[Show abstract][Hide abstract] ABSTRACT: As insulin entry into muscle interstitium is rate-limiting for its overall peripheral action, defining the route and regulation of its entry is critical. Caveolin-1 is required for caveola formation in vascular endothelial cells (ECs) and for EC insulin uptake. Whether this requirement reflects simply the need for caveola availability or involves a more active role for caveolae/caveolin-1 is not known. Here, we examined the role of insulin-stimulated tyrosine 14 (Tyr(14))-caveolin-1 phosphorylation in mediating EC insulin uptake and the role of cellular Src-kinase (cSrc), TNF-α/IL-6 and high fat diet (HFD) in regulating this process.
Freshly isolated ECs from normal or HFD-fed rats and/or cultured ECs were treated with FITC-labelled or regular insulin with or without a Src or phosphotidylinositol-3-kinase inhibitor, TNF-α or IL-6, or transfecting FLAG-tagged wild-type (WT) or mutant (Y14F) caveolin-1. Tyr(14)-caveolin-1/Tyr(416) cSrc phosphorylation and FITC-insulin uptake were quantified by immunostaining and/or western blots.
Insulin stimulated Tyr(14)-caveolin-1 phosphorylation during EC insulin uptake. Inhibiting cSrc, but not phosphotidylinositol-3-kinase, reduced insulin-stimulated caveolin-1 phosphorylation. Furthermore, inhibiting cSrc reduced FITC-insulin uptake by ∼50%. Overexpression of caveolin-1Y14F inhibited, while overexpression of WT caveolin-1 increased, FITC-insulin uptake. Exposure of ECs to TNF-α or IL-6, or to 1-week HFD feeding eliminated insulin-stimulated caveolin-1 phosphorylation and inhibited FITC-insulin uptake to a similar extent.
Insulin stimulation of its own uptake requires caveolin-1 phosphorylation and Src-kinase activity. HFD in vivo and proinflammatory cytokines in vitro both inhibit this process.
[Show abstract][Hide abstract] ABSTRACT: Insulin resistance strongly associates with decreased nitric oxide (NO) bioavailability and endothelial dysfunction. In the vasculature NO mediates multiple processes affecting insulin delivery, including dilating both resistance and terminal arterioles in skeletal muscle in vivo. However, whether NO directly regulates vascular endothelial cell (EC) insulin uptake and its trans-endothelial transport (TET) is unknown. Here we report that L-NAME pre-treatment blocked while L-arginine or sodium nitroprusside (SNP), each enhanced EC uptake of fluoroisothiocyanate (FITC)-labeled insulin. SNP also partly or fully reversed the inhibition of EC insulin uptake caused by L-NAME, wortmannin, the Src inhibitor PP1 or TNFα. In addition, SNP promoted [(125)I]Tyr(A14)insulin transendothelial transport by ∼40%. Treatment with insulin ± SNP did not affect EC cGMP levels, and the cGMP analogue 8-bromo- cGMP did not affect FITC-insulin uptake. In contrast, treatment with insulin and SNP significantly increased EC protein S-nitrosylation, the co-localization of S-NO and protein-tyrosine phosphatase 1B (PTP1B) and Akt phosphorylation at Ser(473) and inhibited PTP1B activity. Moreover, a high-fat diet significantly inhibited EC insulin-stimulated Akt phosphorylation and FITC-insulin uptake that was partially reversed by SNP in rats. Finally, inhibition of S-nitrosylation by knock-down of thioredoxin-interacting protein completely eliminated SNP-enhanced FITC-insulin uptake. We conclude that NO directly promotes EC insulin transport by enhancing protein S-nitrosylation. NO also inhibits PTP1B activity thereby enhancing insulin signaling.
[Show abstract][Hide abstract] ABSTRACT: Insulin's trans-endothelial transport (TET) is critical for its metabolic action on muscle and involves trafficking of insulin bound to its receptor (or at high insulin concentrations, the IGF-I receptor) via caveolae. However, whether caveolae-mediated insulin TET involves actin cytoskeleton organization is unknown. Here we address whether insulin regulates actin filament organization in bovine aortic endothelial cells (bAEC) and whether this affects insulin uptake and TET. We found that insulin induced extensive cortical actin filament remodeling within 5 min. This remodeling was inhibited not only by disruption of actin microfilament organization but also by inhibition of phosphatidylinositol 3-kinase (PI3K) or by disruption of lipid rafts using respective specific inhibitors. Knockdown of either caveolin-1 or Akt using specific small interfering RNA also eliminated the insulin-induced cortical actin filament remodeling. Blocking either actin microfilament organization or PI3K pathway signaling inhibited both insulin uptake and TET. Disruption of actin microfilament organization also reduced the caveolin-1, insulin receptor, and IGF-I receptor located at the plasma membrane. Exposing bAEC for 6 h to either TNFα or IL-6 blocked insulin-induced cortical actin remodeling. Extended exposure (24 h) also inhibited actin expression at both mRNA and protein levels. We conclude that insulin-induced cortical actin filament remodeling in bAEC is required for insulin's TET in a PI3K/Akt and plasma membrane lipid rafts/caveolae-dependent fashion, and proinflammatory cytokines TNFα and IL-6 block this process.
No preview · Article · Jun 2012 · Molecular Endocrinology
[Show abstract][Hide abstract] ABSTRACT: Insulin, at physiological concentrations, regulates the volume of microvasculature perfused within skeletal and cardiac muscle. It can also, by relaxing the larger resistance vessels, increase total muscle blood flow. Both of these effects require endothelial cell nitric oxide generation and smooth muscle cell relaxation, and each could increase delivery of insulin and nutrients to muscle. The capillary microvasculature possesses the greatest endothelial surface area of the body. Yet, whether insulin acts on the capillary endothelial cell is not known. Here, we review insulin's actions at each of three levels of the arterial vasculature as well as recent data suggesting that insulin can regulate a vesicular transport system within the endothelial cell. This latter action, if it occurs at the capillary level, could enhance insulin delivery to muscle interstitium and thereby complement insulin's actions on arteriolar endothelium to increase insulin delivery. We also review work that suggests that this action of insulin on vesicle transport depends on endothelial cell nitric oxide generation and that insulin's ability to regulate this vesicular transport system is impaired by inflammatory cytokines that provoke insulin resistance.
No preview · Article · May 2011 · AJP Endocrinology and Metabolism
[Show abstract][Hide abstract] ABSTRACT: As insulin's movement from plasma to muscle interstitium is rate limiting for its metabolic action, defining the regulation of this movement is critical. Here, we address whether caveolin-1 is required for the first step of insulin's transendothelial transport, its uptake by vascular endothelial cells (ECs), and whether IL-6 and TNFα affect insulin uptake or caveolin-1 expression. Uptake of FITC-labeled insulin was measured using confocal microscopy in control bovine aortic ECs (bAECs), in bAECs in which caveolin-1 was either knocked down or overexpressed, in murine ECs from caveolin-1(-/-) mice and in bAECs exposed to inflammatory cytokines. Knockdown of caveolin-1 expression in bAECs using specific caveolin-1 siRNA reduced caveolin-1 mRNA and protein expression by ∼ 70%, and reduced FITC-insulin uptake by 67% (P < 0.05 for each). Over-expression of caveolin-1 increased insulin uptake (P < 0.05). Caveolin-1-null mouse aortic ECs did not take up insulin and re-expression of caveolin-1 by transfecting these cells with FLAG-tagged caveolin-1 DNA rescued FITC-insulin uptake. Knockdown of caveolin-1 significantly reduced both insulin receptor protein level and insulin-stimulated Akt1 phosphorylation. Knockdown of caveolin-1 also inhibited insulin-induced caveolin-1 and IGF-1 receptor translocation to the plasma membrane. Compared with controls, IL-6 or TNFα (20 ng/ml for 24 h) inhibited FITC-insulin uptake as well as the expression of caveolin-1 mRNA and protein (P < 0.05 for each). IL-6 or TNFα also significantly reduced plasma membrane-associated caveolin-1. Thus, we conclude that insulin uptake by ECs requires expression of caveolin-1 supporting a role for caveolae mediating insulin uptake. Proinflammatory cytokines may inhibit insulin uptake, at least in part, by inhibiting caveolin-1 expression.
Preview · Article · Oct 2010 · AJP Endocrinology and Metabolism
[Show abstract][Hide abstract] ABSTRACT: Endothelial nitric oxide synthase (eNOS) activity is tightly regulated by posttranscriptional modification and its subcellular localization. Here we examined whether insulin modulates nitric oxide (NO) production by regulating eNOS subcellular localization. We used confocal microscopy and immunoblots to examine the time course for 1) subcellular targeting/association of eNOS and caveolin-1 (CAV-1); 2) eNOS Ser(1179) phosphorylation; and 3) NO production in cultured bovine aorta endothelial cells. Serum starvation increased eNOS/CAV-1 localization to the perinuclear region. Adding insulin provoked their prompt translocation to and association at the plasma membrane (PM). Specific monoclonal antibodies against either CAV-1 or eNOS coimmunoprecipitated the other from bovine aorta endothelial cell membrane extracts, and insulin increased this interaction. Insulin stimulated NO production transiently despite a persistent eNOS Ser(1179) phosphorylation. The decline of NO production correlated temporally to insulin-induced translocation of eNOS and CAV-1 to PM. Knockdown of CAV-1 expression with a specific small interfering RNA duplex resulted in eNOS redistributing to the perinuclear region and nearly doubled insulin-induced NO production. Inhibition of phosphatidylinositol 3-kinase activity with wortmannin not only significantly inhibited insulin-induced translocation of eNOS and CAV-1 to PM but also blocked insulin-induced interaction of CAV-1 with eNOS at PM. Insulin increased incorporation of [(3)H]palmitic acid into eNOS immunoprecipitates by approximately 140%. Insulin-induced translocation of eNOS and CAV-1 to PM was palmitoylation dependent. Inhibiting eNOS and CAV-1 palmitoylation enhanced the NO production while blocking the translocation of eNOS and CAV-1 to PM induced by insulin. These data show that insulin acutely regulates eNOS and CAV-1 trafficking to PM of vascular endothelial cells where their interaction can regulate eNOS activity.
No preview · Article · Aug 2009 · Molecular Endocrinology
[Show abstract][Hide abstract] ABSTRACT: In vivo evidence suggests that insulin entry into skeletal muscle is rate limiting for its overall metabolic action. Although there has been controversy regarding whether insulin crosses the endothelium by a passive (transcellular or paracellular) or mediated process, accumulating data favor the latter. Here, we addressed whether insulin signaling within the endothelial cell is required for the first step of transendothelial insulin transport: its uptake by the endothelial cell.
Bovine aortic endothelial cells (bAECs) were incubated in serum-free medium for 6 h before addition of 50 nmol/l fluoroisothiocyanate (FITC)-labeled insulin for 30 min, and uptake of FITC insulin was quantified by confocal immunocytochemistry.
Cellular insulin uptake was temperature dependent, being greater at 37 vs. 4 degrees C (P < 0.05). Inhibiting phosphatidylinositol 3-kinase (PI 3-kinase) (wortmannin), mitogen-activated protein kinase kinase (MEK) (PD98059), the cSrc-family tyrosine kinase (PP1), or the insulin receptor tyrosine kinase (genistein) markedly diminished FITC insulin uptake (P < 0.05 for each). In contrast, inhibiting the phosphotyrosine phosphatase protein tyrosine phosphatase 1B further stimulated insulin uptake (P < 0.05). Addition of the inflammatory cytokine 5 ng/ml tumor necrosis factor-alpha (TNF-alpha) for 6 h before adding 50 nmol/l FITC insulin diminished insulin uptake significantly (P < 0.05). This inhibitory effect of TNF-alpha could be partially reversed by a specific p38 MAPK inhibitor (SB203580).
Insulin uptake by bAECs requires intact insulin signaling via both the PI 3-kinase and MEK signaling cascades and the cSrc-family tyrosine kinases, and endothelial cell insulin uptake is sensitive to cytokine-induced insulin resistance.
[Show abstract][Hide abstract] ABSTRACT: Obstructive sleep apnea is characterized by intermittent obstruction of the upper airway, which leads to intermittent hypoxia. Myocardial glycogen is a major energy resource for heart during hypoxia. Previous studies have demonstrated that intermittent hypoxia rapidly degrades myocardial glycogen and activates glycogen synthase (GS). However, the underlying mechanisms remain undefined. Because sleep apnea/intermittent hypoxia usually happens at night, whether intermittent hypoxia leads to GS activation in the postabsorptive state is not known. In the present study, male adult rats were studied after either an overnight fast or ad libitum feeding with or without intermittent ventilatory arrest (3 90-s periods at 10-min intervals). Hearts were quickly excised and freeze-clamped. Intermittent hypoxia induced a significant decrease in myocardial glycogen content in fed rats and stimulated GS in both fasted and fed rats. However, the portion of GS in the active form increased by approximately 38% in fasted rats compared with a larger, approximately 130% increase in fed rats. The basal G-6-P content was comparable in fasted and fed animals and increased approximately threefold after hypoxia. The basal phosphorylation states of Akt and GSK-3beta and the activity of protein phosphatase 1 (PP1) were comparable between fasted and fed control rats. Hypoxia significantly increased Akt phosphorylation and PP1 activity only in fed rats. In contrast, hypoxia did not induce significant change in GSK-3beta phosphorylation in either fasted or fed rats. We conclude that hypoxia activates GS in fed rat myocardium through a combination of rapid glycogenolysis, elevated local G-6-P content, and increased PP1 activity, and fasting attenuates this action independent of local G-6-P content.
Preview · Article · Mar 2007 · AJP Endocrinology and Metabolism
[Show abstract][Hide abstract] ABSTRACT: Human lemur (Lmr) kinases are predicted to be Tyr kinases based on sequences and are related to neurotrophin receptor Trk kinases. This study used homogeneous recombinant KPI-2 (Lmr2, LMTK2, Cprk, brain-enriched protein kinase) kinase domain and a library of 1,154 peptides on a microarray to analyze substrate specificity. We found that KPI-2 is strictly a Ser/Thr kinase that reacts with Ser either preceded by or followed by Pro residues but unlike other Pro-directed kinases does not strictly require an adjacent Pro residue. The most reactive peptide in the library corresponds to Ser-737 of cystic fibrosis transmembrane conductance regulator, and the recombinant R domain of cystic fibrosis transmembrane conductance regulator was a preferred substrate. Furthermore the KPI-2 kinase phosphorylated peptides corresponding to the single site in phosphorylase and purified phosphorylase b, making this only the second known phosphorylase b kinase. Phosphorylase was used as a specific substrate to show that KPI-2 is inhibited in living cells by addition of nerve growth factor or serum. The results demonstrate the utility of the peptide library to probe specificity and discover kinase substrates and offer a specific assay that reveals hormonal regulation of the activity of this unusual transmembrane kinase.
[Show abstract][Hide abstract] ABSTRACT: Insulin activates signaling pathways in target tissues through the insulin receptor and Tyr phosphorylation of intracellular proteins. Vanadate mimics insulin and enhances its actions through inhibition of protein Tyr phosphatases. Chromium is a micronutrient that enhances insulin action to normalize blood glucose, but the mechanism is not understood. Here we show that either vanadate or chromium stimulates Tyr phosphorylation of insulin receptor in mouse 3T3-L1 adipocytes compared to insulin alone, but a combination of vanadate and chromium is not additive. Phosphorylation of MAPK or 4E-BP1 as markers for insulin signaling is stimulated by vanadate plus insulin, and chromium does not enhance the effects. Vanadate robustly activates glucose uptake by 3T3-L1 adipocytes even without added insulin and increases insulin-stimulated glucose uptake. Chromium pretreatment of adipocytes slightly enhances glucose uptake in response to insulin, but significantly increases glucose uptake above that induced by insulin plus vanadate. These data show that chromium enhances glucose uptake even when Tyr phosphorylation levels are elevated by vanadate plus insulin, suggesting separate mechanisms of action for vanadate and chromium.
No preview · Article · Oct 2006 · Biochemical and Biophysical Research Communications
[Show abstract][Hide abstract] ABSTRACT: The pathways by which insulin exits the vasculature to muscle interstitium have not been characterized. In the present study, we infused FITC-labeled insulin to trace morphologically (using confocal immunohistochemical methods) insulin transport into rat skeletal muscle. We biopsied rectus muscle at 0, 10, 30, and 60 min after beginning a continuous (10 mU x min(-1) x kg(-1)), intravenous FITC-insulin infusion (with euglycemia maintained). The FITC-insulin distribution was compared with that of insulin receptors (IR), IGF-I receptors (IGF-IR), and caveolin-1 (a protein marker for caveolae) in skeletal muscle vasculature. We observed that muscle endothelium stained strongly for FITC-insulin within 10 min, and this persisted to 60 min. Endothelium stained more strongly for FITC-insulin than any other cellular elements in muscle. IR, IGF-IR, and caveolin-1 were also detected immunohistochemically in muscle endothelial cells. We further compared their intracellular distribution with that of FITC-insulin in cultured bovine aortic endothelial cells (bAECs). Considerable colocalization of IR or IGF-IR with FITC-insulin was noted. There was some but less overlap of IR or IGF-IR or FITC-insulin with caveolin-1. Immunoprecipitation of IR coprecipitated caveolin-1, and conversely the precipitation of caveolin-1 brought down IR. Furthermore, insulin increased the tyrosine phosphorylation of caveolin-1, and filipin (which inhibits caveolae formation) blocked insulin uptake. Finally, the ability of insulin, IGF-I, and IGF-I-blocking antibody to diminish insulin transport across bAECs grown on transwell plates suggested that IGF-IR, in addition to IR, can also mediate transendothelial insulin transit. We conclude that in vivo endothelial cells rapidly take up and concentrate insulin relative to plasma and muscle interstitium and that IGF-IR, like IR, may mediate insulin transit through endothelial cells in a process involving caveolae.
[Show abstract][Hide abstract] ABSTRACT: In muscle, physiologic hyperinsulinemia, presumably acting on endothelial cells (ECs), dilates arterioles and regulates both total blood flow and capillary recruitment, which in turn influences glucose disposal. In cultured ECs, however, supraphysiological (e.g. >or=10 nM) insulin concentrations are typically used to study insulin receptor (IR) signaling pathways and nitric oxide generation. IGF-I receptors (IGF-IRs) are more abundant than IR in ECs, and they also respond to high concentrations of insulin. To address whether IR mediates responses to physiologic insulin stimuli, we examined the insulin concentration dependence of IR and IGF-IR-mediated insulin signaling in bovine aortic ECs (bAECs). We also assessed whether insulin/IGF-I hybrid receptors were present in bAECs. Insulin, at 100-500 pM, significantly stimulated the phosphorylation of IRbeta, Akt1, endothelial isoform of nitric oxide synthase, and ERK 1/2 but not the IGF-IRbeta subunit. At concentrations 1-5 nm or greater, insulin dose-dependently enhanced the tyrosine phosphorylation of IGF-IRbeta, and this was inhibited by IGF-IR neutralizing antibody. In addition, immunoprecipitation of IRbeta pulled down the IGF-IRbeta, and the IRbeta immunocytochemically colocalized with IGF-IRbeta, suggesting that ECs have insulin/IGF-I hybrid receptors. We conclude that: 1) insulin at physiological concentrations selectively activates IR signaling in bAECs; 2) bAECs express IGF-IR and insulin/IGF-I hybrid receptors in addition to IR; 3) high concentrations of insulin (>or=1-5 nM) activate IGF-IR and hybrid receptors as well as IR; and 4) this crossover activation can confound interpretation of studies of insulin action in ECs when high insulin concentrations are used.
[Show abstract][Hide abstract] ABSTRACT: Chromium has been recognized for decades as a nutritional factor that improves glucose tolerance by enhancing in vivo insulin action, but the molecular mechanism is unknown. Here we report pretreatment of CHO-IR cells with chromium enhances tyrosine phosphorylation of the insulin receptor. Different chromium(III) compounds were effective at enhancing insulin receptor phosphorylation in intact cells, but did not directly activate recombinant insulin receptor kinase. The level of insulin receptor phosphorylation in cells can be increased by inhibition of the opposing protein tyrosine phosphatase (PTP1B), a target for drug development. However, chromium did not inhibit recombinant human PTP1B using either p-nitrophenyl phosphate or the tyrosine-phosphorylated insulin receptor as the substrate. Chromium also did not alter reversible redox regulation of PTP1B. Purified plasma membranes exhibited insulin-dependent kinase activity in assays using substrate peptides mimicking sites of Tyr phosphorylation in the endogenous substrate IRS-1. Plasma membranes prepared from chromium-treated cells had higher specific activity of insulin-dependent kinase relative to controls. We conclude that cellular chromium potentiates insulin signaling by increasing insulin receptor kinase activity, separate from inhibition of PTPase. Our results suggest that nutritional and pharmacological therapies may complement one another to combat insulin resistance, a hallmark of type 2 diabetes.