Modulation by protein kinase C of the hormonal responsiveness of hepatocytes from lean (Fa/fa?) and obese (fa/fa) Zucker rats.

The effect of phorbol myristate acetate (PMA) on the hormonal responsiveness of hepatocytes from lean and obese Zucker rats was studied. Phenylephrine-stimulated phosphatydylinositol labeling and phosphorylase activation were antagonized by PMA in cells from obese and lean animals; bigger residual effects were observed in cells from obese animals even at high PMA concentrations. Cyclic AMP accumulation induced by isoproterenol, glucagon, forskolin and cholera toxin was higher in cells from lean animals than in those from obese rats. PMA diminished glucagon- and cholera toxin-induced cyclic AMP accumulation; cells from lean animals were more sensitive to PMA. Two groups of isoforms of protein kinase C (PKC) were observed in hepatocytes from Zucker rats using DEAE-cellulose column chromatography: PKC 1 and PKC 2. The PKC 1 isozymes were separated into four peaks using hydroxylapatite: aa, 1a (PKC-beta), 1b (PKC-alpha) and 1c. Short treatment with PMA decreased the activity of PKC 1 (peaks 1b (PKC-alpha) and 1c) and to a lesser extent of PKC 2; cells from lean animals were more sensitive to PMA than those obtained from obese rats. Our results indicate that cells from genetically obese Zucker rats are in general less sensitive to this activator of protein kinase C than those from their lean littermates. The possibility that alterations in the phosphorylation/dephosphorylation cycles, that control metabolism and hormonal responsiveness, may contribute to this obese state is suggested.

Induction of hepatic glucose-6-phosphatase gene expression by lipid infusion.

The distal enzymatic step in the process of glucose output is catalyzed by the glucose-6-phosphatase (Glc-6-Pase) complex. The recently cloned catalytic unit of this complex has been shown to be regulated by insulin, dexamethasone, cAMP, and glucose. Using a combination of intralipid and/or nicotinic acid infusions and a pancreatic clamp technique, we maintained plasma free fatty acids (FFAs) at three different levels (0.26 +/- 0.07, 0.56 +/- 0.09, and 1.59 +/- 0.12 mmol/l) in the presence of well-controlled hormonal and metabolic conditions. An increase in the plasma FFA concentration within the physiological range caused a rapid, greater than threefold increase in the mRNA and protein levels of the catalytic subunit of Glc-6-Pase in the liver. These data indicate that the in vivo gene expression of Glc-6-Pase in the liver is regulated by circulating lipids independent of insulin and thus that prolonged hyperlipidemia may contribute to the increased production of glucose via increased expression of this protein.

Regulation of the glucose-6-phosphatase gene by glucose occurs by transcriptional and post-transcriptional mechanisms. Differential effect of glucose and xylitol.

To understand how glucose regulates the expression of the glucose-6-phosphatase gene, the effect of glucose was studied in primary cultures of rat hepatocytes. Glucose-6-phosphatase mRNA levels increased about 10-fold when hepatocytes were incubated with 20 mm glucose. The rate of transcription of the glucose-6-phosphatase gene increased about 3-fold in hepatocytes incubated with glucose. The half-life of glucose-6-phosphatase mRNA was estimated to be 90 min in the absence of glucose and 3 h in its presence. Inhibition of the oxidative and the nonoxidative branches of the pentose phosphate pathway blocked the stimulation of glucose-6-phosphatase expression by glucose but not by xylitol or carbohydrates that enter the glycolytic/gluconeogenic pathways at the level of the triose phosphates. These results indicate that (i) the glucose induction of the mRNA for the catalytic unit of glucose-6-phosphatase occurs by transcriptional and post-transcriptional mechanisms and that (ii) xylitol and glucose increase the expression of this gene through different signaling pathways.

Effects of fasting on hepatic and peripheral glucose metabolism in conscious rats with near-total fat depletion.

Experimental diabetes and fasting are both associated with hypoinsulinaemia and share several other metabolic features. We investigated hepatic and peripheral glucose metabolism in young rats after near-total depletion of their fat mass. Conscious rats were fasted for 72 h (n = 13), while 6 h-fasted animals (n = 14) served as controls. Rats were studied either during saline infusion or insulin (18 m-units/kg per min)-clamp studies. In fasting, despite a 2-fold increase in hepatic glucose-6-phosphatase (Glc-6-Pase) Vmax. (from 16 +/- 2 mumol/g of liver per min in control; P < 0.001), the basal hepatic glucose production (HGP) decreased by 47% [from 88 +/- 3 mumol/kg lean body mass (LBM) per min in control; P < 0.01]. The decreased HGP in fasting was associated with a 70% decrease in the hepatic levels of glucose 6-phosphate (Glc-6-P) (from 366 +/- 53 nmol/g wet wt. in control; P < 0.01). Thus Glc-6-Pase activity assayed in the presence of the Glc-6-P levels found in vivo was decreased by 44%. During hyperinsulinaemia, peripheral glucose uptake was decreased by 15% with 3 days of fasting (from 272 +/- 17 mumol/kg LBM per min in control; P < 0.01). This was completely accounted for by a 42% decrease in whole-body glycolysis (P < 0.01), while the rate of glycogen synthesis was unchanged. Thus fasting (after near-total fat depletion) differs from experimental diabetes because: (1) despite markedly increased Glc-6-Pase, HGP is decreased in fasting, due to a marked decrease in the substrate level (Glc-6-P) in vivo; and (2) the impairment in peripheral insulin sensitivity in fasting is due to a decrease in the glycolytic, and not the glycogen-synthetic, pathway.

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.

Quantitation of hepatic glucose fluxes and pathways of hepatic glycogen synthesis in conscious mice.

Mice were studied with the euglycemic hyperinsulinemic and the hyperglycemic clamp techniques after a 6-h fast: 1) euglycemic (6.7 +/- 0.2 mM) hyperinsulinemia (approximately 800 microU/ml); 2) hyperglycemic (15.3 +/- 0.4 mM) hyperinsulinemia (approximately 800 microU/ml). All mice received an infusion of [3-3H]glucose and [U-14C]lactate. Basal hepatic glucose production (HGP) averaged approximately 170 mumol.kg-1.min-1 in both groups. During euglycemic and hyperglycemic hyperinsulinemia, HGP decreased by 53% (to 76.7 +/- 11.1 mumol.kg-1.min-1; P < 0.01) and 74% (to 43.3 +/- 7.2 mumol.kg-1.min-1; P < 0.01), respectively. Hyperglycemia increased glucose cycling (by 2.1-fold; P < 0.01) and the contribution of gluconeogenesis to HGP (88 vs. 43%; P < 0.01) while decreasing that of glycogenolysis (12 vs. 57%; P < 0.01). The percentage of neosynthetized hepatic glycogen formed via the direct pathway was markedly increased during hyperglycemia (53 +/- 2% vs. 23 +/- 3%; P < 0.01): These data indicate that the assessment of hepatic glucose fluxes can be accomplished in conscious unrestrained mice and that, in the presence of hyperinsulinemia, hyperglycemia causes 1) a further inhibition of HGP mainly via inhibition of glycogenolysis and increase in hepatic glucose cycling; and 2) about a fivefold stimulation in the direct pathway of hepatic glycogen formation.

Glucose regulates in vivo glucose-6-phosphatase gene expression in the liver of diabetic rats.

Overproduction of glucose by the liver is the major cause of fasting hyperglycemia in both insulin-dependent and non-insulin-dependent diabetes mellitus. The distal enzymatic step in the process of glucose output is catalyzed by the glucose-6-phosphatase complex. We show here that 90% partially pancreatectomized diabetic rats have a >5-fold increase in the messenger RNA and a 3-4-fold increase in the protein level of the catalytic subunit of glucose-6-phosphatase in the liver. Normalization of the plasma glucose concentration in diabetic rats with either insulin or the glycosuric agent phlorizin normalized the hepatic glucose-6-phosphatase messenger RNA and protein within approximately 8 h. Conversely, phlorizin failed to decrease hepatic glucose-6-phosphatase gene expression in diabetic rats when the fall in the plasma glucose concentration was prevented by glucose infusion. These data indicate that in vivo gene expression of glucose-6-phosphatase in the diabetic liver is regulated by glucose independently from insulin, and thus prolonged hyperglycemia may result in overproduction of glucose via increased expression of this protein.

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.

Demonstration of a glycogen/glucose 1-phosphate cycle in hepatocytes from fasted rats. Selective inactivation of phosphorylase by 2-deoxy-2-fluoro-alpha-D-glucopyranosyl fluoride.

In search for a nonmetabolized, superior glucose analogue to study the mechanism of glucose-induced glycogen synthesis, we have tested 2-deoxy-2-fluoro-alpha-D-glucopyranosyl fluoride, which inhibits muscle phosphorylase beta 10-fold better than dose glucose (Street, I.P., Armstrong, C.R., and Withers, S.G. (1986) Biochemistry 25, 6021-6027). In a gel-filtered liver extract, 0.6 mM analogue and 10 mM glucose equally accelerated the inactivation of phosphorylase and shortened the latency before the activation of glycogen synthase. The analogue was not measurably defluorinated or phosphorylated by intact hepatocytes, as monitored by 19F NMR. When added to isolated hepatocytes, 10 mM analogue inactivated phosphorylase more extensively than did 50 mM glucose, but unlike glucose, it did not result in the activation of glycogen synthase. Therefore, the binding of glucose to phosphorylase alpha can account for the inactivation of phosphorylase, but the metabolism of glucose (probably to Glc-6-P) appears to be required to achieve activation of glycogen synthase. The livers of overnight-fasted, anesthetized mice contained appreciable amounts of both phosphorylase alpha and glycogen synthase alpha, without net glycogen accumulation. Likewise, hepatocytes isolated from fasted rats and incubated with 10 mM glucose contained 41% of phosphorylase and 32% of glycogen synthase in the alpha form, and these values remained stable for 1 h, while glycogen accumulated at only 22% of the rate expected from the glycogen synthase activity. The addition of 10 mM analogue decreased phosphorylase alpha to 10% without significant change in glycogen synthase alpha (38%), but with a 4-fold increased rate of glycogen accumulation. These findings imply that synthase alpha is fully active in the liver of the fasted animal and that the absence of net glycogen synthesis is due to continuous glycogenolysis by phosphorylase alpha.

Short term effects of leptin on hepatic gluconeogenesis and in vivo insulin action.

Long term administration of leptin decreases caloric intake and fat mass and improves glucose tolerance. Here we examine whether leptin acutely regulates peripheral and hepatic insulin action. Recombinant mouse leptin (0.3 mg/kg.h, Leptin +) or vehicle (Leptin -) were administered for 6 h to 4-month-old rats (n = 20), and insulin (3 milliunits/kg.min) clamp studies were performed. During physiologic hyperinsulinemia (plasma insulin approximately 65 microunits/ml), the rates of whole body glucose uptake, glycolysis, and glycogen synthesis and the rates of 2-deoxyglucose uptake in individual tissues were similar in Leptin - and Leptin +. Post-absorptive hepatic glucose production (HGP) was similar in the two groups. However, leptin enhanced insulin's inhibition of HGP (4.1 +/- 0.7 and 6.2 +/- 0.7 mg/kg.min; p < 0.05). The decreased HGP in the Leptin + group was due to a marked suppression of hepatic glycogenolysis (0.7 +/- 0.1 versus 4.1 +/- 0.6 mg/kg.min, in Leptin + versus Leptin -, respectively; p < 0.001), whereas the % contribution of gluconeogenesis to HGP was markedly increased (82 +/- 3% versus 36 +/- 4% in Leptin + and Leptin -, respectively; p < 0.001). At the end of the 6-h leptin infusion, the hepatic abundance of glucokinase mRNA was decreased, whereas that of phosphoenolpyruvate carboxykinase mRNA was increased compared with Leptin -. We conclude that an acute increase in plasma leptin 1) enhances insulin's ability to inhibit HGP, 2) does not affect peripheral insulin action, and 3) induces a redistribution of intrahepatic glucose fluxes and changes in the gene expression of hepatic enzymes that closely resemble those of fasting.

Proteomic method identifies proteins nitrated in vivo during inflammatory challenge.

Inflammation in asthma, sepsis, transplant rejection, and many neurodegenerative diseases associates an up-regulation of NO synthesis with increased protein nitration at tyrosine. Nitration can cause protein dysfunction and is implicated in pathogenesis, but few proteins that appear nitrated in vivo have been identified. To understand how this modification impacts physiology and disease, we used a proteomic approach toward targets of protein nitration in both in vivo and cell culture inflammatory disease models. This approach identified more than 40 nitrotyrosine-immunopositive proteins, including 30 not previously identified, that became modified as a consequence of the inflammatory response. These targets include proteins involved in oxidative stress, apoptosis, ATP production, and other metabolic functions. Our approach provides a means toward obtaining a comprehensive view of the nitroproteome and promises to broaden understanding of how NO regulates cellular processes.

Carbon flux via the pentose phosphate pathway regulates the hepatic expression of the glucose-6-phosphatase and phosphoenolpyruvate carboxykinase genes in conscious rats.

Hepatic gene expression of P-enolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (Glc-6-Pase) is regulated in response to changes in the availability of substrates, in particular glucose (Glc; Massillon, D., Barzilai, N., Chen, W., Hu, M., and Rossetti, L. (1996) J. Biol. Chem. 271, 9871-9874). We investigated the mechanism(s) in conscious rats. Hyperglycemia per se caused a rapid and marked increase in Glc-6-Pase mRNA abundance and protein levels. By contrast, hyperglycemia decreased the abundance of PEPCK mRNA. Importantly, inhibition of glucokinase activity by glucosamine infusion blunted both the stimulation of Glc-6-Pase and the inhibition of PEPCK gene expression by Glc, suggesting that an intrahepatic signal (metabolite) generated by the metabolism of glucose at or beyond Glc-6-P was responsible for the regulatory effect of Glc. The effect of Glc on the L-type pyruvate kinase gene is mediated by xylulose-5-P (Doiron, B., Cuif, M., Chen, R., and Kahn, A. (1996) J. Biol. Chem. 271, 5321-5324). Thus, we next investigated whether an isolated increase in the hepatic concentration of this metabolite can also reproduce the effects of Glc on Glc-6-Pase and PEPCK gene expression in vivo. Xylitol, which is directly converted to xylulose-5-P in the liver, was infused to raise the hepatic concentration of xylulose-5-P by approximately 3-fold. Xylitol infusion did not alter the levels of Glc-6-P and of fructose-2,6-biphosphate. However, it replicated the effects of hyperglycemia on Glc-6-Pase and PEPCK gene expression and resulted in a 75% increase in the in vivo flux through Glc-6-Pase (total glucose output).