Mechanism of protein kinase B activation by cyclic AMP-dependent protein kinase.

Activation of protein kinase B (PKB) by growth factors and hormones has been demonstrated to proceed via phosphatidylinositol 3-kinase (PI3-kinase). In this report, we show that PKB can also be activated by PKA (cyclic AMP [cAMP]-dependent protein kinase) through a PI3-kinase-independent pathway. Although this activation required phosphorylation of PKB, PKB is not likely to be a physiological substrate of PKA since a mutation in the sole PKA consensus phosphorylation site of PKB did not abolish PKA-induced activation of PKB. In addition, mechanistically, this activation was different from that of growth factors since it did not require phosphorylation of the S473 residue, which is essential for full PKB activation induced by insulin. These data were supported by the fact that mutation of residue S473 of PKB to alanine did not prevent it from being activated by forskolin. Moreover, phosphopeptide maps of overexpressed PKB from COS cells showed differences between insulin- and forskolin-stimulated cells that pointed to distinct activation mechanisms of PKB depending on whether insulin or cAMP was used. We looked at events downstream of PKB and found that PKA activation of PKB led to the phosphorylation and inhibition of glycogen synthase kinase-3 (GSK-3) activity, a known in vivo substrate of PKB. Overexpression of a dominant negative PKB led to the loss of inhibition of GSK-3 in both insulin- and forskolin-treated cells, demonstrating that PKB was responsible for this inhibition in both cases. Finally, we show by confocal microscopy that forskolin, similar to insulin, was able to induce translocation of PKB to the plasma membrane. This process was inhibited by high concentrations of wortmannin (300 nM), suggesting that forskolin-induced PKB movement may require phospholipids, which are probably not generated by class I or class III PI3-kinase. However, high concentrations of wortmannin did not abolish PKB activation, which demonstrates that translocation per se is not important for PKA-induced PKB activation.     

Activation of translation initiation factor eIF2B by insulin requires phosphatidyl inositol 3-kinase

Eukaryotic initiation factor eIF2B mediates a key regulatory step in peptide-chain initiation and is acutely activated by insulin, although it is not clear how. Inhibitors of phosphatidylinositide 3-kinase blocked activation of eIF2B, although rapamycin, which inhibits the p70 S6 kinase pathway, did not. Furthermore, a dominant negative mutant of PI 3-kinase also prevented activation of eIF2B, while a Sos-mutant, which blocks MAP kinase activation, did not. The data demonstrate that a pathway distinct from MAP and p70 S6 kinases regulates eIF2B. Glycogen synthase kinase-3 (GSK-3) phosphorylates and inactivates eIF2B. In all cases, eIF2B and GSK-3 were regulated reciprocally. Dominant negative PI 3-kinase abolished the insulin-induced inhibition of GSK-3. These data strongly support the hypothesis that insulin activates eIF2B through a signalling pathway involving PI 3-kinase and inhibition of GSK-3.     

Glycogen synthase kinase-3 couples AKT-dependent signaling to the regulation of p21Cip1 degradation.

Signaling via the phosphoinositide 3-kinase (PI3K)/AKT pathway is crucial for the regulation of endothelial cell (EC) proliferation and survival, which involves the AKT-dependent phosphorylation of the DNA repair protein p21(Cip1) at Thr-145. Because p21(Cip1) is a short-lived protein with a high proteasomal degradation rate, we investigated the regulation of p21(Cip1) protein levels by PI3K/AKT-dependent signaling. The PI3K inhibitors Ly294002 and wortmannin reduced p21(Cip1) protein abundance in human umbilical vein EC. However, mutation of the AKT site Thr-145 into aspartate (T145D) did not increase its protein half-life. We therefore investigated whether a kinase downstream of AKT regulates p21(Cip1) protein levels. In various cell types, AKT phosphorylates and inhibits glycogen synthase kinase-3 (GSK-3). Upon serum stimulation of EC, GSK-3beta was phosphorylated at Ser-9. Site-directed mutagenesis revealed that GSK-3 in vitro phosphorylated p21(Cip1) specifically at Thr-57 within the Cdk binding domain. Overexpression of GSK-3beta decreased p21(Cip1) protein levels in EC, whereas the specific inhibition of GSK-3 with lithium chloride interfered with p21(Cip1) degradation and increased p21(Cip1) protein about 10-fold in EC and cardiac myocytes (30 mm, p < 0.001). These data indicate that GSK-3 triggers p21(Cip1) degradation. In contrast, stimulation of AKT increases p21(Cip1) via inhibitory phosphorylation of GSK-3.     

PLCgamma participates in insulin stimulation of glucose uptake through activation of PKCzeta in brown adipocytes

Based on recent studies showing that PLCgamma associates to insulin receptor, we investigated its role in insulin stimulation of glucose transport in brown adipocytes. Insulin stimulation induced rapid PLCgamma association to phosphorylated insulin receptor, and activation of PLCgamma, as assessed by the mobilization of Ca(2+) from intracellular stores and by the production of the second messenger DAG. Both events are dependent on activation of PI3-kinase. Inhibition of PLCgamma activity either with the chemical compound U73122 or with an inhibitor peptide precluded insulin stimulation of glucose uptake, GLUT4 translocation, and actin reorganization, as wortmannin did. In contrast, the inactive analog U73343 did not have an inhibitory effect. Furthermore, translocation of GLUT4-GFP in response to insulin was completely abolished by cotransfection with a PLCgamma-inactive mutant in HeLa cells, a cell model sensitive to insulin that express PLCgamma. U73122 did not affect PI3-kinase nor Akt activation, but impaired PKCzeta activation by insulin, as wortmannin did. PLC activity renders two products, IP(3) and DAG, and DAG can be metabolized to PA by the action of DAG-kinase. Using the compound R54494, a DAG-kinase inhibitor, insulin-induced PKCzeta activation was also suppressed, this activity being restored by addition of PA. In summary, these data indicate that PLCgamma, activated at least partially by PI3-kinase, is a link between insulin receptor and PKCzeta through the production of PA and could mediate insulin-induced glucose uptake and GLUT4 translocation.     

Inhibition of glycogen-synthase kinase 3 stimulates glycogen synthase and glucose transport by distinct mechanisms in 3T3-L1 adipocytes

The role of glycogen-synthase kinase 3 (GSK3) in insulin-stimulated glucose transport and glycogen synthase activation was investigated in 3T3-L1 adipocytes. GSK3 protein was clearly present in adipocytes and was found to be more abundant than in muscle and liver cell lines. The selective GSK3 inhibitor, LiCl, stimulated glucose transport and glycogen synthase activity (20 and 65%, respectively, of the maximal (1 microm) insulin response) and potentiated the responses to a submaximal concentration (1 nm) of insulin. LiCl- and insulin-stimulated glucose transport were abolished by the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, wortmannin; however, LiCl stimulation of glycogen synthase was not. In contrast to the rapid stimulation of glucose transport by insulin, transport stimulated by LiCl increased gradually over 3-5 h reaching 40% of the maximal insulin-stimulated level. Both LiCl- and insulin-stimulated glycogen synthase activity were maximal at 25 min. However, insulin-stimulated glycogen synthase activity returned to basal after 2 h, coincident with reactivation of GSK3. After a 2-h exposure to insulin, glycogen synthase was refractory to restimulation with insulin, indicating selective desensitization of this pathway. However, LiCl could partially stimulate glycogen synthase in desensitized cells. Furthermore, coincubation with LiCl during the 2 h exposure to insulin completely blocked desensitization of glycogen synthase activity. In summary, inhibition of GSK3 by LiCl: 1) stimulated glycogen synthase activity directly and independently of PI3-kinase, 2) stimulated glucose transport at a point upstream of PI3-kinase, 3) stimulated glycogen synthase activity in desensitized cells, and 4) prevented desensitization of glycogen synthase due to chronic insulin treatment. These data are consistent with GSK3 playing a central role in the regulation of glycogen synthase activity and a contributing factor in the regulation of glucose transport in 3T3-L1 adipocytes.     

Reciprocal regulation of glycogen phosphorylase and glycogen synthase by insulin involving phosphatidylinositol-3 kinase and protein phosphatase-1 in HepG2 cells

The effect of insulin on glycogen synthesis and key enzymes of glycogen metabolism, glycogen phosphorylase and glycogen synthase, was studied in HepG2 cells. Insulin stimulated glycogen synthesis 1.83-3.30 fold depending on insulin concentration in the medium. Insulin caused a maximum of 65% decrease in glycogen phosphorylase 'a' and 110% increase in glycogen synthase activities in 5 min. Although significant changes in enzyme activities were observed with as low as 0.5 nM insulin level, the maximum effects were observed with 100 nM insulin. There was a significant inverse correlation between activities of glycogen phosphorylase 'a' and glycogen synthase 'a' (R² = 0.66, p < 0.001). Addition of 30 mM glucose caused a decrease in phosphorylase 'a' activity in the absence of insulin and this effect was additive with insulin up to 10 nM concentration. The inactivation of phosphorylase 'a' by insulin was prevented by wortmannin and rapamycin but not by PD98059. The activation of glycogen synthase by insulin was prevented by wortmannin but not by PD98059 or rapamycin. In fact, PD98059 slightly stimulated glycogen synthase activation by insulin. Under these experimental conditions, insulin decreased glycogen synthase kinase-3 activity by 30-50% and activated more than 4-fold particulate protein phosphatase-1 activity and 1.9-fold protein kinase B activity; changes in all of these enzyme activities were abolished by wortmannin. The inactivation of GSK-3 and activation of PKB by insulin were associated with their phosphorylation and this was also reversed by wortmannin. The addition of protein phosphatase-1 inhibitors, okadaic acid and calyculin A, completely abolished the effects of insulin on both enzymes. These data suggest that stimulation of glycogen synthase by insulin in HepG2 cells is mediated through the PI-3 kinase pathway by activating PKB and PP-1G and inactivating GSK-3. On the other hand, inactivation of phosphorylase by insulin is mediated through the PI-3 kinase pathway involving a rapamycin-sensitive p70^s6k and PP-1G. These experiments demonstrate that insulin regulates glycogen phosphorylase and glycogen synthase through (i) a common signaling pathway at least up to PI-3 kinase and bifurcates downstream and (ii) that PP-1 activity is essential for the effect of insulin.     

Class II phosphoinositide 3-kinase is activated by insulin but not by contraction in skeletal muscle.

While the role of the class IA phosphoinositide 3-kinase (PI 3-kinase) in insulin signaling is well established, little is known about the role of the class II PI 3-kinases. We show that insulin stimulation of intact rat soleus and epitrochlearis muscles causes a 3- to 4-fold increase in the activity of the wortmannin-resistant alpha isoform of the class II PI 3-kinase (PI3K-C2alpha). This activation is rapid and parallels the insulin-induced activation of the class IA PI 3-kinase associated with IRS-1 in these muscles. However, while contraction activated p38 Map kinase, it did not stimulate the activity of the class II PI 3-kinase. Therefore, activation of class II PI 3-kinase is unlikely to provide a mechanism that explains the fact that exercise-induced activation of glucose uptake is not blocked by wortmannin. However, the results suggest that activation of class II PI 3-kinase is likely to play a role in insulin signaling pathways in skeletal muscle.     

H-ras Induces Glucose Uptake in Brown Adipocytes in an Insulin- and Phosphatidylinositol 3-Kinase-Independent Manner

Fetal brown adipocytes (parental cells) expressed mainly Glut4 mRNA glucose transporter, the expression of Glut1 mRNA being much lower. At physiological doses, insulin stimulation for 15 min increased 3-fold glucose uptake and doubled the amount of Glut4 protein located at the plasma membrane. Moreover, phosphatidylinositol (PI) 3-kinase activity was induced by the presence of insulin in those cells, glucose uptake being precluded by PI 3-kinase inhibitors such as wortmannin or LY294002. H-ras^lys12-transformed brown adipocytes showed a 10-fold higher expression of Glut1 mRNA and protein than parental cells, Glut4 gene expression being completely down-regulated. Glucose uptake increased by 10-fold in transformed cells compared to parental cells; this uptake was unaltered in the presence of insulin and/or wortmannin. Transient transfection of parental cells with a dominant form of active Ras increased basal glucose uptake by 5-fold, no further effects being observed in the presence of insulin. However, PI 3-kinase activity (immunoprecipitated with anti-αp85 subunit of PI 3-kinase) remained unaltered in H-ras permanent and transient transfectants. Our results indicate that activated Ras induces brown adipocyte glucose transport in an insulin-independent manner, this induction not involving PI 3-kinase activation.     

Phosphatidylinositol 3-kinase is required for integrin-stimulated AKT and Raf-1/mitogen-activated protein kinase pathway activation.

Cell attachment to fibronectin stimulates the integrin-dependent interaction of p85-associated phosphatidylinositol (PI) 3-kinase with integrin-dependent focal adhesion kinase (FAK) as well as activation of the Ras/mitogen-activated protein (MAP) kinase pathway. However, it is not known if this PI 3-kinase-FAK interaction increases the synthesis of the 3-phosphorylated phosphoinositides (3-PPIs) or what role, if any, is played by activated PI 3-kinase in integrin signaling. We demonstrate here the integrin-dependent accumulation of the PI 3-kinase products, PI 3,4-bisphosphate [PI(3,4)P2] and PI(3,4,5)P3, as well as activation of AKT kinase, a serine/threonine kinase that can be stimulated by binding of PI(3,4)P2. The PI 3-kinase inhibitors wortmannin and LY294002 significantly decreased the integrin-induced accumulation of the 3-PPIs and activation of AKT kinase, without having significant effects on the levels of PI(4,5)P2 or tyrosine phosphorylation of paxillin. These inhibitors also reduced cell adhesion/spreading onto fibronectin but had no effect on attachment to polylysine. Interestingly, integrin-mediated Erk-2, Mek-1, and Raf-1 activation, but not Ras-GTP loading, was inhibited at least 80% by wortmannin and LY294002. In support of the pharmacologic results, fibronectin activation of Erk-2 and AKT kinases was completely inhibited by overexpression of a dominant interfering p85 subunit of PI 3-kinase. We conclude that integrin-mediated adhesion to fibronectin results in the accumulation of the PI 3-kinase products PI(3,4)P2 and PI(3,4,5)P3 as well as the PI 3-kinase-dependent activation of the kinases Raf-1, Mek-1, Erk-2, and AKT and that PI 3-kinase may function upstream of Raf-1 but downstream of Ras in integrin activation of Erk-2 MAP and AKT kinases.     

GSK-3beta regulation in skeletal muscles by adrenaline and insulin: evidence that PKA and PKB regulate different pools of GSK-3

We have recently shown that while adrenaline alone has no effect on the activation of Protein Kinase B (PKB) in rat soleus muscle, it greatly potentiates the effects of insulin (Brennesvik et al., Cellular Signalling 17: 1551-1559, 2005). In the current study we went on to investigate whether this was paralleled by a similar effect on GSK-3, which is a major PKB target. Surprisingly adrenaline alone increased phosphorylation of GSK-3beta Ser9 and GSK-3alpha Ser21 and adrenaline's effects were additive with those of insulin but did not synergistically potentiate insulin action. Dibutyryl-cAMP (5 mM) and the PKA specific cAMP analogue N6-Benzoyl-cAMP (2 mM) increased GSK-3beta Ser9 phosphorylation, whereas the Epac specific cAMP analogue 8-(4-chlorophenylthio)-2'-O-methyl-cAMP (1 mM) did not. Wortmannin (PI 3-kinase inhibitor; 1 microM) blocked insulin-stimulated GSK-3 phosphorylation completely, but adrenaline increased GSK-3beta Ser9 phosphorylation in the presence of wortmannin. The PKA inhibitor H89 (50 microM) reduced adrenaline-stimulated GSK-3beta Ser9 phosphorylation but did not influence the effects of insulin. Insulin-stimulated GSK-3 Ser9 phosphorylation was paralleled by decreased glycogen synthase phosphorylation at the sites phosphorylated by GSK-3 as expected. However, adrenaline-stimulated GSK-3 Ser9 phosphorylation was paralleled by increased glycogen synthase phosphorylation indicating this pool of GSK-3 may not be directly involved in phosphorylation of glycogen synthase. Our results indicate the existence of at least two distinct pools of GSK-3beta in soleus muscle, one phosphorylated by PKA and another by PKB. Further, we hypothesise that each of these pools is involved in the control of different cellular processes.     

Immune-mediated signaling in intestinal goblet cells via PI3-kinase- and AKT-dependent pathways

In the intestinal epithelium, activation of phosphatidylinositol 3-kinase (PI3-kinase)/AKT pathways, via growth factor-mediated signaling, has been shown to regulate cell proliferation and inhibit apoptosis. An immune-activated receptor critical for Th2 immune responses, IL-4Ralpha can also activate PI3-kinase via insulin receptor substrate (IRS)-dependent signaling. Here, using the intestinal goblet cell-specific gene RELMbeta, we investigated the effect of PI3-kinase activation via Th2 immune responses on the goblet cell phenotype. IL-13 stimulation activated PI3-kinase and AKT signal transduction in LS174T cells. Not only did pharmacological inhibition of PI3-kinase and AKT1/2 inhibit RELMbeta induction by IL-13, but AKT inhibition also significantly reduced constitutive basal expression of RELMbeta, a response reproduced by the simultaneous pharmacological inhibition of both epidermal growth factor receptor and IGF-I receptor signaling. In vivo, the disruption of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), an inhibitor of PI3-kinase activation, led to the activation of RELMbeta expression in the small intestine. Furthermore, induction of an intestinal Th2 immune response by infection with a small intestinal nematode parasite, Heligmosomoides polygyrus, led to enhanced epithelial cell proliferation, activation of AKT as demonstrated by the loss of Foxo1 nuclear localization, and robust induction of RELMbeta expression in wild-type, but not IL-4Ralpha knockout, mice. These results demonstrate that Th2 immune responses can regulate goblet cell responses by activation of PI3-kinase and AKT pathways via IL-4Ralpha.     

Association of phosphatidylinositol 3-kinase with the insulin receptor: compartmentation in rat liver

Phosphatidylinositol 3-kinase (PI 3-kinase) plays an important role in a variety of hormone and growth factor-mediated intracellular signaling cascades and has been implicated in the regulation of a number of metabolic effects of insulin, including glucose transport and glycogen synthase activation. In the present study we have examined 1) the association of PI 3-kinase with the insulin receptor kinase (IRK) in rat liver and 2) the subcellular distribution of PI 3-kinase-IRK interaction. Insulin treatment promoted a rapid and pronounced recruitment of PI 3-kinase to IRKs located at the plasma membrane, whereas no increase in association with endosomal IRKs was observed. In contrast to IRS-1-associated PI 3-kinase activity, association of PI 3-kinase with the plasma membrane IRK did not augment the specific activity of the lipid kinase. With use of the selective PI 3-kinase inhibitor wortmannin, our data suggest that the cell surface IRK beta-subunit is not a substrate for the serine kinase activity of PI 3-kinase. The functional significance for the insulin-stimulated selective recruitment of PI 3-kinase to cell surface IRKs remains to be elucidated.     

Phosphatidylinositol 3-kinase is required for insulin-stimulated tyrosine phosphorylation of Shc in 3T3-L1 adipocytes

The interactions between the phosphatidylinositol 3-kinase (PI 3-kinase) and Ras/MAPK kinase pathways have been the subject of considerable interest. In the current studies, we find that epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) lead to rapid phosphorylation of Shc (maximum at 1-2 min), whereas insulin-mediated Shc phosphorylation is relatively delayed (maximum at 5-10 min), suggesting that an intermediary step may be necessary for insulin stimulation of Shc phosphorylation. The Src homology-2 (SH2) domain of Shc is necessary for PDGF- and EGF-mediated Shc phosphorylation, whereas the phosphotyrosine binding (PTB) domain is critical for the actions of insulin. Because the Shc PTB domain can interact with phospholipids, we postulated that PI 3-kinase might be a necessary intermediary step facilitating insulin-stimulated phosphorylation of Shc. In support of this, we found that the PI 3-kinase inhibitors, wortmannin and LY294002, blocked insulin-stimulated but not EGF- or PDGF-stimulated Shc phosphorylation. Furthermore, overexpression of a dominant negative PI 3-kinase construct (p85N-SH2) blocked insulin, but not EGF- or PDGF-induced Shc phosphorylation. All three growth factors cause localization of Shc to the plasma membrane, but only the effect of insulin was inhibited by wortmannin, supporting the view that PI 3-kinase-generated phospholipids mediate insulin-stimulated Shc phosphorylation. Consistent with this, expression of a constitutively active PI 3-kinase (p110(C)(AAX)) increased membrane localization of Shc, and this was completely blocked by wortmannin. A mutant Shc with a disrupted PTB domain (Shc S154) did not localize to the membrane in p110(C)(AAX)-expressing cells or after insulin stimulation and was not phosphorylated by insulin. In summary, 1) PI 3-kinase is a necessary early step in insulin-stimulated Shc phosphorylation, whereas the effects of EGF and PDGF on Shc phosphorylation are independent of PI 3-kinase. 2) PI 3-kinase-stimulated generation of membrane phospholipids can localize Shc to the plasma membrane through the Shc PTB domain facilitating phosphorylation by the insulin receptor.     

Identification of nonsteroidal anti-inflammatory drug-activated gene (NAG-1) as a novel downstream target of phosphatidylinositol 3-kinase/AKT/GSK-3beta pathway

The signaling pathway of phosphatidylinositol 3-kinase (PI3K)/AKT, which is involved in cell survival, proliferation, and growth, has become a major focus in targeting cancer therapeutics. Nonsteroidal anti-inflammatory drug-activated gene (NAG-1) was previously identified as a gene induced by several anti-tumorigenic compounds including nonsteroidal anti-inflammatory drugs, peroxisome proliferator-activated receptor gamma ligands, and dietary compounds. NAG-1 has been shown to exhibit anti-tumorigenic and/or pro-apoptotic activities in vivo and in vitro. In this report, we showed a PI3K/AKT/glycogen synthase kinase-3beta (GSK-3beta) pathway regulates NAG-1 expression in human colorectal cancer cells as assessed by the inhibition of PI3K, AKT, and GSK-3beta. PI3K inhibition by LY294002 showed an increase in NAG-1 protein and mRNA expression, and 1l-6-hydroxymethyl-chiro-inositol 2(R)-2-O-methyl-3-O-octadecylcarbonate (AKT inhibitor) also induced NAG-1 expression. LY294002 caused increased apoptosis, cell cycle, and cell growth arrest in HCT-116 cells. Inhibition of GSK-3beta, which is negatively regulated by AKT, using AR-A014418 and lithium chloride completely abolished LY294002-induced NAG-1 expression as well as the NAG-1 promoter activity. Furthermore, the down-regulation of GSK-3 gene using small interference RNA resulted in a decline of the NAG-1 expression in the presence of LY294002. These data suggest that expression of NAG-1 is regulated by PI3K/AKT/GSK-3beta pathway in HCT-116 cells and may provide a further understanding of the important role of PI3K/AKT/GSK-3beta pathway in tumorigenesis.     

Reconstitution of phosphoinositide 3-kinase-dependent insulin signaling in a cell-free system

Early insulin signaling events were examined in a novel cell-free assay utilizing subcellular fractions derived from 3T3-L1 adipocytes. The following cellular processes were observed in vitro in a manner dependent on insulin, time of incubation, and exogenous ATP: 1) autophosphorylation and activation of the insulin receptor; 2) tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1); 3) association of tyrosine-phosphorylated IRS-1 with phosphoinositide 3-kinase; 4) activation of the kinase Akt via its phosphorylation on Thr-308 and Ser-473; and 5) phosphorylation of glycogen synthase kinase-3 by activated Akt. The activation of Akt in vitro was abolished in the presence of the phosphoinositide 3-kinase inhibitor, wortmannin, thus recapitulating the most notable regulatory feature of Akt observed in vivo. Evidence is presented indicating that the critical spatial compartmentalization of signaling molecules necessary for efficient signal transduction is likely to be preserved in the cell-free system. Additionally, data are provided demonstrating that full Akt activation in this system is dependent on plasma membrane-associated IRS-1, cannot be mediated by robust cytosol-specific tyrosine phosphorylation of IRS-1, and occurs in the complete absence of detectable IRS-2 phosphorylation in the cytosol and plasma membrane.     

Potential mechanism(s) involved in the regulation of glycogen synthesis by insulin

Stimulation of glycogen synthesis is one of the major physiological responses modulated by insulin. Although, details of the precise mechanism by which insulin action on glycogen synthesis is mediated remains uncertain, significant advances have been made to understand several steps in this process. Most importantly, recent studies have focussed on the possible role of glycogen synthase kinase-3 (GSK-3) and glycogen bound protein phosphatase-1 (PP-1G) in the activation of glycogen synthase (GS) - a key enzyme of glycogen metabolism. Evidence is also accumulating to establish a link between insulin receptor induced signaling pathway(s) and glycogen synthesis. This article summarizes the potential contribution of various elements of insulin signaling pathway such as mitogen activated protein kinase (MAPK), protein kinase B (PKB), and phosphatidyl inositol 3-kinase (PI3-K) in the activation of GS and glycogen synthesis.     

Regulation of Glycogen Content in Astrocytes via Cav-1/PTEN/AKT/GSK-3β Pathway by Three Anti-bipolar Drugs

Here we present the data indicating that chronic treatment with three antibipolar drugs, lithium, carbamazepine and valproic acid regulates Cav-1/PTEN/PI3K/AKT/GSK-3β signalling pathway and glycogen content in primary cultured astrocytes. All three drugs down-regulate gene expression of Caveoline 1 (Cav-1), decrease membrane content of phosphatase and tensin homolog (PTEN), increase activity of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) and serine-threonine kinase (AKT), and elevate glycogen synthase kinase 3β (GSK-3β) phosphorylation thus suppressing its activity. As expected, treatment with any of these three drugs increases glycogen content in astrocytes. Our findings indicate that regulation of glycogen content via Cav-1/PTEN/AKT/GSK-3β pathway by the three anti-bipoar drugs may be responsible for therapeutic effects of these drugs, and Cav-1 is an important signal element that may contribute to pathogenesis of various CNS diseases and regulation of its gene expression may be one of the underlying mechanisms of drug action for antibipolar drugs and antidepressants currently in clinical use.     

Regulation of glycogen synthase in rat hepatocytes. Evidence for multiple signaling pathways.

We examined the signaling pathways regulating glycogen synthase (GS) in primary cultures of rat hepatocytes. The activation of GS by insulin and glucose was completely reversed by the phosphatidylinositol 3-kinase inhibitor wortmannin. Wortmannin also inhibited insulin-induced phosphorylation and activation of protein kinase B/Akt (PKB/Akt) as well as insulin-induced inactivation of GS kinase-3 (GSK-3), consistent with a role for the phosphatidylinositol 3-kinase/PKB-Akt/GSK-3 axis in insulin-induced GS activation. Although wortmannin completely inhibited the significantly greater level of GS activation produced by the insulin-mimetic bisperoxovanadium 1,10-phenanthroline (bpV(phen)), there was only minimal accompanying inhibition of bpV(phen)-induced phosphorylation and activation of PKB/Akt, and inactivation of GSK-3. Thus, PKB/Akt activation and GSK-3 inactivation may be necessary but are not sufficient to induce GS activation in rat hepatocytes. Rapamycin partially inhibited the GS activation induced by bpV(phen) but not that effected by insulin. Both insulin- and bpV(phen)-induced activation of the atypical protein kinase C (zeta/lambda) (PKC (zeta/lambda)) was reversed by wortmannin. Inhibition of PKC (zeta/lambda) with a pseudosubstrate peptide had no effect on GS activation by insulin, but substantially reversed GS activation by bpV(phen). The combination of this inhibitor with rapamycin produced an additive inhibitory effect on bpV(phen)-mediated GS activation. Taken together, our results indicate that the signaling components mammalian target of rapamycin and PKC (zeta/lambda) as well as other yet to be defined effector(s) contribute to the modulation of GS in rat hepatocytes.