Insulin recruits GLUT4 from distinct compartments via distinct traffic pathways with differential microtubule dependence in rat adipocytes

In the present study, we investigated the physiological significance of the microtubules in the subcellular localization and trafficking of GLUT4 in rat primary adipocytes. Morphological and biochemical analyses revealed a dose- and time-dependent disruption of the microtubules by treatment with nocodazole. With nearly complete disruption of the microtubules, the insulin-stimulated glucose transport activity was inhibited by 55%. This inhibition was concomitant with a comparable inhibition of GLUT4 translocation measured by the subcellular fractionation and the cell-surface GLUT4 labeling by trypsin cleavage. In addition, the time-course of insulin stimulation of the glucose transport activity was significantly delayed by microtubule disruption (t(1/2) were 7 and 2.3 min in nocodazole-treated and control cells, respectively), while the rate of GLUT4 endocytosis was little affected. The impaired insulin-stimulated glucose transport activity was not fully restored to the level in control cells by blocking GLUT4 endocytosis, suggesting that the inhibition was due to the existence of a microtubule-dependent subpopulation in the insulin-responsive GLUT4 pool. On the other hand, nocodazole partially inhibited insulin-induced translocation of the insulin-regulated aminopeptidase and the vesicle-associated membrane protein (VAMP)-2 without affecting GLUT1 and VAMP-3. In electrically permeabilized adipocytes, the insulin-stimulated glucose transport was inhibited by 40% by disruption of the microtubules whereas that stimulated with GTP gamma S was not affected. Intriguingly, the two reagents stimulated glucose transport to the comparable level by disruption of the microtubules. These data suggest that insulin recruits GLUT4 to the plasma membrane from at least two distinct intracellular compartments via distinct traffic routes with differential microtubule dependence in rat primary adipocytes.     

Mapping of R-SNARE function at distinct intracellular GLUT4 trafficking steps in adipocytes

The functional trafficking steps used by soluble NSF attachment protein receptor (SNARE) proteins have been difficult to establish because of substantial overlap in subcellular localization and because in vitro SNARE-dependent binding and fusion reactions can be promiscuous. Therefore, to functionally identify the site of action of the vesicle-associated membrane protein (VAMP) family of R-SNAREs, we have taken advantage of the temporal requirements of adipocyte biosynthetic sorting of a dual-tagged GLUT4 reporter (myc-GLUT4-GFP) coupled with small interfering RNA gene silencing. Using this approach, we confirm the requirement of VAMP2 and VAMP7 for insulin and osmotic shock trafficking from the vesicle storage sites, respectively, and fusion with the plasma membrane. Moreover, we identify a requirement for VAMP4 for the initial biosynthetic entry of GLUT4 from the Golgi apparatus into the insulin-responsive vesicle compartment, VAMP8, for plasma membrane endocytosis and VAMP2 for sorting to the specialized insulin-responsive compartment after plasma membrane endocytosis.     

Modulation of GLUT4 and GLUT1 recycling by insulin in rat adipocytes: kinetic analysis based on the involvement of multiple intracellular compartments

The trafficking kinetics of GLUT4 and GLUT1 in rat epididymal adipocytes were analyzed by a four-compartment model based upon steady-state pool sizes of three intracellular fractions and one plasma membrane fraction separated and assessed under both basal and insulin-stimulated states. The steady-state compartment sizes provided relative values of the kinetic coefficients characterizing the rate of each process in the loop. Absolute values of these coefficients were obtained by matching the simulated half-times to those observed experimentally and reported in the literature for both basal and insulin-stimulated states. Our analysis revealed that insulin modulates the GLUT4 trafficking at multiple steps in the rat adipocyte, not only reducing the endocytotic rate constant 3-4-fold and increasing the exocytotic rate 8-24-fold but also increasing the two rate coefficients coupling the three intracellular compartments 2-6-fold each. Furthermore, GLUT1 was completely segregated from GLUT4 in two of the three intracellular compartments, and its steady-state distribution is consistent with a four-compartment model of GLUT1 recycling involving an insulin sensitive endocytosis step in common with the GLUT4 system, but with all other processes being insensitive to insulin.     

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 stimulates actin comet tails on intracellular GLUT4-containing compartments in differentiated 3T3L1 adipocytes

Incubation of isolated GLUT4-containing vesicles with Xenopus oocyte extracts resulted in a guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) and sodium orthovanadate stimulation of actin comet tails. The in vitro actin-based GLUT4 vesicle motility was inhibited by both latrunculin B and a dominant-interfering N-WASP mutant, N-WASP/Delta VCA. Preparations of gently sheared (broken) 3T3L1 adipocytes also displayed GTP gamma S and sodium orthovanadate stimulation of actin comet tails on GLUT4 intracellular compartments. Furthermore, insulin pretreatment of intact adipocytes prior to gently shearing also resulted in a marked increase in actin polymerization and actin comet tailing on GLUT4 vesicles. In addition, the insulin stimulation of actin comet tails was completely inhibited by Clostridum difficile toxin B, demonstrating a specific role for a Rho family member small GTP-binding protein. Expression of N-WASP/Delta VCA in intact cells had little effect on adipocyte cortical actin but partially inhibited insulin-stimulated GLUT4 translocation. Taken together, these data demonstrate that insulin can induce GLUT4 vesicle actin comet tails that are necessary for the efficient translocation of GLUT4 from intracellular storage sites to the plasma membrane.     

GLUT4 retention in adipocytes requires two intracellular insulin-regulated transport steps

Insulin regulates glucose uptake into fat and muscle by modulating the distribution of the GLUT4 glucose transporter between the surface and interior of cells. The GLUT4 trafficking pathway overlaps with the general endocytic recycling pathway, but the degree and functional significance of the overlap are not known. In this study of intact adipocytes, we demonstrate, by using a compartment-specific fluorescence-quenching assay, that GLUT4 is equally distributed between two intracellular pools: the transferrin receptor-containing endosomes and a specialized compartment that excludes the transferrin receptor. These pools of GLUT4 are in dynamic communication with one another and with the cell surface. Insulin-induced redistribution of GLUT4 to the surface requires mobilization of both pools. These data establish a role for the general endosomal system in the specialized, insulin-regulated trafficking of GLUT4. Trafficking through the general endosomal system is regulated by rab11. Herein, we show that rab11 is required for the transport of GLUT4 from endosomes to the specialized compartment and for the insulin-induced translocation to the cell surface, emphasizing the importance of the general endosomal pathway in the specialized trafficking of GLUT4. Based on these findings we propose a two-step model for GLUT4 trafficking in which the general endosomal recycling compartment plays a specialized role in the insulin-regulated traffic of GLUT4. This compartment-based model provides the framework for understanding insulin-regulated trafficking at a molecular level.     

Transient changes in four GLUT4 compartments in rat adipocytes during the transition,insulin-stimulated to basal: implications for the GLUT4 trafficking pathway

In rat adipocytes, insulin-induced GLUT4 recruitment to the plasma membrane (PM) is associated with characteristic changes in the GLUT4 contents of three distinct endosomal fractions, T, H, and L. The organelle-specific marker distribution pattern suggests that these endosomal GLUT4 compartments are sorting endosomes (SR), GLUT4-storage endosomes (ST), and GLUT4 exocytotic vesicules (EV), respectively, prompting us to analyze GLUT4 recycling based upon a four-compartment kinetic model. Our analysis revealed that insulin modulates GLUT4 trafficking at multiple steps, including not only the endocytotic and exocytotic rates, but also the two rate coefficients coupling the three intracellular compartments. This analysis assumes that GLUT4 cycles through PM T, H,L, and back to PM, in that order, with transitions characterized by four first-order coefficients. Values assigned to these coefficients are based upon the four steady-state GLUT4 pool sizes assessed under both basal and insulin stimulated states and the transition time courses observed in the plasma membrane GLUT4 pool. Here we present the first reported experimental measurements of transient changes in each of the four GLUT4 compartments during the insulin-stimulated to basal transition in rat adipocytes and compare these experimental results with the corresponding model simulations. The close correlation of these results offers clear support for the general validity of the assumed model structure and the assignment of the T compartment to the sorting endosome GLUT4 pool. Variations in the recycling pathway from that of an unbranched cyclic topography are also considered in the light of these experimental observations. The possibility that H is a coupled GLUT4 storage compartment lying outside the direct cyclic pathway is contraindicated by the data. Okadaic acid-induced GLUT4 recruitment is accompanied by modulation of the rate coefficients linking individual endosomal GLUT4 compartments, further demonstrating a significant role of the endosomal pathways in GLUT4 exocytosis.     

Proteinase inhibitors in rat serum. Purification and partial characterization of three functionally distinct trypsin inhibitors.

Three different serine proteinase inhibitors were isolated from rat serum and purified to apparent homogeneity. One of the inhibitors appears to be homologous to alpha 1-proteinase inhibitor isolated from man and other species, but the other two, designated rat proteinase inhibitor I and rat proteinase inhibitor II, seem to have no human counterpart. alpha 1-Proteinase inhibitor (Mr 55000) inhibits trypsin, chymotrypsin and elastase, the three serine proteinases tested. Rat proteinase inhibitor I (Mr 66000) is active towards trypsin and chymotrypsin, but is inactive towards elastase. Rat proteinase inhibitor II (Mr 65000) is an effective inhibitor of trypsin only. Their contributions to the trypsin-inhibitory capacity of rat serum are about 68, 14 and 18% for alpha 1-proteinase inhibitor, rat proteinase inhibitor I and rat proteinase inhibitor II respectively.     

Subcellular fractionation, partial purification and characterization of neutral triacylglycerol lipase from pig liver.

The subcellular distributions of acidic (pH 4.5) and neutral (pH 7.5) longchain triacylglycerol lipases (glycerol ester hydrolase, EC 3.1.1.3) of pig liver have been determined. The distribution of the acidic lipase closely paralleled that of the lysosomal marker enzyme, cathepsin D. Approx. 60% of the neutral lipolytic activity resided in the soluble fraction;the distribution of this activity failed to parallel that of marker enzymes for mitochondria, lysosomes, microsomes, or plasma membranes. A method has been developed for purification of the neutral lipase from the soluble fraction by ultracentrifugation. An approximate 90-fold purification was achieved, with recovery of 16% of the initial activity. The partially purified neutral lipase exhibited a pH optimum between 7.25 and 7.5. It required 30 mM emulsified triolein for optimal activity and ceased to liberate fatty acids after 30 min of incubation. The enzymatic activity was destroyed by heating at 60 degrees C. Neutral lipase was inhibited by sodium deoxycholate, Triton X-100 and iodoacetamide. The activity was not inhibited by sodium taurocholate, EDTA, heparin and diethyl-p-nitrophenyl phosphate. Neutral lipase failed to exhibit activity in assay systems specific for lipoprotein lipase, monoolein hydrolase, tributyrinase, and methyl butyrate esterase and showed little or no capacity to hydrolyze chyle chylomicrons or plasma very low density lipoproteins. It is suggested that the function of neutral lipase may be to supply the liver with fatty acids liberated from endogenously synthesized or stored triacylglycerols.     

Separation of insulin signaling into distinct GLUT4 translocation and activation steps

GLUT4 (glucose transporter 4) plays a pivotal role in insulin-induced glucose uptake to maintain normal blood glucose levels. Here, we report that a cell-permeable phosphoinositide-binding peptide induced GLUT4 translocation to the plasma membrane without inhibiting IRAP (insulin-responsive aminopeptidase) endocytosis. However, unlike insulin treatment, the peptide treatment did not increase glucose uptake in 3T3-L1 adipocytes, indicating that GLUT4 translocation and activation are separate events. GLUT4 activation can occur at the plasma membrane, since insulin was able to increase glucose uptake with a shorter time lag when inactive GLUT4 was first translocated to the plasma membrane by pretreating the cells with this peptide. Inhibition of phosphatidylinositol (PI) 3-kinase activity failed to inhibit GLUT4 translocation by the peptide but did inhibit glucose uptake when insulin was added following peptide treatment. Insulin, but not the peptide, stimulated GLUT1 translocation. Surprisingly, the peptide pretreatment inhibited insulin-induced GLUT1 translocation, suggesting that the peptide treatment has both a stimulatory effect on GLUT4 translocation and an inhibitory effect on insulin-induced GLUT1 translocation. These results suggest that GLUT4 requires translocation to the plasma membrane, as well as activation at the plasma membrane, to initiate glucose uptake, and both of these steps normally require PI 3-kinase activation.     

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.     

Effects of insulin on steady state kinetics of GLUT4 subcellular distribution in rat adipocytes. Evidence of constitutive GLUT4 recycling.

We labeled rat adipocyte cell surface glucose transporters with an impermeable, photoreactive glucose analogue, 1,3-bis-(3-deoxy-D-glucopyranose-3-yloxy)-2-propyl 4-benzoylbenzoate (B3GL) and its radioactive tracer [3H]B3GL. The labeling did not affect glucose transporter subcellular distribution in basal and insulin-stimulated adipocytes. When basal or insulin-stimulated adipocytes were labeled with [3H]B3GL and incubated at 37 degrees C in steady state, labeled GLUT4 was rapidly reduced at the cell surface and stoichiometrically recovered in microsomes without any change in GLUT4 protein levels in either pool. The labeled GLUT4 equilibrium exchange was found to be a simple first order process describable by two first order rate constants, one for internalization (k(in)) and the other for externalization (kex). Insulin affected both rate constants, reducing k(in) by 2.8-fold and increasing kex by 3.3-fold. It is concluded that GLUT4 constantly and rapidly recycles in adipocytes between the cell surface and its storage pool, and insulin increases the cell surface GLUT4 level in rat adipocytes by modulating both the internalization and the externalization steps of constitutively recycling GLUT4.     

Dehydroascorbic acid transport by GLUT4 in Xenopus oocytes and isolated rat adipocytes

Dehydroascorbic acid (DHA), the first stable oxidation product of vitamin C, was transported by GLUT1 and GLUT3 in Xenopus laevis oocytes with transport rates similar to that of 2-deoxyglucose (2-DG), but due to inherent difficulties with GLUT4 expression in oocytes it was uncertain whether GLUT4 transported DHA (Rumsey, S. C. , Kwon, O., Xu, G. W., Burant, C. F., Simpson, I., and Levine, M. (1997) J. Biol. Chem. 272, 18982-18989). We therefore studied DHA and 2-DG transport in rat adipocytes, which express GLUT4. Without insulin, rat adipocytes transported 2-DG 2-3-fold faster than DHA. Preincubation with insulin (0.67 micrometer) increased transport of each substrate similarly: 7-10-fold for 2-DG and 6-8-fold for DHA. Because intracellular reduction of DHA in adipocytes was complete before and after insulin stimulation, increased transport of DHA was not explained by increased internal reduction of DHA to ascorbate. To determine apparent transport kinetics of GLUT4 for DHA, GLUT4 expression in Xenopus oocytes was reexamined. Preincubation of oocytes for >4 h with insulin (1 micrometer) augmented GLUT4 transport of 2-DG and DHA by up to 5-fold. Transport of both substrates was inhibited by cytochalasin B and displayed saturable kinetics. GLUT4 had a higher apparent transport affinity (K(m) of 0.98 versus 5.2 mm) and lower maximal transport rate (V(max) of 66 versus 880 pmol/oocyte/10 min) for DHA compared with 2-DG. The lower transport rate for DHA could not be explained by binding differences at the outer membrane face, as shown by inhibition with ethylidene glucose, or by transporter trans-activation and therefore was probably due to substrate-specific differences in transporter/substrate translocation or release. These novel data indicate that the insulin-sensitive transporter GLUT4 transports DHA in both rat adipocytes and Xenopus oocytes. Alterations of this mechanism in diabetes could have clinical implications for ascorbate utilization.     

Two compartments for insulin-stimulated exocytosis in 3T3-L1 adipocytes defined by endogenous ACRP30 and GLUT4.

Insulin stimulates adipose cells both to secrete proteins and to translocate the GLUT4 glucose transporter from an intracellular compartment to the plasma membrane. We demonstrate that whereas insulin stimulation of 3T3-L1 adipocytes has no effect on secretion of the alpha3 chain of type VI collagen, secretion of the protein hormone adipocyte complement related protein of 30 kD (ACRP30) is markedly enhanced. Like GLUT4, regulated exocytosis of ACRP30 appears to require phosphatidylinositol-3-kinase activity, since insulin-stimulated ACRP30 secretion is blocked by pharmacologic inhibitors of this enzyme. Thus, 3T3-L1 adipocytes possess a regulated secretory compartment containing ACRP30. Whether GLUT4 recycles to such a compartment has been controversial. We present deconvolution immunofluorescence microscopy data demonstrating that the subcellular distributions of ACRP30 and GLUT4 are distinct and nonoverlapping; in contrast, those of GLUT4 and the transferrin receptor overlap. Together with supporting evidence that GLUT4 does not recycle to a secretory compartment via the trans-Golgi network, we conclude that there are at least two compartments that undergo insulin-stimulated exocytosis in 3T3-L1 adipocytes: one for ACRP30 secretion and one for GLUT4 translocation.     

Subcellular localization and partial characterization of insulin proteolytic activity in rat liver

Using (A14-125I)-insulin as a tracer, insulin proteolytic activity in rat liver was found to be localized both to the cytosol and the endoplasmic reticulum. The membrane-associated activity was highly latent (70-80%). Both cytosolic and particulate activities had similar Km values and Mr of approx. 300 000 by gel filtration. Both were strongly inhibited by diamide (90%), but were unaffected by leupeptin or pepstatin. A comparison of the subcellular distributions with various 125I-isomers of insulin as tracers showed that both particulate and cytosolic activities were highest with (A14-125I)-insulin.     

Syntaxin 16 controls the intracellular sequestration of GLUT4 in 3T3-L1 adipocytes

The regulated delivery of Glut4-containing vesicles to the plasma membrane is a specialised example of regulated membrane trafficking. Present models favour the transporter trafficking through two inter-related endosomal cycles. The first is the proto-typical endosomal system. This is a fast trafficking event that, in the absence of insulin, serves to internalise Glut4 from the plasma membrane. Once in this pathway, Glut4 is further sorted into a slowly recycling pathway that operates between recycling endosomes, the trans Golgi network, and a population of vesicles often referred to as Glut4-storage vesicles. Little is known about the molecules that regulate these distinct sorting steps. Here, we have studied the role of Stx16 in Glut4 trafficking. Using two independent strategies, we show that Stx16 plays a crucial role in Glut4 traffic in 3T3-L1 adipocytes. Over-expression of a mutant form of Stx16 devoid of a transmembrane anchor was found to significantly slow the reversal of insulin-stimulated glucose transport. Depletion of Stx16 using antisense approaches profoundly reduced insulin-stimulated glucose transport but was without effect on cell surface transferrin receptor levels, and also reduced the extent of Glut4 translocation to the plasma membrane in response to insulin. These data support a model in which Stx16 is crucial in the sorting of Glut4 from the fast cycling to the slow cycling intracellular trafficking pathways in adipocytes.     

Phenylarsine oxide causes an insulin-dependent, GLUT4-specific degradation in rat adipocytes

An incubation of rat adipocytes with phenylarsine oxide (PAO) and then with insulin caused an inhibition of 3-O-methylglucose equilibrium exchange flux and a parallel reduction in cellular GLUT4 content detected by Western blots. Both the transport inhibition and the GLUT4 reduction were saturable with an increasing concentration of PAO showing essentially an identical Ki value of 35 microM. Both effects were not observed in the absence of insulin or if cells were incubated with insulin first. The reduction was specific to GLUT4; the immunoreactivities of GLUT1, insulin receptor, and clathrin were not affected in these experiments. The GLUT4 reduction occurred only in intact cells and was not observed in homogenized cells or fractionated membranes. GLUT4 in both the microsomal storage pool and the plasma membrane pool were affected with no indication of insulin-induced recruitment impairment. GLUT4 reduction was not observed in the presence of chloroquine or at 18 degrees C suggesting involvement of the lysosomal pathway. Based on these results, we propose that there is a PAO-sensitive protein mechanism that controls an insulin-dependent GLUT4 degradation pathway in adipocytes. This protein mechanism and the GLUT4 degradation pathway may play an important role in determining the steady-state GLUT4 level in the insulin-sensitive peripheral tissues in normal and diseased states.     

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.     

Cadmium induces impaired glucose tolerance in rat by down-regulating GLUT4 expression in adipocytes

Cadmium (Cd) has been known to cause hyperglycemia with diabetes-related complications in experimental animals; however, the molecular basis underlying this Cd-induced hyperglycemia is not known. Here, we report the novel finding that the impaired glucose tolerance (IGT) in rats induced by CdCl(2) is accompanied by a drastic (by as much as 90%) and dose-dependent reduction in GLUT4 protein and GLUT4 mRNA levels in adipocytes. The effect was specific to GLUT4; neither GLUT1 nor insulin-responsive aminopeptidase in adipocytes was affected. GLUT2 in hepatocytes was also not affected. Interestingly, the effect on GLUT4 was also specific to adipocytes; the muscle tissues of the Cd-treated rats showed only a slight (<25%) reduction in GLUT4 protein level with no change in GLUT4 message level, and again with no change in GLUT1 protein and its message levels. Although the insulin-induced GLUT4 translocation in adipocytes was not affected by the Cd treatment, the 3-O-methy-D-glucose flux in insulin-stimulated adipocytes of Cd-treated rat was drastically reduced. Together these findings clearly demonstrate that Cd induces IGT in rats by selectively down-regulating GLUT4 expression in adipocytes.