Osmotic shock stimulates GLUT4 translocation in 3T3L1 adipocytes by a novel tyrosine kinase pathway

Insulin acutely increases glucose transporter 1 on plasma membranes and glucose uptake in an AKT-dependent manner in chicken adipocytes

Avian glucose transporters (GLUT) responsible for insulin-responsive glucose uptake into adipocytes remain poorly characterized. We aimed to identify the insulin-responsive GLUT using primary culture of chicken adipocytes. Acute stimulation with 1 μM insulin for 20 min increased 2-deoxyglucose uptake, AKT protein phosphorylation, and GLUT1 protein levels on the plasma membrane of the chicken adipocytes, whereas pretreatment with 10 μM triciribine, an AKT inhibitor, canceled these effects. Furthermore, the insulin stimulation did not affect GLUT12 protein levels on the plasma membrane of the chicken adipocytes. Our results suggest that GLUT1 is an insulin-responsive GLUT in chicken adipocytes.

GLUT12 expression and regulation in murine small intestine and human Caco-2 cells

GLUT12 was cloned from the mammary cancer cell line MCF-7, but its physiological role still needs to be elucidated. To gain more knowledge of GLUT12 function in the intestine, we investigated GLUT12 subcellular localization in the small intestine and its regulation by sugars, hormones, and intracellular mediators in Caco-2 cells and mice. Immunohistochemical methods were used to determine GLUT12 subcellular localization in human and murine small intestine. Brush border membrane vesicles were isolated for western blot analyses. Functional studies were performed in Caco-2 cells by measuring α-methyl-d-glucose (αMG) uptake in the absence of sodium. GLUT12 is located in the apical cytoplasm, below the brush border membrane, and in the perinuclear region of murine and human enterocytes. In Caco-2 cells, GLUT12 translocation to the apical membrane and α-methyl- d-glucose uptake by the transporter are stimulated by protons, glucose, insulin, tumor necrosis factor-α (TNF-α), protein kinase C, and AMP-activated protein kinase. In contrast, hypoxia decreases GLUT12 expression in the apical membrane. Upregulation of TNF-α and hypoxia-inducible factor-1α ( HIF-1α) genes is found in the jejunal mucosa of diet-induced obese mice. In these animals, GLUT12 expression in the brush border membrane is slightly decreased compared with lean animals. Moreover, an intraperitoneal injection of insulin does not induce GLUT12 translocation to the membrane, as it occurs in lean animals. GLUT12 rapid translocation to the enterocytes' apical membrane in response to glucose and insulin could be related to GLUT12 participation in sugar absorption during postprandial periods. In obesity, in which insulin sensitivity is reduced, the contribution of GLUT12 to sugar absorption is affected.

Isoalantolactone derivative promotes glucose utilization in skeletal muscle cells and increases energy expenditure in db/db mice via activating AMPK-dependent signaling

Augmenting glucose utilization and energy expenditure in skeletal muscle via AMP-activated protein kinase (AMPK) is an imperative mechanism for the management of type 2 diabetes. Chemical derivatives (2a-2h, 3, 4a-4d, 5) of the isoalantolactone (K007), a bioactive molecule from roots of Inula racemosa were synthesized to optimize the bioactivity profile to stimulate glucose utilization in skeletal muscle cells. Interestingly, 4a augmented glucose uptake, driven by enhanced translocation of glucose transporter 4 (GLUT4) to cell periphery in L6 rat skeletal muscle cells. The effect of 4a was independent to phosphatidylinositide-3-kinase (PI-3-K)/Akt pathway, but mediated through Liver kinase B1 (LKB1)/AMPK-dependent signaling, leading to activation of downstream targets acetyl coenzyme A carboxylase (ACC) and sterol regulatory element binding protein 1c (SREBP-1c). In db/db mice, 4a administration decreased blood glucose level and improved body mass index, lipid parameters and glucose tolerance associated with elevation of GLUT4 expression in skeletal muscle. Moreover, 4a increased energy expenditure via activating substrate utilization and upregulated the expression of thermogenic transcription factors and mitochondrial proteins in skeletal muscle, suggesting the regulation of energy balance. These findings suggest the potential implication of isoalantolactone derivatives for the management of diabetes.

Transient activation of fission yeast AMPK is required for cell proliferation during osmotic stress

Transient activation of the cellular energy sensor AMPK during osmotic stress requires its energy-sensing subunit. Cellular ATP levels decrease during osmotic stress, which triggers energy stress, which in turn requires dynamic activation of AMPK.

The heterotrimeric kinase AMPK acts as an energy sensor to coordinate cell metabolism with environmental status in species from yeast through humans. Low intracellular ATP leads to AMPK activation through phosphorylation of the activation loop within the catalytic subunit. Other environmental stresses also activate AMPK, but it is unclear whether cellular energy status affects AMPK activation under these conditions. Fission yeast AMPK catalytic subunit Ssp2 is phosphorylated at Thr-189 by the upstream kinase Ssp1 in low-glucose conditions, similar to other systems. Here we find that hyperosmotic stress induces strong phosphorylation of Ssp2-T189 by Ssp1. Ssp2-pT189 during osmotic stress is transient and leads to transient regulation of AMPK targets, unlike sustained activation by low glucose. Cells lacking this activation mechanism fail to proliferate after hyperosmotic stress. Activation during osmotic stress requires energy sensing by AMPK heterotrimer, and osmotic stress leads to decreased intracellular ATP levels. We observed mitochondrial fission during osmotic stress, but blocking fission did not affect AMPK activation. Stress-activated kinases Sty1 and Pmk1 did not promote AMPK activation but contributed to subsequent inactivation. Our results show that osmotic stress induces transient energy stress, and AMPK activation allows cells to manage this energy stress for proliferation in new osmotic states.

Distinct functions of Ulk1 and Ulk2 in the regulation of lipid metabolism in adipocytes

ULK1 (unc-51 like kinase 1) is a serine/threonine protein kinase that plays a key role in regulating the induction of autophagy. Recent studies using autophagy-defective mouse models, such as atg5- or atg7-deficient mice, revealed an important function of autophagy in adipocyte differentiation. Suppression of adipogenesis in autophagy-defective conditions has made it difficult to study the roles of autophagy in metabolism of differentiated adipocytes. In this study, we established autophagy defective-differentiated 3T3-L1 adipocytes, and investigated the roles of Ulk1 and its close homolog Ulk2 in lipid and glucose metabolism using the established adipocytes. Through knockdown approaches, we determined that Ulk1 and Ulk2 are important for basal and MTORC1 inhibition-induced autophagy, basal lipolysis, and mitochondrial respiration. However, unlike other autophagy genes (Atg5, Atg13, Rb1cc1/Fip200, and Becn1) Ulk1 was dispensable for adipogenesis without affecting the expression of CCAAT/enhancer binding protein ? (CEBPA) and peroxisome proliferation-activated receptor gamma (PPARG). Ulk1 knockdown reduced fatty acid oxidation and enhanced fatty acid uptake, the metabolic changes that could contribute to adipogenesis, whereas Ulk2 knockdown had opposing effects. We also found that the expression levels of insulin receptor (INSR), insulin receptor substrate 1 (IRS1), and glucose transporter 4 (SLC2A4/GLUT4) were increased in Ulk1-silenced adipocytes, which was accompanied by upregulation of insulin-stimulated glucose uptake. These results suggest that ULK1, albeit its important autophagic role, regulates lipid metabolism and glucose uptake in adipocytes distinctly from other autophagy proteins.

PPIP5K1 modulates ligand competition between diphosphoinositol polyphosphates and PtdIns(3,4,5)P3 for polyphosphoinositide-binding domains

We describe new signalling consequences for PPIP5K1 (diphosphoinositol pentakisphosphate kinase type 1)-mediated phosphorylation of InsP6 and 5-InsP7 to 1-InsP7 and InsP8. In NIH 3T3 cells, either hyperosmotic stress or receptor activation by PDGF (platelet-derived growth factor) promoted translocation of PPIP5K1 from the cytoplasm to the plasma membrane. The PBD1 (polyphosphoinositide-binding domain) in PPIP5K1 recapitulated that translocation. Mutagenesis of PBD1 to reduce affinity for PtdIns(3,4,5)P3 prevented translocation. Using surface plasmon resonance, we found that PBD1 association with vesicular PtdIns(3,4,5)P3 was inhibited by InsP6 and diphosphoinositol polyphosphates. However, the inhibition by PPIP5K1 substrates (IC50: 5-InsP7=5 μM and InsP6=7 μM) was substantially more potent than that of the PPIP5K1 products (IC50: InsP8=32 μM and 1-InsP7=43 μM). This rank order of ligand competition with PtdIns(3,4,5)P3 was also exhibited by the PH (pleckstrin homology) domains of Akt (also known as protein kinase B), GRP1 (general receptor for phosphoinositides 1) and SIN1 (stress-activated protein kinase-interaction protein 1). We propose that, in vivo, PH domain binding of InsP6 and 5-InsP7 suppresses inappropriate signalling ('noise') from stochastic increases in PtdIns(3,4,5)P3. That restraint may be relieved by localized depletion of InsP6 and 5-InsP7 at the plasma membrane following PPIP5K1 recruitment. We tested this hypothesis in insulin-stimulated L6 myoblasts, using mTOR (mechanistic/mammalian target of rapamycin)-mediated phosphorylation of Akt on Ser473 as a readout for SIN1-mediated translocation of mTORC (mTOR complex) 2 to the plasma membrane [Zoncu, Efeyan and Sabatini (2011) Nat. Rev. Mol. Cell Biol. 12, 21-35]. Knockdown of PPIP5K1 expression was associated with a 40% reduction in Ser473 phosphorylation. A common feature of PtdIns(3,4,5)P3-based signalling cascades may be their regulation by PPIP5K1.

NAADP mediates insulin-stimulated glucose uptake and insulin sensitization by PPARγ in adipocytes

Insulin stimulates glucose uptake through the membrane translocation of GLUT4 and GLUT1. Peroxisome proliferator-activated receptor γ (PPARγ) enhances insulin sensitivity. Here, we demonstrate that insulin stimulates GLUT4 and GLUT1 translocation, and glucose uptake, by activating the signaling pathway involving nicotinic acid adenine dinucleotide phosphate (NAADP), a calcium mobilizer, in adipocytes. We also demonstrate that PPARγ mediates insulin sensitization by enhancing NAADP production through upregulation of CD38, the only enzyme identified for NAADP synthesis. Insulin produced NAADP by both CD38-dependent and -independent pathways, whereas PPARγ produced NAADP by CD38-dependent pathway. Blocking the NAADP signaling pathway abrogated both insulin-stimulated and PPARγ-induced GLUT4 and GLUT1 translocation, thereby inhibiting glucose uptake. CD38 knockout partially inhibited insulin-stimulated glucose uptake. However, CD38 knockout completely blocked PPARγ-induced glucose uptake in adipocytes and PPARγ-mediated amelioration of glucose tolerance in diabetic mice. These results demonstrated that the NAADP signaling pathway is a critical molecular target for PPARγ-mediated insulin sensitization.

Cell volume regulation and signaling in 3T3-L1 pre-adipocytes and adipocytes: on the possible roles of caveolae, insulin receptors, FAK and ERK1/2

Caveolae have been implicated in sensing of cell volume perturbations, yet evidence is still limited and findings contradictory. Here, we investigated the possible role of caveolae in cell volume regulation and volume sensitive signaling in an adipocyte system with high (3T3-L1 adipocytes); intermediate (3T3-L1 pre-adipocytes); and low (cholesterol-depleted 3T3-L1 pre-adipocytes) caveolae levels. Using large-angle light scattering, we show that compared to pre-adipocytes, differentiated adipocytes exhibit several-fold increased rates of volume restoration following osmotic cell swelling (RVD) and osmotic cell shrinkage (RVI), accompanied by increased swelling-activated taurine efflux. However, caveolin-1 distribution was not detectably altered after osmotic swelling or shrinkage, and caveolae integrity, as studied by cholesterol depletion or expression of dominant negative Cav-1, was not required for either RVD or RVI in pre-adipocytes. The insulin receptor (InsR) localizes to caveolae and its expression dramatically increases upon adipocyte differentiation. In pre-adipocytes, InsR and its effectors focal adhesion kinase (FAK) and extracellular signal regulated kinase (ERK1/2) localized to focal adhesions and were activated by a 5 min exposure to insulin (100 nM). Osmotic shrinkage transiently inhibited InsR Y(146)-phosphorylation, followed by an increase at t=15 min; a similar pattern was seen for ERK1/2 and FAK, in a manner unaffected by cholesterol depletion. In contrast, cell swelling had no detectable effect on InsR, yet increased ERK1/2 phosphorylation. In conclusion, differentiated 3T3-L1 adipocytes exhibit greatly accelerated RVD and RVI responses and increased swelling-activated taurine efflux compared to pre-adipocytes. Furthermore, in pre-adipocytes, Cav-1/caveolae integrity is not required for volume regulation. Given the relationship between hyperosmotic stress and insulin signaling, the finding that cell volume regulation is dramatically altered upon adipocyte differentiation may be relevant for the understanding of insulin resistance and metabolic syndrome.

The differential role of Hif1β/Arnt and the hypoxic response in adipose function, fibrosis, and inflammation

In obesity, adipocytes distant from vasculature become hypoxic and dysfunctional. This hypoxic response is mediated by hypoxia-inducible factors (Hif1α, Hif2α, and Hif3α) and their obligate partner, Hif1β (Arnt). We show that mice lacking Hif1β in fat (FH1βKO) are lean, exhibit reduced adipocyte size, and are protected from age- and diet-induced glucose intolerance. There is also reduced Vegf and vascular permeability in FH1βKO fat, but diet-induced inflammation and fibrosis is unchanged. Adipocytes from FH1βKO mice have reduced glucose uptake due to decreased Glut1 and Glut4, which is mirrored in 3T3-L1 adipocytes with Hif1β knockdown. Hif1β knockdown cells also fail to respond appropriately to hypoxia with reduced cellular respiration and reduced mitochondrial gene expression. Some, but not all, of these effects are reproduced by Hif1α knockdown. Thus, Hif1β/Arnt regulates glucose uptake, mitochondrial gene expression, and vascular permeability to control adipose mass and function, providing a target for obesity therapy.

Next-generation Akt inhibitors provide greater specificity: effects on glucose metabolism in adipocytes

Many human tumours exhibit activation of the PI3K (phosphoinositide 3-kinase)/Akt pathway, and inhibition of this pathway slows tumour growth. This led to the development of specific Akt inhibitors for in vivo use. However, activation of Akt is also necessary for processes including glucose metabolism. Therefore a potential complication of such anticancer drugs is insulin resistance and/or diabetes. In the process of characterizing the metabolic effects of early-phase Akt inhibitors, we discovered an off-target inhibitory effect on mammalian facilitative glucose transporters. In view of the crucial role of glucose transport for all mammalian cells, such an off-target effect would have major implications for further development of this family of compounds. In the present study, we have characterized a next-generation Akt inhibitor, MK-2206. MK-2206 is an orally active allosteric Akt inhibitor under development for treating solid tumours. We report that MK-2206 potently inhibits Thr308Akt and Ser473Akt phosphorylation in 3T3-L1 adipocytes (IC50 0.11 and 0.18 μM respectively) as well as downstream effects of insulin on GLUT4 (glucose transporter 4) translocation (IC50 0.47 μM) and glucose transport (IC50 0.14 μM). Notably, the potency of MK-2206 is approximately 1 log higher than previous inhibitors and its specificity is significantly improved with modest inhibitory effects on glucose transport in GLUT4-expressing adipocytes and GLUT1-rich human erythrocytes, independently of Akt. Nevertheless, MK-2206 clearly has potent effects on Akt2, the principal isoform involved in peripheral insulin action, in which case insulin resistance will probably be a major complication following in vivo administration. We conclude that MK-2206 provides an optimal tool for studying the effects of Akt in vitro.

Akt inhibitors reduce glucose uptake independently of their effects on Akt

The protein kinase Akt is involved in various cellular processes, including cell proliferation, growth and metabolism. Hyperactivation of Akt is commonly observed in human tumours and so this pathway has been the focus of targeted drug discovery. However, Akt also plays an essential role in other physiological processes, such as the insulin-regulated transport of glucose into muscle and fat cells. This process, which is essential for whole-body glucose homoeostasis in mammals, is thought to be mediated via Akt-dependent movement of GLUT4 glucose transporters to the plasma membrane. In the present study, we have investigated the metabolic side effects of non-ATP-competitive allosteric Akt inhibitors. In 3T3-L1 adipocytes, these inhibitors caused a decrease in the Akt signalling pathway concomitant with reduced glucose uptake. Surprisingly, a similar reduction in GLUT4 translocation to the plasma membrane was not observed. Further investigation revealed that the inhibitory effects of these compounds on glucose uptake in 3T3-L1 adipocytes were independent of the Akt signalling pathway. The inhibitors also inhibited glucose transport into other cell types, including human erythrocytes and T-47D breast cancer cells, suggesting that these effects are not specific to GLUT4. We conclude that these drugs may, at least in part, inhibit tumorigenesis through inhibition of tumour cell glucose transport.

Volume-sensitive outwardly rectifying chloride channel in white adipocytes from normal and diabetic mice

The volume-sensitive outwardly rectifying (VSOR) chloride channel is ubiquitously expressed and involved in cell volume regulation after osmotic swelling, called regulatory volume decrease (RVD), in various cell types. In adipocytes, the expression of the VSOR channel has not been explored to date. Here, by employing the whole-cell patch-clamp technique, we examined whether or not the VSOR channel is expressed in white adipocytes freshly isolated from epididymal fat pads of normal (C57BL/6 or KK) and diabetic (KKA(y)) mice. Whole cell voltage-clamp recordings revealed that Cl(-) currents were gradually activated upon cell swelling induced by application of a hypotonic solution, both in normal and diabetic adipocytes. Although both the mean cell size (or cell capacitance) and the current magnitude in KKA(y) adipocytes were larger than those in C57BL/6 cells, the current density was significantly lower in KKA(y) adipocytes (23.32 +/- 1.94 pA in C57BL/6 adipocytes vs. 13.04 +/- 2.41 pA in KKA(y) adipocytes at +100 mV). Similarly, the current density in diabetic KKA(y) adipocytes was lower than that in adipocytes from KK mice (a parental strain of KKA(y) mice), which do not present diabetes until an older age. The current was inhibited by Cl(-) channel blockers, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and glibenclamide, or hypertonic solution, and showed outward rectification and inactivation kinetics at large positive potentials. These electrophysiological and pharmacological properties are consistent with those of the VSOR channel in other cell types. Moreover, adipocytes showed RVD, which was inhibited by NPPB. In KKA(y) adipocytes, RVD was significantly slower (tau; 8.42 min in C57BL/6 adipocytes vs. 11.97 min in KKA(y) adipocytes) and incomplete during the recording period (25 min). It is concluded that the VSOR channel is functionally expressed and involved in volume regulation in white adipocytes. RVD is largely impaired in adipocytes from diabetic mice, presumably as a consequence of the lower density of the functional VSOR channel in the plasma membrane.

Role of PKCzeta translocation in the development of type 2 diabetes in rats following continuous glucose infusion

AIM

We investigated the molecular mechanisms of hyperglycaemia-induced insulin resistance and type 2 diabetes in rats receiving a continuous glucose infusion (GI).

METHODS

Female Wistar rats were infused with either 2.8 mol/L glucose or saline (2 mL/h) for durations varying from 0 to 15 days. Blood samples were analysed daily to determine glucose and insulin dynamics. Subsets of animals were sacrificed and soleus muscles were extracted for determination of protein expression, subcellular location, and activities of insulin-signalling proteins.

RESULTS

Rats accommodated this systemic glucose oversupply and developed insulin resistance on day 5 (normoglycaemia/hyperinsulinaemia) and type 2 diabetes on day 15 (hyperglycaemia/normoinsulinaemia). The effect of GI on protein kinase Czeta (PKCzeta) activity was independent of changes in phosphatidylinositol 3-kinase activity, and occurred in parallel with an increase in PDK1 activity. Activated PKCzeta was mainly located in the cytosol after 5 days of GI that was coincident with the translocation of GLUT4 to the plasma membrane, and normoglycaemia. After 15 days of GI, PKCzeta translocated from the cytosol to the plasma membrane with a concomitant decrease in PDK1 activity. This caused an increase in the association between PKCzeta and PKB and a decrease in PDK1-PKB reactions at the plasma membrane, leading to reduced PKB activity. The activity of PKCzeta per se was also compromised. The PKCzeta and PKB activity reduction and the blunted insulin-stimulated GLUT4 translocation eventually led to hyperglycaemia and diabetes.

CONCLUSION

Translocation of PKCzeta may play a central role in the development of type 2 diabetes.

Effects of contraction on localization of GLUT4 and v-SNARE isoforms in rat skeletal muscle

In skeletal muscle, contractions increase glucose uptake due to a translocation of GLUT4 glucose transporters from intracellular storage sites to the surface membrane. Vesicle-associated membrane proteins (VAMPs) are believed to play an important role in docking and fusion of the GLUT4 transporters at the surface membrane. However, knowledge about which VAMP isoforms colocalize with GLUT4 vesicles in mature skeletal muscle and whether they translocate during muscle contractions is incomplete. The aim of the present study was to further identify VAMP isoforms, which are associated with GLUT4 vesicles and examine which VAMP isoforms translocate to surface membranes in skeletal muscles undergoing contractions. VAMP2, VAMP3, VAMP5, and VAMP7 were enriched in immunoprecipitated GLUT4 vesicles. In response to 20 min of in situ contractions, there was a redistribution of GLUT4 (+64 +/- 13%), transferrin receptor (TfR; +75 +/- 22%), and insulin-regulated aminopeptidase (IRAP; +70 +/- 13%) to fractions enriched in heavy membranes away from low-density membranes (-32 +/- 7%; -18 +/- 12%; -33 +/- 9%; respectively), when compared with the resting contralateral muscle. Similarly, there was a redistribution of VAMP2 (+240 +/- 40%), VAMP5 (+79 +/- 9%), and VAMP7 (+79 +/- 29%), but not VAMP3, to fractions enriched in heavy membranes away from low-density membranes (-49 +/- 10%, -54 +/- 9%, -14 +/- 11%, respectively) in contracted vs. resting muscle. In summary, VAMP2, VAMP3, VAMP5, and VAMP7 coimmunoprecipitate with intracellular GLUT4 vesicles in muscle, and VAMP2, VAMP5, VAMP7, but not VAMP3, translocate to the cell surface membranes similar to GLUT4, TfR, and IRAP in response to muscle contractions. These findings suggest that VAMP2, VAMP5, and VAMP7 may be involved in translocation of GLUT4 during muscle contractions.

Retinal insulin receptor signaling in hyperosmotic stress

In the diabetic eye, the increased accumulation of sorbitol in the retina has been implicated in the pathogenesis of diabetic retinopathy (DR). Neurodegeneration is an important component of DR as demonstrated by increased neural apoptosis in the retina during experimental and human diabetes. Insulin receptor (IR) activation has been shown to rescue retinal neurons from apoptosis through a phosphoinositide 3-kinase and protein kinase B (Akt) survival cascade. In this study, we examined the IR signaling in sorbitol-induced hyperosmotic stressed retinas.

A cell-based screening identifies compounds from the stem of Momordica charantia that overcome insulin resistance and activate AMP-activated protein kinase

Treatment of insulin resistance is a critical strategy in the prevention and management of type 2 diabetes. The crude extracts from all parts of Momordica charantia L. have been reported by many studies for the effective treatment of diabetes and related complications. However, the exact ingredients responsible for the hypoglycemic effect and the underlying mechanism of their actions have not been well characterized because of the lack of a proper assay and screening system. A new cell-based, nonradioactive, and nonfluorescent screening method was demonstrated in this study to screen for natural products from the stem of M. charantia, aiming to identify hypoglycemic components that can overcome cellular insulin resistance. The results suggest triterpenoids being potential hypoglycemic components of the plant and the mechanism underlying their action involving AMP-activated protein kinase.

Phospho-Site-Specific Antibody Microarray to Study the State of Protein Phosphorylation in the Retina

Neurodegeneration is an important component of diabetic retinopathy as demonstrated by increased neural apoptosis in the retina during experimental and human diabetes. Accumulation of sorbitol and fructose and the generation or enhancement of oxidative stress has been reported in the whole retina of diabetic animals. Aldose reductase (AR), the first and the rate limiting enzyme in the pathway reduces glucose to sorbitol and the diabetic complications are prevented by drugs that inhibit AR. In this study we examined the phosphorylation state of various retinal proteins in response to sorbitol-treatment by phosphor-site-specific antibody microarray. Our results suggest that various retinal protein kinases and cytoskeletal proteins either activated or down regulated in response to sorbitol treatment. Further, our study also indicates the activation of retinal insulin- and insulin growth factor 1 receptor and their downstream signaling proteins such as phosphoinositide 3-kinanse and protein kinase B (Akt). Understanding the regulation of retinal proteins involved in polyol (sorbitol) pathway would help to design therapeutic agents for the treatment of diabetic retinopathy.

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.

Tyrosine phosphorylation of Munc18c regulates platelet-derived growth factor-stimulated glucose transporter 4 translocation in 3T3L1 adipocytes

Platelet-derived growth factor (PDGF) stimulation of skeletal muscle, cultured myotubes, and 3T3L1 adipocytes results in glucose transporter 4 (Glut4) translocation, albeit to a reduced level compared with insulin. To address the mechanism of PDGF action, we have determined that the Syntaxin 4 negative regulatory protein, Munc18c, undergoes PDGF-stimulated phosphorylation on tyrosine residue 521. The tyrosine phosphorylation of Munc18c on Y521 occurred concomitant with the dissociation of the Munc18c protein from Syntaxin 4 in a time frame consistent with Glut4 translocation. Moreover, expression of the wild-type Munc18c protein did not inhibit PDGF-induced Glut4 translocation, whereas expression of Y521A-Munc18c mutant was inhibitory and failed to dissociate from Syntaxin 4. In contrast, expression of either wild-type Munc18c or the Y521A-Munc18c mutant both resulted in a marked inhibition of insulin-stimulated Glut4 translocation. Together, these data demonstrate that one mechanism accounting for the PDGF induction of Glut4 translocation is the suppression of the Munc18c negative regulation of Syntaxin 4 function.

Involvement of vesicular H+-ATPase in insulin-stimulated glucose transport in 3T3-F442A adipocytes

In secretory cells, osmotic swelling of secretory granules is proposed to be an intermediate step in exocytic fusion of the granules with the plasma membrane. For osmotic swelling of the granule, a H (+) gradient generated by vacuolar-type H (+) -ATPase (V-ATPase) may be a driving force for accumulation of K (+) via its exchange with H (+) , concurrent with accumulation of Cl (-) and H(2)O. Here, we investigated whether a similar chemiosmotic mechanism is involved in the insulin-stimulated recruitment of GLUT4 to the plasma membrane in 3T3-F442A adipocytes. Incubating cells in a hypo-osmotic medium significantly increased 2-deoxy glucose (2-DG) uptake and the plasma membrane GLUT4 content (possibly via induction of osmotic swelling of GLUT4-containing vesicles (G4V)) and also potentiated the insulin-stimulated 2-DG uptake. Promotion of the G4V membrane ionic permeability using nigericin, an electroneutral K (+) /H (+) exchange ionophore, increased 2-DG uptake and the plasma membrane GLUT4 content. However, co-treatment with nigericin and insulin did not show an additive effect. Bafilomycin A(1), a diagnostically specific inhibitor of V-ATPase, inhibited insulin- and nigericin-stimulated 2-DG uptake. Immunoadsorption plus immunoblotting demonstrated that GLUT4 and V-ATPase co-localize in the same intracellular membranes. Together, these results indicate that V-ATPases in the G4V membrane may play an important role in the insulin-stimulated exocytic fusion of G4V with the plasma membrane via its participation in osmotic swelling of the vesicle.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Sub-cellular localization of insulin-regulated membrane aminopeptidase, IRAP to vesicles in neurons

Angiotensin IV and LVV-hemorphin 7 promote robust enhancing effects on learning and memory. These peptides are also competitive inhibitors of the insulin-regulated membrane aminopeptidase, suggesting that the biological actions of these peptides may result from inhibition of IRAP activity. However, the normal function of IRAP in the brain is yet to be determined. The present study investigated the sub-cellular distribution of IRAP in four neuronal cell lines and in the mouse brain. Using sub-cellular fractionation, IRAP was found to be enriched in low density microsomes, while lower levels of IRAP were also present in high density microsomes, plasma membrane and mitochondrial fractions. Dual-label immunohistochemistry confirmed the presence of IRAP in vesicles co-localized with the vesicular maker VAMP2, in the trans Golgi network co-localized with TGN 38 and in endosomes co-localized with EEA1. Finally using electron microscopy, IRAP specific immunoreactivity was predominantly associated with large 100-200 nm vesicles in hippocampal neurons. The location, appearance and size of these vesicles are consistent with neurosecretory vesicles. IRAP precipitate was also detected in intracellular structures including the rough endoplasmic reticulum, Golgi stack and mitochondrial membranes. The sub-cellular localization of IRAP in neurons demonstrated in the present study bears striking parallels with distribution of IRAP in insulin responsive cells, where the enzyme plays a role in insulin-regulated glucose uptake. Therefore, we propose that the function of IRAP in neurons may be similar to that in insulin responsive cells.

Insulin-stimulated exocytosis of GLUT4 is enhanced by IRAP and its partner tankyrase

The glucose transporter GLUT4 and the aminopeptidase IRAP (insulin-responsive aminopeptidase) are the major cargo proteins of GSVs (GLUT4 storage vesicles) in adipocytes and myocytes. In the basal state, most GSVs are sequestered in perinuclear and other cytosolic compartments. Following insulin stimulation, GSVs undergo exocytic translocation to insert GLUT4 and IRAP into the plasma membrane. The mechanisms regulating GSV trafficking are not fully defined. In the present study, using 3T3-L1 adipocytes transfected with siRNAs (small interfering RNAs), we show that insulin-stimulated IRAP translocation remained intact despite substantial GLUT4 knockdown. By contrast, insulin-stimulated GLUT4 translocation was impaired upon IRAP knockdown, indicating that IRAP plays a role in GSV trafficking. We also show that knockdown of tankyrase, a Golgi-associated IRAP-binding protein that co-localizes with perinuclear GSVs, attenuated insulin-stimulated GSV translocation and glucose uptake without disrupting insulin-induced phosphorylation cascades. Moreover, iodixanol density gradient analyses revealed that tankyrase knockdown altered the basal-state partitioning of GLUT4 and IRAP within endosomal compartments, apparently by shifting both proteins toward less buoyant compartments. Importantly, the afore-mentioned effects of tankyrase knockdown were reproduced by treating adipocytes with PJ34, a general PARP (poly-ADP-ribose polymerase) inhibitor that abrogated tankyrase-mediated protein modification known as poly-ADP-ribosylation. Collectively, these findings suggest that physiological GSV trafficking depends in part on the presence of IRAP in these vesicles, and that this process is regulated by tankyrase and probably its PARP activity.

Berberine stimulates glucose transport through a mechanism distinct from insulin

Berberine exerts a hypoglycemic effect, but the mechanism remains unknown. In the present study, the effect of berberine on glucose uptake was characterized in 3T3-L1 adipocytes. It was revealed that berberine stimulated glucose uptake in 3T3-L1 adipocytes in a dose- and time-dependent manner with the maximal effect at 12 hours. Glucose uptake was increased by berberine in 3T3-L1 preadipocytes as well. Berberine-stimulated glucose uptake was additive to that of insulin in 3T3-L1 adipocytes, even at the maximal effective concentrations of both components. Unlike insulin, the effect of berberine on glucose uptake was insensitive to wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and SB203580, an inhibitor of p38 mitogen-activated protein kinase. Berberine activated extracellular signal-regulated kinase (ERK) 1/2, but PD98059, an ERK kinase inhibitor, only decreased berberine-stimulated glucose uptake by 32%. Berberine did not induce Ser473 phosphorylation of Akt nor enhance insulin-induced phosphorylation of Akt. Meanwhile, the expression and cellular localization of glucose transporter 4 (GLUT4) were not altered by berberine. Berberine did not increase GLUT1 gene expression. However, genistein, a tyrosine kinase inhibitor, completely blocked berberine-stimulated glucose uptake in 3T3-L1 adipocytes and preadipocytes, suggesting that berberine may induce glucose transport via increasing GLUT1 activity. In addition, berberine increased adenosine monophosphate-activated protein kinase and acetyl-coenzyme A carboxylase phosphorylation. These findings suggest that berberine increases glucose uptake through a mechanism distinct from insulin, and activated adenosine monophosphate-activated protein kinase seems to be involved in the metabolic effect of berberine.

Endothelin-1 impairs glucose transporter trafficking via a membrane-based mechanism

Endothelin-1 (ET-1) disrupts insulin-regulated glucose transporter GLUT4 trafficking. Since the negative consequence of chronic ET-1 exposure appears to be independent of signal disturbance along the insulin receptor substrate-1/phosphatidylinositol (PI) 3-kinase (PI3K)/Akt-2 pathway of insulin action, we tested if ET-1 altered GLUT4 regulation engaged by osmotic shock, a PI3K-independent stimulus that mimics insulin action. Regulation of GLUT4 by hyperosmotic stress was impaired by ET-1. Because of the mutual disruption of both insulin- and hyperosmolarity-stimulated GLUT4 translocation, we tested whether shared signaling and/or key phosphatidylinositol 4,5-bisphosphate (PIP2)-regulated cytoskeletal events of GLUT4 trafficking were targets of ET-1. Both insulin and hyperosmotic stress signaling to Cbl were impaired by ET-1. Also, plasma membrane PIP2 and cortical actin levels were reduced in cells exposed to ET-1. Exogenous PIP2, but not PI 3,4,5-bisphosphate, restored actin structure, Cbl activation, and GLUT4 translocation. These data show that ET-1-induced PIP2/actin disruption impairs GLUT4 trafficking elicited by insulin and hyperosmolarity. In addition to showing for the first time the important role of PIP2-regulated cytoskeletal events in GLUT4 regulation by stimuli other than insulin, these studies reveal a novel function of PIP2/actin structure in signal transduction.

Insulin receptor signals regulating GLUT4 translocation and actin dynamics

In skeletal muscle and adipose tissue, insulin-stimulated glucose uptake is dependent upon translocation of the insulin-responsive glucose transporter GLUT4 from intracellular storage compartments to the plasma membrane. This insulin-induced redistribution of GLUT4 protein is achieved through a series of highly organized membrane trafficking events, orchestrated by insulin receptor signals. Recently, several key molecules linking insulin receptor signals and membrane trafficking have been identified, and emerging evidence supports the importance of subcellular compartmentalization of signaling components at the right time and in the right place. In addition, the translocation of GLUT4 in adipocytes requires insulin stimulation of dynamic actin remodeling at the inner surface of the plasma membrane (cortical actin) and in the perinuclear region. This results from at least two independent insulin receptor signals, one leading to the activation of phosphatidylinositol (PI) 3-kinase and the other to the activation of the Rho family small GTP-binding protein TC10. Thus, both spatial and temporal regulations of actin dynamics, both beneath the plasma membrane and around endomembranes, by insulin receptor signals are also involved in the process of GLUT4 translocation.

Cell signaling, the essential role of O-GlcNAc!

An increasing body of evidence points to a central regulatory role for glucose in mediating cellular processes and expands the role of glucose well beyond its traditional role(s) in energy metabolism. Recently, it has been recognized that one downstream effector produced from glucose is UDP-GlcNAc. Levels of UDP-GlcNAc, and the subsequent addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) to Ser/Thr residues, is involved in regulating nuclear and cytoplasmic proteins in a manner analogous to protein phosphorylation. O-GlcNAc protein modification is essential for life in mammalian cells, highlighting the importance of this simple post-translational modification in basic cellular regulation. Recent research has highlighted key roles for O-GlcNAc serving as a nutrient sensor in regulating insulin signaling, the cell cycle, and calcium handling, as well as the cellular stress response.

Cell hydration and mTOR-dependent signalling

Insulin- and amino acid-induced signalling by the mammalian target of rapamycin (mTOR) involves hyperphosphorylation of the p70 ribosomal S6 protein kinase (p70S6-kinase) and the eukaryotic initiation factor 4E (eIF4E) binding protein 4E-BP1 and contributes to regulation of protein metabolism. This review considers the impact of cell hydration on mTOR-dependent signalling. Although hypoosmotic hepatocyte swelling in some instances activates p70S6-kinase, the hypoosmolarity-induced proteolysis inhibition in perfused rat liver is insensitive to mTOR inhibition by rapamycin. Likewise, swelling-dependent proteolysis inhibition by insulin and swelling-independent proteolysis inhibition by leucine, a potent activator of p70S6-kinase and 4E-BP1 hyperphosphorylation, in perfused rat liver is insensitive to rapamycin, indicating that at least rapamycin-sensitive mTOR signalling is not involved. Hyperosmotic dehydration in different cell types produces inactivation of signalling components around mTOR, thereby attenuating insulin-induced glucose uptake, glycogen synthesis, and lipogenesis in adipocytes, and MAP-kinase phosphatase MKP-1 expression in hepatoma cells. Direct inactivation of mTOR, stimulation of the AMP-activated protein kinase, and the destabilization of individual proteins may impair mTOR signalling under dehydrating conditions. Further investigation of the crosstalk between the mTOR pathway(s) and hyperosmotic signalling will improve our understanding about the contribution of cell hydration changes in health and disease and will provide further rationale for fluid therapy of insulin-resistant states.

Sennidin stimulates glucose incorporation in rat adipocytes

A novel small molecule compound which exerts insulin mimetic is desirable. Dozens of natural products that have quinone, naphthoquinone, or anthraquinone structure, were tested by a glucose incorporation assay. We found that sennidin A, anthraquinone derivative, stimulated glucose incorporation to near level of maximal insulin-stimulated and sennidin B, a stereoisomer of sennidin A, also stimulated, but the activity of sennidin B was lower than sennidin A. Sennidin A-stimulated glucose incorporation was completely inhibited by wortmannin. Sennidin A did not induce tyrosine phosphorylation of insulin receptor (IR) and insulin receptor substrate-1 (IRS-1), but induced phosphorylation of Akt and glucose transporter 4 (GLUT4) translocation. Our results suggest that in rat adipocytes, sennidin A stimulates glucose incorporation in the phosphatidylinositol 3-kinase (PI3K)- and Akt-dependent, but in the IR/IRS1-independent manner.

Are tyrosine kinases involved in mediating contraction-stimulated muscle glucose transport?

Muscle contractions and insulin stimulate glucose transport into muscle by separate pathways. The contraction-mediated increase in glucose transport is mediated by two mechanisms, one involves the activation of 5'-AMP-activated protein kinase (AMPK) and the other involves the activation of calcium/calmodulin-dependent protein kinase II (CAMKII). The steps leading from the activation of AMPK and CAMKII to the translocation of GLUT4 to the cell surface have not been identified. Studies with the use of the tyrosine kinase inhibitor genistein suggest that one or more tyrosine kinases could be involved in contraction-stimulated glucose transport. The purpose of the present study was to determine the involvement of tyrosine kinases in contraction-stimulated glucose transport in rat soleus and epitrochlearis muscles. Contraction-stimulated glucose transport was completely prevented by pretreatment with genistein (100 microM) and the related compound butein (100 microM). However, the structurally distinct tyrosine kinase inhibitors 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyridine and herbimycin did not reduce contraction-stimulated glucose transport. Furthermore, genistein and butein inhibited glucose transport even when muscles were exposed to these compounds after being stimulated to contract. Muscle contractions did not result in increases in tyrosine phosphorylation of proteins such as proline-rich tyrosine kinase and SRC. These results provide evidence that tyrosine kinases do not mediate contraction-stimulated glucose transport and that the inhibitory effects of genistein on glucose transport result from direct inhibition of the glucose transporters at the cell surface.

Adipose tissue from pregnant women with and without gestational diabetes mellitus: insulin-sensitive but resistant to hyperosomolarity

OBJECTIVE

We sought to determine the contribution of adipose tissue to the insulin resistance of pregnancy. We also investigated whether hyperosmolar stress (induced by sorbitol) stimulates glucose uptake in human adipose tissue and, if so, whether this effect is altered in pregnancy and gestational diabetes mellitus.

STUDY DESIGN

Subcutaneous and omental adipose tissue biopsy specimens were obtained at elective abdominal surgery or cesarean delivery from 16 normal glucose-tolerant pregnant women, 13 pregnant women with gestational diabetes mellitus, and 19 body mass index-matched nonpregnant control subjects. Basal, insulin (100 nmol/L)-, and sorbitol (250 mmol/L)-stimulated glucose uptake levels were measured.

RESULTS

Basal and insulin-stimulated glucose uptake into adipose tissue was not impaired in pregnancy or gestational diabetes mellitus compared with control subjects. Hyperosmolarity stimulated glucose uptake in human adipose tissue from the subcutaneous, but not omental depot, and not in adipose tissue from pregnant subjects.

CONCLUSION

There is no significant difference in insulin sensitivity in adipose tissue from pregnant or nonpregnant women; hyperosmolarity stimulates glucose uptake in subcutaneous adipose tissue from nonpregnant women, and adipose tissue from pregnant women is sorbitol resistant. These findings suggest the phosphotidylinositol 3-kinase-independent pathway may have pathophysiologic relevance to glucose uptake in human adipose tissue and may be impaired in pregnancy.

GLUT8 subcellular localization and absence of translocation to the plasma membrane in PC12 cells and hippocampal neurons

GLUT8 is a high-affinity glucose transporter present mostly in testes and a subset of brain neurons. At the cellular level, it is found in a poorly defined intracellular compartment in which it is retained by an N-terminal dileucine motif. Here we assessed GLUT8 colocalization with markers for different cellular compartments and searched for signals, which could trigger its cell surface expression. We showed that when expressed in PC12 cells, GLUT8 was located in a perinuclear compartment in which it showed partial colocalization with markers for the endoplasmic reticulum but not with markers for the trans-Golgi network, early endosomes, lysosomes, and synaptic-like vesicles. To evaluate its presence at the plasma membrane, we generated a recombinant adenovirus for the expression of GLUT8 containing an extracellular myc epitope. Cell surface expression was evaluated by immunofluorescence microscopy of transduced PC12 cells or primary hippocampal neurons exposed to different stimuli. Those included substances inducing depolarization, activation of protein kinase A and C, activation or inhibition of tyrosine kinase-linked signaling pathways, glucose deprivation, AMP-activated protein kinase stimulation, and osmotic shock. None of these stimuli-induced GLUT8 cell surface translocation. Furthermore, when GLUT8myc was cotransduced with a dominant-negative form of dynamin or GLUT8myc-expressing PC-12 cells or neurons were incubated with an anti-myc antibody, no evidence for constitutive recycling of the transporter through the cell surface could be obtained. Thus, in cells normally expressing it, GLUT8 was associated with a specific intracellular compartment in which it may play an as-yet-uncharacterized role.

Activators of AMP-activated protein kinase enhance GLUT4 translocation and its glucose transport activity in 3T3-L1 adipocytes

To determine whether the increase in glucose uptake following AMP-activated protein kinase (AMPK) activation in adipocytes is mediated by accelerated GLUT4 translocation into plasma membrane, we constructed a chimera between GLUT4 and enhanced green fluorescent protein (GLUT4-eGFP) and transferred its cDNA into the nucleus of 3T3-L1 adipocytes. Then, the dynamics of GLUT4-eGFP translocation were visualized in living cells by means of laser scanning confocal microscopy. It was revealed that the stimulation with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) and 2,4-dinitrophenol (DNP), known activators of AMPK, promptly accelerates its translocation within 4 min, as was found in the case of insulin stimulation. The insulin-induced GLUT4 translocation was markedly inhibited after addition of wortmannin (P < 0.01). However, the GLUT4 translocation through AMPK activators AICAR and DNP was not affected by wortmannin. Insulin- and AMPK-activated translocation of GLUT4 was not inhibited by SB-203580, an inhibitor of p38 mitogen-activated protein kinase (MAPK). Glucose uptake was significantly increased after addition of AMPK activators AICAR and DNP (P < 0.05). AMPK- and insulin-stimulated glucose uptake were similarly suppressed by wortmannin (P < 0.05-0.01). In addition, SB-203580 also significantly prevented the enhancement of glucose uptake induced by AMPK and insulin (P < 0.05). These results suggest that AMPK-activated GLUT4 translocation in 3T3-L1 adipocytes is mediated through the insulin-signaling pathway distal to the site of activated phosphatidylinositol 3-kinase or through a signaling system distinct from that activated by insulin. On the other hand, the increase of glucose uptake dependent on AMPK activators AICAR and DNP would be additionally due to enhancement of the intrinsic activity in translocated GLUT4 protein, possibly through a p38 MAPK-dependent mechanism.

Physical exercise increases the expression of TNFα and GLUT 1 in muscle tissue of diabetes prone Psammomys obesus

INTRODUCTION

Tumor necrosis factor-alpha (TNFalpha) is a major mediator of insulin resistance. On the other hand, it has been suggested that TNFalpha may facilitate glucose uptake through GLUT 1 expression. We recently found that physical exercise prevented the progression to type 2 diabetes mellitus in diabetes prone Psammomys obesus (sand rat).

AIM

The aim of the present study was to characterize the influence of physical exercise on the expression of TNFalpha, its receptor R1 and GLUT 1 in muscle tissue of this animal model.

METHODS

Animals were assigned for 4 weeks to four groups: high-energy diet (HC), high-energy diet and exercise (HE), low-energy diet (LC), low-energy diet and exercise (LE). TNFalpha, R1 and GLUT 1 expression were analyzed using Western blot technique.

RESULTS

None of the animals in the HE group became diabetic while all the animals in the HC group became diabetic. TNFalpha, its receptor (R1) and GLUT 1 expressions were significantly higher in the two exercising groups (LE and HE) and significantly lower in the HC group compared to the control LC group.

CONCLUSIONS

Physical exercise augments the expression of TNFalpha, its receptor R1 and the glucose transporter GLUT 1 in muscle tissue. We suggest that this mechanism may improve glucose uptake through pathways parallel and unrelated to insulin signaling that may include MAPK and/or NO. These biochemical processes contribute to the beneficial effects of physical exercise on the prevention of type 2 diabetes mellitus.

Role of PYK2 in the development of obesity and insulin resistance

Non-receptor proline-rich tyrosine kinase-2 (PYK2), which is activated by phosphorylation of one or more of its tyrosine residues, has been implicated in the regulation of GLUT4 glucose transporter translocation and glucose transport. Some data favor a positive role of PYK2 in stimulating glucose transport, whereas other studies suggest that PYK2 may participate in the induction of insulin resistance. To ascertain the importance of PYK2 in the setting of obesity and insulin resistance, we (1) evaluated the regulation of PYK2 in mice fed a high-fat diet and (2) characterized body and glucose homeostasis in wild type (WT) and PYK2(-/-) mice on different diets. We found that both PYK2 expression and phosphorylation were significantly increased in liver and adipose tissues harvested from high-fat diet fed mice. Wild type and PYK2(-/-) mice were fed a high-fat diet for 8 weeks to induce insulin resistance/obesity. Surprisingly, in response to this diet PYK2(-/-) mice gained significantly more weight than WT mice (18.7+/-1.2g vs. 9.5+/-0.6g). Fasting serum leptin and insulin and blood glucose levels were significantly increased in high-fat diet fed mice irrespective of the presence of PYK2 protein. There was a close correlation between serum leptin and body weight. Intraperitoneal glucose tolerance tests revealed that as expected, the high-fat diet resulted in increased blood glucose levels following glucose administration in wild type mice compared to those fed normal chow. An even greater increase in blood glucose levels was observed in PYK2(-/-) mice compared to wild type mice. These results demonstrate that a lack of PYK2 exacerbates weight gain and development of glucose intolerance/insulin resistance induced by a high-fat diet, suggesting that PYK2 may play a role in slowing the development of obesity, insulin resistance, and/or frank diabetes.

Akt Activation Is Required at a Late Stage of Insulin-Induced GLUT4 Translocation to the Plasma Membrane

Insulin stimulates the translocation of glucose transporter GLUT4 from intracellular vesicles to the plasma membrane (PM). This involves multiple steps as well as multiple intracellular compartments. The Ser/Thr kinase Akt has been implicated in this process, but its precise role is ill defined. To begin to dissect the role of Akt in these different steps, we employed a low-temperature block. Upon incubation of 3T3-L1 adipocytes at 19 C, GLUT4 accumulated in small peripheral vesicles with a slight increase in PM labeling concomitant with reduced trans-Golgi network labeling. Although insulin-dependent translocation of GLUT4 to the PM was impaired at 19 C, we still observed movement of vesicles toward the surface. Strikingly, insulin-stimulated Akt activity, but not phosphatidylinositol 3 kinase activity, was blocked at 19 C. Consistent with a multistep process in GLUT4 trafficking, insulin-stimulated GLUT4 translocation could be primed by treating cells with insulin at 19 C, whereas this was not the case for Akt activation. These data implicate two insulin-regulated steps in GLUT4 translocation: 1) redistribution of GLUT4 vesicles toward the cell cortex—this process is Akt-independent and is not blocked at 19 C; and 2) docking and/or fusion of GLUT4 vesicles with the PM—this process may be the major Akt-dependent step in the insulin regulation of glucose transport.

Acquisition of Unprecedented Phosphatidylinositol 3,5-Bisphosphate Rise in Hyperosmotically Stressed 3T3-L1 Adipocytes, Mediated by ArPIKfyve-PIKfyve Pathway

Unlike yeast, where hyperosmotic stress induces a dramatic increase in phosphatidylinositol 3,5-bisphosphate (PtdIns 3,5-P(2)) synthesis, in mammalian cells, although activating a complex array of signaling events, hyperosmotic stress fails to up-regulate PtdIns 3,5-P(2), indicating the PtdIns 3,5-P(2) pathway is not involved in mammalian osmo-protective responses. Here we report an unexpected and marked PtdIns 3,5-P(2) increase in response to hyperosmotic stress in differentiated 3T3-L1 adipocytes. Because this effect was not observed in the precursor preadipocytes, a specific role during acquisition of the adipocyte phenotype and transition into insulin-responsive cells could be suggested. However, acute insulin action did not result in a measurable PtdIns 3,5-P(2) rise, indicating the PtdIns 3,5-P(2) pathway is a specific hyperosmotically activated signaling cascade selectively operating in differentiated 3T3-L1 adipocytes. Hyperosmolarity activates different components of several kinase cascades, including p38 mitogen-activated protein and tyrosine kinases, but these appear to be separate from the activated PtdIns 3,5-P(2) pathway. Because PtdIns 3,5-P(2) is primarily produced by PIKfyve-catalyzed synthesis and requires the upstream activator hVac14 (called herein ArPIKfyve) that physically associates with and activates PIKfyve, we examined the contribution of ArPIKfyve-PIKfyve for the hyperosmotic stress-induced rise in PtdIns 3,5-P(2). Small interfering RNA-directed gene silencing to selectively deplete ArPIKfyve or PIKfyve in 3T3-L1 adipocytes determined the ArPIKfyve-PIKfyve axis fully accountable for the hyperosmotically activated PtdIns 3,5-P(2). Together these results reveal a previously uncharacterized PtdIns 3,5-P(2) pathway activated selectively in hyperosmotically stressed 3T3-L1 adipocytes and suggest a plausible role for PtdIns 3,5-P(2) in the osmo-protective response mechanism in this cell type.

Calcium regulates the mode of exocytosis induced by hypotonic shock in isolated neuronal presynaptic endings

A decrease in the osmolarity of incubation medium is accompanied by calcium influx in neuronal presynaptic endings. We studied the influence of Ca2+ on exocytosis induced by hypotonic shock using the hydrophilic fluorescent dye acridine orange and the hydrophobic fluorescent dye FM2-10. It was shown using acridine orange that lowering of osmolarity to 230 mOsm/l induces exocytosis both in calcium-containing and calcium-free medium. By contrast, we were able to demonstrate calcium-dependence of exocytosis using styryl dye FM2-10. Lowering of osmolarity leads to increase of [3H]D-aspartate and [3H]GABA release in calcium-free medium. Addition of calcium inhibits hypotonic-induced neurotransmitter release. Decreasing of NaCl concentration to 92 mM in isotonic medium is able to induce d-aspartate and GABA release. Thus, our data suggest that hypotonic swelling induces calcium-independent exocytosis possibly by a "kiss and run" mechanism. Calcium influx mediated by stretch channels is able to provoke full fusion between plasma membrane and synaptic vesicles. [3H]D-aspartate and [3H]GABA released by hypotonic shock is determined by sodium lowering rather than by osmolarity decreasing itself.

Genistein directly inhibits GLUT4-mediated glucose uptake in 3T3-L1 adipocytes

The isoflavone-derivative genistein is commonly applied as an inhibitor of tyrosine kinases. In this report we analyze the effect of genistein on insulin-stimulated glucose uptake in 3T3-L1 adipocytes. In these cells insulin-induced glucose uptake is primarily mediated by the GLUT4 glucose transporter. We observed that pre-treatment with genistein did not affect insulin-induced tyrosine kinase activity of the insulin receptor or activation of protein kinase B. On the other hand, genistein acted as a direct inhibitor of insulin-induced glucose uptake in 3T3-L1 adipocytes with an IC(50) of 20 microM. We conclude that apart from acting as a general tyrosine kinase inhibitor, genistein also affects the function of other proteins such as the GLUT4 transporter. These data suggest that caution must be applied when interpreting data on the involvement of tyrosine kinase activity in glucose uptake in 3T3-L1 adipocytes.

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.

Requirement of protein tyrosine kinase and phosphatase activities for human sperm exocytosis

The acrosome is a membrane-limited granule that overlies the nucleus of the mature spermatozoon. In response to physiological or pharmacological stimuli, sperm undergo calcium-dependent exocytosis termed the acrosome reaction, which is an absolute prerequisite for fertilization. Protein tyrosine phosphorylation and dephosphorylation are a mechanisms by which multiple cellular events are regulated. Here we report that calcium induces tyrosine phosphorylation in streptolysin O (SLO)-permeabilized human sperm. As expected, pretreatment with tyrphostin A47-a tyrosine kinase inhibitor-abolishes the calcium effect. Interestingly, the calcium-induced increase in tyrosine phosphorylation has a functional correlate in sperm exocytosis. Masking of phosphotyrosyl groups with a specific antibody or inhibition of tyrosine kinases with genistein, tyrphostin A47, and tyrphostin A51 prevent the acrosome reaction. By reversibly sequestering intra-acrosomal calcium with a photo-inhibitable chelator, we show a requirement for protein tyrosine phosphorylation late in the exocytotic pathway, after the efflux of intra-acrosomal calcium. Both mouse and human sperm contain highly active tyrosine phosphatases. Importantly, this activity declines when sperm are incubated under capacitating conditions. Inhibition of tyrosine phosphatases with pervanadate, bis(N,N-dimethylhydroxoamido)hydroxovanadate, ethyl-3,4-dephostatin, and phenylarsine oxide prevents the acrosome reaction. Our results show that both tyrosine kinases and phosphatases play a central role in sperm exocytosis.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Positive and negative regulation of glucose uptake by hyperosmotic stress

This review will provide insight on the current understanding of the intracellular signaling mechanisms by which hyperosmolarity mimics insulin responses such as Glut 4 translocation and glucose transport but also antagonizes insulin effects. Glucose uptake induced by insulin is largely dependent on the PI 3-kinase/PKB pathway. In both adipocyte and muscle cells, hyperosmolarity promotes glucose uptake by multiple mechanisms which do not require PI 3-kinase/PKB pathway but are dependent on the cell type. In muscle, osmotic stress induces glucose uptake by stimulation of AMP-Kinase and/or inhibition of Glut 4 endocytosis. In adipocytes, activation of Gab1-dependent signaling pathway plays an important role in osmotic stress-mediated glucose uptake. Apart of its insulin-like effects, hyperosmolarity can lead to cellular insulin resistance mediated by both prevention of PKB activation and inhibition of the Insulin Receptor Substrate-1 (IRS1) function. Serine phosphorylation and degradation of IRS1 negatively regulate its functions. Understanding how osmotic stress induces glucose transport or mediates insulin resistance may provide novel targets for strategies to enhance glucose transport or to prevent insulin resistance.

Aldose reductase induced by hyperosmotic stress mediates cardiomyocyte apoptosis: differential effects of sorbitol and mannitol

Cells adapt to hyperosmotic conditions by several mechanisms, including accumulation of sorbitol via induction of the polyol pathway. Failure to adapt to osmotic stress can result in apoptotic cell death. In the present study, we assessed the role of aldose reductase, the key enzyme of the polyol pathway, in cardiac myocyte apoptosis. Hyperosmotic stress, elicited by exposure of cultured rat cardiac myocytes to the nonpermeant solutes sorbitol and mannitol, caused identical cell shrinkage and adaptive hexose uptake stimulation. In contrast, only sorbitol induced the polyol pathway and triggered stress pathways as well as apoptosis-related signaling events. Sorbitol resulted in activation of the extracellular signal-regulated kinase (ERK), p54 c-Jun N-terminal kinase (JNK), and protein kinase B. Furthermore, sorbitol treatment resulting in induction and activation of aldose reductase, decreased expression of the antiapoptotic protein Bcl-xL, increased DNA fragmentation, and glutathione depletion. Apoptosis was attenuated by aldose reductase inhibition with zopolrestat and also by glutathione replenishment with N-acetylcysteine. In conclusion, our data show that hypertonic shrinkage of cardiac myocytes alone is not sufficient to induce cardiac myocyte apoptosis. Hyperosmolarity-induced cell death is sensitive to the nature of the osmolyte and requires induction of aldose reductase as well as a decrease in intracellular glutathione levels.

Sphingomyelinase activates GLUT4 translocation via a cholesterol-dependent mechanism

A basis for the insulin mimetic effect of sphingomyelinase on glucose transporter isoform GLUT4 translocation remains unclear. Because sphingomyelin serves as a major determinant of plasma membrane cholesterol and a relationship between plasma membrane cholesterol and GLUT4 levels has recently become apparent, we assessed whether GLUT4 translocation induced by sphingomyelinase resulted from changes in membrane cholesterol content. Exposure of 3T3-L1 adipocytes to sphingomyelinase resulted in a time-dependent loss of sphingomyelin from the plasma membrane and a concomitant time-dependent accumulation of plasma membrane GLUT4. Degradation products of sphingomyelin did not mimic this stimulatory action. Plasma membrane cholesterol amount was diminished in cells exposed to sphingomyelinase. Restoration of membrane cholesterol blocked the stimulatory effect of sphingomyelinase. Increasing concentrations of methyl-beta-cyclodextrin, which resulted in a dose-dependent reversible decrease in membrane cholesterol, led to a dose-dependent reversible increase in GLUT4 incorporation into the plasma membrane. Although increased plasma membrane GLUT4 content by cholesterol extraction with concentrations of methyl-beta-cyclodextrin above 5 mM most likely reflected decreased GLUT4 endocytosis, translocation stimulated by sphingomyelinase or concentrations of methyl-beta-cyclodextrin below 2.5 mM occurred without any visible changes in the endocytic retrieval of GLUT4. Furthermore, moderate loss of cholesterol induced by sphingomyelinase or low concentrations of methyl-beta-cyclodextrin did not alter membrane integrity or increase the abundance of other plasma membrane proteins such as the GLUT1 glucose transporter or the transferrin receptor. Regulation of GLUT4 translocation by moderate cholesterol loss did not involve known insulin-signaling proteins. These data reveal that sphingomyelinase enhances GLUT4 exocytosis via a novel cholesterol-dependent mechanism.

Arsenite stimulated glucose transport in 3T3-L1 adipocytes involves both Glut4 translocation and p38 MAPK activity

The protein-modifying agent arsenite stimulates glucose uptake in 3T3-L1 adipocytes. In the current study we have analysed the signalling pathways that contribute to this response. By subcellular fractionation we observed that arsenite, like insulin, induces translocation of the GLUT1 and GLUT4 glucose transporters from the low-density membrane fraction to the plasma membrane. Arsenite did not activate early steps of the insulin receptor (IR)-signalling pathway and the response was insensitive to inhibition of phosphatidylinositol-3'-kinase (PI-3') kinase by wortmannin. These findings indicate that the 'classical' IR-IR substrate-PI-3' kinase pathway, that is essential for insulin-induced GLUT4 translocation, is not activated by arsenite. However, arsenite-treatment did induce tyrosine-phosphorylation of c-Cbl. Furthermore, treatment of the cells with the tyrosine kinase inhibitor, tyrphostin A25, abolished arsenite-induced glucose uptake, suggesting that the induction of a tyrosine kinase by arsenite is essential for glucose uptake. Both arsenite and insulin-induced glucose uptake were inhibited partially by the p38 MAP kinase inhibitor, SB203580. This compound had no effect on the magnitude of translocation of glucose transporters indicating that the level of glucose transport is determined by additional factors. Arsenite- and insulin-induced glucose uptake responded in a remarkably similar dose-dependent fashion to a range of pharmacological- and peptide-inhibitors for atypical PKC-lambda, a downstream target of PI-3' kinase signalling in insulin-induced glucose uptake. These data show that in 3T3-L1 adipocytes both arsenite- and insulin-induced signalling pathways project towards a similar cellular response, namely GLUT1 and GLUT4 translocation and glucose uptake. This response to arsenite is not functionally linked to early steps of the IR-IRS-PI-3' kinase pathway, but does coincide with c-Cbl phosphorylation, basal levels of PKC-lambda activity and p38 MAPK activation.

Push/pull mechanisms of GLUT4 traffic in muscle cells

AIMS

Understanding the mechanisms by which insulin regulates glucose transporter 4 (GLUT4) traffic in skeletal muscle has been a major challenge since the discoveries of glucose transporter's translocation and the cloning of GLUT4. Here we summarize our work of the past 5 years on the regulation of GLUT4 traffic in skeletal muscle cells.

METHODS

L6 cells overexpressing GLUT4 harbouring an exofacial myc epitope gave us the opportunity to perform dynamic assessments of GLUT4 exocytosis, endocytosis, as well as a means to follow GLUT4 molecules along their journey through intracellular compartments.

RESULTS

We found that insulin stimulation, which results in the expected gain in surface GLUT4, is mostly attributed to enhanced GLUT4 exocytosis, and does not significantly affect the initial rate of internalization. Two mechanisms by which insulin enhances GLUT4 exocytosis are described: 'Pull' relates to actin remodelling-based segregation of the insulin signalling molecules and the directed recruitment of GLUT4/VAMP2 containing vesicles. 'Push' is the accelerated inter-endosomal transit of endocytosed GLUT4 molecules through the recycling endosome. The interface between the two types of regulatory input by insulin is suggested to be the budding of GLUT4 from the transferrin receptor (TfR)-containing, recycling endosome.

CONCLUSIONS

We propose a model on the identity of the GLUT4 pools responsible for GLUT4 recruitment to the plasma membrane in the basal state, or following insulin or hyperosmolarity stimuli.

Hyperosmotic stress inhibits insulin receptor substrate-1 function by distinct mechanisms in 3T3-L1 adipocytes

In 3T3-L1 adipocytes, hyperosmotic stress was found to inhibit insulin signaling, leading to an insulin-resistant state. We show here that, despite normal activation of insulin receptor, hyperosmotic stress inhibits both tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and IRS-1-associated phosphoinositide 3 (PI 3)-kinase activity in response to physiological insulin concentrations. Insulin-induced membrane ruffling, which is dependent on PI 3-kinase activation, was also markedly reduced. These inhibitory effects were associated with an increase in IRS-1 Ser307 phosphorylation. Furthermore, the mammalian target of rapamycin (mTOR) inhibitor rapamycin prevented the osmotic shock-induced phosphorylation of IRS-1 on Ser307. The inhibition of mTOR completely reversed the inhibitory effect of hyperosmotic stress on insulin-induced IRS-1 tyrosine phosphorylation and PI 3-kinase activation. In addition, prolonged osmotic stress enhanced the degradation of IRS proteins through a rapamycin-insensitive pathway and a proteasome-independent process. These data support evidence of new mechanisms involved in osmotic stress-induced cellular insulin resistance. Short-term osmotic stress induces the phosphorylation of IRS-1 on Ser307 by an mTOR-dependent pathway. This, in turn, leads to a decrease in early proximal signaling events induced by physiological insulin concentrations. On the other hand, prolonged osmotic stress alters IRS-1 function by inducing its degradation, which could contribute to the down-regulation of insulin action.

Evidence that nitroprusside stimulates glucose uptake in isolated rat cardiomyocytes via mitogen-activated protein kinase

Sodium nitroprusside (SNP), a nitric oxide (NO.) donor, stimulates glucose uptake in skeletal muscle. We investigated the stimulatory effect of SNP on glucose uptake in cardiomyocytes and the possible role of soluble guanylate cyclase, phosphatidylinositol-3-kinase (PI-3-kinase) and the mitogen-activated protein kinases (MAPKs). Cardiomyocytes were isolated from adult male Wistar rats by trypsin/collagenase perfusion and glucose uptake determined from the accumulation of 3H-2-deoxyglucose. SNP caused a dose-dependent increase in glucose uptake with 200-300% increase at 30 mM. Cytochalasin B completely prevented the SNP-induced increase in glucose uptake. 8-Br-cGMP (100 microM) and the NO. donor spermineNONOate (100 microM) were without effect on basal glucose uptake. SNP-stimulated glucose uptake was not inhibited by the guanylate cyclase inhibitor ODQ (10 microM). Sodium ferrocyanide (Na4Fe(CN)6), a compound structurally related to SNP, but without any NO. group, also stimulated glucose uptake in cardiomyocytes suggesting that the effect of SNP could be unrelated to liberation of NO. Wortmannin, an inhibitor of PI-3-kinase, inhibited insulin-stimulated glucose uptake completely but did not affect SNP-stimulated glucose uptake. SNP-stimulated glucose uptake was inhibited by 50 microM PD 098059 (inhibitor of the MAPK-kinases that activate external regulated kinase [ERK1/2]) and by 50 microM SB203580 (inhibitor of p38MAPK). In conclusion, high SNP concentrations dose-dependently stimulate glucose uptake in cardiomyocytes and our data suggest a role for MAPK signalling, but not PI-3-kinase and soluble guanylate cyclase, in stimulation of glucose uptake.

Glucosamine-induced insulin resistance is coupled to O-linked glycosylation of Munc18c

Evidence suggests that glucosamine inhibits distal components regulating insulin-stimulated GLUT4 translocation to the plasma membrane. Here we assessed whether key membrane docking and fusion events were targeted. Consistent with a plasma membrane-localized effect, 3T3-L1 adipocytes exposed to glucosamine displayed an increase in cell-surface O-linked glycosylation and a simultaneously impaired mobilization of GLUT4 by insulin. Analysis of syntaxin 4 and SNAP23, plasma membrane-localized target receptor proteins (t-SNAREs) for the GLUT4 vesicle, showed that they were not cell-surface targets of O-linked glycosylation. However, the syntaxin 4 binding protein, Munc18c, was targeted by O-linked glycosylation. This occurred concomitantly with a block in insulin-stimulated association of syntaxin 4 with its cognate GLUT4 vesicle receptor protein (v-SNARE), VAMP2. In conclusion, our data suggest that the mechanism by which glucosamine inhibits insulin-stimulated GLUT4 translocation involves modification of Munc18c.