Effects of high-sucrose feeding on insulin resistance and hemodynamic responses to insulin in spontaneously hypertensive rats

Nutrigenetic Interaction of Spontaneously Hypertensive Rat Chromosome 20 Segment and High-Sucrose Diet Sensitizes to Metabolic Syndrome

Several corresponding regions of human and mammalian genomes have been shown to affect sensitivity to the manifestation of metabolic syndrome via nutrigenetic interactions. In this study, we assessed the effect of sucrose administration in a newly established congenic strain BN.SHR20, in which a limited segment of rat chromosome 20 from a metabolic syndrome model, spontaneously hypertensive rat (SHR), was introgressed into Brown Norway (BN) genomic background. We mapped the extent of the differential segment and compared the genomic sequences of BN vs. SHR within the segment in silico. The differential segment of SHR origin in BN.SHR20 spans about 9 Mb of the telomeric portion of the short arm of chromosome 20. We identified non-synonymous mutations e.g., in ApoM, Notch4, Slc39a7, Smim29 genes and other variations in or near genes associated with metabolic syndrome in human genome-wide association studies. Male rats of BN and BN.SHR20 strains were fed a standard diet for 18 weeks (control groups) or 16 weeks of standard diet followed by 14 days of high-sucrose diet (HSD). We assessed the morphometric and metabolic profiles of all groups. Adiposity significantly increased only in BN.SHR20 after HSD. Fasting glycemia and the glucose levels during the oral glucose tolerance test were higher in BN.SHR20 than in BN groups, while insulin levels were comparable. The fasting levels of triacylglycerols were the highest in sucrose-fed BN.SHR20, both compared to the sucrose-fed BN and the control BN.SHR20. The non-esterified fatty acids and total cholesterol concentrations were higher in BN.SHR20 compared to their respective BN groups, and the HSD elicited an increase in non-esterified fatty acids only in BN.SHR20. In a new genetically defined model, we have isolated a limited genomic region involved in nutrigenetic sensitization to sucrose-induced metabolic disturbances.

Role of FAT/CD36 in novel PKC isoform activation in heart of spontaneously hypertensive rats

Disruption to the sensitive balance of long-chain fatty acids and glucose in the heart could cause cardiovascular diseases. Searching for a possible role of novel protein kinase C (nPKC) in heart with disrupted energy balance, we compared the insulin-resistant spontaneously hypertensive rats (SHR), which carry a nonfunctional variant of the fatty acid transporter FAT/CD36, with the less insulin-resistant congenic strain SHR-4 that is genetically identical except for a segment on chromosome 4 including a wild-type gene for a functional FAT/CD36. We analyzed expression of the nPKC-δ and -ε isoforms plus triacylglycerols (TAG) content in the myocardium of both FAT/CD36 strains and after a high sucrose diet (HSD). Two weeks before killing, males of both strains were randomly divided into two groups and fed either a standard laboratory chow or an HSD. PKC was determined by Western blotting in particulate and cytosolic fractions from left ventricles. The SHR-4 rats exhibited lower serum levels of insulin and free fatty acids than did SHR rats and higher amounts of PKC-ε in the heart particulate fraction. HSD caused accumulation of heart TAG in SHR but not in SHR-4. HSD increased PKC-δ and decreased PKC-ε expression in particulate fraction from left ventricles of SHR-4 while having no effects in SHR. These results demonstrate that reduced insulin resistance in SHR-4 rats with wild-type FAT/CD36 is associated with the insulin signaling pathway involving nPKCs.

THE AUTODIGESTION HYPOTHESIS AND RECEPTOR CLEAVAGE IN DIABETES AND HYPERTENSION

One of the key features of cardiovascular complications, such as hypertension or diabetes, is that they often appear at the same time in the same individual together with other forms of co-morbidities. While clinically a recognized phenomenon, no molecular mechanism for such co-morbidities has received universal acceptance. We propose a new hypothesis that provides a molecular basis for co-morbidities in hypertension due to unchecked proteolytic activity and receptor destruction. Testing of the hypothesis in the spontaneously hypertensive rat reveals an unchecked matrix metalloproteinase and serine protease activity in plasma and on several cardiovascular and parenchymal cells. The elevated proteolytic activity causes extracellular cleavage of multiple receptor types, such that cleavage of one receptor type leads to loss of the function carried out by this receptor. Proteolytic cleavage of the extracellular domain of the β(2) adrenergic receptor in arteries and arterioles causes vasoconstriction and elevation of the central blood pressure while cleavage of the extracellular domain of the insulin receptor leads to insulin resistance and lack of transmembrane glucose transport. A diverse set of cell dysfunctions in the spontaneously hypertensive rat are accompanied by cleavage of the membrane receptors that are involved in these functions. Chronic inhibition of the unchecked protease activity in the spontaneously hypertensive rat serves to restore the extracellular receptor density and alleviates the corresponding cell dysfunctions. The mild unchecked proteolytic activity in the spontaneously hypertensive rat points towards a chronic autodigestion process as a contributor to the end organ injury encountered in this rat strain. The presence of various soluble receptors, which consist of extracellular fragments of membrane receptors, in the plasma of hypertensive and diabetic patients suggest that the autodigestion process may also be present in man.

Fructose: metabolic, hedonic, and societal parallels with ethanol

Rates of fructose consumption continue to rise nationwide and have been linked to rising rates of obesity, type 2 diabetes, and metabolic syndrome. Because obesity has been equated with addiction, and because of their evolutionary commonalities, we chose to examine the metabolic, hedonic, and societal similarities between fructose and its fermentation byproduct ethanol. Elucidation of fructose metabolism in liver and fructose action in brain demonstrate three parallelisms with ethanol. First, hepatic fructose metabolism is similar to ethanol, as they both serve as substrates for de novo lipogenesis, and in the process both promote hepatic insulin resistance, dyslipidemia, and hepatic steatosis. Second, fructosylation of proteins with resultant superoxide formation can result in hepatic inflammation similar to acetaldehyde, an intermediary metabolite of ethanol. Lastly, by stimulating the "hedonic pathway" of the brain both directly and indirectly, fructose creates habituation, and possibly dependence; also paralleling ethanol. Thus, fructose induces alterations in both hepatic metabolism and central nervous system energy signaling, leading to a "vicious cycle" of excessive consumption and disease consistent with metabolic syndrome. On a societal level, the treatment of fructose as a commodity exhibits market similarities to ethanol. Analogous to ethanol, societal efforts to reduce fructose consumption will likely be necessary to combat the obesity epidemic.

NaCl plus chitosan as a dietary salt to prevent the development of hypertension in spontaneously hypertensive rats

The effect of NaCl plus 3% chitosan on the systolic blood pressure of spontaneously hypertensive rats (SHR) were evaluated and compared with NaCl plus KCl (NaCl, 49.36% + KCl 49.36%) and chitosan or NaCl treatment alone. In SHR, administration of NaCl plus chitosan (44 mM Na/day) for two months significantly decreased the systolic blood pressure greater than of NaCl plus KCl and NaCl alone. NaCl plus chitosan resulted, though not statistically significant, in decreased urinary Na⁺ excretion and decreased blood urea nitrogen levels. Urinary creatinine of NaCl plus chitosan was slightly decreased compared to 3 treated groups. Serum electrolytes levels, however, remained unchanged. The combination of NaCl and chitosan may be superior to the conventional use of NaCl plus KCl or NaCl alone in the prevention of hypertension. Even though these supplementary diets have demonstrated potential anti-hypertensive effects in the experimental animal model, further research is needed before any recommendations can be made.

Probiotics reduce the inflammatory response induced by a high-fat diet in the liver of young rats

Nonalcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease in the pediatric population. Preliminary evidence suggests a potential therapeutic utility of probiotics for this condition. Here, we tested the potential effect of the probiotic VSL#3 (a multistrain preparation composed of Streptococcus thermophilus and several species of Lactobacillus and Bifidobacteria) on oxidative and inflammatory damage induced by a high-fat diet in the liver of young rats. At weaning, young male Sprague-Dawley rats were randomly divided into 3 groups (n = 6) fed a standard, nonpurified diet (Std; 5.5% of energy from fat) or a high-fat liquid diet (HFD; 71% of energy from fat). One of the HFD groups received by gavage VSL#3 (13 x 10(9) bacteria x kg(-1) x d(-1)). After 4 wk, the HFD rats had greater body weight gain, fat mass, serum aminotransferase, and liver weight than rats fed the Std diet. The HFD induced liver lipid peroxidation, tumor necrosis factor (TNFalpha) production, protein S-nitrosylation, inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2 expression, and metalloproteinase (MMP) activity. Moreover, in the HFD group, PPARalpha expression was less than in rats fed the Std diet. In rats fed the HFD diet and treated with VSL#3, liver TNFalpha levels, MMP-2 and MMP-9 activities, and expression of iNOS and COX-2 were significantly lower than in the HFD group. In VSL#3-treated rats, PPARalpha expression was greater than in the HFD group. A modulation of the nuclear factor-kappaB pathway by VSL#3 was also demonstrated. Our data suggest that VSL#3 administration could limit oxidative and inflammatory liver damage in patients with NAFLD.

Perinatal taurine depletion increases susceptibility to adult sugar-induced hypertension in rats

This study tests the hypothesis that perinatal taurine depletion produces autonomic nervous system dysregulation and increases arterial pressure in young male rats maintained on a high sugar diet. Sprague-Dawley dams were either taurine depleted (beta-alanine 3% in water) or left untreated from conception to weaning. Their male offspring were fed normal rat chow with or without 5% glucose. At 7-8 weeks of age, the male offspring were either tested in a conscious, unrestrained state or after anesthetia. Body weight was slightly lower in the taurine-depleted rats although their heart or kidneys to body weight ratios were similar. Plasma potassium, blood urea nitrogen, plasma creatinine, hematocrit, fasting blood glucose concentrations and glucose tolerance were all similar. In the taurine-depleted, high glucose group, mean arterial pressure and sympathetic nervous system activity were increased while baroreflex function was impaired. These findings suggest that in this model perinatal taurine depletion causes autonomic nervous system dysfunction that may contribute to dietary high sugar-induced hypertension.

Endothelial and vascular dysfunctions and insulin resistance in rats fed a high-fat, high-sucrose diet

This study was designed to examine the effects of a high-fat, high-sucrose (HFHS) diet on vascular and metabolic actions of insulin. Male rats were randomized to receive an HFHS or regular chow diet for 4 wk. In a first series of experiments, the rats had pulsed Doppler flow probes and intravascular catheters implanted to measure blood pressure, heart rate, and regional blood flows. Insulin sensitivity and vascular responses to insulin were assessed during a euglycemic hyperinsulinemic clamp performed in conscious rats. In a second series of experiments, new groups of rats were used to examine skeletal muscle glucose transport activity and to determine in vitro vascular reactivity, endothelial nitric oxide synthase (eNOS) protein expression in muscle and vascular tissues and endothelin content, nitrotyrosine formation, and NAD(P)H oxidase protein expression in vascular tissues. The HFHS-fed rats displayed insulin resistance, hyperinsulinemia, hypertriglyceridemia, hyperlipidemia, elevated blood pressure, and impaired insulin-mediated renal and skeletal muscle vasodilator responses. A reduction in endothelium-dependent vasorelaxation, accompanied by a decreased eNOS protein expression in muscles and blood vessel endothelium, and increased vascular endothelin-1 protein content were also noted in HFHS-fed rats compared with control rats. Furthermore, the HFHS diet induced a reduced insulin-stimulated glucose transport activity in muscles and increased levels of NAD(P)H oxidase protein and nitrotyrosine formation in vascular tissues. These findings support the importance of eNOS protein in linking metabolic and vascular disease and indicate the ability of a Westernized diet to induce endothelial dysfunction and to alter metabolic and vascular homeostasis.

Hepatic effects of a fructose diet in the stroke-prone spontaneously hypertensive rat

BACKGROUND

Feeding stroke-prone spontaneously hypertensive rats (SHRSP) a diet rich in fructose results in a profound glucose intolerance not observed in the normotensive Wistar Kyoto (WKY) strain. The aim of this study was to investigate the role of the liver in the underlying mechanisms in the SHRSP.

METHODS

SHRSP and WKY rats were fed either 60% fructose or regular chow for 2 weeks with blood pressure being measured using tail-cuff plethysmography and radiotelemetry. Intraperitoneal glucose tolerance tests were performed and livers harvested for analysis of expression of inflammatory mediators and antioxidant proteins by western blotting and quantitative reverse transcriptase-PCR. The serum triglyceride content and fatty acid profiles were also measured.

RESULTS

Feeding SHRSP and WKY on 60% fructose for 2 weeks resulted in glucose intolerance with no increases in levels of blood pressure. Serum triglycerides were increased in both strains of fructose-fed rats with the highest levels being observed in the SHRSP. The serum fatty acid profiles were changed with large increases in the amounts of oleic acid (18.1) and reductions in linoleic acid (18.2). Levels of expression of c-jun N-terminal kinase/stress activated protein kinase (JNK/SAPK), and nuclear factor kappaB (NF-kappaB) were shown to be unchanged between the livers of the chow and fructose-fed groups. In contrast, protein levels of the three isoforms of superoxide dismutase (SOD) were upregulated in liver of SHRSP fed on fructose while only manganese SOD (MnSOD) was upregulated in fructose-fed WKY rats.

CONCLUSIONS

These results demonstrate that the major contribution of the liver in the early pathogenesis of metabolic syndrome may be an increased secretion of triglyceride containing altered proportions of fatty acid pools. Feeding rats a diet rich in fructose does not affect hepatic expression of inflammatory pathways and the increased hepatic SOD expression may constitute an early protective mechanism.