Activation of hepatic glycogenolysis by phagocytic stimulation

Infusion of latex beads into isolated perfused rat livers transiently increased glucose output, perfusate lactate/pyruvate ratio and portal vein pressure, mimicking hepatic effects of heat-aggregated IgG (HAG). Indomethacin attenuated hepatic responses to latex beads, and extracellular calcium was required for full expression of hepatic responses. Prior infusion of HAG inhibited the glycogenolytic response to latex beads, supporting a common mechanism of action for the two agents.     

抵抗素在胰岛素抵抗中的作用研究进展

抵抗素是近年来新近发现的一种脂肪组织特异性分泌的细胞因子,与肥胖、胰岛素抵抗和2型糖尿病有着密切关系.众多研究表明,抵抗素通过影响机体糖脂代谢,诱导肝脏、脂肪及肌肉组织产生胰岛素抵抗.本文简要介绍抵抗素和肝脏的胰岛素抵抗,并进一步分析抵抗素对肝脏糖、脂代谢及肝细胞胰岛素信号转导三方面的调节,重点说明抵抗素在肝脏胰岛素抵抗的作用.     

Stimulation of hepatic glycogenolysis by acetylglyceryl ether phosphorylcholine

1-O-Alkyl-2-acetyl-sn-glyceryl 3-phosphorylcholine or acetylglyceryl ether phosphorylcholine (AGEPC) stimulated glycogenolysis in perfused livers from fed rats at concentrations as low as 10(-11) M. At the lower AGEPC concentrations, e.g. 2 X 10(-10) M, a single transient phase of enhanced hepatic glucose output was elicited upon infusion of this agonist. At higher concentrations, e.g. 2 X 10(-8) M, a sharp transient spike of glucose output was observed, followed by a stable elevated steady state rate of glucose output until the AGEPC infusion was terminated. Increased rates of lactate and acetoacetate output and a diminished hepatic oxygen consumption were characteristic of the response of the livers to AGEPC at 2 X 10(-10) M. Neither alpha- nor beta-adrenergic antagonists blocked the glycogenolytic response of AGEPC. Repeated infusion of AGEPC led to homologous desensitization of the response, but the response of the liver to the alpha-adrenergic agonist, phenylephrine, or to glucagon, subsequent to AGEPC stimulation, was unaffected. Increasing the period of perfusion between successive additions of AGEPC, from 7 to 30 min, resulted in an increased glycogenolytic response to this agonist. When the perfusate calcium concentration was reduced from 1.25 to 0.05 mM, the glycogenolytic response to AGEPC was markedly diminished; calcium efflux from the liver following stimulation with AGEPC was not observed. The data presented in this study illustrate a potent agonist effect of AGEPC on the glycogenolytic system in the rat liver.     

Cyclic AMP-mediated activation of hepatic glycogenolysis by fructose

Isolated livers from fed and fasted rats were perfused for 30 min with recirculating blood-buffer medium containing no added substrate and then switched to a flow-through perfusion using the same medium for an additional 5, 10 and 30 min. Continous infusion of fructose for the final 5, 10 or 30 min resulted in activation of glycogen phosphorylase, an increase in the activity of protein kinase, elevated levels of tissue adenosine 3′,5′-monphosphate (cylic AMP), and no consistent effect on glycogen synthase. Infusion of glucose under the same conditions resulted in activation of glycogen synthase, inactivation of glycogen phosphorylase, no change in protein kinase, and no consistent change in tissue cyclic AMP. These results demonstrate that while glucose promotes hepatic glycogen synthesis, fructose promotes activation of the enzymatic cascade responsible for glycogen breakdown.     

Potassium-mediated stimulation of hepatic glycogenolysis

Increased extracellular potassium concentrations ([K+]o) stimulated transient increases in glucose release and 45Ca2+ washout in the perfused rat liver. Stimulated glucose release had a K0.5 of about 26 mM for [K+]o, was not desensitized by successive infusion intervals of increased [K+]o, was not affected by altering the direction of perfusion, was absolutely dependent on the presence of [Ca2+]o, and was blocked by 2 mM cobalt or 10 microM verapamil. The increase in 45Ca2+ washout resulting from increased [K+]o also was blocked by 2 mM cobalt or 10 microM verapamil. Inhibitors of vascular tone (nitroprusside, atriopeptin II), arachidonic acid metabolism (indomethacin, nordihydroguaiaretic acid), and alpha- or beta-adrenergic or muscarinic nerve stimulation/secretion (phentolamine, propranolol, atropine) were unable to inhibit the [K+]o-stimulated glucose release. ATP, ADP, and AMP concentrations in tissue freeze-clamped 2 min after the onset of infusion of 50 mM K+ were not significantly different from control tissue. Glucose release from freshly isolated suspensions or primary cultured monolayers of hepatocytes or from liver slices, all of which responded to glucagon or phenylephrine, did not respond to increased [K+]o. The results indicate that glycogenolysis stimulated by depolarizing gradients of K+ is dependent on an intact perfused vasculature and may be mediated by potential-sensitive Ca2+ channels present in the vascular endothelium of the liver.     

Synergistic inhibition of hepatic glycogenolysis in the presence of insulin and a cAMP antagonist

The effects of insulin on the ability of the specific intracellular cAMP-dependent protein kinase antagonist, the Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate, to inhibit glycogenolysis induced by the Sp diastereomer was studied in hepatocytes isolated from fed rats. Addition of the cAMP agonist, (Sp)-cAMPS, to hepatocytes resulted in a concentration-dependent increase in glycogenolytic glucose production concomitant with the cAMP-dependent activation of phosphorylase and inhibition of glycogen synthase. Activity curves were shifted to the right in the presence of the cAMP antagonist, (Rp)-cAMPS. Preincubation of the hepatocytes with a maximally effective concentration of insulin did not affect the concentration of (Sp)-cAMPS required for half-maximal activation of phosphorylase but did result in a 10-fold shift in the concentration of (Sp)-cAMPS required for half-maximal inactivation of glycogen synthase. Preincubation of hepatocytes with a combination of the cAMP antagonist, (Rp)-cAMPS, and insulin resulted in synergistic inhibition of (Sp)-cAMPS-induced phosphorylase activation, glycogen synthase inactivation, and glycogenolytic glucose production. Since neither phosphorothioate diastereomer was hydrolyzed significantly during the course of the experiments, the synergistic effects of insulin are postulated to be working through a mechanism subsequent to the phosphodiesterase activation step.     

Cyclic AMP-mediated activation of hepatic glycogenolysis by fructose.

Isolated livers from fed and fasted rats were perfused for 30 min with recirculating blood-buffer medium containing no added substrate and then switched to a flow-through perfusion using the same medium for an additional 5, 10 and 30 min. Continuous infusion of fructose for the final 5, 10 or 30 min resulted in activation of glycogen phosphorylase, an increase in the activity of protein kinase, elevated levels of tissue adenosine 3', 5'-monophosphate (cyclic AMP), and no consistent effect on glycogen synthase. Infusion of glucose under the same conditions resulted in activation of glycogen synthase, inactivation of glycogen phosphorylase, no change in protein kinase, and no consistent change in tissue cyclic AMP. These results demonstrate that while glucose promotes hepatic glycogen synthesis, fructose promotes activation of the enzymatic cascade responsible for glycogen breakdown.     

Endoplasmic reticulum stress causes insulin resistance by inhibiting delivery of newly synthesized insulin receptors to the cell surface

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates a signaling network known as the unfolded protein response (UPR). Here we characterize how ER stress and the UPR inhibit insulin signaling. We find that ER stress inhibits insulin signaling by depleting the cell surface population of the insulin receptor. ER stress inhibits proteolytic maturation of insulin proreceptors by interfering with transport of newly synthesized insulin proreceptors from the ER to the plasma membrane. Activation of AKT, a major target of the insulin signaling pathway, by a cytosolic, membrane-bound chimera between the AP20187-inducible F_V2E dimerization domain and the cytosolic protein tyrosine kinase domain of the insulin receptor was not affected by ER stress. Hence, signaling events in the UPR, such as activation of the JNK mitogen-activated protein (MAP) kinases or the pseudokinase TRB3 by the ER stress sensors IRE1α and PERK, do not contribute to inhibition of signal transduction in the insulin signaling pathway. Indeed, pharmacologic inhibition and genetic ablation of JNKs, as well as silencing of expression of TRB3, did not restore insulin sensitivity or rescue processing of newly synthesized insulin receptors in ER-stressed cells.     

Liver-specific inducible nitric-oxide synthase expression is sufficient to cause hepatic insulin resistance and mild hyperglycemia in mice

Inducible nitric-oxide synthase (iNOS), a major mediator of inflammation, plays an important role in obesity-induced insulin resistance. Inhibition of iNOS by gene disruption or pharmacological inhibitors reverses or ameliorates obesity-induced insulin resistance in skeletal muscle and liver in mice. It is unknown, however, whether increased expression of iNOS is sufficient to cause insulin resistance in vivo. To address this issue, we generated liver-specific iNOS transgenic (L-iNOS-Tg) mice, where expression of the transgene, iNOS, is regulated under mouse albumin promoter. L-iNOS-Tg mice exhibited mild hyperglycemia, hyperinsulinemia, insulin resistance, and impaired insulin-induced suppression of hepatic glucose output, as compared with wild type (WT) littermates. Insulin-stimulated phosphorylation of insulin receptor substrate-1 (IRS-1) and -2, and Akt was significantly attenuated in liver, but not in skeletal muscle, of L-iNOS-Tg mice relative to WT mice without changes in insulin receptor phosphorylation. Moreover, liver-specific iNOS expression abrogated insulin-stimulated phosphorylation of glycogen synthase kinase-3β, forkhead box O1, and mTOR (mammalian target of rapamycin), endogenous substrates of Akt, along with increased S-nitrosylation of Akt relative to WT mice. However, the expression of insulin receptor, IRS-1, IRS-2, Akt, glycogen synthase kinase-3β, forkhead box O1, protein-tyrosine phosphatase-1B, PTEN (phosphatase and tensin homolog), and p85 phosphatidylinositol 3-kinase was not altered by iNOS transgene. Hyperglycemia was associated with elevated glycogen phosphorylase activity and decreased glycogen synthase activity in the liver of L-iNOS-Tg mice, whereas phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and proliferator-activated receptor γ coactivator-1α expression were not altered. These results clearly indicate that selective expression of iNOS in liver causes hepatic insulin resistance along with deranged insulin signaling, leading to hyperglycemia and hyperinsulinemia. Our data highlight a critical role for iNOS in the development of hepatic insulin resistance and hyperglycemia.     

Contribution of hepatic glycogenolysis and gluconeogenesis in the defense against short-term insulin induced hypoglycemia in rats

In this study, the contribution of liver glycogenolysis and gluconeogenesis in the defense against short-term insulin induced hypoglycemia (IIH) was investigated. For this purpose, we used an experimental model in which IIH was obtained by administering an IP injection of a pharmacological dose (1 U/kg) of regular insulin to rats that had been deprived of food for a period of six hours. This experimental model is suitable to study the simultaneous participation of glycogen breakdown and gluconeogenesis in the defense against IIH. The livers of IIH rats showed insignificant changes in the glycogen concentration, total phosphorylase, active phosphorylase, and percent of active phosphorylase. Our results also indicated that the livers of IIH rats that received the concentration of L-alanine, L-glutamine, L-lactate, or glycerol found in the blood during IIH (basal values) showed negligible glucose production. Nonetheless, glucose, urea, and pyruvate production increased (P<0.05) if the livers were perfused with a saturating concentration of gluconeogenic precursors. In agreement with these results, IIH rats that received intragastric L-alanine, L-glutamine, or L-lactate showed increased (P<0.05) glycemia 30 min after the administration of these substances. However, when using glycerol, higher glycemia (P<0.05) was observed at 2 and 5 min, but not 30 min after the administration of this hepatic gluconeogenic precursor. Thus, we can conclude that the oral availability of gluconeogenic precursors could allow for their use as important antidote in the defense against IIH.     

Regulation of immune-aggregate-stimulated hepatic glycogenolysis and vasoconstriction by vicinal dithiols.

Evidence suggesting that vicinal dithiols regulate immune-aggregate-induced vasoconstriction and glycogenolysis in the perfused rat liver was obtained. Phenylarsine oxide (PhAsO) and other tervalent organic arsenicals inhibited in a dose-dependent manner hepatic glycogenolysis, vasoconstriction, Ca2+ mobilization and the stimulated O2 consumption caused by immune-aggregate infusion. Polar tervalent and quinquivalent arsenicals were less effective than hydrophobic arsenicals. Prior infusion of Fc- but not Fab-fragments of IgG prevented partially immune-aggregate-stimulated hepatic metabolism, suggesting that immune aggregates elicit hepatic metabolic responses through Fc gamma receptors. The inhibitory action of PhAsO on immune-aggregate-stimulated hepatic glycogenolysis was unique; inhibition of glycogenolysis was not observed when phenylephrine, isoprenaline or glucagon was used as a stimulant. Although PhAsO might be expected to sequester cellular thiols, no significant change in the oxidation-reduction state of the major cellular thiol, glutathione, was found during PhAsO infusion. In addition, PhAsO exerted its effects without producing changes in hepatic adenine nucleotides and cyclic AMP. Evidence suggesting the involvement of vicinal dithiols was obtained through thiol-competition experiments using mono- and di-thiols. PhAsO inhibition of IgG-aggregate-stimulated hepatic vasoconstriction and glycogenolysis was reversed significantly by infusion of 2,3-dimercaptopropan-1-ol at 3-fold molar excess, whereas 2-mercaptoethanol at 40-fold molar excess was ineffective. The results of the present study provide evidence documenting the participation of vicinal dithiols during the coupling of hepatic immune-aggregate clearance by Kupffer cells with vasoconstriction of the hepatic vasculature (e.g. endothelial cells) and glycogenolysis (e.g. parenchymal cells).     

Stimulation of hepatic glycogenolysis by phorbol 12-myristate 13-acetate.

In isolated perfused rat livers, infusion of phorbol 12-myristate 13-acetate (PMA) (150 nM) resulted in a 3-fold stimulation of the rate of glucose production. This response was maximal at a perfusate PMA concentration of 150 nM, and was significantly diminished at higher concentrations of PMA (e.g. 300 nM). Stimulation of glycogenolysis by PMA was greatly decreased in livers perfused with Ca2+-free medium. PMA infusion into livers perfused in the absence of Ca2+ did not result in Ca2+ efflux from the livers. Additionally, in hepatocytes isolated from livers of fed rats, neither PMA nor 1-oleoyl-2-acetyl-rac-glycerol stimulated the rate of glucose production. Although indomethacin has been demonstrated to block PMA-stimulated hepatic glycogenolysis [Garcia-Sainz & Hernandez-Sotomayor (1985) Biochem. Biophys. Res. Commun. 132, 204-209], infusion of PMA into perfused rat livers did not alter the rates of production of either prostaglandin E2 or 6-oxo-prostaglandin F1 alpha in the livers. These data, along with the observed increases in the perfusion pressure and decrease in O2 consumption in isolated perfused livers suggest that phorbol-ester-stimulated glycogenolysis is not a consequence of a direct effect of phorbol ester on liver parenchymal cells.     

On the mechanism of hepatic glycogenolysis induced by anoxia or cyanide

Addition of glucagon to isolated hepatocytes increased glycogenolysis and phosphorylase a in a proportional manner. KCN caused slightly more glycogenolysis at considerably lower levels of phosphorylase a; the discrepancy was most pronounced after pretreatment of the hepatocytes with EGTA. When incubated with tagatose, the hepatocytes accumulated tagatose 1-phosphate, a presumed inhibitor of phosphorylase a. In these conditions the glucagon-induced glycogenolysis was blocked, but the glycogen loss caused by KCN or anoxia was not affected. Cyanide and anoxia may allow phosphorylase b and a to become equally active, or they may trigger a non-phosphorolytic glycogenolysis.     

Hypertriglyceridemia: cause or consequence of insulin resistance?

An overproduction of VLDL by the liver and a slower clearance of these lipoproteins are usually seen in diabetic patients. There is correlation between insulin resistance and plasma triglyceride concentration. Triglyceride may influence an early step in the insulin action pathway and alternatively, insulin resistance may cause hypertriglyceridemia. Hyperinsulinemia and/or hypertriglyceridemia may play a strong role in the cardiovascular risk of patients with type II diabetes. There is an important need to conduct trials to define therapy that can reduce the risk of cardiovascular complications.     

The problem of establishing relationships between hepatic steatosis and hepatic insulin resistance

Excessive deposition of fat in the liver (hepatic steatosis) is frequently accompanied by hepatic insulin resistance. Whether this correlation is due to a causal relationship between the conditions has been the subject of considerable debate, and the literature abounds with conflicting data and theories. Here we provide a perspective by defining the problem and its challenges, analyzing the possible causative relationships, and drawing some conclusions.     

Hemorrhage induces the rapid development of hepatic insulin resistance

Hyperglycemia is an early metabolic response to trauma and hemorrhage. The role of hepatic insulin resistance to the development of this hyperglycemia is not well understood. The aim of this study was to determine whether the liver becomes insulin resistant and to identify the particular hepatic insulin signaling pathways that may be compromised following trauma and hemorrhage. Male adult rats were bled to a mean arterial pressure of 40 mmHg and maintained at that pressure for 90 min followed by resuscitation with Ringer lactate. Data showed that trauma and hemorrhage rapidly induced profound hyperinsulinemia in combination with significant hyperglycemia, suggesting the development of insulin resistance. After trauma and hemorrhage, hepatic insulin signaling via the insulin-induced phosphatidylinositol 3 (PI3)-kinase-Akt pathway was abolished, whereas ERK1/2 signaling was relatively normal. The regulation (inhibition) of a hepatic-, insulin-, and the PI3-kinase-dependent gene, IGF binding protein-1, was also lost. The present study provides convincing evidence of a rapid onset hepatic insulin resistance following a combination of trauma and hemorrhage.