Regulation of prostaglandin endoperoxide H synthase-2 expression by 2,3,7,8,-tetrachlorodibenzo-p-dioxin.

We have examined the molecular mechanisms for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-stimulated prostaglandin synthesis in Mardin Darvey canine kidney cells (MDCK). TCDD stimulates prostaglandin synthesis in these cells, at least in part, by elevating prostaglandin endoperoxide H2 synthase-2 (PGHS-2) levels. TCDD-stimulated transcription of the PGHS-2 gene was maximal (6-fold) within 2 h and resulted in a 100-fold increase in PGHS-2 mRNA and a 25-fold increase in PGHS-2 protein levels by 4 h. Transient transfection experiments using luciferase-reporter plasmids demonstrated that control element(s) responsible for TCDD activation of the murine PGHS-2 promoter in MDCK cells are located in the first 965 nucleotides upstream from the PGHS-2 transcriptional initiation site. A canonical xenobiotic response element, similar to those that control transcription of other well-known TCDD-sensitive genes, is present at position -157, but does not appear to be sufficient for halogenated aromatic hydrocarbon (HAH) activation of the PGHS-2 promoter. TCDD failed to stimulate transcription from the PGHS-2 promoter when reporter plasmids were transfected into Hepa 1c1c7 cells, a line which contains the functional aryl hydrocarbon receptor. It seems likely that inappropriate expression of PGHS-2 may contribute to the toxic effects of TCDD and other HAHs. In particular, PGHS-2 expression may affect those toxic reactions that involve inappropriate cellular growth, such as dermal hyperplasia and tumor formation. It is also likely that elevated synthesis of prostaglandins, which are potent regulators of immune function, could play a role in the immunotoxicity associated with HAH exposure.