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

Plasma membrane phosphatidylinositol (4,5)-bisphosphate promotes Weibel-Palade body exocytosis

Phosphatidylinositol (4,5)-bisphosphate transiently accumulates at sites of Weibel–Palade body–plasma membrane fusion and promotes agonist-evoked exocytosis of endothelial von-Willebrand factor.

Weibel–Palade bodies (WPB) are specialized secretory organelles of endothelial cells that control vascular hemostasis by regulated, Ca²⁺-dependent exocytosis of the coagulation-promoting von-Willebrand factor. Some proteins of the WPB docking and fusion machinery have been identified but a role of membrane lipids in regulated WPB exocytosis has so far remained elusive. We show here that the plasma membrane phospholipid composition affects Ca²⁺-dependent WPB exocytosis and von-Willebrand factor release. Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂] becomes enriched at WPB–plasma membrane contact sites at the time of fusion, most likely downstream of phospholipase D1-mediated production of phosphatidic acid (PA) that activates phosphatidylinositol 4-phosphate (PI4P) 5-kinase γ. Depletion of plasma membrane PI(4,5)P₂ or down-regulation of PI4P 5-kinase γ interferes with histamine-evoked and Ca²⁺-dependent WPB exocytosis and a mutant PI4P 5-kinase γ incapable of binding PA affects WPB exocytosis in a dominant-negative manner. This indicates that a unique PI(4,5)P₂-rich environment in the plasma membrane governs WPB fusion possibly by providing interaction sites for WPB-associated docking factors.

A novel method for simulating insulin mediated GLUT4 translocation

Glucose transport in humans is a vital process which is tightly regulated by the endocrine system. Specifically, the insulin hormone triggers a cascade of intracellular signals in target cells mediating the uptake of glucose. Insulin signaling triggers cellular relocalization of the glucose transporter protein GLUT4 to the cell surface, which is primarily responsible for regulated glucose import. Pathology associated with the disruption of this pathway can lead to metabolic disorders, such as type II diabetes mellitus, characterized by the failure of cells to appropriately uptake glucose from the blood. We describe a novel simulation tool of the insulin intracellular response, incorporating the latest findings regarding As160 and GEF interactions. The simulation tool differs from previous computational approaches which employ algebraic or differential equations; instead, the tool incorporates statistical variations of kinetic constants and initial molecular concentrations which more accurately mimic the intracellular environment. Using this approach, we successfully recapitulate observed in vitro insulin responses, plus the effects of Wortmannin-like inhibition of the pathway. The developed tool provides insight into transient changes in molecule concentrations throughout the insulin signaling pathway, and may be employed to identify or evaluate potentially critical components of this pathway, including those associated with insulin resistance. In the future, this highly tractable platform may be useful for simulating other complex cell signaling pathways. Biotechnol. Bioeng. 2014;111: 2454-2465. © 2014 Wiley Periodicals, Inc.

A novel light-dependent activation of DAGK and PKC in bovine photoreceptor nuclei

In this work, we describe a selective light-dependent distribution of the lipid kinase 1,2-diacylglycerol kinase (EC 2.7.1.107, DAGK) and the phosphorylated protein kinase C alpha (pPKCα) in a nuclear fraction of photoreceptor cells from bovine retinas. A nuclear fraction enriched in small nuclei from photoreceptor cells (PNF), was obtained when a modified nuclear isolation protocol developed by our laboratory was used. We measured and compared DAGK activity as phosphatidic acid (PA) formation in PNF obtained from retinas exposed to light and in retinas kept in darkness using [γ-(32)P]ATP or [(3)H]DAG. In the absence of exogenous substrates and detergents, no changes in DAGK activity were observed. However, when DAGK activity assays were performed in the presence of exogenous substrates, such as stearoyl arachidonoyl glycerol (SAG) or dioleoyl glycerol (DOG), and different detergents (used to make different DAGK isoforms evident), we observed significant light effects on DAGK activity, suggesting the presence of several DAGK isoforms in PNF. Under conditions favoring DAGKζ activity (DOG, Triton X-100, dioleoyl phosphatidylserine and R59022) we observed an increase in PA formation in PNF from retinas exposed to light with respect to those exposed to darkness. In contrast, under conditions favoring DAGKɛ (SAG, octylglucoside and R59022) we observed a decrease in its activity. These results suggest different physiological roles of the above-mentioned DAGK isoforms. Western blot analysis showed that whereas light stimulation of bovine retinas increases DAGKζ nuclear content, it decreases DAGKɛ and DAGKβ content in PNF. The role of PIP2-phospholipase C in light-stimulated DAGK activity was demonstrated using U73122. Light was also observed to induce enhanced pPKCα content in PNF. The selective distribution of DAGKζ and ɛ in PNF could be a light-dependent mechanism that in vertebrate retina promotes selective DAG removal and PKC regulation.

Metabolic and mitogenic effects of IGF-II in rainbow trout (Oncorhynchus mykiss) myocytes in culture and the role of IGF-II in the PI3K/Akt and MAPK signalling pathways

Primary cultures of rainbow trout skeletal muscle cells were used to examine the role of insulin-like growth factor II (IGF-II) in fish muscle metabolism and growth, and to compare its main signal transduction pathways with those of IGF-I. IGF-II stimulated 2-deoxy-d-glucose (2-DG) uptake in trout myocytes at concentrations of between 5 and 100 nM, with similar maximal effects and temporal pattern to IGF-I (100 nM). The results of incubation with inhibitors (Wortmannin and CKB) indicated that IGF-II stimulates glucose uptake through the same mechanisms as IGF-I. In addition, IGF-II stimulated myoblast DNA synthesis (measured by thymidine incorporation) at relatively low concentrations (0.1-10 nM), with the maximum increase at 1 nM (167+/-17% with respect to control values). The cells were immunoreactive against ERK 1/2 MAPK and Akt/PKB, components of the two main signal transduction pathways for the IGF-I receptor. IGF-II stimulated the phosphorylation of the protein MAPK, especially at the proliferation stage (increases of up to 125.7+/-16.9% and 125.3+/-3.3% with respect to control in IGF-II- and IGF-I-treated cells, respectively). In contrast, the effects of both IGFs on the activation of the PI3K/Akt pathway were stronger in fully differentiated myocytes and in early-formed fibres (up to 359+/-18.5% in IGF-II-treated cells with respect to control). These results indicate that IGF-II has both mitogenic and metabolic effects in trout muscle cells, which are equivalent to those found in response to IGF-I. Both IGFs exert these effects though the same signalling pathways (MAPK and PI3K/Akt).

Pharmacological investigations of the anti-diabetic effect of Cortex Moutan and its active component paeonol

Cortex Moutan (CM, root bark of Paeonia suffruticosa Andr.) is one of the common herbs found in anti-diabetic traditional Chinese medicine formulae. To study the potential anti-diabetic mechanisms of CM, four in vitro models (intestinal brush border membrane vesicles (BBMV), rat hepatoma cell line H4IIE, human skin fibroblasts cell line Hs68 and mouse adipocytes 3T3-L1) were used. CM showed significant in vitro anti-diabetic effects by inhibiting glucose uptake of BBMV and enhancing glucose uptake into Hs68 and 3T3-L1 cells. Using bioassay-guided fractionation, paeonol was confirmed to be one of the active constituents for inhibiting BBMV glucose uptake. With neonatal-streptozotocin diabetic rats, paeonol (200 and 400mg/kgbody wt.) was found to improve oral glucose tolerance in vivo. To the best of our knowledge, this is the first report on the anti-diabetic effect of paeonol.

Protein kinase B/Akt is a novel cysteine string protein kinase that regulates exocytosis release kinetics and quantal size

Protein kinase B/Akt has been implicated in the insulin-dependent exocytosis of GLUT4-containing vesicles, and, more recently, insulin secretion. To determine if Akt also regulates insulin-independent exocytosis, we used adrenal chromaffin cells, a popular neuronal model. Akt1 was the predominant isoform expressed in chromaffin cells, although lower levels of Akt2 and Akt3 were also found. Secretory stimuli in both intact and permeabilized cells induced Akt phosphorylation on serine 473, and the time course of Ca2+-induced Akt phosphorylation was similar to that of exocytosis in permeabilized cells. To determine if Akt modulated exocytosis, we transfected chromaffin cells with Akt constructs and monitored catecholamine release by amperometry. Wild-type Akt had no effect on the overall number of exocytotic events, but slowed the kinetics of catecholamine release from individual vesicles, resulting in an increased quantal size. This effect was due to phosphorylation by Akt, because it was not seen in cells transfected with kinase-dead mutant Akt. As overexpression of cysteine string protein (CSP) results in a similar alteration in release kinetics and quantal size, we determined if CSP was an Akt substrate. In vitro 32P-phosphorylation studies revealed that Akt phosphorylates CSP on serine 10. Using phospho-Ser10-specific antisera, we found that both transfected and endogenous cellular CSP is phosphorylated by Akt on this residue. Taken together, these findings reveal a novel role for Akt phosphorylation in regulating the late stages of exocytosis and suggest that this is achieved via the phosphorylation of CSP on serine 10.

Insulin regulates the membrane arrival, fusion, and C-terminal unmasking of glucose transporter-4 via distinct phosphoinositides

Insulin increases glucose uptake into muscle via glucose transporter-4 (GLUT4) translocation to the cell membrane, but the regulated events in GLUT4 traffic are unknown. Here we focus on the role of class IA phosphatidylinositol (PI) 3-kinase and specific phosphoinositides in the steps of GLUT4 arrival and fusion with the membrane, using L6 muscle cells expressing GLUT4myc. To this end, we detected the availability of the myc epitope at the cell surface or intravesicular spaces and of the cytosol-facing C-terminal epitope, in cells and membrane lawns derived from them. We observed the following: (a) Wortmannin and LY294002 at concentrations that inhibit class IA PI 3-kinase reduced but did not abate the C terminus gain, yet the myc epitope was unavailable for detection unless lawns or cells were permeabilized, suggesting the presence of GLUT4myc in docked, unfused vesicles. Accordingly, GLUT4myc-containing vesicles were detected by immunoelectron microscopy of membranes from cells pretreated with wortmannin and insulin, but not insulin or wortmannin alone. (b) Insulin caused greater immunological availability of the C terminus than myc epitopes, suggesting that C terminus unmasking had occurred. Delivering phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) to intact cells significantly increased lawn-associated myc signal without C terminus gain. Conversely, phosphatidylinositol 3-phosphate (PI3P) increased the detection of C terminus epitope without any myc gain. We propose that insulin regulates GLUT4 membrane arrival, fusion, and C terminus unmasking, through distinct phosphoinositides. PI(3,4,5)P(3) causes arrival and fusion without unmasking, whereas PI3P causes arrival and unmasking without fusion.

Wortmannin induces zebrafish cardia bifida through a mechanism independent of phosphoinositide 3-kinase and myosin light chain kinase

Cardia bifida is an anomaly of the embryonic heart in which the bilateral myocardial rudiments fail to travel to the midline, resulting in the formation of two separate hearts in lateral positions. In zebrafish, eight loci responsible for the cardia bifida phenotype were identified in the large-scale genetic screen. Wortmannin has been reported to be a highly selective inhibitor of phosphoinositide 3-kinase and myosin light chain kinase activity. We provide the first evidence that wortmannin treatment of zebrafish embryos can induce cardia bifida in a dose-dependent manner and that wortmannin alters cardiac development between 6 and 16 h post-fertilization. In addition, we demonstrate that wortmannin induces zebrafish cardia bifida through a mechanism independent of phosphoinositide 3-kinase and myosin light chain kinase. Our findings may provide new insights into the cardiomyocyte function and disfunction.

Endofacial competitive inhibition of glucose transporter-4 intrinsic activity by the mitogen-activated protein kinase inhibitor SB203580

The translocation of glucose transporter-4 (GLUT4) to the cell surface is a complex multistep process that involves movement of GLUT4 vesicles from a reservoir compartment, and docking and fusion of the vesicles with the plasma membrane. It has recently been proposed that a p38 mitogen-activated protein kinase (MAPK)-dependent step may lead to intrinsic activation of the transporters exposed at the cell surface. In contrast to data obtained in muscle and adipocyte cell lines, we found that no insulin activation of p38 MAPK occurred in rat adipose cells. However, the p38 MAPK inhibitor SB203580 consistently inhibited transport activity after preincubation with the adipose cells. These apparently contradictory findings led us to hypothesize that the inhibitor may have a direct effect on the transport catalytic activity of GLUT4 that was independent of inhibition of the kinase. Kinetic analysis of 3-O-methyl-d-glucose transport activity revealed that SB203580 was a noncompetitive inhibitor of zero-trans (substrate outside but not inside) transport, but was a competitive inhibitor of equilibrium-exchange (substrate inside and outside) transport. This pattern of inhibition of GLUT4 was also observed with cytochalasin B. The pattern of inhibition is consistent with interaction at the endofacial surface, but not the exofacial surface of the transporter. Occupation of the endofacial substrate site reduces maximum velocity under zero-trans conditions, because return of the substrate site to the outside is blocked, and no substrate is present inside to displace the inhibitor. Under equilibrium-exchange conditions, internal substrate competitively displaces the inhibitor, and the transport K(m) is increased.

On the mechanism for neomycin reversal of wortmannin inhibition of insulin stimulation of glucose uptake

Although a number of studies and approaches have indicated that activation of the Ser/Thr kinase called Akt/protein kinase B is critical for the insulin-stimulated increase of glucose uptake in adipocytes, other studies have indicated that this enzyme may play an ancillary role. For example, a recent study indicated that neomycin would allow insulin-stimulated Glut4 translocation and glucose transport in the presence of the phosphatidylinositol (PI) 3-kinase inhibitor, wortmannin, a known inhibitor of Akt activation (James, D. J., Salaun, C., Brandie, F. M., Connell, J. M. C., and Chamberlain, L. H. (2004) J. Biol. Chem. 279, 20567-20570). To better understand this observation, we examined a number of downstream targets of Akt. As previously reported, treatment of 3T3-L1 adipocytes with neomycin prevented the wortmannin inhibition of insulin-stimulated glucose transport. However, in the presence of neomycin, wortmannin did not inhibit the insulin-stimulated phosphorylation of several downstream targets of Akt including a proline-rich Akt substrate of 40 kDa, ribosomal protein S6, and glycogen synthase kinase-3. In addition, neomycin did not prevent the ability of a structurally unrelated PI 3-kinase inhibitor, LY294002, to inhibit the insulin-stimulated activation of glucose uptake. Moreover, neomycin reversed the inhibitory effect of wortmannin but not LY294002 on insulin stimulation of Akt kinase activity. Finally, neomycin was found to inactivate in vitro the PI 3-kinase inhibitory actions of wortmannin but not LY294002. These results indicate that the effects of neomycin in adipocytes are not mediated via its ability to sequester phosphatidylinositol 4,5-bisphosphate but are instead caused by the ability of neomycin to inactivate wortmannin.

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.