Florian Kahles

Osteopontin: A Novel Regulator at the Cross Roads of Inflammation, Obesity and Diabetes

Since its first description more than 20 years ago osteopontin has emerged as an active player in many physiological and pathological processes, including biomineralization, tissue remodeling and inflammation. As an extracellular matrix protein and proinflammatory cytokine osteopontin is thought to facilitate the recruitment of monocytes/macrophages and to mediate cytokine secretion in leukocytes. Modulation of immune cell response by osteopontin has been associated with various inflammatory diseases and may play a pivotal role in the development of adipose tissue inflammation and insulin resistance. Here we summarize recent findings on the role of osteopontin in metabolic disorders, particularly focusing on diabetes and obesity.

GLP-1 secretion is increased by inflammatory stimuli in an IL-6-dependent manner, leading to hyperinsulinemia and blood glucose lowering.

Hypoglycemia and hyperglycemia are both predictors for adverse outcome in critically ill patients. Hyperinsulinemia is induced by inflammatory stimuli as a relevant mechanism for glucose lowering in the critically ill. The incretine hormone GLP-1 was currently found to be induced by endotoxin, leading to insulin secretion and glucose lowering under inflammatory conditions in mice. Here, we describe GLP-1 secretion to be increased by a variety of inflammatory stimuli, including endotoxin, interleukin-1β (IL-1β), and IL-6. Although abrogation of IL-1 signaling proved insufficient to prevent endotoxin-dependent GLP-1 induction, this was abolished in the absence of IL-6 in respective knockout animals. Hence, we found endotoxin-dependent GLP-1 secretion to be mediated by an inflammatory cascade, with IL-6 being necessary and sufficient for GLP-1 induction. Functionally, augmentation of the GLP-1 system by pharmacological inhibition of DPP-4 caused hyperinsulinemia, suppression of glucagon release, and glucose lowering under endotoxic conditions, whereas inhibition of the GLP-1 receptor led to the opposite effect. Furthermore, total GLP-1 plasma levels were profoundly increased in 155 critically ill patients presenting to the intensive care unit (ICU) in comparison with 134 healthy control subjects. In the ICU cohort, GLP-1 plasma levels correlated with markers of inflammation and disease severity. Consequently, GLP-1 provides a novel link between the immune system and the gut with strong relevance for metabolic regulation in context of inflammation.

Glucagon-like peptide-1 (GLP-1) and its split products GLP-1(9-37) and GLP-1(28-37) stabilize atherosclerotic lesions in apoe−/− mice

BACKGROUND [corrected] Glucagon-like peptide-1 (GLP-1) based therapies are new treatment options for patients with type 2 diabetes. Recent reports suggest vasoprotective actions of GLP-1. Similar beneficial effects might be reached by GLP-1(9-37) and the c-terminal GLP-1 split product (28-37) although both peptides do not activate the GLP-1 receptor. We therefore investigated the actions of GLP-1(7-37), GLP-1(9-37) as well as GLP-1(28-37) on vascular lesion formation in a mouse model of atherosclerosis. METHODS AND RESULTS GLP-1(7-37), GLP-1(9-37) and GLP-1(28-37) and LacZ (control) were overexpressed for a period of 12 weeks in apoe(-/-) mice on high-fat diet (n = 10/group) using an adeno-associated viral vector system. Neither of the constructs changed overall lesion size. However, GLP-1(7-37), GLP-1(9-37) and GLP-1(28-37) significantly reduced plaque macrophage infiltration (GLP-1(7-37): 40.6%, GLP-1(9-37): 47.0%, GLP-1(28-37): 40.1% decrease, p < 0.05) and plaque MMP-9 expression (GLP-1(7-37): 41.6%, GLP-1(9-37): 50.2%, GLP-1(28-37): 44.0% decrease, p < 0.05) compared to LacZ in the aortic arch. Moreover, all GLP-1 constructs increased plaque collagen content (GLP-1(7-37): 49.3%, GLP-1(9-37): 86.0%, GLP-1(28-37): 81.9% increase, p < 0.05) and increased fibrous cap thickness (GLP-1(7-37): 188.0%, GLP-1(9-37): 179.9% GLP-1(28-37): 111.0% increase, p < 0.05). These effects of GLP-1(7-37), GLP-1(9-37) and GLP-1(28-37) on plaque macrophage infiltration, MMP-9 expression and plaque collagen content were confirmed in the aortic root. CONCLUSION GLP-1(7-37), GLP-1(9-37) and GLP-1(28-37) reduce plaque inflammation and increase phenotypic characteristics of plaque stability in a murine model of atherosclerosis. Future studies are needed to determine whether these effects translate into improved plaque stability and less cardiovascular events in high-risk patients with type 2 diabetes.

Epigenetic Regulation of Vascular Smooth Muscle Cell Function in Atherosclerosis

Epigenetics involve heritable and acquired changes in gene transcription that occur independently of the DNA sequence. Epigenetic mechanisms constitute a hierarchic upper-level of transcriptional control through complex modifications of chromosomal components and nuclear structures. These modifications include, for example, DNA methylation or post-translational modifications of core histones; they are mediated by various chromatin-modifying enzymes; and ultimately they define the accessibility of a transcriptional complex to its target DNA. Integrating epigenetic mechanisms into the pathophysiologic concept of complex and multifactorial diseases such as atherosclerosis may significantly enhance our understanding of related mechanisms and provide promising therapeutic approaches. Although still in its infancy, intriguing scientific progress has begun to elucidate the role of epigenetic mechanisms in vascular biology, particularly in the control of smooth muscle cell phenotypes. In this review, we will summarize epigenetic pathways in smooth muscle cells, focusing on mechanisms involved in the regulation of vascular remodeling.

PDE4 inhibition reduces neointima formation and inhibits VCAM-1 expression and histone methylation in an Epac-dependent manner

Phosphodiesterase 4 (PDE4) activity mediates cAMP-dependent smooth muscle cell (SMC) activation following vascular injury. In this study we have investigated the effects of specific PDE4 inhibition with roflumilast on SMC proliferation and inflammatory activation in vitro and neointima formation following guide wire-induced injury of the femoral artery in mice in vivo. In vitro, roflumilast did not affect SMC proliferation, but diminished TNF-α induced expression of the vascular cell adhesion molecule 1 (VCAM-1). Specific activation of the cAMP effector Epac, but not PKA activation mimicked the effects of roflumilast on VCAM-1 expression. Consistently, the reduction of VCAM-1 expression was rescued following inhibition of Epac. TNF-α induced NFκB p65 translocation and VCAM-1 promoter activity were not altered by roflumilast in SMCs. However, roflumilast treatment and Epac activation repressed the induction of the activating epigenetic histone mark H3K4me2 at the VCAM-1 promoter, while PKA activation showed no effect. Furthermore, HDAC inhibition blocked the inhibitory effect of roflumilast on VCAM-1 expression. Both, roflumilast and Epac activation reduced monocyte adhesion to SMCs in vitro. Finally, roflumilast treatment attenuated femoral artery intima-media ratio by more than 50% after 4weeks. In summary, PDE4 inhibition regulates VCAM-1 through a novel Epac-dependent mechanism, which involves regulatory epigenetic components and reduces neointima formation following vascular injury. PDE4 inhibition and Epac activation might represent novel approaches for the treatment of vascular diseases, including atherosclerosis and in-stent restenosis.

Glucose-dependent insulinotropic peptide secretion is induced by inflammatory stimuli in an interleukin-1-dependent manner in mice

Recently, glucagon‐like peptide‐1 (GLP‐1) levels have been found to be increased in response to inflammatory stimuli, leading to insulin secretion and prevention of hyperglycaemia during endotoxemia in mice. In the present study, we assess the relevance of the other incretin hormone, glucose‐dependent insulinotropic peptide (GIP), as a regulator of glucose metabolism under inflammatory conditions. We found that lipopolysaccharide (LPS) increased GIP secretion in a time‐ and dose‐dependent manner in C57BL/6J mice. To elucidate the underlying mechanisms, mice were injected with inflammatory cytokines known to be released by LPS. Circulating GIP levels significantly increased in response to interleukin (IL)‐1β but not IL‐6 or tumour necrosis factor (TNF)‐α administration. Using respective knockout mice we found that LPS‐mediated GIP secretion was selectively dependent on IL‐1 signalling. To evaluate the functional relevance of inflammatory GIP secretion we pretreated mice with the GIP‐receptor antagonist (Pro3)GIP. This blunted LPS‐induced TNF‐α and IL‐6 secretion but did not affect LPS‐induced insulin secretion or blood glucose‐lowering. In conclusion, GIP provides a novel link between the immune system and the gut, with proinflammatory‐immune modulatory function but minor glucose regulatory relevance in the context of acute endotoxemia.

The PDE4 inhibitor roflumilast reduces weight gain by increasing energy expenditure and leads to improved glucose metabolism.

To investigate the metabolic effects of the phosphodiesterase‐4 (PDE4) inhibitor roflumilast, a clinically approved anti‐inflammatory drug used for the treatment of chronic obstructive pulmonary disease.

Does osteopontin induce adipose tissue inflammation by local macrophage proliferation

In this issue of Molecular Metabolism, Tardelli and colleagues analyzed the impact of osteopontin (OPN) on monocyte and macrophage proliferation in the context of obesity-driven adipose tissue inflammation [1]. Chronic low-grade inflammation of adipose tissue during obesity, a crucial contributor to insulin resistance, type 2 diabetes, and subsequent cardiovascular disease, is associated with abnormal cytokine production, monocyte infiltration, and activation of inflammatory signaling pathways [2]. Recent evidence could also show that, in addition to blood monocyte recruitment, in situ proliferation of adipose tissue macrophages is an important pathological feature of macrophage accumulation contributing to obesity-associated adipose tissue inflammation [3]. OPN is a secreted matricellular protein involved in many physiological and pathological processes, including biomineralization, tissue remodeling, and inflammation [4]. OPN is a proinflammatory immune regulator and has been identified as a key mediator of adipose tissue inflammation, insulin resistance, and type 2 diabetes [4]. Using OPNneutralizing antibodies and knockout mice, the groups of Stulnig and Bruemmer showed that OPN mediates obesity-induced adipose tissue macrophage infiltration and insulin resistance in obese mice [5,6]. Furthermore, OPN was found to stimulate inflammatory signaling pathways and secretion of cytokines in adipose tissue macrophages [7]. However, the impact of OPN on the emerging role of local macrophage proliferation remained elusive. In the current study, Tardelli and colleagues showed for the first time that OPN directly increases proliferation of human monocytes. To address the question whether OPN might also influence proliferation of mature macrophages, human blood monocytes were differentiated into macrophages in the presence of M-CSF for 6 days. Surprisingly, using a live cell movie analyzer, the researchers were able to show that OPN also activates motility and proliferation of mature macrophages. It is known that OPN expression is drastically up-regulated in adipose tissue macrophages of high-fat diet-induced and genetically obese mice without corresponding differences in OPN plasma levels [8]. Together, these findings suggest, that OPN exercises its actions locally in adipose tissue rather than systemically. To investigate the question of whether OPN affects local adipose tissue macrophage proliferation in obesity, the authors performed diet-induced

The incretin hormone GIP is upregulated in patients with atherosclerosis and stabilizes plaques in ApoE−/− mice by blocking monocyte/macrophage activation

Objective The incretin hormones GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic peptide) are secreted by the gut after food intake leading to pancreatic insulin secretion and glucose lowering. Beyond its role in glucose control, GLP-1 was found in mice and men to beneficially modulate the process of atherosclerosis, which has been linked to improved cardiovascular outcome of patients with diabetes at high cardiovascular risk treated with GLP-1 receptor agonists. However, little is known on the role of the other main incretin in the cardiovascular system. The aim of this study was to characterize GIP in atherosclerotic cardiovascular disease. Methods and results Serum concentrations of GIP were assessed in 731 patients who presented for elective coronary angiography at the University Hospital Aachen. While GIP concentrations were not associated with coronary artery disease (CAD), we found 97 patients with PAD (peripheral artery disease) vs. 634 without PAD to have higher circulating GIP levels (413.0 ± 315.3 vs. 332.7 ± 292.5 pg/mL, p = 0.0165). GIP levels were independently related to PAD after multivariable adjustment for CAD, age, sex, BMI, hypertension, diabetes, CRP, WBC, and smoking. To investigate the functional relevance of elevated GIP levels in human atherosclerotic disease, we overexpressed GIP (1–42) in ApoE−/− mice fed a Western diet for 12 weeks using an adeno-associated viral vector system. GIP overexpression led to reduced atherosclerotic plaque macrophage infiltration and increased collagen content compared to control (LacZ) with no change in overall lesion size, suggesting improved plaque stability. Mechanistically, we found GIP treatment to reduce MCP-1-induced monocyte migration under In vitro conditions. Additionally, GIP prevented proinflammatory macrophage activation leading to reduced LPS-induced IL-6 secretion and inhibition of MMP-9 activity, which was attributable to GIP dependent inhibition of NfκB, JNK-, ERK, and p38 in endotoxin activated macrophages. Conclusion Elevated concentrations of the incretin hormone GIP are found in patients with atherosclerotic cardiovascular disease, while GIP treatment attenuates atherosclerotic plaque inflammation in mice and abrogates inflammatory macrophage activation in vitro. These observations identified GIP as a counterregulatory vasoprotective peptide, which might open new therapeutic avenues for the treatment of patients with high cardiovascular risk.

Myocardial infarction is sufficient to increase GLP-1 secretion, leading to improved left ventricular contractility and mitochondrial respiratory capacity

Myocardial infarction causes rapid impairment of left ventricular function and requires a hypercontractile response of non‐infarcted tissue areas to maintain haemodynamic stability. This compensatory adaptation is mediated by humoral, inflammatory and neuronal signals. GLP‐1 is an incretin hormone with glucoregulatory and cardioprotective capacities and is secreted in response to nutritional and inflammatory stimuli. Inactivation of GLP‐1 is caused by the ubiquitously present enzyme DPP‐4. In this study, circulating concentrations of GLP‐1 were assessed after myocardial infarction and were evaluated in the light of metabolism, left ventricular contractility and mitochondrial function. Circulating GLP‐1 concentrations were markedly increased in patients with acute myocardial infarction. Experimental myocardial infarction by permanent LAD ligation proved sufficient to increase GLP‐1 secretion in mice. This took place in a time‐dependent manner, which coincided with the capacity of DPP‐4 inhibition, by linagliptin, to augment left ventricular contractility in a GLP‐1 receptor‐dependent manner. Mechanistically, DPP‐4 inhibition increased AMPK activity and stimulated the mitochondrial respiratory capacity of non‐infarcted tissue areas. We describe a new functional relevance of inflammatory GLP‐1 secretion for left ventricular contractility during myocardial infarction.