Low density lipoprotein oxidation by ferritin at lysosomal pH

Ligustrazine as an Extract from Medicinal and Edible Plant Chuanxiong Encapsulated in Liposome–Hydrogel Exerting Antioxidant Effect on Preventing Skin Photoaging

Long-term sunlight exposure will cause the accumulation of free radicals in the skin and lead to oxidative damage and aging, antioxidant drugs have gradually become the focus of research, but there is little research on antioxidant drugs for percutaneous treatment. The purpose of this study was to prepare ligustrazine hydrochloride (TMPZ)-loaded liposome–hydrogel (TMPZ-LG), evaluate its antioxidant properties, and apply it on the skin of mice to observe whether it had preventive and therapeutic effect on the irradiation under the ultraviolet rays, in an attempt to make it into a new kind of delivery through the skin. TMPZ-LG was prepared by the combination of film dispersion and sodium carboxymethylcellulose (2%, CMC-Na) natural swelling method. The release rates in vitro permeation across the dialysis membrane and ex vivo transdermal had both reached 40%; the scavenging effect of TMPZ-LG on 1,1-diphenyl-2-picrylhydrazyl (DPPH) and H₂O₂ were 65.57 ± 4.13% and 73.06 ± 5.65%; the inhibition rate of TMPZ-LG on malondialdehyde (MDA) production in liver homogenate and anti-low density lipoprotein (LDL) oxidation experiments ex vivo were 15.03 ± 0.9% and 21.57 ± 1.2%. Compared with untreated mice, the skin pathological symptoms of mice coated with TMPZ-LG were significantly reduced after ultraviolet irradiation, and there was statistical significance. The results showed TMPZ-LG could exert good antioxidant activity in vitro and ex vivo; therefore, it is feasible to prevent and treat skin oxidation.

Effects of lysosomal low density lipoprotein oxidation by ferritin on macrophage function

We have previously demonstrated that low density lipoprotein (LDL) can be oxidised by iron in the lysosomes of macrophages. Some of the iron content of lysosomes might be delivered through autophagy of ferritin (the main iron-storage protein in the body). We have now investigated the effects of ferritin-mediated LDL oxidation on macrophage function. The addition of ferritin to human THP-1 cells and human monocyte-derived macrophages increased lysosomal lipid peroxidation, as shown by LPO-Foam, a fluorescent probe targetted to lysosomes. Incubating THP-1 cells with ferritin and native LDL or LDL aggregated by sphingomyelinase, to allow their endocytosis and delivery to lysosomes, led to the formation of lysosomal ceroid (an advanced lipid oxidation product), indicative of lysosomal LDL oxidation. Incubating THP-1 cells with ferritin and LDL caused metabolic activation of the cells, as shown by increased extracellular acidification and oxygen consumption measured by a Seahorse analyser. LDL oxidised by ferritin in lysosomes might be released from macrophages when the cells die and lyse and affect neighbouring cells in atherosclerotic lesions. Adding LDL oxidised by ferritin at lysosomal pH (pH 4.5) to macrophages increased their intracellular reactive oxygen species formation, shown using dihydroethidium, and increased apoptosis. Ferritin might therefore contribute to LDL oxidation in the lysosomes of macrophages and have atherogenic effects.

Vitamins E and C do not effectively inhibit low density lipoprotein oxidation by ferritin at lysosomal pH

Low density lipoprotein (LDL) might be oxidized by iron in the lysosomes of macrophages in atherosclerotic lesions. We have shown previously that the iron-storage proteinferritin can oxidize LDL at lysosomal pH. We have now investigated the roles of the most important antioxidant contained in LDL, α-tocopherol (the main form of vitamin E) and of ascorbate (vitamin C), a major water-soluble antioxidant, on LDL oxidation by ferritin at lysosomal pH (pH 4.5). We incubated LDL with ferritin at pH 4.5 and 37 °C and measured its oxidation by monitoring the formation of conjugated dienes at 234 n min a spectrophotometer. α-Tocopherol is well known to inhibit LDL oxidation at pH 7.4, but enrichment of LDL with α-tocopherol was unable to inhibit LDL oxidation by ferritin at pH 4.5. Ascorbate had a complex effect on LDL oxidation by ferritin at lysosomal pH and exhibited both antioxidant and pro-oxidant effects. It had no antioxidant effect on partially oxidized LDL, only a pro-oxidant effect. Ascorbate completely inhibited LDL oxidation by copper at pH 7.4 for a long period, but in marked contrast did not inhibit LDL oxidation by copper at lysosomal pH. Dehydroascorbate, the oxidation product of ascorbate, had a pronounced pro-oxidant effect on LDL incubated with ferritin at pH 4.5. The inability of α-tocopherol and ascorbate to effectively inhibit LDL oxidation by ferritin at lysosomal pH might help to explain why the large clinical trials with these vitamins failed to show protection against cardiovascular diseases.

Recognition of lipoproteins by scavenger receptor class A members

Scavenger receptor class A (SR-A) proteins are type II transmembrane glycoproteins that form homotrimers on the cell surface. This family has five known members (SCARA1 to 5, or SR-A1 to A5) that recognize a variety of ligands and are involved in multiple biological pathways. Previous reports have shown that some SR-A family members can bind modified low-density lipoproteins (LDLs); however, the mechanisms of the interactions between the SR-A members and these lipoproteins are not fully understood. Here, we systematically characterize the recognition of SR-A receptors with lipoproteins and report that SCARA1 (SR-A1, CD204), MARCO (SCARA2), and SCARA5 recognize acetylated or oxidized LDL and very-low-density lipoprotein in a Ca2+-dependent manner through their C-terminal scavenger receptor cysteine-rich (SRCR) domains. These interactions occur specifically between the SRCR domains and the modified apolipoprotein B component of the lipoproteins, suggesting that they might share a similar mechanism for lipoprotein recognition. Meanwhile, SCARA4, a SR-A member with a carbohydrate recognition domain instead of the SRCR domain at the C terminus, shows low affinity for modified LDL and very-low-density lipoprotein but binds in a Ca2+-independent manner. SCARA3, which does not have a globular domain at the C terminus, was found to have no detectable binding with these lipoproteins. Taken together, these results provide mechanistic insights into the interactions between SR-A family members and lipoproteins that may help us understand the roles of SR-A receptors in lipid transport and related diseases such as atherosclerosis.

Lysosome (Dys)function in Atherosclerosis-A Big Weight on the Shoulders of a Small Organelle

Atherosclerosis is a progressive insidious chronic disease that underlies most of the cardiovascular pathologies, including myocardial infarction and ischemic stroke. The malfunctioning of the lysosomal compartment has a central role in the etiology and pathogenesis of atherosclerosis. Lysosomes are the degradative organelles of mammalian cells and process endogenous and exogenous substrates in a very efficient manner. Dysfunction of these organelles and consequent inefficient degradation of modified low-density lipoproteins (LDL) and apoptotic cells in atherosclerotic lesions have, therefore, numerous deleterious consequences for cellular homeostasis and disease progression. Lysosome dysfunction has been mostly studied in the context of the inherited lysosomal storage disorders (LSDs). However, over the last years it has become increasingly evident that the consequences of this phenomenon are more far-reaching, also influencing the progression of multiple acquired human pathologies, such as neurodegenerative diseases, cancer, and cardiovascular diseases (CVDs). During the formation of atherosclerotic plaques, the lysosomal compartment of the various cells constituting the arterial wall is under severe stress, due to the tremendous amounts of lipoproteins being processed by these cells. The uncontrolled uptake of modified lipoproteins by arterial phagocytic cells, namely macrophages and vascular smooth muscle cells (VSMCs), is the initial step that triggers the pathogenic cascade culminating in the formation of atheroma. These cells become pathogenic "foam cells," which are characterized by dysfunctional lipid-laden lysosomes. Here, we summarize the current knowledge regarding the origin and impact of the malfunctioning of the lysosomal compartment in plaque cells. We further analyze how the field of LSD research may contribute with some insights to the study of CVDs, particularly how therapeutic approaches that target the lysosomes in LSDs could be applied to hamper atherosclerosis progression and associated mortality.

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

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.

Cysteamine inhibits lysosomal oxidation of low density lipoprotein in human macrophages and reduces atherosclerosis in mice

Background and aims

We have shown previously that low density lipoprotein (LDL) aggregated by vortexing is internalised by macrophages and oxidised by iron in lysosomes to form the advanced lipid/protein oxidation product ceroid. We have now used sphingomyelinase-aggregated LDL, a more pathophysiological form of aggregated LDL, to study lysosomal oxidation of LDL and its inhibition by antioxidants, including cysteamine (2-aminoethanethiol), which concentrates in lysosomes by several orders of magnitude. We have also investigated the effect of cysteamine on atherosclerosis in mice.

Methods

LDL was incubated with sphingomyelinase, which increased its average particle diameter from 26 to 170 nm, and was then incubated for up to 7 days with human monocyte-derived macrophages. LDL receptor-deficient mice were fed a Western diet (19–22 per group) and some given cysteamine in their drinking water at a dose equivalent to that used in cystinosis patients. The extent of atherosclerosis in the aortic root and the rest of the aorta was measured.

Results

Confocal microscopy revealed lipid accumulation in lysosomes in the cultured macrophages. Large amounts of ceroid were produced, which colocalised with the lysosomal marker LAMP2. The antioxidants cysteamine, butylated hydroxytoluene, amifostine and its active metabolite WR-1065, inhibited the production of ceroid. Cysteamine at concentrations well below those expected to be present in lysosomes inhibited the oxidation of LDL by iron ions at lysosomal pH (pH 4.5) for prolonged periods. Finally, we showed that the extent of atherosclerotic lesions in the aortic root and arch of mice was significantly reduced by cysteamine.

Conclusions

These results support our hypothesis that lysosomal oxidation of LDL is important in atherosclerosis and hence antioxidant drugs that concentrate in lysosomes might provide a novel therapy for this disease.

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The drug cysteamine, which accumulates in lysosomes, inhibited the oxidation of LDL by iron at pH 4.5 (the pH of lysosomes).

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Cysteamine inhibited the lysosomal oxidation of LDL inside cultured macrophages.

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Cysteamine reduced atherosclerosis in LDL receptor knockout mice.

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These results support our hypothesis that lysosomal oxidation of LDL is important in atherosclerosis.

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Antioxidant drugs that concentrate in lysosomes might provide a novel therapy for this disease.