Diminished Cartilage-Lubricating Ability of Human Osteoarthritic Synovial Fluid Deficient in Proteoglycan 4 Restoration Through Proteoglycan 4 Supplementation

Objective. The purposes of this study were 1) to quantify the proteoglycan 4 (PRG4) and hyaluronan (HA) content in synovial fluid (SF) from normal donors and from patients with chronic osteoarthritis (OA) and 2) to assess the cartilage boundary-lubricating ability of PRG4-deficient OA SF as compared to that of normal SF, with and without supplementation with PRG4 and/or HA. Methods. OA SF was aspirated from the knee joints of patients with symptomatic chronic knee OA prior to therapeutic injection. PRG4 concentrations were measured using a custom sandwich enzyme-linked immunosorbent assay (ELISA), and HA concentrations were measured using a commercially available ELISA. The molecular weight distribution of HA was measured by agarose gel electrophoresis. The cartilage boundary-lubricating ability of PRG4-deficient OA SF, PRG4-deficient OA SF supplemented with PRG4 and/or HA, and normal SF was assessed using a cartilage-on-cartilage friction test. Two friction coefficients (mu) were calculated: static (mu(static,) (Neq)) and kinetic () (where (Neq) represents equilibrium axial load and angle brackets indicate that the value is an average). Results. The mean +/- SEM PRG4 concentration in normal SF was 287.1 +/- 31.8 mu g/ml. OA SF samples deficient in PRG4 (146.5 +/- 28.2 mu g/ml) as compared to normal were identified and selected for lubrication testing. The HA concentration in PRG4-deficient OA SF (mean +/- SEM 0.73 +/- 0.08 mg/ml) was not significantly different from that in normal SF (0.54 +/- 0.09 mg/ml). In PRG4-deficient OA SF, the molecular weight distribution of HA was shifted toward the lower range. The cartilage boundary-lubricating ability of PRG4-deficient OA SF was significantly diminished as compared to normal (mean +/- SEM = 0.043 +/- 0.008 versus 0.025 +/- 0.002; P < 0.05) and was restored when supplemented with PRG4 ( = 0.023 +/- 0.003; P < 0.05). Conclusion. These results indicate that some OA SF may have decreased PRG4 levels and diminished cartilage boundary-lubricating ability as compared to normal SF and that PRG4 supplementation can restore normal cartilage boundary lubrication function to these OA SF.