Mechanical strain activates estrogen response elements in bone cells

The involvement of the estrogen receptor in the early responses of bone cells to mechanical strain was investigated by subjecting subconfluent monolayer cultures of ROS.SMER #14 cells (ROS 17/2.8 cells stably transfected with additional ERα) to 17β-estradiol or a single short period of dynamic mechanical strain (600 cycles, 1 Hz). The basal proliferation rate of ROS.SMER #14 cells was similar to ROS 17/2.8 cells, whose proliferative responsiveness to strain and estrogen is similar to that of primary cultures of rat long bone-derived osteoblasts. At peak strains of 3400 μ , strain-related proliferation in ROS.SMER #14 cells was 1.4 times that of ROS 17/2.8 cells. At 10−8 mol/L, 17β-estradiol-related proliferation was nearly twice greater. The ROS.SMER #14 cells were transiently transfected with an estrogen-responsive reporter, 2ERE-pS2-CAT, containing two consensus estrogen response elements (ERE) linked to a chloroamphenicol acetyl transferase gene. Strain increased normalized ERE-CAT activity threefold and estradiol (10−8 mol/L) sixfold. Both strain-related and estradiol-related increases in proliferation and ERE-CAT activity were blocked by the estrogen antagonist ICI 182,780 (10−6 mol/L). These data show that strain as well as estrogen stimulates increased proliferation in ROS 17/2.8 cells and increased ERα-related ERE activity in ROS cells transfected with ERα. Proliferation is greater in the cells with more estrogen receptors. Both strain- and estrogen-related proliferation and ERE activity are blocked by the estrogen antagonist ICI 182,780. This indicates that ROS cells’ early responses to mechanical strain involve ERα and estrogen-responsive genes.