FoxO6 transcriptional activity is regulated by Thr26 and Ser184, independent of nucleo-cytoplasmic shuttling

Forkhead box O proteins: steering the course of stem cell fate

Stem cells are pivotal players in the intricate dance of embryonic development, tissue maintenance, and regeneration. Their behavior is delicately balanced between maintaining their pluripotency and differentiating as needed. Disruptions in this balance can lead to a spectrum of diseases, underscoring the importance of unraveling the complex molecular mechanisms that govern stem cell fate. Forkhead box O (FOXO) proteins, a family of transcription factors, are at the heart of this intricate regulation, influencing a myriad of cellular processes such as survival, metabolism, and DNA repair. Their multifaceted role in steering the destiny of stem cells is evident, as they wield influence over self-renewal, quiescence, and lineage-specific differentiation in both embryonic and adult stem cells. This review delves into the structural and regulatory intricacies of FOXO transcription factors, shedding light on their pivotal roles in shaping the fate of stem cells. By providing insights into the specific functions of FOXO in determining stem cell fate, this review aims to pave the way for targeted interventions that could modulate stem cell behavior and potentially revolutionize the treatment and prevention of diseases.

Mechanistic regulation of FOXO transcription factors in the nucleus

FOXO proteins represent evolutionarily conserved transcription factors (TFs) that play critical roles in responding to various physiological signals or pathological stimuli, either through transcription-dependent or -independent mechanisms. Dysfunction of these proteins have been implicated in numerous diseases, including cancer. Although the regulation of FOXO TFs shuttling between the cytoplasm and the nucleus has been extensively studied and reviewed, there's still a lack of a comprehensive review focusing on the intricate interactions between FOXO, DNA, and cofactors in the regulation of gene expression. In this review, we aim to summarize recent advances and provide a detailed understanding of the mechanism underlying FOXO proteins binding to target DNA. Additionally, we will discuss the challenges associated with pharmacological approaches in modulating FOXO function, and explore the dynamic association between TF, DNA, and RNA on chromatin. This review will contribute to a better understanding of mechanistic regulations of eukaryotic TFs within the nucleus.

Homology Modeling, Screening, and Identification of Potential FOXO6 Inhibitors Curtail Gastric Cancer Progression: an In Silico Drug Repurposing Approach

Gastric cancer is the world's second leading cause of cancer-related fatalities, with the epidemiology changing over the previous several decades. FOXOs are the O subfamily of the forkhead box (FOX) transcription factor family, which consists of four members: FOXO1, FOXO3, FOXO4, and FOXO6. FOXO6 mRNA and protein levels are increased in gastric cancer tissues. FOXO6 forced overexpression enhances gastric cancer cell growth, while knockdown decreases proliferation. In our study, the GEPIA, Kaplan-Meier, KEGG, and STRING databases were used to determine FOXO6 mRNA expression, overall survival ratio, interactive pathways, and top 10 associated proteins in gastric cancer respectively. Due to the lack of a solved structure for FOXO6, homology modeling was performed to obtain a 3D structure model, and we used anti-cancer drugs and small molecules to target FOXO6 for identifying a potential selective FOXO6 inhibitor. The chemical composition of the proteins and ligands has a significant impact on docking procedure performance. With this in mind, a critical evaluation of the performance of three regularly used docking routines was carried out: MVD, AutoDock Vina in PyRx, and ArgusLab. The binding affinities, docking scores, and intermolecular interactions were used as assessment criteria. In the study, the porfimer sodium showed excellent binding affinity to the FOXO6 protein. The major three docking software packages were used to analyze the scoring/H-bonding energy and intermolecular interactions. Based on the results, we concluded that FOXO6 was upregulated in gastric cancer and the ligand porfimer sodium emerges as a promising potential FOXO6 inhibitor to curtail gastric cancer progression.

FOXO family isoforms

FOXO family of proteins are transcription factors involved in many physiological and pathological processes including cellular homeostasis, stem cell maintenance, cancer, metabolic, and cardiovascular diseases. Genetic evidence has been accumulating to suggest a prominent role of FOXOs in lifespan regulation in animal systems from hydra, C elegans, Drosophila, and mice. Together with the observation that FOXO3 is the second most replicated gene associated with extreme human longevity suggests that pharmacological targeting of FOXO proteins can be a promising approach to treat cancer and other age-related diseases and extend life and health span. However, due to the broad range of cellular functions of the FOXO family members FOXO1, 3, 4, and 6, isoform-specific targeting of FOXOs might lead to greater benefits and cause fewer side effects. Therefore, a deeper understanding of the common and specific features of these proteins as well as their redundant and specific functions in our cells represents the basis of specific targeting strategies. In this review, we provide an overview of the evolution, structure, function, and disease-relevance of each of the FOXO family members.

FoxO1 as a tissue-specific therapeutic target for type 2 diabetes

Forkhead box O (FoxO) proteins are transcription factors that mediate many aspects of physiology and thus have been targeted as therapeutics for several diseases including metabolic disorders such as type 2 diabetes mellitus (T2D). The role of FoxO1 in metabolism has been well studied, but recently FoxO1's potential for diabetes prevention and therapy has been debated. For example, studies have shown that increased FoxO1 activity in certain tissue types contributes to T2D pathology, symptoms, and comorbidities, yet in other tissue types elevated FoxO1 has been reported to alleviate symptoms associated with diabetes. Furthermore, studies have reported opposite effects of active FoxO1 in the same tissue type. For example, in the liver, FoxO1 contributes to T2D by increasing hepatic glucose production. However, FoxO1 has been shown to either increase or decrease hepatic lipogenesis as well as adipogenesis in white adipose tissue. In skeletal muscle, FoxO1 reduces glucose uptake and oxidation, promotes lipid uptake and oxidation, and increases muscle atrophy. While many studies show that FoxO1 lowers pancreatic insulin production and secretion, others show the opposite, especially in response to oxidative stress and inflammation. Elevated FoxO1 in the hypothalamus increases the risk of developing T2D. However, increased FoxO1 may mitigate Alzheimer's disease, a neurodegenerative disease strongly associated with T2D. Conversely, accumulating evidence implicates increased FoxO1 with Parkinson's disease pathogenesis. Here we review FoxO1's actions in T2D conditions in metabolic tissues that abundantly express FoxO1 and highlight some of the current studies targeting FoxO1 for T2D treatment.

Cognitive Impairment in Multiple Sclerosis

Almost three million individuals suffer from multiple sclerosis (MS) throughout the world, a demyelinating disease in the nervous system with increased prevalence over the last five decades, and is now being recognized as one significant etiology of cognitive loss and dementia. Presently, disease modifying therapies can limit the rate of relapse and potentially reduce brain volume loss in patients with MS, but unfortunately cannot prevent disease progression or the onset of cognitive disability. Innovative strategies are therefore required to address areas of inflammation, immune cell activation, and cell survival that involve novel pathways of programmed cell death, mammalian forkhead transcription factors (FoxOs), the mechanistic target of rapamycin (mTOR), AMP activated protein kinase (AMPK), the silent mating type information regulation 2 homolog 1 (Saccharomyces cerevisiae) (SIRT1), and associated pathways with the apolipoprotein E (APOE-ε4) gene and severe acute respiratory syndrome coronavirus (SARS-CoV-2). These pathways are intertwined at multiple levels and can involve metabolic oversight with cellular metabolism dependent upon nicotinamide adenine dinucleotide (NAD+). Insight into the mechanisms of these pathways can provide new avenues of discovery for the therapeutic treatment of dementia and loss in cognition that occurs during MS.

The discrete roles of individual FOXO transcription factor family members in B-cell malignancies

Forkhead box (FOX) class O (FOXO) proteins are a dynamic family of transcription factors composed of four family members: FOXO1, FOXO3, FOXO4 and FOXO6. As context-dependent transcriptional activators and repressors, the FOXO family regulates diverse cellular processes including cell cycle arrest, apoptosis, metabolism, longevity and cell fate determination. A central pathway responsible for negative regulation of FOXO activity is the phosphatidylinositol-3-kinase (PI3K)-AKT signalling pathway, enabling cell survival and proliferation. FOXO family members can be further regulated by distinct kinases, both positively (e.g., JNK, AMPK) and negatively (e.g., ERK-MAPK, CDK2), with additional post-translational modifications further impacting on FOXO activity. Evidence has suggested that FOXOs behave as 'bona fide' tumour suppressors, through transcriptional programmes regulating several cellular behaviours including cell cycle arrest and apoptosis. However, an alternative paradigm has emerged which indicates that FOXOs operate as mediators of cellular homeostasis and/or resistance in both 'normal' and pathophysiological scenarios. Distinct FOXO family members fulfil discrete roles during normal B cell maturation and function, and it is now clear that FOXOs are aberrantly expressed and mutated in discrete B-cell malignancies. While active FOXO function is generally associated with disease suppression in chronic lymphocytic leukemia for example, FOXO expression is associated with disease progression in diffuse large B cell lymphoma, an observation also seen in other cancers. The opposing functions of the FOXO family drives the debate about the circumstances in which FOXOs favour or hinder disease progression, and whether targeting FOXO-mediated processes would be effective in the treatment of B-cell malignancies. Here, we discuss the disparate roles of FOXO family members in B lineage cells, the regulatory events that influence FOXO function focusing mainly on post-translational modifications, and consider the potential for future development of therapies that target FOXO activity.

Metformin alleviates the cognitive impairment induced by benzo[a]pyrene via glucolipid metabolism regulated by FTO/FoxO6 pathway in mice

Benzo[a]pyrene (B[a]P) is neurotoxic; however, the mechanism and prevention are still unclear. In this study, we assessed the intervention effect of metformin (MET) on cognitive dysfunction in mice induced by B[a]P from the perspective of glucolipid metabolism. Forty-two male healthy ICR mice were randomly categorized into 6 groups and were gavaged with B[a]P (0, 2.5, 5, or 10 mg/kg), 45 times for 90 days. The controls were gavaged with edible peanut oil, and the intervention groups were co-treated with B[a]P (10 mg/kg) and MET (200 or 300 mg/kg). We assessed the cognitive function of mice, observed the pathomorphological and ultrastructural changes, and detected neuronal apoptosis and glucolipid metabolism. Results showed that B[a]P dose-dependently induced cognitive impairment, neuronal damage, glucolipid metabolism disorder in mice, and enhanced proteins of fat mass and obesity-associated protein (FTO) and forkhead box protein O6 (FoxO6) in the cerebral cortex and liver, which were alleviated by the MET intervention. The findings indicated the critical role of glucolipid metabolism disorder in the cognitive impairment in mice caused by B[a]P and the prevention of MET against B[a]P neurotoxicity by regulating glucolipid metabolism via restraining FTO/FoxO6 pathway. The finding provides a scientific basis for the neurotoxicity and prevention strategies of B[a]P.

A FOXO1-dependent transcription network is a targetable vulnerability of mantle cell lymphomas

Targeting lineage-defined transcriptional dependencies has emerged as an effective therapeutic strategy in cancer treatment. Through screening for molecular vulnerabilities of mantle cell lymphoma (MCL), we identified a set of transcription factors (TFs) including FOXO1, EBF1, PAX5, and IRF4 that are essential for MCL propagation. Integrated chromatin immunoprecipitation and sequencing (ChIP-Seq) with transcriptional network reconstruction analysis revealed FOXO1 as a master regulator that acts upstream in the regulatory TF hierarchy. FOXO1 is both necessary and sufficient to drive MCL lineage commitment through supporting the lineage-specific transcription programs. We further show that FOXO1, but not its close paralog FOXO3, can reprogram myeloid leukemia cells and induce B-lineage gene expression. Finally, we demonstrate that cpd10, a small molecule identified from an enriched FOXO1 inhibitor library, induces a robust cytotoxic response in MCL cells in vitro and suppresses MCL progression in vivo. Our findings establish FOXO1 inhibition as a therapeutic strategy targeting lineage-driven transcriptional addiction in MCL.

mTORC2 Is the Major Second Layer Kinase Negatively Regulating FOXO3 Activity

Forkhead box O (FOXO) proteins are transcription factors involved in cancer and aging and their pharmacological manipulation could be beneficial for the treatment of cancer and healthy aging. FOXO proteins are mainly regulated by post-translational modifications including phosphorylation, acetylation and ubiquitination. As these modifications are reversible, activation and inactivation of FOXO factors is attainable through pharmacological treatment. One major regulatory input of FOXO signaling is mediated by protein kinases. Here, we use specific inhibitors against different kinases including PI3K, mTOR, MEK and ALK, and other receptor tyrosine kinases (RTKs) to determine their effect on FOXO3 activity. While we show that inhibition of PI3K efficiently drives FOXO3 into the cell nucleus, the dual PI3K/mTOR inhibitors dactolisib and PI-103 induce nuclear FOXO translocation more potently than the PI3Kδ inhibitor idelalisib. Furthermore, specific inhibition of mTOR kinase activity affecting both mTORC1 and mTORC2 potently induced nuclear translocation of FOXO3, while rapamycin, which specifically inhibits the mTORC1, failed to affect FOXO3. Interestingly, inhibition of the MAPK pathway had no effect on the localization of FOXO3 and upstream RTK inhibition only weakly induced nuclear FOXO3. We also measured the effect of the test compounds on the phosphorylation status of AKT, FOXO3 and ERK, on FOXO-dependent transcriptional activity and on the subcellular localization of other FOXO isoforms. We conclude that mTORC2 is the most important second layer kinase negatively regulating FOXO activity.

Mechanistic insights into the role of FOXO in diabetic retinopathy

Diabetes mellitus (DM), a metabolic disorder characterized by insulin-deficiency or insulin-resistant conditions. The foremost microvascular complication of diabetes is diabetic retinopathy (DR). This is a multifaceted ailment mainly caused by the enduring adverse effects of hyperglycaemia. Inflammation, oxidative stress, and advanced glycation products (AGES) are part and parcel of DR pathogenesis. In regulating many cellular and biological processes, the family of fork-head transcription factors plays a key role. The current review highlights that FOXO is a requisite regulator of pathways intricate in diabetic retinopathy on account of its effect on microvascular cells inflammatory and apoptotic gene expression, and FOXO also has the foremost province in regulating cell cycle, proliferation, apoptosis, and metabolism. Blockage of insulin turns into an exaggerated level of glucose in the bloodstream and can upshot into the exaggerated triggering of FOXO1, which can ultimately uplift the production of several factors of apoptosis and inflammation, such as TNF-α, NF-kB, and various others, as well as reactive oxygen species, which can also come up with diabetic retinopathy. The current review also focuses on various therapies which can be used in the future, like SIRT1 signalling, resveratrol, retinal VEGF, etc., which can be used to suppress FOXO over activation and can prevent the progression of diabetic complications viz. diabetic retinopathy.

FOXO transcription factors in antioxidant defense

Forkhead box, class O (FOXO) family proteins are widely expressed and highly conserved transcriptional regulators that modulate cellular fuel metabolism, stress resistance and cell death. FOXO target genes include genes encoding antioxidant proteins, thus likely contributing to the key role FOXOs play in the cellular response to oxidative stress and supporting the cellular strategies of antioxidant defense, that is, prevention (of the formation of reactive oxygen species), interception (of reactive species prior to their reaction with cellular components), repair (of damaged biomolecules), and adaptation (i.e., the stimulation of signaling pathways allowing for the expression of protective proteins). FOXOs themselves are regulated by redox processes at several levels, including expression of FOXO genes and enzymatic as well as nonenzymatic posttranslational modifications of FOXO proteins. The latter include modifications of FOXO cysteine residues. Here, an overview is provided on (i) the contribution of FOXO target genes to cellular antioxidative strategies, and (ii) on the impact of thiol homeostasis and thiol modification on FOXO activity.

Forkhead box family transcription factors as versatile regulators for cellular reprogramming to pluripotency

Forkhead box (Fox) transcription factors play important roles in mammalian development and disease. However, their function in mouse somatic cell reprogramming remains unclear. Here, we report that FoxD subfamily and FoxG1 accelerate induced pluripotent stem cells (iPSCs) generation from mouse fibroblasts as early as day4 while FoxA and FoxO subfamily impede this process obviously. More importantly, FoxD3, FoxD4 and FoxG1 can replace Oct4 respectively and generate iPSCs with germline transmission together with Sox2 and Klf4. On the contrary, FoxO6 almost totally blocks reprogramming through inhibiting cell proliferation, suppressing the expression of pluripotent genes and hindering the process of mesenchymal to epithelial transition (MET). Thus, our study uncovers unexpected roles of Fox transcription factors in reprogramming and offers new insights into cell fate transition.

FOXO transcription factor family in cancer and metastasis

Forkhead box O (FOXO) transcription factors regulate diverse biological processes, affecting development, metabolism, stem cell maintenance and longevity. They have also been increasingly recognised as tumour suppressors through their ability to regulate genes essential for cell proliferation, cell death, senescence, angiogenesis, cell migration and metastasis. Mechanistically, FOXO proteins serve as key connection points to allow diverse proliferative, nutrient and stress signals to converge and integrate with distinct gene networks to control cell fate, metabolism and cancer development. In consequence, deregulation of FOXO expression and function can promote genetic disorders, metabolic diseases, deregulated ageing and cancer. Metastasis is the process by which cancer cells spread from the primary tumour often via the bloodstream or the lymphatic system and is the major cause of cancer death. The regulation and deregulation of FOXO transcription factors occur predominantly at the post-transcriptional and post-translational levels mediated by regulatory non-coding RNAs, their interactions with other protein partners and co-factors and a combination of post-translational modifications (PTMs), including phosphorylation, acetylation, methylation and ubiquitination. This review discusses the role and regulation of FOXO proteins in tumour initiation and progression, with a particular emphasis on cancer metastasis. An understanding of how signalling networks integrate with the FOXO transcription factors to modulate their developmental, metabolic and tumour-suppressive functions in normal tissues and in cancer will offer a new perspective on tumorigenesis and metastasis, and open up therapeutic opportunities for malignant diseases.

Role of Forkhead Transcription Factors of the O Class (FoxO) in Development and Progression of Alzheimer's Disease

In the Central Nervous System (CNS), a specific loss of focal neurons leads to mental and neurological disorders like dementia, Alzheimer's Disease (AD), Huntington's disease, Parkinson's disease, etc. AD is a neurological degenerative disorder, which is progressive and irreversible in nature and is the widely recognized reason for dementia in the geriatric populace. It affects 10% of people above the age of 65 and is the fourth driving reason for death in the United States. Numerous evidence suggests that the neuronal compartment is not the only genesis of AD, but transcription factors also hold significant importance in the occurrence and advancement of the disease. It is the need of the time to find the novel molecular targets and new techniques for treating or slowing down the progression of neurological disorders, especially AD. In this article, we summarised a conceivable association between transcriptional factors and their defensive measures against neurodegeneration and AD. The mammalian forkhead transcription factors of the class O (FoxO) illustrate one of the potential objectives for the development of new methodologies against AD and other neurocognitive disorders. The presence of FoxO is easily noticeable in the "cognitive centers" of the brain, specifically in the amygdala, hippocampus, and the nucleus accumbens. FoxO proteins are the prominent and necessary factors in memory formation and cognitive functions. FoxO also assumes a pertinent role in the protection of multiple cells in the brain by controlling the involving mechanism of autophagy and apoptosis and also modulates the process of phosphorylation of the targeted protein, thus FoxO must be a putative target in the mitigation of AD. This review features the role of FoxO as an important biomarker and potential new targets for the treatment of AD.

The PI3K pathway impacts stem gene expression in a set of glioblastoma cell lines

BACKGROUND

The PI3K pathway controls diverse cellular processes including growth, survival, metabolism, and apoptosis. Nuclear FOXO factors were observed in cancers that harbor constitutively active PI3K pathway output and stem signatures. FOXO1 and FOXO3 were previously published to induce stem genes such as OCT4 in embryonic stem cells. Here, we investigated FOXO-driven stem gene expression in U87MG glioblastoma cells.

METHODS

PI3K-activated cancer cell lines were investigated for changes in gene expression, signal transduction, and clonogenicity under conditions with FOXO3 disruption or exogenous expression. The impact of PI3K pathway inhibition on stem gene expression was examined in a set of glioblastoma cell lines.

RESULTS

We found that CRISPR-Cas9-mediated FOXO3 disruption in U87MG cells caused decreased OCT4 and SOX2 gene expression, STAT3 phosphorylation on tyrosine 705 and clonogenicity. FOXO3 over expression led to increased OCT4 in numerous glioblastoma cancer cell lines. Strikingly, treatment of glioblastoma cells with NVP-BEZ235 (a dual inhibitor of PI3K and mTOR), which activates FOXO factors, led to robust increases OCT4 gene expression. Direct FOXO factor recruitment to the OCT4 promoter was detected by chromatin immunoprecipitation analyses using U87MG extracts.

DISCUSSION

We show for the first time that FOXO transcription factors promote stem gene expression glioblastoma cells. Treatment with PI3K inhibitor NVP-BEZ235 led to dramatic increases in stem genes in a set of glioblastoma cell lines.

CONCLUSION

Given that, PI3K inhibitors are actively investigated as targeted cancer therapies, the FOXO-mediated induction of stem genes observed in this study highlights a potential hazard to PI3K inhibition. Understanding the molecular underpinnings of stem signatures in cancer will allow refinements to therapeutic strategies. Targeting FOXO factors to reduce stem cell characteristics in concert with PI3K inhibition may prove therapeutically efficacious.

FoxO6 regulates Hippo signaling and growth of the craniofacial complex

The mechanisms that regulate post-natal growth of the craniofacial complex and that ultimately determine the size and shape of our faces are not well understood. Hippo signaling is a general mechanism to control tissue growth and organ size, and although it is known that Hippo signaling functions in neural crest specification and patterning during embryogenesis and before birth, its specific role in postnatal craniofacial growth remains elusive. We have identified the transcription factor FoxO6 as an activator of Hippo signaling regulating neonatal growth of the face. During late stages of mouse development, FoxO6 is expressed specifically in craniofacial tissues and FoxO6-/- mice undergo expansion of the face, frontal cortex, olfactory component and skull. Enlargement of the mandible and maxilla and lengthening of the incisors in FoxO6-/- mice are associated with increases in cell proliferation. In vitro and in vivo studies demonstrated that FoxO6 activates Lats1 expression, thereby increasing Yap phosphorylation and activation of Hippo signaling. FoxO6-/- mice have significantly reduced Hippo Signaling caused by a decrease in Lats1 expression and decreases in Shh and Runx2 expression, suggesting that Shh and Runx2 are also linked to Hippo signaling. In vitro, FoxO6 activates Hippo reporter constructs and regulates cell proliferation. Furthermore PITX2, a regulator of Hippo signaling is associated with Axenfeld-Rieger Syndrome causing a flattened midface and we show that PITX2 activates FoxO6 expression. Craniofacial specific expression of FoxO6 postnatally regulates Hippo signaling and cell proliferation. Together, these results identify a FoxO6-Hippo regulatory pathway that controls skull growth, odontogenesis and face morphology.

FoxO transcription factors in cancer metabolism

FoxO transcription factors serve as the central regulator of cellular homeostasis and are tumor suppressors in human cancers. Recent studies have revealed that, besides their classic functions in promoting cell death and inducing cell cycle arrest, FoxOs also regulate cancer metabolism, an emerging hallmark of cancer. In this review, we summarize the regulatory mechanisms employed to control FoxO activities in the context of cancer biology, and discuss FoxO function in metabolism reprogramming in cancer and interaction with other key cancer metabolism pathways. A deeper understanding of FoxOs in cancer metabolism may reveal novel therapeutic opportunities in cancer treatment.

Involvement of the FOXO6 transcriptional factor in breast carcinogenesis

In mammals, FOXO transcriptional factors form a family of four members (FOXO1, 3, 4, and 6) involved in the modulation proliferation, apoptosis, and carcinogenesis. The role of the FOXO family in breast cancer remains poorly elucidated. According to the cellular context and the stage of the disease, FOXOs can have opposite effects on carcinogenesis. To study the role of FOXOs in breast carcinogenesis in more detail, we examined their expression in normal tissues, breast cell lines, and a large series of breast tumours of human origin. We found a very low physiological level of FOXO6 expression in normal adult tissues and high levels of expression in foetal brain. FOXO gene expressions fluctuate specifically in breast cancer cells compared to normal cells, suggesting that these genes may have different roles in breast carcinogenesis. For the first time, we have shown that, among the various FOXO genes, only FOXO6 was frequently highly overexpressed in breast cell lines and tumours. We also found that inhibition of the endogenous expression of FOXO6 by a specific siRNA inhibited the growth of the human breast cell lines MDA-MB-468 and HCC-38. FACS and Western blot analysis showed that inhibition of endogenous expression of FOXO6 induced accumulation of cells in G0/G1 phase of the cell cycle, but not apoptosis. These results tend to demonstrate that the overexpression of the human FOXO6 gene that we highlighted in the breast tumors stimulates breast carcinogenesis by activating breast cancer cell proliferation.

Regulation of FOXO Factors in Mammalian Cells

Forkhead box O (FOXO) transcription factors are central regulators of cellular homeostasis. FOXOs respond to a wide range of external stimuli, including growth factor signaling, oxidative stress, genotoxic stress, and nutrient deprivation. These signaling inputs regulate FOXOs through a number of posttranslational modifications, including phosphorylation, acetylation, ubiquitination, and methylation. Covalent modifications can affect localization, DNA binding, and interactions with other cofactors in the cell. FOXOs integrate the various modifications to regulate cell type-specific gene expression programs that are essential for metabolic homeostasis, redox balance, and the stress response. Together, these functions are critical for coordinating a response to environmental fluctuations in order to maintain cellular homeostasis during development and to support healthy aging.

Posttranscriptional regulation of FOXO expression: microRNAs and beyond

Forkhead box, class O (FOXO) transcription factors are major regulators of diverse cellular processes, including fuel metabolism, oxidative stress response and redox signalling, cell cycle progression and apoptosis. Their activities are controlled by multiple posttranslational modifications and nuclear-cytoplasmic shuttling. Recently, post-transcriptional regulation of FOXO synthesis has emerged as a new regulatory level of their functions. Accumulating evidence suggests that this post-transcriptional mode of regulation of FOXO activity operates in response to stressful stimuli, including oxidative stress. Here, we give a brief overview on post-transcriptional regulation of FOXO synthesis by microRNAs (miRNAs) and by RNA-binding regulatory proteins, human antigen R (HuR) and quaking (QKI). Aberrant post-transcriptional regulation of FOXOs is frequently connected with various disease states. We therefore discuss characteristic examples of FOXO regulation at the post-transcriptional level under various physiological and pathophysiological conditions, including oxidative stress and cancer. The picture emerging from this summary points to a diversity of interactions between miRNAs/miRNA-induced silencing complexes and RNA-binding regulatory proteins. Better insight into these complexities of post-transcriptional regulatory interactions will add to our understanding of the mechanisms of pathological processes and the role of FOXO proteins.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

Neuronal functions of FOXO/DAF-16

The FOXO family of transcription factors plays a conserved role in longevity and tissue homeostasis across species. In the mammalian nervous system, emerging evidence has implicated FOXOs in cognitive performance, stem cell maintenance, regeneration, and protection against stress. Much of what we know about neuronal functions of FOXO emerged from recent studies in C. elegans. Similar to mammalian FOXO, the worm FOXO ortholog, called DAF-16, regulates learning and memory, regeneration, and stress resistance in neurons. Here, we discuss the current state of our knowledge of FOXO’s functions in neurons in mammals and invertebrates, and highlight areas where our understanding is limited. Defining the function of FOXO factors in the healthy, aged, and diseased brain may have important implications for improving healthspan and treating neurodegenerative disease.

Forkhead Transcription Factors: Formulating a FOXO Target for Cognitive Loss

BACKGROUND

With almost 47 million individuals worldwide suffering from some aspect of dementia, it is clear that cognitive loss impacts a significant proportion of the global population. Unfortunately, definitive treatments to resolve or prevent the onset of cognitive loss are limited. In most cases such care is currently non-existent prompting the need for novel treatment strategies.

METHODS

Mammalian forkhead transcription factors of the O class (FoxO) are one such avenue of investigation that offer an exciting potential to bring new treatments forward for disorders that involve cognitive loss. Here we examine the background, structure, expression, and function of FoxO transcription factors and their role in cognitive loss, programmed cell death in the nervous system with apoptosis and autophagy, and areas to target FoxOs for dementia and specific disorders such as Alzheimer's disease.

RESULTS

FoxO proteins work in concert with a number of other cell survival pathways that involve growth factors, such as erythropoietin and neurotrophins, silent mating type information regulation 2 homolog 1 (Saccharomyces cerevisiae) (SIRT1), Wnt1 inducible signaling pathway protein 1 (WISP1), Wnt signaling, and cancer-related pathways. FoxO transcription factors oversee proinflammatory pathways, affect nervous system amyloid (Aβ) production and toxicity, lead to mitochondrial dysfunction, foster neuronal apoptotic cell death, and accelerate the progression of degenerative disease. However, under some scenarios such as those involving autophagy, FoxOs also can offer protection in the nervous system and reduce toxic intracellular protein accumulations and potentially limit Aβ toxicity.

CONCLUSION

Given the ability of FoxOs to not only promote apoptotic cell death in the nervous system, but also through the induction of autophagy offer protection against degenerative disease that can lead to dementia, a fine balance in the activity of FoxOs may be required to target cognitive loss in individuals. Future work should yield exciting new prospects for FoxO proteins as new targets to treat the onset and progression of cognitive loss and dementia.

FoxO6 affects Plxna4-mediated neuronal migration during mouse cortical development

The forkhead transcription factor FoxO6 is prominently expressed during development of the murine neocortex. However, its function in cortical development is as yet unknown. We now demonstrate that cortical development is altered in FoxO6^+/- and FoxO6^-/- mice, showing migrating neurons halted in the intermediate zone. Using a FoxO6-directed siRNA approach, we substantiate the requirement of FoxO6 for a correct radial migration in the developing neocortex. Subsequent genome-wide transcriptome analysis reveals altered expression of genes involved in cell adhesion, axon guidance, and gliogenesis upon silencing of FoxO6 We then show that FoxO6 binds to DAF-16-binding elements in the Plexin A4 (Plxna4) promoter region and affects Plxna4 expression. Finally, ectopic Plxna4 expression restores radial migration in FoxO6^+/- and siRNA-mediated knockdown models. In conclusion, the presented data provide insights into the molecular mechanisms whereby transcriptional programs drive cortical development.

Forkhead transcription factors: new considerations for alzheimer's disease and dementia

Life expectancy of individuals in both developed and undeveloped nations continues to rise at an unprecedented rate. Coupled to this increase in longevity for individuals is the rise in the incidence of chronic neurodegenerative disorders that includes Alzheimer's disease (AD). Currently, almost ten percent of the population over the age of 65 suffers from AD, a disorder that is presently without definitive therapy to prevent the onset or progression of cognitive loss. Yet, it is estimated that AD will continue to significantly increase throughout the world to impact millions of individuals and foster the escalation of healthcare costs. One potential target for the development of novel strategies against AD and other cognitive disorders involves the mammalian forkhead transcription factors of the O class (FoxOs). FoxOs are present in "cognitive centers" of the brain to include the hippocampus, the amygdala, and the nucleus accumbens and may be required for memory formation and consolidation. FoxOs play a critical role in determining survival of multiple cell types in the nervous system, drive pathways of apoptosis and autophagy, and control stem cell proliferation and differentiation. FoxOs also interface with multiple cellular pathways that include growth factors, Wnt signaling, Wnt1 inducible signaling pathway protein 1 (WISP1), and silent mating type information regulation 2 homolog 1 (Saccharomyces cerevisiae) (SIRT1) that ultimately may control FoxOs and determine the fate and function of cells in the nervous system that control memory and cognition. Future work that can further elucidate the complex relationship FoxOs hold over cell fate and cognitive function could yield exciting prospects for the treatment of a number of neurodegenerative disorders including AD.

Redox regulation of FoxO transcription factors

Transcription factors of the forkhead box, class O (FoxO) family are important regulators of the cellular stress response and promote the cellular antioxidant defense. On one hand, FoxOs stimulate the transcription of genes coding for antioxidant proteins located in different subcellular compartments, such as in mitochondria (i.e. superoxide dismutase-2, peroxiredoxins 3 and 5) and peroxisomes (catalase), as well as for antioxidant proteins found extracellularly in plasma (e.g., selenoprotein P and ceruloplasmin). On the other hand, reactive oxygen species (ROS) as well as other stressful stimuli that elicit the formation of ROS, may modulate FoxO activity at multiple levels, including posttranslational modifications of FoxOs (such as phosphorylation and acetylation), interaction with coregulators, alterations in FoxO subcellular localization, protein synthesis and stability. Moreover, transcriptional and posttranscriptional control of the expression of genes coding for FoxOs is sensitive to ROS. Here, we review these aspects of FoxO biology focusing on redox regulation of FoxO signaling, and with emphasis on the interplay between ROS and FoxOs under various physiological and pathophysiological conditions. Of particular interest are the dual role played by FoxOs in cancer development and their key role in whole body nutrient homeostasis, modulating metabolic adaptations and/or disturbances in response to low vs. high nutrient intake. Examples discussed here include calorie restriction and starvation as well as adipogenesis, obesity and type 2 diabetes.

FoxO proteins in the nervous system

Acute as well as chronic disorders of the nervous system lead to significant morbidity and mortality for millions of individuals globally. Given the ability to govern stem cell proliferation and differentiated cell survival, mammalian forkhead transcription factors of the forkhead box class O (FoxO) are increasingly being identified as potential targets for disorders of the nervous system, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and auditory neuronal disease. FoxO proteins are present throughout the body, but they are selectively expressed in the nervous system and have diverse biological functions. The forkhead O class transcription factors interface with an array of signal transduction pathways that include protein kinase B (Akt), serum- and glucocorticoid-inducible protein kinase (SgK), IκB kinase (IKK), silent mating type information regulation 2 homolog 1 (S. cerevisiae) (SIRT1), growth factors, and Wnt signaling that can determine the activity and integrity of FoxO proteins. Ultimately, there exists a complex interplay between FoxO proteins and their signal transduction pathways that can significantly impact programmed cell death pathways of apoptosis and autophagy as well as the development of clinical strategies for the treatment of neurodegenerative disorders.

Inhibition of mutant EGFR in lung cancer cells triggers SOX2-FOXO6-dependent survival pathways

Treatment of EGFR-mutant lung cancer with erlotinib results in dramatic tumor regression but it is invariably followed by drug resistance. In characterizing early transcriptional changes following drug treatment of mutant EGFR-addicted cells, we identified the stem cell transcriptional regulator SOX2 as being rapidly and specifically induced, both in vitro and in vivo. Suppression of SOX2 sensitizes cells to erlotinib-mediated apoptosis, ultimately decreasing the emergence of acquired resistance, whereas its ectopic expression reduces drug-induced cell death. We show that erlotinib relieves EGFR-dependent suppression of FOXO6, leading to its induction of SOX2, which in turn represses the pro-apoptotic BH3-only genes BIM and BMF. Together, these observations point to a physiological feedback mechanism that attenuates oncogene addiction-mediated cell death associated with the withdrawal of growth factor signaling and may therefore contribute to the development of resistance.

DOI:http://dx.doi.org/10.7554/eLife.06132.001

Tumors can form when cells gain mutations in genes that enable them to grow and divide rapidly. In some human lung cancers, genetic mutations are found in a gene that makes a protein called EGFR. This protein encourages cells to divide and the mutations can lead to the cancer cells producing more EGFR, or producing a form of the protein that is more active.

Treating these cancers with a drug called erlotinib inhibits EGFR and makes the tumors shrink dramatically, but the tumors will usually re-grow because any tumor cells that survive often become resistant to the drug. There are several ways that the tumor cells can become resistant, which makes the task of developing a solution to this problem more difficult. It has been suggested that the tumor cells may enter a temporary ‘drug-tolerant’ state that helps them to survive and makes it more likely that they will develop resistance to the drug. However, it is not clear how this drug-tolerant state might work.

To address this question, Rothenberg et al. examined which genes are switched on (or ‘expressed’) in tumor cells with a mutant version of EGFR after they were treated with the erlotinib drug. The experiments show that a gene called SOX2 is expressed in these cells. Cells that had lower levels of SOX2 expression were more sensitive to the effects of the drug and fewer cells developed resistance. On the other hand, cells that had higher levels of SOX2 expression were less sensitive to the drug and resistance was more likely to develop.

A protein called FOXO6—which is usually suppressed by EGFR—activates the SOX2 gene in these cells. Therefore, using erlotinib to inhibit EGFR to kill the cancer cells increases the activity of FOXO6, which in turn promotes the survival of some of the cells by activating the SOX2 gene. A better understanding of the ways in which cancer cells adapt to erlotinib and other drugs may help us to design more effective treatments with better outcomes for patients.

DOI:http://dx.doi.org/10.7554/eLife.06132.002

Concise review: forkhead pathway in the control of adult neurogenesis

New cells are continuously generated from immature proliferating cells in the adult brain in two neurogenic niches known as the subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus and the sub-ventricular zone (SVZ) of the lateral ventricles. However, the molecular mechanisms regulating their proliferation, differentiation, migration and functional integration of newborn neurons in pre-existing neural network remain largely unknown. Forkhead box (Fox) proteins belong to a large family of transcription factors implicated in a wide variety of biological processes. Recently, there has been accumulating evidence that several members of this family of proteins play important roles in adult neurogenesis. Here, we describe recent advances in our understanding of regulation provided by Fox factors in adult neurogenesis, and evaluate the potential role of Fox proteins as targets for therapeutic intervention in neurodegenerative diseases.

FoxO6 integrates insulin signaling with MTP for regulating VLDL production in the liver

Excessive production of triglyceride-rich very low-density lipoproteins (VLDL-TG) contributes to hypertriglyceridemia in obesity and type 2 diabetes. To understand the underlying mechanism, we studied hepatic regulation of VLDL-TG production by (forkhead box O6) FoxO6, a forkhead transcription factor that integrates insulin signaling to hepatic metabolism. We showed that transgenic mice expressing a constitutively active FoxO6 allele developed hypertriglyceridemia, culminating in elevated VLDL-TG levels and impaired postprandial TG clearance. This effect resulted in part from increased hepatic VLDL-TG production. We recapitulated these findings in cultured HepG2 cells and human primary hepatocytes, demonstrating that FoxO6 promoted hepatic VLDL-TG secretion. This action correlated with the ability of FoxO6 to stimulate hepatic production of microsomal triglyceride transfer protein (MTP), a molecular chaperone that catalyzes the rate-limiting step in VLDL-TG assembly and secretion. FoxO6 was shown to bind to the MTP promoter and stimulate MTP promoter activity in HepG2 cells. This effect was inhibited by insulin, consistent with the ability of insulin to promote FoxO6 phosphorylation and disable FoxO6 DNA-binding activity. Mutations of the FoxO6 target site within the MTP promoter abrogated FoxO6-mediated induction of MTP promoter activity. Hepatic FoxO6 expression became deregulated in insulin-resistant mice with obesity and type 2 diabetes. FoxO6 inhibition in insulin-resistant liver suppressed hepatic MTP expression and curbed VLDL-TG overproduction, contributing to the amelioration of hypertriglyceridemia in obese and diabetic db/db mice. These results characterize FoxO6 as an important signaling molecule upstream of MTP for regulating hepatic VLDL-TG production.

FOXO3 promotes quiescence in adult muscle stem cells during the process of self-renewal

Skeletal muscle stem cells, or “satellite cells” (SCs), are required for the regeneration of damaged muscle tissue. Although SCs self-renew during regeneration, the mechanisms that govern SC re-entry into quiescence remain elusive. We show that FOXO3, a member of the forkhead family of transcription factors, is expressed in quiescent SCs (QSCs). Conditional deletion of Foxo3 in QSCs impairs self-renewal and increases the propensity of SCs to adopt a differentiated fate. Transcriptional analysis of SCs lacking FOXO3 revealed a downregulation of Notch signaling, a key regulator of SC quiescence. Conversely, overexpression of Notch intracellular domain (NICD) rescued the self-renewal deficit of FOXO3-deficient SCs. We show that FOXO3 regulates NOTCH1 and NOTCH3 receptor expression and that decreasing expression of NOTCH1 and NOTCH3 receptors phenocopies the effect of FOXO3 deficiency in SCs. We demonstrate that FOXO3, perhaps by activating Notch signaling, promotes the quiescent state during SC self-renewal in adult muscle regeneration.

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FOXO3 is expressed in quiescent adult SCs

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FOXO3 is required for self-renewal of SCs

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FOXO3-deficient SCs display an increased propensity to differentiate

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FOXO3 promotes Notch signaling, a key regulator of quiescence in adult SCs

Current overview of functions of FoxO proteins, with special regards to cellular homeostasis, cell response to stress, as well as inflammation and aging

FoxO transcription factors act at the interconnections between metabolic pathways inducible by many important signal transducers and mediators, such as p53, Ikk-β, NFKB, Akt, sirtuins, PTEN, and others. This may account for a crucial significance of disruptions in FoxO functions both in many kinds of diseases (including cancer, chronic inflammatory diseases, degenerative diseases, obesity, polymetabolic syndrome) and in some disease-like conditions (such as inflammaging, cachexia related to chronic inflammation, cancer-promotion by some chronic inflammatory responses, and the aging process itself). This paper reviews complex interactions between FoxOs and other signal transducers, trying to pinpoint how exactly disruptions of FoxO functions may occur, and how they may contribute to occurrence, development or complications of the conditions mentioned above.

FoxO6 in glucose metabolism (FoxO6)

The forkhead box O (FoxO) subfamily has four members, namely FoxO1, FoxO3, FoxO4, and FoxO6. Unlike the other three members of the FoxO family, FoxO6 has garnered considerably less attention because of earlier reports that FoxO6 expression was limited to the brain. Recent data indicate that FoxO6 is produced in the liver of both rodents and humans. Hepatic FoxO6 activity, which remains at low basal levels in fed states, is markedly induced in fasted mice. FoxO6 activity becomes abnormally higher in the liver of mice with dietary obesity or type 2 diabetes (T2D). Genetically engineered mice with elevated FoxO6 activity in the liver exhibit prediabetes, culminating in the development of glucose intolerance, fasting hyperglycemia, and hyperinsulinemia. Conversely, inhibition of FoxO6 activity in the insulin-resistant liver results in a reduction in fasting hyperglycemia, contributing to the amelioration of hyperinsulinemia in T2D mice. These new data suggest that FoxO6 is an important regulator of hepatic glucose metabolism in response to insulin or physiological cues. Insulin inhibits FoxO6 activity by promoting its phosphorylation and disabling its activity in the nucleus without altering its subcellular distribution via a mechanism that is distinct from other members of the FoxO subfamily. In this article, we comprehensively review the role of FoxO6 in glucose metabolism in health and disease. We also address whether FoxO6 dysregulation is a contributing factor for the pathogenesis of fasting hyperglycemia and discuss whether FoxO6 is a potential therapeutic target for improving fasting hyperglycemia in T2D.

FoxO6 and PGC-1α form a regulatory loop in myogenic cells

Transcription factors of the FoxO (forkhead box O) family regulate a wide range of cellular physiological processes, including metabolic adaptation and myogenic differentiation. The transcriptional activity of most FoxO members is inhibitory to myogenic differentiation and overexpression of FoxO1 inhibits the development of oxidative type I fibres in vivo. In this study, we found that FoxO6, the last discovered FoxO family member, is expressed ubiquitously in various tissues but with higher expression levels in oxidative tissues, such as brain and oxidative muscles. Both the expression level and promoter activity of FoxO6 were found to be enhanced by PGC-1α (peroxisome-proliferator-activated receptor γ co-activator 1α), thus explained its enriched expression in oxidative tissues. We further demonstrated that FoxO6 represses the expression of PGC-1α via direct binding to an upstream A/T-rich element (AAGATATCAAAACA,−2228–2215) in the PGC-1α promoter. Oxidative low-intensity exercise induced PGC-1α but reduced FoxO6 expression levels in hind leg muscles, and the binding of FoxO6 to PGC-1α promoter was also prevented by exercise. As FoxO6 promoter can be co-activated by PGC-1α and its promoter in turn can be repressed by FoxO6, it suggests that FoxO6 and PGC-1α form a regulatory loop for setting oxidative metabolism level in the skeletal muscle, which can be entrained by exercise.

FoxO6 regulates memory consolidation and synaptic function

The FoxO family of transcription factors is known to slow aging downstream from the insulin/IGF (insulin-like growth factor) signaling pathway. The most recently discovered FoxO isoform in mammals, FoxO6, is highly enriched in the adult hippocampus. However, the importance of FoxO factors in cognition is largely unknown. Here we generated mice lacking FoxO6 and found that these mice display normal learning but impaired memory consolidation in contextual fear conditioning and novel object recognition. Using stereotactic injection of viruses into the hippocampus of adult wild-type mice, we found that FoxO6 activity in the adult hippocampus is required for memory consolidation. Genome-wide approaches revealed that FoxO6 regulates a program of genes involved in synaptic function upon learning in the hippocampus. Consistently, FoxO6 deficiency results in decreased dendritic spine density in hippocampal neurons in vitro and in vivo. Thus, FoxO6 may promote memory consolidation by regulating a program coordinating neuronal connectivity in the hippocampus, which could have important implications for physiological and pathological age-dependent decline in memory.

Inhibition of H2O2-induced cell death through FOXO1 modulation by EUK-172 in SK-N-MC cells

It has been suggested that excess accumulation of reactive oxygen species, termed oxidative stress, may lead to neuronal death resulting in neurodegenerative disorders such as Parkinson's and Alzheimer's diseases. In oxidative stress-induced cell death numerous transcription factors are thought to be involved. One of them is Forkhead box protein O1 (FOXO1) that governs many genes involved in oxidative stress resistance, DNA repair, cell cycle arrest, proliferation and apoptosis. Apparently, FOXO1 activity is tightly linked to post translational modifications including phosphorylation and acetylation, which are modulated by many factors such as oxidative stress. Reactive oxygen species, as the major players in oxidative stress, guide FOXO1 nuclear localization at least by simultaneous c-Jun N-terminal kinase (JNK) activation and Akt/PKB activity suppression. Here, we showed that a synthetic salen-manganese derivative (EUK-172) with strong catalase activity reduced oxidative stress evident through marked reduction in intracellular reactive oxygen species, protein carbonylation and lipid peroxidation. In addition, our results indicated that EUK-172 not only reduced the FOXO1 protein content, but also it inhibited FOXO1 nuclear translocation in H(2)O(2)-exposed SK-N-MC cells. These events attenuated caspase-3 activity and bax/Bcl-2 ratio leading to higher viability of the H(2)O(2)-treated SK-N-MC cells.

The Interface between BCR-ABL-Dependent and -Independent Resistance Signaling Pathways in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a clonal hematopoietic disorder characterized by the presence of the Philadelphia chromosome which resulted from the reciprocal translocation between chromosomes 9 and 22. The pathogenesis of CML involves the constitutive activation of the BCR-ABL tyrosine kinase, which governs malignant disease by activating multiple signal transduction pathways. The BCR-ABL kinase inhibitor, imatinib, is the front-line treatment for CML, but the emergence of imatinib resistance and other tyrosine kinase inhibitors (TKIs) has called attention for additional resistance mechanisms and has led to the search for alternative drug treatments. In this paper, we discuss our current understanding of mechanisms, related or unrelated to BCR-ABL, which have been shown to account for chemoresistance and treatment failure. We focus on the potential role of the influx and efflux transporters, the inhibitor of apoptosis proteins, and transcription factor-mediated signals as feasible molecular targets to overcome the development of TKIs resistance in CML.

Behavioral stress-induced activation of FoxO3a in the cerebral cortex of mice

BACKGROUND

The transcription factor FoxO3a is highly expressed in brain, but little is known about the response of FoxO3a to behavioral stress and its impact in the associated behavioral changes.

METHODS

We tested the response of brain FoxO3a in the learned helplessness (LH) paradigm and tested signaling pathways that mediate the response of FoxO3a.

RESULTS

A single session of inescapable shocks (IES) in mice reduced FoxO3a phosphorylation at the Akt-regulating serine/threonine residues and induced prolonged nuclear accumulation of FoxO3a in the cerebral cortex, both indicating activation of FoxO3a in brain. The response of FoxO3a is accompanied by a transient inactivation of Akt and a prolonged activation of glycogen synthase kinase-3beta (GSK3β). Noticeably, FoxO3a formed a protein complex with GSK3β in the cerebral cortex, and the interaction between the two proteins was stronger in IES-treated mice. Inhibition of glycogen synthase kinase-3 was able to abolish IES-induced LH behavior, disrupt IES-induced GSK3β-FoxO3a interaction, and reduce nuclear FoxO3a accumulation. In vitro approaches further revealed that the interaction between GSK3β and FoxO3a was strongest when both were active; FoxO3a was phosphorylated by recombinant GSK3β; and glycogen synthase kinase-3 inhibitors effectively reduced FoxO3a transcriptional activity. Importantly, IES-induced LH behavior was markedly diminished in FoxO3a-deficient mice that had minimal FoxO3a expression and reduced levels of FoxO3a-inducible genes.

CONCLUSIONS

FoxO3a is activated in response to IES by interacting with GSK3β, and inhibition of GSK3β or reducing FoxO3a expression promotes resistance to stress-induced behavioral disturbance by disrupting this signaling mechanism.

Recent progress toward understanding the molecular mechanisms that regulate skeletal muscle mass

The maintenance of muscle mass is critical for health and issues associated with the quality of life. Over the last decade, extensive progress has been made with regard to our understanding of the molecules that regulate skeletal muscle mass. Not surprisingly, many of these molecules are intimately involved in the regulation of protein synthesis and protein degradation [e.g. the mammalian target of rapamycin (mTOR), eukaryotic initiation factor 2B (eIF2B), eukaryotic initiation factor 3f (eIF3f) and the forkhead box O (FoxO) transcription factors]. It is also becoming apparent that molecules which sense, or control, the energetic status of the cell play a key role in the regulation of muscle mass [e.g. AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor gamma coactivator-1 α (PGC1α)]. In this review we will attempt to summarize the current knowledge of how these molecules regulate skeletal muscle mass.

FoxO6 integrates insulin signaling with gluconeogenesis in the liver

OBJECTIVE

Excessive endogenous glucose production contributes to fasting hyperglycemia in diabetes. This effect stems from inept insulin suppression of hepatic gluconeogenesis. To understand the underlying mechanisms, we studied the ability of forkhead box O6 (FoxO6) to mediate insulin action on hepatic gluconeogenesis and its contribution to glucose metabolism.

RESEARCH DESIGN AND METHODS

We characterized FoxO6 in glucose metabolism in cultured hepatocytes and in rodent models of dietary obesity, insulin resistance, or insulin-deficient diabetes. We determined the effect of FoxO6 on hepatic gluconeogenesis in genetically modified mice with FoxO6 gain- versus loss-of-function and in diabetic db/db mice with selective FoxO6 ablation in the liver.

RESULTS

FoxO6 integrates insulin signaling to hepatic gluconeogenesis. In mice, elevated FoxO6 activity in the liver augments gluconeogenesis, raising fasting blood glucose levels, and hepatic FoxO6 depletion suppresses gluconeogenesis, resulting in fasting hypoglycemia. FoxO6 stimulates gluconeogenesis, which is counteracted by insulin. Insulin inhibits FoxO6 activity via a distinct mechanism by inducing its phosphorylation and disabling its transcriptional activity, without altering its subcellular distribution in hepatocytes. FoxO6 becomes deregulated in the insulin-resistant liver, accounting for its unbridled activity in promoting gluconeogenesis and correlating with the pathogenesis of fasting hyperglycemia in diabetes. These metabolic abnormalities, along with fasting hyperglycemia, are reversible by selective inhibition of hepatic FoxO6 activity in diabetic mice.

CONCLUSIONS

Our data uncover a FoxO6-dependent pathway by which the liver orchestrates insulin regulation of gluconeogenesis, providing the proof-of-concept that selective FoxO6 inhibition is beneficial for curbing excessive hepatic glucose production and improving glycemic control in diabetes.

Characterization of intracellular translocation of Forkhead transcription factor O (FoxO) members induced by NGF in PC12 cells

Nuclear translocation of Forkhead transcription factors of the O class (FoxOs) is important for the action of growth factors. However it is not known if all members of the FOXO family have the same translocation properties. We examined the effects of nerve growth factor (NGF) on nuclear/cytoplasmic shuttling of FoxO1, FoxO3a and FoxO6 in PC12 cells and determined their translocation kinetics. Our data demonstrated that NGF could induce the nuclear exclusion of FoxO1-GFP and FoxO3a-GFP in PC12 cells with different properties, but had no effect on FoxO6-GFP's nuclear localization and FoxO6-GFP showed an exclusive nuclear localization. Translocat ould be blocked by K252a and LY294002 but not by PD98059. Moreover, FoxO3a returned to cytoplasm at a higher rate than FoxO1 after NGF stimulation and it was more sensitive than FoxO1 to NGF stimulation.

PDK1-Foxo1 in agouti-related peptide neurons regulates energy homeostasis by modulating food intake and energy expenditure

Insulin and leptin intracellular signaling pathways converge and act synergistically on the hypothalamic phosphatidylinositol-3-OH kinase/3-phosphoinositide-dependent protein kinase 1 (PDK1). However, little is known about whether PDK1 in agouti-related peptide (AGRP) neurons contributes to energy homeostasis. We generated AGRP neuron-specific PDK1 knockout (AGRPPdk1^−/−) mice and mice with selective expression of transactivation-defective Foxo1 (Δ256Foxo1^AGRPPdk1^−/−). The AGRPPdk1^−/− mice showed reductions in food intake, body length, and body weight. The Δ256Foxo1^AGRPPdk1^−/− mice showed increased body weight, food intake, and reduced locomotor activity. After four weeks of calorie-restricted feeding, oxygen consumption and locomotor activity were elevated in AGRPPdk1^−/− mice and reduced in Δ256Foxo1^AGRPPdk1^−/− mice. In vitro, ghrelin-induced changes in [Ca²⁺]_i and inhibition of ghrelin by leptin were significantly attenuated in AGRPPdk1^−/− neurons compared to control neurons. However, ghrelin-induced [Ca²⁺]_i changes and leptin inhibition were restored in Δ256Foxo1^AGRPPdk1^−/− mice. These results suggested that PDK1 and Foxo1 signaling pathways play important roles in the control of energy homeostasis through AGRP-independent mechanisms.

Foxo1 mediates insulin-like growth factor 1 (IGF1)/insulin regulation of osteocalcin expression by antagonizing Runx2 in osteoblasts

In this study, we determined the molecular mechanisms whereby forkhead transcription factor Foxo1, a key downstream signaling molecule of insulin-like growth factor 1 (IGF1)/insulin actions, regulates Runx2 activity and expression of the mouse osteocalcin gene 2 (Bglap2) in osteoblasts in vitro. We showed that Foxo1 inhibited Runx2-dependent transcriptional activity and osteocalcin mRNA expression and Bglap2 promoter activity in MC-4 preosteoblasts. Co-immunoprecipitation assay showed that Foxo1 physically interacted with Runx2 via its C-terminal region in osteoblasts or when co-expressed in COS-7 cells. Electrophoretic mobility shift assay demonstrated that Foxo1 suppressed Runx2 binding to its cognate site within the Bglap2 promoter. IGF1 and insulin prevented Foxo1 from inhibiting Runx2 activity by promoting Foxo1 phosphorylation and nuclear exclusion. In contrast, a neutralizing anti-IGF1 antibody decreased Runx2 activity and osteocalcin expression in osteoblasts. Chromatin immunoprecipitation assay revealed that IGF1 increased Runx2 interaction with a chromatin fragment of the proximal Bglap2 promoter in a PI3K/AKT-dependent manner. Conversely, knockdown of Foxo1 increased Runx2 interaction with the promoter. This study establishes that Foxo1 is a novel negative regulator of osteoblast-specific transcription factor Runx2 and modulates IGF1/insulin-dependent regulation of osteocalcin expression in osteoblasts.

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

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.

A fork in the path: Developing therapeutic inroads with FoxO proteins

Advances in clinical care for disorders involving any system of the body necessitates novel therapeutic strategies that can focus upon the modulation of cellular proliferation, metabolism, inflammation and longevity. In this respect, members of the mammalian forkhead transcription factors of the O class (FoxOs) that include FoxO1, FoxO3, FoxO4 and FoxO6 are increasingly being recognized as exciting prospects for multiple disorders. These transcription factors govern development, proliferation, survival and longevity during multiple cellular environments that can involve oxidative stress. Furthermore, these transcription factors are closely integrated with several novel signal transduction pathways, such as erythropoietin and Wnt proteins, that may influence the ability of FoxOs to act as a “double-edge sword” to sometimes promote cell survival, but at other times lead to cell injury. Here we discuss the fascinating but complex role of FoxOs during cellular injury and oxidative stress, progenitor cell development, fertility, angiogenesis, cardiovascular function, cellular metabolism and diabetes, cell longevity, immune surveillance and cancer.

Energy metabolism in adult neural stem cell fate

The adult mammalian brain contains a population of neural stem cells that can give rise to neurons, astrocytes, and oligodendrocytes and are thought to be involved in certain forms of memory, behavior, and brain injury repair. Neural stem cell properties, such as self-renewal and multipotency, are modulated by both cell-intrinsic and cell-extrinsic factors. Emerging evidence suggests that energy metabolism is an important regulator of neural stem cell function. Molecules and signaling pathways that sense and influence energy metabolism, including insulin/insulin-like growth factor I (IGF-1)-FoxO and insulin/IGF-1-mTOR signaling, AMP-activated protein kinase (AMPK), SIRT1, and hypoxia-inducible factors, are now implicated in neural stem cell biology. Furthermore, these signaling modules are likely to cooperate with other pathways involved in stem cell maintenance and differentiation. This review summarizes the current understanding of how cellular and systemic energy metabolism regulate neural stem cell fate. The known consequences of dietary restriction, exercise, aging, and pathologies with deregulated energy metabolism for neural stem cells and their differentiated progeny will also be discussed. A better understanding of how neural stem cells are influenced by changes in energy availability will help unravel the complex nature of neural stem cell biology in both the normal and diseased state.

FoxO1 links hepatic insulin action to endoplasmic reticulum stress

Forkhead box O1 (FoxO1) is a transcription factor that mediates the inhibitory effect of insulin on target genes in hepatic metabolism. Hepatic FoxO1 activity is up-regulated to promote glucose production during fasting and is suppressed to limit postprandial glucose excursion after meals. Increased FoxO1 activity augments the expression of insulin receptor (IR) and IR substrate (IRS)2, which in turn inhibits FoxO1 activity in response to reduced insulin action. To address the underlying physiology of such a feedback loop for regulating FoxO1 activity, we delivered FoxO1-ADA by adenovirus-mediated gene transfer into livers of adult mice. FoxO1-ADA is a constitutively active allele that is refractory to insulin inhibition, allowing us to determine the metabolic effect of a dislodged FoxO1 feedback loop in mice. We show that hepatic FoxO1-ADA production resulted in significant induction of IR and IRS2 expression. Mice with increased FoxO1-ADA production exhibited near glycogen depletion. Unexpectedly, hepatic FoxO1-ADA production elicited a profound unfolded protein response, culminating in the induction of hepatic glucose-regulated protein 78 (GRP78) expression. These findings were recapitulated in primary human and mouse hepatocytes. FoxO1 targeted GRP78 gene for trans-activation via selective binding to an insulin responsive element in the GRP78 promoter. This effect was counteracted by insulin. Our studies underscore the importance of an IR and IRS2-dependent feedback loop to keep FoxO1 activity in check for maintaining hepatic glycogen homeostasis and promoting adaptive unfolded protein response in response to altered metabolism and insulin action. Excessive FoxO1 activity, resulting from a dislodged FoxO1 feedback loop in insulin resistant liver, is attributable to hepatic endoplasmic reticulum stress and metabolic abnormalities in diabetes.

FoxO, autophagy, and cardiac remodeling

In response to changes in workload, the heart grows or shrinks. Indeed, the myocardium is capable of robust and rapid structural remodeling. In the setting of normal, physiological demand, the heart responds with hypertrophic growth of individual cardiac myocytes, a process that serves to maintain cardiac output and minimize wall stress. However, disease-related stresses, such as hypertension or myocardial infarction, provoke a series of changes that culminate in heart failure and/or sudden death. At the other end of the spectrum, cardiac unloading, such as occurs with prolonged bed rest or weightlessness, causes the heart to shrink. In recent years, considerable strides have been made in deciphering the molecular and cellular events governing pro- and anti-growth events in the heart. Prominent among these mechanisms are those mediated by FoxO (Forkhead box-containing protein, O subfamily) transcription factors. In many cell types, these proteins are critical regulators of cell size, viability, and metabolism, and their importance in the heart is just emerging. Also in recent years, evidence has emerged for a pivotal role for autophagy, an evolutionarily conserved pathway of lysosomal degradation of damaged proteins and organelles, in cardiac growth and remodeling. Indeed, evidence for activated autophagy has been detected in virtually every form of myocardial disease. Now, it is clear that FoxO is an upstream regulator of both autophagy and the ubiquitin-proteasome system. Here, we discuss recent advances in our understanding of cardiomyocyte autophagy, its governance by FoxO, and the roles each of these plays in cardiac remodeling.

A FOXO-Pak1 transcriptional pathway controls neuronal polarity

Neuronal polarity is essential for normal brain development and function. However, cell-intrinsic mechanisms that govern the establishment of neuronal polarity remain to be identified. Here, we report that knockdown of endogenous FOXO proteins in hippocampal and cerebellar granule neurons, including in the rat cerebellar cortex in vivo, reveals a requirement for the FOXO transcription factors in the establishment of neuronal polarity. The FOXO transcription factors, including the brain-enriched protein FOXO6, play a critical role in axo-dendritic polarization of undifferentiated neurites, and hence in a switch from unpolarized to polarized neuronal morphology. We also identify the gene encoding the protein kinase Pak1, which acts locally in neuronal processes to induce polarity, as a critical direct target gene of the FOXO transcription factors. Knockdown of endogenous Pak1 phenocopies the effect of FOXO knockdown on neuronal polarity. Importantly, exogenous expression of Pak1 in the background of FOXO knockdown in both primary neurons and postnatal rat pups in vivo restores the polarized morphology of neurons. These findings define the FOXO proteins and Pak1 as components of a cell-intrinsic transcriptional pathway that orchestrates neuronal polarity, thus identifying a novel function for the FOXO transcription factors in a unique aspect of neural development.