Growth hormone-binding protein levels over one year in healthy prepubertal children: intraindividual variation and correlation with height velocity

Broad variability in pharmacokinetics of GH following rhGH injections in children

OBJECTIVE

Daily subcutaneous self-injection of GH is used worldwide to treat short stature in childhood; longitudinal data on the impact of this regimen on GH-uptake are lacking.

DESIGN

Children with/without GH-deficiency participating in clinical trials were followed prospectively (≤8 times). Blood was sampled pre-GH-injection (dose GH³³/GH⁶⁷ μg/kg) and either every 30 min thereafter for 24 h (Experimental-setting; 59 GH-curves/15 children); or every 2 h thereafter for 16 h (Clinical-setting; 429 GH-curves/117 children). Pharmacokinetics were estimated by time T_max (h) of maximal GH-concentration (C_max, mU/L) and area under the curve for 16 h (AUC, mU/L ∗ h).

RESULTS

In the Clinical-setting, median C_max was 71 mU/L and AUC was 534 mU/L ∗ h, with coefficients of variation for intra-individual variation of 39% and 36%, respectively, and inter-individual variation of 44% and 42%, respectively. 43% of C_max and AUC variability was explained by GH-dose and proxies for injection depth (baseline GH-level, GH_peakwidth, BMI_SDS). In the Experimental- versus Clinical-setting, 85% and 40% of GH-curves, respectively, reached zero-levels within 24 h. A longer duration was found following a more superficial GH-injection. Spontaneous GH-peaks were identified already 6 h after the GH-injection in about half of the curves of both GHD and non-GHD patients.

CONCLUSION

Very broad intra-individual and inter-individual variability was found. A high GH-peak will optimize growth effects; the highest C_max was found after a deep injection of GH at the higher dose and concentration. In as many as 60% of the children, GH remained detectable in serum after 24 h; a constant GH-level will promote IGF-I and metabolic effects.

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.

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.

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.

Growth hormone binding protein correlates strongly with leptin and percentage body fat in GH-deficient adults, is increased by GH replacement but does not predict IGF-I response

GH-binding protein (GHBP) corresponds to the extracellular domain of the GH receptor (GHR) and has been shown to be closely related to body fat. This study aimed to examine the inter-relationship between GHBP, leptin and body fat, and to test the hypothesis that GHBP is modified by GH replacement in GH-deficient adults and predicts IGF-I response. Twenty adults, mean age 47 years (range 20-69) with proven GH deficiency were randomly allocated to either GH (up to 0.25 U/kg/week in daily doses) or placebo for 3 months before cross-over to the opposite treatment. Plasma GHBP and leptin were measured at baseline and 2, 4, 8 and 12 weeks after each treatment. Whole body composition was measured at baseline by dual-energy X-ray absorptiometry (DEXA). There was a strong correlation between baseline leptin and GHBP (r = 0.88, P < 0.0001) and between baseline GHBP and percentage body fat, (r = 0.83, P < 0.0001). Mean GHBP levels were higher on GH compared with placebo, 1.53 +/- 0.28 vs 1.41 +/- 0.25nM, P = 0.049. There was no correlation between baseline IGF-I and GHBP (r = -0.049, P = 0.84), and GHBP did not predict IGF-I response to GH replacement. The close inter-relationship between GHBP, leptin and body fat suggests a possible role for GHBP in the regulation of body composition. GHBP is increased by GH replacement in GH-deficient adults, but does not predict biochemical response to GH replacement.