Regulation of mouse CYP1A1 gene expression by dioxin: requirement of two cis-acting elements during induction.

Abstract

The mouse cytochrome P,450 (CYPIAI) gene is responsible for the metabolism of numerous carcinogens and toxic chemicals. Induction by the environmental contaminant tetrachlorodibenzo-p-dioxin (TCDD) requires a functional aromatic hydrocarbon (Ah) receptor. We examined the 5'-flanking region of the CYPIAI gene in mouse hepatoma Hepa-1 wild-type cells and a mutant line having a defect in chromatin binding of the TCDD-receptor complex. We identified two cis-acting elements (distal, -1071 to -901 region; proximal, -245 to -50 region) required for constitutive and TCDD-inducible CYPIAI gene expression. Three classes of DNA-protein complexes binding to the distal element were identified: class I, found only in the presence of TCDD and a functional Ah receptor, that was heat labile and not competed against by simian virus 40 (SV40) early promoter DNA; class II, consisting of at least three constitutive complexes that were heat stable and bound to SV40 DNA; and class III, composed of at least three constitutive complexes that were thermolabile and were not competed against by SV40 DNA. Essential contacts for these proteins were centered at -993 to -990 for the class I complex, -987, -986, or both for the class II complexes, and -938 to -927 for the class III complexes. The proximal element was absolutely essential for both constitutive and TCDD-inducible CYPIAI gene expression, and at least two constitutive complexes bound to this region. These data are consistent with the proximal element that binds proteins being necessary but not sufficient for inducible gene expression; interaction of these proteins with those at the distal element was found to be required for full CYPIAJ induction by TCDD.The cytochromes P450 represent a ubiquitous superfamily of heme-containing enzymes that play a unique role in the metabolism of many endogenous substrates as well as innumerable drugs, chemical carcinogens, and other environmental pollutants (26). To date, more than 72 P450 genes in 14 families have been characterized in organisms ranging from bacteria to mammals (26b).More is known about the regulation of the CYPIA7 gene than that of any other P450 gene (D. W. Nebert and J. E. Jones, Int. J. Biochem., in press). CYPIAI encodes P1450, an enzyme involved in the metabolism of polycyclic aromatic hydrocarbons such as benzo(a)pyrene. The gene is inducible by plant flavones; detoxification of plant metabolites in animals may have been the reason for the evolution of this induction process (26). Interestingly, the CYPIAI gene is also inducible by tetrachlorodibenzo-p-dioxin (TCDD), one of the most potent teratogens and tumor promoters known (32).A series of events is known to occur during the CYPIAI induction process (26 and references therein). The polycyclic aromatic inducer enters the cell passively and binds to the aromatic hydrocarbon (Ah) receptor with an apparent K<, of <1 nM. The inducer-receptor complex requires a temperature-dependent step in order to gain chromatin-binding properties. Elevated CYPIAl mRNA is known to be a cons...

Identification and expression profile of a new cytochrome P450 isoform (CYP414A1) in the hepatopancreas of Venerupis (Ruditapes) philippinarum exposed to benzo[a]pyrene, cadmium and copper

“…Additionally, mRNA expression of CYP1A in Boreogadus saida and Danio rerio also showed significant inductions following B[a]P exposure (Nahrgang et al, 2009;Thompson et al, 2010). This well-characterized induction is perhaps mediated by promoting the binding of the liganded aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT) to aromatic hydrocarbon-responsive element (AHREs) located in the 5 -flanking regions of the CYP genes (Neuhold et al, 1989;Whitlock, 1999). Cd has been demonstrated to stimulate or inhibit the liver and kidney cytochrome P450 isoforms in rat (Plewka et al, 1999(Plewka et al, , 2004.…”

DNA methylation of the CYP1A1 enhancer is associated with smoking‐induced genetic alterations in human lung

“…The promoter is silent in the absence of the enhancer region, and AHR complex binding is necessary for CYP1A1 gene expression. 17,18 In vitro, methylation of cytosine-phosphate-guanosine (CpG) in the XRE inhibits binding of the AHR complex and DNA methylation may regulate CYP1A1 expression in different human tissues. [19][20][21][22][23] We hypothesized that DNA methylation could be involved in inter-individual differences in CYP1A1 gene expression in human lung and play a role in lung cancer development.…”

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.

“…However, upon exposure to ligand in vivo, the dioxin-receptor complex undergoes an activation process involving a poorly understood structural alteration that enables it to translocate to the nucleus (29). In vitro, the ligand-activated dioxin receptor has been shown to represent a DNA-binding protein (references 5 and 47 and references therein) which specifically recognizes dioxin-responsive positive control elements (xenobiotic response elements [XREs]) found in the promoter and upstream regions of the cytochrome P-450IA1 and glutathione S-transferase Ya genes (9,16,17,21,31,33 and 48) in that (i) both categories of receptors directly transduce an extracellular signal to the target transcription units they regulate and (ii) the dioxin and steroid receptors require ligand for function.A striking property of the dioxin receptor is that the ligand strictly controls the activation of the receptor from a latent species to a DNA-binding form also under cell-free conditions (7,17,30). In the case of steroid hormone receptors, the glucocorticoid response element of the rat tyrosine aminotransferase gene is protected against methylation by dimethyl sulfate in hepatoma cells only after hormone administration (3), supporting the hypothesis that the glucocorticoid receptor interacts in vivo with its target sequence in a ligand-dependent manner.…”