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

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Abstract
Objective The receptor activator of nuclear factor κB (RANK) is a member of the tumor necrosis factor receptor family. It is activated by the secreted or cell surface–bound RANK ligand (RANKL). Osteoprotegerin (OPG) is a soluble nonsignaling receptor for RANKL and interferes with RANK activation. 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 regulation of chondrocyte function. Methods Normal and osteoarthritic (OA) human articular cartilage was used for explant tissue culture or for isolation of chondrocytes and cell culture. Expression of RANK, RANKL, and OPG was analyzed by immunohistochemistry, Western blotting, or reverse transcription–polymerase chain reaction. Recombinant RANKL was added to cartilage or chondrocyte cultures, and gene expression, collagenase and nitric oxide production, and NF‐κB 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 midzone of OA cartilage. OPG was detected by Western blotting, and was increased in response to interleukin‐1β stimulation. OPG, RANK, and RANKL protein were also detected in cultured chondrocytes. Addition of exogenous RANKL did not activate NF‐κB, induce expression of genes encoding proinflammatory mediators in chondrocytes, or stimulate the production of collagenase and nitric oxide. Conclusion These results demonstrate the expression of OPG, RANK, and RANKL in cartilage. However, RANKL does not activate human articular chondrocytes.