Impact of Oxysterols in Age-Related Disorders and Strategies to Alleviate Adverse Effects

Oxysterols or cholesterol oxidation products are a class of molecules with the sterol moiety, derived from oxidative reaction of cholesterol through enzymatic and non-enzymatic processes. They are widely reported in animal-origin foods and prove significant involvement in the regulation of cholesterol homeostasis, lipid transport, cellular signaling, and other physiological processes. Reports of oxysterol-mediated cytotoxicity are in abundance and thus consequently implicated in several age-related and lifestyle disorders such as cardiovascular diseases, bone disorders, pancreatic disorders, age-related macular degeneration, cataract, neurodegenerative disorders such as Alzheimer's and Parkinson's disease, and some types of cancers. In this chapter, we attempt to review a selection of physiologically relevant oxysterols, with a focus on their formation, properties, and roles in health and disease, while also delving into the potential of natural and synthetic molecules along with bacterial enzymes for mitigating oxysterol-mediated cell damage.

Attenuation of 7-ketocholesterol- and 7β-hydroxycholesterol-induced oxiapoptophagy by nutrients, synthetic molecules and oils: Potential for the prevention of age-related diseases

Age-related diseases for which there are no effective treatments include cardiovascular diseases; neurodegenerative diseases such as Alzheimer's disease; eye disorders such as cataract and age-related macular degeneration; and, more recently, Severe Acute Respiratory Syndrome (SARS-CoV-2). These diseases are associated with plasma and/or tissue increases in cholesterol derivatives mainly formed by auto-oxidation: 7-ketocholesterol, also known as 7-oxo-cholesterol, and 7β-hydroxycholesterol. The formation of these oxysterols can be considered as a consequence of mitochondrial and peroxisomal dysfunction, leading to increased in oxidative stress, which is accentuated with age. 7-ketocholesterol and 7β-hydroxycholesterol cause a specific form of cytotoxic activity defined as oxiapoptophagy, including oxidative stress and induction of death by apoptosis associated with autophagic criteria. Oxiaptophagy is associated with organelle dysfunction and in particular with mitochondrial and peroxisomal alterations involved in the induction of cell death and in the rupture of redox balance. As the criteria characterizing 7-ketocholesterol- and 7β-hydroxycholesterol-induced cytotoxicity are often simultaneously observed in major age-related diseases (cardiovascular diseases, age-related macular degeneration, Alzheimer's disease) the involvement of these oxysterols in the pathophysiology of the latter seems increasingly likely. It is therefore important to better understand the signalling pathways associated with the toxicity of 7-ketocholesterol and 7β-hydroxycholesterol in order to identify pharmacological targets, nutrients and synthetic molecules attenuating or inhibiting the cytotoxic activities of these oxysterols. Numerous natural cytoprotective compounds have been identified: vitamins, fatty acids, polyphenols, terpenes, vegetal pigments, antioxidants, mixtures of compounds (oils, plant extracts) and bacterial enzymes. However, few synthetic molecules are able to prevent 7-ketocholesterol- and/or 7β-hydroxycholesterol-induced cytotoxicity: dimethyl fumarate, monomethyl fumarate, the tyrosine kinase inhibitor AG126, memantine, simvastatine, Trolox, dimethylsufoxide, mangafodipir and mitochondrial permeability transition pore (MPTP) inhibitors. The effectiveness of these compounds, several of which are already in use in humans, makes it possible to consider using them for the treatment of certain age-related diseases associated with increased plasma and/or tissue levels of 7-ketocholesterol and/or 7β-hydroxycholesterol.

Cellular crosstalk in cardioprotection: Where and when do reactive oxygen species play a role?

A well-balanced intercellular communication between the different cells within the heart is vital for the maintenance of cardiac homeostasis and function. Despite remarkable advances on disease management and treatment, acute myocardial infarction remains the major cause of morbidity and mortality worldwide. Gold standard reperfusion strategies, namely primary percutaneous coronary intervention, are crucial to preserve heart function. However, reestablishment of blood flow and oxygen levels to the infarcted area are also associated with an accumulation of reactive oxygen species (ROS), leading to oxidative damage and cardiomyocyte death, a phenomenon termed myocardial reperfusion injury. In addition, ROS signaling has been demonstrated to regulate multiple biological pathways, including cell differentiation and intercellular communication. Given the importance of cell-cell crosstalk in the coordinated response after cell injury, in this review, we will discuss the impact of ROS in the different forms of inter- and intracellular communication, as well as the role of gap junctions, tunneling nanotubes and extracellular vesicles in the propagation of oxidative damage in cardiac diseases, particularly in the context of ischemia/reperfusion injury.

Lipid Peroxidation in Atherosclerotic Cardiovascular Diseases

Significance: Atherosclerotic cardiovascular diseases (ACVDs) continue to be a primary cause of mortality worldwide in adults aged 35-70 years, occurring more often in countries with lower economic development, and they constitute an ever-growing global burden that has a considerable socioeconomic impact on society. The ACVDs encompass diverse pathologies such as coronary artery disease and heart failure (HF), among others. Recent Advances: It is known that oxidative stress plays a relevant role in ACVDs and some of its effects are mediated by lipid oxidation. In particular, lipid peroxidation (LPO) is a process under which oxidants such as reactive oxygen species attack unsaturated lipids, generating a wide array of oxidation products. These molecules can interact with circulating lipoproteins, to diffuse inside the cell and even to cross biological membranes, modifying target nucleophilic sites within biomolecules such as DNA, lipids, and proteins, and resulting in a plethora of biological effects. Critical Issues: This review summarizes the evidence of the effect of LPO in the development and progression of atherosclerosis-based diseases, HF, and other cardiovascular diseases, highlighting the role of protein adduct formation. Moreover, potential therapeutic strategies targeted at lipoxidation in ACVDs are also discussed. Future Directions: The identification of valid biomarkers for the detection of lipoxidation products and adducts may provide insights into the improvement of the cardiovascular risk stratification of patients and the development of therapeutic strategies against the oxidative effects that can then be applied within a clinical setting.

Polycystic Kidney Disease Ryanodine Receptor Domain (PKDRR) Proteins in Oomycetes

In eukaryotes, two sources of Ca²⁺ are accessed to allow rapid changes in the cytosolic levels of this second messenger: the extracellular medium and intracellular Ca²⁺ stores, such as the endoplasmic reticulum. One class of channel that permits Ca²⁺ entry is the transient receptor potential (TRP) superfamily, including the polycystic kidney disease (PKD) proteins, or polycystins. Channels that release Ca²⁺ from intracellular stores include the inositol 1,4,5-trisphosphate/ryanodine receptor (ITPR/RyR) superfamily. Here, we characterise a family of proteins that are only encoded by oomycete genomes, that we have named PKDRR, since they share domains with both PKD and RyR channels. We provide evidence that these proteins belong to the TRP superfamily and are distinct from the ITPR/RyR superfamily in terms of their evolutionary relationships, protein domain architectures and predicted ion channel structures. We also demonstrate that a hypothetical PKDRR protein from Phytophthora infestans is produced by this organism, is located in the cell-surface membrane and forms multimeric protein complexes. Efforts to functionally characterise this protein in a heterologous expression system were unsuccessful but support a cell-surface localisation. These PKDRR proteins represent potential targets for the development of new "fungicides", since they are of a distinctive structure that is only found in oomycetes and not in any other cellular organisms.

Healthy adiposity and extended lifespan in obese mice fed a diet supplemented with a polyphenol-rich plant extract

Obesity may not be consistently associated with metabolic disorders and mortality later in life, prompting exploration of the challenging concept of healthy obesity. Here, the consumption of a high-fat/high-sucrose (HF/HS) diet produces hyperglycaemia and hypercholesterolaemia, increases oxidative stress, increases endotoxaemia, expands adipose tissue (with enlarged adipocytes, enhanced macrophage infiltration and the accumulation of cholesterol and oxysterols), and reduces the median lifespan of obese mice. Despite the persistence of obesity, supplementation with a polyphenol-rich plant extract (PRPE) improves plasma lipid levels and endotoxaemia, prevents macrophage recruitment to adipose tissues, reduces adipose accumulation of cholesterol and cholesterol oxides, and extends the median lifespan. PRPE drives the normalization of the HF/HS-mediated functional enrichment of genes associated with immunity and inflammation (in particular the response to lipopolysaccharides). The long-term limitation of immune cell infiltration in adipose tissue by PRPE increases the lifespan through a mechanism independent of body weight and fat storage and constitutes the hallmark of a healthy adiposity trait.

The CNGRC-GG-D(KLAKLAK)2 peptide induces a caspase-independent, Ca2+-dependent death in human leukemic myeloid cells by targeting surface aminopeptidase N/CD13

The CD13 antigen's binding site for the Asn-Gly-Arg (NGR) motif enables NGR-containing chemotherapeutic drugs to be delivered to CD13-positive tumours. Human CD13-positive acute myeloid leukemia (AML) cells proliferate abnormally and escape death. Here, we show that the CNGRC-GG-D(KLAKLAK)2 peptide induces death in AML cell lines (U937, THP-1, NB4, HL-60) and primary blood cells from AML patients. Cell death was characterized as a caspase-independent mechanism, without DNA fragmentation, but phosphatidylserine externalization and membrane disruption. Our results demonstrate in U937 cells that (i) the NGR-peptide triggers the loss of mitochondrial potential(ΔΨm) and generates superoxide anion (O2-), (ii) N-acetyl-L-cysteine (NAC) and extra/intracellular Ca2+ chelators (BAPTA) prevent both O2- production and cell death, (iii) the Ca2+-channel blocker nifedipine prevents cell death (indicating that Ca2+ influx is the initial death trigger), and (iv) BAPTA, but not NAC, prevents ΔΨm loss (suggesting O2- is a mitochondrial downstream effector). AML cell lines and primary blasts responding to the lethal action of NGR-peptide express promatrix metalloproteinase-12 (proMMP-12) and its substrate progranulin (an 88 kDa cell survival factor). A cell-free assay highlighted proMMP-12 activation by O2-. Accordingly, NGR-peptide's downregulation of 88 kDa progranulin protein was prevented by BAPTA and NAC. Conversely, AML blast resistance to NGR-peptide is associated with the expression of a distinct, 105 kDa progranulin isoform. These results indicate that CNGRC-GG-D(KLAKLAK)2 induces death in AML cells through the Ca2+-mitochondria-O2.-pathway, and support the link between proMMP-12 activation and progranulin cleavage during cell death. Our findings may have implications for the understanding of tumour biology and treatment.

Oxysterols and 4-hydroxy-2-nonenal contribute to atherosclerotic plaque destabilization

A growing bulk of evidence suggests that cholesterol oxidation products, known as oxysterols, and 4-hydroxy-2-nonenal (HNE), the major proatherogenic components of oxidized low density lipoproteins (oxLDLs), significantly contribute to atherosclerotic plaque progression and destabilization, with eventual plaque rupture. These oxidized lipids are involved in various key steps of this complex process, mainly thanks to their ability to induce inflammation, oxidative stress, and apoptosis. This review summarizes the current knowledge of the effects induced by these compounds on vascular cells, after their accumulation in the arterial wall and in the atherosclerotic plaque.

Mitochondrial dysfunctions in 7-ketocholesterol-treated 158N oligodendrocytes without or with α-tocopherol: Impacts on the cellular profil of tricarboxylic cycle-associated organic acids, long chain saturated and unsaturated fatty acids, oxysterols, cholesterol and cholesterol precursors

In multiple sclerosis (MS) a process of white matter degradation leading to demyelination is observed. Oxidative stress, inflammation, apoptosis, necrosis and/or autophagy result together into a progressive loss of oligodendrocytes. 7-ketocholesterol (7KC), found increased in the cerebrospinal fluid of MS patients, triggers a rupture of RedOx homeostasis associated with mitochondrial dysfunctions, aptoptosis and autophagy (oxiapoptophagy) in cultured murine oligodendrocytes (158N). α-tocopherol is able to mild the alterations induced by 7KC partially restoring the cellular homeostasis. In presence of 7KC, the amount of adherent 158N cells was decreased and oxidative stress was enhanced. An increase of caspase-3 and PARP degradation (evidences of apoptosis), and an increased LC3-II/LC3-I ratio (criterion of autophagy), were detected. These events were associated with a decrease of the mitochondrial membrane potential (ΔΨ_m) and by a decrease of oxidative phosphorylation revealed by reduced NAD⁺ and ATP. The cellular lactate was higher while pyruvate, citrate, fumarate, succinate (tricarboxylic acid (TCA) cycle intermediates) were significantly reduced in exposed cells, suggesting that an impairment of mitochondrial respiratory functions could lead to an increase of lactate production and to a reduced amount of ATP and acetyl-CoA available for the anabolic pathways. The concentration of sterol precursors lathosterol, lanosterol and desmosterol were significantly reduced together with satured and unsatured long chain fatty acids (C16:0 - C18:0, structural elements of membrane phospholipids). Such reductions were milder with α-tocopherol. It is likely that the cell death induced by 7KC is associated with mitochondrial dysfunctions, including alterations of oxidative phosphorylation, which could result from lipid anabolism dysfunctions, especially on TCA cycle intermediates. A better knowledge of mitochondrial associated dysfunctions triggered by 7KC will contribute to bring new information on the demyelination processes which are linked with oxidative stress and lipid peroxidation, especially in MS.

Phytochemical indicaxanthin suppresses 7-ketocholesterol-induced THP-1 cell apoptosis by preventing cytosolic Ca(2+) increase and oxidative stress

7-Ketocholesterol (7-KC)-induced apoptosis of macrophages is considered a key event in the development of human atheromas. In the present study, the effect of indicaxanthin (Ind), a bioactive pigment from cactus pear fruit, on 7-KC-induced apoptosis of human monocyte/macrophage THP-1 cells was investigated. A pathophysiological condition was simulated by using amounts of 7-KC that can be reached in human atheromatous plaque. Ind was assayed within a micromolar concentration range, consistent with its plasma level after dietary supplementation with cactus pear fruit. Pro-apoptotic effects of 7-KC were assessed by cell cycle arrest, exposure of phosphatidylserine at the plasma membrane, variation of nuclear morphology, decrease of mitochondrial trans-membrane potential, activation of Bcl-2 antagonist of cell death and poly(ADP-ribose) polymerase-1 cleavage. Kinetic measurements within 24 h showed early formation of intracellular reactive oxygen species over basal levels, preceding NADPH oxidase-4 (NOX-4) over-expression and elevation of cytosolic Ca²⁺, with progressive depletion of total thiols. 7-KC-dependent activation of the redox-sensitive NF-κB was observed. Co-incubation of 2·5 μm of Ind completely prevented 7-KC-induced pro-apoptotic events. The effects of Ind may be ascribed to inhibition of NOX-4 basal activity and over-expression, inhibition of NF-κB activation, maintaining cell redox balance and Ca homeostasis, with prevention of mitochondrial damage and consequently apoptosis. The findings suggest that Ind, a highly bioavailable dietary phytochemical, may exert protective effects against atherogenetic toxicity of 7-KC at a concentration of nutritional interest.

Macrophage oxysterols and their binding proteins: roles in atherosclerosis

PURPOSE OF REVIEW

To offer a comprehensive review on the roles that oxysterols synthesized or engulfed by macrophages, or oxysterol-binding proteins in these cells, play in the development and progression of atherosclerotic lesions.

RECENT FINDINGS

Oxysterols abundant within the plaque have the capacity to potentiate macrophage proinflammatory signaling and to induce cell death. These activities may contribute to formation of the complex lesion, expansion of the necrotic core, and to plaque rupture. On the contrary, several endogenous oxysterols generated by cholesterol hydroxylases act as ligands of liver X receptors, stimulate macrophage cholesterol efflux, repress proinflammatory signaling, and promote macrophage survival, counteracting lesion progression. Cytoplasmic oxysterol-binding proteins represent a family of sterol and phosphoinositide sensors that may contribute to the regulatory impact of these bioactive lipids on processes relevant in the context of atherogenesis.

SUMMARY

The generation and deposition of oxysterols within the developing plaque is envisioned to modulate macrophage lipid metabolism, to affect the delicate balance of proinflammatory and anti-inflammatory processes, and to impact cell fate decisions, thus, determining whether the lesion remains benign or whether it develops into a hazardous, vulnerable plaque.

OSBP-related proteins (ORPs) in human adipose depots and cultured adipocytes: evidence for impacts on the adipocyte phenotype

Oxysterol-binding protein (OSBP) homologues, ORPs, are implicated in lipid homeostatic control, vesicle transport, and cell signaling. We analyzed here the quantity of ORP mRNAs in human subcutaneous (s.c.) and visceral adipose depots, as well as in the Simpson-Golabi-Behmel syndrome (SGBS) adipocyte cell model. All of the ORP mRNAs were present in the s.c and visceral adipose tissues, and the two depots shared an almost identical ORP mRNA expression pattern. SGBS adipocytes displayed a similar pattern, suggesting that the adipose tissue ORP expression pattern mainly derives from adipocytes. During SGBS cell adipogenic differentiation, ORP2, ORP3, ORP4, ORP7, and ORP8 mRNAs were down-regulated, while ORP11 was induced. To assess the impacts of ORPs on adipocyte differentiation, ORP3 and ORP8, proteins down-regulated during adipogenesis, were overexpressed in differentiating SGBS adipocytes, while ORP11, a protein induced during adipogenesis, was silenced. ORP8 overexpression resulted in reduced expression of the aP2 mRNA, while down-regulation of adiponectin and aP2 was observed in ORP11 silenced cells. Furthermore, ORP8 overexpression or silencing of ORP11 markedly decreased cellular triglyceride storage. These data identify the patterns of ORP expression in human adipose depots and SGBS adipocytes, and provide the first evidence for a functional impact of ORPs on the adipocyte phenotype.

Oxysterols and their cellular effectors

Oxysterols are oxidized 27-carbon cholesterol derivatives or by-products of cholesterol biosynthesis, with a spectrum of biologic activities. Several oxysterols have cytotoxic and pro-apoptotic activities, the ability to interfere with the lateral domain organization, and packing of membrane lipids. These properties may account for their suggested roles in the pathology of diseases such as atherosclerosis, age-onset macular degeneration and Alzheimer’s disease. Oxysterols also have the capacity to induce inflammatory responses and play roles in cell differentiation processes. The functions of oxysterols as intermediates in the synthesis of bile acids and steroid hormones, and as readily transportable forms of sterol, are well established. Furthermore, their actions as endogenous regulators of gene expression in lipid metabolism via liver X receptors and the Insig (insulin-induced gene) proteins have been investigated in detail. The cytoplasmic oxysterol-binding protein (OSBP) homologues form a group of oxysterol/cholesterol sensors that has recently attracted a lot of attention. However, their mode of action is, as yet, poorly understood. Retinoic acid receptor-related orphan receptors (ROR) α and γ, and Epstein-Barr virus induced gene 2 (EBI2) have been identified as novel oxysterol receptors, revealing new physiologic oxysterol effector mechanisms in development, metabolism, and immunity, and evoking enhanced interest in these compounds in the field of biomedicine.

Oxysterols and calcium signal transduction

Ionised calcium (Ca(2+)) is a key second messenger, regulating almost every cellular process from cell death to muscle contraction. Cytosolic levels of this ion can be increased via gating of channel proteins located in the plasma membrane, endoplasmic reticulum and other membrane-delimited organelles. Ca(2+) can be removed from cells by extrusion across the plasma membrane, uptake into organelles and buffering by anionic components. Ca(2+) channels and extrusion mechanisms work in concert to generate diverse spatiotemporal patterns of this second messenger, the distinct profiles of which determine different cellular outcomes. Increases in cytoplasmic Ca(2+) concentration are one of the most rapid cellular responses upon exposure to certain oxysterol congeners or to oxidised low-density lipoprotein, occurring within seconds of addition and preceding increases in levels of reactive oxygen species, or changes in gene expression. Furthermore, exposure of cells to oxysterols for periods of hours to days modulates Ca(2+) signal transduction, with these longer-term alterations in cellular Ca(2+) homeostasis potentially underlying pathological events within atherosclerotic lesions, such as hyporeactivity to vasoconstrictors observed in vascular smooth muscle, or ER stress-induced cell death in macrophages. Despite their candidate roles in physiology and disease, little is known about the molecular mechanisms that couple changes in oxysterol concentrations to alterations in Ca(2+) signalling. This review examines the ways in which oxysterols could influence Ca(2+) signal transduction and the potential roles of this in health and disease.

Sphingosine kinase regulates oxidized low density lipoprotein-mediated calcium oscillations and macrophage survival

We recently reported that oxidized LDL (oxLDL) induces an oscillatory increase in intracellular calcium ([Ca(2+)](i)) levels in macrophages. Furthermore, we have shown that these [Ca(2+)](i) oscillations mediate oxLDL's ability to inhibit macrophage apoptosis in response to growth factor deprivation. However, the signal transduction pathways by which oxLDL induces [Ca(2+)](i) oscillations have not been elucidated. In this study, we show that these oscillations are mediated in part by intracellular mechanisms, as depleting extracellular Ca(2+) did not completely abolish the effect. Inhibiting sarco-endoplasmic reticulum ATPase (SERCA) completely blocked [Ca(2+)](i) oscillations, suggesting a role for Ca(2+) reuptake by the ER. The addition of oxLDL resulted in an almost immediate activation of sphingosine kinase (SK), which can increase sphingosine-1-phosphate (S1P) levels by phosphorylating sphingosine. Moreover, S1P was shown to be as effective as oxLDL in blocking macrophage apoptosis and producing [Ca(2+)](i) oscillations. This suggests that the mechanism in which oxLDL generates [Ca(2+)](i) oscillations may be 1) activation of SK, 2) SK-mediated increase in S1P levels, 3) S1P-mediated Ca(2+) release from intracellular stores, and 4) SERCA-mediated Ca(2+) reuptake back into the ER.