Forkhead box protein O1

FOXO1-dependent inhibition of adipogenesis

FOXO1 negatively regulates adipogenesis.[9] Presently, the exact mechanism by which this is accomplished is not entirely understood. In the currently accepted model, FOXO1 negatively regulates adipogenesis by binding to the promoter sites of PPARG and preventing its transcription. Rising levels of PPARG are required to initiate adipogenesis; by preventing its transcription, FOXO1 is preventing the onset of adipogenesis. During stimulation by insulin, FOXO1 is excluded from the nucleus and is subsequently unable to prevent transcription of PPARG and inhibit adipogenesis.[10] However, there is substantial evidence to suggest that there are other factors that mediate the interaction between FOXO1 and the PPARG promoter, and that inhibition of adipogenesis is not entirely dependent on FOXO1 preventing transcription of PPARG.[11] The failure to commit to adipogenesis is primarily due to active FOXO1 arresting the cell in G0/G1 through activation of yet unknown downstream targets, with a putative target being SOD2.[12]

FOXO1 belongs to the forkhead family of transcription factors that are characterized by a distinct fork head domain. The specific function of this gene has not yet been determined; however, it may play a role in myogenic growth and differentiation.[13] FOXO1 is essential for the maintenance of human ESC pluripotency. This function is probably mediated through direct control by FOXO1 of OCT4 and SOX2 gene expression through occupation and activation of their respective promoters.[14] In hepatic cells this transcription factor seems to increase the expression of PEPCK and glycogen-6-phosphatase (the same enzymes that are blocked via the metformin/AMPK/SHP pathway). Blocking this transcription factor offers an opportunity for novel therapies for diabetes mellitus.[15] In pancreatic alpha-cells FOXO1 is important in regulating prepro-glucagon expression.[16] In pancreatic beta cells FOXO1 mediates glucagon-like peptide-1 effects on pancreatic beta-cell mass.[17]

Depicts insulin-regulated nuclear exclusion of FOXO1 and its effect on transcription of glucose-6 phosphatase

When the level of blood glucose is high, the pancreas releases insulin into the bloodstream. Insulin then causes the activation of PI3K, which subsequently phosphorylates Akt. Akt then phosphorylates FOXO1, causing nuclear exclusion.[18][19] This phosphorylated FOXO1 is then ubiquitinated and degraded by the proteosome.[20] The phosphorylation of FOXO1 is irreversible; this prolongs insulin's inhibitory effect on glucose metabolism and hepatic glucose production. Transcription of glucose 6-phosphatase subsequently decreases, which consequently decreases the rates of gluconeogenesis and glycogenolysis.[21] FOXO1 also activates transcription of phosphoenolpyruvate carboxykinase, which is required for gluconeogenesis.[22] The activity of FOXO1 is also regulated through CBP induced acetylation[23] on Lys-242, Lys-245, and Lys-262. These lysine residues are located within the DNA-binding domain; acetylation inhibits the ability of FOXO1 to interact with the glucose-6 phosphatase promoter by decreasing the stability of the FOXO1-DNA complex. Additionally, this acetylation increases the rate of phosphorylation on Ser-253 by Akt. Mutating Ser-253 to Ala-253 makes FOXO1 constitutively active. SIRT1 reverses this acetylation process; however, the exact mechanism by which SIRT1 deacetylates FOXO1 is still under investigation; presently, acetylation is thought to mitigate the transcriptional activity of FOXO1 and thereby provide an additional level of metabolic regulation that is independent of the insulin/PI3K pathway.[24]

FOXO1 may play an important role in apoptosis because it is phosphorylated and inhibited by AKT.[25] When FOXO1 is over expressed in human LNCaP prostate cancer cells, it causes apoptosis.[25] Also, FOXO1 regulates TNF-related apoptosis-inducing ligand (TRAIL), which caused FOXO1-induced apoptosis in the human prostate cancer cell line LAPC4 when FOXO1 adenovirus-mediated overexpression was used.[25] FOXO1 upregulates Fas ligand (FasL) transcriptionally resulting in apoptotic cell death.[25] Additionally, FOXO1 trans-activate Bim protein, which a member of the Bcl-2 family that promotes apoptosis and plays a role in the intrinsic mitochondrial apoptotic pathway.[25] Further, it was revealed that DNA damage-induced cell death in p53-deficient and p53-proficient cells was reduced when human FOXO1 is silenced by siRNA.[25] In type 2 diabetes the beta cells of the pancreas, which normally produce insulin undergo apoptosis, which greatly reduces insulin production. Fatty acids in the beta cells activate FOXO1, resulting in apoptosis of the beta cells.[26]

FOXO1 activation plays a role in cell cycle progression regulation.[25] The transcription and half- life of cyclin-dependent kinase inhibitor p27KIP1 rises when FOXO1 is active.[25] A study detects that FOXO1 regulates the nuclear localization of p27KIP1 in porcine granulosa cells and impacts cell cycle progression.[25] Furthermore, FOXO1-mediated cell cycle arrest is linked with cyclin D1 and cyclin D2 suppression in mammals.[25] It was detected that human FOXO1 is linked with the cyclin D1 promoter using chromatin immunoprecipitation assays (ChIP assays).[25] H215R is a human FOXO1 mutant, which cannot bind to the canonical FRE to induce expression of p27KIP1, repress cyclin D1 and cyclin D2 promoter activity and encourages cell cycle arrest at cyclin G1 (CCNG1).[25] As a result of that, activation of FOXO1 prevents the cell-division cycle at cyclin G1 (CCNG1) out of one of two ways stimulating or suppressing gene transcription.[25]

Phosphorylation of the FOXO1 protein is a result of the activation of the PI3K /AKT pathway.[27] Serum and glucocorticoid-inducible kinase SGK can also phosphorylate and inactivate FOXO1 transcription factor.[25] FOXO1 translocate from the nucleus to cytoplasm and inactivate through phosphorylation at well-defined sites by AKT/SGK1 protein kinases.[27] FOXO1 transcription factor can phosphorylate directly by AKT/SGK1 on three sites T24, S256 and S319.[28] Additionally, FOXO1 loses its interactions with DNA when phosphorylated by AKT/SGK1 because S256, which is one of the three AKT/SGK sites, changes the DNA-binding domain charge from a positive charge to a negative charge.[27]

Insulin signaling substrates 1 and 2 of the insulin-signaling cascade also regulate FOXO1 through phosphorylation by AKT.[27] AKT, which is referred to as protein kinase B, phosphorylates FOXO1 and accumulates in the cytosol.[27]

Casein kinase 1, a growth factor-activated protein kinase, also phosphorylates and potentiates FOXO1 and translocates FOXO1 to the cytoplasm.[27]