Insulin and okadaic acid induce Rab4 redistribution in adipocytes

Ameliorating effect on glycolipid metabolism and chemical profile of Millettia speciosa champ. extract

ETHNOPHARMACOLOGICAL RELEVANCE

Millettia speciosa Champ., also called Niu dali, is a fabaceous medicinal plant mainly distributed in southeast China, where it is a functional food for developing physical strength, and often used traditionally in medicinal treatment of numbness of the wrists, diabetes, hepatitis, and so on.

AIM OF THE STUDY

To investigate the chemical profile, ameliorating effects of MSC on glycolipid metabolism in diabetic mice and to identify the possible mechanism of action.

MATERIALS AND METHODS

High-performance liquid chromatography coupled with electrospray ionization quadrupole time of flight mass spectrometry (HPLC-ESI-QTOF-MS) was applied to analyze the chemical compositions from M. speciosa extract (MSC). MSC was orally administered to high-fat diet and STZ-induced diabetic mice at doses of 4.55, 9.10 and 13.65 mg/(kg·d) respectively for 10 weeks. Indices of glycolipid metabolism, including fasting blood glucose (FBG), fasting insulin, insulin resistance index (IRI), blood lipids, HPA-axis hormones, and related gene expressions were evaluated.

RESULTS

86 compounds were tentatively identified from MSC, counting for 91.97% of the total extract, mainly including 23 alkaloids (including 2 cyanogenetic glycosides firstly identified in this species, total content accounted to 39.71%), 23 flavonoids (11.91%), 17 acids (including 3 amino acids, 9 phenolic acids and 5 organic acids; 9.2%), 9 terpenoids and steroids (20.13%), 7 esters (3.33%), 3 lignans (3.73%), 3 saccharides (4.0%) and 1 anthraquinone (0.18%). MSC could ameliorate the glycolipid disorder in diabetic mice markedly, and significant regulations on CRH and ACTH hormones were observed. Moreover, the cellular morphology of liver and pancreas were significantly improved and the expressions of IRS2, PI3K, Akt and GLUT4 were significantly up-regulated by MSC treatment.

CONCLUSION

This was the first time to study the chemical profile and ameliorating effect on glycolipid metabolism of M. speciosa. It was found to be rich in flavonoids and alkaloids, which might support the potential relation of material foundation and the activity in regulating glycolipid metabolism. The ameliorating effect on glycolipid disorder in diabetic mice might be associated to the regulation of related hormones of the HPA axis and the IRS2/PI3K/Akt/GLUT4 signalling pathway. It was of great significance for advanced directed separation and pharmacological activity research of MSC.

Rab14 limits the sorting of Glut4 from endosomes into insulin-sensitive regulated secretory compartments in adipocytes

Insulin increases glucose uptake by increasing the rate of exocytosis of the facilitative glucose transporter isoform 4 (Glut4) relative to its endocytosis. Insulin also releases Glut4 from highly insulin-regulated secretory compartments (GSVs or Glut4 storage vesicles) into constitutively cycling endosomes. Previously it was shown that both overexpression and knockdown of the small GTP-binding protein Rab14 decreased Glut4 translocation to the plasma membrane (PM). To determine the mechanism of this perturbation, we measured the effects of Rab14 knockdown on the trafficking kinetics of Glut4 relative to two proteins that partially co-localize with Glut4, the transferrin (Tf) receptor and low-density-lipoprotein-receptor-related protein 1 (LRP1). Our data support the hypothesis that Rab14 limits sorting of proteins from sorting (or 'early') endosomes into the specialized GSV pathway, possibly through regulation of endosomal maturation. This hypothesis is consistent with known Rab14 effectors. Interestingly, the insulin-sensitive Rab GTPase-activating protein Akt substrate of 160 kDa (AS160) affects both sorting into and exocytosis from GSVs. It has previously been shown that exocytosis of GSVs is rate-limited by Rab10, and both Rab10 and Rab14 are in vitro substrates of AS160. Regulation of both entry into and exit from GSVs by AS160 through sequential Rab substrates would provide a mechanism for the finely tuned 'quantal' increases in cycling Glut4 observed in response to increasing concentrations of insulin.

Glut4 Is Sorted from a Rab10 GTPase-independent Constitutive Recycling Pathway into a Highly Insulin-responsive Rab10 GTPase-dependent Sequestration Pathway after Adipocyte Differentiation

The RabGAP AS160/TBC1D4 controls exocytosis of the insulin-sensitive glucose transporter Glut4 in adipocytes. Glut4 is internalized and recycled through a highly regulated secretory pathway in these cells. Glut4 also cycles through a slow constitutive endosomal pathway distinct from the fast transferrin (Tf) receptor recycling pathway. This slow constitutive pathway is the only Glut4 cycling pathway in undifferentiated fibroblasts. The α2-macroglobulin receptor LRP1 cycles with Glut4 and the Tf receptor through all three exocytic pathways. To further characterize these pathways, the effects of knockdown of AS160 substrates on the trafficking kinetics of Glut4, LRP1, and the Tf receptor were measured in adipocytes and fibroblasts. Rab10 knockdown decreased cell surface Glut4 in insulin-stimulated adipocytes by 65%, but not in basal adipocytes or in fibroblasts. This decrease was due primarily to a 62% decrease in the rate constant of Glut4 exocytosis (kex), although Rab10 knockdown also caused a 1.4-fold increase in the rate constant of Glut4 endocytosis (ken). Rab10 knockdown in adipocytes also decreased cell surface LRP1 by 30% by decreasing kex 30-40%. There was no effect on LRP1 trafficking in fibroblasts or on Tf receptor trafficking in either cell type. These data confirm that Rab10 is an AS160 substrate that limits exocytosis through the highly insulin-responsive specialized secretory pathway in adipocytes. They further show that the slow constitutive endosomal (fibroblast) recycling pathway is Rab10-independent. Thus, Rab10 is a marker for the specialized pathway in adipocytes. Interestingly, mathematical modeling shows that Glut4 traffics predominantly through the specialized Rab10-dependent pathway both before and after insulin stimulation.

Potential Bioactive Compounds from Seaweed for Diabetes Management

Diabetes mellitus is a group of metabolic disorders of the endocrine system characterised by hyperglycaemia. Type II diabetes mellitus (T2DM) constitutes the majority of diabetes cases around the world and are due to unhealthy diet, sedentary lifestyle, as well as rise of obesity in the population, which warrants the search for new preventive and treatment strategies. Improved comprehension of T2DM pathophysiology provided various new agents and approaches against T2DM including via nutritional and lifestyle interventions. Seaweeds are rich in dietary fibres, unsaturated fatty acids, and polyphenolic compounds. Many of these seaweed compositions have been reported to be beneficial to human health including in managing diabetes. In this review, we discussed the diversity of seaweed composition and bioactive compounds which are potentially useful in preventing or managing T2DM by targeting various pharmacologically relevant routes including inhibition of enzymes such as α-glucosidase, α-amylase, lipase, aldose reductase, protein tyrosine phosphatase 1B (PTP1B) and dipeptidyl-peptidase-4 (DPP-4). Other mechanisms of action identified, such as anti-inflammatory, induction of hepatic antioxidant enzymes’ activities, stimulation of glucose transport and incretin hormones release, as well as β-cell cytoprotection, were also discussed by taking into consideration numerous in vitro, in vivo, and human studies involving seaweed and seaweed-derived agents.

Hypoxia inhibits Cavin-1 and Cavin-2 expression and down-regulates caveolae in adipocytes

During obesity, a hypoxic state develops within the adipose tissue, resulting in insulin resistance. To understand the underlying mechanism, we analyzed the involvement of caveolae because they play a crucial role in the activation of insulin receptors. In the present study, we demonstrate that in 3T3-L1 adipocytes, hypoxia induces the disappearance of caveolae and inhibits the expression of Cavin-1 and Cavin-2, two proteins necessary for the formation of caveolae. In mice, hypoxia induced by the ligature of the spermatic artery results in the decrease of cavin-1 and cavin-2 expression in the epididymal adipose tissue. Down-regulation of the expression of cavins in response to hypoxia is dependent on hypoxia-inducible factor-1. Indeed, the inhibition of hypoxia-inducible factor-1 restores the expression of cavins and caveolae formation. Expression of cavins regulates insulin signaling because the silencing of cavin-1 and cavin-2 impairs insulin signaling pathway. In human, cavin-1 and cavin-2 are decreased in the sc adipose tissue of obese diabetic patients compared with lean subjects. Moreover, the expression of cavin-2 correlates negatively with the homeostatic model assessment index of insulin resistance and glycated hemoglobin level. In conclusion, we propose a new mechanism in which hypoxia inhibits cavin-1 and cavin-2 expression, resulting in the disappearance of caveolae. This leads to the inhibition of insulin signaling and the establishment of insulin resistance.

Anti-diabetic effects of brown algae derived phlorotannins, marine polyphenols through diverse mechanisms

Marine algae are popular and abundant food ingredients mainly in Asian countries, and also well known for their health beneficial effects due to the presence of biologically active components. The marine algae have been studied for biologically active components and phlorotannins, marine polyphenols are among them. Among marine algae, brown algae have extensively studied for their potential anti-diabetic activities. Majority of the investigations on phlorotannins derived from brown algae have exhibited their various anti-diabetic mechanisms such as α-glucosidase and α-amylase inhibitory effect, glucose uptake effect in skeletal muscle, protein tyrosine phosphatase 1B (PTP 1B) enzyme inhibition, improvement of insulin sensitivity in type 2 diabetic db/db mice, and protective effect against diabetes complication. In this review, we have made an attempt to discuss the various anti-diabetic mechanisms associated with phlorotannins from brown algae that are confined to in vitro and in vivo.

Rab10 and myosin-Va mediate insulin-stimulated GLUT4 storage vesicle translocation in adipocytes

Rab proteins are important regulators of insulin-stimulated GLUT4 translocation to the plasma membrane (PM), but the precise steps in GLUT4 trafficking modulated by particular Rab proteins remain unclear. Here, we systematically investigate the involvement of Rab proteins in GLUT4 trafficking, focusing on Rab proteins directly mediating GLUT4 storage vesicle (GSV) delivery to the PM. Using dual-color total internal reflection fluorescence (TIRF) microscopy and an insulin-responsive aminopeptidase (IRAP)-pHluorin fusion assay, we demonstrated that Rab10 directly facilitated GSV translocation to and docking at the PM. Rab14 mediated GLUT4 delivery to the PM via endosomal compartments containing transferrin receptor (TfR), whereas Rab4A, Rab4B, and Rab8A recycled GLUT4 through the endosomal system. Myosin-Va associated with GSVs by interacting with Rab10, positioning peripherally recruited GSVs for ultimate fusion. Thus, multiple Rab proteins regulate the trafficking of GLUT4, with Rab10 coordinating with myosin-Va to mediate the final steps of insulin-stimulated GSV translocation to the PM.

Rab4b is a small GTPase involved in the control of the glucose transporter GLUT4 localization in adipocyte

Background

Endosomal small GTPases of the Rab family, among them Rab4a, play an essential role in the control of the glucose transporter GLUT4 trafficking, which is essential for insulin-mediated glucose uptake. We found that adipocytes also expressed Rab4b and we observed a consistent decrease in the expression of Rab4b mRNA in human and mice adipose tissue in obese diabetic states. These results led us to study this poorly characterized Rab member and its potential role in glucose transport.

Methodology/Principal Findings

We used 3T3-L1 adipocytes to study by imaging approaches the localization of Rab4b and to determine the consequence of its down regulation on glucose uptake and endogenous GLUT4 location. We found that Rab4b was localized in endosomal structures in preadipocytes whereas in adipocytes it was localized in GLUT4 and in VAMP2-positive compartments, and also in endosomal compartments containing the transferrin receptor (TfR). When Rab4b expression was decreased with specific siRNAs by two fold, an extent similar to its decrease in obese diabetic subjects, we observed a small increase (25%) in basal deoxyglucose uptake and a more sustained increase (40%) in presence of submaximal and maximal insulin concentrations. This increase occurred without any change in GLUT4 and GLUT1 expression levels and in the insulin signaling pathways. Concomitantly, GLUT4 but not TfR amounts were increased at the plasma membrane of basal and insulin-stimulated adipocytes. GLUT4 seemed to be targeted towards its non-endosomal sequestration compartment.

Conclusion/Significance

Taken our results together, we conclude that Rab4b is a new important player in the control of GLUT4 trafficking in adipocytes and speculate that difference in its expression in obese diabetic states could act as a compensatory effect to minimize the glucose transport defect in their adipocytes.

Insulin action on glucose transporters through molecular switches, tracks and tethers

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. Work done over more than two decades has contributed to mapping the insulin signalling and GLUT4 vesicle trafficking events underpinning this response. In spite of this intensive scientific research, there are outstanding questions that continue to challenge us today. The present review summarizes the knowledge in the field, with emphasis on the latest breakthroughs in insulin signalling at the level of AS160 (Akt substrate of 160 kDa), TBC1D1 (tre-2/USP6, BUB2, cdc16 domain family member 1) and their target Rab proteins; in vesicle trafficking at the level of vesicle mobilization, tethering, docking and fusion with the membrane; and in the participation of the cytoskeleton to achieve optimal temporal and spatial location of insulin-derived signals and GLUT4 vesicles.

Rab proteins in endocytosis and Glut4 trafficking

The intracellular trafficking of numerous proteins requires a tight control to fulfil their physiological functions. It is the case of the adipocyte and muscle glucose transporter Glut4 that is retained intracellularly until insulin induces its recruitment to the plasma membrane. Rabs are evolutionarily conserved small GTPases that control intracellular traffic events from yeast to mammalian cells. In the past few decades, considerable progresses have been made in identifying the route of Glut4, the Rabs involved in controlling it, and more recently the connection between insulin signalling and Glut4 trafficking through Rab activity control.

Small G proteins in insulin action: Rab and Rho families at the crossroads of signal transduction and GLUT4 vesicle traffic

Insulin stimulates glucose uptake into muscle and adipose tissues through glucose transporter 4 (GLUT4). GLUT4 cycles between the intracellular compartments and the plasma membrane. GLUT4 traffic-regulating insulin signals are largely within the insulin receptor-insulin receptor substrate-phosphatidylinositol 3-kinase (IR-IRS-PI3K) axis. In muscle cells, insulin signal bifurcates downstream of the PI3K into one arm leading to the activation of the Ser/Thr kinases Akt and atypical protein kinase C, and another leading to the activation of Rho family protein Rac1 leading to actin remodelling. Activated Akt inactivates AS160, a GTPase-activating protein for Rab family small G proteins. Here we review the roles of Rab and Rho proteins, particularly Rab substrates of AS160 and Rac1, in insulin-stimulated GLUT4 traffic. We discuss: (1) how distinct steps in GLUT4 traffic may be regulated by discrete Rab proteins, and (2) the importance of Rac1 activation in insulin-induced actin remodelling in muscle cells, a key element for the net gain in surface GLUT4.

FIP2 and Rip11 specify Rab11a-mediated cellular distribution of GLUT4 and FAT/CD36 in H9c2-hIR cells

Rab11a has been shown to be involved in different vesicle trafficking processes. To further define the functional role of Rab11a in vesicle movement we knocked down gene expression of Rab11a and two of its effectors, Rip11 and FIP2, in H9c2-hIR cells and measured the cell surface abundance of GLUT4myc and FAT/CD36. We observed that by knocking down Rab11a, both GLUT4myc and FAT/CD36 abundance at the plasma membrane were substantially increased. In the case of GLUT4myc, the in vitro knockdown of FIP2 also increased the cell surface abundance of GLUT4myc. Knockdown of both FIP2 and Rip11 increase the abundance of FAT/CD36 at the plasma membrane. Stimulated translocation of GLUT4myc and FAT/CD36 is not altered after gene knockdown of Rab11a. These data therefore show that (i) Rab11a regulates cell surface abundance of both GLUT4 and FAT/CD36 and that (ii) both Rab11a-dependent processes are differently regulated by Rab11a effector proteins.

A novel method to monitor insulin-stimulated GTP-loading of Rab11a in cardiomyocytes

As a member of the Rab small GTPase family, Rab11a has been shown to be involved in different vesicle trafficking processes. In earlier work we identified Rab11a to be present in GLUT4-containing vesicles after insulin stimulation and showed its involvement in insulin-dependent glucose uptake. However, it remained elusive if Rab11a is directly activated by the insulin signalling cascade and at which step a potential activation occurs. To examine the GTP-loading of Rab11a, we introduced a biotinylated GTP-analog into H9c2-hIR cells, transiently overexpressing HA-tagged Rab11a, and measured its binding to the GTPase after insulin stimulation. We observed that Rab11a is transiently GTP-loaded after insulin stimulation with a 2.3 (+/-0.3) fold activation (n=5), reaching its maximum after 4 min and declining back to basal after additional 2 min. The activation of Rab11a is phosphatidylinositol 3-kinase (PI3-kinase) dependent and downstream of Akt, as shown by in vitro knockdown of this kinase. These data show that Rab11a is directly activated by insulin and represents an element of the GLUT4 trafficking machinery.

Regulation of small GTP-binding proteins by insulin

Several members of the extensive family of small GTP-binding proteins are regulated by insulin, and have been implicated in insulin action on glucose uptake. These proteins are themselves negatively regulated by a series of specific GAPs (GTPase-activating proteins). Interestingly, there is increasing evidence to suggest that PKB (protein kinase B)-dependent phosphorylation of some GAPs may relieve this negative regulation and so lead to the activation of the target small GTP-binding protein. We review recent evidence that this may be the case, and place specific emphasis on the role of these pathways in insulin-stimulated glucose uptake.

Crosstalk between Rab GTPases and cell junctions

For the past several years, studies from other laboratories, as well as ours, have begun to unravel the mechanism of germ cell movement in the testis by using several in vitro and in vivo models of tight and adherens junction assembly and disassembly, two cellular phenomena that confer cell movement. However, for cell movement to be fully appreciated, the importance of "intracellular" cell movements, such as those involving actin and microtubule filaments, must be better understood. Recent research on Rab GTPases has shown that members of this superfamily function in the trafficking of vesicles containing cargo to distinct subcellular sites such as the plasma membrane while utilizing actin and microtubule filaments as tracks. In this mini-review, we provide an overview of Rab GTPase structure, function, and regulation, while placing added emphasis on the role of Rabs in cell junction dynamics in the testis.

AS160, the Akt substrate regulating GLUT4 translocation, has a functional Rab GTPase-activating protein domain

Recently, we described a 160 kDa protein (designated AS160, for Akt substrate of 160 kDa) with a predicted Rab GAP (GTPase-activating protein) domain that is phosphorylated on multiple sites by the protein kinase Akt. Phosphorylation of AS160 in adipocytes is required for insulin-stimulated translocation of the glucose transporter GLUT4 to the plasma membrane. The aim of the present study was to determine whether AS160 is in fact a GAP for Rabs, and, if so, what its specificity is. We first identified a group of 16 Rabs in a preparation of intracellular vesicles containing GLUT4 by MS. We then prepared the recombinant GAP domain of AS160 and examined its activity against many of these Rabs, as well as several others. The GAP domain was active against Rabs 2A, 8A, 10 and 14. There was no significant activity against 14 other Rabs. GAP activity was further validated by the finding that the recombinant GAP domain with the predicted catalytic arginine residue replaced by lysine was inactive. Finally, it was found by immunoblotting that Rabs 2A, 8A and 14 are present in GLUT4 vesicles. These results indicate that AS160 is a Rab GAP, and suggest novel Rabs that may participate in GLUT4 translocation.

Functional role of Rab11 in GLUT4 trafficking in cardiomyocytes

We have recently shown the co-localization of Rab11 and the glucose transporter GLUT4 in cardiac muscle and an insulin-stimulated increase of Rab11 in GLUT4-containing vesicles in this tissue. We now assessed the effect of Rab11 wt and a dominant-negative mutant (N124I) on GLUT4 trafficking in the cardiomyoblast cell line H9c2 stably overexpressing the insulin receptor (H9c2-E2) and in human primary skeletal myotubes. These cells were used for transient cotransfection or adenoviral co-infection with GLUT4myc and Rab11 wt or N124I with subsequent determination of 2-deoxyglucose (2-DOG) uptake and GLUT4myc translocation. Concomitant overexpression of GLUT4myc and Rab11 wt in cardiomyocytes decreased the amount of GLUT4myc at the cell surface by about 50%, an effect not observed for Rab11 N124I. However, the dominant-negative mutant reduced the efficiency of insulin to promote glucose uptake and GLUT4 translocation in both cardiac and skeletal muscle cells to about one half. The level of Akt phosphorylation does not vary after cotransfection indicating that insulin signalling remained unaffected under these conditions. In conclusion, our data show that Rab11 (i) mediates endocytosis of GLUT4 and (ii) plays a pivotal role in insulin-regulated translocation of this transporter to the plasma membrane.

EHD2 interacts with the insulin-responsive glucose transporter (GLUT4) in rat adipocytes and may participate in insulin-induced GLUT4 recruitment

Insulin-induced GLUT4 recruitment to the plasma membrane involves GLUT4 trafficking through multiple subcellular compartments regulated by multiple proteins, many of which are yet to be identified. Here we describe a 65 kDa protein found in purified GLUT4 vesicles of rat adipocytes as a potential GLUT4 traffic regulatory protein. On the basis of MALDI-TOF MS, RT-PCR, gene cloning, protein sequencing, and immunoreactivity data, we identified this protein as EHD2, a member of the EH domain-containing proteins that have been implicated in vesicle trafficking. EHD2 in rat adipocytes was 85% membrane-associated, including approximately 10% in immunopurified GLUT4 vesicles. This association of EHD2 with GLUT4 vesicles occurred in PM and three distinct endosomal fractions and was not significantly affected by cellular insulin treatment. In co-immunoprecipitation experiments, however, EHD2 physically interacted with GLUT4 in each of these fractions, and cellular insulin treatment selectively enhanced this interaction in an endosomal fraction thought to contain GLUT4 exocytic vesicles. EHD2 also interacted with the clathrin adaptor middle chain subunit micro(1), micro(2), and rCALM in GST pull-down experiments. Significantly, an affinity-purified EHD2 antibody and a peptide corresponding to the EHD2 sequence Glu(428)-Glu(535) drastically (by 75% and 35%, respectively) suppressed the insulin-induced increase in the plasma membrane GLUT4 contents in SLO-permeabilized rat adipocytes without affecting the basal GLUT4 distribution. These findings strongly suggest that EHD2 interacts with GLUT4 in rat adipocytes and may play a key role in insulin-induced GLUT4 recruitment to the plasma membrane.

Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Since the discovery of insulin roughly 80 yr ago, much has been learned about how target cells receive, interpret, and respond to this peptide hormone. For example, we now know that insulin activates the tyrosine kinase activity of its cell surface receptor, thereby triggering intracellular signaling cascades that regulate many cellular processes. With respect to glucose homeostasis, these include the function of insulin to suppress hepatic glucose production and to increase glucose uptake in muscle and adipose tissues, the latter resulting from the translocation of the glucose transporter 4 (GLUT4) to the cell surface membrane. Although simple in broad outline, elucidating the molecular intricacies of these receptor-signaling pathways and membrane-trafficking processes continues to challenge the creative ingenuity of scientists, and many questions remain unresolved, or even perhaps unasked. The identification and functional characterization of specific molecules required for both insulin signaling and GLUT4 vesicle trafficking remain key issues in our pursuit of developing specific therapeutic agents to treat and/or prevent this debilitating disease process. To this end, the combined efforts of numerous research groups employing a range of experimental approaches has led to a clearer molecular picture of how insulin regulates the membrane trafficking of GLUT4.

N-Acetylated alpha-linked acidic dipeptidase expressed in rat adipocytes is localized in the insulin-responsive glucose transporter (GLUT4) intracellular compartments and involved in the insulin-stimulated GLUT4 recruitment

The GLUT4-containing vesicles purified from rat adipocyte contain many protein species of unknown identity, some of which are likely to play a critical role in the trafficking of GLUT4. Presently, we describe an 85-kDa protein in GLUT4-vesicles of rat adipocytes as a potential GLUT4 traffic regulatory protein. MALDI-TOF MS, RT-PCR, gene cloning, protein sequence analysis, and immunoreactivity assay have identified this protein as N-acetylated alpha-linked acidic dipeptidase (NAALADase) expressed in rat adipocytes. NAALADase in rat adipocytes was mostly membrane-associated and colocalized in discrete GLUT4-compartments with enrichment in putative GLUT4-sorting endosomes (G4G(L)). Total cell lysates of adipocytes exhibited NAALADase activity. Next, we treated rat adipocytes with 2-[phosphonomethy]pentanedionic acid (2-PMPA), a potent NAALADase inhibitor, and studied its effect on the distribution of GLUT4 and 3-O-methyl glucose (3OMG) flux. In 2-PMPA-treated adipocytes, there was a significant reduction (by 40%) in the insulin-stimulated GLUT4 translocation to the plasma membrane. The 3OMG flux in insulin-stimulated adipocytes was also delayed (51% of control) by 2-PMPA treatment, indicating that 2-PMPA impairs insulin-stimulated GLUT4 recruitment and the uptake of glucose. It is suggested that NAALADase may function as a regulator required for the insulin-stimulated GLUT4 vesicle movement and/or its exocytosis, thus may regulate insulin-induced GLUT4 recruitment in rat adipocytes.

Insulin stimulation of GLUT-4 translocation: a model for regulated recycling

Insulin stimulates glucose transport in muscle and fat cells by causing the redistribution of a facilitative glucose transporter, GLUT-4, from an intracellular compartment to the cell surface. But what is this intracellular GLUT-4 compartment? It may be a specialized compartment, perhaps analogous to synaptic vesicles, or may simply be part of the endosomal system. Other constituents of this compartment might be regulators of GLUT-4 movement to the cell surface, and their identification should make it possible to find the link between the insulin signal transduction pathway and GLUT-4 translocation.

Receptor activation regulates cortical, but not vesicular localization of NDP kinase

We used immunofluorescence techniques to determine the localization of nucleoside diphosphate (NDP) kinase in NIH-3T3 fibroblasts. We found that cytoplasmic NDP kinase can be separated into two populations according to subcellular localization and response to extracellular stimuli. Specifically, within minutes of stimulation of resting fibroblasts with serum, growth factors or bombesin, a portion of NDP kinase becomes associated with membrane ruffles and lamellipodia. Another pool of NDP kinase accumulates independently of stimulation around intracellular vesicles. Transfection of cells with activated Rac mimics, whereas expression of dominant negative Rac inhibits, the effects of extracellular stimulation on the translocation of NDP kinase to the cell cortex. Neither Rac mutant affects the vesicle-associated pool. Association of NDP kinase with vesicles depends on microtubule integrity and is disrupted by nocodazole. In cell-free assays NDP kinase binds tightly to membrane vesicles associated with taxol-stabilized microtubules. Binding of NDP kinase to this fraction is reduced by ATP and abolished by GTP, as well as guanine nucleotides that are NDP kinase substrates. Thus, the localization of the two NDP kinase pools identified here is regulated independently by distinct cellular components: the appearance of cortical NDP kinase is a consequence of Rac activation, whereas vesicular NDP kinase is responsive to microtubule dynamics and nucleotides, in particular GTP. These results suggest that in fibroblasts NDP kinase participates in Rac-related cortical events and in GTP-dependent processes linked to intracellular vesicle trafficking.

Co-localization of rab4 with endocytosis-related proteins in the rat parotid glands

Small GTP-binding proteins have been implicated in the regulation of vesicular traffic. We investigated the localization of Rab4 in the rat parotid glands by Western blotting and light-microscopic immunohistochemistry. Rab4 was localized mainly on the intracellular membranes in the subapical-actin terminal web, but was not present in the basolateral region both in acinar and ductal cells. Actin, alpha-adaptin, Rab5A and aquaporin5 were detected in the Rab4-containing intracellular membrane fraction prepared using anti-Rab4 antibody covalently coupled to magnetic beads. Detection of actin indicated that the Rab4-containing intracellular membranes were attached to the actin filaments. Although alpha-adaptin was immunohistochemically distributed along the plasma membrane, this protein coexisted with Rab4 only at the apical region. Rab5A immunoreactivity was distributed all around the cytoplasm. These findings suggested that Rab4 participates in endocytosis at the apical membrane of parotid glands.

CD2AP/CMS regulates endosome morphology and traffic to the degradative pathway through its interaction with Rab4 and c-Cbl

The small GTPase Rab4 is involved in endocytosis through sorting and recycling early endosomes. To better understand the role of Rab4 in regulation of vesicular trafficking, we searched for effectors that specifically interact with Rab4-Q67L, the GTP-bound form of Rab4. We cloned an ubiquitous 80-kDa protein, identical to CD2-associated protein/Cas ligand with multiple SH3 domains (CD2AP/CMS), that interacts with Rab4-Q67L in the yeast two-hybrid system and in vitro. CD2AP/CMS expressed in mammalian cells was localized to punctate structures and along actin filaments. None of the known markers of early endosomes [Early Endosomes Antigen 1 (EEA1), Rab5 and Rab11] colocalized with the CD2AP/CMS-positive vesicles. However, coexpression of Rab4-Q67L with CD2AP/CMS induces a significant enlargement of EEA1-positive early endosomes. Rab4, CD2AP/CMS and Rab7 colocalized in these modified endosomes. Coexpression of c-Cbl and CD2AP/CMS also resulted in an enlargement of early endosomes. Using various truncated forms of CD2AP/CMS, we demonstrate that early endosomes enlargement requires that CD2AP/CMS interacts with both Rab4 and c-Cbl. The expression of a truncated form of CD2AP/CMS that retains the ability to interact with Rab4 but not c-Cbl inhibits ligand-induced PDGF receptor degradation. We propose that CD2AP/CMS, through interactions with Rab4 and c-Cbl, controls early endosome morphology and may play a role in traffic between early and late endosomes, and thus in the degradative pathway.

Acyl-coenzyme A dehydrogenases are localized on GLUT4-containing vesicles via association with insulin-regulated aminopeptidase in a manner dependent on its dileucine motif

Insulin-regulated aminopeptidase (IRAP, also termed vp165) is known to be localized on the GLUT4-containing vesicles and to be recruited to the plasma membrane after stimulation with insulin. The cytoplasmic region of IRAP contains two dileucine motifs and acidic regions, one of which (amino acid residues 55-82) is reportedly involved in retention of GLUT4-containing vesicles. The region of IRAP fused with glutathione-S-transferase [GST-IRAP(55-82)] was incubated with lysates from 3T3-L1 adipocytes, leading to identification of long-chain, medium-chain, and short-chain acyl-coenzyme A dehydrogenases (ACDs) as the proteins associated with IRAP. The association was nearly abolished by mutation of the dileucine motif of IRAP. Immunoblotting of fractions prepared from sucrose gradient ultracentrifugation and vesicles immunopurified with anti-GLUT4 antibody revealed these ACDs to be localized on GLUT4-containing vesicles. Furthermore, 3-mercaptopropionic acid and hexanoyl-CoA, inhibitors of long-chain and medium-chain ACDs, respectively, induced dissociation of long-chain acyl-coenzyme A dehydrogenase and/or medium-chain acyl-coenzyme A dehydrogenase from IRAP in vitro as well as recruitment of GLUT4 to the plasma membrane and stimulation of glucose transport activity in permeabilized 3T3-L1 adipocytes. These findings suggest that ACDs are localized on GLUT4-containing vesicles via association with IRAP in a manner dependent on its dileucine motif and play a role in retention of GLUT4-containing vesicles to an intracellular compartment.

Isolation and characterization of the two major intracellular Glut4 storage compartments

In rat adipose cells, intracellular Glut4 resides in two distinct vesicular populations one of which contains cellugyrin whereas another lacks this protein (Kupriyanova, T. A., and Kandror, K. V. (2000) J. Biol. Chem. 275, 36263--36268). Cell surface biotinylated MPR and (125)I-labeled transferrin are accumulated in cellugyrin-positive vesicles and to a lesser extent in cellugyrin-negative vesicles. An average cellugyrin-positive vesicle carries not more than one molecule of either Glut4, insulin-responsive aminopeptidase (IRAP), or transferrin receptor (TfR), whereas cellugyrin-negative vesicles contain five to six molecules of Glut4, more than 10 molecules of IRAP, and still one molecule of TfR per vesicle. Cellugyrin-negative vesicles are translocated to the cell surface after insulin stimulation, whereas cellugyrin-positive vesicles maintain intracellular localization both in the absence and in the presence of insulin and, therefore, may be involved in interendosomal protein transport. Both cellugyrin-positive and cellugyrin-negative vesicles are present in extracts of non-homogenized cells and therefore may represent the major form of Glut4 storage in vivo.

Diet and exercise in type 2 diabetes mellitus

The question is no longer whether diet and exercise can benefit the individual with type 2 diabetes. Rather, the type and duration of exercise the magnitude of the effects on glycemic control, insulin sensitivity, and on risk factors for cardiovascular disease must be considered in determining the feasibility and acceptability of an intervention program. It is now clear that regular physical exercise is important in both the prevention and treatment of type 2 diabetes. The benefits of exercise are many and include increased energy expenditure, which, combined with dietary restriction, leads to decreased body fat, increased insulin sensitivity, improved long-term glycemic control, improved lipid profiles, lower blood pressure, and increased cardiovascular fitness. Persons with type 2 diabetes often find it difficult to exercise and are at increased risk for injury or exacerbation of underlying diseases or diabetic complications. Therefore, before starting an exercise program, all patients with type 2 diabetes should have a complete history and physical examination, with particular attention to evaluation of cardiovascular disease, medications that may affect glycemic control during or after exercise, and diabetic complications including retinopathy, nephropathy, and neuropathy. Exercise programs should be designed to start slowly, build up gradually, and emphasize moderately intense exercise performed at least three times a week and preferably five to seven times a week for best results.

The role of small G-proteins in the regulation of glucose transport (review)

Insulin increases the rate of glucose transport into fat and muscle cells by stimulating the translocation of intracellular Glut 4-containing vesicles to the plasma membrane. This results in a marked increase in the amount of the facilitative glucose transporter Glut 4 at the cell surface, allowing for an enhanced glucose uptake. This process requires a continuous cycling through the early endosomes, a Glut 4 specific storage compartment and the plasma membrane. The main effect of insulin is to increase the rate of Glut 4 trafficking from its specific storage compartment to the plasma membrane. The whole phenomenon involves signal transduction from the insulin receptor, vesicle trafficking (sorting and fusion processes) and actin cytoskeleton modifications, which are all supposed to require small GTPases. This review describes the potential role of the various members of the Ras, Rad, Rho, Arf and Rab families in the traffic of the Glut 4-containing vesicles.

GLUT4 ablation in mice results in redistribution of IRAP to the plasma membrane

Glucose transporter (GLUT) 4 is the insulin responsive glucose transporter in adipose tissue, skeletal muscle, and heart. Insulin elicits increased glucose uptake by recruiting GLUT4 from a specialized intracellular storage site to the cell surface. Expression of various proteins that colocalize with GLUT4 and/or are involved in insulin-stimulated GLUT4 translocation was examined in adipocytes as well as skeletal and cardiac muscles from GLUT4 null mice. Our data demonstrate that expression of insulin-regulated aminopeptidase (IRAP) is divergently regulated in GLUT4 null tissues, e.g., upregulated 1.6-fold in GLUT4 null adipocytes and downregulated in GLUT4 null skeletal muscle (40%) and heart (60%). IRAP exhibited abnormal subcellular distribution and impaired insulin-stimulated translocation in GLUT4-deficient tissues. We propose the compartment containing IRAP and proteins normally associated with GLUT4 vesicle traffics constitutively to the cell surface in GLUT4 null adipocytes and skeletal muscle.

Direct interaction of Rab4 with syntaxin 4

In the present study, we examined the possible interaction between Rab4 and syntaxin 4, both having been implicated in insulin-induced GLUT4 translocation. Rab4 and syntaxin 4 were coimmunoprecipitated from the lysates of electrically permeabilized rat adipocytes. The interaction between the two proteins was reduced by insulin treatment and increased by the addition of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). An in vitro binding assay revealed that the bacterially expressed Rab4 was bound to a glutathione S-transferase fusion protein containing the cytoplasmic domain of syntaxin 4 (GST-syntaxin 4-(1-273)) but not to syntaxin 1A or vesicle-associated membrane protein-2. The interaction between Rab4 and syntaxin 4 seemed to be regulated by the guanine nucleotide status of Rab4, because 1) GTPgammaS treatment of the cells significantly increased, but guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS) treatment decreased the amount of Rab4 pulled down with GST-syntaxin 4-(1-273) from the cell lysates; 2) GTPgammaS loading on Rab4 caused a marked increase in the affinity of Rab4 to syntaxin 4 whereas GDPbetaS loading had little effect; and 3) a GTPase-deficient mutant of Rab4 (Rab4(Q67L)), but not a GTP-binding-defective mutant (Rab4(S22N)), was bound to GST-syntaxin 4-(1-273). Although insulin stimulated [gamma-(32)P]GTP binding to Rab4 in a time-dependent fashion, its effect on the Rab4 interaction with syntaxin 4 was apparently biphasic; an initial increase in Rab4 associated with syntaxin 4 was followed by a gradual dissociation of the GTPase from syntaxin 4. Finally, the binding of Rab4(Q67L) to GST-syntaxin 4-(1-273) was inhibited by munc-18c in a dose-dependent manner, indicating that GTP-loaded Rab4 binds to syntaxin 4 in the open conformation. These results suggest that 1) Rab4 interacts with syntaxin 4 in a direct and specific manner, and 2) the interaction is regulated by the guanine nucleotide status of Rab4 as well as by the conformational status of syntaxin 4.

Peroxovanadate induces tyrosine phosphorylation of phosphoinositide-dependent protein kinase-1 potential involvement of src kinase

Phosphoinositide-dependent protein kinase-1 (PDK1) is a recently identified kinase that phosphorylates and activates protein kinase B (PKB). Activation of PKB by insulin is linked to its translocation from the cytosol to the plasma membrane. However, no data are available yet concerning the localization of PDK1 in insulin-sensitive tissue. Using isolated adipocytes, we studied the effect of insulin and of an insulin-mimicking agent peroxovanadate on the subcellular localization of PDK1. In unstimulated adipocytes, overexpressed PDK1 was mostly cytosolic with a low amount associated to membranes. Peroxovanadate stimulation induced the redistribution of PDK1 to the membranes while insulin was without effect. This peroxovanadate effect was dependent on phosphatidylinositol 3,4,5 triphosphate [PtdIns(3,4,5)P3] production as inhibition of PtdIns 3-kinase by wortmannin or deletion of the PH domain of PDK1 prevented the peroxovanadate-induced translocation of PDK1. Further, peroxovanadate-treatment induced a tyrosine phosphorylation of PDK1 which was wortmannin insensitive and did not require the PH domain of PDK1. An inhibitor of Src kinase (PP2) decreased the peroxovanadate-induced PDK1 tyrosine phosphorylation and overexpression of v-Src stimulated this phosphorylation. Mutation of tyrosine 373 of PDK1 abolished the v-Src induced PDK1 tyrosine phosphorylation and partially reduced the effect of peroxovanadate. Our findings suggest that PDK1 could be a substrate for tyrosine kinases and identify Src kinase as one of the tyrosine kinases able to phosphorylate PDK1.

Mechanism and regulation of GLUT-4 vesicle fusion in muscle and fat cells

Twenty years ago it was shown that recruitment of glucose transporters from an internal membrane compartment to the plasma membrane led to increased glucose uptake into fat and muscle cells stimulated by insulin. The final step of this process is the fusion of glucose transporter 4 (GLUT-4)-containing vesicles with the plasma membrane. The identification of a neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex as a requirement for synaptic vesicle-plasma membrane fusion led to the search for homologous complexes outside the nervous system. Indeed, isoforms of the neuronal SNAREs were identified in muscle and fat cells and were shown to be required for GLUT-4 incorporation into the cell membrane. In addition, proteins that bind to nonneuronal SNAREs were cloned and proposed to regulate vesicle fusion. We have summarized the molecular mechanisms leading to membrane fusion in nonneuronal systems, focusing on the role of SNAREs and accessory proteins (Munc18c, synip, Rab4, and VAP-33) in incorporation of GLUT-4 into the plasma membrane. Potential modes of regulation of this process are discussed, including SNARE phosphorylation and interaction with the cytoskeleton.

Rab3D, a small GTP-binding protein implicated in regulated secretion, is associated with the transcytotic pathway in rat hepatocytes

Rab3 isotypes are expressed in regulated secretory cells. Here, we report that rab3D is also expressed in rat hepatocytes, classic models for constitutive secretion. Using reverse transcriptase polymerase chain reaction (RT-PCR) with primers specific for rat rab3D, we amplified a 151 base pair rab3D fragment from total RNA extracted from primary cultures of rat hepatocytes. Immunoblot analysis using polyclonal antibodies to peptides representing the N- and C-terminal hypervariable regions of murine rab3D recognized a protein of approximately 25 kd in hepatocyte lysates, hepatic subcellular fractions, and tissue extracts. The distribution of rab3D was primarily cytosolic; however, only membrane-associated rab3D significantly bound guanosine triphosphate (GTP) in overlay assays. Several lines of investigation indicate that rab3D is associated with the transcytotic pathway. First, rab3D was enriched in a crude vesicle carrier fraction (CVCF), which includes transcytotic carriers. Vesicular compartments immunoisolated from the CVCF on magnetic beads coated with anti-rab3D antibody were enriched in the transcytosed form of the polymeric IgA receptor (pIgA-R), but lacked not only the pIgA-R precursor form associated with the secretory pathway, but also a Golgi marker protein. Second, indirect immunofluorescence on frozen liver sections and in polarized cultured hepatocytes localized rab3D-positive sites at or near the apical plasma membrane and to the pericanalicular cytoplasm. Finally, cholestasis induced by bile duct ligation (BDL), a manipulation known to slow transcytosis, caused rab3D to accumulate in the pericanalicular cytoplasm of cholestatic hepatocytes. Our results indicate that rab3D plays a role in the regulation of apically directed transcytosis in rat hepatocytes.

Association of carboxyl esterase with facilitative glucose transporter isoform 4 (GLUT4) intracellular compartments in rat adipocytes and its possible role in insulin-induced GLUT4 recruitment

Facilitative glucose transporter isoform 4 (GLUT4) in rat adipocytes is largely sequestered in intracellular sites, and insulin recruits GLUT4 from these sites to the cell surface. The process is known to involve multiple intracellular compartments and associated proteins, many of which are yet to be identified. Recently, we purified three distinct insulin-sensitive intracellular GLUT4 compartments (G4T(L), G4H, and G4L) in rat adipocytes and unraveled several new resident proteins in these compartments. Here, we describe one of them, a 62-kDa protein, purified and identified as rat adipose tissue carboxyl esterase (p62/CE) by matrix-assisted laser desorption/ionization time of flight mass spectroscopy, reverse transcription-polymerase chain reaction, gene cloning, and immunological and enzymatic activity measurements. p62/CE in rat adipocytes was 80% cytosolic and 20% microsome-associated. It was found in all of the three insulin-sensitive intracellular GLUT4 compartments, and particularly enriched in G4T(L,) a compartment thought to represent GLUT4 endocytic vesicles. Significantly, an antibody against p62/CE introduced into rat adipocytes completely abolished the insulin-induced GLUT4 recruitment to the plasma membrane in host cells without affecting the basal GLUT4 distribution. Together, these findings suggest that p62/CE plays a key role in insulin-induced GLUT4 recruitment in rat adipocytes, probably by hydrolyzing acylglycerols or acyl-CoA esters to the respective free acids that are required for GLUT4 transport vesicle budding and/or fusion.

An evaluation of the expression, subcellular localization, and function of rab4 in the exocrine pancreas

The small GTP-binding protein, rab4, is involved in recycling of transferrin receptors and translocation of GLUT4. Recent studies suggest that rab4 controls regulated exocytosis in the exocrine pancreas. We conducted the present study to further investigate the role of rab4 in the exocrine pancreas. We found that the exocrine pancreas expresses two rab4 immunoanalogs, one of approximately 28 kDa identified previously in neonatal glands, and one of approximately 24 kDa which is similar to rab4 characterized in other systems. The latter species was mostly membrane-anchored and localized to endosome-like structures in a supranuclear region that was immunopositive for the transferrin receptor. The approximately 24-kDa rab4 form also localized to the apical plasmamembrane, and this immunofluorescence increased greatly in tissue challenged with a secretagogue. We propose that the approximately 24-kDa rab4 species is involved in compensatory membrane retrieval following regulated exocytosis, and that rab4-positive endocytic vesicles move through a supranuclear recycling compartment.

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.

Separation and partial characterization of three distinct intracellular GLUT4 compartments in rat adipocytes. Subcellular fractionation without homogenization

Insulin recruits GLUT4 from an intracellular location to the plasma membrane in rat adipocytes. The process involves multiple intracellular compartments and multiple protein functions, details of which are largely unknown partly due to our inability to separate individual GLUT4 compartments. Here, by hypotonic lysis, differential centrifugation, and glycerol density gradient sedimentation, we separated intracellular GLUT4 compartments in rat adipocytes into three fractions: plasma membrane-containing fraction T and plasma membrane-free fractions H and L. The GLUT4 contents in fractions T, H, and L were approximately 25, 56, and 18% of total GLUT4, respectively, in basal adipocytes and 55, 42, and 3-4% in insulin-stimulated adipocytes. The plasma membrane GLUT4 contents estimated separately further revealed that intracellular GLUT4 in fraction T amounts to approximately 20% in both basal and insulin-stimulated adipocytes. Organelle-specific marker and membrane traffic-related protein distribution data suggested that intracellular GLUT4 in fraction T represents sorting endosomes, whereas GLUT4 in fractions H and L represents storage endosomes and exocytic vesicles, respectively. The subcellular fractionation without homogenization described here should be useful in identifying the role of the individual GLUT4 compartments and the associated proteins in insulin-induced GLUT4 recruitment in rat adipocytes.

Role of the actin cytoskeleton in insulin action

Insulin has diverse effects on cells, including stimulation of glucose transport, gene expression, and alterations of cell morphology. The hormone mediates these effects by activation of signaling pathways which utilize, 1) adaptor molecules such as the insulin receptor substrates (IRS), the Src and collagen homologs (Shc), and the growth factor receptor binding protein 2 (Grb2); 2) lipid kinases such as phosphatidylinositol 3-kinase (PI 3-Kinase); 3) small G proteins; and 4) serine, threonine, and tyrosine kinases. The activation of such signaling molecules by insulin is now well established, but we do not yet fully understand the mechanisms integrating these seemingly diverse pathways. Here, we discuss the involvement of the actin cytoskeleton in the propagation and regulation of insulin signals. In muscle cells in culture, insulin induces a rapid actin filament reorganization that coincides with plasma membrane ruffling and intense accumulation of pinocytotic vesicles. Initiation of these effects of insulin requires an intact actin cytoskeleton and activation of PI 3-kinase. We observed recruitment PI 3-kinase subunits and glucose transporter proteins to regions of reorganized actin. In both muscle and adipose cells, actin disassembly inhibited early insulin-induced events such as recruitment of glucose transporters to the cell surface and enhanced glucose transport. Additionally, actin disassembly inhibited more prolonged effects of insulin, including DNA synthesis and expression of immediate early genes such as c-fos. Intact actin filaments appear to be essential for mediation of early events such as association of Shc with Grb2 in response to insulin, which leads to stimulation of gene expression. Preliminary observations support a role for focal adhesion signaling complexes in insulin action. These observations suggest that the actin cytoskeleton facilitates propagation of the morphological, metabolic, and nuclear effects of insulin by regulating proper subcellular distribution of signaling molecules that participate in the insulin signaling pathway.

Expression and localization of Rab3D in rat parotid gland

Rab3 proteins (isoforms A, B, C and D) are low molecular weight GTP-binding proteins proposed to be involved in regulated exocytosis. In the present study, Rab3 protein expression and localization was examined in rat parotid gland by reverse transcription (rt) PCR, Western blotting and immunocytochemistry. An approximately 200 bp PCR product was obtained from parotid RNA by rtPCR and this fragment was cloned and sequenced. Nucleotide and deduced amino acid sequences obtained from five clones were identical to rab3D. Membrane and cytosolic fractions prepared from parotid acini were immunoblotted with antisera specific for each of the four Rab3 isoforms. A 28 kDa protein was detected with Rab3D-specific antisera in both fractions with staining being more intense in the membrane fraction. No other Rab3 isoforms were detected by immunoblotting, a result consistent with those obtained by rtPCR. Rab3D was enriched in zymogen granule membranes and Triton X-114 extraction revealed that this isoform is predominantly lipid-modified in parotid. Localization of Rab3D was done on frozen sections of parotid gland by immunofluorescence microscopy. Staining was observed primarily in the acinar cells and was adjacent to the acinar lumen. Incubation of dispersed acini with isoproterenol and substance P stimulated amylase secretion 4- and 2-fold above basal, respectively. Isoproterenol, but not substance P, induced redistribution of Rab3D from the cytosol to the membrane fraction in dispersed parotid acini. Consistent with these findings, isoproterenol injections into fasted rats also resulted in increased membrane-associated Rab3D in the parotid acini. These results indicate that Rab3D is: (1) the major Rab3 isoform expressed in rat parotid gland; (2) localized to zymogen granule membranes; and (3) involved with regulated enzyme secretion in acinar cells.

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.

From insulin receptor signalling to Glut 4 translocation abnormalities in obesity and insulin resistance

Insulin resistance is commonly associated with obesity in rodents. Using mice made obese with goldthioglucose (GTG-obese mice), we have shown that insulin resistance results from defects at the level of the receptor and from intracellular alterations in insulin signalling pathway, without major alteration in the number of the Glut 4 glucose transporter. Activation of phosphatidylinositol 3-kinase (PI 3-kinase) was found to be profoundly affected in response to insulin. This defect appears very early in the development of obesity, together with a marked decrease in IRS 1 tyrosine phosphorylation. In order to better understand the abnormalities in glucose transport in insulin resistance, we have studied the pathway leading from the insulin receptor kinase stimulation to the translocation of the Glut 4 containing vesicles. This stimulation involves the activation of PI 3-kinase, which in turns activates protein kinase B. We have then focussed at the mechanism of vesicle exocytosis, and more specifically at the role of the small GTPase Rab4 in this process. We have shown that Rab4 participates, first in the intracellular retention of the Glut 4 containing vesicles, second in the insulin signalling pathway leading to glucose transporter translocation.

Early steps in insulin action

The biological effects of insulin are initiated by the binding of insulin to the insulin receptor. Insulin binds to the extracellular domain of the insulin receptor and induces conformational changes in the receptor, leading to autophosphorylation of the receptor on intracellular tyrosine residues. These phosphorylated tyrosine residues act as binding sites for proteins which subsequently may be phosphorylated by the insulin receptor. As a result, yet other proteins can be recruited to form larger complexes and, in the case of enzymes, changes in their activity may take place. By a combination of these processes, the activated insulin receptor initiates cascades of biochemical events which are regulated mainly by specific phosphorylation or dephosphorylation reactions. Intermediates which are involved in the normal insulin signalling pathway are subjects of expanding research.