Effects of Chronic Estrogen Treatment on Modulating Age-Related Bone Loss in Female Mice

While female mice do not have the equivalent of a menopause they do undergo reproductive senescence Thus to dissociate the effects of aging versus estrogen deficiency on age-related bone loss we sham operated ovariectomized or ovariectomized and estrogen replaced female C57/BL6 mice at 6 months of age and followed them to age 18 to 22 months Lumbar spines and femurs were excised for analysis and bone marrow hematopoietic lineage negative (lin-) cells (enriched for osteoprogenitor cells) were isolated for gene expression studies Six-month-old intact control mice were euthanized to define baseline parameters Compared with young mice aged/sham-operated mice had a 42% reduction in lumbar spine bone volume/total volume (BV/TV) and maintaining constant estrogen levels over life in ovariectomized/estrogen treated mice did not prevent age-related trabecular bone loss at this site By contrast lifelong estrogen treatment of ovariectomized mice completely prevented the age related reduction in cortical volumetric bone mineral density (vBMD) and thickness at the tibial diaphysis present in the aged/sham operated mice As compared with cells from young mice lin- cells from aged/sham-operated mice expressed significantly higher mRNA levels for osteoblast differentiation and proliferation marker genes These data thus demonstrate that in mice age-related loss of cortical bone in the appendicular skeleton but not loss of trabecular bone in the spine can be prevented by maintaining constant estrogen levels over life The observed increase in osteoblastic differentiation and proliferation marker gene expression in progenitor bone marrow cells from aged versus young mice may represent a compensatory mechanism in response to ongoing bone loss (C) 2010 American Society for Bone and Mineral Research