Objective: To study the effect of anterior cruciate ligament (ACL) injury on lubricin concentrations in synovial fluid (SF) and its correlation with time postinjury, inflammatory cytokines, lubricin‐degrading enzymes, and SF proteoglycan content.Methods: SF samples were obtained from both knees of 30 patients with unilateral ACL insufficiency, 32–364 days postinjury. Lubricin, inflammatory cytokines (interleukin‐1β [IL‐1β], tumor necrosis factor α [TNFα], and IL‐6), and catabolic enzymes (procathepsin B and neutrophil elastase) were measured in SF from injured and contralateral (uninjured) joints, by enzyme‐linked immunosorbent assay. Sulfated glycosaminoglycan (sGAG) levels in the SF were measured by Alcian blue binding assay.Results: SF lubricin concentrations were significantly (P < 0.001) reduced at an early stage following ACL injury when compared with those in the contralateral joint. Within 12 months, the lubricin concentration in the injured knee (slope = 0.006, SE = 0.00010, P < 0.001) approached that in the contralateral knee, which did not change with time (slope = −0.0002, SE = 0.00050, P = 0.71). TNFα levels showed a significant negative relationship with log₂ lubricin levels. IL‐1β, TNFα, IL‐6, procathepsin B, and neutrophil elastase concentrations in SF from injured knees were greater in samples from recently injured knees compared with those that were chronically injured. There were no detectable cytokines or enzymes in the SF of contralateral joints. Concentrations of sGAG were significantly (P = 0.0002) higher in the SF from injured knees compared with the contralateral joints.Conclusion: The decrease in SF lubricin concentrations following ACL injury may place the joint at an increased risk of wear‐induced damage as a consequence of lack of boundary lubrication, potentially leading to secondary osteoarthritis. The decrease in SF lubricin was associated with an increase in levels of inflammatory cytokines.

Osteoarthritis is a complex disease involving the mechanical breakdown of articular cartilage in the presence of altered joint mechanics and chondrocyte death, but the connection between these factors is not well established. Lubricin, a mucinous glycoprotein encoded by the PRG4 gene, provides boundary lubrication in articular joints. Joint friction is elevated and accompanied by accelerated cartilage damage in humans and mice that have genetic deficiency of lubricin. Here, we investigated the relationship between coefficient of friction and chondrocyte death using ex vivo and in vitro measurements of friction and apoptosis. We observed increases in whole-joint friction and cellular apoptosis in lubricin knockout mice compared with wild-type mice. When we used an in vitro bovine explant cartilage-on-cartilage bearing system, we observed a direct correlation between coefficient of friction and chondrocyte apoptosis in the superficial layers of cartilage. In the bovine explant system, the addition of lubricin as a test lubricant significantly lowered the static coefficient of friction and number of apoptotic chondrocytes. These results demonstrate a direct connection between lubricin, boundary lubrication, and cell survival and suggest that supplementation of synovial fluid with lubricin may be an effective treatment to prevent cartilage deterioration in patients with genetic or acquired deficiency of lubricin.

Objective: The glycoprotein lubricin (encoded by the gene Prg4) is secreted by surface chondrocytes and synovial cells, and has been shown to reduce friction in vitro. In contrast to man‐made bearings, mammalian diarthrodial joints must endogenously produce friction‐reducing agents. This study was undertaken to investigate whether friction is associated with wear.Methods: The lubricating ability of synovial fluid (SF) samples from humans with genetic lubricin deficiency was tested in vitro. The coefficient of friction in the knee joints of normal and lubricin‐null mice was measured ex vivo; these joints were also studied by light and electron microscopy. Atomic force microscopy was used to image and measure how lubricin reduces friction in vitro.Results: SF lacking lubricin failed to reduce friction in the boundary mode. Joints of lubricin‐null mice showed early wear and higher friction than joints from their wild‐type counterparts. Lubricin self‐organized and reduced the work of adhesion between apposing asperities.Conclusion: These data show that friction is coupled with wear at the cartilage surface in vivo. They imply that acquired lubricin degradation occurring in inflammatory joint diseases predisposes the cartilage to damage. Lastly, they suggest that lubricin, or similar biomolecules, will have applications in man‐made devices in which reducing friction is essential.

Objective: To study the relationship between the boundary‐lubricating ability of synovial fluid (SF) and articular cartilage damage in a rabbit knee injury model, to correlate collagen markers of such damage with SF boundary‐lubricating ability and elastase activity, and to examine the lubricating ability of SF, together with collagen markers of articular cartilage damage, under the inflammatory conditions of knee joint synovitis (KJS) and rheumatoid arthritis (RA).Methods: SF was aspirated weekly from the affected knee joints of 10 adult rabbits following transection of the anterior and posterior cruciate ligaments. The boundary‐lubricating ability of SF was determined in vitro using a previously described friction apparatus. Lubricin concentrations and type II collagen (CII) peptides were quantified by sandwich enzyme‐linked immunosorbent assays (ELISAs). Levels of the C‐terminal neoepitope 9A4 (derived from collagenase degradation of CI, CII, and CIII) and of epitope 5‐D‐4 of keratan sulfate (a marker of proteoglycan depletion) were quantified by inhibition ELISAs. Elastase activity was measured spectrophotometrically. The sensitivity of purified human lubricin to digestion by neutrophil elastase (NE) was examined by Western blotting.Results: The lubricating ability of SF from injured rabbit knees was significantly decreased at weeks 2 and 3 compared with week 1 after injury. Lubricin concentrations were significantly higher at week 1 than at weeks 2 and 3. CII peptide concentrations increased significantly at weeks 2 and 3 compared with week 1, while 9A4 neoepitope concentrations increased significantly at week 3 compared with weeks 1 and 2. There were no significant differences in epitope 5‐D‐4 concentrations among the 3 weeks. Elastase activity in SF increased significantly at weeks 2 and 3 compared with week 1. Elastase activity correlated significantly with diminishing lubrication at weeks 1, 2, and 3. SF from patients with KJS or RA exhibited deficient lubrication and elevated levels of CII peptides compared with SF from normal controls. NE was shown to completely degrade purified human lubricin in vitro.Conclusion: Loss of boundary‐lubricating ability of SF after injury is associated with damage to the articular cartilage matrix. This can be attributed to inflammatory processes resulting from the injury, particularly in the early phases. This association also exists in patients with acute knee injuries or progressive chronic inflammatory arthritis.

Objective: To investigate whether cartilage degeneration is prevented or minimized following intraarticular injections of lubricin derived from human synoviocytes in culture, recombinant human PRG4 (rhPRG4), or human synovial fluid (SF) in a rat model of anterior cruciate ligament (ACL) injury.Methods: Unilateral ACL transection (ACLT) was performed in Lewis rats (n = 45). Nine animals were left untreated. The remaining rats were given intraarticular injections (50 μl/injection) of either phosphate buffered saline (PBS) (n = 9), human synoviocyte lubricin (200 μg/ml; n = 9), rhPRG4 (200 μg/ml; n = 9), or human SF lubricin (200 μg/ml; n = 9) twice weekly beginning on day 7 after injury. Joints were harvested on day 32 after injury. Histologic analysis was performed using Safranin O–fast green staining, and articular cartilage degeneration was graded using the Osteoarthritis Research Society International (OARSI)–modified Mankin criteria. Histologic specimens were immunoprobed for lubricin and sulfated glycosaminoglycans. A 24‐hour urine collection was performed on days 17 and 29 postinjury, and urinary C‐terminal telopeptide of type II collagen (CTX‐II) levels were measured.Results: Treatment with human synoviocyte lubricin resulted in significantly lower OARSI scores for cartilage degeneration compared with no treatment or PBS treatment (P < 0.05). Increased immunostaining for lubricin in the superficial zone chondrocytes and on the surface of cartilage was observed in lubricin‐treated, but not untreated or PBS‐treated, joints. On day 17, urinary CTX‐II levels in human synoviocyte lubricin– and human SF lubricin–treated animals were significantly lower than those in untreated animals (P = 0.005 and P = 0.002, respectively) and in PBS‐treated animals (P = 0.002 and P < 0.001, respectively).Conclusion: After treatment with any of the 3 types of lubricin evaluated in this study, a reduction in cartilage damage following ACLT was evident, combined with a reduction in type II collagen degradation. Our findings indicate that intraarticular lubricin injection following an ACL injury may be beneficial in retarding the degeneration of cartilage and the development of posttraumatic OA.