Roles of Nrf2 in cell proliferation and differentiation.

The Keap1-Nrf2 system plays pivotal roles in defense mechanisms by regulating cellular redox homeostasis. Nrf2 is an inducible transcription factor that activates a battery of genes encoding antioxidant proteins and phase II enzymes in response to oxidative stress and electrophilic xenobiotics. The activity of Nrf2 is regulated by Keap1, which promotes the ubiquitination and subsequent degradation of Nrf2 under normal conditions and releases the inhibited Nrf2 activity upon exposure to the stresses. Though an impressive contribution of the Keap1-Nrf2 system to the protection from exogenous and endogenous electrophilic insults has been well established, a line of evidence has suggested that the Keap1-Nrf2 system has various novel functions, particularly in cell proliferation and differentiation. Because the proliferation and differentiation of diverse cell types are often influenced and modulated by the cellular redox balance, Nrf2 has been considered to control these cellular processes by regulating the cellular levels of reactive oxygen species (ROS). In addition, analyses of the genome-wide distribution of Nrf2 have identified new sets of Nrf2 target genes whose products are involved in cell proliferation and differentiation but not necessarily in the regulation of oxidative stress. Considering the most characteristic features of Nrf2 as an inducible transcription factor, a newly emerged concept proposes that the Keap1-Nrf2 system translates environmental stresses into regulatory network signals in cell fate determination. In this review, we introduce the contribution of Nrf2 to lineage-specific differentiation, maintenance and differentiation of stem cells, and proliferation of normal and cancer cells, and we discuss how the response to fluctuating environments modulates cell behavior through the Keap1-Nrf2 system.