| 초록 |
Receptor activator of nuclear factor- ${ kappa}B$ ligand (RANKL) is an osteoblast/stromal cell-derived essential factor for osteoclastogenesis. During endochondral bone formation, hypertrophic chondrocytes calcify cartilage matrix that is subsequently resorbed by osteoclasts in order to be replaced by new bone. Hypoxia-induced upregulation of RANKL expression has been previously demonstrated in an in vitro system using osteoblasts; however, the involved mechanism remains unclear in chondrocytes. In the present study, we investigated whether hypoxia regulates RANKL expression in ATDC5 cells, a murine chondrogenic cell line, and hypoxia-inducible factor- $1{ alpha}$ (HIF- $1{ alpha}$ ) mediates hypoxia-induced RANKL expression by transactivating the RANKL promoter. The expression levels of RANKL mRNA and protein, as well as HIF- $1{ alpha}$ protein, were significantly increased in ATDC5 cells under hypoxic condition. Constitutively active HIF- $1{ alpha}$ alone significantly increased the levels of RANKL expression under normoxic conditions, whereas dominant negative HIF- $1{ alpha}$ reduced hypoxia-induced RANKL expression. HIF- $1{ alpha}$ increased RANKL promoter reporter activity in a HIF- $1{ alpha}$ binding element-dependent manner in ATDC5 cells. Hypoxia-induced RANKL levels were much higher in differentiated ATDC5 cells, as compared to proliferating ATDC5 cells. These results suggested that under hypoxic conditions, HIF- $1{ alpha}$ mediates induction of RANKL expression in chondrocytes; in addition, hypoxia plays a role in osteoclastogenesis during endochondral bone formation, at least in part, through the induction of RANKL expression in hypertrophic chondrocytes. |
