Glucose Transporters And Insulin Action Pdf

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Randhawa, Amira Klip; Insulin action on glucose transporters through molecular switches, tracks and tethers. Biochem J 15 July ; 2 : — Glucose entry into muscle cells is precisely regulated by insulin, through recruitment of GLUT4 glucose transporter-4 to the membrane of muscle and fat cells.

Signaling pathways in insulin action: molecular targets of insulin resistance

The ability of insulin to stimulate glucose uptake into muscle and adipose tissue is central to the maintenance of whole-body glucose homeostasis. Deregulation of insulin action manifests itself as insulin resistance, a key component of type II diabetes mellitus T2DM. Both forms of diabetes confer an increased risk of major lifelong complications. In the case of insulin resistance, this includes a fivefold increased risk of coronary vascular disease. The need for an effective treatment for both forms of diabetes as well as for the development of early detection methodologies has, therefore, become increasingly important.

Yet for this to be possible, we must first understand the mechanism through which insulin regulates glucose uptake and identify the key molecular players involved. In , the glucose transporter GLUT4 was cloned and shown to be the major isoform responsible for enhanced glucose uptake into muscle and adipose tissue following the secretion of insulin into the bloodstream Birnbaum , Charron et al.

Evidence that GLUT4 redistributes from an intracellular membrane pool to the plasma membrane upon insulin stimulation has since been confirmed using a variety of different methods. These have included subcellular fractionation Marette et al. Identifying the nature of the intracellular compartment s in insulin-responsive tissues where GLUT4 resides in the absence of insulin is key to understanding the mechanism through which insulin promotes the redistribution of this GLUT4 pool to the cell surface.

These observations were supported by immunofluorescence and electron microscopy studies in insulin-responsive tissues, which demonstrated that GLUT4 is localised within complex tubulovesicular structures at the perinuclear region of the cell, to small peripheral vesicles in the cytoplasm and to clathrin-coated pits at the cell surface Friedman et al.

How insulin stimulates the net translocation of GLUT4 to the plasma membrane has been the subject of intense investigation. To explore this, we will first discuss how GLUT4 is sequestered away from the plasma membrane in the basal state, and how insulin action may impact on these processes.

We will then discuss insulin signalling itself in more detail, before moving on to a detailed discussion of the trafficking events that occur at the plasma membrane because our understanding of this area of GLUT4 biology has increased significantly in the past 2—3 years. The existence of an insulin-responsive GSV pool to which GLUT4 is targeted in the absence of insulin is supported by many studies and is widely accepted within the field.

However, the mechanism by which GLUT4 is actively retained within this compartment and thereby sequestered away from the plasma membrane in the basal state remains controversial. Currently, evidence exists to support two entirely different mechanisms of GLUT4 sequestration. Studies in both 3T3-L1 adipocytes Martin et al. Additional studies in 3T3-L1 adipocytes have suggested that the slow recycling of GLUT4 is due to a dynamic sorting process that involves the continuous budding and fusion of GLUT4-containing vesicles between the endosomal compartment and the GSVs Karylowski et al.

This study also supports the hypothesis that GSVs are a post-endosomal pool, a theory formulated based on the observation that the disruption of the function of certain proteins including Rab11 Zeigerer et al. In this model, as illustrated in Fig. Citation: Journal of Endocrinology , 1; Govers et al.

It is likely that this model involves the translocation of GLUT4 to the plasma membrane via a pathway akin to route 2 Fig. The fundamental discrepancy between these two models has been recently addressed in an interesting study by Muretta et al. They demonstrated that the process of replating transfected 3T3-L1 adipocytes before analysis, as performed by Martin et al. On the basis of these observations, the authors have concluded that the static retention model predominates over the dynamic retention model in 3T3-L1 adipocytes.

It is also important to note that 3T3-L1 adipocytes possess a very different cellular architecture to the native adipocyte in situ. In 3T3-L1 adipocytes, much of the intracellular GLUT4 is found within the perinuclear region in addition to small peripheral cytoplasmic vesicles, meaning that GLUT4 is often located a significant distance away from the plasma membrane. Thus, there is no obvious requirement for long-range translocation of GLUT4 from deep intracellular compartments to the plasma membrane in native adipocytes.

To what extent, therefore, the dynamic and static retention models play a role in primary and native adipocytes is not known, nor is the degree to which these models account for GLUT4 translocation in intact muscle where GLUT4 translocates to both the sarcolemma and t-tubules Lauritzen et al.

The static retention model would require that the insulin signal enter deep into the cell and trigger the release of GLUT4 from the sequestered pool s into the continuously cycling system, and from there traverse towards the plasma membrane. On this basis, insulin would stimulate flux through route 2 Fig. If correct, then the mechanism by which insulin might release GLUT4 from the sequestered store in this model is not known. Once released into the cycling pool, GLUT4 must then traverse the cytoplasm towards the plasma membrane before docking and fusing with it.

Some studies have reported that vesicles containing GFP-tagged GLUT4 translocate towards the plasma membrane along microtubules and that this process is regulated by insulin in a PI3-kinase-independent but kinesin-dependent manner Semiz et al. However, while we also observe microtubule-driven movements of GLUT4 in 3T3-L1 adipocytes, these are predominantly towards the perinuclear region from the plasma membrane Fletcher et al.

This observation is consistent with other studies showing that microtubule depolymerising agents disperse GLUT4 from the perinuclear region Molero et al.

Taken together, the data suggest that microtubules are unlikely to play a major role in the directed movement of GLUT4-containing vesicles to the plasma membrane.

Under the dynamic retention model, insulin could stimulate the net translocation of GLUT4 to the cell surface via a fundamentally different mechanism. In this model, the majority of intracellular GLUT4 in the basal state is in dynamic equilibrium with the plasma membrane and so must be on vesicles that are constantly arriving at and fusing with the plasma membrane before the GLUT4 is endocytosed and recycled back into intracellular compartments.

In this case, there would be no need to invoke an effect of insulin on the release of GLUT4 from deep intracellular compartments. Insulin also stimulates GLUT1 and TfR translocation to the plasma membrane, although the magnitude of the insulin effect twofold or less is considerably lower than that observed for GLUT4 and TfR translocation, at least in adipocytes.

However, the static and dynamic retention models will also need to take account of the mechanism by which insulin stimulates GLUT1 and TfR translocation. At present, this is a particularly poorly understood area. Clearly much further work is required to clarify which of the dynamic and static retention models are operational in 3T3-L1 adipocytes, rat adipocytes, and human fat and muscle cells, before we can be sure where insulin acts on GLUT4 trafficking.

This area must therefore remain a priority for further investigation. Activation of the insulin receptor by insulin elicits a wide range of cellular responses that together coordinate a shift in the steady-state distribution of GLUT4 to the cell surface. However, despite an ever-growing list of potential molecules that appear to be required for insulin-stimulated GLUT4 translocation, numerous gaps remain in our understanding of the complete signal transduction pathway that begins at the insulin receptor and culminates with the fusion of GLUT4 vesicles with the plasma membrane.

The activation of the insulin receptor by insulin is thought to stimulate two independent signal transduction pathways that converge at the level of GLUT4 trafficking to bring about the redistribution of the glucose transporter to the cell surface Fig. The transient increase in plasma membrane PIP 3 following insulin receptor activation results in the recruitment of proteins containing pleckstrin homology PH domains to the cell surface.

This figure illustrates the major signalling pathways implicated in insulin action of glucose transport. These include the PI3-kinase-dependent black and -independent grey pathways. See text for further details. A wortmannin-insensitive pathway has also been proposed that functions in parallel to PI3K activation.

The importance of this pathway in insulin-stimulated glucose uptake is however, controversial due to conflicting siRNA and mouse knockout studies. In a study by Chang et al. Furthermore, while the generation of Cbl knockout mice does not result in a significant difference in insulin sensitivity Molero et al.

Finally, studies performed in muscle cells indicate that TC10 signalling may not be required for cortical actin rearrangements as has been shown in adipocytes JeBailey et al. The generation of a phosphorylation mutant called AS 4P , in which four of the six phosphorylation sites are mutated to alanine residues, dominantly inhibits the insulin-stimulated translocation of a GLUT4 reporter construct to the plasma membrane when co-expressed in 3T3-L1 adipocytes Sano et al.

This hypothesis is supported by siRNA studies that demonstrate that following the ablation of AS from adipocytes, plasma membrane GLUT4 levels become elevated in the absence of insulin Eguez et al. However, it should be noted that over-expression of the AS 4P mutant, while dominant inhibitory, does not completely block insulin-stimulated glucose uptake into adipocytes.

Furthermore, knockdown of AS using siRNA-mediated ablation has a relatively weak stimulatory effect on basal glucose uptake.

These data have been taken to suggest that the AS pathway is not the only pathway by which insulin regulates glucose uptake, although this remains to be firmly established.

The phosphorylation of AS is thought to render the GAP domain inactive and therefore relieve the inhibitory effect of AS on its target Rab s ; however, it remains unclear how this is achieved mechanistically. This would suggest that the phosphorylation of AS leads to its dissociation from GLUT4 vesicles by facilitating its binding to proteins that recognise their target proteins by binding to phospho-threonine or phospho-serine residues.

Interestingly, studies have revealed that in adipose cells, Rab10 appears to be the isoform regulated by AS Sano et al. These findings are based on: a their ability, when over-expressed as GTP-locked mutants, to rescue the inhibitory effects on GLUT4 trafficking of either the dominant-negative AS 4P mutant or of siRNA-mediated AS knockdown, and b the effect of siRNA-mediated depletion of the Rab isoform on insulin-stimulated glucose transport in adipocytes or muscle cells.

However, given that Rab proteins are involved in a plethora of intracellular trafficking events, both these experimental approaches should be approached with some caution since the perturbation of any intracellular GLUT4 trafficking step, whether directly involved in the delivery of GLUT4 to the plasma membrane or not, could inhibit insulin-stimulated GLUT4 translocation. Since it is difficult to assess the integrity of this compartment, and until technologies for studying GLUT4 trafficking improve, indirect effects on GLUT4 translocation are going to be difficult to eliminate from any particular enquiry.

However, for a more comprehensive review of the role of Rabs in insulin action, the reader is directed to two recent reviews Ishikura et al. AS is expressed in tissue and muscle, as well as several other tissues at least at the level of mRNA.

AS possesses a total of up to eight different phosphorylation sites, whereas TBC1D1 possesses just two, at least in cells stimulated with insulin, AMP kinase activators and insulin-like growth factors Geraghty et al. Furthermore, although both proteins possess binding sites for proteins, the binding sites involved differ significantly Geraghty et al. In addition to a role in insulin-stimulated glucose uptake in muscle, as discussed above, TBC1D1 is likely to play an important role in the stimulation of muscle glucose uptake in response to contraction and activators of AMP kinase Taylor et al.

Our understanding of the role of AS and TBC1D1 would greatly benefit from the development of knockout and knockin in which phosphorylation sites in AS and TBC1D1 are knocked out individually or in combinations mouse models. Such mice have yet to be reported; however, a mouse strain SJL resistant to high-fat diet-induced obesity has been recently shown to possess a mutation in TBC1D1 that causes premature termination of protein translation Chadt et al. Insulin-stimulated glucose uptake in extensor digitorum longus muscles isolated from SJL mice was reduced compared with that in the control congenic strain, but basal uptake rates were not different.

This is a surprising result given that siRNA-mediated ablation of AS in cell lines increases basal glucose uptake Eguez et al. However, the SJL mouse may still express a truncated version of TBC1D1 lacking GAP activity and this may act as a dominant-negative inhibitor of glucose uptake into muscles, so complicating the interpretation of the phenotype. SJL mice exhibit increased muscle palmitate uptake and oxidation, when compared with a congenic control strain that is susceptible to high-fat diet-induced obesity.

However, whether this is related to the obesity-resistant phenotype of the SJL strain was not established. As TBC1D1 is expressed in the hypothalamus, a possible role for the protein in appetite control cannot be discounted. Again, the molecular basis for this genotype—phenotype relationship is not known. Several studies now point to an important role of the phosphoinositide 3Pkinase, PIKfyve, in insulin action on glucose uptake.

This is consistent with a recent report that insulin stimulates PI 3,5 P 2 production in intracellular membranes of 3T3-L1 adipocytes Ikonomov et al. We showed that insulin stimulates the phosphorylation of Ser in cells and that this phosphorylation plays a role in GLUT4 trafficking because over-expression of the non-activatable PIKfyve[SA] mutant in 3T3-L1 adipocytes enhanced insulin-stimulated GLUT4 vesicle translocation to the plasma membrane Berwick et al.

This observation is consistent with recent studies showing that pharmacologic inhibition using the selective PIKfyve inhibitor YM Jefferies et al. Together, however, these observations are not consistent with Shisheva's studies Ikonomov et al. Clearly, further work is required to understand these observations. This is consistent with recent studies demonstrating that PIKfyve is predominantly associated with early endosomes, from where it regulates retrograde membrane trafficking of the mannosephosphate receptor to the TGN Rutherford et al.

Interestingly, we have found that the PIKfyve[SA] mutant inhibits the trafficking of several metabolite transporters and ion channels to the cell surface Seebohm et al.

That these proteins are invariably found in the recycling endosomal system, rather than a compartment equivalent to the GSV pool containing GLUT4, probably underlies the reason for the inhibitory rather than enhancing effect of the PIKfyve[SA] mutant observed in these studies.

Thus, the effect of insulin is not due to autophosphorylation by PIKfyve itself, as claimed by Shisheva Ikonomov et al.

Hyperosmotic stress is well known to stimulate glucose uptake in 3T3-L1 adipocytes, also in a PI3-kinase-independent manner Sakaue et al. This in turn could be the reason for the apparent stimulatory effect of phosphorylation on 5-kinase activity we observe i.

The possibility that a similar phenomenon takes place in PIKfyve warrants investigation. While insulin signalling may regulate the release of GLUT4 from sequestered intracellular storage pools, as discussed above, it has recently become apparent that the hormone also has substantial effects on the docking and fusion of GLUT4 vesicles with the plasma membrane.

When used in conjunction with sophisticated image analysis software, this has enabled detailed kinetic analysis of the behaviour of individual GLUT4-containing vesicles through the various stages of their arrival, docking and fusion with the plasma membrane.

Glucose transporters and insulin action--implications for insulin resistance and diabetes mellitus

Type 2 diabetes mellitus T2DM is one of the most severe public health problems in the world. In recent years, evidences show a commonness of utilization of alternative medicines such as phytomedicine for the treatment of T2DM. Phenolic acids are the most common compounds in non-flavonoid group of phenolic compounds and have been suggested to have a potential to lower the risk of T2DM. Skeletal muscle is the major organ that contributes to the pathophysiology of T2DM. Studies have shown that several phenolic acids caffeic acid, chlorogenic acid, gallic acid, salicylic acid, p-coumaric acid, ferulic acid, sinapic acid have antidiabetic effects, and these compounds have been implicated in the regulation of skeletal muscle glucose metabolism, especially glucose transport.

Insulin Signaling and the Regulation of Glucose Transport

Obesity associated with systemic inflammation induces insulin resistance IR , with consequent chronic hyperglycemia. These pathways promote greater translocation of GLUT4 and consequent glucose uptake by the skeletal muscle. In this sense, the association between autophagy and exercise has also demonstrated a relevant role in the uptake of muscle glucose. Insulin, in turn, uses a phosphoinositide 3-kinase PI3K -dependent mechanism, while exercise signal may be triggered by the release of calcium from the sarcoplasmic reticulum.

The ability of insulin to stimulate glucose uptake into muscle and adipose tissue is central to the maintenance of whole-body glucose homeostasis. Deregulation of insulin action manifests itself as insulin resistance, a key component of type II diabetes mellitus T2DM. Both forms of diabetes confer an increased risk of major lifelong complications. In the case of insulin resistance, this includes a fivefold increased risk of coronary vascular disease. The need for an effective treatment for both forms of diabetes as well as for the development of early detection methodologies has, therefore, become increasingly important.

Metrics details. Gaps remain in our understanding of the precise molecular mechanisms by which insulin regulates glucose uptake in fat and muscle cells.

International Scholarly Research Notices

Insulin stimulates glucose uptake in muscle and adipose cells primarily by recruiting GLUT4 from an intracellular storage pool to the plasma membrane. Dysfunction of this process known as insulin resistance causes hyperglycemia, a hallmark of diabetes and obesity. Thus the understanding of the mechanisms underlying this process at the molecular level may give, an insight into the prevention and treatment of these health problems. GLUT4 in rat adipocytes, for example, constantly recycles between the cell surface and an intracellular pool by endocytosis and exocytosis, each of which is regulated by an insulin-sensitive and GLUT4-selective sorting mechanism. Our working hypothesis has been that this sorting mechanism includes a specific interaction of a cytosolic protein with the GLUT4 cytoplasmic domain. Relevance of these observations to a novel euglycemic drug design is discussed. This is a preview of subscription content, access via your institution.

GLUT4, the major isoform in insulin-responsive tissue, translocates from an intracellular pool to the cell surface and as such determines insulin-stimulated glucose uptake. However, despite intensive research over 50 years, the insulin-dependent and -independent pathways that mediate GLUT4 translocation are not fully elucidated in any species. Insulin resistance IR is one of the hallmarks of equine metabolic syndrome and is the most common metabolic predisposition for laminitis in horses.

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5 Response
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    Whether any of these interactions is actually responsible for the insulin-induced GLUT regulation is yet to be determined. Introduction. Glucose transport in animal.

  2. Pascaline R.

    Insulin stimulates glucose uptake in muscle and adipose cells primarily by recruiting GLUT4 from an intracellular storage pool to the plasma membrane. Dysf.

  3. Lydia N.

    Although diabetes has been identified as a major risk factor for atrial fibrillation, little is known about glucose metabolism in the healthy and diabetic atria.

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