Anti-atherogenic effect of coenzyme Q₁₀ in apolipoprotein E gene knockout mice¹

Abstract

Oxidation of low-density lipoprotein (LDL) lipid is implicated in atherogenesis and certain antioxidants inhibit atherosclerosis. Ubiquinol-10 (CoQ₁₀H₂) inhibits LDL lipid peroxidation in vitro although it is not known whether such activity occurs in vivo, and, if so, whether this is anti-atherogenic. We therefore tested the effect of ubiquinone-10 (CoQ₁₀) supplemented at 1% (w/w) on aortic lipoprotein lipid peroxidation and atherosclerosis in apolipoprotein E–deficient (apoE−/−) mice fed a high-fat diet. Hydroperoxides of cholesteryl esters and triacylglycerols (together referred to as LOOH) and their corresponding alcohols were used as the marker for lipoprotein lipid oxidation. Atherosclerosis was assessed by morphometry at the aortic root, proximal and distal arch, and the descending thoracic and abdominal aorta. Compared to controls, CoQ₁₀-treatment increased plasma coenzyme Q, ascorbate, and the CoQ₁₀H₂ : CoQ₁₀ + CoQ₁₀H₂ ratio, decreased plasma α-tocopherol (α-TOH), and had no effect on cholesterol and cholesterylester alcohols (CE-OH). Plasma from CoQ₁₀-supplemented mice was more resistant to ex vivo lipid peroxidation. CoQ₁₀ treatment increased aortic coenzyme Q and α-TOH and decreased the absolute concentration of LOOH, whereas tissue cholesterol, cholesteryl esters, CE-OH, and LOOH expressed per bisallylic hydrogen-containing lipids were not significantly different. CoQ₁₀-treatment significantly decreased lesion size in the aortic root and the ascending and the descending aorta. Together these data show that CoQ₁₀ decreases the absolute concentration of aortic LOOH and atherosclerosis in apoE−/− mice.

Introduction

Oxidative modifications of low-density lipoprotein (LDL) in the intima are thought to represent early and pro-atherogenic events [1]. This argument is supported by the presence of oxidized proteins [2], [3], [4], [5], [6] and lipids [7], [8], [9] in human atherosclerotic lesions. For example, ∼5–10% of aortic cholesteryl linoleate (C18:2) are present as hydroperoxides, alcohols, or oxo-derivatives [9] and these oxidation products of C18:2 are evenly distributed among all classes of lipoproteins present in diseased vessels [10]. Both LDL-like particles isolated from human lesions and in vitro oxidized LDL have potential pro-atherogenic activities in vitro and in vivo [11]. However, it is not clear to what extent in vitro oxidized LDL reflects the biological properties of LDL-like particles of lesions. In addition to oxidation, lesion LDL is aggregated and also modified by nonoxidative processes [12], [13] and interactions with proteoglycans [14], [15] that may contribute to foam cell formation. Furthermore, the relative importance of absolute versus the fractional content of aortic oxidized lipids and how these parameters correlate with disease progression remain unknown.

Lipid peroxidation is thought to contribute to the oxidative modification of LDL’s apolipoprotein B-100 via degradation of lipid hydroperoxides [1], [16], the primary products formed during the initial stages of lipoprotein oxidation [17]. As a result, antioxidants that inhibit lipoprotein lipid peroxidation in vitro are considered as potential anti-atherogenic compounds. Indeed, several antioxidants commonly used [18], [19], [20] or designed as potential anti-atherogenic agents [21], [22] inhibit atherosclerosis in various animal models of the disease. However, not all antioxidants attenuate atherosclerosis [23], [24], [25] for reason(s) largely unknown. One problem is that at present there is no suitable surrogate in vitro method to assess in vivo LDL oxidation (discussed in [25]). Also, there is recent evidence that in LDL receptor–deficient rabbits, inhibition of aortic lipoprotein lipid peroxidation is not sufficient for inhibition of atherosclerosis [25].

Ubiquinol-10 (CoQ₁₀H₂) is a lipid-soluble antioxidant (reviewed in [26], [27]) that potently inhibits LDL lipid peroxidation in vitro [28]. CoQ₁₀H₂ acts as a chain-breaking radical scavenger [29], [30], synergizes with vitamin E (α-TOH) via reduction of the α-TOH–derived phenoxyl radical [31], [32], and thus inhibits tocopherol-mediated peroxidation [33]. CoQ₁₀H₂ is also the first lipid-soluble antioxidant consumed when isolated LDL or human plasma is exposed to a vast array of oxidants (reviewed in [27]). Therefore, CoQ₁₀H₂ is a candidate antioxidant for in vivo inhibition of lipoprotein lipid oxidation and hence a potential inhibitor of atherosclerosis, although this has not been investigated to date.

We therefore used apolipoprotein E–deficient (apoE−/−) mice to test whether CoQ₁₀ supplementation inhibits aortic lipid oxidation and/or atherosclerosis. CoQ₁₀ was used rather than CoQ₁₀H₂ because the former is stable and effectively converted into the antioxidant active CoQ₁₀H₂ upon intestinal uptake [34], [35]. ApoE−/− mice were used as they have been suggested to represent a useful tool to assess the contribution of lipoprotein oxidation to atherogenesis, as oxidation-specific epitopes [36] and oxidized lipids are present in lesions [37], [38]. Furthermore, hydroperoxides and their corresponding alcohols of cholesteryl esters accumulate in the aorta together with α-TOH as lesion size increases, while lipid-adjusted concentrations of CoQ decrease [39].