The osteoprotegerin/receptor activator of nuclear factor kappa B/receptor activator of nuclear factor kappa B ligand system in cartilage

Objective. The receptor activator of nuclear factor kappaB (RANK) is a memb er of the tumor necrosis factor receptor family. It is activated by the sec reted or cell surface-bound RANK ligand (RANKL). Osteoprotegerin (OPG) is a soluble nonsignaling receptor for RANKL and interferes with RANK activatio n. This receptor-ligand system regulates the differentiation of osteoclasts and dendritic cells. The present study examined human articular cartilage for the expression of these molecules and the role of RANKL in the regulati on of chondrocyte function. Methods. Normal and osteoarthritic (OA) human articular cartilage was used for explant tissue culture or for isolation of chondrocytes and cell cultur e. Expression of RANK, RANKL, and OPG was analyzed by, immunohistochemistry , Western blotting, or reverse transcription-polymerase chain reaction. Rec ombinant RANKL was added to cartilage or chondrocyte cultures, and gene exp ression, collagenase and nitric oxide production, and NF-kappaB activation were determined. Results. RANK, RANKL, and OPG messenger RNA (mRNA) were expressed in normal cartilage. By immunohistochemistry, RANK, RANKL, and OPG were detected in the superficial zone of normal cartilage. OA cartilage contained increased levels of OPG mRNA, and expression of the 3 proteins extended into the midz one of OA cartilage. OPG was detected by Western blotting, and was increase d in response to interleukin-1 beta stimulation. OPG, RANK, and RANKL prote in were also detected in cultured chondrocytes. Addition of exogenous RANKL did not activate NF-kappaB, induce expression of genes encoding proinflamm atory mediators in chondrocytes, or stimulate the production of collagenase and nitric oxide. Conclusion. These results demonstrate the expression of OPG, RANK, and RANK L in cartilage. However, RANKL does not activate human articular chondrocyt es.