We have shown recently that by acting on the thyroid-stimulating hormone (TSH) receptor (TSHR), TSH negatively regulates osteoclast differentiation. Both heterozygotic and homozygotic TSHR null mice are osteopenic with evidence of enhanced osteoclast differentiation. Here, we report that the accompanying elevation of TNF␣, an osteoclastogenic cytokine, causes the increased osteoclast differentiation. This enhancement in TSHR ؊/؊ and TSHR ؉/؊ mice is abrogated in compound TSHR ؊͞؊ ͞TNF␣ ؊/؊ and TSHR ؉/؊ ͞ TNF␣ ؉/؊ mice, respectively. In parallel studies, we find that TSH directly inhibits TNF␣ production, reduces the number of TNF␣-producing osteoclast precursors, and attenuates the induction of bone remodeling ͉ osteoclast ͉ macrophage ͉ cytokine A nterior pituitary hormones have long been thought just to stimulate the secretion of master hormones from target endocrine glands, except for our recent demonstration of direct effects of thyroid-stimulating hormone (TSH) and folliclestimulating hormone on the skeleton (1). Thus, until recently, TSH was considered solely to regulate thyroid follicular cell differentiation and thyroid hormone secretion by binding to a seven transmembrane, glycosylated G protein-coupled receptor, the TSH receptor (TSHR). Previous studies had identified TSHRs in other tissues and cells, including the pituitary, thymus, testes, kidney, brain, lymphocytes, adipocytes, and fibroblasts (2, 3), but their functional significance has remained uncertain.Gene ablation studies in mice revealed that TSHR haploinsufficiency did not affect thyroid gland development or function, whereas the total absence of the TSHR expectedly disrupted thyroid follicular structure (4). However, bone mass was reduced not only in homozygote mice but also in the haploinsufficient heterozygotes (5). That TSHR haploinsufficiency, in the absence of a thyroid defect, resulted in osteoporosis established a primary role for the TSHR in bone metabolism. Furthermore, supplementation of TSHR Ϫ/Ϫ mice with thyroid extract to render them euthyroid corrected all hypothyroid abnormalities, including runting, but not reductions in bone mass (5) or sodium-iodide symporter expression (4). The latter observation confirmed that the osteoporosis arose from TSHR deficiency rather than altered thyroid hormone levels. Consistent with this notion, the genetic ablation of the ␣1͞ thyroid hormone receptor has been shown not to result in a bone remodeling defect (6).We found that the osteoporosis in TSHR knockout mice was the result of an enhancement in osteoclast differentiation. Consistent with the low bone mass, ex vivo cultures of bone marrow cell precursors from both heterozygote and homozygote mice showed increased osteoclast formation and the enhanced expression of an osteoclast marker tartrate-resistant acid phosphatase (TRAP) (5). This enhanced osteoclast formation was not associated with increased receptor activator of NF-B ligand (RANKL) production but instead with a several-fold increase in the synthesis and release of TNF␣, another...
