The kinase DYRK1A phosphorylates the transcription factor FKHR at Ser329 in vitro, a novel in vivo phosphorylation site

DYRK1A, a Dosage-Sensitive Gene Involved in Neurodevelopmental Disorders, Is a Target for Drug Development in Down Syndrome

Down syndrome (DS) is one of the leading causes of intellectual disability, and patients with DS face various health issues, including learning and memory deficits, congenital heart disease, Alzheimer's disease (AD), leukemia, and cancer, leading to huge medical and social costs. Remarkable advances on DS research have been made in improving cognitive function in mouse models for future therapeutic approaches in patients. Among the different approaches, DYRK1A inhibitors have emerged as promising therapeutics to reduce DS cognitive deficits. DYRK1A is a dual-specificity kinase that is overexpressed in DS and plays a key role in neurogenesis, outgrowth of axons and dendrites, neuronal trafficking and aging. Its pivotal role in the DS phenotype makes it a prime target for the development of therapeutics. Recently, disruption of DYRK1A has been found in Autosomal Dominant Mental Retardation 7 (MRD7), resulting in severe mental deficiency. Recent advances in the development of kinase inhibitors are expected, in the near future, to remove DS from the list of incurable diseases, providing certain conditions such as drug dosage and correct timing for the optimum long-term treatment. In addition the exact molecular and cellular mechanisms that are targeted by the inhibition of DYRK1A are still to be discovered.

Systematic diversification of benzylidene heterocycles yields novel inhibitor scaffolds selective for Dyrk1A, Clk1 and CK2

The dual-specificity tyrosine-regulated kinase 1A (Dyrk1A) has gathered much interest as a pharmacological target in Alzheimer's disease (AD), but it plays a role in malignant brain tumors as well. As both diseases are multi-factorial, further protein kinases, such as Clk1 and CK2, were proposed to contribute to the pathogenesis. We designed a new class of α-benzylidene-γ-butyrolactone inhibitors that showed low micromolar potencies against Dyrk1A and/or Clk1 and a good selectivity profile among the most frequently reported off-target kinases. A systematic replacement of the heterocyclic moiety gave access to further inhibitor classes with interesting selectivity profiles, demonstrating that the benzylidene heterocycles provide a versatile tool box for developing inhibitors of the CMGC kinase family members Dyr1A/1B, Clk1/4 and CK2. Efficacy for the inhibition of Dyrk1A-mediated tau phosphorylation was demonstrated in a cell-based assay. Multi-targeted but not non-specific kinase inhibitors were also obtained, that co-inhibited the lipid kinases PI3Kα/γ. These compounds were shown to inhibit the proliferation of U87MG cells in the low micromolar range. Based on the molecular properties, the inhibitors described here hold promise for CNS activity.

DYRK1A Controls HIV-1 Replication at a Transcriptional Level in an NFAT Dependent Manner

BACKGROUND

Transcription of the HIV-1 provirus is regulated by both viral and host proteins and is very important in the context of viral latency. In latently infected cells, viral gene expression is inhibited as a result of the sequestration of host transcription factors and epigenetic modifications.

RESULTS

In our present study we analyzed the effect of host factor dual specificity tyrosine-phosphorylation-regulated kinase 1A (DYRK1A) on HIV-1 replication. We show that DYRK1A controls HIV-1 replication by regulating provirus transcription. Downregulation or inhibition of DYRK1A increased LTR-driven transcription and viral replication in cell lines and primary PBMC. Furthermore, inhibition of DYRK1A resulted in reactivation of latent HIV-1 provirus to a similar extent as two commonly used broad-spectrum HDAC inhibitors. We observed that DYRK1A regulates HIV-1 transcription via the Nuclear Factor of Activated T-cells (NFAT) by promoting its translocation from the nucleus to the cytoplasm. Therefore, inhibition of DYRK1A results in increased nuclear levels of NFAT and increased NFAT binding to the viral LTR and thus increasing viral transcription.

CONCLUSIONS

Our data indicate that host factor DYRK1A plays a role in the regulation of viral transcription and latency. Therefore, DYRK1A might be an attractive candidate for therapeutic strategies targeting the viral reservoir.

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.

Truncation and Activation of Dual Specificity Tyrosine Phosphorylation-regulated Kinase 1A by Calpain I: A MOLECULAR MECHANISM LINKED TO TAU PATHOLOGY IN ALZHEIMER DISEASE

Hyperphosphorylation and dysregulation of exon 10 splicing of Tau are pivotally involved in pathogenesis of Alzheimer disease (AD) and/or other tauopathies. Alternative splicing of Tau exon 10, which encodes the second microtubule-binding repeat, generates Tau isoforms containing three and four microtubule-binding repeats, termed 3R-Taus and 4R-Taus, respectively. Dual specificity tyrosine-phosphorylation-regulated kinase 1A (Dyrk1A) lies at the Down syndrome critical region of chromosome 21. Overexpression of this kinase may contribute to the early Tau pathology in Down syndrome via phosphorylation of Tau and dysregulation of Tau exon 10. Here, we report that Dyrk1A was truncated at the C terminus and was associated with overactivation of calpain I in AD brain. Calpain I proteolyzed Dyrk1A in vitro first at the C terminus and further at the N terminus and enhanced its kinase activity toward Tau via increased Vmax but not Km. C-terminal truncation of Dyrk1A resulted in stronger activity than its full-length protein in promotion of exon 10 exclusion and phosphorylation of Tau. Dyrk1A was truncated in kainic acid-induced excitotoxic mouse brains and coincided with an increase in 3R-Tau expression and phosphorylation of Tau via calpain activation. Moreover, truncation of Dyrk1A was correlated with an increase in the ratio of 3R-Tau/4R-Tau and Tau hyperphosphorylation in AD brain. Collectively, these findings suggest that truncation/activation of Dyrk1A by Ca(2+)/calpain I might contribute to Tau pathology via promotion of exon 10 exclusion and hyperphosphorylation of Tau in AD brain.

Forkhead followed by disordered tail: The intrinsically disordered regions of FOXO3a

Forkhead box Class O is one of 19 subfamilies of the Forkhead box family, comprising 4 human transcription factors: FOXO1, FOXO3a, FOXO4, and FOXO6, which are involved in many crucial cellular processes. FOXO3a is a tumor suppressor involved in multiple physiological and pathological processes, and plays essential roles in metabolism, cell cycle arrest, DNA repair, and apoptosis. In its role as a transcription factor, the FOXO3a binds a consensus Forkhead response element DNA sequence, and recruits transcriptional coactivators to activate gene transcription. FOXO3a has additional functions, such as regulating p53-mediated apoptosis and activating kinase ATM. With the exception of the structured DNA-binding forkhead domain, most of the FOXO3a sequence comprises intrinsically disordered regions (IDRs), including 3 regions (CR1-3) that are conserved within the FOXO subfamily. Numerous studies have demonstrated that these IDRs directly mediate many of the diverse functions of FOXO3a. These regions contain post-translational modification and protein-protein interaction sites that integrate upstream signals to maintain homeostasis. Thus, the FOXO3a IDRs are emerging as key mediators of diverse regulatory processes, and represent an important target for the future development of therapeutics for FOXO3a-related diseases.

Akt signaling dynamics in individual cells

The protein kinase Akt (for which there are three isoforms) is a key intracellular mediator of many biological processes, yet knowledge of Akt signaling dynamics is limited. Here, we have constructed a fluorescent reporter molecule in a lentiviral delivery system to assess Akt kinase activity at the single cell level. The reporter, a fusion between a modified FoxO1 transcription factor and clover, a green fluorescent protein, rapidly translocates from the nucleus to the cytoplasm in response to Akt stimulation. Because of its long half-life and the intensity of clover fluorescence, the sensor provides a robust readout that can be tracked for days under a range of biological conditions. Using this reporter, we find that stimulation of Akt activity by IGF-I is encoded into stable and reproducible analog responses at the population level, but that single cell signaling outcomes are variable. This reporter, which provides a simple and dynamic measure of Akt activity, should be compatible with many cell types and experimental platforms, and thus opens the door to new insights into how Akt regulates its biological responses.

Kinase and channel activity of TRPM6 are co-ordinated by a dimerization motif and pocket interaction

Mutations in the gene that encodes the atypical channel-kinase TRPM6 (transient receptor potential melastatin 6) cause HSH (hypomagnesaemia with secondary hypocalcaemia), a disorder characterized by defective intestinal Mg²⁺ transport and impaired renal Mg²⁺ reabsorption. TRPM6, together with its homologue TRPM7, are unique proteins as they combine an ion channel domain with a C-terminally fused protein kinase domain. How TRPM6 channel and kinase activity are linked is unknown. Previous structural analysis revealed that TRPM7 possesses a non-catalytic dimerization motif preceding the kinase domain. This interacts with a dimerization pocket lying within the kinase domain. In the present study, we provide evidence that the dimerization motif in TRPM6 plays a critical role in regulating kinase activity as well as ion channel activity. We identify mutations within the TRPM6 dimerization motif (Leu¹⁷¹⁸ and Leu¹⁷²¹) or dimerization pocket (L1743A, Q1832K, A1836N, L1840A and L1919Q) that abolish dimerization and establish that these mutations inhibit protein kinase activity. We also demonstrate that kinase activity of a dimerization motif mutant can be restored by addition of a peptide encompassing the dimerization motif. Moreover, we observe that mutations that disrupt the dimerization motif and dimerization pocket interaction greatly diminish TRPM6 ion channel activity, in a manner that is independent of kinase activity. Finally, we analyse the impact on kinase activity of ten disease-causing missense mutations that lie outwith the protein kinase domain of TRPM6. This revealed that one mutation lying nearby the dimerization motif (S1754N), found previously to inhibit channel activity, abolished kinase activity. These results provide the first evidence that there is structural co-ordination between channel and kinase activity, which is mediated by the dimerization motif and pocket interaction. We discuss that modulation of this interaction could comprise a major regulatory mechanism by which TRPM6 function is controlled.

We show that TRPM6 kinase activity is linked to channel activity. This occurs through a kinase-independent mechanism involving the dimerization motif binding to a pocket within the kinase domain. A disease-causing mutation (S1754N) lying nearby the dimerization pocket inactivates kinase activity.

Dyrk1a haploinsufficiency induces diabetes in mice through decreased pancreatic beta cell mass

AIMS/HYPOTHESIS

Growth factors and nutrients are important regulators of pancreatic beta cell mass and function. However, the signalling pathways by which these factors modulate these processes have not yet been fully elucidated. DYRK1A (also named minibrain/MNB) is a member of the dual-specificity tyrosine phosphorylation-regulated kinase (DYRK) family that has been conserved across evolution. A significant amount of data implicates DYRK1A in brain growth and function, as well as in neurodegenerative processes in Alzheimer's disease and Down's syndrome. We investigated here whether DYRK1A would be an attractive candidate for beta cell growth modulation.

METHODS

To study the role of DYRK1A in beta cell growth, we used Dyrk1a-deficient mice.

RESULTS

We show that DYRK1A is expressed in pancreatic islets and provide evidence that changes in Dyrk1a gene dosage in mice strongly modulate glycaemia and circulating insulin levels. Specifically, Dyrk1a-haploinsufficient mice show severe glucose intolerance, reduced beta cell mass and decreased beta cell proliferation.

CONCLUSIONS/INTERPRETATION

Taken together, our data indicate that DYRK1A is a critical kinase for beta cell growth as Dyrk1a-haploinsufficient mice show a diabetic profile.

The WNK-regulated SPAK/OSR1 kinases directly phosphorylate and inhibit the K+-Cl- co-transporters

Precise homoeostasis of the intracellular concentration of Cl- is achieved via the co-ordinated activities of the Cl- influx and efflux. We demonstrate that the WNK (WNK lysine-deficient protein kinase)-activated SPAK (SPS1-related proline/alanine-rich kinase)/OSR1 (oxidative stress-responsive kinase 1) known to directly phosphorylate and stimulate the N[K]CCs (Na+-K+ ion co-transporters), also promote inhibition of the KCCs (K+-Cl- co-transporters) by directly phosphorylating a recently described C-terminal threonine residue conserved in all KCC isoforms [Site-2 (Thr1048)]. First, we demonstrate that SPAK and OSR1, in the presence of the MO25 regulatory subunit, robustly phosphorylates all KCC isoforms at Site-2 in vitro. Secondly, STOCK1S-50699, a WNK pathway inhibitor, suppresses SPAK/OSR1 activation and KCC3A Site-2 phosphorylation with similar efficiency. Thirdly, in ES (embryonic stem) cells lacking SPAK/OSR1 activity, endogenous phosphorylation of KCC isoforms at Site-2 is abolished and these cells display elevated basal activity of 86Rb+ uptake that was not markedly stimulated further by hypotonic high K+ conditions, consistent with KCC3A activation. Fourthly, a tight correlation exists between SPAK/OSR1 activity and the magnitude of KCC3A Site-2 phosphorylation. Lastly, a Site-2 alanine KCC3A mutant preventing SPAK/OSR1 phosphorylation exhibits increased activity. We also observe that KCCs are directly phosphorylated by SPAK/OSR1, at a novel Site-3 (Thr5 in KCC1/KCC3 and Thr6 in KCC2/KCC4), and a previously recognized KCC3-specific residue, Site-4 (Ser96). These data demonstrate that the WNK-regulated SPAK/OSR1 kinases directly phosphorylate the N[K]CCs and KCCs, promoting their stimulation and inhibition respectively. Given these reciprocal actions with anticipated net effects of increasing Cl- influx, we propose that the targeting of WNK-SPAK/OSR1 with kinase inhibitors might be a novel potent strategy to enhance cellular Cl- extrusion, with potential implications for the therapeutic modulation of epithelial and neuronal ion transport in human disease states.

A high-performance liquid chromatography assay for Dyrk1a, a Down syndrome-associated kinase

Down syndrome is the most common aneuploidy. It is caused by the presence of an extra copy of chromosome 21. Several studies indicate that aberrant expression of the kinase Dyrk1a (dual-specificity tyrosine phosphorylation-regulated kinase 1a) is implicated in Down syndrome, in particular in the onset of mental retardation. Moreover, elevated Dyrk1a activity may also be a risk factor for other neurodegenerative disorders such as Alzheimer's disease. Over the past years, Dyrk1a has appeared as a potential drug target. Availability of sensitive and quantitative enzyme assays is of prime importance to understand the role of Dyrk1a and to develop specific inhibitors. Here, we describe a new method to measure Dyrk1a activity based on the separation and quantification of specific fluorescent peptides (substrate and phosphorylated product) by high-performance liquid chromatography (HPLC). Kinetic and mechanistic analyses using well-known inhibitors of Dyrk1a confirmed the reliability of this approach. In addition, this assay was further validated using brain extracts of mice models expressing different copies of the Dyrk1a gene. Our results indicate that this novel Dyrk1a assay is simple, sensitive, and specific. It avoids the use of radioactivity-based approaches that, until now, have been widely employed to measure Dyrk1a activity.

Insulin and LiCl synergistically rescue myogenic differentiation of FoxO1 over-expressed myoblasts

Most recent studies reported that FoxO1 transcription factor was a negative regulator of myogenesis under serum withdrawal condition, a situation not actually found in vivo. Therefore, the role of FoxO1 in myogenesis should be re-examined under more physiologically relevant conditions. Here we found that FoxO1 was preferentially localized to nucleus in proliferating (PMB) and confluent myoblasts (CMB) and its nuclear exclusion was a prerequisite for formation of multinucleated myotubes (MT). The nuclear shuttling of FoxO1 in PMB could be prevented by leptomycin B and we further found that cytoplasmic accumulation of FoxO1 in myotubes was caused by the blockade of its nuclear import. Although over-expression of wildtype FoxO1 in C2C12 myoblasts significantly blocked their myogenic differentiation under serum withdrawal condition, application of insulin and LiCl, an activator of Wnt signaling pathway, to these cells successfully rescued their myogenic differentiation and generated myotubes with larger diameters. Interestingly, insulin treatment significantly reduced FoxO1 level and also delayed nuclear re-accumulation of FoxO1 triggered by mitogen deprivation. We further found that FoxO1 directly repressed the promoter activity of myogenic genes and this repression can be relieved by insulin and LiCl treatment. These results suggest that FoxO1 inhibits myogenesis in serum withdrawal condition but turns into a hypertrophy potentiator when other myogenic signals, such as Wnt and insulin, are available.

Insulin signaling, resistance, and the metabolic syndrome: insights from mouse models into disease mechanisms

Insulin resistance is a major underlying mechanism responsible for the 'metabolic syndrome', which is also known as insulin resistance syndrome. The incidence of the metabolic syndrome is increasing at an alarming rate, becoming a major public and clinical problem worldwide. The metabolic syndrome is represented by a group of interrelated disorders, including obesity, hyperglycemia, hyperlipidemia, and hypertension. It is also a significant risk factor for cardiovascular disease and increased morbidity and mortality. Animal studies have demonstrated that insulin and its signaling cascade normally control cell growth, metabolism, and survival through the activation of MAPKs and activation of phosphatidylinositide-3-kinase (PI3K), in which the activation of PI3K associated with insulin receptor substrate 1 (IRS1) and IRS2 and subsequent Akt→Foxo1 phosphorylation cascade has a central role in the control of nutrient homeostasis and organ survival. The inactivation of Akt and activation of Foxo1, through the suppression IRS1 and IRS2 in different organs following hyperinsulinemia, metabolic inflammation, and overnutrition, may act as the underlying mechanisms for the metabolic syndrome in humans. Targeting the IRS→Akt→Foxo1 signaling cascade will probably provide a strategy for therapeutic intervention in the treatment of type 2 diabetes and its complications. This review discusses the basis of insulin signaling, insulin resistance in different mouse models, and how a deficiency of insulin signaling components in different organs contributes to the features of the metabolic syndrome. Emphasis is placed on the role of IRS1, IRS2, and associated signaling pathways that are coupled to Akt and the forkhead/winged helix transcription factor Foxo1.

Modification of the Drosophila model of in vivo Tau toxicity reveals protective phosphorylation by GSK3β

Hyperphosphorylation of the microtubule associated protein, Tau, is the hallmark of a group of neurodegenerative disorders known as the tauopathies which includes Alzheimer's disease. Precisely how and why Tau phosphorylation is increased in disease is not fully understood, nor how individual sites modify Tau function. Several groups have used the Drosophila visual system as an in vivo model to examine how the toxicity of Tau varies with phosphorylation status. This system relies on overexpression of Tau from transgenes but is susceptible to position effects altering expression and activity of the transgenes. We have refined the system by eliminating position effects through the use of site-specific integration. By standardising Tau expression levels we have been able to compare directly the toxicity of different isoforms of Tau and Tau point mutants that abolish important phosphorylation events. We have also examined the importance of human kinases in modulating Tau toxicity in vivo. We were able to confirm that human GSK3β phosphorylates Tau and increases toxicity but, unexpectedly, we identified that preventing phosphorylation of Ser404 is a protective event. When phosphorylation at this site is prevented, Tau toxicity in the Drosophila visual system is increased in the presence of GSK3β. Our data suggest that not all phosphorylation events on Tau are associated with toxicity.

The function of FOXO1 in the late phases of the cell cycle is suppressed by PLK1-mediated phosphorylation

Polo-like kinase 1 (PLK1) plays crucial roles in multiple stages of cell division. Our previous studies suggest that global transcriptional regulation by PLK1 may contribute to its multiple functions. PLK1 depletion is associated with a decrease in cell viability and the induction of apoptosis; however, the underlying mechanisms are not completely understood. Here, we report that forkhead box protein O1 (FOXO1) is a novel physiological substrate of PLK1. FOXO1 is at the interface of crucial cellular processes, orchestrating programs of gene expression that regulate apoptosis, cell cycle progression, and oxidative-stress resistance. PLK1 interacts with and phosphorylates FOXO1, mainly at the G 2/M phase of the cell cycle. PLK1-mediated phosphorylation leads to the impairment of FOXO1's transcriptional activity in an Akt-independent manner. By immunofluorescence staining and subcellular fractionation, we demonstrate that PLK1-induced FOXO1 phosphorylation causes its nuclear exclusion. Furthermore, PLK1-mediated phosphorylation of FOXO1 negatively regulates its pro-apoptotic function and abrogates its ability to delay entry into and progression through G 2/M transition. Therefore, our results suggest that PLK1 abrogates the inhibitory effects of FOXO1 on cell growth and survival to ensure timely cell cycle progression. This study not only reveals a novel and major regulatory mechanism of FOXO1 at the late phases of the cell cycle, but also provides new insight into the molecular mechanisms by which PLK1 inhibition leads to growth arrest and cell death.

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.

The role of FOXO1 in the decidual transformation of the endometrium and early pregnancy

Successful pregnancy requires coordination of embryo development, decidualization of endometrium, and placenta formation. Decidualization denotes the transformation of endometrial stromal cells into specialized secretory cells, a process further characterized with influx of specialized immune cells into stroma, predominantly uterine natural killer cells and macrophages, and vascular remodeling. This differentiation process depends on the convergence of the cyclic adenosine monophosphate and progesterone signaling pathways. The decidual process is indispensable for the formation of a functional feto-maternal interface as it controls tissue homeostasis during endovascular trophoblast invasion and bestows tissue resistance to environmental stress signals, including protection against oxidative cell death. FOXO proteins have emerged as key mediators of cell fate because of their ability to regulate either pro-apoptotic genes or genes involved in differentiation, cell cycle arrest, oxidative defenses, and DNA repair. In the endometrium, FOXO1 is of particular importance as a critical regulator of progesterone-dependent differentiation, menstrual shedding, and protection of the feto-maternal against oxidative damage during pregnancy.

FOXOs: signalling integrators for homeostasis maintenance

Forkhead box O (FOXO) transcription factors are involved in the regulation of the cell cycle, apoptosis and metabolism. In model organisms, FOXO activity also affects stem cell maintenance and lifespan as well as age-related diseases, such as cancer and diabetes. Multiple upstream pathways regulate FOXO activity through post-translational modifications and nuclear-cytoplasmic shuttling of both FOXO and its regulators. The diversity of this upstream regulation and the downstream effects of FOXOs suggest that they function as homeostasis regulators to maintain tissue homeostasis over time and coordinate a response to environmental changes, including growth factor deprivation, metabolic stress (starvation) and oxidative stress.

Kinetics of nuclear-cytoplasmic translocation of Foxo1 and Foxo3A in adult skeletal muscle fibers

In skeletal muscle, the transcription factors Foxo1 and Foxo3A control expression of proteins that mediate muscle atrophy, making the nuclear concentration and nuclear-cytoplasmic movements of Foxo1 and Foxo3A of therapeutic interest in conditions of muscle wasting. Here, we use Foxo-GFP fusion proteins adenovirally expressed in cultured adult mouse skeletal muscle fibers to characterize the time course of nuclear efflux of Foxo1-GFP in response to activation of the insulin-like growth factor-1 (IGF-1)/phosphatidylinositol-3-kinase (PI3K)/Akt pathway to determine the time course of nuclear influx of Foxo1-GFP during inhibition of this pathway and to show that Akt mediates the efflux of nuclear Foxo1-GFP induced by IGF-1. Localization of endogenous Foxo1 in muscle fibers, as determined via immunocytochemistry, is consistent with that of Foxo1-GFP. Inhibition of the nuclear export carrier chromosome region maintenance 1 by leptomycin B (LMB) traps Foxo1 in the nucleus and results in a relatively rapid rate of Foxo1 nuclear accumulation, consistent with a high rate of nuclear-cytoplasmic shuttling of Foxo1 under control conditions before LMB application, with near balance of unidirectional influx and efflux. Expressed Foxo3A-GFP shuttles ∼20-fold more slowly than Foxo1-GFP. Our approach allows quantitative kinetic characterization of Foxo1 and Foxo3A nuclear-cytoplasmic movements in living muscle fibers under various experimental conditions.

FoxO1 inhibits sterol regulatory element-binding protein-1c (SREBP-1c) gene expression via transcription factors Sp1 and SREBP-1c

Induction of lipogenesis in response to insulin is critically dependent on the transcription factor, sterol regulatory element-binding protein-1c (SREBP-1c). FoxO1, a forkhead box class-O transcription factor, is an important mediator of insulin action, but its role in the regulation of lipid metabolism has not been clearly defined. We examined the effects of FoxO1 on srebp1 gene expression in vivo and in vitro. In vivo studies showed that constitutively active (CA) FoxO1 (CA-FoxO1) reduced basal expression of SREBP-1c mRNA in liver by ∼60% and blunted induction of SREBP-1c in response to feeding. In liver-specific FoxO knock-out mice, SREBP-1c expression was increased ∼2-fold. Similarly, in primary hepatocytes, CA-FoxO1 suppressed SREBP1-c expression and inhibited basal and insulin-induced SREBP-1c promoter activity. SREBP-1c gene expression is induced by the liver X receptor (LXR), but CA-FoxO1 did not block the activation of SREBP-1c by the LXR agonist TO9. Insulin stimulates SREBP-1c transcription through Sp1 and via "feed forward" regulation by newly synthesized SREBP-1c. CA-FoxO1 inhibited SREBP-1c by reducing the transactivational capacity of both Sp1 and SREBP-1c. In addition, chromatin immunoprecipitation assays indicate that FoxO1 can associate with the proximal promoter region of the srebp1 gene and disrupt the assembly of key components of the transcriptional complex of the SREBP-1c promoter. We conclude that FoxO1 inhibits SREBP-1c transcription via combined actions on multiple transcription factors and that this effect is exerted at least in part through reduced transcriptional activity of Sp1 and SREBP-1c and disrupted assembly of the transcriptional initiation complex on the SREBP-1c promoter.

DYRK1A: a master regulatory protein controlling brain growth

Copy number variation in a small region of chromosome 21 containing DYRK1A produces morphological and cognitive alterations in human. In mouse models, haploinsufficiency results in microcephaly, and a human DYRK1A gain-of-function model (three alleles) exhibits increased brain volume. To investigate these developmental aspects, we used a murine BAC clone containing the entire gene to construct an overexpression model driven by endogenous regulatory sequences. We compared this new model to two other mouse models with three copies of Dyrk1a, YACtgDyrk1a and Ts65Dn, as well as the loss-of-function model with one copy (Dyrk1a(+/-)). Growth, viability, brain weight, and brain volume depended strongly upon gene copy number. Brain region-specific variations observed in gain-of-function models mirror their counterparts in the loss-of-function model. Some variations, such as increased volume of the superior colliculus and ventricles, were observed in both the BAC transgenic and Ts65Dn mice. Using unbiased stereology we found that, in the cortex, neuron density is inversely related to Dyrk1a copy number but, in thalamic nuclei, neuron density is directly related to copy number. In addition, six genes involved either in cell division (Ccnd1 and pAkt) or in neuronal machinery (Gap43, Map2, Syp, Snap25) were regulated by Dyrk1a throughout development, from birth to adult. These results imply that Dyrk1a expression alters different cellular processes during brain development. Dyrk1a, then, has two roles in the development process: shaping the brain and controlling the structure of neuronal components.

Discovery of potent small molecule inhibitors of DYRK1A by structure-based virtual screening and bioassay

In this study, six novel dual-specificity tyrosine phosphorylation regulated kinase 1A (DYRK1A) inhibitors with IC(50) values ranging from 1.51 to 88.13 μM were successfully identified through virtual screening and in vitro plus cell based bioassay. Compound 5 with IC(50) value of 1.51 μM is the most potent hit against DYRK1A in vitro, while compound 3 exhibited the most potent activity in cultured cells. The inhibition mechanism was explored by molecular docking approach. This study may provide a start point for further mechanism based study as well as discovery of drug candidate against Down syndrome (DS).

Increased activation of the PI3K/AKT pathway compromises decidualization of stromal cells from endometriosis

CONTEXT

Endometriosis affects approximately 10% of women in the United States and causes pain and infertility. Decidualization of endometrial stromal cells from women with endometriosis is aberrant.

OBJECTIVE

The objective of this study was to investigate a potential mechanism for the inadequate decidual response in stromal cells from ovarian endometriomas.

DESIGN

Stromal cells of the endometrium from women without endometriosis (HSC) or from ovarian endometriomas (OsisSC) were grown in culture and treated with 10 μm LY294002 or 250 nm MK2206, 100 nm medroxyprogesterone acetate (M), and 0.5 mm dibutyryl cAMP (A) or infection with 100 multiplicity of infection adenoviral constructs containing wild-type Forkhead box O1 or triple-mutant FOXO1. Real-time PCR was used to measure the expression of FOXO1, IGF binding protein-1 (IGFBP1), and prolactin (PRL) mRNA, and Western blot and immunohistochemical staining were used to detect the levels of progesterone receptor (PR), FOXO1, AKT, and p(Ser473)-AKT protein in vitro or in vivo.

RESULTS

Expression of the decidua-specific genes, IGFBP1 and PRL, were significantly lower in OsisSC compared with normal HSC in response to M+A treatment. Basal expression levels of PRA, PRB, and FOXO1 proteins were dramatically lower in OsisSC. Overexpression of triple-mutant FOXO1 increased mRNA levels of IGFBP1 and PRL in OsisSC in the presence of M+A, whereas the overexpression of wild-type FOXO1 had no effect. AKT was highly phosphorylated in OsisSC compared with HSC and inhibition of phosphatidylinositol 3-kinase, with LY294002, increased levels of FOXO1 protein as well as IGFBP1 mRNA in the presence of M+A. Moreover, inhibition of AKT with MK2206, an allosteric AKT inhibitor, dramatically increased the accumulation of nuclear FOXO1 as well as expression of IGFBP1. Finally, immunohistochemical staining demonstrated higher p(Ser473)-AKT and lower FOXO1 levels in endometriosis tissues, compared with normal endometrial tissues.

CONCLUSIONS

In endometriotic stromal cells, overactivation of the phosphatidylinositol 3-kinase/AKT signaling pathway contributes to the reduced expression of the decidua-specific gene, IGFBP1, potentially through reduced levels of nuclear FOXO1.

Role of transcription factor modifications in the pathogenesis of insulin resistance

Non-alcoholic fatty liver disease (NAFLD) is characterized by fat accumulation in the liver not due to alcohol abuse. NAFLD is accompanied by variety of symptoms related to metabolic syndrome. Although the metabolic link between NAFLD and insulin resistance is not fully understood, it is clear that NAFLD is one of the main cause of insulin resistance. NAFLD is shown to affect the functions of other organs, including pancreas, adipose tissue, muscle and inflammatory systems. Currently efforts are being made to understand molecular mechanism of interrelationship between NAFLD and insulin resistance at the transcriptional level with specific focus on post-translational modification (PTM) of transcription factors. PTM of transcription factors plays a key role in controlling numerous biological events, including cellular energy metabolism, cell-cycle progression, and organ development. Cell type- and tissue-specific reversible modifications include lysine acetylation, methylation, ubiquitination, and SUMOylation. Moreover, phosphorylation and O-GlcNAcylation on serine and threonine residues have been shown to affect protein stability, subcellular distribution, DNA-binding affinity, and transcriptional activity. PTMs of transcription factors involved in insulin-sensitive tissues confer specific adaptive mechanisms in response to internal or external stimuli. Our understanding of the interplay between these modifications and their effects on transcriptional regulation is growing. Here, we summarize the diverse roles of PTMs in insulin-sensitive tissues and their involvement in the pathogenesis of insulin resistance.

Deciphering the role of forkhead transcription factors in cancer therapy

Forkhead O transcription factors (FOXO) are critical for the regulation of cell cycle arrest, cell death, and DNA damage repair. Inactivation of FOXO proteins may be associated with tumorigenesis, including breast cancer, prostate cancer, glioblastoma, rhabdomyosarcoma, and leukemia. Accumulated evidence shows that activation of oncogenic pathways such as phosphoinositide-3-kinase/AKT/IKK or RAS/mitogen-activated protein kinase suppresses FOXO transcriptional activity through the phosphorylation of FOXOs at different sites that ultimately leads to nuclear exclusion and degradation of FOXOs. In addition, posttranslational modifications of FOXOs such as acetylation, methylation and ubiquitination also contribute to modulating FOXO3a functions. Several anti-cancer drugs like paclitaxel, imatinib, and doxorubicin activate FOXO3a by counteracting those oncogenic pathways which restrain FOXOs functions. In this review, we will illustrate the regulation of FOXOs and reveal potential therapeutics that target FOXOs for cancer treatment.

FoxO transcription factors; Regulation by AKT and 14-3-3 proteins

The forkhead box O (FoxO) transcription factor family is a key player in an evolutionary conserved pathway downstream of insulin and insulin-like growth factor receptors. The mammalian FoxO family consists of FoxO1, 3, 4 and 6, which share high similarity in their structure, function and regulation. FoxO proteins are involved in diverse cellular and physiological processes including cell proliferation, apoptosis, reactive oxygen species (ROS) response, longevity, cancer and regulation of cell cycle and metabolism. The regulation of FoxO protein function involves an intricate network of posttranslational modifications and protein-protein interactions that provide integrated cellular response to changing physiological conditions and cues. AKT was identified in early genetic and biochemical studies as a main regulator of FoxO function in diverse organisms. Though other FoxO regulatory pathways and mechanisms have been delineated since, AKT remains a key regulator of the pathway. The present review summarizes the current knowledge of FoxO regulation by AKT and 14-3-3 proteins, focusing on its mechanistic and structural aspects and discusses its crosstalk with the other FoxO regulatory mechanisms. This article is part of a Special Issue entitled: PI3K-AKT-FoxO axis in cancer and aging.

The human T-cell leukemia virus type 1 oncoprotein tax controls forkhead box O4 activity through degradation by the proteasome

Activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway by the viral Tax oncoprotein plays a pivotal role in clonal expansion of human T-cell leukemia virus type 1 (HTLV-1)-infected cells. As the Forkhead box O (FoxO) tumor suppressors act as downstream effectors of PI3K/Akt, they represent good candidate targets whose dysregulation by Tax might be involved in HTLV-1-mediated activation and transformation of infected cells. In this report, we provide evidence showing that Tax induces a dose-dependent degradation of FoxO4 by the ubiquitin-proteasome pathway. Consistent with that, we demonstrate that Tax expression increases the interaction between FoxO4 and Mdm2 E3 ligase, leading to a strong FoxO4 polyubiquitination. These processes require the phosphorylation of FoxO4 by Akt, since a mutant of FoxO4 with mutations on its three Akt phosphorylation sites appears to be resistant to Tax-mediated degradation and ubiquitination. In addition, we show that Tax expression is associated with degradation and phosphorylation of endogenous FoxO4 in Jurkat T cells. Finally, we demonstrate that Tax represses FoxO4 transcriptional activity. Our study demonstrates that Tax can control FoxO4 protein stability and transcriptional activity and provides new insight into the subversion of cell signaling pathways during HTLV-1 infection.

The role of mirk kinase in sarcomas

Targeting the tyrosine kinase KIT in gastrointestinal stromal tumors has led to improved treatment. Other kinases might serve as therapeutic targets in the more common forms of sarcoma. The kinase Mirk/dyrk1B is highly expressed in the vast majority of osteosarcomas and rhabdomyosarcomas and mediates their growth, as depletion of Mirk led to tumor cell apoptosis. Mirk is known to increase the expression of a series of antioxidant genes, which scavenge reactive oxygen species (ROS) within various tumor cells, mediating their survival. As a result, depleting Mirk led to increased levels of damaging ROS. Tumor cells depleted of Mirk were also sensitized to low levels of chemotherapeutic drugs that increase ROS levels. In contrast, Mirk expression is quite low in most normal cells, and Mirk depletion or embryonic knockout of Mirk did not detectably affect cell survival. Thus targeting Mirk for intervention in sarcomas might spare most normal tissues.

Dyrk1A Positively Stimulates ASK1-JNK Signaling Pathway during Apoptotic Cell Death

Dual-specificity tyrosine (Y)-phosphorylation-regulated protein kinase 1A (Dyrk1A) is the mammalian homologue of Drosophila melanogaster minibrain and its human gene is mapped to the Down syndrome critical region of chromosome 21. Dyrk1A phosphorylates several transcription factors, including NFAT and CREB and a number of cytosolic proteins such as APP, tau, and α-synuclein. Although Dyrk1A is involved in the control of cell growth and postembryonic neurogenesis, its potential role during cell death and signaling pathway is not clearly understood. In the present study, we show that Dyrk1A is activated under the condition of apoptotic cell death. In addition, Dyrk1A is coupled to JNK1 activation, and directly interacts with apoptosis signal-regulating kinase 1 (ASK1). Moreover, Dyrk1A positively regulates ASK1-mediated JNK1-signaling, and appears to directly phosphorylate ASK1. These data indicate that Dyrk1A regulates cell death through facilitating ASK1-mediated signaling events.

Genome-wide association study identifies single nucleotide polymorphism in DYRK1A associated with replication of HIV-1 in monocyte-derived macrophages

BACKGROUND

HIV-1 infected macrophages play an important role in rendering resting T cells permissive for infection, in spreading HIV-1 to T cells, and in the pathogenesis of AIDS dementia. During highly active anti-retroviral treatment (HAART), macrophages keep producing virus because tissue penetration of antiretrovirals is suboptimal and the efficacy of some is reduced. Thus, to cure HIV-1 infection with antiretrovirals we will also need to efficiently inhibit viral replication in macrophages. The majority of the current drugs block the action of viral enzymes, whereas there is an abundance of yet unidentified host factors that could be targeted. We here present results from a genome-wide association study identifying novel genetic polymorphisms that affect in vitro HIV-1 replication in macrophages.

METHODOLOGY/PRINCIPAL FINDINGS

Monocyte-derived macrophages from 393 blood donors were infected with HIV-1 and viral replication was determined using Gag p24 antigen levels. Genomic DNA from individuals with macrophages that had relatively low (n = 96) or high (n = 96) p24 production was used for SNP genotyping with the Illumina 610 Quad beadchip. A total of 494,656 SNPs that passed quality control were tested for association with HIV-1 replication in macrophages, using linear regression. We found a strong association between in vitro HIV-1 replication in monocyte-derived macrophages and SNP rs12483205 in DYRK1A (p = 2.16 × 10(-5)). While the association was not genome-wide significant (p<1 × 10(-7)), we could replicate this association using monocyte-derived macrophages from an independent group of 31 individuals (p = 0.0034). Combined analysis of the initial and replication cohort increased the strength of the association (p = 4.84 × 10(-6)). In addition, we found this SNP to be associated with HIV-1 disease progression in vivo in two independent cohort studies (p = 0.035 and p = 0.0048).

CONCLUSIONS/SIGNIFICANCE

These findings suggest that the kinase DYRK1A is involved in the replication of HIV-1, in vitro in macrophages as well as in vivo.

Integrating opposing signals toward Forkhead box O

Transcription factors are the common convergence points of signal transduction pathways to affect gene transcription. Signal transduction activity results in posttranslational modification (PTM) of transcription factors and the sum of these modifications at any given time point will determine the action of the transcription factor. It has been suggested that these PTMs provide a transcription factor code analogous to the histone code. However, the number and variety of these modifications and the lack of knowledge in general of their dynamics precludes at present a concise view of how combinations of PTMs affect transcription factor function. Also, a single type of PTM such as phosphorylation can have opposing effects on transcription factor activity. Transcription factors of the Forkhead box O (FOXO) class are predominantly regulated through signaling, by phosphoinositide 3-kinase/protein kinase B (also known as AKT) pathway and a reactive oxygen species/c-Jun N-terminal kinase pathway. Both pathways result in increased FOXO phosphorylation yet with opposing result. Whereas PKB-mediated phosphorylation inactivates FOXO, c-Jun N-terminal kinase-mediated phosphorylation results in activation of FOXO. Here we discuss regulation of FOXO transcription factors by phosphorylation as an example for understanding integration of signal transduction at the level of transcription activity.

MNB/DYRK1A as a multiple regulator of neuronal development

MNB/DYRK1A is a member of the dual-specificity tyrosine phosphorylation-regulated kinase (DYRK) family that has been strongly conserved across evolution. There are substantial data implicating MNB/DYRK1A in brain development and adult brain function, as well as in neurodegeneration and Down syndrome pathologies. Here we review our current understanding of the neurodevelopmental activity of MNB/DYRK1A. We discuss how MNB/DYRK1A fulfils several sequential roles in neuronal development and the molecular mechanisms possibly underlying these functions. We also summarize the evidence behind the hypotheses to explain how the imbalance in MNB/DYRK1A gene dosage might be implicated in the neurodevelopmental alterations associated with Down syndrome. Finally, we highlight some research directions that may help to clarify the mechanisms and functions of MNB/DYRK1A signalling in the developing brain.

Activation, regulation, and inhibition of DYRK1A

Dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) is a protein kinase with diverse functions in neuronal development and adult brain physiology. Higher than normal levels of DYRK1A are associated with the pathology of neurodegenerative diseases and have been implicated in some neurobiological alterations of Down syndrome, such as mental retardation. It is therefore important to understand the molecular mechanisms that control the activity of DYRK1A. Here we review the current knowledge about the initial self-activation of DYRK1A by tyrosine autophosphorylation and propose that this mechanism presents an ancestral feature of the CMGC group of kinases. However, tyrosine phosphorylation does not appear to regulate the enzymatic activity of DYRK1A. Control of DYRK1A may take place on the level of gene expression, interaction with regulatory proteins and regulated nuclear translocation. Finally, we compare the properties of small molecule inhibitors that target DYRK1A and evaluate their potential application and limitations. The β-carboline alkaloid harmine is currently the most selective and potent inhibitor of DYRK1A and has proven very useful in cellular assays.

Direct association of Sprouty-related protein with an EVH1 domain (SPRED) 1 or SPRED2 with DYRK1A modifies substrate/kinase interactions

The mammalian SPRED (Sprouty-related protein with an EVH1 domain) proteins include a family of three members, SPRED1-3. Currently, little is known about their biochemistry. The best described, SPRED1, has been shown to inhibit the Ras/ERK pathway downstream of Ras. All three SPREDs have a cysteine-rich domain (CRD) that has high homology to the CRD of the Sprouty family of proteins, several of which are also Ras/ERK inhibitors. In the belief that binding partners would clarify SPRED function, we assayed for their associated proteins. Here, we describe the direct and endogenous interaction of SPRED1 and SPRED2 with the novel kinase, DYRK1A. DYRK1A has become the subject of recent research focus as it plays a central role in Caenorhabditis elegans oocyte maturation and egg activation, and there is strong evidence that it could be involved in Down syndrome in humans. Both SPRED1 and SPRED2 inhibit the ability of DYRK1A to phosphorylate its substrates, Tau and STAT3. This inhibition occurs via an interaction of the CRD of the SPREDs with the kinase domain of DYRK1A. DYRK1A substrates must bind to the kinase to enable phosphorylation, and SPRED proteins compete for the same binding site to modify this process. Our accumulated evidence indicates that the SPRED proteins are likely physiological modifiers of DYRK1A.

Epigallocatechin gallate-induced modulation of FoxO signaling in mammalian cells and C. elegans: FoxO stimulation is masked via PI3K/Akt activation by hydrogen peroxide formed in cell culture

The green tea flavonoid epigallocatechin gallate (EGCG) is demonstrated in this study to modulate FoxO transcription factors in human skin fibroblasts in culture. EGCG at 1 microM stimulated FoxO transcription factor nuclear accumulation and DNA binding activity. This effect was masked at higher EGCG concentrations (100 microM) by EGCG-derived hydrogen peroxide generated in cell culture media that stimulates phosphoinositide-3'-kinase (PI3K)/Akt signaling to attenuate FoxO activity, involving FoxO phosphorylation, nuclear exclusion and attenuation of DNA binding activity. Like low concentrations of EGCG, harmine, an inhibitor of the FoxO kinase DYRK1a, stimulated FoxO nuclear accumulation and DNA binding activity. Exposure of Caenorhabditis elegans worms to EGCG caused nuclear accumulation of the FoxO ortholog, DAF-16, and enhanced expression of the DAF-16 target gene, sod-3. In line with the role of FoxO/DAF-16 in the control of life span, C. elegans mean and maximum life span were enhanced by 20% and 13%, respectively, by EGCG.

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.

Highly phosphorylated FOXO3A is an adverse prognostic factor in acute myeloid leukemia

PURPOSE

The Forkhead transcription factors (FOXO) are tumor suppressor genes regulating differentiation, metabolism, and apoptosis that functionally interact with signal transduction pathways shown to be deregulated and prognostic in acute myelogenous leukemia (AML). This study evaluated the level of expression and the prognostic relevance of total and phosphorylated FOXO3A protein in AML.

EXPERIMENTAL DESIGN

We used reverse-phase protein array methods to measure the level of total and phosphoprotein expression of FOXO3A, in leukemia-enriched protein samples from 511 newly diagnosed AML patients.

RESULTS

The expression range was similar to normal CD34+ cells and similar in blood and marrow. Levels of total FOXO3A were higher at relapse compared with diagnosis. Levels of pFOXO3A or the ratio of phospho to total (PT) were not associated with karyotpe but were higher in patients with FLT3 mutations. Higher levels of pFOXO3A or PT-FOXO3A were associated with increased proliferation evidenced by strong correlation with higher WBC, percent marrow, and blood blasts and by correlation with higher levels of Cyclins B1, D1 and D3, pGSK3, pMTOR, and pStat5. Patients with High levels of pFOXO3A or PT-FOXO3A had higher rates of primary resistance and shorter remission durations, which combine to cause an inferior survival experience (P = 0.0002). This effect was independent of cytogenetics. PT-FOXO3A was a statistically significant independent predictor in multivariate analysis.

CONCLUSIONS

High levels of phosphorylation of FOXO3A is a therapeutically targetable, independent adverse prognostic factor in AML.

The FoxO family in cardiac function and dysfunction

The Forkhead family of transcription factors mediates many aspects of physiology, including stress response, metabolism, commitment to apoptosis, and development. The Forkhead box subfamily O (FoxO) proteins have garnered particular interest due to their involvement in the modulation of cardiovascular biology. In this review, we discuss the mechanisms of FoxO regulation and outcomes of FoxO signaling under normal and pathological cardiovascular contexts.

Age-associated motor and visuo-spatial learning phenotype in Dyrk1A heterozygous mutant mice

Dual-specificity tyrosine-regulated kinase 1A (DYRK1A) is a candidate gene for the Down syndrome neurological defects and may be involved in the progression of Alzheimer's disease. Heterozygous mice for Dyrk1A (Dyrk1A+/-) exhibit decreased brain size, motor abnormalities and cognitive deficits in the adult. However, there is no information about the mutant phenotype in old ages. Here we analyze the impact of Dyrk1A dosage reduction on motor performance and hippocampal-dependent learning and memory in aged Dyrk1A+/- mice. Whereas motor tests showed marked alterations in traction ability, prehensile reflex and balance, heterozygous mice only present a slight impairment of visuo-spatial memory even though they show a robust decrease of CA1-CA3 and dentate gyrus cells.

Dyrk1A binds to multiple endocytic proteins required for formation of clathrin-coated vesicles

In spite of a nuclear targeting sequence, a substantial amount of dual-specificity tyrosine phosphorylation-regulated kinase (Dyrk1A) is located within the cytoplasm of neurons. Analysis of fractionated rat brains revealed that the majority of Dyrk1A was in the postnuclear precipitate. The kinase in this fraction was resistant to high salt and Triton X-100 extraction at pH 6.5. Hypothesizing that Dyrk1A binds tightly with cell constituents, we searched for Dyrk1A binding proteins in the Triton X-100-insoluble fraction extracted with urea and fractionated by column chromatography. An overlay assay using the recombinant kinase revealed that multiple proteins are capable of binding to Dyrk1A. Among them, we identified clathrin heavy chain and dynamin 1 as potential candidates. An overlay assay using purified and partially purified proteins showed the binding of Dyrk1A with both proteins. Under native conditions, Dyrk1A precipitated with newly formed clathrin cages and with dynamin via the GST-amphiphysin SH3 domain. We also identified another endocytic protein, endophilin 1, as an additional Dyrk1A binding protein. We then tested whether the clathrin-coated vesicle (CCV)-associated proteins could be phosphorylated by Dyrk1A. Multiple proteins apparently distinctive from the known substrates were phosphorylated in the brain CCV. Our findings suggest a role for Dyrk1A in controlling synaptic vesicle recycling processes.

Insulin acts through FOXO3a to activate transcription of plasminogen activator inhibitor type 1

Plasminogen activator inhibitor-1 (PAI-1) is an important regulator of fibrinolysis. PAI-1 levels are elevated in type 2 diabetes, and this elevation correlates with macro- and microvascular complications of diabetes. However, the mechanistic link between insulin and up-regulation of PAI-1 is unclear. Here we demonstrate that overexpression of Forkhead-related transcription factor (Fox)O1, FoxO3a, and FoxC1 augment insulin's ability to activate the PAI-1 promoter. In addition, insulin treatment promotes the phosphorylation of nuclear and cytoplasmic Fox03a and an increase of cytoplasmic Fox03a. In contrast, insulin treatment led to the accumulation of phospho-Fox01 only in the cytoplasm. Furthermore, insulin also increased the ability of chimeric LexA-FoxO1, LexA-FoxO3a, and LexA-FoxC1 proteins to increase the activity of a LexA reporter, suggesting that the effect of insulin on FoxO3a was direct. Using small interfering RNA to specifically deplete each of the Fox transcription factors tested, we demonstrate that only reduction of FoxO3a inhibits insulin-increased PAI-1-Luc expression and PAI-1 mRNA accumulation. Finally, chromatin immunoprecipitation assays confirm the presence of FoxO3a on the PAI-1 promoter. These results suggest that FoxO3a mediates insulin-increased PAI-1 gene expression.

Differential response of glioma cells to FOXO1-directed therapy

Gliomas are the most common adult primary brain tumors, and the most malignant form, glioblastoma multiforme, is invariably fatal. The phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway is altered in most glioblastoma multiforme. PTEN, an important negative regulator of the PI3K-Akt pathway, is also commonly mutated in glioma, leading to constitutive activation of Akt. One ultimate consequence is phosphorylation and inactivation of FOXO forkhead transcription factors that regulate genes involved in apoptosis, cell cycle arrest, nutrient availability, DNA repair, stress, and angiogenesis. We tested the ability of a mutant FOXO1 factor that is not subject to Akt phosphorylation to overcome dysregulated PI3K-Akt signaling in two PTEN-null glioma cell lines, U87 and U251. Adenovirus-mediated gene transfer of the mutant FOXO1 successfully restored cell cycle arrest and induced cell death in vitro and prolonged survival in vivo in xenograft models of human glioma (33% survival at 1 year of animals bearing U251 tumors). However, U87 were much more resistant than U251 to mutant FOXO1-induced death, showing evidence of increased nuclear export and Akt-independent phosphorylation of FOXO1 at S249. A cyclin-dependent kinase 2 inhibitor decreased phosphorylation of S249 and rendered U87 cells significantly more susceptible to mutant FOXO1-induced death. Our results indicate that targeting FOXO1, which is at the convergence point of several growth factor receptor tyrosine kinase pathways, can effectively induce glioma cell death and inhibit tumor growth. They also highlight the importance of Akt-independent phosphorylation events in the nuclear export of FOXO1.

DYRK1A is a novel negative regulator of cardiomyocyte hypertrophy

Activation of the phosphatase calcineurin and its downstream targets, transcription factors of the NFAT family, results in cardiomyocyte hypertrophy. Recently, it has been shown that the dual specificity tyrosine (Y) phosphorylation-regulated kinase 1A (DYRK1A) is able to antagonize calcineurin signaling by directly phosphorylating NFATs. We thus hypothesized that DYRK1A might modulate the hypertrophic response of cardiomyocytes. In a model of phenylephrine-induced hypertrophy, adenovirus-mediated overexpression of DYKR1A completely abrogated the hypertrophic response and significantly reduced the expression of the natriuretic peptides ANF and BNP. Furthermore, DYRK1A blunted cardiomyocyte hypertrophy induced by overexpression of constitutively active calcineurin and attenuated the induction of the hypertrophic gene program. Conversely, knockdown of DYRK1A, utilizing adenoviruses encoding for a specific synthetic miRNA, resulted in an increase in cell surface area accompanied by up-regulation of ANF- mRNA. Similarly, treatment of cardiomyocytes with harmine, a specific inhibitor of DYRK1A, revealed cardiomyocyte hypertrophy on morphological and molecular level. Moreover, constitutively active calcineurin led to robust induction of an NFAT-dependent luciferase reporter, whereas DYRK1A attenuated calcineurin-induced reporter activation in cardiomyocytes. Conversely, both knockdown and pharmacological inhibition of DYRK1A significantly augmented the effect of calcineurin in this assay. In summary, we identified DYRK1A as a novel negative regulator of cardiomyocyte hypertrophy. Mechanistically, this effect appears to be mediated via inhibition of NFAT transcription factors.

Novel benzylidene-thiazolidine-2,4-diones inhibit Pim protein kinase activity and induce cell cycle arrest in leukemia and prostate cancer cells

The Pim protein kinases play important roles in cancer development and progression, including prostate tumors and hematologic malignancies. To investigate the potential role of these enzymes as anticancer drug targets, we have synthesized novel benzylidene-thiazolidine-2,4-diones that function as potent Pim protein kinase inhibitors. With IC(50) values in the nanomolar range, these compounds block the ability of Pim to phosphorylate peptides and proteins in vitro and, when added to DU145 prostate cancer cells overexpressing Pim, inhibit the ability of this enzyme to phosphorylate a known substrate, the BH(3) protein BAD. When added to prostate cancer cell lines, including PC3, DU145, and CWR22Rv1, and human leukemic cells, MV4;11, K562, and U937 cells, these compounds induce G(1)-S cell cycle arrest and block the antiapoptotic effect of the Pim protein kinase. The cell cycle arrest induced by these compounds is associated with an inhibition of cyclin-dependent kinase 2 and activity and translocation of the Pim-1 substrate p27(Kip1), a cyclin-dependent kinase 2 inhibitory protein, to the nucleus. Furthermore, when added to leukemic cells, these compounds synergize with the mammalian target of rapamycin inhibitor rapamycin to decrease the phosphorylation level of the translational repressor 4E-BP1 at sites phosphorylated by mammalian target of rapamycin. Combinations of rapamycin and the benzylidene-thiazolidine-2,4-diones synergistically block the growth of leukemic cells. Thus, these agents represent novel Pim inhibitors and point to an important role for the Pim protein kinases in cell cycle control in multiple types of cancer cells.

Green tea polyphenols rescue of brain defects induced by overexpression of DYRK1A

Individuals with partial HSA21 trisomies and mice with partial MMU16 trisomies containing an extra copy of the DYRK1A gene present various alterations in brain morphogenesis. They present also learning impairments modeling those encountered in Down syndrome. Previous MRI and histological analyses of a transgenic mice generated using a human YAC construct that contains five genes including DYRK1A reveal that DYRK1A is involved, during development, in the control of brain volume and cell density of specific brain regions. Gene dosage correction induces a rescue of the brain volume alterations. DYRK1A is also involved in the control of synaptic plasticity and memory consolidation. Increased gene dosage results in brain morphogenesis defects, low BDNF levels and mnemonic deficits in these mice. Epigallocatechin gallate (EGCG) - a member of a natural polyphenols family, found in great amount in green tea leaves - is a specific and safe DYRK1A inhibitor. We maintained control and transgenic mice overexpressing DYRK1A on two different polyphenol-based diets, from gestation to adulthood. The major features of the transgenic phenotype were rescued in these mice.

FGF15/FGFR4 integrates growth factor signaling with hepatic bile acid metabolism and insulin action

The current studies show FGF15 signaling decreases hepatic forkhead transcription factor 1 (FoxO1) activity through phosphatidylinositol (PI) 3-kinase-dependent phosphorylation. The bile acid receptor FXR (farnesoid X receptor) activates expression of fibroblast growth factor (FGF) 15 in the intestine, which acts through hepatic FGFR4 to suppress cholesterol-7alpha hydroxylase (CYP7A1) and limit bile acid production. Because FoxO1 activity and CYP7A1 gene expression are both increased by fasting, we hypothesized CYP7A1 might be a FoxO1 target gene. Consistent with recently reported results, we show CYP7A1 is a direct target of FoxO1. Additionally, we show that the PI 3-kinase pathway is key for both the induction of CYP7A1 by fasting and the suppression by FGF15. FGFR4 is the major hepatic FGF receptor isoform and is responsible for the hepatic effects of FGF15. We also show that expression of FGFR4 in liver was decreased by fasting, increased by insulin, and reduced by streptozotocin-induced diabetes, implicating FGFR4 as a primary target of insulin regulation. Because insulin and FGF both target the PI 3-kinase pathway, these observations suggest FoxO1 is a key node in the convergence of FGF and insulin signaling pathways and functions as a key integrator for the regulation of glucose and bile acid metabolism.

Effect of hyperhomocysteinemia on the protein kinase DYRK1A in liver of mice

Hyperhomocysteinemia due to cystathionine beta synthase (CBS)-deficiency confers diverse clinical manifestations, notably liver diseases. Even if hyperhomocysteinemia in liver of CBS-deficient mice, a murine model of hyperhomocysteinemia, promotes mitochondrial oxidative stress and pro-apoptotic signals, protective signals may counteract these pro-apoptotic signals, leading to chronic inflammation. As DYRK1A, a serine/threonine kinase, has been described as a candidate antiapoptotic factor, we have analyzed the expression of DYRK1A in liver of CBS-deficient mice. We found that DYRK1A protein level was reduced in liver of CBS-deficient mice, which was not observed at the gene expression level. Moreover, the use of primary hepatocytes/Kupffer cells co-culture showed that degradation of DYRK1A induced by hyperhomocysteinemia requires calpain activation. Our results demonstrate a deleterious effect of hyperhomocysteinemia on DYRK1A protein expression, and emphasize the role of hyperhomocysteinemia on calpain activation.

New insights for FOXO and cell-fate decision in HIV infection and HIV associated neurocognitive disorder

Human immunodeficiency virus Type 1 (HIV-1) infection and associated diseases continue to represent major health problem worldwide. FOXO transcriptional factors play an important role in the regulation of cell apoptosis, cell cycle arrest, stress resistance, metabolism and differentiation. This chapter will discuss the diverse functions of FOXO in different cell types including T-cells, macrophages, neurons and astrocytes within the context of HIV-1 infection. Given the overwhelming evidence that FOXO proteins influence the cell fate of immune cells and involve in the homeostasis of the central nervous system (CNS), we will also discuss the potential role of FOXO factors in HIV-1-associated neurological disorders.