TNF mediates the skeletal effects of thyroid-stimulating hormone

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 TNF expression by IL-1, TNF , and receptor activator of NFB ligand. TSH also suppresses osteoclast formation in murine macrophages and RAW-C3 cells. The suppression is more profound in cells that overexpress the TSHR than those transfected with empty vector. The overexpression of ligand-independent, constitutively active TSHR abrogates osteoclast formation even under basal conditions and in the absence of TSH. Finally, IL-1 TNF and receptor activator of NFB ligand fail to stimulate AP-1 and NFB binding to DNA in cells transfected with TSHR or constitutively active TSHR. The results suggest that TNF is the critical cytokine mediating the downstream antiresorptive effects of TSH on the skeleton.