Brain cholesterol metabolite 24-hydroxycholesterol modulates inotropic responses to β-adrenoceptor stimulation: The role of NO and phosphodiesterase

25-hydroxycholesterol triggers antioxidant signaling in mouse atria

Oxysterol, 25-hydroxycholesterol (25HC), is a potent regulator of immune reactions, its synthesis greatly increases by macrophages during inflammation. We hypothesize that 25HC can have cardioprotective effects by limiting consequences of excessive β-adrenoceptor (βAR) stimulation, particularly reactive oxygen species (ROS) production, in mouse atria. Isoproterenol, a βAR agonist, increased extra- and intracellular levels of ROS. This enhancement of ROS production was suppressed by NADPH oxidase antagonists as well as 25HC. Inhibition of β3ARs, Gi protein and protein kinase Cε prevented the effect of 25HC on isoproterenol-dependent ROS synthesis. Furthermore, 25HC suppressed isoproterenol-induced lipid peroxidation and mitochondrial ROS generation as well as ROS-dependent component of positive inotropic response to isoproterenol. Additionally, 25HC decreased mitochondrial ROS production and lipid peroxidation induced by antimycin A, a mitochondrial poison. Thus, 25HC exerts antioxidant properties alleviating mitochondrial dysfunction-induced and βAR-dependent cardiac oxidative damage. In the latter case, 25HC can act via signaling mechanism engaging β3ARs, Gi protein and protein kinase Cε.

The mechanism of 25-hydroxycholesterol-mediated suppression of atrial β1-adrenergic responses

25-Hydroxycholesterol (25HC) is a biologically active oxysterol, whose production greatly increases during inflammation by macrophages and dendritic cells. The inflammatory reactions are frequently accompanied by changes in heart regulation, such as blunting of the cardiac β-adrenergic receptor (AR) signaling. Here, the mechanism of 25HC-dependent modulation of responses to β-AR activation was studied in the atria of mice. 25HC at the submicromolar levels decreased the β-AR-mediated positive inotropic effect and enhancement of the Ca2+ transient amplitude, without changing NO production. Positive inotropic responses to β1-AR (but not β2-AR) activation were markedly attenuated by 25HC. The depressant action of 25HC on the β1-AR-mediated responses was prevented by selective β3-AR antagonists as well as inhibitors of Gi protein, Gβγ, G protein-coupled receptor kinase 2/3, or β-arrestin. Simultaneously, blockers of protein kinase D and C as well as a phosphodiesterase inhibitor did not preclude the negative action of 25HC on the inotropic response to β-AR activation. Thus, 25HC can suppress the β1-AR-dependent effects via engaging β3-AR, Gi protein, Gβγ, G protein-coupled receptor kinase, and β-arrestin. This 25HC-dependent mechanism can contribute to the inflammatory-related alterations in the atrial β-adrenergic signaling.

Oxysterols in Central and Peripheral Synaptic Communication

Cholesterol is a key molecule for synaptic transmission, and both central and peripheral synapses are cholesterol rich. During intense neuronal activity, a substantial portion of synaptic cholesterol can be oxidized by either enzymatic or non-enzymatic pathways to form oxysterols, which in turn modulate the activities of neurotransmitter receptors (e.g., NMDA and adrenergic receptors), signaling molecules (nitric oxide synthases, protein kinase C, liver X receptors), and synaptic vesicle cycling involved in neurotransmitters release. 24-Hydroxycholesterol, produced by neurons in the brain, could directly affect neighboring synapses and change neurotransmission. 27-Hydroxycholesterol, which can cross the blood-brain barrier, can alter both synaptogenesis and synaptic plasticity. Increased generation of 25-hydroxycholesterol by activated microglia and macrophages could link inflammatory processes to learning and neuronal regulation. Amyloids and oxidative stress can lead to an increase in the levels of ring-oxidized sterols and some of these oxysterols (4-cholesten-3-one, 5α-cholestan-3-one, 7β-hydroxycholesterol, 7-ketocholesterol) have a high potency to disturb or modulate neurotransmission at both the presynaptic and postsynaptic levels. Overall, oxysterols could be used as "molecular prototypes" for therapeutic approaches. Analogs of 24-hydroxycholesterol (SGE-301, SGE-550, SAGE718) can be used for correction of NMDA receptor hypofunction-related states, whereas inhibitors of cholesterol 24-hydroxylase, cholestane-3β,5α,6β-triol, and cholest-4-en-3-one oxime (olesoxime) can be utilized as potential anti-epileptic drugs and (or) protectors from excitotoxicity.

Molecular basis for the recognition of 24-(S)-hydroxycholesterol by integrin αvβ3

A growing body of evidence suggests that oxysterols such as 25-hydroxycholesterol (25HC) are biologically active and involved in many physiological and pathological processes. Our previous study demonstrated that 25HC induces an innate immune response during viral infections by activating the integrin-focal adhesion kinase (FAK) pathway. 25HC produced the proinflammatory response by binding directly to integrins at a novel binding site (site II) and triggering the production of proinflammatory mediators such as tumor necrosis factor-α (TNF) and interleukin-6 (IL-6). 24-(S)-hydroxycholesterol (24HC), a structural isomer of 25HC, plays a critical role in cholesterol homeostasis in the human brain and is implicated in multiple inflammatory conditions, including Alzheimer's disease. However, whether 24HC can induce a proinflammatory response like 25HC in non-neuronal cells has not been studied and remains unknown. The aim of this study was to examine whether 24HC produces such an immune response using in silico and in vitro experiments. Our results indicate that despite being a structural isomer of 25HC, 24HC binds at site II in a distinct binding mode, engages in varied residue interactions, and produces significant conformational changes in the specificity-determining loop (SDL). In addition, our surface plasmon resonance (SPR) study reveals that 24HC could directly bind to integrin αvβ3, with a binding affinity three-fold lower than 25HC. Furthermore, our in vitro studies with macrophages support the involvement of FAK and NFκB signaling pathways in triggering 24HC-mediated production of TNF. Thus, we have identified 24HC as another oxysterol that binds to integrin αvβ3 and promotes a proinflammatory response via the integrin-FAK-NFκB pathway.

Selected plasma oxysterols as a potential multi-marker biosignature panel for Behçet's Disease

Clinical, genetic, and medical evidence has shown the inflammatory vasculitis aspect of Behçet's Disease (BD). Whereas oxysterols are vital factors in inflammation and oxidative stress, it is still unknown whether they are involved in the pathophysiology of BD. The current study aims to explore the profile of oxysterols in plasma of BD patients. Thirty patients diagnosed with BD and forty healthy controls matched for age and gender were included. Results showed that the cholestane-3β,5α,6β-triol, 27-hydroxycholesterol (27-OHC) and cholestanol levels were higher in BD than controls. In addition, plasma levels of 7-ketocholesterol (7-KC) and 25-hydroxycholesterol (25-OHC) were lower in BD patient. However, levels of 24S-hydroxycholesterol (24-OHC) did not significantly differ. For BD patients, the plasma 7-KC level was negatively correlated with the BD activity index (BDAI) while 27-OHC was positively correlated with high-sensitivity C-reactive protein (hs-CRP) in patients with active course of the disease. According to ROC analysis, a remarkable increase in the area under the curve (AUC) with a higher sensitivity (Se) and specificity (Sp) for 7-KC, 25-OHC and 27-OHC combined markers was observed. The present study indicated that the identification of the predictive value of these three-selected biomarkers related to oxidative stress and inflammation in patients should lead to a better identification of the etiological mechanism of BD.

25-Hydroxycholesterol as a Signaling Molecule of the Nervous System

Cholesterol is an essential component of plasma membrane and precursor of biological active compounds, including hydroxycholesterols (HCs). HCs regulate cellular homeostasis of cholesterol; they can pass across the membrane and vascular barriers and act distantly as para- and endocrine agents. A small amount of 25-hydroxycholesterol (25-HC) is produced in the endoplasmic reticulum of most cells, where it serves as a potent regulator of the synthesis, intracellular transport, and storage of cholesterol. Production of 25-HC is strongly increased in the macrophages, dendrite cells, and microglia at the inflammatory response. The synthesis of 25-HC can be also upregulated in some neurological disorders, such as Alzheimer’s disease, amyotrophic lateral sclerosis, spastic paraplegia type 5, and X-linked adrenoleukodystrophy. However, it is unclear whether 25-HC aggravates these pathologies or has the protective properties. The molecular targets for 25-HC are transcriptional factors (LX receptors, SREBP2, ROR), G protein-coupled receptor (GPR183), ion channels (NMDA receptors, SLO1), adhesive molecules (α5β1 and ανβ3 integrins), and oxysterol-binding proteins. The diversity of 25-HC-binding proteins points to the ability of HC to affect many physiological and pathological processes. In this review, we focused on the regulation of 25-HC production and its universal role in the control of cellular cholesterol homeostasis, as well as the effects of 25-HC as a signaling molecule mediating the influence of inflammation on the processes in the neuromuscular system and brain. Based on the evidence collected, it can be suggested that 25-HC prevents accumulation of cellular cholesterol and serves as a potent modulator of neuroinflammation, synaptic transmission, and myelinization. An increased production of 25-HC in response to a various type of damage can have a protective role and reduce neuronal loss. At the same time, an excess of 25-HC may exert the neurotoxic effects.

Effects of Oxysterols on Immune Cells and Related Diseases

Oxysterols are the products of cholesterol oxidation. They have a wide range of effects on several cells, organs, and systems in the body. Oxysterols also have an influence on the physiology of the immune system, from immune cell maturation and migration to innate and humoral immune responses. In this regard, oxysterols have been involved in several diseases that have an immune component, from autoimmune and neurodegenerative diseases to inflammatory diseases, atherosclerosis, and cancer. Here, we review data on the participation of oxysterols, mainly 25-hydroxycholesterol and 7α,25-dihydroxycholesterol, in the immune system and related diseases. The effects of these oxysterols and main oxysterol receptors, LXR and EBI2, in cells of the immune system (B cells, T cells, macrophages, dendritic cells, oligodendrocytes, and astrocytes), and in immune-related diseases, such as neurodegenerative diseases, intestinal diseases, cancer, respiratory diseases, and atherosclerosis, are discussed.

Immune-related oxysterol modulates neuromuscular transmission via non-genomic liver X receptor-dependent mechanism

Inflammatory reactions induce changes in the neuromuscular system. The mechanisms underlying this link are unclear. Besides cytokines and reactive oxygen species (ROS), production of an antiviral oxysterol 25-hydroxycholesterol (25HC) by immune cells is quickly increased in response to inflammation. Hypothetically, 25HC could contribute to regulation of neuromuscular activity as well as redox status. We found that 25HC (0.01-10 μM) can bidirectionally modulate neurotransmission in mice diaphragm, the main respiratory muscle. Low concentrations (≤0.1 μM) of 25HC reduced involvement of synaptic vesicles (SVs) into exocytosis during 20-Hz activity, whereas higher inflammatory-related concentrations (≥1 μM) had a profound potentiating effect on SV mobilization. The latter stimulatory action of 25HC was accompanied by increase in Ca²⁺ release from intracellular stores via IP₃ receptors. Both increase in SV mobilization and [Ca²⁺]_in were suppressed by a specific antagonist of liver X receptors (LXRs). These receptors formed clusters within the synaptic membranes in a lipid raft-dependent manner. Either raft disruption or intracellular Ca²⁺ chelation prevented 25HC-mediated acceleration of the exocytotic rate. The same action had inhibition of estrogen receptor α, Gi-protein, Gβγ, phospholipase C and protein kinase C. Additionally, 1 μM 25HC upregulated ROS production in a Ca²⁺-dependent way and an antioxidant partially decreased the exocytosis-promoting effect of 25HC. Thus, 25HC has prooxidant properties and it is a potent regulator of SV mobilization via activation of lipid raft-associated LXRs which can trigger signaling via estrogen receptor α - Gi-protein - Gβγ - phospholipase C - Ca²⁺ - protein kinase C pathway. 25HC-mediated increase in ROS may modulate this signaling.

24S-hydroxycholesterol: Cellular effects and variations in brain diseases

The adult brain exhibits a characteristic cholesterol homeostasis, with low synthesis rate and active catabolism. Brain cholesterol turnover is possible thanks to the action of the enzyme cytochrome P450 46A1 (CYP46A1) or 24-cholesterol hydroxylase, that transforms cholesterol into 24S-hydroxycholesterol (24S-HC). But before crossing the blood-brain barrier (BBB), this oxysterol, that is the most abundant in the brain, can act locally, affecting the functioning of neurons, astrocytes, oligodendrocytes, and vascular cells. The first part of this review addresses different aspects of 24S-HC production and elimination from the brain. The second part concentrates in the effects of 24S-HC at the cellular level, describing how this oxysterol affects cell viability, amyloid β production, neurotransmission, and transcriptional activity. Finally, the role of 24S-HC in Alzheimer, Huntington and Parkinson diseases, multiple sclerosis and amyotrophic lateral sclerosis, as well as the possibility of using this oxysterol as predictive and/or evolution biomarker in different brain disorders is discussed.