A Lipidomic Perspective on Intermediates in Cholesterol Synthesis as Indicators of Disease Status

Oxysterols as Biomarkers of Aging and Disease

Oxysterols derive from either enzymatic or non-enzymatic oxidation of cholesterol. Even though they are produced as intermediates of bile acid synthesis pathway, they are recognised as bioactive compounds in cellular processes. Therefore, their absence or accumulation have been shown to be associated with disease phenotypes. This chapter discusses the contribution of oxysterol to ageing, age-related diseases such as neurodegeneration and various disorders such as cancer, cardiovascular disease, diabetes, metabolic and ocular disorders. It is clear that oxysterols play a significant role in development and progression of these diseases. As a result, oxysterols are being investigated as suitable markers for disease diagnosis purposes and some drug targets are in development targeting oxysterol pathways. However, further research will be needed to confirm the suitability of these potentials.

Vitamin D Status of Mice Deficient in Scavenger Receptor Class B Type 1, Cluster Determinant 36 and ATP-Binding Cassette Proteins G5/G8

Classical lipid transporters are suggested to modulate cellular vitamin D uptake. This study investigated the vitamin D levels in serum and tissues of mice deficient in SR-B1 (Srb1^-/-), CD36 (Cd36^-/-) and ABC-G5/G8 (Abcg5/g8^-/-) and compared them with corresponding wild-type (WT) mice. All mice received triple-deuterated vitamin D₃ (vitamin D₃-d₃) for six weeks. All knockout mice vs. WT mice showed specific alterations in their vitamin D concentrations. Srb1^-/- mice had higher levels of vitamin D₃-d₃ in the serum, adipose tissue, kidney and heart, whereas liver levels of vitamin D₃-d₃ remained unaffected. Additionally, Srb1^-/- mice had lower levels of deuterated 25-hydroxyvitamin D₃ (25(OH)D₃-d₃) in the serum, liver and kidney compared to WT mice. In contrast, Cd36^-/- and WT mice did not differ in the serum and tissue levels of vitamin D₃-d₃, but Cd36^-/- vs. WT mice were characterized by lower levels of 25(OH)D₃-d₃ in the serum, liver and kidney. Finally, Abcg5/g8^-/- mice tended to have higher levels of vitamin D₃-d₃ in the serum and liver. Major alterations in Abcg5/g8^-/- mice were notably higher levels of 25(OH)D₃-d₃ in the serum and kidney, accompanied by a higher hepatic mRNA abundance of Cyp27a1 hydroxylase. To conclude, the current data emphasize the significant role of lipid transporters in the uptake, tissue distribution and activation of vitamin D.

Inhibition of Niemann-Pick C1-like protein 1 by ezetimibe reduces uptake of deuterium-labeled vitamin D in mice

For a long time, orally ingested vitamin D was assumed to enter the body exclusively via simple passive diffusion. Recent data from in vitro experiments have described Niemann-Pick C1-like protein 1 (Npc1l1) as an important sterol transporter for vitamin D absorption. However, short-term applications of ezetimibe, which inhibits Npc1l1, were not associated with reduced vitamin D uptake in animals and humans. The current study aimed to elucidate the effect of long-term inhibition of Npc1l1 by ezetimibe on the uptake and storage of orally administered triple deuterated vitamin D₃ (vitamin D₃-d₃). Therefore, 30 male wild-type mice were randomly assigned into three groups and received diets with 25 μg/kg of vitamin D₃-d₃ that contained 0 (control group), 50 or 100 mg/kg ezetimibe for six weeks. Mice fed diets with 50 or 100 mg/kg ezetimibe had lower circulating levels of cholesterol than control mice (-12 %, -15 %, P < 0.01). In contrast, the concentrations of 7-dehydrocholesterol in serum (P < 0.001) and liver (P < 0.05) were higher in mice treated with ezetimibe than in control mice, indicating an increased sterol synthesis to compensate for cholesterol reduction. Long-term application of ezetimibe significantly reduced the concentrations of vitamin D₃-d₃ in the serum and tissues of mice. The magnitude of vitamin D₃ reduction was comparable between the two ezetimibe groups. In comparison to the control group, mice treated with ezetimibe had lower concentrations of deuterated vitamin D₃ compared with the control group in serum (62 %, P < 0.001), liver (79 %, P < 0.001), kidney (54 %, P < 0.001), adipose tissues (55 %, P < 0.001) and muscle (41 %, P < 0.001). Surprisingly, the serum concentration of deuterated 25-hydroxyvitamin D₃ was higher in the group fed 100 mg/kg ezetimibe than in the control group (P < 0.05). The protein expression of the vitamin D hydroxylases Cyp2r1, Cyp27a1, Cyp3a11, Cyp24a1 and Cyp2j3 in liver and Cyp27b1 and Cyp24a1 in kidney remained largely unaffected by ezetimibe. To conclude, Npc1l1 appears to be crucial for the uptake of orally ingested vitamin D because long-term inhibition of Npc1l1 by ezetimibe strongly reduced the levels of deuterium-labeled vitamin D in the body; the observed rise in deuterated 25-hydroxyvitamin D₃ in serum of these mice can not be explained by the expression levels of the key enzymes involved in vitamin D hydroxylation.

Omics analysis of oxysterols to better understand their pathophysiological role

High amounts of cholesterol have been definitely associated with the pathogenesis of several diseases, including metabolic and neurodegenerative disorders, cardiovascular diseases, and cancer. In all these pathologies the exacerbation of pro-oxidant and inflammatory responses is a consistent feature. In this scenario, species derived from enzymatic and non-enzymatic cholesterol oxidation, namely oxysterols, are strongly suspected to play a primary role. The consideration of these bioactive lipids is therefore helpful in investigating pathological mechanisms and may also acquire clinical value for the diagnosis and treatment of diseases. For this purpose and considering that a great number of oxysterols may be present together in the body, the employment of lipidomics technology certainly represents a powerful strategy for the simultaneous detection and characterization of these compounds in biological specimens. In this review, we will discuss the applicability of the lipidomics approach in the study of the association between oxysterols and diseases.

Sterol Analysis by Quantitative Mass Spectrometry

Analysis of sterols by mass spectrometry is a fundamental technique allowing for both qualitative and quantitative characterization of sterol molecular lipid species. Lipids are isolated from matrix or matrices by homogenization and solvent extraction, and converted into species amenable for ionization either by derivatization or adduct formation. Chromatogaphy (either gas or liquid phase) can assist with the resolution of sterols. Tandem mass spectrometry allows the precise identification of sterol lipid species, while comparison to internal standards added during extraction enables accurate quantification.

Oxysterols: Old Tale, New Twists

Oxysterols have long been known for their important role in cholesterol homeostasis, where they are involved in both transcriptional and posttranscriptional mechanisms for controlling cholesterol levels. However, they are increasingly associated with a wide variety of other, sometimes surprising cell functions. They are activators of the Hedgehog pathway (important in embryogenesis), and they act as ligands for a growing list of receptors, including some that are of importance to the immune system. Oxysterols have also been implicated in several diseases such as neurodegenerative diseases and atherosclerosis. Here, we explore the latest research into the roles oxy-sterols play in different areas, and we evaluate the current evidence for these roles. In addition, we outline critical concepts to consider when investigating the roles of oxysterols in various situations, which includes ensuring that the concentration and form of the oxysterol are relevant in that context--a caveat with which many studies have struggled.