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

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-kappa B (NF-kappa B) and its DNA binding through induction of I kappa B kinase alpha (IKK alpha) and IKK beta 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 I kappa B alpha, thereby inducing NF-kappa B 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]