Chemotherapy and chemoprevention have been two of the most important means to control cancer incidence and mortality, and the cellular defensive machinery against oxidative/electrophilic stresses plays significant roles in both means. This defensive system is composed of cytoprotective enzymes that metabolize and eliminate oxidative/electrophitic species. The transcription factor nuclear factor erythroid 2-related factor 2 （Nrf2） controls the basal and inducible expression of many cytoprotective genes, and plays a pivotal role in coordinating cellular defensive responses. Under basal conditions, the activity of Nrf2 is inhibited by binding to Kelch-like ECH-associated protein 1 （Keap 1）, which is capable of sensing oxidative/electrophilic signals. Upon oxidative/electrophilic stresses, the binding of Nrf2 to Keapl is disrupted, leading to activation of Nrf2 and induction of cytoprotective enzymes. Thus, Nrf2 has emerged as an important target of chemopreventive drugs. However, activation of Nrf2 could lead to very different outcomes depending on the cellular context. The indiscriminative protective effects of Nrf2 lead to its undesired functions in carcinogenesis and chemoresistance of cancer cells. Activation of Nrf2 provides neoplastic cells with growth advantages and protects cancer cells from chemotherapeutic drugs, resulting in poor clinical outcomes. In this means, inhibitors of Nrf2 signaling can enhance the efficacy of chemotherapeutic drugs and deserve further development. A better understanding of the regulation and functions of Nrf2 would be helpful for researches in both chemoprevention and chemotherapy of cancer.
