Regulation and function of FoxO transcription factors in normal and cancer stem cells: what have we learned?

Forkhead FoxO transcription factors exert critical biological functions in response to genotoxic stress. In mammals four FoxOs proteins are known. FoxOs induce cell cycle arrest, repair damaged DNA, or initiate apoptosis by modulating genes that control these processes. In particular, FoxO proteins are critical regulators of oxidative stress by modulating the expression of several anti-oxidant enzyme genes. This function of FoxO is essential for the regulation of stem and progenitor cell pool in the hematopoietic system and possibly other stem cells. Overall functions of FoxOs are consistent with their role as tumor suppressors as has been shown in animal models. As such, FoxOs are suppressed in various cancer cells. However, recent reports strongly suggest that FoxOs are critical for the maintenance of leukemic stem cells. The diverse functions of FoxOs are orchestrated by tight regulations of expression and activity of its family members. Here we discuss the recent progress in understanding the function of FoxOs specifically in normal and cancer stem cells and what is known about the regulation of these proteins in various cell types and tissues including in the physiological setting of primary cells in vivo. These studies underscore the importance of regulation of FoxO proteins and whether these factors play distinct or redundant functions. Understanding how FoxOs are modulated is critical for devising novel therapies based on targeted restoration/or inhibition of FoxO function in cancer and in other diseased cells in which FoxOs have a key function.