ADP-ribosylation factor 6 delineates separate pathways used by endothelin 1 and insulin for stimulating glucose uptake in 3T3-L1 adipocytes

Does coupling to ADP ribosylation factor 6 explain differences between muscarinic and other receptors in interaction with β-adrenoceptor-mediated smooth muscle relaxation?

Numerous studies in airways, ileum, and urinary bladder have demonstrated that relaxation by β-adrenoceptor agonists has lower potency and/or efficacy when contraction was elicited by muscarinic receptor agonists as compared to other G-protein-coupled receptors, KCl, or basal tone, but the molecular mechanisms behind this relative resistance remain unclear. A paper by Huang et al. in this issue demonstrates that NAV2729, an inhibitor of ADP ribosylation factor 6, inhibits contraction of isolated blood vessels elicited by muscarinic receptor agonists, but not by α₁-adrenoceptor agonists or KCl. Against this background, we discuss the role of ADP ribosylation factor 6 in cellular responses to G-protein-coupled receptor stimulation. While ADP ribosylation factor 6 apparently is the only promising molecular explanation for the relative resistance of smooth muscle contraction elicited by muscarinic agonists, the existing data are insufficient for a robust conclusion.

Adipocyte Gq signaling is a regulator of glucose and lipid homeostasis in mice

Obesity is the major driver of the global epidemic in type 2 diabetes (T2D). In individuals with obesity, impaired insulin action leads to increased lipolysis in adipocytes, resulting in elevated plasma free fatty acid (FFA) levels that promote peripheral insulin resistance, a hallmark of T2D. Here we show, by using a combined genetic/biochemical/pharmacologic approach, that increased adipocyte lipolysis can be prevented by selective activation of adipocyte Gq signaling in vitro and in vivo (in mice). Activation of this pathway by a Gq-coupled designer receptor or by an agonist acting on an endogenous adipocyte Gq-coupled receptor (CysLT2 receptor) greatly improved glucose and lipid homeostasis in obese mice or in mice with adipocyte insulin receptor deficiency. Our findings identify adipocyte Gq signaling as an essential regulator of whole-body glucose and lipid homeostasis and should inform the development of novel classes of GPCR-based antidiabetic drugs.

Impaired Glucose Tolerance and Reduced Plasma Insulin Precede Decreased AKT Phosphorylation and GLUT3 Translocation in the Hippocampus of Old 3xTg-AD Mice

Several studies have demonstrated that mouse models of Alzheimer's disease (AD) can exhibit impaired peripheral glucose tolerance. Further, in the APP/PS1 mouse model, this is observed prior to the appearance of AD-related neuropathology (e.g., amyloid-β plaques; Aβ) or cognitive impairment. In the current study, we examined whether impaired glucose tolerance also preceded AD-like changes in the triple transgenic model of AD (3xTg-AD). Glucose tolerance testing (GTT), insulin ELISAs, and insulin tolerance testing (ITT) were performed at ages prior to (1-3 months and 6-8 months old) and post-pathology (16-18 months old). Additionally, we examined for altered insulin signaling in the hippocampus. Western blots were used to evaluate the two-primary insulin signaling pathways: PI3K/AKT and MAPK/ERK. Since the PI3K/AKT pathway affects several downstream targets associated with metabolism (e.g., GSK3, glucose transporters), western blots were used to examine possible alterations in the expression, translocation, or activation of these targets. We found that 3xTg-AD mice display impaired glucose tolerance as early as 1 month of age, concomitant with a decrease in plasma insulin levels well prior to the detection of plaques (∼14 months old), aggregates of hyperphosphorylated tau (∼18 months old), and cognitive decline (≥18 months old). These alterations in peripheral metabolism were seen at all time points examined. In comparison, PI3K/AKT, but not MAPK/ERK, signaling was altered in the hippocampus only in 18-20-month-old 3xTg-AD mice, a time point at which there was a reduction in GLUT3 translocation to the plasma membrane. Taken together, our results provide further evidence that disruptions in energy metabolism may represent a foundational step in the development of AD.

Genome-Wide Meta-Analysis Unravels Interactions between Magnesium Homeostasis and Metabolic Phenotypes

Magnesium (Mg²⁺) homeostasis is critical for metabolism. However, the genetic determinants of the renal handling of Mg²⁺, which is crucial for Mg²⁺ homeostasis, and the potential influence on metabolic traits in the general population are unknown. We obtained plasma and urine parameters from 9099 individuals from seven cohorts, and conducted a genome-wide meta-analysis of Mg²⁺ homeostasis. We identified two loci associated with urinary magnesium (uMg), rs3824347 (P=4.4×10^-13) near TRPM6, which encodes an epithelial Mg²⁺ channel, and rs35929 (P=2.1×10^-11), a variant of ARL15, which encodes a GTP-binding protein. Together, these loci account for 2.3% of the variation in 24-hour uMg excretion. In human kidney cells, ARL15 regulated TRPM6-mediated currents. In zebrafish, dietary Mg²⁺ regulated the expression of the highly conserved ARL15 ortholog arl15b, and arl15b knockdown resulted in renal Mg²⁺ wasting and metabolic disturbances. Finally, ARL15 rs35929 modified the association of uMg with fasting insulin and fat mass in a general population. In conclusion, this combined observational and experimental approach uncovered a gene-environment interaction linking Mg²⁺ deficiency to insulin resistance and obesity.

EGFR/ARF6 regulation of Hh signalling stimulates oncogenic Ras tumour overgrowth

Multiple signalling events interact in cancer cells. Oncogenic Ras cooperates with Egfr, which cannot be explained by the canonical signalling paradigm. In turn, Egfr cooperates with Hedgehog signalling. How oncogenic Ras elicits and integrates Egfr and Hedgehog signals to drive overgrowth remains unclear. Using a Drosophila tumour model, we show that Egfr cooperates with oncogenic Ras via Arf6, which functions as a novel regulator of Hh signalling. Oncogenic Ras induces the expression of Egfr ligands. Egfr then signals through Arf6, which regulates Hh transport to promote Hh signalling. Blocking any step of this signalling cascade inhibits Hh signalling and correspondingly suppresses the growth of both, fly and human cancer cells harbouring oncogenic Ras mutations. These findings highlight a non-canonical Egfr signalling mechanism, centered on Arf6 as a novel regulator of Hh signalling. This explains both, the puzzling requirement of Egfr in oncogenic Ras-mediated overgrowth and the cooperation between Egfr and Hedgehog.

EGFR signalling is required for oncogenic Ras driven tumorigenesis. In this study, using a Drosophila tumour model the authors demonstrate that depletion of Arf6, a Ras-related GTP-binding protein activated by EGFR, supresses oncogenic Ras driven overgrowth via modulation of Hedgehog signalling.

S6 Kinase Reflects and Regulates Ethanol-Induced Sedation

Alcohol use disorders (AUDs) affect people at great individual and societal cost. Individuals at risk for AUDs are sensitive to alcohol's rewarding effects and/or resistant to its aversive and sedating effects. The molecular basis for these traits is poorly understood. Here, we show that p70 S6 kinase (S6k), acting downstream of the insulin receptor (InR) and the small GTPase Arf6, is a key mediator of ethanol-induced sedation in Drosophila. S6k signaling in the adult nervous system determines flies' sensitivity to sedation. Furthermore, S6k activity, measured via levels of phosphorylation (P-S6k), is a molecular marker for sedation and overall neuronal activity: P-S6k levels are decreased when neurons are silenced, as well as after acute ethanol sedation. Conversely, P-S6k levels rebound upon recovery from sedation and are increased when neuronal activity is enhanced. Reducing neural activity increases sensitivity to ethanol-induced sedation, whereas neuronal activation decreases ethanol sensitivity. These data suggest that ethanol has acute silencing effects on adult neuronal activity, which suppresses InR/Arf6/S6k signaling and results in behavioral sedation. In addition, we show that activity of InR/Arf6/S6k signaling determines flies' behavioral sensitivity to ethanol-induced sedation, highlighting this pathway in acute responses to ethanol.

SIGNIFICANCE STATEMENT

Genetic factors play a major role in the development of addiction. Identifying these genes and understanding their molecular mechanisms is a necessary first step in the development of targeted therapeutic intervention. Here, we show that signaling from the insulin receptor in Drosophila neurons determines flies' sensitivity to ethanol-induced sedation. We show that this signaling cascade includes the small GTPase Arf6 and S6 kinase (S6k). In addition, activity of S6k is regulated by acute ethanol exposure and by neuronal activity. S6k activity is therefore both an acute target of ethanol exposure and a regulator of ethanol's effects on behavior.

A novel regulatory function of sweet taste-sensing receptor in adipogenic differentiation of 3T3-L1 cells

BACKGROUND

Sweet taste receptor is expressed not only in taste buds but also in nongustatory organs such as enteroendocrine cells and pancreatic beta-cells, and may play more extensive physiological roles in energy metabolism. Here we examined the expression and function of the sweet taste receptor in 3T3-L1 cells.

METHODOLOGY/PRINCIPAL FINDINGS

In undifferentiated preadipocytes, both T1R2 and T1R3 were expressed very weakly, whereas the expression of T1R3 but not T1R2 was markedly up-regulated upon induction of differentiation (by 83.0 and 3.8-fold, respectively at Day 6). The α subunits of Gs (Gαs) and G14 (Gα14) but not gustducin were expressed throughout the differentiation process. The addition of sucralose or saccharin during the first 48 hours of differentiation considerably reduced the expression of peroxisome proliferator activated receptor γ (PPARγ and CCAAT/enhancer-binding protein α (C/EBPα at Day 2, the expression of aP2 at Day 4 and triglyceride accumulation at Day 6. These anti-adipogenic effects were attenuated by short hairpin RNA-mediated gene-silencing of T1R3. In addition, overexpression of the dominant-negative mutant of Gαs but not YM-254890, an inhibitor of Gα14, impeded the effects of sweeteners, suggesting a possible coupling of Gs with the putative sweet taste-sensing receptor. In agreement, sucralose and saccharin increased the cyclic AMP concentration in differentiating 3T3-L1 cells and also in HEK293 cells heterologously expressing T1R3. Furthermore, the anti-adipogenic effects of sweeteners were mimicked by Gs activation with cholera toxin but not by adenylate cyclase activation with forskolin, whereas small interfering RNA-mediated knockdown of Gαs had the opposite effects.

CONCLUSIONS

3T3-L1 cells express a functional sweet taste-sensing receptor presumably as a T1R3 homomer, which mediates the anti-adipogenic signal by a Gs-dependent but cAMP-independent mechanism.

Generation and characterization of Tmeff2 mutant mice

TMEFF2 is a single-transmembrane protein containing one EGF-like and two follistatin-like domains. Some studies implicated TMEFF2 as a tumor suppressor for prostate and other cancers, whereas others reported TMEFF2 functioning as a growth factor for neurons and other cells. To gain insights into the apparently conflicting roles of TMEFF2, we generated a null allele of Tmeff2 gene by replacing its first coding exon with human placental alkaline phosphatase cDNA (Tmeff2(PLAP)). Tmeff2(PLAP/PLAP) homozygous mutant mice are born normal, but show growth retardation and die around weaning age. Tmeff2 is widely expressed in the nervous system, and the Tmeff2(PLAP) knock-in allele enables the visualization of neuronal innervations of skin and internal organs with a simple alkaline phosphatase staining. Tmeff2 is also highly expressed in prostate gland and white adipose tissues (WAT). However, with the exception of reduced WAT mass, extensive anatomical and molecular analyses failed to detect any structural or molecular abnormalities in the brain, the spinal cord, the enteric nervous system, or the prostate in the Tmeff2 mutants. No tumors were found in Tmeff2-mutant mice. The Tmeff2(PLAP/PLAP) knock-in mouse is an useful tool for studying the in vivo biological functions of TMEFF2.

Lipid-induced insulin resistance mediated by the proinflammatory receptor TLR4 requires saturated fatty acid-induced ceramide biosynthesis in mice

Obesity is associated with an enhanced inflammatory response that exacerbates insulin resistance and contributes to diabetes, atherosclerosis, and cardiovascular disease. One mechanism accounting for the increased inflammation associated with obesity is activation of the innate immune signaling pathway triggered by TLR4 recognition of saturated fatty acids, an event that is essential for lipid-induced insulin resistance. Using in vitro and in vivo systems to model lipid induction of TLR4-dependent inflammatory events in rodents, we show here that TLR4 is an upstream signaling component required for saturated fatty acid-induced ceramide biosynthesis. This increase in ceramide production was associated with the upregulation of genes driving ceramide biosynthesis, an event dependent of the activity of the proinflammatory kinase IKKβ. Importantly, increased ceramide production was not required for TLR4-dependent induction of inflammatory cytokines, but it was essential for TLR4-dependent insulin resistance. These findings suggest that sphingolipids such as ceramide might be key components of the signaling networks that link lipid-induced inflammatory pathways to the antagonism of insulin action that contributes to diabetes.

Role of insulin-like growth factor-binding protein 5 (IGFBP5) in organismal and pancreatic beta-cell growth

A family of IGF-binding proteins (IGFBP) exerts biological actions both dependent on and independent of IGF-I. A major effector of the insulin/IGF-I signaling pathway, the serine/threonine protein kinase Akt, mediates cellular processes such as glucose uptake, protein synthesis, cell survival, and growth. IGF-I is required for normal organismal growth, and in the pancreatic beta-cell, the insulin/IGF-I signaling pathway is critical for normal and adaptive maintenance of beta-cell mass. Expression of myrAkt1, an activated form of Akt, in the endocrine pancreas drives beta-cell expansion through dramatic increases in both islet and beta-cell size and number. Herein we present a comparative expression profiling of myrAkt1 transgenic islets that demonstrates the increased abundance of transcripts encoding proteins associated with growth, suppression of apoptosis, RNA processing, and metabolism. Although IGFBP5 is identified as a gene induced by Akt1 activation in the beta-cell, Igfbp5 expression is not necessary for myrAkt1 to augment beta-cell size or mass in vivo. However, in the absence of Igfbp5, mice demonstrate an increase in size and mild glucose intolerance. This is accentuated during diet-induced obesity, when Igfbp5-deficient mice have increased adiposity compared with wild-type mice on the same diet. These studies reveal a novel role for Igfbp5 in the control of growth and metabolism.

The transcription factors Egr1 and Egr2 have opposing influences on adipocyte differentiation

The zinc finger-containing transcription factors Egr1 (Krox24) and Egr2 (Krox20) have been implicated in the proliferation and differentiation of many cell types. Egr2 has earlier been shown to play a positive role in adipocyte differentiation, but the function of Egr1 in this context is unknown. We compared the roles of Egr1 and Egr2 in the differentiation of murine 3T3-L1 adipocytes. Egr1 protein was rapidly induced after addition of differentiation cocktail, whereas Egr2 protein initially remained low before increasing on days 1 and 2, concomitant with the disappearance of Egr1. In marked contrast to the effects of Egr2, differentiation was inhibited by ectopic expression of Egr1 and potentiated by knockdown of Egr1. The pro-adipogenic effects of Egr1 knockdown were particularly notable when isobutylmethylxanthine (IBMX) was omitted from the differentiation medium. However, knockdown of Egr1 did not affect CCAAT/enhancer binding protein (C/EBP)beta protein expression or phosphorylation of CREB Ser133. Further, Egr1 did not directly affect the activity of promoters for the master adipogenic transcription factors, C/EBPalpha or peroxisome proliferator-activated receptor-gamma2, as assessed in luciferase reporter assays. These data indicate that Egr1 and Egr2 exert opposing influences on adipocyte differentiation and that the careful regulation of both is required for maintaining appropriate levels of adipogenesis. Further, the pro-differentiation effects of IBMX involve suppression of the inhibitory influence of Egr1.

Isoform-specific regulation of adipocyte differentiation by Akt/protein kinase Balpha

The phosphatidylinositol 3-kinase (PI3K)/Akt pathway tightly regulates adipose cell differentiation. Here we show that loss of Akt1/PKBalpha in primary mouse embryo fibroblast (MEF) cells results in a defect of adipocyte differentiation. Adipocyte differentiation in vitro and ex vivo was restored in cells lacking both Akt1/PKBalpha and Akt2/PKBbeta by ectopic expression of Akt1/PKBalpha but not Akt2/PKBbeta. Akt1/PKBalpha was found to be the major regulator of phosphorylation and nuclear export of FoxO1, whose presence in the nucleus strongly attenuates adipocyte differentiation. Differentiation-induced cell division was significantly abrogated in Akt1/PKBalpha-deficient cells, but was restored after forced expression of Akt1/PKBalpha. Moreover, expression of p27(Kip1), an inhibitor of the cell cycle, was down regulated in an Akt1/PKBalpha-specific manner during adipocyte differentiation. Based on these data, we suggest that the Akt1/PKBalpha isoform plays a major role in adipocyte differentiation by regulating FoxO1 and p27(Kip1).

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.

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.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

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.

The GDP-bound form of Arf6 is located at the plasma membrane

The function of Arf6 has been investigated largely by using the T27N and the Q67L mutants, which are thought to be blocked in GDP- and GTP-bound states, respectively. However, these mutants have been poorly characterized biochemically. Here, we found that Arf6(T27N) is not an appropriate marker of the inactive GDP-bound form because it has a high tendency to lose its nucleotide in vitro and to denature. As a consequence, most of the protein is aggregated in vivo and localizes to detergent-insoluble structures. However, a small proportion of Arf6(T27N) is able to form a stable complex with its exchange factor EFA6 at the plasma membrane, accounting for its dominant-negative phenotype. To define the cellular localization of Arf6-GDP, we designed a new mutant, Arf6(T44N). In vitro, this mutant has a 30-fold decreased affinity for GTP. In vivo, it is mostly GDP bound and, in contrast to the wild type, does not switch to the active conformation when expressed with EFA6. This GDP-locked mutant is found at the plasma membrane, where it localizes with EFA6 and Ezrin in actin- and phosphatidylinositol (4,5)-bisphosphate-enriched domains. From these results, we conclude that the Arf6 GDP-GTP cycle takes place at the plasma membrane.

Neomycin Prevents the Wortmannin Inhibition of Insulin-stimulated Glut4 Translocation and Glucose Transport in 3T3-L1 Adipocytes

Insulin stimulates the movement of the facilitative glucose transporter glucose transporter-4 (Glut4) from an intracellular compartment to the plasma membrane in adipocytes and muscle cells, resulting in an increased rate of glucose uptake. Insulin-stimulated Glut4 translocation and glucose transport are abolished by wortmannin, a specific inhibitor of phosphatidylinositol 3'-kinase (PI3K). Here, we demonstrate that neomycin, a drug that masks the cellular substrate of PI3K, phosphatidylinositol 4,5-bisphosphate (PIP), prevents wortmannin inhibition of insulin-stimulated (2)Glut4 translocation and glucose transport without activating protein kinase B, a downstream effector of PI3K. These results suggest that PIP(2) may have an important regulatory function in insulin-stimulated Glut4 translocation and glucose transport.

Phosphatidylinositol 4,5-bisphosphate regulates adipocyte actin dynamics and GLUT4 vesicle recycling

To investigate the potential role of phosphatidylinositol 4, 5-bisphosphate (PI(4,5)P2) in the regulation of actin polymerization and GLUT4 translocation, the type I phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) were expressed in 3T3L1 adipocytes. In preadipocytes (fibroblasts) PIP5K expression promoted actin polymerization on membrane-bound vesicles to form motile actin comets. In contrast, expression of PIP5K in differentiated 3T3L1 adipocytes resulted in the formation of enlarged vacuole-like structures coated with F-actin, cortactin, dynamin, and N-WASP. Treatment with either latrunculin B (an inhibitor for actin polymerization) or Clostridium difficile toxin B (a general Rho family inhibitor) resulted in a relatively slower disappearance of coated F-actin from these vacuoles, but the vacuoles themselves remained unaffected. Functionally, the increased PI(4,5)P2 levels resulted in an inhibition of transferrin receptor and GLUT4 endocytosis and a slow accumulation of these proteins in the PI(4,5)P2-enriched vacuoles along with the non-clathrin-derived endosome marker (caveolin) and the AP-2 adaptor complex. However, these structures were devoid of early endosome markers (EEA1, clathrin) and the biosynthetic membrane secretory machinery markers p115 (Golgi) and syntaxin 6 (trans-Golgi Network). Taken together, these data demonstrate that PI(4,5)P2 has distinct morphologic and functional properties depending upon specific cell context. In adipocytes, altered PI(4,5)P2 metabolism has marked effects on GLUT4 endocytosis and intracellular vesicle trafficking due to the derangement of actin dynamics.

Isoform-specific Regulation of Insulin-dependent Glucose Uptake by Akt/Protein Kinase B

Recent data have implicated the serine/threonine protein kinase Akt/protein kinase B (PKB) in a diverse array of physiological pathways, raising the question of how biological specificity is maintained. Partial clarification derived from the observation that mice deficient in either of the two isoforms, Akt1/PKBalpha or Akt2/PKBbeta, demonstrate distinct abnormalities, i.e. reduced organismal size or insulin resistance, respectively. However, the question still persists as to whether these divergent phenotypes are due exclusively to tissue-specific differences in isoform expression or distinct capacities for signaling intrinsic to the two proteins. Here we show that Akt2/PKBbeta-/- adipocytes derived from immortalized mouse embryo fibroblasts display significantly reduced insulin-stimulated hexose uptake, clearly establishing that the partial defect in glucose disposal in these mice derives from lack of a cell autonomous function of Akt2/PKBbeta. Moreover, in adipocytes differentiated from primary fibroblasts or immortalized mouse embryo fibroblasts, and brown preadipocytes the absence of Akt2/PKBbeta resulted in reduction of insulin-induced hexose uptake and glucose transporter 4 (GLUT4) translocation, whereas Akt1/PKBalpha was dispensable for this effect. Most importantly, hexose uptake and GLUT4 translocation were completely restored after re-expression of Akt2/PKBbeta in Akt2/PKBbeta-/- adipocytes, but overexpression of Akt1/PKBalpha at comparable levels was ineffective at rescuing insulin action to normal. These results show that the Akt1/PKBalpha and Akt2/PKBbeta isoforms are uniquely adapted to preferentially transmit distinct biological signals, and this property is likely to contribute significantly to the ability of Akt/PKB to play a role in diverse processes.

Recycling of Raft-associated prohormone sorting receptor carboxypeptidase E requires interaction with ARF6

Little is known about the molecular mechanism of recycling of intracellular receptors and lipid raft-associated proteins. Here, we have investigated the recycling pathway and internalization mechanism of a transmembrane, lipid raft-associated intracellular prohormone sorting receptor, carboxypeptidase E (CPE). CPE is found in the trans-Golgi network (TGN) and secretory granules of (neuro)endocrine cells. An extracellular domain of the IL2 receptor alpha-subunit (Tac) fused to the transmembrane domain and cytoplasmic tail of CPE (Tac-CPE25) was used as a marker to track recycling of CPE. We show in (neuro)endocrine cells, that upon stimulated secretory granule exocytosis, raft-associated Tac-CPE25 was rapidly internalized from the plasma membrane in a clathrin-independent manner into early endosomes and then transported through the endocytic recycling compartment to the TGN. A yeast two-hybrid screen and in vitro binding assay identified the CPE cytoplasmic tail sequence S472ETLNF477 as an interactor with active small GTPase ADP-ribosylation factor (ARF) 6, but not ARF1. Expression of a dominant negative, inactive ARF6 mutant blocked this recycling. Mutation of residues S472 or E473 to A in the cytoplasmic tail of CPE obliterated its binding to ARF6, and internalization from the plasma membrane of Tac-CPE25 mutated at S472 or E473 was significantly reduced. Thus, CPE recycles back to the TGN by a novel mechanism requiring ARF6 interaction and activity.

Role of AMP-activated protein kinase in cyclic AMP-dependent lipolysis In 3T3-L1 adipocytes

AMP-activated protein kinase (AMPK) is a phylogenetically conserved intracellular energy sensor that has been implicated as a major regulator of glucose and lipid metabolism in mammals. However, its possible role in mediating or influencing the adrenergic control of lipolysis in adipocytes remains uncertain. In this study, we utilized the murine cultured preadipocyte line 3T3-L1 to examine this question. Treatment of adipocytes with isoproterenol or forskolin promoted the phosphorylation of AMPK at a critical activating Thr-172 residue in a dose- and time-dependent manner. This correlated well with a stimulation of the activity of AMPK, as measured in the immune complex. Analogs of cAMP mimicked the effect of isoproterenol and forskolin on AMPK phosphorylation. Treatment of adipocytes with insulin reduced both basal and forskolin-induced AMPK phosphorylation via a pathway dependent on phosphatidylinositol 3'-kinase. Overexpression of a dominant-inhibitory mutant of AMPK blocked isoproterenol-induced lipolysis by approximately 50%. These data indicate that there exists a novel pathway by which cAMP can lead to the activation of AMPK, and in adipocytes, this is required for maximal activation of lipolysis.

ADP-ribosylation factor 6 regulates insulin secretion through plasma membrane phosphatidylinositol 4,5-bisphosphate

ADP-ribosylation factor 6 (ARF6) is a small GTP-binding protein that regulates peripheral vesicular trafficking and actin cytoskeletal dynamics, and it has been implicated as critical to regulated secretion. Expression of a dominant-inhibitory ARF6 mutant, ARF6(T27N), impaired glucose-, depolarization-, and gamma-thio-GTP-stimulated insulin secretion in the pancreatic beta cell line, MIN6. In response to depolarization, MIN6 cells expressing ARF6(T27N) displayed an unaltered initial fast phase but an impaired subsequent slow phase of insulin secretion. Actin cytoskeletal disassembly with latrunculin A enhanced insulin secretion, whereas stabilization with jasplakinolide inhibited secretion, consistent with the actin cytoskeleton serving as a barrier to exocytosis in these cells. ARF6(T27N) led to a depolarization-dependent reduction in the levels of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] with a time course that paralleled the inhibition of secretion. Moreover, blockade of PI(4,5)P2-dependent events by expression of a lipid-binding protein resulted in inhibition of depolarization-induced secretion in a manner identical to ARF6(T27N). These results indicate that ARF6 is required to sustain adequate levels of PI(4,5)P2 during periods of increased PI(4,5)P2 metabolism such as regulated secretion.

Platelet-derived growth factor (PDGF) stimulates glucose transport in 3T3-L1 adipocytes overexpressing PDGF receptor by a pathway independent of insulin receptor substrates

Insulin is unique among growth factors and hormones in its ability to control metabolic functions such as the stimulation of glucose uptake and glucose transporter (GLUT4) translocation in physiological target tissues, such as muscle and adipose cells. Nonetheless, the mechanisms underlying this specificity have remained incompletely understood, particularly in view of the ability of some growth factors to mimic insulin-dependent early signaling events. In this study, we have probed the basis of insulin specificity by overexpressing in hormone-responsive 3T3-L1 adipocytes wild-type platelet-derived growth factor (PDGF) receptor (PDGFR)-beta and selected, informative mutant receptor proteins. We show that such adipocytes overexpressing wild-type PDGFR on exposure to cognate growth factor activate glucose transport, GLUT4 translocation, and the serine-threonine protein kinase Akt/protein kinase B to a degree comparable with that produced in response to insulin. In addition, PDGF elicits the robust generation of phosphatidylinositol-3,4,5-trisphosphate in vivo in PDGFR-overexpressing 3T3-L1 adipocytes. Expression of PDGFR-beta mutant proteins demonstrates that these responses require the presence of an intact phosphatidylinositol 3-kinase (PI3K)-binding site on the overexpressed PDGF receptor. Furthermore, PDGF stimulates these effects independent of insulin receptor substrate(IRS)-1 or IRS-2 tyrosine phosphorylation or docking to activated PI3K. These data demonstrate that 1) the basis of insulin-specific glucose transport in cultured adipocytes is the low level of receptors for other growth factors and 2) in the presence of adequate receptors, PDGF is fully capable of activating glucose transport in a manner requiring PI3K and subsequent phosphatidylinositol-3,4,5-trisphosphate accumulation but independent of insulin, insulin receptor, and IRS proteins.

Endothelin-1-stimulated glucose uptake is desensitized by tumor necrosis factor-alpha in 3T3-L1 adipocytes

Tumor necrosis factor-alpha (TNF-alpha) is a potent inducer of insulin resistance, and increased TNF-alpha expression is associated with impaired glucose disposal. Although insulin is the primary regulator of glucose transport in adipose, endothelin-1, a vasoconstrictor peptide that signals through the heterotrimeric G proteins Galphaq/11, potently stimulates glucose uptake in 3T3-L1 adipocytes by a mechanism independent of phosphatidylinositol (PI) 3-kinase. Here, we report that exposure of 3T3-L1 adipocytes to TNF-alpha for 48 h dose-dependently decreased endothelin-1-stimulated glucose uptake and translocation of GLUT4 to the plasma membrane. TNF-alpha exposure had no effect on endothelin-1 receptor number at the cell surface. In contrast, TNF-alpha treatment reduced the quantity of Galphaq/11 and proline-rich tyrosine kinase 2 (PYK2) and decreased endothelin-1-stimulated PYK2-Tyr402 tyrosine phosphorylation. Taken together, these results suggest that TNF-alpha-induced desensitization of endothelin-1-stimulated GLUT4 translocation and glucose uptake in 3T3-L1 adipocytes is due, at least in part, to a decreased expression of Galphaq/11, leading to a suppression in tyrosine phosphorylation of PYK2.

Dynamics of phosphoinositides in membrane retrieval and insertion

The phosphoinositides PtdIns(4,5)P2 and PtdIns(3,4,5)P3 are concentrated in plasma membranes of eukaryotic cells, and excluded from endosomes, whereas PtdIns(3)P is formed in these latter intracellular membranes and is apparently excluded from the plasma membrane. The logic of this asymmetric disposition is now revealed by the nature of the effector proteins that selectively bind these lipids through specific modules and by the processes that they catalyze. PtdIns(3,4,5)P3 has a role in directing exocytosis, in addition to many other signaling events, whereas PtdIns(4,5)P2 directs endocytosis through its ability to anchor several coat proteins to the plasma membrane. Remarkably, the elimination of PtdIns(4,5)P2 from forming endosomes may be required for membrane fission to occur. Thus membrane insertion and retrieval can be regulated by plasma membrane concentrations of PtdIns(3,4,5)P3 and PtdIns(4,5)P2, whereas PtdIns(3)P directs the downstream trafficking and recycling of intracellular membranes through its attraction of proteins that catalyze these processes. The phosphoinositides thereby control many cell features that depend upon protein sorting, including the composition of the plasma membrane itself, which in turn determines the cell's responses to its environment.

Cdc42 is a Rho GTPase family member that can mediate insulin signaling to glucose transport in 3T3-L1 adipocytes

We investigated the role of cdc42, a Rho GTPase family member, in insulin-induced glucose transport in 3T3-L1 adipocytes. Microinjection of anti-cdc42 antibody or cdc42 siRNA led to decreased insulin-induced and constitutively active G(q) (CA-G(q); Q209L)-induced GLUT4 translocation. Adenovirus-mediated expression of constitutively active cdc42 (CA-cdc42; V12) stimulated 2-deoxyglucose uptake to 56% of the maximal insulin response, and this was blocked by treatment with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, wortmannin, or LY294002. Both insulin and CA-G(q) expression caused an increase in cdc42 activity, showing that cdc42 is activated by insulin and is downstream of G alpha(q/11) in this activation pathway. Immunoprecipitation experiments showed that insulin enhanced a direct association of cdc42 and p85, and both insulin treatment and CA-cdc42 expression stimulated PI3-kinase activity in immunoprecipitates with anti-cdc42 antibody. Furthermore, the effects of insulin, CA-G(q), and CA-cdc42 on GLUT4 translocation or 2-deoxyglucose uptake were inhibited by microinjection of anti-protein kinase C lambda (PKC lambda) antibody or overexpression of a kinase-deficient PKC lambda construct. In summary, activated cdc42 can mediate 1) insulin-stimulated GLUT4 translocation and 2) glucose transport in a PI3-kinase-dependent manner. 3) Insulin treatment and constitutively active G(q) expression can enhance the cdc42 activity state as well as the association of cdc42 with activated PI3-kinase. 4) PKC lambda inhibition blocks CA-cdc42, CA-G(q), and insulin-stimulated GLUT4 translocation. Taken together, these data indicate that cdc42 can mediate insulin signaling to GLUT4 translocation and lies downstream of G alpha(q/11) and upstream of PI3-kinase and PKC lambda in this stimulatory pathway.

Epithelial cell cytoskeleton and intracellular trafficking V. Confluence of membrane trafficking and motility in epithelial cell models

Migration of epithelial cells occurs in a variety of important biological processes including tissue morphogenesis, wound healing, and the metastasis of epithelial tumors. In some instances, the cells remain attached to each other and migrate together as a sheet, maintaining epithelial integrity. In others (e.g., metastasis), junctional complexes are disrupted and cells migrate individually. In both cases, motility involves the extension of membranous protrusions (filopodia and lamellipodia) in the direction of movement and the transient assembly and disassembly of integrin-mediated adhesions with the extracellular matrix. The driving force for these events is provided by regulated changes in the organization of the actin cytoskeleton, which are thought to be coordinated with alterations in intracellular membrane traffic. In this themes article, I review current hypotheses about how these processes are integrated and attempt to identify fruitful areas for future research.

An adipocentric view of signaling and intracellular trafficking

Adipocytes have traditionally been considered to be the primary site for whole body energy storage mainly in the form of triglycerides and fatty acids. This occurs through the ability of insulin to markedly stimulate both glucose uptake and lipogenesis. Conventional wisdom held that defects in fuel partitioning into adipocytes either because of increased adipose tissue mass and/or increased lipolysis and circulating free fatty acids resulted in dyslipidemia, obesity, insulin resistance and perhaps diabetes. However, it has become increasingly apparent that loss of adipose tissue (lipodystrophies) in both animal models and humans also leads to metabolic disorders that result in severe states of insulin resistance and potential diabetes. These apparently opposite functions can be resolved by the establishment of adipocytes not only as a fuel storage depot but also as a critical endocrine organ that secretes a variety of signaling molecules into the circulation. Although the molecular function of these adipocyte-derived signals are poorly understood, they play a central role in the maintenance of energy homeostasis by regulating insulin secretion, insulin action, glucose and lipid metabolism, energy balance, host defense and reproduction. The diversity of these secretory factors include enzymes (lipoprotein lipase (LPL) and adipsin), growth factors [vascular endothelial growth factor (VEGF)], cytokines (tumor necrosis factor-alpha, interleukin 6) and several other hormones involved in fatty acid and glucose metabolism (leptin, Acrp30, resistin and acylation stimulation protein). Despite the large number of molecules secreted by adipocytes, our understanding of the pathways and mechanisms controlling intracellular trafficking and exocytosis in adipocytes is poorly understood. In this article, we will review the current knowledge of the trafficking and secretion processes that take place in adipocytes, focusing our attention on two of the best characterized adipokine molecules (leptin and adiponectin) and on one of the most intensively studied regulated membrane proteins, the GLUT4 glucose transporter.

A phosphatidylinositol 3-kinase-independent insulin signaling pathway to N-WASP/Arp2/3/F-actin required for GLUT4 glucose transporter recycling

Recruitment of intracellular glucose transporter 4 (GLUT4) to the plasma membrane of fat and muscle cells in response to insulin requires phosphatidylinositol (PI) 3-kinase as well as a proposed PI 3-kinase-independent pathway leading to activation of the small GTPase TC10. Here we show that in cultured adipocytes insulin causes acute cortical localization of the actin-regulatory neural Wiskott-Aldrich syndrome protein (N-WASP) and actin-related protein-3 (Arp3) as well as cortical F-actin polymerization by a mechanism that is insensitive to the PI 3-kinase inhibitor wortmannin. Expression of the dominant inhibitory N-WASP-DeltaWA protein lacking the Arp and actin binding regions attenuates the cortical F-actin rearrangements by insulin in these cells. Remarkably, the N-WASP-DeltaWA protein also inhibits insulin action on GLUT4 translocation, indicating dependence of GLUT4 recycling on N-WASP-directed cortical F-actin assembly. TC10 exhibits sequence similarity to Cdc42 and has been reported to bind N-WASP. We show the inhibitory TC10 (T31N) mutant, which abrogates insulin-stimulated GLUT4 translocation and glucose transport, also inhibits both cortical localization of N-WASP and F-actin formation in response to insulin. These findings reveal that N-WASP likely functions downstream of TC10 in a PI 3-kinase-independent insulin signaling pathway to mobilize cortical F-actin, which in turn promotes GLUT4 responsiveness to insulin.

PYK2 as a mediator of endothelin-1/G alpha 11 signaling to GLUT4 glucose transporters

Endothelin-1 (ET-1) signaling through G alpha(q/11) stimulates translocation of intracellular GLUT4 glucose transporters to the plasma membrane of 3T3-L1 adipocytes by an unknown mechanism that requires protein tyrosine phosphorylation and ADP-ribosylation factor 6 (ARF6) but is independent of phosphatidylinositol 3 (PI3)-kinase. In contrast, insulin action on this process requires PI3-kinase but not ARF6. Here we report the identification of two proteins selectively tyrosine-phosphorylated in response to ET-1 but not insulin: the Ca(2+)-activated tyrosine kinase PYK2 and its physiological substrate, the adhesion scaffold protein paxillin. Endogenous paxillin as well as expressed Myc-tagged PYK2 or a Myc-tagged kinase-deficient PYK2 protein were acutely directed to F-actin-rich adhesion sites from the adipocyte cytoplasm in response to ET-1 but not insulin. CADTK-related non-kinase (CRNK) is a dominant negative form of PYK2 containing the C-terminal portion of the protein, which binds paxillin but lacks the PYK2 autophosphorylation site (Tyr(402)). CRNK expression in 3T3-L1 adipocytes inhibited ET-1-mediated F-actin polymerization and translocation of Myc-tagged GLUT4-enhanced green fluorescent protein (EGFP) to the plasma membrane without disrupting insulin action on these processes. These data reveal the tyrosine kinase PYK2 as a required signaling element in the regulation of GLUT4 recycling in 3T3-L1 adipocytes by ET-1, whereas insulin signaling is directed through a different pathway.

beta-Arrestin-mediated ADP-ribosylation factor 6 activation and beta 2-adrenergic receptor endocytosis

beta-Arrestins are multifunctional adaptor proteins known to regulate internalization of agonist-stimulated G protein-coupled receptors by linking them to endocytic proteins such as clathrin and AP-2. Here we describe a previously unappreciated mechanism by which beta-arrestin orchestrates the process of receptor endocytosis through the activation of ADP-ribosylation factor 6 (ARF6), a small GTP-binding protein. Involvement of ARF6 in the endocytic process is demonstrated by the ability of GTP-binding defective and GTP hydrolysis-deficient mutants to inhibit internalization of the beta(2)-adrenergic receptor. The importance of regulation of ARF6 function is shown by the ability of the ARF GTPase-activating protein GIT1 to inhibit and of the ARF nucleotide exchange factor, ARNO, to enhance receptor endocytosis. Endogenous beta-arrestin is found in complex with ARNO. Upon agonist stimulation of the receptor, beta-arrestin also interacts with the GDP-liganded form of ARF6, thereby facilitating ARNO-promoted GTP loading and activation of the G protein. Thus, the agonist-driven formation of a complex including beta-arrestin, ARNO, and ARF6 provides a molecular mechanism that explains how the agonist-stimulated receptor recruits a small G protein necessary for the endocytic process and controls its activation.

G(alpha)11 signaling through ARF6 regulates F-actin mobilization and GLUT4 glucose transporter translocation to the plasma membrane

The action of insulin to recruit the intracellular GLUT4 glucose transporter to the plasma membrane of 3T3-L1 adipocytes is mimicked by endothelin 1, which signals through trimeric G(alpha)q or G(alpha)11 proteins. Here we report that murine G(alpha)11 is most abundant in fat and that expression of the constitutively active form of G(alpha)11 [G(alpha)11(Q209L)] in 3T3-L1 adipocytes causes recruitment of GLUT4 to the plasma membrane and stimulation of 2-deoxyglucose uptake. In contrast to the action of insulin on GLUT4, the effects of endothelin 1 and G(alpha)11 were not inhibited by the phosphatidylinositol 3-kinase inhibitor wortmannin at 100 nM. Signaling by insulin, endothelin 1, or G(alpha)11(Q209L) also mobilized cortical F-actin in cultured adipocytes. Importantly, GLUT4 translocation caused by all three agents was blocked upon disassembly of F-actin by latrunculin B, suggesting that the F-actin polymerization caused by these agents may be required for their effects on GLUT4. Remarkably, expression of a dominant inhibitory form of the actin-regulatory GTPase ARF6 [ARF6(T27N)] in cultured adipocytes selectively inhibited both F-actin formation and GLUT4 translocation in response to endothelin 1 but not insulin. These data indicate that ARF6 is a required downstream element in endothelin 1 signaling through G(alpha)11 to regulate cortical actin and GLUT4 translocation in cultured adipocytes, while insulin action involves different signaling pathways.