Mode of Growth Hormone Action in Osteoblasts

Abstract

Growth hormone (GH) affects bone size and mass in part through stimulating insulin-like growth factor type 1 (IGF-1) production in liver and bone. Whether GH acts independent of IGF-1 in bone remains unclear. To define the mode of GH action in bone, we have used a Cre/loxP system in which the type 1 IGF-1 receptor (Igf1r) has been disrupted specifically in osteoblasts in vitro and in vivo. Calvarial osteoblasts from mice homozygous for the floxed IGF-1R allele (IGF-1R flox/flox ) were infected with adenoviral vectors expressing Cre. Disruption of IGF-1R mRNA (>90%) was accompanied by near elimination of IGF-1R protein but retention of GHR protein. GH-induced STAT5 activation was consistently greater in osteoblasts with an intact IGF-1R. Osteoblasts lacking IGF-1R retained GH-induced ERK and Akt phosphorylation and GH-stimulated IGF-1 and IGFBP-3 mRNA expression. GH-induced osteoblast proliferation was abolished by Cre-mediated disruption of the IGF-1R or co-incubation of cells with an IGF-1-neutralizing antibody. By contrast, GH inhibited apoptosis in osteoblasts lacking the IGF-1R. To examine the effects of GH on osteoblasts in vivo, mice wild type for the IGF-1R treated with GH subcutaneously for 7 days showed a doubling in the number of osteoblasts lining trabecular bone, whereas osteoblast numbers in similarly treated mice lacking the IGF-1R in osteoblasts were not significantly affected. These results indicate that although direct IGF-1R-independent actions of GH on osteoblast apoptosis can be demonstrated in vitro, IGF-1R is required for anabolic effects of GH in osteoblasts in vivo.The process of osteogenesis and remodeling of the skeleton is orchestrated by a constellation of local growth factors, cytokines, and systemic hormones (1, 2), of which growth hormone (GH) 2 and insulin-like growth factor type 1 (IGF-1) are key components. GH belongs to a family of cytokine peptides (3) and is produced and stored by somatotroph cells within the anterior pituitary. GH actions are mediated by binding to the transmembrane GHR, thereby triggering increased association with and activation of Janus kinases (JAKs) (4 -6) to activate signal transducers and activators of transcription (STATs) (7-15), phosphatidylinositol 3-kinase/Akt (16,17),. Growth hormone exerts many, but not all (21), of its effects by stimulation of IGF-1 from liver and peripheral tissues. IGF-1 is a small polypeptide with homology to pro-insulin that is produced by a number of cell types. IGF-1 signals via the type 1 IGF-1 receptor (IGF-1R), engaging ERK and phosphatidylinositol 3-kinase pathways through Src homology 2 domain-containing proteins and insulin receptor substrates-1 and 2 (22, 23). The effects of IGF-1 on bone have been well documented. IGF-1 has been shown to induce proliferation of MC3T3 osteoblast-like cells (24) and is an important survival factor for many mammalian cell types, including osteoblasts. IGF-1 production increases during the initial phases of fetal rat calvarial osteoblast differentiation in vitro and then declines wi...

Autocrine and Paracrine Actions of IGF-I Signaling in Skeletal Development

“…Indeed, GH can exert both IGF-Idependent and IGF-I-independent actions to regulate bone cell and chondrocyte functions. For example, GH could stimulate ERK and Akt phosphorylation and inhibit apoptosis in cultures of osteoblasts lacking IGF-IR expression, but fail to induce cell proliferation in the same cultures (70). Moreover, we (50) showed that GH could stimulate bone formation even in the global IGF-I KO mice, although not to the extent of IGF-I replacement.…”

“…Studies by Gan and colleagues showed that GH can promote the formation of GHR/JAK2/IGF-IR complex to facilitate GHR signaling in osteoblasts (71), indicating a novel complementation between GHR and IGF-IR signaling, which could be a basis for some IGF-IR-dependent GH actions. The fact that OCN IGF-IR KO mice failed to respond to GH treatment to increase bone mass suggests that IGF-IR-dependent actions of GH are the predominant responses to promote skeletal anabolism (70). It, however, remains unclear regarding the sources of IGF-I that respond to GH and promote osteoblast functions in the bone.…”

Osteoblasts in osteoporosis: past, emerging, and future anabolic targets

“…A role of IGF/IGFBP signaling in age-related bone loss is supported by the finding that high levels of IGFBP2 correlate with increased bone turnover in aged men and women (100). Although GH promotes osteoblastic cell proliferation and differentiation (101), its effects on osteoblasts are mainly mediated by IGF1 (102). Genetic models in mice indicate that endogenous IGF1 increases bone formation and bone mass, which is more significant at the cortical than at the trabecular bone level (103,104).…”

Promotion of Osteoblast Differentiation in Mesenchymal Cells Through Cbl-Mediated Control of STAT5 Activity

“…Growth hormone (GH)/IGF-1 signaling is known to play a major role in osteoblastogenesis and bone formation [53,57,58]. The actions of GH in osteoblasts are in part mediated by STAT5 activation which upregulates IGF-1 expression [51,59]. IGF-1 signaling in turn activates RUNX2 activity and promotes osteoblast differentiation in MSC [60][61][62].…”