Reduction of friction by recombinant human proteoglycan 4 in IL‐1α stimulated bovine cartilage explants

Abstract

A boundary lubricant attaches and protects sliding bearing surfaces by preventing interlocking asperity-asperity contact. Proteoglycan-4 (PRG4) is a boundary lubricant found in the synovial fluid that provides chondroprotection to articular surfaces. Inflammation of the diarthrodial joint modulates local PRG4 concentration. Thus, we measured the effects of inflammation, with Interkeukin-1α (IL-1α) incubation, upon boundary lubrication and PRG4 expression in bovine cartilage explants. We further aimed to determine whether the addition of exogenous human recombinant PRG4 (rhPRG4) could mitigate the effects of inflammation on boundary lubrication and PRG4 expression in vitro. Cartilage explants, following a 7-day incubation with IL-1α, were tested in a disc-on-disc configuration using either rhPRG4 or saline (PBS control) as a lubricant. Following mechanical testing, explants were studied immunohistochemically or underwent RNA extraction for RT-PCR. We found that static coefficient of friction (COF) significantly decreased to 0.14 ±0.065 from 0.21 ±0.059 (p=0.014) in IL-1α stimulated explants lubricated with rhPRG4, as compared to PBS. PRG4 expression was significantly up regulated from 30.8 ± 19 copies in control explants lubricated with PBS to 3330 ± 1760 copies in control explants lubricated with rhPRG4 (p<0.001). Explants stimulated with IL-1α displayed no increase in PRG4 expression upon lubrication with rhPRG4, but with PBS as the lubricant, IL-1α stimulation significantly increased PRG4 expression compared to the control condition from 30.8 ± 19 copies to 401 ± 340 copies (p=0.015). Overall, these data suggest that exogenous rhPRG4 may provide a therapeutic option for reducing friction in transient inflammatory conditions and increasing PRG4 expression.

Two compartment pharmacokinetic model describes the intra‐articular delivery and retention of rhprg4 following ACL transection in the Yucatan mini pig

“…Human recombinant PRG4 also displays anti‐inflammatory properties through inhibition of the innate immunity receptors, TLR2, and TLR4 . Suppressing inflammation by itself may play a positive role since incubation of bovine cartilage explants with IL‐1β in vitro has been shown to suppress native PRG4 expression . Figures and together support the notion that rhPRG4 densely coated cartilage surfaces on Days 1 and 3 post injection and subsequently diminished over time.…”

“…Sections were probed for surface accumulation and nascent expression of PRG4 within chondrocytes using monoclonal antibody 9G3 (EMD Millipore; Darmstadt, Germany) which reacts with the mucin domain of PRG4 using methods described previously …”

“…However, this value must be reconciled with radioactivity still present on the articular surface up to 28 days following IA injection. Lubricin persists in traumatized cartilage and contributes to protect joints from mechanical injury and may mitigate PTOA. Our current focus is to gather data in a large animal model, particularly at early time points in an injury model, to enable pharmacokinetic modeling that will be predictive of lubricin residence time in the human knee by allometric scaling .…”

Friction-Induced Mitochondrial Dysregulation Contributes to Joint Deterioration in Prg4 Knockout Mice

“…Physiological loading and lubricated movement of apposed cartilage surfaces promotes lubricin expression both in vitro [22,23,24] and in vivo [25]. While normal joint loading is required for maintaining cartilage health [26,27], abnormal loading, either due to blunt trauma [28,29], strenuous exercise or friction vis-a-vis the absence of lubricin, leads to chondrocyte apoptosis and osteoarthrosis [14,30].…”

Early genetic restoration of lubricin expression in transgenic mice mitigates chondrocyte peroxynitrite release and caspase-3 activation

“…This scenario would recapitulate the effects of lubricin down regulation and later restoration of lubricin expression in joints that have undergone trauma or sports injury. [41] These models will provide a better understanding of post-traumatic osteoarthritis and how it might be mitigated via lubricin supplementation. [12]…”