Abstract

Exposure of the human breast epithelial cell line MCF10A to ≥1 μg/ml cycloheximide (CHX)-induced accumulations of CYP1A1 mRNA 6-fold greater than that achieved with only 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Cotreatment with CHX and TCDD caused superinduction of CYP1A1 with accumulations of CYP1A1 mRNA 30-fold greater than that achieved with only TCDD. Similar results were obtained with the protein translation inhibitors anisomycin (ANS) and puromycin (PUR). Intra- and interinhibitor comparisons of dose/concentration response curves demonstrated the absence of a quantitative relationship between [³H]leucine incorporation and CYP1A1 induction/superinduction. The inducing/superinducing activities of CHX were suppressed by coincubation with the aryl hydrocarbon receptor (AhR) antagonists α-naphthoflavone and 3′-methoxy-4′-nitroflavone (PD168641). Electrophoretic mobility shift assays demonstrated that nuclear extracts from CHX-treated and CHX + TCDD cotreated cultures formed ∼58 and ∼340% of the AhR/DNA complexes obtained with TCDD-treated cultures, respectively. In contrast, rat liver extracts did not form AhR/DNA complexes after in vitro transformation with CHX. AhR turnover in TCDD-treated hepatoma 1c1c7 cultures was suppressed by cotreatment with CHX. In contrast, CHX or ANS treatment of MCF10A cultures induced AhR loss and enhanced AhR loss in cultures cotreated with TCDD. Cotreatment with N-benzoyloxycarbonyl-(Z)-Leu-Leu-leucinal (MG132) but not leptomycin B suppressed AhR loss. Hence, in MCF10A cells, CHX is not an AhR agonist but can superinduce CYP1A1 via an AhR-dependent mechanism; CYP1A1 superinduction by translation inhibitors is neither quantitatively related to effects on protein synthesis nor due to a generalized prevention of AhR proteolysis, and proteasome-mediated degradation of the activated AhR can occur in the nucleus.