Neutrophil gelatinase–associated lipocalin is expressed in osteoarthritis and forms a complex with matrix metalloproteinase 9

Abstract Objective Expression of matrix metalloproteinase 9 (MMP‐9) is up‐regulated in osteoarthritis (OA) and usually presents as multiple bands when synovial fluid (SF) from OA patients is analyzed by zymography. Among these bands is an ∼125–130–kd band for high molecular weight (HMW) gelatinase, which has not been characterized. This study was undertaken to characterize the HMW MMP activity in OA SF. Methods MMP activity in OA SF was determined by gelatin zymography. Recombinant MMPs were used to identify MMP activity on the zymogram. Western immunoblotting, immunoprecipitation, and immunodepletion analyses were performed using antibodies specific for human MMP‐9 and human neutrophil gelatinase–associated lipocalin (NGAL). Human cartilage matrix degradation was determined by dimethylmethylene blue assay. Results Zymographic analysis showed that the HMW gelatinase in OA SF comigrated with a purified NGAL–MMP‐9 complex. Results of Western immunoblotting showed that the HMW gelatinase was also recognized by antibodies specific for human NGAL or human MMP‐9. These same antibodies also immunoprecipitated the HMW gelatinase activity from OA SF. The NGAL–MMP‐9 complex was reconstituted in vitro in gelatinase buffer. In the presence of NGAL, MMP‐9 activity was stabilized; in the absence of NGAL, rapid loss of MMP‐9 activity occurred. MMP‐9–mediated release of cartilage matrix proteoglycans was significantly higher in the presence of NGAL ( P < 0.05). Conclusion Our findings demonstrate that the HMW gelatinase activity in OA SF represents a complex of NGAL and MMP‐9. The ability of NGAL to protect MMP‐9 activity is relevant to cartilage matrix degradation in OA and may represent an important mechanism by which NGAL may contribute to the loss of cartilage matrix proteins in OA.