Membrane-targeted phosphatidylinositol 3-kinase mimics insulin actions and induces a state of cellular insulin resistance

Signature pattern of gene expression and signaling pathway in premature diabetic patients uncover their correlation to early age coronary heart disease

BACKGROUND

Coronary Heart Disease (CHD) is the leading cause of death in industrialized countries. There is currently no direct relation between CHD and type 2 diabetes mellitus (T2D), one of the major modifiable risk factors for CHD. This study was carried out for genes expression profiling of T2D associated genes to identify related biological processes/es and modulated signaling pathway/s of male subjects with CHD.

METHOD

the subjects were divided into four groups based on their disease, including control, type 2 diabetes mellitus (T2D), CHD, and CHD + T2D groups. The RNA was extracted from their blood, and RT² Profiler™ PCR Array was utilized to determine gene profiling between groups. Finally, the PCR Array results were validated by using Q-RT-PCR in a more extensive and independent population.

RESULT

PCR Array results revealed that the T2D and T2D + CHD groups shared 11 genes significantly up-regulated in both groups. Further analysis showed that the mRNA levels of AKT2, IL12B, IL6, IRS1, IRS2, MAPK14, and NFKB1 increased. Consequently, the mRNA levels of AQP2, FOXP3, G6PD, and PIK3R1 declined in the T2D + CHD group compared to the T2D group. Furthermore, in silico analysis indicated 36 Gene Ontology terms and 59 signaling pathways were significantly enriched in both groups, which may be a culprit in susceptibility of diabetic patients to CHD development.

CONCLUSION

Finally, the results revealed six genes as a hub gene in altering various biological processes and signaling pathways. The expression trend of these identified genes might be used as potential markers and diagnostic tools for the early identification of the vulnerability of T2D patients to develop premature CHD.

Association of Delta-6-Desaturase Expression with Aggressiveness of Cancer, Diabetes Mellitus, and Multiple Sclerosis: A Narrative Review

Background: The phosphatidylinositol 3-kinase/ protein kinase B /mammalian target of rapamycin (PI3K/Akt/ mTOR) signaling regulates multiple cellular processes and organizes cell proliferation, survival, and differentiation with the available nutrients, in particular, fatty acids. Polyunsaturated fatty acids (PUFAs) are cytotoxic to cancer cells and play a critical role in the treatment of multiple sclerosis (MS) and diabetes mellitus (DM). PUFAs are produced in the body by desaturases and elongases from dietary essential fatty acids (EFAs), primarily involving delta-6-desaturase (D6D). D6D is a rate-limiting enzyme for maintaining many aspects of lipid homeostasis and normal health. D6D is important to recognize the mechanisms that regulate the expression of this enzyme in humans. A lower level of D6D was seen in breast tumors compared to normal tissues. Interestingly, the elevated serum level of D6D was seen in MS and DM, which explains the critical role of D6D in inflammatory diseases. Methods: We searched databases of PubMed, Web of Science (WOS), Google Scholar, Scopus and related studies by predefined eligibility criteria. We assessed their quality and extracted data. Results: Regarding the mTOR signaling pathway, there is remarkable contributions of many inflammatory diseases to attention to common metabolic pathways are depicted. Of course, we need to have the insights into each disorder and their pathological process. The first step in balancing the intake of EFAs is to prevent the disruption of metabolism and expression of the D6D enzyme. Conclusions: The ω6 and ω3 pathways are two major pathways in the biosynthesis of PUFAs. In both of these, D6D is a vital bifunctional enzyme desaturating linoleic acid or alpha-linolenic acid. Therefore, if ω6 and ω3 EFAs are given together in a ratio of 2: 1, the D6D expression will be down-regulated and normalized.

Sonic hedgehog signaling instigates high-fat diet-induced insulin resistance by targeting PPARγ stability

Obesity is a major risk for patients with chronic metabolic disorders including type 2 diabetes. Sonic hedgehog (Shh) is a morphogen that regulates the pancreas and adipose tissue formation during embryonic development. Peroxisome proliferator-activated receptor γ (PPARγ) is a member of the nuclear receptor superfamily and one of the most important regulators of insulin action. Here, we evaluated the role and mechanism of Shh signaling in obesity-associated insulin resistance and characterized its effect on PPARγ. We showed that Shh expression was up-regulated in subcutaneous fat from obese mice. In differentiated 3T3-L1 and primary cultured adipocytes from rats, recombinant Shh protein and SAG (an agonist of Shh signaling) activated an extracellular signal-regulated kinase (ERK)-dependent noncanonical pathway and induced PPARγ phosphorylation at serine 112, which decreased PPARγ activity. Meanwhile, Shh signaling degraded PPARγ protein via binding of PPARγ to neural precursor cell-expressed developmentally down-regulated protein 4-1 (NEDD4-1). Furthermore, vismodegib, an inhibitor of Shh signaling, attenuated ERK phosphorylation induced by a high fat diet (HFD) and restored PPARγ protein level, thus ameliorating glucose intolerance and insulin resistance in obese mice. Our finding suggests that Shh in subcutaneous fat decreases PPARγ activity and stability via activation of an ERK-dependent noncanonical pathway, resulting in impaired insulin action. Inhibition of Shh may serve as a potential therapeutic approach to treat obesity-related diabetes.

Suppression of Nkx3.2 by phosphatidylinositol-3-kinase signaling regulates cartilage development by modulating chondrocyte hypertrophy

Phosphatidylinositol-3-kinase (PI3K) is a key regulator of diverse biological processes including cell proliferation, migration, survival, and differentiation. While a role of PI3K in chondrocyte differentiation has been suggested, its precise mechanisms of action are poorly understood. Here we show that PI3K signaling can down-regulate Nkx3.2 at both mRNA and protein levels in various chondrocyte cultures in vitro. In addition, we have intriguingly found that p85β, not p85α, is specifically employed as a regulatory subunit for PI3K-mediated Nkx3.2 suppression. Furthermore, we found that regulation of Nkx3.2 by PI3K requires Rac1-PAK1, but not Akt, signaling downstream of PI3K. Finally, using embryonic limb bud cultures, ex vivo long bone cultures, and p85β knockout mice, we demonstrated that PI3K-mediated suppression of Nkx3.2 in chondrocytes plays a role in the control of cartilage hypertrophy during skeletal development in vertebrates.

Regulation of nucleotide metabolism by mutant p53 contributes to its gain-of-function activities

Mutant p53 (mtp53) is an oncogene that drives cancer cell proliferation. Here we report that mtp53 associates with the promoters of numerous nucleotide metabolism genes (NMG). Mtp53 knockdown reduces NMG expression and substantially depletes nucleotide pools, which attenuates GTP dependent protein (GTPase) activity and cell invasion. Addition of exogenous guanosine or GTP restores the invasiveness of mtp53 knockdown cells, suggesting that mtp53 promotes invasion by increasing GTP. Additionally, mtp53 creates a dependency on the nucleoside salvage pathway enzyme deoxycytidine kinase (dCK) for the maintenance of a proper balance in dNTP pools required for proliferation. These data indicate that mtp53 harboring cells have acquired a synthetic sick or lethal phenotype relationship with the nucleoside salvage pathway. Finally, elevated expression of NMG correlates with mutant p53 status and poor prognosis in breast cancer patients. Thus, mtp53’s control of nucleotide biosynthesis has both a driving and sustaining role in cancer development.

Insulin-dependent transcriptional control in L6 rat myotubes is associated with modulation of histone acetylation and accumulation of the histone variant H2A.Z in the proximity of the transcriptional start site

Besides its direct metabolic effects, insulin induces transcriptional alterations in its target tissues. However, whether such changes are accompanied by epigenetic changes on the chromatin template encompassing insulin responsive genes is unclear. Here, mRNA levels of insulin-responsive genes hexokinase 2 (Hk2), insulin receptor substrate (Irs2), and the PI3K subunit p85β (Pik3r2) were compared in control versus insulin-stimulated L6 myotubes. Chromatin immunoprecipitation (ChIP) was performed with antibodies directed to histone H2A, histone variant H2A.Z, acetylated histone H3 on lysines 9/14, and acetylated H2A.Z. Insulin induced a more than 2-fold Hk2 mRNA increase, while Irs2 and Pik3r2 were downregulated. ChIP to H2A and H2A.Z showed higher H2A.Z accumulation around the transcriptional start site (TSS) of these insulin-modulated genes, while H2A.Z accumulation was lower distally to the TSS in the Hk2 promoter. H2A.Z levels and H3K9/14 acetylation correlated on several loci along the Hk2 gene, and H3K9/14 as well as H2A.Z acetylation was enhanced by insulin treatment. On the contrary, reduced H3K9/14 acetylation was observed in insulin-repressed Irs2 and Pik3r2, and recovery of acetylation by treatment with the histone deacetylase inhibitor trichostatin A reverted insulin-induced Irs2 downregulation. The chromatin regions encompassing selected insulin-responsive genes are thus featured by accumulation of H2A.Z around the TSS. H2A.Z accumulation facilitates insulin-dependent modulation of pharmacologically treatable H3K9/14 and H2A.Z acetylations. Indeed, inhibition of histone deacetylases by TSA treatment reverted insulin induced Irs2 gene downregulation. Dysregulated histone acetylation may thus be potentially targeted with histone deacetylase inhibitors.

A new synthetic chalcone derivative, 2-hydroxy-3',5,5'-trimethoxychalcone (DK-139), suppresses the Toll-like receptor 4-mediated inflammatory response through inhibition of the Akt/NF-κB pathway in BV2 microglial cells

Microglial cells are the resident innate immune cells that sense pathogens and tissue injury in the central nervous system (CNS). Microglial activation is critical for neuroinflammatory responses. The synthetic compound 2-hydroxy-3',5,5'-trimethoxychalcone (DK-139) is a novel chalcone-derived compound. In this study, we investigated the effects of DK-139 on Toll-like receptor 4 (TLR4)-mediated inflammatory responses in BV2 microglial cells. DK-139 inhibited lipopolysaccharide (LPS)-induced TLR4 activity, as determined using a cell-based assay. DK-139 blocked LPS-induced phosphorylation of IκB and p65/RelA NF-κB, resulting in inhibition of the nuclear translocation and trans-acting activity of NF-κB in BV2 microglial cells. We also found that DK-139 reduced the expression of NF-κB target genes, such as those for COX-2, iNOS, and IL-1β, in LPS-stimulated BV2 microglial cells. Interestingly, DK-139 blocked LPS-induced Akt phosphorylation. Inhibition of Akt abrogated LPS-induced phosphorylation of p65/RelA, while overexpression of dominant-active p110CAAX enhanced p65/RelA phosphorylation as well as iNOS and COX2 expression. These results suggest that DK-139 exerts an anti-inflammatory effect on microglial cells by inhibiting the Akt/IκB kinase (IKK)/NF-κB signaling pathway.

Vascular smooth muscle insulin resistance, but not hypertrophic signaling, is independent of angiotensin II-induced IRS-1 phosphorylation by JNK

Angiotensin II (ANG II) has been implicated in the pathogenesis of diabetic micro- and macrovascular disease. In vascular smooth muscle cells (VSMCs), ANG II phosphorylates and degrades insulin receptor substrate-1 (IRS-1). While the pathway responsible for IRS-1 degradation in this system is unknown, c-Jun NH(2)-terminal kinase (JNK) has been linked with serine phosphorylation of IRS-1 and insulin resistance. We investigated the role of JNK in ANG II-induced IRS-1 phosphorylation, degradation, Akt activation, glucose uptake, and hypertrophic signaling, focusing on three IRS-1 phosphorylation sites: Ser302, Ser307, and Ser632. Maximal IRS-1 phosphorylation on Ser632 occurred at 5 min, on Ser307 at 30 min, and on Ser302 at 60 min. The JNK inhibitor SP600125 reduced ANG II-induced IRS-1 Ser307 phosphorylation (by 80%), IRS-1 Ser302 phosphorylation (by 70%), and IRS-1 Ser632 phosphorylation (by 50%). However, JNK inhibition had no effect on ANG II-mediated IRS-1 degradation, nor did it reverse the ANG II-induced decrease in Akt phosphorylation or glucose uptake. Transfection of VSMCs with mutants S307A, S302A, or S632A of IRS-1 did not block ANG II-mediated IRS-1 degradation. In contrast, JNK inhibition attenuated insulin-induced upregulation of collagen and smooth muscle α-actin in ANG II-pretreated cells. We conclude that phosphorylation of Ser307, Ser302, and Ser632 of IRS-1 is not involved in ANG II-mediated IRS-1 degradation, and that JNK alone does not mediate ANG II-stimulated IRS-1 degradation, but rather is responsible for the hypertrophic effects of insulin on smooth muscle.

PI3K/Akt signaling requires spatial compartmentalization in plasma membrane microdomains

Spatial compartmentalization of signaling pathway components generally defines the specificity and enhances the efficiency of signal transduction. The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is known to be compartmentalized within plasma membrane microdomains; however, the underlying mechanisms and functional impact of this compartmentalization are not well understood. Here, we show that phosphoinositide-dependent kinase 1 is activated in membrane rafts in response to growth factors, whereas the negative regulator of the pathway, phosphatase and tensin homolog deleted on chromosome 10 (PTEN), is primarily localized in nonraft regions. Alteration of this compartmentalization, either by genetic targeting or ceramide-induced recruitment of PTEN to rafts, abolishes the activity of the entire pathway. These findings reveal critical steps in raft-mediated PI3K/Akt activation and demonstrate the essential role of membrane microdomain compartmentalization in enabling PI3K/Akt signaling. They further suggest that dysregulation of this compartmentalization may underlie pathological complications such as insulin resistance.

Blocking O-linked GlcNAc cycling in Drosophila insulin-producing cells perturbs glucose-insulin homeostasis

A dynamic cycle of O-linked GlcNAc (O-GlcNAc) addition and removal is catalyzed by O-GlcNAc transferase and O-GlcNAcase, respectively, in a process that serves as the final step in a nutrient-driven "hexosamine-signaling pathway." Evidence points to a role for O-GlcNAc cycling in diabetes and insulin resistance. We have used Drosophila melanogaster to determine whether O-GlcNAc metabolism plays a role in modulating Drosophila insulin-like peptide (dilp) production and insulin signaling. We employed transgenesis to either overexpress or knock down Drosophila Ogt(sxc) and Oga in insulin-producing cells (IPCs) or fat bodies using the GAL4-UAS system. Knockdown of Ogt decreased Dilp2, Dilp3, and Dilp5 production, with reduced body size and decreased phosphorylation of Akt in vivo. In contrast, knockdown of Oga increased Dilp2, Dilp3, and Dilp5 production, increased body size, and enhanced phosphorylation of Akt in vivo. However, knockdown of either Ogt(sxc) or Oga in the IPCs increased the hemolymph carbohydrate concentration. Furthermore, phosphorylation of Akt stimulated by extraneous insulin in an ex vivo cultured fat body of third instar larvae was diminished in strains subjected to IPC knockdown of Ogt or Oga. Knockdown of O-GlcNAc cycling enzymes in the fat body dramatically reduced neutral lipid stores. These results demonstrate that altered O-GlcNAc cycling in Drosophila IPCs modulates insulin production and influences the insulin responsiveness of peripheral tissues. The observed phenotypes in O-GlcNAc cycling mimic pancreatic β-cell dysfunction and glucose toxicity related to sustained hyperglycemia in mammals.

BCR-ABL but not JAK2 V617F inhibits erythropoiesis through the Ras signal by inducing p21CIP1/WAF1

BCR-ABL is a causative tyrosine kinase (TK) of chronic myelogenous leukemia (CML). In CML patients, although myeloid cells are remarkably proliferating, erythroid cells are rather decreased and anemia is commonly observed. This phenotype is quite different from that observed in polycythemia vera (PV) caused by JAK2 V617F, whereas both oncogenic TKs activate common downstream molecules at the level of hematopoietic stem cells (HSCs). To clarify this mechanism, we investigated the effects of BCR-ABL and JAK2 V617F on erythropoiesis. Enforced expression of BCR-ABL but not of JAK2 V617F in murine LSK (Lineage(-)Sca-1(hi)CD117(hi)) cells inhibited the development of erythroid cells. Among several signaling molecules downstream of BCR-ABL, an active mutant of N-Ras (N-RasE12) but not of STAT5 or phosphatidylinositol 3-kinase (PI3-K) inhibited erythropoiesis, while N-RasE12 enhanced the development of myeloid cells. BCR-ABL activated Ras signal more intensely than JAK2 V617F, and inhibition of Ras by manumycin A, a farnesyltransferase inhibitor, ameliorated erythroid colony formation of CML cells. As for the mechanisms of Ras-induced suppression of erythropoiesis, we found that GATA-1, an erythroid-specific transcription factor, blocked Ras-mediated mitogenic signaling at the level of MEK through the direct interaction. Furthermore, enforced expression of N-RasE12 in LSK cells derived from p53-, p16(INK4a)/p19(ARF)-, and p21(CIP1/WAF1)-null/wild-type mice revealed that suppressed erythroid cell growth by N-RasE12 was restored only by p21(CIP1/WAF1) deficiency, indicating that a cyclin-dependent kinase (CDK) inhibitor, p21(CIP1/WAF1), plays crucial roles in Ras-induced suppression of erythropoiesis. These data would, at least partly, explain why respective oncogenic TKs cause different disease phenotypes.

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.

Transcription factor AP-2beta inhibits expression and secretion of leptin, an insulin-sensitizing hormone, in 3T3-L1 adipocytes

BACKGROUND

We have previously reported an association between the activator protein-2beta (AP-2beta) transcription factor gene and type 2 diabetes. This gene is preferentially expressed in adipose tissue, and subjects with a disease-susceptible allele of AP-2beta showed stronger AP-2beta expression in adipose tissue than those without the susceptible allele. Furthermore, overexpression of AP-2beta led to lipid accumulation and induced insulin resistance in 3T3-L1 adipocytes.

RESULT

We found that overexpression of AP-2beta in 3T3-L1 adipocytes decreased the promoter activity of leptin, and subsequently decreased both messenger RNA (mRNA) and protein expression and secretion. Furthermore, knockdown of endogenous AP-2beta by RNA-interference increased mRNA and protein expression of leptin. Electrophoretic mobility shift and chromatin immunoprecipitation assays revealed specific binding of AP-2beta to leptin promoter regions in vitro and in vivo. In addition, site-directed mutagenesis of the AP-2-binding site located between position +34 and +42 relative to the transcription start site abolished the inhibitory effect of AP-2beta. Our results clearly showed that AP-2beta directly inhibited insulin-sensitizing hormone leptin expression by binding to its promoter.

CONCLUSION

AP-2beta modulated the expression of leptin through direct interaction with its promoter region.

Amino acids and insulin are regulators of muscle protein synthesis in neonatal pigs

The stage of development between birth and weaning in mammals is a period of very rapid growth that is crucial for the long-term well-being of the animal. The rate of protein deposition in neonatal animals is very high because dietary protein is efficiently utilized to increase body protein mass. Our studies in neonatal pigs have shown that this high efficiency of protein deposition is largely due to the marked increase in protein synthesis after feeding, and this response is particularly profound in the skeletal muscle. The enhanced stimulation of muscle protein synthesis in neonates after feeding is independently mediated by the rise in insulin and amino acids and this response declines with age. Intracellular signaling components that respond to the postprandial rise in amino acids and insulin have been identified and their activation has been shown to be elevated in skeletal muscle of neonatal pigs after a meal and to decrease with development. The enhanced activation of these components in the amino acid and insulin signaling pathways in neonatal muscle contributes to the high rate of muscle protein synthesis and rapid gain in skeletal muscle mass in newborn pigs, which are essential determinants of efficient growth during development.

PI(3,4,5)P3 potentiates phospholipase C-beta activity

Phospholipase C-beta (PLC-beta) isozymes are key effectors in G protein-coupled signaling pathways. Previously, we showed that PLC-beta1 and PLC-beta3 bound immobilized PIP(3). In this study, PIP(3) was found to potentiate Ca(2+)-stimulated PLC-beta activities using an in vitro reconstitution assay. LY294002, a specific PI 3-kinase inhibitor, significantly inhibited 10 min of agonist-stimulated total IP accumulation. Both LY294002 and wortmannin inhibited 90 sec of agonist-stimulated IP(3) accumulation in intact cells. Moreover, transfected p110CAAX, a constitutively activated PI 3-kinase catalytic subunit, increased 90 sec of oxytocin-stimulated IP(3) accumulation. Receptor-ligand binding assays indicated that LY294002 did not affect G protein-coupled receptors directly, suggesting a physiological role for PIP(3) in directly potentiating PLC-beta activity. When coexpressed with p110CAAX, fluorescence-tagged PLC-beta3 was increasingly localized to the plasma membrane. Additional observations suggest that the PH domain of PLC-beta is not important for p110CAAX-induced membrane association.

Phosphorylation of IRS proteins Yin-Yang regulation of insulin signaling

Growing evidence reveals that insulin signal pathway is not static, but is rather a dynamic, flexible, and fed in by negative (Yin) and positive (Yang) regulation in response to environmental changes. Normal insulin response reflects the balance between Yin and Yang regulation acting upon insulin signaling pathway. Conceivably, imbalance between the Yin and Yang results in abnormal insulin sensitivity such as insulin resistance. IRS-proteins are insulin receptor substrates that mediate insulin signaling via multiple tyrosyl phosphorylations. However, they are also substrates for many serine/threonine kinases downstream of other signaling network and become serine phosphorylated in response to various conditions such as inflammation, stress and over nutrients. The serine phosphorylation of IRS-proteins alters the capacities of IRS-proteins to be phosphorylated on tyrosyl, therefore, able to mediate insulin signaling. The unique structure of IRS-proteins render them idea molecules to fulfill the task to sense the environmental cues and integrate them into insulin sensitivity through serine/threonine phosphorylation. This review intends to summarize the role of IRS-proteins in insulin signaling with focuses on the role of Yin and Yang regulation of insulin signaling pathway. Understanding the dynamic of these complicated regulation net work not only provide us a complete picture of what happens in the normal conditions, but also pathaphysiological conditions such as obesity and insulin resistance.

Postreceptoral adipocyte insulin resistance induced by nelfinavir is caused by insensitivity of PKB/Akt to phosphatidylinositol-3,4,5-trisphosphate

Adipocyte insulin resistance can be caused by proximal insulin signaling defects but also from postreceptor mechanisms, which in large are poorly characterized. Adipocytes exposed for 18 h to the HIV protease inhibitor nelfinavir manifest insulin resistance characterized by normal insulin-stimulated tyrosine phosphorylation of the insulin receptor and insulin receptor substrate proteins, preserved in vitro phosphatidylinositol 3-kinase (PI 3-kinase) assay activity but impaired activation of PKB/Akt and stimulation of glucose uptake. Here we aimed to assess whether impaired PKB/Akt activation is indeed rate limiting for insulin signaling propagation in response to nelfinavir and the mechanism for defective PKB/Akt activation. Nelfinavir treatment of 3T3-L1 adipocytes impaired the insulin-stimulated translocation and membrane fusion of myc-glucose transporter (GLUT)-4-green fluorescent protein (GFP) reporter. Phosphorylation of PKB/Akt substrates including glycogen synthase kinase-3 and AS160 decreased in response to nelfinavir, and this remained true, even in cells with forced generation of phosphatidylinositol-3,4,5-trisphohphate (PIP(3)) by a membrane-targeted active PI 3-kinase, confirming that impaired PKB/Akt activation was rate limiting for insulin signal propagation. Cells expressing a GFP-tagged pleckstrin homology domain of general receptors for phosphoinositides 1, which binds PIP(3), revealed intact PIP(3)-mediated plasma membrane translocation of this reporter in nelfinavir-treated cells. However, expression of a membrane-targeted catalytic subunit of PI 3-kinase failed to induce myc-GLUT4-GFP translocation in the absence of insulin, as it did in control cells. Conversely, a membrane-targeted and constitutively active PKB/Akt mutant was normally phosphorylated on S473 and T308, confirming intact PKB/Akt kinases activity, and induced myc-GLUT4-GFP translocation. Collectively, nelfinavir uncovers a postreceptor mechanism for insulin resistance, caused by interference with the sensing of PIP(3) by PKB/Akt, leading to impaired GLUT4 translocation and membrane fusion.

Transcription factor activating protein-2beta: a positive regulator of monocyte chemoattractant protein-1 gene expression

We previously reported an association between the activating protein (AP)-2beta transcription factor gene and type 2 diabetes. This gene is preferentially expressed in adipose tissue, and subjects with a disease-susceptible allele of AP-2beta showed stronger AP-2beta expression in adipose tissue than those without the susceptible allele. Furthermore, overexpression of AP-2beta leads to lipid accumulation by enhancing glucose transport and inducing insulin resistance in 3T3-L1 adipocytes. In this study, we found that overexpression of AP-2beta in 3T3-L1 adipocytes accelerated the promoter activity of monocyte chemoattractant protein-1 (MCP-1) and subsequently increased both mRNA and protein expression and protein secretion. Furthermore, knockdown of endogenous AP-2beta by RNA interference reduced the mRNA and the protein expression of MCP-1. EMSAs and chromatin immunoprecipitation assays revealed specific binding of AP-2beta to MCP-1 promoter regions, in vitro and in vivo. Additionally, site-directed mutagenesis of the AP-2 binding site located at -137 to -129 relative to the transcription start site markedly diminished MCP-1 promoter activity, whereas other putative AP-2 binding sites did not. Our results clearly show that AP-2beta directly enhanced MCP-1 secretion by binding to its promoter. Thus, we propose that AP-2beta positively regulates MCP-1 expression; subsequently contributes to the infiltration of macrophages to adipose tissue; and leads to insulin resistance, type 2 diabetes, and cardiovascular diseases.

Regulation of muscle growth in neonates

PURPOSE OF REVIEW

This review reports recent findings on the multiple factors that regulate skeletal muscle growth in neonates.

RECENT FINDINGS

Skeletal muscle is the fastest growing protein mass in neonates. The high rate of neonatal muscle growth is due to accelerated rates of protein synthesis accompanied by the rapid accumulation of muscle nuclei. Feeding profoundly stimulates muscle protein synthesis in neonates and the response decreases with age. The feeding-induced stimulation of muscle protein synthesis is modulated by enhanced sensitivity to the postprandial rise in insulin and amino acids. Insulin and amino acid signaling components have been identified that are involved in the feeding-induced stimulation of protein synthesis in neonatal muscle. The enhanced activation of these signaling components in skeletal muscle of the neonate contributes to the high rate of muscle protein synthesis and rapid gain in muscle protein mass in neonates.

SUMMARY

Recent findings suggest that the immature muscle has a heightened capacity to activate signaling cascades that promote translation initiation in response to the postprandial rise in insulin and amino acids thereby enabling their efficient utilization for muscle growth. This capacity is further supported by enhanced satellite cell proliferation, but how these two processes are linked remains to be established.

Signaling by small GTPases in the immune system

The Ras superfamily consists of over 50 low-molecular-weight proteins that cycle between an inactive guanosine diphosphate-bound state and an active guanosine triphosphate (GTP)-bound state. They are involved in a variety of signal transduction pathways that regulate cell growth, intracellular trafficking, cell migration, and apoptosis. Several methods have been devised to measure the activation state of Ras proteins, defined as the percent of Ras molecules in the active GTP-bound state. We have previously developed a quantitative biochemical method that can be applied to animal and human tissues and have used it to measure the activation state of Ras, Rap1, Rheb, and Rho proteins in cultured cells and in animal and human tumors. Ras, Rac, and Rho all play roles in regulating the functions of T and B lymphocytes and dendritic cells, and these proteins are clearly important in maintaining normal immune system function.

Transcription factor activating enhancer-binding protein-2beta. A negative regulator of adiponectin gene expression

We previously reported the association between the activating enhancer-binding protein-2beta (AP-2beta) transcription factor gene and type 2 diabetes. This gene is preferentially expressed in adipose tissue, and subjects with the disease-susceptible allele of AP-2beta showed stronger expression in adipose tissue than those without the susceptible allele. Furthermore, overexpression of AP-2beta leads to lipid accumulation by enhancing glucose transport and inducing insulin resistance in 3T3-L1 adipocytes. In this study we demonstrated that overexpression of AP-2beta in 3T3-L1 adipocytes decreased the expression and secretion of adiponectin and increased those of interleukin-6 (IL-6). Interestingly, the effects of AP-2beta on the expressions of adiponectin and IL-6 and the mechanisms by which AP-2beta modulated their expressions were different. We found that the promoter activity of adiponectin gene was inhibited by AP-2beta overexpression and enhanced by knockdown of endogenous AP-2beta, whereas IL-6 was unaffected. Electrophoretic mobility shift assays revealed the existence of putative responsive elements for AP-2beta and NF-YA in human and mouse adiponectin promoter regions, and mutation of this AP-2beta binding site abolished the inhibitory effect of AP-2beta. Furthermore, chromatin immunoprecipitation assays demonstrated that AP-2beta and NF-YA competitively bind to the same region of the adiponectin promoter. Our results clearly demonstrated that AP-2beta directly inhibits adiponectin gene expression by displacing NF-YA and binding to its promoter. We conclude that AP-2beta might modulate the expression of adiponectin by directly inhibiting its transcriptional activity.

Control of dendritic arborization by the phosphoinositide-3'-kinase-Akt-mammalian target of rapamycin pathway

The molecular mechanisms that determine the size and complexity of the neuronal dendritic tree are unclear. Here, we show that the phosphoinositide-3' kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signaling pathway promotes the growth and branching of dendrites in cultured hippocampal neurons. Constitutively active mutants of Ras, PI3K, and Akt, or RNA interference (RNAi) knockdown of lipid phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome Ten), induced growth and elaboration of dendrites that was blocked by mTOR inhibitor rapamycin and/or by overexpression of eIF-4E binding protein 1 (4E-BP1), which inhibits translation of 5' capped mRNAs. The effect of PI3K on dendrites was lost in more mature neurons (>14 d in vitro). Dendritic complexity was reduced by inhibition of PI3K and by RNAi knockdown of mTOR or p70 ribosomal S6 kinase (p70S6K, an effector of mTOR). A rapamycin-resistant mutant of mTOR "rescued" the morphogenetic effects of PI3K in the presence of rapamycin. By regulating global and/or local protein translation, and as a convergence point for multiple signaling pathways, mTOR could play a central role in the control of dendrite growth and branching during development and in response to activity.

Bridging the GAP between insulin signaling and GLUT4 translocation

Upon binding and activating its cell-surface receptor, insulin triggers signaling cascades that regulate many cellular processes. Regarding glucose homeostasis, insulin suppresses hepatic glucose production and increases glucose transport into muscle and adipose tissues. At the cellular level, glucose uptake results from the insulin-stimulated translocation of the glucose transporter 4 (GLUT4) from intracellular storage sites to the plasma membrane. Although the signaling molecules that function proximal to the activated insulin receptor have been well characterized, it is not known how the distal insulin-signaling cascade interfaces with and mobilizes GLUT4-containing compartments. Recently, several candidate signaling molecules, including AS160, PIKfyve and synip, have been identified that might provide functional links between the insulin signaling cascade and GLUT4 compartments. Future work will focus on delineating the precise GLUT4 trafficking steps regulated by these molecules.

Suppression of Egr-1 transcription through targeting of the serum response factor by oncogenic H-Ras

The transcription factor Egr-1 functions as a key regulator in cellular growth, differentiation, and apoptosis. The loss of Egr-1 expression is closely associated with tumor development, although the molecular mechanism behind the suppression of Egr-1 is largely unknown. In this report, we show that growth factor-induced transcriptional activation of Egr-1 gene is downregulated by chronic expression of oncogenic H-Ras in NIH3T3 fibroblasts. Our results demonstrate that phosphoinositide 3-kinase (PI3K) signaling is necessary for oncogenic H-Ras-mediated reduction of Egr-1 gene expression. Aberrant activation of PI3K signaling by oncogenic Ras decreased the level of serum response factor (SRF) protein through the acceleration of proteolysis, which resulted in decreased SRF binding to the serum response element (SRE) sites within the Egr-1 promoter, leading to the suppression of Egr-1 transcription. Inhibition of PI3K signaling restored the downregulation of SRF and Egr-1 expression caused by oncogenic Ras. Our findings suggest a novel signaling mechanism by which prolonged activation of oncogenic H-Ras can trigger the loss of tumor suppressor Egr-1 through the PI3K pathway in NIH3T3 fibroblast model cell lines.

Englitazone administration to late pregnant rats produces delayed body growth and insulin resistance in their fetuses and neonates

The level of maternal circulating triacylglycerols during late pregnancy has been correlated with the mass of newborns. PPARgamma (peroxisome-proliferator-activated receptor gamma) ligands, such as TZDs (thiazolidinediones), have been shown to reduce triacylglycerolaemia and have also been implicated in the inhibition of tissue growth and the promotion of cell differentiation. Therefore TZDs might control cell proliferation during late fetal development and, by extension, body mass of pups. To investigate the response to EZ (englitazone), a TZD, on perinatal development, 0 or 50 mg of englitazone/kg of body mass was given as an oral dose to pregnant rats daily from day 16 of gestation until either day 20 for the study of their fetuses, or until day 21 of gestation for the study of neonates. EZ decreased maternal triacylglycerol levels at day 20 of gestation and neonatal mass, but not fetal mass. Fetuses and neonates from EZ-treated mothers exhibited high levels of insulin and were found to be hyperglycaemic. The apparent insulin-resistant state in neonates from EZ-treated pregnant rats was corroborated, since they showed higher plasma NEFA [non-esterified ('free') fatty acid] levels, ketonaemia and liver LPL (lipoprotein lipase) activity and lower plasma IGF-I (type 1 insulin-like growth factor) levels, in comparison with those from control mothers. Moreover, at the molecular level, an increase in Akt phosphorylation was found in the liver of neonates from EZ-treated mothers, which confirms that the insulin pathway was negatively affected. Thus the response of fetuses and neonates to maternal antidiabetic drug treatment is the opposite of what would be expected, and can be justified by the scarce amount of adipose tissue impeding a normal response to PPARgamma ligands and by hyperinsulinaemia as being responsible for a major insulin-resistant condition.

JNK and tumor necrosis factor-alpha mediate free fatty acid-induced insulin resistance in 3T3-L1 adipocytes

Lipid infusion and high fat feeding are established causes of systemic and adipose tissue insulin resistance. In this study, we treated 3T3-L1 adipocytes with a mixture of free fatty acids (FFAs) to investigate the molecular mechanisms underlying fat-induced insulin resistance. FFA treatment impaired insulin receptor-mediated signal transduction and decreased insulin-stimulated GLUT4 translocation and glucose transport. FFAs activated the stress/inflammatory kinases c-Jun N-terminal kinase (JNK) and IKKbeta, and the suppressor of cytokine signaling protein 3, increased secretion of the inflammatory cytokine tumor necrosis factor (TNF)-alpha, and decreased secretion of adiponectin into the medium. RNA interference-mediated down-regulation of JNK blocked JNK activation and prevented most of the FFA-induced defects in insulin action. Blockade of TNF-alpha signaling with neutralizing antibodies to TNF-alpha or its receptors or with a dominant negative TNF-alpha peptide had a partial effect to inhibit FFA-induced cellular insulin resistance. We found that JNK activation by FFAs was not inhibited by blocking TNF-alpha signaling, whereas the FFA-induced increase in TNF-alpha secretion was inhibited by RNA interference-mediated JNK knockdown. Together, these results indicate that 1) JNK can be activated by FFAs through TNF-alpha-independent mechanisms, 2) activated JNK is a major contributor to FFA-induced cellular insulin resistance, and 3) TNF-alpha is an autocrine/paracrine downstream effector of activated JNK that can also mediate insulin resistance.

Insulin regulates the membrane arrival, fusion, and C-terminal unmasking of glucose transporter-4 via distinct phosphoinositides

Insulin increases glucose uptake into muscle via glucose transporter-4 (GLUT4) translocation to the cell membrane, but the regulated events in GLUT4 traffic are unknown. Here we focus on the role of class IA phosphatidylinositol (PI) 3-kinase and specific phosphoinositides in the steps of GLUT4 arrival and fusion with the membrane, using L6 muscle cells expressing GLUT4myc. To this end, we detected the availability of the myc epitope at the cell surface or intravesicular spaces and of the cytosol-facing C-terminal epitope, in cells and membrane lawns derived from them. We observed the following: (a) Wortmannin and LY294002 at concentrations that inhibit class IA PI 3-kinase reduced but did not abate the C terminus gain, yet the myc epitope was unavailable for detection unless lawns or cells were permeabilized, suggesting the presence of GLUT4myc in docked, unfused vesicles. Accordingly, GLUT4myc-containing vesicles were detected by immunoelectron microscopy of membranes from cells pretreated with wortmannin and insulin, but not insulin or wortmannin alone. (b) Insulin caused greater immunological availability of the C terminus than myc epitopes, suggesting that C terminus unmasking had occurred. Delivering phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) to intact cells significantly increased lawn-associated myc signal without C terminus gain. Conversely, phosphatidylinositol 3-phosphate (PI3P) increased the detection of C terminus epitope without any myc gain. We propose that insulin regulates GLUT4 membrane arrival, fusion, and C terminus unmasking, through distinct phosphoinositides. PI(3,4,5)P(3) causes arrival and fusion without unmasking, whereas PI3P causes arrival and unmasking without fusion.

Bidirectional regulation of monocyte chemoattractant protein-1 gene at distinct sites of its promoter by nitric oxide in vascular smooth muscle cells

We have previously reported that chronic activation of phosphatidylinositol 3-kinase (PI3-kinase) by the overexpression of membrane-targeted p110CAAX induced proinflammatory gene expression in rat vascular smooth muscle cells (VSMCs) through the induction of CCAAT/enhancer binding protein-beta (C/EBP-beta) and C/EBP-delta. To examine the anti-inflammatory effect of nitric oxide (NO) on proinflammatory gene expression, we have investigated the effects of sodium nitroprusside (SNP) on the monocyte chemoattractant protein-1 (MCP-1) gene expression in VSMCs under chronic activation of PI3-kinase. At low concentrations (0.05 mM) of SNP, but not at high concentrations (0.5-1.0 mM), MCP-1 mRNA and protein expression as well as its transcriptional activity were significantly reduced. We found that SNP induced C/EBP homologous protein (CHOP) expression, which inhibited C/EBP binding activity and reduced the C/EBP activity induced by chronic activation of PI3-kinase in a dose-dependent manner up to 1.0 mM. Consistently, the increase in CHOP expression significantly reduced the MCP-1 promoter activity induced by PI3-kinase. However, the overexpression of CHOP alone upregulated MCP-1 promoter activity in a dose-dependent manner up to high concentrations. Deletion analysis of MCP-1 promoter and electrophoretic mobility shift assay identified the CHOP-response element (CHOP-RE) at the region between -190 and -179 bp of MCP-1 promoter. By using CHOP-RE as a decoy, we significantly suppressed the increase in promoter activity of MCP-1 induced by either CHOP or SNP. Thus CHOP induced by an NO donor has bidirectional effects on MCP-1 gene expression: it decreases gene expression by inhibition of C/EBPs, and it increases the gene expression through CHOP-RE.

Membrane localization of all class I PI 3-kinase isoforms suppresses c-Myc-induced apoptosis in Rat1 fibroblasts via Akt

Phosphoinositide 3'-kinases (PI3Ks) constitute a family of lipid kinases implicated in signal transduction through tyrosine kinase receptors and heterotrimeric G protein-linked receptors. PI3Ks are heterodimers made up of four different 110-kDa catalytic subunits (p110alpha, p110beta, p110gamma, and p110delta) and a smaller regulatory subunit. Despite a clear implication of PI3Ks in survival signaling, the contribution of the individual PI3K isoforms has not been elucidated. To address this issue, we generated Rat1 fibroblasts that co-express c-Myc and membrane targeted derivates of the different p110 isoforms. Here we present data for the first time showing that activation of PI3-kinase signaling through membrane localization of p110beta, p110gamma, and p110delta protects c-Myc overexpressing Rat1 fibroblasts from apoptosis caused by serum deprivation like it has been described for p110alpha. Expression of each p110 isoform reduces significantly caspase-3 like activity in this apoptosis model. Decreased caspase-3 activity correlates with the increase in Akt phosphorylation in cells that contain one of the myristoylated p110 isoforms. p110 isoform-mediated protection from cell death was abrogated upon expression of a kinase-negative version of Akt.

Acetylation of insulin receptor substrate-1 is permissive for tyrosine phosphorylation

Background

Insulin receptor substrate (IRS) proteins are key moderators of insulin action. Their specific regulation determines downstream protein-protein interactions and confers specificity on growth factor signalling. Regulatory mechanisms that have been identified include phosphorylation of IRS proteins on tyrosine and serine residues and ubiquitination of lysine residues. This study investigated other potential molecular mechanisms of IRS-1 regulation.

Results

Using the sos recruitment yeast two-hybrid system we found that IRS-1 and histone deacetylase 2 (HDAC2) interact in the cytoplasmic compartment of yeast cells. The interaction mapped to the C-terminus of IRS-1 and was confirmed through co-immunoprecipitation in vitro of recombinant IRS-1 and HDAC2. HDAC2 bound to IRS-1 in mammalian cells treated with phorbol ester or after prolonged treatment with insulin/IGF-1 and also in the livers of ob/ob mice but not PTP1B knockout mice. Thus, the association occurs under conditions of compromised insulin signalling. We found that IRS-1 is an acetylated protein, of which the acetylation is increased by treatment of cells with Trichostatin A (TSA), an inhibitor of HDAC activity. TSA-induced increases in acetylation of IRS-1 were concomitant with increases in tyrosine phosphorylation in response to insulin. These effects were confirmed using RNA interference against HDAC2, indicating that HDAC2 specifically prevents phosphorylation of IRS-1 by the insulin receptor.

Conclusions

Our results show that IRS-1 is an acetylated protein, a post-translational modification that has not been previously described. Acetylation of IRS-1 is permissive for tyrosine phosphorylation and facilitates insulin-stimulated signal transduction. Specific inhibition of HDAC2 may increase insulin sensitivity in otherwise insulin resistant conditions.

Regulation of Vascular L-type Ca2+Channels by Phosphatidylinositol 3,4,5-Trisphosphate

Modulation of voltage-gated L-type Ca2+ channels by phosphoinositide 3-kinase (PI3K) regulates Ca2+ entry and plays a crucial role in vascular excitation-contraction coupling. Angiotensin II (Ang II) activates Ca2+ entry by stimulating L-type Ca2+ channels through Gbeta-sensitive PI3K in portal vein myocytes. Moreover, PI3K and Ca2+ entry activation have been reported to be necessary for receptor tyrosine kinase-coupled and G protein-coupled receptor-induced DNA synthesis in vascular cells. We have previously shown that tyrosine kinase-regulated class Ia and G protein-regulated class Ib PI3Ks are able to modulate vascular L-type Ca2+ channels. PI3Ks display 2 enzymatic activities: a lipid-kinase activity leading to the formation of phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3 or PIP3] and a serine-kinase activity. Here we show that exogenous PIP3 applied into the cell through the patch pipette is able to reproduce the Ca2+ channel-stimulating effect of Ang II and PI3Ks. Moreover, the Ang II-induced PI3K-mediated stimulation of Ca2+ channel and the resulting increase in cytosolic Ca2+ concentration are blocked by the anti-PIP3 antibody. Mutants of PI3K transfected into vascular myocytes also revealed the essential role of the lipid-kinase activity of PI3K in Ang II-induced Ca2+ responses. These results suggest that PIP3 is necessary and sufficient to activate a Ca2+ influx in vascular myocytes stimulated by Ang II.

Insulin stimulation of GLUT4 exocytosis, but not its inhibition of endocytosis, is dependent on RabGAP AS160

Insulin maintains whole body blood glucose homeostasis, in part, by regulating the amount of the GLUT4 glucose transporter on the cell surface of fat and muscle cells. Insulin induces the redistribution of GLUT4 from intracellular compartments to the plasma membrane, by stimulating a large increase in exocytosis and a smaller inhibition of endocytosis. A considerable amount is known about the molecular events of insulin signaling and the complex itinerary of GLUT4 trafficking, but less is known about how insulin signaling is transmitted to GLUT4 trafficking. Here, we show that the AS160 RabGAP, a substrate of Akt, is required for insulin stimulation of GLUT4 exocytosis. A dominant-inhibitory mutant of AS160 blocks insulin stimulation of exocytosis at a step before the fusion of GLUT4-containing vesicles with the plasma membrane. This mutant, however, does not block insulin-induced inhibition of GLUT4 endocytosis. These data support a model in which insulin signaling to the exocytosis machinery (AS160 dependent) is distinct from its signaling to the internalization machinery (AS160 independent).

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.

Modulation of insulin action

Insulin is a key hormone regulating the control of metabolism and the maintenance of normoglycaemia and normolipidaemia. Insulin acts by binding to its cell surface receptor, thus activating the receptor's intrinsic tyrosine kinase activity, resulting in receptor autophosphorylation and phosphorylation of several substrates. Tyrosine phosphorylated residues on the receptor itself and on subsequently bound receptor substrates provide docking sites for downstream signalling molecules, including adapters, protein serine/threonine kinases, phosphoinositide kinases and exchange factors. Collectively, those molecules orchestrate the numerous insulin-mediated physiological responses. A clear picture is emerging of the way in which insulin elicits several intracellular signalling pathways to mediate its physiologic functions. A further challenge, being pursued by several laboratories, is to understand the molecular mechanisms that underlie insulin action at the peripheral level, deregulation of which ultimately leads to hyperglycaemia and Type 2 diabetes. We review how circulating factors such as insulin itself, TNF-alpha, interleukins, fatty acids and glycation products influence insulin action through insulin signalling molecules themselves or through other pathways ultimately impinging on the insulin-signalling pathway. Understanding how the mechanism by which molecular insulin action is modulated by these factors will potentially provide new targets for pharmacological agents, to enable the control of altered glucose and lipid metabolism and diabetes.

Akt-mediated cardiomyocyte survival pathways are compromised by G alpha q-induced phosphoinositide 4,5-bisphosphate depletion

Expression of the wild type alpha subunit of Gq (GqWT) in cardiomyocytes induces hypertrophy, whereas a constitutively active G alpha q subunit (GqQ209L) induces apoptosis. Akt phosphorylation increases with GqWT expression but is markedly attenuated in cardiomyocytes expressing GqQ209L or in those expressing GqWT and treated with agonist. A membrane-targeted Akt rescues GqQ209L-expressing cardiomyocytes from apoptotic cell death. In contrast, leukemia inhibitory factor fails to activate Akt or promote cell survival in these cells. Association of Akt and PDK-1 with the membrane is also diminished in GqQ209L-expressing cardiomyocytes. Phosphatidylinositol 3,4,5-trisphosphate (PIP3), the primary regulator of Akt, increases significantly in GqWT-expressing cells but not in cardiomyocytes expressing GqQ209L. Levels of phosphatidylinositol 4,5-bisphosphate (PIP2), the immediate precursor of PIP3, are also markedly lower in GqQ209L-expressing compared to control cells. Expression of a GqQ209L mutant that has diminished capacity to activate phospholipase C does not decrease PIP2 or Akt or induce apoptosis. In transgenic mice with cardiac G alpha q overexpression, heart failure and increased cardiomyocyte apoptosis develop during the peripartal period. Akt phosphorylation and PIP2 levels decrease concomitantly. Our findings suggest that an Akt-mediated cell survival pathway is compromised by the diminished availability of PIP2 elicited by pathological levels of Gq activity.

Platelet-derived growth factor (PDGF) stimulates glucose transport in 3T3-L1 adipocytes overexpressing PDGF receptor by a pathway independent of insulin receptor substrates

Insulin is unique among growth factors and hormones in its ability to control metabolic functions such as the stimulation of glucose uptake and glucose transporter (GLUT4) translocation in physiological target tissues, such as muscle and adipose cells. Nonetheless, the mechanisms underlying this specificity have remained incompletely understood, particularly in view of the ability of some growth factors to mimic insulin-dependent early signaling events. In this study, we have probed the basis of insulin specificity by overexpressing in hormone-responsive 3T3-L1 adipocytes wild-type platelet-derived growth factor (PDGF) receptor (PDGFR)-beta and selected, informative mutant receptor proteins. We show that such adipocytes overexpressing wild-type PDGFR on exposure to cognate growth factor activate glucose transport, GLUT4 translocation, and the serine-threonine protein kinase Akt/protein kinase B to a degree comparable with that produced in response to insulin. In addition, PDGF elicits the robust generation of phosphatidylinositol-3,4,5-trisphosphate in vivo in PDGFR-overexpressing 3T3-L1 adipocytes. Expression of PDGFR-beta mutant proteins demonstrates that these responses require the presence of an intact phosphatidylinositol 3-kinase (PI3K)-binding site on the overexpressed PDGF receptor. Furthermore, PDGF stimulates these effects independent of insulin receptor substrate(IRS)-1 or IRS-2 tyrosine phosphorylation or docking to activated PI3K. These data demonstrate that 1) the basis of insulin-specific glucose transport in cultured adipocytes is the low level of receptors for other growth factors and 2) in the presence of adequate receptors, PDGF is fully capable of activating glucose transport in a manner requiring PI3K and subsequent phosphatidylinositol-3,4,5-trisphosphate accumulation but independent of insulin, insulin receptor, and IRS proteins.

Insulin/insulin-like growth factor-I hybrid receptor abundance decreases with development in suckling pigs

The activation of the insulin signaling pathway that leads to translation initiation is enhanced in skeletal muscle of neonates, and decreases with development in parallel with the developmental decline in muscle protein synthesis. Because the elevated expression of insulin receptor (IR)/insulin-like growth factor-I receptor (IGF-IR) hybrids has been associated with insulin resistance in some studies, we hypothesized that IR/IGF-IR hybrid abundance and binding affinity increase with development. To test this hypothesis, we determined the abundances and binding affinities of the IR, IGF-IR and hybrid receptor in skeletal muscle of 7- and 26-d-old pigs. We found that the abundances of IR, IGF-IR and hybrid receptor were higher in muscle of 7- than 26-d-old pigs. However, the relative proportions of hybrid receptor abundance compared with IR abundance and IGF-IR abundance were similar at both ages. The binding affinities of the IR, IGF-IR and hybrid receptor also were similar at both ages. Overall, the results suggest that insulin/IGF-I hybrid receptor abundance and binding affinity do not contribute to the developmental decline in the activation of the insulin signaling pathway.

Differing roles of Akt and serum- and glucocorticoid-regulated kinase in glucose metabolism, DNA synthesis, and oncogenic activity

Serum- and glucocorticoid-regulated kinase (SGK) is a serine kinase that has a catalytic domain homologous to that of Akt, but lacks the pleckstrin homology domain present in Akt. Akt reportedly plays a key role in various cellular actions, including glucose transport, glycogen synthesis, DNA synthesis, anti-apoptotic activity, and cell proliferation. In this study, we attempted to reveal the different roles of SGK and Akt by overexpressing active mutants of Akt and SGK. We found that adenovirus-mediated overexpression of myristoylated (myr-) forms of Akt resulted in high glucose transport activity in 3T3-L1 adipocytes, phosphorylated glycogen synthase kinase-3 (GSK3) and enhanced glycogen synthase activity in hepatocytes, and the promotion of DNA synthesis in interleukin-3-dependent 32D cells. In addition, stable transfection of myr-Akt in NIH3T3 cells induced an oncogenic transformation in soft agar assays. The active mutant of SGK (D-SGK, substitution of Ser422 with Asp) and myr-SGK were shown to phosphorylate GSK3 and to enhance glycogen synthase activity in hepatocytes in a manner very similar to that observed for myr-Akt. However, despite the comparable degree of GSK3 phosphorylation between myr-Akt and d-SGK or myr-SGK, d-SGK and myr-SGK failed to enhance glucose transport activity in 3T3-L1 cells, DNA synthesis in 32D cells, and oncogenic transformation in NIH3T3 cells. Therefore, the different roles of SGK and Akt cannot be attributed to ability or inability to translocate to the membrane thorough the pleckstrin homology domain, but rather must be attributable to differences in the relatively narrow substrate specificities of these kinases. In addition, our observations strongly suggest that phosphorylation of GSK3 is either not involved in or not sufficient for GLUT4 translocation, DNA synthesis, or oncogenic transformation. Thus, the identification of substrates selectively phosphorylated by Akt, but by not SGK, may provide clues to clarifying the pathway leading from Akt activation to these cellular activities.

In situ electroporation of radioactive compounds into adherent cells

We previously developed a technique, termed in situ electroporation, where nonpermeant molecules are introduced through an electrical pulse into adherent cells, while they grow on electrically conductive, optically transparent, indium-tin oxide (ITO). Careful control of the electric field intensity results in essentially 100% of the cells taking up the introduced material, without any detectable effect upon the physiology of the cell, presumably because the pores reseal rapidly so that the cellular interior is restored to its original state. Electroporation of radioactive material is faced with two important considerations: (1) potential for exposure of personnel to irradiation, and (2) the requirement for electroporation of a large number of cells. In this report, we describe a modification in the geometry of the slides and electrodes which permits the use of inexpensive ITO-coated glass of lower conductivity that can be discarded after use, to electroporate large numbers of cells using a minimum volume of radioactive nucleotide solution. The results demonstrate that, using this assembly, the determination of the Ras-bound GTP/GTP+GDP ratios through electroporation of [alpha32P]GTP can be conducted using approximately five times lower amounts of isotope than in previous designs. Moreover, this assembly permits efficient upscaling, which makes the determination of Ras-GTP binding in cells which are deficient in Ras activity possible. In addition, we demonstrate the labeling of two viral phosphoproteins--the Simian Virus 40 Large Tumor antigen, and Adenovirus E1A--through [gamma32P]ATP electroporation using this setup. In both cases, electroporation of the nucleotide can achieve a great increase in the efficiency and specificity of labeling compared to the addition of [32P]-orthophosphate to the culture medium, presumably because the immediate phosphate donor nucleotide itself is introduced, which can directly bind to the target proteins.

Cdc42 is a Rho GTPase family member that can mediate insulin signaling to glucose transport in 3T3-L1 adipocytes

We investigated the role of cdc42, a Rho GTPase family member, in insulin-induced glucose transport in 3T3-L1 adipocytes. Microinjection of anti-cdc42 antibody or cdc42 siRNA led to decreased insulin-induced and constitutively active G(q) (CA-G(q); Q209L)-induced GLUT4 translocation. Adenovirus-mediated expression of constitutively active cdc42 (CA-cdc42; V12) stimulated 2-deoxyglucose uptake to 56% of the maximal insulin response, and this was blocked by treatment with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, wortmannin, or LY294002. Both insulin and CA-G(q) expression caused an increase in cdc42 activity, showing that cdc42 is activated by insulin and is downstream of G alpha(q/11) in this activation pathway. Immunoprecipitation experiments showed that insulin enhanced a direct association of cdc42 and p85, and both insulin treatment and CA-cdc42 expression stimulated PI3-kinase activity in immunoprecipitates with anti-cdc42 antibody. Furthermore, the effects of insulin, CA-G(q), and CA-cdc42 on GLUT4 translocation or 2-deoxyglucose uptake were inhibited by microinjection of anti-protein kinase C lambda (PKC lambda) antibody or overexpression of a kinase-deficient PKC lambda construct. In summary, activated cdc42 can mediate 1) insulin-stimulated GLUT4 translocation and 2) glucose transport in a PI3-kinase-dependent manner. 3) Insulin treatment and constitutively active G(q) expression can enhance the cdc42 activity state as well as the association of cdc42 with activated PI3-kinase. 4) PKC lambda inhibition blocks CA-cdc42, CA-G(q), and insulin-stimulated GLUT4 translocation. Taken together, these data indicate that cdc42 can mediate insulin signaling to GLUT4 translocation and lies downstream of G alpha(q/11) and upstream of PI3-kinase and PKC lambda in this stimulatory pathway.

Sumoylation of Pdx1 is associated with its nuclear localization and insulin gene activation

Pancreatic duodenal homeobox-1 (Pdx1) is a transcription factor, and its phosphorylation is thought to be essential for activation of insulin gene expression. This phosphorylation is related to a concomitant shift in molecular mass from 31 to 46 kDa. However, we found that Pdx1 was modified by SUMO-1 (small ubiquitin-related modifier 1) in beta-TC-6 and COS-7 cells, which were transfected with Pdx1 cDNA. This modification contributed to the increase in molecular mass of Pdx1 from 31 to 46 kDa. Additionally, sumoylated Pdx1 localized in the nucleus. The reduction of SUMO-1 protein by use of RNA interference (SUMO-iRNAs) resulted in a significant decrease in Pdx1 protein in the nucleus. A 34-kDa form of Pdx1 was detected by the cells exposed to SUMO-iRNAs in the presence of lactacystin, a proteasome inhibitor. Furthermore, the reduced nuclear sumoylated Pdx1 content was associated with significant lower transcriptional activity of the insulin gene. These findings indicate that SUMO-1 modification is associated with both the localization and stability of Pdx1 as well as its effect on insulin gene activation.

Granulocyte-macrophage colony-stimulating factor signals for increased glucose transport via phosphatidylinositol 3-kinase- and hydrogen peroxide-dependent mechanisms

Granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulates cellular glucose uptake by decreasing the apparent K(m) for substrate transport through facilitative glucose transporters on the plasma membrane. Little is known about this signal transduction pathway and the role of the alpha subunit of the GM-CSF receptor (alpha GMR) in modulating transporter activity. We examined the function of phosphatidylinositol 3-kinase (PI 3-kinase) in GM-CSF-stimulated glucose uptake and found that PI 3-kinase inhibitors, wortmannin and LY294002, completely blocked the GM-CSF-dependent increase of glucose uptake in Xenopus oocytes expressing the low affinity alpha GMR and in human cells expressing the high affinity alpha beta GMR complex. We identified a Src homology 3 domain-binding motif in alpha GMR at residues 358-361 as a potential interaction site for the PI 3-kinase regulatory subunit, p85. Physical evidence for p85 binding to alpha GMR was obtained by co-immunoprecipitation with antibodies to alpha GMR and p85, and an alpha GMR mutant with alteration of the Src homology 3 binding domain lost the ability to bind p85. Experiments with a construct eliminating most of the intracellular portion of alpha GMR showed a 50% reduction in GM-CSF-stimulated glucose uptake with residual activity blocked by wortmannin. Searching for a proximally generated diffusible factor capable of activating PI 3-kinase, we identified hydrogen peroxide (H(2)O(2)), generated by ligand or antibody binding to alpha GMR, as the initiating factor. Catalase treatment abrogated GM-CSF- or anti-alpha GMR antibody-stimulated glucose uptake in alpha GMR-expressing oocytes, and H(2)O(2) activated PI 3-kinase and led to some stimulation of glucose uptake in uninjected oocytes. Human myeloid cell lines and primary explant human lymphocytes expressing high affinity GM-CSF receptors responded to alpha GMR antibody with increased glucose uptake. These results identify the early events in the stimulation of glucose uptake by GM-CSF as involving local H(2)O(2) generation and requiring PI 3-kinase activation. Our findings also provide a mechanistic explanation for signaling through the isolated alpha subunit of the GM-CSF receptor.

Protein kinase B/Akt is essential for the insulin- but not progesterone-stimulated resumption of meiosis in Xenopus oocytes

In the present study, we have characterized the Xenopus Akt expressed in oocytes from the African clawed frog Xenopus laevis and tested whether its activity is required for the insulin- and progesterone-stimulated resumption of meiosis. A cDNA encoding the Xenopus Akt was isolated and sequenced, and its expression in the Xenopus oocyte was confirmed by reverse transcription PCR and Northern blotting. Using phosphospecific antibodies and enzyme assays, a large and rapid activation of the Xenopus Akt was observed upon insulin stimulation of the oocytes. In contrast, progesterone caused a modest activation of this kinase with a slower time course. To test whether the activation of Akt was required in the stimulation of the resumption of meiosis, we have utilized two independent approaches: a functional dominant negative Akt mutant and an inhibitory monoclonal antibody. Both the mutant Akt, as well as the inhibitory monoclonal antibody, completely blocked the insulin-stimulated resumption of meiosis. In contrast, both treatments only partially inhibited (by approx. 30%) the progesterone-stimulated resumption of meiosis when submaximal doses of this hormone were utilized. These data demonstrate a crucial role for Akt in the insulin-stimulated cell cycle progression of Xenopus oocytes, whereas Akt may have an ancillary function in progesterone signalling.

A PI3-kinase signaling code for insulin-triggered insertion of glucose transporters into the plasma membrane

Activation of phosphatidyl-inositol-3'-OH-kinase (PI3K) and the resulting production of phosphatidyl-inositol-3,4,5-trisphosphate (PIP3) are ubiquitous signaling steps that link various cell surface receptors to multiple intracellular targets. In fat and muscle cells, the same PI3K pathway that regulates metabolic enzymes, proliferation, and differentiation has also been shown to be involved in insulin-triggered insertion of glucose transporter GLUT4 into the plasma membrane. The multiple PI3K functions raise the question of how the same PI3K pathway can be selectively used for different cell functions. Here we developed a dual-color evanescent wave microscopy method to simultaneously measure PIP3 production and GLUT4 insertion in individual 3T3L1 adipocytes. Activation of PI3K was found to be both necessary and sufficient for triggering GLUT4 insertion, but transporter insertion was markedly suppressed for small-amplitude, persistent PIP3 signals and for large-amplitude, short PIP3 signals. The rejection of these common PI3K signaling responses may explain the selective advantage of insulin over platelet-derived growth factor and other stimuli for inducing GLUT4 insertion. Our study suggests that the same PI3K pathway can control specific cell functions by relying on effector systems that respond to particular receptor-encoded time courses and amplitudes of PIP3 signals.

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

It is reported that 3-phosphoinositide-dependent protein kinase-1 (PDK-1) is activated in a phosphatidylinositol 3,4,5-trisphosphate-dependent manner and phosphorylates Akt, p70S6 kinase, and atypical protein kinase C (PKC), but its function on insulin signaling is still unclear. We cloned a full-length pdk-1 cDNA from a human brain cDNA library, and the adenovirus to overexpress wild type PDK-1 (PDK-1WT) or membrane-targeted PDK-1 (PDK-1CAAX) was constructed. Overexpressed PDK-1WT existed mainly at cytosol, and PDK-1CAAX was located at the plasma membrane. In 3T3-L1 adipocytes, insulin induced mobility shift of PDK-1 protein, but overexpressed PDK-1WT and CAAX were shifted at the basal state. Insulin stimulated tyrosine phosphorylation of PDK-1WT, but PDK-1CAAX was already tyrosine-phosphorylated at the basal state. Overexpression of PDK-1WT led to a full activation of PKC zeta/lambda without insulin stimulation but showed only the minimum effects to stimulate phosphorylation of Akt and GSK-3. In contrast, the overexpression of PDK-1CAAX caused phosphorylation of Akt and GSK-3 more strongly without insulin stimulation. However, PDK-1CAAX did not affect 2-deoxyglucose uptake and inhibited glycogen synthesis, surprisingly. Finally, PDK-1CAAX expression inhibited insulin-induced ERK1/2 phosphorylation in a dose-dependent manner. Taken together, the translocation of PDK-1 from cytosol to the plasma membrane is critical for Akt and GSK-3 activation. On the other hand, only atypical PKC and Akt activation was insufficient for stimulation of glucose transport, and constitutive activation of Akt-GSK-3 pathway may inhibit glycogen synthesis and MAPK cascade in 3T3-L1 adipocytes.

Insulin activates CCAAT/enhancer binding proteins and proinflammatory gene expression through the phosphatidylinositol 3-kinase pathway in vascular smooth muscle cells

Phosphatidylinositol 3-kinase (PI3K) is a key molecule mediating signals of insulin in vascular smooth muscle cells (VSMCs). To examine the effect of chronic activation of PI3K on the gene expression of VSMCs, membrane-targeted p110CAAX, a catalytic subunit of PI3K, was overexpressed in rat VSMCs by adenovirus-mediated gene transfer. Similar to insulin's effects, cells overexpressing p110CAAX exhibited a 10- to 15-fold increase in monocyte chemoattractant protein-1 (MCP-1) mRNA expression as compared with the control cells. Electrophoretic mobility shift assay analysis showed that the overexpression of p110CAAX activated neither the NF-kappaB binding nor the activator protein (AP-1) binding activities. We found that two CCAAT/enhancer binding protein (C/EBP) binding sites located between 2.6 and 3.6 kb upstream of the MCP-1 gene were responsible for the induction by p110CAAX. The overexpression of C/EBP-beta and C/EBP-delta but not C/EBP-alpha caused 6- to 8-fold induction of MCP-1 promoter activity. Consistently, the overexpression of p110CAAX as well as insulin induced mRNA expression and nuclear expression of C/EBP-beta and C/EBP-delta in VSMCs. These results clearly indicate that the activation of PI3K induced proinflammatory gene expression through activating C/EBP-beta and C/EBP-delta but not NF-kappaB, which may explain the proinflammatory effect of insulin in the insulin-resistant state.