High-mobility group box proteins modulate tumor necrosis factor-α expression in osteoclastogenesis via a novel deoxyribonucleic acid sequence

We have previously shown that mice lacking the TSH receptor ( TSHR) exhibit osteoporosis due to enhanced osteoclast formation. The fact that this enhancement is not observed in double-null mice of TSHR and TNF alpha suggests that TNF alpha overexpression in osteoclast progenitors ( macrophages) may be involved. It is unknown how TNF alpha expression is regulated in osteoclastogenesis. Here, we describe a receptor activator for nuclear factor-kappa B ligand ( RANKL)-responsive sequence ( CCG AGA CAG AGG TGT AGG GCC), spanning from -157 to -137 bp of the 5'-flanking region of the TNF alpha gene, which functions as a cis-acting regulatory element. We further show how RANKL treatment stimulates the high-mobility group box proteins ( HMGB) HMGB1 and HMGB2 to bind the RANKL-responsive sequence and up-regulates TNF alpha transcription. Exogenous HMGB elicits the expression of cytokines, including TNF alpha, as well as osteoclast formation. Conversely, TSH inhibits the expression of HMGB and TNF alpha and the formation of osteoclasts. These results suggest that HMGB play a pivotal role in osteoclastogenesis. We also show a direct correlation between the expression of HMGB and TNF alpha and osteoclast formation in TSHR-null mice and TNF alpha-null mice. Taken together, we conclude that HMGB and TNF alpha play critical roles in the regulation of osteoclastogenesis and the remodeling of bone.