20. Investigating the Pathological Mechanisms and Stem Cell Changes Associated With Early-onset Osteoporosis in a Transfemale Mice Model

PURPOSE: Clinical observations have revealed a significant incidence of osteopenia or osteoporosis in a considerable percentage of transwomen despite undergoing regular postoperative hormone replacement therapy with estrogen (E2). The leptin receptor-mesenchymal stem cell (Lepr-MSC) plays a crucial role in determining the fate of osteogenesis and adipogenesis, with a high percentage of the Lepr-MSC cluster expressing abundant androgen receptors. Our study aimed to investigate changes in bone density in a transfemale mouse model receiving PHRT and to explore alterations in the Lepr-MSC cluster. METHODS: Twelve-week-old C57BL/6 mice were divided into four groups: the sham group, the OP group (bilateral orchiectomy), the OP + low E2 group (bilateral orchiectomy + subcutaneous E2 at 0.2 µg/g twice a week), and the OP + high E2 group (bilateral orchiectomy + subcutaneous E2 at 0.6 µg/g twice a week). After 30 days of E2 treatment, the mice were sacrificed, and their femur and tibia marrow plugs were collected for microCT and flow cytometry analysis. Additionally, the male osteosarcoma cell line MG63 was used to assess the impact of E2 on calcification in vitro. RESULTS: The OP group showed a significant reduction in bone volume, percent bone volume, trabecular thickness, and trabecular bone number. E2 treatment, at both low and high doses, only marginally improved these bone structures compared to the OP group. In vitro studies indicated that E2 treatment tends to enhance calcium deposition in cultured cells. Additionally, there was a notable decrease in Lepr-MSCs in the OP group, with a further decline observed in both the OP + low E2 and OP + high E2 groups. CONCLUSIONS: The results demonstrated that a high dose of E2 treatment may offer limited benefit in reducing trabecular separation compared to the OP group, but it remained significantly different from the sham group. Additionally, E2 treatment showed no benefit in increasing the amount of LepR-MSCs.