G13 and Rho signaling in thrombin-mediated synovial proliferation and S-phase progression in RA SFs. Rheumatoid arthritis (RA) synovial fibroblasts (SFs) infected or not infected with adenoviruses encoding GFP (control vector), the C-terminal regions of Gα12 (Gα12-ct), Gα13-ct, or p115RGS were cultured for 48 hours in DMEM containing 1% FCS and then stimulated with 10 units/ml thrombin. At 24 hours after the thrombin stimulation, proliferation assay and cell-cycle analysis of RA SFs were performed. (a) Effect of Rho signaling inhibition on thrombin-induced cell proliferation. Numbers represent the optical density (OD) as measured by ELISA plate reader at 450 nm. (b) Effect of Rho signaling inhibition on thrombin-induced cell-cycle progression. Numbers represent the percentage of cells that exhibited mean channel fluorescence (FL2-H) in the S/G2/M phase. Data are expressed as mean ± standard deviation of five experiments, using five independent donors. (-), cells without infection; p115RGS, regulator of G-protein signaling domain of p115Rho guanine nucleotide exchange factor. *P < 0.05, **P < 0.01, in comparison with thrombin stimulation.