The isozyme-specific effects of cyclooxygenase-deficiency on bone in mice

Prostaglandin E2 (PGE2) plays a critical role in skeletal physiology and bone loss. PGE2 production is regulated in vivo by at least two cyclooxygenase (COX) isozymes, COX-1 and COX-2. The purpose of this study was to investigate the in vivo effects of the selective deletion of COX-1 or COX-2 on bone mineral density (BMD), bone microarchitecture and bone strength in wild type (WT), COX-1−/− and COX-2−/− mice. Using a LUNAR PIXImus, BMD was measured in 18 (WT), 18 COX-1−/− and 16 COX-2−/− mice. COX-1−/− mice exhibited significantly higher BMD (0.0506 g/cm2 ± 0.0014 g/cm2) than either WT (0.0493 g/cm2 ± 0.0019, P ≤ 0.05) or COX-2−/− (0.0473 g/cm2 ± 0.0034, P ≤ 0.01) mice. COX-2−/− mice had significantly lower BMD than WT (P ≤ 0.01) or COX-1−/− (P ≤ 0.01). Flexure stress of the femurs, determined by breaking the bones with three-point bending, correlated with bone density. Although plasma levels of both Ca2+ and PTH were comparable in wild type and COX-1−/− mice, both were elevated in COX-2−/− mice consistent with primary hyperparathyroidism. These studies suggest that COX enzymes are important regulators of BMD and bone strength in mice. The beneficial effect of absence of the COX-1 enzyme on skeletal parameters may be secondary to decreases in PGE2. On the other hand, primary hyperparathyroidism and lower bone magnesium content may account for the lower BMD and impairments in bone strength of COX-2−/− mice. Further elucidation of the effects of the COX pathway on bone remodeling may provide important information on potential therapeutic targets for preventing and/or treating osteoporosis.