Berberine, an isoquinoline alkaloid, has a wide range of pharmacological effects, including anti-inflammation, yet the exact mechanism is unknown. Because cyclooxygenase-2 (COX-2) plays a key role in prostaglandins (PGs) synthesis, which is elevated in inflammation, we examined whether the anti-inflammatory mechanism of berberine is mediated through COX-2 regulation. In oral cancer cell line OC2 and KB cells, a 12 h berberine treatment (1, 10, and 100 microM) reduced prostaglandin E2 (PGE2) production dose-dependently with or without 12-O-tetradecanoylphorbol-13-acetate (TPA, 10 nM) induction. This berberine induced effect occurred rapidly (3 h) as a result of reduced COX-2 protein, but not enzyme activity. The electrophoretic mobility shift assay revealed that activator protein 1 (AP-1) binding was decreased in oral cancer cells treated with berberine for 2 h. Further analysis showed that berberine inhibited AP-1 binding directly. These anti-inflammatory effects paralleled to the in vivo results where berberine pretreatment of Wistar rat inhibited the production of exudates and PGE2 in carrageenan induced air pouch.