Estrogen receptor α in osteocytes regulates trabecular bone formation in female mice.

Estrogens are well known steroid hormones necessary to maintain bone health. In addition, mechanical loading, in which estrogen signaling may intersect with the Wnt/ß-catenin pathway, is essential for bone maintenance. As osteocytes are known as the major mechanosensory cells embedded in mineralized bone matrix, osteocyte ERα deletion mice (ERα(ΔOcy/ΔOcy)) were generated by mating ERα floxed mice with Dmp1-Cre mice to determine the role of ERα in osteocytes. Trabecular bone mineral density of female, but not male ERα(ΔOcy/ΔOcy) mice was significantly decreased. Bone formation parameters in ERα(ΔOcy/ΔOcy) were significantly decreased while osteoclast parameters were unchanged. This suggests that ERα in osteocytes exerts osteoprotective function by positively controlling bone formation. To identify potential targets of ERα, gene array analysis of Dmp1-GFP osteocytes sorted by FACS from ERα(ΔOcy/ΔOcy) and control mice was performed. Gene expression microarray followed by gene ontology analyses revealed that osteocytes from ERα(ΔOcy/ΔOcy) highly expressed genes categorized in 'Secreted' when compared to control osteocytes. Among them, expression of Mdk and Sostdc1, both of which are Wnt inhibitors, was significantly increased without alteration of expression of the mature osteocyte markers such as Sost and ß-catenin. Moreover, hindlimb suspension experiments showed that trabecular bone loss due to unloading was greater in ERα(ΔOcy/ΔOcy) mice without cortical bone loss. These data suggest that ERα in osteocytes has osteoprotective functions in trabecular bone formation through regulating expression of Wnt antagonists, but conversely plays a negative role in cortical bone loss due to unloading.

Vitamin D receptor in osteoblasts is a negative regulator of bone mass control.

The physiological and beneficial actions of vitamin D in bone health have been experimentally and clinically proven in mammals. The active form of vitamin D [1α,25(OH)(2)D(3)] binds and activates its specific nuclear receptor, the vitamin D receptor (VDR). Activated VDR prevents the release of calcium from its storage in bone to serum by stimulating intestinal calcium absorption and renal reabsorption. However, the direct action of VDR in bone tissue is poorly understood because serum Ca(2+) homeostasis is maintained through tightly regulated ion transport by the kidney, intestine, and bone. In addition, conventional genetic approaches using VDR knockout (VDR-KO, VDR(-/-)) mice could not identify VDR action in bone because of the animals' systemic defects in calcium metabolism. In this study, we report that systemic VDR heterozygous KO (VDR(+/L-)) mice generated with the Cre/loxP system as well as conventional VDR heterozygotes (VDR(+/-)) showed increased bone mass in radiological assessments. Because mineral metabolism parameters were unaltered in both types of mice, these bone phenotypes imply that skeletal VDR plays a role in bone mass regulation. To confirm this assumption, osteoblast-specific VDR-KO (VDR(ΔOb/ΔOb)) mice were generated with 2.3 kb α1(I)-collagen promoter-Cre transgenic mice. They showed a bone mass increase without any dysregulation of mineral metabolism. Although bone formation parameters were not affected in bone histomorphometry, bone resorption was obviously reduced in VDR(ΔOb/ΔOb) mice because of decreased expression of receptor activator of nuclear factor kappa-B ligand (an essential molecule in osteoclastogenesis) in VDR(ΔOb/ΔOb) osteoblasts. These findings establish that VDR in osteoblasts is a negative regulator of bone mass control.