Possible involvement of granulocyte oxidative burst in Nrf2 signaling in cancer.

The activation process of granulocytes is accompanied by the intense production of reactive oxygen species (ROS). Overproduction of ROS is cytotoxic, damages macromolecules and can lead to the occurrence of lipid peroxidation. Cellular defense against the toxicity of ROS is enhancement of detoxifying enzymes activation. Regulation of many detoxifying enzymes is mediated by the antioxidant response element (ARE) that is located in the promoter region of related genes. In eukaryotes, there are only few transcription factors known to be activated by ROS. One of them is NF-E2-related factor 2 (Nrf2). Normally, Nrf2 is present in the cytoplasm as an inactive Keap1-Nrf2 complex. However, after direct attack by ROS, Nrf2 is released from Keap1 repression and translocated into nucleus where it binds with ARE sequence to initiate gene expression. ROS may also influence nuclear factor-kappaB (NF-kappaB) intracellular signaling repressing the Nrf2-ARE pathway at transcriptional level. Since ROS are crucial in granulocyte-mediated tumor cell lysis the induction of NF-kappaB signaling pathway may be an important mechanism in suppressing the tumor growth.