*2.2. Cells*

Human osteoarthritic (OA) cartilage (Mankin score 7–9) was obtained from total knee arthroplasty samples (n = 4) from patients, after informed consent signature [18] before inclusion in the study. The study was approved by the Ethics Committee of the Saarland Physicians Council (*Ärztekammer des Saarlandes*, reference number Ha06/08). All procedures were in conformity with the Helsinki Declaration. Human OA chondrocytes (passage 1–2) were isolated by collagenase digestion of cartilage slices as previously described [18,22] and cultured in DMEM, 10% FBS, 100 U/mL penicillin G, 100 μL/mL streptomycin (growth medium) prior to the studies, without cell dedifferentiation.
