**1. Introduction**

Cardiovascular diseases resulting from atherosclerosis have become the most important cause of mortality and morbidity in the general population [1]. Although atherosclerosis develops as a consequence of multiple risk factors such as hypertension, dyslipidemia, diabetes, aging and smoking, the common pathway for its development is endothelial dysfunction and vascular inflammation [2]. In the last two decades, the role of advanced glycation end products (AGEs) in the development of endothelial dysfunction has gained increasing interest [3–5], initially as a possible molecular mechanism of diabetic cardiovascular complications [3], and, in the last years, as an independent risk factor of vascular injury [6].

The aim of this chapter is to review the results of our laboratory and others on the molecular mechanisms triggered by AGEs in the endothelium that could participate in the atherosclerotic process. These mechanisms and molecular pathways could help in the development of

Impact of Advanced Glycation End Products on Endothelial Function and Their Potential Link…

http://dx.doi.org/10.5772/intechopen.73025

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It is generally accepted that AGEs target cells by three main mechanisms. First, proteins modified by AGEs have altered biological function, either enzymatic activity, binding properties or structural conformation. Second, extracellular matrix components modified by AGEs interact abnormally with other matrix components and with matrix receptors, such as integrins. This includes also the formation of new links or the alteration of those previously existing, between proteins, which may alter the physical properties of extracellular matrix and cell environment. Third, plasma proteins modified by AGEs bind to cell surface receptors, of which the receptor for AGEs (RAGE) is acknowledged to be the most important, activating intracellular

Binding of AGEs to RAGE is responsible for the generation of reactive oxygen species (ROS) and the activation of transcription factors such as nuclear factor-κB (NF-κB), with subsequent changes in the expression of many genes involved in vascular inflammation and endothelial dysfunction [18–20]. Besides from the involvement of AGEs-RAGE axis, the precursors of AGEs, like Amadori products or early glycated products also have a role in the global response of non-enzymatic glycation of proteins, so we will also discuss their effects on endothelial cells.

One of the first and best studied actions of AGEs on endothelial cells is the induction of ROS. The suggested mechanisms for this action are several and range from the activation of ROS-producing enzymes to the reduction of ROS-neutralizing enzymes. In the first group of enzymes or enzyme complexes are nicotinamide adenine dinucleotide phosphate (NADPH) oxidase [21] and mitochondria [22], whereas in the second, there are endothelial nitric oxide (NO) synthase (eNOS) [23], superoxide dismutase (SOD) and glutathione peroxidase [24, 25]. The molecular mecha-

ROS production in endothelial cells has important consequences on endothelial activation. In brain microvascular endothelial cells, AGEs-induced ROS production enhances vascular endothelial growth factor (VEGF) expression, which mediates an increase in cell permeability [28], and platelet tissue factor up-regulation [29]. Other mechanisms of AGEs on endothelial cells promoting endothelial activation or dysfunction are the generation of asymmetric dimethylarginine (ADMA, a metabolic by-product of natural protein modification processes in the cytoplasm of cells, that acts as a competitive inhibitor of NOS) [30], or impaired calcium signaling [31].

It is important to note that the effects of AGEs' precursors (i.e. Amadori products or glycated proteins) on endothelial cells, differ from the effects of AGEs themselves. Several works have focused on this issue (see, for a review, [32]). Amadori products modify eNOS activity and gene expression, promoting apoptosis of endothelial cells [33, 34]. A recent study

nisms of these actions have been related to the activation of NF-κB via RAGE [26, 27].

new therapeutic targets against atherosclerosis or vascular disease.

signaling pathways and various cellular responses.

**2.1. AGEs-induced ROS production in the endothelium**

**2. Molecular mechanisms triggered by AGEs in the endothelium**

AGEs are products of non-enzymatic molecular modifications of proteins and lipids that affect the structure and function of the target molecule. They are produced endogenously by spontaneous reactions, but pathophysiological conditions may accelerate their formation and they also contribute to disease by different mechanisms.

AGEs comprise a heterogeneous group: the most studied are pentosidine and Nε-carboxymethyllysine (CML) and quantitatively, the most important in the tissues are the hydroimidazolones like CML [7]. AGEs are formed by a combination of glycation, oxidation, and/or carbonylation reactions both in the extra- and in the intracellular space. Other processes involving lipid peroxidations in the cell membranes lead to the formation of advanced lipid end products, as for example, malondialdehyde [8]. The classical mechanism of AGE formation is the slow Maillard reaction between glucose or reducing sugars and proteins [9]. The interaction between the carbonyl groups of reducing sugars and amino groups of proteins results in the formation of a Schiff base within a few hours. Intramolecular rearrangement of the Schiff base results in more stable Amadori products [9]. An example of these types of products is glycated hemoglobin or glycated albumin, the former is widely used in clinical practice for diagnosis and follow-up of diabetes mellitus and the last could be regarded as a smart alternative to modified hemoglobin for the same purposes, with less dependence on hematological diseases and intracellular conditions. Finally, the process of oxidation of the Amadori products leads to reactive carbonyl compounds and subsequently to the formation of AGEs within weeks to months. AGEs can also be formed intracellularly. Glucose is altered into reactive carbonyl compounds during glycolysis pathway, of which the best-known is methylglyoxal. The chemical reaction between these carbonyl compounds and proteins can result in AGEs [10].

Absorption of exogenous AGEs also contributes to their accumulation in tissues. Tobacco smoke contains highly reactive glycation products which rapidly form AGEs *in vitro* and *in vivo* and therefore, increase the serum AGEs levels in smokers compared to non-smokers [11]. The content of AGEs in food depends on the temperature at which food products are prepared, with oven frying being the most severe inducer [12]. Approximately 10% of the ingested AGEs are absorbed from the gastrointestinal tract into the blood [13]. The final level of AGEs accumulation depends on their clearance and the metabolic mechanisms by the kidney and liver, respectively. Increased level of AGEs can be found in patients with either renal [13] or liver failure [14].

The role of AGEs in cardiovascular diseases is a matter of interest in the last years [15], and the strong association between the axis of action of AGEs and their receptor (RAGE) and atherosclerosis or cardiovascular ischemic disease [3, 16, 17] has attracted increased attention. The aim of this chapter is to review the results of our laboratory and others on the molecular mechanisms triggered by AGEs in the endothelium that could participate in the atherosclerotic process. These mechanisms and molecular pathways could help in the development of new therapeutic targets against atherosclerosis or vascular disease.
