Phosphatidylinositol 3-kinase, but not p70/p85 ribosomal S6 protein kinase, is required for the regulation of phosphoenolpyruvate carboxykinase (PEPCK) gene expression by insulin. Dissociation of signaling pathways for insulin and phorbol ester regulation of PEPCK gene expression

Sirt1 Mediates Vitamin D Deficiency-Driven Gluconeogenesis in the Liver via mTorc2/Akt Signaling

As an active form of vitamin D (VD), 1,25-dihydroxyvitamin D (1,25(OH)₂D₃) is involved in the development of many metabolic diseases, such as diabetes, autoimmune diseases, and tumours. While prospective epidemiological studies have consistently implicated VD deficiency in the regulation of glucose metabolism and insulin sensitivity, the specific mechanism remains unclear. Here, we generated 1α(OH)ase-null mice (targeted ablation of the 25-hydroxyvitamin D 1α hydroxylase enzyme) and found that these mice developed hepatic glucose overproduction, glucose intolerance, and hepatic insulin resistance accompanied by reduced Sirtuin 1 (Sirt1) expression. The chromatin immunoprecipitation (ChIP) and a luciferase reporter assay revealed that 1,25(OH)₂D₃-activated VD receptor (VDR) directly interacted with one VD response element (VDRE) in the Sirt1 promoter to upregulate Sirt1 transcription, triggering a cascade of serine/threonine kinase (AKT) phosphorylation at S473 and FOXO1 phosphorylation at S256. This phosphorylation cascade reduced the expression of gluconeogenic genes, eventually attenuating glucose overproduction in the liver. In addition, a signaling pathway was found to modulate gluconeogenesis involving VDR, Sirt1, Rictor (a component of mTOR complex 2 [mTorc2]), AKT, and FOXO1, and Sirt1 and FOXO1 were identified as key modulators of dysregulated gluconeogenesis due to VD deficiency.

Intricate Regulation of Phosphoenolpyruvate Carboxykinase (PEPCK) Isoforms in Normal Physiology and Disease

BACKGROUND

The phosphoenolpyruvate carboxykinase (PEPCK) isoforms are considered as rate-limiting enzymes for gluconeogenesis and glyceroneogenesis pathways. PEPCK exhibits several interesting features such as a) organelle-specific isoforms (cytosolic and a mitochondrial) in vertebrate clade, b) tissue-specific expression of isoforms and c) organism-specific requirement of ATP or GTP as a cofactor. In higher organisms, PEPCK isoforms are intricately regulated and activated through several physiological and pathological stimuli such as corticoids, hormones, nutrient starvation and hypoxia. Isoform-specific transcriptional/translational regulation and their interplay in maintaining glucose homeostasis remain to be fully understood. Mounting evidence indicates the significant involvement of PEPCK isoforms in physiological processes (development and longevity) and in the progression of a variety of diseases (metabolic disorders, cancer, Smith-Magenis syndrome).

OBJECTIVE

The present systematic review aimed to assimilate existing knowledge of transcriptional and translational regulation of PEPCK isoforms derived from cell, animal and clinical models.

CONCLUSION

Based on current knowledge and extensive bioinformatics analysis, in this review we have provided a comparative (epi)genetic understanding of PCK1 and PCK2 genes encompassing regulatory elements, disease-associated polymorphisms, copy number variations, regulatory miRNAs and CpG densities. We have also discussed various exogenous and endogenous modulators of PEPCK isoforms and their signaling mechanisms. A comprehensive review of existing knowledge of PEPCK regulation and function may enable identification of the underlying gaps to design new pharmacological strategies and interventions for the diseases associated with gluconeogenesis.

Genistein represses PEPCK-C expression in an insulin-independent manner in HepG2 cells and in alloxan-induced diabetic mice

Genistein has been reported to exert beneficial effects on type 2 diabetes mellitus (T2DM); however, the underlying molecular mechanisms involved therein have not been clearly elucidated. To address this question, the effect of genistein on the expression of phosphoenolpyruvate carboxykinase (PEPCK), and glucose production in HepG2 cells and in alloxan-induced diabetic mice was investigated. HepG2 cells were exposed to different concentration of genistein in presence or absence of modulators, and the expression of cytosolic PEPCK (PEPCK-C) and the signaling pathways was studied. Further, the biological relevance of the in vitro study was tested in alloxan-induced diabetic mice. Genistein lowered PEPCK-C expression and glucose production in HepG2 cells accompanied with increased in phosphorylation states of AMPK, MEK½, ERK½, and CRTC2. Treatment with the AMPK inhibitor (compound C) enhanced genistein-induced MEK½ and ERK½ activity indicating a potential cross-talk between the two signaling pathways. In vivo, genistein also reduced fasting glucose levels accompanied with reduced PEPCK-C expression and increased in AMPK and ERK½ phosphorylation states in the liver of genistein-treated alloxan-induced diabetic mice. Genistein fulfills the criteria of a suitable anti-diabetic agent by reducing glucose production and inhibiting PEPCK-C expression in HepG2 cells and also in alloxan-induced diabetic mice. These results indicate that genistein is an effective candidate for preventing T2DM through the modulation of AMPK-CRTC2 and MEK/ERK signaling pathways, which may allow a novel approach to modulate dysfunction in hepatic gluconeogenesis in T2DM.

Regulation of Glucose Homeostasis by Glucocorticoids

Glucocorticoids are steroid hormones that regulate multiple aspects of glucose homeostasis. Glucocorticoids promote gluconeogenesis in liver, whereas in skeletal muscle and white adipose tissue they decrease glucose uptake and utilization by antagonizing insulin response. Therefore, excess glucocorticoid exposure causes hyperglycemia and insulin resistance. Glucocorticoids also regulate glycogen metabolism. In liver, glucocorticoids increase glycogen storage, whereas in skeletal muscle they play a permissive role for catecholamine-induced glycogenolysis and/or inhibit insulin-stimulated glycogen synthesis. Moreover, glucocorticoids modulate the function of pancreatic α and β cells to regulate the secretion of glucagon and insulin, two hormones that play a pivotal role in the regulation of blood glucose levels. Overall, the major glucocorticoid effect on glucose homeostasis is to preserve plasma glucose for brain during stress, as transiently raising blood glucose is important to promote maximal brain function. In this chapter we will discuss the current understanding of the mechanisms underlying different aspects of glucocorticoid-regulated mammalian glucose homeostasis.

Control of hepatic gluconeogenesis by the promyelocytic leukemia zinc finger protein

The promyelocytic leukemia zinc finger (PLZF) protein is involved in major biological processes including energy metabolism, although its role remains unknown. In this study, we demonstrated that hepatic PLZF expression was induced in fasted or diabetic mice. PLZF promoted gluconeogenic gene expression and hepatic glucose output, leading to hyperglycemia. In contrast, hepatic PLZF knockdown improved glucose homeostasis in db/db mice. Mechanistically, peroxisome proliferator-activated receptor γ coactivator 1α and the glucocorticoid receptor synergistically activated PLZF expression. We conclude that PLZF is a critical regulator of hepatic gluconeogenesis. PLZF manipulation may benefit the treatment of metabolic diseases associated with gluconeogenesis.

Transcription factor TIP27 regulates glucose homeostasis and insulin sensitivity in a PI3-kinase/Akt-dependent manner in mice

BACKGROUND/OBJECTIVES

Juxtaposed with another zinc-finger gene 1 (TIP27 or JAZF1) is a 27-kDa transcription factor, and genome-wide association studies have recently revealed TIP27 to be associated with type 2 diabetes. However, little is known about its role in the regulation of metabolism. In this study, we investigated the effects of TIP27 overexpression on glucose homeostasis and insulin signaling in high-fat diet (HFD)-fed TIP27 transgenic (TIP27-Tg) mice and db/db mice.

METHODS

We assessed the effects of TIP27 overexpression in both TIP27-Tg mice and db/db mice on glucose metabolism and changes in insulin sensitivity during glucose (GTT) and insulin (ITT) tolerance tests. A hyperinsulinemic-euglycemic clamp was performed on TIP27-Tg mice. Real-time quantitative PCR and western blotting were used to assess mRNA and protein expressions.

RESULTS

TIP27 overexpression in TIP27-Tg mice and in db/db mice led to reduced total cholesterol and fasting plasma insulin levels, and enhanced glucose tolerance and insulin sensitivity during GTT and ITT. Hyperinsulinemic-euglycemic clamp experiments demonstrated that HFD-fed TIP27-Tg mice had lower hepatic glucose production and higher insulin sensitivity compared with nontransgenic littermates. In addition, the hepatic expressions of phosphoenolpyruate carboxykinase (PEPCK), glucose-6-phosphatase (G6Pase) mRNAs and proteins were significantly decreased, whereas the phosphorylation of insulin receptor, insulin receptor substrate-1, adenosine monophosphate-activated protein kinase and Akt kinase (Akt) in the liver was significantly increased in HFD-fed TIP27-Tg mice compared with nontransgenic littermates. Adenovirus-mediated TIP27 overexpression in db/db mice also decreased the expression of gluconeogenic genes and increased the phosphorylation of insulin signaling molecules in the liver compared with controls. Finally, LY294002, a phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, abolished the suppressive effect of TIP27 overexpression on PEPCK and G6Pase expression.

CONCLUSIONS

TIP27 has an important role in glucose homeostasis through the regulation of hepatic glucose metabolism and insulin sensitivity. Furthermore, this regulation requires activation of PI3-kinase.

Vanin-1 is a key activator for hepatic gluconeogenesis

Vanin-1 (VNN1) is a liver-enriched oxidative stress sensor that has been implicated in the regulation of multiple metabolic pathways. Clinical investigations indicated that the levels of VNN1 were increased in the urine and blood of diabetic patients, but the physiological significance of this phenomenon remains unknown. In this study, we demonstrated that the hepatic expression of VNN1 was induced in fasted mice or mice with insulin resistance. Gain- and loss-of-function studies indicated that VNN1 increased the expression of gluconeogenic genes and hepatic glucose output, which led to hyperglycemia. These effects of VNN1 on gluconeogenesis were mediated by the regulation of the Akt signaling pathway. Mechanistically, vnn1 transcription was activated by the synergistic interaction of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and hepatocyte nuclear factor-4α (HNF-4α). A chromatin immunoprecipitation analysis indicated that PGC-1α was present near the HNF-4α binding site on the proximal vnn1 promoter and activated the chromatin structure. Taken together, our results suggest an important role for VNN1 in regulating hepatic gluconeogenesis. Therefore, VNN1 may serve as a potential therapeutic target for the treatment of metabolic diseases caused by overactivated gluconeogenesis.

Repression of glucocorticoid-stimulated angiopoietin-like 4 gene transcription by insulin

Angiopoietin-like 4 (Angptl4) is a glucocorticoid receptor (GR) primary target gene in hepatocytes and adipocytes. It encodes a secreted protein that inhibits extracellular LPL and promotes adipocyte lipolysis. In Angptl4 null mice, glucocorticoid-induced adipocyte lipolysis and hepatic steatosis are compromised. Markedly, insulin suppressed glucocorticoid-induced Angptl4 transcription. To unravel the mechanism, we utilized small molecules to inhibit insulin signaling components and found that phosphatidylinositol 3-kinase and Akt were vital for the suppression in H4IIE cells. A forkhead box transcription factor response element (FRE) was found near the 15 bp Angptl4 glucocorticoid response element (GRE). Mutating the Angptl4 FRE significantly reduced glucocorticoid-induced reporter gene expression in cells. Moreover, chromatin immunoprecipitation revealed that GR and FoxO1 were recruited to Angptl4 GRE and FRE in a glucocorticoid-dependent manner, and cotreatment with insulin abolished both recruitments. Furthermore, in 24 h fasted mice, significant occupancy of GR and FoxO1 at the Angptl4 GRE and FRE was found in the liver. In contrast, both occupancies were diminished after 24 h refeeding. Finally, overexpression of dominant negative FoxO1 mutant abolished glucocorticoid-induced Angptl4 expression, mimicking the insulin suppression. Overall, we demonstrate that both GR and FoxO1 are required for Angptl4 transcription activation, and that FoxO1 negatively mediates the suppressive effect of insulin.

Exercise training decreases mitogen-activated protein kinase phosphatase-3 expression and suppresses hepatic gluconeogenesis in obese mice

Insulin plays an important role in the control of hepatic glucose production. Insulin resistant states are commonly associated with excessive hepatic glucose production, which contributes to both fasting hyperglycaemia and exaggerated postprandial hyperglycaemia. In this regard, increased activity of phosphatases may contribute to the dysregulation of gluconeogenesis. Mitogen-activated protein kinase phosphatase-3 (MKP-3) is a key protein involved in the control of gluconeogenesis. MKP-3-mediated dephosphorylation activates FoxO1 (a member of the forkhead family of transcription factors) and subsequently promotes its nuclear translocation and binding to the promoters of gluconeogenic genes such as phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). In this study, we investigated the effects of exercise training on the expression of MKP-3 and its interaction with FoxO1 in the livers of obese animals. We found that exercised obese mice had a lower expression of MKP-3 and FoxO1/MKP-3 association in the liver. Further, the exercise training decreased FoxO1 phosphorylation and protein levels of Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and gluconeogenic enzymes (PEPCK and G6Pase). These molecular results were accompanied by physiological changes, including increased insulin sensitivity and reduced hyperglycaemia, which were not caused by reductions in total body mass. Similar results were also observed with oligonucleotide antisense (ASO) treatment. However, our results showed that only exercise training could reduce an obesity-induced increase in HNF-4α protein levels while ASO treatment alone had no effect. These findings could explain, at least in part, why additive effects of exercise training treatment and ASO treatment were not observed. Finally, the suppressive effects of exercise training on MKP-3 protein levels appear to be related, at least in part, to the reduced phosphorylation of Extracellular signal-regulated kinases (ERK) in the livers of obese mice.

The anti-neurodegenerative agent clioquinol regulates the transcription factor FOXO1a

Many diseases of aging including AD (Alzheimer's disease) and T2D (Type 2 diabetes) are strongly associated with common risk factors, suggesting that there may be shared aging mechanisms underlying these diseases, with the scope to identify common cellular targets for therapy. In the present study we have examined the insulin-like signalling properties of an experimental AD 8-hydroxyquinoline drug known as CQ (clioquinol). The IIS [insulin/IGF-1 (insulin-like growth factor-1) signalling] kinase Akt/PKB (protein kinase B) inhibits the transcription factor FOXO1a (forkhead box O1a) by phosphorylating it on residues that trigger its exit from the nucleus. In HEK (human embryonic kidney)-293 cells, we found that CQ treatment induces similar responses. A key transcriptional response to IIS is the inhibition of hepatic gluconeogenic gene expression, and, in rat liver cells, CQ represses expression of the key gluconeogenic regulatory enzymes PEPCK (phosphoenolpyruvate carboxykinase) and G6Pase (glucose-6-phosphatase). The effects on FOXO1a and gluconeogenic gene expression require the presence of Zn2+ ions, reminiscent of much earlier studies examining diabetogenic properties of 8-hydroxyquinolines. Comparative investigation of the signalling properties of a panel of these compounds demonstrates that CQ alone exhibits FOXO1a regulation without diabetogenicity. Our results suggest that Zn2+-dependent regulation of FOXOs and gluconeogenesis may contribute to the therapeutic properties of this drug. Further investigation of this signalling response might illuminate novel pharmacological strategies for the treatment of age-related diseases.

Hepatic Sirt1 deficiency in mice impairs mTorc2/Akt signaling and results in hyperglycemia, oxidative damage, and insulin resistance

Insulin resistance is a major risk factor for type 2 diabetes mellitus. The protein encoded by the sirtuin 1 (Sirt1) gene, which is a mouse homolog of yeast Sir2, is implicated in the regulation of glucose metabolism and insulin sensitivity; however, the underlying mechanism remains elusive. Here, using mice with a liver-specific null mutation of Sirt1, we have identified a signaling pathway involving Sirt1, Rictor (a component of mTOR complex 2 [mTorc2]), Akt, and Foxo1 that regulates gluconeogenesis. We found that Sirt1 positively regulates transcription of the gene encoding Rictor, triggering a cascade of phosphorylation of Akt at S473 and Foxo1 at S253 and resulting in decreased transcription of the gluconeogenic genes glucose-6-phosphatase (G6pase) and phosphoenolpyruvate carboxykinase (Pepck). Liver-specific Sirt1 deficiency caused hepatic glucose overproduction, chronic hyperglycemia, and increased ROS production. This oxidative stress disrupted mTorc2 and impaired mTorc2/Akt signaling in other insulin-sensitive organs, leading to insulin resistance that could be largely reversed with antioxidant treatment. These data delineate a pathway through which Sirt1 maintains insulin sensitivity and suggest that treatment with antioxidants might provide protection against progressive insulin resistance in older human populations.

How porphyrinogenic drugs modeling acute porphyria impair the hormonal status that regulates glucose metabolism. Their relevance in the onset of this disease

This work deals with the study of how porphyrinogenic drugs modeling acute porphyrias interfere with the status of carbohydrate-regulating hormones in relation to key glucose enzymes and to porphyria, considering that glucose modulates the development of the disease. Female Wistar rats were treated with 2-allyl-2-isopropylacetamide (AIA) and 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) using different doses of AIA (100, 250 and 500mg/kg body weight) and a single dose of DDC (50mg DDC/kg body weight). Rats were sacrificed 16h after AIA/DDC administration. In the group treated with the highest dose of AIA (group H), hepatic 5-aminolevulinic acid synthase (ALA-S) increased more than 300%, phosphoenolpyruvate carboxykinase (PEPCK) and glycogen phosphorylase (GP) activities were 43% and 46% lower than the controls, respectively, plasmatic insulin levels exceeded normal values by 617%, and plasmatic glucocorticoids (GC) decreased 20%. GC results are related to a decrease in corticosterone (CORT) adrenal production (33%) and a significant reduction in its metabolization by UDP-glucuronosyltransferase (UGT) (62%). Adrenocorticotropic hormone (ACTH) stimulated adrenal production 3-fold and drugs did not alter this process. Thus, porphyria-inducing drugs AIA and DDC dramatically altered the status of hormones that regulate carbohydrate metabolism increasing insulin levels and reducing GC production, metabolization and plasmatic levels. In this acute porphyria model, gluconeogenic and glycogenolytic blockages caused by PEPCK and GP depressed activities, respectively, would be mainly a consequence of the negative regulatory action of insulin on these enzymes. GC could also contribute to PEPCK blockage both because they were depressed by the treatment and because they are positive effectors on PEPCK. These disturbances in carbohydrates and their regulation, through ALA-S de-repression, would enhance the porphyria state promoted by the drugs on heme synthesis and destruction. This might be the mechanism underlying the "glucose effect" observed in hepatic porphyrias. The statistical correlation study performed showed association between all the variables studied and reinforce these conclusions.

Dynamic glucoregulation and mammalian-like responses to metabolic and developmental disruption in zebrafish

Zebrafish embryos are emerging as models of glucose metabolism. However, patterns of endogenous glucose levels, and the role of the islet in glucoregulation, are unknown. We measured absolute glucose levels in zebrafish and mouse embryos, and demonstrate similar, dynamic glucose fluctuations in both species. Further, we show that chemical and genetic perturbations elicit mammalian-like glycemic responses in zebrafish embryos. We show that glucose is undetectable in early zebrafish and mouse embryos, but increases in parallel with pancreatic islet formation in both species. In zebrafish, increasing glucose is associated with activation of gluconeogenic phosphoenolpyruvate carboxykinase1 (pck1) transcription. Non-hepatic Pck1 protein is expressed in mouse embryos. We show using RNA in situ hybridization, that zebrafish pck1 mRNA is similarly expressed in multiple cell types prior to hepatogenesis. Further, we demonstrate that the Pck1 inhibitor 3-mercaptopicolinic acid suppresses normal glucose accumulation in early zebrafish embryos. This shows that pre- and extra-hepatic pck1 is functional, and provides glucose locally to rapidly developing tissues. To determine if the primary islet is glucoregulatory in early fish embryos, we injected pdx1-specific morpholinos into transgenic embryos expressing GFP in beta cells. Most morphant islets were hypomorphic, not a genetic, but embryos still exhibited persistent hyperglycemia. We conclude from these data that the early zebrafish islet is functional, and regulates endogenous glucose. In summary, we identify mechanisms of glucoregulation in zebrafish embryos that are conserved with embryonic and adult mammals. These observations justify use of this model in mechanistic studies of human metabolic disease.

MAPK phosphatase-3 promotes hepatic gluconeogenesis through dephosphorylation of forkhead box O1 in mice

Insulin resistance results in dysregulated hepatic gluconeogenesis that contributes to obesity-related hyperglycemia and progression of type 2 diabetes mellitus (T2DM). Recent studies show that MAPK phosphatase-3 (MKP-3) promotes gluconeogenic gene transcription in hepatoma cells, but little is known about the physiological role of MKP-3 in vivo. Here, we have shown that expression of MKP-3 is markedly increased in the liver of diet-induced obese mice. Consistent with this, adenovirus-mediated MKP-3 overexpression in lean mice promoted gluconeogenesis and increased fasting blood glucose levels. Conversely, shRNA knockdown of MKP-3 in both lean and obese mice resulted in decreased fasting blood glucose levels. In vitro experiments identified forkhead box O1 (FOXO1) as a substrate for MKP-3. MKP-3-mediated dephosphorylation of FOXO1 at Ser256 promoted its nuclear translocation and subsequent recruitment to the promoters of key gluconeogenic genes. In addition, we showed that PPARγ coactivator-1α (PGC-1α) acted downstream of FOXO1 to mediate MKP-3-induced gluconeogenesis. These data indicate that MKP-3 is an important regulator of hepatic gluconeogenesis in vivo and suggest that inhibition of MKP-3 activity may provide new therapies for T2DM.

Generation, validation and humanisation of a novel insulin resistant cell model

Insulin resistance is a characteristic of type 2 diabetes and is a major independent risk factor for progression to the disease. In particular, insulin resistance associates with increased body fat and almost certainly contributes to the dramatic increase in risk of type 2 diabetes associated with obesity. Therefore, in order to design truly effective insulin sensitising agents, targeted at the mechanism of disease development, we aimed to generate an obesity-related insulin resistant cell model. Rat hepatoma cells were grown in the presence of serum isolated from obese rodents or obese human volunteers, and the insulin sensitivity of the cells monitored over time by measuring a well-characterised insulin regulated gene promoter. Higher insulin concentrations were required to fully repress the gene in the cells grown in obese rodent serum compared with those grown in serum from lean rodents (almost a 10-fold shift in insulin sensitivity). This was reversed by restoration of normal growth medium, while the insulin resistance was prevented by pioglitazone or metformin. Meanwhile, growth of cells in serum collected from obese human volunteers with diabetes also reduced the insulin sensitivity of the rat cells. No clinical marker predicted the degree of insulin resistance that was generated by the human serum. We have developed a novel insulin resistant cell model for the study of the molecular development of obesity-linked insulin resistance, screen for compounds to overcome obesity-related insulin resistance and potentially search for novel serum biomarkers of insulin resistance.

Bifurcation of insulin signaling pathway in rat liver: mTORC1 required for stimulation of lipogenesis, but not inhibition of gluconeogenesis

The livers of insulin-resistant, diabetic mice manifest selective insulin resistance, suggesting a bifurcation in the insulin signaling pathway: Insulin loses its ability to block glucose production (i.e., it fails to suppress PEPCK and other genes of gluconeogenesis), yet it retains its ability to stimulate fatty acid synthesis (i.e., continued enhancement of genes of lipogenesis). Enhanced lipogenesis is accompanied by an insulin-stimulated increase in the mRNA encoding SREBP-1c, a transcription factor that activates the entire lipogenic program. Here, we report a branch point in the insulin signaling pathway that may account for selective insulin resistance. Exposure of rat hepatocytes to insulin produced a 25-fold increase in SREBP-1c mRNA and a 95% decrease in PEPCK mRNA. Insulin-mediated changes in both mRNAs were blocked by inhibitors of PI3K and Akt, indicating that these kinases are required for both pathways. In contrast, subnanomolar concentrations of rapamycin, an inhibitor of the mTORC1 kinase, blocked insulin induction of SREBP-1c, but had no effect on insulin suppression of PEPCK. We observed a similar selective effect of rapamycin in livers of rats and mice that experienced an insulin surge in response to a fasting-refeeding protocol. A specific inhibitor of S6 kinase, a downstream target of mTORC1, did not block insulin induction of SREBP-1c, suggesting a downstream pathway distinct from S6 kinase. These results establish mTORC1 as an essential component in the insulin-regulated pathway for hepatic lipogenesis but not gluconeogenesis, and may help to resolve the paradox of selective insulin resistance in livers of diabetic rodents.

Effects of pulsatile secretion of growth hormone (GH) on fat deposition in human GH transgenic rats

Growth hormone (GH) is an endocrine regulator of glucose and lipid metabolism as well as body growth. GH levels are decreased and a unique pulsatile secretory pattern becomes obvious after puberty particularly in males. Coincidentally with this, males tend to deposit body fat. Experimental and clinical evidence has accumulated that obesity is associated with a decrease in GH levels. A strain of transgenic rats has been generated with severe obesity but normal nose-to-tail length, which has low circulating GH levels without pulsatility (human growth hormone (hGH) transgenic rats). The present review mainly focuses on recent and current work analysing the relationship between the occurrence of obesity and low GH levels and/or the absence of GH pulsatility in this transgenic animal model. This model has elevated blood glucose, non-esterified fatty acid, insulin and leptin levels associated with hyperphagia, suggesting that these rats also carry insulin- and leptin-resistant characteristics. hGH transgenic rats were subjected to a pair-feeding treatment to normalize food intake and chronic GH replacement to normalize GH levels. While the pair-feeding for 8 weeks successfully suppressed body-weight gain, the fat pad : body weight ratio remained very similar to freely-eating control hGH transgenic rats, which indicates the hyperphagia is not the sole contributor to the excess fat accumulation in this model. However, continuous elevation of peripheral hGH levels (approximately 2-fold) for 8 weeks by means of a slow-release vehicle resulted in a significant decrease in the fat mass : body weight ratios by 30 %. This GH treatment altered neither food intake nor body-weight gain. Thus, two characteristic phenotypes observed in the hGH transgenic rats, hyperphagia and obesity, seem to be closely related to GH levels and GH secretory pattern. This relationship might be working in the regulation of changes in seasonal body composition in wild animals.

Zinc-dependent effects of small molecules on the insulin-sensitive transcription factor FOXO1a and gluconeogenic genes

Metal-binding compounds have recently been reported to have anti-hyperglycaemic properties in vivo. In the current study, we have investigated the ability of these compounds and related structures to induce insulin-like signal transduction to downstream effectors such as the transcription factor FOXO1a and the key gluconeogenic regulatory enzymes phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase). Our results indicate that β-thujaplicin, diethyldithiocarbamate (DEDTC) and its clinically-used dimer disulfiram, induce insulin-like dose-dependent effects on signalling to FOXO1a in a manner that is strictly dependent on the presence of zinc ions, as other ions including aluminium, cobalt, copper, lithium and manganese cannot substitute. The most potent compound tested on gluconeogenesis is disulfiram, which in the presence of 10 μM zinc, inhibited both PEPCK and G6Pase with an IC50 of 4 μM. Our results demonstrate that metal-binding compounds with diverse structures can induce zinc-dependent insulin-like effects on signal transduction and gene expression.

Beta-glucans in the treatment of diabetes and associated cardiovascular risks

Diabetes mellitus is characterized by high blood glucose level with typical manifestations of thirst, polyuria, polydipsia, and weight loss. It is caused by defects in insulin-mediated signal pathways, resulting in decreased glucose transportation from blood into muscle and fat cells. The major risk is vascular injury leading to heart disease, which is accelerated by increased lipid levels and hypertension. Management of diabetes includes: control of blood glucose level and lipids; and reduction of hypertension. Dietary intake of beta-glucans has been shown to reduce all these risk factors to benefit the treatment of diabetes and associated complications. In addition, beta-glucans also promote wound healing and alleviate ischemic heart injury. However, the mechanisms behind the effect of beta-glucans on diabetes and associated complications need to be further studied using pure beta-glucan.

Role of the liver in glucose homeostasis in PI 3-kinase p85alpha-deficient mice

Phosphoinositide 3-kinase (PI3K) p85alpha-deficient mice exhibit hypoglycemia as a result of increased insulin sensitivity and glucose uptake in peripheral tissues. Although PI3K is central to the metabolic actions of insulin, its mechanism of action in liver is not well understood. In the present study, we investigated hepatic insulin signaling and glucose homeostasis in p85alpha-deficient and wild-type mice. In the livers of p85alpha-deficient mice, p50alpha played a compensatory role in insulin-stimulated PI3K activation by binding to insulin receptor substrate (IRS)-1/2. In p85alpha-deficient mice, the ratio of p50alpha over p110 catalytic subunit of PI3K in the liver was higher than in the muscles. PI3K activity associated with IRS-1/2 was not affected by the lack of p85alpha in the liver. Insulin-stimulated Akt and phosphatase and tensin homologue deleted on chromosome 10 (PTEN) activities in the liver were similar in p85alpha-deficient and wild-type mice. A hyperinsulinemic-euglycemic clamp study revealed that the glucose infusion rate and the rate of disappearance were higher in p85alpha-deficient mice than in wild-type mice but that endogenous glucose production tended to be higher in p85alpha-deficient mice than in wild-type mice. Consistent with this finding, the expression of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase in livers after fasting was higher in p85alpha-deficient mice than in wild-type mice. After mice were fasted, the intrahepatic glucose-6-phosphate level was almost completely depleted in p85alpha-deficient mice. The glycogen content fell to nearly zero as a result of glycogenolysis shortly after the initiation of fasting in p85alpha-deficient mice. The absence of an increase in insulin-stimulated PI3K activation in the liver of p85alpha-deficient mice, unlike the muscles, may be associated with the molecular balance between the regulatory subunit and the catalytic subunit of PI3K. Gluconeogenesis was rather elevated in p85alpha-deficient mice, compared with in wild-type mice, and the liver seemed to partially compensate for the increase in glucose uptake in peripheral tissues.

Factors That Control the Tissue-Specific Transcription of the Gene for Phosphoenolpyruvate Carboxykinase-C

Transcription of the gene for PEPCK-C occurs in a number of mammalian tissues, with highest expression occurring in the liver, kidney cortex, and white and brown adipose tissue. Several hormones and other factors, including glucagon, epinephrine, insulin, glucocorticoids and metabolic acidosis, control this process in three responsive tissues, liver, adipose tissue, and kidney cortex. Expression of the gene in these three tissues in regulated in a different manner, responding to the specific physiological role of the tissue. The PEPCK-C gene promoter has been extensively studied and a number of regulatory regions identified that bind key transcription factors and render the gene responsive to hormonal and dietary stimuli. This review will focus on the control of transcription for the gene, with special emphasis on our current understanding of the transcription factors that are involved in the response of PEPCK-C gene in specific tissues. We have also reviewed the biological function of PEPCK-C in each of the tissues discussed in this review, in order to place the control of PEPCK-C gene transcription in the appropriate physiological context. Because of its extraordinary importance in mammalian metabolism and its broad pattern of tissue-specific expression, the PEPCK-C gene has become a model for studying the biological basis of the control of gene transcription.

Role of phosphatidylinositol 3-kinase activation on insulin action and its alteration in diabetic conditions

Inositol phospholipids phosphorylated on D3-position of their inositol rings (3-phosphoinositides) are known to play important roles in various cellular events. Activation of PI (phosphatidylinositol) 3-kinase is essential for aspects of insulin-induced glucose metabolism, including translocation of GLUT4 to the cell surface and glycogen synthesis. The enzyme exists as a heterodimer containing a regulatory subunit and one of two widely-distributed isoforms of the p110 catalytic subunit: p110alpha or p110beta. Activation of PI 3-kinase and its downstream AKT has been demonstrated to be essential for almost all of the insulin-induced glucose and lipid metabolism such as glucose uptake, glycogen synthesis, suppression of glucose output and triglyceride synthesis as well as insulin-induced mitogenesis. Accumulated PI(3,4,5)P(3) activates several serine/threonine kinases containing a PH (pleckstrin homology) domain, including Akt, atypical PKCs, p70S6 kinase and GSK. In the obesity-induced insulin resistant condition, JNK and p70S6K are activated and phosphorylate IRS-proteins, which diminishes the insulin-induced tyrosine phosphorylation of IRS-proteins and thereby impairs the PI 3-kinase/AKT activations. Thus, the drugs which restore the impaired insulin-induced PI 3-kinase/AKT activation, for example, by suppressing JNK or p70S6K, PTEN or SHIP2, could be novel agents to treat diabetes mellitus.

Role of the PI3-kinase/mTor pathway in the regulation of the stearoyl CoA desaturase (SCD1) gene expression by insulin in liver

The stearoyl-CoA desaturase 1 (SCD1) catalyzes the synthesis of monounsaturated fatty acids. This enzyme is a critical control point regulating hepatic lipogenesis and lipid oxidation. Therefore SCD1 may be a potential therapeutic target in the treatment of obesity and metabolic syndrome. Regulation of SCD1 expression occurs primarily at the level of transcription. In the present study, we characterized the insulin response elements (IREs) and the insulin signaling pathway mediating the regulation of SCD1 gene transcription in liver. In chicken embryo hepatocytes (CEH) and HepG2 cells, insulin stimulates SCD1 promoter activity by 2.5 folds. This activation is mediated by two different IREs on the chicken promoter, one localized between -1,975 and -1,610 bp and one between -372 and -297 bp. The latter binds both NF-Y and SREBP-1 transcription factors in response to insulin. We also demonstrated that insulin induction of SCD1 gene expression and promoter activity is abolished by pre-incubation of cells with specific inhibitors of both PI3-kinase (LY294002) and mTor (Rapamycin) or by over-expression of a dominant negative mutant of PI3-kinase. The PI3-kinase and mTor pathway mediates the insulin response on both IREs. In summary, insulin activates SCD1 gene expression in liver via a signaling pathway that involves PI3-kinase and mTor and the downstream transcription factors NF-Y and SREBP-1. Sentence summary: Insulin regulates SCD1 gene expression via two different IREs. The most 3' IRE is localized between -372 and -297 bp and binds the NF-Y and SREBP-1 transcription factors in response to insulin. PI3-kinase and mTor mediate the action of insulin on both IREs.

Characterization of a protein kinase B inhibitor in vitro and in insulin-treated liver cells

OBJECTIVE

Abnormal expression of the hepatic gluconeogenic genes (glucose-6-phosphatase [G6Pase] and PEPCK) contributes to hyperglycemia. These genes are repressed by insulin, but this process is defective in diabetic subjects. Protein kinase B (PKB) is implicated in this action of insulin. An inhibitor of PKB, Akt inhibitor (Akti)-1/2, was recently reported; however, the specificity and efficacy against insulin-induced PKB was not reported. Our aim was to characterize the specificity and efficacy of Akti-1/2 in cells exposed to insulin and then establish whether inhibition of PKB is sufficient to prevent regulation of hepatic gene expression by insulin.

RESEARCH DESIGN AND METHODS

Akti-1/2 was assayed against 70 kinases in vitro and its ability to block PKB activation in cells exposed to insulin fully characterized.

RESULTS

Akti-1/2 exhibits high selectivity toward PKBalpha and PKBbeta. Complete inhibition of PKB activity is achieved in liver cells incubated with 1-10 mumol/l Akti-1/2, and this blocks insulin regulation of PEPCK and G6Pase expression. Our data demonstrate that only 5-10% of maximal insulin-induced PKB is required to fully repress PEPCK and G6Pase expression. Finally, we demonstrate reduced insulin sensitivity of these gene promoters in cells exposed to submaximal concentrations of Akti-1/2; however, full repression of the genes can still be achieved by high concentrations of insulin.

CONCLUSIONS

This work establishes the requirement for PKB activity in the insulin regulation of PEPCK, G6Pase, and a third insulin-regulated gene, IGF-binding protein-1 (IGFBP1); suggests a high degree of functional reserve; and identifies Akti-1/2 as a useful tool to delineate PKB function in the liver.

Transcriptional regulation by insulin: from the receptor to the gene

Insulin, after binding to its receptor, regulates many cellular processes and the expression of several genes. For a subset of genes, insulin exerts a negative effect on transcription; for others, the effect is positive. Insulin controls gene transcription by modifying the binding of transcription factors on insulin-response elements or by regulating their transcriptional activities. Different insulin-signaling cascades have been characterized as mediating the insulin effect on gene transcription. In this review, we analyze recent data on the molecular mechanisms, mostly in the liver, through which insulin exerts its effect. We first focus on the key transcription factors (viz. Foxo, sterol-response-element-binding protein family (SREBP), and Sp1) involved in the regulation of gene transcription by insulin. We then present current information on the way insulin downregulates and upregulates gene transcription, using as examples of downregulation phosphoenolpyruvate carboxykinase (PEPCK) and insulin-like growth factor binding protein 1 (IGFBP-1) genes and of upregulation the fatty acid synthase and malic enzyme genes. The last part of the paper focuses on the signaling cascades activated by insulin in the liver, leading to the modulation of gene transcription.

Regulation of hepatic insulin-like growth factor-binding protein-1 gene expression by insulin: central role for mammalian target of rapamycin independent of forkhead box O proteins

The expression of IGF-binding protein-1 (IGFBP-1) is induced in rat liver by dexamethasone and glucagon and is completely inhibited by 100 nM insulin. Various studies have implicated phosphatidylinositol 3-kinase, protein kinase B (Akt), phosphorylation of the transcription factors forkhead in rhabdomyosarcoma 1 (Foxo1)/Foxo3, and the mammalian target of rapamycin (mTOR) in insulin's effect. In this study we examined insulin regulation of IGFBP-1 in both subconfluent and confluent hepatocytes. In subconfluent hepatocytes, insulin inhibition of IGFBP-1 mRNA levels was blocked by inhibiting PI3 kinase activation, and there was a corresponding inhibition of Foxo1/Foxo3 phosphorylation. In these same cells, inhibition of the insulin effect by rapamycin occurred in the presence of insulin-induced Foxo1/Foxo3 phosphorylation. In confluent hepatocytes, insulin could not activate the phosphatidylinositol 3-kinase (PI3 kinase)-Akt-Foxo1/Foxo3 pathway, but still inhibited IGFBP-1 gene expression in an mTOR-dependent manner. In subconfluent hepatocytes, the serine/threonine phosphatase inhibitor okadaic acid (100 nM) partially inhibited IGFBP-1 gene expression by 40%, but did not produce phosphorylation of either Akt or Foxo proteins. In contrast, 1 nm insulin inhibited the IGFBP-1 mRNA level by 40% and correspondingly activated Akt and Foxo1/Foxo3 phosphorylation to a level comparable to that observed with 100 nM insulin. These results suggest a potential role for a serine/threonine phosphatase(s) in the regulation of IGFBP-1 gene transcription, which is not downstream of mTOR and is independent of Akt. In conclusion, we have found that in rat liver, insulin inhibition of IGFBP-1 mRNA levels can occur in the absence of the phosphorylation of Foxo1/Foxo3, whereas activation of the mTOR pathway is both necessary and sufficient.

A novel regulation of IRS1 (insulin receptor substrate-1) expression following short term insulin administration

Reduced insulin-mediated glucose transport in skeletal muscle is a hallmark of the pathophysiology of T2DM (Type II diabetes mellitus). Impaired intracellular insulin signalling is implicated as a key underlying mechanism. Attention has focused on early signalling events such as defective tyrosine phosphorylation of IRS1 (insulin receptor substrate-1), a major target for the insulin receptor tyrosine kinase. This is required for normal induction of signalling pathways key to many of the metabolic actions of insulin. Conversely, increased serine/threonine phosphorylation of IRS1 following prolonged insulin exposure (or in obesity) reduces signalling capacity, partly by stimulating IRS1 degradation. We now show that IRS1 levels in human muscle are actually increased 3-fold following 1 h of hyperinsulinaemic euglycaemia. Similarly, transient induction of IRS1 (3-fold) in the liver or muscle of rodents occurs following feeding or insulin injection respectively. The induction by insulin is also observed in cell culture systems, although to a lesser degree, and is not due to reduced proteasomal targeting, increased protein synthesis or gene transcription. Elucidation of the mechanism by which insulin promotes IRS1 stability will permit characterization of the importance of this novel signalling event in insulin regulation of liver and muscle function. Impairment of this process would reduce IRS1 signalling capacity, thereby contributing to the development of hyperinsulinaemia/insulin resistance prior to the appearance of T2DM.

Regulators for Blood Glucose Level Affect Gene Expression of Aquaporin 3

Aquaporin 3 (AQP3), a membrane protein, is known to permeabilize water and other small molecules such as glycerol and urea and is localized in the bowel, skin, kidney, and erythrocytes. Since glycerol is a nutrient and serves as a source material in glycolytic metabolism, absorption of glycerol in the gastrointestinal tract may be under some control. Therefore we first investigated whether insulin regulating the glycolytic pathway took part in glycerol transport through AQP3 in the gastrointestinal tract and found that insulin significantly suppressed mRNA and protein expressions of AQP3 in Caco-2 cells. The antidiabetic drugs troglitazone and tolbutamide were also observed to suppress significantly AQP3 expression, but the biguanides metformin and buformin did not induce such suppression. Epinephrine was found to increase expression of AQP3, although glucagon showed no change of expression. Wortmannin and rapamycin were demonstrated to deactivate suppression of AQP3 expression by insulin and troglitazone, suggesting that the signal transducers, phosphoinositide 3 kinase (PI3K) and the mammalian target of rapamycin (mTOR), are involved in the signal pathway for regulating transcription of AQP3.

Deficiency of PDK1 in liver results in glucose intolerance, impairment of insulin-regulated gene expression and liver failure

The liver plays an important role in insulin-regulated glucose homoeostasis. To study the function of the PDK1 (3-phosphoinositide-dependent protein kinase-1) signalling pathway in mediating insulin's actions in the liver, we employed CRE recombinase/loxP technology to generate L(liver)-PDK1-/- mice, which lack expression of PDK1 in hepatocytes and in which insulin failed to induce activation of PKB in liver. The L-PDK1-/- mice were not insulin-intolerant, possessed normal levels of blood glucose and insulin under normal feeding conditions, but were markedly glucose-intolerant when injected with glucose. The L-PDK1-/- mice also possessed 10-fold lower levels of hepatic glycogen compared with control littermates, and were unable to normalize their blood glucose levels within 2 h after injection of insulin. The glucose intolerance of the L-PDK1-/- mice may be due to an inability of glucose to suppress hepatic glucose output through the gluconeogenic pathway, since the mRNA encoding hepatic PEPCK (phosphoenolpyruvate carboxykinase), G6Pase (glucose-6-phosphatase) and SREBP1 (sterol-regulatory-element-binding protein 1), which regulate gluconeogenesis, are no longer controlled by feeding. Furthermore, three other insulin-controlled genes, namely IGFBP1 (insulin-like-growth-factor-binding protein-1), IRS2 (insulin receptor substrate 2) and glucokinase, were regulated abnormally by feeding in the liver of PDK1-deficient mice. Finally, the L-PDK1-/- mice died between 4-16 weeks of age due to liver failure. These results establish that the PDK1 signalling pathway plays an important role in regulating glucose homoeostasis and controlling expression of insulin-regulated genes. They suggest that a deficiency of the PDK1 pathway in the liver could contribute to development of diabetes, as well as to liver failure.

Existence of two parallel mechanisms for glucose uptake in heterotrophic plant cells

The implied existence of two mechanisms for glucose uptake into heterotrophic plant cells was investigated using the fluorescent glucose derivative 2-NBDG (2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose), two membrane impermeable fluorescent markers (3000 mol. wt. dextran-Texas Red (d-TR) and Alexa-488), hexose carrier and endocytic inhibitors (phloridzin and wortmannin-A, respectively), and fluorescent and confocal microscopy. Both phloridzin and wortmannin-A significantly reduced the uptake of 2-NBDG into sycamore cultured cells, which was confirmed by fluorescent microscopy. Phloridzin prevented 2-NBDG uptake exclusively into the cytosol, whereas the wortmannin-A effect was more general, with 2-NBDG uptake into the vacuole being the more affected. Simultaneous incubation of cells in the membrane-impermeable fluorescent probes Alexa-488 and d-TR for 24 h resulted in co-localization of the labelling in the central vacuole and other endosomal compartments. Cytoplasts, cells devoid of vacuoles, were instrumental in demonstrating the transport of 2-NBDG by separate uptake mechanisms. In cytoplasts incubated simultaneously in 2-NBDG and d-TR for 2 h, a green fluorescent cytosol was indicative of transport of hexoses across the plasmalemma, while the co-localization of 2-NBDG and d-TR in internal vesicles demonstrated transport via an endocytic system. The absence of vesicles when cytoplasts were pre-incubated in wortmannin-A authenticated the endocytic vesicular nature of the co-shared 2-NBDG and d-TR fluorescent structures. In summary, uptake of 2-NBDG occurs by two separate mechanisms: (i) a plasmalemma-bound carrier-mediated system that facilitates 2-NBDG transport into the cytosol, and (ii) an endocytic system that transports most of 2-NBDG directly into the vacuole.

Insulin-mediated down-regulation of apolipoprotein A5 gene expression through the phosphatidylinositol 3-kinase pathway: role of upstream stimulatory factor

The apolipoprotein A5 gene (APOA5) has been repeatedly implicated in lowering plasma triglyceride levels. Since several studies have demonstrated that hyperinsulinemia is associated with hypertriglyceridemia, we sought to determine whether APOA5 is regulated by insulin. Here, we show that cell lines and mice treated with insulin down-regulate APOA5 expression in a dose-dependent manner. Furthermore, we found that insulin decreases human APOA5 promoter activity, and subsequent deletion and mutation analyses uncovered a functional E box in the promoter. Electrophoretic mobility shift and chromatin immunoprecipitation assays demonstrated that this APOA5 E box binds upstream stimulatory factors (USFs). Moreover, in transfection studies, USF1 stimulates APOA5 promoter activity, and the treatment with insulin reduced the binding of USF1/USF2 to the APOA5 promoter. The inhibition of the phosphatidylinositol 3-kinase (PI3K) pathway abolished insulin's effect on APOA5 gene expression, while the inhibition of the P70 S6 kinase pathway with rapamycin reversed its effect and increased APOA5 gene expression. Using an oligonucleotide precipitation assay for USF from nuclear extracts, we demonstrate that phosphorylated USF1 fails to bind to the APOA5 promoter. Taken together, these data indicate that insulin-mediated APOA5 gene transrepression could involve a phosphorylation of USFs through the PI3K and P70 S6 kinase pathways that modulate their binding to the APOA5 E box and results in APOA5 down-regulation. The effect of exogenous hyperinsulinemia in men showed a decrease in the plasma ApoAV level. These results suggest a potential contribution of the APOA5 gene in hypertriglyceridemia associated with hyperinsulinemia.

SHARP-2/Stra13/DEC1 as a potential repressor of phosphoenolpyruvate carboxykinase gene expression

The influence of the enhancer of split- and hairy-related protein-2 (SHARP-2) transcriptional repressor on the expression of rat phosphoenolpyruvate carboxykinase (PEPCK) gene was examined. When H4IIE cells were treated with epigallocatechin gallate, a green tea constituent, an increase in SHARP-2 mRNA levels and a decrease in PEPCK mRNA levels were observed. The adenovirus-mediated overexpression of SHARP-2 in H4IIE cells and primary cultured rat hepatocytes led to a decrease in the levels of PEPCK mRNA. Finally, when a SHARP-2 expression plasmid was transiently transfected with various reporter plasmids into MH1C1 cells, the promoter activity of a PEPCK reporter plasmid was specifically decreased. Based on these findings, we conclude that SHARP-2 is a potential repressor of PEPCK gene expression.

An extract of Syzygium aromaticum represses genes encoding hepatic gluconeogenic enzymes

Insulin action is impaired in diabetic patients, which leads to increased hepatic glucose production. Plants and herbs have been used for medicinal purposes, including the treatment of diabetes, for centuries. Since dietary management is a starting point for the treatment of diabetes, it is important to recognize the effect of plant-based compounds on tissues that regulate glucose metabolism, such as the liver. In a recent study, several herbs and spices were found to increase glucose uptake into adipocytes, an insulin-like effect. Our data reveal that Syzygium aromaticum (L.) Merrill and Perry (Myrtaceae) (commonly referred to as clove) extract acts like insulin in hepatocytes and hepatoma cells by reducing phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase) gene expression. Much like insulin, clove-mediated repression is reversed by PI3K inhibitors and N-acetylcysteine (NAC). A more global analysis of gene expression by DNA microarray analysis reveals that clove and insulin regulate the expression of many of the same genes in a similar manner. These results demonstrate that consumption of certain plant-based diets may have beneficial effects for the treatment of diabetes and indicate a potential role for compounds derived from clove as insulin-mimetic agents.

Hepatic insulin resistance precedes the development of diabetes in a model of intrauterine growth retardation

Intrauterine growth retardation (IUGR) has been linked to the development of type 2 diabetes in adulthood. We developed an IUGR model in rats whereby at age 3-6 months the animals develop a diabetes that is associated with insulin resistance. Hyperinsulinemic-euglycemic clamp studies were performed at age 8 weeks, before the onset of obesity and diabetes. Basal hepatic glucose production (HGP) was significantly higher in IUGR than in control rats (14.6 +/- 0.4 vs. 12.3 +/- 0.3 mg. kg(-1). min(-1); P < 0.05). Insulin suppression of HGP was blunted in IUGR versus control rats (10.4 +/- 0.6 vs. 6.5 +/- 1.0 mg. kg(-1). min(-1); P < 0.01); however, rates of glucose uptake and glycogenolysis were similar between the two groups. Insulin-stimulated insulin receptor substrate 2 and Akt-2 phosphorylation were significantly blunted in IUGR rats. PEPCK and glucose-6-phosphatase mRNA levels were increased at least threefold in liver of IUGR compared with control rats. These studies suggest that an aberrant intrauterine milieu permanently impairs insulin signaling in the liver so that gluconeogenesis is augmented in the IUGR rat. These processes occur early in life, before the onset of hyperglycemia, and indicate that uteroplacental insufficiency causes a primary defect in gene expression and hepatic metabolism that leads to the eventual development of overt hyperglycemia.

Glycogen synthase kinase-3 regulates IGFBP-1 gene transcription through the thymine-rich insulin response element

BACKGROUND

Hepatic expression of several gene products involved in glucose metabolism, including phosphoenolpyruvate carboxykinase (PEPCK), glucose-6-phosphatase (G6Pase) and insulin-like growth factor binding protein-1 (IGFBP-1), is rapidly and completely inhibited by insulin. This inhibition is mediated through the regulation of a DNA element present in each of these gene promoters, that we call the Thymine-rich Insulin Response Element (TIRE). The insulin signalling pathway that results in the inhibition of these gene promoters requires the activation of phosphatidylinositol 3-kinase (PI 3-kinase). However, the molecules that connect PI 3-kinase to these gene promoters are not yet fully defined. Glycogen Synthase Kinase 3 (GSK-3) is inhibited following activation of PI 3-kinase. We have shown previously that inhibitors of GSK-3 reduce the activity of two TIRE-containing gene promoters (PEPCK and G6Pase), whose products are required for gluconeogenesis.

RESULTS

In this report we demonstrate that in H4IIE-C3 cells, four distinct classes of GSK-3 inhibitor mimic the effect of insulin on a third TIRE-containing gene, IGFBP-1. We identify the TIRE as the minimum requirement for inhibition by these agents, and demonstrate that the target of GSK-3 is unlikely to be the postulated TIRE-binding protein FOXO-1. Importantly, overexpression of GSK-3 in cells reduces the insulin regulation of TIRE activity as well as endogenous IGFBP-1 expression.

CONCLUSIONS

These results implicate GSK-3 as an intermediate in the pathway from the insulin receptor to the TIRE. Indeed, this is the first demonstration of an absolute requirement for GSK-3 inhibition in insulin regulation of gene transcription. These data support the potential use of GSK-3 inhibitors in the treatment of insulin resistant states such as Type 2 diabetes mellitus, but suggest that it will be important to identify all TIRE-containing genes to assess potential side effects of these agents.

Wasting and the substrate-to-energy controlled pathway: a role for insulin resistance and amino acids

Amino acids contained in proteins can be transformed either in glucose precursors or in acetate, the end product of free fatty acid (FFA) oxidation. The dynamics of glucose, FFA, and amino acid competition for entry into the citric acid cycle (tricarboxylic acid [TCA] cycle) are very complex and not fully understood. Conditions where glucose is insufficiently driven to full oxidation are characterized by lowest efficiency in energy production per mole of oxygen consumed. Moreover, acetate provided by oxidation of FFA increases consumption of amino acids as precursors of the oxaloacetate required for condensation with acetate and for maintenance of citrate synthesis. Increased consumption of amino acids in the TCA cycle, if not matched by adequate intake, leads to muscular wasting and cachexia. Therefore, amino acid needs are very complex, and their intake must provide a balanced ratio of glucogenic and ketogenic precursors suitable to trigger entry of glucose to full oxidation and blunt the level of FFA utilization. Optimization of substrate entry into energy production must also be coupled with sufficient availability of amino acids in ratios suitable for maintaining protein synthesis, inhibiting the catabolic drive, and promoting integrity of cellular proteic structures. Alimentary proteins have a content of amino acids that is far from the stoichiometric ratios of essential amino acids required by humans. An amino acid formulation suitable to match energy needs, control carbohydrate and lipid flow into the TCA cycle, and promote protein synthesis in contracting cells is detailed in this article.

Aldosterone stimulates gene expression of hepatic gluconeogenic enzymes through the glucocorticoid receptor in a manner independent of the protein kinase B cascade

Primary aldosteronism is associated with glucose intolerance and diabetes, which is due in part to impaired insulin release caused by reduction of potassium, although other possibilities remain to be elucidated. To evaluate the in vivo effects of aldosterone on glucose metabolism, a single dose of aldosterone was administered to mice, which resulted in elevation of the blood glucose level. In primary cultured mouse hepatocytes, the gene expression of gluconeogenic enzymes such as glucose-6-phosphatase (G6Pase), fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase increased in response to aldosterone in a dose-dependent manner even at a concentration similar to a physiological condition (10(-9) M). The inhibitory effect of insulin on G6Pase gene expression was partially suppressed by aldosterone. Furthermore, aldosterone enhanced G6Pase promoter activity in human hepatoma cell line HepG2, which was prevented by co-treatment with a glucocorticoid antagonist RU-486, but not a mineralocorticoid antagonist spironolactone. In contrast, aldosterone had no effects on major insulin signaling pathways including insulin receptor substrate-1, protein kinase B, and forkhead transcription factor. These results suggest that aldosterone may affect the inhibitory effect of insulin on hepatic gluconeogenesis through the glucocorticoid receptor, which may be one of the causes of impaired glucose metabolism in primary aldosteronism.

Novel concepts in insulin regulation of hepatic gluconeogenesis

The regulation of hepatic gluconeogenesis is an important process in the adjustment of the blood glucose level, and pathological changes in the glucose production of the liver are a central characteristic in type 2 diabetes. The pharmacological intervention in signaling events that regulate the expression of the key gluconeogenic enzymes phosphoenolpyruvate carboxykinase (PEPCK) and the catalytic subunit glucose-6-phosphatase (G-6-Pase) is regarded as a potential strategy for the treatment of metabolic aberrations associated with this disease. However, such intervention requires a detailed understanding of the molecular mechanisms involved in the regulation of this process. Glucagon and glucocorticoids are known to increase hepatic gluconeogenesis by inducing the expression of PEPCK and G-6-Pase. The coactivator protein PGC-1 has been identified as an important mediator of this regulation. In contrast, insulin is known to suppress both PEPCK and G-6-Pase gene expression by the activation of PI 3-kinase. However, PI 3-kinase-independent pathways can also lead to the inhibition of gluconeogenic enzymes. This review focuses on signaling mechanisms and nuclear events that transduce the regulation of gluconeogenic enzymes.

Molecular mechanisms of insulin resistance in IRS-2-deficient hepatocytes

To assess the role of insulin receptor (IR) substrate (IRS)-2 in insulin action and resistance in the liver, immortalized neonatal hepatocyte cell lines have been generated from IRS-2(-/-), IRS-2(+/-), and wild-type mice. These cells maintained the expression of the differentiated liver markers albumin and carbamoyl phosphate synthetase, as well as bear a high number of IRs. The lack of IRS-2 did not result in enhanced IRS-1 tyrosine phosphorylation or IRS-1-associated phosphatidylinositol (PI) 3-kinase activity on insulin stimulation. Total insulin-induced PI 3-kinase activity was decreased by 50% in IRS-2(-/-) hepatocytes, but the translocation of PI-3,4,5-trisphosphate to the plasma membrane in these cells was almost completely abolished. Downstream PI 3-kinase, activation of Akt, glycogen synthase kinase (GSK)-3 (alpha and beta isoforms), Foxo1, and atypical protein kinase C were blunted in insulin-stimulated IRS-2(-/-) cells. Reconstitution of IRS-2(-/-) hepatocytes with adenoviral IRS-2 restored activation of these pathways, demonstrating that IRS-2 is essential for functional insulin signaling in hepatocytes. Insulin induced a marked glycogen synthase activity in wild-type and heterozygous primary hepatocytes; interestingly, this response was absent in IRS-2(-/-) cells but was rescued by infection with adenoviral IRS-2. Regarding gluconeogenesis, the induction of phosphoenolpyruvate carboxykinase and glucose 6-phosphatase by dibutyryl cAMP and dexamethasone was observed in primary hepatocytes of all genotypes. However, insulin was not able to suppress gluconeogenic gene expression in primary hepatocytes lacking IRS-2, but when IRS-2 signaling was reconstituted, these cells recovered this response to insulin. Suppression of gluconeogenic gene expression in IRS-2-deficient primary hepatocytes was also restored by infection with dominant negative Delta 256Foxo1.

Different mechanisms are used by insulin to repress three genes that contain a homologous thymine-rich insulin response element

Insulin rapidly and completely inhibits expression of the hepatic insulin-like growth factor binding protein-1 (IGFBP-1), phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) genes. This inhibition is mediated through a phosphatidyl inositol 3-kinase-dependent regulation of a DNA element, termed the thymine-rich insulin response element, found within the promoters of each of these genes. This has led to the conclusion that these three promoters are regulated by insulin using the same molecular mechanism. However, we recently found that the regulation of the IGFBP1 but not the PEPCK or G6Pase genes by insulin was sensitive to rapamycin, an inhibitor of mTOR. Here, we present further evidence that different regulatory pathways mediate the insulin regulation of these promoters. Importantly, we identify a protein phosphatase activity in the pathway connecting mTOR to the IGFBP-1 promoter. These data have major implications for the development of molecular therapeutics for the treatment of insulin-resistant states such as diabetes and hypertension.

Dual specificity mitogen-activated protein (MAP) kinase phosphatase-4 plays a potential role in insulin resistance

Insulin is the key hormone that controls glucose homeostasis. Dysregulation of insulin function causes diabetes mellitus. Among the two major forms of diabetes, type 2 diabetes accounts for over 90% of the affected population. The incidence of type 2 diabetes is highly related to obesity. To find novel proteins potentially involved in obesity-related insulin resistance and type 2 diabetes, a functional expression screen was performed to search for genes that negatively regulate insulin signaling. Specifically, a reporter system comprised of the PEPCK promoter upstream of alkaline phosphatase was used in a hepatocyte cell-based assay to screen an expression cDNA library for genes that reverse insulin-induced repression of PEPCK transcription. The cDNA library used in this study was derived from the white adipose tissue of ob/ob mice, which are highly insulin-resistant. The mitogen-activated dual specificity protein kinase phosphatase 4 (MKP-4) was identified as a candidate gene in this screen. Here we show that MKP-4 is expressed in insulin-responsive tissues and that the expression levels are up-regulated in obese insulin-resistant rodent models. Heterologous expression of MKP-4 in preadipocytes significantly blocked insulin-induced adipogenesis, and overexpression of MKP-4 in adipocytes inhibited insulin-stimulated glucose uptake. Our data suggest that MKP-4 negatively regulates insulin signaling and, consequently, may contribute to the pathogenesis of insulin resistance.

Insulin induces expression of adenosine kinase gene in rat lymphocytes by signaling through the mitogen-activated protein kinase pathway

The activity of adenosine kinase (AK) was significantly impaired in splenocytes isolated from diabetic rats. Administration of insulin to diabetic animals restored AK activity, protein, and mRNA levels in diabetic splenocytes. Experiments performed on cultured rat lymphocytes demonstrated that insulin did not change the stability of AK mRNA. Insulin induced AK gene expression in a dose- and time-dependent manner. Maximal increases in AK mRNA (3.9-fold) and activity level (3.7-fold) were observed at the fourth and fifth hours of cell incubation with 10 nM insulin, respectively. The insulin effect on AK expression was not influenced by dibutyryl cAMP (dcAMP). On the other hand dcAMP weakly increased (1.7-fold) basal expression of AK. Exposure of rat lymphocytes to wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), or rapamycin, an inhibitor of mTOR, did not affect the ability of insulin to stimulate expression of AK. Prior treatment of the cells with 10 microM PD98059, an inhibitor of mitogen-activated protein kinase (MAPK) kinase (MEK) completely blocked insulin-stimulated expression of AK gene. Insulin produced a significant transient increase in the tyrosine phosphorylation of ERK1/2, and PD98059 inhibited this phosphorylation. Furthermore exposure of cells to insulin has resulted in transient phosphorylation of Elk-1 on Ser-383 and sustained elevation of c-Jun and c-Fos protein. The maximal phosphorylation of Elk-1 was observed at 15 min, and was blocked by PD98059. We concluded that insulin stimulates AK gene expression through a series of events occurring sequentially. This includes activation of the MAPK cascade and subsequent phosphorylation of Elk-1 followed by increased expression of c-fos and c-jun genes.

The three insulin response sequences in the glucose-6-phosphatase catalytic subunit gene promoter are functionally distinct

Glucose-6-phosphatase catalyzes the terminal step in the gluconeogenic and glycogenolytic pathways. In HepG2 cells, the maximum repression of basal glucose-6-phosphatase catalytic subunit (G6Pase) gene transcription by insulin requires two distinct promoter regions, designated A (located between -231 and -199) and B (located between -198 and -159), that together form an insulin response unit. Region A binds hepatocyte nuclear factor-1, which acts as an accessory factor to enhance the effect of insulin, mediated through region B, on G6Pase gene transcription. We have previously shown that region B binds the transcriptional activator FKHR (FOXO1a) in vitro. Chromatin immunoprecipitation assays demonstrate that FKHR also binds the G6Pase promoter in situ and that insulin inhibits this binding. Region B contains three insulin response sequences (IRSs), designated IRS 1, 2, and 3, that share the core sequence T(G/A)TTTT. However, detailed analyses reveal that these three G6Pase IRSs are functionally distinct. Thus, FKHR binds IRS 1 with high affinity and IRS 2 with low affinity but it does not bind IRS 3. Moreover, in the context of the G6Pase promoter, IRS 1 and 2, but not IRS 3, are required for the insulin response. Surprisingly, IRS 3, as well as IRS 1 and IRS 2, can each confer an inhibitory effect of insulin on the expression of a heterologous fusion gene, indicating that, in this context, a transcription factor other than FKHR, or its orthologs, can also mediate an insulin response through the T(G/A)TTTT motif.

Elements of the glucocorticoid and retinoic acid response units are involved in cAMP-mediated expression of the PEPCK gene

Although many genes are regulated by the concerted action of several hormones, hormonal signaling to gene promoters has generally been studied one hormone at a time. The phosphoenolpyruvate carboxykinase (PEPCK) gene is a case in point. Transcription of this gene is induced by glucagon (acting by the second messenger, cAMP), glucocorticoids, and retinoic acid, and it is dominantly repressed by insulin. These hormonal responses require the presence of different hormone response units (HRUs), which consist of constellations of DNA elements and associated transcription factors. These include the glucocorticoid response unit (GRU), cAMP response unit (CRU), retinoic acid response unit (RARU), and the insulin response unit. HRUs are known to have functional overlap. In particular, the cAMP response element of the CRU is also a component of the GRU. The purpose of this study was to determine whether known GRU or RARU elements or transcription factors function as components of the CRU. We show here that the glucocorticoid accessory factor binding site 1 and glucocorticoid accessory factor binding site 3 elements, which are components of both the GRU and RARU, are an important part of the CRU. Furthermore, we find that the transcription factor, chicken ovalbumin upstream promoter-transcription factor, and two coactivators, cAMP response element-binding protein-binding protein and steroid receptor coactivator-1, participate in both the cAMP and glucocorticoid responses. This provides a further illustration of how the PEPCK gene promoter integrates different hormone responses through overlapping HRUs that utilize some of the same transcription factors and coactivators.

Effect of spontaneous gestational diabetes on fetal and postnatal hepatic insulin resistance in Lepr(db/+) mice

Infant macrosomia is a classic feature of a gestational diabetes mellitus (GDM) pregnancy and is associated with increased risk of adult obesity and type II diabetes mellitus, however mechanisms linking GDM and later disease remain poorly understood. The heterozygous leptin receptor-deficient (Lepr(db/+)) mouse develops spontaneous GDM and the fetuses display characteristics similar to infants of GDM mothers. We examined the effects of GDM on maternal insulin resistance, fetal growth, and postnatal development of hepatic insulin resistance. Fetal body weight on d 18 of gestation was 6.5% greater (p < 0.05) in pups from ad libitum-fed db/+ mothers compared with wild-type (WT) controls. Pair-feeding db/+ mothers to the intake of WT mothers normalized fetal weight despite less than normal maternal insulin sensitivity. More stringent caloric restriction reduced insulin and glucose levels below WT controls and resulted in fetal intrauterine growth restriction. The level of hepatic insulin receptor protein was decreased by 28% to 31% in both intrauterine growth restriction and fetuses from ad libitum-fed GDM mothers compared with offspring from WT mothers. In 24-wk-old adult offspring from GDM mothers, body weight was similar to WT offspring, however, the females from GDM mothers were fatter and hyperinsulinemic compared with offspring from WT mothers. Insulin-stimulated phosphorylation of Akt, a key intermediate in insulin signaling, was severely decreased in the livers of adult GDM offspring. Hepatic glucose-6-phosphatase activity was also inappropriately increased in the adult offspring from GDM mothers. These results suggest that spontaneous GDM in the pregnant Lepr(db/+) mouse is triggered by overfeeding, and this effect results in obesity and insulin resistance in the livers of the adult offspring. The specific decrease in Akt phosphorylation in livers of adult offspring suggests that this may be a mechanism for reduced insulin-dependent physiologic events, such as suppression of hepatic glucose production, a defect associated with susceptibility to type II diabetes mellitus.

Reduction of protein tyrosine phosphatase 1B increases insulin-dependent signaling in ob/ob mice

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of insulin receptor (IR) signal transduction and a drug target for treatment of type 2 diabetes. Using PTP1B antisense oligonucleotides (ASOs), effects of decreased PTP1B levels on insulin signaling in diabetic ob/ob mice were examined. Insulin stimulation, prior to sacrifice, resulted in no significant activation of insulin signaling pathways in livers from ob/ob mice. However, in PTP1B ASO-treated mice, in which PTP1B protein was decreased by 60% in liver, similar stimulation with insulin resulted in increased tyrosine phosphorylation of the IR and IR substrate (IRS)-1 and -2 by threefold, fourfold, and threefold, respectively. IRS-2-associated phosphatidylinositol 3-kinase activity was also increased threefold. Protein kinase B (PKB) serine phosphorylation was increased sevenfold in liver of PTP1B ASO-treated mice upon insulin stimulation, while phosphorylation of PKB substrates, glycogen synthase kinase (GSK)-3alpha and -3beta, was increased more than twofold. Peripheral insulin signaling was increased by PTP1B ASO, as evidenced by increased phosphorylation of PKB in muscle of insulin-stimulated PTP1B ASO-treated animals despite the lack of measurable effects on muscle PTP1B protein. These results indicate that reduction of PTP1B is sufficient to increase insulin-dependent metabolic signaling and improve insulin sensitivity in a diabetic animal model.

Insulin signal transduction pathways and insulin-induced gene expression

Insulin regulates metabolic activity, gene transcription, and cell growth by modulating the activity of several intracellular signaling pathways. Insulin activation of one mitogen-activated protein kinase cascade, the MEK/ERK kinase cascade, is well described. However, the effect of insulin on the parallel p38 pathway is less well understood. The present work examines the effect of inhibiting the p38 signaling pathway by use of specific inhibitors, either alone or in combination with insulin, on the activation of ERK1/2 and on the regulation of gene transcription in rat hepatoma cells. Activation of ERK1/2 was induced by insulin and was dependent on the activation of MEK1, the kinase upstream of ERK in this pathway. Treatment of cells with p38 inhibitors also induced ERK1/2 activation/phosphorylation. The addition of p38 inhibitors followed by insulin addition resulted in a greater than additive activation of ERK1/2. The two genes studied, c-Fos and Pip92, are immediate-early genes that are dependent on the ERK1/2 pathway for insulin-regulated induction because the insulin effect was inhibited by pretreatment with a MEK1 inhibitor. The addition of p38 inhibitors induced transcription of both genes in a dose-dependent manner, and insulin stimulation of both genes was enhanced by prior treatment with p38 inhibitors. The ability of the p38 inhibitors to induce ERK1/2 and gene transcription, both alone and in combination with insulin, was abolished by prior inhibition of MEK1. These data suggest possible cross-talk between the p38 and ERK1/2 signaling pathways and a potential role of p38 in insulin signaling.

Activating transcription factor-2 mediates transcriptional regulation of gluconeogenic gene PEPCK by retinoic acid

All-trans-retinoic acid (RA) is known to increase the rate of transcription of the PEPCK gene upon engagement of the RA receptor (RAR). RA also mediates induction of specific gene transcription via several signaling pathways as a nongenomic effect. Here we show that RA upregulation of PEPCK promoter activity requires the cAMP response element (CRE)-1 in addition to the RA-response element and that activating transcription factor-2 (ATF-2) binds the CRE element to mediate this effect. Furthermore, we show that RA treatment potentiates ATF-2-dependent transactivation by inducing specific phosphorylation of ATF-2 by p38beta kinase. ATF-2 activation by RA blocked the inhibitory intramolecular interaction of ATF-2 amino and carboxyl terminal domains in a p38beta kinase-dependent manner. Consistent with these results, RA treatment increased the DNA binding activity of ATF-2 on the PEPCK CRE-1 sequence. Taken together, the data suggest that RA activates the p38beta kinase pathway leading to phosphorylation and activation of ATF-2, thereby enhancing PEPCK gene transcription and glucose production.

Role of Akt/protein kinase B in metabolism

Since its discovery more than a decade ago, the Ser/Thr kinase Akt/PKB (protein kinase B) has been recognized as being remarkably well conserved across a broad range of species and involved in a diverse array of cellular processes. Among its many roles, Akt appears to be common to signaling pathways that mediate the metabolic effects of insulin in several physiologically important target tissues. Refining our understanding of those pivotal molecular components that normally coordinate insulin action throughout the body is essential for a full understanding of insulin resistance in diabetes mellitus and ultimately the successful treatment of this disease.