Osteoarthritis (OA) is a degenerative joint disease inducing the degradation of the articular cartilage. Syndecan-4 (Sdc4) is a heparan sulfate proteoglycan, expressed under inflammatory conditions and by chondrocytes during OA. Little is known about Sdc4 shedding and its regulation in OA. Therefore, we investigated the regulation of Sdc4 shedding and underlying shedding mechanisms under OA conditions.
Articular cartilage, serum, synovial fluid and synovial membrane from OA patients with different radiological severity were analyzed. ELISA, RT-qPCR and IHC for Sdc4, MMP-2 and -9 were performed. MMP inhibitors and siRNA were evaluated for their effect on Sdc4 shedding by ELISA and on IL-1 signaling by western blot (pERK/ERK).
Shed Sdc4 was increased in synovial fluid of OA patients, but not in the serum and is a good predictor (AUC = 0.72) for OA severity with a sensitivity of 67.5% and specificity 65.2%. MMP-9, but not MMP-2, was increased in cartilage and synovial membrane at mRNA levels and in the synovial fluid at protein levels. Shed Sdc4 correlated with the amount of MMP-9 in synovial fluid. Further, the inhibition and knock-down of MMP-9 decreased the amount of shed Sdc4 in vitro. Increased Sdc4 shedding resulted in less phosphorylation of ERK upon IL-1β stimulation.
Shed Sdc4 might be a good prognostic biomarker for OA mediated cartilage degradation. MMP-9 seems to be the relevant sheddase for Sdc4 under OA conditions, desensitizing chondrocytes towards IL-1 signaling.