Oral Presentation 1st Asia Pacific Herbert Fleisch Workshop 2025

Circadian glucocorticoid rhythm timing and amplitude regulate bone remodeling (#19)

Annelies E Smit 1 , Jan Kroon 1 , Sander Kooijman 1 , Maaike Schilperoort 1 , Bram C.J. van der Eerden 2 , Marijke Koedam 2 , Onno C Meijer 1 , Elizabeth M Winter 3
  1. Department of Medicine - Division of Endocrinology & Einthoven Laboratory , Leiden University Medical Center, Leiden, The Netherlands
  2. Department of Internal Medicine, Erasmus Medical Center, Rotterdam, The Netherlands
  3. Department of Medicine - Division of Endocrinology, Einthoven Laboratory & Center for Bone Quality, Leiden University Medical Center, Leiden, The Netherlands

Background

Synthetic glucocorticoids increase fracture risk. Beyond elevated glucocorticoid levels, loss of circadian rhythmicity may contribute, as flattened glucocorticoid rhythm induces osteoporosis in mice. This study investigated how glucocorticoid rhythm amplitude and timing affect bone health.

Methods

We implanted 7.5% corticosterone (CORT) pellets in female C57Bl/6J mice. This blunts the natural circadian CORT rhythm without elevating CORT exposure. To assess the role of glucocorticoid troughs and peaks, we injected CORT pellet-implanted mice with glucocorticoid receptor (GR) antagonist RU486 timed at the natural glucocorticoid trough (p.m.), or with CORT at the natural glucocorticoid peak (a.m.) for one week. To examine the importance of trough-timing, CORT pellet-implanted mice received daily a.m. or p.m. RU486 injections for 7 weeks. We analyzed plasma bone formation marker P1NP levels by ELISA, gene expression by qPCR and bone microarchitecture by micro-CT.

Results

Flattening the CORT rhythm for one week reduced plasma P1NP by -38%, compared to vehicle (p<0.01). Reinstating a trough normalized P1NP (+2% ns), whereas reinstating a peak did not (-57% p<0.001). After 7 weeks, P1NP peaked at the time of RU486 injection, regardless of injection timing, indicating that GR-signaling troughs drive bone formation. Mechanistically, CORT pellets flattened the diurnal expression amplitude of circadian regulation gene Bmal1 (morning vs evening, ns), which was restored by RU486 at either time point (morning vs evening, p<0.05). Regarding bone microarchitecture, 7 weeks of flattened CORT rhythm reduced cortical bone thickness (-13% p<0.001) and trabecular bone volume (-32% p<0.05). RU486 injection at either timepoint rescued cortical thinning (a.m. -2% ns; p.m. -1% ns). Notably, a.m. RU486 injections preserved trabecular bone volume (+3% ns), while p.m. injections failed to prevent loss (-31% p<0.05), demonstrating that trough-timing matters.

Conclusion

Reinstating a well-timed GR trough prevents osteoporosis in mice. Thus, maintaining GC withdrawals during glucocorticoid-treated patients may be promising to prevent osteoporosis.