Mechanistic study of polychlorinated biphenyl 126-induced CYP11B1 and CYP11B2 up-regulation

Although polychlorinated biphenyls (PCBs) have been shown to accumulate in the adrenal, little is known about the effects of these endocrine disruptors on adrenal steroidogenesis. Our previous studies showed that high concentrations of PCB126 stimulated CYP11B1 and CYP11B2 mRNA expression and consequently raised cortisol and aldosterone synthesis in the human adrenocortical H295R cells, respectively. In this study, we further investigated the mechanism underlying the PCB126-induced steroidogenic alterations. We first examined the role of the PCB126 nuclear receptor aryl hydrocarbon receptor (AhR) using a potent antagonist 3',4'-dimethoxyflavone (3',4'-DMF). Although 3',4'-DMF abolished AhR-dependent transcriptional activity, it could not block PCB126-stimulated CYP11B1 and CYP11B2 induction. Conversely, 3',4'-DMF synergistically increased the stimulatory effects of PCB126. Furthermore, PCB39, -77, -132, -156, and -169, whether AhR ligands or not, all could increase CYP11B1 and CYP11B2 mRNA accumulation. Promoter analyses demonstrated that PCB126 had little effects on the transcription rate of both genes, whereas RNA degradation assays showed that PCB126 protected both transcripts from degradation. In contrast, 3',4'-DMF exhibited positive effects on transcription but no influence on transcript stability. The synergistic induction of CYP11B1 and CYP11B2 mRNA levels by the PCB126/3',4'-DMF cotreatment might result from the combination of transcriptional regulation by 3',4'-DMF and posttranscriptional regulation by PCB126. This study also demonstrated that an internal region of CYP11B1 mRNA (nucleotides 881-1285) was important for PCB126-mediated transcript stabilization. From these findings, we concluded that PCB126 up-regulated steroidogenic CYP11B1 and CYP11B2 mRNA expression not via AhR-mediated transcriptional activation but by increasing posttranscriptional mRNA stability.