Mechanical Strain and Estrogen Activate Estrogen Receptor α in Bone Cells

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Abstract
Bone cells' early responses to estrogen and mechanical strain were investigated in the ROS 17/2.8 cell line. Immunoblotting with antiphosphorylated estrogen receptor α (ER-α) antibody showed that when these cells were exposed for 10 minutes to estrogen (10−8 M) or a single period of cyclic dynamic strain (peak 3400 μϵ, 1Hz, 600 cycles), there was an increase in the intensity of a 66-kDa band, indicating phosphorylation of ser122 in the amino terminus of ER-α. Increased phosphorylation was detected within 5 minutes of exposure to estrogen and 5 minutes after the end of the period of strain. Estrogen and strain also activated the mitogen-activated protein kinase (MAPK) family member extracellular regulated kinase-1 (ERK-1). Increases in ERK activation coincided with increased ER-α phosphorylation. Activation of ERK-1 and the phosphorylation of ER-α, by both estrogen and strain, were prevented by the MAP kinase kinase (MEK) inhibitor U0126 and the protein kinase A (PKA) inhibitor (PKI). These data support previous suggestions that resident bone cells' early responses to strain and estrogen share a common pathway, which involves ER-α. This pathway also appears to involve PKA and ERK-mediated phosphorylation of ser122 within the amino terminus of ER-α. Reduced availability of this pathway when estrogen levels are reduced could explain diminished effectiveness of mechanically related control of bone architecture after the menopause.

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