Reduced osteoprotegerin expression by osteocytes may contribute to rebound resorption after denosumab discontinuation
Qiang Fu, Nancy C. Bustamante-Gomez, Humberto Reyes-Pardo, Igor Gubrij, Diana Escalona-Vargas, Jeff D. Thostenson, Michela Palmieri, Joseph J. Goellner, Intawat Nookaew, C. Lowry Barnes, Jeffrey B. Stambough, Elena Ambrogini, Charles A. O’Brien
Qiang Fu, Nancy C. Bustamante-Gomez, Humberto Reyes-Pardo, Igor Gubrij, Diana Escalona-Vargas, Jeff D. Thostenson, Michela Palmieri, Joseph J. Goellner, Intawat Nookaew, C. Lowry Barnes, Jeffrey B. Stambough, Elena Ambrogini, Charles A. O’Brien
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Research Article Bone biology

Reduced osteoprotegerin expression by osteocytes may contribute to rebound resorption after denosumab discontinuation

Abstract

Denosumab is an anti-RANKL Ab that potently suppresses bone resorption, increases bone mass, and reduces fracture risk. Discontinuation of denosumab causes rapid rebound bone resorption and bone loss, but the molecular mechanisms are unclear. We generated humanized RANKL mice and treated them with denosumab to examine the cellular and molecular conditions associated with rebound resorption. Denosumab potently suppressed both osteoclast and osteoblast numbers in cancellous bone in humanized RANKL mice. The decrease in osteoclast number was not associated with changes in osteoclast progenitors in bone marrow. Long-term, but not short-term, denosumab administration reduced osteoprotegerin (OPG) mRNA in bone. Localization of OPG expression revealed that OPG mRNA is produced by a subpopulation of osteocytes. Long-term denosumab administration reduced osteocyte OPG mRNA, suggesting that OPG expression declines as osteocytes age. Consistent with this, osteocyte expression of OPG was more prevalent near the surface of cortical bone in humans and mice. These results suggest that new osteocytes are an important source of OPG in remodeling bone and that suppression of remodeling reduces OPG abundance by reducing new osteocyte formation. The lack of new osteocytes and the OPG they produce may contribute to rebound resorption after denosumab discontinuation.

Authors

Qiang Fu, Nancy C. Bustamante-Gomez, Humberto Reyes-Pardo, Igor Gubrij, Diana Escalona-Vargas, Jeff D. Thostenson, Michela Palmieri, Joseph J. Goellner, Intawat Nookaew, C. Lowry Barnes, Jeffrey B. Stambough, Elena Ambrogini, Charles A. O’Brien

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Figure 1

Creation of hRANKL mice.

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Creation of hRANKL mice. (A) Amino acid sequence comparison of the regio...
(A) Amino acid sequence comparison of the region of the RANKL protein harboring the 9 amino acid epitope bound by denosumab, identified by Schieferdecker et al. (15). The epitope is indicated by the horizontal blue line. The 4 amino acids encoded by the murine gene that were modified to match the human sequence are shown in light blue. (B) Femoral cortical thickness (Cortical Th) and cancellous bone volume (BV/TV) of untreated, 12-week-old, female WT and hRANKL mice. P values determined by 2-tailed, unpaired t test. WT mice, n = 5; hRANKL mice, n = 7. (C) Four-month-old male WT or homozygous hRANKL mice were injected once per week with denosumab at a dose of 10 mg/kg BW for 3 weeks. Blood was collected on the day of the first injection and 1 week after the last injection, and serum TRAP5b and CTX-1 were measured by ELISA. P values determined by 2-tailed, unpaired t test and Holm-Šídák adjustment for multiple comparisons. WT mice, n = 3, hRANKL mice, n = 4. (D) Gene expression in cortical bone of the same mice used in C. P values determined by 2-tailed, unpaired t test. Bars and whiskers represent the mean ± SD.