Nonresponsiveness of normal human fibroblasts to dioxin correlates with the presence of a constitutive xenobiotic response element-binding factor

Polychlorinated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzofuran (TCDF) have been shown to induce transcription of the cytochrome P-450IA1 gene by activating an intracellular receptor protein (the Ah- or dioxin receptor) to bind to specific DNA sequences, termed xenobiotic response elements (XREs). However, the expression and inducibility of the cytochrome P-450IA1 activity exhibit tissue-specific differences. With regard to the TCDF induction response, we have examined three human cell types of endodermal (the hepatoma cell line HepG2), ectodermal (normal keratinocytes), and mesodermal origin (normal fibroblasts). DNase I hypersensitivity analysis of the 5' flank and first intron of the P-450IA1 gene showed that in the nonresponsive fibroblasts the chromatin structure lacked open regions while in the two responsive cell types (keratinocytes and HepG2) several constitutive hypersensitive sites as well as TCDF-induced alterations in the chromatin structure could be detected. This observation might correlate with the fact that the XRE, in either the context of the P-450IA1 gene sequences or in front of a heterologous promoter, was inefficient in directing a TCDF induction response in fibroblasts. In in vitro DNA binding studies, the dioxin receptor was activated to a DNA-binding nuclear form in all three cell types. However, in fibroblast nuclear extracts two novel constitutive protein-XRE complexes were detected. The fibroblast factor(s) were immunochemically distinct from the receptor but exhibited indistinguishable DNA binding specificity. These data are compatible with a model where the P-450IA1 is noninducible in fibroblasts due to the presence of a putative repressor(s) which may compete effectively with the receptor for binding to the response element as indicated by in vitro DNA-binding off-rate experiments.