A Circadian Trough in Glucocorticoid Signaling Is Essential for Bone Health in Mice

We previously demonstrated that flattening circadian glucocorticoid (GC) rhythmicity without increasing overall GC exposure induces an osteoporotic phenotype in mice. Here, we aimed to further elucidate the importance of the amplitude and timing of circadian GC oscillations for bone health. C57Bl/6J mice were implanted with vehicle or corticosterone slow-releasing pellets to flatten the circadian GC rhythm. To differentiate between the importance of circadian GC peaks or troughs, mice with flattened GC rhythm received daily CORT injections at the time of the natural GC peak, or glucocorticoid receptor (GR) antagonist (RU486) injections at the time of the trough. One week of flattened GC rhythm reduced serum bone formation marker P1NP levels. Reinstating a trough with RU486 rescued this loss, whereas reinstating a GC peak did not. In an experiment in which mice with flattened GC rhythm received prolonged treatment for 7 weeks with RU486 at Zeitgeber time (ZT) 1 versus 11, we found that P1NP levels peaked with RU486 regardless of time of injection, altogether suggesting that bone formation depends on transient withdrawals from GCs. Seven weeks of flattened CORT rhythm reduced total lean mass and induced cortical bone thinning and trabecular bone loss. RU486 injection at either timepoint prevented cortical bone decline. Notably, trabecular bone volume was only preserved when RU486 was injected at the time of the natural GC trough at ZT1. In conclusion, reinstating a trough in GR signaling at its natural time of the day suffices to prevent osteoporosis in mice under conditions of flattened GC rhythm.