**4. Conclusions**

It is worth mentioning that, most of the above-mentioned works did not elucidate the precise AGE(s) that trigger signal transduction mechanisms upon interacting with RAGE. Kislinger et al. [54] studied the effect of CML-adducts and showed that CML-mediated VCAM-1 expression on HUVECs was also suppressed in the presence of excess sRAGE or anti-RAGE IgG. Nevertheless, they suggest that the findings presented in their work do not rule out other specific AGE products of glycation or oxidation, such as pentosidine, pyralline, methylglyoxal, and imidazolone [55–57], which are present in our modified albumins. Additionally, they also specified that their findings do not rule out either the presence of other receptors or cellular interaction sites for CML adducts, being possible that other receptors for AGE [58–60] may also engage CML- and AGE-modified adducts. These situations might explain why no reduction in the up-regulation of adhesion molecules is observed after pre-treatment with

Additionally, Amadori-modified albumin stimulates adhesion of monocytes to endothelial cells through enhanced transcription of the cell surface adhesion molecules E-selectin, VCAM-1 and ICAM-1 [61], implicating an initial endothelial cell activation occurring at atherosclerosis-prone vascular sites [62, 63]. However, Amadori products do not compete with AGE-albumin for binding to AGE receptors such as RAGE [64]. Aortic endothelial cells express specific receptors for Amadori-modified albumin [37, 65]. Although less information is available for the receptor for Amadori products and signaling through Amadori-modified albumin receptors remains obscure, calnexin [66] and nucleophosmin [67, 68] have been reported to be the fructosyl-lysine specific binding proteins [66–68]. Binding of Amadori-modified albumin to calnexin-like receptors may participate in degradation and/or activation of signal transduction processes involved in mediating the biologic activities of Amadori-modified albumin [66]. The E-selectin expression induced by Amadori-modified albumin was 10 or 20 times higher than that induced with three types of AGEs-HSAs and was not suppressed by anti-RAGE antibody [69]. This would explain why RAGE antagonism would not counteract the increase in adhesion molecules expression. In agreement with this hypothesis, Esposito et al. [70] found that anti-RAGE antibody completely prevented leukocyte adhesion to endothelial cells grown for 8 weeks in high-glucosecontaining media, but it did not reduce the adhesion at 24 h. These results demonstrate that AGEs are important mediators of high-glucose-induced endothelial dysfunction after longterm exposure, whereas the same changes in acute exposure occur with the action of mediators other than AGEs. As the formation of Amadori products is highly probable after 24 h incubation in high glucose medium, but not the formation of AGEs, the effects on the inflammation parameters observed by Esposito et al. [70], and not prevented by anti-RAGE antibod-

sRAGE and anti-RAGE antibody under our experimental conditions.

222 Endothelial Dysfunction - Old Concepts and New Challenges

ies, might be due to the effect of the early glycated products, and not AGEs.

Besides from directly blocking RAGE, alternative pharmacological approaches might turn out to be more promising. Namely, it has been shown that both RAGE and sRAGE can be regulated by currently available pharmacological agents [71]. Other drugs currently in use for diabetic complications have been shown to have an effect on AGE accumulation. These include the antihypertensive angiotensin-converting enzyme inhibitor (ACEI) ramipril [72] and the glucose-lowering drug metformin [73], which both reduce AGE. Forbes et al. [74] demonstrated that compared with placebo, the ACEI perindopril increased human plasma sRAGE levels and reduced plasma AGE concentrations, suggesting an additional mechanistic effect of ACE inhibition in the treatment and prevention of vascular disease. The inhibition of ACE in Oxidative stress induction by AGEs at endothelium triggers molecular signaling pathways that produce an inflammatory response or even endothelial dysfunction. Adhesion molecules expression at the membrane surface of endothelial cells as a consequence of this response or induced by AGEs by other mechanisms mediates the adhesion of leukocytes to endothelium. This adhesion is a key step in the atherogenesis process and the possible involvement of AGE-RAGE axis in it should be considered as potential therapeutic target. Finally, possible pharmacological modulation of AGE-RAGE axis activity at the endothelium is suggested, but specific pharmacological tools available nowadays are not efficient enough; momentarily, drugs used for cardiovascular and metabolic problems could be helpful in modulating the AGE-RAGE axis.
