Diluted Mainstream Cigarette Smoke Condensates Activate Estrogen Receptor and Aryl Hydrocarbon Receptor-Mediated Gene Transcription

Background: Epidemiological data indicate that in females cigarette smoking exerts antiestrogenic effects that manifest clinically in an increased incidence of osteoporosis, earlier menopause, increased spot bleeding, and decreased risk of endometrial cancer for female smokers. The molecular mechanism of this effect is unclear; however, decreased serum estrogen levels in female smokers have been correlated with increased concentrations of the metabolite 2-hydroxyestrogen in females who smoke. Induction of estrogen metabolizing enzymes, CYP1A1 and 1A2, is one mechanism by which increased 2-hydroxyestrogen concentrations may occur. It has also been suggested that the estrogen receptor (ER) may contribute to this anti-estrogenic effect by binding to antagonist(s) in cigarette smoke. Methods: Gel retardation analysis was employed to determine if diluted mainstream cigarette smoke condensates (DMCSCs) could activate the aryl hydrocarbon receptor (AhR). AhR-regulated ethoxyresorufin-O-deethylase (EROD) activity and dioxin response element (DRE)-mediated luciferase induction were assessed in Hepa1c1c7 mouse hepatoma cells. A competitive ligand binding assay was utilized to determine if DMCSCs could bind to the ER. ER-dependent luciferase activity was assessed in MCF-7 cells. Results: In gel retardation assays, DMCSCs induced a protein–DNA complex when incubated with a radiolabeled wild-type DRE oligonucleotide. The complex was effectively competed by excess unlabeled DRE but not by excess unlabeled mutant DRE. In Hepa1c1c7 mouse hepatoma cells transiently transfected with a DRE-regulated luciferase reporter gene, pGudluc1.1, treatment with DMCSCs resulted in a 23- and 25-fold increase in luciferase activity (P<0.01) and an 8.5- and 10.5-fold (P<0.01) induction in EROD activity, respectively. DMCSCs completely displaced bound tritiated E2 from the ER in a dose-dependent manner and induced ER-regulated luciferase activity significantly 6-fold (P<0.01), representing 86% of the maximal induction observed with E2. Conclusions: DMCSCs can bind to and transcriptionally activate the AhR and ER nuclear receptors and cause induction of DRE- and ER-regulated genes. Further study is required to identify the specific compound(s) responsible for these activities.