Comparative proteomic characterization of articular cartilage tissue from normal donors and patients with osteoarthritis

Objective. To identify potential molecular mediators and biomarkers for osteoarthritis (OA), through comparative proteomic analysis of articular cartilage tissue obtained from normal donors without OA (n = 7) and patients with OA (n = 7). Methods. The proteomic analyses comprised extraction of soluble proteins from cartilage, separation of the protein mixtures by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by in-gel digestion, and subsequent nano-liquid chromatography-tandem mass spectrometry analysis in conjunction with a database search for protein identification and semi-quantitation. Results. A total of 814 distinct proteins were identified with high confidence from 14 samples; 420 of these proteins were detected with >= 3 unique peptides in at least 4 samples from the same group. Using stringent criteria, 59 proteins were found to be differentially expressed in OA cartilage. Gene Ontology and Ingenuity pathway analysis tools were used to characterize these proteins into functional categories. One of the up-regulated proteins, HtrA1, a serine protease, was detected at high levels in cartilage. Conclusion. Altered protein expression in the disease state is associated with many aspects of the pathogenesis of OA, such as increased proteolysis, lipid metabolism, immune response, and decreased signal transduction. To our knowledge, this is the first time that a large portion of these proteins and their expression patterns were identified in cartilage, thus providing new insights for finding novel pathologic mediators and biomarkers of OA.