Frauke Adams

Retinol-Binding Protein 4 in Human Obesity

Studies in mice suggest that adipocytes serve as glucose sensors and regulate systemic glucose metabolism through release of serum retinol-binding protein 4 (RBP4). This model has not been validated in humans. RBP4 was highly expressed in isolated mature human adipocytes and secreted by differentiating human adipocytes. In contrast to the animal data, RBP4 mRNA was downregulated in subcutaneous adipose tissue of obese women, and circulating RBP4 concentrations were similar in normal weight, overweight, and obese women (n = 74). RBP4 was positively correlated with GLUT4 expression in adipose tissue, independent of any obesity-associated variable. Five percent weight loss slightly decreased adipose RBP4 expression but did not influence circulating RBP4. In another set of experiments, we stratified patients (n = 14) by low or high basal fasting interstitial glucose concentrations, as determined by the microdialysis technique. Venous glucose concentrations were similar throughout oral glucose tolerance testing, and basal RBP4 expression in adipose tissue and serum RBP4 concentrations were similar in the groups with higher and lower interstitial glucose levels. Our findings point to profound differences between rodents and humans in the regulation of adipose or circulating RBP4 and challenge the notion that glucose uptake by adipocytes has a dominant role in the regulation of RBP4.

Atrial Natriuretic Peptide Induces Postprandial Lipid Oxidation in Humans

OBJECTIVE—Atrial natriuretic peptide (ANP) regulates arterial blood pressure. In addition, ANP has recently been shown to promote human adipose tissue lipolysis through cGMP-mediated hormone-sensitive lipase activation. We hypothesized that ANP increases postprandial free fatty acid (FFA) availability and energy expenditure while decreasing arterial blood pressure. RESEARCH DESIGN AND METHODS—We infused human ANP (25 ng · kg−1 · min−1) in 12 men (age 32 ± 0.8 years, BMI 23.3 ± 0.4 kg/m2) before, during, and 2 h after ingestion of a standardized high-fat test meal in a randomized, double-blind, cross-over fashion. Cardiovascular changes were monitored by continuous electrocardiogram and beat-by-beat blood pressure recordings. Metabolism was monitored through venous blood sampling, intramuscular and subcutaneous abdominal adipose tissue microdialysis, and indirect calorimetry. RESULTS—ANP infusion decreased mean arterial blood pressure by 4 mmHg during the postprandial phase (P < 0.01 vs. placebo). At the same time, ANP induced lipolysis systemically (P < 0.05 vs. placebo) and locally in subcutaneous abdominal adipose tissue (P < 0.0001 vs. placebo), leading to a 50% increase in venous glycerol (P < 0.01) and FFA (P < 0.05) concentrations compared with placebo. The increase in FFA availability with ANP was paralleled by a 15% increase in lipid oxidation rates (P < 0.05 vs. placebo), driving a substantial increase in postprandial energy expenditure (P < 0.05 vs. placebo). CONCLUSIONS—Our data identify the ANP system as a novel pathway regulating postprandial lipid oxidation, energy expenditure, and concomitantly arterial blood pressure. The findings could have therapeutic implications.

Adipose Tissue Metabolism and CD11b Expression on Monocytes in Obese Hypertensives

At a given degree of adiposity, metabolic and cardiovascular risk varies markedly between individuals. Animal studies suggest that differentially expressed systemic activation of monocytes contributes to the obesity-associated risk variability. We tested the hypothesis that systemic monocyte activation is associated with changes in adipose tissue and skeletal muscle metabolism. In 17 obese hypertensive patients, we assessed CD11b expression on circulating monocytes, gene expression in adipose tissue biopsies, and obtained blood samples and adipose tissue and skeletal muscle microdialysis samples in the fasted state and during a glucose load. Patients were stratified into groups with higher and lower CD11b expression on monocytes. Expression of the macrophage marker CD68 was increased markedly in adipose tissue of subjects with higher CD11b expression. Although no differences in systemic insulin sensitivity were found between both groups, patients with higher peripheral CD11b expression showed a markedly augmented increase in dialysate glucose in adipose tissue during oral glucose tolerance testing and increased adipose tissue lipolysis as well. Our data demonstrate that human monocyte activation is associated with tissue-specific changes in glucose and lipid metabolism. These findings may be explained in part by monocyte/macrophage infiltration of adipose tissue, which appears to interfere with insulin responsiveness.

Tissue-Specific Response to Interstitial Angiotensin II in Humans

Abstract—Angiotensin II is synthesized locally in various tissues; however, the role of interstitial angiotensin II in the regulation of regional metabolism and tissue perfusion is not clear. We characterized the effect of interstially applied angiotensin II in skeletal muscle and subcutaneous adipose tissue of young, normal-weight, healthy subjects by using the microdialysis technique. Furthermore, we tested the hypothesis that the effect of interstitial angiotensin II is modulated by nitric oxide. Tissues were perfused with 0.01, 0.1, and 1 &mgr;mol/L angiotensin II in the presence of the l- or d-isomer of NG-nitro-arginine-methyl ester (L- or D-NAME), the effective and noneffective isomer, respectively, for blocking nitric oxide synthase. Dialysate ethanol, glycerol, glucose, lactate, and pyruvate concentrations were measured to assess changes in blood flow (ethanol dilution technique), lipolysis, and glycolysis, respectively. Baseline blood flow and dialysate concentrations of the metabolites were similar with L- and D-NAME in both tissues. Blood flow and dialysate glucose and lactate did not change significantly in both tissues during perfusion with angiotensin II. Dialysate glycerol dose-dependently increased in adipose tissue (P <0.0438) but decreased in muscle (P <0.007). In muscle, dialysate pyruvate increased (P <0.0002), whereas lactate/pyruvate ratio decreased (P <0.001), both dose-dependently. All effects were similar with L- and D-NAME and could be reversed by nitroprusside. We conclude that in contrast to the profound hemodynamic effect of intravascular angiotensin II, interstitial angiotensin II has a minimal acute effect on blood flow in both tissues. However, interstitial angiotensin II modulates lipid and carbohydrate metabolism in a tissue specific fashion. Thus, the physiology of interstitial angiotensin II cannot be predicted from intravascular studies.

Metabolic Actions Could Confound Advantageous Effects of Combined Angiotensin II Receptor and Neprilysin Inhibition

To the Editor: Dual angiotensin II receptor and neprilysin inhibition with LCZ696 provides additive blood pressure lowering in hypertensive patients, likely because of the well-characterized cardiovascular and renal natriuretic peptide actions.1 However, increased natriuretic peptide availability through neprilysin inhibition could also affect human lipid metabolism. The response could be exploited therapeutically. Yet, adverse effects on metabolic and cardiovascular risk cannot be excluded. Natriuretic peptides stimulate lipolysis in human adipocytes through natriuretic peptide receptor-A activation.2 The response is not attenuated by β-adrenergic receptor blockade in vitro and in vivo. Atrial and brain natriuretic peptides are more potent lipolytic agents than the prototypical β-adrenoreceptor agonist isoproterenol. Systemic atrial natriuretic peptide infusion dose-dependently increases circulating free fatty acid and glycerol concentrations3 and improves postprandial lipid oxidation in human subjects.4 Therefore, medications raising systemic natriuretic peptide levels, such as LCZ696, could augment lipid mobilization and lipid oxidation. Improved lipid mobilization may be beneficial in overweight and obese patients in terms of weight loss. On the other hand, excessive lipid mobilization could promote muscular and hepatic ectopic fat storage and, thus, insulin resistance. …

Influences of levodopa on adipose tissue and skeletal muscle metabolism in patients with idiopathic Parkinson’s disease

ObjectiveThe substantial weight loss in Parkinson’s patients may be related to direct influences of levodopa treatment on fat mobilization/oxidation. We assessed systemic and local metabolic responses to levodopa/benserazide in patients with idiopathic Parkinson’s disease.MethodsWe studied 10 Parkinson’s disease patients and examined adipose tissue and skeletal muscle metabolism directly with microdialysis. We monitored dialysate concentrations of ethanol, glucose, lactate, pyruvate, and glycerol to assess tissue blood flow and metabolism before and after levodopa/benserazide intake. We also conducted in vitro studies on adipocytes from healthy women.ResultsLevodopa/benserazide increased serum levodopa, 3,4-dihydroxyphenylacetic acid (DOPAC), and norepinephrine (P < 0.01). Serum adipose tissue and skeletal muscle glycerol did not change or decreased. Adipose tissue glycerol was inversely correlated with serum levodopa concentrations (P < 0.05). In isolated adipocytes, levodopa attenuated isoproterenol-induced glycerol release (P < 0.05).ConclusionLevodopa/benserazide elicits pronounced metabolic changes in both adipose tissue and skeletal muscle with a switch from lipid to carbohydrate metabolism. In adipose tissue, levodopa/benserazide failed to activate lipolysis. Therefore, we suggest that levodopa/benserazide does not induce fat wasting through direct and acute influences on adipose tissue metabolism.

Hemodynamic and metabolic responses to interstitial angiotensin II in normal weight and obese men

Objective The expression of the AT1 receptor in adipose tissue is not decreased or even increased in obese subjects despite systemic activation of the renin–angiotensin system. Therefore, we hypothesized that peripheral tissues of obese subjects are hypersensitive to angiotensin (Ang) II. Methods We characterized the effect of locally applied Ang II in skeletal muscle and subcutaneous adipose tissue of healthy non-obese (n = 12) and obese (n = 11) men using the microdialysis technique. Tissues were perfused with Ringers solution + ethanol and incremental doses of Ang II (0.01, 0.1 and 1 μmol/l). Dialysate ethanol, glycerol, glucose, lactate, and pyruvate concentrations were measured to assess changes in blood flow (ethanol dilution technique), lipolysis and glycolysis, respectively. Results In adipose tissue, basal ethanol ratio was significantly higher and dialysate metabolite concentrations were significantly lower in obese versus non-obese men. In muscle, basal dialysate glycerol was significantly higher in obese versus non-obese men. Ang II elicited small increases in ethanol ratio and decreases in dialysate glucose in adipose tissue and skeletal muscle in both non-obese and obese men. Dialysate lactate increased significantly in both tissues of obese, but not non-obese men. Dialysate glycerol increased in adipose tissue of non-obese (+ 40%) but not of obese and remained almost unchanged in muscle of both groups. Conclusions Interstitially applied Ang II elicits subtle changes in tissue perfusion and metabolism. However, we did not find a major increase in interstitial Ang II responsiveness in obese men.