Abstract
It is well known that thyroid hormone excess causes bone loss. However, the precise mechanism of bone loss by thyroid hormone still remains unclear. When T~3~ was added to unfractionated bone cells after degeneration of pre‐existent osteoclasts, T~3~ (1 pM–100 nM) dose‐dependently stimulated osteoclast‐like cell formation, irrespective of the presence of indomethacin and IL‐6 Ab. T~3~ increased the expression of osteoprotegerin (OPG) messenger RNA (mRNA), but not of receptor activator of nuclear factor κB ligand (RANKL) in unfractionated bone cells, suggesting that the stimulatory effect of T~3~ on osteoclast formation was not mediated by the RANKL/OPG system. We next examined the direct effect of T~3~ on osteoclast precursors in the absence of osteoblasts, using hemopoietic blast cells derived from spleen cells. T~3~ (1 pM–100 nM) dose‐dependently stimulated osteoclast‐like cell formation from osteoclast precursors. OPG did not inhibit T~3~‐induced osteoclast formation from osteoclast precursor cells. The polymerase chain reaction (PCR) product corresponding in size to the mouse T~3~ receptor α1 cDNA was detected in osteoclast precursors from mouse hemopoietic blast cells as well as mouse heart and mouse osteoblastic cell line MC3T3‐E1 cells, suggesting that T~3~ directly stimulated osteoclast‐like cell formation from osteoclast precursors in the absence of osteoblasts. Further, T~3~ increased the expression of c‐Fos mRNA at 15 min and 24 h and Fra‐1 mRNA at 2 and 6 h in osteoclast precursors. Consistent with the increased expression of c‐Fos mRNA observed by RT‐PCR, the activation of c‐Fos occurred in osteoclast precursor cells stimulated by T~3~, while the activation of neither NF‐κB nor MAPKs was observed by immunoblot analysis. Antisense oligodeoxynucleotides (as‐ODN) complementary to c‐Fos mRNA at 1 μM significantly inhibited T~3~‐induced osteoclast‐like cell formation from osteoclast precursors in the absence of stromal cells while sense‐ODN did not affect T~3~‐induced osteoclast‐like cell formation. These results indicate that T~3~ directly stimulates osteoclast differentiation at least in part by up‐regulation of c‐fos protein in osteoclast precursor cells. J. Cell. Physiol. 201: 17–25, 2004. © 2004 Wiley‐Liss, Inc.