Serum binding proteins for growth hormone: origins, regulation of gene expression and possible roles

Involvement of somatostatin receptor subtypes in membrane ion channel modification by somatostatin in pituitary somatotropes

1. Growth hormone (GH) secretion from pituitary somatotropes is mainly regulated by two hypothalamic hormones, GH-releasing hormone (GHRH) and somatotrophin releasing inhibitory factor (SRIF). 2. Somatotrophin releasing inhibitory factor inhibits GH secretion via activation of specific membrane receptors, somatostatin receptors (SSTRs) and signalling transduction systems in somatotropes. 3. Five subtypes of SSTRs, namely SSTR1, 2, 3, 4 and 5, have been identified, with the SSTR2 subtype divided into SSTR2A and SSTR2B. All SSTRs are G-protein-coupled receptors. 4. Voltage-gated Ca(2+) and K(+) channels on the somatotrope membrane play an important role in regulating GH secretion and SRIF modifies both channels to reduce intracellular free Ca(2+) concentration and GH secretion. 5. Using specific SSTR subtype-specific agonists, it has been found that reduction in Ca(2+) currents by SRIF is mediated by SSTR2 and an increase in K(+) currents is mediated by both SSTR2 and SSTR4 in rat somatotropes.

Regulation of growth-hormone-receptor gene expression by growth hormone and pegvisomant in human mesangial cells

BACKGROUND

Mice transgenic for growth hormone develop mesangial proliferation, glomerular hypertrophy, and progressive glomerular sclerosis suggesting that the growth hormone-insulin-like growth factor I (IGF-I) pathway plays an important role. Therefore, we studied the impact of variable concentrations of 22 kD, 20 kD growth hormone, as well as of the growth hormone receptor antagonist pegvisomant (B2036-PEG), on both the growth hormone receptor (GHR/GHBP) gene expression and growth hormone binding protein (GHBP) formation in a human glomerular mesangial cell line. Further, the impact on collagen, IGF-I and IGF binding protein-1 (IGFBP-1) formation was studied.

METHODS

In order to assess transcription, quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used.

RESULTS

Physiologic doses of 22 kD or 20 kD growth hormone caused a dose-dependent and significant (P < 0.01) up-regulation of GHR/GHBP gene transcription, whereas supraphysiologic doses (50 and 500 ng/mL) resulted in down-regulation (P < 0.001). Whenever pegvisomant was used, there was no increase in GHR/GHBP expression. These data were confirmed using run-on experiments. Further, the assessment of GHBP presented a constant, dose-dependent increase, which was completely abolished in the experiments where pegvisomant was used.

CONCLUSION

We present data showing that growth hormone has a direct impact on GHR/GHPB gene transcription and that pegvisomant is a potent growth hormone receptor antagonist in human mesangial cells. In addition, although the GHR/GHBP gene transcription is down-regulated by supraphysiologic growth hormone concentrations, this effect was not found when GHBP levels were measured. This finding may reflect a self-inhibitory effect of growth hormone on the level of GHR/GHBP gene transcription, which does not involve the regulation of the shedding of GHBP and may, therefore, be of physiologic interest.

Altered endogenous growth hormone secretory kinetics and diurnal GH-binding protein profiles in adults with chronic liver disease

OBJECTIVE

Increased serum GH concentrations and GH responses to a variety of stimuli have been reported in patients with chronic liver disease (CLD). We investigated the pulsatile pattern of endogenous GH release and GH-binding protein (GHBP) and insulin-like growth factor-I (IGF-I) diurnal profiles in adults with cirrhosis, in comparison with healthy, matched control subjects.

DESIGN

Case-control, cross-sectional.

PATIENTS

Seven patients with biopsy proven cirrhosis, and sex, age, height, weight and oestrogen status matched controls.

MEASUREMENTS

Serum immunoreactive GH concentrations in samples collected at 20-minute intervals for 24 hours were analysed using a multi-parameter deconvolution method to simultaneously resolve endogenous GH secretory and disappearance rates. Diurnal patterns of GHBP (specific immunoprecipitation method) and serum IGF-I (RIA after acid-ethanol extraction) were assessed.

RESULTS

The mean daily GH secretion rate in patients with CLD was increased (210 +/- 93 vs 100 +/- 55 mU/I/day; P = 0.025), and GH disappearance half-time was prolonged (43 +/- 10 vs 24 +/- 9 min; P = 0.006) compared to controls. Detectable GH secretory bursts were more frequent in patients with CLD (10 +/- 1 vs 6 +/- 3/day; P = 0.038), but of similar mean mass (21 +/- 10 vs 17 +/- 8 mU/I) compared to controls. In patients with CLD, mean serum GHBP was slightly lower (63 +/- 36 vs 71 +/- 14% pooled control; P > 0.1). Fasting serum IGF-I concentrations (after size-exclusion HPLC) were lower in the patients with CLD (13 +/- 5 vs 21 +/- 2 nmol/l; P < 0.0001). Multiple regression analysis showed that GH secretion rate was increased in patients with CLD with higher Child's classifications (R2 = 0.86; P = 0.002) and with lower serum IGF-I concentrations measured after HPLC (R2 = 0.11; P = 0.044).

CONCLUSIONS

Adults with chronic liver disease have (1) increased total daily GH secretion rates, which appear to be influenced by disease severity and diminished serum IGF-I-mediated negative feedback; (2) markedly impaired endogenous GH clearance, possibly reflecting changes in hepatic GH-receptor status; and (3) GHBP levels which do not correlate with GH kinetics or serum IGF-I concentrations.

Regulation of the growth hormone (GH) receptor and GH-binding protein by GH pulsatility

Growth hormone (GH) secretion is pulsatile in man and every other mammalian species that has been studied. The magnitude of pulses, their frequency, and their regularity vary. The receptor, on its part, undergoes cycles of internalization and recycling that are in synchrony with the frequency of GH pulses. This sequence of events has been shown to be advantageous to growth. It is suggested that changes in GH secretion dynamics secondarily lead to most of the changes in GH receptor abundance and GH-binding protein (GH-BP) abundance. Across a wide scope of comparative studies, ontogenesis data, experimental systems, physiologic conditions, nutritional states, and disease situations, the pulsatility of serum GH is negatively correlated with cell membrane GH receptors and serum levels of GH-BP. It is suggested that these conditions regulate primarily the pattern of GH pulsatility, which in turn regulates the GH receptor/GH-BP, and thereby exert the specific effects on target cells to promote or suppress growth or to express distinct metabolic actions.

The turnover of growth hormone (GH)-binding protein and GH receptor in rabbit and rat

The present study was undertaken to further explore the comparative dynamics of growth hormone-binding protein (GH-BP) in relation to the turnover of the GH-receptor (GH-R) in vivo in rabbits and rats. The strategy used was to examine the time course of hepatic GH-R turnover over a 3 h period after cycloheximide treatment, with simultaneous measurements of serum GH-BP level. In the rabbit we sampled multiple liver biopsies and serum samples consecutively from each animal. In the rat, experiments on individual animals were conducted for each time point. In the rat, both liver GH-R and serum GH-BP declined after cycloheximide injection following first-order kinetics. The t 1/2 values for GH-R and GH-BP were 29.7-44.5 and 82.7-119.5 min (95% confidence limits), respectively. A significant positive correlation was found between rat liver GH-R and serum GH-BP (r = 0.85; p < 0.001). In contrast, the decline in rabbit liver GH-R, following cycloheximide treatment was accompanied by simultaneous time-dependent accumulation of serum GH-BP. The t 1/2 for rabbit serum GH-BP accumulation was 30.4-67.6 min. Scatchard analysis of [125I]hGH binding to rabbit GH-BP indicated that the binding capacity increased from 2818 +/- 538 fmol/ml, at time zero, to 5236 +/- 419 fmol/ml following 60 min cycloheximide treatment (p < 0.05). No significant changes in affinity were observed.(ABSTRACT TRUNCATED AT 250 WORDS)