Pharmacokinetic and pharmacogenomic modelling of the CYP3A activity marker 4β-hydroxycholesterol during efavirenz treatment and efavirenz/rifampicin co-treatment

OBJECTIVES

To assess the effect of the major efavirenz metabolizing enzyme (CYP2B6) genotype and the effects of rifampicin co-treatment on induction of CYP3A by efavirenz.

PATIENTS AND METHODS

Two study arms (arm 1, n = 41 and arm 2, n = 21) were recruited into this study. In arm 1, cholesterol and 4β-hydroxycholesterol were measured in HIV treatment-naive patients at baseline and then at 4 and 16 weeks after initiation of efavirenz-based antiretroviral therapy. In arm 2, cholesterol and 4β-hydroxycholesterol were measured among patients taking efavirenz during rifampicin-based tuberculosis (TB) treatment (efavirenz/rifampicin) just before completion of TB treatment and then serially following completion of TB treatment (efavirenz alone). Non-linear mixed-effect modelling was performed.

RESULTS

A one-compartment, enzyme turnover model described 4β-hydroxycholesterol kinetics adequately. Efavirenz treatment in arm 1 resulted in 1.74 (relative standard error = 15%), 3.3 (relative standard error = 33.1%) and 4.0 (relative standard error = 37.1%) average fold induction of CYP3A for extensive (CYP2B6*1/*1), intermediate (CYP2B6*1/*6) and slow (CYP2B6*6/*6) efavirenz metabolizers, respectively. The rate constant of 4β-hydroxycholesterol formation [mean (95% CI)] just before completion of TB treatment [efavirenz/rifampicin co-treatment, 7.40 × 10(-7) h(-1) (5.5 × 10(-7)-1.0 × 10(-6))] was significantly higher than that calculated 8 weeks after completion [efavirenz alone, 4.50 × 10(-7) h(-1) (4.40 × 10(-7)-4.52 × 10(-7))]. The CYP3A induction dropped to 62% of its maximum by week 8 of completion.

CONCLUSIONS

Our results indicate that efavirenz induction of CYP3A is influenced by CYP2B6 genetic polymorphisms and that efavirenz/rifampicin co-treatment results in higher induction than efavirenz alone.

Pharmacogenetic and pharmacokinetic aspects of CYP3A induction by efavirenz in HIV patients

We investigated the effects of pharmacogenetic variations and efavirenz pharmacokinetics on inter-individual differences in the extent of CYP3A induction by efavirenz using 4β-hydroxycholesterol/cholesterol (4β-OHC/Chol) as a marker for CYP3A induction. Plasma 4β-hydroxycholesterol and cholesterol concentrations were determined at baseline, and at the 4th, 16th and 48th week of efavirenz-based highly active antiretroviral therapy in antiretroviral therapy-naive HIV patients (n=77). Efavirenz plasma concentrations were quantified at weeks 4 and 16. CYP2B6, CYP3A5, ABCB1, UGT2B7 genotyping were done. Compared with baseline, the median plasma 4β-OHC/Chol ratio increased at the 4th (257%), 16th (291%) and 48th (165%) week (P<0.0001). CYP2B6*6 genotype significantly influenced 4β-OHC/Chol ratio at weeks 16 (P=0.02) and 48 (P=0.04) being highest in CYP2B6*6/*6>*1/*6>*1/*1. There were positive correlations between plasma efavirenz and 4β-OHC/Chol ratios (week 4: P=0.02, week 16: P=0.001). CYP3A enzyme induction by efavirenz is pronounced in CYP2B6 slow metabolizers who have high efavirenz plasma exposure.

High levels of 15-oxygenated steroids in circulation of patients with multiple sclerosis: fact or fiction?

15-Oxygenated cholesterol species such as 5α-cholest-8(14)ene-3β,15α-diol (15HC) and 3β-hydroxy-5α-cholest-8(14)-en-15-one (15KC) are commercially available synthetic products unlikely to occur in biological systems. Surprisingly, Farez et al. recently reported that these two steroids occur in human circulation at levels considerably higher than those of any other endogenous oxysterol [Farez, M. et al. 2009. Toll-like receptor 2 and poly(ADP-ribose) polymerase 1 promote central nervous system neuroinflammation in progressive EAE. Nat. Immunol. 10: 958-964]. The levels were reported to be increased in patients with multiple sclerosis in a progressive phase and the authors suggested that this could be utilized diagnostically. Based on extensive in vitro experiments exposing cells to the same high levels of 15HC as found in vivo (1000 ng/ml) the authors concluded that 15HC may be an important pathogenetic factor in multiple sclerosis. Using combined gas chromatography-mass spectrometry we fail to detect significant plasma levels of 15HC either in healthy controls or in patients with multiple sclerosis (levels < 2 ng/ml). If 15KC is present in these plasma samples, the concentration of it must be <10 ng/ml. Our failure to detect significant levels of the above steroids could not be due to loss during hydrolysis and work-up because recovery of the added two oxysterols was close to 100%. Autoxidation of lipoprotein-bound cholesterol resulted in extensive conversion of cholesterol into 7-oxygenated but not 15-oxygenated sterols. We conclude that if present there are trace amounts only of the above 15-oxygenated steroids in human circulation and that the role of such oxysterols as pathogenetic factors and biomarkers must be reconsidered.

Effects of high-dose itraconazole treatment on lipoproteins in men

OBJECTIVE

Epidemiological studies have convincingly demonstrated a positive association between LDL-cholesterol (LDL-C) and coronary artery disease but, in the case of HDL-C, there is an inverse association. Administration of high doses of the antifungal agent ketoconazole (800 mg/d) reduces serum concentrations of total cholesterol and LDL-C and there is a tendency for an increase in HDL-C. Our goal was to examine whether high-dose itraconazole raises HDL-C in subjects with normal levels of cholesterol.

PATIENTS AND METHODS

8 male patients with onychomycosis received 2 one-week cycles of treatment with itraconazole at a dose of 400 mg once daily in an open, prospective exploratory trial. Serum levels of itraconazole and its active metabolite hydroxyitraconazole were determined using high-performance liquid chromatography at the end of each treatment cycle. Fasting levels of serum lipoproteins and triglycerides were measured twice using routine enzymatic assays at the beginning and end of each cycle. The effects of itraconazole and hydroxyitraconazole on HDL-C metabolism were assessed in vitro using a human Caco-2 cell line and analyzing apoA-I levels with an enzyme-linked immunosorbent assay.

RESULTS

During itraconazole treatment total cholesterol and LDL-C decreased on average by 12% (p < 0.001) and 17% (p < 0.001), respectively, whereas HDL-C increased by 21% (p < 0.001). The ratio LDL: HDL-C, an index of atherogenic risk, decreased by 30% (p < 0.001). Incubation of Caco-2 cells in the presence of itraconazole and hydroxyitraconazole for 3 hours resulted in a significant increase in apoA-I concentration in the medium (913 and 412%, respectively) compared with control.

CONCLUSION

In addition to its inhibitory effect on cholesterol synthesis, high-dose itraconazole (400 mg/d) causes a significant decrease in serum LDL-C and, in contrast to ketoconazole, a significant increase in HDL-C. In vitro studies with Caco-2 cells indicate that the latter observation might be caused by an increase in apoA-I levels.

Oxysterols and Alzheimer's disease

There is a clear link between cholesterol turnover and neurodegenerative diseases and hypercholesterolemia is an established risk factor for Alzheimer's disease (AD). The failure to demonstrate a transfer of cholesterol from the circulation into the brain in humans and experimental animals makes it difficult to explain the link between hypercholesterolemia and AD. In contrast to cholesterol itself, side-chain oxidized cholesterol metabolites such as 24S-hydroxycholesterol and 27-hydroxycholesterol are able to pass the blood-brain barrier (BBB). Formation of 24S-hydroxycholesterol is the quantitatively most important mechanism for elimination of cholesterol from the brain and we recently demonstrated a significant net uptake of 27-hydroxycholesterol by the brain from the circulation. We have also shown that patients with AD have increased brain levels of 27-hydroxycholesterol, which may affect the production of beta-amyloid in the brain. The levels of 27-hydroxycholesterol in the circulation are correlated with the levels of cholesterol and the possibility must be considered that the flux of 27-hydroxycholesterol into the brain is the missing link between hypercholesterolemia and Alzheimer's disease. Current knowledge about the role of the two oxysterols for cholesterol homeostasis in the brain as well as their diagnostic potential are reviewed.

Measurement of oxysterols and alpha-tocopherol in plasma and tissue samples as indices of oxidant stress status

Oxidant stress seems to play a role in several setting of human pathology, such as atherosclerosis, cancer, and aging. The study of oxidant stress in human disease should be based on the evaluation of either sensitive and specific markers of enhanced oxidant stress, such as oxysterols, or antioxidant defense, by measuring alpha-tocopherol. We have developed a rapid method to measure the oxysterols 7beta-hydroxycholesterol and 7-ketocholesterol in plasma (50 healthy subjects) and tissue as an index of oxidant stress in vivo, and from the same sample alpha-tocopherol content. The mean plasma concentration of 7beta-hydroxycholesterol and 7-ketocholesterol was 4.6+/-1.1 and 13.4+/-7.6 ng/mL, respectively. Plasma alpha-tocopherol concentration was 5.8+/-1.0 micromol/mol cholesterol. Samples from atherosclerotic plaques contained 20 times more cholesterol, about 45 times higher oxysterols levels, and 600 times more alpha-tocopherol compared to normal arteries. No significant difference in cholesterol and oxysterol content was observed between cirrhotic and normal liver. However, cirrhotic liver contained significantly smaller concentration of alpha-tocopherol compared to normal liver. In conclusion, we have developed a rapid and reliable method for the assay of cholesterol oxidation products and alpha-tocopherol in plasma and tissue useful for estimation of oxidant stress/antioxidant balance.

On the anti-atherogenic effect of the antioxidant BHT in cholesterol-fed rabbits: inverse relation between serum triglycerides and atheromatous lesions

We have shown that inclusion of the antioxidant butylated hydroxytoluene (BHT) in the diet protects against development of atherosclerotic lesions in cholesterol-fed rabbits. In parallel, BHT treatment results in increased plasma triglyceride levels. The present study explores the relationship between the triglyceride-inducing and protective effects of BHT in two different studies. The combined material contains 22 rabbits fed cholesterol and 18 rabbits fed cholesterol in combination with 1% BHT. In the BHT group there was an inverse relationship between triglyceride exposure/cholesterol exposure and extent of lesions with r=0.74 (P=0.0005). Our results show that increased triglyceride exposure parallels the anti-atherogenic effect of BHT. There was no significant correlation between atheromatosis and serum BHT levels. beta-very low density lipoprotein (beta-VLDL) from cholesterol and BHT animals was triglyceride-enriched and smaller compared to beta-VLDL from cholesterol-fed animals, but there was no significant association between the anti-atherogenic effect of BHT and particle size or apolipoprotein pattern of LDL or beta-VLDL. LDL isolated from rabbits treated with cholesterol and BHT was less sensitive to oxidative modification than LDL isolated from rabbits treated with cholesterol only. In conclusion, our results demonstrate that the degree of triglyceride exposure may be an important modulator of the anti-atherogenic effect of an antioxidant.

On the turnover of brain cholesterol in patients with Alzheimer's disease. Abnormal induction of the cholesterol-catabolic enzyme CYP46 in glial cells

Evidence is accumulating for a link between cerebral cholesterol metabolism and Alzheimer's disease (AD). Here we focus on a possible relationship between AD and a newly discovered mechanism for cholesterol efflux from the brain, involving conversion of brain cholesterol into 24S-hydroxycholesterol by the neuronal oxidative enzyme CYP46. There was a marked difference in the distribution of CYP46 in brains of control and AD patients. The neuronal cells were less stained in AD brains than in controls while marked positive staining was found in glial cells in AD but not in controls. The dynamic changes in the mechanisms for cholesterol efflux from the brain are of interest in relation to the link between brain cholesterol and amyloid beta-protein in AD.

Antiepileptic drugs increase plasma levels of 4beta-hydroxycholesterol in humans: evidence for involvement of cytochrome p450 3A4

The major cholesterol oxidation products in the human circulation are 27-hydroxycholesterol, 24-hydroxycholesterol, and 7alpha-hydroxycholesterol. These oxysterols are formed from cholesterol by specific cytochrome P450 enzymes, CYP27, CYP46, and CYP7A, respectively. An additional oxysterol present in concentrations comparable with 7alpha- and 24-hydroxycholesterol is 4beta-hydroxycholesterol. We now report that patients treated with the antiepileptic drugs phenobarbital, carbamazepine, or phenytoin have highly elevated levels of plasma 4beta-hydroxycholesterol. When patients with uncomplicated cholesterol gallstone disease were treated with ursodeoxycholic acid, plasma 4beta-hydroxycholesterol increased by 45%. Ursodeoxycholic acid, as well as the antiepileptic drugs, are known to induce cytochrome P450 3A. Recombinant CYP3A4 was shown to convert cholesterol to 4beta-hydroxycholesterol, whereas no conversion was observed with CYP1A2, CYP2C9, or CYP2B6. The concentration of 4alpha-hydroxycholesterol in plasma was lower than the concentration of 4beta-hydroxycholesterol and not affected by treatment with the antiepileptic drugs or ursodeoxycholic acid. Together, these data suggest that 4beta-hydroxycholesterol in human circulation is formed by a cytochrome P450 enzyme.

From brain to bile. Evidence that conjugation and omega-hydroxylation are important for elimination of 24S-hydroxycholesterol (cerebrosterol) in humans

The brain is the almost exclusive site of formation of 24S-hydroxycholesterol in man, and there is a continuous flux of this oxysterol across the blood-brain barrier into the circulation. The hepatic metabolism of 24S-hydroxycholesterol was studied here by three different approaches: incubation of tritium-labeled 24S-hydroxycholesterol with human primary hepatocytes, administration of tritium-labeled 24S-hydroxycholesterol to a human volunteer, and quantitation of free and conjugated 24S-hydroxycholesterol and its neutral metabolites in ileocecal fluid from patients with ileal fistulae. 24S-Hydroxycholesterol as well as 24R-hydroxycholesterol were converted into bile acids by human hepatocytes at a rate of about 40% of that of the normal intermediate in bile acid synthesis, 7 alpha-hydroxycholesterol. There was also a conversion of 24S-hydroxycholesterol into conjugate(s) of 5-cholestene-3 beta,24S,27-triol at a rate similar to the that of conversion into bile acids. When administered to a human volunteer, labeled 24S-hydroxycholesterol was converted into bile acids at about half the rate of simultaneously administered labeled 7 alpha-hydroxycholesterol. Free, sulfated, and glucuronidated 24S-hydroxycholesterol and 5-cholestene-3 beta,24,27-triol were identified in ileocecal fluid. The excretion of these steroids was about 3.5 mg/24 h, amounting to more than 50% of the total estimated flux of 24S-hydroxycholesterol from the brain. It is concluded that 24S-hydroxycholesterol is a less efficient precursor to bile acids and that about half of it is conjugated and eliminated in bile as such or as a conjugate of a 27-hydroxylated metabolite. The less efficient metabolism of 24S-hydroxycholesterol may explain the surprisingly high levels of this oxysterol in the circulation and is of interest in relation to the suggested role of 24S-hydroxycholesterol as a regulator of cholesterol homeostasis.

Bioavailability of vitamin E as function of food intake in healthy subjects: effects on plasma peroxide-scavenging activity and cholesterol-oxidation products

Clinical trials with vitamin E have yielded contrasting results. In these trials, the amount of vitamin E given was different, and the compliance was not assessed in all studies. In addition, the modality of intake, ie, in relation to food, was not specified in any trial. Vitamin E is lipophilic, and its absorption is expected to be increased by food. We studied the bioavailability of vitamin E in relation to food intake and the effect on the lipid peroxide-scavenging activity of plasma and on 7beta-hydroxycholesterol and 7-ketocholesterol (oxysterols) as markers of oxidant stress. Twenty healthy Italian subjects were randomly assigned to take vitamin E at 300 mg/d on an empty stomach (group A) or during dinner (group B) for 15 days. Plasma vitamin E markedly increased in group B (84%) compared with group A (29%). The lipid peroxide-scavenging activity of plasma increased significantly in group B (14%, P=0.005) but did not change in group A. All subjects showed very low levels of plasma oxysterols, which were not affected by vitamin E supplementation in either group. This study shows that plasma concentration of vitamin E and plasma antioxidant activity in response to oral supplementation are markedly affected by food intake. Healthy Italian subjects show very low levels of cholesterol oxidation products; these low levels are possibly related to the Mediterranean diet. To obtain maximal absorption, vitamin E must be given at meals. These data should be taken into account in clinical trials with vitamin E.

Acyl-CoA thioesterases belong to a novel gene family of peroxisome proliferator-regulated enzymes involved in lipid metabolism

Acyl-CoA thioesterases hydrolyze acyl-CoAs to the corresponding free fatty acid plus coenzyme A. The activity is strongly induced in rat and mouse liver after feeding the animals peroxisome proliferators (PPs). To elucidate the role of these enzymes in lipid metabolism, the authors have cloned the cDNAs corresponding to the inducible cytosolic and mitochondrial type I enzymes (CTE-I and MTE-I), and studied tissue expression and nutritional regulation of expression of the mRNAs in mice. The constitutive expression of both mRNAs was low in liver, with CTE-I expressed mainly in kidney and brown adipose tissue, and MTE-I expressed in brown adipose tissue and heart. As expected, the expression in liver of both the CTE-I and MTE-I mRNAs were strongly induced (> 50-fold) by treatment with clofibrate. A similar level of induction was observed by fasting and a time-course study showed that the CTE-I and MTE-I mRNAs were increased already at 6 h after removal of the diet. Refeeding normal chow diet to mice fasted for 24 h normalized the mRNA levels with a T1/2 of about 3-4 h. Feeding mice a fat-free diet further decreased the expression, possibly indicating repression of expression. The strong expression of MTE-I and CTE-I in the heart was increased about 10-fold by fasting. To further characterize these highly regulated enzymes, the authors have cloned the corresponding genes and promoter regions. The structures of the two genes were found to be very similar, consisting of three exons and two introns. Exon-intron borders conform to general consensus sequences, and, especially, the first exon appears to be highly conserved. The promoter regions of both the CTE-I and MTE-I genes contain putative PP response elements, suggesting an involvement of PP-activated receptors in the regulation of these genes.

Hepatic cholesterol metabolism and resistance to dietary cholesterol in LXRbeta-deficient mice

The nuclear oxysterol-receptor paralogues LXRalpha and LXRbeta share a high degree of amino acid identity and bind endogenous oxysterol ligands with similar affinities. While LXRalpha has been established as an important regulator of cholesterol catabolism in cholesterol-fed mice, little is known about the function of LXRbeta in vivo. We have generated mouse lines with targeted disruptions of each of these LXR receptors and have compared their responses to dietary cholesterol. Serum and hepatic cholesterol levels and lipoprotein profiles of cholesterol-fed animals revealed no significant differences between LXRbeta(-/-) and wild-type mice. Steady-state mRNA levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase, farnesyl diphosphate synthase, and squalene synthase were increased in LXRbeta(-/-) mice compared with LXRbeta(+/+) mice, when fed standard chow. The mRNA levels for cholesterol 7alpha-hydroxylase, oxysterol 7alpha-hydroxylase, sterol 12alpha-hydroxylase, and sterol 27-hydroxylase, respectively, were comparable in these strains, both on standard and 2% cholesterol chow. Our results indicate that LXRbeta(-/-) mice - in contrast to LXRalpha(-/-) mice - maintain their resistance to dietary cholesterol, despite subtle effects on the expression of genes coding for enzymes involved in lipid metabolism. Thus, our data indicate that LXRbeta has no complete overlapping function compared with LXRalpha in the liver.

Oxysterols in the circulation of patients with the Smith-Lemli-Opitz syndrome: abnormal levels of 24S- and 27-hydroxycholesterol

Infants with the cholesterol synthesis defect Smith- Lemli-Opitz syndrome (SLO) have reduced activity of the enzyme 7-dehydrocholesterol-7-reductase and accumulate 7-dehydrocholesterol, with the highest concentration in the brain. As a result of the generally reduced content of cholesterol, plasma levels of oxysterols would be expected to be reduced. 24S-hydroxycholesterol is almost exclusively formed in the brain, whereas 27-hydroxycholesterol is mainly formed from extrahepatic and extracerebral cholesterol. In accordance with the expectations, sterol-correlated plasma levels of 24S-hydroxycholesterol were reduced in infants with SLO (by about 50%). In contrast, the sterol-correlated levels of 27-hydroxycholesterol in the circulation were markedly increased. No side-chain oxidized metabolites of 7-dehydrocholesterol were detected in the circulation. Recombinant human CYP27 had markedly lower 27-hydroxylase activity toward 7-dehydrocholesterol than towards cholesterol. HEK293 cells expressing 24S-hydroxylase active toward cholesterol had no significant activity towards 7-dehydrocholesterol. The plasma levels of 3 beta,7 alpha-dihydroxy-5-cholestenoic in the patients acid were reduced, suggesting a generally reduced metabolism of 27-oxygenated steroids. It is concluded that the accumulation of 7-dehydrocholesterol in the brains of patients with SLO reduces formation of 24S-hydroxycholesterol. The condition is associated with markedly increased circulating levels of 27-hydroxycholesterol, most probably due to reduced metabolism of this oxysterol. We discuss the possibility that the circulating levels of 24S-hydroxycholesterol may be used as a marker for the severity of the disease.--Björkhem, I., L. Starck, U. Andersson, D. Lütjohann, S. von Bahr, I. Pikuleva, A. Babiker, and U. Diczfaulsy. Oxysterols in the circulation of patients with the Smith-Lemli-Opitz syndrome: abnormal levels of 24S- and 27-hydroxycholesterol. J. Lipid Res. 2001. 42: 366--371.

Evidence that the major oxysterols in human circulation originate from distinct pools of cholesterol: a stable isotope study

The major oxysterols in human circulation are 7 alpha-, 27-, and (24S)-hydroxycholesterol. Two unique experiments were performed to elucidate their origin and kinetics. A volunteer was exposed to (18)O(2)-enriched air. A rapid incorporation of (18)O in both 7 alpha- and 27-hydroxycholesterol and disappearance of label after exposure were observed. The half-life was estimated to be less than 1 h. Incorporation of (18)O in (24S)-hydroxycholesterol was not significant. In the second experiment a volunteer was infused with liposomes containing 10 g of [(2)H(6)]cholesterol. This resulted in an enrichment of plasma cholesterol with (2)H of up to 13%, and less than 0.5% in cerebrospinal fluid cholesterol. The content of (2)H in circulating 7 alpha-hydroxycholesterol remained approximately equal to that of plasma cholesterol and decreased with a half-life of about 13 days. The (2)H content of circulating 27-hydroxycholesterol was initially lower than that of cholesterol but in the last phase of the experiment it exceeded that of cholesterol. No significant incorporation of (2)H in (24S)-hydroxycholesterol was observed. It is evident that 7 alpha-hydroxycholesterol must originate from a rapidly miscible pool, about 80% of 27-hydroxycholesterol from a more slowly exchangeable pool, and more than 90% of (24S)-hydroxycholesterol from a nonexchangeable pool, presumably the brain. The results are discussed in relation to the role of oxysterols in cholesterol homeostasis and their use as markers for pathological conditions. - Meaney, S., M. Hassan, A. Sakinis, D. Lütjohann, K. von Bergmann, A. Wennmalm, U. Diczfalusy, and I. Björkhem. Evidence that the major oxysterols in human circulation originate from distinct pools of cholesterol: a stable isotope study. J. Lipid Res. 2001. 42: 70;-78.

Plasma levels of 24S-hydroxycholesterol in patients with neurological diseases

The brain is the exclusive or almost exclusive site of formation of 24S-hydroxycholesterol and we have shown that the circulating level of 24S-hydroxycholesterol is dependent upon the relation between cerebral production and hepatic clearance. In the present work we determined plasma levels of 24S-hydroxycholesterol in patients with various neurological diseases. Eleven subjects with brain death occurring 6-10 h before collection of the plasma samples had markedly reduced circulating levels of 24S-hydroxycholesterol (-43%, P<0.001). Patients with advanced Alzheimer's disease and cerebral inflammatory diseases had slightly lower levels of 24S-hydroxycholesterol in plasma when compared to matched controls. Patients with acute ischemic stroke, multiple sclerosis and primary brain tumors had levels not significantly different from those of controls. The conditions leading to reduced plasma levels of 24S-hydroxycholesterol had no significant effect on plasma levels of another side-chain oxidized oxysterol, 27-hydroxycholesterol. Except for conditions characterized by very marked destruction of the central nervous system, different severe neurological diseases seem to have relatively small effects on the flux of 24S-hydroxycholesterol from the brain.

Long-term effects of vitamin E, vitamin C, and combined supplementation on urinary 7-hydro-8-oxo-2'-deoxyguanosine, serum cholesterol oxidation products, and oxidation resistance of lipids in nondepleted men

We studied the long-term effects of vitamins E and C and their combination on lipid peroxidation in vivo and in vitro. The Antioxidant Supplementation in Atherosclerosis Prevention (ASAP) trial is a double-masked placebo-controlled randomized clinical trial to study the effects of vitamin C (500 mg of slow release ascorbate per day), vitamin E (182 mg of RRR-alpha-tocopherol acetate per day), and the combination of both antioxidants. Lipid peroxidation measurements were carried out for 48 male participants at entry and at 12 and 36 months. Compared with placebo, vitamin E and the vitamin combination increased plasma lipid-standardized alpha-tocopherol during the first 12 months by 68.2% and 65.2% (P:<0. 001 for both), respectively, and reduced serum 7beta-hydroxycholesterol by 50.4% (P:=0.013) and 44.0% (P:=0.041), respectively. The net change of lipid standardized alpha-tocopherol was 63.8% after 36 months of vitamin E supplementation and 43.3% for the combination. Vitamin C supplementation elevated plasma total ascorbate level by 30.1% (P:=0.043) in 12 months and by 91.1% (P:=0. 001) in 36 months. Neither vitamin E, vitamin C, nor the combination influenced the urinary excretion rate of 7-hydro-8-oxo-2'-deoxyguanosine or the antioxidative capacity of plasma. Vitamin E and the combination of vitamins E and C enhanced the oxidation resistance of isolated lipoproteins and total serum lipids. Our data indicate that long-term supplementation of nondepleted men with a reasonable dose of vitamin E alone or in combination with slow release vitamin C reduces lipid peroxidation in vitro and in vivo, whereas a relatively high dose of vitamin C alone does not.

Formation of oxysterols from different pools of cholesterol as studied by stable isotope technique: cerebral origin of most circulating 24S-hydroxycholesterol in rats, but not in mice

In order to study the origin of different oxysterols in the circulation, in particular 24S-hydroxycholesterol, different pools of cholesterol in rat and mouse were labelled by feeding the animals with a diet supplemented with 0.3 or 0.5% hexadeuterium-labelled cholesterol, respectively, for 10 days. The incorporation of deuterium label in cholesterol and different oxysterols was measured by combined gas chromatography-mass spectrometry in selected tissues and in the circulation. In both rat and mouse, a high incorporation of label was found in cholesterol present in serum and liver (up to 77%). Incorporation of label was similar in 7 alpha- and 7 beta-hydroxycholesterol of the same origin. There was no significant incorporation of deuterium in brain cholesterol, and little or no incorporation in the brain oxysterols investigated, in both animals. In the testis, the incorporation of the deuterium label in cholesterol was less than half of that in the liver, with similarly reduced labelling of the testicular oxysterols. 24S-Hydroxycholesterol in the circulation contained a deuterium content that was about 50% of that of serum and liver cholesterol in the mouse experiment and about 30% in the rat experiment. Thus, about 50% of circulating 24S-hydroxycholesterol in the mouse and about 70% of this fraction in the rat must originate from pools of cholesterol that are not in equilibrium with plasma and liver cholesterol. The liver is probably responsible for a considerable part of the extracerebral formation of 24S-hydroxycholesterol, since this organ contained detectable amounts of 24S-hydroxycholesterol with a relatively high incorporation of deuterium in both animal species. The results are consistent with a cerebral origin of more than half of the 24S-hydroxycholesterol in the circulation of rats, but not in mice.

Involvement of the peroxisome proliferator-activated receptor alpha in regulating long-chain acyl-CoA thioesterases

Long-chain acyl-CoA thioesterases catalyze the hydrolysis of acyl-CoAs to the corresponding free fatty acid and CoA. We recently cloned four members of a novel multi-gene family of peroxisome proliferator-induced genes encoding cytosolic (CTE-I), mitochondrial (MTE-I), and peroxisomal (PTE-Ia and PTE-Ib) acyl-CoA thioesterases (Hunt et al. 1999. J. Biol. Chem. 274: 34317-34326). As the peroxisome proliferator-activated receptor alpha (PPARalpha) plays a central role in regulating genes involved in lipid metabolism, we examined the involvement of this receptor in regulation of the thioesterases, particularly CTE-I and MTE-I. Northern blot analysis shows that the induction of these thioesterases by clofibrate is mediated through a strictly PPARalpha-dependent mechanism. All four acyl-CoA thioesterases are induced at mRNA level by fasting and using PPARalpha-null mice, it is evident that the increase in CTE-I due to fasting is mainly independent of the PPARalpha in liver and heart. The CTE-I gene responds rapidly to fasting, with induction of mRNA and protein evident after 6 h. This fasting effect is rapidly reversible, with CTE-I mRNA returning almost to control levels after 3 h refeeding, and being further repressed to 20% of control after 9 h refeeding. Although CTE-I mRNA shows a low basal expression in liver, it can be suppressed 90% by feeding a fat-free diet. These data demonstrate that the nutritional regulation of the thioesterases involves the PPARalpha and other signaling pathways responsible for activation and repression. Putative physiological functions for the acyl-CoA thioesterases are discussed.

Plasma levels of 24S-hydroxycholesterol reflect the balance between cerebral production and hepatic metabolism and are inversely related to body surface

We have previously presented evidence that most of the 24S-hydroxycholesterol present in the circulation originates from the brain and that most of the elimination of this oxysterol occurs in the liver. Plasma 24S-hydroxycholesterol levels decline by a factor of about 5 during the first decades of life. The concentration of the enzyme cholesterol 24S-hydroxylase in the brain is, however, about constant from the first year of life, and reduced enzyme levels thus cannot explain the decreasing plasma levels during infancy. In the present work we tested the hypothesis that the plasma levels of 24S-hydroxycholesterol may reflect the size of the brain relative to the capacity of the liver to eliminate the substance. It is shown here that the age-dependent changes in absolute as well as cholesterol-related plasma level of 24S-hydroxycholesterol closely follow the changes in the ratio between estimated brain weight and estimated liver volume. The size of the brain is increased only about 50% whereas the size of the liver is increased by about 6-fold after the age of 1 year. Liver volume is known to be highly correlated to body surface, and in accordance with this the absolute as well as the cholesterol-related plasma level of 24S-hydroxycholesterol was found to be highly inversely correlated to body surface in 77 healthy subjects of varying ages (r(2) = 0.74). Two chondrodystrophic dwarves with normal size of the brain but with markedly reduced body area had increased levels of 24S-hydroxycholesterol when related to age but normal levels when related to body surface. It is concluded that the balance between cerebral production and hepatic metabolism is a critical determinant for plasma levels of 24S-hydroxycholesterol at different ages and that endocrinological factors are less important. The results are discussed in relation to the possibility to use 24S-hydroxycholesterol in the circulation as a marker for cholesterol homeostasis in the brain.

Cloning and regulation of peroxisome proliferator-induced acyl-CoA thioesterases from mouse liver

1.1. Acyl-CoA thioesterases hydrolyze acyl-CoAs to the corresponding free fatty acid plus CoASH. The activity is strongly induced in rat and mouse liver after feeding the animals peroxisome proliferators. To elucidate the role of these enzymes in lipid metabolism, we have cloned the cDNAs corresponding to the inducible cytosolic and mitochondrial type I enzymes (CTE-I and MTE-I) and studied tissue expression and nutritional regulation of expression of the mRNAs in mice. The constitutive expression of both mRNAs was low in liver, with CTE-I being expressed mainly in kidney and brown adipose tissue and MTE-I expressed in brown adipose tissue and heart. As expected, the expression in liver of both the CTE-I and MTE-I mRNAs was strongly induced (> 50-fold) by treatment with clofibrate. A similar level of induction was observed by fasting and a time-course study showed that both mRNAs were increased already at 6 hours after removal of the diet. Refeeding normal chow diet to mice fasted for 24 hours normalized the mRNA levels with a T1/2 of about 3-4 hours. Feeding mice a fat-free diet further decreased the expression, possibly indicating repression of expression. The strong expression of MTE-I and CTE-I in the heart was increased about 10-fold by fasting. To further characterize these highly regulated enzymes, we have cloned the corresponding genes and promoter regions. The structures of the two genes were found to be very similar, consisting of three exons and two introns. Exon-intron borders conform to general consensus sequences and especially the first exon appears to be highly conserved. The promoter regions of both the CTE-I and MTE-I genes contain putative peroxisome proliferator response elements (PPREs), suggesting an involvement of peroxisome proliferator-activated receptors in the regulation of these genes.

Elimination of cholesterol as cholestenoic acid in human lung by sterol 27-hydroxylase: evidence that most of this steroid in the circulation is of pulmonary origin

Human alveolar macrophages have exceptionally high capacity to convert cholesterol into 27-hydroxycholesterol and cholestenoic acid by the sterol 27-hydroxylase mechanism. It is shown here that the human lung has a higher content of 27-hydroxycholesterol relative to cholesterol than any other organ. In order to evaluate the importance of the sterol 27-hydroxylase mechanism for cholesterol homeostasis in the lung, the production of cholestenoic acid by human lung was investigated. Removal of one lung reduced the level of cholestenoic acid in the circulation by 48 +/- 4% (P < 0.005). The levels of cholestenoic acid in the pulmonary artery and in the pulmonary vein showed significant differences (P < 0.002) with higher levels in the pulmonary vein (108 +/- 16 and 104 +/- 16 ng/mL, respectively). This corresponds to a net flux of cholestenoic acid from the lung of about 14 mg/day, which is more than 80% of the reported removal of this oxysterol and its metabolites from the circulation by the liver per day. Bypassing the lung for 60 min led to a reduction in circulating cholestenoic acid (30%) that fits with a pulmonary origin when taking into account the half-life of cholestenoic acid. The level of circulating cholestenoic acid was found to be less in patients with different lung diseases. It is evident that most of the cholestenoic acid in the circulation is of pulmonary origin. The present results suggest that the sterol 27-hydroxylase in the lung is responsible for at least half of the total flux of 27-oxygenated cholesterol metabolites to the liver and that this enzyme system may be of importance for cholesterol homeostasis in the lung.

Strong association between malnutrition, inflammation, and atherosclerosis in chronic renal failure

BACKGROUND

Atherosclerotic cardiovascular disease and malnutrition are widely recognized as leading causes of the increased morbidity and mortality observed in uremic patients. C-reactive protein (CRP), an acute-phase protein, is a predictor of cardiovascular mortality in nonrenal patient populations. In chronic renal failure (CRF), the prevalence of an acute-phase response has been associated with an increased mortality.

METHODS

One hundred and nine predialysis patients (age 52 +/- 1 years) with terminal CRF (glomerular filtration rate 7 +/- 1 ml/min) were studied. By using noninvasive B-mode ultrasonography, the cross-sectional carotid intima-media area was calculated, and the presence or absence of carotid plaques was determined. Nutritional status was assessed by subjective global assessment (SGA), dual-energy x-ray absorptiometry (DXA), serum albumin, serum creatinine, serum urea, and 24-hour urine urea excretion. The presence of an inflammatory reaction was assessed by CRP, fibrinogen (N = 46), and tumor necrosis factor-alpha (TNF-alpha; N = 87). Lipid parameters, including Lp(a) and apo(a)-isoforms, as well as markers of oxidative stress (autoantibodies against oxidized low-density lipoprotein and vitamin E), were also determined.

RESULTS

Compared with healthy controls, CRF patients had an increased mean carotid intima-media area (18.3 +/- 0.6 vs. 13.2 +/- 0.7 mm2, P < 0.0001) and a higher prevalence of carotid plaques (72 vs. 32%, P = 0.001). The prevalence of malnutrition (SGA 2 to 4) was 44%, and 32% of all patients had an acute-phase response (CRP > or = 10 mg/liter). Malnourished patients had higher CRP levels (23 +/- 3 vs. 13 +/- 2 mg/liter, P < 0.01), elevated calculated intima-media area (20.2 +/- 0.8 vs. 16.9 +/- 0.7 mm2, P < 0.01) and a higher prevalence of carotid plaques (90 vs. 60%, P < 0.0001) compared with well-nourished patients. During stepwise multivariate analysis adjusting for age and gender, vitamin E (P < 0.05) and CRP (P < 0.05) remained associated with an increased intima-media area. The presence of carotid plaques was significantly associated with age (P < 0.001), log oxidized low-density lipoprotein (oxLDL; P < 0.01), and small apo(a) isoform size (P < 0.05) in a multivariate logistic regression model.

CONCLUSION

These results indicate that the rapidly developing atherosclerosis in advanced CRF appears to be caused by a synergism of different mechanisms, such as malnutrition, inflammation, oxidative stress, and genetic components. Apart from classic risk factors, low vitamin E levels and elevated CRP levels are associated with an increased intima-media area, whereas small molecular weight apo(a) isoforms and increased levels of oxLDL are associated with the presence of carotid plaques.

Increased plasma 7 beta-hydroxycholesterol concentrations in a population with a high risk for cardiovascular disease

The mortality in coronary heart disease among 50- to 54-year-old men is 4 times higher in Lithuania than in Sweden. It was recently suggested that traditional risk factors could not explain this mortality difference. LDL of Lithuanian men showed, however, a lower resistance to oxidation than that of Swedish men. In addition, the plasma concentration of gamma-tocopherol, lycopene, and beta-carotene were lower in Lithuanian men. In the present investigation, we determined plasma oxysterols in men from Lithuania and Sweden and found that the plasma concentration of 7 beta-hydroxycholesterol was higher in Lithuanian men, 12+/-5 versus 9+/-8 (SD) ng/mL (P=0.0011). This oxysterol is a cholesterol autoxidation product and there is no indication that it should have an enzymatic origin. Mean LDL oxidation lag time was shorter in Lithuanian men (75+/-14 versus 90+/-13 minutes, P<0.0001) and the concentration of LDL linoleic acid was lower (249+/-56 versus 292+/-54 microgram/mg of LDL protein, P<0.0001). Lipid corrected gamma-tocopherol was 0.07+/-0.02 mg/mL in Vilnius men and 0.12+/-0. 04 mg/mL (P<0.0001) in Linköping men. There was a negative correlation between the concentration of 7 beta-hydroxycholesterol and lag time (R=-0.31, P=0.0023). It is suggested that the higher 7 beta-hydroxycholesterol concentration in Lithuanian men is an indication of an increased in vivo lipid peroxidation.

Removal of cholesterol from extrahepatic sources by oxidative mechanisms

Sterol 27-hydroxylase is an evolutionarily old cytochrome P450 species that is critical for oxidation of the side chain of cholesterol in connection with bile acid biosynthesis in the liver. The wide tissue and organ distribution of the enzyme suggests that it may also have other functions. It was recently shown that some cells (e.g. macrophages) have a high capacity to convert cholesterol into both 27-hydroxycholesterol and cholestenoic acid and that there is a significant flux of these steroids from extrahepatic sources to the liver where they are further oxidized into bile acids. The magnitude of this flux is such that it may be of importance for overall homeostasis of cholesterol. Very recently it was shown that the brain utilizes a similar mechanism for removal of cholesterol. A unique brain-specific 24S-hydroxylase converts cholesterol into 24S-hydroxycholesterol that is transported over the blood-brain barrier much more rapidly than unmetabolized cholestero. When 24S-hydroxycholesterol has reached the circulation it is taken up by the liver and further metabolized, most probably into bile acids. This flux is likely to be of importance for cholesterol homeostasis in the brain. This review summarizes our current knowledge regarding oxidative mechanisms for removal of extrahepatic cholesterol. It is evident that some cells utilize these mechanisms as alternatives or complements to the classical HDL-dependent reverse cholesterol transport.

Unsaturated fatty acids increase plasminogen activator inhibitor-1 expression in endothelial cells

In vivo studies have demonstrated a strong positive correlation between plasma very low density lipoprotein (VLDL) triglyceride and plasma plasminogen activator inhibitor-1 (PAI-1) activity levels. Furthermore, VLDL has been shown to induce PAI-1 secretion from cultured endothelial cells. In contrast, no or variable effects on PAI-1 secretion have been reported for native low density lipoprotein. It could be speculated that fatty acids derived from VLDL triglycerides are the actual mediators, resulting in an enhanced secretion of PAI-1. In the present study, we have analyzed the effects of both saturated and unsaturated fatty acids on PAI-1 expression and secretion by endothelial cells. Addition of 0 to 50 micromol/L of either palmitic acid or stearic acid had no effect on PAI-1 secretion from human umbilical vein endothelial cells or EA. hy926 cells. In contrast, addition of oleic acid, linoleic acid, linolenic acid, and eicosapentaenoic acid resulted in a significant increase in PAI-1 secretion from both cell types. Northern blot analysis of PAI-1 mRNA levels was in agreement with these findings. Transfection experiments demonstrated that addition of linolenic acid and eicosapentaenoic acid significantly increased PAI-1 transcription. The fatty acid response region was localized to a previously described VLDL-inducible region of the PAI-1 promoter. Electromobility shift assays demonstrated that unsaturated fatty acids induced the same complex as did VLDL, whereas saturated fatty acids had no effect. Furthermore, it was demonstrated that the activation procedure did not involve fatty acid oxidation to any significant extent. In conclusion, the present study demonstrates that unsaturated fatty acids increase PAI-1 transcription and secretion by endothelial cells in vitro. The effect appears to be mediated by a previously described VLDL-inducible transcription factor.

A study of plasmalogen as an index of oxidative stress in patients with chronic renal failure. Evidence of increased oxidative stress in malnourished patients

BACKGROUND

The incidence of premature cardiovascular disease is high in patients with chronic renal failure (CRF). Free-radical-induced tissue damage is thought to play a major role in the pathogenesis of atherosclerosis and several reports indicate increased oxidative stress in patients with CRF. However, the cause of such stress in CRF is not exactly known. Plasmalogens, a group of phospholipids with a vinyl ether bond in the sn-1 position, are supposed to be sensitive markers of oxidative stress.

METHODS

The fasting relative plasmalogen levels of erythrocyte membranes (DMA 16/C16:0 and DMA 18/C18:0), as well as of vitamin E and serum lipids, were determined in a cohort of 105 patients (mean age 51+/-2 years) with advanced CRF (creatinine clearance 9+/-1 ml/min) before starting dialysis treatment. Twenty-nine healthy controls (47+/-2 years) were also investigated.

RESULTS

Significantly lower relative plasmalogen levels (DMA 16/C16:0 and DMA 18/C18:0) were found in erythrocytes of predialysis patients than in controls. When the patients were divided on the basis of subjective global assessment of nutritional status (SGA), the malnourished patients (SGA 2-4) had significantly (P<0.05) lower relative plasmalogen levels than the well-nourished predialysis patients (SGA 1). In the prospective part of the study, we found that a 12-month dialysis treatment in 38 patients was associated with significant increases in both erythrocyte DMA 16/C16:0 (P<0.001) and DMA18/C18:0 (P<0.05) ratios.

CONCLUSION

The present results suggest that predialysis patients are exposed to an augmented oxidative stress which is partially reversed by 12 months of dialysis treatment. The present study also demonstrates lower relative plasmalogen levels in erythrocyte membranes in malnourished than in well-nourished predialysis patients. One could speculate that an increased oxidative stress may be a factor contributing to the high prevalence of cardiovascular disease documented in malnourished CRF patients.

Cholesterol homeostasis in human brain: turnover of 24S-hydroxycholesterol and evidence for a cerebral origin of most of this oxysterol in the circulation

We have previously demonstrated that the brain contains about 80% of the 24S-hydroxycholesterol in the human body and that there is a net flux of this steroid from the brain into the circulation (Lütjohann, D. et al. 1996. Proc. Natl. Acad. Sci. USA. 93: 9799-9804). Combining previous data with new data on 12 healthy volunteers, the arteriovenous difference between levels of this oxysterol in the internal jugular vein and in a peripheral artery was found to be -10.2 +/- 2.8 ng/ml (mean +/- SEM) corresponding to a net flux of 24S-hydroxycholesterol from the brain of about 6.4 mg/24 h. The arteriovenous difference between levels of 24S-hydroxycholesterol in the hepatic vein and a peripheral artery of 12 other volunteers was found to be 7.4 +/- 2.2 ng/ml, corresponding to a hepatic uptake of about 7.6 mg/24 h. The concentrations of 24S-hydroxycholesterol in the renal vein were about the same as those in a peripheral artery, indicating that a renal elimination is not of importance. Intravenously injected deuterium-labeled racemic 24-hydroxycholesterol was eliminated from the circulation of two human volunteers with half-lives of 10 h and 14 h, respectively. A positive correlation was found between the levels of circulating cholesterol and 24S-hydroxycholesterol. The results are consistent with a cerebral origin of most of the circulating 24S-hydroxycholesterol and suggest that the liver is the major eliminating organ. It is concluded that conversion into 24S-hydroxycholesterol is a quantitatively important mechanism for elimination of cholesterol from human brain. The possibility is discussed that circulating levels of 24S-hydroxycholesterol can be used as a marker for pathological and/or developmental changes in the brain.

Transport of side-chain oxidized oxysterols in the human circulation

To study the transport of 24-hydroxycholesterol, 27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid in the circulation, the distribution of these oxysterols was determined in plasma, very low density lipoprotein, low density lipoprotein, high density lipoprotein, and lipoprotein-free plasma. An accurate method based on isotope dilution-mass spectrometry with use of individual deuterium labeled internal standards was used. 24-Hydroxycholesterol and 27-hydroxycholesterol were found to be associated mainly with HDL and LDL, whereas 3beta-hydroxy-5-cholestenoic acid was found predominantly in the lipoprotein-free fraction. While both 24-hydroxycholesterol and 27-hydroxycholesterol are present mainly in esterified form in plasma, 3beta-hydroxy-5-cholestenoic acid was present as free acid only. For reasons of comparison, a number of other oxysterols were determined in plasma and in isolated lipoprotein fractions. Significant amounts of these oxysterols were formed by cholesterol autoxidation during fractionation of plasma. It was therefore not possible to calculate the distribution of these oxysterols in the different plasma fractions. The present results are consistent with our previous finding that the less polar cholesterol metabolite 27-hydroxycholesterol competes with cholesterol for transport out of cells using HDL as an acceptor molecule, whereas the transport of the more polar compound 3beta-hydroxy-5-cholestenoic acid is facilitated by albumin.

An animal model to study local oxidation of LDL and its biological effects in the arterial wall

Oxidized LDL (oxLDL) is present in atherosclerotic lesions and is believed to play a key role in atherogenesis. Mainly on the basis of cell culture studies, oxLDL has been shown to produce many biological effects that influence the atherosclerotic process. To study LDL oxidation in vivo, we have established a model in which Sprague-Dawley rats are given a single injection of unmodified human LDL (> or = 4 mg/kg body weight). Within 6 hours, an accumulation of apolipoprotein B and epitopes present on oxLDL are detected in the arterial endothelium and media. The presence of oxLDL is associated with activation of the transcription factor nuclear factor-kappaB in the endothelium as well as endothelial expression of intercellular adhesion molecule-1. Injection of LDL enriched with the antioxidant probucol resulted in arterial accumulation of apolipoprotein B, but the expression of oxLDL-specific epitopes was reduced at 24 hours. Thus, this simple model has the potential to analyze the mechanisms behind and biological effects of LDL oxidation in vivo.

Sterol 27-hydroxylase- and apoAI/phospholipid-mediated efflux of cholesterol from cholesterol-laden macrophages: evidence for an inverse relation between the two mechanisms

Cholesterol-laden, human monocyte-derived macrophages were found to contain 27-hydroxycholesterol in proportion to their content of cholesterol ester. In accordance with previous work with human lung alveolar macrophages, there was a significant efflux of 27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid from the cultured cells. The efflux of 27-hydroxycholesterol was proportional to the cellular content of this steroid. Incubation of cholesterol-laden macrophages with reconstituted discoidal complexes made from apolipoprotein A-I and phospholipids resulted in a decrease in total cellular cholesterol, an increase in the efflux of free cholesterol, and a concomitant decrease in the total production and efflux of 27-oxygenated steroids, in particular, 3beta-hydroxy-5-cholestenoic acid. Reconstituted discoidal complexes with the Milano variant of apolipoprotein A-I gave virtually identical results, whereas high density lipoprotein was less efficient. These results suggest that cultured cholesterol-laden cells can export some of their excess cholesterol in the form of 27-hydroxycholesterol, 3beta-hydroxy-5-cholestenoic acid, and free cholesterol. In the presence of exogenous cholesterol acceptors, export of free cholesterol becomes more effective, resulting in less cholesterol exported via the 27-hydroxylase pathway. The balance between the two mechanisms for removal of cholesterol from macrophages may be of importance for formation of foam cells and development of atherosclerosis.

Elimination of cholesterol in macrophages and endothelial cells by the sterol 27-hydroxylase mechanism. Comparison with high density lipoprotein-mediated reverse cholesterol transport

Cultured macrophages and endothelial cells have been reported to secrete 27-oxygenated metabolites of cholesterol. This mechanism was compared with the classical high density lipoprotein (HDL)-dependent reverse cholesterol transport. Under standard conditions, macrophage preparations had considerably higher capacity to secrete 27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid than had endothelial cells and fibroblasts. Western blotting showed that lung macrophages contained the most sterol 27-hydroxylase protein of the cells tested. The relative amounts of 3beta-hydroxy-5-cholestenoic acid produced by the macrophages were also highest. Macrophages derived from monocytes of patients with sterol 27-hydroxylase deficiency did not secrete 27-oxygenated products, demonstrating that sterol 27-hydroxylase is the critical enzyme for the conversion of cholesterol into the 27-oxygenated steroids. That sterol 27-hydroxylase is responsible not only for 27-hydroxylation of cholesterol but also for the further oxidation of this steroid into 3beta-hydroxy-5-cholestenoic acid was shown with use of tritium-labeled 27-hydroxycholesterol and an inhibitor of sterol 27-hydroxylase. Secretion of 27-oxygenated products by the cultured macrophages as well as the ratio between the alcohol and the acid appeared to be dependent upon total 27-hydroxylase activity, the availability of substrate cholesterol, and the presence of an acceptor for 27-hydroxycholesterol in the medium. With albumin as extracellular acceptor, the major secreted product was 3beta-hydroxy-5-cholestenoic acid. Under such conditions, secretion of labeled 27-oxygenated products was higher than that of labeled cholesterol from lung alveolar macrophages preloaded with [4-14C]cholesterol. With HDL as acceptor, 27-hydroxycholesterol was the major secreted product, and the total secretion of labeled 27-oxygenated products was only about 10% of that of labeled cholesterol. Thus, 27-hydroxycholesterol and cholesterol may compete for HDL-mediated efflux from the cells. The results support the contention that the sterol 27-hydroxylase-mediated elimination of cholesterol is more important in macrophages than in endothelial cells. This mechanism may be an alternative and/or a complement to the classical HDL-mediated reverse cholesterol transport in macrophages, in particular when the concentration of HDL is low.

Ca2+ channel blockers verapamil and nifedipine inhibit apoptosis induced by 25-hydroxycholesterol in human aortic smooth muscle cells

We have characterized the death of human aortic smooth muscle cells induced by 25-hydroxycholesterol, an oxidation product of cholesterol. Chromatin condensation characteristic of apoptosis was observed by enzymatic (TUNEL) staining of chromatin, and by electron microscopy. Fourteen percent of cells treated with 5 microg/ml of 25-hydroxycholesterol for 24 h displayed chromatin degradation as determined by positive TUNEL staining. Addition of TNF alpha (10 ng/ml) and IFN gamma (20 ng/ml) increased the proportion of TUNEL positive cells to 30%, whereas the cytokines alone were without effect. After 48 h, 40% of the cells treated with 5 microg/ml of 25-hydroxycholesterol were TUNEL positive, and 21% of the cells displayed chromatin condensation. Oligonucleosomal DNA fragmentation typical of apoptosis was demonstrated by agarose gel electrophoresis. Furthermore, activation of the ICE-like protease caspase 3 (CPP32) was observed in cells treated with 25-hydroxycholesterol. Addition of the Ca2+ entry blockers verapamil or nifedipine to the culture medium inhibited apoptosis by more than 70% and reduced cytotoxicity, while removal of Ca2+ from culture medium reduced apoptosis by 42%. Within a few minutes after addition, 25-hydroxycholesterol induced intracellular Ca2+ oscillations with a frequency of approximately 0.3-0.4 min(-1). Thus it appears that Ca2+ influx through plasma membrane channels is an important signal in oxysterol-induced apoptosis. Addition of TNF alpha and IFN gamma enhanced cytotoxicity and resulted in a higher proportion of apoptotic cells, suggesting that inflammatory cytokines can increase the cytotoxicity of lipid oxidation products.

Effect of the oxidation state of LDL on the modulation of arterial vasomotor response in vitro

Although it is established that highly oxidized LDL modify both vasodilator and vasoconstrictor responses in normal and atherosclerotic arterial tissue, there is a paucity of data on the relationship between the degree of the oxidative modification of LDL and vasomotor response. We therefore compared the impact of native LDL (Nat-LDL), and of partially (P-oxLDL), of moderately (M-oxLDL) and of highly oxidized LDL (H-oxLDL) on the vasomotor response of isolated human internal mammary artery and of rat thoracic aorta. Copper-mediated oxidative modification for up to 24 h at 37 degrees C was characterised by a progressive increase in the net negative electrical charge of LDL, and in the content of oxysterols; by contrast, lipid hydroperoxide and TBARS content peaked in M-oxLDL at 6 h. Neither basal vascular tone nor vasoconstriction induced by KCl (100 mmol/l) were modified significantly in arterial segments in relation to the degree of LDL oxidation. While Nat-LDL did not modify the contractile response of rat aorta to norepinephrine, increase in the degree of oxidative modification of LDL progressively and significantly shifted the norepinephrine response curve to the right (EC50 values for Nat-LDL, M-oxLDL and H-oxLDL: 1.2+/-0.5x10(-8), 3.5+/-1x10(-7), 1.3+/-0.4x10(-6) mol/l respectively) with reduction in the maximal effect (74.5+/-12.2 and 100.1+/-6.2% for H-oxLDL and M-oxLDL respectively, P < 0.05 versus controls). Similar findings were made in human arteries treated with H-oxLDL (P < 0.05 for EC50 and maximal response versus controls). The acetylcholine-induced, endothelial-dependent relaxation of rat aortic segments was significantly and progressively impaired with increase in the degree of LDL oxidation, maximal relaxation with H-oxLDL being 3-fold less (P < 0.05) than Nat-LDL at the same protein concentration (100 microg/ml). Acetylated LDL was without effect. Our data indicate that the increase in the degree of copper-mediated, oxidative modification of LDL parallels progressive reduction in the vasomotor response of the arterial wall to norepinephrine-induced contraction and to acetylcholine-induced relaxation subsequent to precontraction. Our data are consistent with the hypothesis that the major oxysterols (7-ketocholesterol, 7beta-hydroxycholesterol) present in Ox-LDL underlie such effects.

Vitamin C reduces cholesterol-induced microcirculatory changes in rabbits

The microcirculation was studied for 10 weeks in untreated rabbits (n = 12) and in rabbits treated with vitamin C in their drinking water (0.5 g/d; n = 6), a 1% cholesterol diet (n = 12), or a combination of the two treatments (n = 11). The studies were performed by direct intravital microscopic imaging of the conjunctiva of both eyes to evaluate blood flow velocity, microvessel diameter, and microhemorheologic conditions. As we reported previously, changes occurred in all of the aforementioned variables as a consequence of cholesterol feeding. After 3 and 6 weeks of feeding, there was a marked and significant (P < .0001) decrease in blood flow velocity in third-order arterioles, which was accompanied by stasis and erythrocyte aggregation in the smaller conjunctival vessels. When cholesterol treatment was combined with vitamin C, blood flow was almost identical to that of controls and significantly (P < .0001) higher than that of rabbits treated with cholesterol alone. All other changes were also significantly reduced by the addition of vitamin C treatment to the cholesterol diet. Cholesterol-treated rabbits developed macroscopic arterial lesions that were not significantly reduced by vitamin C treatment. Neither circulating oxysterol levels nor atheromas were reduced by vitamin C treatment, which also had no significant effect on lipid or circulating vitamin E levels. We have previously shown that the lipid-soluble antioxidant BHT is able to prevent both cholesterol-induced microcirculatory changes and the development of arterial lesions in rabbits. This phenomenon is compatible with a critical oxidation step occurring in the lipid phase that is common to both processes. The finding that microcirculatory changes can be prevented by a water-soluble antioxidant is compatible with a role for water-soluble oxidants in this context. The possibility is discussed that vitamin C might also be important for the microcirculation in humans.

Vitamin A concentration in the liver decreases with age in patients with cystic fibrosis

BACKGROUND

Vitamin A deficiency is a common manifestation in cystic fibrosis (CF), but high levels of vitamin A in the liver have also been described. High levels of vitamin A in the liver are toxic, while normal levels might be protective against liver damage. In order to investigate whether liver damage in patients with CF is related to vitamin A content of the liver, vitamin A status was investigated in 15 patients with CF aged 8 to 34 years.

METHODS

Liver biopsy was performed on clinical indication and the vitamin A concentration in the liver was determined as retinylpalmitate. Serum levels of retinol and retinol-binding protein were investigated on the morning of the biopsy. Eight patients had morphologic signs of cirrhosis. Eight patients had been on treatment with ursodeoxycholic acid for 1 to 3 years. All but three patients had been on vitamin A supplementation for years.

RESULTS

Five patients had serum concentrations of retinol below the reference range and seven patients had decreased serum levels of retinol-binding protein. There was a strong correlation between serum levels of retinol and retinol-binding protein (rs = 0.90, p = 0.01), but no correlations with age, Shwachamn score, or genotype. Six of the patients had vitamin A concentrations in the liver < 40 micrograms/g wet weight, and the concentrations decreased significantly with age (rs = 0.77, p = 0.01), without correlation to clinical score or liver disease. There was no indication of hypervitaminosis, although younger patients had been or were being treated with vitamin A in fat-water emulsion.

CONCLUSIONS

Our results indicate that the risk of vitamin a deficiency in cystic fibrosis increases with age. The data do not support the view that patients are at risk for hypervitaminosis by long-term supplementation with vitamin A. No correlation was found between the severity of liver disease and the vitamin A content in the liver.

Localization of sterol 27-hydroxylase immuno-reactivity in human atherosclerotic plaques

It has recently been shown that extrahepatic cells can eliminate intracellular cholesterol by enzymatic conversion into 27-hydroxy-cholesterol and 3 beta-hydroxy-5-cholestenoic acid. Using immunohistochemical methods, we studied the presence of the enzyme responsible for these conversions, sterol 27-hydroxylase, in human carotid atherosclerotic plaques. All plaques examined were found to contain sterol 27-hydroxylase immuno-reactive cells. While some endothelial cells stained for sterol 27-hydroxylase, the majority of the immunoreactive cells co-localized with macrophages. Accumulation of sterol 27-hydroxylase-positive cells were often observed in macrophage-rich core regions of complicated lesions. High concentrations of 27-hydroxycholesterol were found in plaques, while the concentration in non-atherosclerotic human vessels was lower by two orders of magnitude. The rabbit, which is particularly sensitive to dietary cholesterol and easily develops fatty streaks, had low plasma levels of 27-hydroxycholesterol, 3 ng/ml compared to 150 ng/ml in humans. The concentration of 27-hydroxycholesterol in the atherosclerotic rabbit vessels was also lower compared to human atherosclerotic plaques. The results are consistent with our hypothesis that sterol 27-hydroxylase may be utilized by human macrophages as a defence towards a high cholesterol load. This mechanism may be less important in some other species.

Lipoprotein oxidation and progression of carotid atherosclerosis

BACKGROUND

Epidemiological studies and animal experiments have provided evidence supporting the role of lipid peroxidation in atherogenesis and cardiovascular diseases. Direct evidence linking lipid oxidation to atherosclerotic progression in humans, however, has been lacking. We investigated the association of lipid oxidation products with the progression of early carotid atherosclerosis in hypercholesterolemic men from eastern Finland.

METHODS AND RESULTS

Twenty subjects with a fast progression and 20 with no progression of carotid atherosclerosis in 3 years were selected from > 400 participants in the Kuopio Atherosclerosis Prevention Study. Progression of carotid atherosclerosis was assessed by high-resolution B-mode ultrasonography. Serum 7 beta-hydroxycholesterol, a major oxidation product of cholesterol in membranes and lipoproteins, and seven other cholesterol oxidation products were measured by isotope dilution-mass spectrometry, lipid hydroperoxides in LDL fluorometrically as thiobarbituric acid-reactive substances (TBARS) and oxidation susceptibility of LDL and VLDL kinetically. High concentrations of serum 7 beta-hydroxycholesterol (beta = 47, P = .0005), cigarette smoking (beta = .35, P = .0167), and LDL TBARS (beta = .23, P = .0862) and an increased oxidation susceptibility of VLDL + LDL (beta = .22 P = .1114) were the strongest predictors of a 3-year increase in carotid wall thickness of more than 30 variables tested in step-up least-squares regression models. A 10-variable model explained 60% of the atherosclerotic progression. In a multivariate logistic model, the risk of experiencing a fast progression increased by 80% (P = .013) per unit (microgram/L) of 7 beta-hydroxycholesterol.

CONCLUSIONS

The findings of this study provide further evidence to support an association between lipid oxidation and atherogenesis in humans.

Formation of oxysterols during oxidation of low density lipoprotein by peroxynitrite, myoglobin, and copper

Oxidation of low density lipoprotein (LDL) in the artery wall leads to the formation of cholesterol oxidation products that may result in cytotoxicity. Different mechanisms could contribute to LDL oxidation in vivo resulting in characteristic and specific modification of the cholesterol molecule. Alternatively, attack on cholesterol by chain propagating peroxyl radicals could result in the same distribution of oxidation products irrespective of the initial pro-oxidant mechanism. To distinguish between these possibilities we have monitored the formation of nine oxysterols during LDL oxidation, promoted by copper, myoglobin, peroxynitrite, or azo bis amidino propane. Regardless of the oxidant used, the pattern of oxysterol formation was essentially the same. The yields of products identified decreased in the order 7-oxocholesterol > 7 beta-hydroxycholesterol > 7 alpha-hydroxycholesterol > 5,6 beta-epoxycholesterol > 5,6 alpha-epoxycholesterol except in the case of peroxynitrite in which case a higher yield of 5, 6 beta-epoxycholesterol relative to 7-oxocholesterol was found. No formation of cholestane 3 beta, 5 alpha, 6 beta-triol, or the 24-,25-,27-hydroxycholesterols was seen. Concentration of 7-oxocholesterol levels in LDL was positively correlated with the degree of protein modification. Endogenous alpha-tocopherol in LDL or supplementation with butylated hydroxytoluene prevented oxysterol formation. Taken together these data indicate that the oxidation of cholesterol and protein in LDL occur as secondary oxidation events consequent on the attack of fatty acid peroxyl/alkoxyl radicals on the 7-position of cholesterol, and with amino acids on apoB. Furthermore, oxidant processes with atherogenic potential, such as peroxynitrite, copper, and myoglobin are capable of producing oxidized LDL containing cytotoxic mediators.

Cholesterol homeostasis in human brain: evidence for an age-dependent flux of 24S-hydroxycholesterol from the brain into the circulation

We have investigated whether side chain-hydroxylated cholesterol species are important for elimination of cholesterol from the brain. Plasma concentrations of 24-hydroxycholesterol (24-OH-Chol) in the internal jugular vein and the brachial artery in healthy volunteers were consistent with a net flux of this steroid from the brain into the circulation, corresponding to elimination of approximately 4 mg cholesterol during a 24-h period in adults. Results of experiments with rats exposed to 18O2 were also consistent with a flux of 24-OH-Chol from the brain into the circulation. No other oxysterol measured showed a similar behavior as 24-OH-Chol. These results and the finding that the concentration of 24-OH-Chol was 30- to 1500-fold higher in the brain than in any other organ except the adrenals indicate that the major part of 24-OH-Chol present in the circulation originates from the brain. Both the 24-OH-Chol present in the brain and in the circulation were the 24S-stereoisomer. In contrast to other oxysterols, levels of plasma 24-OH-Chol were found to be markedly dependent upon age. The ratio between 24-OH-Chol and cholesterol in plasma was approximately 5 times higher during the first decade of life than during the sixth decade. There was a high correlation between levels of 24-OH-Chol in plasma and cerebrospinal fluid. It is suggested that the flux of 24-OH-Chol from the brain is important for cholesterol homeostasis in this organ.

The oxysterols cholest-5-ene-3 beta,4 alpha-diol, cholest-5-ene-3 beta,4 beta-diol and cholestane-3 beta,5 alpha,6 alpha-triol are formed during in vitro oxidation of low density lipoprotein, and are present in human atherosclerotic plaques

Isolated human low density lipoprotein (LDL) was oxidized with either cupric ions or soybean lipoxygenase and linoleic acid. Cholesterol oxidation products (oxysterols) were determined by isotope dilution gas chromatography-mass spectrometry. A new cholestane-3,5,6-triol isomer, cholestane-3 beta,5 alpha,6 alpha-triol, which has not previously been recognized as a cholesterol autoxidation product, was found at similar concentrations as the well-known cytotoxic cholestane-3 beta,5 alpha,6 beta-triol during both copper- and lipoxygenase-mediated LDL oxidation. Furthermore, two epimeric cholest-5-ene-3 beta,4-diols were identified in the oxidized LDL at similar concentrations. These two isomers were also identified in human atherosclerotic tissue in a ratio of 1:1 at a concentration more than 10-times higher than in non-atherosclerotic vessels. In vitro oxidation of LDL under an 18O2 atmosphere revealed that molecular oxygen was the only source of the oxygen functions at C-4 in the cholest-5-ene-3 beta,4-diols. Taken together, these findings suggest that the cholest-5-ene-3 beta,4-diols in atherosclerotic plaques are formed by autoxidation.

Importance of a novel oxidative mechanism for elimination of intracellular cholesterol in humans

We have recently demonstrated that cultured human alveolar macrophages efficiently convert cholesterol into excretable 27-oxygenated products. We show here that increasing the intracellular concentration of cholesterol by a factor of 10 leads to about a twofold increase in the excretion of 27-oxygenated products from cultured macrophages. Inhibition of the sterol 27-hydroxylase caused a significant intracellular accumulation of cholesterol. A direct comparison was made between flux of cholesterol and 27-oxygenated products from macrophages preloaded with [4-14C]cholesterol. Under the specific conditions employed with fetal calf serum in the culture medium, the flux of 27-oxygenated products was about 10% of that of cholesterol. Since the sterol 27-hydroxylase, which converts cholesterol to 27-oxygenated products, is present in many cell types, we suggest that 27-oxygenation is a general mechanism for removal of intracellular cholesterol. To evaluate this hypothesis, we measured the net uptake by the human liver of circulating 27-oxygenated products, which was found to be about 20 mg/24 h. This uptake corresponds to approximately 4% of the bile acid production, assuming quantitative conversion into bile acids. It is concluded that the 27-hydroxylase pathway is of significance for elimination of extrahepatic cholesterol.

Oxysterols present in atherosclerotic tissue decrease the expression of lipoprotein lipase messenger RNA in human monocyte-derived macrophages

The presence of oxysterols in macrophages isolated from atherosclerotic tissue and the effect of oxysterols on the regulation of lipoprotein lipase (LPL) mRNA were studied. Both rabbit and human macrophages, freshly isolated from atherosclerotic aorta, show about the same distribution of oxysterols, analyzed by isotope dilution mass spectrometry, except that all three preparations of human arterial-derived macrophages contained high levels of 27-hydroxycholesterol, which was not found in rabbit macrophages. To determine if oxysterols regulate LPL expression, human monocyte-derived macrophages were incubated with different oxysterols. Incubation with 7 beta-hydroxycholesterol and 25-hydroxycholesterol resulted in a 70-75% reduction of LPL mRNA, analyzed by quantitative RT-PCR. Cholesterol and other tested oxysterols showed no effect on macrophage LPL mRNA expression compared with control. LPL activity in the medium was also reduced after exposure of the macrophages to 7 beta-hydroxycholesterol and 25-hydroxycholesterol. In conclusion, we have demonstrated accumulation of oxysterols in macrophage-derived foam cells isolated from atherosclerotic aorta. There was suppression of LPL mRNA in human monocyte-derived macrophages after incubation with 7 beta-hydroxycholesterol and 25-hydroxycholesterol. It is tempting to suggest that an exposure to oxysterols may explain our earlier observation of a low level of LPL mRNA in arterial foam cells.

Clofibrate treatment increases stearoyl-CoA desaturase mRNA level and enzyme activity in mouse liver

Stearoyl-CoA desaturase activity is encoded by two highly homologous genes, SCD1 and SCD2, which show tissue-specific expression and regulation. SCD1, which is expressed in the liver, showed a marked diurnal variation with the highest expression during the feeding period. Treatment of mice with the peroxisome proliferator clofibrate, which induces several lipid metabolizing enzymes, increased both the enzyme activity and mRNA level in the liver, indicating regulation at the transcriptional level. The highest expression of both SCD1 and SCD2 was found in brown adipose tissue, which was slightly down-regulated by feeding a fat-free diet.

Time course of oxysterol formation during in vitro oxidation of low density lipoprotein

Cholesterol oxidation products (oxysterols) have been implicated in several aspects of atherogenesis; they affect key enzymes in cholesterol homeostasis, induce calcification in vascular cells and possess cytotoxic properties. Oxysterols are formed during oxidative modification of low density lipoprotein (LDL). Using a recently developed method based on isotope dilution-mass spectrometry, the kinetics of formation of oxysterols during oxidation of LDL by cupric ions or soybean lipoxygenase was studied. The same products, mainly 7- and 5-oxygenated cholesterol, were formed by the two oxidation methods. Virtually no side-chain oxidized oxysterols were formed. During the oxidations, preferentially esterified cholesterol was consumed and consumption of polyunsaturated fatty acids and formation of conjugated dienes preceded the appearance of oxysterols. Cholesterol 7-hydroperoxides potential cytotoxins, were present in LDL oxidized by copper or lipoxygenase.

Determination of cholesterol oxidation products in human plasma by isotope dilution-mass spectrometry

A method based on isotope dilution-mass spectrometry was developed for the determination of nine cholesterol oxidation products in human plasma. The cholesterol oxidation products determined were cholest-5-ene-3 beta,7 alpha-diol, cholest-5-ene-3 beta,7 beta-diol (7 alpha- and 7 beta-hydroxycholesterol, respectively), 3 beta-hydroxycholest-5-en-7-one(7-oxocholesterol),5,6 alpha-epoxy-5 alpha- cholestan-3 beta-ol (cholesterol-5 alpha,6 alpha-epoxide),5,6 beta-epoxy-5 beta-cholestan-3 beta-ol (cholesterol-5 beta,6 beta-epoxide), (cholesterol-5 beta,6 beta-epoxide), cholestane-3 beta,5 alpha,6 beta-triol, cholest-5-ene-3 beta,24-diol (24-hydroxycholesterol), cholest-5-ene-3 beta,25-diol (25-hydroxycholesterol), and cholest-5-ene-3 beta,27-diol (27-hydroxycholesterol). A corresponding deuterium-labeled internal standard, containing 3 to 6 deuterium atoms, was synthesized for each cholesterol oxidation product except 5 beta,6 beta-epoxycholesterol which was determined using the internal standard for 5 alpha,6 alpha-epoxycholesterol. Plasma from 31 healthy volunteers was analyzed by the new method and 27-, 24-, and 7 alpha-hydroxycholesterol were the most abundant cholesterol oxidation products (mean values 154, 64, and 43 ng/ml, respectively). The other oxysterols determined were present in concentrations lower than 30 ng/ml. Males had higher 27-hydroxycholesterol concentrations in plasma than females. The 5,6-oxygenated products were present mainly unesterified while the other oxidation products were mostly in esterified form.