title: Cobaltous chloride and hypoxia inhibit aryl hydrocarbon receptor-mediated responses in breast cancer cells.
abstract: The aryl hydrocarbon receptor (AhR) is expressed in estrogen receptor (ER)-positive ZR-75 breast cancer cells. Treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces CYP1A1 protein and mRNA levels and also activates inhibitory AhR-ERalpha crosstalk associated with hormone-induced reporter gene expression. In ZR-75 cells grown under hypoxia, induction of these AhR-mediated responses by TCDD was significantly inhibited. This was not accompanied by decreased nuclear AhR levels or decreased interaction of the AhR complex with the CYP1A1 gene promoter as determined in a chromatin immunoprecipitation assay. Hypoxia-induced loss of Ah-responsiveness was not associated with induction of hypoxia-inducible factor-1alpha or other factors that sequester the AhR nuclear translocation (Arnt) protein, and overexpression of Arnt under hypoxia did not restore Ah-responsiveness. The p65 subunit of NFkappaB which inhibits AhR-mediated transactivation was not induced by hypoxia and was primarily cytosolic in ZR-75 cells grown under hypoxic and normoxic conditions. In ZR-75 cells maintained under hypoxic conditions for 24 h, BRCA1 (an enhancer of AhR-mediated transactivation in breast cancer cells) was significantly decreased and this contributed to loss of Ah-responsiveness. In cells grown under hypoxia for 6 h, BRCA1 was not decreased, but induction of CYP1A1 by TCDD was significantly decreased. Cotreatment of ZR-75 cells with TCDD plus the protein synthesis inhibitor cycloheximide for 6 h enhanced CYP1A1 expression in cells grown under hypoxia and normoxia. These results suggest that hypoxia rapidly induces protein(s) that inhibit Ah-responsiveness and these may be similar to constitutively expressed inhibitors of Ah-responsiveness (under normoxia) that are also inhibited by cycloheximide.
authors.affiliation: Department of Veterinary Physiology and Pharmacology, Texas A&M University, College Station, TX 77843-4466, USA.
authors.forename: Shaheen
authors.initials: S
authors.lastname: Khan
authors.forename: Shengxi
authors.initials: S
authors.lastname: Liu
authors.forename: Matthew
authors.initials: M
authors.lastname: Stoner
authors.forename: Stephen
authors.initials: S
authors.lastname: Safe
mesh_terms: D016587:Antimutagenic Agents / Q000633:toxicity*; D015152:Blotting, Northern; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism; D015687:Cell Hypoxia / Q000502:physiology; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D003035:Cobalt / Q000633:toxicity*; D003513:Cycloheximide / Q000494:pharmacology; D019363:Cytochrome P-450 CYP1A1 / Q000096:biosynthesis / Q000235:genetics; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D004336:Drug Antagonism; D004785:Environmental Pollutants / Q000633:toxicity; D004790:Enzyme Induction / Q000187:drug effects; D005260:Female; D015870:Gene Expression / Q000187:drug effects; D006801:Humans; D000072317:Polychlorinated Dibenzodioxins / Q000633:toxicity; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism; D018336:Receptors, Aryl Hydrocarbon / Q000037:antagonists & inhibitors* / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism
chemical_list: D016587:Antimutagenic Agents; D004785:Environmental Pollutants; D000072317:Polychlorinated Dibenzodioxins; D011500:Protein Synthesis Inhibitors; D012333:RNA, Messenger; D018336:Receptors, Aryl Hydrocarbon; D003035:Cobalt; D003513:Cycloheximide; D019363:Cytochrome P-450 CYP1A1; C111164:BRAP protein, human; D044767:Ubiquitin-Protein Ligases; C018021:cobaltous chloride