Characterization of estrogen receptor alpha in mouse osteoblasts

Abstract

Although they are mostly involved in coordinating the reproductive function estrogens also affect the physiology of non-reproductive tissues such as bone. As a result they impact on the function of the two cell types operating bone homeostasis, osteoblasts and osteoclasts. Estrogens effects are entirely mediated by two nuclear receptors, Estrogen Receptor (ER) alpha and ER beta. ERs are ligand induced transcription factors, which upon estrogen binding activate the transcription target genes, thereby affecting cell fate. Osteoblasts express ER alpha and are believed to be the direct target of estrogens, mediating most of the effects estrogens have on bone. We therefore sought to characterize ER alpha in osteoblasts. Using mouse primary osteoblasts and mesenchymal cell lines we found that ER alpha is expressed in osteoblasts and that it can mediate transcriptional activation. The expression level of ER alpha is however very low as compared to reproductive tissues. Surprisingly, ER alpha in oteoblasts has a high residual transcriptional activity. This basal activity rather than being genuinely ligand-independent was found to depend on residual estrogens found in the growth medium. ER alpha expression increases during differentiation of mouse osteoblasts. This was thought to reflect the specificity of ER alpha expression in osteoblasts. However we also show that ER alpha expression is not specific to osteoblasts as differentiation of a mesenchymal cell line into myoblasts also increases the levels of ER alpha expression. Finally, Estrogens signalling was found not to influence osteoblast differentiation. Although they confirm the presence of a functional ER in osteoblasts, these results challenge the view that osteoblasts can act as direct mediators of estrogen actions on bone.