Mechanism of Control of Hepatic Glycogenesis by Insulin

A systematic review of the biological mediators of fat taste and smell

Taste and smell play a key role in our ability to perceive foods. Overconsumption of highly palatable energy-dense foods can lead to increased caloric intake and obesity. Thus there is growing interest in the study of the biological mediators of fat taste and associated olfaction as potential targets for pharmacologic and nutritional interventions in the context of obesity and health. The number of studies examining mechanisms underlying fat taste and smell has grown rapidly in the last 5 years. Therefore, the purpose of this systematic review is to summarize emerging evidence examining the biological mechanisms of fat taste and smell. A literature search was conducted of studies published in English between 2014 and 2021 in adult humans and animal models. Database searches were conducted using PubMed, EMBASE, Scopus, and Web of Science for key terms including fat/lipid, taste, and olfaction. Initially, 4,062 articles were identified through database searches, and a total of 84 relevant articles met inclusion and exclusion criteria and are included in this review. Existing literature suggests that there are several proteins integral to fat chemosensation, including cluster of differentiation 36 (CD36) and G protein-coupled receptor 120 (GPR120). This systematic review will discuss these proteins and the signal transduction pathways involved in fat detection. We also review neural circuits, key brain regions, ingestive cues, postingestive signals, and genetic polymorphism that play a role in fat perception and consumption. Finally, we discuss the role of fat taste and smell in the context of eating behavior and obesity.

Insulin's other life: an autoantigen in type 1 diabetes

One hundred years ago, Frederick Banting, John Macleod, Charles Best and James Collip, and their collaborators, discovered insulin. This discovery paved the way to saving countless lives and ushered in the "Insulin Era." Since the discovery of insulin, we have made enormous strides in understanding its role in metabolism and diabetes. Insulin has played a dramatic role in the treatment of people with diabetes; particularly type 1 diabetes (T1D). Insulin replacement is a life-saving therapy for people with T1D and some with type 2 diabetes. T1D is an autoimmune disease caused by the T-cell-mediated destruction of the pancreatic insulin-producing beta cells that leads to a primary insulin deficiency. It has become increasingly clear that insulin, and its precursors preproinsulin (PPI) and proinsulin (PI), can play another role-not as a hormone but as an autoantigen in T1D. Here we review the role played by the products of the INS gene as autoantigens in people with T1D. From many elegant animal studies, it is clear that T-cell responses to insulin, PPI and PI are essential for T1D to develop. Here we review the evidence that autoimmune responses to insulin and PPI arise in people with T1D and discuss the recently described neoepitopes derived from the products of the insulin gene. Finally, we look forward to new approaches to deliver epitopes derived from PPI, PI and insulin that may allow immune tolerance to pancreatic beta cells to be restored in people with, or at risk of, T1D.

904 nm Low-Level Laser Irradiation Decreases Expression of Catabolism-Related Genes in White Adipose Tissue of Wistar Rats: Possible Roles of Laser on Metabolism

Background: Adipose tissue is the main energy storage tissue in the body. Its catabolic and anabolic responses depend on several factors, such as nutritional status, metabolic profile, and hormonal signaling. There are few studies addressing the effects of laser photobiomodulation (PBM) on adipose tissue and results are controversial. Objective: Our purpose was to investigate the metabolic effects of PBM on adipose tissue from Wistar rats supplemented or not with caffeine. Materials and methods: Wistar rats were divided into four groups: control (CTL), laser-treated [CTL (L)], caffeine (CAF), and caffeine+PBM [CAF (L)]. Blood was extracted for quantification of triglyceride and cholesterol levels and white adipose tissues were collected for analysis. We evaluated gene expression in the adipose tissue for the leptin receptor, lipase-sensitive hormone, tumor necrosis factor alpha, and beta adrenergic receptor. Results: We demonstrated that the low-level laser irradiation was able to increase the feed intake of the animals and the relative mass of the adipose tissue in the CTL (L) group compared with CTL. Laser treatment also increases serum triglycerides [CTL = 46.99 ± 5.87; CTL (L) = 57.46 ± 14.38; CAF = 43.98 ± 5.17; and CAF (L) = 56.9 ± 6.12; p = 0.007] and total cholesterol (CTL = 70.62 ± 6.80; CTL (L) = 79.41 ± 13.07; CAF = 71.01 ± 5.52; and CAF (L) = 79.23 ± 6.881; p = 0.003). Conclusions: Laser PBM decreased gene expression of the studied genes in the adipose tissue, indicating that PBM is able to block the catabolic responses of this tissue. Interestingly, the CAF (L) and CAF animals presented the same CLT (L) phenotype, however, without increasing the feed intake and the relative weight of the adipose tissue. The description of these phenomena opens a new perspective for the study of the action of low-level laser in adipose tissue.

Molecular regulation of insulin granule biogenesis and exocytosis

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by hyperglycemia, insulin resistance and hyperinsulinemia in early disease stages but a relative insulin insufficiency in later stages. Insulin, a peptide hormone, is produced in and secreted from pancreatic β-cells following elevated blood glucose levels. Upon its release, insulin induces the removal of excessive exogenous glucose from the bloodstream primarily by stimulating glucose uptake into insulin-dependent tissues as well as promoting hepatic glycogenesis. Given the increasing prevalence of T2DM worldwide, elucidating the underlying mechanisms and identifying the various players involved in the synthesis and exocytosis of insulin from β-cells is of utmost importance. This review summarizes our current understanding of the route insulin takes through the cell after its synthesis in the endoplasmic reticulum as well as our knowledge of the highly elaborate network that controls insulin release from the β-cell. This network harbors potential targets for anti-diabetic drugs and is regulated by signaling cascades from several endocrine systems.

Regulation of liver development: implications for liver biology across the lifespan

The liver serves a spectrum of essential metabolic and synthetic functions that are required for the transition from fetal to postnatal life. Processes essential to the attainment of adequate liver mass and function during fetal life include cell lineage specification early in development, enzymic and other functional modes of differentiation throughout gestation, and ongoing cell proliferation to achieve adequate liver mass. Available data in laboratory rodents indicate that the signaling networks governing these processes in the fetus differ from those that can sustain liver function and mass in the adult. More specifically, fetal hepatocytes may develop independent of key mitogenic signaling pathways, including those involving the Erk mitogen-activated protein kinases MAPK1/3 and the mechanistic target of rapamycin (mTOR). In addition, the fetal liver is subject to environmental influences that, through epigenetic mechanisms, can have sustained effects on function and, by extension, contribute to the developmental origin of adult metabolic disease. Finally, the mitogen-independent phenotype of rat fetal hepatocytes in late gestation makes these cells suitable for cell-based therapy of liver injury. In the aggregate, studies on the mechanisms governing fetal liver development have implications not only for the perinatal metabolic transition but also for the prevention and treatment of liver disorders throughout the lifespan.

Pancreatic regulation of glucose homeostasis

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

Glucose and fatty acid metabolism in a 3 tissue in-vitro model challenged with normo- and hyperglycaemia

Nutrient balance in the human body is maintained through systemic signaling between different cells and tissues. Breaking down this circuitry to its most basic elements and reconstructing the metabolic network in-vitro provides a systematic method to gain a better understanding of how cross-talk between the organs contributes to the whole body metabolic profile and of the specific role of each different cell type. To this end, a 3-way connected culture of hepatocytes, adipose tissue and endothelial cells representing a simplified model of energetic substrate metabolism in the visceral region was developed. The 3-way culture was shown to maintain glucose and fatty acid homeostasis in-vitro. Subsequently it was challenged with insulin and high glucose concentrations to simulate hyperglycaemia. The aim was to study the capacity of the 3-way culture to maintain or restore normal circulating glucose concentrations in response to insulin and to investigate the effects these conditions on other metabolites involved in glucose and lipid metabolism. The results show that the system’s metabolic profile changes dramatically in the presence of high concentrations of glucose, and that these changes are modulated by the presence of insulin. Furthermore, we observed an increase in E-selectin levels in hyperglycaemic conditions and increased IL-6 concentrations in insulin-free-hyperglycaemic conditions, indicating, respectively, endothelial injury and proinflammatory stress in the challenged 3-way system.

Insulin and the stimulation of glycogen synthesis. The road from glycogen structure to glycogen synthase to cyclic AMP-dependent protein kinase to insulin mediators

The enhanced phosphorylations via cAMP, Ca2+ mobilization, and diacyl glycerol formation via the activation of the respective kinases is now classical. The decreased phosphorylation via inhibition of adenylate cyclase via the alpha adrenergic receptor is also becoming understood. What the insulin studies on the control of glycogen synthesis have taught us is that the rate limiting enzyme glycogen synthase is regulated by multiple covalent phosphorylation in an elegant but complex manner. The overall pattern of dephosphorylation is influenced by effecting both phosphatase and kinase activities in a set of interrelated mechanisms. In the presence of glucose, in muscle, fat, and liver under physiological conditions G-6-P acts as a signal to stimulate the phosphatase. An additional stimulation could occur via a novel insulin phosphatase stimulatory mediator. The phosphatase is also stimulated by at least three covalent mechanisms involving altered phosphorylation state. In one there is a decreased phosphorylation of the phosphatase inhibitor 1 potentially related to decreased cAMP-dependent protein kinase activity. In the second, there is decreased phosphorylation of the deinhibitor also potentially related to decreased cAMP-dependent protein kinase phosphorylation. In the third, an increased activity of casein kinase 2 could activate the ATP-Mg dependent phosphatase by an increased phosphorylation of phosphatase inhibitor 2 (modulatory subunit). In the liver, allosteric control of the phosphatase by G-6-P and nucleotides is of great importance. Insulin also stimulates the phosphatase in long-term experiments via increased protein synthesis. It is clear that future work will be required to determine which species of the various classes of phosphatases are regulated in short-term and long-term regulation by insulin. In terms of kinases, the effects of insulin to inactivate and desensitize the cAMP-dependent protein kinase are established. The molecular mechanisms of this effect remain to be worked out. The enhanced activity of MAP and S-6 kinase would appear to be part of a cascade of reactions perhaps originating in the autophosphorylation and activation of the insulin receptor tyrosine kinase. The mechanism of the short-term activation of casein kinase 2 remains to be elucidated. A cAMP-dependent protein kinase inhibitory mediator, which also inhibits adenylate cyclase is an important element in the regulation of kinase and adenylate cyclase activity by insulin. Its physiological significance must be established in the future, in terms of its control of glycogen synthase activation by insulin. Clearly this kinase inhibitor as well as the phosphatase stimulator are potential regulators of glycogen synthase activity by insulin.

Acetylcholine affects rat liver metabolism via type 3 muscarinic receptors in hepatocytes

Although the role of acetylcholine (Ach) in hepatic glucose metabolism is well elucidated, it is still unclear if it influences gluconeogenesis, glycogenolysis and high-energy phosphate metabolism, and if it does what the mechanisms of this influence are. Therefore, using isolated perfused rat liver as a model, we have studied the effect of Ach on oxygen consumption, synthesis of glucose from lactate and pyruvate, glycogen formation, mitochondrial oxidative phosphorylation and ATP-synthesis. We have established that effects of Ach on oxygen consumption depend on its concentration. When used at a concentration of 10(-7) M, Ach exerts maximum stimulatory effect, while its infusion at 10(-6) M causes a decrease of oxygen consumption by the liver. Moreover, when used at a concentration of 10(-6) M or 10(-7) M, Ach increases rates of glucose production from the gluconeogenic substrates lactate and pyruvate, leading to enhanced glycogen content in perfused liver. It was also shown that Ach possesses a stimulating effect on alanine and aspartate aminotransferases. As detected by 31P NMR spectroscopy, continuous liver perfusion with pyruvate and lactate in the presence of Ach leads to a significant decrease of ATP level, implying enhanced energy requirements for gluconeogenesis under these conditions. Elimination of the described effects of Ach by atropine, the antagonist of muscarinic receptors, and identification of the type 3 muscarinic receptors (m3) in isolated hepatocytes as well as in whole liver, imply that Ach may exert its effect on liver metabolism through m3 receptors.

Proliferation and hypertrophy of liver cells surrounding islet grafts in diabetic recipient rats

The liver offers an adequate site for the metabolic function of pancreatic islet implants. Little is known about the effects of the islet grafts on the host organ. This study examines liver tissue of normal or streptozotocin (STZ)-diabetic rats at different intervals following intraportal injection of syngeneic islets. Implantation of 800-islet-grafts, containing 0.9 million beta cells, normalized overt diabetes within 14 days. This period of metabolic normalization was characterized by a specific sequence of alterations in the implant area. During the first days after transplantation, islet cells migrated into the liver lobules, whereby tight hepatocyte-islet cell contacts were established. Hepatocytes surrounding grafts showed massive lipid accumulation and hypertrophy (cellular profile area 603 +/- 72 microns 2 in diabetic islet recipients vs. 382 +/- 42 microns 2 in diabetic controls; P < .005). The implant area also contained significantly more liver cells in proliferative activity than hepatic tissue in normal controls (bromodeoxyuridine labeling index of peri-islet hepatocytes 6.2%, 4.6%, and 0.9% on posttransplantation days 2, 4, and 14, respectively, compared with 0.02% in normal controls). The cellular hypertrophy and hyperplasia explain the sudden increase in liver weight of diabetic recipients (from 8.0 +/- 1.1 g to 13.8 +/- 2.2 g on posttransplantation day 2; P < .005). Both alterations can be attributed to the massive local discharge of insulin in an insulin-deficient organ containing an excess of extra-cellular nutrients. Progressive revascularization of the implant sites and overall metabolic normalization are thought to explain the return of a normal liver histology by the third week after transplantation. In conclusion, intraportal islet grafts exert profound effects on the liver of diabetic rat recipients. The morphological features of the implant sites may serve as markers for the function of the islet grafts as well as for the adaptive capacity of the recipient liver.

Indomethacin treatment causes loss of insulin action in rats: involvement of prostaglandins in the mechanism of insulin action

Glucose tolerance tests in rats showed that after indomethacin treatment plasma insulin levels rose five-fold higher than in untreated controls. Accordingly, the pancreatic islets of indomethacin-treated rats secreted insulin at a threefold higher rate. Glucose tolerance tests additionally showed that indomethacin treatment led to a retarded disposal of the elevated blood glucose. Both effects appear to be caused by an attenuation of the hormone responsiveness for insulin and noradrenaline (alpha-adrenoceptor action) by indomethacin. The following observations support this view: insulin and adrenaline (alpha-adrenoceptor action) lost their ability to lower cyclic adenosine monophosphate (AMP) levels in hepatocytes; the glycogen content of liver and skeletal muscle was reduced by 95% and 65%, respectively; in adipocytes the stimulation of glucose transport by insulin was reduced by 60%. These effects of indomethacin can be reversed by the addition of exogenous prostaglandin E (PGE), as elevated cyclic AMP synthesis was again sensitive to alpha-adrenergic inhibition in the liver. These results indicate a relationship between prostaglandins and insulin action. These effects of indomethacin could result from reduced synthesis of cyclic PIP (prostaglandylinositol cyclic phosphate), a proposed second messenger for insulin and alpha-adrenoceptor action, whose synthesis was decreased by indomethacin treatment and increased by the addition of exogenous PGE. Stimulation of glucose transport by cyclic PIP was unaffected by indomethacin treatment, in contrast to the stimulation by insulin. Inhibition of PGE and cyclic PIP synthesis resulted in a metabolic state comparable to insulin resistance in non-insulin-dependent diabetes mellitus.

Glucokinase expression in rat hepatoma cells induces glucose uptake and is rate limiting in glucose utilization

In contrast to hepatocytes, hepatoma cells lack glucokinase activity and show increased aerobic glycolysis. FTO-2B and H4IIE rat hepatoma cell lines were obtained in which the rat glucokinase gene was expressed (FTOGK and H4GK). These lines were generated by infection of the hepatoma cells with a retroviral vector carrying the phosphoenolpyruvate carboxykinase (PEPCK)-glucokinase chimeric gene. Both the FTOGK and H4GK cells expressed the chimeric gene in a regulated manner, like the endogenous PEPCK gene. Glucokinase activity was detected in both FTOGK and H4GK. These cells lines showed a marked increase in glucose uptake with 18.5 mM glucose in the incubation medium. FTOGK and H4GK showed an increase in the content of glucose 6-phosphate, and were able to accumulate high levels of glycogen, in contrast to FTO-2B cells, which were unable to store the polysaccharide. In addition, cells expressing glucokinase showed high concentration of fructose 2,6-bisphosphate and substantial lactate production, which was related to the glucose concentration in the medium and the time of incubation. These results suggest that glucose phosphorylation is rate limiting for glucose uptake and utilization in FTO-2B and H4IIE cells.

Attenuation of insulin actions in primary rat hepatocyte cultures by phenylarsine oxide

Phenylarsine oxide (PAO), a trivalent arsenical which complexes vicinal dithiols, prevented the action of insulin in primary cultured adult rat hepatocytes. Simultaneous short-term treatment of 48-h old cells with insulin and 2 microM PAO resulted in complete attenuation of the insulin-dependent increase in the level of fructose 2,6-bisphosphate and the activation of phosphofructokinase 2, pyruvate kinase, glucokinase flux and glycolysis. Basal rates of glucose transport and glycolysis were not affected. PAO also abolished stimulation of glycogen synthesis and amino-acid transport and the decrease of glycogenolysis evoked by insulin. The 20-fold activation of the insulin receptor tyrosine kinase by insulin was, however, not reduced by PAO. The data suggest that in differentiated hepatocytes insulin signal transduction involves vicinal sulhydryls located at a post-receptor step.

Insulin resistance in septic rats--a study by the euglycemic clamp technique

Male Wistar rats weighing 250 +/- 30g were made septic by cecum ligation and perforation. Peripheral and hepatic sensitivity to insulin was assessed by the euglycemic glucose clamp technique with simultaneous [3H]glucose infusion. Hepatic glucose output was not suppressed by the insulin infusion in the septic rats in contrast with the controls. Glucose utilization by the peripheral tissues was not significantly different between the septic and control rats. Counterregulatory hormone levels were higher in the septic group. Our data suggest that the liver is the site of insulin resistance in the septic state.

Relationship of substrate level to turnover rate in fasted adult and newborn dogs

Glucose turnover, clearance and response to insulin were determined in fasted newborn and adult dogs. Fasting levels of glucose and insulin and rates of glucose turnover and clearance were not different between the two groups. Blood glucose correlated with basal glucose turnover in newborn pups but not in adult dogs. Glucose turnover was not related to fasting plasma insulin levels. Glucose clearance was an inverse function of blood glucose levels among newborn but not adult dogs. Glucose clearance and blood glucose levels were not related to insulin concentrations. In response to euglycemic hyperinsulinemia, glucose metabolism increased 4-fold among adults but only 1.7-fold in pups. Hyperglycemic hyperinsulinemia increased glucose metabolism in both groups but to a much greater extent in the pups. Euglycemic hyperinsulinemia increased the metabolic clearance rate of glucose 4.2-fold among adults but only 1.8-fold in newborn dogs. In response to hyperglycemic hyperinsulinemia glucose clearance rates were now similar. Despite euglycemic hyperinsulinemia, the newborn dog had an attenuated response to insulin, demonstrating lower rates of glucose metabolism and glucose clearance. The response to the hyperglycemic stimuli suggests that maximal glucose uptake was not achieved during hyperinsulinemia alone. This response supports the concept of glucose-mediated regulation of glucose disposal in newborn animals.

Glycogen synthase kinetics in isolated human adipocytes: an in vitro model for the effects of insulin on glycogen synthase

Glycogen synthase which catalyzes the incorporation of uridine dipophosphate glucose into glycogen is found in muscle, liver, and fat. The activity of this enzyme is increased by insulin through a dephosphorylation mechanism. Because of the critical role of glycogen synthase in glucose storage and overall glucose metabolism, it is important to assess the status of the activity of this enzyme in normal humans as well as in individuals with pathological conditions, such as non-insulin-dependent diabetes mellitus. However, in human subjects, studies of the regulation of glycogen synthase in vivo are time consuming and tedious. The present study was, therefore, undertaken to establish whether adipocytes isolated from subcutaneous adipose tissue biopsies from normal human subjects could be used to assess the effect of insulin in vitro on glycogen synthase activity. Regulation of glycogen synthase in human adipocytes by glucose 6-phosphate and uridine disphosphate glucose was found to be somewhat different than that reported for the regulation of this enzyme in tissues from other species. The adipocyte was found to be a sensitive model for insulin activation of this enzyme. Glycogen synthase was stimulated twofold by an insulin concentration of as low as 1 ng/ml, while half-maximal activation of enzyme activity occurred at 0.4 +/- 0.1 ng insulin/ml. The present studies indicate that the isolated human subcutaneous adipocyte may serve as a useful model for in vitro investigation of the effects of insulin on glycogen synthase.

Liver glycogen metabolism in endotoxin shock. I. Endotoxin administration decreases glycogen synthase activities in dog livers

The effects of E. coli endotoxin administration on hepatic glycogen content and glycogen synthase activities in dogs were studied. Liver glycogen content was decreased by 80% 2 hr after endotoxin injection. When enzyme preparations were preincubated at 25 degrees C for 3 hr prior to their assays, 75% of total glycogen synthase was in I form in control dogs. Under such conditions, endotoxin administration decreased the percentage I activity from 75 to 37%; decreased the Vmax and Km for UDP-glucose for total glycogen synthase by 62.2 and 35.3%, respectively; decreased the Vmax and Km for UDP-glucose for glycogen synthase I by 75.6 and 15.6%, respectively; increased the A0.5 for glucose-6-P for the activation of glycogen synthase D by 126% at high (10 mM) and by 18-fold at low (1 mM) UDP-glucose concentration; increased the percentage D activity from 24 to 72%; decreased the I50 for ATP for the inhibition of total glycogen synthase by 49.7%; decreased the I50 for ATP for the inhibition of glycogen synthase I by 26.4%; and decreased the percentage I activity from 78 to 33% at ATP concentrations below 6 mM. When enzyme preparations were not preincubated prior to their assays, 90% of total glycogen synthase was in D form in control dogs. Under such conditions, endotoxin administration decreased the Vmax and Km for UDP-glucose for total glycogen synthase by 47.1 and 33.3%, respectively, and increased the A0.5 for glucose-6-P for the activation of glycogen synthase D by 24.2% at high (10 mM) and by 106% at low (1 mM) UDP-glucose concentration. From these results, it is clear that endotoxin administration greatly impaired hepatic glycogenesis by decreasing the activity of glycogen synthase; this impairment is at least in part responsible for the depletion of liver glycogen content in endotoxin shock. Kinetic analyses revealed that the decrease in the activity of glycogen synthase in endotoxic shock is a result of a decrease in the interconversion of this enzyme from inactive to active form and an increase in the interconversion from active to inactive form.

Mechanisms of hormonal regulation of hepatic glucose metabolism

Acute hormonal regulation of liver carbohydrate metabolism mainly involves changes in the cytosolic levels of cAMP and Ca2+. Epinephrine, acting through beta 2-adrenergic receptors, and glucagon activate adenylate cyclase in the liver plasma membrane through a mechanism involving a guanine nucleotide-binding protein that is stimulatory to the enzyme. The resulting accumulation of cAMP leads to activation of cAMP-dependent protein kinase, which, in turn, phosphorylates many intracellular enzymes involved in the regulation of glycogen metabolism, gluconeogenesis, and glycolysis. These are (1) phosphorylase b kinase, which is activated and, in turn, phosphorylates and activates phosphorylase, the rate-limiting enzyme for glycogen breakdown; (2) glycogen synthase, which is inactivated and is rate-controlling for glycogen synthesis; (3) pyruvate kinase, which is inactivated and is an important regulatory enzyme for glycolysis; and (4) the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase bifunctional enzyme, phosphorylation of which leads to decreased formation of fructose 2,6-P2, which is an activator of 6-phosphofructo-1-kinase and an inhibitor of fructose 1,6-bisphosphatase, both of which are important regulatory enzymes for glycolysis and gluconeogenesis. In addition to rapid effects of glucagon and beta-adrenergic agonists to increase hepatic glucose output by stimulating glycogenolysis and gluconeogenesis and inhibiting glycogen synthesis and glycolysis, these agents produce longer-term stimulatory effects on gluconeogenesis through altered synthesis of certain enzymes of gluconeogenesis/glycolysis and amino acid metabolism. For example, P-enolpyruvate carboxykinase is induced through an effect at the level of transcription mediated by cAMP-dependent protein kinase. Tyrosine amino-transferase, serine dehydratase, tryptophan oxygenase, and glucokinase are also regulated by cAMP, in part at the level of specific messenger RNA synthesis. The sympathetic nervous system and its neurohumoral agonists epinephrine and norepinephrine also rapidly alter hepatic glycogen metabolism and gluconeogenesis acting through alpha 1-adrenergic receptors. The primary response to these agonists is the phosphodiesterase-mediated breakdown of the plasma membrane polyphosphoinositide phosphatidylinositol 4,5-P2 to inositol 1,4,5-P3 and 1,2-diacylglycerol. This involves a guanine nucleotide-binding protein that is different from those involved in the regulation of adenylate cyclase. Inositol 1,4,5-P3 acts as an intracellular messenger for Ca2+ mobilization by releasing Ca2+ from the endoplasmic reticulum.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of glycogen synthesis in health and disease

Investigations in our laboratory have shown that the activity of glycogen synthase phosphatase in the liver is shared by at least two functionally distinct proteins: a G-component, which is tightly associated with glycogen particles, and a soluble S-component. Most preparations of glycogen synthase-b that are isolated from the liver of fed glucagon-treated animals require the presence of both components in order to be converted to synthase-a. The G-component is subject to control mechanisms that do not affect the S-component. Its activity is strongly inhibited by phosphorylase-a. This feature explains why glycogen synthesis and glycogenolysis do not normally occur simultaneously, except in the glycogen-depleted liver, where a futile cycle may occur. Experiments in vitro have shown that a minimal glycogen concentration is required to ensure the interaction between the G-component and phosphorylase-a. The G-component is also selectively inhibited by Ca2+, and the magnitude of this inhibition depends markedly on the glycogen concentration. The latter inhibition is probably one of the mechanisms by which cyclic adenosine monophosphate (cAMP)-independent glycogenolytic agents achieve the inactivation of glycogen synthase in the liver. Glucocorticoid hormones and insulin are required for the induction and/or maintenance of the G-component in the liver. During the development of the fetal rat, glucocorticoids induce the G-component in the liver. This is an essential event in the glucocorticoid-triggered deposition of glycogen in the fetal liver. A functional adrenal cortex is also required in the adult animal to prevent a loss of the capacity for hepatic glycogen storage during starvation. The latter capacity depends on the concentration of functional G-component in the liver. Chronic diabetes causes a similar functional loss. However, the effect of glucocorticoids is not mediated by a putative secretion of insulin.

Reversal of glucose-induced inhibition of newborn rat liver mitochondrial maturation by administration of alkylxanthines at birth

A glucose injection given immediately after birth delays the maturation which normally occurs in rat liver mitochondria and which increases the rate of ATP synthesis coupled to succinate oxidation from a low value at birth to the adult value a few hours after birth [R. Meister, J. Comte, L. Baggetto, C. Godinot and D. C. Gautheron, Biochim. biophys. Acta 722, 36 (1983)]. Alkylxanthine (pentoxifylline, HWA 285) administration at birth has no effect on the maturation of mitochondria prepared from 2-hr-old rat livers while DBcAMP administration increases their RCR and their rate of ATP synthesis. On the contrary, both alkylxanthines and DBcAMP reverse the glucose-induced inhibition of mitochondrial maturation. This DBcAMP effect cannot be mimicked by butyrate and is therefore related to cAMP. The cAMP content of rat liver increases during this postnatal period in both control and glucose-treated rats, although glucose administration tends to decrease the level of cAMP. Alkylxanthine administration restores after 2 hr the cAMP level in glucose-treated animals. The variations of RCR could not be completely correlated with the level of cAMP. The possible involvement of other factors in the mitochondrial maturation and the glucose effect is discussed.

Insulin-induced increases in the activity of the spontaneously active and ATP.Mg-dependent forms of phosphatase-1 in alloxan-diabetic rat liver

Liver supernatant from normal and alloxan-diabetic rats was fractionated by DEAE-cellulose chromatography and the separated phosphoprotein phosphatase fractions were assayed with [32P]histone f2b, [32P]phosphorylase a and [32P]phosphorylase kinase as substrates. In diabetic rat liver, one of the phosphatase fractions found in the normal liver was significantly reduced. This fraction was identified as a mixture of the spontaneously active form and the ATP . Mg-dependent form of phosphoprotein phosphatase-1 (Fc) based on sensitivity to inhibitor-2, substrate specificity, and the fact that it could be activated 42-70% by glycogen synthase kinase-3 in the presence of ATP . Mg. Further analysis of this fraction showed that liver cytosol from diabetic rats contained 62-79% lower spontaneously active phosphatase-1 activity and 40-51% lower combined spontaneously active and ATP . Mg-dependent protein phosphatase-1 (Fc) activity. Insulin administration increased the spontaneously active and the ATP . Mg-dependent protein phosphatase-1 activities approximately 45% and 36%, respectively, in alloxan-diabetic rats. These data imply that the lower levels of spontaneously active phosphatase-1 activity in diabetic rat liver cannot be explained by presuming phosphatase-1 to have been present as Fc, the inactive form. Moreover, insulin restored the total activity of the spontaneously active and activatable forms of phosphatase-1 to those present in normal liver implying that both forms of phosphatase-1 activity are under hormonal control.

Regulation of hepatic glycogen metabolism: effects of diabetes, insulin infusion, and pancreatic islet transplantation

Insulin-deficient diabetes mellitus results in diminished capacity of the liver to accumulate glycogen. One site of metabolic lesion in the diabetic liver is at the level of the synthase-activating enzyme, synthase phosphatase. This activity is progressively diminished with increasing severity of chemically induced diabetes in both soluble and smooth endoplasmic reticulum (SER) associated subfractions. Insulin administration via an implanted miniosmotic pump or via intrahepatic islet transplantation increased synthase phosphatase activity, particularly in SER. Hepatic glycogen synthesis and accumulation was enhanced as well. The data support a role for insulin in maintenance of the ability of the liver to synthesize and accumulate glycogen mediated either directly or indirectly through SER-synthase phosphatase activity.

Effect of starvation-refeeding and an exogenous glucocorticoid on carbohydrate metabolism in chick liver

Broiler chicks, 4 weeks of age, were subjected to a regimen of 48-hr starvation and 24-hr refeeding as a means of inducing hepatic glycogen supercompensation. A synthetic glucocorticoid (prednisolone) and transcription inhibitor (actinomycin D) treatment were superimposed on the starvation-refeeding regimen to examine the effect of an exogenous glucocorticoid and the necessity for de novo protein synthesis during glycogen supercompensation. Starvation decreased plasma glucose, immunoreactive insulin and liver glycogen. These parameters returned to, or overshot prefasting levels after a 48-hr refeeding period. Prednisolone magnified the overshoot response but some de novo protein synthesis was required. Glycogen synthase a activity was opposite that of liver glycogen content. A possible nonhormone stimulated glycogen synthetic mechanism in the starvation-refeeding response of the chick was noted.

Glycogen synthesis in serum-free cultured hepatocytes in response to insulin and dexamethasone

Liver parenchymal cells cultured in serum-free medium may retain their ability to synthesize glycogen in response to insulin. Specific hormone requirements are needed by hepatocytes to retain the biochemical pattern of mature cells. Insulin supplementation of culture medium seems to be essential to maintain the glycogen synthesis rate of cultured hepatocytes. The continuous presence of dexamethasone amplified the insulin-induced glycogen synthesis. Cytophotometric analysis showed differences in the way that individual cells accumulate glycogen in response to insulin stimulus, which indicates that liver parenchymal cells in culture are functionally heterogeneous.

In vivo insulin sensitivity in the rat determined by euglycemic clamp

Our aim was to develop the glucose clamp (GC) technique in the conscious rat for assessment of in vivo insulin sensitivity. A 2-h euglycemic GC could be performed in chronically cannulated rats using 625 microliter blood. Overnight-fasted rats were infused with porcine insulin (1.67 mU . kg-1 . h-1). Insulin levels of 41 +/- 2 (SE) mU/liter were produced in rats aged 91 +/- 4 days with a 60- to 120-min glucose infusion rate (GIR60-120) of 10.6 +/- 0.6 mg . kg-1 . min-1 (n = 9) during euglycemia. GIR60-120 was significantly (P less than 0.025) reduced in rats aged greater than 130 days (mean, 169 +/- 16 days) to 7.7 +/- 1.2 mg . kg-1 . min-1 (n = 7). Metabolic clearance rate of porcine insulin (46 +/- 3 ml . kg-1 . min-1) and GIR60-120 compared with plateau plasma insulin levels are higher than values reported in humans. The latter may be due to suppression of a higher basal hepatic glucose production or increased potency of porcine compared with native insulin. We conclude that the GC can be accomplished in the rat. When combined with tracer administration and subsequent killing, it should provide a quantitative in vivo measurement of insulin sensitivity in individual tissues.

Effects of insulin-glucagon interactions on glycogenolysis and protein kinase activity in rat hepatocytes

The effects of insulin and glucagon on cAMP accumulation, protein kinase activation, and glycogenolysis were investigated in isolated rat hepatocytes. Glucagon (0.01 nM to 10 micro M) increased the activation state of protein kinase and the rate of glucose accumulation. Addition of 1.0 nM insulin to cells preincubated with 0.1 nM glucagon attenuated the rate of glucose accumulation, but did not alter the protein kinase activity ratio. Addition of 0.1 nM glucagon to cells preincubated with 1.0 nM insulin caused a rapid activation of protein kinase; however, glycogenolysis was not immediately affected. These effects were enhanced with pharmacological concentrations of glucagon and insulin. These data indicate that the degree of protein kinase activation does not always correlate temporally or quantitatively with rates of glycogenolysis in liver cells exposed to insulin and glucagon.

Binding, degradation, and bioactivity of insulin in primary cultures of rat hepatocytes

Hepatocytes from fasted rats, previously maintained as a monolayer in a serum-free glucagon-containing culture medium are demonstrated to provide a useful model system for the study of receptor-mediated mechanisms of insulin action. The cultured liver cells show glucagon- and insulin-responsive biological effects. These cultures show the long-term effects of insulin on the syntheses of protein ([3H]leucine incorporation), glycogen, and lipids (conversion of [3H]glucose) in a dose-dependent manner in the physiological range of insulin concentrations. The order of the effects of different analogues of insulin on each of the bioactivities studied at 37 degrees C is the same as their order to compete at 25 degrees C for binding to insulin-specific receptors. The characterization of radioactive products of 125I-insulin using gel filtration and anti-insulin A chain antibody has shown a transient accumulation of insulin A chain, indicating that the sequential pathway of insulin degradation is operative in the anchored, cultured hepatocytes. Whereas the synthesis of proteins proceeds linearly after insulin inoculation, the syntheses of glycogen and lipids first occur after a lag period of about 10 and 12 h, respectively. None of the three 125I-labeled fragments or products of 125I-insulin released in the culture mediums showed any biological activity (glycogenesis) in cultured hepatocytes although a high-molecular-weight 125I-product isolated from cells could not be tested because of its insolubility. The possibility that the insulin effects might have been mediated via a nonradioactive fragment of insulin or another chemical agent remains open.

Adverse effects of fructose in perfused livers of diabetic rats

Livers isolated from both fed normal and alloxan diabetic rats were perfused for 30 min using Krebs-Henseleit bicarbonate blood buffer medium followed by 10 min flow-through infusions with either 5 mM or 28 mM fructose concentrations. In livers of normal and diabetic rats, both 5 mM and 28 mM fructose concentrations produced an elevation in tissue cyclic AMP levels, activation of glycogen phosphorylase, increased protein kinase activity, decreased tissue ATP levels, large increases in tissue fructose-1-phosphate, and variable effects upon glycogen synthase. These results are consistent with previously reported cyclic AMP mediated activation of glycogen phosphorylase by fructose via protein kinase in normal rat liver. In addition, both 5 mM and 28 mM fructose infusion resulted in large decreases in normal and diabetic synthase phosphatase activity. Therefore, these results in both normal and diabetic livers are inconsistent with a direct beneficial effect of fructose in the isolated perfused rat liver.