Circulating markers for biosynthesis of cholesterol and bile acids are not depressed in asymptomatic gallstone subjects

Non-Cholesterol Sterol Concentrations as Biomarkers for Cholesterol Absorption and Synthesis in Different Metabolic Disorders: A Systematic Review

Non-cholesterol sterols are validated biomarkers for intestinal cholesterol absorption and endogenous cholesterol synthesis. However, their use in metabolic disturbances has not been systematically explored. Therefore, we conducted a systematic review to provide an overview of non-cholesterol sterols as markers for cholesterol metabolism in different metabolic disorders. Potentially relevant studies were retrieved by a systematic search of three databases in July 2018 and ninety-four human studies were included. Cholesterol-standardized levels of campesterol, sitosterol and cholestanol were collected to reflect cholesterol absorption and those of lathosterol and desmosterol to reflect cholesterol synthesis. Their use as biomarkers was examined in the following metabolic disorders: overweight/obesity (n = 16), diabetes mellitus (n = 15), metabolic syndrome (n = 5), hyperlipidemia (n = 11), cardiovascular disease (n = 17), and diseases related to intestine (n = 16), liver (n = 22) or kidney (n = 2). In general, markers for cholesterol absorption and synthesis displayed reciprocal patterns, showing that cholesterol metabolism is tightly regulated by the interplay of intestinal absorption and endogenous synthesis. Distinctive patterns for cholesterol absorption or cholesterol synthesis could be identified, suggesting that metabolic disorders can be classified as 'cholesterol absorbers or cholesterol synthesizers'. Future studies should be performed to confirm or refute these findings and to examine whether this information can be used for targeted (dietary) interventions.

Gallbladder bile supersaturated with cholesterol in gallstone patients preferentially develops from shortage of bile acids

Gallstone (GS) formation requires that bile is supersaturated with cholesterol, which is estimated by a cholesterol saturation index (CSI) calculated from gallbladder (GB) total lipids and the mol% (mole percent) of bile acids (BAs), cholesterol, and phospholipids (PLs). Whereas CSI indicates GS risk, we hypothesized that additional comparisons of GB lipid mol% data are inappropriate to identify why CSI is increased in GS disease. We anticipated that GB lipid mmol/l (millimole per liter) levels should instead identify that, and therefore retrieved GB mmol/l data for BAs, cholesterol, and PLs from a study on 145 GS and 87 GS-free patients and compared them with the corresponding mol% data. BA and PL mmol/l levels were 33% and 31% lower in GS patients, while cholesterol was unaltered. CSI was higher in GS patients and correlated inversely with GB levels of BAs and PLs, but not with cholesterol. A literature search confirmed, in 13 studies from 11 countries, that GB BA levels and, to a certain extent, PLs are strongly reduced in GS patients, while cholesterol levels are not elevated. Our findings show that a shortage of BAs is a major reason why GB bile is supersaturated with cholesterol in GS patients. These results are sustainable because they are also valid from a global perspective.

Acute caloric restriction counteracts hepatic bile acid and cholesterol deficiency in morbid obesity

BACKGROUND

Bile acid (BA) synthesis is regulated by BA signalling in the liver and by fibroblast growth factor 19 (FGF19), synthesized and released from the intestine. In morbid obesity, faecal excretion and hepatic synthesis of BAs and cholesterol are strongly induced and caloric restriction reduces their faecal excretion considerably. We hypothesized that the high intestinal food mass in morbidly obese subjects promotes faecal excretion of BAs and cholesterol, thereby creating a shortage of both BAs and cholesterol in the liver.

METHODS

Ten morbidly obese women (BMI 42 ± 2.6 kg m^-2 ) were monitored on days 0, 3, 7, 14 and 28 after beginning a low-calorie diet (800-1100 kcal day^-1 ). Serum was collected and liver size and fat content determined. Synthesis of BAs and cholesterol was evaluated from serum markers, and the serum levels of lipoproteins, BAs, proprotein convertase subtilisin/kexin type 9 (PCSK9), insulin, glucose and FGF19 were monitored. Fifty-four nonobese women (BMI <25 kg m^-2 ) served as controls.

RESULTS

At baseline, synthesis of both BAs and cholesterol and serum levels of BAs and PCSK9 were elevated in the obese group compared to controls. Already after 3 days on a low-calorie diet, BA and cholesterol synthesis and serum BA and PCSK9 levels normalized, whereas LDL cholesterol increased. FGF19 and triglyceride levels were unchanged, and liver volume was reduced by 10%.

CONCLUSIONS

The results suggest that hepatic BAs and cholesterol are deficient in morbid obesity. Caloric restriction rapidly counteracts these deficiencies, normalizing BA and cholesterol synthesis and circulating PCSK9 levels, indicating that overproduction of cholesterol in enlarged peripheral tissues cannot explain this phenotype. We propose that excessive food intake promotes faecal loss of BAs and cholesterol contributing to their hepatic deficiencies.

Age-associated alterations in cholesterol homeostasis: evidence from a cross-sectional study in a Northern Italy population

BACKGROUND

The modifications of cholesterol metabolism associated with aging are ill-defined. The objective of this study was to define age-associated alterations of the different metabolic pathways controlling cholesterol homeostasis by analyzing circulating sterols.

METHODS

We analyzed serum samples collected from 201 adult (75 male, 126 female) subjects within the epidemiological MICOL study (Multicentrica Italiana Colelitiasi). The age range was 38-79 years; 103 had evidence of gallstones. The concentrations of the different sterols, recognized as markers of the main pathways of cholesterol homeostasis, were analyzed by gas chromatography-mass spectrometry, including lathosterol (synthesis), campesterol and sitosterol (absorption), and 7α-hydroxy-4-cholesten-3-one (degradation to bile acids).

RESULTS

A significant direct correlation was detected between age and cholesterol levels (r =0.34, P<0.01). The lathosterol/cholesterol ratio was lower in older age quartiles (P<0.05 by analysis of variance), with an inverse correlation between the lathosterol/cholesterol ratio and age (r=-0.32, P<0.01). Such correlation was particularly evident in females. The campesterol/cholesterol and sitosterol/cholesterol ratios were inversely correlated with aging in control, but not in gallstone patients. The levels of 7α-hydroxy-4-cholesten-3-one were not correlated with age.

CONCLUSION

These data show a reduction of cholesterol synthesis with aging which is associated with increased circulating cholesterol levels. The finding might be related to a reduced metabolic need for cholesterol in advancing age, leading to a downregulation of the main mechanisms of cholesterol intake in the liver. A different age-related behavior was observed in gallstone-free versus gallstone patients regarding cholesterol absorption. The possible implications in terms of the pharmacological management of hypercholesterolemia in the elderly remain to be defined.

Pronounced variation in bile acid synthesis in humans is related to gender, hypertriglyceridaemia and circulating levels of fibroblast growth factor 19

BACKGROUND

Bile acid (BA) synthesis is essential in cholesterol and lipid homoeostasis.

METHODS

Serum samples from 435 normal and 23 cholecystectomized subjects were obtained after overnight fasting and assayed for markers of BA and cholesterol synthesis, as well as cholesterol absorption. We determined whether BA synthesis was related to fibroblast growth factor 19 (FGF19; a circulating metabolic regulator that is thought to inhibit BA synthesis), gender, age and serum lipids.

RESULTS

Bile acid synthesis varied more than 9-fold in normal individuals and was 29% higher in men than in women. Whilst low-density lipoprotein cholesterol increased with age, BA and cholesterol synthesis were stable. BA production was positively correlated with serum triglycerides (TGs), and 35% of individuals with a high level (>95th percentile) of BA synthesis had hypertriglyceridaemia (HTG) (>95th percentile). Serum FGF19 levels varied by 7-fold in normal individuals and were related inversely to BA synthesis but were not related to gender, plasma lipids or history of cholecystectomy.

CONCLUSIONS

Bile acid synthesis has a wide inter-individual variation, is lower in women than in men and is correlated positively with serum TGs. High BA production is frequently linked to HTG. Age-related hypercholesterolaemia is not associated with changes in BA or cholesterol production, nor to an increase in cholesterol absorption. In humans, the circulating level of FGF19 may regulate hepatic BA production under fasting conditions.

Biliary sphincterotomy does not relate to diarrhoea or major changes in bile acid synthesis or plasma lipids

OBJECTIVES

Bile acid (BA) malabsorption may occur after cholecystectomy. Bile may flow more freely into the duodenum after endoscopic sphincterotomy (EST), in part resembling the situation after cholecystectomy. The (75)SeCHAT test used to diagnose BA malabsorption correlates inversely with synthesis and faecal excretion of BAs. The BA intermediate 7alpha-hydroxy-4-cholesten-3-one (C4) mirrors BA and lathosterol cholesterol synthesis. The aim was to study whether EST causes BA diarrhoea and alterations in BA synthesis or lipid profiles.

MATERIAL AND METHODS

Twelve patients underwent the (75)SeHCAT test prior to and 3 months after undergoing EST and a further 22 only after EST. The Gastrointestinal Symptom Rating Scale (GSRS), 1 week daily stool frequency and consistency, C4, lathosterol, cholesterol and triglycerides were investigated. The (75)SeHCAT values of 29 healthy subjects served as controls.

RESULTS

Stool frequency (median 1/day, IQR (interquartile range): 0.7) and consistency (median: 3, IQR: 0.65) were normal and none reported diarrhoea after EST (n=34). The GSRS scores were normal. There was no significant change in (75)SeHCAT (median 22%, IQR 29% versus 19.5%, IQR 25, n=12). There was a trend towards lower (75)SeHCAT after EST compared with the controls (median 26%, IQR 32, n=34 versus median 38%, IQR 19.5, n=29, p=0.075) and higher lathosterol (median 47.1 mg/mole, IQR 32.7 versus median 52.5 mg/mole, IQR 35.6, n=14, p=0.055). The C4 and lipids did not change significantly.

CONCLUSIONS

EST did not induce diarrhoea and in line with this BA synthesis and serum lipids are unaltered.

Fecal bile acid excretion and messenger RNA expression levels of ileal transporters in high risk gallstone patients

BACKGROUND

Cholesterol gallstone disease (GS) is highly prevalent among Hispanics and American Indians. In GS, the pool of bile acids (BA) is decreased, suggesting that BA absorption is impaired. In Caucasian GS patients, mRNA levels for ileal BA transporters are decreased. We aimed to determine fecal BA excretion rates, mRNA levels for ileal BA transporter genes and of regulatory genes of BA synthesis in Hispanic GS patients.

RESULTS

Excretion of fecal BA was measured in seven GS females and in ten GS-free individuals, all with a body mass index < 29. Participants ingested the stool marker Cr2O3 (300 mg/day) for 10 days, and fecal specimens were collected on the last 3 days. Chromium was measured by a colorimetric method, and BA was quantitated by gas chromatography/mass spectroscopy. Intake of calories, nutrients, fiber and cholesterol were similar in the GS and GS-free subjects. Mean BA excretion levels were 520 +/- 80 mg/day for the GS-free group, and 461 +/- 105 mg/day for the GS group. Messenger RNA expression levels were determined by RT-PCR on biopsy samples obtained from ileum during diagnostic colonoscopy (14 GS-free controls and 16 GS patients) and from liver during surgery performed at 8 and 10 AM (12 GS and 10 GS-free patients operated on for gastrointestinal malignancies), all with a body mass index < 29. Messenger RNA level of the BA transporter genes for ileal lipid binding protein, multidrug resistance-associated protein 3, organic solute transporter alpha, and organic solute transporter beta were similar in GS and GS-free subjects. Messenger RNA level of Cyp27A1, encoding the enzyme 27alpha-hydroxylase, the short heterodimer partner and farnesoid X receptor remained unchanged, whereas the mRNA level of Cyp7A1, the rate limiting step of BA synthesis, was increased more than 400% (p < 0.01) in the liver of GS compared to GS-free subjects.

CONCLUSION

Hispanics with GS have fecal BA excretion rates and mRNA levels of genes for ileal BA transporters that are similar to GS-free subjects. However, mRNA expression levels of Cyp7A1 are increased in GS, indicating that regulation of BA synthesis is abnormal in Hispanics with GS.

Reduced ileal expression of OSTalpha-OSTbeta in non-obese gallstone disease

Cholelithiasis is a multifactorial process, and several mechanisms have been postulated. A decreased expression of the ileal apical sodium-dependent bile acid transporter (ASBT) and of the cytosolic ileal lipid binding protein (ILBP) was recently described in female non-obese patients. The role of the recently identified organic solute transporters alpha and beta (OSTalpha, OSTbeta) in gallstone pathogenesis remains unclear. Therefore, we performed analysis of OSTalpha-OSTbeta in gallstone patients according to body weight. Ileal mucosal biopsies were collected during routine colonoscopy from female gallstone carriers (n = 19) and controls (n = 34). OSTalpha-OSTbeta mRNA expression was measured using the LightCycler sequence detection system; protein was analyzed by immunohistochemistry and Western blot. The mRNA expression of OSTalpha-OSTbeta was significantly reduced (OSTalpha: 3.3-fold, P = 0.006; OSTbeta: 2.6-fold, P = 0.03) in normal-weight but not overweight gallstone carriers compared with controls. OSTalpha-OSTbeta protein levels also showed a reduction by 40-67%. The expression of OSTalpha-OSTbeta correlated positively with ASBT (r = 0.65, 0.58, respectively), ILBP (r = 0.77, 0.67), and the farnesoid X receptor (r = 0.58, 0.50). Fibroblast growth factor-19 showed a 2.8-fold reduction (P = 0.06), and liver receptor homolog-1 showed a 2-fold reduction (P = 0.04) in non-obese patients. In conclusion, an impaired function of all three ileal bile acid transporters may lead to low ileal bile acid reabsorption and an altered bile acid pool composition and therefore may contribute to the formation of gallstones in non-obese patients.

Determination of key intermediates in cholesterol and bile acid biosynthesis by stable isotope dilution mass spectrometry

For more than a decade, we have developed stable isotope dilution mass spectrometry methods to quantify key intermediates in cholesterol and bile acid biosynthesis, mevalonate and oxysterols, respectively. The methods are more sensitive and reproducible than conventional radioisotope (RI), gas-chromatography (GC) or high-performance liquid chromatography (HPLC) methods, so that they are applicable not only to samples from experimental animals but also to small amounts of human specimens. In this paper, we review the development of stable isotope dilution mass spectrometry for quantifying mevalonate and oxysterols in biological materials, and demonstrate the usefulness of this technique.

Increased activity of hepatic microsomal triglyceride transfer protein and bile acid synthesis in gallstone disease

A strong interrelationship exists between the regulation of bile acid (BA) metabolism and hepatic very low density lipoprotein (VLDL) production. We have recently shown that BA synthesis is increased in gallstone disease. We investigated the activity of hepatic microsomal triglyceride transfer protein (MTTP) as a surrogate of VLDL production, BA synthesis, and mRNA expression levels of proteins that regulate fatty acid (FA) metabolism in the liver of gallstone (GS) patients compared with GS-free patients. Twenty-seven volunteers subjected to elective surgery; 9 were GS-free and 18 with GS agreed to have a liver biopsy. We quantified by a fluorescence assay the activity of MTTP and by quantitative reverse-transcription PCR (RT-PCR) the mRNA content of hepatic MTTP and genes that regulate hepatic sterol and FA metabolism. Plasma was assayed for lathosterol and 7alpha-hydroxy-4-cholesten-3-one. Liver histology was normal in GS and GS-free patients. Serum VLDL triglycerides and apoB were significantly increased in GS. Hepatic triglycerides tripled in GS (P<0.001) compared with GS-free. MTTP activity increased 70% (P<0.001). Serum lathosterol and hepatic cholesterol concentrations, and mRNA expressions of MTTP, CD36, and FABP1 were similar in GS-free and GS patients. Hepatic mRNA expression of hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) and 3-hydroxyl-3-methyl-glutaryl-CoA synthase (HMGS) were significantly decreased--40% and 27%, respectively--in GS. Serum 7alpha-hydroxy-4-cholesten-3-one was 75% higher, and mRNA expression of CYP7A1 was increased sevenfold (P<0.001) in GS.

CONCLUSION

Hepatic MTTP activity and BA synthesis are increased in GS. Results suggest that hepatic VLDL production and trafficking of BA are increased in gallstone patients.

Apical sodium bile acid transporter and ileal lipid binding protein in gallstone carriers

Although a cholesterol supersaturation of gallbladder bile has been identified as the underlying pathophysiologic defect, the molecular pathomechanism of gallstone formation in humans remains poorly understood. A deficiency of the apical sodium bile acid transporter (ASBT) and ileal lipid binding protein (ILBP) in the small intestine may result in bile acid loss into the colon and might promote gallstone formation by reducing the bile acid pool and increasing the amount of hydrophobic bile salts. To test this hypothesis, protein levels and mRNA expression of ASBT and ILBP were assessed in ileal mucosa biopsies of female gallstone carriers and controls. Neither ASBT nor ILBP levels differed significantly between gallstone carriers and controls. However, when study participants were subgrouped by body weight, ASBT and ILBP protein were 48% and 67% lower in normal weight gallstone carriers than in controls (P < 0.05); similar differences were found for mRNA expression levels. The loss of bile transporters in female normal weight gallstone carriers was coupled with a reduction of protein levels of hepatic nuclear factor 1alpha and farnesoid X receptor. In conclusion, in normal weight female gallstone carriers, the decreased expression of ileal bile acid transporters may form a molecular basis for gallstone formation.

Common polymorphisms in the CYP7A1 gene do not contribute to variation in rates of bile acid synthesis and plasma LDL cholesterol concentration

Transcriptional regulation of the cholesterol 7alpha-hydroxylase (CYP7AI) gene is of critical importance for bile acid and cholesterol metabolism. We evaluated the physiological significance of two common polymorphisms (-203C/A and -469T/C) in the promoter region of the CYP7AI gene. No evidence was found for physiological differences between either the -203C and -203A alleles or the -469T and -469C alleles in transient transfection studies using native 834bp promoter constructs. Moreover, no association was observed between the CYP7AI promoter polymorphisms and the hepatic cholesterol 7alpha-hydroxylase activity and parameters of bile acid synthesis rates, as analyzed in subjects with gallstone disease. In addition, no relationships were found between the promoter polymorphisms and plasma LDL cholesterol concentration in association studies conducted in three different groups of middle-aged Swedish men. Finally, near complete allelic association was found between the two promoter polymorphisms and the IVS6+363G/A polymorphism at the 3' end of the CYP7AI gene (|D'|=0.98), indicating strong linkage disequilibrium across the whole CYP7AI gene. It is concluded that common polymorphisms of the CYP7A1 gene do not contribute to variation in cholesterol 7alpha-hydroxylase activity, rates of bile acid synthesis and plasma LDL cholesterol concentration in humans.

Bile acid synthesis is increased in Chilean Hispanics with gallstones and in gallstone high-risk Mapuche Indians

BACKGROUND & AIMS

Gallstone disease is an important, costly health-care problem in Western societies. It is still unclear whether hepatic lipid regulatory enzymes play primary or secondary roles in gallstone formation. In this study, the aim was to investigate whether the synthesis of bile acids and cholesterol is increased in gallstone disease and to test whether such a metabolic change, if present, might occur before gallstone formation.

METHODS

A total of 125 Chilean Hispanic women (80 without gallstones and 45 with gallstones) matched for age and body mass index were investigated, along with 40 Chilean Mapuche Indian women (20 without gallstones and 20 with gallstones), a population group in which the prevalence for gallstone disease is very high. Fasting blood plasma samples were assayed for 7 alpha-hydroxy-4-cholesten-3-one and lathosterol, 2 strong indicators for hepatic bile acid and body cholesterol synthesis, respectively.

RESULTS

Plasma 7 alpha-hydroxy-4-cholesten-3-one levels, corrected for plasma cholesterol, were significantly increased by 50% in Hispanic women with gallstones as compared with gallstone-free Hispanics (P < 0.006). As compared with Hispanic women without gallstones, plasma 7 alpha-hydroxy-4-cholesten-3-one levels were increased by > or =100% (P < 0.002) in Mapuche Indian women, independently of whether gallstones were present. Plasma lathosterol, corrected for plasma cholesterol, was significantly increased by 22% in Hispanic women with gallstones and in Mapuche Indian women compared with Hispanic women.

CONCLUSIONS

The results indicate that the synthesis of bile acids and cholesterol is induced in gallstone disease and precedes gallstone development. These inductions presumably occur as a response to an increased intestinal loss of bile acids.

Monitoring hepatic cholesterol 7alpha-hydroxylase activity by assay of the stable bile acid intermediate 7alpha-hydroxy-4-cholesten-3-one in peripheral blood

We describe an accurate method for monitoring the enzymatic activity of hepatic cholesterol 7alpha-hydroxylase (C7alphaOH; CYP7A1), the rate-limiting and major regulatory enzyme in the synthesis of bile acids. Assay of 7alpha-hydroxy-4-cholesten-3-one (C4), an intermediate in bile acid synthesis, revealed that the level of C4 in peripheral blood serum or plasma showed a strong correlation to the enzymatic activity of hepatic C7alphaOH, both at steady-state conditions (r = 0.929) as well as during the rapid changes that occur during the diurnal phases. This assay should be of value in clarifying the regulation of bile acid synthesis in vivo in laboratory animals and humans since it allows for the monitoring of hepatic C7alphaOH activity using peripheral blood samples.

Effects of combined treatment with pravastatin and ursodeoxycholic acid on hepatic cholesterol metabolism

BACKGROUND

Treatment with ursodeoxycholic acid and also, to some degree, statins reduces cholesterol saturation of bile. The present study aimed [1] to study the effects of combined treatment with ursodeoxycholic acid and pravastatin on hepatic cholesterol metabolism and [2] to evaluate if the addition of pravastatin to ursodeoxycholic acid treatment has beneficial effects on the lipid composition of gallbladder bile in gallstone patients.

MATERIALS AND METHODS

Nineteen patients with cholesterol gallstones were subjected to combined treatment with ursodeoxycholic acid (500 mg bid) and pravastatin (20 mg bid) for three weeks before cholecystectomy. Eleven patients received ursodeoxycholic acid only and 20 untreated gallstone patients served as controls. Gallbladder bile was collected, and for both the patients receiving combined treatment and the controls a liver biopsy was also obtained peroperatively.

RESULTS

The cholesterol saturation of bile averaged 59% in the patients on combined treatment, 60% in the ursodeoxycholic acid-treated patients, and 130% in the untreated controls. In the patients receiving ursodeoxycholic acid, this bile salt constituted approximately 60% of all bile salts. The patients receiving combined treatment had reduced cholesterol synthesis, as reflected by a 45% reduction in serum lathosterol. The activity and the mRNA levels of cholesterol 7 alpha-hydroxylase and the mRNA levels for the low density lipoprotein-receptor were not significantly affected.

CONCLUSIONS

Pravastatin does not further reduce the cholesterol saturation of bile in gallstone patients treated with ursodeoxycholic acid, although hepatic cholesterol synthesis is inhibited. The study supports the important concept that de novo synthesized cholesterol is not particularly important for biliary cholesterol secretion in humans.

Chromosomal organization of candidate genes involved in cholesterol gallstone formation: a murine gallstone map

Epidemiologic and family studies indicate that cholesterol gallstone formation is in part genetically determined. The major contribution to our current understanding of gallstone genes derives from animal studies, particularly cross-breeding experiments in inbred mouse strains that differ in genetic susceptibility to cholesterol gallstone formation (quantitative trait loci mapping). In this review we summarize how the combined use of genomic strategies and phenotypic studies in inbred mice has proven to be a powerful means of dissecting the complex pathophysiology of this common disease. We present a "gallstone map" for the mouse, consisting of all genetic loci that have been identified to confer gallstone susceptibility as well as putative candidate genes. Translation of the genetic loci and genes between mouse and human predicts chromosomal regions in the human genome that are likely to harbor gallstone genes. Both the number and the precise understanding of gallstone genes are expected to further increase with rapid progress of the genome projects, and multiple new targets for early diagnosis and prevention of gallstone disease should become possible.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.