Stratified control of IGF-I expression by hypoxia and stress hormones in osteoblasts

Abstract

Bone cells respond to the integrated effects of local and systemic regulation. Here we show that hypoxia and the stress hormones PGE2 and glucocorticoid interact in complex ways in osteoblasts, converging on insulin like growth factor I (IGF-I) expression. Whereas hypoxia alone rapidly increased transcription factor HIF activity, it suppressed DNA synthesis, had no significant effects on protein synthesis or alkaline phosphatase activity, and drove discrete changes in a panel of osteoblast mRNAs. Notably, hypoxia increased expression of the acute phase response transcription factors C/EBPδ which can induce IGF-I in response to PGE2, but conversely prevented the stimulatory effect of PGE2 on IGF-I mRNA. However, unlike its effect on C/EBPδ, hypoxia suppressed expression of the obligate osteoblast transcription factor Runx2, which can activate an upstream response element in the IGF-I gene promoter. Hypoxic inhibition of IGF-I and Runx2 were enforced by glucocorticoid, and continued with prolonged exposure. Our studies thus reveal that IGF-I expression is stratified by two critical transcriptional elements in osteoblasts, which are resolved by the individual and combined effects of hypoxic stress and stress hormones. In so doing, hypoxia suppresses Runx2, limits the enhancing influence of PGE2, and interacts with glucocorticoid to reduce IGF-I expression by osteoblasts.

Skeletal effects of growth hormone and insulin-like growth factor-I therapy

“…These changes are associated with a fall in production of both IGF-I and stimulatory IGF binding proteins (Canalis, 2005; Cheng et al, 1998; Chevalley et al, 1996) as well as decreased basal and IGF-induced chondrocyte proliferation (Jux et al, 1998). Another stress hormone, prostaglandin E2 (PGE2), can increase IGF-I production via increased cAMP synthesis (Thomas et al, 1996), while hypoxic stress can interact with stress hormones to ultimately reduce IGF-I expression (McCarthy et al, 2014). …”

Role of Alternatively Spliced Messenger RNA (mRNA) Isoforms of the Insulin-Like Growth Factor 1 (IGF1) in Selected Human Tumors

“…In both IGF1 promoters, multiple specificity protein 1 (SP1) binding elements are present, which indicates that SP1 can be an important IGF1 transcription regulator [ 48 ]. Other study revealed that IGF1 promoter and transcription factor Runx2, activating an upstream response element involved in IGF1 promoter activity, were both reduced by prolonged hypoxia in cortisol treated and prostaglandin E2 (PGE 2 )-stimulated rat osteoblasts [ 71 ].…”

“…GH-IGF1 axis is important in the control of linear growth, as an anabolic factor for skeletal development and bone function [ 100 , 113 , 114 , 115 , 116 , 117 ]. IGF1 signaling is required in normal osteoblasts, especially during bone formation and remodeling [ 71 , 78 , 118 ].…”

Androgen receptor activation integrates complex transcriptional effects in osteoblasts, involving the growth factors TGF-β and IGF-I, and transcription factor C/EBPδ

“…By contrast to Runx2, PKA activated C/EBPδ activity is suppressed when it associates with 17β-E bound ERα, thereby restraining C/EBPδ in the cytoplasm and limiting its gene activation potential (Chang et al, 2005). Moreover, the IGF-I gene promoter also contains Runx and Smad transcription factor binding sites further upstream, which may act in concert with the downstream C/EBP binding site, especially in cells where hormone and growth factor activations have occurred (McCarthy et al, 2014). Even so, both Runx2 and C/EBPδ suppress the direct effects of ligand bound ERα by association it its own DNA binding domain (McCarthy, 2003;Centrella and McCarthy, 2012).…”

“…The sequences of the probes are as previously reported ((McCarthy and Centrella, 2010;McCarthy et al, 2014). Three µg of nuclear protein, pre-incubated with 2 µg of poly(dI:dC), were supplemented with 5 X 10 4 cpm of probe (0.2 ng), and fractionated through a 5% nondenaturing A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT polyacrylamide gel.…”