DAF-16 recruits the CREB-binding protein coactivator complex to the insulin-like growth factor binding protein 1 promoter in HepG2 cells

Silent information regulator 2 potentiates Foxo1-mediated transcription through its deacetylase activity

Longevity regulatory genes include the Forkhead transcription factor FOXO and the NAD-dependent histone deacetylase silent information regulator 2 (Sir2). Genetic studies demonstrate that Sir2 acts to extend lifespan in Caenorhabditis elegans upstream of DAF-16, a member of the FOXO family, in the insulin-like signaling pathway. However, the molecular mechanisms underlying the requirement of DAF-16 activity in Sir2-mediated longevity remain unknown. Here we show that reversible acetylation of Foxo1 (also known as FKHR), the mouse DAF-16 ortholog, modulates its transactivation function. cAMP-response element-binding protein (CREB)-binding protein binds and acetylates Foxo1 at the K242, K245, and K262 residues, the modification of which is involved in the attenuation of Foxo1 as a transcription factor. Conversely, Sir2 binds and deacetylates Foxo1 at residues acetylated by cAMP-response element-binding protein-binding protein. Sir2 is recruited to insulin response sequence-containing promoter and increases the expression of manganese superoxide dismutase and p27(kip1) in a deacetylase-activity-dependent manner. Our findings establish Foxo1 as a direct and functional target for Sir2 in mammalian systems.

FOXO4 is acetylated upon peroxide stress and deacetylated by the longevity protein hSir2(SIRT1)

FOXO transcription factors have important roles in metabolism, cellular proliferation, stress tolerance, and aging. FOXOs are negatively regulated by protein kinase B/c-Akt-mediated phosphorylation. Here we show that FOXO factors are also subject to regulation by reversible acetylation. We provide evidence that the acetyltransferase CREB-binding protein (CBP) binds FOXO resulting in acetylation of FOXO. This acetylation inhibits FOXO transcriptional activity. Binding of CBP and acetylation are induced after treatment of cells with peroxide stress. Deacetylation of FOXOs involves binding of the NAD-dependent deacetylase hSir2(SIRT1). Accordingly, hSir2(SIRT1)-mediated deacetylation precludes FOXO inhibition through acetylation and thereby prolongs FOXO-dependent transcription of stress-regulating genes. These data demonstrate that acetylation functions in a second pathway of negative control for FOXO factors and provides a novel mechanism whereby hSir2(SIRT1) can promote cellular survival and increase lifespan.

Mammalian SIRT1 represses forkhead transcription factors

The NAD-dependent deacetylase SIR2 and the forkhead transcription factor DAF-16 regulate lifespan in model organisms, such as yeast and C. elegans. Here we show that the mammalian SIR2 ortholog SIRT1 deacetylates and represses the activity of the forkhead transcription factor Foxo3a and other mammalian forkhead factors. This regulation appears to be in the opposite direction from the genetic interaction of SIR2 with forkhead in C. elegans. By restraining mammalian forkhead proteins, SIRT1 also reduces forkhead-dependent apoptosis. The inhibition of forkhead activity by SIRT1 parallels the effect of this deacetylase on the tumor suppressor p53. We speculate how down-regulating these two classes of damage-responsive mammalian factors may favor long lifespan under certain environmental conditions, such as calorie restriction.

Foxo Transcription Factors Induce the Atrophy-Related Ubiquitin Ligase Atrogin-1 and Cause Skeletal Muscle Atrophy

Skeletal muscle atrophy is a debilitating response to fasting, disuse, cancer, and other systemic diseases. In atrophying muscles, the ubiquitin ligase, atrogin-1 (MAFbx), is dramatically induced, and this response is necessary for rapid atrophy. Here, we show that in cultured myotubes undergoing atrophy, the activity of the PI3K/AKT pathway decreases, leading to activation of Foxo transcription factors and atrogin-1 induction. IGF-1 treatment or AKT overexpression inhibits Foxo and atrogin-1 expression. Moreover, constitutively active Foxo3 acts on the atrogin-1 promoter to cause atrogin-1 transcription and dramatic atrophy of myotubes and muscle fibers. When Foxo activation is blocked by a dominant-negative construct in myotubes or by RNAi in mouse muscles in vivo, atrogin-1 induction during starvation and atrophy of myotubes induced by glucocorticoids are prevented. Thus, forkhead factor(s) play a critical role in the development of muscle atrophy, and inhibition of Foxo factors is an attractive approach to combat muscle wasting.

Protein kinase B-alpha inhibits human pyruvate dehydrogenase kinase-4 gene induction by dexamethasone through inactivation of FOXO transcription factors

Starvation and diabetes increase pyruvate dehydrogenase kinase-4 (PDK4) expression, which conserves gluconeogenic substrates by inactivating the pyruvate dehydrogenase complex. Mechanisms that regulate PDK4 gene expression, previously established to be increased by glucocorticoids and decreased by insulin, were studied. Treatment of HepG2 cells with dexamethasone increases the relative abundance of PDK4 mRNA, and insulin blocks this effect. Dexamethasone also increases human PDK4 (hPDK4) promoter activity in HepG2 cells, and insulin partially inhibits this effect. Expression of constitutively active PKB alpha abrogates dexamethasone stimulation of hPDK4 promoter activity, while coexpression of constitutively active FOXO1a or FOXO3a, which are mutated to alanine at the three phosphorylation sites for protein kinase B (PKB), disrupts the ability of PKB alpha to inhibit promoter activity. A glucocorticoid response element for glucocorticoid receptor (GR) binding and three insulin response sequences (IRSs) that bind FOXO1a and FOXO3a are identified in the hPDK4 promoter. Mutation of the IRSs reduces the ability of glucocorticoids to stimulate PDK4 transcription. Transfection studies with E1A, which binds to and inactivates p300/CBP, suggest that interactions between p300/CBP and GR as well as FOXO factors are important for glucocorticoid-stimulated hPDK4 expression. Insulin suppresses the hPDK4 induction by glucocorticoids through inactivation of the FOXO factors.

Characterization of cis-elements mediating the stimulation of glucose-6-phosphate transporter promoter activity by glucocorticoids

The endoplasmatic glucose-6-phosphate transporter is involved in the control of hepatic glucose production and blood glucose homeostasis. In this study, the expression of a luciferase reporter gene under the control of the glucose-6-phosphate transporter gene promoter was examined in transiently transfected hepatoma cells. The promoter activity was stimulated approximately 2.5-fold by dexamethasone. Mutational analyses demonstrated that the regions nucleotide (nt) -215/-209 and nt -197/-183 relative to the translation start site were critical for this regulation. In gel electrophoretic mobility shift assays the transcription factor Fox O1, also called forkhead in rhabdomyosarcoma (FKHR), overexpressed in 293 cells, bound to a probe with the sequence nt -215/-209. The overexpression of Fox O1 stimulated the induction of glucose-6-phosphate transporter promoter activity by dexamethasone via nt -215/-209 in hepatoma cells. Recombinant glucocorticoid receptor DNA binding domain protein bound to a probe with the sequence of nt -197/-183 in gel electrophoretic mobility shift assays and an oligonucleotide with this sequence transferred glucocorticoid responsiveness to a heterologous promoter. The data indicate that the glucose-6-phosphate transporter promoter contains a glucocorticoid response unit consisting of binding sites for Fox O1 and the glucocorticoid receptor.

Convergence of peroxisome proliferator-activated receptor gamma and Foxo1 signaling pathways

The forkhead factor Foxo1 (or FKHR) was identified in a yeast two-hybrid screen as a peroxisome proliferator-activated receptor (PPAR) gamma-interacting protein. Foxo1 antagonized PPARgamma activity and vice versa indicating that these transcription factors functionally interact in a reciprocal antagonistic manner. One mechanism by which Foxo1 antagonizes PPARgamma activity is through disruption of DNA binding as Foxo1 inhibited the DNA binding activity of a PPARgamma/retinoid X receptor alpha heterodimeric complex. The Caenorhabditis elegans nuclear hormone receptor, DAF-12, interacted with the C. elegans forkhead factor, DAF-16, paralleling the interaction between PPARgamma and Foxo1. daf-12 and daf-16 have been implicated in C. elegans insulin-like signaling pathways, and PPARgamma and Foxo1 likewise have been linked to mammalian insulin signaling pathways. These results suggest a convergence of PPARgamma and Foxo1 signaling that may play a role in insulin action and the insulinomimetic properties of PPARgamma ligands. A more general convergence of nuclear hormone receptor and forkhead factor pathways may be important for multiple biological processes and this convergence may be evolutionarily conserved.

Structure and developmental expression of Strongyloides stercoralis fktf-1, a proposed ortholog of daf-16 in Caenorhabditis elegans

A forkhead transcription factor gene, fktf-1, which we propose to be orthologous to the Caenorhabditis elegans dauer-regulatory gene daf-16 has been discovered in the parasitic nematode Strongyloides stercoralis. Genomic and cDNA sequences from both species predict alternately spliced a and b message isoforms. In contrast to C. elegans, where two a isoforms, daf-16a1 and daf-16a2, are found, a single fktf-1a isoform is found in S. stercoralis. Five of the 10 introns found in the C. elegans gene are found in the proposed S. stercoralis ortholog. Functional motifs common to DAF-16 and several mammalian forkhead transcription factors are conserved in FKTF-1. These include the forkhead DNA binding domain, four Akt/protein kinase B phosphorylation sites and a C-terminal domain that may associate with factors such as the steroid receptor coactivator and other factors necessary for transcriptional regulation. An N-terminal serine-rich domain found in DAF-16A is greatly expanded in FKTF-1A. This domain is missing in DAF-16B, FKTF-1B and all mammalian orthologs. FKTF-1 shows the closest phylogenetic relationship to DAF-16 among all known mammalian and nematode forkhead transcription factors. Like its proposed Caenorhabditis ortholog, the fktf-1 message is expressed at all stages of the life cycle examined thus far. Discovery of fktf-1 indicates the presence of an insulin-like signalling pathway in S. stercoralis similar to that known to regulate dauer development in C. elegans. This pathway is a likely candidate to control infective larval arrest and reactivation as well as regulation of the switch between parasitic and free-living development in the parasite.

FOXO transcription factors as regulators of immune homeostasis: molecules to die for?

Regulation of phosphatidylinositol 3-kinase (PI3K) activity has been demonstrated to be critical for correct lymphocyte function. The molecular targets of this lipid kinase have been the subject of extensive research, and many functional effects of PI3K activation are thought to be mediated by the serine-threonine kinase protein kinase B (PKB/c-akt). Genetic analyses in the nematode worm Caenorhabditis elegans have identified a novel PI3K-regulated signaling pathway that regulates organism lifespan through inhibition of a Forkhead (FOX) transcription factor, DAF-16. Recent studies have subsequently revealed an evolutionarily conserved signaling module in higher eukaryotes in which PKB can directly phosphorylate and inactive a family of Forkhead box class O (FOXO) transcription factors. Phosphorylation results in nuclear exclusion and inhibition of transcription. FOXO transcription factors have been found to play critical roles in regulation of proliferation, apoptosis and control of oxidative stress. This occurs through both activation and repression of target gene expression by multiple mechanisms. Here the regulation and function of these transcription factors is discussed with specific relevance to immune homeostasis. A greater understanding of the regulation and function of this signaling pathway in lymphocytes may provide novel therapeutic opportunities for immune diseases.

Control of cell number by Drosophila FOXO: downstream and feedback regulation of the insulin receptor pathway

The Drosophila insulin receptor (dInR) regulates cell growth and proliferation through the dPI3K/dAkt pathway, which is conserved in metazoan organisms. Here we report the identification and functional characterization of the Drosophila forkhead-related transcription factor dFOXO, a key component of the insulin signaling cascade. dFOXO is phosphorylated by dAkt upon insulin treatment, leading to cytoplasmic retention and inhibition of its transcriptional activity. Mutant dFOXO lacking dAkt phosphorylation sites no longer responds to insulin inhibition, remains in the nucleus, and is constitutively active. dFOXO activation in S2 cells induces growth arrest and activates two key players of the dInR/dPI3K/dAkt pathway: the translational regulator d4EBP and the dInR itself. Induction of d4EBP likely leads to growth inhibition by dFOXO, whereas activation of dInR provides a novel transcriptionally induced feedback control mechanism. Targeted expression of dFOXO in fly tissues regulates organ size by specifying cell number with no effect on cell size. Our results establish dFOXO as a key transcriptional regulator of the insulin pathway that modulates growth and proliferation.

Characterization of insulin inhibition of transactivation by a C-terminal fragment of the forkhead transcription factor Foxo1 in rat hepatoma cells

The transcription factor Foxo1 controls the expression of genes involved in fundamental cellular processes. In keeping with its important physiological roles, Foxo1 activity is negatively regulated in response to growth factors and cytokines that activate a phosphatidylinositol 3-kinase (PI 3-kinase) protein kinase B (PKB)/Akt pathway. PKB/Akt-mediated phosphorylation of Foxo1 has been shown to result in the inhibition of target gene transcription and to trigger the export of Foxo1 from the nucleus, which is generally believed to explain the subsequent decrease of transcription. In the present study, using a chimeric protein in which a C-terminal fragment of Foxo1 (amino acids 208-652) containing the transactivation domain is fused to the yeast Gal4 DNA binding domain, we present evidence showing that insulin can directly regulate transactivation by Foxo1 in H4IIE rat hepatoma cells. Insulin inhibition of Foxo1-(208-652)-stimulated transactivation is mediated by PI 3-kinase but in contrast to full-length Foxo1, does not require either of the two PKB/Akt phosphorylation sites (Ser253 and Ser316) present in the protein fragment. Using mutational and deletion studies, we identify two potential phosphorylation sites, Ser319 and Ser499, as well as a 15-amino acid region located between residues 350 and 364 that are critical for insulin inhibition of transactivation by Foxo1-(208-652). We conclude that the transcriptional activity of Foxo1 is regulated at different levels by insulin: transactivation, as well as DNA binding and nuclear exclusion. These different regulatory mechanisms allow the precise control of transcription of Foxo1 target genes by insulin.

Decisions on life and death: FOXO Forkhead transcription factors are in command when PKB/Akt is off duty

Forkhead transcription factors of the FOXO family are important downstream targets of protein kinase B (PKB)/Akt, a kinase shown to play a decisive role in cell proliferation and cell survival. Direct phosphorylation by PKB/Akt inhibits transcriptional activation by FOXO factors, causing their displacement from the nucleus into the cytoplasm. Work from recent years has shown that this family of transcription factors regulates the expression of a number of genes that are crucial for the proliferative status of a cell, as well as a number of genes involved in programmed cell death. As such, these transcription factors appear to play an essential role in many of the effects of PKB/Akt on cell proliferation and survival. Indeed, in cells of the hematopoietic system, mere activation of a FOXO factor is sufficient to activate a variety of proapoptotic genes and to trigger apoptosis. In contrast, in most other cell types, activation of FOXO blocks cellular proliferation and drives cells into a quiescent state. In such cell types, FOXO factors also provide the protective mechanisms that are required to adapt to the altered metabolic state of quiescent cells. Thus, as PKB/Akt signaling is switched off, FOXO factors take over to determine the fate of a cell, long-term survival in a quiescent state, or programmed cell death. This review summarizes our current understanding of the mechanisms by which PKB/Akt and FOXO factors regulate these decisions.

Akt modulates STAT3-mediated gene expression through a FKHR (FOXO1a)-dependent mechanism

The phosphatidylinositol 3-kinase/Akt pathway plays an important role in the signaling of insulin and other growth factors, which reportedly attenuate the interleukin-6 (IL-6)-mediated stimulation of acute phase plasma protein genes. We investigated the effect of the protein kinase Akt on IL-6-mediated transcriptional activation. The transient expression of constitutively active Akt inhibited the IL-6-dependent activity of the alpha(2)-macroglobulin promoter in HepG2 cells, whereas expression of an inactive mutant of phosphatidylinositol-dependent kinase 1 had the opposite effect. Since Akt is known to regulate gene expression through inactivation of the transcription factor FKHR (forkhead in rhabdomyosarcoma), we examined the effect of FKHR on STAT3-mediated transcriptional regulation. Indeed, the overexpression of FKHR specifically enhanced the activity of STAT3-dependent promoters but not that of a STAT5-responsive promoter. The effect of FKHR required the presence of functional STAT3 and was abrogated by the expression of dominant negative STAT3 mutants. Furthermore, FKHR and STAT3 were shown to coimmunoprecipitate and to colocalize in the nuclear regions of IL-6-treated HepG2 cells. Our results indicate that FKHR can modulate the IL-6-induced transcriptional activity by acting as a coactivator of STAT3.

A forkhead transcription factor FKHR up-regulates lipoprotein lipase expression in skeletal muscle

Lipoprotein lipase (LPL) plays a role in lipid usage in skeletal muscle by hydrolyzing plasma triglycerides into fatty acids, which are further utilized for beta-oxidation. Lipid usage is stimulated during fasting, diabetes mellitus and exercise, concomitant with enhanced LPL expression in skeletal muscle. Here we show that the forkhead type transcription factor FKHR is strongly induced in skeletal muscle in fasting mice, in mice with streptozotocin-induced diabetes and in mice after treadmill running. Ectopic expression of FKHR enhanced LPL gene expression in C2C12 muscle cells in culture. These results implicate FKHR as an important modulator of lipid metabolism in skeletal muscle.

DYRK1 is a co-activator of FKHR (FOXO1a)-dependent glucose-6-phosphatase gene expression

Expression of glucose-6-phosphatase (G6Pase), one of the rate-limiting enzymes of hepatic gluconeogenesis, has recently been shown to be transactivated by the transcription factor FKHR. One of the proteins known to directly interact with FKHR is the nuclear protein kinase DYRK1A. In order to study the effects of DYRK1A on G6Pase gene expression, we generated a H4IIEC3 rat hepatoma cell line stably expressing DYRK1A by retroviral infection. Overexpression of DYRK1A increased the expression of G6Pase about threefold, as determined by Northern blotting. In transiently transfected HepG2 cells, co-expression of DYRK1A and a G6Pase promoter construct increased G6Pase promoter activity about twofold. This effect of DYRK1A was independent of its kinase activity, since a kinase-dead DYRK1A mutant as well as a point mutant of the phosphorylation site of DYRK1A in FKHR (Ser329Ala) failed to affect the effect of DYRK1A on the G6Pase expression. The effect of DYRK on the G6Pase promoter activity was produced by the isoforms DYRK1A and DYRK1B, which are localized in the nucleus, but not by DYRK2. Mutations of the FKHR-binding sites in the G6Pase promoter markedly reduced the effect of DYRK1 on the G6Pase promoter activity. In summary, the data suggest that DYRK1 is a specific co-activator of FKHR, independent of its kinase activity.

Common mechanism for oncogenic activation of MLL by forkhead family proteins

The mixed lineage leukemia (MLL) gene undergoes fusions with a diverse set of genes as a consequence of chromosomal translocations in acute leukemias. Two of these partner genes code for members of the forkhead subfamily of transcription factors designated FKHRL1 and AFX. We demonstrate here that MLL-FKHRL1 enhances the self-renewal of murine myeloid progenitors in vitro and induces acute myeloid leukemias in syngeneic mice. The long latency (mean = 157 days), reduced penetrance, and hematologic features of the leukemias were very similar to those observed for the forkhead fusion protein MLL-AFX and contrasted with the more aggressive features of leukemias induced by MLL-AF10. Transformation mediated by MLL-forkhead fusion proteins required 2 conserved transcriptional effector domains (CR2 and CR3), each of which alone was not sufficient to activate MLL. A synthetic fusion of MLL with FKHR, a third mammalian forkhead family member that contains both effector domains, was also capable of transforming hematopoietic progenitors in vitro. A comparable requirement for 2 distinct transcriptional effector domains was also displayed by VP16, which required its proximal minimal transactivation domain (MTD/H1) and distal H2 domain to activate the oncogenic potential of MLL. The functional importance of CR2 was further demonstrated by its ability to substitute for H2 of VP16 in domain-swapping experiments to confer oncogenic activity on MLL. Our results, based on bona fide transcription factors as partners for MLL, unequivocally establish a transcriptional effector mechanism to activate its oncogenic potential and further support a role for fusion partners in determining pathologic features of the leukemia phenotype.

Human immunodeficiency virus type 1 (HIV-1) accessory protein Vpr induces transcription of the HIV-1 and glucocorticoid-responsive promoters by binding directly to p300/CBP coactivators

The accessory Vpr protein of human immunodeficiency virus type 1 (HIV-1) is a promiscuous activator of viral and cellular promoters. We report that Vpr enhances expression of the glucocorticoid receptor-induced mouse mammary tumor virus (MMTV) promoter and of the Tat-induced HIV-1 long terminal repeat promoter by directly binding to p300/CBP coactivators. In contrast, Vpr does not bind to p/CAF or to members of the p160 family of nuclear receptor coactivators, such as steroid receptor coactivator 1a and glucocorticoid receptor (GR)-interacting protein 1. Vpr forms a stable complex with p300 and also interacts with the ligand-bound glucocorticoid receptor in vivo. Mutation analysis showed that the C-terminal part of Vpr binds to the C-terminal portion of p300/CBP within amino acids 2045 to 2191. The same p300 region interacts with the p160 coactivators and with the adenovirus E1A protein. Accordingly, E1A competed for binding to p300 in vitro. Coexpression of E1A or of small fragments of p300 containing the Vpr binding site resulted in inhibition of Vpr's transcriptional effects. The C-terminal part of p300 containing the transactivating region is required for Vpr transactivation, whereas the histone acetyltransferase enzymatic region is dispensable. Vpr mutants that bind p300 but not the GR did not activate expression of the MMTV promoter and had dominant-negative effects. These results indicate that Vpr activates transcription by acting as an adapter linking transcription components and coactivators.

Regulation of insulin action and pancreatic beta-cell function by mutated alleles of the gene encoding forkhead transcription factor Foxo1

Type 2 diabetes results from impaired action and secretion of insulin. It is not known whether the two defects share a common pathogenesis. We show that haploinsufficiency of the Foxo1 gene, encoding a forkhead transcription factor (forkhead box transcription factor O1), restores insulin sensitivity and rescues the diabetic phenotype in insulin-resistant mice by reducing hepatic expression of glucogenetic genes and increasing adipocyte expression of insulin-sensitizing genes. Conversely, a gain-of-function Foxo1 mutation targeted to liver and pancreatic beta-cells results in diabetes arising from a combination of increased hepatic glucose production and impaired beta-cell compensation due to decreased Pdx1 expression. These data indicate that Foxo1 is a negative regulator of insulin sensitivity in liver, adipocytes and pancreatic beta-cells. Impaired insulin signaling to Foxo1 provides a unifying mechanism for the common metabolic abnormalities of type 2 diabetes.NOTE: In the AOP version of this article, the name of the fourth author was misspelled as W K Cavanee rather than the correct spelling: W K Cavenee. This has been corrected in the full-text online version of the article. The name will appear correctly in the print version.

MLL-AFX requires the transcriptional effector domains of AFX to transform myeloid progenitors and transdominantly interfere with forkhead protein function

MLL-AFX is a fusion gene created by t(X;11) chromosomal translocations in a subset of acute leukemias of either myeloid or lymphoid derivation. It codes for a chimeric protein consisting of MLL fused to AFX, a forkhead transcription factor that normally regulates genes involved in apoptosis and cell cycle progression. We demonstrate here that forced expression of MLL-AFX enhances the self-renewal of hematopoietic progenitors in vitro and induces acute myeloid leukemias after long latencies in syngeneic recipient mice. MLL-AFX interacts with the transcriptional coactivator CBP, which is also a fusion partner for MLL in human leukemias. A potent minimal transactivation domain (CR3) at the C terminus of AFX mediates interactions with the KIX domain of CBP and is necessary for transformation of myeloid progenitors by MLL-AFX. However, CR3 alone is not sufficient, suggesting that simple acquisition of a transactivation domain per se does not activate the oncogenic potential of MLL. Rather, two conserved transcriptional effector domains (CR2 and CR3) of AFX are required for full oncogenicity of MLL-AFX and also endow it with the potential to competitively interfere with transcription and apoptosis mediated by wild-type forkhead proteins. Furthermore, a dominant-negative mutant of AFX containing CR2 and CR3 enhances the growth of myeloid progenitors in vitro, although considerably less effectively than does MLL-AFX. Taken together, these data suggest that recruitment of transcriptional cofactors utilized by forkhead proteins is a critical requirement for oncogenic action of MLL-AFX, which may impact both MLL- and forkhead-dependent transcriptional pathways.

Interference between the PHA-4 and PEB-1 transcription factors in formation of the Caenorhabditis elegans pharynx

PHA-4 is a forkhead/winged helix transcription factor that acts as an organ identity factor in the development of the Caenorhabditis elegans pharynx. PEB-1 is a novel DNA-binding protein also involved in pharyngeal morphogenesis. PHA-4 and PEB-1 bind at overlapping sites on the C183 sequence element that controls pharynx-specific expression of the C. elegans myo-2 gene. It has been suggested that PHA-4 and PEB-1 act cooperatively on the C183 sequence. In this study, we test this model and assess the C183-dependent transcriptional activity of PHA-4 and PEB-1, both individually and in combination. We show that PHA-4 and PEB-1 are both modest transcriptional activators in yeast but that co-expression of the two factors does not result in significantly increased expression of a C183-regulated reporter gene. Electrophoretic mobility-shift assays provide no evidence for the formation of a PHA-4/PEB-1 complex in vitro but rather show that PHA-4 and PEB-1 cannot bind C183 simultaneously. As we have reported previously, ectopic expression of PHA-4 in C. elegans causes ectopic expression of a C183-regulated reporter gene. We show that ectopic expression of PEB-1 cannot cause ectopic expression of the same reporter but rather ectopic PEB-1 inhibits reporter gene activation by PHA-4. Overall, our results do not support a model in which PHA-4 and PEB-1 synergize in vivo but rather support a model in which PEB-1 may negatively modulate PHA-4's ability to activate transcription through C183 during formation of the C. elegans pharynx.

The transcription factors GATA4 and dHAND physically interact to synergistically activate cardiac gene expression through a p300-dependent mechanism

An intricate array of heterogeneous transcription factors participate in programming tissue-specific gene expression through combinatorial interactions that are unique to a given cell-type. The zinc finger-containing transcription factor GATA4, which is widely expressed in mesodermal and endodermal derived tissues, is thought to regulate cardiac myocyte-specific gene expression through combinatorial interactions with other semi-restricted transcription factors such as myocyte enhancer factor 2, nuclear factor of activated T-cells, serum response factor, and Nkx2.5. Here we determined that GATA4 also interacts with the cardiac-expressed basic helix-loop-helix transcription factor dHAND (also known as HAND2). GATA4 and dHAND synergistically activated expression of cardiac-specific promoters from the atrial natriuretic factor gene, the b-type natriuretic peptide gene, and the alpha-myosin heavy chain gene. Using artificial reporter constructs this functional synergy was shown to be GATA site-dependent, but E-box site-independent. A mechanism for the transcriptional synergy was suggested by the observation that the bHLH domain of dHAND physically interacted with the C-terminal zinc finger domain of GATA4 forming a higher order complex. This transcriptional synergy observed between GATA4 and dHAND was associated with p300 recruitment, but not with alterations in DNA binding activity of either factor. Moreover, the bHLH domain of dHAND directly interacted with the CH3 domain of p300 suggesting the existence of a higher order complex between GATA4, dHAND, and p300. Taken together with previous observations, these results suggest the existence of an enhanceosome complex comprised of p300 and multiple semi-restricted transcription factors that together specify tissue-specific gene expression in the heart.

Cell cycle and death control: long live Forkheads

The FOXO family of Forkhead transcription factors, FKHR (FOXO1), FKHR-L1 (FOXO3a) and AFX (FOXO4), are regulated by the phosphoinositide-3-kinase-protein-kinase-B (PI3K-PKB/c-Akt) pathway. Direct phosphorylation by PKB results in cytoplasmic retention and inactivation, inhibiting the expression of FOXO-regulated genes, which control the cell cycle, cell death, cell metabolism and oxidative stress. This pathway appears to be well conserved throughout evolution. In the nematode Caenorhabditis elegans, it affects lifespan and controls dauer formation. Recent discoveries about FOXO regulation by PI3K-PKB signalling suggest that the PI3K-PKB-FOXO pathway might participate in similar processes in higher eukaryotes.

Cyclic AMP-induced forkhead transcription factor, FKHR, cooperates with CCAAT/enhancer-binding protein beta in differentiating human endometrial stromal cells

Decidual transformation of human endometrial stromal (ES) cells requires sustained activation of the protein kinase A (PKA) pathway. In a search for novel transcriptional mediators of this process, we used differential display PCR analysis of undifferentiated primary ES cells and cells stimulated with 8-bromo-cAMP (8-Br-cAMP). We now report on the role of forkhead homologue in rhabdomyosarcoma (FKHR), a recently described member of the forkhead/winged-helix transcription factor family, as a mediator of endometrial differentiation. Sustained 8-Br-cAMP stimulation resulted in the induction and nuclear accumulation of FKHR in differentiating ES cells. Immunohistochemical studies revealed that endometrial stromal expression of FKHR in vivo is confined to decidualizing cells during the late secretory phase of the cycle and coincides with the expression of CCAAT/enhancer-binding protein beta (C/EBPbeta). Reporter gene studies showed that FKHR potently enhances PKA-dependent activation of the tissue-specific decidual prolactin (dPRL) promoter, a major differentiation marker in human ES cells. Transcriptional augmentation by FKHR was effected through functional cooperation with C/EBPbeta and binding to a composite FKHR-C/EBPbeta response unit in the proximal promoter region. Furthermore, FKHR and C/EBPbeta were shown to interact directly in a glutathione S-transferase pull-down assay. These results provide the first evidence of regulated expression of FKHR and demonstrate that FKHR has an integral role in PKA-dependent endometrial differentiation through its ability to bind and functionally cooperate with C/EBPbeta.

Phosphorylation of forkhead transcription factors by erythropoietin and stem cell factor prevents acetylation and their interaction with coactivator p300 in erythroid progenitor cells

The mammalian forkhead transcription factors, FOXO3a (FKHRL1), FOXO1a (FKHR) and FOXO4 (AFX) are negatively regulated by PKB/Akt kinase. In the present study we examined the engagement of forkhead family of transcription factors in erythropoietin (Epo)- and stem cell factor (SCF)-mediated signal transduction. Our data show that all three forkhead family members, FOXO3a, FOXO1a and FOXO4 are phosphorylated in human primary erythroid progenitors. Experiments performed to determine various upstream signaling pathways contributing to phosphorylation of forkhead family members show that only PI-3-kinase pathway is required for inactivation of FOXO3a. Our data also demonstrate that during Epo deprivation FOXO3a interacts with the transcriptional coactivator p300 and such interaction is disrupted by stimulation of cells with Epo. To determine the domains in FOXO3a, mediating its interaction with p300, we performed GST pull-down assays and found that the N-terminus region containing the first 52 amino acids was sufficient for binding p300. Finally, our data demonstrate that FOXO3a and FOXO1a are acetylated during growth factor deprivation and such acetylation is reversed by stimulation with Epo. Thus mammalian forkhead transcription factors are involved in Epo and SCF signaling in primary erythroid progenitors and may play a role in the induction of apoptotic and mitogenic signals.

The genetics of aging

The genetic analysis of life span has only begun in mammals, invertebrates, such as Caenorhabditis elegans and Drosophila, and yeast. Even at this primitive stage of the genetic analysis of aging, the physiological observations that rate of metabolism is intimately tied to life span is supported. In many examples from mice to worms to flies to yeast, genetic variants that affect life span also modify metabolism. Insulin signaling regulates life span coordinately with reproduction, metabolism, and free radical protective gene regulation in C. elegans. This may be related to the findings that caloric restriction also regulates mammalian aging, perhaps via the modulation of insulin-like signaling pathways. The nervous system has been implicated as a key tissue where insulin-like signaling and free radical protective pathways regulate life span in C. elegans and Drosophila. Genes that determine the life span could act in neuroendocrine cells in diverse animals. The involvement of insulin-like hormones suggests that the plasticity in life spans evident in animal phylogeny may be due to variation in the timing of release of hormones that control vitality and mortality as well as variation in the response to those hormones. Pedigree analysis of human aging may reveal variations in the orthologs of the insulin pathway genes and coupled pathways that regulate invertebrate aging. Thus, genetic approaches may identify a set of circuits that was established in ancestral metazoans to regulate their longevity.

Distinct and overlapping functions of insulin and IGF-I receptors

Targeted gene mutations have established distinct, yet overlapping, developmental roles for receptors of the insulin/IGF family. IGF-I receptor mediates IGF-I and IGF-II action on prenatal growth and IGF-I action on postnatal growth. Insulin receptor mediates prenatal growth in response to IGF-II and postnatal metabolism in response to insulin. In rodents, unlike humans, insulin does not participate in embryonic growth until late gestation. The ability of the insulin receptor to act as a bona fide IGF-II-dependent growth promoter is underscored by its rescue of double knockout Igf1r/Igf2r mice. Thus, IGF-II is a true bifunctional ligand that is able to stimulate both insulin and IGF-I receptor signaling, although with different potencies. In contrast, the IGF-II/cation-independent mannose-6-phosphate receptor regulates IGF-II clearance. The growth retardation of mice lacking IGF-I and/or insulin receptors is due to reduced cell number, resulting from decreased proliferation. Evidence from genetically engineered mice does not support the view that insulin and IGF receptors promote cellular differentiation in vivo or that they are required for early embryonic development. The phenotypes of insulin receptor gene mutations in humans and in mice indicate important differences between the developmental roles of insulin and its receptor in the two species.

The forkhead transcription factor Foxo1 (Fkhr) confers insulin sensitivity onto glucose-6-phosphatase expression

Type 2 diabetes is characterized by the inability of insulin to suppress glucose production in the liver and kidney. Insulin inhibits glucose production by indirect and direct mechanisms. The latter result in transcriptional suppression of key gluconeogenetic and glycogenolytic enzymes, phosphoenolpyruvate carboxykinase (Pepck) and glucose-6-phosphatase (G6p). The transcription factors required for this effect are incompletely characterized. We report that in glucogenetic kidney epithelial cells, Pepck and G6p expression are induced by dexamethasone (dex) and cAMP, but fail to be inhibited by insulin. The inability to respond to insulin is associated with reduced expression of the forkhead transcription factor Foxo1, a substrate of the Akt kinase that is inhibited by insulin through phosphorylation. Transduction of kidney cells with recombinant adenovirus encoding Foxo1 results in insulin inhibition of dex/cAMP-induced G6p expression. Moreover, expression of dominant negative Foxo1 mutant results in partial inhibition of dex/cAMP-induced G6p and Pepck expression in primary cultures of mouse hepatocyes and kidney LLC-PK1-FBPase(+) cells. These findings are consistent with the possibility that Foxo1 is involved in insulin regulation of glucose production by mediating the ability of insulin to decrease the glucocorticoid/cAMP response of G6p.

The forkhead transcription factor Foxo1 (Fkhr) confers insulin sensitivity onto glucose-6-phosphatase expression

Type 2 diabetes is characterized by the inability of insulin to suppress glucose production in the liver and kidney. Insulin inhibits glucose production by indirect and direct mechanisms. The latter result in transcriptional suppression of key gluconeogenetic and glycogenolytic enzymes, phosphoenolpyruvate carboxykinase (Pepck) and glucose-6-phosphatase (G6p). The transcription factors required for this effect are incompletely characterized. We report that in glucogenetic kidney epithelial cells, Pepck and G6p expression are induced by dexamethasone (dex) and cAMP, but fail to be inhibited by insulin. The inability to respond to insulin is associated with reduced expression of the forkhead transcription factor Foxo1, a substrate of the Akt kinase that is inhibited by insulin through phosphorylation. Transduction of kidney cells with recombinant adenovirus encoding Foxo1 results in insulin inhibition of dex/cAMP-induced G6p expression. Moreover, expression of dominant negative Foxo1 mutant results in partial inhibition of dex/cAMP-induced G6p and Pepck expression in primary cultures of mouse hepatocyes and kidney LLC-PK1-FBPase(+) cells. These findings are consistent with the possibility that Foxo1 is involved in insulin regulation of glucose production by mediating the ability of insulin to decrease the glucocorticoid/cAMP response of G6p.

Gene- and activation-specific mechanisms for insulin inhibition of basal and glucocorticoid-induced insulin-like growth factor binding protein-1 and phosphoenolpyruvate carboxykinase transcription. Roles of forkhead and insulin response sequences

The insulin response sequence (IRS) of the phosphoenolpyruvate carboxykinase (PEPCK) promoter, located within the glucocorticoid response unit, was first characterized by its ability to mediate insulin inhibition when inserted into a thymidine kinase promoter. The IRSs of the PEPCK and insulin-like growth factor binding protein-1 (IGFBP-1) promoters have been proposed to contribute to regulation by glucocorticoids and insulin. Forkhead (FKHR) recognizes IRS sequences, is phosphorylated in response to insulin, and mediates insulin inhibition of basal IGFBP-1 transcription in an IRS-dependent manner. Here, we investigate the contributions of FKHR and IRSs to insulin inhibition of basal and glucocorticoid-induced transcription of PEPCK and IGFBP-1. Expression of T/S/S, in which three putative protein kinase B (PKB) sites in FKHR are mutated, reduced insulin inhibition of basal expression of IGFBP-1 but not PEPCK. Mutation of the IGFBP-1 IRSs abolished insulin inhibition in the presence of T/S/S. Mutation of the PEPCK IRS had no effect on insulin inhibition in the presence of T/S/S, indicating that insulin inhibits PEPCK transcription independently of the IRS or of the putative PKB phosphorylation sites in FKHR. Mutations in the IRS or FKHR had no effect on insulin inhibition of glucocorticoid-induced transcription of either the PEPCK or IGFBP-1 gene. Thus, insulin uses gene- and activation-specific mechanisms to regulate the basal and glucocorticoid-induced activity of these genes.

Differential regulation of endogenous glucose-6-phosphatase and phosphoenolpyruvate carboxykinase gene expression by the forkhead transcription factor FKHR in H4IIE-hepatoma cells

The insulin responsive H4IIEC3 rat hepatoma cell line (H4 cells) was used in order to determine the role of the transcription factor FKHR in the regulation of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). Both PEPCK and G6Pase contain putative FKHR binding sites in their promoter sequence. Using a retroviral expression system, we stably overexpressed FKHR in H4-cells. FKHR was phosphorylated in a PI 3-kinase- and Akt-dependent manner, and was translocated from the nucleus to the cytoplasm in response to insulin. Furthermore, overexpression of FKHR markedly increased the expression of the catalytic subunit of G6Pase (basal about 2.5-fold, dexamethasone/cAMP stimulated about fivefold, respectively). In contrast, both basal and dexamethasone/cAMP-induced levels of PEPCK mRNA were unaffected by FKHR-overexpression. These data suggest a specific function for FKHR in the regulation of hepatic gluconeogenesis at the level of G6Pase, but not PEPCK gene expression.

Phosphatidylinositol 3-kinase signaling inhibits DAF-16 DNA binding and function via 14-3-3-dependent and 14-3-3-independent pathways

In Caenorhabditis elegans, an insulin-like signaling pathway to phosphatidylinositol 3-kinase (PI 3-kinase) and AKT negatively regulates the activity of DAF-16, a Forkhead transcription factor. We show that in mammalian cells, C. elegans DAF-16 is a direct target of AKT and that AKT phosphorylation generates 14-3-3 binding sites and regulates the nuclear/cytoplasmic distribution of DAF-16 as previously shown for its mammalian homologs FKHR and FKHRL1. In vitro, interaction of AKT- phosphorylated DAF-16 with 14-3-3 prevents DAF-16 binding to its target site in the insulin-like growth factor binding protein-1 gene, the insulin response element. In HepG2 cells, insulin signaling to PI 3-kinase/AKT inhibits the ability of a GAL4 DNA binding domain/DAF-16 fusion protein to activate transcription via the insulin-like growth factor binding protein-1-insulin response element, but not the GAL4 DNA binding site, which suggests that insulin inhibits the interaction of DAF-16 with its cognate DNA site. Elimination of the DAF-16/1433 association by mutation of the AKT/14-3-3 sites in DAF-16, prevents 14-3-3 inhibition of DAF-16 DNA binding and insulin inhibition of DAF-16 function. Similarly, inhibition of the DAF-16/14-3-3 association by exposure of cells to the PI 3-kinase inhibitor LY294002, enhances DAF-16 DNA binding and transcription activity. Surprisingly constitutively nuclear DAF-16 mutants that lack AKT/14-3-3 binding sites also show enhanced DNA binding and transcription activity in response to LY294002, pointing to a 14-3-3-independent mode of regulation. Thus, our results demonstrate at least two mechanisms, one 14-3-3-dependent and the other 14-3-3-independent, whereby PI 3-kinase signaling regulates DAF-16 DNA binding and transcription function.

Insulin inhibits transcription of IRS-2 gene in rat liver through an insulin response element (IRE) that resembles IREs of other insulin-repressed genes

Recent data indicate that sustained elevations in plasma insulin suppress the mRNA for IRS-2, a component of the insulin signaling pathway in liver, and that this deficiency contributes to hepatic insulin resistance and inappropriate gluconeogenesis. Here, we use nuclear run-on assays to show that insulin inhibits transcription of the IRS-2 gene in the livers of intact rats. Insulin also inhibited transcription of a reporter gene driven by the human IRS-2 promoter that was transfected into freshly isolated rat hepatocytes. The human promoter contains a heptanucleotide sequence, TGTTTTG, that is identical to the insulin response element (IRE) identified previously in the promoters of insulin-repressed genes. Single base pair substitutions in this IRE decreased transcription of the IRS-2-driven reporter in the absence of insulin and abolished insulin-mediated repression. We conclude that insulin represses transcription of the IRS-2 gene by blocking the action of a positive factor that binds to the IRE. Sustained repression of IRS-2, as occurs in chronic hyperinsulinemia, contributes to hepatic insulin resistance and accelerates the development of the diabetic state.

Insulin suppresses transactivation by CAAT/enhancer-binding proteins beta (C/EBPbeta). Signaling to p300/CREB-binding protein by protein kinase B disrupts interaction with the major activation domain of C/EBPbeta

CAAT/enhancer-binding proteins (C/EBPs) play an important role in the regulation of gene expression in insulin-responsive tissues. We have found that a complex containing C/EBPbeta interacts with an insulin response sequence in the insulin-like growth factor-binding protein-1 (IGFBP-1) gene and that a C/EBP-binding site can mediate effects of insulin on promoter activity. Here, we examined mechanisms mediating this effect of insulin. The ability of insulin to suppress promoter activity via a C/EBP-binding site is blocked by LY294002, a phosphatidylinositol 3-kinase inhibitor, but not by rapamycin, which blocks activation of p70(S6 kinase). Dominant negative phosphatidylinositol 3-kinase and protein kinase B (PKB) block the effect of insulin, while activated PKB suppresses promoter function via a C/EBP-binding site, mimicking the effect of insulin. Coexpression studies indicate that insulin and PKB suppress transactivation by C/EBPbeta, but not C/EBPalpha, and that N-terminal transactivation domains in C/EBPbeta are required. Studies with Gal4 fusion proteins reveal that insulin and PKB suppress transactivation by the major activation domain in C/EBPbeta (AD II), located between amino acids 31 and 83. Studies with E1A protein indicate that interaction with p300/CBP is required for transactivation by AD II and the effect of insulin and PKB. Based on a consensus sequence, we identified a PKB phosphorylation site (Ser(1834)) within the region of p300/CBP known to bind C/EBPbeta. Mammalian two-hybrid studies indicate that insulin and PKB disrupt interactions between this region of p300 and AD II and that Ser(1834) is critical for this effect. Signaling by PKB and phosphorylation of Ser(1834) may play an important role in modulating interactions between p300/CBP and transcription factors and mediate effects of insulin and related growth factors on gene expression.