Deletion of IGF-I Receptor (IGF-IR) in Primary Osteoblasts Reduces GH-Induced STAT5 Signaling

Abstract

GH promotes longitudinal growth and regulates multiple cellular functions in humans and animals. GH signals by binding to GH receptor (GHR) to activate the tyrosine kinase, Janus kinase 2 (JAK2), and downstream pathways including signal transducer and activator of transcription 5 (STAT5), thereby regulating expression of genes including IGF-I. GH exerts effects both directly and via IGF-I, which signals by activating the IGF-I receptor (IGF-IR). IGF-IR is a cell surface receptor that contains intrinsic tyrosine kinase activity within its intracellular domain. In this study, we examined the potential role of IGF-IR in facilitating GH-induced signal transduction, using mouse primary calvarial osteoblasts with Lox-P sites flanking both IGF-IR alleles. These cells respond to both GH and IGF-I and in vitro infection with an adenovirus that drives expression of Cre recombinase (Ad-Cre) dramatically reduces IGF-IR abundance without affecting the abundance of GHR, JAK2, STAT5, or ERK. Notably, infection with Ad-Cre, but not a control adenovirus, markedly inhibited acute GH-induced STAT5 activity (more than doubling the ED(50) and reducing the maximum activity by nearly 50%), while sparing GH-induced ERK activity, and markedly inhibited GH-induced transactivation of a STAT5-dependent luciferase reporter. The effect of Ad-Cre on GH signaling was specific, as platelet-derived growth factor-induced signaling was unaffected by Ad-Cre-mediated reduction of IGF-IR. Ad-Cre-mediated inhibition of GH signaling was reversed by adenoviral reexpression of IGF-IR, but not by infection with an adenovirus that drives expression of a hemagglutination-tagged somatostatin receptor, which drives expression of the unrelated somatostatin receptor, and Ad-Cre infection of nonfloxed osteoblasts did not affect GH signaling. Notably, infection with an adenovirus encoding a C-terminally truncated IGF-IR that lacks the tyrosine kinase domain partially rescued both acute GH-induced STAT5 activity and GH-induced IGF-I gene expression in cells in which endogenous IGF-IR was reduced. These data, in concert with our earlier findings that GH induces a GHR-JAK2-IGF-IR complex, suggest a novel function for IGF-IR. In addition to its role as a key IGF-I signal transducer, this receptor may directly facilitate acute GH signaling. The implications of these findings are discussed.

Human GH Receptor-IGF-1 Receptor Interaction: Implications for GH Signaling

“…Furthermore, the deletion or silencing of IGF-1R in osteoblasts and ␤-cells desensitizes the cells to GH-induced STAT5 activation and downstream gene expression (30 -32). This diminished GH-induced signaling in osteoblasts was specifically rescued by the reexpression of IGF-1R or an IGF-1R lacking most of its ICD (31) but not by an IGF-1R/insulin receptor (IR) chimera in which the regions of the IGF-1R ECD ␣-chain were replaced by those of the related IR (34). These findings suggest that the GHR-IGF-1R interaction via the ECDs allows augmented GH signaling and that the ICD kinase activity of IGF-1R is not required.…”

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

“…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).…”

The growth hormone receptor mediated oncogenesis

“…shown that deletion of IGF-1R markedly inhibits acute GH-induced STAT5 activity, while sparing GH-induced ERK activity in primary osteoblasts (Gan et al, 2010). Similar results on functional collaboration of GHR and IGF-1R but not insulin receptors were obtained in another study on mouse calvarial cells (Gan et al, 2013).…”

“…Recently it has been shown that both mitogens have a much closer interaction than previously thought. Huang et al have reported that GH can induce a functional complex between GHR, Jak2 and IGF-1R which is at least partially independent of the kinase activity of IGF-1R (and therefore IGF-1) (Gan et al, 2010). In addition by using a Cre-loxP system these authors have…”