Annelies Smit¹, Jan Kroon¹, Sander Kooijman¹, Maaike Schilperoort², Salwa Afkir¹, Bram van der Eerden³, Marijke Koedam³, Onno Meijer¹, Elizabeth Winter^1,4.

¹Leiden University Medical Center, Department of Medicine- Division of Endocrinology, Leiden, Netherlands.
²Columbia University Irving Medical Center, Department of Medicine, New York, United States.
³Erasmus Medical Center, Department of Internal Medicine, Rotterdam, Netherlands.
⁴Leiden University Medical Center, Department of Medicine- Center for Bone Quality, Leiden, Netherlands.

Purpose:

Synthetic glucocorticoid (GC) use increases fracture risk. Aside from GC excess, this may also be due to the absence of endogenous GC circadian rhythm, as a flattened GC rhythm induces osteoporosis in mice. This study explored whether reinstating the circadian GC rhythm could prevent osteoporosis.

Methods & Results:

We implanted female C57Bl/6J mice with slow-release, 7.5% corticosterone (CORT) pellets, the natural GC in mice, to blunt CORT rhythm without elevating overall CORT exposure. One week of flattened CORT rhythm reduced serum levels of bone formation marker Procollagen Type I N-terminal propeptide (P1NP) (-38% p<0.01). Reinstating a GC trough by injecting GC receptor antagonist RU486 rescued P1NP levels (+2% ns), whereas reinstating a GC peak by CORT injections did not (-57% p<0.001). To elucidate the long-term importance of a GC trough, and the relevance of timing, mice with 7.5% CORT pellets received daily RU486 injections during the natural GC trough (AM) or peak (PM). 7 weeks of flattened CORT rhythm reduced cortical bone thickness (-13% p<0.001) and trabecular bone volume (-32% p<0.05), as determined by micro-CT. RU486 injection at either timepoint rescued cortical thinning (AM -2% ns; PM -1% ns). Trabecular bone volume was preserved by AM RU486 (+3% ns), but not PM RU486 (-31% p<0.05). At 2 weeks, flattened CORT rhythm increased bone resorption marker Tartrate-resistant acid phosphatase (TRAP) levels (+25% p<0.01) without altering P1NP levels (+3% ns), indicating a shift toward resorption. In contrast, RU486-injected mice showed increased TRAP (+19% p<0.01) and P1NP (+28% p<0.01) levels, suggesting overall increased remodeling. At 7 weeks, RU486 AM mice showed a morning peak in P1NP levels, while RU486 PM mice had an evening P1NP peak, suggesting that bone remodeling synchronizes to the GC trough.

Conclusion:

Reinstating a well-timed GC trough prevents osteoporosis in mice, by preserving the balance in bone remodeling. Thus, maintenance of a GC trough may be a promising strategy to prevent GC therapy-induced osteoporosis.