Targeting Forkhead box O1 from the concept to metabolic diseases: lessons from mouse models

Abstract

Forkhead box O (FOXO) transcription factors have been implicated in regulating the metabolism, cellular proliferation, stress resistance, apoptosis, and longevity. Through the insulin receptor substrate → phosphoinositide 3-kinase → Akt signal cascade, FOXO integrates insulin action with the systemic nutrient and energy homeostasis. Activation of FOXO1 in liver induces gluconeogenesis via phosphoenolpyruvate carboxykinase (PEPCK)/glucose 6-phosphate pathway, and disrupts mitochondrial metabolism and lipid metabolism via heme oxygenase 1/sirtuin 1/Ppargc1α pathway. In skeletal muscle, FOXO1 activation underpins the carbohydrate/lipid switch during fasting state. Inhibition of FOXO1 under physiological conditions accounts for maintenance of skeletal muscle mass/function and adipose differentiation. In pancreatic β-cells, nuclear translocation of FOXO1 antagonizes pancreatic and duodenal homeobox 1 and attenuates β-cells proliferation and insulin secretion. Regardless, FOXO1 promotes the proliferation of β-cells through induction of Cyclin D1 in low nutrition, and elicits antioxidant mechanism to protect against β-cell failure during oxidative insults. In the brain, FOXO1 controls food intake through transcriptional regulation of the orexigenic neuropeptide Y, agouti-related protein, and carboxypeptidase E. In this article, we review the role of FOXO1 in the regulation of metabolism and energy expenditure based on recent findings from mouse models, and discuss the therapeutic value of targeting FOXO1 in metabolic diseases.

FIG. 1.

The posttranslational regulation of FOXO1. Hormonal and stress stimuli regulate FOXO1 via phosphorylation, acetylation, ubiquitination, methylation, and glucose-derived O-GlcNAc modification. The phosphorylation induced by Akt, Sgk, and IKK is inhibitory (dark gray), whereas MST1 and AMPK phosphorylate and activate FOXO1 (white background). Methylation of FOXO1 by arginine methyltransferase PRMT1 blocks Akt-mediated phosphorylation and thus promotes FOXO1 nuclear redistribution. The role of CBP-catalyzed acetylation and SirT1/2-catalyzed deacetylation in FOXO1 activity is controversial, being reported positive and negative, or vice versa, in the literature. O-GlcNAc regulates FOXO1 activation in response to glucose, resulting in paradoxically increased expression of gluconeogenic genes while concomitantly inducing expression of genes encoding enzymes that detoxify reactive oxygen species. Akt, protein kinase B; AMPK, AMP-activated protein kinase; CBP, CREB (cAMP response element-binding) binding protein; FOXO, forkhead box O; IKK, IκB (inhibitor of NF-κB) kinase; IRS, insulin receptor substrates; MST1, mammalian sterile 20-like kinase-1; O-GlcNAc, O-linked β-N-acetyl-glucosamine; O-GlcNAcT = O-linked β-N-acetyl-glucosamine transferase; PDK1, 3-Phosphoinositide-dependent kinase 1; PRMT1, protein arginine methyltransferase 1; Sgk, serum/glucocorticoid-inducible protein kinase; SirT1/2, sirtuin 1 or 2.

FIG. 2.

The effects of FOXO1 activation and ablation on gene expression in the liver. Normalized expression of liver genes was analyzed in fasted (16 h) and fed (4 h) 6-week-old control (CNTR), Irs1/2 double-knockout (DKO), and Irs1/2 and FOXO1 TKO mice using Affymetrix GeneChips. Liver genes that had been changed significantly (false discovery rate FDR <0.05) were further analyzed for either a positive (+) or negative (−) correlation with principal component using the NIA Array Analysis Tool. The analyses indicate that 9824 significantly changed probe sets corresponded to 5756 annotated genes of which 420 displayed a maximal change of at least 1.5-fold. The principal components (PC)—with a positively and negatively correlated gene cluster—accounted for 86% of the total expression variance, including 3531 displaying increased and 593 displaying decreased expression in the DKO liver. Dysregulated expression of these genes in the DKO liver was largely restored in the TKO liver to the normal range displayed by the control liver. Data were presented as average normalized expression (log2 scale) of gene clusters positively correlated (•) and negatively correlated (▪) with principal component. The error bars represent the standard deviation. Adapted from Dong et al. (25) with permission. TKO, triple knockout.

FIG. 3.

The regulation of hepatic glucose and lipid metabolism by FOXO1. Under ordinary conditions, feeding stimulates insulin secretion from pancreatic β-cells, and FOXO1 is inhibited by insulin signal via IRS-PI3K-Akt cascade. In fasting state, insulin signal is weak and FOXO1 is activated/translocated into the nuclei to trigger gluconeogenesis for glucose supply. Under insulin resistance conditions, however, hyperactive FOXO1 promotes gluconeogenesis in such an uncontrolled way that it leads to hyperglycemia. Moreover, FOXO1 induces Hmox1 that disrupts electron transport chain and impairs mitochondrial metabolism (including fatty acid oxidation). FOXO1 might also enhance downstream of insulin signal (increasing Akt phosphorylation) by acting on Trb3 and p38, which may eventually promote lipogenesis. Regardless, it remains unclear how insulin drives lipogenesis under insulin-resistant conditions when insulin fails to initiate the IR-IRS-PIK-Akt cascade. FA, fatty acid; FAO, fatty acid oxidation; G6P, glucose 6-phosphate; Hmox1, heme oxygenase 1; MTP, microsomal triglyceride transfer protein; PEPCK, phosphoenolpyruvate carboxykinase; PI3K, phosphoinositide 3-kinase; SREBP-1c, sterol-regulatory element binding protein 1c; TG, triglyceride; Trb3, Tribbles homolog 3; VLDL, very-low-density lipoprotein.

FIG. 4.

Metabolic regulation by FOXO1 in skeletal muscle. Under feeding state FOXO1 is inactivated, and insulin drives protein synthesis through Akt-TSC1/2-mTOR pathway. In fasting states, FOXO1 induces PDK4 that inhibits PDH and blunts glycolysis (pyruvate → tricarboxylic acid cycle). On the other hand, FOXO1 induces LPL and CD36: the former promoting the hydrolysis of lipoprotein into FFA, and the latter promoting the FFA uptake by muscle cells. Moreover, FOXO1 disrupts the RXRα/LXR(complex and the downstream SREBP-1c-mediated lipogenesis in muscle cells. Thus, FOXO1 plays a key role in the carbohydrate/lipid metabolic switch in skeletal muscle during fasting/feed cycle. Under insulin resistance conditions, FOXO1 is hyperactivated and induces autophagy-related proteins degradation through Atp1 and MuRF1, which causes atrophy (muscle loss) and disturbs metabolic homeostasis. Atg1, atrogin 1 (F-box protein 32); FFA, free fatty acid; LPL, lipoprotein lipase; LXRα, liver X receptor α; mTOR, mammalian target of rapamycin; MuRF1, muscle-specific RING finger protein 1; PDH, pyruvate dehydrogenase; PDK4, pyruvate dehydrogenase kinase 4; RXRα, retinoid X receptor α; TSC1/2, tuberous sclerosis complex 1 and 2.

FIG. 5.

Regulation of adipocyte differentiation by FOXO1. Activation of FOXO1 facilitate its nuclear re-distribution and binding to PPARγ. The association of FOXO1 with PPARγ lead to 2 consequences: (a) it disrupts the PPARγ-RXRα complex, which is required for the binding of PPARγ to DNA (the PPRE) to elicit expression for genes that are responsible for adipocyte differentiation; (b) it transrepresses the PPARγ transactivation of target gene expression and inhibit adipocyte differentiation. The entry of active FOXO1 into nuclei also induces p21 (a cell cycle inhibitor) and blocks adipocyte differentiation. SirT2 and high-fat diet that hyperactivate FOXO1 can disrupt lipid storage and energy homeostasis in adipose tissue. HFD, high-fat diet; PPARγ, peroxisome proliferator-activated receptor γ; PPRE, PPAR response element.

FIG. 6.

The regulation of β-cell function by FOXO1. FOXO1 competes with FOXA2 for the PDX1 promoter and represses expression of PDX1 and probably NGN3 and NKX61, the factors responsible for β-cell proliferation. FOXO1 also excludes PDX1 protein from the nuclei in a reciprocal way. Thus, activation and nuclear redistribution of FOXO1 suppresses PDX1, NGN3, and NKX61, resulting in the inhibition of β-cell proliferation. This effect was enhanced during insulin resistance and ER stress or high FFA influx. In low nutrition, however, FOXO1 is activated due to the silence of PI3K-Akt or MAPK inhibitory pathways, which promotes β-cell proliferation through Cyclin D1. Moreover, FOXO1 plays an important role in protecting β-cell from oxidative stress, by forming complex with PML body and SirT1 to undergoes activation and induce antioxidant enzymes like MnSOD and catalase. FOXA2, forkhead box A2; JNK, c-jun-N-terminal kinase; MAPK, mitogen-activated protein kinase; MnSOD, manganese superoxide dismutase; NGN3, neurogenin 3; PDX1, pancreatic and duodenal homeobox 1; PML, promyelocytic leukemia.