The role of endogenous glucocorticoids in bone remodeling

To elucidate the role of endogenous glucocorticoid signaling in bone, we previously developed Col2.3-HSD2 and Col3.6-HSD2 transgenic mice in which a 2.3-kb or 3.6-kb Colla1 promoter fragment drives expression of 11β-hydroxysteroid dehydrogenase type 2 (HSD2) in mature and early osteoblasts, respectively. In the first study, we first characterized the bone phenotype of Col3.6-HSD2 mice. Col3.6-HSD2 mice had decreased trabecular bone in vertebra and decreased cortical bone in femur and tibia. Transgenic calvarial osteoblast and bone marrow stromal cultures had decreased alkaline phosphatase and mineral staining, and reduced Colla1, bone sialoprotein and osteocalcin mRNA expression. Cell growth and proliferation were decreased in transgenic cultures. Transgenic bone marrow cells showed more osteoclast formation in vitro. However, osteoclast resorptive activity was decreased in vitro and in vivo. Microarray analysis showed that multiple signaling pathways were affected in transgenic osteoblasts including cell cycle and integrin. These data suggest that endogenous glucocorticoid signaling is required for optimal bone mass acquisition by affecting both the osteoblast and osteoclast lineages.^ Second, we studied whether murine osteoclast lineage cells express 11β-hydroxysteroid dehydrogenase type 1 (HSD1). Osteoclast precursors from bone marrow macrophages (BMMs), RAW264.7 cells, and sorted precursors based on surface markers (CD11b- CD45R- CD3-CD115+) were treated with M-CSF and RANKL to induce osteoclast formation. HSD1 was expressed in progenitor cells and significantly reduced in osteoclasts, while HSD2 expression was not changed. When the progenitor cells were treated with M-CSF or up to 4 days, M-CSF caused a time- and dose-dependent reduction of HSD1 mRNA, with the greatest reduction after only one day treatment. To study the function of HSD1 in osteoclast formation, BMMs were treated with M-CSF and RANKL in the presence of 11-DHC and its active metabolite corticosterone for up to 5 days. Both steroids caused a dose- and time-dependent suppression of osteoclast formation. The major inhibition of osteoclastogenesis occurred at the early stage of osteoclast formation, and the inhibition was blunted by pre-treatment with the HSD inhibitor carbenoxolone (CBX). These data show that osteoclast precursors express HSD1 as a means of generating locally active glucocorticoids and this system declines during osteoclast formation in vitro.