Regulation of prostaglandin endoperoxide H synthase-2 induction by dioxin in rat hepatocytes: possible c-Src-mediated pathway.

The tumor promoter 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to increase the expression of prostaglandin endoperoxide H synthase (PGHS)-2. This study focused on the regulatory mechanism of TCDD-mediated transcriptional activation of PGHS-2. Treatment of rat hepatocytes with TCDD led to a dose-dependent induction of PGHS-2 mRNA levels associated with an increased synthesis of prostaglandin E(2), whereas expression of PGHS-1 was not affected. In vitro experiments with c-Src inhibitors, such as herbimycin A and geldanamycin, and in vivo studies with c-Src-deficient mice indicated that up-regulation of PGHS-2 but not the cytochrome P450 gene CYP1A1 by TCDD is mediated via a c-Src-dependent pathway. Transient transfection studies with different reporter constructs of the murine PGHS-2 promoter mutated in the xenobiotic-responsive element (XRE) or CCAAT/enhancer binding protein (C/EBP) element revealed that a C/EBP-binding site is an important regulatory cis-acting factor for trans-activation of the PGHS-2 gene by TCDD. Consistent with transfection studies, gel mobility shift assays showed that TCDD led to an enhanced DNA-binding activity of C/EBP beta transcription factor. The experimental data presented in this article reveal a XRE-independent and c-Src-mediated activation of the PGHS-2 gene by TCDD through the C/EBP response element located in its promoter region.