FOXO1

Forkhead box protein O1 (FOXO1) also known as forkhead in rhabdomyosarcoma is a protein that in humans is encoded by the FOXO1 gene. FOXO1 is a transcription factor that plays important roles in regulation of gluconeogenesis and glycogenolysis by insulin signaling, and is also central to the decision for a preadipocyte to commit to adipogenesis. It is primarily regulated through phosphorylation on multiple residues; its transcriptional activity is dependent on its phosphorylation state.

Mechanism of action

In its un-phosphorylated state, FOXO1 is localized to the nucleus, where it binds to the insulin response sequence located in the promoter for glucose 6-phosphatase and increases its rate of transcription. FOXO1, through increasing transcription of glucose-6-phosphatase, indirectly increases the rate of hepatic glucose production. However, when FOXO1 is phosphorylated by Akt on Thr-24, Ser-256, and Ser-319, it is excluded from the nucleus, where it is then ubiquitinated and degraded. The phosphorylation of FOXO1 by Akt subsequently decreases the hepatic glucose production through a decrease in transcription of glucose 6-phosphatase.

FOXO3

Forkhead box O3, also known as FOXO3 or FOXO3a, is a human protein encoded by the FOXO3 gene.

Function

FOXO3 belongs to the O subclass of the forkhead family of transcription factors which are characterized by a distinct fork head DNA-binding domain. There are three other FoxO family members in humans, FOXO1, FOXO4 and FOXO6. These transcription factors share the ability to be inhibited and translocated out of the nucleus on phosphorylation by proteins such as Akt/PKB in the PI3K signaling pathway (aside from FOXO6, which may be constitutively nuclear). Other post-translational modifications including acetylation and methylation are seen and can result in increased or altered FOXO3a activity.

This protein likely functions as a trigger for apoptosis through upregulation of genes necessary for cell death, such as Bim and PUMA, or downregulation of anti-apoptotic proteins such as FLIP.

Gopinath et al.(2014) demonstrate a functional requirement for FOXO3 as a regulator of Notch signaling pathway (an essential regulator of quiescence in adult stem cells) in the self-renewal of stem cells during muscle regeneration.

FOXO4

Forkhead box protein O4 is a protein that in humans is encoded by the FOXO4 gene. It is located on the long arm of the X chromosome from base pair 71,096,148 to 71,103,533.

Structure and function

FOXO4 is a member of the forkhead family transciption factors O subclass, which is characterized by a winged helix domain used for DNA binding. There are 4 members of the FOXO family, including FOXO1, FOXO3, and FOXO6. Their activity is modified by many post translational activities, such as phosphorylation, ubiquitination, and acetylation. Depending on this modified state, FOXO4 binding affinity for DNA is altered, allowing for FOXO4 to regulate many cellular pathways including oxidative stress signaling, longevity, insulin signaling, cell cycle progression, and apoptosis. Two of the main upstream regulators of FOXO4 activity are phosphoinositide 3- kinase (PI3K) and serine/threonine kinase AKT/PKB. Both PI3K and AKT modify FOXO4 and prevent it from translocating to the nucleus, effectively preventing the transcription of the downstream FOXO targets.