Skeletal impact of 17β-estradiol in T cell-deficient mice: age-dependent bone effects and osteosarcoma formation.

Estrogen (E ₂ )-dependent ER+ breast cancer, the most common breast cancer subtype, is also the most likely to metastasize to bone and form osteolytic lesions. However, ER+ breast cancer bone metastasis human xenograft models in nude mice are rarely studied due to complexities associated with distinguishing possible tumoral vs. bone microenvironmental effects of E ₂ . To address this knowledge gap, we systematically examined bone effects of E ₂ in developing young (4-week-old) vs. skeletally mature (15-week-old) female Foxn1 ^nu nude mice supplemented with commercial 60-day slow-release E ₂ pellets and doses commonly used for ER+ xenograft models. E ₂ pellets (0.05-0.72 mg) were implanted subcutaneously and longitudinal changes in hind limb bones (vs. age-matched controls) were determined over 6 weeks by dual-energy X-ray absorptiometry (DXA), microCT, radiographic imaging, and histology, concurrent with assessment of serum levels of E ₂ and bone turnover markers. All E ₂ doses tested induced significant and identical increases in bone density (BMD) and volume (BV/TV) in 4-week-old mice with high bone turnover, increasing bone mineral content (BMC) while suppressing increases in bone area (BA). E ₂ supplementation, which caused dose-dependent changes in circulating E ₂ that were not sustained, also led to more modest increases in BMD and BV/TV in skeletally mature 15-week-old mice. Notably, E ₂ -supplementation induced osteolytic osteosarcomas in a subset of mice independent of age. These results demonstrate that bone effects of E ₂ supplementation should be accounted for when assessing ER+ human xenograft bone metastases models.