Up-regulation of liver glucose-6-phosphatase in x-linked hypophosphatemic mice

We previously showed that a phosphate-deficient diet resulting in hypophosphatemia upregulated the catalytic subunit p36 of rat liver glucose-6-phosphatase, which is responsible for hepatic glucose production. A possible association between phosphate and glucose homeostasis was now further evaluated in the Hyp mouse, a murine homologue of human X-linked hypophosphatemia. We found that in the Hyp mouse as in the dietary Pi deficiency model, serum insulin was reduced while glycemia was increased, and that liver glucose-6-phosphatase activity was enhanced as a consequence of increased mRNA and protein levels of p36. In contrast, the Hyp model had decreased mRNA and protein levels of the putative glucose-6-phosphate translocase p46 and liver cyclic AMP was not increased as in the phosphate-deficient diet rats. It is concluded that in genetic as in dietary hypophosphatemia, elevated glucose-6-phosphatase activity could be partially responsible for the impaired glucose metabolism albeit through distinct mechanisms.

Probing into the function of the gene product responsible for glycogen storage disease type Ib

This study aimed at directly assessing glucose 6-phosphate (G6P) transport by intact rat liver microsomes. Tracer uptake from labeled G6P occurred with T(1/2) values that proved insensitive to unlabeled G6P or 100 microM vanadate, and could not be activated over background levels by intravesicular phosphate in the complete absence of G6P hydrolysis. [(32)P]Phosphate efflux was similarly unaffected by G6P or phosphate in the incubation medium. We conclude that the gene product responsible for glycogen storage disease type Ib is functionally distinct from the bacterial hexose phosphate transporter, which operates as an obligatory phosphate:phosphate or G6P:phosphate exchanger.

An integrated view of the kinetics of glucose and phosphate transport, and of glucose 6-phosphate transport and hydrolysis in intact rat liver microsomes

The dynamics of the glucose 6-phosphatase system were investigated in intact rat liver microsomes using a fast-sampling, rapid-filtration apparatus. Glucose and phosphate transport followed single exponential kinetics, appeared to be homogeneous, was unaffected by unlabeled substrate concentrations up to 100 mM, proved insensitive to various potential inhibitors, and demonstrated similarly low energies of activation. The extent of tracer accumulation from glucose 6-phosphate depended on which of the glucose or phosphate moieties was the labeled species in the parent molecule. The rates of tracer equilibration reflected those of glucose or phosphate transport but similar initial rates of uptake were observed for the glucose and phosphate products of hydrolysis. However, the latter accounted for only 12-13% of the steady-state rate of total glucose production. It is concluded that tracer uptake cannot represent substrate transport, that labeled glucose 6-phosphate at best represents a tiny fraction of the intramicrosomal glucose or phosphate pools, and that glucose 6-phosphate transport is not an obligatory prerequisite to its hydrolysis. The latter conclusion invalidates a major postulate of the substrate transport-catalytic unit concept but proves compatible with a conformational model whereby glucose 6-phosphate transport and hydrolysis are tightly coupled processes while glucose and phosphate share, along with water and a variety of other organic and inorganic solutes, a common porelike structure for their transport through the microsomal membrane.

Dietary P(i) deprivation in rats affects liver cAMP, glycogen, key steps of gluconeogenesis and glucose production

We previously reported [Xie, Li, Méchin and van de Werve (1999) Biochem. J. 343, 393-396] that dietary phosphate deprivation for 2 days up-regulated both the catalytic subunit and the putative glucose-6-phosphate translocase of the rat liver microsomal glucose-6-phosphatase system, suggesting that increased hepatic glucose production might be responsible for the frequent clinical association of hypophosphataemia and glucose intolerance. We now show that liver cAMP was increased in rats fed with a diet deficient in P(i) compared with rats fed with a control diet. Accordingly, in the P(i)-deficient group pyruvate kinase was inactivated, the concentration of phosphoenolpyruvate was increased and fructose 2, 6-bisphosphate concentration was decreased. Phosphoenolpyruvate carboxykinase activity was marginally increased and glucokinase activity was unchanged by P(i) deprivation. The liver glycogen concentration decreased in the P(i)-deficient group. In the fed state, plasma glucose concentration was increased and plasma P(i) and insulin concentrations were substantially decreased in the P(i)-deficient group. All of these changes, except decreased plasma P(i), were cancelled in the overnight fasted P(i)-deficient group. In the fasted P(i)-deficient group, immediately after a glucose bolus, the plasma glucose level was elevated and the inhibition of endogenous glucose production was decreased. However, this mild glucose intolerance was not sufficient to affect the rate of fall of the glucose level after the glucose bolus. Taken together, these changes are compatible with a stimulation of liver gluconeogenesis and glycogenolysis by the P(i)-deficient diet and further indicate that the liver might contribute to impaired glucose homeostasis in P(i)-deficient states.

Glucose-6-phosphate transporter and receptor functions of the glucose 6-phosphatase system analyzed from a consensus defined by multiple alignments

The cDNA encoding the protein (P46) that is mutated in glycogen storage disease type-1b (GSD-1b) has been previously cloned by homology with bacterial sequences of the uhp (upper hexose phosphate) system. Hydropathic profiles, transmembrane-prediction analysis, and a multiple alignment of 14 sequences related to P46 (with percentage of identity around 30%) allowed to identify two large domains in the proteins linked by a large variable loop. Highly conserved transmembrane (TM) segments, TM1 and TM4 in the first domain and TM5 in the second one, were identified almost in all the integral proteins related to P46. The multiple alignment allowed definition of a consensus involving the 14 sequences related to P46. The detailed comparison of the consensus with the UhpT (the bacterial G6P transporter) and with UhpC (the bacterial G6P receptor) sequences reveals that the P46 protein could carry both G6P receptor and transporter functions.

Chronic intraperitoneal insulin delivery, as compared with subcutaneous delivery, improves hepatic glucose metabolism in streptozotocin diabetic rats

We have previously shown that chronic insulin treatment by the intraperitoneal route normalizes the elevated glucose production (GP) in streptozotocin (STZ) diabetic rats, while insulin delivered by the subcutaneous route only partially normalizes GP. To investigate the biochemical mechanism of the effect of chronic insulin delivery by either route on hepatic glucose metabolism, we measured the hepatic activity of glucose 6-phosphatase (G6Pase) and glucokinase (GK). Four groups of rats were used: (1) nondiabetic rats (N, n = 7), (2) untreated STZ diabetic rats (D, n = 8), (3) diabetic rats treated intraperitoneally (IP, n = 6), or (4) subcutaneously (SC, n = 8) (both 3 U of insulin/d). Glucose levels, higher in D, were normalized by insulin treatment regardless of route. Peripheral insulin levels were lowest in D and highest in SC as expected (N, 162 +/- 18 pmol/L; D, 66 +/- 12; IP, 360 +/- 96; SC, 798 +/- 198). STZ diabetes resulted in a 10-fold decrease in GK (P < .001), and a 2-fold increase in G6Pase activity (P < .01). Both intraperitoneal and subcutaneous treatments normalized G6Pase activity. In contrast, with subcutaneous but not intraperitoneal treatment, GK activity was still 35% less than normal (SC v N, P < .05). Glucose 6-phosphate (G6P) levels did not differ among the groups. In summary: (1) the increase in GP in D reflected increased activity of G6Pase and reduced activity of GK, (2) the partial suppression of GP with subcutaneous insulin treatment reflected correction of increased G6Pase activity, but only partial correction of low GK activity, and (3) the normalization of GP with intraperitoneal insulin treatment reflected correction of both increased G6Pase activity and low GK activity. Our current studies indicate that chronic intraperitoneal insulin treatment is superior to subcutaneous treatment with regard to hepatic glucose metabolism.

Ontogeny of the catalytic subunit and putative glucose-6-phosphate transporter proteins of the rat microsomal liver glucose-6-phosphatase system

The catalytic subunit (p36) and putative glucose-6-phosphate (G6P) transporter (p46) protein levels of the rat glucose-6-phosphatase (G6Pase) system were studied in relation to G6Pase hydrolytic activity and G6P uptake in liver microsomes during the fetal to neonatal period. The mean G6P hydrolytic activity in liver microsomes increased significantly from the 20th to 21st day of gestation (from 6 to 22 mU/mg protein) and was further enhanced by 3-fold 6 hours after birth, with a maximal activity at 1 day of age (112 mU/mg protein). In contrast, G6P uptake into the vesicles was undetectable before birth, appeared after day 1 (656 pmol/mg protein), and decreased after day 2 (about 330 pmol/mg protein). Immunoblot analysis showed that the mean p36 protein level was low (< 1.6 arbitrary units [AU]) during gestation, increased sharply (to about 4.0 AU) during the first day, and remained stable afterward. Unlike p36, p46 protein was present before birth at values comparable to those postpartum. P46 increased from 3.2 AU at 20 days to 4.6 AU at 21 days of gestation, and decreased transiently after birth. These results show that (1) G6Pase hydrolytic activity before birth can occur without detectable G6P uptake function; (2) the presence of the putative G6P transporter protein is not sufficient to elicit G6P uptake; and (3) full G6Pase activity requires optimal expression of both p36 and p46 proteins. These data are discussed in relation to the function of G6Pase.

Distinct hormone stimulation and counteraction by insulin of the expression of the two components of glucose 6-phosphatase in HepG2 cells

We found recently (J. Biol. Chem. 274, 33866-33869, 1999) that the expression of the catalytic subunit (p36) and putative glucose 6-phosphate translocase (p46) of the liver glucose 6-phosphatase system was stimulated by cyclic AMP and glucose and repressed by insulin. We now further show in HepG2 cells that whereas insulin (0.01-10 nM) suppressed p36 mRNA, it only reduced p46 mRNA by half at 1 microM. Cyclic AMP (0.01-100 microM) caused a 2.7-fold increase in p36 mRNA but barely increased p46 mRNA. In contrast, dexamethasone (0.1-100 nM) increased both p36 and p46 mRNA by more than 3-fold. The effects of cyclic AMP and dexamethasone were counteracted by 1 microM insulin. The endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin (1-100 nM) increased p36 mRNA by 2-fold but not p46 mRNA. It thus appears that the hormonal changes which affect p36 alone concur with known modifications in glucose production; those that affect both p36 and p46 are rather consistent with glucose storage.

New lessons in the regulation of glucose metabolism taught by the glucose 6-phosphatase system

The operation of glucose 6-phosphatase (EC 3.1.3.9) (Glc6Pase) stems from the interaction of at least two highly hydrophobic proteins embedded in the ER membrane, a heavily glycosylated catalytic subunit of m 36 kDa (P36) and a 46-kDa putative glucose 6-phosphate (Glc6P) translocase (P46). Topology studies of P36 and P46 predict, respectively, nine and ten transmembrane domains with the N-terminal end of P36 oriented towards the lumen of the ER and both termini of P46 oriented towards the cytoplasm. P36 gene expression is increased by glucose, fructose 2,6-bisphosphate (Fru-2,6-P2) and free fatty acids, as well as by glucocorticoids and cyclic AMP; the latter are counteracted by insulin. P46 gene expression is affected by glucose, insulin and cyclic AMP in a manner similar to P36. Accordingly, several response elements for glucocorticoids, cyclic AMP and insulin regulated by hepatocyte nuclear factors were found in the Glc6Pase promoter. Mutations in P36 and P46 lead to glycogen storage disease (GSD) type-1a and type-1 non a (formerly 1b and 1c), respectively. Adenovirus-mediated overexpression of P36 in hepatocytes and in vivo impairs glycogen metabolism and glycolysis and increases glucose production; P36 overexpression in INS-1 cells results in decreased glycolysis and glucose-induced insulin secretion. The nature of the interaction between P36 and P46 in controling Glc6Pase activity remains to be defined. The latter might also have functions other than Glc6P transport that are related to Glc6P metabolism.

Insulin-induced Ca(2+) entry in hepatocytes is important for PI 3-kinase activation, but not for insulin receptor and IRS-1 tyrosine phosphorylation

Insulin produces an influx of Ca(2+) into isolated rat hepatocyte couplets that is important to couple its tyrosine kinase receptor to MAPK activity (Benzeroual et al., Am. J. Physiol. 272, (1997) G1425-G1432. In the present study, we have examined the implication of Ca(2+) in the phosphorylation state of the insulin receptor (IR) beta-subunit and of insulin receptor substrate-1 (IRS-1), as well as in the stimulation of PI 3-kinase activity in cultured hepatocytes. External Ca(2+) chelation (EGTA 4 mM) or administration of Ca(2+) channel inhibitors gadolinium 50 microM or nickel 500 microM inhibited insulin-induced PI 3-kinase activation by 85, 50 and 50%, respectively, whereas 200 microM verapamil was without effect. In contrast, the insulin-induced tyrosine phosphorylation of IR beta-subunit and of IRS-1 was not affected by any of the experimental conditions. Our data demonstrate that the stimulation of PI 3-kinase activity by the activated insulin receptor, but not the phosphorylation of IR beta-subunit and IRS-1, requires an influx of Ca(2+). Ca(2+) thus appears to play an important role as a second messenger in insulin signaling in liver cells.

Diabetes affects similarly the catalytic subunit and putative glucose-6-phosphate translocase of glucose-6-phosphatase

The effect of streptozocin diabetes on the expression of the catalytic subunit (p36) and the putative glucose-6-phosphate translocase (p46) of the glucose-6-phosphatase system (G6Pase) was investigated in rats. In addition to the documented effect of diabetes to increase p36 mRNA and protein in the liver and kidney, a approximately 2-fold increase in the mRNA abundance of p46 was found in liver, kidney, and intestine, and a similar increase was found in the p46 protein level in liver. In HepG2 cells, glucose caused a dose-dependent (1-25 mM) increase (up to 5-fold) in p36 and p46 mRNA and a lesser increase in p46 protein, whereas insulin (1 microM) suppressed p36 mRNA, reduced p46 mRNA level by half, and decreased p46 protein by about 33%. Cyclic AMP (100 microM) increased p36 and p46 mRNA by >2- and 1.5-fold, respectively, but not p46 protein. These data suggest that insulin deficiency and hyperglycemia might each be responsible for up-regulation of G6Pase in diabetes. It is concluded that enhanced hepatic glucose output in insulin-dependent diabetes probably involves dysregulation of both the catalytic subunit and the putative glucose-6-phosphate translocase of the liver G6Pase system.

Evidence that the transit of glucose into liver microsomes is not required for functional glucose-6-phosphatase

We show that the production of glucose from glucose-6-phosphate hydrolysis outside microsomes is a function of glucose-6-phosphatase independent of its property to form glucose inside microsomes. Indeed, during development (before 1 day of age), mouse liver microsomes had glucose-6-phosphatase producing glucose solely outside microsomes. Furthermore, in vivo treatment of rats with the glucocorticoid analogue triamcinolone resulted in increased glucose-6-phosphatase activity outside but not inside microsomes and without change in the catalytic subunit 40 kDa glucose-6-phosphatase mRNA abundance or protein level, indicating that other factors induced by triamcinolone (e.g., altered membrane lipid environment and/or a regulatory protein) were responsible for the activity change. Triamcinolone treatment also lessened the inhibition of glucose-6-phosphatase by pyridoxal 5'-phosphate (PLP), but this effect was not due to an interaction of PLP with the active site. Accordingly, reversal of the inhibition was observed after permeabilization of the microsomes. The two distinct orientations of liver microsomal glucose-6-phosphate phosphohydrolase suggest different physiological roles played by this enzyme in the endoplasmic reticulum membrane.

Dexfenfluramine modulates hepatic glycogen metabolism by a calcium-dependent pathway

In this study, the mechanism of action of dexfenfluramine (DEXF) at the hepatic level was investigated. The drug is shown to bind to the alpha 1-adrenergic receptor and to increase intracellular calcium in isolated rat hepatocytes, thereby activating phosphorylase via a calcium-dependent mechanism. Moreover, phosphorylase activation by DEXF was inhibited by different agents that interfere with the alpha 1-adrenergic signalling system: prazosin, phorbol 12 alpha-myristate 13 beta-acetate (PMA), and DEXF itself. We also show that phosphorylase activation induced by catecholamines and analogues (epinephrine, phenylephrine), whose actions are mediated by a calcium-dependent mechanism, was counteracted by the drug in the submillimolar range (0.1-1 mM). The activation of glycogenolysis by the drug is accompanied by a stimulation of the glycolytic flux (54% increase in lactate plus pyruvate accumulation), consistent with an increase in fructose-2,6-bisphosphate (F-2,6-BP) levels (36%). These results indicate that the interaction of DEXF with the alpha 1-adrenergic receptor channels glucose 6-phosphate derived from glycogen away from glucose production into the glycolytic pathway.

Insulin induces Ca2+ influx into isolated rat hepatocyte couplets

Isolated rat hepatocyte couplets were used to study the direct effect of insulin on intracellular Ca2+ homeostasis. Insulin induced a dose-dependent increase in hepatocellular Ca2+ that was gradual, generally monophasic, and reversible. Chelation of extracellular Ca2+ abolished the insulin-induced Ca2+ response, and this suppression was not related to an effect on insulin binding, as indicated by displacement studies. We thus tested the effect of several Ca2+ channel inhibitors on insulin-induced Ca2+ influx. Verapamil at 20 or 200 microM was without effect, whereas 500 microM nickel and 50 microM gadolinium strongly inhibited insulin-induced Ca2+ entry. Finally, we tested whether insulin-induced Ca2+ movements were implicated in the stimulation of mitogen-activated protein kinase (MAPK) activity, which we measured with the use of an immune-complex assay. Verapamil was without effect on the insulin-dependent stimulation of p44mapk activity, whereas addition of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, nickel, or gadolinium strongly inhibited the effect of the peptide hormone. Our results indicate that insulin triggers Ca2+ influx into hepatocytes, possibly through the opening of channels on the plasma membrane, and that this effect is important for insulin activation of MAPK.

Activation of mitogen-activated protein kinase in freshly isolated rat hepatocytes by both a calcium- and a protein kinase C-dependent pathway

In the present study, we investigated the role of calcium and protein kinase C (PKC) in the activation of mitogen-activated protein kinase (MAPK) in isolated rat hepatocytes. We found that the glycogenolytic hormone norepinephrine (NE), acting through the alpha1-adrenergic receptor and the G protein Gq, was able to induce a dose- and time-dependent activation of MAPK in hepatocytes. Vasopressin, which acts through a different receptor but also through stimulation of the Gq-dependent pathway, also caused a twofold activation of MAPK. Activation of MAPK by both agonists required the presence of free extracellular calcium and was blocked by the specific PKC inhibitor, Ro 31-8220. MAPK activation was also induced by phorbol myristate acetate (PMA), confirming that a PKC-dependent pathway exists for MAPK activation in liver. Furthermore, calcium-mobilizing agents such as thapsigargin and ionomycin were able to induce an activation of MAPK by a PKC-independent pathway that was totally abolished by preincubation of cells with EGTA. A second pathway for MAPK activation that relies solely on calcium may therefore exist. Ro 31-8220 did not affect phosphorylase activation by NE, vasopressin, thapsigargin, and ionomycin, indicating that PKC inhibition did not interfere with the signaling pathway leading to inositol-1,4,5-triphosphate (IP3)-induced calcium mobilization or with changes in calcium fluxes. The role of MAPK activation by NE and vasopressin in the regulation of hepatic carbohydrate metabolism is discussed.

Evidence for a membrane exchangeable glucose pool in the functioning of rat liver glucose-6-phosphatase

We have investigated the kinetics of tracer uptake into rat liver microsomes in relation to [14C]glucose 6-phosphate (Glu-6-P) hydrolysis by glucose 6-phosphatase (Glu-6-Pase). 1) The steady-state levels of intravesicular tracer accumulated during the rapid (AMP1) and slow (AMP2) phases of uptake both demonstrate Michaelis-Menten kinetics relative to outside Glu-6-P concentrations with Km values similar to those observed for the initial burst (Vi) and steady-state (VSS) rates of Glu-6-P hydrolysis. 2) The AMP1/AMP2 ratio is constant (mean value = 0.105 +/- 0.018) over the whole range of outside Glu-6-P concentrations and is equal to the AMP1max/AMP2max ratio (0.109 +/- 0.032). 3) Linear relationships are observed between the initial rates of glucose transport during the slow uptake phase (V alpha 2) and [AMP1], and between [VSS] and [AMP2]. 4) The value of Vss max exceeds by more than 10-fold that of V alpha 2 max. 5) It is concluded that the substrate transport model is incompatible with those results and that AMP1 represents a membrane exchangeable glucose pool. 6) We propose a new version of the conformational model in which the catalytic site lies deep within a hydrophilic pocket of an intrinsic membrane protein and communicates with the extra- and intravesicular spaces through channels with different glucose permeabilities.

Glucose transport and glucose 6-phosphate hydrolysis in intact rat liver microsomes

Glucose transport was investigated in rat liver microsomes in relation to glucose 6-phosphatase (Glu-6-Pase) activity using a fast sampling, rapid filtration apparatus. 1) The rapid phase in tracer uptake and the burst phase in glucose 6-phosphate (Glu-6-P) hydrolysis appear synchronous, while the slow phase of glucose accumulation occurs during the steady-state phase of glucose production. 2) [14C]Glucose efflux from preloaded microsomes can be observed upon addition of either cold Glu-6-P or Glu-6-Pase inhibitors, but not cold glucose. 3) Similar steady-state levels of intramicrosomal glucose are observed under symmetrical conditions of Glu-6-P or vanadate concentrations during influx and efflux experiments, and those levels are directly proportional to Glu-6-Pase activity. 4) The rates of both glucose influx and efflux are characterized by t1/2 values that are independent of Glu-6-P concentrations. 5) Glucose efflux in the presence of saturating concentrations of vanadate was not blocked by 1 mM phloretin, and the initial rates of efflux appear directly proportional to intravesicular glucose concentrations. 6) It is concluded that glucose influx into microsomes is tightly linked to Glu-6-Pase activity, while glucose efflux may occur independent of hydrolysis, so that microsomal glucose transport appears unidirectional even though it can be accounted for by diffusion only over the accessible range of sugar concentrations.

Histone II-A stimulates glucose-6-phosphatase and reveals mannose-6-phosphatase activities without permeabilization of liver microsomes

The effect of histone II-A on glucose-6-phosphatase and mannose-6-phosphatase activities was investigated in relation to microsomal membrane permeability. It was found that glucose-6-phosphatase activity in histone II-A-pretreated liver microsomes was stimulated to the same extent as in detergent-permeabilized microsomes, and that the substrate specificity of the enzyme for glucose 6-phosphate was lost in histone II-A-pretreated microsomes, as [U-14C]glucose-6-phosphate hydrolysis was inhibited by mannose 6-phosphate and [U-14C]mannose 6-phosphate hydrolysis was increased. The accumulation of [U-14C]glucose from [U-14C]glucose 6-phosphate into untreated microsomes was completely abolished in detergent-treated vesicles, but was increased in histone II-A-treated microsomes, accounting for the increased glucose-6-phosphatase activity, and demonstrating that the microsomal membrane was still intact. The stimulation of glucose-6-phosphatase and mannose-6-phosphatase activities by histone II-A was found to be reversed by EGTA. It is concluded that the effects of histone II-A on glucose-6-phosphatase and mannose-6-phosphatase are not caused by the permeabilization of the microsomal membrane. The measurement of mannose-6-phosphatase latency to evaluate the intactness of the vesicles is therefore inappropriate.

Modulation of glucagon-induced glucose production by dexfenfluramine in rat hepatocytes

The mechanism of the antihyperglycaemic action of dexfenfluramine (DEXF) was investigated in isolated rat hepatocytes exposed to glucagon. Preincubation of hepatocytes with DEXF caused a dose-dependent inhibition of cyclic AMP formation by 100 nM glucagon (Ki = 0.29 mM) that was almost complete at 1 mM DEXF. Surprisingly, glucagon-induced phosphorylase activation was not affected by DEXF despite the significant drop in cyclic AMP levels. Glucose production stimulated by glucagon was inhibited by up to 48% by 1 mM DEXF, and the rate of glucose production correlated positively with the steady-state concentration of glucose 6-phosphate. DEXF also partially restored lactate + pyruvate production which was abolished by an optimal concentration of glucagon. Although DEXF was not able to prevent the inactivation of pyruvate kinase by glucagon, the lack of further accumulation of phosphoenolpyruvate in DEXF-treated cells supports the conclusion that the flux through pyruvate kinase is stimulated, probably via the increase in fructose 2,6-bisphosphate, thereby increasing glycolysis. Our results thus indicate that DEXF counteracts the inhibition of glycolysis by glucagon and that this property might contribute to the antihyperglycaemic effect of this drug. Furthermore, this study shows that, in the presence of the drug, glucagon caused phosphorylase activation and pyruvate kinase inactivation without a significant increase in cyclic AMP levels.

Importance of substrate changes in the decrease of hepatic glucose cycling during insulin infusion and declining glycemia in the depancreatized dog

We wished to determine whether the elevated glucose cycling (GC) between glucose and glucose-6-phosphate (G<-->G6P) in diabetes can be reversed with acute insulin treatment. In six insulin-deprived, anesthetized, depancreatized dogs, insulin was infused for 6-9 h at a starting dose of 45-150 pmol.kg-1.min-1 to normalize plasma glucose from 23.9 +/- 1.4 to 5.0 +/- 0.4 mmol/l and gradually decreased to and maintained at a basal rate (1.7 +/- 1.0 pmol.kg-1.min-1) during the last 3 h. GC, measured with [2-3H]- and [6-3H]glucose, fell markedly from 15.3 +/- 2.7 and normalized at 1.3 +/- 0.6 mumol.kg-1.min-1 (P < 0.001). This occurred because total hepatic glucose output fell much more (from 41.2 +/- 3.1 to 11.6 +/- 1.2) than did glucose production (from 25.9 +/- 1.9 to 10.3 +/- 1.0 mumol.kg-1.min-1) (both P < 0.01). Freeze-clamped liver biopsies were taken at timed intervals for measurements of hepatic enzymes and substrates. The elevated hepatic hexose-6-phosphate levels decreased with insulin infusion (151 +/- 24 vs. 71 +/- 13 nmol/g, P < 0.01). Maximal activities of glucose-6-phosphatase (G6Pase) (from 17.6 +/- 0.8 to 19.6 +/- 2.6 U/g) and glucokinase (from 1.1 +/- 0.2 to 1.0 +/- 0.2 U/g) did not change. Insulin infusion resulted in a threefold increase (P < 0.05) in the activity of glycogen synthase (active form), but had no effect on hepatic glycogen content.(ABSTRACT TRUNCATED AT 250 WORDS)

A conformational model for the human liver microsomal glucose-6-phosphatase system: evidence from rapid kinetics and defects in glycogen storage disease type 1

Rapid kinetics of glucose-6-phosphate (G6P) uptake and hydrolysis as well as of orthophosphate uptake were investigated in microsomes prepared from normal and glycogen storage disease type 1a (GSD 1a) human livers using a fast sampling, rapid filtration apparatus and were compared to those of rat liver microsomes. As shown before with rat microsomes, the production of [U-14C]glucose from 0.2 mmol/L [U-14C]G6P by untreated normal human microsomes was characterized by a burst in activity during the first seconds of incubation, followed by a slower linear rate. The initial velocity of the burst was equal to the rate of glucose production in detergent-treated microsomes. In untreated and detergent-treated GSD 1a microsomes, no glucose-6-phosphatase activity was observed. When untreated normal human or rat microsomes were incubated in the presence of 0.2 mmol/L [U-14C]G6P, an accumulation of [U-14C]glucose was observed, whereas no radioactive compound (G6P and/or glucose) was taken up by GSD 1a microsomes. Orthophosphate uptake was, however, detectable in both GSD 1a and normal untreated vesicles. These results do not support a rate-limiting transport of G6P in untreated normal human microsomes and further show that in this case of GSD 1a, no distinct G6P transport activity is present.

Absence of glucose uptake by liver microsomes: an explanation for the complete latency of glucose dehydrogenase

The permeability of rat liver microsomes to glucose was investigated in relation to the hexose-6-phosphate dehydrogenase system (EC 1.1.1.47). It was found that glucose-6-phosphate dehydrogenase activity could be assayed with NADP as coenzyme in both untreated and detergent-treated microsomes. However, when glucose was used as substrate, activity was only measurable in detergent-treated microsomes. Moreover, radioactive glucose added to microsomes in a variety of experimental conditions was never taken up by the vesicles. Our results indicate that NADP (or NAD) availability is probably not the reason for the absence of glucose dehydrogenase activity in untreated microsomes but rather membrane impermeability to glucose would account for the complete latency observed. This finding calls for a reevaluation of glucose transport in relation to other enzymes of the endoplasmic reticulum, such as glucose-6-phosphatase.

Effect of hepatic portal infusion of pyruvate on pancreatic hormone response during exercise

The purpose of the present investigation was to evaluate the effects of a small infusion of pyruvate into the hepatic portal vein on the pancreatic hormone response during exercise (30-min treadmill run; 26 m/min, 0% grade) in adrenodemedullated rats. Resting and exercising rats were infused with either pyruvate (5% solution; 0.016 ml/min) into the portal vein, pyruvate into the jugular vein, or saline into the portal vein. Peripheral and portal blood glucose concentrations were decreased (P < 0.01) similarly in all groups after the exercise period. Peripheral insulin, glucagon, and norepinephrine levels, either at rest or after exercise, were not significantly affected by the infusions. The response of portal pancreatic hormone concentrations to exercise was, however, reduced by the pyruvate infused into the portal and jugular veins for insulin and into the portal vein only for glucagon. The normal increase in peripheral glucagon-insulin molar ratio during exercise was shut down by the infusion of pyruvate into the portal vein but not by the infusion of pyruvate into the jugular vein or by the infusion of saline. These results indicate that a small blood infusion of pyruvate, even in the presence of a decreasing blood glucose level, can attenuate substantially the pancreatic hormone response during exercise in adrenodemedullated rats.

Identification of the glycogenic compound 5-iodotubercidin as a general protein kinase inhibitor

Addition of micromolar concentrations of the adenosine derivative 5-iodotubercidin (Itu) initiates glycogen synthesis in isolated hepatocytes by causing inactivation of phosphorylase and activation of glycogen synthase [Flückiger-Isler and Walter (1993) Biochem. J. 292, 85-91]. We report here that Itu also antagonizes the effects of saturating concentrations of glucagon and vasopressin on these enzymes. The Itu-induced activation of glycogen synthase could not be explained by the removal of phosphorylase a (a potent inhibitor of the glycogen-associated synthase phosphatase). When tested on purified enzymes, Itu did not affect the activities of the major Ser/Thr-specific protein phosphatases (PP-1, PP-2A, PP-2B and PP-2C), but it inhibited various Ser/Thr-specific protein kinases as well as the tyrosine kinase activity of the insulin receptor (IC50 between 0.4 and 28 microM at 10-15 microM ATP). Tubercidin, which did not affect glycogen synthase or phosphorylase in liver cells, was 300 times less potent as a protein kinase inhibitor. Kinetic analysis of the inhibition of casein kinase-1 and protein kinase A showed that Itu acts as a competitive inhibitor with respect to ATP, and as a mixed-type inhibitor with respect to the protein substrate. We propose that Itu inactivates phosphorylase and activates glycogen synthase by inhibiting phosphorylase kinase and various glycogen synthase kinases. Consistent with the broad specificity of Itu in vitro, this compound decreased the phosphorylation level of numerous phosphopolypeptides in intact liver cells. Our data suggest that at least some of the biological effects of Itu can be explained by an inhibition of protein kinases.

Opposite effects of hyperglycemia and insulin deficiency on liver glycogen synthase phosphatase activity in the diabetic rat

The specific effect of hyperglycemia on the reported decrease in liver glycogen synthase phosphatase activity was studied in STZ-induced diabetic rats with normal fasting insulinemia. Four groups of animals were investigated: control (nondiabetic), diabetic hyperglycemic (STZ), diabetic normoglycemic (STZ followed by 3-day phloridzin treatment), and a diabetic normoglycemic group injected with glucose to reinstate hyperglycemia. None of the treatments significantly altered fasting plasma insulin and glucagon concentrations. We found that hepatic synthase phosphatase activity decreased in STZ-induced diabetic rats and was further markedly reduced when glycemia was normalized in the diabetic animals. This additional decrease in phosphatase activity was almost fully reversed when hyperglycemia was restored by acute glucose infusion of the normoglycemic diabetic rats. In parallel, the levels of liver G6P and F6P were markedly reduced in the diabetic normoglycemic rats and restored with reinstatement of hyperglycemia. In contrast, liver microsomal glucose-6-phosphatase activity was enhanced and glucokinase activity was lowered in all diabetic groups, regardless of glycemia. Our results indicate that hyperglycemia per se counteracts part of the loss of hepatic synthase phosphatase in diabetic animals and provokes the stable conversion of synthase phosphatase from a less active to a more active form.

Interaction of mannose-6-phosphate with the hysteretic transition in glucose-6-phosphate hydrolysis in intact liver microsomes

We showed previously that glucose-6-phosphatase activity was characterised in intact liver microsomes by a hysteretic transition between a rapid and a slower catalytic form of the enzyme. We have now further investigated the substrate specificity of these two kinetic forms. It was found that the pre-incubation of intact microsomes with mannose-6-phosphate or glucose-6-phosphate (50 microM for 30 s) suppressed the burst in glucose-6-phosphatase activity, that the hysteretic transition was reversible and that mannose-6-phosphate inhibited glucose-6-phosphate hydrolysis during the first seconds of incubation, but not anymore after the burst. Our results indicate (i) that mannose-6-phosphate is recognised by the enzyme and can promote the hysteretic transition and (ii) that the transient phase is part of the catalytic mechanism itself.

Increased synthase phosphatase activity is responsible for the super-activation of glycogen synthase in hepatocytes from fasted obese Zucker rats

Addition of 60 mM glucose caused a similar partial activation of glycogen synthase in hepatocytes isolated from overnight fasted Wistar rats and from fasted lean Zucker (Fa/fa?) rats. In contrast, the activation went rapidly to completion in cells from fasted obese (fa/fa) rats. Subsequent addition of 4 microM microcystin, a potent inhibitor of type 1 and type 2A protein phosphatases, induced a rapid inactivation of glycogen synthase, which occurred at a similar rate in all three types of hepatocytes. This suggests that the super-activation of glycogen synthase in hepatocytes from fasted obese rats is not due to a lower synthase kinase activity. Glycogen synthase phosphatase was quantitatively assayed in broken-cell preparations from the same livers, with exogenous synthase b as substrate. The synthase phosphatase activity in the fa/fa livers was 3-fold higher than that in the livers from both lean Zucker rats and Wistar rats. This difference has to be attributed to an increased synthase phosphatase activity of the glycogen-bound protein phosphatase-1 in livers of fasted obese rats. The results suggest that in the latter animals the available insulin exceeds the insulin resistance of the liver. The resulting overexpression of the insulin-dependent synthase-phosphatase-1G activity may explain the super-activation of glycogen synthase in response to a glucose challenge.

Hormone-stimulated glucose production from glycogen in hepatocytes from streptozotocin diabetic rats

The contribution of hormone-stimulated glycogenolysis to hepatic glucose production was studied in hepatocytes from streptozotocin diabetic rats. To this end, the activation of glycogen phosphorylase by glucagon, vasopressin, and the alpha 1-adrenergic agonist phenylephrine was compared in hepatocytes from normal and diabetic rats and related to glycogen content, glucose production, and microsomal glucose-6-phosphatase activity. Streptozotocin-induced diabetes reduced the glycogen content and the amount of total (a + b) phosphorylase in hepatocytes proportionally to the severity of the disease. In cells from severely diabetic rats (group 1), the responsiveness of activation of phosphorylase to the hormones was reduced by about half, consistent with a 45% reduction in total phosphorylase. In addition, the sensitivity of phosphorylase activation to all hormones investigated was decreased by about 1 order of magnitude or more in cells of this group. In hepatocytes from rats with milder diabetes (group 2), maximal phosphorylase activation reached an intermediate value between that of the control group and of group 1. In response to all hormones investigated, group 2 diabetic rat hepatocytes produced less glucose than control rat liver cells, while in group 1 there was no increase in glucose production at all, presumably because glycogen concentration was too low. However, in group 2 diabetic rat hepatocytes, glucagon-stimulated glucose production, unlike phosphorylase activation, did not show decrease sensitivity, presumably because glucose-6-phosphatase activity is increased by diabetes. Our results thus indicate that hormone-stimulated liver glycogenolysis is unlikely to contribute to enhanced glucose production in insulin-deficient diabetes, despite increased glucose-6-phosphatase activity.

Rapid kinetics of liver microsomal glucose-6-phosphatase. Evidence for tight-coupling between glucose-6-phosphate transport and phosphohydrolase activity

Rapid kinetics of both glucose-6-P uptake and hydrolysis in fasted rat liver microsomes were investigated with a recently developed fast-sampling, rapid-filtration apparatus. Experiments were confronted with both the substrate transport and conformational models currently proposed for the glucose-6-phosphatase system. Accumulation in microsomes of 14C products from [U-14C]glucose-6-P followed biexponential kinetics. From the inside to outside product concentrations, it could be inferred that mostly glucose should accumulate inside the vesicles. While biexponential kinetics are compatible with the mathematical predictions of a simplified substrate transport model, the latter fails in explaining the "burst" in total glucose production over a similar time scale to that used for the uptake measurements. Since the initial rate of the burst phase in untreated microsomes exactly matched the steady-state rate of glucose production in detergent-treated vesicles, it can be definitely concluded that the substrate transport model does not describe adequately our results. While the conformational model accounts for both the burst of glucose production and the kinetics of glucose accumulation into the vesicles, it cannot explain the burst in 32Pi production from [32P]glucose-6-P measured under the same conditions. Since the amplitude of the observed bursts is not compatible with a presteady state in enzyme activity, we propose that a hysteretic transition best explains our results in both untreated and permeabilized microsomes, thus providing a new rationale to understand the molecular mechanism of the glucose-6-phosphatase system.

A test to evaluate the effect of individual components of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid buffers on enzymatic activity

A simple dilution test for evaluating the individual effect on enzymatic activity of [Ca2+], [EGTA], or [Ca.EGTA] variations in Ca-EGTA buffers is presented. We verified that a 50-fold dilution of the buffer (25-0.5 mM) at constant pH did not affect [Ca2+] (measured with fura-2), whereas [EGTA] and [Ca.EGTA] varied. Therefore the test can be applied to evaluate the proper effect of Ca2+ in a Ca-EGTA buffer on enzyme activity because such an effect is expected to remain unchanged upon dilution of the buffer. Applications of the test are shown for three enzymes apparently sensitive to Ca2+ but found to be effectively influenced only by Ca.EGTA (liver glucose-6-phosphatase), EGTA (intestinal mucosa phosphatase), or indeed Ca2+ (brain cyclic nucleotide phosphodiesterase).

Fasting enhances glycogen synthase activation in hepatocytes from insulin-resistant genetically obese (fa/fa) rats

Glycogen synthase activation and phosphorylase inactivation by glucose were studied in hepatocytes isolated from fed or overnight-fasted lean or genetically obese (fa/fa) rats. In cells from fed animals, both the time course and dose-response to glucose of synthase activation were the same in both groups, despite higher levels of phosphorylase a in hepatocytes from obese animals. In contrast, in cells from fasted obese animals synthase activation with or without glucose was enhanced severalfold over that of lean controls, despite similar levels of phosphorylase a and of total (a + b) synthase activities. In both nutritional conditions glucose 6-phosphate concentrations were 2-3-fold higher in obese-rat hepatocytes than in lean-rat cells. In addition, synthase activation was transient in the fasted lean group, but was sustained in obese-rat hepatocytes. The rate of synthase activation was, however, comparable in lean- and obese-rat liver Sephadex G-25 filtrates, irrespective of the nutritional state of the donor rats. It is concluded that enhanced synthase activation in hepatocytes from starved obese rats might be due to an unbalanced synthase interconversion brought about by elevated glucose 6-phosphate concentrations and impaired kinase [van de Werve & Massillon (1990) Biochem. J. 269, 795-799], rather than to an intrinsic change in synthase phosphatase.

Altered regulation of glycogen metabolism by vasopressin and phenylephrine in hepatocytes from insulin-resistant obese (fa/fa) rats. Role of protein kinase C

The hormonal control of glycogen synthase and phosphorylase interconversion was investigated in hepatocytes isolated from lean and genetically obese (fa/fa) rats. In cells from obese animals, the inactivation of synthase by 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA), phospholipase C, vasopressin and the alpha 1-adrenergic agonist phenylephrine was markedly impaired, and the property of PMA to counteract phosphorylase activation by phenylephrine was attenuated. The maximal response of phosphorylase activation to phenylephrine and vasopressin was increased in obese-rat hepatocytes, but the sensitivity to these hormones was similar to that in lean-rat hepatocytes. These observations indicate that the defect in protein kinase C that we reported previously in heart of insulin-resistant fa/fa rats [van de Werve, Zaninetti, Lang, Vallotton & Jeanrenaud (1987) Diabetes 36, 310-319] is probably also expressed in liver.

Liver glucose-6-phosphatase activity is modulated by physiological intracellular Ca2+ concentrations

The effect of varying concentrations of free Ca2+ on the formation of Pi from mannose-6-P or of Pi and [U-14C]glucose from [U-14C]glucose-6-P was investigated in isolated fasted rat hepatocytes made permeable by freezing and in liver microsomes. Free Ca2+ concentration was adjusted by the use of Ca-EGTA buffers. In permeabilized cells, glucose-6-phosphatase (EC 3.1.3.9) activity was inhibited up to 50% and in intact microsomes up to 70% by increasing free Ca2+ concentrations from 0.01 to 10 microM. The inhibition was reversible and competitive with respect to glucose-6-P. Treatment of microsomes with 0.4% deoxycholate exposed 90% of latent mannose-6-phosphatase activity which was insensitive to Ca2+. The results indicate that Ca2+ affects the glucose-6-P translocase rather than the phosphohydrolase component. It is concluded that the glucose-6-phosphatase system is modulated by changes in Ca2+ concentrations in the range of those occurring in the liver cell upon hormonal stimulation.

Identification of a major defect in insulin-resistant tissues of genetically obese (fa/fa) rats. Impaired protein kinase C

In perfused lean rat hearts, the activator of protein kinase C phorbol myristate acetate (PMA), when present alone, stimulates glucose transport but inhibits the insulin stimulation of this transport. PMA also inactivates glycogen synthase in hepatocytes. In contrast, none of these effects are observed in hearts and hepatocytes of obese animals, indicating an impaired protein kinase C activation in these tissues, which are insulin resistant. Direct measurements of protein kinase C activity in lean rat hearts revealed that PMA provokes a translocation of the enzyme from a soluble to a particulate fraction. In obese rat hearts, the basal distribution of protein kinase C is altered (more activity is found in the soluble and less in the particulate fraction), and the translocation induced by PMA is impaired. Pretreatment of lean rats with PMA in vivo, aimed at downregulating protein kinase C, induces the same defects (i.e., insulin resistance and unresponsiveness to PMA) as those observed in hearts of untreated obese animals. The results indicate that part of the insulin resistance might be the consequence of altered modulation of insulin action by protein kinase C.

The onset of liver glycogen synthesis in fasted-refed lean and genetically obese (fa/fa) rats

Lean and genetically obese (fa/fa) rats were fed ad libitum, or fasted for 17 h and then meal-fed for varying time intervals. During refeeding, glucose-6-phosphatase activity of lean rats declined to the low value that was present in livers of fasted obese rats and which remained unchanged in the obese group during the meal. Refeeding also resulted in increases in hepatic concentrations of glucose-6-phosphate and fructose-6-phosphate, fructose 1,6-bisphosphate, fructose-2,6-bisphosphate, alpha-glycerophosphate, pyruvate and lactate in lean and obese rats, absolute values being higher in the fasted obese than in the fasted lean group. Obese animals had higher postprandial portal blood insulin, glucose and lactate concentrations than lean animals. In spite of this, the rate of hepatic glycogen deposition was the same in both groups and was accompanied by similar glycogen synthase a levels. Following refeeding, phosphorylase was transiently inactivated in livers of lean but not of obese animals, while glycogen synthase was inactivated in both groups. The data suggest that in lean animals refeeding was associated with a stimulation of liver glycolysis, presumably by insulin; in fasted obese rats hepatic glycolysis was already in a stimulated state and was only slightly enhanced further after the meal, in keeping with their unaltered hyperinsulinaemia; there was an increased turnover of liver glycogen or a resistance to insulin stimulation of glycogen synthesis in fa/fa rats during refeeding.

Control of glycogen phosphorylase interconversion by phorbol esters, diacylglycerols, Ca2+ and hormones in isolated rat hepatocytes

In isolated rat hepatocytes: phosphorylase activation by the ionophore A23187 was enhanced in the presence of tumour-promoting phorbol esters and 1,2- (but not 1,3-) diacylglycerols (dioleoyl- and oleoylacetyl-glycerol), with a similar dose-dependency; the activation of phosphorylase by phenylephrine (1 microM) (but not by vasopressin or glucagon) was inhibited both by tumour-promoting phorbol esters and diacylglycerols, but with a different dose-dependency: complete inhibition was achieved with concentrations of phorbol esters two orders of magnitude lower than those of diacylglycerol; binding of the alpha 1-adrenergic antagonist [3H]prazosin and its displacement by unlabelled prazosin was not significantly affected in the presence of the phorbol esters. The possible involvement of protein kinase C in the control of phosphorylase interconversion is discussed.

Tumour-promoting phorbol esters increase basal and inhibit insulin-stimulated lipogenesis in rat adipocytes without decreasing insulin binding

In isolated rat adipocytes, tumour-promoting phorbol esters caused (1) dose-dependent stimulation of lipogenesis in the absence of insulin and (2) inhibition of the lipogenic effect of submaximal concentrations of insulin, but without affecting insulin binding. The possible involvement of protein kinase C in insulin action is discussed.

Altered liver glycogen metabolism in fed genetically obese mice

The cyclic AMP and glycogen concentrations and the activities of phosphorylase kinase, phosphorylase a and glycogen synthase a were not different in livers from lean or ob/ob mice despite increased plasma glucose and insulin in the obese group. The liver water content was decreased by 10% in the obese mice. In hepatocytes isolated from lean mice and incubated with increasing glucose concentrations (14-112 mM), a sequential inactivation of phosphorylase and activation of glycogen synthase was observed. In hepatocytes from obese mice the inactivation of phosphorylase was not followed by an activation of synthase. The inactivation of phosphorylase occurred more rapidly and was followed by an activation of synthase in hepatocytes isolated from both groups of mice when in the incubation medium Na+ was replaced by K+ or when Ca2+ was omitted and 2.5 mM-EGTA included. The inactivation of phosphorylase and activation of synthase were not different in broken-liver-cell preparations from lean and obese animals. The re-activation of phosphorylase in liver filtrates in the presence of 0.1 microM-cyclic AMP and MgATP was inhibited by about 70% by EGTA and stimulated by Ca2+ and was always greater in preparations from ob/ob mice. The apparent paradox between the impairment of glycogen metabolism in isolated liver preparations and the situation in vivo in obese mice is discussed.

Lack of vasopressin receptors in liver, but not in kidney, of ob/ob mice

The activity of phosphorylase a was measured in isolated hepatocytes from fed lean and ob/ob mice after addition of vasopressin, angiotensin, phenylephrine and glucagon. The binding of these hormones to purified liver plasma membranes was also determined. In hepatocytes of ob/ob mice, no increase in phosphorylase a was measured after addition of vasopressin, whereas the other hormones promoted an increase in the activity of the enzyme. No specific vasopressin receptors could be measured on purified liver plasma membrane of ob/ob mice. A decrease in the number of receptors for angiotensin and glucagon, without modification of the affinity, was also observed. No restoration of the number of vasopressin receptors was observed in liver of ob/ob mice starved for 3 days or in younger (5-6 weeks) animals. Vasopressin receptors and vasopressin-stimulated adenylate cyclase, measured on purified kidney medulla membranes, were similar in both lean and ob/ob mice. The data indicate a selective lack of vasopressin receptors and metabolic response in liver of the ob/ob mouse.

Fructose 2,6-bisphosphate in livers of genetically obese rats

Livers of genetically obese (fa/fa) rats, starved for 24 h, contained more fructose 2,6-bisphosphate, glucose 6-phosphate, fructose 6-phosphate and glycogen, and more pyruvate kinase and phosphofructokinase 2 activities, than livers of control lean rats. These changes may be explained in terms of cyclic AMP concentration, which was decreased in livers of obese starved rats.

Isolation and characteristics of hepatocytes

The preparation of isolated hepatocytes for the investigation of liver metabolism in the intact cell is described, including some criteria to assess their metabolic competence. A comparison is made between parameters in the perfused liver and in hepatocytes. Freshly isolated hepatocytes are not readily suitable for metabolic studies.