Effect of vitamin E on low density lipoprotein oxidation at lysosomal pH

Hadeel K M Alboaklah,David S Leake

doi:10.1080/10715762.2020.1817912

Effect of vitamin E on low density lipoprotein oxidation at lysosomal pH

Hadeel K M Alboaklah, David S Leake

Open Access

https://doi.org/10.1080/10715762.2020.1817912

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Abstract

Many cholesterol-laden foam cells in atherosclerotic lesions are macrophages and much of their cholesterol is present in their lysosomes and derived from low density lipoprotein (LDL). LDL oxidation has been proposed to be involved in the pathogenesis of atherosclerosis. We have shown previously that LDL can be oxidised in the lysosomes of macrophages. α-Tocopherol has been shown to inhibit LDL oxidation in vitro, but did not protect against cardiovascular disease in large clinical trials. We have therefore investigated the effect of α-tocopherol on LDL oxidation at lysosomal pH (about pH 4.5). LDL was enriched with α-tocopherol by incubating human plasma with α-tocopherol followed by LDL isolation by ultracentrifugation. The α-tocopherol content of LDL was increased from 14.4 ± 0.2 to 24.3 ± 0.3 nmol/mg protein. LDL oxidation was assessed by measuring the formation of conjugated dienes at 234 nm and oxidised lipids (cholesteryl linoleate hydroperoxide and 7-ketocholesterol) by HPLC. As expected, LDL enriched with α-tocopherol was oxidised more slowly than control LDL by Cu2+ at pH 7.4, but was not protected against oxidation by Cu2+ or Fe3+ or a low concentration of Fe2+ at pH 4.5 (it was sometimes oxidised faster by α-tocopherol with Cu2+ or Fe3+ at pH 4.5). α-Tocopherol-enriched LDL reduced Cu2+ and Fe3+ into the more pro-oxidant Cu+ and Fe2+ faster than did control LDL at pH 4.5. These findings might help to explain why the large clinical trials of α-tocopherol did not protect against cardiovascular disease.

Highlights

Atherosclerosis is a chronic inflammatory disease of the large and medium-sized arteries and the underlying cause of coronary heart disease and thrombotic strokes, which are major causes of morbidity and mortality worldwide [1]
We have shown previously that low density lipoprotein (LDL) can be oxidised in the lysosomes of macrophages. a-Tocopherol has been shown to inhibit LDL oxidation in vitro, but did not protect against cardiovascular disease in large clinical trials
We enriched LDL with a-tocopherol by incubating plasma with a-tocopherol disolved in dimethyl sulfoxide and isolating LDL [28]. (Adding a-tocopherol to LDL in a simple buffer does not lead to its incorporation into the LDL particles [28].) The a-tocopherol content of LDL was increased from 14.4 ± 0.2 to 24.3 ± 0.3 nmol/mg protein

Summary

Introduction

Atherosclerosis is a chronic inflammatory disease of the large and medium-sized arteries and the underlying cause of coronary heart disease and thrombotic strokes, which are major causes of morbidity and mortality worldwide [1]. There has been much interest in the oxidation of low density lipoprotein (LDL) as one of the key mechanisms of atherogenesis [4,5]. Epidemiological studies have suggested that dietary antioxidants, including vitamin E, might diminish the risk of coronary heart disease [7] and many experimental studies, almost all of which have been conducted at pH 7.4, have shown that a-tocopherol inhibits LDL oxidation in vitro [8,9]. A-Tocopherol might affect atherosclerosis by non-antioxidant mechanisms [10]. The large clinical trials, failed to show that vitamin E decreases cardiovascular disease [11,12,13,14,15,16,17,18,19,20]

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