Regulation of EGFR endocytic trafficking by rab proteins.

The Epidermal Growth Factor Receptor (EGFR) is a member of the receptor tyrosine kinase family and has important roles in development and cancer. Through ligand stimulation, the EGFR initiates a number of biochemical pathways that integrate to form specific physiological responses. In addition to these signaling pathways, the ligand stimulation also causes the EGFR to internalize and be transported through the endocytic pathway. The endocytic pathway regulates the rate of EGFR degradation and recycling, as well as the signaling mediated by the EGFR. In this review, the role of rabs, a family of small molecular weight guanine nucleotide binding proteins, is examined in how they regulate endocytic trafficking.

Expression of constitutively active Rab5 uncouples maturation of the Salmonella-containing vacuole from intracellular replication.

The enteric bacterial pathogen Salmonella typhimurium enters and proliferates within both phagocytic and non-phagocytic host cells. Upon entry, the bacteria reside in membrane-bound vacuoles (SCVs) that mature with time, as evidenced by the sequential loss of early endosomal markers, followed by the selective recruitment of a number of lysosomal membrane glycoproteins (LAMPs). This remodelling process renders the SCVs non-fusogenic with lysosomes and is also thought to create a vacuolar environment permissive for replication. We demonstrate that disruption of the endocytic pathway by the expression of a constitutively active form of the small GTPase rab5 (rab5Q79L) significantly altered the biogenesis of the SCVs without inhibiting bacterial replication in HeLa cells. Expression of rab5Q79L caused the retention of early endosomal markers on SCVs and early acquisition of LAMP2, and led to an increase in the kinetics of intracellular replication. We also demonstrate that a significant fraction of LAMP2 in SCVs is derived from the cell surface via endocytosis rather than via the biosynthetic route. Further, in fibroblasts lacking a functional AP3 adaptor complex, in which all newly synthesized LAMP is delivered to the cell surface, recruitment of LAMP to the SCVs remained unaffected. These findings raise the possibility that all the SCV-associated LAMP could be derived by endocytosis from the cell surface.

Receptor and membrane recycling can occur with unaltered efficiency despite dramatic Rab5(q79l)-induced changes in endosome geometry.

Current models for sorting in the endosomal compartment suggest that endosomal geometry plays a significant role as membrane-bound proteins accumulate in tubular regions for recycling, and lumenal markers accumulate in large vacuolar portions for delivery to lysosomes. Rab5, a small molecular weight GTPase, functions in the formation and maintenance of the early/sorting endosome. Overexpression of the constitutively active form, Rab5(Q79L), leads to enhanced endosome fusion resulting in the enlargement of early endosomes. Using an adenoviral expression system to regulate the time and level of Rab5(Q79L) overexpression in HeLa cells, we find that although endosomes are dramatically enlarged, the rates of transferrin receptor-mediated endocytosis and recycling are unaffected. Moreover, despite the enlarged endosome phenotype, neither the rate of internalization of a fluid phase marker nor the rate of recycling of a bulk lipid marker were affected. These results suggest that GTP hydrolysis by Rab5 is rate-limiting for endosome fusion but not for endocytic trafficking and that early endosome geometry may be a less critical determinant of sorting efficiencies than previously thought.

Munc18c regulates insulin-stimulated glut4 translocation to the transverse tubules in skeletal muscle.

To examine the intracellular trafficking and translocation of GLUT4 in skeletal muscle, we have generated transgenic mouse lines that specifically express a GLUT4-EGFP (enhanced green fluorescent protein) fusion protein under the control of the human skeletal muscle actin promoter. These transgenic mice displayed EGFP fluorescence restricted to skeletal muscle and increased glucose tolerance characteristic of enhanced insulin sensitivity. The GLUT4-EGFP protein localized to the same intracellular compartment as the endogenous GLUT4 protein and underwent insulin- and exercise-stimulated translocation to both the sarcolemma and transverse-tubule membranes. Consistent with previous studies in adipocytes, overexpression of the syntaxin 4-binding Munc18c isoform, but not the related Munc18b isoform, in vivo specifically inhibited insulin-stimulated GLUT4-EGFP translocation. Surprisingly, however, Munc18c inhibited GLUT4 translocation to the transverse-tubule membrane without affecting translocation to the sarcolemma membrane. The ability of Munc18c to block GLUT4-EGFP translocation to the transverse-tubule membrane but not the sarcolemma membrane was consistent with substantially reduced levels of syntaxin 4 in the transverse-tubule membrane. Together, these data demonstrate that Munc18c specifically functions in the compartmentalized translocation of GLUT4 to the transverse-tubules in skeletal muscle. In addition, these results underscore the utility of this transgenic model to directly visualize GLUT4 translocation in skeletal muscle.

Regulation of signal transduction by endocytosis.

Endocytosis of ligand-activated receptors has generally been considered a mechanism to attenuate signaling. There is now a growing body of evidence suggesting that this process is much more sophisticated and that endocytic membrane trafficking regulates both the intensity of signaling and the co-localization of activated receptors with downstream signaling molecules.

Synip: a novel insulin-regulated syntaxin 4-binding protein mediating GLUT4 translocation in adipocytes.

Insulin-stimulated glucose transport and GLUT4 translocation require regulated interactions between the v-SNARE, VAMP2, and the t-SNARE, syntaxin 4. We have isolated a novel syntaxin 4-binding protein, Synip, which specifically interacts with syntaxin 4. Insulin induces a dissociation of the Synip:syntaxin 4 complex due to an apparent decrease in the binding affinity of Synip for syntaxin 4. In contrast, the carboxyterminal domain of Synip does not dissociate from syntaxin 4 in response to insulin stimulation but inhibits glucose transport and GLUT4 translocation. These data implicate Synip as an insulin-regulated syntaxin 4-binding protein directly involved in the control of glucose transport and GLUT4 vesicle translocation.

Regulation of insulin-stimulated GLUT4 translocation by Munc18c in 3T3L1 adipocytes.

Insulin stimulates glucose transporter (GLUT) 4 vesicle translocation from intracellular storage sites to the plasma membrane in 3T3L1 adipocytes through a VAMP2- and syntaxin 4-dependent mechanism. We have observed that Munc18c, a mammalian homolog of the yeast syntaxin-binding protein n-Sec1p, competed for the binding of VAMP2 to syntaxin 4. Consistent with an inhibitory function for Munc18c, expression of Munc18c, but not the related Munc18b isoform, prevented the insulin stimulation of GLUT4 and IRAP/vp165 translocation to the plasma membrane without any significant effect on GLUT1 trafficking. As expected, overexpressed Munc18c was found to co-immunoprecipitate with syntaxin 4 in the basal state. However, these complexes were found to dissociate upon insulin stimulation. Furthermore, endogenous Munc18c was predominantly localized to the plasma membrane and its distribution was not altered by insulin stimulation. Although expression of enhanced green fluorescent protein-Munc18c primarily resulted in a dispersed cytosolic distribution, co-expression with syntaxin 4 resulted in increased localization to the plasma membrane. Together, these data suggest that Munc18c inhibits the docking/fusion of GLUT4-containing vesicles by blocking the binding of VAMP2 to syntaxin 4. Insulin relieves this inhibition by inducing the dissociation of Munc18c from syntaxin 4 and by sequestering Munc18c to an alternative plasma membrane binding site.

Expression of a dominant interfering dynamin mutant in 3T3L1 adipocytes inhibits GLUT4 endocytosis without affecting insulin signaling.

To examine the role of clathrin-coated vesicle endocytosis in insulin receptor signaling and GLUT4 trafficking, we used recombinant adenovirus to express a dominant interfering mutant of dynamin (K44A/dynamin) in 3T3L1 adipocytes. Functional expression of K44A/dynamin, as measured by inhibition of transferrin receptor internalization, did not affect insulin-stimulated insulin receptor autophosphorylation, Shc tyrosine phosphorylation, or mitogen-activated protein kinase activation. Although the tyrosine phosphorylation of insulin receptor substrate-1 was slightly reduced, correlating with a 25% decrease in insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity, insulin-stimulated Akt kinase activation was unaffected. In contrast, expression of K44A/dynamin resulted in the cell-surface accumulation of GLUT4 under basal conditions and an inhibition of GLUT4 endocytosis without affecting insulin-stimulated GLUT4 exocytosis. These data demonstrate that disruption of clathrin-mediated endocytosis does not significantly perturb insulin receptor signal transduction pathways. Furthermore, K44A/dynamin expression causes an accumulation of GLUT4 at the cell surface, suggesting that GLUT4 vesicles exist in at least two distinct intracellular compartments, one that undergoes continuous recycling and a second that is responsive to insulin.

Inhibition of clathrin-mediated endocytosis selectively attenuates specific insulin receptor signal transduction pathways.

To examine the role of clathrin-dependent insulin receptor internalization in insulin-stimulated signal transduction events, we expressed a dominant-interfering mutant of dynamin (K44A/dynamin) by using a recombinant adenovirus in the H4IIE hepatoma and 3T3L1 adipocyte cell lines. Expression of K44A/dynamin inhibited endocytosis of the insulin receptor as determined by both cell surface radioligand binding and trypsin protection analysis. The inhibition of the insulin receptor endocytosis had no effect on either the extent of insulin receptor autophosphorylation or insulin receptor substrate 1 (IRS1) tyrosine phosphorylation. In contrast, expression of K44A/dynamin partially inhibited insulin-stimulated Shc tyrosine phosphorylation and activation of the mitogen-activated protein kinases ERK1 and -2. Although there was an approximately 50% decrease in the insulin-stimulated activation of the phosphatidylinositol 3-kinase associated with IRS1, insulin-stimulated Akt kinase phosphorylation and activation were unaffected. The expression of K44A/dynamin increased the basal rate of amino acid transport, which was additive with the effect of insulin but had no effect on the basal or insulin-stimulated DNA synthesis. In 3T3L1 adipocytes, expression of K44A/dynamin increased the basal rate of glucose uptake, glycogen synthesis, and lipogenesis without any significant effect on insulin stimulation. Together, these data demonstrate that the acute actions of insulin are largely independent of insulin receptor endocytosis and are initiated by activation of the plasma membrane-localized insulin receptor.

Insulin regulation of the Ras activation/inactivation cycle.

In addition to mediating a number of metabolic functions, insulin also uses mitogenic pathways to maintain cellular homeostasis. Many of these mitogenic responses are mediated by signals through the small molecular weight guanine nucleotide binding protein, Ras. In the last decade, great progress has been made in understanding the molecular mechanisms which regulate the insulin mediated conversion of Ras from its inactive, GDP-bound state, to the activated GTP-bound form. More recently, it has been appreciated that insulin also regulates the inactivation of this pathway, namely by uncoupling the protein complexes whose formation is required for Ras activation. This review addresses molecular mechanism which both positively and negatively regulate this mitogenic signalling pathway.

The 66-kDa Shc isoform is a negative regulator of the epidermal growth factor-stimulated mitogen-activated protein kinase pathway.

In addition to tyrosine phosphorylation of the 66-, 52-, and 46-kDa Shc isoforms, epidermal growth factor (EGF) treatment of Chinese hamster ovary cells expressing the human EGF receptor also resulted in the serine/threonine phosphorylation of approximately 50% of the 66-kDa Shc proteins. The serine/threonine phosphorylation occurred subsequent to tyrosine phosphorylation and was prevented by pretreatment of the cells with the MEK-specific inhibitor PD98059. Surprisingly, only the gel-shifted 66-kDa Shc isoform (serine/threonine phosphorylated) was tyrosine phosphorylated and associated with Grb2. In contrast, only the non-serine/threonine-phosphorylated fraction of 66-kDa Shc was associated with the EGF receptor. To assess the relationship between the three Shc isoforms in EGF-stimulated signaling, the cDNA encoding the 66-kDa Shc species was cloned from a 16-day-old mouse embryo library. Sequence alignment confirmed that the 66-kDa Shc cDNA resulted from alternative splicing of the primary Shc transcript generating a 110-amino acid extension at the amino terminus. Co-immunoprecipitation of Shc and Grb2 from cells overexpressing the 52/46-kDa Shc isoforms versus the 66-kDa Shc species directly demonstrated a competition of binding for a limited pool of Grb2 proteins. Furthermore, expression of the 66-kDa Shc isoform markedly accelerated the inactivation of ERK following EGF stimulation. Together, these data indicate that the serine/threonine phosphorylation of 66-kDa Shc impairs its ability to associate with the tyrosine-phosphorylated EGF receptor and can function in a dominant-interfering manner by inhibiting EGF receptor downstream signaling pathways.

Signal transducer and activator of transcription-3 serine phosphorylation by insulin is mediated by a Ras/Raf/MEK-dependent pathway.

We recently reported that insulin stimulation results in the serine phosphorylation of STAT3 (signal transducer and activator of transcription-3). In the present study, we identified serine 727 as the site of insulin-stimulated STAT3 serine phosphorylation. This phosphorylation event occurs independent of tyrosine phosphorylation. Furthermore, interleukin-6-induced tyrosine phosphorylation can occur independent of serine phosphorylation, demonstrating that these two phosphorylation pathways are mechanistically unrelated. Selective activation of the JNK and p38 family of mitogen-activated protein (MAP) kinases by anisomycin treatment did not result in the phosphorylation of STAT3. In contrast, activation of the ERK MAP kinase pathway with both insulin and osmotic shock resulted in the serine phosphorylation of STAT3. In addition, expression of a dominant-interfering Ras mutant (N17Ras) or treatment with the specific MEK inhibitor (PD98059) prevented the insulin stimulation of STAT3 serine phosphorylation. Blockade of ERK activation by expression of the MAP kinase phosphatase (MKP-1) had no effect on insulin-stimulated STAT3 serine phosphorylation. Together, these data demonstrate that the insulin-stimulated serine phosphorylation of STAT3 occurs by a MEK-dependent pathway that is independent of ERK activation.

Insulin stimulates the serine phosphorylation of the signal transducer and activator of transcription (STAT3) isoform.

Insulin stimulation of Chinese hamster ovary cells expressing the human insulin receptor and differentiated 3T3L1 adipocytes resulted in a time-dependent reduction in the SDS-polyacrylamide gel electrophoretic mobility of STAT3. The decreased STAT3 mobility initially occurred by 2 min and was quantitative by 5 min. In addition, the change in STAT3 mobility was concentration-dependent and was detectable at 0.3 nm insulin with maximal effect between 1 and 3 nm. Although both these cell types also express the STAT1 alpha, STAT1 beta, STAT5, and STAT6 isoforms, only STAT3 was observed to undergo an insulin-dependent reduction in mobility. Immuno-precipitation of STAT1 and STAT3 from 32P-labeled cells demonstrated that only STAT3 was phosphorylated in response to insulin whereas phosphoamino acid analysis indicated that this phosphorylation event occurred exclusively on serine residues. Furthermore, treatment of cell extracts with alkaline phosphatase reversed the insulin-stimulated decrease in STAT3 mobility. Together, these data demonstrate that insulin is a specific activator of STAT3 serine phosphorylation without affecting the other STAT isoforms.

Mutation of an aspartate residue highly conserved among G-protein-coupled receptors results in nonreciprocal disruption of alpha 2-adrenergic receptor-G-protein interactions. A negative charge at amino acid residue 79 forecasts alpha 2A-adrenergic receptor sensitivity to allosteric modulation by monovalent cations and fully effective receptor/G-protein coupling.

We have mutated the aspartate residue in the putative second transmembrane spanning domain of the alpha 2A-adrenergic receptor (alpha 2AAR) to the non-negatively charged asparagine (D79N) and glutamine (D79Q) and the negatively charged glutamate (D79E) residue in an effort to better characterize the role of this residue, highly conserved among G-protein-coupled receptors, in Na+ regulation of ligand binding and in receptor G-protein coupling. Allosteric modulation of receptor-ligand interactions by Na+ is retained by the D79E alpha 2AAR but lost upon mutation to the uncharged D79N and D79Q residues. Loss of allosteric effects of Na+ is paralleled by a complete loss of retrograde information transfer from G-proteins to alpha 2AAR in AtT20 cells, measured via the sensitivity of radiolabeled agonist binding to Gpp(NH)p. In contrast to the complete elimination of retrograde signaling via the D79N and D79Q alpha 2AAR, anterograde information transfer from receptor to G-protein is modified in a more subtle quantitative way, since agonist-stimulated GTPase activity via D79N and D79Q alpha 2AAR, although apparently attenuated compared to wild type and D79E alpha 2AAR, is no less than the GTPase activity elicited by endogenous somatostatin receptors in AtT20 cells. These data indicate that a negative charge at amino acid residue 79 forecasts sensitivity to allosteric regulation by monovalent cations and its mutation to non-negatively charged residues elicits a nonparallel modulation of receptor-->G-protein versus G-protein-->receptor communication between alpha 2AAR and pertussis toxin-sensitive GTP-binding proteins.