Baseline Oxysterols and Other Markers of Oxidative Stress, Inflammation and Malnutrition in the Vitamin E and Intima Media Thickness Progression in End-Stage Renal Disease (VIPER) Cohort

Oxysterols: From redox bench to industry

More and more attention is nowadays given to the possible translational application of a great number of biochemical and biological findings with the involved molecules. This is also the case of cholesterol oxidation products, redox molecules over the last years deeply investigated for their implication in human pathophysiology. Oxysterols of non-enzymatic origin, the excessive increase of which in biological fluids and tissues is of toxicological relevance for their marked pro-oxidant and pro-inflammatory properties, are increasingly applied in clinical biochemistry as molecular markers in the diagnosis and monitoring of several human and veterinary diseases. Conversely, oxysterols of enzymatic origin, the production of which is commonly under physiological regulation, could be considered and tested as promising pharmaceutical agents because of their antiviral, pro-osteogenic and antiadipogenic properties of some of them. Very recently, the quantification of oxysterols of non-enzymatic origin has been adopted in a systematic way to evaluate, monitor and improve the quality of cholesterol-based food ingredients, that are prone to auto-oxidation, as well as their industrial processing and the packaging and the shelf life of the finished food products. The growing translational value of oxysterols is here reviewed in its present and upcoming applications in various industrial fields.

Localisation of oxysterols at the sub-cellular level and in biological fluids

Oxysterols are oxidized derivatives of cholesterol that are formed enzymatically or via reactive oxygen species or both. Cholesterol or oxysterols ingested as food are absorbed and packed into lipoproteins that are taken up by hepatic cells. Within hepatic cells, excess cholesterol is metabolised to form bile acids. The endoplasmic reticulum acts as the main organelle in the bile acid synthesis pathway. Metabolised sterols originating from this pathway are distributed within other organelles and in the cell membrane. The alterations to membrane oxysterol:sterol ratio affects the integrity of the cell membrane. The presence of oxysterols changes membrane fluidity and receptor orientation. It is well documented that hydroxylase enzymes located in mitochondria facilitate oxysterol production via an acidic pathway. More recently, the presence of oxysterols was also reported in lysosomes. Peroxisomal deficiencies favour intracellular oxysterols accumulation. Despite the low abundance of oxysterols compared to cholesterol, the biological actions of oxysterols are numerous and important. Oxysterol levels are implicated in the pathogenesis of multiple diseases ranging from chronic inflammatory diseases (atherosclerosis, Alzheimer's disease and bowel disease), cancer and numerous neurodegenerative diseases. In this article, we review the distribution of oxysterols in sub-cellular organelles and in biological fluids.

Chronic Kidney Disease and Disproportionally Increased Cardiovascular Damage: Does Oxidative Stress Explain the Burden?

Chronic kidney disease (CKD) patients are among the groups at the highest risk for cardiovascular disease and significantly shortened remaining lifespan. CKD enhances oxidative stress in the organism with ensuing cardiovascular damage. Oxidative stress in uremia is the consequence of higher reactive oxygen species (ROS) production, whereas attenuated clearance of pro-oxidant substances and impaired antioxidant defenses play a complementary role. The pathophysiological mechanism underlying the increased ROS production in CKD is at least partly mediated by upregulation of the intrarenal angiotensin system. Enhanced oxidative stress in the setting of the uremic milieu promotes enzymatic modification of circulating lipids and lipoproteins, protein carbamylation, endothelial dysfunction via disruption of nitric oxide (NO) pathways, and activation of inflammation, thus accelerating atherosclerosis. Left ventricular hypertrophy (LVH) and heart failure are hallmarks of CKD. NADPH oxidase activation, xanthine oxidase, mitochondrial dysfunction, and NO-ROS are the main oxidative pathways leading to LVH and the cardiorenal syndrome. Finally, a subset of antioxidant enzymes, the paraoxonases (PON), deserves special attention due to abundant clinical evidence accumulated regarding reduced serum PON1 activity in CKD as a contributor to the increased burden of cardiovascular disease. Future, meticulously designed studies are needed to assess the effects of antioxidant therapy on patients with CKD.

Low-density lipoprotein oxidation biomarkers in human health and disease and effects of bioactive compounds

Based on the significance of oxidized low-density lipoprotein (LDL) in health and disease, this review focuses on human studies addressing oxidation of LDL, including three lines of biomarkers, (i) ex vivo LDL resistance to oxidation, a "challenge test" model, (ii) circulating oxidized LDL, indicating the "current in vivo status", and (iii) autoantibodies against oxidized LDL as fingerprints of an immune response to oxidized LDL, along with circulating oxysterols and 4-hydroxynonenal as biomarkers of lipid peroxidation. Lipid peroxidation and oxidized LDL are hallmarks in the development of various metabolic, cardiovascular and other diseases. Changes further occur across life stages from infancy to older age as well as in athletes and smokers. Given their responsiveness to targeted nutritional interventions, markers of LDL oxidation have been employed in a rapidly growing number of human studies for more than 2 decades. There is growing interest in foods, which, besides providing energy and nutrients, exert beneficial effects on human health, such as protection of DNA, proteins and lipids from oxidative damage. Any health claim, however, needs to be substantiated by supportive evidence derived from human studies, using reliable biomarkers to demonstrate such beneficial effects. A large body of evidence has accumulated, demonstrating protection of LDL from oxidation by bioactive food compounds, including vitamins, other micronutrients and secondary plant ingredients, which will facilitate the selection of oxidation biomarkers for future human intervention studies and health claim support.

Modified Lipids and Lipoproteins in Chronic Kidney Disease: A New Class of Uremic Toxins

Chronic kidney disease (CKD) is associated with an enhanced oxidative stress and deep modifications in lipid and lipoprotein metabolism. First, many oxidized lipids accumulate in CKD and were shown to exert toxic effects on cells and tissues. These lipids are known to interfere with many cell functions and to be pro-apoptotic and pro-inflammatory, especially in the cardiovascular system. Some, like F2-isoprostanes, are directly correlated with CKD progression. Their accumulation, added to their noxious effects, rendered their nomination as uremic toxins credible. Similarly, lipoproteins are deeply altered by CKD modifications, either in their metabolism or composition. These impairments lead to impaired effects of HDL on their normal effectors and may strongly participate in accelerated atherosclerosis and failure of statins in end-stage renal disease patients. This review describes the impact of oxidized lipids and other modifications in the natural history of CKD and its complications. Moreover, this review focuses on the modifications of lipoproteins and their impact on the emergence of cardiovascular diseases in CKD as well as the appropriateness of considering them as actual mediators of uremic toxicity.

Wishful Thinking: The Surprisingly Sparse Evidence for a Relationship between Oxidative Stress and Cardiovascular Disease in Hemodialysis Patients

The increased frequency of cardiovascular disease observed in hemodialysis patients is secondary to the combination of many traditional (age, male sex, hypertension, smoking, diabetes mellitus, and dyslipidemia) and novel and uremia-related (inflammation, uremic toxins, adipokine imbalance, coagulation disorders, protein-energy wasting, volume overload, endothelial dysfunction, hyperparathyroidism, and subclinical hypothyroidism) risk factors. Usually, in the latter group, oxidative stress is included. However, after decades of research, it remains essentially unknown if oxidative stress has a causative role in the development of cardiovascular disease in long-term hemodialysis patients because adequate longitudinal studies are lacking. Data deriving from cross-sectional studies suggest that biomarkers of oxidative stress are associated with cardiovascular disease prevalence. Conversely, conflicting and inconclusive results have been obtained on the association between oxidative stress and coronary artery calcification, atherosclerosis, and all-cause and cardiovascular disease-related outcome. It is desirable that further studies are conducted on this topic in the near future.

Circulating plasma antioxidants, inflammatory markers and asymptomatic carotid atherosclerosis in end-stage renal disease patients: a case control study

Few studies have been conducted on the relationship between antioxidant plasma vitamin concentrations, inflammatory markers and carotid atherosclerosis with inconclusive results in endstage renal disease (ESRD) patients. A case-control study was performed to investigate the relationship between plasma antioxidant concentrations, inflammatory markers, and carotid intima-media thickness (CIMT) in healthy subjects and in patients undergoing hemodialysis (HD). We enrolled 40 subjects (20 healthy, 20 with ESRD) asymptomatic for carotid atherosclerosis. After carotid ultrasound investigation (CUI), medical history data, physical examination, venous blood samples were collected. These were analyzed for concentrations of antioxidant vitamins (A, E), carotenoids (lycopene, beta-carotene), inflammatory markers (C-reactive protein, fibrinogen), and lipid profile. Low concentrations of vitamin A, vitamin E, lycopene, and beta-carotene were significantly associated with carotid atherosclerosis in patients with ESRD (p less than 0.001). In addition, high concentration of low density lipoprotein cholesterol and total cholesterol (p less than 0.01), C-reactive protein and fibrinogen (p less than 0.001) were also associated with carotid atherosclerosis, while other laboratory parameters considered (high density lipoprotein cholesterol and triglycerides) were not significantly associated with carotid atherosclerosis. A regular intake of foods rich in antioxidant vitamins with low fat concentrations may slow the progression of atherosclerotic process in this group of patients.

Oxysterol as a marker of atherogenic dyslipidemia in adolescence

CONTEXT

Oxysterols represent potentially important oxidative stress biomarkers in adolescence.

OBJECTIVE

The objective of the study was to examine the relationship between the concentrations of serum enzymatically and nonenzymatically generated oxysterols, measures of obesity, and metabolic components including insulin resistance and levels of blood pressure and serum lipids.

DESIGN

This was a cross-sectional study.

SETTING

All subjects were examined between 2003 and 2005 at a hospital, a part of a follow-up evaluation mother-daughter pairs representing pregnancies affected or unaffected by gestational diabetes that resulted in the deliveries in 1989-1991.

SUBJECTS

Subjects included a subset (n=89) of the total study population of 189 adolescent girls with a mean age of 15.32+/-0.65 yr and body mass index of 22.54+/-3.98 kg/m2.

MAIN OUTCOME MEASURES

Measures included serum levels of the oxysterols 7alpha-hydroxy-cholesterol, 7beta-hydroxycholesterol, and 7-ketocholesterol; and body mass index, homeostasis model assessment insulin resistance index, fasting insulin, fasting glucose, blood pressure, total cholesterol, non-high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, triglycerides, and apolipoprotein B (ApoB).

RESULTS

Serum oxysterol concentrations in the adolescent cohort correlated positively with insulin (P<0.05), total cholesterol (P<0.05), non-high-density lipoprotein cholesterol (P<0.05), low-density lipoprotein cholesterol (P<0.05), and ApoB (P<0.01). ApoB and fasting insulin were found to be the major determinants of serum oxysterols after adjustment for body mass index. Being a daughter of gestational diabetes pregnancy alone did not seem to be a predisposing factor to increased oxidative stress in our cohort.

CONCLUSION

Serum oxysterol concentrations increase with obesity, insulin, and ApoB, which are established derangements associated with the metabolic syndrome.

In vivo interconversion of 7beta-hydroxycholesterol and 7-ketocholesterol, potential surrogate markers for oxidative stress

The oxysterols 7beta-hydroxycholesterol and 7-ketocholesterol are cholesterol autoxidation products. These two oxysterols are formed as a result of low density lipoprotein oxidation and in a study on biomarkers for oxidative stress in patients with atherosclerosis, 7beta-hydroxycholesterol was found to be the strongest predictor of progression of carotid atherosclerosis. Interconversion of 7beta-hydroxycholesterol and 7-ketocholesterol in vitro has been reported recently, using recombinant 11beta-hydroxysteroid dehydrogenase or rodent liver microsomes. In this study deuterium-labeled 7beta-hydroxycholesterol or 7-ketocholesterol was administered intravenously to two healthy volunteers and blood samples were collected at different time points. The mean half-life for elimination of 7beta-hydroxycholesterol from the circulation was estimated to be 1.9 h. The corresponding half-life for 7-ketocholesterol was estimated to be 1.5 h. Infusion of deuterium-labeled 7-ketocholesterol resulted in labeling of 7beta-hydroxycholesterol and vice versa. In addition, the biological within-day and between-day variations of the two oxysterols were determined. In summary, the present investigation clearly shows an interconversion of 7beta-hydroxycholesterol and 7-ketocholesterol in humans.