Cyclooxygenase-2 selective inhibition suppresses restoration of tibial trabecular bone formation in association with restriction of osteoblast maturation in skeletal reloading after hindlimb elevation of mice

To clarify the role of cyclooxygenase-2 (COX-2) in acute recovery of trabecular bone in reloaded hindlimbs of tail-suspended mice, we administered a COX-2 selective inhibitor in the mice during the reloading period after unloading. Experiments were conducted on 140 male C57BL/6J mice (8 weeks old). They were divided into ground control (GC) and unloading by tail suspension (UL) groups. On day 7, Group GC was divided into Groups GC + Vehicle (Veh) and GC + Celecoxib (Cel), while Group UL mice were fed on the ground [reloading (RL)] after 7-day unloading and were then divided into Groups RL + Veh and RL + Cel. Bone histomorphometry, osteogenic cell development, and mRNA expression of osteogenic molecules were assessed. At 5 days after reloading, the increase of bone formation rate and the ratio of osteocalcin mRNA expression per CFU-f colony in Group RL + Cel were significantly decreased compared with those in Group RL + Veh, while alkaline phosphatase-positive (ALP+) CFU-f formation and the ratios of cbfa-1, osterix, and type 1 collagen mRNA expression per CFU-f colony increased to the same levels in both RL groups. At 14 days after reloading, decreased bone volume by unloading in RL + Veh recovered to the same level as that of GC + Veh, but that in RL + Cel did not recover completely. The increase of c-fos mRNA expression in bone marrow cells at 1, 24, and 48 h after reloading, osteocalcin mRNA at 6 h, and osterix mRNA at 24 h were suppressed by COX-2 inhibitor. These data indicate that the COX-2 selective inhibitor celecoxib suppresses the restoration of tibial trabecular bone formation and the acute recovery of trabecular bone. These actions are closely related to restriction of c-fos and osteocalcin mRNA expressions and osteoblast differentiation in bone marrow cells.