Estrogen actions in the growth plate cartilage

Abstract: Estrogens may influence bone growth locally or systemically via the estrogen receptors alpha (ERα), beta (ERβ) and G protein-coupled estrogen receptor 1 (GPER-1). In Paper I, our study showed that the treatment of ovariectomized C57BL/6 mice with a selective ERα agonist 1,3,5-tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT) reduced growth plate height and hypertrophic zone height suggesting that the effect was induced via ERα. Furthermore, chondrocyte proliferation in the growth plate was also inhibited by 17β-estradiol (E2) or PPT as evaluated by proliferating cell nuclear antigen (PCNA) staining. Furthermore, tibiae and femur bones were shorter in E2- or PPT-treated mice when compared to vehicle-treated controls. In contrast, bone lengths in mice treated with a selective ERβ agonist 2,3-bis(4- hydroxyphenyl)-propionitrile (DPN) were similar to bone lengths in controls. These results showed that estrogenic effects on bone growth and growth plate maturation are mainly mediated via ERα. In contrast, the selective GPER-1 agonist G1 had no effects on either metatarsal bone growth ex vivo or tibia and femur growth in treated metatarsals or mice when compared to control groups. Thus, the results from Paper I and II demonstrated that, ligand stimulation of GPER-1 and ERβ does not influence bone growth in mice. In Paper III, target genes and signaling pathways affected by E2 were identified. The enriched pathways inhibited by E2 included estrogen response early and late, apoptosis, epithelial mesenchymal transition and angiogenesis. Also, the mammalian target of rapamycin (mTOR) signaling pathway, which regulates chondrocyte proliferation and differentiation, was significantly inhibited by E2. Among the most strongly affected genes, the expression of peptide YY, a negative regulator of bone formation and mineral density, was inhibited by E2 treatment. Furthermore, epidermal growth factor and oxidative phosphorylation signalling pathways and subgroups of genes regulated by Myc and genes important for mitotic spindle assembly were among the enriched pathways upregulated by E2. Our data showed that E2 actions on bone growth and growth plate maturation are mainly mediated via ERα. In contrast, ligand stimulation of either ERβ or GPER-1 did not influence bone growth in mice. Also, our study has identified target genes and pathways influenced by E2 in the growth plate. Further studies are required to determine the specific mechanisms involving E2-regulated genes. Our findings may have direct implications for the development of new and more selective treatment modalities of extreme tall stature using selective ER modulators that may have fewer side effects than high-dose E2 treatment.