title: 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.
authors.affiliation: Garvan Institute of Medical Research, St. Vincent's Hospital, Sydney, New South Wales, Australia.
authors.forename: K
authors.initials: K
authors.lastname: Leung
authors.forename: I A
authors.initials: IA
authors.lastname: Rajkovic
authors.forename: E
authors.initials: E
authors.lastname: Peters
authors.forename: I
authors.initials: I
authors.lastname: Markus
authors.forename: J J
authors.initials: JJ
authors.lastname: Van Wyk
authors.forename: K K
authors.initials: KK
authors.lastname: Ho
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D000906:Antibodies / Q000494:pharmacology; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D015536:Down-Regulation; D004347:Drug Interactions; D005246:Feedback; D013006:Growth Hormone / Q000378:metabolism / Q000502:physiology; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology* / Q000502:physiology; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D008954:Models, Biological; D010006:Osteoblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017526:Receptor, IGF Type 1 / Q000276:immunology / Q000502:physiology; D011986:Receptors, Somatotropin / Q000096:biosynthesis* / Q000187:drug effects / Q000276:immunology; D011994:Recombinant Proteins / Q000494:pharmacology
chemical_list: D000906:Antibodies; D007328:Insulin; D011986:Receptors, Somatotropin; D011994:Recombinant Proteins; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D017526:Receptor, IGF Type 1