Background/Purpose: Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease characterized by synovial inflammation and joint destruction. Monocytes and synovial macrophages are key players in inflammatory process of RA. Enolase-1 (ENO1) is a multifunctional glycolytic enzyme in cytoplasm of cells and it is also found on the cell surface as plasminogen receptor. The majority of cells expressing ENO1 in peripheral blood mononuclear cells (PBMCs) and synovial fluid mononuclear cells (SFMCs) derived from RA patients were known to be CD14-positive monocytes and macrophages. This study was aimed to discover and investigate a novel ligand of cell surface expressed ENO1 and biological role of the interaction between ENO1 and its novel ligand, apolipoprotein B (apoB).

Methods: ENO1 binding protein was identified present in RA synovial fluid (SF) using affinity-base mass spectrometry analysis. The interaction between ENO1 and apoB was evaluated using physical characterization, such as ligand blotting assay, ligand binding assay, surface plasmon resonance (SPR), and confocal microscopy. The production of pro-inflammatory cytokines in PBMCs from RA or healthy control (HC) after stimulation with apoB were evaluated using cytokines ELISA. The pro-inflammatory effect of apoB was evaluated in K/BxN serum transfer arthritis mouse model.

Results: Characterization of physical interaction between ENO1 and apoB using various binding assay, ligand blotting assay, ligand binding assay, SPR, and confocal microscopy showed that apoB is a novel ligand of ENO1. Interaction between surface ENO1 and apoB induced higher levels of pro-inflammatory cytokines in RA PBMCs than HC PBMCs. When surface ENO1 expression was down-regulated after transfection with ENO1-specific siRNA, production of inflammatory cytokines by RA PBMCs in response to apoB stimulation decreased. Moreover, in K/BxN serum transfer arthritis model, mice were immunized with K/BxN serum and apoB induced exacerbation of arthritis with increased ankle thickness, arthritis score, and production of pro-inflammatory cytokines. LDLR knockout mice are known to have high levels of serum LDL and apoB. The arthritis score and ankle thickness were markedly higher in low density lipoproteins receptor (LDLR) knockout mice compared to wild type after K/BxN serum transfer.

Conclusion: In this study, we discovered apoB as a novel ligand of ENO1. ApoB may enhance chronic inflammation in RA patients.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/apolipoprotein-b-binds-to-enolase-1-and-aggravates-inflammation-in-rheumatoid-arthritis/