Membrane localization of 3-phosphoinositide-dependent protein kinase-1 stimulates activities of Akt and atypical protein kinase C but does not stimulate glucose transport and glycogen synthesis in 3T3-L1 adipocytes

Insulin-like growth factor 1 promotes the gonadal development of Pampus argenteus by regulating energy metabolism†

Insulin-like growth factor 1 (Igf1) is known to promote ovarian maturation by interacting with other hormones. However, the limited research on the role of Igf1 in the energy metabolism supply of gonads has hindered further exploration. To explore the role of Igf1 in gonadal development of silver pomfret, we analyzed the expression levels and the localization of igf1 mRNA and protein during testicular and ovarian development of silver pomfret. The results of the study showed upregulation of Igf1 in the critical period of vitellogenesis and sperm meiosis, which was found to be mainly expressed in the somatic cells of the gonads. Upon adding E2 and Igf1 to cultured gonadal tissues, the expression of energy-related genes was significantly increased, along with the E2-enhanced effect of Igf1 in the testis. Importantly, stimulation of both ovaries and testes with E2 and Igf1 led to a remarkable increase in the expression of vitellogenesis and meiosis-related genes. Therefore, we conclude that Igf1 promotes vitellogenesis and sperm meiosis by regulating gonadal energy production. Moreover, the expression of Igf1 in gonads is significantly regulated by E2. These findings provide new insights for the research of Igf1 in fish breeding, thus allowing the regulation of energy metabolism between growth and reproduction for successful reproductive outcomes.

O-linked N-acetylglucosamine modification is essential for physiological adipose expansion induced by high-fat feeding

Adipose tissues accumulate excess energy as fat and heavily influence metabolic homeostasis. O-linked N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation), which involves the addition of N-acetylglucosamine to proteins by O-GlcNAc transferase (Ogt), modulates multiple cellular processes. However, little is known about the role of O-GlcNAcylation in adipose tissues during body weight gain due to overnutrition. Here, we report on O-GlcNAcylation in mice with high-fat diet (HFD)-induced obesity. Mice with knockout of Ogt in adipose tissue achieved using adiponectin promoter-driven Cre recombinase (Ogt-FKO) gained less body weight than control mice under HFD. Surprisingly, Ogt-FKO mice exhibited glucose intolerance and insulin resistance, despite their reduced body weight gain, as well as decreased expression of de novo lipogenesis genes and increased expression of inflammatory genes, resulting in fibrosis at 24 weeks of age. Primary cultured adipocytes derived from Ogt-FKO mice showed decreased lipid accumulation. Both primary cultured adipocytes and 3T3-L1 adipocytes treated with OGT inhibitor showed increased secretion of free fatty acids. Medium derived from these adipocytes stimulated inflammatory genes in RAW 264.7 macrophages, suggesting that cell-to-cell communication via free fatty acids might be a cause of adipose inflammation in Ogt-FKO mice. In conclusion, O-GlcNAcylation is important for healthy adipose expansion in mice. Glucose flux into adipose tissues may be a signal to store excess energy as fat.NEW & NOTEWORTHY We evaluated the role of O-GlcNAcylation in adipose tissue in diet-induced obesity using adipose tissue-specific Ogt knockout mice. We found that O-GlcNAcylation in adipose tissue is essential for healthy fat expansion and that Ogt-FKO mice exhibit severe fibrosis upon long-term overnutrition. O-GlcNAcylation in adipose tissue may regulate de novo lipogenesis and free fatty acid efflux to the degree of overnutrition. We believe that these results provide new insights into adipose tissue physiology and obesity research.

Trilobatin ameliorates insulin resistance through IRS-AKT-GLUT4 signaling pathway in C2C12 myotubes and ob/ob mice

Background

Trilobatin, a natural compound, has been found to exhibit anti-diabetic properties in high-fat diet (HFD) and streptozotocin (STZ) induced type 2 diabetic mice. But up to now no research has been reported on the effect of trilobatin on insulin resistance in peripheral tissues. Herein, we determined the effects of trilobatin on insulin resistance in palmitate-treated C2C12 myotubes and ob/ob mice.

Methods

Male ob/ob mice (8-10 weeks) and same background C57BL/6 mice were used to evaluate the role of trilobatin on insulin resistance; protein expression and phosphorylation were measured by western blot; glucose uptake was determined a fluorescent test.

Results

Treatment with trilobatin prevented palmitate-induced insulin resistance by enhancing glucose uptake and the phosphorylation of insulin resistance substrate 1 (IRS1) and protein Kinase B, (PKB/AKT), recovered the translocation of GLUT4 from cytoplasm to membrane, but preincubation with LY294002, an inhibitor of PI3K, blocked the effects of trilobatin on glucose uptake and the distribution of GLUT4 in C2C12 myotubes. Furthermore, administration with trilobatin for 4 weeks significantly improved insulin resistance by decreasing fasting blood glucose and insulin in serum, enhancing the phosphorylation of IRS1 and AKT, and recovering the expression and translocation of GLUT4 in ob/ob mice.

Conclusions

IRS-AKT-GLUT4 signaling pathway might be involved in trilobatin ameliorating insulin resistance in skeletal muscle of obese animal models.

Emerging Roles for MicroRNAs in Diabetic Microvascular Disease: Novel Targets for Therapy

Chronic, low-grade systemic inflammation and impaired microvascular function are critical hallmarks in the development of insulin resistance. Accordingly, insulin resistance is a major risk factor for type 2 diabetes and cardiovascular disease. Accumulating studies demonstrate that restoration of impaired function of the diabetic macro- and microvasculature may ameliorate a range of cardiovascular disease states and diabetes-associated complications. In this review, we focus on the emerging role of microRNAs (miRNAs), noncoding RNAs that fine-tune target gene expression and signaling pathways, in insulin-responsive tissues and cell types important for maintaining optimal vascular homeostasis and preventing the sequelae of diabetes-induced end organ injury. We highlight current pathophysiological paradigms of miRNAs and their targets involved in regulating the diabetic microvasculature in a range of diabetes-associated complications such as retinopathy, nephropathy, wound healing, and myocardial injury. We provide an update of the potential use of circulating miRNAs diagnostically in type I or type II diabetes. Finally, we discuss emerging delivery platforms for manipulating miRNA expression or function as the next frontier in therapeutic intervention to improve diabetes-associated microvascular dysfunction and its attendant clinical consequences.

Insulin-Sensitive Protein Kinases (Atypical Protein Kinase C and Protein Kinase B/Akt): Actions and Defects in Obesity and Type II Diabetes

Glucose transport into muscle is the initial process in glucose clearance and is uniformly defective in insulin-resistant conditions of obesity, metabolic syndrome, and Type II diabetes mellitus. Insulin regulates glucose transport by activating insulin receptor substrate-1 (IRS-1)-dependent phosphatidylinositol 3-kinase (PI3K) which, via increases in PI-3, 4, 5-triphosphate (PIP3), activates atypical protein kinase C (aPKC) and protein kinase B (PKB/Akt). Here, we review (i) the evidence that both aPKC and PKB are required for insulin-stimulated glucose transport, (ii) abnormalities in muscle aPKC/PKB activation seen in obesity and diabetes, and (iii) mechanisms for impaired aPKC activation in insulin-resistant conditions. In most cases, defective muscle aPKC/PKB activation reflects both impaired activation of IRS-1/PI3K, the upstream activator of aPKC and PKB in muscle and, in the case of aPKC, poor responsiveness to PIP3, the lipid product of PI3K. Interestingly, insulin-sensitizing agents (e.g., thiazolidinediones, metformin) improve aPKC activation by insulin in vivo and PIP3 in vitro, most likely by activating 5′-adenosine monophosphate-activated protein kinase, which favorably alters intracellular lipid metabolism. Differently from muscle, aPKC activation in the liver is dependent on IRS-2/PI3K rather than IRS-1/PI3K and, surprisingly, the activation of IRS-2/PI3K and aPKC is conserved in high-fat feeding, obesity, and diabetes. This conservation has important implications, as continued activation of hepatic aPKC in hyperinsulinemic states may increase the expression of sterol regulatory element binding protein-1c, which controls genes that increase hepatic lipid synthesis. On the other hand, the defective activation of IRS-1/PI3K and PKB, as seen in diabetic liver, undoubtedly and importantly contributes to increases in hepatic glucose output. Thus, the divergent activation of aPKC and PKB in the liver may explain why some hepatic actions of insulin (e.g., aPKC-dependent lipid synthesis) are increased while other actions (e.g., PKB-dependent glucose metabolism) are diminished. This may explain the paradox that the liver secretes excessive amounts of both very low density lipoprotein triglycerides and glucose in Type II diabetes. Previous reviews from our laboratory that have appeared in the Proceedings have provided essentials on phospholipid-signaling mechanisms used by insulin to activate several protein kinases that seem to be important in mediating the metabolic effects of insulin. During recent years, there have been many new advances in our understanding of how these lipid-dependent protein kinases function during insulin action and why they fail to function in states of insulin resistance. The present review will attempt to summarize what we believe are some of the more important advances.

The regulation of reproductive neuroendocrine function by insulin and insulin-like growth factor-1 (IGF-1)

The mammalian reproductive hormone axis regulates gonadal steroid hormone levels and gonadal function essential for reproduction. The neuroendocrine control of the axis integrates signals from a wide array of inputs. The regulatory pathways important for mediating these inputs have been the subject of numerous studies. One class of proteins that have been shown to mediate metabolic and growth signals to the CNS includes Insulin and IGF-1. These proteins are structurally related and can exert endocrine and growth factor like action via related receptor tyrosine kinases. The role that insulin and IGF-1 play in controlling the hypothalamus and pituitary and their role in regulating puberty and nutritional control of reproduction has been studied extensively. This review summarizes the in vitro and in vivo models that have been used to study these neuroendocrine structures and the influence of these growth factors on neuroendocrine control of reproduction.

From pluripotency to islets: miRNAs as critical regulators of human cellular differentiation

MicroRNAs (miRNAs) actively regulate differentiation as pluripotent cells become cells of pancreatic endocrine lineage, including insulin-producing β cells. The process is dynamic; some miRNAs help maintain pluripotency, while others drive cell fate decisions. Here, we survey the current literature and describe the biological role of selected miRNAs in maintenance of both mouse and human embryonic stem cell (ESC) pluripotency. Subsequently, we review the increasing evidence that miRNAs act at selected points in differentiation to regulate decisions about early cell fate (definitive endoderm and mesoderm), formation of pancreatic precursor cells, endocrine cell function, as well as epithelial to mesenchymal transition.

The Feynman trajectories: determining the path of a protein using fixed-endpoint assays

Richard Feynman postulated in 1948 that the path of an electron can be best described by the sum or functional integral of all possible trajectories rather than by the notion of a single, unique trajectory. As a consequence, the position of an electron does not harbor any information about the paths that contributed to this position. This observation constitutes a classical endpoint observation. The endpoint assay is the desired type of experiment for high-throughput screening applications, mainly because of limitations in data acquisition and handling. Quite contrary to electrons, it is possible to extract information about the path of a protein using endpoint assays, and these types of applications are reviewed in this article.

Role of PKCzeta translocation in the development of type 2 diabetes in rats following continuous glucose infusion

AIM

We investigated the molecular mechanisms of hyperglycaemia-induced insulin resistance and type 2 diabetes in rats receiving a continuous glucose infusion (GI).

METHODS

Female Wistar rats were infused with either 2.8 mol/L glucose or saline (2 mL/h) for durations varying from 0 to 15 days. Blood samples were analysed daily to determine glucose and insulin dynamics. Subsets of animals were sacrificed and soleus muscles were extracted for determination of protein expression, subcellular location, and activities of insulin-signalling proteins.

RESULTS

Rats accommodated this systemic glucose oversupply and developed insulin resistance on day 5 (normoglycaemia/hyperinsulinaemia) and type 2 diabetes on day 15 (hyperglycaemia/normoinsulinaemia). The effect of GI on protein kinase Czeta (PKCzeta) activity was independent of changes in phosphatidylinositol 3-kinase activity, and occurred in parallel with an increase in PDK1 activity. Activated PKCzeta was mainly located in the cytosol after 5 days of GI that was coincident with the translocation of GLUT4 to the plasma membrane, and normoglycaemia. After 15 days of GI, PKCzeta translocated from the cytosol to the plasma membrane with a concomitant decrease in PDK1 activity. This caused an increase in the association between PKCzeta and PKB and a decrease in PDK1-PKB reactions at the plasma membrane, leading to reduced PKB activity. The activity of PKCzeta per se was also compromised. The PKCzeta and PKB activity reduction and the blunted insulin-stimulated GLUT4 translocation eventually led to hyperglycaemia and diabetes.

CONCLUSION

Translocation of PKCzeta may play a central role in the development of type 2 diabetes.

Myosin 5a is an insulin-stimulated Akt2 (protein kinase Bbeta) substrate modulating GLUT4 vesicle translocation

Phosphatidylinositol 3-kinase activation of Akt signaling is critical to insulin-stimulated glucose transport and GLUT4 translocation. However, the downstream signaling events following Akt activation which mediate glucose transport stimulation remain relatively unknown. Here we identify an Akt consensus phosphorylation motif in the actin-based motor protein myosin 5a and show that insulin stimulation leads to phosphorylation of myosin 5a at serine 1650. This Akt-mediated phosphorylation event enhances the ability of myosin 5a to interact with the actin cytoskeleton. Small interfering RNA-induced inhibition of myosin 5a and expression of dominant-negative myosin 5a attenuate insulin-stimulated glucose transport and GLUT4 translocation. Furthermore, knockdown of Akt2 or expression of dominant-negative Akt (DN-Akt) abolished insulin-stimulated phosphorylation of myosin 5a, inhibited myosin 5a binding to actin, and blocked insulin-stimulated glucose transport. Taken together, these data indicate that myosin 5a is a newly identified direct substrate of Akt2 and, upon insulin stimulation, phosphorylated myosin 5a facilitates anterograde movement of GLUT4 vesicles along actin to the cell surface.

Role of SGK1 kinase in regulating glucose transport via glucose transporter GLUT4

Insulin stimulates glucose transport into muscle and fat cells by enhancing GLUT4 abundance in the plasma membrane through activation of phosphatidylinositol 3-kinase (PI3K). Protein kinase B (PKB) and PKCzeta are known PI3K downstream targets in the regulation of GLUT4. The serum- and glucocorticoid-inducible kinase SGK1 is similarly activated by insulin and capable to regulate cell surface expression of several metabolite transporters. In this study, we evaluated the putative role of SGK1 in the modulation of GLUT4. Coexpression of the kinase along with GLUT4 in Xenopus oocytes stimulated glucose transport. The enhanced GLUT4 activity was paralleled by increased transporter abundance in the plasma membrane. Disruption of the SGK1 phosphorylation site on GLUT4 ((S274A)GLUT4) abrogated the stimulating effect of SGK1. In summary, SGK1 promotes glucose transporter membrane abundance via GLUT4 phosphorylation at Ser274. Thus, SGK1 may contribute to the insulin and GLUT4-dependent regulation of cellular glucose uptake.

Essential role of PDK1 in regulating endothelial cell migration

The serine/threonine protein kinase phosphoinositide-dependent kinase 1 (PDK1) plays a central role in cellular signaling by phosphorylating members of the AGC family of kinases, including PKB/Akt. We now present evidence showing that PDK1 is essential for the motility of vascular endothelial cells (ECs) and that it is involved in the regulation of their chemotaxis. ECs differentiated from mouse embryonic stem cells lacking PDK1 completely lost their ability to migrate in vitro in response to vascular endothelial growth factor-A (VEGF-A). In addition, PDK1^−/− embryoid bodies exhibit evident developmental and vascular defects that can be attributed to a reduced cell migration. Moreover, the overexpression of PDK1 increased the EC migration induced by VEGF-A. We propose a model of spatial distribution of PDK1 and Akt in which the synthesis of phosphatidylinositol 3,4,5 triphosphate at plasma membrane by activation of phosphoinositide 3-kinase recruits both proteins at the leading edge of the polarized ECs and promotes cell chemotaxis. These findings establish a mechanism for the spatial localization of PDK1 and its substrate Akt to regulate directional migration.

Multiple genes and factors associated with bipolar disorder converge on growth factor and stress activated kinase pathways controlling translation initiation: implications for oligodendrocyte viability

Famine and viral infection, as well as interferon therapy have been reported to increase the risk of developing bipolar disorder. In addition, almost 100 polymorphic genes have been associated with this disease. Several form most of the components of a phosphatidyl-inositol signalling/AKT1 survival pathway (PIK3C3, PIP5K2A, PLCG1, SYNJ1, IMPA2, AKT1, GSK3B, TCF4) which is activated by growth factors (BDNF, NRG1) and also by NMDA receptors (GRIN1, GRIN2A, GRIN2B). Various other protein products of genes associated with bipolar disorder either bind to or are affected by phosphatidyl-inositol phosphate products of this pathway (ADBRK2, HIP1R, KCNQ2, RGS4, WFS1), are associated with its constituent elements (BCR, DUSP6, FAT, GNAZ) or are downstream targets of this signalling cascade (DPYSL2, DRD3, GAD1, G6PD, GCH1, KCNQ2, NOS3, SLC6A3, SLC6A4, SST, TH, TIMELESS). A further pathway relates to endoplasmic reticulum-stress (HSPA5, XBP1), caused by problems in protein glycosylation (ALG9), growth factor receptor sorting (PIK3C3, HIP1R, SYBL1), or aberrant calcium homoeostasis (WFS1). Key processes relating to these pathways appear to be under circadian control (ARNTL, CLOCK, PER3, TIMELESS). DISC1 can also be linked to many of these pathways. The growth factor pathway promotes protein synthesis, while the endoplasmic reticulum stress pathway, and other stress pathways activated by viruses and cytokines (IL1B, TNF, Interferons), oxidative stress or starvation, all factors associated with bipolar disorder risk, shuts down protein synthesis via control of the EIF2 alpha and beta translation initiation complex. For unknown reasons, oligodendrocytes appear to be particularly prone to defects in the translation initiation complex (EIF2B) and the convergence of these environmental and genomic signalling pathways on this area might well explain their vulnerability in bipolar disorder.

Insulin receptor signals regulating GLUT4 translocation and actin dynamics

In skeletal muscle and adipose tissue, insulin-stimulated glucose uptake is dependent upon translocation of the insulin-responsive glucose transporter GLUT4 from intracellular storage compartments to the plasma membrane. This insulin-induced redistribution of GLUT4 protein is achieved through a series of highly organized membrane trafficking events, orchestrated by insulin receptor signals. Recently, several key molecules linking insulin receptor signals and membrane trafficking have been identified, and emerging evidence supports the importance of subcellular compartmentalization of signaling components at the right time and in the right place. In addition, the translocation of GLUT4 in adipocytes requires insulin stimulation of dynamic actin remodeling at the inner surface of the plasma membrane (cortical actin) and in the perinuclear region. This results from at least two independent insulin receptor signals, one leading to the activation of phosphatidylinositol (PI) 3-kinase and the other to the activation of the Rho family small GTP-binding protein TC10. Thus, both spatial and temporal regulations of actin dynamics, both beneath the plasma membrane and around endomembranes, by insulin receptor signals are also involved in the process of GLUT4 translocation.

The transcription factor AP-2beta causes cell enlargement and insulin resistance in 3T3-L1 adipocytes

We have reported the association of variations in the activating protein-2beta (AP-2beta) transcription factor gene with type 2 diabetes. This gene was preferentially expressed in 3T3-L1 adipocytes in a differentiation stage-dependent manner, and preliminary experiments showed that subjects with the disease-susceptible allele showed stronger expression in adipose tissue than those without the susceptible allele. Thus, we overexpressed the AP-2beta gene in 3T3-L1 adipocytes to clarify whether AP-2beta might play a crucial role in the pathogenesis of type 2 diabetes through dysregulation of adipocyte function. In cells overexpressing AP-2beta, cells increased in size by accumulation of triglycerides accompanied by enhanced glucose uptake. On the contrary, suppression of AP-2beta expression by small interfering RNA inhibited glucose uptake. Enhancement of glucose uptake by AP-2beta overexpression was attenuated by inhibitors of phospholipase C (PLC) and atypical protein kinase Czeta/lambda (PKCzeta/lambda), but not by a phosphatidylinositol 3-kinase (PI3-K) inhibitor. Consistently, we found activation of PLC and atypical PKC, but not PI3-K, by AP-2beta expression. Furthermore, overexpression of PLCgamma enhanced glucose uptake, and this activation was inhibited by an atypical PKC inhibitor, suggesting that the enhanced glucose uptake may be mediated through PLC and atypical PKCzeta/lambda, but not PI3-K. Moreover, we observed the increased tyrosine phosphorylation of Grb2-associated binder-1 (Gab1) and its association with PLCgamma, indicating that Gab1 may be involved in AP-2beta-induced PLCgamma activation. Finally, AP-2beta overexpression was found to relate to the impaired insulin signaling. We propose that AP-2beta is a candidate gene for producing adipocyte hypertrophy and may relate to the abnormal characteristics of adipocytes observed in obesity.

Subcellular localization of glucose transporter 4 in the hypothalamic arcuate nucleus of ob/ob mice under basal conditions

Glucose transporter (GLUT) 4 plays an important role in insulin-induced glucose uptake in skeletal muscle and white adipose tissue. Although GLUT4 is abundant in the hypothalamus as well as in these peripheral tissues, little is known about the role of GLUT4 in the hypothalamus. In this study, we examined the subcellular localization of GLUT4 and the activation of insulin signaling pathways in the hypothalamic arcuate nucleus of ob/ob mice under basal conditions. The expression of GLUT4 in the arcuate nucleus of ob/ob mice was higher than that in lean mice. Interestingly, GLUT4 on the plasma membrane increased significantly in neurons of the arcuate nucleus of ob/ob mice when compared to that in lean mice. Because serum insulin levels of ob/ob mice were very high, we hypothesized that insulin strongly stimulates GLUT4 translocation in the arcuate nucleus of ob/ob mice. Unexpectedly, tyrosine phosphorylation of IR and insulin receptor substrate-1 (IRS-1) was faint in the hypothalamus of lean and ob/ob mice. In addition, phosphorylation of IRS-1 at Ser307 in the hypothalamus of ob/ob mice was higher when compared to that in lean mice, suggesting that insulin signaling is impaired by phosphorylation of IRS-1 at Ser307 in the hypothalamus of ob/ob mice. However, serine phosphorylation of Akt in the arcuate nucleus of ob/ob mice increased significantly when compared to that in lean mice. Furthermore, the expression of brain-derived neurotrophic factor, an activator of PI3K-Akt pathway in neurons, increased significantly in the ventromedial hypothalamus of ob/ob mice. We discuss the possibility of novel pathways which induce the translocation of GLUT4 in the arcuate nucleus of ob/ob mice.

Insulin regulation of GnRH gene expression through MAP kinase signaling pathways

In mammals, reproduction is acutely regulated by metabolic status. Insulin is an important nutritional signal from the periphery that may regulate the reproductive axis. To determine whether insulin acts directly on the GnRH neuron, we performed studies in mouse-derived GnRH-expressing cell lines. Both insulin receptor protein and mRNA were detected in these cells. A saturation radioligand binding assay revealed high affinity, low capacity binding sites for insulin in GnRH neurons. Insulin also stimulated GnRH promoter activity in GnRH neurons. This effect was blocked by pretreatment with the MEK inhibitor, PD98059, indicating a role for MAP kinase signaling. In transient transfection studies, insulin treatment stimulated expression of a 1250 bp mouse GnRH gene promoter fragment four-fold when compared to promoter activity in untreated cells. In contrast, insulin did not stimulate activity of a 587 bp fragment of the mGnRH gene promoter, indicating that the promoter elements mediating insulin stimulation of the GnRH promoter are located between -1250 and -587 bp. Our studies suggest that insulin may regulate reproductive function by direct effects on the GnRH neurons and specifically by stimulating GnRH gene expression.

Role of the PDK1-PKB-GSK3 pathway in regulating glycogen synthase and glucose uptake in the heart

In order to investigate the importance of the PDK1-PKB-GSK3 signalling network in regulating glycogen synthase (GS) in the heart, we have employed tissue specific conditional knockout mice lacking PDK1 in muscle (mPDK1-/-), as well as knockin mice in which the protein kinase B (PKB) phosphorylation site on glycogen synthase kinase-3alpha (GSK3alpha) (Ser21) and GSK3beta (Ser9) is changed to Ala. We demonstrate that in hearts from mPDK1-/- or double GSK3alpha/GSK3beta knockin mice, insulin failed to stimulate the activity of GS or induce its dephosphorylation at residues that are phosphorylated by GSK3. We also establish that in the heart, both GSK3 isoforms participate in the regulation of GS, with GSK3beta playing a more prominent role. This contrasts with skeletal muscle where GSK3beta is the major regulator of insulin-induced GS activity. Despite the inability of insulin to stimulate glycogen synthesis in hearts from the mPDK1-/- or double GSK3alpha/GSK3beta knockin mice, these animals possessed normal levels of cardiac glycogen, demonstrating that total glycogen levels are regulated independently of insulin's ability to stimulate GS in the heart and that mechanisms such as allosteric activation of GS by glucose-6-phosphate and/or activation of GS by muscle contraction, could operate to maintain normal glycogen levels in these mice. We also demonstrate that in cardiomyocytes derived from the mPDK1-/- hearts, although the levels of glucose transporter type 4 (GLUT4) are increased 2-fold, insulin failed to stimulate glucose uptake, providing genetic evidence that PDK1 plays a crucial role in enabling insulin to promote glucose uptake in cardiac muscle.

Protein phosphatase 2A negatively regulates insulin's metabolic signaling pathway by inhibiting Akt (protein kinase B) activity in 3T3-L1 adipocytes

Protein phosphatase 2A (PP2A) is a multimeric serine/threonine phosphatase which has multiple functions, including inhibition of the mitogen-activated protein (MAP) kinase pathway. Simian virus 40 small t antigen specifically inhibits PP2A function by binding to the PP2A regulatory subunit, interfering with the ability of PP2A to associate with its cellular substrates. We have reported that the expression of small t antigen inhibits PP2A association with Shc, leading to augmentation of insulin and epidermal growth factor-induced Shc phosphorylation with enhanced activation of the Ras/MAP kinase pathway. However, the potential involvement of PP2A in insulin's metabolic signaling pathway is presently unknown. To assess this, we overexpressed small t antigen in 3T3-L1 adipocytes by adenovirus-mediated gene transfer and found that the phosphorylation of Akt and its downstream target, glycogen synthase kinase 3beta, were enhanced both in the absence and in the presence of insulin. Furthermore, protein kinase C lambda (PKC lambda) activity was also augmented in small-t-antigen-expressing 3T3-L1 adipocytes. Consistent with this result, both basal and insulin-stimulated glucose uptake were enhanced in these cells. In support of this result, when inhibitory anti-PP2A antibody was microinjected into 3T3-L1 adipocytes, we found a twofold increase in GLUT4 translocation in the absence of insulin. The small-t-antigen-induced increase in Akt and PKC lambda activities was not inhibited by wortmannin, while the ability of small t antigen to enhance glucose transport was inhibited by dominant negative Akt (DN-Akt) expression and Akt small interfering RNA (siRNA) but not by DN-PKC lambda expression or PKC lambda siRNA. We conclude that PP2A is a negative regulator of insulin's metabolic signaling pathway by promoting dephosphorylation and inactivation of Akt and PKC lambda and that most of the effects of PP2A to inhibit glucose transport are mediated through Akt.

Phophatidylinositol-3 Kinase/Mammalian Target of Rapamycin/p70S6KRegulates Contractile Protein Accumulation in Airway Myocyte Differentiation

Increased airway smooth muscle in airway remodeling results from myocyte proliferation and hypertrophy. Skeletal and vascular smooth muscle hypertrophy is induced by phosphatidylinositide-3 kinase (PI(3) kinase) via mammalian target of rapamycin (mTOR) and p70S6 kinase (p70S6K). We tested the hypothesis that this pathway regulates contractile protein accumulation in cultured canine airway myocytes acquiring an elongated contractile phenotype in serum-free culture. In vitro assays revealed a sustained activation of PI(3) kinase and p70S6K during serum deprivation up to 12 d, with concomitant accumulation of SM22 and smooth muscle myosin heavy chain (smMHC) proteins. Immunocytochemistry revealed that activation of PI3K/mTOR/p70S6K occurred almost exclusively in myocytes that acquire the contractile phenotype. Inhibition of PI(3) kinase or mTOR with LY294002 or rapamycin blocked p70S6K activation, prevented formation of large elongated contractile phenotype myocytes, and blocked accumulation of SM22 and smMHC. Inhibition of MEK had no effect. Steady-state mRNA abundance for SM22 and smMHC was unaffected by blocking p70S6K activation. These studies provide primary evidence that PI(3) kinase and mTOR activate p70S6K in airway myocytes leading to the accumulation of contractile apparatus proteins, differentiation, and growth of large, elongated contractile phenotype airway smooth muscle cells.

The adaptor protein Grb14 regulates the localization of 3-phosphoinositide-dependent kinase-1

The metabolic actions of insulin are transduced through the phosphatidylinositol 3-kinase pathway. A critical component of this pathway is 3-phosphoinositide-dependent kinase-1 (PDK-1), a PH domain-containing enzyme that catalyzes the activating phosphorylation for many AGC kinases, including Akt and protein kinase C isozymes. We used a directed proteomics-based approach to identify the adaptor protein Grb14, which binds the insulin receptor through an SH2 domain, as a novel PDK-1 binding partner. Interaction of these two proteins is constitutive and mediated by a PDK-1 binding motif on Grb14. Disruption of this motif by point mutation or deletion of the Grb14 SH2 domain prevents the insulin-triggered membrane translocation of PDK-1. The interaction of PDK-1 with Grb14 facilitates Akt function: disruption of the interaction by overexpression of a construct of Grb14 mutated in the PDK-1 binding motif significantly decreases insulin-dependent activation of Akt. Thus, Grb14 serves as an adaptor protein to recruit PDK-1 to activated insulin receptor, thus promoting Akt phosphorylation and transduction of the insulin signal.

Nelfinavir-induced insulin resistance is associated with impaired plasma membrane recruitment of the PI 3-kinase effectors Akt/PKB and PKC-zeta

AIMS/HYPOTHESIS

Chronic exposure of 3T3-L1 adipocytes to the HIV protease inhibitor nelfinavir induces insulin resistance, recapitulating key metabolic alterations of adipose tissue in the lipodystrophy syndrome induced by these agents. Our goal was to identify the defect in the insulin signal transduction cascade leading to nelfinavir-induced insulin resistance.

METHODS

Fully differentiated 3T3-L1 adipocytes were exposed to 30 micro mol/l nelfinavir for 18 h, after which the amount, the phosphorylation and the localisation of key proteins in the insulin signalling cascade were evaluated.

RESULTS

Insulin-induced interaction of phosphatidylinositol 3'-kinase (PI 3-kinase) with IRS proteins was normal in cells treated with nelfinavir, as was IRS-1-associated PI 3-kinase activity. Yet insulin-induced phosphorylation of Akt/protein kinase B (PKB), p70S6 kinase and extracellular signal-regulated kinase 1/2 was significantly impaired. This could not be attributed to increased protein phosphatase 2A activity or to increased expression of phosphoinositide phosphatases (SHIP2 or PTEN). However, insulin failed to induce translocation of the PI 3-kinase effectors Akt/PKB and protein kinase C-zeta (PKC-zeta) to plasma membrane fractions of nelfinavir-treated adipocytes.

CONCLUSIONS/INTERPRETATION

We therefore conclude that nelfinavir induces a defect in the insulin signalling cascade downstream of the activation of PI 3-kinase. This defect manifests itself by impaired insulin-mediated recruitment of Akt/PKB and PKC-zeta to the plasma membrane.

Protein Phosphatase-2Cα as a Positive Regulator of Insulin Sensitivity through Direct Activation of Phosphatidylinositol 3-Kinase in 3T3-L1 Adipocytes

During differentiation, expression of protein phosphatase-2Calpha (PP2Calpha) is increased in 3T3-L1 adipocytes. To elucidate the role of PP2Calpha in insulin signaling, we overexpressed wild-type (WT) PP2Calpha by adenovirus-mediated gene transfer in 3T3-L1 adipocytes. Overexpression of PP2Calpha-WT enhanced the insulin sensitivity of glucose uptake without any changes in the early steps of insulin signaling. Infection with adenovirus 5 expressing PP2Calpha-WT increased phosphatidylinositol 3-kinase (PI3K) activities in the immunoprecipitate using antibody against the p85 or p110 subunit under both basal and insulin-stimulated conditions, followed by activation of downstream steps in the PI3K pathway, such as phosphorylation of Akt, glycogen synthase kinase-3, and atypical protein kinase C. In contrast, overexpression of the phosphatase-defective mutant PP2Calpha(R174G) did not produce such effects. Furthermore, overexpression of PP2Calpha-WT (but not PP2Calpha(R174G)) decreased the (32)P-labeled phosphorylation state as well as the gel mobility shift of the p85 subunit, suggesting that dephosphorylation of the p85 subunit by PP2Calpha activation might stimulate PI3K catalytic activity. Moreover, knockdown of PP2Calpha by transfection of small interfering RNA led to a significant decrease in Akt phosphorylation. In addition, microinjection of anti-PP2Calpha antibody or PP2Calpha small interfering RNA led to decreased insulin-stimulated GLUT4 translocation. In conclusion, PP2Calpha is a new positive regulator of insulin sensitivity that acts through a direct activation of PI3K in 3T3-L1 adipocytes.

Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Since the discovery of insulin roughly 80 yr ago, much has been learned about how target cells receive, interpret, and respond to this peptide hormone. For example, we now know that insulin activates the tyrosine kinase activity of its cell surface receptor, thereby triggering intracellular signaling cascades that regulate many cellular processes. With respect to glucose homeostasis, these include the function of insulin to suppress hepatic glucose production and to increase glucose uptake in muscle and adipose tissues, the latter resulting from the translocation of the glucose transporter 4 (GLUT4) to the cell surface membrane. Although simple in broad outline, elucidating the molecular intricacies of these receptor-signaling pathways and membrane-trafficking processes continues to challenge the creative ingenuity of scientists, and many questions remain unresolved, or even perhaps unasked. The identification and functional characterization of specific molecules required for both insulin signaling and GLUT4 vesicle trafficking remain key issues in our pursuit of developing specific therapeutic agents to treat and/or prevent this debilitating disease process. To this end, the combined efforts of numerous research groups employing a range of experimental approaches has led to a clearer molecular picture of how insulin regulates the membrane trafficking of GLUT4.

Increased atypical PKC activity in endurance-trained human skeletal muscle

Exercise training may modulate protein content and enzyme activities in skeletal muscle. However, it is not known whether atypical protein kinase C (aPKC) is affected by training. Thus, we investigated aPKC, extracellular-regulated protein kinase 1/2 (ERK 1/2), and P38 mitogen-activated protein kinase (P38 MAPK) activities and expression in skeletal muscle from untrained and endurance-trained subjects at rest and after 20min of cycle exercise (80% of VO(2peak)). Activities of aPKC (P<0.05) and ERK 1/2 (P=0.06), but not phosphorylation of P38 MAPK, were higher in trained than in sedentary subjects at rest. Exercise increased the activities of ERK 1/2 (P<0.01) and aPKC (P<0.05) and the phosphorylation (Thr180/Tyr182) of P38 MAPK (P<0.01) similarly in muscle from trained and sedentary subjects. Protein expression of the kinases was similar in trained and sedentary muscle. The increased aPKC activity in exercise-trained subjects could be important in explaining the enhanced insulin action in these individuals.

Effect of hyperglycemia on signal transduction in skeletal muscle from diabetic Goto-Kakizaki rats

We determined basal and insulin-stimulated responses on signaling intermediates in soleus skeletal muscle from male Wistar and diabetic Goto-Kakizaki (GK) rats. Rats were infused with glucose (5 or 20 mm) for 3 h, followed by a continuous infusion of saline or insulin (3 U/kg.h) for 20 min. Under euglycemic and hyperglycemic conditions, basal and insulin-stimulated action on phosphatidylinositol (PI) 3-kinase, protein kinase B/Akt, and ERK were reduced in GK rats, whereas insulin-stimulated protein kinase C (PKC)zeta activity was not altered. Interestingly, basal PKCzeta activity was increased under hyperglycemic conditions in GK and Wistar rats. This finding of increased PKCzeta activity was confirmed in vitro in isolated soleus muscle exposed to high extracellular glucose, and occurred concomitant with an increase in PI-dependent kinase 1 (PDK-1) activity. The glucose effects were not specific to PKCzeta, because an increase in phosphorylation of PKCalpha/beta and PKCdelta, but not PKCtheta, in isolated soleus muscle exposed to 25 mm glucose was observed. In conclusion, insulin signaling defects in diabetic GK rats are not corrected by an acute normalization of glycemia. Interestingly, acute hyperglycemia leads to a parallel increase in PDK-1, PKCalpha/beta, PKCdelta, and PKCzeta phosphorylation/activity via a PI 3-kinase-protein kinase B/Akt-independent mechanism. The long-term consequence of elevated PDK-1 and PKC phosphorylation/activity should be considered in the context of diabetes mellitus, as hyperglycemia is a clinical feature of this disease.

Requirement for 3-phosphoinositide-kependent dinase-1 (PDK-1) in insulin-induced glucose uptake in immortalized brown adipocytes

To provide insight into the physiological importance of 3-phosphoinositide-dependent kinase-1 (PDK-1) in the metabolic actions of insulin, we have generated mice that harbor a PDK-1 gene containing LoxP sites (PDK-1(lox/lox) mice) and established immortalized brown preadipocyte cell lines both from these animals and from wild-type mice. Exposure to appropriate hormonal inducers resulted in the differentiation of >80% of the immortalized brown preadipocytes derived from both types of mice into mature adipocytes. Introduction of the Cre recombinase with the use of adenovirus-mediated gene transfer induced a dose-dependent decrease in the abundance of PDK-1 in PDK-1(lox/lox) adipocytes but not in the wild-type cells. In Cre-expressing PDK-1(lox/lox) adipocytes in which the abundance of PDK-1 was reduced by approximately 85%, the insulin-induced phosphorylation both of Akt on threonine 308 and of p70 S6 kinase on threonine-389 was markedly inhibited. The phosphorylation both of Akt on serine 473 and of p42 and p44 isoforms of mitogen-activated protein kinase induced by insulin was not affected by Cre expression, indicating that the latter specifically inhibits PDK-1-dependent signaling. Both glucose uptake and the translocation of glucose transporter 4 to the plasma membrane induced by insulin as well as glucose uptake induced by a constitutively active form of phosphoinositide 3-kinase were also greatly inhibited by Cre expression in PDK-1(lox/lox) adipocytes. Phosphorylation of AMP-activated protein kinase and glucose uptake induced by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) were not affected by Cre expression in PDK-1(lox/lox) adipocytes. These results indicate that PDK-1 is essential for insulin-induced glucose uptake in adipocytes.

Lipid Raft targeting of the TC10 amino terminal domain is responsible for disruption of adipocyte cortical actin

Overexpression of the Rho family member TC10alpha, disrupts adipocyte cortical actin structure and inhibits insulin-stimulated GLUT4 translocation when targeted to lipid raft microdomains. This appears to be independent of effecter domain function because overexpression of the wild-type (TC10/WT), constitutively GTP-bound (TC10/Q75L), and constitutively GDP bound (TC10/T31N) all inhibit adipocyte cortical actin structure and GLUT4 translocation. To examine the structural determinants responsible for these effects, we generated a series of chimera proteins between TC10 with that of H-Ras and K-Ras. Chimera containing the 79 (TC10-79/H-Ras), 41 (TC10-41/H-Ras), or 16 (TC10-16/H-Ras) amino acids of the TC10 amino terminal extension fused to H-Ras disrupted cortical actin and inhibited insulin-stimulated GLUT4 translocation. In contrast, the same amino terminal TC10 extensions fused to K-Ras had no significant effect on either GLUT4 translocation or cortical actin structure. Similarly, expression of TC10beta was without effect, whereas fusion of the amino terminal 8 amino acid of TC10alpha onto TC10beta resulted in an inhibition of insulin-stimulated GLUT4 translocation. Within the amino terminal extension point mutation analysis demonstrated that both a GAG and GPG sequences when lipid raft targeted was essential for these effects. Furthermore, expression of the amino terminal TC10 deletions DeltaNT-TC10/WT or DeltaNT-TC10/T31N had no detectable effect on cortical actin organization and did not perturb insulin-stimulated GLUT4 translocation. Surprisingly, however, expression of DeltaNT-TC10/Q75L remained fully capable of inhibiting insulin-stimulated GLUT4 translocation without affecting cortical actin. These data demonstrate that inhibitory effect of TC10 overexpression on adipocyte cortical actin organization is due to the specific lipid raft targeting of the unusual TC10 amino terminal extension.

Use of RNA interference-mediated gene silencing and adenoviral overexpression to elucidate the roles of AKT/protein kinase B isoforms in insulin actions

Insulin plays a central role in the regulation of glucose homeostasis in part by stimulating glucose uptake and glycogen synthesis. The serine/threonine protein kinase Akt has been proposed to mediate insulin signaling in several processes. However, it is unclear whether Akt is involved in insulin-stimulated glucose uptake and which isoforms of Akt are responsible for each insulin action. We confirmed that expression of a constitutively active Akt, using an adenoviral expression vector, promoted translocation of glucose transporter 4 (GLUT4) to plasma membrane, 2-deoxyglucose (2-DG) uptake, and glycogen synthesis in both Chinese hamster ovary cells and 3T3-L1 adipocytes. Inhibition of Akt either by adenoviral expression of a dominant negative Akt or by the introduction of synthetic 21-mer short interference RNA against Akt markedly reduced insulin-stimulated GLUT4 translocation, 2-DG uptake, and glycogen synthesis. Experiments with isoform-specific short interference RNA revealed that Akt2, and Akt1 to a lesser extent, has an essential role in insulin-stimulated GLUT4 translocation and 2-DG uptake in both cell lines, whereas Akt1 and Akt2 contribute equally to insulin-stimulated glycogen synthesis. These data suggest a prerequisite role of Akt in insulin-stimulated glucose uptake and distinct functions among Akt isoforms.

Intracellular segregation of phosphatidylinositol-3,4,5-trisphosphate by insulin-dependent actin remodeling in L6 skeletal muscle cells

Insulin stimulates glucose uptake by recruiting glucose transporter 4 (GLUT4) from an intracellular pool to the cell surface through a mechanism that is dependent on phosphatidylinositol (PI) 3-kinase (PI3-K) and cortical actin remodeling. Here we test the hypothesis that insulin-dependent actin filament remodeling determines the location of insulin signaling molecules. It has been shown previously that insulin treatment of L6 myotubes leads to a rapid rearrangement of actin filaments into submembrane structures where the p85 regulatory subunit of PI3-K and organelles containing GLUT4, VAMP2, and the insulin-regulated aminopeptidase (IRAP) colocalize. We now report that insulin receptor substrate-1 and the p110alpha catalytic subunit of PI3-K (but not p110beta) also colocalize with the actin structures. Akt-1 was also found in the remodeled actin structures, unlike another PI3-K effector, atypical protein kinase C lambda. Transiently transfected green fluorescent protein (GFP)-tagged pleckstrin homology (PH) domains of general receptor for phosphoinositides-1 (GRP1) or Akt (ligands of phosphatidylinositol-3,4,5-trisphosphate [PI-3,4,5-P(3)]) migrated to the periphery of the live cells; in fixed cells, they were detected in the insulin-induced actin structures. These results suggest that PI-3,4,5-P(3) is generated on membranes located within the actin mesh. Actin remodeling and GLUT4 externalization were blocked in cells highly expressing GFP-PH-GRP1, suggesting that PI-3,4,5-P(3) is required for both phenomena. We propose that PI-3,4,5-P(3) leads to actin remodeling, which in turn segregates p85alpha and p110alpha, thus localizing PI-3,4,5-P(3) production on membranes trapped by the actin mesh. Insulin-stimulated actin remodeling may spatially coordinate the localized generation of PI-3,4,5-P(3) and recruitment of Akt, ultimately leading to GLUT4 insertion at the plasma membrane.