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Postea, Otilia Adina (2005): Influence of homocysteine on the interaction between circulating monocytes and endothelial cells. Dissertation, LMU München: Faculty of Medicine
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Abstract

Mild hyperhomocysteinemia is an independent risk factor for the development of coronary artery disease, cerebrovascular disease and peripheral arterial disease. The mechanisms by which hyperhomocysteinemia promotes vascular disease are not completely understood yet. An increasing body of evidence has implicated oxidative stress as being contributory to homocysteine’s deleterious effects on the vasculature. Elevated levels of homocysteine lead to increased generation of superoxide anion in endothelial cells by a biochemical mechanism involving nitric oxide synthase, and, to a lesser extent, by an increase in the chemical oxidation rate of homocysteine and other aminothiols in the circulation. Furthermore, homocysteine has been shown to inhibit the activity of important cellular antioxidant enzymes, like the cellular isoform of glutathione peroxidase or superoxide dismutase, which may contribute to homocysteine’s induced oxidant stress. The resulting increase in reactive oxygen species leads to decreased bioavailability of the endothelium-derived signaling molecule nitric oxide via oxidative inactivation and thereby induces endothelial dysfunction. This seems to play a central role in the molecular mechanisms underlying the effects of homocysteine on vascular function. Hyperhomocysteinemia not only leads to endothelial dysfunction but also promotes the development and propagation of atherosclerotic lesion in atherosclerosis-prone animal models. As the recruitment of circulating monocytes to the vessel wall plays a crucial role in the process of atherosclerosis, the purpose of this study was to examine the influence of homocysteine on the interaction of endothelial cells with monocytes. Exposure of endothelial monolayers to D,L- and L-homocysteine resulted in a time- and dose-dependent increase in adherent THP-1 cells by upregulating ICAM-1 expression on endothelial cells. L-cysteine and D-homocysteine had no effects. This indicates that the stimulatory effect is specific for the naturally occurring L-stereoisomer and rather a biochemical than a chemical effect. The increased endothelial expression of ICAM-1 seems to be mediated by increased activation of the nuclear transcription factor NF-kB, as shown by increased nuclear translocation of NF-kB in homocysteine-incubated endothelial cells. In accordance, inhibition of NF-kB translocation by a synthetic inhibitor Bay 11-7082 significantly diminished homocysteine-induced ICAM-1 expression and adhesion of monocytes to endothelial cells. In addition, incubation of monocytes with D,L- homocysteine and L-homocysteine resulted in significant increase in the number of adhering monocytes to unstimulated endothelial monolayer by upregulating the expression of beta-2 integrins. Furthermore, homocysteine-incubation of endothelial cells and monocytes resulted in a dose-dependent and significant increase in the intracellular generation of reactive oxygen species. In support of the role of increased oxidant stress for the above mentioned effects, treatment of endothelial cells with the superoxide scavengers MnTBAP or Tiron together with homocysteine abolished homocysteine-induced monocyte adhesion, ICAM-1 expression and the nuclear translocation of NF-kB. Incubation of THP-1 monocytes with Tiron abolished homocysteine-induced beta-2 integrin expression on these cells and adhesion to unstimulated endothelial cells. These findings suggest that superoxide anion radicals mediate homocysteine’s effects on endothelium-monocyte interactions. In addition to previous studies that indicated that a significant source of reactive oxygen species in homocysteine-treated endothelial cells might be endothelial nitric oxide synthase, experiments using inhibitors of nitric oxide synthase in THP-1 cells indicated that nitric oxide synthase-dependent generation of superoxide anion also occurs in homocysteine-incubated THP-1 cells. This mechanism may contribute to homocysteine-induced oxidant stress. The information generated from these studies may be helpful in designing intervention strategies aimed at inhibiting the generation of reactive oxygen species in the vasculature that is associated with signaling events of monocyte recruitment and infiltration involved in atherosclerosis.