Fatty acid nitration in human low-density lipoprotein
Introduction
Atherosclerosis represents one of the main causes of morbidity and mortality of occidental countries [1]. The disease affects key arteries due to the accumulation of lipids and fibrous elements into the arterial wall. In advanced stages, a chronic inflammatory condition linked with oxidative stress is observed [2] which is characterized by artery wall lipid and protein oxidation and deposition along with endothelium loss of function [3]. Native low-density lipoprotein (LDL) does not present pro-atherogenic effects [4,5]. In fact, cholesterol accumulation in atherosclerotic lesions is not due to cellular uptake of native LDL through the LDL receptor, but due to the uptake of an oxidized-modified form by scavenger receptors present in monocytes/macrophages and smooth muscle cells (SMC) membranes [6]. In contrast to native LDL, oxidized LDL (oxLDL) uptake by scavenger receptors in macrophages or SMC is not under a negative feedback regulation. Therefore this process results in massive uptake and intracellular accumulation of cholesterol and its oxidation products which determine the formation of foam cells (lipid-filled cells), major components of the atheroma plaque [7]. Since LDL oxidation, elevated cell adhesion and altered metabolism are key events related to lesion development, oxidative modifications (e.g. oxidation/nitration) of the apolipoproteinB-100 (apoB-100) and/or the lipid components of the LDL confers the particle its pro-atherogenic features [8]. In addition to promoting foam cell formation, oxLDL alters the endothelial production and bioavailability of nitric oxide (•NO), stimulates endothelial cells apoptosis and has direct chemotactic effects on monocytes. Moreover, it induces vascular cell expression and production of growth, adhesion, and chemotactic factors, overall promoting the formation of an inflammatory focus in the arterial intima [9,10].
General consensus states that LDL oxidation does not take place during circulation due to the presence of antioxidants (i.e. tocopherol, ascorbate, uric acid) [11]. By contrast, most oxidation reactions occur following LDL infiltration towards sub-endothelial space, where lower antioxidant concentrations are present. Sites of the inflammatory vascular lesion are prone to be permeable to LDL particles along with recruited leukocytes. In this context, activated cells produce reactive oxygen and nitrogen species that convert native LDL into oxLDL [[12], [13], [14]]. Reactive species comprise, among others, superoxide (O2•-), hydroxyl (•OH) and •NO radicals. In addition, the oxidant and nitrating agent peroxynitrite, produced by •NO and O2•- diffusion-limited reaction, along with hydrogen peroxide (H2O2) is also likely to be produced by vascular cells and macrophages [[15], [16], [17]]. NADPH oxidase, xanthine oxidase, nitric oxide synthase (NOS), myeloperoxidase and lipooxygenase represent some of the enzymes responsible for the generation of the above mentioned reactive species present in atherosclerotic lesions [11]. As previously mentioned, both the lipid and protein content of LDL is susceptible to oxidative modifications. In particular, numerous studies report and characterize the oxidation products of the lipid fraction of LDL and their biological actions (revised in Ref. [11]). Reactive nitrogen species (RNS) such as peroxynitrite-derived radicals may react with polyunsaturated fatty acids generating lipid-derived-products which modify the lipoprotein and alters its recognition by normal cell receptors [12,18,19]. Formation of lysophospholipids, sphingophospholipids or free aldehydes is associated with the induction of monocyte adhesion, cytokines production, and muscle cell mitogenesis or enzymes expression such as cyclooxygenase 2. Oxidative modifications of apoB-100 have also been reported, especially at Lys, Cys, His, Trp and Tyr residues and are associated with oxLDL antigenicity, scavenger receptor recognition and loss of normal receptor recognition.
Oxidative modifications of LDL by reactive oxygen species (ROS) and RNS include not only oxidation but also nitration of both lipid and protein fractions. Although lipid and protein oxidation has been extensively studied, this review focuses mostly on nitration processes in LDL, including nitrotyrosine and nitro-fatty acid (NFA) formation. We also discuss the biological conditions responsible for switching the pro-atherogenic oxidized LDL to a putative anti-inflammatory phenotype where NFA formation is enhanced, keeping lipid oxidation into a minimum.
Section snippets
Nitric oxide inhibition of LDL oxidation
Although •NO-derived metabolites may exert oxidative modifications in LDL through peroxynitrite, nitrogen dioxide (•NO2) and/or the NO2−-myeloperoxidase system [20,21], •NO itself inhibits lipid oxidation-dependent processes. Nitric oxide is not reactive enough to initiate lipid oxidation, however, it is highly reactive with lipid-derived radicals generated during the propagation stage, such as alkoxyl (LO•) or peroxyl (LOO•) radicals (k = 1-3 x 109 M−1s−1) [22], yielding a variety of
Oxidation versus nitration of LDL
Oxidative modifications of LDL are necessary for the acquisition of the pro-inflammatory properties present during the initiation and progression of atherosclerosis. The LDL particle consists of an apolar core of cholesteryl esters and triglycerides, surrounded by a monolayer of phospholipids, free cholesterol and one molecule of apoB-100 (4,536 amino acids) that present many oxidizable amino acids including His, Lys, Trp, Cys and Tyr [31]. Both the lipid and the protein components of LDL could
Conclusions
Nitration of the lipid fraction of LDL is a feasible process that may occur during nitro-oxidative stress conditions when high concentrations of nitrating species derived from •NO and O2 enzymatic and non-enzymatic sources are formed. Oxidized LDL represents a footprint of oxidation processes that may induce a pro-inflammatory response. However, the anti-inflammatory and signaling role that NFA have, may open new questions about the plausible role of NFA in modulating the inflammatory response
Acknowledgments
This work was supported by grants from Comisión Sectorial de Investigación Científica (CSIC)-Grupos No 536 to HR and AT, Comisión Sectorial de Investigación Científica (CSIC)-I+D 2016 to AT, and Comisión Sectorial de Investigación Científica (CSIC)-Iniciación ID:416 to MM. MM was supported by a post-graduate fellowship by Agencia Nacional de Investigación e Innovación (ANII)- Uruguay.
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