Insulin Stimulates Dephosphorylation of Phosphorylase in Rat Epitrochlearis Muscles

An IFNγ-dependent immune-endocrine circuit lowers blood glucose to potentiate the innate antiviral immune response

Viral infection makes us feel sick as the immune system alters systemic metabolism to better fight the pathogen. The extent of these changes is relative to the severity of disease. Whether blood glucose is subject to infection-induced modulation is mostly unknown. Here we show that strong, nonlethal infection restricts systemic glucose availability, which promotes the antiviral type I interferon (IFN-I) response. Following viral infection, we find that IFNγ produced by γδ T cells stimulates pancreatic β cells to increase glucose-induced insulin release. Subsequently, hyperinsulinemia lessens hepatic glucose output. Glucose restriction enhances IFN-I production by curtailing lactate-mediated inhibition of IRF3 and NF-κB signaling. Induced hyperglycemia constrained IFN-I production and increased mortality upon infection. Our findings identify glucose restriction as a physiological mechanism to bring the body into a heightened state of responsiveness to viral pathogens. This immune-endocrine circuit is disrupted in hyperglycemia, possibly explaining why patients with diabetes are more susceptible to viral infection.

The role of glycogen phosphorylase in glycogen biogenesis in skeletal muscle after exercise

Initially it was believed that phosphorylase was responsible for both glycogen breakdown and synthesis in the living cell. The discovery of glycogen synthase and McArdle's disease (lack of phosphorylase activity), together with the high P_i/glucose 1-P ratio in skeletal muscle, demonstrated that glycogen synthesis could not be attributed to reversal of the phosphorylase reaction. Rather, glycogen synthesis was attributable solely to the activity of glycogen synthase, subsequent to the transport of glucose into the cell. However, the well-established observation that phosphorylase was inactivated (i.e., dephosphorylated) during the initial recovery period after prior exercise, when the rate of glycogen accumulation is highest and independent of insulin, suggested that phosphorylase could play an active role in glycogen accumulation. But the quantitative contribution of phosphorylase inactivation was not established until recently, when studying isolated murine muscle preparations during recovery from repeated contractions at temperatures ranging from 25 to 35 °C. Thus, in both slow-twitch, oxidative and fast-twitch, glycolytic muscles, inactivation of phosphorylase accounted for 45%-75% of glycogen accumulation during the initial hours of recovery following repeated contractions. Such data indicate that phosphorylase inactivation may be the most important mechanism for glycogen accumulation under defined conditions. These results support the initial belief that phosphorylase plays a quantitative role in glycogen formation in the living cell. However, the mechanism is not via activation of phosphorylase, but rather via inactivation of the enzyme.

Insulin: The master regulator of glucose metabolism

Insulin is the master regulator of glucose, lipid, and protein metabolism. Following ingestion of an oral glucose load or mixed meal, the plasma glucose concentration rises, insulin secretion by the beta cells is stimulated and the hyperinsulinemia, working in concert with hyperglycemia, causes: (i) suppression of endogenous (primarily reflects hepatic) glucose production, (ii) stimulation of glucose uptake by muscle, liver, and adipocytes, (iii) inhibition of lipolysis leading to a decline in plasma FFA concentration which contributes to the suppression of hepatic glucose production and augmentation of muscle glucose uptake, and (iv) vasodilation in muscle, which contributes to enhanced muscle glucose disposal. Herein, the integrated physiologic impact of insulin to maintain normal glucose homeostasis is reviewed and the molecular basis of insulin's diverse actions in muscle, liver, adipocytes, and vasculature are discussed.

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

A cyclic guanosine monophosphate-dependent pathway can regulate net hepatic glucose uptake in vivo

We previously showed that hepatic nitric oxide regulates net hepatic glucose uptake (NHGU), an effect that can be eliminated by inhibiting hepatic soluble guanylate cyclase (sGC), suggesting that the sGC pathway is involved in the regulation of NHGU. The aim of the current study was to determine whether hepatic cyclic guanosine monophosphate (cGMP) reduces NHGU. Studies were performed on conscious dogs with transhepatic catheters. A hyperglycemic-hyperinsulinemic clamp was established in the presence of portal vein glucose infusion. 8-Br-cGMP (50 µg/kg/min) was delivered intraportally, and either the glucose load to the liver (CGMP/GLC; n = 5) or the glucose concentration entering the liver (CGMP/GCC; n = 5) was clamped at 2× basal. In the control group, saline was given intraportally (SAL; n = 10), and the hepatic glucose concentration and load were doubled. 8-Br-cGMP increased portal blood flow, necessitating the two approaches to glucose clamping in the cGMP groups. NHGU (mg/kg/min) was 5.8 ± 0.5, 2.7 ± 0.5, and 4.8 ± 0.3, whereas the fractional extraction of glucose was 11.0 ± 1, 5.5 ± 1, and 8.5 ± 1% during the last hour of the study in SAL, CGMP/GLC, and CGMP/GCC, respectively. The reduction of NHGU in response to 8-Br-cGMP was associated with increased AMP-activated protein kinase phosphorylation. These data indicate that changes in liver cGMP can regulate NHGU under postprandial conditions.

Adrenaline potentiates insulin-stimulated PKB activation in the rat fast-twitch epitrochlearis muscle without affecting IRS-1-associated PI 3-kinase activity

We have previously shown in the rat slow-twitch soleus muscle that adrenaline greatly potentiates insulin-stimulated protein kinase B (PKB) phosphorylation without having an effect alone. However, insulin signalling capacity through the PKB pathway is higher in soleus than in fast-twitch muscles, whereas adrenaline activates phosphorylase more strongly in epitrochlearis. Therefore, the aim of the present study was to investigate the interaction between adrenaline and insulin signalling in the fast-twitch epitrochlearis muscle. Insulin increased insulin receptor substrate-1 (IRS-1)-associated phosphoinositide (PI) 3-kinase activity threefold, and adrenaline did not influence basal or insulin-stimulated PI 3-kinase activity. Insulin but not adrenaline increased PKB activity and phosphorylation of Ser(473) and Thr(308). It is interesting to note that adrenaline potentiated insulin-stimulated PKB activity and PKB Ser(473) and Thr(308) phosphorylation. These effects were mimicked by dibutyryl-cyclic adenosine monophosphate (db-cAMP). Adrenaline and db-cAMP increased glycogen synthase kinase (GSK)-3beta Ser(9) phosphorylation independently of PKB activation and enhanced insulin-stimulated GSK-3beta Ser(9) phosphorylation. Although adrenaline increased GSK-3 phosphorylation (inhibiting activity), phosphorylation of its target sites on glycogen synthase was increased, and adrenaline blocked insulin-stimulated glycogen synthase dephosphorylation of Ser(641) and Ser(645,649,653,657), glycogen synthase activation and glycogen synthesis. Insulin-stimulated glucose transport was not influenced by adrenaline despite the increased PKB activation. In conclusion, as in the slow-twitch soleus muscle, adrenaline potentiates insulin-stimulated PKB activation in the fast-twitch glycolytic epitrochlearis muscle without increasing IRS-1-associated PI 3-kinase activity. Furthermore, adrenaline induces phosphorylation of a pool of GSK-3 that is not involved in the regulation of glycogen metabolism. These results indicate that the combination of adrenaline and insulin may activate novel signalling molecules rather than just summing up their effects on linear pathways.

Effects of malathion subchronic exposure on rat skeletal muscle glucose metabolism

Regarding the widespread use of organophosphorous pesticides (OP) especially malathion in environment and reported cases of muscle disturbances in human and animal, the present work was undertaken to explore effects of malathion subchronic exposure on rat leg skeletal muscle glucose metabolism by measuring key enzymes of glycogenolysis and glycolysis. Malathion was administered through food for 4 weeks at concentrations of 100, 200, and 400ppm to rats. Activities of enzymes including glycogen phosphorylase (GP), hexokinase (HK), and phosphofructokinase-1 (PFK) were measured in skeletal muscle homogenate of exposed rats. Levels of glucose and insulin were measured in blood. Four weeks administration of malathion at doses of 200 and 400ppm increased blood glucose concentrations to 44.4 and 60.6% of control, respectively. Malathion at doses of 200 and 400ppm increased blood insulin concentration to 36.6 and 143.2% of control, respectively. Malathion at doses of 100, 200, and 400ppm increased muscle PFK activity to 40.4, 53.5, and 83.1% of control, respectively. Malathion at doses of 400ppm increased skeletal muscle GP to 91.6% of control. Skeletal muscle HK was not influenced by malathion treatment. It is concluded that malathion influences muscle glycogenolysis and glycolysis as well as secretion of insulin from pancreas which all may explain diabetic potential of malathion.

Effects of insulin and transgenic overexpression of UDP-glucose pyrophosphorylase on UDP-glucose and glycogen accumulation in skeletal muscle fibers

UDP-glucose (UDP-Glc) and glycogen levels in skeletal muscle fibers of defined fiber type were measured using microanalytical methods. Infusing rats with insulin increased glycogen in both Type I and Type II fibers. Insulin was without effect on UDP-Glc in Type I fibers but decreased UDP-Glc by 35-40% in Type IIA/D and Type IIB fibers. The reduction in UDP-Glc suggested that UDP-Glc pyrophosphorylase (PPL) activity might limit glycogen synthesis in response to insulin. To explore this possibility, we generated mice overexpressing a UDP-Glc PPL transgene in skeletal muscle. The transgene increased both UDP-Glc PPL activity and levels of UDP-Glc in skeletal muscles by approximately 3-fold. However, overexpression of UDP-Glc PPL was without effect on either the levels of skeletal muscle glycogen or glucose tolerance in vivo. The transgene was also without effect on either control or insulin-stimulated rates of (14)C-glucose incorporation into glycogen in muscles incubated in vitro. The results indicate that UDP-Glc PPL activity is not limiting for glycogen synthesis.

Insulin and insulin antagonists evoke phosphorylation of P20 at serine 157 and serine 16 respectively in rat skeletal muscle

We show here that insulin and insulin antagonists differentially modify phosphorylation of three phospho-isoforms of P20 (termed S1, S2 and S3) in rat skeletal muscle. Precise phosphorylation sites of S1 and S2 were mapped to serine 157 and serine 16 respectively. Insulin evoked phosphorylation of P20 at serine 157 through the phosphatidylinositol (PI) 3-kinase pathway. Epinephrine and calcitonin gene-related peptide decreased phosphorylation at serine 157 and increased phosphorylation at serine 16 and other unidentified sites. These results demonstrate that the PI-3-kinase pathway of anabolic insulin and the cAMP pathway of catabolic hormones converge on P20 and suggest a potential role of this protein in regulating energy metabolism of skeletal muscle.

On the mechanism of inactivation of muscle glycogen phosphorylase by insulin

Glucose 6-phosphate, an allosteric inhibitor of skeletal muscle phosphorylase b, inhibits at physiological concentrations and conditions the phosphorylation and activation of the enzyme by phosphorylase b kinase. AMP inhibits the dephosphorylation of phosphorylase a, but is without effect on the phosphorylation of phosphorylase b. Glucose 6-phosphate has no effect on the activity of phosphorylase a and does not affect its dephosphorylation by phosphatases 1 or 2A. The inhibition of the phosphorylation of phosphorylase b by glucose 6-phosphate may explain the reported decreased phosphorylation of phosphorylase in muscle following insulin treatment, which elevates intracellular levels of glucose 6-phosphate.

Phenylarsine oxide inhibits insulin-stimulated protein phosphatase 1 activity and GLUT-4 translocation

Phenylarsine oxide (PAO) has previously been shown to inhibit insulin-stimulated glucose transport without affecting insulin binding and tyrosine kinase activity of insulin receptor (S. C. Frost and M. D. Lane. J. Biol. Chem. 260: 2646-2652, 1985). This study examines the effect of PAO on insulin's ability to activate adipocyte protein phosphatase 1 (PP-1) and dephosphorylate GLUT-4, the insulin-sensitive glucose transporter. In particulate fractions, insulin stimulated PP-1 activity (40% increase over basal with phosphorylase a) in a time- and dose-dependent manner (half-maximal effect of 0.89 nM in 1 min). Insulin did not alter cytosolic PP-1 activity. With GLUT-4 as a substrate, insulin caused more than twofold stimulation of particulate PP-1 activity. Addition of PAO (5 microM) before or after insulin treatment abolished insulin's effect on PP-1 activation. The presence of 2,3-dimercaptopropanol (200 microM) prevented the effect of PAO on PP-1 activation and glucose uptake. In addition, PAO significantly increased GLUT-4 phosphorylation, blocked insulin-stimulated dephosphorylation, and partially diminished insulin-stimulated translocation of GLUT-4. We conclude that PAO may interfere with the components of insulin signal transduction pathways that lead to the activation of PP-1 and this may be responsible for the observed inhibition in insulin action.

Glucose analogue inhibitors of glycogen phosphorylase: the design of potential drugs for diabetes

The T-state crystal structure of the glucose-phosphorylase b complex has been used as a model for the design of glucose analogue inhibitors that may be effective in the regulation of blood glucose levels. Modeling studies indicated room for additional atoms attached at the C1-beta position of glucose and some scope for additional atoms at the C1-alpha position. Kinetic parameters were determined for alpha-D-glucose: Ki = 1.7 mM, Hill coefficient n = 1.5, and alpha (synergism with caffeine) = 0.2. For beta-D-glucose, Ki = 7.4 mM, n = 1.5, and alpha = 0.4. More than 20 glucose analogues have been synthesized and tested in kinetic experiments. Most were less effective inhibitors than glucose itself and the best inhibitor was alpha-hydroxymethyl-1-deoxy-D-glucose (Ki = 1.5 mM, n = 1.3, alpha = 0.4). The binding of 14 glucose analogues to glycogen phosphorylase b in the crystal has been studied at 2.4-A resolution and the structure have been refined to crystallographic R values of less than 0.20. The kinetic and crystallographic studies have been combined to provide rationalizations for the apparent affinities of glucose and the analogues. The results show the discrimination against beta-D-glucose in favor of alpha-D-glucose is achieved by an additional hydrogen bond made in the alpha-glucose complex through water to a protein group and an unfavorable environment for a polar group in the beta pocket. The compound alpha-hydroxymethyl-1-deoxy-D-glucose has an affinity similar to that of glucose and makes a direct hydrogen bond to a protein group. Comparison of analogues with substituent atoms that have flexible geometry (e.g., 1-hydroxyethyl beta-D-glucoside) with those whose substituent atoms are more rigid (e.g., beta-azidomethyl-1-deoxyglucose or beta-cyanomethyl-1-deoxyglucose) indicates that although all three compounds make similar polar interactions with the enzyme, those with more rigid substituent groups are better inhibitors. In another example, alpha-azidomethyl-1-deoxyglucose was a poor inhibitor. In the crystal structure the compound made several favorable interactions with the enzyme but bound in an unfavorable conformation, thus providing an explanation for its poor inhibition. Attempts to utilize a contact to a buried aspartate group were partially successful for a number of compounds (beta-aminoethyl, beta-mesylate, and beta-azidomethyl analogues). The beta pocket was shown to bind gentiobiose (6-O-beta-D-glucopyranosyl-D-glucose), indicating scope for binding of larger side groups for future studies.

Amylin activates glycogen phosphorylase in the isolated soleus muscle of the rat

The pancreatic beta-cell hormone amylin acts in isolated rat skeletal muscle to decrease insulin-stimulated incorporation of glucose into glycogen. It also increases blood levels of lactate and glucose in fasted rats in vivo. However, it remained uncertain whether amylin exerts direct effects to stimulate muscle glycogenolysis. We now report that amylin caused a dose-dependent increase in activity of muscle glycogen phosphorylase in isolated rat soleus muscle by stimulating phosphorylase a. Insulin inhibited amylin-stimulated activation of phosphorylase. Effects of amylin to stimulate muscle glycogenolysis are consistent with observed effects of amylin in vivo and could be a major mechanism whereby amylin modulates carbohydrate metabolism.

The molecular mechanism by which insulin stimulates glycogen synthesis in mammalian skeletal muscle

The ability of insulin to promote the phosphorylation of some proteins and the dephosphorylation of others is paradoxical. An insulin-stimulated protein kinase is shown to activate the type-1 protein phosphatase that controls glycogen metabolism, by phosphorylating its regulatory subunit at a specific serine. Furthermore, the phosphorylation of this residue is stimulated by insulin in vivo. Increased and decreased phosphorylation of proteins by insulin can therefore be explained through the same basic underlying mechanism.