Greg R. Collier

Neural mechanisms in human obesity-related hypertension.

OBJECTIVE Two hypotheses concerning mechanisms of weight gain and of blood pressure elevation in obesity were tested. The first hypothesis is that in human obesity sympathetic nervous system underactivity is present, as a metabolic basis for the obesity. The second hypothesis, attributable to Landsberg, is that sympathetic nervous activation occurs with chronic overeating, elevating blood pressure. These are not mutually exclusive hypotheses, since obesity is a heterogeneous disorder. DESIGN AND METHODS Whole body and regional sympathetic nervous system activity, in the kidneys and heart, was measured at rest using noradrenaline isotope dilution methodology in a total of 86 research voluteers in four different subject groups, in lean and in obese people who either did, or did not, have high blood pressure. RESULTS In the lean hypertensive patients, noradrenaline spillover for the whole body, and from the heart and kidneys was substantially higher than in the healthy lean volunteers. In normotensive obesity, the whole body noradrenaline spillover rate was normal, mean renal noradrenaline spillover was elevated (twice normal), and cardiac noradrenaline spillover reduced by approximately 50%. In obesity-related hypertension, there was elevation of renal noradrenaline spillover, comparable to that present in normotensive obese individuals but not accompanied by suppression of cardiac noradrenaline spillover, which was more than double that of normotensive obese individuals (P<0.05), and 25% higher than in healthy volunteers. There was a parallel elevation of heart rate in hypertensive obese individuals. CONCLUSIONS The sympathetic underactivity hypothesis of obesity causation now looks untenable, as based on measures of noradrenaline spillover, sympathetic nervous system activity was normal for the whole body and increased for the kidneys; the low sympathetic activity in the heart would have only a trifling impact on total energy balance. The increase in renal sympathetic activity in obesity may possibly be a necessary cause for the development of hypertension in obese individuals, although clearly not a sufficient cause, being present in both normotensive and hypertensive obese individuals. The discriminating feature of obesity-related hypertension was an absence of the suppression of the cardiac sympathetic outflow seen in normotensive obese individuals. Sympathetic nervous changes in obesity-related hypertension conformed rather closely to those expected from the Landsberg hypothesis.

Genetic variation in selenoprotein S influences inflammatory response

Chronic inflammation has a pathological role in many common diseases and is influenced by both genetic and environmental factors. Here we assess the role of genetic variation in selenoprotein S (SEPS1, also called SELS or SELENOS), a gene involved in stress response in the endoplasmic reticulum and inflammation control. After resequencing SEPS1, we genotyped 13 SNPs in 522 individuals from 92 families. As inflammation biomarkers, we measured plasma levels of IL-6, IL-1β and TNF-α. Bayesian quantitative trait nucleotide analysis identified associations between SEPS1 polymorphisms and all three proinflammatory cytokines. One promoter variant, −105G → A, showed strong evidence for an association with each cytokine (multivariate P = 0.0000002). Functional analysis of this polymorphism showed that the A variant significantly impaired SEPS1 expression after exposure to endoplasmic reticulum stress agents (P = 0.00006). Furthermore, suppression of SEPS1 by short interfering RNA in macrophage cells increased the release of IL-6 and TNF-α. To investigate further the significance of the observed associations, we genotyped −105G → A in 419 Mexican American individuals from 23 families for replication. This analysis confirmed a significant association with both TNF-α (P = 0.0049) and IL-1β (P = 0.0101). These results provide a direct mechanistic link between SEPS1 and the production of inflammatory cytokines and suggest that SEPS1 has a role in mediating inflammation.

Regulation of the selenoprotein SelS by glucose deprivation and endoplasmic reticulum stress - SelS is a novel glucose-regulated protein.

SelS is a newly identified selenoprotein and its gene expression is up‐regulated in the liver of Psammomys obesus after fasting. We have examined whether SelS is regulated by glucose deprivation and endoplasmic reticulum (ER) stress in HepG2 cells. Glucose deprivation and the ER stress inducers tunicamycin and thapsigargin increased SelS gene expression and protein content several‐fold in parallel with glucose‐regulated protein 78. The overexpression of SelS increased Min6 cell resistance to oxidative stress‐induced toxicity. These results indicate that SelS is a novel member of the glucose‐regulated protein family and its function is related to the regulation of cellular redox balance.

Leptin in human plasma is derived in part from the brain, and cleared by the kidneys

Leptin is believed to exert a regulatory influence on food intake and use of energy after gaining access to the brain, possibly via a specific transfer mechanism. It has been suggested that in obesity the primary flaw may be an inability of leptin to enter the central nervous system. We tested, with simultaneous arterial and internal jugular venous sampling, whether uptake of leptin occurs in the brain, and whether this process might be defective in obesity. In 39 fasting men aged 44 (SE 3) years, age range 19–76 years, and of body mass index (BMI) 27·2 (0·7) kg/m (range 19·6–38·5), arteriovenous plasma concentration gradients of leptin were measured across the brain (n=15), kidneys (13), gut and liver (11), heart (nine), and forearm (15), by way of simultaneous venous and radial or brachial arterial blood sampling. With repositioning of the central venous catheter, sampling was done in up to four venous sites. 20 men were obese (BMI >28) and 19 were lean (BMI <26). Central venous catheterisation and measurement of internal jugular and renal plasma flows was performed with our previously described techniques, with appropriate consent by the volunteers, and institutional ethics committee review. Leptin was measured by a radioimmunoassay (RIA; Linco, St Charles, MO, USA), with a sensitivity limit of 0·5 ng/mL and intra-assay coefficient of variation 5%. The concentration of leptin in arterial plasma overall was 7·2 (1·1) ng/mL. The leptin concentration in internal jugular venous plasma was on average 21% higher than the corresponding arterial value (6·58 vs 5·53 ng/mL [p<0·03]), which with the existing mean plasma flow of 205 mL/min equated to a unilateral overflow of leptin into the internal jugular vein of 208 (126) ng/min. In contrast, there was a mean extraction of leptin from plasma of 17% in passage through the kidneys, with a mean removal rate of 1833 ng/min, equivalent to a renal plasma clearance of 145 (92) mL/min at the existing mean renal plasma flow of 832 mL/min. There was no detectable net flux of leptin to or from plasma in passage through the heart, the gut and liver, or the forearm. In the five obese men in which this was measured, cerebral leptin overflow was 451 (349) ng/min, compared with 86 (24) ng/min in ten lean men (p<0·05). Renal clearance of leptin was unaffected by obesity. Our results suggest that the brain is a source of leptin circulating in plasma. Alternative explanations, such as modification of the extent to which leptin is bound to plasma protein during passage through the cerebrovascular circulation are less likely. This is an unexpected finding, as tissue expression of leptin to date has been described only in adipose tissue and placenta. Release of leptin from the brain to the circulation was higher in obese than lean men, suggesting that the rise in plasma leptin concentration in human obesity is not entirely attributable to increased release of leptin from adipose tissue.

Characterization of obesity phenotypes in Psammomys obesus (Israeli sand rats).

Psammomys obesus (the Israeli sand rat) has been well studied as an animal model of Type 2 diabetes. However, obesity phenotypes in these animals have not been fully characterized. We analyzed phenotypic data including body weight, percentage body fat, blood glucose and plasma insulin concentration for over 600 animals from the Psammomys obesus colony at Deakin University to investigate the relationships between body fat, body weight and Type 2 diabetes using regression analysis and general linear modelling. The body weight distribution in Psammomys obesus approximates a normal distribution and closely resembles that observed in human populations. Animals above the 75th percentile for body weight had increased body fat content and a greater risk of developing diabetes. Increased visceral fat content .was also associated with elevated blood glucose and plasma insulin concentrations in these animals. A familial effect was also demonstrated in Psammomys obesus, and accounted for 51% of the variation in body weight, and 23–26% of the variation in blood glucose and plasma insulin concentrations in these animals. Psammomys obesus represents an excellent animal model of.obesity and Type 2 diabetes that exhibits a phenotypic pattern closely resembling that observed in human population studies. The obesity described in these animals was familial in nature and was significantly associated with Type 2 diabetes.

Regulation of skeletal muscle oxidative capacity and insulin signaling by the mitochondrial rhomboid protease PARL.

Type 2 diabetes mellitus (T2DM) and aging are characterized by insulin resistance and impaired mitochondrial energetics. In lower organisms, remodeling by the protease pcp1 (PARL ortholog) maintains the function and lifecycle of mitochondria. We examined whether variation in PARL protein content is associated with mitochondrial abnormalities and insulin resistance. PARL mRNA and mitochondrial mass were both reduced in elderly subjects and in subjects with T2DM. Muscle knockdown of PARL in mice resulted in malformed mitochondrial cristae, lower mitochondrial content, decreased PGC1alpha protein levels, and impaired insulin signaling. Suppression of PARL protein in healthy myotubes lowered mitochondrial mass and insulin-stimulated glycogen synthesis and increased reactive oxygen species production. We propose that lower PARL expression may contribute to the mitochondrial abnormalities seen in aging and T2DM.

Lifestyle factors fail to explain the variation in plasma leptin concentrations in women

To assess the relationship between circulating leptin concentrations, metabolic parameters, and lifestyle factors such as alcohol intake, physical activity level, smoking habits, and reproductive history, a cohort of 359 women was drawn from a population-based study conducted in Victoria, Australia. The parameters measured included body mass index (BMI); waist and hip circumference; blood pressure; and fasting glucose, insulin, triacylglycerol, cholesterol, and leptin concentrations. In addition, a self-administered questionnaire was used to assess reproductive history, physical activity level, alcohol intake, and smoking habits. Our results demonstrated that BMI, body weight, waist circumference, and hip circumference were all strongly correlated with circulating leptin concentrations in this population (r > 0.56, P < 0.001 in all cases). Waist/hip ratio, triacylglycerols, insulin, glucose, and cholesterol were also associated with leptin (P < 0.05), but there was no association between leptin and age, height, or blood pressure. When these associations were adjusted for BMI, age, glucose, and waist circumference were significantly associated with leptin. The lifestyle factors examined did not help to explain the observed variation in leptin concentrations between individuals when results were adjusted for degree of adiposity and age.

Quantitation of tryptophan and other plasma amino acids by automated pre-column o-phthaldialdehyde derivatization high-performance liquid chromatography: improved sample preparation

Pre-column derivatization with o-phthaldialdehyde is a rapid and sensitive method for the quantitation of amino acids in biological fluids. This method uses acetonitrile as a deproteinizing reagent which gives improved recovery of tryptophan compared with 5-sulfosalicylic acid and permits the measurement of aspartic acid which coelutes with 5-sulfosalicylic acid. The method is automated to increase reproducibility and convenience. Mean coefficients of variation for peak areas relative to internal standard were 3.2 and 5.2% for amino acid standards and plasma samples, respectively. The presence of nitrilotriacetic acid stabilized the o-phthaldialdehyde reagent which is important in an automated system. The method is suitable for the analysis of large numbers of plasma samples where total tryptophan and aspartic acid are of interest.

Decorin is a secreted protein associated with obesity and type 2 diabetes

Objective:To characterize the expression of the small leucine-rich glycoprotein decorin in adipose tissue.Design:Real-time PCR was used to measure decorin gene expression in adipose tissue from normal glucose tolerant (NGT), impaired glucose tolerant and type 2 diabetic (T2D) Psammomys obesus. Adipose tissue was fractionated to determine which cells were responsible for decorin expression. The location of decorin protein expression in adipose tissue was determined using immunohistochemistry. Real-time PCR was used to measure decorin mRNA levels in human adipose tissue from 16 insulin-sensitive, 16 insulin-resistant and 6 T2D human subjects. Circulating plasma decorin concentrations were measured by enzyme-linked immunosorbent assay in 145 NGT and 141 T2D human individuals from a large-scale epidemiological study in Mauritius.Results:Decorin mRNA was found to be highly expressed in adipose tissue, and decorin gene expression was significantly higher in visceral than that in subcutaneous adipose tissue depots in both P. obesus and human subjects (P=0.002 and P=0.001, respectively). Decorin mRNA was predominantly expressed by stromal/vascular cells of adipose tissue, and decorin protein in adipose tissue was primarily detected adjacent to blood vessels. Circulating plasma decorin levels in humans were elevated by 12% in T2D (P=0.049) compared to NGT subjects. There was a significant independent correlation between plasma decorin levels and waist-to-hip ratio (WHR, P=0.024). In male subjects, plasma decorin levels were significantly correlated with WHR (P=0.006), and fasting and 2-h glucose levels in an oral glucose tolerance test (P=0.027 and P=0.001, respectively).Conclusions:Decorin expression in adipose tissue was markedly upregulated in the obese state and may therefore play a role in adipose tissue homeostasis or in pathophysiology associated with obesity.

Clinical efficacy of metformin against insulin resistance parameters : Sinking the iceberg

It has been increasingly recognised in recent years that type 2 (non-insulin-dependent) diabetes is part of a cluster of cardiovascular risk factors known as the metabolic syndrome, but also endorsed with such names as the deadly quartet, syndrome X and the insulin resistance syndrome. Atherosclerosis is the most common complication of type 2 diabetes among Europeans, and coronary artery, cerebrovascular and peripheral vascular disease are 2 to 5 times more common in people with this condition than in those without diabetes.These observations indicate that the treatment of type 2 diabetes requires agents that do more than simply lower blood glucose levels, and a therapy with both antihyperglycaemic effects and beneficial effects on dyslipidaemia, hypertension, obesity, hyperinsulinaemia and insulin resistance is likely to be most useful. In this respect, metformin has an important and established role: this drug has been shown to lower blood glucose and triglyceride levels, and to assist with weight reduction and to reduce hyperinsulinaemia and insulin resistance.Studies in the Israeli sand rat, Psammomys obesus, have indicated hyperinsulinaemia/insulin resistance to be the initial and underlying metabolic disorder in obesity and type 2 diabetes. Thus, the well established effect of metformin in reducing insulin resistance makes this drug an excellent candidate for the prevention of progression of impaired glucose tolerance to type 2 diabetes, and for the reduction of mortality associated with cardiovascular disease.