Cyclooxygenase (COX) is the rate-limiting enzyme for the conversion of prostaglandins from arachidonic acid. Upregulation of COX-2 has been well documented during tumorigenesis and metastasis of breast cancer. Isoliquiritigenin (ILN), a flavonoid isolated from licorice (the rhizomes of GLYCYRRHIZA GLABRA, a member of the bean plant family), is known to be a potential suppressor of COX-2 expression. This study focuses on phorbol ester-induced COX-2 expression in the non-tumorigenic MCF-10A cells. Real-time PCR and Western blotting indicated that ILN at 5 microM or above significantly inhibited phorbol 12-myristate 13-acetate (PMA)-induced COX-2 expression in the breast cells. The activated PKC alpha appeared to be not affected, whereas its downstream mitogen-activated protein kinase (MAPK) ERK-1/2 was deactivated. ERK can activate the transcriptional factor binding of AP-1 or CRE, which can be located at the COX-2 promoter region (- 72/- 53). Electrophoretic mobility shift assays illustrated that ILN suppressed DNA binding at this region. The shifted bands could be competed off with consensus sequences of AP-1 and CRE, and the supershift assay demonstrated that CREB-1 instead of c-Jun was responsible for the binding. This study showed that ILN downregulated PMA-induced COX-2 expression by modulating ERK-1/2 signaling, a finding that may be relevant to the disease prevention properties of licorice.
Georg Thieme Verlag KG Stuttgart New York.