Benzo[a]pyrene diol epoxide suppresses retinoic acid receptor-β2 expression by recruiting DNA (cytosine-5-)-methyltransferase 3A

Tobacco smoke is an important cause of human cancers, as it contains more than 60 carcinogens [1-4], which are major risk factors for cancers of the head and neck, lung, esophagus, pancreas, and bladder [5-9]. Benzo [a]pyrene diol epoxide (BPDE), a carcinogen present in tobacco smoke and environmental pollution, has been shown to induce gene mutations (such as in p53 and KRAS genes) in vitro [10-13]. Previously, we identified and cloned several BPDE-binding genes (such as ATM and BRCA2) and the cytosine-phosphate-guanine (CpG) islands of various gene promoters [14]. Cigarette smoke has been shown to cause morphologic changes and the loss of retinoic acid receptor-beta2 (RAR-β2) expression in the lung tissues of experimental animals [15]. Cigarette smoke, specifically the tobacco carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, has also been shown to induce the methylation of the RAR-β2 gene promoters in murine lung cancer models [16]. We have also previously shown that RAR-β2 expression is suppressed in premalignant and malignant esophageal cells [17-19]. Consequently, expression of epidermal growth factor receptor (EGFR), extracellular signal-regulated protein kinases 1/2 (ERK1/2), activated protein-1 (AP-1), and cyclooxygenase-2 (COX-2) are induced by BPDE [19]. Numerous studies have demonstrated that RAR-β2 expression is frequently and progressively lost in premalignant and malignant tissues of the head and neck, lung, esophagus, pancreas, mammary gland, prostate, and other sites [20-22]. Lost expression of RAR-β2 in these various human cancers has been shown to be due to hypermethylation of its gene promoter [23-27]. However, it is still unknown if, and if so, how BPDE suppresses the expression of RAR-β2 and induces methylation of its gene promoter.In this study, we first confirmed our previous finding [17,19] that BPDE treatment inhibited RAR-β2 expression and after 24 h BPDE treatment, RAR-β2 was reexpressed (Figure 1A) and then we showed that BPDE i