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 i ncrease the expression of prostaglandin endoperoxide H synthase (PGHS)-2. T his study focused on the regulatory mechanism of TCDD-mediated transcriptio nal activation of PGHS-2, Treatment of rat hepatocytes,vith TCDD led to a d ose-dependent induction of PGHS-2 mRNA levels associated with an increased synthesis of prostaglandin E-2, whereas expression of PGHS-1 was not affect ed, In vitro experiments with c-Src inhibitors, such as herbimycin A and ge ldanamycin, 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 me diated via a c-Src-dependent pathway, Transient transfection studies with d ifferent reporter constructs of the murine PGHS-2 promoter mutated in the x enobiotic-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 t o 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 respon se element located in its promoter region.