The expression of RANKL and OPG in the various grades of osteoarthritic cartilage

The objective of the study was to determine whether cartilage expression of the bone regulating molecules receptor activator of nuclear factor κB ligand (RANKL) and osteoprotegerin (OPG) varies between the different grades of osteoarthritis (OA). Cartilage samples were obtained from 30 patients undergoing total hip/knee replacement surgery. Tissue sections were stained with Safranin O and graded. Immunohistochemical staining was then performed, and levels of RANKL and OPG expression were assessed using a semi-quantitative scoring system. In addition, levels of mRNA encoding for RANKL and OPG were determined by a relative real-time reverse transcription–polymerase chain reaction technique. We found that expression of RANKL protein, mRNA expression, and the ratio of RANKL: OPG mRNA was greater in grade 2 cartilage in comparison with grade 0 cartilage (P < 0.05). Increased RANKL staining in the grade 2 cartilage was predominantly in the peri-cellular region of the middle and deep zones as well as in the matrix of the superficial zone. OPG mRNA expression was greater in grade 3 cartilage in comparison with grade 0 cartilage (P < 0.05). Cartilage and subchondral bone are in close proximity and soluble proteins produced in the cartilage are likely to move from one compartment to the other. Our finding of increased expression of RANKL in grade 2 OA cartilage might explain the increase in bone turnover reported in the subchondral bone of OA patients. The changes seen in the different grades of tissue may also indicate that this effect occurs during the early stages of OA development.