Diurnal variation of cholesterol precursors squalene and methyl sterols in human plasma lipoproteins

Day-night and seasonal variation of human gene expression across tissues

Circadian and circannual cycles trigger physiological changes whose reflection on human transcriptomes remains largely uncharted. We used the time and season of death of 932 individuals from GTEx to jointly investigate transcriptomic changes associated with those cycles across multiple tissues. Overall, most variation across tissues during day-night and among seasons was unique to each cycle. Although all tissues remodeled their transcriptomes, brain and gonadal tissues exhibited the highest seasonality, whereas those in the thoracic cavity showed stronger day-night regulation. Core clock genes displayed marked day-night differences across multiple tissues, which were largely conserved in baboon and mouse, but adapted to their nocturnal or diurnal habits. Seasonal variation of expression affected multiple pathways, and it was enriched among genes associated with the immune response, consistent with the seasonality of viral infections. Furthermore, they unveiled cytoarchitectural changes in brain regions. Altogether, our results provide the first combined atlas of how transcriptomes from human tissues adapt to major cycling environmental conditions. This atlas may have multiple applications; for example, drug targets with day-night or seasonal variation in gene expression may benefit from temporally adjusted doses.

Hypoxia truncates and constitutively activates the key cholesterol synthesis enzyme squalene monooxygenase

Cholesterol synthesis is both energy- and oxygen-intensive, yet relatively little is known of the regulatory effects of hypoxia on pathway enzymes. We previously showed that the rate-limiting and first oxygen-dependent enzyme of the committed cholesterol synthesis pathway, squalene monooxygenase (SM), can undergo partial proteasomal degradation that renders it constitutively active. Here, we show hypoxia is a physiological trigger for this truncation, which occurs through a two-part mechanism: (1) increased targeting of SM to the proteasome via stabilization of the E3 ubiquitin ligase MARCHF6 and (2) accumulation of the SM substrate, squalene, which impedes the complete degradation of SM and liberates its truncated form. This preserves SM activity and downstream pathway flux during hypoxia. These results uncover a feedforward mechanism that allows SM to accommodate fluctuating substrate levels and may contribute to its widely reported oncogenic properties.

Emerging Insights on the Diverse Roles of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) in Chronic Liver Diseases: Cholesterol Metabolism and Beyond

Chronic liver diseases are commonly associated with dysregulated cholesterol metabolism. Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a serine protease of the proprotein convertase family that is mainly synthetized and secreted by the liver, and represents one of the key regulators of circulating low-density lipoprotein (LDL) cholesterol levels. Its ability to bind and induce LDL-receptor degradation, in particular in the liver, increases circulating LDL-cholesterol levels in the blood. Hence, inhibition of PCSK9 has become a very potent tool for the treatment of hypercholesterolemia. Besides PCSK9 limiting entry of LDL-derived cholesterol, affecting multiple cholesterol-related functions in cells, more recent studies have associated PCSK9 with various other cellular processes, including inflammation, fatty acid metabolism, cancerogenesis and visceral adiposity. It is increasingly becoming evident that additional roles for PCSK9 beyond cholesterol homeostasis are crucial for liver physiology in health and disease, often contributing to pathophysiology. This review will summarize studies analyzing circulating and hepatic PCSK9 levels in patients with chronic liver diseases. The factors affecting PCSK9 levels in the circulation and in hepatocytes, clinically relevant studies and the pathophysiological role of PCSK9 in chronic liver injury are discussed.

Reduction of High Cholesterol Levels by a Preferably Fixed-Combination Strategy as the First Step in the Treatment of Hypertensive Patients with Hypercholesterolemia and High/Very High Cardiovascular Risk: A Consensus Document by the Italian Society of Hypertension

The primary and secondary prevention strategies of atherosclerotic cardiovascular disease (ASCVD) largely rely on the management of arterial hypertension and hypercholesterolemia, two major risk factors possibly linked in pathophysiological terms by the renin-angiotensin system activation and that often coexist in the same patient synergistically increasing cardiovascular risk. The classic pharmacologic armamentarium to reduce hypercholesterolemia has been based in the last two decades on statins, ezetimibe, and bile acid sequestrants. More recently numerous novel, additive resources targeting different pathways in LDL cholesterol metabolism have emerged. They include drugs targeting the proprotein convertase subtilisin/kexin type 9 (PCSK9) (inhibitory antibodies; small-interfering RNAs), the angiopoietin-like protein 3 (inhibitory antibodies), and the ATP-citrate lyase (the inhibitory oral prodrug, bempedoic acid), with PCSK9 inhibitors and bempedoic acid already approved for clinical use. With the potential of at least halving LDL cholesterol levels faster and more effectively with the addition of ezetimibe than with high-intensity statin alone, and even more with the addition of the novel available drugs, this document endorsed by the Italian Society of Hypertension proposes a novel paradigm for the treatment of the hypertensive patient with hypercholesterolemia at high and very high ASCVD risk. Our proposal is based on the use as a first-line of a preferably fixed combination of lipid-lowering drugs, under the motto “Our goal: achieve control. No setback: combine and check”.

A key mammalian cholesterol synthesis enzyme, squalene monooxygenase, is allosterically stabilized by its substrate

Cholesterol biosynthesis is a high-cost process and, therefore, tightly regulated by both transcriptional and posttranslational negative feedback mechanisms in response to the level of cellular cholesterol. Squalene monooxygenase (SM, also known as squalene epoxidase or SQLE) is a rate-limiting enzyme in the cholesterol biosynthetic pathway and catalyzes epoxidation of squalene. The stability of SM is negatively regulated by cholesterol via its N-terminal regulatory domain (SM-N100). In this study, using a SM-luciferase fusion reporter cell line, we performed a chemical genetics screen that identified inhibitors of SM itself as up-regulators of SM. This effect was mediated through the SM-N100 region, competed with cholesterol-accelerated degradation, and required the E3 ubiquitin ligase MARCH6. However, up-regulation was not observed with statins, well-established cholesterol biosynthesis inhibitors, and this pointed to the presence of another mechanism other than reduced cholesterol synthesis. Further analyses revealed that squalene accumulation upon treatment with the SM inhibitor was responsible for the up-regulatory effect. Using photoaffinity labeling, we demonstrated that squalene directly bound to the N100 region, thereby reducing interaction with and ubiquitination by MARCH6. Our findings suggest that SM senses squalene via its N100 domain to increase its metabolic capacity, highlighting squalene as a feedforward factor for the cholesterol biosynthetic pathway.

Diurnal Variation of Markers for Cholesterol Synthesis, Cholesterol Absorption, and Bile Acid Synthesis: A Systematic Review and the Bispebjerg Study of Diurnal Variations

Human studies have shown diurnal rhythms of cholesterol and bile acid synthesis, but a better understanding of the role of the circadian system in cholesterol homeostasis is needed for the development of targeted interventions to improve metabolic health. Therefore, we performed a systematic literature search on the diurnal rhythms of cholesterol synthesis and absorption markers and of bile acid synthesis markers. We also examined the diurnal rhythms of the cholesterol synthesis markers lathosterol and desmosterol, and of the cholesterol absorption markers cholestanol, campesterol, and sitosterol in serum samples from the Bispebjerg study. These samples were collected every three hours over a 24-h period in healthy males (n = 24) who consumed low-fat meals. The systematic search identified sixteen papers that had examined the diurnal rhythms of the cholesterol synthesis markers lathosterol (n = 3), mevalonate (n = 9), squalene (n = 2), or the bile acid synthesis marker 7α-hydroxy-4-cholesten-3-one (C4) (n = 4). Results showed that lathosterol, mevalonate, and squalene had a diurnal rhythm with nocturnal peaks, while C4 had a diurnal rhythm with daytime peaks. Furthermore, cosinor analyses of the serum samples showed a significant diurnal rhythm for lathosterol (cosinor p < 0.001), but not for desmosterol, campesterol, sitosterol, and cholestanol (cosinor p > 0.05). In conclusion, cholesterol synthesis and bile acid synthesis have a diurnal rhythm, though no evidence for a diurnal rhythm of cholesterol absorption was found under highly standardised conditions. More work is needed to further explore the influence of external factors on the diurnal rhythms regulating cholesterol homeostasis.

Circadian regulation of glucose, lipid, and energy metabolism in humans

The circadian system orchestrates metabolism in daily 24-hour cycles. Such rhythms organize metabolism by temporally separating opposing metabolic processes and by anticipating recurring feeding-fasting cycles to increase metabolic efficiency. Although animal studies demonstrate that the circadian system plays a pervasive role in regulating metabolism, it is unclear how, and to what degree, circadian research in rodents translates into humans. Here, we review evidence that the circadian system regulates glucose, lipid, and energy metabolism in humans. Using a range of experimental protocols, studies in humans report circadian rhythms in glucose, insulin, glucose tolerance, lipid levels, energy expenditure, and appetite. Several of these rhythms peak in the biological morning or around noon, implicating earlier in the daytime is optimal for food intake. Importantly, disruptions in these rhythms impair metabolism and influence the pathogenesis of metabolic diseases. We therefore also review evidence that circadian misalignment induced by mistimed light exposure, sleep, or food intake adversely affects metabolic health in humans. These interconnections among the circadian system, metabolism, and behavior underscore the importance of chronobiology for preventing and treating type 2 diabetes, obesity, and hyperlipidemia.

Familial longevity is characterized by high circadian rhythmicity of serum cholesterol in healthy elderly individuals

The biological clock, whose function deteriorates with increasing age, determines bodily circadian (i.e. 24h) rhythms, including that of cholesterol metabolism. Dampening of circadian rhythms has been associated with aging and disease. Therefore, we hypothesized that individuals with a familial predisposition for longevity have a higher amplitude circadian serum cholesterol concentration rhythm. The aim of this study was to investigate circadian rhythmicity of serum cholesterol concentrations in offspring of nonagenarian siblings and their partners. Offspring from nonagenarian siblings (n = 19), and their partners as controls (n = 18), were recruited from the Leiden Longevity Study. Participants (mean age 65 years) were studied in a controlled in‐hospital setting over a 24‐h period, receiving three isocaloric meals at 9:00 h, 12:00 h and 18:00 h. Lights were off between 23:00 h and 8:00 h. Serum total cholesterol (TC), HDL cholesterol (HDL‐C), non‐HDL‐C and triglycerides (TG) were determined every 30 min over a 24‐h period. Serum TC concentrations were higher during day than during night in offspring (5.2 vs. 4.7 mm, P < 0.001) and in controls (5.3 vs. 5.0 mm, P < 0.001). The difference in TC concentrations between day and night tended to be greater in offspring than in controls (0.5 vs. 0.3 mm, P = 0.109), reaching statistical significance in females (P = 0.045). Notably, the day–night serum differences in non‐HDL‐C were twofold greater in offspring than in controls (0.43 vs. 0.21 mm, P = 0.044) and most explicit in females (0.53 vs. 0.22, P = 0.078). We conclude that familial longevity is characterized by a high circadian rhythmicity of non‐HDL‐C in healthy elderly offspring from nonagenarian siblings.

Biochemistry of Statins

Cardiovascular disease (CVD) is the leading cause of morbidity and mortality worldwide. Elevated blood lipids may be a major risk factor for CVD. Due to consistent and robust association of higher low-density lipoprotein (LDL)-cholesterol levels with CVD across experimental and epidemiologic studies, therapeutic strategies to decrease risk have focused on LDL-cholesterol reduction as the primary goal. Current medication options for lipid-lowering therapy include statins, bile acid sequestrants, a cholesterol-absorption inhibitor, fibrates, nicotinic acid, and omega-3 fatty acids, which all have various mechanisms of action and pharmacokinetic properties. The most widely prescribed lipid-lowering agents are the HMG-CoA reductase inhibitors, or statins. Since their introduction in the 1980s, statins have emerged as the one of the best-selling medication classes to date, with numerous trials demonstrating powerful efficacy in preventing cardiovascular outcomes (Kapur and Musunuru, 2008 [1]). The statins are commonly used in the treatment of hypercholesterolemia and mixed hyperlipidemia. This chapter focuses on the biochemistry of statins including their structures, pharmacokinetics, and mechanism of actions as well as the potential adverse reactions linked to their clinical uses.

A circadian gene expression atlas in mammals: implications for biology and medicine

To characterize the role of the circadian clock in mouse physiology and behavior, we used RNA-seq and DNA arrays to quantify the transcriptomes of 12 mouse organs over time. We found 43% of all protein coding genes showed circadian rhythms in transcription somewhere in the body, largely in an organ-specific manner. In most organs, we noticed the expression of many oscillating genes peaked during transcriptional "rush hours" preceding dawn and dusk. Looking at the genomic landscape of rhythmic genes, we saw that they clustered together, were longer, and had more spliceforms than nonoscillating genes. Systems-level analysis revealed intricate rhythmic orchestration of gene pathways throughout the body. We also found oscillations in the expression of more than 1,000 known and novel noncoding RNAs (ncRNAs). Supporting their potential role in mediating clock function, ncRNAs conserved between mouse and human showed rhythmic expression in similar proportions as protein coding genes. Importantly, we also found that the majority of best-selling drugs and World Health Organization essential medicines directly target the products of rhythmic genes. Many of these drugs have short half-lives and may benefit from timed dosage. In sum, this study highlights critical, systemic, and surprising roles of the mammalian circadian clock and provides a blueprint for advancement in chronotherapy.

Serum and lipoprotein sitostanol and non-cholesterol sterols after an acute dose of plant stanol ester on its long-term consumption

PURPOSE

Chronic inhibition of cholesterol absorption with large doses of plant stanol esters (staest) alters profoundly cholesterol metabolism, but it is unknown how an acute inhibition with a large staest dose alters the postprandial serum and lipoprotein cholesterol precursor, plant sterol, and sitostanol contents.

METHODS

Hypercholesterolemic subjects, randomly and double-blind divided into control (n = 18) and intervention groups (n = 20), consumed experimental diet without and with staest (plant stanols 8.8 g/day) for 10 weeks. Next morning after a fasting blood sample (0 h), the subjects had a breakfast without or with staest (4.5 g of plant stanols). Blood sampling was repeated 4 h later. Lipoproteins were separated with ultracentrifugation, and sterols were measured with gas-liquid chromatography.

RESULTS

In 0-h chylomicrons and VLDL, plant sterols were lower in staest than in controls. Postprandially, cholestenol (cholesterol synthesis marker) was reduced in chylomicrons in staest compared with controls (-0.13 ± 0.04 μg/dL vs. 0.01 ± 0.08 μg/dL, P < 0.05). Staest decreased postprandially avenasterol in chylomicrons (P < 0.05 from 0 h). Sitostanol was high at 0 h by chronic staest in serum and VLDL but not in chylomicrons. Postprandial sitostanol was increased by staest in VLDL only.

CONCLUSIONS

Chronic cholesterol absorption inhibition with large amount of plant stanol esters decreases plant sterols in triglyceride-rich lipoproteins. Acute plant stanol ester consumption increases sitostanol content in triglyceride-rich lipoproteins but suggests to decrease the risk of plant sterol and plant stanol accumulation into vascular wall by chylomicrons.

Circulating Proprotein Convertase Subtilisin Kexin Type 9 Has a Diurnal Rhythm Synchronous With Cholesterol Synthesis and Is Reduced by Fasting in Humans

Objective— To gain insight into the function of proprotein convertase subtilisin kexin type 9 (PCSK9) in humans by establishing whether circulating levels are influenced by diurnal, dietary, and hormonal changes.

Methods and Results— We monitored circulating PCSK9 in a set of dynamic human experiments and could show that serum PCSK9 levels display a diurnal rhythm that closely parallels that of cholesterol synthesis, measured as serum lathosterol. In contrast to these marked diurnal changes in cholesterol metabolism, serum low-density lipoprotein (LDL) cholesterol levels remained stable during the diurnal cycle. Depletion of liver cholesterol by treatment with the bile acid–binding resin, cholestyramine, abolished the diurnal rhythms of both PCSK9 and lathosterol. Fasting (>18 hours) strongly reduced circulating PCSK9 and lathosterol levels, whereas serum LDL levels remained unchanged. Growth hormone, known to be increased during fasting in humans, reduced circulating PCSK9 in parallel to LDL cholesterol levels.

Conclusion— Throughout the day, and in response to fasting and cholesterol depletion, circulating PCSK9 displays marked variation, presumably related to oscillations in hepatic cholesterol that modify its activity in parallel with cholesterol synthesis. In addition to this sterol-mediated regulation, additional effects on LDL receptors may be mediated by hormones directly influencing PCSK9.

Evaluation of methods for the determination of cholesterol absorption and synthesis in humans

Cardiovascular disease risk and its associated complications correlate positively with circulating cholesterol levels. Plasma cholesterol levels are maintained by reciprocally related endogenous cholesterol synthesis and cholesterol absorption from dietary and biliary sources. Numerous in vivo clinical methods exist to quantify the absorption and synthesis of cholesterol in humans. This review summarizes these different methods available to study cholesterol absorption and synthesis, highlighting each method's strengths and weaknesses, as well as their applicability in different types of trials.

Cholesterol absorption status and fasting plasma cholesterol are modulated by the microsomal triacylglycerol transfer protein -493 G/T polymorphism and the usual diet in women

An important inter-individual variability in cholesterol absorption has been reported. It could result from polymorphisms in genes coding for proteins involved in the absorption process and in interaction with dietary intakes. To assess whether the extent of cholesterol absorption or synthesis is modified in adult women according to the -493 G/T polymorphism in the microsomal triglyceride transfer protein gene (MTP) and/or the habitual diet. Cholestanol and sitosterol, as well as desmosterol and lathosterol, surrogate markers of cholesterol absorption or synthesis, respectively, were analyzed in the fasting plasma of 69 middle-aged women under a Western-type diet (WD) and after 3 months on a low-saturated fat, low-cholesterol/Mediterranean-type diet (LFLCD). Genotypes for MTP -493G/T polymorphism were determined. Under an usual WD, subjects homozygous for the MTP -493 T allele exhibited higher (P < 0.05) fasting serum concentrations of cholestanol (199.0 ± 30.0 vs. 133 ± 7.4 × 10(2 )mmol/mol cholesterol) and lathosterol (188.7 ± 21.8 vs. 147.6 ± 9.1 × 10(2) mmol/mol cholesterol), as well as total cholesterol (7.32 ± 0.22 vs. 6.63 ± 0.12 mmol/l) compared to G carrier subjects. After 3 months on a LFLCD, level of absorption markers decreased in TT subjects with no change in synthesis ones, leading to values comparable to those measured in G carriers. The lowering of plasma total and LDL cholesterol due to dietary change was 2.4- and 2.3-fold greater in TT women than in G carriers. The polymorphism -493G/T in MTP modulates the level of cholesterol absorption but not synthesis in women under a WD, an effect abolished under a prudent LFLCD.

Efficacy of alternate day versus daily dosing of rosuvastatin

BACKGROUND

Compared with other statins, rosuvastatin has a relatively long half-life, which may allow for the administration of this medication on an alternate day basis.

OBJECTIVE

To compare the efficacy of administering rosuvastatin on a daily basis versus on an alternate day basis for the treatment of dyslipidemia.

METHODS

In the present crossover study, 45 patients with documented hypercholesterolemia requiring pharmacotherapy were administered either 20 mg of rosuvastatin on alternate days or 10 mg of rosuvastatin daily for six weeks. After a four-week washout period, patients were then switched to the other regimen for another six weeks. The primary end point was the percentage reduction of low-density lipoprotein cholesterol (LDL-C).

RESULTS

LDL-C decreased by 48.5% versus 40.9% with daily and alternate day dosing, respectively. This represented an additional absolute reduction of LDL-C of 7.6% (95% CI 1.8% to 13.4%, P=0.012) with the daily dosing regimen. Both dosing regimens provided similar improvements in high-density lipoprotein cholesterol and triglycerides.

CONCLUSIONS

Compared with alternate day dosing, daily dosing of rosuvastatin provides a statistically significant advantage in LDL-C reduction. However, the alternate day regimen may be a viable option for those patients in whom cost is a limitation to compliance.

Changes in plasma lipids with psychosocial stress are related to hypertension status and the norepinephrine stress response

Hypertension is a known risk factor for cardiovascular disease. Hypertensive individuals show exaggerated norepinephrine (NE) reactivity to stress. Norepinephrine is a known lipolytic factor. It is unclear if, in hypertensive individuals, stress-induced increases in NE are linked with the elevations in stress-induced circulating lipid levels. Such a mechanism could have implications for atherosclerotic plaque formation. In a cross-sectional, quasi-experimentally controlled study, 22 hypertensive and 23 normotensive men (mean +/- SEM, 45 +/- 3 years) underwent an acute standardized psychosocial stress task combining public speaking and mental arithmetic in front of an audience. We measured plasma NE and the plasma lipid profile (total cholesterol [TC], low-density-lipoprotein cholesterol [LDL-C], high-density-lipoprotein cholesterol, and triglycerides) immediately before and after stress and at 20 and 60 minutes of recovery. All lipid levels were corrected for stress hemoconcentration. Compared with normotensives, hypertensives had greater TC (P = .030) and LDL-C (P = .037) stress responses. Independent of each other, mean arterial pressure (MAP) upon screening and immediate increase in NE predicted immediate stress change in TC (MAP: beta = .41, P = .003; NE: beta = .35, P = .010) and LDL-C (MAP: beta = .32, P = .024; NE: beta = .38, P = .008). Mean arterial pressure alone predicted triglycerides stress change (beta = .32, P = .043) independent of NE stress change, age, and BMI. The MAP-by-NE interaction independently predicted immediate stress change of high-density-lipoprotein cholesterol (beta = -.58, P < .001) and of LDL-C (beta = -.25, P < .08). We conclude that MAP and NE stress reactivity may elicit proatherogenic changes of plasma lipids in response to acute psychosocial stress, providing one mechanism by which stress might increase cardiovascular risk in hypertension.

The influence of sleep and sleep loss upon food intake and metabolism

The present review investigates the role of sleep and its alteration in triggering metabolic disorders. The reduction of the amount of time sleeping has become an endemic condition in modern society and the current literature has found important associations between sleep loss and alterations in nutritional and metabolic aspects. Studies suggest that individuals who sleep less have a higher probability of becoming obese. It can be related to the increase of ghrelin and decrease of leptin levels, generating an increase of appetite and hunger. Sleep loss has been closely associated with problems in glucose metabolism and a higher risk for the development of insulin resistance and diabetes, and this disturbance may reflect decreased efficacy of the negative-feedback regulation of the hypothalamic–pituitary–adrenal axis. The period of sleep is also associated with an increase of blood lipid concentrations, which can be intensified under conditions of reduced sleep time, leading to disorders in fat metabolism. Based on a review of the literature, we conclude that sleep loss represents an important risk factor for weight gain, insulin resistance, type 2 diabetes and dyslipidaemia. Therefore, an adequate sleep pattern is fundamental for the nutritional balance of the body and should be encouraged by professionals in the area.

Postprandial behavior of plasma squalene and non-cholesterol sterols in men with varying cholesterol absorption

BACKGROUND

The purpose of this study was to investigate, whether low vs. high absorption of cholesterol affects the postprandial lipid clearance (squalene as the surrogate marker) and postprandial cholesterol metabolism evaluated with plasma levels of cholesterol absorption (cholestanol and plant sterols) and synthesis markers (desmosterol and lathosterol).

METHODS

Fifteen normo- or mildly hypercholesterolemic men were divided into low or high cholesterol absorbers on the basis of plasma cholestanol to cholesterol ratio and they volunteered to an oral fat load test containing fat 35 g/m(2) body surface.

RESULTS

Plasma squalene to cholesterol ratio did not differ between the groups throughout the postprandial follow-up of 8 h. The level differences in the plasma absorption and synthesis markers seen at baseline remained between the groups, so that in high absorbers the absorption markers remained high and synthesis markers low throughout the postprandial follow-up. The postprandial response curves of desmosterol (p<0.05) and lathosterol (p=0.052) to cholestanol decreased linearly in the low, but not in the high absorbers.

CONCLUSIONS

Low vs. high absorption of cholesterol does not affect the first 8-h postprandial lipid clearance. The metabolic profile of cholesterol is maintained postprandially. The postprandial decrease in cholesterol synthesis differs in low vs. high absorbers especially through the desmosterol pathway.

Vaccines with the MF59 adjuvant do not stimulate antibody responses against squalene

Squalene is a naturally occurring oil which has been used in the development of vaccine adjuvants, such as the oil-in-water emulsion MF59. In past years, by use of noncontrolled and nonvalidated assays, a claim was made that antisqualene antibodies were detectable in the sera of individuals with the so-called Gulf War syndrome. Using a validated enzyme-linked immunosorbent assay for the quantitation of immunoglobulin G (IgG) and IgM antibodies against squalene, we demonstrated that antisqualene antibodies are frequently detectable at very low titers in the sera of subjects who were never immunized with vaccines containing squalene. More importantly, vaccination with a subunit influenza vaccine with the MF59 adjuvant neither induced antisqualene antibodies nor enhanced preexisting antisqualene antibody titers. In conclusion, antisqualene antibodies are not increased by immunization with vaccines with the MF59 adjuvant. These data extend the safety profile of the MF59 emulsion adjuvant.

Physiological and therapeutic factors affecting cholesterol metabolism: does a reciprocal relationship between cholesterol absorption and synthesis really exist?

Cholesterol absorption and synthesis contribute to maintaining cholesterol homeostasis. Several physiological and therapeutic factors affect cholesterol homeostasis, including: genetics, circadian rhythm, body weight, plant sterols, ezetimibe, and statin therapy. The present objective is to determine the main vector, i.e. cholesterol absorption or synthesis, affected by each of these factors, and to examine whether an alteration in one vector is linked to a reciprocal change in the other. Current techniques used to assess cholesterol absorption and synthesis are also reviewed. Review of physiological factors affecting cholesterol metabolism suggest a reciprocal relationship between these two vectors. Carriers of the E2 isoform of apolipoprotein E and ATP binding cassette (ABC) G8 19H (exon 1 mutation) show a decrease in cholesterol absorption accompanied by a corresponding increase in synthesis. Circadian rhythm affects cholesterol synthesis, however, its effect on absorption has yet to be established. Obese subjects show an increase in cholesterol synthesis with a subsequent decrease in cholesterol absorption. Weight loss down regulates cholesterol synthesis, but has little or no effect on absorption. In the case of therapeutic factors, plant sterols and stanols inhibit cholesterol absorption, which results in a compensatory increase in synthesis. Ezetimibe also decreases intestinal absorption, while reciprocally increasing synthesis. Statin therapy down regulates synthesis, which is accompanied by a rise in absorption. These findings suggest that a change in one vector, fairly consistently, results in a compensatory and opposing change in the other. An understanding of this reciprocal relationship between cholesterol absorption and synthesis may allow for the development of more effective interventions for dyslipidemic disorders.

Stigmasterol reduces plasma cholesterol levels and inhibits hepatic synthesis and intestinal absorption in the rat

Plant sterols compete with cholesterol (cholest-5-en-3beta-ol) for intestinal absorption to limit absorption and lower plasma concentrations of cholesterol. Stigmasterol (24-ethyl-cholesta-5,22-dien-3beta-ol; Delta(22) derivative of sitosterol [24-ethyl-cholest-5-en-3beta-ol]), but not campesterol (24-methyl-cholest-5-en-3beta-ol) and sitosterol, is reported to inhibit cholesterol biosynthesis via inhibition of sterol Delta(24)-reductase in human Caco-2 and HL-60 cell lines. We studied the effect of feeding 0.5% stigmasterol on plasma and liver sterols and intestinal cholesterol and sitosterol absorption in 12 wild-type Kyoto (WKY) and 12 Wistar rats. After 3 weeks of feeding, cholesterol and sitosterol absorption was determined in 6 rats from each group by plasma dual-isotope ratio method. After 3 more weeks, plasma and hepatic sterols and hepatic enzyme activities were determined in all rats. After feeding stigmasterol, baseline plasma cholesterol was 1.3 times and plant sterols 3 times greater in WKY compared with Wistar rats. Stigmasterol feeding lowered plasma cholesterol by approximately 11%, whereas plasma campesterol and sitosterol levels were virtually unchanged in both rat strains, and stigmasterol constituted 3.2% of plasma sterols in WKY rats and 1% in Wistar rats. After 6 weeks of feeding, cholesterol and sitosterol absorption decreased 23% and 30%, respectively, in WKY, and 22% and 16%, respectively, in the Wistar rats as compared with untreated rats. The intestinal bacteria in both rat strains metabolized stigmasterol to mainly the 5beta-H stanol (>40%), with only small amounts of 5alpha-H derivative (approximately 1.5%), whereas the C-22 double bond was resistant to bacterial metabolism. Hepatic stigmasterol levels increased from 11 microg/g liver tissue to 104 mug/g in WKY rats and from 5 microg/g liver tissue to 21 microg/g in Wistar rats. 3-Hydroxy-3-methylglutaryl coenzyme A reductase activity was suppressed 4-fold in the WKY and almost 1.8-fold in Wistar rats, cholesterol 7alpha-hydroxylase activity was suppressed 1.6-fold in the WKY and 3.5-fold in Wistar rats, whereas cholesterol 27-hydroxylase activity was unchanged after feeding. In conclusion, stigmasterol, when fed, lowers plasma cholesterol levels, inhibits intestinal cholesterol and plant sterol absorption, and suppresses hepatic cholesterol and classic bile acid synthesis in Wistar as well as WKY rats. However, plasma and hepatic incorporation of stigmasterol is low.

Bile acid synthesis in humans has a rapid diurnal variation that is asynchronous with cholesterol synthesis

BACKGROUND & AIMS

The conversion of cholesterol to bile acids by the liver is an important regulator of body cholesterol homeostasis. In rodents, both cholesterol and bile acid synthesis have marked diurnal rhythms that peak synchronously at midnight. The aim of this study was to establish whether such diurnal rhythms are also present in healthy humans.

METHODS

Serum levels of the markers 7alpha-hydroxy-4-cholesten-3-one (C4) monitoring bile acid biosynthesis and lathosterol reflecting cholesterol synthesis were determined at 90-minute intervals in 8 human volunteers during standardized dietary conditions.

RESULTS

Serum C4 showed 2 distinct peaks (2- to 4-fold above baseline) during a 24-hour period, the first at 1:00 pm and the second at 9:00 pm. During the night, C4 levels declined, and they returned to baseline levels the next morning. In contrast, serum lathosterol levels peaked at night, between midnight and 4:00 am. The diurnal changes of C4 were not synchronous with serum lipid changes or with the postprandial increase in serum bile acids and were maintained in cholecystectomized subjects.

CONCLUSIONS

Bile acid synthesis in humans has a diurnal rhythm, with 2 peaks during the daytime, that is opposite from the circadian rhythm of cholesterol synthesis. This is completely different from the pattern in rodents and indicates the presence of an important species variation in the regulation of cholesterol homeostasis.

Effect of high plant sterol-enriched diet and cholesterol absorption inhibitor, SCH 58235, on plant sterol absorption and plasma concentrations in hypercholesterolemic wild-type Kyoto rats

BACKGROUND AND AIMS

Plant sterols are widely distributed in human diet but are poorly absorbed so that their plasma levels are very low. However, when fed in large amounts, they lower plasma cholesterol levels by interfering with cholesterol absorption. We have studied the effect of 4 weeks of feeding a chow diet supplemented with 1% plant sterols [brassicasterol (6.3%), campesterol (28.5%), stigmasterol (15.6%) and sitosterol (49.6%)], with or without SCH 58235 (a derivative of ezetimibe), 30 mg/kg per day, known to suppress intestinal cholesterol absorption, on plasma, tissue, biliary, and fecal sterols in Wistar and wild-type Kyoto (WKY) rats, and their metabolism by intestinal bacteria.

METHODS

After 2 weeks of feeding control or experimental diet, rats were given [3alpha-(3)H]sitosterol intravenously and [4-(14)C]sitosterol by mouth, and blood was collected after 1, 2, 3, and 5 days after labeling to determine sitosterol absorption. Feces were collected during the last 3 days and freeze dried. At the end of feeding, bile fistulas were created in 3 rats of each strain and bile was collected for 1 hour. All rats were then sacrificed and plasma and liver were collected for sterol measurements and activities of hepatic HMG-CoA reductase, cholesterol 7alpha-hydroxylase, and cholesterol 27-hydroxylase.

RESULTS

Wild-type Kyoto rats were hypercholesterolemic compared to Wistar rats and had increased plant sterols in the plasma. Plasma cholesterol tended to be lower in WKY rats after feeding with plant sterol-enriched diet whereas plant sterol levels rose to approximately 31% of plasma sterols in WKY and 14% in Wistar rats. However, brassicasterol and stigmasterol, with a double bond at C-22, constituted less than 3.5% of total plasma plant sterols. After feeding, biliary plant sterols increased 2.25-fold in Wistar and 1.5-fold in WKY rats, suggesting less hepatic clearance in WKY rats. SCH 58235 feeding significantly increased plasma as well as biliary cholesterol levels in both the untreated and plant sterol-fed WKY rats, and the plasma plant sterols showed a tendency to increase but did not reach significant level. Intestinal bacteria in both rat strains metabolized all plant sterols to mainly the 5beta-H-stanols. However, the C-22 double bond was stable to bacterial degradation. Intestinal absorption of sitosterol and cholesterol was increased 1.5- and 1.3-fold, respectively, in the WKY rats as compared to the Wistar rats, and plant sterol feeding lowered absorption of these sterols in both strains. Absorption of both these sterols was also lowered in SCH 58235-treated rats in both strains and was further lowered when SCH 58235 and plant sterols were simultaneously fed. The activity of the rate-limiting enzyme, HMG-CoA reductase, was increased 1.57-fold in Wistar rats and 1.27-fold in WKY rats that were fed plant sterols as compared to untreated rats.

CONCLUSIONS

(1) Plant sterol absorption was increased whereas hepatic elimination of all sterols was diminished in WKY rats accounting for elevated cholesterol and plant sterol levels. (2) The 1% plant sterol-enriched diet tended to lower plasma cholesterol levels whereas SCH 58235 feeding significantly increased plasma cholesterol levels in the WKY rats. (3) Intestinal absorption of sterols with C-22 double bond is diminished and the side-chain double bond is resistant to intestinal bacteria.

Detection of antibodies to squalene

An ELISA-based assay is described for the measurement of antibodies to squalene (SQE) in human serum and plasma. The assay was adapted from the previously described assay for murine antibodies to SQE (J. Immunol. Methods 267 (2002) 119). Like the murine SQE antibody assay, the human antibody assay used sterile cell culture 96-well plates coated with SQE (20 nmol/well). Phosphate-buffered saline (PBS)-0.5% casein was used as both a blocking agent and dilution buffer. The assay has a high through-put capacity and is reproducible and quantitative. This assay was used to evaluate samples from three different human cohorts. The first cohort was retired employees of the United States Army Medical Research Institute of Infectious Diseases (USAMRIID alumni). The mean age was 68 (N=40; range 58-82). Most were vaccinated with the U.S. licensed anthrax vaccine (AVA) and most had received several other vaccines through a USAMRIID special immunization program. The second cohort was of similar age (N=372; mean age 67; range 54-97) from the normal population of Frederick, MD and were not vaccinated with AVA. The third cohort (N=299) was from Camp Memorial Blood Center, United States Army Medical Department Activities, Fort Knox, KY. (No additional volunteer information is available.) Using this new ELISA method, antibodies to SQE were detected in all three of the cohorts. IgG antibodies to SQE were detected in 7.5% and 15.1% of the samples from the USAMRIID alumni and Frederick cohorts, respectively. These differences were not significantly different (chi((1))(2)=1.69, p=0.19). In contrast, no IgG antibodies to SQE were detected in the Fort Knox cohort which is significantly different than the Frederick cohort (chi((1))(2)=49.25, p<0.0001). IgM antibodies to SQE were detected in 37.5% and 32.3% of the samples from the USAMRIID and Frederick cohorts, respectively, but there was no significant difference between the cohorts. In the Fort Knox cohort, 19.4% of the samples were positive for IgM antibodies to SQE, which was significantly different from the Frederick cohort (chi((1))(2)=14.23, p=0.0002). Although the age of the volunteers from the Fort Knox cohort is unknown, the demographic of the donors at the blood bank volunteers is 85% 17-21 years of age. This suggested that the prevalence of antibodies to SQE may increase with age. This was confirmed with mouse studies in which the presence of antibodies was monitored as a function of time. No antibodies to SQE were detected in female BALB/c, B10.Br and C57BL/6 mice at 2 months of age, but they reached a maximum prevalence with 100% and 89% of animals testing positive for IgG and IgM antibodies to SQE, respectively, in the C57Bl/6 mice at 18 months of age. BALB/c and B10.Br mice also developed antibodies to SQE over time, but were at a lower prevalence than those observed in the C57BL/6 mice. Thirty-five of the 40 volunteers in the USAMRIID were vaccinated with AVA (mean no. doses=26; range 3-47). Comparison of the prevalence of antibodies to SQE from the AVA immunized group with the Frederick cohort revealed that there was no statistical differences for IgG (chi((1))(2)=2.3, p=0.13) or IgM (chi((1))(2)=0.33, p=0.56). When the data from the USAMRIID and Frederick cohorts were combined and analyzed for the presence of antibodies to SQE with respect to the sex of the volunteer, females (40.8%) were found to have a higher prevalence of IgM antibodies to SQE than men (28.4%) (chi((1))(2)=6.59, p=0.01). No significant difference was observed in the prevalence for IgG antibodies to SQE in females (17.7%) and males (12.5%). We conclude that antibodies to SQE occur naturally in humans; have an increased prevalence in females; are not correlated with vaccination with AVA; and appear to increase in prevalence with age.

Endogenous cholesterol synthesis is associated with VLDL-2 apoB-100 production in healthy humans

Subjects with high plasma cholesterol levels exhibit a high production of VLDL apolipoprotein B-100 (apoB-100), suggesting that cholesterol is a mediator for VLDL production. The objective of the study was to examine whether endogenous cholesterol synthesis, reflected by the lathosterol-cholesterol ratio (L-C ratio), affects the secretory rates of different VLDL subfractions. Ten healthy subjects were studied after overnight fasting. During a 10 h primed, constant infusion of 13C-valine (15 micromol/kg/h), enrichment was determined in apoB-100 from ultracentrifugally isolated VLDL-1 and VLDL-2 by gas chromatography mass spectrometry. The synthesis rates of VLDL-1 apoB-100 and VLDL-2 apoB-100, catabolism, and transfer were estimated by compartmental analysis. Mean VLDL-1 apoB-100 pool size was 90 +/- 15 mg, and mean VLDL-2 apoB-100 pool size was 111 +/- 14 mg. Absolute synthesis rate of VLDL-1 apoB-100 was 649 +/- 127 mg/day and 353 +/- 59 mg/day for VLDL-2 apoB-100. There was a strong association between the absolute synthesis rate of VLDL-2 apoB-100 and L-C ratio (r 2 = 0.61, P < 0.01). In contrast, no correlation was observed between L-C ratio and absolute synthesis rate of VLDL-1 apoB-100 (r 2 = 0.302, P = 0.09). In conclusion, these data provide additional support for an independent regulation of VLDL-1 apoB-100 and VLDL-2 apoB-100 production. Endogenous cholesterol synthesis is correlated only with the VLDL-2 apoB-100 production.

The effects of lovastatin and simvastatin on the diurnal periodicity of plasma mevalonate concentrations in patients with heterozygous familial hypercholesterolemia

Animal and human studies have shown that the biosynthesis of cholesterol exhibits diurnal periodicity with nocturnal increases in the level of cholesterol precursors. Dietary cholesterol, which increases the intracellular pool of cholesterol and plasma cholesterol levels, has been shown to blunt the nocturnal increases in cholesterol biosynthesis. Patients with heterozygous familial hypercholesterolemia (FH) have very high levels of plasma low-density lipoprotein cholesterol (LDL) due to their reduced ability to metabolize LDL particles. The present studies were carried out to determine whether diurnal variations in cholesterol synthesis occur in FH patients and to test the effects of 3-hydroxy-3-methyl glutaryl CoA (HMG CoA) reductase inhibitors on the diurnal cycle of cholesterol biosynthesis in these patients. Diurnal rates of cholesterol synthesis were assessed by measuring the plasma concentrations of mevalonate, an intermediate in the pathway of cholesterol biosynthesis. Female FH patients exhibited a diurnal pattern in plasma mevalonate levels similar to that previously reported in controls with peak values occurring at night. Treatment with lovastatin and simvastatin (40 mg b.i.d.) significantly reduced 24-h mean plasma mevalonate levels from baseline values. Administration of lovastatin in the evening reduced the nocturnal increases in mevalonate levels, and the administration of simvastatin completely abolished the nighttime rise. These results demonstrate that inhibition of cholesterol biosynthesis by lovastatin and simvastatin modifies the normal diurnal rhythm of cholesterol biosynthesis in female FH patients.

Induction and detection of antibodies to squalene. II. Optimization of the assay for murine antibodies

An improved high throughput assay for measuring murine antibodies to squalene (SQE) is described. The assay is highly reproducible and sensitive and can detect 80 ng/ml of antibody to SQE. The assay, an ELISA, is similar to our previously described assay in which plates containing PVDF membranes were used [J. Immunol. Methods 245 (2000) 1]. The PVDF plates worked well for detection of murine monoclonal antibodies (mAbs) to SQE, but substantial PVDF plate variation was observed, resulting in significant loss of signal and reproducibility between different lots of plates. In the new assay, the PVDF plates were replaced with Costar round bottom 96-well sterile tissue culture plates. These latter plates, which are not normally used for ELISA assay, gave high absorbances for monoclonal antibodies and anti-SQE serum binding to SQE and low absorbances for solvent-treated wells. Other commercially available polystyrene ELISA plates were unsuitable, in that either the background was high or the absorbance for antibodies binding to SQE was low, or both. This change in plate from PVDF to polystyrene allowed the use of an ELISA plate washer, which dramatically increased the throughput rate over the hand-washed PVDF plates. The improved assay also replaced fetal bovine serum (FBS), which contained SQE in lipoproteins, with fatty acid-free bovine serum albumin (BSA) as the blocker/diluent. Fifteen nanomoles of SQE were selected as the optimal amount of SQE to add to the wells. The binding of monoclonal antibodies and anti-SQE serum was dependent upon both the amount of antibody added to the wells and the amount of SQE added to the wells. Antibody concentration curves were hyperbolic in shape, as seen with most other antibodies. Antibody binding first increased with SQE amount and then reached a plateau around 10 nmol of SQE/well. At high SQE amounts (>75 nmol/well), antibody binding decreased with the amount of SQE added. Using 3H-SQE, the amount of SQE bound to the wells increased linearly, up to 50 nmol of SQE added. Approximately 90% of the added SQE bound to the well. When amounts greater than 100 nmol of SQE were added, the amount of SQE bound to the wells was greatly reduced to approximately 5-10% of the added SQE. The assay was highly reproducible both from lot to lot of plates and from experiment to experiment.

Induction and detection of antibodies to squalene

An enzyme-linked immunosorbent assay (ELISA) utilizing antigen coated on hydrophobic polyvinyldiene fluoride (PVDF) membranes is described for detecting antibodies that bind to squalene (SQE). Because of the prior lack of availability of validated antibodies to SQE, positive controls for the assay were made by immunization with formulations containing SQE to create monoclonal antibodies (mAbs) that reacted with SQE. Among eight immunogens tested, only two induced detectable murine antibodies to SQE: liposomes containing dimyristoyl phosphatidylcholine, dimyristoyl phosphatidylglycerol, 71% SQE, and lipid A [L(71% SQE+LA)], and, to a much lesser extent, an oil-in-water emulsion containing SQE, Tween 80, Span 85, and lipid A. In each case, lipid A served as an adjuvant, but neither SQE alone, SQE mixed with lipid A, liposomes containing 43% SQE and lipid A, nor several other emulsions containing both SQE and lipid A, induced antibodies that reacted with SQE. Monoclonal antibodies produced after immunizing mice with [L(71% SQE+LA)] served as positive controls for developing the ELISA. Monoclonal antibodies were produced that either recognized SQE alone but did not recognize squalane (SQA, the hydrogenated form of SQE), or that recognized both SQE and SQA. As found previously with other liposomal lipid antigens, liposomes containing lipid A also induced antibodies that reacted with the liposomal phospholipids. However, mAbs were also identified that reacted with SQE on PVDF membranes, but did not recognize either SQA or liposomal phospholipid. The polyclonal antiserum produced by immunizing mice with [L(71% SQE+LA)] therefore contained a mixed population of antibody specificities and, as expected, the ELISA of polyclonal antiserum with PVDF membranes detected antibodies both to SQE and SQA. We conclude that SQE is a weak antigen, but that antibodies that specifically bind to SQE can be readily induced by immunization with [L(71% SQE+LA)] and detected by ELISA with PVDF membranes coated with SQE.

The rebound of lipoproteins after LDL-apheresis. Kinetics and estimation of mean lipoprotein levels

We studied the rebound of lipoproteins in 20 hypercholesterolemic men [mean total cholesterol (TC) levels 9.6+/-1.8 mmol/l] after LDL-apheresis (LA) to determine the rate of recovery and the change in cholesterol synthesis, and to find a uniform estimation for time-averaged levels. After 10-20 months on biweekly LA using dextran sulfate cellulose columns and concomitant simvastatin administration, time-averaged levels (+/-SD) measured by integration of the area under the curve were as follows: TC 4.4+/-1.0 mmol/l, LDL cholesterol (LDL-C) 2.5+/-1.0 mmol/l, apolipoprotein B (apo B) 1. 3+/-0.3 g/l, triglycerides (TG) 1.7+/-0.7 mmol/l, HDL-C 1.1+/-0.2 mmol/l, and lipoprotein(a) [Lp(a)] 53.7+/-49.4 mg/dl. Mean acute reductions in TC, LDL-C, apo B, Lp(a), and TG were 61, 77, 75, 76, and 62%, respectively. HDL-C levels were not influenced. Median recovery half times for TC, LDL-C, apo B, and Lp(a) were 3.0, 4.0, 2. 3, and 3.5 days, respectively. The rebound of Lp(a) was identical to LDL-C, in 12 and 13 days post-treatment, respectively, whereas apo B and TC returned to pre-treatment levels in 7.5 and 10 days, respectively, due to the fast rebound of VLDL particles. Notwithstanding these differences, time-averaged levels (C(AVG)) could be estimated uniformly for the four latter parameters with the formula: C(AVG)=C(MIN)+0.73(C(MAX)-C(MIN)), where C(MAX) and C(MIN) are the immediate pre- and post-treatment levels. During long-term treatment the whole-body cholesterol synthesis was increased as measured by the ratio lathosterol to cholesterol of 3.24+/-1.49 mmol/mmol, whereas no further transient increase in the recovery period after LA was found. In conclusion, long-term LA and simvastatin therapy induced acute and chronic changes in lipids and lipoproteins showing the feasibility of biweekly treatment. It was shown that time-averaged levels, as a measure for the effective plasma levels, can be accurately estimated from pre- and post-treatment levels only.

Dietary squalene increases cholesterol synthesis measured with serum non-cholesterol sterols after a single oral dose in humans

Studies considering long-term squalene consumption have revealed no consistent effects on serum cholesterol levels but the immediate effect of dietary squalene on cholesterol synthesis has not been studied. Thus, the effect of a single dose of dietary squalene on postprandial lipid metabolism was studied in 16 male volunteeers aged 22-79 years. Two oral fat meals a week apart were administered to every subject, one without (control) and the other with 500 mg of squalene. Lipids, retinyl palmitate, squalene and non-cholesterol sterols were measured at baseline and after 3, 4, 6, 9, 12 and 24 h postprandially in plasma, chylomicron, VLDL and VLDL bottom and, in six randomly chosen subjects, also in IDL, LDL and HDL. In the fasting samples, squalene was mainly transported in LDL and HDL, whereas in squalene-supplemented postprandium most of squalene was carried in the triglyceride-rich lipoproteins. Postprandial squalene and retinyl palmitate curves closely resembled each other. After the squalene-enriched dietary fat load, squalene was significantly increased compared to control fat loads in plasma, chylomicrons, VLDL and IDL. Squalene addition increased significantly lathosterol/campesterol ratio in chylomicrons and VLDL at 12 h and in VLDL bottom at 9-12 h, and increased significantly VLDL lanosterol/campesterol ratio at 12 h, indicating enhanced cholesterol synthesis caused by squalene. Plasma plant sterol levels remained unchanged. In conclusion, a single oral dose of squalene representing a potential daily dietary amount increases cholesterol synthesis within 9-12 h detected in chylomicrons, VLDL and VLDL bottom.

Deuterium uptake and plasma cholesterol precursor levels correspond as methods for measurement of endogenous cholesterol synthesis in hypercholesterolemic women

To assess the validity of two techniques used to measure human cholesterol synthesis, the rate of uptake of deuterium (D) into plasma free cholesterol (FC), and plasma cholesterol precursor (squalene, lanosterol, desmosterol and lathosterol) levels were compared in 14 women [65-71 yr with low density lipoprotein-cholesterol (LDL-C) > or = 3.36 mmol x L(-1)]. Subjects consumed each of six diets for 5-wk periods according to a randomized crossover design. The experimental diets included a baseline diet (39% energy as fat, 164 mg chol x 4.2 MJ(-1)) and five reduced-fat diets (30% of energy as fat), where two-thirds of the fat was either soybean oil; squeeze, tub or stick margarines; or butter. Fractional and absolute synthesis rates (FSR and ASR) of FC were determined using the deuterium incorporation (DI) method, while cholesterol precursor levels were measured using gas-liquid chromatography. Data were pooled across diets for each variable and correlation coefficients were calculated to determine if associations were present. There was good agreement among levels of the various cholesterol precursors. In addition, FSR in pools/d (p x d(-1)) and ASR in grams/d (g x d(-1)) were strongly associated with lathosterol (r= 0.72 and 0.71, P= 0.0001), desmosterol (r= 0.75 and 0.75, P = 0.0001), lanosterol (r = 0.67 and 0.67), and squalene (r = 0.69 and 0.68) when levels of the precursors were expressed as micromol x mmol(-1) C. Significant but lower correlations were observed between the D uptake and plasma cholesterol precursor levels when the latter were expressed in absolute amounts (micromol x L(-1)). The wide range of fatty acid profiles of the experimental diets did not influence the degree of association between methods. In conclusion, the DI method and levels of some cholesterol precursors correspond as methods for shortterm measurement of cholesterol synthesis.

Independent association of serum squalene and noncholesterol sterols with coronary artery disease in postmenopausal women

OBJECTIVES

The purpose of the study was to investigate whether cholesterol metabolism is associated with coronary artery disease (CAD) in postmenopausal women.

BACKGROUND

Although hypercholesterolemia, a predominant risk factor of CAD, is related to cholesterol metabolism, the association between cholesterol metabolism and CAD is not well known.

METHODS

In addition to conventional coronary risk factors, fasting serum squalene, delta8-cholestenol, desmosterol, lathosterol (indicators of cholesterol synthesis), cholestanol, campesterol and sitosterol (indicators of cholesterol absorption) were measured in 48 50- to 55-year-old consecutive women with angiographically verified CAD and in 61 age-matched healthy controls.

RESULTS

The coronary patients had elevated ratios of squalene (p < 0.001), desmosterol (p = 0.005), campesterol (p = 0.028) and sitosterol (p = 0.022) to cholesterol, but had lower respective lathosterol value (p = 0.041) compared with the controls, despite similar serum cholesterol levels. Adjusted for age, body mass index, family history of CAD, smoking, hypertension, serum triglycerides, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol level and glycosylated hemoglobin A1c% (GHbA1c), the ratios of squalene (odds ratio, 1.36; 95% confidence interval, 1.17 to 1.57), lathosterol (0.98; 0.97 to 0.99), campesterol (1.01; 1.00 to 1.01) and sitosterol (1.01; 1.00 to 1.03) were significantly associated with the risk of CAD. In addition, family history of CAD and GHbA1c% were also independently related to the presence of CAD.

CONCLUSIONS

The results suggest that women with elevated ratios of serum squalene, campesterol and sitosterol to cholesterol and low respective lathosterol values have enhanced risk for CAD. Thus, enhanced absorption and reduced synthesis of cholesterol may be related to coronary atherosclerosis.

Serum squalene in postmenopausal women without and with coronary artery disease

Squalene, found in earlier studies in human atherosclerotic plaques, was measured in the serum of postmenopausal women with coronary artery disease (CAD) (n = 25) and randomly chosen age-matched healthy controls (n = 30). The squalene concentrations of the whole population ranged from 37.5 to 115.5 microg/dl, and were higher in serum of the CAD than healthy women (91.4+/-2.6 versus 65.2+/-2.6 microg/dl, P = 0.000), a finding observed also in relation to cholesterol (43.8+/-1.8 versus 32.9+/-1.1 10(2)x mmol/mol of cholesterol, P = 0.000). The squalene concentration was also increased in chylomicrons, VLDL and d>1.006 g/ml lipoproteins, and the proportions to cholesterol in VLDL and d>1.006 g/ml lipoproteins. The respective squalene and cholesterol concentrations were related to each other in serum, VLDL and d>1.006 g/ml lipoproteins (r = 0.52, 0.85 and 0.55, respectively), whereas the correlation with triglycerides was seen only in VLDL (r = 0.84) over the whole population. Besides enhanced intestinal secretion, it remains to be shown whether higher serum squalene in postmenopausal coronary women is due to increased cholesterol synthesis or a defect in squalene conversion to lanosterol.

Upregulation of cholesterol synthesis after acute myocardial infarction--is cholesterol a positive acute phase reactant?

Acute myocardial infarction is associated with profound alterations in the plasma lipoprotein profile. The mechanism of these alterations is not clear, and both cholesterol biosynthesis up- and downregulation could possibly be a consequence of acute myocardial infarction. We determined plasma lipids, lipoproteins, apolipoproteins, and lathosterol-which is regarded as an estimate of whole body cholesterol biosynthesis in humans-concentrations in 34 patients (age 68+/-10 years, 24 male, 10 female) admitted to our hospital with acute MI and with onset of symptoms within the last 12 h. Samples were taken immediately after admission to the hospital, and 1, 2, and 10 days after admission. On the first day after admission there was a decrease in total cholesterol (C) by 14.1%, (P = 0.01), in LDL-C by 14.4% (P = 0.03), in HDL-C by 9.3% (NS), and in triglycerides by 19.5% (NS). Apolipoprotein B100 was reduced by 18.3% (P = 0.008), and apolipoprotein AI by 12.3% (NS). The lathosterol/cholesterol ratio was increased by 23.1% after 1 day, and by 28.7% after 2 days (P = 0.05). After 10 days, all variables except the apolipoproteins had essentially returned to baseline values. In conclusion, the changes in the plasma lipid profile after acute myocardial infarction are associated with a profound increase of whole body cholesterol biosynthesis as judged by the lathosterol/cholesterol ratio. These changes may possibly enhance the delivery of cholesterol to cells involved in tissue repair mechanisms after acute myocardial infarction.

Fat-modified diets influence serum concentrations of cholesterol precursors and plant sterols in hypercholesterolemic subjects

Serum noncholesterol sterols, cholesterol precursors and plant sterols, are indicators of cholesterol absorption and synthesis. Serum plant sterol concentrations correlate positively with cholesterol absorption, but have also been found to correlate with dietary unsaturated to saturated fatty acid ratios. We studied the concentration of serum noncholesterol sterols during four different fat-modified diets, (1) high-fat, saturated fat-enriched (control), (2) reduced-fat, sunflower oil-enriched (SO-enriched), (3) rapeseed oil-enriched (RO-enriched), and (4) reduced-fat, saturated fat-enriched (reduced-fat), followed for 6 months in hypercholesterolemic subjects in a parallel design. The proportion of lathosterol (micrograms per 100 mg cholesterol), a precursor of cholesterol synthesis, increased significantly (P < .05) in both SO-enriched (mean +/- SD 147 +/- 57 v 167 +/- 76, 0 v 6 months) and RO-enriched (147 +/- 54 v 157 +/- 52) groups, where the reduction in low-density lipoprotein (LDL) cholesterol was also significant. The proportion of sitosterol, a plant sterol, decreased significantly in the control group (137 +/- 48 v 122 +/- 42), and the proportion of another plant sterol, campesterol, increased in the RO-enriched group (280 +/- 141 v 333 +/- 162), reflecting changes in the use of vegetable oils in these two groups rather than increased cholesterol absorption. In the whole study population, the proportion of linoleic and alpha-linolenic acid (a marker of the use of RO) in cholesterol esters (CEs) correlated (P < .001) with the proportion of sitosterol (r = .43) and campesterol (r = .36) in serum at the end of the study. In conclusion, serum cholesterol precursors were found to be useful indicators of cholesterol metabolism, but changes in serum plant sterols reflected dietary changes rather than cholesterol metabolism during long-term dietary intervention with fat-modified diets.

Direct association between the hepatic secretion of very-low-density lipoprotein apolipoprotein B-100 and plasma mevalonic acid and lathosterol concentrations in man

Apolipoprotein B-100 (apo B) is the principal structural and functional protein of the pro-atherogenic lipoproteins, but its homeostasis in man has not been clearly established. The hepatic availability of cholesterol substrate may be a determining factor. We examined whether there was a direct correlation between plasma concentrations of mevalonic acid (MVA) and lathosterol (indices of in vivo cholesterol synthesis) and hepatic secretion of very-low-density lipoprotein (VLDL) apo B in 13 normolipidaemic, healthy male subjects. The secretion of VLDL apo B was measured using a primed constant intravenous infusion of 1-[13C]-leucine (1 mg/kg per h) over 8 h. Gas-chromatography mass spectrometry (GCMS) was used to derive isotopic enrichment of apo B and fractional turnover rate was calculated using a monoexponential function. There was a highly significant positive correlation between the absolute secretion rate (ASR) of VLDL apo B and the plasma concentrations of mevalonic acid (r = 0.72, P = 0.005) and lathosterol (r = 0.81, P = 0.001) and the lathosterol:cholesterol ratio (r = 0.79, P = 0.001). In multiple regression analysis, these correlations remained significant after adjusting for waist circumference, age, apolipoprotein E genotype and dietary fat intake. The data further support the notion that the availability of cholesterol substrate regulates the hepatic secretion rate of apo B.

Determination of farnesyl pyrophosphate in dog and human plasma by high-performance liquid chromatography with fluorescence detection

A nonradioisotopic method has been developed for the determination of all-trans-farnesyl pyrophosphate (FPP), the common intermediate at the branch point of the biosynthesis of cholesterol and nonsterol end products, in dog and human plasma. FPP was cleaved to the parent alcohol, farnesol, by the direct addition of alkaline phosphatase to plasma. Farnesol extracted from plasma was converted into a fluorescent derivative with 9-anthroylcyanide. After the excess reagent was removed using an NH2-bonded phase cartridge, the derivative was separated by a column-switching high-performance liquid chromatographic system, followed by fluorescence detection. A linear response was obtained over the range of 2-18 ng/ml, when FPP was added to dog plasma in which the endogenous FPP concentrations had been lowered to undetectable levels by treatment with an HMG-CoA reductase inhibitor. The endogenous plasma FPP levels in the morning in dog and human detected for the first time by our method were 5.2 and 6.6 ng/ml, respectively. The method was utilized to examine the circadian rhythm of FPP in dog plasma, and different rhythms were observed between feeding and fasting dogs.

Diurnal rhythmicity of human cholesterol synthesis: normal pattern and adaptation to simulated "jet lag"

The diurnal rhythm of cholesterol synthesis was determined by deuterium incorporation from body water in five normolipemic men studied during a 24-h baseline period and on the 1st, 2nd, and 4th days of a simulated 12-h time zone shift achieved by delaying sleep times and, starting on the 2nd day, meal-times. Profiles of plasma cortisol and thyrotropin (TSH) were obtained simultaneously. Under baseline conditions, cholesterol synthetic rates varied from essentially zero in the morning to maximal values around midnight. On the 1st shifted day, this diurnal variation was unaltered despite sleep-wake reversal. The diurnal pattern of cholesterol synthesis, however, was shifted 5 h on the 2nd shifted day and approximately 12 h on the 4th. The diurnal variation of synthetic rate cholesterol fractional synthesis and plasma cortisol levels was negatively correlated on both the baseline day and the 1st shifted day. A positive correlation with the TSH rhythm was found on the 1st day only. During the 2nd and 4th days, the rhythm of cholesterol synthesis adapted faster than the rhythms of cortisol and TSH. These findings indicate that cholesterol synthesis is not acutely entrained by the sleep-wake cycle nor is it primarily entrained by the circadian clock.

Postprandial vitamin A and squalene clearances and cholesterol synthesis off and on lovastatin treatment in type III hyperlipoproteinemia

Postprandial fat clearance and absorption, fecal elimination and synthesis of cholesterol, bile acid synthesis, and cholesterol precursors and plant sterols in serum were studied in five patients with type III dyslipoproteinemia off and on lovastatin. The basal values were related to those in nontreated normolipidemic control subjects with apolipoprotein E3/3 phenotype (apo E3 controls, n = 16). On regular home diets, cholesterol precursor concentrations and cholesterol precursor/cholesterol ratios were high in the type III group. However, cholesterol absorption efficiency, bile acid and cholesterol synthesis measured with sterol balance technique and the precursor sterol/plant sterol ratios in serum were similar to the control values, suggesting that cholesterol absorption and metabolism was normal in these subjects. Lovastatin normalized the increased lipoprotein concentrations and reduced biliary cholesterol secretion, absolute absorption of cholesterol, precursor sterol/cholesterol and precursor sterol/plant sterol ratios in serum, fecal neutral and total sterol outputs and cholesterol synthesis. Lovastatin had no effect on cholesterol absorption efficiency or bile acid synthesis. Despite normalization of the triglyceride-rich lipoprotein levels by lovastatin, the postprandial vitamin A and squalene peak concentrations and the areas under the curves remained above the control ranges. The findings show that in type III hyperlipidemia, the precursor sterol/cholesterol ratios do not predict cholesterol synthesis. The latter, bile acid synthesis, precursor sterol/plant sterol ratios in serum, and cholesterol absorption are normal under basal conditions. The normalization of increased lipids by lovastatin is mainly due to reduced synthesis and absolute absorption of cholesterol, while the retarded postprandial fat clearance was not normalized by the drug.