title: Activation of the aromatic hydrocarbon receptor pathway is not sufficient for transcriptional repression of BRCA-1: requirements for metabolism of benzo[a]pyrene to 7r,8t-dihydroxy-9t,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene.
abstract: Reduction of BRCA-1 expression through nonmutational events may be a predisposing event in the onset of sporadic breast cancer. In this study, we investigated the mechanisms through which the environmental carcinogen benzo[a]pyrene (B[a]P) lowered BRCA-1 mRNA levels in breast cancer MCF-7 cells. We report that B[a]P does not compromise the stability of BRCA-1 mRNA, but represses transcriptional activity of a 1.69-kb BRCA-1 (pGL3-BRCA-1) promoter fragment that contains both exon-1A and exon-1B transcription start sites. The loss of BRCA-1 promoter activity was accompanied by accumulation of CYP1A1 and BAX-alpha mRNA and p53 and p21 protein, whereas levels of Bcl-2 mRNA were reduced. The aromatic hydrocarbon receptor ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which is not metabolized, did not affect BRCA-1 promoter activity or the cellular levels of BRCA-1 and p53 protein, but it did induce a CYP1A1-like promoter. Conversely, treatment with the B[a]P metabolite 7r,8t-dihydroxy-9t,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) repressed BRCA-1 promoter activity and protein, while increasing p53 and p21 protein levels. Transient expression of dominant-negative p53 ((175)Arg-->His) counteracted the detrimental effects of BPDE on BRCA-1 promoter activity and protein levels. Similarly, treatment with B[a]P, TCDD, or BPDE failed to repress transcription from the pGL3-BRCA-1 construct transfected into ZR75.1 breast cancer cells containing mutated p53 ((152)Pro-->Leu). We conclude that activation of the aromatic hydrocarbon receptor is not sufficient for down-regulation of BRCA-1 transcription, which is, however, inhibited by the B[a]P metabolite BPDE through a p53-dependent pathway.
authors.affiliation: Cancer Biology Interdisciplinary Program, Department of Nutritional Sciences, The University of Arizona, Tucson, Arizona 85721, USA.
authors.forename: Brandon D
authors.initials: BD
authors.lastname: Jeffy
authors.forename: Ryan B
authors.initials: RB
authors.lastname: Chirnomas
authors.forename: Eddy J
authors.initials: EJ
authors.lastname: Chen
authors.forename: Jean M
authors.initials: JM
authors.lastname: Gudas
authors.forename: Donato F
authors.initials: DF
authors.lastname: Romagnolo
mesh_terms: D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / Q000378:metabolism* / Q000633:toxicity; D019313:BRCA1 Protein / Q000096:biosynthesis* / Q000235:genetics; D001564:Benzo(a)pyrene / Q000378:metabolism* / Q000633:toxicity; D001711:Biotransformation; D001943:Breast Neoplasms / Q000139:chemically induced / Q000235:genetics* / Q000378:metabolism*; D002273:Carcinogens / Q000378:metabolism* / Q000633:toxicity; D015536:Down-Regulation; D020869:Gene Expression Profiling; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000502:physiology; D019398:Genes, BRCA1 / Q000187:drug effects; D006801:Humans; D000072317:Polychlorinated Dibenzodioxins / Q000633:toxicity; D011401:Promoter Regions, Genetic / Q000187:drug effects; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D018336:Receptors, Aryl Hydrocarbon / Q000378:metabolism / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D014158:Transcription, Genetic / Q000187:drug effects* / Q000502:physiology; D014407:Tumor Cells, Cultured; D016159:Tumor Suppressor Protein p53 / Q000235:genetics / Q000502:physiology
chemical_list: D019313:BRCA1 Protein; D002273:Carcinogens; D000072317:Polychlorinated Dibenzodioxins; D012333:RNA, Messenger; D018336:Receptors, Aryl Hydrocarbon; D016159:Tumor Suppressor Protein p53; D001564:Benzo(a)pyrene; D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide