Characterization of elements mediating regulation of phosphoenolpyruvate carboxykinase gene transcription by protein kinase A and insulin. Identification of a distinct complex formed in cells that mediate insulin inhibition

Gluconeogenesis in cancer cells - Repurposing of a starvation-induced metabolic pathway?

Cancer cells constantly face a fluctuating nutrient supply and interference with adaptive responses might be an effective therapeutic approach. It has been discovered that in the absence of glucose, cancer cells can synthesize crucial metabolites by expressing phosphoenolpyruvate carboxykinase (PEPCK, PCK1 or PCK2) using abbreviated forms of gluconeogenesis. Gluconeogenesis, which in essence is the reverse pathway of glycolysis, uses lactate or amino acids to feed biosynthetic pathways branching from glycolysis. PCK1 and PCK2 have been shown to be critical for the growth of certain cancers. In contrast, fructose-1,6-bisphosphatase 1 (FBP1), a downstream gluconeogenesis enzyme, inhibits glycolysis and tumor growth, partly by non-enzymatic mechanisms. This review sheds light on the current knowledge of cancer cell gluconeogenesis and its role in metabolic reprogramming, cancer cell plasticity, and tumor growth.

Endotoxin-Induced IL-6 Promoter Activation in Skeletal Muscle Requires an NF-κB Site

Previous studies in monocytes and other cell types have provided evidence of a role for the NF-κB pathway in IL-6 induction. The purpose of the present study was to examine the involvement of NF-κB in the induction of the IL-6 promoter in skeletal muscle cells by endotoxin (LPS), TNFα or IL-1α. Transfection of C2C12 mouse myocytes with a luciferase reporter under the control of the IL-6 promoter indicated each immunomodulator enhanced IL-6 promoter activity. Mutation and inhibitor studies indicate this response was dependent on the IL-6 NF-κB binding site, but independent of NF-IL6, AP-1, CREB or C/EBP. Cotransfection with an expression vector which constitutively activates the RelA pathway increased IL-6 promoter activity, and activity could not be further enhanced by cytokines or LPS. However, cotransfecting various dominant negative upstream NF-κB kinase expression vectors which inhibited RelA or RelB pathways either individually or in combination had no effect on LPS-induced activation of the IL-6 promoter, but abolished induction from a NF-κB-based promoter. This lack of effect was not due to a lack of NF-κB pathway activation in C2C12 myocytes because both Western analysis and EMSA supershifting showed an LPS-induced increase in nuclear RelA and RelA phosphorylation. However, another protein was observed bound to the IL-6 NF-κB site that does not bind to a consensus NF-κB site. The present findings provide novel insights regarding inflammation-induced stimulation of IL-6 promoter activity in skeletal muscle which is an important but non-traditional component of the innate immune system.

The 3′-untranslated region of the Ste20-like kinase SLK regulates SLK expression

Ste20-like kinase, SLK, a germinal center kinase found in kidney epithelial cells, signals to promote apoptosis. Expression of SLK mRNA and protein and kinase activity are increased during kidney development and recovery from ischemic acute renal failure. The 3′-untranslated region (3′-UTR) of SLK mRNA contains multiple adenine and uridine-rich elements, suggesting that 3′-UTR may regulate mRNA stability. This was confirmed in COS cell transient transfection studies, which showed that expression of the SLK open-reading frame plus 3′-UTR mRNA was reduced by 35% relative to the open-reading frame alone. To further characterize the SLK-3′-UTR, this nucleotide sequence was subcloned downstream of enhanced green fluorescent protein (EGFP) cDNA. In COS, 293T, and glomerular epithelial cells, expression of EGFP mRNA and protein was markedly reduced in the presence of the SLK-3′-UTR. After transfection and subsequent addition of actinomycin D, EGFP mRNA remained stable in cells for at least 6 h, whereas EGFP-SLK-3′-UTR mRNA decayed with a half-life of ∼4 h. A region containing five AUUUA motifs within the SLK-3′-UTR destabilized EGFP mRNA. Deletion of this region from the SLK-3′-UTR, in part, restored mRNA stability. By UV cross-linking and SDS-PAGE, the SLK-3′-UTR bound to protein(s) of ∼30 kDa in extracts of COS cells, glomerular epithelial cells, and kidney. Cotransfection of HuR (a RNA binding protein of ∼30 kDa) increased the steady-state mRNA level of EGFP-SLK-3′-UTR but not EGFP. Thus the SLK-3′-UTR may interact with kidney RNA-binding proteins to regulate expression of SLK mRNA during kidney development and after ischemic injury.

Comparative acute effects of leptin and insulin on gluconeogenesis and ketogenesis in perfused rat liver

The acute effects of physiological levels of leptin (10 ng ml(-1)) and insulin (20 microU ml(-1)) on hepatic gluconeogenesis and ketogenesis were compared. Leptin or insulin alone decreased (p<0.05) the activation of hepatic glucose, L-lactate and urea production from L-alanine. However, the hepatic glucose production was not modified if leptin was combined with insulin. These results indicated that both, i.e. leptin and insulin, could promote a non-additive reduction in the rate of catabolism of L-alanine. However, in contrast with insulin (p<0.05), leptin did not inhibit the activation of hepatic glucose production from pyruvate or glycerol. On the other hand, activation of hepatic production of acetoacetate and beta-hydroxybutyrate from octanoate was not affected by leptin or insulin. Thus, our data demonstrate that the acute effect of leptin on hepatic metabolism was partially similar to insulin (activation of glucose production from L-alanine and activation of acetoacetate or beta-hydroxybutyrate production from octanoate) and partially different from insulin (activation of glucose production from pyruvate or glycerol).

3',5'-cyclic adenosine monophosphate response element-binding protein and CCAAT enhancer-binding protein are dispensable for insulin inhibition of phosphoenolpyruvate carboxykinase transcription and for its synergistic induction by protein kinase A and glucocorticoids

Phosphoenolpyruvate carboxykinase (PEPCK) transcription is induced by cAMP/protein kinase A (PKA) and glucocorticoids [dexamethasone (Dex)] and is inhibited by insulin to regulate blood glucose. Recent reports suggested that CCAAT enhancer binding protein (C/EBP) binding to the PEPCK cAMP response element (CRE) plays a role in Dex induction and that insulin-induces inhibitory forms of C/EBPbeta to inhibit transcription. Here, we assessed the roles of CRE-binding protein (CREB) and C/EBP factors in mediating hormone-regulated transcription. Neither cAMP nor insulin regulated the phosphorylation of C/EBP. Cycloheximide did not block insulin inhibition, indicating that alternate translation of C/EBPbeta is not required. Dominant-negative CREB or C/EBP blocked induction by PKA, but neither affected regulation by Dex or insulin. Tethering the activation domains of CREB or C/EBP to a CRE-->Gal4 (G4) site mediated varying extents of basal and PKA-inducible activity, but neither activation domain affected induction by Dex or inhibition by insulin. Surprisingly, synergistic induction by PKA and Dex did not require the CRE and was unaffected by dominant-negative CREB or C/EBP. PKA and Dex also synergistically induced a minimal 3 x glucocorticoid response element promoter, but inhibited Dex induction of the mouse mammary tumor virus and IGF-binding protein 1 promoters, even though PKA alone did not regulate these promoters. These results suggest that PKA modifies the activity of other factors involved in Dex induction to mediate synergistic induction or inhibition in a promoter-specific manner. Our data indicate that the roles of CREB and C/EBP are restricted to mediating PEPCK induction by PKA, and that other factors mediate PEPCK induction by Dex, synergism between PKA and Dex, and inhibition by insulin.

Hepatic nuclear factor 3 and nuclear factor 1 regulate 5-aminolevulinate synthase gene expression and are involved in insulin repression

Although the negative regulation of gene expression by insulin has been widely studied, the transcription factors responsible for the insulin effect are still unknown. The purpose of this work was to explore the molecular mechanisms involved in the insulin repression of the 5-aminolevulinate synthase (ALAS) gene. Deletion analysis of the 5'-regulatory region allowed us to identify an insulin-responsive region located at -459 to -354 bp. This fragment contains a highly homologous insulin-responsive (IRE) sequence. By transient transfection assays, we determined that hepatic nuclear factor 3 (HNF3) and nuclear factor 1 (NF1) are necessary for an appropriate expression of the ALAS gene. Insulin overrides the HNF3beta or HNF3beta plus NF1-mediated stimulation of ALAS transcriptional activity. Electrophoretic mobility shift assay and Southwestern blotting indicate that HNF3 binds to the ALAS promoter. Mutational analysis of this region revealed that IRE disruption abrogates insulin action, whereas mutation of the HNF3 element maintains hormone responsiveness. This dissociation between HNF3 binding and insulin action suggests that HNF3beta is not the sole physiologic mediator of insulin-induced transcriptional repression. Furthermore, Southwestern blotting assay shows that at least two polypeptides other than HNF3beta can bind to ALAS promoter and that this binding is dependent on the integrity of the IRE. We propose a model in which insulin exerts its negative effect through the disturbance of HNF3beta binding or transactivation potential, probably due to specific phosphorylation of this transcription factor by Akt. In this regard, results obtained from transfection experiments using kinase inhibitors support this hypothesis. Due to this event, NF1 would lose accessibility to the promoter. The posttranslational modification of HNF3 would allow the binding of a protein complex that recognizes the core IRE. These results provide a potential mechanism for the insulin-mediated repression of IRE-containing promoters.

Mechanisms of basal and kinase-inducible transcription activation by CREB

The cAMP response element (CRE)-binding protein (CREB) stimulates basal transcription of CRE-containing genes and mediates induction of transcription upon phosphorylation by protein kinases. The basal activity of CREB maps to a carboxy-terminal constitutive activation domain (CAD), whereas phosphorylation and inducibility map to a central, kinase-inducible domain (KID). The CAD interacts with and recruits the promoter recognition factor TFIID through an interaction with a specific TATA-binding-protein-associated factor (TAF), dTAFII110/ hTAFII135. Interaction between the TAF and the CAD is mediated by a central cluster of hydrophobic amino acids, mutation of which disrupts TAF binding, polymerase recruitment, and transcription activation. Assessment of the contributions of the CAD and KID to recruitment of the polymerase complex versus enhancement of subsequent reaction steps (isomerization, promoter clearance, and reinitiation) showed that the CAD and P-KID act in a concerted mechanism to stimulate transcription. The CAD, but not the KID, mediated recruitment of a complex containing components of a transcription initiation complex, including pol II, IIB, and IID. However, the CAD was relatively ineffective in stimulating subsequent steps in the reaction mechanism. In contrast, phosphorylation of the KID in CREB effectively stimulated isomerization of the recruited polymerase complex and multiple-round transcription. A model for the activation of transcription by phosphorylated CREB is proposed, in which the polymerase is recruited by interaction of the CAD with TFIID and the recruited polymerase is activated further by phosphorylation of the KID in CREB.

Short-term leptin-dependent inhibition of hepatic gluconeogenesis is mediated by insulin receptor substrate-2

Leptin has both insulin-like and insulin-antagonistic effects on glucose metabolism. To test whether leptin interferes directly with insulin signaling, we perfused isolated rat livers with leptin (0.1, 0.5, 5, and 25 nmol/liter), leptin + insulin (5 nmol/liter + 10 nmol/liter), insulin (10 nmol/liter), or vehicle (control). Leptin reduced L-lactate-(10 mmol/liter)-stimulated glucose production by 39-66% (P < 0.006 vs. control) and phosphoenolpyruvate carboxykinase (PEPCK) activity by 22-52% (P < 0.001). Physiological leptin concentrations (0.1-5 nmol/liter) stimulated the tyrosine phosphorylation (pY) of insulin receptor substrate-2 (IRS-2) (280-954%; P < 0.05) and its associated phosphatidylinositol-3 kinase activity (122-621%; P < 0.003). Leptin (0.5-25 nmol/liter) inhibited IRS-1 pY and its associated phosphatidylinositol-3 kinase activity (20-89%; P < 0.03) but stimulated janus kinase-2 pY (272-342%; P < 0.001). Leptin also down-regulated its short receptor isoform in a time- and concentration-dependent manner (28-54%; P < 0.05). Exposure to leptin + insulin additively reduced glucose production and PEPCK activity (approximately 50%; P < 0.001 vs. control) and doubled IRS-2 pY (P < 0.01 vs. insulin). However, leptin + insulin decreased IRS-1 pY by 57% (P < 0.01 vs. insulin). Insulin alone (P < 0.01), but not leptin, increased autophosphorylation of nonreceptor tyrosine kinases (pp59(Lyn) + pp125(Fak)). In conclusion, leptin both alone and in combination with insulin reduces hepatic glucose production by decreasing the synthesis of the key enzyme of gluconeogenesis, PEPCK, which results mainly from the stimulation of the IRS-2 pathway.

The role of C/EBP in nutrient and hormonal regulation of gene expression

C/EBPs are a family of transcription factors that play important roles in energy metabolism. Although initially thought to be constitutive regulators of transcription, an increasing amount of evidence indicates that their transactivating capacity within the cell can be modulated by nutrients and hormones. There are several mechanisms whereby this occurs. First, hormones/nutrients are known to directly alter the expression of C/EBPs. Second, hormones/nutrients may cause an alteration in the phosphorylation state of C/EBPs, which can affect their DNA-binding activity or transactivating capacity. Third, C/EBPs can function as accessory factors on gene promoters within a hormone response unit, interacting with other transcription factors to enhance the degree of responsiveness to specific hormones. Given their role in regulating genes involved in a wide variety of metabolic events, advancing our understanding of the molecular mechanism of action of C/EBPs will undoubtedly further our appreciation for the role these transcription factors play in both health and disease.

Phosphatidylinositol 3-kinase and Ras/mitogen-activated protein kinase signaling pathways are required for the regulation of 5-aminolevulinate synthase gene expression by insulin

Insulin regulates the expression of several hepatic genes. Although the general definition of insulin signaling has progressed dramatically, the elucidation of the complete signaling pathway from insulin receptor to transcription factors involved in the regulation of a specific gene remains to be established. In fact, recent works suggest that multiple divergent insulin signaling pathways regulate the expression of distinct genes. 5-Aminolevulinate synthase (ALAS) is a mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of heme biosynthesis. It has been reported that insulin caused the rapid inhibition of housekeeping ALAS transcription, but the mechanism involved in this repression has not been explored. The present study investigates the role of phosphatidylinositol 3-kinase (PI3-kinase) and mitogen-activated protein kinase pathways in insulin signaling relevant to ALAS inhibition. To explore this, we combined the transient overexpression of regulatory proteins involved in these pathways and the use of small cell permeant inhibitors in rat hepatocytes and HepG2 cells. Wortmannin and LY294002, PI3-kinase inhibitors, as well as lovastatin and PD152440, Ras farnesylation inhibitors, and MEK inhibitor PD98059 abolished the insulin repression of ALAS transcription. The inhibitor of mTOR/p70(S6K) rapamycin had no effect whatsoever upon hormone action. The overexpression of vectors encoding constitutively active Ras, MEK, or p90(RSK) mimicked the inhibitory action of insulin. Conversely, negative mutants of PKB, Ras, or MEK impaired insulin inhibition of ALAS promoter activity. Furthermore, inhibition of one of the pathways blocks the inhibitory effect produced by the activation of the other. Our findings suggest that factors involved in two signaling pathways that are often considered to be functionally separate during insulin action, the Ras/ERK/p90(RSK) pathway and the PI3K/PKB pathway, are jointly required for insulin-mediated inhibition of ALAS gene expression in rat hepatocytes and human hepatoma cells.

Selective tonic inhibition of G-6-Pase catalytic subunit, but not G-6-P transporter, gene expression by insulin in vivo

The regulation of glucose-6-phosphatase (G-6-Pase) catalytic subunit and glucose 6-phosphate (G-6-P) transporter gene expression by insulin in conscious dogs in vivo and in tissue culture cells in situ were compared. In pancreatic-clamped, euglycemic conscious dogs, a 5-h period of hypoinsulinemia led to a marked increase in hepatic G-6-Pase catalytic subunit mRNA; however, G-6-P transporter mRNA was unchanged. In contrast, a 5-h period of hyperinsulinemia resulted in a suppression of both G-6-Pase catalytic subunit and G-6-P transporter gene expression. Similarly, insulin suppressed G-6-Pase catalytic subunit and G-6-P transporter gene expression in H4IIE hepatoma cells. However, the magnitude of the insulin effect was much greater on G-6-Pase catalytic subunit gene expression and was manifested more rapidly. Furthermore, cAMP stimulated G-6-Pase catalytic subunit expression in H4IIE cells and in primary hepatocytes but had no effect on G-6-P transporter expression. These results suggest that the relative control strengths of the G-6-Pase catalytic subunit and G-6-P transporter in the G-6-Pase reaction are likely to vary depending on the in vivo environment.

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.

The homeodomain proteins PBX and MEIS1 are accessory factors that enhance thyroid hormone regulation of the malic enzyme gene in hepatocytes

Triiodothyronine (T3) stimulates a robust increase (>40-fold) in transcription of the malic enzyme gene in chick embryo hepatocytes. Previous work has shown that optimal T3 regulation of malic enzyme transcription is dependent on the presence of an accessory element (designated as region E) that immediately flanks a cluster of five T3 response elements in the malic enzyme gene. Here, we have analyzed the binding of nuclear proteins to region E and investigated the mechanism by which region E enhances T3 responsiveness. In nuclear extracts from hepatocytes, region E binds heterodimeric complexes consisting of the homeodomain proteins PBX and MEIS1. Region E contains four consecutive PBX/MEIS1 half-sites. PBX-MEIS1 heterodimers bind the first and second half-sites, the third and fourth half-sites, and the first and fourth half-sites. The configuration conferring the greatest increase in T3 responsiveness consists of the first and fourth half-sites that are separated by 7 nucleotides. Stimulation of T3 response element functions by region E does not require the presence of additional malic enzyme sequences. In pull-down experiments, PBX1a and PBX1b specifically bind the nuclear T3 receptor-alpha, and this interaction is enhanced by the presence of T3. A T3 receptor-alpha region containing the DNA binding domain plus flanking sequences (amino acids 21-157) is necessary and sufficient for binding to PBX1a and PBX1b. These results indicate that PBX-MEIS1 complexes interact with nuclear T3 receptors to enhance T3 regulation of malic enzyme transcription in hepatocytes.

A nucleoprotein complex containing CCAAT/enhancer-binding protein beta interacts with an insulin response sequence in the insulin-like growth factor-binding protein-1 gene and contributes to insulin-regulated gene expression

Highly related insulin response sequences (IRSs) mediate effects of insulin on the expression of multiple genes in the liver, including insulin-like growth factor binding protein-1 (IGFBP-1) and phosphoenolpyruvate carboxykinase (PEPCK). Gel shift studies reveal that oligonucleotide probes containing an IRS from the IGFBP-1 or PEPCK gene form a similar complex with hepatic nuclear proteins. Unlabeled competitors containing the IGFBP-1 or PEPCK IRS or a binding site for C/EBP proteins inhibit the formation of this complex. Antibody against C/EBPbeta (but not other C/EBP proteins) supershifts this complex, and Western blotting of affinity purified proteins confirms that C/EBPbeta is present in this complex. Studies with affinity purified and recombinant protein indicate that C/EBPbeta does not interact directly with the IRS, but that other factors are required. Gel shift assays and reporter gene studies with constructs containing point mutations within the IRS reveal that the ability to interact with factors required for the formation of this complex correlates well with the ability of insulin to regulate promoter activity via this IRS (r = 0.849, p < 0.01). Replacing the IRS in reporter gene constructs with a C/EBP-binding site (but not an HNF-3/forkhead site or cAMP response element) maintains the effect of insulin on promoter activity. Together, these findings indicate that a nucleoprotein complex containing C/EBPbeta interacts with IRSs from the IGFBP-1 and PEPCK genes in a sequence-specific fashion and may contribute to the ability of insulin to regulate gene expression.

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.

Regulation of glucose-6-phosphatase gene expression by protein kinase Balpha and the forkhead transcription factor FKHR. Evidence for insulin response unit-dependent and -independent effects of insulin on promoter activity

Glucose-6-phosphatase plays an important role in the regulation of hepatic glucose production, and insulin suppresses glucose-6-phosphatase gene expression. Recent studies indicate that protein kinase B and Forkhead proteins contribute to insulin-regulated gene expression in the liver. Here, we examined the role of protein kinase B and Forkhead proteins in mediating effects of insulin on glucose-6-phosphatase promoter activity. Transient transfection studies with reporter gene constructs demonstrate that insulin suppresses both basal and dexamethasone/cAMP-induced activity of the glucose-6-phosphatase promoter in H4IIE hepatoma cells. Both effects are partially mimicked by coexpression of protein kinase Balpha. Coexpression of the Forkhead transcription factor FKHR stimulates the glucose-6-phosphatase promoter activity via interaction with an insulin response unit (IRU), and this activation is suppressed by protein kinase B. Coexpression of a mutated form of FKHR that cannot be phosphorylated by protein kinase B abolishes the regulation of the glucose-6-phosphatase promoter by protein kinase B and disrupts the ability of insulin to regulate the glucose-6-phosphatase promoter via the IRU. Mutation of the insulin response unit of the glucose-6-phosphatase promoter also prevents the regulation of promoter activity by FKHR and protein kinase B but only partially impairs the ability of insulin to suppress both basal and dexamethasone/cAMP-stimulated promoter function. Taken together, these results indicate that signaling by protein kinase B to Forkhead proteins can account for the ability of insulin to regulate glucose-6-phosphatase promoter activity via the IRU and that other mechanisms that are independent of the IRU, protein kinase B, and Forkhead proteins also are important in mediating effects of in insulin on glucose-6-phosphatase gene expression.