Fig. 4.

db-cAMP and forskolin do not activate transcription of Cyp1a1 but interfere with TCDD-dependent Cyp1a1 induction. Reporter constructs for Cyp1a1 were as follows: p1646P1Luc3 (A and C), containing the Cyp1a1 natural promoter with six copies of DRE and the negative regulatory element, and pAhRDtkLuc3 (B and D), lacking the negative regulatory element but containing three copies of DRE. These constructs were transfected into Hepa1 cells. At 24 h posttransfection, cells were stimulated as indicated, and reporter activity was determined. RLU, relative light units; pGL-3 basic, empty vector.