Reactive oxygen species in cancer biology and anticancer therapy.

Reactive oxygen species (ROS) are a group of highly reactive chemicals under tight control of intracellular antioxidants. The balance in oxidation-antioxidation is essential for maintaining normal cell functions, and any imbalance could lead to a wide range of diseases including cancer. The intracellular level of ROS is generally elevated in cancer cells, revealing a critical role of ROS in the process of carcinogenesis and cancer progression. Conversely, there is also evidence showing that ROS can act as cancer suppressors. This may be due to the varying antioxidant capacities of different cancers. These findings indicate a complex redox state in cancer cells. In this review we summarize the main features of ROS and their functions with respect to cancer initiation, hallmarks of cancer, and signaling in cancer cells. ROS-elevating and ROS-depleting anticancer strategies and their mechanisms are thoroughly discussed. We argue that the rationale for therapy choice depends on a complete understanding of cancer cell redox state, namely, the "redox signaling signature" of cancer.