FOXO1 negatively regulatesadipogenesis.[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 ofPPARG 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 beingSOD2.[12]
FOXO1 belongs to theforkhead family oftranscription factors that are characterized by a distinctfork head domain. The specific function of this gene has not yet been determined; however, it may play a role inmyogenic 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 ofPEPCK andglycogen-6-phosphatase (the same enzymes that are blocked via themetformin/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 mediatesglucagon-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 releasesinsulin into the bloodstream. Insulin then causes the activation ofPI3K, which subsequently phosphorylatesAkt. 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 ofglucose 6-phosphatase subsequently decreases, which consequently decreases the rates ofgluconeogenesis andglycogenolysis.[21] FOXO1 also activates transcription ofphosphoenolpyruvate carboxykinase, which is required for gluconeogenesis.[22] The activity of FOXO1 is also regulated throughCBP induced acetylation[23] onLys-242, Lys-245, and Lys-262. These lysine residues are located within theDNA-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 byAkt. 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 inapoptosis because it is phosphorylated and inhibited byAKT.[25] When FOXO1 is over expressed in humanLNCaP prostatecancer cells, it causesapoptosis.[25] Also, FOXO1 regulates TNF-related apoptosis-inducing ligand (TRAIL), which caused FOXO1-inducedapoptosis in the humanprostate cancer cell lineLAPC4 when FOXO1 adenovirus-mediated overexpression was used.[25] FOXO1 upregulates Fas ligand (FasL) transcriptionally resulting inapoptotic cell death.[25] Additionally, FOXO1 trans-activateBim protein, which a member of theBcl-2 family that promotesapoptosis and plays a role in the intrinsic mitochondrial apoptotic pathway.[25] Further, it was revealed thatDNA damage-induced cell death inp53-deficient andp53-proficient cells was reduced when human FOXO1 is silenced bysiRNA.[25] Intype 2 diabetes thebeta cells of the pancreas, which normally produceinsulin undergo apoptosis, which greatly reduces insulin production.Fatty acids in the beta cells activate FOXO1, resulting in apoptosis of the beta cells.[26] FOXO1 can also supprot survival of malignant B cells by inducing the activity of GAB1-PI3K axis and mTORC2-pAKT axis.[27][28]
FOXO1 activation plays a role incell cycle progression regulation.[25] The transcription and half- life ofcyclin-dependent kinase inhibitorp27Kip1 rises when FOXO1 is active.[25] A study detects that FOXO1 regulates the nuclear localization of p27Kip1 in porcine granulosa cells and impactscell cycle progression.[25] Furthermore, FOXO1-mediated cell cycle arrest is linked withcyclin D1 andcyclin D2 suppression in mammals.[25] It was detected that human FOXO1 is linked with thecyclin 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, represscyclin D1 andcyclin D2 promoter activity and encouragescell cycle arrest at cyclin G1 (CCNG1).[25] As a result of that, activation of FOXO1 prevents thecell-division cycle at cyclin G1 (CCNG1) out of one of two ways stimulating or suppressing gene transcription.[25]
In its un-phosphorylated state, FOXO1 is localized to the nucleus, where it binds to the insulin response sequence located in thepromoter forglucose 6-phosphatase and increases its rate of transcription. FOXO1, through increasing transcription of glucose-6-phosphatase, indirectly increases the rate of hepatic glucose production.[22] However, when FOXO1 is phosphorylated byAkt on Thr-24, Ser-256, and Ser-319, it is excluded from the nucleus, where it is thenubiquitinated and degraded. Thephosphorylation of FOXO1 by Akt subsequently decreases the hepatic glucose production through a decrease in transcription of glucose 6-phosphatase.
Phosphorylation of the FOXO1 protein is a result of the activation of thePI3K /AKT pathway.[29] Serum and glucocorticoid-inducible kinaseSGK can also phosphorylate and inactivate FOXO1 transcription factor.[25] FOXO1 translocate from thenucleus tocytoplasm and inactivate throughphosphorylation at well-defined sites by AKT/SGK1 protein kinases.[29] FOXO1 transcription factor can phosphorylate directly byAKT/SGK1 on three sites T24, S256 and S319.[30] Additionally, FOXO1 loses its interactions withDNA when phosphorylated byAKT/SGK1 because S256, which is one of the three AKT/SGK sites, changes theDNA-binding domain charge from a positive charge to a negative charge.[29]
Insulin signaling substrates 1 and 2 of the insulin-signaling cascade also regulate FOXO1 through phosphorylation byAKT.[29]AKT, which is referred to as protein kinase B, phosphorylates FOXO1 and accumulates in thecytosol.[29]
Casein kinase 1, a growth factor-activated protein kinase, also phosphorylates and potentiates FOXO1 and translocates FOXO1 to thecytoplasm.[29]
Because FOXO1 provides a link between transcription and metabolic control by insulin, it is also a potential target for genetic control oftype 2 diabetes. In the insulin-resistant murine model, there is increased hepatic glucose production due to a loss in insulin sensitivity; the rates of hepatic gluconeogenesis and glycogenolysis are increased when compared to normal mice; this is presumably due to un-regulated FOXO1. When the same experiment was repeated with haploinsufficient FOXO1, insulin sensitivity was partially restored, and hepatic glucose production subsequently decreased.[31] Similarly, in mice fed with a high fat diet (HFD), there is increased insulin resistance in skeletal and liver cells. However, when haploinsufficient FOXO1 mice were treated with the same HFD, there was a notable decrease in insulin resistance in both skeletal and liver cells. This effect was significantly augmented by the simultaneous administration ofrosiglitazone, which is a commonly prescribed anti-diabetic drug.[32] These results create an opportunity for a novel gene therapy based approach to alleviating insulin desensitization in type 2 diabetes.
In diabetes (both type 1 and type 2), gluconeogenesis in the kidney contributes more to blood glucose than it does in normal subjects.[33] Enhancing suppression of FOXO1 by insulin can reduce gluconeogenesis in both the liver and kidney.[33]
In HFD-fed mice, the combination of FOXO1 and Notch-1 haploinsufficiency was more effective at restoring insulin sensitivity than FOXO1 haploinsufficiency alone.[34]
Insulin-producing cells could be generated through the inhibition of FOXO1 in intestinalorganoids generated from intestinal stem cells isolated from adult tissue.[35]
FOXO1 can also have "oncogenic" functions and contribute to the non-genetic adaptation of leukemic or lymphoma cells to targeted therapy, which makes it a potential therapeutic target in specific diseases.[36][27]
FOXO1 plays a role in the protection of cells fromoxidative stress.[29] It seems to promote cell death whenoxidative stress is high in tissues that are involved in diabetic complications.[29] In such situations, it has a destructive role instead of a protective role.[29]
InAdaptive Immunity system, FOXO1 regulates the return of peripheralB cells by upregulation of L-section and controls class-switch recombination of peripheralB cells and inT cells it enhances survival ofCD8 memory.[29]
InCarcinogenesis, FOXO1 plays a role of a tumor suppressor and its inactivation has been documented in many kinds of humancancer.[29] It suppresses survival oftumor cells by inducingapoptosis in prostate cancer cells and glioma cells by upregulating the proapoptotic factors.[29] Increased activation of FOXO1 may inhibit themetastasis of the prostate cancer cells to other organs by suppressing the migration and invasion or suppressing theRunt-domain containingRunx2 transcriptional activity.[29]
^"Human PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^abGalili N, Davis RJ, Fredericks WJ, Mukhopadhyay S, Rauscher FJ, Emanuel BS, Rovera G, Barr FG (November 1993). "Fusion of a fork head domain gene to PAX3 in the solid tumour alveolar rhabdomyosarcoma".Nature Genetics.5 (3):230–5.doi:10.1038/ng1193-230.PMID8275086.S2CID12374322.
^Nagashima T, Shigematsu N, Maruki R, Urano Y, Tanaka H, Shimaya A, Shimokawa T, Shibasaki M (November 2010). "Discovery of novel forkhead box O1 inhibitors for treating type 2 diabetes: improvement of fasting glycemia in diabetic db/db mice".Molecular Pharmacology.78 (5):961–70.doi:10.1124/mol.110.065714.PMID20736318.S2CID212576.
