Adenosine receptor expression in rheumatoid synovium: a basis for methotrexate action

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 (ADORA(1), ADORA(2A), ADORA(2B), ADORA(3 )and ADORA(3variant)) in rheumatoid synovial tissue and any influence of MTX exposure on this expression. Furthermore, we investigated whether polymorphisms within ADORA(3 )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 ADORA(3 )receptor gene. Double immunofluorescence was used to identify cells expressing ADOR protein. RESULTS: All ADOR genes were expressed in all synovial samples. ADORA(3 )and A(3variant )were the dominant subtypes expressed irrespective of MTX therapy. Expression of ADORA(2A )and ADORA(2B )was increased in patients receiving MTX compared to those not receiving MTX. There was no association between the ADORA(3 )rs1544224 SNP and high and low disease activity or MTX-associated adverse effects. ADORA(2B )protein expression was most obvious in vascular endothelial cells whereas ADORA(3 )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 ADORA(3 )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.