Dexamethasone-induced insulin resistance in 3T3-L1 adipocytes is due to inhibition of glucose transport rather than insulin signal transduction

Glucocorticoids reportedly induce insulin resistance. In this study, we investigated the mechanism of glucocorticoid-induced insulin resistance using 3T3-L1 adipocytes in which treatment with dexamethasone has been shown to impair the insulin-induced increase in glucose uptake. In 3T3-L1 adipocytes treated with dexamethasone, the GLUT1 protein expression level was decreased by 30%, which possibly caused decreased basal glucose uptake. On the other hand, dexamethasone treatment did not alter the amount of GLUT4 protein in total cell lysates but decreased the insulin-stimulated GLUT4 translocation to the plasma membrane, which possibly caused decreased insulin-stimulated glucose uptake. Dexamethasone did not alter tyrosine phosphorylation of insulin receptors, and it significantly decreased protein expression and tyrosine phosphorylation of insulin receptor substrate (IRS)-1. Interestingly, however, protein expression and tyrosine phosphorylation of IRS-2 were increased. To investigate whether the reduced IRS-1 content is involved in insulin resistance, IRS-1 was overexpressed in dexamethasone-treated 3T3-L1 adipocytes using an adenovirus transfection system. Despite protein expression and phosphorylation levels of IRS-1 being normalized, insulin-induced 2-deoxy-D-[3H]glucose uptake impaired by dexamethasone showed no significant improvement. Subsequently, we examined the effect of dexamethasone on the glucose uptake increase induced by overexpression of GLUT2-tagged p110alpha, constitutively active Akt (myristoylated Akt), oxidative stress (30 mU glucose oxidase for 2 h), 2 mmol/l 5-aminoimidazole-4-carboxamide ribonucleoside for 30 min, and osmotic shock (600 mmol/l sorbitol for 30 min). Dexamethasone treatment clearly inhibited the increases in glucose uptake produced by these agents. Thus, in conclusion, the GLUT1 decrease may be involved in the dexamethasone-induced decrease in basal glucose transport activity, and the mechanism of dexamethasone-induced insulin resistance in glucose transport activity (rather than the inhibition of phosphatidylinositol 3-kinase activation resulting from a decreased IRS-1 content) is likely to underlie impaired glucose transporter regulation.

Overexpression of SH2-containing inositol phosphatase 2 results in negative regulation of insulin-induced metabolic actions in 3T3-L1 adipocytes via its 5'-phosphatase catalytic activity

Phosphatidylinositol (PI) 3-kinase plays an important role in various metabolic actions of insulin including glucose uptake and glycogen synthesis. Although PI 3-kinase primarily functions as a lipid kinase which preferentially phosphorylates the D-3 position of phospholipids, the effect of hydrolysis of the key PI 3-kinase product PI 3,4,5-triphosphate [PI(3,4,5)P3] on these biological responses is unknown. We recently cloned rat SH2-containing inositol phosphatase 2 (SHIP2) cDNA which possesses the 5'-phosphatase activity to hydrolyze PI(3,4,5)P3 to PI 3,4-bisphosphate [PI(3,4)P2] and which is mainly expressed in the target tissues of insulin. To study the role of SHIP2 in insulin signaling, wild-type SHIP2 (WT-SHIP2) and 5'-phosphatase-defective SHIP2 (Delta IP-SHIP2) were overexpressed in 3T3-L1 adipocytes by means of adenovirus-mediated gene transfer. Early events of insulin signaling including insulin-induced tyrosine phosphorylation of the insulin receptor beta subunit and IRS-1, IRS-1 association with the p85 subunit, and PI 3-kinase activity were not affected by expression of either WT-SHIP2 or Delta IP-SHIP2. Because WT-SHIP2 possesses the 5'-phosphatase catalytic region, its overexpression marked by decreased insulin-induced PI(3,4,5)P3 production, as expected. In contrast, the amount of PI(3,4,5)P3 was increased by the expression of Delta IP-SHIP2, indicating that Delta IP-SHIP2 functions in a dominant-negative manner in 3T3-L1 adipocytes. Both PI(3,4,5)P3 and PI(3,4)P2 were known to possibly activate downstream targets Akt and protein kinase C lambda in vitro. Importantly, expression of WT-SHIP2 inhibited insulin-induced activation of Akt and protein kinase C lambda, whereas these activations were increased by expression of Delta IP-SHIP2 in vivo. Consistent with the regulation of downstream molecules of PI 3-kinase, insulin-induced 2-deoxyglucose uptake and Glut4 translocation were decreased by expression of WT-SHIP2 and increased by expression of Delta IP-SHIP2. In addition, insulin-induced phosphorylation of GSK-3beta and activation of PP1 followed by activation of glycogen synthase and glycogen synthesis were decreased by expression of WT-SHIP2 and increased by the expression of Delta IP-SHIP2. These results indicate that SHIP2 negatively regulates metabolic signaling of insulin via the 5'-phosphatase activity and that PI(3,4,5)P3 rather than PI(3,4)P2 is important for in vivo regulation of insulin-induced activation of downstream molecules of PI 3-kinase leading to glucose uptake and glycogen synthesis.

14-3-3 protein binds to insulin receptor substrate-1, one of the binding sites of which is in the phosphotyrosine binding domain

Insulin binding to its receptor induces the phosphorylation of cytosolic substrates, insulin receptor substrate (IRS)-1 and IRS-2, which associate with several Src homology-2 domain-containing proteins. To identify unique IRS-1-binding proteins, we screened a human heart cDNA library with 32P-labeled recombinant IRS-1 and obtained two isoforms (epsilon and zeta) of the 14-3-3 protein family. 14-3-3 protein has been shown to associate with IRS-1 in L6 myotubes, HepG2 hepatoma cells, Chinese hamster ovary cells, and bovine brain tissue. IRS-2, a protein structurally similar to IRS-1, was also shown to form a complex with 14-3-3 protein using a baculovirus expression system. The amount of 14-3-3 protein associated with IRS-1 was not affected by insulin stimulation but was increased significantly by treatment with okadaic acid, a potent serine/threonine phosphatase inhibitor. Peptide inhibition experiments using phosphoserine-containing peptides of IRS-1 revealed that IRS-1 contains three putative binding sites for 14-3-3 protein (Ser-270, Ser-374, and Ser-641). Among these three, the motif around Ser-270 is located in the phosphotyrosine binding domain of IRS-1, which is responsible for the interaction with the insulin receptor. Indeed, a truncated mutant of IRS-1 consisting of only the phosphotyrosine binding domain retained the capacity to bind to 14-3-3 protein in vivo. Finally, the effect of 14-3-3 protein binding on the insulin-induced phosphorylation of IRS-1 was investigated. Phosphoamino acid analysis revealed that IRS-1 coimmunoprecipitated with anti-14-3-3 antibody to be weakly phosphorylated after insulin stimulation, on tyrosine as well as serine residues, compared with IRS-1 immunoprecipitated with anti-IRS-1 antibody. Thus, the association with 14-3-3 protein may play a role in the regulation of insulin sensitivity by interrupting the association between the insulin receptor and IRS-1.

p85alpha gene generates three isoforms of regulatory subunit for phosphatidylinositol 3-kinase (PI 3-Kinase), p50alpha, p55alpha, and p85alpha, with different PI 3-kinase activity elevating responses to insulin

Phosphatidylinositol 3-kinase (PI 3-kinase) is stimulated by association with a variety of tyrosine kinase receptors and intracellular tyrosine-phosphorylated substrates. We isolated a cDNA that encodes a 50-kDa regulatory subunit of PI 3-kinase with an expression cloning method using 32P-labeled insulin receptor substrate-1 (IRS-1). This 50-kDa protein contains two SH2 domains and an inter-SH2 domain of p85alpha, but the SH3 and bcr homology domains of p85alpha were replaced by a unique 6-amino acid sequence. Thus, this protein appears to be generated by alternative splicing of the p85alpha gene product. We suggest that this protein be called p50alpha. Northern blotting using a specific DNA probe corresponding to p50alpha revealed 6.0- and 2.8-kb bands in hepatic, brain, and renal tissues. The expression of p50alpha protein and its associated PI 3-kinase were detected in lysates prepared from the liver, brain, and muscle using a specific antibody against p50alpha. Taken together, these observations indicate that the p85alpha gene actually generates three protein products of 85, 55, and 50 kDa. The distributions of the three proteins (p85alpha, p55alpha, and p50alpha), in various rat tissues and also in various brain compartments, were found to be different. Interestingly, p50alpha forms a heterodimer with p110 that can as well as cannot be labeled with wortmannin, whereas p85alpha and p55alpha associate only with p110 that can be wortmannin-labeled. Furthermore, p50alpha exhibits a markedly higher capacity for activation of associated PI 3-kinase via insulin stimulation and has a higher affinity for tyrosine-phosphorylated IRS-1 than the other isoforms. Considering the high level of p50alpha expression in the liver and its marked responsiveness to insulin, p50alpha appears to play an important role in the activation of hepatic PI 3-kinase. Each of the three alpha isoforms has a different function and may have specific roles in various tissues.

Altered expression levels and impaired steps in the pathway to phosphatidylinositol 3-kinase activation via insulin receptor substrates 1 and 2 in Zucker fatty rats

To elucidate the mechanism of obesity-related insulin resistance, we investigated the impaired steps in the processes of phosphatidylinositol (PI) 3-kinase activation through binding with insulin receptor substrates 1 and 2 (IRS-1 and IRS-2) in liver and muscle of Zucker fatty rats. The expressions of IRS-1 and IRS-2 were shown to be downregulated in both liver and muscle in fatty rats (hepatic IRS-1, 83%; hepatic IRS-2, 45%; muscle IRS-1, 60%; muscle IRS-2, 78%), resulting in decreased tyrosine phosphorylation in response to insulin stimulation. Despite the decrease in the tyrosine phosphorylation levels of hepatic IRS-1 and IRS-2 being mild to moderate, associated PI 3-kinase activities were dramatically decreased in fatty rats (IRS-1, 14%; IRS-2, 10%), which may suggest alteration in the sites of phosphorylated tyrosine residues of hepatic IRS-1 and IRS-2. In addition, we demonstrated that the expressions of p85alpha and p55alpha regulatory subunits of PI 3-kinase were reduced (p85alpha, 67%; p55alpha, 54%), and that the p50alpha regulatory subunit was markedly upregulated (176%) in the livers of fatty rats without apparent alterations in expressions of the catalytic subunits p110alpha and p110beta. These alterations may reflect the obesity-related insulin resistance commonly observed in human NIDDM.

MKK6/3 and p38 MAPK pathway activation is not necessary for insulin-induced glucose uptake but regulates glucose transporter expression

p38 mitogen-activated protein kinase (MAPK), which is situated downstream of MAPK kinase (MKK) 6 and MKK3, is activated by mitogenic or stress-inducing stimuli, as well as by insulin. To clarify the role of the MKK6/3-p38 MAPK pathway in the regulation of glucose transport, dominant negative p38 MAPK and MKK6 mutants and constitutively active MKK6 and MKK3 mutants were overexpressed in 3T3-L1 adipocytes and L6 myotubes using an adenovirus-mediated transfection procedure. Constitutively active MKK6/3 mutants up-regulated GLUT1 expression and down-regulated GLUT4 expression, thereby significantly increasing basal glucose transport but diminishing transport induced by insulin. Similar effects were elicited by chronic (24 h) exposure to tumor necrosis factor alpha, interleukin-1beta, or 200 mm sorbitol, all activate the MKK6/3-p38 MAPK pathway. SB203580, a specific p38 MAPK inhibitor, attenuated these effects, further confirming that both MMK6 and MMK3 act via p38 MAPK, whereas they had no effect on the increase in glucose transport induced by a constitutively active MAPK kinase 1 (MEK1) mutant or by myristoylated Akt. In addition, suppression of p38 MAPK activation by overexpression of a dominant negative p38 MAPK or MKK6 mutant did not diminish insulin-induced glucose uptake by 3T3-L1 adipocytes. It is thus apparent that activation of p38 MAPK is not essential for insulin-induced increases in glucose uptake. Rather, p38 MAPK activation leads to a marked down-regulation of insulin-induced glucose uptake via GLUT4, which may underlie cellular stress-induced insulin resistance caused by tumor necrosis factor alpha and other factors.

A novel 55-kDa regulatory subunit for phosphatidylinositol 3-kinase structurally similar to p55PIK Is generated by alternative splicing of the p85alpha gene

Phosphatidylinositol 3-kinase, which is composed of a 110-kDa catalytic subunit and a regulatory subunit, plays important roles in various cellular signaling mechanisms. We screened a rat brain cDNA expression library with 32P-labeled human IRS-1 protein and cloned cDNAs that were very likely to be generated by alternative splicing of p85alpha gene products. These cDNAs were demonstrated to encode a 55-kDa protein (p55alpha) containing two SH2 domains and an inter-SH2 domain of p85alpha but neither a bcr domain nor a SH3 homology domain. Interestingly, p55 alpha contains a unique 34-amino acid sequence at its NH2 terminus, which is not included in the p85alpha amino acid sequence. This 34-amino acid portion was revealed to be comparable with p55PIK (p55gamma) in length, with a high homology between the two, suggesting that these NH2-terminal domains of p55alpha and p5 gamma may have a specific role that p85 does not. The expression of p55alpha mRNA is most abundant in the brain, but expression is ubiquitous in most rat tissues. Furthermore, it should be noted that the expression of p85alpha mRNA in muscle is almost undetectably low by Northern blotting with a cDNA probe coding for the p85alpha SH3 domain, while the expression of p55alpha can be readily detected. These results suggest that p55 alpha may play an unique regulatory role for phosphatidylinositol 3-kinase in brain and muscle.

Cell permeant polyphosphoinositide-binding peptides that block cell motility and actin assembly

Polyphosphoinositides (PPIs) affect the localization and activities of many cellular constituents, including actin-modulating proteins. Several classes of polypeptide sequences, including pleckstrin homology domains, FYVE domains, and short linear sequences containing predominantly hydrophobic and cationic residues account for phosphoinositide binding by most such proteins. We report that a ten-residue peptide derived from the phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding region in segment 2 of gelsolin, when coupled to rhodamine B has potent PIP(2) binding activity in vitro; crosses the cell membrane of fibroblasts, platelets, melanoma cells, and neutrophils by a process not involving endocytosis; and blocks cell motility. This peptide derivative transiently disassembles actin filament structures in GFP-actin-expressing NIH3T3 fibroblasts and prevents thrombin- or chemotactic peptide-stimulated actin assembly in platelets and neutrophils, respectively, but does not block the initial [Ca(2+)] increase caused by these agonists. The blockage of actin assembly and motility is transient, and cells recover motility within an hour after their immobilization by 5-20 microm peptide. This class of reagents confirms the critical relation between inositol lipids and cytoskeletal structure and may be useful to probe the location and function of polyphosphoinositides in vivo.

Identification and characterization of a novel inositol polyphosphate 5-phosphatase

We have identified a cDNA encoding a novel inositol polyphosphate 5-phosphatase. It contains two highly conserved catalytic motifs for 5-phosphatase, has a molecular mass of 51 kDa, and is ubiquitously expressed and especially abundant in skeletal muscle, heart, and kidney. We designated this 5-phosphatase as SKIP (Skeletal muscle and Kidney enriched Inositol Phosphatase). SKIP is a simple 5-phosphatase with no other motifs. Baculovirus-expressed recombinant SKIP protein exhibited 5-phosphatase activities toward inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, phosphatidylinositol (PtdIns) 4,5-bisphosphate, and PtdIns 3,4, 5-trisphosphate but has 6-fold more substrate specificity for PtdIns 4,5-bisphosphate (K(m) = 180 microM) than for inositol 1,4, 5-trisphosphate (K(m) = 1.15 mM). The ectopic expression of SKIP protein in COS-7 cells and immunostaining of neuroblastoma N1E-115 cells revealed that SKIP is expressed in cytosol and that loss of actin stress fibers occurs where the SKIP protein is concentrated. These results imply that SKIP plays a negative role in regulating the actin cytoskeleton through hydrolyzing PtdIns 4,5-bisphosphate.

Enhanced insulin-stimulated activation of phosphatidylinositol 3-kinase in the liver of high-fat-fed rats

Insulin receptor substrate (IRS)-1 and IRS-2, which mediate phosphatidylinositol (PI) 3-kinase activation, play essential roles in insulin-induced translocation of GLUT4 and in glycogen synthesis. In this study, we investigated the process of PI 3-kinase activation via binding with IRS-1 and -2 in liver, muscle, and fat of high-fat-fed rats, a model of insulin-resistant diabetes. In the liver of high-fat-fed rats, insulin increased the PI 3-kinase regulatory subunit p85alpha and the PI 3-kinase activities associated with IRS-1 3.6- and 2.4-fold, and with IRS-2, 4.7- and 3.0-fold, respectively, compared with those in control rats. The tyrosine phosphorylation levels of IRS-1 and IRS-2 were not significantly altered, however. In contrast with the liver, tyrosine phosphorylation levels and associated PI 3-kinase proteins and activities were decreased in the muscle and adipose tissue of high-fat-fed rats. Thus, high-fat feeding appears to cause insulin resistance in the liver by a mechanism different from the impaired PI 3-kinase activation observed in muscle and adipose tissue. Taking into consideration that hepatic PI 3-kinase activation is severely impaired in obese diabetic models such as Zucker fatty rats, it is possible that the mechanism by which a high-fat diet causes insulin resistance is quite different from that associated with obesity and overeating due to abnormality in the leptin system. This is the first report to show increased PI 3-kinase activation by insulin in an insulin-resistant diabetic animal model. These findings may be important for understanding the mechanism of insulin resistance in human NIDDM, since a high-fat diet is considered to be one of the major factors exacerbating insulin insensitivity in humans.

Imidapril, an angiotensin-converting enzyme inhibitor, improves insulin sensitivity by enhancing signal transduction via insulin receptor substrate proteins and improving vascular resistance in the Zucker fatty rat

Angiotensin-converting enzyme (ACE) inhibitors are antihypertensive agents, that inhibit the conversion of angiotensin I to angiotensin II, resulting in smooth-muscle relaxation and a reduction of vascular resistance. Recently, it has been suggested that ACE inhibitors improve insulin resistance in diabetic patients. To investigate the effect of an ACE inhibitor on insulin sensitivity, insulin signaling, and circulation, imidapril was administered orally or intraduodenally to Zucker fatty rats. Oral administration of imidapril improved insulin sensitivity based on the results of an oral glucose tolerance test (OGTT) and a decrease in urinary glucose secretion. Phosphatidylinositol 3-kinase (PI 3-kinase) activity associated with hepatic insulin receptor substrate-1 (IRS-1) in the insulin-stimulated condition was significantly enhanced 110% without a significant alteration in tyrosine phosphorylation of IRS-1 in the imidapril-treated group. In muscle, IRS-1 tyrosine phosphorylation and PI 3-kinase activity associated with IRS-1 in the insulin-stimulated condition were enhanced 70% and 20%, respectively, in the imidapril-treated group. In contrast, an alteration of the IRS-2 pathway was observed only in liver; a significant insulin-induced increase in the IRS-2-associated PI 3-kinase over the basal level was observed in the imidapril-treated group but not in the control. In addition, treatment with imidapril was shown to significantly reduce blood pressure and increase blood flow in the liver and muscle. These results suggest that the ACE inhibitor imidapril may improve insulin sensitivity not only by acting directly on the insulin signaling pathway but also by increasing blood flow in tissues via normalization of vascular resistance, a major cause of hypertension.

Insulin receptor substrate (IRS)-2 is dephosphorylated more rapidly than IRS-1 via its association with phosphatidylinositol 3-kinase in skeletal muscle cells

Insulin receptor substrate (IRS)-2 is structurally and functionally similar to IRS-1. Indeed, stimulation with insulin or insulin-like growth factor I led to the rapid tyrosine phosphorylation of both IRS-1 and IRS-2, which in turn activated phosphatidylinositol (PI) 3-kinase in L6 cells and rat skeletal muscle. However, IRS-2 was rapidly dephosphorylated (3-10 min after the addition of insulin/insulin-like growth factor I), whereas IRS-1 phosphorylation continued for at least 60 min. The time courses of the PI 3-kinase activity associated with IRS-1 and IRS-2 paralleled the tyrosine phosphorylation of these proteins. Preincubation with sodium orthovanadate, an inhibitor of protein tyrosine phosphatase, blocked the rapid dephosphorylation of IRS-2, suggesting the involvement of tyrosine phosphatase. The activation of PI 3-kinase apparently plays an important role in the rapid dephosphorylation of IRS-2, as IRS-2 dephosphorylation was inhibited markedly by suppressing PI 3-kinase activity with wortmannin or overexpression of the dominant negative p85 subunit of PI 3-kinase, which cannot bind the p110 catalytic subunit. In addition, platelet-derived growth factor stimulation prior to insulin stimulation decreased IRS-associated PI 3-kinase and significantly inhibited the dephosphorylation of IRS-2. Taken together, these observations suggest that IRS-2 plays a unique role in mediating the signals from the insulin receptor to downstream molecules and that this effect is more transient than that of IRS-1. Tyrosine phosphatase and IRS-associated PI 3-kinase activity thus contribute to the rapid dephosphorylation of IRS-2.

Different subcellular distribution and regulation of expression of insulin receptor substrate (IRS)-3 from those of IRS-1 and IRS-2

Adipocytes contain three major substrate proteins of the insulin receptor, termed IRS-1, IRS-2, and IRS-3. We demonstrated that IRS-1 and IRS-2 are located mainly in the low density microsome (LDM) fraction and are tyrosine phosphorylated in response to insulin stimulation, leading to phosphatidylinositol (PI) 3-kinase activation. In contrast, IRS-3 is located mainly in the plasma membrane (PM) fraction and contributes to PI 3-kinase activation in the PM fraction. The different cellular localizations of IRS proteins may account for the mechanism of insulin resistance induced by a high fat diet, considering that PI 3-kinase activation in the LDM fraction is reportedly essential for the translocation of GLUT4 in adipocytes. High fat feeding in rats increased both protein and mRNA levels of IRS-3 but decreased those of IRS-1 and IRS-2 in epididymal adipocytes. As a result, selective impairment of insulin-induced PI 3-kinase activation was observed in the LDM fraction, whereas PI 3-kinase activation was conserved in the PM fraction. This is the first report showing that different IRS proteins function in different subcellular compartments, which may contribute to determining the insulin sensitivity in adipocytes.

Inhibitory effect of hyperglycemia on insulin-induced Akt/protein kinase B activation in skeletal muscle

To determine the molecular mechanism underlying hyperglycemia-induced insulin resistance in skeletal muscles, postreceptor insulin-signaling events were assessed in skeletal muscles of neonatally streptozotocin-treated diabetic rats. In isolated soleus muscle of the diabetic rats, insulin-stimulated 2-deoxyglucose uptake, glucose oxidation, and lactate release were all significantly decreased compared with normal rats. Similarly, insulin-induced phosphorylation and activation of Akt/protein kinase B (PKB) and GLUT-4 translocation were severely impaired. However, the upstream signal, including phosphorylation of the insulin receptor (IR) and insulin receptor substrate (IRS)-1 and -2 and activity of phosphatidylinositol (PI) 3-kinase associated with IRS-1/2, was enhanced. The amelioration of hyperglycemia by T-1095, a Na(+)-glucose transporter inhibitor, normalized the reduced insulin sensitivity in the soleus muscle and the impaired insulin-stimulated Akt/PKB phosphorylation and activity. In addition, the enhanced PI 3-kinase activation and phosphorylation of IR and IRS-1 and -2 were reduced to normal levels. These results suggest that sustained hyperglycemia impairs the insulin-signaling steps between PI 3-kinase and Akt/PKB, and that impaired Akt/PKB activity underlies hyperglycemia-induced insulin resistance in skeletal muscle.

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.

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.

Oligomer formation of histamine H2 receptors expressed in Sf9 and COS7 cells

A histamine H2 receptor, which had been mutated at its glycosylation site and tagged at its N-terminus with an HA tag (HA-H2 receptor), was expressed in Sf9 cells and COS7 cells. Immunoprecipitation and immunoblotting of HA-H2 receptors with alphaHA antibody revealed four bands of 31.5 +/- 2.5 kDa, 59.0 +/- 6.0 kDa, 80.5 +/- 4.5 kDa and 120 kDa. These bands were also detected by immunoblot using anti-H2 receptor serum (C-terminus). In addition, H2 receptors without the HA-tag coimmunoprecipitated with HA-tagged H2 receptors devoid of the 51 C-terminal amino acids, via immunoprecipitation with alphaHA antibody, when the two receptors were coexpressed. These results suggest that H2 receptors are present as receptor oligomers and that the C-terminal portion is not involved in the formation of these oligomers.

No correlation of plasma cell 1 overexpression with insulin resistance in diabetic rats and 3T3-L1 adipocytes

Membrane glycoprotein plasma cell 1 (PC-1) has been shown to be increased in type 2 diabetes and involved in insulin resistance through inhibiting the insulin receptor tyrosine kinase, which was demonstrated using cultured breast cancer cells. However, other reports have shown contradictory results in Chinese hamster ovary cells and in vitro kinase assay. Thus, we considered it necessary to investigate the effect of PC-1 using highly insulin-sensitive cells. Here, we used two of the following approaches: 1) investigating PC-1 expression levels in insulin-responsive tissues in rat models of diabetes and 2) overexpressing PC-1 in 3T3-L1 adipocytes. We found that PC-1 was highly expressed in insulin-responsive tissues, such as liver and adipose tissue, in normal rats. However, high-fat feeding or streptozotocin-induced diabetes did not change its expression levels in liver, adipose tissue, and skeletal muscle. Thus, PC-1 expression levels were not associated with high-fat-diet-induced insulin resistance or hyperglycemia. Although PC-1 was increased in adipose tissue in Zucker fatty rats (protein level, by 50%; mRNA level, by 90%), its expression levels in liver and skeletal muscle, tissues that are more responsible for whole body glucose metabolism than adipose tissue, did not significantly differ from those in normal rats. Next, we overexpressed PC-1 in 3T3-L1 adipocytes using an adenovirus transfection system. PC-1 expression was markedly increased to a level 16-fold greater than that in normal human adipose tissue, which is higher than the previously reported levels in diabetic patients. However, insulin-induced tyrosine phosphorylation of the insulin receptor and insulin receptor substrate 1, activation of phosphatidylinositol 3-kinase, and glucose uptake were not affected by PC-1 overexpression. These results strongly suggest that increased PC-1 expression is not causally related to insulin resistance.

Role of the C terminus in histamine H2 receptor signaling, desensitization, and agonist-induced internalization

To evaluate the role of the histamine H2 receptor C terminus in signaling, desensitization, and agonist-induced internalization, canine H2 receptors with truncated C termini were generated. Wild-type (WT) and truncated receptors were tagged at their N termini with a hemagglutinin (HA) epitope and expressed in COS7 cells. Most of the C-terminal intracellular tail could be truncated (51 of 70 residues, termed T308 mutant) without loss of functions: cAMP production, tiotidine binding, and plasma membrane targeting. In fact, the T308 mutant produced more cAMP than the WT when cell-surface expression per cell was equivalent. Pretreatment of cells with 10(-5) M histamine desensitized cAMP productions via WT and T308 receptors to similar extents. Incubation of cells expressing WT receptors with 10(-5) M histamine reduced cell-surface anti-HA antibody binding by approximately 30% (by 30 min, t1/2 approximately 15 min), but did not affect the Bmax of tiotidine in membrane fractions, which represents total receptor amounts, suggesting that WT receptors were internalized from the cell surface. In contrast, no internalization was observed with T308 receptors following histamine treatment. A mutant with a deletion of the 30 C-terminal amino acids, termed T329, was functional but was as potent as the WT in terms of cAMP production. Apart from being desensitized by histamine, the internalization of the receptor was indistinguishable from that of the WT. Internalization was observed in the T320 but not in T313 mutant, narrowing the region involved in internalization to that between Glu314 and Asn320 (ETSLRSN). Of these seven residues, either Thr315, Ser316, or both, were replaced with Ala. Thr315 and Ser316 are conserved among species. The mutation at Thr315 (but not that at Ser316) abolished internalization. Taken together, these results demonstrate that Thr315 is involved in agonist-induced internalization. Furthermore, the finding that T308 receptors were desensitized in the absence of internalization suggests that internalization and desensitization are meditated by independent mechanisms.

Five isoforms of the phosphatidylinositol 3-kinase regulatory subunit exhibit different associations with receptor tyrosine kinases and their tyrosine phosphorylations

There are five isoforms of the regulatory subunit for the heterodimeric type of phosphatidylinositol 3-kinase. These five regulatory subunit isoforms were overexpressed using an adenovirus transfection system, and their own tyrosine phosphorylations and associations with various tyrosine kinase receptors were investigated. When overexpressed in CHO-PDGFR cells, the associations of these regulatory subunit isoforms with the platelet-derived growth factor receptor were similar. However, when overexpressed in CHO-IR cells, p55gamma exhibited a significantly lower ability to bind with IRS-1 upon insulin stimulation, as compared with other regulatory subunit isoforms. Furthermore, p55alpha and p55gamma were found to be tyrosine-phosphorylated. Finally, interestingly, when overexpressed in CHO-EGFR cells or A431 cells and stimulated with epidermal growth factor (EGF), phosphorylated EGF receptor was detected in p85alpha, p85beta and p50alpha immunoprecipitates, but not in p55alpha and p55gamma immunoprecipitates. In addition, EGF-induced tyrosine phosphorylation was observed in p85alpha, p85beta, p55alpha and p55gamma, but not in p50alpha, immunoprecipitates. Thus, each regulatory subunit exhibits specific responses regarding both the association with tyrosine-phosphorylated substrates and its own tyrosine phosphorylation. These results suggest that each isoform possesses specific roles in signal transduction, based on its individual tyrosine kinase receptor.

Replacement of both tryptophan residues at 388 and 412 completely abolished cytochalasin B photolabelling of the GLUT1 glucose transporter

A mutated GLUT1 glucose transporter, a Trp-388, 412 mutant whose tryptophans 388 and 412 were both replaced by leucines, was constructed by site-directed mutagenesis and expressed in Chinese hamster ovary cells. Glucose transport activity was decreased to approx. 30% in the Trp-388, 412 mutant compared with that in the wild type, a similar decrease in transport activity had been observed previously in the Trp-388 mutant and the Trp-412 mutant which had leucine at 388 and 412 respectively. Cytochalasin B labelling of the Trp-388 mutant was only decreased rather than abolished, a result similar to that obtained previously for the Trp-412 mutant. Cytochalasin B labelling was finally abolished completely in the Trp-388, 412 mutant, while cytochalasin B binding to this mutant was decreased to approx. 30% of that of the wild-type GLUT1 at the concentration used for photolabelling. This level of binding is thought to be adequate to detect labelling, assuming that the labelling efficiency of these transporters is similar. These findings suggest that cytochalasin B binds to the transmembrane domain of the glucose transporter in the vicinity of helix 10-11, and is inserted covalently by photoactivation at either the 388 or the 412 site.

Hyperglycemia impairs the insulin signaling step between PI 3-kinase and Akt/PKB activations in ZDF rat liver

Akt/PKB activation is reportedly essential for insulin-induced glucose metabolism in the liver. During the hypoinsulinemic and hyperglycemic phase in the Zucker diabetic fatty (ZDF) rat liver, insulin-induced phosphorylations of the insulin receptor (IR) and insulin receptor substrate (IRS)-1/2 were significantly enhanced. Similarly, phosphatidylinositol (PI) 3-kinase activities associated with IRS-1/2 were markedly increased in ZDF rat liver compared with those in the control lean rat liver. However, interestingly, insulin-induced phosphorylation and kinase activation of Akt/PKB were severely suppressed. The restoration of normoglycemia by sodium-dependent glucose transporter (SGLT) inhibitor to ZDF rats normalized elevated PI 3-kinase activation and phosphorylation of IR and IRS-1/2 to lean control rat levels. In addition, impaired insulin-induced Akt/PKB activation was also normalized. These results suggest that chronic hyperglycemia reduces the efficiency of the activation step from PI 3-kinase to Akt/PKB kinase and that this impairment is the molecular mechanism underlying hyperglycemia-induced insulin resistance in the liver.

Palmitoylation of the canine histamine H2 receptor occurs at Cys(305) and is important for cell surface targeting

To determine the presence and functional role of the histamine H2 receptor (H2R) palmitoylation, a receptor with a Cys(305) to Ala (A(305) receptor) mutation was generated. Wild-type (WT) and A(305) receptors were tagged at their N-termini with a hemagglutinin (HA) epitope. WT, but not A(305), receptors incorporated [3H]palmitate by metabolic labeling, indicating that the H2R is palmitoylated at Cys(305). Immunocytochemistry of WT and A(305) receptors expressed in COS7 cells revealed WT receptors to be distributed at the plasma membrane, while the majority of A(305) receptors were localized intracellularly with only a small portion being at the plasma membrane. However, the affinity of the A(305) receptor for tiotidine was comparable to that of the WT receptor. In addition, when the amounts of cell surface receptors as determined by anti-HA antibody binding were equivalent, A(305) receptors mediated production of more cAMP than WT receptors. Preincubation of COS7 cells expressing each receptor with 10(-5) M histamine for 30 min reduced subsequent cAMP production in response to histamine via the receptors to similar extents, indicating that palmitoylation is not necessary for desensitization. In addition, cell surface A(305) receptors were capable of being internalized from the cell surface at a rate and extent similar to those of WT receptors. Finally, CHO cell lines stably expressing either WT or A(305) receptors were incubated with 10(-5) M histamine for 1, 6, 12 and 24 h. Total amounts of WT and A(305) receptors, as determined by tiotidine binding, were reduced by incubation, indicating downregulation. Downregulation of the A(305) receptor was more extensive than that of the WT receptor. Thus, palmitoylation of the H2R might be important for targeting to the cell surface and stability.

Peretinoin, an acyclic retinoid, inhibits hepatocarcinogenesis by suppressing sphingosine kinase 1 expression in vitro and in vivo

Sphingosine-1-phospate is a potent bioactive lipid metabolite that regulates cancer progression. Because sphingosine kinase 1 and sphingosine kinase 2 (SPHK 1/2) are both essential for sphingosine-1-phospate production, they could be a therapeutic target in various cancers. Peretinoin, an acyclic retinoid, inhibits post-therapeutic recurrence of hepatocellular carcinoma via unclear mechanisms. In this study, we assessed effects of peretinoin on SPHK expression and liver cancer development in vitro and in vivo. We examined effects of peretinoin on expression, enzymatic and promoter activity of SPHK1 in a human hepatoma cell line, Huh-7. We also investigated effects of SPHK1 on hepatocarcinogenesis induced by diethylnitrosamine using SPHK1 knockout mice. Peretinoin treatment of Huh-7 cells reduced mRNA levels, protein expression and enzymatic activity of SPHK1. Peretinoin reduced SPHK1 promoter activity; this effect of peretinoin was blocked by overexpression of Sp1, a transcription factor. Deletion of all Sp1 binding sites within the SPHK1 promoter region abolished SPHK1 promoter activity, suggesting that peretinoin reduced mRNA levels of SPHK1 via Sp1. Additionally, diethylnitrosamine-induced hepatoma was fewer and less frequent in SPHK1 knockout compared to wild-type mice. Our data showed crucial roles of SPHK1 in hepatocarcinogenesis and suggests that peretinoin prevents hepatocarcinogenesis by suppressing mRNA levels of SPHK1.

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.