Chronic N-acetylcysteine prevents fructose-induced insulin resistance and hypertension in rats

The fructose-fed rat: a review on the mechanisms of fructose-induced insulin resistance and hypertension

The metabolic syndrome is an important public health concern that predisposes individuals to the development of cardiovascular disease and/or Type 2 diabetes. The fructose-fed rat is an animal model of acquired systolic hypertension that displays numerous features of the metabolic syndrome. This animal model is used to study the relationship between insulin resistance/compensatory hyperinsulinemia and the development of hypertension. Several mechanisms have been proposed to mediate the link between insulin resistance and hypertension. In this review, we have addressed the role of sympathetic nervous system overactivation, increased production of vasoconstrictors, such as endothelin-1 and angiotensin II, and prostanoids in the development of hypertension in fructose-fed rats. The roles of nitric oxide, impaired endothelium-dependent relaxation and sex hormones in the pathogenesis of the fructose-fed induced hypertensive rats have also been highlighted. More recently, increased formation of reactive oxygen species and elevated levels of uric acid have been reported to contribute to fructose-induced hypertension.

L-cysteine supplementation lowers blood glucose, glycated hemoglobin, CRP, MCP-1, and oxidative stress and inhibits NF-kappaB activation in the livers of Zucker diabetic rats

This study examined the hypothesis that l-cysteine supplementation can lower insulin resistance, glycemia, oxidative stress, and markers of vascular inflammation in type 2 diabetes using Zucker diabetic fatty (ZDF) rats as a model. Starting at the age of 6 weeks, ZDF rats were supplemented orally (daily gavage, 8 weeks) with saline placebo (D) or l-cysteine (LC; 1 mg/kg bw) and fed a high-calorie diet. Six-week-old rats without any supplementation were considered baseline (BL) rats. D rats showed elevated fasting blood glucose, glycated Hb, CRP, and MCP-1 compared with BL rats in which there was no onset of diabetes. LC supplementation significantly lowered blood levels of glucose (18%, p= 0.05), glycated Hb (8%, p= 0.02), CRP (23%, p= 0.02), MCP-1 (32%, p= 0.01), and insulin resistance (25%) compared with levels seen in saline-supplemented D rats. There was a decrease in plasma protein oxidation levels (p< 0.01); however, GSH levels were similar in LC and D groups. Although LC did not change blood hematocrit or levels of transaminases, it did lower alkaline phosphatase (29%, p= 0.01) levels in comparison to D. Western blotting analyses of liver showed increased activation of NF-kappaB and Akt (50% pNF-kappaB and 20% pAkt) in D compared with BL rats. LC supplementation inhibited these effects (17% pAkt, 18% pNF-kappaB). This is the first report showing that l-cysteine supplementation can lower glycemia and markers of vascular inflammation in diabetes apparently by preventing NF-kappaB activation in a diabetic animal model.

Chronic etanercept treatment prevents the development of hypertension in fructose-fed rats

The purpose of this study was to investigate the effect of chronic treatment with etanercept (a soluble recombinant fusion protein consisting of the extracellular ligand-binding domain of tumor necrosis factor receptor type 2) on the development of hypertension in fructose-fed rats (FFR). High fructose feeding and treatment with etanercept (0.3 mg/kg, three times per week) was initiated simultaneously in male Wistar rats. Systolic blood pressure, fasted plasma parameters, insulin sensitivity, vascular reactivity, plasma angiotensin II (Ang II), and norepinephrine were determined following 9 weeks of treatment. FFR exhibited insulin resistance, hyperinsulinemia, hypertriglyceridemia, endothelial dysfunction, and hypertension. Treatment with etanercept prevented the rise in blood pressure without affecting insulin levels, insulin sensitivity, triglycerides, or Ang II levels in FFR. Etanercept treatment improved acetylcholine-induced relaxation and normalized endothelial nitric oxide synthase expression in aortas from FFR. The results of this study suggest that treatment with etanercept prevented the development of hypertension by improving vascular function and restoring endothelial nitric oxide synthase expression in FFR.

Effects of N-acetylcysteine on alcohol abstinence and alcohol-induced adverse effects in rats

Alcoholism is rampant in modern society and some antioxidant compound could perhaps be useful to reduce the damage done by alcohol consumption and abstinence. The present study was undertaken to investigate the association of N-acetylcysteine (NAC) intake, alcoholism, and alcohol abstinence on lipid profile, in vivo low-density lipoprotein (LDL) oxidation, oxidative stress, and antioxidant status in serum and liver of rats. Initially, male Wistar 30 rats were divided into two groups: (C, N=6) given standard chow and water; (E, N=24) receiving standard chow and aqueous ethanol solution in semi-voluntary research. After 30 days of ethanol exposure, (E) group was divided into four subgroups (N=6/group): (E-E) continued drinking 30% ethanol solution; (E-NAC) drinking ethanol solution containing 2 g/L NAC; (AB) changed ethanol solution to water; (AB-NAC) changed ethanol to aqueous solution 2 g/L NAC. After 15 days of the E-group division, E-E rats had higher serum alanine transaminase, lower body weight, and surface area, despite higher energy intake than C. E-E rats had also lower feed efficiency, dyslipidemia with enhanced triacylglycerol, very low-density lipoprotein (VLDL), lipid hydroperoxide (LH) and in vivo oxidized-LDL (ox-LDL). AB, E-NAC, and AB-NAC rats ameliorated serum oxidative stress markers and normalized serum lipids. E-E rats had higher hepatic LH and lower reduced glutathione (GSH)/oxidized glutathione (GSSG) ratio than C, indicating hepatic oxidative stress. AB and E-NAC rats normalized hepatic LH, GSSG, and the GSH/GSSG ratio, compared to E-E. AB-NAC rats had the lowest serum ox-LDL, hepatic LH levels, and the highest GSH reductase activity in hepatic tissue. In conclusion, the present study brought new insights into alcohol consumption, because ethanol exposure enhanced serum in vivo ox-LDL, as well as serum and hepatic oxidative stress. N-acetylcysteine offers promising therapeutic value to inhibit ethanol-induced adverse effects. Ethanol withdrawal had beneficial effects on serum lipids, but was more effective when coupled with NAC supplementation. Ethanol abstinence and NAC intake interact synergistically, improving serum lipids and hepatic antioxidant defenses.

Positive and negative regulation of insulin signaling by reactive oxygen and nitrogen species

Regulated production of reactive oxygen species (ROS)/reactive nitrogen species (RNS) adequately balanced by antioxidant systems is a prerequisite for the participation of these active substances in physiological processes, including insulin action. Yet, increasing evidence implicates ROS and RNS as negative regulators of insulin signaling, rendering them putative mediators in the development of insulin resistance, a common endocrine abnormality that accompanies obesity and is a risk factor of type 2 diabetes. This review deals with this dual, seemingly contradictory, function of ROS and RNS in regulating insulin action: the major processes for ROS and RNS generation and detoxification are presented, and a critical review of the evidence that they participate in the positive and negative regulation of insulin action is provided. The cellular and molecular mechanisms by which ROS and RNS are thought to participate in normal insulin action and in the induction of insulin resistance are then described. Finally, we explore the potential usefulness and the challenges in modulating the oxidant-antioxidant balance as a potentially promising, but currently disappointing, means of improving insulin action in insulin resistance-associated conditions, leading causes of human morbidity and mortality of our era.

Fructose modifies the hormonal response and modulates lipid metabolism during aerobic exercise after glucose supplementation

The metabolic response when aerobic exercise is performed after the ingestion of glucose plus fructose is unclear. In the present study, we administered two beverages containing GluF (glucose+fructose) or Glu (glucose alone) in a randomized cross-over design to 20 healthy aerobically trained volunteers to compare the hormonal and lipid responses provoked during aerobic exercise and the recovery phase. After ingesting the beverages and a 15-min resting period, volunteers performed 30 min of moderate aerobic exercise. Urinary and blood samples were taken at baseline (t(-15)), during the exercise (t(0), t(15) and t(30)) and during the recovery phase (t(45), t(75) and t(105)). Plasma insulin concentrations were higher halfway through the exercise period and during acute recuperation (t(15) and t(75); P<0.05) following ingestion of GluF than after Glu alone, without any differences between the effects of either intervention on plasma glucose concentrations. Towards the end of the exercise period, urinary catecholamine concentrations were lower following GluF (t(45); P<0.05). Plasma triacylglycerol (triglyceride) concentrations were higher after the ingestion of GluF compared with Glu (t(15), t(30), t(45) and t(105); P<0.05). Furthermore, with GluF, we observed higher levels of lipoperoxides (t(15), t(30), t(45) and t(105); P<0.05) and oxidized LDL (low-density lipoprotein; t(30); P<0.05) compared with after the ingestion of Glu alone. In conclusion, hormonal and lipid alterations are provoked during aerobic exercise and recovery by the addition of a dose of fructose to the pre-exercise ingestion of glucose.

Hypothesis: could excessive fructose intake and uric acid cause type 2 diabetes?

We propose that excessive fructose intake (>50 g/d) may be one of the underlying etiologies of metabolic syndrome and type 2 diabetes. The primary sources of fructose are sugar (sucrose) and high fructose corn syrup. First, fructose intake correlates closely with the rate of diabetes worldwide. Second, unlike other sugars, the ingestion of excessive fructose induces features of metabolic syndrome in both laboratory animals and humans. Third, fructose appears to mediate the metabolic syndrome in part by raising uric acid, and there are now extensive experimental and clinical data supporting uric acid in the pathogenesis of metabolic syndrome. Fourth, environmental and genetic considerations provide a potential explanation of why certain groups might be more susceptible to developing diabetes. Finally, we discuss the counterarguments associated with the hypothesis and a potential explanation for these findings. If diabetes might result from excessive intake of fructose, then simple public health measures could have a major impact on improving the overall health of our populace.

Comparative effects of N-acetyl-l-cysteine and ramipril on arterial hypertension, insulin resistance, and oxidative stress in chronically glucose-fed rats

Beneficial effects of an antioxidant (N-acetyl-l-cysteine, NAC) and an angiotensin I-converting enzyme (ACE) inhibitor (ramipril) were assessed in a rat model of insulin resistance induced by 10% glucose feeding for 20 weeks. Treatments with NAC (2 g/kg per day) and ramipril (1 mg/kg per day) were initiated at 16 weeks in the drinking fluid. Systolic blood pressure, plasma levels of insulin and glucose, and insulin resistance were significantly higher in rats treated with glucose for 20 weeks. This was associated with a higher production of superoxide anion and NADPH oxidase activity in aorta and liver and with a marked reduction in protein expression of skeletal muscle insulin receptor substrate-1 (IRS-1) in the gastrocnemius muscle. NAC prevented all these alterations. Although ramipril also reversed high blood pressure, it had a lesser effect on insulin resistance (including IRS-1) and blocked superoxide anion production only in aorta. Ramipril, in contrast to NAC, did not reduce NADPH oxidase activity in aorta and liver or plasma levels of 4-hydroxynonenal and malondialdehyde. Results suggest that the inhibition of the oxidative stress in hypertensive and insulin-resistant states contributes to the therapeutic effects of NAC and ramipril. Whereas NAC exerts effective antioxidant activity in multiple tissues, ramipril appears to preferentially target the vasculature.

Renal antioxidant enzymes and glutathione redox status in leptin-induced hypertension

Previously, we have demonstrated that leptin increases blood pressure (BP) in the rats through two oxidative stress-dependent mechanisms: stimulation of extracellular signal-regulated kinases (ERK) by H(2)O(2) and scavenging of nitric oxide (NO) by superoxide (O(2-.)). Herein, we examined if renal glutathione system and antioxidant enzymes determine the mechanism of prohypertensive effect of leptin. Leptin administered at 0.5 mg/kg/day for 4 or 8 days increased BP and renal Na(+),K(+)-ATPase activity and reduced fractional sodium excretion; these effects were prevented by NADPH oxidase inhibitor, apocynin. Superoxide scavenger, tempol, abolished the effect of leptin on BP and renal Na(+) pump in rats receiving leptin for 8 days, whereas ERK inhibitor, PD98059, was effective in animals treated with leptin for 4 days. Leptin administered for 4 days decreased glutathione (GSH) and increased glutathione disulfide (GSSG) in the kidney. In animals receiving leptin for 8 days GSH returned to normal level, which was accompanied by up-regulation of gamma-glutamylcysteine synthetase (gamma-GCS), a rate-limiting enzyme of the GSH biosynthetic pathway. In addition, superoxide dismutase (SOD) activity was decreased, whereas glutathione peroxidase (GPx) was increased in rats receiving leptin for 8 days. Cotreatment with gamma-GCS inhibitor, buthionine sulfoximine (BSO), accelerated, whereas GSH precursor, N-acetylcysteine (NAC), attenuated leptin-induced changes in gamma-GCS, SOD, and GPx. In addition, coadministration of BSO changed the mechanism of BP elevation from H(2)O(2)-ERK to (O(2-.))-NO dependent in animals receiving leptin for 4 days, whereas NAC had the opposite effect in rats treated with leptin for 8 days. These results suggest that initial change in GSH redox status induces decrease in SOD/GPx ratio, which results in greater amount of (O)2-.)) versus H(2)O(2) in later phase of leptin treatment, thus shifting the mechanism of BP elevation from H(2)O(2)-ERK to (O(2-.))-NO dependent.

Blockade of sensory abnormalities and kinin B(1) receptor expression by N-acetyl-L-cysteine and ramipril in a rat model of insulin resistance

Glucose-fed rat is a model of insulin resistance that displays sensory polyneuropathy and hypertension. This study aimed at comparing the beneficial effects of N-acetyl-L-cysteine (NAC, antioxidant) and ramipril (angiotensin-1 converting enzyme inhibitor) on tactile and cold allodynia induced by chronic glucose feeding. Impact of these treatments was also assessed on hypertension, plasma glucose and insulin concentrations, insulin resistance and kinin B(1) receptor expression. Male Wistar rats (50-75 g) were given 10% d-glucose in their drinking water for 11 weeks or tap water (controls). Glucose-fed rats were treated either with NAC (1 g/kg/day, gavage), ramipril (1 mg/kg/day in drinking water) or no drug during the last 5 weeks. Glucose feeding for 6 weeks induced a significant increase in systolic blood pressure and hyperglycaemia which was accompanied by tactile and cold allodynia. At 11 weeks, plasma insulin, insulin resistance (HOMA index), kinin B(1) receptor mRNA in spinal cord and renal cortex and B(1) receptor binding sites in spinal cord were enhanced in glucose-fed rats. NAC and ramipril caused a progressive to complete inhibition of tactile and cold allodynia from 6 to 11 weeks. High systolic blood pressure, hyperinsulinemia, insulin resistance and kinin B(1) receptor expression were also normalized or attenuated in glucose-fed rats by either treatment. Results suggest that chronic treatment with an antioxidant or an ACE inhibitor provides similar beneficial effects on sensory polyneuropathy, hypertension and insulin resistance in glucose-fed rats. Both therapies were associated with a reduction of the expression of the pro-nociceptive kinin B(1) receptor.

Dietary fructose and the metabolic syndrome

PURPOSE OF REVIEW

Fructose, a naturally found sugar in many fruits, is now commonly used as an industrial sweetener and is excessively consumed in Western diets. High fructose intake is increasingly recognized as causative in development of prediabetes and metabolic syndrome. The mechanisms underlying fructose-induced metabolic disturbances are unclear but are beginning to be unravelled. This review presents recent findings in this field and an overall mechanistic insight into the metabolic effects of dietary fructose and its role in metabolic syndrome.

RECENT FINDINGS

Recent animal studies have confirmed the link between fructose feeding and increased plasma uric acid, a potentially causative factor in metabolic syndrome. Advanced glycation end products are also implicated because of their direct protein modifications and indirect effects on inflammation and oxidative stress. Human studies have demonstrated fructose's ability to change metabolic hormonal response, possibly contributing to decreased satiety.

SUMMARY

There is much evidence from both animal models and human studies supporting the notion that fructose is a highly lipogenic nutrient that, when consumed in high quantities, contributes to tissue insulin insensitivity, metabolic defects, and the development of a prediabetic state. Recently evidence has helped to decipher the mechanisms involved in these metabolic changes.

Effect of L-carnitine on skeletal muscle lipids and oxidative stress in rats fed high-fructose diet

There is evidence that high-fructose diet induces insulin resistance, alterations in lipid metabolism, and oxidative stress in rat tissues. The purpose of this study was to evaluate the effect of L-carnitine (CAR) on lipid accumulation and peroxidative damage in skeletal muscle of rats fed high-fructose diet. Fructose-fed animals (60 g/100 g diet) displayed decreased glucose/insulin (G/I) ratio and insulin sensitivity index (ISI_0,120) indicating the development of insulin resistance. Rats showed alterations in the levels of triglycerides, free fatty acids, cholesterol, and phospholipids in skeletal muscle. The condition was associated with oxidative stress as evidenced by the accumulation of lipid peroxidation products, protein carbonyls, and aldehydes along with depletion of both enzymic and nonenzymic antioxidants. Simultaneous intraperitoneal administration of CAR (300 mg/kg/day) to fructose-fed rats alleviated the effects of fructose. These rats showed near-normal levels of the parameters studied. The effects of CAR in this model suggest that CAR supplementation may have some benefits in patients suffering from insulin resistance.

Involvement of kinin B1 receptor and oxidative stress in sensory abnormalities and arterial hypertension in an experimental rat model of insulin resistance

Diabetes Mellitus leads to pain neuropathy and cardiovascular complications which remain resistant to current therapies involving the control of glycaemia. This study aims at defining the contribution of kinin B(1) receptor (B(1)R) and the oxidative stress on sensory abnormalities and arterial hypertension in a rat model of insulin resistance. Rats were fed with 10% d-glucose for a chronic period of 12-14 weeks and the impact of a diet supplemented with alpha-lipoic acid, a potent antioxidant, was determined on tactile and cold allodynia, arterial hypertension and the expression of kinin B(1)R (real-time PCR and autoradiography) in several tissues. Acute effects of brain penetrant (LF22-0542) and peripherally acting (R-715) B(1)R antagonists were also assessed. Glucose-fed rats exhibited tactile and cold allodynia along with increases in systolic blood pressure between 4 and 12 weeks; these alterations were alleviated by alpha-lipoic acid. The latter regimen also decreased significantly increased plasma levels of insulin and glucose and insulin resistance (HOMA index) at 14 weeks. B(1)R mRNA was virtually absent in liver, aorta, lung, kidney and spinal cord isolated from control rats, yet B(1)R mRNA was markedly increased in all tissues in glucose-fed rats. Up-regulated B(1)R mRNA and B(1)R binding sites (spinal cord) were significantly reduced by alpha-lipoic acid in glucose-fed rats. LF22-0542 reduced tactile and cold allodynia (3h) and reversed arterial hypertension (3-48h) in glucose-fed rats. R-715 abolished tactile and cold allodynia but had not effect on blood pressure. Data suggest that the oxidative stress contributes to the induction and up-regulation of B(1)R in the model of insulin resistance induced by glucose feeding. The over expressed B(1)R contributes centrally to arterial hypertension and in the periphery to sensory abnormalities.

Is the fructose index more relevant with regards to cardiovascular disease than the glycemic index?

The glycemic index (G.I.) is a means for categorizing carbohydrates based on their ability to raise blood glucose, subsequently this index has been popularized as a way for selecting foods to reduce the risk for obesity, diabetes, and cardiovascular disease. We suggest that the G.I. is better aimed at identifying foods that stimulate insulin secretion rather than foods that stimulate insulin resistance. In this regard, fructose has a low G.I. but may be causally linked with the obesity and cardiovascular disease epidemic. The reported association of high G.I. with cardiovascular disease may be due to the association of sugar intake which contains fructose, but which has a high G.I. due to its glucose content. We propose the use of a fructose index to categorize foods and propose studies to determine the effect of low fructose diets as a means to prevent obesity, diabetes, and cardiovascular disease in the population.

Effect of Chronic N-Acetyl Cysteine Administration on Oxidative Status in the Presence and Absence of Induced Oxidative Stress in Rat Striatum

Antioxidants have possible therapeutic value in neurodegenerative disorders, although they may have pro-oxidant effects under certain conditions. Glutathione (GSH) is a key free radical scavenger. N-acetylcysteine (NAC) bolsters GSH and intracellular cysteine and also has effective free radical scavenger properties. The effects of chronic NAC administration (50 mg/kg/day, 500 mg/kg/day, 1500 mg/kg/day x 21 days) on cellular markers of oxidative status was studied in striatum of healthy male Sprague-Dawley rats as well as in animals with apparent striatal oxidative stress following chronic haloperidol treatment (1.5 mg/kg/day x 3 weeks). In non-haloperidol treated animals, NAC 50 and 500 mg/kg did not affect oxidative status, although NAC 1,500 mg/kg significantly increased striatal superoxide levels, decreased lipid peroxidation and increased consumption of reduced glutathione (GSH). Haloperidol alone evoked a significant increase in superoxide and lipid peroxidation. All NAC doses blocked haloperidol induced increases in superoxide levels, while NAC 500 mg/kg and 1,500 mg/kg prevented haloperidol-associated lipid peroxidation levels and also increased the GSSG/GSH ratio. NAC may protect against conditions of striatal oxidative stress, although possible pro-oxidative actions at high doses in otherwise healthy individuals, e.g. to offset worsening of neurodegenerative illness, should be viewed with caution.

The kinin B1 receptor antagonist SSR240612 reverses tactile and cold allodynia in an experimental rat model of insulin resistance

BACKGROUND AND PURPOSE

Diabetes causes sensory polyneuropathy with associated pain in the form of tactile allodynia and thermal hyperalgesia which are often intractable and resistant to current therapy. This study tested the beneficial effects of the non-peptide and orally active kinin B(1) receptor antagonist SSR240612 against tactile and cold allodynia in a rat model of insulin resistance.

EXPERIMENTAL APPROACH

Rats were fed with 10% D-glucose for 12 weeks and effects of orally administered SSR240612 (0.3-30 mg kg(-1)) were determined on the development of tactile and cold allodynia. Possible interference of SSR240612 with vascular oxidative stress and pancreatic function was also addressed.

KEY RESULTS

Glucose-fed rats exhibited tactile and cold allodynia, increases in systolic blood pressure and higher plasma levels of insulin and glucose, at 12 weeks. SSR240612 blocked tactile and cold allodynia at 3 h (ID(50)=5.5 and 7.1 mg kg(-1), respectively) in glucose-fed rats but had no effect in control rats. The antagonist (10 mg kg(-1)) had no effect on plasma glucose and insulin, insulin resistance (HOMA index) and aortic superoxide anion production in glucose-fed rats.

CONCLUSIONS AND IMPLICATIONS

We provide the first evidence that the B(1) receptors are involved in allodynia in an experimental rat model of insulin resistance. Allodynia was alleviated by SSR240612 most likely through a direct inhibition of B(1) receptors affecting spinal cord and/or sensory nerve excitation. Thus, orally active non-peptide B(1) receptor antagonists should have clinical therapeutic potential in the treatment of sensory polyneuropathy.

Crocetin attenuates palmitate-induced insulin insensitivity and disordered tumor necrosis factor-alpha and adiponectin expression in rat adipocytes

BACKGROUND AND PURPOSE

A number of studies have implicated adipocyte-derived factors in the development of insulin resistance. Intracellular redox status has been reported to play a significant role in the modulation of insulin action. This study was designed to investigate the potential of crocetin, a potent antioxidant, to protect adipocytes against the induction of insulin insensitivity and disordered expression of tumor necrosis factor (TNF)-alpha and adiponectin in vitro.

EXPERIMENTAL APPROACH

We used palmitate to induce insulin resistance in freshly isolated rat adipocytes, and observed the effect of crocetin, N-acetylcysteine, diphenyleneiodonium, rotenone and oxypurinol. Insulin sensitivity was measured using 2-deoxy-D-[1-(3)H]-glucose uptake assay. Levels of glucose transporter 4, TNF-alpha and adiponectin were evaluated by immunoblot analysis, and levels of mRNA for TNF-alpha and adiponectin by reverse transcription-polymerase chain reaction analysis. Intracellular production of reactive oxygen species (ROS) was determined spectrofluorometrically using 2',7'-dichlorofluorescin diacetate.

KEY RESULTS

Palmitate induced a 45% decrease in insulin-stimulated glucose uptake in adipocytes. The mRNA and protein expression of TNF-alpha were enhanced by 64% and 59% respectively whereas the mRNA and protein expression of adiponectin were reduced by 43% and 36% respectively by palmitate treatment. These changes were accompanied by a 54% increase in intracellular ROS levels. Crocetin, N-acetylcysteine and diphenyleneiodonium were found to attenuate these abnormalities.

CONCLUSIONS AND IMPLICATIONS

Crocetin blocked the impaired insulin-stimulated glucose uptake and disordered TNF-alpha and adiponectin expression induced by palmitate in rat adipocytes. Inactivation of NADPH oxidase may account for these observations.

Fructose-mediated stress signaling in the liver: implications for hepatic insulin resistance

Organisms reprogram metabolic pathways to adapt to changes in nutrient availability. This requires that nutrient-based stimuli are sensed, signals are transmitted, and highly specific responses are engaged. We propose that in the liver, the mitogen-activated protein kinase, c-jun N-terminal kinase (JNK), links excessive nutrient metabolism with impaired insulin regulation of glucose production. The liver, by virtue of its anatomic position and selective regulatory features, buffers and is highly responsive to changes in nutrient delivery. In particular, sugars such as sucrose and fructose uniquely regulate and are selectively metabolized by the liver. We propose that when hepatic fructose uptake exceeds requirements for glycogen and energy (hepatic sugar excess), the JNK-signaling pathway is engaged as part of the adaptive response.

Metabolic effects of fructose

PURPOSE OF REVIEW

Fructose is consumed in significant amounts in Western diets. An increase in fructose consumption over the past 10-20 years has been linked with a rise in obesity and metabolic disorders. Fructose/sucrose produces deleterious metabolic effects in animal models. This raises concern regarding the short-term and long-term effects of fructose and its risk in humans.

RECENT FINDINGS

In rodents, fructose stimulates lipogenesis and leads to hepatic and extrahepatic insulin resistance, dyslipidaemia and high blood pressure. Insulin resistance appears to be related to ectopic lipid deposition. In humans, short-term fructose feeding increases de-novo lipogenesis and blood triglycerides and causes hepatic insulin resistance. There is presently no evidence for fructose-induced muscle insulin resistance in humans. The cellular mechanisms underlying the metabolic effects of fructose involve production of reactive oxygen species, activation of cellular stress pathways and possibly an increase in uric acid synthesis.

SUMMARY

Consuming large amounts of fructose can lead to the development of a complete metabolic syndrome in rodents. In humans, fructose consumed in moderate to high quantities in the diet increases plasma triglycerides and alters hepatic glucose homeostasis, but does not appear to cause muscle insulin resistance or high blood pressure in the short term. Further human studies are required to delineate the effects of fructose in humans.