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

Short-term fasting differentially regulates PI3K/AkT/mTOR and ERK signalling in the rat hypothalamus

It is known that insulin secreted by pancreatic β-cells enters the brain by crossing the blood-brain barrier. However, it was demonstrated that insulin expression occurs in various brain regions as well. Albeit the list of insulin actions in the brain is long and it includes control of energy homeostasis, neuronal survival, maintenance of synaptic plasticity and cognition, not much is known about the adaptive significance of insulin synthesis in brain. We previously reported that short-term fasting promotes insulin expression and subsequent activation of insulin receptor in the rat periventricular nucleus. In order to uncover a physiological importance of the fasting-induced insulin expression in hypothalamus, we analyzed the effect of short-term food deprivation on the expression of several participants of PI3K/AKT/mTOR and Ras/MAPK signaling pathways that are typically activated by this hormone. We found that the hypothalamic content of total and activated IRS1, IRS2, PI3K, and mTOR remained unchanged, but phosphorylated AKT1/2/3 was decreased. The levels of activated ERK1/2 were increased after six-hour fasting. Moreover, activated ERK1/2 was co-expressed with activated insulin receptor in the nucleus arcuatus. Our previously published and current findings suggest that the ERK activation in hypothalamus was at least partially initiated by the centrally produced insulin.

Refining insulin signaling in the proximal tubule at the level of the substrate

Insulin has many varied actions in the proximal tubule. Two distinct activities include upregulation of sodium/bicarbonate reabsorption and downregulation of gluconeogenesis. The inability to perform these 2 tasks simultaneously under fed and fasted conditions can lead to hyper- or hypoglycemia, acidosis, and/or impaired extracellular fluid regulation. Nakamura and colleagues illuminate our understanding of this process, which appears to be managed in part by recruitment of different insulin receptor substrates under different physiological conditions.

Effects of Agriophyllum squarrosum extracts on glucose metabolism in KKAy mice and the associated underlying mechanisms

ETHNOPHARMACOLOGICAL RELEVANCE

Agriophyllum squarrosum (L.) Moq. is a traditional Mongol medicine commonly used in the treatment of diabetes.

AIM OF THE STUDY

To examine the effects of Agriophyllum squarrosum extract (ASE) on glucose metabolism in type 2 diabetic KKAy mice, and to investigate the mechanisms underlying these effects.

MATERIAL AND METHODS

KKAy mice were divided into a model control group (MCG), a low-dose Agriophyllum squarrosum extract group (LASEG), a medium-dose Agriophyllum squarrosum extract group (MASEG), a high-dose Agriophyllum squarrosum extract group (HASEG), and a metformin group (MEG). Syngeneic C57BL/6 mice were used as a normal control group (NCG). Drugs were administered to all mice by gavage for 8 weeks. Random blood glucose levels were measured in the mice at baseline and after 2, 4, and 8 weeks of treatment. Glucose tolerance was measured after 6 weeks of drug administration. After 8 weeks, glycated serum proteins (GSP) and advanced glycation end-products (AGEs) in the serum of all mice were measured. Sections of mouse liver tissues were used for periodic acid-Schiff staining (PAS) and the content of hepatic glycogen was determined. Immunohistochemistry was used to determine the effects of ASE on liver phospho-insulin receptor substrate 2 (P-IRS2) protein expression. Western blotting was used to quantify the protein expression levels of phosphatidylinositol 3-kinase (PI3K), AKT, phospho-AKT (S473) (P-AKT), glycogen synthase kinase 3β (GSK3β), and glucose transporters 4 (GLUT4), while PCR was used to quantify the mRNA expression levels of insulin receptor substrate 2 (IRS2), PI3K, AKT, GSK3β, and GLUT4.

RESULTS

ASE treatment decreased random blood glucose levels in type 2 diabetic KKAy mice; increased glucose tolerance; decreased serum GSP and AGEs content; increased glycogen synthesis in liver tissues; upregulated the protein expression levels of PI3K, AKT, GLUT4, and P-IRS2; downregulated the protein expression level of GSK3β in liver tissues; upregulated the mRNA expression levels of IRS2, PI3K, AKT, and GLUT4; and downregulated the mRNA expression level of GSK3β in liver tissues.

CONCLUSION

ASE treatment may increase glucose metabolism in KKAy mice and improve glucose tolerance. The underlying mechanisms of the beneficial effects of ASE may be associated with the increase of glycogen synthesis, the inhibition of AGEs production, the upregulation of IRS2, PI3K, AKT, and GLUT4 protein and mRNA expression, and the downregulation of GSK3β protein and mRNA expression.

A low-protein diet eliminates the circadian rhythm of serum insulin and hepatic lipid metabolism in mice

Insulin is a key molecule that synchronizes peripheral clocks, such as that in the liver. Although we previously reported that mice fed a low-protein diet showed altered expression of lipid-related genes in the liver and induction of hepatic steatosis, it is unknown whether a low-protein diet impairs insulin secretion and modifies the hepatic circadian rhythm. Therefore, we investigated the effects of the intake of a low-protein diet on the circadian rhythm of insulin secretion and hepatic lipid metabolism in mice. Under 12-h light/12-h dark cycle, mice fed a low-protein diet for 7 days displayed enhanced food intake at the end of the light phase, although central and peripheral PER2 expression rhythm was maintained. Serum insulin levels in mice fed a low-protein diet remained low during the day, and the insulin secretion in OGTT was also markedly lower than in normal mice. In mice fed low-protein diet, hepatic TG accumulation was observed during the nighttime, with relatively high levels of ACC1 mRNA and total ACC proteins. Although there were no differences in the activity rhythm of hepatic mTOR between mice fed a normal or low-protein diet, hepatic IRS-2 expression in mice fed a low-protein diet remained low during the day, with no increase at the beginning of the light period. These results suggested that the low-protein diet eliminated the circadian rhythm of serum insulin and hepatic lipid metabolism in mice, providing insights into our understanding of the mechanisms of hepatic disorders of lipid metabolism.

Distinct signalling properties of insulin receptor substrate (IRS)-1 and IRS-2 in mediating insulin/IGF-1 action

Insulin/IGF-1 action is driven by a complex and highly integrated signalling network. Loss-of-function studies indicate that the major insulin/IGF-1 receptor substrate (IRS) proteins, IRS-1 and IRS-2, mediate different biological functions in vitro and in vivo, suggesting specific signalling properties despite their high degree of homology. To identify mechanisms contributing to the differential signalling properties of IRS-1 and IRS-2 in the mediation of insulin/IGF-1 action, we performed comprehensive mass spectrometry (MS)-based phosphoproteomic profiling of brown preadipocytes from wild type, IRS-1^-/- and IRS-2^-/- mice in the basal and IGF-1-stimulated states. We applied stable isotope labeling by amino acids in cell culture (SILAC) for the accurate quantitation of changes in protein phosphorylation. We found ~10% of the 6262 unique phosphorylation sites detected to be regulated by IGF-1. These regulated sites included previously reported substrates of the insulin/IGF-1 signalling pathway, as well as novel substrates including Nuclear Factor I X and Semaphorin-4B. In silico prediction suggests the protein kinase B (PKB), protein kinase C (PKC), and cyclin-dependent kinase (CDK) as the main mediators of these phosphorylation events. Importantly, we found preferential phosphorylation patterns depending on the presence of either IRS-1 or IRS-2, which was associated with specific sets of kinases involved in signal transduction downstream of these substrates such as PDHK1, MAPK3, and PKD1 for IRS-1, and PIN1 and PKC beta for IRS-2. Overall, by generating a comprehensive phosphoproteomic profile from brown preadipocyte cells in response to IGF-1 stimulation, we reveal both common and distinct insulin/IGF-1 signalling events mediated by specific IRS proteins.

Differential control of ageing and lifespan by isoforms and splice variants across the mTOR network

Ageing can be defined as the gradual deterioration of physiological functions, increasing the incidence of age-related disorders and the probability of death. Therefore, the term ageing not only reflects the lifespan of an organism but also refers to progressive functional impairment and disease. The nutrient-sensing kinase mTOR (mammalian target of rapamycin) is a major determinant of ageing. mTOR promotes cell growth and controls central metabolic pathways including protein biosynthesis, autophagy and glucose and lipid homoeostasis. The concept that mTOR has a crucial role in ageing is supported by numerous reports on the lifespan-prolonging effects of the mTOR inhibitor rapamycin in invertebrate and vertebrate model organisms. Dietary restriction increases lifespan and delays ageing phenotypes as well and mTOR has been assigned a major role in this process. This may suggest a causal relationship between the lifespan of an organism and its metabolic phenotype. More than 25 years after mTOR's discovery, a wealth of metabolic and ageing-related effects have been reported. In this review, we cover the current view on the contribution of the different elements of the mTOR signalling network to lifespan and age-related metabolic impairment. We specifically focus on distinct roles of isoforms and splice variants across the mTOR network. The comprehensive analysis of mouse knockout studies targeting these variants does not support a tight correlation between lifespan prolongation and improved metabolic phenotypes and questions the strict causal relationship between them.

The Role of PPARγ in Cardiovascular Diseases

The peroxisome proliferator-activated receptors (PPAR) belong to the nuclear superfamily of ligand-activated transcription factors. PPARγ acts as a nutrient sensor that regulates several homeostatic functions. Its disruption can lead to vascular pathologies, disorders of fatty acid/lipid metabolism and insulin resistance. PPARγ can modulate several signaling pathways connected with blood pressure regulation. Firstly, it affects the insulin signaling pathway and endothelial dysfunction by modulation of expression and/or phosphorylation of signaling molecules through the PI3K/Akt/eNOS or MAPK/ET-1 pathways. Secondly, it can modulate gene expression of the renin- angiotensin system – cascade proteins, which potentially slow down the progression of atherosclerosis and hypertension. Thirdly, it can modulate oxidative stress response either directly through PPAR or indirectly through Nrf2 activation. In this context, activation and functioning of PPARγ is very important in the regulation of several disorders such as diabetes mellitus, hypertension and/or metabolic syndrome.

Targeting type 2 diabetes: lessons from a knockout model of insulin receptor substrate 2

Insulin receptor substrate 2 (IRS2) is a widely expressed protein that regulates crucial biological processes including glucose metabolism, protein synthesis, and cell survival. IRS2 is part of the insulin – insulin-like growth factor (IGF) signaling pathway and mediates the activation of the phosphotidylinositol 3-kinase (PI3K)–Akt and the Ras–mitogen-activated protein kinase (MAPK) cascades in insulin target tissues and in the pancreas. The best evidence of this is that systemic elimination of the Irs2 in mice (Irs2−/−) recapitulates the pathogenesis of type 2 diabetes (T2D), in that diabetes arises as a consequence of combined insulin resistance and beta-cell failure. Indeed, work using this knockout mouse has confirmed the importance of IRS2 in the control of glucose homeostasis and especially in the survival and function of pancreatic beta-cells. These studies have shown that IRS2 is critically required for beta-cell compensation in conditions of increased insulin demand. Importantly, islets isolated from T2D patients exhibit reduced IRS2 expression, which supports the likely contribution of altered IRS2-dependent signaling to beta-cell failure in human T2D. For all these reasons, the Irs2−/− mouse has been and will be essential for elucidating the inter-relationship between beta-cell function and insulin resistance, as well as to delineate therapeutic strategies to protect beta-cells during T2D progression.

Phosphatidylinositol 3-kinase (PI3K) activity bound to insulin-like growth factor-I (IGF-I) receptor, which is continuously sustained by IGF-I stimulation, is required for IGF-I-induced cell proliferation

Continuous stimulation of cells with insulin-like growth factors (IGFs) in G(1) phase is a well established requirement for IGF-induced cell proliferation; however, the molecular components of this prolonged signaling pathway that is essential for cell cycle progression from G(1) to S phase are unclear. IGF-I activates IGF-I receptor (IGF-IR) tyrosine kinase, followed by phosphorylation of substrates such as insulin receptor substrates (IRS) leading to binding of signaling molecules containing SH2 domains, including phosphatidylinositol 3-kinase (PI3K) to IRS and activation of the downstream signaling pathways. In this study, we found prolonged (>9 h) association of PI3K with IGF-IR induced by IGF-I stimulation. PI3K activity was present in this complex in thyrocytes and fibroblasts, although tyrosine phosphorylation of IRS was not yet evident after 9 h of IGF-I stimulation. IGF-I withdrawal in mid-G(1) phase impaired the association of PI3K with IGF-IR and suppressed DNA synthesis the same as when PI3K inhibitor was added. Furthermore, we demonstrated that Tyr(1316)-X-X-Met of IGF-IR functioned as a PI3K binding sequence when this tyrosine is phosphorylated. We then analyzed IGF signaling and proliferation of IGF-IR(-/-) fibroblasts expressing exogenous mutant IGF-IR in which Tyr(1316) was substituted with Phe (Y1316F). In these cells, IGF-I stimulation induced tyrosine phosphorylation of IGF-IR and IRS-1/2, but mutated IGF-IR failed to bind PI3K and to induce maximal phosphorylation of GSK3β and cell proliferation in response to IGF-I. Based on these results, we concluded that PI3K activity bound to IGF-IR, which is continuously sustained by IGF-I stimulation, is required for IGF-I-induced cell proliferation.

Pre- and postnatal calorie restriction perturbs early hypothalamic neuropeptide and energy balance

Energy balance is regulated by circulating leptin concentrations and hypothalamic leptin receptor (ObRb) signaling via STAT3 but is inhibited by SOCS3 and PTP1B. Leptin signaling enhances anorexigenic neuropeptides and receptor (POMC, MC3-R, MC4-R) activation while suppressing orexigenic neuropeptides (NPY, AgRP). We investigated in a sex-specific manner the early (PN2) and late (PN21) postnatal hypothalamic mechanisms in response to intrauterine (IUGR), postnatal (PNGR), and combined (IPGR) calorie and growth restriction. At PN2, both male and female IUGR were hypoleptinemic, but hypothalamic leptin signaling in females was activated as seen by enhanced STAT3. In addition, increased SOCS3 and PTP1B supported early initiation of leptin resistance in females that led to elevated AgRP but diminished MC3-R and MC4-R. In contrast, males demonstrated leptin sensitivity seen as a reduction in PTP1B and MC3-R and MC4-R with no effect on neuropeptide expression. At PN21, with adequate postnatal caloric intake, a sex-specific dichotomy in leptin concentrations was seen in IUGR, with euleptinemia in males indicative of persisting leptin sensitivity and hyperleptinemia in females consistent with leptin resistance, both with normal hypothalamic ObRb signaling, neuropeptides, and energy balance. In contrast, superimposition of PNGR upon IUGR (IPGR) led to diminished leptin concentrations with enhanced PTP1B and an imbalance in arcuate nuclear NPY/AgRP and POMC expression that favored exponential hyperphagia and diminished energy expenditure postweaning. We conclude that IUGR results in sex-specific leptin resistance observed mainly in females, whereas PNGR and IPGR abolish this sex-specificity, setting the stage for acquiring obesity after weaning.

HSP90 interacting with IRS-2 is involved in cAMP-dependent potentiation of IGF-I signals in FRTL-5 cells

Prolonged stimulation of FRTL-5 thyroid cells with cAMP-generating agents including thyroid-stimulating hormone (TSH) or cAMP analogues potentiates tyrosine phosphorylation of insulin receptor substrate (IRS)-2 triggered by insulin-like growth factor (IGF)-I, leading to enhancement of IGF-I-dependent proliferation. Because we identified HSP90 as an IRS-2-interacting protein, the roles of HSP90 in potentiation of IGF signals through IRS-2 were investigated. We found that prolonged dibutyryl cAMP treatment induced serine/threonine phosphorylation of IRS-2. Using a specific inhibitor of HSP90 chaperone activity, geldanamycin, or small interfering RNA against HSP90, we showed that HSP90 mediates cAMP-induced serine/threonine phosphorylation of IRS-2. Furthermore, inhibition of HSP90 by geldanamycin during dibutyryl cAMP pretreatment of cells for 24h suppressed cAMP-dependent potentiation of tyrosine phosphorylation of IRS-2 induced by IGF-I. Taking together, we conclude that HSP90 interacting with IRS-2 mediates cAMP-dependent serine/threonine phosphorylation of IRS-2 via its chaperone activity, leading to potentiation of tyrosine phosphorylation of IRS-2 induced by IGF-I.

Paraquat-induced oxidative stress represses phosphatidylinositol 3-kinase activities leading to impaired glucose uptake in 3T3-L1 adipocytes

Accumulated evidence indicates that oxidative stress causes and/or promotes insulin resistance; however, the mechanism by which this occurs is not fully understood. This study was undertaken to elucidate the molecular mechanism by which oxidative stress induced by paraquat impairs insulin-dependent glucose uptake in 3T3-L1 adipocytes. We confirmed that paraquat-induced oxidative stress decreased glucose transporter 4 (GLUT4) translocation to the cell surface, resulting in repression of insulin-dependent 2-deoxyglucose uptake. Under these conditions, oxidative stress did not affect insulin-dependent tyrosine phosphorylation of insulin receptor, insulin receptor substrate (IRS)-1 and -2, or binding of the phosphatidylinositol 3'-OH kinase (PI 3-kinase) p85 regulatory subunit or p110alpha catalytic subunit to each IRS. In contrast, we found that oxidative stress induced by paraquat inhibited activities of PI 3-kinase bound to IRSs and also inhibited phosphorylation of Akt, the downstream serine/threonine kinase that has been shown to play an essential role in insulin-dependent translocation of GLUT4 to the plasma membrane. Overexpression of active form Akt (myr-Akt) restored inhibition of insulin-dependent glucose uptake by paraquat, indicating that paraquat-induced oxidative stress inhibits insulin signals upstream of Akt. Paraquat treatment with and without insulin treatment decreased the activity of class Ia PI 3-kinases p110alpha and p110beta that are mainly expressed in 3T3-L1 adipocytes. However, paraquat treatment did not repress the activity of the PI 3-kinase p110alpha mutated at Cys(90) in the p85 binding region. These results indicate that the PI 3-kinase p110 is a possible primary target of paraquat-induced oxidative stress to reduce the PI 3-kinase activity and impaired glucose uptake in 3T3-L1 adipocytes.

The role of blood vessels, endothelial cells, and vascular pericytes in insulin secretion and peripheral insulin action

The pathogenesis of type 2 diabetes is intimately intertwined with the vasculature. Insulin must efficiently enter the bloodstream from pancreatic beta-cells, circulate throughout the body, and efficiently exit the bloodstream to reach target tissues and mediate its effects. Defects in the vasculature of pancreatic islets can lead to diabetic phenotypes. Similarly, insulin resistance is accompanied by defects in the vasculature of skeletal muscle, which ultimately reduce the ability of insulin and nutrients to reach myocytes. An underappreciated participant in these processes is the vascular pericyte. Pericytes, the smooth muscle-like cells lining the outsides of blood vessels throughout the body, have not been directly implicated in insulin secretion or peripheral insulin delivery. Here, we review the role of the vasculature in insulin secretion, islet function, and peripheral insulin delivery, and highlight a potential role for the vascular pericyte in these processes.

Effect of paraquat-induced oxidative stress on insulin regulation of insulin-like growth factor-binding protein-1 gene expression

Oxidative stress is thought to play a role in the development of insulin resistance. In order to elucidate the molecular effect of oxidative stress on liver insulin signaling, we analyzed the effect of paraquat (1,1-dimethyl-4,4-dipyridynium; PQ)-derived oxidative stress on the expression of insulin-dependent genes and activation of liver insulin signaling pathway. Incubation of primary cultured rat hepatocytes with 2 mM PQ for 6 h impaired the suppressive effect of insulin on insulin-like growth factor-binding protein-1 (IGFBP-1) gene expression, but did not influence glucose-6-phosphatase gene expression. Insulin-dependent phosphorylation or activation of insulin receptor, insulin receptor substrate-1 and -2, phosphatidylinositol 3-kinase, Akt and forkhead in rhabdomyosarcoma were not affected by PQ pre-treatment. In contrast, PQ treatment impaired insulin-dependent phosphorylation of mammalian target of rapamycin (mTOR). These results indicate that PQ-induced oxidative stress impairs insulin-dependent mTOR activation and that this impairment probably causes inhibition of insulin-dependent repression of IGFBP-1 expression.

The vascular actions of insulin control its delivery to muscle and regulate the rate-limiting step in skeletal muscle insulin action

Evidence suggests that insulin delivery to skeletal muscle interstitium is the rate-limiting step in insulin-stimulated muscle glucose uptake and that this process is impaired by insulin resistance. In this review we examine the basis for the hypothesis that insulin acts on the vasculature at three discrete steps to enhance its own delivery to muscle: (1) relaxation of resistance vessels to increase total blood flow; (2) relaxation of pre-capillary arterioles to increase the microvascular exchange surface perfused within skeletal muscle (microvascular recruitment); and (3) the trans-endothelial transport (TET) of insulin. Insulin can relax resistance vessels and increase blood flow to skeletal muscle. However, there is controversy as to whether this occurs at physiological concentrations of, and exposure times to, insulin. The microvasculature is recruited more quickly and at lower insulin concentrations than are needed to increase total blood flow, a finding consistent with a physiological role for insulin in muscle insulin delivery. Microvascular recruitment is impaired by obesity, diabetes and nitric oxide synthase inhibitors. Insulin TET is a third potential site for regulating insulin delivery. This is underscored by the consistent finding that steady-state insulin concentrations in plasma are approximately twice those in muscle interstitium. Recent in vivo and in vitro findings suggest that insulin traverses the vascular endothelium via a trans-cellular, receptor-mediated pathway, and emerging data indicate that insulin acts on the endothelium to facilitate its own TET. Thus, muscle insulin delivery, which is rate-limiting for its metabolic action, is itself regulated by insulin at multiple steps. These findings highlight the need to further understand the role of the vascular actions of insulin in metabolic regulation.

AMP-activated protein kinase activation increases phosphorylation of glycogen synthase kinase 3beta and thereby reduces cAMP-responsive element transcriptional activity and phosphoenolpyruvate carboxykinase C gene expression in the liver

AMP-activated protein kinase (AMPK) activation reportedly suppresses transcriptional activity of the cAMP-responsive element (CRE) in the phosphoenolpyruvate carboxykinase C (PEPCK-C) promoter and reduces hepatic PEPCK-C expression. Although a previous study found TORC2 phosphorylation to be involved in the suppression of AMPK-mediated CRE transcriptional activity, we herein present evidence that glycogen synthase kinase 3beta (GSK3beta) phosphorylation induced by AMPK also plays an important role. We initially found that injecting fasted mice with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) markedly increased Ser-9 phosphorylation of hepatic GSK3beta within 15 min. Stimulation with AICAR or the GSK3beta inhibitor SB-415286 strongly inhibited CRE-containing promoter activity in HepG2 cells. Using the Gal4-based transactivation assay system, the transcriptional activity of cAMP-response element-binding protein (CREB) was suppressed by both AICAR and SB415286, whereas that of TORC2 was repressed significantly by AICAR but very slightly by SB415286. These results show inactivation of GSK3beta to directly inhibit CREB but not TORC2. Importantly, the AICAR-induced suppression of PEPCK-C expression was shown to be blunted by overexpression of GSK3beta(S9G) but not wild-type GSK3beta. In addition, AICAR stimulation decreased, whereas Compound C (AMPK inhibitor) increased CREB phosphorylation (Ser-129) in HepG2 cells. The time-courses of decreased CREB phosphorylation (Ser-129) and increased GSK3beta phosphorylation were very similar. Furthermore, AMPK-mediated GSK3beta phosphorylation was inhibited by an Akt-specific inhibitor in HepG2 cells, suggesting involvement of the Akt pathway. In summary, phosphorylation (Ser-9) of GSK3beta is very likely to be critical for AMPK-mediated PEPCK-C gene suppression. Reduced CREB phosphorylation (Ser-129) associated with inactivation of GSK3beta by Ser-9 phosphorylation may be the major mechanism underlying PEPCK-C gene suppression by AMPK-activating agents such as biguanide.

Hepatic overexpression of a dominant negative form of raptor enhances Akt phosphorylation and restores insulin sensitivity in K/KAy mice

Several serine/threonine kinases reportedly phosphorylate serine residues of IRS-1 and thereby induce insulin resistance. In this study, to investigate the effect of mTOR/raptor on insulin signaling and metabolism in K/KAy mice with genetic obesity-associated insulin resistance, a dominant negative raptor, COOH-terminally deleted raptor (raptor-DeltaC(T)), was overexpressed in the liver via injection of its adenovirus into the circulation. Hepatic raptor-DeltaC(T) expression levels were 1.5- to 4-fold that of endogenously expressed raptor. Glucose tolerance in raptor-DeltaC(T)-overexpressing mice improved significantly compared with that of LacZ-overexpressing mice. Insulin-induced activation of p70S6 kinase (p70(S6k)) was significantly suppressed in the livers of raptor-DeltaC(T) overexpressing mice. In addition, insulin-induced IRS-1, Ser(307), and Ser(636/639) phosphorylations were significantly suppressed in the raptor-DeltaC(T)-overexpressing liver, whereas tyrosine phosphorylation of IRS-1 was increased. PI 3-kinase activation in response to insulin stimulation was increased approximately twofold, and Akt phosphorylation was clearly enhanced under both basal and insulin-stimulated conditions in the livers of raptor-DeltaC(T) mice. Thus, our data indicate that suppression of the mTOR/p70(S6k) pathway leads to improved glucose tolerance in K/KAy mice. These observations may contribute to the development of novel antidiabetic agents.

Effects of free fatty acids on carbohydrate metabolism and insulin signalling in perfused rat liver

BACKGROUND

Elevated circulating free fatty acids (FFAs) induce insulin resistance and play a crucial role in the development of type 2 diabetes, in which fasting hepatic glucose production (HGP) is increased. However, direct effects of FFAs on fasting HGP are still unclear because indirect endocrine and metabolic effects contribute to FFA action. Thus, we aimed to investigate acute direct effects of specific FFAs on fasting HGP, lactate uptake, and insulin signalling.

MATERIALS AND METHODS

Isolated livers obtained from 20 h fasted rats were perfused with albumin-bound palmitate or oleate (200 micromol L(-1) each) or vehicle (control) for 180 min (n = 5-7/group).

RESULTS

Compared to control, hepatic lactate uptake was increased by palmitate and oleate (~+40%; P < 0.05), while HGP from lactate (~3 mmol L(-1)) and liver glycogen content were similar. Tyrosine phosphorylation (pY) of insulin-receptor-substrate-(IRS)-2 and p70S6-kinase phosphorylation were not affected by FFAs. Palmitate decreased insulin-receptor-beta pY, IRS-1 pY and phosphoinositol-3-kinase expression by 46 +/- 16%, 46 +/- 11% and 20 +/- 9%, respectively (P < 0.03), while oleate reduced Akt phosphorylation by 85 +/- 7% (P < 0.006).

CONCLUSIONS

Isolated liver perfusion with saturated or unsaturated FFAs reduced insulin signalling protein phosphorylation at different sites and increased lactate uptake without affecting HGP or glycogen content. These results suggest that at fasting, both saturated and unsaturated FFAs increase hepatic glucose precursor uptake and may, independently of insulin's presence, accelerate protein dephosphorylation of the insulin signalling cascade at different sites.

Disruption of the insulin-like growth factor type 1 receptor in osteoblasts enhances insulin signaling and action

Defective bone formation is common in patients with diabetes, suggesting that insulin normally exerts anabolic actions in bone. However, because insulin can cross-activate the insulin-like growth factor type 1 receptor (IGF-1R), which also functions in bone, it has been difficult to establish the direct (IGF-1-independent) actions of insulin in osteoblasts. To overcome this problem, we examined insulin signaling and action in primary osteoblasts engineered for conditional disruption of the IGF-1 receptor (DeltaIGF-1R). Calvarial osteoblasts from mice carrying floxed IGF-1R alleles were infected with adenoviral vectors expressing the Cre recombinase (Ad-Cre) or green fluorescent protein (Ad-GFP) as control. Disruption of IGF-1R mRNA (>90%) eliminated IGF-1R without affecting insulin receptor (IR) mRNA and protein expression and eliminated IGF-1R/IR hybrids. In DeltaIGF-1R osteoblasts, insulin signaling was markedly increased as evidenced by increased phosphorylation of insulin receptor substrate 1/2 and enhanced ERK/Akt activation. Microarray analysis of RNA samples from insulin-treated, DeltaIGF-1R osteoblasts revealed striking changes in several genes known to be downstream of ERK including Glut-1 and c-fos. Treatment of osteoblasts with insulin induced Glut-1 mRNA, increased 2-[1,2-(3)H]-deoxy-d-glucose uptake, and enhanced proliferation. Moreover, insulin treatment rescued the defective differentiation and mineralization of DeltaIGF-1R osteoblasts, suggesting that IR signaling can compensate, at least in part, for loss of IGF-1R signaling. We conclude that insulin exerts direct anabolic actions in osteoblasts by activation of its cognate receptor and that the strength of insulin-generated signals is tempered through interactions with the IGF-1R.

Increased hepatic expression of ganglioside-specific sialidase, NEU3, improves insulin sensitivity and glucose tolerance in mice

Membrane microdomains rich in gangliosides are recognized as being critical for proper compartmentalization of insulin signaling. Plasma membrane-associated sialidase, NEU3, is a key enzyme for ganglioside hydrolysis. We previously reported that mice overexpressing NEU3 mainly in muscles developed severe insulin-resistant diabetes. To examine the possible contributions of NEU3 to in vivo insulin sensitivity and glucose tolerance, NEU3 was expressed by using adenoviral vectors in the livers of C57BL/6 mice on standard and high-fat diets, and insulin-resistant KKAy mice on standard diets. Hepatic NEU3 overexpression paradoxically improved glucose tolerance and insulin sensitivity in the C57BL/6 mice fed standard diets, and glucose tolerance in the C57BL/6 mice fed high-fat diets and in KKAy mice. Hepatic NEU3 overexpression increased hepatic glycogen deposition and triglyceride accumulation, and enhanced the hepatic peroxisome proliferator-activated receptor gamma and fetuin expression in the C57BL/6 mice on standard and high-fat diets, and in KKAy mice. Thin-layer chromatographic analysis demonstrated increased levels of GM1 and markedly reduced GM3 in the livers of mice with hepatic NEU3 overexpression (NEU3 mice). Basal and insulin-stimulated tyrosine phosphorylations of insulin receptor substrate 1 were significantly increased, but tyrosine phosphorylations of the insulin receptor and insulin receptor substrate 2 in the NEU3 liver were unchanged. Insulin-stimulated tyrosine phosphorylations of the insulin receptor were increased in adipose tissues of NEU3 mice. These results suggest that hepatic NEU3 overexpression improves insulin sensitivity and glucose tolerance through modification of ganglioside composition and peroxisome proliferator-activated receptor gamma signaling. Our findings also provide further evidence that NEU3 is an important regulator of insulin sensitivity and glucose tolerance.

The novel roles of liver for compensation of insulin resistance in human growth hormone transgenic rats

Chronic excess of GH is known to cause hyperinsulinemia and insulin resistance. We developed human GH transgenic (TG) rats, which were characterized by high plasma levels of human GH and IGF-I. These TG rats showed higher levels of plasma insulin, compared with control littermates, whereas plasma glucose concentrations were normal. Insulin-dependent glucose uptake into adipocytes and muscle was impaired, suggesting that these rats developed insulin resistance. In contrast, insulin-independent glucose uptake into hepatocytes from TG rats was significantly increased, and glycogen and lipid levels in livers of TG rats were remarkably high. Because the role of liver in GH-induced insulin resistance is poorly understood, we studied insulin signaling at early stages and insulin action in liver and primary cultures of hepatocytes prepared from TG rats. There was no difference in insulin receptor kinase activity induced by insulin between TG and control rats; however, insulin-dependent insulin receptor substrate-2 tyrosine phosphorylation, glycogen synthase activation, and expression of enzymes that induce lipid synthesis were potentiated in hepatocytes of TG rats. These results suggest that impairment of insulin-dependent glucose uptake by GH excess in adipose tissue and muscle is compensated by up-regulation of glucose uptake in liver and that potentiation of insulin signaling through insulin receptor substrate-2 in liver experiencing GH excess causes an increase in glycogen and lipid synthesis from incorporated glucose, resulting in accumulation of glycogen and lipids in liver. This novel mechanism explains normalization of plasma glucose levels at least in part in a GH excess model.

Angiotensin II modulates tyr-phosphorylation of IRS-4, an insulin receptor substrate, in rat liver membranes

Angiotensin II (Ang II), a major regulator of blood pressure, is also involved in the control of cellular proliferation and hypertrophy and might exhibit additional actions in vivo by modulating the signaling of other hormones. As hypertension and Insulin (Ins) resistance often coexist and are risk factors for cardiovascular diseases, Ang II and Insulin signaling cross-talk may have an important role in hypertension development. The effect of Ins on protein tyrosine phosphorylation was assayed in rat liver membrane preparations, a rich source of Ins receptors. Following stimulation, Ins (10(-7) M) induced tyr-phosphorylation of different proteins. Insulin consistently induced tyr-phosphorylation of a 160 kDa protein (pp160) with maximum effect between 1 and 3 min. The pp160 protein was identified by anti-IRS-4 but not by anti-IRS-1 antibody. Pre-stimulation with Ang II (10(-7) M) diminishes tyr-phosphorylation level of pp160/IRS-4 in a dose-dependent manner. Okadaic acid, the PP1A and PP2A Ser/Thr phosphatase inhibitor, increases pp160 phosphorylation induced by Ins and prevents the inhibitory effect of Ang II pre-stimulation. Genistein, a tyrosine kinase inhibitor, diminishes tyr-phosphorylation level of IRS-4. PI3K inhibitors Wortmanin and LY294002, both increase tyr-phosphorylation of IRS-4, either in the presence of Ins alone or combined with Ang II. These results suggest that Ins and Ang II modulate IRS-4 tyr-phosphorylation in a PI3K-dependent manner. In summary, we showed that Ins induces tyr-phosphorylation of IRS-4, an effect modulated by Ang II. Assays performed in the presence of different inhibitors points toward a PI3K involvement in this signaling pathway.

Tissue-specific roles of IRS proteins in insulin signaling and glucose transport

In type 2-diabetes and impaired glucose tolerance, the muscle, fat and liver become resistant to insulin, and recent developments place dysregulation of insulin receptor substrate (IRS) expression and activation at the center of such defects. IRS1 and IRS2 are the major insulin receptor substrates leading to glucose homeostasis, and have distinct and overlapping roles in diverse organs. The majority of the published literature in this field suggests that IRS1 is the major substrate leading to stimulation of glucose transport in muscle and adipose tissues, whereas in liver, IRS1 and IRS2 have complementary roles in insulin signaling and metabolism.

Critical nodes in signalling pathways: insights into insulin action

Physiologically important cell-signalling networks are complex, and contain several points of regulation, signal divergence and crosstalk with other signalling cascades. Here, we use the concept of 'critical nodes' to define the important junctions in these pathways and illustrate their unique role using insulin signalling as a model system.

Resistin-like Molecule β Activates MAPKs, Suppresses Insulin Signaling in Hepatocytes, and Induces Diabetes, Hyperlipidemia, and Fatty Liver in Transgenic Mice on a High Fat Diet

Resistin and resistin-like molecules (RELMs) are a family of proteins reportedly related to insulin resistance and inflammation. Because the serum concentration and intestinal expression level of RELMbeta were elevated in insulin-resistant rodent models, in this study we investigated the effect of RELMbeta on insulin signaling and metabolism using transgenic mice and primary cultured hepatocytes. First, transgenic mice with hepatic RELMbeta overexpression were shown to exhibit significant hyperglycemia, hyperlipidemia, fatty liver, and pancreatic islet enlargement when fed a high fat diet. Hyperinsulinemic glucose clamp showed a decreased glucose infusion rate due to increased hepatic glucose production. In addition, the expression levels of IRS-1 and IRS-2 proteins as well as the degrees of insulin-induced phosphatidylinositol 3-kinase and Akt activations were attenuated in RELMbeta transgenic mice. Similar down-regulations of IRS-1 and IRS-2 proteins were observed in primary cultured hepatocytes chronically treated (for 24 h) with RELMbeta, suggesting the insulin resistance-inducing effect of RELMbeta to be direct. Furthermore, it was shown that RELMbeta acutely and markedly activates ERK and p38, while weakly activating JNK, in primary cultured hepatocytes. This increased basal p38 phosphorylation level was also observed in the livers of RELMbeta transgenic mice. In conclusion, RELMbeta, a gut-derived hormone, impairs insulin signaling probably via the activations of classic MAPKs, and increased expression of RELMbeta may be involved in the pathogenesis of glucose intolerance and hyperlipidemia in some insulin-resistant models. Thus, RELMbeta is a potentially useful marker for assessing insulin resistance and may also be a target for future novel anti-diabetic agents.

MEK kinase 1 is essential for Bcr–Abl-induced STAT3 and self-renewal activity in embryonic stem cells

BCR-ABL oncogene, the molecular hallmark of chronic myelogenous leukemia, arises in a primitive hematopoietic stem cell that has the capacity for both differentiation and self-renewal. Its product, Bcr-Abl protein, has been shown to activate signal transducers and activators of transcription 3 (STAT3) and to promote self-renewal in embryonic stem (ES) cells, even in the absence of leukemia inhibitory factor (LIF). MEK kinase 1 (MEKK1) is a 196-kDa mitogen-activated protein kinase (MAPK) kinase kinase involved in Bcr-Abl signal transduction. To investigate the role of MEKK1 in Bcr-Abl-induced transformation of stem cells, p210 Bcr-Abl was stably transfected into wild-type (WT(p210)) and MEKK1-/- (MEKK1-/-(p210)) ES cells. Bcr-Abl enhanced MEKK1 expression in ES transfectants, as it does in other Bcr-Abl-transformed cells. In the absence of LIF, WT(p210) cells showed constitutive STAT3 activation and formed rounded, compact colonies having strong alkaline phosphatase activity, a characteristic phenotype of undifferentiated ES cells. MEKK1-/-(p210) cells, by contrast, showed less STAT3 activity than WT(p210) cells and formed large, flattened colonies having weak alkaline phosphatase activity, a phenotype of differentiated ES cells. These results indicate that MEKK1 plays a key role in Bcr-Abl-induced STAT3 activation and in ES cells' capacity for LIF-independent self-renewal, and may thus be involved in Bcr-Abl-mediated leukemogenesis in stem cells.

Complementary roles of IRS-1 and IRS-2 in the hepatic regulation of metabolism

Hepatic insulin resistance is a critical component in the development of type 2 diabetes mellitus. In many cases, insulin resistance in liver is associated with reduced expression of both major insulin receptor substrate (IRS) proteins, IRS-1 and IRS-2. To investigate the specific functions of IRS-1 and IRS-2 in regulating liver function in vivo, we developed an adenovirus-mediated RNA interference technique in which short hairpin RNAs (shRNAs) are used to knock down IRS-1, IRS-2, or both, by 70-80% in livers of WT mice. The knockdown of IRS-1 resulted in an upregulation of the gluconeogenic enzymes glucose-6 phosphatase and phosphoenolpyruvate carboxykinase, as well as a marked increase in hepatic nuclear factor-4 alpha. Decreased IRS-1 was also associated with a decrease in glucokinase expression and a trend toward increased blood glucose, whereas knockdown of IRS-2 resulted in the upregulation of lipogenic enzymes SREBP-1c and fatty acid synthase, as well as increased hepatic lipid accumulation. The concomitant injection of IRS-1 and IRS-2 adenoviral shRNAs resulted in systemic insulin resistance, glucose intolerance, and hepatic steatosis. The alterations in the dual-knockdown mice were associated with defective Akt activation and Foxo1 phosphorylation. Taken together, our results demonstrate that hepatic IRS-1 and IRS-2 have complementary roles in the control of hepatic metabolism, with IRS-1 more closely linked to glucose homeostasis and IRS-2 more closely linked to lipid metabolism.

Differential contribution of insulin receptor substrates 1 versus 2 to insulin signaling and glucose uptake in l6 myotubes

Insulin receptor substrates-1 and 2 (IRS-1 and IRS-2) are pivotal in relaying insulin signaling in insulin-responsive tissues such as muscle. However, the precise contribution of IRS-1 vis-a-vis IRS-2 in insulin-mediated metabolic and mitogenic responses has not been compared directly in differentiated muscle cells. This study aimed to determine the relative contribution of IRS-1 versus IRS-2 in these responses, using small interfering RNA (siRNA)-mediated specific gene silencing. In L6 myotubes, transfection of siRNA targeted specifically against IRS-1 (siIRS-1) or IRS-2 (siIRS-2) reduced the cognate protein expression by 70-75%. Insulin-induced ERK phosphorylation was much more sensitive to IRS-2 than IRS-1 ablation, whereas p38MAPK phosphorylation was reduced by 43 or 62% in myotubes treated with siIRS-1 or siIRS-2, respectively. Insulin-induced Akt1 and Akt2 phosphorylation was reduced in myotubes treated with siIRS-1, but only Akt2 phosphorylation was reduced in myotubes treated with siIRS-2. In contrast, siIRS-1 treatment caused a marked reduction in insulin-induced actin remodeling, glucose uptake, and GLUT4 translocation, and siIRS-2 was without effect on these responses. Notably, combined siIRS-1 and siIRS-2, although reducing each IRS by around 75%, caused no further drop in glucose uptake than that achieved with siIRS-1 alone, but abolished p38MAPK phosphorylation. We conclude that insulin-stimulated Akt1 phosphorylation, actin remodeling, GLUT4 translocation, and glucose uptake are regulated mainly by IRS-1, whereas IRS-2 contributes selectively to ERK signaling, and Akt2 and p38MAPK lie downstream of both IRS in muscle cells.

Protein tyrosine phosphatases: their role in insulin action and potential as drug targets

Protein tyrosine phosphatases (PTPases) are the enzymes responsible for the selective dephosphorylation of tyrosine residues. PTPases function to regulate a wide array of biological responses mediated by growth factors and other stimuli by balancing the cellular level of phosphotyrosine in concert with their counterparts, protein tyrosine kinases. The important roles which PTPases play in regulating intracellular signalling and, ultimately, biological function along with the recent availability of information regarding their structural features has highlighted them as potential targets for pharmacological modulation. This is demonstrated by the increased level of activity directed towards the identification of novel small-molecule PTPase inhibitors. The rationale and potential utility of this drug discovery approach is discussed here, with particular emphasis on its application for the treatment of insulin resistance and Type 2 diabetes.

Dissipating excess energy stored in the liver is a potential treatment strategy for diabetes associated with obesity

For examining whether dissipating excess energy in the liver is a possible therapeutic approach to high-fat diet-induced metabolic disorders, uncoupling protein-1 (UCP1) was expressed in murine liver using adenoviral vectors in mice with high-fat diet-induced diabetes and obesity, and in standard diet-fed lean mice. Once diabetes with obesity developed, hepatic UCP1 expression increased energy expenditure, decreased body weight, and reduced fat in the liver and adipose tissues, resulting in markedly improved insulin resistance and, thus, diabetes and dyslipidemia. Decreased expressions of enzymes for lipid synthesis and glucose production and activation of AMP-activated kinase in the liver seem to contribute to these improvements. Hepatic UCP1 expression also reversed high-fat diet-induced hyperphagia and hypothalamic leptin resistance, as well as insulin resistance in muscle. In contrast, intriguingly, in standard diet-fed lean mice, hepatic UCP1 expression did not significantly affect energy expenditure or hepatic ATP contents. Furthermore, no alterations in blood glucose levels, body weight, or adiposity were observed. These findings suggest that ectopic UCP1 in the liver dissipates surplus energy without affecting required energy and exerts minimal metabolic effects in lean mice. Thus, enhanced UCP expression in the liver is a new potential therapeutic target for the metabolic syndrome.

Insulin-like growth factor-I signaling in human neuroblastoma cells

Neuroblastoma is a heterogeneous tumor consisting of N (neuronal) and S (stromal) cells. We report that more tumorigenic and motile N cells express higher levels of IGF-I receptor (IGF-IR) than less tumorigenic, more adherent S cells. Shc, one of the two major docking partners of IGF-IR, is equally expressed in N and S cell lines. IGF-I treatment phosphorylates Shc in N cells, but only weakly activates Shc in S cells. Expression of the second partner, insulin receptor substrate (IRS), is cell type specific. S cells exclusively express IRS-1 that undergoes sustained phosphorylation by IGF-I. In contrast, N cells express IRS-2 that is transiently phosphorylated by IGF-I. Downstream of IRS-2 and Shc, IGF-I treatment results in strong activation of Akt and MAPK in N cells and activation of both pathways is required for IGF-I-mediated differentiation. Only IGF-IR activation of phosphatidylinositol-3 kinase is required for tumor edge ruffling in N and S cells, with stimulation of focal adhesion kinase (FAK) and paxillin. This detailed understanding of the 'biochemical signature' of N and S cells provides the background needed to target and disrupt specific IGF signaling pathways in an attempt to develop more effective therapies.

Deficiency of adrenomedullin induces insulin resistance by increasing oxidative stress

Hypertension, insulin resistance, and obesity are common age-related metabolic disorders that are often associated with increased oxidative stress and the resultant vascular damage. Underlying mechanisms have been suggested, and age-related overproduction of oxidative stress is one possible candidate. Since we recently found a vasoactive peptide, adrenomedullin, to be an endogenous antioxidant that potently inhibits oxidative stress-induced vascular damage, in the current study we evaluated oxidative stress-induced changes in aged mice. Insulin sensitivities in young and aged adrenomedullin-deficient mice were measured by means of the hyperinsulinemic-euglycemic clamp method; insulin resistance was apparent in aged adrenomedullin-deficient mice with increased urinary excretion of 8-iso-prostaglandin F2alpha, a marker of oxidative stress, but not in young adrenomedullin-deficient mice. Concomitantly, only aged adrenomedullin-deficient mice not only showed increased production of muscular reactive oxygen species, as demonstrated by the electron spin resonance method, but also had significantly decreased insulin-stimulated glucose uptake into the soleus muscle associated with impairment of insulin signals such as insulin receptor substrate-1,2 and phosphatidylinositol-3 kinase activities. In turn, these abnormalities could be nearly reversed by either treatment with 4-hydroxy-2,2,6,6-tetramethyl-piperidine-N-oxyl, a membrane-permeable superoxide dismutase mimetic, or adrenomedullin supplementation. Evidence presented in this report suggests that age-related accumulation of oxidative stress is involved in blood pressure regulation and insulin resistance in aged adrenomedullin-deficient mice, and adrenomedullin is thus an endogenous substance counteracting oxidative stress-induced insulin resistance associated with aging.

Defective signaling through Akt-2 and -3 but not Akt-1 in insulin-resistant human skeletal muscle: potential role in insulin resistance

Recent evidence has shown that activation of phosphatidyinositol-3-kinase (PI3K) and Akt, necessary for insulin stimulation of glucose transport, is impaired in insulin resistance. It is unknown, however, which Akt isoform shows impaired activation in insulin resistance. Additionally, related growth factors (epidermal or platelet-derived vascular) also stimulate PI3K, but it is unknown whether production of 3,4,5 phosphatidyinositol is sufficient to stimulate glucose transport in insulin-resistant muscle. Moreover, these studies were performed in rodents, and little data exists from humans. Hence, we investigated the stimulation of PI3K and Akt-1, -2, and -3 by insulin and epidermal growth factors (EGFs) in skeletal muscles from lean and obese insulin-resistant humans. Insulin activated all Akt isoforms in lean muscles, whereas only Akt-1 was activated in obese muscles. Insulin receptor substrate (IRS)-1 was associated with PI3K activity, which is necessary for Akt activation by insulin, and was reduced in obese muscles, and this was accompanied by decreased IRS-1 expression. In contrast, insulin- or EGF-stimulated phosphotyrosine-associated PI3K activity was not different between lean and obese muscles. These results show that a defect in the ability of insulin to activate Akt-2 and -3 may explain the impaired insulin-stimulated glucose transport in insulin resistance. Additionally, these data also show that different upstream or downstream signals may regulate the activity of the various Akt isoforms.

MEK kinase 1 interacts with focal adhesion kinase and regulates insulin receptor substrate-1 expression

MEK kinase 1 (MEKK1) has been shown to contribute to the regulation of cell migration, whereas focal adhesion kinase (FAK) is a major player involved in both cell migration and integrin signaling. Here we show that MEKK1 and FAK are co-immunoprecipitated from mouse fibroblasts. Moreover, the association between MEKK1 and FAK appears to be physiologically relevant, as it is enhanced by treatment with epidermal growth factor (EGF). Targeting FAK to the membrane also enhanced its association with MEKK1, indicating that MEKK1 is localized to a membrane-related subcellular domain, perhaps focal adhesions. Interestingly, the expression of insulin receptor substrate-1 (IRS-1) was diminished in MEKK1-deficient fibroblasts, which is similar to an earlier finding in FAK-deficient fibroblasts. Insulin-like growth factor 1 (IGF-1)-induced ERK activation was diminished in MEKK1-deficient cells, but phosphatidylinositol 3-kinase/Akt activation was not. Although integrin reportedly regulates the transcription of the IRS-1 gene via FAK-mediated JNK activation, no impairment of fibronectin-stimulated activation of FAK, ERK, or JNK was observed in MEKK1-deficient cells. Reconstitution of MEKK1 expression restored IRS-1 expression as well as IGF-1-induced ERK activation. Taken together, these findings indicate that MEKK1 interacts with FAK in focal adhesions and regulates IRS-1 expression.

Regulation of PPAR gamma transcriptional activity in 3T3-L1 adipocytes

PPAR gamma is a member of the nuclear hormone receptor superfamily and functions as a transcriptional regulator of genes linked to adipogenesis and lipid metabolism. The regulation of PPAR gamma activity by insulin signaling molecules in adipocytes has yet to be clarified. Therefore, it is important to measure endogenous PPAR gamma transcriptional activities in response to various stimuli in adipocytes. Herein, with a transcription reporter assay using recombinant adenovirus vectors expressing PPRE (PPAR responsive elements)-reporter genes, we established a novel system for measuring endogenous PPAR gamma transcriptional activity in 3T3-L1 adipocytes. By means of this system, a marked increase (8.5-fold) in PPAR gamma transcriptional activity was detected after treatment with 10(-6)M pioglitazone, a thiazolidinedione (TZD), indicating that this system can measure PPAR gamma activity accurately. Furthermore, MAPK activation, achieved by overexpressing constitutively activated MEK1, inhibited PPAR gamma transcriptional activity. In contrast, treatment with PKA stimulators markedly increased PPAR gamma activity. Interestingly, PI 3-kinase overexpression resulted in a marked decrease in PPAR gamma activity. These observations have important implications for understanding the regulation of PPAR gamma transcriptional activity.

Angiotensin II subtype 2 receptor activation inhibits insulin-induced phosphoinositide 3-kinase and Akt and induces apoptosis in PC12W cells

In the present study, we identified novel negative cross-talk between the angiotensin II subtype 2 (AT2) receptor and insulin receptor signaling in the regulation of phosphoinositide 3-kinase (PI3K), Akt, and apoptosis in rat pheochromocytoma cell line, PC12W cells, which exclusively express AT2 receptor. We demonstrated that insulin-mediated insulin receptor substrate (IRS)-2-associated PI3K activity was inhibited by AT2 receptor stimulation, whereas IRS-1-associated PI3K activity was not significantly influenced. AT2 receptor stimulation did not change insulin-induced tyrosine phosphorylation of IRS-2 or its association with the p85alpha subunit of PI3K, but led to a significant reduction of insulin-induced p85alpha phosphorylation. AT2 receptor stimulation increased the association of a protein tyrosine phosphatase, SHP-1, with IRS-2. Moreover, we demonstrated that AT2 receptor stimulation inhibited insulin-induced Akt phosphorylation and that insulin-mediated antiapoptotic effect was also blocked by AT2 receptor activation. Overexpression of a catalytically inactive dominant negative SHP-1 markedly attenuated the AT2 receptor- mediated inhibition of IRS-2-associated PI3K activity, Akt phosphorylation, and antiapoptotic effect induced by insulin. Taken together, these results indicate that AT2 receptor-mediated activation of SHP-1 and the consequent inhibition IRS-2-associated PI3K activity contributed at least partly to the inhibition of Akt phosphorylation, thereby inducing apoptosis.

Unique phosphorylation mechanism of Gab1 using PI 3-kinase as an adaptor protein

Grb2-associated binder-1 (Gab1) undergoes tyrosine phosphorylation in response to stimulation by growth factors and hormones including insulin, epidermal growth factor (EGF), nerve growth factor (NGF), and hepatocyte growth factor (HGF). However, the HGF receptor is the only one known to associate directly with Gab1. Herein, we explore the mechanism of Gab1 phosphorylation by other receptor protein-tyrosine kinases unable to bind to Gab1 directly. The Src homology 2 (SH2) domain of the phosphatidylinositol 3-kinase (PI3K) regulatory subunit binds Gab1 in a phosphorylation-independent manner. Moreover, the regulatory subunit of PI3K can mediate the association of Gab1 and receptor protein-tyrosine kinases including the insulin, EGF, and NGF receptors, all of which phosphorylate Gab1. Thus, it appears that the PI3K regulatory subunit acts as an adaptor protein via a phosphotyrosyl-independent SH2 interaction, allowing Gab1 to serve as a substrate for several tyrosine kinases. This is a new role for the PI3K regulatory subunit.

Defects of the insulin receptor substrate (IRS) system in human metabolic disorders

Insulin receptor substrate (IRS) molecules are key mediators in insulin signaling and play a central role in maintaining basic cellular functions such as growth, survival, and metabolism. They act as docking proteins between the insulin receptor and a complex network of intracellular signaling molecules containing Src homology 2 (SH2) domains. Four members (IRS-1, IRS-2, IRS-3, IRS-4) of this family have been identified that differ as to tissue distribution, subcellular localization, developmental expression, binding to the insulin receptor, and interaction with SH2 domain-containing proteins. Results from targeted disruption of the IRS genes in mice have provided important clues to the functional differences among these related molecules, suggesting they play different and specific roles in vivo. The available data are consistent with the notion that IRS-1 and IRS-2 are not functionally interchangeable in tissues that are responsible for glucose production (liver), glucose uptake (skeletal muscle and adipose tissue), and insulin production (pancreatic beta cells). In fact, IRS-1 appears to have its major role in skeletal muscle whereas IRS-2 appears to regulate hepatic insulin action as well as pancreatic beta cell development and survival. By contrast, IRS-3 and IRS-4 genes appear to play a redundant role in the IRS signaling system. Defects in muscle IRS-1 expression and function have been reported in insulin-resistant states such as obesity and type 2 diabetes. Several polymorphisms in the IRS genes have been identified, but only the Gly-->Arg972 substitution of IRS-1, interacting with environmental factors, seems to have a pathogenic role in the development of type 2 diabetes. In contrast, polymorphisms of the other IRS genes do not appear to contribute to type 2 diabetes.

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.

Insulin resistance with enhanced insulin signaling in high-salt diet-fed rats

Previous clinical studies showed an apparent correlation between hypertension and insulin resistance, and patients with diabetes are known to have increased blood pressure responsiveness to salt loading. To investigate the effect of high salt intake on insulin sensitivity and the insulin signaling pathway, a high-salt diet (8% NaCl) or a normal diet was given to 7-week-old SD rats for 2 weeks. High salt-fed rats developed slightly but significantly higher systolic blood pressure than controls (133 +/- 2 vs. 117 +/- 2 mmHg, P < 0.001), with no change in food intake or body weight. High salt-fed rats were slightly hyperglycemic (108.5 +/- 2.8 vs. 97.8 +/- 2.5 mg/dl, P = 0.01) and slightly hyperinsulinemic (0.86 +/- 0.07 vs. 0.61 +/- 0.06 ng/ml, P = 0.026) in the fasting condition, as compared with controls. Hyperinsulinemic-euglycemic clamp study revealed a 52.7% decrease in the glucose infusion rate and a 196% increase in hepatic glucose production in high salt-fed rats, which also showed a 66.4% decrease in 2-deoxyglucose uptake into isolated skeletal muscle and a 44.5% decrease in insulin-induced glycogen synthase activation in liver, as compared with controls. Interestingly, despite the presence of insulin resistance, high salt-fed rats showed enhanced insulin-induced tyrosine phosphorylation of insulin receptor substrate (IRS)-1, IRS-2 (liver and muscle), and IRS-3 (liver only). Phosphatidylinositol (PI) 3-kinase activities associated with IRS and phosphotyrosine in the insulin-stimulated condition increased 2.1- to 4.1-fold, as compared with controls. Insulin-induced phosphorylation of Ser-473 of Akt and Ser-21 of glycogen synthase kinase-3 also increased 2.9- and 2-fold, respectively, in the liver of the high salt-fed rats. Therefore, in both the liver and muscle of high salt-fed rats, intracellular insulin signaling leading to PI 3-kinase activation is enhanced and insulin action is attenuated. The hyperinsulinemic-euglycemic clamp study showed that decreased insulin sensitivity induced with a high-salt diet was not reversed by administration of pioglitazone. The following can be concluded: 1) a high-salt diet may be a factor promoting insulin resistance, 2) the insulin-signaling step impaired by high salt intake is likely to be downstream from PI 3-kinase or Akt activation, and 3) this unique insulin resistance mechanism may contribute to the development of diabetes in patients with hypertension.

Insulin signal transduction in rat small intestine: role of MAP kinases in expression of mucosal hydrolases

The postreceptor events regulating the signal of insulin downstream in rat intestinal cells have not yet been analyzed. Our objectives were to identify the nature of receptor substrates and phosphorylated proteins involved in the signaling of insulin and to investigate the mechanism(s) by which insulin enhances intestinal hydrolases. In response to insulin, the following proteins were rapidly phosphorylated on tyrosine residues: 1) insulin receptor substrates-1 (IRS-1), -2, and -4; 2) phospholipase C-isoenzyme-gamma; 3) the Ras-GTPase-activating protein (GAP) associated with Rho GAP and p62(Src); 4) the insulin receptor beta-subunit; 5) the p85 subunits of phosphatidylinositol 3-kinase (PI 3-kinase); 6) the Src homology 2 alpha-collagen protein; 7) protein kinase B; 8) mitogen-activated protein (MAP) kinase-1 and -2; and 9) growth receptor-bound protein-2. Compared with controls, insulin enhanced the intestinal activity of MAP kinase-2 and protein kinase B by two- and fivefold, respectively, but did not enhance p70/S6 ribosomal kinase. Administration of an antireceptor antibody or MAP-kinase inhibitor PD-98059 but not a PI 3-kinase inhibitor (wortmannin) to sucklings inhibited the effects of insulin on mucosal mass and enzyme expression. We conclude that normal rat enterocytes express all of the receptor substrates and mediators involved in different insulin signaling pathways and that receptor binding initiates a signal enhancing brush-border membrane hydrolase, which appears to be regulated by the cascade of MAP kinases but not by PI 3-kinase.

Substrates of semicarbazide-sensitive amine oxidase co-operate with vanadate to stimulate tyrosine phosphorylation of insulin-receptor-substrate proteins, phosphoinositide 3-kinase activity and GLUT4 translocation in adipose cells

It has been shown that the combination of benzylamine or tyramine and low concentrations of vanadate markedly stimulates glucose transport in rat adipocytes by a mechanism that requires semicarbazide-sensitive amine oxidase (SSAO) activity and H(2)O(2) formation. Here we have further analysed the insulin-like effects of the combination of SSAO substrates and vanadate and we have studied the signal-transduction pathway activated in rat adipocytes. We found that several SSAO substrates (benzylamine, tyramine, methylamine, n-decylamine, histamine, tryptamine or beta-phenylethylamine), in combination with low concentrations of vanadate, stimulate glucose transport in isolated rat adipocytes. Furthermore, SSAO substrates together with vanadate stimulated the recruitment of GLUT4 to the cell surface in isolated rat adipocytes. Benzylamine plus vanadate also stimulated glucose transport and GLUT4 translocation in 3T3-L1 adipocytes. Benzylamine or tyramine in combination with vanadate potently stimulated the tyrosine phosphorylation of both insulin receptor substrate (IRS)-1 and IRS-3. In contrast, benzylamine and vanadate caused only a weak stimulation of insulin receptor kinase. Benzylamine or tyramine in combination with vanadate also stimulated phosphoinositide 3-kinase activity; wortmannin abolished the stimulatory effect of benzylamine and vanadate on glucose transport in adipose cells. Furthermore, the administration of benzylamine and vanadate in vivo caused a rapid lowering of plasma glucose levels, which took place in the absence of alterations in plasma insulin. On the basis of these results we propose that SSAO activity regulates glucose transport in adipocytes. SSAO oxidative activity stimulates glucose transport via the translocation of GLUT4 carriers to the cell surface, resulting from a potent tyrosine phosphorylation of IRS-1 and IRS-3 and phosphoinositide 3-kinase activation. Our results also indicate that substrates of SSAO might regulate glucose disposal in vivo.

Contraction inhibits insulin-stimulated insulin receptor substrate-1/2-associated phosphoinositide 3-kinase activity, but not protein kinase B activation or glucose uptake, in rat muscle

The initial stages of insulin-stimulated glucose uptake are thought to involve tyrosine phosphorylation of insulin receptor substrates (IRSs), which recruit and activate phosphoinositide 3-kinase (PI 3-kinase), leading to the activation of protein kinase B (PKB) and other downstream effectors. In contrast, contraction stimulates glucose uptake via a PI 3-kinase-independent mechanism. The combined effects of insulin and contraction on glucose uptake are additive. However, it has been reported that contraction causes a decrease in insulin-stimulated IRS-1-associated PI 3-kinase activity. To investigate this paradox, we have examined the effects of contraction on insulin-stimulated glucose uptake and proximal insulin-signalling events in isolated rat epitrochlearis muscle. Stimulation by insulin or contraction produced a 3-fold increase in glucose uptake, with the effects of simultaneous treatment by insulin and contraction being additive. Wortmannin completely blocked the additive effect of insulin in contracting skeletal muscle, indicating that this is a PI 3-kinase-dependent effect. Insulin-stimulated recruitment of PI 3-kinase to IRS-1 was unaffected by contraction; however, insulin produced no discernible increase in PI 3-kinase activity in IRS-1 or IRS-2 immunocomplexes in contracting skeletal muscle. Consistent with this, contraction inhibited insulin-stimulated p70(S6K) activation. In contrast, insulin-stimulated activation of PKB was unaffected by contraction. Thus, in contracting skeletal muscle, insulin stimulates glucose uptake and activates PKB, but not p70(S6K), by a PI 3-kinase-dependent mechanism that is independent of changes in IRS-1- and IRS-2-associated PI 3-kinase activity.

Insulin receptor substrate polymorphisms and type 2 diabetes mellitus

Insulin receptor substrate (IRS) molecules are key mediators in insulin signalling and play a central role in maintaining basic cellular functions, such as growth, survival and metabolism. They act as docking proteins for the insulin receptor and a complex network of intracellular signalling molecules containing Src homology 2 (SH2) domains. Four members (IRS-1, IRS-2, IRS-3 and IRS-4) of this family have been identified that differ in tissue distribution, subcellular localisation, developmental expression, binding to the insulin receptor and interaction with SH2 domain-containing proteins. Results from targeted disruption of the IRS genes in mice have provided important clues as to the functional differences among these related molecules and suggest that they play very different roles in vivo. The available data are consistent with the notion that both IRS-1 and IRS-2 are important for insulin action and glucose homeostasis in vivo, whereas IRS-and IRS-4 appear to play a redundant role in the IRS signalling system. Considering their key role in both insulin action and insulin secretion, IRS-1 and IRS-2 molecules have been considered plausible candidate genes involved in the pathogenesis of Type 2 diabetes. Several polymorphisms in the IRS genes have been identified, but only the Gly --> Arg72 substitution of IRS-1, acting with environmental factors, seems to have a pathogenic role in the development of Type 2 diabetes. In contrast, polymorphisms of the other IRS genes do not appear to contribute to Type 2 diabetes.

Muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase in human skeletal muscle

Physiological stress associated with muscle damage results in systemic insulin resistance. However, the mechanisms responsible for the insulin resistance are not known; therefore, the present study was conducted to elucidate the molecular mechanisms associated with insulin resistance after muscle damage. Muscle biopsies were obtained before (base) and at 1 h during a hyperinsulinemic-euglycemic clamp (40 mU x kg(-1) x min(-1)) in eight young (age 24+/-1 yr) healthy sedentary (maximal O(2) consumption, 49.7+/-2.4 ml x kg(-1) x min(-1)) males before and 24 h after eccentric exercise (ECC)-induced muscle damage. To determine the role of cytokines in ECC-induced insulin resistance, venous blood samples were obtained before (control) and 24 h after ECC to evaluate ex vivo endotoxin-induced mononuclear cell secretion of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and IL-1beta. Glucose disposal was 19% lower after ECC (P<0.05). Insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation was 45% lower after ECC (P<0.05). Insulin-stimulated phosphatidylinositol (PI) 3-kinase, Akt (protein kinase B) serine phosphorylation, and Akt activity were reduced 34, 65, and 20%, respectively, after ECC (P < 0.05). TNF-alpha, but not IL-6 or IL-1beta production, increased 2.4-fold 24 h after ECC (P<0.05). TNF-alpha production was positively correlated with reduced insulin action on PI 3-kinase (r = 0.77, P = 0.04). In summary, the physiological stress associated with muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase, presumably leading to decreased insulin-mediated glucose uptake. Although more research is needed on the potential role for TNF-alpha inhibition of insulin action, elevated TNF-alpha production after muscle damage may impair insulin signal transduction.

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.

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.

Ceramide generation is sufficient to account for the inhibition of the insulin-stimulated PKB pathway in C2C12 skeletal muscle cells pretreated with palmitate

We have employed C2C12 myotubes to investigate lipid inhibition of insulin-stimulated signal transduction and glucose metabolism. Cells were preincubated for 18 h in the absence or presence of free fatty acids (FFAs) and stimulated with insulin, and the effects on glycogen synthesis and signaling intermediates were determined. While the unsaturated FFAs oleate and linoleate inhibited both basal and insulin-stimulated glycogen synthesis, the saturated FFA palmitate reduced only insulin-stimulated glycogen synthesis, and was found to inhibit insulin-stimulated phosphorylation of glycogen synthase kinase-3 and protein kinase B (PKB). However, no effect of palmitate was observed on tyrosine phosphorylation, p85 association, or phosphatidylinositol 3-kinase activity in IRS-1 immunoprecipitates. In contrast, palmitate promoted phosphorylation of mitogen-activated protein MAP) kinases. Ceramide, a derivative of palmitate, has recently been associated with similar inhibition of PKB, and here, ceramide levels were found to be elevated 2-fold in palmitate-treated C2C12 cells. Incubation of C2C12 cells with ceramide closely reproduced the effects of palmitate, leading to inhibition of glycogen synthesis and PKB and to stimulation of MAP kinase. We conclude that palmitate-induced insulin resistance occurs by a mechanism distinct from that of unsaturated FFAs, and involves elevation of ceramide by de novo synthesis, leading to PKB inhibition without affecting IRS-1 function.

p85/p110-type phosphatidylinositol kinase phosphorylates not only the D-3, but also the D-4 position of the inositol ring

Activation of p85/p110-type phosphatidylinositol (PI) kinase has been implicated in various cellular activities. This PI kinase phosphorylates the D-4 position with a similar or higher efficiency than the D-3 position when trichloroacetic acid-treated cell membrane is used as a substrate, although it phosphorylates almost exclusively the D-3 position of the inositol ring in phosphoinositides when purified PI is used as a substrate. Furthermore, the lipid kinase activities of p110 for both the D-3 and D-4 positions were completely abolished by introducing kinase-dead point mutations in their lipid kinase domains (DeltaKinalpha and DeltaKinbeta, respectively). In addition, both PI 3- and PI 4-kinase activities of p110alpha and p110beta immunoprecipitates were similarly inhibited by either wortmannin or LY294002, specific inhibitors of p110. Insulin induced phosphorylation of not only the D-3 position, but also the D-4 position. Indeed, overexpression of p110 in Sf9 or 3T3-L1 cells induced marked phosphorylation of the D-4 position to a level comparable to or much greater than that of D-3, whereas inhibition of endogenous p85/p110-type PI kinase via overexpression of dominant-negative p85alpha (Deltap85alpha) in 3T3-L1 adipocytes abolished insulin-induced synthesis of both. Thus, p85/p110-type PI kinase phosphorylates the D-4 position of phosphoinositides more efficiently than the D-3 position in vivo, and each of the D-3- or D-4-phosphorylated phosphoinositides may transmit signals downstream.