Simona Gargiulo

Cholesterol oxidation products in the vascular remodeling due to atherosclerosis

Like the other oxidation products of the lipid moiety of plasma low density lipoproteins (LDL), cholesterol oxidation products are consistently found within the characteristic lesions of atherosclerosis, both in experimental animals and in man. A growing bulk of evidence suggests that oxysterols make a significant contribution to the vascular remodeling that occurs in atherosclerosis, being involved in various key steps of this complex process: endothelial cell dysfunction, adhesion of circulating blood cells, foam cell and fibrous cap formation, modulation of the extracellular matrix (ECM), vascular cell apoptosis and plaques instability. Moreover, oxysterols have been demonstrated to be at least one or two orders of magnitude more reactive than unoxidized cholesterol in exerting pro-inflammatory, pro-apoptotic, and pro-fibrogenic effects. Thus, a pathological level of cholesterol oxidation in the vasculature may be the missing molecular link between hypercholesterolemia and the formation of atherosclerotic lesions.

Inflammation-related gene expression by lipid oxidation-derived products in the progression of atherosclerosis.

Vascular areas of atherosclerotic development persist in a state of inflammation, and any further inflammatory stimulus in the subintimal area elicits a proatherogenic response; this alters the behavior of the artery wall cells and recruits further inflammatory cells. In association with the inflammatory response, oxidative events are also involved in the development of atherosclerotic plaques. It is now unanimously recognized that lipid oxidation-derived products are key players in the initiation and progression of atherosclerotic lesions. Oxidized lipids, derived from oxidatively modified low-density lipoproteins (LDLs), which accumulate in the intima, strongly modulate inflammation-related gene expression, through involvement of various signaling pathways. In addition, considerable evidence supports a proatherogenic role of a large group of potent bioactive lipids called eicosanoids, which derive from oxidation of arachidonic acid, a component of membrane phospholipids. Of note, LDL lipid oxidation products might regulate eicosanoid production, modulating the enzymatic degradation of arachidonic acid by cyclooxygenases and lipoxygenases; these enzymes might also directly contribute to LDL oxidation. This review provides a comprehensive overview of current knowledge on signal transduction pathways and inflammatory gene expression, modulated by lipid oxidation-derived products, in the progression of atherosclerosis.

The link between altered cholesterol metabolism and Alzheimer's disease

Alzheimers disease (AD), the most common form of dementia, is characterized by the progressive loss of neurons and synapses, and by extracellular deposits of amyloid‐β (Aβ) as senile plaques, Aβ deposits in the cerebral blood vessels, and intracellular inclusions of hyperphosphorylated tau in the form of neurofibrillary tangles. Several mechanisms contribute to AD development and progression, and increasing epidemiological and molecular evidence suggests a key role of cholesterol in its initiation and progression. Altered cholesterol metabolism and hypercholesterolemia appear to play fundamental roles in amyloid plaque formation and tau hyperphosphorylation. Over the last decade, growing evidence supports the idea that cholesterol oxidation products, known as oxysterols, may be the missing link between altered brain cholesterol metabolism and AD pathogenesis, as their involvement in neurotoxicity, mainly by interacting with Aβ peptides, is reported.

Interaction between 24-hydroxycholesterol, oxidative stress, and amyloid-β in amplifying neuronal damage in Alzheimer’s disease: three partners in crime

All three cholesterol oxidation products implicated thus far in the pathogenesis of Alzheimer’s disease, 7β‐hydroxycholesterol, 24‐hydroxycholesterol, and 27‐hydroxycholesterol, markedly enhance the binding of amyloid‐beta (Aβ) to human differentiated neuronal cell lines (SK‐N‐BE and NT‐2) by up‐regulating net expression and synthesis of CD36 and β1‐integrin receptors. However, only 24‐hydroxycholesterol markedly potentiates the pro‐apoptotic and pro‐necrogenic effects of Aβ1–42 peptide on these cells: 7β‐hydroxycholesterol and 27‐hydroxycholesterol, like unoxidized cholesterol, show no potentiating effect. This peculiar behavior of 24‐hydroxycholesterol at physiologic concentrations (1 μm) depends on its strong enhancement of the intracellular generation of NADPH oxidase–dependent reactive oxygen species (ROS), mainly H2O2, and the consequent impairment of neuronal cell redox equilibrium, measured in terms of the GSSG/GSH ratio. Cell incubation with antioxidants quercetin or genistein prevents 24‐hydroxycholesterol’s pro‐oxidant effect and potentiation of Aβ‐induced necrosis and apoptosis. Thus, the presence of 24‐hydroxycholesterol in the close vicinity of amyloid plaques appears to enhance the adhesion of large amounts of Aβ to the plasma membrane of neurons and then to amplify the neurotoxic action of Aβ by locally increasing ROS steady‐state levels. This report further supports a primary involvement of altered brain cholesterol metabolism in the complex pathogenesis of Alzheimer’s disease.

Relation between TLR4/NF-κB signaling pathway activation by 27-hydroxycholesterol and 4-hydroxynonenal, and atherosclerotic plaque instability.

It is now thought that atherosclerosis, although due to increased plasma lipids, is mainly the consequence of a complicated inflammatory process, with immune responses at the different stages of plaque development. Increasing evidence points to a significant role of Toll‐like receptor 4 (TLR4), a key player in innate immunity, in the pathogenesis of atherosclerosis. This study aimed to determine the effects on TLR4 activation of two reactive oxidized lipids carried by oxidized low‐density lipoproteins, the oxysterol 27‐hydroxycholesterol (27‐OH) and the aldehyde 4‐hydroxynonenal (HNE), both of which accumulate in atherosclerotic plaques and play a key role in the pathogenesis of atherosclerosis. Secondarily, it examined their potential involvement in mediating inflammation and extracellular matrix degradation, the hallmarks of high‐risk atherosclerotic unstable plaques. In human promonocytic U937 cells, both 27‐OH and HNE were found to enhance cell release of IL‐8, IL‐1β, and TNF‐α and to upregulate matrix metalloproteinase‐9 (MMP‐9) via TLR4/NF‐κB‐dependent pathway; these actions may sustain the inflammatory response and matrix degradation that lead to atherosclerotic plaque instability and to their rupture. Using specific antibodies, it was also demonstrated that these inflammatory cytokines increase MMP‐9 upregulation, thus enhancing the release of this matrix‐degrading enzyme by macrophage cells and contributing to plaque instability. These innovative results suggest that, by accumulating in atherosclerotic plaques, the two oxidized lipids may contribute to plaque instability and rupture. They appear to do so by sustaining the release of inflammatory molecules and MMP‐9 by inflammatory and immune cells, for example, macrophages, through activation of TLR4 and its NF‐κB downstream signaling.

Oxidized cholesterol as the driving force behind the development of Alzheimer’s disease

Alzheimer’s disease (AD), the most common neurodegenerative disorder associated with dementia, is typified by the pathological accumulation of amyloid Aβ peptides and neurofibrillary tangles (NFT) within the brain. Considerable evidence indicates that many events contribute to AD progression, including oxidative stress, inflammation, and altered cholesterol metabolism. The brain’s high lipid content makes it particularly vulnerable to oxidative species, with the consequent enhancement of lipid peroxidation and cholesterol oxidation, and the subsequent formation of end products, mainly 4-hydroxynonenal and oxysterols, respectively from the two processes. The chronic inflammatory events observed in the AD brain include activation of microglia and astrocytes, together with enhancement of inflammatory molecule and free radical release. Along with glial cells, neurons themselves have been found to contribute to neuroinflammation in the AD brain, by serving as sources of inflammatory mediators. Oxidative stress is intimately associated with neuroinflammation, and a vicious circle has been found to connect oxidative stress and inflammation in AD. Alongside oxidative stress and inflammation, altered cholesterol metabolism and hypercholesterolemia also significantly contribute to neuronal damage and to progression of AD. Increasing evidence is now consolidating the hypothesis that oxidized cholesterol is the driving force behind the development of AD, and that oxysterols are the link connecting the disease to altered cholesterol metabolism in the brain and hypercholesterolemia; this is because of the ability of oxysterols, unlike cholesterol, to cross the blood brain barrier (BBB). The key role of oxysterols in AD pathogenesis has been strongly supported by research pointing to their involvement in modulating neuroinflammation, Aβ accumulation, and cell death. This review highlights the key role played by cholesterol and oxysterols in the brain in AD pathogenesis.

Plaque oxysterols induce unbalanced up-regulation of matrix metalloproteinase-9 in macrophagic cells through redox-sensitive signaling pathways: Implications regarding the vulnerability of atherosclerotic lesions

An imbalance in the matrix metalloproteinases/tissue inhibitors of metalloproteinases (MMPs/TIMPs) contributes to atherosclerotic plaque destabilization and rupture. Here we determined whether oxysterols accumulating in advanced atherosclerotic lesions play a role in plaque destabilization. In human promonocytic U937 cells, we investigated the effects of an oxysterol mixture of composition similar to that in advanced human carotid plaques on the expression and synthesis of MMP-9 and its endogenous inhibitors TIMP-1 and TIMP-2. A marked increment of MMP-9 gene expression, but not of its inhibitors, was observed by real-time RT-PCR; MMP-9 gelatinolytic activity was also found increased by gel zymography. Consistently, a net increment of MMP-9 protein level was also observed by immunoblotting. Using antioxidants or specific inhibitors or siRNAs, we demonstrated that the oxysterol mixture induces MMP-9 expression through: (i) overproduction of reactive oxygen species, probably by NADPH-oxidase and mitochondria; (ii) up-regulation of mitogen-activated protein kinase signaling pathways via protein kinase C; and (iii) up-regulation of activator protein-1- and nuclear factor-κB-DNA binding. These results suggest, for the first time, that oxysterols accumulating in advanced atherosclerotic lesions significantly contribute to plaque vulnerability by promoting MMP-9/TIMP-1/2 imbalance in phagocytic cells.

Oxidation as a crucial reaction for cholesterol to induce tissue degeneration: CD36 overexpression in human promonocytic cells treated with a biologically relevant oxysterol mixture.

Oxidative stress, inflammation and altered cholesterol metabolism and levels are among the pathogenetic mechanisms of cognitive impairment that may accompany aging. Within the research area of hypercholesterolemia and age‐related disease processes, the molecular mechanisms of cholesterol interaction with the inflammatory cells of the macrophage lineage are yet to be elucidated. We thus investigated the effect of both non‐oxidized and oxidized cholesterol on monocytic cell differentiation and foam cell formation, as it occurs within vascular lesions during progression of atherosclerosis. In vitro experiments performed on human U937 promonocytic cells showed that a biologically representative mixture of oxysterols markedly stimulated CD36 expression and synthesis. In contrast, non‐oxidized cholesterol did not exert any effect on CD36 mRNA and protein levels. Furthermore, the oxysterol‐induced up‐regulation of CD36 appeared to be based on the subsequent activation of protein kinase Cδ (PKCδ), extracellular signal‐regulated kinase 1/2 (ERK1/2) and peroxisome proliferator‐activated receptor γ (PPARγ). Cells overexpressing CD36 were indeed able to actively take up oxidized low‐density lipoproteins, and become foam cells. The essential role of ERK pathway and CD36 receptor in oxysterol‐induced foam cell formation was proved by the prevention of the latter event when monocytic cells were incubated in the presence of MEK1/2 selective inhibitor or anti‐CD36 specific antibody. These experimental findings point to cholesterol oxidation as an essential reaction for this sterol to exert cellular stress and tissue damage in age‐related diseases in which inflammation represents a main driving force.

Up-regulation of β-amyloidogenesis in neuron-like human cells by both 24- and 27-hydroxycholesterol: Protective effect of N-acetyl-cysteine

An abnormal accumulation of cholesterol oxidation products in the brain of patients with Alzheimers disease (AD) would further link an impaired cholesterol metabolism in the pathogenesis of the disease. The first evidence stemming from the content of oxysterols in autopsy samples from AD and normal brains points to an increase in both 27‐hydroxycholesterol (27‐OH) and 24‐hydroxycholesterol (24‐OH) in the frontal cortex of AD brains, with a trend that appears related to the disease severity. The challenge of differentiated SK‐N‐BE human neuroblastoma cells with patho‐physiologically relevant amounts of 27‐OH and 24‐OH showed that both oxysterols induce a net synthesis of Aβ1‐42 by up‐regulating expression levels of amyloid precursor protein and β‐secretase, as well as the β‐secretase activity. Interestingly, cell pretreatment with N‐acetyl‐cysteine (NAC) fully prevented the enhancement of β‐amyloidogenesis induced by the two oxysterols. The reported findings link an impaired cholesterol oxidative metabolism to an excessive β‐amyloidogenesis and point to NAC as an efficient inhibitor of oxysterols‐induced Aβ toxic peptide accumulation in the brain.

Molecular signaling operated by a diet-compatible mixture of oxysterols in up-regulating CD36 receptor in CD68 positive cells.

Oxidation of dietary cholesterol during food storage and processing, and/or that of endogenous cholesterol in the presence of increased steady-state levels of reactive oxygen species, leads to the production of derivatives, termed oxysterols. Among the biochemical effects exerted by an oxysterol mixture, it has recently been observed that marked up-regulation of CD36 scavenger receptor on macrophage cells plays a primary role in foam cell formation. This article reports evidence of a significant co-localization of CD36 receptor with cells of the macrophage lineage, i.e. CD68 positive cells, LDL apoprotein B100 and lipids in human advanced atherosclerotic lesions. In addition, it provides a comprehensive analysis of the molecular signaling operated by a nutritionally relevant mixture of oxysterols in overexpressing CD36 receptor in cells of the macrophage lineage. The involvement of a G protein, Src, phospholipase C cascade and peroxisome proliferator-activated receptor gamma in oxysterol-mediated signaling was demonstrated by using selective inhibitors, while the central role of the downstream protein kinase Cdelta and extracellular signal-regulated kinase pathways in oxysterol-induced enhancement of CD36 was conclusively proved by means of small interfering RNA (siRNA) technology.