Liver cells contain constitutive DNase I-hypersensitive sites at the xenobiotic response elements 1 and 2 (XRE1 and -2) of the rat cytochrome P-450IA1 gene and a constitutive, nuclear XRE-binding factor that is distinct from the dioxin receptor

Oct-1 interacts with conserved motifs in the human thyroid transcription factor 1 gene minimal promoter

The homeodomain containing thyroid transcription factor 1 (TTF-1) is a lung- and thyroid-enriched protein implicated in the regulation of a number of pulmonary specific genes. Within the lung TTF-1 is expressed within the epithelial cells. Although the molecular mechanisms that govern this tight cell-type-specific distribution are unclear, transient transfection studies have suggested that tissue specificity is conferred in part by regions of the proximal promoter. Further studies have shown that two functionally important regions (BS1 and BS2) are sites for activation of the TTF-1 gene by the homeodomain protein HoxB3, raising the possibility that Hox proteins might function in the regulation of TTF-1 in vivo. The different cellular distributions of the two proteins within the lung suggest, however, that proteins distinct from HoxB3 might be the mediators of expression through these sites. In the present study we have used gel-mobility-shift experiments to show that in a pulmonary adenocarcinoma cell line (NCI-H441) that expresses TTF-1, the same single protein binds to both of these sites. The binding of this protein is competed for specifically by the addition of oligonucleotides containing a range of octamer-binding sites but not by a variety of non-related binding sites. Using specific antiserum we have identified this protein as being the ubiquitously expressed POU-domain protein Oct-1. Reverse transcriptase-PCR performed with degenerated primers suggests that Oct-1 is the major POU-domain-containing protein expressed in H441 cells. These results suggest that BS1 and BS2 are functional octamer sites and might therefore be implicated in the basal rather than the tissue-restricted expression of the TTF-1 gene.

Aryl hydrocarbon-induced interactions at multiple DNA elements of diverse sequence--a multicomponent mechanism for activation of cytochrome P4501A1 (CYP1A1) gene transcription

In vivo footprinting experiments, augmented with gel shift and transfection analyses suggest that activation of the CYP1A1 gene by aryl hydrocarbons may be a multicomponent process. During the first 30 minutes of exposure to aryl hydrocarbon carcinogens and environmental contaminants, in vivo footprints appear at nine distinct sites within a 281 bp region centered 950 bp upstream of the CYP1A1 transcription start site. Six of these sites are unrelated in sequence to the three xenobiotic response elements (XREs) within this region, at which the aryl hydrocarbon (AH) receptor is known to bind. These six display a variety of footprint patterns, are diverse in sequence and range in G-C content from 60 to 75%. This diversity suggests that multiple nuclear factors may be responsible for these six in vivo footprints. These observations are consistent with competition gel shift experiments showing that the nuclear factors binding at two of these sites are different from each other, as well as from the AH receptor. Gel shifts also indicate that the sequence-specific factors binding at these sites are expressed constitutively. This is consistent with a model in which in vivo footprints are induced at these six sites, not through direct activation or de novo synthesis of DNA-binding factors, but through a two phase mechanism in which binding of the nuclear AH receptor complex to XREs facilitates the binding of constitutive factors at these sites. This facilitation could be mediated either through specific protein-protein interactions or through alterations in chromatin structure that make these sites accessible to constitutive nuclear factors. A function for the sequences at which aryl hydrocarbons induce in vivo footprints is suggested by transfection experiments showing that one of these sequences cooperates with a weak XRE to confer on a reporter gene responsiveness to aryl hydrocarbons.

Dioxin receptor and C/EBP regulate the function of the glutathione S-transferase Ya gene xenobiotic response element

The rat glutathione S-transferase Ya gene xenobiotic response element (XRE) has both constitutive and xenobiotic-inducible activity. We present evidence that the XRE is regulated by both the constitutive C/EBP transcription factor and the xenobiotic-activated dioxin receptor. A ligand-activated XRE-binding protein was shown to be dioxin receptor by specific antibody immunodepletion and binding of highly purified receptor. Identification of C/EBP alpha as the constitutive binding protein was demonstrated by competition with a C/EBP binding site, protein-DNA cross-linking to determine the molecular weight of the constitutive protein(s), specific antibody immunodepletion, and binding of purified bacterially expressed C/EBP alpha. Mutational analysis of the XRE revealed that the constitutive factor (C/EBP alpha) shares a nearly identical overlapping binding site with the dioxin receptor. In functional testing of the putative C/EBP-XRE interaction, cotransfected C/EBP alpha activated an XRE test promoter in the non-xenobiotic-responsive HeLa cell line. Unexpectedly, cotransfected C/EBP alpha had no effect on basal activity but significantly increased the xenobiotic response of the XRE test promoter in the xenobiotic-responsive, C/EBP-positive HepG2 cell line. Furthermore, inhibition of C/EBP-binding protein(s) in HepG2 cells by transfection of C/EBP oligonucleotides suppressed the xenobiotic response. These results suggest that C/EBP alpha and dioxin receptor recognize the same DNA sequence element and that transcriptional regulation can occur by cooperative interactions between these two transcription factors.

Dioxin receptor and C/EBP regulate the function of the glutathione S-transferase Ya gene xenobiotic response element

The rat glutathione S-transferase Ya gene xenobiotic response element (XRE) has both constitutive and xenobiotic-inducible activity. We present evidence that the XRE is regulated by both the constitutive C/EBP transcription factor and the xenobiotic-activated dioxin receptor. A ligand-activated XRE-binding protein was shown to be dioxin receptor by specific antibody immunodepletion and binding of highly purified receptor. Identification of C/EBP alpha as the constitutive binding protein was demonstrated by competition with a C/EBP binding site, protein-DNA cross-linking to determine the molecular weight of the constitutive protein(s), specific antibody immunodepletion, and binding of purified bacterially expressed C/EBP alpha. Mutational analysis of the XRE revealed that the constitutive factor (C/EBP alpha) shares a nearly identical overlapping binding site with the dioxin receptor. In functional testing of the putative C/EBP-XRE interaction, cotransfected C/EBP alpha activated an XRE test promoter in the non-xenobiotic-responsive HeLa cell line. Unexpectedly, cotransfected C/EBP alpha had no effect on basal activity but significantly increased the xenobiotic response of the XRE test promoter in the xenobiotic-responsive, C/EBP-positive HepG2 cell line. Furthermore, inhibition of C/EBP-binding protein(s) in HepG2 cells by transfection of C/EBP oligonucleotides suppressed the xenobiotic response. These results suggest that C/EBP alpha and dioxin receptor recognize the same DNA sequence element and that transcriptional regulation can occur by cooperative interactions between these two transcription factors.

Mechanism of dioxin action: receptor-enhancer interactions in intact cells

We have used a ligation-mediated polymerase chain reaction technique to analyze protein-DNA interactions at a dioxin-responsive enhancer upstream of the CYP1A1 gene in intact mouse hepatoma cells. In its inactive state, the enhancer binds few, if any, proteins within the major DNA groove in vivo. Thus, the inactive enhancer is relatively inaccessible to DNA-binding proteins. Exposure of cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin leads to the binding of the liganded Ah receptor at six sites within the major DNA groove of the enhancer. The receptor-enhancer interactions occur rapidly and do not require ongoing transcription, consistent with their role in regulating CYP1A1 gene expression. The liganded receptor, which is a heteromer composed of at least two basic helix-loop-helix proteins, is probably the only DNA-binding transcription factor necessary to activate the enhancer in vivo. The small size and irregular distribution of receptor binding sites suggest that chromatin structure imposes substantial steric constraints upon the function of the receptor-enhancer system in intact cells.

Transcription-dependent and transcription-independent nucleosome disruption induced by dioxin

In mouse hepatoma cells, both the regulatory and the transcribed regions of the cyp1a1 gene assume a nucleosomal configuration when the gene is silent; two nucleosomes occupy specific sites at the transcriptional promoter. Activation of transcription by 2,3,7,8-tetrachlorodibenzo-p-dioxin is accompanied by changes in chromatin structure, which depend upon a functional aromatic hydrocarbon (Ah) receptor. In the transcribed region of the gene, nucleosome disruption occurs as a consequence of RNA elongation. In contrast, at the promoter, loss of positioned nucleosome sis independent of transcription and represents an event in the mechanism by which the liganded Ah receptor enhances transcriptional initiation.