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

The role of adhesion molecules in osteocalcin-induced effects on glucose and lipid metabolism in adipocytes

Recent findings suggest that uncarboxylated osteocalcin (GluOC) promotes glucose and lipid metabolism via its putative receptor GPRC6A; however, its direct effect on adipocytes remains elusive. In this study, we elucidated the effects of GluOC on adipocytes, with an emphasis on the role of cell adhesion molecules. We determined that GluOC promoted the expression of adipocyte adhesion molecule (ACAM) and its transcription factor Krüppel-like factor 4 and enhanced the cortical actin filament assembly, which ameliorated lipid droplet hypertrophy. Additionally, GluOC upregulated the expression of integrin αVβ3 and activation of focal adhesion kinase (FAK) and prevented insulin receptor substrate 1 (IRS1) degradation by inhibiting the ubiquitin-proteasome system via the FAK-PLC-PKC axis, which activated IRS1-Akt-mediated glucose transporter 4 (GLUT4) transport. Furthermore, we showed that GluOC elevated the expression of the insulin-independent glucose transporters GLUT1 and GLUT8, which facilitated insulin stimulation-independent glucose transport. The GluOC-induced activation of integrin αVβ3 signaling promoted microtubule assembly, which improved glucose and lipid metabolism via its involvement in intracellular vesicular transport. GluOC treatment also suppressed collagen type 1 formation, which might prevent adipose tissue fibrosis in obese individuals. Overall, our results imply that GluOC promotes glucose and lipid metabolism via ACAM, integrin αVβ3, and GLUT1 and 8 expression, directly affecting adipocytes.

Veneer Is a Webtool for Rapid, Standardized, and Transparent Interpretation, Annotation, and Reporting of Mammalian Cell Surface N-Glycocapture Data

Currently, no consensus exists regarding criteria required to designate a protein within a proteomic data set as a cell surface protein. Most published proteomic studies rely on varied ontology annotations or computational predictions instead of experimental evidence when attributing protein localization. Consequently, standardized approaches for analyzing and reporting cell surface proteome data sets would increase confidence in localization claims and promote data use by other researchers. Recently, we developed Veneer, a web-based bioinformatic tool that analyzes results from cell surface N-glycocapture workflows─the most popular cell surface proteomics method used to date that generates experimental evidence of subcellular location. Veneer assigns protein localization based on defined experimental and bioinformatic evidence. In this study, we updated the criteria and process for assigning protein localization and added new functionality to Veneer. Results of Veneer analysis of 587 cell surface N-glycocapture data sets from 32 published studies demonstrate the importance of applying defined criteria when analyzing cell surface proteomics data sets and exemplify how Veneer can be used to assess experimental quality and facilitate data extraction for informing future biological studies and annotating public repositories.

T1R3 homomeric sweet taste receptor negatively regulates insulin-induced glucose transport through Gαs-mediated microtubules disassembly in 3T3-L1 adipocytes

T1R3 is a class C G protein-coupled receptor family member that forms heterodimeric umami and sweet taste receptors with T1R1 and T1R2, respectively, in the taste cells of taste buds. T1R3 is expressed in 3T3-L1 cells in homomeric form and negatively regulates adipogenesis in a Gαs-dependent but cAMP-independent manner. Although T1R3 expression is markedly upregulated during adipogenesis, its physiological role in mature adipocytes remains obscure. Here, we show that stimulation of T1R3 with sucralose or saccharin induces microtubule disassembly in differentiated 3T3-L1 adipocytes. The effect was reproduced by treatment with cholera toxin or isoproterenol but not with forskolin. Treatment with sucralose or saccharin for 3 h inhibited insulin-stimulated glucose uptake by 32% and 45% in differentiated adipocytes, respectively, similar to the inhibitory effect of nocodazole (by 33%). Isoproterenol treatment inhibited insulin-stimulated glucose transport by 26%, whereas sucralose did not affect the intrinsic activity of the glucose transporter, indicating that it inhibited insulin-induced GLUT4 translocation to the plasma membrane. Immunostaining analysis showed that insulin-stimulated GLUT4 accumulation on the plasma membrane was abrogated in sucralose-treated cells, in association with depolymerization of microtubules. Sucralose-mediated inhibition of GLUT4 translocation was reversed by the overexpression of dominant-negative Gαs (Gαs-G226A) or knockdown of Gαs. Additionally, membrane fractionation analysis showed that sucralose treatment reduced GLUT4 levels in the plasma membrane fraction from insulin-stimulated adipocytes. We have identified a novel non-gustatory role for homomeric T1R3 in adipocytes, and activation of the T1R3 receptor negatively regulates insulin action of glucose transport via Gαs-dependent microtubule disassembly.

Microtubules in insulin action: what's on the tube?

Microtubules (MT) have a role in the intracellular response to insulin stimulation and subsequent glucose transport by glucose transporter 4 (GLUT4), which resides in specialized storage vesicles that travel through the cell. Before GLUT4 is inserted into the plasma membrane for glucose transport, it undergoes complex trafficking through the cell via the integration of cytoskeletal networks. In this review, we highlight the importance of MT elements in insulin action in adipocytes through a summary of MT depolymerization studies, MT-based GLUT4 movement, molecular motor proteins involved in GLUT4 trafficking, as well as MT-related phenomena in response to insulin and links between insulin action and MT-associated proteins.

Hippocampal Proteome of Rats Subjected to the Li-Pilocarpine Epilepsy Model and the Effect of Carisbamate Treatment

In adult rats, the administration of lithium–pilocarpine (LiPilo) reproduces most clinical and neuropathological features of human temporal lobe epilepsy (TLE). Carisbamate (CRS) possesses the property of modifying epileptogenesis in this model. Indeed, about 50% of rats subjected to LiPilo status epilepticus (SE) develop non-convulsive seizures (NCS) instead of motor seizures when treated with CRS. However, the mechanisms underlying these effects remain unknown. The aim of this study was to perform a proteomic analysis in the hippocampus of rats receiving LiPilo and developing motor seizures or NCS following CRS treatment. Fifteen adult male Sprague–Dawley rats were used. SE was induced by LiPilo injection. CRS treatment was initiated at 1 h and 9 h after SE onset and maintained for 7 days, twice daily. Four groups were studied after video-EEG control of the occurrence of motor seizures: a control group receiving saline (CT n = 3) and three groups that underwent SE: rats treated with diazepam (DZP n = 4), rats treated with CRS displaying NCS (CRS-NCS n = 4) or motor seizures (CRS-TLE n = 4). Proteomic analysis was conducted by 2D-SDS-PAGE. Twenty-four proteins were found altered. In the CRS-NCS group, proteins related to glycolysis and ATP synthesis were down-regulated while proteins associated with pyruvate catabolism were up-regulated. Moreover, among the other proteins differentially expressed, we found proteins related to inflammatory processes, protein folding, tissue regeneration, response to oxidative stress, gene expression, biogenesis of synaptic vesicles, signal transduction, axonal transport, microtubule formation, cell survival, and neuronal plasticity. Our results suggest a global reduction of glycolysis and cellular energy production that might affect brain excitability. In addition, CRS seems to modulate proteins related to many other pathways that could significantly participate in the epileptogenesis-modifying effect observed.

PKA-independent cAMP stimulation of white adipocyte exocytosis and adipokine secretion: modulations by Ca2+ and ATP

We examined the effects of cAMP, Ca(2+) and ATP on exocytosis and adipokine release in white adipocytes by a combination of membrane capacitance patch-clamp recordings and biochemical measurements of adipokine secretion. 3T3-L1 adipocyte exocytosis proceeded even in the complete absence of intracellular Ca(2+) ([Ca(2+)]i; buffered with BAPTA) provided cAMP (0.1 mm) was included in the intracellular (pipette-filling) solution. Exocytosis typically plateaued within ∼10 min, probably signifying depletion of a releasable vesicle pool. Inclusion of 3 mm ATP in combination with elevation of [Ca(2+)]i to ≥700 nm augmented the rate of cAMP-evoked exocytosis ∼2-fold and exocytosis proceeded for longer periods (≥20 min) than with cAMP alone. Exocytosis was stimulated to a similar extent upon substitution of cAMP by the Epac (exchange proteins activated by cAMP) agonist 8-Br-2'-O-Me-cAMP (1 mm included in the pipette solution). Inhibition of protein kinase A (PKA) by addition of Rp-cAMPS (0.5 mm) to the cAMP-containing pipette solution was without effect. A combination of the adenylate cyclase activator forskolin (10 μm) and the phosphodiesterase inhibitor IBMX (200 μm; forsk-IBMX) augmented adiponectin secretion measured over 30 min 3-fold and 2-fold in 3T3-L1 and human subcutaneous adipocytes, respectively. This effect was unaltered by pre-loading of cells with the Ca(2+) chelator BAPTA-AM and 2-fold amplified upon inclusion of the Ca(2+) ionophore ionomycin (1 μm) in the extracellular solution. Adiponectin release was also stimulated by the membrane-permeable Epac agonist 8-Br-2'-O-Me-cAMP-AM but unaffected by inclusion of the membrane-permeable PKA inhibitor Rp-8-Br-cAMPS (200 μm). The adipokines leptin, resistin and apelin were present in low amounts in the incubation medium (1-6% of measured adiponectin). Adipsin was secreted in substantial quantities (50% of adiponectin concentration) but release of this adipokine was unaffected by forsk-IBMX. We propose that white adipocyte exocytosis is stimulated by cAMP/Epac-dependent but Ca(2+)- and PKA-independent release of vesicles residing in a readily releasable pool and that the release is augmented by a combination of Ca(2+) and ATP. We further suggest that secreted vesicles chiefly contain adiponectin.

Expression of the glucose-sensing receptor T1R3 in pancreatic islet: changes in the expression levels in various nutritional and metabolic states

We reported recently that the taste type 1 receptor 3 (T1R3), a subunit of the sweet taste receptor, functions as a cell-surface glucose-sensing receptor in pancreatic β-cells. In the present study, we investigated the expression of T1R3 in pancreatic islets. mRNA for T1R2 and T1R3 was detected in mouse pancreatic islets. Quantitatively, the mRNA expression level of T1R2 was less than 1% of that of T1R3. Immunohistochemically, T1R3 was abundantly expressed in mouse islets whereas T1R2 was barely detected. Most immunoreactive T1R3 was colocalized with insulin and almost all β-cells were positive for T1R3. In addition, T1R3 was expressed in some portion of α-cells. Immunoreactivity of T1R3 in β-cells was markedly reduced in fed mice compared to those in fasting mice. In contrast, mRNA for T1R3 was not different in islets of fasting and fed mice. Glucose-induced insulin-secretion was higher in islets obtained from fasting mice compared to those from fed mice. The expression of T1R3 was markedly reduced in islets of ob/ob mice compared to those of control mice. Similarly, the expression of T1R3 was reduced in islet of db/db mice. In addition, the expression of T1R3 was markedly reduced in β-cells of fatty diabetic rats and GK rats, models of obese and non-obese type 2 diabetes, respectively. These results indicate that T1R3 is expressed mainly in β-cells and the expression levels are different depending upon the nutritional and metabolic conditions.

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

Prolonged insulin stimulation down-regulates GLUT4 through oxidative stress-mediated retromer inhibition by a protein kinase CK2-dependent mechanism in 3T3-L1 adipocytes

Although insulin acutely stimulates glucose uptake by promotion of GLUT4 translocation from intracellular compartments to the plasma membrane in adipocytes and muscles, long term insulin stimulation causes GLUT4 depletion that is particularly prominent in the insulin-responsive GLUT4 storage compartment. This effect is caused mainly by accelerated lysosomal degradation of GLUT4, although the mechanism is not fully defined. Here we show that insulin acutely induced dissociation of retromer components from the low density microsomal membranes of 3T3-L1 adipocytes that was accompanied by disruption of the interaction of Vps35 with sortilin. This insulin effect was dependent on the activity of protein kinase CK2 but not phosphatidylinositol 3-kinase or extracellular signal-regulated kinase 1/2. Knockdown of Vps26 decreased GLUT4 to a level comparable with that with insulin stimulation for 4 h. Vps35 with a mutation in the CK2 phosphorylation motif (Vps35-S7A) was resistant to insulin-induced dissociation from the low density microsomal membrane, and its overexpression attenuated GLUT4 down-regulation with insulin. Furthermore, insulin-generated hydrogen peroxide was an upstream mediator of the insulin action on retromer and GLUT4. These results suggested that insulin-generated oxidative stress switches the GLUT4 sorting direction to lysosomes through inhibition of the retromer function in a CK2-dependent manner.

Sites of glucose transporter-4 vesicle fusion with the plasma membrane correlate spatially with microtubules

In adipocytes, vesicles containing glucose transporter-4 (GLUT4) redistribute from intracellular stores to the cell periphery in response to insulin stimulation. Vesicles then fuse with the plasma membrane, facilitating glucose transport into the cell. To gain insight into the details of microtubule involvement, we examined the spatial organization and dynamics of microtubules in relation to GLUT4 vesicle trafficking in living 3T3-L1 adipocytes using total internal reflection fluorescence (TIRF) microscopy. Insulin stimulated an increase in microtubule density and curvature within the TIRF-illuminated region of the cell. The high degree of curvature and abrupt displacements of microtubules indicate that substantial forces act on microtubules. The time course of the microtubule density increase precedes that of the increase in intensity of fluorescently-tagged GLUT4 in this same region of the cell. In addition, portions of the microtubules are highly curved and are pulled closer to the cell cortex, as confirmed by Parallax microscopy. Microtubule disruption delayed and modestly reduced GLUT4 accumulation at the plasma membrane. Quantitative analysis revealed that fusions of GLUT4-containing vesicles with the plasma membrane, detected using insulin-regulated aminopeptidase with a pH-sensitive GFP tag (pHluorin), preferentially occur near microtubules. Interestingly, long-distance vesicle movement along microtubules visible at the cell surface prior to fusion does not appear to account for this proximity. We conclude that microtubules may be important in providing spatial information for GLUT4 vesicle fusion.

Endocytosis, recycling, and regulated exocytosis of glucose transporter 4

Glucose transporter 4 (GLUT4) is responsible for the uptake of glucose into muscle and adipose tissues. Under resting conditions, GLUT4 is dynamically retained through idle cycling among selective intracellular compartments, from whence it undergoes slow recycling to the plasma membrane (PM). This dynamic retention can be released by command from intracellular signals elicited by insulin and other stimuli, which result in 2-10-fold increases in the surface level of GLUT4. Insulin-derived signals promote translocation of GLUT4 to the PM from a specialized compartment termed GLUT4 storage vesicles (GSV). Much effort has been devoted to the characterization of the intracellular compartments and dynamics of GLUT4 cycling and to the signals by which GLUT4 is sorted into, and recruited from, GSV. This review summarizes our understanding of intracellular GLUT4 traffic during its internalization from the membrane, its slow, constitutive recycling, and its regulated exocytosis in response to insulin. In spite of specific differences in GLUT4 dynamic behavior in adipose and muscle cells, the generalities of its endocytic and exocytic itineraries are consistent and an array of regulatory proteins that regulate each vesicular traffic event emerges from these cell systems.

Molecular signatures reveal circadian clocks may orchestrate the homeorhetic response to lactation

Genes associated with lactation evolved more slowly than other genes in the mammalian genome. Higher conservation of milk and mammary genes suggest that species variation in milk composition is due in part to the environment and that we must look deeper into the genome for regulation of lactation. At the onset of lactation, metabolic changes are coordinated among multiple tissues through the endocrine system to accommodate the increased demand for nutrients and energy while allowing the animal to remain in homeostasis. This process is known as homeorhesis. Homeorhetic adaptation to lactation has been extensively described; however how these adaptations are orchestrated among multiple tissues remains elusive. To develop a clearer picture of how gene expression is coordinated across multiple tissues during the pregnancy to lactation transition, total RNA was isolated from mammary, liver and adipose tissues collected from rat dams (n = 5) on day 20 of pregnancy and day 1 of lactation, and gene expression was measured using Affymetrix GeneChips. Two types of gene expression analysis were performed. Genes that were differentially expressed between days within a tissue were identified with linear regression, and univariate regression was used to identify genes commonly up-regulated and down-regulated across all tissues. Gene set enrichment analysis showed genes commonly up regulated among the three tissues enriched gene ontologies primary metabolic processes, macromolecular complex assembly and negative regulation of apoptosis ontologies. Genes enriched in transcription regulator activity showed the common up regulation of 2 core molecular clock genes, ARNTL and CLOCK. Commonly down regulated genes enriched Rhythmic process and included: NR1D1, DBP, BHLHB2, OPN4, and HTR7, which regulate intracellular circadian rhythms. Changes in mammary, liver and adipose transcriptomes at the onset of lactation illustrate the complexity of homeorhetic adaptations and suggest that these changes are coordinated through molecular clocks.

Compartmentalization and regulation of insulin signaling to GLUT4 by the cytoskeleton

One of the early events in the development of Type 2 diabetes appears to be an inhibition of insulin-mediated GLUT4 redistribution to the cell surface in tissues that express GLUT4. Understanding this process, and how it begins to breakdown in the development of insulin resistance is quite important as we face treatment and prevention of metabolic diseases. Over the past few years, and increasing number of laboratories have produced compelling data to demonstrate a role for both the actin and microtubule networks in the regulation of insulin-mediated GLUT4 redistribution to the cell surface. In this review, we explore this process from insulin-signal transduction to fusion of GLUT4 membrane vesicles, focusing on studies that have implicated a role for the cytoskeleton. We see from this body of work that both the actin network and the microtubule cytoskeleton play roles as targets of insulin action and effectors of insulin signaling leading to changes in GLUT4 redistribution to the cell surface and insulin-mediated glucose uptake.

Microtubule disruption with BAPTA and dimethyl BAPTA by a calcium chelation-independent mechanism in 3T3-L1 adipocytes

While the physiological role for calcium in the insulin action on glucose transport has been disputed, it was reassessed in a recent study by using a calcum chelator, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl) ester (BAPTA-AM). Although BAPTA has been widely used to study the role for calcium in a variety of cell functions, it has also been suggested to have properties unrelated to the calcium chelating activity. Here, we investigated the effects of BAPTA and dimethyl BAPTA on the cytoskeletons in 3T3-L1 adipocytes. Both calcium chelators were successfully loaded in 3T3-L1 adipocytes and inhibited endothelin-1-induced cytosolic calcium elevation. Confocal fluorescence microscopy revealed that BAPTA and dimethyl BAPTA caused profound depolymerization of the microtubules without affecting the cortical actin filaments in 3T3-L1 adipocytes. Biochemical quantification also showed that BAPTA and dimethyl BAPTA significantly decreased the amount of polymerized tubulin but had little effect on filamentous actin. Consistent with these results, GLUT4-positive perinuclear compartments were dispersed throughout the cytoplasm in BAPTA- or dimethyl BAPTA-loaded adipocytes. Intriguingly, these calcium chelators did not disrupt the microtubules in undifferentiated preadipocytes. The microtubule-depolymerizing property of BAPTA and dimethyl BAPTA is unrelated to calcium chelation, since the microtubules were resistant to depletion of cytosolic calcium by using a calcium ionophore A23187. Insulin-stimulated glucose transport was not affected by cytosolic calcium depletion with A23187, but significantly inhibited with BAPTA and dimethyl BAPTA to the extent similar to that with nocodazole. BAPTA and its derivatives should be used with caution in studies of cytoskeleton-related cell functions.

The effect of nocodazole on the transfection efficiency of lipid-bilayer coated gold nanoparticles

Nonviral vectors are safer than viral systems for gene therapy applications. However, the limited efficacy always prevents their being widely used in clinical practice. Aside from searching new gene nonviral vectors, many researchers focus on finding out new substances to improve the transfection efficiency of existent vectors. In this work, we found a transfection enhancer, nocodazole (NCZ), for dimethyldioctadecylammonium (DODAB, a cationic lipid) bilayer coated gold nanoparticles (AuNPs) mediated gene delivery. It was found that NCZ produces 3-fold transfection enhancement to HEK 293T cells assessed by flow cytometry (FCM). The result was further confirmed by luciferase assay, in which NCZ induced more than 5 times improvement in transfection efficiency after 48 h of transfection. The results from the inductively coupled plasma mass spectrometry (ICP-MS) and FCM showed that NCZ did not affect the internalization of DODAB-AuNPs/DNA complexes. The trafficking of the complexes by transmission electron microscopy (TEM) indicated that the interrupted transportation of the complexes to the lysosomes contributed greatly to the transfection enhancement. Therefore, NCZ can be used as a transfection enhancer in DODAB-AuNPs mediated transfection system. This work also gave an insight to improving the efficiency of lipid-mediated transfection: modifying lipid on gold nanoparticles and pre-treating cells by NCZ before the transfection.

Mechanism of action of the stimulatory effect of apigenin-6-C-(2''-O-alpha-l-rhamnopyranosyl)-beta-L-fucopyranoside on 14C-glucose uptake

There has been a growing interest in hypoglycemic agents from natural products, particularly those derived from plants. Flavonoids are naturally occurring phenolic compounds with a broad range of biological activities and the beneficial effects of flavonoids have been studied in relation to diabetes mellitus, either through their capacity to avoid glucose absorption or to improve glucose tolerance. The purpose of this study was to investigate the mechanism of action of the stimulatory effect of apigenin-6-C-(2''-O-alpha-L-rhamnopyranosyl)-beta-L-fucopyranoside (1), isolated from Averrhoa carambola L. (Oxalidaceae) leaves, on (14)C-glucose uptake. This compound (1) was found to have an acute effect on blood glucose lowering in diabetic rats and stimulated glucose-induced insulin secretion after oral treatment in hyperglycemic rats. A significant stimulatory effect of compound 1 on (14)C-glucose uptake was observed at 50 and 100 microM. The effect of compound 1 on glucose uptake was completely nullified by wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), RO318220, an inhibitor of protein kinase C (PKC), PD98059, a specific inhibitor of mitogen-activated protein kinase (MEK), cycloheximide, an inhibitor of protein synthesis, and colchicine, a microtubule-depolymerizing agent. Compound 1 (100 microM) and insulin (10 nM) did not show any synergistic effect on glucose uptake. These results suggest that the flavonoid may have a dual target of action, as an insulin-secretagogue and also as an insulin-mimetic agent.

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

Glucose entry into muscle cells is precisely regulated by insulin, through recruitment of GLUT4 (glucose transporter-4) to the membrane of muscle and fat cells. Work done over more than two decades has contributed to mapping the insulin signalling and GLUT4 vesicle trafficking events underpinning this response. In spite of this intensive scientific research, there are outstanding questions that continue to challenge us today. The present review summarizes the knowledge in the field, with emphasis on the latest breakthroughs in insulin signalling at the level of AS160 (Akt substrate of 160 kDa), TBC1D1 (tre-2/USP6, BUB2, cdc16 domain family member 1) and their target Rab proteins; in vesicle trafficking at the level of vesicle mobilization, tethering, docking and fusion with the membrane; and in the participation of the cytoskeleton to achieve optimal temporal and spatial location of insulin-derived signals and GLUT4 vesicles.

A pre-docking role for microtubules in insulin-stimulated glucose transporter 4 translocation

Insulin stimulates glucose uptake by inducing translocation of glucose transporter 4 (GLUT4) from intracellular resides to the plasma membrane. How GLUT4 storage vesicles are translocated from the cellular interior to the plasma membrane remains to be elucidated. In the present study, intracellular transport of GLUT4 storage vesicles and the kinetics of their docking at the plasma membrane were comprehensively investigated at single vesicle level in control and microtubule-disrupted 3T3-L1 adipocytes by time-lapse total internal reflection fluorescence microscopy. It is demonstrated that microtubule disruption substantially inhibited insulin-stimulated GLUT4 translocation. Detailed analysis reveals that microtubule disruption blocked the recruitment of GLUT4 storage vesicles to underneath the plasma membrane and abolished the docking of them at the plasma membrane. These data suggest that transport of GLUT4 storage vesicles to the plasma membrane takes place along microtubules and that this transport is obligatory for insulin-stimulated GLUT4 translocation.

The mass action hypothesis: formation of Glut4 storage vesicles, a tissue-specific, regulated exocytic compartment

Insulin stimulates glucose uptake into the target tissues of fat and muscle by recruiting or translocating Glut4 glucose transport proteins to their functional location at the cell surface. In the basal state, Glut4 is sequestered intracellularly in several vesicular compartments, one of which has come to be known as Glut4 storage vesicles (GSVs). The GSVs represent a tissue-specific compartment that is an ultimate target of the insulin signalling cascade. Glut4 translocation has been extensively studied because of its intrinsic scientific importance to cell biology as well as its relevance to the pathology of type 2 diabetes mellitus. I review herein the ontogeny of GSVs and their composition as it relates to a tissue-specific, hormone-sensitive exocytic compartment and propose a mechanism for their formation.

A critical role of cavin (polymerase I and transcript release factor) in caveolae formation and organization

Cavin (PTRF) has been shown to be a highly abundant protein component of caveolae, but its functional role there is unknown. Here, we confirm that cavin co-localizes with caveolin-1 in adipocytes by confocal microscopy and co-distributes with caveolin-1 in lipid raft fractions by sucrose gradient flotation. However, cavin does not directly associate with caveolin-1 as solubilization of caveolae disrupts their interaction. Cholesterol depletion with beta-cyclodextrin causes a significant down-regulation of cavin from plasma membrane lipid raft fractions. Overexpression of cavin in HEK293-Cav-1 cells and knockdown of cavin in 3T3-L1 adipocytes enhances and diminishes caveolin-1 levels, respectively, indicating an important role for cavin in maintaining the level of caveolin-1. A truncated form of cavin, eGFP-cavin-1-322, which lacks 74 amino acids from the C-terminal, reveals a microtubular network localization by confocal microscopy. Disruption of cytoskeletal elements with latrunculin B or nocodazole diminishes cavin expression without affecting the caveolin-1 amount. We propose that the presence of cavin on the inside surface of caveolae stabilizes these structures, probably through interaction with the cytoskeleton, and cavin therefore plays an important role in caveolae formation and organization.

The SUMO conjugating enzyme Ubc9 is a regulator of GLUT4 turnover and targeting to the insulin-responsive storage compartment in 3T3-L1 adipocytes

The small ubiquitin-related modifier (SUMO) conjugating enzyme Ubc9 has been shown to upregulate GLUT4 in L6 myoblast cells, although the mechanism of action has remained undefined. Here we investigated the physiological significance of Ubc9 in GLUT4 turnover and subcellular targeting by adenovirus vector-mediated overexpression and by small interfering RNA (siRNA)-mediated gene silencing of Ubc9 in 3T3-L1 adipocytes. Overexpression of Ubc9 resulted in an inhibition of GLUT4 degradation and promoted its targeting to the unique insulin-responsive GLUT4 storage compartment (GSC), leading to an increase in GLUT4 amount and insulin-responsive glucose transport in 3T3-L1 adipocytes. Overexpression of Ubc9 also antagonized GLUT4 downregulation and its selective loss in GSC induced by long-term insulin stimulation. By contrast, siRNA-mediated depletion of Ubc9 accelerated GLUT4 degradation and decreased the amount of the transporter, concurrent with its selective loss in GSC, which resulted in attenuated insulin-responsive glucose transport. Intriguingly, overexpression of the catalytically inactive mutant Ubc9-C93A produced effects indistinguishable from those with wild-type Ubc9, suggesting that Ubc9 regulates GLUT4 turnover and targeting to GSC by a mechanism independent of its catalytic activity. Thus, Ubc9 is a pivotal regulator of the insulin sensitivity of glucose transport in adipocytes.

Bi-directional transport of GLUT4 vesicles near the plasma membrane of primary rat adipocytes

Insulin stimulates glucose uptake into adipocytes by mobilizing intracellular membrane vesicles containing GLUT4 proteins to the plasma membrane. Here we applied time-lapse total internal reflection fluorescence microscopy to study moving parameters and characters of exogenously expressed GLUT4 vesicles in basal, insulin and nocodazole treated primary rat adipocytes. Our results showed that microtubules were essential for long-range transport of GLUT4 vesicles but not obligatory for GLUT4 distribution in rat adipocytes. Insulin reduced the mobility of the vesicles, made them tethered/docked to the PM and finally had constitutive exocytosis. Moreover, long-range bi-directional movements of GLUT4 vesicles were visualized for the first time by TIRFM. It is likely that there are interactions between insulin signaling and microtubules, to regulating GLUT4 translocation in rat adipocytes.

Myosin 5a is an insulin-stimulated Akt2 (protein kinase Bbeta) substrate modulating GLUT4 vesicle translocation

Phosphatidylinositol 3-kinase activation of Akt signaling is critical to insulin-stimulated glucose transport and GLUT4 translocation. However, the downstream signaling events following Akt activation which mediate glucose transport stimulation remain relatively unknown. Here we identify an Akt consensus phosphorylation motif in the actin-based motor protein myosin 5a and show that insulin stimulation leads to phosphorylation of myosin 5a at serine 1650. This Akt-mediated phosphorylation event enhances the ability of myosin 5a to interact with the actin cytoskeleton. Small interfering RNA-induced inhibition of myosin 5a and expression of dominant-negative myosin 5a attenuate insulin-stimulated glucose transport and GLUT4 translocation. Furthermore, knockdown of Akt2 or expression of dominant-negative Akt (DN-Akt) abolished insulin-stimulated phosphorylation of myosin 5a, inhibited myosin 5a binding to actin, and blocked insulin-stimulated glucose transport. Taken together, these data indicate that myosin 5a is a newly identified direct substrate of Akt2 and, upon insulin stimulation, phosphorylated myosin 5a facilitates anterograde movement of GLUT4 vesicles along actin to the cell surface.

Focal adhesion kinase regulates insulin resistance in skeletal muscle

AIMS/HYPOTHESIS

On the basis of our previous studies, we investigated the possible role of focal adhesion kinase (FAK) in the development of insulin resistance in skeletal muscle, a major organ responsible for insulin-stimulated glucose uptake.

MATERIALS AND METHODS

Insulin-resistant C2C12 skeletal muscle cells were transfected with FAK wild-type or FAK mutant plasmids, knocked down using small interfering RNA (siRNA), and their effects on the levels and activities of insulin-signalling molecules and on glucose uptake were determined.

RESULTS

A significant decrease in tyrosine phosphorylation of FAK in insulin-resistant C2C12 cells was observed. A similar decrease was observed in skeletal muscle obtained from insulin-resistant Sprague-Dawley rats fed a high-fat diet. Increased levels of FAK in insulin-resistant C2C12 skeletal muscle cells increased insulin sensitivity and glucose uptake. These effects were reversed by an increase in the level of kinase activity mutant FAK or suppression of endogenous FAK by siRNA. FAK was also found to interact downstream with insulin receptor substrate-1, phosphatidylinositol 3-kinase and protein kinase C and glycogen synthase kinase 3beta, leading to translocation of glucose transporter 4 and resulting in the regulation of glucose uptake.

CONCLUSIONS/INTERPRETATION

The present study provides strong evidence that the modulation of FAK level regulates the insulin sensitivity of skeletal muscle cells. The results demonstrate a direct role of FAK in insulin-resistant skeletal muscle cells for the first time.

Actin disruption inhibits endosomal traffic of P-glycoprotein-EGFP and resistance to daunorubicin accumulation

Intracellular traffic of human P-glycoprotein (P-gp), a membrane transporter responsible for multidrug resistance in cancer chemotherapy, was investigated using a P-gp and enhanced green fluorescent fusion protein (P-gp-EGFP) in human breast cancer MCF-7 cells. The stably expressed P-gp-EGFP from a clonal cell population was functional as a drug efflux pump, as demonstrated by the inhibition of daunorubicin accumulation and the conferring of resistance of the cells to colchicine and daunorubicin. Colocalization experiments demonstrated that a small fraction of the total P-gp-EGFP expressed was localized intracellularly and was present in early endosome and lysosome compartments. P-gp-EGFP traffic was shown to occur via early endosome transport to the plasma membrane. Subsequent movement of P-gp-EGFP away from the plasma membrane occurred by endocytosis to the early endosome and lysosome. The component of the cytoskeleton responsible for P-gp-EGFP traffic was demonstrated to be actin rather than microtubules. In functional studies it was shown that in parallel with the interruption of the traffic of P-gp-EGFP, cellular accumulation of the P-gp substrate daunorubicin was increased after cells were treated with actin inhibitors, and cell proliferation was inhibited to a greater extent than in the presence of daunorubicin alone. The actin dependence of P-gp traffic and the parallel changes in cytotoxic drug accumulation demonstrated in this study delineates the pathways of P-gp traffic and may provide a new approach to overcoming multidrug resistance in cancer chemotherapy.

Role of insulin-dependent cortical fodrin/spectrin remodeling in glucose transporter 4 translocation in rat adipocytes

Fodrin or nonerythroid spectrin is an abundant component of the cortical cytoskeletal network in rat adipocytes. Fodrin has a highly punctate distribution in resting cells, and insulin causes a dramatic remodeling of fodrin to a more diffuse pattern. Insulin-mediated remodeling of actin occurs to a lesser extent than does that of fodrin. We show that fodrin interacts with the t-soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) syntaxin 4, and this interaction is increased by insulin stimulation and decreased by prior latrunculin A treatment. Latrunculin A disrupts all actin filaments, inhibits glucose transporter 4 (GLUT4) translocation, and causes fodrin to partially redistribute from the plasma membrane to the cytosol. In contrast, cytochalasin D disrupts only the short actin filament signal, and cytochalasin D neither inhibits GLUT4 translocation nor fodrin redistribution in adipocytes. Together, our data suggest that insulin induces remodeling of the fodrin-actin network, which is required for the fusion of GLUT4 storage vesicles with the plasma membrane by permitting their access to the t-SNARE syntaxin 4.

Inflammation as a risk factor for myocardial infarction

Myocardial infarction (MI) is one of the common diseases whose pathogenesis includes genetic factors. To reveal genetic backgrounds of this clinically heterogeneous disorder, we started our case-control association study by examining large-scale gene-based single nucleotide polymorphism (SNP) sets for approximately 1,000 patients and controls. As a core genotyping method, a combination of multiplex PCR and Invader method was used, and after genotyping approximately 65,000 SNPs, we found two SNPs located within lymphotoxin-alpha (LTA) gene showing significant association with MI. LTA is one of the cytokines produced in the early stages of vascular inflammatory processes. These SNPs seem to be involved in inflammation by both qualitatively and quantitatively modifying the function of LTA protein, thereby conferring a risk of MI. The genetic association was further confirmed by other researchers using white European trios. To further understand the roles of LTA protein in the pathogenesis of MI, we searched for proteins that interact directly with LTA protein and identified galectin-2 protein as a binding partner of LTA protein. It is a member of galactose-binding lectin family whose function has not been well characterized. Genetic association study again revealed that an SNP in LGALS2 encoding galectin-2 was also associated with susceptibility to MI. This genetic substitution seemed to affect the transcriptional level of galectin-2, which led to altered secretion of LTA, thereby affecting the degree of inflammation. Thus, our findings indicate the importance of inflammation, especially the LTA cascade, in the pathogenesis of MI. Also, combined strategy of genetic and molecular-cellular biological approaches may be useful for clarification of the pathogenesis of common diseases in general.

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.

The role of cytoskeleton in glucose regulation

Cytoskeleton plays an important role in glucose regulation, mainly in the following three aspects. First, cytoskeleton regulates insulin secretion by guiding intracellular transport of insulin-containing vesicles and regulating release of insulin. Second, cytoskeleton is involved in insulin action by regulating distribution of insulin receptor substrate, GLUT4 translocation, and internalization of insulin receptor. In addition, cytoskeleton directs the intracellular distribution of glucose metabolism related enzymes including glycogen synthase and many glycolysis enzymes.

The subcellular fractionation properties and function of insulin receptor substrate-1 (IRS-1) are independent of cytoskeletal integrity

Efficient insulin action requires spatial and temporal coordination of signaling cascades. The prototypical insulin receptor substrate, IRS-1 plays a central role in insulin signaling. By subcellular fractionation IRS-1 is enriched in a particulate fraction, termed the high speed pellet (HSP), and its redistribution from this fraction is associated with signal attenuation and insulin resistance. Anecdotal evidence suggests the cytoskeleton may underpin the localization of IRS-1 to the HSP. In the present study we have taken a systematic approach to examine whether the cytoskeleton contributes to the subcellular fractionation properties and function of IRS-1. By standard microscopy or immunoprecipitation we were unable to detect evidence to support a specific interaction between IRS-1 and the major cytoskeletal components actin (microfilaments), vimentin (intermediate filaments), and tubulin (microtubules) in 3T3-L1 adipocytes or in CHO.IR.IRS-1 cells. Pharmacological disruption of microfilaments and microtubules, individually or in combination, was without effect on the subcellular distribution of IRS-1 or insulin-stimulated tyrosine phosphorylation in either cell type. Phosphorylation of Akt was modestly reduced (20-35%) in 3T3-L1 adipocytes but not in CHO.IR.IRS-1 cells. In cells lacking intermediate filaments (Vim(-/-)) IRS-1 expression, distribution and insulin-stimulated phosphorylation appeared normal. Even after depolymerisation of microfilaments and microtubules, insulin-stimulated phosphorylation of IRS-1 and Akt were maintained in Vim(-/-) cells. Taken together these data indicate that the characteristic subcellular fractionation properties and function of IRS-1 are unlikely to be mediated by cytoskeletal networks and that proximal insulin signaling does not require an intact cytoskeleton.

Disruption of microtubules ablates the specificity of insulin signaling to GLUT4 translocation in 3T3-L1 adipocytes

Although the cytoskeletal network is important for insulin-induced glucose uptake, several studies have assessed the effects of microtubule disruption on glucose transport with divergent results. Here, we investigated the effects of microtubule-depolymerizing reagent, nocodazole and colchicine, on GLUT4 translocation in 3T3-L1 adipocytes. After nocodazole treatment to disrupt microtubules, GLUT4 vesicles were dispersed from the perinuclear region in the basal state, and insulin-induced GLUT4 translocation was partially inhibited by 20-30%, consistent with other reports. We found that platelet-derived growth factor (PDGF), which did not stimulate GLUT4 translocation in intact cells, was surprisingly able to enhance GLUT4 translocation to approximately 50% of the maximal insulin response, in nocodazole-treated cells with disrupted microtubules. This effect of PDGF was blocked by pretreatment with wortmannin and attenuated in cells pretreated with cytochalasin D. Using confocal microscopy, we found an increased co-localization of GLUT4 and F-actin in nocodazole-treated cells upon PDGF stimulation compared with control cells. Furthermore, microinjection of small interfering RNA targeting the actin-based motor Myo1c, but not the microtubule-based motor KIF3, significantly inhibited both insulin- and PDGF-stimulated GLUT4 translocation after nocodazole treatment. In summary, our data suggest that 1) proper perinuclear localization of GLUT4 vesicles is a requirement for insulin-specific stimulation of GLUT4 translocation, and 2) nocodazole treatment disperses GLUT4 vesicles from the perinuclear region allowing them to engage insulin and PDGF-sensitive actin filaments, which can participate in GLUT4 translocation in a phosphatidylinositol 3-kinase-dependent manner.

Dynamic tracking and mobility analysis of single GLUT4 storage vesicle in live 3T3-L1 cells

Glucose transporter 4 (GLUT4) is responsible for insulin-stimulated glucose transporting into the insulin-sensitive fat and muscle cells. The dynamics of GLUT4 storage vesicles (GSVs) remains to be explored and it is unclear how GSVs are arranged based on their mobility. We examined this issue in 3T3-L1 cells via investigating the three-dimensional mobility of single GSV labeled with EGFP-fused GLUT4. A thin layer of cytosol right adjacent to the plasma membrane was illuminated and successively imaged at 5 Hz under a total internal reflection fluorescence microscope with a penetration depth of 136 nm. Employing single particle tracking, the three-dimensional subpixel displacement of single GSV was tracked at a spatial precision of 22 nm. Both the mean square displacement and the diffusion coefficient were calculated for each vesicle. Tracking results revealed that vesicles moved as if restricted within a cage that has a mean radius of 160 nm, suggesting the presence of some intracellular tethering matrix. By constructing the histogram of the diffusion coefficients of GSVs, we observed a smooth distribution instead of the existence of distinct groups. The result indicates that GSVs are dynamically retained in a continuous and wide range of mobility rather than into separate classes.

Recycling of the Membrane-anchored Chemokine, CX3CL1

CX(3)CL1 (fractalkine) plays an important role in inflammation by acting as both chemoattractant and as an adhesion molecule. As for other chemokines, expression of CX(3)CL1 is known to be regulated at the level of transcription and translation. The unique transmembrane structure of CX(3)CL1 raises the possibility of additional functional regulation by altering its abundance at the cell surface. This could be accomplished in principle by changes in traffic between subcellular compartments. To analyze this possibility we examined the subcellular distribution of CX(3)CL1 in human ECV-304 cells stably expressing untagged or green fluorescent protein-tagged forms of the chemokine. CX(3)CL1 was present in two distinct compartments, diffusely on the plasma membrane and in a punctate juxtanuclear compartment. The latter shared some features with, yet was distinct from the conventional endocytic pathway and may represent a specialized recycling subcompartment. Accordingly, surface CX(3)CL1 was found to be in dynamic equilibrium with the juxtanuclear vesicular compartment. Intracellular CX(3)CL1 co-localized with the SNARE (soluble N-ethylmaleimide factor attachment protein receptor) proteins syntaxin-13 and VAMP-3. Cleavage of VAMP-3 by tetanus toxin or impairment of syntaxin-13 function by expression of a dominant-negative allele inhibited the ability of internalized CX(3)CL1 to traffic back to the plasma membrane. These data demonstrate the existence of a dynamic, SNARE-mediated recycling of CX(3)CL1 from the cell surface to and from an endomembrane storage compartment. The intracellular storage depot may serve as a source of the chemokine that could be rapidly mobilized by stimuli.

Microtubules Regulate Angiotensin II Type 1 Receptor and Rac1 Localization in Caveolae/Lipid Rafts

Objective— Microtubules are important in signal transduction temporal–spatial organization. Full expression of angiotensin II (Ang II) signaling in vascular smooth muscle cells (VSMCs) is dependent on the reactive oxygen species (ROS) derived from nicotinamide-adenine dinucleotide phosphate (NAD(P)H) oxidase and the dynamic association of the Ang II type 1 receptor (AT 1 R) with caveolae/lipid rafts. Translocation of the small GTPase Rac1 to the plasma membrane is an essential step for activation of NAD(P)H oxidase; however, its precise localization in the plasma membrane after agonist stimulation and how it is targeted are unknown. We hypothesized that microtubules are involved in regulating multiphasic Ang II signaling events in VSMC.

Methods and Results— We show that Ang II promotes Rac1 and AT 1 R trafficking into caveolae/lipid rafts, which is blocked by disruption of microtubules with nocodazole. As a consequence, nocodazole significantly inhibits Ang II–stimulated H 2 O 2 production, its downstream ROS-dependent epidermal growth factor receptor transactivation, Akt phosphorylation, and vascular hypertrophy without affecting Rac1 activation or ROS-independent extracellular signal-regulated kinase 1/2 phosphorylation.

Conclusions— These results suggest that proper Rac1 and AT 1 R trafficking into caveolae/lipid rafts requires the integrity of microtubules and provide insight into the essential role of microtubules for the spatial–temporal organization of ROS-dependent and caveolae/lipid rafts–dependent AT 1 R signaling linked to vascular hypertrophy.

Rapid trafficking of the neuronal glutamate transporter, EAAC1: evidence for distinct trafficking pathways differentially regulated by protein kinase C and platelet-derived growth factor

The neuronal glutamate transporter, EAAC1, appears to both limit spillover between excitatory synapses and provide precursor for the synthesis of the inhibitory neurotransmitter, gamma-aminobutyric acid. There is evidence for a large intracellular pool of EAAC1 from which transporter is redistributed to the cell surface following activation of protein kinase C (PKC) or platelet-derived growth factor (PDGF) receptor by seemingly independent pathways. A variety of biotinylation strategies were employed to measure trafficking of EAAC1 to and from the plasma membrane and to examine the effects of phorbol ester and PDGF on these events. Biotinylation of cell surface protein under trafficking-permissive conditions (37 degrees C) resulted in a 2-fold increase in the amount of biotinylated EAAC1 within 15 min in C6 glioma and in primary neuronal cultures, suggesting that EAAC1 has a half-life of approximately 5-7 min for residence at the plasma membrane. Both phorbol ester and PDGF increased the amount of transporter labeled under these conditions. Using a reversible biotinylation strategy, a similarly rapid internalization of EAAC1 was observed in C6 glioma. Phorbol ester, but not PDGF, blocked this measure of internalization. Incubation at 18 degrees C, which blocks some forms of intracellular membrane trafficking, inhibited PKC- and PDGF-dependent redistribution of EAAC1 but had no effect on basal trafficking of EAAC1. These studies suggest that both PKC and PDGF accelerate delivery of EAAC1 to the cell surface and that PKC has an additional effect on endocytosis. The data also suggest that basal and regulated pools of EAAC1 exist in distinct compartments.

Functional variation in LGALS2 confers risk of myocardial infarction and regulates lymphotoxin-α secretion in vitro

Myocardial infarction (MI) has become one of the leading causes of death in the world. Its pathogenesis includes chronic formation of plaque inside the vessel wall of the coronary artery and acute rupture of the artery, implicating a number of inflammation-mediating molecules, such as the cytokine lymphotoxin-alpha (LTA). Functional variations in LTA are associated with susceptibility to MI. Here we show that LTA protein binds to galectin-2, a member of the galactose-binding lectin family. Our case-control association study in a Japanese population showed that a single nucleotide polymorphism in LGALS2 encoding galectin-2 is significantly associated with susceptibility to MI. This genetic substitution affects the transcriptional level of galectin-2 in vitro, potentially leading to altered secretion of LTA, which would then affect the degree of inflammation; however, its relevance to other populations remains to be clarified. Smooth muscle cells and macrophages in the human atherosclerotic lesions expressed both galectin-2 and LTA. Our findings thus suggest a link between the LTA cascade and the pathogenesis of MI.

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.

Localization of low density lipoprotein receptor-related protein 1 to caveolae in 3T3-L1 adipocytes in response to insulin treatment

The insulin-induced translocation of low density lipoprotein receptor-related protein 1 (LRP1) from intracellular membranes to the cell surface in 3T3-L1 adipocytes was differentiation-dependent and did not occur in 3T3-L1 fibroblasts. Prompted by findings that the plasma membrane of 3T3-L1 adipocytes was rich in caveolae, we determined whether LRP1 became caveolae-associated upon insulin stimulation. The caveolae domain was isolated by the well characterized detergent solubilization and sucrose density ultracentrifugation methodology. Under basal conditions, only a trace amount of LRP1 was caveolae-associated despite the markedly elevated caveolin-1 and caveolae after adipocytic cell differentiation. Upon insulin treatment, the amount of LRP1 associated with caveolae was increased by 4-fold within 10 min, which was blocked completely by pretreatment with wortmannin prior to insulin. The caveolar localization of LRP1 in adipocytes was specific to insulin; treatment with platelet-derived growth factor-bb isoform did not promote but rather decreased caveolar localization of LRP1 below basal levels. The insulin-induced caveolar localization of LRP1 was also observed in 3T3-L1 fibroblasts where translocation of LRP1 from intracellular membranes to the cell surface was absent, suggesting that association of LRP1 with caveolae was achieved, at least in part, through lateral transmigration along the plane of plasma membranes. Immunocytochemistry studies revealed partial co-localization of LRP1 (either endogenous LRP1 or an epitope-tagged minireceptor) with caveolin-1 in cells treated with insulin, which was confirmed by co-immunoprecipitation of LRP1 with caveolin-1 in cells treated with insulin but not platelet-derived growth factor-bb. These results suggest that the localization of LRP1 to caveolae responds selectively to extracellular signals.