Abstract

Previous studies showed that TGF-β down-regulates aryl hydrocarbon (AhR) expression in human lung carcinoma cells A549. Here we analyzed the molecular mechanisms by which TGF-β modulates AhR expression. A 5799-nucleotide 5′-flanking region of human AhR gene was isolated. Transient transfection studies of full-length (hAhRP) and deletion promoter constructs indicate the requirement of acis-regulatory element encompassing −1980 to −1892 for full constitutive activity. Basal hAhRP activity occurs in a cell-specific manner; human hepatoma HepG2 cells possess a 10-fold higher activity compared with A549 cells. TGF-β exerts cell-specific effects on hAhRP activity. Treatment of cells with 100 pM TGF-β leads to a 50% inhibition in A549 and a 3-fold induction in HepG2 cells. Deletion mutagenesis identified a TGF-β-responsive sequence containing a functional conserved Smad-binding element. Transient overexpression of Smad 2, 3, and 4 indicates that these signal transducers modulate hAhRP activity. The down-regulation of AhR by TGF-β is modulated by 5′-TG-3′-interacting factor (TGIF). Transient overexpression of TGIF in MDA-MB231 and HepG2 cells led to inhibition of hAhRP activity and a similar decrease of AhR mRNA expression. Our findings indicate that Smad proteins are involved in the cell-specific regulation of AhR expression by TGF-β.