Maria E. Trujillo

Visceral Fat Adipokine Secretion Is Associated With Systemic Inflammation in Obese Humans

Although excess visceral fat is associated with noninfectious inflammation, it is not clear whether visceral fat is simply associated with or actually causes metabolic disease in humans. To evaluate the hypothesis that visceral fat promotes systemic inflammation by secreting inflammatory adipokines into the portal circulation that drains visceral fat, we determined adipokine arteriovenous concentration differences across visceral fat, by obtaining portal vein and radial artery blood samples, in 25 extremely obese subjects (mean ± SD BMI 54.7 ± 12.6 kg/m2) during gastric bypass surgery at Barnes-Jewish Hospital in St. Louis, Missouri. Mean plasma interleukin (IL)-6 concentration was ∼50% greater in the portal vein than in the radial artery in obese subjects (P = 0.007). Portal vein IL-6 concentration correlated directly with systemic C-reactive protein concentrations (r = 0.544, P = 0.005). Mean plasma leptin concentration was ∼20% lower in the portal vein than in the radial artery in obese subjects (P = 0.0002). Plasma tumor necrosis factor-α, resistin, macrophage chemoattractant protein-1, and adiponectin concentrations were similar in the portal vein and radial artery in obese subjects. These data suggest that visceral fat is an important site for IL-6 secretion and provide a potential mechanistic link between visceral fat and systemic inflammation in people with abdominal obesity.

Adiponectin – journey from an adipocyte secretory protein to biomarker of the metabolic syndrome

Adiponectin is an adipocyte‐derived hormone that was discovered in 1995. Unlike leptin, which was identified around the same time, the clinical relevance of adiponectin remained obscure for a number of years. However, starting in 2001, several studies were published from different laboratories that highlighted the potential antidiabetic, antiatherosclerotic and anti‐inflammatory properties of this protein complex. Methods to measure the protein with high throughput assays in clinical samples were developed shortly thereafter, and as a result hundreds of clinical studies have been published over the past 3 years describing the role of adiponectin in endocrine and metabolic dysfunction. Furthermore, adiponectin research has expanded to include a role for adiponectin in cancer and other disease areas. Although it is an impossible task to summarize the findings from all these studies in a single review, we aim to demonstrate the utility of circulating adiponectin as a biomarker of the metabolic syndrome. Evidence for this relationship will include how decreased levels of plasma adiponectin (‘hypoadiponectinaemia’) are associated with increased body mass index (BMI), decreased insulin sensitivity, less favourable plasma lipid profiles, increased levels of inflammatory markers and increased risk for the development of cardiovascular disease. Therefore, adiponectin levels hold great promise for use in clinical application serving as a potent indicator of underlying metabolic complications.

Hypoxia-Inducible Factor 1α Induces Fibrosis and Insulin Resistance in White Adipose Tissue

ABSTRACT Adipose tissue can undergo rapid expansion during times of excess caloric intake. Like a rapidly expanding tumor mass, obese adipose tissue becomes hypoxic due to the inability of the vasculature to keep pace with tissue growth. Consequently, during the early stages of obesity, hypoxic conditions cause an increase in the level of hypoxia-inducible factor 1α (HIF1α) expression. Using a transgenic model of overexpression of a constitutively active form of HIF1α, we determined that HIF1α fails to induce the expected proangiogenic response. In contrast, we observed that HIF1α initiates adipose tissue fibrosis, with an associated increase in local inflammation. “Trichrome- and picrosirius red-positive streaks,” enriched in fibrillar collagens, are a hallmark of adipose tissue suffering from the early stages of hypoxia-induced fibrosis. Lysyl oxidase (LOX) is a transcriptional target of HIF1α and acts by cross-linking collagen I and III to form the fibrillar collagen fibers. Inhibition of LOX activity by β-aminoproprionitrile treatment results in a significant improvement in several metabolic parameters and further reduces local adipose tissue inflammation. Collectively, our observations are consistent with a model in which adipose tissue hypoxia serves as an early upstream initiator for adipose tissue dysfunction by inducing a local state of fibrosis.

Adipocyte, Adipose Tissue, and Infectious Disease

The adipocyte and its secretory products have been implicated in a vast array of physiological processes. Work in many laboratories is focusing on the specific contributions of the adipocyte in the areas of metabolism, inflammation, and cancer in order to gain a comprehensive understanding of the systemic influence of the adipocyte under normal and pathophysiological conditions. Until recently, adipose tissue has been considered to be a mere storage compartment of triglycerides. It is now clear that adipocytes are highly active endocrine cells that play a central role in overall energy homeostasis and are important contributors to some aspects of the immune system. They do so not only by influencing systemic lipid homeostasis but also through the production and release of a host of adipocyte-specific and adipocyte-enriched hormonal factors, cytokines, and extracellular matrix components (commonly referred to as “adipokines”). Little attention has been given to the role of adipose tissue in infectious disease. However, the strong proinflammatory potential of adipose tissue suggests an important role in the systemic innate immune response. Furthermore, the adipocyte proper serves as an important target for the intracellular parasite Trypanosoma cruzi, the cause of Chagas’ disease. In chronic Chagas’ disease, adipocytes may represent an important long-term reservoir for parasites from which relapse of infection can occur. In other cases, such as with certain subtypes of adenoviruses, infection with viral particles leads to long-term hyperplasia and hyperproliferation of adipocytes, associating these adenoviral infections with a high propensity for subsequent obesity. Here, we discuss briefly the systemic contributions of adipose tissue to the inflammatory response to infection and provide a brief overview of infectious agents that have a specific impact on adipose tissue. Adipose tissue is composed of many different cell types, but adipocytes are among the most predominant cells. Since different adipose tissue depots show distinct gene expression patterns and vary widely in size and proximity to neighboring organs, individual depots could be viewed as “mini-organs.” Despite differences between the different pads, the depots share similarity with respect to their ability to store lipids and secrete adipose tissue-derived hormones. Adipose tissue stores lipid in the form of triglycerides and cholesterol esters within the lipid droplets that represent specialized organelles inside the adipocyte. Since the lipid droplet is such a large component of the adipocyte (95% of the mass of the adipocyte), changes in the amount of lipid stored within the adipocyte affect fat cell size (ranging from 25 to 250 m). Due to the enormous size of the lipid droplet within a normal adipocyte, these cells were viewed originally merely as lipid storage cells. However, it is now known that adipose tissue acts as an endocrine organ regulating a variety of physiological functions, placing the study of adipokines, or adipose tissue-derived secretory products, at the forefront of research in the field of adipose tissue biology.

Obese Mice Lacking Inducible Nitric Oxide Synthase Are Sensitized to the Metabolic Actions of Peroxisome Proliferator–Activated Receptor-γ Agonism

OBJECTIVE—Synthetic ligands for peroxisome proliferator–activated receptor-γ (PPAR-γ) improve insulin sensitivity in obesity, but it is still unclear whether inflammatory signals modulate their metabolic actions. In this study, we tested whether targeted disruption of inducible nitric oxide (NO) synthase (iNOS), a key inflammatory mediator in obesity, modulates the metabolic effects of rosiglitazone in obese mice. RESEARCH DESIGN AND METHODS—iNOS−/− and iNOS+/+ were subjected to a high-fat diet or standard diet for 18 weeks and were then treated with rosiglitazone for 2 weeks. Whole-body insulin sensitivity and glucose tolerance were determined and metabolic tissues harvested to assess activation of insulin and AMP-activated protein kinase (AMPK) signaling pathways and the levels of inflammatory mediators. RESULTS—Rosiglitazone was found to similarly improve whole-body insulin sensitivity and insulin signaling to Akt/PKB in skeletal muscle of obese iNOS−/− and obese iNOS+/+ mice. However, rosiglitazone further improved glucose tolerance and liver insulin signaling only in obese mice lacking iNOS. This genotype-specific effect of rosiglitazone on glucose tolerance was linked to a markedly increased ability of the drug to raise plasma adiponectin levels. Accordingly, rosiglitazone increased AMPK activation in muscle and liver only in obese iNOS−/− mice. PPAR-γ transcriptional activity was increased in adipose tissue of iNOS−/− mice. Conversely, treatment of 3T3-L1 adipocytes with a NO donor blunted PPAR-γ activity. CONCLUSIONS—Our results identify the iNOS/NO pathway as a critical modulator of PPAR-γ activation and circulating adiponectin levels and show that invalidation of this key inflammatory mediator improves the efficacy of PPAR-γ agonism in an animal model of obesity and insulin resistance.

Systemic pan-AMPK activator MK-8722 improves glucose homeostasis but induces cardiac hypertrophy

Hitting a dozen enzymes with one drug The adenosine monophosphate-activated protein kinase (AMPK) controls cellular energy status. AMPK is activated when energy levels fall. This stimulates adenosine triphosphate (ATP)-generating pathways that promote glucose uptake and inhibits ATP-consuming pathways associated with glucose synthesis. In principle, these effects would be beneficial in metabolic diseases, including diabetes. Pharmacological activation of AMPK has been challenging, however, because in mammals, the enzyme exists as 12 distinct complexes. Myers et al. describe an orally available compound (MK-8722) that activates all 12 complexes (see the Perspective by Hardie). In animal models, MK-8722 ameliorated diabetes, but it also caused enlargement of the heart. MK-8722 may be a useful tool compound for laboratory research on AMPK function. Science, this issue p. 507; see also p. 455 In animals, a drug activating all 12 isoforms of the energy regulator AMPK benefits metabolism but may pose heart risks. 5′-Adenosine monophosphate–activated protein kinase (AMPK) is a master regulator of energy homeostasis in eukaryotes. Despite three decades of investigation, the biological roles of AMPK and its potential as a drug target remain incompletely understood, largely because of a lack of optimized pharmacological tools. We developed MK-8722, a potent, direct, allosteric activator of all 12 mammalian AMPK complexes. In rodents and rhesus monkeys, MK-8722–mediated AMPK activation in skeletal muscle induced robust, durable, insulin-independent glucose uptake and glycogen synthesis, with resultant improvements in glycemia and no evidence of hypoglycemia. These effects translated across species, including diabetic rhesus monkeys, but manifested with concomitant cardiac hypertrophy and increased cardiac glycogen without apparent functional sequelae.

Apoptosis through targeted activation of caspase8 ("ATTAC-mice"): Novel mouse models of inducible and reversible tissue ablation

Recent studies identifying obesity as a significant and increasingly more common cause of morbidity and mortality have intensified research efforts aimed at increasing our understanding of adipose tissue biology. These efforts have culminated in the discovery of several adipokines, or adipose tissue-derived hormones, that have been implicated in the regulation of multiple physiological functions, as well as the realization that adipose tissue dysfunction plays an important role in the pathogenesis of diseases such as obesity and diabetes. To better understand the role of adipose tissue in these physiological/pathological events, several studies have employed transgenic strategies to eliminate adipose tissue. However, these mouse models of congenital lipoatrophy/lipodystrophy exhibit severe metabolic and somatic cell dysfunction. To circumvent this limitation, we have characterized the first inducible fatless mouse. The FAT-ATTAC mouse is a transgenic model whereby expression of a myristoylated caspase 8-FKBP fusion protein enables selective ablation of adipocytes via induction of apoptosis that occurs upon treatment with a chemical dimerizer. The FAT-ATTAC mouse model not only has the advantage that adipocyte ablation be induced at any time during development, but it is also fully reversible, as adipose tissue regenerates after cessation of dimerizer treatment. The inducibility of this fatless mouse model holds potential for revealing novel physiological roles for adipose tissue as well as its contribution to the etiology and pathogenesis of various disease states. Here we describe several ongoing areas of research employing the FAT-ATTAC mouse; in addition we describe potential uses of the targeted transgenic apoptotic approach to study other cell types of interest.

Discovery of MK-1421, a Potent, Selective sstr3 Antagonist, as a Development Candidate for Type 2 Diabetes

The imidazolyl-tetrahydro-β-carboline class of sstr3 antagonists have demonstrated efficacy in a murine model of glucose excursion and may have potential as a treatment for type 2 diabetes. The first candidate in this class caused unacceptable QTc interval prolongation in oral, telemetrized cardiovascular (CV) dogs. Herein, we describe our efforts to identify an acceptable candidate without CV effects. These efforts resulted in the identification of (1R,3R)-3-(4-(5-fluoropyridin-2-yl)-1H-imidazol-2-yl)-1-(1-ethyl-pyrazol-4-yl)-1-(3-methyl-1,3,4-oxadiazol-3H-2-one-5-yl)-2,3,4,9-tetrahydro-1H-β-carboline (17e, MK-1421).

The Discovery of MK-4256, a Potent SSTR3 Antagonist as a Potential Treatment of Type 2 Diabetes

A structure-activity relationship study of the imidazolyl-β-tetrahydrocarboline series identified MK-4256 as a potent, selective SSTR3 antagonist, which demonstrated superior efficacy in a mouse oGTT model. MK-4256 reduced glucose excursion in a dose-dependent fashion with maximal efficacy achieved at doses as low as 0.03 mg/kg po. As compared with glipizide, MK-4256 showed a minimal hypoglycemia risk in mice.

GPR40 reduces food intake and body weight through GLP-1

G protein-coupled receptor 40 (GPR40) partial agonists lower glucose through the potentiation of glucose-stimulated insulin secretion, which is believed to provide significant glucose lowering without the weight gain or hypoglycemic risk associated with exogenous insulin or glucose-independent insulin secretagogues. The class of small-molecule GPR40 modulators, known as AgoPAMs (agonist also capable of acting as positive allosteric modulators), differentiate from partial agonists, binding to a distinct site and functioning as full agonists to stimulate the secretion of both insulin and glucagon-like peptide-1 (GLP-1). Here we show that GPR40 AgoPAMs significantly increase active GLP-1 levels and reduce acute and chronic food intake and body weight in diet-induced obese (DIO) mice. These effects of AgoPAM treatment on food intake are novel and required both GPR40 and GLP-1 receptor signaling pathways, as demonstrated in GPR40 and GLP-1 receptor-null mice. Furthermore, weight loss associated with GPR40 AgoPAMs was accompanied by a significant reduction in gastric motility in these DIO mice. Chronic treatment with a GPR40 AgoPAM, in combination with a dipeptidyl peptidase IV inhibitor, synergistically decreased food intake and body weight in the mouse. The effect of GPR40 AgoPAMs on GLP-1 secretion was recapitulated in lean, healthy rhesus macaque demonstrating that the putative mechanism mediating weight loss translates to higher species. Together, our data indicate effects of AgoPAMs that go beyond glucose lowering previously observed with GPR40 partial agonist treatment with additional potential for weight loss.