Objective: The lipid peroxidation product 4-hydroxynonenal (HNE) is prominently produced in osteoarthritic (OA) synovial cells, but its specific contribution to cartilage destruction is not understood. This study was designed to test whether HNE signaling and binding are involved in OA cartilage degradation through type II collagen (CII) and matrix metalloproteinase 13 (MMP-13) modulation.
Methods: HNE levels in synovial fluid and in isolated OA chondrocytes treated with free radical donors were determined by enzyme-linked immunosorbent assay. The formation of the HNE/CII adducts was measured in cartilage explants by immunoprecipitation. Levels of CII and MMP-13 messenger RNA and protein were determined by reverse transcription-polymerase chain reaction, Western blotting, and by the use of commercial kits.
Results: Levels of HNE/protein adducts were higher in OA synovial fluid compared with normal synovial fluid and were higher in OA chondrocytes treated with free radical donors compared with untreated cells. In cartilage explants, HNE induced CII cleavage, as established by the generation of neoepitopes. The level of HNE/CII adducts was increased in OA cartilage explants incubated with free radical donors. Modification of CII by HNE accelerated its degradation by active MMP-13. In isolated OA chondrocytes, HNE inhibited the expression of CII and tissue inhibitor of metalloproteinases 1 and induced MMP-13 mainly through activation of p38 MAPK. In vitro, HNE binding to MMP-13 activated this enzyme at a molar ratio of 1:100 (MMP-13 to HNE).
Conclusion: The increased level of HNE in OA cartilage and the ability of HNE to induce transcriptional and posttranslational modifications of CII and MMP-13 suggest that this aldehyde could play a role in OA.