Bone Abstracts

Thyrotoxicosis results in osteoporosis, and thyroid hormone (T3) stimulates osteoclastic bone resorption by unknown mechanisms. We previously demonstrated that knockout mice lacking thyroid hormone receptor α (TRα^0/0) are euthyroid but have high bone mass, whereas mice lacking TRβ (TRβ^−/−) are thyrotoxic and osteoporotic. Tartrate resistant acid phosphatase (TRAcP) staining revealed osteoclast numbers were reduced by 13% (P<0.05) in TRα^0/0 mice, but increased by 20% (P<0.05) in TRβ^−/− mice, suggesting T3 acts via TRα to stimulate osteoclastogenesis and bone resorption.

To investigate this hypothesis, we treated WT, TRα^0/0 and TRβ^−/− bone marrow (BM) with M-CSF (25 ng/ml) and RANKL (10 ng/ml) in the absence or presence of T3 (100 nM). T3 treatment had no effect on the number, size or survival of osteoclasts from any genotype, indicating T3 does not exert direct actions in osteoclasts. Nevertheless, threefold more osteoclasts formed in TRα^0/0 BM cultures compared to WT or TRβ^−/− (P<0.001), suggesting that osteoclast precursor cell differentiation is impaired in vivo in TRα^0/0 mice. We then examined interactions between osteoblasts and osteoclasts by co-culturing WT osteoblasts with WT, TRα^0/0 or TRβ^−/− BM in the absence and presence of T3. In T3-treated cultures there was an increase in osteoclast number (40–90%, P<0.05), TRAcP activity (76–91%, P<0.05) and resorption (73–80%, P<0.05) irrespective of BM genotype, thus indicating that the impaired osteoclastogenesis in TRα^0/0 mice is not due to an osteoclast defect. We next co-cultured WT BM with WT, TRα^0/0 or TRβ^−/− osteoblasts in the absence or presence of T3. T3 treatment increased osteoclast number (45–66%, P<0.05) and TRAcP activity (21–80%, P<0.05) in co-cultures containing either WT or TRβ^−/− osteoblasts. By contrast, almost no osteoclasts formed in co-cultures containing TRα^0/0 osteoblasts and WT BM in either the absence or presence of T3.

Overall, these data demonstrate that T3 stimulates osteoclastic bone resorption indirectly via TRα-dependent actions in osteoblasts.