Abstract
The pathogenesis of Graves' disease (GD) is not fully understood. However, aberrant inflammatory cytokine production following stimulation of thyroid-stimulating hormone (TSH) receptors by autoantibodies has been implicated. Osteopontin, a multifunctional extracellular matrix protein, has recently been identified as a biomarker for various autoimmune diseases. This study addressed the role of osteopontin in GD.
Osteopontin, interferon-α (IFN-α) and interferon-γ (IFN-γ) were measured by enzyme-linked immunosorbent assay in serum from GD patients (n = 76) and healthy controls (n = 65). mRNA expression of osteopontin and its receptors (integrins), cytokines and chemokines in peripheral blood mononuclear cells (PBMC) from GD patients and controls was assessed by quantitative reverse transcriptase-polymerase chain reaction. The effect of osteopontin on activating nuclear factor-κB (NF-κB) activation was determined by plasmid transfection of HeLa cells using a luciferase reporter assay.
Serum osteopontin concentrations correlated significantly with typical biochemical features of GD including thyroxine (FT4) (r = 0.43; P = 0.004), triiodothyronine (FT3) (r = 0.36; P = 0.02), TSH receptor antibody (r = 0.54; P < 0.001) and TSH concentrations (r = −0.56; P < 0.001). Furthermore, serum osteopontin concentrations and mRNA levels in PBMC from GD patients were significantly higher than healthy controls (P < 0.001). Serum IFN-α and IFN-γ were significantly increased in GD patients (P < 0.001). There was a selective elevation in expression of osteopontin receptors αv, β1, β3 and CD44, and the proinflammatory cytokines IL-1β and IL-8, and chemokines CCL2 and CCL3, in PBMC from GD patients (all P < 0.05). The expression of these NF-κB downstream target genes correlated significantly with osteopontin concentration. In vitro assays showed that serum from GD patients enhanced NF-κB activation, which was suppressed by osteopontin abrogation using a monoclonal antibody.
This clinical correlation between serum osteopontin concentration and GD suggests that osteopontin could serve as a novel biomarker for GD. The authors speculate that osteopontin may affect GD development via NF-κB activation and the subsequent change in inflammatory milieu. Therefore, osteopontin might be a potential therapeutic target in GD and other autoimmune diseases.