Purpose

We recently have shown that a flattened glucocorticoid (GC) circadian rhythm in itself induces an osteoporotic phenotype in mice. Here, we investigated whether the diurnal trough in GC signalling is essential to preserve bone mass and structure.

Methods

Female C57Bl/6J mice were implanted with either vehicle or slow-releasing 7.5 mg corticosterone (CORT) pellets (7 weeks) to flatten GC circadian rhythm. Additionally, mice were given daily subcutaneous injections with either vehicle or the non-specific glucocorticoid receptor antagonist RU-486 to reintroduce a diurnal trough in GC signalling. To investigate the effect of timing, mice were injected either at the time of the endogenous GC trough (ZT1) or GC peak (ZT11). Readouts were bone turnover markers (n=12/group) and bone microarchitecture (using micro-CT) (n=12/group).

Results

The various interventions did not affect serum tartrate-resistant acid phosphatase (TRAP) levels. RU-486 injection at ZT11 notably reversed serum procollagen type I N-terminal propeptide (P1NP) rhythm compared to vehicle, while injection at ZT1 induced a similar P1NP pattern to vehicle. Furthermore, flattened CORT-induced loss of lean mass was rescued by RU-486 injection at both ZT1 and ZT11, suggesting prevention of bone wasting. CORT pellets significantly reduced cortical thickness (-13.0%), cortical bone area (-10.0%) and trabecular bone volume (-31.8%), as compared to vehicle. RU-486 injection at either timepoint significantly improved cortical thickness (ZT1 +10.5%; ZT11 +9.2%) and cortical bone area (ZT1 +8.7%; ZT11 +9.5%), as compared to CORT mice. Notably, RU-486 injection at ZT1 prevented trabecular bone volume loss, while RU-486 injection at ZT11 did not (-30.6%, as compared to vehicle).

Conclusion

Reinstating a diurnal trough in GC signalling under condition of flattened GC levels is sufficient to rescue bone mass and structure. The fact that these effects were more pronounced when the GR antagonist was injected at the time of the endogenous GC trough, suggests that also the circadian context of bone tissue is important.