The pattern of fragments was similar in all cell lines but different from that produced by the circulating proteases. Six out of nine cell lines produced protease(s) which degraded IGFBP-1 in addition to IGFBP-3. Whilst all the characterized enzymes tested also fragmented IGFBP-3 and plasmin cleaved IGFBP-1, none of these acted in the same way as either circulating or cell line-derived proteolytic activity. The activity associated with the characterized enzymes and cell lines was inhibited in the presence of serum from normal healthy subjects. These results demonstrate that the serum of pregnant women, post-operative patients and patients with cancer contain circulating proteases which cause fragmentation of IGFBP-3 but have little effect on IGFBP-1. Cell-derived proteases were shown to act on IGFBP-3 and IGFBP-1 in a number of instances but are not active in the presence of circulating inhibitors. These proteases may play an important role in regulating the availability of IGFs to normal and neoplastic tissues.

The regulation of insulin-like growth factor binding protein (IGFBP)-1 during prolonged fasting

OBJECTIVE

Insulin-like growth factor binding protein (IGFBP)-1 levels increase overnight, being inversely related to changes in insulin. With prolonged fasting IGFBP-1 levels increase further. In animal studies high IGFBP-1 levels increase plasma glucose levels possibly by regulating the insulin-like actions of 'bio-available' plasma IGF. Following prolonged fasting, there is an increase in insulin requirement. A proportion of this reversible insulin resistance may be due to inhibitory effects of high IGFBP-1 levels on IGF action. This study examined the regulation of IGFBP-1 in the presence of reversible insulin resistance.

SUBJECTS

Nine normal adult volunteers, seven female and two male (mean age 27.6 +/- SD 2.6 years, range 21.7-46.0 years) of normal body mass index were studied.

METHODS

Subjects fasted from 2200 h day 0 to 0900 h day 3 (59 hours), the fast being completed with a 75-g glucose meal. At least one week later, an 11-hour overnight fast was performed, followed by a repeat glucose meal. Blood samples were taken at regular intervals from 0900 h day 1 and for 5 hours during both glucose meal studies via an indwelling cannula.

MEASUREMENTS

Serum levels of IGFBP-1, insulin, GH, glucose, IGF-I and cortisol were measured at varying intervals during the fast and both glucose meal studies.

RESULTS

Following the initial 11-hour overnight fast IGFBP-1 levels rose from (mean +/- SEM) 32 +/- 5 micrograms/l to reach a maximum of 144 +/- 24 micrograms/l after 32 hours of fasting. IGFBP-1 levels then fluctuated, falling in the morning (93 +/- 8 micrograms/l) and then rising overnight (126 +/- 9 micrograms/l), but not regaining the initial peak levels. The increase of IGFBP-1 from overnight fasting levels was associated with a fall in plasma insulin from 5.7 +/- 0.7 to 2.2 +/- 0.2 mU/l. In comparison, 30 minutes after termination of the fast with the glucose meal, IGFBP-1 levels fell from 120 +/- 11 to 24 +/- 2 micrograms/l within 4 hours. After an overnight fast IGFBP-1 levels fell from 35 +/- 5 to 13 +/- 2 micrograms/l within 3 hours. There was glucose intolerance and increased insulin levels following the glucose meal preceded by the 59-hour fast when compared with the overnight fast. The fall of IGFBP-1 levels after the glucose meal was best expressed, taking into account subject variation, by the following regression equations: Glucose meal preceded by 11-hour fast: log [IGFBP-1] = 1.64-0.255 log [1 h previous insulin] (R2 0.51); Glucose meal preceded by 59-hour fast: log [IGFBP-1] = 1.41-0.265 log [1 h previous insulin] + 0.557 log [current glucose] (R2 0.82).

CONCLUSION

In man, insulin appears to regulate circulating IGFBP-1 levels in all circumstances, this regulation being unaffected by the resistance to insulin action induced by prolonged fasting. The high IGFBP-1 levels were statistically related to the higher glucose levels and may have directly contributed to the increased insulin requirement observed after prolonged fasting.

Glucose does not influence the insulin-like growth factor (JGF) binding to carrier proteins (IGFBPs): analysis of rat and human serum by western ligand blotting

The insulin-like growth factors (IGFs) circulate bound to specific proteins (termed IGFBP-1 through IGFBP-6) that modulate IGF bioactivity in tissues. The aim of this study was to analyse the effects of glucose on IGF binding to IGFBPs in rat and human serum by means of western ligand blotting. Serum samples were incubated with increasing concentrations of glucose (0 to 50 mmol/l), and EDTA (25 mmol/l) was added to inhibit protease activity. To analyse the effect of glucose on protection of IGFBPs from protease activity, serum from pregnant women (reported to be very rich in proteolytic activity against IGFBPs) was added to rat serum previously incubated with glucose. Glucose did not affect the 125I-IGF binding to rat and human serum IGFBPs. The intensity of IGFBP-3 bands decreased considerably during the incubation. This appeared to be due to endogenous protease activity, since the decrease was blocked by addition of EDTA. The incubation of rat serum with pregnant human serum produced a marked attenuation of IGFBP-3 and disappearance of IGFBP-4 bands. In conclusion, our study shows that glucose does not influence the IGF binding to IGFBP-3 either in rat or in human serum, confirms the presence of endogenous proteolytic activity in normal non-pregnant serum, and demonstrates that glucose has no protective action against protease activity.

The effect of recombinant human insulin-like growth factor-I treatment on growth hormone secretion in two subjects with growth hormone insensitivity (Laron syndrome)

OBJECTIVE

Growth hormone (GH) secretion is increased in conditions of GH insensitivity such as Laron syndrome, with elevation of both basal and peak levels. We have studied the effect of recombinant IGF-I therapy on the pattern of GH secretion in two subjects with GH insensitivity.

SUBJECTS

Two pubertal subjects with GH insensitivity (female, 16.4 years, breast stage 3; male 13.6 years, genital stage 2) were investigated after 6 months of IGF-I therapy (120 micrograms/kg twice daily s.c. at 0800 and 1900 h). GH profiles taken before the start of IGF-I therapy, when both subjects were prepubertal (aged 14.0 and 11.5 years respectively), were used for comparison.

METHODS

GH profiles were performed with blood samples taken every 20 minutes between 2000 and 0800 h from an indwelling cannula.

MEASUREMENTS

Serum samples were assayed for GH by immunoradiometric assay and IGF-I, IGFBP-1 and insulin by radioimmunoassay.

RESULTS

Before IGF-I therapy, GH profile studies demonstrated pulsatile GH secretion. Basal GH was elevated with no value falling below the limit of detection of the assay and an increase in peak levels (maximum 203 and 206 mU/l at 0000 h and 0020 h respectively). After 6 months IGF-I therapy, the GH profiles were significantly different. With the onset of puberty a further increase in GH secretion would have been expected; nevertheless, following administration of IGF-I at 1900 h, GH secretion decreased with a reduction in mean overnight GH levels from 65 to 33 mU/l and 53 to 11 mU/l respectively. GH pulsatility was also suppressed in the two subjects, for the first 3.5 and 6 hours overnight respectively. Pulsatile GH secretion then returned with peak levels reaching 130 and 63 mU/l respectively. Prior to therapy IGF-I levels were at the lower limit of assay detection. On IGF-I therapy serum IGF-I levels reached a peak within 3 hours (298 and 438 micrograms/l) coinciding with the suppression of GH secretion. IGF-I levels fell rapidly overnight to 92 and 101 micrograms/l at 0800 h prior to the next injection. The fall in serum IGF-I coincided with the return of GH secretion. IGFBP-1 levels increased overnight both before and during IGF-I therapy, rising from 24 to 83 and 22 to 110 micrograms/l before therapy and 13 to 60 and 13 to 71 micrograms/l during therapy. This rise in IGFBP-1 appeared to be inversely related to the fall in serum insulin levels overnight and appeared not to be affected by IGF-I therapy.

CONCLUSION

GH secretion is suppressed by exogenous IGF-I therapy in GH insensitive subjects. The failure to maintain high serum IGF-I levels overnight, presumably due to a persisting defect in serum IGFBP-3 levels, was associated with an early return of GH secretion. These findings may have implications for the dose and regimen of IGF-I therapy in subjects with growth hormone insensitivity.

Insulin like growth factor-I, insulin like growth factor binding protein-1, and insulin in childhood Crohn's disease

Twenty nine children with Crohn's disease were studied before and after treatment with steroids or an elemental diet to assess the effect of disease activity and treatment on serum insulin like growth factor I (IGF-I), insulin like growth factor binding protein (IGFBP-1), and insulin concentrations. The median serum IGF-I concentration was lower in patients with active disease than in matched controls, and lower in stunted than well grown patients, but insulin and IGFBP-1 concentrations were not significantly different between any group. After four weeks of either treatment there was an increase in the median serum IGF-I concentration; this was greater in the steroid group than the elemental diet group. The median serum insulin concentration increased and median serum IGFBP-1 concentration decreased in the steroid treated group but not in the elemental diet group. These changes were accompanied by a greater and more sustained increase in energy intake in the steroid group. Despite this the median height velocity SD score was greater in the elemental diet group than in the steroid group.

Suppression of endogenous insulin secretion regulates the rapid rise of insulin-like growth factor binding protein (IGFBP)-1 levels following acute hypoglycaemia

OBJECTIVES

Recent animal studies have suggested that insulin-like growth factor binding protein (IGFBP)-1 may regulate the insulin-like actions of the circulating IGFs. In man, IGFBP-1 levels change rapidly with nutritional status and are inversely related to changes in insulin. In-vitro studies have shown that both insulin and glucose independently regulate IGFBP-1 secretion in an inverse manner. A rapid rise of serum IGFBP-1 levels following insulin-induced hypoglycaemia suggested that glucose or glucose availability, rather than insulin, may be the major regulator of IGFBP-1.

DESIGN

Three separate experiments both in patients and in normal volunteers were designed to examine the possibility that in these extreme circumstances glucose rather than insulin was the predominant regulator of IGFBP-1.

METHODS

Insulin tolerance tests (ITT) were performed during the routine assessment of pituitary function in seven patients, four female and three male (mean age +/- SEM 36.8 +/- 6.3 years, range 20.7-69.3 years) with pituitary disease. Hypoglycaemic glucose clamp studies (insulin 2 mU/kg/min for 180 minutes) were performed in five normal volunteers, two female and three male (mean age 33.6 +/- 2.2 years, range 23.5-42.0 years). A three-part infusion study was performed in five volunteers, three female and two male (mean age 22.9 +/- 0.9 years, range 20.8-25.0 years) who received for 45 minutes on three occasions separated by at least 7 days either octreotide (long-acting somatostatin analogue) (1 microgram/min), adrenaline (3 micrograms/min) or control.

MEASUREMENTS

Serum levels of IGFBP-1, insulin, glucose, C-peptide and cortisol were measured at varying intervals during the three studies by radioimmunoassay (RIA).

RESULTS

Symptomatic hypoglycaemia (1.0 +/- 0.1 mmol/l) occurred at 30 minutes in all patients during the ITT. Serum IGFBP-1 levels rose from 28 +/- 7 to 86 +/- 15 micrograms/l at 180 minutes. During the hypoglycaemic glucose clamp study plasma glucose fell from 4.8 +/- 0.3 to 2.2 +/- 0.3 mmol/l. In contrast to the response observed during ITT, IGFBP-1 levels fell from 22 +/- 6 to 10 +/- 1 microgram/l by 180 minutes. During the octreotide infusion study there was no change in plasma glucose and plasma insulin levels fell from 5.8 +/- 1.9 to < 2.0 mU/l. Serum IGFBP-1 levels rose from 21 +/- 2 to 68 +/- 5 micrograms/l by 180 minutes. There was no change in IGFBP-1 during either the adrenaline infusion or the control study. The rise in IGFBP-1 following the octreotide infusion (68 +/- 5 micrograms/l) was similar to that in the patients undergoing the ITT (86 +/- 15 micrograms/l) (P = 0.3).

CONCLUSION

The rapid rise of serum IGFBP-1 levels induced by acute hypoglycaemia could be reproduced in euglycaemic conditions with octreotide when insulin secretion was suppressed, whereas IGFBP-1 levels did not rise with hypoglycaemia induced by a prolonged insulin infusion. These findings suggest that the surprising rise of IGFBP-1 levels observed during ITT is not secondary to changes in glucose. The rapid removal from the portal circulation of endogenous insulin with its inhibitory effect on IGFBP-1 secretion therefore appears to be the likely cause for the rapid rise of IGFBP-1 following an ITT. This conclusion supports the hypothesis that IGFBP-1 may inhibit the insulin-like actions of 'free' IGF when insulin secretion is low and so directly link the availability and hence actions of IGFs to acute but temporary changes in nutritional status.

Two site-specific radioimmunoassays which demonstrate the presence of proteolytically modified insulin-like growth factor-binding protein-3 in the circulation

The presence of proteolytic activity in the circulation directed against insulin-like growth factor binding protein-3 (IGFBP-3) was originally described in pregnancy but has subsequently been described in a number of catabolic and other pathological conditions. However, detection of this proteolytically modified IGFBP-3 has been demonstrated, in most cases, following separation using SDS-PAGE and this has led to speculation that its occurrence may be an artefact of the harsh conditions employed. Using two site-specific antisera, one of which recognizes as its antigenic site a region of IGFBP-3 which is close to that of the IGF-binding domain, we have developed two radioimmunoassays which, when compared, can reveal alterations in the IGF-binding domain of IGFBP-3. The presence of IGFBP-3 modified by the circulating protease is denoted in this system by a divergence in the IGFBP-3 levels observed; this divergence has been shown to coincide with protease activity as determined by Western ligand blotting. The use of these assays has confirmed that the IGF-binding site of IGFBP-3 undergoes proteolytic modification in the circulation during a number of pathologies.

Cytokines modulate the sensitivity of human fibroblasts to stimulation with insulin-like growth factor-I (IGF-I) by altering endogenous IGF-binding protein production

Human dermal fibroblasts produce a number of insulin-like growth factor-binding proteins (IGFBPs) including the main circulating form, IGFBP-3. It has been suggested that the regulation of IGFBP secretion may play a major role in modulating insulin-like growth factor (IGF) bioactivity. We have quantified the effects of two cytokines, transforming growth factor-beta 1 (TGF-beta 1) and tumour necrosis factor-alpha (TNF-alpha) which have opposing actions on fibroblast IGFBP-3 production, and examined their subsequent role in IGF-I mitogenesis. TGF-beta 1 caused a dose-dependent increase in IGFBP-3 in serum-free fibroblast-conditioned media. TGF-beta 1 (1 microgram/l) resulted in immunoreactive IGFBP-3 levels reaching 286.5 +/- 22.4% of control after 20 h, the increase being confirmed by Western ligand blot. TNF-alpha caused a dose-dependent decrease in fibroblast IGFBP-3 secretion, 1 microgram TNF-alpha/l reducing IGFBP-3 levels to 32.1 +/- 11.% of control. This effect was not due to cytotoxicity and was not cell-density-dependent. Fibroblast proliferation was examined using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric cytochemical bioassay. The addition of IGF-I resulted in dose-dependent growth stimulation after 48 h, the effective range being 20-100 micrograms/l. The IGF-I analogue Long-R3-IGF-I which has little affinity for the IGFBPs was approximately 20-fold more potent in this assay, and was unaffected by exogenous IGFBP-3.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of insulin-like growth factor-II in human plasma using a specific monoclonal antibody-based two-site immunoradiometric assay

An immunoradiometric assay (IRMA) for the measurement of insulin-like growth factor-II (IGF-II) in human plasma has been developed, optimized and evaluated clinically in normal subjects and patients with disorders of the GH/IGF-I axis. Six monoclonal antibodies (MAbs) to recombinant human IGF-II (rhIGF-II) were produced, all of which had low cross-reactivity with rhIGF-I (< 0.01%) and insulin (< 0.01%). Compatibility of pairs of MAbs was tested in two-site IRMAs using three radioiodinated MAbs and three MAbs linked to Sephacryl S-300 (with separation of bound and free radiolabelled MAb by sucrose layering). Seven pairs of MAbs bound rhIGF-II and the combination of 125I-labelled W3D9 and W2H1 linked to solid phase was selected. The optimized assay had a completion time of 4 h, a minimum detection limit of 30 ng/ml (2.5 standard deviations from the zero standard) and detected a single peak of endogenous IGF-II in normal plasma which co-eluted with rhIGF-II after acid gel chromatography. IGF-II was measured in formic acid/acetone extracts of plasma from 16 normal subjects (mean 685, range 516-1008 micrograms/l), four acromegalic patients (mean 637, range 553-700 micrograms/l), fourteen patients with type-1 diabetes (mean 635, range 247-753 micrograms/l), nine patients with uraemia (mean 423, range 78-850 micrograms/l), and three patients with Laron-type GH insensitivity (75, 35 and 36 micrograms/l). No significant fluctuations were detected between samples obtained hourly from 08.00 to 19.00 h in normal subjects. Low levels of IGF-binding proteins (IGFBPs) remaining in plasma extracts may interfere with the measurement of IGF-II and give rise to falsely elevated IGF-II levels in radioimmunoassays or falsely suppressed levels in IRMAs. Such interference did not occur with the IRMA when used to measure IGF-II in extracts from normal subjects, acromegalic patients and patients with type-1 diabetes, and the addition of excess rhIGF-I in order to displace IGF-II from residual IGFBPs had no effect on IGF-II measurements in these samples. However, levels of IGF-II measured in extracts from patients with Laron-type GH insensitivity and patients with uraemia increased markedly after preincubation with excess rhIGF-I. The accurate measurement of IGF-II by IRMA in extracts from these subjects therefore requires the displacement of IGF-II from IGFBPs prior to assay. We conclude that, in contrast to radioimmunoassays, the two-site IRMA developed here provides a practical, rapid and specific method for the measurement of IGF-II in human plasma.

The effects of dexamethasone treatment on immunoreactive and bioactive insulin-like growth factors (IGFs) and IGF-binding proteins in normal male volunteers

Glucocorticoids inhibit somatic growth in man and laboratory animals, and have long been regarded as suppressors of both stimulated GH secretion and insulin-like growth factor (IGF) activity. Recent evidence suggests, however, that glucocorticoids can be potent GH secretagogues in their own right with concomitant increases in circulating IGF-I levels. IGFs circulate tightly bound to a group of high-affinity binding proteins (IGFBPs) which modulate their actions. In order to investigate the effects of glucocorticoids on serum levels of IGFs and IGFBPs, normal male volunteers were sampled over 24-h periods before and directly after treatment with dexamethasone (2 mg twice daily) for 96 h. Following dexamethasone administration, all volunteers showed a marked increase in mean +/- S.E.M. IGF-I levels over the 24-h sampling period (292.2 +/- 31.8 before dexamethasone, 425.9 +/- 37 micrograms/l after dexamethasone, P < 0.005); there was no change in mean IGF-II levels. Integrated mean insulin levels were raised by dexamethasone treatment (50 +/- 4.6 before dexamethasone, 117 +/- 13.4 mU/l after dexamethasone, P = 0.002) and IGFBP-1 was significantly suppressed (42.9 +/- 8.2 before dexamethasone, 28.0 +/- 7.9 micrograms/l after dexamethasone, P < 0.001). IGFBP-2 levels were similarly suppressed after dexamethasone (319.5 +/- 24.5 before dexamethasone, 214.8 +/- 8.5 micrograms/l after dexamethasone, P = 0.002), and there was a significant increase in IGFBP-3 levels from 3.24 +/- 0.25 to 3.67 +/- 0.32 mg/l (P = 0.0153). Mean IGF bioactivity over the sampling period after dexamethasone was reduced by approximately 60% (0.93 +/- 0.39 before dexamethasone, 0.39 +/- 0.05 U/ml after dexamethasone, P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of insulin-like growth factor-binding proteins on steroidogenesis by human granulosa cells in culture

The effects of insulin-like growth factor-binding proteins (IGFBPs) 1 and 3 on steroidogenesis by human granulosa cells has been examined. Both IGFBP-1 and IGFBP-3 produced a dose-related inhibition of IGF-I-stimulated oestradiol accumulation in granulosa cell-conditioned medium with complete reversal of the effects of IGF-I in the presence of a molar excess of binding protein. IGFBPs 1 and 3 also exerted a small (25-40%) but significant and consistent inhibition of oestradiol secretion in response to follicle-stimulating hormone (FSH) alone. The progesterone response to IGF-I was inhibited by IGFBPs 1 and 3 but there was no effect on FSH-stimulated progesterone production. These data support the concept of a physiologically important intraovarian IGF system in the human ovary and demonstrate an unequivocally inhibitory effect of IGFBPs 1 and 3 on IGF-I-stimulated granulosa cell steroidogenesis.

Insulin-like growth factor binding protein 1 (IGFBP-1) levels in Turner syndrome

We tested whether IGFBP-1, a modulator of IGF-I action, would play a role in the pathogenesis of growth failure and metabolic picture of Turner syndrome. Fasting serum levels of IGFBP-1 were assessed in nineteen girls with Turner syndrome (aging 6.5 to 17.2 years) by radioimmunoassay. Our patients showed normal values of IGFBP-1 (mean +/- SD: 68.6 +/- 32.5 micrograms/l, range: 16 to 134 micrograms/l; range for age and pubertal stage-matched normal children: 15 to 180 micrograms/l). IGFBP-1 levels inversely correlated with bone age (p < 0.05), weight (p < 0.001), percentage of ideal body weight (p < 0.002) and body mass index (BMI) (p < 0.001). Our results seem to rule out a role of IGFBP-1 in the pathogenesis of growth failure in Turner syndrome. The close inverse relationship between IGFBP-1 levels and BMI suggests the serum concentrations of IGFBP-1 to be regulated by the nutritional status. Due to IGFBP-1 inhibiting action on IGF biological activity, the reduction of IGFBP-1 levels in overweight subjects might represent a mechanism to enhance the IGF insulin-like activity, thus supplementing the insulin action.

The induction of specific proteases for insulin-like growth factor-binding proteins following major heart surgery

Insulin-like growth factors (IGF-I and IGF-II) circulate bound to specific high-affinity binding proteins (IGFBPs). Recent evidence has shown that in pregnancy and severe illness, specific proteases modify these binding proteins, reducing their affinity for IGFs. We have studied 12 patients, undergoing elective coronary artery vein-bypass graft surgery, for the appearance of these proteases and have demonstrated the induction of two independent, heat-labile, cation-dependent proteases. Proteolytic activity directed against IGFBP-3 was detected in all patients between 24 h and 5 days after surgery; the second IGFBP-4 specific protease was active 1 h after sternotomy. The total IGF-I levels were found to decrease following surgery, with the IGF-I distribution in the plasma being radically altered from that seen prior to the operation. One day after the operation the majority of the IGF-I, instead of being bound in the relatively inert 150 kDa complex, was associated with the smaller binding proteins which are more readily accessible to the tissues. These findings are in contrast to pregnancy where, despite similar proteases, the majority of the IGF-I remains in the 150 kDa complex. The alteration seen in IGF-I distribution after surgery did not appear to be a direct result of the IGFBP-3 proteolytic activity or an effect of the addition of heparin to the circulation. The potential increase in bioavailability of IGFs caused by the alteration in carrier protein may play a pivotal role in countering the catabolic state induced by surgery.

Sex hormone binding globulin levels in adolescent subjects with diabetes mellitus

In normal adolescents there is a pubertal fall in circulating levels of sex hormone binding globulin (SHBG) in both sexes which is not explained by classically accepted mechanisms of control of SHBG. Recent in vitro and in vivo evidence has suggested that SHBG is inversely regulated by insulin. In view of this we have compared SHBG levels in 80 adolescent subjects with Type 1 diabetes to those in 61 normal adolescents. In both normals and in Type 1 diabetic subjects there was a pubertal fall in SHBG levels. Contrary to expectations, SHBG levels were not elevated in those with diabetes, but prepubertally were significantly lower in both sexes (boys mean +/- SD, 70 +/- 28 nmol l-1, normals 130 +/- 52 nmol l-1, p less than 0.001; girls, 61 +/- 17 nmol l-1, normals 110 +/- 23 nmol l-1, p = 0.01). In pubertal subjects no differences in SHBG levels were seen between the two groups, or between either sex within any group. In subjects with Type 1 diabetes SHBG levels were unrelated to metabolic control as reflected by HbA1 but were inversely related to pubertal stage (r = 0.55, p less than 0.001). In prepubertal subjects with diabetes, in whom abnormal SHBG levels were found, these levels were weakly related to insulin dose (r = 0.33, p less than 0.05); no such relationship was found in the other groups. The significance of the abnormal SHBG levels in prepubertal children with diabetes and its relationship to any irregularities of their sexual development is unclear.

The insulin-like growth factor (IGF)-binding proteins and IGF bioactivity in Laron-type dwarfism

Laron-type dwarfism (LTD) is caused by a variable defect in the GH receptor gene and is, therefore, an ideal model to study the physiology of the insulin-like growth factors (IGFs) and their binding proteins (IGFBPs) in the complete absence of GH action. In this study we examined the overnight variation of the IGFs, IGFBPs, and IGF bioactivity in two prepubertal subjects with LTD. Subject 1 was a 14-yr-old female, 103 cm tall (-8.3 SD), and subject 2 was a 11.5-yr-old male, 103.6 cm tall (-5.9 SD). Both had serum IGF-I levels below 0.07 U/mL and low constant serum IGF-II levels overnight (185 +/- 10 and 232 +/- 8 micrograms/L), despite high serum GH levels [mean GH, 65 (32.5 micrograms/L) and 53 mU/L (26.5 micrograms/L)]. Serum IGFBP-1 levels increased overnight (from 24 and 22 micrograms/L at 2000 h to 83 and 110 micrograms/L at 0800 h) as serum insulin levels fell [from 19 (136 pmol/L) and 17 mU/L (122 pmol/L) at 2000 h to less than 2 (less than 14 pmol/L) and 5 mU/L (36 pmol/L) at 0800 h] in subjects 1 and 2, respectively. Serum IGFBP-2 levels remained constant overnight, as assessed on Western Ligand blotting and, despite the changes in IGFBP-1, remained the most prominent IGFBP throughout. On size separation, most of the IGF-II (greater than 60%) eluted with IGFBP-2 and the other low mol wt IGFBPs. Serum IGFBP-3 levels were reduced, and IGFBP-3 was not the major IGF carrier in LTD serum, in contrast to normal serum. An IGFBP-3-specific protease that was heat sensitive and cation dependent was identified as the cause of an apparent overnight rise of serum IGFBP-3 levels. No IGFBP-3 variation and no proteolytic activity was seen in normal serum or rapidly separated LTD plasma. Serum IGF bioactivity, measured in a porcine cartilage bioassay, was 0.18 and 0.55 U/mL in subjects 1 and 2; differences in bioactivity between subjects did not relate to serum IGF-II levels, but, rather, to differences in IGFBP-3 levels. Serum IGF bioactivity was not constant overnight and varied in a similar fashion in both subjects 1 and 2, with reduction in bioactivity between 0600-0800 h by 55% and 32%, suggesting the presence of inhibitory factors in the LTD serum; this decrease coincided with the rise in serum IGFBP-1 levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin and growth factors adaptation to normal puberty

Insulin and IGF-1 production were measured throughout childhood and puberty and into adult life. Basal insulin and IGF-1 secretion increased during puberty, falling back to prepubertal levels in adulthood. However, fasting glucose levels remained constant, implying an increase in tissue resistance to insulin coinciding with puberty.

The insulin-like growth factors and their binding proteins in a case of non-islet-cell tumour-associated hypoglycaemia

Non-islet-cell tumours which induce hypoglycaemia are rare. Insulin-like growth factor-II (IGF-II) produced by some tumours is thought to be responsible for the hypoglycaemia and other systemic effects, despite normal or even low serum IGF-II levels. We studied a 44-year-old woman presenting with symptomatic hypoglycaemia associated with a large intraabdominal haemangiopericytoma. The serum IGF-II level was 455 micrograms/l when measured after acid-ethanol extraction (normal range (NR) 450-750 micrograms/l) and 1063 micrograms/l after acid chromatography (normal human serum pool 1068 micrograms/l). Levels of fasting plasma insulin, C-peptide, glucose and serum IGF-I levels were low before the operation (less than 2 mU/l (NR less than 2-14), 0.23 nmol/l (NR 0.4-1.2), 3.1 mmol/l, (NR 3.7-5.9) and 0.02 U/ml respectively). After tumour removal, the symptoms resolved rapidly and the patient made a full recovery. Secretion of both insulin and growth hormone was suppressed before the operation in response to a 75 g glucose meal and to an infusion of 100 micrograms GH-releasing hormone (GHRH) respectively in comparison with studies after the operation. Serum IGF-II levels 6 weeks and 12 weeks after the operation fell to 385 micrograms/l (777 micrograms/l; acid chromatography) and 280 micrograms/l (647 micrograms/l; acid chromatography) and serum IGF-I levels increased to 0.35 U/ml and 0.26 U/ml. Serum before the operation and tumour extract contained chiefly a large molecular weight precursor IGF-II (molecular weight 15,000-20,000) which disappeared from the serum after the operation. The IGF-binding proteins (IGFBP-1, IGFBP-2, IGFBP-3 and IGFBP-4) were examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Levels of GH binding activity, IGFBP-1, insulin, blood glucose and cortisol in intensive care patients

OBJECTIVE

To investigate levels of serum GH binding activity, insulin-like growth factor binding protein-1 (IGFBP-1), blood glucose, serum insulin, and cortisol in patients on the Intensive Therapy Unit.

DESIGN

Case-control study of severely ill patients admitted to the Intensive Therapy Unit.

PATIENTS

Six critically ill patients (51-78 years) who required ventilatory and nutritional support and six healthy age, sex, height and weight matched controls.

MEASUREMENTS

Patients and controls were studied for two 24-hour periods; the patients before and after commencing parenteral nutrition, the controls whilst fasted and on a second occasion when fed a diet equal in protein and calories to that of the patients' parenteral nutrition. Samples were taken hourly for measurement of IGFBP-1, blood glucose, serum insulin and cortisol. Growth hormone binding activity was measured at 0 hours.

RESULTS

Blood glucose levels were higher in the patients than controls in both the fasted (mean +/- SEM 5.1 +/- 0.5 vs 3.8 +/- 0.2 mmol/l, P = 0.04) and fed states (10.1 +/- 1.6 vs 5.0 +/- 0.1 mmol/l, P = 0.02) and patients' insulin levels were also higher when fed (81.5 +/- 31.6 vs 24.2 +/- 4.8 mU/l, P = 0.046) although there were no significant differences between patients and controls when fasted. IGFBP-1 levels were inversely related to insulin levels in both the patients and controls; mean IGFBP-1 concentrations were higher in fasted patients than in controls (123 +/- 38 vs 52 +/- 9, P = 0.046) but when fed, both groups had similar mean levels. Serum GH binding activity was low in the patients and did not change with feeding. Mean 24-hour cortisol levels were higher in the patients than in controls, whether fasted or fed, and showed no nyctohemeral rhythm.

CONCLUSIONS

We have previously reported that critically ill patients have low levels of IGF-I with augmented basal levels of GH. The present results demonstrate that these changes in the GH-IGF-I axis are associated with insulin resistance with respect to blood glucose and high levels of IGFBP-1 when patients are fasted. However, when fed, the inverse relationship of IGFBP-1 to insulin is preserved. Patients have low levels of GH binding activity and increased mean cortisol levels. Interventional studies in this patient group with GH and IGF-I must take account of these changes in binding protein and cortisol levels.

The induction of a specific protease for insulin-like growth factor binding protein-3 in the circulation during severe illness

The insulin-like growth factors (IGF-I and IGF-II) are almost completely bound in the circulation to specific binding proteins (IGFBPs). These IGFBPs appear to play a pivotal role in maintaining circulating levels and modulating the delivery of the IGFs to the tissues. A large proportion of the circulating IGFs are bound with high affinity to one of the binding proteins. IGFBP-3. The mechanism by which these IGFs are transferred from the circulatory pool to the tissue receptors is at present unclear. Recent studies in late pregnancy have demonstrated the presence of specific proteases which may modify the IGFBPs such that their affinities for the IGFs are reduced. In this paper, we have demonstrated the presence of a heat-sensitive cation-dependent proteolytic enzyme specific for IGFBP-3 in the serum of five severely ill patients. The activity of this protease was found to vary in these patients, becoming more apparent during fasting than when studied after commencement of parenteral nutrition, indicating that one of the influencing factors in the activity of this protease is the nutritional intake of the patient. Age- and sex-matched healthy adults were also studied in a similar protocol, but no proteolytic modification of any of the IGFBPs was found in any of the samples examined. As the levels of both IGF-I and IGF-II were found to be low in the patients, the presence of a circulatory protease suggests that this may be an adaptive response to increase the bioavailability of the IGFs and possibly to improve the nitrogen retention and counter the catabolic state in severe illness.