Ruben Rodriguez

Angiotensin receptor blockade increases pancreatic insulin secretion and decreases glucose intolerance during glucose supplementation in a model of metabolic syndrome.

Renin-angiotensin system blockade improves glucose intolerance and insulin resistance, which contribute to the development of metabolic syndrome. However, the contribution of impaired insulin secretion to the pathogenesis of metabolic syndrome is not well defined. To assess the contributions of angiotensin receptor type 1 (AT₁) activation and high glucose intake on pancreatic function and their effects on insulin signaling in skeletal muscle and adipose tissue, an oral glucose tolerance test (oGTT) was performed in five groups (n = 10/group) of rats: 1) lean strain-control 2) obese Otsuka Long-Evans Tokushima Fatty (OLETF), 3) OLETF + angiotensin receptor blocker (ARB; 10 mg/kg · d olmesartan for 6 wk; OLETF ARB), 4) OLETF + 5% glucose water (HG) for 6 wk (OLETF HG), and 5) OLETF + HG + ARB (OLETF HG/ARB). The glucose response to the oGTT increased 58% in OLETF compared with lean-strain control, whereas glucose supplementation increased it an additional 26%. Blockade of angiotensin receptor reduced the oGTT response 19% in the ARB-treated groups and increased pancreatic insulin secretion 64 and 113% in OLETF ARB and OLETF HG/ARB, respectively. ARB treatment in OLETF ARB and OLETF HG/ARB did not have an effect on insulin signaling proteins in skeletal muscle; however, it reduced pancreatic AT₁ protein expression 20 and 27%, increased pancreatic glucagon-like peptide-1 (GLP-1) receptor protein expression 41 and 88%, respectively, and increased fasting plasma GLP-1 approximately 2.5-fold in OLETF ARB. The results suggest that improvement of glucose intolerance is independent of an improvement in muscle insulin signaling, but rather by improved glucose-stimulated insulin secretion associated with decreased pancreatic AT₁ activation and increased GLP-1 signaling.

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.

Decreased expression of adipose CD36 and FATP1 are associated with increased plasma non-esterified fatty acids during prolonged fasting in northern elephant seal pups (Mirounga angustirostris)

SUMMARY The northern elephant seal pup (Mirounga angustirostris) undergoes a 2–3 month post-weaning fast, during which it depends primarily on the oxidation of fatty acids to meet its energetic demands. The concentration of non-esterified fatty acids (NEFAs) increases and is associated with the development of insulin resistance in late-fasted pups. Furthermore, plasma NEFA concentrations respond differentially to an intravenous glucose tolerance test (ivGTT) depending on fasting duration, suggesting that the effects of glucose on lipid metabolism are altered. However, elucidation of the lipolytic mechanisms including lipase activity during prolonged fasting in mammals is scarce. To assess the impact of fasting and glucose on the regulation of lipid metabolism, adipose tissue and plasma samples were collected before and after ivGTTs performed on early (2 weeks, N=5) and late (6–8 weeks; N=8) fasted pups. Glucose administration increased plasma triglycerides and NEFA concentrations in late-fasted seals, but not plasma glycerol. Fasting decreased basal adipose lipase activity by 50%. Fasting also increased plasma lipase activity twofold and decreased the expressions of CD36, FAS, FATP1 and PEPCK-C by 22–43% in adipose tissue. Plasma acylcarnitine profiling indicated that late-fasted seals display higher incomplete LCFA β-oxidation. Results suggest that long-term fasting induces shifts in the regulation of lipolysis and lipid metabolism associated with the onset of insulin resistance in northern elephant seal pups. Delineation of the mechanisms responsible for this shift in regulation during fasting can contribute to a more thorough understanding of the changes in lipid metabolism associated with dyslipidemia and insulin resistance in mammals.

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.

Insulin and GLP-1 infusions demonstrate the onset of adipose-specific insulin resistance in a large fasting mammal: potential glucogenic role for GLP-1

Prolonged food deprivation increases lipid oxidation and utilization, which may contribute to the onset of the insulin resistance associated with fasting. Because insulin resistance promotes the preservation of glucose and oxidation of fat, it has been suggested to be an adaptive response to food deprivation. However, fasting mammals exhibit hypoinsulinemia, suggesting that the insulin resistance‐like conditions they experience may actually result from reduced pancreatic sensitivity to glucose/capacity to secrete insulin. To determine whether fasting results in insulin resistance or in pancreatic dysfunction, we infused early‐ and late‐fasted seals (naturally adapted to prolonged fasting) with insulin (0.065 U/kg), and a separate group of late‐fasted seals with low (10 pmol/L per kg) or high (100 pmol/L per kg) dosages of glucagon‐like peptide‐1 (GLP‐1) immediately following a glucose bolus (0.5 g/kg), and measured the systemic and cellular responses. Because GLP‐1 facilitates glucose‐stimulated insulin secretion, these infusions provide a method to assess pancreatic insulin‐secreting capacity. Insulin infusions increased the phosphorylation of insulin receptor and Akt in adipose and muscle of early‐ and late‐fasted seals; however, the timing of the signaling response was blunted in adipose of late‐fasted seals. Despite the dose‐dependent increases in insulin and increased glucose clearance (high dose), both GLP‐1 dosages produced increases in plasma cortisol and glucagon, which may have contributed to the glucogenic role of GLP‐1. Results suggest that fasting induces adipose‐specific insulin resistance in elephant seal pups, while maintaining skeletal muscle insulin sensitivity, and therefore suggests that the onset of insulin resistance in fasting mammals is an evolved response to cope with prolonged food deprivation.

Ethnic and Gender Disparities in Adolescent Obesity and Elevated Systolic Blood Pressure in a Rural US Population

A cross-sectional study was conducted to assess the prevalence of overweight, obesity, and elevated systolic blood pressure (SBP) in ethnically diverse adolescents (1064 males; 974 females; 13-17 years) in a rural community. Prevalence of overweight was 20.4% in females and 17.5% in males. In contrast, the prevalence of obesity was 29.4% in males and 14.5% in females. African American males had the highest prevalence of obesity (33.3%) compared with non-Hispanic whites (26.3%). Prevalence of elevated SBP was higher than pre-elevated SBP in males regardless of race/ethnicity, but not in females. Obese females had 4-fold and 9-fold greater odds of developing pre-elevated SBP and elevated SBP, respectively, than their normal weight cohorts. Prevalence of obesity is almost twice that of overweight in males in our rural population suggesting that adolescent males from disadvantaged, rural populations are potentially at a greater risk for metabolic disorders than those in major metropolitan areas.

Prolonged fasting activates Nrf2 in post-weaned elephant seals

SUMMARY Elephant seals naturally experience prolonged periods of absolute food and water deprivation (fasting). In humans, rats and mice, prolonged food deprivation activates the renin–angiotensin system (RAS) and increases oxidative damage. In elephant seals, prolonged fasting activates RAS without increasing oxidative damage likely due to an increase in antioxidant defenses. The mechanism leading to the upregulation of antioxidant defenses during prolonged fasting remains elusive. Therefore, we investigated whether prolonged fasting activates the redox-sensitive transcription factor Nrf2, which controls the expression of antioxidant genes, and if such activation is potentially mediated by systemic increases in RAS. Blood and skeletal muscle samples were collected from seals fasting for 1, 3, 5 and 7 weeks. Nrf2 activity and nuclear content increased by 76% and 167% at week 7. Plasma angiotensin II (Ang II) and transforming growth factor β (TGF-β) were 5000% and 250% higher at week 7 than at week 1. Phosphorylation of Smad2, an effector of Ang II and TGF signaling, increased by 120% at week 7 and by 84% in response to intravenously infused Ang II. NADPH oxidase 4 (Nox4) mRNA expression, which is controlled by smad proteins, increased 430% at week 7, while Nox4 protein expression, which can activate Nrf2, was 170% higher at week 7 than at week 1. These results demonstrate that prolonged fasting activates Nrf2 in elephant seals and that RAS stimulation can potentially result in increased Nox4 through Smad phosphorylation. The results also suggest that Nox4 is essential to sustain the hormetic adaptive response to oxidative stress in fasting seals.

Increased Physical Activity Reduces the Odds of Elevated Systolic BloodPressure Independent of Body Mass or Ethnicity in Rural Adolescents

Objectives: Reduced Physical Activity (PA) has been implicated in the increased prevalence of adolescent obesity and Systolic Blood Pressure (SBP). The present study provides a robust examination of these relationships in Hispanic adolescents from a rural population for which data are scarce. Methods: We compared PA levels, SBP and body mass categories (normal weight, overweight and obese) between non-Hispanic white and Hispanic adolescents (15 ± 0.1 yrs; n=983 males, 911 females) using odds ratio and path analyses. Results: When groups (by gender & ethnicity) were categorized by body mass independent of SBP, prevalence of elevated SBP for obese compared to normal weight cohorts was 3.5- and 12-fold greater for non-Hispanic white males and females, respectively, and 2- and 3-fold greater for Hispanic males and females, respectively. When categorized by SBP independent of Body Mass Index (BMI), prevalence of obesity for adolescents with elevated SBP compared to normotensive cohorts was 3.5- and 6-fold greater for non-Hispanic white males and females, respectively, and 2-fold greater for both Hispanic males and females. Conclusions: Path analyses suggest that both reduced PA and increased BMI are simultaneous predictors of the observed elevation in SBP. Odds ratio analyses revealed that 6+hr PA/wk reduced the probability of developing elevated SBP 3-fold in both genders independent of body mass category or ethnicity, identifying increased PA as a critical behavioral element to target to alleviate the consequences of obesity-related increases in SBP in young people regardless of ethnicity or gender.

Simultaneous GLP-1 receptor activation and angiotensin receptor blockade increase natriuresis independent of altered arterial pressure in obese OLETF rats.

Obesity is associated with an inappropriately activated renin–angiotensin–aldosterone system, suppressed glucagon-like peptide-1 (GLP-1), increased renal Na+ reabsorption, and hypertension. To assess the link between GLP-1 and angiotensin receptor type 1 (AT1) signaling on obesity-associated impairment of urinary Na+ excretion (UNaV) and elevated arterial pressure, we measured mean arterial pressure (MAP) and heart rate by radiotelemetry and metabolic parameters for 40 days. We tested the hypothesis that stimulation of GLP-1 signaling provides added benefit to blockade of AT1 by increasing UNaV and further reducing arterial pressure in the following groups: (1) untreated Long–Evans Tokushima Otsuka (LETO) rats (n = 7); (2) untreated Otsuka Long–Evans Tokushima Fatty (OLETF) rats (n = 9); (3) OLETF + ARB (ARB; 10 mg olmesartan/kg/day; n = 9); (4) OLETF + GLP-1 receptor agonist (EXE; 10 µg exenatide/kg/day; n = 7); and (5) OLETF + ARB + EXE (Combo; n = 6). On day 2, UNaV was 60% and 62% reduced in the EXE and Combo groups, respectively, compared with that in the OLETF rats. On day 40, UNaV was increased 69% in the Combo group compared with that in the OLETF group. On day 40, urinary angiotensinogen was 4.5-fold greater in the OLETF than in the LETO group and was 56%, 62%, and 58% lower in the ARB, EXE, and Combo groups, respectively, than in the OLETF group. From day 2 to the end of the study, MAP was lower in the ARB and Combo groups than in the OLETF rats. These results suggest that GLP-1 receptor activation may reduce intrarenal angiotensin II activity, and that simultaneous blockade of AT1 increases UNaV in obesity; however, these beneficial effects do not translate to a further reduction in MAP.

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.