17 beta-estradiol potently suppresses cAMP-induced insulin-like growth factor-I gene activation in primary rat osteoblast cultures

Yale University School of Medicine, Section of Plastic Surgery, New Haven, Connecticut 06520-8041, USA.
Journal of Biological Chemistry (Impact Factor: 4.6). 08/1997; 272(29):18132-9. DOI: 10.1074/jbc.272.29.18132
Source: PubMed

ABSTRACT Insulin-like growth factor-I (IGF-I) is a key factor in bone remodeling. In osteoblasts, IGF-I synthesis is enhanced by parathyroid hormone and prostaglandin E2 (PGE2) through cAMP-activated protein kinase. In rats, estrogen loss after ovariectomy leads to a rise in serum IGF-I and an increase in bone remodeling, both of which are reversed by estrogen treatment. To examine estrogen-dependent regulation of IGF-I expression at the molecular level, primary fetal rat osteoblasts were co-transfected with the estrogen receptor (hER, to ensure active ER expression), and luciferase reporter plasmids controlled by promoter 1 of the rat IGF-I gene (IGF-I P1), used exclusively in these cells. As reported, 1 microM PGE2 increased IGF-I P1 activity by 5-fold. 17beta-Estradiol alone had no effect, but dose-dependently suppressed the stimulatory effect of PGE2 by up to 90% (ED50 approximately 0.1 nM). This occurred within 3 h, persisted for at least 16 h, required ER, and appeared specific, since 17alpha-estradiol was 100-300-fold less effective. By contrast, 17beta-estradiol stimulated estrogen response element (ERE)-dependent reporter expression by up to 10-fold. 17beta-Estradiol also suppressed an IGF-I P1 construct retaining only minimal promoter sequence required for cAMP-dependent gene activation, but did not affect the 60-fold increase in cAMP induced by PGE2. There is no consensus ERE in rat IGF-I P1, suggesting novel downstream interactions in the cAMP pathway that normally enhances IGF-I expression in skeletal cells. To explore this, nuclear extract from osteoblasts expressing hER were examined by electrophoretic mobility shift assay using the atypical cAMP response element in IGF-I P1. Estrogen alone did not cause DNA-protein binding, while PGE2 induced a characteristic gel shift complex. Co-treatment with both hormones caused a gel shift greatly diminished in intensity, consistent with their combined effects on IGF-I promoter activity. Nonetheless, hER did not bind IGF-I cAMP response element or any adjacent sequences. These results provide new molecular evidence that estrogen may temper the biological effects of hormones acting through cAMP to regulate skeletal IGF-I expression and activity.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Estrogen receptor (ER) activation has complex effects on bone cells, and loss of circulating estradiol adversely affects skeletal status in women. Hormone replacement therapy effectively circumvents bone loss after menopause, but enhances disease risk in other tissues. Here we show that prostaglandin E2 (PGE2) augments the activity of an osteoblast-derived selective ER modulator, ObSERM. The stimulatory effect of PGE2 is replicated in part by either the PG receptor EP3 agonist 17-phenyl trinor PGE2 or by the PG receptor FP agonist PGF2α. Whereas activation of the various PG receptors induces multiple downstream signals, the response to PGE2 was mimicked by activators of protein kinase C, and suppressed by inhibition of protein kinase C but not by inhibition of protein kinase A. Moreover, inhibition of nitric oxide synthesis and activation of the PTH and Wnt pathways increases ObSERM activity. Our studies therefore reveal that ObSERM activity is controlled in distinct ways and revise our understanding of ER activation within bone by agents or events associated with PG expression. They also predict ways to sustain or improve bone formation, fracture repair, and surgical healing without adding the risk of disease in other tissues where ER activation also has important biological functions. J. Cell. Physiol. © 2014 Wiley Periodicals, Inc.
    Journal of Cellular Physiology 10/2014; 230(5). DOI:10.1002/jcp.24842 · 3.87 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    Gene 01/2014; DOI:10.1016/j.gene.2014.01.011 · 2.08 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The effects of 24-months of recreational artistic gymnastics participation on plasma insulin-like growth factor-I (IGF-I), insulin-like growth factor binding protein-3 (IGFBP-3) and bone were assessed in prepubertal females, four to eight years of age, with no history of sports participation. Over 24-months, log IGF-I, IGFBP-3 and IGF-I/IGFBP-3 increased in gymnasts (n=48) and controls (n=66). When gymnasts and controls were combined, the 24-month log IGF-I and total body bone area, bone mineral content (BMC) and areal bone mineral density (aBMD), lumbar spine BMC and aBMD, femur BMC and aBMD, narrow neck cross sectional area (CSA), intertrochanteric CSA and section modulus (Z) and shaft Z were correlated. Negative correlations were observed between hours of gymnastics participation and the 24-month gain in log IGF-I, IGFBP-3 and IGF-I/IFGBP-3. The lack of a marked IGF response and the small bone response to participation in recreational artistic gymnastics in the current project may be partially related to the young age of participants.