Identification of Fibroblast Growth Factor-18 as a Molecule to Protect Adult Articular Cartilage by Gene Expression Profiling*

Background: Prevention of adult articular cartilage and treatment of its lesions are essential. Results: Microarray analyses identified Fgf18, which inhibits aggrecan release from cartilage and enhances proliferation of chondrocytes. The intra-articular injection of rhFGF18 prevented cartilage degeneration in a rat osteoarthritis model. Conclusion: Fgf18 protects adult articular cartilage through Timp1 expression. Significance: Fgf18 may represent a therapeutic agent for osteoarthritis. To identify genes that maintain the homeostasis of adult articular cartilage and regenerate its lesions, we initially compared four types of chondrocytes: articular (AA) versus growth plate (AG) cartilage chondrocytes in adult rats, and superficial layer (IS) versus deep layer (ID) chondrocytes of epiphyseal cartilage in infant rats. Microarray analyses revealed that 40 and 186 genes had 10-fold higher expression ratios of AA/AG and IS/ID, respectively, and 16 genes showed 10-fold of both AA/AG and IS/ID ratios. The results were validated by real-time RT-PCR analysis. Among them, Hoxd1, Fgf18, and Esm1 were expressed more strongly in AA than in IS. Fgf18 was the extracellular and secreted factor that decreased glycosaminoglycan release and depletion from the cartilage, and enhanced proliferation of articular chondrocytes. Fgf18 was strongly expressed in the articular cartilage chondrocytes of adult rats. In a surgical rat osteoarthritis model, a once-weekly injection of recombinant human FGF18 (rhFGF18) given 3 weeks after surgery prevented cartilage degeneration in a dose-dependent manner at 6 and 9 weeks after surgery, with significant effect at 10 g/week of rhFGF18. As the underlying mechanism, rhFGF18 strongly up-regulated Timp1 expression in the cell and organ cultures, and inhibition of aggrecan release by rhFGF18 was restored by addition of an antibody to Timp1. In conclusion, we have identified Fgf18 as a molecule that protects articular cartilage by gene expression profiling, and the anticatabolic effects may at least partially be mediated by the Timp1 expression.