DIFFERENTIAL REGULATION OF MOUSE AH RECEPTOR GENE-EXPRESSION IN CELL-LINES OF DIFFERENT TISSUE ORIGINS

The dioxin-binding Ah receptor (AHR) is a ligand-activated transcripti on factor that regulates the expression of several drug-metabolizing e nzymes and has been implicated in immunosuppression, teratogenesis, ce ll-specific hyperplasia, and certain types of malignancies and toxicit ies. In order to examine tissue-specific regulation of the mouse Ah re ceptor gene (Ahr), we studied chimeric deletion constructs, containing the Ahr 5' flanking region and the firefly luciferase reporter gene ( Luc). Transient transfection assays were performed in five established mouse cell lines: Hepa-1c1c7 (derived from hepatoma), JB6-C1 41-5a (e pidermis), MLE-12 (lung epithelium), F9 (embryonal carcinoma), and NIH /3T3 (fibroblasts). Treatment of the cell lines included: dioxin (2,3, 7,8-tetrachlorodibenzo-p-dioxin), retinoic acid (RA), cyclic adenosine 3':5'-monophosphate (cAMP), or 12-O-tertrdecanoylphorbol 13-acetate ( TPA). Expression levels another, this finding was also confirmed by me asurements of AHR mRNA steady-state levels. In all cell lines except F 9 cells, maximal constitutive expression was observed with constructs containing 78 bp of Ahr promoter sequences, which include several puta tive binding sites for the transcription factor Sp1. In contrast, in F 9 cells, inclusion of sequences between -174 and -78 resulted in a fou rfold stimulation of constitutive expression, suggesting that other tr anscription factors are important in Ahr gene expression in these cell s. In MLE-12 and 41-5a cells, expression was significantly decreased b y treatment with dioxin, RA, cAMP, or TPA. A similar inhibitory effect was observed in cAMP-treated MLE-12 and F9 cells; this result was con firmed by RT-PCR measurements of AHR mRNA steady-state levels. These r esults indicate that both up- and down-regulation of the Ahr gene occu r and exhibit tissue- and cell-type specificity. (C) 1996 Academic Pre ss, Inc.