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-Erα 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-1α 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 NFκB 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 hr, 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 hr, 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 hr 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 Ahresponsiveness (under normoxia) that are also inhibited by cycloheximide.

Transcriptional and posttranscriptional regulation of CYP1A1 by vanadium in human hepatoma HepG2 cells

“…However, previous studies have demonstrated that V 5+ compounds exert protective effects against chemical-induced carcinogenesis mainly by modifying various xenobiotic-metabolizing enzymes (Evangelou 2002). Data from our laboratory and others showed that heavy metals other than V 5+ are capable of modifying the carcinogen-metabolizing enzyme, Cyp1a1, at different stages of its regulatory pathway (Bessette et al 2005;Khan et al 2007;Korashy and El-Kadi 2008).…”

Down-Regulation of the Carcinogen-Metabolizing Enzyme Cytochrome P450 1a1 by Vanadium

“…However, previous studies have demonstrated that V 5ϩ compounds exert protective effects against chemical-induced carcinogenesis mainly by modifying various xenobiotic-metabolizing enzymes (Evangelou, 2002). Data from our laboratory and others showed that heavy metals other than V 5ϩ are capable of modifying the carcinogen-metabolizing enzyme, Cyp1a1, at different stages of its regulatory pathway (Bessette et al, 2005;Korashy and El-Kadi, 2005;Elbekai and El-Kadi, 2007;Khan et al, 2007).…”

TCDD mediates inhibition of p53 and activation of ERα signaling in MCF-7 cells at moderate hypoxic conditions

“…On the other hand hypoxia did not change TCDD-mediated activation of AhR signaling in MCF-7 cells. Lower oxygen concentrations (1%) were shown to inhibit the AhR signaling in human breast cancer cells (22). Several other studies also described that the activation of AhR pathway inhibits activation of HIF-1· pathway and vice versa in rats (23), mice (23) and multiple cell lines (24).…”