4-Hydroxyestradiol Induces Anchorage-Independent Growth of Human Mammary Epithelial Cells via Activation of IκB Kinase: Potential Role of Reactive Oxygen Species Available to Purchase

Estrogen is converted by cytochrome P450 1B1 to 4-hydroxyestradiol (4-OHE₂), a putative carcinogenic metabolite of estrogen. This catechol estrogen metabolite is oxidized further to produce a reactive quinone via semiquinone. Redox cycling between 4-OHE₂ and its quinoid generates reactive oxygen species (ROS). ROS not only causes oxidative DNA damage but also promotes neoplastic transformation of initiated cells. In the present study, 4-OHE₂ induced anchorage-independent colony formation in human mammary epithelial cells (MCF-10A). MCF-10A cells treated with 4-OHE₂ exhibited increased accumulation of intracellular ROS. The antioxidant N-acetyl-l-cysteine inhibited the neoplastic transformation induced by 4-OHE₂. ROS overproduced by 4-OHE₂ increased the nuclear translocation of nuclear factor-κB (NF-κB) and its DNA binding through induction of IκB kinase α (IKKα) and IKKβ activities. The inhibition of the IKK activities with Bay 11-7082 significantly reduced the anchorage-independent growth induced by 4-OHE₂. The 4-OHE₂–induced activation of extracellular signal-regulated kinase and Akt resulted in enhanced IKK activities and phosphorylation of IκBα, thereby inducing NF-κB activation and anchorage-independent growth of MCF-10A cells. In conclusion, ROS, concomitantly overproduced during redox cycling of 4-OHE₂, activates IKK signaling, which may contribute to neoplastic transformation of MCF-10A cells. [Cancer Res 2009;69(6):2416–24]