Suppression of insulin-stimulated phosphatidylinositol 3-kinase activity by the beta3-adrenoceptor agonist CL316243 in rat adipocytes

Insulin increased 2-deoxyglucose (2-DG) uptake via the translocation of glucose transporter (GLUT) 4 to the plasma membrane fraction in rat adipocytes. The stimulatory actions of insulin were accompanied by both an increase in the immunoreactive p85 subunit of phosphatidylinositol (PI) 3-kinase in the plasma membrane fractions and PI 3-kinase activation by tyrosine phosphorylation of the p85 subunit. The beta3-adrenoceptor agonist CL316243 (CL) suppressed all the insulin actions in adenosine deaminase (ADA)-treated cells, but was without effect in non-ADA-treated cells. The inhibitory effects of CL on GLUT 4 translocation and PI 3-kinase activation were abolished by the addition of N6-phenylisopropyl adenosine. Cholera toxin treatment, which markedly increased intracellular cAMP levels, suppressed increases in the levels of GLUT 4 and PI 3-kinase in the plasma membrane fractions in response to insulin. In addition, dibutyryl (Bt2) cAMP also impaired the activation of PI 3-kinase by insulin. These results indicated that CL suppressed insulin-stimulated glucose transport under conditions where cAMP levels were markedly increased (approximately 12-fold). The inhibitory actions of PI 3-kinase activation by insulin were exerted even when cAMP, 8-bromo-cAMP, or Bt2 cAMP was added to immunoprecipitates of the p85 subunit of PI 3-kinase, after treating the cells with insulin. These results suggest that CL suppressed insulin-stimulated PI 3-kinase activity via a cAMP-dependent mechanism, at least in part, direct cAMP action in ADA-treated adipocytes, by which PI 3-kinase activation was inhibited, resulting in the decrease in GLUT 4 translocation and subsequent 2-DG uptake in response to insulin.     

Regulation of insulin-stimulated GLUT4 translocation by Munc18c in 3T3L1 adipocytes

Insulin stimulates glucose transporter (GLUT) 4 vesicle translocation from intracellular storage sites to the plasma membrane in 3T3L1 adipocytes through a VAMP2- and syntaxin 4-dependent mechanism. We have observed that Munc18c, a mammalian homolog of the yeast syntaxin-binding protein n-Sec1p, competed for the binding of VAMP2 to syntaxin 4. Consistent with an inhibitory function for Munc18c, expression of Munc18c, but not the related Munc18b isoform, prevented the insulin stimulation of GLUT4 and IRAP/vp165 translocation to the plasma membrane without any significant effect on GLUT1 trafficking. As expected, overexpressed Munc18c was found to co-immunoprecipitate with syntaxin 4 in the basal state. However, these complexes were found to dissociate upon insulin stimulation. Furthermore, endogenous Munc18c was predominantly localized to the plasma membrane and its distribution was not altered by insulin stimulation. Although expression of enhanced green fluorescent protein-Munc18c primarily resulted in a dispersed cytosolic distribution, co-expression with syntaxin 4 resulted in increased localization to the plasma membrane. Together, these data suggest that Munc18c inhibits the docking/fusion of GLUT4-containing vesicles by blocking the binding of VAMP2 to syntaxin 4. Insulin relieves this inhibition by inducing the dissociation of Munc18c from syntaxin 4 and by sequestering Munc18c to an alternative plasma membrane binding site.     

A novel, multifuntional c-Cbl binding protein in insulin receptor signaling in 3T3-L1 adipocytes

The protein product of the c-Cbl proto-oncogene is prominently tyrosine phosphorylated in response to insulin in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. After insulin-dependent tyrosine phosphorylation, c-Cbl specifically associates with endogenous c-Crk and Fyn. These results suggest a role for tyrosine-phosphorylated c-Cbl in 3T3-L1 adipocyte activation by insulin. A yeast two-hybrid cDNA library prepared from fully differentiated 3T3-L1 adipocytes was screened with full-length c-Cbl as the target protein in an attempt to identify adipose-specific signaling proteins that interact with c-Cbl and potentially are involved in its tyrosine phosphorylation in 3T3-L1 adipocytes. Here we describe the isolation and the characterization of a novel protein that we termed CAP for c-Cbl-associated protein. CAP contains a unique structure with three adjacent Src homology 3 (SH3) domains in the C terminus and a region showing significant sequence similarity with the peptide hormone sorbin. Both CAP mRNA and proteins are expressed predominately in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. CAP associates with c-Cbl in 3T3-L1 adipocytes independently of insulin stimulation in vivo and in vitro in an SH3-domain-mediated manner. Furthermore, we detected the association of CAP with the insulin receptor. Insulin stimulation resulted in the dissociation of CAP from the insulin receptor. Taken together, these data suggest that CAP represents a novel c-Cbl binding protein in 3T3-L1 adipocytes likely to participate in insulin signaling.     

Presenilin 1 associates with glycogen synthase kinase-3beta and its substrate tau

Families bearing mutations in the presenilin 1 (PS1) gene develop Alzheimer's disease. Previous studies have shown that the Alzheimer-associated mutations in PS1 increase production of amyloid beta protein (Abeta1-42). We now show that PS1 also regulates phosphorylation of the microtubule-associated protein tau. PS1 directly binds tau and a tau kinase, glycogen synthase kinase 3beta (GSK-3beta). Deletion studies show that both tau and GSK-3beta bind to the same region of PS1, residues 250-298, whereas the binding domain on tau is the microtubule-binding repeat region. The ability of PS1 to bring tau and GSK-3beta into close proximity suggests that PS1 may regulate the interaction of tau with GSK-3beta. Mutations in PS1 that cause Alzheimer's disease increase the ability of PS1 to bind GSK-3beta and, correspondingly, increase its tau-directed kinase activity. We propose that the increased association of GSK-3beta with mutant PS1 leads to increased phosphorylation of tau.     

Erythropoietin induces the association of phosphatidylinositol 3'-kinase with a tyrosine-phosphorylated protein complex containing the erythropoietin receptor

Stimulation of sensitive cells with erythropoietin results in rapid induction of protein tyrosine phosphorylation. Other than tyrosine phosphorylation of one chain of the erythropoietin receptor, the identities of the remaining tyrosine-phosphorylated proteins are undefined. In this report, we demonstrate that the stimulation of the erythropoietin-sensitive human UT7 cells by erythropoietin rapidly resulted in the appearance of phosphatidylinositol 3-kinase activity in anti-phosphotyrosine immunoprecipitates. Erythropoietin action was rapid, detectable after as early as 1 min stimulation, transient, returning to control level after 30 min stimulation and was observed using the erythropoietin concentrations able to stimulate the cell proliferation. Anti-(phosphatidylinositol 3-kinase) antibodies specifically immunoprecipitated 125I-erythropoietin bound to its receptor, strongly suggesting that phosphatidylinositol 3-kinase associated with a protein complex containing the activated erythropoietin receptor. To confirm this result, phosphatidylinositol 3-kinase was immunoprecipitated from erythropoietin-stimulated cells using mild conditions followed by Western analysis using anti-phosphotyrosine antibodies. Five tyrosine phosphorylated proteins were revealed: the cloned chain of the erythropoietin receptor, the regulatory subunit of phosphatidylinositol 3-kinase and three unidentified proteins of 111, 97 and 64 kDa. None of these tyrosine phosphorylated proteins was detected in anti-(phosphatidylinositol 3-kinase) immunoprecipitates from unstimulated cells. Thus, our results show that phosphatidylinositol 3-kinase associates with a tyrosine-phosphorylated protein complex containing the activated erythropoietin receptor.     

The regulatory mechanism of phosphatidylinositol 3-kinase by insulin in 3T3 L1 fibroblasts: phosphorylation-independent activation of phosphatidylinositol 3-kinase

Progress in understanding the basis of resistance to rifampicin (RifR) has allowed molecular tests for the detection of drug-resistant tuberculosis to be developed. One hundred thirteen strains of Mycobacterium tuberculosis isolated from patients with multidrug resistant tuberculosis (MDR-TB) were investigated for genotypic analysis of RifR by polymerase chain reaction-heteroduplex formation (PCR-HDF) and characterization of mutations by automated DNA sequencing of the rpoB gene. A subset of isolates (22) representative of different mutations as confirmed by sequence analysis were also evaluated by the Line Probe Assay (LiPA). In 106 of the RifR strains, 24 mutations within an 81-bp region of the rpoB gene affecting 13 amino acids were observed. Most isolates (7/8) harboring Leu533 --> Pro codon mutation required minimum inhibitory concentrations (MICs) of < or = 8 microg/ml. There was geographic variation in the frequency of occurrence of particular rpoB mutations, with the Ser531 --> Leu/Trp codon mutation found in 59/113 of isolates. Although there are certain limitations in the use of both the rapid PCR-HDF diagnostic assay and the LiPA for the detection of rifampicin susceptibility of M. tuberculosis, these provide important and convenient tools for identifying and managing patients with MDR-TB.     

The insulin receptor substrate 1 associates with phosphotyrosine phosphatase SHPTP2 in liver and muscle of rats

OBJECTIVE: To determine efficacy of a vaccine containing modified-live bovine viral diarrhea virus (BVDV) type 1 for protecting pregnant cows and their fetuses against virulent heterologous BVDV type 1. DESIGN: Randomized controlled cohort study. ANIMALS: 18 yearling beef heifers seronegative for BVDV and negative when tested for BVDV by virus isolation. PROCEDURE: Cattle were randomly assigned to control (unvaccinated; n = 6) or vaccinated (12) groups. Vaccinated heifers were given a combination vaccine containing modified-live BVDV type 1 comprising a cytopathic (NADL) strain. All 18 heifers were then bred and challenge-exposed between 70 and 75 days of gestation with BVDV type 1, administered intranasally. Cattle were monitored, and infection status of offspring was determined after parturition. Antibody concentrations of vaccinated and control heifers were also monitored. RESULTS: All 6 calves from control heifers had positive results on multiple virus isolation tests and were considered persistently infected. In comparison, only 2 calves from vaccinated cows had positive results on virus isolation tests and were considered persistently infected. One vaccinated heifer aborted, but the fetus was not persistently infected, and the abortion was not attributed to BVDV infection. CLINICAL IMPLICATIONS: Analysis of these data indicated that a single dose of a modified-live NADL-derived BVDV type 1 vaccine will confer protection to dams and their fetuses against challenge-exposure to heterologous BVDV type 1 organisms.     

Comparison of the signaling abilities of the cytoplasmic domains of the insulin receptor and the insulin receptor-related receptor in 3T3-L1 adipocytes

In the present work a chimeric receptor containing the intracellular domain of the insulin receptor-related receptor (IRR) and the extracellular domain of the colony stimulating factor-1 (CSF-1) receptor was expressed in 3T3-L1 adipocytes and compared with the parallel chimeric receptor containing the cytoplasmic domain of the insulin receptor (IR). Both chimeric receptors exhibited CSF-stimulated tyrosine kinase activity when assayed in vitro after in vivo activation comparable to that of the endogenous IR present in these cells. No cross-activation of the expressed chimeric and endogenous receptors was observed. The cytoplasmic domain of the IRR was found to 1) mediate activation of the Ser/Thr kinase Akt/PKB, 2) stimulate glucose uptake, 3) inhibit lipolysis, and 4) stimulate glycogen synthase, all with a potency comparable to those of the expressed CSF-1R/IR chimera and the endogenous insulin receptors. These results indicate that despite the extensive differences in sequence between the cytoplasmic domains of the IRR and IR, the elements required for insulin-specific responses have been conserved in this distinct member of the insulin receptor family.     

Mechanism of down-regulation of c-kit receptor. Roles of receptor tyrosine kinase, phosphatidylinositol 3'-kinase, and protein kinase C

The receptor tyrosine kinase Kit and Kit ligand (KL), encoded at the murine white spotting (W) and steel (Sl) loci, respectively, function in hematopoiesis, melanogenesis, and gametogenesis. To understand the mechanism of turnover of Kit in mast cells, mutant receptors generated in vitro were heterologously expressed in Wsb/Wsh mast cells lacking endogenous c-kit expression, and the effects of mutations on KL-induced internalization and ubiquitination/degradation of Kit were studied. Upon binding of KL, KL.Kit receptor complexes were rapidly internalized, and the turnover was accelerated by ubiquitin-mediated degradation. Inactivation of the Kit kinase resulted in a reduced rate of internalization of KL.Kit complexes, degradation of kinase-inactive receptor complexes was relatively slow, and receptor ubiquitination was absent. But abolishment of KL-induced receptor association and activation of phosphatidylinositol 3'-kinase and of tyrosine 821 autophosphorylation did not affect KL-induced internalization and ubiquitination/degradation of Kit. Furthermore, Kit receptors can be down-regulated by proteolytic cleavage induced by either activation of protein kinase C or by isopropyl alcohol. In summary, KL-induced internalization of KL.Kit complexes and ubiquitination/degradation require an active kinase. By contrast, proteolytic cleavage of Kit mediated by protein kinase C activation is independent of kinase activity.     

Interaction of insulin receptor substrate-1 with the sigma3A subunit of the adaptor protein complex-3 in cultured adipocytes

Signaling through the insulin receptor tyrosine kinase involves its autophosphorylation in response to insulin and the subsequent tyrosine phosphorylation of substrate proteins such as insulin receptor substrate-1 (IRS-1). In basal 3T3-L1 adipocytes, IRS-1 is predominantly membrane-bound, and this localization may be important in targeting downstream signaling elements that mediate insulin action. Since IRS-1 localization to membranes may occur through its association with specific membrane proteins, a 3T3-F442A adipocyte cDNA expression library was screened with non-tyrosine-phosphorylated, baculovirus-expressed IRS-1 in order to identify potential IRS-1 receptors. A cDNA clone that encodes sigma3A, a small subunit of the AP-3 adaptor protein complex, was demonstrated to bind IRS-1 utilizing this cloning strategy. The specific interaction between IRS-1 and sigma3A was further verified by in vitro binding studies employing baculovirus-expressed IRS-1 and a glutathione S-transferase (GST)-sigma3A fusion protein. IRS-1 and sigma3A were found to co-fractionate in a detergent-resistant population of low density membranes isolated from basal 3T3-L1 adipocytes. Importantly, the addition of exogenous purified GST-sigma3A to low density membranes caused the release of virtually all of the IRS-1 bound to these membranes, while GST alone had no effect. These results are consistent with the hypothesis that sigma3A serves as an IRS-1 receptor that may dictate the subcellular localization and the signaling functions of IRS-1.     

SHP-1 associates with both platelet-derived growth factor receptor and the p85 subunit of phosphatidylinositol 3-kinase

The Src homology 2 (SH2)-containing protein tyrosine phosphatase 1, SHP-1, is highly expressed in all hematopoietic cells as well as in many non-hematopoietic cells, particularly in some malignant epithelial cell lines. In hematopoietic cells, SHP-1 negatively regulates multiple cytokine receptor pathways. The precise function and the targets of SHP-1 in non-hematopoietic cells, however, are largely unknown. Here we demonstrate that SHP-1 associates with both the tyrosine-phosphorylated platelet-derived growth factor (PDGF) receptor and the p85 subunit of phosphatidylinositol 3-kinase in MCF-7 and TRMP cells. Through the use of mutant PDGF receptors and performing peptide competition for immunoprecipitation, it was determined that SHP-1 independently associates with the PDGF receptor and p85 and that its N-terminal SH2 domain is directly responsible for the interactions. Overexpression of SHP-1 in TRMP cells transfected with the PDGF receptor markedly inhibited PDGF-induced c-fos promoter activation, whereas the expression of three catalytically inactive SHP-1 mutants increased the c-fos promoter activation in response to PDGF stimulation. These results indicate that SHP-1 might negatively regulate PDGF receptor-mediated signaling in these cells. Identification of the association of SHP-1 with the PDGF receptor and p85 in MCF-7 and TRMP cells furthers our understanding of the function of SHP-1 in non-hematopoietic cells.     

Estrogen stimulation of creatine kinase B specific activity in 3T3L1 adipocytes after their differentiation in culture: dependence on estrogen receptor

We have demonstrated previously that rat adipose tissue showed sex and depot-specific responses to gonadal steroids. The epididymal fat pad in males responded exclusively to androgens by increased specific activity of the brain type isozyme of creatine kinase (CK). In females, the parametrial adipose tissue responded exclusively to estrogens. The present study was undertaken to follow the responsiveness to steroid hormones, and the presence of estrogen receptors (ER), in 3T3L1 cells during their differentiation from pre-adipocytes to adipocytes. In pre-adipocytes in which the basal CK specific activity is low, there was no CK response to 17beta estradiol (E2) or dihydrotestosterone (DHT). Differentiation of the cells into adipocytes was accompanied by increased basal CK activity which was stimulated by E2, but not by DHT. Responsiveness to E2 began 5 days after switching pre-adipocytes to differentiation medium. Upon differentiation, ER became demonstrable in the cell nuclei by staining with FITC labeled anti-idiotypic antibody (clone 1D5) directed against the steroid binding domain of ER. The response to E2 was time-dependent and blocked completely by cycloheximide or actinomycin D. 1D5 itself, which has an estrogen mimetic effect, stimulated CK activity in the cells similarly to E2. The antiestrogen tamoxifen which also stimulated CK activity in the adipocytes, completely blocked E2 action. The 'pure' antagonist of E2, ICI 164,384 and the tissue-selective antiestrogens, raloxifene or tamoxifen methiodide were also complete antagonists with no agonistic effects. The response of the 3T3L1 adipocytes to E2 was upregulated by 1,25(OH)2D3. Moreover, IGF1 was also a potent stimulator of CK in these cells, and therefore may mediate partially the stimulation by E2. Transient transfection of the pre-adipocytes with ER permitted E2 induction of CK. Thus, the appearance of ER and concomitant responsiveness to E2 is another hormone-related change occurring in 3T3L1 cells during differentiation, in addition to changes such as development of insulin responsiveness. The interactions in this system provide a useful in vitro model for investigating the development of responsiveness to E2.     

Association of the insulin receptor with phospholipase C-gamma (PLCgamma) in 3T3-L1 adipocytes suggests a role for PLCgamma in metabolic signaling by insulin

Phospholipase C-gamma (PLCgamma) is the isozyme of PLC phosphorylated by multiple tyrosine kinases including epidermal growth factor, platelet-derived growth factor, nerve growth factor receptors, and nonreceptor tyrosine kinases. In this paper, we present evidence for the association of the insulin receptor (IR) with PLCgamma. Precipitation of the IR with glutathione S-transferase fusion proteins derived from PLCgamma and coimmunoprecipitation of the IR and PLCgamma were observed in 3T3-L1 adipocytes. To determine the functional significance of the interaction of PLCgamma and the IR, we used a specific inhibitor of PLC, U73122, or microinjection of SH2 domain glutathione S-transferase fusion proteins derived from PLCgamma to block insulin-stimulated GLUT4 translocation. We demonstrate inhibition of 2-deoxyglucose uptake in isolated primary rat adipocytes and 3T3-L1 adipocytes pretreated with U73122. Antilipolytic effect of insulin in 3T3-L1 adipocytes is unaffected by U73122. U73122 selectively inhibits mitogen-activated protein kinase, leaving the Akt and p70 S6 kinase pathways unperturbed. We conclude that PLCgamma is an active participant in metabolic and perhaps mitogenic signaling by the insulin receptor in 3T3-L1 adipocytes.     

Stimulation of IRS-1-associated phosphatidylinositol 3-kinase and Akt/protein kinase B but not glucose transport by beta1-integrin signaling in rat adipocytes

The signal transduction pathway by which insulin stimulates glucose transport is not understood, but a role for complexes of insulin receptor substrate (IRS) proteins and phosphatidylinositol (PI) 3-kinase as well as for Akt/protein kinase B (PKB) has been proposed. Here, we present evidence suggesting that formation of IRS-1/PI 3-kinase complexes and Akt/PKB activation are insufficient to stimulate glucose transport in rat adipocytes. Cross-linking of beta1-integrin on the surface of rat adipocytes by anti-beta1-integrin antibody and fibronectin was found to cause greater IRS-1 tyrosine phosphorylation, IRS-1-associated PI 3-kinase activity, and Akt/PKB activation, detected by anti-serine 473 antibody, than did 1 nM insulin. Clustering of beta1-integrin also significantly potentiated stimulation of insulin receptor and IRS-1 tyrosine phosphorylation, IRS-associated PI 3-kinase activity, and Akt/PKB activation caused by submaximal concentrations of insulin. In contrast, beta1-integrin clustering caused neither a change in deoxyglucose transport nor an effect on the ability of insulin to stimulate deoxyglucose uptake at any concentration along the entire dose-response relationship range. The data suggest that (i) beta1-integrins can engage tyrosine kinase signaling pathways in isolated fat cells, potentially regulating fat cell functions and (ii) either formation of IRS-1/PI 3-kinase complexes and Akt/PKB activation is not necessary for regulation of glucose transport in fat cells or an additional signaling pathway is required.     

Effect of aging on insulin receptor, insulin receptor substrate-1, and phosphatidylinositol 3-kinase in liver and muscle of rats

Insulin stimulates the tyrosine kinase activity of its receptor, resulting in the phosphorylation of its cytosolic substrate, insulin receptor substrate-1 (IRS-1), which, in turn, associates with phosphatidylinositol 3-kinase (PI 3-kinase), thereby activating the latter. Aging is associated with insulin resistance, but the exact molecular mechanism is unknown. In the present study, we examined the levels and phosphorylation status of the insulin receptor and IRS-1 as well as the association between IRS-1 and PI 3-kinase in the liver and muscle of 2-, 5-, 12-, and 20-month-old rats. There were no changes in the insulin receptor concentration in the liver and muscle of rats 2-. 5-, 12-, and 20-month rats. There were no changes in the insulin receptor concentration in the liver and muscle of rats 2-20 months old, as determined by immunoblotting using antibody to the COOH-terminus of the receptor. However, insulin stimulation of receptor autophosphorylation, as determined by immunoblotting with antiphosphotyrosine antibody was reduced by 25% (P < 0.05) in the liver and muscle of rats at 20 months. Interestingly, IRS-1 protein levels decrease at an early stage (5 months) by 58 +/- 9%, (P < 0.01) and remained at low levels thereafter in muscle, but not in liver. In samples previously immunoprecipitated with anti-IRS-1 antibody and blotted with antiphosphotyrosine antibody, there were 60 +/- 9% (P < 0.001) and 92 +/- 4% (P < 0.001) decreases in the insulin-stimulated IRS-1 association with PI 3-kinase was decreased by 70 +/- 2% in the liver and muscle, respectively, of 20-month rats. The insulin-stimulated IRS-1 association with PI 3-kinase was decreased by 70 +/- 2% in the liver (P < 0.001) and by 98 +/- 3% (P < 0.001) in the muscle of 20-month-old rats, with no change in the PI 3-kinase protein levels. The phosphotyrosine-associated PI 3-kinase activity after insulin stimulation was dramatically reduced in liver and muscle of 20-month-old rats compared to that in 2-month-old rats. Finally, by immunoprecipitation, the detection of insulin-stimulated IRS-2 phosphorylation followed the same pattern as that for IRS-1 in both liver of 2- and 20-month-old rats. These data suggest that changes in the early steps of insulin signal transduction may have an important role in the insulin resistance observed in old animals.     

Fatty acid induced insulin resistance in rat-1 fibroblasts overexpressing human insulin receptors: impaired insulin-stimulated mitogen-activated protein kinase activity

Saturated fatty acids cause insulin resistance but the underlying molecular mechanism is still unknown. We examined the effect of saturated nonesterified fatty acids on insulin binding and action in transfected Rat-1 fibroblasts, which over-expressed human insulin receptors. Incubation with 1.0 mmol/l palmitate for 1-4 h did not affect insulin binding, insulin receptor autophosphorylation, insulin-stimulated tyrosine kinase activity toward poly(Glu4:Tyr1), pp185 and Shc phosphorylation and PI3-kinase activity in these cells. However, the dose response curve of insulin-stimulated glucose transport was right-shifted. Palmitate inhibited the maximally insulin-stimulated mitogen activated protein (MAP) kinase activity toward synthetic peptide to 7% that of control. The palmitate treatment influenced neither cytosolic protein kinase A activity nor cAMP levels. These results suggested that 1) palmitate did not inhibit the early steps of insulin action from insulin binding to pp185 or Shc phosphorylation but inhibited insulin-stimulated MAP kinase, and that 2) palmitate decreased insulin sensitivity as manifested by inhibited insulin-stimulated glucose uptake. In conclusion, the mechanism of saturated non-esterified fatty acid induced insulin resistance in glucose uptake may reside at post PI3-kinase or Shc steps, including the level of MAP kinase activation.     

Glucosamine-induced insulin resistance in 3T3-L1 adipocytes is caused by depletion of intracellular ATP

Glucosamine, which enters the hexosamine pathway downstream of the rate-limiting step, has been routinely used to mimic the insulin resistance caused by high glucose and insulin. We investigated the effect of glucosamine on insulin-stimulated glucose transport in 3T3-L1 adipocytes. The Delta-insulin (insulin-stimulated minus basal) value for 2-deoxyglucose uptake was dramatically inhibited with increasing concentrations of glucosamine with an ED50 of 1.95 mM. Subcellular fractionation experiments demonstrated that reduction in insulin-stimulated 2-deoxyglucose uptake by glucosamine was due to an inhibition of translocation of both Glut 1 and Glut 4 from the low density microsomes (LDM) to the plasma membrane. Analysis of the insulin signaling cascade revealed that glucosamine impaired insulin receptor autophosphorylation, insulin receptor substrate (IRS-1) phosphorylation, IRS-1-associated PI 3-kinase activity in the LDM, and AKT-1 activation by insulin. Measurement of intracellular ATP demonstrated that the effects of glucosamine were highly correlated with its ability to reduce ATP levels. Reduction of intracellular ATP using azide inhibited Glut 1 and Glut 4 translocation from the LDM to the plasma membrane, insulin receptor autophosphorylation, and IRS-1 tyrosine phosphorylation. Additionally, both the reduction in intracellular ATP and the effects on insulin action caused by glucosamine could be prevented by the addition of inosine, which served as an alternative energy source in the medium. We conclude that direct administration of glucosamine can rapidly lower cellular ATP levels and affect insulin action in fat cells by mechanisms independent of increased intracellular UDP-N-acetylhexosamines and that increased metabolism of glucose via the hexosamine pathway may not represent the mechanism of glucose toxicity in fat cells.     

Novel T cell antigen 4-1BB associates with the protein tyrosine kinase p56lck1

4-1BB is a 30-kDa inducible T cell Ag, and is expressed predominantly as a 55-kDa dimer on both CD4+ and CD8+ T lymphocytes. The cytoplasmic tail of 4-1BB contains the sequence Cys-Arg-Cys-Pro, which is similar to the sequence Cys-X-Cys-Pro, which mediates the binding of the CD4 and CD8 molecules to the protein tyrosine kinase p56lck. An anti-4-1BB mAb, 53A2, was used to determine whether 4-1BB may associate with p56lck. The 53A2 mAb specifically recognized 4-1BB on a CD8+ T cell line, CTLL-2, and coimmunoprecipitated a 56-kDa protein along with 4-1BB. Peptide mapping indicated that the 56-kDa phosphoprotein was identical to p56lck. The coimmunoprecipitation of p56lck with 4-1BB also occurred in nonlymphoid cells such as insect (Sf-21) and HeLa cells when the two recombinant proteins were coexpressed. Analysis of mutant p56lck recombinant proteins showed that two cysteine residues critical for p56lck-CD4 (or -CD8) complex formation are also required for the p56lck-4-1BB interaction. These studies establish that 4-1BB physically associates with p56lck.     

Requirement of atypical protein kinase clambda for insulin stimulation of glucose uptake but not for Akt activation in 3T3-L1 adipocytes

Phosphoinositide (PI) 3-kinase contributes to a wide variety of biological actions, including insulin stimulation of glucose transport in adipocytes. Both Akt (protein kinase B), a serine-threonine kinase with a pleckstrin homology domain, and atypical isoforms of protein kinase C (PKCzeta and PKClambda) have been implicated as downstream effectors of PI 3-kinase. Endogenous or transfected PKClambda in 3T3-L1 adipocytes or CHO cells has now been shown to be activated by insulin in a manner sensitive to inhibitors of PI 3-kinase (wortmannin and a dominant negative mutant of PI 3-kinase). Overexpression of kinase-deficient mutants of PKClambda (lambdaKD or lambdaDeltaNKD), achieved with the use of adenovirus-mediated gene transfer, resulted in inhibition of insulin activation of PKClambda, indicating that these mutants exert dominant negative effects. Insulin-stimulated glucose uptake and translocation of the glucose transporter GLUT4 to the plasma membrane, but not growth hormone- or hyperosmolarity-induced glucose uptake, were inhibited by lambdaKD or lambdaDeltaNKD in a dose-dependent manner. The maximal inhibition of insulin-induced glucose uptake achieved by the dominant negative mutants of PKClambda was approximately 50 to 60%. These mutants did not inhibit insulin-induced activation of Akt. A PKClambda mutant that lacks the pseudosubstrate domain (lambdaDeltaPD) exhibited markedly increased kinase activity relative to that of the wild-type enzyme, and expression of lambdaDeltaPD in quiescent 3T3-L1 adipocytes resulted in the stimulation of glucose uptake and translocation of GLUT4 but not in the activation of Akt. Furthermore, overexpression of an Akt mutant in which the phosphorylation sites targeted by growth factors are replaced by alanine resulted in inhibition of insulin-induced activation of Akt but not of PKClambda. These results suggest that insulin-elicited signals that pass through PI 3-kinase subsequently diverge into at least two independent pathways, an Akt pathway and a PKClambda pathway, and that the latter pathway contributes, at least in part, to insulin stimulation of glucose uptake in 3T3-L1 adipocytes.     

Reconstitution of insulin signaling pathways in rat 3Y1 cells lacking insulin receptor and insulin receptor substrate-1. Evidence that activation of Akt is insufficient for insulin-stimulated glycogen synthesis or glucose uptake in rat 3Y1 cells

Rat 3Y1 cells have endogenous insulin-like growth factor-1 receptors and insulin receptor substrate (IRS)-2, but lack both insulin receptor (IR) and IRS-1. To investigate the role of IR and IRS-1 in effects of insulin, we transfected IR and IRS-1 expression plasmids into cells and reconstituted the insulin signaling pathways. 3Y1 cells stably expressing the c-myc epitope-tagged glucose transporter type 4 (3Y1-GLUT4myc) exhibit no effects of insulin, at physiological concentrations. The 3Y1-GLUT4myc-IR cells expressing GLUT4myc and IR responded to phosphatidylinositol 3,4, 5-trisphosphate (PI-3,4,5-P3) accumulation, Akt activation, the stimulation of DNA synthesis, and membrane ruffling but not to glycogen synthesis, glucose uptake, or GLUT4myc translocation. The further expression of IRS-1 in 3Y1-GLUT4myc-IR cells led to stimulation of glycogen synthesis but not to glucose uptake or GLUT4myc translocation in response to insulin, although NaF or phorbol 12-myristate 13-acetate did trigger GLUT4myc translocation in the cells. These results suggest that, in rat 3Y1 cells, (i) IRS-1 is essential for insulin-stimulated glycogen synthesis but not for DNA synthesis, PI-3,4,5-P3 accumulation, Akt phosphorylation, or membrane ruffling, and (ii) the accumulation of PI-3,4,5-P3 and activation of Akt are insufficient for glycogen synthesis, glucose uptake or for GLUT4 translocation.