Insulin receptor function is inhibited by guanosine 5'-[gamma-thio]triphosphate (GTP[S])

Cross-Talk Between Insulin Signaling and G Protein-Coupled Receptors

Diabetes is a major risk factor for the development of heart failure. One of the hallmarks of diabetes is insulin resistance associated with hyperinsulinemia. The literature shows that insulin and adrenergic signaling is intimately linked to each other; however, whether and how insulin may modulate cardiac adrenergic signaling and cardiac function remains unknown. Notably, recent studies have revealed that insulin receptor and β2 adrenergic receptor (β2AR) forms a membrane complex in animal hearts, bringing together the direct contact between 2 receptor signaling systems, and forming an integrated and dynamic network. Moreover, insulin can drive cardiac adrenergic desensitization via protein kinase A and G protein-receptor kinases phosphorylation of the β2AR, which compromises adrenergic regulation of cardiac contractile function. In this review, we will explore the current state of knowledge linking insulin and G protein-coupled receptor signaling, especially β-adrenergic receptor signaling in the heart, with emphasis on molecular insights regarding its role in diabetic cardiomyopathy.

Ligand-dependent autophosphorylation of the insulin receptor is positively regulated by Gi-proteins

Previously, we have shown that the human insulin receptor (IR) interacts with G(i)2, independent of tyrosine kinase activity and stimulates NADPH oxidase via the Galpha subunit of G(i)2. We have now investigated the regulatory role of G(i)2-proteins in IR function. For the experiments, isolated IRs from plasma membranes of human fat cells were used. The activation of IR autophosphorylation by insulin was blocked by G-protein inactivation through GDPbetaS (guanosine 5'-[beta-thio]disphosphate). Consistently, activation of G-proteins by micromolar concentrations of GTPgammaS (guanosine 5'-[gamma-thio]triphosphate) induced receptor autophosphorylation 5-fold over baseline and increased insulin-induced autophosphorylation by 3-fold. In the presence of 10 microM GTPgammaS, insulin was active at picomolar concentrations, indicating that insulin acted via its cognate receptor. Pretreatment of the plasma membranes with pertussis toxin prevented insulin- and GTPgammaS-induced autophosphorylation, but did not disrupt the IR-G(i)2 complex. The functional nature of the IR-G(i)2 complex was made evident by insulin's ability to increase association of G(i)2 with the IR. This leads to an augmentation of maximal receptor autophosphorylation induced by insulin and GTPgammaS. The specificity of this mechanism was further demonstrated by the use of isolated preactivated G-proteins. Addition of G(i)2alpha and Gbetagamma mimicked maximal response of insulin, whereas Galphas or Galphao had no stimulatory effect. These results define a novel mechanism by which insulin signalling mediates tyrosine kinase activity and autophosphorylation of the IR through recruitment of G(i)-proteins.

The role of G proteins in insulin signalling

Insulin modulates many intracellular processes including cellular metabolism, cell proliferation and cell differentiation. Some of these processes involve significant changes in the traffic of intracellular vesicles or in the structural organization of the cell. These phenomena have been linked to the activity of regulatory GTP-binding proteins. Most, if not all functions, of the insulin receptor are associated with its tyrosine kinase activity. Thus, over the past few years, a significant effort has been dedicated to elucidate the cross-talk between the tyrosine kinase activity of the receptor and the regulation of G protein-mediated pathways. Recent progress indicates that G proteins may mediate the control of several of insulin's intracellular functions. These include the regulation of the MAP kinase pathway, the activation of phospholipase D and the regulation of glucose uptake. This article discusses some recent advances in this area.

Insulin-induced activation of NADPH-dependent H2O2 generation in human adipocyte plasma membranes is mediated by Galphai2

Human fat cells possess a multireceptor-linked H2O2-generating system that is activated by insulin. Previous studies revealed that manganese was the sole cofactor required for a hormonal regulation of NADPH-dependent H2O2 generation in vitro. In this report it is shown that the synergistic activation of NADPH-dependent H2O2 generation by Mn2+ and insulin was blocked by GDPbetaS (guanosine 5'-O-(2-thiodiphosphate)), pertussis toxin and COOH-terminal anti-Galphai1-2 or the corresponding peptide. Consistently, manganese could be replaced by micromolar concentrations of GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), which increased NADPH-dependent H2O2 generation by 20-40%. Insulin shifted the dose response curve for GTPgammaS to the left (>10-fold) and increased the maximal response. In the presence of 10 microM GTPgammaS, the hormone was active at picomolar concentrations, indicating that insulin acted via its cognate receptor. The insulin receptor and Gi were co-adsorbed on anti-Galphai and anti-insulin receptor beta-subunit (anti-IRbeta) affinity columns. Partially purified insulin receptor preparations contained Galphas, Galphai2, and Gbetagamma (but no Galphai1 or Galphai3). The functional nature of the insulin receptor-Gi2 complex was made evident by insulin's ability to modulate labeling of Gi by bacterial toxins. Insulin action was mimicked by activated Galphai, but not by Galphao or Gbetagamma, indicating that insulin's signal was transduced via Galphai2. Thus, NADPH oxidase is the first example of an effector system that is coupled to the insulin receptor via a heterotrimeric G protein.

Guanine nucleotide binding regulatory proteins in liver from obese humans with and without type II diabetes: evidence for altered "cross-talk" between the insulin receptor and Gi-proteins

A novel pathway for physiological "cross-talk" between the insulin receptor and the regulatory Gi-protein has been demonstrated. We tested the hypothesis that a coupling defect between Gi and the insulin receptor is present in the liver of obese patients with and without type II diabetes. Insulin 1 x 10(-9) M (approximately ED50) and 1 x 10(-7) M (Max) inhibited pertussis toxin-catalyzed ADP ribosylation of Gi in human liver plasma membranes from lean and obese nondiabetic patients. However, 1 x 10(-7) M insulin was without effect in membranes from patients with type II diabetes. This coupling defect was not intrinsic to Gi, since Mg2+ and GTP gamma S inhibited pertussis toxin-catalyzed ADP ribosylation in both diabetic and nondiabetic patients. Binding of insulin of the alpha-subunit and activation of the tyrosine kinase intrinsic to the beta-subunit of the insulin receptor are not responsible for the coupling defect. 125I insulin binding is the same in obese patients with or without diabetes. Tyrosine kinase of the insulin receptor is decreased in diabetes. However, a monoclonal antibody to the insulin receptor (MA-20) at equimolar concentrations with insulin equally inhibits pertussis toxin-catalyzed ADP ribosylation of Gi without activating tyrosine kinase or insulin receptor autophosphorylation. Immunodetection of G-proteins suggested that Gi3 alpha was normal in diabetes and Gi1-2 alpha was decreased by 40% in the diabetic group as compared to the obese nondiabetic group but was normal when compared to the lean non diabetic group. We conclude that the novel pathway of insulin signaling involving the regulatory Gi proteins via biochemical mechanisms not directly involving the tyrosine kinase of the insulin receptor is altered in obese type II diabetes and offers a new target for the search of the mechanism(s) of insulin resistance.

Guanosine 5'-(gamma-thio) triphosphate (GTP gamma S) inhibits phosphorylation of insulin receptor and a novel GTP-binding protein, Gir, from human placenta

A novel 66 kDa GTP-binding protein, designated Gir, has been partially purified along with insulin receptor (IR) from human placenta. This protein binds 8-azido-GTP, is ADP-ribosylated by pertussis toxin, phosphorylated by IR tyrosine kinase and cross-reacts with antibodies against synthetic peptides from the GTP-binding domain of Gz alpha(P960). Phosphorylation of IR-beta subunit and Gir by IR tyrosine kinase was almost completely inhibited by 100 microM GTP gamma S, > 75% by 50 microM and 20-30% by 1 microM, while GDP at these concentrations had no significant effect on the phosphorylation. IR tyrosine kinase phosphorylated Gir at the tyrosine residues. These studies indicate regulation of IR tyrosine kinase activity by guanosine phosphates and involvement of Gir in insulin action.

Photoaffinity labelling of cardiac membrane GTP-binding proteins in response to insulin

Plasma membranes from rat cardiac ventricular tissue and insulin receptors partially purified by wheat-germ-agglutinin chromatography were subjected to direct photoaffinity labelling with [alpha-32P]GTP in order to elucidate the presence of insulin-receptor-coupled GTP-binding proteins. In plasma membranes three proteins have been identified that exhibit an enhanced photolabelling with the nucleotide in response to insulin. The apparent molecular masses of these proteins were found to be 56, 60 and 74 kDa. Photolabelling of partially purified insulin receptors showed the copurification of the 60-kDa species, whereas the 56-kDa and 74-kDa proteins could not be detected. Furthermore, the 60-kDa G-protein was found to be specifically co-immunoprecipitated with the insulin receptor. Incubation of insulin receptors with insulin increased the labelling of the 60-kDa band to 205 +/- 27% (n = 5) of control. Immuno- and ligand-blotting experiments revealed the additional presence of a 39-kDa G(o)-like protein and two G-proteins with molecular masses of 24 and 26 kDa in the receptor preparation. Under basal conditions the insulin receptor and the 60-kDa G-protein exhibited an apparent inverse distribution between plasma and microsomal membranes with the G-protein being extensively labelled in the microsomal fraction. In conclusion, our data show that, in its native environment, the cardiac insulin receptor couples to at least three GTP-binding proteins. Out of these, a 60-kDa species of microsomal origin, copurifies with the insulin receptor. It is suggested that this G-protein is associated with the insulin receptor and may be involved in insulin receptor signalling in target cells.

Insulin stimulates GDP release from G proteins in the rat and human liver plasma membranes

Plasma membranes (1-2 mg protein) prepared from the livers of adult male rats and human organ donors were incubated with 0.6 microM [alpha-32P] guanosine triphosphate (GTP) in an adenosine triphosphate (ATP)-regenerating buffer at 37 degrees C for 1 h; during this incubation, the [32P]GTP is hydrolyzed and the nucleotide that is predominantly bound to the membranes is [32P] guanosine diphosphate (GDP). [32P]GDP release from the liver membranes was proportional to the protein concentration and increased as a function of time. At 5 mM, Ca2+, Mg2+, Mn2+, and Zn2+ maximally inhibited GDP release by 80-90%, whereas, 5 mM Cu2+ maximally stimulated the reaction by 100%. Therefore, cations were not included in the buffer used in the GDP release step. One microM Gpp(NH)p (5'-guanylylimidodiphosphate), a nonhydrolyzable analog of GTP, maximally stimulated [32P]GDP release in the liver membranes by up to 30%. Although 10 nM Gpp(NH)p had no effect on GDP release, it appeared to stabilize the hormonal effect by blocking further GDP/GTP exchange. In the rat membranes, 1-100 nM glucagon (used as a positive control) stimulated [32P]GDP release by about 17% (P < .05); similarly, 0.1-100 nM insulin stimulated [32P]GDP release by 10-13% (P < .05). In the human membranes, 10 pM to 100 nM insulin stimulated [32P]GDP release by 7-10%. In the rat membranes, 10 nM insulin stimulated [32P]GDP release by 17 and 24% at 2 and 4 min, respectively (P < .05); in the human membranes, 10 nM insulin stimulated [32P]GDP release by about 9% at 2 and 4 min.(ABSTRACT TRUNCATED AT 250 WORDS)

An insulin receptor peptide (1135-1156) stimulates guanosine 5'-[gamma-thio]triphosphate binding to the 67 kDa G-protein associated with the insulin receptor

Peptides representing two putative G-protein-binding motifs (GPBP1 and GPBP2) derived from insulin-receptor sequences were tested for their ability to stimulate guanosine 5'-[gamma-thio]-triphosphate (GTP[S]; 'GTP gamma S') binding to a preparation containing the 41 and 67 kDa G-proteins that are associated with the insulin receptor [Jo, Cha, Davis and McDonald (1992) Endocrinology (Baltimore) 131, 2855-2861]. GPBP2 (residues 1135-1156) specifically stimulated GTP[S] binding, whereas GPBP1 (1319-1333) did not. Substitution of Arg-1152 with Gln in GPBP2 corresponding to a mutation site in insulin-resistant patients [Cocozza, Porcellini, Riccardi, Monticelli, Condorelli, Ferrera, Pianese, Miele, Capaldo, Beguinot and Varrone (1992) Diabetes 41, 521-526] attenuated the stimulatory potency of GPBP2. Size-exclusion chromatography and studies with purified 67 kDa G-protein revealed that GPBP2 stimulated GTP[S] binding only to the 67 kDa G-protein. These studies provide evidence for a potential regulatory site for G-protein interaction with the insulin receptor in the tyrosine kinase domain.

The mechanism of insulin action

Insulin was isolated over 70 years ago, but the intracellular transduction of the insulin signal has not been elucidated. Significant progress has been made, particularly in the last 10 years, with the characterization of the insulin receptor and its intrinsic tyrosine kinase. However, no mechanism has been proposed that accounts for all the actions of insulin. Furthermore, all the mechanisms discussed in this brief overview contain major inadequacies. Despite these gaps in our knowledge, substantial evidence indicates that receptor tyrosine kinase activity is essential for insulin action and multiple pathways almost certainly participate. Hopefully, continued dissection of the signalling pathways will soon yield the mechanism of insulin action.

Stimulation of glucose transport by guanine nucleotides in permeabilized rat adipocytes

Effects of guanine nucleotides on glucose transport were studied in permeabilized rat epididymal fat cells. GTP gamma S and Gpp(NH)p, but not App(NH)p, stimulated 3-O-methylglucose transport. Effect of GTP gamma S was dose-dependent, being detectable at 0.1 mM, and 1.0 mM GTP gamma S stimulated glucose transport to the same extent as insulin. GTP gamma S (0.3 mM) enhanced insulin-stimulated glucose transport while 1 mM GTP gamma S did not affect insulin-mediated transport. GDP beta S had no effect on glucose transport by itself but rather enhanced insulin action. NaF, which is known to activate trimeric G proteins, increased glucose transport to the same extent as insulin. Likewise, mastoparan augmented glucose transport. These results indicate that a certain type of trimeric G protein(s) is involved in the regulation of glucose transport.

The generation of inositolglycan mediators from rat liver plasma membranes: the role of guanine nucleotide binding proteins

The guanine nucleotide dependence for the generation of inositolglycan second messengers from rat liver plasma membranes has been investigated. Plasma membranes, when treated with insulin release a soluble mediator substance which activates pyruvate dehydrogenase (PDH). Guanosine 5'-[3-thio]triphosphate (GTP gamma S) was found to be as potent as insulin in stimulating mediator release. The stimulatory effects of GTP gamma S required the presence of magnesium and following preincubation of membranes with guanosine 5'-[2-thio]diphosphate (GDP beta S) the stimulation of mediator release by either insulin or GTP gamma S was blocked. The activation of PDH by mediator fractions produced in response to either insulin or GTP gamma S was abolished following treatment of the fractions with anti-inositolglycan antibodies. The significance of these observations with respect to the possible involvement of a regulatory guanine-nucleotide binding protein (G-protein) in the generation of insulin mediators is discussed.

Guanosine nucleotides regulate hormone binding of insulin receptors

Insulin receptors in turkey erythrocyte and rat adipocyte plasma membranes display non-linear hormone binding by Scatchard analysis. This result is consistent with evidence that the insulin-binding sites are heterogeneous and have at least two affinities for the hormone. Mild reduction of plasma membranes with dithiothreitol, before insulin binding, increased the fraction of hormone binding with high affinity without significantly changing the total number of receptor-binding sites. In the presence of guanosine 5'-[gamma-thio]triphosphate, the amount of receptor with high affinity for insulin in the reduced membranes decreased to that present in the absence of reduction; the effect of the nucleotide was concentration- and temperature-dependent. This decrease in insulin binding was specific for guanine nucleotides.

Cholera and pertussis toxins modify regulation of glucose transport activity in rat adipose cells: evidence for mediation of a cAMP-independent process by G-proteins

Adenylyl cyclase in rat adipose cells is stimulated by ligands for Rs receptors (e.g. isoproterenol) and inhibited by ligands for Ri receptors (e.g. adenosine). In contrast, Rs receptors mediate inhibition and Ri receptors mediate augmentation of insulin-stimulated glucose transport activity by a process independent of changes in cellular cAMP-dependent protein kinase activity [Kuroda M., Honnor R. C., Cushman S. W., Londos C. and Simpson I. A. (1987) J. biol. Chem. 262, 245-253]. The present study examines the possible role of G-proteins in the regulation of insulin-stimulated glucose transport activity by Rs and Ri receptors. First, conditions were established that permit intoxication of isolated rat adipocytes by cholera and pertussis toxins without compromising cell integrity. Effectiveness of toxin treatment was monitored by examining adenylyl cyclase activity in isolated plasma membranes. Secondly, neither toxin interfered with the ability of a maximal concentration insulin to initiate the glucose transport response. Thirdly, pertussis toxin eliminated the augmenting effects of adenosine on insulin-stimulated glucose transport activity, but enhanced the inhibitory effects of isoproterenol. Findings with ligands for other Ri receptors (nicotinic acid and prostaglandin E2) mirrored those with adenosine. Finally, cholera toxin elicited a modest depression of transport activity, and only in the absence of an Ri ligand (e.g. adenosine). Furthermore, in contrast to the enhanced stimulation of adenylyl cyclase by isoproterenol and GTP, cholera toxin eliminated the inhibitory effect of isoproterenol on transport activity. The augmentative effects of adenosine on transport activity were unchanged. Measurements of (-/+cAMP) cAMP-dependent protein kinase activity ratios reinforce the notion that modulation of glucose transport activity is independent of changes in cAMP. We conclude that regulation of glucose transport activity by Rs and Ri receptors is mediated by the G-proteins, Gs and Gi (or other toxin substrates), respectively. Inasmuch as such regulation occurs at the plasma membrane and appears to be cAMP-independent, it is suggested that glucose transporters may be direct targets for receptor: G-protein interactions.

Pertussis toxin inhibits autophosphorylation and activation of the insulin receptor kinase

Pertussis toxin is an ADP-ribosyltransferase which alters the function of some of the GTP-binding proteins and inhibits some actions of insulin. In vivo, pertussis toxin (2 micrograms/ml/2h) inhibited insulin-stimulated tyrosyl autophosphorylation of the insulin receptor by 50% in FaO cells, and nearly completely inhibited phosphorylation of the cellular insulin receptor substrate pp185. Similarly, insulin-stimulated autophosphorylation and kinase activity of the insulin receptor purified on wheat germ agglutinin-agarose from pertussis toxin-treated FaO cells was diminished 50%; however, treatment of cells with the catalytically inactive B-oligomer of the toxin had no effect on receptor tyrosine kinase activity in vitro. Pertussis toxin did not alter insulin binding or the cellular levels of ATP, cAMP, and cGMP. Furthermore, immunoprecipitation of the insulin receptor from intact cells with anti-insulin receptor antibodies showed that pertussis toxin did not increase the phosphorylation of serine or threonine residues in the insulin receptor. These results suggest that pertussis toxin can modulate signal transduction of insulin at the level of the insulin receptor kinase.

Insulin activates guanosine 5'-[gamma-thio] triphosphate (GTP gamma S) binding to a novel GTP-binding protein, GIR, from human placenta

A novel GTP-binding protein, GIR, along with insulin receptor (IR), has been partially purified from human placenta. A non-hydrolyzable substrate, GTP gamma S which is known to bind to GTP-binding proteins with high affinity, reduces insulin binding to IR-GIR fraction by approximately 29% and 100 nM insulin stimulates GTP gamma S binding to IR-GIR fraction by approximately five-fold. The molecular weight of the protein (may be subunit) that binds to 8-azido-GTP, a photoaffinity label for G-proteins, is approximately 66,000.