gms | German Medical Science

Objectives: Previous studies revealed an anti-inflammatory effect of the endogenous nucleoside adenosine and its metabolite inosine in arthritic synovial tissue after binding to specific adenosine receptors (AR). In the human body, CD39 converts ATP to AMP, which in turn is metabolized to adenosine by CD73. The transmembrane Equilibrative Nucleoside Transporters ENT1 and ENT2 regulate the transfer of adenosine between intra- and extracellular space modulating its extracellularly acting levels. We investigated the adenosine synthesizing and transporting machinery in OA and RA synoviocytes to elucidate their contribution to inflammatory processes.

Methods: Synovial tissue was obtained from patients with rheumatoid arthritis (RA) or osteoarthritis (OA). Fixed tissue and mixed synovial cells were analyzed regarding their adenosine deaminase (ADA), ENT1 and ENT2 as well as CD39 and CD73 expression via immunohistochemical staining and FACS. Spontaneous nucleoside release was determined by HPLC. In addition, synoviocytes were treated with the CD73 inhibitor A, B-Methylenadenosine 5'diphosphate (AMPCP), the ADA inhibitor 1 Deazaadenosine (DAA) and different selective A1-, A2A-, A2B- and A3AR agonists. After treatment, IL-6 release was quantified.

Results and Conclusion: ADA, ENT1, ENT2 a well as all AR subtypes were expressed in OA and RA synovium. While A1- and A3AR levels were similar in both tissues, A2A- and A2BAR expression was more pronounced in RA samples. Mixed synoviocytes from both OA and RA patients co-expressed CD39 and CD73 and released adenosine and inosine spontaneously. However, OA synovial cells released nucleosides in significantly higher concentrations compared to RA cells (adenosine: OA: 23.8 ± 8.7 ng/ml, RA 4.6 ± 4.7 ng/ml; inosine: OA 73.4 ± 37.5 ng/ml, RA 10.4 ± 8.3 ng/ml). AMPCP increased IL-6 release significantly in both OA and RA mixed synoviocytes. Contrary, DAA significantly reduced IL-6 concentrations. The A2BAR agonist BAY 60-6583 significantly inhibited IL-6 expression in both cell types. The A2AAR agonist CGS 21680 hydrochloride exhibited the same effect only in RA cells.

OA and RA synoviocytes express the complete enzymatic machinery to synthesize adenosine and inosine endogenously. However, RA synovial cells produced lower nucleoside levels, likely due to a decreased functionality of CD39 and CD73 or an elevated expression of adenosine and inosine degrading enzymes in an inflamed microenvironment. CD37 inhibition increased IL-6 release and ADA inhibition resulted in the opposite suggesting that mainly adenosine is responsible for anti-inflammatory effects in the synovium. Taken together, induction of the CD39/CD73 enzymatic machinery with simultaneous inhibition of ADA activity in synovial tissue might be a promising anti-inflammatory therapeutic strategy for OA and RA patients.