Juergen Eckel

Adaptive immunity in obesity and insulin resistance

Obesity is the hallmark of the metabolic syndrome and predisposes patients to the development of major chronic metabolic diseases including type 2 diabetes mellitus. Adipose tissue expansion in obesity is characterized by increasing infiltration of proinflammatory immune cells into adipose tissue causing chronic, low-grade inflammation. Phenotypic switching of macrophages is an important mechanism of adipose tissue inflammation, and there is involvement of cells from the adaptive immune system in this process. T-cell phenotype changes and recruitment of B cells and T cells precedes macrophage infiltration. Cytokines and chemokines produced by immune cells influence localized and systemic inflammation, which is a pathogenic link between obesity and insulin resistance. Antigens absorbed from the gut might contribute to T-cell activation and recruitment into visceral adipose tissue in obesity. This Review summarizes, in the context of obesity, the evidence for infiltration of adipose tissue by cells of the adaptive immune system, how adaptive system cells affect innate cell populations and the influence of adaptive immune cells on the development of insulin resistance.

Evidence against a Beneficial Effect of Irisin in Humans

Brown adipose tissue has gained interest as a potential target to treat obesity and metabolic diseases. Irisin is a newly identified hormone secreted from skeletal muscle enhancing browning of white fat cells, which improves systemic metabolism by increasing energy expenditure in mice. The discovery of irisin raised expectations of its therapeutic potential to treat metabolic diseases. However, the effect of irisin in humans is unclear. Analyses of genomic DNA, mRNA and expressed sequence tags revealed that FNDC5, the gene encoding the precursor of irisin, is present in rodents and most primates, but shows in humans a mutation in the conserved start codon ATG to ATA. HEK293 cells transfected with a human FNDC5 construct with ATA as start codon resulted in only 1% full-length protein compared to human FNDC5 with ATG. Additionally, in vitro contraction of primary human myotubes by electrical pulse stimulation induced a significant increase in PGC1α mRNA expression. However, FNDC5 mRNA level was not altered. FNDC5 mRNA expression in muscle biopsies from two different human exercise studies was not changed by endurance or strength training. Preadipocytes isolated from human subcutaneous adipose tissue exhibited differentiation to brite human adipocytes when incubated with bone morphogenetic protein (BMP) 7, but neither recombinant FNDC5 nor irisin were effective. In conclusion, our findings suggest that it is rather unlikely that the beneficial effect of irisin observed in mice can be translated to humans.

The role of epicardial and perivascular adipose tissue in the pathophysiology of cardiovascular disease.

•  Introduction •  Fat depots around the heart and vasculature ‐  Definitions ‐  Epicardial adipose tissue ‐  Perivascular adipose tissue ‐  Pericardial adipose tissue •  Physiological function of epicardial adipose tissue •  Visualization of epicardial adipose tissue •  Epicardial fat thickness as diagnostic marker ‐  Association of epicardial fat with insulin resistance and T2DM ‐  Association of epicardial fat with cardiovascular dysfunction ‐  Association of epicardial fat with the metabolic syndrome ‐  Association of epicardial fat with adipokines ‐  Effect of weight loss on epicardial adipose tissue •  Visualization of perivascular adipose tissue •  Expression and secretion of fatty acids and adipokines from epicardial and perivascular fat depots ‐  Fatty acids ‐  Adipokines •  Cross‐talk between secretory products from EAT and the myocardium •  Cross‐talk between secretory products from PVAT and the vasculature ‐  Vasorelaxation ‐  Vasoconstriction ‐  Vascular remodelling ‐  Alterations in obesity and insulin resistance •  Perspectives and conclusions

Inflammation and metabolic dysfunction: links to cardiovascular diseases

Abdominal obesity is a major risk factor for cardiovascular disease, and recent studies highlight a key role of adipose tissue dysfunction, inflammation, and aberrant adipokine release in this process. An increased demand for lipid storage results in both hyperplasia and hypertrophy, finally leading to chronic inflammation, hypoxia, and a phenotypic change of the cellular components of adipose tissue, collectively leading to a substantially altered secretory output of adipose tissue. In this review we have assessed the adipo-vascular axis, and an overview of adipokines associated with cardiovascular disease is provided. This resulted in a first list of more than 30 adipokines. A deeper analysis only considered adipokines that have been reported to impact on inflammation and NF-κB activation in the vasculature. Out of these, the most prominent link to cardiovascular disease was found for leptin, TNF-α, adipocyte fatty acid-binding protein, interleukins, and several novel adipokines such as lipocalin-2 and pigment epithelium-derived factor. Future work will need to address the potential role of these molecules as biomarkers and/or drug targets.

DPP4 in Diabetes

Dipeptidyl-peptidase 4 (DPP4) is a glycoprotein of 110 kDa, which is ubiquitously expressed on the surface of a variety of cells. This exopeptidase selectively cleaves N-terminal dipeptides from a variety of substrates, including cytokines, growth factors, neuropeptides, and the incretin hormones. Expression of DPP4 is substantially dysregulated in a variety of disease states including inflammation, cancer, obesity, and diabetes. Since the incretin hormones, glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide (GIP), are major regulators of post-prandial insulin secretion, inhibition of DPP4 by the gliptin family of drugs has gained considerable interest for the therapy of type 2 diabetic patients. In this review, we summarize the current knowledge on the DPP4–incretin axis and evaluate most recent findings on DPP4 inhibitors. Furthermore, DPP4 as a type II transmembrane protein is also known to be cleaved from the cell membrane involving different metalloproteases in a cell-type-specific manner. Circulating, soluble DPP4 has been identified as a new adipokine, which exerts both para- and endocrine effects. Recently, a novel receptor for soluble DPP4 has been identified, and data are accumulating that the adipokine-related effects of DPP4 may play an important role in the pathogenesis of cardiovascular disease. Importantly, circulating DPP4 is augmented in obese and type 2 diabetic subjects, and it may represent a molecular link between obesity and vascular dysfunction. A critical evaluation of the impact of circulating DPP4 is presented, and the potential role of DPP4 inhibition at this level is also discussed.

Browning of white fat: does irisin play a role in humans?

The discovery of irisin as an exercise-regulated myokine inducing browning of WAT has gained interest as a potential new strategy to combat obesity and its associated disorders, such as type 2 diabetes. However, there are inconsistencies regarding the relevance of irisin in humans. The regulation of FNDC5 mRNA expression by exercise and contraction could not be reproduced by a number of human studies using several exercise protocols and in vitro approaches. Furthermore, the nature of FNDC5 fragments and the presence of irisin in humans are questionable and probably contribute to conflicting data obtained with commercially available ELISA kits. Most importantly, the information regarding the concentration of circulating irisin in humans is not clear, as different studies using different kits measure irisin levels in a wide range. Data about the role of irisin in states of human obesity and metabolic diseases are conflicting and, in some cases, changes in irisin levels have been observed; they were only moderate in 10-20%. Independent of the presence and regulation of FNDC5/irisin in humans, the application of recombinant irisin could still represent a therapeutic strategy to fight obesity. However, the current data obtained from human cell models reveal that FNDC5/irisin has no effect on browning of the major WAT depots in humans and is likely to selectively target a small subpopulation of adipocytes, which are located in classical BAT regions, such as the supraclavicular adipose tissue. Thus, other candidates, such as BMP7 or CNPs, seem to be more prominent candidates as inducers of browning in humans.

Secretory Type II Phospholipase A2 Is Produced and Secreted by Epicardial Adipose Tissue and Overexpressed in Patients with Coronary Artery Disease

CONTEXT Epicardial adipose tissue (EAT) is a visceral adipose tissue in close contact with coronary vessels, the excess of which is associated with coronary artery disease (CAD). OBJECTIVE Our goal was to identify candidate molecule(s) characterizing EAT that could intervene in the pathogenesis of CAD. DESIGN An approach combining microarrays and bioinformatic sequence analysis tools for predicting secreted proteins (TargetP) was applied to paired biopsies of sc adipose tissue (SAT) and EAT, obtained from patients with or without CAD (NCAD). RESULTS were validated in three independent groups of subjects by quantitative RT-PCR, Western blot, immunohistochemistry, and explant secretion. RESULTS Secretory type II phospholipase A2 (sPLA2-IIA) ranked as the highest gene coding for potentially secreted proteins with the highest overexpression in EAT in both CAD and NCAD. Quantitative RT-PCR confirmed its increased expression in EAT (P < 0.01) as well as EAT from CAD as compared with NCAD (49.3 +/- 13 vs. 17.4 +/- 9.7 P < 0.01). sPLA2-IIA protein levels were higher in EAT than SAT (P < 0.001). EAT explants also showed significantly higher sPLA2-IIA secretion levels than SAT ones (4.37 +/- 2.7 vs. 0.67 +/- 0.28 ng/ml to 1 per gram tissue per 24 h, P < 0.03). sPLA2-IIA labeling was seen in the stroma vascular fraction between adipocytes and in connective capsules in EAT, with no immunostaining of the adipocytes. SAT was weakly labeled following the same process. CONCLUSION We have shown for the first time an increased expression of sPLA2-IIA in EAT in patients with CAD. sPLA2-IIA is a phospholipase, which has been shown to be an independent risk factor for CAD. These findings suggest that EAT has a potentially pathophysiological role in CAD.

Functional annotation of the human fat cell secretome.

Context: Recent secretome analyses suggest that human fat cells secrete hundreds of proteins (adipokines). Objective: We made an overall analysis of their potential functional importance. Materials and methods: A secretome of 347 adipokines was evaluated by in silico analysis of their expression during adipocyte differentiation, regulation by obesity and adipose region. The gene expression in human adipose tissue was investigated in microarray studies using samples from different adipose depots from lean or obese patients. Results: 60% of the adipokines were regulated by obesity and 50% between visceral and subcutaneous adipose region. Eight adipokines, all novel, scored particularly high in the in silico analysis. Among those, four were both regulated by obesity and adipose region, namely WNT1-inducible-signaling pathway protein 2, transmembrane glycoprotein NMB, inter-alpha-trypsin inhibitor heavy chain H5, and complement C4-A. Furthermore, many adipokines were extracellular matrix proteins. Conclusion: Several novel adipokines have potential important functional features warranting in depth analysis.

Cardioprotective properties of omentin-1 in type 2 diabetes: evidence from clinical and in vitro studies.

Context Adipokines are linked to the development of cardiovascular dysfunction in type 2 diabetes (DM2). In DM2-patients, circulating levels of omentin-1, an adipokine preferentially expressed in epicardial adipose tissue, are decreased. This study investigated whether omentin-1 has a cardioprotective function. Methods Omentin-1 levels in plasma and cardiac fat depots were determined in DM2-patients versus controls. Moreover, the relation between omentin-1 levels and cardiac function was examined in men with uncomplicated DM2. Finally, we determined whether omentin-1 could reverse the induction of cardiomyocyte dysfunction by conditioned media derived from epicardial adipose tissue from patients with DM2. Results Omentin-1 was highly expressed and secreted by epicardial adipose tissue, and reduced in DM2. Circulating omentin-1 levels were lower in DM2 versus controls, and positively correlated with the diastolic parameters early peak filling rate, early deceleration peak and early deceleration mean (all P<0.05). The improved diastolic function following pioglitazone treatment associated with increases in omentin-1 levels (P<0.05). In vitro, exposure of cardiomyocytes to conditioned media derived from epicardial adipose tissue from patients with DM2 induced contractile dysfunction and insulin resistance, which was prevented by the addition of recombinant omentin. Conclusion These data identify omentin-1 as a cardioprotective adipokine, and indicate that decreases in omentin-1 levels could contribute to the induction of cardiovascular dysfunction in DM2.

Role of lipid-derived mediators in skeletal muscle insulin resistance

Imbalance between nutritional intake and energy expenditure has been described to culminate in obesity, which predisposes to insulin resistance and type 2 diabetes mellitus. In such states of energy oversupply, excess amounts of lipids are available in tissues and circulation. Over the past years, an increasingly important role in development of skeletal muscle (SkM) insulin resistance has been attributed to lipids and impaired fatty acid metabolism. In this review, we reflect the current state of knowledge about the effects of various lipid-derived mediators on SkM insulin sensitivity. Furthermore, potential mechanisms underlying the biogenesis of intramyocellular ectopic lipid stores are discussed. Previously, a pivotal role was attributed to mitochondrial dysfunction. However, results of recent studies have suggested an important role for exercise deficiency, accompanied by decreased expression levels of peroxisome proliferator-activated receptor-γ coactivator-1α and subsequent, incomplete β-oxidation. Additionally, we summarize the implications of increased levels of lipid-derived endocannabinoids (ECs) for metabolic control in peripheral tissue and highlight the benefits of targeting the EC system.