Possible Role of Mitochondrial DNA Mutations in Chronification of Inflammation: Focus on Atherosclerosis

Abstract

Chronification of inflammation is the process that lies at the basis of several human diseases that make up to 80% of morbidity and mortality worldwide. It can also explain a great deal of processes related to aging. Atherosclerosis is an example of the most important chronic inflammatory pathology in terms of public health impact. Atherogenesis is based on the inflammatory response of the innate immunity arising locally or focally. The main trigger for this response appears to be modified low-density lipoprotein (LDL), although other factors may also play a role. With the quick resolution of inflammation, atherosclerotic changes in the arterial wall do not occur. However, a violation of the innate immunity response can lead to chronification of local inflammation and, as a result, to atherosclerotic lesion formation. In this review, we discuss possible mechanisms of the impaired immune response with a special focus on mitochondrial dysfunction. Some mitochondrial dysfunctions may be due to mutations in mitochondrial DNA. Several mitochondrial DNA mutations leading to defective mitophagy have been identified. The regulatory role of mitophagy in the immune response has been shown in recent studies. We suggest that defective mitophagy promoted by mutations in mitochondrial DNA can cause innate immunity disorders leading to chronification of inflammation.

Keywords: atherosclerosis; chronification of inflammation; defective mitophagy; innate immunity; mitochondrial dysfunction; modified LDL.

Figure 1

Phases of inflammation in relation to atherosclerotic lesion development. The commonly accepted course of atherosclerotic lesion progression generally corresponds to the consecutive stages of the inflammatory response, with the formation of stable, fibrous plaque being comparable to scarification.

Figure 2

Schematic presentation of the arterial intima. Proteoglycan-rich layer contains several cell types, including stellate pericyte-like cells and modified smooth muscular cells with reduced contractility.

Figure 3

Cascade of multiple atherogenic modification of low-density lipoprotein (LDL). Multiple atherogenic modifications of LDL particles have been detected in human blood plasma: desialylation was the first event, followed by loss of free cholesterol and cholesterol esters, phospholipids and triglycerides, increase in particle density and decrease in its size; next, negative charge of particles was increased, leading to the formation of electronegative LDL fraction, in which misfolded apolipoprotein B (apoB) was reported; at later stages, increased oxidation and decreased antioxidant content were observed; finally, large highly atherogenic complexes can be formed due to self-association of modified LDL particles and the formation of autoantibodies.

Figure 4

Involvement of subendothelial cells in foam cell formation. From the blood flow, both LDL and modified LDL enter the vessel wall, where they can be internalized by macrophages, pericytes and vascular smooth muscle cells (vascular smooth muscle a-actin (SMA)-positive cells) via scavenger receptors or by phagocytosis or pinocytosis. These macrophages and SMA-positive cells with the taken-up lipid contents in their cytoplasm become foam cells. Adapted from [37], with permission.

Figure 5

Role of inflammatory cytokines in triggering foam cell formation. Adapted from [37], with permission.

Figure 6

Schematic overview of initiation of atherosclerotic lesion formation. Adopted from [40], with permission.

Figure 7

Morphological mapping of the aorta samples. Presented are two examples of morphological mapping of the aortic wall: (A) and (B). Segments of the vascular wall were divided according to morphological characteristics into 68 (A) and 70 (B) regions containing atherosclerotic lesions of varying severity (fatty infiltration, fatty streak, lipofibrous plaque, fibrous plaque) or unaffected tissue. These and other aorta samples were further analyzed for the mutational burden in mtDNA. Adapted from [119] with permission.

Figure 8

Co-localization of mtDNA mutation and atherosclerosis in human aortic intima. A significant correlation between mtDNA mutation burden and atherosclerosis burden was observed: r = 0.131, p = 0.034 (Spearman’s rho). The area under received operating characteristic (ROC)-curve for mtDNA mutation burden accounted for 0.587 (95% confidence interval (CI) 0.506–0.668, p = 0.041), the positive actual state was the presence of advanced atherosclerotic lesions.

Figure 9

Ability of THP-1 cells and cybrid lines to form innate immune tolerance. (A) MtDNA genotyping of THP-1 cells and cybrid lines was performed and mtDNA heteroplasmy index was measured for 10 mtDNA mutations. (B) Innate immune tolerance formation ability of THP-1 cells and cybrid lines. First, 1 µg/mL of lipopolysaccharides (LPS) was added to the cells cultured in suspension in RPMI medium (10% FBS) for 16 h (1st LPS). Then, cells were washed by sterile PBS and fresh RPMI medium with or without 1 µg/mL of LPS was added for 4 h (2nd LPS). Finally, the secretion of tumor necrosis factor (TNF) and IL-1β was evaluated by ELISA. A statistical analysis of the results of three independent experiments was carried out using the IBM SPSS Statistics 21 software package. The significance of p < 0.05 according to the results of t-test for paired samples is marked with an asterisk. Ns - no significance, i.e., p > 0.05.

Figure 10

The effect of mitophagy modulators on the ability of human monocytes being activated in response to LPS. Monocytes were isolated from patients using magnetic CD14+ separation. Isolated cells were placed into 24-well plates (106 cells per 1ml of serum free X-VIVO media) with or without mitophagy modulators and had been incubating with or without 1 µg/mL of LPS for 24h. Then secretion level of TNF and IL-1β was measured using ELISA. The values of cytokine secretion by LPS-treated monocytes were taken as 1. Statistical analysis of the results of three independent experiments was carried out using the IBM SPSS Statistics 21 software package. Significance p < 0.05 is marked with an asterisk and estimates the difference between Control and added mitophagy modulators according to the results of t-test for paired samples. Ns – no significance, p > 0.05. Mitophagy modulators: FCCP, Carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone, 10-7 M; Carbamazepine, 8 µg/mL; AICAR, 5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside, 0.3 mM; 3-MA, 3-Methyladenine, 2.5 mM.

Figure 11

Impaired mitochondrial function and deficient mitophagy promote atherosclerotic lesion formation.