Background/Purpose: Rheumatoid arthritis is an inflammatory arthritis characterized by joint erosions and destruction. This damage is mediated in part by invasion of synovial fibroblasts into cartilage and bone. Cellular invasion has been extensively studied in cancer cells, but we are just beginning to understand the elements necessary for synovial fibroblast invasion. Focal adhesion kinase (FAK) is a protein scaffold and tyrosine kinase important for cancer cell migration and invasion. FAK may also play a role in synovial fibroblast invasion and arthritis. Phosphorylated FAK is increased in rheumatoid synovial tissue and localizes to focal adhesions in rheumatoid synovial fibroblasts. Also, FAK mediates IL-6 induction and is expressed in endothelial and immune cells. FAK inhibitors are in clinical trials for cancer, so if FAK were important for synovial fibroblast invasion and arthritis, FAK inhibitors might also be of benefit for treating rheumatoid arthritis.

Methods: Synovial fibroblasts derived from the synovial fluid of patients with rheumatoid arthritis were treated with vehicle control or two different FAK inhibitors, PF 562271 and PF 573228, and plated on Matrigel invasion chambers to assess the role of FAK in invasion. To dissect which steps of invasion require FAK activity, we assessed migration using fibronectin coated transwells and focal matrix degradation by plating synovial fibroblasts on coverslips coated with fluorescent gelatin. To determine the role of FAK in arthritis, we crossed established lines of mice to generate mice that overexpress TNFα, express tamoxifen inducible Cre recombinase, and carry FAK flanked by LoxP sites. These TNF⁺Cre⁺FAK^f/fmice spontaneously get arthritis due to overexpression of TNFα and can be induced to delete FAK by treatment with tamoxifen. Western blots were done to assess loss of FAK protein in splenocytes after tamoxifen treatment. Arthritis was assessed in FAK deficient and control arthritic mice by scoring paw swelling and deformity as well as testing grip strength from 2 to 5 months of age.

Results: Inhibition of FAK using either PF 562271 or PF 573228 resulted in decreased rheumatoid synovial fibroblast invasion through Matrigel and decreased migration through fibronectin coated transwells compared to vehicle control. Focal degradation of gelatin still occurs when FAK is inhibited. TNF⁺Cre⁺FAK^f/fmice treated with tamoxifen were found to have good deletion of FAK in splenocytes. Arthritic mice deficient in FAK did not have significant alterations in arthritis by clinical scoring compared to arthritic mice without FAK deletion.

Conclusion: Consistent with findings in cancer cells, activated FAK is an important component of synovial fibroblast migration and the migratory component of invasion, but not focal matrix degradation. According to clinical arthritis scores, FAK does not appear to play a critical role in arthritis. Studies are ongoing to address if loss of FAK protein, as opposed to FAK inhibition, alters focal gelatin degradation and if joint erosions are altered in arthritis in the absence of FAK, which may be a more specific sign of synovial fibroblast dysfunction than clinical scores.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/role-of-focal-adhesion-kinase-in-synovial-fibroblast-invasion-and-arthritis/