Measurement and Clinical Significance of Lipid Peroxidation as a Biomarker of Oxidative Stress: Oxidative Stress in Diabetes, Atherosclerosis, and Chronic Inflammation

Abstract

Endothelial dysfunction is one of the initial steps in the pathogenesis of atherosclerosis and development of cardiovascular disease in patients with diabetes mellitus. Several risk factors are associated with endothelial dysfunction and atherosclerosis, such as hypertension, dyslipidaemia, inflammation, oxidative stress, and advanced glycation-end products. Among these risk factors, oxidative stress is the largest contributor to the formation of atherosclerotic plaques. Measurement of reactive oxygen species (ROS) is still difficult, and assays for the measurement of ROS have failed to show a consistent correlation between pathological states and oxidative stress. To solve this problem, this review summarizes the current knowledge on biomarkers of oxidative stress, especially lipid peroxidation, and discusses the roles of oxidative stress, as measured by indices of lipid peroxidation, in diabetes mellitus, atherosclerosis, and chronic inflammation.

Keywords: 8-isoprostaglandin F2α; Fe-ROMs test; atherosclerosis; cardiovascular disease; chronic inflammation; diabetes mellitus; endothelial dysfunction; lipid peroxidation; malondialdehyde; oxidative stress; oxidative stress biomarkers; oxidized high-density lipoprotein.

Figure 1

Formation of arachidonic acid hydroperoxides. Abstraction of the bis-allylic hydrogen leads to the attack of oxygen to either of two positions and the formation of arachidonic acid peroxyl radical (LOO•). The peroxyradical (5-20:4-OO•) is converted to arachidonic acid hydroperoxides (5-20:4-OOH) and propagates the reaction as the peroxyl radical abstracts a second hydrogen atom from the polyunsaturated fatty acid (PUFA). On the other hand, the peroxyradical (9-20:4-OO•) shows a tendency to generate its corresponding endoperoxide rather than arachidonic acid hydroperoxide (9-20:4-OOH).

Figure 2

Formation of various arachidonic acid hydroperoxides. As described in Figure 1, abstraction of the bis-allylic hydrogen attached to the –C– atom at position 7 leads to the formation of arachidonic acid peroxides (5-20:4-OO• or 9-20:4-OO•). Abstraction of the bis-allylic hydrogen at position 10 and 13 generates arachidonic acid hydroperoxides (8-20:4-OO• or 12-20:4-OO•) and hydroperoxides (11-20:4-OO• or 15-20:4-OO•), respectively. Arachidonic acid hydroperoxides (5-20:4-OOH and 15-20:4-OOH) are generated from arachidonic acid peroxides (5-20:4-OO• and 15-20:4-OO•), respectively. On the other hand, arachidonic acid peroxides (8-20:4-OO•, 9-20:4-OO•, 11-20:4-OOH and 12-20:4-OO•) are converted to their corresponding endoperoxides, respectively.

Figure 3

Formation of 8-isoprostaglandin F₂α (8-isoPGF₂α) and malondialdehyde from arachidonic acid. 11-Peroxy radical (11-20:4-OO•) is generated from arachidonic acid and then undergoes subsequent cyclization to generate the bicyclic endoperoxide (4). The unstable bicycloendoperoxide (4) is then reduced to the 8-isoPGF₂α or cleaved to produce malondialdehyde (indicated in red).

Figure 4

Formation of four classes of F₂-isoprostanes (IsoPs) from arachidonic acid. 15-series, 5-series, 8-series, and 12-series F₂-IsoPs are produced from 11-peroxy radicals, 9-peroxy radicals, 12-peroxy radicals, and 8-peroxy radicals, respectively. 8-IsoPGF₂α is classified into 15-series F2-IsoPs. PGF₂α is synthesized from arachidonic acid via a cyclooxygenase (COX)-dependent pathway. The structural difference between PGF₂α and the 15-series F_2-IsoPs is that the former is an optically pure compound, whereas IsoPs are racemic. The structural distinction from PGF₂α and 8-isoPGF₂α is that PGF₂α contains side chains that are oriented trans to the prostane ring (indicated in the red circle).

Figure 5

Time course of changes in plasma glucose, oxidative stress, and endothelial function. (a) Plasma glucose levels increase after a meal and then return to the baseline. (b) Changes in oxidative stress can be assessed by periodic measurement of plasma levels of malondialdehyde (MDA), 8-isoPGF₂α, or nitrotyrosine after a meal. These markers of oxidative stress increase and return to baseline in a time course manner similar to the changes in plasma glucose level. (c) Plasma 8-isoPGF₂α is gradually excreted into urine. Therefore, the excretion rate is expected to increase and thereafter decline, whereas accumulated urinary 8-isoPGF₂α reaches a maximum. (d) Endothelial function is assessed by venous occlusion plethysmography (VOP) and flow-mediated dilatation (FMD) (FMD and VOP will be explained later in the text). Endothelial function is impaired concomitantly with the increase in plasma glucose levels and oxidative stress.

Figure 6

Reactive oxygen species (ROS) are major players in the development of vascular endothelial dysfunction in diabetes. Diabetes mellitus is associated with hyperglycemia, hypertension, obesity, inflammation, and dyslipidemia, all of which lead to vascular damage via ROS-mediated reactions. Details of the ROS-mediated reactions are described in the text (Section 5.3, Section 5.4 and Section 5.5).

Figure 7

Cause-effect relationship between oxidative stress and type 2 diabetes mellitus (T2DM). Insulin stimulates insulin receptor signaling pathway, including the phosphorylation of insulin receptor substrate-1 (IRS-1), PI3-kinase, and PIP2 and the activation of Akt, eventually leading to the translocation of glucose transporter 4 (GLUT4) to the plasma membrane. Obesity-associated inflammation of the adipose tissue and liver induces macrophage infiltration and increase in pro-inflammatory cytokines and ROS generation. Increased ROS inhibits the insulin receptor signaling pathway, leading to insulin resistance and hyperinsulinemia.

Figure 8

Protein nitration and chlorination caused by the invasion of leukocytes into inflammatory lesions. Myeloperoxidase (MPO), released locally by activated leukocytes, works in concert with ROS and reactive nitrogen species (RNS) and increases protein nitration and chlorination at inflammatory lesions. MPO mediates chlorination and nitration of ApoA1 residues in the pro-inflammatory environment of human atherosclerotic plaque.

Figure 9

Changes in brachial-ankle pulse wave velocity (baPWV), high-sensitivity C-reactive protein (hs-CRP), and oxidative stress parameters in non-metabolic syndrome (MetS) and MetS subjects. Kim et al. [149] divided subjects into non-MetS and MetS groups. Each group was further subdivided into four groups according to age, the 19-34 (I), 35-44 (II), 45-54 (III) and 55-79 (IV) years groups. BaPWV, hs-CRP, ox-LDL, 8-isoPGF₂α and MDA levels were measured in non-MetS (I, II, III, and IV) and MetS (I, II, III, and IV) subjects. The line chart was created by using the numerical values provided in [149].

Figure 10

Usefulness of oxidized HDL (oxHDL) as a biomarker of oxidative stress. Local inflammation, which results from increased adipose tissue mass and local vascular changes, induces ROS generation and synthesis and secretion of pro-inflammatory cytokines such as IL-6. IL-6 stimulates hepatic C-reactive protein (CRP) production, whereas increased ROS lead to hydroperoxidation of HDL. Oxidation of HDL impairs its atheroprotective functions, leading to vascular injury. Therefore, oxidative stress (ROS generation and subsequent HDL oxidation) is associated with obesity, inflammation (CRP production), and vascular damage. Oxidative stress can be assessed by the Fe-reactive oxygen metabolites (ROMs) test, and the ratio of oxHDL to HDL is a more reliable biomarker for monitoring oxidative stress than oxHDL. The CRP also causes endothelial cell dysfunction and progression of atherosclerosis, possibly by decreasing nitric oxide synthesis [150].