Androgen regulation of growth hormone binding protein

Growth hormone binding protein - physiological and analytical aspects

A significant proportion of total circulating growth hormone (GH) is bound to a high affinity growth hormone binding protein (GHBP). Several low affinity binding proteins have also been described. Significant differences between species exist with respect to origin and regulation of GHBP, but generally it resembles the extracellular domain of the GH receptor. Concentrations are associated with GH status, body composition and other factors. Although the clinical relevance of GHBP is not fully understood it is suggested that concentrations indirectly reflect GH receptor status. This is supported by cases of Laron's syndrome where a molecular defect in the extracellular domain of the GH receptor is associated with low or unmeasurable GHBP concentrations. Methods to measure GHBP have evolved from chromatographic, activity based procedures to direct immunoassays. In clinical practice, measurement of GHBP can be helpful to differentiate between GH deficiency and GH insensitivity, particularly if GHBP is absent.

Growth hormone receptor modulators

Growth hormone (GH) regulates somatic growth, substrate metabolism and body composition. Its actions are elaborated through the GH receptor (GHR). GHR signalling involves the role of at least three major pathways, STATs, MAPK, and PI3-kinase/Akt. GH receptor function can be modulated by changes to the ligand, to the receptor or by factors regulating signal transduction. Insights on the physico-chemical basis of the binding of GH to its receptor and the stoichiometry required for activation of the GH receptor-dimer has led to the development of novel GH agonists and antagonists. Owing to the fact that GH has short half-life, several approaches have been taken to create long-acting GHR agonists. This includes the pegylation, sustained release formulations, and ligand-receptor fusion proteins. Pegylation of a GH analogue (pegvisomant) which binds but not activate signal transduction forms the basis of a new successful approach to the treatment of acromegaly. GH receptors can be regulated at a number of levels, by modifying receptor expression, surface availability and signalling. Insulin, thyroid hormones and sex hormones are among hormones that modulate GHR through some of these mechanisms. Estrogens inhibit GH signalling by stimulating the expression of SOCS proteins which are negative regulators of cytokine receptor signalling. This review of GHR modulators will cover the effects of ligand modification, and of factors regulating receptor expression and signalling.

Effect of aromatizable and unaromatizable androgen replacement in hypogonadal men on GH responsiveness

OBJECTIVES

Although studies have clearly demonstrated that oestrogen replacement affects GH responsiveness by causing relative GH resistance, the effect of androgen replacement is unknown. Circumstantial evidence only suggests that androgen replacement may increase GH sensitivity and/or responsiveness. To examine the impact of androgens on GH responsiveness, hypogonadal men underwent the IGF-1 generation test in the unreplaced state, replaced with testosterone (T) and also replaced with dihydrotestosterone (DHT), its nonaromatizable metabolite.

DESIGN AND PATIENTS

Twelve hypogonadal men with a normal GH axis were recruited. Each subject in random order had 4 weeks off T (NoRx), 4 weeks on T gel (TG) and 4 weeks on DHT gel (DHTG) applied daily, with 1 week washout between each preparation. An IGF-1 generation test using a subcutaneous injection of 7 mg of GH was performed at the end of each of these 4-week phases.

MEASUREMENTS

Serum GHBP, total and free IGF-1, IGFBP-3 and acid-labile subunit (ALS) levels were measured at baseline and 24 h (peak) after GH administration.

RESULTS

Despite a decrease in GHBP during the TG and DHTG phases, there were no observed differences in baseline, peak or increment (peak - baseline) total or free IGF-1 between the NoRx, TG or DHTG phases.

CONCLUSIONS

There is no evidence of fluctuation in GH responsiveness in hypogonadal men, untreated or replaced with T or DHT alone. This implies that the increased level of oestradiol as a consequence of T replacement in hypogonadal men does not impact significantly on GH responsiveness, nor is there evidence of an androgen effect with elevated DHT levels as a consequence of either T or DHT replacement.

Physiology and pathophysiology of growth hormone-binding protein: methodological and clinical aspects

Circulating GH is partly bound to a high-affinity binding protein (GHBP), which in humans is derived from cleavage of the extracellular domain of the GH receptor. The precise biological function GHBP is unknown, although a regulation of GH bioactivity appears plausible. GHBP levels are determined by GH secretory status, body composition, age, and sex hormones, but the cause-effect relationships remain unclarified. In addition to the possible in vivo significance of GHBP, the interaction between GH and GHBP has methodological implications for both GH and GHBP assays. The present review concentrates on methodological aspects of GHBP measurements, GHBP levels in certain clinical conditions with a special emphasis on disturbances in the GH-IGF axis, and discusses the possible relationship between plasma GHBP and GH receptor status in peripheral tissues.

Ethinylestradiol and testosterone have divergent effects on circulating IGF system components in adolescents with constitutional tall stature

OBJECTIVE

Pharmacological doses of estrogens or testosterone are used to limit the final height of girls or boys with constitutional tall stature but the mechanism behind this growth inhibition is still debated. We therefore studied the changes in the circulating components of the insulin-like growth factor (IGF) system during high dose sex steroid therapy.

DESIGN AND METHODS

Twenty three girls and twenty boys with constitutional tall stature were treated with 100 microg ethinylestradiol per day or 250 mg testosterone ester every 14 days respectively. In 19 girls and 18 boys, the levels of IGF-I, free IGF-I, IGF-II, acid-labile subunit (ALS) and IGF binding proteins (IGFBP)-2 to -6 were measured before and 3-6 months after the start of therapy (group 1). In 18 girls and 11 boys, samples were collected at the end of therapy and 3 to 6 months afterwards (group 2). Fourteen girls and nine boys belonged to both groups. All parameters were measured by radioimmunoassay or ELISA.

RESULTS

Levels of IGF-I were decreased significantly by estrogen treatment but remained unchanged during testosterone treatment. Free IGF-I decreased during estrogen treatment but increased during testosterone therapy. Estrogens increased IGF-II and testosterone reduced it. The important reduction of IGFBP-2 during estrogen therapy is not reproduced by androgen therapy, neither is the stimulation by estrogens of IGFBP-4. IGFBP-3 is not modulated by either sex steroid. We found that IGFBP-6 is up-regulated by testosterone but not by estrogens; the reverse is true for ALS, which increased during estrogen treatment but remained unchanged during testosterone treatment.

CONCLUSIONS

Our findings demonstrate that androgens and estrogens exert differential effects on the circulating levels of several IGF components.

Androgen and estrogen treatment, alone or in combination, differentially influences bone maturation and hypothalamic mechanisms that time puberty in the male rhesus monkey (Macaca mulatta)

In higher primates, the mechanisms that govern the ontogeny of gonadotropin-releasing hormone pulse generator activity and that, therefore, dictate the timing of the onset of puberty remain intriguingly elusive. Groups of three infant agonadal male monkeys were treated with sex steroids [17beta-estradiol (E(2)), testosterone (T), or dihydrotestosterone (DHT)] for the first year of life to advance bone age (BA). E(2) and T resulted in a significant advancement of BA, and a pubertal BA of 130 wk was attained at a mean chronological age of 64 and 67 wk, respectively. In contrast, DHT failed to advance BA during treatment but stimulated linear growth. All animals exhibited a pubertal resurgence in LH secretion, but the timing of this developmental event did not differ between treatment and control groups (the mean for all animals was 117.7 +/- 8.9 wk). Two of the three T-treated animals, however, displayed a pubertal LH resurgence at a remarkably young age (70 and 76 wk of age) that coincided with T withdrawal. During the period of steroid treatment, all three groups were significantly heavier than the controls. The rate of body weight gain was most rapid in the DHT-treated group. Steroid treatments also resulted in accelerated linear growth. Body weight gain and linear growth continued at the same rate as controls after withdrawal of treatment. These data indicate that attainment of a pubertal BA may be a necessary but not a sufficient factor to trigger the onset of puberty. The results not only are consistent with the view that androgen-induced skeletal maturation in males is mediated by estrogen receptor activity but also indicate that androgen receptor activity contributes to the pubertal growth spurt in males.

Expression of insulin-like growth factor-I (IGF-I) in aneurysmal bone cyst

The effects of insulin-like growth factor-I on bone tissue and its role in bone development have been extensively investigated, but there is little information on its role in the pathogenesis of aneurysmal bone cyst. Therefore, using the techniques of immunohistochemistry and in situ hybridization, the authors studied the expression of insulin-like growth factor-I in 19 specimens of aneurysmal bone cyst. Insulin-like growth factor-I or specific mRNA sequences encoding for insulin-like growth factor-I were detectable in all specimens tested and were mainly localized in multinucleate giant cells. In contrast, only insignificant levels of insulin-like growth factor-I expression were detectable in normal human bone tissue. Taken together with the previously reported role of insulin-like growth factor-I in the pathogenesis of giant cell tumor, the findings of this study suggest that insulin-like growth factor-I may play a role in the pathogenesis of aneurysmal bone cyst.

Gender difference in insulin-like growth factor I response to growth hormone (GH) treatment in GH-deficient adults: role of sex hormone replacement

GH production in healthy women is about thrice that in men. Yet insulin-like growth factor I (IGF-I) levels are similar, suggesting a lower responsivity to GH in women. In untreated GH-deficient adults, basal IGF-I levels are reportedly lower in females than in males, and the therapeutic recombinant human GH (rhGH) dose required to achieve optimal IGF-I levels is higher in the former, suggesting a pivotal role of estrogens on rhGH requirement in GH-deficient patients. We, therefore, analyzed our 2-yr data on the effect of rhGH on serum IGF-I in 77 GH-deficient patients (33 men, mean +/- SD age, 37.2 +/- 13.8 yr; 44 women, mean +/- SD age, 36.9 +/- 11.9 yr) with due attention to gender differences and to the effects of sex hormone replacement. Of the 44 women, 33 had estrogen substitution. Of the 33 men, 23 were on androgen replacement. Patients (11 premenopausal women and 10 men) not on hormonal replacement were eugonadal. Basal IGF-I levels in untreated GH-deficient women were significantly lower than in men (8.8 +/- 0.7 nmol/L vs. 12.2 +/- 0.9 nmol/L; P < 0.01), despite similar basal GH levels. The daily rhGH dose per kg body weight required to normalize IGF-I in women was higher than in men, the difference being statistically significant at all time points (P < 0.05-0.01). The IGF-I increase (delta) per IU GH/day x kg over the 24-month period was about twice higher in men than in women. Also calculated on a weight basis, rhGH responsivity (rhGH responsivity = (deltaIGF1(nmol/L)/dose (IU/day/kg)) was higher in men than in women at all time intervals (P < 0.05-0.01). Estrogen replacement in women significantly increased rhGH requirement. The rhGH dose per kg body weight required in estrogen-substituted women was significantly higher than in nonestrogen-substituted women (P < 0.01 at t = 18 and 24 months, respectively). In women on estrogen substitution, rhGH responsivity plateaued from 6 months on, whereas in eugonadal women without estrogen substitution the responsivity for rhGH increased over time. In men, the reverse was true; rhGH responsivity increased over time in men on androgen substitution, but plateaued in men without androgen substitution. The mechanisms underlying this gender difference are not known. Differential influences of estrogens and androgens on the expression of the GH receptor gene and IGF-I messenger RNA may be operative. The present study confirms short-term data published in the literature on a sex difference in rhGH dose requirement in GH-deficient patients. It furthers extends the data by demonstrating that this sex difference in GH responsivity persists and changes during the 24 months of the study. Moreover, it shows that estrogen replacement blunts the IGF-I response to rhGH in women, whereas in men with androgen substitution the responsivity increases over time, thus bearing a risk of undertreatment in women and overtreatment in men.

Serum levels of growth hormone binding protein in children with normal and precocious puberty: relation to age, gender, body composition and gonadal steroids

AIM

To study the regulation of GHBP serum levels by gonadal steroids in normal and precocious puberty.

STUDY PROTOCOL

We studied GHBP levels in relation to age, sex, pubertal maturation, body composition as well as to circulating IGF-I and gonadal steroid levels in 320 healthy children. Furthermore, we studied the regulation of circulating GHBP in 33 girls with central precocious puberty before and during gonadal suppression with GnRH agonist.

METHODS

GHBP was determined by a time-resolved fluoroimmunoassay (GHBP TR-FIA) based on a commercially available immunoassay for GH, the DELFIA GH assay.

RESULTS

In healthy children GHBP levels were significantly higher in normal girls compared with boys, and there was no significant increase in GHBP in puberty in both sexes. GHBP levels did not correlate with height (SDS), age, pubertal stage, IGF-I or testosterone/oestradiol levels in boys and girls, respectively. There were significant correlations between BMI and GHBP in boys and girls (R 2 = 0.14 and R 2 = 0.12, both P < 0.0001). Furthermore, GHBP correlated highly significantly with the percentage body fat, determined by BIA in 43 healthy girls (R 2 = 0. 40, P < 0.0001). GHBP levels were significantly higher in girls with central precocious puberty (CPP) (1.31 SDS (1.26), mean (SD)) compared to prepubertal controls (P < 0.0001), and above + 2 SD in 10 out of 33 patients. In girls with CPP, GHBP correlated inversely with oestradiol before treatment (R 2 = 0.26, P < 0.01) and there was a tendency towards a positive correlation with BMI (R 2 = 0.13, P = 0.078). By contrast, there were no signficant correlations between GHBP and IGF-I or height SDS. Gonadal suppression with GnRH agonist treatment caused a transient significant increase of 0.57 SD after 2 months of treatment (P < 0.001), but decreased to baseline levels hereafter.

CONCLUSION

We conclude that in children, as in adults, body fat is the primary determinant for the circulating level of GHBP, and that the difference in body fat is probably the main factor for the higher levels of serum GHBP in girls compared with boys, as well as for the negative influence of testosterone levels in boys and of oestrogen levels in girls. The elevation in GHBP levels observed in girls with central precocious puberty is probably due their higher body fat content.

Insulin-Like growth factor I: implications in aging

According to the somatomedin model, growth hormone (GH)-dependent hepatic synthesis is responsible for maintaining circulating insulin-like growth factor (IGF)-I levels. On the other hand, the local autocrine/paracrine IGF-I expression in peripheral tissue is generally GH-independent and reflects the effects of various and tissue-specific trophic hormones. Circulating IGF-I levels undergo important age-related variations increasing at puberty and decreasing, thereafter, to low levels in the elderly. Low IGF-I levels in the elderly mainly reflect impaired somatotroph secretion but the decline in gonadal sex steroid levels, some protein and micronutrients malnutrition as well as age-dependent variations in IGF-binding proteins may also play a role in the age-related decrease in IGF-I activity. This, in turn, partially accounts for age-related changes in bones, muscles, cardiovascular system, central nervous system and the immune system. However, it is currently unclear whether treatment with exogenous IGF-I can retard or reverse age-related changes in body structure and function.

Reduced androgen levels in adult turner syndrome: influence of female sex steroids and growth hormone status

BACKGROUND AND OBJECTIVES

In girls with Turner syndrome androgen levels are reduced. In order to assess androgen status in women with Turner syndrome, we compared untreated adult women with Turner syndrome with a group of normal women. In addition, the effects of female sex hormone replacement therapy and GH status on the levels of circulating androgens in Turner syndrome was examined.

DESIGN

All patients were receiving female hormone replacement therapy (HRT), which was discontinued four months prior to the initial examination. Patients were studied before and during HRT. Following the initial evaluation, patients were given cyclical HRT for six months consisting of either oral substitution (17beta-oestradiol with norethisterone from day 13-22), or transdermal oestrogen substitution (17beta-oestradiol) with 1 mg norethisterone administered orally from day 13-22. Control subjects were studied once in the early follicular stage of the menstrual cycle.

SUBJECTS

The study group consisted of 27 (33.2 +/- 7.9 years) patients with Turner syndrome and an age matched control group of 24 (32.7 +/- 7.6 years) normal women.

MEASUREMENTS

Body composition measures, SHBG, testosterone (T), free testosterone (FT), dihydrotestosterone (DHT), alpha-4-androstendione (A), dehydroepiandrosterone sulphate (DHEAS), 17beta-oestradiol (E2), oestrone (E1), oestrone sulphate (ES), 24 h integrated GH concentration (ICGH), insulin-like growth factor I (IGF-I), insulin-like growth factor binding protein (IGFBP-3) were determined at baseline and after six months in women with Turner syndrome, and at baseline in control women.

RESULTS

Circulating levels of A, T, FT, DHT, and SHBG were reduced by 25-40% in comparison with age matched normal women. The level of DHEAS was normal. The level of E2 was undetectable and levels of E1 and ES were very low in untreated Turner women. Treatment with 17beta-oestradiol and norethisterone increased oestrogen to levels comparable to those of normal women, while further decreasing FT (P = 0.02), DHT (P = 0.04), and T (P = 0.1). In untreated women with Turner syndrome IGF-I correlated significantly with DHEAS (R = 0.503, P < 0.01), while in normal women IGF-I correlated with A (R = 0.637, P < 0.01), T (R = 0.536, P < 0.01), and FT (R = 0.700, P < 0.01). During hormonal replacement in women with Turner syndrome IGF-I correlated significantly with DHEAS (R = 0.547, P < 0.01). Employing multiple regression analysis IGFBP-3, ICGH, DHEAS and fat free mass explained 85% (adjusted R = 0.92, P < 0.0005) of the variation in the level of IGF-I in untreated Turner syndrome. In treated Turners IGFBP-3, ICGH, SHBG, T, and FT explained 78% (adjusted R = 0.88, P < 0.0005). In controls IGFBP-3, SHBG, BMI and age explained 74% (adjusted R = 0.86, P < 0.0005) of the variation in IGF-I, while GH status did not contribute at all.

CONCLUSION

The present study shows that many adults with Turner syndrome have reduced levels of circulating androgens, compared with an age-matched group of normal women. Conditions associated with Turner syndrome such as increased prevalence of sexual problems, reduced bone mineral content, osteoporosis, and an increased incidence of fractures and alterations in body composition could perhaps be alleviated or abolished by substitution with a low dose of androgens. Treatment with female hormonal replacement therapy is associated with a decrease in testosterone, free testosterone and dihydrotestosterone, possibly mediated by the androgenic effect of norethisterone. Furthermore significant differences in sex steroid levels, GH status and indices of body composition can be compatible with comparable levels of IGF-I in two very different groups of individuals.

Effects of growth hormone and testosterone on cortical bone formation and bone density in aged orchiectomized rats

Osteoporosis in men is a disease that is increasing in incidence, and with an increasing elderly population it poses a serious health problem. Since both testosterone (T) and growth hormone (GH) have an anabolic effect on bone and both decrease with aging, we were prompted to test whether the administration of these hormones in combination would increase bone mass in orchiectomized (orx) senile rats more than administration of either agent alone. Twenty-month-old male Wistar rats were divided into five groups with seven animals each: (a) age-matched intact control, (b) orx, (c) orx+GH (2.5 mg/kg/day), (d) orx+T [10 mg/kg, subcutaneous (s.c.), injection given twice a week], and (e) orx+GH+T. Testosterone and GH were given subcutaneously for 4 weeks. Bone histomorphometry of the tibial shaft showed that the orx group had lower cortical bone area than the intact control group. The decrease in cortical bone area was due to increased intracortical porosis as well as decreased periosteal bone formation rate (BFR). Administration of T to the orx animals prevented the development of the porosis and the decrease in periosteal BFR. The bone mineral content (BMC) and bone mineral density (BMD) of the femur as tested by dual-energy X-ray absorptiometry were significantly higher in the orx+T than in the orx group and were not significantly different from that of the intact control group. Administration of GH to the orx rats increased periosteal BFR significantly; however, the BMC and BMD measured were not increased significantly in comparison to the orx group. When GH and T were combined in treatment, the cortical bone area, periosteal BFR, and femoral BMD were all significantly higher than that of the orx and even higher than the intact control rats. Two-way analysis of variance shows that the individual effect of GH and T treatment on the periosteal BFR and cortical bone area was significant. The effect of T, but not GH, on femoral BMC and BMD was also significant; however, there is no synergistic interaction between the two treatments. Four weeks of orx with or without GH or T administration had no significant effect on tibial metaphyseal cancellous bone volume. In conclusion, this short-term study suggests that the combined intervention of GH and T in androgen-deficient aged male rats may have an independent effect in preventing osteopenia. The significant effect of GH+T may be attributed to the prevention of intracortical porosis, and an increase in periosteal bone formation and cortical bone mass.

Determinants of serum insulin-like growth factor I in growth hormone deficient adults as compared to healthy subjects

OBJECTIVE

Growth hormone status is an important determinant of serum IGF-I but it is well known that hypopituitary adults with pronounced GH-deficiency (GHDA) may exhibit normal IGF-I levels. To elucidate possible causes of this apparent paradox we compared the significance of putative IGF-I predictors in GHDA and normal subjects.

DESIGN

A cross-sectional study.

SUBJECTS

Twenty-seven GHDA (9 females, 18 males, mean +/- SE age 44 +/- 1 years) and 27 healthy control subjects (9 females, 18 males, mean +/- SE age 43 +/- 2 years).

RESULTS

Serum IGF-I and IGFBP-3 were significantly lower in GHDAs, but a considerable overlap existed (IGF-I (microgram/l) 87 +/- 12 (GHDA) vs 177 +/- 10 (Control) (P < 0.001)). In both Controls and GHDA, IGF-I was higher in males than females (Control: 196 +/- 12 vs 138 +/- (P = 0.004); GHDA: 97 +/- 16 vs 56 +/- 11 (P = 0.05)). In GHDA, males on testosterone substitution had the highest IGF-I concentrations. The molar IGF-I:IGFBP-3 ratio was significantly lower in GHDAs (0.18 +/- 0.01 vs 0.23 +/- 0.02 (P = 0.002)). IGFBP-1 (microgram/l) was significantly elevated in GHDAs (6.28 +/- 1.11 vs 3.07 +/- 0.32 (P < 0.001)) despite comparable fasting insulin levels. Percentage total body fat (TBF, DEXA, waist/hip ratio, and intra-abdominal fat (CT) were all elevated in GHDAs. IGF-I correlated positively with lean body mass (DEXA) and negatively with TBF and IGFBP-1 in both groups. IGF-I correlated negatively with age in CON but not in GHDAs, whereas IGF-I correlated positively with IGFBP-3 only in GHDAs. Multiple regression analysis revealed that age and IGFBP-1 were the only significant predictors of IGF-I in CON, whereas IGFBP-3 and, to a lesser extent TBF, were the only independent predictors of IGF-I in GHDAs. Neither peak stimulated GH, nor physical fitness contributed in any equations in the two groups.

CONCLUSIONS

1) IGF-I levels are regulated by several variables in addition to GH status 2) age per se is an independent negative determinant in healthy subjects but not in GHDA 3) it is probable that some cases of paradoxically high IGF-I levels in GHDA are secondary to inappropriately elevated IGFBP-3 levels. 4) in mid-adulthood males have higher IGF-I levels than females and it is likely that testosterone directly stimulates IGF-I. The influence of gender and sex steroids must therefore be accounted for when comparing IGF-I levels between hypopituitary and healthy subjects.