Identification of a distal GLUT4 trafficking event controlled by actin polymerization

C2 domain-containing phosphoprotein CDP138 regulates GLUT4 insertion into the plasma membrane

The protein kinase B(β) (Akt2) pathway is known to mediate insulin-stimulated glucose transport through increasing glucose transporter GLUT4 translocation from intracellular stores to the plasma membrane (PM). Combining quantitative phosphoproteomics with RNAi-based functional analyses, we show that a previously uncharacterized 138 kDa C2 domain-containing phosphoprotein (CDP138) is a substrate for Akt2, and is required for optimal insulin-stimulated glucose transport, GLUT4 translocation, and fusion of GLUT4 vesicles with the PM in live adipocytes. The purified C2 domain is capable of binding Ca(2+) and lipid membranes. CDP138 mutants lacking the Ca(2+)-binding sites in the C2 domain or Akt2 phosphorylation site S197 inhibit insulin-stimulated GLUT4 insertion into the PM, a rate-limiting step of GLUT4 translocation. Interestingly, CDP138 is dynamically associated with the PM and GLUT4-containing vesicles in response to insulin stimulation. Together, these results suggest that CDP138 is a key molecule linking the Akt2 pathway to the regulation of GLUT4 vesicle-PM fusion.

Glycemic index differences of high-fat diets modulate primarily lipid metabolism in murine adipose tissue

A low vs. high glycemic index of a high-fat (HF) diet (LGI and HGI, respectively) significantly retarded adverse health effects in adult male C57BL/6J mice, as shown recently (Van Schothorst EM, Bunschoten A, Schrauwen P, Mensink RP, Keijer J. FASEB J 23: 1092–1101, 2009). The LGI diet enhanced whole body insulin sensitivity and repressed HF diet-induced body and white adipose tissue (WAT) weight gain, resulting in significantly reduced serum leptin and resistin levels and increased adiponectin levels. We questioned how WAT is modulated and characterized the molecular mechanisms underlying the glycemic index-mediated effects using whole genome microarrays. This showed that the LGI diet mainly exerts its beneficial effects via substrate metabolism, especially fatty acid metabolism. In addition, cell adhesion and cytoskeleton remodeling showed reduced expression, in line with lower WAT mass. An important transcription factor showing enhanced expression is PPAR-γ. Furthermore, serum levels of triglycerides, total cholesterol, and HDL- and LDL-cholesterol were all significantly reduced by LGI diet, and simultaneously muscle insulin sensitivity was significantly increased as analyzed by protein kinase B/Akt phosphorylation. Cumulatively, even though these mice were fed an HF diet, the LGI diet induced significantly favorable changes in metabolism in WAT. These effects suggest a partial overlap with pharmacological approaches by thiazolidinediones to treat insulin resistance and statins for hypercholesterolemia. It is therefore tempting to speculate that such a dietary approach might beneficially support pharmacological treatment of insulin resistance or hypercholesterolemia in humans.

Inhibition of the mTOR/p70S6K pathway is not involved in the insulin-sensitizing effect of AMPK on cardiac glucose uptake

The AMP-activated protein kinase (AMPK) is known to increase cardiac insulin sensitivity on glucose uptake. AMPK also inhibits the mammalian target of rapamycin (mTOR)/p70 ribosomal S6 kinase (p70S6K) pathway. Once activated by insulin, mTOR/p70S6K phosphorylates insulin receptor substrate-1 (IRS-1) on serine residues, resulting in its inhibition and reduction of insulin signaling. AMPK was postulated to act on insulin by inhibiting this mTOR/p70S6K-mediated negative feedback loop. We tested this hypothesis in cardiomyocytes. The stimulation of glucose uptake by AMPK activators and insulin correlated with AMPK and protein kinase B (PKB/Akt) activation, respectively. Both treatments induced the phosphorylation of Akt substrate 160 (AS160) known to control glucose uptake. Together, insulin and AMPK activators acted synergistically to induce PKB/Akt overactivation, AS160 overphosphorylation, and glucose uptake overstimulation. This correlated with p70S6K inhibition and with a decrease in serine phosphorylation of IRS-1, indicating the inhibition of the negative feedback loop. We used the mTOR inhibitor rapamycin to confirm these results. Mimicking AMPK activators in the presence of insulin, rapamycin inhibited p70S6K and reduced IRS-1 phosphorylation on serine, resulting in the overphosphorylation of PKB/Akt and AS160. However, rapamycin did not enhance the insulin-induced stimulation of glucose uptake. In conclusion, although the insulin-sensitizing effect of AMPK on PKB/Akt is explained by the inhibition of the insulin-induced negative feedback loop, its effect on glucose uptake is independent of this mechanism. This disconnection revealed that the PKB/Akt/AS160 pathway does not seem to be the rate-limiting step in the control of glucose uptake under insulin treatment.

Loss of AS160 Akt substrate causes Glut4 protein to accumulate in compartments that are primed for fusion in basal adipocytes

The Akt substrate AS160 (TCB1D4) regulates Glut4 exocytosis; shRNA knockdown of AS160 increases surface Glut4 in basal adipocytes. AS160 knockdown is only partially insulin-mimetic; insulin further stimulates Glut4 translocation in these cells. Insulin regulates translocation as follows: 1) by releasing Glut4 from retention in a slowly cycling/noncycling storage pool, increasing the actively cycling Glut4 pool, and 2) by increasing the intrinsic rate constant for exocytosis of the actively cycling pool (k(ex)). Kinetic studies were performed in 3T3-L1 adipocytes to measure the effects of AS160 knockdown on the rate constants of exocytosis (k(ex)), endocytosis (k(en)), and release from retention into the cycling pool. AS160 knockdown released Glut4 into the actively cycling pool without affecting k(ex) or k(en). Insulin increased k(ex) in the knockdown cells, further increasing cell surface Glut4. Inhibition of phosphatidylinositol 3-kinase or Akt affected both k(ex) and release from retention in control cells but only k(ex) in AS160 knockdown cells. Glut4 vesicles accumulate in a primed pre-fusion pool in basal AS160 knockdown cells. Akt regulates the rate of exocytosis of the primed vesicles through an AS160-independent mechanism. Therefore, there is an additional Akt substrate that regulates the fusion of Glut4 vesicles that remain to be identified. Mathematical modeling was used to test the hypothesis that this substrate regulates vesicle priming (release from retention), whereas AS160 regulates the reverse step by stimulating GTP turnover of a Rab protein required for vesicle tethering/docking/fusion. Our analysis indicates that fusion of the primed vesicles with the plasma membrane is an additional non-Akt-dependent insulin-regulated step.

Dual-mode of insulin action controls GLUT4 vesicle exocytosis

Insulin stimulates translocation of GLUT4 storage vesicles (GSVs) to the surface of adipocytes, but precisely where insulin acts is controversial. Here we quantify the size, dynamics, and frequency of single vesicle exocytosis in 3T3-L1 adipocytes. We use a new GSV reporter, VAMP2-pHluorin, and bypass insulin signaling by disrupting the GLUT4-retention protein TUG. Remarkably, in unstimulated TUG-depleted cells, the exocytic rate is similar to that in insulin-stimulated control cells. In TUG-depleted cells, insulin triggers a transient, twofold burst of exocytosis. Surprisingly, insulin promotes fusion pore expansion, blocked by acute perturbation of phospholipase D, which reflects both properties intrinsic to the mobilized vesicles and a novel regulatory site at the fusion pore itself. Prolonged stimulation causes cargo to switch from approximately 60 nm GSVs to larger exocytic vesicles characteristic of endosomes. Our results support a model whereby insulin promotes exocytic flux primarily by releasing an intracellular brake, but also by accelerating plasma membrane fusion and switching vesicle traffic between two distinct circuits.

Endocytosis, recycling, and regulated exocytosis of glucose transporter 4

Glucose transporter 4 (GLUT4) is responsible for the uptake of glucose into muscle and adipose tissues. Under resting conditions, GLUT4 is dynamically retained through idle cycling among selective intracellular compartments, from whence it undergoes slow recycling to the plasma membrane (PM). This dynamic retention can be released by command from intracellular signals elicited by insulin and other stimuli, which result in 2-10-fold increases in the surface level of GLUT4. Insulin-derived signals promote translocation of GLUT4 to the PM from a specialized compartment termed GLUT4 storage vesicles (GSV). Much effort has been devoted to the characterization of the intracellular compartments and dynamics of GLUT4 cycling and to the signals by which GLUT4 is sorted into, and recruited from, GSV. This review summarizes our understanding of intracellular GLUT4 traffic during its internalization from the membrane, its slow, constitutive recycling, and its regulated exocytosis in response to insulin. In spite of specific differences in GLUT4 dynamic behavior in adipose and muscle cells, the generalities of its endocytic and exocytic itineraries are consistent and an array of regulatory proteins that regulate each vesicular traffic event emerges from these cell systems.

Mapping insulin/GLUT4 circuitry

One of the most important metabolic actions of insulin is catalysing glucose uptake into skeletal muscle and adipose tissue. This is accomplished via activation of the phosphatidylinositol-3-kinase/Akt signalling pathway and subsequent translocation of GLUT4 from intracellular storage vesicles to the plasma membrane. As such, this represents an ideal system for studying the convergence of signal transduction and protein trafficking. The GLUT4 translocation process is complex, but can be dissected into at least four discrete trafficking steps. This raises the question as to which of these is the major regulated step in insulin-stimulated GLUT4 translocation. Numerous molecules have been reported to regulate GLUT4 trafficking. However, with the exception of TBC1D4, the molecular details of these distal signalling arms of the insulin signalling network and how they modify distinct steps of GLUT4 trafficking have not been established. We discuss the need to adopt a more global approach to expand and deepen our understanding of the molecular processes underpinning this system. Strategies that facilitate the generation of detailed models of the entire insulin signalling network will enable us to identify the critical nodes that control GLUT4 traffic and decipher emergent properties of the system that are not currently apparent.

Fatty acid 2-hydroxylase mediates diffusional mobility of Raft-associated lipids, GLUT4 level, and lipogenesis in 3T3-L1 adipocytes

Straight chain fatty acid alpha-oxidation increases during differentiation of 3T3-L1 adipocytes, leading to a marked accumulation of odd chain length fatty acyl moieties. Potential roles of this pathway in adipocyte differentiation and lipogenesis are unknown. Mammalian fatty acid 2-hydroxylase (FA2H) was recently identified and suggested to catalyze the initial step of straight chain fatty acid alpha-oxidation. Accordingly, we examined whether FA2H modulates adipocyte differentiation and lipogenesis in mature adipocytes. FA2H level markedly increases during differentiation of 3T3-L1 adipocytes, and small interfering RNAs against FA2H inhibit the differentiation process. In mature adipocytes, depletion of FA2H inhibits basal and insulin-stimulated glucose uptake and lipogenesis, which are partially rescued by the enzymatic product of FA2H, 2-hydroxy palmitic acid. Expression of fatty-acid synthase and SCD1 was decreased in FA2H-depleted cells, and levels of GLUT4 and insulin receptor proteins were reduced. 2-Hydroxy fatty acids are enriched in cellular sphingolipids, which are components of membrane rafts. Accelerated diffusional mobility of raft-associated lipids was shown to enhance degradation of GLUT4 and insulin receptor in adipocytes. Consistent with this, depletion of FA2H appeared to increase raft lipid mobility as it significantly accelerated the rates of fluorescence recovery after photobleaching measurements of lipid rafts labeled with Alexa 488-conjugated cholera toxin subunit B. Moreover, the enhanced recovery rates were partially reversed by treatment with 2-hydroxy palmitic acid. In conclusion, our findings document the novel role of FA2H in adipocyte lipogenesis possibly by modulation of raft fluidity and level of GLUT4.

Identification of three distinct functional sites of insulin-mediated GLUT4 trafficking in adipocytes using quantitative single molecule imaging

Insulin stimulation of glucose uptake is achieved by redistribution of insulin-responsive glucose transporters, GLUT4, from intracellular storage compartment(s) to the plasma membrane in adipocytes and muscle cells. Although GLUT4 translocation has been investigated using various approaches, GLUT4 trafficking properties within the cell are largely unknown. Our novel method allows direct analysis of intracellular GLUT4 dynamics at the single molecule level by using Quantum dot technology, quantitatively establishing the behavioral nature of GLUT4. Our data demonstrate the predominant mechanism for intracellular GLUT4 sequestration in the basal state to be "static retention" in fully differentiated 3T3L1 adipocytes. We also directly defined three distinct insulin-stimulated GLUT4 trafficking processes: 1) release from the putative GLUT4 anchoring system in storage compartment(s), 2) the speed at which transport GLUT4-containing vesicles move, and 3) the tethering/docking steps at the plasma membrane. Intriguingly, insulin-induced GLUT4 liberation from its static state appeared to be abolished by either pretreatment with an inhibitor of phosphatidylinositol 3-kinase or overexpression of a dominant-interfering AS160 mutant (AS160/T642A). In addition, our novel approach revealed the possibility that, in certain insulin-resistant states, derangements in GLUT4 behavior can impair insulin-responsive GLUT4 translocation.

An emerging role for IQGAP1 in regulating protein traffic

IQGAP1, an effector of CDC42p GTPase, is a widely conserved, multifunctional protein that bundles F-actin through its N-terminus and binds microtubules through its C-terminus to modulate the cell architecture. It has emerged as a potential oncogene associated with diverse human cancers. Therefore, IQGAP1 has been heavily investigated; regardless, its precise cellular function remains unclear. Work from yeast suggests that IQGAP1 plays an important role in directed cell growth, which is a conserved feature crucial to morphogenesis, division axis, and body plan determination. New evidence suggests a conserved role for IQGAP1 in protein synthesis and membrane traffic, which may help to explain the diversity of its cellular functions. Membrane traffic mediates infections by intracellular pathogens and a range of degenerative human diseases arise from dysfunctions in intracellular traffic; thus, elucidating the mechanisms of cellular traffic will be important in order to understand the basis of a wide range of inherited and acquired human diseases. Recent evidence suggests that IQGAP1 plays its role in cell growth through regulating the conserved mTOR pathway. The mTOR signaling cascade has been implicated in membrane traffic and is activated in nearly all human cancers, but clinical response to the mTOR-specific inhibitor rapamycin has been disappointing. Thus, understanding the regulators of this pathway will be crucial in order to identify predictors of rapamycin sensitivity. In this review, I discuss emerging evidence that supports a potential role of IQGAP1 in regulating membrane traffic via regulating the mTOR pathway.

GLUT4 is sorted to vesicles whose accumulation beneath and insertion into the plasma membrane are differentially regulated by insulin and selectively affected by insulin resistance

GLUT4 is sorted to vesicles whose accumulation beneath and insertion into the plasma membrane are differentially regulated by insulin and selectively affected by insulin resistance.

Insulin stimulates glucose transport by recruiting the GLUT4 glucose transporter to the plasma membrane. Here we use total internal reflection fluorescence microscopy to show that two trafficking motifs of GLUT4, a FQQI motif and a TELE-based motif, target GLUT4 to specialized vesicles that accumulate adjacent to the plasma membrane of unstimulated adipocytes. Mutations of these motifs redistributed GLUT4 to transferrin-containing recycling vesicles adjacent to the plasma membrane, and the degree of redistribution correlated with the increases of the GLUT4 mutants in the plasma membrane of basal adipocytes. These results establish that GLUT4 defaults to recycling endosomes when trafficking to specialized vesicles is disrupted, supporting the hypothesis that the specialized vesicles are derived from an endosomal compartment. Insulin stimulates both the accumulation of GLUT4 in the evanescent field and the fraction of this GLUT4 that is inserted into the plasma membrane. Unexpectedly, these two steps are differentially affected by the development of insulin resistance. We ascribe this selective insulin resistance to inherent differences in the sensitivities of GLUT4 vesicle accumulation and insertion into the plasma membrane to insulin. Differences in insulin sensitivities of various processes may be a general mechanism for the development of the physiologically important phenomenon of selective insulin resistance.

Exocytotic vesicle behaviour assessed by total internal reflection fluorescence microscopy

The regulated trafficking or exocytosis of cargo-containing vesicles to the cell surface is fundamental to all cells. By coupling the technology of fluorescently tagged fusion proteins with total internal reflection fluorescence microscopy (TIRFM), it is possible to achieve the high spatio-temporal resolution required to study the dynamics of sub-plasma membrane vesicle trafficking and exocytosis. TIRFM has been used in a number of cell types to visualize and dissect the various steps of exocytosis revealing how molecules identified via genetic and/or biochemical approaches are involved in the regulation of this process. Here, we summarize the contribution of TIRFM to our understanding of the mechanism of exocytosis and discuss the novel methods of analysis that are required to exploit the large volumes of data that can be produced using this technique.

Cdc42p is activated during vacuole membrane fusion in a sterol-dependent subreaction of priming

Cdc42p is a Rho GTPase that initiates signaling cascades at spatially defined intracellular sites for many cellular functions. We have previously shown that Cdc42p is localized to the yeast vacuole where it initiates actin polymerization during membrane fusion. Here we examine the activation cycle of Cdc42p during vacuole membrane fusion. Expression of either GTP- or GDP-locked Cdc42p mutants caused several morphological defects including enlarged cells and fragmented vacuoles. Stimulation of multiple rounds of fusion enhanced vacuole fragmentation, suggesting that cycles of Cdc42p activation, involving rounds of GTP binding and hydrolysis, are required to propagate Cdc42p signaling. We developed an assay to directly examine Cdc42p activation based on affinity to a probe derived from the p21-activated kinase effector, Ste20p. Cdc42p was rapidly activated during vacuole membrane fusion, which kinetically coincided with priming subreaction. During priming, Sec18p ATPase activity dissociates SNARE complexes and releases Sec17p, however, priming inhibitors such as Sec17p and Sec18p ligands did not block Cdc42p activation. Therefore, Cdc42p activation seems to be a parallel subreaction of priming, distinct from Sec18p activity. Specific mutants in the ergosterol synthesis pathway block both Sec17p release and Cdc42p activation. Taken together, our results define a novel sterol-dependent subreaction of vacuole priming that activates cycles of Cdc42p activity to facilitate membrane fusion.

Cluster analysis of insulin action in adipocytes reveals a key role for Akt at the plasma membrane

The phosphatidylinositol 3-kinase/Akt pathway regulates many biological processes, including insulin-regulated GLUT4 insertion into the plasma membrane. However, Akt operates well below its capacity to facilitate maximal GLUT4 translocation. Thus, reconciling modest changes in Akt expression or activity as a cause of metabolic dysfunction is complex. To resolve this, we examined insulin regulation of components within the signaling cascade in a quantitative kinetic and dose-response study combined with hierarchical cluster analysis. This revealed a strong relationship between phosphorylation of Akt substrates and GLUT4 translocation but not whole cell Akt phosphorylation. In contrast, Akt activity at the plasma membrane strongly correlated with GLUT4 translocation and Akt substrate phosphorylation. Additionally, two of the phosphorylated sites in the Akt substrate AS160 clustered separately, with Thr(P)-642 grouped with other Akt substrates. Further experiments suggested that atypical protein kinase Czeta phosphorylates AS160 at Ser-588 and that these two sites are mutually exclusive. These data indicate the utility of hierarchical cluster analysis for identifying functionally related biological nodes and highlight the importance of subcellular partitioning of key signaling components for biological specificity.