Insulin-mimetic signaling by the sulfonylurea glimepiride and phosphoinositolglycans involves distinct mechanisms for redistribution of lipid raft components

CB1 Receptor-Dependent and Independent Induction of Lipolysis in Primary Rat Adipocytes by the Inverse Agonist Rimonabant (SR141716A)

(1) Background: Acute administration of the cannabinoid receptor 1 (CB1R) inverse agonist Rimonabant (SR141716A) to fed Wistar rats was shown to elicit a rapid and short-lasting elevation of serum free fatty acids. (2) Methods: The effect of Rimonabant on lipolysis in isolated primary rat adipocytes was studied to raise the possibility for direct mechanisms not involving the (hypothalamic) CB1R. (3) Results: Incubation of these cells with Rimonabant-stimulated lipolysis to up to 25% of the maximal isoproterenol effect, which was based on both CB1R-dependent and independent mechanisms. The CB1R-dependent one was already effective at Rimonabant concentrations of less than 1 µM and after short-term incubation, partially additive to β-adrenergic agonists and blocked by insulin and, in part, by adenosine deaminase, but not by propranolol. It was accompanied by protein kinase A (PKA)-mediated association of hormone-sensitive lipase (HSL) with lipid droplets (LD) and dissociation of perilipin-1 from LD. The CB1R-independent stimulation of lipolysis was observed only at Rimonabant concentrations above 1 µM and after long-term incubation and was not affected by insulin. It was recapitulated by a cell-free system reconstituted with rat adipocyte LD and HSL. Rimonabant-induced cell-free lipolysis was not affected by PKA-mediated phosphorylation of LD and HSL, but abrogated by phospholipase digestion or emulsification of the LD. Furthermore, LD isolated from adipocytes and then treated with Rimonabant (>1 µM) were more efficient substrates for exogenously added HSL compared to control LD. The CB1R-independent lipolysis was also demonstrated in primary adipocytes from fed rats which had been treated with a single dose of Rimonabant (30 mg/kg). (4) Conclusions: These data argue for interaction of Rimonabant (at high concentrations) with both the LD surface and the CB1R of primary rat adipocytes, each leading to increased access of HSL to LD in phosphorylation-independent and dependent fashion, respectively. Both mechanisms may lead to direct and acute stimulation of lipolysis at peripheral tissues upon Rimonabant administration and represent targets for future obesity therapy which do not encompass the hypothalamic CB1R.

Analysis of Direct Effects of the CB1 Receptor Antagonist Rimonabant on Fatty Acid Oxidation and Glycogenolysis in Liver and Muscle Cells in vitro

Recent pharmacological findings regarding rimonabant, an anorectic and cannabinoid type 1 receptor (CB1R) antagonist, strongly suggest that some of its effects on the metabolic parameters and energy balance in rats are not related to the centrally mediated reduction in caloric intake. Instead, they may be associated with acute induction of glycogenolysis in the liver, in combination with transient increase in glucose oxidation and persistent increase in fat oxidation. It is possible that rimonabant produced direct short- or long-term stimulatory effect on these processes in primary and cultured rat cells. Rimonabant slightly stimulated β-oxidation of long-chain fatty acids in cultured rat myocytes overexpressing glucose transporter isoform 4, as well as activated phosphorylation of adenosine monophosphate-dependent protein kinase (AMPK) in primary rat hepatocytes upon long-term incubation. However, short-term action of rimonabant failed to stimulate β-oxidation in myocytes, myotubes, and hepatocytes, as well as to upregulate AMPK phosphorylation, glycogenolysis, and cAMP levels in hepatocytes. As a consequence, the acute effects of rimonabant on hepatic glycogen content (reduction) and total energy expenditure (increase) in rats fed with a standard diet cannot be explained by direct stimulation of glycogenolysis and fatty acid oxidation in muscles and liver. Rather, these effects seem to be centrally mediated.

Mechanism of action of camphoryl-benzene sulfonamide derivative on glucose uptake in adipose tissue

The aim of the present study was to investigate the mechanism of action of a sulfonamide derivative on glucose uptake in adipose tissue, as well as to characterize the effects of this compound on intestinal disaccharidases and advanced glycation end-products (AGEs) formation. Camphoryl-benzene sulfonamide (CS) was able to stimulate glucose uptake in isolated adipocytes, adipose tissue, and in soleus muscle. The stimulatory effect of the compound (10 μM) on glucose uptake on adipose tissue was blocked by diazoxide, wortmannin, U73122, colchicine, and N-ethylmaleimide. On the other hand, the effects of CS were not blocked by glibenclamide, an inhibitor of the K⁺ -ATP channel, or even by the inhibitor of protein p38 MAPK, SB 203580. In vivo, this compound reduced intestinal disaccharidase activity, while, in vitro, CS reduced the formation of AGEs at 7, 14, and 28 days of incubation. The stimulatory effect of CS on glucose uptake requires the activation of the K⁺ -ATP channel, translocation, and fusion of GLUT4 vesicles to the plasma membrane on adipocytes for glucose homeostasis. In addition, the inhibition of disaccharidase activity contributes to the glucose homeostasis in a short-term as well as the remarkable reduction in AGE formation indicates that the CS may prevent of complications of late diabetes.

Focal adhesion kinase-promoted tumor glucose metabolism is associated with a shift of mitochondrial respiration to glycolysis

Cancer cells often gains a growth advantage by taking up glucose at a high rate and undergoing aerobic glycolysis through intrinsic cellular factors that reprogram glucose metabolism. Focal adhesion kinase (FAK), a key transmitter of growth factor and anchorage stimulation, is aberrantly overexpressed or activated in most solid tumors, including pancreatic ductal adenocarcinomas (PDACs). We determined whether FAK can act as an intrinsic driver to promote aerobic glycolysis and tumorigenesis. FAK inhibition decreases and overexpression increases intracellular glucose levels during unfavorable conditions, including growth factor deficiency and cell detachment. Amplex glucose assay, fluorescence and carbon-13 tracing studies demonstrate that FAK promotes glucose consumption and glucose-to-lactate conversion. Extracellular flux analysis indicates that FAK enhances glycolysis and decreases mitochondrial respiration. FAK increases key glycolytic proteins, including enolase, pyruvate kinase M2 (PKM2), lactate dehydrogenase and monocarboxylate transporter. Furthermore, active/tyrosine-phosphorylated FAK directly binds to PKM2 and promotes PKM2-mediated glycolysis. On the other hand, FAK-decreased levels of mitochondrial complex I can result in reduced oxidative phosphorylation (OXPHOS). Attenuation of FAK-enhanced glycolysis re-sensitizes cancer cells to growth factor withdrawal, decreases cell viability and reduces growth of tumor xenografts. These observations, for the first time, establish a vital role of FAK in cancer glucose metabolism through alterations in the OXPHOS-to-glycolysis balance. Broadly targeting the common phenotype of aerobic glycolysis and more specifically FAK-reprogrammed glucose metabolism will disrupt the bioenergetic and biosynthetic supply for uncontrolled growth of tumors, particularly glycolytic PDAC.

Glimepiride promotes osteogenic differentiation in rat osteoblasts via the PI3K/Akt/eNOS pathway in a high glucose microenvironment

Our previous studies demonstrated that glimepiride enhanced the proliferation and differentiation of osteoblasts and led to activation of the PI3K/Akt pathway. Recent genetic evidence shows that endothelial nitric oxide synthase (eNOS) plays an important role in bone homeostasis. In this study, we further elucidated the roles of eNOS, PI3K and Akt in bone formation by osteoblasts induced by glimepiride in a high glucose microenvironment. We demonstrated that high glucose (16.5 mM) inhibits the osteogenic differentiation potential and proliferation of rat osteoblasts. Glimepiride activated eNOS expression in rat osteoblasts cultured with two different concentrations of glucose. High glucose-induced osteogenic differentiation was significantly enhanced by glimepiride. Down-regulation of PI3K P85 levels by treatment with LY294002 (a PI3K inhibitor) led to suppression of P-eNOS and P-AKT expression levels, which in turn resulted in inhibition of RUNX2, OCN and ALP mRNA expression in osteoblasts induced by glimepiride at both glucose concentrations. ALP activity was partially inhibited by 10 µM LY294002. Taken together, our results demonstrate that glimepiride-induced osteogenic differentiation of osteoblasts occurs via eNOS activation and is dependent on the PI3K/Akt signaling pathway in a high glucose microenvironment.

Synthesis of Substituted Ethyl Argiosulfonylcarbamates

A series of substituted ethyl argiosulfonylcarbamates were synthesized from substituted aromatics through three steps including chlorosulfonation, ammonolysis, nucleophilic addition and their structures were confirmed by1H NMR and MS spectrum. The results showed that the yields of compounds were higher than 80%.

Glimepiride reduces CD14 expression and cytokine secretion from macrophages

Background

Activated microglia are associated with deposits of aggregated proteins within the brains of patients with Alzheimer’s disease (AD), Parkinson’s disease (PD) and prion diseases. Since the cytokines secreted from activated microglia are thought to contribute to the pathogenesis of these neurodegenerative diseases, compounds that suppress cytokine production have been identified as potential therapeutic targets. CD14 is a glycosylphosphatidylinositol (GPI)- anchored protein that is part of a receptor complex that mediates microglial responses to peptides that accumulate in prion disease (PrP82-146), AD (amyloid-β (Aβ)₄₂) and PD (α-synuclein (αSN)). As some GPI-anchored proteins are released from cells by treatment with glimepiride, a sulphonylurea used for the treatment of diabetes, the effects of glimepiride upon CD14 expression and cytokine production from cultured macrophages were studied.

Methods

RAW 264 cells and microglial cells were treated with glimepiride or phosphatidylinositol (PI)-phospholipase C (PLC) and the expression of cell receptors was analysed by ELISA and immunoblot. Treated cells were subsequently incubated with Aβ₄₂, αSN, PrP82-146 or lipopolysaccharide (LPS) and the amounts of Toll-like receptor (TLR)-4, tumour necrosis factor (TNF), interleukin (IL)-1 and IL-6 measured.

Results

Glimepiride released CD14 from RAW 264 cells and microglial cells. Pre-treatment with glimepiride significantly reduced TNF, IL-1 and IL-6 secretion from RAW 264 and microglial cells incubated with LPS, Aβ₄₂, αSN and PrP82-146. Glimepiride also reduced the LPS, Aβ₄₂, αSN and PrP82-146-induced translocation of TLR-4 into membrane rafts that is associated with cell activation. These effects of glimepiride were also seen after digestion of RAW 264 cells with PI-phospholipase C (PLC). In addition, the effects of glimepiride were blocked by pharmacological inhibition of GPI-PLC. The cytokine production was CD14-dependent; it was reduced in microglia from CD14 knockout mice and was blocked by antiserum to CD14.

Conclusions

RAW 264 and microglial cell responses to Aβ_1–42, αSN, PrP82-146 and LPS are dependent upon CD14 expression. Glimepiride induced the shedding of CD14 from cells by activation of GPI-PLC and consequently reduced cytokine production in response to Aβ₄₂, αSN, PrP82-146 and LPS. These results suggest that glimepiride acts as a novel anti-inflammatory agent that could modify the progression of neurodegenerative diseases.

Evaluation of the effects of glimepiride (Amaryl) and repaglinide (novoNorm) on atherosclerosis progression in high cholesterol-fed male rabbits

BACKGROUND

Atherosclerosis is an inflammatory disease of the blood vessel wall, characterized in early stages by endothelial dysfunction, recruitment and activation of monocyte/macrophages. Glimepiride is one of the third generation sulphonylurea drugs, useful for control of diabetes mellitus type two and it may exert anti inflammatory activity, by induction of nitric oxide production or through selective suppression of the cyclooxygenase pathway. Repaglinide is a new hypoglycemic agent, and a member of the carbamoylmethyl benzoic acid family. Some results from the literature demonstrate that repaglinide has favorable effects on the parameters of antioxidative balance.

OBJECTIVES

The objective of the present study was to assess the effect of glimepiride and repaglinide on atherosclerosis via interfering with the inflammatory and oxidative pathways.

MATERIALS AND METHODS

Twenty four local domestic male rabbits were involved in this study. The animals were randomly divided into four groups; Group I rabbits fed normal chow (oxiod) diet for 10 weeks. Group II rabbits were fed with 1% cholesterol enriched diet. Group III rabbits were fed with 1% cholesterol enriched diet together with Glimepiride (0.1 mg/kg once daily before morning feed). Group IV rabbits were fed with 1% cholesterol enriched diet together with Repaglinide (0.3 mg/kg once daily before morning feed). Blood samples were collected before (0 time) and every two weeks of experimental diets for measurement of serum triglycerides (TG), total cholesterol (TC), High-density lipoprotein cholesterol (HDL-C), high sensitive C - reactive protein (hsCRP), Interleukin - 6 (IL-6) and Tumor Necrosis Factor alpha (TNF-α) levels. At the end of 10 weeks, the aorta was removed for measurement of aortic Malondialdehyde (MDA), reduced glutathione (GSH) and aortic intimal thickness.

RESULTS

Glimepiride and repaglinide treatment did show significant effect on lipid parameters compared with induced untreated group (P < 0.05). Also, they significantly reduced the elevation in hsCRP, IL-6, TNF-α, aortic MDA and aortic intimal thickness compared with induced untreated group (P < 0.05), and they helped to restore the aortic GSH levels (P < 0.05).

CONCLUSIONS

Glimepiride and repaglinide may reduce atherosclerosis progression in hypercholesterolemic rabbits by interfering with the inflammatory and oxidative pathways without affecting lipid parameters.

Insulin receptors and downstream substrates associate with membrane microdomains after treatment with insulin or chromium(III) picolinate

We have examined the association of insulin receptors (IR) and downstream signaling molecules with membrane microdomains in rat basophilic leukemia (RBL-2H3) cells following treatment with insulin or tris(2-pyridinecarbxylato)chromium(III) (Cr(pic)(3)). Single-particle tracking demonstrated that individual IR on these cells exhibited reduced lateral diffusion and increased confinement within 100 nm-scale membrane compartments after treatment with either 200 nM insulin or 10 μM Cr(pic)(3). These treatments also increased the association of native IR, phosphorylated insulin receptor substrate 1 and phosphorylated AKT with detergent-resistant membrane microdomains of characteristically high buoyancy. Confocal fluorescence microscopic imaging of Di-4-ANEPPDHQ labeled RBL-2H3 cells also showed that plasma membrane lipid order decreased following treatment with Cr(pic)(3) but was not altered by insulin treatment. Fluorescence correlation spectroscopy demonstrated that Cr(pic)(3) did not affect IR cell-surface density or compete with insulin for available binding sites. Finally, Fourier transform infrared spectroscopy indicated that Cr(pic)(3) likely associates with the lipid interface in reverse-micelle model membranes. Taken together, these results suggest that activation of IR signaling in a cellular model system by both insulin and Cr(pic)(3) involves retention of IR in specialized nanometer-scale membrane microdomains but that the insulin-like effects of Cr(pic)(3) are due to changes in membrane lipid order rather than to direct interactions with IR.

MLR-1023 is a potent and selective allosteric activator of Lyn kinase in vitro that improves glucose tolerance in vivo

2(1H)-pyrimidinone,5-(3-methylphenoxy) (MLR-1023) is a candidate for the treatment of type 2 diabetes. The current studies were aimed at determining the mechanism by which MLR-1023 mediates glycemic control. In these studies, we showed that MLR-1023 reduced blood glucose levels without increasing insulin secretion in vivo. We have further determined that MLR-1023 did not activate peroxisome proliferator-activated α, δ, and γ receptors or glucagon-like peptide-1 receptors or inhibit dipeptidyl peptidase-4 or α-glucosidase enzyme activity. However, in an in vitro broad kinase screen MLR-1023 activated the nonreceptor-linked Src-related tyrosine kinase Lyn. MLR-1023 increased the V(max) of Lyn with an EC(50) of 63 nM. This Lyn kinase activation was ATP binding site independent, indicating that MLR-1023 regulated the kinase through an allosteric mechanism. We have established a link between Lyn activation and blood glucose lowering with studies showing that the glucose-lowering effects of MLR-1023 were abolished in Lyn knockout mice, consistent with existing literature linking Lyn kinase and the insulin-signaling pathway. In summary, these studies describe MLR-1023 as a unique blood glucose-lowering agent and show that MLR-1023-mediated blood glucose lowering depends on Lyn kinase activity. These results, coupled with other results (J Pharmacol Exp Ther 342:23-32, 2012), suggest that MLR-1023 and Lyn kinase activation may be a new treatment modality for type 2 diabetes.

A glycosylphosphatidylinositol analogue reduced prion-derived peptide mediated activation of cytoplasmic phospholipase A2, synapse degeneration and neuronal death

The pathogenesis of prion diseases includes synapse degeneration and neuronal death. Here we report that pre-treatment with glucosamine-phosphatidylinositol (glucosamine-PI), a synthetic analogue of the glycosylphosphatidylinositol (GPI) anchor that attaches the prion protein (PrP(C)) to plasma membranes, increased the resistance of cultured cortical neurones to the toxic effects of the prion-derived peptide PrP82-146. Pre-treatment with glucosamine-PI reduced the PrP82-146 induced activation of cytoplasmic phospholipase A(2) (cPLA(2)), activation of caspase-3 and synapse degeneration. The addition of glucosamine-PI significantly increased the amount of cholesterol within neuronal membranes consistent with the hypothesis that GPI anchors sequester cholesterol. Whereas in untreated neurones PrP82-146 was found within lipid rafts, in glucosamine-PI treated neurones most PrP82-146 was found in the normal cell membrane and was rerouted into the lysosomes. Complex GPI anchors isolated from PrP(C), Thy-1 or CD55 were also protective against PrP82-146. We conclude that glucosamine-PI, or isolated GPI anchors, can modify local membrane micro-environments that are important in the initiation of signalling events that mediate PrP82-146 induced neurodegeneration.

Glimepiride induces proliferation and differentiation of rat osteoblasts via the PI3-kinase/Akt pathway

Glimepiride is a third-generation sulfonylurea agent and is widely used in the treatment of type 2 diabetes mellitus. In addition to the stimulatory effects on pancreatic insulin secretion, glimepiride has also been reported to have extrapancreatic functions including activation of PI3 kinase (PI3K) and Akt in rat adipocytes and skeletal muscle. PI3-kinase and Akt are important signaling molecules in the regulation of proliferation and differentiation in various cells. This study investigated the actions of glimepiride in rat osteoblasts and the role of PI3K/Akt pathway. Cell proliferation was determined by measuring absorbance at 550 nm. Supernatant assay was used for measuring alkaline phosphatase activity. Western blot analysis was used for determining collagen I, insulin receptor substrate-1/2, PI3K/Akt, and endothelial nitric oxide synthase expression. We found that glimepiride significantly enhanced proliferation and differentiation of osteoblasts and led to activation of several key signaling molecules including insulin receptor substrate-1/2, PI3K/Akt, and endothelial nitric oxide synthase. Furthermore, a specific inhibitor of PI3K abolished the stimulatory effects of glimepiride on proliferation and differentiation. Taken together, these observations provide concrete evidence that glimepiride activates the PI3K/Akt pathway; and this activation is likely required for glimepiride to stimulate proliferation and differentiation of rat osteoblasts.

Lysosomal targeting and trafficking of acid sphingomyelinase to lipid raft platforms in coronary endothelial cells

OBJECTIVE

The purpose of this study was to determine whether lysosome trafficking and targeting of acid sphingomyelinase (ASMase) to this organelle contribute to the formation of lipid raft (LR) signaling platforms in the membrane of coronary arterial endothelial cells (CAECs).

METHODS AND RESULTS

By measurement of fluorescent resonance energy transfer (FRET), it was found that in FasL-stimulated CAECs, membrane lamp1 (a lysosome marker protein) or Fas and GM1 (a LR marker) were trafficking together. Cofocal colocalization assay showed that ceramide was enriched in these LR platforms. Further studies demonstrated that these ceramide molecules in LR platforms were colocalized with ASMase, a ceramide producing enzyme. Fluorescence imaging of living CAECs loaded with lysosomal specific dyes demonstrated that lysosomes fused with membrane on FasL stimulation. In the presence of lysosome function inhibitors, bafilomycin (Baf) or glycyl-L-phenylalanine-beta-naphthylamide (GPN), these FasL-induced changes were abolished. Moreover, this FasL-induced formation of LR platforms was also blocked in ECs transfected with siRNA of sortilin, an intracellular transporter for targeting of ASMase to lysosomes. Functionally, FasL-induced impairment of vasodilator response was reversed by lysosomal inhibitors or sortilin gene silencing.

CONCLUSIONS

Lysosomal trafficking and targeting of ASMase are importantly involved in LRs clustering in ECs membrane, leading to the formation of signaling platforms or signalosomes.

Coordinated regulation of esterification and lipolysis by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in rat adipocytes

Inhibition of lipolysis by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in isolated rat adipocytes has previously been shown to rely on the degradation of cyclic adenosine monophosphate by the phosphodiesterase, Gce1, and the 5'-nucleotidase, CD73. These glycosylphosphatidylinositol (GPI)-anchored proteins are translocated from plasma membrane lipid rafts to intracellular lipid droplets upon H2O2-induced activation of a GPI-specific phospholipase C (GPI-PLC) in response to palmitate and glimepiride in intact adipocytes and, as demonstrated here, in cell-free systems as well. The same agents are also known to stimulate the incorporation of fatty acids into triacylglycerol. Here the involvement of H2O2 production, GPI-PLC activation and translocation of Gce1 and CD73 in the agent-induced esterification and accompanying lipid droplet formation was tested in rat adipocytes using relevant inhibitors. The results demonstrate that upregulation of the esterification and accumulation of triacylglycerol by glimepiride depends on the sequential H2O2-induced GPI-PLC activation and GPI-protein translocation as does inhibition of lipolysis. In contrast, stimulation of the esterification and triacylglycerol accumulation by palmitate relies on insulin-independent tyrosine phosphorylation and thus differs from its anti-lipolytic mechanism. As expected, insulin regulates lipid metabolism via typical insulin signalling independent of H2O2 production, GPI-PLC activation and GPI-protein translocation, albeit these processes are moderately stimulated by insulin. In conclusion, triacylglycerol and lipid droplet formation in response to glimepiride and H2O2 may involve the hydrolysis of cyclic adenosine monophosphate by lipid droplet-associated Gce1 and CD73 which may regulate lipid droplet-associated triacylglycerol-synthesizing and hydrolyzing enzymes in coordinated and inverse fashion.

Glimepiride activates eNOS with a mechanism Akt but not caveolin-1 dependent

Insulin stimulates caveolin-1 and eNOS phosphorylation. The sulfonylurea glimepiride mimics several insulin actions by mechanisms that are poorly understood. Glimepiride induces caveolin-1 phosphorylation and activates PI3K and Akt in rat adipocytes. In this paper, we investigated the effect of glimepiride on eNOS activation in human endothelial cells. We found that glimepiride induces caveolin-1 and eNOS phosphorylation. To better understand the role of caveolin-1 in glimepiride action, we downregulated caveolin-1 expression by specific siRNA transfection. Caveolin-1 silencing did not change eNOS and Akt phosphorylation induced by glimepiride. On the contrary, LY294002, a specific PI3K inhibitor, blocked eNOS serine 1177 phosphorylation. These findings suggest that glimepiride induces eNOS phosphorylation in endothelial cells through an Akt-dependent mechanism, not regulated by caveolin-1.

Glimepiride induces nitric oxide production in human coronary artery endothelial cells via a PI3-kinase-Akt dependent pathway

Diabetes mellitus is one of the major risk factors for coronary artery disease (CAD). A recent study reported that glimepiride, a new third-generation sulfonylurea, inhibited the formation of atheromatous plaques in high-cholesterol fed rabbits. However, the mechanism by which glimepiride induces atheroprotection remains unknown. In the present study, we tested the hypothesis that glimepiride may stimulate NO production in vascular endothelial cells. Human coronary artery endothelial cells (HCAECs) were treated with glimepiride, glibenclamide or vehicle, and NO release was measured. Akt phosphorylation was evaluated by Western blot. The effects of LY294002, a specific PI3-kinase inhibitor, and antisense oligonucleotides directed to Akt, on glimepiride-induced NO production were examined. Glimepiride (0.1-10 microM), but not glibenclamide, induced NO production, significantly increasing it by 1.8-fold (n=6, p<0.05). LY294002 inhibited glimepiride-induced NO production by 68%. Akt was rapidly phosphorylated by glimepiride and antisense oligonucleotides directed to Akt completely inhibited glimepiride-induced NO production. These data demonstrate that glimepiride induces NO production in HCAECs by activating PI3-kinase and Akt, and also suggest that use of glimepiride in type 2 diabetes may show promise for preventing CAD in addition to lowering glucose levels.

Phosphorylation of glycosyl-phosphatidylinositol by phosphatidylinositol 3-kinase changes its properties as a substrate for phospholipases

Phosphatidylinositol 3-kinases (PI3K) phosphorylate the 3-position of the inositol ring of phosphatidylinositol-4,5-bisphosphate to produce phosphatidylinositol-3,4,5-trisphosphate. It is not clear whether PI3K can phosphorylate the inositol group in other biomolecules. We sought to determine whether PI3K was able to use glycosyl-phosphatidylinositol (GPI) as a substrate. This phospholipid may exist either in free form (GPIfree) or forming a lipid anchor (GPIanchor) for the attachment of extracellular proteins to the plasma membrane. We demonstrate the specific PI3K-mediated phosphorylation of the inositol 3-hydroxyl group within both types of GPI by incubating this phospholipid with immunoprecipitated PI3K. The phosphorylated product behaves in HPLC as a derivative of a PI3K lipid product. To our knowledge, this is the first demonstration that PI3K uses lipid substrates other than phosphoinositides. Further, we show that this has potential functional consequences. When GPIfree is phosphorylated, it becomes a poorer substrate for GPI-specific phospholipase D, but a better substrate for phosphatidylinositol-specific phospholipase C. These phosphorylation events may constitute the basis of a previously undescribed signal transduction mechanism.

Phosphatidylinositol 3-kinase activation is required for sulfonylurea stimulation of glucose transport in rat skeletal muscle

Sulfonylureas are drugs widely used in the treatment of patients with type 2 diabetes mellitus. In addition to their pancreatic effect of stimulating insulin secretion, many studies suggest that sulfonylureas also have extrapancreatic actions. We have previously reported that gliclazide, a second-generation sulfonylurea, stimulates the glucose uptake by rat hindquarter skeletal muscle directly and immediately by promoting the translocation of glucose transporter 4 to the plasma membrane. The aim of our study was to approach the gliclazide intracellular signaling pathway. For this purpose, we incubated clamped and isolated soleus muscle from rat with gliclazide. The following results were obtained: 1) gliclazide stimulates insulin receptor substrate (IRS)-1-phosphatidylinositol 3 (PI3)-kinase-associated activity, and this activity is necessary for gliclazide-stimulated glucose transport; 2) gliclazide treatment produces a gradual translocation of the diacylglycerol (DAG)-dependent isoforms protein kinase C (PKC) alpha, theta, and epsilon from cytosolic to membrane fraction that is dependent on PI3-kinase and phospholipase C (PLC)-gamma activation; and 3) PKC and PLC-gamma activation is necessary for gliclazide-stimulated glucose transport. We propose a hypothetical signaling pathway by which gliclazide could stimulate IRS-1 that would allow its association with PI3-kinase, promoting its activation. PI3-kinase products could induce PLC-gamma activation, whose hydrolytic activity could activate the DAG-dependent isoforms PKC alpha, theta, and epsilon.

GPI-anchored protein cleavage in the regulation of transmembrane signals

The structure of covalently-linked glycosylphosphatidylinositol (GPI) anchors of membrane proteins displayed on the cell surface is described. Evidence of how the GPI-anchors are sorted into membrane rafts in the plasma membrane is reviewed. Proteins are released by hydrolysis of the linkage to the GPI anchor and phospholipases from different sources involved in this process are characterised. The regulation of protein conformation and function resulting from phospholipase cleavage of the GPI anchor is discussed in the context of its role in signal transduction by insulin. In this signalling system, re-distribution of critical membrane components, including GPI-anchored proteins and non-receptor tyrosine kinases, between different raft domains appears to play a central role in the signal transduction pathway.

Insulin receptor substrate-4 is expressed in muscle tissue without acting as a substrate for the insulin receptor

Insulin receptor substrate (IRS) proteins represent key elements of the insulin-signaling cascade. IRS-4 is the most recently characterized member of the IRS family with an undefined in vivo function. In contrast to IRS-1 and IRS-2, IRS-4 exhibits a limited tissue expression, and IRS-4 protein has not been detected in any mouse or primary human tissue so far. The purpose of the present study was to analyze the expression of IRS-4 in rat muscle and human skeletal muscle cells and assess involvement of IRS-4 in initial insulin signaling. Using immunoblotting and immunoprecipitation, the specific expression of IRS-4 protein could be demonstrated in rat soleus and cardiac muscle and human skeletal muscle cells, but it was not significantly detectable in quadriceps and gastrocnemius. A prominent down-regulation of IRS-4 was observed in heart and soleus muscle of WOKW rats, an animal model of the metabolic syndrome. In human skeletal muscle cells, both IRS-1 and IRS-2 are rapidly phosphorylated on tyrosine in response to insulin, whereas essentially no tyrosine phosphorylation of IRS-4 was observed in response to both insulin and IGF-I. Instead, a 2-fold increase in IRS-4 tyrosine phosphorylation was observed in myocytes subjected to osmotic stress. In conclusion, IRS-4 protein is expressed in heart and skeletal muscle in a fiber type specific fashion. Our data suggest that IRS-4 does not function as a substrate of the insulin and the IGF-I receptor in primary muscle cells but may be involved in nonreceptor tyrosine kinase signaling.

Interaction of phosphoinositolglycan(-peptides) with plasma membrane lipid rafts of rat adipocytes

Insulin receptor-independent activation of the insulin signal transduction cascade in insulin-responsive target cells by phosphoinositolglycans (PIG) and PIG-peptides (PIG-P) is accompanied by redistribution of glycosylphosphatidylinositol (GPI)-anchored plasma membrane proteins (GPI proteins) and dually acylated nonreceptor tyrosine kinases from detergent/carbonate-resistant glycolipid-enriched plasma membrane raft domains of high-cholesterol content (hcDIGs) to rafts of lower cholesterol content (lcDIGs). Here we studied the nature and localization of the primary target of PIG(-P) in isolated rat adipocytes. Radiolabeled PIG-P (Tyr-Cys-Asn-NH-(CH(2))(2)-O-PO(OH)O-6Manalpha1(Manalpha1-2)-2Manalpha1-6Manalpha1-4GluN1-6Ino-1,2-(cyclic)-phosphate) prepared by chemical synthesis or a radiolabeled lipolytically cleaved GPI protein from Saccharomyces cerevisiae, which harbors the PIG-P moiety, bind to isolated hcDIGs but not to lcDIGs. Binding is saturable and abolished by pretreatment of intact adipocytes with trypsin followed by NaCl or with N-ethylmaleimide, indicating specific interaction of PIG-P with a cell surface protein. A 115-kDa polypeptide released from the cell surface by the trypsin/NaCl-treatment is labeled by [(14)C]N-ethylmaleimide. The labeling is diminished upon incubation of adipocytes with PIG-P which can be explained by direct binding of PIG-P to the 115-kDa protein and concomitant loss of its accessibility to N-ethylmaleimide. Binding of PIG-P to hcDIGs is considerably increased after pretreatment of adipocytes with (glycosyl)phosphatidylinositol-specific phospholipases compatible with lipolytic removal of endogenous ligands, such as GPI proteins/lipids. These data demonstrate that in rat adipocytes synthetic PIG(-P) as well as lipolytically cleaved GPI proteins interact specifically with hcDIGs. The interaction depends on the presence of a trypsin/NaCl/NEM-sensitive 115-kDa protein located at hcDIGs which thus represents a candidate for a binding protein for exogenous insulin-mimetic PIG(-P) and possibly endogenous GPI proteins/lipids.

Interaction of phosphatidylinositolglycan(-peptides) with plasma membrane lipid rafts triggers insulin-mimetic signaling in rat adipocytes

The phosphoinositolglycan(-peptide) (PIG-P) portion of glycosylphosphatidylinositol-anchored plasma membrane (GPI) proteins or synthetic PIG(-P) molecules interact with proteinaceous binding sites which are located in high-cholesterol-containing detergent/carbonate-insoluble glycolipid-enriched raft domains (hcDIGs) of the plasma membrane. In isolated rat adipocytes, PIG(-P) induce the redistribution of GPI proteins from hcDIGs to low-cholesterol-containing DIGs (lcDIGs) and concomitantly provoke insulin-mimetic signaling and metabolic action. Using a set of synthetic PIG(-P) derivatives we demonstrate here that their specific binding to hcDIGs and their insulin-mimetic signaling/metabolic activity strictly correlate with respect to (i) translocation of the GPI proteins, Gce1 and 5(')-nucleotidase, from hcDIGs to lcDIGs, (ii) dissociation of the nonreceptor tyrosine kinase, pp59(Lyn), from caveolin residing at hcDIGs, (iii) translocation of pp59(Lyn) from hcDIGs to lcDIGs, (iv) activation of pp59(Lyn), (v) tyrosine phosphorylation of insulin receptor substrate proteins-1/2, and finally (vi) stimulation of glucose transport. The natural PIG(-P) derived from the carboxy-terminal tripeptide of Gce1, YCN-PIG, exhibits the highest potency followed by a combination of the separate peptidylethanolamidyl and PIG constituents. We conclude that efficient positive cross-talk of PIG(-P) to the insulin signaling cascade requires their interaction with hcDIGs. We suggest that PIG(-P) thereby displace GPI proteins from binding to hcDIGs leading to their release from hcDIGs for lateral movement to lcDIGs which initiates signal transduction from DIGs via caveolin and pp59(Lyn) to the insulin receptor substrate proteins of the insulin signaling pathway.

Dynamics of plasma membrane microdomains and cross-talk to the insulin signalling cascade

The critical role of the heterogeneous nature of cellular plasma membranes in transmembrane signal transduction has become increasingly appreciated during the past decade. Areas of relatively disordered, loosely packed phospholipids are disrupted by hydrophobic detergent/carbonate-insoluble glycolipid-enriched raft microdomains (DIGs) of highly ordered (glyco)sphingolipids and cholesterol. DIGs exhibit low buoyant density and are often enriched in glycosylphosphatidylinositol-anchored plasma membrane proteins (GPI proteins), dually acylated signalling proteins, such as non-receptor tyrosine kinases (NRTKs), and caveolin. At least two types of DIGs, hcDIGs and lcDIGs, can be discriminated on basis of higher and lower content, respectively, of these typical DIGs components. In quiescent differentiated cells, GPI proteins and NRTKs are mainly associated with hcDIGs, however, in adipose cells certain insulin-mimetic stimuli trigger redistribution of subsets of GPI proteins and NRTKs from hcDIGs to lcDIGs. Presumably, these stimuli induce displacement of GPI proteins from a GPI receptor located at hcDIGs whereas simultaneously NRTKs dissociate from a complex with caveolin located at hcDIGs, too. NRTKs are thereby activated and, in turn, modulate intracellular signalling pathways, such as stimulation of metabolic insulin signalling in insulin-sensitive cells. The apparent dynamics of DIGs may provide a target mechanism for regulating the activity of lipid-modified signalling proteins by small drug molecules, as exemplified by the sulfonylurea, glimepiride, which lowers blood glucose in an insulin-independent fashion, in part.