Background/Purpose: Fibroblast-like synoviocytes (FLS) in the synovial intimal lining are key mediators of inflammation and joint destruction in rheumatoid arthritis (RA). In RA these cells assume a tumor-like phenotype, aggressively invading the extracellular matrix and producing cartilage-degrading proteases and inflammatory cytokines. The behavior of synovial fibroblasts is controlled by multiple interconnected signal transduction pathways involving reversible protein phosphorylation. However, little is known about the role of protein tyrosine phosphatases (PTPs) in FLS function. The objective of this study was to identify PTPs involved in the aggressive phenotype of RA FLS. 

Methods: Comparative screening was conducted of PTP expression in FLS from patients with RA or osteoarthritis (OA) by quantitative polymerase chain reaction (PCR). The functional effect on RA FLS of protein tyrosine phosphatase kappa (RPTPκ), a transmembrane PTP that was up-regulated in RA, was then analyzed by knockdown using cell-permeable antisense oligonucleotides (ASO). Transwell FLS invasion assays were performed with Matrigel-coated inserts, using fetal bovine serum or platelet-derived growth factor as chemoattractants. FLS adhesion and spreading was assessed by F-actin staining with phalloidin and visualized by immunofluorescence microscopy. Expression levels of matrix metalloproteinases (MMPs) were quantified by qPCR. Western blotting of cell lysates using phosphospecific antibodies was used to assess activation of signaling pathways. Cell survival was measured by flow cytometry following staining with propidium iodide and Annexin V.

Results: Expression of PTPRK, encoding the protein tyrosine phosphatase RPTPκ, was increased in RA (n=16) compared to OA (n=15) FLS (1.99±0.68 fold increase, p<0.05). RPTPκ, a proposed transforming growth factor β (TGFβ) dependent gene, belongs to a family of PTPs that mediate cell-cell adhesion through homophilic interaction between their extracellular domains. We found that PTPRK expression is induced by stimulation of RA FLS with 50 ng/ml TGFβ1 for 24 hours (1.91±0.45 fold increase, p<0.05). Subsequent studies focused on the functional role of RPTPκ in RA FLS. Knockdown of this PTP with ASO led to impaired cell invasion (67.8±16.5% decrease, p<0.05). Although cell adhesion to an extracellular matrix was unaffected, RPTPκ deficiency disrupted cell spreading (40.2±12.7% decrease, p<0.05). RPTPκ-deficient RA FLS displayed more than 2-fold decreased TNF (50 ng/ml) or TGFβ-mediated induction of MMP1, MMP8, MMP10, MMP13 and MMP14, which correlated with reduced induction of JNK phosphorylation.

Conclusion: These findings indicate a novel role for RPTPκ as a key mediator of FLS function. RPTPκ promotes the invasiveness of RA FLS and, due to its higher expression in RA compared with OA FLS, could contribute to the unique “transformed” phenotype of rheumatoid cells. Therefore RPTPκ could be a novel therapeutic target for RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-transmembrane-protein-tyrosine-phosphatase-kappa-promotes-aggressiveness-of-rheumatoid-arthritis-fibroblast-like-synoviocytes/