FoxO1 [NCBI Accession:NM002015] belongs to the forkhead box O family that is characterized by a highly conserved DNA binding motif, known as forkhead box or winged helix domain (Accili and Arden 2004). This family contains four distinct isoforms namely FoxO1, FoxO3, FoxO4, and FoxO6, all of which are ubiquitously expressed in the body. FoxO1, also known as FKHR (forkhead in rhabdomyosarcoma), is initially described by Galili et al. (1993) as the product of the t(2;13)(q35;q14) translocation associated with alveolar rhabdomyosarcoma, a cancer of connective tissue that usually develops in children. Subsequent studies characterize FoxO1 as a key nuclear factor that mediates the inhibitory effect of insulin or insulin-like growth factor 1 (IGF-1) on the expression of genes, whose functions are instrumental for cell growth, differentiation, oxidative stress, survival, and metabolism in mammals (Accili and Arden 2004). FoxO1 orthologues dFoxO in Drosophila melanogaster and DAF16 in C. elegans contribute to the regulation of longevity (Hwangbo et al. 2004; Lee et al. 2003).
FoxO1 Is a Nutrient Sensor
FoxO1 is abundantly expressed in the brain and is shown to be an important regulator of the feeding behavior. In response to fasting, hypothalamic FoxO1 activity becomes upregulated, and this effect promotes hunger and stimulates food intake. In response to refeeding, hypothalamic FoxO1 activity is attenuated, contributing to the inhibition of food intake. Consistent with this notion, genetically modified mice with the FoxO1 activity in the hypothalamus are associated with excessive weight gain secondary to increased food intake (Kitamura et al. 2006; Kim et al. 2006). In contrast, inhibition of hypothalamic FoxO1 activity suppresses food intake and attenuates weight gain, resulting in leanness with improved glucose metabolism and increased sensitivity to insulin in mice (Ren et al. 2012). Thus, FoxO1 appears as a nutrient sensor for regulating food intake.
FoxO1 Is an Insulin Signaling Transducer
It is noteworthy that insulin-dependent inhibition of FoxO1 activity can ensue without necessarily altering FoxO1 subcellular redistribution, (Accili and Arden 2004), consistent with the notion that FoxO1 nuclear export is not a prerequisite for insulin-dependent inhibition of target gene expression. It is now clear that insulin inhibits FoxO1 activity via Akt-dependent phosphorylation. Phosphorylation distorts FoxO1 DNA binding domain, preventing its cognate binding to target promoters. Consistent with this notion is the presence of both Thr24 and Ser256 phosphorylation sites with the FoxO1 DNA-binding domain (Accili and Arden 2004). Further biochemical underpinning of this notion derives from the studies of FoxO6 – another member of the FoxO family. Distinct from other FoxO members (FoxO1, FoxO3, FoxO4), FoxO6 does not undergo insulin-stimulated nucleocytoplasmic shuttling. Instead, insulin inhibits nuclear FoxO6 activity by promoting its phosphorylation and disabling its DNA-binding activity in the nucleus (Kim et al. 2011). Thus, insulin-mediated FoxO1 nuclear export is not necessary for insulin inhibition of FoxO1 activity in cells.
FoxO1 Regulation of Gluconeogenesis and Glucose Metabolism
FoxO1 plays a pivotal role in mediating insulin action on gluconeogenesis, a metabolic pathway that takes place mainly in the liver for the generation of glucose from noncarbohydrate substrates including lactate, glycerol, and amino acids. A life-sustaining process, gluconeogenesis provides the sole fuel source for the brain, testes, and erythrocytes during a prolonged fast or exercise. Gluconeogenesis takes place mainly in the liver and to a much lesser extent in the kidney (Edgerton et al. 2009; Wahren and Ekberg 2007). In healthy individuals, hepatic gluconeogenesis accounts for up to 80% of total endogenous glucose production during a prolonged fast (Ekberg et al. 1999). Gluconeogenesis is controlled by phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). PEPCK catalyzes the conversion of oxaloacetate into phosphoenolpyruvate, the rate-limiting step of gluconeogenesis. G6Pase carries out the final step of gluconeogenesis by converting glucose-6-phosphate to glucose. FoxO1 is characterized as a key transcription factor that integrates insulin signaling to gluconeogenesis in the liver. FoxO1 physically binds to the promoters of PEPCK and G6Pase genes, and functionally stimulates PEPCK and G6Pase expression in the liver. This effect is enhanced by glucagon and inhibited by insulin (Accili and Arden 2004). As a result, increased FoxO1 activity is associated with augmented hepatic gluconeogenesis, contributing to the development of hyperglycemia in mice (Matsumoto et al. 2007; Qu et al. 2006). Conversely, FoxO1 deficiency is associated with diminished hepatic gluconeogenesis, lowering blood sugar levels in mice (Altomonte et al. 2003).
FoxO1-dependent regulation of gluconeogenesis provides a mechanism for the liver to adjust the rate of glucose production. Under the fed condition, FoxO1 activity is inhibited by insulin to suppress hepatic gluconeogenesis. This effect limits glucose production and prevents excessive postprandial blood glucose excursion. Under fasting condition, FoxO1 activity is increased to stimulate hepatic gluconeogenesis, resulting in increased glucose output from liver (Accili and Arden 2004). Such a reciprocal mechanism of hepatic insulin action is crucial for rapid adaptation of liver to metabolic shift between fed and fasting states to maintain blood glucose levels within the physiological range (Qu et al. 2006). An inept ability of insulin to inhibit FoxO1 activity in the liver, resulting from insulin resistance, accounts in part for unrestrained gluconeogenesis and fasting hyperglycemia in obesity and type 2 diabetes.
Although hepatic FoxO1 activity is swiftly inhibited by insulin via the phosphorylation-dependent mechanism, this pathway contributes to but does not account for the acute inhibitory action of insulin to switch off hepatic glucose production in response to postprandial glucose intake. There is new evidence that hepatic acetyl-CoA, derived from white adipose tissue, serves as a significant regulator of hepatic glucose production. Under fasting or insulin resistant condition, hepatic acetyl-CoA levels become higher, fueling hepatic gluconeogenesis. This effect is acutely inhibited by insulin. Insulin suppresses lipolysis in adipose tissues to limit hepatic acetyl-CoA levels, contributing to the inhibition of glucose production in the liver (Perry et al. 2015).
FoxO1 Regulation of Triglyceride Metabolism
FoxO1 also plays a significant role in regulating lipid metabolism. This is reflected by the ability of FoxO1 to mediate insulin action on hepatic expression of microsomal triglyceride transfer protein (MTP) and apolipoprotein C-III (ApoC-III), two hepatic proteins in triglyceride (TG) metabolism. MTP (MW, 88 kDa) is a molecular chaperone that is responsible for transporting lipid droplets to nascent apolipoprotein B (apoB) for very low-density lipoprotein (VLDL) production in the liver (Kamagate et al. 2008). MTP deficiency is associated with abnormal production of triglyceride-rich particles, resulting in abetalipoproteinemia or Bassen-Kornzweig syndrome, a rare autosomal recessive disorder in humans (Berriot-Varoqueaux et al. 2000). APOC3 functions as an inhibitor of lipoprotein lipase (LPL) and hepatic lipase (HL), key enzymes responsible for the hydrolysis of TG in VLDL and chylomicrons in the postabsorption phase (Wang et al. 1985; McConathy et al. 1992; Kinnunen and Ehnholm 1976). Elevated APOC3 levels also perturb hepatic uptake and clearance of TG-rich lipoprotein remnants (Wuarfordt et al. 1982; Mann et al. 1997). This action is mediated by the low-density lipoprotein (LDL) family receptors independently of LPL (Gordts et al. 2016). Apart from its extracellular functions, APOC3 plays an intracellular role in facilitating VLDL-TG assembly and secretion from the liver (Qin et al. 2011; Chan et al. 2006; Taskinen et al. 2011; Cohn et al. 2004). Elevated ApoC-III levels are associated with impaired clearance of TG-rich particles, leading to the accumulation of TG-rich lipoprotein remnants in plasma (Altomonte et al. 2004). FoxO1 is shown to mediate insulin-dependent regulation of both MTP and ApoC-III production in the liver (Altomonte et al. 2004). This effect correlates with the ability of FoxO1 to bind to the IRE motif within both MTP and ApoC-III promoters (Altomonte et al. 2004).
Elucidation of FoxO1-dependent regulation of hepatic MTP and ApoC-III production provides mechanistic insight into the pathophysiology of hypertriglyceridemia - the most common lipid disorder that is characterized by increased production of very low-density lipoprotein (VLDL) and/or decreased clearance of TG-rich particles (VLDL and chylomicrons) in human subjects with obesity and type 2 diabetes. Checked FoxO1 activity, stemming from insulin resistance in the liver, promotes hepatic MTP and ApoC-III overproduction, contributing to increased hepatic VLDL-TG production and decreased systemic clearance of triglyceride-rich particles. Selective inhibition of FoxO1 activity in insulin resistant liver is predicted to curb excessive VLDL-TG production, improve TG catabolism, and ameliorate hypertriglyceridemia in obesity and type 2 diabetes.
FoxO1 in the Pathogenesis of Nonalcoholic Fatty Liver Disease
Apart from its role in regulating glucose and triglyceride production in the liver, FoxO1 plays an important part in the pathogenesis of nonalcoholic fatty liver disease (NAFLD) – dubbed as hepatic manifestation of metabolic syndrome. NAFLD is characterized by excessive lipid deposition in the liver of subjects with little alcohol consumption. Patients with NALFD are at heightened risk of developing steatohepatitis with potential complications of fibrosis and cirrhosis. Elevated FoxO1 production in the liver is associated with NAFLD in mice (Qu et al. 2006). Patients with steatohepatitis also have significantly elevated FoxO1 activity in the liver (Valenti et al. 2008). To dissect the underlying mechanism, Zhang et al. (Zhang et al. 2006) show that hepatic lipogenic genes including SREBP-1c, FAS, and ACC are upregulated in transgenic mice expressing a constitutively active FoxO1 allele. These findings are reproduced by two independent studies showing that adenovirus-mediated FoxO1 production results in augmented lipogenesis with concomitant fat accumulation in the liver (Qu et al. 2006; Matsumoto et al. 2006). It appears that unchecked FoxO1 activity contributes to the upregulation of lipogenic gene expression, accounting in part for the development of NAFLD under insulin resistant conditions.
This notion is buttressed by several independent studies. First, FoxO1 gain-of-function, resulting from adenovirus-mediated FoxO1 production or transgenic overexpression of its constitutively active allele, is associated with impaired insulin action in the liver, accompanied by inappropriately increased hepatic glucose production (Accili and Arden 2004). Second, FoxO1 loss-of-function, caused by hepatic expression of its dominant-negative allele or antisense oligonucleotide-mediated FoxO1 knockdown in the liver, is associated with improved insulin action and blood glucose profiles in insulin resistant obese mice (Altomonte et al. 2003). Third, FoxO1 haploinsufficiency protects mice from developing high fat diet-induced insulin resistance and rescues the diabetic phenotype in insulin receptor substrate 2 (IRS2)-deficient diabetic mice (Accili and Arden 2004). Fourth, liver-specific FoxO1 knockout results in near normalization of hepatic glucose and lipid metabolism in insulin resistant mice with simultaneous IRS1 and IRS2 depletion or alternatively IR depletion in the liver (Dong et al. 2008; I OS 2015). These results underscore the significance of FoxO1 in integrating insulin signaling with hepatic lipid metabolism. Hepatic FoxO1 dysregulation is deleterious to hepatic insulin signaling, contributing to abnormal lipid metabolism in diabetes.
FoxO1 Regulation of Cellular Autophagy
Autophagy, rhetorically called “programmed cell survival,” as opposed to apoptosis that is dubbed “programmed cell death,” is essential for maintaining energy homeostasis for cell survival in response to nutrient deprivation or oxidative stress. Autophagy is an intracellular process in which cytoplasmic constituents including damaged organelles and misfolded or damaged proteins were delivered in the autophagosome to the lysosome for degradation. Defective autophagy is associated with human diseases (Choi et al. 2013). FoxO1 is shown to promote cardiomyocyte survival in response to oxidative stress (Sengupta et al. 2011). During muscle atrophy, FoxO1 and FoxO3 activities are upregulated to stimulate the expression of a number of key genes involved in antioxidative stress and muscle autophagy and atrophy. These effects serve as a cytoprotective mechanism for preventing muscle loss (Milan et al. 2015). There is also evidence that FoxO1 mediates autophagy via its protein-protein interaction with Atg7 (autophagy-related protein 7) in response to cellular stress. This action, which takes place in the cytoplasm independently of FoxO1 transcription activity, seems to impact autophagy-mediated cell death in cancer cells (Zhao et al. 2010). This observation is intriguing, defying the longstanding notion that FoxO1, after trafficking from the nucleus to the cytoplasm, is ubiquitinated and is destined for proteasome-mediated degradation. This study unfolds a new functional facet of cytosolic FoxO1 proteins in regulating autophagy.
FoxO1 Regulation of Oxidative Stress
FoxO1 processes antioxidative function. This is exemplified in C. elegans, in which the FoxO1 ortholog Daf16 is upregulated along with its nuclear localization in response to oxidative stress in C. elegans (Honda and Honda 1999; Essers et al. 2005). This effect contributes to the induction of superoxide dismutase (Sod) expression, protecting C. elegans from oxidative damage and extending its life-span (Murphy et al. 2003). Such an antioxidative function of FoxO1 is conserved from C. elegans to mammals. Of particular significance is the illustration of the role of FoxO1 in regulating oxidative stress in β-cells – a cell type that is poorly equipped with antioxidative function, due to low expression of antioxidative enzymes such as SOD, catalase (CAT), and glutathione peroxidase (GPX). As a result, β-cells are vulnerable to oxidative stress secondary to glucolipotoxicity, accounting in part for the etiology of β-dysfunction in diabetes. FoxO1 is shown to stimulate β-cell expression of SOD1, CAT, and GPX1–three key enzymes for defending oxidative stress. These effects improve β-cell survival and function in the face of oxidative stress in dietary obese mice (Zhang et al. 2016). FoxO1 is also shown to promote the expression of DNA repair enzyme GADD45α, protecting β-cells from nitric oxide-elicited DNA damage (Kitamura et al. 2005). FoxO1 also protects β-cells from glucolipotoxicity by inhibiting the expression of thioredoxin-interacting protein (TXNIP), whose overproduction is linked to β-cell apoptosis in diabetes (Kibbe et al. 2013). Thus, FoxO1 confers an evolutionally conserved cytoprotective effect on cell survival and function.
FoxO1 Regulation of Endoplasmic Reticulum Stress
Apart from its role in oxidative stress, FoxO1 plays a significant role in regulating endoplasmic reticulum (ER) stress, also known as unfolded protein response (UPR), that is triggered in response to the accumulation of misfolded or unfolded proteins in the ER lumen. Such an adaptive mechanism serves to resolve ER stress by halting protein synthesis to favor the production of molecular chaperones for protein folding. Unresolved ER stress results in cell apoptosis. FoxO1 is shown to integrate insulin signaling to expression of the ER chaperone glucose-regulated protein (GRP78) in the liver, and this effect acts to resolve ER stress in hepatocytes (Kamagate et al. 2010). An ER resident protein, GRP78, remains bound to IRE1, PERK, and ATF6 in unstressed cells. In response to ER stress, GRP78 dissociates from IRE1, PERK, and ATF6, triggering UPR and promoting the induction of genes encoding ER chaperones GRP78 and GRP94. FoxO1 signaling through GRP78 is critical for maintaining hepatic glycogen homeostasis. Hepatic production of a FoxO1 constitutively active allele that is not inhibited by insulin consequently results in undue ER stress and glycogen depletion secondary to excessive FoxO1-induced glycogenolysis in hepatocytes (Kamagate et al. 2010).
Likewise, FoxO1 activity is upregulated in β-cells when cultured in the presence of free fatty acids. This effect precedes the induction of ER stress and β-cell apoptosis, raising the idea of inhibiting FoxO1 activity for mitigating ER stress and preventing β-cell dysfunction in type 2 diabetes (Martinez et al. 2008).
Hormonal Regulation of FoxO1 Activity
FoxO1 is subject to hormonal regulation. Hepatic FoxO1 expression is stimulated by glucagon and inhibited by insulin, consistent with the shifting of the metabolic state from fasting to refeeding. Insulin inhibits FoxO1 activity by stimulating its phosphorylation and promoting its trafficking from the nucleus to cytoplasm. Consistent with this model, FoxO1 contains three highly conserved amino acid residues (Thr-24, Ser-256, Ser-319) that are phosphorylated by Akt. Insulin-dependent regulation of FoxO1 activity provides an effective mechanism for suppressing hepatic production of both glucose and VLDL-TG after meals. This mechanism serves to prevent excessive postprandial glucose and triglyceride excursion for maintaining blood glucose and lipid levels within the physiological range in normal individuals. However, in insulin resistant subjects, hepatic FoxO1 expression becomes deregulated, due to the impaired ability of insulin to inhibit FoxO1 activity in the liver. This effect promotes excessive production of both glucose and triglyceride, contributing to the dual pathogenesis of fasting hyperglycemia and hyperlipidemia in type 2 diabetes (Matsumoto et al. 2006). Apart from its acute regulation by insulin, FoxO1 activity is regulated at multiple levels. At the transcriptional level, FoxO1 mRNA expression is induced in response to glucagon during fasting, correlating with its enhanced activity in stimulating hepatic glucose production (Qu et al. 2006).
FoxO1 Is Regulated via Its Feedback Loop
FoxO1 regulates its transcriptional activity via a feedback mechanism. In response to nutrient deprivation, the Drosophila FoxO1 ortholog dFoxO is stimulated to promote the expression of its upstream effector gene encoding insulin receptor (dIR) (Puig and Tjian 2005). A similar mechanism applies to mammalian cells, in which FoxO1 stimulates IR and IRS2 expression, which in turn suppresses FoxO1 expression. This finding is reproduced in both the liver and β-cells (Kamagate et al. 2010; Tsunekawa et al. 2011). Unchecked FoxO1 activity, resulting from molecular defects in the FoxO1 feedback loop, is deleterious to hepatic metabolism, culminating in unrestrained glycogen breakdown and excessive ER stress in the liver. Likewise, excessive ER stress, caused by FoxO1 overproduction, is associated with β-cell dysfunction and apoptosis (Martinez et al. 2008). Thus, FoxO1 feedback loop serves as a safeguarding mechanism for keeping FoxO1 activity in check to avert hepatic glycogen depletion and prevent ER stress in cells.
Posttranslational Regulation of FoxO1 Activity
FoxO1 is subject to posttranslational modification. In addition to insulin-stimulated protein phosphorylation, FoxO1 activity is regulated by other signaling molecules, such as cyclin-dependent kinases (CDKs). CDK1 catalyzes FoxO1 phosphorylation at Ser249. This effect disrupts FoxO1 association with 14-3-3 proteins, promoting FoxO1 nuclear retention and augmenting FoxO1-dependent transcription, leading to apoptosis in neuronal cells (Yuan et al. 2008). In contrast, CDK2-mediated phosphorylation of FoxO1 at Ser249 promotes its cytoplasmic localization, resulting in inhibition of FoxO1 transcriptional activity. This effect favors cell survival in response to DNA damage in human prostate cancer cells (Huang et al. 2006).
Furthermore, FoxO1 activity is regulated by kinase (ERK) and p38, two members of the mitogen-activated protein kinase (MAPK) family. MAPK-mediated phosphorylation of FoxO1 enhances its transcriptional activity in stimulating vascular endothelial growth factor receptor 2 (VEGFR2) in endothelial cells (Asada et al. 2007). This effect underscores the importance of FoxO1 in angiogenesis and vascular remodeling. Indeed, FoxO1-null mice die at embryonic day 11.5 due to defective angiogenesis in the embryos (Furuyama et al. 2004).
Acetylation and Deacetylation Regulation of FoxO1 Activity
FoxO1 activity is dynamically regulated by acetylation and deacetylation. As shown in Fig. 1, FoxO1 is acetylated by cAMP response element-binding (CREB)-binding protein (CBP) and/or p300 factor. This effect enhances FoxO1 activity in cells (Perrot and Rechler 2005). In contrast, FoxO1 acetylation at the basic residues (Lys-242, Lys-245, and Lys-262) attenuates FoxO1 activity by disabling FoxO1 binding to target promoters (Matsuzaki et al. 2005). FoxO1 activity can be enhanced or suppressed depending on the sites of acetylation.
Deacetylation of FoxO1 protein also modulates its trans-activation activity. FoxO1 is a substrate of SIRT1, the mammalian ortholog of the yeast Sir2 deacetylase (Motta et al. 2004). FoxO1 contains a consensus site (459-LXXLL-463) within its carboxyl trans-activation domain that is responsible for SIRT1 binding. Mutations in the LXXLL motif (L462A and L463A) disrupt FoxO1 binding to SIRT1 and abolish SIRT1-mediated deacetylation (Nakae et al. 2006). FoxO1 deacetylation by SIRT1 plays a key role in modulating hepatic expression of gluconeogenic genes PEPCK and G6Pase in the liver (Nakae et al. 2006).
Likewise, FoxO1 is subject to deacetylation by the Class IIa histone deacetylases (HDACs) (Mihaylova et al. 2011). In response to fasting, HDACs are dephosphorylated and translocated to the nucleus, where HDACs recruit HDAC3 for deacetylating FoxO1 in hepatocytes. This effect enhances FoxO1 activity in stimulating gluconeogenesis in the liver in the fasting state (Mihaylova et al. 2011).
Ubiquitination and Proteolysis of FoxO1 Proteins
FoxO1, when phosphorylated in response to insulin or insulin-like growth factor 1 (IGF-1), is translocated from the nucleus to the cytoplasm. Phosphorylated FoxO1 proteins undergo ubiquitination, a posttranslational modification that is mediated by the E3 ligases such as SKP2 and MDM2 (Matsuzaki et al. 2003). This effect seems to be activated by CRY1, a cytoplasmic protein that acts to promote FoxO1 binding to E3 ligases (Jang et al. 2016). Ubiquitinated FoxO1 proteins are targeted for proteolytic degradation. COP1 also possess E3 ligase activity to stimulate cytosolic FoxO1 ubiquitination and degradation (Kato et al. 2008). Similarly, FoxO1 is associated with Itch – another E3 ligase that promotes FoxO1 ubiquitination and degradation in immune cells (Xiao et al. 2014). E3 ligase-mediated ubiquitination serves as a distinct mechanism for regulating FoxO1 stability and activity in cells. Such posttranslational modification mechanism is shared by other FoxO members including FoxO3 and FoxO4 (Fu et al. 2009).
Dephosphorylation and Recycle of FoxO1 Proteins in Cells
It is generally thought that FoxO1, upon its phosphorylation and nuclear export, is destined for proteosome-mediated degradation in the cytoplasm. This view is being challenged by Yan et al. (Yan et al. 2008), who have shown that FoxO1 is a substrate of protein phosphatase 2A (PP2A), which dephosphorylates FoxO1 and promotes FoxO1 nuclear localization in cultured lymphoid FL5.12 cells. Inhibition of PP2A protects FoxO1 from dephosphorylation and perturbs FoxO1 nuclear trafficking (Yan et al. 2008). FoxO1 dephosphorylation and reverse translocation may serve as a fine-tuning mechanism for recycling cytosolic FoxO1 and promoting FoxO1 reentry to the nucleus for function.
Methylation Regulation of FoxO1 Activity
FoxO1 methylation constitutes a distinct mechanism for posttranslational modification of FoxO1 activity in cells. FoxO1 is shown to be a substrate of the protein arginine methyltransferase 1 (PRMT1), a ubiquitously expressed enzyme that catalyzes the methylation of terminal nitrogens of guanidinium side chains at Arg residues of target proteins (Yamagata et al. 2008). PRMT1 methylates FoxO1 at Arg248 and Arg250 within a consensus motif for Akt phosphorylation (Fig. 1). This effect blocks Akt-mediated phosphorylation of FoxO1 at Ser253, favoring FoxO1 retention of the nucleus and enhancing FoxO1 activity. Arg methylation of FoxO1 is considered an antagonistic mechanism for counterbalancing Akt/PKB-mediated phosphorylation and inhibition of FoxO1 activity in cells. Likewise, FoxO3 also undergoes methylation at Lys271, a specific residue that is methylated by the methyltransferase Set9. This action increases FoxO3 activity in cells (Calnan et al. 2012).
O-Linked Glycosylation-Dependent Regulation of FoxO1 Activity
Another layer of posttranslational modification is the O-linked glycosylation, an enzymatic process that is catalyzed by O-N-acetylglucosamine (O-GlcNAc) transferase for the addition of sugar moieties to Ser and Thr residues in target proteins. This posttranslational modification takes place on Ser or Thr, amino acid residues that are often the sites of phosphorylation by Ser/Thr kinases. As a result, O-linked glycosylation interferes with phosphorylation, and this effect can affect protein stability, protein folding, or subcellular distribution in cells. FoxO1 is shown to undergo O-linked glycosylation, resulting in skewed FoxO1 nuclear localization and contributing to increased FoxO1 activity in cells at high glucose levels (Kuo et al. 2008a, b; Housley et al. 2008). O-linked glycosylation of FoxO1 becomes abnormally higher, correlated with increased hepatic glucose production in diabetes (Housley et al. 2008). O-linked glycosylation provides a mechanism for modulating FoxO1 activity in response to metabolic stress.
Association of FoxO1 Polymorphism with Diabetes
Despite its functional importance in insulin signaling and carbohydrate metabolism, FOXO1 has not been assigned as a susceptible gene for diabetes in humans. Nonetheless, there is clinical evidence that genetic FOXO1 variants are associated with β-cell dysfunction, impaired glucose tolerance, and type 2 diabetes in Finnish men (Mussig et al. 2009). Likewise, genetic FOXO1 mutations are also associated with an increased risk of developing obesity and type 2 diabetes in Pima Indians (Muller et al. 2015). In contrast, it is also reported that such a close association between FOXO1 polymorphism and type 2 diabetes is lacking in other ethical groups including Caucasian and African-Americans (Karim et al. 2006). The available clinical data do not support the notion that genetic mutations in the FOXO1 locus would entail a significant risk of developing type 2 diabetes in humans.
Characterization of FoxO1 signaling has provided important insights into the mechanism by which insulin (or IGF-1) regulates the expression of target genes in cell growth, differentiation, metabolism, oxidative stress, and aging in response to nutritional cues. FoxO1 is ubiquitously expressed with diverse functions in different cell types. Due to space limitation, this article focuses on reviewing of research progress made toward our understanding of the insulin-Akt-FoxO1 signaling pathway in regulating glucose and lipid metabolism in the liver, as well as on reviewing of the mechanisms by which FoxO1 activity is regulated at both transcriptional and posttranslational levels. Insulin signaling through FoxO1 is instrumental for adjusting the rate of hepatic glucose and triglyceride production according to the physiological state. FoxO1 activity is subject to insulin inhibition. Loss of insulin inhibition is associated with augmented FoxO1 activity in both insulin-deficient and insulin-resistant livers. This effect contributes to hepatic overproduction of glucose and triglycerides, accounting in part for the dual pathogenesis of hyperglycemia and hypertriglyceridemia in both type 1 and type 2 diabetes. FoxO1 expression becomes deregulated in the liver of rodents with diabetes and humans with steatohepatitis. The available preclinical studies and clinical data support the notion of targeting FoxO1 for improving glucose and lipid metabolism in diabetes.
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