The effects of wortmannin, a potent inhibitor of phosphatidylinositol 3-kinase, on insulin-stimulated glucose transport, GLUT4 translocation, antilipolysis, and DNA synthesis

Flow cytometry protocol for GLUT4-myc detection on cell surfaces

Insulin and muscle contraction trigger GLUT4 translocation to the plasma membrane, which increases glucose uptake by muscle cells. Insulin resistance and Type 2 diabetes are the result of impaired GLUT4 translocation. Quantifying GLUT4 translocation is essential for comprehending the intricacies of both physiological and pathophysiological processes involved in glucose metabolism. The most commonly used methods for measuring GLUT4 translocation are the ELISA-type assay and the immunofluorescence assay. While some reports suggest that flow cytometry could be useful in quantifying GLUT4 translocation, this technique is not frequently used. Much of our current understanding of the regulation of GLUT4 has been based on experiments using the rat myoblast cell line (L6 cell) which expresses GLUT4 with a myc epitope on the exofacial loop. In the present study, we use the L6-GLUT4myc cell line to develop a flow cytometry-based approach to detect GLUT4 translocation. Flow cytometry offers the advantages of both immunofluorescence and ELISA-based assays. It allows easy identification of separate cell populations in the sample, similar to immunofluorescence, while providing results based on a population-level analysis of multiple individual cells, like an ELISA-based assay. Our results demonstrate a 0.6-fold increase with insulin stimulation compared with basal conditions. Finally, flow cytometry consistently yielded results across different experiments and exhibited sensitivity under the tested conditions.

Acute effectors of GLUT1 glucose transporter subcellular targeting in CIT3 mouse mammary epithelial cells

Lactogenic hormones cause intracellular targeting of glucose transporter 1 (GLUT1) for transport of glucose to the site of lactose synthesis in mammary glands. Our aim was to study the intracellular trafficking mechanisms involved in GLUT1 targeting and recycling in CIT3 mouse mammary epithelial cells. Fusion proteins of GLUT1 and enhanced green fluorescent protein (EGFP) were expressed in CIT3 cells maintained in growth medium (GM), or exposed to secretion medium (SM), containing prolactin. Agents acting on Golgi and related subcellular compartments and on GLUT1 and GLUT4 targeting in muscle and fat cells were studied. Wortmannin and staurosporine effects on internalization of GLUT1 were not specific, supporting a basal constitutive GLUT1 membrane-recycling pathway between an intracellular pool and the cell surface in CIT3 cells, which targets most GLUT1 to the plasma membrane in GM. Upon exposure to prolactin in SM, GLUT1 was specifically targeted intracellularly to a brefeldin A-sensitive compartment. Arrest of endosomal acidification by bafilomycin A1 disrupted this prolactin-induced GLUT1 intracellular trafficking with central coalescence of GLUT1-EGFP signal, suggesting that it is via endosomal pathways. This machinery offers another level of regulation of lactose synthesis by altering GLUT1 targeting within minutes to hours.

Lipolysis is stimulated by PEGylated conjugated linoleic acid through the cyclic adenosine monophosphate-independent signaling pathway in 3T3-L1 cells: activation of MEK/ERK MAPK signaling pathway and hyper-secretion of adipo-cytokines

We previously reported that PEGylated conjugated linoleic acid (PCLA) as a pro-drug treatment of cultures of 3T3-L1 cells containing differentiated adipocytes caused de-differentiation by downregulation of PPARgamma2-induced adipogenesis, and cell apoptosis induced by PCLA was lower than that induced by conjugated linoleic acid (CLA) owing to the biocompatible and hydrophilic properties of poly(ethylene glycol) (PEG). To further investigate our previous observations, the present study is designed to evaluate the lipolytic action of PCLA and its role in biochemical signaling pathways of 3T3-L1 cells when compared to the CLA itself. Although both CLA and PCLA stimulated lipolysis, our results indicated a sensitivity difference between CLA and PCLA treatment: a time-dependent effect on lipolysis and p-extracellular signal-related kinases (ERK) expression was observed for PCLA-treated, but not for CLA-treated cultures. Also, the induction by PCLA of mitogen-activated protein kinase kinase (MEK)/ERK mitogen-activated protein kinase (MAPK) activation was linked to secretion of adipo-cytokines, interleukin-6 (IL-6), and interleukin-8 (IL-8), in time-dependent manners. Interestingly, adenylyl cyclase inhibitor, 2', 5'-dideoxyadenosine (DDA), pre-treatment did not prevent PCLA-stimulated lipolysis. In fact, isoproterenol, but not PCLA, caused a significant increase in cyclic adenosine monophosphate (cAMP) levels, suggesting that the PCLA-induced lipolysis was not mediated in the conventional cAMP-dependent pathway and the cAMP was the intracellular mediator for isoproterenol-induced lipolysis. Overall, our findings provide support for a role for PCLA as a pro-drug in the regulation of metabolism in adipose tissue.

Fate of a bioactive fluorescent wortmannin derivative in cells

Here, we report on NBD-Wm, a fluorescent wortmannin (Wm) probe that maintains the bioactivity of Wm as an inhibitor of PI3 kinase and as an antiproliferative agent. The attachment of the NBD fluorochrome permits NBD-Wm in cells to be monitored by NBD fluorescence-based methods such as FACS or fluorescence microscopy or with an anti-NBD antibody. The fluorescence of NBD-Wm treated cells reached a peak at 1.5 h and then decreased because of the extrusion of a fluorescent compound into the culture media. Cells accumulated NBD-Wm to levels about 30-fold higher than those in the media. NBD-Wm modified five major proteins, with the modification of the catalytic subunit of PI3 kinase being a minor band. The bioactivity of NBD-Wm, coupled with a variety of techniques available for determining its disposition, suggest that NBD-Wm can be a useful tool in understanding the mechanism of action of viridins.

Covalent reactions of wortmannin under physiological conditions

Wortmannin (Wm), a steroid-like molecule of 428.4 Da, appears to be unstable in biological fluids (apparent chemical instability), yet it exhibits an antiproliferative activity in assays employing a 48 hr incubation period (prolonged bioactivity), a situation we refer to as the "wortmannin paradox." Under physiological conditions, Wm covalently reacts with nucleophiles such as the side chains of cysteine, N-methyl hexanoic acid, lysine, or proline at the C20 position on the furan ring. Like Wm, WmC20 amino acid derivatives had significant antiproliferative activities. Three Wm derivatives, WmC20-proline, WmC20-cysteine, and a WmC20-N-methyl hexanoic acid, generated Wm that then reacted with lysine in an exchange-type reaction. This unusual, reversible, covalent reaction of Wm with nucleophiles under physiological conditions provides an explanation for the wortmannin paradox.

Berberine-stimulated glucose uptake in L6 myotubes involves both AMPK and p38 MAPK

Berberine is a plant alkaloid used in traditional Chinese medicine and has been reported to have antihyperglycemic activity in NIDDM patients. However, the molecular basis for this action is yet to be elucidated. Here we investigate the effects and signaling pathways of berberine on L6 rat skeletal muscles. Our study demonstrates that berberine stimulates glucose uptake in a time- and dose-dependent manner. Intriguingly, berberine-stimulated glucose uptake does not vary as insulin concentration increases, and could not be blocked by the PI 3-kinase inhibitor wortmannin. Berberine only weakly stimulates the phosphorylation of Akt/PKB, a key molecule in the insulin signaling pathway, but strongly promotes the phosphorylation of AMPK and p38 MAPK. The effects of berberine are not a result of pro-oxidant action, but a consequence of an increased cellular AMP:ATP ratio. Moreover, berberine-stimulated glucose uptake is inhibited by the AMPK inhibitor Compound C and the p38 MAPK inhibitor SB202190. Inhibition of AMPK reduces p38 MAPK phosphorylation, suggesting that AMPK lies upstream of p38 MAPK. These results suggest that berberine circumvents insulin signaling pathways and stimulates glucose uptake through the AMP-AMPK-p38 MAPK pathway, which may account for the antihyperglycemic effects of this drug.

Lipid phosphatases as drug discovery targets for type 2 diabetes

The soaring incidence of type 2 diabetes has created pressure for new pharmaceutical strategies to treat this devastating disease. With much of the focus on overcoming insulin resistance, investigation has focused on finding ways to restore activation of the phosphatidylinositol 3'-kinase pathway, which is diminished in many patients with type 2 diabetes. Here we review the evidence that lipid phosphatases, specifically PTEN and SHIP2, attenuate this important insulin signalling pathway. Both in vivo and in vitro studies indicate their role in regulating whole-body energy metabolism, and possibly weight gain as well. The promise and challenges presented by this new class of drug discovery targets will also be discussed.

Effect of endothelin-1 on lipolysis in rat adipocytes

OBJECTIVE

To explore the role of endothelin-1 (ET-1) on lipid metabolism, we examined the effect of ET-1 on lipolysis in rat adipocytes.

RESEARCH METHODS AND PROCEDURE

Adipocytes isolated from male Sprague-Dawley rats, weighing 400 to 450 grams, were incubated in Krebs-Ringer buffer with or without 10(-7) M ET-1 for various times or with various concentrations of ET-1 for 4 hours; then glycerol release into the incubation medium was measured. In addition, selective ET(A)R and ET(B)R blockers were used to identify the ET receptor subtype involved. We also explored the involvement of cyclic adenosine monophosphate (cAMP) in ET-1-stimulated lipolysis using an adenylyl cyclase inhibitor and by measuring changes in intracellular cAMP levels in response to ET-1 treatment. To further explore the underlying mechanism of ET-1 action, we examined the involvement of the extracellular signal-regulated kinase (ERK)-mediated pathways.

RESULTS

Our results showed that ET-1 caused lipolysis in rat adipocytes in a time- and dose-dependent manner. BQ610, a selective ET(A)R blocker, blocked this effect. The adenylyl cyclase inhibitor, 2',5'-dideoxyadenosine, had no effect on ET-1-stimulated lipolysis. ET-1 did not induce an increase in intracellular cAMP levels. In addition, ET-1-induced lipolysis was blocked by inhibition of ERK activation using PD98059. Coincubation of cells with ET-1 and insulin suppressed ET-1-stimulated lipolysis.

DISCUSSION

These findings show that ET-1 stimulates lipolysis in rat adipocytes through the ET(A)R and activation of the ERK pathway. The underlying mechanism is cAMP-independent. However, this non-conventional lipolytic effect of ET-1 is inhibited by the anti-lipolytic effect of insulin.

Insulin but not PDGF relies on actin remodeling and on VAMP2 for GLUT4 translocation in myoblasts

Insulin promotes the translocation of glucose transporter 4 (GLUT4) from intracellular pools to the surface of muscle and fat cells via a mechanism dependent on phosphatidylinositol (PtdIns) 3-kinase, actin cytoskeletal remodeling and the v-SNARE VAMP2. The growth factor PDGF-BB also robustly activates PtdIns 3-kinase and induces actin remodeling, raising the question of whether it uses similar mechanisms to insulin in mobilizing GLUT4. In L6 myoblasts stably expressing Myc-tagged GLUT4, neither stimulus affected the rate of GLUT4 endocytosis, confirming that they act primarily by enhancing exocytosis to increase GLUT4 at the cell surface. Although surface GLUT4myc in response to insulin peaked at 10 minutes and remained steady for 30 minutes, PDGF action was transient, peaking at 5 minutes and disappearing by 20 minutes. These GLUT4myc translocation time courses mirrored that of phosphorylation of Akt by the two stimuli. Interestingly, insulin and PDGF caused distinct manifestations of actin remodeling. Insulin induced discrete, long (>5 μm) dorsal actin structures at the cell periphery, whereas PDGF induced multiple short (<5 μm) dorsal structures throughout the cell, including above the nucleus. Latrunculin B, cytochalasin D and jasplakinolide, which disrupt actin dynamics, prevented insulin- and PDGF-induced actin remodeling but significantly inhibited GLUT4myc translocation only in response to insulin (75-85%, P<0.05), not to PDGF (20-30% inhibition). Moreover, transfection of tetanus toxin light chain, which cleaves the v-SNAREs VAMP2 and VAMP3, reduced insulin-induced GLUT4myc translocation by >70% but did not affect the PDGF response. These results suggest that insulin and PDGF rely differently on the actin cytoskeleton and on tetanus-toxin-sensitive VAMPs for mobilizing GLUT4.

Intracellular delivery of phosphatidylinositol (3,4,5)-trisphosphate causes incorporation of glucose transporter 4 into the plasma membrane of muscle and fat cells without increasing glucose uptake

Insulin stimulates glucose uptake into muscle and fat cells by translocating glucose transporter 4 (GLUT4) to the cell surface, with input from phosphatidylinositol (PI) 3-kinase and its downstream effector Akt/protein kinase B. Whether PI 3,4,5-trisphosphate (PI(3,4,5)P(3)) suffices to produce GLUT4 translocation is unknown. We used two strategies to deliver PI(3,4,5)P(3) intracellularly and two insulin-sensitive cell lines to examine Akt activation and GLUT4 translocation. In 3T3-L1 adipocytes, the acetoxymethyl ester of PI(3,4,5)P(3) caused GLUT4 migration to the cell periphery and increased the amount of plasma membrane-associated phospho-Akt and GLUT4. Intracellular delivery of PI(3,4,5)P(3) using polyamine carriers also induced translocation of myc-tagged GLUT4 to the surface of intact L6 myoblasts, demonstrating membrane insertion of the transporter. GLUT4 translocation caused by carrier-delivered PI(3,4,5)P(3) was not reproduced by carrier-PI 4,5-bisphosphate or carrier alone. Like insulin, carrier-mediated delivery of PI(3,4,5)P(3) elicited redistribution of perinuclear GLUT4 and Akt phosphorylation at the cell periphery. In contrast to its effect on GLUT4 mobilization, delivered PI(3,4,5)P(3) did not increase 2-deoxyglucose uptake in either L6GLUT4myc myoblasts or 3T3-L1 adipocytes. The ability of exogenously delivered PI(3,4,5)P(3) to augment plasma membrane GLUT4 content without increasing glucose uptake suggests that input at the level of PI 3-kinase suffices for GLUT4 translocation but is insufficient to stimulate glucose transport.

Obesity--is it a genetic disorder?

Obesity is one of the most pressing problems in the industrialized world. Twin, adoption and family studies have shown that genetic factors play a significant role in the pathogenesis of obesity. Rare mutations in humans and model organisms have provided insights into the pathways involved in body weight regulation. Studies of candidate genes indicate that some of the genes involved in pathways regulating energy expenditure and food intake may play a role in the predisposition to obesity. Amongst these genes, sequence variations in the adrenergic receptors, uncoupling proteins, peroxisome proliferator-activated receptor, and the leptin receptor genes are of particular relevance. Results that have been replicated in at least three genome-wide scans suggest that key genes are located on chromosomes 2p, 3q, 5p, 6p, 7q, 10p, 11q, 17p and 20q. We conclude that the currently available evidence suggests four levels of genetic determination of obesity: genetic obesity, strong genetic predisposition, slight genetic predisposition, and genetically resistant. This growing body of research may help in the development of anti-obesity agents and perhaps genetic tests to predict the risk for obesity.

Differential regulation of lipogenesis and leptin production by independent signaling pathways and rosiglitazone during human adipocyte differentiation

Since leptin levels are independently correlated with risk of coronary heart disease, we have identified signaling pathways important in mediating leptin production and lipogenesis in human preadipocytes. We used inhibitors of p70(S6) kinase, p42/44 mitogen-activated protein kinase (MAPK), p38 MAPK, and phosphatidylinositol 3-kinase (PI3K). Human preadipocytes were induced to differentiate in insulin, dexamethasone, triiodothyronine, and 3-isobutyl-1-methylxanthine in the presence or absence of inhibitors and the peroxisome proliferator-activated receptor (PPAR)-gamma activator rosiglitazone. Differentiation was assessed by measuring leptin secretion, lipid content, and lipogenic activity. Rosiglitazone increased cell protein by 15%, the lipid content of the cell layer was doubled, and the lipogenic activity increased sevenfold but did not stimulate leptin secretion. None of the inhibitors significantly inhibited protein content over 20 days, but lipid content and lipogenic activity were inhibited by p70(S6) kinase and p38 MAPK inhibition but not by p42/44 MAPK or PI3K inhibition. All of the inhibitors significantly decreased leptin secretion, and these inhibitory effects were increased by coincubation with rosiglitazone. We conclude that PI3K and p42/44 MAPK pathways are not critical to the differentiation program leading to lipid accumulation, but stimulation of leptin secretion is dependent on these as well as the p70(S6) kinase and p38 MAPK signaling pathways.

Regulation of phosphoinositide metabolism, Akt phosphorylation, and glucose transport by PTEN (phosphatase and tensin homolog deleted on chromosome 10) in 3T3-L1 adipocytes

To investigate the roles of PTEN (phosphatase and tensin homolog deleted on chromosome 10) in the regulation of 3-position phosphorylated phosphoinositide metabolism as well as insulin-induced Akt phosphorylation and glucose metabolism, wild-type PTEN and its phosphatase-dead mutant (C124S) with or without an N-terminal myristoylation tag were overexpressed in Sf-9 cells and 3T3-L1 adipocytes using baculovirus and adenovirus systems, respectively. When expressed in Sf-9 cells together with the p110alpha catalytic subunit of phosphoinositide 3-kinase, myristoylated PTEN markedly reduced the accumulations of both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate induced by p110alpha. In contrast, overexpression of the C124S mutants apparently increased these accumulations. In 3T3-L1 adipocytes, insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate were markedly suppressed by overexpression of wild-type PTEN with the N-terminal myristoylation tag, but not by that without the tag. On the contrary, the C124S mutants of PTEN enhanced insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. Interestingly, the phosphorylation level of Akt at Thr308 (Akt2 at Thr309), but not at Ser473 (Akt2 at Ser474), was revealed to correlate well with the accumulation of phosphatidylinositol 3,4,5-trisphosphate modified by overexpression of these PTEN proteins. Finally, insulin-induced increases in glucose transport activity were significantly inhibited by the overexpression of myristoylated wild-type PTEN, but were not enhanced by expression of the C124S mutant of PTEN. Therefore, in conclusion, 1) PTEN dephosphorylates both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate in vivo, and the C124S mutants interrupt endogenous PTEN activity in a dominant-negative manner. 2) The membrane targeting process of PTEN may be important for exerting its function. 3) Phosphorylations of Thr309 and Ser474 of Akt2 are regulated differently, and the former is regulated very sensitively by the function of PTEN. 4) The phosphorylation level of Ser474, but not that of Thr309, in Akt2 correlates well with insulin-stimulated glucose transport activity in 3T3-L1 adipocytes. 5) The activity of endogenous PTEN may not play a major role in the regulation of glucose transport activity in 3T3-L1 adipocytes.

Physiological functions of protein kinase inhibitors

Protein kinases are key regulatory enzymes involved in a multitude of biochemical pathways. This chapter will describe the current research on targeting specific protein kinases with inhibitors in attempts to disrupt flux through specific pathways. Targeting specific kinases presents a distinct challenge as there are hundreds of individual kinase enzymes that use ATP as a substrate to phosphorylate specific target molecules. The challenge clearly lies in obtaining specificity for a given kinase, thus allowing inhibition or activation of a specific pathway. This chapter will focus on two areas of kinase inhibitors, those that target the MAP kinase pathway and those directed against the phosphatidylinositol-3 kinase (PI-3K) related kinase family. The cellular and physiological effects of inhibition of the various pathways controlled by these kinases will be reviewed.

Demonstration of insulin-responsive trafficking of GLUT4 and vpTR in fibroblasts

Insulin-responsive trafficking of the GLUT4 glucose transporter and the insulin-regulated aminopeptidase (IRAP) in adipose and muscle cells is well established. Insulin regulation of GLUT4 trafficking in these cells underlies the role that adipose tissue and muscle play in the maintenance of whole body glucose homeostasis. GLUT4 is expressed in a very limited number of tissues, most highly in adipose and muscle, while IRAP is expressed in many tissues. IRAP's physiological role in any of the tissues in which it is expressed, however, is unknown. The fact that IRAP, which traffics by the same insulin-regulated pathway as GLUT4, is expressed in ‘non-insulin responsive’ tissues raises the question of whether these other cell types also have a specialized insulin-regulated trafficking pathway. The existence of an insulin-responsive pathway in other cell types would allow regulation of IRAP activity at the plasma membrane as a potentially important physiological function of insulin. To address this question we use reporter molecules for both GLUT4 and IRAP trafficking to measure insulin-stimulated translocation in undifferentiated cells by quantitative fluorescence microscopy. One reporter (vpTR), a chimera between the intracellular domain of IRAP and the extracellular and transmembrane domains of the transferrin receptor, has been previously characterized. The other is a GLUT4 construct with an exofacial HA epitope and a C-terminal GFP. By comparing these reporters to the transferrin receptor, a marker for general endocytic trafficking, we demonstrate the existence of a specialized, insulin-regulated trafficking pathway in two undifferentiated cell types, neither of which normally express GLUT4. The magnitude of translocation in these undifferentiated cells (approximately threefold) is similar to that reported for the translocation of GLUT4 in muscle cells. Thus, undifferentiated cells have the necessary retention and translocation machinery for an insulin response that is large enough to be physiologically important.

p110beta is up-regulated during differentiation of 3T3-L1 cells and contributes to the highly insulin-responsive glucose transport activity

Activation of p85/p110 type phosphatidylinositol kinase is essential for aspects of insulin-induced glucose metabolism, including translocation of GLUT4 to the cell surface and glycogen synthesis. The enzyme exists as a heterodimer containing a regulatory subunit (e.g. p85alpha) and one of two widely distributed isoforms of the p110 catalytic subunit: p110alpha or p110beta. In the present study, we compared the two isoforms in the regulation of insulin action. During differentiation of 3T3-L1 cells into adipocytes, p110beta was up-regulated approximately 10-fold, whereas expression of p110alpha was unaltered. The effects of the increased p110 expression were further assessed by expressing epitope tagged p110beta and p110alpha in 3T3-L1 cells using adenovirus transduction systems, respectively. In vitro, the basal lipid kinase activity of p110beta was lower than that of p110alpha. When p110alpha and p110beta were overexpressed in 3T3-L1 adipocytes, exposing cells to insulin induced each of the subunits to form complexes with p85alpha and tyrosine-phosphorylated IRS-1 with similar efficiency. However, whereas the kinase activity of p110beta, either endogenous or exogeneous, was markedly enhanced by insulin stimulation, only very small increases of the activity of p110alpha were observed. Interestingly, overexpression of p110beta increased insulin-induced glucose uptake by 3T3-L1 cells without significantly affecting basal glucose transport, whereas overexpression of p110alpha increased both basal and insulin-stimulated glucose uptake. Finally, microinjection of anti-p110beta neutralizing antibody into 3T3-L1 adipocytes abolished insulin-induced translocation of GLUT4 to the cell surface almost completely, whereas anti-p110alpha neutralizing antibody did only slightly. Together, these findings suggest that p110beta plays a crucial role in cellular activities evoked acutely by insulin.

GH induced lipolysis stimulation in 3T3-L1 adipocytes stably expressing hGHR: analysis on signaling pathway and activity of 20K hGH

We have constructed a cell line of 3T3-L1 which can efficiently express human GHR (3T3-L1-hGHR) after differentiation to adipocytes. The expressed hGHR was detected as two bands with approximate molecular sizes of 120K by Western analysis using hGHR specific monoclonal antibody. Maximum lipolytic activity induced by hGH in the 3T3-L1-hGHR was enhanced 10-fold as compared to that in 3T3-L1, suggesting that expressed hGHR is functionally active. Comparative analysis using bGH and hGH revealed that 70% of lipolysis stimulation by 1-10 ng/ml hGH could be attributed to hGHR-mediated response. Analyses on inhibition and phosphorylation of signaling molecules suggested that GH-induced lipolysis stimulation is dependent on gene expression and not mediated through PKA-, PKC-, PLA-, PLC-, nor MAPK-pathway but possibly through JAK-STATs pathway. Duration of STAT5 activation by hGH continued up to 48 h. We also revealed that 22 K hGH isoform, 20K hGH which has been reported as a weaker agonist for GH-induced lipolysis stimulation, possesses equipotent activity and shows stronger action in the presence of hGHBP as compared to 22 K hGH. Taken together we conclude that the hGH-induced lipolysis was not mediated through MAP-, PKA-, PKC-, nor PLA-pathway but might be mediated through STAT pathway and that 20K hGH might show higher lipolytic activity than 22 K hGH in adipose tissue that produces a large amount of GHBP.

Structure and function of phosphatidylinositol-3,4 kinase

Activation of phosphatidylinositol (PI)-kinase is involved in the regulation of a wide array of cellular activities. The enzyme exists as a dimer, consisting of a catalytic and a regulatory subunit. Five isoforms of the regulatory subunit have been identified and classified into three groups comprising respectively 85-kDa, 55-kDa, and 50-kDa proteins. Structural differences in the N-terminal regions of the different group members contribute to defining their binding specificity, their subcellular distributions, and their capacity to activate the 110-kDa catalytic subunit. Two widely distributed isoforms of the catalytic subunit have been identified-p110alpha and p110beta. Despite the fact that they bind to the p85alpha regulatory subunit similarly, p110alpha and p110beta appear to have separate functions within cells and to be activated by different stimuli. Moreover, although p85/p110 PI-kinase almost exclusively phosphorylates the D-3 position of the inositol ring in phosphoinositides when purified PI is used as a substrate in vitro, it appears to phosphorylate the D-4 position with similar or higher efficiency in vivo. Thus, it is highly probable that p85/p110 PI-kinase transmits signals to downstream targets via both D-3- and D-4-phosphorylated phosphoinositides.

Characterization of the insulin-regulated endocytic recycling mechanism in 3T3-L1 adipocytes using a novel reporter molecule

The endocytic trafficking of the GLUT4 glucose transporter and the insulin-regulated aminopeptidase (IRAP) are regulated by insulin. We have used a chimera between the intracellular domain of IRAP and the extracellular and transmembrane domains of the transferrin receptor (vpTR) to characterize IRAP-like trafficking in 3T3-L1 adipocytes. Our data demonstrate that the cytoplasmic domain of IRAP is sufficient to target vpTR to the insulin-regulated, slow recycling pathway in adipocytes and that the dynamic retention of vpTR is dependent on a di-leucine motif. Our kinetic analysis demonstrates that vpTR recycles as a single kinetic pool and that vpTR is very efficiently sorted from endosomes to the insulin-regulated recycling pathway. An implication of these findings is that the key step in the dynamic retention of vpTR occurs within the early endosomal system. We have previously shown that vpTR is trafficked by an insulin-regulated pathway in Chinese hamster ovary cells (Johnson, A. O., Subtil, A., Petrush, R., Kobylarz, K., Keller, S., and Mc Graw, T. E. (1998) J. Biol. Chem. 273, 17968-17977). The behavior of vpTR in Chinese hamster ovary cells is similar to its behavior in 3T3-L1 adipocytes. The main difference is that insulin has a larger effect on the trafficking of vpTR in the adipocytes. We concluded that the insulin-regulated slow recycling endocytic mechanism is expressed in many different cell types and therefore is not a unique characteristic of cells that express GLUT4.

Human rhabdomyosarcoma cells retain insulin-regulated glucose transport activity through glucose transporter 1

We evaluated the expression of glucose transporter (glut) isoforms and its function in RD cells, human rhabdomyosarcoma, which retain the potential to differentiate into muscle. Gluts 1, 3, and 4 were expressed in RD cells, as detected by reverse-transcription polymerase chain reaction and immunocytochemistry. Supraphysiological concentration (1 microM) of insulin treatment increased 2-deoxy glucose transport by up to 1.68-fold together with concomitant tyrosine phosphorylation of the insulin receptor beta subunit and of insulin receptor substrate 1. Suppression of glut 1 mRNA by 38% by antisense oligonucleotide transfection led to a reduction of basal and insulin-stimulated 2-deoxy glucose transport by 38 and 55%, respectively. Suppression of gluts 3 and 4 by antisense oligonucleotide transfection did not affect both basal and insulin-stimulated 2-deoxy glucose transport. Thus, glut 1 accounts for the major part of basal and insulin-stimulated glucose transport in RD cells. Next, we transfected expression vectors carrying human gluts 1 and 4 cDNAs into RD cells to add further support for the role of glut 1 in glucose transport. Overexpression of glut 1 stimulated basal and insulin-stimulated 2-deoxy glucose transport by 1.66- and 1.43-fold, respectively. Glut 4 overexpression did not affect basal and insulin-stimulated 2-deoxy glucose transport. Western blot analysis using glut 1 antibody showed that glut 1 was redistributed from intracellular membrane to plasma membrane. These observations support the notion that RD cells, with the potential to differentiate into muscle, retain insulin responsiveness. As human muscle cell lines are not available at this point, RD cells can serve as a useful alternative to human muscle for studies related to insulin signal transduction and glucose transport.

Identification of the APS protein as a novel insulin receptor substrate

In order to identify novel substrates involved in insulin receptor signaling, a yeast two-hybrid 3T3-L1 adipocyte cDNA library was screened with the cytoplasmic domain of the human insulin receptor as bait. Here we describe the isolation and characterization of an interacting protein, APS, which contains pleckstrin homology and Src homology 2 domains and several potential tyrosine phosphorylation sites. APS mRNA and protein are expressed primarily in skeletal muscle, heart, and adipose tissue, and in differentiated 3T3-L1 adipocytes. We show that APS associates with phosphotyrosines situated within the activation loop of the insulin receptor via the APS Src homology 2 domain. Insulin stimulation of 3T3-L1 adipocytes resulted in rapid tyrosine phosphorylation of endogenous APS on tyrosine 618, whereas platelet-derived growth factor treatment resulted in no APS phosphorylation. In summary, we have identified a new insulin receptor substrate that is primarily expressed in insulin-responsive tissues and in 3T3-L1 adipocytes whose phosphorylation shows insulin receptor specificity. These findings suggest a potential role for APS in insulin-regulated metabolic signaling pathways.

Glucose-dependent insulinotropic polypeptide stimulation of lipolysis in differentiated 3T3-L1 cells: wortmannin-sensitive inhibition by insulin

GIP is an important insulinotropic hormone (incretin) that has also been implicated in fat metabolism. There is controversy regarding the actions of GIP on adipocytes. In the current study, the existence of GIP receptors and effects of GIP on lipolysis were studied in differentiated 3T3-L1 cells. GIP receptor messenger RNA was detected by RT-PCR and RNase protection assay. Receptors were detected in binding studies (IC50 26.7 +/- 0.7 nM). GIP stimulated glycerol release with an EC50 of 3.28 +/- 0.63 nM. GIP (10(-9)-10(-7) M) +/- IBMX increased cAMP production by 1180-2246%. The adenylyl cyclase inhibitor MDL 12330A (10(-4) M) inhibited GIP-induced glycerol production by >90%, and reduced cAMP responses to basal. Preincubation of 3T3-L1 cells with insulin inhibited glycerol responses to GIP, and the inhibitory effect of insulin was blocked by the phosphatidylinositol 3'-kinase inhibitor, wortmannin. It is concluded that GIP stimulates glycerol release in 3T3-L1 cells primarily via stimulation of cAMP production, and that insulin antagonizes GIP-induced lipolysis in a wortmannin-sensitive fashion. It is suggested that effects of GIP on fat metabolism in vivo may depend upon the circulating insulin level, and that meal-released GIP may elevate circulating fatty acids, thus optimizing pancreatic beta-cell responsiveness to stimulation by glucose and GIP.

Autosomal genomic scan for loci linked to obesity and energy metabolism in Pima Indians

An autosomal genomic scan to search for linkage to obesity and energy metabolism was completed in Pima Indians, a population prone to obesity. Obesity was assessed by percent body fat (by hydrodensitometry) and fat distribution (the ratio of waist circumference to thigh circumference). Energy metabolism was measured in a respiratory chamber as 24-h metabolic rate, sleeping metabolic rate, and 24-h respiratory quotient (24RQ), an indicator of the ratio of carbohydrate oxidation to fat oxidation. Five hundred sixteen microsatellite markers with a median spacing of 6.4 cM were analyzed, in 362 siblings who had measurements of body composition and in 220 siblings who had measurements of energy metabolism. These comprised 451 sib pairs in 127 nuclear families, for linkage analysis to obesity, and 236 sib pairs in 82 nuclear families, for linkage analysis to energy metabolism. Pointwise and multipoint methods for regression of sib-pair differences in identity by descent, as well as a sibling-based variance-components method, were used to detect linkage. LOD scores >=2 were found at 11q21-q22, for percent body fat (LOD=2.1; P=.001), at 11q23-q24, for 24-h energy expenditure (LOD=2.0; P=.001), and at 1p31-p21 (LOD=2.0) and 20q11.2 (LOD=3.0; P=.0001), for 24RQ, by pointwise and multipoint analyses. With the variance-components method, the highest LOD score (LOD=2.3 P=.0006) was found at 18q21, for percent body fat, and at 1p31-p21 (LOD=2.8; P=.0003), for 24RQ. Possible candidate genes include LEPR (leptin receptor), at 1p31, and ASIP (agouti-signaling protein), at 20q11.2.

Action of insulin on the surface morphology of hepatocytes: role of phosphatidylinositol 3-kinase in insulin-induced shape change of microvilli

In previous studies we have shown that the insulin-responding glucose transporter isoform of 3T3-L1 adipocytes, GluT4, is almost completely located on microvilli. Furthermore, insulin caused the integration of these microvilli into the plasma membrane, suggesting that insulin-induced stimulation of glucose uptake may be due to the destruction of the cytoskeletal diffusion barrier formed by the actin filament bundle of the microvillar shaft regions [Lange et al. (1990) FEBS Lett. 261, 459-463; Lange et al. (1990) FEBS Lett. 276, 39-41]. Similar shape changes in microvilli were observed when the transport rates of adipocytes were modulated by glucose feeding or starvation. Here we demonstrate that the action of insulin on the surface morphology of hepatocytes is identical to that on 3T3L1 adipocytes; small and narrow microvilli on the surface of unstimulated hepatocytes were rapidly shortened and dilated on top of large domed surface areas. The aspect and mechanism of this effect are closely related to "membrane ruffling" induced by insulin and other growth factors. Pretreatment of hepatocytes with the PI 3-kinase inhibitor wortmannin (100 nM), which completely prevents transport stimulation by insulin in adipocytes and other cell types, also inhibited insulin-induced shape changes in microvilli on the hepatocyte surface. In contrast, vasopressin-induced microvillar shape changes in hepatocytes [Lange et al. (1997) Exp. Cell Res. 234, 486-497] were insensitive to wortmannin pretreatment. These findings indicate that PI 3-kinase products are necessary for stimulation of submembrane microfilament dynamics and that cytoskeletal reorganization is critically involved in insulin stimulation of transport processes. The mechanism of the insulin-induced cytoskeletal reorganization can be explained on the basis of the recent finding of Lu et al. [Biochemistry 35(1996) 14027-14034] that PI 3-kinase products exhibit much higher affinity for the profilin-actin complex than the primary products, PIP and PIP2. Thus, activated PI 3-kinase may direct a flux of profilin-actin complexes to the membrane locations of activated insulin receptors, where, due to the release of actin monomers after binding of profilactin to PI(3,4)P2 and PI(3,4,5)P3, massive actin polymerization is initiated. As a consequence, PI 3-kinase activation initiates a vectorial reorganization of the cellular actin system to membrane sites neighboring activated insulin receptors, giving rise to local membrane stress as visualized by extensive surface deformations and shortening of microvilli. In addition, extensive high-affinity binding of F-actin-barbed endcapping proteins enhances the cytoplasmic concentration of rapidly polymerizing filament ends. Consequently, the actin monomer concentration is lowered and the (cytoplasmic) pointed ends of the microvillar shaft bundle depolymerize and become shorter. The observations presented strengthen the previously postulated diffusion-barrier concept of glucose- and ion-uptake regulation and provide a mechanistic basis for explaining the action of insulin and other growth factors on transport processes across the plasma membrane.

Autophagic proteolysis: control and specificity

The rate of proteolysis is an important determinant of the intracellular protein content. Part of the degradation of intracellular proteins occurs in the lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very active and almost completely accounts for starvation-induced proteolysis. Factors inhibiting this process include amino acids, cell swelling and insulin. In the mechanisms controlling macroautophagy, protein phosphorylation plays an important role. Activation of a signal transduction pathway, ultimately leading to phosphorylation of ribosomal protein S6, accompanies inhibition of macroautophagy. Components of this pathway may include a heterotrimeric Gi3-protein, phosphatidylinositol 3-kinase and p70S6 kinase. Recent evidence indicates that lysosomal protein degradation can be selective and occurs via ubiquitin-dependent and -independent pathways.

Wortmannin inhibits both insulin- and contraction-stimulated glucose uptake and transport in rat skeletal muscle

The role of phosphatidylinositol (PI) 3-kinase for insulin- and contraction-stimulated muscle glucose transport was investigated in rat skeletal muscle perfused with a cell-free perfusate. The insulin receptor substrate-1-associated PI 3-kinase activity was increased sixfold upon insulin stimulation but was unaffected by contractions. In addition, the insulin-stimulated PI 3-kinase activity and muscle glucose uptake and transport in individual muscles were dose-dependently inhibited by wortmannin with one-half maximal inhibition values of approximately 10 nM and total inhibition at 1 microM. This concentration of wortmannin also decreased the contraction-stimulated glucose transport and uptake by approximately 30-70% without confounding effects on contractility or on muscle ATP and phosphocreatine concentrations. At higher concentrations (3 and 10 microM), wortmannin completely blocked the contraction-stimulated glucose uptake but also decreased the contractility. In conclusion, inhibition of PI 3-kinase with wortmannin in skeletal muscle coincides with inhibition of insulin-stimulated glucose uptake and transport. Furthermore, in contrast to recent findings in incubated muscle, wortmannin also inhibited contraction-stimulated glucose uptake and transport. The inhibitory effect of wortmannin on contraction-stimulated glucose uptake may be independent of PI 3-kinase activity or due to inhibition of a subfraction of PI 3-kinase with low sensitivity to wortmannin.