Background/Purpose: Rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS) promote disease pathogenesis by aggressively invading the joint extracellular matrix. The focal adhesion kinase (FAK) signaling pathway is emerging as a contributor to the anomalous behavior of FLS in this disease. Because the receptor protein tyrosine phosphatase α (RPTPα), encoded by the PTPRA gene, is a key promoter of FAK signaling, we investigated whether RPTPα mediates FLS aggressiveness and joint damage.

Methods: Through RPTPα knockdown with cell-permeable morpholino antisense oligonucleotides, we assessed RA FLS gene expression in response to tumor necrosis factor (TNF) and interleukin 1β (IL-1β) by quantitative PCR, invasion in transwell assays in response to platelet-derived growth factor (PDGF), and activation of signaling pathways by Western blotting of FLS lysates. Arthritis development was examined in mice lacking Ptpra using the K/BxN passive serum transfer model. The contribution of radiosensitive and radioresistant cells to disease was evaluated by reciprocal bone-marrow transplantation. Pharmacological inhibition of RPTPα activity was achieved using a novel cell-permeable peptide.

Results: We found that RPTPα was enriched in the RA synovial lining and expressed in RA FLS. Upon RPTPα knockdown, PDGF-induced invasion of RA FLS was decreased by 49±8% (p<0.0001, Mann-Whitney test). RPTPα knockdown in RA FLS also decreased TNF-induced expression of C-X-C motif chemokine 10 (CXCL10) and matrix metalloproteinase 13 (MMP13) by >80% (p<0.001, Mann-Whitney test), and decreased IL-1β-induced production of IL6, MMP3 and MMP13 by ≥50% (p<0.0001, Mann-Whitney test). These phenotypes correlated with >3-fold increase in phosphorylation of SRC on inhibitory Y527 and decreased phosphorylation of FAK on stimulatory Y397. Treatment of RA FLS with inhibitors of SRC and FAK mimicked the effect of RPTPα knockdown. Ptpra-deficient mice were protected from arthritis development (58±3% decrease in area under the curve [AUC] of ankle swelling, p<0.0001, paired t-test), which we confirmed to be due to radioresistant cells. Importantly, pharmacological inhibition of RPTPα activity decreased TNF-induced expression of CXCL10 in RA FLS by 65±16% and reduced arthritis in mice (32±1% decrease in AUC of ankle swelling, p<0.05, paired t-test).

Conclusion: These data suggest that RPTPα mediates pro-inflammatory and pro-invasive signaling in RA FLS by regulating phosphorylation of SRC and FAK. We also find that knockdown or pharmacological inhibition of RPTPα significantly ameliorates arthritis in an FLS-dependent model of RA, suggesting that inhibition of RPTPα could be a novel therapeutic strategy to target the aggressiveness of FLS in RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/receptor-protein-tyrosine-phosphatase-alpha-enhances-rheumatoid-synovial-fibroblast-signaling-and-promotes-arthritis-in-mice/