gms | German Medical Science

Objectives: Nuclear factor erythroid 2-related factor 2 (Nrf2) is a crucial transcription factor to maintain cellular redox homeostasis, but is also affecting bone metabolism. However, the association between Nrf2 and osteoporotic bone loss in elderly females is incompletely understood. Our aim was to elucidate a potential age-dependent contribution of Nrf2 to female osteoporosis using a murine model.

Methods: Eighteen female wild type (WT) and 12 Nrf2-knockout (KO) mice were sacrificed at different ages (i.e., 12 weeks, "young mature adult", and 90 weeks, "old") to analyze their femurs. The morphological properties of cortical and trabecular femoral bones were determined by micro-computed tomography (µCT) imaging and were compared to gold standard histochemistry. Quasi-static compression tests were performed to calculate the mechanical properties of bones from different groups (i.e., WT and KO animals). We further used digital image correlation (DIC) to analyze the full-field strain distribution between the two groups of bones and compared them with numerical models obtained from voxel-based finite element analyses. Additionally, the population of bone resorbing cells and aromatase expression by osteocytes was immunohistochemically evaluated between both groups. Furthermore, empty osteocyte lacunae were counted in the cortex. The µCT, histopathology, and mechanical properties were compared using Wilcoxon rank sum test. Statistical significance was set at p<0.05.

Results and conclusion: Old Nrf2-KO mice revealed a significantly reduced trabecular bone mineral density (BMD), cortical thickness (Ct.Th), cortical area (Ct.Ar), and cortical bone fraction (Ct.Ar/Tt.Ar) as compared to old WT mice. In contrast, in skeletally mature young adult mice there was no such difference between WT and KO mice. Specifically, while all old WT mice showed thin metaphyseal trabeculae, trabecular bone was completely absent in 60% of old KO mice. Additionally, old KO mice showed significantly higher numbers of osteoclast-like cells and fewer aromatase-positive osteocytes when compared to age-matched WT mice. In contrast, the occurrence of empty osteocyte lacunae did not differ between both groups. Female Nrf2-KO mice further showed age-dependently reduced fracture resilience when compared to age-matched WT mice.

Biomechanical, histological, and µCT analyses together confirmed lower bone quantity and quality as well as a higher number of bone resorbing cells in old female Nrf2-KO mice, when compared to WT controls. Additionally, aromatase expression in osteocytes of old Nrf2-KO mice hints towards a lack of estrogen, which suggest that chronic Nrf2 loss can lead to age-dependent progression of female osteoporosis.