PPARβ/δ prevents endoplasmic reticulum stress-associated inflammation and insulin resistance in skeletal muscle cells through an AMPK-dependent mechanism

AIM/HYPOTHESIS

Endoplasmic reticulum (ER) stress, which is involved in the link between inflammation and insulin resistance, contributes to the development of type 2 diabetes mellitus. In this study, we assessed whether peroxisome proliferator-activated receptor (PPAR)β/δ prevented ER stress-associated inflammation and insulin resistance in skeletal muscle cells.

METHODS

Studies were conducted in mouse C2C12 myotubes, in the human myogenic cell line LHCN-M2 and in skeletal muscle from wild-type and PPARβ/δ-deficient mice and mice exposed to a high-fat diet.

RESULTS

The PPARβ/δ agonist GW501516 prevented lipid-induced ER stress in mouse and human myotubes and in skeletal muscle of mice fed a high-fat diet. PPARβ/δ activation also prevented thapsigargin- and tunicamycin-induced ER stress in human and murine skeletal muscle cells. In agreement with this, PPARβ/δ activation prevented ER stress-associated inflammation and insulin resistance, and glucose-intolerant PPARβ/δ-deficient mice showed increased phosphorylated levels of inositol-requiring 1 transmembrane kinase/endonuclease-1α in skeletal muscle. Our findings demonstrate that PPARβ/δ activation prevents ER stress through the activation of AMP-activated protein kinase (AMPK), and the subsequent inhibition of extracellular-signal-regulated kinase (ERK)1/2 due to the inhibitory crosstalk between AMPK and ERK1/2, since overexpression of a dominant negative AMPK construct (K45R) reversed the effects attained by PPARβ/δ activation.

CONCLUSIONS/INTERPRETATION

Overall, these findings indicate that PPARβ/δ prevents ER stress, inflammation and insulin resistance in skeletal muscle cells by activating AMPK.

Oleate prevents saturated-fatty-acid-induced ER stress, inflammation and insulin resistance in skeletal muscle cells through an AMPK-dependent mechanism

AIMS/HYPOTHESIS

Although the substitution of saturated fatty acids with oleate has been recommended in the management of type 2 diabetes mellitus, the mechanisms by which oleate improves insulin resistance in skeletal muscle cells are not completely known. Here, we examined whether oleate, through activation of AMP-activated protein kinase (AMPK), prevented palmitate-induced endoplasmic reticulum (ER) stress, which is involved in the link between lipid-induced inflammation and insulin resistance.

METHODS

Studies were conducted in mouse C2C12 myotubes and in the human myogenic cell line LHCN-M2. To analyse the involvement of AMPK, activators and inhibitors of this kinase and overexpression of a dominant negative AMPK construct (K45R) were used.

RESULTS

Palmitate increased the levels of ER stress markers, whereas oleate did not. In palmitate-exposed cells incubated with a lower concentration of oleate, the effects of palmitate were prevented. The induction of ER stress markers by palmitate was prevented by the presence of the AMPK activators AICAR and A-769662. Moreover, the ability of oleate to prevent palmitate-induced ER stress and inflammation (nuclear factor-kappa B [NF-κB] DNA-binding activity and expression and secretion of IL6) as well as insulin-stimulated Akt phosphorylation and 2-deoxyglucose uptake was reversed in the presence of the AMPK inhibitor compound C or by overexpression of a dominant negative AMPK construct. Finally, palmitate reduced phospho-AMPK levels, whereas this was not observed in oleate-exposed cells or in palmitate-exposed cells supplemented with oleate.

CONCLUSIONS/INTERPRETATION

Overall, these findings indicate that oleate prevents ER stress, inflammation and insulin resistance in palmitate-exposed skeletal muscle cells by activating AMPK.

Plasma PTX3 protein levels inversely correlate with insulin secretion and obesity, whereas visceral adipose tissue PTX3 gene expression is increased in obesity

Plasma acutephase protein pentraxin 3 (PTX3) concentration is dysregulated in human obesity and metabolic syndrome. Here, we explore its relationship with insulin secretion and sensitivity, obesity markers, and adipose tissue PTX3 gene expression. Plasma PTX3 protein levels were analyzed in a cohort composed of 27 lean [body mass index (BMI) ≤ 25 kg/m(2)] and 48 overweight (BMI 25-30 kg/m(2)) men (cohort 1). In this cohort, plasma PTX3 was negatively correlated with fasting triglyceride levels and insulin secretion after intravenous and oral glucose administration. Plasma PTX3 protein and PTX3 gene expression in visceral (VAT) and subcutaneous (SAT) whole adipose tissue and adipocyte and stromovascular fractions were analyzed in cohort 2, which was composed of 19 lean, 28 overweight, and 15 obese subjects (BMI >30 kg/m(2)). An inverse association with body weight and waist/hip ratio was observed in cohort 2. In VAT depots, PTX3 mRNA levels were higher in subjects with BMI >25 kg/m(2) than in lean subjects, positively correlated with IL-1β mRNA levels, and higher in the adipocyte than stromovascular fraction. Human preadipocyte SGBS cell line was used to study PTX3 production in response to factors that obesity entails. In SGBS adipocytes, PTX3 gene expression was enhanced by IL-1β and TNFα but not IL-6 or insulin. In conclusion, the negative correlation between PTX3 and glucose-stimulated insulin secretion suggests a role for PTX3 in metabolic control. PTX3 gene expression is upregulated in VAT depots in obesity, despite lower plasma PTX3 protein, and by some proinflammatory cytokines in cultured adipocytes.

Differential pattern of glycogen accumulation after protein phosphatase 1 glycogen-targeting subunit PPP1R6 overexpression, compared to PPP1R3C and PPP1R3A, in skeletal muscle cells

BACKGROUND

PPP1R6 is a protein phosphatase 1 glycogen-targeting subunit (PP1-GTS) abundant in skeletal muscle with an undefined metabolic control role. Here PPP1R6 effects on myotube glycogen metabolism, particle size and subcellular distribution are examined and compared with PPP1R3C/PTG and PPP1R3A/G(M).

RESULTS

PPP1R6 overexpression activates glycogen synthase (GS), reduces its phosphorylation at Ser-641/0 and increases the extracted and cytochemically-stained glycogen content, less than PTG but more than G(M). PPP1R6 does not change glycogen phosphorylase activity. All tested PP1-GTS-cells have more glycogen particles than controls as found by electron microscopy of myotube sections. Glycogen particle size is distributed for all cell-types in a continuous range, but PPP1R6 forms smaller particles (mean diameter 14.4 nm) than PTG (36.9 nm) and G(M) (28.3 nm) or those in control cells (29.2 nm). Both PPP1R6- and G(M)-derived glycogen particles are in cytosol associated with cellular structures; PTG-derived glycogen is found in membrane- and organelle-devoid cytosolic glycogen-rich areas; and glycogen particles are dispersed in the cytosol in control cells. A tagged PPP1R6 protein at the C-terminus with EGFP shows a diffuse cytosol pattern in glucose-replete and -depleted cells and a punctuate pattern surrounding the nucleus in glucose-depleted cells, which colocates with RFP tagged with the Golgi targeting domain of β-1,4-galactosyltransferase, according to a computational prediction for PPP1R6 Golgi location.

CONCLUSIONS

PPP1R6 exerts a powerful glycogenic effect in cultured muscle cells, more than G(M) and less than PTG. PPP1R6 protein translocates from a Golgi to cytosolic location in response to glucose. The molecular size and subcellular location of myotube glycogen particles is determined by the PPP1R6, PTG and G(M) scaffolding.

Activation of peroxisome proliferator-activated receptor-{delta} by GW501516 prevents fatty acid-induced nuclear factor-{kappa}B activation and insulin resistance in skeletal muscle cells

Elevated plasma free fatty acids cause insulin resistance in skeletal muscle through the activation of a chronic inflammatory process. This process involves nuclear factor (NF)-kappaB activation as a result of diacylglycerol (DAG) accumulation and subsequent protein kinase Ctheta (PKCtheta) phosphorylation. At present, it is unknown whether peroxisome proliferator-activated receptor-delta (PPARdelta) activation prevents fatty acid-induced inflammation and insulin resistance in skeletal muscle cells. In C2C12 skeletal muscle cells, the PPARdelta agonist GW501516 prevented phosphorylation of insulin receptor substrate-1 at Ser(307) and the inhibition of insulin-stimulated Akt phosphorylation caused by exposure to the saturated fatty acid palmitate. This latter effect was reversed by the PPARdelta antagonist GSK0660. Treatment with the PPARdelta agonist enhanced the expression of two well known PPARdelta target genes involved in fatty acid oxidation, carnitine palmitoyltransferase-1 and pyruvate dehydrogenase kinase 4 and increased the phosphorylation of AMP-activated protein kinase, preventing the reduction in fatty acid oxidation caused by palmitate exposure. In agreement with these changes, GW501516 treatment reversed the increase in DAG and PKCtheta activation caused by palmitate. These effects were abolished in the presence of the carnitine palmitoyltransferase-1 inhibitor etomoxir, thereby indicating that increased fatty acid oxidation was involved in the changes observed. Consistent with these findings, PPARdelta activation by GW501516 blocked palmitate-induced NF-kappaB DNA-binding activity. Likewise, drug treatment inhibited the increase in IL-6 expression caused by palmitate in C2C12 and human skeletal muscle cells as well as the protein secretion of this cytokine. These findings indicate that PPARdelta attenuates fatty acid-induced NF-kappaB activation and the subsequent development of insulin resistance in skeletal muscle cells by reducing DAG accumulation. Our results point to PPARdelta activation as a pharmacological target to prevent insulin resistance.

A glucose response element from the S. cerevisiae hexose transporter HXT1 gene is sensitive to glucose in human fibroblasts

Glucose is an essential nutrient, and a regulator of gene expression in eukaryotic cells. Here, a comparative, function-based genomic approach has been used to identify glucose regulatory elements and transduction pathways common to both yeast and mammalian cells. We have isolated a region in the promoter of the Saccharomyces cerevisiae hexose transporter gene HXT1 that conferred glucose sensitivity in yeast, when located upstream of the minimal CYC1 promoter. This element contained binding motifs for Rgt1, a transcriptional modulator involved in the yeast glucose-induction pathway, that were sufficient to elicit glucose responsiveness. The HXT1 regulatory element was then fused to the minimal cytomegalovirus promoter (HXT1-MIN) and inserted into an adenovirus for delivery to human fibroblasts, where it exhibited glucose-dependent transcriptional activation. Glucose action was mimicked by fructose and unrelated to glucose 6-P content, whilst non-metabolizable glucose analogues showed no effect. Activation of AMP kinase by 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranosanide blocked glucose induction, revealing parallels with the yeast glucose-repressing pathway. In contrast, delivery of Rgt1 to fibroblasts did not modify HXT1-MIN responsiveness. Thus, elements of the S.cerevisiae HXT1 gene conserve glucose regulation in human fibroblasts equivalent to the metabolism-dependent, glucose-repressing pathway in yeast. These data suggest that the instructions carried within gene regulatory elements controlling nutrient regulation of gene expression have been conserved throughout evolution.

Overexpression of UCP3 in cultured human muscle lowers mitochondrial membrane potential, raises ATP/ADP ratio, and favors fatty acid vs. glucose oxidation

The skeletal muscle mitochondrial uncoupling protein-3 (UCP3) promotes substrate oxidation, but direct evidence for its metabolic role is lacking. Here, we show that UCP3 overexpression in cultured human muscle cells decreased mitochondrial membrane potential (DYm). Despite this, the ATP content was not significantly decreased compared with control cells, whereas ADP content was reduced and thus the ATP/ADP ratio raised. This finding was contrasts with the effect caused by the chemical protonophoric uncoupler, CCCP, which lowered DYm, ATP, and the ATP/ADP ratio. UCP3-overexpression enhanced oxidation of oleate, regardless of the presence of glucose, whereas etomoxir, which blocks fatty acid entry to mitochondria, suppressed the UCP3 effect. Glucose oxidation was stimulated in UCP3-overexpressing cells, but this effect was inhibited by oleate. UCP3 caused weak increase of both 2-Deoxyglucose uptake and glycolytic rate, which differed from the marked stimulation by CCCP. We concluded that UCP3 promoted nutrient oxidation by lowering DYm and enhanced fatty acid-dependent inhibition of glucose oxidation. Unlike the uncoupler CCCP, however, UCP3 raised the ATP/ADP ratio and modestly increased glucose uptake and glycolysis. We propose that this differential effect provides a biological significance to UCP3, which is up-regulated in metabolic stress situations where it could be involved in nutrient partitioning.

Hepatic glycogen synthesis is highly sensitive to phosphorylase activity: evidence from metabolic control analysis

We used metabolic control analysis to determine the flux control coefficient of phosphorylase on glycogen synthesis in hepatocytes by titration with a specific phosphorylase inhibitor (CP-91149) or by expression of muscle phosphorylase using recombinant adenovirus. The muscle isoform was used because it is catalytically active in the b-state. CP-91149 inactivated phosphorylase with sequential activation of glycogen synthase. It increased glycogen synthesis by 7-fold at 5 mm glucose and by 2-fold at 20 mm glucose with a decrease in the concentration of glucose causing half-maximal rate (S(0.5)) from 26 to 19 mm. Muscle phosphorylase was expressed in hepatocytes mainly in the b-state. Low levels of phosphorylase expression inhibited glycogen synthesis by 50%, with little further inhibition at higher enzyme expression, and caused inactivation of glycogen synthase that was reversed by CP-91149. At endogenous activity, phosphorylase has a very high (greater than unity) negative control coefficient on glycogen synthesis, regardless of whether it is determined by enzyme inactivation or overexpression. This high control is attenuated by glucokinase overexpression, indicating dependence on other enzymes with high control. The high control coefficient of phosphorylase on glycogen synthesis affirms that phosphorylase is a strong candidate target for controlling hyperglycemia in type 2 diabetes in both the absorptive and postabsorptive states.

Treatment based on a combination of the CYP2B1/cyclophosphamide system and p53 delivery enhances tumour regression in human pancreatic cancer

BACKGROUND

Strategies based on the introduction of pro-drug activating enzymes or the restoration of tumour suppressor genes have been proposed as encouraging methods to improve the efficiency of treatments in pancreatic cancer. The in situ bioactivation of cyclophosphamide by cytochrome p450-2B1 and subsequent p53 delivery were examined.

MATERIALS AND METHODS

NP-18 cell line derived from a human pancreatic adenocarcinoma was treated in vitro with a combination of the Adenovirus-CYP2B1/cyclophosphamide and adenoviral-mediated wt-p53 reintroduction. Cell viability and cytometric cell cycle profiles were analyzed to evaluate the sensitivity of NP-18 cells to this treatment. The efficiency of this combination was assessed in an in vivo model consisting of xenografts into the subcutaneous tissue of Balb/c mice by tumour growth, histological analysis and cell cycle determinations.

RESULTS

Ad-CYP2B1/cyclophosphamide or Ad-p53 treatments led to a marked decrease in cell viability of NP-18 cells. Combination of both treatments elicited a higher loss of cell viability and marked increases in sub-G1 population in cell cycle profiles. Animals treated with the combination strategy showed a quick reduction of tumour volumes due to the bioactivation of cyclophosphamide by CYP2B1 and sustained growth inhibition throughout the period evaluated after p53 delivery. Only this group of animals presented statistically significant differences with respect to control and cyclophosphamide-treated groups (P < 0.05).

CONCLUSIONS

These results indicate that in situ bioactivation of cyclophosphamide by CYP2B1 and the recognition of the damaged DNA by p53 increase tumour regressions and may be a promising therapy for solid tumour therapy in man.

DAG accumulation from saturated fatty acids desensitizes insulin stimulation of glucose uptake in muscle cells

The increased availability of saturated lipids has been correlated with development of insulin resistance, although the basis for this impairment is not defined. This work examined the interaction of saturated and unsaturated fatty acids (FA) with insulin stimulation of glucose uptake and its relation to the FA incorporation into different lipid pools in cultured human muscle. It is shown that basal or insulin-stimulated 2-deoxyglucose uptake was unaltered in cells preincubated with oleate, whereas basal glucose uptake was increased and insulin response was impaired in palmitate- and stearate-loaded cells. Analysis of the incorporation of FA into different lipid pools showed that palmitate, stearate, and oleate were similarly incorporated into phospholipids (PL) and did not modify the FA profile. In contrast, differences were observed in the total incorporation of FA into triacylglycerides (TAG): unsaturated FA were readily diverted toward TAG, whereas saturated FA could accumulate as diacylglycerol (DAG). Treatment with palmitate increased the activity of membrane-associated protein kinase C, whereas oleate had no effect. Mixture of palmitate with oleate diverted the saturated FA toward TAG and abolished its effect on glucose uptake. In conclusion, our data indicate that saturated FA-promoted changes in basal glucose uptake and insulin response were not correlated to a modification of the FA profile in PL or TAG accumulation. In contrast, these changes were related to saturated FA being accumulated as DAG and activating protein kinase C. Therefore, our results suggest that accumulation of DAG may be a molecular link between an increased availability of saturated FA and the induction of insulin resistance.

Overexpression of protein targeting to glycogen in cultured human muscle cells stimulates glycogen synthesis independent of glycogen and glucose 6-phosphate levels

There is growing evidence that glycogen targeting subunits of protein phosphatase-1 play a critical role in regulation of glycogen metabolism. In the current study, we have investigated the effects of adenovirus-mediated overexpression of a specific glycogen targeting subunit known as protein targeting to glycogen (PTG) in cultured human muscle cells. PTG was overexpressed both in muscle cells cultured at high glucose (glycogen replete) or in cells incubated for 18 h in the absence of glucose and then incubated in high glucose (glycogen re-synthesizing). In both glycogen replete and glycogen resynthesizing cells, PTG overexpression caused glycogen to be synthesized at a linear rate 1-5 days after viral treatment, while in cells treated with a virus lacking a cDNA insert (control virus), glycogen content reached a plateau at day 1 with no further increase. In the glycogen replete PTG overexpressing cells, glycogen content was 20 times that in controls at day 5. Furthermore, in cells undergoing glycogen resynthesis, PTG overexpression caused a doubling of the initial rate of glycogen synthesis over the first 24 h relative to cells treated with control virus. In both sets of experiments, the effects of PTG on glycogen synthesis were correlated with a 2-3-fold increase in glycogen synthase activity state, with no changes in glycogen phosphorylase activity. The alterations in glycogen synthase activity were not accompanied by changes in the intracellular concentration of glucose 6-phosphate. We conclude that PTG overexpression activates glycogen synthesis in a glucose 6-phosphate-independent manner in human muscle cells while overriding glycogen-mediated inhibition. Our findings suggest that modulation of PTG expression in muscle may be a mechanism for enhancing muscle glucose disposal and improving glucose tolerance in diabetes.

Increased glucose disposal induced by adenovirus-mediated transfer of glucokinase to skeletal muscle in vivo

In non-insulin-dependent diabetes mellitus, insulin-stimulated glucose uptake is impaired in muscle, contributing in a major way to development of hyperglycemia. We previously showed that expression of the glucose phosphorylating enzyme glucokinase (GK) in cultured human myocytes improved glucose storage and disposal, suggesting that GK delivery to muscle in situ could potentially enhance glucose clearance. Here we have tested this idea directly by intramuscular delivery of an adenovirus containing the liver GK cDNA (AdCMV-GKL) into one hind limb. We injected an adenovirus containing the beta-galactosidase gene (AdCMV-lacZ) into the hind limb of newborn rats. beta-Galactosidase activity was localized in muscle for as long as 1 month after delivery, with a large percentage of fibers staining positive in the gastrocnemius. Using the same approach with AdCMV-GKL, GK protein content was increased from zero to 50-400% of the GK in normal liver sample, and total glucose phosphorylating activity was increased in GK-expressing muscles relative to the counterpart uninfected muscle. Expression of GK in muscle improved glucose tolerance rather than changing basal glycemic control. Glucose levels were reduced by approximately 35% 10 min after administration of a glucose bolus to fed animals treated with AdCMV-GKL relative to AdCMV-lacZ-treated controls. The enhanced rate of glucose clearance was reflected in increases in muscle 2-deoxy glucose uptake and blood lactate levels. We conclude that restricted expression of GK in muscle leads to an enhanced capacity for muscle glucose disposal and whole body glucose tolerance under conditions of maximal glucose-insulin stimulation, suggesting that under these conditions glucose phosphorylation becomes rate-limiting. Our findings also show that gene delivery to a fraction of the whole body is sufficient to improve glucose disposal, providing a rationale for the development of new therapeutic strategies for treatment of diabetes.-Jiménez-Chillarón, J. C., Newgard, C. B., Gómez-Foix, A. M. Increased glucose disposal induced by adenovirus-mediated transfer of glucokinase to skeletal muscle in vivo.

Genetic background determines the response to adenovirus-mediated wild-type p53 expression in pancreatic tumor cells

The development of new therapies is particularly urgent with regard to pancreatic tumors. Gene therapy approaches involving p53 replacement are promising due to the central role of p53 in the cellular response to DNA damage and the high incidence of p53 mutations in pancreatic tumors. Adenoviruses containing wild-type (wt) p53 cDNA (Ad5CMV-p53) were introduced into four human pancreatic cell lines to examine the impact caused by exogenous wt p53 on these cells. Introduction of wt p53 in mutant p53 cells (NP-9, NP-18, and NP-31) caused marked falls in cell proliferation and rises in the level of apoptosis. In contrast, overexpression of p53 did not induce apoptosis in NP-29 (wt p53). The presence of p16 contributes to the induction of apoptosis, as demonstrated by introduction of the wt p16 gene (Ad5RSV-p16). Analysis of cell cycle and apoptosis in etoposide-treated cells corroborated the inability of NP-29 to die by apoptosis, suggesting that this wt p53 cell line lacks p53 downstream functions in the apoptosis pathway. Taken together, our results indicate that the effects elicited by exogenous p53 protein depend upon the molecular alterations related to p53 actions on cell cycle and apoptosis. Therefore, knowledge of the genetic background of tumor cells is crucial to the development of efficient therapies based on the introduction of tumor suppressor genes.

Glycogen depletion rather than glucose 6-P increments controls early glycogen recovery in human cultured muscle

In glycogen-containing muscle, glycogenesis appears to be controlled by glucose 6-phosphate (6-P) provision, but after glycogen depletion, an autoinhibitory control of glycogen could be a determinant. We analyzed in cultured human muscle the contribution of glycogen depletion versus glucose 6-P in the control of glycogen recovery. Acute deglycogenation was achieved by engineering cells to overexpress glycogen phosphorylase (GP). Cells treated with AdCMV-MGP adenovirus to express 10 times higher active GP showed unaltered glycogen relative to controls at 25 mM glucose, but responded to 6-h glucose deprivation with more extensive glycogen depletion. Glycogen synthase (GS) activity ratio was double in glucose-deprived AdCMV-MGP cells compared with controls, despite identical glucose 6-P. The GS activation peak (30 min) induced by glucose reincubation dose dependently correlated with glucose 6-P concentration, which reached similar steady-state levels in both cell types. GS activation was significantly blunted in AdCMV-MGP cells, whereas it strongly correlated, with an inverse relationship, with glycogen content. An initial (0-1 h) rapid insulin-independent glycogen resynthesis was observed only in AdCMV-MGP cells, which progressed up to glycogen levels approximately 150 micrograms glucose/mg protein; control cells, which did not deplete glycogen below this concentration, showed a 1-h lag time for recovery. In summary, acute deglycogenation, as achieved by GP overexpression, caused the activation of GS, which inversely correlated with glycogen replenishment independent of glucose 6-P. During glycogen recovery, the activation promoted by acute deglycogenation rendered GS effective for controlling glycogenesis, whereas the transient activation of GS induced by the glucose 6-P rise had no impact on the resynthesis rate. We conclude that the early insulin-independent glycogen resynthesis is dependent on the activation of GS due to GP-mediated exhaustion of glycogen rather than glucose 6-P provision.

Enhanced pancreatic tumor regression by a combination of adenovirus and retrovirus-mediated delivery of the herpes simplex virus thymidine kinase gene

We have evaluated the effectiveness of combining the different characteristics of retrovirus and adenovirus to apply the herpes simplex virus thymidine kinase gene (HSVtk) and ganciclovir (GCV) treatment for gene therapy of pancreatic cancer. Transduction of NP-18 human pancreatic cells in culture by either the adenoviral vector (ADV/tk) or the retroviral vector (Rv/tk) followed by GCV treatment resulted in a GCV dose-dependent cytotoxic effect. A bystander effect was determined, both in NP-18 cultures and in xenogeneic cell mixtures of NP-18 and PA317 cells. Studies in vivo indicated that the effectiveness of tumor regression after HSVtk gene transfer and GCV treatment was dependent first on the tumor size at the time of viral injection and secondly, in large tumors, on the type of virus administered. The administration of the viral combination (ADV/tk + vector producer cells VPC-Rv/tk) was the best approach tested and resulted in a dramatic reduction in tumor mass after 4 days of GCV treatment which was maintained for the treatment period. Remarkably, two animals presented a complete eradication of the tumor. Thus, the HSVtk/GCV system when administered using a viral combination (ADV/tk + VPC-Rv/tk), may be a promising suicide gene therapy for pancreatic carcinomas.

Expression of glucokinase in cultured human muscle cells confers insulin-independent and glucose concentration-dependent increases in glucose disposal and storage

Insulin resistance, as is found in skeletal muscle of individuals with obesity and NIDDM, appears to involve a reduced capacity of the hormone to stimulate glucose uptake and/or phosphorylation. The glucose phosphorylation step, as catalyzed by hexokinase II, has been described as rate limiting for glucose disposal in muscle, but overexpression of this enzyme under control of a muscle-specific promoter in transgenic mice has had limited metabolic impact. In the current study, we investigated in a cultured muscle model whether expression of glucokinase, which in contrast to hexokinase II is not inhibited by glucose-6-phosphate (G-6-P), would have a pronounced metabolic impact. We used a recombinant adenovirus containing the cDNA-encoding rat liver glucokinase (AdCMV-GKL) to increase the glucose phosphorylating activity in cultured human muscle cells by fourfold. G-6-P levels increased in AdCMV-GKL-treated cells in a glucose concentration-dependent manner over the range of 1-30 mmol/l, whereas the much smaller increases in G-6-P in control cells were maximal at glucose concentrations <5 mmol/l. Further, cells expressing glucokinase accumulated 17 times more 2-deoxyglucose-6-phosphate than control cells. In AdCMV-GKL-treated cells, the time-dependent rise in G-6-P correlated with an increase in the activity ratio of glycogen synthase. AdCMV-GKL-treated cells also exhibited a 2.5- to 3-fold increase in glycogen content and a four- to fivefold increase in glycolytic flux, proportional to the increase in glucose phosphorylating capacity. All of these observations were made in the absence of insulin. Thus we concluded that expression of glucokinase in cultured human muscle cells results in proportional increases in insulin-independent glucose disposal, and that muscle glucose storage and utilization becomes controlled in a glucose concentration-dependent manner in AdCMV-GKL-treated cells. These results encourage testing whether delivery of glucokinase to muscle in vivo has an impact on glycemic control, which could be a method for circumventing the failure of insulin to stimulate glucose uptake and/or phosphorylation in muscle normally in insulin-resistant subjects.

Overexpression of glycogen phosphorylase increases GLUT4 expression and glucose transport in cultured skeletal human muscle

Skeletal muscle glucose utilization, a major factor in the control of whole-body glucose tolerance, is modulated in accordance with the muscle metabolic demand. For instance, it is increased in chronic contraction or exercise training in association with elevated expression of GLUT4 and hexokinase II (HK-II). In this work, the contribution of increased metabolic flux to the regulation of the glucose transport capacity was analyzed in cultured human skeletal muscle engineered to overexpress glycogen phosphorylase (GP). Myocytes treated with an adenovirus-bearing muscle GP cDNA (AdCMV-MGP) expressed 10 times higher GP activity and exhibited a twofold increase in the Vmax for 2-deoxy-D-[3H]glucose (2-DG) uptake, with no effect on the apparent Km. The stimulatory effect of insulin on 2-DG uptake was also markedly enhanced in AdCMV-MGP-treated cells, which showed maximal insulin stimulation 2.8 times higher than control cells. No changes in HKII total activity or the intracellular compartmentalization were found. GLUT4, protein, and mRNA were raised in AdCMV-MGP-treated cells, suggesting pretranslational activation. GLUT4 was immunodetected intracellularly with a perinuclear predominance. Culture in glucose-free or high-glucose medium did not alter GLUT4 protein content in either control cells or AdCMV-MGP-treated cells. Control and GP-overexpressing cells showed similar autoinhibition of glucose transport, although they appeared to differ in the mechanism(s) involved in this effect. Whereas GLUT1 protein increased in control cells when they were switched from a high-glucose to a glucose-free medium, GLUT1 remained unaltered in GP-expressing cells upon glucose deprivation. Therefore, the increased intracellular metabolic (glycogenolytic-glycolytic) flux that occurs in muscle cells overexpressing GP causes an increase in GLUT4 expression and enhances basal and insulin-stimulated glucose transport, without significant changes in the autoinhibition of glucose transport. This mechanism of regulation may be operative in the postexercise situation in which GLUT4 expression is upregulated in coordination with increased glycolytic flux and energy demand.

Reduction of islet amylin expression and basal secretion by adenovirus-mediated delivery of amylin antisense cDNA

Reduction of amylin content and secretion in rat islets was attempted by transduction with an adenovirus bearing a 0.2-kb fragment of rat amylin cDNA inserted in the antisense orientation (AdCMV-alpha amylin). Exposure of islets to AdCMV-alpha amylin at a multiplicity of infection (moi) of 200 (1.2 x 10(7) pfu/ml) reduced amylin mRNA levels by 37 +/- 5% (p < 0.005), whereas infection with an adenovirus expressing the reporter gene of beta-galactosidase (AdCMV-lacz) did not modify amylin expression. Transduction with the antisense construct was specifically associated with the decrease (30 +/- 6%; p < 0.001) in the amylin content. Insulin content was unaltered in AdCMV-alpha amylin islets compared to AdCMV-lacz-transduced or untransduced cells. Basal amylin secretion (2.8 mM glucose) was 36 +/- 3% (p < 0.005) lower in AdCMV-alpha amylin islets than in untransduced or AdCMV-lacz-transduced islets. In contrast, no difference in amylin secretion in response to high glucose concentrations (16.7 mM) was detected in AdCMV-alpha amylin-transduced islets. Thus, a reduction of amylin content and basal secretion in islet cells can be achieved by the adenovirus-mediated expression of antisense RNA.

Metabolic impact of adenovirus-mediated overexpression of the glucose-6-phosphatase catalytic subunit in hepatocytes

Glucose-6-phosphatase (G6Pase) catalyzes the hydrolysis of glucose 6-phosphate (Glu-6-P) to free glucose and, as the last step in gluconeogenesis and glycogenolysis in liver, is thought to play an important role in glucose homeostasis. G6Pase activity appears to be conferred by a set of proteins localized to the endoplasmic reticulum, including a glucose-6-phosphate translocase, a G6Pase phosphohydrolase or catalytic subunit, and glucose and inorganic phosphate transporters in the endoplasmic reticulum membrane. In the current study, we used a recombinant adenovirus containing the cDNA encoding the G6Pase catalytic subunit (AdCMV-G6Pase) to evaluate the metabolic impact of overexpression of the enzyme in primary hepatocytes. We found that AdCMV-G6Pase-treated liver cells contain significantly less glycogen and Glu-6-P, but unchanged UDP-glucose levels, relative to control cells. Further, the glycogen synthase activity state was closely correlated with Glu-6-P levels over a wide range of glucose concentrations in both G6Pase-overexpressing and control cells. The reduction in glycogen synthesis in AdCMV-G6Pase-treated hepatocytes is therefore not a function of decreased substrate availability but rather occurs because of the regulatory effects of Glu-6-P on glycogen synthase activity. We also found that AdCMV-G6Pase-treated-cells had significantly lower rates of lactate production and [3-3H]glucose usage, coupled with enhanced rates of gluconeogenesis and Glu-6-P hydrolysis. We conclude that overexpression of the G6Pase catalytic subunit alone is sufficient to activate flux through the G6Pase system in liver cells. Further, hepatocytes treated with AdCMV-G6Pase exhibit a metabolic profile resembling that of liver cells from patients or animals with non-insulin-dependent diabetes mellitus, suggesting that dysregulation of the catalytic subunit of G6Pase could contribute to the etiology of the disease.

Adenovirus-mediated gene transfer into human hepatocytes: analysis of the biochemical functionality of transduced cells

The use of replication-defective adenoviruses to deliver transgenes into hepatocytes seems to be a promising approach to human liver gene therapy. However, the effects that the adenovirus-mediated expression of a foreign gene could have on the expression of other hepatic characteristic genes have not yet been properly examined. We have investigated this problem by using human hepatocytes infected with a recombinant E-1 defective adenovirus that carried a modified lacZ gene. The analysis of the biochemical functionality of transduced cells showed that the use of adenovirus: (1) was a very efficient way to introduce a foreign gene into human hepatocytes (80% transduced cells after 1 h contact, at an MOI of 15; approximately 100% transduced cells at an MOI of 20); (2) allowed the expression of the transgene to levels that enabled cells effectively to use lactose as an energy source; (3) does not affect urea synthesis, plasma protein synthesis and xenobiotic biotransformation activities (1A2, 2A1, 2B6, 3A3/5). Glycolysis was moderately increased (approximately 20%), while gluconeogenesis decreased (approximately 20%) in transduced hepatocyte; moreover, (4) the expression of inducible genes (acute-phase plasma proteins, CYPs) was not impaired in transduced human hepatocytes upon stimulation with IL-6 or methylcholantrene. The results of this research support the idea that efficient expression of transgenes can be achieved in human hepatocytes by means of adenoviral transduction, without altering these characteristic hepatic biochemical functions.

Glucose 6-phosphate produced by glucokinase, but not hexokinase I, promotes the activation of hepatic glycogen synthase

In a previous study (O'Doherty, R. M., Lehman, D. L., Seoane, J., Gómez-Foix, A. M., Guinovart, J. J., and Newgard, C.B. (1996) J. Biol. Chem. 271, 20524-20530), we demonstrated that adenovirus-mediated overexpression of glucokinase but not hexokinase I has a potent enhancing effect on glycogen synthesis in primary hepatocytes. In an effort to understand the underlying mechanism of this differential effect of the two hexokinase isoforms, we have investigated changes in key intracellular metabolites and the activation state of glycogen synthase in cells treated with recombinant adenoviruses expressing the liver isoform of glucokinase (AdCMV-GKL) or hexokinase I (AdCMV-HKI). Glucose 6-phosphate (Glu-6-P) levels are elevated from approximately 1.5 nmol/mg protein to 8-10 nmol/mg protein in both AdCMV-GKL- and AdCMV-HKI-treated hepatocytes as glucose is raised from 1 to 5 mM, levels four times higher than those in untreated cells. In AdCMV-GKL-treated cells, Glu-6-P continues to accumulate at glucose levels greater than 5 mM, reaching a maximum of 120 nmol/mg protein in cells incubated at 25 mM glucose, a value 10 and 50 times greater than the maximal levels achieved in AdCMV-HKI-treated and untreated cells, respectively. In parallel with the changes observed in Glu-6-P levels, increases in UDP-Glc in AdCMV-HKI- and AdCMV-GKL-treated cells were most pronounced at low (1-5 mM) and high (25 mM) glucose levels, respectively. Despite the significant increases in Glu-6-P and UDP-Glc achieved in AdCMV-HKI-treated cells, only AdCMV-GKL-treated cells exhibited increases in glycogen synthase activity ratio and translocation of the enzyme from a soluble to a particulate form relative to untreated control cells. We conclude that Glu-6-P produced by overexpressed glucokinase is glycogenic because it effectively promotes activation of glycogen synthase. Glu-6-P produced by overexpressed hexokinase, in contrast, appears to be unable to exert the same regulatory effects, probably due to the different subcellular distribution of the two glucose-phosphorylating enzymes.

Differential metabolic effects of adenovirus-mediated glucokinase and hexokinase I overexpression in rat primary hepatocytes

The first step of glucose metabolism is the phosphorylation of glucose, catalyzed by the hexokinase family of enzymes. To address the metabolic impact of increasing glucose phosphorylation capacity in liver, rat primary hepatocytes were treated with recombinant adenoviruses containing the cDNAs encoding either rat liver glucokinase (AdCMV-GKL) or rat hexokinase I (AdCMV-HKI). Maximal glucose phosphorylation in AdCMV-GKL- and AdCMV-HKI-treated hepatocytes was increased 7.1 +/- 1.2- and 6.3 +/- 0.8-fold, respectively, over hepatocytes treated with an adenovirus expressing beta-galactosidase. Glucose usage (measured with 3 and 20 m 2-[3H]glucose and 5-[3H]glucose) was significantly increased in AdCMV-GKL-treated cells preincubated in 1 or 25 mM glucose. Treatment of hepatocytes with AdCMV-HKI also caused enhanced glucose utilization, but the increases were smaller and were less apparent in cells preincubated in high (25 mM) glucose. AdCMV-GKL-treated hepatocytes incubated for 48 h in the presence of variable glucose concentrations had glycogen levels that were maximally 15.0 +/- 0. 6-fold greater than levels in corresponding control cells. AdCMV-HKI-treated hepatocytes incubated under similar conditions had unchanged glycogen levels relative to controls. In AdCMV-GKL-treated cells, lactate output was increased to a maximum of 3.0 +/- 0.4-fold (at 25 mM glucose), glucose oxidation was increased 3.5 +/- 0.3-fold, and triglyceride production was unchanged relative to untreated cells. Among these three parameters, only lactate production was increased in AdCMV-HKI-treated cells, and then only at low glucose concentrations. We conclude that overexpression of glucokinase has potent effects on glucose storage and utilization in hepatocytes and that these effects are not matched by overexpression of hexokinase I.

Direct activating effects of dexamethasone on glycogen metabolizing enzymes in primary cultured rat hepatocytes

The direct effects of dexamethasone on glycogen synthase and phosphorylase and glycogen content have been investigated in primary cultured rat hepatocytes. Dexamethasone induced the transient translocation of glycogen synthase from the soluble to the 10000xg pelletable fraction and the activation of this enzyme, although more significant, longer-standing activation was achieved in the pelletable fraction. Neither total glycogen synthase content nor glycogen synthase mRNA levels were modified. Dexamethasone also caused the sustained activation (up to 6h) of glycogen phosphorylase, which was not accompanied by an increase in its mRNA level. Glycogen cell content and the incorporation of (14C) glucose into glycogen decreased after dexamethasone treatment. The data show that dexamethasone, unlike other glycogenolytic hormones, at concentrations of 10 nM or higher, stimulate hepatocyte glycogenolysis without inducing the inverse coupling of synthase and phosphorylase. The co-existence of active forms of both glycogen synthase and phosphorylase promoted by dexamethasone leads to a situation that is analogous to that of the fasted liver.

Overexpression of muscle glycogen phosphorylase in cultured human muscle fibers causes increased glucose consumption and nonoxidative disposal

The effect of increased expression of glycogen phosphorylase on glucose metabolism in human muscle was examined in primary cultured fibers transduced with recombinant adenovirus AdCMV-MGP encoding muscle glycogen phosphorylase. Increments of 20-fold in total enzyme activity and of 14-fold of the active form of the enzyme were associated with a 30% reduction in basal glycogen levels. Total glycogen synthase activity was doubled in AdCMV-MGP-transduced cells even though the activity ratio was decreased. Incubation with forskolin, which inactivated glycogen synthase and activated glycogen phosphorylase, induced greater net glycogenolysis in engineered cells. In unstimulated fibers, lactate production was three times higher in AdCMV-MGP fibers as compared with controls, despite similar rates of glycogenolysis. In transduced fibers incubated with 2-deoxyglucose, the level of 2-deoxyglucose 6-phosphate was about 8-fold elevated over the control even though hexokinase activity was unmodified in AdCMV-MGP fibers. Overexpression of glycogen phosphorylase also led to enhancement of [U-14C]glucose incorporation into glycogen, lactate, and lipid. Accordingly, determination of lipid cell content revealed that engineered cells were accumulating lipids. Furthermore, 14CO2 formation from [U-14C]glucose was 1.6-fold higher, whereas 14CO2 formation from [6-14C]glucose was unmodified, in AdCMV-MGP fibers. Our data show that in human skeletal muscle cells in culture, the increase in glycogen phosphorylase activity is able to up-regulate glycogen synthase activity indicating the enhancement of glycogen turnover. We suggest that the increase in glycogen phosphorylase and, thereby, in glycogen metabolism, is sufficient to enhance glucose uptake in the muscle cell. Glucose taken up by engineered muscle cells is essentially disposed of through nonoxidative metabolism and converted into lactate and lipid.

Adenovirus-mediated delivery into myocytes of muscle glycogen phosphorylase, the enzyme deficient in patients with glycogen-storage disease type V

The feasibility of using adenovirus as a vector for the introduction of glycogen phosphorylase activity into myocytes has been examined. We used the C2C12 myoblast cell line to assay the impact of phosphorylase gene transfer on myocyte glycogen metabolism and to reproduce in vitro the two strategies proposed for the treatment of muscle genetic diseases, myoblast transplantation and direct DNA delivery. In this study, a recombinant adenovirus containing the muscle glycogen phosphorylase cDNA transcribed from the cytomegalovirus promoter (AdCMV-MGP) was used to transduce both differentiating myoblasts and nondividing mature myotube cells. Muscle glycogen phosphorylase mRNA levels and total phosphorylase activity were increased in both cell types after viral treatment although more efficiently in the differentiated myotubes. The increase in phosphorylase activity was transient (15 days) in myoblasts whereas in myotubes higher levels of phosphorylase gene expression and activity were reached, which remained above control levels for the duration of the study (20 days). The introduction of muscle phosphorylase into myotubes enhanced their glycogenolytic capacity. AdCMV MGP-transduced myotubes had lower glycogen levels under basal conditions. In addition, these engineered cells showed more extensive glycogenolysis in response to both adrenaline, which stimulates glycogen phosphorylase phosphorylation, and carbonyl cyanide m-chlorophenylhydrazone, a metabolic uncoupler. In conclusion, transfer of the muscle glycogen phosphorylase cDNA into myotubes confers an enhanced and regulatable glycogenolytic capacity. Thus this system might be useful for delivery of muscle glycogen phosphorylase and restoration of glycogenolysis in muscle cells from patients with muscle phosphorylase deficiency (McArdle's disease).

Amylin impairment of insulin effects on glycogen synthesis and phosphoenolpyruvate carboxykinase gene expression in rat primary cultured hepatocytes

The ability of amylin to impair hepatic insulin action is controversial. We have found that the effect of amylin in primary cultured hepatocytes is strongly dependent on the culture conditions. Only in hepatocytes preincubated in the presence of fetal serum did amylin, at concentrations ranging from 1 to 100 nM, reduce insulin-stimulated glycogen synthesis rate and glycogen accumulation without showing direct effects. Neither basal glycogen synthase nor glycogen phosphorylase activity was modified by amylin treatment. Nevertheless, amylin (100 nM) blocked the activation of glycogen synthase by insulin. Amylin also proved capable of opposing the reduction in the expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene induced by insulin, whereas the basal mRNA level of PEPCK was unaffected by amylin treatment. Thus, these results show that, in cultured rat hepatocytes, amylin is indeed able to interfere with insulin regulation of glycogenesis and PEPCK gene expression, favouring the hypothesis that amylin may modulate liver sensitivity to insulin.

Stimulation of glucose utilization by fructose in isolated rat hepatocytes

In rat hepatocytes fructose at low concentrations (below 1 mM) stimulated the glycolytic flux as measured by the release of 3H2O from [3-3H]glucose and increased fructose 2,6-bisphosphate [Fru(2,6)P2] levels, without modifying the activity of 6-phosphofructo-2-kinase. Maximal stimulation of the glycolytic pathway by 0.1 mM fructose was observed when hepatocytes were incubated in the presence of physiological concentrations of glucose (8 mM). The rise in Fru(2,6)P2 levels was probably due to an increase in glucose 6-phosphate, which in turn resulted from the stimulation of glucose phosphorylation as measured by the formation of 3H2O from [2-3H]glucose. Furthermore, no effects of low doses of fructose on the glycolytic flux or on glucose phosphorylation were observed in hepatocytes from streptozocin-diabetic rats in which glucokinase is almost absent, or in hepatocytes incubated in the presence of mannoheptulose where glucokinase is inhibited. These results suggest that fructose at low concentrations increases the glycolytic flux by raising Fru(2,6)P2 levels solely as a consequence of the stimulation of glucose phosphorylation.

Adenovirus-mediated transfer of the muscle glycogen phosphorylase gene into hepatocytes confers altered regulation of glycogen metabolism

The muscle isozyme of glycogen phosphorylase is potently activated by the allosteric ligand AMP, whereas the liver isozyme is not. In this study we have investigated the metabolic impact of expression of muscle phosphorylase in liver cells. To this end, we constructed a replication-defective, recombinant adenovirus containing the muscle glycogen phosphorylase cDNA (termed AdCMV-MGP) and used this system to infect hepatocytes in culture. AMP-activatable glycogen phosphorylase activity was increased 46-fold 6 days after infection of primary liver cells with AdCMV-MGP. Despite large increases in phosphorylase activity, glycogen levels were only slightly reduced in AdCMV-MGP-infected liver cells compared to uninfected cells or cells infected with wild-type adenovirus. The lack of correlation of phosphorylase activity and glycogen content suggests that the liver cell environment can inhibit the muscle phosphorylase isozyme. This inhibition can be overcome, however, by addition of carbonyl cyanide m-chlorophenylhydrazone (CCCP), which increases AMP levels by 30-fold and causes a much larger decrease in glycogen levels in AdCMV-MGP-infected cells than in uninfected or wild-type adenovirus-infected controls. CCCP treatment also caused a preferential decrease in glycogen content relative to glucagon treatment in AdCMV-MGP-infected hepatocytes (74% versus 11%, respectively), even though the two drugs caused equal increases in phosphorylase a activity. Introduction of muscle phosphorylase into hepatocytes therefore confers a capacity for glycogenolytic response to effectors that is not provided by the endogenous liver phosphorylase isozyme. The remarkable efficiency of adenovirus-mediated gene transfer into primary hepatocytes and the demonstration of altered regulation of glycogen metabolism as a consequence of expression of a non-cognate phosphorylase isozyme may have implications for gene therapy of glycogen storage diseases.

Activation by vanadate of glycolysis in hepatocytes from diabetic rats

In hepatocytes from starved streptozocin-induced diabetic rats, vanadate increases the glycolytic flux because it raises the levels of fructose-2,6-bisphosphate (Fru-2,6-P2), the main regulatory metabolite of this pathway. This effect of vanadate on Fru-2,6-P2 levels is time and dose dependent, and it remains in cells incubated in a calcium-depleted medium. Vanadate is also able to counteract the decrease on Fru-2,6-P2 levels produced by glucagon, colforsin, or exogenous cAMP. However, vanadate does not modify 6-phosphofructo-2-kinase and pyruvate kinase activities, but it does counteract the inactivation of these enzymes induced by glucagon. Likewise, Fru-2,6-P2ase activity is also not affected by vanadate. In addition, vanadate is able to increase the production of both lactate and CO2 in hepatocytes from streptozocin-induced diabetic rats incubated in the presence of glucose in the medium. Vanadate behaves as a glycolytic effector in these cells, and this effect may be related to its ability to normalize blood glucose levels in diabetic animals.

Anti-insulin effects of amylin and calcitonin-gene-related peptide on hepatic glycogen metabolism

To evaluate the effects of amylin and calcitonin-gene-related peptide (CGRP) as anti-insulin agents in hepatic tissue, we have studied whether these two agents counteracted the action of insulin on glycogen metabolism in isolated rat hepatocytes. In this system insulin stimulates [14C]glucose incorporation into glycogen and activates glycogen synthase. Incubation of the cells with insulin in the presence of amylin or CGRP markedly blocked the insulin stimulation of these two parameters, whereas amylin or CGRP acting alone did not induce any effect. We also examined the ability of amylin and CGRP to modify the anti-glucagon effects of insulin. In the presence of 100 nM-amylin or -CGRP, 10 nM-insulin was almost unable to counteract the inactivation of glycogen synthase and the activation of phosphorylase induced by glucagon. In contrast, neither amylin nor CGRP modified the effect of glucagon on these two enzymes. Our results indicate that amylin and CGRP are able to impair the action of insulin on hepatic glycogen metabolism.

Effects of vanadate on protein kinases in rat hepatocytes

In rat hepatocytes, vanadate modifies neither the intracellular concentration of cyclic AMP nor the --cyclic AMP/+cyclic AMP activity ratio for cyclic AMP-dependent protein kinase. Vanadate can, however, counteract the increase in cyclic AMP and the increase in the --cyclic AMP/+cyclic AMP activity ratio of cyclic AMP-dependent protein kinase induced by glucagon. On the other hand, vanadate treatment of hepatocytes can produce a time- and concentration-dependent increase in cyclic AMP- and Ca2+-independent casein kinase activity. Maximal activation at the optimal time with 5 mM-vanadate was about 70% over control. A clear relationship was observed between the activation of casein kinase and the inactivation of glycogen synthase after vanadate treatment. These results suggest that casein kinase activity may be involved in vanadate actions in rat hepatocytes.

Oxytocin inactivates and phosphorylates rat hepatocyte glycogen synthase

Incubation of isolated rat hepatocytes with oxytocin produces a time- and dose-dependent inactivation of glycogen synthase. Such inactivation is associated with an increase in the phosphorylation state of the 88 kDa subunit of the enzyme, as observed after electrophoretic analysis of the 32P-labelled enzyme isolated by immunoprecipitation from cells incubated with [32P]phosphate. CNBr cleavage of the immunoprecipitated glycogen synthase showed that multiple sites were phosphorylated after exposure of the cells to the hormone. The effect of oxytocin on hepatocyte glycogen synthase activity was not observed in the absence of extracellular Ca2+. Inactivation of glycogen synthase by oxytocin was partially abolished in the presence of insulin. These results indicate that the effects of oxytocin on glycogen synthase from rat hepatocytes are similar to those observed for other Ca2+-mediated glycogenolytic hormones, such as vasopressin.

Prostaglandins E2 and F2 alpha affect glycogen synthase and phosphorylase in isolated hepatocytes

Prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) inactivated glycogen synthase and activated glycogen phosphorylase in rat hepatocytes in a dose- and time-dependent manner. These effects were dependent on the presence of Ca2+ in the incubation medium. When glycogen synthase was immunoprecipitated from cells incubated with [32P]Pi and then treated with PGE2 or PGF2 alpha, there was increased phosphorylation of the 88 kDa subunit of the enzyme. This phosphorylation affected two CNBr fragments of the glycogen synthase, CB-1 and CB-2, the same fragments that are phosphorylated by different glycogenolytic hormones. No phosphorylation of glycogen synthase by prostaglandins was observed in the absence of Ca2+. Thus the effect of PGE2 and PGF2 alpha on these glycogen-metabolizing enzymes supports a role for regulation by prostaglandins of glucose metabolism in parenchymal liver cells.

Control of glycogen synthase and phosphorylase in hepatocytes from diabetic rats. Effects of glucagon, vasopressin, and vanadate

Although glycogen synthase is present in a highly inactivated state in hepatocytes from streptozocin-induced diabetic rats, glucagon, vasopressin, and vanadate are still able to further decrease the basal activity of the enzyme. This inactivation was observed with the low-to-high glucose 6-phosphate activity ratio assay. The inactivation of glycogen synthase occurred concomitantly with the activation of glycogen phosphorylase. When hepatocytes from diabetic rats were incubated with [32P]phosphate and then with the agents and when the 32P-labeled glycogen synthase was immunoprecipitated, we observed that the 32P bound to the 88,000-Mr subunit increased in all cases. All the [32P]phosphate was located in two cyanogen bromide fragments of the enzyme, indicating that the enzyme was phosphorylated at multiple sites. The fragments were precisely those phosphorylated by glycogenolytic hormones in hepatocytes from normal rats. These results demonstrated that hepatic glycogen synthase, although highly inactive, is under potential hormonal control in diabetes and that the enzyme has not reached its maximal level of phosphorylation. Furthermore, they indicated that vanadate behaves as a glycogenolytic agent regarding its effects on glycogen-metabolizing enzymes in hepatocytes from diabetic rats.

Vanadate raises fructose 2,6-bisphosphate concentrations and activates glycolysis in rat hepatocytes

In rat hepatocytes, vanadate increases fructose 2,6-bisphosphate (Fru-2,6-P2) in a time- and dose-dependent manner, and counteracts the decrease in this metabolite caused by glucagon, forskolin or exogenous cyclic AMP. Vanadate does not directly modify the activity of 6-phosphofructo-2-kinase, even though it can counteract the inactivation of this enzyme caused by glucagon. Furthermore, vanadate raises the yield of 3H2O from [3-3H]glucose, indicating that it increases the flux through 6-phosphofructo-1-kinase. Moreover, vanadate in hepatocytes incubated in the presence of glucose increases the production of both lactate and CO2. Therefore vanadate has insulin-like effects on the glycolytic pathway in rat hepatocytes. These results clearly contrast with our previous observation that vanadate exerts glycogenolytic non-insulin-like effects on glycogen synthase and phosphorylase.

Glycogenolytic, noninsulin-like effects of vanadate on rat hepatocyte glycogen synthase and phosphorylase

Vanadate inactivated rat hepatocyte glycogen synthase and activated glycogen phosphorylase in a dose- and time-dependent manner. These effects were observed in hepatocytes from both fasted as well as fed rats. When rat hepatocytes were preincubated with [32P]phosphate and then with vanadate, and the 32P-labeled glycogen synthase was specifically immunoprecipitated, it was observed that vanadate stimulated the phosphorylation of the 88,000-dalton subunit of glycogen synthase. All of the phosphate was located in the same two CNBr fragments of the enzyme which are phosphorylated by glucagon and other glycogenolytic hormones. In cells incubated in a calcium-depleted medium, vanadate was still able to inactivate glycogen synthase but its effects on phosphorylase were essentially lost. These results demonstrate that, in the hepatocyte, vanadate exerts opposite effects than in the adipocyte and skeletal muscle, where vanadate has an insulin-like action.

Effects of lithium ions on glycogen synthase and phosphorylase in rat hepatocytes

Incubation of hepatocytes from fasted rats with LiCl provoked a concentration- and time-dependent activation of glycogen synthase. This effect was observed in the absence of glucose in the incubation medium. No changes in the intracellular concentrations of ATP or glucose-6-phosphate were detected. Lithium was also able to activate glycogen synthase in the absence of extracellular calcium. If hepatocytes were incubated with lithium and insulin, an additive effect of both agents on glycogen synthase activity was observed. LiCl was also effective in activating the enzyme in hepatocytes obtained from fed rats. When hepatocytes were incubated with [33P]phosphate and then treated with LiCl, a decrease in the amount of [32P]phosphate incorporated in the enzyme was observed. This dephosphorylation affected two CNBr fragments of the enzyme (CB-2 and CB-1), suggesting that several phosphorylation sites were involved. Lithium was also able to activate glycogen phosphorylase from both fasted and fed rats. Phosphorylase activation was concentration- and time-dependent, either in the presence or absence of calcium in the incubation medium. These findings demonstrate that although lithium appears to mimic the effects of insulin on glycogen synthase activity, its mechanism of action must be different from that of the hormone.