Formation of invadopodia-like structures by synovial cells promotes cartilage breakdown in collagen-induced arthritis: involvement of the protein tyrosine kinase Src.

Objective: Invasive synovial fibroblasts are suggested to be the major effectors of cartilage and bone destruction, and this aggressive phenotype can lead to irreversible damage. In cancer cells, invasion across tissue boundaries and metastasis have recently been shown to depend on the capacity of the cells to breach the basement membrane, a process that was linked to the formation of the actin-rich cell protrusions called invadopodia. This study was undertaken to investigate whether arthritic synovial cells use invadopodia to invade and degrade cartilage components.

Methods: Fibroblast-like synoviocytes (FLS) from control rats or rats with collagen-induced arthritis (CIA) were cultured on fluorescent matrix in the presence of Src inhibitors or were transfected with wild-type or variants of Src kinases. The in vivo effect of Src inhibition on cartilage degradation and invasion was studied in a rat model of CIA.

Results: FLS from rats with CIA produced more invadopodia-like structures than did FLS from control rats, leading to increased extracellular matrix degradation. Furthermore, c-Src activation was increased in synovial cells from rats with CIA, and Src activity was found to mediate the formation of invadopodia. Pharmacologic blockade of Src activity by PP2 or intraarticular expression of a c-Src-specific short hairpin RNA in the CIA model reduced synovial membrane hyperplasia and cartilage degradation, an event linked to decreased invadopodia formation by synovial fibroblasts.

Conclusion: This study demonstrates that inhibition of invadopodia formation in arthritic synovial cells leads to a direct effect on extracellular matrix degradation in vitro and in vivo, making invadopodia a relevant therapeutic target for interfering with this process.