Max Thorwald

Angiotensin receptor-mediated oxidative stress is associated with impaired cardiac redox signaling and mitochondrial function in insulin-resistant rats

Activation of angiotensin receptor type 1 (AT1) contributes to NADPH oxidase (Nox)-derived oxidative stress during metabolic syndrome. However, the specific role of AT1 in modulating redox signaling, mitochondrial function, and oxidative stress in the heart remains more elusive. To test the hypothesis that AT1 activation increases oxidative stress while impairing redox signaling and mitochondrial function in the heart during diet-induced insulin resistance in obese animals, Otsuka Long Evans Tokushima Fatty (OLETF) rats (n = 8/group) were treated with the AT1 blocker (ARB) olmesartan for 6 wk. Cardiac Nox2 protein expression increased 40% in OLETF compared with age-matched, lean, strain-control Long Evans Tokushima Otsuka (LETO) rats, while mRNA and protein expression of the H₂O₂-producing Nox4 increased 40-100%. ARB treatment prevented the increase in Nox2 without altering Nox4. ARB treatment also normalized the increased levels of protein and lipid oxidation (nitrotyrosine, 4-hydroxynonenal) and increased the redox-sensitive transcription factor Nrf2 by 30% and the activity of antioxidant enzymes (SOD, catalase, GPx) by 50-70%. Citrate synthase (CS) and succinate dehydrogenase (SDH) activities decreased 60-70%, whereas cardiac succinate levels decreased 35% in OLETF compared with LETO, suggesting that mitochondrial function in the heart is impaired during obesity-induced insulin resistance. ARB treatment normalized CS and SDH activities, as well as succinate levels, while increasing AMPK and normalizing Akt, suggesting that AT1 activation also impairs cellular metabolism in the diabetic heart. These data suggest that the cardiovascular complications associated with metabolic syndrome may result from AT1 receptor-mediated Nox2 activation leading to impaired redox signaling, mitochondrial activity, and dysregulation of cellular metabolism in the heart.

Angiotensin Receptor Blockade Recovers Hepatic UCP2 Expression and Aconitase and SDH Activities and Ameliorates Hepatic Oxidative Damage in Insulin Resistant Rats

Metabolic syndrome (MetS) is commonly associated with elevated renin-angiotensin system, oxidative stress, and steatohepatitis with down-regulation of uncoupling proteins (UCPs). However, the mechanisms linking renin-angiotensin system, steatosis, and UCP2 to hepatic oxidative damage during insulin resistance are not described. To test the hypothesis that angiotensin receptor activation contributes to decreased hepatic UCP2 expression and aconitase activity and to increased oxidative damage after increased glucose intake in a model of MetS, lean and obese Long Evans rats (n = 10/group) were randomly assigned to the following groups: 1) untreated Long Evans Tokushima Otsuka (lean, strain control), 2) untreated Otsuka Long Evans Tokushima Fatty (OLETF) (MetS model), 3) OLETF + angiotensin receptor blocker (ARB) (10 mg olmesartan/kg·d × 6 wk), 4) OLETF + high glucose (HG) (5% in drinking water × 6 wk), and 5) OLETF + ARB + HG (ARB/HG × 6 wk). HG increased body mass (37%), plasma triglycerides (TGs) (35%), plasma glycerol (87%), plasma free fatty acids (28%), and hepatic nitrotyrosine (74%). ARB treatment in HG decreased body mass (12%), plasma TG (15%), plasma glycerol (23%), plasma free fatty acids (14%), and hepatic TG content (42%), suggesting that angiotensin receptor type 1 (AT1) activation and increased adiposity contribute to the development of obesity-related dyslipidemia. ARB in HG also decreased hepatic nitrotyrosine and increased hepatic UCP2 expression (59%) and aconitase activity (40%), as well as antioxidant enzyme activities (50-120%), suggesting that AT1 activation also contributes to protein oxidation, impaired lipid metabolism, and antioxidant metabolism in the liver. Thus, in addition to promoting obesity-related hypertension, AT1 activation may also impair lipid metabolism and antioxidant capacity, resulting in steatosis via decreased UCP2 and tricarboxylic acid cycle activity.

Angiotensin and mineralocorticoid receptor antagonism attenuates cardiac oxidative stress in angiotensin II-infused rats.

Angiotensin II (Ang II) and aldosterone contribute to hypertension, oxidative stress and cardiovascular damage, but the contributions of aldosterone during Ang II‐dependent hypertension are not well defined because of the difficulty to assess each independently. To test the hypothesis that during Ang II infusion, oxidative and nitrosative damage is mediated through both the mineralocorticoid receptor (MR) and angiotensin type 1 receptor (AT1), five groups of Sprague–Dawley rats were studied: (i) control; (ii) Ang II infused (80 ng/min × 28 days); (iii) Ang II + AT1 receptor blocker (ARB; 10 mg losartan/kg per day × 21 days); (iv) Ang II + mineralocorticoid receptor (MR) antagonist (Epl; 100 mg eplerenone/day × 21 days); and (v) Ang II + ARB + Epl (Combo; × 21 days). Both ARB and combination treatments completely alleviated the Ang II‐induced hypertension, whereas eplerenone treatment only prolonged the onset of the hypertension. Eplerenone treatment exacerbated the Ang II‐mediated increase in plasma and heart aldosterone 2.3‐ and 1.8‐fold, respectively, while ARB treatment reduced both. Chronic MR blockade was sufficient to ameliorate the AT1‐mediated increase in oxidative damage. All treatments normalized protein oxidation (nitrotyrosine) levels; however, only ARB and Combo treatments completely reduced lipid peroxidation (4‐hydroxynonenal) to control levels. Collectively, these data suggest that receptor signalling, and not the elevated arterial blood pressure, is the principal culprit in the oxidative stress‐associated cardiovascular damage in Ang II‐dependent hypertension.

Angiotensin receptor blockade improves cardiac mitochondrial activity in response to an acute glucose load in obese insulin resistant rats

Hyperglycemia increases the risk of oxidant overproduction in the heart through activation of a multitude of pathways. Oxidation of mitochondrial enzymes may impair their function resulting in accumulation of intermediates and reverse electron transfer, contributing to mitochondrial dysfunction. Furthermore, the renin-angiotensin system (RAS) becomes inappropriately activated during metabolic syndrome, increasing oxidant production. To combat excess oxidant production, the transcription factor, nuclear factor erythriod-2- related factor 2 (Nrf2), induces expression of many antioxidant genes. We hypothesized that angiotensin II receptor type 1 (AT1) blockade improves mitochondrial function in response to an acute glucose load via upregulation of Nrf2. To address this hypothesis, an oral glucose challenge was performed in three groups prior to dissection (n = 5–8 animals/group/time point) of adult male rats: 1) Long Evans Tokushima Otsuka (LETO; lean strain-control), 2) insulin resistant, obese Otsuka Long Evans Tokushima Fatty (OLETF), and 3) OLETF + angiotensin receptor blocker (ARB; 10 mg olmesartan/kg/d × 6 weeks). Hearts were collected at T0, T60, and T120 minutes post-glucose infusion. ARB increased Nrf2 binding 32% compared to OLETF at T60. Total superoxide dismutase (SOD) and catalase (CAT) activities were increased 45% and 66% respectively in ARB treated animals compared to OLETF. Mitochondrial enzyme activities of aconitase, complex I, and complex II increased by 135%, 33% and 66%, respectively in ARB compared to OLETF. These data demonstrate the protective effects of AT1 blockade on mitochondrial function during the manifestation of insulin resistance suggesting that the inappropriate activation of AT1 during insulin resistance may impair Nrf2 translocation and subsequent antioxidant activities and mitochondrial function.

Glucoregulatory and Cardiometabolic Profiles of Almond vs. Cracker Snacking for 8 Weeks in Young Adults: A Randomized Controlled Trial

The transition to nutritional independence makes new college students vulnerable to alterations in eating patterns, which can increase the risk of cardiometabolic disorders. The aim of the study was to examine the potential benefits of almond vs. cracker snacking in improving glucoregulatory and cardiometabolic profiles in new college students. A randomized controlled, parallel-arm, 8-week intervention of 73 college students (BMI: 18–41 kg/m2) with no cardiometabolic disorders was conducted. Participants were randomized into either an almond snack group (56.7 g/day; 364 kcal; n = 38) or Graham cracker control group (77.5 g/day; 338 kcal/d; n = 35). Chronic, static changes were assessed from fasting serum/plasma samples at baseline, and after 4 and 8 weeks. Acute, dynamic effects were assessed during a 2-h oral glucose tolerance test (OGTT) at 8 weeks. Almond snacking resulted in a smaller decline in HDL cholesterol over 8 weeks (13.5% vs. 24.5%, p < 0.05), 13% lower 2-h glucose area under the curve (AUC), 34% lower insulin resistance index (IRI) and 82% higher Matsuda index (p < 0.05) during the OGTT, despite similar body mass gains over 8 weeks compared with the cracker group. In general, both almond and cracker snacking reduced fasting glucose, and LDL cholesterol. Conclusions: Incorporating a morning snack in the dietary regimen of predominantly breakfast-skipping, first-year college students had some beneficial effects on glucoregulatory and cardiometabolic health. Almond consumption has the potential to benefit postprandial glucoregulation in this cohort. These responses may be influenced by cardiometabolic risk factor status.

Cardiac Glutathione Levels Increase in Response to Glucose in Diabetic Rats

ABSTRACT Diabetic hearts are susceptible to damage from inappropriate activation of the renin angiotensin system (RAS) and hyperglycemic events both of which contribute to increased oxidant production. Prolonged elevation of oxidants impairs mitochondrial enzyme function, further contributing to metabolic derangement. Nuclear factor erythriod‐2‐related factor 2 (Nrf2) induces antioxidant genes including those for glutathione (GSH) synthesis following translocation to the nucleus. We hypothesized that an acute elevation in glucose impairs Nrf2‐related gene expression in diabetic hearts, while AT1 antagonism would aid in Nrf2‐mediated antioxidant production and energy replenishment. We used four groups (n = 6–8/group) of 25‐week‐old rats: 1) LETO (lean strain‐control), 2) type II diabetic OLETF, 3) OLETF + angiotensin receptor blocker (ARB; 10 mg olmesartan/kg/d × 8 wks), and 4) ARBM (4 weeks on ARB, 4 weeks off) to study the effects of acutely elevated glucose on cardiac mitochondrial function and Nrf2 signaling in the diabetic heart. Animals were gavaged with a glucose bolus (2 g/kg) and groups were dissected at T0, T180, and T360 minutes. Nrf2 mRNA was 32% lower in OLETF rats compared to LETO and remained suppressed in response to glucose. LETO Nrf2 mRNA increased 25% at T360 in response to glucose while no changes were observed in diabetic hearts. GCLC and GCLM mRNA decreased in diabetic hearts 33% and 44% respectively and remained suppressed in response to glucose while ARB treatment increased GCLM transcripts 90% at T180. These data illustrate that during T2DM and in response to glucose, cardiac Nrf2′s adaptive response to environmental stressors such as glucose is impaired in diabetic hearts and that ARB treatment may aid Nrf2′s impaired dynamic response. Graphical abstract Figure. No Caption available. HighlightsT2DM impairs cardiac Nrf2‐related gene response during a glucose challenge.Blockade of the AT1 receptor confers some benefit to impaired Nrf2‐related gene transcripts during T2DM.Removal of AT1 blockade impairs GSH’s dynamic response to a glucose challenge.

Angiotensin receptor and tumor necrosis factor-α activation contributes to glucose intolerance independent of systolic blood pressure in obese rats

Pathological activation of the renin-angiotensin system and inflammation are associated with hypertension and the development of metabolic syndrome (MetS). The contributions of angiotensin receptor type 1 (AT1) activation, independent of blood pressure, and inflammation to glucose intolerance and renal damage are not well defined. Using a rat model of MetS, we hypothesized that the onset of glucose intolerance is primarily mediated by AT1 activation and inflammation independent of elevated systolic blood pressure (SBP). To address this hypothesis, we measured changes in SBP, adiposity, plasma glucose and triglyceride levels, and glucose tolerance in six groups of rats: 1) lean, strain control Long-Evans Tokushima Otsuka (LETO; n = 5), 2) obese Otsuka Long-Evans Tokushima Fatty (OLETF; n = 8), 3) OLETF + angiotensin receptor blocker (ARB; 10 mg olmesartan/kg; n = 8), 4) OLETF + tumor necrosis factor-α (TNF-α) inhibitor (ETAN; 1.25 mg etanercept/kg; n = 6), 5) OLETF + TNF-α inhibitor + angiotensin receptor blocker (ETAN+ARB; 1.25 mg etanercept/kg + 10 mg olmesartan/kg; n = 6), and 6) OLETF + calcium channel blocker (CCB; 5 mg amlodipine/kg; n = 7). ARB and ETAN+ARB were most effective at decreasing SBP in OLETF, and ETAN did not offer any additional reduction. Glucose tolerance improved in ARB, ETAN, and ETAN+ARB compared with OLETF, whereas CCB had no detectable effect. Furthermore, all treatments reduced adiposity, whereas ETAN alone normalized urinary albumin excretion. These results suggest that AT1 activation and inflammation are primary factors in the development of glucose intolerance in a setting of MetS and that the associated increase in SBP is primarily mediated by AT1 activation.