A, Serum-starved RSF were stimulated with or without PGE2 (1 μM) for 60 min in the presence or absence of 3-Isobutyl-1-methylxanthine (IBMX, 100 μM). Lysates from the cells were subjected to Rap1 pull down assay. B, cAMP levels in cell lysates from RSF stimulated with or without PGE2 (1 μM) for 30 min in the presence or absence of 3-Isobutyl-1-methylxanthine (IBMX, 100 μM) were analyzed by ELISA. C, Rap1 activation was analyzed by detecting GTP-Rap1 with lysates from RSF stimulated with forskolin (10 μM) for 60 min. D, cAMP levels in cell lysates from RSF stimulated with or without forskolin (10 μM) for 30 min were analyzed by ELISA. Blots are representative data and values are expressed as means ± standard errors from the number of patients with RA indicated in each panel. A significant difference between two groups (B & D) determined by Student’s t-test is indicated with a single asterisk (P < 0.05).