Rab4 is phosphorylated by the insulin-activated extracellular-signal-regulated kinase ERK1

Amino acid starvation-induced LDLR trafficking accelerates lipoprotein endocytosis and LDL clearance

Mammalian cells utilize Akt‐dependent signaling to deploy intracellular Glut4 toward cell surface to facilitate glucose uptake. Low‐density lipoprotein receptor (LDLR) is the cargo receptor mediating endocytosis of apolipoprotein B‐containing lipoproteins. However, signaling‐controlled regulation of intracellular LDLR trafficking remains elusive. Here, we describe a unique amino acid stress response, which directs the deployment of intracellular LDLRs, causing enhanced LDL endocytosis, likely via Ca²⁺ and calcium/calmodulin‐dependent protein kinase II‐mediated signalings. This response is independent of induction of autophagy. Amino acid stress‐induced increase in LDL uptake in vitro is comparable to that by pravastatin. In vivo, acute AAS challenge for up to 72 h enhanced the rate of hepatic LDL uptake without changing the total expression level of LDLR. Reducing dietary amino acids by 50% for 2 to 4 weeks ameliorated high fat diet‐induced hypercholesterolemia in heterozygous LDLR‐deficient mice, with reductions in both LDL and VLDL fractions. We suggest that identification of signaling‐controlled regulation of intracellular LDLR trafficking has advanced our understanding of the LDLR biology, and may benefit future development of additional therapeutic strategies for treating hypercholesterolemia.

RAF-MEK-ERK pathway in cancer evolution and treatment

The RAF-MEK-ERK signaling cascade is a well-characterized MAPK pathway involved in cell proliferation and survival. The three-layered MAPK signaling cascade is initiated upon RTK and RAS activation. Three RAF isoforms ARAF, BRAF and CRAF, and their downstream MEK1/2 and ERK1/2 kinases constitute a coherently orchestrated signaling module that directs a range of physiological functions. Genetic alterations in this pathway are among the most prevalent in human cancers, which consist of numerous hot-spot mutations such as BRAF^V600E. Oncogenic mutations in this pathway often override otherwise tightly regulated checkpoints to open the door for uncontrolled cell growth and neoplasia. The crosstalk between the RAF-MEK-ERK axis and other signaling pathways further extends the proliferative potential of this pathway in human cancers. In this review, we summarize the molecular architecture and physiological functions of the RAF-MEK-ERK pathway with emphasis on its dysregulations in human cancers, as well as the efforts made to target the RAF-MEK-ERK module using small molecule inhibitors.

Tyrosine phosphorylation of Rab7 by Src kinase

The small molecular weight GTPase Rab7 is a key regulator for late endosomal/lysosomal membrane trafficking, it was known that Rab7 is phosphorylated, but the corresponding kinase and the functional regulation of Rab7 phosphorylation remain unclear. We provide evidence here that Rab7 is a substrate of Src kinase, and is tyrosine-phosphorylated by Src, withY183 residue of Rab7 being the optimal phosphorylation site for Src. Further investigations demonstrated that the tyrosine phosphorylation of Rab7 depends on the guanine nucleotide binding activity of Rab7 and the activity of Src kinase. The tyrosine phosphorylation of Rab7 is physiologically induced by EGF, and impairs the interaction of Rab7 with RILP, consequently inhibiting EGFR degradation and sustaining Akt signaling. These results suggest that the tyrosine phosphorylation of Rab7 may be involved in coordinating membrane trafficking and cell signaling.

NME genes in epithelial morphogenesis

The NME family of genes encodes highly conserved multifunctional proteins that have been shown to participate in nucleic acid metabolism, energy homeostasis, cell signaling, and cancer progression. Some family members, particularly isoforms 1 and 2, have attracted extensive interests because of their potential anti-metastasis activity. Unfortunately, there have been few consensus mechanistic explanations for this critical function because of the numerous molecular functions ascribed to these proteins, including nucleoside diphosphate kinase, protein kinase, nuclease, transcription factor, growth factor, among others. In addition, different studies showed contradictory prognostic correlations between NME expression levels and tumor progression in clinical samples. Thus, analyses using pliable in vivo systems have become critical for unraveling at least some aspects of the complex functions of this family of genes. Recent works using the Drosophila genetic system have suggested a role for NME in regulating epithelial cell motility and morphogenesis, which has also been demonstrated in mammalian epithelial cell culture. This function is mediated by promoting internalization of growth factor receptors in motile epithelial cells, and the adherens junction components such as E-cadherin and β-catenin in epithelia that form the tissue linings. Interestingly, NME genes in epithelial cells appear to function in a defined range of expression levels. Either down-regulation or over-expression can perturb epithelial integrity, resulting in different aspects of epithelial abnormality. Such biphasic functions provide a plausible explanation for the documented anti-metastatic activity and the suspected oncogenic function. This review summarizes these recent findings and discusses their implications.

The extracellular signal-regulated kinase: multiple substrates regulate diverse cellular functions

The extracellular signal-regulated kinase (ERK) cascade is a central pathway that transmits signals from many extracellular agents to regulate cellular processes such as proliferation, differentiation and cell cycle progression. The signaling via the ERK cascade is mediated by sequential phosphorylation and activation of protein kinases in the different tiers of the cascade. Although the main core phosphorylation chain of the cascade includes Raf kinases, MEK1/2, ERK1/2 (ERKs) and RSKs, other alternatively spliced forms and distinct components exist in the different tiers, and participate in ERK signaling under specific conditions. These components enhance the complexity of the ERK cascade and thereby, enable the wide variety of functions that are regulated by it. Another factor that is important for the dissemination of ERKs' signals is the multiplicity of the cascade's substrates, which include transcription factors, protein kinases and phosphatases, cytoskeletal elements, regulators of apoptosis, and a variety of other signaling-related molecules. About 160 substrates have already been discovered for ERKs, and the list of these substrates, as well as the function and mechanism of activation of representative substrates, are described in the current review. Many of these substrates are localized in the nucleus, and seem to participate in the regulation of transcription upon stimulation. However, other substrates are found in the cytosol as well as in other cellular organelles, and those are responsible for processes such as translation, mitosis and apoptosis. Understanding of these processes may provide a full picture of the distinct, and even opposing cellular processes that are regulated by the ERK cascade.

Signal transduction gRABs attention

Rab proteins are small GTPases involved in the regulation of vesicular membrane traffic. Research done in the past years has demonstrated that some of these proteins are under the control of signal transduction pathways. Still, several recent papers point out to a new unexpected role for this family of Ras-related proteins, as potential regulators of intracellular signaling pathways. In particular, several evidence indicate that members of the Rab family of small GTPases, through their effectors, are key molecules participating to the regulation of numerous signal transduction pathways profoundly influencing cell proliferation, cell nutrition, innate immune response, fragmentation of compartments during mitosis and apoptosis. Even more surprisingly, direct involvement of Rab proteins in signaling to the nucleus has been demonstrated. This review will focus on aspects of Rab proteins function connected to signal transduction and, in particular, connections between membrane traffic and other cell pathways will be examined.

Endocytosis and signaling cascades: a close encounter

Internalization of receptors and other cell surface components is well known to occur via clathrin-mediated endocytosis, although other less well characterized pathways are also involved. Internalized receptors are then delivered to early endosomes, where they are sorted to be recycled back to the plasma membrane for reutilization or transported to late endosomes/lysosomes for degradation. Endocytosis has long been considered as a constitutive, housekeeping function of animal cells that occurs independently of the cellular environment in contrast to regulated secretion. Here, we will discuss recent studies that are uncovering the existence of cross-talk between signaling molecules and components of the transport machinery, indicating that endocytosis can be modulated by signaling pathways.

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

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

Rab2 requires PKC iota/lambda to recruit beta-COP for vesicle formation

The small GTPase Rab2 initiates the recruitment of soluble components necessary for protein sorting and recycling from pre-Golgi intermediates. Our previous studies showed that Rab2 required protein kinase C (PKC) or a PKC-like protein to recruit beta-COP to membrane (Tisdale EJ, Jackson M. Rab2 protein enhances coatomer recruitment to pre-Golgi intermediates. J Biol Chem 1998;273: 17269-17277). We investigated the role of PKC in Rab2 function by first determining the active isoform that associates with membranes used in our assay. Western blot analysis detected three isoforms: PKC alpha, gamma and iota/lambda. A quantitative binding assay was used to measure recruitment of these kinases when incubated with Rab2. Only PKC iota/lambda translocated to membrane in a dose-dependent manner. Microsomes treated with anti-PKC iota/lambda lost the ability to bind beta-COP, suggesting that Rab2 requires PKC iota/lambda for beta-COP recruitment. The recruitment of beta-COP to membranes is not regulated by PKC iota/lambda kinase activity. However, PKC iota/lambda activity was necessary for Rab2-mediated vesicle budding. We found that the addition of either a kinase-deficient PKC iota/lambda mutant or atypical PKC pseudosubstrate peptide to the binding assay drastically reduced vesicle formation. These data suggest that Rab2 causes translocation of PKC iota/lambda to vesicular tubular clusters (VTCs), which promotes the recruitment of COPI to generate retrograde-transport vesicles.

Enhanced prenyltransferase activity and Rab content in rat liver regeneration

Rabs are small GTP-binding proteins with a regulatory role in intracellular vesicular traffic. The modulation of their levels and activity in different physiological situations is poorly understood. During the first cell cycle of rat liver regeneration we observed a differential regulation of some Rabs, with a progressive increase of those involved in exocytosis and a progressive decrease of one involved in endocytosis. This could be related with the need of exposing growth factor receptors and prolonging the transduction of their signal in preparation for mitosis. Moreover, we observed an increased activity of protein prenyltransferases, the enzymes responsible for the prenylation of several proteins involved in crucial processes of proliferation, without a corresponding increase in the amount of prenyltransferase protein.

Expression of a prenylation-deficient Rab4 interferes with propagation of insulin signaling through insulin receptor substrate-1

Rab proteins are small GTP-binding proteins of the Ras superfamily that function in the regulation of vesicle transport processes. The Rab4 isoform has been implicated in insulin action. For instance, overexpression of a prenylation-deficient form of Rab4 has been shown to inhibit insulin-dependent GLUT4 translocation. Other steps affected by Rab4 in the cascade of events resulting from insulin receptor activation have not been elucidated. In the present studies, we measured effects on insulin-signaling proteins in 3T3-L1 adipocytes transiently expressing cytoplasmic forms of Rab4 and Rab5. Expression of a mutant Rab4 lacking a prenylation site resulted in reduced insulin-dependent phosphorylation ofcytoplasmic and internal membrane-associated insulin receptor substrate-1, leading to decreased insulin receptor substrate-1-associated phosphatidylinositol 3'-OH kinase activation and decreased Akt activation. These effects were not observed upon introduction of a similar mutant form of Rab5. These data indicate that Rab4 or a Rab4-associated protein is involved at one or more steps in propagating the insulin signal, in addition to any role it may play in the regulation of GLUT4 vesicle translocation. Our results support models of insulin signaling in which regulation of internal membrane trafficking plays a role in transduction of the insulin signal.

Expression of rab11a N124I in gastric parietal cells inhibits stimulatory recruitment of the H+-K+-ATPase

Stimulation of the gastric parietal cell results in a massive redistribution of H+-K+-ATPase from cytoplasmic tubulovesicles to the apical plasma membrane. Previous studies have implicated the small GTPase rab11 in this process. Using matrix-assisted laser desorption mass spectrometry, we confirmed that rab11 is associated with H+-K+-ATPase-enriched gastric microsomes. A stoichiometry of one rab11 per six copies of H+-K+-ATPase was estimated. Furthermore, rab11 exists in at least three forms on rabbit gastric microsomes: the two most prominent resemble rab11a, whereas the third resembles rab11b. Using an adenoviral expression system, we expressed the dominant negative mutant rab11a N124I in primary cultures of rabbit parietal cells under the control of the tetracycline transactivator protein (tTA). The mutant was well expressed with a distribution similar to that of the H+-K+-ATPase. Stimulation of these cultures with histamine and IBMX was assessed by measuring the aminopyrine (AP) uptake relative to resting cells (AP index). In experiments on six culture preparations, stimulated uninfected cells gave an AP index of 10.0 +/- 2.9, whereas parallel cultures expressing rab11a N124I were poorly responsive to stimulation, with a mean AP index of 3.2 +/- 0. 9. Control cultures expressing tTA alone or tTA plus actin responded equally well to stimulation, giving AP index values of 9.0 +/- 3.1 and 9.6 +/- 0.9, respectively. Thus inhibition by rab11a N124I is not simply due to adenoviral infection. The AP uptake data were confirmed by immunocytochemistry. In uninfected cells, H+-K+-ATPase demonstrated a broad cytoplasmic distribution, but it was cleared from the cytoplasm and associated with apically derived membranes on stimulation. In cells expressing rab11a N124I, H+-K+-ATPase maintained its resting localization on stimulation. Furthermore, this effect could be alleviated by culturing infected cells in the presence of tetracycline, which prevents expression of the mutant rab11. We therefore conclude that rab11a is the prominent GTPase associated with gastric microsomes and that it plays a role in parietal cell activation.

The small GTPases Rab5a, Rab5b and Rab5c are differentially phosphorylated in vitro

Rab GTPases play a fundamental role in the regulation of membrane traffic. Three different Rab5 isoforms have been reported but no differences in their function in endocytosis have been discovered. As the Rab5 isoforms show a conserved consensus site for Ser/Thr phosphorylation, we investigated whether this site was phosphorylated. Here, we report that the three Rab5 proteins are differentially recognized by different kinases. Rab5a is efficiently phosphorylated by extracellular-regulated kinase 1 but not by extracellular-regulated kinase 2, while cdc2 kinase preferentially phosphorylates Ser-123 of Rab5b. These findings strongly suggest that phosphorylation could be important to differentially regulate the function of the Rab5 isoforms.

The role of G proteins in insulin signalling

Insulin modulates many intracellular processes including cellular metabolism, cell proliferation and cell differentiation. Some of these processes involve significant changes in the traffic of intracellular vesicles or in the structural organization of the cell. These phenomena have been linked to the activity of regulatory GTP-binding proteins. Most, if not all functions, of the insulin receptor are associated with its tyrosine kinase activity. Thus, over the past few years, a significant effort has been dedicated to elucidate the cross-talk between the tyrosine kinase activity of the receptor and the regulation of G protein-mediated pathways. Recent progress indicates that G proteins may mediate the control of several of insulin's intracellular functions. These include the regulation of the MAP kinase pathway, the activation of phospholipase D and the regulation of glucose uptake. This article discusses some recent advances in this area.

Insulin promotes phosphorylation and activation of geranylgeranyltransferase II. Studies with geranylgeranylation of rab-3 and rab-4

Rab proteins play a crucial role in the trafficking of intracellular vesicles. Rab proteins are GTPases that cycle between an inactive GDP-bound form and an active GTP-bound conformation. A prerequisite to Rab activation by GTP loading is its post-translational modification by the addition of geranylgeranyl moieties to highly conserved C-terminal cysteine residues. We examined the effect of insulin on the activity of geranylgeranyltransferase II (GGTase II) in 3T3-L1 fibroblasts and adipocytes. In fibroblasts, insulin increased the enzymatic activity of GGTase II 2.5-fold after 1 h of incubation, an effect that is blocked by perillyl alcohol, an inhibitor of prenyltransferases, but not by the geranylgeranyltransferase I inhibitor, GGTI-298, or the farnesyltransferase inhibitor, alpha-hydroxyfarnesylphosphonic acid. Concomitantly, insulin stimulated the phosphorylation of the GGTase II alpha-subunit without any effect on the GGTase II beta-subunit. At the same time, insulin also increased the amounts of geranylgeranylated Rab-3 in 3T3-L1 fibroblasts from 44 +/- 1.2% in control cells to 63 +/- 3.8 and 64 +/- 6.1% after 1 and 24 h of incubation, respectively. In adipocytes, insulin increased the amounts of geranylgeranylated Rab-4 from 38 +/- 0.6% in control cells to 56 +/- 1.7 and 60 +/- 2.6% after 1 and 24 h of incubation, respectively. In both fibroblasts and adipocytes, the presence of perillyl alcohol blocked the ability of insulin to increase geranylgeranylation of Rab-4, whereas GGTI-298 and alpha-hydroxyfarnesylphosphonic acid were without effect, indicating that insulin activates GGTase II. In summary, insulin promotes phosphorylation and activation of GGTase II in both 3T3 L1 fibroblasts and adipocytes and increases the amounts of geranylgeranylated Rab-3 and Rab-4 proteins.

Akt-2 binds to Glut4-containing vesicles and phosphorylates their component proteins in response to insulin

Glut4-containing vesicles immunoadsorbed from primary rat adipocytes possess endogenous protein kinase activity and phosphorylation substrates. Phosphorylation of several vesicle proteins including Glut4 itself is rapidly activated by insulin. Wortmannin blocks the effect of insulin when added to cells in vivo prior to insulin administration. By means of MonoQ chromatography and Western blot analysis, vesicle-associated protein kinase is identified as Akt-2, a lipid-binding protein kinase involved in insulin signaling. Akt-2 is found to be recruited to Glut4-containing vesicles in response to insulin.

A cell cycle regulated MAP kinase with a possible role in cytokinesis in tobacco cells

Mitogen-activated protein (MAP) kinases have been demonstrated to have a role in meiosis but their involvement in mitotic events is less clear. Using a peptide antibody raised against the tobacco MAP kinase p43(Ntf6) and extracts from synchronized tobacco cell suspension cultures, we show that this kinase is activated specifically during mitosis. Entry into mitosis appears to be necessary for the activation of the kinase, which occurs as a post-translational event. The activation of the kinase occurs in late anaphase/early telophase. The p43(Ntf6) protein shows a transient localization to the cell plate in anaphase cells, in the middle of the two microtubule arrays characteristic of the phragmoplast, a plant-specific structure involved in laying down the new cell wall. The combined data support a role for the MAP kinase p43(Ntf6) in cytokinesis in tobacco cells.

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

To delineate the signaling pathway leading to glucose transport protein (GLUT4) translocation, we examined the effect of microinjection of the nonhydrolyzable GTP analog, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), into 3T3-L1 adipocytes. Thirty minutes after the injection of 5 mM GTPgammaS, 40% of injected cells displayed surface GLUT4 staining indicative of GLUT4 translocation compared with 55% for insulin-treated cells and 10% in control IgG-injected cells. Treatment of the cells with the phosphatidylinositol 3-kinase inhibitor wortmannin or coinjection of GST-p85 SH2 fusion protein had no effect on GTPgammaS-mediated GLUT4 translocation. On the other hand, coinjection of antiphosphotyrosine antibodies (PY20) blocked GTPgammaS-induced GLUT4 translocation by 65%. Furthermore, microinjection of GTPgammaS led to the appearance of tyrosine-phosphorylated proteins around the periphery of the plasma membrane, as observed by immunostaining with PY20. Treatment of the cells with insulin caused a similar phosphotyrosine-staining pattern. Electroporation of GTPgammaS stimulated 2-deoxy-D-glucose transport to 70% of the extent of insulin stimulation. In addition, immunoblotting with phosphotyrosine antibodies after electroporation of GTPgammaS revealed increased tyrosine phosphorylation of several proteins, including 70- to 80-kDa and 120- to 130-kDa species. These results suggest that GTPgammaS acts upon a signaling pathway either downstream of or parallel to activation of phosphatidylinositol 3-kinase and that this pathway involves tyrosine-phosphorylated protein(s).

Role of Rab GTPases in membrane traffic

Small GTPases of the Rab subfamily have been known to be key regulators of intracellular membrane traffic since the late 1980s. Today this protein group amounts to more than 40 members in mammalian cells which localize to distinct membrane compartments and exert functions in different trafficking steps on the biosynthetic and endocytic pathways. Recent studies indicate that cycles of GTP binding and hydrolysis by the Rab proteins are linked to the recruitment of specific effector molecules on cellular membranes, which in turn impact on membrane docking/fusion processes. Different Rabs may, nevertheless, have slightly different principles of action. Studies performed in yeast suggest that connections between the Rabs and the SNARE machinery play a central role in membrane docking/fusion. Further elucidation of this linkage is required in order to fully understand the functional mechanisms of Rab GTPases in membrane traffic.

Association of cytosolic Rab4 with GDI isoforms in insulin-sensitive 3T3-L1 adipocytes

Translocation of an intracellular pool of GLUT4 glucose transporters to the fat and muscle cell surface is thought to involve small GTP-binding proteins such as the Rab4 protein. The cycling of Rab proteins between cytosol and intracellular membranes necessary for their function appears to be regulated by GDP-dissociation inhibitors (GDI), three of which have been cloned thus far. Previous data suggest that Rab4 binds two of these isoforms of GDI (1 and 2) similarly when purified proteins are employed [Shisheva, A., et al. (1994) Mol. Cell. Biol. 14, 3459-3468]. In the present study, we have analyzed the cytosolic Rab4 in complexes with GDI-1 or GDI-2 in intact cells using a coprecipitation technique. We show here that in insulin-sensitive 3T3-L1 adipocytes and other cultured cells, Rab4 simultaneously forms stable cytosolic complexes with both GDI-1 and GDI-2. Acute insulin treatment of the cultured adipocytes significantly increases cytosolic levels of Rab4 which can be quantitatively immunoprecipitated with anti-Rab4 antibodies. Surprisingly, the increased cytosolic Rab4 due to insulin action is predominantly associated with cytosolic GDI-1. The levels of cytosolic Rab4-GDI-2 complexes were virtually unaltered by insulin. Insulin-dependent alterations of Rab4 and GDI-1 phosphorylation were not detected in 32P-labeled 3T3-L1 adipocytes, suggesting another mechanism accounts for the specificity of Rab4 binding to GDI-1. Taken together, these data suggest there is selective formation of Rab4-GDI-1 complexes in response to insulin which plays a role in the action of insulin on membrane trafficking.

Rab 3D in rat adipose cells and its overexpression in genetic obesity (Zucker fatty rat)

Members of the Rab 3 subfamily of low-molecular-mass GTP-binding proteins have been functionally implicated in regulated exocytosis. The aim of the present study was to examine the subcellular distribution of a member of this family, Rab 3D, in rat adipose cells, given the hypothesis that this protein might be involved in insulin-stimulated GLUT4 exocytosis. We show that Rab 3D immunoreactivity is associated predominantly with the high-density microsomal fraction, where the signal intensity is 3- and 7-fold greater than that in plasma membranes and low-density microsomes respectively. Rab 3D does not co-localize with GLUT4 on immuno-isolated intracellular vesicles and, unlike GLUT4, it is not redistributed in response to insulin. Thus, if Rab 3D plays a role in GLUT4 trafficking, it relies on mechanisms independent of relocation. We observed that Rab 3D is overexpressed in adipose cells of obese (fa/fa) Zucker rats, in a tissue- and isoform-specific manner. The pathophysiological significance of this defect remains elusive. This could form the molecular basis for altered adipose secretory function in obesity.

Potential role of Rab4 in the regulation of subcellular localization of Glut4 in adipocytes

A role for Rab4 in the translocation of the glucose transporter Glut4 induced by insulin has been recently proposed. To study more directly the role of this small GTPase, freshly isolated adipocytes were transiently transfected with the cDNAs of both an epitope-tagged Glut4-myc and Rab4, a system which allows direct measurement of the concentration of Glut4 molecules at the cell surface. When cells were cotransfected with Glut4-myc and Rab4, the concentration of Glut4-myc at the cell surface decreased in parallel with the increased expression of Rab4, suggesting that Rab4 participates in the intracellular retention of Glut4. In parallel, the amount of Rab4 associated with the Glut4-containing vesicles increased. When Rab4 was moderately overexpressed, the number of Glut4-myc molecules recruited to the cell surface in response to insulin was similar to that observed in mock-transfected cells, and thus the insulin efficiency was increased. When Rab4 was expressed at a higher level, the amount of Glut4-myc present at the cell surface in response to insulin decreased. Since the overexpressed protein was predominantly cytosolic, this suggests that the cytosolic Rab4 might complex some factor(s) necessary for insulin action. This hypothesis was strengthened by the fact that Rab4 deltaCT, a Rab4 mutant lacking the geranylgeranylation sites, inhibited insulin-induced recruitement of Glut4-myc to the cell surface, even when moderately overexpressed. Rab3D was without effect on Glut4-myc subcellular distribution in basal or insulin-stimulated conditions. While two mutated proteins unable to bind GTP did not decrease the number of Glut4-myc molecules in basal or insulin-stimulated conditions at the plasma membrane, the behavior of a mutated Rab4 protein without GTPase activity was similar to that of the wild-type Rab4 protein, indicating that GTP binding but not its hydrolysis was required for the observed effects. Altogether, our results suggest that Rab4, but not Rab3D, participates in the molecular mechanism involved in the subcellular distribution of the Glut4 molecules both in basal and in insulin-stimulated conditions in adipocytes.

A synthetic peptide corresponding to the Rab4 hypervariable carboxyl-terminal domain inhibits insulin action on glucose transport in rat adipocytes

The present study was conducted to examine the involvement of Rab4, a low molecular weight GTP-binding protein, in the action of insulin on glucose transport. A synthetic peptide corresponding to the Rab4 hypervariable carboxyl-terminal domain, Rab4-(191-210), was successfully transferred into rat adipocytes by electroporation and inhibited insulin-stimulated glucose transport by about 50% without affecting the basal transport activity. In contrast, synthetic peptides corresponding to the Rab3C and Rab3D carboxyl-terminal hypervariable domain had little effect on insulin action on glucose transport. The Rab4-(191-210) peptide also reduced insulin-induced GLUT4 translocation from the intracellular pool to the plasma membrane. Furthermore, the Rab4-(191-210) peptide reduced both insulin-induced glucose transport and GLUT4 translocation in the presence of a major histocompatibility complex class I antigen-derived peptide, D(k)-(62-85), which is a potent inhibitor of GLUT4 internalization, suggesting that the peptide inhibited exocytotic recruitment of GLUT4-containing vesicles. The Rab4-(191-210) peptide also inhibited GTP gamma S-stimulated glucose transport. In addition, insulin-stimulated glucose transport was inhibited by the addition of anti-Rab4 antibody. These results suggest that Rab4 protein plays a crucial role in insulin action on GLUT4 translocation, especially in exocytotic recruitment by the hormone of the glucose transporter to the plasma membrane from the intracellular retention pool.

Failure of insulin-regulated recruitment of the glucose transporter GLUT4 in cardiac muscle of obese Zucker rats is associated with alterations of small-molecular-mass GTP-binding proteins

Cardiac ventricular tissue of lean and genetically obese (fa/fa) Zucker rats was used to study the expression, subcellular distribution and insulin-induced recruitment of the glucose transporter GLUT4 and to elucidate possible molecular alterations of the translocation process. Hearts were removed from basal and insulin-treated (20 min) lean and obese Zucker rats, and processed for subcellular fractionation and Western blotting of proteins. In obese rats, the total GLUT4 content in a crude membrane fraction was reduced to 75 +/- 8% (P = 0.019) of lean controls. In contrast, GLUT4 abundance in plasma membranes was not significantly different between lean and obese rats with a concomitant decrease (47 +/- 3%) in the microsomal fraction of obese animals. In plasma membranes of lean animals insulin was found to increase the GLUT4 abundance to 294 +/- 43% of control with a significantly (P = 0.009) reduced effect in the obese group (139 +/- 10% of control). In these animals insulin failed to recruit GLUT4 from the microsomal fraction, whereas the hormone induced a significant decrease (41 +/- 4%) of microsomal GLUT4 in lean controls. In GLUT4-enriched membrane vesicles, obtained from cardiac microsomes of lean rats, a 24 kDa GTP-binding protein could be detected, whereas no significant labelling of this species was observed in GLUT4 vesicles prepared from obese animals. In addition to the translocation of GLUT4, insulin was found to promote the movement of the small GTP-binding protein rab4A from the cytosol (decrease to 61 +/- 13% of control) to the plasma membrane (increase to 177 +/- 19% of control) in lean rats with no effect of the hormone on rab4A redistribution in the obese group. In conclusion, cardiac glucose uptake of insulin-resistant obese Zucker rats is subject to multiple cellular abnormalities involving a reduced expression, altered redistribution and defective recruitment of GLUT4. We show here an association of the latter defect with alterations at the level of small GTP-binding proteins possibly leading to an impaired trafficking of GLUT4 in the insulin-resistant state.

Does mitogen-activated-protein kinase have a role in insulin action? The cases for and against

The discovery of the mitogen-activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk-1 and Erk-2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth-promoting effects of insulin and the regulation of Glut-1 and c-fos expression, and AP-1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut-4 translocation), glycogen synthesis, acetyl-CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

Signalling through the insulin receptor and the insulin-like growth factor-I receptor

The insulin receptor and the insulin-like growth factor I receptor belong to the family of tyrosine kinase receptors. Both receptors appear as a disulphide-linked dimer; each half of the dimer consisting of a 130 k M(r) alpha-subunit linked to a 90 k M(r) beta-subunit. Both halves of the dimer are linked together by disulphide bonds to form an alpha 2 beta 2 structure. The insulin receptor functions as an allosteric enzyme in which the binding of the hormone to the alpha-subunit leads to a series of conformational changes resulting in activation of the beta-subunit tyrosine kinase. Upon multisite autophosphorylation the latter becomes competent to phosphorylate cellular substrates resulting in the biological responses of insulin. Recent findings have recognized the mitogen activated protein kinase cascade as a central signalling circuitry linking cell surface receptors, such as the insulin receptor, to the nucleus, and playing a role in regulation of metabolism, growth and differentiation.

Parallel changes in Glut 4 and Rab4 movements in two insulin-resistant states

Insulin-induced Glut 4 and Rab4 movements were studied in two insulin-resistant states. In adipocytes from streptozotocin diabetic rats, the amount of Glut 4 was decreased by 60%. The remaining Glut 4 molecules were translocated in response to insulin, and in parallel, Rab4 left the intracellular compartment. In contrast, in 3T3-L1 adipocytes rendered insulin-resistant by a prolonged insulin treatment, both Rab4 and Glut 4 remained in the intracellular compartment following an acute insulin stimulation. Those results illustrate a similar behavior of Glut 4 and Rab4 in two situations where insulin resistance results from different mechanisms, and add further support for a role of Rab4 in Glut 4 translocation.

Protein-tyrosine-phosphatase 2C is phosphorylated and inhibited by 44-kDa mitogen-activated protein kinase

Protein-tyrosine-phosphatase 2C (PTP2C, also named SHPTP2, SHPTP3, or PTP1D) is a cytosolic enzyme with two Src homology 2 domains. We have investigated its regulation by phosphorylation in PC12 rat pheochromocytoma cells. In untreated cells, PTP2C was phosphorylated predominantly on serine residues. A 5-min treatment with epidermal growth factor (EGF) induced an increase in phosphorylation on threonine and, to a lesser degree, on serine. After 45 min of exposure to EGF, PTP2C phosphorylation returned to basal levels. Using an in vitro kinase assay, we found that the 44-kDa mitogen-activated protein kinase, p44mapk, phosphorylated PTP2C on serine and threonine residues. This phosphorylation resulted in a pronounced inhibition of PTP2C enzyme activity measured with phosphorylated EGF receptors as substrate. Moreover, in intact PC12 cells, PTP2C was also inhibited following a short EGF treatment, but its activity returned to normal when the exposure to EGF was maintained for 45 min. The profile of this response to EGF can be inversely correlated to that of the stimulatory action of EGF on p44mapk. These data suggest that the EGF-induced regulation of PTP2C activity is mediated by p44mapk. These findings provide evidence for an additional role of the mitogen-activated protein kinase cascade--namely, the regulation of a PTP.