Thrombin induces IL-6 secretion via Rho-mediated signaling in RA synovial fibroblasts. Rheumatoid arthritis (RA) synovial fibroblasts (SFs) were or were not infected with adenoviruses encoding green-fluorescent protein (control vector), the C-terminal regions of Gα12 (Gα12-ct), Gα13-ct, or p115RGS. Cells were cultured for 48 hours in DMEM containing 1% FCS, then stimulated with the indicated amount of thrombin. At 3 to 24 hours after thrombin stimulation, the supernatants of cultured cells were collected and assayed for IL-6 using commercial ELISA kits. (a) Dose-dependent IL-6 production by RA SFs at 12 hours after thrombin stimulation. (b) Time course of IL-6 secretion by RA SFs stimulated with 10 units/ml thrombin. (c) Effect of Rho signaling inhibition on thrombin-induced IL-6 secretion at 12 hours after thrombin stimulation. Data are expressed as mean ± standard deviation of five experiments, using five independent donors. (-), cells without infection; p115RGS, regulator of the G-protein signaling domain of p115Rho guanine nucleotide exchange factor. *P < 0.05, **P < 0.01, in comparison with (a) time 0, (b) no thrombin stimulation, and (c) the indicated data.