Thrombin induces HO-1 expression in human synovial fibroblasts. (A) Total protein was extracted from normal synovial fibroblasts (lines 1 and 2) and osteoarthritis synovial fibroblast (OASF) cells (lines 3 to 6), and subjected to Western blot analysis for thrombin. (B; left panel) Human synovial fibroblasts were cultured for 48 hours, and media were collected to measure thrombin. (B; right panel) Synovial fluid was obtained from normal (n = 10) or osteoarthritis patients (n = 15) and examined with ELISA for the expression of thrombin. OASFs (six OA lines) were incubated with various concentrations of thrombin for 24 hours (C, E) or with thrombin (3 U/ml) for 6, 12, or 24 hours (D, F). The HO-1 expression was examined with Western blotting and qPCR. Results are expressed as the mean ± SEM. *P < 0.05 as compared with basal level. #P < 0.05 as compared with the thrombin-treated group.