Paeoniflorin-6’-O-benzene sulfonate (CP-25) is a novel compound derived from paeoniflorin that has been demonstrated to have therapeutic effects in a rat model of rheumatoid arthritis (RA).However,the underlying mechanism has not been elucidated to date.We explored this mechanism in the present study by treating rats with adjuvant arthritis (AA) with CP-25.We found that the membrane EP4 protein level was downregulated;whereas,GRK2 was upregulated,in fibroblast-like synoviocyte (FLS)s of AA rats.Prostaglandin (PGE)2 stimulated FLS proliferation and enhanced the membrane EP4 receptor protein level;the latter was reversed by the administration of an EP4 receptor agonist,whereas the membrane GRK2 protein level gradually increased.The changes in the EP4 receptor and GRK2 expression were enhanced by TNF-α,and the former was accompanied by an alteration in the cyclic (c)AMP level.The EP4 receptor agonist stimulation increased the association between GRK2 and the EP4 receptor.GRK2 knockdown abrogated the abnormalities in FLS proliferation.The CP-25 treatment (100 mg/kg) suppressed joint inflammation with an efficacy that was similar to that of methotrexate.This finding was associated with EP4 upregulation and GRK2 downregulation in FLSs.Thus,GRK2 plays an important role in the abnormal FLS proliferation observed in AA possibly by promoting EP4 receptor desensitization and decreasing the cAMP level.Our results demonstrate that CP-25 has therapeutic potential for the treatment of human RA via GRK2 regulation.