Effect of growth hormone on the low level of growth hormone binding protein in idiopathic short stature

Growth hormone (GH) and 17beta-estradiol regulation of the expression of mouse GH receptor and GH-binding protein in cultured mouse hepatocytes

In the present study, primary mouse hepatocytes from 8- to 10-week-old virgin female Swiss-Webster mice were perfused with collagenase (100 U/ml) using the two-step method. Isolated hepatocytes were plated in a rat tail type I collagen sandwich configuration to examine the regulation of GH receptor (GHR) and GH-binding protein (GHBP) expression by GH and 17beta-estradiol (E2). After 48 h of initial plating, hepatocytes were divided into groups of five replicates and treated for 24 h with medium containing no hormones (controls), GH (100 ng/ml), E2 (10(-9) M), E2 (10(-9) M) plus GH (100 ng/ml), or E2 plus GH and ICI 182-780 at different concentrations. Treatment of hepatocytes with GH or E2 alone did not have any effect on the cellular concentrations of GHBP and GHR. However, the combination of E2 and GH up-regulated the cellular concentrations of GHBP and GHR 2- to 3-fold. GHBP and GHR messenger RNA concentrations were also up-regulated 2- to 3-fold. ICI 182-780, a competitive inhibitor of E2 for the estrogen receptor (ER), at different concentrations inhibited the E2 and GH-induced stimulation of GHBP and GHR. Furthermore, ER concentrations increased 5- to 7-fold in hepatocytes treated with E2 and GH compared with those in untreated cells or cells treated with either E2 or GH alone. In the present study we have shown that in cultured hepatocytes from virgin female mice, GH or E2 alone did not affect the concentrations of GHBP and GHR. However, E2 and GH together significantly up-regulated GHR and GHBP expression.

Analytical performance and clinical usefulness of two binding assays for growth hormone binding protein (GHBP) measurement: high performance liquid chromatography (HPLC)-gel filtration and dextran-coated charcoal adsorption

We compared two binding assays for growth hormone binding protein (GHBP) measurements, which differ in the method of bound and free GH separation: HPLC-gel filtration or dextran coated-charcoal adsorption (DCC). Two pools of sera (high and medium GHBP activity) were used for quality-control assessment. Moreover, 62 samples from 34 children and 28 adults with different nutritional status were studied. Total, between- and intra-iodination coefficients of variation (CVs) from the two methods were not different. Although percentage binding measured in the pool sera significantly differed, the concentrations assessed by Scatchard plot were comparable. Results obtained by the two methods in the 62 sera were significantly correlated (r = 0.77, P < 0.001). With both methods GHBP activity correlated with chronological age and body mass index (BMI) and differed among groups with different nutritional status. Although HPLC and DCC separation methods for GHBP measurement differ in their practicability, our study demonstrates that performance and the clinical usefulness of the two methods are comparable.

Partial growth hormone insensitivity in childhood

There appears to be a continuum of GH-responsiveness, ranging from complete GH-resistance in Laron syndrome to normal GH-sensitivity. Partial GHI is suggested by findings at both ends of this spectrum; some patients with Laron-type dwarfism are less severely affected by their GHI than others (Savage et al, 1993; for a review, see Savage et al, 1995) and some short, non-GH-deficient, children have reduced responsiveness to GH. Among children with ISS, we have identified a subgroup where defects at the level of the GH receptor lead to a partial-GHI syndrome (Carlsson et al, 1994; Attie et al, 1995; Goddard et al, 1995). Partial-GHI may explain the growth failure in some children who do not meet the criteria for GHD. So far, the clinical evaluation of short children has been focused on the exclusion or demonstration of GHD. The diagnosis of GHD at present requires that stimulated or spontaneous GH concentrations should fail to reach a certain, arbitrarily determined level. This assumes that GH-sensitivity is equal in all subjects, with the exception of rare cases with Laron syndrome. The diagnosis of GHD is in itself controversial, and it has been suggested that GH testing should be supplemented by other measures such as auxological evaluation and measurement of other components of the GH/IGF-I axis (Rosenfeld et al, 1995). However, the fact that some short children have partial-GHI suggests that both GH secretion and GH responsiveness should be taken into consideration when investigating the cause of short stature (Figure 5). Many short children do not have GHD per se, but may be short due to inadequate GH stimulation because of reduced GH-sensitivity.

Serum growth hormone-binding protein is decreased in prepubertal children with idiopathic short stature

The causes for growth failure in children with idiopathic short stature (ISS) are not definitely established. Peripheral GH resistance due to changes at the level of the GH receptor has been suggested as one of the most probable explanation. In this study, we have selected a group of prepubertal children with ISS to evaluate the GHBP/receptor status by measuring the GH binding protein (GHBP) activity in plasma. Thirty prepubertal children with ISS (18 boys and 12 girls; age range: 4.79 to 11.33 yr) and twenty age-matched children with normal growth (11 boys and 9 girls) were studied. The ISS group presented growth retardation of -2.3 +/- 0.43 SD score (mean +/- SD) and normal GH secretion. Plasma IGF-I levels were below or in the low normal range (mean +/- SD: 136.3 +/- 62.3 micrograms/l, a concentration that was significantly different from IGF-I levels in the normal group (mean +/- SD: 187 +/- 57.5 micrograms/l p < 0.005). Plasma GHBP activity using a GH-binding/gel chromatography assay showed significantly lower values in ISS group (mean +/- SD: 7.17 +/- 1.5%) as compared with those of the control group (mean +/- SD: 12.02 +/- 2.04%; p < 0.001). There were no significant age- or sex-related differences in GHBP values in either group. The decreased GHBP levels observed in this group of children with ISS suggest that they may present a certain degree of GH insensitivity, probably due to a defect at the GH-receptor level.

Growth responses to patterned GH delivery

We have investigated the effects of different patterns of administration of recombinant human growth hormone (rhGH) on weight gain, organ growth, serum GH binding protein (GHBP) and insulin-like growth factor-l (IGF-1) levels in a series of studies using hypophysectomized (Hx) or GH-deficient dwarf (dw/dw) rats. Animals were given rhGH either by subcutaneous (s.c.) injections (1 or 2 per day) or s.c. infusions and rhlGF-1 (2 mg/kg/day) by s.c. infusion. In Hx rats, all rhGH regimes increased body weight, tibial epiphyseal plate width, and organ weights in a dose-related manner. Dwarf rats showed a smaller growth response to rhGH than Hx rats, whereas rhGH induced greater elevations in serum GHBP in drarf rats. Growth responses depended on the pattern of rhGH administration (twice daily injections > continuous infusions > daily injections). The shape of the body growth curves also differed; rhGH injections increased weight gain linearly, whereas infusions gave an initial rapid weight gain which slowed with time (a curvilinear response). For both regimens, tibial epiphyseal plate width increased linearly with rhGH dose but infusions were 5-fold more potent than daily injections. Spleen and thymus weights were markedly increased by rhGH and were also affected by the pattern of GH exposure. At 5 mg rhGH/kg/day, thymus weights were 390±35 mg for injectionsvs. 613 ± 34 mg for infusions (P<0.001) compared with 248 ± 16 mg in vehicle-treated Hx controls. Infusions of rhlGF-1 also stimulated specific organ growth but caused less weight gain. RhlGF-1 additively increased the weight gain caused by rhGH injections but not by rhGH infusions. Circulating IGF-1 and GHBP levels were increased in a dose-dependent manner by rhGH infusion, whereas daily injections were ineffective. Thus, differential organ growth could be related to the higher serum IGF-1 concentrations induced by continuous rhGH administration. These studies show that whole body growth is best maintained by intermittent rhGH exposure, whereas, paradoxically, differential organ growth is most pronounced with continuous rhGH administration.

Long-term results of growth hormone therapy in children with short stature, subnormal growth rate and normal growth hormone response to secretagogues. Dutch Growth Hormone Working Group

BACKGROUND AND OBJECTIVE

Growth hormone treatment in children with idiopathic short stature (ISS) leads to growth acceleration in the first years, but the effect on final height is still poorly documented. We therefore studied the long-term effect of GH therapy in children with idiopathic short stature.

DESIGN

We have treated 27 prepubertal children with ISS with recombinant human GH (rhGH) in an initial dosage of 2 IU/m2 body surface/day subcutaneously, which was doubled either after the first year if the height velocity increment was less than 2 cm/year, or thereafter if height velocity fell below the P50 for bone age. Growth and bone maturation of the treatment group (ISS group, n = 21) were compared to those of an untreated control group with ISS (ISS controls, n = 27) and of a group of rhGH treated children with isolated GH deficiency (GHD group, n = 7).

RESULTS

In 9 patients of the ISS group still on treatment, height standard deviation score (HSDS) for chronological age increased from -3.8 +/- 0.7 to -2.3 +/- 0.9 (mean +/- standard deviation) over 6 years, while in matched ISS controls HSDS for age did not change. HSDS for age in the GHD group increased from -3.9 +/- 0.6 to -1.8 +/- 0.7 after 4 years, significantly more than the ISS group. Bone maturation was accelerated in the ISS and GHD groups. HSDS for bone age and predicted adult height did not change in either group. Final height in 12 children of the ISS group was -2.6 +/- 1.0 SDS. In the untreated controls final height was similar. A low integrated GH concentration over 24 hours, a low GH peak to provocative stimuli, and minimal initial BA delay predicted a favourable outcome.

CONCLUSION

rhGH treatment in this group of children with idiopathic short stature did not increase average final height. Part of the heterogeneity of the response can be attributed to the variation in endogenous GH secretion and initial bone age delay.

The use of recombinant human growth hormone in short children with chronic renal failure

Regulation of the somatotropic axis is altered in chronic renal failure (CRF) resulting in a secondary syndrome of growth hormone (GH) insensitivity. Secretion of growth hormone estimated by deconvolution analysis is low normal in prepubertal patients and reduced in late pubertal children with CRF. Basal and integrated GH serum concentration measured by RIA is increased due to reduced renal metabolic clearance, whereas the fractional urinary excretion is increased due to damage of renal tubular cells. GH receptor mRNA is decreased (rat) and the serum concentration of GH binding protein (BP) activity is low (man). Insulin-like growth factor (IGF)-1 production rate is reduced, whereas serum concentrations of IGFBPs are increased secondary to reduced renal metabolic clearance. This results in a reduction of free, active IGF-1. Treatment with GH induces a rise in serum IGF-1 concentration and normalizes IGF bioactivity. Clinical studies in prepubertal children demonstrated a dramatic rise in height velocity during the first treatment year and to a lesser extent during the following years. In children on conservative treatment prior to dialysis, mean height SDS improved by 1.5 within two years and by 2.0 within four years. Patients with renal allografts responded in a similar way. Age and pretreatment height velocity SDS are confounding variables for the response to GH. Renal function seems not be altered by recombinant human (rh) GH in patients with CRF, and the number of renal allograft rejection crises seems not to be substantially increased under rhGH treatment in allograft recipients.