Evidence suggests that estrogen affects the pulmonary response to carcinogenic pollutants, such as dioxins. In this study, we examined dioxin and estrogen signaling cross-talk in lung adenocarcinoma cell lines that were engineered to exhibit different aryl hydrocarbon receptor (AhR)/estrogen receptor (ER) a phenotypes. Data showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) weakly antagonized estrogen-activated ERa activity in cells expressing abundant ERa, but little AhR. Increase of AhR expression or presence of a dioxin-responsive element in proximity silenced the antiestrogenic effect of TCDD. AhR was bound to dioxin-responsive element and transcriptionally active in both TCDD-untreated and -treated lung adenocarcinoma cells. 17b-estradiol (E2) reduced basal and TCDDinduced AhR activity only in ERa-positive cells. AhR and ERa exhibited a protein-protein interaction in the presence of E2. Cotreatment with TCDD moderated this protein interaction. Colocalization of ERa and AhR at the estrogen-responsive site under E2 and TCDD/E2 treatments implied that E2 $ ERa might hijack AhR away from the dioxinresponsive site. Increasing the relative expression of AhR to ERa counteracted inhibition of AhR activity by E2 $ ERa. When AhR and ERa were both highly expressed, TCDD and E2 up-regulated expression of dual-responsive genes cytochrome P450 (CYP) 1A1 and CYP1B1 in a cumulative manner, increasing the danger of metabolic activation of carcinogens. Whereas TCDD $ AhR and E2 $ ERa appeared to regulate CYP1B1 separately through their binding sites, E2 $ ERa increased the TCDD responsiveness and mRNA expression of CYP1A1 in a noncanonical way. In conclusion, AhR/ERa expression pattern, estrogen level, and promoter context determine the genomic action of dioxin in lung adenocarcinoma cells.Keywords: estrogen; estrogen receptor a; dioxin; aryl hydrocarbon receptor; signaling cross-talk Dioxins and dioxin-like furans and polycyclic aromatic hydrocarbons (PAHs) formed during incomplete combustion have been and continue to be a major threat to lung health. Exposure to dioxins has been found to increase lung cancer death in several industrial cohorts and their respective high-exposure subcohorts. Cumulative dose-response trends for cancer are also seen in lowexposure, long-duration studies (1). Although some dioxin-like PAHs, such as benzo[a]pyrene and anthanthrene, act as a complete carcinogen to induce tumor formation after administration to the lung, dioxins increase tumor incidence only in cases of prior exposure to a known carcinogen in animal carcinogenicity tests (2, 3). Dioxins have no direct genotoxic effect, but they may alter the genotoxicity of endogenous and exogenous compounds by increasing metabolic activation catalyzed by cytochrome P450 (CYP) enzymes CYP1A1 and CYP1B1 (1, 2). Indeed, dioxins, as well as dioxin-like compounds, can modulate a spectrum of biological processes, including cell division and differentiation, by activation of a transcription factor, named aryl hydrocarbon receptor (AhR), as a ligand....