Circulating growth hormone binding proteins

Growth hormone proteoformics atlas created to promote predictive, preventive, and personalized approach in overall management of pituitary neuroendocrine tumors

Human growth hormone (GH) is the indispensable hormone for the maintenance of normal physiological functions of the human body, including the growth, development, metabolism, and even immunoregulation. The GH is synthesized, secreted, and stored by somatotroph cells in adenohypophysis. Abnormal GH is associated with various GH-related diseases, such as acromegaly, dwarfism, diabetes, and cancer. Currently, some studies found there are dozens or even hundreds of GH proteoforms in tissue and serum as well as a series of GH-binding protein (GHBP) proteoforms and GH receptor (GHR) proteoforms were also identified. The structure-function relationship of protein hormone proteoforms is significantly important to reveal their overall physiological and pathophysiological mechanisms. We propose the use of proteoformics to study the relationship between every GH proteoform and different physiological/pathophysiological states to clarify the pathogenic mechanism of GH-related disease such as pituitary neuroendocrine tumor and conduct precise molecular classification to promote predictive preventive personalized medicine (PPPM / 3P medicine). This article reviews GH proteoformics in GH-related disease such as pituitary neuroendocrine tumor, which has the potential role to provide novel insight into pathogenic mechanism, discover novel therapeutic targets, identify effective GH proteoform biomarker for patient stratification, predictive diagnosis, and prognostic assessment, improve therapy method, and further accelerate the development of 3P medicine.

Nonalcoholic fatty liver disease is associated with increased GHBP and reduced GH/IGF-I levels

INTRODUCTION

Nonalcoholic fatty liver disease (NAFLD) has been described in adult GH deficiency syndrome. Furthermore, chronic liver disease can be associated with significant changes in levels of IGF-I, GH-binding protein (GHBP), IGF-binding proteins (IGFBPs) and acid-labile subunit (ALS). However, the effect of liver steatosis on the GHBP production has not been investigated yet.

AIM OF THE STUDY

To explore whether GH secretion and/or levels of IGF-I, IGFBP-3, ALS and GHBP could be altered in obese patients in relation to the presence of liver steatosis.

MATERIALS AND METHODS

A total of 115 obese patients (BMI > 30) were enrolled in the protocol (65 patients with liver steatosis and 50 age- and BMI-matched controls). In all patients, the following parameters were studied: serum levels of glucose, insulin, the HOMA index, IGF-I, GHBP, IGFBP-3, ALS and GH after GHRH and arginine stimulation test.

RESULTS

As expected, patients with NAFLD had blood glucose, insulin, HOMA-R significantly higher than controls, indicating a more severe insulin-resistance state in NAFLD. Furthermore, patients with NAFLD had higher levels of GHBP and IGFBP-3 and lower GH peak and IGF-I levels as compared to controls. No difference was found in ALS levels between the groups. In a multivariate analysis, GHBP was positively associated with hepatic steatosis while IGF-1 was negatively associated with hepatic steatosis.

CONCLUSIONS

This study demonstrates that in patients with NAFLD, the GHBP levels are increased, and that the GH/IGF-I axis is significantly altered probably leading to reduced IGF-I bioavailability at tissue level.

Maturation of the growth axis in marsupials occurs gradually during post-natal life and over an equivalent developmental stage relative to eutherian species

The separation of a nutrition-responsive insulin-like growth factor (IGF) system and a growth hormone (GH) responsive IGF system to control pre- and post-natal growth of developing mammals may originate from the constraints imposed by intra-uterine development. In eutherian species that deliver relatively precocial young, maturation of the GH regulatory system is coincident with the time of birth. We measured the hepatic expression of the four key growth axis genes GH-receptor, IGF-1 and -2, and IGFBBP-3, and plasma protein concentrations of IGF-1 from late fetal life through to adult stages of a marsupial, the tammar wallaby. The data clearly show that maturation of GH-regulated growth in marsupials occurs gradually over the course of post-natal life at an equivalent developmental stage to that of precocial eutherian mammals. This suggests that the timing of GH-regulated growth in marsupials is not related to parturition but instead to the relative developmental stage.

Insulin resistance and changes in plasma concentration of TNFalpha, IGF1, and NEFA in dogs during weight gain and obesity

Obesity-induced insulin resistance (IR) is a common problem in humans as well as domestic dogs. It is well-known that this syndrome is associated with many modifications but it is still unclear if the changes are alterations or adaptations. The purpose of this study was to develop obesity-induced IR in dogs, through a long-term overfeeding period, and to explore hormonal and metabolic disturbances associated with the development of this syndrome. Dogs were overfed for 7 months. Body weight increased by 43 +/- 5%, and insulin sensitivity decreased by 44 +/- 5%. Plasma insulin-like growth factor 1 (IGF1), tumour necrosis factor alpha (TNFalpha), and non-esterified fatty acids (NEFA) concentrations progressively increased during the overfeeding period (IGF1: 111 +/- 13 to 266 +/- 32 ng/ml, p < 0.001; TNFalpha: 5 +/- 5 to 134 +/- 41 pg/ml; NEFA: 0.974 +/- 0.094 to 1.590 +/- 0.127 mmol/l, p < 0.05). These metabolic and hormonal impairments are associated with IR, in obese dogs, and could explain, at least in part, the outbreak of this syndrome.

Soluble leptin receptor and soluble receptor-bound fraction of leptin in the metabolic syndrome

OBJECTIVE

In obesity, plasma leptin is high and soluble leptin receptor (sOb-R) levels are low, resulting in a low fraction of bound leptin. The aim of this study was to investigate the influence of insulin resistance (IR) and the metabolic syndrome (MS) on sOb-R concentration and the bound-free ratio of leptin.

RESEARCH METHODS AND PROCEDURES

sOb-R, leptin levels, and homeostasis model assessment (HOMA) index for IR were determined in 76 middle-aged obese or overweight men.

RESULTS

Concentration of sOb-R and soluble receptor-bound fraction of leptin were lowest in the highest tertile of HOMA-IR. sOb-R and the bound-free ratio of leptin correlated with HOMA-IR, leptin concentration, and waist-to-hip ratio independently of age, BMI, and fat mass. Leptin and waist-to-hip ratio were the sole independent determinants of sOb-R concentration, and BMI, HOMA-IR, and visceral adipose tissue were independent determinants of the bound fractin of leptin. sOb-R concentration and the bound fraction of leptin decreased with increasing numbers of components of the MS, resulting in lower sOb-R concentration and a lower fraction of bound leptin in men with the MS.

DISCUSSION

IR and abdominal obesity are associated with low sOb-R concentration and low bound-free ratio of leptin independent of fat mass. Low sOb-R concentration and low bound-free ratio of leptin segregate with components of the MS. We suggest that low sOb-R levels and a low fraction of specifically bound leptin are markers of leptin resistance, which is independently associated with IR and abdominal obesity and may constitute an additional component of the MS.

Inability of heterologous growth hormone (GH) to regulate GH binding protein in GH-transgenic swine

Transgenic pigs expressing bovine, ovine, or human growth hormone (GH) structural genes fused to mouse metallothionein-I (mMT-bGH), ovine MT (oMT-oGH), or mouse transferrin (mTf-hGH) promoters were used to study the effects of GH on the regulation of serum GH-binding protein (GHBP). In the 14 transgenic pigs studied, circulating concentrations of heterologous GH ranged from 15 to 2,750 ng/mL. Using chromatographic methods, specific binding of GH was detected in serum from normal pigs but was undetectable in serum from all the transgenic pigs used, probably as a result of the high serum concentrations of heterologous GH present in these animals. Thus, to avoid interference of binding by high GH concentrations, serum samples were subjected to immunoblotting using a specific anti-GHBP antibody. A specific 54-kDa band was detected in normal pig serum as well as in sera from mMT-bGH, oMT-oGH, and mTf-hGH pigs. Additionally, sera from transgenic mMT-bGH pigs and their sibling controls were subjected to immunoprecipitation with an anti-GHBP antibody followed by immunoblotting with the same antibody. With this technique, we detected two specific bands of 53 and 45 kDa that could represent different degrees of glycosylation of GHBP. As determined by densitometric analysis the amount of GHBP in transgenic pig sera was similar to that detected in sera of the respective control animals. The amount of circulating GHBP remained unchanged even in oMT-oGH and mTf-hGH pigs that were exposed from birth to circulating concentrations of GH as high as 2,750 ng/mL. Thus, we conclude that heterologous GH do not act as modulators ofthe serum GHBP in pigs.

GH/IGF-I axis in anorexia nervosa

Patients with anorexia nervosa (AN) may develop multiple endocrine abnormalities, including amenorrhea, hyperactivity of the hypothalamus-pituitary-adrenal axis, hypothyroidism and particular changes in the activity of the growth hormone (GH)/insulin-like growth factor I (IGF-I) axis. Exaggerated GH secretion and reduced IGF-I levels are usually found in AN, as well as in conditions of malnutrition and malabsorption, insulin-dependent diabetes mellitus, liver cirrhosis and catabolic states. In AN, GH hypersecretion at least partially reflects malnutrition-induced peripheral GH resistance, which leads to reduced IGF-I synthesis and release; this implies an impairment of the negative IGF-I feedback action on GH secretion. On the other hand, primary alterations in the neural control of GH secretion cannot be ruled out. The neuroendocrine alterations include enhanced somatotroph responsiveness to growth hormone releasing hormone (GHRH) and impaired GH response to most central nervous system-mediated stimuli. Particular resistance to cholinergic manipulation has also been demonstrated, thus suggesting a somewhat specific alteration in the somatostatin (SS)-mediated cholinergic influence on GH secretion. Moreover, paradoxical GH responses to glucose load, thyrotropin releasing hormone (TRH) and luteinizing hormone releasing hormone (LHRH) have also been reported. The effect of reduced leptin levels on GH hypersecretion in AN is still unclear, but ghrelin (the gastric hormone that is a natural ligand of the GH secretagogue receptor and strongly stimulates somatotroph secretion) is thought to play a major role. Regardless of the supposed central and peripheral alterations, it has to be emphasised that the activity of the GH/IGF-I axis in AN is generally restored by nutritional and stable weight gain. It therefore reflects an impaired nutritional state and cannot be considered a primary hallmark of the disease.

Activity of GH/IGF-I axis in trauma and septic patients during artificial nutrition: different behavior patterns?

The aim of this study was to compare several parameters of GH/IGF-I axis activity in septic and trauma patients during Intensive Care Unit (ICU) stay. To this goal, 13 patients with sepsis (SEP) and 16 with trauma (TRA) were studied. Thirty-three adult subjects (AS) were studied as controls. Serum IGF-I and -II, IGFBP-1, -2 and -3, GH and GHBP levels were studied on day 1, 3, 5 and 7 after ICU admission, during comparable artificial nutrition in SEP and TRA and basally in AS. In 5 patients with SEP and 6 with TRA, the GH response to GHRH was evaluated on day 3. On ICU day 1, IGF-I and -II and IGFBP-3 in SEP were lower (p<0.05) than in TRA which, in turn, were lower (p<0.01) than in AS. IGF-I increased (p<0.05) both in SEP and TRA, but, on ICU day 7, those in SEP persisted lower than in TRA, which became similar to those in AS. IGF-II levels increased (p<0.05) in SEP only, persisting lower (p<0.05) than in TRA. On ICU day 1, GH in SEP and TRA were similar and did not vary until day 7, overlapping those in AS. The GH response to GHRH in SEP and TRA was similar and lower (p<0.01) than in AS. These findings indicate that IGF-I activity is impaired more in septic than in trauma patients. Reduced IGF-I activity probably reflects peripheral GH resistance though basal and GHRH-induced GH levels were not increased in these conditions.

Activity of GH/IGF-1 axis in burn patients: comparison with normal subjects and patients with GH deficiency

The aim of this study was to clarify the activity of GH/IGF-1 axis as well as the variations of nutritional parameters following a thermal injury in man. To this goal, in 22 patients with burn [BURN, age (mean+/-SE): 46.5+/-3.4 yr, BMI: 25.0+/-0.8 kg/m2, % burn surface area: 26.0+/-3.0%, ROI score: 0.22+/-0.1] we evaluated IGF-1, IGF binding protein (IGFBP-3), GH, GH binding protein (GHBP), pre-albumin (pre-A), albumin (A) and transferrin (TRA) levels on days 1, 3, 7, 14 and 28 after intensive care unit (ICU) admission. IGF-1, IGFBP-3, GH and GHBP levels were also assayed basally in 29 normal subjects (Ns) (Ns, age: 47.5+/-2.8 yr, BMI: 22.0+/-1.4 kg/m2) and in 34 panhypopituitary patients with severe GH deficiency (GHD, age: 42.7+/-2.5 yr, BMI: 25.6+/-0.8 kg/m2). On ICU day 1, IGF-1 and IGFBP-3 in BURN were higher than those in GHD (p<0.05 for both, respectively) and lower than those in Ns (p<0.05) while GH levels in BURN did not differ from those in Ns and higher than GHD (p<0.01). In BURN, IGF-I and IGFBP-3 levels showed a progressive decline (p<0.05) with nadir on day 14, when they overlapped those in GHD, and then an increase on day 28, though persisting lower than in Ns, while GH levels did not vary during ICU stay. IGF-I levels were associated neither to burn extension nor to ROI score. On ICU day 1 pre-A, A and TRA levels were similar to those in Ns, but underwent a progressive decrease with nadir on day 7 (p<0.001) for pre-A and TRA, and later, on day 14 (p<0.05) for A; pre-A and TRA but not A showed a rebound increase (p<0.01) on day 14, though persistingly lower than in Ns. In conclusion, our present data firstly show the time course variation of IGF-I levels in burn patients as function of nutritional and hormonal variables. It has to be emphasized that in the most critical phase after burn injuries, IGF-1 levels are as low as in hypopituitary patients with severe GHD. The physiological basis which leads to the impairment of this endogenous anabolic drive in this phase is, however, not clear yet.

Identification of a region critical for proteolysis of the human growth hormone receptor

Release of soluble growth hormone binding protein (GHBP) corresponding to the extracellular domain of the GH receptor (GHR) occurs via distinct mechanisms depending on species. In human, proteolysis of full length GHR results in liberation of GHBP into the extracellular medium. A putative protease responsive for GHR cleavage has been identified, however, the residues involved are still unknown. In this study, using the mutational approach to the extracellular domain of the human GHR, we demonstrated that deletion of three residues located close to the transmembrane domain abolishes constitutive GHBP shedding without change in cellular GH binding. Deletion also significantly decreased the phorbol 12-myristate 13-acetate (PMA)-induced release of GHBP and the accumulation of membrane-anchored remnant proteins. Taken together, these results suggest that integrity of the juxtamembrane region of GHR is necessary for its biochemical cleavage and that a common mechanism is involved in constitutive and PMA-induced shedding.

Intersubject responsiveness of high-affinity growth hormone (GH)-binding protein (GHBP) to long-term GH replacement therapy

In adult growth hormone deficiency (GHD) syndrome responsiveness to GH replacement therapy is reported to vary considerably. The underlying mechanisms, however, are not well understood. The aim of this study was to investigate which baseline variables determine the reported variable intersubject responsiveness of high-affinity GH-binding protein (GHBP) to GH replacement therapy. In the setting of a double blind study over 12 months with placebo control over the first 6 months, we analyzed the interrelationship between a number of baseline variables, which vary considerably amongst subjects, and the GHBP response to GH replacement in 31 GHD adults (21 males and 10 females). The following variables were investigated: age, gender, duration of GHD, body composition, serum levels of high-affinity GHBP, insulin-like growth factor-1 (IGF-1), and IGF-binding protein-3 (IGFBP-3). The results showed that in the 6 months treated group of 16 patients (11 males, 5 females), serum IGF-1 increased from 87 ng/ml (range: 26 to 173) to 250 (range: 62 to 467) (p<0.01) and GHBP increased from 1,302 pmol/l (range: 845 to 1,m960) to 1418 (range: 941 to 2,025) (p=0.04). Both parameters showed a significant time effect (within-subjects) (p<0.001). In the 12 months treated group of 15 patients (10 males, 5 females), serum IGF-1 increased from 92 ng/ml (range: 20 to 180) to 272 (range: 45 to 491) (p<0.01), whereas GHBP did not show a significant change: from 1,186 pmol/l (range: 660 to 1,690) to 1,252 (range: 580 to 1,890) (p=0.87). Also no significant time effect (within-subjects) was observed for GHBP (p=0.06). Step-wise multiple regression analyses revealed that during the 6 months placebo period baseline GHBP explained 83% of the variance in post-placebo GHBP, whereas the variance in post-treatment GHBP could be accurately predicted (adjusted R2=0.93) from baseline GHBP and body fat mass, irrespective of the duration of GH treatment. No other baseline variables contributed independently to the GHBP response, with the exception of IGFBP-3, which showed a small, but significant contribution in females, but not in males. These findings indicate that the variable intersubject responsiveness of GHBP to GH replacement therapy is mainly due to differences in baseline body fat mass amongst adult GHD patients, and that in female patients a relatively low baseline IGFBP-3 contributes to a rise in serum GHBP after GH treatment. The clinical relevance of measuring GHBP in adult GHD patients is limited to the first screening step to diagnose GHD, because long-term GH therapy tends to restore serum GHBP to pretreatment levels.

Role of ecto-kinase in phorbol ester-enhanced growth hormone-binding protein release from human IM-9 cells

Previously we reported that a phorbol ester, phorbol 12, 13-dibutyrate (PDBu), increased the release of human growth hormone-binding protein (hGH-BP) in IM-9 cells, and that this phorbol ester-enhanced release was mediated by protein kinase Ca (PKCalpha). In the present study, the mechanisms of the phorbol ester-enhanced hGH-BP release were further investigated. Treatment of IM-9 cells with PDBu did not increase hGH-BPs (55-60 kDa) in the intracellular soluble fraction. When the cells were treated with trypsin to remove human growth hormone receptors (hGHRs) on the cell surface after stimulation, no hGH-BPs were detected in the culture supernatants, nor did treatment with bafilomycin A1 or chloroquine affect the PDBu-enhanced hGH-BP release. These results suggest that hGH-BPs released by PDBu stimulation are derived from cell surface hGHRs and not generated within the cells. Protein kinase inhibitors with broad specificities, K-252a and K-252b, inhibited the PDBu-enhanced release with almost the same dose-dependency, although only a trace amount of K-252b was incorporated into IM-9 cells than K-252a, suggesting that K-252b probably inhibits an ecto-kinase extracellularly. PDBu actually enhanced the phosphorylation of several extracellular proteins, and this enhanced phosphorylation was completely inhibited by K-252b treatment. Moreover, the PKCalpha-specific inhibitor bisindolylmaleimide III which inhibits PDBu enhanced hGH-BP release inhibited the PDBu-enhanced phosphorylation of extracellular proteins. On the other hand, the impermeable PKC inhibitor PKC inhibitor peptide 19-31 did not inhibit PDBu-enhanced release, suggesting that the target PKCalpha for PDBu is not present on the extracellular surface. Taken together, these results suggest that, in addition to intracellular PKCalpha, activation of an undefined ecto-kinase may also be involved in the PDBu-enhanced hGH-BP release.

Dose-response study of GH effects on circulating IGF-I and IGFBP-3 levels in healthy young men and women

The aim of our study was to define the dose-response effect of a short-term treatment with different recombinant human growth hormone (rhGH) doses (1.25, 2.5, 5.0, 10.0, and 20.0 micrograms . kg-1. day-1 for 4 days) on insulin-like growth factor I (IGF-I) and insulin-like growth factor-binding protein (IGFBP)-3 levels in 21 normal young adults of both sexes. The dose of 1.25 microgram/kg rhGH did not modify IGF-I levels. The dose of 2.5 micrograms/kg rhGH significantly increased IGF-I levels in men (P < 0.05) but not in women, whereas the higher doses increased IGF-I levels in both sexes (P < 0.002). IGFBP-3 levels were not modified by 1.25 or 2.5 micrograms/kg rhGH in either sex. On the other hand, 5.0 micrograms/kg increased IGFBP-3 levels in men (P < 0.05) but not in women, whereas the higher doses increased IGFBP-3 levels similarly in both sexes (P < 0.02). In conclusion, our results demonstrate that IGF-I and IGFBP-3 responses to rhGH are dose and sex dependent. However, IGFBP-3 is less sensitive than IGF-I to rhGH stimulation.

Assessment of GH/IGF-I axis in obesity by evaluation of IGF-I levels and the GH response to GHRH+arginine test

The GH response to provocative stimuli in obese is often as low as in panhypopituitaric patients with severe GHD; however, IGF-I levels are normal or slightly reduced. In 53 patients with simple obesity (11 M and 42 F, age: 40.3+/-1.6 yr, BMI: 39.1+/-1.0 Kg/m2), we evaluated the GH response to GHRH (1 microg/kg iv)+arginine (ARG, 0.5 g/kg iv), and total IGF-I levels. The mean (+/-SE) GH peak after GHRH+ARG was markedly lower (74% reduction, p<0.0001) in obese (16.8+/-2.0 microg/l) than in normal subjects (62.7+/-4.3 microg/l). IGF-I levels in obese patients (134.0+/-7.6 microg/l) were lower (33% reduction, p<0.001) than in normal subjects (200.8+/-5.7 microg/l). Taking into account the 3rd centile limit of normal response, the GH response to GHRH+ARG was reduced in 62.3% (33/53) of the obese patients, and 21.2% (7/33) of them had low IGF-I levels. Assuming the 1st centile limit, it was reduced in 33.9% (18/53) obese subjects, and 22% (4/18) of them had low IGF-I levels. Considering 3.0 microg/L as arbitrary cut-off, the GH response was reduced in 5.7% (3/53) of the obese patients, and still one of them had low IGF-I levels. Our findings: a) confirm that the secretory capacity of somatotroph cells is often deeply impaired in obesity; b) demonstrate that IGF-I assay generally rules out severe impairment of GH/IGF-I axis in obese patients with marked reduction of the GH secretion; c) indicate that the percentage of obese patients with concomitant reduction of GH secretion and IGF-I levels is not negligible. Thus, IGF-I assay should be routinely performed in obese patients; those presenting with low IGF-I levels should undergo further evaluation of their hypothalamo-pituitary function and morphology, particularly in the presence of empty sella.

Pharmacokinetics of radioiodinated growth hormones in the turtle Chrysemys dorbigni

Growth hormone binding proteins (GHBP) have been identified in the blood of many species. The aim of the present work is to study the physiological role of the GHBP in the turtle serum which we recently described. Binding studies were carried out using in vivo pharmacokinetic and chromatographic techniques as well as in vitro methods. When (125)I-GH was injected in physiological concentration into Chrysemys dorbigni turtles, the first step of pharmacokinetics was the binding of a significant fraction of the labeled GH by the GHBPs present in serum. The decay curve followed a three compartments model and gave the equation: Ae(-alphat) + Be(-betat) + Ce(-gammat). The fast compartment with t(1/2) of 14.4 min or 25.2 min, for hGH and bGH represents 30.3% and 18.9% of total radioactivity, respectively, at hypothetical time zero (not experi mental). Chromatographic studies reveal that this rapid compartment represents free GH. The second and third compartments represent complex forms between GH and GHBPs present in the turtle serum, and represent 70% and 80% of total radioactivity for hGH and bGH, respectively. In vitro chromatographic studies showed direct evidence of the presence of GHBPs in the turtle serum. The presence of these GHBPs changed the pharmacokinetics of labeled GH in plasma and the subsequent liver uptake of GH. The labeled hGH or bGH binds to turtle serum in similar proportion, but maximal liver uptake of these hormones are completely different (L/B ratio of 9.2 +/- 0.6 (n = 5) for ( 125)I-hGH and 4.8 +/- 0.3 (n = 7) for (125)I-bGH). The reasons for these differences could be that human GH binds to lactogenic and somatotropic receptors and bovine GH binds only to somatotropic receptors.

High-affinity growth hormone (GH)-binding protein (GHBP), body fat mass, and insulin-like growth factor-binding protein-3 predict the GHBP response to GH therapy in adult GH deficiency syndrome

The study objective was to investigate which baseline factors can accurately predict plasma high-affinity growth hormone (GH)-binding protein (GHBP) levels after GH replacement therapy in patients with GH deficiency (GHD). The study group consisted of 36 GHD patients (22 men and 14 women; mean age, 43.1 years; (range, 21 to 60) known to have adult-onset GHD for many years (range, 4 to 22). They were randomly divided into a GH-treated group (n = 19) and a placebo group (n = 17). Body composition (assessed by bioelectrical impendance analysis [BIA]), plasma GHBP (fast protein liquid chromatography [FPLC] size-exclusion gel chromatography), insulin-like growth factor-I (IGF-I), and IGF-binding protein-3 ([IGFBP-3] radioimmunoassays) were measured before and after 6 months. A stepwise multiple linear regression analysis with the plasma GHBP level after 6 months as the dependent variable was used to unravel significant explanatory (or predictor) variables. In contrast to placebo therapy, GH replacement therapy increased the mean plasma levels of IGF-I and IGFBP-3 to the normal range, whereas a small but statistically significant increase in plasma GHBP was observed. The combination of baseline plasma GHBP, body fat mass, and IGFBP-3 predicts posttreatment GHBP levels accurately (adjusted R2 = .97), indicating that baseline variables such as age, gender, fat-free mass, and IGF-I have no contribution. Furthermore, reliability analysis showed that the observed and predicted values for GHBP fit a strict parallel model. These findings indicate that the variations in body fat mass and IGFBP-3 among adult GHD subjects explain the reported variable response of GHBP to GH replacement therapy.

The IGF-I response to very low rhGH doses is preserved in human ageing

OBJECTIVES

The activity of the GH/IGF-I axis varies during life and is clearly reduced in the elderly. In fact, GH, IGF-I and IGFBP-3 levels in older people are clearly reduced and similar to those observed in patients with GH deficiency. The declining activity of the GH/IGF-I axis with advancing age may contribute to changes in body composition, structure, function and metabolism. In fact, treatment with pharmacological doses of rhGH restored plasma IGF-I levels, increased lean body mass and muscle strength while decreased adipose tissue mass in healthy elderly subjects. At present it is unclear whether peripheral GH sensitivity is preserved in aging. To clarify this point, we aimed to verify the effect of both single dose and short term treatment with very low rhGH doses on the IGF-I levels in normal elderly subjects. Normal young adults were studied as controls.

DESIGN

We studied the IGF-I response to rhGH administration after single (20 micrograms/kg s.c.) or repeated administrations (5 micrograms/kg s.c. for 4 days) in two groups of young and elderly subjects.

SUBJECTS

Twenty-seven healthy elderly (ES, 14 F and 13 M, age mean +/- SEM: 69.4 +/- 1.3 years, BMI: 23.9 +/- 0.5 kg/m2) and 21 young adult subjects (YS, 12 F and 9 M, 29.8 +/- 1.2 years, 23.8 +/- 0.5 kg/m2) were studied, divided into two groups.

MEASUREMENTS

Group 1: blood samples for IGF-I and IGFBP-3 assay were drawn basally and 12 h after rhGH administration (20 micrograms/kg). Group 2: blood samples for IGF-I, IGFBP-3, glucose and insulin assays were drawn basally, 12 h after the first and the last rhGH administration (5 micrograms/kg). Free T3 (fT3), free T4 (fT4) and TSH levels were also assayed basally and after the last rhGH administration; oestradiol and testosterone levels were measured basally.

RESULTS

Basal IGF-I levels were lower in ES (whole group) than in YS (whole group) (123.1 +/- 8.9 vs. 230.4 +/- 16.1 micrograms/l, P < 0.001) while IGFBP-3 levels in the two groups were similar (2.7 +/- 0.2 vs. 3.1 +/- 0.2 mg/l). No sex-related differences in IGF-I and IGFBP-3 levels were recorded in either group. Group 1: the single administration of 20 micrograms/kg rhGH induced a significant (P < 0.001) IGF-I rise both in YS (318.0 +/- 25.3 vs. 256.0 +/- 21.6 micrograms/l) and ES (187.2 +/- 16.8 vs. 100.4 +/- 9.5 micrograms/l). IGF-I levels after rhGH in ES persisted lower than those in YS (P < 0.001), but the percentage IGF-I increase after rhGH was higher (P < 0.001) in ES (91.6 +/- 12.9%) than in YS (23.9 +/- 5.0%) subjects. Both in YS and ES IGFBP-3 levels were significantly increased to the same extent by 20 micrograms/kg rhGH (3.0 +/- 0.2 vs. 2.3 +/- 0.2 mg/l; 2.9 +/- 0.2 vs. 2.6 +/- 0.2 mg/l, P < 0.001 vs. baseline). Group 2: basal glucose, insulin, fT3, fT4 and TSH levels in YS and ES were similar; testosterone levels in aged and young men were similar while oestradiol levels in aged women were lower (P < 0.01) than in the young ones. IGF-I levels were significantly increased 12 h after the first administration of 5 micrograms/kg rhGH both in ES (166.6 +/- 15.7 vs. 138.3 +/- 12.1 micrograms/l, P < 0.03) and YS (272.2 +/- 16.1 vs. 230.4 +/- 16.1 micrograms/l, P < 0.001). Twelve hours after the last rhGH administration IGF-I levels were further increased (P < 0.001) both in ES (208.7 +/- 21.1 micrograms/l) and YS (301.7 +/- 17.6 micrograms/l). IGF-I levels in ES persisted lower than those in YS at each time point (P < 0.001); however, the percentage IGF-I increase after rhGH in ES and YS was similar (after the first administration: 22.4 +/- 5.1 vs. 21.7 +/- 5.1%; after the last administration: 52.9 +/- 9.5 vs. 39.5 +/- 9.9%). No significant variation in IGFBP-3, glucose, insulin, fT3, fT4 or TSH levels was recorded in either ES or YS.

CONCLUSIONS

Our data demonstrate that IGF-I levels in aging are reduced but the peripheral sensitivity to rhGH is preserved. In fact, in aged subjects the percentage rhGH-induced IGF-I increase is similar or even highe

Activation of protein kinase C alpha enhances human growth hormone-binding protein release

The effect of phorbol ester on human growth hormone-binding protein (hGH-BP) release was investigated. The hGH-BP release from human IM-9 cells measured by immunoblotting was dose-dependently enhanced by a phorbol ester, phorbol 12, 13-dibutyrate (PDBu), and reached plateau at 100 nM. The increased hGH-BP release was shown after 10 min incubation with PDBu and reached a plateau at 60 min after stimulation. Similarly, a diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol, enhanced hGH-BP release. The enhancement was not inhibited by cycloheximide pretreatment, suggesting that the enhanced hGH-BP release does not require de novo protein synthesis. The PDBu-enhanced hGH-BP release was strongly inhibited by extracellular EDTA, and was dose-dependently inhibited by protein kinase C (PKC)-specific inhibitor, Ro 31-8220. These results suggest that activation of PKC mediates the PDBu-enhanced hGH-BP release. Of the 11 known PKC isoforms in human cells, PKCalpha, delta, mu and iota were detected in IM-9 cells by immunoblotting. Of these isoforms, PKCalpha, delta and mu were present in the membrane fraction, which is a known activation marker of PKC. Furthermore, when several PKC-specific inhibitors (Gö 6976, GF 109203X or bisindolylmaleimide III) with different specificities for each isoform were used, there was a good correlation between inhibition of the enhancement of hGH-BP release and inhibition of the phosphorylation of PKC isoforms, another activation marker of PKC, in PKCalpha but not in PKCdelta and mu. These results suggest that activation of PKCalpha is involved in PDBu-enhanced hGH-BP release.

Specific interactions of growth hormone (GH) with GH-receptors and GH-binding proteins in vivo in genetically GH-deficient Ames dwarf mice

The fate of exogenous radiolabeled growth hormone (125I-hGH) was studied in Ames dwarf mice, which do not express growth hormone (GH) or prolactin (PRL) genes. Labeled GH was injected in low amounts that did not exceed the normal physiological GH concentration in mice. Binding of most of the injected 125I-hGH by the GH-binding proteins (GHBPs) present in plasma represents the first step in the handling of this material in vivo. The decay curve followed a two-compartment model and gave the equation: Conc = 2.807e-0067t + 15301e-0.0647t (coefficient of determination 0.9986+/-0.0019), while in normal mice, GH decay followed a three-compartment model as we have previously reported. The fast compartment with t1/2 of 1-2 min was virtually absent in dwarf mice, and chromatographic studies revealed the disappearance of free GH in these mice. We also present evidence of the labeled GH-forming complexes, presumably with GHBPs under in vivo conditions. The second step of processing labeled GH in vivo is the uptake by the liver, which was slower in dwarf than in normal mice (30-45 vs 15 min). Moreover, a lower GH uptake was found in dwarf than in normal mice (UB ratio of 1.75+/-0.29 [30 min] vs L/B ratio of 3.68+/-0.33 [15 min], respectively) due to lower concentration of free GH in plasma and to the reduced number of GH-receptors (GHRs). The radioactive material present in the liver was compatible with 125I-hGH-GHR complexes with Stokes radius of 59A. In summary, we provide evidence that plasma of dwarf mice contains proteins capable of binding GH in vivo and probably representing GHBPs not complexed with GH. The presence of these proteins modified the pharmacokinetics of 125I-hGH in plasma and its subsequent uptake by the liver. The presence of these binding proteins in the absence of endogenous GH suggests that a fraction of total GHBPs (one class?) is independent of GH concentration.

Growth hormone binding protein activity in obese children

We evaluated growth hormone binding protein (GHBP) activity in a group of obese children (12 boys and 12 girls, age 3.1-14.7 years, BMI 21.1-33.3, 11 prepubertal and 13 early pubertal) and in 26 age-matched normal weight children (14 boys and 12 girls, age 2.1-16.0 years, BMI 14.2-21.4, 18 prepubertal and 8 early pubertal). All children were of normal stature. GHBP activity was significantly higher in the obese (39.1 +/- 1.1%) than in the control children (28.3 +/- 1.0%, p < 0.0001). Mean serum GHBP was not different between boys and girls or between prepubertal and pubertal subjects. A positive correlation was found between BMI and GHBP levels only in the normal weight children (r = 0.425, p < 0.05). Baseline insulin concentrations in the obese children were 97.6 +/- 7.9 pmol/l (normal values, 45.0 +/- 18.6 pmol/l), and the mean insulin AUC following OGTT in the obese was 811.3 +/- 160.7 pmol/l (normal values, 373.1 +/- 150.1 pmol/l). Serum GHBP activity in the obese was not correlated with baseline serum insulin concentrations or with the insulin AUC following OGTT. In conclusion, we found that obese children have elevated GHBP activity, and speculate that this phenomenon may serve to compensate for their reduced GH secretion and accelerated GH clearance.

Growth hormone-binding protein enhances growth hormone activity in vivo

The decay curve of labeled growth hormone (GH) in the plasma followed a three-compartment model and could be described by the equation: concentration = Ae-alpha t + Be-beta t + Ce-gamma t, where A, B, and C are y-intercepts and alpha, beta, and gamma are compartments. When 125I-labeled ovine prolactin (oPRL) was injected, the decay curve could be described by the equation: concentration = Ae-alpha t + Ce-gamma t. Formation of 125I-labeled bovine-GH-binding protein (GHBP) complexes with somatogenic characteristics was demonstrated in the serum of both normal and GH transgenic mice. In contrast, 125I-oPRL was unable to form complexes of this type in any of the mice studied. Receptor-mediated liver uptake was found to be faster for PRL than for GH (5-6 min vs. 15-20 min). Liver uptake of radioactivity was significantly lower for PRL than for GH [liver to blood ratio (L/B) of 1.7 +/- 0.3 at 6 min vs. L/B of 3.7 +/- 0.6 at 20 min, respectively]. The presence of binding proteins for GH substantially reduces the clearance of this hormone and consequently increases the liver uptake of GH (mediated by GH receptors). This suggests that GHBPs act to increase the biological activity of GH in vivo.

Identification and initial characterization of serum growth hormone binding protein in the turtle Chrysemys dorbigni

Proteins that bind growth hormone (GHBP) have been identified in the blood of many mammalian and avian species, but not in reptilian species. We carried out binding studies with the serum of turtles using chromatographic techniques as well as the dextran-charcoal separation method. As in other species, we found at least two different GHBPs: one with high MW and low affinity and the other with lower MW and higher affinity. The high affinity GHBP was partially purified using gel filtration and affinity chromatography, reaching a degree of purification of 11,000 times (0.17 nmol/g of serum protein in the serum vs 1900 nmol/g protein in the purified material). When the high affinity GHBP was characterized, it was found to have a dissociation constant (Kd: 2.6 +/- 0.7 nM) similar to those described for mouse or rat, but lower than those for chicken, rabbit or man. The binding capacity (Bmax) was 120 +/- 43 fmoles/mg of protein, which can be also expressed as 1.08 +/- 0.38 pmol/ml of serum. A preliminary MW estimation of 50-60 kDa was obtained for turtle higher affinity GHBP. The specificity of this high affinity GHBP is somatogenic, since bovine GH competes as well as human GH for 125I-hGH bound to binding protein, while ovine PRL competes only partially and with low affinity. Unrelated hormones, as insulin and glucagon, can not displace the 125I-hGH bound to turtle GHBP. A very important seasonal variation in turtle GHBP activity was observed: maximum binding was found in November (springtime), followed by a continuous decline over March and May.

Visceral adipose tissue is associated with circulating high affinity growth hormone-binding protein

Recent data show that body fat distribution, specifically visceral fat accumulation, is associated with the regulation of GH secretion. To our knowledge no studies have been performed with regard to the relationship between plasma high affinity GH-binding protein (GHBP) levels and fat distribution in humans. To address this question, we measured plasma GHBP and insulin-like growth factor I levels as well as visceral, sc abdominal, and hip adipose tissue (AT) areas by using magnetic resonance imaging scanning in 12 patients with GH deficiency (GHD) and in 12 age- and sex-matched healthy subjects. The GHD patients were subsequently treated with GH replacement therapy. Regardless of the GH status of the subjects, body mass index and visceral AT area were positively correlated to plasma GHBP (r = 0.70; P < 0.01 and r = 0.73; P < 0.01, respectively), whereas the sc AT areas at the abdominal level tended to correlate positively with GHBP levels, but did not reach significance (r = 0.44; P = 0.07). The sc AT areas at the hip level were not correlated with plasma GHBP levels. In the GHD patients the pretreatment visceral and abdominal sc AT areas were positively correlated with the change in GHBP levels after GH replacement (r = 0.82; P < 0.01 and r = 0.75; P < 0.01, respectively). The pretreatment sc AT area at the hip level was not associated with the therapy-induced changes in plasma GHBP (r = 0.28; P > 0.10). In summary, this study shows that visceral fat is associated with circulating GHBP levels, suggesting that visceral fat mass may be involved in the regulation of the plasma GHBP level. Further, the amount of abdominal fat in GHD patients may partially determine the plasma GHBP response to GH replacement therapy.

Growth hormone binding protein and free growth hormone in chronic renal failure

Chronic renal failure in children is associated with growth failure. While the pathogenesis of uremic growth failure is multifactorial, an abnormal growth hormone/ insulin-like growth factor (GH-IGF) axis is an important contributory element. Patients with uremia exhibit insensitivity to the action of GH, as exemplified by high plasma GH levels, low IGF-I activity, and poor somatic growth. This insensitivity can be overcome by supraphysiological doses of exogenous GH. Plasma GH binding protein (GHBP, the circulating ectodomain of the GH receptor) levels are decreased in patients with renal failure, as are hepatic GH receptor levels in animal models. Since GHBP levels are thought to reflect GH receptor levels in tissues, it is likely that the uremic GH insensitivity in humans is mediated by a decreased number of GH receptors. Another implication of the low plasma GHBP is a disproportionate elevation of free plasma GH (the biologically active moiety) relative to total GH, lending additional support to the concept of GH insensitivity in uremia. GH kinetics are altered in renal failure because of: (1) inability to excrete GH and (2) changes in the bound fraction of GH in the circulation.