The relationships of phytosterols and oxyphytosterols in plasma and aortic valve cusps in patients with severe aortic stenosis

Advances and Challenges in Plant Sterol Research: Fundamentals, Analysis, Applications and Production

Plant sterols (PS) are cholesterol-like terpenoids widely spread in the kingdom Plantae. Being the target of extensive research for more than a century, PS have topped with evidence of having beneficial effects in healthy subjects and applications in food, cosmetic and pharmaceutical industries. However, many gaps in several fields of PS's research still hinder their widespread practical applications. In fact, many of the mechanisms associated with PS supplementation and their health benefits are still not fully elucidated. Furthermore, compared to cholesterol data, many complex PS chemical structures still need to be fully characterized, especially in oxidized PS. On the other hand, PS molecules have also been the focus of structural modifications for applications in diverse areas, including not only the above-mentioned but also in e.g., drug delivery systems or alternative matrixes for functional foods and fats. All the identified drawbacks are also superimposed by the need of new PS sources and technologies for their isolation and purification, taking into account increased environmental and sustainability concerns. Accordingly, current and future trends in PS research warrant discussion.

Plasma oxyphytosterols most likely originate from hepatic oxidation and subsequent spill-over in the circulation

We evaluated oxyphytosterol (OPS) concentrations in plasma and various tissues of two genetically modified mouse models with either increased cholesterol (apoE KO mice) or increased cholesterol and plant sterol (PS) concentrations (apoExABCG8 dKO mice). Sixteen female apoE KO and 16 dKO mice followed the same standard, low OPS-chow diet. Animals were euthanized at 36 weeks to measure PS and OPS concentrations in plasma, brain, liver and aortic tissue. Cholesterol and oxysterol (OS) concentrations were analyzed as reference for sterol oxidation in general. Plasma campesterol (24.1 ± 4.3 vs. 11.8 ± 3.0 mg/dL) and sitosterol (67.4 ± 12.7 vs. 4.9 ± 1.1 mg/dL) concentrations were severely elevated in the dKO compared to the apoE KO mice (p < 0.001). Also, in aortic and brain tissue, PS levels were significantly elevated in dKO. However, plasma, aortic and brain OPS concentrations were comparable or even lower in the dKO mice. In contrast, in liver tissue, both PS and OPS concentrations were severely elevated in the dKO compared to apoE KO mice (sum OPS: 7.4 ± 1.6 vs. 4.1 ± 0.8 ng/mg, p < 0.001). OS concentrations followed cholesterol concentrations in plasma and all tissues suggesting ubiquitous oxidation. Despite severely elevated PS concentrations, OPS concentrations were only elevated in liver tissue, suggesting that OPS are primarily formed in the liver and plasma concentrations originate from hepatic spill-over into the circulation.

Safety of the extension of use of plant sterol esters as a novel food pursuant to Regulation (EU) 2015/2283

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver an opinion on the safety of an extension of use of the novel food 'plant sterol esters' when added to vegetable fat spreads and to liquid vegetable fat-based emulsions for cooking and baking purposes pursuant to Regulation (EU) 2015/2283. Member States expressed concerns in relation to plant sterol oxidation products (POP) and consumption by non-target population groups. The median (0.5%) and P90 (2.28%) value of the oxidation rates of plant sterols determined by a wide range of cooking experiments were used together with exposure estimates for plant sterol when added and cooked with vegetable fat spreads and liquids. The no-observed adverse effect level (NOAEL) of a subchronic rat study and an applied default uncertainty factor of 200 served to derive levels (i.e. 0.64 mg POP/kg body weight (bw) per day) considered safe for humans. This safe level of exposure would be exceeded at the P95 by all age groups when considering the P90 oxidation rate and using EFSA's comprehensive food consumption database for assessing the potential exposure. When considering the median oxidation rate, the safe level of 0.64 mg POP/kg bw per day would be exceeded at the highest P95 intake estimates in children below 9 years of age. When considering an intake of the maximum authorised use level of 3 g plant sterols/person per day and oxidation rates of 0.5% and 2.28%, the resulting daily POP intakes per kg bw by an adult weighing 70 kg would be 0.21 and 0.98 mg/kg bw per day, respectively, the latter value exceeding 0.64 mg/kg bw per day. The Panel concludes that the safety of the intended extension of use of plant sterol esters under the proposed conditions of use has not been established.

Methodological Aspects of Phytosterol Measurements in Biological Samples

Phytosterol measurement has gained a lot of interest during the last two decades after foods and supplements with added 4-desmethyl phytosterols were recognized and used as effective and safe non-pharmacologic hypocholesterolemic agents, and also after the mechanisms of intestinal absorption and hepatic excretion of sterols were unraveled. In addition, the wide use of serum phytosterols as biomarkers of cholesterol absorption has increased the interest in their measurement. In this review, the basic methods are discussed without going into details of the practical operations. The analysis includes first lipid extraction and saponification from various biologic matrices such as serum/plasma, feces, or tissues, after which the individual sterols are separated by adsorption chromatography (gas-liquid or liquid or high performance liquid chromatography) based on the polarity of the various sterols. We also deal with some specific aspects of phytosterol measurements in biological samples such as the need of harmonization of their analysis in biological samples, the discrepancies in the results of sitosterol and campesterol concentrations between different studies, and what is known about their biological day-to-day fluctuation. Phytosterols have a remarkable role in human health, so that their complicated and time consuming measurements call attention to routine ways of standardization between the sterol research laboratories.

Serum Concentration of Plant Sterol Oxidation Products (POP) Compared to Cholesterol Oxidation Products (COP) after Intake of Oxidized Plant Sterols: A Randomised, Placebo-Controlled, Double-Blind Dose‒Response Pilot Study

Plant sterols (PS) are oxidized to PS oxidation products (POP). This study quantified the change in serum POP compared to cholesterol oxidation products (COP) after the intake of increasing POP doses. This was a double-blind, randomized, placebo-controlled, dose‒response pilot study with healthy individuals in four groups (15 per group). The control group received products with no added PS or POP and treatment groups received daily 20–25 g margarine with added PS (mean 3 g/d) and two cookies (~28 g) for six weeks. Cookies delivered 8.7 (low-dose), 15.2 (medium-dose), or 37.2 (high-dose) mg/d POP. Fasting serum POP and COP were measured at the baseline, days 14, 28, and 42 in all participants and days 7, 21, and 35 in a subset. Sixty individuals completed the study; 52 were included in per protocol analysis. Serum POP increased with increasing POP intake and plateaued at dose >15 mg/d. Stabilized POP concentrations were (mean ± SD) 38.9 ± 6.9, 91.0 ± 27.9, 144.4 ± 37.9 and 203.0 ± 63.7 nmol/L, for control, low-, medium-, and high-dose POP groups, respectively. For all groups, the serum COP ranged from 213 to 262 nmol/L and the average POP/COP ratio was <1. Serum POP concentrations increased non-linearly, reaching stabilized concentrations in <7 days, and remained below COP concentrations after the intake of increasing POP doses.

Plasma oxyphytosterol concentrations are not associated with CVD status in Framingham Offspring Study participants

Dietary plant sterols, such as campesterol and sitosterol, reduce plasma cholesterol concentrations, but any relationship to plaque development and CVD remains unclear. Some epidemiologic studies have suggested that elevated plasma plant sterol concentrations are atherogenic, including the Framingham Offspring Study that identified a positive association between plant sterol concentrations and CVD status. We hypothesized that this suggested atherogenicity relates to the oxidation status of plant sterols (i.e., concentrations of plasma oxyphytosterols). Therefore, in the Framingham Offspring Study cohort, we measured plasma oxyphytosterol concentrations in 144 patients with documented CVD and/or more than 50% carotid stenosis and 383 matched controls. We analyzed plasma oxyphytosterol concentrations by GC/MS/MS and performed conditional logistic regression analysis to determine associations between plasma plant sterol or oxyphytosterol concentrations and CVD status. We found that higher total cholesterol (TC)-standardized campesterol concentrations [odds ratio (OR): 2.36; 95% CI: 1.60, 3.50] and higher sitosterol concentrations (OR: 1.47; 95% CI: 1.09, 1.97) were significantly associated with increased CVD risk, as in the earlier study. However, the sum of absolute oxyphytosterol concentrations (OR: 0.99; 95% CI: 0.81, 1.21) and the sum of TC-standardized oxyphytosterol concentrations (OR: 0.98; 95% CI: 0.80, 1.19) were not associated with an increased CVD risk. Results were comparable for individual absolute and TC-standardized oxycampesterol and oxysitosterol concentrations. Plasma nonoxidized TC-standardized sitosterol and campesterol concentrations showed weak or no correlations with oxyphytosterol concentrations, while all individual plasma concentrations of oxyphytosterol correlated with each other. In conclusion, circulating plasma oxyphytosterols are not associated with CVD risk in the Framingham Offspring Study.

Plasma levels of the oxyphytosterol 7α-hydroxycampesterol are associated with cardiovascular events

BACKGROUND AND AIMS

There are safety issues regarding plant sterol ester-enriched functional food. Oxidized plant sterols, also called oxyphytosterols, are supposed to contribute to plant sterol atherogenicity. This study aimed to analyze associations of plasma oxyphytosterol levels with cardiovascular events.

METHODS

Plasma cholesterol was measured by gas chromatography-flame ionization detection. Plasma campesterol and sitosterol and their 7-oxygenated metabolites were analyzed by gas chromatography-mass selective detection.

RESULTS

In 376 patients admitted for elective coronary angiography, who were not on lipid-lowering drugs, 82 cardiovascular events occurred during a follow-up period of 4.2 ± 1.8 years. Patients with cardiovascular events had significantly higher 7α-hydroxycampesterol plasma levels (median, 0.46; [interquartile range (IQR) 0.22-0.81] nmol/L vs. median, 0.25 [IQR, 0.17-0.61] nmol/L; p = 0.003) and 7α-hydroxycampesterol-to-cholesterol ratios (median 0.08 [IQR, 0.04-0.14] nmol/mmol vs. median, 0.05 [IQR 0.03-0.11] nmol/mmol; p = 0.005) than controls without such events. Patients above the median were characterized by higher cumulative event rates in Kaplan-Meier-analysis (Logrank-test p = 0.084 and p = 0.025) for absolute and cholesterol corrected 7α-hydroxycampesterol, respectively. After adjustment for influencing factors and related lipids, the hazard ratios per one standard deviation of the log-transformed variables (HR) were 1.19 [95% confidence interval (CI), 0.95-1.48], p = 0.132 for 7α-hydroxycampesterol and HR, 1.18 [95% CI, 0.94-1.48], p = 0.154 for 7α-hydroxycampesterol-to-cholesterol ratio. None of the other investigated oxyphytosterols showed an association with cardiovascular events.

CONCLUSIONS

In patients not on lipid-lowering drugs, absolute plasma levels of 7α-hydroxycampesterol and their ratios to cholesterol are associated with cardiovascular events. Further research is required to elucidate the role of OPS in cardiovascular diseases.

The effects of vitamin E or lipoic acid supplementation on oxyphytosterols in subjects with elevated oxidative stress: a randomized trial

Despite increased serum plant sterol concentrations after consumption of plant sterol enriched margarines, plasma oxyphytosterol concentrations were not increased in healthy subjects. Here, we assessed plasma oxyphytosterol concentrations and whether they are affected by antioxidants in subjects with elevated oxidative stress. Twenty subjects with impaired glucose tolerance (IGT) or type 2 diabetes (DM2) consumed for 4 weeks placebo, vitamin E (804 mg/d) or lipoic acid capsules (600 mg/d). Plasma and blood cell oxyphytosterol and oxycholesterol concentrations were determined in butylated hydroxytoluene-enriched EDTA plasma via GC-MS. Also, markers reflecting oxidative stress and antioxidant capacity were measured. Plasma oxycampesterol and oxysitosterol concentrations were 122% and 83% higher in IGT or DM2 subjects than in healthy subjects, as determined in an earlier study. Vitamin E or lipoic acid supplementation did not reduce plasma oxyphytosterol and oxycholesterol concentrations, or other markers reflecting oxidative stress or antioxidative capacity. Concentrations of different oxyphytosterols correlated within plasma, and within red blood cells and platelets. However, plasma and blood cell oxyphytosterol levels did not correlate. Although plasma oxyphytosterol concentrations are higher in IGT or DM2 subjects than in healthy subjects, 4-weeks vitamin E or lipoic acid supplementation does not lower plasma oxycholesterol or oxyphytosterol concentrations.

The effects of amoxicillin and vancomycin on parameters reflecting cholesterol metabolism

BACKGROUND

Changes in the microbiota composition have been implicated in the development of obesity and type 2 diabetes. However, not much is known on the involvement of gut microbiota in lipid and cholesterol metabolism. In addition, the gut microbiota might also be a potential source of plasma oxyphytosterol and oxycholesterol concentrations (oxidation products of plant sterols and cholesterol). Therefore, the aim of this study was to modulate the gut microbiota by antibiotic therapy to investigate effects on parameters reflecting cholesterol metabolism and oxyphytosterol concentrations.

DESIGN

A randomized, double blind, placebo-controlled trial was performed in which 55 obese, pre-diabetic men received oral amoxicillin (broad-spectrum antibiotic), vancomycin (antibiotic directed against Gram-positive bacteria) or placebo (microcrystalline cellulose) capsules for 7days (1500mg/day). Plasma lipid and lipoprotein, non-cholesterol sterol, bile acid and oxy(phyto)sterol concentrations were determined at baseline and after 1-week intervention.

RESULTS

Plasma secondary bile acids correlated negatively with cholestanol (marker for cholesterol absorption, r=-0.367; P<0.05) and positively with lathosterol concentrations (marker for cholesterol synthesis, r=0.430; P<0.05). Fasting plasma secondary bile acid concentrations were reduced after vancomycin treatment as compared to placebo treatment (-0.24±0.22μmol/L vs. -0.08±0.29μmol/L; P<0.01). Vancomycin and amoxicillin treatment did not affect markers for cholesterol metabolism, plasma TAG, total cholesterol, LDL-C or HDL-C concentrations as compared to placebo. In addition, both antibiotic treatments did not affect individual isoforms or total plasma oxyphytosterol or oxycholesterol concentrations.

CONCLUSION

Despite strong correlations between plasma bile acid concentrations and cholesterol metabolism (synthesis and absorption), amoxicillin and vancomycin treatment for 7days did not affect plasma lipid and lipoprotein, plasma non-cholesterol sterol and oxy(phyto)sterol concentrations in obese, pre-diabetic men.

Plant sterol ester diet supplementation increases serum plant sterols and markers of cholesterol synthesis, but has no effect on total cholesterol levels

This double-blind, randomized, placebo-controlled, cross-over intervention-study was conducted in healthy volunteers to evaluate the effects of plant sterol ester supplemented margarine on cholesterol, non-cholesterol sterols and oxidative stress in serum and monocytes. Sixteen volunteers, average age 34 years, with no or mild hypercholesterolemia were subjected to a 4 week period of daily intake of 3g plant sterols per day supplied via a supplemented margarine on top of regular eating habits. After a wash-out period of one week, volunteers switched groups. Compared to placebo, a diet supplementation with plant sterols increased serum levels of plant sterols such as campesterol (+0.16±0.19mg/dL, p=0.005) and sitosterol (+0.27±0.18mg/dL, p<0.001) and increased markers of cholesterol synthesis such as desmosterol (+0.05±0.07mg/dL, p=0.006) as well as lathosterol (+0.11±0.16mg/dL, p=0.012). Cholesterol serum levels, however, were not changed significantly (+18.68±32.6mg/dL, p=0.052). These findings could not be verified in isolated circulating monocytes. Moreover, there was no effect on monocyte activation and no differences with regard to redox state after plant sterol supplemented diet. Therefore, in a population of healthy volunteers with no or mild hypercholesterolemia, consumption of plant sterol ester supplemented margarine results in increased concentrations of plant sterols and cholesterol synthesis markers without affecting total cholesterol in the serum, activation of circulating monocytes or redox state.

Oxidation of sitosterol and transport of its 7-oxygenated products from different tissues in humans and ApoE knockout mice

The most common phytosterols in the human diet are sitosterol and campesterol, which originate exclusively from plant derived food. These phytosterols are taken up by NPC1L1 transport from the intestine into the enterocytes together with cholesterol and other xenosterols. Phytosterols are selectively pumped back from the enterocytes into the intestinal lumen and on the liver site from hepatocytes into bile by heterodimeric ABCG5/G8 transporters. Like cholesterol, both phytosterols are prone to ring and side chain oxidation. It could be shown that oxyphytosterols, found in atherosclerotic tissue, are most likely of in situ oxidation (Schött et al.; Biochem. Biophys. Res. Commun. 2014 Apr 11;446(3):805-10). However, up to now, the entire mechanism of phytosterol oxidation is not clearly understood. Here, we provide further information about the oxidation of sitosterol and the transport of its oxidation products out of tissue. Our survey includes data of 104 severe aortic stenosis patients that underwent an elective aortic valve cusp replacement. We studied their phytosterol concentrations, as well as absolute and substrate corrected oxyphytosterol levels in plasma and valve cusp tissue. In addition, we also examined phytosterol and oxyphytosterol concentrations in plasma and tissues (from brain and liver) of 10 male ApoE knockout mice. The ratio of 7-oxygenated-sitosterol-to-sitosterol exceeds the ratio for 7-oxygenated-campesterol-to-campesterol in plasma and tissue of both humans and mice. This finding indicates that sitosterol is oxidized to a higher amount than campesterol and that a selective oxidative mechanism might exist which can differentiate between certain phytosterols. Secondly, the concentrations of oxyphytosterols found in plasma and tissue support the idea that oxysitosterols are preferably transported out of individual tissues. Selective oxidation of sitosterol and preferred transport of sitosterol oxidation products out of tissue seem to be a metabolic pathway of forced sitosterol clearance from tissue compartments.

Increased plant sterol deposition in vascular tissue characterizes patients with severe aortic stenosis and concomitant coronary artery disease

The aim of the study was to evaluate the relationship between phytosterols, oxyphytosterols, and other markers of cholesterol metabolism and concomitant coronary artery disease (CAD) in patients with severe aortic stenosis who were scheduled for elective aortic valve replacement. Markers of cholesterol metabolism (plant sterols and cholestanol as markers of cholesterol absorption and lathosterol as an indicator of cholesterol synthesis) and oxyphytosterols were determined in plasma and aortic valve tissue from 104 consecutive patients with severe aortic stenosis (n=68 statin treatment; n=36 no statin treatment) using gas chromatography-flame ionization and mass spectrometry. The extent of CAD was determined by coronary angiography prior to aortic valve replacement. Patients treated with statins were characterized by lower plasma cholesterol, cholestanol, and lathosterol concentrations. However, statin treatment did not affect the sterol concentrations in cardiovascular tissue. The ratio of campesterol-to-cholesterol was increased by 0.46±0.34μg/mg (26.0%) in plasma of patients with CAD. The absolute values for the cholesterol absorption markers sitosterol and campesterol were increased by 18.18±11.59ng/mg (38.8%) and 11.40±8.69ng/mg (30.4%) in the tissues from patients with documented CAD compared to those without concomitant CAD. Campesterol oxides were increased by 0.06±0.02ng/mg (17.1%) in the aortic valve cusps and oxidized sitosterol-to-cholesterol ratios were up-regulated by 0.35±0.2ng/mg (22.7%) in the plasma of patients with CAD. Of note, neither cholestanol nor the ratio of cholestanol-to-cholesterol was associated with CAD. Patients with concomitant CAD are characterized by increased deposition of plant sterols, but not cholestanol in aortic valve tissue. Moreover, patients with concomitant CAD were characterized by increased oxyphytosterol concentrations in plasma and aortic valve cusps.

7β-Hydroxysitosterol crosses the blood-brain barrier more favored than its substrate sitosterol in ApoE-/- mice

In this study, we compare the distribution of intraperitoneally injected sitosterol, 7β-hydroxysitosterol or vehicle only (control) for 28days in male ApoE-/- mice. Furthermore we examine its impact on surrogate markers of cholesterol biosynthesis and sterol absorption rate in plasma, brain and liver tissues from these animals. Injection of sitosterol revealed a 32.1% (P=0.013) lower plasma total cholesterol compared with control. Cholesterol corrected plasma and absolute brain and liver levels of sitosterol are 4.1-, 1.7-, and 7.2-fold (P<0.001 for all) higher, respectively. This is in accordance with a reduced plasma campesterol to cholesterol ratio (-16.2%; P=0.018) together with a 24.1% (P=0.047) lower concentration of hepatic lathosterol. After injection of 7β-hydroxysitosterol the concentrations of 7β-hydroxysitosterol in plasma, brain and liver are 21.0-, 65.8- and 42.7-fold (P<0.001 for all) higher, respectively, compared with control. Injection of 7β-hydroxysitosterol revealed significantly lower plasma cholesterol corrected cholestanol and campesterol (-44.2%; P=0.001 and -24.5; P=0.004) as well as lower absolute liver cholestanol levels (-31.9%; P<0.001) compared with control. Intraperitoneally injected sitosterol and 7β-hydroxysitosterol differently influence cholesterol metabolism in plasma and liver. We conclude that the polar 7β-hydroxysitosterol compound can pass the blood brain barrier with higher efficacy than its substrate, sitosterol. Though present in higher amounts in the brain, both, sitosterol and 7β-hydroxysitosterol do not influence cholesterol metabolism in the brain as proven by our surrogate markers.

Postprandial plasma oxyphytosterol concentrations after consumption of plant sterol or stanol enriched mixed meals in healthy subjects

Epidemiological studies have reported inconsistent results on the relationship between increased plant sterol concentrations with cardiovascular risk, which might be related to the formation of oxyphytosterols (plant sterol oxidation products) from plant sterols. However, determinants of oxyphytosterol formation and metabolism are largely unknown. It is known, however, that serum plant sterol concentrations increase after daily consumption of plant sterol enriched products, while concentrations decrease after plant stanol consumption. Still, we have earlier reported that fasting oxyphytosterol concentrations did not increase after consuming a plant sterol- or a plant stanol enriched margarine (3.0g/d of plant sterols or stanols) for 4weeks. Since humans are in a non-fasting state for most part of the day, we have now investigated effects on oxyphytosterol concentrations during the postprandial state. For this, subjects consumed a shake (50g of fat, 12g of protein, 67g of carbohydrates), containing no, or 3.0g of plant sterols or plant stanols. Blood samples were taken up to 8h and after 4h subjects received a second shake (without plant sterols or plant stanols). Serum oxyphytosterol concentrations were determined in BHT-enriched EDTA plasma via GC-MS/MS. 7β-OH-campesterol and 7β-OH-sitosterol concentrations were significantly higher after consumption of a mixed meal enriched with plant sterol esters compared to the control and plant stanol ester meal. These increases were seen only after consumption of the second shake, illustrative for a second meal effect. Non-oxidized campesterol and sitosterol concentrations also increased after plant sterol consumption, in parallel with 7β-OH concentrations and again only after the second meal. Apparently, plant sterols and oxyphytosterols follow the same second meal effect as described for dietary cholesterol. However, the question remains whether the increase in oxyphytosterols in the postprandial phase is due to absorption or endogenous formation.