2-Chloroadenosine but not adenosine induces apoptosis in rheumatoid fibroblasts independently of cell surface adenosine receptor signalling

1. The apoptotic effect of adenosine and its analogues was studied in fibroblast-like synoviocytes derived from rheumatoid arthritis patients (RA-FLSs). Evoked cell death was quantitatively examined by assessing DNA fragmentation using an enzyme-liked immunosorbent assay and by measuring phosphatidylserine exposure through flow cytometric analysis of annexin V binding. 2. Exposing cells for 24 h to 2-chloroadenosine (2-CADO), a nonspecific, Adenosine Deaminase (ADA)-resistant, Adenosine Receptor (AdoR) agonist, induced DNA fragmentation, and thus Apoptosis, in RA-FLSs at concentrations > or =50 microM. By contrast, incubation with adenosine for up to 72 h did not evoke DNA fragmentation, even in the presence of ADA inhibitor coformycin and nucleoside transporter inhibitor nitrobenzylmercaptopurin (NBMPR). Transcription of all four AdoR isoforms was detected in RA-FLSs; nevertheless selective AdoR agonists similarly failed to induce DNA fragmentation. 3. DNA fragmentation evoked by 2-CADO was inhibited by NBMPR and by 5'-iodotubercidin, an Adenosine Kinase Inhibitor, but not by xanthine amine congener, an A(1) and A(2) receptor antagonist, or by selective AdoR antagonists. 4. The nonspecific Caspase Inhibitor benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone abolished the apoptotic effect of 2-CADO. 5. These results suggest that 2-CADO induces Apoptosis in RA-FLSs independently of AdoR-mediated signalling. Instead, 2-CADO, but not adenosine, is taken up into RA-FLSs via human equilibrative nucleoside transporter-1, where it is phosphorylated by Adenosine Kinase. The resultant phospho-2-CADO induces DNA fragmentation by activating a Caspase pathway.