Homocysteine increases monocyte and T-cell adhesion to human aortic endothelial cells
Introduction
Hyperhomocysteinemia is recognized as an independent risk factor for atherosclerosis [1] and may be a key factor in atherogenesis [2]. The mechanisms by which homocysteinemia leads to atherosclerosis are not well understood. Administration of homocysteine (HCY) has been reported to cause vascular injury and thrombosis in animals [3]. Previous studies have shown that HCY may contribute to the pathogenesis of atherosclerosis through altering endothelial functions, including injury to endothelial cells [4], promoting thrombosis by enhancing factor V activation [5], altering tissue plasminogen activator binding to endothelium [6], and decreasing production of prostacyclin [7] and nitric oxide [8].
Modulation of immune-endothelial cells interaction is another potential mechanism by which HCY may contribute to the pathogenesis of atherosclerosis. Chemotaxis and accumulation of leukocytes in the arterial wall are known to be critical events in inflammatory process associated with the development of atherosclerosis [9], [10]. The vascular endothelium is regarded as an important component of the immune system, regulating the production of chemoattractants and the adhesion and migration of leukocytes [9], [11], [12], [13], [14], [15]. The presence of high levels of HCY in the circulation may affect not only vascular endothelial function, but also the cells of the immune system. The adhesion of polymorphonuclear cells, such as neutrophils, to endothelial cells is the main event in acute inflammation [16], while the adhesion of monocytes and lymphocytes, such as T cells, to endothelial cells is suggested to be an important step in the initiation and development of the inflammatory processes associated with the formation of fatty streaks and development of atherosclerosis [9], [10], [17], [18]. In addition, several inflammatory cytokines, including interleukin (IL)-1, tumor necrosis factor (TNF), and interferon, produced by activated monocytes and macrophages, may stimulate the endothelium to up-regulate gene encoding for chemokines, other cytokines, and adhesion molecules which mediate attraction and adhesion of leukocytes to the endothelium [19].
HCY has been shown in vivo and in vitro to promote inflammatory processes such as the adhesion of neutrophils to endothelial cells [20], [21], as well as the release of the inflammatory cytokine IL-8 and monocyte chemoattractant protein-1 (MCP-1) [22]. However, relatively little is known about the effect of HCY on the monocyte and T-cell interactions with endothelial cells under inflammatory cytokine stimulation. In this in vitro study, HCY was shown to enhance the cytokine-stimulated expression of endothelial cell adhesion molecules and monocyte and T-cell adhesion to endothelial cells, mechanisms by which HCY may contribute to the pathogenesis of atherosclerosis.
Section snippets
Cell culture
Human aortic endothelial cells (HAEC) were obtained from Clonetics (San Diego, CA), and passages 4 to 7 were used in this study. The HAEC were cultured in MCDB-131 medium (Sigma, St Louis, MO) supplemented with 2% fetal bovine serum (FBS; Gibco, Grand Island, NY), 10 mg/l human epidermal growth factor (Clonetics), 9 mg/l bovine brain extract (Clonetics), 0.5 mg/l hydrocortisone (Clonetics), 100 000 U/l penicillin (Gibco), 100 mg/l streptomycin (Gibco), and 1.25 mg/l amphotericin B (Sigma). The
HCY increases immune cell adhesion to HAEC
In this study, we examined whether HCY could influence monocyte and T-cell adhesion to endothelial cells. As shown in Fig. 1A, pretreatment of HAEC with increasing doses of HCY (0–100 μmol/l) for 20 h increased U937 cell adhesion to IL-1β-stimulated HAEC. HCY also slightly, but not significantly, increased U937 cell adhesion to unstimulated HAEC (data not shown). When U937 cells were treated with HCY (0–100 μmol/l) for 20 h and then incubated with IL-1β-stimulated HAEC without HCY treatment,
Discussion
Exposure of endothelial cells to inflammatory cytokines, oxidized LDL, lipopolysaccharides, and oxidative stress induces leukocyte adhesion by increasing the surface expression of the various CAMs and contributes to the formation of fatty streaks and the development of atherosclerosis [9], [10], [11], [27], [28], [29], [30], [31]. The results from this study suggest that HCY may contribute to the development of atherosclerosis through increasing endothelial cell interaction with monocytes and T
Acknowledgements
This material is based upon work supported by the US Department of Agriculture, under agreement No. 58-1950-9-001. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the US Department of Agriculture.Takuro Koga is a visiting scientist in the Vascular Biology Laboratory, JM USDA Human Nutrition Research Center on Aging at Tufts University. Dr Koga is supported by the Noda Institute for
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