No abstract

We show the systemic administration of low levels of TSH increases bone volume and improves bone microarchitecture and strength in aged OVX rats. TSH's actions are mediated by its inhibitory effects on RANKL-induced osteoclast formation and bone resorption coupled with stimulatory effects on osteoblast differentiation and bone formation, suggesting TSH directly affects bone remodeling in vivo.Introduction: Thyroid-stimulating hormone (TSH) receptor haploinsufficient mice with normal circulating thyroid hormone levels have reduced bone mass, suggesting that TSH directly affects bone remodeling. We examined whether systemic TSH administration restored bone volume in aged ovariectomized (OVX) rats and influenced osteoclast formation and osteoblast differentiation in vitro. Materials and Methods: Sprague-Dawley rats were OVX at 6 months, and TSH therapy was started immediately after surgery (prevention mode; n ‫ס‬ 80) or 7 mo later (restoration mode; n ‫ס‬ 152). Hind limbs and lumbar spine BMD was measured at 2-or 4-wk intervals in vivo and ex vivo on termination at 8-16 wk. Long bones were subjected to CT, histomorphometric, and biomechanical analyses. The direct effect of TSH was examined in osteoclast and osteoblast progenitor cultures and established rat osteosarcoma-derived osteoblastic cells. Data were analyzed by ANOVA Dunnett test. Results: In the prevention mode, low doses (0.1 and 0.3 g) of native rat TSH prevented the progressive bone loss, and importantly, did not increase serum triiodothyroxine (T3) and thyroxine (T4) levels in aged OVX rats. In restoration mode, animals receiving 0.1 and 0.3 g TSH had increased BMD (10-11%), trabecular bone volume (100-130%), trabecular number (25-40%), trabecular thickness (45-60%), cortical thickness (5-16%), mineral apposition and bone formation rate (200-300%), and enhanced mechanical strength of the femur (51-60%) compared with control OVX rats. In vitro studies suggest that TSH's action is mediated by its inhibitory effects on RANKL-induced osteoclast formation, as shown in hematopoietic stem cells cultivated from TSH-treated OVX rats. TSH also stimulates osteoblast differentiation, as shown by effects on alkaline phosphatase activity, osteocalcin expression, and mineralization rate. Conclusions: These results show for the first time that systemically administered TSH prevents bone loss and restores bone mass in aged OVX rats through both antiresorptive and anabolic effects on bone remodeling.

Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: tumor-induced osteomalacia (TIO); parathyroid hormone (PTH); parathyroid hormonerelated protein (PTHrP); opossum kidney (OK); serial analysis of gene expression (SAGE); secreted frizzled-related protein-4 (sFRP-4); mean arterial blood pressure (MAP); thyro-parathyroidectomized (TPTX); phosphate diet (Pi); fractional excretion of phosphate (FEPi); fractional sodium excretion (FENa); fractional excretion of calcium (FECa); X-linked hypophosphatemic rickets (XLH); autosomal dominant hypophosphatemic rickets (ADHR).

Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: tumor-induced osteomalacia (TIO); parathyroid hormone (PTH); parathyroid hormonerelated protein (PTHrP); opossum kidney (OK); serial analysis of gene expression (SAGE); secreted frizzled-related protein-4 (sFRP-4); mean arterial blood pressure (MAP); thyro-parathyroidectomized (TPTX); phosphate diet (Pi); fractional excretion of phosphate (FEPi); fractional sodium excretion (FENa); fractional excretion of calcium (FECa); X-linked hypophosphatemic rickets (XLH); autosomal dominant hypophosphatemic rickets (ADHR).

SUMMARYThe phosphaturic activity of intact, full-length, fibroblast growth factor-23 (FGF-23) is well documented. FGF-23 circulates as the intact protein, and as fragments generated as the result of proteolysis of the full-length protein. To assess whether short fragments of FGF-23 are phosphaturic, we compared the effect of acute, equimolar infusions of full-length FGF-23 and various FGF-23 fragments carboxyl-terminal to amino acid 176. In rats, intravenous infusions of full-length FGF-23, and FGF-23 176-251, significantly and equivalently increased fractional phosphate excretion (FE Pi)) from 14±3 to 32±5% and 15±2 to 33±2% (p <0.001), respectively. reduced Na + -dependent Pi uptake and enhanced internalization of the Na + -Pi IIa co-transporter. We conclude that carboxyl terminal fragments of FGF-23 are phosphaturic, and that a short, 26 amino acid fragment of FGF-23 retains significant phosphaturic activity.