Signal transduction and protein phosphorylation in the regulation of cellular metabolism by insulin

Structure and Regulation of Glycogen Synthase in the Brain

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

Modulation of the transcriptional activity of peroxisome proliferator-activated receptor gamma by protein-protein interactions and post-translational modifications

Peroxisome proliferator-activated receptor gamma (PPARγ) belongs to a nuclear receptor superfamily; members of which play key roles in the control of body metabolism principally by acting on adipose tissue. Ligands of PPARγ, such as thiazolidinediones, are widely used in the treatment of metabolic syndromes and type 2 diabetes mellitus (T2DM). Although these drugs have potential benefits in the treatment of T2DM, they also cause unwanted side effects. Thus, understanding the molecular mechanisms governing the transcriptional activity of PPARγ is of prime importance in the development of new selective drugs or drugs with fewer side effects. Recent advancements in molecular biology have made it possible to obtain a deeper understanding of the role of PPARγ in body homeostasis. The transcriptional activity of PPARγ is subject to regulation either by interacting proteins or by modification of the protein itself. New interacting partners of PPARγ with new functions are being unveiled. In addition, post-translational modification by various cellular signals contributes to fine-tuning of the transcriptional activities of PPARγ. In this review, we will summarize recent advancements in our understanding of the post-translational modifications of, and proteins interacting with, PPARγ, both of which affect its transcriptional activities in relation to adipogenesis.

Effects of bis(alpha-furancarboxylato)oxovanadium(IV) on non-diabetic and streptozotocin-diabetic rats

BACKGROUND

Bis(alpha-furancarboxylato)oxovanadium(IV) (BFOV), a new orally active anti-diabetic vanadium complex with organic agent, has been synthesized and characterized. The current study examined the stability in aqueous solution and effects of the complex on carbohydrate and lipid metabolism in non-diabetic and streptozotocin-induced diabetic rats.

METHODS

Diabetic rats were induced by a single dose injection of streptozotocin (STZ, 50 mg/kg body weight, i.p.). The rats were randomly divided into non-diabetic (control, CON), diabetic (DM) and BFOV (0.2 mmol/kg body weight)-treated, diabetic-BFOV (0.1, 0.2 and 0.4 mmol/kg body weight) groups. All substances were given intragastrically to non-diabetic and STZ-induced diabetic rats for 4 weeks. Blood glucose concentration was monitored during administration and, at the end of experiment glycosylated hemoglobin, serum insulin, lipid concentrations and glycogen content were observed.

RESULTS

Administration of BFOV to STZ-diabetic rats dose-dependently reduced blood glucose concentration when compared to diabetic rats (P<0.01), but it did not influence blood glucose in non-diabetic rats. Serum insulin concentrations were not increased in the BFOV-treated diabetic groups and, in contrast, significantly lowered in the 0.2 mmol/kg body weight BFOV-treated non-diabetic group at the end of experiment. Moreover, BFOV markedly reduced glycosylated hemoglobin concentration and improved dyslipidemia in STZ-diabetic rats, in a dose-dependent manner (P<0.05, P<0.01), but had no significant effect on non-diabetic rats.

CONCLUSION

The organic vanadium complex was found to effectively attenuate diabetic alterations in STZ-diabetic rats.

Differential protein phosphorylation in 3T3-L1 adipocytes in response to insulin versus platelet-derived growth factor. No evidence for a phosphatidylinositide 3-kinase-independent pathway in insulin signaling

Insulin regulates glucose metabolism in adipocytes via a phosphatidylinositide 3-kinase (PI3K)-dependent pathway that appears to involve protein phosphorylation. However, the generation of phosphoinositides is not sufficient for insulin action, and it has been suggested that insulin regulation of glucose metabolism may involve both PI3K-dependent and -independent pathways, the latter being insulin specific. To test this hypothesis, we have designed a phosphoprotein screen to study insulin-specific phosphoproteins that may be either downstream or in parallel to PI3K. Nineteen insulin-regulated phosphospots were detected in the cytosol and high speed pellet fractions, only six of which were significantly regulated by platelet-derived growth factor. Importantly, almost all (92%) of the insulin-specific phosphoproteins identified using this approach were sensitive to the PI3K inhibitor wortmannin. Thus, we obtained no evidence for an insulin-specific, PI3K-independent signaling pathway. A large proportion (62%) of the insulin-specific phosphoproteins were enriched in the same high speed pellet fraction to which PI3K was recruited in response to insulin. Thus, our data suggest that insulin specifically stimulates the phosphorylation of a novel subset of downstream targets and this may in part be because of the unique localization of PI3K in response to insulin in adipocytes.

Insulin-stimulated phosphorylation of the protein phosphatase-1 striated muscle glycogen-targeting subunit and activation of glycogen synthase

Protein phosphatase-1 (PP-1) in heart and skeletal muscle binds to a glycogen-targeting subunit (G(M)) in the sarcoplasmic reticulum. Phosphorylation of G(M) has been postulated to govern activity of PP1 in response to adrenaline and insulin. In this study, we used biochemical assays and G(M) expression in living cells to examine the effects of insulin on the phosphorylation of G(M), and the binding of PP-1 to G(M). We also assayed glycogen synthase activation in cells expressing wild type G(M) and G(M) mutated at the phosphorylation sites. In biochemical assays kinase(s) prepared from insulin-stimulated Chinese hamster ovary (CHO-IR) cells and C2C12 myotubes phosphorylated a glutathione S-transferase (GST) fusion protein, GST-G(M)(1-240), at both site 1 (Ser(48)) and site 2 (Ser(67)). Phosphorylation of both sites was dependent on activation of the mitogen-activated protein kinase pathway, involving in particular ribosomal protein S6 kinase. Full-length G(M) was expressed in CHO-IR cells and metabolic (32)P labeling at sites 1 and 2 was increased by insulin treatment. The G(M) expressed in CHO-IR cells or in C2C12 myotubes co-immunoprecipitated endogenous PP-1, and association was transiently lost following treatment of the cells with insulin. In contrast PP-1 binding to G(M)(S67T), a version of G(M) not phosphorylated at site 2, was unaffected by insulin treatment. Expression of G(M) increased basal activity of endogenous glycogen synthase in CHO-IR cells. Insulin stimulated glycogen synthase activity the same extent in cells expressing wild type G(M) or G(M) mutated to eliminate phosphorylation site 1 and/or site 2. Phosphorylation of G(M) is stimulated by insulin, but this phosphorylation is not involved in insulin control of glycogen metabolism. We speculate that other functions of G(M) at the sarcoplasmic reticulum membrane might be affected by insulin.

Second messenger-regulated protein kinases in the brain: their functional role and the action of antidepressant drugs

Depression has been treated pharmacologically for over three decades, but the views regarding the mechanism of action of antidepressant drugs have registered recently a major change. It was increasingly appreciated that adaptive changes in postreceptor signaling pathways, rather than primary action of drugs on monoamine transporters, metabolic enzymes, and receptors, are connected to therapeutic effect. For some of the various signaling pathways affected by antidepressant treatment, it was shown that protein phosphorylation, which represents an obligate step for most pathways, is markedly affected by long-term treatment. Changes were reported to be induced in the function of protein kinase C, cyclic AMP-dependent protein kinase, and calcium/calmodulin-dependent protein kinase. For two of these kinases (cyclic AMP- and calcium/calmodulin-dependent), the changes have been studied in isolated neuronal compartments (microtubules and presynaptic terminals). Antidepressant treatment activates the two kinases and increases the endogenous phosphorylation of selected substrates (microtubule-associated protein 2 and synaptotagmin). These modifications may be partly responsible for the changes induced by antidepressants in neurotransmission. The changes in protein phosphorylation induced by long-term antidepressant treatment may contribute to explain the therapeutic action of antidepressants and suggest new strategies of pharmacological intervention.

In vivo regulation of protein-serine kinases by insulin in skeletal muscle of fructose-hypertensive rats

The effects of tail-vein insulin injection (2 U/kg) on the regulation of protein-serine kinases in hindlimb skeletal muscle were investigated in hyperinsulinemic hypertensive fructose-fed (FF) animals that had been fasted overnight. Basal protein kinase B (PKB) activity was elevated about twofold in FF rats and was not further stimulated by insulin. Phosphatidylinositol 3-kinase (PI3K), which lies upstream of PKB, was increased approximately 3.5-fold within 2-5 min by insulin in control rats. Basal and insulin-activated PI3K activities were further enhanced up to 2-fold and 1.3-fold, respectively, in FF rats. The 70-kDa S6 kinase (S6K) was stimulated about twofold by insulin in control rats. Both basal and insulin-stimulated S6K activity was further enhanced up to 1.5-fold and 3.5-fold, respectively, in FF rats. In control rats, insulin caused a 40-50% reduction of the phosphotransferase activity of the beta-isoform of glycogen synthase kinase 3 (GSK-3beta), which is a PKB target in vitro. Basal GSK-3beta activity was decreased by approximately 40% in FF rats and remained unchanged after insulin treatment. In summary, 1) the PI3K --> PKB --> S6K pathway was upregulated under basal conditions, and 2) insulin stimulation of PI3K and S6K activities was enhanced, but both PKB and GSK-3 were refractory to the effects of insulin in FF rats.

Inhibitor-1 is not required for the activation of glycogen synthase by insulin in skeletal muscle

Glycogen synthase is an excellent in vitro substrate for protein phosphatase-1 (PP1), which is potently inhibited by the phosphorylated forms of DARPP-32 (dopamine- and cAMP-regulated phosphoprotein, M(r) = 32,000) and Inhibitor-1. To test the hypothesis that the activation of glycogen synthase by insulin is due to a decrease in the inhibition of PP1 by the phosphatase inhibitors, we have investigated the effects of insulin on glycogen synthesis in skeletal muscles from wild-type mice and mice lacking Inhibitor-1 and DARPP-32 as a result of targeted disruption of the genes encoding the two proteins. Insulin increased glycogen synthase activity and the synthesis of glycogen to the same extent in wild-type and knockout mice, indicating that neither Inhibitor-1 nor DARPP-32 is required for the full stimulatory effects of insulin on glycogen synthase and glycogen synthesis in skeletal muscle.

Glucose and glycogen utilisation in myocardial ischemia--changes in metabolism and consequences for the myocyte

Experimentally, enhanced glycolytic flux has been shown to confer many benefits to the ischemic heart, including maintenance of membrane activity, inhibition of contracture, reduced arrhythmias, and improved functional recovery. While at moderate low coronary flows, the benefits of glycolysis appear extensive, the controversy arises at very low flow rates, in the absence of flow; or when glycolytic substrate may be present in excess, such that high glucose concentrations with or without insulin overload the cell with deleterious metabolites. Under conditions of total global ischemia, glycogen is the only substrate for glycolytic flux. Glycogenolysis may only be protective until the accumulation of metabolites (lactate, H+, NADH, sugar phosphates and Pi ) outweighs the benefit of the ATP produced. The possible deleterious effects associated with increased glycolysis cannot be ignored, and may explain some of the controversial findings reported in the literature. However, an optimal balance between the rate of ATP production and rate of accumulation of metabolites (determined by the glycolytic flux rate and the rate of coronary washout), may ensure optimal recovery. In addition, the effects of glucose utilisation must be distinguished from those of glycogen, differences which may be explained by functional compartmentation within the cell.

Insulin regulates lipoprotein lipase activity in rat adipose cells via wortmannin- and rapamycin-sensitive pathways

Lipoprotein lipase (LPL) hydrolyzes the triacylglycerol component of circulating lipoprotein particles, mediating the uptake of fatty acids into adipose tissue and muscle. Insulin is the principal factor responsible for regulating LPL activity in adipose tissue, yet the mechanisms whereby insulin controls LPL expression are unknown. The current studies used wortmannin, a specific inhibitor of phosphatidylinositol (PI) 3-kinase, and rapamycin, a specific inhibitor of activation of phosphoprotein 70 ribosomal protein S6 kinase (p70s6k), to explore some of the components of the insulin signaling pathway controlling LPL activity in adipose cells. Preincubation of isolated rat adipose cells with wortmannin completely abrogated the stimulation of LPL activity by insulin, while preincubation with rapamycin caused approximately a 60% inhibition of insulin-stimulated LPL activity. Thus, the current studies show that the regulation of adipose tissue LPL by insulin is mediated via a wortmannin-sensitive pathway, most likely PI 3-kinase, and that a rapamycin-sensitive pathway, most likely p705s6k, constitutes an important downstream component in the insulin signaling pathway through which LPL is regulated.

Multiple signaling pathways involved in the metabolic effects of insulin

The metabolic effects of insulin are initiated by the binding of insulin to the extracellular domain of the insulin receptor within the plasma membrane of muscle and adipose and liver cells. The subsequent activation of the intracellular tyrosine protein kinase activity of the receptor leads to autophosphorylation of the receptor as well as phosphorylation of a number of intracellular proteins. This gives rise to the activation of Ras and phosphatidylinositol 3-kinase and hence to the activation of a number of serine/threanine protein kinases. Many of these kinases appear to be arranged in cascades, including a cascade that results in the activation of mitogen-activated protein kinase and another that may result in the activation of protein kinase B, leading to the inhibition of glycogen synthase kinase-3 and the activation of the 70 kiloDalton ribosomal S6 protein kinase (p70 S6 kinase). We have explored the role of these early events in the the stimulation of glycogen, fatty acid, and protein synthesis by insulin in rat epididymal fat cells. Comparisons have been made between the metabolic effects of insulin and those of epidermal growth factor, since these 2 agents have contrasting effects on p70 S6 kinase and mitogen-activated protein kinase. The effects of wortmannin (which inhibits phosphatidylinositol 3-kinase), and rapamycin (which blocks the activation of p70 S6 kinase) have also been studied. These and other studies indicate that the mitogen-activated protein kinase cascade is probably not important in the acute metabolic effects of insulin, but may have a role in the regulation of gene transcription and hence the more long-term effects of insulin. The short-term metabolic effects of insulin appear to involve at least 3 distinct signaling pathways: (1) those leading to increases in glucose transport and the activation of glycogen synthase, acetyl-CoA carboxylase, eukaryotic initiation factor-2B, and phosphodiesterase, which may involve phosphatidylinositol 3-kinase and protein kinase B; (2) those leading to some of the effects of insulin on protein synthesis (formation of eukaryotic initiation factor-4F complex, S6 phosphorylation, and activation of eukaryotic elongation factor-2), which may involve phosphatidylinositol 3-kinase and p70 S6 kinase; and finally, (3) that leading to the activation of pyruvate dehydrogenase, which is unique in apparently not requiring activation of phosphatidylinositol 3-kinase.

Early bioenergetic changes in hepatocarcinogenesis: preneoplastic phenotypes mimic responses to insulin and thyroid hormone

Biochemical and molecular biological approaches in situ have provided compelling evidence for early bioenergetic changes in hepatocarcinogenesis. Hepatocellular neoplasms regularly develop from preneoplastic foci of altered hepatocytes, irrespective of whether they are caused by chemicals, radiation, viruses, or transgenic oncogenes. Two striking early metabolic aberrations were discovered: (1) a focal excessive storage of glycogen (glycogenosis) leading via various intermediate stages to neoplasms, the malignant phenotype of which is poor in glycogen but rich in ribosomes (basophilic), and (2) an accumulation of mitochondria in so-called oncocytes and amphophilic cells, giving rise to well-differentiated neoplasms. The metabolic pattern of human and experimentally induced focal hepatic glycogenosis mimics the phenotype of hepatocytes exposed to insulin. The conversion of the highly differentiated glycogenotic hepatocytes to the poorly differentiated cancer cells is usually associated with a reduction in gluconeogenesis, an activation of the pentose phosphate pathway and glycolysis, and an ever increasing cell proliferation. The metabolic pattern of preneoplastic amphophilic cell populations has only been studied to a limited extent. The few available data suggest that thyromimetic effects of peroxisomal proliferators and hepadnaviral infection may be responsible for the emergence of the amphophilic cell lineage of hepatocarcinogenesis. The actions of both insulin and thyroid hormone are mediated by intracellular signal transduction. It is, thus, conceivable that the early changes in energy metabolism during hepatocarcinogenesis are the consequence of alterations in the complex network of signal transduction pathways, which may be caused by genetic as well as epigenetic primary lesions, and elicit adaptive metabolic changes eventually resulting in the malignant neoplastic phenotype.

Effects of exercise and feeding on the hexosamine biosynthetic pathway in rat skeletal muscle

Products of the hexosamine biosynthesis pathway (HSNP) have been implicated in glucose-induced insulin resistance. We measured the major products of HSNP, UDP-N-acetyl hexosamines (UDP-HexNAc), and the activity of L-glutamine: D-fructose-6-phosphate amidotransferase (GFAT, rate-limiting enzyme) in rat hindlimb muscles immediately after exercise and 1, 3, and 16 h postexercise (swimming) in fed and fasted rats and sedentary controls. Muscle glycogen decreased by 50-75% postexercise. In sedentary rats, muscle GFAT activity decreased by approximately 30% (P < 0.002) after an 18-h fast. GFAT activity was not affected by exercise under any condition. Muscle UDP-HexNAc increased approximately 30% postexercise (P < 0.01) in ad libitum-fed but not in fasted rats. UDP-HexNAc remained elevated (approximately 30%, P < 0.002) for 16 h if animals were fed postexercise. Concentrations of UDP-hexoses, GDP-mannose, and UDP were unchanged postexercise. Conclusions are that muscle GFAT activity is regulated by the nutritional state but not by acute exercise. Glucose flux via HNSP may be increased postexercise in muscles of ad libitum-fed rats. Increased HSNP products may serve as negative feedback regulators to limit excessive muscle glycogen deposition postexercise.

Inhibition of adipogenesis through MAP kinase-mediated phosphorylation of PPARgamma

Adipocyte differentiation is an important component of obesity and other metabolic diseases. This process is strongly inhibited by many mitogens and oncogenes. Several growth factors that inhibit fat cell differentiation caused mitogen-activated protein (MAP) kinase-mediated phosphorylation of the dominant adipogenic transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) and reduction of its transcriptional activity. Expression of PPARgamma with a nonphosphorylatable mutation at this site (serine-112) yielded cells with increased sensitivity to ligand-induced adipogenesis and resistance to inhibition of differentiation by mitogens. These results indicate that covalent modification of PPARgamma by serum and growth factors is a major regulator of the balance between cell growth and differentiation in the adipose cell lineage.

Increased glycogen accumulation in transgenic mice overexpressing glycogen synthase in skeletal muscle

To investigate the role of glycogen synthase in controlling glycogen accumulation, we generated three lines of transgenic mice in which the enzyme was overexpressed in skeletal muscle by using promoter-enhancer elements derived from the mouse muscle creatine kinase gene. In all three lines, expression was highest in muscles composed primarily of fast-twitch fibers, such as the gastrocnemius and anterior tibialis. In these muscles, glycogen synthase activity was increased by as much as 10-fold, with concomitant increases (up to 5-fold) in the glycogen content. The uridine diphosphoglucose concentrations were markedly decreased, consistent with the increase in glycogen synthase activity. Levels of glycogen phosphorylase in these muscles increased (up to 3-fold), whereas the amount of the insulin-sensitive glucose transporter 4 either remained unchanged or decreased. The observation that increasing glycogen synthase enhances glycogen accumulation supports the conclusion that the activation of glycogen synthase, as well as glucose transport, contributes to the accumulation of glycogen in response to insulin in skeletal muscle.

Cell volume and the metabolic actions of insulin

Insulin increases the volume of isolated hepatocytes and cells in perfused livers, but effects of the hormone on the volume of fat or muscle cells have not been demonstrated. Exogenous amino acids may stimulate swelling of liver cells and induce insulin-like effects on hepatic protein metabolism; however, swelling of liver cells can be induced by some treatment that do not induce insulin-like metabolic responses. Exogenous amino acids also influence protein metabolism of fat and muscle cells, but no relationship with cell volume has been established and no corresponding effects on metabolism of carbohydrates or lipids have been observed. Three families of mitogen-activated protein kinases are activated after changes in extracellular osmolarity but they appear to play little or no role in the metabolic actions of insulin. Direct evidence against a metabolic role for the extracellular signal-regulated kinases ERK-1 and ERK-2 is discussed. The c-Jun N-terminal kinases (also called stress-activated protein kinases) and the mammalian homologs of the yeast Hog protein kinase are strongly activated by environmental stresses associated with catabolic metabolism. We conclude that cell volume and protein metabolism may be correlated in liver but there is no compelling evidence that the effects of insulin on metabolism of liver, fat, or muscle cells can be accounted for by changes in cell volume. The effects of insulin on cell volume may represent a discrete aspect of the complete physiological response rather than an obligatory intermediate step in metabolic signalling.

Regulation of both glycogen synthase and PHAS-I by insulin in rat skeletal muscle involves mitogen-activated protein kinase-independent and rapamycin-sensitive pathways

Incubating rat diaphragm muscles with insulin increased the glycogen synthase activity ratio (minus glucose 6-phosphate/plus glucose 6-phosphate) by approximately 2-fold. Insulin increased the activities of mitogen-activated protein (MAP) kinase and the Mr = 90,000 isoform of ribosomal protein S6 kinase (Rsk) by approximately 1.5-2.0-fold. Epidermal growth factor (EGF) was more effective than insulin in increasing MAP kinase and Rsk activity, but in contrast to insulin, EGF did not affect glycogen synthase activity. The activation of both MAP kinase and Rsk by insulin was abolished by incubating muscles with the MAP kinase kinase (MEK) inhibitor, PD 098059; however, the MEK inhibitor did not significantly reduce the effect of insulin on activating glycogen synthase. Incubating muscles with concentrations of rapamycin that inhibited activation of p70S6K abolished the activation of glycogen synthase. Insulin also increased the phosphorylation of PHAS-I (phosphorylated heat- and acid-stable protein) and promoted the dissociation of the PHAS-I*eIF-4E complex. Increasing MAP kinase activity with EGF did not mimic the effect of insulin on PHAS-I phosphorylation, and the effect of insulin on increasing MAP kinase could be abolished with the MEK inhibitor without decreasing the effect of insulin on PHAS-I. The effects of insulin on PHAS-I were attenuated by rapamycin. Thus, activation of the MAP kinase/Rsk signaling pathway appears to be neither necessary nor sufficient for insulin action on glycogen synthase and PHAS-I in rat skeletal muscle. The results indicate that the effects of insulin on increasing the synthesis of glycogen and protein in skeletal muscle, two of the most important actions of the hormone, involve a rapamycin-sensitive mechanism that may include elements of the p70S6K signaling pathway.

Insulin stimulation of mitogen-activated protein kinase, p90rsk, and p70 S6 kinase in skeletal muscle of normal and insulin-resistant mice. Implications for the regulation of glycogen synthase

Skeletal muscles from mice stimulated with insulin in vivo were used to evaluate relationships between the insulin receptor tyrosine kinase, mitogen-activated protein (MAP) kinase, p90rsk, p70 S6 kinase (p70S6k), and glycogen synthase. Two models of insulin resistance were also evaluated: (a) transgenic mice with a severe insulin receptor defect and (b) gold thioglucose (GTG) mice (obesity with minimal insulin receptor dysfunction). In normal mice, insulin stimulated MAP kinase (6-fold), p90rsk (RSK2, 5-fold), p70S6k (10-fold), and glycogen synthase (30-50% increase in fractional velocity). In transgenic mice, stimulation of MAP kinase and RSK2 were not detectable, whereas activation of p70S6k and glycogen synthase were preserved. In GTG mice, activation of MAP kinase, RSK2, p70S6k, and glycogen synthase were impaired. Since p70S6k and glycogen synthase were correlated, rapamycin was used to block p70S6k, and glycogen synthase activation was unaffected in normal mice; however, it was partially impaired in transgenic mice.

CONCLUSIONS

(a) stimulation of p70S6k and glycogen synthase are selectively preserved in muscles with a severe insulin receptor kinase defect, indicating signal amplification in pathways leading to these effects; (b) MAP kinase-RSK2 and p70S6k activation are impaired in obese mice, suggesting multiple loci for postreceptor insulin resistance; (c) glycogen synthase was dissociated from MAP kinase and RSK2, indicating that they are not required for this effect of insulin; and (d) p70S6k is not essential for glycogen synthase activation, but it may participate in redundant signaling pathways leading to this effect of insulin.

Differential activation of mitogen-activated protein kinase and S6 kinase signaling pathways by 12-O-tetradecanoylphorbol-13-acetate (TPA) and insulin. Evidence for involvement of a TPA-stimulated protein-tyrosine kinase

AG-18, an inhibitor of protein-tyrosine kinases, was employed to study the role of tyrosine-phosphorylated proteins in insulin- and phorbol ester-induced signaling cascades. When incubated with Chinese hamster ovary cells overexpressing the insulin receptor, AG-18 reversibly inhibited insulin-induced tyrosine phosphorylation of insulin receptor substate-1, with minimal effects either on receptor autophosphorylation or on phosphorylation of Shc64. Under these conditions, AG-18 inhibited insulin-stimulated phosphorylation of the ribosomal protein S6, while no inhibition of insulin-induced activation of mitogen-activated protein kinase (MAPK) kinase or MAPK was detected. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced activation of MAPK kinase and MAPK and phosphorylation of S6 were inhibited by AG-18. This correlated with inhibition of TPA-stimulated tyrosine phosphorylation of several proteins, the most prominent ones being pp114 and pp120. We conclude that Tyr-phosphorylated insulin receptor substrate-1 is the main upstream regulator of insulin-induced S6 phosphorylation by p70s6k, whereas MAPK signaling seems to be activated in these cells primarily through the adaptor molecule Shc. In contrast, TPA-induced S6 phosphorylation is mediated by the MAPK/p90rsk cascade. A key element of this TPA-stimulated signaling pathway is an AG-18-sensitive protein-tyrosine kinase.

Multiple signalling pathways involved in the stimulation of fatty acid and glycogen synthesis by insulin in rat epididymal fat cells

We have investigated the signalling pathways involved in the stimulation of glycogen and fatty acid synthesis by insulin in rat fat cells using wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and rapamycin, which blocks activation of p70 ribosomal S6 protein kinase (p70S6K). Insulin produced a decrease in the activity of glycogen synthase kinase-3 which is likely to be important in the observed stimulation of glycogen synthase. Both of these actions were found to be sensitive to inhibition by wortmannin. Activation of three processes is involved in the stimulation of fatty acid synthesis from glucose by insulin, namely glucose uptake, acetyl-CoA carboxylase and pyruvate dehydrogenase. Whereas wortmannin largely abolished the effects of insulin on glucose utilization and acetyl-CoA carboxylase activity, it was without effect on the stimulation of pyruvate dehydrogenase. Although epidermal growth factor stimulated mitogen-activated protein kinase to a greater extent than insulin, it was unable to mimic the effect of insulin on glycogen synthase, glycogen synthase kinase-3, glucose utilization, acetyl-CoA carboxylase or pyruvate dehydrogenase. Rapamycin also failed to have any appreciable effect on stimulation of these parameters by insulin, although it did block the effect of insulin on p70S6K. We conclude that the activity of phosphatidylinositol 3-kinase is required for the effects of insulin on glycogen synthesis, glucose uptake and acetyl-Co-AN carboxylase, but is not involved in signalling to pyruvate dehydrogenase. Activation of mitogen-activated protein kinase or p70S6K, however, does not appear to be sufficient to bring about the stimulation of fatty acid or glycogen synthesis. Altogether is seems likely that at least four distinct signalling pathways are involved in the effects of insulin on rat fat cells.

ATP-sensitive binding of a 70-kDa cytosolic protein to the glucose transporter in rat adipocytes

We have identified a 70-kDa cytosolic protein (GTBP70) in rat adipocytes that binds to glutathione S-transferase fusion proteins corresponding to the cytoplasmic domains of the facilitative glucose transporter isoforms Glut1, Glut2, and Glut4. GTBP70 did not bind to irrelevant fusion proteins, indicating that the binding is specific to the glucose transporter. GTBP70 binding to the glucose transporter showed little isoform specificity but was significantly subdomain-specific; it bound to the C-terminal domain and the central loop, but not to the N-terminal domain of Glut4. The GTBP70 binding to Glut4 was not affected by the presence of 2 mM EDTA, 2.4 mM Ca2+, or 150 mM K+. The binding was inhibited by ATP in a dose-dependent manner, with 50% inhibition at 10 mM ATP. This inhibition was specific to ATP, as ADP and AMP-PCP (adenosine 5'-(beta, gamma-methylenetriphosphate)) were without effect. GTBP70 did not react with antibodies against phosphotyrosine, phosphothreonine, or phosphoserine, suggesting that it is not a phosphoprotein. The binding of GTBP70 to Glut4 was not affected by the pretreatment of adipocytes with insulin. When these experiments were repeated using rat hepatocyte cytosols, no ATP-sensitive 70-kDa protein binding to the glucose transporter fusion proteins was evident, suggesting that either GTBP70 expression or its function is cell-specific. These findings strongly suggest the possibility that GTBP70 may play a key role in glucose transporter regulation in insulin target cells such as adipocytes.

Okadaic acid stimulates glucose transport in rat adipocytes by increasing the externalization rate constant of GLUT4 recycling

GLUT4, the major insulin-responsive glucose transporter isoform in rat adipocytes, rapidly recycles between the cell surface and an intracellular pool with two first order rate constants, one for internalization (kin) and the other for externalization (kex). Insulin decreases kin by 2.8-fold and increases kex by 3.3-fold, thus increasing the steady-state cell surface GLUT4 level by approximately 8-fold (Jhun, B. H., Rampal, A. L., Liu, H., Lachaal, M., and Jung, C. (1992) J. Biol. Chem. 267, 17710-17715). To gain an insight into the biochemical mechanisms that modulate these rate constants, we studied the effects upon them of okadaic acid (OKA), a phosphatase inhibitor that exerts a insulin-like effect on glucose transport in adipocytes. OKA stimulated 3-O-methylglucose transport maximally 3.1-fold and increased the cell surface GLUT4 level 3.4-fold. When adipocytes were pulse-labeled with an impermeant, covalently reactive glucose analog, [3H]1,3-bis-(3-deoxy-D-glucopyranose-3-yloxy)-2-propyl 4-benzoylbenzoate, and the time course of labeled GLUT4 recycling was followed, the kex was found to increase 2.8-fold upon maximal stimulation by OKA, whereas the kin remained unchanged within experimental error. These findings demonstrate that OKA mimics the insulin effect on only GLUT4 externalization and suggest that insulin stimulates GLUT4 externalization by increasing the phosphorylation state of a serine/threonine phosphoprotein, probably by inhibiting protein phosphatase 1 or 2A.

The tripartite DNA element responsible for diet-induced rat fatty acid synthase (FAS) regulation

We investigated which region of the 5'-flanking sequence of the rat fatty acid synthase (FAS)-encoding gene could be responsible for its nutritionally regulated expression. Diet-induced differences in chromatin structure were determined by DNase I treatment of intact nuclei from hepatic tissue. A low-fat diet results in a different pattern of DNase I-hypersensitive sites (HS) in the chromatin of the FAS promoter (pFAS) from that seen when the nuclear extract was prepared from the livers of normally fed rats. The protein-binding properties of the region defined by DNase I hypersensitivity were tested by gel retardation. A putative cis-acting element with a tripartite structure, 5'-GCCT, 6-bp spacer and a 3'-palindrome, could be localized between bp -518 to -495 in pFAS. Competition experiments with oligodeoxyribonucleotides (oligos) representing subfragments of this cis-element showed that the requirement for structure is stricter than that for sequence. This element could be one of the termini of the insulin-induced signal cascade.

Parallel changes in Glut 4 and Rab4 movements in two insulin-resistant states

Insulin-induced Glut 4 and Rab4 movements were studied in two insulin-resistant states. In adipocytes from streptozotocin diabetic rats, the amount of Glut 4 was decreased by 60%. The remaining Glut 4 molecules were translocated in response to insulin, and in parallel, Rab4 left the intracellular compartment. In contrast, in 3T3-L1 adipocytes rendered insulin-resistant by a prolonged insulin treatment, both Rab4 and Glut 4 remained in the intracellular compartment following an acute insulin stimulation. Those results illustrate a similar behavior of Glut 4 and Rab4 in two situations where insulin resistance results from different mechanisms, and add further support for a role of Rab4 in Glut 4 translocation.

Signal transduction mechanism for the stimulation of the sarcolemmal Na(+)-Ca2+ exchanger by insulin

The signal transduction pathway for insulin-mediated activation of sarcolemmal Na(+)-Ca2+ exchange was examined. Insulin stimulated Na(+)-Ca2+ exchanger activity in a dose-dependent manner, with the EC50 being about 0.7 U/l. The insulin effect was blocked by the protein kinase inhibitor, staurosporine, indicating possible involvement of a protein kinase in insulin action. Also, the relationship between the insulin effect and activation of a G protein was examined by testing the effects of 5' guanylyl imidodiphosphate (Gpp(NH))p) on Na(+)-Ca2+ exchange in the presence and absence of insulin. When exchanger activity was assayed at a calcium concentration of 40 microM, insulin alone had no effect whereas ATP and Gpp(NH)p increased exchanger activity. However, insulin responsiveness was restored in vesicles preloaded with either ATP or Gpp(NH)p, suggesting that insulin may act through a combination of G protein coupling and protein phosphorylation to enhance Na(+)-Ca2+ exchanger activity. We conclude that calcium overload in the diabetic heart may involve a defect in acute activation of the exchanger by insulin.

ATP citrate-lyase and glycogen synthase kinase-3 beta in 3T3-L1 cells during differentiation into adipocytes

ATP citrate-lyase (CL), acetyl-CoA carboxylase (ACC) and glycogen synthase kinase-3 beta (GSK-3 beta) levels were measured in cytosol from 3T3-L1 cells during differentiation from fibroblasts into fat-cells. Protein levels were estimated from immunoblots using specific antisera. Cytosol from confluent cells contain significant amounts of GSK-3 beta, which fell during differentiation of these cells into adipocytes. CL from confluent cells was found to be mostly in the form of a single protein band of apparent mass 110 kDa. Levels of CL and ACC increased during cell differentiation into adipocytes. During the first 3 days of differentiation, CL migration changed, and it was expressed as a complex of protein bands of apparent mass 110 kDa, 113 kDa and 115 kDa. At later stages of differentiation, when these cells had assumed the phenotype of fat-cells, they expressed CL mainly as protein bands of 110 and 113 kDa. When samples containing these bands were treated with alkaline phosphatase, the 113 kDa protein band collapsed into the 110 kDa species. This suggests that the slower-migrating species of CL is a higher-order phosphorylation state of the same protein. Furthermore, when purified CL, mostly expressed as the 110 kDa species, was phosphorylated with cyclic AMP-dependent protein kinase alone or together with GSK-3 and resolved by SDS/PAGE, the phosphorylated CL now migrated more slowly as the 113 kDa and 115 kDa forms. CL phosphorylation was hormone-regulated, since, in samples from fat-cells that had the complex two-band pattern, when cultured in medium without serum or hormones, CL migration reverted to a single band of 110 kDa, similar to confluent cells. Treatment of these 'down-regulated' cells with insulin rapidly induced substantial amounts of the 113 kDa species, with a concomitant decrease in the 110 kDa species.

Ras signaling in the activation of glucose transport by insulin

An approach involving microinjection and microanalysis has been developed to investigate signal-transduction pathways involved in the hormonal control of metabolism. We have applied this strategy to investigate the role of Ras signaling in the acute activation of glucose transport by insulin in cardiac myocytes. Glucose transport activity was assessed by measuring the initial rate of accumulation of 2-deoxyglucose 6-phosphate (dGlc6P) in individual cells after incubation in 2-deoxyglucose. Insulin increased accumulation of dGlc6P by 3- to 4-fold, consistent with its stimulatory effect on glucose transport. Accumulation of dGlc6P was increased severalfold by microinjecting the nonhydrolyzable GTP analogue, guanosine 5'-[gamma-thio]triphosphate, which activates members of the Ras superfamily of GTP-binding proteins. Injecting activated Ha-Ras protein also mimicked insulin by increasing dGlc6P; whereas, injecting a Ras protein lacking the COOH-terminal site of fatty acylation required for Ras function was without effect. Introducing the neutralizing Ras antibody Y13-259 into cells attenuated the effect of insulin. These findings implicate Ras in the acute regulation of metabolism by insulin.

Development of the mammary gland and lactation

Many hormones, growth factors, and protooncogene products involved in mammary development and lactation have been identified; however, the mechanism of their concerted action remains to be explained. In addition to these regulatory factors, normal mammary development and lactation require the cell-cell interaction of stromal and parenchymal elements in the mammary gland. Recent studies indicate that PRL effects on target cells are likely transmitted into cells via activation of tyrosine kinase associated with a protein-designated JAK-2, while other studies have identified stimulatory and inhibitory factors that interact with the 5' promotor regions of milk-product genes.

Molecular cloning and tissue distribution of PHAS-I, an intracellular target for insulin and growth factors

Although the actions of insulin and a number of growth factors that signal via protein-tyrosine kinase receptors are believed to involve increased phosphorylation of key intracellular proteins, relatively few of the downstream phosphoproteins have been identified. In this report we describe a cDNA encoding one of the most prominent insulin-stimulated phosphoproteins in rat adipocytes. The cDNA encodes a protein, designated PHAS-I, which has 117 amino acids and a M(r) of 12,400. When translated in vitro and subjected to SDS/PAGE, PHAS-I migrates anomalously, having an apparent M(r) of 21,000. The predicted amino acid composition is interesting in that approximately 45% of the PHAS-I protein is accounted for by only four amino acids--serine, threonine, proline, and glycine. The PHAS-I gene is expressed in a variety of tissues, although the highest levels of mRNA are present in fat and skeletal muscle, two of the most insulin-responsive tissues. The nucleotide and deduced amino acid sequences of PHAS-I differ from any that have been reported, and homology screening provided no clues concerning the function of the protein. However, in view of its tissue distribution and the fact that the protein is phosphorylated in response to insulin, we speculate that PHAS-I is important in insulin action.

Glucose transport and phosphorylation in single cardiac myocytes: rate-limiting steps in glucose metabolism

Microanalytic methods were used to investigate the regulation of glucose metabolism by insulin in single myocytes isolated from adult rat ventricles. Cultured myocytes were incubated with or without insulin and, with either glucose or 2-deoxyglucose (2-DG), rinsed, and freeze-dried. Individual cells were weighed and levels of 2-DG-6-phosphate (2-DG-6-P) or glucose and glucose 6-phosphate (G-6-P) were determined after enzymatic amplification. In cells incubated with 2-DG, insulin increased the level of 2-DG-6-P by as much as 30-fold, indicative of dramatic activation of glucose transport. In cells incubated with glucose, insulin increased the levels of G-6-P by approximately threefold. Increasing extracellular glucose without insulin also increased G-6-P; however, intracellular glucose concentrations were not increased, indicating that glucose transport is rate limiting in nonstimulated myocytes. In contrast, intracellular glucose concentrations were increased by over an order of magnitude by insulin, reaching 60% of the extracellular glucose concentration. Measurements of glucose and G-6-P in the same insulin-treated cells revealed that accumulation of G-6-P reached a plateau when extracellular glucose was increased > 2 mM. At this point the estimated intracellular glucose concentration was 300 microM, or approximately 10 times the Michaelis constant of hexokinase for glucose. These results indicate that in the presence of insulin and physiological concentrations of glucose, hexokinase is saturated with glucose. Consequently, the rate-limiting step for insulin-stimulated glucose utilization is glucose phosphorylation rather than glucose transport.

The insulin receptor: structure, function, and signaling

The insulin receptor is a member of the ligand-activated receptor and tyrosine kinase family of transmembrane signaling proteins that collectively are fundamentally important regulators of cell differentiation, growth, and metabolism. The insulin receptor has a number of unique physiological and biochemical properties that distinguish it from other members of this large well-studied receptor family. The main physiological role of the insulin receptor appears to be metabolic regulation, whereas all other receptor tyrosine kinases are engaged in regulating cell growth and/or differentiation. Receptor tyrosine kinases are allosterically regulated by their cognate ligands and function as dimers. In all cases but the insulin receptor (and 2 closely related receptors), these dimers are noncovalent, but insulin receptors are covalently maintained as functional dimers by disulfide bonds. The initial response to the ligand is receptor autophosphorylation for all receptor tyrosine kinases. In most cases, this results in receptor association of effector molecules that have unique recognition domains for phosphotyrosine residues and whose binding to these results in a biological response. For the insulin receptor, this does not occur; rather, it phosphorylates a large substrate protein that, in turn, engages effector molecules. Possible reasons for these differences are discussed in this review. The chemistry of insulin is very well characterized because of possible therapeutic interventions in diabetes using insulin derivatives. This has allowed the synthesis of many insulin derivatives, and we review our recent exploitation of one such derivative to understand the biochemistry of the interaction of this ligand with the receptor and to dissect the complicated steps of ligand-induced insulin receptor autophosphorylation. We note possible future directions in the study of the insulin receptor and its intracellular signaling pathway(s).

Metformin--an update

Metformin (dimethylbiguanide) is an antihyperglycaemic drug used to treat non-insulin dependent diabetes mellitus. It acts in the presence of insulin to increase glucose utilization and reduce glucose production, thereby countering insulin resistance. The effects of metformin include increased glucose uptake, oxidation and glycogenesis by muscle, increased glucose metabolism to lactate by the intestine, reduced hepatic gluconeogenesis and possibly a reduced rate of intestinal glucose absorption. Metformin appears to facilitate steps in the postreceptor pathways of insulin action, and may exert effects that are independent of insulin. In muscle, metformin increases translocation into the plasma membrane of certain isoforms of the glucose transporter. The effects of metformin are generally moderate, and do not cause clinical hypoglycaemia or increased weight gain. Metformin has an antihypertriglyceridaemic effect and exerts various potentially useful effects on haemostasis. A risk of lactic acidosis is negligible provided that the contraindications, particularly renal incompetence are respected.

Okadaic acid, insulin, and denervation effects on glucose and amino acid transport and glycogen synthesis in muscle

Effects of okadaic acid (OKA) and calyculin A, cell-permeating specific inhibitors of phosphoprotein phosphatases-1 and -2A, were studied in isolated rat hemidiaphragms. OKA stimulated glucose transport (half-maximum = approximately 0.1 microM; maximum = approximately 1 microM) but was less effective than 6 nM insulin. Insulin and OKA effects were not additive. OKA diminished or abolished glucose transport-stimulation by insulin. System A amino acid transport was also stimulated by OKA, insulin was more effective, and preexposure to OKA inhibited insulin stimulation. Calyculin A affected both transport systems similarly to OKA. OKA did not affect basal glycogen synthesis but abolished its stimulation by insulin. Denervated muscles develop post-receptor insulin resistance. Glucose transport and glycogen synthesis were essentially unresponsive to insulin 3 days postdenervation; however, glucose transport was stimulated by OKA similarly to controls. OKA did not affect glycogen synthesis in denervated muscle except for abolishing a small insulin effect. The data suggest similar acute regulation of glucose and system A amino acid transport in muscle. Enhanced Ser/Thr phosphorylation of unidentified protein(s) stimulates both processes but inhibits their full stimulation by insulin. Postdenervation insulin resistance likely reflects impaired signal transduction.