Metabolic effects of troglitazone on fructose-induced insulin resistance in the rat

Fasting serum fructose is associated with risk of gestational diabetes mellitus

Objective

To investigate the association of fasting serum fructose concentrations and the incidence of GDM.

Research design and methods

Five hundred twenty six pregnant women who attended the obstetric clinic of Xinhua Hospital, Chongming Branch were recruited prospectively from September 2019 to November 2020. Fasting serum fructose concentrations were measured by a validated liquid chromatography–tandem mass spectrometry method. GDM was diagnosed according to the criteria of the IADPSG. Independent sample t-test was used to compare the differences between groups. Multiple stepwise regression analysis was used to estimate the associations of serum fructose and other variables. Multivariate logistic regression models were adopted to evaluate the odds ratios (ORs) for GDM.

Results

Of the 526 pregnant women, 110 were diagnosed with GDM. Fasting fructose concentrations were increased significantly in GDM patients compared to those without GDM (1.30 ug/ml vs 1.16 ug/ml, p<0.001). Fasting fructose concentration was independently associated with GDM after adjusting the potential confounders, 1 ug/ml increase in fasting serum fructose level was associated with an 81.1% increased risk of GDM (1.811, [1.155-2.840]). Taking fructose <1.036 ug/ml as the reference, the OR for GDM was significantly higher in fructose ≥1.036 ug/ml group (OR, 1.669; 95% CI, 1.031–2.701) after all the potential confounders were adjusted.

Conclusions

Increased fasting serum fructose levels were independently associated with the incidence of GDM.

Triglyceride glucose index for predicting cardiovascular outcomes in patients with coronary artery disease

Background

Triglyceride glucose (TyG) index is a novel marker for metabolic disorders and recently it has been reported to be associated with cardiovascular disease (CVD) risk in apparently healthy individuals. However, the prognostic value of TyG index in patients with stable coronary artery disease (CAD) is not determined.

Methods

We conducted a nested case-control study among 3,745 patients with stable CAD. Patients were followed up for 11,235 person-years. The cardiovascular events (CVEs) were defined as all-cause death, non-fatal myocardial infarction (MI), stroke and post-discharge revascularization [percutaneous coronary intervention (PCI) coronary artery bypass grafting (CABG)]. In total, 290 (7.7%) patients with CVEs and 1,450 controls were matched according to age, gender, previous history of PCI or CABG and the duration of follow-up. TyG index was calculated as formula: ln[fasting triglycerides (mg/dL) × fasting plasma glucose (mg/dL)/2].

Results

Multivariable Cox proportional hazards models revealed that TyG index was positively associated with CVEs risk (hazard ratio: 1.364, 95% confidence interval: 1.100-1.691, P=0.005). The Kaplan-Meier analysis indicated that patients within the highest quartile of TyG index presented the lowest event-free survival (P=0.029). Moreover, a 1-standard deviation (SD) increment in TyG index was associated with 23.2% [hazard ratio (HR): 1.232, 95% confidence interval (95% CI): 1.084-1.401] higher risk of CVEs, which was superior to other triglyceride or glycemic related markers.

Conclusions

The present study, firstly, showed that TyG index was positively associated with future CVEs, suggesting that TyG may be a useful marker for predicting clinical outcomes in patients with CAD.

Nutritional and Lipid Modulation of PCSK9: Effects on Cardiometabolic Risk Factors

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a serine protease involved in the regulation of LDL receptor (LDLR) expression and apolipoprotein B lipoprotein cholesterol metabolism. Hepatic PCSK9 protein expression, activity, and secretion have been shown to affect cholesterol homeostasis. An upregulation of hepatic PSCK9 protein leads to increased LDLR degradation, resulting in decreased uptake of apoB lipoproteins and a consequent increase in the plasma concentration of these lipoproteins, including LDL and chylomicron remnants. Hence, PCSK9 has become a novel target for lipid-lowering therapies. The aim of this review is to outline current findings on the metabolic and dietary regulation of PCSK9 and effects on cholesterol, apoB lipoprotein metabolism, and cardiovascular disease (CVD) risk. PCSK9 gene and protein expression have been shown to be regulated by metabolic status and the diurnal pattern. In the fasting state, plasma PCSK9 is reduced via modulation of the nuclear transcriptional factors, including sterol regulatory element-binding protein (SREBP) 1c, SREBP2, and hepatocyte nuclear factor 1α. Plasma PCSK9 concentrations are also known to be positively associated with plasma insulin and homeostasis model assessment of insulin resistance, and appear to be regulated by SREBP1c independently of glucose status. Plasma PCSK9 concentrations are stable in response to high-fat or high-protein diets in healthy individuals; however, this response may differ in altered metabolic conditions. Dietary n-3 polyunsaturated fatty acids have been shown to reduce plasma PCSK9 concentration and hepatic PCSK9 mRNA expression, consistent with their lipid-lowering effects, whereas dietary fructose appears to upregulate PCSK9 mRNA expression and plasma PCSK9 concentrations. Further studies are needed to elucidate the mechanisms of how dietary components regulate PCSK9 and effects on cholesterol and apoB lipoprotein metabolism, as well as to delineate the clinical impact of diet on PCSK9 in terms of CVD risk.

Mechanisms by which the thiazolidinedione troglitazone protects against sucrose-induced hepatic fat accumulation and hyperinsulinaemia

BACKGROUND AND PURPOSE

Thiazolidinediones (TZD) are known to ameliorate fatty liver in type 2 diabetes. To date, the underlying mechanisms of their hepatic actions remain unclear.

EXPERIMENTAL APPROACH

Hepatic triglyceride content and export rates were assessed in 2 week high-sucrose-fed Wistar rats treated with troglitazone and compared with untreated high-sucrose rodent controls. Fractional de novo lipogenesis (DNL) contributions to hepatic triglyceride were quantified by analysis of triglyceride enrichment from deuterated water. Hepatic insulin clearance and NO status during a meal tolerance test were also evaluated.

KEY RESULTS

TZD significantly reduced hepatic triglyceride (P < 0.01) by 48%, decreased DNL contribution to hepatic triglyceride (P < 0.01) and increased postprandial non-esterified fatty acids clearance rates (P < 0.01) in comparison with the high-sucrose rodent control group. During a meal tolerance test, plasma insulin AUC was significantly lower (P < 0.01), while blood glucose and plasma C-peptide levels were not different. Insulin clearance was increased (P < 0.001) by 24% and was associated with a 22% augmentation of hepatic insulin-degrading enzyme activity (P < 0.05). Finally, hepatic NO was decreased by 24% (P < 0.05).

CONCLUSIONS

Overall, TZD show direct actions on liver by reducing hepatic DNL and increasing hepatic insulin clearance. The alterations in hepatic insulin clearance were associated with changes in insulin-degrading enzyme activity, with possible modulation of NO levels.

Added fructose: a principal driver of type 2 diabetes mellitus and its consequences

Data from animal experiments and human studies implicate added sugars (eg, sucrose and high-fructose corn syrup) in the development of diabetes mellitus and related metabolic derangements that raise cardiovascular (CV) risk. Added fructose in particular (eg, as a constituent of added sucrose or as the main component of high-fructose sweeteners) may pose the greatest problem for incident diabetes, diabetes-related metabolic abnormalities, and CV risk. Conversely, whole foods that contain fructose (eg, fruits and vegetables) pose no problem for health and are likely protective against diabetes and adverse CV outcomes. Several dietary guidelines appropriately recommend consuming whole foods over foods with added sugars, but some (eg, recommendations from the American Diabetes Association) do not recommend restricting fructose-containing added sugars to any specific level. Other guidelines (such as from the Institute of Medicine) allow up to 25% of calories as fructose-containing added sugars. Intake of added fructose at such high levels would undoubtedly worsen rates of diabetes and its complications. There is no need for added fructose or any added sugars in the diet; reducing intake to 5% of total calories (the level now suggested by the World Health Organization) has been shown to improve glucose tolerance in humans and decrease the prevalence of diabetes and the metabolic derangements that often precede and accompany it. Reducing the intake of added sugars could translate to reduced diabetes-related morbidity and premature mortality for populations.

4-Hydroxyisoleucine improves insulin resistance by promoting mitochondrial biogenesis and act through AMPK and Akt dependent pathway

4-Hydroxyisoleucine (4-HIL) is an unusual amino acid isolated from fenugreek seeds (Trigonella foenum graecum L). Various studies have shown that it acts as an antidiabetic agent yet its mechanism of action is not clear. We therefore investigated the effect 4-HIL on the high fructose diet fed streptozotocin induced diabetic rats and L6 myotubes. 4-HIL (50 mg/kg) has improved blood lipid profile, glucose tolerance and insulin sensitivity in a diabetic rat model. It has increased the glucose uptake in L6 myotubes in AMPK-dependent manner and upregulated the expression of genes (PGC-1α, PGC-1β, CPT 1 and CPT 2), which have role in mitochondrial biogenesis and energy metabolism in the liver, skeletal muscles as well as in L6 myotubes. Interestingly, it also increased the AMPK and Akt expression along with their phosphorylated forms in the liver and muscle tissues of treated animals. Altogether we concluded that 4-HIL acts to improve insulin resistance by promoting mitochondrial biogenesis in high fructose diet fed STZ induced diabetic rats.

Dipeptidyl peptidase IV inhibition upregulates GLUT4 translocation and expression in heart and skeletal muscle of spontaneously hypertensive rats

The purpose of the current study was to test the hypothesis that the dipeptidyl peptidase IV (DPPIV) inhibitor sitagliptin, which exerts anti-hyperglycemic and anti-hypertensive effects, upregulates GLUT4 translocation, protein levels, and/or mRNA expression in heart and skeletal muscle of spontaneously hypertensive rats (SHRs). Ten days of treatment with sitagliptin (40 mg/kg twice daily) decreased plasma DPPIV activity in both young (Y, 5-week-old) and adult (A, 20-week-old) SHRs to similar extents (~85%). However, DPPIV inhibition only lowered blood pressure in Y-SHRs (119 ± 3 vs. 136 ± 4 mmHg). GLUT4 translocation, total protein levels and mRNA expression were decreased in the heart, soleus and gastrocnemius muscle of SHRs compared to age-matched Wistar Kyoto (WKY) normotensive rats. These differences were much more pronounced between A-SHRs and A-WKY rats than between Y-SHRs and Y-WKY rats. In Y-SHRs, sitagliptin normalized GLUT4 expression in the heart, soleus and gastrocnemius. In A-SHRs, sitagliptin increased GLUT4 expression to levels that were even higher than those of A-WKY rats. Sitagliptin enhanced the circulating levels of the DPPIV substrate glucagon-like peptide-1 (GLP-1) in SHRs. In addition, stimulation of the GLP-1 receptor in cardiomyocytes isolated from SHRs increased the protein level of GLUT4 by 154 ± 13%. Collectively, these results indicate that DPPIV inhibition upregulates GLUT4 in heart and skeletal muscle of SHRs. The underlying mechanism of sitagliptin-induced upregulation of GLUT4 in SHRs may be, at least partially, attributed to GLP-1.

Antihyperglycemic, hypolipidemic and antioxidant activities of ethanolic extract of Commiphora mukul gum resin in fructose-fed male Wistar rats

High fructose feeding (66 % of fructose) induces type-2 diabetes in rats, which is associated with the insulin resistance, hyperinsulinemia, hypertriglyceridemia and oxidative stress. The present study was undertaken to evaluate the effect of ethanol extract of Commiphora mukul gum resin (CMEE) on blood glucose, plasma insulin, lipid profiles, reduced glutathione, lipid peroxidation, protein oxidation and enzymatic antioxidants like superoxide dismutase, catalase, glutathione reductase, glutathione peroxidase, glutathione-S-transferase in fructose-induced type-2 diabetic rats. A significant gain in body weight, hyperglycemia, hyperinsulinemia, increased lipid profiles, lipid peroxidation, protein oxidation and decreased reduced glutathione, activities of enzymatic antioxidants and insulin sensitivity (increased homeostasis assessment assay) were observed in high-fructose-induced diabetic rats. The administration of CMEE (200 mg/kg/day) daily for 60 days in high-fructose-induced diabetic rats reversed the above parameters significantly. CMEE has the ability to improve insulin sensitivity and delay the development of insulin resistance, aggravate antioxidant status in diabetic rats and may be used as an adjuvant therapy for patients with insulin resistance.

Inhibition of matrix metalloproteinase-2 improves endothelial function and prevents hypertension in insulin-resistant rats

BACKGROUND AND PURPOSE

Insulin resistance is often found to be associated with high blood pressure. We propose that in insulin-resistant hypertension, endothelial dysfunction is the consequence of increased activity of vascular MMP-2. As MMP-2 proteolytically cleaves a number of extracellular matrix proteins, we hypothesized that MMP-2 impairs endothelial function by proteolytic degradation of endothelial NOS (eNOS) or its cofactor, heat shock protein 90 (HSP90).

EXPERIMENTAL APPROACH

We tested our hypothesis in bovine coronary artery endothelial cells and fructose-fed hypertensive rats (FHR), a model of acquired systolic hypertension and insulin resistance.

KEY RESULTS

Treatment of FHRs with the MMP inhibitor doxycycline, preserved endothelial function as well as prevented the development of hypertension, suggesting that MMPs impair endothelial function. Furthermore, incubating endothelial cells in vitro with a recombinant MMP-2 decreased NO production in a dose-dependent manner. Using substrate cleavage assays and immunofluorescence microscopy studies, we found that MMP-2 not only cleaves and degrades HSP90, an eNOS cofactor but also co-localizes with both eNOS and HSP90 in endothelial cells, suggesting that MMPs functionally interact with the eNOS system. Treatment of FHRs with doxycycline attenuated the decrease in eNOS and HSP90 expression but did not improve insulin sensitivity.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that increased activity of MMP-2 in FHRs impairs endothelial function and promotes hypertension. Inhibition of MMP-2 could be a potential therapeutic strategy for the management of hypertension.

Fructose diet treatment in mice induces fundamental disturbance of cardiomyocyte Ca2+ handling and myofilament responsiveness

High fructose intake has been linked to insulin resistance and cardiac pathology. Dietary fructose-induced myocardial signaling and morphological alterations have been described, but whether cardiomyocyte function is influenced by chronic high fructose intake is yet to be elucidated. The goal of this study was to evaluate the cardiomyocyte excitation-contraction coupling effects of high dietary fructose and determine the capacity for murine cardiomyocyte fructose transport. Male C57Bl/6J mice were fed a high fructose diet for 12 wk. Fructose- and control-fed mouse cardiomyocytes were isolated and loaded with the fura 2 Ca(2+) fluorescent dye for analysis of twitch and Ca(2+) transient characteristics (4 Hz stimulation, 37°C, 2 mM Ca(2+)). Myocardial Ca(2+)-handling protein expression was determined by Western blot. Gene expression of the fructose-specific transporter, GLUT5, in adult mouse cardiomyocytes was detected by real-time and conventional RT-PCR techniques. Diastolic Ca(2+) and Ca(2+) transient amplitude were decreased in isolated cardiomyocytes from fructose-fed mice relative to control (16 and 42%, respectively), coincident with an increase in the time constant of Ca(2+) transient decay (24%). Dietary fructose increased the myofilament response to Ca(2+) (as evidenced by a left shift in the shortening-Ca(2+) phase loop). Protein expression of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a), phosphorylated (P) phospholamban (Ser(16)), and P-phospholamban (Thr(17)) was reduced, and protein phosphatase 2A expression increased, in fructose-fed mouse hearts. Hypertension and cardiac hypertrophy were not evident. These findings demonstrate that fructose diet-associated myocardial insulin resistance induces profound disturbance of cardiomyocyte Ca(2+) handling and responsiveness in the absence of altered systemic loading conditions.

Repeated Application of Low-Frequency Electroacupuncture Improves High-Fructose Diet-Induced Insulin Resistance in Rats

Background

Insulin resistance is frequently present in obesity and during the development of type 2 diabetes mellitus.

Objective

The purpose of the present study was to investigate the effect of electroacupuncture (EA) on high-fructose diet (HFD)-induced insulin resistance.

Methods

Male Wistar rats were fed HFD for 4 weeks and developed insulin resistance. Insulin sensitivity was assessed by clamp. The number of animals was seven, eight and seven in the control, HFD and HFD+EA groups, respectively. AMP-activated protein kinase (AMPK) and glucose transporter 4 (GLUT4) in skeletal muscle were measured by Western blotting analysis (n=7 in each group). EA stimulation was carried out 12 times over 4 weeks at an intensity of 1–3 mA and a frequency of 2/15 Hz in a conscious state without restraint.

Results

There was no significant difference in mean body weight and fasting blood glucose concentration between groups at the end of the experiment. The mean glucose infusion rate during the clamp was significantly lower in the HFD group than in controls (p<0.05). There was no significant difference in expression of GLUT4 in skeletal muscle in the control and each group. Phosphorylated AMPKα (Thr172) in skeletal muscle showed a significant increase immediately after the final EA stimulation when compared with the control group (p<0.05).

Conclusion

Repeated application of EA is capable of improving diet-induced insulin resistance, probably through activation of AMPK signalling pathways in skeletal muscle. These results suggest that repeated application of EA may have beneficial effects on diet-induced insulin resistance.

Rosiglitazone and fenofibrate additive effects on lipids

Background. To evaluate the effect of rosiglitazone, fenofibrate, or their combined use on plasma lipids in normoglycemic healthy adults. Methods and Results. Subjects were randomized in a double-blind fashion to rosiglitazone + placebo, fenofibrate + placebo, rosiglitazone + fenofibrate, or matching double placebo. The between-group difference in the change in fasting TG, high-density lipoprotein cholesterol (HDL-C), LDL-C, and plasma apolipoproteins A-I, A-II, and C-III level were compared after 12 weeks of treatment. A total of 548 subjects were screened and 41 met the inclusion criteria. After 12 weeks of therapy, the median change in the triglyceride levels showed a significant reduction ranging from 47 to 55 mg per deciliter in the fenofibrate only and rosiglitazone/fenofibrate groups compared with placebo (P = 0.0496). However, the rosiglitazone only group did not show significant change in triglyceride level. The change in the Apo AII showed increase in all the treatment groups compared with placebo (P = 0.009). There was also significant change in the Apo CIII that showed reduction of its level in the fenofibrate only and rosiglitazone/fenofibrate groups (P = 0.0003). Conclusion. Rosiglitazone does not appear to modulate hypertriglyceridemia in patients with elevated triglycerides independent of glucose metabolism.

Hypoglycemic effect of aqueous shallot and garlic extracts in rats with fructose-induced insulin resistance

The present study has been carried out to investigate the effect of aqueous extract of shallot (Allium ascalonicum) and garlic (Allium satium) on the fasting insulin resistance index (FIRI) and intraperitoneal glucose tolerance test (IPGTT) of fructose-induced insulin resistance rats. Male albino Wistar rats were fed either normal or high-fructose diet for a period of eight weeks. Fasting blood glucose level, fasting blood triglyceride level, FIRI, and the area under the glucose tolerance curve were significantly elevated in fructose-fed animals. Fructose-induced insulin resistance rats treated by aqueous shallot or garlic extract (500 mg/kg body weight/day, i.p.) for duration of eight weeks. Control animals only received normal saline (0.9%). The results showed that neither shallot nor garlic extracts significantly altered the FIRI and the IPGTT at the fourth week after treatment. The fasting blood glucose in fructose-induced insulin resistance animals has been significantly decreased in 8-week treated animals by both shallot and garlic extracts. Shallot extract administration, but not garlic extract, for a period of eight weeks can significantly improve the intraperitoneal glucose tolerance and diminish the FIRI. These results indicate that shallot and garlic extracts have a hypoglycemic influence on the fructose-induced insulin resistance animals and aqueous shallot extract is a stronger hypoglycemic agent than the garlic extract.

Mendelian randomization studies do not support a role for raised circulating triglyceride levels influencing type 2 diabetes, glucose levels, or insulin resistance

OBJECTIVE

The causal nature of associations between circulating triglycerides, insulin resistance, and type 2 diabetes is unclear. We aimed to use Mendelian randomization to test the hypothesis that raised circulating triglyceride levels causally influence the risk of type 2 diabetes and raise normal fasting glucose levels and hepatic insulin resistance.

RESEARCH DESIGN AND METHODS

We tested 10 common genetic variants robustly associated with circulating triglyceride levels against the type 2 diabetes status in 5,637 case and 6,860 control subjects and four continuous outcomes (reflecting glycemia and hepatic insulin resistance) in 8,271 nondiabetic individuals from four studies.

RESULTS

Individuals carrying greater numbers of triglyceride-raising alleles had increased circulating triglyceride levels (SD 0.59 [95% CI 0.52-0.65] difference between the 20% of individuals with the most alleles and the 20% with the fewest alleles). There was no evidence that the carriers of greater numbers of triglyceride-raising alleles were at increased risk of type 2 diabetes (per weighted allele odds ratio [OR] 0.99 [95% CI 0.97-1.01]; P = 0.26). In nondiabetic individuals, there was no evidence that carriers of greater numbers of triglyceride-raising alleles had increased fasting insulin levels (SD 0.00 per weighted allele [95% CI -0.01 to 0.02]; P = 0.72) or increased fasting glucose levels (0.00 [-0.01 to 0.01]; P = 0.88). Instrumental variable analyses confirmed that genetically raised circulating triglyceride levels were not associated with increased diabetes risk, fasting glucose, or fasting insulin and, for diabetes, showed a trend toward a protective association (OR per 1-SD increase in log(10) triglycerides: 0.61 [95% CI 0.45-0.83]; P = 0.002).

CONCLUSIONS

Genetically raised circulating triglyceride levels do not increase the risk of type 2 diabetes or raise fasting glucose or fasting insulin levels in nondiabetic individuals. One explanation for our results is that raised circulating triglycerides are predominantly secondary to the diabetes disease process rather than causal.

Different roles of zinc plus arachidonic acid on insulin sensitivity between high fructose- and high fat-fed rats

AIMS

This study was to determine the effects of zinc plus arachidonic acid (ZA) treatment on the insulin action in the specific ZA target organs using hyperinsulinemic euglycemic clamp method.

MAIN METHODS

18 Sprague-Dawley rats weighing ~130 g were divided into 3 groups of 6 rats and treated them with 1) normal rat chow, 2) high fructose (60.0%) diet only, or 3) the same fructose diet plus drinking water containing 10mg zinc plus 50mg arachidonic acid (AA)/L. In a separate study, male Wistar rats weighing ~250 g were fed normal rat chow (n=4) or high fat (66.5%) diet with drinking water containing zero (n=9) or 10mg AA plus 20mg zinc /L (n=9). After 4 week treatment, insulin action was assessed using the hyperinsulinemic eguglycemic clamp technique.

KEY FINDINGS

High fructose feeding impaired suppression of hepatic glucose output by insulin compared to controls during the clamp procedure (4.39 vs. 2.35 mg/kg/min; p<0.05). However, ZA treatment in high fructose-fed rats showed a significant improvement of hepatic insulin sensitivity compared to non-treatment controls (4.39 vs. 2.18 mg/kg/min; p<0.05). Glucose infusion rates in Wistar rats maintained on a high fat diet (HFD) were significantly lower compared to control rats (22.8 ± 1.3 vs. 31.9 ± 1.4 mg/kg/min; p<0.05). ZA treatment significantly improved (~43%) peripheral tissue insulin sensitivity in HFD fed animals (26.7 ± 1.3 [n=9] vs. 22.8 ± 1.3mg/kg/min; p<0.05).

SIGNIFICANCE

These data demonstrate that ZA treatment is effective in improving glucose utilization in hyperglycemic rats receiving either a high-fructose or a high-fat diet.

Gum Guar fiber associated with fructose reduces serum triacylglycerol but did not improve the glucose tolerance in rats

INTRODUCTION

The increased intake of dietary fructose can be associated with alterations on energy homeostasis and lipid/carbohydrate metabolism, such as insulin resistance and dislipidemia. On the other hand, the ingestion of soluble fiber gum guar could improve benefic mechanism on glucose tolerance and lipids profile.

OBJECTIVE

The aim of the present study were to investigate the effects of the supplemental feeding partially hydrolyzed gum guar on glucose and lipid homeostasis, in rats fed with fructose solution.

METHODS

The study was performed on thirty day-old male Wistar rats randomly assigned into four groups: control(C) or treated with fructose (F-20%), fiber (FB-5%), or fructose plus fiber (F-20% + FB-5% = FF) solution for 30 days on glucose tolerance (OGTT), triacylglycerol concentration in the liver by chloroform/methanol method, glucose, triacylglycerol and total cholesterol serum concentration by assayed by enzymatic colorimetric method, insulin receptor (IR) concentration in the liver by Western Blotting.

RESULTS

The total body weight gain was not different between groups; in regards of total caloric intake, in the F group was significantly higher and in the FB group was lower than other groups. The triacylglycerol concentration in the liver of FF group was significantly higher than F group, the triacylglycerol concentration in the serum was higher the F group compared with other groups. The OGTT reveal impaired on glucose tolerance in the F, FB, FF compared with C. The IR concentration in the liver was lower in the F, FB, FF compared with C, no significant difference was observed between groups for IR concentration in the gastrocnemius muscle. No significant difference was observed between groups for carcass fat content and serum total cholesterol.

CONCLUSION

Fructose induced important alterations on glucose tolerance and lipid metabolism, despite of fiber showed reversion of part this alterations. The association fructose plus fiber to seem decrease insulin receptor concentration in the liver, with consequent impair on glucose tolerance.

Asymmetric syncytial expression of GLUT9 splice variants in human term placenta and alterations in diabetic pregnancies

Glucose transport from the maternal to fetal side of the placenta is critical for fetal growth and development due to the absence of fetal gluconeogenesis. Human GLUT9, existing as 2 isoforms, is a novel member of the transporter family. This study investigated the localization and relative expression levels of these isoforms in the human term placenta from both control and diabetic patients. Placenta samples were collected from normal pregnancies and those complicated by maternal diabetes (White classifications A1, A2, and B). Antibodies specific for the different isoforms were used to detect expression. Both forms of the protein are expressed in syncytiotrophoblast cells. Subcellular fractionation revealed an asymmetrical expression pattern with GLUT9a on basal membranes, whereas GLUT9b localizes to microvillus membranes. Expression of both isoforms is significantly increased in placental tissue from diabetic pregnancies. Altered expression of GLUT9 in the placenta may play a role in the fetal pathophysiology associated with diabetes-complicated pregnancies.

SirT1 knockdown in liver decreases basal hepatic glucose production and increases hepatic insulin responsiveness in diabetic rats

Hepatic gluconeogenesis is a major contributing factor to hyperglycemia in the fasting and postprandial states in type 2 diabetes mellitus (T2DM). Because Sirtuin 1 (SirT1) induces hepatic gluconeogenesis during fasting through the induction of phosphoenolpyruvate carboxylase kinase (PEPCK), fructose-1,6-bisphosphatase (FBPase), and glucose-6-phosphatase (G6Pase) gene transcription, we hypothesized that reducing SirT1, by using an antisense oligonucleotide (ASO), would decrease fasting hyperglycemia in a rat model of T2DM. SirT1 ASO lowered both fasting glucose concentration and hepatic glucose production in the T2DM rat model. Whole body insulin sensitivity was also increased in the SirT1 ASO treated rats as reflected by a 25% increase in the glucose infusion rate required to maintain euglycemia during the hyperinsulinemic-euglycemic clamp and could entirely be attributed to increased suppression of hepatic glucose production by insulin. The reduction in basal and clamped rates of glucose production could in turn be attributed to decreased expression of PEPCK, FBPase, and G6Pase due to increased acetylation of signal transducer and activator of transcription 3 (STAT3), forkhead box O1 (FOXO1), and peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha), known substrates of SirT1. In addition to the effects on glucose metabolism, SirT1 ASO decreased plasma total cholesterol, which was attributed to increased cholesterol uptake and export from the liver. These results indicate that inhibition of hepatic SirT1 may be an attractive approach for treatment of T2DM.

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.

Activation of AMP-activated protein kinase, inhibition of pyruvate dehydrogenase activity, and redistribution of substrate partitioning mediate the acute insulin-sensitizing effects of troglitazone in skeletal muscle cells

The aim of this study was to investigate the acute effects of troglitazone on several pathways of glucose and fatty acid (FA) partitioning and the molecular mechanisms involved in these processes in skeletal muscle. Exposure of L6 myotubes to troglitazone for 1 h significantly increased phosphorylation of AMPK and ACC, which was followed by approximately 30% and approximately 60% increases in palmitate oxidation and carnitine palmitoyl transferase-1 (CPT-1) activity, respectively. Troglitazone inhibited basal ( approximately 25%) and insulin-stimulated ( approximately 35%) palmitate uptake but significantly increased basal and insulin-stimulated glucose uptake by approximately 2.2- and 2.7-fold, respectively. Pharmacological inhibition of AMPK completely prevented the effects of troglitazone on palmitate oxidation and glucose uptake. Interestingly, even though troglitazone exerted an insulin sensitizing effect, it reduced basal and insulin-stimulated rates of glycogen synthesis, incorporation of glucose into lipids, and glucose oxidation to values corresponding to approximately 30%, approximately 60%, and 30% of the controls, respectively. These effects were accompanied by an increase in basal and insulin-stimulated phosphorylation of Akt(Thr308), Akt(Ser473), and GSK3alpha/beta. Troglitazone also powerfully suppressed pyruvate decarboxylation, which was followed by a significant increase in basal ( approximately 3.5-fold) and insulin-stimulated ( approximately 5.5-fold) rates of lactate production by muscle cells. In summary, we provide novel evidence that troglitazone exerts acute insulin sensitizing effects by increasing FA oxidation, reducing FA uptake, suppressing pyruvate dehydrogenase activity, and shifting glucose metabolism toward lactate production in muscle cells. These effects seem to be at least partially dependent on AMPK activation and may account for potential acute PPAR-gamma-independent anti-diabetic effects of thiazolidinediones in skeletal muscle.

Nonclinical Safety Evaluation of Muraglitazar, a Novel PPARα/γ Agonist

The toxicity of muraglitazar, an oxybenzylglycine, nonthiazolidinedione peroxisome proliferator-activated receptor (PPAR) alpha/gamma agonist, was evaluated in a comprehensive nonclinical toxicology program that included single-dose oral toxicity studies in mice, rats, and monkeys; repeat-dose toxicity studies in rats, dogs, and monkeys; a battery of in vitro and in vivo genetic toxicity studies; carcinogenicity studies in mice and rats; reproductive and developmental toxicity studies in rats and rabbits; and studies to investigate species-specific findings. Pharmacologically mediated changes, similar to those observed with other PPARgamma agonists, were observed following chronic administration and included subcutaneous edema, hematologic/hematopoietic and serum chemistry alterations, and morphologic findings in the heart and adipose tissue in rats and monkeys. In dogs, a species highly sensitive to PPARgamma agonists, muraglitazar caused pronounced species-specific clinical toxicity and degenerative changes in the brain, spinal cord, and testes at high doses and exposures. Muraglitazar was nongenotoxic in the standard battery of genotoxicity studies. Gallbladder adenomas in male mice and adipocyte neoplasms in male and female rats were seen at suprapharmacologic exposures, whereas urinary bladder tumors occurred in male rats at lower exposures. Subsequent investigative studies established that the urinary bladder carcinogenic effect was mediated by urolithiasis rather than a direct pharmacologic effect on urothelium. Muraglitazar had no effects on reproductive function in male and female rats at high systemic exposures, was not teratogenic in rats or rabbits, and demonstrated no selective developmental toxicity. Overall, there were no nonclinical findings that precluded the safe administration of muraglitazar to humans.

Suppression of PPAR-gamma attenuates insulin-stimulated glucose uptake by affecting both GLUT1 and GLUT4 in 3T3-L1 adipocytes

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) plays a critical role in regulating insulin sensitivity and glucose homeostasis. In this study, we identified highly efficient small interfering RNA (siRNA) sequences and used lentiviral short hairpin RNA and electroporation of siRNAs to deplete PPAR-gamma from 3T3-L1 adipocytes to elucidate its role in adipogenesis and insulin signaling. We show that PPAR-gamma knockdown prevented adipocyte differentiation but was not required for maintenance of the adipocyte differentiation state after the cells had undergone adipogenesis. We further demonstrate that PPAR-gamma suppression reduced insulin-stimulated glucose uptake without affecting the early insulin signaling steps in the adipocytes. Using dual siRNA strategies, we show that this effect of PPAR-gamma deletion was mediated by both GLUT4 and GLUT1. Interestingly, PPAR-gamma-depleted cells displayed enhanced inflammatory responses to TNF-alpha stimulation, consistent with a chronic anti-inflammatory effect of endogenous PPAR-gamma. In summary, 1) PPAR-gamma is essential for the process of adipocyte differentiation but is less necessary for maintenance of the differentiated state, 2) PPAR-gamma supports normal insulin-stimulated glucose transport, and 3) endogenous PPAR-gamma may play a role in suppression of the inflammatory pathway in 3T3-L1 cells.

Hypoglycemic effects of exopolysaccharides produced by mycelial cultures of two different mushrooms Tremella fuciformis and Phellinus baumii in ob/ob mice

The anti-diabetic activities of the exopolysaccharides (EPS) produced by submerged mycelial culture of two different mushrooms, Tremella fuciformis and Phellinus baumii, in ob/ob mice were investigated. All the animals were randomly divided into three groups with seven animals in each group: The control group received 0.9% NaCl solution; the diabetic groups were treated with EPS from T. fuciformis (Tf EPS) and P. baumii (Pb EPS) at the level of 200 mg/kg body weight using an oral zoned daily for 52 days. The plasma glucose levels in the EPS-fed mice were substantially reduced by about 52% (Tf EPS) and 32% (Pb EPS), respectively, as compared to control mice. The results of oral glucose tolerance test (OGTT) revealed that both EPS-fed groups significantly increased the glucose disposal after 52 days of EPS treatments. Furthermore, higher food efficiency ratios and reduced blood triglyceride levels were observed in the EPS-treated groups. Because peroxisome proliferator-activated receptor gamma (PPAR-gamma) is indeed a key regulator of insulin action, we investigated the expression pattern of adipose tissue PPAR-gamma messenger RNA (mRNA) and plasma levels of PPAR-gamma. It was revealed that PPAR-gamma was significantly activated in response to EPS treatments. The results suggested that both EPS exhibited considerable hypoglycemic effect and improved insulin sensitivity possibly through regulating PPAR-gamma-mediated lipid metabolism. Our results indicated that two mushroom-derived EPS might be developed as potential oral hypoglycemic agents or functional foods for the management of non-insulin-dependent diabetes mellitus.

Long-term Pharmacological Activation of PPARγDoes not Prevent Left Ventricular Remodeling in Dogs with Advanced Heart Failure

BACKGROUND

Peroxisome proliferator-activated receptor gamma (PPARgamma) activators affect the myocardium through inhibition of inflammatory cytokines and metabolic modulation but their effect in the progression of heart failure is unclear. In the present study, we examined the effects of the PPARgamma activator, GW347845 (GW), on the progression of heart failure.

METHODS AND RESULTS

Heart failure was produced in 21 dogs by intracoronary microembolizations to LV ejection fraction (EF) less than 30% and randomized to 3 months of therapy with high-dose GW (10 mg/Kg daily, n = 7), low-dose GW (3 mg/Kg daily, n = 7), or no therapy (control, n = 7). In control dogs, EF significantly decreased (28 +/- 1 vs. 22 +/- 1%, p < 0.001) and end-diastolic volume (EDV) and end-systolic volume (ESV) increased during the 3 months of the follow-up period (64 +/- 4 vs. 76 +/- 5; p = 0.003, 46 +/- 3 vs. 59 +/- 4 ml, p = 0.002, respectively). In dogs treated with low-dose GW, EDV increased significantly (69 +/- 4 vs.81 +/- 5 ml, p = 0.01), whereas ESV remained statistically unchanged (50 +/- 3 vs. 54 +/- 3 ml, p = 0.10) resulting in modestly increased ejection fraction (27 +/- 1 vs. 32 +/- 3%, p = 0.05). In dogs treated with high-dose GW, both EDV and ESV increased (72 +/- 4 vs. 79 +/- 5 ml, p = 0.04; 53 +/- 3 vs. 62 +/- 5 ml, p = 0.04) and EF decreased (26 +/- 1 vs. 23 +/- 1%, p = 0.04) as with control dogs. There was significantly increased myocardial hypertrophy as evidenced by increased LV weight to body weight ratio and myocyte cross-section area in the GW treated animals compared to controls. Compared to control, treatment with GW had no effect on mRNA expression of PPARgamma, inflammatory cytokines, stretch response proteins, or transcription factors that may induce hypertrophy.

CONCLUSIONS

Long-term PPARgamma activation with GW did not prevent progressive LV remodeling in dogs with advanced heart failure.

Effects of a palatinose-based liquid diet (Inslow) on glycemic control and the second-meal effect in healthy men

Postprandial hyperglycemia induces prolonged hyperinsulinemia, which is a risk factor for type 2 diabetes mellitus. Foods with a low glycemic index blunt the rapid rise in postprandial plasma glucose and insulin levels. We herein investigated the effects of a novel, palatinose-based liquid diet (Inslow, Meiji Dairy Products, Tokyo, Japan) on postprandial plasma glucose and insulin levels and on the rate of substrate oxidation in 7 healthy men. Furthermore, to examine the effects of Inslow on the second-meal effect, we quantified our subjects' postprandial plasma glucose, insulin, and free fatty acid levels for up to 7 hours after they ingested a breakfast containing Inslow or control formula, followed by a standard lunch 5 hours later. Our results showed that peak plasma glucose and insulin levels 30 minutes after Inslow loading were lower than after control formula loading. Postprandial fat oxidation rates in the Inslow group were higher than in the control formula group (P < .05). In the second-meal effect study, plasma glucose and insulin levels after lunch in the Inslow group were lower than in the control formula group (P < .01), although the peak levels in these groups were not different. The free fatty acid concentration in the Inslow group immediately before lunch was significantly lower than in the control formula group (P < .05). In conclusion, consumption of Inslow at breakfast appears to improve patient glycemic control by reducing their postprandial plasma glucose and insulin levels after lunch (second-meal effect).

Enalapril increases the local extravasation of macromolecules and nitric oxide synthase in pancreas of the fructose-fed insulin-resistant rat model

OBJECTIVES

Angiotensin-converting enzyme (ACE) inhibitors have been associated with an increased risk of acute pancreatitis. The pathogenesis of this condition remains unclear, but an activation of the kinin system and a resultant localized angioedema have been implicated in the initial step leading to acute pancreatic damage. The goal of the present study was to explore the impact of ACE inhibition on pancreatic microcirculation and capillary permeability in normal and insulin-resistant rats.

METHODS

Chow- or fructose-fed Sprague-Dawley rats were treated with enalapril (dosage, 10 mg.kg.d) or vehicle for 4 weeks before measuring in vivo the extravasation of Evans blue (EB) dye in pancreas. Unanesthetized animals (n = 10-17 per group) were injected with EB 20 mg.kg in the caudal vein 10 minutes before killing, and EB dye was extracted from each pancreas by using formamide.

RESULTS

Relative to controls, enalapril-treated animals showed a 5-fold increase in pancreatic extravasation of EB in the fructose-fed rat model (P < 0.001); smaller changes (2-fold) were observed in the chow-fed animals treated with enalapril (P < 0.001). The increase in pancreatic vasopermeability observed with enalapril in the fructose-fed animals was accompanied by a significant increase in total pancreatic nitric oxide synthase (NOS) activity compared to controls (Delta = +128%; P < 0.001). This increase in NOS activity seemed to be solely attributable to an upregulation of the endothelial NOS isoform because only the eNOS immunoreactive mass (as opposed to nNOS) seemed to be increased in the pancreas of these animals. Treatment with enalapril was not associated with any increase in serum amylase concentrations in either animal subgroup.

CONCLUSIONS

Enalapril increases capillary permeability (extravasation of macromolecules) in the pancreas of the fructose-fed rat model. This suggests that ACE inhibition upregulates the eNOS isoform locally, increases vasopermeability of the pancreas, and can therefore result in local edema in the fructose-fed insulin-resistant rat model.

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.

Peroxisome Proliferator-Activated Receptor-??

Insulin resistance and obesity is a common health problem in the industrialized world. As a result of the availability of high-calorie food and a reduction in energy expenditure, maladaptive metabolic processes may interfere with the action of insulin and increase susceptibility for the development of atherosclerotic cardiovascular diseases. With the advent of peroxisome proliferator-activated receptors (PPARs), the mechanisms of this maladaptation and its relationship to insulin resistance syndrome components have become less obscure, promising new therapeutic approaches for this common problem. In this review we first focus on the molecular structure and cellular mechanisms of action of these receptors and then discuss how PPAR-gamma, a PPAR isoform, provides a link between adiposity, insulin resistance, and atherosclerosis.

Troglitazone Inhibits Long-Term Glycation and Oxidation of Low-Density Lipoprotein

Troglitazone (T) is a member of a new class of antidiabetic drugs termed thiazolidinediones (TZDs), which has previously been used as an anti-diabetic agent. In this study we investigated the influence of T, a ligand for PPAR-gamma receptor, on copper-catalyzed or cell-mediated oxidation of native, glycated, and glycoxidated low-density lipoprotein (LDL). A dose-dependent inhibition of copper-mediated low-density lipoprotein-oxidation, as monitored by the formation of oxidation-specific fluorescence, was observed for both native and glycated low-density lipoprotein. At the concentration of 20 microg/mL the inhibition amounted from 14.7% to 64.7% by all low-density lipoprotein forms. For glycated low-density lipoprotein we obtained the highest oxidation rate, but the most pronounced inhibition by T was found for glycoxidated low-density lipoprotein (goLDL). Inhibitory effects of T were also investigated by measurement of relative electrophoretic mobility (REM) in the concentration range of 0 to 20 microg/mL. The inhibition of 4h oxidation of native low-density lipoprotein was found in the entire concentration range, but significance was seen at 10 microg/mL. The long-term glycation and glycoxidation of low-density lipoprotein as measured by 5-hydroxymethyl-2-furaldehyde (5-HMF) formation and binding of fructosamine was found to be inhibited by T. In endothelial cell-mediated oxidation of low-density lipoprotein cytotoxicity of T in the concentration range of 0 to 160 microg/mL during 2 to 24 h oxidation was investigated. In the non-cytotoxic concentration range of 5 to 20 microg/mL, a significantly reduced liberation of isoprostane 8-epi-PGF2alpha during 24 h cell-mediated oxidation of low-density lipoprotein and its modifications was found. This inhibitory action of T was most significant in the case of goLDL and amounted to approximately 20% to 60% inhibition at 5 to 20 microg/mL T, respectively. In the concentration range of 40 to 160 microg/mL, however, T showed an increasing cytotoxic action, as evidenced by loss of cell adhesion, loss of cellular protein, morphological changes, and cell disintegration as well as by strongly enhanced troglitazone-mediated isoprostane 8-IP liberation (fivefold to sixfold). T may be used as a model to explore the thiazolidinediones' mechanism on oxidation in a more general aspect for treatment for T2DM, because T is not clinically available.

Dietary fructose: implications for dysregulation of energy homeostasis and lipid/carbohydrate metabolism

Fructose intake and the prevalence of obesity have both increased over the past two to three decades. Compared with glucose, the hepatic metabolism of fructose favors lipogenesis, which may contribute to hyperlipidemia and obesity. Fructose does not increase insulin and leptin or suppress ghrelin, which suggests an endocrine mechanism by which it induces a positive energy balance. This review examines the available data on the effects of dietary fructose on energy homeostasis and lipid/carbohydrate metabolism. Recent publications, studies in human subjects, and areas in which additional research is needed are emphasized.

Peroxisome proliferator-activated receptor-gamma agonist rosiglitazone reduces clinical inflammatory responses in type 2 diabetes with coronary artery disease after coronary angioplasty

Rosiglitazone, an agonist of peroxisome proliferator-activated receptor-gamma (PPAR gamma ), is an insulin-sensitizing antidiabetic agent and inhibits restenosis in animal blood vessels. However, its benefit for patients with type 2 diabetes and coronary artery disease (CAD) after percutaneous coronary intervention is unknown. Patients with diabetes and CAD who had undergone percutaneous coronary intervention were randomized to either receive or not receive rosiglitazone (4 mg/d) for 6 months. After 6 months of rosiglitazone treatment, the plasma levels of fasting glucose and insulin and those of hemoglobin A1C and homeostasis model assessment of insulin resistance were significantly decreased in the rosiglitazone group as compared with baseline levels and those in the control group. After 2 and 6 months of rosiglitazone treatment, the plasma level of high-density lipoprotein was significantly increased in the rosiglitazone group. In addition, plasma levels of monocyte chemoattractant protein-1 and C-reactive protein and hyperresponsiveness of low-dose lipopolysaccharide-induced monocyte chemoattractant protein-1 secretion from monocytes were reduced. Furthermore, the occurrence of coronary events was significantly decreased in the rosiglitazone group at 6-month follow-up. Our data indicate that rosiglitazone may protect the vascular wall through not only improving the features of metabolic disorders but also reducing proinflammatory responses and the occurrence of coronary events in patients with diabetes and CAD after percutaneous coronary intervention.

Mechanisms and clinical effects of thiazolidinediones

Studies of pioglitazone, troglitazone, BRL 49653 and other thiazolidinediones in preclinical animal models of non-insulin dependent diabetes mellitus (NIDDM) and obesity led to the observation that these compounds were effective in reducing hyperglycaemia and hyperlipidaemia. In these models, animals treated with thiazolidinediones had notable improvements in blood glucose levels, hepatic glucose output, peripheral insulin resistance, and serum lipid levels. Mechanistic studies indicate that thiazolidinediones act at many intracellular sites and can influence several processes to increase cell sensitivity to insulin. These include influence on insulin receptor kinase activity, control of insulin receptor phosphorylation, change in number of insulin receptors, quantity and activity of GLUT-4, modulation of tumour necrosis factor (TNF) activity, activation of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and alteration of hepatic glucose metabolism. Available data on pioglitazone and troglitazone from clinical studies support the efficacy and safety of this class of compounds in reducing hyperglycaemia, hypertriglyceridaemia and insulin resistance associated with NIDDM. Currently, only troglitazone is approved for use in the United States and only in combination with insulin. This new pharmacological class of drugs has great promise for the treatment of NIDDM and also as a valuable research tool to further the understanding of the mechanisms that underlie NIDDM and insulin resistance syndrome.

Effect of pioglitazone on the early stage of type 2 diabetic nephropathy in KK/Ta mice

Pioglitazone (PIO) has preventive effects on impaired glucose tolerance (IGT) and urinary albumin excretion in diabetes. These effects in the early stage of diabetic nephropathy have not been fully described. Endothelial constitutive nitric oxide synthase (ecNOS) might be one of the mechanisms of glomerular hyperfiltration. The objective of the present study was to evaluate the effect of PIO, including the role of ecNOS on the early stage of diabetic nephropathy in KK/Ta mice. KK/Ta mice were given PIO (10 mg/kg/d) started at 12 or 16 weeks of age for 8 or 4 weeks, respectively. They were divided into 3 groups as follows: early treatment (n = 8), late treatment (n = 8), and control group (n = 12). The urinary albumin/creatinine ratio (ACR), fasting and casual blood glucose levels, ratio of glomerular and Bowman's capsule volume (GB ratio), and systemic blood pressure were measured as phenotypic characterizations. The ecNOS and iNOS protein expression in glomeruli were evaluated by immunofluorescence. PIO, especially early treatment, improved the ACR and the GB ratio, and ecNOS protein expression was decreased in the endothelium of glomerular vessels. The iNOS protein was not detectable. There were no significant changes in the levels of fasting and casual blood glucose and systemic blood pressure among all groups. We conclude that the effect of PIO on microalbuminuria might not be due to changing systemic blood pressure and blood glucose levels. It appears that the decrease of urinary albumin excretion might be related to improvement of glomerular enlargement, including hyperfiltration, since the levels of ecNOS protein were reduced by PIO in the glomerular vessels.

Insulin receptors and renal sodium handling in hypertensive fructose-fed rats

BACKGROUND

Insulin resistance and hypertension are present in Sprague-Dawley rats fed a fructose-enriched diet. In these rats, insulin might elevate blood pressure via an antinatriuretic action.

METHODS

To investigate the sodium-insulin interaction in fructose-fed rats, we compared insulin sensitivity, insulin receptor binding, and insulin receptor mRNA levels in the kidney and skeletal muscle of rats that were fed standard rat chow or a fructose-enriched diet (66%) with either low (0.07%), normal (0.3%), or high (7.5%) NaCl concentrations for 3 weeks.

RESULTS

Systolic blood pressure increased in the fructose-fed rats receiving the normal and high-salt diet, but not the low-salt diet. When the rats were fed the low-salt diet, the rate of glucose infusion required to maintain euglycemia during a hyperinsulinemic clamp and insulin receptor number and mRNA levels in skeletal muscle were lower in fructose-fed than control rats. High-salt diet decreased significantly the rate of glucose disposal during the clamp and muscular insulin receptor number and mRNA levels in control, but not fructose-fed rats. During the low-salt diet, renal insulin receptor number and mRNA levels were comparable in fructose-fed and control rats and hyperinsulinemia had comparable acute antinatriuretic effects in the two groups; when the rats were maintained on the high-salt diet, the expected decrease in renal insulin receptor number and mRNA levels occurred in control but not fructose-fed rats and, consistent with this finding, the antinatriuretic response to hyperinsulinemia was blunted only in controls. An inverse relationship between dietary NaCl content and renal insulin receptor mRNA levels was observed in control but not fructose-fed rats.

CONCLUSION

Fructose-fed rats appear to have lost the feedback mechanism that limits insulin-induced sodium retention through a down-regulation of the renal insulin receptor when the dietary NaCl content is increased. This abnormality might possibly contribute to the elevation of blood pressure in these rats.

Desaturase activities in rat model of insulin resistance induced by a sucrose-rich diet

A sucrose-rich diet, as compared with a similar starch diet, induces a time-dependent typical noninsulin-dependent diabetes syndrome characterized by insulin resistance in rats. Within the first 3 wk, there was glucose intolerance associated with hyperinsulinemia, hypertriglyceridemia, and high plasma FFA. In this study, we examined the effect of the sucrose-rich diet vs. the starch diet during short- (3 wk) and longterm treatment (6 mon) on hepatic delta9, delta6, and delta5 desaturases. These enzymes modulate monounsaturated FA and PUFA biosynthesis, respectively. Sucrose feeding (3 wk) caused an initial hyperinsulinemia that was normalized within 6 mon. In the early period (3 wk), stearoyl-CoA desaturase-1 (SCD-1) mRNA and activity were decreased, whereas delta6 desaturase mRNA abundance and delta6 and delta5 desaturase activities remained unchanged. After 6 mon of sucrose feeding, activities of the delta9, delta6, and delta5 desaturases were each increased. The SCD-1 and delta6 desaturase mRNA were also correspondingly higher. These increases were consistent with an increase in oleic acid, the 20:4/18:2 ratio, and 22:4n-6 and 22:5n-6 acids in liver and muscle lipids. On the other hand, the percentage of 22:6n-3 acid was decreased. In conclusion, a sucrose-rich diet after 6 mon induces an increase in rat liver SCD-1 and delta6 desaturase mRNA and enzymatic activities that are opposite to the changes reported in insulin-dependent diabetes mellitus. It appears that neither blood insulin levels nor insulin resistance is a factor affecting the delta9, delta6, and delta5 desaturase changes in mRNA and activity found with the sucrose-rich diet.

Expression and function of PPARgamma in rat placental development

Peroxisome proliferator-activated receptor (PPAR) gamma is a nuclear receptor known to regulate adipogenesis. Deletion of the PPARgamma gene in the mouse results in death by embryonic day 10.0 (E10.0) due to the failure of establishment of a labyrinth layer in the placenta, which suggests that PPARgamma is involved in trophoblast differentiation. To define PPARgamma function further in placental development, the expression and localization of the PPARgamma gene in the rat placenta was investigated. RT-PCR analysis shows the presence of PPARgamma mRNA in the placenta of day 11 of pregnancy (d11). The expression level is higher at d13 and then later decreased. Immunohistochemistry detects both PPARgamma and its putative intrinsic ligand, 15-deoxy-Delta(12,14)-prostaglandin J(2), in the trophoblast of layer I which lined the maternal sinus. Oral administration of troglitazone, an agonist of PPARgamma, to pregnant rats between d9 and d11 increases the expression level of PPARgamma in the placenta and reduces the mortality of the fetuses by half. These results suggest that PPARgamma is required not only for trophoblast differentiation but also trophoblast maturation to establish maternal-fetal transport.

The effect of nitric oxide synthase inhibitor on improved insulin action by pioglitazone in high-fructose-fed rats

The present study was performed to investigate whether nitric oxide synthase (NOS) inhibition influences the increased whole-body insulin action by pioglitazone in high-fructose-fed rats. Male Wistar rats aged 6 weeks were randomly divided into 3 groups and each group was fed one of the following diets for 3 weeks: standard chow diet (control group), high-fructose diet (fructose-fed group), and high-fructose diet plus pioglitazone (pioglitazone-treated group). The control and pioglitazone-treated groups were further divided into 2 subgroups respectively, and some rats of each subgroup were infused the NOS inhibitor, N(G)-monomethyl-l-arginine (L-NMMA), during the euglycemic clamp studies. In vivo insulin action was determined by the 2-step (3 and 30 mU/kg body weight [BW]/min low- and high-dose, respectively) hyperinsulinemic euglycemic clamp procedure in the awake condition. Glucose infusion rate (GIR) was considered as the index of insulin action. Endothelium-type NOS (eNOS) and inducible NOS (iNOS) in skeletal muscle were also measured. At the low-dose clamp, high-fructose feeding produced a marked decrease in GIR compared with the control group. Pioglitazone-treated animals showed a significant increase in GIR, reaching a similar level as the control group. However, the improved GIR was decreased to the level of the fructose-fed group by L-NMMA infusion. The GIR of the control group was not affected by L-NMMA infusion. The same tendency as the low-dose clamp was found at the high-dose clamp. In skeletal muscle, eNOS and iNOS protein content were not affected by high-fructose feeding and/or pioglitazone treatment. These results suggest that NOS inhibition can decrease the improved insulin resistance by pioglitazone in high-fructose-fed rats. Therefore, although NOS protein content is not changed by high-fructose feeding and/or pioglitazone treatment, it could be concluded that nitric oxide (NO) plays an important role in the improvement of insulin action by pioglitazone.

Adipose-specific peroxisome proliferator-activated receptor gamma knockout causes insulin resistance in fat and liver but not in muscle

Syndrome X, typified by obesity, insulin resistance (IR), dyslipidemia, and other metabolic abnormalities, is responsive to antidiabetic thiazolidinediones (TZDs). Peroxisome proliferator-activated receptor (PPAR) gamma, a target of TZDs, is expressed abundantly in adipocytes, suggesting an important role for this tissue in the etiology and treatment of IR. Targeted deletion of PPARgamma in adipose tissue resulted in marked adipocyte hypocellularity and hypertrophy, elevated levels of plasma free fatty acids and triglyceride, and decreased levels of plasma leptin and ACRP30. In addition, increased hepatic glucogenesis and IR were observed. Despite these defects, blood glucose, glucose and insulin tolerance, and insulin-stimulated muscle glucose uptake were all comparable to those of control mice. However, targeted mice were significantly more susceptible to high-fat diet-induced steatosis, hyperinsulinemia, and IR. Surprisingly, TZD treatment effectively reversed liver IR, whereas it failed to lower plasma free fatty acids. These results suggest that syndrome X may be comprised of separable PPARgamma-dependent components whose origins and therapeutic sites may reside in distinct tissues.

Sugars, insulin sensitivity, and the postprandial state

Since insulin resistance was first associated with type 2 diabetes and later with cardiovascular disease and hypertension, there has been considerable interest in the role of dietary and environmental factors. Sucrose and fructose have been a particular research focus. Research on animals, particularly rodents, has shown a clear and consistent effect of high-sucrose and high-fructose diets in decreasing insulin sensitivity. Experiments in humans have produced very conflicting results, with limited evidence for a negative effect on insulin sensitivity at higher intakes of fructose or sucrose (generally > 30% of daily energy from sucrose and > 15% of daily energy from fructose). Observation studies in humans have not shown a link between sucrose consumption and insulin sensitivity independent of other dietary factors. This is in contrast with several small studies that showed an improvement in insulin sensitivity after subjects followed dietary advice to lower the glycemic index of their food choices (where sugars were not a target for change). However, the pattern of postprandial glucose and insulin responses elicited by sucrose and fructose differs substantially from that elicited by starches, with lower troughs elicited by sucrose and fructose 2-3 h after eating. These differences in the pattern of postprandial responses offer a potential explanation for the conflicting results on insulin sensitivity, with the possibility that increases in insulin exposure may affect insulin sensitivity through down-regulation of insulin action.

Regulation of the glyoxalase pathway in human brain microvascular endothelium: effects of troglitazone and tertiary butylhydroperoxide

The glyoxalase system, comprised of glyoxalase-I and glyoxalase-II with glutathione as the cofactor, plays an important role in the detoxification of methylglyoxal and other alpha-oxo-aldehydes. Such aldehydes, which increase with hyperglycemia, give rise to advanced glycation end products. The objective of this research was to examine the glyoxalase system in human cerebromicrovascular cells. The hypothesis tested was that this pathway would be regulated by phase 2 enzyme inducers such as t-butylhydroquinone and modulated by the insulin-sensitizing drug troglitazone. Human cerebromicrovascular endothelial cells were cultured and exposed to varying concentrations of t-butylhydroquinone or troglitazone. The activity of glyoxylase-I in human endothelial cells was similar to the activity present in hepatocytes. The phase 2 enzyme inducer t-butylhydroquinone had no effect on the glyoxalase enzymes activities but significantly increased glutathione levels and glutathione reductase activity, indicating that phase 2 enzyme inducers might promote alpha-oxo-aldehyde scavenging. Troglitazone decreased the activities of glyoxalase-I and -II and decreased glyoxalase-I mRNA. Troglitazone had no effect on glutathione levels or on the activity of glutathione reductase or glutathione peroxidase. We conclude that phase 2 enzyme inducers may promote scavenging of alpha-oxoaldehydes in endothelial cells.

Effect of heterozygous PPARgamma deficiency and TZD treatment on insulin resistance associated with age and high-fat feeding

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is the target receptor for thiazolidinedione (TZD) compounds, which are a class of insulin-sensitizing drugs used in the treatment of type 2 diabetes. Paradoxically, however, mice deficient in PPARgamma (PPARgamma(+/-)) are more insulin sensitive than their wild-type (WT) littermates, not less, as would be predicted. To determine whether PPARgamma deficiency could prevent the development of the insulin resistance associated with increasing age or high-fat (HF) feeding, insulin sensitivity was assessed in PPARgamma(+/-) and WT mice at 2, 4, and 8 mo of age and in animals fed an HF diet. Because TZDs elicit their effect through PPARgamma receptor, we also examined the effect of troglitazone (a TZD) in these mice. Glucose metabolism was assessed by hyperinsulinemic euglycemic clamp and oral glucose tolerance test. Insulin sensitivity declined with age for both groups. However, the decline in the PPARgamma(+/-) animals was substantially less than that of the WT animals, such that, by 8 mo of age, the PPARgamma(+/-) mice were markedly more insulin sensitive than the WT mice. This greater sensitivity in PPARgamma(+/-) mice was lost with TZD treatment. HF feeding led to marked adipocyte hypertrophy and peripheral tissue and hepatic insulin resistance in WT mice but also in PPARgamma(+/-) mice. Treatment of these mice with troglitazone completely prevented the adipocyte hypertrophy and normalized insulin action. In conclusion, PPARgamma deficiency partially protects against age-related insulin resistance but does not protect against HF diet-induced insulin resistance.