Insulin-like growth factor I and insulin down-regulate growth hormone (GH) receptors in rat osteoblasts: evidence for a peripheral feedback loop regulating GH action.

Abstract

The anabolic actions of GH are mediated by the production of insulin-like growth factor I (IGF-I) from the liver and by local production of IGF-I in extrahepatic tissues. Insulin facilitates the hepatic production of IGF-I by up-regulating GH receptors (GHRs) in the liver and augmenting the IGF-I response to GH. Although GHRs have also been identified in extrahepatic tissues that produce IGF-I, the possibility that IGF-I and insulin might partake in GHR regulation, thereby modulating the effects of GH locally has not received detailed study. The aim of this study was to investigate whether IGF-I and insulin are involved in the local regulation of GHRs, using osteoblasts as a model of GH-responsive extrahepatic tissues. We have used UMR106.06, a well differentiated rat osteoblast-like cell line that expresses GHRs and exhibits a mitogenic response to GH. IGF-I and insulin (0-10 nM) increased cell number and reduced [125I]GH binding in a concentration-dependent manner, with ED50 values of 0.8 and 0.3 nM, respectively. Although IGF-I increased cell number maximally by 36.9 +/- 1.2% (mean +/- SE) above the control value and insulin by 104.8 +/- 5.7% (P < 0.001), they decreased GH binding to 47.0 +/- 9.3% (P < 0.01) and 29.8 +/- 8.7% of the control value (P < 0.001), respectively. Scatchard analysis revealed that the down-regulation of GH binding was attributed to reduced receptor numbers and not binding affinity. The effects of IGF-I and insulin at submaximal concentrations were additive, although the combined effects did not exceed the maximal effect of either growth factor alone. Addition of an anti-IGF-I receptor antibody (alpha IR3) reversed the inhibition of GH binding induced by IGF-I, but not that caused by insulin; similarly, an antiinsulin receptor antibody (29B4) attenuated the inhibitory effect of insulin only. Addition of alpha IR3 alone or an ant-IGF-I antibody (Sm1.2) decreased cell number and increased GH binding in a concentration-dependent mode. GH at 1.5 nM significant increased cell number by 19.3 +/- 2.4% above the control level (P < 0.01), an increase that was reversed by alpha IR3. GH increased GH binding by 32.4 +/- 7.2% (P < 0.05) in cells treated with alpha IR3 to remove the secondary effect of IGF-I. In summary, IGF-I and insulin acted via specific receptors to stimulate cell proliferation and down-regulate GHRs in osteoblasts. GH stimulated cell proliferation, an action mediated by local production of IGF-I, and GH enhanced its own binding. The collective data suggest the presence of a peripheral negative feedback loop that allows IGF-I to limit locally the response of extrahepatic tissues to circulating GH.

Age-related changes in effects of insulin-like growth factor I on human osteoblast-like cells

“…These observations are consistent with IGF-I's promoting osteoblast differentiation. IGF-I has been shown to stimulate proliferation in numerous osteoblast-like model systems [10][11][12][13][14][15][16][17]. The present studies present an apparent paradox in the effect of rhIGF-I on stimulating DNA synthesis and decreasing lag time while slowing the maximal growth rate attained by an osteoblast strain and altering extracellular matrix expression.…”

“…The most frequently reported effect of IGF-I on osteoblast-like cells in itro has been the stimulation of cellular proliferation. Proliferation was increased by IGF-I in MC3T3-E1 cells [10], the human OHS-4 cell line [11], the human TE89 cell line [12], MG63 osteosarcoma cells [13], SAOS-2 cells [14], UMR 106-06 cells [15] and primary fetal rat calvaria cells [16]. IGF-I stimulated, in a dose-and time-dependent manner, the specific activity of creatine kinase (a marker of skeletal cell division) in both female and male rat calvarial bone cells, in ROS 17\2.8 cells and in epiphyseal cartilage cell cultures [17].…”

Expression of growth hormone receptor by peripheral blood lymphocytes in children: evaluation in clinical conditions of impaired growth

“…Our data indicate that the higher expression of GHR is GH receptor expression on human lymphocytes 333 ᭧ 1997 Blackwell Science Ltd, Clinical Endocrinology, 47, 329-335 found in subjects who are growing slowly (Group A and B short children) or not at all (adults). The upregulation of GHR might be induced (Leung et al, 1996) by the lower GH levels or the lowered IGF-1 levels, which have been reported in adults (Zadik et al, 1985) and idiopathic short children (Carlsson et al, 1994). Despite the fact that stimulated GH peaks were above the arbitrary cut-off level of 10 mg/l (20 mU/l) in Group A patients, however, no significant difference in GHR expression was found between these patients and Group B children, in whom peak GH levels were < 10 mg/l (20 mU/l).…”

Regulation of bone mass by growth hormone

“…Curve B demonstrates normal bone mass maintenance, but failure to acquire a normal peak bone mass. osteoblasts and to interfere with GH receptor function (in rats), suggesting a local feedback loop [34,35].…”