Glucose overutilization in diabetes: evidence from studies on the changes in hexokinase, the pentose phosphate pathway and glucuronate-xylulose pathway in rat kidney cortex in diabetes

AcDCXR Is a Cowpea Aphid Effector With Putative Roles in Altering Host Immunity and Physiology

Cowpea, Vigna unguiculata, is a crop that is essential to semiarid areas of the world like Sub-Sahara Africa. Cowpea is highly susceptible to cowpea aphid, Aphis craccivora, infestation that can lead to major yield losses. Aphids feed on their host plant by inserting their hypodermal needlelike flexible stylets into the plant to reach the phloem sap. During feeding, aphids secrete saliva, containing effector proteins, into the plant to disrupt plant immune responses and alter the physiology of the plant to their own advantage. Liquid chromatography tandem mass spectrometry (LC-MS/MS) was used to identify the salivary proteome of the cowpea aphid. About 150 candidate proteins were identified including diacetyl/L-xylulose reductase (DCXR), a novel enzyme previously unidentified in aphid saliva. DCXR is a member of short-chain dehydrogenases/reductases with dual enzymatic functions in carbohydrate and dicarbonyl metabolism. To assess whether cowpea aphid DCXR (AcDCXR) has similar functions, recombinant AcDCXR was purified and assayed enzymatically. For carbohydrate metabolism, the oxidation of xylitol to xylulose was tested. The dicarbonyl reaction involved the reduction of methylglyoxal, an α-β-dicarbonyl ketoaldehyde, known as an abiotic and biotic stress response molecule causing cytotoxicity at high concentrations. To assess whether cowpea aphids induce methylglyoxal in plants, we measured methylglyoxal levels in both cowpea and pea (Pisum sativum) plants and found them elevated transiently after aphid infestation. Agrobacterium-mediated transient overexpression of AcDCXR in pea resulted in an increase of cowpea aphid fecundity. Taken together, our results indicate that AcDCXR is an effector with a putative ability to generate additional sources of energy to the aphid and to alter plant defense responses. In addition, this work identified methylglyoxal as a potential novel aphid defense metabolite adding to the known repertoire of plant defenses against aphid pests.

Diacetyl/l-Xylulose Reductase Mediates Chemical Redox Cycling in Lung Epithelial Cells

Reactive carbonyls such as diacetyl (2,3-butanedione) and 2,3-pentanedione in tobacco and many food and consumer products are known to cause severe respiratory diseases. Many of these chemicals are detoxified by carbonyl reductases in the lung, in particular, dicarbonyl/l-xylulose reductase (DCXR), a multifunctional enzyme important in glucose metabolism. DCXR is a member of the short-chain dehydrogenase/reductase (SDR) superfamily. Using recombinant human enzyme, we discovered that DCXR mediates redox cycling of a variety of quinones generating superoxide anion, hydrogen peroxide, and, in the presence of transition metals, hydroxyl radicals. Redox cycling activity preferentially utilized NADH as a cosubstrate and was greatest for 9,10-phenanthrenequinone and 1,2-naphthoquinone, followed by 1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone (menadione). Using 9,10-phenanthrenequinone as the substrate, quinone redox cycling was found to inhibit DCXR reduction of l-xylulose and diacetyl. Competitive inhibition of enzyme activity by the quinone was observed with respect to diacetyl (K_i = 190 μM) and l-xylulose (K_i = 940 μM). Abundant DCXR activity was identified in A549 lung epithelial cells when diacetyl was used as a substrate. Quinones inhibited reduction of this dicarbonyl, causing an accumulation of diacetyl in the cells and culture medium and a decrease in acetoin, the reduced product of diacetyl. The identification of DCXR as an enzyme activity mediating chemical redox cycling suggests that it may be important in generating cytotoxic reactive oxygen species in the lung. These activities, together with the inhibition of dicarbonyl/l-xylulose metabolism by redox-active chemicals, as well as consequent deficiencies in pentose metabolism, are likely to contribute to lung injury following exposure to dicarbonyls and quinones.

Effect of lower doses of vanadate in combination with Azadirachta indica leaf extract on hepatic and renal antioxidant enzymes in streptozotocin-induced diabetic rats

The present study was undertaken to investigate short-term (21 days) effects of oral administration of Azadirachta indica leaf extract and vanadate, separately and in combination, on the activities of antioxidant enzymes in streptozotocin-induced diabetic rats. Vanadate is a remarkable antidiabetic agent and shows insulin mimetic effect. However, severe toxicity is associated with vanadate when used in high concentration while at lower concentration the hypoglycemic property of vanadate is reduced. So, we used a low dose of vanadate in combination with A. indica leaf extract and evaluated their effect on the antioxidant defense system. Streptozotocin-diabetic rats were treated separately with insulin, vanadate (0.6 mg/ml), A. indica, and with combined dose of vanadate (0.2 mg/ml) and A. indica. At the end of the experiment, rats were sacrificed and serum glucose levels and activities of superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase were determined in cytosolic fraction of liver and kidney. Diabetic rats showed hyperglycemic condition and alteration in antioxidant enzyme activities. Treatment with antidiabetic compounds resulted in the reduction of glucose levels and restoration of enzyme activities to normal. Results showed that combined treatment of vanadate and A. indica leaf extract was the most effective in normalizing altered antioxidant enzyme system.

(-)-Epigallocatechin-3-gallate, a potential inhibitor to human dicarbonyl/L-xylulose reductase

Dicarbonyl/l-xylulose reductase (DCXR), mainly catalysing the reduction of α-dicarbonyl compounds and l-xylulose, belongs to the short-chain dehydrogenase/reductase superfamily. Its enzyme activity can be inhibited by short-chain fatty acids. In this study, a novel DCXR inhibitor named (-)-epigallocatechin-3-gallate (EGCG) was reported. First, we overexpressed recombinant human DCXR in Escherichia coli, purified the enzyme by affinity chromatography and measured its activity. The inhibition effects of EGCG and its analogues on DCXR were determined subsequently, and EGCG showed the strongest inhibition with 50% inhibition concentration value of 78.8 μM. The surface plasmon resonance analysis also indicated that the equilibrium dissociation constant (KD) reached to 7.11 × 10(-8) M, which implied a high affinity between EGCG and DCXR. From enzyme kinetic analysis, EGCG acted as a mixed inhibitor against its forward and reverse substrates and the coenzyme, reduced nicotinamide adenine dinucleotide phosphate (NADPH). However, the inhibition is pH dependent. The molecular docking finally showed that EGCG formed several hydrogen bonds with the Thr190 residue of DCXR, and the model was further verified by site-directed mutagenesis. Therefore, EGCG is a potential inhibitor to human DCXR.

Beneficial influence of fungal metabolite nigerloxin on diabetes-induced oxidative stress in experimental rats

Oxidative stress plays a key role in the progression of diabetes and its complications. In this study, the beneficial influence of the fungal metabolite nigerloxin, a new aldose reductase inhibitor and a free-radical scavenger, was investigated on oxidative stress in streptozotocin-induced diabetic rats. Groups of diabetic rats were orally administered nigerloxin for 30 days at a dose of 25 and 100 mg·(kg body mass)−1·day−1. Diabetic rats showed significantly increased lipid peroxide levels in blood and liver, which was accompanied by lowered concentrations of antioxidant molecules and activities of antioxidant enzymes in blood and liver. Administration of nigerloxin for 30 days at a daily dose of 100 mg∙(kg body mass)–1 to diabetic rats significantly decreased plasma and liver lipid peroxides, elevated the nonenzymatic antioxidants ascorbic acid, reduced glutathione, and total thiols, and elevated the activities of antioxidant enzymes in blood and liver. Nigerloxin showed a tendency to counter lipid abnormalities in diabetic animals, while fasting glucose and body mass were unaffected by nigerloxin treatment. Thus, this animal study has indicated the beneficial influence of nigerloxin on oxidative stress associated with diabetes that may have an implication in delaying or ameliorating the secondary complications.

Impaired glucose tolerance plus hyperlipidaemia induced by diet promotes retina microaneurysms in New Zealand rabbits

With the increasing prevalence of diabetes mellitus and metabolic syndrome worldwide, experimental models are required to better understand the pathophysiology and therapeutic approaches to preserve pancreatic beta cells, attenuate atherosclerosis and protect target organs. The aims of this study were to develop an experimental model of impaired glucose tolerance combined with hypercholesterolaemia induced by diet and assess metabolic alterations and target organ lesions. New Zealand male rabbits were fed high-fat/high-sucrose (10/40%) and cholesterol-enriched diet for 24 weeks, when they were sacrificed. Biochemistry, fundus photographs with fluorescein angiography and pathological analyses were performed. Cholesterol-fed and normal animals of same age were compared.

RESULTS

The animals with diet-induced impaired glucose tolerance combined with hypercholesterolaemia gained weight, increased blood glucose, total cholesterol, LDL-C and triglycerides and decreased HDL-C (P < 0.05 vs. baseline). Fructosamine levels and the homeostasis model assessment of insulin resistance (HOMA-IR) index were increased, while there was a reduction in the HOMA-β (P < 0.05 for all vs. baseline). Histomorphologic findings of this model were aortic atherosclerosis, hepatic steatofibrosis and glomerular macrophage infiltration. Early clinical features of diabetic retinopathy with hyperfluorescent dots consistent with presence of retina microaneurysms were seen since week 12, progressing up to the end of the experiment (P < 0.0005 vs. baseline and 12 weeks). Our model reproduced several metabolic characteristics of human diabetes mellitus and promoted early signs of retinopathy. This non-expensive model is suitable for studying mechanistic pathways and allowing novel strategic approaches.

POSSIBLE ANTIDIABETIC AND ANTIHYPERLIPIDAEMIC EFFECT OF FERMENTED PARKIA BIGLOBOSA (JACQ) EXTRACT IN ALLOXAN-INDUCED DIABETIC RATSs

1. The hypoglycaemic effect of fermented seeds of Parkia biglobosa (PB; African locust bean), a natural nutritional condiment that features frequently in some African diets as a spice, was investigated in the present study in alloxan-induced diabetic rats. Its effect was compared with that of glibenclamide (Daonil; Sanofi-Aventis, Paris, France), a reference antidiabetic drug. The effects of PB on lipid profiles were also examined. 2. In order to assess the hypoglycaemic and hypolipidaemic effects of aqueous and methanolic extracts of PB on experimental animals, fasting plasma glucose (FPG), total cholesterol, triglyceride, high-density lipoprotein (HDL) and low-density lipoprotein (LDL) were determined. In addition, the weight of each animal was determined to assess any possible weight gain or loss in the experimental animals (diabetic rats treated with Daonil (group C), the aqueous extract of PB (group D) or the methanolic extract of PB (group E)) compared with control groups (non-diabetic (group A) and non-treated diabetic (group B)). 3. A single dose of 120 mg/kg, i.v., alloxan administered to rats resulted in significant increases in the FPG (P < 0.001) of test animals compared with controls. However, dietary supplementation with PB (6 g/kg extract for 4 weeks administered orally using an intragastric tube) ameliorated the alloxan-induced diabetes in a manner comparable with that of the reference antidiabetic drug glibenclamide. Aqueous and methanolic extracts of PB (6% w/w) elicited 69.2% and 64.4% reductions, respectively, in FPG compared with 70.4% in 0.01 mg/150 g glibenclamide-treated rats. 4. Although animals treated with the aqueous extract of PB gained weight in manner similar to normal controls, animals given the methanolic extract and glibenclamide lost weight in manner similar to non-treated diabetic rats. In addition, high levels of HDL and low LDL were observed in animals treated with the aqueous extract of PB, a pattern similar to that seen in normal controls. Low levels of HDL and high levels of LDL were observed in animals treated with the methanolic extract of PB, a pattern similar to that seen in non-treated diabetic controls. 5. The results of the present study demonstrate that both aqueous and methanolic extracts of fermented seeds of PB exert a hypoglycaemic effect; hence, PB has an antidiabetic property. However, only the aqueous extract of PB ameliorated the loss of bodyweight usually associated with diabetes. Although the aqueous extract has a favourable lipid profile, which is probably an indication of its possible anti-arteriogenic property (hypertension and ischaemic heart diseases being common complications in diabetes mellitus), the methanolic extract shows possible contraindication to ischaemic heart diseases.

Intestinal disaccharidases and some renal enzymes in streptozotocin-induced diabetic rats fed sapogenin extract from bitter yam (Dioscorea polygonoides)

In this study, the effects of bitter yam sapogenin extract or commercial diosgenin on intestinal disaccharidases and some renal enzymes in diabetic rats were investigated. Diabetic male Wistar rats were fed diets supplemented with 1% sapogenin extract or commercial diosgenin for 3 weeks. Plasma glucose, intestinal disaccharidases and the activities of transaminases, acid phosphatase, glucose-6-phosphatase, ATP citrate lyase, glucose-6-phosphate dehydrogenase and pyruvate kinase were assessed for the level of metabolic changes in the kidney of diabetic rats. Sapogenin extract or commercial diosgenin supplementation resulted in a significant decrease in lactase and maltase activities in all three regions of the intestine compared to the diabetic control group. However, the test diets significantly reduced intestinal sucrase activity in the proximal and mid regions. Test diets supplementation resulted in a significant decrease in the activities of the transaminases compared to the normal and diabetic control groups. The activity of glucose-6-phosphatase was significantly increased while the activities of ATP citrate lyase, pyruvate kinase and glucose-6-phosphate dehydrogenase were significantly reduced in the kidney of the diabetic control rats compared to the normal group. Test diets supplementation did not significantly alter glucose-6-phosphatase, ATP citrate lyase and pyruvate kinase activities compared to the diabetic control. However, there was a significant increase in glucose-6-phosphate dehydrogenase activity toward the normal group. In conclusion, the consumption of bitter yam sapogenin extract or commercial diosgenin demonstrated hypoglycemic properties, which are beneficial in diabetes by reducing intestinal disaccharidases activities; however, bitter yam sapogenin extract may adversely affect the integrity of kidney membrane.

Structure of the tetrameric form of human L-Xylulose reductase: Probing the inhibitor-binding site with molecular modeling and site-directed mutagenesis

L-Xylulose reductase (XR) is a member of the short-chain dehydrogenase/reductase (SDR) superfamily. In this study we report the structure of the biological tetramer of human XR in complex with NADP(+) and a competitive inhibitor solved at 2.3 A resolution. A single subunit of human XR is formed by a centrally positioned, seven-stranded, parallel beta-sheet surrounded on either side by two arrays of three alpha-helices. Two helices located away from the main body of the protein form the variable substrate-binding cleft, while the dinucleotide coenzyme-binding motif is formed by a classical Rossmann fold. The tetrameric structure of XR, which is held together via salt bridges formed by the guanidino group of Arg203 from one monomer and the carboxylate group of the C-terminal residue Cys244 from the neighboring monomer, explains the ability of human XR to prevent the cold inactivation seen in the rodent forms of the enzyme. The orientations of Arg203 and Cys244 are maintained by a network of hydrogen bonds and main-chain interactions of Gln137, Glu238, Phe241, and Trp242. These interactions are similar to those defining the quaternary structure of the closely related carbonyl reductase from mouse lung. Molecular modeling and site-directed mutagenesis identified the active site residues His146 and Trp191 as forming essential contacts with inhibitors of XR. These results could provide a structural basis in the design of potent and specific inhibitors for human XR.

Up-regulation of Human myo-Inositol Oxygenase by Hyperosmotic Stress in Renal Proximal Tubular Epithelial Cells

myo-Inositol oxygenase (MIOX) catalyzes the oxidative cleavage of myo-inositol (MI) to give d-glucuronic acid, a committed step in MI catabolism. d-Glucuronic acid is further metabolized to xylitol via the glucuronate-xylulose pathway. Although accumulation of polyols such as xylitol and sorbitol is associated with MI depletion in diabetic complications, no causal relationship has been established. Therefore we are examining the role of MIOX in diabetic nephropathy. Here we present evidence that the basis for the depletion of MI in diabetes is likely to be mediated by the increased expression of MIOX, which is induced by sorbitol, mannitol, and xylitol in a porcine renal proximal tubular epithelial cell line, LLC-PK1. To understand the molecular mechanism of regulation of MIOX expression by polyols, we have cloned the human MIOX gene locus of 10 kb containing 5.6 kb of the 5' upstream sequence. Analysis of the 5' upstream sequence led to the identification of an osmotic response element (ORE) in the promoter region, which is present approximately 2 kb upstream of the translation start site. Based on luciferase reporter and electrophoretic mobility shift assays, polyols increased the ORE-dependent expression of MIOX. In addition, we demonstrate that the activity of the promoter is dependent on the binding of the transcription factor, tonicity element-binding protein, or osmotic response element-binding protein, to the ORE site. These results suggest that the expression of MIOX is up-regulated by a positive feedback mechanism where xylitol, one of the products of MI catabolism via the glucuronate-xylulose pathway, induces an overexpression of MIOX.

Increased renal glucose metabolism in Type 1 diabetes mellitus

AIMS

In poorly controlled diabetes, increased renal glucose uptake has been implicated in the pathogenesis of diabetic nephropathy by promoting nonenzymatic glycosylation of proteins, activation of protein kinase C, and increased polyol pathway flux. However, whether glucose uptake by the diabetic kidney is actually increased, especially in patients with Type 1 diabetes, is unclear.

METHODS

To examine this question, we used a combination of net balance and isotopic techniques to compare renal glucose uptake in 12 subjects with Type 1 diabetes before and after restoration of near normoglycaemia by infusion of insulin with that in 15 postabsorptive nondiabetic volunteers.

RESULTS

Prior to insulin infusion, the diabetic subjects were markedly hyperglycaemic (arterial glucose 15.8 +/- 0.9 vs. 4.4 +/- 0.1 mm) and their renal tissue glucose uptake (i.e. total glucose disappearance across the kidney minus glycosuria) was increased more than 2 1/2-fold (388 +/- 43 vs. 148 +/- 12 micromol/min, P < 0.001). This was wholly explained by the mass action effects of hyperglycaemia since the diabetic subjects had normal renal blood flow (1575 +/- 82 vs. 1492 +/- 68 mL/min, P = 0.46) and reduced renal tissue glucose fractional extraction (1.7 +/- 0.2 vs. 2.3 +/- 0.1%, P = 0.027). Insulin infusion for three hours, which restored near normoglycaemia (arterial glucose 7.6 +/- 0.7 mm), reduced renal tissue glucose uptake toward normal (258 +/- 41 micromol/min, P = 0.006) without altering renal blood flow (1557 +/- 110, P = 0.63) or renal tissue glucose fractional extraction (2.1 +/- 0.3%, P = 0.35). Renal and hepatic glucose release, which had been increased (419 +/- 49 and 960 +/- 54 vs. 204 +/- 9 and 734 +/- 32 micromol/min, both P < 0.001), were suppressed by insulin to 138 +/- 22 and 520 +/- 53 micromol/min, respectively (both P < 0.001).

CONCLUSIONS

In poorly controlled Type 1 diabetes, renal glucose uptake is markedly increased, which provides a link between hyperglycaemia and biochemical processes implicated in the pathogenesis of diabetic nephropathy. Its reversal by restoration of near normoglycaemia with insulin may explain the benefit of intensive insulin therapy in preventing diabetic nephropathy.

Trigonella foenum graecum seed powder protects against histopathological abnormalities in tissues of diabetic rats

Trigonella foenum graecum is a well-known hypoglycemic agent used in traditional Indian medicines. It was previously reported that oral administration of its seed powder for 3 weeks to alloxan diabetic rats stabilized glucose homeostasis and free radical metabolism in liver and kidney. In the present study, we further investigated the effects of 3 weeks alloxan induced diabetes on the histological structure and function of liver and kidney and the protective effect of T. foenum graecum seed powder (TSP) oral administration to the diabetic rats utilizing enzyme analysis and light and transmission electron microscopy. The activity of the enzyme, glutamate dehydrogenase was significantly higher whereas the activity of D-beta-hydroxybutyrate dehydrogenase enzyme was significantly lower in liver and kidney of alloxan-induced diabetic rats. Histopathological studies showed liver degenerative and early nephropathic changes in diabetic rats. Ultrastructure of the diabetic liver revealed a reduction in the rough endoplasmic reticulum and swelling of mitochondria in the hepatocytes. TSP treatment to the diabetic rats effectively prevented the alteration in the activities of the two enzymes and partially prevented the structural abnormalities thus suggesting a protective effect of TSP on the liver and kidney of the diabetic rats. The role of TSP in reversing the diabetic state at the cellular level besides the metabolic normalization further proves its potential as an antidiabetic agent.

Structure-based discovery of human l-xylulose reductase inhibitors from database screening and molecular docking

Human L-xylulose reductase (XR) is an enzyme of the glucuronic acid/uronate cycle of glucose metabolism and is a possible target for treatment of the long-term complications of diabetes. In this study we utilised the molecular modelling program DOCK to analyse the 249,071 compounds of the National Cancer Institute Database and retrieved those compounds with high predicted affinity for XR. Several carboxylic acid-based compounds were tested and shown to inhibit XR. These included nicotinic acid (IC50=100 microM), benzoic acid (IC50=29 microM) and their derivatives. These results extend and improve upon the activities of known, commercially available inhibitors of XR such as the aliphatic fatty acid n-butyric acid (IC50=64 microM). To optimise the interaction between the inhibitor and the holoenzyme, the program GRID was used to design de novo compounds based on the inhibitor benzoic acid. The inclusion of a hydroxy-phenyl group and a phosphate to the benzoic acid molecule increased the net binding energy by 1.3- and 2.4-fold, respectively. The resultant compounds may produce inhibitors with improved specificity for XR.

Regulation of mesangial cell hexokinase activity and expression by heparin-binding epidermal growth factor-like growth factor: epidermal growth factors and phorbol esters increase glucose metabolism via a common mechanism involving classic mitogen-activated protein kinase pathway activation and induction of hexokinase II expression

Heparin-binding epidermal growth factor -like growth factor (HB-EGF) expression and hexokinase (HK) activity are increased in various pathologic renal conditions. Although the mitogenic properties of HB-EGF have been well characterized, its effects on glucose (Glc) metabolism have not. We therefore examined the possibility that HB-EGF might regulate HK activity and expression in glomerular mesangial cells, which constitute the principal renal cell type affected by a variety of pathologic conditions. Protein kinase C (PKC)-dependent classic mitogen-activated protein kinase (MAPK) pathway activation has been associated with increased HK activity in this cell type, so we also examined dependence upon these signaling intermediates. HB-EGF (> or =10 nm) increased total HK activity over 50% within 12-24 h, an effect mimicked by other EGF receptor agonists, but not by IGF-1 or elevated Glc. EGF receptor and classic MAPK pathway antagonists prevented this increase, as did general inhibitors of gene transcription and protein synthesis. Both HB-EGF and phorbol esters activated the classic MAPK pathway, albeit via PKC-independent and PKC-dependent mechanisms, respectively. Both stimuli were associated with increased HK activity, selectively increased HKII isoform expression, and increased Glc metabolism via both the glycolytic-tricarboxylic acid cycle route and the pentose phosphate pathway. HB-EGF thus constitutes a novel regulator of mesangial cell HK activity and Glc metabolism. HKII is the principal regulated isoform in these cells, as it is in insulin-sensitive peripheral tissues, such as muscle. However, the uniform requirement for classic MAPK pathway activation distinguishes HKII regulation in mesangial cells from that observed in muscle. These findings suggest a novel mechanism whereby growth factors may couple metabolism to glomerular injury.

Thrombin is a novel regulator of hexokinase activity in mesangial cells

BACKGROUND

Hexokinase (HK) activity is fundamentally important to cellular glucose uptake and metabolism. Phorbol esters increase both HK activity and glucose utilization in cultured mesangial cells via a protein kinase C (PKC)- and extracellular signal-regulated kinases 1 and 2 (ERK1/2)-dependent mechanism. In adult kidneys, increased HK activity has been reported in both glomerular injury and in diabetes, but the mechanisms responsible for these changes are unknown. Thrombin, a known activator of both PKC and ERK1/2, is increased in the settings of renal injury and diabetes. Thus, thrombin may contribute to the observed changes in HK activity in vivo.

METHODS

Thrombin and thrombin receptor agonists were tested for the ability to increase HK activity and glucose metabolism in murine mesangial (SV40 MES 13) cells. ERK1/2 activation was also evaluated in parallel. Thrombin inhibition (hirudins), PKC depletion, Ser-Thr kinase inhibition (H-7), MEK1/2 inhibition (PD98059), pertussis toxin (PTX), and general inhibitors of transcription or translation were then tested for the ability to attenuate these effects.

RESULTS

Thrombin (>/=0.01 U/mL) mimicked the effect of phorbol esters, increasing HK activity> 50% within 12 to 24 hours (P < 0.05). This effect was inhibited by hirudins, mimicked by thrombin receptor agonists, and accompanied by increased Glc utilization. H-7, PD98059, and general inhibitors of transcription or translation-but not PTX-prevented thrombin-induced HK activity at 24 hours. PKC depletion and PD98059 also blocked the associated phosphorylation and activation of ERK1/2.

CONCLUSIONS

Thrombin increases mesangial cell HK activity via a PTX-insensitive mechanism involving thrombin receptor activation, PKC-dependent activation of ERK1/2, and both ongoing gene transcription and de novo protein synthesis. As such, thrombin is a novel regulator of HK activity in mesangial cells and may play a role in coupling renal injury to metabolism.

Pyridine nucleotides in glucose metabolism and diabetes: a review

Nicotinamide adenine dinucleotide (NAD) and its derivatives NADH, NADP and NADPH have regulatory functions in the generation of triose phosphates and pyruvate from glucose. In many studies of the influence of the diabetic state on relationships between pyridine nucleotide and glucose metabolism, the focus has been on the sorbitol pathway. Less attention has been paid to other aspects of the role of pyridine nucleotides in pyruvate formation from glucose, in particular the effects of the NAD precursors nicotinamide and nicotinic acid on glucose metabolism. This paper reviews current knowledge of the involvement of pyridine nucleotides and their precursors in glucose catabolism in the normal and diabetic state. Reference is also made to the following three current hypotheses for mechanisms underlying diabetic microangiopathy: 1. Chronic glucose overutilization, caused by hyperglycemia, in tissues which lack insulin receptors and therefore are freely permeable to glucose. 2. Enhancement of sorbitol pathway activity with an ensuing decrease in the ratio of NAD/NADH. 3. Enhanced utilization of both glucose and pyridine nucleotides in formation of triose phosphates and pyruvate. Therapy with NAD precursors like nicotinamide might have corrective effects on these proposed biochemical aberrations, thereby retarding progression of microangiopathy.

Regulation of mesangial cell hexokinase activity by PKC and the classic MAPK pathway

Phorbol esters increase glucose (Glc) uptake and utilization in a variety of cell types, and, in some cells, these changes have been attributed to increased Glc phosphorylation and better functional coupling of hexokinases (HKs) to facilitative Glc transporters. Phorbol esters are potent mesangial cell mitogens, but their effects on HK-catalyzed Glc phosphorylation and metabolism are unknown. When examined in murine mesangial cells, active, but not inactive, phorbol esters increased HK activity in a time- and dose-dependent manner. Maximal induction of HK activity at 12-24 h was accompanied by parallel increases in both Glc utilization and lactate production and was blocked by the specific MEK1/2 inhibitor PD-98059 (IC(50) approximately 3 microM). This effect involved early activation of protein kinase C (PKC), MEK1/2, and ERK1/2, and the prolonged time course of subsequent HK induction was attributable, in part, to requirements for ongoing gene transcription and de novo protein synthesis. Mesangial cell HK activity thus exhibits novel regulatory behavior involving both PKC and classic MAPK pathway activation, suggesting specific mechanisms whereby PKC activation may influence Glc metabolism.

Abnormal renal and hepatic glucose metabolism in type 2 diabetes mellitus

Release of glucose by liver and kidney are both increased in diabetic animals. Although the overall release of glucose into the circulation is increased in humans with diabetes, excessive release of glucose by either their liver or kidney has not as yet been demonstrated. The present experiments were therefore undertaken to assess the relative contributions of hepatic and renal glucose release to the excessive glucose release found in type 2 diabetes. Using a combination of isotopic and balance techniques to determine total systemic glucose release and renal glucose release in postabsorptive type 2 diabetic subjects and age-weight-matched nondiabetic volunteers, their hepatic glucose release was then calculated as the difference between total systemic glucose release and renal glucose release. Renal glucose release was increased nearly 300% in diabetic subjects (321+/-36 vs. 125+/-15 micromol/min, P < 0.001). Hepatic glucose release was increased approximately 30% (P = 0.03), but increments in hepatic and renal glucose release were comparable (2.60+/-0.70 vs. 2.21+/-0.32, micromol.kg-1.min-1, respectively, P = 0.26). Renal glucose uptake was markedly increased in diabetic subjects (353+/-48 vs. 103+/-10 micromol/min, P < 0.001), resulting in net renal glucose uptake in the diabetic subjects (92+/-50 micromol/ min) versus a net output in the nondiabetic subjects (21+/-14 micromol/min, P = 0.043). Renal glucose uptake was inversely correlated with renal FFA uptake (r = -0.51, P < 0.01), which was reduced by approximately 60% in diabetic subjects (10. 9+/-2.7 vs. 27.0+/-3.3 micromol/min, P < 0.002). We conclude that in type 2 diabetes, both liver and kidney contribute to glucose overproduction and that renal glucose uptake is markedly increased. The latter may suppress renal FFA uptake via a glucose-fatty acid cycle and explain the accumulation of glycogen commonly found in the diabetic kidney.

Pyrimidine nucleotide synthesis in the rat kidney in early diabetes

Early renal hypertrophy of diabetes is associated with increases in the tissue content of RNA, DNA, and sugar nucleotides involved in the formation of carbohydrate-containing macromolecules. We have previously reported an increase in the activity of enzymes of the de novo and salvage pathways of purine synthesis in early diabetes; the present communication explores the changes in the pathways of pyrimidine synthesis. Measurements have been made of key enzymes of the de novo and salvage pathways at 3, 5, and 14 days after induction of diabetes with streptozotocin (STZ), phosphoribosyl pyrophosphate (PPRibP), and some purine and pyrimidine bases. Carbamoyl-phosphate synthetase II, the rate-limiting enzyme of the de novo route, did not increase in the first 5 days after STZ treatment, the period of most rapid renal growth; a significant rise was seen at 14 days (+38%). Dihydroorotate dehydrogenase, a mitochondrial enzyme, showed the most marked rise (+147%) at 14 days. The conversion of orotate to UMP, catalyzed by the enzymes of complex II, was increased at 3 days (+42%), a rise sustained to 14 days. The salvage route enzyme, uracil phosphoribosyltransferase (UPRTase), showed a pattern of change similar to complex II. The effect of the decreased concentration of PPRibP on the activities of CPSII, for which it is an allosteric activator, and on activities of OPRTase and UPRTase, for which it is an essential substrate, is discussed with respect to the relative Ka and Km values for PPRibP and the possibility of metabolite channeling.

Capillaries phosphorylate glucose in a concentration-dependent manner through a glucokinase-like enzyme: a study in the eel

Glucose phosphorylation was studied in a pure capillary preparation obtained from the rete mirabile of the eel swimbladder. In the 3000g supernatant of capillary homogenates, the glucose phosphorylating activity did not reach the maximum at low glucose concentration (1 mmole/liter), as it occurs in most tissues, but increased with the increase in glucose concentration and approached the maximum at very high (300 mmole/liter) glucose levels, with values (mean +/- SEM, n = 10) of 5.85 +/- 0.94 nmole.min-1.mg-1 protein and 19.97 +/- 1.89 at 1 and 300 mmole/liter glucose, respectively. The apparent Km value for glucose was about 50 mmole/liter, i.e., at supraphysiological glucose concentration, like the enzyme glucokinase, typically present in the liver but absent from most other tissues. This new enzyme did not phosphorylate fructose (similar to glucokinase from liver, which is rather specific for glucose) but was not inhibited by N-acetyl-glucosamine (in contrast to hepatic glucokinase). Thus, capillaries phosphorylate glucose in a concentration-dependent manner, which suggests that they are equipped with a glucokinase-like enzyme. This may explain the reported increase in glucose uptake during capillary exposure to high glucose concentrations and would suggest that the hyperglycemia of the diabetic state may be associated with increased glucose utilization, which may play a role in the development of microangiopathy.

Glucose transport and metabolism in rat renal proximal tubules: multicomponent effects of insulin

Glucose transport and metabolism, and the effect of insulin thereon, was studied using suspensions of rat renal tubules enriched in the proximal component. [U-14C]Glucose oxidation is a saturable process (Km 3.1 +/- 0.2 mM; Vmax 14 +/- 0.2 mumole 14CO2 formed/g tissue protein per h). Glucose oxidation and [14C]lactate formation from glucose are inhibited in part by phlorizin and phloretin: the data suggest that the rate-limiting entry of glucose into the cell metabolic pool occurs by both the Na-glucose cotransport system (at the brush border) and the equilibrating, phloretin-sensitive system (at the basal-lateral membrane). Raising external glucose from 5 to 30 mM markedly increases aerobic and anaerobic lactate formation. Gluconeogenesis from lactate is not affected by variations of glucose concentrations. 24 h after streptozotocin administration, aerobic lactate formation is enhanced, as is the uptake of methyl alpha-D-glucoside by the tubules, while anaerobic glycolysis is depressed. Streptozotocin treatment (ST) increases both the Km and Vmax of glucose oxidation; gluconeogenesis and lactate oxidation are not affected. The effect of streptozotocin treatment on lactate formation are abolished by 1 mU/ml insulin. Streptozotocin treatment increases tissue hexokinase activity, decreases glucose-6-phosphatase, but has no significant effect on fructose-1,6-diphosphatase; phosphoenolpyruvate carboxykinase and pyruvate dehydrogenase. The data demonstrate fast streptozotocin-induced changes in cellular enzymes of carbohydrate metabolism. The enhancing effect of streptozotocin on methyl alpha-glucoside uptake is transient: 8 days after administration of the agent, no significant difference from controls is found. It is concluded that under the given experimental conditions insulin enhances the equilibrating glucose entry by the phloretin-sensitive pathway at the basal-lateral membrane, and transiently inhibits the Na-glucose cotransport system.

Anabolic response of some tissues to diabetes

In contrast to liver, adipose tissue, and muscle, in which the diabetic state is associated with a "catabolic response," some tissues, typically the kidney and perhaps the intestinal mucosa and some vascular cell types, show an "anabolic response" to diabetes, with enhanced activity of the anabolic pathways and diminished activity of the catabolic ones. The kidney of alloxan or streptozotocin diabetic rats is hypertrophied, and shows enrichment in intracellular glycogen and abundant accumulation of glycoprotein material at the basement membrane level. Accordingly, protein synthesis and the enzymes of glucose utilization as well as those engaged in UDP sugar formation or in the hydroxylation and glycosylation processes (required for glycoprotein synthesis) show increased activity in the diabetic kidney, while the catabolic, lysosomal enzymes (cathepsin D and several glycosidases) are depressed. We observed a reduction of -24% in the activity of cathepsin D and -23% in that of galactosidase in the kidney of streptozotocin diabetic mice, as opposed to increases of +135 and +32%, respectively, found in liver. It is not known which factor(s) may be responsible for such an anabolic response of some tissues to diabetes, but persistent hyperglycemia and/or some hormonal abnormalities may be involved. The above data refer to changes in tissue enzyme content caused by induction-repression mechanisms, but rapid (activation-inhibition) effects may also occur. We observed that preincubation of slices of mouse kidney cortex for 10 min with 20.8 mmole/liter glucose resulted in a 80% activation of phosphofructokinase, as assayed in the tissue homogenate at physiological (50 mumole/liter) concentration of the substrate fructose-6-P, suggesting that hyperglycemia may be responsible for some of the metabolic changes occurring in the diabetic kidney.

Glucose metabolism in renal tubular function

Heterogeneity of metabolic activity along the nephron points to a very varied relationship between glucose metabolism and ion transport. Glycolysis is linked closely to free-water clearance and possibly to sodium, potassium, and hydrogen ion transport. Glucose oxidation, while not the major source of renal energy, is crucial in sodium, potassium, and phosphate reabsorption. Gluconeogenesis recovers carbon compounds generated during the process of renal ammoniagenesis. Glucose synthesis and active sodium transport appear to compete for renal ATP, although no regulatory function for this competition has been identified. Glucose formed in the proximal tubule may support free-water clearance in adjacent distal tubule, but is not thought to contribute to any medullary function. The complex network of biosynthetic and catabolic pathways of glucose metabolism may have evolved in the kidney to protect the organism against wide variations in glucose demand which would otherwise be unavoidable during the course of rapidly fluctuating renal electrolyte loads.

Changes in activity of enzymes related to glycolysis, gluconeogenesis and lipogenesis in placentae from diabetic women

The activity of enzymes with a regulatory function in the pathways of glycolysis, gluconeogenesis and NADP-generation was investigated in 50 placentae from normal pregnancies and deliveries, 23 placentae from women with gestational diabetes, and 12 placentae from insulin-dependent patients. In placentae from the gestational diabetic group, the activity of pyruvate kinase and of NADPH-generating enzymes was raised and the activity of enzymes connected to glucogenesis was unchanged. These alterations were attributed to the oversupply of glucose and insulin to morphologically normal and well-oxygenated placental tissue. In the placentae from the insulin-dependent group, the activity of pyruvate kinase was reduced, the activity of NADPH-generating enzymes was enhanced and the activity of those related to the gluconeogenesis was unchanged. It is suggested that this pattern of enzyme changes reflects a reduction in the glycolytic capacity in these placentae, which may be due to inhibition by products of enhanced fatty acid oxidation in diabetes, amino acids and/or by long-term anoxia as a result of uteroplacental circulatory disturbances. The possible relation of reduced energy-forming capacity of the placenta in diabetes to its transport function is discussed.

Effect of diabetes on the sugar nucleotides in several tissues of the rat

Studies have been carried out to determine the effect of diabetes on the concentrations of sugar nucleotides in several tissues of the rat. This represents one aspect of an investigation aimed at evaluating the role which alterations in the metabolism of glycoproteins or other glycoconjugates may play in the development of the long-term complications of this disease. Measurements were made of the nucleotides of hexoses, N-acetylhexosamines, hexuronic acids, and sialic acid in liver, kidney, skeletal muscle, testis and heart of alloxan-diabetic rats and age-matched controls. Of the intermediates studied, only UDP-glucose and UDP-galactose showed significant alterations in diabetes. The direction of the changes depended on the tissue, with the levels in liver and skeletal muscle being decreased, those in kidney and testis increased and the concentrations in heart being unchanged. In the diabetic liver, the concentrations of UDP-glucose was reduced to 0.75 that of normal, while in skeletal muscle both UDP-glucose and UDP-galactose were significantly decreased (diabetic to normal ratios of 0.67 and 0.64, respectively). Kidney and testis, on the other hand, showed elevations of both UDP-glucose and UDP-galactose, with the diabetic levels being 1.2 to 1.3 those of normal levels. The direction of change in UDP-glucose in a tissue appeared to reflect its known ability to synthesize glycogen in diabetes. The finding of elevated UDP-glucose and UDP-galactose concentrations in diabetic kidney is considered to be potentially of great importance to the increased synthesis of basement membrane collagen by this tissue.

Changes in metabolism of rat kidney and liver caused by experimental diabetes and by dietary sucrose

Two groups of rats were fed diets in which the carbohydrate components was either starch or sucrose. A third group was fed on a stock diet. Half of the animals in each group were made diabetic by injection of either streptozotocin, in two of the groups, or alloxan, in the third group. Both diabetes and sucrose-feeding increased renal gluconeogenesis as indicated by increased activities of fructose-1,6-diphosphatase and glucose-6-phosphatase. Sucrose-feeding increased fatty acid synthesis both in the liver and kidney. However, the effect of diabetes on fatty acid synthesis was different at the two tissue sites. Diabetes, whether induced by streptozotocin or alloxan, decreased fatty acid synthesis in the liver but increased the rate in the kidney. The latter response was obtained for each diet but was additive with the effect of sucrose. We conclude that the effect of diabetes on renal lipid metabolism may reflect, in part, the accelerated glucose flux. The response to both diabetes and sucrose-feeding is also possibly associated with the increased lipid required for the membrane synthesis reported previously.

Phosphoribosylpyrophosphate bioavailability in diabetic rat renal cortex in vivo

Experimental diabetes induces increased content of RNA and UTP in the renal cortex. Studies were designed to assess the bioavailability of 5-phosphoribosyl-1-pyrophosphate (PRPP) in the diabetic renal cortex because PRPP is an important determinant of the de novo synthesis of nucleotides. The tissue bioavailability of PRPP determines the effects of orotate or adenine administration on UTP, ATP, and GTP content and on the incorporation of labeled precursors into UTP and ATP. Diabetic and control rats with chronic intravenous cannulas were infused over 2.5-24 h with orotate or adenine. Orotate administration induced greater decreases in ATP and GTP and in labeled adenine incorporation into ATP concomitant with smaller increases in UTP in controls than in diabetic animals. Adenine administration induced a greater decrease of labeled orotate incorporation into UTP and a smaller increase in ATP in controls than in diabetic animals. Prolonging the adenine infusion resulted in disappearance of these differences. The results are compatible with greater initial bioavailability of PRPP in the diabetic renal cortex than in controls but with a rate of maximal PRPP generation that is the same in both tissues.