Plasma-induced endothelial activation associated with incident atherosclerosis: prospective results from the Bruneck Study

The role of immune activation in endotoxin-induced atherogenesis

Some infectious agents may contribute to atherosclerosis by maintaining a heightened state of inflammatory response. Although the risk for atherosclerosis was associated with elevated plasma levels of endotoxin, it is difficult to firmly establish what place endotoxin assumes in the etiology of this disease. As the ability for endotoxin to promote disease may depend on its ability to initiate an inflammatory response, it may be controlled by additional regulatory factors. We measured plasma levels of endotoxin and serum levels of neopterin and soluble interleukin-2 receptor in a random population of 402 men and women, 50—79 years old at the 1990 baseline evaluation (Bruneck Study). End point of the prospective survey was incident (early) atherosclerosis in the carotid arteries as assessed with duplex ultrasound. Subjects with high endotoxin levels (90th percentile) in combination with low neopterin or soluble interleukin-2 receptor levels (below median) did not differ from those with low endotoxin in their risk of incident atherosclerosis. The risk associated with high endotoxin, however, was markedly elevated in subjects with high (above median) neopterin or soluble interleukin-2 receptor levels. The study provides epidemiological evidence that the atherogenic potential of endotoxemia is affected by concomitant immune activation.

Potential Role of Endotoxin as a Proinflammatory Mediator of Atherosclerosis

Atherosclerosis is increasingly recognized as a chronic inflammatory disease. Although a variety of inflammatory markers (ie, C-reactive protein) have been associated with atherosclerosis and its consequences, it is important to identify principal mediators of the inflammatory responses. One potentially important source of vascular inflammation in atherosclerosis is bacterial endotoxin. Mutations in Toll-like receptor 4 (TLR-4), an integral component of the endotoxin signaling complex, are fairly common in the Caucasian population and have recently been associated with reduced incidence of atherosclerosis and other cardiovascular diseases in some studies. Moreover, epidemiological studies suggest that endotoxemia at levels as low as 50 pg/mL constitutes a strong risk factor for the development of atherosclerosis. Endotoxin concentrations in this range may be produced by a variety of common subclinical Gram-negative infections. In this article, we outline the main elements of the endotoxin signaling receptor complex that initiates proinflammatory signaling (lipopolysaccharide binding protein [LBP], CD14, TLR-4, and MD-2) and discuss how changes in expression of these molecules may affect proatherogenic responses in the vessel wall. We also describe some of the proinflammatory effects of endotoxin that may be relevant to atherosclerosis, and discuss how serum lipoproteins, especially high-density lipoprotein, may modulate endotoxin-induced inflammatory responses. Further, we discuss recent findings suggesting that the lipid-lowering statins may have an additional protective role in blocking at least some of these proinflammatory signaling pathways. Finally, we discuss species diversity with regard to endotoxin signaling that should be considered when extrapolating experimental data from animal models to humans.

Influence of diabetes on the exacerbation of an inflammatory response in cardiovascular tissue

Coronary artery disease results from an inflammatory process in blood vessels of afflicted individuals. This process is accelerated with diabetes for reasons that are largely unknown. Recent evidence indicates that infection at sites remote from the heart leads to bacteremia and endotoxemia, thereby stimulating systemic inflammation, which represents an important risk factor for atherosclerosis. We examined the inflammatory response of the heart/aorta of diabetic db/db mice that develop type II diabetes. Subcutaneous inoculation of lipopolysaccharide was used to mimic a local infection. This stimulated an up-regulation of adhesion molecules, cytokines, and chemokines via an endotoxemia that was significantly more rapid and more pronounced in the diabetic compared with normal mice. The 13- to 30-fold induction of key proinflammatory molecules in the heart/aorta of diabetic mice even exceeded that at the site of inoculation. Given that infection, bacteremia, and endotoxemia are relatively frequent events in humans, these results identify a putative mechanism for increased cardiovascular heart disease in diabetes.

Inhibition of plasma-dependent monocyte chemokinesis and cytokine-triggered endothelial activation for neutrophil transmigration by administration of clopidogrel in man

Mediators released by spontaneously activated platelets may contribute to alterations in endothelial and leukocyte dysfunctions. We investigated the roles of clopidogrel and aspirin in ex vivo endothelial activation for interactions with leukocytes. Eight healthy volunteers received clopidogrel or aspirin for 8 days. Blood samples were taken before, during, and after treatment. Levels of adhesion molecules and platelet-derived mediators in these samples were measured using commercially available test kits, and effects of plasma on endothelial cells and leukocytes were investigated in neutrophil transendothelial migration, monocyte-endothelial adhesion and leukocyte migration assays. Plasma samples from clopidogrel-treated persons induced diminished chemokinesis of monocytes. Tumour necrosis factor-induced priming of endothelial cells for enhanced neutrophil transmigration was also diminished by pretreatment of endothelial cells, but not of neutrophils, with plasma derived from subjects during clopidogrel treatment. Plasma from the aspirin group had no such effects. Administration of clopidogrel but not aspirin significantly decreased serum levels of soluble intercellular adhesion molecule-1, whereas no changes in levels of soluble vascular cell adhesion molecule-1, P-selectin, L-selectin, von Willebrand factor, platelet-derived growth factor, vascular-endothelial growth factor, and transforming growth factor-beta were observed. Inhibition of plasma-promoted endothelial activation by clopidogrel may indicate a novel role in the prevention of atherosclerosis.