Ligand-independent GLUT4 translocation induced by guanosine 5'-O-(3-thiotriphosphate) involves tyrosine phosphorylation

Insulin stimulation of Akt/PKB phosphorylation in the placenta of preeclampsia patients

CONTEXT AND OBJECTIVE

Preeclampsia is a multi-systemic disease and one of the most frequent severe health problems during pregnancy. Binding of insulin triggers phosphorylation and activates cytoplasmic substrates such as phosphatidylinositol 3 kinase (PI3K). Phosphorylation of membrane phosphoinositide 2 (PIP2) to phosphoinositide 3 (PIP3) by PI3K starts Akt/PKB activation. Defects in phosphorylation of the insulin receptor and its substrates have an important role in insulin resistance. Studies have shown that insulin resistance is associated with preeclampsia and its pathophysiology. The aim here was to investigate insulin stimulation of the Akt/PKB pathway in the placenta, in normal and preeclampsia parturients.

DESIGN AND SETTING

Cross-sectional study in a tertiary public university hospital.

METHODS

Placentas were collected from 12 normal and 12 preeclampsia patients. These were stimulated and analyzed using Western blot to quantify the Akt/PKB phosphorylation.

RESULTS

The insulin stimulation was confirmed through comparing the stimulated group (1.14 ± 0.10) with the non-stimulated group (0.91 ± 0.08; P < 0.001). The phosphorylation of Akt/PKB did not differ between the placenta of the normal patients (1.26 ± 0.16) and those of the preeclampsia patients (1.01 ± 0.11; P = 0.237).

CONCLUSIONS

In vitro insulin stimulation of the human placenta has been well established. There was no difference in Akt/PKB phosphorylation, after stimulation with insulin, between placentas of normal and preeclampsia patients. Nevertheless, it cannot be ruled out that the Akt/PKB signaling pathway may have a role in the pathophysiology of preeclampsia, since the substrates of Akt/PKB still need to be investigated.

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.

SKIP negatively regulates insulin-induced GLUT4 translocation and membrane ruffle formation

Skeletal muscle and kidney enriched inositol phosphatase (SKIP) is an inositol polyphosphate 5-phosphatase that hydrolyzes phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] to downregulate intracellular levels. In this study, we show that SKIP inhibits phosphoinositide 3-kinase signaling in insulin-stimulated CHO cells. Ectopic expression of SKIP did not inhibit insulin-induced PI(3,4,5)P3 generation but did rapidly decrease insulin-induced intracellular PI(3,4,5)P3 levels compared with those in control cells. Further, insulin-induced phosphorylation of some downstream targets such as Akt and p70 S6 kinase was markedly inhibited by the ectopic expression of SKIP, whereas phosphorylation of mitogen-activated protein kinase was not. In contrast, downregulation of intracellular SKIP levels by antisense oligonucleotides dramatically enhanced Akt (protein kinase B) phosphorylation in response to insulin, suggesting that endogenous SKIP downregulates insulin signaling. SKIP also markedly inhibited GLUT4 translocation and membrane ruffle formation. We conclude that SKIP preferentially regulates glucose transport and actin cytoskeletal rearrangement among a variety of PI(3,4,5)P3 downstream events.

Differential effects of phosphatidylinositol 3-kinase inhibition on intracellular signals regulating GLUT4 translocation and glucose transport

Phosphatidylinositol (PI) 3-kinase is required for insulin-stimulated translocation of GLUT4 to the surface of muscle and fat cells. Recent evidence suggests that the full stimulation of glucose uptake by insulin also requires activation of GLUT4, possibly via a p38 mitogen-activated protein kinase (p38 MAPK)-dependent pathway. Here we used L6 myotubes expressing Myc-tagged GLUT4 to examine at what level the signals regulating GLUT4 translocation and activation bifurcate. We compared the sensitivity of each process, as well as of signals leading to GLUT4 translocation (Akt and atypical protein kinase C) to PI 3-kinase inhibition. Wortmannin inhibited insulin-stimulated glucose uptake with an IC(50) of 3 nm. In contrast, GLUT4myc appearance at the cell surface was less sensitive to inhibition (IC(50) = 43 nm). This dissociation between insulin-stimulated glucose uptake and GLUT4myc translocation was not observed with LY294002 (IC(50) = 8 and 10 microm, respectively). The sensitivity of insulin-stimulated activation of PKC zeta/lambda, Akt1, Akt2, and Akt3 to wortmannin (IC(50) = 24, 30, 35, and 60 nm, respectively) correlated closely with inhibition of GLUT4 translocation. In contrast, insulin-dependent p38 MAPK phosphorylation was efficiently reduced in cells pretreated with wortmannin, with an IC(50) of 7 nm. Insulin-dependent p38 alpha and p38 beta MAPK activities were also markedly reduced by wortmannin (IC(50) = 6 and 2 nm, respectively). LY294002 or transient expression of a dominant inhibitory PI 3-kinase construct (Delta p85), however, did not affect p38 MAPK phosphorylation. These results uncover a striking correlation between PI 3-kinase, Akt, PKC zeta/lambda, and GLUT4 translocation on one hand and their segregation from glucose uptake and p38 MAPK activation on the other, based on their wortmannin sensitivity. We propose that a distinct, high affinity target of wortmannin, other than PI 3-kinase, may be necessary for activation of p38 MAPK and GLUT4 in response to insulin.

Recovery of flagellar dynein function in a Chlamydomonas actin/dynein-deficient mutant upon introduction of muscle actin by electroporation

Flagellar and ciliary inner-arm dyneins contain actin as a subunit; however, the function of this actin subunit remains unknown. As a first step toward experimental manipulation of actin in dynein, we developed a method for introducing exogenous actin into Chlamydomonas cells by electroporation. A non-motile mutant, ida5oda1, lacking inner-arm dyneins due to the absence of conventional actin, was electroporated in the presence of rabbit skeletal muscle actin. About 20% of the electroporated cells recovered motility under optimal conditions. In addition, by taking advantage of their phototactic behavior, the rescued cells could be concentrated. Motility was also recovered with fluorescently labeled actin; in this case, axonemes became fluorescent after electroporation, suggesting that actin was in fact incorporated as a dynein subunit. The feasibility of incorporating a substantial amount of macromolecules by electroporation will be useful not only for studying actin function, but also for a variety of studies using Chlamydomonas in which no efficient methods have been developed for expressing or introducing foreign proteins and other macromolecules.

The tumor suppressor PTEN negatively regulates insulin signaling in 3T3-L1 adipocytes

PTEN is a tumor suppressor with sequence homology to protein-tyrosine phosphatases and the cytoskeleton protein tensin. PTEN is capable of dephosphorylating phosphatidylinositol 3,4, 5-trisphosphate in vitro and down-regulating its levels in insulin-stimulated 293 cells. To study the role of PTEN in insulin signaling, we overexpressed PTEN in 3T3-L1 adipocytes approximately 30-fold above uninfected or control virus (green fluorescent protein)-infected cells, using an adenovirus gene transfer system. PTEN overexpression inhibited insulin-induced 2-deoxy-glucose uptake by 36%, GLUT4 translocation by 35%, and membrane ruffling by 50%, all of which are phosphatidylinositol 3-kinase-dependent processes, compared with uninfected cells or cells infected with control virus. Microinjection of an anti-PTEN antibody increased basal and insulin stimulated GLUT4 translocation, suggesting that inhibition of endogenous PTEN function led to an increase in intracellular phosphatidylinositol 3,4,5-trisphosphate levels, which stimulates GLUT4 translocation. Further, insulin-induced phosphorylation of downstream targets Akt and p70S6 kinase were also inhibited significantly by overexpression of PTEN, whereas tyrosine phosphorylation of the insulin receptor and IRS-1 or the phosphorylation of mitogen-activated protein kinase were not affected, suggesting that the Ras/mitogen-activated protein kinase pathway remains fully functional. Thus, we conclude that PTEN may regulate phosphatidylinositol 3-kinase-dependent insulin signaling pathways in 3T3-L1 adipocytes.

The trimeric GTP-binding protein (G(q)/G(11)) alpha subunit is required for insulin-stimulated GLUT4 translocation in 3T3L1 adipocytes

To investigate the potential role of trimeric GTP-binding proteins regulating GLUT4 translocation in adipocytes, wild type and constitutively active G(q) (G(q)/Q209L), G(i) (G(i)/Q205L), and G(s) (G(s)/Q227L) alpha subunit mutants were expressed in 3T3L1 adipocytes. Although expression of neither the wild type nor G(i)/Q205L and G(s)/Q227L alpha subunit mutants had any effect on the basal or insulin-stimulated translocation of a co-expressed GLUT4-enhanced green fluorescent protein (EGFP) fusion protein, expression of G(q)/Q209L resulted in GLUT4-EGFP translocation in the absence of insulin. In contrast, microinjection of an inhibitory G(q)/G(11) alpha subunit-specific antibody but not a G(i) or G(s) alpha subunit antibody prevented insulin-stimulated endogenous GLUT4 translocation. Consistent with a required role for GTP-bound G(q)/G(11), expression of the regulators of G protein signaling (RGS4 and RGS16) also attenuated insulin-stimulated GLUT4-EGFP translocation. To assess the relationship between G(q)/G(11) function with the phosphatidylinositol 3-kinase dependent pathway, expression of a dominant-interfering p85 regulatory subunit, as well as wortmannin treatment inhibited insulin-stimulated but not G(q)/Q209L-stimulated GLUT4-EGFP translocation. Furthermore, G(q)/Q209L did not induce the in vivo accumulation of phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), whereas expression of the RGS proteins did not prevent the insulin-stimulated accumulation of PIP(3). Together, these data demonstrate that insulin stimulation of GLUT4 translocation requires at least two independent signal transduction pathways, one mediated through the phosphatidylinositol 3-kinase and another through the trimeric GTP-binding proteins G(q) and/or G(11).

Endothelin-1-induced GLUT4 translocation is mediated via Galpha(q/11) protein and phosphatidylinositol 3-kinase in 3T3-L1 adipocytes

Endothelin-1 (ET-1) can stimulate insulin-responsive glucose transporter (GLUT4) translocation in 3T3-L1 adipocytes (Wu-Wong, J. R., Berg, C. E., Wang, J., Chiou, W. J., and Fissel, B. (1999) J. Biol. Chem. 274, 8103-8110), and in the current study, we have evaluated the signaling pathway leading to this response. First, we inhibited endogenous Galpha(q/11) function by single-cell microinjection using anti-Galpha(q/11) antibody or RGS2 protein (a GTPase activating protein for Galpha(q)) followed by immunostaining to quantitate GLUT4 translocation in 3T3-L1 adipocytes. ET-1-stimulated GLUT4 translocation was markedly decreased by 70 or 75% by microinjection of Galpha(q/11) antibody or RGS2 protein, respectively. Pretreatment of cells with the Galpha(i) inhibitor (pertussis toxin) or microinjection of a Gbetagamma inhibitor (glutathione S-transferase-beta-adrenergic receptor kinase (GST-BARK)) did not inhibit ET-1-induced GLUT4 translocation, indicating that Galpha(q/11 )mediates ET-1 signaling to GLUT4 translocation. Next, we found that ET-1-induced GLUT4 translocation was inhibited by the phosphatidylinositol (PI) 3-kinase inhibitors wortmannin or LY294002, but not by the phospholipase C inhibitor U-73122. ET-1 stimulated the PI 3-kinase activity of the p110alpha subunit (5.5-fold), and microinjection of anti-p110alpha or PKC-lambda antibodies inhibited ET-stimulated GLUT4 translocation. Finally, we found that Galpha(q/11) formed immunocomplexes with the type-A endothelin receptor and the 110alpha subunit of PI 3-kinase and that ET-1 stimulation enhances tyrosine phosphorylation of Galpha(q/11). These results indicate that: 1) ET-1 signaling to GLUT4 translocation is dependent upon Galpha(q/11) and PI 3-kinase; and 2) Galpha(q/11) can transmit signals from the ET(A) receptor to the p110alpha subunit of PI 3-kinase, as does insulin, subsequently leading to GLUT4 translocation.

G alpha-q/11 protein plays a key role in insulin-induced glucose transport in 3T3-L1 adipocytes

We evaluated the role of the G alpha-q (Galphaq) subunit of heterotrimeric G proteins in the insulin signaling pathway leading to GLUT4 translocation. We inhibited endogenous Galphaq function by single cell microinjection of anti-Galphaq/11 antibody or RGS2 protein (a GAP protein for Galphaq), followed by immunostaining to assess GLUT4 translocation in 3T3-L1 adipocytes. Galphaq/11 antibody and RGS2 inhibited insulin-induced GLUT4 translocation by 60 or 75%, respectively, indicating that activated Galphaq is important for insulin-induced glucose transport. We then assessed the effect of overexpressing wild-type Galphaq (WT-Galphaq) or a constitutively active Galphaq mutant (Q209L-Galphaq) by using an adenovirus expression vector. In the basal state, Q209L-Galphaq expression stimulated 2-deoxy-D-glucose uptake and GLUT4 translocation to 70% of the maximal insulin effect. This effect of Q209L-Galphaq was inhibited by wortmannin, suggesting that it is phosphatidylinositol 3-kinase (PI3-kinase) dependent. We further show that Q209L-Galphaq stimulates PI3-kinase activity in p110alpha and p110gamma immunoprecipitates by 3- and 8-fold, respectively, whereas insulin stimulates this activity mostly in p110alpha by 10-fold. Nevertheless, only microinjection of anti-p110alpha (and not p110gamma) antibody inhibited both insulin- and Q209L-Galphaq-induced GLUT4 translocation, suggesting that the metabolic effects induced by Q209L-Galphaq are dependent on the p110alpha subunit of PI3-kinase. In summary, (i) Galphaq appears to play a necessary role in insulin-stimulated glucose transport, (ii) Galphaq action in the insulin signaling pathway is upstream of and dependent upon PI3-kinase, and (iii) Galphaq can transmit signals from the insulin receptor to the p110alpha subunit of PI3-kinase, which leads to GLUT4 translocation.

Osmotic shock inhibits insulin signaling by maintaining Akt/protein kinase B in an inactive dephosphorylated state

We have previously reported that insulin and osmotic shock stimulate an increase in glucose transport activity and translocation of the insulin-responsive glucose transporter isoform GLUT4 to the plasma membrane through distinct pathways in 3T3L1 adipocytes (D. Chen, J. S. Elmendorf, A. L. Olson, X. Li, H. S. Earp, and J. E. Pessin, J. Biol. Chem. 272:27401-27410, 1997). In investigations of the relationships between these two signaling pathways, we have now observed that these two stimuli are not additive, and, in fact, osmotic shock pretreatment was found to completely prevent any further insulin stimulation of glucose transport activity and GLUT4 protein translocation. In addition, osmotic shock inhibited the insulin stimulation of lipogenesis and glycogen synthesis. This inhibition of insulin-stimulated downstream signaling occurred without any significant effect on insulin receptor autophosphorylation or tyrosine phosphorylation of insulin receptor substrate 1 (IRS1). Furthermore, there was no effect on either the insulin-stimulated association of the p85 type I phosphatidylinositol (PI) 3-kinase regulatory subunit with IRS1 or phosphotyrosine antibody-immunoprecipitated PI 3-kinase activity. In contrast, osmotic shock pretreatment markedly inhibited the insulin stimulation of protein kinase B (PKB) and p70S6 kinase activities. In addition, the dephosphorylation of PKB was prevented by pretreatment with the phosphatase inhibitors okadaic acid and calyculin A. These data support a model in which osmotic shock-induced insulin resistance of downstream biological responses results from an inhibition of insulin-stimulated PKB activation.

An SH2 domain-containing 5' inositolphosphatase inhibits insulin-induced GLUT4 translocation and growth factor-induced actin filament rearrangement

Tyrosine kinase receptors lead to rapid activation of phosphatidylinositol 3-kinase (PI3 kinase) and the subsequent formation of phosphatidylinositides (PtdIns) 3,4-P2 and PtdIns 3,4, 5-P3, which are thought to be involved in signaling for glucose transporter GLUT4 translocation, cytoskeletal rearrangement, and DNA synthesis. However, the specific role of each of these PtdIns in insulin and growth factor signaling is still mainly unknown. Therefore, we assessed, in the current study, the effect of SH2-containing inositol phosphatase (SHIP) expression on these biological effects. SHIP is a 5' phosphatase that decreases the intracellular levels of PtdIns 3,4,5-P3. Expression of SHIP after nuclear microinjection in 3T3-L1 adipocytes inhibited insulin-induced GLUT4 translocation by 100 +/- 21% (mean +/- the standard error) at submaximal (3 ng/ml) and 64 +/- 5% at maximal (10 ng/ml) insulin concentrations (P < 0.05 and P < 0.001, respectively). A catalytically inactive mutant of SHIP had no effect on insulin-induced GLUT4 translocation. Furthermore, SHIP also abolished GLUT4 translocation induced by a membrane-targeted catalytic subunit of PI3 kinase. In addition, insulin-, insulin-like growth factor I (IGF-I)-, and platelet-derived growth factor-induced cytoskeletal rearrangement, i.e., membrane ruffling, was significantly inhibited (78 +/- 10, 64 +/- 3, and 62 +/- 5%, respectively; P < 0.05 for all) in 3T3-L1 adipocytes. In a rat fibroblast cell line overexpressing the human insulin receptor (HIRc-B), SHIP inhibited membrane ruffling induced by insulin and IGF-I by 76 +/- 3% (P < 0.001) and 68 +/- 5% (P < 0.005), respectively. However, growth factor-induced stress fiber breakdown was not affected by SHIP expression. Finally, SHIP decreased significantly growth factor-induced mitogen-activated protein kinase activation and DNA synthesis. Expression of the catalytically inactive mutant had no effect on these cellular responses. In summary, our results show that expression of SHIP inhibits insulin-induced GLUT4 translocation, growth factor-induced membrane ruffling, and DNA synthesis, indicating that PtdIns 3,4,5-P3 is the key phospholipid product mediating these biological actions.

Rapid stimulation of glucose transport by mitochondrial uncoupling depends in part on cytosolic Ca2+ and cPKC

2,4-Dinitrophenol (DNP) uncouples the mitochondrial oxidative chain from ATP production, preventing oxidative metabolism. The consequent increase in energy demand is, however, contested by cells increasing glucose uptake to produce ATP via glycolysis. In L6 skeletal muscle cells, DNP rapidly doubles glucose transport, reminiscent of the effect of insulin. However, glucose transport stimulation by DNP does not require insulin receptor substrate-1 phosphorylation and is wortmannin insensitive. We report here that, unlike insulin, DNP does not activate phosphatidylinositol 3-kinase, protein kinase B/Akt, or p70 S6 kinase. However, chelation of intra- and extracellular Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid-AM in conjunction with EGTA inhibited DNP-stimulated glucose uptake by 78.9 +/- 3.5%. Because Ca2+-sensitive, conventional protein kinase C (cPKC) can activate glucose transport in L6 muscle cells, we examined whether cPKC may be translocated and activated in response to DNP in L6 myotubes. Acute DNP treatment led to translocation of cPKCs to plasma membrane. cPKC immunoprecipitated from plasma membranes exhibited a twofold increase in kinase activity in response to DNP. Overnight treatment with 4-phorbol 12-myristate 13-acetate downregulated cPKC isoforms alpha, beta, and gamma and partially inhibited (45.0 +/- 3.6%) DNP- but not insulin-stimulated glucose uptake. Consistent with this, the PKC inhibitor bisindolylmaleimide I blocked PKC enzyme activity at the plasma membrane (100%) and inhibited DNP-stimulated 2-[3H]deoxyglucose uptake (61.2 +/- 2.4%) with no effect on the stimulation of glucose transport by insulin. Finally, the selective PKC-beta inhibitor LY-379196 partially inhibited DNP effects on glucose uptake (66.7 +/- 1.6%). The results suggest interfering with mitochondrial ATP production acts on a signal transduction pathway independent from that of insulin and partly mediated by Ca2+ and cPKCs, of which PKC-beta likely plays a significant role.

Phosphoinositide 3-kinase: the key switch mechanism in insulin signalling

Insulin plays a key role in regulating a wide range of cellular processes. However, until recently little was known about the signalling pathways that are involved in linking the insulin receptor with downstream responses. It is now apparent that the activation of class 1a phosphoinositide 3-kinase (PI 3-kinase) is necessary and in some cases sufficient to elicit many of insulin's effects on glucose and lipid metabolism. The lipid products of PI 3-kinase act as both membrane anchors and allosteric regulators, serving to localize and activate downstream enzymes and their protein substrates. One of the major ways these lipid products of PI 3-kinase act in insulin signalling is by binding to pleckstrin homology (PH) domains of phosphoinositide-dependent protein kinase (PDK) and protein kinase B (PKB) and in the process regulating the phosphorylation of PKB by PDK. Using mechanisms such as this, PI 3-kinase is able to act as a molecular switch to regulate the activity of serine/threonine-specific kinase cascades important in mediating insulin's effects on endpoint responses.