A Novel Ligand-independent Function of the Estrogen Receptor Is Essential for Osteocyte and Osteoblast Mechanotransduction

Abstract

Bone senses and adapts to meet mechanical needs by means of an extensive mechanotransduction network comprising osteocytes (former osteoblasts entrapped in mineral) and their cytoplasmic projections through which osteocytes communicate with osteoblasts and osteoclasts on the bone surface. Mechanical stimulation promotes osteocyte (and osteoblast) survival by activating the extracellular signal-regulated kinases, ERKs. Estrogens have similar effects and, intriguingly, the adaptive response of bone to mechanical forces is defective in mice lacking estrogen receptor (ER) ␣ or ER␤. We report that ERKs are not activated by stretching in osteocytic and osteoblastic cells in which both ER␣ and ER␤ have been knocked out or knocked down and this is reversed partially by transfection of either one of the two human ERs and fully by transfection of both receptors. ERK activation in response to stretching is also recovered by transfecting the ligand-binding domain (E) of either receptor or an ER␣ mutant that does not bind estrogens. Furthermore, mechano-responsiveness is restored by transfecting the E␣ targeted to the plasma membrane, but not to the nucleus, whereas ER␣ mutants with impaired plasma membrane localization or binding to caveolin-1 fail to confer ERK activation in response to stretching. Lastly, the ER antagonist ICI 182,780 abrogates ERK activation and the anti-apoptotic effect of mechanical stimulation. We conclude that in addition to their role as ligand-dependent mediators of the effects of estrogens, the ERs participate in the transduction of mechanical forces into pro-survival signaling in bone cells, albeit in a ligand-independent manner.That the skeleton adapts to meet mechanical needs was first recognized by Wolff (1) and later expanded by Frost (2) in the mechanostat hypothesis. Bone adjusts to load by changing its mass, shape, or microarchitecture (3, 4), and it responds differently depending on the magnitude of strain. Whereas insufficient or excessive levels of strain induce bone resorption, physiological levels of strain maintain bone mass (5). Osteocytes (former osteoblasts buried in the mineral) are thought to be the cells acting as mechanosensors. Osteoblasts and osteoclasts, the executive cells for bone formation and resorption, are present on bone for relatively short periods and occur in low number and only in locations that undergo remodeling at a given time point, which represent ϳ10% of the bone surface. On the other hand, osteocytes are by far the most abundant resident cells and are present throughout the entire bone tissue. Importantly, osteocytes are the core of a functional syncytium that extends from the mineralized bone matrix to the bone surface and the bone marrow, all the way to the blood vessels. This strategic location permits the detection of variations in the level of strain as well as the dispersion of the signals leading to adaptive responses. Through such network, osteocytes might continually compare present mechanical strains to usual levels of strain (the "set point" of ...

Crosstalk between Caveolin-1/Extracellular Signal-regulated Kinase (ERK) and β-Catenin Survival Pathways in Osteocyte Mechanotransduction

“…Consistent with this earlier work, we showed previously that caveolin-1 physically interacts with integrin ␤1 and ERKs in osteocytic cells and that pharmacologic disruption of caveolae abolishes ERK activation and the antiapoptotic effect of mechanical stimulation (24). Moreover, we found that the caveolin-interacting domain of estrogen receptor ␣ is required for these mechanoresponses (40). Our current findings suggest that caveolin-1 also interacts with ␤-catenin in osteocytic cells, as expression of caveolin-1 is required for ␤-catenin membrane localization upon mechanical stimulation.…”

Different Roles of Estrogen Receptors α and β in the Regulation of E-Cadherin Protein Levels in a Mouse Mammary Epithelial Cell Line

“…46). ERs colocalize with caveolin-1 and are necessary for ligand-independent extracellular signal-regulated kinase activation in osteocytic and osteoblastic cells (47). However, it remains to be established if physical association of ERs to caveolae occurs in HC11 cells.…”

Sex hormones and their receptors in bone homeostasis: insights from genetically modified mouse models

“…Recently, membrane-dependent estrogen responses through G-protein-coupled receptor 30 were shown to be necessary for E2-stimulated insulin release in both female and male mice [28] and for normal estrogenic responses on the growth plate and thus bone growth but not on other bone responses [29]. Finally, it appears that the effects of mechanical loading and strain on bone may also involve ER signaling, perhaps via phosphorylation of the ERs, resulting in activation of the receptor (and upregulation of the activity of EREs) even in the absence of estrogen [30][31][32]. Mice in whom the only ERα present could not directly bind DNA (nonclassical ER knockin or NERKI) but could activate nonclassical signaling pathways were generated [33].…”