TNFα 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 TNFα, an osteoclastogenic cytokine, causes the increased osteoclast differentiation. This enhancement in TSHR−/− and TSHR+/− mice is abrogated in compound TSHR−/−/TNFα−/− and TSHR+/−/TNFα+/− mice, respectively. In parallel studies, we find that TSH directly inhibits TNFα production, reduces the number of TNFα-producing osteoclast precursors, and attenuates the induction of TNFα expression by IL-1, TNFα, and receptor activator of NF-κB 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/TNFα and receptor activator of NF-κB ligand fail to stimulate AP-1 and NF-κB binding to DNA in cells transfected with TSHR or constitutively active TSHR. The results suggest that TNFα is the critical cytokine mediating the downstream antiresorptive effects of TSH on the skeleton.