Hormone-induced effects on the rat liver microsomal glucose-6-phosphatase system in vitro

Free fatty acids increase basal hepatic glucose production and induce hepatic insulin resistance at different sites

To investigate the sites of the free fatty acid (FFA) effects to increase basal hepatic glucose production and to impair hepatic insulin action, we performed 2-h and 7-h Intralipid + heparin (IH) and saline infusions in the basal fasting state and during hyperinsulinemic clamps in overnight-fasted rats. We measured endogenous glucose production (EGP), total glucose output (TGO, the flux through glucose-6-phosphatase), glucose cycling (GC, index of flux through glucokinase = TGO - EGP), hepatic glucose 6-phosphate (G-6-P) content, and hepatic glucose-6-phosphatase and glucokinase activities. Plasma FFA levels were elevated about threefold by IH. In the basal state, IH increased TGO, in vivo glucose-6-phosphatase activity (TGO/G-6-P), and EGP (P < 0.001). During the clamp compared with the basal experiments, 2-h insulin infusion increased GC and in vivo glucokinase activity (GC/TGO; P < 0.05) and suppressed EGP (P < 0.05) but failed to significantly affect TGO and in vivo glucose-6-phosphatase activity. IH decreased the ability of insulin to increase GC and in vivo glucokinase activity (P < 0.01), and at 7 h, it also decreased the ability of insulin to suppress EGP (P < 0.001). G-6-P content was comparable in all groups. In vivo glucose-6-phosphatase and glucokinase activities did not correspond to their in vitro activities as determined in liver tissue, suggesting that stable changes in enzyme activity were not responsible for the FFA effects. The data suggest that, in overnight-fasted rats, FFA increased basal EGP and induced hepatic insulin resistance at different sites. 1) FFA increased basal EGP through an increase in TGO and in vivo glucose-6-phosphatase activity, presumably due to a stimulatory allosteric effect of fatty acyl-CoA on glucose-6-phosphatase. 2) FFA induced hepatic insulin resistance (decreased the ability of insulin to suppress EGP) through an impairment of insulin's ability to increase GC and in vivo glucokinase activity, presumably due to an inhibitory allosteric effect of fatty acyl-CoA on glucokinase and/or an impairment in glucokinase translocation.

Glucose 6-phosphate hydrolysis is activated by glucagon in a low temperature-sensitive manner

Glucagon affects liver glucose metabolism mainly by activating glycogen breakdown and by inhibiting pyruvate kinase, whereas a possible effect on glucose-6-phosphatase has also been suggested. Although such a target is of physiological importance for liver glucose production it was never proven. By using a model of liver cells, perifused with dihydroxyacetone, we show here that the acute stimulation of gluconeogenesis by glucagon (10(-7) m) was not related to the significant inhibition of pyruvate kinase but to a dramatic activation of the hydrolysis of glucose 6-phosphate. We failed to find an acute change in glucose-6-phosphatase activity by glucagon, but the increase in glucose 6-phosphate hydrolysis was abolished at 21 degrees C; conversely the effect on pyruvate kinase was not affected by temperature. The activation of glucose 6-phosphate hydrolysis by glucagon was confirmed in vivo, in postabsorptive rats receiving a constant infusion of glucagon, by the combination of a 2-fold increase in hepatic glucose production and a 60% decrease in liver glucose 6-phosphate concentration. Besides the description of a novel effect of glucagon on glucose 6-phosphate hydrolysis by a temperature-sensitive mechanism, this finding could represent an important breakthrough in the understanding of type II diabetes, because glucose 6-phosphate is proposed to be a key molecule in the transcriptional effect of glucose.

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.

Hepatocyte nuclear factor 1alpha is an accessory factor required for activation of glucose-6-phosphatase gene transcription by glucocorticoids

Deficiency of glucose-6-phosphatase (G6Pase), a key enzyme in glucose homeostasis, causes glycogen storage disease type 1a (GSD-1a), also know as von Gierke disease. Expression of the G6Pase gene is regulated by multiple hormones, including glucocorticoids. The synthetic glucocorticoid dexamethasone increased G6Pase mRNA abundance and gene transcription in H4-IIE hepatoma cells. Transient transfection assays demonstrated that the G6Pase promoter was active in H4-IIE cells only in the presence of dexamethasone. The minimal G6Pase promoter was contained within nucleotides -234/+3, which has two putative glucocorticoid response elements (GREs) at nucleotides -178/-164 (site 1) and -154/-140 (site 2). Electromobility shift and transient transfection assays showed that only GRE site 1 was required for glucocorticoid-activated transcription from the G6Pase promoter. Deletion analysis demonstrated that the DNA elements absolutely essential for glucocorticoid-stimulated transcription from the G6Pase promoter were contained within nucleotides -234/-212, encompassing binding motifs for hepatocyte nuclear factors (HNFs) 1 (-226/-212) and 4 (-231/-220). Electromobility shift and cotransfection assays showed that HNF1alpha bound to its cognate site and mediated transcription activation of the G6Pase gene by glucocorticoids.

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.

The diabetogenic effects of glucocorticoids are more pronounced in low- than in high-insulin responders

We investigated in six low- and six high-insulin responders (LIR and HIR) the effect of dexamethasone (Dex, 15 mg orally during 48 hr) on oral glucose tolerance (OGTT), glucose turnover under basal conditions and during glucose infusion of 2 mg.kg-1.min-1, and insulin response during hyperglycemic clamp. Dex increased fasting glucose more in LIR (P less than 0.05). During OGTT, Dex caused a more prominent increment in glucose in LIR, whereas the increment in insulin was less in LIR (P less than 0.05). After Dex, in three LIR but in no HIR, a diabetic OGTT was observed. Dex significantly increased basal hepatic glucose production (turnover measured with [6-3H]glucose), hepatic total glucose output (turnover measured with [2-3H]glucose), and glucose cycling (hepatic total glucose output--hepatic glucose production) only in LIR. Dex decreased basal glucose metabolic clearance to the same extent in LIR and HIR. Hyperglycemic clamp revealed that Dex induced a significant increase (P less than 0.05) in insulin response only in HIR. Dex effects on insulin release during hyperglycemic clamp were negatively correlated with the glucose area during Dex OGTT (P less than 0.01). Thus, the double tracer method provided a new insight into the pathogenesis of the steroid effect on carbohydrate tolerance. Dex increased basal glycemia more in LIR because only in LIR was glucose production increased. During OGTT, the LIR who were not able to counteract the effects of Dex by an appropriate enhancement in insulin secretion developed a decreased OGTT. The evaluation of insulin response after Dex may thus allow differentiation of the subset of LIR that run an increased risk of non-insulin-dependent diabetes mellitus.

Effect of superfused insulin on cerebral cortical glucose utilization in awake goats

The effect on cortical cerebral glucose utilization (CMRglu) of intracerebral insulin administration in awake goats was studied. The insulin was superfused in a mock cerebrospinal fluid (CSF) solution employing chronically implanted cranial windows. Two windows were implanted bilaterally: one window over an equivalent portion of each parietal cortex. With one window used to deliver insulin/CSF and the other used to simultaneously deliver CSF alone (control), changes in CMRglu were assessed using a modification of a sequential 2-[3H]- then 2-[14C]deoxy-D-glucose (2DG) technique originally described by Altenau and Agranoff (Brain Res. 153: 375-381, 1978). Initial experiments employing 125I-insulin demonstrated that the superfusion procedure increased insulin levels only in the outer 1 mm of cortical tissue exposed to insulin containing perfusate. Additional preliminary evaluations, using conditions known to alter CMRglu, generally established that present methods were adequate to induce and detect CMRglu changes. However, it was also shown experimentally and using a mathematical model that 2-[3H]DG test/control tissue ratios could be influenced by subsequent changes in CMRglu and the dephosphorylation rate. Thus 3H ratios could not be used to establish preexperimental test/control CMRglu relationships as the originally devised model assumed but could be employed to indicate changes in dephosphorylation. The mathematical model allowed for improved estimates of CMRglu changes from 2-[14C]DG/2-[3H]DG test over control tissue ratios. Even with these corrections, insulin was estimated to cause no more than an 8-15% increase in cortical CMRglu. A very limited role for insulin, at least in cerebral cortical metabolic regulation, is thus indicated.

Sub-compartmentation of the 'cytosolic' glucose 6-phosphate pool in cultured rat hepatocytes

[14C]Glucose release either from endogenous 14C-prelabelled glycogen or from added 14C-labelled glucose 6-phosphate was measured in filipin-treated, permeabilized hepatocytes in 48 h culture. [14C]Glucose output from prelabelled glycogen was not altered by the addition of 5 mM glucose 6-phosphate to the incubation medium. Conversely, [14C]glucose release from 5 mM labelled glucose 6-phosphate was not influenced by different glycogen concentrations in the cells. Moreover, in the permeabilized cells the anion transport inhibitor DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) inhibited only the liberation of [14C]glucose from labelled glucose 6-phosphate but not from glycogen. It is therefore concluded that there exist at least 2 separate, mutually non-accessible glucose 6-phosphate pools in cultured rat hepatocytes, one linked to glycogenolysis and the other to gluconeogenesis.

Increased glucose turnover and glucose cycling in acromegalic patients with normal glucose tolerance

To characterize the diabetogenic effects of growth hormone, we simultaneously measured glucose turnover with 2-3H- and 6-3H-glucose in six acromegalic patients with normal fasting blood glucose and oral glucose tolerance tests. Eight healthy volunteers served as controls. All subjects were studied under both basal conditions and during glucose infusion (2 mg X kg-1 X min-1). We determined true glucose production and irreversible glucose uptake using 6-3H-glucose and glucose cycling (difference between 2-3H- and 6-3H-glucose). After an overnight fast, glucose production was higher than normal in the acromegalic patients (2.18 +/- 0.15 vs 1.85 +/- 0.03 mg X kg-1 X min-1, p less than 0.05) despite hyperinsulinaemia. The metabolic clearance rate was normal. During the glucose infusion, glucose production was not suppressed as effectively in the acromegalic patients as in controls nor was glucose uptake augmented, while metabolic clearance rate was decreased. In acromegaly, basal glucose cycling was increased (0.44 +/- 0.08 vs 0.25 +/- 0.07 mg X kg-1 X min-1, p less than 0.05). Furthermore cycling of endogenous glucose measured during glucose infusion was also augmented (0.41 +/- 0.05 vs 0.24 +/- 0.05 mg X kg-1 X min-1, p less than 0.05). Hence the increase of glucose cycling (70%) was much more pronounced than that of glucose production (17%). In conclusion, small defects in glucose metabolism in acromegaly can be detected with sensitive tracer methods. These derangements are confined to the liver under fasting conditions, but are of both hepatic and extrahepatic origin during glucose loading.

Antagonistic regulation of the glucose/glucose 6-phosphate cycle by insulin and glucagon in cultured hepatocytes

Flux through the glucose/glucose 6-phosphate cycle in cultured hepatocytes was measured with radiochemical techniques. Utilization of [2-3H]glucose was taken as a measure of glucokinase flux. Liberation of [14C]glucose from [U-14C]glycogen and from [U-14C]lactate, as well as the difference between the utilization of [2-3H]glucose and of [U-14C]glucose, were taken as measures of glucose-6-phosphatase flux. At constant 5 mM-glucose and 2 mM-lactate concentrations insulin increased glucokinase flux by 35%; it decreased glucose-6-phosphatase flux from glycogen by 50%, from lactate by 15% and reverse flux from external glucose by 65%, i.e. overall by 40%. Glucagon had essentially no effect on glucokinase flux; it enhanced glucose-6-phosphatase flux from glycogen by 700%, from lactate by 45% and reverse flux from external glucose by 20%, i.e. overall by 110%. At constant glucose concentrations cellular glucose 6-phosphate concentrations were essentially not altered by insulin, but were increased by glucagon by 230%. In conclusion, under basic conditions without added hormones the glucose/glucose 6-phosphate cycle showed only a minor net glucose uptake, of 0.03 mumol/min per g of hepatocytes; this flux was increased by insulin to a net glucose uptake of 0.21 mumol/min per g and reversed by glucagon to a net glucose release of 0.22 mumol/min per g. Since the glucose 6-phosphate concentrations after hormone treatment did not correlate with the glucose-6-phosphatase flux, it is suggested that the hormones influenced the enzyme activity directly.

Effects of fasting and refeeding on the activity of hepatic glucose-6-phosphatase in rats

The activities of glucose-6-phosphate hydrolase and glucose-6-phosphate translocase were determined in rats fasted for 1-3 days and in animals fasted for one day and then either refed with mixed pellet or given oral or intraperitoneal glucose. The assay was based on the colorimetric measurement of the released inorganic phosphate. Fasting over 24 h significantly increased both the translocase and the hydrolase activity of glucose-6-phosphatase. These parameters showed a further increase when rats were fasted for another 24 h. In animals fasted for 24 h and then refed with standardized pellet diet, a progressive fall of enzyme activity was noticed. However, even 72 h of refeeding did not lead to complete normalization. Glucose given orally or intraperitoneally also suppressed the enzyme activity, although the effect was somewhat delayed. As expected, in fasting rats glucose and insulin levels were significantly decreased. Normoglycaemia was established after just 24 h, regardless of refeeding with pellets or with glucose. The former group exhibited hyper- and the latter hypo-insulinaemic pattern. We speculate that augmented activity of hepatic glucose-6-phosphatase during fasting stimulates the metabolism of glucose through the glucose cycle and is thereby at least partially responsible for insulin resistance accompanying the fasting state.

Hormonal regulation of key gluconeogenic enzymes and glucose release in cultured hepatocytes: effects of dexamethasone and gastrointestinal hormones on glucagon action

Hormonal regulation of key gluconeogenic enzymes and glucose release by glucagon, dexamethasone, secretin and somatostatin was evaluated in maintenance cultured rat hepatocytes. (i) Phosphoenolpyruvate (PEP)-carboxykinase activity declined rapidly during the first 24 h in serum- and hormone-free culture with a further slight decay during the following 2 days. Dexamethasone and glucagon independently increased PEP-carboxykinase and acted synergistically when added in combination. Glucose-6-phosphatase activity declining linearly during hormone-free culture was stimulated by glucagon. Dexamethasone itself was without significant effects but completely abolished glucagon action. Fructose-1,6-diphosphatase was maintained at its initial level during the first day under control conditions and declined thereafter. Neither glucagon nor dexamethasone affected total activity or substrate (fructose-1,6-diphosphate) affinity of this enzyme. In short-term experiments on cells cultured under control conditions, protein synthesis-dependent stimulation of PEP-carboxykinase by glucagon and the permissive action of dexamethasone was demonstrated. Glucose-6-phosphatase and fructose-1,6-diphosphatase were not altered by hormones within this period. (ii) Stimulation by glucagon of gluconeogenesis was independent of its action on PEP-carboxykinase. Dexamethasone inhibited glycogenolysis but maintained glucose release at control levels probably by stimulation of gluconeogenesis. When added in combination, the glycogen-preserving action of dexamethasone acutely reduced the glucose release in response to glucagon. Glucagon sensitivity remained unchanged. (iii) The gastrointestinal hormones secretin and somatostatin were ineffective in modulating basal or glucagon-stimulated glucose release and gluconeogenic key enzymes. They are therefore unlikely to play a physiological role in hepatic glucose metabolism.

A putative second messenger of insulin action regulates hepatic microsomal glucose-6-phosphatase

Physiological concentrations of insulin suppressed rat liver microsomal glucose-6-phosphatase activity in vitro. To attest a hypothesis that a putative second messenger of insulin action (insulin mediator) mediated this process, we isolated the low molecular factor from insulin-treated plasma membranes of rat liver, which was acid- and heat-stable substance of a peptide nature. The insulin mediator which was proved to activate the mitochondria pyruvate dehydrogenase suppressed microsomal glucose-6-phosphatase. The insulin mediator was linked to suppression of the gluconeogenic enzyme through the control of non-specific phosphohydroxylase.

Effect of 5 alpha-dihydrocorticoids on enzymes of gluconeogenesis in rat liver

5 alpha-Dihydrocortisol (11 beta, 17, 21-trihydroxy-5 alpha-pregnane-3,20-dione), 5 alpha-dihydrocorticosterone (11 beta, 21-dihydroxy-5 alpha-pregnane-3,20-dione) as well as cortisol (11 beta, 17, 21-trihydroxy-4-pregnene-3,20-dione) and corticosterone (11 beta, 21-dihydroxy-4-pregnene-3,20-dione) were administered for seven days to male rats. Blood glucose increased in cortisol- and corticosterone-treated rats and blood insulin decreased after 5 alpha-dihydrocorticosteroid treatment. In the liver, total protein was elevated after cortisol, corticosterone and 5 alpha-dihydrocorticosterone application. Phosphoenolpyruvate carboxykinase and fructose-1,6-diphosphatase activities in liver were significantly lowered after treatment with 5 alpha-dihydrocortisol and 5 alpha-dihydrocorticosterone.

Changes in glucose and lipid metabolism in starved or starved-refed Japanese quail (coturnix coturnix japonica) in relation to fine structure of liver cells

1. Metabolic response of adult quail to fasting or refeeding was studied by measuring the main blood and hepatic metabolites. Moreover, the fine structure of hepatocytes in these physiological conditions was described. 2. Starvation or refeeding did not affect glycemia in male as in female quails. 3. Fasting had no effect on plasma free fatty acids in female quails, whereas plasma triglycerides were markedly decreased. 4. In fasted quails, there was an active ketogenesis with a high 3-hydroxybutyrate/acetoacetate ratio. 5. Ultrastructural aspect of liver parenchymal cells from fasted quails revealed alterations in the quantity of glycogen, smooth endoplasmic reticulum, lysosomes and in the form of the rough endoplasmic reticulum. 6. The significance of these morphological changes was discussed in relation to an hormonal stimulation.