Inflammation is a major cause of cartilage destruction and leads to the imbalance of metabolic activities in the arthritic joint. Pigment epithelium-derived factor (PEDF) has been reported to have both pro- and anti-inflammatory activities in various cell types and to be upregulated in the arthritic joint, but its role in joint destruction is unclear. Our aim was to investigate the role of PEDF in cartilage degeneration under inflammatory conditions.
We showed that PEDF protein levels were higher in human osteoarthritis samples compared to normal samples. We demonstrated that ectopic PEDF expression in primary human articular chondrocytes exacerbated catabolic gene expression in the presence of IL-1β. In whole bone organ cultures, IL-1β induced MMP-1, MMP-3 and MMP-13 protein production, and caused significant cartilage matrix loss. Interestingly, Toluidine Blue staining showed that PEDF-deficient bones from 29 week old animals, but not 10 week old animals, had reduced matrix loss in response to IL-1β compared to their wild type counterparts. In addition, PEDF-deficiency in 29 week old animals preserved matrix integrity and protected against cell loss in the MIA joint destruction model in vivo.
In arthritic disease pathogenesis, the role of PEDF is unclear as even its expression in the joint is controversial. In one report, PEDF expression is found in normal articular chondrocytes and upregulated in human OA cartilage samples . Another recent study shows that PEDF is not expressed in normal or OA articular chondrocytes, but is only upregulated predominantly in osteophytic chondrocytes . While it was shown that PEDF contributes to the terminal differentiation of the endochondral ossification process of osteophyte formation , its role in the inflammatory process in joint destruction remains unclear. In this study, we sought to investigate PEDF protein in articular chondrocytes, and address whether PEDF plays a role in cartilage degeneration by evaluating its effects on cartilage in an ex vivo culture system under inflammatory stimuli and in an inflammatory animal model of joint destruction.
In this study, we demonstrated that PEDF protein is expressed in adult knee articular chondrocytes where it potentiates inflammatory stimuli-induced joint cartilage damage. PEDF enhanced the catabolic profile of primary human articular chondrocytes under inflammatory stimuli. In explant cultures using PEDF-deficient mice, loss of PEDF expression protected the explants cultured from older mice against matrix loss. The protective effect was associated with a decrease in MMP protein levels. In an in vivo inflammatory joint destruction model, PEDF loss preserved matrix levels and cellularity. These data are consistent with the notion that PEDF renders chondrocytes more responsive to inflammatory stimuli over time.
Osteoarthritis (OA) is a prevalent disease characterized by chronic joint degeneration and low-grade localized inflammation. There is no available treatment to delay OA progression. We report that in human primary articular chondrocytes, erythromycin, a well-known macrolide antibiotic, had the ability to inhibit pro-inflammatory cytokine Interleukin 1β (IL-1β)-induced catabolic gene expression and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. Furthermore, erythromycin inhibited monosodium iodoacetate (MIA)-induced joint inflammation and cartilage matrix destruction in mice, an arthritis model that reflects the inflammatory and cartilage matrix loss aspects of OA. EM900, an erythromycin-derivative lacking antibiotic function, had the same activity as erythromycin in vitro and in vivo, indicating distinct anti-inflammatory and antibiotic properties. Using an antibody against erythromycin, we found erythromycin was present on chondrocytes in a dose-dependent manner. The association of erythromycin with chondrocytes was diminished in ghrelin receptor null chondrocytes, and administration of the ghrelin ligand prevented the association of erythromycin with chondrocytes. Importantly, the anti-inflammatory activity of erythromycin was diminished in ghrelin receptor null chondrocytes. Moreover, erythromycin could not exert its chondroprotective effect in ghrelin receptor null mice, and the loss of ghrelin receptor further augmented joint damage upon MIA-injection. Therefore, our study identified a novel pharmacological mechanism for how erythromycin exerts its chondroprotective effect. This mechanism entails ghrelin receptor signaling, which is necessary for alleviating inflammation and joint destruction.