To investigate a role for insulin-resistant pathways in inflammation and therapeutic targeting for disease modification in rheumatoid arthritis (RA).
RA disease activity and cardiovascular risk factors, including insulin resistance and body mass index (BMI), were assessed in an Irish RA cohort. Glucose transporter 1 (GLUT-1) and GLUT-4 activity in RA and osteoarthritis (OA) synovial tissue was examined using immunohistochemistry. Spontaneous release of proinflammatory mediators from ex vivo RA synovial explants and primary synovial fibroblast (SF) cell culture supernatants was quantified by enzyme-linked immunosorbent assay. Phosphorylated AMP-activated protein kinase (p-AMPK) and GLUT-1 protein expression was analyzed by Western blotting. Cellular glycolytic and oxidative phosphorylation was assessed using extracellular flux analysis.
Insulin resistance was independently associated with both BMI (unstandardized coefficient B 0.113 [95% confidence interval (95% CI) 0.059-0.167]; P < 0.001) (n = 61) and swollen joint count in 28 joints (SJC28) (B 0.114 [95% CI 0.032-0.197]; P = 0.008) (n = 61). Increased GLUT-1 expression in RA synovium (n = 26) versus OA synovium (n = 16) was demonstrated (P = 0.0003), with increased expression in the lining, sublining, and vascular regions. In contrast, decreased GLUT-4 expression in the RA lining layer (n = 21) versus the OA lining layer (n = 8) was observed (P = 0.0358). Decreased GLUT-1 protein expression was observed in parallel with increased p-AMPK protein expression in SFs in the presence of metformin (n = 4). Metformin increased glycolytic activity and decreased oxidative phosphorylation in RASFs (n = 7) (P < 0.05 for both). Metformin or aminoimidazole carboxamide ribonucleotide presence decreased spontaneous production of interleukin-6 (IL-6), IL-8, and monocyte chemotactic protein 1 in RA synovial explants and SFs (n = 5-7).
Insulin resistance is significantly associated with BMI and synovitis in RA, suggesting distinct interplay between glucose availability and inflammation in RA. Furthermore, the effect of metformin on proinflammatory mechanisms suggests a role for AMPK-modifying compounds in the treatment of RA.