Osteoprotegerin and receptor activator of nuclear factor kappaB ligand (RANKL) regulate osteoclast formation by cells in the human rheumatoid arthritic joint

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Abstract
Objective. This study investigated the involvement of the recently identified regulators of osteoclast formation RANKL [receptor activator of nuclear factor kappaB (RANK) ligand, osteoclast differentiation factor, TRANCE, osteoprotegerin ligand] and its natural inhibitor, osteoprotegerin (OPG), in the bone erosion of rheumatoid arthritis (RA). Methods. mRNA was extracted from cells isolated from the pannus and synovial membrane regions of joints of 11 RA patients. Semiquantitative reverse transcription–polymerase chain reaction was carried out, and the isolated cells were also cultured to determine their ability to form osteoclasts. Results. mRNAs encoding RANKL, RANK, OPG and macrophage‐colony stimulating factor were expressed by cells isolated from RA joints. In addition, mRNA encoding for tumour necrosis factor apoptosis‐inducing ligand and the osteoclast markers tartrate‐resistant acid phosphatase and calcitonin receptor were also often expressed. Osteoclasts capable of forming resorption lacunae were generated from cells in the RA joints. At 50 ng/ml, recombinant OPG completely inhibited the resorptive activity of these cells. There was a significant correlation between the ratio of RANKL mRNA to OPG mRNA and the number of resorption pits produced (P = 0.028). Conclusion. These data suggest that RANKL is an essential factor for osteoclast formation by cells in the rheumatic joint and that OPG may prevent the bone erosion seen in RA joints.