Thrombin exerts its effects through interaction with specific PAR1, PAR 3, and PAR4 receptors [
26]. To investigate the role of PAR1, PAR3, and PAR4 subtype receptors in thrombin-mediated increase of HO-1 expression, cells were treated with PAR1-, 3-, and 4-specific agonist peptides and then examined for expression levels of HO-1. Of the agonist peptides tested, the PAR1-selective receptor agonist peptide, SFLLRN-NH
2 (100 μ
M) and TFRGAP-NH
2 (PAR3 agonist peptide; 100 μ
M) significantly increased the expression of HO-1 (Figure
2A). In contrast, GYPGQV-NH
2 (PAR4 agonist peptide; 100 μ
M) failed to upregulate HO-1 expression. Furthermore, thrombin, SFLLRN-NH
2, and TFRGAP-NH
2 only slightly increased HO-1 expression in normal synovial fibroblasts (Figure
2B), indicating that PAR agonist-induced HO-1 expression is more important during OA pathogenesis. To confirm that PAR1 and PAR3 subtype receptors are involved in the thrombin-mediated increase of HO-1 expression, specific inhibition of PAR-receptor expression was accomplished with siRNA (Figure
2C). It was found that PAR1 and PAR3 receptor-specific siRNA but not PAR4-receptor siRNA significantly blocked the thrombin-mediated increase of HO-1 expression (Figure
2D, E), indicating that interactions between thrombin and PAR1/PAR3 are important for HO-1 expression in human OASF cells.