Phorbol esters stimulate phosphatidylinositol 3,4,5-trisphosphate production in 3T3-L1 adipocytes: implications for stimulation of glucose transport

PKCβ phosphorylates PI3Kγ to activate it and release it from GPCR control

All class I phosphoinositide 3-kinases (PI3Ks) associate tightly with regulatory subunits through interactions that have been thought to be constitutive. PI3Kγ is key to the regulation of immune cell responses activated by G protein-coupled receptors (GPCRs). Remarkably we find that PKCβ phosphorylates Ser582 in the helical domain of the PI3Kγ catalytic subunit p110γ in response to clustering of the high-affinity IgE receptor (FcεRI) and/or store-operated Ca²⁺- influx in mast cells. Phosphorylation of p110γ correlates with the release of the p84 PI3Kγ adapter subunit from the p84-p110γ complex. Ser582 phospho-mimicking mutants show increased p110γ activity and a reduced binding to the p84 adapter subunit. As functional p84-p110γ is key to GPCR-mediated p110γ signaling, this suggests that PKCβ-mediated p110γ phosphorylation disconnects PI3Kγ from its canonical inputs from trimeric G proteins, and enables p110γ to operate downstream of Ca²⁺ and PKCβ. Hydrogen deuterium exchange mass spectrometry shows that the p84 adaptor subunit interacts with the p110γ helical domain, and reveals an unexpected mechanism of PI3Kγ regulation. Our data show that the interaction of p110γ with its adapter subunit is vulnerable to phosphorylation, and outline a novel level of PI3K control.

A drug targeting only p110α can block phosphoinositide 3-kinase signalling and tumour growth in certain cell types

Genetic alterations in PI3K (phosphoinositide 3-kinase) signalling are common in cancer and include deletions in PTEN (phosphatase and tensin homologue deleted on chromosome 10), amplifications of PIK3CA and mutations in two distinct regions of the PIK3CA gene. This suggests drugs targeting PI3K, and p110α in particular, might be useful in treating cancers. Broad-spectrum inhibition of PI3K is effective in preventing growth factor signalling and tumour growth, but suitable inhibitors of p110α have not been available to study the effects of inhibiting this isoform alone. In the present study we characterize a novel small molecule, A66, showing the S-enantiomer to be a highly specific and selective p110α inhibitor. Using molecular modelling and biochemical studies, we explain the basis of this selectivity. Using a panel of isoform-selective inhibitors, we show that insulin signalling to Akt/PKB (protein kinase B) is attenuated by the additive effects of inhibiting p110α/p110β/p110δ in all cell lines tested. However, inhibition of p110α alone was sufficient to block insulin signalling to Akt/PKB in certain cell lines. The responsive cell lines all harboured H1047R mutations in PIK3CA and have high levels of p110α and class-Ia PI3K activity. This may explain the increased sensitivity of these cells to p110α inhibitors. We assessed the activation of Akt/PKB and tumour growth in xenograft models and found that tumours derived from two of the responsive cell lines were also responsive to A66 in vivo. These results show that inhibition of p110α alone has the potential to block growth factor signalling and reduce growth in a subset of tumours.

Characterization and performance of a near-infrared 2-deoxyglucose optical imaging agent for mouse cancer models

Malignant neoplasms exhibit an elevated rate of glycolysis over normal cells. This characteristic can be exploited for optical imaging of tumors in mice. A near-infrared fluorophore, IRDye 800CW, emission maximum 794 nm, was conjugated to 2-deoxyglucose (2-DG). An immunofluorescent cell-based assay was used to evaluate specificity and sensitivity of the conjugate in cultured cell monolayers. Dose-dependent uptake was established with increasing concentrations of IRDye 800CW 2-DG for epithelial and prostate carcinomas. IRDye 800CW 2-DG was specifically blocked by an antibody against GLUT1 glucose transporter, and by excess unlabeled 2-DG or d-glucose. Signal was increased by a phorbol ester activator of glucose transport. Fluorescence microscopy data confirmed localization of the conjugate in the cytoplasm. Subsequent in vivo studies optimized dose, clearance, and timing for signal capture in nude mouse xenografts. In all cases, tumors were clearly imaged with good signal-to-noise characteristics. These data indicate that IRDye 800CW 2-DG is a broadly applicable optical imaging agent for in vivo imaging of neoplasms in mice.

Phosphoinositides in insulin action on GLUT4 dynamics: not just PtdIns(3,4,5)P3

Accumulated evidence over the last several years indicates that insulin regulates multiple steps in the overall translocation of GLUT4 vesicles to the fat/muscle cell surface, including formation of an intracellular storage pool of GLUT4 vesicles, its movement to the proximity of the cell surface, and the subsequent docking/fusion with the plasma membrane. Insulin-stimulated formation of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3); and in some cases, of its catabolite PtdIns(3,4)P(2)] plays a pivotal role in this process. PtdIns(3,4,5)P(3) is synthesized by the activated wortmannin-sensitive class IA phosphoinositide (PI) 3-kinase and controls the rate-limiting cell surface terminal stages of the GLUT4 journey. However, recent research is consistent with the conclusion that signals by each of the remaining five PIs, i.e., PtdIns(3)P, PtdIns(4)P, PtdIns(5)P, PtdIns(3,5)P(2), and PtdIns(4,5)P(2), may act in concert with that of PtdIns(3,4,5)P(3) in integrating the insulin receptor-issued signals with GLUT4 surface translocation and glucose transport activation. This review summarizes the experimental evidence supporting the complementary function of these PIs in insulin responsiveness of fat and muscle cells, with particular reference to mechanistic insights and functional significance in the regulation of overall GLUT4 vesicle dynamics.

Protein kinase C attenuates β-adrenergic receptor-mediated lipolysis, probably through inhibition of the β1-adrenergic receptor system

Lipolysis in rat white adipocytes is stimulated by beta-adrenergic agonists. Phorbol 12-myristate 13-acetate (PMA) attenuated the receptor-mediated lipolysis by causing a shift of the dose-response curve to the higher concentrations of norepinephrine and isoproterenol. Although the adipocytes possess beta1-, beta2-, and beta3-adrenergic receptor subtypes, the effect of PMA was observed only when a beta1-agonist (dobutamine) was used. No lipolysis-attenuating effect of PMA was found when cells were exposed to a beta2-agonist (procaterol) and beta3-agonists (BRL 37344 and CL 316243), or to forskolin and 8-bromo cAMP. CGP 20712A (beta1-antagonist) efficiently inhibited lipolysis by norepinephrine, isoproterenol, and dobutamine, but did not affect lipolysis by the beta2- and beta3-agonists. ICI 118551 (beta2-antagonist) had no significant effect on lipolysis by the beta-agonists examined. CGP 20712A abolished the lipolysis-attenuating effect of PMA, but ICI 118551 did not. The protein kinase C (PKC) inhibitors, GF 109203X or Gö 6976, suppressed the effect of PMA. Pretreatment of adipocytes with PMA for 6 h caused downregulation of conventional and novel PKCs in association with a decrease in the lipolysis-attenuating effect of PMA. These results indicate that conventional and novel PKCs attenuate lipolysis mediated by beta-adrenergic receptors, probably through inhibition of the beta1-adrenergic receptor system.

Regulation of GLUT1-mediated glucose uptake by PKClambda-PKCbeta(II) interactions in 3T3-L1 adipocytes

Members of the PKC (protein kinase C) superfamily play key regulatory roles in glucose transport. How the different PKC isotypes are involved in the regulation of glucose transport is still poorly defined. PMA is a potent activator of conventional and novel PKCs and PMA increases the rate of glucose uptake in many different cell systems. In the present study, we show that PMA treatment increases glucose uptake in 3T3-L1 adipocytes by two mechanisms: a mitogen-activated protein kinase kinase-dependent increase in GLUT1 (glucose transporter 1) expression levels and a PKClambda-dependent translocation of GLUT1 towards the plasma membrane. Intriguingly, PKClambda co-immunoprecipitated with PKCbeta(II) and did not with PKCbeta(I). Previously, we have described that down-regulation of PKCbeta(II) protein levels or inhibiting PKCbeta(II) by means of the myristoylated PKCbetaC2-4 peptide inhibitor induced GLUT1 translocation towards the plasma membrane in 3T3-L1 adipocytes. Combined with the present findings, these results suggest that the liberation of PKClambda from PKCbeta(II) is an important factor in the regulation of GLUT1 distribution in 3T3-L1 adipocytes.

Repression of 5-aminolevulinate synthase gene by the potent tumor promoter, TPA, involves multiple signal transduction pathways

The potent tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) induces activator protein-1 (AP-1) transcription factors, early response genes involved in a diverse set of transcriptional regulatory processes, and protein kinase C (PKC) activity. This work was designed to explore the signal transduction pathways involved in TPA regulation of 5-aminolevulinate synthase (ALAS) gene expression, the mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of heme biosynthesis. We have previously reported that TPA causes repression of ALAS gene, but the signaling pathways mediating this effect remain elusive. The present study investigates the role of different cascades often implicated in the propagation of phorbol ester signaling. To explore this, we combined the transient overexpression of regulatory proteins involved in these pathways and the use of small cell permeant inhibitors in human hepatoma HepG2 cells. In these experimental conditions, we analyzed TPA action upon endogenous ALAS mRNA levels, as well as the promoter activity of a fusion reporter construct, harboring the TPA-responsive region of ALAS gene driving chloramphenicol acetyl transferase gene expression. We demonstrated that the participation of alpha isoform of PKC, phosphatidylinositol 3-kinase (PI3K), extracellular-signal regulated kinase (ERK1/2), and c-Jun N-terminal kinase (JNK) is crucial for the end point response. Remarkably, in this case, ERK activation is achieved in a Ras/Raf/MEK-independent manner. We also propose that p90RSK would be a convergent point between PI3K and ERK pathways. Furthermore, we elucidated the crosstalk among the components of the cascades taking part in TPA-mediated ALAS repression. Finally, by overexpression of a constitutively active p90RSK and the coactivator, cAMP-response element protein (CREB)-binding protein (CBP), we reinforced our previous model, that implies competition between AP-1 and CREB for CBP.

alpha1A-adrenoceptors activate glucose uptake in L6 muscle cells through a phospholipase C-, phosphatidylinositol-3 kinase-, and atypical protein kinase C-dependent pathway

The role of alpha1-adrenoceptor activation on glucose uptake in L6 cells was investigated. The alpha1-adrenoceptor agonist phenylephrine [pEC50 (-log10 EC50), 5.27 +/- 0.30] or cirazoline (pEC50, 5.00 +/- 0.23) increased glucose uptake in a concentration-dependent manner, as did insulin (pEC50, 7.16 +/- 0.21). The alpha2-adrenoceptor agonist clonidine was without any stimulatory effect on glucose uptake. The stimulatory effect of cirazoline was inhibited by the alpha1-adrenoceptor antagonist prazosin, but not by the beta-adrenoceptor antagonist propranolol. RT-PCR showed that the alpha1A-adrenoceptor was the sole alpha1-adrenoceptor subtype expressed in L6 cells. Cirazoline- or insulin-mediated glucose uptake was inhibited by the phosphatidylinositol-3 kinase inhibitor LY294002, suggesting a possible interaction between the alpha1-adrenoceptor and insulin pathways. Cirazoline or insulin stimulated phosphatidylinositol-3 kinase activity, but alpha1-adrenoceptor activation did not phosphorylate Akt. Both cirazoline- and insulin-mediated glucose uptake were inhibited by protein kinase C (PKC), phospholipase C, and p38 kinase inhibitors, but not by Erk1/2 inhibitors (despite both treatments being able to phosphorylate Erk1/2). Insulin and cirazoline were able to activate and phosphorylate p38 kinase. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate and the calcium ionophore A23187 produced significant increases in glucose uptake, indicating roles for PKC and calcium in glucose uptake. Down-regulation of conventional PKC isoforms inhibited glucose uptake mediated by 12-O-tetradecanoylphorbol-13-acetate, but not by insulin or cirazoline. This study demonstrates that alpha1-adrenoceptors mediate increases in glucose uptake in L6 muscle cells. This effect appears to be related to activation of phospholipase C, phosphatidylinositol-3 kinase, p38 kinase, and PKC.

Inhibition of Insulin Sensitivity by Free Fatty Acids Requires Activation of Multiple Serine Kinases in 3T3-L1 Adipocytes

Insulin receptor substrate (IRS) has been suggested as a molecular target of free fatty acids (FFAs) for insulin resistance. However, the signaling pathways by which FFAs lead to the inhibition of IRS function remain to be established. In this study, we explored the FFA-signaling pathway that contributes to serine phosphorylation and degradation of IRS-1 in adipocytes and in dietary obese mice. Linoleic acid, an FFA used in this study, resulted in a reduction in insulin-induced glucose uptake in 3T3-L1 adipocytes. This mimics insulin resistance induced by high-fat diet in C57BL/6J mice. The reduction in glucose uptake is associated with a decrease in IRS-1, but not IRS-2 or GLUT4 protein abundance. Decrease in IRS-1 protein was proceeded by IRS-1 (serine 307) phosphorylation that was catalyzed by serine kinases inhibitor kappaB kinase (IKK) and c-JUN NH2-terminal kinase (JNK). IKK and JNK were activated by linoleic acid and inhibition of the two kinases led to prevention of IRS-1 reduction. We demonstrate that protein kinase C (PKC) theta is expressed in adipocytes. In 3T3-L1 adipocytes and fat tissue, PKCtheta was activated by fatty acids as indicated by its phosphorylation status, and by its protein level, respectively. Activation of PKCtheta contributes to IKK and JNK activation as inhibition of PKCtheta by calphostin C blocked activation of the latter kinases. Inhibition of either PKCtheta or IKK plus JNK by chemical inhibitors resulted in protection of IRS-1 function and insulin sensitivity in 3T3-L1 adipocytes. These data suggest that: 1) activation of PKCtheta contributes to IKK and JNK activation by FFAs; 2) IKK and JNK mediate PKCtheta signals for IRS-1 serine phosphorylation and degradation; and 3) this molecular mechanism may be responsible for insulin resistance associated with hyperlipidemia.

Amino acid- and lipid-induced insulin resistance in rat heart: molecular mechanisms

Lipids compete with glucose for utilization by the myocardium. Amino acids are an important energetic substrate in the heart but it is unknown whether they reduce glucose disposal. The molecular mechanisms by which lipids and amino acids impair insulin-mediated glucose disposal in the myocardium are unknown. We evaluated the effect of lipids and amino acids on the insulin stimulated glucose uptake in the isolated rat heart and explored the involved target proteins. The hearts were perfused with 16 mM glucose alone or with 6% lipid or 10% amino acid solutions at the rate of 15 ml/min. After 1 h of perfusion (basal period), insulin (240 nmol/l) was added and maintained for an additional hour. Both lipids and amino acids blocked the insulin effect on glucose uptake (P<0.01) and reduced the activity of the IRSs/PI 3-kinase/Akt/GSK3 axis leading to the activation of glucose transport and glycogen synthesis. Amino acids, but not lipids, increased the activity of the p70 S6 kinase leading to the stimulation of protein synthesis. Amino acids induce myocardial insulin resistance recruiting the same molecular mechanisms as lipids. Amino acids retain an insulin-like stimulatory effect on p70 S6 kinase, which is independent from the PI 3-Kinase downstream effectors.

Mammalian cell morphology and endocytic membrane homeostasis require enzymatically active phosphoinositide 5-kinase PIKfyve

The dual specificity mammalian enzyme PIKfyve phosphorylates in vitro position d-5 in phosphatidylinositol (PtdIns) and PtdIns 3-P, itself or exogenous protein substrates. Here we have addressed the crucial questions for the identity of the lipid products and the role of PIKfyve enzymatic activity in mammalian cells. CHO, HEK293, and COS cells expressing PIKfyve(WT) at high levels and >90% efficiencies increased selectively the intracellular PtdIns 3,5-P(2) production by 30--55%. In these cell types PtdIns 5-P was undetectable. A kinase-deficient point mutant, PIKfyve(K1831E), transiently transfected into these or other cells elicited a dramatic dominant phenotype. Subsequent to a dilation of the PIKfyve-containing vesicles, PIKfyve(K1831E)-expressing cells progressively accumulated multiple swollen lucent vacuoles of endosomal origin, first in the perinuclear cytoplasm and then toward the cell periphery. Despite their drastically altered morphology, the PIKfyve(K1831E)-expressing cells were viable and functionally active, evidenced by several criteria. This phenotype was completely reversed by introducing PIKfyve(WT) into the PIKfyve(K1831E)-transfected cells. Disruptions of the localization signal in the PIKfyve kinase-deficient mutant yielded a PIKfyve(K1831E Delta fyve) protein, incompetent to vacuolate cells, implying that an active PIKfyve enzyme at distinct late endocytic membranes is crucial for normal cell morphology. This was further substantiated by examining the vacuolation-induced potency of several pharmacological stimuli in cells expressing high PIKfyve(WT) levels. Together, the results indicate that PIKfyve enzymatic activity, possibly through the generation of PtdIns 3,5-P(2), and/or yet to be identified endogenous phosphoproteins, is critical for cell morphology and endomembrane homeostasis.

Insulin-sensitive phospholipid signaling systems and glucose transport. Update II

Insulin provokes rapid changes in phospholipid metabolism and thereby generates biologically active lipids that serve as intracellular signaling factors that regulate glucose transport and glycogen synthesis. These changes include: (i) activation of phosphatidylinositol 3-kinase (PI3K) and production of PIP3; (ii) PIP3-dependent activation of atypical protein kinase Cs (PKCs); (iii) PIP3-dependent activation of PKB; (iv) PI3K-dependent activation of phospholipase D and hydrolysis of phosphatidylcholine with subsequent increases in phosphatidic acid (PA) and diacylglycerol (DAG); (v) PI3K-independent activation of glycerol-3-phosphate acylytansferase and increases in de novo synthesis of PA and DAG; and (vi) activation of DAG-sensitive PKCs. Recent findings suggest that atypical PKCs and PKB serve as important positive regulators of insulin-stimulated glucose metabolism, whereas mechanisms that result in the activation of DAG-sensitive PKCs serve mainly as negative regulators of insulin signaling through PI3K. Atypical PKCs and PKB are rapidly activated by insulin in adipocytes, liver, skeletal muscles, and other cell types by a mechanism requiring PI3K and its downstream effector, 3-phosphoinositide-dependent protein kinase-1 (PDK-1), which, in conjunction with PIP3, phosphorylates critical threonine residues in the activation loops of atypical PKCs and PKB. PIP3 also promotes increases in autophosphorylation and allosteric activation of atypical PKCs. Atypical PKCs and perhaps PKB appear to be required for insulin-induced translocation of the GLUT 4 glucose transporter to the plasma membrane and subsequent glucose transport. PKB also appears to be the major regulator of glycogen synthase. Together, atypical PKCs and PKB serve as a potent, integrated PI3K/PDK-1-directed signaling system that is used by insulin to regulate glucose metabolism.

Association of protein kinase C(lambda) with adducin in 3T3-L1 adipocytes

There is evidence that the atypical protein kinases C (PKC(lambda), PKC(zeta)) participate in signaling from the insulin receptor to cause the translocation of glucose transporters from an intracellular location to the plasma membrane in adipocytes. In order to search for downstream effectors of these PKCs, we identified the proteins that were immunoprecipitated by an antibody against PKC(lambda/zeta) from lysates of 3T3-L1 adipocytes through peptide sequencing by mass spectrometry. The data show that PKC(lambda) is the major atypical PKC in these cells. Moreover, an oligomeric complex consisting of alpha- and gamma-adducin, which are cytoskeletal proteins, coimmunoprecipitated with PKC(lambda). Association of the adducins with PKC(lambda) was further indicated by the finding that the adducins coimmunoprecipitated proportionally with PKC(lambda) in repeated rounds of immunoprecipitation. Such an association is consistent with literature reports that the adducins contain a single major site for PKC phosphorylation in their carboxy termini. Using antibody against the phospho form of this site for immunoblotting, we found that insulin caused little or no increase in the phosphorylation of this site on the adducins in a whole cell lysate or on the small portion of the adducins that coimmunoprecipitated with PKC(lambda). PKC(lambda) and the adducins were located in both the cytosol and subcellular membranous fractions. The binding of PKC(lambda) to adducin may function to localize PKC(lambda) in 3T3-L1 adipocytes.

Ischemic preconditioning activates phosphatidylinositol-3-kinase upstream of protein kinase C

The present study is designed to test whether phosphatidylinositol 3-kinase (PI3-kinase) has a role in the signaling pathway in ischemic preconditioning (PC) and whether it is proximal or distal to protein kinase C (PKC). Before 20 minutes of global ischemia, Langendorff-perfused rat hearts were perfused for 20 minutes (control); preconditioned with 4 cycles of 5-minute ischemia and 5-minute reflow (PC); treated with either wortmannin (WM) or LY 294002 (LY), each of which is a PI3-kinase inhibitor, for 5 minutes before and throughout PC; treated with 1,2-dioctanoyl-sn-glycerol (DOG), an activator of PKC for 10 minutes (DOG); treated identically to the DOG group except with WM added 10 minutes before and during perfusion with DOG; or treated with either WM or LY for 25 minutes. Recovery of left ventricular developed pressure (LVDP; percentage of initial preischemic LVDP), measured after 30 minutes of reflow, was improved by PC (72+/-2% versus 36+/-4% in control; P<0.001), and this was blocked by WM and LY (41+/-4% and 43+/-5%, respectively; P<0.05 compared with PC). DOG addition improved postischemic LVDP (67+/-6%; P<0.001 compared with control), but in contrast to its effect on PC, WM did not completely eliminate the protective effect of DOG (52+/-4%; P>0.05 compared with DOG; P<0.05 compared with control). PC induced phosphorylation of protein kinase B and translocation of PKC epsilon, and it increased NO production, and these effects were blocked by WM, which suggests a role for PI3-kinase in PC upstream of PKC and NO.

A MAP kinase-signaling pathway mediates neurite outgrowth on L1 and requires Src-dependent endocytosis

The neural cell adhesion molecule L1 mediates the axon outgrowth, adhesion, and fasciculation necessary for proper development of synaptic connections. Mutations of human L1 cause an X-linked mental retardation syndrome termed CRASH (corpus callosum hypoplasia, retardation, aphasia, spastic paraplegia, and hydrocephalus), and L1 knock-out mice display defects in neuronal process extension resembling the CRASH phenotype. Little is known about the biochemical or cellular mechanism by which L1 performs neuronal functions. Here it is demonstrated that clustering of L1 with antibodies or L1 protein in rodent B35 neuroblastoma and cerebellar neuron cultures induced the phosphorylation/activation of the mitogen-activated protein kinases (MAPKs) and extracellular signal-regulated kinases 1 and 2. MAPK activation was essential for L1-dependent neurite outgrowth, because chemical inhibitors [2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one and 1,4-diamino-2, 3-dicyano-1,4-bis(2-aminophenylthio)butadiene] of the MAPK kinase MEK strongly suppressed neurite outgrowth by cerebellar neurons on L1. The nonreceptor tyrosine kinase pp60(c-src) was required for L1-triggered MAPK phosphorylation, as shown in src-minus cerebellar neurons and by expression of the kinase-inactive mutant Src(K295M) in B35 neuroblastoma cells. Phosphatidylinositol 3-kinase (PI3-kinase) and the small GTPase p21(rac) were identified as signaling intermediates to MAPK by phosphoinositide and Rac-GTP assays and expression of inhibitory mutants. Antibody-induced endocytosis of L1, visualized by immunofluorescence staining and confocal microscopy of B35 cells, was blocked by expression of kinase-inactive Src(K295M) and dominant-negative dynamin(K44A) but not by inhibitors of MEK or PI3-kinase. Dynamin(K44A) also inhibited L1 antibody-triggered MAPK phosphorylation. This study supports a model in which pp60(c-src) regulates dynamin-mediated endocytosis of L1 as an essential step in MAPK-dependent neurite outgrowth on an L1 substrate.

A MAP kinase-signaling pathway mediates neurite outgrowth on L1 and requires Src-dependent endocytosis

The neural cell adhesion molecule L1 mediates the axon outgrowth, adhesion, and fasciculation necessary for proper development of synaptic connections. Mutations of human L1 cause an X-linked mental retardation syndrome termed CRASH (corpus callosum hypoplasia, retardation, aphasia, spastic paraplegia, and hydrocephalus), and L1 knock-out mice display defects in neuronal process extension resembling the CRASH phenotype. Little is known about the biochemical or cellular mechanism by which L1 performs neuronal functions. Here it is demonstrated that clustering of L1 with antibodies or L1 protein in rodent B35 neuroblastoma and cerebellar neuron cultures induced the phosphorylation/activation of the mitogen-activated protein kinases (MAPKs) and extracellular signal-regulated kinases 1 and 2. MAPK activation was essential for L1-dependent neurite outgrowth, because chemical inhibitors [2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one and 1,4-diamino-2, 3-dicyano-1,4-bis(2-aminophenylthio)butadiene] of the MAPK kinase MEK strongly suppressed neurite outgrowth by cerebellar neurons on L1. The nonreceptor tyrosine kinase pp60(c-src) was required for L1-triggered MAPK phosphorylation, as shown in src-minus cerebellar neurons and by expression of the kinase-inactive mutant Src(K295M) in B35 neuroblastoma cells. Phosphatidylinositol 3-kinase (PI3-kinase) and the small GTPase p21(rac) were identified as signaling intermediates to MAPK by phosphoinositide and Rac-GTP assays and expression of inhibitory mutants. Antibody-induced endocytosis of L1, visualized by immunofluorescence staining and confocal microscopy of B35 cells, was blocked by expression of kinase-inactive Src(K295M) and dominant-negative dynamin(K44A) but not by inhibitors of MEK or PI3-kinase. Dynamin(K44A) also inhibited L1 antibody-triggered MAPK phosphorylation. This study supports a model in which pp60(c-src) regulates dynamin-mediated endocytosis of L1 as an essential step in MAPK-dependent neurite outgrowth on an L1 substrate.

Distinct signalling pathways mediate insulin and phorbol ester-stimulated eukaryotic initiation factor 4F assembly and protein synthesis in HEK 293 cells

Stimulation of serum-starved human embryonic kidney (HEK) 293 cells with either the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), or insulin resulted in increases in the phosphorylation of 4E-BP1 and p70 S6 kinase, eIF4F assembly, and protein synthesis. All these effects were blocked by rapamycin, a specific inhibitor of mTOR. Phosphatidylinositol 3-kinase and protein kinase B were activated by insulin but not by TPA. Therefore TPA can induce eIF4F assembly, protein synthesis, and the phosphorylation of p70 S6 kinase and 4E-BP1 independently of both phosphatidylinositol 3-kinase and protein kinase B. Using two structurally unrelated inhibitors of MEK (PD098059 and U0126), we provide evidence that Erk activation is important in TPA stimulation of eIF4F assembly and the phosphorylation of p70 S6 kinase and 4E-BP1 and that basal MEK activity is important for basal, insulin, and TPA-stimulated protein synthesis. Transient transfection of constitutively active mitogen-activated protein kinase interacting kinase 1 (the eIF4E kinase) indicated that inhibition of protein synthesis and eIF4F assembly by PD098059 is not through inhibition of eIF4E phosphorylation but of other signals emanating from MEK. This report also provides evidence that increased eIF4E phosphorylation alone does not affect the assembly of the eIF4F complex or general protein synthesis.

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.

Role of protein kinase C isoforms in phorbol ester-induced vascular endothelial growth factor expression in human glioblastoma cells

Aberrant expression of the potent angiogenic cytokine, vascular endothelial growth factor (VEGF), has been demonstrated to be associated with most human solid tumors. Both transcriptional and post-transcriptional mechanisms have been shown to modulate VEGF expression in a multitude of cell types. Here we report that when protein kinase C (PKC) pathways were activated in human glioblastoma U373 cells by phorbol 12-myristate 13-acetate (PMA), VEGF mRNA expression was up-regulated via a post-transcriptional mRNA stabilization mechanism. PMA treatment exhibited no increase in VEGF-specific transcriptional activation as determined by run-off transcription assays and VEGF promoter-luciferase reporter assays. However, PMA increased VEGF mRNA half-life from 0.8 to 3.6 h which was blocked by PKC inhibitors but not by protein kinase A or cyclic nucleotide-dependent protein kinase inhibitors. When U373 cells were transfected with antisense oligonucleotide sequences to the translation start sites of PKC-alpha, -beta, -gamma, -delta, -epsilon, or -zeta isoforms, both PKC-alpha and -zeta antisense oligonucleotides showed substantial inhibition of PMA-induced VEGF mRNA. In addition, overexpression of PKC-zeta resulted in a strong constitutive up-regulation of VEGF mRNA expression. This study demonstrates for the first time that specific PKC isoforms regulate VEGF mRNA expression through post-transcriptional mechanisms.

Proliferative defect and embryonic lethality in mice homozygous for a deletion in the p110alpha subunit of phosphoinositide 3-kinase

Phosphatidylinositol 3,4,5-trisphosphate is a phospholipid signaling molecule involved in many cellular functions including growth factor receptor signaling, cytoskeletal organization, chemotaxis, apoptosis, and protein trafficking. Phosphorylation at the 3 position of the inositol ring is catalyzed by many different 3-kinases (classified as types IA, IB, II, and III), but the physiological roles played by each of the different 3-kinase isozymes during embryonic development and in homeostasis in animals is incompletely understood. Mammalian type IA kinase isozymes are heterodimers that are active at 37 degrees C when the catalytic 110-kDa subunit interacts through an amino-terminal binding domain with a regulatory 85- or 55-kDa subunit. Using gene targeting in embryonic stem cells, we deleted this binding domain in the gene encoding the alpha isoform of the 110-kDa catalytic subunit (Pik3ca) of the alpha isozyme of the type IA kinases, leading to loss of expression of the p110 catalytic subunit. We show that Pik3cadel/del embryos are developmentally delayed at embryonic day (E) 9.5 and die between E9.5 and E10.5. E9. 5 Pik3cadel/del embryos have a profound proliferative defect but no increase in apoptosis. A proliferative defect is supported by the observation that fibroblasts from Pik3cadel/del embryos fail to replicate in Dulbecco's modified Eagle's medium and fetal calf serum, even with supplemental growth factors.

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.

Multiple signaling pathways regulate cell surface expression and activity of the excitatory amino acid carrier 1 subtype of Glu transporter in C6 glioma

Neuronal and glial sodium-dependent transporters are crucial for the control of extracellular glutamate levels in the CNS. The regulation of these transporters is relatively unexplored, but the activity of other transporters is regulated by protein kinase C (PKC)- and phosphatidylinositol 3-kinase (PI3K)-mediated trafficking to and from the cell surface. In the present study the C6 glioma cell line was used as a model system that endogenously expresses the excitatory amino acid carrier 1 (EAAC1) subtype of neuronal glutamate transporter. As previously observed, phorbol 12-myristate 13-acetate (PMA) caused an 80% increase in transporter activity within minutes that cannot be attributed to the synthesis of new transporters. This increase in activity correlated with an increase in cell surface expression of EAAC1 as measured by using a membrane-impermeant biotinylation reagent. Both effects of PMA were blocked by the PKC inhibitor bisindolylmaleimide II (Bis II). The putative PI3K inhibitor, wortmannin, decreased L-[3H]-glutamate uptake activity by >50% within minutes. Wortmannin decreased the Vmax of L-[3H]-glutamate and D-[3H]-aspartate transport, but it did not affect Na+-dependent [3H]-glycine transport. Wortmannin also decreased cell surface expression of EAAC1. Although wortmannin did not block the effects of PMA on activity, it prevented the PMA-induced increase in cell surface expression. This trafficking of EAAC1 also was examined with immunofluorescent confocal microscopy, which supported the biotinylation studies and also revealed a clustering of EAAC1 at cell surface after treatment with PMA. These studies suggest that the trafficking of the neuronal glutamate transporter EAAC1 is regulated by two independent signaling pathways and also may suggest a novel endogenous protective mechanism to limit glutamate-induced excitotoxicity.

Multiple signaling pathways regulate cell surface expression and activity of the excitatory amino acid carrier 1 subtype of Glu transporter in C6 glioma

Neuronal and glial sodium-dependent transporters are crucial for the control of extracellular glutamate levels in the CNS. The regulation of these transporters is relatively unexplored, but the activity of other transporters is regulated by protein kinase C (PKC)- and phosphatidylinositol 3-kinase (PI3K)-mediated trafficking to and from the cell surface. In the present study the C6 glioma cell line was used as a model system that endogenously expresses the excitatory amino acid carrier 1 (EAAC1) subtype of neuronal glutamate transporter. As previously observed, phorbol 12-myristate 13-acetate (PMA) caused an 80% increase in transporter activity within minutes that cannot be attributed to the synthesis of new transporters. This increase in activity correlated with an increase in cell surface expression of EAAC1 as measured by using a membrane-impermeant biotinylation reagent. Both effects of PMA were blocked by the PKC inhibitor bisindolylmaleimide II (Bis II). The putative PI3K inhibitor, wortmannin, decreased L-[3H]-glutamate uptake activity by >50% within minutes. Wortmannin decreased the Vmax of L-[3H]-glutamate and D-[3H]-aspartate transport, but it did not affect Na+-dependent [3H]-glycine transport. Wortmannin also decreased cell surface expression of EAAC1. Although wortmannin did not block the effects of PMA on activity, it prevented the PMA-induced increase in cell surface expression. This trafficking of EAAC1 also was examined with immunofluorescent confocal microscopy, which supported the biotinylation studies and also revealed a clustering of EAAC1 at cell surface after treatment with PMA. These studies suggest that the trafficking of the neuronal glutamate transporter EAAC1 is regulated by two independent signaling pathways and also may suggest a novel endogenous protective mechanism to limit glutamate-induced excitotoxicity.

Autophagic proteolysis: control and specificity

The rate of proteolysis is an important determinant of the intracellular protein content. Part of the degradation of intracellular proteins occurs in the lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very active and almost completely accounts for starvation-induced proteolysis. Factors inhibiting this process include amino acids, cell swelling and insulin. In the mechanisms controlling macroautophagy, protein phosphorylation plays an important role. Activation of a signal transduction pathway, ultimately leading to phosphorylation of ribosomal protein S6, accompanies inhibition of macroautophagy. Components of this pathway may include a heterotrimeric Gi3-protein, phosphatidylinositol 3-kinase and p70S6 kinase. Recent evidence indicates that lysosomal protein degradation can be selective and occurs via ubiquitin-dependent and -independent pathways.