Sclerostin Blockade and Zoledronic Acid Improve Bone Mass and Strength in Male Mice With Exogenous Hyperthyroidism

Abstract

Hyperthyroidism in mice is associated with low bone mass, high bone turnover, and high concentrations of sclerostin, a potent Wnt inhibitor. Here, we explored the effects of either increasing bone formation with sclerostin antibodies (Scl-Ab) or reducing bone turnover with bisphosphonates on bone mass and strength in hyperthyroid mice. Twelve-week-old C57BL/6 male mice were rendered hyperthyroid using l-thyroxine (T4; 1.2 µg/mL added to the drinking water) and treated with 20 mg/kg Scl-Ab twice weekly or 100 µg/kg zoledronic acid (ZOL) once weekly or phosphate-buffered saline for 4 weeks. Hyperthyroid mice displayed a lower trabecular bone volume at the spine (-42%, P < 0.05) and the distal femur (-55%, P < 0.05) compared with euthyroid controls. Scl-Ab and ZOL treatment of hyperthyroid mice increased trabecular bone volume at the spine by threefold and twofold, respectively. Serum bone formation and resorption markers were increased in hyperthyroid mice and suppressed by treatment with ZOL but not Scl-Ab. Trabecular bone stiffness at the lumbar vertebra was 63% lower in hyperthyroid mice (P < 0.05) and was increased fourfold by Sci-Ab (P < 0.001) and threefold by ZOL treatment (P < 0.01). Bone strength based on ultimate load, which was 10% lower in hyperthyroidism, was increased by Scl-Ab by 71% and ZOL by 22% (both P < 0.001). Increased proportion of low mineralized bone seen in hyperthyroid mice was restored by treatment with Scl-Ab and ZOL. Thus, bone-forming and antiresorptive drugs prevent bone loss in hyperthyroid mice via different mechanisms.

Disruption of BMP Signaling Prevents Hyperthyroidism-Induced Bone Loss in Male Mice

“…turnover state with subsequent bone loss due to predominant bone resorption. (6)(7)(8)(9)21) Hence, bone-forming osteoblasts as well as bone-resorbing osteoclasts are targeted by TH. (3,(41)(42)(43)(44)(45) Similar to previous studies, (10)(11)(12)(13)(14) our results demonstrate that T 3 augments osteoblast differentiation and function in vitro and in vivo.…”

“…Total T 4 and T 3 serum concentrations were determined using radioimmunoassays (RIA-4524 and RIA-4525 respectively; DRG Instruments, Marburg, Germany) as previously described. (9,21) The lower limit of analysis was 10 nM and 0.22 nM for T 4 and T 3 , respectively. Intra-assay coefficients of variations were below 7.5%.…”

“…Hyperthyroidism was induced as described previously. (21,22) Mice were randomized into two groups either rendered hyperthyroid (Hyper) by adding L-thyroxine (T 4 , 1.2 μg/ mL, Sigma-Aldrich, Munich, Germany) into their drinking water or remained euthyroid (Co) by receiving tap water. All animals had ad libitum access to their respective drinking water.…”

“…Hyperthyroidism leads to high bone turnover-induced bone loss due to predominant bone resorption. (9,21) Thus, we tested whether blockade of BMP signaling by scavenging BMP ligands may counteract the accelerated bone turnover and subsequent bone loss. To that end, we treated male hyperthyroid mice with ALK3-Fc, which was reported to sequester ALK3-targeting ligands such as BMP2 and BMP4.…”

“…We previously reported that exogenously applied TH enhances bone turnover with an upregulation of both bone formation and predominantly bone resorption, leading to high turnover bone loss in male mice. (9,21) However, in spite of elevated numbers of osteoblasts, Wnt signaling was rather decreased, showing a high expression of the Wnt inhibitors sclerostin and dickkopf-1 and downregulation of Wnt targets Axin2 and osteoprotegerin in bone tissue. (9) Moreover, deletion of dickkopf-1 did not rescue hyperthyroidism-induced bone loss in mice.…”