Abstract-TheKey Words: angiogenesis Ⅲ endothelial cells Ⅲ forkhead Ⅲ transcription factors Ⅲ vascular biology F orkhead/winged helix proteins are characterized by a conserved 100 -amino acid domain called the "forkhead box." 1 These proteins function as transcription factors, and they participate in diverse biological processes in development and disease. 2,3 The forkhead family in humans consists of 39 distinct members, which have been divided into 19 subgroups (Forkhead box [FOX] A to S). The FoxO subgroup of transcription factors has received the most attention because of their recently discovered roles in controlling detoxification of reactive oxygen species (ROS), 5,6 cell cycle, 7 apoptosis, 8,9 DNA repair, 7 glucose metabolism, 10 -14 and the regulation of cell size. 15-17a Thus, this review will primarily focus on the FoxO subgroup in cardiovascular cells. The reader should note that the role of forkhead factors in endothelial cells and angiogenesis has been reviewed recently. 17b
FoxO OverviewThe forkhead transcription factors of the O subgroup (FoxO) consist of 4 members, FoxO1 (FKHR), FoxO3 (FKHRL1), FoxO4 (AFX), and FoxO6. FoxO1, FoxO3, and FoxO4 are expressed in most tissues to varying degrees, 18,19 FoxO1 is abundantly expressed in adipose tissues, FoxO3 is abundant in cardiac and neuronal tissues, and FoxO4 is abundant in skeletal and cardiac muscle. FoxO6 is predominantly expressed in a specific region of the brain. 20 Interest in the FoxO subfamily developed when it was reported that alveolar rhabdomyosarcomas resulted from a genetic translocation forming a fusion gene composed of sequences from the "paired box" (Pax) and "forkhead in rhabdomyosarcoma" (FKHR) transcription factors. 21 The fusion protein contained an intact DNA binding domain from Pax3 and part of the DNA binding and C-terminal domains of FKHR (later to be termed FoxO1 1 ). In further studies, another forkhead related factor termed AFX1, because of its location on the X chromosome (later to be termed FoxO4), was shown to participate in translocations that resulted in human malignancies involving fusion with the myeloid-lymphoid leukemia (MLL) gene. 22 Further cloning studies based on sequence similarities to FKHR resulted in the isolation of the gene FKHRL1, 23 later to be termed FoxO3. In more recent studies, the FoxO6 gene has been assigned to the FoxO group because it displays sequence similarity to the other members, although its regulatory properties have diverged from those of the other Original received September 13, 2007; revision received November 13, 2007; accepted November 15, 2007 FoxO factors. 24 For the purposes of this review, the generic term FoxO is used to describe properties identified in one or more of FoxO1, FoxO3, and FoxO4, unless otherwise specified. The functional diversification of the different FoxO isoforms was revealed by targeted gene disruption in mice (Table). FoxO1-null embryos die on embryonic day 10.5 as a consequence of incomplete vascular development. 25,26 FoxO3-null mice survive to adulth...