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 (αIR3) 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 αIR3 alone or an anti-IGF-I antibody (Sm1.2) decreased cell number and increased GH binding in a concentration-dependent mode. GH at 1.5 nM significantly increased cell number by 19.3 ± 2.4% above the control level (P < 0.01), an increase that was reversed by αIR3. GH increased GH binding by 32.4 ± 7.2% (P < 0.05) in cells treated with αIR3 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.