Differential effects of constitutively active phosphatidylinositol 3-kinase on glucose transport, glycogen synthase activity, and DNA synthesis in 3T3-L1 adipocytes

Machine Learning Reveals Lipidome Remodeling Dynamics in a Mouse Model of Ovarian Cancer

Ovarian cancer (OC) is one of the deadliest cancers affecting the female reproductive system. It may present little or no symptoms at the early stages and typically unspecific symptoms at later stages. High-grade serous ovarian cancer (HGSC) is the subtype responsible for most ovarian cancer deaths. However, very little is known about the metabolic course of this disease, particularly in its early stages. In this longitudinal study, we examined the temporal course of serum lipidome changes using a robust HGSC mouse model and machine learning data analysis. Early progression of HGSC was marked by increased levels of phosphatidylcholines and phosphatidylethanolamines. In contrast, later stages featured more diverse lipid alterations, including fatty acids and their derivatives, triglycerides, ceramides, hexosylceramides, sphingomyelins, lysophosphatidylcholines, and phosphatidylinositols. These alterations underscored unique perturbations in cell membrane stability, proliferation, and survival during cancer development and progression, offering potential targets for early detection and prognosis of human ovarian cancer.

Machine Learning Reveals Lipidome Remodeling Dynamics in a Mouse Model of Ovarian Cancer

Ovarian cancer (OC) is one of the deadliest cancers affecting the female reproductive system. It may present little or no symptoms at the early stages, and typically unspecific symptoms at later stages. High-grade serous ovarian cancer (HGSC) is the subtype responsible for most ovarian cancer deaths. However, very little is known about the metabolic course of this disease, particularly in its early stages. In this longitudinal study, we examined the temporal course of serum lipidome changes using a robust HGSC mouse model and machine learning data analysis. Early progression of HGSC was marked by increased levels of phosphatidylcholines and phosphatidylethanolamines. In contrast, later stages featured more diverse lipids alterations, including fatty acids and their derivatives, triglycerides, ceramides, hexosylceramides, sphingomyelins, lysophosphatidylcholines, and phosphatidylinositols. These alterations underscored unique perturbations in cell membrane stability, proliferation, and survival during cancer development and progression, offering potential targets for early detection and prognosis of human ovarian cancer.

Teaser

Time-resolved lipidome remodeling in an ovarian cancer model is studied through lipidomics and machine learning.

Metabolic Role of PTEN in Insulin Signaling and Resistance

Phosphatase and tensin homolog (PTEN) is most prominently known for its function in tumorigenesis. However, a metabolic role of PTEN is emerging as a result of its altered expression in type 2 diabetes (T2D), which results in impaired insulin signaling and promotion of insulin resistance during the pathogenesis of T2D. PTEN functions in regulating insulin signaling across different organs have been identified. Through the use of a variety of models, such as tissue-specific knockout (KO) mice and in vitro cell cultures, PTEN's role in regulating insulin action has been elucidated across many cell types. Herein, we will review the recent advancements in the understanding of PTEN's metabolic functions in each of the tissues and cell types that contribute to regulating systemic insulin sensitivity and discuss how PTEN may represent a promising therapeutic strategy for treatment or prevention of T2D.

Fetal and trophoblast PI3K p110α have distinct roles in regulating resource supply to the growing fetus in mice

Studies suggest that placental nutrient supply adapts according to fetal demands. However, signaling events underlying placental adaptations remain unknown. Here we demonstrate that phosphoinositide 3-kinase p110α in the fetus and the trophoblast interplay to regulate placental nutrient supply and fetal growth. Complete loss of fetal p110α caused embryonic death, whilst heterozygous loss resulted in fetal growth restriction and impaired placental formation and nutrient transport. Loss of trophoblast p110α resulted in viable fetuses, abnormal placental development and a failure of the placenta to transport sufficient nutrients to match fetal demands for growth. Using RNA-seq we identified genes downstream of p110α in the trophoblast that are important in adapting placental phenotype. Using CRISPR/Cas9 we showed loss of p110α differentially affects gene expression in trophoblast and embryonic stem cells. Our findings reveal important, but distinct roles for p110α in the different compartments of the conceptus, which control fetal resource acquisition and growth.

PRR14 is a novel activator of the PI3K pathway promoting lung carcinogenesis

Chromosomal focal amplifications often cause an increase in gene copy number, contributing to the pathogenesis of cancer. PRR14 overexpression is associated with recurrent locus amplification in lung cancer, and it correlates with a poor prognosis. We show that increased PRR14 expression promoted and reduced PRR14 expression impeded lung cancer cell proliferation. Interestingly, PRR14 cells were markedly enlarged in size and exhibited an elevated activity of the PI3-kinase/Akt/mTOR pathway, which was associated with a heightened sensitivity to the inhibitors of PI3K and mammalian target of rapamycin (mTOR). Biochemical analysis revealed that PRR14, as a proline-rich protein, binds to the Src homology 3 (SH3) domains of GRB2 resulting in PI3K activation. Significantly, two cancer patient-derived PRR14 mutants displayed considerably augmented GRB2-binding and an enhanced ability of promoting cell proliferation. Together with the in vivo data demonstrating a strong tumor-promoting activity of PRR14 and the mutants, our work uncovered this proline-rich protein as a novel activator of the PI3K pathway that promoted tumorigenesis in lung cancer.

Disruption of cytochrome c oxidase function induces the Warburg effect and metabolic reprogramming

Defects in mitochondrial oxidative phosphorylation complexes, altered bioenergetics and metabolic shift are often seen in cancers. Here we show a role for the dysfunction of electron transport chain component, cytochrome c oxidase (CcO) in cancer progression. We show that genetic silencing of the CcO complex by shRNA expression and loss of CcO activity in multiple cell types from the mouse and human sources resulted in metabolic shift to glycolysis, loss of anchorage dependent growth and acquired invasive phenotypes. Disruption of CcO complex caused loss of transmembrane potential and induction of Ca²⁺/Calcineurin-mediated retrograde signaling. Propagation of this signaling, includes activation of PI3-kinase, IGF1R and Akt, Ca²⁺ sensitive transcription factors and also, TGFβ1, MMP16, periostin that are involved in oncogenic progression. Whole genome expression analysis showed up regulation of genes involved in cell signaling, extracellular matrix interactions, cell morphogenesis, cell motility and migration. The transcription profiles reveal extensive similarity to retrograde signaling initiated by partial mtDNA depletion, though distinct differences are observed in signaling induced by CcO dysfunction. The possible CcO dysfunction as a biomarker for cancer progression was supported by data showing that esophageal tumors from human patients show reduced CcO subunits IVi1 and Vb in regions that were previously shown to be hypoxic core of the tumors. Our results show that mitochondrial electron transport chain defect initiates a retrograde signaling. These results suggest that a defect in CcO complex can potentially induce tumor progression.

Insulin receptor-independent upregulation of cellular glucose uptake

BACKGROUND

Cellular glucose uptake can be enhanced by upregulating Ras signaling in either insulin-dependent or -independent manner. In presence of insulin and intact insulin signaling, Ras has a negligible role in glucose uptake. Conversely, when insulin signaling is impaired in obesity or diabetes, the insulin-independent Ras pathway may be valuable for enhancing glucose disposal. We previously reported that Ad36, a human adenovirus, enhances cellular glucose uptake by upregulating the Ras/Glut4 pathway. Here, we investigated if Ad36-upregulated Ras via the insulin-independent pathway, to enhance glucose uptake. Furthermore, uncontrolled upregulation of Ras is linked with oncogenic cell transformation, if the tumor-suppressor gene p53 is also downregulated. Hence, we determined if upregulation of Ras by Ad36 would induce oncogenic cell transformation. Finally, we determined the relevance of Ad36 to insulin resistance in humans.

METHODS

Insulin receptor (IR) was knocked down with small interfering RNA in 3T3-L1 adipocytes, to determine if Ad36 increases the Ras/Glut4 pathway and glucose uptake without IR-signaling. Next, the effects of Ad36 on cell transformation and p53 abundance were determined. Finally, overweight or obese women were screened for seropositivity to Ad36, as an indicator of natural Ad36 infection. Associations of Ad36 infection with adiposity and C-reactive proteins (CRPs)-two key markers of insulin resistance, and with glucose disposal, were determined.

RESULTS

Unaffected by IR knock-down, Ad36 significantly increased the Ras pathway, Glut4 translocation and glucose uptake in 3T3-L1 adipocytes. Despite Ras upregulation, Ad36 did not transform 3T3-L1 cells. This may be because Ad36 significantly increased p53 protein in 3T3-L1 cells or mice adipose tissue. Ad36 seropositivity was associated with greater adiposity and CRP levels, yet a significantly higher systemic glucose disposal rate.

CONCLUSIONS

Overall, the study offers Ras/Glut4 pathway as an alternate to enhance glucose disposal when insulin signaling is impaired, and, importantly, provides Ad36 as a tool to understand the modulation of that pathway.

Insulin-stimulated glucose uptake occurs in specialized cells within the cumulus oocyte complex

The oocyte exists within the mammalian follicle surrounded by somatic cumulus cells. These cumulus cells metabolize the majority of the glucose within the cumulus oocyte complex and provide energy substrates and intermediates such as pyruvate to the oocyte. The insulin receptor is present in cumulus cells and oocytes; however, it is unknown whether insulin-stimulated glucose uptake occurs in either cell type. Insulin-stimulated glucose uptake is thought to be unique to adipocytes, skeletal and cardiac muscle, and the blastocyst. Here, we show for the first time that many of the components required for insulin signaling are present in both cumulus cells and oocytes. We performed a set of experiments on mouse cumulus cells and oocytes and human cumulus cells using the nonmetabolizable glucose analog 2-deoxy-d-glucose to measure basal and insulin-stimulated glucose uptake. We show that insulin-stimulated glucose uptake occurs in both compact and expanded cumulus cells of mice, as well as in human cumulus cells. Oocytes, however, do not display insulin-stimulated glucose uptake. Insulin-stimulated glucose uptake in cumulus cells is mediated through phosphatidylinositol 3-kinase signaling as shown by inhibition of insulin-stimulated glucose uptake and Akt phosphorylation with the specific phosphatidylinositol 3-kinase inhibitor, LY294002. To test the effect of systemic in vivo insulin resistance on insulin sensitivity in the cumulus cell, cumulus cells from high fat-fed, insulin-resistant mice and women with polycystic ovary syndrome were examined. Both sets of cells displayed blunted insulin-stimulated glucose uptake. Our studies identify another tissue that, through a classical insulin-signaling pathway, demonstrates insulin-stimulated glucose uptake. Moreover, these findings suggest insulin resistance occurs in these cells under conditions of systemic insulin resistance.

Regulation of glucose transport by ROCK1 differs from that of ROCK2 and is controlled by actin polymerization

A role of Rho-associated coiled-coil-containing protein kinase (ROCK)1 in regulating whole-body glucose homeostasis has been reported. However, cell-autonomous effects of ROCK1 on insulin-dependent glucose transport in adipocytes and muscle cells have not been elucidated. To determine the specific role of ROCK1 in glucose transport directly, ROCK1 expression in 3T3-L1 adipocytes and L6 myoblasts was biologically modulated. Here, we show that small interfering RNA-mediated ROCK1 depletion decreased insulin-induced glucose transport in adipocytes and myoblasts, whereas adenovirus-mediated ROCK1 expression increased this in a dose-dependent manner, indicating that ROCK1 is permissive for glucose transport. Inhibition of ROCK1 also impaired glucose transporter 4 translocation in 3T3-L1 adipocytes. Importantly, the ED₅₀ of insulin for adipocyte glucose transport was reduced when ROCK1 was expressed, leading to hypersensitivity to insulin. These effects are dependent on actin cytoskeleton remodeling, because inhibitors of actin polymerization significantly decreased ROCK1's effect to promote insulin-stimulated glucose transport. Unlike ROCK2, ROCK1 binding to insulin receptor substrate (IRS)-1 was not detected by immunoprecipitation, although cell fractionation demonstrated both ROCK isoforms localize with IRS-1 in low-density microsomes. Moreover, insulin's ability to increase IRS-1 tyrosine 612 and serine 632/635 phosphorylation was attenuated by ROCK1 suppression. Replacing IRS-1 serine 632/635 with alanine reduced insulin-stimulated phosphatidylinositol 3-kinase activation and glucose transport in 3T3-L1 adipocytes, indicating that phosphorylation of these serine residues of IRS-1, which are substrates of the ROCK2 isoform in vitro, are crucial for maximal stimulation of glucose transport by insulin. Our studies identify ROCK1 as an important positive regulator of insulin action on glucose transport in adipocytes and muscle cells.

Selective PPARγ modulator INT131 normalizes insulin signaling defects and improves bone mass in diet-induced obese mice

INT131 is a potent non-thiazolidinedione (TZD)-selective peroxisome proliferator-activated receptor-γ modulator being developed for the treatment of type 2 diabetes. In preclinical studies and a phase II clinical trial, INT131 has been shown to lower glucose levels and ameliorate insulin resistance without typical TZD side effects. To determine whether the insulin-sensitizing action of INT131 is mediated by effects on insulin-mediated glucose homeostasis and insulin signaling, high-fat diet-induced obese (DIO) insulin-resistant mice treated with INT131 were studied. INT131's effects on bone density were also investigated. Treatment with INT131 enhanced systemic insulin sensitivity, as revealed by lower insulin levels in the fasted state and an increase in the area above the curve during an insulin tolerance test. These effects were independent of changes in adiposity. Insulin-stimulated PI3K activity in skeletal muscle and adipose tissue of DIO mice was significantly reduced ∼50-65%, but this was restored completely by INT131 therapy. The INT131 effects on PI3K activity are most likely due to increased IRS-1 tyrosine phosphorylation. Concurrently, insulin-mediated Akt phosphorylation also increased after INT131 treatment in DIO mice. Importantly, INT131 therapy caused a significant increase in bone mineral density without alteration in circulating osteocalcin in these mice. These data suggest that a newly developed insulin-sensitizing agent, INT131, normalizes obesity-related defects in insulin action on PI3K signaling in insulin target tissues by a mechanism involved in glycemic control. If these data are confirmed in humans, INT131 could be used for treating type 2 diabetes without loss in bone mass.

Sodium/myo-inositol cotransporter 1 and myo-inositol are essential for osteogenesis and bone formation

myo-Inositol (MI) plays an essential role in several important processes of cell physiology, is involved in the neural system, and provides an effective treatment for some psychiatric disorders. Its role in osteogenesis and bone formation nonetheless is unclear. Sodium/MI cotransporter 1 (SMIT1, the major cotransporter of MI) knockout (SMIT1(-/-)) mice with markedly reduced tissue MI levels were used to characterize the essential roles of MI and SMIT1 in osteogenesis. SMIT1(-/-) embryos had a dramatic delay in prenatal mineralization and died soon after birth owing to respiratory failure, but this could be rescued by maternal MI supplementation. The rescued SMIT1(-/-) mice had shorter limbs, decreased bone density, and abnormal bone architecture in adulthood. Deletion of SMIT1 resulted in retarded postnatal osteoblastic differentiation and bone formation in vivo and in vitro. Continuous MI supplementation partially restored the abnormal bone phenotypes in adult SMIT1(-/-) mice and strengthened bone structure in SMIT1(+/+) mice. Although MI content was much lower in SMIT1(-/-) mesenchymal cells (MSCs), the I(1,4,5)P(3) signaling pathway was excluded as the means by which SMIT1 and MI affected osteogenesis. PCR expression array revealed Fgf4, leptin, Sele, Selp, and Nos2 as novel target genes of SMIT1 and MI. SMIT1 was constitutively expressed in multipotential C3H10T1/2 and preosteoblastic MC3T3-E1 cells and could be upregulated during bone morphogenetic protein 2 (BMP-2)-induced osteogenesis. Collectively, this study demonstrated that deficiency in SMIT1 and MI has a detrimental impact on prenatal skeletal development and postnatal bone remodeling and confirmed their essential roles in osteogenesis, bone formation, and bone mineral density (BMD) determination.

Role of calcineurin, hnRNPA2 and Akt in mitochondrial respiratory stress-mediated transcription activation of nuclear gene targets

Pathophysiological conditions causing mitochondrial dysfunction and altered transmembrane potential (psim) initiate a mitochondrial respiratory stress response, also known as mitochondrial retrograde response, in a variety of mammalian cells. An increase in the cytosolic Ca2+ [Ca2+]c as part of this signaling cascade activates Ca2+ responsive phosphatase, calcineurin (Cn). Activation of IGF1R accompanied by increased glycolysis, invasiveness, and resistance to apoptosis is a phenotypic hallmark of C2C12 skeletal muscle cells subjected to this stress. The signaling is associated with activation and increased nuclear translocation of a number of transcription factors including a novel NFkappaB (cRel:p50) pathway, NFAT, CREB and C/EBPdelta. This culminates in the upregulation of a number of nuclear genes including Cathepsin L, RyR1, Glut4 and Akt1. We observed that stress regulated transcription activation of nuclear genes involves a cooperative interplay between NFkappaB (cRel:p50), C/EBPdelta, CREB, and NFAT. Our results show that the functional synergy of these factors requires the stress-activated heterogeneous nuclear ribonucleoprotein, hnRNPA2 as a transcriptional coactivator. We report here that mitochondrial stress leads to induced expression and activation of serine threonine kinase Akt1. Interestingly, we observe that Akt1 phosphorylates hnRNPA2 under mitochondrial stress conditions, which is a crucial step for the recruitment of this coactivator to the stress target promoters and culmination in mitochondrial stress-mediated transcription activation of target genes. We propose that mitochondrial stress plays an important role in tumor progression and emergence of invasive phenotypes.

Effect of ghrelin on glucose-insulin homeostasis: therapeutic implications

Ghrelin is a 28-amino-acid peptide that displays a strong growth hormone- (GH-) releasing activity through the activation of the growth hormone secretagogue receptor (GHSR). The first studies about role of ghrelin were focused on its orexigenic ability, but despite indisputable pharmacological data, the evidence for a physiological role for ghrelin in the control of appetite is much less clear. Mice with targeted deletion of either ghrelin or the GHSR exhibit an essentially normal metabolic phenotype when fed a regular chow diet, suggesting that ghrelin may have a redundant role in the regulation of food intake. RNAs for ghrelin as well as GHSR are expressed in the pancreas of rats and humans and several studies propose that ghrelin could have an important function in glucose homeostasis and insulin release, independent of GH secretion. Low plasma ghrelin levels are associated with elevated fasting insulin levels and insulin resistance, suggesting both physiological and pathophysiological roles for ghrelin. For this reason, at least theoretically, ghrelin and/or its signalling manipulation could be useful for the treatment or prevention of diseases of glucose homeostasis such as type 2 diabetes.

Adipose tissue plasticity during catch-up fat driven by thrifty metabolism: relevance for muscle-adipose glucose redistribution during catch-up growth

OBJECTIVE

Catch-up growth, a risk factor for later type 2 diabetes, is characterized by hyperinsulinemia, accelerated body-fat recovery (catch-up fat), and enhanced glucose utilization in adipose tissue. Our objective was to characterize the determinants of enhanced glucose utilization in adipose tissue during catch-up fat.

RESEARCH DESIGN AND METHODS

White adipose tissue morphometry, lipogenic capacity, fatty acid composition, insulin signaling, in vivo glucose homeostasis, and insulinemic response to glucose were assessed in a rat model of semistarvation-refeeding. This model is characterized by glucose redistribution from skeletal muscle to adipose tissue during catch-up fat that results solely from suppressed thermogenesis (i.e., without hyperphagia).

RESULTS

Adipose tissue recovery during the dynamic phase of catch-up fat is accompanied by increased adipocyte number with smaller diameter, increased expression of genes for adipogenesis and de novo lipogenesis, increased fatty acid synthase activity, increased proportion of saturated fatty acids in triglyceride (storage) fraction but not in phospholipid (membrane) fraction, and no impairment in insulin signaling. Furthermore, it is shown that hyperinsulinemia and enhanced adipose tissue de novo lipogenesis occur concomitantly and are very early events in catch-up fat.

CONCLUSIONS

These findings suggest that increased adipose tissue insulin stimulation and consequential increase in intracellular glucose flux play an important role in initiating catch-up fat. Once activated, the machinery for lipogenesis and adipogenesis contribute to sustain an increased insulin-stimulated glucose flux toward fat storage. Such adipose tissue plasticity could play an active role in the thrifty metabolism that underlies glucose redistribution from skeletal muscle to adipose tissue.

Insulin action on glucose transporters through molecular switches, tracks and tethers

Glucose entry into muscle cells is precisely regulated by insulin, through recruitment of GLUT4 (glucose transporter-4) to the membrane of muscle and fat cells. Work done over more than two decades has contributed to mapping the insulin signalling and GLUT4 vesicle trafficking events underpinning this response. In spite of this intensive scientific research, there are outstanding questions that continue to challenge us today. The present review summarizes the knowledge in the field, with emphasis on the latest breakthroughs in insulin signalling at the level of AS160 (Akt substrate of 160 kDa), TBC1D1 (tre-2/USP6, BUB2, cdc16 domain family member 1) and their target Rab proteins; in vesicle trafficking at the level of vesicle mobilization, tethering, docking and fusion with the membrane; and in the participation of the cytoskeleton to achieve optimal temporal and spatial location of insulin-derived signals and GLUT4 vesicles.

A distinctive physiological role for IkappaBbeta in the propagation of mitochondrial respiratory stress signaling

The NFkappaBs regulate an array of physiological and pathological processes, including propagation of mitochondrial respiratory stress signaling in mammalian cells. We showed previously that mitochondrial stress activates NFkappaB using a novel calcineurin-requiring pathway that is different from canonical or non-canonical pathways. This study shows that IkappaBbeta is essential for the propagation of mitochondrial stress signaling. Knock down of IkappaBbeta, but not IkappaBalpha, mRNA reduced the mitochondrial stress-mediated activation and nuclear translocation of cRel:p50, inhibiting expression of nuclear target genes RyR1 and cathepsin L. IkappaBbeta mRNA knock down also reduced resistance to staurosporine-induced apoptosis and decreased in vitro invasiveness. Induced receptor switching to insulin-like growth factor-1 receptor and increased glucose uptake are hallmarks of mitochondrial stress. IkappaBbeta mRNA knock down selectively abrogated the receptor switch and altered tubulin cytoskeletal organization. These results show that mitochondrial stress signaling uses an IkappaBbeta-initiated NFkappaB pathway that is distinct from the other known NFkappaB pathways. Furthermore, our results demonstrate the distinctive physiological roles of the two inhibitory proteins IkappaBbeta and IkappaBalpha.

Insulin activates ATP-sensitive potassium channels via phosphatidylinositol 3-kinase in cultured vascular smooth muscle cells

The effects of insulin on the vasculature are significant because insulin resistance is associated with hypertension. To increase the understanding of the effects of insulin on the vasculature, we analyzed changes in potassium ion transport in cultured vascular smooth muscle cells (VSMCs). Using the potential-sensitive fluorescence dye bis-(1,3-dibutylbarbituric acid)trimethine oxonol [DiBAC4(3)], we found that insulin induced membrane hyperpolarization after 2 min in A10 cells. Insulin-induced hyperpolarization was suppressed by glibenclamide, an ATP-sensitive potassium (K(ATP)) channel blocker. Using a cell-attached patch clamp experiment, the K(ATP) channel was activated by insulin in both A10 cells and isolated VSMCs from rat aortas, indicating that insulin causes membrane hyperpolarization via K(ATP) channel activation. These effects were not dependent on intracellular ATP concentration, but wortmannin, a phosphatidylinositol 3-kinase (PI3-K) inhibitor, significantly suppressed insulin-induced K(ATP) channel activation. In addition, insulin enhanced phosphorylation of insulin receptor, insulin receptor substrate (IRS)-1 and protein kinase B (Akt) after 2 min. These data suggest that K(ATP) channel activation by insulin is mediated by PI3-K. Furthermore, using a nitric oxide synthase (NOS) inhibitor, we found that NOS might play an important role downstream of PI3-K in insulin-induced K(ATP) channel activation. This study may contribute to our understanding of mechanisms of insulin resistance-associated hypertension.

Activation of a novel calcineurin-mediated insulin-like growth factor-1 receptor pathway, altered metabolism, and tumor cell invasion in cells subjected to mitochondrial respiratory stress

We have previously shown that disruption of mitochondrial membrane potential by depletion of mitochondrial DNA (mtDNA) or treatment with a mitochondrial ionophore, carbonyl cyanide m-chlorophenylhydrazone, initiates a stress signaling, which causes resistance to apoptosis, and induces invasive behavior in C2C12 myocytes and A549 cells. In the present study we show that calcineurin (Cn), activated as part of this stress signaling, plays an important role in increased glucose uptake and glycolysis. Here we report that, although both insulin and insulin-like growth factor-1 receptor levels (IR and IGF1R, respectively) are increased in response to mitochondrial stress, autophosphorylation of IGF1R was selectively increased suggesting a shift in receptor pathways. Using an approach with FK506, an inhibitor of Cn, and mRNA silencing by small interference RNA we show that mitochondrial stress-activated Cn is critical for increased GLUT 4 and IGF1R expression and activation. The importance of the IGF1R pathway in cell survival under mitochondrial stress is demonstrated by increased apoptosis either by IGF1R mRNA silencing or by treatment with IGF1R inhibitors (AG1024 and picropodophyllin). This study describes a novel mechanism of mitochondrial stress-induced metabolic shift involving Cn with implications in resistance to apoptosis and tumor proliferation.

Phosphatidylinositol 3-phosphate [PtdIns3P] is generated at the plasma membrane by an inositol polyphosphate 5-phosphatase: endogenous PtdIns3P can promote GLUT4 translocation to the plasma membrane

Exogenous delivery of carrier-linked phosphatidylinositol 3-phosphate [PtdIns(3)P] to adipocytes promotes the trafficking, but not the insertion, of the glucose transporter GLUT4 into the plasma membrane. However, it is yet to be demonstrated if endogenous PtdIns(3)P regulates GLUT4 trafficking and, in addition, the metabolic pathways mediating plasma membrane PtdIns(3)P synthesis are uncharacterized. In unstimulated 3T3-L1 adipocytes, conditions under which PtdIns(3,4,5)P3 was not synthesized, ectopic expression of wild-type, but not catalytically inactive 72-kDa inositol polyphosphate 5-phosphatase (72-5ptase), generated PtdIns(3)P at the plasma membrane. Immunoprecipitated 72-5ptase from adipocytes hydrolyzed PtdIns(3,5)P2, forming PtdIns(3)P. Overexpression of the 72-5ptase was used to functionally dissect the role of endogenous PtdIns(3)P in GLUT4 translocation and/or plasma membrane insertion. In unstimulated adipocytes wild type, but not catalytically inactive, 72-5ptase, promoted GLUT4 translocation and insertion into the plasma membrane but not glucose uptake. Overexpression of FLAG-2xFYVE/Hrs, which binds and sequesters PtdIns(3)P, blocked 72-5ptase-induced GLUT4 translocation. Actin monomer binding, using latrunculin A treatment, also blocked 72-5ptase-stimulated GLUT4 translocation. 72-5ptase expression promoted GLUT4 trafficking via a Rab11-dependent pathway but not by Rab5-mediated endocytosis. Therefore, endogenous PtdIns(3)P at the plasma membrane promotes GLUT4 translocation.

GABAA receptor-associated phosphoinositide 3-kinase is required for insulin-induced recruitment of postsynaptic GABAA receptors

Type A gamma-aminobutyric acid (GABAA) receptors mediate most of the fast inhibitory synaptic transmission within the vertebrate brain. The regulation of this inhibition is vital in modulating neural activity. One regulator of GABAA receptor function is insulin, which can serve to enhance GABAA receptor-mediated miniature inhibitory postsynaptic currents, via an increase in the number of receptors at the plasma membrane. We set out to investigate the molecular mechanisms involved in the insulin-induced potentiation of GABAA receptor-mediated responses, by examining the role of phosphoinositide 3-kinase (PI3-K), a key mediator of the insulin response within the brain. We found that PI3-K associates with the GABAA receptor, and this interaction is increased following insulin treatment. Additionally, the beta2 subunit of the GABAA receptor appears to mediate the insulin-stimulated association with the N-terminal SH2 domain of the p85 subunit of PI3-K. Our results imply a mechanism whereby insulin can regulate changes in synaptic transmission through its downstream actions on the GABAA receptor.

Berberine, a natural plant product, activates AMP-activated protein kinase with beneficial metabolic effects in diabetic and insulin-resistant states

Berberine has been shown to have antidiabetic properties, although its mode of action is not known. Here, we have investigated the metabolic effects of berberine in two animal models of insulin resistance and in insulin-responsive cell lines. Berberine reduced body weight and caused a significant improvement in glucose tolerance without altering food intake in db/db mice. Similarly, berberine reduced body weight and plasma triglycerides and improved insulin action in high-fat-fed Wistar rats. Berberine downregulated the expression of genes involved in lipogenesis and upregulated those involved in energy expenditure in adipose tissue and muscle. Berberine treatment resulted in increased AMP-activated protein kinase (AMPK) activity in 3T3-L1 adipocytes and L6 myotubes, increased GLUT4 translocation in L6 cells in a phosphatidylinositol 3' kinase-independent manner, and reduced lipid accumulation in 3T3-L1 adipocytes. These findings suggest that berberine displays beneficial effects in the treatment of diabetes and obesity at least in part via stimulation of AMPK activity.

PI3K is required for insulin-stimulated but not EGF-stimulated ERK1/2 activation

The Ras/Raf/extracellular signal-regulated kinase 1 and 2 (ERK1/2) signaling pathway is known to cross-talk with other signaling pathways, including phosphatidylinositol 3-kinase (PI3K)/Akt pathway. However, the role of PI3K in ERK-1/2 activation induced by tyrosine kinase receptors was not fully understood. Here, we report that two structurally distinct PI3K inhibitors, wortmannin and LY294002, inhibited insulin-induced activation of ERK1/2 but had no effect on EGF-induced activation of ERK1/2 in hepatocellular carcinoma BEL-7402 and SMMC-7721 cells, breast cancer MCF-7 cells, and prostate cancer LNCaP cells. Although protein kinase C could act as a mediator between PI3K and ERK1/2, protein kinase C inhibitor chelerythrine chloride did not inhibit insulin-induced ERK1/2 activation. Both insulin- and EGF-induced ERK1/2 activation are strictly dependent on Ras activation, however, wortmannin only inhibited insulin-induced, but not EGF-induced Ras activation. These results indicate that PI3K plays different roles in the activation of Ras/ERK1/2 signaling by insulin and EGF, and that insulin-stimulated, but not EGF-stimulated, ERK1/2 and Akt signalings diverge at PI3K.

Possible mode of action for insulinomimetic activity of vanadyl(IV) compounds in adipocytes

Vanadyl(IV) ions (+4 oxidation state of vanadium) and their complexes have been shown to have in vitro insulinomimetic activity and to be effective in treating animals with diabetes mellitus. Although, researchers have proposed many vanadyl compounds for the treatment of diabetes patients, the mode of action of vanadyl compounds remains controversial. In order to evaluate the mode of action of these compounds, we examined the insulinomimetic activity of VOSO4, bis(picolinato)oxovanadyl(IV), and bis(maltolato)oxovanadyl(IV) in the presence of several inhibitors relevant to the glucose metabolism. After confirming that these vanadyl compounds were incorporated in the adipocytes as estimated by ESR method, we evaluated the mode of action by examining free fatty acids (FFA) release in the adipocytes. Inhibition of FFA release by these vanadyl compounds was found to be reversed by the addition of inhibitors, typically by cytochalasin B (glucose transporter 4 (GLUT4) inhibitor), cilostamide (phosphodiesterase inhibitor), HNMPA-(AM)3 (tyrosine kinase inhibitor), and wortmannin (PI3-k inhibitor), indicating that these compounds affect primarily GLUT4 and phosphodiesterase, as named "ensemble mechanism". Based on these results, we suggest that vanadyl compounds act on at least four sites relevant to the glucose metabolism, and on GLUT4 and phosphodiesterase in particular in rat adipocytes, which in turn normalizes the blood glucose levels of diabetic animals. The obtained results provide evidence for the role of vanadyl ion and its complexes in stimulation of the uptake and degeneration of glucose.

Activators of AMP-activated protein kinase enhance GLUT4 translocation and its glucose transport activity in 3T3-L1 adipocytes

To determine whether the increase in glucose uptake following AMP-activated protein kinase (AMPK) activation in adipocytes is mediated by accelerated GLUT4 translocation into plasma membrane, we constructed a chimera between GLUT4 and enhanced green fluorescent protein (GLUT4-eGFP) and transferred its cDNA into the nucleus of 3T3-L1 adipocytes. Then, the dynamics of GLUT4-eGFP translocation were visualized in living cells by means of laser scanning confocal microscopy. It was revealed that the stimulation with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) and 2,4-dinitrophenol (DNP), known activators of AMPK, promptly accelerates its translocation within 4 min, as was found in the case of insulin stimulation. The insulin-induced GLUT4 translocation was markedly inhibited after addition of wortmannin (P < 0.01). However, the GLUT4 translocation through AMPK activators AICAR and DNP was not affected by wortmannin. Insulin- and AMPK-activated translocation of GLUT4 was not inhibited by SB-203580, an inhibitor of p38 mitogen-activated protein kinase (MAPK). Glucose uptake was significantly increased after addition of AMPK activators AICAR and DNP (P < 0.05). AMPK- and insulin-stimulated glucose uptake were similarly suppressed by wortmannin (P < 0.05-0.01). In addition, SB-203580 also significantly prevented the enhancement of glucose uptake induced by AMPK and insulin (P < 0.05). These results suggest that AMPK-activated GLUT4 translocation in 3T3-L1 adipocytes is mediated through the insulin-signaling pathway distal to the site of activated phosphatidylinositol 3-kinase or through a signaling system distinct from that activated by insulin. On the other hand, the increase of glucose uptake dependent on AMPK activators AICAR and DNP would be additionally due to enhancement of the intrinsic activity in translocated GLUT4 protein, possibly through a p38 MAPK-dependent mechanism.

Acute activation of Erk1/Erk2 and protein kinase B/akt proceed by independent pathways in multiple cell types

We used two inhibitors of the signaling enzyme phosphatidylinositol 3-kinase (PtdIns3K), wortmannin and LY294002, to evaluate the potential involvement of PtdIns3K in the activation of the MAP kinases (MAPK), Erk1 and Erk2. In dose-response studies carried out on six different cell lines and a primary cell culture, we analyzed the ability of the inhibitors to block phosphorylation of protein kinase B/akt (PKB/akt) at Ser473 as a measure of PtdIns3K activity, or the phosphorylation of Erk1/2 at activating Thr/Tyr sites as a measure of the extent of activation of MAPK/Erk kinase (MEK/Erk). In three different hemopoietic cell lines stimulated with cytokines, and in HEK293 cells, stimulated with serum, either wortmannin or LY294002, but never both, could partially block phosphorylation of Erks. The same observations were made in a B-cell line and in primary fibroblasts. In only one cell type, the A20 B cells, was there a closer correlation between the PtdIns3K inhibition by both inhibitors, and their corresponding effects on Erk phosphorylation. However, this stands out as an exception that gives clues to the mechanism by which cross-talk might occur. In all other cells, acute activation of the pathway leading to Erk phosphorylation could proceed independently of PtdIns3K activation. In a biological assay comparing these two pathways, the ability of LY294002 and the MEK inhibitor, U0126, to induce apoptosis were tested. Whereas LY294002 caused death of cytokine-dependent hemopoietic cells, U0126 had little effect, but both inhibitors together had a synergistic effect. The data show that these two pathways are regulating very different downstream events involved in cell survival.

Expression of Class A Scavenger Receptor Is Enhanced by High Glucose in Vitro and under Diabetic Conditions in Vivo

In the early stage of atherosclerosis, macrophages take up chemically modified low density lipoproteins (LDL) through the scavenger receptors, leading to foam cell formation in atherosclerotic lesions. To get insight into a role of the scavenger receptors in diabetes-enhanced atherosclerotic complications, the effects on class A scavenger receptor (SR-A) of high glucose exposure in vitro as well as the diabetic conditions in vivo were determined in the present study. The in vitro experiments demonstrated that high glucose exposure to human monocyte-derived macrophages led to an increased SR-A expression with a concomitant increase in the endocytic uptake of acetylated LDL and oxidized LDL. The endocytic process was significantly suppressed by an anti-SR-A neutralizing antibody. Stability analyses revealed a significant increased stability of SR-A at a mRNA level but not a protein level, indicating that high glucose-induced up-regulation of SR-A is due largely to increased stability of SR-A mRNA. High glucose-enhanced SR-A expression was prevented by protein kinase C and NAD(P)H oxidase inhibitors as well as antioxidants. High glucose-enhanced production of intracellular peroxides was visualized in these cells, which was attenuated by an antioxidant. The in vivo experiments demonstrated that peritoneal macrophages from streptozotocin-induced diabetic mice increased SR-A expression when compared with those from nondiabetic mice. Endocytic degradation of acetylated LDL and oxidized LDL were also increased with these macrophages but not with the corresponding macrophages from diabetic SR-A knock-out mice. These in vitro and in vivo results probably suggest that reactive oxygen species generated from a protein kinase C-dependent NAD(P)H oxidase pathway plays a role in the high glucose-induced up-regulation of SR-A, leading to the increased endocytic degradation of modified LDL for foam cell formation. This could be one mechanism for an increased rate of atherosclerosis in patients with diabetes.

Mechanisms regulating phosphoinositide 3-kinase signalling in insulin-sensitive tissues

A great deal of evidence has accumulated indicating that the activity of PI 3-kinase is necessary, and in some cases sufficient, for a wide range of insulin's actions in the cell. Most biochemical, genetic and pharmacological studies have focused on identifying potential roles for the class-Ia PI 3-kinases which are rapidly activated following insulin stimulation. However, recent evidence indicates the alpha isoform of class-II PI 3-kinase (PI3K-C2alpha) may also play a role as insulin causes a very rapid activation of this as well. The basic mechanisms by which insulin activates the various members of the PI 3-kinase family are increasingly well understood and these studies reveal multiple mechanisms for modulating the activity and functionality of PI 3-kinase and for down regulating the signals they generate. These include inhibitory phosphorylation events, lipid phosphatases such as PTEN and SHIP2 and inhibitor proteins of the suppressors of cytokine signalling (SOCS) family. The current review will focus on these mechanisms and how defects in these might contribute to the development of insulin resistance.

Reduction of PTP1B induces differential expression of PI3-kinase (p85alpha) isoforms

Protein tyrosine phosphatase 1B (PTP1B) inhibition increases insulin sensitivity and normalizes blood glucose levels in animals. The molecular events associated with PTP1B inhibition that increase insulin sensitivity remain controversial. Insulin resistant, diabetic ob/ob mice, dosed with PTP1B antisense for 3 weeks exhibited a decrease in PTP1B protein levels and a change in the expression level of p85alpha isoforms in liver, characterized by a reduction in p85alpha and an upregulation of the p50alpha and p55alpha isoforms. Transfection of mouse hepatocytes with PTP1B antisense caused a downregulation PTP1B and p85alpha protein levels. Furthermore, transfection of mouse hepatocytes with PTP1B siRNA downregulated p85alpha protein expression and enhanced insulin-induced PKB phosphorylation. Treatment of mouse hepatocytes with p85alpha antisense oligonucleotide caused a reduction of p85alpha and an increase in p50alpha and p55alpha isoforms and enhanced insulin-stimulated PKB activation. These results demonstrate that PTP1B inhibition causes a direct differential regulation of p85alpha isoforms of PI3-kinase in liver and that reduction of p85alpha may be one mechanism by which PTP1B inhibition improves insulin sensitivity and glucose metabolism in insulin-resistant states.

Protein Phosphatase-2Cα as a Positive Regulator of Insulin Sensitivity through Direct Activation of Phosphatidylinositol 3-Kinase in 3T3-L1 Adipocytes

During differentiation, expression of protein phosphatase-2Calpha (PP2Calpha) is increased in 3T3-L1 adipocytes. To elucidate the role of PP2Calpha in insulin signaling, we overexpressed wild-type (WT) PP2Calpha by adenovirus-mediated gene transfer in 3T3-L1 adipocytes. Overexpression of PP2Calpha-WT enhanced the insulin sensitivity of glucose uptake without any changes in the early steps of insulin signaling. Infection with adenovirus 5 expressing PP2Calpha-WT increased phosphatidylinositol 3-kinase (PI3K) activities in the immunoprecipitate using antibody against the p85 or p110 subunit under both basal and insulin-stimulated conditions, followed by activation of downstream steps in the PI3K pathway, such as phosphorylation of Akt, glycogen synthase kinase-3, and atypical protein kinase C. In contrast, overexpression of the phosphatase-defective mutant PP2Calpha(R174G) did not produce such effects. Furthermore, overexpression of PP2Calpha-WT (but not PP2Calpha(R174G)) decreased the (32)P-labeled phosphorylation state as well as the gel mobility shift of the p85 subunit, suggesting that dephosphorylation of the p85 subunit by PP2Calpha activation might stimulate PI3K catalytic activity. Moreover, knockdown of PP2Calpha by transfection of small interfering RNA led to a significant decrease in Akt phosphorylation. In addition, microinjection of anti-PP2Calpha antibody or PP2Calpha small interfering RNA led to decreased insulin-stimulated GLUT4 translocation. In conclusion, PP2Calpha is a new positive regulator of insulin sensitivity that acts through a direct activation of PI3K in 3T3-L1 adipocytes.

Platelet-derived growth factor (PDGF) stimulates glucose transport in 3T3-L1 adipocytes overexpressing PDGF receptor by a pathway independent of insulin receptor substrates

Insulin is unique among growth factors and hormones in its ability to control metabolic functions such as the stimulation of glucose uptake and glucose transporter (GLUT4) translocation in physiological target tissues, such as muscle and adipose cells. Nonetheless, the mechanisms underlying this specificity have remained incompletely understood, particularly in view of the ability of some growth factors to mimic insulin-dependent early signaling events. In this study, we have probed the basis of insulin specificity by overexpressing in hormone-responsive 3T3-L1 adipocytes wild-type platelet-derived growth factor (PDGF) receptor (PDGFR)-beta and selected, informative mutant receptor proteins. We show that such adipocytes overexpressing wild-type PDGFR on exposure to cognate growth factor activate glucose transport, GLUT4 translocation, and the serine-threonine protein kinase Akt/protein kinase B to a degree comparable with that produced in response to insulin. In addition, PDGF elicits the robust generation of phosphatidylinositol-3,4,5-trisphosphate in vivo in PDGFR-overexpressing 3T3-L1 adipocytes. Expression of PDGFR-beta mutant proteins demonstrates that these responses require the presence of an intact phosphatidylinositol 3-kinase (PI3K)-binding site on the overexpressed PDGF receptor. Furthermore, PDGF stimulates these effects independent of insulin receptor substrate(IRS)-1 or IRS-2 tyrosine phosphorylation or docking to activated PI3K. These data demonstrate that 1) the basis of insulin-specific glucose transport in cultured adipocytes is the low level of receptors for other growth factors and 2) in the presence of adequate receptors, PDGF is fully capable of activating glucose transport in a manner requiring PI3K and subsequent phosphatidylinositol-3,4,5-trisphosphate accumulation but independent of insulin, insulin receptor, and IRS proteins.

Roles of mitogen-activated protein kinase and phosphoinositide 3'-kinase in ErbB2/ErbB3 coreceptor-mediated heregulin signaling

ErbB2/HER2 and ErbB3/HER3, two members of the ErbB/HER family, together constitute a heregulin coreceptor complex that elicits a potent mitogenic and transforming signal. Among known intracellular effectors of the ErbB2/ErbB3 heregulin coreceptor are mitogen-activated protein kinase (MAPK) and phosphoinositide (PI) 3-kinase. Activation of the distinct MAPK and PI 3-kinase signaling pathways by the ErbB2/ErbB3 coreceptor in response to heregulin and their relative contributions to the mitogenic and transformation potentials of the activated coreceptor were investigated here. To this end, cDNAs encoding the wild-type ErbB3 protein (ErbB3-WT) and ErbB3 proteins with amino acid substitutions in either the Shc-binding site (ErbB3-Y1325F), the six putative PI 3-kinase-binding sites (ErbB3-6F), or both (ErbB3-7F) were generated and expressed in NIH-3T3 cells to form functional ErbB2/ErbB3 heregulin coreceptors. While the coreceptor incorporating ErbB3-WT activated both the MAPK and the PI 3-kinase signaling pathways, those incorporating ErbB3-Y1325F or ErbB3-6F activated either PI 3-kinase or MAPK, respectively. The ErbB2/ErbB3-7F coreceptor activated neither. Elimination of either signaling pathway lowered basal and eliminated heregulin-dependent expression of cyclin D1, which was in each case accompanied by an attenuated mitogenic response. Selective elimination of the PI 3-kinase pathway severely impaired the ability of heregulin to transform cells expressing the coreceptor, whereas attenuation of the MAPK pathway had a lesser effect. Thus, while both pathways contributed in a roughly additive manner to the mitogenic response elicited by the activated ErbB2/ErbB3 coreceptor, the PI 3-kinase pathway predominated in the induction of cellular transformation.

Effects of sterculic acid on stearoyl-CoA desaturase in differentiating 3T3-L1 adipocytes

The effects of sterculic acid on cell size, adiposity, and fatty acid composition of differentiating 3T3-L1 adipocytes are correlated with stearoyl-CoA desaturase (SCD) expression (mRNA and protein levels) and enzyme activity. Fluorescence-activated cell scanning (FACS) analysis showed that adipocytes differentiated with methylisobutylxanthine, dexamethasone, and insulin (MDI) plus 100 microM sterculic acid comprised a population of predominantly large cells with reduced adiposity compared to MDI-treated cells. Although both groups had similar amounts of total fat, their fatty acid profiles were strikingly different: MDI-treated cells had high levels of the unsaturated palmitoleic (Delta(9)-16:1) and oleic (Delta(9)-18:1) acids, whereas the cells cultured with MDI plus sterculic acid accumulated palmitic (16:0) and stearic (18:0) acids together with a marked reduction in Delta(9)-16:1. Although the cells treated with MDI plus sterculic acid had similar levels of scd1 and scd2 mRNAs and antibody-detectable SCD protein as the MDI-treated cells, the SCD enzyme activity was inhibited more than 90%. The accumulation of 16:0 and 18:0, together with normal levels of fatty acid synthase (FAS) and aP2 mRNAs, shows that de novo synthesis and elongation of fatty acids, as well as cell differentiation, were not affected by sterculic acid. Because of the increase in cell size in the sterculic acid-treated cells, the insulin-stimulated 2-deoxyglucose (2-DOG) uptake was determined. Compared to MDI-treated cells, the 2-DOG uptake in the cells treated with sterculic acid was not affected. These results indicate that sterculic acid directly inhibits SCD activity, possibly by a turnover-dependent reaction, without affecting the processes required for adipocyte differentiation, scd gene expression or SCD protein translation.

Promoter polymorphisms -359T/C and -303A/G of the catalytic subunit p110beta gene of human phosphatidylinositol 3-kinase are not associated with insulin secretion or insulin sensitivity in finnish subjects

OBJECTIVE

Phosphatidylinositol (PI) 3-kinase activity is required for insulin-stimulated translocation of GLUT4 transporters and glucose uptake and utilization. Therefore, genes encoding the subunits of PI 3-kinase are promising candidate genes for insulin resistance and type 2 diabetes. We recently cloned the catalytic subunit p110beta gene of human PI 3-kinase and reported two nucleotide polymorphisms, -359T/C and -303A/G, in the promoter region of this gene. In this study, we determined the effects of these polymorphisms on insulin secretion and insulin sensitivity.

RESEARCH DESIGN AND METHODS

We studied two separate groups of Finnish nondiabetic subjects. Insulin secretion was evaluated by intravenous glucose tolerance test and insulin sensitivity by hyperinsulinemic-euglycemic clamp.

RESULTS

Our results showed that the -359T/C and -303A/G polymorphisms did not have a significant effect on fasting plasma insulin levels, insulin secretion, or insulin sensitivity.

CONCLUSIONS

It is unlikely that the promoter polymorphisms -359T/C and -303A/G of the catalytic subunit p110beta gene of human PI 3-kinase have a major impact on insulin secretion, insulin sensitivity, or the risk of type 2 diabetes in Finnish subjects.

Role of PKC isoforms in glucose transport in 3T3-L1 adipocytes: insignificance of atypical PKC

To elucidate the involvement of protein kinase C (PKC) isoforms in insulin-induced and phorbol ester-induced glucose transport, we expressed several PKC isoforms, conventional PKC-alpha, novel PKC-delta, and atypical PKC isoforms of PKC-lambda and PKC-zeta, and their mutants in 3T3-L1 adipocytes using an adenovirus-mediated gene transduction system. Endogenous expression and the activities of PKC-alpha and PKC-lambda/zeta, but not of PKC-delta, were detected in 3T3-L1 adipocytes. Overexpression of each wild-type PKC isoform induced a large amount of PKC activity in 3T3-L1 adipocytes. Phorbol 12-myristrate 13-acetate (PMA) activated PKC-alpha and exogenous PKC-delta but not atypical PKC-lambda/zeta. Insulin also activated the overexpressed PKC-delta but not PKC-alpha. Expression of the wild-type PKC-alpha or PKC-delta resulted in significant increases in glucose transport activity in the basal and PMA-stimulated states. Dominant-negative PKC-alpha expression, which inhibited the PMA activation of PKC-alpha, decreased in PMA-stimulated glucose transport. Glucose transport activity in the insulin-stimulated state was increased by the expression of PKC-delta but not of PKC-alpha. These findings demonstrate that both conventional and novel PKC isoforms are involved in PMA-stimulated glucose transport and that other novel PKC isoforms could participate in PMA-stimulated and insulin-stimulated glucose transport. Atypical PKC-lambda/zeta was not significantly activated by insulin, and expression of the wild-type, constitutively active, and dominant-negative mutants of atypical PKC did not affect either basal or insulin-stimulated glucose transport. Thus atypical PKC enzymes do not play a major role in insulin-stimulated glucose transport in 3T3-L1 adipocytes.

Role of PDK1 in insulin-signaling pathway for glucose metabolism in 3T3-L1 adipocytes

To investigate the role of 3-phosphoinositide-dependent protein kinase 1 (PDK1) in the insulin-signaling pathway for glucose metabolism, wild-type (wt), the kinase-dead (kd), or the plecstrin homology (PH) domain deletion (DeltaPH) mutant of PDK1 was expressed using an adenovirus gene transduction system in 3T3-L1 adipocytes. wt-PDK1 and kd-PDK1 were found in both membrane and cytosol fractions, whereas DeltaPH-PDK1, which exhibited PDK1 activity similar to that of wt-PDK1, was detected exclusively in the cytosol fraction. Insulin dose dependently activated protein kinase B (PKB) but did not change atypical protein kinase C (aPKC) activity in control cells. aPKC activity was not affected by expression of wt-, kd-, or DeltaPH-PDK1 in either the presence or the absence of insulin. Overexpression of wt-PDK1 enhanced insulin-induced activation of PKB as well as insulin-induced phosphorylation of glycogen synthase kinase (GSK)3alpha/beta, a direct downstream target of PKB, although insulin-induced glycogen synthesis was not significantly enhanced by wt-PDK1 expression. Neither DeltaPH-PDK1 nor kd-PDK1 expression affected PKB activity, GSK3 phosphorylation, or glycogen synthesis. Thus membrane localization of PDK1 via its PH domain is essential for insulin signaling through the PDK1-PKB-GSK3alpha/beta pathway. Glucose transport activity was unaffected by expression of wt-PDK1, kd-PDK1, or DeltaPH-PDK1 in either the presence or the absence of insulin. These findings suggest the presence of a signaling pathway for insulin-stimulated glucose transport in which PDK1 to PKB or aPKC is not involved.

Hyperglycemia and insulin resistance: possible mechanisms

Sustained hyperglycemia impairs insulin-stimulated glucose utilization and glycogen synthesis in human and rat skeletal muscles, a phenomenon referred to clinically as glucose toxicity. In rat extensor digitorum longus (EDL) muscle preparations preincubated for 2-4 h in a hyperglycemic medium (25 mM vs. 0 mM glucose), we have shown that the ability of insulin to stimulate glucose incorporation into glycogen is impaired. Interestingly, this was associated with a decreased activation of Akt/PKB, but not its upstream regulator, PI3-kinase. A similar pattern of signaling abnormalities has been observed in adipocytes, L6 muscle cells, C2C12 cells, and (as reported here) EDL incubated with C(2)-ceramide. On the other hand, no increase was observed in ceramide mass in EDL incubated with 25 mM glucose. Hyperglycemia-induced insulin resistance also has been described in adipocytes, where it has been linked to activation of novel and conventional protein kinase C isoforms that phosphorylate the insulin receptor and IRS. In addition, we have recently shown that hyperglycemia causes insulin resistance in cultured human umbilical vein endothelial cells (HUVEC). Here, it was associated with an increased propensity to apoptosis and, as in muscle, with an impaired ability of insulin to activate Akt. Interestingly, these effects of hyperglycemia and an increase in diacylglycerol synthesis, which is also caused, were prevented by adding AICAR, an activator of AMP-activated protein kinase (AMPK), to the incubation medium. These results suggest that hyperglycemia causes insulin resistance in cells other than those in classic insulin target tissues. Whether AMPK activation can reverse or prevent insulin resistance in all of these cells remains to be determined.

Acyl-coenzyme A dehydrogenases are localized on GLUT4-containing vesicles via association with insulin-regulated aminopeptidase in a manner dependent on its dileucine motif

Insulin-regulated aminopeptidase (IRAP, also termed vp165) is known to be localized on the GLUT4-containing vesicles and to be recruited to the plasma membrane after stimulation with insulin. The cytoplasmic region of IRAP contains two dileucine motifs and acidic regions, one of which (amino acid residues 55-82) is reportedly involved in retention of GLUT4-containing vesicles. The region of IRAP fused with glutathione-S-transferase [GST-IRAP(55-82)] was incubated with lysates from 3T3-L1 adipocytes, leading to identification of long-chain, medium-chain, and short-chain acyl-coenzyme A dehydrogenases (ACDs) as the proteins associated with IRAP. The association was nearly abolished by mutation of the dileucine motif of IRAP. Immunoblotting of fractions prepared from sucrose gradient ultracentrifugation and vesicles immunopurified with anti-GLUT4 antibody revealed these ACDs to be localized on GLUT4-containing vesicles. Furthermore, 3-mercaptopropionic acid and hexanoyl-CoA, inhibitors of long-chain and medium-chain ACDs, respectively, induced dissociation of long-chain acyl-coenzyme A dehydrogenase and/or medium-chain acyl-coenzyme A dehydrogenase from IRAP in vitro as well as recruitment of GLUT4 to the plasma membrane and stimulation of glucose transport activity in permeabilized 3T3-L1 adipocytes. These findings suggest that ACDs are localized on GLUT4-containing vesicles via association with IRAP in a manner dependent on its dileucine motif and play a role in retention of GLUT4-containing vesicles to an intracellular compartment.

Liver X receptors as insulin-mediating factors in fatty acid and cholesterol biosynthesis

The nuclear receptor liver X receptor (LXR) alpha, an important regulator of cholesterol and bile acid metabolism, was analyzed after insulin stimulation in liver in vitro and in vivo. A time- and dose-dependent increase in LXRalpha steady-state mRNA level was seen after insulin stimulation of primary rat hepatocytes in culture. A maximal induction of 10-fold was obtained when hepatocytes were exposed to 400 nm insulin for 24 h. Cycloheximide, a potent inhibitor of protein synthesis, prevented induction of LXRalpha mRNA expression by insulin, indicating that the induction is dependent on de novo synthesis of proteins. Stabilization studies using actinomycin D indicated that insulin stimulation increased the half-life of LXRalpha transcripts in cultured primary hepatocytes. Complementary studies where rats and mice were injected with insulin induced LXRalpha mRNA levels and confirmed our in vitro studies. Furthermore, deletion of both the LXRalpha and LXRbeta genes (double knockout) in mice markedly suppressed insulin-mediated induction of an entire class of enzymes involved in both fatty acid and cholesterol metabolism. The discovery of insulin regulation of LXR in hepatic tissue as well as gene targeting studies in mice provide strong evidence that LXRs plays a central role not only in cholesterol homeostasis, but also in fatty acid metabolism. Furthermore, LXRs appear to be important insulin-mediating factors in regulation of lipogenesis.

Clenbuterol prevents epinephrine from antagonizing insulin-stimulated muscle glucose uptake

In the present study, we investigated the effects of chronic clenbuterol treatment on insulin-stimulated glucose uptake in the presence of epinephrine in isolated rat skeletal muscle. Insulin (50 microU/ml) increased glucose uptake in both fast-twitch (epitrochlearis) and slow-twitch (soleus) muscles. In the presence of 24 nM epinephrine, insulin-stimulated glucose uptake was completely suppressed. This suppression of glucose uptake by epinephrine was accompanied by an increase in the intracellular concentration of glucose 6-phosphate and a decrease in insulin-receptor substrate-1-associated phosphatidylinositol 3-kinase (IRS-1/PI3-kinase) activity. Clenbuterol treatment had no direct effect on insulin-stimulated glucose uptake. However, after clenbuterol treatment, epinephrine was ineffective in attenuating insulin-stimulated muscle glucose uptake. This ineffectiveness of epinephrine to suppress insulin-stimulated glucose uptake occurred in conjunction with its inability to increase the intracellular concentration of glucose 6-phosphate and attenuate IRS-1/PI3-kinase activity. Results of this study indicate that the effectiveness of epinephrine to inhibit insulin-stimulated glucose uptake is severely diminished in muscle from rats pretreated with clenbuterol.

The roles of phosphatidylinositol 3-kinase and protein kinase Czeta for thrombopoietin-induced mitogen-activated protein kinase activation in primary murine megakaryocytes

Thrombopoietin (TPO) stimulates a network of intracellular signaling pathways that displays extensive cross-talk. We have demonstrated previously that the ERK/mitogen-activated protein kinase pathway is important for TPO-induced endomitosis in primary megakaryocytes (MKs). One known pathway by which TPO induces ERK activation is through the association of Shc with the penultimate phosphotyrosine within the TPO receptor, Mpl. However, several investigators found that the membrane-proximal half of the cytoplasmic domain of Mpl is sufficient to activate ERK in vitro and support base-line megakaryopoiesis in vivo. Using BaF3 cells expressing a truncated Mpl (T69Mpl) as a tool to identify non-Shc/Ras-dependent signaling pathways, we describe here novel mechanisms of TPO-induced ERK activation mediated, in part, by phosphoinositide 3-kinase (PI3K). Similar to cells expressing full-length receptor, PI3K was activated by its incorporation into a complex with IRS2 or Gab2. Furthermore, the MEK-phosphorylating activity of protein kinase Czeta (PKCzeta) was also enhanced after TPO stimulation of T69Mpl, contributing to ERK activity. PKCzeta and PI3K also contribute to TPO-induced ERK activation in MKs, confirming their physiological relevance. Like in BaF3 cells, a TPO-induced signaling complex containing p85PI3K is detectable in MKs expressing T61Mpl and is probably responsible for PI3K activation. These data demonstrate a novel role of PI3K and PKCzeta in steady-state megakaryopoiesis.

Differential activation of protein kinase B and p70(S6)K by glucose and insulin-like growth factor 1 in pancreatic beta-cells (INS-1)

It has been shown that IGF-1-induced pancreatic beta-cell proliferation is glucose-dependent; however, the mechanisms responsible for this glucose dependence are not known. Adenoviral mediated expression of constitutively active phosphatidylinositol 3-kinase (PI3K) in the pancreatic beta-cells, INS-1, suggested that PI3K was not necessary for glucose-induced beta-cell proliferation but was required for IGF-1-induced mitogenesis. Examination of the signaling components downstream of PI3K, 3-phosphoinositide-dependent kinase 1, protein kinase B (PKB), glycogen synthase kinase-3, and p70-kDa-S6-kinase (p70(S6K)), suggested that a major part of glucose-dependent beta-cell proliferation requires activation of mammalian target of rapamycin/p70(S6K), independent of phosphoinositide-dependent kinase 1/PKB activation. Adenoviral expression of the kinase-dead form of PKB in INS-1 cells decreased IGF-1-induced beta-cell proliferation. However, a surprisingly similar decrease was also observed in adenoviral wild type and constitutively active PKB-infected cells. Upon analysis of extracellular signal-regulated protein kinase 1 and 2 (ERK1/ERK2), an increase in ERK1/ERK2 phosphorylation activation by glucose and IGF-1 was observed in kinase-dead PKB-infected cells, but this phosphorylation activation was inhibited in the constitutively active PKB-infected cells. Hence, there is a requirement for the activation of both ERK1/ERK2 and mammalian target of rapamycin/p70(S6K) signal transduction pathways for a full commitment to glucose-induced pancreatic beta-cell mitogenesis. However, for IGF-1-induced activation, these pathways must be carefully balanced, because chronic activation of one (PI3K/PKB) can lead to dampening of the other (ERK1/2), reducing the mitogenic response.

Constitutively active phosphatidylinositol 3-kinase and AKT are sufficient to stimulate the epithelial Na+/H+ exchanger 3

Phosphatidylinositol 3-kinase (PI 3-kinase) is a cytoplasmic signaling molecule that is recruited to activated growth factor receptors and has been shown to be involved in regulation of stimulated exocytosis and endocytosis. One of the downstream signaling molecules activated by PI 3-kinase is the protein kinase Akt. Previous studies have indicated that PI 3-kinase is necessary for basal Na(+)/H(+) exchanger 3 (NHE3) transport and for fibroblast growth factor-stimulated NHE3 activity in PS120 fibroblasts. However, it is not known whether activation of PI 3-kinase is sufficient to stimulate NHE3 activity or whether Akt is involved in this PI 3-kinase effect. We used an adenoviral infection system to test the possibility that activation of PI 3-kinase or Akt alone is sufficient to stimulate NHE3 activity. This hypothesis was investigated in PS120 fibroblasts stably expressing NHE3 after somatic gene transfer using a replication-deficient recombinant adenovirus containing constitutively active catalytic subunit of PI 3-kinase or constitutively active Akt. The adenovirus construct used was engineered with an upstream ecdysone promoter to allow time-regulated expression. Adenoviral infection was nearly 100% at 48 h after infection. Forty-eight hours after infection (24 h after activation of the ecdysone promoter), PI 3-kinase and Akt amount and activity were increased. Increases in both PI 3-kinase activity and Akt activity stimulated NHE3 transport. In addition, a membrane-permeant synthetic 10-mer peptide that binds polyphosphoinositides and increases PI 3-kinase activity similarly enhanced NHE3 transport activity and also increased the percentage of NHE3 on the plasma membrane. The magnitudes of stimulation of NHE3 by constitutively active PI 3-kinase, PI 3-kinase peptide, and constitutively active Akt were similar to each other. These results demonstrate that activation of PI 3-kinase or Akt is sufficient to stimulate NHE3 transport activity in PS120/NHE3 cells.

Insulin resistance: cellular and clinical concepts

Insulin resistance is defined as a clinical state in which a normal or elevated insulin level produces an attenuated biologic response. Specifically, the biologic response most studied is insulin-stimulated glucose disposal, yet the precise cellular mechanism responsible is not yet known. However, the presence of insulin resistance is observed many years before the onset of clinical hyperglycemia and the diagnosis of Type 2 diabetes. Insulin resistance at this stage appears to be significantly associated with a clustering of cardiovascular risk factors predisposing the individual to accelerated cardiovascular disease. An overview of insulin resistance and the associated clinical insulin resistant state will be discussed.

The hepatitis B virus-X protein activates a phosphatidylinositol 3-kinase-dependent survival signaling cascade

The hepatitis B virus-X (HBx) protein is known as a multifunctional protein that not only coactivates transcription of viral and cellular genes but coordinates the balance between proliferation and programmed cell death, by inducing or blocking apoptosis. In this study the role of the HBx protein in activation of phosphatidylinositol 3-kinase (PI3K) was investigated as a possible cause of anti-apoptosis in liver cells. HBx relieved serum deprivation-induced and pro-apoptic stimuli-induced apoptosis in Chang liver (CHL) cells. Treatment with 1-d-3-deoxy-3-fluoro-myo-inositol, an antagonist to PI3K, which blocks the formation of 3'-phosphorylated phosphatidyl inositol in CHL cells transformed by HBx (CHL-X) but not normal Chang liver (CHL) cells, showed a marked loss of viability with evidence of apoptosis. Similarly, treatment with wortmannin, an inhibitor of PI3K, stimulated apoptosis in HBx-transformed CHL cells but not in normal cells, confirming that HBx blocks apoptosis through the PI3K pathway. The serine 47 threonine kinase, Akt, one of the downstream effectors of PI3K-dependent survival signaling was 2-fold higher in HBx-transformed CHL (CHL-X) cells than CHL cells. Phosphorylation of Akt at serine 473 and Bad at serine 136 were induced by HBx, which were specifically blocked by wortmannin and dominant negative mutants of Akt and Bad, respectively. We also demonstrated that HBx inhibits caspase 3 activity and HBx down-regulation of caspase 3 activity was blocked by the PI3K inhibitor. Regions required for PI3K phosphorylation on the HBx protein overlap with the known transactivation domains. HBx blocks apoptosis induced by serum withdrawal in CHL cells in a p53-independent manner. The results indicate that, unlike other DNA tumor viruses that block apoptosis by inactivating p53, the hepatitis B virus achieves protection from apoptotic death through a HBx-PI3K-Akt-Bad pathway and by inactivating caspase 3 activity that is at least partially p53-independent in liver cells. Moreover, these data suggest that modulation of the PI3K activity may represent a potential therapeutic strategy to counteract the occurrence of apoptosis in human hepatocellular carcinoma.

Expression of a prenylation-deficient Rab4 inhibits the GLUT4 translocation induced by active phosphatidylinositol 3-kinase and protein kinase B

The small GTPase Rab4 has been shown to participate in the subcellular distribution of GLUT4 under both basal and insulin-stimulated conditions in adipocytes. In the present work, we have characterized the effect of Rab4 DeltaCT, a prenylation-deficient and thus cytosolic form of Rab4, in this process. We show that the expression of Rab4 DeltaCT in freshly isolated adipocytes inhibits insulin-induced GLUT4 translocation, but only when this protein is in its GTP-bound active form. Further, it not only blocks the effect of insulin, but also that of a hyperosmotic shock, but does not interfere with the effect of zinc ions on GLUT4 translocation. Rab4 DeltaCT was then shown to prevent GLUT4 translocation induced by the expression of an active form of phosphatidylinositol 3-kinase or of protein kinase B, without altering the activities of the enzymes. Our results are consistent with a role of Rab4 DeltaCT acting as a dominant negative protein towards Rab4, possibly by binding to Rab4 effectors.

Zinc has an insulin-like effect on glucose transport mediated by phosphoinositol-3-kinase and Akt in 3T3-L1 fibroblasts and adipocytes

Zinc has insulin-like effects on cells, including promotion of both lipogenesis and glucose transport. The relationship between zinc and the stimulation of glucose transport is unclear. We hypothesize that zinc affects the insulin-signaling pathway. In this study, the effect of zinc on glucose transport and insulin signaling was examined in 3T3-L1-preadipocytes and -adipocytes. Treatment of cells with up to 200 micromol/L zinc significantly increased glucose transport (P < 0.05). The effect of zinc on adipocytes was greater than on preadipocytes, and the effect of zinc plus insulin was greater than that of either insulin or zinc alone. Cytochalasin D, which disrupts actin filaments, attenuated the increase of glucose transport induced by zinc or insulin (P < 0.05). At 100 nmol/L, wortmannin, the phosphoinositide (PI) 3-kinase inhibitor, decreased basal glucose transport and blocked zinc-stimulated glucose transport in both cell types (P < 0.05). H7, an inhibitor of protein kinase C, did not reduce basal glucose transport but decreased zinc-induced glucose transport (P < 0.05). Zinc increased tyrosine phosphorylation of the insulin receptor beta subunit of both preadipocytes and adipocytes after 5-10 min of treatment (P < 0.05). Zinc at 200 micromol/L did not affect tyrosine phosphorylation of insulin receptor substrate (IRS)-1 or -2; further, there was no effect of zinc on the association of the p85 subunit of PI 3-kinase and IRS-1. Zinc significantly increased serine-473 phosphorylation of Akt in both preadipocytes and adipocytes (P < 0.05). The PI 3-kinase inhibitor, wortmannin, totally blocked the effect of zinc on Akt activation. Hence, it appears that zinc can induce an increase in glucose transport into cells and potentiate insulin-induced glucose transport, likely acting through the insulin-signaling pathway.