Transient effect of platelet-derived growth factor on GLUT4 translocation in 3T3-L1 adipocytes

α1-Antitrypsin A treatment attenuates neutrophil elastase accumulation and enhances insulin sensitivity in adipose tissue of mice fed a high-fat diet

Neutrophils accumulate in insulin-sensitive tissues during obesity and may play a role in impairing insulin sensitivity. The major serine protease expressed by neutrophils is neutrophil elastase (NE), which is inhibited endogenously by α1-antitrypsin A (A1AT). We investigated the effect of exogenous (A1AT) treatment on diet-induced metabolic dysfunction. Male C57Bl/6j mice fed a chow or a high-fat diet (HFD) were randomized to receive intraperitoneal injections three times weekly of either Prolastin (human A1AT; 2 mg) or vehicle (PBS) for 10 wk. Prolastin treatment did not affect plasma NE concentration, body weight, glucose tolerance, or insulin sensitivity in chow-fed mice. In contrast, Prolastin treatment attenuated HFD-induced increases in plasma and white adipose tissue (WAT) NE without affecting circulatory neutrophil levels or increases in body weight. Prolastin-treated mice fed a HFD had improved insulin sensitivity, as assessed by insulin tolerance test, and this was associated with higher insulin-dependent IRS-1 (insulin receptor substrate) and Akt^Ser473 phosphorylation, and reduced inflammation markers in WAT but not liver or muscle. In 3T3-L1 adipocytes, Prolastin reversed recombinant NE-induced impairment of insulin-stimulated glucose uptake and IRS-1 phosphorylation. Furthermore, PDGF mediated p-Akt^Ser473 activation and glucose uptake (which is independent of IRS-1) was not affected by recombinant NE treatment. Collectively, our findings suggest that NE infiltration of WAT during metabolic overload contributes to insulin resistance by impairing insulin-induced IRS-1 signaling.NEW & NOTEWORTHY Neutrophils accumulate in peripheral tissues during obesity and are critical coordinators of tissue inflammatory responses. Here, we provide evidence that inhibition of the primary neutrophil protease, neutrophil elastase, with α1-antitrypsin A (A1AT) can improve insulin sensitivity and glucose homeostasis of mice fed a high-fat diet. This was attributed to improved insulin-induced IRS-1 phosphorylation in white adipose tissue and provides further support for a role of neutrophils in mediating diet-induced peripheral tissue insulin resistance.

Impaired intracellular trafficking of sodium-dependent vitamin C transporter 2 contributes to the redox imbalance in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a glutamine expansion at the first exon of the huntingtin gene. Huntingtin protein (Htt) is ubiquitously expressed and it is localized in several organelles, including endosomes. HD is associated with a failure in energy metabolism and oxidative damage. Ascorbic acid is a powerful antioxidant highly concentrated in the brain where it acts as a messenger, modulating neuronal metabolism. It is transported into neurons via the sodium-dependent vitamin C transporter 2 (SVCT2). During synaptic activity, ascorbic acid is released from glial reservoirs to the extracellular space, inducing an increase in SVCT2 localization at the plasma membrane. Here, we studied SVCT2 trafficking and localization in HD. SVCT2 is decreased at synaptic terminals in YAC128 male mice. Using cellular models for HD (STHdhQ7 and STHdhQ111 cells), we determined that SVCT2 trafficking through secretory and endosomal pathways is altered in resting conditions. We observed Golgi fragmentation and SVCT2/Htt-associated protein-1 mis-colocalization. Additionally, we observed altered ascorbic acid-induced calcium signaling that explains the reduced SVCT2 translocation to the plasma membrane in the presence of extracellular ascorbic acid (active conditions) described in our previous results. Therefore, SVCT2 trafficking to the plasma membrane is altered in resting and active conditions in HD, explaining the redox imbalance observed during early stages of the disease.

Metabolic Coordination of Pericyte Phenotypes: Therapeutic Implications

Pericytes are mural vascular cells found predominantly on the abluminal wall of capillaries, where they contribute to the maintenance of capillary structural integrity and vascular permeability. Generally quiescent cells in the adult, pericyte activation and proliferation occur during both physiological and pathological vascular and tissue remodeling. A considerable body of research indicates that pericytes possess attributes of a multipotent adult stem cell, as they are capable of self-renewal as well as commitment and differentiation into multiple lineages. However, pericytes also display phenotypic heterogeneity and recent studies indicate that lineage potential differs between pericyte subpopulations. While numerous microenvironmental cues and cell signaling pathways are known to regulate pericyte functions, the roles that metabolic pathways play in pericyte quiescence, self-renewal or differentiation have been given limited consideration to date. This review will summarize existing data regarding pericyte metabolism and will discuss the coupling of signal pathways to shifts in metabolic pathway preferences that ultimately regulate pericyte quiescence, self-renewal and trans-differentiation. The association between dysregulated metabolic processes and development of pericyte pathologies will be highlighted. Despite ongoing debate regarding pericyte classification and their functional capacity for trans-differentiation in vivo, pericytes are increasingly exploited as a cell therapy tool to promote tissue healing and regeneration. Ultimately, the efficacy of therapeutic approaches hinges on the capacity to effectively control/optimize the fate of the implanted pericytes. Thus, we will identify knowledge gaps that need to be addressed to more effectively harness the opportunity for therapeutic manipulation of pericytes to control pathological outcomes in tissue remodeling.

MDM2-related responses in 3T3-L1 adipocytes exposed to cooling and subsequent rewarming

Insulin-like growth factor-I and insulin induce the production of phospho-Ser-166 MDM2, a target of Akt, and influence the formation of the MDM2 complex. The glycolipid hormone insulin differentially activates phosphatidylinositol 3-kinase (PI3K)/Akt pathways in 3T3-L1 (L1) adipocytes incubated at 19 °C. Responses of L1 adipocytes to different temperature changes and their regulatory mechanisms are poorly understood. We exposed L1 adipocytes to cooling and subsequent rewarming in the presence or absence of wortmannin, a PI3K inhibitor, or mithramycin A, a transcription inhibitor, and examined the induction of phospho-Ser-166 MDM2 and MDM2 and the subcellular formation of the MDM2 complex using western blot analysis. Exposure to 28 and 18 °C induced phospho-MDM2 in cells and increased the level of MDM2 in the plasma membrane of cells. These temperatures did not affect the total MDM2 level. Similar results were obtained when the cells were treated with insulin. Exposure to 4 °C increased the total MDM2 level and did not induce phospho-MDM2, which was induced by rewarming at 37 °C after cooling at 4°C without any alteration in the protein level. Mithramycin A (10 μM) did not alter the increase in protein level induced at 4 °C. The induction of phospho-molecules at 28 and 18 °C was impaired slightly by 1 μM of wortmannin but not by 0.1 μM of wortmannin. This low concentration of wortmannin completely blocked the induction of phospho-MDM2 by rewarming. Our results indicate that temperature changes induce MDM2-related responses, including those that are stimulated by receptor responses and dependent on a kinase inhibitor, in L1 adipocytes.

Astragalus polysaccharides alleviates glucose toxicity and restores glucose homeostasis in diabetic states via activation of AMPK

AIM

To establish the mechanism underlying the improvement of glucose toxicity by Astragalus polysaccharide (APS), which occurred via an AMP activated protein kinase (AMPK)-dependent pathway.

METHODS

In vivo and in vitro effects of APS on glucose homeostasis were examined in a type 2 diabetes mellitus (T2DM) rat model. The T2DM rat model was duplicated by a high-fat diet (58% fat, 25.6% carbohydrate, and 16.4% protein) and a small dose of streptozotocin (STZ, 25 mg/kg, ip). After APS therapy (700 mg.kg(-1).d(-1), ig) for 8 weeks, blood glucose, glycosylated hemoglobin, and serum insulin were measured. Insulin sensitivity was evaluated by the comprehensive analysis of oral glucose tolerance tests (OGTT) and HOMA IR index. Hepatic glycogen was observed by the PAS staining method. The expression and activity of skeletal muscle AMPKalpha and acetyl-CoA carboxylase (ACC), and the phosphorylation of hepatic glycogen synthase (GS), the glycogen synthase (GS),were measured by Western blotting. Glucose uptake was measured with the 2-deoxy-[(3)H]-D-glucose method in C2C12 cells.

RESULTS

The hyperglycemia status, insulin sensitivity, glucose uptake, and activation level of AMPK in diabetic rats were improved in response to APS administration. APS could also alleviate glucose toxicity in cultured mouse cells by the activation of AMPK.

CONCLUSION

APS can alleviate glucose toxicity by increasing liver glycogen synthesis and skeletal muscle glucose translocation in the T2DM rat model, via activation of AMPK.

The Rab GTPase-activating protein AS160 as a common regulator of insulin- and Galphaq-mediated intracellular GLUT4 vesicle distribution

Akt substrate of 160kDa (AS160) is a Rab GTPase activating protein (GAP) and was recently identified as a component of the insulin signaling pathway of glucose transporter type 4 (GLUT4) translocation. We and others, previously reported that the activation of Galphaq protein-coupled receptors (GalphaqPCRs) also stimulated GLUT4 translocation and glucose uptake in several cell lines. Here, we report that the activation of GalphaqPCRs also promoted phosphorylation of AS160 by the 5'-AMP activated protein kinase (AMPK). The suppression of AS160 phosphorylation by the siRNA mediated AMPKalpha1 subunit knockdown promoted GLUT4 vesicle retention in intracellular compartments. This suppression did not affect the ratio of non-induced cell surface GLUT4 to Galphaq-induced it. Rat 3Y1 cells lacking AS160 did not show insulin-induced GLUT4 translocation. The cells stably expressing GLUT4 revealed GLUT4 vesicles that were mainly localized in the perinuclear region and less frequently on the cell surface. After expression of exogenous AS160, GLUT4 on the cell surface decreased and GLUT4 vesicles were redistributed throughout the cytoplasm. Although PMA-induced or sodium fluoride-induced GLUT4 translocation was significantly increased in these cells, insulin did not affect GLUT4 translocation. These results suggest that AS160 is a common regulator of insulin- and GalphaqPCR activation-mediated GLUT4 distribution in the cells.

Tyrosine phosphorylation of Munc18c regulates platelet-derived growth factor-stimulated glucose transporter 4 translocation in 3T3L1 adipocytes

Platelet-derived growth factor (PDGF) stimulation of skeletal muscle, cultured myotubes, and 3T3L1 adipocytes results in glucose transporter 4 (Glut4) translocation, albeit to a reduced level compared with insulin. To address the mechanism of PDGF action, we have determined that the Syntaxin 4 negative regulatory protein, Munc18c, undergoes PDGF-stimulated phosphorylation on tyrosine residue 521. The tyrosine phosphorylation of Munc18c on Y521 occurred concomitant with the dissociation of the Munc18c protein from Syntaxin 4 in a time frame consistent with Glut4 translocation. Moreover, expression of the wild-type Munc18c protein did not inhibit PDGF-induced Glut4 translocation, whereas expression of Y521A-Munc18c mutant was inhibitory and failed to dissociate from Syntaxin 4. In contrast, expression of either wild-type Munc18c or the Y521A-Munc18c mutant both resulted in a marked inhibition of insulin-stimulated Glut4 translocation. Together, these data demonstrate that one mechanism accounting for the PDGF induction of Glut4 translocation is the suppression of the Munc18c negative regulation of Syntaxin 4 function.

Acute effectors of GLUT1 glucose transporter subcellular targeting in CIT3 mouse mammary epithelial cells

Lactogenic hormones cause intracellular targeting of glucose transporter 1 (GLUT1) for transport of glucose to the site of lactose synthesis in mammary glands. Our aim was to study the intracellular trafficking mechanisms involved in GLUT1 targeting and recycling in CIT3 mouse mammary epithelial cells. Fusion proteins of GLUT1 and enhanced green fluorescent protein (EGFP) were expressed in CIT3 cells maintained in growth medium (GM), or exposed to secretion medium (SM), containing prolactin. Agents acting on Golgi and related subcellular compartments and on GLUT1 and GLUT4 targeting in muscle and fat cells were studied. Wortmannin and staurosporine effects on internalization of GLUT1 were not specific, supporting a basal constitutive GLUT1 membrane-recycling pathway between an intracellular pool and the cell surface in CIT3 cells, which targets most GLUT1 to the plasma membrane in GM. Upon exposure to prolactin in SM, GLUT1 was specifically targeted intracellularly to a brefeldin A-sensitive compartment. Arrest of endosomal acidification by bafilomycin A1 disrupted this prolactin-induced GLUT1 intracellular trafficking with central coalescence of GLUT1-EGFP signal, suggesting that it is via endosomal pathways. This machinery offers another level of regulation of lactose synthesis by altering GLUT1 targeting within minutes to hours.

Insulin receptor signals regulating GLUT4 translocation and actin dynamics

In skeletal muscle and adipose tissue, insulin-stimulated glucose uptake is dependent upon translocation of the insulin-responsive glucose transporter GLUT4 from intracellular storage compartments to the plasma membrane. This insulin-induced redistribution of GLUT4 protein is achieved through a series of highly organized membrane trafficking events, orchestrated by insulin receptor signals. Recently, several key molecules linking insulin receptor signals and membrane trafficking have been identified, and emerging evidence supports the importance of subcellular compartmentalization of signaling components at the right time and in the right place. In addition, the translocation of GLUT4 in adipocytes requires insulin stimulation of dynamic actin remodeling at the inner surface of the plasma membrane (cortical actin) and in the perinuclear region. This results from at least two independent insulin receptor signals, one leading to the activation of phosphatidylinositol (PI) 3-kinase and the other to the activation of the Rho family small GTP-binding protein TC10. Thus, both spatial and temporal regulations of actin dynamics, both beneath the plasma membrane and around endomembranes, by insulin receptor signals are also involved in the process of GLUT4 translocation.

Sennidin stimulates glucose incorporation in rat adipocytes

A novel small molecule compound which exerts insulin mimetic is desirable. Dozens of natural products that have quinone, naphthoquinone, or anthraquinone structure, were tested by a glucose incorporation assay. We found that sennidin A, anthraquinone derivative, stimulated glucose incorporation to near level of maximal insulin-stimulated and sennidin B, a stereoisomer of sennidin A, also stimulated, but the activity of sennidin B was lower than sennidin A. Sennidin A-stimulated glucose incorporation was completely inhibited by wortmannin. Sennidin A did not induce tyrosine phosphorylation of insulin receptor (IR) and insulin receptor substrate-1 (IRS-1), but induced phosphorylation of Akt and glucose transporter 4 (GLUT4) translocation. Our results suggest that in rat adipocytes, sennidin A stimulates glucose incorporation in the phosphatidylinositol 3-kinase (PI3K)- and Akt-dependent, but in the IR/IRS1-independent manner.

Platelet-derived growth factor stimulates glucose transport in skeletal muscles of transgenic mice specifically expressing platelet-derived growth factor receptor in the muscle, but it does not affect blood glucose levels

Insulin stimulates the disposal of blood glucose into skeletal muscle and adipose tissues by the translocation of GLUT4 from intracellular pools to the plasma membrane, and consequently the concentration of blood glucose levels decreases rapidly in vivo. Phosphatidylinositol (PI) 3-kinase and Akt play a pivotal role in the stimulation of glucose transport by insulin, but detailed mechanisms are unknown. We and others reported that not only insulin but also platelet-derived growth factor (PDGF) and epidermal growth factor facilitate glucose uptake through GLUT4 translocation by activation of PI 3-kinase and Akt in cultured cells. However, opposite results were also reported. We generated transgenic mice that specifically express the PDGF receptor in skeletal muscle. In these mice, PDGF stimulated glucose transport into skeletal muscle in vitro and in vivo. Thus, PDGF apparently shares with insulin some of the signaling molecules needed for the stimulation of glucose transport. The degree of glucose uptake in vivo reached approximately 60% of that by insulin injection in skeletal muscle, but blood glucose levels were not decreased by PDGF in these mice. Therefore, PDGF-induced disposal of blood glucose into skeletal muscle is insufficient for rapid decrease of blood glucose levels.

Insulin but not PDGF relies on actin remodeling and on VAMP2 for GLUT4 translocation in myoblasts

Insulin promotes the translocation of glucose transporter 4 (GLUT4) from intracellular pools to the surface of muscle and fat cells via a mechanism dependent on phosphatidylinositol (PtdIns) 3-kinase, actin cytoskeletal remodeling and the v-SNARE VAMP2. The growth factor PDGF-BB also robustly activates PtdIns 3-kinase and induces actin remodeling, raising the question of whether it uses similar mechanisms to insulin in mobilizing GLUT4. In L6 myoblasts stably expressing Myc-tagged GLUT4, neither stimulus affected the rate of GLUT4 endocytosis, confirming that they act primarily by enhancing exocytosis to increase GLUT4 at the cell surface. Although surface GLUT4myc in response to insulin peaked at 10 minutes and remained steady for 30 minutes, PDGF action was transient, peaking at 5 minutes and disappearing by 20 minutes. These GLUT4myc translocation time courses mirrored that of phosphorylation of Akt by the two stimuli. Interestingly, insulin and PDGF caused distinct manifestations of actin remodeling. Insulin induced discrete, long (>5 μm) dorsal actin structures at the cell periphery, whereas PDGF induced multiple short (<5 μm) dorsal structures throughout the cell, including above the nucleus. Latrunculin B, cytochalasin D and jasplakinolide, which disrupt actin dynamics, prevented insulin- and PDGF-induced actin remodeling but significantly inhibited GLUT4myc translocation only in response to insulin (75-85%, P<0.05), not to PDGF (20-30% inhibition). Moreover, transfection of tetanus toxin light chain, which cleaves the v-SNAREs VAMP2 and VAMP3, reduced insulin-induced GLUT4myc translocation by >70% but did not affect the PDGF response. These results suggest that insulin and PDGF rely differently on the actin cytoskeleton and on tetanus-toxin-sensitive VAMPs for mobilizing GLUT4.

Growth hormone induces cellular insulin resistance by uncoupling phosphatidylinositol 3-kinase and its downstream signals in 3T3-L1 adipocytes

Growth hormone (GH) is well known to induce in vivo insulin resistance. However, the molecular mechanism of GH-induced cellular insulin resistance is largely unknown. In this study, we demonstrated that chronic GH treatment of differentiated 3T3-L1 adipocytes reduces insulin-stimulated 2-deoxyglucose (DOG) uptake and activation of Akt (also known as protein kinase B), both of which are downstream effects of phosphatidylinositol (PI) 3-kinase, despite enhanced tyrosine phosphorylation of insulin receptor substrate (IRS)-1, association of IRS-1 with the p85 subunit of PI 3-kinase, and IRS-1-associated PI 3-kinase activity. In contrast, chronic GH treatment did not affect 2-DOG uptake and Akt activation induced by overexpression of a membrane-targeted form of the p110 subunit of PI 3-kinase (p110(CAAX)) or Akt activation stimulated by platelet-derived growth factor. Fractionation studies indicated that chronic GH treatment reduces insulin-stimulated translocation of Akt from the cytosol to the plasma membrane. Interestingly, chronic GH treatment increased insulin-stimulated association of IRS-1 with p85 and IRS-1-associated PI 3-kinase activity preferentially in the cytosol. These results indicate that cellular insulin resistance induced by chronic GH treatment in 3T3-L1 adipocytes is caused by uncoupling between activation of PI 3-kinase and its downstream signals, which is specific to the insulin-stimulated PI 3-kinase pathway. This effect of GH might result from the altered subcellular distribution of IRS-1-associated PI 3-kinase.

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.

Differentiated 3T3L1 adipocytes are composed of heterogenous cell populations with distinct receptor tyrosine kinase signaling properties

Various studies have demonstrated that the platelet-derived growth factor (PDGF) receptor in adipocytes can activate PI 3-kinase activity without affecting insulin-responsive glucose transporter (GLUT4) translocation. To investigate this phenomenon of receptor signaling specificity, we utilized single cell analysis to determine the cellular distribution and signaling properties of PDGF and insulin in differentiated 3T3L1 adipocytes. The insulin receptor was highly expressed in a large percentage of the cell population (>95%) that also expressed caveolin 2 and GLUT4 with very low levels of the PDGF receptor. In contrast, the PDGF receptor was only expressed in approximately 10% of the differentiated 3T3L1 cell population with relatively low levels of the insulin receptor, caveolin 2, and GLUT4. Consistent with this observation, insulin stimulated the phosphorylation of Akt in the caveolin 2- and GLUT4-positive cells, whereas PDGF primarily stimulated Akt phosphorylation in the caveolin 2- and GLUT4-negative cell population. Furthermore, transfection of the PDGF receptor in the insulin receptor-, GLUT4-, and caveolin 2-positive cells resulted in the ability of PDGF to stimulate GLUT4 translocation. These data demonstrate that differentiated 3T3L1 adipocytes are not a homogeneous population of cells, and the lack of PDGF receptor expression in the GLUT4-positive cell population accounts for the inability of the endogenous PDGF receptor to activate GLUT4 translocation.

Evaluation of early response to SU101 target-based therapy in patients with recurrent supratentorial malignant gliomas using FDG PET and Gd-DTPA MRI

Changes in [18F]-2-fluoro-2-deoxyglucose (FDG) uptake and gadopentetate dimeglumine (Gd-DTPA) enhancement before and after the first course of treatment with a cytostatic agent SU101 (N-[(4-trifluoromethyl)-phenyl]-5-methylisoxazole-4-carboxamide, SUGEN) were assessed using positron emission tomography (PET) and magnetic resonance imaging (MRI) in a pilot study of 8 patients with recurrent supratentorial malignant gliomas. The localization and the volume of Gd-DTPA enhancement and FDG hypermetabolism were analyzed. PET and MRI studies were performed one week before and 7.6+/-3.7 weeks after administration of SU101. The ratios of mean tumor activity to mean contralateral white matter and ipsilateral cerebellar activity were calculated for tumor regions, and SUV values corrected to the subjects' body surface area and glucose level (SUVbsa*glu) were calculated for nontumor regions. Five patients had a substantial increase of tumor volume on both PET and MRI during the first course of SU101. PET and MRI showed roughly equivalent volume changes. Large tumor volume increases were associated with a short time to clinical progression. The metabolic change in the tumor following the first course of SU101 varied from patient to patient, ranging from a 31% reduction to a 43% increase in FDG uptake ratio. Changes in FDG uptake were not predictive of time to progression or survival. In 2 patients with marked clinical deterioration and rapid tumor growth, there were differences in localization of Gd-DTPA enhancement and FDG hypermetabolism suggesting that hypermetabolism beyond the area of contrast enhancement may be of value in predicting rapid progression of high-grade glioma. SU101 did not induce any appreciable changes in SUVbsa*glu for non-tumor brain in 6 of 8 patients.

Signalling through the insulin receptor

The insulin receptor substrates function at the heart of the insulin signalling network. It has recently become apparent that the intracellular localisation of these molecules is regulated in a precise manner that is critical for both the generation and the termination of the insulin signal. Some insulin receptor substrate isoforms appear to be associated with an insoluble matrix that resembles the cytoskeleton. When inappropriately dissociated from this matrix the signalling network collapses concomitant with loss of insulin sensitivity.

Platelet-derived growth factor rapidly increases activity and cell surface expression of the EAAC1 subtype of glutamate transporter through activation of phosphatidylinositol 3-kinase

Na(+)-dependent glutamate transporters are the primary mechanism for removal of excitatory amino acids (EAAs) from the extracellular space of the central nervous system and influence both physiologic and pathologic effects of these compounds. Recent evidence suggests that the activity and cell surface expression of a neuronal subtype of glutamate transporter, EAAC1, are rapidly increased by direct activation of protein kinase C and are decreased by wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K). We hypothesized that this regulation could be analogous to insulin-induced stimulation of the GLUT4 subtype of glucose transporter, which is dependent upon activation of PI3-K. Using C6 glioma, a cell line that endogenously and selectively expresses EAAC1, we report that platelet-derived growth factor (PDGF) increased Na(+)-dependent L-[(3)H]-glutamate transport activity within 30 min. This effect of PDGF was not due to a change in total cellular EAAC1 immunoreactivity but was instead correlated with an increase cell surface expression of EAAC1, as measured using a membrane impermeant biotinylation reagent combined with Western blotting. A decrease in nonbiotinylated intracellular EAAC1 was also observed. These studies suggest that PDGF causes a redistribution of EAAC1 from an intracellular compartment to the cell surface. These effects of PDGF were accompanied by a 35-fold increase in PI3-K activity and were blocked by the PI3-K inhibitors, wortmannin and LY 294002, but not by an inhibitor of protein kinase C. Other growth factors, including insulin, nerve growth factor, and epidermal growth factor had no effect on glutamate transport nor did they increase PI3-K activity. These studies suggest that, as is observed for insulin-mediated translocation of GLUT4, EAAC1 cell surface expression can be rapidly increased by PDGF through activation of PI3-K. It is possible that this PDGF-mediated increase in EAAC1 activity may contribute to the previously demonstrated neuroprotective effects of PDGF.