Introduction: Methotrexate (MTX) exerts at least part of its anti-inflammatory effects through adenosine receptors (ADOR). The aims of this study were to determine the expression of all four adenosine receptor genes (ADORA1, ADORA2A, ADORA2B, ADORA3 and ADORA3variant) in rheumatoid synovial tissue and any influence of MTX exposure on this expression. Furthermore, we investigated whether polymorphisms within ADORA3 were associated with response and/or adverse effects associated with MTX.
Methods: Adenosine receptor gene expression was undertaken using PCR in 20 rheumatoid arthritis (RA) synovial samples. A separate cohort of 225 RA patients receiving MTX was genotyped for SNPs in the ADORA3 receptor gene. Double immunofluorescence was used to identify cells expressing ADOR protein.
Results: All ADOR genes were expressed in all synovial samples. ADORA3 and A3variant were the dominant subtypes expressed irrespective of MTX therapy. Expression of ADORA2A and ADORA2B was increased in patients receiving MTX compared to those not receiving MTX. There was no association between the ADORA3 rs1544224 SNP and high and low disease activity or MTX-associated adverse effects. ADORA2B protein expression was most obvious in vascular endothelial cells whereas ADORA3 protein was more abundant and expressed by synovial fibroblasts.
Conclusions: We have shown that adenosine receptors are expressed in RA synovium. There is differential expression of receptors such that ADORA3 is expressed at significantly higher levels. This evidence demonstrates the potential for MTX to exert its anti-inflammatory effects at the primary site of pathology within the joints of patients with RA.