**6. Reactive oxygen species – Implications in cardiovascular pathology**

Atherosclerosis (ATS) and its notable complication, coronary heart disease, still represent the major cause of premature death worldwide. Several lines of evidence suggest that the major risk factors (hypertension, diabetes mellitus, hyperlipemia, smoking) elicit oxidative stress at the luminal surface of vascular wall that will be further responsible for the oxidative damage of lipoproteins, formation of lipid peroxides, platelet aggregation and activation of macrophages [10]. LDL lipoproteins are the easiest to be oxidized because of their high PUFA content; at variance from native LDL, oxidatively modified LDLs are more avidly taken up by macrophages via the scavenger receptor thus generating the well-known "foam cells" of the atherosclerotic plaques. Experimental studies demonstrated that LDL can be oxidized by all of the major cells of the arterial wall (macrophages, endothelial cells, smooth muscle cells). Besides its rapid uptake by macrophages, oxidized LDL elicit a chemoatractive effect facilitating monocyte adhesion to the endothelium and a toxic affect at the level of endothelial cells by inhibiting the release of nitric oxide. *In vivo* identification of oxidized LDL in atherosclerotic plaques clearly established in the late 80s the oxidative-modification theory of ATS. Much effort was further directed towards identification of factors that influence the susceptibility of LDL particles to oxidation. Among these, the presence of small dense LDS particles, of preformed lipid peroxides, as well as glycation or binding of LDLs to proteoglycans were proven to facilitate oxidation [12].

Highly reactive aldehydes are one of the major causative factors in oxidative related cardiovascular pathology and ageing. Specific aldehydes (e.g., 4-hydroxynonenal acetaldehyde, acrolein) were reported to be transiently increased in the settings of heart failure and ischemia-reperfusion injury [13] and to interfere with transcriptional regulation of endogenous anti-oxidant networks in mitochondria [1]. Recently, accumulation of reactive aldehydes was studied from the point of view of the subsequent protein carbonylation and its implication in cardiovascular pathophysiology [4].

34 Lipid Metabolism

decrease.

accepted today.

proven to facilitate oxidation [12].

lactoferrin are antioxidants.

in hypoxia, leukocyte activation or in muscle tissue during strenuous physical exercise. Another source of free Fe2+ is represented by hemoglobin, which at low concentrations acts as a pro-oxidant favoring PUFA peroxidation. Proteins that bind Fe2+ have a different action: thus, ferritin has a pro-oxidant capacity, while hemosiderin and

Bilirubin, resulting from the metabolism of hemoglobin, as transition metal ions, causes alterations in the membrane structure by initiating PUFA peroxidation. Bilirubin crosses the blood-brain barrier, inhibits oxidative phosphorylation and decreases AMPc and GSH concentration. Thus, the encephalopathy caused by intense hemolytic jaundice in neonates is correlated with elevated levels of bilirubin, blood lipid peroxides and GSH

The same changes were observed in hepatitis of various etiologies (viral, ethanolic) and were correlated with graded morphological changes of the steatosis type, up to the

The bilirubin has an antioxidant effect, enhanced by binding to albumin, its plasma transport form. This different behavior of bilirubin depends on the concentration and the environment, like ascorbic acid, which features a pro- and antioxidant character, widely

**6. Reactive oxygen species – Implications in cardiovascular pathology** 

Atherosclerosis (ATS) and its notable complication, coronary heart disease, still represent the major cause of premature death worldwide. Several lines of evidence suggest that the major risk factors (hypertension, diabetes mellitus, hyperlipemia, smoking) elicit oxidative stress at the luminal surface of vascular wall that will be further responsible for the oxidative damage of lipoproteins, formation of lipid peroxides, platelet aggregation and activation of macrophages [10]. LDL lipoproteins are the easiest to be oxidized because of their high PUFA content; at variance from native LDL, oxidatively modified LDLs are more avidly taken up by macrophages via the scavenger receptor thus generating the well-known "foam cells" of the atherosclerotic plaques. Experimental studies demonstrated that LDL can be oxidized by all of the major cells of the arterial wall (macrophages, endothelial cells, smooth muscle cells). Besides its rapid uptake by macrophages, oxidized LDL elicit a chemoatractive effect facilitating monocyte adhesion to the endothelium and a toxic affect at the level of endothelial cells by inhibiting the release of nitric oxide. *In vivo* identification of oxidized LDL in atherosclerotic plaques clearly established in the late 80s the oxidative-modification theory of ATS. Much effort was further directed towards identification of factors that influence the susceptibility of LDL particles to oxidation. Among these, the presence of small dense LDS particles, of preformed lipid peroxides, as well as glycation or binding of LDLs to proteoglycans were

irreversible ones, cirrhosis, caused by exceeding the protective antioxidant systems.

On the other hand, decreased antioxidant defense further contributes to the oxidative damage. Low concentration of GSH-peroxidase in the vascular wall creates conditions favorable to the actions of hydrogen peroxide and other FR on lipids and lipoproteins [28]. In physiological conditions, nitric oxide acts as an antioxidant, inhibiting LDL peroxidation and their destructive effect on interstitial proteoglycans. With the increased production of FR, NO may become a prooxidant factor, stimulating LDL peroxidation by a mechanism involving myoglobin. Deficiency of other protective factors will favor oxidative injury. Lipid-soluble antioxidants such as tocopherols and ubiquinol are present in the hydrophobic environment of the lipoproteins in order to protect PUFA from FR attack. *In vitro* experimental data showed that: i) exposure of LDL to oxidative stress will trigger lipid peroxidation only after the loss of its above mentioned antioxidants and ii) enrichment of LDL with vitamin E will make LDL oxidation more difficult [6].

Accordingly, the beneficial role of antioxidant supplementation has been extensively investigated in the past decades in a variety of animal models. Most investigators reported beneficial effects, i.e., prevention of atherosclerotic lesions with vitamin E supplementation, yet an early study by Keaney et al. mentioned a deleterious effect of high doses of tocopherol on endothelial-dependent relaxation in cholesterol fed rabbits [11]. Unfortunately, despite the promising observational experimental data, several prospective, double-blind, placebo-controlled trials did not support a causal relationship between vitamin C and E supplementation and a lower risk of coronary heart disease [21]. Similarly, lack of beneficial effect with long term vitamin E supplementation was recently reported in large clinical trial (the Women's Health Study) that addressed the role of antioxidant therapy in the primary prevention of heart failure [2].

These negative results may be related to the fact that antioxidant supplements could abolish the physiological role of ROS as signaling molecules [18], especially when considering that most cardiovascular patients are treated with "pleiotropic" drugs such as statins, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, that besides their major effects are reported to reduce ROS formation [23]. Indeed, a large body of evidence demonstrated unequivocally that reduced amounts of reactive oxgen species, most probably of mitochondrial origin [17] but not exclusively, are essential in regulating cardiovascular homeostasis [19] as well as the powerful mechanisms of endogenous cardioprotection at postischemic reperfusion, namely pre- and postconditioning [20].

#### 36 Lipid Metabolism

In conclusion, increasing the level of endogenous antioxidants, as recently suggested via the supplimentation of weak "pro-oxidants" [8], and not chronic supplementation with large dose of exogenous antioxidants could become in the future a more appropriate approach to treat diseases that share oxidative stress as a common denominator.

Oxidative Stress and Lipid Peroxidation – A Lipid Metabolism Dysfunction 37

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